| The BCR-ABL oncoprotein is essential in the pathogenesis of chronic myeloid leukemia (CML). BCR-ABL's elevated tyrosine kinase activity contributes to the activation of multiple signal transduction pathways, some of which have been proposed to contribute to leukemogenesis. The development of small molecule inhibitors such as imatinib mesylate, which block BCR-ABL tyrosine kinase activity, has yielded promising results in the treatment of early stage CML. Despite this, the investigation of the molecular mechanisms underlying BCR-ABL-mediated leukemogenesis continues to be essential, as it is has become evident that kinase inhibition does not eliminate leukemic disease, and/or that resistance to tyrosine kinase inhibitors occurs. Given that evasion of apoptosis is a major characteristic in tumorigenesis, and that anti-cancer therapies commonly rely on the induction of apoptosis, a major goal of this study was to elucidate novel molecular mechanisms by which BCR-ABL suppresses apoptotic pathways, and thereby promotes aberrant survival.;We demonstrate that BCR-ABL negatively regulates the pro-apoptotic FOXO3a transcription factor in a PI3-K dependent manner, maintaining FOXO3a phosphorylation and cytoplasmic retention, and hence interfering with FOXO3a-mediated up-regulation of pro-apoptotic gene expression. In particular, our studies indicate that FOXO3a targets the TNF-related apoptosis inducing ligand (TRAIL) promoter, BCR-ABL suppresses TRAIL expression, and that one mechanism for TRAIL suppression is through negative regulation of FOXO3a. We found that BCR-ABL-mediated inhibition of FOXO3a also involves enhanced proteasome-dependent suppression of FOXO3a. We observed that the clinically relevant proteasome inhibitor, bortezomib, restored nuclear FOXO3a expression and induced apoptosis in imatinib-sensitive and -resistant BCR-ABL-transformed cells. In vivo, the suppression of FOXO3a, TRAIL, and another FOXO3a target, BIM, was associated with BCR-ABL-induced leukemic disease in a murine CML model, whereas bortezomib treatment inhibited symptoms of leukemic disease, prolonged survival, and restored normal expression of FOXO3a, TRAIL, and BIM. Finally, our studies revealed that another mechanism of FOXO3a regulation occurs at the mRNA level, in which BCR-ABL was found to suppress FOXO3a mRNA levels. Overall, the studies in this thesis demonstrate the significance and mechanisms of FOXO3a regulation by BCR-ABL, as well as yield novel insights into the molecular effects of bortezomib. |
