A 60 kd MDM2 isoform is produced by caspase cleavage in non-apoptotic tumor cells

The MDM2 oncogene product is a regulator of the p53 tumor suppressor. MDM2 is cleaved by Caspase 3 (CPP32) during apoptosis after aspartic acid-361, generating a 60 kd fragment. Here we report that human tumor cell lines often express high levels of a 60 kd MDM2 isoform (p60) in the absence of apoptosis. We demonstrate that p60 is a product of caspase cleavage of full length MDM2 after residue 361. The protease that cleaves MDM2 in non-apoptotic cells appears to be distinct from the apoptosis-specific Caspase 3, since Caspase 3 substrate poly(ADP-ribose) polymerase (PARP) is not cleaved in cells producing p60. The p60 form of MDM2 is a significant fraction of the p53-bound MDM2 protein in certain tumor cells, suggesting that it functions in the regulation of p53. p60 is also detected in breast tumors overexpressing MDM2. These observations suggest that MDM2 is regulated by caspase processing in non-apoptotic cells, and may account for the MDM2 proteins of similar mobility seen in tumors and other cell lines.