Phosphorylation of Ser1452 on BRG1 inhibits the function of the SWI/SNF complex in chromatin activation

BRG1, one of core subunits of the SWI/SNF chromatin remodeling complex, is frequently mutated in cancers. Previously, we reported significant downregulation of the phosphorylation level of BRG1 on Ser¹⁴⁵² (<10%) in cell lines derived from ovarian clear cell carcinoma with frequent recurrence and acquired drug resistance. In this study, we tried to elucidate the roles of BRG1 phosphorylation, using cell lines expressing wild-type, phosphorylation-mimic (brg1-S¹⁴⁵²D), or non-phosphorylatable (brg1-S¹⁴⁵²A) BRG1. Quantitative proteomic analyses revealed upregulation of proteins and phosphoproteins related to linker histone H1s, histone methylation, and protein ubiquitylation in brg1-S¹⁴⁵²D cells, which may coordinately promote the chromatin inactivation and ubiquitin-dependent degradation of target proteins. Consistent with these results, brg1-S¹⁴⁵²D cells exhibited an increase in condensed chromatin and polyubiquitylated proteins. In brg1-S¹⁴⁵²D cells, we also detected downregulation of various cancer-related proteins (e.g., EGFR and MET) as well as decreased migration, proliferation, and sensitivity to taxanes and oxaliplatin. Together, our results reveal that BRG1 phosphorylation drives tumor malignancy by inhibiting the functions of SWI/SNF complex in chromatin activation, thereby promoting expression of various cancer-related proteins. SIGNIFICANCE: For the first time we demonstrated that the mutation on Ser¹⁴⁵² phosphorylation site of BRG1, a component of SWI/SNF chromatin remodeling complex, changed protein and phosphoprotein levels of linker histone H1s, binding competitor of histone H1s, and histone methylase/demethylase involved in the heterochromatic histone modifications to promote the chromatin inactivation. In phosphorylation-mimic mutant, significant decrease of various cancer-related proteins as well as migration, proliferation, and sensitivity to specific antitumor agents were detected. Our results reveal that BRG1 phosphorylation drives tumor malignancy by inhibiting the functions of SWI/SNF complex in chromatin activation, thereby promoting expression of various cancer-related proteins.