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      Biochemistry of adipose tissue: an endocrine organ

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          Abstract

          Adipose tissue is no longer considered to be an inert tissue that stores fat. This tissue is capable of expanding to accommodate increased lipids through hypertrophy of existing adipocytes and by initiating differentiation of pre-adipocytes. Adipose tissue metabolism exerts an impact on whole-body metabolism. As an endocrine organ, adipose tissue is responsible for the synthesis and secretion of several hormones. These are active in a range of processes, such as control of nutritional intake (leptin, angiotensin), control of sensitivity to insulin and inflammatory process mediators (tumor necrosis factor α (TNF-α), interleukin-6 (IL-6), resistin, visfatin, adiponectin, among others) and pathways (plasminogen activator inhibitor 1 (PAI-1) and acylation stimulating protein (ASP) for example). This paper reviews some of the biochemical and metabolic aspects of adipose tissue and its relationship to inflammatory disease and insulin resistance.

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          Most cited references44

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          Interleukin-6 increases insulin-stimulated glucose disposal in humans and glucose uptake and fatty acid oxidation in vitro via AMP-activated protein kinase.

          Although interleukin-6 (IL-6) has been associated with insulin resistance, little is known regarding the effects of IL-6 on insulin sensitivity in humans in vivo. Here, we show that IL-6 infusion increases glucose disposal without affecting the complete suppression of endogenous glucose production during a hyperinsulinemic-euglycemic clamp in healthy humans. Because skeletal muscle accounts for most of the insulin-stimulated glucose disposal in vivo, we examined the mechanism(s) by which IL-6 may affect muscle metabolism using L6 myotubes. IL-6 treatment increased fatty acid oxidation, basal and insulin-stimulated glucose uptake, and translocation of GLUT4 to the plasma membrane. Furthermore, IL-6 rapidly and markedly increased AMP-activated protein kinase (AMPK). To determine whether the activation of AMPK mediated cellular metabolic events, we conducted experiments using L6 myotubes infected with dominant-negative AMPK alpha-subunit. The effects described above were abrogated in AMPK dominant-negative-infected cells. Our results demonstrate that acute IL-6 treatment enhances insulin-stimulated glucose disposal in humans in vivo, while the effects of IL-6 on glucose and fatty acid metabolism in vitro appear to be mediated by AMPK.
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            Visfatin, an adipocytokine with proinflammatory and immunomodulating properties.

            Adipocytokines are mainly adipocyte-derived cytokines regulating metabolism and as such are key regulators of insulin resistance. Some adipocytokines such as adiponectin and leptin affect immune and inflammatory functions. Visfatin (pre-B cell colony-enhancing factor) has recently been identified as a new adipocytokine affecting insulin resistance by binding to the insulin receptor. In this study, we show that recombinant visfatin activates human leukocytes and induces cytokine production. In CD14(+) monocytes, visfatin induces the production of IL-1beta, TNF-alpha, and especially IL-6. Moreover, it increases the surface expression of costimulatory molecules CD54, CD40, and CD80. Visfatin-stimulated monocytes show augmented FITC-dextran uptake and an enhanced capacity to induce alloproliferative responses in human lymphocytes. Visfatin-induced effects involve p38 as well as MEK1 pathways as determined by inhibition with MAPK inhibitors and we observed activation of NF-kappaB. In vivo, visfatin induces circulating IL-6 in BALB/c mice. In patients with inflammatory bowel disease, plasma levels of visfatin are elevated and its mRNA expression is significantly increased in colonic tissue of Crohn's and ulcerative colitis patients compared with healthy controls. Macrophages, dendritic cells, and colonic epithelial cells might be additional sources of visfatin as determined by confocal microscopy. Visfatin can be considered a new proinflammatory adipocytokine.
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              Adiponectin stimulates AMP-activated protein kinase in the hypothalamus and increases food intake.

              Adiponectin has been shown to stimulate fatty acid oxidation and enhance insulin sensitivity through the activation of AMP-activated protein kinase (AMPK) in the peripheral tissues. The effects of adiponectin in the central nervous system, however, are still poorly understood. Here, we show that adiponectin enhances AMPK activity in the arcuate hypothalamus (ARH) via its receptor AdipoR1 to stimulate food intake; this stimulation of food intake by adiponectin was attenuated by dominant-negative AMPK expression in the ARH. Moreover, adiponectin also decreased energy expenditure. Adiponectin-deficient mice showed decreased AMPK phosphorylation in the ARH, decreased food intake, and increased energy expenditure, exhibiting resistance to high-fat-diet-induced obesity. Serum and cerebrospinal fluid levels of adiponectin and expression of AdipoR1 in the ARH were increased during fasting and decreased after refeeding. We conclude that adiponectin stimulates food intake and decreases energy expenditure during fasting through its effects in the central nervous system.
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                Author and article information

                Journal
                Arch Med Sci
                Arch Med Sci
                AMS
                Archives of Medical Science : AMS
                Termedia Publishing House
                1734-1922
                1896-9151
                10 February 2013
                20 April 2013
                : 9
                : 2
                : 191-200
                Affiliations
                [1 ]Ciências Químicas e das Biomoléculas e Unidade de Mecanismos Moleculares da Doença do Centro de Investigação em Saúde e Ambiente, Escola Superior de Tecnologia da Saúde do Porto, Instituto Politécnico do Porto, Portugal
                [2 ]Centro de Farmacologia e Biopatologia Química (U38-FCT), Faculdade de Medicina da Universidade do Porto, Portugal
                Author notes
                Corresponding author: Prof. Ruben Fernandes, ESTSP, Rua Valente Perfeito 322, 4400-330 Vila Nova de Gaia, Portugal. Phone: + 351 222 061 004. E-mail: rpf@ 123456estsp.ipp.pt
                Article
                20246
                10.5114/aoms.2013.33181
                3648822
                23671428
                2068b244-604c-4059-8127-8c7678484bf2
                Copyright © 2013 Termedia & Banach

                This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License, permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 08 June 2012
                : 02 July 2012
                : 04 July 2012
                Categories
                State of the Art Paper

                Medicine
                adipose tissue,adipocytes,adipokines
                Medicine
                adipose tissue, adipocytes, adipokines

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