Mitogen requirement for cell cycle progression in the absence of pocket protein activity

Primary mouse embryonic fibroblasts lacking expression of all three retinoblastoma protein family members (TKO MEFs) have lost the G(1) restriction point. However, in the absence of mitogens these cells become highly sensitive to apoptosis. Here, we show that TKO MEFs that survive serum depletion pass G, but completely arrest in G(2). p21(CIP1) and p27(KIP1) inhibit Cyclin A-Cdk2 activity and sequester Cyclin B1-Cdk1 in inactive complexes in the nucleus. This response is alleviated by mitogen restimulation or inactivation of p53. Thus, our results disclose a cell cycle arrest mechanism in G2 that restricts the proliferative capacity of mitogen-deprived cells that have lost the G, restriction point. The involvement of p53 provides a rationale for the synergism between loss of Rb and p53 in tumorigenesis.