Sara Janssen

Bitter taste receptors and α-gustducin regulate the secretion of ghrelin with functional effects on food intake and gastric emptying

Ghrelin is a hunger hormone with gastroprokinetic properties but the factors controlling ghrelin secretion from the stomach are unknown. Bitter taste receptors (T2R) and the gustatory G proteins, α-gustducin (gust) and α-transducin, are expressed in the gut and are involved in the chemosensation of nutrients. This study aimed to investigate whether T2R-agonists affect (i) ghrelin release via α-gustducin and (ii) food intake and gastric emptying via the release of ghrelin. The mouse stomach contains two ghrelin cell populations: cells containing octanoyl and desoctanoyl ghrelin, which were colocalized with α-gustducin and α-transducin, and cells staining for desoctanoyl ghrelin. Gavage of T2R-agonists increased plasma octanoyl ghrelin levels in WT mice but the effect was partially blunted in gust−/− mice. Intragastric administration of T2R-agonists increased food intake during the first 30 min in WT but not in gust−/− and ghrelin receptor knockout mice. This increase was accompanied by an increase in the mRNA expression of agouti-related peptide in the hypothalamus of WT but not of gust−/− mice. The temporary increase in food intake was followed by a prolonged decrease (next 4 h), which correlated with an inhibition of gastric emptying. The delay in emptying, which was partially counteracted by ghrelin, was not mediated by cholecystokinin and GLP-1 but involved a direct inhibitory effect of T2R-agonists on gastric contractility. This study is unique in providing functional evidence that activation of bitter taste receptors stimulates ghrelin secretion. Modulation of endogenous ghrelin levels by tastants may provide novel therapeutic applications for the treatment of weight -and gastrointestinal motility disorders.

Ghrelin modulates fatty acid synthase and related transcription factor mRNA levels in a tissue-specific manner in neonatal broiler chicks.

The endogenous ligand for the growth hormone (GH) secretagogue receptor ghrelin is a peptide secreted by the stomach of mammals and stimulates food intake and enhances adiposity. In avian species, ghrelin is mainly produced by the proventriculus but reduces food intake whereas its effect on lipogenesis in different tissues is unknown. We therefore investigated the effects of a single intravenous injection of 2.8 microg (1 nmol per chick) recombinant chicken ghrelin in neonatal broiler chicks. Besides food intake and plasma corticosterone levels, mRNA levels of the key lipogenic enzyme fatty acid synthase (FAS) and its related transcription factors sterol regulatory element binding protein-1 (SREBP-1) and peroxisome proliferator-activated receptor-gamma (PPARgamma) were determined in diencephalon, liver and quadriceps femoris muscle before, and 15, 30, and 60 min after injection. Chicken ghrelin administration induced a significant short-term (<30 min) reduction in food intake and markedly elevated plasma corticosterone levels. In diencephalon, FAS, SREBP-1 and PPARgamma mRNA levels were significantly increased within 15 min after ghrelin injection. These observations suggest that central fatty acid metabolism is involved in the anorectic effects of ghrelin. In contrast, hepatic mRNA levels of FAS and both transcription factors were significantly reduced within 30 min after ghrelin injection. In muscle, FAS and transcription factor gene expression was very low and not affected by ghrelin. Overall, our results indicate that ghrelin has opposite effects on FAS and transcription factor mRNA amounts with increased levels in diencephalon (central anorectic effect) and decreased levels in liver (peripheral anti-lipogenic effect) in chickens.

The role of alpha-gustducin, in the effect of T2R-agonists on the secretion and physiological effects of ghrelin.

concentrations were higher in the healed group suggesting that the activity of elastase in these wounds was under more stringent control than in those patients that did not heal. Discussion: These results suggest that the higher SLPI concentrations found in healing wounds are integral to the prevention of inappropriate elastase activity at wound sites and thus the prevention of chronic wound development. This work was funded by a grant from the Dunhill Medical Trust.

Role of the Energy Sensor AMP-Activated Protein Kinase (AMPK) and Its Downstream Effector Uncoupling Protein 2 (UCP2) in the Orexigenic Effect of Endogenous Ghrelin

Background. Ghrelin released from the stomach, stimulates food intake through stimulation of neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons in the hypothalamus. Several studies proposed a pivotal role for the energy sensor AMPK and UCP2 in ghrelins effects on NPY/AgRP expression and food intake stimulation, although most of these studies focused on the effects of exogenous ghrelin. Aim. To investigate whether a rise in endogenous ghrelin levels is able to influence NPY/AgRP expression via activation of AMPK activity and UCP2 expression. Methods. Endogenous ghrelin levels were increased in wildtype (GHS-R+/+) and ghrelin receptor knockout (GHS-R-/-) mice by fasting (24h) or by induction of streptozotocin (STZ)-diabetes (15 days). Plasma octanoylated ghrelin levels were determined by radioimmunoassay. The mRNA expression of AgRP, NPY and UCP2 in the hypothalamus was measured by real-time PCR. Hypothalamic AMPK activity in tissue lysates was measured with an immunoprecipitation kinase assay. Results.Octanoylated ghrelin levels peaked (4.5fold, P<0.01) after 24h of fasting and declined thereafter due to a decrease (1.7-fold, P<0.01) in the mRNA expression of ghrelin-O-acyl transferase (GOAT). GHS-R+/+ mice showed a significant increase in AgRP mRNA (from 0.33±0.03 to 1.03±0.09; P<0.001) and NPY mRNA (from 0.54 ± 0.08 to 0.88 ± 0.05; P<0.01) expression after 24h-fasting. In GHS-R-/mice no significant increase in AgRP and NPY mRNA expression was observed. Fasting did not affect AMPK activity in both genotypes but increased UCP2 mRNA expression (GHS-R+/+: from 0.35±0.05 to 0.50±0.04; P<0.05 and GHS-R-/-: from 0.41±0.04 to 0.55±0.08; P=0.058). The hyperghrelinemia associated with the induction of STZ-diabetes was accompanied by a significant increase in the expression of NPY and AgRP in GHS-R+/+ mice compared to nondiabetic controls (AgRP: from 0.22±0.12 to 2.74±0.45; P<0.001 and NPY: from 0.73±0.06 to 2.23±0.64; P<0.05) but not in GHS-R-/mice. AMPK activity in the hypothalamus of GHSR+/+ mice after induction of diabetes (20.2±1.6 pmol/min/mg) was decreased (P<0.05) compared to non-diabetic littermates (16.3±1.3 pmol/min/mg) and there was no genotypic difference. Similarly, UCP2 mRNA levels were decreased after the induction of STZ-diabetes compared to control mice in both genotypes (GHS-R+/+: from 0.68±0.02 to 0.26±0.03; P<0.001 and GHS-R-/-: from 0.61±0.05 to 0.28±0.05; P<0.01). Conclusion. Increasing endogenous ghrelin levels by fasting and by induction of diabetes stimulates the expression of AgRP and NPY in the hypothalamus via interaction with the GHS-R. The change in AMPK activity and its downstream effector, UCP2, accompanying these changes in endogenous ghrelin levels occur independently from the GHS-R suggesting that AMPK and UCP2 do not play a major role in the orexigenic effect of endogenous ghrelin.