Differential regulation of the p21/WAF-1 and mdm2 genes after high-dose UV irradiation: p53-Dependent and p53-independent regulation of the mdm2 gene

Background: DNA damage in mammalian cells stabilizes the p53 protein which then functions as a cell cycle checkpoint by leading to growth arrest or apoptosis. p53 is a transcription factor and positively regulates the expression of the p21/WAF-1 gene and the mdm2 gene. After high-dose UV irradiation, p53 increases the expression of the p21/WAF-1 gene immediately (2 to 5 hours after irradiation) while the induction of the rr?dmd gene is delayed (8 to 12 hours after irradiation). Experiments presented here explore this differential expression of two different p53-regulated genes. Materials and Methods: IP-Western (protein) and Northern (mRNA) blot experiments are used to follow mdm2 and p21/WAF-1 expression in primary rat or mouse cells after a low-dose (4 J/m(2)) or a high-dose (20 J/M-2) of UV irradiation. Northern blot and nuclear run-on experiments are employed to study mRNA stability as well as transcription rates of selected genes. Results: After high-dose UV irradiation, p53 is rapidly stabilized and the expression of p21/WAF1 is immediately increased. By contrast, both protein and mRNA levels of mdm2 first decrease in a p53-independent manner,and later increase in a p53-dependent manner. The initial decline of mdm2 expression following high-dose UV irradiation is W-dosage dependent and regulated at the level of transcription. Conclusion: p53 regulates two genes, p21/WAF1 (blocks cell cycle progression) and mdm2 (reverses p53 activity), that mediate opposite actions. This process is regulated in a temporal fashion after high-dose UV irradiation, so that cell cycle progression can be halted while DNA repair continues prior to reversal of p53-mediated arrest by mdm2.