INDUCTION OF WAF1 CIP1 BY A P53-INDEPENDENT PATHWAY/

The p53-inducible gene WAF1/CIP1 encodes a M(r) 21,000 protein (p21) t hat has been shown to arrest cell growth by inhibition of cyclin-depen dent kinases. Induction of WAF1/CIP1 in cells undergoing p53-dependent G, arrest or apoptosis supports the idea that WAF1/CIP1 is a critical downstream effector of p53. In the present study, we used embryonic f ibroblasts from p53 ''knock-out'' mice to demonstrate p53-independent induction of WAF1/CIP1. We show that serum or individual growth factor s such as platelet-derived growth factor, fibroblast growth factor, an d epidermal growth factor but not insulin are able to induce WAF1/ CIP 1 in quiescent p53-deficient cells as well as in normal cells. The kin etics of this transient induction, which is enhanced by cycloheximide, demonstrates that WAF1/CIP1 is an immediate-early gene the transcript of which reaches a peak at approximately 2 h following serum or growt h factor stimulation. On the other hand, DNA damage elicited by gamma- irradiation induces WAF1/CIP1 in normal human and mouse fibroblasts bu t does not affect WAF1/CIP1 expression in p53-deficient cells. These r esults suggest the existence of two separate pathways for the inductio n of WAF1/ CIP1, a p53-dependent one activated by DNA damage and a p53 -independent one activated by mitogens at the entry into the cell cycl e. The possible function of p21 at this early stage is discussed.