The tumour suppressor and chromatin‐remodelling factor BRG1 antagonizes Myc activity and promotes cell differentiation in human cancer

Abstract

BRG1, a member of the SWI/SNF complex, is mutated in cancer, but it is unclear how it promotes tumourigenesis. We report that re-expression of BRG1 in lung cancer cells up-regulates lung-specific transcripts, restoring the gene expression signature of normal lung. Using cell lines from several cancer types we found that those lacking BRG1 do not respond to retinoic acid (RA) or glucocorticoids (GC), while restoration of BRG1 restores sensitivity. Conversely, in SH-SY5Y cells, a paradigm of RA-dependent differentiation, abrogation of BRG1 prevented the response to RA. Further, our data suggest an antagonistic functional connection between BRG1 and MYC, whereby, refractoriness to RA and GC by BRG1 inactivation involves deregulation of MYC activity. Mechanistically, some of these effects are mediated by BRG1 binding to MYC and MYC-target promoters. The BRG1-MYC antagonism was also evident in primary tumours. Finally, BRG1 restoration significantly dampened invasion and progression and decreased MYC in lung cancer cells orthotopically implanted in nude mice. Thus, BRG1 inactivation enables cancer cells to sustain undifferentiated gene expression programs and prevent its response to environmental stimuli.

MAX Inactivation in Small Cell Lung Cancer Disrupts MYC–SWI/SNF Programs and Is Synthetic Lethal with BRG1

“…For example, MYC physically interacts with the SWI/ SNF component, SMARCB1 ( 26 ), and BRG1 is required to regulate the expression of MYC and MYC target genes ( 16 , 27 ). In tumors carrying BRG1 mutations, this regulation is abolished, thereby preventing cell differentiation and promoting cell growth ( 16 ). Here, we provide evidence that BRG1 also regulates the levels of MAX , stimulated by the presence of glucocorticoids.…”

“…Here, we found that most of the MYC targets that were upregulated by MAX were inversely associated with the gene expression signatures of sh BRG1 cells and of embryonic lungs from mice overexpressing Nmyc and Cmyc . In contrast, there was a direct association with the expression profi le after BRG1 reconstitution in lung cancer cells ( 16 ). These fi ndings imply that BRG1, MAX, and MYC orchestrate the transcriptional regulation of a common set of genes and suggest that MYC represses cell differentiationrelated transcripts in a MAX-independent manner.…”

“…In addition, the findings were reproduced in cells derived from the H1299 lung cancer cells, which are BRG1 deficient. These were the isogenic cells H1299tr-BRG1wt and H1299tr-BRG1mut, which express the wild-type and a mutant version of BRG1, respectively ( 16 ). The level of expression of ectopic MAX from the 5′-UTR-MAX was very low in the BRG1 -mutant cells, in a hormone-free environment, and increased slightly upon the addition of glucocorticoids.…”

“…Two of these shRNAs had previously been shown to deplete BRG1 expression effi ciently and specifically (depleted BRG1 but not BRM expression; ref. 16 ). A scramble shRNA (Sigma MISSION shRNA non-mammalian control SHC002) was used as a control.…”

“…ChIP assays were performed as previously described ( 16 ). Briefl y, preliminary fi xation experiments were performed over a predetermined period.…”