The relationship between G(1) checkpoint function and rapamycininduced apoptosis was examined using two human rhabdomyosarcoma cell lines, Rh1 and Rh30, that express mutated p53 alleles. Serum-starved tumor cells became apoptotic when exposed to rapamycin, but were completely protected by expression of a rapamycin-resistant mutant mTOR. Exposure to rapamycin (100 ng/ml) for 24 h significantly increased the proportion of Rh1 and Rh30 cells in G(1) phase, although there were no significant changes in expression of cyclins D1, E, or A in drug-treated cells. To determine whether apoptosis was associated with continued slow progression through G(1) to S phase, cells were exposed to rapamycin for 24 h, then labeled with bromodeoxyuridine (BrdUrd). Histochemical analysis showed that >90% of cells with morphological signs of apoptosis had incorporated BRDURD: To determine whether restoration of G(1) arrest could protect cells from rapamycin-induced apoptosis, cells were infected with replication-defective adenovirus expressing either p53 or p21(CIP1). Infection of Rh30 cells with either Ad-p53 or Ad-p21, but not control virus (Ad-beta-gal), induced G(1) accumulation, up-regulation of p21(CIP1), and complete protection of cells from rapamycin-induced apoptosis. Within 24 h of infection of Rh1 cells with Ad-p21, expression of cyclin A was reduced by >90%. Similar results were obtained after Ad-p53 infection of Rh30 cells. Consistent with these data, incorporation of [(3)H]thymidine or BrdUrd into DNA was significantly inhibited, as was cyclin-dependent kinase 2 activity. These data indicate that rapamycin-induced apoptosis in tumor cells is a consequence of continued G(1) progression during mTOR inhibition and that arresting cells in G(1) phase, by overexpression of p53 or p21(CIP1), protects against apoptosis. The response to rapamycin was next examined in wild-type or murine embryo fibroblasts nullizygous for p53or p21(CIP1). Under serum-free conditions, rapamycin-treated wild-type MEFs showed no increase in apoptosis compared to controls. In contrast, rapamycin significantly induced apoptosis in cells deficient in p53 ( approximately 2.4-fold) or p21(CIP1) ( approximately 5.5-fold). Infection of p53(-/-) MEFs with Ad-p53 or Ad-p21 completely protected against rapamycin-induced apoptosis. Under serum-containing conditions, rapamycin inhibited incorporation of BrdUrd significantly more in wild-type murine embryo fibroblasts (MEFs) than in those lacking p53 or p21(CIP1). When BrdUrd was added 24 h after rapamycin, almost 90% and 70% of cells lacking p53 or p21(CIP1), respectively, incorporated nucleoside. In contrast, only 19% of wild-type cells incorporated BrdUrd in the presence of rapamycin. Western blot analysis of cyclin levels showed that rapamycin had little effect on levels of cyclins D1 or E in any MEF strain. However, cyclin A was reduced to very low levels by rapamycin in wild-type cells, but remained high in cells lacking p53 or p21(CIP1). Taken together, the data suggest that p53 cooperates in enforcing G(1) cell cycle arrest, leading to a cytostatic response to rapamycin. In contrast, in tumor cells, or MEFs, having deficient p53 function the response to this agent may be cell cycle progression and apoptosis.