Sensing of Fatty Acids for Octanoylation of Ghrelin Involves a Gustatory G-Protein

Abstract

BackgroundGhrelin is an important regulator of energy – and glucose homeostasis. The octanoylation at Ser3 is essential for ghrelin’s biological effects but the mechanisms involved in the octanoylation are unknown. We investigated whether the gustatory G-protein, α-gustducin, and the free fatty acid receptors GPR40 and GPR120 are involved in the fatty acid sensing mechanisms of the ghrelin cell.MethodsWild-type (WT) and α-gustducin knockout (gust−/−) mice were fed a glyceryl trioctanoate-enriched diet (OD) during 2 weeks. Ghrelin levels and gastric emptying were determined. Co-localization between GPR40, GPR120 and ghrelin or α-gustducin/α-transducin was investigated by immunofluorescence staining. The role of GPR120 in the effect of medium and long chain fatty acids on the release of ghrelin was studied in the ghrelinoma cell line, MGN3-1. The effect of the GPR40 agonist, MEDICA16, and the GPR120 agonist, grifolic acid, on ghrelin release was studied both in vitro and in vivo. ResultsFeeding an OD specifically increased octanoyl ghrelin levels in the stomach of WT mice but not of gust−/− mice. Gastric emptying was accelerated in WT but not in gust−/− mice. GPR40 was colocalized with desoctanoyl but not with octanoyl ghrelin, α-gustducin or α-transducin positive cells in the stomach. GPR120 only colocalized with ghrelin in the duodenum. Addition of octanoic acid or α-linolenic acid to MGN3-1 cells increased and decreased octanoyl ghrelin levels, respectively. Both effects could not be blocked by GPR120 siRNA. MEDICA16 and grifolic acid did not affect ghrelin secretion in vitro but oral administration of grifolic acid increased plasma ghrelin levels.ConclusionThis study provides the first evidence that α-gustducin is involved in the octanoylation of ghrelin and shows that the ghrelin cell can sense long- and medium-chain fatty acids directly. GPR120 but not GPR40 may play a role in the lipid sensing cascade of the ghrelin cell.

Inhibitory Effect of Oleic Acid on Octanoylated Ghrelin Production

“…The second is the suppression of octanoylated ghrelin secretion regulated by signaling via G protein-coupled receptor 120 GPR120 and α-gustducin. GPR120, which functions mainly as a receptor for unsaturated longchain fatty acids 30 , could play a role in the lipid sensing cascade in ghrelin-producing cells 31 . The decrease in octanoylated ghrelin secretion after feeding is partially induced by long-chain fatty acids that act directly on gastric GPR120-expressing ghrelin cells 32 .…”

“…The decrease in octanoylated ghrelin secretion after feeding is partially induced by long-chain fatty acids that act directly on gastric GPR120-expressing ghrelin cells 32 . In addition, it was reported that the G protein α-gustducin is also involved in ghrelin octanoylation 31 and that α-gustducin could couple GPR120 33 . In our study, stearic acid, oleic acid, linoleic acid, and α-linolenic acid could have suppressed the release of octanoylated ghrelin via GPR120 and/or α-gustducin-mediated mechanism.…”

Low glucose-induced ghrelin secretion is mediated by an ATP-sensitive potassium channel

“…Therefore, ghrelin secretion might be regulated by hormonal, neuronal, and metabolite signals that arise from nutrient sensing and absorption in the intestine. In fact, in support of this view, recent studies have shown that nutrients (i.e., fatty acids and glucose) and hormones (i.e., insulin and oxytocin) regulate ghrelin secretion in fluorescence-activated cell sorting-separated primary mouse ghrelin cells and ghrelinoma cell lines (i.e., SG-1, PG-1, and mouse ghrelinoma 3-1 (MGN3-1)) (Janssen et al 2012, Sakata et al 2012.…”

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

“…Free fatty-acid receptors are involved in control of various gastrointestinal hormones by fatty acids [80], and that also seems to be the case with ghrelin. The long-chain fatty-acid receptor FFAR4 (GPR120), but not FFAR1 (GPR40), is expressed in ghrelin-producing cells [32, [127][128][129]. Knockdown of FFAR4 attenuates α-linolenic acid-induced suppression of ghrelin secretion from MGN3-1 or SG-1 cells [128,129].…”

“…The long-chain fatty-acid receptor FFAR4 (GPR120), but not FFAR1 (GPR40), is expressed in ghrelin-producing cells [32, [127][128][129]. Knockdown of FFAR4 attenuates α-linolenic acid-induced suppression of ghrelin secretion from MGN3-1 or SG-1 cells [128,129]. The FFAR4 agonists GW-9508 and compound B suppress ghrelin secretion in vitro [32,129], but not in gastric mucosal cells of FFAR4-knockout mice [32].…”