New concepts in white adipose tissue physiology

Numerous studies address the physiology of adipose tissue (AT). The interest surrounding the physiology of AT is primarily the result of the epidemic outburst of obesity in various contemporary societies. Briefly, the two primary metabolic activities of white AT include lipogenesis and lipolysis. Throughout the last two decades, a new model of AT physiology has emerged. Although AT was considered to be primarily an abundant energy source, it is currently considered to be a prolific producer of biologically active substances, and, consequently, is now recognized as an endocrine organ. In addition to leptin, other biologically active substances secreted by AT, generally classified as cytokines, include adiponectin, interleukin-6, tumor necrosis factor-alpha, resistin, vaspin, visfatin, and many others now collectively referred to as adipokines. The secretion of such biologically active substances by AT indicates its importance as a metabolic regulator. Cell turnover of AT has also recently been investigated in terms of its biological role in adipogenesis. Consequently, the objective of this review is to provide a comprehensive critical review of the current literature concerning the metabolic (lipolysis, lipogenesis) and endocrine actions of AT.

Adipocitocinas: uma nova visão do tecido adiposo / Adipokines: a new view of adipose tissue

A identificação da leptina, hormônio secretado pelos adipócitos, cujo efeito sobre o sistema nervoso simpático e a função endócrina confere participação ativa no controle do dispêndio energético, bem como do apetite, acrescentou às funções do tecido adiposo no organismo humano o papel de órgão multifuncional, produtor e secretor de inúmeros peptídeos e proteínas bioativas, denominadas adipocitocinas. Alterações na quantidade de tecido adiposo, como ocorrem na obesidade, afetam a produção da maioria desses fatores secretados pelos adipócitos. Ainda que essas alterações estejam freqüentemente associadas às inúmeras disfunções metabólicas e ao aumento do risco de doenças cardiovasculares, permanece sob investigação o envolvimento do tecido adiposo no desenvolvimento dessas complicações, considerada a sua função endócrina. As concentrações de várias adipocitocinas elevam-se na obesidade e têm sido relacionadas à hipertensão (angiotensinogênio), ao prejuízo da fibrinólise (inibidor do ativador de plasminogênio-1) e à resistência à insulina (proteína estimuladora de acilação, fator de necrose tumoral-alfa, interleucina-6 e resistina). De outro modo, leptina e adiponectina têm efeitos sobre a sensibilidade à insulina. Na obesidade, a resistência insulínica também está relacionada à resistência à leptina e aos teores plasmáticos reduzidos de adiponectina. Leptina e adiponectina ainda exercem efeitos orgânicos adicionais distintos: frente à participação da leptina no controle da ingestão alimentar, a adiponectina apresenta potente ação anti-aterogênica. Algumas drogas utilizadas no controle do diabetes elevam a produção endógena de adiponectina, em roedores e humanos, indicando que o desenvolvimento de novos medicamentos com alvo nas adipocitocinas pode representar uma alternativa terapêutica de prevenção da resistência insulínica e da aterosclerose em indivíduos obesos.

Leptin is a hormone secreted by adipocytes whose effect on the sympathetic nervous system and endocrine function confers active participation in the control of energy expenditure and appetite. Its identification added to the fat tissues in the human body the role of a multifunctional organ that produces and secretes a number of bioactive peptides and proteins, called adipocytokines. Changes in the amount of fat tissue, such as the ones that occur in obesity, affect the production of most of these factors secreted by adipocytes. Even if these changes are frequently associated with many metabolic disorders and increased risk for cardiovascular diseases, the role of fat tissue in the development of these complications, considered its endocrine function, continue to be investigated. The concentration of various adipocytokines increase in obesity and have been associated with hypertension (angiotensinogen), fibrinolysis impairment (plasminogen activator inhibitor-1) and insulin resistance (protein that stimulates acylation, tumor necrosis factor-alpha, interleukine-6 and resistin). On the other hand, leptin and adiponectin affect insulin sensitivity. In obesity, insulin resistance is also associated with leptin resistance and reduced plasma levels of adiponectin. Leptin and adiponectin still have complementary and distinct organic functions: adiponectin has potent antiatherogenic activity while leptin participates in the control of food intake. Some medications used to control diabetes increase adiponectin production in rodents and humans, suggesting that the development of new medications that target the adipocytokines can represent a new therapeutic alternative to prevent insulin resistance and atherosclerosis in obese individuals.