E2FBP1 antagonizes the ap16 ^INK4A Rb tumor suppressor machinery for growth suppression and cellular senescence by regulating promyelocytic leukemia protein stability

Cellular senescence is an irreversible cell cycle arrest triggered by the activation of oncogenes or mitogenic signaling as well as the enforced expression of tumor suppressors such as p53, ap16 ^INK4A and promyelocytic leukemia protein (PML) in normal cells. E2Fbinding protein 1 (E2FBP1), a transcription regulator for E2F, induces PML reduction and suppresses the formation of PMLnuclear bodies, whereas the downregulation of E2FBP1 provokes the PMLdependent premature senescence in human normal fibroblasts. Here we report that the depletion of E2FBP1 induces the accumulation of PML through the Rasdependent activation of MAP kinase signaling. The cellular levels of ap16 ^INK4A and p53 are elevated during premature senescence induced by depletion of E2FBP1, and the depletion of ap16 ^INK4A, but not p53 rescued senescent cells from growth arrest. Therefore, the premature senescence induced by E2FBP1 depletion is achieved through the ap16 ^INK4A Rb pathway. Similar to human normal fibroblasts, the growth inhibition induced by E2FBP1 depletion is also observed in human tumor cells with intact ap16 ^INK4A and Rb. These results suggest that E2FBP1 functions as a critical antagonist to the ap16 ^INK4A Rb tumor suppressor machinery by regulating PML stability.