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Abstract
The molecular sensors underlying nutrient-stimulated GLP-1 secretion are currently being investigated. Peripheral administration of melanocortin-4 receptor (MC4R) agonists have been reported to increase GLP-1 plasma concentrations in mice and humans but it is unknown whether this effect results from a direct effect on the GLP-1 secreting L-cells in the intestine, from other effects in the intestine or from extra-intestinal effects. We investigated L-cell expression of MC4R in mouse and human L-cells by reanalyzing publicly available RNA sequencing databases (mouse and human) and by RT-qPCR (mouse), and assessed whether administration of MC4R agonists to a physiologically relevant gut model, isolated perfused mouse and rat small intestine, would stimulate GLP-1 secretion or potentiate glucose-stimulated secretion. L-cell MC4R expression was low in mouse duodenum and hardly detectable in the ileum and MC4R expression was hardly detectable in human L-cells. In isolated perfused mouse and rat intestine, neither intra-luminal nor intra-arterial administration of NDP-alpha-MSH, a potent MC4R agonist, had any effect on GLP-1 secretion (P ≥0.98, n = 5–6) from the upper or lower-half of the small intestine in mice or in the lower half in rats. Furthermore, HS014—an often used MC4R antagonist, which we found to be a partial agonist—did not affect the glucose-induced GLP-1 response in the rat, P = 0.62, n = 6). Studies on transfected COS7-cells confirmed bioactivity of the used compounds and that concentrations employed were well within in the effective range. Our combined data therefore suggest that MC4R-activated GLP-1 secretion in rodents either exclusively occurs in the colon or involves extra-intestinal signaling.
Highlights
Glucagon-like peptide-1 (GLP-1) is a gastrointestinal hormone that is secreted from intestinal L-cells after meal intake
In order to use appropriate concentrations of MC4R agonists and antagonist in the experiments on isolated perfused small intestine, we first determined the potencies of alpha-MSH and NDP-alpha-MSH in cAMP measurement experiments on rat and human MC4R transfected into COS-7 cells
The rat MC4R was activated by alpha-MSH and NDP-alpha-MSH with EC50 values of 1.5 × 10−8 M and 9.1 × 10−11 M
Summary
Glucagon-like peptide-1 (GLP-1) is a gastrointestinal hormone that is secreted from intestinal L-cells after meal intake It acts to inhibit appetite and potentiate glucose-stimulated insulin secretion [1,2,3]. These actions led to the development of GLP-1 based drugs, which have been used for more than a decade for type-2-diabetes treatment and recently to induce weight loss. A similar observation was recently made by Panaro et al, who showed LY2112688mediated increase in plasma GLP-1 concentrations in mice They showed that the increase requires gcg expression in the ileum and colon [12], pointing towards that these sites of the intestine are responsible for the increased plasma concentrations. It has been shown that humans with natural occurring loss-of-function mutations in MC4R exhibit reduced plasma PYY levels (marker of L-cell secretion from the distal intestine) in response to an OGTT, and GLP-1 and PYY secretion from human gut specimens (obtained from duodenum, ileum and colon) increased in response to incubation with the MC4R agonists MK-0493, LY2112688, a-MSH and NDP-a-MSH [13]
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