A UV-RESPONSIVE G(2) CHECKPOINT IN RODENT CELLS

We have studied the effect of UV irradiation on the cell cycle progres sion of synchronized Chinese hamster ovary cells. Synchronization of c ells in S or G(2) phase was accomplished by the development of a novel protocol using mimosine, which blocks: cell cycle progression at the G(1)/S boundary. After removal of mimosine, cells proceed synchronousl y through the S and G(2) phases, allowing manipulation of cells at spe cific points in either phase. Synchronization of cells in G(1) was ach ieved by release of cells after a period of serum starvation. Cells sy nchronized by these methods were UV irradiated at defined points in G( 1), S, and G(2), and their subsequent progression through the cell cyc le was monitored. UV irradiation of G(1)-synchronized cells caused a d ose-dependent delay in entry into S phase. Irradiation of S-phase-sync hronized cells inhibited progression through S phase and then resulted in accumulation of cells for a prolonged interval in G(2). Apoptosis of a subpopulation of cells during this extended period was noted, UV irradiation of G(2)-synchronized cells caused a shorter G(2) arrest. T he arrest itself and its duration were dependent upon the timing (with in G(2) phase) of the irradiation and the UV dose, respectively We hav e thus defined a previously undescribed (in mammalian cells) UV-respon sive checkpoint in G(2) phase. The implications of these findings with respect to DNA metabolism are discussed.