mTOR Signaling in X/A-Like Cells Contributes to Lipid Homeostasis in Mice

Mechanosensing by Piezo1 in gastric ghrelin cells contributes to hepatic lipid homeostasis in mice

Ghrelin is an orexigenic peptide released by gastric ghrelin cells that contributes to obesity and hepatic steatosis. The mechanosensitive ion channel Piezo1 in gastric ghrelin cells inhibits the synthesis and secretion of ghrelin in response to gastric mechanical stretch. We sought to modulate hepatic lipid metabolism by manipulating Piezo1 in gastric ghrelin cells. Mice with a ghrelin cell-specific deficiency of Piezo1 (Ghrl-Piezo1-/-) had hyperghrelinemia and hepatic steatosis when fed a low-fat or high-fat diet. In these mice, hepatic lipid accumulation was associated with changes in gene expression and in protein abundance and activity expected to increase hepatic fatty acid synthesis and decrease lipid β-oxidation. Pharmacological inhibition of the ghrelin receptor improved hepatic steatosis in Ghrl-Piezo1-/- mice, thus confirming that the phenotype of these mice was due to overproduction of ghrelin caused by inactivation of Piezo1. Gastric implantation of silicone beads to induce mechanical stretch of the stomach inhibited ghrelin synthesis and secretion, thereby helping to suppress fatty liver development induced by a high-fat diet in wild-type mice but not in Ghrl-Piezo1-/- mice. Our study elucidates the mechanism by which Piezo1 in gastric ghrelin cells regulate hepatic lipid accumulation, providing insights into potential treatments for fatty liver.

New antidepressant mechanism of Yueju Pill: Increasing ghrelin level by inhibiting gastric mTOR/S6K signaling pathway and sensitizing hippocampal GHS-R

Background and aim

Yueju Pill (YJ) not only has good antidepressant effect but also can effectively treat digestive system diseases. However,it remains unclear whether the mechanism of antidepressant action of YJ is related to the peripheral digestive system. The purpose of this study was to elucidate the antidepressant mechanism of YJ on ghrelin level based on gastric mTOR/S6K signal pathway and sensitized hippocampal Ghrelin/GHS-R system in CUMS mice.

Experimental procedure

The depression model was induced by chronic unpredictable mild stress (CUMS) and social isolation. The antidepressant effect of YJ was observed by behavioral experiment and hemodynamic experiments. Ghrelin levels in in hippocampus and blood were measured by Elisa kit, and the mRNA of ghrelin in mice stomach was measured by Real-time Quantitative PCR (RT-qPCR). The activation level of gastric mTOR/S6K signal pathway was detected by Western Blot (WB). Rapamycin (Rapa) and L-Leucine (L-Leu) were used to verify the effects of YJ on the synthesis and release of ghrelin. The activity of GHS-R in hippocampus was observed by immunofluorescence. Hippocampal neuronal damage was evaluated by HE staining and Nissl staining. The level of central neurotransmitter was measured by liquid chromatograph mass spectrometer (LC-MS).

Results and conclusion

YJ ameliorates CUMS-induced depressive-like behavior by inhibiting the gastric mTOR/S6K signaling pathway and increasing GHR expression in the mouse stomach. However, these effects of YJ could be resisted by L-Leu (a mTOR receptor agonist). Further studies have shown that YJ can sensitize the Ghrelin/GHS-R system in the hippocampus, with significant neuroprotective effects, and is also involved in regulating the levels of key neurotransmitters (5-hydroxytryptamine, Dopamine and γ-aminobutyric acid) in depressive-like states.

Gastric mechanosensitive channel Piezo1 regulates ghrelin production and food intake

Ghrelin, produced mainly by gastric X/A-like cells, triggers a hunger signal to the central nervous system to stimulate appetite. It remains unclear whether X/A-like cells sense gastric distention and thus regulate ghrelin production. Here we show that PIEZO1 expression in X/A-like cells decreases in patients with obesity when compared to controls, whereas it increases after sleeve gastrectomy. Male and female mice with specific loss of Piezo1 in X/A-like cells exhibit hyperghrelinaemia and hyperphagia and are more susceptible to overweight. These phenotypes are associated with impairment of the gastric CaMKKII/CaMKIV-mTOR signalling pathway. Activation of PIEZO1 by Yoda1 or gastric bead implantation inhibits ghrelin production, decreases energy intake and induces weight loss in mice. Inhibition of ghrelin production by Piezo1 through the CaMKKII/CaMKIV-mTOR pathway can be recapitulated in a ghrelin-producing cell line mHypoE-42. Our study reveals a mechanical regulation of ghrelin production and appetite by PIEZO1 of X/A-like cells, which suggests a promising target for anti-obesity therapy.

Signaling pathways in obesity: mechanisms and therapeutic interventions

Obesity is a complex, chronic disease and global public health challenge. Characterized by excessive fat accumulation in the body, obesity sharply increases the risk of several diseases, such as type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, and is linked to lower life expectancy. Although lifestyle intervention (diet and exercise) has remarkable effects on weight management, achieving long-term success at weight loss is extremely challenging, and the prevalence of obesity continues to rise worldwide. Over the past decades, the pathophysiology of obesity has been extensively investigated, and an increasing number of signal transduction pathways have been implicated in obesity, making it possible to fight obesity in a more effective and precise way. In this review, we summarize recent advances in the pathogenesis of obesity from both experimental and clinical studies, focusing on signaling pathways and their roles in the regulation of food intake, glucose homeostasis, adipogenesis, thermogenesis, and chronic inflammation. We also discuss the current anti-obesity drugs, as well as weight loss compounds in clinical trials, that target these signals. The evolving knowledge of signaling transduction may shed light on the future direction of obesity research, as we move into a new era of precision medicine.

Lower ghrelin levels does not impact the metabolic benefit induced by Roux-en-Y gastric bypass

OBJECTIVE

Roux-en-Y gastric bypass is an effective intervention for metabolic disorder. We aim to elucidate whether ghrelin contributes to weight reduction, and glycemic and lipid control after Roux-en-Y gastric bypass (RYGB).

DESIGN

Four-week-old WT and Ghrl-TSC1^-/- mice were fed high fat diet for 12 weeks before surgery, and continued to be on the same diet for 3 weeks after surgery. Body weight, food intake, glycemic and lipid metabolism were analyzed before and after surgery.

RESULTS

Gastric and circulating ghrelin was significantly increased in mice with RYGB surgery. Hypoghrelinemia elicited by deletion of TSC1 to activate mTOR signaling in gastric X/A like cells demonstrated no effect on weight reduction, glycemic and lipid control induced by Roux-en-Y gastric bypass surgery.

CONCLUSION

Lower ghrelin levels does not impact the metabolic benefit induced by Roux-en-Y gastric bypass.

Ghrelin signalling is dysregulated in male but not female offspring in a rat model of maternal vertical sleeve gastrectomy

Bariatric surgery is the most effective and durable means of treating obesity and its comorbidities. Women make up 80% of those receiving weight loss surgery and they experience improvements in fertility. Unfortunately, bariatric surgery in the context of pregnancy is associated with complications, including growth restriction and small-for-gestational age offspring (SGA). SGA offspring have a greater risk for obesity in adulthood, although the mechanism for this SGA-induced obesity is unknown. In a rat model of vertical sleeve gastrectomy (VSG), we previously identified reductions during pregnancy in ghrelin, a stomach-derived hormone that increases appetite and induces growth hormone secretion. Here, we hypothesise that VSG offspring will have altered ghrelin signalling compared to offspring of Sham dams as a result of reduced in utero ghrelin. At postnatal day (PND)21, male and female offspring of dams that have previously received VSG have an increase in mRNA expression for the ghrelin receptor in the hypothalamus compared to Sham offspring, and the expression of GOAT is lower in females compared to males. Liver expression of endogenous ghrelin antagonist, LEAP2, is elevated at PND60 in VSG offspring. Expression of other genes in the growth hormone system (growth hormone-releasing hormone and growth hormone) were not altered. Plasma levels of total ghrelin at PND21 are also not different between VSG and Sham pups. In adult pups, 1-hour chow intake of male but not female VSG offspring given is less than Sham offspring when given 50 µg kg-1 of exogenous ghrelin by i.p. injection. These results indicate that maternal VSG surgery has an impact on ghrelin signalling in offspring and that, as adults, male VSG offspring may be functionally less responsive to ghrelin than controls.

Ghrelin Based Therapy of Metabolic Diseases

BACKGROUND

Ghrelin, a unique 28 amino acid peptide hormone secreted by the gastric X/A like cells, is an endogenous ligand of the growth hormone secretagogue receptor (GHSR). Ghrelin-GHSR signaling has been found to exert various physiological functions, including stimulation of appetite, regulation of body weight, lipid and glucose metabolism, and increase of gut motility and secretion. This system is thus critical for energy homeostasis.

OBJECTIVE

The objective of this review is to highlight the strategies of ghrelin-GHSR based intervention for therapy of obesity and its related metabolic diseases.

RESULTS

Therapeutic strategies of metabolic disorders targeting the ghrelin-GHSR pathway involve neutralization of circulating ghrelin by antibodies and RNA spiegelmers, antagonism of ghrelin receptor by its antagonists and inverse agonists, inhibition of ghrelin O-acyltransferase (GOAT), as well as potential pharmacological approach to decrease ghrelin synthesis and secretion.

CONCLUSION

Various compounds targeting the ghrelin-GHSR system have shown promising efficacy for the intervention of obesity and relevant metabolic disorders in animals and in vitro. Further clinical trials to validate their efficacy in human beings are urgently needed.

Ghrelin ameliorates nonalcoholic steatohepatitis induced by chronic low-grade inflammation via blockade of Kupffer cell M1 polarization

Whether the stomach influences the progression of nonalcoholic steatohepatitis (NASH) remains largely unknown. Ghrelin, a 28-amino acid gastric hormone, is critical for the regulation of energy metabolism and inflammation. We investigated whether ghrelin affects the progression of NASH. NASH was induced with lipopolysaccharide (LPS; 240 μg/kg/day) in male C57BL/6J mice with high-fat diet (HFD). Ghrelin (11 nmol/kg/day) was administrated by a subcutaneous mini-pump. Liver steatosis, inflammation, and fibrosis were assessed. Kupffer cells and hepatocytes isolated from wild type, GHSR1a^-/- or PPARγ^+/- mice were cocultured to determine the cellular and molecular mechanism by which ghrelin ameliorates NASH. A low concentration of LPS activates the Kupffer cells, leading to the development of NASH in mice fed HFD. Ghrelin blocked the progression of NASH induced by LPS via GHSR1a-mediated attenuation of Kupffer cells M1 polarization. GHSR1a was detected in Kupffer cells isolated from wild-type mice but not in GHSR1a deficient animals. Upon binding with ghrelin, internalization of GHSR1a occurred. Ghrelin reduced levels of tumor necrosis factor-α and inducible nitricoxide synthase while increasing Arg1 in Kupffer cells treated with LPS. Ghrelin markedly attenuated the upregulation of lipid accumulation induced by the supernatant of Kupffer cells under both basal and LPS-treated conditions. Deficiency of PPARγ significantly reduced the effect of LPS on the hepatic steatosis in mice and in cultured hepatocytes. Our studies indicate that the stomach may improve the development of NASH via ghrelin. Ghrelin may serve as a marker and therapeutic target for NASH.

mTOR Pathway is Involved in Energy Homeostasis Regulation as a Part of the Gut-Brain Axis

Mammalian, or mechanic, target of rapamycin (mTOR) signaling is a crucial factor in the regulation of the energy balance that functions as an energy sensor in the body. The present review explores how the mTOR/S6k intracellular pathway is involved in modulating the production of different signals such as ghrelin and nesfatin-1 in the gastrointestinal tract to regulate food intake and body weight. The role of gastric mTOR signaling in different physiological processes was studied in depth through different genetic models that allow the modulation of mTOR signaling in the stomach and specifically in gastric X/A type cells. It has been described that mTOR signaling in X/A-like gastric cells has a relevant role in the regulation of glucose and lipid homeostasis due to its interaction with different organs such as liver and adipose tissue. These findings highlight possible therapeutic strategies, with the gut–brain axis being one of the most promising targets in the treatment of obesity.

DOCK5 regulates energy balance and hepatic insulin sensitivity by targeting mTORC1 signaling

The dedicator of cytokinesis 5 (DOCK5) is associated with obesity. However, the mechanism by which DOCK5 contributes to obesity remains completely unknown. Here, we show that hepatic DOCK5 expression significantly decreases at a state of insulin resistance (IR). Deletion of DOCK5 in mice reduces energy expenditure, promotes obesity, augments IR, dysregulates glucose metabolism, and activates the mTOR (Raptor)/S6K1 pathway under a high-fat diet (HFD). The overexpression of DOCK5 in hepatocytes inhibits gluconeogenic gene expression and increases the level of insulin receptor (InsR) and Akt phosphorylation. DOCK5 overexpression also inhibits mTOR/S6K1 phosphorylation and decreases the level of raptor protein expression. The opposite effects were observed in DOCK5-deficient hepatocytes. Importantly, in liver-specific Raptor knockout mice and associated hepatocytes, the effects of an adeno-associated virus (AAV8)- or adenovirus-mediated DOCK5 knockdown on glucose metabolism and insulin signaling are largely eliminated. Additionally, DOCK5-Raptor interaction is indispensable for the DOCK5-mediated regulation of hepatic glucose production (HGP). Therefore, DOCK5 acts as a regulator of Raptor to control hepatic insulin activity and glucose homeostasis.

Physiological Effect of Ghrelin on Body Systems

Ghrelin is a relatively novel multifaceted hormone that has been found to exert a plethora of physiological effects. In this review, we found/confirmed that ghrelin has effect on all body systems. It induces appetite; promotes the use of carbohydrates as a source of fuel while sparing fat; inhibits lipid oxidation and promotes lipogenesis; stimulates the gastric acid secretion and motility; improves cardiac performance; decreases blood pressure; and protects the kidneys, heart, and brain. Ghrelin is important for learning, memory, cognition, reward, sleep, taste sensation, olfaction, and sniffing. It has sympatholytic, analgesic, antimicrobial, antifibrotic, and osteogenic effects. Moreover, ghrelin makes the skeletal muscle more excitable and stimulates its regeneration following injury; delays puberty; promotes fetal lung development; decreases thyroid hormone and testosterone; stimulates release of growth hormone, prolactin, glucagon, adrenocorticotropic hormone, cortisol, vasopressin, and oxytocin; inhibits insulin release; and promotes wound healing. Ghrelin protects the body by different mechanisms including inhibition of unwanted inflammation and induction of autophagy. Having a clear understanding of the ghrelin effect in each system has therapeutic implications. Future studies are necessary to elucidate the molecular mechanisms of ghrelin actions as well as its application as a GHSR agonist to treat most common diseases in each system without any paradoxical outcomes on the other systems.

Mechanisms controlling hormone secretion in human gut and its relevance to metabolism

The homoeostatic regulation of metabolism is highly complex and involves multiple inputs from both the nervous and endocrine systems. The gut is the largest endocrine organ in our body and synthesises and secretes over 20 different hormones from enteroendocrine cells that are dispersed throughout the gut epithelium. These hormones include GLP-1, PYY, GIP, serotonin, and CCK, each of whom play pivotal roles in maintaining energy balance and glucose homeostasis. Some are now the basis of several clinically used glucose-lowering and weight loss therapies. The environment in which these enteroendocrine cells exist is also complex, as they are exposed to numerous physiological inputs including ingested nutrients, circulating factors and metabolites produced from neighbouring gut microbiome. In this review, we examine the diverse means by which gut-derived hormones carry out their metabolic functions through their interactions with different metabolically important organs including the liver, pancreas, adipose tissue and brain. Furthermore, we discuss how nutrients and microbial metabolites affect gut hormone secretion and the mechanisms underlying these interactions.

ETV5 regulates GOAT/ghrelin system in an mTORC1-dependent manner

Ghrelin, a 28 amino acid peptide hormone, regulates multiple important metabolic functions. Its acylation by ghrelin-O-acyl-transferase enzyme (GOAT) is required for binding to and activating its receptor, the growth hormone secretagogue receptor 1a. Mechanism underlying the regulation of GOAT and acyl ghrelin remains unclear. The present study demonstrated that ETV5 could transactivate GOAT promoter region and increase its expression, leading to subsequent increase in the production of acyl ghrelin. mTORC1 modulated ETV5 expression levels, likely via altering its protein stability, in the murine hypothalamic CLU122 cells and in mice. Moreover, ETV5 mediated the effects of mTORC1 signaling on the expression level of acyl ghrelin. Our study suggests a novel mTORC1-ETV5-GOAT/ghrelin axis in the regulation of ghrelin system. ETV5 may be a key regulator of mTORC1-GOAT/ghrelin axis in ghrelin producing cells and a potential therapeutic target for organism energy imbalance.