CAFFEINE OVERRIDES THE S-PHASE CELL-CYCLE BLOCK IN SEA-URCHIN EMBRYOS

During the early mitotic cell cycles of the sea urchin embryo, the cel l oscillates between S-phase and M-phase. In the presence of aphidicol in, a DNA synthesis inhibitor, a checkpoint control blocks the activat ion of the p34(cdc2) protein kinase, by keeping it in the inactive, ty rosine phosphorylated form, and the embryos do not enter mitosis. Caff eine has been shown to bypass the G(2)/M-phase checkpoint in mammalian cells and in cycling Xenopus extracts and to induce mitosis despite t he presence of damaged or unreplicated DNA. In this study we show that caffeine also induces mitosis and cell division in sea urchin embryos , in the presence of unreplicated DNA, by stimulating the tyrosine dep hosphorylation of p34(cdc2) and switching on its protein kinase activi ty. We also show that the caffeine-induced activation of the p34(cdc2) protein kinase is not mediated by either of the two second messengers , calcium and cAMP or by inhibition of the p34(cdc2) tyrosine kinase. Thus, none of the mechanisms proposed for caffeine's action can explai n how it overrides the S-phase checkpoint in the early cell cycles of the sea urchin embryo.