BRG1 Is Required for Formation of Senescence-Associated Heterochromatin Foci Induced by Oncogenic RAS or BRCA1 Loss

Abstract

bCellular senescence is an important tumor suppression mechanism. We have previously reported that both oncogene-induced dissociation of BRCA1 from chromatin and BRCA1 knockdown itself drive senescence by promoting formation of senescenceassociated heterochromatin foci (SAHF). However, the molecular mechanism by which BRCA1 regulates SAHF formation and senescence is unclear. BRG1 is a chromatin-remodeling factor that interacts with BRCA1 and pRB. Here we show that BRG1 is required for SAHF formation and senescence induced by oncogenic RAS or BRCA1 loss. The interaction between BRG1 and BRCA1 is disrupted during senescence. This correlates with an increased level of chromatin-associated BRG1 in senescent cells. BRG1 knockdown suppresses the formation of SAHF and senescence, while it has no effect on BRCA1 chromatin dissociation induced by oncogenic RAS, indicating that BRG1 functions downstream of BRCA1 chromatin dissociation. Furthermore, BRG1 knockdown inhibits SAHF formation and senescence induced by BRCA1 knockdown. Conversely, BRG1 overexpression drives SAHF formation and senescence in a DNA damage-independent manner. This effect depends upon BRG1's chromatin-remodeling activity as well as the interaction between BRG1 and pRB. Indeed, the interaction between BRG1 and pRB is enhanced during senescence. Chromatin immunoprecipitation analysis revealed that BRG1's association with the human CDKN2A and CDKN1A gene promoters was enhanced during senescence induced by oncogenic RAS or BRCA1 knockdown. Consistently, knockdown of pRB, p21 CIP1 , and p16 INK4a , but not p53, suppressed SAHF formation induced by BRG1. Together, these studies reveal the molecular underpinning by which BRG1 acts downstream of BRCA1 to promote SAHF formation and senescence.

RC-6 ribonuclease induces caspase activation, cellular senescence and neuron-like morphology in NT2 embryonal carcinoma cells

“…Previous studies indicated that β-galactosidase activity is a major characteristic found in senescence cells and is generally applied to senescence detection (65)(66)(67). In addition, many studies have reported that p16 and p21 protein levels are increased in senescence cells (50)(51)(52)(53). At present, a small fraction of RC-6-treated NT2 cells can express β-galactosidase activity.…”

“…Thus, RC-6 induced some cells to give rise to cellular senescence. Previous studies indicated that p16, p21 and p27 are related to cellular senescence in senescence cells (49)(50)(51)(52)(53). Therefore, these proteins were determined in the present study.…”

p16 loss rescues functional decline of Brca1-deficient mammary stem cells

“…[14][15][16][17] The finding that loss of p16 rescues the functional decline of mammary stem cells and MEC senescence with Brca1 deficiency suggests that p16 blocks these cells from entering an active cell cycle. These results indicate that, in addition to the p53-p21 pathway that is activated by BRCA1 loss, [14][15][16] p16 is also a critical downstream target of BRCA1 in controlling mammary cell proliferation and senescence. In line with these data, it was recently reported in vitro that BRCA1 knockdown enhances the association of BRG1, a chromatinremodeling factor that interacts with BRCA1, with the p16 promoter, leading to activation of its transcription and senescence.…”

“…In line with these data, it was recently reported in vitro that BRCA1 knockdown enhances the association of BRG1, a chromatinremodeling factor that interacts with BRCA1, with the p16 promoter, leading to activation of its transcription and senescence. 16 However, whether BRCA1 directly interacts with p16 promoter remains to be determined. More recently, it was found that human mammary epithelial cells from BRCA1-mutation carriers exhibit senescence, which is triggered by pRb pathway activation.…”

“…Indeed, it was recently reported that p16Ink4a is induced by Brca1 deficiency in fibroblasts and mammary glands, and that haploid loss of BRCA1 in human mammary epithelial cells (MECs) activates the Rb, not the p53, pathway leading to senescence. [16][17][18] We have recently discovered that heterozygous germline deletion or specific depletion of Brca1 in mouse epithelium induce senescence in MECs. 19,20 Whether Brca1 loss affects mammary stem cell function is poorly understood.…”