Objective: The G-protein-coupled receptor Gpr40 is expressed in beta-cells where it contributes to free fatty acid (FFA) enhancement of glucose-stimulated insulin secretion. However, other sites of Gpr40 expression, including the intestine, have been suggested. The transcription factor IPF1/PDX1 was recently shown to bind to an enhancer element within the 5'-flanking region of Gpr40, implying that IPF1/PDX1 might regulate Gpr40 expression. Here, we addressed whether 1) Gpr40 is expressed in the intestine and 2) Ipf1/Pdx1 function is required for Gpr40 expression.
Research design and methods: In the present study, Gpr40 expression was monitored by X-gal staining using Gpr40 reporter mice and by in situ hybridization. Ipf1/Pdx1-null and beta-cell specific mutants were used to investigate whether Ipf1/Pdx1 controls Gpr40 expression. Plasma insulin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucose levels in response to acute oral fat diet were determined in Gpr40 mutant and control mice.
Results: Here, we show that Gpr40 is expressed in endocrine cells of the gastrointestinal tract, including cells expressing the incretin hormones GLP-1 and GIP, and that Gpr40 mediates FFA-stimulated incretin secretion. We also show that Ipf1/Pdx1 is required for expression of Gpr40 in beta-cells and endocrine cells of the anterior gastrointestinal tract.
Conclusions: Together, our data provide evidence that Gpr40 modulates FFA-stimulated insulin secretion from beta-cells not only directly but also indirectly via regulation of incretin secretion. Moreover, our data suggest a conserved role for Ipf1/Pdx1 and Gpr40 in FFA-mediated secretion of hormones that regulate glucose and overall energy homeostasis.