The differential staurosporine-mediated G1 arrest in normal versus tumor cells is dependent on the retinoblastoma protein

Previously, we reported that breast cancer cells with retinoblastoma (pRb) pathway–defective checkpoints can be specifically targeted with chemotherapeutic agents, following staurosporine-mediated reversible growth inhibition in normal cells. Here we set out to determine if the kinetics of staurosporine-mediated growth inhibition is specifically targeted to the G 1 phase of cells, and if such G 1 arrest requires the activity of wild-type pRb. Normal human mammary epithelial and immortalized cells with intact pRb treated with low concentrations of staurosporine arrested in the G 1 phase of the cell cycle, whereas pRb-defective cells showed no response. The duration of G 1 and transition from G 1 to S phase entry were modulated by staurosporine in Rb-intact cells. In pRb + cells, but not in Rb − cells, low concentrations of staurosporine also resulted in a significant decrease in cyclin-dependent kinase 4 (CDK4) expression and activity. To directly assess the role of pRb in staurosporine-mediated G 1 arrest, we subjected wild-type (Rb +/+ ) and pRb −/− mouse embryo fibroblasts (MEFs) to staurosporine treatments. Our results show that whereas Rb +/+ MEFs were particularly sensitive to G 1 arrest mediated by staurosporine, pRb −/− cells were refractory to such treatment. Additionally, CDK4 expression was also inhibited in response to staurosporine only in Rb +/+ MEFs. These results were recapitulated in breast cancer cells treated with siRNA to pRb to down-regulate the pRb expression. Collectively, our data suggest that treatment of cells with nanomolar concentrations of staurosporine resulted in down-regulation of CDK4, which ultimately leads to G 1 arrest in normal human mammary epithelial and immortalized cells with an intact pRb pathway, but not in pRb-null/defective cells. (Cancer Res 2006; 66(19): 9744-53)