S-phase arrest by nucleoside analogues and abrogation of survival without cell cycle progression by 7-hydroxystaurosporine

The mechanisms of resistance to nucleoside analogues established in preclin ical models are rarely found in primary tumors resistant to therapy with th ese agents. We tested the hypothesis that cells sense sublethal incorporati on of analogues into DNA during replication and react by arresting further DNA synthesis and cell cycle progression, After removal of drug, cells may be able to repair damaged DNA and continue proliferation, thus escaping nuc leoside analogue toxicity. As a corollary, we evaluated whether dysregulati on of this mechanism causes cell death. Using gemcitabine as a model of S-p hase-specific nucleoside analogues in human acute myelogenous leukemia ML-1 cells, we found that DNA synthesis decreased, cells arrested in S-phase tr ansit, and 60-70% of the population accumulated in S-phase in response to c ytostatic conditions. proliferation continued after washing the cells into drug-free medium. S-phase-arrested cells were then treated with otherwise n ontoxic concentrations of UCN-01, which caused rapid onset of apoptosis wit hout cell cycle progression specifically in cells with an S-phase DNA conte nt. Thus, S-phase arrest by nucleoside analogues sensitizes cells to UCN-01 , which appears to activate signaling for death mechanisms and/or inhibit s urvival pathways, These results differ from those in cells arrested at the G(2) checkpoint, in which UCN-01 abrogates cell cycle arrest, permitting ce lls to progress in the fell cycle before apoptosis.