Genetic cooperation between p21Cip1 and INK4 inhibitors in cellular senescence and tumor suppression

Cell-cycle inhibitors of the Cip/Kip and INK4 families are involved in cellular senescence and tumor suppression. Some of these proteins, p21^Cip1, p16^INK4a and p15^INK4b, are coexpressed in response to antiproliferative signals such as cellular senescence resulting in cell-cycle arrest. To understand the roles of these inhibitors and their synergistic effect, we have characterized the growth properties and senescent behavior of primary cells deficient in p21^Cip1 and expressing an endogenous Cdk4^R24C (cyclin-dependent kinase) mutant (Cdk4^R24C knock-in cells) insensitive to INK4 proteins. Inactivation of both p21^Cip1 and INK4 pathways strongly cooperate in suppressing cellular senescence in vitro. These double mutant cells behavior as immortal cultures and display high sensitivity to cellular transformation by oncogenes. Moreover, mice double mutant in the INK4 and p21^Cip1 pathways (Cdk4^R24C; p21^Cip1-null mice) display an increased incidence of specific sarcomas, suggesting a significant cooperation between these two families of cell-cycle inhibitors in senescence responses and tumor suppression in vivo.