Requirements for cell cycle arrest by p16(INK4a)

Analysis of tumor-derived mutations has led to the suggestion that p16(INK4 a), cyclin D1, cdk4, and the retinoblastoma protein (pRB) are components of a regulatory pathway that is inactivated in most tumor cells. Cell cycle a rrest induced by p16(INK4a), inhibitor of cyclin D-dependent kinases, requi res pRB, and it has been proposed that this G1 arrest is mediated by pRB-E2 F repressor complexes. By comparing the properties of primary mouse embryon ic fibroblasts specifically lacking pRB-family members, we find that PRE is insufficient for a p16(INK4a)-induced arrest. In addition to pRB, a second function provided by either p107 or p130, two pRB-related proteins, is req uired for p16(INK4a) to block DNA synthesis. We infer that p16(INK4a)-induc ed arrest is not mediated exclusively by pRB, but depends on the nonredunda nt functions of at least two pRB-family members.