Abstract 1094: Transcriptional activation by pRB/p107 and coordination with SWI/SNF

Abstract

The retinoblastoma susceptibility gene, Rb1, is closely linked with osteosarcoma as well as retinoblastoma. The basis of such ties between individual susceptibility genes and tumors of specific tissue origin is a central question in cancer biology. Functional analysis of pRB and the closely related family members, p107 and p130, has tended to focus on repression of proliferation, but genetic evidence from our lab and others indicates an active requirement for pRB in osteoblast differentiation that might relate more directly with osteosarcoma susceptibility. Our previous studies linked pRB with activation of the early osteoblast marker, alkaline phosphatase (the product of the Akp2 gene). Here we have investigated the functional relationships between pRB and the SWI/SNF complex in Akp2 activation. Chromatin immunoprecipitation (ChIP) assays reveal the Akp2 promoter is targeted directly by all three pRB family members. Only p130 occupies the promoter prior to induction, while pRB and p107 target Akp2 directly during activation, and concurrent with association of the global co-activator p300. Akp2 is also a direct target of SWI/SNF chromatin remodeling complexes. These ATPase-powered complexes help orchestrate the programmatic shifts in gene expression necessary for progression from a self-renewing precursor state to a state of terminal differentiation. In osteoblasts, SWI/SNF complexes that repress osteogenic gene expression in order to maintain the precursor state are characterized by inclusion of the BRM ATPase. The alternative ATPase, BRG1, characterizes complexes that activate osteogenic genes during terminal differentiation. Thus, the onset of osteoblast differentiation is marked by a switch from BRM to BRG1 complexes on the Akp2 promoter. Our analysis shows that the BRM to BRG1 switch is accomplished in a precise two-step mechanism, with dissociation of BRM-containing SWI/SNF dependent on p300, and association of BRG1-containing SWI/SNF dependent on pRB/p107. Binding of RNA polymerase-II is dependent in turn on BRG1. Thus, in contrast to their widely studied role in cell cycle repression, pRB/p107 play a directly activating role in osteogenic gene expression, specifically in recruitment of the activating SWI/SNF complex.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1094.