The nuclear death domain protein p84N5 activates a G(2)/M cell cycle checkpoint prior to the onset of apoptosis

In contrast to extracellular signals, the mechanisms utilized to transduce nuclear apoptotic signals are not well understood. Characterizing these mec hanisms is important for predicting how tumors will respond to genotoxic ra diation or chemotherapy. The retinoblastoma (Rb) tumor suppressor protein c an regulate apoptosis triggered by DNA damage through an unknown mechanism. The nuclear death domain-containing protein p84N5 can induce apoptosis tha t is inhibited by association with Rb, The pattern of caspase and NF-kappaB activation during p84N5-induced apoptosis is similar to p53-independent ce llular responses to DNA damage. One hallmark of this response is the activa tion of a G(2)/M cell cycle checkpoint. In this report, we characterize the effects of p84N5 on the cell cycle. Expression of p84N5 induces changes in cell cycle distribution and kinetics that are consistent with the activati on of a G(2)/M cell cycle checkpoint. Like the radiation-induced checkpoint , caffeine blocks p84N5-induced G(2)/M arrest but not subsequent apoptotic cell death, The p84N5-induced checkpoint is functional in ataxia telangiect asia-mutated kinase-deficient cells. We conclude that p84N5 induces an atax ia telangiectasia-mutated kinase (ATM)independent, caffeine-sensitive G(2)/ dM cell cycle arrest prior to the onset of apoptosis. This conclusion is co nsistent with the hypotheses that p84N5 functions in an Rb-regulated cellul ar response that is similar to that triggered by DNA damage.