Cdk2 is dispensable for cell cycle inhibition and tumor suppression mediated by p27(Kip1) and p21(Cip1)

p27(Kip1) and p21(Cip1) are thought to suppress tumor growth and prevent cell cycle progression by inhibiting Cdk2-cyclin E/A kinases. Since Cdk2 is dispensable for mitotic cell division, we analyzed the activity of these inhibitors in Cdk2-deficient cells. Ectopic expression of p27(Kip1) or p21(Cip1) efficiently inhibits cell cycle progression of Cdk2(-/-) fibroblasts. Loss of p27(Kip1) or p21(Cip1) confers similar proliferative advantages to Cdk2(+/+) and Cdk2(-/-) cells. Moreover, Cdk2 is dispensable for p21(Cip1)-induced cell cycle arrest after DNA damage. Finally, ablation of Cdk2 in p27(Kip1) null mice does not suppress their phenotypic defects, including development of pituitary tumors. These results indicate that Cdk2 is not an essential target for p27(Kip1) and p21(Cip1) in cell cycle inhibition and tumor suppression.