The Microphthalmia-associated Transcription Factor Requires SWI/SNF Enzymes to Activate Melanocyte-specific Genes

Abstract

The microphthalmia transcription factor (Mitf ) activates melanocyte-specific gene expression, is critical for survival and proliferation of melanocytes during development, and has been described as an oncogene in malignant melanoma. SWI/SNF complexes are ATP-dependent chromatin-remodeling enzymes that play a role in many developmental processes. To determine the requirement for SWI/SNF enzymes in melanocyte differentiation, we introduced Mitf into fibroblasts that inducibly express dominant negative versions of the SWI/SNF ATPases, Brahma or Brahma-related gene 1 (BRG1). These dominant negative SWI/SNF components have been shown to inhibit gene activation events that normally require SWI/SNF enzymes. We found that Mitf-mediated activation of a subset of endogenous melanocyte-specific genes required SWI/SNF enzymes but that cell-cycle regulation occurred independently of SWI/SNF function. Activation of tyrosinase-related protein 1, a melanocytespecific gene, correlated with SWI/SNF-dependent changes in chromatin accessibility at the endogenous locus. Both BRG1 and Mitf could be localized to the tyrosinase-related protein 1 and tyrosinase promoters by chromatin immunoprecipitation, whereas immunofluorescence and immunoprecipitation experiments indicate that Mitf and BRG1 co-localized in the nucleus and physically interacted. Together these results suggest that Mitf can recruit SWI/SNF enzymes to melanocyte-specific promoters for the activation of gene expression via induced changes in chromatin structure at endogenous loci.Melanocytes are pigment-producing cells that are developmentally derived from the neural crest and that comprise 1-2% of the epidermis (1). They are also present on the epithelial surfaces of mucous membranes, hair follicles, the cochlea of the inner ear, and both the uvea and conjunctiva of the eye (2, 3). On the skin, they play a photoprotective role by synthesizing and distributing melanin (4). Excessive exposure to UV radiation has been linked to the transformation of cutaneous melanocytes to melanoma, a cancer that has steadily increased in frequency and is difficult to treat (5, 6).Microphthalmia-associated transcription factor (Mitf ) 2 is the "master regulator" of melanocyte differentiation and was elegantly shown to convert fibroblasts into dendritic cells that express melanocyte-specific genes (7). It is important for the commitment, proliferation, and survival of melanocytes during neural crest cell migration, and null mutations of the mouse Mitf gene result in complete absence of melanocytes and lack of pigmentation in the skin, eyes, and inner ear (8, 9). Two human diseases resulting from mutations in the MITF gene are Waardenburg type 2 syndrome and Tietz syndrome, both of which are characterized by pigmentary disturbances and sensineural deafness (8).Mitf is a basic helix-loop-helix leucine zipper transcription factor that binds DNA either as a homodimer or as a heterodimer with TFE3, TFEB, or TFEC to conserved E boxes (CAC(G/A)TG) in the promoters of its target genes, which incl...

Epigenetic Mechanisms Modulate Thyroid Transcription Factor 1-mediated Transcription of the Surfactant Protein B Gene

“…These results, thus, demonstrate that Brg1-mediated chromatin remodeling activity is a critical component of Sftpb activation. Consistent with the present observations, Brg1 has been reported to modulate the expression of a subset of genes through interactions with specific transcription factors (34,35,78). For example, Brg1 interacts with Smad3 to selectively increase the expression of a subset of TGF-␤-inducible genes (34) and is a critical factor for the functional activity of glucocorticoid receptor (79).…”

Up-regulation of MET Expression by α-Melanocyte-stimulating Hormone and MITF Allows Hepatocyte Growth Factor to Protect Melanocytes and Melanoma Cells from Apoptosis

“…The tyrosinase gene, as well as the genes encoding other melanogenic proteins, appears to have a nonpermissive chromatin conformation. Indeed, MITF needs to recruit the SWI/SNF enzymes that allow chromatin remodelling and melanogenic gene transcription (29). HIF1A, BCL2, and MET, which do not require SWI/SNF enzymes, appear to be in an active genomic region.…”

Dual roles of lineage restricted transcription factors

“…It was also described that MITF recruits the SWI/SNF complex to the promoters of differentiation-related targets Tyrosinase and TRP1, but not to cell maintenance genes TBX2 and BCL2. 27,28 This mechanism is suggested to drive selective expression of MITF target genes. SWI/SNF complexes are ATP-dependent chromatinremodeling enzymes that alter the position of nucleosomes along the chromosome and, as a consequence, affect promoter accessibility to regulatory factors.…”