p2jcipi js j^j ReqU¡redfor fj,e Early G2 Checkpoint Response to Ionizing Radiation

We have previously reported that the immediate G2 checkpoint delay of normal human fibroblasts in response to ionizing radiation is correlated with inhibition of p34CDC2/cyclin B kinase activity. Here, we observed increased amounts of the cyclin-dependent protein kinase inhibitor p21l IPI associated with p34CDC2/cyclin B protein complexes from irradi ated normal human fibroblasts. Since wild-type p53 function is not re quired for the early G2 checkpoint response to ionizing radiation, we investigated whether a p53-independent induction of p21clpl was required for the ( ;, checkpoint. Early passage human fibroblasts 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 func tion in these cells by E6 protein does not alter their intact early G2 checkpoint response to y-rays. p21CIP> was found to be undetectable in p34' '" 2/cyclin B protein complexes and in total extracts from the E6-expressing cells, with or without exposure to ionizing radiation. These data indicate that p21CIPI is not required for the immediate G2 checkpoint response and is not induced by a p53-independent pathway in G2 phase following exposure to y-rays.