G1 S CELL-CYCLE BLOCKERS AND INHIBITORS OF CYCLIN-DEPENDENT KINASES SUPPRESS CAMPTOTHECIN-INDUCED NEURONAL APOPTOSIS/

Previous studies have demonstrated that G1/S cell cycle blockers and i nhibitors of cyclin-dependent kinases (CDKs) prevent the death of nerv e growth factor (NGF)-deprived PC12 cells and sympathetic neurons, sug gesting that proteins normally involved in the cell cycle may also ser ve to regulate neuronal apoptosis. Past findings additionally demonstr ate that DNA-damaging agents, such as the DNA topoisomerase (topo-1) i nhibitor camptothecin, also induce neuronal apoptosis. In the present study, we show that camptothecin-induced apoptosis of PC12 cells, symp athetic neurons, and cerebral cortical neurons is suppressed by the G1 /S blockers deferoxamine and mimosine, as well as by the CDK-inhibitor s flavopiridol and olomoucine. In each case, the IC50 values were simi lar to those reported for inhibition of death induced by NGF-deprivati on. In contrast, other agents that arrest DNA synthesis, such as aphid icolin and N-acetylcysteine, failed to block death. This suggests that the inhibition of DNA synthesis per se is insufficient to provide pro tection from camptothecin. We find additionally that the cysteine aspa rtase family protease inhibitor zVAD-fmk inhibits apoptosis evoked by NGF-deprivation but not camptothecin treatment. Thus, despite their sh ared sensitivity to G1/S blockers and CDK inhibitors, the apoptotic pa thways triggered by these two causes of death diverge at the level of the cysteine aspartase. In summary, neuronal apoptosis induced by the DNA-damaging agent camptothecin appears to involve signaling pathways that normally control the cell cycle. The consequent death signals of such deregulation, however, are different from those that result from trophic factor deprivation.