Heterozygosity of p21 Enhances Tumor Cell Proliferation and Cyclin D1-associated Kinase Activity in a Murine Mammary Cancer Model

The p21 cyclin-dependent kinase (cdk) inhibitor is a regulator of the G1-S cell cycle checkpoint. Despite the importance of p21 in cell cycle inhibition, its role as a tumor suppressor is uncertain. p21 mutations are infrequent in human tumors, and p21 null mice exhibit no increased tumor incidence. To ascertain whether p21 could influence tumor formation or progression in the context of other oncogenic stimuli, we crossed p21-deficient mice with mammary tumor susceptible Wnt-1 transgenic mice. The p211/1, p211/2, and p212/2 Wnt-1 transgenic female offspring were monitored for mammary tumor incidence and growth rates. p21 status had no effect on the age at which mammary tumors formed. However, p211/2 mammary tumors grew significantly faster than p211/1 and p212/2 mammary tumors. The increased growth rates were confirmed by mitotic index counts and by BrdUrd labelling assays, indicating that a significantly higher percentage of p211/2 tumor cells were in S phase and mitosis than their p211/1 and p212/2 counterparts. Moreover, cyclin D1associated phosphorylation of retinoblastoma protein was significantly increased in p211/2 tumor lysates compared with p211/1 and p212/2 lysates. These results are consistent with data indicating that reduced levels of p21 can facilitate cyclin/cdk complex formation while enhancing cdk activity. Thus, a reduction of p21 dosage may promote tumor progression in the presence of other oncogenic initiators. The dependence of p21 on prior oncogenic stimuli for its tumor-promoting activities suggests that it may behave as a tumor modifier gene rather than as a tumor suppressor gene.