CLEAVAGE OF P21(CIP1 WAF1) AND P27(KIP1) MEDIATES APOPTOSIS IN ENDOTHELIAL-CELLS THROUGH ACTIVATION OF CDK2 - ROLE OF A CASPASE CASCADE/

Apoptosis of human endothelial cells after growth factor deprivation i s associated with rapid and dramatic up-regulation of cyclin A-associa ted cyclin-dependent kinase 2 (cdk2) activity. In apoptotic cells, the C termini of the cdk inhibitors p21(Cip1/Waf1) and p27(Kip1) are trun cated by specific cleavage. The enzyme involved in this cleavage is CP P32 and/or a CPP32-like caspase. After cleavage, p21(Cip1/Waf1) loses its nuclear localization sequence and exits the nucleus. Cleavage of p 21(Cip1/Waf1) and p27(Kip1) results in, a substantial reduction in the ir association with nuclear cyclin-cdk2 complexes, leading to a dramat ic induction of cdk2 activity. Dominant-negative cdk2, as well as a mu tant of p21(Cip1/Waf1) resistant to caspase cleavage, partially suppre ss apoptosis. These data suggest that cdk2 activation, through caspase -mediated cleavage of cdk inhibitors, may be instrumental in the execu tion of apoptosis following caspase activation.