Enhanced Apoptosis in Response to Chemotherapeutic Agents in Leukemic Cells Is Associated with Post-Transcriptional Modification of Cyclin A1.

A critical cell cycle regulatory protein, cyclin A1 has previously been demonstrated to initiate acute myeloid leukemia (AML) in transgenic mice. We have recently shown that the cellular level and distribution of cyclin A1 correlates with the status of the leukemic phenotype. The aim of our present study is to investigate the role of cyclin A1 in mediating the response of leukemic cells to drug-induced apoptosis, and to define the molecular pathways that cooperate with cyclin A1 to enhance drug-induced apoptosis. We have shown that apoptosis was enhanced in U-937 cells when treated with chemotherapeutic agents including staurosporine and TNF. Interestingly, the increased rate of apoptosis was associated with concomitant increase in cyclin A1 expression. Treatment with a general caspase inhibitor blocked the cleavage of caspase-3 but did not prevent the up-regulation of cyclin A1. We have found that cyclin A1 protein is normally undergoing ubiquitin-mediated degradation in myeloid cells. However, in cells undergoing apoptosis induced by TNF, cyclin A1 protein is prevented from degradation and ubiquitination, suggesting that post-transcriptional modification of cyclin A1 protein is involved. Two dimensional gel analyses further verified that cyclin A1 protein is modified following the increased rate of apoptosis induced by TNF. We have further established a link between cyclin A1 and CDC25C, which might contribute to the protein modification of cyclin A1 and thus contribute to the enhanced apoptosis in response to TNF in leukemic cells. To further define the functional role of cyclin A1 in drug-induced apoptosis, we are currently using myeloid cells from cyclin A1 deficient mice and wild-type controls.