Insulin and Norepinephrine Regulate Ghrelin Secretion from a Rat Primary Stomach Cell Culture

Abstract

Ghrelin is a peptide hormone primarily produced in the previously unidentified X/A endocrine cells of the stomach. Extensive studies have focused on the effects of ghrelin on growth hormone release and appetite regulation. However, the mechanisms regulating ghrelin secretion are less understood. In the present study, we developed a primary culture of newborn rat stomach cells to investigate the mechanisms regulating ghrelin synthesis and secretion. We demonstrated that this cell preparation secretes ghrelin in a regulated manner through the increase of cAMP, intracellular calcium, and activation of protein kinase C. Norepinephrine (NE) (0.1-10 μm) stimulated ghrelin secretion through the β1-adrenergic receptor via increased cAMP and protein kinase A activity, whereas acetylcholine had no effect. Because circulating ghrelin levels were previously shown to be inversely correlated with insulin levels, we investigated the effect of insulin on ghrelin secretion. We first demonstrated that ghrelin cells express the insulin receptor α- and β-subunits. Next, we determined that insulin (1-10 nm) inhibited both basal and NE-stimulated ghrelin secretion, caused an increase in phosphorylated serine-threonine kinase (AKT) and a reduction in intracellular cAMP, but did not alter proghrelin mRNA levels. The inhibitory effect of insulin was blocked by inhibiting phospho-inositol-3 kinase and AKT but not MAPK. Higher dose insulin (100 nm) did not suppress ghrelin secretion, which prompted the investigation of cellular insulin resistance by pretreating the cells with 100 nm insulin for 24 h. This caused a reduction in insulin receptor expression and prevented the insulin-mediated AKT activation and the suppression of ghrelin secretion with no impact on NE-stimulated ghrelin secretion. Our findings highlight the role of the sympathetic nervous system, insulin, and insulin resistance in the regulation of ghrelin secretion.

Circulating acylghrelin levels are suppressed by insulin and increase in response to hypoglycemia in healthy adult volunteers

“…Reciprocal effects of hypoglycemia and hyperinsulinemia on AG secretion may explain the lack of correlation during hyperinsulinemia and hypoglycemia. The molecular mechanisms regulating ghrelin secretion have been investigated in an in vitro study in newborn rat stomach cells (17). In that study, insulin incubation directly inhibited ghrelin secretion through an increase in phosphorylated serine-threonine kinase (AKT) and a reduction in intracellular cAMP.…”

β1-Adrenergic receptor deficiency in ghrelin-expressing cells causes hypoglycemia in susceptible individuals

“…Supportive of this assertion, ghrelin secretion increases when sympathetic nerves are stimulated artificially or when adrenergic agents are infused locally into the gastric submucosa (29,30). Norepinephrine, epinephrine, and isoproterenol all stimulate ghrelin secretion from ghrelinoma cell lines and from primary cultures of gastric mucosal cells (26,(31)(32)(33)(34)(35)(36). Reserpine, which depletes norepinephrine from sympathetic Ghrelin is an orexigenic gastric peptide hormone secreted when caloric intake is limited.…”

Reduction in total plasma ghrelin levels following catecholamine depletion: Relation to bulimic and depressive symptoms

“…In fact, there is increasing in vitro and in vivo evidence for a stimulatory effect of catecholamines on ghrelin (Gagnon and Anini, 2012;Iwakura et al, 2011;Schulpis et al, 2004;Zhao et al, 2010). An instructive paradigm to further evaluate the influence of catecholamines on ghrelin is to transiently deplete catecholamine stores by oral administration of alpha-methyl-paratyrosine (AMPT) (Berman et al, 1999;Hasler et al, 2008).…”