Dose-dependent effects of DNA-damaging agents on p53-mediated cell cycle arrest

We examined the dose-dependent effects of DNA-damaging agents on G1 arrest in isogenic human cell lines differing in their p53 status. As expected, 5 or 20 Gy of ionizing radiation induced a p53-dependent G1 arrest. In contrast, UV light or actinomycin D induced a modest G1 arrest that was p53-dependent only at lower doses. At higher doses, cells were arrested in G1 in a p53-independent manner coinciding with inhibition of RNA synthesis and abolished cyclin E expression. Interestingly, expression of cyclin E was enhanced after exposure to moderate doses of UV light and actinomycin D, and this enhancement was suppressed by wild-type p53. We propose that agents inducing transcription-blocking DNA lesions will at higher doses inhibit the progression of cells into S phase by a p53-independent mechanism involving the attenuation of E2F-mediated transcription of genes, such as cyclin E.