Role of ghrelin in metabolic and muscle disorders

Abstract

Ghrelin gene products namely unacylated ghrelin (UnAG), acylated ghrelin (AG) and obestatin are key metabolic molecules being studied for their therapeutic potentials. The aim of this thesis was to investigate the role of ghrelin axis in cardiometabolic disorders including metabolic syndrome (MetS), hypertension, central obesity and pressure ulcer. The first cardiometabolic disorder being studied in this thesis was MetS which is a cluster of cardiometabolic risk factors including central obesity, elevated blood pressure, elevated triglycerides, elevated fasting blood glucose, and reduced high-density lipoprotein cholesterol (HDL-C), that predisposes individuals to cardiovascular diseases and type 2 diabetes mellitus. The influence of the interaction of central obesity with the other MetS components including blood pressure, fasting blood glucose, triglycerides, and HDL-C on circulatory UnAG, AG, obestatin, growth hormone (GH), insulin-like growth factor-1 (IGF-1) and nesfatin-1 was examined in 133 adults. The results of this study revealed that 1) obestatin, obestatin/ghrelin ratios, and GH were influenced by the interaction of central obesity with the other MetS risk factors 2) central obese subjects with or without the cluster of the other MetS risk factors had higher obestatin level but lower GH level 3) UnAG, AG and GH were lower in subjects with the cluster of MetS risk factors whereas obestatin was higher in subjects with the cluster of MetS risk factors and 4) ghrelin gene products and GH were correlated with the risk factors of MetS. Besides providing evidence for prioritizing central obesity as the main MetS risk factor, this study clarified the synergistic or differential role of central obesity among the other MetS risk factors. In the second study, obesity and hypertension, two independent cardiometabolic risk factors that increase morbidity and mortality, were investigated. In this study, the interacting influence of central obesity with hypertension on UnAG, AG, obestatin, ghrelin/obestatin ratio, and GH. Fasting plasma abundances of UnAG, AG, obestatin and GH were determined in 387 female adults. Blood samples were selected based on a 2 x 2 factorial design for central obesity and hypertension from an archived sample pool of 1492 participants who were previously screened for MetS. Our results revealed a significant interaction effect of hypertension with central obesity on obestatin. Obestatin was found to be significantly higher in central obese-only subjects when compared to subjects with neither hypertension nor central obesity and central obese subjects with hypertension. UnAG, AG, total ghrelin, ratios of UnAG/obestatin, AG/obestatin, total ghrelin/obestatin and GH were higher in subjects with neither hypertension nor central obesity when compared to central obese subjects with hypertension. This thesis is the first to examine the complex interaction of hypertension with central obesity on ghrelin axis. Our results suggest that ghrelin axis might play unique role in the regulation of blood pressure and control of body fat mass. Cardiometabolic risk factors of MetS negatively affect overall quality of life, Exercise and pharmacological interventions have been recommended to arrest the progression of the cardiometabolic risk factors of MetS. In the third study, we examined the effects of 1-year yoga training on circulatory peptides associated with ghrelin axis including UnAG, AG, obestatin, GH and insulin as well as homeostatic model assessment (HOMA) indices and disposition index in adults with MetS. Sera and data of 79 MetS subjects who had participated in a 1-year program of Hatha yoga exercise training or served as non-exercise control were analyzed. Our results revealed a significant interaction effect of group and time on UnAG, total ghrelin, obestatin and GH, but not AG and insulin. According to our results, UnAG and total ghrelin tended to be higher, GH was higher whereas obestatin was lower in the yoga group when compared to control group after the 1-year experimental period. Furthermore, our results demonstrated that AG was lower in the yoga group when compared to control group whereas insulin, HOMA indices and disposition index were unaltered at the end of the 1-year experimental period. These findings suggest that 1-year yoga training alters circulatory ghrelin gene products and GH in adults with MetS. The study supports a role of ghrelin axis in the pathogenesis of MetS. Poor circulation, a consequence of hypertension, is aggravated by central obesity, dyslipidaemia, and diabetes. Poor circulation resulting from cardiometabolic disorders reduces the nutrients delivered to skeletal muscle cells leading to apoptosis, necrosis and oxidative stress which are characteristics of pressure. Pressure ulcers which are damages to the skin and underlying tissue are associated with increased morbidity and mortality particularly in the aged and individuals with restricted movements. UnAG, the predominant form of circulating ghrelin, has been shown to protect myotubes from cell death which is a known attribute of pressure ulcer. In the fourth study, we investigated whether UnAG could protect skeletal muscle from pressure-induced deep tissue injury by abolishing necroptosis and apoptosis signaling and whether these effects are mediated by SIRT1 pathway. Fifteen adult Sprague Dawley rats were randomly assigned to receive saline or UnAG with or without EX527 (a SIRT1 inhibitor). Animals underwent two 6-hour compression cycles with 100 mmHg static pressure applied over the mid-tibialis region of the right limb whereas the left uncompressed limb served as the intra-animal control. Muscle tissues underneath the compression region and at the similar region of the opposite uncompressed limb were collected for analysis. Our results showed that UnAG 1) attenuated the compression-induced muscle pathohistological alterations including rounding contour of myofibers, extensive nucleus accumulation in the interstitial space, and increased interstitial space 2) abolished the increase in necroptosis proteins including RIP1 and RIP3 and 3) attenuated the elevation of apoptotic proteins including p53, Bax and AIF in the compressed muscle. The protective effects of UnAG were diminished in rats co-treated with EX527. Our results demonstrated that UnAG protected skeletal muscle from compression-induced injury and that the myoprotective effects of UnAG on pressure-induced tissue injury are associated with SIRT1 signaling. These findings have important implications in the prevention and treatment of pressure ulcer. Conclusively, the current thesis has demonstrated that obestatin and the ratios of ghrelin gene products were influenced by the interaction of central obesity with the other four MetS risk factors as well as the interaction of hypertension with central obesity. UnAG and AG decreased in the presence of MetS risk factors whereas obestatin increased in the presence of MetS risk factors. 1-year yoga intervention, a non-pharmacological treatment, decreased obestatin but tended to increase UnAG. Finally, the action of UnAG in abolishing pressure ulcer-related apoptosis and necroptosis as demonstrated in thesis has proved that UnAG may serve a future therapeutic target in the management of pressure ulcers. Taken together, this thesis suggests that ghrelin axis is uniquely affected by biochemical pathways related to cardiovascular and energy-related disorders and might play indispensable roles in cardiometabolic disorders.