MITF-Independent Pro-Survival Role of BRG1-Containing SWI/SNF Complex in Melanoma Cells

Abstract

Metastasized malignant melanoma has a poor prognosis because of its intrinsic resistance to chemotherapy and radiotherapy. The central role in the melanoma transcriptional network has the transcription factor MITF (microphthalmia-associated transcription factor). It has been shown recently that the expression of MITF and some of its target genes require the SWI/SNF chromatin remodeling complex. Here we demonstrate that survival of melanoma cells requires functional SWI/SNF complex not only by supporting expression of MITF and its targets and but also by activating expression of prosurvival proteins not directly regulated by MITF. Microarray analysis revealed that besides the MITF-driven genes, expression of proteins like osteopontin, IGF1, TGFß2 and survivin, the factors known to be generally associated with progression of tumors and the antiapoptotic properties, were reduced in acute BRG1-depleted 501mel cells. Western blots and RT-PCR confirmed the microarray findings. These proteins have been verified to be expressed independently of MITF, because MITF depletion did not impair their expression. Because these genes are not regulated by MITF, the data suggests that loss of BRG1-based SWI/SNF complexes negatively affects survival pathways beyond the MITF cascade. Immunohistochemistry showed high expression of both BRM and BRG1 in primary melanomas. Exogenous CDK2, osteopontin, or IGF1 each alone partly relieved the block of proliferation imposed by BRG1 depletion, implicating that more factors, besides the MITF target genes, are involved in melanoma cell survival. Together these results demonstrate an essential role of SWI/SNF for the expression of MITF-dependent and MITF-independent prosurvival factors in melanoma cells and suggest that SWI/SNF may be a potential and effective target in melanoma therapy.

Insights into the Regulation of Survivin Expression in Tumors

“…Since the survivin promoter is also regulated negatively, mainly through the p53 protein, the positive transactivation effect of the HPV protein E6 on survivin is mediated via the p53 degradation [32]. Survivin expression was completely abolished by the knockdown of BRG1, a critical ATPase of the SWI/SNF-chromatin remodeling complex, indicating a direct regulation of survivin by an epigenetic mechanism [33]. Interestingly, Xu et al [2] identified several polymorphisms in the survivin gene promoter, one of which is located at the repressor CDE/CHR element.…”

SWI/SNF Chromatin Remodeling Enzymes in Melanocyte Differentiation and Melanoma

“…59 A recent study reported heterogeneous staining of BRG1 in primary melanoma and occasional lack of detectable BRG1 expression in some tumors. 36 Yet another study indicated that BRG1 messenger RNA levels were significantly higher in stage IV melanoma samples compared to normal and stage III melanoma samples. 60 Interestingly, recent exome sequencing studies have detected BRG1 mutations at a low frequency in patient-derived melanoma samples 53 (Table 1).…”

“…Interestingly, downregulation of the BRG1 subunit of SWI/SNF enzymes in melanoma cells compromises SOX10 expression. 36 However, it is not known whether BRG1 or other components of the SWI/SNF complex interact with SOX10 during melanocyte differentiation or in melanoma.…”

“…24,46 The requirement for SWI/SNF activity is associated with MITF-dependent and -independent functions. 24,36,46,47,60 …”

“…However, BRM expression was found to be infrequently downregulated in patient-derived melanoma samples. 36,51 …”