Requirements for Cell Cycle Arrest by p16 INK4a

Abstract

Analysis of tumor-derived mutations has led to the suggestion that p16 INK4a, cyclin D1, cdk4, and the retinoblastoma protein (pRB) are components of a regulatory pathway that is inactivated in most tumor cells. Cell cycle arrest induced by p16 INK4a, an inhibitor of cyclin D–dependent kinases, requires pRB, and it has been proposed that this G1 arrest is mediated by pRB–E2F repressor complexes. By comparing the properties of primary mouse embryonic fibroblasts specifically lacking pRB-family members, we find that pRB 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 required for p16 INK4a to block DNA synthesis. We infer that p16 INK4a-induced arrest is not mediated exclusively by pRB, but depends on the nonredundant functions of at least two pRB-family members.