Targeting 14-3-3 Sensitizes Native and Mutant BCR-ABL to Inhibition with U0126, Rapamycin and Bcl-2 Inhibitor GX15-070.

Recurring chromosomal abnormalities in human leukemias result in expression of a wide spectrum of constitutively activated fusion tyrosine kinases, including BCR-ABL associated with t(9;22)(q34;q22) chronic myelogenous leukemia (CML). Small molecule tyrosine kinase inhibitors, such as imatinib, are effective therapies for BCR-ABL-mediated human leukemias. However, clinical drug resistance occurs, which warrants development of alternative and/or complementary therapeutic strategies to target critical downstream signaling molecules. We recently demonstrated that disrupting 14-3-3/ligand association by a peptide-based 14-3-3 competitive antagonist, R18 induces significant apoptosis, partially through reactivation of AKT-inhibited pro-apoptotic FOXO3a in FGFR1 fusion transformed hematopoietic cells [Dong et al, Blood, 2007, 110(1):360–9]. Here we report that targeting 14-3-3 by R18 effectively induced significant apoptosis in Ba/F3 and K562 cells expressing BCR-ABL, through liberation and reactivation of FOXO3a, but not by affecting 14-3-3/BCR-ABL association or BCR-ABL kinase activity. In addition, co-immunoprecipitation experiments revealed that R18 was not able to disrupt 14-3-3/BAD association, suggesting a model that targeting 14-3-3 by R18 induces apoptosis in BCR-ABL transformed cells, at least in part, through liberation and reactivation of FOXO3a phosphorylated and inhibited by activated AKT, but not BAD inhibited by ERK. Thus, we hypothesize that targeting 14-3-3 by R18 may potentiate the inhibition of BCR-ABL transformed cells induced by blocking the parallel MEK1/ERK pathway, pro-survival Bcl-2 proteins, or other signaling effectors downstream of AKT. Indeed, R18 sensitized BCR-ABL transformed cells to inhibition with MEK1 inhibitor U0126, Bcl-2 inhibitor GX15-070, or mTOR inhibitor rapamycin, and the combined treatment of R18 with these anti-cancer reagents synergistically induced apoptotic cell death. Moreover, treatment of these reagents potentiates R18-induced reactivation of pro-apoptotic FOXO3a with enhanced expression of downstream transcription targets including p27kip1 and Bim1, suggesting a potential molecular mechanism of the combined therapy. Furthermore, R18 induced apoptotic cell death in cells expressing diverse imatinib-resistant BCR-ABL mutants, including T315I. This inhibition was enhanced by treatment of R18 in combination with U0126 and rapamycin. Together, our findings suggest a novel therapeutic strategy that targeting 14-3-3 may potentiate the effects of conventional therapy for BCR-ABL associated hematopoietic malignancies, and overcome drug-resistance.