Role for BRG1 in Cell Cycle Control and Tumor Suppression

Abstract

Human BRG1, a subunit of the Swi/Snf chromatin remodeling apparatus, has been implicated in regulation of cellular proliferation and is a candidate tumor suppressor. Reintroduction of BRG1 into a breast tumor cell line, ALAB, carrying a defined mutation in the BRG1 gene, induced growth arrest. Gene expression data revealed that the arrest may in part be accounted for by down-regulation of select E2F target genes such as cyclin E, but more dramatically, by up-regulation of mRNAs for the cyclin-dependent kinase inhibitors p21 and p15. Protein levels of both p15 and p21 were induced, and p21 protein was recruited to a complex with cyclin-dependent kinase, CDK2, to inhibit its activity. BRG1 can associate with the p21 promoter in a p53-independent manner, suggesting that the induction of p21 by BRG1 may be direct. Further, using microarray and real-time PCR analysis we identified several novel BRG1-regulated genes. Our work provides further evidence for a role for BRG1 in the regulation of several genes involved in key steps in tumorigenesis and has revealed a potential mechanism for BRG1-induced growth arrest.The evolutionarily conserved Swi/Snf chromatin remodeling apparatus has functions in both transcriptional activation and repression (29,36,55). The Swi/Snf complex has been shown to alter the arrangement of nucleosomes along DNA in an ATPdependent manner in vitro, thus allowing access for transcription factors to bind their target DNA sequences (29). The mammalian Swi/Snf complex is a 1.5-to 2.0-MDa multisubunit complex, the ATPase activity being generated by BRG1, a 190-kDa component, or a related protein, hBRM (58).In mammalian cells BRG1 is known to play a role in the regulation of cellular proliferation (8,49). This activity of BRG1 has been shown to be dependent on its association with the retinoblastoma tumor suppressor gene product (pRB) and, consequently, via the repression of E2F target gene expression (29). Specifically, it was demonstrated that BRG1/pRB complexes repress certain E2F target genes such as cyclin E, cyclin A, and CDC2 (48, 61). The temporally ordered association of BRG1 with histone deacetylase and pRB complexes ensures sequential activation of cyclin E and then cyclin A and maintains the order of G 1 and S phases of the cell cycle (61).Studies have now revealed that BRG1 is frequently deleted or mutated in a variety of tumor cell lines (59), implicating BRG1 as a potential tumor suppressor gene in its own right. The tumor suppressor function of BRG1 is supported by the generation of mice harboring a BRG1 null mutation. Heterozygotes for BRG1 are susceptible to neoplasia and display large subcutaneous tumors, suggesting that a partial reduction in cellular BRG1 level leads to critical loss of proliferation control (5). More recent work has shown loss of expression of BRG1/BRM expression in a subset of primary lung tumors associated with poor prognosis (39). Several studies have also revealed that the INI1/SNF5 conserved component of the SWI/ SNF complex is also mutated in human tumors,...

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).…”

“…Complexes containing BRG1 have been shown to be required for cell cycle control, apoptosis and differentiation in several biological systems (Dunaief et al, 1994;Murphy et al, 1999;Bultman et al, 2000;Reisman et al, 2002;Martens and Winston, 2003;Hendricks et al, 2004;de la Serna et al, 2006). Homozygous knockout mice for the Brg1 gene die during the embryonic stage, whereas heterozygotic survivors are prone to tumors (Bultman et al, 2000;Saha et al, 2006;de la Serna et al, 2006).…”

“…Homozygous knockout mice for the Brg1 gene die during the embryonic stage, whereas heterozygotic survivors are prone to tumors (Bultman et al, 2000;Saha et al, 2006;de la Serna et al, 2006). BRG1 protein can interact with different proteins involved in regulation of transcription, such as factors involved in myeloid, erythrocyte, lymphocyte, muscle, neural and adipocyte commitment and/or differentiation (Dunaief et al, 1994;Murphy et al, 1999;Bultman et al, 2000;Reisman et al, 2002;Martens and Winston, 2003;Hendricks et al, 2004;Saha et al, 2006;de la Serna et al, 2006). The BRG1 chromatin remodeling protein can associate with numerous chromatin-modifying complexes, including transcription co-activators and corepressors.…”

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.…”