Role of oncogenes in resistance of leukemic cells to apoptosis induced by conventional and novel chemotherapeutic agents

The purpose of this project was to investigate the role of two oncoproteins in the response of acute myelogenous leukemia cells (AML) to chemotherapeutic agents. Bcl-2, a mitochondrial protein which inhibits drug-induced apoptosis by preventing cytochrome c release, has been well correlated with resistance to standard chemotherapeutic agents. In contrast, the role of the tumor suppressor protein p53 in the response of cells to apoptotic stimuli has not yet been fully established. The fact that p53 mutations in AML cell lines tend to predict resistance to nucleoside analogues suggests that wild type p53 activity would increase the cytotoxic response.; We chose to investigate two chemotherapeutic agents. Ara-C is a deoxycytidine analogue which is arguably the most effective agent in the treatment of AML. While the ability of BcI-2 to prevent or delay the apoptotic response to ara-C has been well established, the role of p53 in ara-C-mediated apoptosis remains largely unexplored. Flavopiridol (FP), on the other hand, is a novel agent currently generating much interest as an inducer of apoptosis in hematopoietic cell lines. The mechanism by which FP mediates cell death is poorly understood. Early reports indicated that FP might induce apoptosis independently of p53 and Bcl-2, raising the possibility that it might activate an atypical apoptotic pathway.; Our results indicate that ara-C-induced apoptosis is enhanced in the presence of p53, and that enforced expression of p53 enhances the apoptotic response by increasing cytochrome c release. This suggests that loss of p53 may represent a means by which leukemic cells escape the lethal effects of ara-C by avoiding the apoptotic response to DNA damage. In contrast, we further demonstrate that FP potently induced apoptosis in human leukemia cells, an event which was dependent upon release of cytochrome c from the cytoplasm. The cellular apoptotic response to FP proceeds despite the enforced expression of p53, and overcomes Bcl-2 over-expression in a manner dependent upon its N-terminal loop domain. These findings have implications in the treatment of hematopoietic malignancy which may be resistant to standard chemotherapeutic regimens by means of molecular alterations in apoptotic regulatory proteins.