Roles of Brahma and Brahma/SWI2-related gene 1 in hypoxic induction of the erythropoietin gene

Upon hypoxia, the human erythropoietin (EPO) gene is transactivated by the heterodimeric hypoxia-inducible factor 1 (HIF-1). Mammalian SWI/SNF is a chromatin-remodeling complex involved in the modulation of gene expression. We demonstrate that Brahma (Brm) and Brahma/SWI2-related gene 1 (Brg-1), alternative ATPase subunits of SWI/SNF, potentiate reporter gene activation mediated by HIF-1 in an ATPase-dependent manner. Brm was more potent than Brg-1 in the reporter gene assays. Simultaneous depletion of both Brm and Brg-1 by small interfering RNAs significantly compromised the transcription of the endogenous EPO gene triggered by hypoxia. Whereas knocking down Brm alone resulted in a moderate reduction in transcription of the EPO gene, depletion of Brg-1 resulted in an augmentation of transcription of both the EPO gene and the Brm gene, indicating that Brm can compensate for loss of Brg-1. Chromatin immunoprecipitation (ChIP) and sequential ChIP (re-ChIP) analysis showed that both Brm and Brg-1 associate with the enhancer region of the EPO gene in vivo in a hypoxia-dependent fashion and that each is present in a complex with HIF-1. Brm and Brg-1 were also recruited to the promoter of the vascular endothelial growth factor (VEGF) gene in a hypoxia-dependent fashion, although hypoxic induction of VEGF transcription was not affected by depletions of either or both Brm and Brg-1. Together these studies reveal a novel role for SWI/SNF in the activation of transcription of the EPO gene, indicate an important communication and compensation between Brm and Brg-1, and suggest that the requirement for SWI/SNF during hypoxic induction is gene-specific.