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

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Abstract
P16ink4a 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 pRbΔcdk mutant both blocked G1 in short-term experiments, but only p16 imposed a sustained G1 arrest. Unexpectedly, cells conditionally exposed to pRbΔcdk entered S phase after 2 days, followed by endoreduplication between days 4 – 6. The distinct phenotypes evoked by p16 vs pRbΔcdk appear mediated by cyclin E/CDK2 which, while active in the pRbΔcdk-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.