The effect of glutamate on ghrelin release in mice.

Ghrelin is abundantly produced in the stomach. Here, we found that glutamate decreased ghrelin expression and release in ghrelin-producing cells, and decreased levels of food intake and plasma acyl-ghrelin in mice. Treatment with siRNA of G protein-coupled receptor, family C, group 5, member B (GPRC5B) in ghrelin-producing cell lines completely blocked the effect of glutamate-induced ghrelin suppression. In addition, glutamate inhibited ghrelin release via the extracellular signal-regulated kinase (ERK) activity pathway, and stimulated CREB2 mRNA expression in ghrelin-producing cell lines. These results suggest that glutamate inhibits ghrelin release via ERK-CREB2 pathway. These results suggest that the GPRC5B-ERK-CREB2 pathway is involved in the inhibition of ghrelin expression and secretion in ghrelin cells.

The study of ghrelin secretion and acyl-modification using mice and ghrelinoma cell lines.

Ghrelin is a peptide hormone with a unique structure comprising a medium chain fatty acid modification. Ghrelin cells are known to be abundantly localized in the gastric mucosa and are released into the blood stream to exert their multifunctional physiological effects. To elucidate the regulatory mechanisms of ghrelin secretion and acyl-modification, we developed novel ghrelin-producing cell lines. Using ghrelinoma cell lines, we focused on the mechanisms of ghrelin secretion and found that several GPCRs were highly expressed in ghrelin cells. Then, we showed that noradrenaline treatment stimulated ghrelin secretion via ß1-adrenergic receptor, and fasting-induced ghrelin elevation was completely inhibited by the ß1-adrenergic receptor antagonist in mice. In addition, we demonstrated that long chain fatty acids, glucose, and L-glutamate significantly inhibited ghrelin secretion. Furthermore, we recently revealed that the genes involved in fatty acid synthesis and long chain fatty acid metabolism were expressed in ghrelin cells, and that CPT-1 inhibitor treatment dramatically decreased the levels of acyl-modified ghrelin. Here, we introduce the current knowledge of the mechanisms involving ghrelin secretion and its acyl-modification.

ß-Oxidation in ghrelin-producing cells is important for ghrelin acyl-modification.

Ghrelin is a unique fatty acid-modified peptide hormone produced in the stomach and has important roles in energy homeostasis and gastrointestinal motility. However, the medium-chain fatty acid source for ghrelin acyl-modification is not known. We found that a fat-free diet and the removal of intestinal microbiota did not decrease acyl-ghrelin production in the stomach or plasma acyl-ghrelin levels in mice. RT-PCR analysis showed that genes involving fatty acid synthesis, metabolism, and transport were expressed in pancreas-derived ghrelinoma (PG-1) cells. Treatment with an irreversible inhibitor of carnitine palmitoyltransferase-1 (CPT-1) strongly decreased acylated ghrelin levels but did not affect ghrelin or ghrelin o-acyl transferase (GOAT) mRNA levels in PG-1 cells. Our results suggest that the medium-chain fatty acid used for the acyl-modification of ghrelin is produced in ghrelin-producing cells themselves by ß-oxidation of long-chain fatty acids provided from the circulation.

Milk basic protein increases ghrelin secretion and bone mineral density in rodents.

OBJECTIVES: Milk basic protein (MBP), a mixture of proteins isolated from bovine milk, is known to increase bone formation. Ghrelin, a stomach-derived peptide hormone, also has been reported to stimulate osteoblast formation. The aim of this study was to determine whether MBP-induced bone formation is mediated via ghrelin. METHODS: MBP was chronically administered to mice in their drinking water for 3 wk, and body weight, water intake, and bone mineral density were measured. Additionally, plasma bone-specific alkaline phosphatase, tartrate-resistant acid phosphatase isoform 5b, and ghrelin concentrations were determined by enzyme-linked immunosorbent assay. To examine the direct effect of MBP on ghrelin secretion, gastric tissue culture and primary mucosal cells were stimulated by MBP. RESULTS: The in vivo study of young, growing mice showed that chronic MBP intake for 3 wk increased the plasma ghrelin concentration and bone mineral density of the hind limb tibia. In vitro studies using minced rat gastric mucosa tissues and primary murine isolated gastric mucosal cells revealed that MBP stimulated ghrelin release in a dose-dependent manner. Moreover, MBP-induced ghrelin secretion was partly inhibited by adrenergic blockers. CONCLUSIONS: These findings suggest a novel mechanism by which MBP directly acts on ghrelin secretion. Additionally, the elevated ghrelin level induced by MBP may act as a mediator for bone formation.