Cell cycle arrest and DNA endoreduplication following p21Waf1/Cip1 expression

p21(Waf1/Cip1) is a major transcriptional target of p53 and has been shown to be one of the principal mediators of the p53 induced G1 cell cycle arrest. We show that in addition to the G1 block, p21(Waf1/Cip1) can also contribute to a delay in G2 and expression of p21(Waf1/Cip1) gives rise to cell cycle profiles essentially indistinguishable from those obtained following p53 expression. Arrest of cells in G2 likely reflects an inability to induce cyclin B1/cdc2 kinase activity in the presence of p21(Waf1/Cip1), although the inefficient association of p21(Waf1/cip1) and cyclin B1 suggests that the mechanism of inhibition is indirect. Cells released from an S-phase block were not retarded in their ability to progress through S-phase by the presence of p21(Waf1/Cip1), despite efficient inhibition of cyclin E, A and B1 dependent kinase activity, suggesting that p2l(Waf1/Cip1) is inefficient at inhibiting replicative DNA synthesis in vivo. Interestingly, significant numbers of cells released from the p21(Waf1/Cip1) activated G2 block undergo endoreduplication, passing through another S-phase before undergoing mitosis, This supports a function of the mitotic kinases in both entry into mitosis, and also in preventing re-replication of DNA following S-phase and suggests a role for p21(Waf1/Cip1) in coupling DNA synthesis and mitosis, Unlike p53, which induces apoptosis in these cells, extended expression of p21(Waf1/Cip1) resulted in the expression of a senescent-like phenotype in these p53 null, pRB null tumor cells.