Molecular and Cellular Pathobiology Cooperative Activation of Tissue-Specific Genes by pRB and E2F1

The retinoblastoma tumor suppressor protein pRB is conventionally regarded as an inhibitor of the E2F family of transcription factors. Conversely, pRB is also recognized as an activator of tissue-specific gene expression along various lineages including osteoblastogenesis. During osteoblast differentiation, pRB directly targets Alpl and Bglap, which encode the major markers of osteogenesis alkaline phosphatase and osteocalcin. Surprisingly, p130 and repressor E2Fs were recently found to cooccupy and repress Alpl and Bglap in proliferating osteoblast precursors before differentiation. This raises the further question ofwhether these genes convert to E2F activation targets when differentiation begins, which would constitute a remarkable situation wherein pRB and E2F would be cotargeting genes for activation. Chromatin immunoprecipitation analysis in an osteoblast differentiation model shows that Alpl and Bglap are indeed targeted by an activator E2F, i.e., is E2F1. Promoter occupation of Alpl and Bglap by E2F1 occurs specifically during activation, and depletion of E2F1 severely impairs their induction. Mechanistically, promoter occupation by E2F1 and pRB is mutually dependent, and without this cooperative effect, activation steps previously shown to be dependent on pRB, including recruitment of RNApolymerase II, are impaired.Myocyteand adipocyte-specific genes are also cotargeted byE2F1 andpRBduring differentiation along their respective lineages. The finding that pRB and E2F1 cooperate to activate expression of tissue-specific genes is a paradigmdistinct from the classical concept of pRB as an inhibitor of E2F1, but is consistentwith the observed roles of these proteins in physiological models. Cancer Res; 73(7); 2150–8. 2012 AACR.