The G-Protein−Coupled Receptor GPR40 Directly Mediates Long-Chain Fatty Acid−Induced Secretion of Cholecystokinin

Abstract

BACKGROUND & AIMS Long-chain fatty acid receptors G-protein–coupled receptor 40 (GPR40) (FFAR1) and GPR120 have been implicated in the chemosensation of dietary fats. I cells in the intestine secrete cholecystokinin (CCK), a peptide hormone that stimulates digestion of fat and protein, but these cells are rare and hard to identify. We sought to determine whether dietary fat-induced secretion of CCK is directly mediated by GPR40 expressed on I cells. METHODS We used fluorescence-activated cell sorting to isolate a pure population of I cells from duodenal mucosa in transgenic mice that expressed green fluorescent protein under the control of the CCK promoter (CCK–enhanced green fluorescent protein [eGFP] bacterial artificial chromosome mice). CCK-eGFP cells were evaluated for GPR40 expression by quantitative reverse transcription polymerase chain reaction and immunostaining. GPR40−/− mice were bred with CCK-eGFP mice to evaluate functional relevance of GPR40 on long-chain fatty acid–stimulated increases in [Ca2+]i and CCK secretion in isolated CCK-eGFP cells. Plasma levels of CCK after olive oil gavage were compared between GPR40+/+ and GPR40−/− mice. RESULTS Cells that expressed eGFP also expressed GPR40; expression of GPR40 was 100-fold greater than that of cells that did not express eGFP. In vitro, linoleic, oleic, and linolenic acids increased [Ca2+]i; linolenic acid increased CCK secretion by 53% in isolated GPR40+/+ cells that expressed eGFP. In contrast, in GPR40−/− that expressed eGFP, [Ca2+]i response to linoleic acid was reduced by 50% and there was no significant CCK secretion in response to linolenic acid. In mice, olive oil gavage significantly increased plasma levels of CCK compared with pregavage levels: 5.7-fold in GPR40+/+ mice and 3.1-fold in GPR40−/− mice. CONCLUSIONS Long-chain fatty acid receptor GPR40 induces secretion of CCK by I cells in response to dietary fat.