p21-induced cycle arrest in G₁ protects cells from apoptosis induced by UV-irradiation or RNA polymerase II blockage

Abstract

Cells expressing the R273H mutant of p53, which lacks sequence specific DNA binding capacity, do not undergo cell cycle arrest in G₁ following exposure to ionizing or UV radiation because of their inability to induce p21^Waf1/Cip1, a cyclin-dependent kinase inhibitor and downstream mediator of p53-dependent DNA damage-induced growth arrest. Following UV-irradiation or treatment with an inhibitor of RNA pol II, we observed a rapid induction of the apoptotic process, as evidenced by DNA fragmentation and the proteolytic cleavage of poly(ADP-ribose) polymerase. Using mimosine, a p21^Waf1/Cip1 inducer that bypasses the requirement for transcriptional transactivation by p53, we demonstrated that a G₁ cell cycle arrest can prevent apoptosis following UV-irradiation or treatment with an RNA polymerase II inhibitor. Serum starvation, which also synchronized cells in G₁ but did not induce p21^Waf1/Cip1, did not protect cells from apoptosis. These results demonstrate that restoring a late G₁ checkpoint by inducing p21^Waf1/Cip1 expression can protect cells from DNA damage induced apoptosis. Our results suggest that p21^Waf1/Cip1 can interrupt the apoptotic process at a point downstream from p53 accumulation but upstream from caspase-3 activation.