Regulation of the gene encoding GPR40, a fatty acid receptor expressed selectively in pancreatic β Cells

Abstract

GPR40 is a G protein-coupled receptor expressed preferentially in pancreatic β cells. It is activated by long-chain fatty acids and has been implicated in mediating physiological and pathological effects of long-chain fatty acids on β cells. We mapped the GPR40 transcription start site to a location 1044 bp upstream of the translation start site. This permitted definition of the GPR40 core promoter and the organization of the gene, which comprises a 24-bp non-coding exon, a 698-bp intron and a 4402-bp second exon, containing the entire protein coding sequence. Sequence analysis of the GPR40 locus revealed three evolutionarily conserved regions upstream to the translation start site (HR1-HR3). DNase I-hypersensitive sites were present in the HR2 and HR3 regions in β cells but not in non-βcells. The 5′-flanking region of the GPR40 gene was capable of directing transcriptional activity selectively in β cells. An important component of this is attributable to the HR2 region, which showed strong β cell-specific enhancer activity. Systematic mutagenesis of HR2 revealed several important sub-regions. Mutagenesis of sub-regions 4-5, and 9 reduced transcriptional activity by ∼60 and 40%, respectively. These sub-regions can bind the β cell-specific transcription factors PDX1 and BETA2, respectively, both in vitro and in vivo. Thus, cell-specific expression of the GPR40 gene involves a characteristic chromatin organization of the locus and is controlled at the transcriptional level through HR2, a potent β cell-specific enhancer. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.