BRG-1 is required for RB-mediated cell cycle arrest

Abstract

The antiproliferative action of the retinoblastoma tumor suppressor protein, RB, is disrupted in the majority of human cancers. Disruption of RB activity occurs through several disparate mechanisms, including viral oncoprotein binding, deregulated RB phosphorylation, and mutation of the RB gene. Here we report disruption of RB-signaling in tumor cells through loss of a critical cooperating factor. We have previously reported that C33A cells fail to undergo cell cycle inhibition in the presence of constitutively active RB (PSM-RB). To determine how C33A cells evade RB-mediated arrest, cell fusion experiments were performed with RB-sensitive cells. The resulting fusions were arrested by PSM-RB, indicating that C33A cells lack a factor required for RB-mediated cell cycle inhibition. C33A cells are deficient in BRG-1, a SWI͞SNF family member known to stimulate RB activity. Consistent with BRG-1 deficiency underlying resistance to RB-mediated arrest, we identified two other BRG-1-deficient cell lines (SW13 and PANC-1) and demonstrate that these tumor lines are also resistant to cell cycle inhibition by PSM-RB and p16ink4a, which activates endogenous RB. In cell lines lacking BRG-1, we noted a profound defect in RB-mediated repression of the cyclin A promoter. This deficiency in RB-mediated transcriptional repression and cell cycle inhibition was rescued through ectopic coexpression of BRG-1. We also demonstrate that 3T3-derived cells, which inducibly express a dominant-negative BRG-1, arrest by PSM-RB and p16ink4a in the absence of dominant-negative BRG-1 expression; however, cell cycle arrest was abrogated on induction of dominant-negative BRG-1. These findings demonstrate that BRG-1 loss renders cells resistant to RB-mediated cell cycle progression, and that disruption of RB signaling through loss of cooperating factors occurs in cancer cells.cyclins ͉ Cdk ͉ SWI͞SNF T he retinoblastoma tumor suppressor protein (RB) is a critical regulator of cell cycle progression that is functionally inactivated in the majority of human tumors (1-8). RB functions as a protein-binding protein, binding to greater than 50 identified cellular proteins. However, the requirement of these proteins for RB-mediated cell cycle inhibition is largely unknown. Overall, RB-assembled protein complexes lead to the repression of transcription, and this function of RB is critical for cell cycle regulation. The principal target of RB is believed to be the E2F family of transcriptional activators (6, 9-11). E2F controls the expression of numerous genes directly involved in cell cycle progression or in metabolic processes coupled to DNA replication (6, 9-11). RB binding converts E2F from a transcriptional activator to a repressor through a mechanism that involves the recruitment of histone deacetylases (12, 13). RB also mediates the repression of other gene products, such as cyclin A, through complicated mechanisms that are not clearly understood (14).In response to mitogenic signaling, RB is phosphorylated in mid-G 1 by Cdk4͞cyclin D complexes (1-4)....

The BRG1 ATPase of chromatin remodeling complexes is involved in modulation of mesenchymal stem cell senescence through RB–P53 pathways

“…Several reports showed that BRG1 operates through regulation of these key cellular pathways (Strobeck et al, 2000;Lee et al, 2002;Hendricks et al, 2004;Kang et al, 2004). RB expression seemed to remain unaffected by BRG1 silencing.…”

“…Altogether, our data suggest that BRG1 silencing altered the homeostasis of DNA repair pathway. In fact, the H2AX activation and the upregulation in the expression BRG1, RB and P53 crosstalk Several reports suggested that BRG1 binds to and regulates the retinoblastoma protein and the tumor suppressor protein p53 (Strobeck et al, 2000;Lee et al, 2002;Hendricks et al, 2004;Kang et al, 2004). These are master genes that control cell cycle arrest, differentiation, apoptosis and/or senescence (Felsani et al, 2006;Galderisi et al, 2006;Campisi and d'Adda di Fagagna, 2007;Oberdoerffer and Sinclair, 2007).…”

Remodeling of chromatin structure in senescent cells and its potential impact on tumor suppression and aging

“…Other more distantly related enzymes include human hBrm, Brg1 and the CHD family (CHD1-7). Of note, human hBrm and Brg1 are two candidate tumor suppressor genes that are implicated in repression of E2F target genes and induction of cell differentiation and senescence, through physical and functional interactions with the pRB protein (33,38,56,76,85,104,105,115,124,134,142). Since E2F target genes are incorporated into SAHF and formation of SAHF requires an intact pRB pathway (88,140), hBrm and/or Brg1 might be involved in SAHF assembly, perhaps through deposition of macroH2A.…”

The SWI/SNF complex and cancer

“…The observation that E1A blocks BRG1-mediated growth inhibition strongly suggests that SWI/ SNF activity is required for normal growth regulation by RB family members (Dunaief et al, 1994;Strober et al, 1996), and BRG1 and BRM contain the RB-binding motif LxCxE and bind RB as well as RB family members p107 and p130 (Dunaief et al, 1994;Strober et al, 1996;Dahiya et al, 2000). Constitutively active RB does not induce G1 arrest in cells that lack BRG1 and BRM expression (Strobeck et al, 2000(Strobeck et al, , 2001Zhang et al, 2000;Reisman et al, 2002). However, restoration of BRG1 or BRM expression reconstitutes RB growth inhibition in such cells.…”