P21(CIP1) IS NOT REQUIRED FOR THE EARLY G(2) CHECKPOINT RESPONSE TO IONIZING-RADIATION

We have previously reported that the immediate G(2) checkpoint delay o f normal human fibroblasts in response to ionizing radiation is correl ated with inhibition of p34(CDC2)/cyclin B kinase activity. Here, we o bserved increased amounts of the cyclin-dependent protein kinase inhib itor p21(CIP1) associated with p34(CDC2)/cyclin B protein complexes fr om irradiated normal human fibroblasts. Since wild-type p53 function i s not required for the early G(2) checkpoint response to ionizing radi ation, we investigated whether a p53-independent induction of p21(CIP1 ) was required for the G(2) checkpoint. Early passage human fibroblast s expressing the E6 oncoprotein of human papilloma virus-type 16 (NHF4 E6) were analyzed. It has been demonstrated earlier that inactivation of wild-type p53 function in these cells by E6 protein does not alter their intact early G(2) checkpoint response to gamma-rays. p21(CIP1) was found to be undetectable in p34(CDC2)/cyclin B protein complexes a nd in total extracts from the E6-expressing cells, with or without exp osure to ionizing radiation. These data indicate that p21(CIP1) is not required for the immediate G(2) checkpoint response and is not induce d by a p53-independent pathway in G(2) phase following exposure to gam ma-rays.