Inhibition of Cell Growth by Transforming Growth Factor β1 Is Associated with p53‐Independent Induction of p21 in Gastric Carcinoma Cells

Cell cycle regulators such as cyclins, cyclin‐dependent kinases (cdks) and their inhibitors control the growth of cells. SDI1/CIP1/WAF1/p21 is a potent inhibitor of G1 cdks, whose expression is induced by wild‐type p53. To elucidate the mechanism of growth inhibition by transforming growth factor β1 (TGFβ1), we examined the effect of TGPβ1 on the expression of p21, G1 cyclins and cdks by human gastric cancer cell lines. TGFβ1 induced p21 expression and subsequently suppressed cdk2 kinase activity, followed by a reduction in phosphorylation of the product of the retinoblastoma tumor suppressor gene in TMK‐1 cells, which are responsive to TGFβ1. Coimmunoprecipitation analysis demonstrated that TGFβ1 increased the level of p21 protein present in complexes with cdk2. In contrast, TGFβ1 did not induce p21 in TGFβ1‐resistant MKN‐28 cells. TGFβ1 did not affect the levels of p53 mRNA and protein in TMK‐1 and MKN‐28 cells, which contain mutated p53 genes. These mutated p53 complementary DNAs, when overexpressed, failed to activate transcription from the p21 promoter. Furthermore, TGFβ1 caused a reduction in the steady‐state level of cyclin A protein concomitantly with inhibition of cdk2 kinase activity in TMK‐1 cells. These results suggest that the growth inhibition of tumor cells by TGFβ1 is associated with p53‐independent induction of p21, subsequent suppression of cdk activity and a decrease in cyclin A protein in TMK‐1 cells.