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

A long-acting LEAP2 analog reduces hepatic steatosis and inflammation and causes marked weight loss in mice

OBJECTIVE

The number of individuals affected by metabolic dysfunction associated fatty liver disease [1] is on the rise, yet hormonal contributors to the condition remain incompletely described and only a single FDA-approved treatment is available. Some studies suggest that the hormones ghrelin and LEAP2, which act as agonist and antagonist/inverse agonist, respectively, for the G protein coupled receptor GHSR, may influence the development of MAFLD. For instance, ghrelin increases hepatic fat whereas synthetic GHSR antagonists do the opposite. Also, hepatic steatosis is less prominent in standard chow-fed ghrelin-KO mice but more prominent in 42% high-fat diet-fed female LEAP2-KO mice.

METHODS

Here, we sought to determine the therapeutic potential of a long-acting LEAP2 analog (LA-LEAP2) to treat MAFLD in mice. LEAP2-KO and wild-type littermate mice were fed a Gubra-Amylin-NASH (GAN) diet for 10 or 40 wks, with some randomized to an additional 28 or 10 days of GAN diet, respectively, while treated with LA-LEAP2 vs Vehicle. Various metabolic parameters were followed and biochemical and histological assessments of MAFLD were made.

RESULTS

Among the most notable metabolic effects, daily LA-LEAP2 administration to both LEAP2-KO and wild-type littermates during the final 4 wks of a 14 wk-long GAN diet challenge markedly reduced liver weight, hepatic triglycerides, plasma ALT, hepatic microvesicular steatosis, hepatic lobular inflammation, NASH activity scores, and prevalence of higher-grade fibrosis. These changes were accompanied by prominent reductions in body weight, without effects on food intake, and reduced plasma total cholesterol. Daily LA-LEAP2 administration during the final 10 d of a 41.5 wk-long GAN diet challenge also reduced body weight, plasma ALT, and plasma total cholesterol in LEAP2-KO and wild-type littermates and prevalence of higher grade fibrosis in LEAP2-KO mice.

CONCLUSIONS

Administration of LA-LEAP2 to mice fed a MAFLD-prone diet markedly improves several facets of MAFLD, including hepatic steatosis, hepatic lobular inflammation, higher-grade hepatic fibrosis, and transaminitis. These changes are accompanied by prominent reductions in body weight and lowered plasma total cholesterol. Taken together, these data suggest that LEAP2 analogs such as LA-LEAP2 hold promise for the treatment of MAFLD and obesity.

Ghrelin Induces the Production of Hypothalamic NPY Through the AMPK-mTOR Pathway to Alleviate Cancer-induced Bone Pain

BACKGROUND/AIM

Cancer-induced bone pain (CIBP) is one of the most common symptoms of bone metastasis of tumor cells. The hypothalamus may play a pivotal role in the regulation of CIBP. However, little is known about the exact mechanisms.

MATERIALS AND METHODS

First, we established a CIBP model to explore the relationship among hypothalamic ghrelin, NPY and CIBP. Then, we exogenously administered NPY and NPY receptor antagonists to investigate whether hypothalamic NPY exerted an antinociceptive effect through binding to NPY receptors. Finally, we exogenously administered ghrelin to investigate whether ghrelin alleviated CIBP by inducing the production of hypothalamic NPY through the AMPK-mTOR pathway. Body weight, food intake and behavioral indicators of CIBP were measured every 3 days. Hypothalamic ghrelin, NPY and the AMPK-mTOR pathway were also measured.

RESULTS

The expression of hypothalamic ghrelin and NPY was simultaneously decreased in cancer-bearing rats, which was accompanied by CIBP. Intracerebroventricular (i.c.v.) administration of NPY significantly alleviated CIBP in the short term. The antinociceptive effect of NPY was reversed with the i.c.v. administration of the Y1R and Y2R antagonists. The administration of ghrelin activated the AMPK-mTOR pathway and induced hypothalamic NPY production to alleviate CIBP. This effect of ghrelin on NPY and antinociception was reversed with the administration of a GHS-R1α antagonist.

CONCLUSION

Ghrelin could induce the production of hypothalamic NPY through the AMPK-mTOR pathway to alleviate CIBP, which can provide a novel therapeutic mechanism for CIBP.

Dysfunction of autophagy in high-fat diet-induced non-alcoholic fatty liver disease

ABBREVIATIONS

ACOX1: acyl-CoA oxidase 1; ADH5: alcohol dehydrogenase 5 (class III), chi polypeptide; ADIPOQ: adiponectin, C1Q and collagen domain containing; ATG: autophagy related; BECN1: beclin 1; CRTC2: CREB regulated transcription coactivator 2; ER: endoplasmic reticulum; F2RL1: F2R like trypsin receptor 1; FA: fatty acid; FOXO1: forkhead box O1; GLP1R: glucagon like peptide 1 receptor; GRK2: G protein-coupled receptor kinase 2; GTPase: guanosine triphosphatase; HFD: high-fat diet; HSCs: hepatic stellate cells; HTRA2: HtrA serine peptidase 2; IRGM: immunity related GTPase M; KD: knockdown; KDM6B: lysine demethylase 6B; KO: knockout; LAMP2: lysosomal associated membrane protein 2; LAP: LC3-associated phagocytosis; LDs: lipid droplets; Li KO: liver-specific knockout; LSECs: liver sinusoidal endothelial cells; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAP3K5: mitogen-activated protein kinase kinase kinase 5; MED1: mediator complex subunit 1; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin complex 1; NAFLD: non-alcoholic fatty liver disease; NASH: non-alcoholic steatohepatitis; NFE2L2: NFE2 like bZIP transcription factor 2; NOS3: nitric oxide synthase 3; NR1H3: nuclear receptor subfamily 1 group H member 3; OA: oleic acid; OE: overexpression; OSBPL8: oxysterol binding protein like 8; PA: palmitic acid; RUBCNL: rubicon like autophagy enhancer; PLIN2: perilipin 2; PLIN3: perilipin 3; PPARA: peroxisome proliferator activated receptor alpha; PRKAA2/AMPK: protein kinase AMP-activated catalytic subunit alpha 2; RAB: member RAS oncogene family; RPTOR: regulatory associated protein of MTOR complex 1; SCD: stearoyl-CoA desaturase; SIRT1: sirtuin 1; SIRT3: sirtuin 3; SNARE: soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SQSTM1/p62: sequestosome 1; SREBF1: sterol regulatory element binding transcription factor 1;SREBF2: sterol regulatory element binding transcription factor 2; STING1: stimulator of interferon response cGAMP interactor 1; STX17: syntaxin 17; TAGs: triacylglycerols; TFEB: transcription factor EB; TP53/p53: tumor protein p53; ULK1: unc-51 like autophagy activating kinase 1; VMP1: vacuole membrane protein 1.

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.

Taurine Protects against the Fatty Liver Hemorrhagic Syndrome in Laying Hens through the Regulation of Mitochondrial Homeostasis

Metabolic-associated fatty liver disease (MAFLD) is a chronic liver disease caused by fat deposition in the liver of humans and mammals, while fatty liver hemorrhagic syndrome (FLHS) is a fatty liver disease in laying hens which can increase the mortality and cause severe economic losses to the laying industry. Increasing evidence has shown a close relationship between the occurrence of fatty liver disease and the disruption of mitochondrial homeostasis. Studies have proven that taurine can regulate hepatic fat metabolism, reduce hepatic fatty deposition, inhibit oxidative stress, and alleviate mitochondrial dysfunction. However, the mechanisms by which taurine regulates mitochondrial homeostasis in hepatocytes need to be further studied. In this study, we determined the effects and mechanisms of taurine on high-energy low-protein diet-induced FLHS in laying hens and in cultured hepatocytes in free fatty acid (FFA)-induced steatosis. The liver function, lipid metabolism, antioxidant capacity, mitochondrial function, mitochondrial dynamics, autophagy, and biosynthesis were detected. The results showed impaired liver structure and function, mitochondrial damage and dysfunction, lipid accumulation, and imbalance between mitochondrial fusion and fission, mitochondrial autophagy, and biosynthesis in both FLHS hens and steatosis hepatocytes. Taurine administration can significantly inhibit the occurrence of FLHS, protect mitochondria in hepatocytes from disease induced by lipid accumulation and FFA, up-regulate the expression levels of Mfn1, Mfn2, Opa1, LC3I, LC3II, PINK1, PGC-1α, Nrf1, Nrf2, and Tfam, and down-regulate the expression levels of Fis1, Drp1, and p62. In conclusion, taurine can protect laying hens from FLHS through the regulation of mitochondrial homeostasis, including the regulation of mitochondrial dynamics, autophagy, and biosynthesis.

Tactics with Prebiotics for the Treatment of Metabolic Dysfunction-Associated Fatty Liver Disease via the Improvement of Mitophagy

Mitophagy/autophagy plays a protective role in various forms of liver damage, by renovating cellular metabolism linking to sustain liver homeostasis. A characterized pathway for mitophagy is the phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1)/Parkin-dependent signaling pathway. In particular, PINK1-mediated mitophagy could play an indispensable role in improving the metabolic dysfunction-associated fatty liver disease (MAFLD) which could precede to steatohepatitis (NASH), fibrosis, and hepatocellular carcinoma. In addition, the PI3K/AKT/mTOR pathway might regulate the various characteristics of cellular homeostasis including energy metabolism, cell proliferation, and/or cell protection. Therefore, targeting mitophagy with the alteration of PI3K/AKT/mTOR or PINK1/Parkin-dependent signaling to eliminate impaired mitochondria might be an attractive strategy for the treatment of MAFLD. In particular, the efficacy of prebiotics for the treatment of MAFLD has been suggested to be useful via the modulation of the PI3K/AKT/mTOR/AMPK pathway. Additionally, several edible phytochemicals could activate mitophagy for the improvement of mitochondrial damages, which could also be a promising option to treat MAFLD with providing liver protection. Here, the potential therapeutics with several phytochemicals has been discussed for the treatment of MAFLD. Tactics with a viewpoint of prospective probiotics might contribute to the development of therapeutic interventions.

Role of the ghrelin system in alcohol use disorder and alcohol-associated liver disease: A narrative review

Unhealthy alcohol consumption is a global health problem. Adverse individual, public health, and socioeconomic consequences are attributable to harmful alcohol use. Epidemiological studies have shown that alcohol use disorder (AUD) and alcohol-associated liver disease (ALD) are the top two pathologies among alcohol-related diseases. Consistent with the major role that the liver plays in alcohol metabolism, uncontrolled drinking may cause significant damage to the liver. This damage is initiated by excessive fat accumulation in the liver, which can further progress to advanced liver disease. The only effective therapeutic strategies currently available for ALD are alcohol abstinence or liver transplantation. Any molecule with dual-pronged effects at the central and peripheral organs controlling addictive behaviors and associated metabolic pathways are a potentially important therapeutic target for treating AUD and ALD. Ghrelin, a hormone primarily derived from the stomach, has such properties, and regulates both behavioral and metabolic functions. In this review, we highlight recent advances in understanding the peripheral and central functions of the ghrelin system and its role in AUD and ALD pathogenesis. We first discuss the correlation between blood ghrelin concentrations and alcohol use or abstinence. Next, we discuss the role of ghrelin in alcohol-seeking behaviors and finally its role in the development of fatty liver by metabolic regulations and organ crosstalk. We propose that a better understanding of the ghrelin system could open an innovative avenue for improved treatments for AUD and associated medical consequences, including ALD.

Transcriptional targets of senataxin and E2 promoter binding factors are associated with neuro-degenerative pathways during increased autophagic flux

Autophagy is an intracellular recycling process that degrades harmful molecules and enables survival during starvation, with implications for diseases including dementia, cancer and atherosclerosis. Previous studies demonstrate how a limited number of transcription factors (TFs) can increase autophagy. However, this knowledge has not resulted in translation into therapy, thus, to gain understanding of more suitable targets, we utilized a systems biology approach. We induced autophagy by amino acid starvation and mTOR inhibition in HeLa, HEK 293 and SH-SY5Y cells and measured temporal gene expression using RNA-seq. We observed 456 differentially expressed genes due to starvation and 285 genes due to mTOR inhibition (P_FDR < 0.05 in every cell line). Pathway analyses implicated Alzheimer's and Parkinson's diseases (P_FDR ≤ 0.024 in SH-SY5Y and HeLa) and amyotrophic lateral sclerosis (ALS, P_FDR < 0.05 in mTOR inhibition experiments). Differential expression of the Senataxin (SETX) target gene set was predicted to activate multiple neurodegenerative pathways (P_FDR ≤ 0.04). In the SH-SY5Y cells of neuronal origin, the E2F transcription family was predicted to activate Alzheimer's disease pathway (P_FDR ≤ 0.0065). These exploratory analyses suggest that SETX and E2F may mediate transcriptional regulation of autophagy and further investigations into their possible role in neuro-degeneration are warranted.

Red Pepper Seeds Inhibit Hepatic Lipid Accumulation by Inducing Autophagy via AMPK Activation

Although the red pepper and its seeds have been studied for metabolic diseases, the effects and potential mechanisms of red pepper seed extract (RPS) on hepatic lipid accumulation are not yet completely understood. This study aimed to evaluate the inhibitory effect of RPS on hepatic lipid accumulation via autophagy. C57BL/6 mice were fed a high-fat diet (HFD) or a HFD supplemented with RPS. RPS treatment inhibited hepatic lipid accumulation by suppressing lipogenesis, inducing hepatic autophagic flux, and activating AMPK in HFD-fed mice. To investigate the effect of RPS on an oleic acid (OA)-induced hepatic steatosis cell model, HepG2 cells were incubated in a high-glucose medium and OA, followed by RPS treatment. RPS treatment decreased OA-induced lipid accumulation and reduced the expression of lipogenesis-associated proteins. Autophagic flux dramatically increased in the RPS-treated group. RPS phosphorylated AMPK in a dose-dependent manner, thereby dephosphorylated mTOR. Autophagy inhibition with 3-methyladenine (3-MA) antagonized RPS-induced suppression of lipogenesis-related protein expressions. Moreover, the knockdown of endogenous AMPK also antagonized the RPS-induced regulation of lipid accumulation and autophagy. Our findings provide new insights into the beneficial effects of RPS on hepatic lipid accumulation through the AMPK-dependent autophagy-mediated downregulation of lipogenesis.

Intracerebroventricular injection of ghrelin receptor antagonist alleviated NAFLD via improving hypothalamic insulin resistance

OBJECTIVES

Non-alcoholic fatty liver disease (NAFLD) is a hepatic manifestation of clinical metabolic syndrome. Insulin resistance is an important factor in the pathogenesis of NAFLD. Ghrelin, widely distributed in peripheral tissues and the central nervous system, plays a vital role in regulating food intake, energy balance, and substance metabolism. In this study, the effect of intracerebroventricular (ICV) injection of ghrelin receptor antagonist on NAFLD was explored.

MATERIALS AND METHODS

A rat model of NAFLD was established by feeding a high-fat diet, and a selective ghrelin receptor antagonist [D-Lys-3]-GHRP-6 was injected via ventricular intubation implantation. The serum total cholesterol (TC), triglycerides (TGs), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and hepatic TGs were measured using the colorimetric method. Fasting plasma glucose (FPG) and fasting plasma insulin (FPI) were determined to calculate homeostatic model assessment insulin resistance (HOMA-IR). Hematoxylin-eosin (HE) and Oil Red O staining were conducted to observe the pathological changes and lipid accumulation in the liver. Hosphatidylinositide3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway protein expressions were measured using western blot analysis.

RESULTS

ICV injection of [D-Lys-3]-GHRP-6 significantly reduced serum lipids, transaminase, and HOMA-IR, improved liver injury, and inhibited lipid accumulation in the liver of NAFLD rats. Moreover, ICV injection of [D-Lys-3]-GHRP-6 significantly up-regulated the phosphorylation levels of PI3K/Akt/mTOR signaling protein expressions in the hypothalamus, indicating a significant improvement in hypothalamic insulin resistance.

CONCLUSION

Blockade of central ghrelin receptor can treat NAFLD possibly via the hypothalamic PI3K/Akt/mTOR signaling pathway to improve insulin resistance.

mTOR: A Potential New Target in Nonalcoholic Fatty Liver Disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) continues to rise, yet effective treatments are lacking due to the complex pathogenesis of this disease. Although recent research has provided evidence for the “multiple strikes” theory, the classic “two strikes” theory has not been overturned. Therefore, there is a crucial need to identify multiple targets in NAFLD pathogenesis for the development of diagnostic markers and targeted therapeutics. Since its discovery, the mechanistic target of rapamycin (mTOR) has been recognized as the central node of a network that regulates cell growth and development and is closely related to liver lipid metabolism and other processes. This paper will explore the mechanisms by which mTOR regulates lipid metabolism (SREBPs), insulin resistance (Foxo1, Lipin1), oxidative stress (PIG3, p53, JNK), intestinal microbiota (TLRs), autophagy, inflammation, genetic polymorphisms, and epigenetics in NAFLD. The specific influence of mTOR on NAFLD was hypothesized to be divided into micro regulation (the mechanism of mTOR’s influence on NAFLD factors) and macro mediation (the relationship between various influencing factors) to summarize the influence of mTOR on the developmental process of NAFLD, and prove the importance of mTOR as an influencing factor of NAFLD regarding multiple aspects. The effects of crosstalk between mTOR and its upstream regulators, Notch, Hedgehog, and Hippo, on the occurrence and development of NAFLD-associated hepatocellular carcinoma are also summarized. This analysis will hopefully support the development of diagnostic markers and new therapeutic targets in NAFLD.

[Dihydromyricetin reduces lipid accumulation in LO2 cells via AMPK/mTOR-mediated lipophagy pathway and inhibits HepG2 cell proliferation in vitro]

OBJECTIVE

To explore the mechanism underlying the hepatoprotective effect of dihydromyricetin (DMY) against lipid accumulation in light of the lipophagy pathway and the inhibitory effect of DMY on HepG2 cell proliferation.

METHODS

LO2 cells were cultured in the presence of 10% FBS for 24 h and treated with 100 μg/mL DMY, or exposed to 50% FBS for 24 h followed by treatment with 50, 100, or 200 μg/mL DMY; the cells in recovery group were cultured in 50% FBS for 24 h and then in 10% FBS for another 24 h. Oil red O staining was used to observe the accumulation of lipid droplets in the cells, and the levels of TC, TG, and LDL and activities of AST, ALT and LDH were measured. The expression of LC3 protein was detected using Western blotting. AO staining and transmission electron microscopy were used to determine the numbers of autophagolysosomes and autophagosomes, respectively. The formation of autophagosomes was observed with MDC staining, and the mRNA expression levels of LC3, ATG7, AMPK, mTOR, p62 and Beclin1 were determined with q-PCR. Flow cytometry was performed to analyze the effect of 50, 100, and 200 μg/mL DMY on cell cycle and apoptosis of HepG2 cells; DNA integrity in the treated cells was examined with cell DNA fragmentation test.

RESULTS

DMY treatment and pretreatment obviously inhibited lipid accumulation and reduced the levels of TC, TG, LDL and enzyme activities of AST, ALT and LDH in LO2 cells (P < 0.05). In routinely cultured LO2 cells, DMY significantly promoted the formation of autophagosomes and autophagolysosomes and upregulated the expression of LC3 protein. DMY obviously attenuated high FBS-induced inhibition of autophagosome formation in LO2 cells, up- regulated the mRNA levels of LC3, ATG7, Beclin1 and AMPK, and downregulated p62 and mTOR mRNA levels (P < 0.05 or 0.01). In HepG2 cells, DMY caused obvious cell cycle arrest, inhibited cell proliferation, and induced late apoptosis and DNA fragmentation.

CONCLUSION

DMY reduces lipid accumulation in LO2 cells by regulating the AMPK/ mTOR-mediated lipophagy pathway and inhibits the proliferation of HepG2 by causing cell cycle arrest and promoting apoptosis.

LEAP2 deletion in mice enhances ghrelin's actions as an orexigen and growth hormone secretagogue

Objective

The hormone liver-expressed antimicrobial peptide-2 (LEAP2) is a recently identified antagonist and an inverse agonist of the growth hormone secretagogue receptor (GHSR). GHSR's other well-known endogenous ligand, acyl-ghrelin, increases food intake, body weight, and GH secretion and is lowered in obesity but elevated upon fasting. In contrast, LEAP2 reduces acyl-ghrelin-induced food intake and GH secretion and is found elevated in obesity but lowered upon fasting. Thus, the plasma LEAP2/acyl-ghrelin molar ratio could be a key determinant modulating GHSR signaling in response to changes in body mass and feeding status. In particular, LEAP2 may serve to dampen acyl-ghrelin action in the setting of obesity, which is associated with ghrelin resistance. Here, we sought to determine the metabolic effects of genetic LEAP2 deletion.

Methods

We generated the first known LEAP2-KO mouse line. Food intake, GH secretion, and cellular activation (c-fos induction) in different brain regions following s.c. acyl-ghrelin administration in LEAP2-KO mice and wild-type littermates were determined. LEAP2-KO mice and wild-type littermates were submitted to a battery of tests (such as measurements of body weight, food intake, and body composition; indirect calorimetry, determination of locomotor activity, and meal patterning while housed in metabolic cages) over the course of 16 weeks of high-fat diet and/or standard chow feeding. Fat accumulation was assessed in hematoxylin & eosin-stained and oil red O-stained liver sections from these mice.

Results

LEAP2-KO mice were more sensitive to s.c. ghrelin. In particular, acyl-ghrelin acutely stimulated food intake at a dose of 0.5 mg/kg BW in standard chow-fed LEAP2-KO mice while a 2× higher dose was required by wild-type littermates. Also, acyl-ghrelin stimulated food intake at a dose of 1 mg/kg BW in high-fat diet-fed LEAP2-KO mice while not even a 10× higher dose was effective in wild-type littermates. Acyl-ghrelin induced a 90.9% higher plasma GH level and 77.2–119.7% higher numbers of c-fos-immunoreactive cells in the arcuate nucleus and olfactory bulb, respectively, in LEAP2-KO mice than in wild-type littermates. LEAP2 deletion raised body weight (by 15.0%), food intake (by 18.4%), lean mass (by 6.1%), hepatic fat (by 42.1%), and body length (by 1.7%) in females on long-term high-fat diet as compared to wild-type littermates. After only 4 weeks on the high-fat diet, female LEAP2-KO mice exhibited lower O₂ consumption (by 13%), heat production (by 9.5%), and locomotor activity (by 49%) than by wild-type littermates during the first part of the dark period. These genotype-dependent differences were not observed in high-fat diet-exposed males or female and male mice exposed for long term to standard chow diet.

Conclusions

LEAP2 deletion sensitizes lean and obese mice to the acute effects of administered acyl-ghrelin on food intake and GH secretion. LEAP2 deletion increases body weight in females chronically fed a high-fat diet as a result of lowered energy expenditure, reduced locomotor activity, and increased food intake. Furthermore, in female mice, LEAP2 deletion increases body length and exaggerates the hepatic fat accumulation normally associated with chronic high-fat diet feeding.

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A novel line of LEAP2-knockout mice was generated.

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LEAP2 deletion sensitizes mice to the GH secretory effects of administered ghrelin.

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LEAP2 deletion reduces ghrelin resistance in diet-induced obese mice.

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HFD-fed female LEAP2-KO mice eat more and gain more body weight and hepatic fat.

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HFD-fed female LEAP2-KO mice exhibit lowered energy expenditure and activity.

Regulatory effects of ghrelin on endoplasmic reticulum stress, oxidative stress, and autophagy: Therapeutic potential

Ghrelin is a regulatory peptide that is the endogenous ligand of the growth hormone secretagogue 1a (GHS-R1a) which belongs to the G protein-coupled receptor family. Ghrelin and GHS-R1a are widely expressed in the central and peripheral tissues and play therapeutic potential roles in the cytoprotection of many internal organs. Endoplasmic reticulum stress (ERS), oxidative stress, and autophagy dysfunction, which are involved in various diseases. In recent years, accumulating evidence has suggested that ghrelin exerts protective effects by regulating ERS, oxidative stress, and autophagy in diverse diseases. This review article summarizes information about the roles of the ghrelin system on ERS, oxidative stress, and autophagy in multiple diseases. It is suggested that ghrelin positively affects the treatment of diseases and may be considered as a therapeutic drug in many illnesses.

Systemic Overexpression of GDF5 in Adipocytes but Not Hepatocytes Alleviates High-Fat Diet-Induced Nonalcoholic Fatty Liver in Mice

OBJECTIVE

Our recent study demonstrated that growth differentiation factor 5 (GDF5) could promote white adipose tissue thermogenesis and alleviate high-fat diet- (HFD-) induced obesity in fatty acid-binding protein 4- (Fabp4-) GDF5 transgenic mice (TG). Here, we further investigated the effects of systemic overexpression of the GDF5 gene in adipocytes HFD-induced nonalcoholic fatty liver disease (NAFLD).

METHODS

Fabp4-GDF5 TG mice were administered an HFD feeding. NAFLD-related indicators associated with lipid metabolism and inflammation were measured. A GDF5 lentiviral vector was constructed, and the LO2 NAFLD cell model was induced by FFA solution (oleic acid and palmitic acid). The alterations in liver function, liver lipid metabolism, and related inflammatory indicators were analyzed.

RESULTS

The liver weight was significantly reduced in the TG group, which was in accordance with the significantly downregulated expression of TNFα, MCP1, Aim2, and SREBP-1c and significantly upregulated expression of CPT-1α and ACOX2 in TG mouse livers. Compared to that of cells in the FAA-free control group, LO2 cells with in situ overexpression of GDF5 developed lipid droplets after FFA treatment; the levels of triglycerides, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were significantly increased in both the GDF5 lentivirus and control lentivirus groups compared with those of the FAA-free group. Additionally, the levels of FAS, SREBP-1, CPT-1α, and inflammation-associated genes, such as ASC and NLRC4, were unaltered despite GDF5 treatment.

CONCLUSION

Systemic overexpression of GDF5 in adipose tissue in vivo significantly reduced HFD-induced NAFLD liver damage in mice. The overexpression of GDF5 in hepatocytes failed to improve lipid accumulation and inflammation-related reactions induced by mixed fatty acids, suggesting that the protective effect of GDF5 in NAFLD was mainly due to the reduction in adipose tissue and improvements in metabolism. Hence, our study suggests that the management of NAFLD should be targeted to reduce the overall amount of body fat and improve metabolic status before the progression to nonalcoholic steatohepatitis occurs.

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.

Veratrilla baillonii Franch Could Alleviate Lipid Accumulation in LO2 Cells by Regulating Oxidative, Inflammatory, and Lipid Metabolic Signaling Pathways

Based on the pathological theory of lipid metabolism and using network pharmacology, this study was designed to investigate the protective effect of water extract of Veratrilla baillonii (WVBF) on non-alcoholic fatty liver disease (NAFLD) model using LO2 cells and to identify the potential mechanism underlying the effect. The components of V. baillonii were identified from the public database of traditional Chinese medicine systems pharmacology database (TCMSP). Cytoscape software was used to construct the related composite target network. Then, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were carried out for critical nodes. The BioGPS database was used to determine the distribution of the target in tissues and organs. Moreover, the inhibitory effect of V. baillonii was further investigated using an in vitro hepatocyte NAFLD model. Fourteen active components were then selected from the 27 known compounds of V. baillonii. The targets of gene enrichment analysis were mainly distributed in the lipid catabolism-related signaling pathway. Network analysis revealed that five target genes of TNF, MAPK8, mTOR, NF-ĸB, and SREBP-1c were key nodes and played important roles in this process. Organ localization analysis indicated that one of the core target site of V. baillonii was liver tissue. The results of the in vitro study revealed that WVBF can alleviate the inflammatory response and lipid accumulation in LO2 hepatocytes by inhibiting oxidative stress and the adipocytokine signaling pathway. Genes and proteins related to the lipid synthesis, such as SREBP-1C, acetyl-CoA carboxylase (ACC), and fatty acid synthase (FASN), were significantly decreased, and PPARα expression is significantly increased with WVBF administration. In conclusion, V. baillonii may regulate local lipid metabolism and attenuate oxidative stress and inflammatory factors through the PPARα/SREBP-1c signaling pathway. The present study also indicates that multiple components of V. baillonii regulate multiple targets and pathways in NAFLD. The findings highlight the potential of V. baillonii as a promising treatment strategy for nonalcoholic fatty liver injury.

Heal the heart through gut (hormone) ghrelin: a potential player to combat heart failure

Ghrelin, a small peptide hormone (28 aa), secreted mainly by X/A-like cells of gastric mucosa, is also locally produced in cardiomyocytes. Being an orexigenic factor (appetite stimulant), it promotes release of growth hormone (GH) and exerts diverse physiological functions, viz. regulation of energy balance, glucose, and/or fat metabolism for body weight maintenance. Interestingly, administration of exogenous ghrelin significantly improves cardiac functions in CVD patients as well as experimental animal models of heart failure. Ghrelin ameliorates pathophysiological condition of the heart in myocardial infarction, cardiac hypertrophy, fibrosis, cachexia, and ischemia reperfusion injury. This peptide also exerts significant impact at the level of vasculature leading to lowering high blood pressure and reversal of endothelial dysfunction and atherosclerosis. However, the molecular mechanism of actions elucidating the healing effects of ghrelin on the cardiovascular system is still a matter of conjecture. Some experimental data indicate its beneficial effects via complex cellular cross talks between autonomic nervous system and cardiovascular cells, some other suggest more direct receptor-mediated molecular actions via autophagy or ionotropic regulation and interfering with apoptotic and inflammatory pathways of cardiomyocytes and vascular endothelial cells. Here, in this review, we summarise available recent data to encourage more research to find the missing links of unknown ghrelin receptor-mediated pathways as we see ghrelin as a future novel therapy in cardiovascular protection.

Recent progress in the discovery of ghrelin O-acyltransferase (GOAT) inhibitors

Ghrelin is a stomach-derived peptide hormone which stimulates appetite. For ghrelin to exert its orexigenic effect, octanoylation on the serine-3 residue of this gut-brain peptide is essential. The octanoylation of ghrelin is mediated by a unique acyltransferase enzyme known as ghrelin O-acyltransferase (GOAT). Thus modulating this enzyme offers viable approaches to alter feeding behaviors. Over the past decade, several small-molecule based approaches have appeared dealing with the discovery of compounds able to modulate this enzyme for the treatment of obesity and type 2 diabetes. Drug discovery efforts from academic groups and several pharmaceutical companies have fielded compounds having efficacy in altering acylated ghrelin levels in animal models but to date, compounds modulating the activity of the GOAT enzyme do not yet represent clinical options. This mini-review covers the drug discovery approaches of the last decade since the discovery of the GOAT enzyme.

Niacin inhibits the synthesis of milk fat in BMECs through the GPR109A-mediated downstream signalling pathway

AIMS

Previous studies have shown the effect of niacin on dairy cow production, but no study on the role of niacin in milk fat synthesis has been performed. Therefore, the purpose of this study was to examine the effect of niacin on milk fat synthesis and its specific mechanism in BMECs.

MAIN METHODS

In this study, 0.5 mM niacin, a GPR109A-inhibiting plasmid, and an AMPK inhibitor were added to BMECs. Milk fat was measured by a triglyceride kit and BODIPY staining. The protein expression of GPR109A, FASN, SREBP1, AMPK, ACC, mTOR and S6K was measured by Western blotting. The gene expression of GPR109A, FASN, and SREBP1 was analysed by RT-PCR.

KEY FINDINGS

Our results showed that 0.5 mM niacin could significantly reduce milk fat synthesis in BMECs and activate the AMPK/ACC signalling pathway by stimulating GPR109A, reducing the protein expression of p-mTOR and p-S6K, and reducing the expression of SREBP1 and FASN in BMECs.

SIGNIFICANCE

The present study clarified the effect of niacin on milk fat synthesis. The results show that niacin inhibits the synthesis of milk fat in BMECs through the downstream signalling pathway mediated by GPR109A. The function of niacin has been expanded, and knowledge of the new mechanism and signalling pathway will help improve the biosynthesis of milk fat.

The new mechanism of Ghrelin/GHSR-1a on autophagy regulation

Autophagy is associated with several diseases. In recent years, accumulating evidence has suggested that ghrelin pathway exerts a protective effect by regulating autophagy. This review aims to assess the potential role and use of ghrelin as a new treatment for obesity, cardiovascular diseases, nonalcoholic fatty liver disease (NFALD), neurodegenerative diseases, and tissue damage associated with autophagy. Ghrelin reduces the basal expression of autophagy-related genes in obesity-associated type 2 diabetes and ghrelin level changes in obesity, heart failure, and NFALD as well as altered autophagy. Ghrelin and its receptor GHSR-1 activation induce the phosphorylation of ERK1/2 and the induction of PI-3 kinase (PI3 K) and phosphorylation of Akt. In the myocardium and hypothalamic NPY/AgRP neurons, ghrelin increases levels of the intracellular energy sensor AMPK and enhances autophagy, protecting cardiac ischemia and inducing neural stem cells. Nonetheless, ghrelin activates the PI3 K/Akt/Bcl-2 pathway and inhibits the activation of autophagy, such as tissues injured by sepsis or doxorubicin. In conclusion, endogenous ghrelin system could be considered as a new target or treatment for metabolism disorders, cardiac diseases, neurodegenerative diseases, and tissue injuries.

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.

Ghrelin Signaling: GOAT and GHS-R1a Take a LEAP in Complexity

Ghrelin and the growth hormone secretagogue receptor 1a (GHS-R1a) are important targets for disorders related to energy balance and metabolic regulation. Pharmacological control of ghrelin signaling is a promising avenue to address health issues involving appetite, weight gain, obesity, and related metabolic disorders, and may be an option for patients suffering from wasting conditions like cachexia. In this review, we summarize recent developments in the biochemistry of ghrelin and GHS-R1a signaling. These include unravelling the enzymatic transformations that generate active ghrelin and the discovery of multiple proteins that interact with ghrelin and GHS-R1a to regulate signaling. Furthermore, we propose that harnessing these processes will lead to highly selective treatments to address obesity, diabetes, and other metabolism-linked disorders.

The Upstream Pathway of mTOR-Mediated Autophagy in Liver Diseases

Autophagy, originally found in liver experiments, is a cellular process that degrades damaged organelle or protein aggregation. This process frees cells from various stress states is a cell survival mechanism under stress stimulation. It is now known that dysregulation of autophagy can cause many liver diseases. Therefore, how to properly regulate autophagy is the key to the treatment of liver injury. mechanistic target of rapamycin (mTOR)is the core hub regulating autophagy, which is subject to different upstream signaling pathways to regulate autophagy. This review summarizes three upstream pathways of mTOR: the phosphoinositide 3-kinase (PI3K)/protein kinase (AKT) signaling pathway, the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway, and the rat sarcoma (Ras)/rapidly accelerated fibrosarcoma (Raf)/mitogen-extracellular activated protein kinase kinase (MEK)/ extracellular-signal-regulated kinase (ERK) signaling pathway, specifically explored their role in liver fibrosis, hepatitis B, non-alcoholic fatty liver, liver cancer, hepatic ischemia reperfusion and other liver diseases through the regulation of mTOR-mediated autophagy. Moreover, we also analyzed the crosstalk between these three pathways, aiming to find new targets for the treatment of human liver disease based on autophagy.

Pharmacological Modulation of Ghrelin to Induce Weight Loss: Successes and Challenges

PURPOSE OF REVIEW

Obesity is affecting over 600 million adults worldwide and has numerous negative effects on health. Since ghrelin positively regulates food intake and body weight, targeting its signaling to induce weight loss under conditions of obesity seems promising. Thus, the present work reviews and discusses different possibilities to alter ghrelin signaling.

RECENT FINDINGS

Ghrelin signaling can be altered by RNA Spiegelmers, GHSR/Fc, ghrelin-O-acyltransferase inhibitors as well as antagonists, and inverse agonists of the ghrelin receptor. PF-05190457 is the first inverse agonist of the ghrelin receptor tested in humans shown to inhibit growth hormone secretion, gastric emptying, and reduce postprandial glucose levels. Effects on body weight were not examined. Although various highly promising agents targeting ghrelin signaling exist, so far, they were mostly only tested in vitro or in animal models. Further research in humans is thus needed to further assess the effects of ghrelin antagonism on body weight especially under conditions of obesity.

Priming of Hypothalamic Ghrelin Signaling and Microglia Activation Exacerbate Feeding in Rats' Offspring Following Maternal Overnutrition

Maternal overnutrition during pregnancy leads to metabolic alterations, including obesity, hyperphagia, and inflammation in the offspring. Nutritional priming of central inflammation and its role in ghrelin sensitivity during fed and fasted states have not been analyzed. The current study aims to identify the effect of maternal programming on microglia activation and ghrelin-induced activation of hypothalamic neurons leading to food intake response. We employed a nutritional programming model exposing female Wistar rats to a cafeteria diet (CAF) from pre-pregnancy to weaning. Food intake in male offspring was determined daily after fasting and subcutaneous injection of ghrelin. Hypothalamic ghrelin sensitivity and microglia activation was evaluated using immunodetection for Iba-1 and c-Fos markers, and Western blot for TBK1 signaling. Release of TNF-alpha, IL-6, and IL-1β after stimulation with palmitic, oleic, linoleic acid, or C6 ceramide in primary microglia culture were quantified using ELISA. We found that programmed offspring by CAF diet exhibits overfeeding after fasting and peripheral ghrelin administration, which correlates with an increase in the hypothalamic Iba-1 microglia marker and c-Fos cell activation. Additionally, in contrast to oleic, linoleic, or C6 ceramide stimulation in primary microglia culture, stimulation with palmitic acid for 24 h promotes TNF-alpha, IL-6, and IL-1β release and TBK1 activation. Notably, intracerebroventricular (i.c.v.) palmitic acid or LPS inoculation for five days promotes daily increase in food intake and food consumption after ghrelin administration. Finally, we found that i.c.v. palmitic acid substantially activates hypothalamic Iba-1 microglia marker and c-Fos. Together, our results suggest that maternal nutritional programing primes ghrelin sensitivity and microglia activation, which potentially might mirror hypothalamic administration of the saturated palmitic acid.