White adipose tissue has been considered for a long time only to be the place where energy is stored mainly in the form of triglycerides. In the nineties, this view has changed considerably because of the discovery of adipocytokines, a substance secreted by fat cells (adipocytes). In a short time, it was demonstrated that the active role of this tissue is not only lipogenezie or lipolysis, but it also affects the metabolism of the whole organism, and the formation of natural immunity.
At the moment of birth in the human body there are approximately 30 million fat cells. With normal healthy adults, it is estimated that fat constitutes 15 to 25% of body weight, depending on sex. These proportions change significantly with people that are overweight or obese. It leads for example to the development of insulin resistance. Among the clinical implications of this pathology we can perceive many diseases harassing society, such as type 2 diabetes, metabolic syndrome and diseases of the cardiovascular system, which are the scourge of our times.
It has been shown that adipose tissue is an active endocrine organ that releases a large amount of biologically active substances, including tumor necrosis factor (TNF α), transforming growth factor β (TGF β), vascular endothelial growth factor (VEGF), retinol binding protein (RBP ) lipoprotein lipase (LPL), cholesterol ester transfer protein (CETP), apolipoprotein E (ApoE), haptoglobin, metallothionein, interleukins, angiotensinogen, thromboplastin, complement factors, C-reactive protein, adipokines, and many others.
Recent scientific reports focused both on the classic adipocytokines, such as adiponectin, leptin or resistin, but also the newly discovered (omentin, chemerin, vaspin, visfatin, etc.). It showed the modest effects of these substances on metabolism. Thanks to a signal sent to a metabolically important organ, it models function of the brain, liver, skeletal muscle and immune system. They play a role in modifying the course of hemostasis and inflammation and affect the cardiovascular system, including blood pressure. A lot of reports refer to changes caused by adipocytokines in the metabolism of lipids and glucose.
Many of the research concerning adipocytokines were performed on an animal model, but their effects are increasingly well documented in humans. One of the best-studied substances produced by adipose tissue-adiponectin, is well known for its effect on the sensitization of tissues to insulin. Disorder of its concentrations are observed in many pathologies, and the best known is its role in the pathogenesis of obesity and diabetes. Adiponectin also has an anti-inflammatory effect. Hypoadiponectemia is associated with lipid profile, which favors the development of atherosclerosis. Adiponectin plasma levels is the opposite proportional to the body mass index and obesity. Known for many years because of its effect on hunger and satiety center, leptin, also affects the modulation of hepatic gluconeogenesis and the function of pancreatic β-cell. While, resistin is suspected of intensifying insulin resistance. High levels of resistin are associated with an increased risk of cardio-vascular disease, endothelial dysfunction and an atherogenic, inflammatory markers.
Although the studies on the newly discovered adipokines are still ongoing, the current data confirm the important role in the pathogenesis of many diseases. It shows the effect of omentin and chemerin in the improvement of insulin sensitivity by enhancing insulin-stimulated glucose uptake, in the case of visfatin effect was induced by inhibition of apoptosis in the mechanism associated with caspase 3 and 8. Apelin caused a similar effect by increasing adiponectin. Additional operation of apelin shown in the mechanisms responsible for the control of thirst and urine output or on the renin-angiotensin-aldosterone system. It also comes with nitric oxide (NO) vasodilation.
In spite of the fact that the influence of many of adipocytokines is not sufficiently proven yet, the existing studies show that they represent pleiotropic effects, which cause some important metabolic effects. This fact forces us to change our thinking about the fat, which we cannot treat longer only as an ordinary storekeeper, but as a producer of many active substances. Better knowledge of the pathomechanisms of body fat, in particular the effect of the substance produced by it, is a potential clinically important therapeutic target in the treatment of many diseases, particularly diabetes. Understanding the pathogenesis of obesity and metabolic sequences associated with it are the challenges of modern medicine in a world plagued by this disease.
Written by: Dominik Porada, Aneta Szafraniec, Jerzy Bednarski
Source:
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New look at adipose tissue
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