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

Background: Ghrelin is an important regulator of energy - and glucose homeostasis. The octanoylation at Ser(3) is essential for ghrelin's biological effects but the mechanisms involved in the octanoylation are unknown. We investigated whether the gustatory G-protein, alpha-gustducin, and the free fatty acid receptors GPR40 and GPR120 are involved in the fatty acid sensing mechanisms of the ghrelin cell. Methods: Wild-type (WT) and alpha-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 alpha-gustducin/alpha-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. Results: Feeding 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, alpha-gustducin or alpha-transducin positive cells in the stomach. GPR120 only colocalized with ghrelin in the duodenum. Addition of octanoic acid or alpha-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. Conclusion: This study provides the first evidence that alpha-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.