Leucine suppresses glucagon secretion from pancreatic islets by directly modulating alpha-cell cAMP

Abstract

ObjectivePancreatic islets are nutrient sensors that regulate organismal blood glucose homeostasis. Glucagon release from the pancreatic -cell is important under fasted, fed, and hypoglycemic conditions, yet metabolic regulation of -cells remains poorly understood. Here, we identified a previously unexplored role for physiological levels of leucine, which is classically regarded as a {beta}-cell fuel, in the intrinsic regulation of -cell glucagon release. MethodsGcgCreERT:CAMPER and GcgCreERT:GCaMP6s mice were generated to perform dynamic, high-throughput functional measurements of -cell cAMP and Ca2+ within the intact islet. Islet perifusion assays were used for simultaneous, time-resolved measurements of glucagon and insulin release from mouse and human islets. The effects of leucine were compared with glucose and the mitochondrial fuels 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH, non-metabolized leucine analog that activates glutamate dehydrogenase), -ketoisocaproate (KIC, leucine metabolite), and methyl-succinate (complex II fuel). CYN154806 (Sstr2 antagonist), diazoxide (KATP activator, which prevents Ca2+-dependent exocytosis from , {beta}, and {delta}-cells), and dispersed -cells were used to inhibit islet paracrine signaling and identify -cell intrinsic effects. ResultsMimicking the effect of glucose, leucine strongly suppressed amino acid-stimulated glucagon secretion. Mechanistically, leucine dose-dependently reduced -cell cAMP at physiological concentrations, with an IC50 of 57, 440, and 1162 M at 2, 6, and 10 mM glucose, without affecting -cell Ca2+. Leucine also reduced -cell cAMP in islets treated with Sstr2 antagonist or diazoxide, as well as dispersed -cells, indicating an -cell intrinsic effect. The effect of leucine was matched by KIC and the glutamate dehydrogenase activator BCH, but not methyl-succinate, indicating a dependence on mitochondrial anaplerosis. Glucose, which stimulates anaplerosis via pyruvate carboxylase, had the same suppressive effect on -cell cAMP but with lower potency. Similarly to mouse islets, leucine suppressed glucagon secretion from human islets under hypoglycemic conditions. ConclusionsThese findings highlight an important role for physiological levels of leucine in the metabolic regulation of -cell cAMP and glucagon secretion. Leucine functions primarily through an -cell intrinsic effect that is dependent on glutamate dehydrogenase, in addition to the well-established -cell regulation by {beta}/{delta}-cell paracrine signaling. Our results suggest that mitochondrial anaplerosis-cataplerosis facilitates the glucagonostatic effect of both leucine and glucose, which cooperatively suppress -cell tone by reducing cAMP. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=173 SRC="FIGDIR/small/551113v2_ufig1.gif" ALT="Figure 1"> View larger version (44K): org.highwire.dtl.DTLVardef@1af2865org.highwire.dtl.DTLVardef@8508fborg.highwire.dtl.DTLVardef@10c009org.highwire.dtl.DTLVardef@1afb6e9_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LILeucine inhibits glucagon secretion from mouse and human islets C_LIO_LILeucine suppresses -cell cAMP via both direct and paracrine effects C_LIO_LIAnaplerosis via glutamate dehydrogenase is sufficient to suppress -cell cAMP C_LIO_LILeucine suppresses -cell cAMP and glucagon secretion more potently than glucose C_LI