Glucagon Stimulates Ghrelin Secretion Through the Activation of MAPK and EPAC and Potentiates the Effect of Norepinephrine

Abstract

Ghrelin is a stomach-derived orexigenic hormone whose levels in circulation are altered by energy availability. Like ghrelin, the glucotropic hormone glucagon increases in the fasting state and serves to normalize energy levels. We hypothesized that glucagon can directly stimulate stomach ghrelin production. To verify this hypothesis, we used a primary culture of dispersed rat stomach cells. We first demonstrated that stomach ghrelin cells express the glucagon receptor (GluR). Glucagon (1-100 nM) significantly stimulated ghrelin secretion and proghrelin mRNA expression, and co-incubation with a GluR inhibitor prevented glucagon's action. The MAP kinase inhibitor (PD98058) reduced the glucagon-stimulated ghrelin secretion and proghrelin mRNA expression. Furthermore, glucagon treatment increased the phosphorylation of ERK1/2. Glucagon also increased intracellular cAMP levels, and inhibition of adenylate cyclase reduced glucagon's effect on ghrelin secretion. Surprisingly, inhibiting protein kinase A (PKA) (using H89 and phosphorothioate [Rp]-cAMP) did not prevent glucagon-stimulated ghrelin secretion. Instead, inhibiting the exchange protein activated by cAMP (EPAC) with Brefeldin-A was able to significantly reduce glucagon-stimulated ghrelin secretion. Furthermore, the EPAC agonist (8-pCPT) significantly stimulated ghrelin secretion. Depleting endoplasmic reticulum calcium stores or blocking voltage-dependant calcium channels prevented glucagon stimulated ghrelin secretion. Finally, co-incubation with the sympathetic neurotransmitter norepinephrine potentiated the glucagon stimulation of ghrelin secretion. Our findings are the first to show a direct link between glucagon and stomach ghrelin production and secretion and highlight the role of MAPK, the PKA-independent EPAC pathway, and the synergy between norepinephrine and glucagon in ghrelin release.

Effects of ghrelin in energy balance and body weight homeostasis

“…57 Thus, ghrelin directly stimulates glucagon secretion from pancreatic α cells (via GHS-R), 90 while glucagon stimulates ghrelin secretion under nutrient-deficient conditions. 91 Studies on laboratory animals have revealed that under the action of acyl-ghrelin, fat is stored not only in adipose tissue but also the liver, 39 where an increase in triacylglycerol content and genes involved in hepatic lipogenesis was found. 92 A recent study published by Gagnon et al brought up for discussion a new aspect of ghrelin involvement in glucose regulation, the authors demonstrating that ghrelin increases the secretion of GLP-1 (glucagon like peptide 1), thereby improving glucose tolerance.…”

Ghrelin – Physiological Functions and Regulation

“…Although some studies 88 found no correlation between ghrelin and leptin levels in obese children and adolescents, other studies suggest that there is a complex interaction between leptin and ghrelin. 89 Immunoneutralisation of circulating plasma ghrelin with specific immunoglobulin (Ig)-G anti-ghrelin antibodies caused a marked increase in plasma leptin and decrease in food intake.…”

The regulation of circulating ghrelin — with recent updates from cell-based assays [Review]

“…Most human studies [184][185][186][187] except for one [119] have reported that glucagon suppresses circulating ghrelin levels, whereas in rats glucagon stimulates ghrelin secretion [63, 188,189].…”

“…Systemic administration of isoproterenol increases plasma ghrelin levels, whereas atenolol and reserpine suppress fasting-induced elevation of ghrelin [30,225]. Ghrelin-producing cells express the α1 and β1 adrenergic receptors [30,32,55], and adrenaline and noradrenaline directly stimulate ghrelin secretion via the β1 receptor [30,55,62,171,188]. Activation of the β1 receptor results in elevation of intracellular cAMP, followed by Ca 2+ release from ER and granular exocytosis.…”