Regulation by transforming growth factor-beta 1 of G1 cyclin-dependent kinases in human retinal epithelial cells

Transforming growth factor beta (TGFbeta) is a potent inhibitor of epithelial cell proliferation, delaying or arresting cell cycle progression in mid-late G1. In long-term life span cells this growth inhibitory action has been attributed to regulatory events on both the levels and activities of G1 cyclin-dependent kinases (CDKs). CDK inhibitors have been shown to play important role in the TGFbeta-induced inhibition of G1 CDKs. In this work, we have investigated the effect of TGFbeta1 on both cell proliferation and G1 CDK activities in primary cultures of human retinal pigment epithelial (RPE) cells. We show that TGFbeta1 exerts a partial inhibitory effect on RPE cell proliferation by causing a significant increase of the RPE cell number in G1. TGFbeta1 induces an up-regulation of the CDK inhibitor p15(INK4B)with its subsequent association to CDK4, and a decline in CDK4 protein level. In parallel, we have observed a decline of p27(KIP1)associated to CDK4 and a significant increase of the inhibitor associated to CDK2. Finally, we show that TGFbeta1 reduces both CDK4 and CDK2 enzymatic activities. The fact that TGFbeta exerts only partial inhibitions on G1 CDKs and cell cycle progression in RPE cells suggests a propensity of these cells to escape from the anti-proliferative action of the cytokine, a phenomenon which could be reinforced during the development of proliferative vitreoretinopathy.