Transforming growth factor β1 can induce CIP1/WAF1 expression independent of the p53 pathway in ovarian cancer cells

Transforming growth factor β (TGFβ) is an important regulator of cellular proliferation. In normal ovarian epithelial cells, TGFβ acts to inhibit growth. However, in ovarian cancer cell lines, this effect is usually lost. Although the regulatory pathway of TGFβ remains unclear, TGFβ- treated cells arrest late in G₁. This inhibition appears to involve blocking of the cyclin-dependent kinase phosphorylation of the retinoblastoma protein. Recently, a general inhibitor of cyclin-dependent kinases, CIP1/WAF1/p21, was identified.Expression of CIP1 is positively regulated by binding of wild- type p53 to a consensus response element upstream of the CIP1 gene. Overexpression of the CIP1 protein causes growth suppression, analogous to TGFβ and wild-type p53. We have examined the induction of CIP1 by TGFβ1 in ovarian cancer cell lines that have been previously characterized for their proliferative response to TGFβ1. OVCA420, a cell line that is dramatically growth inhibited by TGFβ1, significantly induced CIP1 expression in response to TGFβ1. CIP1 induction was accompanied by a decrease in cdk2 kinase activity and cdk2 protein levels. In three other cell lines that respond weakly to TGFβ1, CIP1 expression was not induced. To determine if TGFβ1 induction occurs via p53, regulation of p53 RNA and protein was examined. No differences in p53 transcription, steady-state protein level, de novo synthesis, phosphorylation, or subcellular accumulation were noted. Furthermore, TGFβ1 could not induce transcription from a consensus p53 DNA binding site in the TGFβ1-responsive cell line. Our results suggest that CIP1 expression is an important mediator of TGFβ growth suppression. However, it appears that TGFβ induces CIP1 independent of the p53 pathway.