INHIBITION OF CELL-GROWTH BY TRANSFORMING GROWTH-FACTOR-BETA-1 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 transform ing growth factor beta 1 (TGF beta 1), we examined the effect of TGF b eta 1 on the expression of p21, G1 cyclins and cdks by human gastric c ancer cell lines. TGF beta 1 induced p21 expression and subsequently s uppressed cdk2 kinase activity, followed by a reduction in phosphoryla tion of the product of the retinoblastoma tumor suppressor gene in TMK -1 cells, which are responsive to TGF beta 1. Coimmunoprecipitation an alysis demonstrated that TGF beta 1 increased the level of p21 protein present in complexes with cdk2. In contrast, TGF beta 1 did not induc e p21 in TGF beta 1-resistant MKN-28 cells. TGF beta 1 did not affect the levels of p53 mRNA and protein in TMK-1 and MKN-28 cells, which co ntain mutated p53 genes. These mutated p53 complementary DNAs, when ov erexpressed, failed to activate transcription from the p21 promoter. F urthermore, TGF beta 1 caused a reduction in the steady-state level of cyclin A protein concomitantly with inhibition of cdk2 kinase activit y in TMK-1 cells. These results suggest that the growth inhibition of tumor cells by TGF beta 1 is associated with p53-independent induction of p21, subsequent suppression of cdk activity and a decrease in cycl in A protein in TMK-1 cells.