The role and regulation of human MDM2 (HDM2) in cell cycle and apoptosis

Overexpression of human MDM2 (HDM2) is found in many human tumors and cancers. HDM2 functions as an E3 ligase for p53 and itself, and is critical for the inhibition of p53-mediated cell cycle regulation and apoptosis. In addition, HDM2 interacts with several other cellular proteins and regulates the cell cycle in a p53-independent manner. In this study, we examined the mechanism whereby HDM2 regulates the cell cycle and apoptosis, particularly the role of HDM2 in the interaction between the cell cycle machinery and apoptotic apparatus.; We show that high levels of cdc2 kinase lead to the activation of caspase-3, which in turn modulates HDM2 function during apoptosis. We demonstrate that HDM2 is specifically cleaved in Xenopus mitotic extracts, but not in interphase extracts. Use of an inhibitor of caspase-3 or a caspase-3 cleavage mutant prevents HDM2 cleavage. In addition, overexpression of cdc2 kinase activates caspase-3 in Xenopus interphase extracts. Moreover, overexpression of cdc2/cyclin B1 in mammalian cells leads to caspase-3 activation and apoptosis. These data indicate that high levels of cdc2 kinase activity trigger an apoptotic pathway that leads to caspase activation and HDM2 cleavage.; In addition, we demonstrate that HDM2 protein expression is cell cycle dependent. HDM2 is stabilized during S, G2 phases and dramatically destabilized at M phase. Overexpression of HDM2 leads to a significant increase in S phase cells. These data reveal that HDM2 expression is cell cycle regulated and imply that the decrease of HDM2 may be important for cell cycle M phase exit.; Overall, our studies reveal a direct link between cdc2 kinase and apoptosis through caspase activation and HDM2 cleavage, and uncover an important regulation of MDM2 during the cell cycle.; Furthermore, we have screened a yeast expression cDNA library from human breast cancer MCF-7 cells in order to identify genomic instability genes. Expression of several individual human cDNA clones obtained in the genetic screen results in a 2 to 6-fold increase of chromosome loss in yeast. This innovative strategy may provide a new approach to identify chromosome instability genes in human cancer.