P53 CONTROLS BOTH THE G(2)/M AND THE G(1) CELL-CYCLE CHECKPOINTS AND MEDIATES REVERSIBLE GROWTH ARREST IN HUMAN FIBROBLASTS

Increased expression of wild-type p53 in response to DNA damage arrests cells late in the G(1) stage of the cell cycle by stimulating the synthesis of inhibitors of cyclin-dependent kinases, such as p21/WAF1. To study the effects of p53 without the complication of DNA damage, we used tetracycline to regulate its expression in MDAH041 human fibroblasts that lack endogenous p53. When p53 is expressed at a level comparable to that induced by DNA damage in other cells, most MDAH041 cells arrested in G(1), but a significant fraction also arrested in G(2)/M. Cells released from a mimosine block early in S phase stopped predominantly in G(2)/M in the presence of p53, confirming that p53 can mediate arrest at this stage, as well as in G(1). In these cells, there was appreciable induction of p21/WAF1. MDAH041 cells arrested by tetracycline-regulated p53 for as long as 20 days resumed growth when the p53 level was lowered, in striking contrast to the irreversible arrest mediated by DNA damage. Therefore, irreversible arrest must involve processes other than or in addition to the interaction of p53-induced p21/WAF1 with G(1) and G(2) cyclin-dependent kinases.