p16INK4a, but not constitutively active pRb, can impose a sustained G1 arrest:: molecular mechanisms and implications for oncogenesis

p16(ink4a) and pRb, two components of a key G1/S regulatory pathway, and tumor suppressors commonly targeted in oncogenesis, are among the candidates for gene therapy of cancer. Wild-type p16 and a constitutively active pRb(Delta cdk) mutant both blocked G1 in shortterm experiments, but only p16 imposed a sustained G1 arrest. Unexpectedly, cells conditionally exposed to pRb(Delta cdk) entered S phase after 2 days, followed by endoreduplication between days 4-6. The distinct phenotypes evoked by p16 vs pRb(Delta cdk) appear mediated by cyclin E/CDK2 which, while active in the pRb(Delta cdk)-expressing cells, became rapidly inhibited through restructuring diverse cyclin/CDK/p21 complexes by p16, These results provide novel insights into the roles of p16, pRb and cyclin E in G1/S control and multistep oncogenesis, with implications for gene therapy strategies.