Anticancer treatment using cytotoxic drugs is considered to mediate cell death by activating key elements of the apoptosis program and the cellular stress response. While proteolytic enzymes (caspases) serve as main effectors of apoptosis, the mechanisms involved in activation of the caspase system are less clear. Two distinct pathways upstream of the caspase cascade have been identified. Death receptors, eg, CD95 (APO-1/Fas), trigger caspase-8, and mitochondria release apoptogenic factors (cytochrome c, Apaf-1, AIF), leading to the activation of caspase-9. The stressed endoplasmic reticulum (ER) contributes to apoptosis by the unfolded protein response pathway, which induces ER chaperones, and by the ER overload response pathway, which produces cytokines via nuclear factor-κB. Multiple other stress-inducible molecules, such as p53, JNK, AP-1, NF-κB, PKC/MAPK/ERK, and members of the sphingomyelin pathway have a profound influence on apoptosis. Understanding the complex interaction between different cellular programs provides insights into sensitivity or resistance of tumor cells and identifies molecular targets for rational therapeutic intervention strategies.

In an overall scenario, the development of malignant tumors results from deregulated proliferation or an inability of cells to undergo apoptotic cell death.1,2Anticancer drugs inhibit proliferation and induce apoptosis in sensitive tumor cells.3 4 The cellular targets for different cytotoxic agents are diverse. Thus, anticancer drugs are classified as DNA-damaging agents (cyclophosphamide, cisplatin, doxorubicin), antimetabolites (methotrexate, 5-fluorouracil), mitotic inhibitors (vincristine), nucleotide analogs (6-mercaptopurine), or inhibitors of topoisomerases (etoposide). The common underlying mechanism for chemotherapy-induced apoptosis might be damage to DNA, lipid components of cell membranes, and cellular proteins causing an imbalance of the cellular homeostasis commonly designated as cellular stress. This in turn initiates a complex cascade of stress-inducible signaling molecules in an attempt to return the cell to its previous equilibrium. As for the response to DNA damage, this may include cell-cycle regulation and repair mechanisms. The type and dose of stress within the cellular context appears to dictate the outcome of the cellular response, which is intimately converted to complex pathways mediating cell-cycle control or cell death. Apoptosis seems to be induced if damage exceeds the capacity of repair mechanisms. Here, we review mechanisms of cellular stress signaling with respect to their integration into apoptosis pathways.

Caspases as death effectors

Apoptosis signaling induced by anticancer drugs converges in the activation of intracellular caspases and their modification of protein substrates within the nucleus and cytoplasm (Figure1). Currently more than 14 caspases have been cloned and partially characterized in mammals, some of which are not involved in apoptosis but rather mediate cytokine processing. Caspases are cysteine proteases produced as inactive zymogens that cleave their substrates at aspartic acid residues contained within a tetrapeptide recognition motif. Activation of initiator caspases (procaspase-8, -9, -10) leads to the proteolytic activation of downstream effector caspases (caspase-3, -6, -7) that cleave specific substrates. For example, cleavage of the nuclear lamin is required for nuclear shrinking and budding. Loss of overall cell shape is probably caused by the cleavage of cytoskeletal proteins, such as fodrin, gelsolin, plectin, actin, and cytokeratin. DNA fragmentation is due to cleavage and inactivation of ICAD, the initiator of CAD (caspase-activated DNase). In addition, the activation of several kinases by caspase cleavage, including PAK2—a member of the p21-activated kinase family—and the Ste20-related kinases MST1 and SLK, contributes to the membrane remodeling and active blebbing observed in apoptotic cells.5-7 Two independent initiator pathways lie immediately upstream of these effector events: cross-linking of death receptors by their ligands and the release of apoptogenic factors from mitochondria.5,8 9 

Fig. 1.

The cell death machinery.

The death receptor pathway (left) is triggered by members of the death receptor superfamily such as CD95. Binding of CD95-L to its receptor induces trimerization of CD95 and formation of a death-inducing complex. This complex recruits, via the adaptor molecule FADD, multiple procaspase-8 molecules, resulting in caspase-8 activation. Caspase-8 activation can be blocked by recruitment of c-FLIP. The mitochondrial death pathway (right) is controlled by members of the Bcl-2 family, including the proapoptotic Bax and Bid proteins and the antiapoptotic Bcl-2 and Bcl-XL proteins. Death stimuli induce the release of cytochrome c, AIF, Apaf-1, Smac/DIABLO, and possibly other factors from mitochondria. Cytochrome c associates with Apaf-1 and caspase-9 to form the apoptosome. The death receptor and mitochondrial pathways converge at the level of caspase-3 activation. Caspase-3 activation and activity is antagonized by the IAP proteins, which themselves are antagonized by the Smac/DIABLO protein released from mitochondria. Active caspase-3 activates downstream caspases, which results in cleavage of cellular substrates and apoptosis. Crosstalk between the death receptor and mitochondrial pathways is provided by Bid, a proapoptotic Bcl-2 family member. Caspase-8–mediated cleavage of Bid greatly increases its prodeath activity and results in its translocation to mitochondria, where it promotes cytochrome c exit.

Fig. 1.

The cell death machinery.

The death receptor pathway (left) is triggered by members of the death receptor superfamily such as CD95. Binding of CD95-L to its receptor induces trimerization of CD95 and formation of a death-inducing complex. This complex recruits, via the adaptor molecule FADD, multiple procaspase-8 molecules, resulting in caspase-8 activation. Caspase-8 activation can be blocked by recruitment of c-FLIP. The mitochondrial death pathway (right) is controlled by members of the Bcl-2 family, including the proapoptotic Bax and Bid proteins and the antiapoptotic Bcl-2 and Bcl-XL proteins. Death stimuli induce the release of cytochrome c, AIF, Apaf-1, Smac/DIABLO, and possibly other factors from mitochondria. Cytochrome c associates with Apaf-1 and caspase-9 to form the apoptosome. The death receptor and mitochondrial pathways converge at the level of caspase-3 activation. Caspase-3 activation and activity is antagonized by the IAP proteins, which themselves are antagonized by the Smac/DIABLO protein released from mitochondria. Active caspase-3 activates downstream caspases, which results in cleavage of cellular substrates and apoptosis. Crosstalk between the death receptor and mitochondrial pathways is provided by Bid, a proapoptotic Bcl-2 family member. Caspase-8–mediated cleavage of Bid greatly increases its prodeath activity and results in its translocation to mitochondria, where it promotes cytochrome c exit.

Close modal

Mitochondria and activation of caspases

Mitochondria are organelles with 2 well-defined compartments: the matrix, surrounded by the inner membrane, and the intermembrane space, surrounded by the outer membrane. Mitochondria are induced to release cytochrome c in response to most anticancer drugs and other cellular stresses, either by opening of channels in the outer membrane or because of the organellar swelling and rupture that occurs following permeability transition pore opening.10,11 Although release of cytochrome c through the outer membrane is mostly associated with a permanent loss of the mitochondrial membrane potential, this may be transient due to resealing of the inner membrane.10Release of cytochrome c into the cytosol results in activation of the caspase adaptor Apaf-1 and procaspase-9, which form a holoenzyme complex termed “apoptosome.” Caspase-9 in context with this holoenzyme activates downstream caspases—most importantly caspase-3, but also caspase-8—which results in DNA fragmentation and apoptosis. Cytochrome c exit is but one of a host of mitochondrial prodeath compounds. Also present in mitochondria and released upon induction of apoptosis is Smac/DIABLO (second mitochondria-derived activator of caspases/direct IAP-binding protein with a low isoelectric point), a molecule of the inhibitors of apoptosis (IAP) family (see below) and apoptosis-inducing factor (AIF), which exhibits a potent but apparently caspase-independent apoptotic activity.5 9 

Bcl-2 family proteins play a central role in controlling the mitochondrial pathway. In humans, more than 20 members of this family have been identified to date, including proteins that suppress (Bcl-2, Bcl-XL, Mcl-1, Bfl-1/A1, Bcl-W, Bcl-G) and proteins that promote (Bax, Bak, Bok, Bad, Bid, Bik, Bim, Bcl-Xs, Krk, Mtd, Nip3, Nix, Noxa, Bcl-B)12-15 apoptosis. Bcl-2 proteins localize or translocate to the mitochondrial membrane and modulate apoptosis by permeabilization of the inner and/or outer membrane, leading to release of cytochrome c or by stabilizing barrier function. Most Bcl-2 family proteins are capable of physically interacting, forming homodimers or heterodimers, and functioning as agonists or antagonists of each other.12 Additionally, Bcl-XL binds and inactivates Apaf-1, whereas proapoptotic members can displace Bcl-XL from Apaf-1, allowing Apaf-1 to activate caspase-9. Thus, Bcl-2 family members can directly influence the response to cellular stress.8,11 Control of chemotherapy-induced apoptosis by Bcl-2 or Bcl-XL has been suggested by a number of experimental and clinical studies. Increased Bax levels in several tumor cells have been associated with favorable responses to chemotherapy in vivo. Human colorectal cancer cells lacking functional Bax genes were found to be partially resistant to the apoptotic effects of chemotherapeutic agents. Vice versa, resistance to chemotherapy was found to be related to increased levels of expression of Bcl-2 and Bcl-XL16,17 in some studies.18 

Death receptors and activation of caspases

The second pathway that leads to direct caspase activation originates from death receptor signaling, eg, through CD95 (APO-1/Fas) and its ligand CD95-L (APO-1–L/Fas-L). CD95 and other death receptors contain an intracellular death domain. CD95-L is a type II membrane protein that can be proteolytically shed into the intercellular space as a soluble form that is less potent in inducing apoptosis than the membrane-bound form.19 Binding of CD95-L and similar tumor necrosis factor (TNF)–family ligands (eg, TNF-α or TNF-related apoptosis-inducing ligand [TRAIL]) to their respective death-inducing receptors (CD95/APO-1/FAS, TNF-R1, DR4 [TRAIL-R1], and DR5 [TRAIL-R2], respectively) leads to receptor trimerization and recruitment of adaptor proteins to the cytoplasmic death domain. This death-inducing signaling complex (DISC) forms within seconds of receptor engagement. First, specific adaptor proteins (FADD [Fas-associated death domain] for CD95 and DR4/5; TRADD [TNF-R–associated death domain] for TNF-R1) bind via their own death domain to the death domain of the respective receptor. FADD carries a death effector domain (DED) and recruits the DED-containing procaspase-8 (FLICE) into the DISC. Next, procaspase-8 is activated proteolytically and cleaves various proteins, including procaspase-3, which results in its activation and the completion of the cell death program.9 

The mitochondrial and caspase apoptotic pathways are intimately connected. For example, caspase-8 cleaves the cytosolic proapoptotic protein Bid. Bid is a member of the BH3 domain–only subgroup of Bcl-2 family members. This set of proapoptotic proteins shares its only sequence homology within the BH3 amphipathic α-helical domain that is essential for killing activity and heterodimerization with other Bcl-2 family members. Upon cleavage, Bid translocates to mitochondrial membranes and binds to Bad, which is another Bcl-2 family protein, and induces release of cytochrome c from mitochondria. Cytochrome c in turn causes Apaf-1 to activate caspase-9. Under most conditions, this crosstalk is minimal, and the 2 pathways operate largely independently of each other.5 

Antiapoptotic DED-containing proteins such as BAR,20Bap31,21 and FLIP22 may compete with adaptor proteins such as FADD for binding to procaspases-8 and -10, thus reducing the amount of caspase processing and activation.

Apoptosis induction through death receptors may be antagonized by a number of decoy receptors (DcR1 [TRAIL-R3], DcR2 [TRAIL-R4], DcR3 [CD95]) that lack the death domain and compete for the cognitive death ligands without inducing apoptosis.23 24 

Inhibitors of caspase-action: IAP proteins

A family of endogenous direct inhibitors of caspases (IAPs) are conserved throughout animal evolution with homologies in viruses, yeast, flies (Drosophila), worms (C elegans),mice, and humans.25-28 All IAPs contain 1 to 3 baculovirus IAP repeat (BIR) domains, which may be involved in the inhibition of caspase activity. In addition, most also possess a carboxy-terminal RING finger motif. The mammalian IAPs, X-IAP, cIAP-1 (MHIB), cIAP-2 (MIHC), ML-IAP, and livin, inhibit active caspase-3 and -7 and the activation of caspase-9 mediated by Apaf-1–cytochrome c, while survivin has been implicated in regulation of cell cycle and mitosis. Because survivin, ML-IAP, and livin are overexpressed in many cancers but not in normal adult tissues, these molecules represent possible targets for the development of drugs that selectively eliminate cancer cells.29-31 

A mammalian IAP inhibitor known as Smac or DIABLO binds to IAP family members and neutralizes their antiapoptotic activity. This regulatory effect seems to be part of a mitochondrial positive feedback loop because Smac/DIABLO is a mitochondrial protein that is released together with cytochrome c into the cytosol in apoptotic cells.5 

Ligation of death receptors and cellular stress–induced apoptosis

Recent data from our own laboratory and others suggested that anticancer drug–induced cell death may involve the CD95 system. CD95 and CD95-L are constitutively expressed in many tissues and further induced by the appropriate stimuli. Mutations or failure to up-regulate CD95 and CD95-L may result in apoptosis defects of tumor cells.32 Mutation of CD95 in humans or lpr mice results in a lymphoproliferative syndrome caused by the inability to delete long-term activated T cells.33-37 Splenocytes from lpr mice have been found to exhibit decreased sensitivity toward γ-irradiation– or heat shock–induced apoptosis38corresponding to a suggested function of the CD95 system in cellular stress–induced apoptosis. Several investigations have shown that cell lines derived from leukemia, hepatoma, neuroblastoma, colon, breast cancer, brain tumors, and small lung cell carcinoma increase expression of CD95-L upon treatment with chemotherapeutic drugs or radiation.39-52 Drugs that have been observed to enhance CD95-L messenger RNA levels include doxorubicin, etoposide, teniposide, methotrexate, cytarabine, cisplatin, bleomycin, and 5-fluorouracil. Elevated levels of CD95-L protein have also been detected after many of these treatments, although the increase sometimes appears smaller in magnitude than the increase in messenger RNA.53Furthermore, expression of the CD95 receptor increased after drug treatment, especially in cells bearing wild-type p53.44-46,50,54,55 A direct correlation was observed between CD95 receptor density and drug sensitivity, and mutant cell lines resistant to agonistic antibodies to CD95 were also resistant to anticancer drugs.43,56-58 The most significant result from these studies was that drug-induced apoptosis was prevented in some instances by soluble blocking CD95 receptors, neutralizing CD95-L antibodies, or dominant negative FADD, which prevents signaling of death receptors.39-43,45-48,50-52 Thus, cellular stress may involve interaction of CD95 with its ligand and may lower the threshold for the induction of apoptotic signals. Although CD95/CD95-L interaction may regulate certain types of stress-induced apoptosis, CD95-L–independent oligomerization of the CD95 receptor by cytotoxic drugs and UV irradiation can be sufficient to activate caspase-8 in a FADD-dependent manner.59-61 Other death systems, such as the TNF or TRAIL system, may be involved in stress-induced apoptosis, thereby contributing to the redundancy of the apoptosis network under conditions in which one system is blocked.47 

However, other results are incompatible with the view that death receptor signaling is essential for drug-induced apoptosis. Comparison of CD95-sensitive and CD95-resistant Jurkat T-lymphoma cells revealed no difference in their sensitivity to a broad range of chemotherapeutic drugs. Blockade of CD95 signaling by antibodies that neutralize either the receptor or CD95-L did not protect lymphoma cells against drug-induced cell death.62,63 Also, CD95-deficient thymocytes from lpr mice or cells that express FLIP or a dominant-negative construct of FADD did not exhibit increased proliferation to γ-irradiation and chemotherapeutic drugs compared with control cells.22,64,65 Furthermore, overexpression of the serpin crmA, which inhibits caspase-8, had no effect on drug-induced apoptosis.63,66 Finally, some groups failed to detect increased levels of CD95-L protein in drug-treated tumor cells,54,67 and there have been problems with the specificity of certain commercially available antibodies to CD95-L.68-71 Other studies in mice containing targeted gene disruptions are consistent with the conclusion that drug-induced apoptosis, at least in nontransformed cell types, is independent of the CD95 system. In particular, FADD−/− and caspase-8−/− fibroblasts are resistant to death mediated by death receptor triggering but not to drug and cellular stress–induced apoptosis.72,73 In contrast, caspase-9−/− embryonic stem cells are sensitive to death receptor–mediated apoptosis but exhibit marked decreases in drug-induced apoptosis.74,75 Likewise, Apaf-1−/− thymocytes are sensitive to CD95 but resistant to cellular stress–induced apoptosis.76,77 Collectively, these observations suggest that most anticancer drugs can trigger apoptosis in the absence of a functional CD95/CD95-L pathway, although the CD95/CD95-L pathway might contribute under certain circumstances to apoptosis in response to drug treatment. In this context, Jiang et al78 recently contributed a surprising finding to the ongoing discussion. HepG2 cells, which do or do not constitutively express CD95, were used. Agonistic CD95 antibody induced apoptosis only in the CD95-expressing cell line. However, apoptosis could be induced by cytotoxic drugs in both CD95+ and CD95cell lines. A blocking anti-CD95 antibody inhibited drug-induced apoptosis in CD95+ but not in CD95 cells. Thus, drug-induced apoptosis may be induced via both CD95-dependent and CD95-independent pathways, eg, mitochondrial death signaling.

The relative contribution of death receptor versus mitochondrial pathways in stress-induced apoptosis may vary depending on the dose and kinetics but may also reflect the existence of 2 different cell types with respect to CD95 signaling: Type I cells undergo CD95-mediated apoptosis without the involvement of mitochondria, whereas type II cells require the release of cytochrome c from mitochondria in order for CD95 to exert its apoptotic effect. At the molecular level, these 2 cell types differ principally in the amount of caspase-8 recruited to CD95 via the adapter molecule FADD to form the DISC. Whereas type I cells contain large amounts of DISC in response to anti-CD95 antibodies, type II cells do not and thus are dependent on stimulation of the intrinsic apoptotic pathway to undergo cell death. Mitochondria are activated in both type I and type II cells but are dispensable for the death of type I cells.5,9,79 With respect to drug-induced apoptosis, a type I response (depending on cross-linked CD95 receptors) has been found in some cell types.80 

Caspase-independent apoptosis

Cell death is generally classified into 2 large categories: apoptosis, representing “active” programmed cell death, and necrosis, representing “passive” cell death without (known) underlying regulatory mechanisms. However, there are forms of cell death that cannot be readily classified as apoptosis or necrosis—for example, when cells die by cytoplasmic and membrane changes seen in apoptosis but do not exhibit DNA and/or nuclear fragmentation.81-83 Also, z-VAD-fmk could not rescue cells from apoptosis induced by the overexpression of Bax, although caspase-3 activation and nuclear fragmentation were clearly blocked.84,85 In addition, a mixture of apoptotic and necrotic morphology has been found in some cells, eg, in TNF- and CD95-induced cytotoxicity.86,87 This necroticlike cell death depends on an intact downstream intracellular signaling pathway most likely involving generation of reactive oxygen species (ROS),88 whereas activation of caspases seems to be dispensable.87 

The cellular components of caspase-independent apoptosis are not identified so far. In most apoptotic systems z-VAD-fmk does not block mitochondrial changes, such as loss of the membrane potential, production of ROS, or the release of apoptogenic factors such as cytochrome c and AIF.8 Thus, despite caspase inhibition, apoptotic morphology may still be induced by 2 factors: AIF and Bax or Bax-like proteins. How AIF and other released proteins trigger apoptosis in the absence of caspases is unknown. These molecules may activate proteases such as the calcium-dependent calpain proteinases or the lysosomal cathepsins, which can partially substitute caspases but in a less efficient way.81 Cathepsins released from lysosomes cleave Bid, which activates the mitochondrial apoptosis pathway,89 whereas calpains cleave Bax, which promotes cytochrome c release.90-92 Furthermore, calpains are known to cleave and thereby inactivate the cytoprotective endoplasmic reticulum (ER) chaperone glycoprotein GRP94, which contributes to apoptosis.93 

JNK signaling and cellular stress–induced apoptosis

Jun N-terminal kinase (JNK) signaling and c-Jun/AP-1 have been implicated in various, often opposing cellular responses, including proliferation, differentiation, and cellular stress–induced apoptosis (Figure 2). The AP-1 family of transcription factors consists of members of the Jun, Fos, and ATF-2 subfamilies.94 Mammalian Jun proteins include c-Jun, Jun B, and Jun D. Fos proteins include c-Fos, FosB, Fra-1, and Fra-2. ATF proteins that participate in forming AP-1 dimers are ATF-2 and ATF-a. Depending on the stimulus and cellular context, the composition of the dimeric AP-1 complex varies between different members of the Jun, Fos, and ATF family. The activity of individual AP-1 subunits can be regulated either by transcription increasing the intracellular concentration of the proteins or by posttranslational modifications and interaction with other proteins such as members of the mitogen-activated protein kinase (MAPK) signaling pathways.

Fig. 2.

JNK and cellular stress–induced apoptosis.

JNK signaling has been implicated in proliferation, differentiation, and cellular stress–induced apoptosis. The effects of JNK on cellular apoptosis depend strongly on the cell type and the context of other regulatory influences. JNK signaling can be turned off by dual-specificity MAPK phosphatases. JNK activation results in phosphorylation of AP-1 transcription factor family members such as c-Jun and ATF-2, which then bind to AP-1 binding sites in the promoters of multiple target genes. JNK may contribute to death receptor transcription-dependent apoptotic signaling via c-Jun/AP-1 (leading to promoter induction of CD95-L, TNF-α, and p53) to transcription-independent apoptotic signaling by phosphorylation-dependent posttranslational proapoptotic processes (leading to cytochrome c release, stabilization of p53 protein, inactivation of Bcl-2, Bcl-XL, and activation of c-myc). These mechanisms may function separately or cooperate in induction of apoptosis. JNK signaling in combination with other factors, eg, the suppression of proliferation pathways, may mediate cellular stress–induced apoptosis.

Fig. 2.

JNK and cellular stress–induced apoptosis.

JNK signaling has been implicated in proliferation, differentiation, and cellular stress–induced apoptosis. The effects of JNK on cellular apoptosis depend strongly on the cell type and the context of other regulatory influences. JNK signaling can be turned off by dual-specificity MAPK phosphatases. JNK activation results in phosphorylation of AP-1 transcription factor family members such as c-Jun and ATF-2, which then bind to AP-1 binding sites in the promoters of multiple target genes. JNK may contribute to death receptor transcription-dependent apoptotic signaling via c-Jun/AP-1 (leading to promoter induction of CD95-L, TNF-α, and p53) to transcription-independent apoptotic signaling by phosphorylation-dependent posttranslational proapoptotic processes (leading to cytochrome c release, stabilization of p53 protein, inactivation of Bcl-2, Bcl-XL, and activation of c-myc). These mechanisms may function separately or cooperate in induction of apoptosis. JNK signaling in combination with other factors, eg, the suppression of proliferation pathways, may mediate cellular stress–induced apoptosis.

Close modal

The MAPK pathway includes the subfamilies extracellular signal–regulated kinase (ERK), JNK, and p38. These different MAPKs are members of separate modules and are regulated by distinct extracellular stimuli. For example, ERKs are activated by receptor tyrosine kinases and provide proliferation or differentiation signals. JNK and p38-type MAPKs are activated predominantly by stress stimuli and pathogenic insults but in some cell types also by mitogens. All 3 classes of MAPKs are involved in the regulation of distinct AP-1 components. c-Jun is regulated by JNK phosphorylation and in some cell types also by ERK-mediated mechanisms. c-Fos is a substrate for regulatory phosphorylations by ERK, and ATF-2 is regulated by JNK and p38 kinases.95 Due to this complex regulation of AP-1 factors, the range of biological responses is broad. In the following, we focus on how activation of JNK and c-Jun/AP-1 contributes to cellular stress–induced apoptosis.

JNK protein kinases are encoded by 3 genes. While Jnk1 and Jnk2 genes are ubiquitously expressed, expression of the Jnk3 gene is restricted to the brain, heart, and testis. Alternative splicing generates at least 10 different JNK isoforms, which might differ in their substrate specificity. JNK signaling can be turned off by dual-specificity MAPK phosphatases, which often function in a negative feedback loop.96-99 

JNK signaling may contribute to apoptosis100-112 or may be dispensable for apoptosis113 and even inhibit apoptosis to promote proliferation and differentiation.114,115 Thus, the effects of JNK on cellular responses appear to depend on the cell type and the context of other signals received by the cell. A clear proapoptotic role of JNK has been demonstrated in studies of mice with targeted disruption of the neuronal gene Jnk3.110JNK3−/− mice are developmentally normal but are defective in the apoptotic response to excitotoxins. Disruption of the ubiquitously expressed Jnk1 or Jnk2 genes in mice causes no obvious phenotype or apoptosis defect. In contrast, compound mutation of Jnk1 plus Jnk2 leads to early embryonic death associated with defects in neuronal apoptosis and exencephaly.116,117 An apoptosis defect was also observed in fibroblasts derived from mice with a mutation in the c-Jun gene.118 119 

Among the proapoptotic targets of c-Jun are the promoters of CD95-L and TNF-α, which both contain essential binding sites for AP-1. Expression of these death-inducing ligands is activated by the sequential signaling of JNK and c-Jun/AP-1 following cellular stress and is involved in the induction of cellular stress–induced apoptosis.102 119-128 

Substrates for JNK activity also include p53. Dependent on the cellular context, JNK either destabilizes p53 by binding, promoting ubiquitin-mediated degradation, or stabilizes p53 by phosphorylation, whereby inhibiting ubiquitin-mediated degradation.129,130Furthermore, JNK may be involved in regulating transcription of the p53 gene because c-Jun can repress the p53 promoter.131 These data suggest that JNK may be important for controlling the level of p53 expression by regulating the half-life of p53, although these data are discussed with controversy.

An additional potential target of proapoptotic signaling by JNK is the transcription factor c-Myc. Recent studies indicate that c-Myc interacts with JNK and is phosphorylated at Ser62 and Thr71.132 Apoptosis induced by ectopic c-Myc expression in serum-starved cells is associated with increased JNK activity, as concluded from dominant-negative experiments leading to inhibition of JNK signaling and c-Myc–stimulated apoptosis. However, because JNK-induced apoptosis does not require either ectopic c-Myc expression or serum starvation, the role of c-Myc phosphorylation by JNK is unclear.

Despite a function of c-Jun in the regulation of CD95-L, TNF-α, p53, and c-Myc, JNK might enable apoptosis by interfering with mitochondria, resulting in the release of cytochrome c.133 Potential targets of JNK that may regulate cytochrome c release include members of the Bcl-2 group of apoptotic regulatory proteins. Exposure of cells to cellular stress resulted in translocation of JNK to mitochondria.134 In vitro, JNK phosphorylates Bcl-2 and Bcl-XL and may thereby inactivate the death protective function,135 causing cytochrome c release following cellular stress.134 However, Bcl-2 and Bcl-XLmay not be physiologic substrates of JNK because this kinase did not phosphorylate Bcl-2 in vivo.96 Primary fibroblasts prepared from Jnk1−/− Jnk2−/− mouse embryos lack expression of both JNK protein and JNK activity107 and represent a powerful model for the analysis of JNK-induced apoptosis. These JNK null cells exhibit profound defects in cellular stress–induced apoptosis (UV irradiation, anisomycin, MMS [methyl methane-sulfonate]). The defect in apoptosis was caused by defective activation of effector caspases, including caspase-3.107 However, CD95-induced apoptosis was intact, indicating that JNK is not essential for signaling downstream of death receptors and caspase-8. In contrast, JNK null fibroblasts exhibited impaired mitochondrial depolarization and release of cytochrome c,107 suggesting that apoptotic JNK signaling is mediated via mitochondria. However, because JNK activates death ligands (which bind to death receptors to induce apoptosis), JNK may be dispensable but contributes to death receptor–mediated apoptosis.

In conclusion, JNK may induce apoptosis by transcription-dependent signaling (leading to secretion of death ligands), by transcription-independent signaling (leading to cytochrome c release from mitochondria), or by phosphorylation-dependent posttranslational proapoptotic signaling yet to be identified. It is possible that these mechanisms may function separately, but these mechanisms may also cooperate to induce death. Taken together, JNK signaling in combination with other factors, such as the suppression of proliferation pathways, may induce apoptosis following cellular stress.

Endoplasmic reticulum and cellular stress–induced apoptosis

As a protein-folding compartment, the ER is exquisitely sensitive to alterations in homeostasis, for example, induced by cellular stress. Different stimuli signal through several protein kinases to up-regulate the protein-folding capacity of the ER by activation of 2 signaling pathways: the unfolded protein response pathway, leading to the induction of ER chaperones such as grp78/Bip via the C/EBP homologous transcription factor CHOP/GADD153,136 and the ER overload response pathway, leading to the production of cytokines via nuclear factor κB (NF-κB) (Figure 3). Both pathways help the cell to cope with incorrectly folded or accumulated proteins in the ER but may also contribute to its elimination when abnormalities become too extensive.137Consistent with this idea, both CHOP/GADD153 138 and NF-κB have been implicated in apoptosis regulation.139Another mediator of death signaling may be caspase-12, which is localized to the ER and is proteolytically activated by ER stress. Mice that are deficient in caspase-12 are resistant to ER stress-induced apoptosis, but their cells undergo apoptosis in response to other death stimuli.140 

Fig. 3.

Endoplasmic reticulum and cellular stress–induced apoptosis.

The endoplasmic reticulum regulates protein synthesis, N-linked glycosylation, trafficking, and intracellular Ca++ levels. Alterations in homeostasis such as induced by cellular stress induce the unfolded protein response and the ER overload response pathways, which may cope with incorrectly folded proteins in the ER but may also contribute to its elimination when abnormalities become too intensive. The unfolded protein response pathway leads to induction of chaperones such as grp78/Bip via the transcription factor CHOP/GADD153. The ER overload response pathway leads to production of cytokines via NF-κB. Several ER membrane proteins interact with Bcl-2 family members, such as the antiapoptotic Bax inhibitor I and Bap31 and the proapoptoticS pombe calnexin chaperone homolog Cnx1, the reticulon proteins (RTN) NSP-C/RTN1-C, and RTN-XS, or the calcium pump SERCA. Calreticulin, an ER luminal protein, promotes the release of cytochrome c from mitochondria, caspase-3 activity, and DNA fragmentation. Stress in the ER also activates JNKs in several cell types. Thus, the ER, via specific components of its luminal environment, may play an important role in the modulation of cell sensitivity to apoptosis.

Fig. 3.

Endoplasmic reticulum and cellular stress–induced apoptosis.

The endoplasmic reticulum regulates protein synthesis, N-linked glycosylation, trafficking, and intracellular Ca++ levels. Alterations in homeostasis such as induced by cellular stress induce the unfolded protein response and the ER overload response pathways, which may cope with incorrectly folded proteins in the ER but may also contribute to its elimination when abnormalities become too intensive. The unfolded protein response pathway leads to induction of chaperones such as grp78/Bip via the transcription factor CHOP/GADD153. The ER overload response pathway leads to production of cytokines via NF-κB. Several ER membrane proteins interact with Bcl-2 family members, such as the antiapoptotic Bax inhibitor I and Bap31 and the proapoptoticS pombe calnexin chaperone homolog Cnx1, the reticulon proteins (RTN) NSP-C/RTN1-C, and RTN-XS, or the calcium pump SERCA. Calreticulin, an ER luminal protein, promotes the release of cytochrome c from mitochondria, caspase-3 activity, and DNA fragmentation. Stress in the ER also activates JNKs in several cell types. Thus, the ER, via specific components of its luminal environment, may play an important role in the modulation of cell sensitivity to apoptosis.

Close modal

Although the effects of Bcl-2 on the mitochondria have been studied intensively, little is known about the effects of Bcl-2 on the ER, where antiapoptotic Bcl-2 family proteins are also localized. Several ER membrane proteins have been reported to interact with Bcl-2 family members to enhance their antiapoptotic effect. Among them is Bax inhibitor I141 and the Bcl-2/Bcl-XL–associated Bap31.21,142,143 Similarly, but in a proapoptotic manner, the Schizosaccharomyces pombe calnexin chaperone homolog Cnx1 interacts with Bak,144 whereas the calcium pump SERCA (sarcoplasmic/endoplasmic reticulum calcium-ATPase) interacts with Bcl-2,145 and members of the ER-anchored reticulon family such as NSP-C and RTN-XS bind to Bcl-XL and Bcl-2,146 thereby contributing to apoptosis.

Apoptotic agents perturbing ER functions such as brefeldin A induce the release of cytochrome c from mitochondria that is blocked by Bcl-2 derived from either mitochondria or ER. Brefeldin A–induced cytochrome c release occurred in a caspase-8– and Bid-independent manner and was followed by caspase-3 activation and DNA/nuclear fragmentation.147 Overexpression of calreticulin, an ER luminal protein, sensitized cells to apoptosis induced by thapsigargin (an agent that promotes ER stress by depletion of luminal calcium stores) and staurosporine (a potent inhibitor of phospholipid/calcium-dependent protein kinase). This correlated with an increased release of cytochrome c from mitochondria. Calreticulin-deficient cells were significantly resistant to apoptosis, correlating to a decreased release of cytochrome c from mitochondria and low levels of caspase-3 activity.148 

ER stress may also activate JNKs.149 Lysates from ER-stressed rat pancreatic cells treated with thapsigargin, tunicamycin (which block protein glycoslyation), or dithiothreitol (which interferes with disulfide bond formation) all exhibited increased JNK activity.150 Activation of JNKs by ER stress, although always present, varies in magnitude depending on cell type and is particularly pronounced in cells with a well-developed ER. Coupling of ER stress to JNK activation may be mediated by a mammalian homolog of yeast IRE1, which activates chaperone genes. Overexpression of IRE1 or its mammalian homolog leads to JNK activation, and IRE1α−/− fibroblasts were impaired in JNK activation by ER stress. The cytoplasmic part of IRE1 binds TNF receptor–associated factor-2 (TRAF2), an adaptor protein that couples plasma membrane receptors such as TNF to JNK activation.150 Another hierarchical model for activation of JNKs by ER stress suggests induction of the JNK pathway in Jurkat cells downstream of cytochrome c release and caspase-3.151 

Together, these findings implicate that the ER, via specific components of its luminal environment or by interaction among ER, mitochondria, and JNK, may play an important role in the modulation of cell sensitivity toward apoptosis.

p53 and cellular stress–induced apoptosis

Various stress stimuli such as cytotoxic drugs, γ-irradiation, heat shock, hypoxia, osmotic shock, and DNA-damaging agents stabilize the tumor suppressor protein p53, which promotes cell-cycle arrest to enable DNA repair or apoptosis to eliminate defective cells152 (Figure 4). However, it is still largely unknown how p53 selects the pathways of G1 arrest or apoptosis. In this context, the proline-rich domain (residues 64-92)153,154 and a recently identified transcriptional activation domain (residues 43-63)155 have been suggested to be necessary for mediation of apoptosis because deletion of either of these 2 domains abolishes this activity. On the other hand, it has been shown that phosphorylation and acetylation play important roles for regulating biological activities of p53.156,157Although the roles of these modifications are not fully characterized, they are likely to play roles in regulating the binding of p53 with its negative regulator, Mdm2. Other negative regulatory mechanisms involve binding of JNK to p53, which mediates ubiquitination and proteolytic removal of p53,129 and the retinoblastoma gene product (Rb), which prevents the apoptotic function of p53.158,159Both p53 inhibitors, Rb and Mdm2, are cleaved by caspases during apoptosis,160-162 suggesting a positive self-regulation of programmed cell death and close connection to key cell-cycle regulators.

Fig. 4.

p53 and cellular stress–induced apoptosis.

Various stress stimuli activate the p53 protein, which promotes cell-cycle arrest to enable DNA repair or apoptosis to eliminate defective cells. A key player in the regulation of p53 is Mdm2, which binds to p53 and inhibits the DNA binding activity as well as the transcription rate of the p53 gene. In a negative feedback loop, p53 binds to the mdm2 gene and stimulates its transcription. Another inhibitor is the Rb protein, which prevents the apoptotic function of p53. p53 induces various target genes, such as the cell-cycle regulators p21Waf1, GADD45, and cyclin G. Proapoptotic p53 target proteins include Bax, CD95, DR5, IGF-BP3, NOXA, p53AIP1, and (in Drosophila) Rpr. ROS production may be mediated by the p53-inducible gene PIG3 and may contribute to cytochrome c release from mitochondria. Taken together, the apoptotic target genes of p53 may need to act in concert by activating parallel apoptotic pathways to mount a full apoptotic response.

Fig. 4.

p53 and cellular stress–induced apoptosis.

Various stress stimuli activate the p53 protein, which promotes cell-cycle arrest to enable DNA repair or apoptosis to eliminate defective cells. A key player in the regulation of p53 is Mdm2, which binds to p53 and inhibits the DNA binding activity as well as the transcription rate of the p53 gene. In a negative feedback loop, p53 binds to the mdm2 gene and stimulates its transcription. Another inhibitor is the Rb protein, which prevents the apoptotic function of p53. p53 induces various target genes, such as the cell-cycle regulators p21Waf1, GADD45, and cyclin G. Proapoptotic p53 target proteins include Bax, CD95, DR5, IGF-BP3, NOXA, p53AIP1, and (in Drosophila) Rpr. ROS production may be mediated by the p53-inducible gene PIG3 and may contribute to cytochrome c release from mitochondria. Taken together, the apoptotic target genes of p53 may need to act in concert by activating parallel apoptotic pathways to mount a full apoptotic response.

Close modal

Whereas we do not entirely understand how p53 exerts its effects on cells, it is clear that the transcriptional activating function of p53 is a major component of its biological effects. Many p53 target genes have been identified, and those functions have been characterized. Cell-cycle arrest that is dependent on p53 requires transactivation of p21Waf1, GADD45, and cyclin G. Proapoptotic p53 target proteins include Bax, PIG genes, CD95, DR5 (a receptor for the death ligand TRAIL), IGF-BP3, Rpr (in Drosophila), Cdc42 (a Ras-like GTPase), Noxa (a Bcl-2 family protein), and p53AIP1.15,152 163-169 

The mechanism of p53-induced apoptosis has been extensively studied and involves activation of the mitochondrial Apaf-1/caspase-9 pathway,170 death receptor signaling,50,171,172 and cleavage of downstream caspases.173 For example, cells expressing mutant p53 fail to induce CD95 and are less sensitive to drug-induced apoptosis.50,174 Independently of transcription, p53 may facilitate the transport of CD95 from Golgi stores to the membrane, leading to death receptor aggregation.171 However, in some cases CD95 is not essential for p53-mediated apoptosis, and p53-dependent up-regulation of CD95 does not induce apoptosis per se.175 An additional route by which p53 may signal apoptosis is through the production of ROS, which influence the mitochondrial membrane potential without involving cytochrome c release.173,176 In particular, the p53-inducible gene PIG3 shares homology with an NADPH-quinone oxidoreductase, which generates ROS. When overexpressed alone, PIG3 failed to initiate apoptosis, implying that other signals must be activated in parallel.177 Recently, p53 itself was shown to cause caspase activation in cell-free extracts from E1A/ras-transformed, but not normal, fibroblasts by a mechanism independent of transcription or presence of Bax or cytochrome c.178 Oncogene-dependent activation of caspases by p53 was also mediated by the c-Myc oncogene, a finding consistent with the requirement of caspase-9 and Apaf-1 in p53-dependent Myc-induced apoptosis.179 Thus, p53 can transduce apoptotic signals through protein-protein interactions, thereby modulating p53-dependent caspase activation. Another mechanism by which p53 promotes apoptosis is through activation of the Ras-like GTPase Cdc42, which activates the JNK1-induced phosphorylation of Bcl-2.168 

Taken together, apoptosis mediated by or involving p53 consists of parallel or sequential activation of a set of different molecules and pathways that may need to act in concert to activate a full death response.

NF-κB and cellular stress–induced apoptosis

NF-κB activity is required for the induction of more than 150 genes involved in cell growth, differentiation, development, apoptosis, and adaptive responses to changes in cellular redox balance (Figure5). A wide variety of external stimuli including cytokines, pathogens, stress, and chemotherapeutic agents can lead to the activation of NF-κB.180 These stimuli induce phosphorylation and subsequent degradation of IκB inhibitory proteins, thereby releasing NF-κB proteins for translocation to the nucleus to function as transcription factors.181Phosphorylation of IκB is mediated by a protein complex containing 2 kinases, IκB kinase α and β (IKK-1 and IKK-2), and a noncatalytic regulatory subunit called IKKγ.182 NF-κB transcription factors are heterodimer and homodimer complexes of related proteins that contain a Rel homology domain involved in specific DNA binding, protein dimerization, and nuclear import.180 The Rel proteins predominantly found in mammalian cells consist of 2 transcriptionally inactive forms, NF-κB1 (p50) and NF-κB2 (p52), and 3 transcriptionally active subunits known as RelA (p65), c-Rel, and RelB.180 

Fig. 5.

NF-κB and cellular stress–induced apoptosis.

NF-κB activity is required for the induction of more than 150 genes involved in cell growth, differentiation, development, apoptosis, and adaptive responses to changes in cellular redox balance. NF-κB is bound by IκB, which prevents NF-κB activity. NF-κB target genes with antiapoptotic function include the IAP family, TRAF1 and TRAF2, thought to suppress caspase-8 activation, the prosurvival Bcl-2 homologs Bfl1/A1 and Bcl-XL, and nitrous oxide synthase–inducible genes. The apoptotic signaling of NF-κB may be due to the promoter activation of death receptors and ligands such as CD95, CD95-L, TNF-α, and the TRAIL receptors DR4 and DR5.

Fig. 5.

NF-κB and cellular stress–induced apoptosis.

NF-κB activity is required for the induction of more than 150 genes involved in cell growth, differentiation, development, apoptosis, and adaptive responses to changes in cellular redox balance. NF-κB is bound by IκB, which prevents NF-κB activity. NF-κB target genes with antiapoptotic function include the IAP family, TRAF1 and TRAF2, thought to suppress caspase-8 activation, the prosurvival Bcl-2 homologs Bfl1/A1 and Bcl-XL, and nitrous oxide synthase–inducible genes. The apoptotic signaling of NF-κB may be due to the promoter activation of death receptors and ligands such as CD95, CD95-L, TNF-α, and the TRAIL receptors DR4 and DR5.

Close modal

Different NF-κB transcription factors may play diverse and even opposing roles in modulating cell death by apoptosis. In certain settings, c-Rel has been associated with promoting apoptosis. Increased expression of c-Rel protein and its accumulation in the nucleus correlate with induction of apoptosis in various tissues.128,183-185 In contrast, a variety of studies in knockout mice have demonstrated the importance of RelA and c-Rel in prevention of apoptosis because mice lacking NF-κB activity die during embryogenesis.186,187 Similarly, overexpression of RelA/NF-κB protects cells from TNF-α or chemotherapy-mediated apoptosis,114,186,188-191 whereas inhibition of NF-κB restored apoptosis sensitivity of drug-resistant primary leukemic cells and leukemic cell lines.189 Thus, activation of NF-κB transcription factors in different settings can control apoptosis in quite opposite manners.

NF-κB target genes that may provide antiapoptotic function include the IAP family of caspase inhibitory proteins, TRAF1 and TRAF2, thought to suppress caspase-8 activation, the prosurvival Bcl-2 homolog proteins Bfl1/A1 and Bcl-XL,192,193 and inducible nitrous oxide synthase genes194 whose metabolites have been linked to inhibiton of apoptosis.195 The apoptotic signaling of NF-κB may be due to the promoter activation of death receptors and death ligands such as CD95, CD95-L,120and the TRAIL receptors DR4 and DR5.179 However, although diverse studies describe the requirement of NF-κB for induction of CD95-L,120,196-198 the NF-κB signaling pathway is not required for CD95-L induction in other circumstances.199 Also, crosstalk between NF-κB and the caspase pathway has been found. For instance, RIP, the adaptor for induction of NF-κB by TNF-R1, can be cleaved by caspase-8 and this cleavage may play a role in regulating the balance between life and death in response to TNF.200 

NF-κB is also able to function in concert with other transcription factors, such as AP-1, whose transcriptional activation involves phosphorylation of JNK. Therefore, the signal transduction cascade following cellular stress results in the activation of parallel kinase cascades regulating AP-1 and NF-κB.201 This dual pathway enhances production of proapoptotic and antiapoptotic proteins dependent on the cellular context.

Ceramide and cellular stress–induced apoptosis

Ceramide, a sphingolipid-derived second messenger molecule, has been described as an important bioeffector molecule involved in cellular stress responses implicated in apoptosis, growth inhibition, and cellular differentiation (Figure 6). Stress stimuli such as TNF, CD95-L, oxidative stress, growth factor withdrawal, chemotherapeutic agents, ionizing or UV radiation, and heat shock induce an elevation in the endogenous levels of ceramide, and exogenous ceramide analogs mimic these biological responses in specific cell types.202-207 

Fig. 6.

Ceramide and cellular stress–induced apoptosis.

Ceramide is involved in cellular stress responses implicated in apoptosis, growth inhibition, and differentiation. The major source of ceramide is hydrolysis of sphingomyelin by SMases. De novo synthesis via ceramide synthase may also lead to the generation of ceramide. Ceramide acts as a catalyst for apoptosis through the consecutive activation of MEKK1, SEK1, JNK, c-Jun, and death-inducing ligands such as CD95-L. BAD, a proapoptotic Bcl-2 family member, is induced by a ceramide-mediated pathway involving CAPK, Ras, c-Raf-1, and MEK1. Other mediators of ceramide-induced apoptosis are ROS and the ganglioside GD3, which both affect mitochondria. Ceramide acts also upstream of the antiapoptotic kinase Akt, leading to a decrease in its activity. Activation of the stress response by ceramide leads to either survival or apoptosis. Members of this signaling cascade are CAPK, Ras, c-Raf-1, MEK1, PKC-ζ, JNK, and NF-κB. SPP results from the catabolic pathway for ceramide and acts as a second messenger in cellular proliferation and survival.

Fig. 6.

Ceramide and cellular stress–induced apoptosis.

Ceramide is involved in cellular stress responses implicated in apoptosis, growth inhibition, and differentiation. The major source of ceramide is hydrolysis of sphingomyelin by SMases. De novo synthesis via ceramide synthase may also lead to the generation of ceramide. Ceramide acts as a catalyst for apoptosis through the consecutive activation of MEKK1, SEK1, JNK, c-Jun, and death-inducing ligands such as CD95-L. BAD, a proapoptotic Bcl-2 family member, is induced by a ceramide-mediated pathway involving CAPK, Ras, c-Raf-1, and MEK1. Other mediators of ceramide-induced apoptosis are ROS and the ganglioside GD3, which both affect mitochondria. Ceramide acts also upstream of the antiapoptotic kinase Akt, leading to a decrease in its activity. Activation of the stress response by ceramide leads to either survival or apoptosis. Members of this signaling cascade are CAPK, Ras, c-Raf-1, MEK1, PKC-ζ, JNK, and NF-κB. SPP results from the catabolic pathway for ceramide and acts as a second messenger in cellular proliferation and survival.

Close modal

Hydrolysis of sphingomyelin, a main lipid in plasma membranes of mammalian cells, is the major source of ceramide. Sphingomyelin hydrolysis may occur via the action of sphingomyelin-specific forms of phospholipase C, termed sphingomyelinases (SMases), which are defined by their pH optima as neutral (nSMase) or acid (aSMase). These enzymes are activated in response to TNF and other cytokines. De novo synthesis via ceramide synthase may also lead to the generation of ceramide.203 205 

The catabolic pathway for ceramide involves deacetylation by ceramidases to generate sphingosine, which is phosphorylated by sphingosine kinase to form sphingosine-1-phosphate (SPP). SPP in turn acts as a second messenger in cellular proliferation and survival induced by platelet-derived growth factor or serum. Previously, a model has been proposed in which the dynamic balance between the intracellular levels of ceramide and SPP is an important factor that determines whether a cell survives or dies. According to this model, stress stimuli such as TNF activate SMases, leading to increased intracellular ceramide levels and thus to increased cell death, whereas platelet-derived growth factor and other growth factors stimulate ceramidase and sphingosine kinase and elevate SPP levels, resulting in cellular survival and proliferation.208 209 

Mechanisms by which ceramide induces multiple signaling pathways involve the sequential activation of different kinases such as ceramide-activated protein kinase (CAPK), phosphorylation of Raf-1, and the MAPK cascade. Protein kinase C-ζ (PKC-ζ) has been identified as another CAPK that is a critical element in ceramide-induced JNK activation and NF-κB translocation.206,210 Thus, ceramide may act as a catalyst for the stress response kinase cascade through the consecutive involvement of MEKK1, SEK1, JNK, and c-Jun.206 Concomitant activation NF-κB may contribute to enhanced expression of proapoptotic TNF superfamily members such as CD95-L, TRAIL, and TNF-α.211,212 Ceramide also influences mitochondrial apoptosis signaling by the proapoptotic Bcl-2 family member BAD through a pathway involving CAPK, Ras, c-Raf-1, and MEK1.213 ROS generated at the ubiquinone site of the mitochondrial respiratory chain seem also to be necessary for ceramide-induced apoptosis and transcription factor activation.214 215 

Another metabolizing pathway for ceramide was proposed by Testi and coworkers.216 Ceramide can be shuttled to the Golgi complex, where it is converted to gangliosides. It was found that CD95 ligation or treatment with ceramide resulted in the accumulation of the ganglioside GD3, an event that was prevented by caspase inhibitors. Antisense oligonucleotides toward GD3 synthetase, which is localized in the Golgi complex, attenuated apoptosis, whereas overexpression of the wild-type enzyme was associated with massive cell death. Thus, GD3 ganglioside may be targeted to mitochondria, where it alters mitochondrial function and causes cell death during CD95-mediated apoptosis.

While the role of ceramide for apoptosis induction, eg, through death receptors, is highly controversial, there is a substantial evidence for a role of ceramide in the initiation of the apoptosis response by cytotoxic drugs and γ-irradiation. Cell lines resistant to γ-irradiation– or doxorubicin-induced apoptosis fail to generate ceramide following these treatments.47,217,218 The phenotype of acid SMase knockout mice resembles the type A form of human Niemann-Pick disease. Lung endothelial cells from knockout mice, as well as lymphoblasts and fibroblasts from Niemann-Pick patients, display defective ceramide generation and are resistant to stress-induced apoptosis although thymocytes are still susceptible.204,212 219 

Despite these findings, much confusion remains about the role of endogenous ceramide in apoptosis. Whereas some publications place ceramide upstream of caspases,108,212,220-222 others suggest that it acts downstream of caspases, because it can be blocked by caspase inhibitors.223-226 A possible reason for the discrepancy on the role of ceramides may lie in methodologic problems. Ceramide production is mostly determined in assays using diacylglycerol kinase. In a recent investigation, no ceramide production in response to CD95 ligation could be detected using mass spectroscopy, whereas an apparent increase of ceramide was measured by the classical diacylglycerol kinase assay.227 It was suggested that lysates from apoptotic cells may stimulate diacylglycerol kinase activity directly, which then increases ceramide production.

Thus, depending on the cell line used and the experimental setup, the effects of ceramide generation ranged from induction of apoptosis and cell-cycle arrest to proliferation and terminal differentiation.

Cytotoxic drugs have been developed for the treatment of leukemia and malignant tumors based on their capacity to inhibit cellular proliferation.228 While an overwhelming amount of data indicate that cytotoxic drugs induce and activate molecules of the apoptosis and cellular stress response pathway, a number of key questions are still open and unresolved. For example, the view that apoptosis represents the main mechanism by which tumor cells are killed by cancer therapy may not be universally true.229,230Problems in identifying apoptosis signaling as a key event may be related to the assay used to detect drug-induced cell death. For example, cells that have received sufficient DNA damage to be unable to proliferate may die because of mitotic disaster, which is not detected in apoptosis assays but only in clonogenicity assays. Key elements of the cell death pathway are closely linked to other complex signaling systems such as the DNA damage response and cell-cycle control, which complicates the identification of individual compounds in the clinical setting.231,232 Most importantly, while in vitro assays have convincingly demonstrated that deregulated expression of apoptosis-mediating molecules may confer drug resistance,17 53 results of clinical studies in patients are less clear. Mutations of caspases are rarely found in tumors, although these molecules are the crucial effectors of the apoptosis response and would be ideal targets for mutations providing a survival advantage for tumor cells.

Taken together, research on regulation of apoptosis, growth arrest, and DNA repair initiated by the cellular stress response has provided a detailed insight into fundamental cellular mechanisms with widespread clinical implications. Given the importance of cell death, including apoptosis as one possible outcome of the stress response in cells treated by cytotoxic drugs, further studies are required to identify the role of individual regulators of stress signaling and cell death for sensitivity to anticancer therapy. These will include analysis of gene expression profiles, novel proteomic approaches, as well as functional in vitro and in vivo studies in malignant cells from patients undergoing cancer therapy.

We apologize to those whose work was not cited or discussed because of space limitations. We thank R. Zwacka for critical reading and V. Krok-Szwed for assistance in preparing the manuscript.

Supported by the Deutsche Forschungsgemeinschaft, Deutsche Krebshilfe, EU grant, Deutsche Leukämieforschungshilfe, and Sander Stiftung.

@ 2001 by The American Society of Hematologycharge

1
Steller
H
Mechanisms and genes of cellular suicide.
Science.
267
1995
1445
1449
2
Thompson
CB
Apoptosis in the pathogenesis and treatment of disease.
Science.
267
1985
1456
1462
3
Fisher
DE
Apoptosis in cancer therapy: crossing the threshold.
Cell.
78
1994
539
542
4
Kaufmann
SH
Induction of endonucleolytic DNA cleavage in human acute myelogenous leukemia cells by etoposide, camptothecin, and other cytotoxic anticancer drugs: a cautionary note.
Cancer Res.
49
1989
5870
5878
5
Hengartner
MO
The biochemistry of apoptosis.
Nature.
407
2000
770
776
6
Nagata
S
Apoptotic DNA fragmentation.
Exp Cell Res.
256
2000
12
18
7
Stegh
AH
Herrmann
H
Lampel
S
et al
Identification of the cytolinker plectin as a major early in vivo substrate for caspase-8 during CD95- and tumor necrosis factor receptor-mediated apoptosis.
Mol Cell Biol.
20
2000
5665
5679
8
Green
D
Reed
JC
Mitochondria and apoptosis.
Science.
281
1998
1309
1312
9
Krammer
PH
CD95's deadly mission in the immune system.
Nature.
407
2000
789
795
10
Kroemer
G
Reed
JC
Mitochondrial control of cell death.
Nat Med.
6
2000
513
519
11
Reed
JC
Mechanisms of apoptosis avoidance in cancer.
Curr Opin Oncol.
11
1999
68
75
12
Antonsson
B
Martinou
J-C
The Bcl-2 protein family.
Exp Cell Res.
256
2000
50
57
13
Guo
B
Godzik
A
Reed
JC
Bcl-G, a novel pro-apoptotic member of the Bcl-2 family.
J Biol Chem.
276
2001
2780
2785
14
Ke
N
Godzik
A
Reed
JC
Bcl-B, a novel Bcl-2 family member that differentially binds and regulates Bax and Bak.
J Biol Chem.
276
2001
12481
12484
15
Oda
E
Ohki
R
Murasawa
H
et al
Noxa, a BH3-only member of the Bcl-2 family and candidate mediator of p53-induced apoptosis.
Science.
288
2000
1053
1058
16
Minn
AJ
Rudin
CM
Boise
LH
Thomson
CB
Expression of Bcl-XL can confer a multidrug resistance phenotype.
Blood.
86
1995
1903
1910
17
Zhang
L
Yu
J
Park
BH
Kinzler
KW
Vogelstein
B
Role of BAX in the apoptotic response to anticancer agents.
Science.
290
2000
989
992
18
Campos
L
Rouault
JP
Sabido
O
et al
High expression of Bcl-2 protein in acute myeloid leukemia cells is associated with poor response to chemotherapy.
Blood.
81
1993
3091
3096
19
Nagata
S
Apoptosis by death factor.
Cell.
88
1997
355
365
20
Zhang
H
Xu
Q
Krajewski
S
et al
BAR: an apoptosis regulator at the intersection of caspases and Bcl-2 family proteins.
Proc Natl Acad Sci U S A.
97
2000
2597
2602
21
Ng
FWH
Nguyen
M
Kwan
T
et al
p28 Bap31, a Bcl-2/Bcl-XL- and procaspase-8-associated protein in the endoplasmic reticulum.
J Cell Biol.
139
1997
327
338
22
Kataoka
T
Schröter
M
Hahne
M
et al
FLIP prevents apoptosis induced by death receptors but not by perforin/granzyme B, chemotherapeutic drugs, and γ irradiation.
J Immunol.
161
1998
3936
3942
23
Krammer
PH
CD95(APO-1/Fas)-mediated apoptosis: live and let die.
Adv Immunol.
71
1999
163
210
24
Pitti
RM
Marsters
SA
Lawrence
DA
et al
Genomic amplification of a decoy receptor for Fas ligand in lung and colon cancer.
Nature.
396
1998
699
703
25
Deveraux
QL
Reed
JC
IAP family proteins—suppressors of apoptosis.
Genes Dev.
13
1999
239
252
26
Deveraux
QL
Takahashi
R
Salvesen
GS
Reed
JC
X-linked IAP is a direct inhibitor of cell-death proteases.
Nature.
388
1997
300
304
27
Los
M
Wesselborg
S
Schulze-Osthoff
KM
The role of caspases in development, immunity, and apoptotic signal transduction: lessons from knockout mice.
Immunity.
10
1999
629
639
28
Uren
AG
Coulson
EJ
Vaux
DL
Conservation of baculovirus inhibitor of apoptosis repeat proteins (BIRPs) in viruses, nematodes, vertebrates and yeasts.
Trends Biochem Sci.
23
1998
159
162
29
Fesik
SW
Insights into programmed cell death through structural biology.
Cell.
103
2000
273
282
30
Kasof
GK
Gomes
BC
Livin, a novel inhibitor of apoptosis protein family member.
J Biol Chem.
276
2001
3238
3246
31
Vucic
D
Stennicke
HR
Pisabarro
MT
Salvesen
GS
Dixit
VM
ML-IAP, a novel inhibitor of apoptosis that is preferentially expressed in human melanomas.
Curr Biol.
10
2000
1359
1366
32
Debatin
K-M
Disturbances of the CD95 (APO-1/Fas) system in disorders of lymphohematopoietic cells.
Cell Death Differ.
3
1996
185
189
33
Fisher
GH
Rosenberg
FJ
Straus
SE
et al
Dominant interfering Fas gene mutations impair apoptosis in a human autoimmune lymphoproliferative syndrome.
Cell.
81
1995
935
946
34
Lynch
DH
Watson
ML
Alderson
MR
et al
The mouse fas-ligand gene is mutated in gld mice and is part of a TNF family gene cluster.
Immunity.
1
1994
131
136
35
Rieux-Laucat
F
Le Deist
F
Hivroz
C
et al
Mutations in Fas associated with human lymphoproliferative syndrome and autoimmunity.
Science.
268
1995
1347
1349
36
Takahashi
T
Tanaka
M
Brannan
CI
et al
Generalized lymphoproliferative disease in mice, caused by a point mutation in the Fas ligand.
Cell.
76
1994
969
976
37
Watanabe-Fukunaga
R
Brannan
CI
Copeland
NG
Jenkins
NA
Nagata
S
Lymphoproliferation disorder in mice explained by defects in Fas antigen that mediates apoptosis.
Nature.
356
1992
314
317
38
Reap
EA
Roof
K
Maynor
K
Cohen
PL
Markedly diminished radiation-induced lymphocyte apoptosis in lpr mice suggests a role for Fas in eliminating damaged cells.
Ann N Y Acad Sci.
5
1997
116
118
39
Belka
C
Marini
P
Budach
W
et al
Radiation-induced apoptosis in human lymphocytes relies on the up-regulation of CD95/Fas/APO-1 ligand.
Radiat Res.
149
1998
588
595
40
Bernassola
F
Scheuerpflug
C
Herr
I
Krammer
PH
Debatin
K-M
Melino
G
Induction of apoptosis by IFNg in human neuroblastoma cell lines through the CD95/CD95L autocrine circuit.
Cell Death Differ.
6
1999
652
660
41
Caricchio
R
Kovalenko
D
Kaufmann
WK
Cohen
P
Apoptosis provoked by the oxidative stress inducer menadione (vitamin K3) is mediated by the Fas/Fas ligand system.
Clin Immunol.
93
1999
65
74
42
Caricchio
R
Reap
E
Cohen
P
Fas/Fas ligand interactions are involved in UV-B-induced human lymphocyte apoptosis.
J Immunol.
161
1998
241
251
43
Friesen
C
Herr
I
Krammer
PH
Debatin
K-M
Involvement of the CD95 (APO-1/Fas) receptor/ligand system in drug-induced apoptosis in leukemia cells.
Nat Med.
2
1996
574
577
44
Fulda
S
Los
M
Friesen
C
Debatin
K-M
Chemosensitivity of solid tumor cells in vitro is related to activation of the CD95 system.
Int J Cancer.
76
1998
105
114
45
Fulda
S
Scaffidi
C
Pietsch
T
Krammer
PH
Peter
ME
Debatin
K-M
Activation of the CD95 (APO-1/Fas) pathway in drug- and γ-irradiation-induced apoptosis of brain tumor cells.
Cell Death Differ.
5
1998
884
893
46
Fulda
S
Sieverts
H
Friesen
C
Herr
I
Debatin
K-M
The CD95(APO-1/Fas) system mediates drug induced apoptosis in neuroblastoma cells.
Cancer Res.
57
1997
3823
3829
47
Herr
I
Wilhelm
D
Bohler
T
Angel
P
Debatin
K-M
JNK/SAPK activity is not sufficient for anticancer therapy-induced apoptosis involving CD95-L, TRAIL and TNF-α.
Int J Cancer.
80
1999
417
424
48
Leverkus
M
Yaar
M
Gilchrest
BA
Fas/Fas ligand interaction contributes to UV-induced apoptosis in human keratinocytes.
Exp Cell Res.
232
1997
255
262
49
Mo
Y-Y
Beck
WT
DNA damage signals induction of Fas ligand in tumor cells.
Mol Pharmacol.
55
1999
216
222
50
Mueller
M
Strand
S
Hug
H
et al
Drug-induced apoptosis in hepatoma cells is mediated by the CD95 (APO-1/Fas) receptor/ligand system and involves activation of wild-type p53.
J Clin Invest.
99
1997
403
413
51
Sheard
MA
Vojtesek
B
Janakova
L
Kovarik
J
Zaloudik
J
Up-regulation of Fas (CD95) in human p53-wild-type cancer cells treated with ionizing radiation.
Int J Cancer.
73
1997
757
762
52
Vogt
M
Bauer
MKA
Ferrari
D
Schulze-Osthoff
K
Oxidative stress and hypoxia/reoxygenation trigger CD95 (APO-1/Fas) ligand expression in microglial cells.
FEBS Lett.
429
1998
67
72
53
Kaufmann
SH
Earnshaw
WC
Induction of apoptosis by cancer chemotherapy.
Exp Cell Res.
256
2000
42
49
54
McGahon
AJ
Costa Pereira
AP
Daly
L
Cotter
TG
Chemotherapeutic drug-induced apoptosis is independent of the Fas (APO-1/CD95) receptor/ligand system.
Br J Haematol.
101
1998
539
547
55
Micheau
O
Solary
E
Hammann
A
Martin
F
Dimanche-Boitrel
M-T
Sensitization of cancer cells treated with cytotoxic drugs to Fas-mediated cytotoxicity.
J Natl Cancer Inst.
89
1997
783
789
56
Friesen
C
Fulda
S
Debatin
K-M
Deficient activation of the CD95 (APO-1/Fas) system in drug-resistant cells.
Leukemia.
11
1997
1833
1841
57
Landowski
TH
Gleason-Guzman
MC
Dalton
WS
Selection for drug resistance results in resistance to Fas-mediated apoptosis.
Blood.
89
1997
1854
1861
58
Los
M
Herr
I
Fulda
S
Friesen
C
Schulze-Osthoff
K
Debatin
K-M
Activation of ICE/Ced-3 proteases by anticancer drugs.
Blood.
90
1997
3118
3129
59
Aragane
Y
Kulms
D
Metze
D
et al
Ultraviolet light induces apoptosis via direct activation of CD95 (Fas/APO-1) independently of its ligand CD95L.
J Cell Biol.
140
1998
171
182
60
Micheau
O
Solary
E
Hammann
A
Dimanche-Boitrel
M-T
Fas ligand-independent, FADD-mediated activation of the Fas death pathway by anticancer drugs.
J Biol Chem.
274
1999
7987
7992
61
Rehemtulla
A
Hamilton
CA
Chinnaiyan
AM
Dixit
VM
Ultraviolet radiation-induced apoptosis is mediated by activation of CD95 (Fas/APO-1).
J Biol Chem.
272
1997
25783
25786
62
Eischen
CM
Kottke
TJ
Martins
LM
et al
Comparison of apoptosis in wild-type and Fas-resistant cells: chemotherapy-induced apoptosis is not dependent on Fas/Fas ligand interactions.
Blood.
3
1997
935
943
63
Villunger
A
Egle
A
Kos
M
et al
Drug-induced apoptosis is associated with enhanced Fas (APO-1/CD95) ligand expression but occurs independently of Fas (APO-1/CD95) signaling in human T-acute lymphatic leukemia cells.
Cancer Res.
57
1997
3331
3334
64
Newton
K
Harris
AW
Bath
ML
Smith
KGC
Strasser
A
A dominant interfering mutant of FADD/MORT1 enhances deletion of autoreactive thymocytes and inhibits proliferation of mature T lymphocytes.
EMBO J.
17
1998
706
718
65
Strasser
A
Harris
AW
Huang
DCS
Krammer
PH
Cory
S
Bcl-2 and Fas/APO-1 regulate distinct pathways to lymphocyte apoptosis.
EMBO J.
14
1995
6136
6147
66
Glaser
T
Wagenknecht
B
Groscurth
P
Krammer
PH
Weller
M
Death ligand/receptor-independent caspase activation mediates drug-induced cytotoxic cell death in malignant glioma cells.
Oncogene.
18
1999
5044
5053
67
Gamen
S
Anel
A
Lasierra
P
et al
Doxorubicin-induced apoptosis in human T-cell leukemia is mediated by caspase-3 activation in a Fas-independent way.
FEBS Lett.
41
1997
360
364
68
Fiedler
P
Schaetzlein
CE
Eibel
H
Constitutive expression of FasL in thyrocytes [comment].
Science.
279
1998
2015
69
Herr
I
Posovsky
C
Bohler
T
Debatin
K-M
mAb 33 from transduction laboratories specifically binds to human CD95-L.
Cell Death Differ.
7
2000
129
130
70
Papoff
G
Stasi
G
De Maria
R
et al
Constitutive expression of FasL in thyrocytes [comment].
Science.
279
1998
2015
71
Stokes
TA
Rymaszewski
M
Arscott
PL
et al
Constitutive expression of FasL in thyrocytes [comment].
Science.
279
1998
2015
72
Varfolomeev
EE
Schuchmann
M
Luria
V
et al
Targeted disruption of the mouse caspase 8 gene ablates cell death induction by the TNF receptors, Fas/Apo1, and DR3 and is lethal prenatally.
Immunity.
9
1998
267
276
73
Yeh
W-C
de la Pompa
JL
McCurrach
ME
et al
FADD: essential for embryo development and signaling from some, but not all, inducers of apoptosis.
Science.
279
1998
1954
1958
74
Hakem
R
Hakem
A
Duncan
GS
et al
Differential requirement for caspase-9 in apoptotic pathways in vivo.
Cell.
94
1998
339
352
75
Kuida
K
Haydar
TF
Kuan
C-Y
et al
Reduced apoptosis and cytochrome c-mediated caspase activation in mice lacking caspase-9.
Cell.
94
1998
325
337
76
Cecconi
F
Alvarez-Bolado
G
Meyer
BI
Roth
KA
Gruss
P
Apaf1 (CED-4 homolog) regulates programmed cell death in mammalian development.
Cell.
94
1998
727
737
77
Yoshida
H
Kong
Y-Y
Yoshida
R
et al
Apaf1 is required for mitochondrial pathways of apoptosis and brain development.
Cell.
94
1998
739
750
78
Jiang
S
Song
MJ
Shin
E-C
Lee
M-O
Kim
SJ
Park
JH
Apoptosis in human hepatoma cell lines by chemotherapeutic drugs via Fas-dependent and Fas-independent pathways.
Hepatology.
29
1999
101
110
79
Roy
S
Nicholson
DW
Cross-talk in cell death signaling.
J Exp Med.
192
2000
F21
F25
80
Fulda
S
Meyer
E
Debatin
KM
Cell type specific involvement of death receptor and mitochondrial pathways in drug-induced apoptosis.
Oncogene.
20
2001
1063
1075
81
Borner
C
Monney
L
Apoptosis without caspases: an inefficient molecular guillotine?
Cell Death Diff.
6
1999
497
507
82
Chautan
M
Chazal
G
Cecconi
F
Gruss
P
Golstein
P
Interdigital cell death can occur through a necrotic and caspase-independent pathway.
Curr Biol.
9
1999
967
970
83
Kitanaka
C
Kuchino
Y
Caspase-independent programmed cell death with necrotic morphology.
Cell Death Differ.
6
1999
508
515
84
Miller
TM
Moulder
KL
Knudson
CM
et al
Bax deletion further orders the cell death pathway in cerebellar granule cells and suggests a caspase-independent pathway to cell death.
J Cell Biol.
139
1997
205
217
85
Xiang
J
Chao
DT
Korsmeyer
SJ
Bax-induced cell death may not require interleukin 1-converting enzyme-like proteases.
Proc Natl Acad Sci U S A.
93
1996
14559
14563
86
Kawahara
A
Ohsawa
Y
Matsumura
H
Uchiyama
Y
Nagata
S
Caspase-independent cell killing by Fas-associated protein with death domain.
J Cell Biol.
143
1998
1353
1360
87
Vercammen
D
Beyaert
R
Denecker
G
et al
Inhibition of caspases increases the sensitivity of L929 cells to necrosis mediated by tumor necrosis factor.
J Exp Med.
187
1998
1477
1485
88
Schulze-Osthoff
K
Krammer
PH
Droege
W
Divergent signaling via APO-1/Fas and the TNF receptor, two homologous molecules involved in physiological cell death.
EMBO J.
13
1994
4587
4596
89
Stoka
V
Turk
B
Schendel
SL
et al
Lysosomal protease pathways to apoptosis.
J Biol Chem.
276
2001
3149
3157
90
Gao
G
Dou
P
N-terminal cleavage of bax by calpain generates a potent proapoptotic 18-kDa fragment that promotes Bcl-2-independent cytochrome c release and apoptotic cell death.
J Cell Biochem.
80
2000
53
72
91
Wolf
BB
Goldstein
JC
Stennicke
HR
et al
Calpain functions in a caspase-independent manner to promote apoptosis-like events during platelet activation.
Blood.
94
1999
1683
1692
92
Wood
DE
Thomas
A
Devi
LA
et al
Bax cleavage is mediated by calpain during drug-induced apoptosis.
Oncogene.
17
1998
1096
1078
93
Reddy
RK
Jun
L
Lee
AS
The endoplasmic reticulum chaperone glycoprotein GRP94 with Ca2+-binding and antiapoptotic properties is a novel proteolytic target of calpain during etoposide-induced apoptosis.
J Biol Chem.
274
1999
28476
28483
94
Angel
P
Karin
M
The role of Jun, Fos and the AP-1 complex in cell-proliferation and transformation.
Biochim Biophys Acta.
1072
1991
129
157
95
Leppä
S
Bohmann
D
Diverse functions of JNK signaling and c-Jun in stress response and apoptosis.
Oncogene.
18
1999
6158
6162
96
Davis
R
Signal transduction by the JNK group of MAP kinases.
Cell.
103
2000
239
252
97
Ip
YT
Davis
RJ
Signal transduction by the c-Jun N-terminal kinase (JNK)—from inflammation to development.
Curr Opin Cell Biol.
10
1998
205
219
98
Minden
A
Karin
M
Regulation and function of the JNK subgroup of MAP kinases.
Biochim Biophys Acta.
1333
1997
85
104
99
Weitzman
JB
Quick guide.
Jnk. Curr Biol.
10
2000
R290
100
Chen
Y-R
Meyer
CF
Tan
T-H
Persistent activation of c-Jun N-terminal kinase 1 (JNK1) in g-radiation-induced apoptosis.
J Biol Chem.
271
1996
631
634
101
Chen
Z
Seimiya
H
Naito
M
et al
ASK1 mediates apoptotic cell death induced by genotoxic stress.
Oncogene.
18
1999
173
180
102
Faris
M
Latinis
KM
Kempiak
SJ
Koretzky
GA
Nel
A
Stress-induced Fas ligand expression in T cells is mediated through a MEK kinase 1-regulated response element in the Fas ligand promoter.
Mol Cell Biol.
18
1998
5414
5424
103
Goillot
E
Raingeaud
J
Ranger
A
et al
Mitogen-activated protein kinase-mediated Fas apoptotic signaling pathway.
Proc Natl Acad Sci U S A.
94
1994
3302
3307
104
Herr
I
Wilhelm
D
Meyer
E
Jeremias
I
Angel
P
Debatin
K-M
JNK/SAPK activity contributes to TRAIL-induced apoptosis.
Cell Death Differ.
6
1999
130
135
105
Johnson
NL
Gardner
AM
Diener
KM
et al
Signal transduction pathways regulated by mitogen-activated/extracellular response kinase kinase kinase induce cell death.
J Biol Chem.
271
1996
3229
3237
106
Minden
A
Lin
A
McMahon
M
et al
Differential activation of ERK and JNK mitogen-activated protein kinases by Raf-1 and MEKK.
Science.
266
1994
1719
1723
107
Tournier
C
Hess
P
Yang
DD
et al
Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway.
Science.
288
2000
870
874
108
Verheij
M
Bose
R
Hua Lin
X
et al
Requirement for ceramide initiated JNK/SAPK signalling in stress-induced apoptosis.
Nature.
380
1996
75
79
109
Xia
Z
Dickens
M
Raingeaud
J
Davis
RJ
Greenberg
ME
Opposing effects of ERK and JNK-p38 MAP kinases on apoptosis.
Science.
270
1995
1326
1331
110
Yang
DD
Kuan
CY
Whitmarsh
AJ
et al
Absence of excitotoxicity-induced apoptosis in the hippocampus of mice lacking the Jnk3 gene.
Nature.
389
1997
865
870
111
Yang
X
Khosravi-Far
R
Chang
HY
Baltimore
D
Daxx, a novel Fas-binding protein that activates JNK and apoptosis.
Cell.
89
1997
1067
1076
112
Zanke
BW
Boudreau
K
Rubie
E
et al
The stress-activated protein kinase pathway mediates cell death following injury induced by cis-platinum, UV irradiation or heat.
Curr Biol.
6
1996
606
613
113
Nishina
H
Fischer
KD
Radvanyl
L
et al
Stress-signalling protects thymocytes from apoptosis mediated by CD95 and CD3.
Nature.
385
1997
350
353
114
Liu
Z-G
Hsu
H
Goeddel
DV
Karin
M
Dissection of TNF receptor 1 effector functions: JNK activation is not linked to apoptosis while NF-κB activation prevents cell death.
Cell.
87
1996
565
576
115
Natoli
G
Costanzo
A
Ianni
A
et al
Activation of SAPK/JNK by TNF receptor 1 through a noncytotoxic TRAF2-dependent pathway.
Science.
275
1997
200
203
116
Kuan
CY
Yang
DD
Samanta Roy
DR
Davis
RJ
Rakic
P
Flavell
RA
The Jnk1 and Jnk2 protein kinases are required for regional specific apoptosis during early brain development.
Neuron.
22
1999
667
676
117
Sabapathy
K
Jochum
W
Hochedlinger
K
Chang
L
Karin
M
Wagner
EF
Defective neural tube morphogenesis and altered apoptosis in the absence of both JNK1 and JNK2.
Mech Dev.
89
1999
115
124
118
Behrens
A
Sibilia
M
Wagner
EF
Amino-terminal phosphorylation of c-Jun regulates stress-induced apoptosis and cellular proliferation.
Nat Gen.
21
1999
326
329
119
Kolbus
A
Herr
I
Schreiber
M
Debatin
K-M
Wagner
EF
Angel
P
c-Jun dependent induction of CD95-L expression is a rate-limiting step in the induction of apoptosis by alkylating agents.
Mol Cell Biol.
20
2000
575
582
120
Kasibhatla
S
Brunner
T
Genestier
L
Echeverri
F
Mahboudi
A
Green
DR
DNA damaging agents induce expression of Fas ligand and subsequent apoptosis in T lymphocytes via the activation of NF-κB and AP-1.
Mol Cell.
1
1998
543
551
121
Faris
M
Kokot
N
Latinis
K
et al
The c-Jun N-terminal kinase cascade plays a role in stress-induced apoptosis in Jurkat cells by up-regulating Fas ligand expression.
J Immunol.
160
1998
134
144
122
Harwood
FG
Kasibhatla
S
Petak
I
Vernes
R
Green
DR
Houghton
JA
Regulation of FasL by NF-κB and AP-1 in Fas-dependent thymineless death of human colon carcinoma cells.
J Biol Chem.
275
2000
10023
10029
123
Le-Niculescu
H
Bonfoco
E
Kasuya
Y
Claret
F-X
Green
DR
Karin
M
Withdrawal of survival factors results in activation of the JNK pathway in neuronal cells leading to Fas ligand induction and cell death.
Mol Cell Biol.
19
1999
751
763
124
Matsui
K
Xiao
S
Fine
A
Ju
S-T
Role of activator protein-1 in TCR-mediated regulation of the murine fasl promoter.
J Immunol.
164
2000
3002
3008
125
Tsai
ET
Jain
J
Pesavento
PA
Rao
A
Goldfeld
AE
Tumor necrosis factor α gene regulation in activated T cells involves ATF-2 and NFAT.
Mol Cell Biol.
16
1996
459
467
126
Zagariya
A
Mungre
S
Lovis
R
et al
Tumor necrosis factor alpha gene regulation: enhancement of C/EBPβ-induced activation by c-Jun.
Mol Cell Biol.
18
1998
2815
2824
127
Zhang
J
Gao
J-X
Salojin
K
et al
Regulation of Fas ligand expression during activation-induced cell death in T cells by p38 mitogen-activated protein kinase and c-Jun NH2-terminal kinase.
J Exp Med.
191
2000
1017
1029
128
Lee
H
Arsura
M
Wu
M
Duyao
M
Buckler
AJ
Sonenshein
GE
Role of Rel-related factors in control of c-myc gene transcription in receptor-mediated apoptosis of the murine B cell WEHI 231 line.
J Exp Med.
181
1995
1169
1177
129
Fuchs
SY
Adler
V
Buschmann
T
et al
JNK targets p53 ubiquitination and degradation in nonstressed cells.
Genes Dev.
12
1998
2658
2663
130
Fuchs
SY
Adler
V
Pincus
MR
Ronai
Z
MEKK1/JNK signaling stabilizes and activates p53.
Proc Natl Acad Sci U S A.
95
1998
10541
10546
131
Schreiber
M
Kolbus
A
Piu
F
et al
Control of cell cycle progression by c-Jun is p53 dependent.
Genes Dev.
13
1999
607
613
132
Noguchi
K
Kitanaka
C
Yamana
H
Kokubu
A
Mochizuki
T
Kuchino
Y
Regulation of c-Myc through phosphorylation at Ser-62 and Ser-71 by c-Jun N-terminal kinase.
J Biol Chem.
274
1999
32580
32587
133
Hatai
T
Matsuzawa
A
Inoshita
S
et al
Execution of ASK1-induced apoptosis by the mitochondria-dependent caspase activation.
J Biol Chem.
275
2000
26576
26581
134
Kharbanda
S
Saxena
S
Yoshida
K
et al
Translocation of SAPK/JNK to mitochondria and interaction with Bcl-XL in response to DNA damage.
J Biol Chem.
275
2000
322
327
135
Park
J
Kim
I
Young
JO
Lee
K-W
Han
P-L
Choi
E-J
Activation of c-Jun N-terminal kinase antagonizes an anti-apoptotic action of bcl-2.
J Biol Chem.
272
1997
16725
16728
136
Wang
X-Z
Lawson
B
Brewer
JW
et al
Signals from the stressed endoplasmic reticulum induce C/EBP-homologous protein (CHOP/GADD153).
Mol Cell Biol.
16
1996
4273
4280
137
Kaufman
RJ
Stress signaling from the lumen of the endoplasmic reticulum: coordination of gene transcriptional and translational controls.
Genes Dev.
13
1999
1211
1233
138
Zinszner
H
Kuroda
M
Wang
XZ
et al
CHOP is implicated in programmed cell death in response to impaired function of the endoplasmic reticulum.
Genes Dev.
12
1998
982
995
139
Baichwal
VR
Bauerle
PA
Activate NF-κB or die?
Curr Biol.
7
1997
R94
R96
140
Nakagawa
T
Zhu
H
Morishima
N
et al
Caspase-12-mediates endoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-beta.
Nature.
403
2000
98
103
141
Xu
Q
Reed
JC
Bax inhibitor-1, a mammalian apoptosis suppressor identified by functional screening in yeast.
Mol Cell.
1
1998
337
346
142
Ng
FWH
Shore
GC
Bcl-XL cooperatively associates with the Bap31 complex in the endoplasmic reticulum, dependent on procaspase-8 and Ced-4 adaptor.
J Biol Chem.
273
1998
3140
3143
143
Nguyen
M
Breckenridge
DG
Ducret
A
Shore
GC
Caspase-resistant BAP31 inhibits Fas-mediated apoptotic membrane fragmentation and release of cytochrome c from mitochondria.
Mol Cell Biol.
20
2000
6731
6740
144
Torgler
CN
de Tiani
M
Raven
T
Aubry
J-P
Brown
R
Meldrum
E
Expression of bak in S. pombe results in a lethality mediated through interaction with the calnexin homologue Cnx1.
Cell Death Differ.
4
1997
263
271
145
Kuo
TH
Kim
HR
Zhu
L
Yu
L
Lin
HM
Tsang
W
Modulation of endoplasmic reticulum calcium pump by Bcl-2.
Oncogene.
17
1998
1903
1910
146
Tagami
S
Eguchi
Y
Kinoshita
M
Takeda
M
Tsujimoto
Y
A novel protein, RTN-xS, interacts with both Bcl-XL and Bcl-2 on endoplasmic reticulum and reduces their anti-apoptotic activity.
Oncogene
19
2000
5736
5746
147
Häcki
J
Egger
L
Monney
L
et al
Apoptotic crosstalk between the endoplasmic reticulum and mitochondria controlled by Bcl-2.
Oncogene.
19
2000
2286
2295
148
Nakamura
K
Bossy-Wetzel
E
Burns
K
et al
Changes in endoplasmic reticulum luminal environment affect cell sensitivity to apoptosis.
J Cell Biol.
150
2000
731
740
149
Kyriakis
JM
Banerjee
P
Nikolakaki
E
et al
The stress-activated protein kinase subfamily of c-Jun kinases.
Nature.
369
1994
156
160
150
Urano
F
Wang
XZ
Bertolotti
A
et al
Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1.
Science.
287
2000
664
666
151
Srivastava
RK
Sollott
SJ
Khan
L
Hansford
R
Lakatta
EG
Longo
DL
Bcl-2 and Bcl-XL block thapsigargin-induced nitric oxide generation, c-Jun NH2-terminal kinase activity, and apoptosis.
Mol Cell Biol.
19
1999
5659
5674
152
Levine
JL
p53, the cellular gatekeeper for growth and division.
Cell.
88
1997
323
331
153
Venot
C
Maratrat
M
Dureuil
C
Conseiller
E
Bracco
L
Debussche
L
The requirement for the p53 proline-rich functional domain for mediation of apoptosis is correlated with specific PIG3 gene transactivation and with transcriptional repression.
EMBO J.
17
1998
4668
4679
154
Walker
KK
Levine
AJ
Identification of a novel p53 functional domain that is necessary for efficient growth suppression.
Proc Natl Acad Sci U S A.
93
1996
15335
15340
155
Zhu
J
Zhou
W
Jiang
J
Cheng
X
Identification of a novel p53 functional domain that is necessary for mediating apoptosis.
J Biol Chem.
273
1998
13030
13036
156
Giaccia
AJ
Kastan
MB
The complexity of p53 modulation: emerging patterns from divergent signals.
Genes Dev.
12
1998
2973
2983
157
Prives
C
Signaling to p53: breaking the MDM2–p53 circuit.
Cell.
95
1998
5
8
158
Haupt
Y
Rowan
S
Shaulian
E
Vousden
KH
Oren
M
Induction of apoptosis in HeLa cells by trans-activation-deficient p53.
Genes Dev.
9
1995
2170
2183
159
Morgenbesser
SD
Williams
BO
Jacks
T
DePinho
RA
p53-dependent apoptosis produced by Rb-deficiency in the developing mouse lens.
Nature.
371
1994
72
74
160
Bing
A
Dou
QP
Cleavage of retinoblastoma protein during apoptosis: an interleukin 1 beta-converting enzyme-like protease as candidate.
Cancer Res.
56
1996
438
442
161
Erhardt
P
Tomaselli
KJ
Cooper
GM
Identification of the MDM2 oncoprotein as a substrate for CPP32-like apoptotic proteases.
J Biol Chem.
272
1997
15049
15052
162
Janicke
RU
Walker
PA
Lin
XY
Porter
AG
Specific cleavage of the retinoblastoma protein by an ICE-like protease in apoptosis.
EMBO J.
15
1996
6969
6978
163
Brodsky
MH
Nordstrom
W
Tsang
G
Kwan
E
Rubin
GM
Abrams
JM
Drosophila p53 binds a damage response element at the reaper locus.
Cell.
101
2000
103
113
164
Mueller
M
Wilder
S
Bannasch
D
et al
p53 activates the CD95 (APO-1/Fas) gene in response to DNA damage by anticancer drugs.
J Exp Med.
188
1998
2033
2045
165
Oda
K
Arakawa
H
Tanaka
T
et al
p53AIP1, a potential mediator of p53-dependent apoptosis, and its regulation by Ser-46-phosphorylated p53.
Cell.
102
2000
849
862
166
Owen-Schaub
LB
Zhang
W
Cusack
JC
et al
Wild-type human p53 and a temperature-sensitive mutant induce Fas/APO-1 expression.
Mol Cell Biol.
6
1995
3032
3040
167
Sheikh
MS
Burns
TF
Huang
Y
et al
p53-dependent and -independent regulation of the death receptor KILLER/DR5 gene expression in response to genotoxic stress and tumor necrosis factor alpha.
Cancer Res.
15
1998
1593
1598
168
Thomas
A
Giesler
T
White
E
p53 mediates Bcl-2 phosphorylation and apoptosis via activation of the Cdc42/JNK1 pathway.
Oncogene.
19
2000
5259
5269
169
Wu
GS
Burns
TF
McDonald
ER
III
et al
Induction of TRAIL receptor KILLER/DR5 in p53dependent apoptosis but not growth arrest.
Oncogene.
11
1999
6411
6418
170
Soengas
MS
Alarcon
RM
Yoshida
H
et al
Apaf-1 and caspase-9 in p53-dependent apoptosis and tumor inhibition.
Science.
284
1999
156
159
171
Bennett
M
MacDonald
K
Chan
SW
Luzio
JP
Simari
R
Weissberg
P
Cell surface trafficking of Fas: a rapid mechanism of p53-mediated apoptosis.
Science.
282
1998
290
293
172
Munsch
D
Watanabe-Fukunaga
R
Bourdon
J-C
et al
Human and mouse Fas (APO-1/CD95) death receptor genes each contain a p53-responsive element that is activated by p53 mutants unable to induce apoptosis.
J Biol Chem.
275
2000
3867
3872
173
Li
P-F
Dietz
R
von Harsdorf
R
p53 regulates mitochondrial membrane potential through reactive oxygen species and induces cytochrome c-independent apoptosis blocked by bcl-2.
EMBO J.
18
1999
6027
6036
174
Newton
K
Strasser
A
Ionizing radiation and chemotherapeutic drugs induce apoptosis in lymphocytes in the absence of Fas or FADD/MORT1 signaling: implications for cancer therapy.
J Exp Med.
191
2000
195
200
175
O'Connor
L
Harris
AW
Strasser
A
CD95 (Fas/APO-1) and p53 signal apoptosis independently in diverse cell types.
Cancer Res.
60
2000
1217
1220
176
Johnson
TM
Yu
ZX
Ferrans
VJ
Lowenstein
RA
Finkel
T
Reactive oxygen species are downstream mediators of p53-dependent apoptosis.
Proc Natl Acad Sci U S A.
93
1996
11848
11852
177
Polyak
K
Xia
Y
Zweier
JL
Kinzler
KW
Vogelstein
B
A model for p53-induced apoptosis.
Nature.
389
1997
300
305
178
Ding
HF
McGill
G
Rowan
S
Schmaltz
C
Shimamura
A
Fisher
DE
Oncogene-dependent regulation of caspase activation by p53 protein in a cell-free system.
J Biol Chem.
273
1998
28378
29283
179
Ravi
R
Bedi
GC
Engstrom
LW
et al
Regulation of death receptor expression and TRAIL/APO2L-induced apoptosis by NF-κB.
Nat Cell Biol.
3
2001
409
416
180
Baeuerle
PA
Baltimore
D
NF-κB: ten years after.
Cell.
87
1996
13
20
181
Verma
IM
Stevenson
JK
Schwarz
EM
Van Antwerp
D
Miyamoto
S
Rel/ NF-κB /IκB family: intimate tales of association and dissociation.
Genes Dev.
9
1995
2723
2735
182
Karin
M
How NF-κB is activated: the role of the IkB kinase (IKK) complex.
Oncogene.
18
1999
6867
6874
183
Abbadie
C
Kabrun
N
Bouali
F
et al
High levels of c-rel expression are associated with programmed cell death in the developing avian embryo and in bone marrow cells in vitro.
Cell.
75
1993
899
912
184
Huguet
C
Enrietto
P
Vandenbunder
B
Abbadie
C
c-Rel: a multifunctional transcription factor?
Cell Death Differ.
1
1994
71
76
185
Bash
J
Zong
WX
Gelinas
C
c-Rel arrests the proliferation of HeLa cells and affects critical regulators of the G1/S-phase transition.
Mol Cell Biol.
17
1997
6526
6536
186
Beg
AA
Baltimore
D
An essential role for NF-κB in preventing TNF-alpha-induced cell death.
Science.
274
1996
782
784
187
Li
Q
Estepa
G
Memet
S
Israel
A
Verma
IM
Complete lack of NF-κB activity in IKK1 and IKK2 double-deficient mice: additional defect in neurulation.
Genes Dev.
14
2000
1729
1733
188
Grossmann
M
Metcalf
D
Merryfull
J
Beg
A
Baltimore
D
Gerondakis
S
The combined absence of the transcription factors Rel and RelA leads to multiple hemopoietic cell defects.
Proc Natl Acad Sci U S A.
96
1999
11848
11853
189
Jeremias
I
Kupatt
C
Baumann
B
Herr
I
Wirth
T
Debatin
K-M
Inhibition of nuclear factor κB activation attenuates apoptosis resistance in lymphoid cells.
Blood.
91
1998
4624
4631
190
Wang
C-Y
Mayo
W
Baldwin
AS
Jr
TNF-α and cancer therapy-induced apoptosis: potentiation by inhibition of NF-κB.
Science.
274
1996
784
787
191
Wu
M
Lee
H
Bellas
RE
et al
Inhibition of NF-κB/Rel induces apoptosis of murine B cells.
EMBO J.
15
1996
4682
4690
192
Pahl
HL
Activators and target genes of Rel/ NF-κB transcription factors.
Oncogene.
18
1999
6853
6866
193
Wang
C-Y
Mayo
MW
Korneluk
RG
Goeddel
DV
Baldwin
AS
Jr
NF-κB antiapoptosis: induction of TRAF1 and TRAF2 and c-IAP1 and c-IAP2 to suppress caspase-8 activation.
Science.
281
1998
1680
1683
194
Grigoridis
G
Zhan
Y
Grumont
RJ
et al
The Rel subunit of NF-κB-like transcription factors is a positive and negative regulator of macrophage gene expression: distinct roles for Rel in different macrophage populations.
EMBO J.
15
1996
7099
7107
195
Mannick
JB
Miao
XQ
Stamler
JS
Nitric oxide inhibits Fas-induced apoptosis.
J Biol Chem.
272
1997
24125
24128
196
Barkett
M
Gilmore
TD
Control of apoptosis by Rel/ NF-κB transcription factors.
Oncogene.
18
1999
6910
6924
197
Chan
H
Bartos
DP
Owen-Schaub
LB
Activation-dependent transcriptional regulation of the human fas promoter requires NF-κB p50–p65 recruitment.
Mol Cell Biol.
19
1999
2098
2108
198
Kasibhatla
S
Genestier
L
Green
DR
Regulation of Fas-ligand expression during activation-induced cell death in T lymphocytes via nuclear factor κB.
J Biol Chem.
274
1999
987
992
199
Rivera-Walsh
I
Cvijic
ME
Xiao
G
Sun
S-C
The NF-κ B signaling pathway is not required for Fas ligand gene induction but mediates protection from activation-induced cell death.
J Biol Chem.
275
2000
25222
25230
200
Schulze-Osthoff
K
Stroh
C
Induced proximity model attracts NF-κB researchers.
Cell Death Differ.
7
2000
1025
1026
201
Tak
PP
Firestein
GS
NF-κB: a key role in inflammatory diseases.
J Clin Invest.
107
2001
7
11
202
Basu
S
Kolesnick
R
Stress signals for apoptosis: ceramide and c-Jun kinase.
Oncogene.
17
1998
3277
3285
203
Hannun
YA
Functions of ceramide in coordinating cellular responses to stress.
Science.
274
1996
1866
1859
204
Hannun
YA
Luberto
C
Ceramide in the eukaryotic stress response.
Trends Cell Biol.
10
2000
73
80
205
Kolesnick
R
Fuks
Z
Ceramide: a signal for apoptosis or mitogenesis?
J Exp Med.
181
1995
1949
1952
206
Malisan
F
Testi
R
Lipid signaling in CD95-mediated apoptosis.
FEBS Lett.
452
1999
100
103
207
Testi
R
Sphingomyelin breakdown and cell fate.
Trends Biochem Sci.
21
1996
468
471
208
Cuvillier
O
Pirianov
G
Kleuser
B
et al
Suppression of ceramide-mediated programmed cell death by sphingosine-1-phosphate.
Nature.
381
1996
800
803
209
Van Brocklyn
JR
Cuvillier
O
Olivera
A
Spiegel
S
Sphingosine-1-phosphate: a lipid second messenger regulating cell growth and survival.
J Liposome Res.
8
1998
135
145
210
Bourbon
NA
Yun
J
Kester
M
Ceramide directly activates protein kinase C ζ to regulate a stress-activated protein kinase signaling complex.
J Biol Chem.
275
2000
35617
35623
211
Herr
I
Martin-Villalba
A
Kurz
E
et al
FK506 prevents stroke-induced generation of ceramide and apoptosis signaling.
Brain Res.
826
1999
210
219
212
Herr
I
Wilhelm
D
Bohler
T
Angel
P
Debatin
K-M
Activation of CD95 (APO-1/Fas) signaling by ceramide mediates cancer therapy-induced apoptosis.
EMBO J.
16
1997
6200
6208
213
Basu
S
Bayoumy
S
Zhang
Y
Lozano
J
Kolesnick
R
BAD enables ceramide to signal apoptosis via Ras and Raf-1.
J Biol Chem.
273
1998
30419
30426
214
Quillet-Mary
A
Jaffrezou
JP
Mansat
V
Bordier
C
Naval
J
Laurent
G
Implication of mitochondrial hydrogen peroxide gereration in ceramide-induced apoptosis.
J Biol Chem.
272
1997
21388
21395
215
Susin
SA
Zamzami
N
Castedo
M
et al
The central executioner of apoptosis: multiple connections between protease activation and mitochondria in Fas/APO-1/CD95- and ceramide-induced apoptosis.
J Exp Med.
186
1997
25
37
216
De Maria
R
Lenti
L
Malisan
F
et al
Requirement for GD3 ganglioside in CD95- and ceramide-induced apoptosis.
Science.
277
1997
1652
1655
217
Chmura
JS
Nodzenski
E
Beckett
MA
Kufe
DW
Quintans
J
Weichselbaum
RR
Loss of ceramide production confers resistance to radiation-induced apoptosis.
Cancer Res.
57
1997
1270
1275
218
Michael
JM
Lavin
MF
Watters
DJ
Resistance to radiation-induced apoptosis in Burkitt's lymphoma cells is associated with defective ceramide signaling.
Cancer Res.
57
1997
3600
3605
219
Santana
P
Pena
LA
Haimovitz-Friedman
A
et al
Acid sphingomyelinase-deficient human lymphoblasts and mice are defective in radiation-induced apoptosis.
Cell.
86
1996
189
199
220
Chinnaiyan
AM
Tepper
CG
Seldin
MF
et al
FADD/MORT1 is a common mediator of CD95 (Fas/APO-1) and tumor necrosis factor receptor-induced apoptosis.
J Biol Chem.
271
1996
4961
4965
221
Mizushima
N
Koike
R
Kohsaka
H
et al
Ceramide induces apoptosis via CPP32 activation.
FEBS Lett.
395
1996
267
271
222
Suzuki
A
Iwasaki
M
Kato
M
Wagai
N
Sequential operation of ceramide synthesis and ICE cascade in CPT-11-initiated apoptotic death signaling.
Exp Cell Res.
233
1997
41
47
223
Brenner
B
Ferlinz
K
Grassme
H
et al
Fas/CD95/APO-1 activates the acidic sphingomyelinase via caspases.
Cell Death Differ.
5
1998
29
37
224
Gamen
S
Marzo
I
Anel
A
Pineiro
A
Naval
J
CPP32 inhibition prevents Fas-induced ceramide generation and apoptosis in human cells.
FEBS Lett.
390
1996
232
237
225
Pronk
GJ
Ramer
K
Amiri
P
Willians
LT
Requirement of an ICE-like protease for induction of apoptosis and ceramide generation by REAPER.
Science.
271
1996
808
810
226
Sillence
DJ
Allan
D
Evidence against an early signaling role for ceramide in Fas-mediated apoptosis.
Biochem J.
324
1997
29
32
227
Watts
JD
Gu
M
Polverino
AJ
Patterson
SD
Aebersold
R
Fas-induced apoptosis of T cells occurs independently of ceramide generation.
Proc Natl Acad Sci U S A.
94
1997
7292
7296
228
Farber
S
Diamon
LK
Mercer
RD
Sylvester
RFJ
Wolff
JA
Temporary remissions in acute leukemia in children produced by folic acid antagonist, 4-aminopteoylglutamic acid (amiopterin).
N Engl J Med.
238
1948
787
793
229
Does cancer therapy trigger cell suicide? [editorial].
Science.
286
1999
2256
2258
230
Schmitt
CA
Rosenthal
CT
Lowe
SW
Genetic analysis of chemoresistance in primary murine lymphomas.
Nat Med.
6
2000
1029
1035
231
Lakin
ND
Jackson
SP
Regulation of p53 in response to DNA damage.
Oncogene.
18
1999
7644
7655
232
Vousden
KH
p53: death star.
Cell.
103
2000
691
694

Author notes

Klaus-Michael Debatin, Universitäts Kinderklinik, Prittwitzstr 43, D-89075 Ulm, Germany; e-mail:klaus-michael.debatin@medizin.uni-ulm.de.

Sign in via your Institution