Acylated ghrelin limits fat accumulation and improves redox state and inflammation markers in the liver of high-fat-fed rats

Ghrelin, Neuroinflammation, Oxidative Stress, and Mood Disorders: What Are the Connections?

Ghrelin is a gut peptide hormone associated with feeding behavior and energy homeostasis. Acylated ghrelin binds to the growth hormone secretagogue receptor 1a subtype (GHS-R1a) in the hippocampus, leading to GH release from the anterior pituitary. However, in recent years, ghrelin and its receptor have also been implicated in other processes, including the regulation of cardiomyocyte function, muscle trophism, and bone metabolism. Moreover, GHS-R1a is distributed throughout the brain and is expressed in brain areas that regulate the stress response and emotional behavior. Consistently, a growing body of evidence supports the role of ghrelin in regulating stress response and mood. Stress has consistently been shown to increase ghrelin levels, and despite some inconsistencies, both human and rodent studies suggested antidepressant effects of ghrelin. Nevertheless, the precise mechanism by which ghrelin influences stress response and mood remains largely unknown. Intriguingly, ghrelin and GHS-R1a were consistently reported to exert anti-inflammatory, antioxidant, and neurotrophic effects both in vivo and in vitro, although this has never been directly assessed in relation to psychopathology. In the present review we will discuss available literature linking ghrelin with the stress response and depressive-like behavior in animal models as well as evidence describing the interplay between ghrelin and neuroinflammation/oxidative stress. Although further studies are required to understand the mechanisms involved in the action of ghrelin on mood, we hypothesize that the antiinflammatory and anti-oxidative properties of ghrelin may give a key contribution.

Inhibition of the MyD88 signaling pathway could upregulates Ghrelin expression to synergistically regulate hepatic Echinococcus multilocularis-infected progression

Introduction

AE and whether the inhibition of the MyD88 inflammatory pathway can enhance Ghrelin expression to collaboratively modulate AE progression remains unclear.

Methods

In this study, we evaluated Ghrelin serum levels and changes in TLR4/MyD88/NF-κB pathway proteins and inflammatory factors in AE patients and E. multilocularis mouse models at different stages of infection (-4, -8, and -12 weeks). Additionally, we administered the MyD88 inhibitor TJ-M2010-5 intraperitoneally to infected mice to evaluate alterations in inflammation and Ghrelin levels, as well as disease progression.

Results

A decrease in serum Ghrelin levels in AE patients, whereas both Ghrelin and GHSR, along with TLR4/MyD88/NF-κB pathway proteins and markers of M1/M2 macrophage polarization, exhibited increased expression in the inflammatory cell zones surrounding hepatic lesions. Similar findings were observed in E. multilocularis-infected mice. M1-type inflammatory expression predominated throughout the infection's progression, with sustained high levels of Ghrelin counteracting inflammation. The TLR4/ MyD88/NF-κB pathway remained suppressed during the first 8 weeks, becoming activated only at 12 weeks. Inhibition of the MyD88 pathway resulted in reduced inflammation levels and upregulated Ghrelin expression, thereby collaboratively regulating the progression of hepatic infection.

Conclusion

These findings suggest an interactive regulation between the MyD88 inflammatory signaling pathway and Ghrelin, indicating that MyD88 inhibition could enhance Ghrelin expression to modulate the progression of E. multilocularis infection.

Ghrelin is involved in regulating the progression of Echinococcus Granulosus-infected liver lesions through suppression of immunoinflammation and fibrosis

BACKGROUND

Cystic Echinococcosis (CE) is a zoonotic disease causing fibrosis and necrosis of diseased livers caused by infection with Echinococcus granulosus (E.g). There is evidence that E.g is susceptible to immune escape and tolerance when host expression of immunoinflammation and fibrosis is suppressed, accelerating the progression of CE. Ghrelin has the effect of suppressing immunoinflammation and fibrosis, and whether it is involved in regulating the progression of E.g-infected liver lesions is not clear.

METHODS

Serum and hepatic Ghrelin levels were observed in E.g-infected mice (4, 12 and 36 weeks) and compared with healthy control groups. Co-localization analysis is performed between protein expression of Ghrelin in and around the hepatic lesions of E.g-infected 12-week mice and protein expression of different hepatic histiocytes by mIHC. HepG2 cells and protoscoleces (PSCs) protein were co-cultured in vitro, as well as PSCs were alone in vitro, followed by exogenously administered of Ghrelin and its receptor blocker, [D-Lys3]-GHRP-6, to assess their regulatory effects on immunoinflammation, fibrosis and survival rate of PSCs.

RESULTS

Serum Ghrelin levels were increased in E.g-infected 4- and 12-week mice, and reduced in 36-week mice. E.g-infected mice consistently recruited Ghrelin in and around the hepatic lesions, which was extremely strongly co-localized with the protein expression of hepatic stellate cells (HSCs), T cells and the TGF-β1/Smad3 pathway. The secretion of Ghrelin was increased with increasing concentrations of PSCs protein in HepG2 cells culture medium. Moreover, Ghrelin could significantly inhibit the secretion of IL-2, INF-γ and TNF-α, as well as the expression of Myd88/NF-κB and TGF-β1/Smad3 pathway protein, and promoted the secretion of IL-4 and IL-10. Blocking Ghrelin receptor could significantly inhibit PSCs growth in in vitro experiment.

CONCLUSION

Ghrelin is highly expressed in the early stages of hepatic E.g infection and may be involved in regulating the progression of liver lesions by suppression immunoinflammation and fibrosis.

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.

Ghrelin regulating liver activity and its potential effects on liver fibrosis and Echinococcosis

Ghrelin widely exists in the central nervous system and peripheral organs, and has biological activities such as maintaining energy homeostasis, regulating lipid metabolism, cell proliferation, immune response, gastrointestinal physiological activities, cognition, memory, circadian rhythm and reward effects. In many benign liver diseases, it may play a hepatoprotective role against steatosis, chronic inflammation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress and apoptosis, and improve liver cell autophagy and immune response to improve disease progression. However, the role of Ghrelin in liver Echinococcosis is currently unclear. This review systematically summarizes the molecular mechanisms by which Ghrelin regulates liver growth metabolism, immune-inflammation, fibrogenesis, proliferation and apoptosis, as well as its protective effects in liver fibrosis diseases, and further proposes the role of Ghrelin in liver Echinococcosis infection. During the infectious process, it may promote the parasitism and survival of parasites on the host by improving the immune-inflammatory microenvironment and fibrosis state, thereby accelerating disease progression. However, there is currently a lack of targeted in vitro and in vivo experimental evidence for this viewpoint.

Molecular and cellular mechanisms underlying the hepatoprotective role of ghrelin against NAFLD progression

The underlying mechanisms for the development and progression of nonalcoholic fatty liver disease (NAFLD) are complex and multifactorial. Within the last years, experimental and clinical evidences support the role of ghrelin in the development of NAFLD. Ghrelin is a gut hormone that plays a major role in the short-term regulation of appetite and long-term regulation of adiposity. The liver constitutes a target for ghrelin, where this gut-derived peptide triggers intracellular pathways regulating lipid metabolism, inflammation, and fibrosis. Interestingly, circulating ghrelin levels are altered in patients with metabolic diseases, such as obesity, type 2 diabetes, and metabolic syndrome, which, in turn, are well-known risk factors for the pathogenesis of NAFLD. This review summarizes the molecular and cellular mechanisms involved in the hepatoprotective action of ghrelin, including the reduction of hepatocyte lipotoxicity via autophagy and fatty acid β-oxidation, mitochondrial dysfunction, endoplasmic reticulum stress and programmed cell death, the reversibility of the proinflammatory phenotype in Kupffer cells, and the inactivation of hepatic stellate cells. Together, the metabolic and inflammatory pathways regulated by ghrelin in the liver support its potential as a therapeutic target to prevent NAFLD in patients with metabolic disorders.

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.

Role of the Ghrelin System in Colitis and Hepatitis as Risk Factors for Inflammatory-Related Cancers

It is not known exactly what leads to the development of colorectal cancer (CRC) and hepatocellular carcinoma (HCC), but there are specific risk factors that increase the probability of their occurrence. The unclear pathogenesis, too-late diagnosis, poor prognosis as a result of high recurrence and metastasis rates, and repeatedly ineffective therapy of both cancers continue to challenge both basic science and practical medicine. The ghrelin system, which is comprised of ghrelin and alternative peptides (e.g., obestatin), growth hormone secretagogue receptors (GHS-Rs), and ghrelin-O-acyl-transferase (GOAT), plays an important role in the physiology and pathology of the gastrointestinal (GI) tract. It promotes various physiological effects, including energy metabolism and amelioration of inflammation. The ghrelin system plays a role in the pathogenesis of inflammatory bowel diseases (IBDs), which are well known risk factors for the development of CRC, as well as inflammatory liver diseases which can trigger the development of HCC. Colitis-associated cancer serves as a prototype of inflammation-associated cancers. Little is known about the role of the ghrelin system in the mechanisms of transformation of chronic inflammation to low- and high-grade dysplasia, and, finally, to CRC. HCC is also associated with chronic inflammation and fibrosis arising from different etiologies, including alcoholic and nonalcoholic fatty liver diseases (NAFLD), and/or hepatitis B (HBV) and hepatitis C virus (HCV) infections. However, the exact role of ghrelin in the progression of the chronic inflammatory lesions into HCC is still unknown. The aim of this review is to summarize findings on the role of the ghrelin system in inflammatory bowel and liver diseases in order to better understand the impact of this system on the development of inflammatory-related cancers, namely CRC and HCC.

Potential role of ghrelin in the regulation of inflammation

Several diseases are caused or progress due to inflammation. In the past few years, accumulating evidence suggests that ghrelin, a gastric hormone of 28-amino acid residue length, exerts protective effects against inflammation by modulating the related pathways. This review focuses on ghrelin's anti-inflammatory and potential therapeutic effects in neurological, cardiovascular, respiratory, hepatic, gastrointestinal, and kidney disorders. Ghrelin significantly alleviates excessive inflammation and reduces damage to different target organs mainly by reducing the secretion of inflammatory cytokines, including interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α), and inhibiting the nuclear factor kappa-B (NF-κB) and NLRP3 inflammasome signaling pathways. Ghrelin also regulates inflammation and apoptosis through the p38 MAPK/c-Jun N-terminal kinase (JNK) signaling pathway; restores cerebral microvascular integrity, and attenuates vascular leakage. Ghrelin activates the phosphoInositide-3 kinase (PI3K)/protein kinase B (Akt) pathway and inhibits inflammatory responses in cardiovascular diseases and acute kidney injury. Some studies show that ghrelin exacerbates colonic and intestinal manifestations of colitis. Interestingly, some inflammatory states, such as non-alcoholic steatohepatitis, inflammatory bowel diseases, and chronic kidney disease, are often associated with high ghrelin levels. Thus, ghrelin may be a potential new therapeutic target for inflammation-related diseases.

The effects of diet and chronic exercise on skeletal muscle ghrelin response

Background

Recent findings indicate that ghrelin, particularly the unacylated form (UnAG), acutely stimulates skeletal muscle fatty acid oxidation (FAO) and can preserve insulin signaling and insulin-stimulated glucose uptake in the presence of high concentrations of saturated fatty acids. However, we recently reported that the stimulatory effect of ghrelin on FAO and subsequent ability to protect insulin stimulated glucose uptake was lost following 6-weeks (6w) of chronic high fat feeding. In the current study we examined the effects of both short-term 5 day (5d) and chronic 6w high-fat diet (HFD) on muscle ghrelin response, and whether exercise training could prevent the development of muscle ghrelin resistance with 6w of HFD

Methods and Results

Soleus muscle strips were isolated from male rats to determine the direct effects of acylated (AG) and UnAG isoforms on FAO and glucose uptake. A 5d HFD did not alter the response of soleus muscle to AG or UnAG. Conversely, 6w of HFD was associated with a loss of ghrelin's ability to stimulate FAO and protect insulin stimulated glucose uptake. Muscle response to UnAG remained intact following the 6w HFD with chronic exercise training. Unexpectedly, muscle response to both AG and UnAG was also lost after 6w of low-fat diet (LFD) consumption. Protein content of the classic ghrelin receptor, GHS-R1a, was not affected by diet or training. Corticotropin-releasing hormone receptor-2 (CRF-2R) content, a putative receptor for ghrelin in muscle, was significantly decreased in soleus from 6w HFD-fed animals and increased following exercise training. This may explain the protection of UnAG response with training in HFD-fed rats but does not explain why ghrelin response was also lost in LFD-fed animals.

Conclusions

UnAG protects muscle glucose uptake during acute lipid oversupply, likely due to its ability to stimulate FAO. This effect is lost in 6w HFD-fed animals but protected with exercise training. Unexpectedly, ghrelin response was lost in 6w LFD-fed animals. The loss of ghrelin response in muscle with a LFD cannot be explained by a change in putative ghrelin receptor content. We believe that the sedentary nature of the animals is a major factor in the development of muscle ghrelin resistance and warrants further research.

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Ghrelin stimulates fatty acid oxidation in skeletal muscle.

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This stimulation is strongly associated with protection from acute fat overload.

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Prolonged sedentary behaviour and a high fat diet impair ghrelin's ability to stimulate fatty acid oxidation.

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Exercise training preserves ghrelin's positive effects on skeletal muscle.

Predictive and Diagnostic Value of Serum Adipokines in Pregnant Women with Intrahepatic Cholestasis

The objective of this study was to assess the value of serum leptin, adiponectin, apelin, and ghrelin as biomarkers for the prediction and diagnosis of intra-hepatic cholestasis (ICP). This prospective study included pregnant women in the third trimester of pregnancy: 63 with ICP, 48 and 15 of whom had mild and severe disease, respectively, and 32 as controls. ICP women had increased median levels of serum leptin, adiponectin, apelin, and ghrelin compared to the controls (p < 0.05). These biomarkers meaningfully changed regarding the severity of ICP: While leptin was reduced, apelin and ghrelin were increased, and adiponectin was increased somewhat. To predict and diagnose ICP, the predictive values of serum leptin, adiponectin, and apelin need to be accepted as comparable, with moderate to high sensitivity and specificity; however, the predictive value of serum ghrelin was somewhat lower. More research is needed to clarify the potential properties of adipokines to gain acceptance as a predictive or diagnostic biomarker for ICP.

Demonstration of the protective effect of ghrelin in the livers of rats with cisplatin toxicity

Despite the various and newly developed chemotherapeutic agents in recent years, cisplatin is still used very frequently as a chemotherapeutic agent, even though cisplatin has toxic effects on many organs. The aim of our study is to show whether ghrelin reduces the liver toxicity of cisplatin in the rat model. Twenty-eight male Sprague Dawley albino mature rats were chosen to be utilized in the study. Group 1 rats (n = 7) were taken as the control group, and no medication was given to them. Group 2 rats (n = 7) received 5 mg/kg/day cisplatin and 1 ml/kg/day of 0.9% NaCl, Group 3 rats (n = 7) received 5 mg/kg/day cisplatin and 10 ng/kg/day ghrelin, Group 4 rats (n = 7) received 5 mg/kg/day cisplatin and 20 ng/kg/day ghrelin for 3 days. Glutathione, malondialdehyde (MDA), superoxide dismutase (SOD), plasma alanine aminotransferase (ALT) levels, and liver biopsy results were measured in rats. It was determined that, especially in the high-dose group, the MDA, plasma ALT, and SOD levels increased less in the ghrelin group as compared to the cisplatin group, and the glutathione level decreased slightly with a low dose of ghrelin, while it increased with a higher dose. In histopathological examination, it was determined that the toxic effect of cisplatin on the liver was reduced with a low dose of ghrelin, and its histopathological appearance was similar to normal liver tissue when given a high dose of ghrelin. These findings show that ghrelin, especially in high doses, can be used to reduce the toxic effect of cisplatin.

Potentiality of ghrelin as antioxidant and protective agent

Background

Oxidative stress is the result of cellular troubles related to aerobic metabolism. Furthermore, this stress is always associated with biological responses evoked by physical, chemical, environmental, and psychological factors. Several studies have developed many approaches of antioxidant defense to diminish the severity of many diseases. Ghrelin was originally identified from the rat stomach, and it is a potent growth hormone-releasing peptide that has pleiotropic functions.

Methods

A systematic review was conducted within PubMed, ScienceDirect, MEDLINE, and Scopus databases using keywords such as ghrelin, antioxidant, oxidative stress, and systemic oxidative stress sensor.

Results

In the last decade, many studies show that ghrelin exhibits protection effects against oxidative stress derived probably from its antioxidant effects. Pieces of evidence demonstrate that systemic oxidative stress increase ghrelin levels in the plasma. The expression of ghrelin and its receptor in ghrelin peripheral tissues and extensively in the central nervous system suggests that this endogenous peptide plays an important role as a systemic oxidative stress sensor

Conclusion

The current evidence confirms that ghrelin and its derived peptides (Desacyl-ghrelin, obestatin) act as a protective antioxidant agent. Therefore, stressor modality, duration, and intensity are the parameters of oxidative stress that must be taken into consideration to determine the role of ghrelin, Desacyl-ghrelin, and obestatin in the regulation of cell death pathways.

Acylated Ghrelin as a Multi-Targeted Therapy for Alzheimer's and Parkinson's Disease

Much thought has been given to the impact of Amyloid Beta, Tau and Alpha-Synuclein in the development of Alzheimer's disease (AD) and Parkinson's disease (PD), yet the clinical failures of the recent decades indicate that there are further pathological mechanisms at work. Indeed, besides amyloids, AD and PD are characterized by the culminative interplay of oxidative stress, mitochondrial dysfunction and hyperfission, defective autophagy and mitophagy, systemic inflammation, BBB and vascular damage, demyelination, cerebral insulin resistance, the loss of dopamine production in PD, impaired neurogenesis and, of course, widespread axonal, synaptic and neuronal degeneration that leads to cognitive and motor impediments. Interestingly, the acylated form of the hormone ghrelin has shown the potential to ameliorate the latter pathologic changes, although some studies indicate a few complications that need to be considered in the long-term administration of the hormone. As such, this review will illustrate the wide-ranging neuroprotective properties of acylated ghrelin and critically evaluate the hormone's therapeutic benefits for the treatment of AD and PD.

Double burden of malnutrition in persons with obesity

A paradoxical double challenge has emerged in the last decades with respect to nutrition and nutrition-related clinical conditions. Hunger-related undernutrition continues to represent an unacceptable burden, although its prevalence has been encouragingly reduced worldwide. On the other hand, the prevalence of overweight and obesity, defined as fat excess accumulation with negative impact on individual health, has dramatically increased due to increasingly pervasive obesogenic lifestyle changes. Undernutrition and obesity may coexist in world regions, Countries and even smaller communities and households, being referred to as double burden of malnutrition. It is however important to point out that fat accumulation and obesity may also induce additional nutritional derangements in affected individuals, both directly through metabolic and body composition changes and indirectly through acute and chronic diseases with negative impact on nutritional status. In the current narrative review, associations between fat accumulation in obesity and malnutrition features as well as their known causes will be reviewed and summarized. These include risk of loss of skeletal muscle mass and function (sarcopenia) that may allow for malnutrition diagnosis also in overweight and obese individuals, thereby introducing a new clinically relevant perspective to the obesity-related double burden of malnutrition concept.

Ghrelin and liver disease

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are two of the most common liver diseases associated with obesity, type 2 diabetes and metabolic syndrome. The prevalence of these conditions are increasingly rising and presently there is not a pharmacological option available in the market. Elucidation of the mechanism of action and the molecular underpinnings behind liver disease could help to better understand the pathophysiology of these illnesses. In this sense, in the last years modulation of the ghrelin system in preclinical animal models emerge as a promising therapeutic tool. In this review, we compile the latest knowledge of the modulation of ghrelin system and its intracellular pathways that regulates lipid metabolism, hepatic inflammation and liver fibrosis. We also describe novel processes implicated in the regulation of liver disease by ghrelin, such as autophagy or dysregulated circadian rhythms. In conclusion, the information displayed in this review support that the ghrelin system could be an appealing strategy for the treatment of liver disease.

Perinatal over- and underfeeding affect hypothalamic leptin and ghrelin neuroendocrine responses in adult rats

BACKGROUND

Changes in the nutritional supply during the perinatal period can lead to metabolic disturbances and obesity in adulthood.

OBJECTIVE

The divergent litter size model was used to investigate the hypothalamic sensitivity to leptin and ghrelin as well as the mechanisms involved in the disruption of food intake and energy expenditure.

METHODS

On postnatal day 3 (P3), male Wistar rats were divided into 3 groups: small litter (SL - 3 pups), normal litter (NL - 10 pups), and large litter (LL - 16 pups). Animals at P60 were intraperitoneally treated with leptin (500 µg/Kg), ghrelin (40 µg/Kg), or vehicle (0.9% NaCl) at 5 pm and the following parameters were assessed: food intake and body weight; immunostaining of p-STAT-3 in the hypothalamus; Western Blotting analysis of p-AMPKα and UCP2 in the mediobasal hypothalamus (MBH), and UCP1 in the interscapular brown adipose tissue (BAT); or heat production, VO₂, VCO₂, and locomotor activity.

RESULTS

SL rats had earlier leptin and ghrelin surges, while LL rats had no variations. At P60, after leptin treatment, LL rats showed hypophagia and increased p-STAT-3 expression in the arcuate nucleus, but SL rats had no response. After ghrelin treatment, LL rats did not have the orexigenic response or AMPKα phosphorylation in the MBH, while SL animals, unexpectedly, decreased body weight gain, without changes in food intake, and increased metabolic parameters and UCP1 expression in the BAT.

CONCLUSIONS

Changes in the nutritional supply at early stages of life modify leptin and ghrelin responsiveness in adulthood, programming metabolic and central mechanisms, which contribute to overweight and obesity in adulthood.

Ghrelin forms in the modulation of energy balance and metabolism

Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. This narrative review aims at presenting current emerging knowledge on the impact of ghrelin forms on energy balance and metabolism. AG represents ~ 10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. Moreover, other metabolic AG-induced effects have been reported, including the modulation of glucose homeostasis with stimulation of liver gluconeogenesis, the increase of fat mass and the improvement of skeletal muscle mitochondrial function. On the other hand, UnAG has no orexigenic effects, however recent reports have shown that it is directly involved in the modulation of skeletal muscle energy metabolism by improving a cluster of interlinked functions including mitochondrial redox activities, tissue inflammation and insulin signalling and action. These findings are in agreement with human studies which show that UnAG circulating levels are positively associated with insulin sensitivity both in metabolic syndrome patients and in a large cohort from the general population. Moreover, ghrelin acylation is regulated by a nutrient sensor mechanism, specifically set on fatty acids availability. These recent findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. While a specific receptor for UnAG still needs to be identified, recent evidence strongly supports the hypothesis that the modulation of ghrelin-related molecular pathways, including those involved in its acylation, may be a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.Level of evidence Level V, narrative review.

Role of ghrelin isoforms in the mitigation of hepatic inflammation, mitochondrial dysfunction, and endoplasmic reticulum stress after bariatric surgery in rats

BACKGROUND/OBJECTIVES

Bariatric surgery improves nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH), but the underlying mechanisms remain elusive. We evaluated the potential role of ghrelin isoforms in the amelioration of hepatic inflammation after sleeve gastrectomy and Roux-en-Y gastric bypass (RYGB).

SUBJECTS/METHODS

Plasma ghrelin isoforms were measured in male Wistar rats (n = 129) subjected to surgical (sham operation, sleeve gastrectomy, or RYGB) or dietary interventions [fed ad libitum a normal (ND) or a high-fat diet (HFD) or pair-fed diet]. The effect of acylated and desacyl ghrelin on markers of inflammation, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress in primary rat hepatocytes under palmitate-induced lipotoxic conditions was assessed.

RESULTS

Plasma desacyl ghrelin was decreased after sleeve gastrectomy and RYGB, whereas the acylated/desacyl ghrelin ratio was augmented. Both surgeries diminished obesity-associated hepatic steatosis, CD68⁺- and apoptotic cells, proinflammatory JNK activation, and Crp, Tnf, and Il6 transcripts. Moreover, a postsurgical amelioration in the mitochondrial DNA content, oxidative phosphorylation (OXPHOS) complexes I and II, and ER stress markers was observed. Specifically, following bariatric surgery GRP78, spliced XBP-1, ATF4, and CHOP levels were reduced, as were phosphorylated eIF2α. Interestingly, acylated and desacyl ghrelin inhibited steatosis and inflammation of palmitate-treated hepatocytes in parallel to an upregulation of OXPHOS complexes II, III, and V, and a downregulation of ER stress transducers IRE1α, PERK, ATF6, their downstream effectors, ATF4 and CHOP, as well as chaperone GRP78.

CONCLUSIONS

Our data suggest that the increased relative acylated ghrelin levels after bariatric surgery might contribute to mitigate obesity-associated hepatic inflammation, mitochondrial dysfunction, and ER stress.

Acylated ghrelin induces but deacylated ghrelin prevents hepatic steatosis and insulin resistance in lean rats: Effects on DAG/ PKC/JNK pathway

This study investigated the molecular effects of acylated (AG) and unacylated ghrelin (UAG) or their combination on hepatic lipogenesis pathways and DAG/PKC/JNK signaling in the livers of lean rats fed standard diet. Male rats (n = 10) were classified as control + vehicle (saline, 200 μl), AG, UAG, and AG + UAG-treated groups. All treatments were given at final doses of 200 ng/kg of for 14 days (twice/day, S.C). Administration of AG significantly enhanced circulatory levels of AG and UAG turning the normal ratio of AG/UAG from 1:2.5 to 1:1.2. However, while UAG didn't affect circulatory levels of AG, administration of UAG alone or in combination with AG resulted in AG/UAG ratios of 1:7 and 1:3, respectively. Independent of food intake nor the development of peripheral IR, AG increased hepatic DAG, TGs and CHOL contents and induced hepatic IR. Mechanism of action include 1) upregulation of mRNA and protein levels of DGAT-2 and mtGPAT-1, SREBP-1 and SCD-1, and 2) inhibition of fatty acids (FAs) oxidation mediated by inhibition of AMPK/ PPAR-α/CPT-1 axis. Consequently, AG induced membranous translocation of PKCδ and PKCε leading to activation of JNK and significant inhibition of insulin signaling under basal and insulin stimulation as evident by decreases in the phosphorylation levels of IRS (Tyr⁶¹²) and Akt (Thr³¹⁸) and increased phosphorylation of IRS (Ser³⁰⁷). However, while UAG only activated FAs oxidation in control rats, it reversed all alterations in all measured biochemical endpoints seen in the AG-treated group, when administered in combination with AG, leading to significant decreases in hepatic fat accumulation and prevention of hepatic IR. In conclusion, while exogenous administration of AG is at high risk of developing steatohepatitis and hepatic IR, co-administration of a balanced dose of UAG reduces this risk and inhibits hepatic lipid accumulation and enhance hepatic insulin signaling.

Inhibition of ghrelin o-acyltransferase attenuated lipotoxicity by inducing autophagy via AMPK-mTOR pathway

Background

Nonalcoholic fatty liver disease (NAFLD) has been considered the most commonly occurring chronic hepatopathy in the world. Ghrelin o-acyltransferase (GOAT) is an acylation enzyme which has an acylated position 3 serine on ghrelin. Recent investigation revealed that activated autophagy could attenuate liver steatosis. The aim of this study was to explore therapeutic roles that inhibit GOAT exerted in NAFLD, and its potential association with autophagy.

Materials and methods

Human LO2 cells were pretreated with siRNA-GOAT to induce liver steatosis using free fatty acids (FFAs). A chronic NAFLD model was established by feeding male mice C57bl/6 with high-fat diet (HFD) for 56 days with GO-CoA-Tat administrated subcutaneously. Lipid droplets were identified by Oil Red O stains. Body weight (BW) of mice was measured every week. Autophagy, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), serum biochemical indicators (glucose [Glu], total cholesterol [TC], triglyceride [TG], aspartate aminotransferase [AST], alanine aminotransferase [ALT]) and signaling pathway proteins of phosphorylated AMPK–mTOR were measured.

Results

The TG contents of the FFA and HFD groups were decreased by the inhibition of GOAT. Among mice treated with GO-CoA-Tat and siRNA-GOAT, IL-6 and TNF-α concentrations were remarkably decreased. Indicators of liver injury such as ALT and AST were also remarkably decreased among mice treated with GO-CoA-Tat. Likewise, GO-CoA-Tat significantly reduced the BW of mice and serum TG, TC and Glu. Autophagy was induced along with reduced lipids in the cells of the FFA and HFD groups. The inhibition of GOAT upregulated autophagy via AMPK–mTOR restoration.

Conclusion

These results indicate that the inhibition of GOAT attenuates lipotoxicity by autophagy stimulation via AMPK–mTOR restoration and offers innovative evidence for using GO-CoA-Tat or siRNA-GOAT in NAFLD clinically.

Ghrelin deletion protects against age-associated hepatic steatosis by downregulating the C/EBPα-p300/DGAT1 pathway

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. NAFLD usually begins as low‐grade hepatic steatosis which further progresses in an age‐dependent manner to nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma in some patients. Ghrelin is a hormone known to promote adiposity in rodents and humans, but its potential role in hepatic steatosis is unknown. We hypothesized that genetic ghrelin deletion will protect against the development of age‐related hepatic steatosis. To examine this hypothesis, we utilized ghrelin knockout (KO) mice. Although no different in young animals (3 months old), we found that at 20 months of age, ghrelin KO mice have significantly reduced hepatic steatosis compared to aged‐matched wild‐type (WT) mice. Examination of molecular pathways by which deletion of ghrelin reduces steatosis showed that the increase in expression of diacylglycerol O‐acyltransferase‐1 (DGAT1), one of the key enzymes of triglyceride (TG) synthesis, seen with age in WT mice, is not present in KO mice. This was due to the lack of activation of CCAAT/enhancer binding protein‐alpha (C/EBPα) protein and subsequent reduction of C/EBPα‐p300 complexes. These complexes were abundant in livers of old WT mice and were bound to and activated the DGAT1 promoter. However, the C/EBPα‐p300 complexes were not detected on the DGAT1 promoter in livers of old KO mice resulting in lower levels of the enzyme. In conclusion, these studies demonstrate the mechanism by which ghrelin deletion prevents age‐associated hepatic steatosis and suggest that targeting this pathway may offer therapeutic benefit for NAFLD.

Microbial Mechanistic Insight into the Role of Inulin in Improving Maternal Health in a Pregnant Sow Model

General consumption of “western diet” characterized by high refined carbohydrates, fat and energy intake has resulted in a global obesity epidemics and related metabolic disturbance even for pregnant women. Pregnancy process is accompanied by substantial hormonal, metabolic and immunological changes during which gut microbiota is also remarkably remodeled. Dietary fiber has been demonstrated to have a striking role in shifting the microbial composition so as to improve host metabolism and health in non-pregnant individuals. The present study was conducted to investigate effects of adding a soluble dietary fiber inulin (0 or 1.5%) to low- or high- fat (0 or 5% fat addition) gestational diet on maternal and neonatal health and fecal microbial composition in a sow model. Results showed that inulin addition decreased the gestational body weight gain and fat accumulation induced by fat addition. Circulating concentrations of pro-inflammatory cytokine IL-6, adipokine leptin and chemerin were decreased by inulin supplementation. Inulin addition remarkably reduced the average BMI of newborn piglets and the within litter BMI distributions (%) ranging between 17 and 20 kg/m², and increased the BMI distribution ranging between 14 and 17 kg/m². 16S rRNA gene sequencing of the V3-V4 region showed that fecal microbial changes at different taxonomic levels triggered by inulin addition predisposed the pregnant sow to be thinner and lower inflammatory. Meanwhile, fecal microbial composition was also profoundly altered by gestation stage with distinct changes occurring at perinatal period. Most representative volatile fatty acid (VFA) producing-related genera changed dramatically when reaching the perinatal period and varied degrees of increases were detected with inulin addition. Fecal VFA concentrations failed to show any significant effect with dietary intervention, however, were markedly increased at perinatal period. Our findings indicate that positive microbial changes resulted by 1.5% soluble fiber inulin addition would possibly be the potential mechanisms under which maternal body weight, metabolic and inflammatory status and neonatal BMI were improved. Besides, distinct changes of microbial community at perinatal period indicated the mother sow is undergoing a catabolic state with increased energy loss and inflammation response at that period compared with other stages of gestation.

Therapeutic Potential of Targeting the Ghrelin Pathway

Ghrelin was discovered in 1999 as the endogenous ligand of the growth-hormone secretagogue receptor 1a (GHSR1a). Since then, ghrelin has been found to exert a plethora of physiological effects that go far beyond its initial characterization as a growth hormone (GH) secretagogue. Among the numerous well-established effects of ghrelin are the stimulation of appetite and lipid accumulation, the modulation of immunity and inflammation, the stimulation of gastric motility, the improvement of cardiac performance, the modulation of stress, anxiety, taste sensation and reward-seeking behavior, as well as the regulation of glucose metabolism and thermogenesis. Due to a variety of beneficial effects on systems’ metabolism, pharmacological targeting of the endogenous ghrelin system is widely considered a valuable approach to treat metabolic complications, such as chronic inflammation, gastroparesis or cancer-associated anorexia and cachexia. The aim of this review is to discuss and highlight the broad pharmacological potential of ghrelin pathway modulation for the treatment of anorexia, cachexia, sarcopenia, cardiopathy, neurodegenerative disorders, renal and pulmonary disease, gastrointestinal (GI) disorders, inflammatory disorders and metabolic syndrome.

An Evaluation of Acylated Ghrelin and Obestatin Levels in Childhood Obesity and Their Association with Insulin Resistance, Metabolic Syndrome, and Oxidative Stress

Background: Ghrelin is a 28-amino acid peptide with an orexigenic property, which is predominantly produced by the stomach. Acylated ghrelin is the active form of this hormone. Obestatin is a 23-amino acid peptide which is produced by post-translational modification of a protein precursor that also produces ghrelin. Obestatin has the opposite effect of ghrelin on food intake. The aim of this study was to evaluate acylated ghrelin and obestatin levels and their ratio in obese and normal-weight children and adolescents, and their association with metabolic syndrome (MetS) parameters. Methods: Serum acyl-ghrelin, obestatin, leptin, insulin, fasting plasma glucose (FPG), lipid profile, and malondialdehyde (MDA) were evaluated in 73 children and adolescents (42 obese and 31 control). Insulin resistance was calculated by a homeostasis model assessment of insulin resistance (HOMA-IR). MetS was determined according to IDF criteria. Results: Acyl-ghrelin levels were significantly lower in obese subjects compared to the control group and lower in obese children with MetS compared to obese subjects without MetS. Obestatin was significantly higher in obese subjects compared to that of the control, but it did not differ significantly among those with or without MetS. Acyl-ghrelin to obestatin ratio was significantly lower in obese subjects compared to that in normal subjects. Acyl-ghrelin showed significant negative and obestatin showed significant positive correlations with body mass index (BMI), BMI Z-score, leptin, insulin, and HOMA-IR. Acyl-ghrelin had a significant negative correlation with MDA as an index of oxidative stress. Conclusion: Ghrelin is decreased and obestatin is elevated in obesity. Both of these hormones are associated with insulin resistance, and ghrelin is associated with oxidative stress. The balance between ghrelin and obestatin seems to be disturbed in obesity.

Prohibitin-induced, obesity-associated insulin resistance and accompanying low-grade inflammation causes NASH and HCC

Obesity increases the risk for nonalcoholic steatohepatitis (NASH) and hepatocarcinogenesis. However, the underlying mechanisms involved in the disease process remain unclear. Recently, we have developed a transgenic obese mouse model (Mito-Ob) by prohibitin mediated mitochondrial remodeling in adipocytes. The Mito-Ob mice develop obesity in a sex-neutral manner, but obesity-associated adipose inflammation and metabolic dysregulation in a male sex-specific manner. Here we report that with aging, the male Mito-Ob mice spontaneously develop obesity-linked NASH and hepatocellular carcinoma (HCC). In contrast, the female Mito-Ob mice maintained normal glucose and insulin levels and did not develop NASH and HCC. The anti-inflammatory peptide ghrelin was significantly upregulated in the female mice and down regulated in the male mice compared with respective control mice. In addition, a reduction in the markers of mitochondrial content and function was found in the liver of male Mito-Ob mice with NASH/HCC development. We found that ERK1/2 signaling was significantly upregulated whereas STAT3 signaling was significantly down regulated in the tumors from Mito-Ob mice. These data provide a proof-of-concept that the metabolic and inflammatory status of the adipose tissue and their interplay at the systemic and hepatic level play a central role in the pathogenesis of obesity-linked NASH and HCC.

Hypothalamic inflammation and food intake regulation during chronic illness

Anorexia is a common symptom in chronic illness. It contributes to malnutrition and strongly affects survival and quality of life. A common denominator of many chronic diseases is an elevated inflammatory status, which is considered to play a pivotal role in the failure of food-intake regulating systems in the hypothalamus. In this review, we summarize findings on the role of hypothalamic inflammation on food intake regulation involving hypothalamic neuropeptide Y (NPY) and pro-opiomelanocortin (POMC). Furthermore, we outline the role of serotonin in the inability of these peptide based food-intake regulating systems to respond and adapt to changes in energy metabolism during chronic disease.

Antifibrotic activity of acylated and unacylated ghrelin

Fibrosis can affect almost all tissues and organs, it often represents the terminal stage of chronic diseases, and it is regarded as a major health issue for which efficient therapies are needed. Tissue injury, by inducing necrosis/apoptosis, triggers inflammatory response that, in turn, promotes fibroblast activation and pathological deposition of extracellular matrix. Acylated and unacylated ghrelin are the main products of the ghrelin gene. The acylated form, through its receptor GHSR-1a, stimulates appetite and growth hormone (GH) release. Although unacylated ghrelin does not bind or activate GHSR-1a, it shares with the acylated form several biological activities. Ghrelin peptides exhibit anti-inflammatory, antioxidative, and antiapoptotic activities, suggesting that they might represent an efficient approach to prevent or reduce fibrosis. The aim of this review is to summarize the available evidence regarding the effects of acylated and unacylated ghrelin on different pathologies and experimental models in which fibrosis is a predominant characteristic.

Differential effects of laparoscopic sleeve gastrectomy and laparoscopic gastric bypass on appetite, circulating acyl-ghrelin, peptide YY3-36 and active GLP-1 levels in non-diabetic humans

Laparoscopic Roux-en-Y gastric bypass (LRYGBP) reduces appetite and induces significant and sustainable weight loss. Circulating gut hormones changes engendered by LRYGBP are implicated in mediating these beneficial effects. Laparoscopic sleeve gastrectomy (LSG) is advocated as an alternative to LRYGBP, with comparable short-term weight loss and metabolic outcomes. LRYGBP and LSG are anatomically distinct procedures causing differential entero-endocrine cell nutrient exposure and thus potentially different gut hormone changes. Studies reporting the comparative effects of LRYGBP and LSG on appetite and circulating gut hormones are controversial, with no data to date on the effects of LSG on circulating peptide YY3-36 (PYY3-36) levels, the specific PYY anorectic isoform. In this study, we prospectively investigated appetite and gut hormone changes in response to LRYGBP and LSG in adiposity-matched non-diabetic patients. Anthropometric indices, leptin, fasted and nutrient-stimulated acyl-ghrelin, active glucagon-like peptide-1 (GLP-1), PYY3-36 levels and appetite were determined pre-operatively and at 6 and 12 weeks post-operatively in obese, non-diabetic females, with ten undergoing LRYGBP and eight adiposity-matched females undergoing LSG. LRYGBP and LSG comparably reduced adiposity. LSG decreased fasting and post-prandial plasma acyl-ghrelin compared to pre-surgery and to LRYGBP. Nutrient-stimulated PYY3-36 and active GLP-1 concentrations increased post-operatively in both groups. However, LRYGBP induced greater, more sustained PYY3-36 and active GLP-1 increments compared to LSG. LRYGBP suppressed fasting hunger compared to LSG. A similar increase in post-prandial fullness was observed post-surgery following both procedures. LRYGBP and LSG produced comparable enhanced satiety and weight loss. However, LSG and LRYGBP differentially altered gut hormone profiles.

The opposing effects of ghrelin on hypothalamic and systemic inflammatory processes are modulated by its acylation status and food intake in male rats

Ghrelin is an endogenous hormone that stimulates appetite and adipose tissue accrual. Both the acylated (AG) and non-acylated (DAG) isoforms of this hormone are also reported to exert anti-inflammatory and protective effects systemically and in the central nervous system. As inflammatory processes have been implicated in obesity-associated secondary complications, we hypothesized that this natural appetite stimulator may protect against negative consequences resulting from excessive food intake. Adult male Wistar rats were treated icv (5 μg/day) with AG, DAG, the ghrelin mimetic GH-releasing peptide (GHRP)-6, AG, and pair-fed with controls (AG-pf) or saline for 14 days. Regardless of food intake AG increased visceral adipose tissue (VAT) and decreased circulating cytokine levels. However, AG reduced cytokine production in VAT only in rats fed ad libitum. Hypothalamic cytokine production was increased in AG-treated rats fed ad libitum and by DAG, but intracellular inflammatory signaling pathways associated with insulin and leptin resistance were unaffected. Gliosis was not observed in response to any treatment as glial markers were either reduced or unaffected. AG, DAG, and GHRP-6 stimulated production of hypothalamic insulin like-growth factor I that is involved in cell protective mechanisms. In hypothalamic astrocyte cell cultures AG decreased tumor necrosis factorα and DAG decreased interleukin-1β mRNA levels, suggesting direct anti-inflammatory effects on astrocytes. Thus, whereas ghrelin stimulates food intake and weight gain, it may also induce mechanisms of cell protection that help to detour or delay systemic inflammatory responses and hypothalamic gliosis due to excess weight gain, as well as its associated pathologies.

Ghrelin gene products in acute and chronic inflammation

Ghrelin gene products--the peptides ghrelin, unacylated ghrelin, and obestatin--have several actions on the immune system, opening new perspectives within neuroendocrinology, metabolism and inflammation. The aim of this review is to summarize the available evidence regarding the less known role of these peptides in the machinery of inflammation and autoimmunity, outlining some of their most promising therapeutic applications.

Inhibition of cisplatin-induced lipid catabolism and weight loss by ghrelin in male mice

Cachexia, defined as an involuntary weight loss ≥ 5%, is a serious and dose-limiting side effect of chemotherapy that decreases survival in cancer patients. Alterations in lipid metabolism are thought to cause the lipodystrophy commonly associated with cachexia. Ghrelin has been proposed to ameliorate the alterations in lipid metabolism due to its orexigenic and anabolic properties. However, the mechanisms of action through which ghrelin could potentially ameliorate chemotherapy-associated cachexia have not been elucidated. The objectives of this study were to identify mechanisms by which the chemotherapeutic agent cisplatin alters lipid metabolism and to establish the role of ghrelin in reversing cachexia. Cisplatin-induced weight and fat loss were prevented by ghrelin. Cisplatin increased markers of lipolysis in white adipose tissue (WAT) and of β-oxidation in liver and WAT and suppressed lipogenesis in liver, WAT, and muscle. Ghrelin prevented the imbalance between lipolysis, β-oxidation, and lipogenesis in WAT and muscle. Pair-feeding experiments demonstrated that the effects of cisplatin and ghrelin on lipogenesis, but not on lipolysis and β-oxidation, were due to a reduction in food intake. Thus, ghrelin prevents cisplatin-induced weight and fat loss by restoring adipose tissue functionality. An increase in caloric intake further enhances the anabolic effects of ghrelin.