BRG1 Controls the Activity of the Retinoblastoma Protein via Regulation of p21^{CIP1/WAF1/SDI}

Abstract

The ubiquitous mammalian chromatin-remodeling SWI/SNF-like BAF complexes play critical roles in tumorigenesis. It was suggested that the direct interaction of BRG1 with the retinoblastoma protein pRB is required for regulation of cell cycle progression by pRB. We present evidence that the BRG1-containing complexes regulate the expression of the cdk inhibitor p21 CIP1/WAF1/SDI . Furthermore, we show that the physical interaction between BRG1 and pRB is not required for induction of cell growth arrest and transcriptional repression of E2F target genes by pRB. Instead, BRG1 activates pRB by inducing its hypophosphorylation through up-regulation of the cdk inhibitor p21. The hypophosphorylation of pRB is reinforced by down-regulation of critical components, including cdk2, cyclin E, and cyclin D, in the pRB regulatory network. We demonstrate that up-regulation of p21 by BRG1 is necessary to induce formation of flat cells, growth arrest, and finally, cell senescence. Our results suggest that the BRG1-containing complexes control cellular proliferation and senescence by modulating the pRB pathway via multiple mechanisms.Increasing genetic evidence indicates that the mammalian chromatin-remodeling SWI/SNF-like BAF or hSWI/SNF complexes (28,32,66) play an important role in controlling cell proliferation and differentiation and in inhibiting cancer formation (reviewed in reference 35). Various homozygous mutations in the INI1/hSNF5/BAF47 subunit are linked to malignant rhabdoid tumors (MRTs), which are aggressive pediatric tumors in children under 5 years of age (14,53,65). Mouse models with targeted disruptions of the BAF complex have also provided evidence supporting a role for the BAF complexes in inhibiting tumorigenesis. While homozygous deletions of the INI1/BAF47 gene in mouse are embryonic lethal, the heterozygous mice develop tumors at a high frequency (23,34,49). Similarly, homozygous deletions of BRG1, the essential ATPase subunit of the BAF complex, are lethal, while the heterozygous mice are predisposed to cancer formation (8). Interestingly, reintroduction of BRG1 into SW-13 cells that do not express detectable levels of BRG1 is sufficient to reverse their transformed phenotype by inducing growth arrest and a flattened shape, which requires the activity of the retinoblastoma protein (pRB) (17).pRB is a major tumor suppressor that is frequently disrupted in cancer cells (55). It is a nuclear phosphoprotein that arrests cells in G 0 /G 1 phase by repressing genes required for the G 1 /S phase transition (68). The transcriptional repression by pRB is mediated by interaction with the E2F family of transcription factors, whose binding sites are found in the promoters of many genes involved in cell cycle progression (reviewed in references 18 and 47). The interaction of pRB with E2F is controlled by the phosphorylation status of several serine and threonine residues. Inactivation of pRB by phosphorylation releases E2F and therefore the repression of its target genes, which allows the cell cycle to progress throug...

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).…”

The SWI/SNF complex and cancer

“…However, restoration of BRG1 or BRM expression reconstitutes RB growth inhibition in such cells. SWI/SNF is required for E2F-dependent transcription (Trouche et al, 1997;Kang et al, 2004;Liu et al, 2004), although all of the effects of SWI/SNF on RB checkpoint control may not require binding of BRM or BRG1 to RB. For example, re-expression of BRG1 induces p21, which inhibits RB phosphorylation by cyclin dependent kinases (CDKs) (Kang et al, 2004).…”

“…SWI/SNF is required for E2F-dependent transcription (Trouche et al, 1997;Kang et al, 2004;Liu et al, 2004), although all of the effects of SWI/SNF on RB checkpoint control may not require binding of BRM or BRG1 to RB. For example, re-expression of BRG1 induces p21, which inhibits RB phosphorylation by cyclin dependent kinases (CDKs) (Kang et al, 2004). Cyclin E has been shown to bind and, in conjunction with CDK2, phosphorylate BRG1 and BAF155 (Shanahan et al, 1999;Brumby et al, 2002).…”

MDM2 Controls the Timely Expression of Cyclin A to Regulate the Cell Cycle

“…5), homologous to a conserved domain of a chromosome-remodeling factor BRG1-associated protein, structurally capable of binding to p53 at the MDM2-binding site (36). BRG1 and its associated factors are known to form a repressor complex that restricts cyclin A expression until released by cyclin E in a cell cycle-dependent fashion (22,37). The tumor suppressor p53 binds with BRG1 and several components of this repressor complex to up-regulate p21 expression (38,39).…”