G1 phase arrest by the phosphatidylinositol 3-kinase inhibitor LY 294002 is correlated to up-regulation of p27Kip1 and inhibition of G1 CDKs in choroidal melanoma cells

We have investigated the effect of the flavonoid derivative LY 294002, a potent and selective phosphatidylinositol 3-kinase inhibitor, on cell cycle progression in human choroidal melanoma cells. We demonstrate that LY 294002 induces a specific G1 block in asynchronously growing cells leading to an almost complete inhibition of cell proliferation after three days of treatment. When melanoma cells are released from a nocodazole-induced G2/M block, LY 294002 is shown to delay and greatly restrain the G1/S transition. The inhibitor is able to exert its action as long as it is added during the G1 progression and before the cells enter in S phase. We report that the LY 294002-induced G1 arrest is closely correlated to inhibition of CDK4 and CDK2 activities leading to the impairment of pRb phosphorylation which normally occurs during G1 progression. While the inhibition of CDK4 may be attributed at least in part to the decline in CDK4 protein level, CDK2 activity reduction is rather due to the up-regulation of the CDK inhibitor p27Kip1 and to its increased association to CDK2.