Staurosporine- and radiation-induced G2-phase cell cycle blocks are equally released by caffeine

We show here that the arrests of cells in G2 phase of the cell cycle induced by either staurosporine or ionizing radiation are closely related phenomena governed by a common kinase signaling pathway. The protein kinase inhibitor staurosporine induces a complete G2-phase arrest in exponentially growing TK6 human lymphoblastoid and V79 Chinese hamster fibroblast cells. Both cell types are equally sensitive to the kinase inhibitor and the arrest is dependent on its continued presence. Caffeine completely abrogates this arrest at concentrations comparable to those which abrogate radiation-induced G2-phase arrest. The kinetics of caffeine-induced release of both kinds of arrest are essentially identical. The activity of p34cdc2 kinase was also found to increase in a parallel fashion after caffeine-induced release of both kinds of arrest. As opposed to those transformed cell types which arrest only in G2 phase in response to staurosporine, immortalized C3H 10T1/2 fibroblasts and Muntjak skin fibroblasts display both G1- and G2-phase arrests. The results suggest that staurosporine and radiation interact with regulatory pathways in the cell cycle, and specifically with a caffeine-sensitive signal transduction pathway which recognizes DNA damage, regulates the G2/M-phase transition, and attenuates the biological consequences of radiation exposure.