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      Oxytocin in metabolic homeostasis: implications for obesity and diabetes management

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          Summary

          Oxytocin was once understood solely as a neuropeptide with a central role in social bonding, reproduction, parturition, lactation and appetite regulation. Recent evidence indicates that oxytocin enhances glucose uptake and lipid utilization in adipose tissue and skeletal muscle, suggesting that dysfunction of the oxytocin system could underlie the pathogenesis of insulin resistance and dyslipidaemia. Murine studies revealed that deficiencies in oxytocin signalling and oxytocin receptor expression lead to obesity despite normal food intake, motor activity and increased leptin levels. In addition, plasma oxytocin concentration is notably lower in obese individuals with diabetes, which may suggest an involvement of the oxytocin system in the pathogenesis of cardiometabolic disease. More recently, small scale studies demonstrated that intranasal administration of oxytocin was associated with significant weight loss as well as improvements in insulin sensitivity and pancreatic β‐cell responsivity in human subjects. The multi‐pronged effects of oxytocin signalling on improving peripheral insulin sensitivity, pancreatic function and lipid homeostasis strongly suggest a role for this system as a therapeutic target in obesity and diabetes management. The complexity of obesity aetiology and the pathogenesis of obesity‐related metabolic complications underscore the need for a systems approach to better understand the role of oxytocin in metabolic function.

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          The oxytocin receptor system: structure, function, and regulation.

          The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.
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            The pathogenesis of insulin resistance: integrating signaling pathways and substrate flux.

            Insulin resistance arises when the nutrient storage pathways evolved to maximize efficient energy utilization are exposed to chronic energy surplus. Ectopic lipid accumulation in liver and skeletal muscle triggers pathways that impair insulin signaling, leading to reduced muscle glucose uptake and decreased hepatic glycogen synthesis. Muscle insulin resistance, due to ectopic lipid, precedes liver insulin resistance and diverts ingested glucose to the liver, resulting in increased hepatic de novo lipogenesis and hyperlipidemia. Subsequent macrophage infiltration into white adipose tissue (WAT) leads to increased lipolysis, which further increases hepatic triglyceride synthesis and hyperlipidemia due to increased fatty acid esterification. Macrophage-induced WAT lipolysis also stimulates hepatic gluconeogenesis, promoting fasting and postprandial hyperglycemia through increased fatty acid delivery to the liver, which results in increased hepatic acetyl-CoA content, a potent activator of pyruvate carboxylase, and increased glycerol conversion to glucose. These substrate-regulated processes are mostly independent of insulin signaling in the liver but are dependent on insulin signaling in WAT, which becomes defective with inflammation. Therapies that decrease ectopic lipid storage and diminish macrophage-induced WAT lipolysis will reverse the root causes of type 2 diabetes.
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              Adipocyte dysfunctions linking obesity to insulin resistance and type 2 diabetes.

              Acquired resistance to the action of insulin to stimulate glucose transport in skeletal muscle is associated with obesity and promotes the development of type 2 diabetes. In skeletal muscle, insulin resistance can result from high levels of circulating fatty acids that disrupt insulin signalling pathways. However, the severity of insulin resistance varies greatly among obese people. Here we postulate that this variability might reflect differences in levels of lipid-droplet proteins that promote the sequestration of fatty acids within adipocytes in the form of triglycerides, thereby lowering exposure of skeletal muscle to the inhibitory effects of fatty acids.
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                Author and article information

                Contributors
                fmagkos@gmail.com
                Journal
                Obes Rev
                Obes Rev
                10.1111/(ISSN)1467-789X
                OBR
                Obesity Reviews
                John Wiley and Sons Inc. (Hoboken )
                1467-7881
                1467-789X
                25 September 2018
                January 2019
                : 20
                : 1 ( doiID: 10.1111/obr.v20.1 )
                : 22-40
                Affiliations
                [ 1 ] Clinical Nutrition Research Centre (CNRC), Singapore Institute for Clinical Sciences (SICS), Agency for Science Technology and Research (A*STAR) and National University Health System Singapore
                [ 2 ] Department of Endocrinology Tan Tock Seng Hospital Singapore
                [ 3 ] Cardiovascular and Metabolic Disorders Program Duke‐NUS Medical School Singapore
                [ 4 ] Department of Physiology, Yong Loo Lin School of Medicine National University of Singapore (NUS) Singapore
                [ 5 ] Lee Kong Chian School of Medicine Nanyang Technological University Singapore
                Author notes
                [*] [* ] *Address for correspondence: F Magkos, PhD, Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore. Mailing address: Brenner Centre for Molecular Medicine, 30 Medical Drive, 117609 Singapore.

                E‐mail: fmagkos@ 123456gmail.com

                Author information
                http://orcid.org/0000-0002-5255-8677
                Article
                OBR12757 OBR-05-18-3439.R3
                10.1111/obr.12757
                7888317
                30253045
                93e1011b-3fce-44aa-aa9d-7c7b716a91f0
                © 2018 The Authors. Obesity Reviews published by John Wiley & Sons Ltd on behalf of World Obesity Federation

                This is an open access article under the terms of the http://creativecommons.org/licenses/by-nc/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

                History
                : 28 May 2018
                : 25 July 2018
                : 25 July 2018
                Page count
                Figures: 1, Tables: 3, Pages: 19, Words: 8972
                Funding
                Funded by: Singapore Institute for Clinical Sciences (SICS), Agency for Science, Technology and Research (A*STAR)
                Award ID: BMSI/16‐07803C‐R20H
                Categories
                Obesity‐Diabetes Management/Etiology and Pathophysiology
                Obesity‐Diabetes Management/Etiology and Pathophysiology
                Custom metadata
                2.0
                January 2019
                Converter:WILEY_ML3GV2_TO_JATSPMC version:5.9.7 mode:remove_FC converted:17.02.2021

                Medicine
                beta cell function,glucose metabolism,insulin sensitivity,lipid metabolism
                Medicine
                beta cell function, glucose metabolism, insulin sensitivity, lipid metabolism

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