P21-INDUCED CYCLE ARREST IN G(1) PROTECTS CELLS FROM APOPTOSIS INDUCED BY UV-IRRADIATION OR RNA-POLYMERASE-II BLOCKAGE

Cells expressing the R273H mutant of p53, which lacks sequence specifi c DNA binding capacity, do not undergo cell cycle arrest in G(1) follo wing exposure to ionizing or UV radiation because of their inability t o induce p21(Waf1/Cip1), a cyclin-dependent kinase inhibitor and downs tream mediator of p53-dependent DNA damage-induced growth arrest. Foll owing UV-irradiation or treatment with an inhibitor of RNA pol II, we observed a rapid induction of the apoptotic process, as evidenced by D NA fragmentation and the proteolytic cleavage of poly(ADP-ribose) poly merase. Using mimosine, a p21(Waf1/Cip1) inducer that bypasses the req uirement for transcriptional transactivation by p53, we demonstrated t hat a G(1) cell cycle arrest can prevent apoptosis following UV-irradi ation or treatment with an RNA polymerase II inhibitor, Serum starvati on, which also synchronized cells in G(1) but did not induce p21(Waf1/ Cip1), did not protect cells from apoptosis, These results demonstrate that restoring a late G(1) checkpoint by inducing p21(Waf1/Cip1) expr ession can protect cells from DNA damage induced apoptosis, Our result s suggest that p21(Waf2/Cip1) can interrupt the apoptotic process at a point downstream from p53 accumulation but upstream from caspase-3 ac tivation.