A cyclin D-Cdk4 activity required for G2 phase cell cycle progression is inhibited in ultraviolet radiation-induced G2 phase delay

Cyclin D-Cdk4 complexes have a demonstrated role in G(1) phase, regulating the function of the retinoblastoma susceptibility gene product (Rb). Previously, we have shown that following treatment with low doses of UV radiation, cell lines that express wild-type pie and Cdk4 responded with a Gz phase cell. cycle delay. The UV-responsive lines contained elevated levels of p16 posttreatment, and the accumulation of pie correlated with the G(2) delay, Here we report that in UV-irradiated HeLa and A2058 cells, pie bound Cdk4 and Cdk6 complexes with increased avidity and inhibited a cyclin D3-Cdk4 compiler normally activated in late S/early G(2) phase. Activation of this complex was correlated with the caffeine-induced release from the UV-induced G(2) delay and a decrease in the level of pie bound to Cdk4, Finally, overexpression of a dominant-negative mutant of Cdk4 blocked cells in G(2) phase. These data indicate that the cyclin D3-Cdk4 activity is necessary for cell cycle progression through G(2) phase into mitosis and that the increased binding of p16 blocks this activity and G(2), phase progression after UV exposure.