A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

At the cellular surface, many different receptors are expressed that allow cellular response to extracellular signals provided by the environment. After ligand binding, receptor activation leads to a large variety of biochemical events in which small guanosine triphosphate hydrolases (GTPases; eg, Ras) are crucial. Ras proteins are prototypical G-proteins that have been shown to play a key role in signal transduction, proliferation, and malignant transformation. G-proteins are a superfamily of regulatory GTP hydrolases that cycle between 2 conformations induced by the binding of either guanosine diphosphate (GDP) or GTP1-3 (Figure1). The Ras-like small GTPases are a superfamily of proteins that include Ras, Rap1, Rap2, R-Ras, TC21, Ral, Rheb, and M-Ras. The RAS gene family consists of 3 functional genes, H-RAS, N-RAS, and K-RAS. The RAS genes encode 21-kd proteins, which are associated with the inner leaflet of the plasma membrane (H-Ras, N-Ras, and the alternatively spliced K-RasA and K-RasB). Whereas H-Ras, N-Ras, and K-RasB are ubiquitously expressed, K-RasA is induced during differentiation of pluripotent embryonal stem cells in vitro.4 

Fig. 1.

The switch function of Ras.

Ras cycles between the active GTP-bound and the inactive GDP-bound state. Mitogenic signals activate guanine GEFs such as SOS and CDC25. GEFs increase the rate of dissociation of GDP and stabilize the nucleotide-free form of Ras, leading to binding of GTP to Ras proteins. Ras can also be activated by the inhibition of the GAPs.

Fig. 1.

The switch function of Ras.

Ras cycles between the active GTP-bound and the inactive GDP-bound state. Mitogenic signals activate guanine GEFs such as SOS and CDC25. GEFs increase the rate of dissociation of GDP and stabilize the nucleotide-free form of Ras, leading to binding of GTP to Ras proteins. Ras can also be activated by the inhibition of the GAPs.

Close modal

Regulatory proteins that control the GTP/GDP cycling rate of Ras include GTPase-activating proteins (GAPs), which accelerate the rate of GTP hydrolysis to GDP, and guanine nucleotide exchange factors (GEFs; eg, SOS and CDC25), which induce the dissociation of GDP to allow association of GTP.3 In the GTP-bound state, Ras couples the signals of activated growth factor receptors to downstream mitogenic effectors. By definition, proteins that interact with the active GTP-bound form of Ras (and thus become GTP-dependently activated) to transmit signals are called Ras effectors.5-8 Mechanisms by which GTP-Ras influences the activity of its effectors include direct activation (eg, B-Raf, PI-3 kinase), recruitment to the plasma membrane (eg, c-Raf-1), and association with substrates (eg, Ral-GDS). Other candidates for Ras effectors include protein kinases, lipid kinases, and GEFs.3 5-8 

Ras proteins are produced as cytoplasmatic precursor proteins and require several posttranslational modifications to acquire full biologic activity. These modifications include prenylation, proteolysis, carboxymethylation, and palmitoylation9-13(Figure 2).

Fig. 2.

Overview of the posttranslational modifications of Ras proteins in cells.

FTase or GGTase I transfers a farnesyl (F) group or a geranylgeranyl group from FPP or GGPP to the thiol group of the cysteine residue in the CAAX motif. The C-terminal tripeptide is removed by a CAAX-specific endoprotease in the endoplasmatic reticulum. A PPMTase attaches the methyl group from S-adenosylmethionine (SAM) to the C-terminal cysteine. Finally, a prenyl protein–specific palmitoyltransferase (PPPTase) attaches palmitoyl groups (P) to cysteines near the farnesylated C-terminus. PPMTI indicates prenyl protein–specific methyltransferase inhibitor.

Fig. 2.

Overview of the posttranslational modifications of Ras proteins in cells.

FTase or GGTase I transfers a farnesyl (F) group or a geranylgeranyl group from FPP or GGPP to the thiol group of the cysteine residue in the CAAX motif. The C-terminal tripeptide is removed by a CAAX-specific endoprotease in the endoplasmatic reticulum. A PPMTase attaches the methyl group from S-adenosylmethionine (SAM) to the C-terminal cysteine. Finally, a prenyl protein–specific palmitoyltransferase (PPPTase) attaches palmitoyl groups (P) to cysteines near the farnesylated C-terminus. PPMTI indicates prenyl protein–specific methyltransferase inhibitor.

Close modal

Prenylation of proteins by intermediates of the isoprenoid biosynthetic pathway represents a newly discovered form of posttranslational modification and is catalyzed by 3 different enzymes: protein farnesyltransferase (FTase), protein geranylgeranyltransferase type I (GGTase I), and geranylgeranyltransferase type II (GGTase II).9-13 Prenylated proteins share characteristic carboxy-terminal consensus sequences and can be separated into the proteins with a CAAX (C, cysteine; A, aliphatic amino acid; X, any amino acid) motif and proteins containing a CC or CXC sequence.14-17 FTase I transfers a farnesyl group from farnesyldiphosphate (FPP), and GGTase I transfers a geranylgeranyl group from geranylgeranyldiphosphate (GGPP) to the cysteine residue of the CAAX motif.18 GGTase II transfers the geranylgeranyl groups from GGPPs to both cysteine residues of CC or CXC motifs.

Farnesylation is the first step in the posttranslational modification of Ras. This modification occurs by covalent attachment of a 15-carbon farnesyl moiety in a thioether linkage to the carboxy-terminal cysteine of proteins that contain the CAAX motif. The reaction is catalyzed by FTase, a heterodimer consisting of a 48-kd and a 45-kd subunit (αF/GGI and βF). Binding sites for the substrates, FPP and the CAAX motif, are located on the αF- and βF-subunits.19-21Substrates for FTase include all known Ras proteins, nuclear lamins A and B, the γ-subunit of the retinal trimeric G-protein transducin, rhodopsin kinase, and a peroxisomal protein termed PxF.9-13 

Farnesylation of Ras proteins is followed by endoproteolytic removal of the 3 carboxy-terminal amino acids (AAX) by a cellular thiol-dependent zinc metallopeptidase.22 This endoproteolytic activity (RACE, or Ras and a-factor converting enzyme) is a composite of 2 different CAAX proteases: a zinc-dependent activity encoded by AFC1 and the type IIb signal peptidase-like RCE1 (Ras converting enzyme 1).23 The final step in the carboxy-terminal modification of proteins with a CAAX motif (eg, Ras) is the methylation of the carboxyl group of the prenylated cysteine residue by an as yet uncharacterized methyltransferase.

Some Ras proteins (H-Ras, N-Ras, Ras2) are further lipidated by palmitoylation at 1 or 2 cysteines near the farnesylated carboxy-terminus.9-13,24-27 Like farnesylation, H-Ras palmitoylation plays an important role for signaling functions in vivo.27 Microinjection experiments in Xenopusoocytes revealed that palmitoylation of H-Ras dramatically enhances its affinity for membranes as well as its ability to activate mitogen-activated protein kinase (MAPK) and initiate meiotic maturation.11,27 Both a Ras-specific protein (palmitoyltransferase) and a palmitoyl-protein (thioesterase) have been characterized.28,29 In contrast to farnesylation and proteolysis, palmitoylation and methylation of Ras are thought to be reversible and may have a regulatory role.11 12 

The MAPK signaling cascades

MAPK pathways are well-conserved major signaling systems involved in the transduction of extracellular signals into cellular responses in a variety of organisms, including mammals.30-35 The core components of the MAPK signaling cascades are 3 sequential kinases, including MAP kinase (MAPK, or extracellular signal-regulated kinase, ERK), MAPK kinase (MAPKK, or MAPK/ERK kinase, MEK), and MAPKK kinase (MAPKKK, or MEK kinase, MEKK) (Figure3). The MAPKs are activated by dual phosphorylation on tyrosine and threonine residues by upstream dual-specificity MAPKKs. MAPKKs are also phosphorylated and activated by serine- and threonine-specific MAPKKKs. At least 6 MAPK cascades have been clearly identified in mammalian cells.30-35 The best characterized MAPK signaling pathways are (1) the Ras-to-MAPK signal transduction pathway (or ERK pathway), which is responsive to signals from receptor tyrosine kinase, hematopoietic growth factor receptors, and some heterotrimeric G-protein–coupled receptors, which promote cell proliferation or differentiation; (2) the stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) pathway, which is activated in response to stresses such as heat, high osmolarity, UV irradiation, and proinflammatory cytokines such as tumor necrosis factor–α and interleukin-1 (IL-1); and (3) the p38 pathway, which is responsive to osmotic stress, heat shock, lipopolysaccharide, tumor necrosis factor–α, and IL-1 (Figure 3).30-35Scaffolding/adapter proteins such as MP-1, JSAP-1, and JIP-1 route MAPK modules in mammals by binding ERK-1 and MEK-1, JNK-3 and SEK-1 and MEKK-1, or JNK and MKK-7 and MLKs, respectively.34 35 

Fig. 3.

The best-characterized MAPK modules are the ERK pathway, the SAPK/JNK pathway, and the p38 pathway.

The classical Ras-to-MAPK cascade is shown in bold. The MAPK cascades consist of a MAPKKK, a MAPKK, and a MAPK. MAPKKKs are activated through a large variety of extracellular signals such as growth factors, differentiation factors, and stress. The activated MAPKKK can phosphorylate and activate 1 or several MAPKKs, which, in turn, phosphorylate and activate a specific MAPK. Activated MAPK phosphorylates and activates various substrates in the cytoplasma and the nucleus of the cell, including transcription factors. These downstream targets control cellular responses (eg, apoptosis, proliferation, and differentiation). Thick arrows connect the signaling proteins with their preferred substrates (effectors). Note the complexity and the potential for crosstalk between the pathways.

Fig. 3.

The best-characterized MAPK modules are the ERK pathway, the SAPK/JNK pathway, and the p38 pathway.

The classical Ras-to-MAPK cascade is shown in bold. The MAPK cascades consist of a MAPKKK, a MAPKK, and a MAPK. MAPKKKs are activated through a large variety of extracellular signals such as growth factors, differentiation factors, and stress. The activated MAPKKK can phosphorylate and activate 1 or several MAPKKs, which, in turn, phosphorylate and activate a specific MAPK. Activated MAPK phosphorylates and activates various substrates in the cytoplasma and the nucleus of the cell, including transcription factors. These downstream targets control cellular responses (eg, apoptosis, proliferation, and differentiation). Thick arrows connect the signaling proteins with their preferred substrates (effectors). Note the complexity and the potential for crosstalk between the pathways.

Close modal

Ras-to-MAPK signaling via receptor tyrosine kinases and cytokine receptors

The Ras-to-MAPK pathway appears to be an essential shared element of mitogenic signaling. The MAPKs ERK-1 and ERK-2 are activated by various mitogens in all cells. Ras functions as a membrane-associated biologic switch that relays signals from ligand-stimulated receptors to cytoplasmatic MAPK cascades. These receptors include G-protein–coupled serpentine receptors, tyrosine kinase receptors (eg, platelet-derived growth factor receptor [PDGFR], epidermal growth factor [EGF] receptor) and cytokine receptors that cause stimulation of associated nonreceptor tyrosine kinases (NRTKs; eg, Src, Lyn, Fes). Ligand binding to the extracellular domain of receptor tyrosine kinases (RTKs) causes receptor dimerization, stimulation of protein tyrosine kinase activity, and autophosphorylation.36-40 Tyrosine autophosphorylation sites in growth factor receptors (eg, EGF receptor) function as high-affinity binding sites for SH-2 (src homology) domains of signaling molecules such as PI-3 kinase (PI-3K), phospholipase C (PLC)-γ, p120-GAP, Shc, and SHP-2 tyrosine phosphatase.39 

In contrast to receptor tyrosine kinases, cytokine receptors (such as the prototypical IL-3, IL-5, GM-CSF receptors) do not contain a kinase domain. These receptors are heterodimers of a ligand-specific α-subunit and a β-subunit that is common to IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors.41,42 The NRTKs Lyn and Fes and the Janus kinase JAK2 are physically associated with the β-subunit. The conserved proline-rich motifs in the α- and β-subunits (eg, IL-3, IL-5, GM-CSF-R, IL-2-R, G-CSF-R, and erythropoietin-R) are critical for JAK2 binding and activation. (Figure 4). After ligand binding and receptor dimerization, the receptor-bound tyrosine kinases become activated and cause a cascade of tyrosine phosphorylations. As in the receptor tyrosine kinases, these phosphotyrosines represent docking sites for many signaling molecules, including adapter proteins (eg, PI-3K, Shc, SHP-2, Grb-2).41 42 

Fig. 4.

The classical Ras-to-MAPK cascade.

(A) Signaling by cytokine receptors. The IL-3, IL-5, and GM-CSF receptors consist of a ligand-specific α-subunit and a common β-subunit. The β-subunit binds the NRTKs Lyn, Fes, and JAK2. After ligand binding, the α- and β-subunits are thought to dimerize, thus activating the receptor-bound NRTKs and subsequently causing a cascade of tyrosine phosphorylations. The phosphotyrosine residues represent docking sites for various signaling molecules (eg, Shc, SHP-2). ERKs are activated via the classical Ras-to-MAPK pathway. In addition, the MAPKs p38 and JNK become activated. The activation pathway is not completely understood, but some lines of evidence support the involvement of Ras or HPK-1 (hematopoietic progenitor kinase, a mammalian Ste20-related protein). Activated JAK2 phosphorylates the STAT (signal transducers and activators of transcription) family of nuclear factors which form heterodimers and homodimers, thus causing their translocation to the nucleus and subsequent binding to γ-activating sequences of the promoter region of various genes.41,42 (B) Signaling by receptor tyrosine kinases. Extracellular stimuli such as mitogens or stress result in the intracellular activation of different MAPK cascades. The ERK-1/2 pathway is activated by mitogens in all cells and is an essential part of mitogenic signaling. Translocation of a fraction of activated ERKs to the nucleus subsequently leads to activation of transcription factors such as Elk-1, CREB, SRF, and Fos.40 The Raf kinases connect upstream tyrosine kinases and Ras with downstream serine/threonine kinases. When Ras becomes GTP-loaded, Rafs bind to Ras. It is unclear if Ras-Raf binding is itself always sufficient to activate the Raf kinases, which subsequently phosphorylate and activate the downstream MEKs. GTP-Ras also binds and activates PI-3K and Ral-GEF. PI-3K produces lipid second messengers, which activate AKT (Akt kinase) and ncPKC. Ral-GEF activates Ral-GTPases by promoting the GTP-bound state of Ral. Ral-GTP binds to Ral-BP1 (a GAP for CDC42 and Rac), phospholipase D (PLD1), and Ca2+ calmodulin (CaCM). (I) Inhibitors of Ras membrane association (eg, FTI, GGTI, PPMTI, and REPI); (II) sulindac; (III) MEK inhibitors (eg, PD098059, U0126, and Ro 09-2110). The thick, black arrows show the classical Ras-to-MAPK cascade. The thick, open arrows represent the Ras-to-Ral and the Ras–to–PI-3K signaling pathways. The STAT pathway is shown on the left.

Fig. 4.

The classical Ras-to-MAPK cascade.

(A) Signaling by cytokine receptors. The IL-3, IL-5, and GM-CSF receptors consist of a ligand-specific α-subunit and a common β-subunit. The β-subunit binds the NRTKs Lyn, Fes, and JAK2. After ligand binding, the α- and β-subunits are thought to dimerize, thus activating the receptor-bound NRTKs and subsequently causing a cascade of tyrosine phosphorylations. The phosphotyrosine residues represent docking sites for various signaling molecules (eg, Shc, SHP-2). ERKs are activated via the classical Ras-to-MAPK pathway. In addition, the MAPKs p38 and JNK become activated. The activation pathway is not completely understood, but some lines of evidence support the involvement of Ras or HPK-1 (hematopoietic progenitor kinase, a mammalian Ste20-related protein). Activated JAK2 phosphorylates the STAT (signal transducers and activators of transcription) family of nuclear factors which form heterodimers and homodimers, thus causing their translocation to the nucleus and subsequent binding to γ-activating sequences of the promoter region of various genes.41,42 (B) Signaling by receptor tyrosine kinases. Extracellular stimuli such as mitogens or stress result in the intracellular activation of different MAPK cascades. The ERK-1/2 pathway is activated by mitogens in all cells and is an essential part of mitogenic signaling. Translocation of a fraction of activated ERKs to the nucleus subsequently leads to activation of transcription factors such as Elk-1, CREB, SRF, and Fos.40 The Raf kinases connect upstream tyrosine kinases and Ras with downstream serine/threonine kinases. When Ras becomes GTP-loaded, Rafs bind to Ras. It is unclear if Ras-Raf binding is itself always sufficient to activate the Raf kinases, which subsequently phosphorylate and activate the downstream MEKs. GTP-Ras also binds and activates PI-3K and Ral-GEF. PI-3K produces lipid second messengers, which activate AKT (Akt kinase) and ncPKC. Ral-GEF activates Ral-GTPases by promoting the GTP-bound state of Ral. Ral-GTP binds to Ral-BP1 (a GAP for CDC42 and Rac), phospholipase D (PLD1), and Ca2+ calmodulin (CaCM). (I) Inhibitors of Ras membrane association (eg, FTI, GGTI, PPMTI, and REPI); (II) sulindac; (III) MEK inhibitors (eg, PD098059, U0126, and Ro 09-2110). The thick, black arrows show the classical Ras-to-MAPK cascade. The thick, open arrows represent the Ras-to-Ral and the Ras–to–PI-3K signaling pathways. The STAT pathway is shown on the left.

Close modal

The SH3 domain of Grb-2 binds to SOS, which is a GEF for Ras and facilitates the replacement of GDP with GTP.3-8,36-40 When Ras becomes GTP-loaded, Ras effectors (such as Rafs, MEKK, PI-3K, and Ral) bind to Ras and become activated. The Raf kinases (A-Raf, B-Raf, c-Raf-1) are important Ras effectors and have been demonstrated to act as MAPKKKs/MEKKs in the Ras-to-MAPK (or ERK) pathway.36-40,43-45 Raf kinases have been shown to selectively phosphorylate and activate MAPKKs MEK-1 and MEK-2.36-40,43-45 Other MEK-1/MEK-2 activators include TPL-2, MEKK-1, and c-Mos.46-48 MEK-1 and MEK-2 are dual-specificity kinases that activate the MAPKs of the ERK subgroup (ERK-1 and ERK-2).30-35,49-52 ERK-1 and ERK-2 are proline-directed protein kinases that phosphorylate Ser/Thr-Pro motifs in the consensus sequence Pro-Xaan-Ser/Thr-Pro, where Xaa is any amino acid and n = 1 or 2. Several cytoplasmatic and nuclear substrates of the ERKs have been identified. The best-characterized ERK substrates are cytoplasmatic phospholipase A2(cPLA2), the ribosomal protein S6 kinases (RSKs), and the transcription factor Elk-1.30,32,53 54 

The Ras-to-Ral and the Ras–to–PI-3K signaling pathways

Since the discovery of Raf as a direct Ras effector, numerous other putative Ras effectors have been identified.3-8Among these, evidence to date best supports “effector” roles for the Ral-GEFs (Ral-GDS, RGL, and RGF) and the p110 subunit of PI-3K3-8,55 56 (Figure 4).

Ral-GEFs are activated via binding to GTP-Ras. Ral-GEFs in turn activate Ral-GTPases by promoting the GTP-bound state of Ral. As members of the Ras subfamily of Ras-related GTPases, Ral proteins (RalA and RalB) also cycle between the active GTP-bound states and inactive GDP-bound states. Ral-GTP binds Ral-BP1 (Ral-binding protein-1 or Rlip1 = Rip1 [Ral-interacting protein-1]), which is a GAP for CDC42 and Rac. These 2 GTPases are involved in the regulation of the actin cytoskeleton, the SAPK/JNK pathway, and the p38 pathway (Figure3).

Ras-GTP also binds to and activates the catalytic domain of PI-3K. The lipid second-messenger molecules produced (eg, phosphatidylinositol phosphates PtdIns 3,4-P2 and PtdIns 3,4,5-P3) activate the phosphoinositide-dependent kinases PDK-1 and PDK-2, which then activate Akt kinase and nonconventional isoforms of protein kinase C (ncPKC). PI-3K has been implicated in 4 apparently distinct cellular functions, including mitogenic signaling (DNA synthesis), inhibition of apoptosis, intracellular vesicle trafficking and secretion, and regulation of actin and integrin functions. These functions are most likely mediated by distinct phosphoinositide products of PI-3K56 (Figure 4).

The constitutive activation of Ras appears to be an important factor for the malignant growth of human cancer cells. Recently, the Ras-related proteins R-Ras, M-Ras, and TC21 have also been shown to possess transforming activities similar to those of Ras.57-59 However, their role in human malignancies is unclear. Mutations of the RAS proto-oncogenes (H-RAS, N-RAS, K-RAS) are frequent genetic aberrations found in 20% to 30% of all human tumors, although the incidences in tumor type vary greatly.60,61 The highest rate of RAS mutations was detected in adenocarcinomas of the pancreas (90%), the colon (50%), and the lung (30%). In follicular and undifferentiated carcinomas of the thyroid, the incidence of RAS mutations is also considerable (50%). The most commonly observed RAS mutations arise at sites critical for Ras regulation—namely, codons 12, 13, and 61. Each of these mutations results in the abrogation of the normal GTPase activity of Ras. While all the Ras mutants still form complexes with GAP, the GTPase reaction of Ras cannot be stimulated by GAP, thus causing an increase in the half-lives of Ras-GTP mutants.1,5Transformation results, at least in part, from unregulated stimulation of the mitogenic signal transduction pathway.60 61 

Ras activation is frequently observed in hematologic malignancies such as myeloid leukemias and multiple myelomas. In about one-third of the myelodysplastic syndromes (MDS) and acute myeloid leukemias (AML),RAS genes are mutationally activated62-73 (Table1). N-RAS is mutated and activated in most of the cases, and the presence of the mutation is not associated with any particular FAB type, cytogenetic abnormality, or clinical feature, including prognosis.71,RASmutations occur in about 40% of newly diagnosed multiple myeloma patients, and the frequency increases with disease progression.74 Mutations in N-RAS—especially codon 61 mutations—are more frequent than K-RASmutations.74-78 

In addition to activation by mutation, Ras is thought to be deregulated by constitutive activation of proto-oncogenes and inactivation of tumor suppressor genes.79,80 Several types of human cancers show oncogenic activation of RTKs or NRTKs. Constitutively activated versions of normal receptor tyrosine kinases contain single point mutations (eg, CSF-1 receptor, the Neu/Erb-B2 receptor, and the c-Kit receptor), duplications of juxtamembrane domain-coding sequences (eg, FLT3 receptor), or deletions of the negative regulatory regions in the ligand binding or the transmembrane domains (eg, Erb-B receptor). Point mutations of the CSF-1 receptor (c-FMS) at codons 301 and 969 were found in 10% to 20% of AML or MDS.81,82 Point mutations in the catalytic domain of the c-Kit receptor are found in some cases of myeloproliferative disorders and in 10% of the patients with mastocytosis.83-85 Furthermore, activating tandem internal duplication of the FLT3 receptor has been reported in 20% of AML.86 The members of the c-Kit/c-FMS receptor kinase family (eg, c-Kit, c-FMS, FLT3) are linked with components of the Ras-to-MAPK signaling pathway (eg, Grb-2 and Shc), suggesting that activating mutations of c-FMS and FLT3 may induce activation of Ras.87 88 

In addition, translocations involving receptor tyrosine kinases produce chimeric proteins in which varying N-terminal portions of either the ligand-binding or the transmembrane domain are replaced with novel protein sequences.79,80 Several of these chimeric proteins have been found in human hematologic malignancies. The Npm-Alk fusion protein, a fusion of the N-terminal portion of Npm with the entire cytoplasmatic domain of the receptor tyrosine kinase Alk, is generated by the t(2;5) chromosomal translocation in anaplastic large cell lymphoma.89,90 Tel-PDGFRβ is a fusion protein consisting of the transcription factor Tel (translocation, Ets, leukemia) and PDGFRβ, a well-known receptor tyrosine kinase.91,92 It is generated by the t(5;12) translocation in a subset of chronic myelomonocytic leukemias that results in receptor dimerization and activation and thus leads to the constitutive activation of the Ras-MAPK pathway.3 Another Tel fusion protein, Tel-Abl, is generated by the t(12;9) translocation in AML.93,94 Abl is an NRTK that is also mutated and activated in chronic myelogenous leukemia.95-97 In Bcr-Abl, the product of the t(9;22) translocation, the N-terminal Bcr portion serves as an oligomerization domain. Bcr-Abl is a constitutively activated cytosolic tyrosine kinase that causes abrogation of growth factor dependence, blockade of differentiation, and direct inhibition of apoptosis. Although Ras mutations are extremely rare in chronic myelogenous leukemia, the involvement of Ras has been demonstrated in Bcr-Abl+ cells by the presence of increased levels of GTP-Ras, which leads to the activation of the Raf kinases and other Ras effectors.95-97 Thus, the deregulation of Ras function appears to be a common theme in the transformation by activated receptor and NRTKs. Ras activation may cause elevated cell cycle progression and inhibition of apoptosis.71,79,80 95-97 

In addition to oncogenes, tumor suppressor genes have also been found to be involved in the deregulation of Ras. Neurofibromin, the product of the NF1 gene, encodes a Ras-GAP and is mutated in the autosomal dominant type 1 neurofibromatosis.98 Interestingly, neurofibromatosis type 1 is associated with an increased tendency to develop myeloid leukemias, especially juvenile myelomonocytic myeloid leukemia (JMML).99-107 About 15% of children with JMML cases have clinical neurofibromatosis.99 Additionally, inactivating mutations of the NF1 gene have been found in 15% of JMML without clinical diagnosis of neurofibromatosis, suggesting the existence of NF1 mutations in approximately 30% of all JMML cases.100,102 The involvement of Ras is demonstrated by the finding that leukemic cells from children with neurofibromatosis type 1 show a moderate elevation in the percentage of GTP-Ras.103-106 Furthermore, 15% to 30% of JMML cases lacking the NF1 mutation have activating RASmutations.107 The observation that human JMML cells exhibit hypersensitivity to GM-CSF suggests a common pathophysiologic mechanism involving downstream Ras signaling.106-108 

The pathophysiologic importance of the Ras-MAPK signaling pathway is underscored by the positioning of several oncogene and tumor suppressor gene products on this pathway (Figure 4). Furthermore, it has recently been demonstrated that mutant N-RAS induces myeloproliferative disorders resembling human chronic myelogenous leukemia, AML, and apoptotic syndromes similar to human MDS in bone marrow–repopulated mice.109 These observations make Ras and the Ras-MAPK pathway an attractive target for the development of new anticancer agents.

Inhibitors of Ras farnesyltransferase

Elimination of Ras function by homologous gene recombination or antisense RNA has demonstrated that expression of activated Ras is necessary for maintaining the transformed phenotype of tumor cells.110-113 Inhibitors of oncogenic Ras activity may therefore prove useful as anticancer agents against Ras-induced tumors. One strategy to impede oncogenic Ras function in vivo is the inhibition of Ras posttranslational modification. It has been demonstrated that mutation of the evolutionarily conserved CAAX motif in Ras abolishes plasma membrane binding as well as transforming activity.114-121 Although Ras undergoes several steps of posttranslational modification, only farnesylation is necessary for its membrane localization and cell-transforming activity.121Therefore, it has been proposed that the activity of oncogenic Ras could be blocked by inhibiting the FTase responsible for this modification. However, many CAAX-containing proteins need additional palmitoylation for stable membrane association.

FTase has become a very attractive target for the development of anticancer agents because control of Ras farnesylation can control the function of oncogenic Ras.114-121 Numerous inhibitors of Ras FTase have been synthesized or identified. These Ras FTase inhibitors can be grouped into 3 classes: (1) FPP analogues such as (α-hydroxyfarnesyl) phosphonic acid, β-ketophosphonic and β-hydroxyphosphonic acid derivatives, and J-104871122-124(Figure 5A); (2) CAAX peptide analogues such as BZA-5B, BZA-2B,125-127 L-731,734, L-731,735, L-739,749,128-132 L-739,787,133L-739,750, L-744,832,129,134-137 B581,138Cys-4-ABA-Met and Cys-AMBA-Met,139 FTI-276, FTI-277,140-143 B956, and its methyl ester B1096144 (Figure 5B); in addition, nonpeptidic, tricyclic FTase inhibitors have been developed such as SCH44342, SCH54429, SCH59228, and SCH66336145-149 (Figure6); and (3) bisubstrate inhibitors such as phosphonic acid analogues, the phosphinate inhibitors BMS-185878 and BMS-186511, the phosphonate inhibitor BMS-184467, phosphinyl acid–based derivatives, and the hydroxamine acid analogues150-152 (Figure 5C).

Fig. 5.

Structures of FPP-, CAAX-based, and bisubstrate inhibitors of FTase.

(A) Chemical structures of FPP and FPP-based inhibitors of FTase and PPMTase. FPP is composed of a hydrophobic farnesyl group and a highly charged pyrophosphate moiety. The basic structural element in the FTase inhibitors is a farnesyl group, a pyrophosphate isostere, and a linker. (B) CAAX-based FTase inhibitors. Structural comparison between CAAX-based FTase inhibitors of the pseudopeptide class and the CAAX tetrapeptides CIFM and CVFM. The potent, nonsubstrate FTase inhibitors CIFM and CVFM were identified by systematic amino acid replacements within the CAAX sequence. In FTI-276 and FTI-277, the AA residues of the CAAX motif have been replaced by a hydrophobic linker. (C) In bisubstrate FTase inhibitors, the farnesyl group of FPP and the tripeptide group of the CAAX motif are connected via a linker.

Fig. 5.

Structures of FPP-, CAAX-based, and bisubstrate inhibitors of FTase.

(A) Chemical structures of FPP and FPP-based inhibitors of FTase and PPMTase. FPP is composed of a hydrophobic farnesyl group and a highly charged pyrophosphate moiety. The basic structural element in the FTase inhibitors is a farnesyl group, a pyrophosphate isostere, and a linker. (B) CAAX-based FTase inhibitors. Structural comparison between CAAX-based FTase inhibitors of the pseudopeptide class and the CAAX tetrapeptides CIFM and CVFM. The potent, nonsubstrate FTase inhibitors CIFM and CVFM were identified by systematic amino acid replacements within the CAAX sequence. In FTI-276 and FTI-277, the AA residues of the CAAX motif have been replaced by a hydrophobic linker. (C) In bisubstrate FTase inhibitors, the farnesyl group of FPP and the tripeptide group of the CAAX motif are connected via a linker.

Close modal
Fig. 6.

Nonpeptidic, tricyclic FTase inhibitors.

FTase inhibitor SCH44342 had no in vivo efficacy. Further substitutions led to SCH66336, a highly potent FTase inhibitor, which was found to have therapeutically useful serum levels and half-lives when given orally to rodents and primates. SCH66336 is being tested in human clinical phase II trials.

Fig. 6.

Nonpeptidic, tricyclic FTase inhibitors.

FTase inhibitor SCH44342 had no in vivo efficacy. Further substitutions led to SCH66336, a highly potent FTase inhibitor, which was found to have therapeutically useful serum levels and half-lives when given orally to rodents and primates. SCH66336 is being tested in human clinical phase II trials.

Close modal

In addition to chemically synthesized compounds, several natural products have been identified as FTase inhibitors. These include limonene,153 manumycin (UCF1-C) and its related compounds UCF1-A and UCF1–B,154-156 chaetomellic acid A and B, zaragozic acids, pepticinnamins, gliotoxin,115 barceloneic acid A,157 RPR113228,158 actinoplanic acids A and B,159 oreganic acid,160 lupane derivatives,161 saquayamycins,162 valinoctin A and its analogues,163 and ganoderic acid A and C.164 

Effects of FTase inhibitors in intact tumor cells.

Several FTase inhibitors were demonstrated to be active in intact cells (Table 2). These compounds have been shown to modulate several critical aspects of Ras transformation in whole cells, including selective inhibition of anchorage-independent growth of Ras-transformed fibroblasts in soft agar, morphologic reversion of the Ras-induced phenotype, and inhibition of oocyte maturation induced by oncogenic Ras without gross cytotoxic effects on normal cells. One of the first FTase inhibitors found to be active in intact tumor cells, the FPP analogue (α-hydroxyfarnesyl) phosphonic acid, only partially inhibited the farnesylation of Ras in H-Ras–transformed NIH3T3 fibroblasts.165 Subsequently, more potent FTase inhibitors have been developed. L-739,749 inhibited growth of Ras-transformed rat fibroblasts and caused rapid morphologic reversion of the transformed phenotype.130 The compound B581 inhibited colony formation of Ras-transformed cells in soft agar.138 BZA-5B and BZA-2B, both benzodiazepine peptidomimetic FTase inhibitors, have been shown to slow the growth of H-Ras–transformed cells at concentrations that do not affect the growth of nontransformed cells.125-127,166,167 The peptidomimetic FTase inhibitor B956 and its methyl ester B1086 inhibited the formation of soft agar colonies of 14 human tumor cell lines expressing oncogenic forms of H-Ras, N-Ras, and K-Ras.144 Five human tumor cell lines expressing wild-type Ras required higher concentrations of the drug to inhibit colony formation. About 50% of K-Ras–transformed cell lines were observed to be as resistant as non–Ras-transformed cell lines. It has been suggested that nontransformed cells may produce a form of Ras that is isoprenylated even in the presence of FTase inhibitors.144Additionally, this phenomenon may be due to functional redundancy within the RAS family. The tricyclic inhibitor SCH44342 specifically blocks morphologic transformation induced by Val12-Ha-Ras-CVLS but not Val12-Ha-Ras-CVLL, a form of Ras engineered to bind to GGTase I, indicating that the compound is a specific inhibitor of H-Ras modification by FTase rather than K-Ras modification by GGTase.145 

Similarly, several bisubstrate inhibitors of FTase were shown to inhibit oncogenic Ras-induced growth in vivo. The phosphinyl acid–based bisubstrate analogue FTase inhibitors Nos. 17 to 19 were found to inhibit the anchorage-independent colony growth of Ha-RAS–transformed NIH3T3 cells.150-152 The bisubstrate FTase inhibitor BMS-186511 inhibited FTase activity in whole cells and produced strong inhibition of Ras-transformed growth. Although both H-Ras– and K-Ras–transformed cells were affected by BMS-186511, K-Ras cells appeared to be less sensitive.151BMS-186511 did not produce any signs of gross, unspecific cytotoxicity in untransformed normal cells.

The FTase inhibitor L-744,832 blocked the anchorage-dependent and -independent growth of 31 of 42 human tumor cell lines.136The origin of the tumor cell and the presence or absence of mutationally activated Ras did not correlate with the response to the FTase inhibitor. Interestingly, cell lines with wild-type Ras and activated receptor tyrosine kinases were also sensitive to L-744,832. In contrast, nontransformed epithelial cell lines were far less sensitive. Recently, L-739,749 and L-744,832 have also been reported to inhibit the colony growth of juvenile myelomonocytic leukemia cells, which are known to exhibit deregulated cytokine signal transduction involving the Ras pathway.132 

Biologic mechanisms of FTase inhibitors in intact cells.

Recent investigations into the biologic mechanism of the growth inhibition of Ras-transformed cells have shown that farnesylation of K-Ras and N-Ras is more resistant to FTase inhibitors than farnesylation of H-Ras.126,144,167,168 In part, this phenomenon is a result of a 10- to 50-fold higher affinity of FTase for K-Ras4B than for other Ras isoforms.126,170 In the absence of FTase inhibitors, all Ras proteins are present only in the farnesylated form. However, K-Ras and N-Ras (but not H-Ras) become geranylgeranylated by GGTase I in vivo in a dose-dependent manner when intracellular farnesylation is inhibited by an FTase inhibitor.126,169-171 Subsequently, both FTase and GGTase I inhibitors are required for inhibition of K-Ras processing.168,172 The lack of growth inhibition and gross cytotoxic effects of FTase inhibitors on normal cells is thought to be a result of the resistance of K-Ras processing to FTase inhibitors.167 

Treatment of Ras-transformed cells with FTase inhibitors results in selective suppression of Ras-dependent oncogenic signaling. This includes the inhibition of Ras processing, which results in a decrease in the relative amount of fully processed Ras; the progressive, dose-dependent cytoplasmatic accumulation of unprocessed Ras and inactive Ras-Raf complexes; inhibition of the Ras-induced constitutive activation of MAPK138,140,141,146,173; and decreased transcriptional activity of both c-Jun and Elk-1.138Transformation by mutationally activated Raf, MEK, Mos, or Fos (all of which are downstream effectors of Ras) is not blocked by FTase inhibitors.129 136 

Although FTase inhibitors block Ras farnesylation and the Ras-induced transformed phenotype, proteins other than Ras may be targets of these compounds.174,175 FTase inhibitors block anchorage-independent growth of many human tumor cell lines in soft agar culture, but there is no correlation between biologic susceptibility and the presence of Ras mutations.136,144In addition, anchorage-independent growth of K-Ras–transformed cells is abrogated by FTase inhibitors even though K-Ras processing is not affected.172 Although it is unclear whether soluble species of oncogenic Ras exert any biologically significant effect in drug-treated cells, it has recently been shown that nonfarnesylated H-Ras proteins can be palmitoylated and thus are biologically active. These proteins bound modestly to the plasma membranes (40%) but were still able to trigger exaggerated differentiation of PC12 cells and potent transformation of NIH3T3 fibroblasts.176 

Recently, it has been suggested that the antitransforming effects of FTase inhibitors are mediated at least in part by alteration of farnesylated Rho proteins, including RhoB.174,175,177,178In contrast to Ras proteins, RhoB exists normally in vivo in a farnesylated (RhoB-FF) and a geranylgeranylated version (RhoB-GG).179 RhoB-GG is essential for the degradation of p27KIP1 and facilitates the progression of cells from G1 to S phase. Treatment with FTase inhibitors results in a loss of RhoB-FF and a gain of RhoB-GG.178 Expression of a mutant RhoB-GG protein induces phenotypic reversion, cell growth inhibition, and activation of the cell cycle kinase inhibitor p21WAF1 in cells sensitive to FTase inhibitors, including Ras-transformed cells.178,180P21WAF1 mediates the inhibition of cyclinE-associated protein kinase activity, pRB hypophosphorylation, and inhibition of DNA replication, which results in G1 arrest.180 In addition to the induction of the G1 block, treatment of tumor cells with FTase inhibitors induces apoptosis by upregulating Bax and Bcl-xs expression and by activating caspases.131 181-183 

Synergy of FTase inhibitors with established anticancer treatments such as radiation and chemotherapeutic treatment was recently reported. Agents that prevent microtubule depolymerization, such as taxol and epothilones, act synergistically with FTase inhibitors to block cell growth. FTase inhibitors cause increased sensitivity to induction of the metaphase block by taxol and epothilones.184 In addition, FTase inhibitors have been shown to increase the radiosensitivity of human tumor cells with activating mutations ofRAS oncogenes.143 

Effects of FTase inhibitors in animal models.

FTase inhibitors have also been shown to inhibit the growth of Ras-induced tumors in mouse xenograft models and, more dramatically, in transgenic mouse models (Table 3). Manumycin was reported to inhibit the growth of K-Ras–transformed fibrosarcoma transplanted into nude mice by approximately 70% compared with untreated controls.154 The CAAX peptide analogue L-739,749 specifically suppressed the tumor growth of H-Ras–, N-Ras–, and K-Ras–induced Rat-1 cell tumors in nude mice by 51% to 66%.129 Interestingly, L-739749 exhibited no evidence of systemic toxicity. The peptidomimetic FTase inhibitors B956 and B1086 were shown to inhibit tumor growth of EJ-1 human bladder carcinoma, HT 1080 human fibrosarcoma and, to a lesser extent, HCT116 human colon carcinoma xenografts in nude mice. Inhibition of Ras processing correlated with the inhibition of the tumor growth by B956.144 Analogues of the tetrapeptide CVFM,189 the compound Nos. 46 and 51, showed inhibition of anchorage-independent growth of stably H-Ras–transformed NIH3T3 fibroblasts as well as antitumor activity in an athymic mouse model implanted with H-Ras–transformed Rat-1 cells.187J-104871, an FPP-competitive FTase inhibitor, suppressed tumor growth in nude mice transplanted with activated H-RAS–transformed NIH3T3 cells.124 In contrast to these results, however, treatment of irradiated mice engrafted with NF-1 deficient hematopoietic cells (−/−) with the FTase inhibitor (FTI) L-744,832 failed to revert a myeloproliferative disorder similar to JMML.190 Although L-744,832 abrogated the GM-CSF–induced growth, H-Ras processing and MAPK activation of NF-1 (−/−) hematopoietic cells, this FTI did not reduce the constitutively elevated MAPK activity levels in these cells. This may be due to the resistance of N-Ras and K-Ras processing to inhibition by the FTI.190 

In addition to the mouse xenograft models, FTase inhibitors have been tested in transgenic mouse models. The CAAX-based FTase inhibitor L-744,832 induced regression of mammary and salivary carcinomas in MMTV-v-Ha-RAS mice. These mice harbor the viral Ha-RAS oncogene under the control of the mouse mammary tumor virus (MMTV) long terminal repeat and develop spontaneous mammary and salivary carcinomas.135 In agreement with earlier observations, no systemic toxicity was observed in these mice. Furthermore, L-744,832 was also effective in mammary and lymphoid tumors overexpressing N-RAS in MMTV transgenic mice.137 In contrast to H-Ras, N-Ras remained mostly processed. Consistent with these findings, the antineoplastic effect was less intense in the N-RAS model than the H-RAS model.135,137 This observation suggests that proteins in addition to Ras may be targets of the compound. More recently, L-744,832 was shown to induce regression of mammary tumors in MMTV–TGF-α and MMTV–TGF-α/neu transgenic mice.183 Because the mammary tumor cells harbor an activated receptor tyrosine kinase but wild-type Ras, a feature common in breast cancer, these mice provide a useful model system for breast cancer research. Tumor regression by L-744,832 was demonstrated biochemically by inhibition of MAPK activity and biologically by an increase in G1-phase, a decrease in S-phase fractions, and induction of apoptosis.183 

In both cell culture and mouse models, there is essentially no cytotoxicity or apparent systemic toxicity at doses capable of reverting Ras-induced transformation or of causing tumor regression. FTase inhibitors seem to selectively target a unique aspect of the transformed cell physiology.

Mechanisms of resistance to FTase inhibitors.

As with any drug, the development of tumor resistance to FTase inhibitors is an important issue. To date, the relative frequency, the mechanisms, and the development of tumor resistance to FTI are unclear.K-RAS–transformed cell lines have been shown to be more resistant to FTase inhibitors than H-RAS– orN-RAS–transformed cells.126,144,167 This phenomenon is thought to be a result of a higher affinity of FTase for K-Ras than for other Ras isoforms.126,167 In addition, K-Ras and N-Ras become geranylgeranylated in the presence of FTI.169,171 Subsequently, both FTI and geranylgeranyltransferase inhibitor (GGTI) are required for inhibition of K-Ras processing.168,172 Recently, a variantRAS-transformed cell line was identified that was resistant to phenotypic reversion by FTI.193 This phenomenon was not due to mutation of the FTase subunits, changes in intracellular drug accumulation, or amplification of the multiple-drug resistance gene. The precise mechanism of resistance in these cells remained unclear. However, mutational alteration of FTase might also lead to resistance toward FTI. The Y361L mutant of FTase has been shown to exhibit increased resistance to FTI while maintaining FTase activity toward substrates possessing CIIS carboxy-termini.194 Withdrawal of FTI from successfully treated tumor-bearing mice led to subsequent tumor growth in the absence of the drug. A second FTI treatment resulted in a second response in some mice, but some tumors were found to become resistant to FTI.135 Therefore, chronic, uninterrupted treatment with FTI might be required.

Inhibitors of geranylgeranyl transferase I

Until recently, the emphasis has been on designing specific FTase inhibitors to block Ras processing. This strategy was employed to avoid possible toxic effects originating from inhibition of GGTase I. Because K-RAS mutations are most common in human cancers,60,61 a critical goal is the development of inhibitors that block the growth of human tumors that harbor K-Ras. The resistance of K-Ras to FTase inhibitors,167 the lack of potency of FTase inhibitors against K-Ras–transformed cells,144 and the observation that K-Ras becomes geranylgeranylated in the presence of FTase inhibitors126,169-172 led to the development of GGTase I inhibitors (Figure 7). GGTI-279, GGTI-287, GGTI-297, and GGTI-298 are CAAL-based peptidomimetics that are selective for GGTase I over FTase.173,195-199 In contrast, FTI-276 and FTI-277 are CAAM-based peptidomimetics that are potent and selective inhibitors of FTase over GGTase I.173H-Ras processing in human tumor cell lines was highly sensitive to FTI-277 and resistant to GGTI-286, whereas K-Ras4B processing was more sensitive to GGTI-286 than FTI-277.173 Processing of H-Ras and N-Ras was inhibited by FTI-277, but inhibition of K-Ras processing required both FTase and GGTase I inhibitors. Whereas FTI-277 preferentially blocks activation of MAPK by oncogenic H-Ras, GGTase inhibitors selectively inhibit the activation of MAPK by oncogenic K-Ras4B.173 Although GGTI-298 had very little effect on soft agar growth of several human tumor cell lines harboring H-RAS, N-RAS, or K-RAS mutations, the combination of FTI-277 and GGTI-298 resulted in significant soft agar growth inhibition.172 Both FTase inhibitors and GGTase inhibitors have been reported to arrest Ras-transformed cells in G0/G1 phase of the cell cycle and to induce apoptosis.142,180,196,198,199 In nude mouse xenografts, the GGTase inhibitor GGTI-297 suppressed human lung A-549 and Calu-1 carcinoma tumor growth by 60%. However, both FTase and GGTase inhibitors were required to inhibit K-Ras processing.168Treatment of cells with GGTI-298 blocks PDGF- and EGF-dependent tyrosine phosphorylation of their respective receptors and induces G0/G1-phase arrest and apoptosis.196-198 GGTI-298 has also been shown to induce the cyclin-dependent kinase inhibitor p21WAF but not p27KIP.199 

Fig. 7.

CAAL-based inhibitors of GGTase I.

GGTase I catalyzes the geranylgeranylation of proteins terminating with CAAX sequences where X is restricted to leucine, isoleucine or, to a lesser extent, phenylalanine. In cells, geranylgeranylation of proteins is far more common than farnesylation. Proteins modified by GGTase I include Rap1A, Rap1B, Rac1, Rac2, G25K, and RhoA.

Fig. 7.

CAAL-based inhibitors of GGTase I.

GGTase I catalyzes the geranylgeranylation of proteins terminating with CAAX sequences where X is restricted to leucine, isoleucine or, to a lesser extent, phenylalanine. In cells, geranylgeranylation of proteins is far more common than farnesylation. Proteins modified by GGTase I include Rap1A, Rap1B, Rac1, Rac2, G25K, and RhoA.

Close modal

Inhibitors of the prenylated protein methyltransferase

The C-terminal prenylated protein methyltransferase (PPMTase) is another potential therapeutically relevant target in the development of inhibitors against the posttranslational processing of Ras. N-acetyl-trans,trans-farnesyl-l-cysteine (AFC) is a substrate for PPMTase and acts as a competitive inhibitor.201Although AFC has been shown to inhibit Ras methylation in Ras-transformed NIH3T3 fibroblasts, it does not inhibit the growth of these cells.201 New farnesyl derivatives of rigid carboxylic acid, eg, S-trans,trans-farnesylthiosalicylic acid (FTS), were found to inhibit the growth of H-Ras–transformed cells and to reverse their transformed morphology by a mechanism unrelated to the inhibition of Ras methylation by PPMTase202,203 (Figure 5). It is thought that FTS specifically interacts with Ras farnesylcysteine binding domains and affects membrane anchorage of Ras.202,203 In addition, it has been reported that FTS dislodges Ras from H-Ras–transformed cell membranes and renders the Ras protein susceptible to proteolytic degradation.188 At the same concentration, growth and morphology of non–Ras-transformed or nontransformed cells were not affected by FTS.203 Despite the lack of FTS-induced cytotoxicity in nontransformed cells, FTS reduced Ras levels in cell membranes and inhibited Ras-dependent cell growth.203 In contrast to FTase inhibitors (eg, BZA-5B), FTS also inhibited the growth signaling of receptor tyrosine kinases.203 FTS was shown to decrease total cellular Ras levels, MAPK activity, Raf-1 activity, and DNA synthesis in Ras-transformed EJ-1 cells. This inhibition was also demonstrated in serum-, EGF-, and thrombin-stimulated, untransformed Rat-1 cells.204,205 S-farnesyl-thioacetic acid (FTA), another competitive inhibitor of PPMTase, has been shown to suppress growth and induce apoptosis in HL-60 cells.206 Five-chloro– and 4- or 5-fluoro–derivatives of FTS and a C20 S-geranylgeranyl derivative of thiosalicyclic acid also cause inhibition of Ras-dependent MAPK activity, DNA synthesis, and EJ-1 cell growth. However, several other derivatives were inactive, suggesting stringent structural requirements for the anti-Ras activity of S-prenyl analogues.207Recently, FTS was shown (1) to reduce the amount of activated N-Ras and wild-type Ras isoforms in human melanoma cells and Rat-1 fibroblasts, (2) to disrupt ERK signaling, (3) to revert their transformed phenotype, and (4) to cause a significant reduction in the growth of human melanoma in SCID mice.188 205 

The dorrigocins are novel antifungal antibiotics that were found to reverse the morphology of Ras-transformed NIH3T3 fibroblasts. Dorrigocin A did not inhibit protein prenylation or protein synthesis but was instead found to inhibit the C-terminal methylation in K-Ras–transformed cells.208 

Selective inhibitors of Ras C-terminal sequence-specific endoprotease

UM96001, TPCK, and BFCCMK are Ras C-terminal sequence-specific endoprotease inhibitors (REPI) and potently inhibit ras-transformed rat kidney cell growth as well as growth of human cancer cells.209 These compounds have been reported to almost completely block the anchorage-independent clonogenic growth of these cancer cells. REPIs may selectively induce apoptosis in these cells.209 

Selective inhibitors of MAPKKs, or MEK

PD098059 is a synthetic inhibitor of the Ras-MAPK pathway that selectively blocks the activation of MEK-1 and, to a lesser extent, the activation of MEK-2.210,211 The inhibition of MEK-1 activation was demonstrated to prevent activation of MAPKs ERK-1/2 and subsequent phosphorylation of MAPK substrates both in vitro and in intact cells. In contrast to FTase inhibitors, PD098059 inhibited stimulation of cell growth by several growth factors.210,211 Furthermore, PD098059 reversed the transformed phenotype of Ras-transformed BALB3T3 mouse fibroblasts and rat kidney cells.211 PD098059 failed to inhibit the stress, and IL-1 stimulated JNK/SAPK and the p38 pathways,210 demonstrating its specificity for the ERK pathway. PD098059 has subsequently been used as a tool to study MAPK signaling in various cell types and in carcinogenesis.

Recently, 2 novel inhibitors of MEK-1 and MEK-2 have been identified: U0126212,213 and Ro 09-2210.214 Ro 09-2210, which was identified by screening microbial broths, exhibits potent antiproliferative effects on activated T cells.214Similarly, U0126 was found to inhibit T-cell proliferation in response to both antigenic stimulation and cross-linked anti-CD3 plus anti-CD28 antibodies.212 U0126 and PD098059 are noncompetitive inhibitors with respect to both MEK substrates (ATP and ERK) and bind to free MEK as well as MEK*ERK and MEK*ATP complexes. U0126 displays significantly higher affinity for all forms of MEK (44- to 357-fold) than does PD098059. U0126 and Ro 09-2210 have an inhibitory concentration of 50% (IC50) of 50 to 70 nmol/L, whereas PD098059 has an IC50 of 5 μmol/L.212-214PD098059 and U0126 impede the growth of Ras-transformed cells in soft agar but show minimal effects on cell growth under normal culture conditions.210,213 In contrast to U0126 and PD098059, Ro 09-2210 is also able to inhibit other dual-specificity kinases such as MKK-4, MKK-6, and MKK-7, albeit at 4- to 10-fold higher IC50 concentrations compared with its effect on MEK-1.214 

Inhibitors of Ras transformation with unknown mechanisms of action

Screening tests for drugs that revert RAS-transformed cells to a normal phenotype led to the identification of a number of compounds, such as azatyrosine, oxanosine, and antipain.215-217 The mechanism by which these compounds revert the RAS-induced phenotype is not understood. The pyrazolo-quinoline compound SCH51344 was identified based on its ability to depress human smooth muscle α-actin promoter activity inRAS-transformed cells. Treatment of v-abl-,v-mos-, v-raf-, RAS-, and mutant active MEK-transformed NIH3T3 cells resulted in growth inhibition of these cells in soft agar.218 SCH51344 had very little effect on the activities of proteins in the ERK pathway. The ability of SCH51344 to inhibit the anchorage-independent growth of RAC-V12–transformed Rat-1 cells suggests that the point of inhibition is downstream from RAC.219 

The nonsteriodal, anti-inflammatory drug sulindac has been demonstrated to attenuate the growth and progression of colonic neoplasms in animal models and in patients with familial adenomatous polyposis.220,221 Recently, it has been shown that sulindac sulfide (the active metabolite of sulindac) inhibits Ras signaling and transformation by noncovalent binding to the Ras protein. Furthermore, it has been demonstrated that sulindac sulfide impairs Ras-Raf binding, Raf activation, and nucleotide exchange on Ras and that it accelerates the Ras-GTPase reaction.222 Sulindac is being investigated in a randomized study for the prevention of colon cancer (protocol RUH-SSH-190-0698, NCI-V98-1425).

Disruption of the Ras-to-MAPK signaling pathway has also been shown for the benzoquinone ansamycin geldanamycin. Geldanamycin binds to HSP90 and disrupts the HSP90–Raf-1 multimolecular complex, which causes destabilization of Raf-1 through enhanced degradation of Raf-1.223 However, the geldanamycin-HSP90 complex also causes depletion of other HSP90 substrates such as protein kinases and nuclear hormone receptors (including mutant p53 and ErbB2).224 Several National Cancer Institute–sponsored clinical phase I trials are currently studying the effects of geldanamycin analogues in patients with advanced malignancies.

FTase and GGTase inhibitors have strong growth inhibitory and antitumor activity in cell culture and animal tumor models without showing nonspecific gross toxicity in animals. The specificity and the lack of nonspecific toxicity contrasts dramatically with the nonspecificity and high toxicity of currently available chemotherapeutic drugs. The recent development of orally bioavailable FTase inhibitors with potent and selective in vivo antitumor activity underscores their potential usefulness in the future treatment of human malignancies. The observation that FTase and GGTase inhibitors induce apoptosis in treated tumor cells as well as a G0-G1 arrest suggests that they are not merely cytostatic but cytotoxic for tumor cells. However, the absence of toxicity due to FTase inhibitors in normal cells and tissues in mice at doses that inhibit tumor growth is poorly understood. Ras knockout experiments have demonstrated that H-RAS– and N-RAS–deficient mice are born and grow normally, whereas K-RAS–deficient embryos die between embryonic day 12.5 and term. This finding suggests a partial functional overlap within the RAS gene family.225-228However, H-RAS and N-RAS cannot compensate for the loss of K-RAS function in K-RAS–deficient mice. Functionally redundant pathways might allow normal cells to tolerate treatment with FTase inhibitors.

Because mutated RAS genes have a high prevalence in human cancers (eg, pancreatic, lung, and colon cancers), inhibitors specific for FTase, GGTase, and MEK were initially designed to block the Ras-to-MAPK signaling in solid tumor cells. More than 90% ofRAS mutations found in human tumors occur in N-RAS or K-RAS. Whereas the reversion of the H-RAS–induced transformation by FTase inhibitors correlates well with the intracellular inhibition of H-Ras processing, N-Ras and K-Ras are cross-prenylated by GGTase I in cells treated with FTase inhibitors. However, many of these N-RAS– or K-RAS–transformed cell lines (and even tumor cell lines that do not harbor RAS mutations) are sensitive to FTase inhibitors. Cell biology studies suggest that FTase and GGTase inhibitors may act at additional levels beyond the inhibition of Ras processing. The exact mechanism of action has emerged as a question of major interest, especially because transformed tumor cells respond to treatment with these inhibitors while normal cells remain largely unaffected. Non-Ras targets of FTase and GGTase inhibitors may include other cellular proteins (eg, Rho) that are farnesylated or geranylgeranylated.174,175,178 229-231 

FTase inhibitors (eg, R115777, L-778,123, and SCH66336) have entered several phase I/II clinical trials (Table4). These trials are still ongoing, and preliminary results have not been published. Because favorable synergistic effects have been described for combinations of FTase inhibitors with traditional anticancer treatments such as radiation and chemotherapy,143 184 it will be interesting to see if these results translate into improved patient outcome in clinical trials. The high prevalence of mutationally activated Ras in solid tumors has been the driving force of Ras inhibitor research. However, recent studies in cell culture and animal models suggest that transformed cells with an activated Ras pathway (eg, via mutations upstream of Ras) are also highly sensitive for FTase inhibitors. The involvement of N-RAS in the molecular pathophysiology of myeloid leukemias and multiple myeloma suggests that these malignancies may also represent promising targets for inhibitors of Ras signaling. While it is impossible to predict the outcome of the clinical trials, the biologic properties of these inhibitors are potentially informative because transformation-specific mechanisms are targeted.

We thank Dr Kristine A. Henningfeld for help with the figures and for critical reading of the manuscript.

Supported by a grant to C.W.M.R. from the German Research Council (Deutsche Forschungsgemeinschaft Re 864/4-1) and a grant from the University of Ulm (P.541).

The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 U.S.C. section 1734.

1
Sprang
SR
G protein mechanisms: insights from structural analysis.
Annu Rev Biochem.
66
1997
639
678
2
Bos
JL
All in the family? New insights and questions regarding interconnectivity of Ras, Rap1 and Ral.
EMBO J.
17
1998
6776
6782
3
Rebollo
A
Martinez
CA
Ras proteins: recent advances and new functions.
Blood.
94
1999
2971
2980
4
Pells
S
Divjak
M
Romanowski
P
et al
Developmentally-regulated expression of murine K-ras isoforms.
Oncogene.
15
1997
1781
1786
5
Wittinghofer
A
Signal transduction via Ras.
Biol Chem.
379
1998
933
937
6
Van Aelst
L
White
M
Wigler
MH
Ras partners.
Cold Spring Harbor Symp Quant Biol.
59
1994
181
186
7
Marshall
CJ
Ras effectors.
Curr Opin Cell Biol.
8
1996
197
204
8
Katz
ME
McCormick
F
Signal transduction from multiple Ras effectors.
Curr Opin Genet Dev.
7
1997
75
79
9
Glomset
JA
Farnsworth
CC
Role of protein modification reactions in programming interactions between Ras-related GTPases and cell membranes.
Annu Rev Cell Biol.
10
1994
181
205
10
Zhang
FL
Casey
PJ
Protein prenylation: molecular mechanisms and functional consequences.
Annu Rev Biochem.
65
1996
241
269
11
Gelb
MH
Protein prenylation, et cetera: signal transduction in two dimensions.
Science.
275
1997
1750
1751
12
Mumby
SM
Reversible palmitoylation of signaling proteins.
Curr Opin Cell Biol.
9
1997
148
154
13
Casey
PJ
Seabra
MC
Protein prenyltransferases.
J Biol Chem.
271
1996
5289
5292
14
Reiss
Y
Goldstein
JL
Seabra
MC
Casey
PJ
Brown
MS
Inhibition of purified p21ras farnesyl protein transferase by cys-AAX tetrapeptides.
Cell.
62
1990
81
88
15
Reiss
Y
Stradley
SJ
Gierasch
LM
Brown
MS
Goldstein
JL
Sequence requirement for peptide recognition by rat brain p21ras protein farnesyltransferase.
Proc Natl Acad Sci U S A.
88
1991
732
736
16
Yokoyama
K
Goodwin
GW
Ghomashchi
F
Glomaset
JA
Gelb
MH
A protein geranylgeranyltransferase from bovine brain: implications for protein prenylation specificity.
Proc Natl Acad Sci U S A.
88
1991
5302
5306
17
Moores
SL
Schaber
MD
Mosser
SD
et al
Sequence dependence of protein isoprenylation.
J Biol Chem.
266
1991
14603
14610
18
Trueblood
CE
Ohya
Y
Rine
J
Genetic evidence for in vivo cross-specificity of the CAAX-box protein prenyltransferases farnesyltransferase and geranylgeranyltransferase I in Saccharomyces cerevisiae.
Mol Cell Biol.
13
1993
4260
4275
19
Pellicena
P
Scholten
JD
Zimmerman
K
Creswell
M
Huang
CC
Miller
WT
Involvement of the alpha subunit of farnesyl-protein transferase in substrate recognition.
Biochemistry.
35
1996
13494
13500
20
Trueblood
CE
Boyartchuk
VL
Rine
J
Substrate specificity determinants in the farnesyltransferase β-subunit.
Proc Natl Acad Sci U S A.
94
1997
10774
10779
21
Park
H-W
Boduluri
SR
Moomaw
JF
Casey
PJ
Beese
LS
Crystal structure of protein farnesyltransferase at 2.25 Angstrom resolution.
Science.
275
1997
1800
1804
22
Akopyan
TN
Couedel
Y
Orlowski
M
Fournie-Zaluski
MC
Roques
BP
Proteolytic processing of farnesylated peptides: assay and partial purification from pig brain membranes of an endopeptidase which has the characteristics of E.C. 3.4.24.15.
Biochem Biophys Res Commun.
198
1994
787
794
23
Boyartchuk
VL
Ashby
MN
Rine
J
Modulation of Ras and a-factor function by carboxyl-terminal proteolysis.
Science.
275
1997
1796
1800
24
Hancock
J
Magee
A
Childs
J
Marshall
C
All ras proteins are polyisoprenylated but only some are palmitoylated.
Cell.
57
1989
1167
1177
25
Milligan
G
Parenti
M
Magee
AI
The dynamic role of palmitoylation in signal transduction.
Trends Biochem Sci.
20
1995
181
187
26
Ross
EM
Palmitoylation in G-protein signaling pathways.
Curr Biol.
5
1995
107
109
27
Dudler
T
Gelb
MH
Palmitoylation of Ha-Ras facilitates membrane binding, activation of downstream effectors and meiotic maturation in Xenopus oocytes.
J Biol Chem.
271
1996
11541
11547
28
Liu
L
Dudler
T
Gelb
MH
Purification of a protein palmitoyltransferase that acts on H-Ras protein and on a C-terminal N-Ras peptide.
J Biol Chem.
271
1996
23269
23276
29
Camp
LA
Verkruyse
LA
Afendis
SJ
Slaughter
CA
Hofmann
SL
Molecular cloning and expressing of palmitoyl-protein thioesterase.
J Biol Chem.
269
1994
23212
23219
30
Treisman
R
Regulation of transcription by MAP kinase cascades.
Curr Opin Cell Biol.
8
1996
205
215
31
Fanger
GR
Gerwins
P
Widmann
C
Jarpe
MB
Johnson
GL
MEKKs, GCKs, MLKs, PAKs, TAKs, and Tpls: upstream regulators of the c-Jun amino-terminal kinases?
Curr Opin Genet Dev.
7
1997
67
74
32
Robinson
MJ
Cobb
MH
Mitogen-activated protein kinase pathways.
Curr Opin Cell Biol.
9
1997
180
186
33
Garrington
TP
Johnson
GL
Organization and regulation of mitogen-activated protein kinase signaling pathways.
Curr Opin Cell Biol.
11
1999
211
218
34
Schaeffer
HJ
Weber
MJ
Mitogen-activated protein kinases: specific messages from ubiquitous messengers.
Mol Cell Biol.
19
1999
2435
2444
35
Elion
EA
Routing MAP kinase cascades.
Science.
281
1998
1625
1626
36
Schlessinger
J
How receptor tyrosine kinases activate Ras.
Trends Biol Sci.
18
1993
273
275
37
Marshall
CJ
Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation.
Cell.
80
1995
179
185
38
Marshall
CJ
Raf gets it together.
Nature.
383
1996
127
128
39
Porter
AC
Vaillancourt
RR
Tyrosine kinase receptor-activated signal transduction pathways which lead to oncogenesis.
Oncogene.
17
1998
13434
13452
40
Pawson
T
Saxton
TM
Signaling networks—do all roads lead to the same genes?
Cell.
97
1999
675
678
41
Adachi
T
Alam
R
The mechanism of IL-5 signal transduction.
Am J Physiol.
275
1998
C623
C633
42
Guthridge
MA
Stomski
FC
Thomas
D
et al
Mechanism of activation of the GM-CSF, IL-3, and IL-5 family of receptors.
Stem Cells.
16
1998
301
313
43
Daum
G
Eisenmann-Tappe
I
Fries
HW
Troppmair
J
Rapp
U
The ins and outs of Raf kinases.
Trends Biol Sci.
19
1994
474
480
44
Catling
AD
Schaeffer
H-J
Reuter
CWM
Reddy
GR
Weber
MJ
A proline-rich sequence unique to MEK1 and MEK2 is required for Raf binding and regulates MEK function.
Mol Cell Biol.
15
1995
5214
5225
45
Reuter
CWM
Catling
AD
Jelinek
T
Weber
MJ
Biochemical analysis of MEK activation in NIH3T3 fibroblasts.
J Biol Chem.
270
1995
7644
7655
46
Patriotis
C
Makris
A
Chernoff
J
Tsichlis
PN
Tpl-2 acts in concert with Ras and Raf-1 to activate mitogen-activated protein kinase.
Proc Natl Acad Sci U S A.
91
1994
9755
9759
47
Sameron
A
Ahmad
TB
Carlile
GW
Pappin
D
Narsimhan
RP
Ley
SC
Activation of MEK-1 and SEK-2 by Tpl-2 proto-oncoprotein, a novel MAP kinase kinase.
EMBO J.
15
1996
817
826
48
Posado
J
Yew
N
Ahn
NG
Vande-Woude
GF
Cooper
JA
Mos stimulates MAP kinase in Xenopus oocytes and activates MAP kinase kinase in vitro.
Mol Cell Biol.
13
1993
2546
2553
49
Bardwell
L
Thorner
J
A conserved motif at the amino termini of MEKs might mediate high affinity interaction with the cognate MAPKs.
Trends Biol Sci.
21
1996
373
374
50
Crews
CM
Alessandrini
A
Erikson
RL
The primary structure of MEK, a protein that phosphorylates the ERK gene product.
Science.
258
1992
478
480
51
Wu
J
Harrison
JK
Dent
P
Lynch
KR
Weber
MJ
Sturgill
TW
Identification and characterization of a new mammalian mitogen-activated protein kinase kinase, MKK2.
Mol Cell Biol.
8
1993
4539
4548
52
Zheng
C-F
Guan
K-L
Cloning and characterization of two distinct human extracellular signal-regulated kinase activator kinases, MEK1 and MEK2.
J Biol Chem.
268
1993
11435
11439
53
Xing
J
Ginty
DD
Greenberg
ME
Coupling of the Ras-MAPK pathway to gene activation by RSK2, a growth factor-regulated CREB kinase.
Science.
273
1996
959
963
54
Jaaro
H
Rubinfeld
H
Hanoch
T
Seger
R
Nuclear translocation of mitogen-activated protein kinase kinase (MEK1) in response to mitogenic stimulation.
Proc Natl Acad Sci U S A.
94
1997
3742
3747
55
Feig
LA
Urano
T
Cantor
S
Evidence for a Ras/Ral signaling cascade.
Trends Biochem Sci.
21
1996
438
441
56
Carpenter
CL
Cantley
LC
Phosphoinositide kinases.
Curr Opin Cell Biol.
8
1996
153
158
57
Graham
SM
Cox
AD
Drivas
G
Rush
MG
D'Eustachio
P
Der
CJ
Aberrant function of the Ras-related protein TC21/R-Ras2 triggers malignant transformation.
Mol Cell Biol.
14
1994
4108
4115
58
Cox
AD
Brtva
TR
Lowe
DG
Der
CJ
R-Ras induces malignant, but not morphologic, transformation of NIH3T3 cells.
Oncogene.
9
1994
3281
3288
59
Quilliam
LA
Castro
AF
Rogers-Graham
KS
Martin
CB
Der
CJ
Bi
C
M-Ras/R-Ras3, a transforming ras protein regulated by SOS1, GRF1, and p120 Ras GTPase-activating protein, interacts with the putative Ras effector AF6.
J Biol Chem.
274
1999
23850
23857
60
Bos
JL
RAS oncogenes in human cancer: a review.
Cancer Res.
49
1989
4682
4689
61
Clark
GJ
Der
CJ
Ras proto-oncogene activation in human malignancy.
Cellular Cancer Markers.
Garrett
CT
Sell
S
1995
17
52
Humana Press
Totowa, NJ
62
Janssen
J
Steenvoorden
A
Lyons
J
et al
Ras gene mutations in acute and chronic myelocytic leukemias, chronic myeloproliferative disorders, and myelodysplastic syndromes.
Proc Natl Acad Sci U S A.
84
1987
9228
9232
63
Bos
JL
Verlaan-de-Vries
M
van-der-Eb
AJ
et al
Mutations in N-Ras predominate in acute myeloid leukemia.
Blood.
69
1987
1237
1241
64
Farr
CJ
Saiki
RK
Erlich
HA
McCormick
F
Marshall
CJ
Analysis of Ras gene mutations in acute myeloid leukemia by polymerase chain reaction and oligonucteotide probes.
Proc Natl Acad Sci U S A.
85
1988
1629
1633
65
Padua
RA
Carter
G
Hughes
D
et al
Ras mutations in myelodysplasia detected by amplification, oligonucteotide hybridization and transformation.
Leukemia.
2
1988
503
510
66
Senn
HP
Tran-Thang
C
Wodnar-Filipowicz
A
et al
Mutational analysis of the N-RAS proto-oncogene in active and remission phase acute leukemias.
Int J Cancer.
41
1988
59
64
67
Toksoz
D
Farr
CJ
Marshall
CJ
Ras genes and acute myeloid leukemia.
Br J Haematol.
71
1989
1
6
68
Browett
PJ
Yaxley
JC
Norton
JD
Activation of Harvey ras oncogene by mutation at codon 12 is very rare in hematopoietic malignancies.
Leukemia.
3
1989
86
88
69
Browett
PJ
Norton
JD
Analysis of RAS gene mutations and methylation state in human leukemias.
Oncogene.
4
1989
1029
1036
70
Parker
J
Mufti
GJ
Ras and myelodysplasia: lessons from the last decade.
Semin Hematol.
33
1996
206
224
71
Byrne
JL
Marshall
CJ
The molecular pathophysiology of myeloid leukaemias: Ras revisited.
Br J Haematol.
100
1998
256
264
72
Vogelstein
B
Civin
CI
Preisinger
AC
et al
Ras gene mutations in childhood acute myeloid leukemia: a Pediatric Oncology Group study.
Genes Chromosomes Cancer.
2
1990
159
162
73
Hirsch-Ginsberg
C
LeMaistre
AC
Kantarjian
H
et al
Ras mutations are rare events in Philadelphia chromosome-negative/bcr gene rearrangement-negative chronic myelogenous leukemia, but are prevalent in chronic myelomonocytic leukemia.
Blood.
76
1990
1214
1219
74
Hallek
M
Leif Bergsagel
P
Anderson
KC
Multiple myeloma: increasing evidence for a multistep transformation process.
Blood.
91
1998
3
21
75
Neri
A
Knowes
DM
Greco
A
McCormick
F
Dalla-Favera
R
Analysis of Ras oncogene mutations in human lymphoid malignancies.
Proc Natl Acad Sci U S A.
85
1988
9268
9272
76
Neri
A
Murphy
JP
Cro
L
et al
Ras oncogene mutation in multiple myeloma.
J Exp Med.
170
1989
1715
1725
77
Tanaka
K
Takechi
M
Asaoku
H
Dohy
H
Kamada
N
A high frequency of N-Ras oncogene mutations in multiple myeloma.
Int J Hematol.
56
1992
119
127
78
Corradini
P
Ladetto
M
Voena
C
et al
Mutational activation of N- and K-RAS oncogenes in plasma cell dyscrasias.
Blood.
81
1993
2708
2713
79
Hunter
T
Oncoprotein networks.
Cell.
88
1997
333
346
80
Sawyers
CL
Denny
CT
Chronic myelomonocytic leukemia: Tel-a-kinase what Ets all about.
Cell.
77
1994
171
173
81
Tobal
K
Pagliuca
A
Bhatt
B
Bailey
N
Layton
DM
Mufti
GJ
Mutation of the human FMS gene (M-CSF receptor) in myelodysplastic syndromes and acute myeloid leukemia.
Leukemia.
4
1990
486
489
82
Padua
RA
Guinn
BA
Al-Sabah
AI
et al
Ras, FMS and p53 mutations and poor clinical outcome in myelodysplasias: a 10-year follow-up.
Leukemia.
12
1998
887
892
83
Nakata
Y
Kimura
A
Katoh
O
et al
c-kit point mutation of extracellular domain in patients with myeloproliferative disorders.
Br J Haematol.
91
1995
661
663
84
Buttner
C
Henz
BM
Welker
P
Sepp
NT
Grabbe
J
Identification of activating c-kit mutations in adult-, but not in childhood-onset indolent mastocytosis: a possible explanation for divergent clinical behavior.
J Invest Dermatol.
111
1998
1227
1231
85
Nagata
H
Worobec
AS
Oh
CK
et al
Identification of a point mutation in the catalytic domain of the protooncogene c-kit in peripheral blood mononuclear cells of patients who have mastocytosis with an associated hematologic disorder.
Proc Natl Acad Sci U S A.
92
1995
10560
10564
86
Kiyoi
H
Naoe
T
Nakano
Y
et al
Prognostic implication of FLT3 and N-Ras gene mutations in acute myeloid leukemia.
Blood.
93
1999
3074
3080
87
Dosil
M
Wang
S
Lemischka
IR
Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells.
Mol Cell Biol.
13
1993
6572
6585
88
Rohrschneider
LR
Bourette
RP
Lioubin
MN
Algate
PA
Myles
GM
Carlberg
K
Growth and differentiation signals regulated by the M-CSF receptor.
Mol Reprod Dev.
46
1997
96
103
89
Elmberger
PG
Lozano
MD
Weisenburger
DD
Sanger
W
Chan
WC
Transcripts of the npm-alk fusion gene in anaplastic large cell lymphoma, Hodgkin's disease, and reactive lymphoid lesions.
Blood.
86
1995
3517
3521
90
Waggott
W
Lo
YM
Bastard
C
et al
Detection of NPM-ALK DNA rearrangement in CD30 positive anaplastic large cell lymphoma.
Br J Haematol.
89
1995
905
907
91
Golub
T
Barker
G
Lovett
M
Gilliland
D
Fusion of PDGF receptor β to a novel ets-like gene, tel, in chronic myelomonocytic leukemia with t(5;12) chromosomal translocation.
Cell.
77
1994
307
316
92
Jousset
C
Carron
C
Boureux
A
et al
A domain of TEL conserved in a subset of ETS proteins defines a specific oligomerization interface essential to the mitogenic properties of the TEL-PDGFRβ oncoprotein.
EMBO J.
16
1997
69
82
93
Papadopoulos
P
Ridge
SA
Boucher
CA
Stocking
C
Wiedemann
LM
The novel activation of abl by fusion to an ets-related gene, tel.
Cancer Res.
55
1995
34
38
94
Golub
TR
Goga
A
Barker
GF
et al
Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia.
Mol Cell Biol.
16
1996
4107
4116
95
Kurzrock
R
Gutterman
J
Talpaz
M
The molecular genetics of Philadelphia chromosome-positive leukemias.
N Engl J Med.
319
1988
990
998
96
Faderl
S
Talpaz
M
Estrow
Z
O'Brien
S
Kurzrock
R
Kantarjian
HM
The biology of chronic myeloid leukemia.
N Engl J Med.
341
1999
164
172
97
Zou
X
Calame
K
Signaling pathways activated by oncogenic forms of Abl tyrosine kinase.
J Biol Chem.
274
1999
18141
18144
98
Xu
G
O'Connell
P
Viskochil
D
et al
The neurofibromatosis type 1 gene encodes a protein related to GAP.
Cell.
62
1990
599
608
99
Niemeyer
CM
Arico
M
Basso
G
et al
Chronic myelomonocytic leukemia in childhood: a retrospective analysis of 110 cases.
Blood.
89
1997
3534
3543
100
Shannon
KM
O'Connell
P
Martin
GA
et al
Loss of the normal NF-1 allele from the bone marrow of children with type 1 neurofibromatosis and malignant myeloid disorders.
N Engl J Med.
330
1994
597
601
101
Stiller
CA
Chessells
JM
Fitchett
M
Neurofibromatosis and childhood leukemia/lymphoma: a population based UKCCSG study.
Br J Cancer.
70
1994
969
972
102
Side
L
Taylor
B
Cayouette
M
et al
Homozygous inactivation of the NF-1 gene in bone marrow cells from children with neurofibromatosis type 1 and malignant myeloid disorders.
N Engl J Med.
336
1997
1713
1720
103
DeClue
JE
Papageorge
AG
Fletcher
JA
et al
Abnormal regulation of mammalian p21ras contributes to malignant tumor growth in von Recklinghausen (type 1) neurofibromatosis.
Cell.
69
1992
265
273
104
Kalra
R
Paderanga
DC
Olson
K
Shannon
KM
Genetic analysis is consistent with the hypothesis that NF-1 limits myeloid cell growth through p21ras.
Blood.
84
1994
3435
3439
105
Bollag
G
Clapp
DW
Shih
S
et al
Loss of NF-1 results in activation of the Ras signaling pathway and leads to aberrant growth in hematopoietic cells.
Nat Genet.
12
1996
144
148
106
Largaespada
DA
Brannan
CI
Jenkins
NA
Copeland
NG
NF-1 deficiency causes Ras-mediated granulocyte/macrophage colony stimulating factor hypersensitivity and chronic myeloid leukemia.
Nat Genet.
12
1996
137
143
107
Miyauchi
J
Asada
M
Sasaki
M
Tsunematsu
Y
Kojima
S
Mizutani
S
Mutations of N-ras gene in juvenile chronic myelogenous leukemia.
Blood.
83
1994
2248
2254
108
Birnbaum
RA
O'Marcaigh
A
Wardak
Z
NF1 and GM-CSF interact in myeloid leukemogenesis.
Mol Cell.
5
2000
189
195
109
MacKencie
KL
Dolnikov
A
Millington
M
Shounan
Y
Symonds
G
Mutant N-ras induces myeloproliferative disorders and apoptosis in bone marrow repopulated mice.
Blood.
93
1999
2043
2056
110
Saison-Behmoaras
T
Tocque
B
Rey
I
Chassignol
M
Thuong
NT
Helene
C
Short modified antisense oligonucleotides directed against Ha-RAS point mutation induce selective cleavage of the mRNA and inhibit T24 cells proliferation.
EMBO J.
10
1991
1111
1118
111
Mukhopadhyah
T
Tainsky
M
Cavender
AC
Roth
JA
Specific inhibition of K-ras expression and tumorigenicity of lung cancer cells by antisense RNA.
Cancer Res.
51
1991
1744
1748
112
Shirasawa
S
Furuse
M
Yokoyama
N
Sasazuki
T
Altered growth of human colon cancer cell lines disrupted at activated Ki-Ras.
Science.
260
1993
85
88
113
Kashani-Sabet
M
Funato
T
Florenes
VA
Fodstad
O
Scanlon
KJ
Suppression of the neoplastic phenotype in vivo by an anti-ras ribozyme.
Cancer Res.
54
1994
900
902
114
Gibbs
JB
Ras C-terminal processing enzymes: new drug targets?
Cell.
65
1991
1
4
115
Tamanoi
F
Inhibitors of Ras farnesyltransferases.
Trends Biochem Sci.
18
1993
349
353
116
Gibbs
JB
Oliff
A
Pharmaceutical research in molecular oncology.
Cell.
79
1994
193
198
117
Gibbs
JB
Oliff
A
Kohl
NE
Farnesyltransferase inhibitors: Ras research yields a potential cancer therapeutic.
Cell.
77
1994
175
178
118
Lowy
DR
Willumsen
BM
Rational cancer therapy.
Nat Med.
1
1995
747
748
119
Gibbs
JB
Oliff
A
The potential of farnesyltransferase inhibitors as cancer chemotherapeutics.
Annu Rev Pharmacol Toxicol.
37
1997
143
166
120
Omer
CA
Kohl
NE
CA1A2X-competitive inhibitors of farnesyltransferase as anti-cancer agents.
Trends Pharmacol Sci.
18
1997
437
444
121
Heimbrook
DC
Oliff
A
Therapeutic intervention and signaling.
Curr Biol.
10
1998
284
288
122
Kato
K
Cox
AD
Hisaka
MM
Graham
SM
Buss
JE
Der
CJ
Isoprenoid addition to Ras protein is the critical modification for its membrane association and transforming activity.
Proc Natl Acad Sci U S A.
89
1992
6403
6407
123
Kang
MS
Stemerick
DM
Zwolshen
JH
Harry
BS
Sunkara
PS
Harrison
BL
Farnesyl-derived inhibitors of Ras farnesyl transferase.
Biochem Biophys Res Commun.
217
1995
245
249
124
Yonemoto
M
Satoh
T
Arakawa
H
et al
J-104,871, a novel farnesyltransferase inhibitor, blocks Ras farnesylation in vivo in a farnesyl pyrophosphate-competitive manner.
Mol Pharmacol.
54
1998
1
7
125
James
GL
Goldstein
JL
Brown
MS
et al
Benzodiazepine peptidomimetics: potent inhibitors of Ras farnesylation in animal cells.
Science.
260
1993
1937
1942
126
James
GL
Goldstein
JL
Brown
MS
Polylysine and CVIM sequences of K-RasB dictate specificity of prenylation and confer resistance to benzodiazepine peptidomimetic in vitro.
J Biol Chem.
270
1995
6221
6226
127
Dalton
MB
Fantle
KS
Bechtold
HA
et al
The farnesyl protein transferase inhibitor BZA-5B blocks farnesylation of nuclear lamins and p21ras but does not affect their function or localization.
Cancer Res.
55
1995
3295
3304
128
Kohl
NE
Mosser
SD
DeSolms
SJ
et al
Selective inhibition of ras-dependent transformation by a farnesyltransferase inhibitor.
Science.
260
1993
1934
1937
129
Kohl
NE
Wilson
FR
Mosser
SD
et al
Protein farnesyltransferase inhibitors block the growth of ras-dependent tumors in nude mice.
Proc Natl Acad Sci U S A.
91
1994
9141
9145
130
Prendergast
GC
Davide
JP
DeSolms
JS
et al
Farnesyltransferase inhibition causes morphological reversion of ras-transformed cells by a complex mechanism that involves regulation of the actin cytoskeleton.
Mol Cell Biol.
14
1994
4193
4202
131
Lebowitz
PF
Sakamuro
D
Prendergast
GC
Farnesyl transferase inhibitors induce apoptosis of Ras-transformed cells denied substratum attachment.
Cancer Res.
57
1997
708
713
132
Emanuel
PD
Snyder
RC
Wiley
T
Gopurala
B
Castleberry
RP
Inhibition of juvenile myelomonocytic leukemia cell growth in vitro by farnesyltransferase inhibitors.
Blood.
95
2000
639
645
133
Koblan
KS
Culberson
JC
DeSolms
SJ
et al
NMR studies of novel inhibitors bound to farnesyl-protein transferase.
Protein Sci.
4
1995
681
688
134
Barrington
RE
Subler
MA
Rands
E
et al
A farnesyltransferase inhibitor induces tumor regression in transgenic mice harboring multiple oncogenic mutations by mediating alterations in both cell cycle control and apoptosis.
Mol Cell Biol.
18
1998
85
92
135
Kohl
NE
Omer
CA
Conner
MW
et al
Inhibition of farnesyltransferase induces regression of mammary and salivary carcinomas in ras transgenic mice.
Nat Med.
1
1995
792
797
136
Sepp-Lorenzino
L
Ma
Z
Rands
E
et al
A peptidomimetic inhibitor of farnesyl: protein transferase blocks the anchorage-dependent and -independent growth of human tumor cell lines.
Cancer Res.
55
1995
5302
5309
137
Mangues
R
Corral
T
Kohl
NE
et al
Antitumor effect of a farnesyl protein transferase inhibitor in mammary and lymphoid tumors overexpressing N-RAS in transgenic mice.
Cancer Res.
58
1998
1253
1259
138
Cox
AD
Garcia
AM
Westwick
JK
et al
The CAAX peptidomimetic compound B581 specifically blocks farnesylated, but not geranylgeranylated or myristylated, oncogenic ras signaling and transformation.
J Biol Chem.
269
1994
19203
19206
139
Qian
Y
Blaskovich
MA
Saleem
M
et al
Design and structural requirements of potent peptidomimetic inhibitors of p21ras farnesyltransferase.
J Biol Chem.
269
1994
12410
12413
140
Sun
J
Qian
Y
Hamilton
AD
Sebti
SM
Ras CAAX peptidomimetic FTI-276 selectively blocks tumor growth in nude mice of a human lung carcinoma with K-Ras mutation and p53 deletion.
Cancer Res.
55
1995
4243
4247
141
Lerner
EC
Qian
Y
Blaskovich
MA
et al
Ras CAAX peptidomimetic FTI-277 selectively blocks oncogenic ras signaling by inducing cytoplasmatic accumulation of inactive Ras-Raf complexes.
J Biol Chem.
270
1995
26802
26806
142
Bredel
M
Pollack
IF
Freund
JM
Hamilton
AD
Sebti
SM
Inhibition of Ras and related G-proteins as a therapeutic strategy for blocking malignant glioma growth.
Neurosurgery.
43
1998
124
131
143
Bernhard
EJ
McKenna
WG
Hamilton
AD
et al
Inhibiting Ras prenylation increases the radiosensitivity of human tumor cell lines with activating mutations of ras oncogenes.
Cancer Res.
58
1998
1754
1761
144
Nagasu
T
Yoshimatsu
K
Rowell
C
Lewis
MD
Garcia
AM
Inhibition of human tumor xenograft growth by treatment with the farnesyl transferase inhibitor B956.
Cancer Res.
55
1995
5310
5314
145
Bishop
WR
Bond
R
Petrin
J
et al
Novel tricyclic inhibitors of farnesyl protein transferase.
J Biol Chem.
270
1995
30611
30618
146
Njoroge
FG
Vibulbhan
B
Pinto
P
et al
Potent, selective, and orally bioavailable tricyclic pyridyl acetamide N-oxide inhibitors of farnesyl protein transferase with enhanced in vivo antitumor activity.
J Med Chem.
41
1998
1561
1567
147
Njoroge
FG
Taveras
AG
Kelly
J
et al
(+)-4-[2-[4-(8-Chloro-3,10-dibromo-6,11-dihydro-5H-benzo[5,6]cycloheptal[1,2b]-pyridin-11(R)-yl)-1-piperidinyl]-2-oxo-ethyl]-1-piperidinecarboxamide (SCH66336): a very potent farnesyl protein transferase inhibitor as a novel antitumor agent.
J Med Chem.
41
1998
4890
4902
148
Mallams
AK
Rossman
RR
Doll
RJ
et al
Inhibitors of farnesyl protein transferase. 4-Amido, 4-carbamoyl, and 4-carboxamido derivatives of 1-(8-chloro-6,11-dihydro-5H-benzo[5,6]-cyclohepta[1,2b]pyridin-11-yl)piperazine and 1-(3-bromo-8-chloro-6,11-dihydro-5H-benzo[5,6]cyclohepta[1,2b]pyridin-11-yl)piperazine.
J Med Chem.
41
1998
877
893
149
Liu
M
Bryant
MS
Chen
J
et al
Effects of SCH59228, an orally bioavailable farnesyl protein transferase inhibitor, on the growth of oncogene-transformed fibroblasts and a human colon carcinoma xenograft in nude mice.
Cancer Chemother Pharmacol.
43
1999
50
58
150
Patel
DV
Gordon
EM
Schmidt
RJ
et al
Phosphinyl acid-based bisubstrate analog inhibitors of ras farnesyl protein transferase.
J Med Chem.
38
1995
435
442
151
Manne
V
Yan
N
Carboni
JM
et al
Bisubstrate inhibitors of farnesyltransferase: a novel class of specific inhibitors of ras-transformed cells.
Oncogene.
10
1995
1763
1779
152
Patel
DV
Young
MG
Robinson
SP
Hunihan
L
Dean
BJ
Gordon
EM
Hydroxamic acid-based bisubstrate analog inhibitors of Ras farnesyl protein transferase.
J Med Chem.
39
1996
4197
4210
153
Gelb
MH
Tamanoi
F
Yokoyama
K
Ghomashchi
F
Esson
K
Gould
MN
The inhibition of protein prenyltransferases by oxygenated metabolites of limonene and perillyl alcohol.
Cancer Lett.
91
1995
169
175
154
Hara
M
Akasaka
K
Akinaga
S
et al
Identification of Ras farnesyltransferase inhibitors by microbial screening.
Proc Natl Acad Sci U S A.
90
1993
2281
2285
155
Nagase
T
Kawata
S
Tamura
S
et al
Manumycin and gliotoxin derivative KT7595 block Ras farnesylation and cell growth but do not disturb lamin farnesylation and localization in human tumour cells.
Br J Cancer.
76
1997
1001
1010
156
Kainuma
O
Asano
T
Hasegawa
M
et al
Inhibition of growth and invasive activity of human pancreatic cancer cells by a farnesyltransferase inhibitor, manumycin.
Pancreas.
15
1997
379
383
157
Jayasuriya
H
Bali
RG
Zink
DL
et al
Barcelonic acid A, a new farnesyl-protein transferase inhibitor from Phoma species.
J Nat Prod.
58
1995
986
991
158
Van der Pyl
D
Cans
P
Debernard
JJ
et al
RPR113228, a novel farnesyl-protein transferase inhibitor produced by Chrysosporium lobatum.
J Antibiot (Tokyo).
48
1995
736
737
159
Silverman
KC
Cascales
C
Genilloud
O
et al
Actinoplanic acids A and B as novel inhibitors of farnesyl-protein transferase.
Appl Microbiol Biotechnol.
43
1995
610
616
160
Silverman
KC
Jayasuriya
H
Cascales
C
et al
Oreganic acid, a potent inhibitor of Ras farnesyl-protein transferase.
Biochem Biophys Res Commun.
232
1997
478
481
161
Sturm
S
Gil
RR
Chai
HB
et al
Lupane derivatives from Lophopetalum wallichi with farnesyl protein transferase inhibitory activity.
J Nat Prod.
59
1996
658
663
162
Sekizawa
R
Iinuma
H
Naganawa
H
et al
Isolation of novel saquayamycins as inhibitors of farnesyl-protein transferase.
J Antibiot (Tokyo).
49
1996
487
490
163
Tsuda
M
Muraoka
Y
Takeuchi
T
Sekizawa
R
Umezawa
K
Stereospecific synthesis of a novel farnesyl protein transferase inhibitor, valinoctin and its analogues.
J Antibiot (Tokyo).
49
1996
1031
1035
164
Lee
S
Park
S
Oh
JW
Yang
C
Natural inhibitors for protein prenyltransferase.
Planta Med.
64
1998
303
308
165
Gibbs
JB
Pompliano
DL
Mosser
SD
et al
Selective inhibition of farnesyl-protein transferase blocks ras processing in vivo.
J Biol Chem.
268
1993
7617
7620
166
James
GL
Brown
MS
Cobb
MH
Goldstein
JL
Benzodiazepine peptidomimetic BZA-5B interrupts the MAP kinase activation pathway in H-Ras-transformed Rat-1 cells, but not in untransformed cells.
J Biol Chem.
269
1994
27705
27714
167
James
G
Goldstein
JL
Brown
MS
Resistance of K-RasBV12 proteins to farnesyltransferase inhibitors in Rat1 cells.
Proc Natl Acad Sci U S A.
93
1996
4454
4458
168
Sun
J
Qian
Y
Hamilton
AD
Sebti
SM
Both farnesyltransferase and geranylgeranyltransferase I inhibitors are required for inhibition of oncogenic K-Ras prenylation but each alone is sufficient to suppress human tumor growth in nude mouse xenografts.
Oncogene.
16
1998
1467
1473
169
Whyte
DB
Kirschmeier
P
Hockenberry
TN
et al
K-Ras and N-Ras are geranylgeranylated in cells treated with farnesyl protein transferase inhibitors.
J Biol Chem.
272
1997
14459
14464
170
Zhang
FL
Kirschmeier
P
Carr
D
et al
Characterization of Ha-ras, N-ras, Ki-Ras4A, and Ki-Ras4B as in vitro substrates for farnesyl protein transferase and geranylgeranyl protein transferase type I.
J Biol Chem.
272
1997
10232
10239
171
Rowell
CA
Kowalczyk
JJ
Lewis
MD
Garcia
AM
Direct demonstration of geranylgeranylation and farnesylation of Ki-Ras in vivo.
J Biol Chem.
272
1997
14093
14097
172
Lerner
EC
Zhang
TT
Knowles
DB
Qian
Y
Hamilton
AD
Sebti
SD
Inhibition of the prenylation of K-RAS, but not H- or N-RAS is highly resistant to CAAX peptidomimetics and requires both a farnesyltransferase and a geranylgeranyltransferase I inhibitor in human tumor cell lines.
Oncogene.
15
1997
1283
1288
173
Lerner
EC
Qian
Y
Hamilton
AD
Sebti
SM
Disruption of oncogenic K-Ras4B processing and signaling by a potent geranylgeranyltransferase I inhibitor.
J Biol Chem.
270
1995
26770
26773
174
Lebowitz
PF
Davide
JP
Prendergast
GC
Evidence that farnesyltransferase inhibitors suppress Ras transformation by interfering with Rho activity.
Mol Cell Biol.
15
1995
66136622
175
Lebowitz
PF
Prendergast
GC
Non-Ras targets of farnesyltransferase inhibitors: focus on Rho.
Oncogene.
17
1998
1439
1445
176
Booden
MA
Baker
TL
Solski
PA
Der
CJ
Punke
SG
Buss
JE
A non-farnesylated Ha-Ras protein can be palmitoylated and trigger potent differentiation and transformation.
J Biol Chem.
274
1999
1423
1431
177
Cox
AD
Der
CJ
Farnesyltransferase inhibitors and cancer treatment: targeting simply Ras?
Biochim Biophys Acta.
1333
1997
F51
F71
178
Du
W
Lebowitz
PF
Prendergast
GC
Cell growth inhibition by farnesyltransferase inhibitors is mediated by gain of geranylgeranylated RhoB.
Mol Cell Biol.
19
1999
1831
1840
179
Adamson
P
Marshall
CJ
Hall
A
Tilbrook
PA
Post-translational modifications of p21rho proteins.
J Biol Chem.
267
1992
2003320038
180
Sepp-Lorenzino
L
Rosen
N
A farnesyl-protein transferase inhibitor induces p21 expression and G1 block in p53 wild type tumor cells.
J Biol Chem.
273
1998
20243
20251
181
Hung
WC
Chuang
LY
Involvement of caspase family proteases in FPT inhibitor II-induced apoptosis in human ovarian cancer cells.
Int J Cancer.
12
1998
1339
1342
182
Hung
WC
Chuang
LY
The farnesyltransferase inhibitor, FPT inhibitor III upregulates Bax and Bcl-xs expression and induces apoptosis in human ovarian cancer cells.
Int J Oncol.
12
1998
137
140
183
Norgaard
P
Law
B
Joseph
H
et al
Treatment with farnesyl-protein transferase inhibitor induces regression of mammary tumors in transforming growth factor (TGF) alpha and TGF alpha/neu transgenic mice by inhibition of mitogenic activity and induction of apoptosis.
Clin Cancer Res.
5
1999
35
42
184
Moasser
MM
Sepp-Lorenzino
L
Kohl
NE
Oliff
A
Balog
A
Su
DS
Farnesyl transferase inhibitors cause enhanced mitotic sensitivity to taxol and epothilones.
Proc Natl Acad Sci U S A.
95
1998
1369
1374
185
DeSolms
SJ
Giuliani
EA
Graham
SL
et al
N-Arylalkyl pseudopeptide inhibitors of farnesyltransferase.
J Med Chem.
41
1998
2651
2656
186
McNamara
DJ
Dobrusin
E
Leonard
DM
et al
C-terminal modifications of histidyl-N-benzylglycinamides to give improved inhibition of Ras farnesyltransferase, cellular activity, and anticancer activity in mice.
J Med Chem.
40
1997
3319
3322
187
Leftheris
K
Kline
T
Vite
GD
et al
Development of potent inhibitors of Ras farnesyltransferase possessing cellular and in vivo activity.
J Med Chem.
39
1996
224
236
188
Jansen
B
Schlagbauer-Wadl
H
Kahr
H
et al
Novel Ras antagonist blocks human melanoma growth.
Proc Natl Acad Sci U S A.
96
1999
14019
14024
189
Brown
MS
Goldstein
JL
Paris
KJ
Burnier
JP
Marsters
JC
Jr
Tetrapeptide inhibitors of protein farnesyltransferase: amino-terminal substitution in phenylalanine-containing tetrapeptides restores farnesylation.
Proc Natl Acad Sci U S A.
89
1992
8313
8316
190
Mahgoub
N
Taylor
BR
Gratiot
M
et al
In vitro and in vivo effects of a farnesyltransferase inhibitor on NF-1-deficient hematopoietic cells.
Blood.
94
1999
2469
2476
191
Ito
T
Kawata
S
Tamura
S
et al
Suppression of human pancreatic cancer growth in BALB/c nude mice by manumycin, a farnesyl:protein transferase inhibitor.
Jpn J Cancer Res.
87
1996
113
116
192
Liu
M
Bryant
MS
Chen
J
et al
Antitumor activity of SCH66336, an orally bioavailable tricyclic inhibitor of farnesyl protein transferase, in human tumor xenograft models and Wap-ras transgenic mice.
Cancer Res.
58
1998
4947
4956
193
Prendergast
GC
Davide
JP
Lebowitz
PF
Wechsler-Reya
R
Kohl
NE
Resistance of a variant ras-transformed cell line to phenotypic reversion by farnesyl transferase inhibitors.
Cancer Res.
56
1996
2626
2632
194
Del Villar
K
Urano
J
Guo
L
Tamanoi
F
A mutant form of human protein farnesyltransferase exhibits increased resistance to farnesyltransferase inhibitors.
J Biol Chem.
274
1999
27010
27017
195
Qian
Y
Vogt
A
Vasudevan
A
Sebti
SM
Hamilton
AD
Selective inhibition of type-I geranylgeranyltransferase in vitro and in whole cells by CAAL peptidomimetics.
Bioorg Med Chem.
6
1998
293
299
196
Vogt
A
Qian
Y
McGuire
TF
Hamilton
AD
Sebti
SM
Protein geranylgeranylation, not farnesylation, is required for the G1 to S phase transition in mouse fibroblasts.
Oncogene.
13
1996
1991
1999
197
McGuire
TF
Qian
Y
Vogt
A
Hamilton
AD
Sebti
SM
Platelet-derived growth factor receptor tyrosine phosphorylation requires protein geranylgeranylation but not farnesylation.
J Biol Chem.
271
1996
27402
27407
198
Miquel
K
Pradines
A
Sun
J
et al
GGTI-298 induces G0–G1 block and apoptosis whereas FTI-277 causes G2-M enrichment in A549 cells.
Cancer Res.
57
1997
1846
1850
199
Vogt
A
Sun
J
Qian
Y
Hamilton
AD
Sebti
SM
The geranylgeranyltransferase-I inhibitor GGTI-298 arrests human tumor cells in G0/G1 and induces p21(WAF1/CIP1/SDI1) in a p53-independent manner.
J Biol Chem.
272
1997
27224
27229
200
Lantry
LE
Zhang
Z
Yao
R
et al
Effect of farnesyltransferase inhibitor FTI-276 on established lung adenomas from A/J mice induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone.
Carcinogenesis.
21
2000
113
116
201
Volker
C
Miller
RA
McCleary
WR
et al
Effects of farnesylcysteine analogs on protein carboxyl methylation and signal transduction.
J Biol Chem.
266
1991
21515
21522
202
Marciano
D
Ben-Baruch
G
Marom
M
Egozi
Y
Haklai
R
Kloog
Y
Farnesyl derivatives of rigid carboxylic acids—inhibitors of Ras-dependent cell growth.
J Med Chem.
38
1995
1267
1272
203
Marom
M
Haklai
R
Ben-Baruch
G
Marciano
D
Egozi
Y
Kloog
Y
Selective inhibition of Ras-dependent cell growth by farnesylthiosalisylic acid.
J Biol Chem.
270
1995
22263
22270
204
Gana-Weisz
M
Haklai
R
Marciano
D
Egozi
Y
Ben-Baruch
G
Kloog
Y
The Ras antagonist S-farnesylthiosalicylic acid induces inhibition of MAPK activation.
Biochem Biophys Res Commun.
239
1997
900
904
205
Haklai
R
Weisz
MG
Elad
G
et al
Dislodgment and accelerated degradation of Ras.
Biochemistry.
37
1998
1306
1314
206
Perez-Sala
D
Gilbert
BA
Rando
RR
Canada
FJ
Analogs of farnesylcysteine induce apoptosis in HL-60 cells.
FEBS Lett.
426
1998
319
324
207
Aharonson
Z
Gana-Weisz
M
Varsano
T
Haklai
R
Marciano
D
Kloog
Y
Stringent structural requirements for anti-Ras activity of S-prenyl analogues.
Biochim Biophys Acta.
1406
1998
40
50
208
Kadam
S
McAlpine
JB
Dorrigocins: novel antifungal antibiotics that change the morphology of ras-transformed NIH/3T3 cells to that of normal cells. III. Biological properties and mechanism of action.
J Antibiot (Tokyo).
47
1994
875
880
209
Chen
Y
Selective inhibition of ras-transformed cell growth by a novel fatty acid–based chloromethyl ketone designed to target Ras endoprotease.
Ann N Y Acad Sci.
886
1999
103
108
210
Alessi
DR
Cuenda
A
Cohen
P
Dudley
DT
Saltiel
AD
PD 098059 is a specific inhibitor of the activation of mitogen-activated protein kinase kinase in vitro and in vivo.
J Biol Chem.
270
1995
27489
27494
211
Dudley
DT
Pang
L
Decker
SJ
Bridges
AJ
Saltiel
AR
A synthetic inhibitor of the mitogen-activated protein kinase cascade.
Proc Natl Acad Sci U S A.
92
1995
7686
7689
212
DeSilva
DR
Jones
EA
Favata
MF
et al
Inhibition of mitogen-activated protein kinase kinase blocks T cell proliferation but does not induce or prevent anergy.
J Immunol.
160
1998
4175
4181
213
Favata
MF
Horiuchi
KJ
Manos
EJ
et al
Identification of a novel inhibitor of mitogen-activated protein kinase kinase.
J Biol Chem.
273
1998
18623
18632
214
Williams
DH
Wilkinson
SE
Purton
T
Lamont
A
Flotow
H
Murray
EJ
Ro 09-2210 exhibits potent anti-proliferative effects on activated T-cells by selectively blocking MKK activity.
Biochemistry.
37
1998
9579
9585
215
Cox
LY
Motz
J
Troll
W
Garte
SJ
Antipain-induced suppression of oncogene expression in H-ras-transformed NIH3T3 cells.
Cancer Res.
51
1991
4810
4814
216
Itoh
O
Kuroiwa
S
Atsumi
S
Umezawa
K
Takeuchi
T
Mar
M
Induction by guanosine analogue oxanosine of reversion toward the normal phenotype of K-ras transformed rat kidney cells.
Cancer Res.
49
1989
996
1000
217
Shindo-Okado
N
Makabe
O
Nagahara
H
Nishimura
S
Permanent conversion of mouse and human cells transformed by activated ras or raf genes to apparently normal cells by treatment with the antibiotic azatyrosine.
Mol Carcinog.
2
1989
159
167
218
Kumar
CC
Prorock-Rogers
C
Kelly
J
et al
SCH51344 inhibits ras transformation by a novel mechanism.
Cancer Res.
55
1995
5106
5117
219
Walsh
AB
Dhanasekaran
M
Bar-Sagi
D
Kumar
CC
SCH51344-induced reversal of Ras-transformation is accompanied by the specific inhibition of the Ras and Rac-dependent cell morphology pathway.
Oncogene.
15
1997
2553
2560
220
Giardiello
FM
Hamilton
SR
Krush
AJ
et al
Treatment of colonic and rectal adenomas with sulindac in familial adenomatous polyposis.
N Engl J Med.
328
1993
1313
1316
221
Verhuel
HM
Panigrahy
D
Yuan
J
D'Amato
RJ
Combination oral antiangiogenic therapy with thalidomide and sulindac inhibits tumour growth in rabbits.
Br J Cancer.
79
1999
114
118
222
Herrmann
C
Block
C
Geisen
C
et al
Sulindac sulfide inhibits Ras signaling.
Oncogene.
17
1998
1769
1776
223
Schulte
TW
Blagosklonny
MV
Romanova
L
et al
Destabilization of Raf-1 by geldanamycin leads to disruption of the Raf-1-MEK-mitogen–activated protein kinase signaling pathway.
Mol Cell Biol.
16
1996
5839
5845
224
Stebbins
CE
Russo
AA
Schneider
C
Rosen
N
Hartl
FU
Pavletich
NP
Crystal structure of an HSP90-geldanamycin complex: targeting of a protein chaperone by an antitumor agent.
Cell.
89
1997
239
250
225
Koera
K
Nakamura
K
Nakao
K
et al
K-ras is essential for the development of the mouse embryo.
Oncogene.
15
1997
1151
1159
226
Johnson
L
Greenbaum
D
Cichowski
K
et al
K-ras is an essential gene in the mouse with partial functional overlap with N-ras.
Genes Dev.
11
1997
2468
2481
227
Umanoff
H
Edelmann
W
Pellicer
A
Kucherlapati
R
The murine N-ras gene is not essential for growth and development.
Proc Natl Acad Sci U S A.
92
1995
1709
1713
228
Casey
S
Dautry
F
Inactivation of the murine N-ras gene by gene targeting.
Oncogene.
12
1992
2525
2528
229
Qiu
RG
Chen
J
McCormick
F
Symons
M
A role for Rho in Ras transformation.
Proc Natl Acad Sci U S A.
92
1995
11781
11785
230
Khosravi-Far
R
Solski
PA
Clark
GJ
Kinch
MS
Der
CJ
Activation of Rac1, RhoA and mitogen-activated protein kinases is required for Ras transformation.
Mol Cell Biol.
15
1995
6443
6453
231
Khosravi-Far
R
White
MA
Westwick
JK
et al
Oncogenic Ras activation of Raf/mitogen-activated protein kinase-independent pathways is sufficient to cause tumorigenic transformation.
Mol Cell Biol.
16
1996
3923
3933

Author notes

C. Reuter, Dept of Internal Medicine III, University of Ulm, Robert-Koch-Str 8, D-89081 Ulm, Germany; e-mail: christoph.reuter@medizin.uni-ulm.de.

Sign in via your Institution