P53-DEPENDENT INHIBITION OF CYCLIN-DEPENDENT KINASE-ACTIVITIES IN HUMAN FIBROBLASTS DURING RADIATION-INDUCED G1 ARREST

Gamma-irradiation of human diploid fibroblasts in the G1 interval caus ed arrest of the cell cycle prior to S phase. This cell cycle block wa s correlated with a lack of activation of both cyclin E-Cyclin-depende nt kinase 2 (Cdk2) and cyclin A-Cdk2 kinases and depended on wild-type p53. Although the accumulation of cyclin A was strongly inhibited in gamma-irradiated cells, cyclin E accumulated and bound Cdk2 at normal levels but remained in an inactive state. We found that both whole-cel l lysates and inactive cyclin E-Cdk2 complexes prepared from irradiate d cells contained an activity capable of inactivating cyclin E-Cdk2 co mplexes. The protein responsible for this activity was shown to be p21 CIP1/WAF1, recently described as a p53-inducible Cdk inhibitor. Our da ta suggest a model in which ionizing radiation confers G1 arrest via t he p53-mediated induction of a Cdk inhibitor protein.