Effects of Ghrelin on Triglyceride Accumulation and Glucose Uptake in Primary Cultured Rat Myoblasts under Palmitic Acid-Induced High Fat Conditions

Regulation of peripheral tissue substrate metabolism by the gut-derived hormone ghrelin

Ghrelin increases in the circulation prior to entrained mealtimes, with the acylated (AG) form functioning to stimulate food intake and growth hormone release. Acutely, AG induces whole-body insulin resistance, potentially to maintain glycemia between meals. Alternatively, chronic administration of both AG and the unacylated isoform of ghrelin (unAG) is associated with improved skeletal muscle insulin sensitivity as well as reduced intramuscular lipids and inflammation. This may be due to effects on lipid metabolism, with ghrelin promoting storage of fat in adipose and liver while stimulating oxidation in skeletal muscle, preventing ectopic lipid accumulation. This is of specific relevance in the handling of meal-derived lipids, as ghrelin rises preprandially with effects persisting for 2-3 h following exposure in skeletal muscle, coinciding with elevated plasma FFAs. We hypothesize that ghrelin acts as a preparatory signal for incoming lipids, as well as a regulatory hormone for their use and storage. The effects of ghrelin on skeletal muscle are lost with high fat diet feeding and physical inactivity, potentially being implicated in the pathogenesis of metabolic disease. This review summarizes the metabolic effects of both ghrelin isoforms on peripheral tissues including the pancreas, adipose, liver, and skeletal muscle. Additionally, we speculate on the physiological relevance of these effects in vivo and suggest that ghrelin may be a key regulatory hormone for nutrient handling in the postprandial state.

Stimulation of fat oxidation in rat muscle by unacylated ghrelin persists for 2-3 hours, but is independent of fatty acid transporter translocation

While a definitive mechanism-of-action remains to be identified, recent findings indicate that ghrelin, particularly the unacylated form (UnAG), stimulates skeletal muscle fatty acid oxidation. The biological importance of UnAG-mediated increases in fat oxidation remains unclear, as UnAG peaks in the circulation before mealtimes, and decreases rapidly during the postprandial situation before increases in postabsorptive circulating lipids. Therefore, we aimed to determine if the UnAG-mediated stimulation of fat oxidation would persist long enough to affect the oxidation of meal-derived fatty acids, and if UnAG stimulated the translocation of fatty acid transporters to the sarcolemma as a mechanism-of-action. In isolated soleus muscle strips from male rats, short-term pre-treatment with UnAG elicited a persisting stimulus on fatty acid oxidation 2 h after the removal of UnAG. UnAG also caused an immediate phosphorylation of AMPK, but not an increase in plasma membrane FAT/CD36 or FABPpm. There was also no increase in AMPK signaling or increased FAT/CD36 or FABPpm content at the plasma membrane at 2 h which might explain the sustained increase in fatty acid oxidation. These findings confirm UnAG as a stimulator of fatty acid oxidation and provide evidence that UnAG may influence the handling of postprandial lipids. The underlying mechanisms are not known.

Acylation, a Conductor of Ghrelin Function in Brain Health and Disease

Acyl-ghrelin (AG) is an orexigenic hormone that has a unique octanoyl modification on its third serine residue. It is often referred to as the “hunger hormone” due to its involvement in stimulating food intake and regulating energy homeostasis. The discovery of the enzyme ghrelin-O-acyltransferase (GOAT), which catalyses ghrelin acylation, provided further insights into the relevance of this lipidation process for the activation of the growth hormone secretagogue receptor (GHS-R) by acyl-ghrelin. Although acyl-ghrelin is predominantly linked with octanoic acid, a range of saturated fatty acids can also bind to ghrelin possibly leading to specific functions. Sources of ghrelin acylation include beta-oxidation of longer chain fatty acids, with contributions from fatty acid synthesis, the diet, and the microbiome. In addition, both acyl-ghrelin and unacyl-ghrelin (UAG) have feedback effects on lipid metabolism which in turn modulate their levels. Recently we showed that whilst acyl-ghrelin promotes adult hippocampal neurogenesis and enhances memory function, UAG inhibits these processes. As a result, we postulated that the circulating acyl-ghrelin:unacyl-ghrelin (AG:UAG) ratio might be an important regulator of neurogenesis and cognition. In this review, we discuss emerging evidence behind the relevance of ghrelin acylation in the context of brain physiology and pathology, as well as the current challenges of identifying the provenance of the acyl moiety.

Post-weight loss changes in fasting appetite- and energy balance-related hormone concentrations and the effect of the macronutrient content of a weight maintenance diet: a randomised controlled trial

PURPOSE

We investigated the effects of the macronutrient composition of diets with differing satiety values on fasting appetite-related hormone concentrations after weight loss and examined whether the hormone secretion adapted to changes in body fat mass (FM) and fat-free mass (FFM) during the weight maintenance period (WM).

METHODS

Eighty-two men and women with obesity underwent a 7-week very-low-energy diet (VLED) and were then randomised to a higher-satiety food (HSF) group or a lower-satiety food (LSF) group during 24-weeks of the WM. The groups consumed isoenergetic foods with different satiety ratings and macronutrient compositions.

RESULTS

During the WM, the HSF group consumed more protein and dietary fibre and less fat than the LSF group, but the groups showed similar changes in body weight and fasting appetite-related hormones. In the whole study sample, VLED induced 12 kg (p < 0.001) weight loss. At the end of the WM, weight regain was 1.3 kg (p = 0.004), ghrelin concentration increased, whereas leptin, insulin, and glucose concentrations decreased compared to pre-VLED levels (p < 0.001 for all). Peptide YY did not differ from pre-VLED levels. Changes in ghrelin levels were inversely associated with changes in FFM during weeks 0-12 of the WM (p = 0.002), while changes in leptin and insulin levels were positively associated with changes in FM during weeks 0-12 (p = 0.015 and p = 0.038, respectively) and weeks 12-24 (p < 0.001 and p = 0.022) of the WM.

CONCLUSIONS

The macronutrient composition of an isoenergetic WM diet did not affect fasting appetite-related hormone concentrations. Leptin and insulin adjusted to the reduced FM, whereas ghrelin reflected FFM during the first months of the WM.

TRIAL REGISTRATION

isrctn.com, ID 67529475.

GIST and Ghrelin: To Be or Not to Be?

BACKGROUND

Ghrelin is the orexigenic hormone secreted mainly by the stomach. Its involvement in neoplastic development has been studied in gastrointestinal adenocarcinomas. Our paper aims to evaluate the influence of the ghrelin axis in gastrointestinal stromal tumors (GISTs).

MATERIALS AND METHODS

The study design included two groups of patients, 46 with gastric GISTs and 30 with obesity. Archived tissue samples were evaluated for the presence of gastritis and H. pylori. Immunohistochemical expression of ghrelin and its receptor (GHS-R) was assessed.

RESULTS

All GISTs showed absent immunohistochemical expression for ghrelin, while GHS-R displayed a particular pattern, with notable differences in intensity (p = 0.0256) and percentage of stained cells (p < 0.00001) in the periphery vs. core of tumors. Positive ghrelin expression was lower in the gastric mucosa of the first group compared to the second group (p < 0.001).

CONCLUSION

The ghrelin axis can influence GISTs carcinogenesis through activation of GHS-R. A previously described direct autocrine/paracrine mechanism is not supported by our findings.

γ-Linolenic Acid Prevents Lipid Metabolism Disorder in Palmitic Acid-Treated Alpha Mouse Liver-12 Cells by Balancing Autophagy and Apoptosis via the LKB1-AMPK-mTOR Pathway

Excessive fat deposition is the main character in nonalcoholic fatty liver disease (NAFLD), while γ-linolenic acid (GLA) is a polyunsaturated fatty acid that can reduce lipid deposition. This study investigated the effect and regulatory mechanism of GLA (100 μM) on lipid metabolism in alpha mouse liver 12 (AML-12) cells treated by 400 μM palmitic acid (PA). GLA reduced lipid content and increased fatty acid β oxidation, as indicated by decreasing triglyceride and cholesterol contents and increasing mRNA and protein expressions of CPT1α and PPARα. GLA relieved oxidative stress caused by PA, upregulated mRNA levels of superoxide dismutase and glutathione peroxidase, and decreased reactive oxygen species content. GLA reduced apoptosis, as indicated by decreases in the BAX/BCL2 expression level and apoptosis percentage. GLA activated autophagy, autophagosome-lysosome fusion, and LKB1-AMPK-mTOR signaling and upregulated mRNA and protein expressions of Beclin-1, autophagy-related 5, and liver kinase B1 (LKB1). These effects of GLA on lipid metabolism disorders of PA-treated hepatocytes were reversed by autophagy inhibitor 3MA and AMPK inhibitor compound C, confirming our conclusions. Overall, GLA can protect AML-12 cells from lipid metabolism disorder caused by PA via balancing autophagy and apoptosis mediated by the LKB1-AMPK-mTOR pathway. Consequently, GLA, as a dietary supplement, can help to prevent and treat NAFLD by regulating lipid metabolism and autophagy.

Ghrelin, via corticotropin-releasing factor receptors, reduces glucose uptake and increases lipid content in mouse myoblasts cells

Ghrelin and the corticotropin-releasing factor (CRF) family are known regulators of cellular metabolism and energy balance. We previously demonstrated that myoblast glucose metabolism is regulated by ghrelin and that this effect is mediated by CRF receptor type 2 (CRF-R2). Here we explored the effect of des-acyl ghrelin, the major circulating isoform of ghrelin, on cellular metabolism in mouse myoblast C2C12 cells, and examined whether CRF family receptors mediate its metabolic effects in muscle cells. C2C12 cells were exposed to des-acyl ghrelin with or without the CRF-R1- and CRF-R2-specific antagonists antalarmin or antisauvagine-30, respectively. Des-acyl ghrelin reduced glucose uptake and expression of the glucose transporter GLUT4, but induced retinol-binding protein 4 (RBP4) expression. Antalarmin and antisauvagine-30 inhibited the induction of glucose uptake by des-acyl ghrelin and its effect on GLUT4 and RBP4 expression. Moreover, treating C2C12 cells with des-acyl ghrelin resulted in cAMP activation in response to the CRF-R1-specific ligand stressin, and the CRF-R2-specific ligand Ucn3. Furthermore, des-acyl ghrelin reduced the expression of uncoupling proteins UCP2 and UCP3. Adding antalarmin or antisauvagine-30 to the medium reversed this effect. Finally, des-acyl ghrelin elevated lipid content and acetyl-CoA carboxylase expression in C2C12 cells. Our results suggest that during food deprivation, des-acyl ghrelin signals the muscle cells that glucose levels are low and that they should switch to fatty acids for their metabolic fuel.

Unacylated ghrelin stimulates fatty acid oxidation to protect skeletal muscle against palmitate-induced impairment of insulin action in lean but not high-fat fed rats

Background

Ghrelin is a gut hormone that spikes in circulation before mealtime. Recent findings suggest that both ghrelin isoforms stimulate skeletal muscle fatty acid oxidation, lending to the possibility that it may regulate skeletal muscle’s handling of meal-derived substrates. It was hypothesized in the current study that ghrelin may preserve muscle insulin response during conditions of elevated saturated fatty acid (palmitate) availability by promoting its oxidation.

Methods and results

Soleus muscle strips were isolated from male rats to determine the direct effects of ghrelin isoforms on fatty acid oxidation, glucose uptake and insulin signaling. We demonstrate that unacylated ghrelin (UnAG) is the more potent stimulator of skeletal muscle fatty acid oxidation. Both isoforms of ghrelin generally protected muscle from impaired insulin-mediated phosphorylation of AKT Ser⁴⁷³ and Thr³⁰⁸, as well as downstream phosphorylation of AS160 Ser⁵⁸⁸ during high palmitate exposure. However, only UnAG was able to preserve insulin-stimulated glucose uptake during exposure to high palmitate concentrations. The use of etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase (CPT-1) abolished this protection, strongly suggesting that UnAG’s stimulation of fatty acid oxidation may be essential to this protection. To our knowledge, we are also the first to investigate the impact of a chronic high-fat diet on ghrelin’s actions in muscle. Following 6 wks of a high-fat diet, UnAG was unable to preserve insulin-stimulated signaling or glucose transport during an acute high palmitate exposure. UnAG was also unable to further stimulate 5′ AMP-activated protein kinase (AMPK) or fatty acid oxidation during high palmitate exposure. Corticotropin-releasing hormone receptor-2 (CRF-2R) content was significantly decreased in muscle from high-fat fed animals, which may partially account for the loss of UnAG’s effects.

Conclusions

UnAG is able to protect muscle from acute lipid exposure, likely due to its ability to stimulation fatty acid oxidation. This effect is lost in high-fat fed animals, implying a resistance to ghrelin at the level of the muscle. The underlying mechanisms accounting for ghrelin resistance in high fat-fed animals remain to be discovered.

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Saturated lipids acutely impair muscle insulin signaling and glucose transport.

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Ghrelin isoforms consistently protect insulin signaling from lipid detriment.

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Unacylated ghrelin more potently stimulates fat oxidation, preserving glucose transport.

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Muscle of chronic high fat-fed rats may be resistant to ghrelin’s metabolic effects.

Acylated Ghrelin and The Regulation of Lipid Metabolism in The Intestine

Acylated ghrelin (AG) is a gastrointestinal (GI) peptide mainly secreted by the stomach that promotes cytosolic lipid droplets (CLD) hypertrophy in adipose tissues and liver. However, the role of AG in the regulation of lipid metabolism in the intestine remains unexplored. This study aimed at determining whether AG influences CLD production and chylomicron (CM) secretion in the intestine. The effects of AG and oleic acid on CLD accumulation and CM secretion were first investigated in cultured Caco-2/15 enterocytes. Intestinal lipid metabolism was also studied in Syrian Golden Hamsters submitted to conventional (CD) or Western (WD) diets for 8 weeks and continuously administered with AG or physiological saline for the ultimate 2 weeks. In cultured Caco-2/15 enterocytes, CLD accumulation influenced CM secretion while AG reduced fatty acid uptake. In WD hamsters, continuous AG treatment amplified chylomicron output while reducing postprandial CLD accumulation in the intestine. The present study supports the intimate relationship between CLD accumulation and CM secretion in the intestine and it underlines the importance of further characterizing the mechanisms through which AG exerts its effects on lipid metabolism in the intestine.

Ghrelin stimulates fatty acid oxidation and inhibits lipolysis in isolated muscle from male rats

Ghrelin is classically known as a central appetite-stimulating hormone but has recently been recognized to have a significant role in peripheral tissue energy metabolism. However, the direct effects of ghrelin on skeletal muscle, a major site for glucose and lipid disposal, remain understudied. We found that the two major ghrelin isoforms, acylated and unacylated ghrelin, were able to significantly increase skeletal muscle fatty acid oxidation (~20%) while incorporation of fatty acids into major lipid pools remained unchanged. The increase in fatty acid oxidation was accompanied by increases in acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK. Ghrelin isoforms had no independent effect on lipolysis under unstimulated conditions, but nearly completely abolished epinephrine-stimulated lipolysis. This effect was generally, but not consistently related to a blunting in the phosphorylation of HSL activation sites, Ser660 and 563. Taken together, these findings suggest that ghrelin isoforms have a direct, acute effect on fatty acid oxidation and lipolysis.

Improvement of Lipid and Glucose Metabolism by Capsiate in Palmitic Acid-Treated HepG2 Cells via Activation of the AMPK/SIRT1 Signaling Pathway

Capsiate, a nonpungent ingredient of CH-19 Sweet, exhibits anti-obesity effects on animals and humans. This study investigated the effects and molecular mechanism of capsiate on lipid and glucose metabolism in PA-treated HepG2 cells. Results showed that compared with the PA-alone group, 100 μM capsiate inhibited lipid accumulation, decreased TG (0.0562 ± 0.0142 vs 0.0381 ± 0.0055 mmol/g of protein; P = 0.024) and TC (0.1087 ± 0.0037 vs 0.0359 ± 0.0059 mmol/g of protein; P = 0.000) levels, and increased the HDL-C level (0.0189 ± 0.0067 vs 0.1050 ± 0.0106 mmol/g of protein; P = 0.000) and glycogen content (0.0065 ± 0.0007 vs 0.0146 ± 0.0008 mg/10⁶ cells; P = 0.000) of PA-treated HepG2 cells; 100 μM capsiate also upregulated the level of CD36 ( P = 0.000), phosphorylation of ACC ( P = 0.034), and expression of CPT1 ( P = 0.013) in PA-treated HepG2 cells, leading to an enhancement of lipid metabolism. Meanwhile, 100 μM capsiate upregulated the levels of GLUT1, GLUT4, GK, and phosphorylation of GS ( P = 0.001, 0.029, 0.000, and 0.045, respectively) and downregulated the PEPCK level ( P = 0.001) to improve glucose metabolism in PA-treated HepG2 cells. Furthermore, the levels of phosphorylation of AMPK and expression of SIRT1 in HepG2 cells were increased by a 100 μM capsiate treatment ( P = 0.001 and 0.000, respectively), while the FGF21 level was decreased ( P = 0.003). Most of these effects were reversed by pretreatment with compound C, a selective AMPK inhibitor. Thus, capsiate might improve lipid and glucose metabolism in HepG2 cells by activating the AMPK/SIRT1 signaling pathway.

Deletion of ghrelin prevents aging-associated obesity and muscle dysfunction without affecting longevity

During aging, decreases in energy expenditure and locomotor activity lead to body weight and fat gain. Aging is also associated with decreases in muscle strength and endurance leading to functional decline. Here, we show that lifelong deletion of ghrelin prevents development of obesity associated with aging by modulating food intake and energy expenditure. Ghrelin deletion also attenuated the decrease in phosphorylated adenosine monophosphate‐activated protein kinase (pAMPK) and downstream mediators in muscle, and increased the number of type IIa (fatigue resistant, oxidative) muscle fibers, preventing the decline in muscle strength and endurance seen with aging. Longevity was not affected by ghrelin deletion. Treatment of old mice with pharmacologic doses of ghrelin increased food intake, body weight, and muscle strength in both ghrelin wild‐type and knockout mice. These findings highlight the relevance of ghrelin during aging and identify a novel AMPK‐dependent mechanism for ghrelin action in muscle.

Acylated and desacyl ghrelin are associated with hepatic lipogenesis, β-oxidation and autophagy: role in NAFLD amelioration after sleeve gastrectomy in obese rats

Bariatric surgery improves non-alcoholic fatty liver disease (NAFLD). Our aim was to investigate the potential role of ghrelin isoforms in the resolution of hepatic steatosis after sleeve gastrectomy, a restrictive bariatric surgery procedure, in diet-induced obese rats. Male Wistar rats (n = 161) were subjected to surgical (sham operation and sleeve gastrectomy) or dietary interventions [fed ad libitum a normal (ND) or a high-fat (HFD) diet or pair-fed]. Obese rats developed hepatosteatosis and showed decreased circulating desacyl ghrelin without changes in acylated ghrelin. Sleeve gastrectomy induced a dramatic decrease of desacyl ghrelin, but increased the acylated/desacyl ghrelin ratio. Moreover, sleeve gastrectomy reduced hepatic triglyceride content and lipogenic enzymes Mogat2 and Dgat1, increased mitochondrial DNA amount and induced AMPK-activated mitochondrial FFA β-oxidation and autophagy to a higher extent than caloric restriction. In primary rat hepatocytes, the incubation with both acylated and desacyl ghrelin (10, 100 and 1,000 pmol/L) significantly increased TG content, triggered AMPK-activated mitochondrial FFA β-oxidation and autophagy. Our data suggest that the decrease in the most abundant isoform, desacyl ghrelin, after sleeve gastrectomy contributes to the reduction of lipogenesis, whereas the increased relative acylated ghrelin levels activate factors involved in mitochondrial FFA β-oxidation and autophagy in obese rats, thereby ameliorating NAFLD.