Mechanisms of regulation of WAF1/Cip1 gene expression in human breast carcinoma: role of p53-dependent and independent signal transduction pathways.

WAF1/Cip1 was recently identified as the wild-type p53 target that appears to mediate the tumor suppressing effects of p53. We investigated the mechanisms of regulation of WAF1/Cip1 gene expression in human breast carcinoma (HBC) cells. Our results demonstrate that the HBC cells harboring wild-type p53 express 26-33-fold higher WAF1/Cip1 mRNA levels than the cells harboring mutant p53. The DNA damaging agent etoposide induced p53 accumulation only in cells harboring wild-type p53 yet it induced WAF1/Cip1 gene expression in cells carrying wild-type or mutant p53, suggesting the involvement of p53-dependent and independent signaling pathways in the regulation of WAF1/Cip1 gene expression. Serum starvation-induced growth arrest although not altering the endogenous p53 levels or its ability to transactivate the reporter gene, induced WAF1/Cip1 gene expression in cells carrying wild-type as well as mutant p53. These results further implicated the involvement of p53-independent signal transduction pathways in WAF1/Cip1 gene regulation. Our data also suggest that WAF1/Cip1 gene expression is tightly associated with cell cycle progression in cells containing either wild-type or mutant p53. WAF1/Cip1 expression was transiently induced in response to serum treatment and declined as the cells passed through the S-phase of the cell cycle. We thus provide evidence that the mechanisms of WAF1/Cip1 gene regulation involve p53-dependent and independent signaling pathways in HBC.