Mammalian SWI/SNF complexes are ATP-dependent chromatin remodeling enzymes that have been implicated in the regulation of gene expression, cell-cycle control and oncogenesis. MyoD is a muscle-specific regulator able to induce myogenesis in numerous cell types. To ascertain the requirement for chromatin remodeling enzymes in cellular differentiation processes, we examined MyoD-mediated induction of muscle differentiation in fibroblasts expressing dominant-negative versions of the human brahma-related gene-1 (BRG1) or human brahma (BRM), the ATPase subunits of two distinct SWI/SNF enzymes. We find that induction of the myogenic phenotype is completely abrogated in the presence of the mutant enzymes. We further demonstrate that failure to induce muscle-specific gene expression correlates with inhibition of chromatin remodeling in the promoter region of an endogenous muscle-specific gene. Our results demonstrate that SWI/SNF enzymes promote MyoD-mediated muscle differentiation and indicate that these enzymes function by altering chromatin structure in promoter regions of endogenous, differentiation-specific loci.

ATP-dependent chromatin-remodeling complexes are conserved among all eukaryotes and function by altering nucleosome structure to allow cellular regulatory factors access to the DNA. Mammalian SWI-SNF complexes contain either of two highly conserved ATPase subunits: BRG1 or BRM. To identify cellular genes that require mammalian SWI-SNF complexes for the activation of gene expression, we have generated cell lines that inducibly express mutant forms of the BRG1 or BRM ATPases that are unable to bind and hydrolyze ATP. The mutant subunits physically associate with at least two endogenous members of mammalian SWI-SNF complexes, suggesting that nonfunctional, dominant negative complexes may be formed. We determined that expression of the mutant BRG1 or BRM proteins impaired the ability of cells to activate the endogenous stress response gene hsp70 in response to arsenite, a metabolic inhibitor, or cadmium, a heavy metal. Activation of hsp70 by heat stress, however, was unaffected. Activation of the heme oxygenase 1 promoter by arsenite or cadmium and activation of the cadmium-inducible metallothionein promoter also were unaffected by the expression of mutant SWI-SNF components. Analysis of a subset of constitutively expressed genes revealed no or minimal effects on transcript levels. We propose that the requirement for mammalian SWI-SNF complexes in gene activation events will be specific to individual genes and signaling pathways.The packaging of eukaryotic DNA into nucleosomes and higher order chromatin structure presents cells with a significant barrier to DNA utilization and necessitates mechanisms by which chromatin structure can be modified so that transcription can occur. Many multiprotein complexes with the ability to modify chromatin structure have been identified. These include histone acetyltransferases and deacetylases, which directly modify histone tail domains, and a class of energy-dependent enzymes that utilize ATP hydrolysis to alter nucleosome structure (reviewed in references 23, 30, 32, 34, 70, 83, and 84). The ATP-dependent chromatin remodeling complexes are conserved among eukaryotes, they share a related subunit that possesses DNA-stimulated ATPase activity, and each has been demonstrated to alter nucleosome structure in vitro in an ATP-dependent manner. Most of these complexes can be classified into two groups, those containing homologues of the yeast SWI2-SNF2 ATPase subunit, including yeast SWI-SNF (7, 12, 55), human SWI-SNF (hSWI-SNF) (24, 35, 82), yeast RSC (8), and Drosophila BRM complexes (54, 71), and those containing homologues of the Drosophila imitation-switch (ISWI) ATPase gene (16), including yeast ISW1 and ISW2 (76), human RSF (39), and the Drosophila NURF, CHRAC, and ACF complexes (25,75,78). A third group can be defined by Xenopus and human complexes containing the Mi2 protein, a related ATPase found in association with histone deacetylase activity (72,81,87,90).Although members of the ATP-dependent class of chromatin remodelers facilitate alterations in nucleosome structure in ...

Cell cycle arrest is critical for muscle differentiation, and the two processes are closely coordinated but temporally separable. SWI/SNF complexes are ATP-dependent chromatin-remodeling enzymes that have been shown to be required for muscle differentiation in cell culture and have also been reported to be required for Rb-mediated cell cycle arrest. We therefore looked more closely at how SWI/SNF enzymes affect the events that occur during MyoD-induced myogenesis, namely, cell cycle regulation and muscle-specific gene expression, in cells that inducibly express dominant negative versions of Brahma (BRM) and Brahma-related gene 1 (BRG1), the ATPase subunits of two distinct SWI/SNF complexes. Although dominant negative BRM and BRG1 inhibited expression of every muscle-specific regulator and structural gene assayed, there was no effect on MyoD-induced activation of cell cycle regulatory proteins, and thus, cells arrested normally. In particular, in the presence or absence of dominant negative BRM or BRG1, MyoD was able to activate expression of p21, cyclin D3, and Rb, all of which are critical for cell cycle withdrawal in the G 1 /G 0 phase of the cell cycle. These findings suggest that at least one basis for the distinct mechanisms that regulate cessation of cell proliferation and musclespecific gene expression during muscle differentiation is that SWI/SNF-mediated chromatin-remodeling enzymes are required only for the latter.