Irradiation induces WAF1 expression through a p53-independent pathway in KG-1 cells.

WAF1 binds to cyclin-Cdk complexes and inhibits their activity, causing cell cycle arrest. Previous studies have shown that expression of WAF1 is induced through the p53-dependent pathway; WAF1 is induced in cells with functional p53 but not in cells with either mutant p53 or no 53. Human myeloblastic leukemia cells KG-1 had no constitutive expression of p53, and irradiation did not induce p53. However, irradiation increased WAF1 expression in KG-1 cells and other cell lines containing mutant p53. The KG-1 cells constitutively produced low levels of tumor necrosis factor (TNF); irradiation markedly increased the production of TNF. Notably, induction of WAF1 mRNA by irradiation was blocked by anti-TNF antibody. Furthermore, exogenously added TNF increased levels of WAF1 mRNA in these cells. Irradiation increased the rate of WAF1 transcription 3-fold, and the half-life (t1/2) of WAF1 mRNA in these cells increased from < 1 h in unirradiated cells to > 4 h in irradiated cells. These findings indicate that increased levels of WAF1 transcripts occur, at least in part, through a pathway of TNF production and that the increase in WAF1 mRNA observed after irradiation is regulated by both transcriptional and posttranscriptional mechanisms. Our present study strongly suggests that an alternative pathway of induction of WAF1 occurs independent of activation by p53.