p21CIP1 is dispensable for the G2 arrest caused by genistein in human melanoma cells

We have investigated the effect of genistein on cell cycle distribution of the human choroidal melanoma cell line OCM-1. We report that this isoflavonoid arrested cells in G2. This effect was correlated with the induction of the CDK inhibitor p21(CIP1). However, while CDK1 activity was markedly reduced following genistein treatment, CDK2 activity was not affected. This was in agreement with the absence of G1 arrest that we observed but caused some doubt about the functionality of p21(CIP1). Attempts to demonstrate mutation or post-translational modification of p21(CIP1) from OCM-1 cells were unsuccessful. In fact, the level of p21(CIP1) induced by genistein was shown to be unsufficient to cause CDK2 inhibition. The role of p21(CIP1) in the inhibition of CDK1 was questionable, as we demonstrated that genistein impaired Tyr15 dephosphorylation of CDK1 and because CDK1-cyclin B1 complexes from treated cells could be reactivated upon exposure to CDC25 phosphatase, Finally, we report that p21(CIP1) was not absolutely required for the genistein-induced G2 arrest, as the isoflavone caused at least partial G2 arrest in pal-deficient Rat-1 fibroblasts as web as in p21-/- mouse embryo fibroblasts. (C) 2000 Academic Press.