Abstract
Aims/hypothesisLipids are a potent stimulus for the secretion of glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic peptide (GIP). Traditionally, this effect was thought to involve the sensing of lipid digestion products by free fatty acid receptor 1 (FFA1) and G-protein coupled receptor 119 (GPR119) on the apical surface of enteroendocrine cells. However, recent evidence suggests that lipids may in fact be sensed basolaterally, and that fatty acid absorption and chylomicron synthesis may be a prerequisite for their stimulatory effect on gut peptide release. Therefore, we investigated the effect of chylomicrons on GLP-1 and GIP secretion in vitro.MethodsThe effect of chylomicrons on incretin secretion was investigated using GLUTag cells and duodenal cultures of both murine and human origin. The role of lipoprotein lipase (LPL) and FFA1 in GLUTag cells was assessed by pharmacological inhibition and small (short) interfering RNA (siRNA)-mediated knockdown. The effect of chylomicrons on intracellular calcium concentration ([Ca2+]i) was determined by imaging GLUTag cells loaded with Fura-2. In the primary setting, the contributions of FFA1 and GPR119 were investigated using L cell-specific Gpr119 knockout cultures treated with the FFA1 antagonist GW1100.ResultsChylomicrons stimulated GLP-1 release from GLUTag cells, and both GLP-1 and GIP secretion from human and murine duodenal cultures. Chylomicron-triggered GLP-1 secretion from GLUTag cells was largely abolished following lipase inhibition with orlistat or siRNA-mediated knockdown of Lpl. In GLUTag cells, both GW1100 and siRNA-mediated Ffar1 knockdown reduced GLP-1 secretion in response to chylomicrons, and, consistent with FFA1 Gq-coupling, chylomicrons triggered an increase in [Ca2+]i. However, LPL and FFA1 inhibition had no significant effect on chylomicron-mediated incretin secretion in murine cultures. Furthermore, the loss of GPR119 had no impact on GLP-1 secretion in response to chylomicrons, even in the presence of GW1100.Conclusions/interpretationChylomicrons stimulate incretin hormone secretion from GLUTag cells as well as from human and murine duodenal cultures. In GLUTag cells, the molecular pathway was found to involve LPL-mediated lipolysis, leading to the release of lipid species that activated FFA1 and elevated intracellular calcium.
Highlights
The upper small intestine harbours a rich population of enteroendocrine cells (EECs) [1], which release gut peptides following the ingestion of a meal
As extracellular signal-regulated kinase (ERK) phosphorylation has previously been linked with gut peptide release [26, 27], the ability of chylomicrons to stimulate glucagon-like peptide (GLP)-1 secretion was tested in the presence or absence of the widely used mitogen-activated protein kinase/ERK kinase (MEK) inhibitor U0126 (Fig. 1b)
glucosedependent insulinotropic peptide (GIP) secretion in murine primary cultures. (a) GLP-1 secretion from L cell-specific Gpr119 knockout (Cre-positive/Gpr119flox) and (b) wildtype-like (Cre-negative/Gpr119flox) murine duodenal cultures treated with the G-protein coupled receptor 119 (GPR119) agonist AR231453 (AR; 100 nmol/l), chylomicrons (CM; 100 μg/ml) or the free fatty acid receptor 1 (FFA1) agonist AM-1638 (1 μmol/l), in the presence or absence of the FFA1 antagonist GW1100 (5 μmol/l, 30 min pretreatment)
Summary
The upper small intestine harbours a rich population of enteroendocrine cells (EECs) [1], which release gut peptides following the ingestion of a meal. Lipids are an established potent stimulus for the secretion of gut peptides including incretins, glucagon-like peptide (GLP)-1 and glucosedependent insulinotropic peptide (GIP) [2,3,4] These peptide hormones serve to facilitate the efficient digestion and absorption of lipids and other nutrients, and to promote satiety. Triacylglycerols are digested by lipases in the small intestinal lumen to generate long-chain fatty acids (LCFAs) and 2monoacylglycerol. These digestion products mediate the stimulatory effect of lipids on gut peptide secretion, acting via Gprotein-coupled receptors (GPCRs) expressed on EECs [5,6,7]. Secondary messages downstream of GPCRs include elevation of cytosolic Ca2+ and cAMP concentrations as well as activation of protein kinases such as extracellular signal-regulated kinase (ERK) [15]
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