Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. AG represents ~10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. UnAG has no orexigenic effects, and its circulating levels are positively associated with insulin sensitivity in metabolic syndrome patients. Skeletal muscle oxidative stress and inflammation are key negative modulators of tissue and whole-body insulin action, and UnAG was recently reported to reduce oxidative stress in non-muscle cells in-vitro. Its potential direct involvement in the regulation of muscle intermediate metabolism in vivo and its clinical impact remain however largely unknown. In the current studies, we first investigated potential associations between plasma AG, UnAG and HOMA insulin resistance index in the general population. In 719 individuals from the North-East-Italy MoMa epidemiological study, TG and UnAG but not of AG were negatively associated with HOMA after adjusting for gender and body mass both at baseline and at 5-year follow-up, and changes in TG and UnAG but not AG were negatively associated with changes in HOMA independently of potential confounders. We next tested the hypothesis that UnAG increases insulin sensitivity by modulating oxidative stress, inflammation and insulin action in skeletal muscle tissue. In healthy male rats, UnAG administration consistently reduced muscle mitochondrial ROS production, reduced inflammation and activated insulin-anabolic signalling. Analogous findings were observed in transgenic mice with systemic UnAG overexpression, and in UnAG-treated myotubes in vitro, thereby supporting a potential direct hormone effect. Importantly, these findings were not observed with AG administration nor in liver samples, thus further indicating independent and tissue specific UnAG actions. Autophagy, the removal of dysfunctional organelles, is an emerging protective mechanism in both cardiac and skeletal muscle. We therefore also hypothyzed that UnAG could reduce ROS production by inducing muscle autophagy. In cardiomiocytes UnAG reduced ROS production in association with increased dysfunctional mitochondria removal and in UnAG but not AG treated myotubes silencing of autophagy promoter ATG5 restored ROS generation. We next investigated the potential clinical relevance of UnAG actions in disease models characterized by muscle oxidative stress, inflammation and insulin resistance. In high-fat diet (HFD)-induced obese and diabetic mice, chronic UnAG overexpression prevented hyperglycemia and whole-body insulin resistance, as well as muscle oxidative stress, inflammation and altered insulin signalling. In rodent chronic kidney disease induced by 5/6 nephrectomy, protein-energy wasting, enhanced skeletal muscle ROS production, increased tissue inflammation and impaired insulin signalling were also completely normalized by UnAG treatment. Importantly, these findings were associated with a recovery of gastrocnemious muscle mass. While a specific receptor for UnAG needs to be identified, our combined findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. Underlying mechanism appear to involve the modulation of mitochondrial function with reduced ROS generation, which could be mediated at least in part by autophagy induction. Data from disease models also suggests that modulation of ghrelin acylation to enhance UnAG availability is a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.

NOVEL METABOLIC ROLES OF UNACYLATED GHRELIN: FROM PATHOPHYSIOLOGY TO DISEASE MODELS TREATMENT

GORTAN CAPPELLARI, GIANLUCA
2016-04-29

Abstract

Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. AG represents ~10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. UnAG has no orexigenic effects, and its circulating levels are positively associated with insulin sensitivity in metabolic syndrome patients. Skeletal muscle oxidative stress and inflammation are key negative modulators of tissue and whole-body insulin action, and UnAG was recently reported to reduce oxidative stress in non-muscle cells in-vitro. Its potential direct involvement in the regulation of muscle intermediate metabolism in vivo and its clinical impact remain however largely unknown. In the current studies, we first investigated potential associations between plasma AG, UnAG and HOMA insulin resistance index in the general population. In 719 individuals from the North-East-Italy MoMa epidemiological study, TG and UnAG but not of AG were negatively associated with HOMA after adjusting for gender and body mass both at baseline and at 5-year follow-up, and changes in TG and UnAG but not AG were negatively associated with changes in HOMA independently of potential confounders. We next tested the hypothesis that UnAG increases insulin sensitivity by modulating oxidative stress, inflammation and insulin action in skeletal muscle tissue. In healthy male rats, UnAG administration consistently reduced muscle mitochondrial ROS production, reduced inflammation and activated insulin-anabolic signalling. Analogous findings were observed in transgenic mice with systemic UnAG overexpression, and in UnAG-treated myotubes in vitro, thereby supporting a potential direct hormone effect. Importantly, these findings were not observed with AG administration nor in liver samples, thus further indicating independent and tissue specific UnAG actions. Autophagy, the removal of dysfunctional organelles, is an emerging protective mechanism in both cardiac and skeletal muscle. We therefore also hypothyzed that UnAG could reduce ROS production by inducing muscle autophagy. In cardiomiocytes UnAG reduced ROS production in association with increased dysfunctional mitochondria removal and in UnAG but not AG treated myotubes silencing of autophagy promoter ATG5 restored ROS generation. We next investigated the potential clinical relevance of UnAG actions in disease models characterized by muscle oxidative stress, inflammation and insulin resistance. In high-fat diet (HFD)-induced obese and diabetic mice, chronic UnAG overexpression prevented hyperglycemia and whole-body insulin resistance, as well as muscle oxidative stress, inflammation and altered insulin signalling. In rodent chronic kidney disease induced by 5/6 nephrectomy, protein-energy wasting, enhanced skeletal muscle ROS production, increased tissue inflammation and impaired insulin signalling were also completely normalized by UnAG treatment. Importantly, these findings were associated with a recovery of gastrocnemious muscle mass. While a specific receptor for UnAG needs to be identified, our combined findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. Underlying mechanism appear to involve the modulation of mitochondrial function with reduced ROS generation, which could be mediated at least in part by autophagy induction. Data from disease models also suggests that modulation of ghrelin acylation to enhance UnAG availability is a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.
BARAZZONI, ROCCO
28
2014/2015
Settore BIO/10 - Biochimica
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11368/2908033
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