Obesity and Adipokines

Abstract

Adipose tissue (AT) is composed of adipocytes and a diverse population of nonadipocytes that are commonly referred to as stromal-vascular cells. Adipose tissue has traditionally been considered a passive storage energy depot that, indeed, does serve as a long-term reservoir for fuel stored as triglyceride. However, laboratory, clinical, and epidemiological studies over the past decade have redefined and greatly expanded our understanding of the physiological role of AT. We now appreciate that AT is an endocrine organ with important roles in maintaining whole-body energy homeostasis and systemic metabolism. This appreciation derives in large part from the identification of multiple AT-secreted factors that modulate central and peripheral processes. These include free fatty acids, which have significant effects on glucose and insulin homeostasis, as well as bioactive peptides termed adipokines. Adipokines act in an autocrine, paracrine, and/or endocrine fashion to promote metabolic homeostasis, and integrate adipose tissue, liver, muscle, and CNS physiology. There are currently more than 50 known adipokines, as well as locally generated hormones and metabolites that, together, affect multiple physiological functions including food intake, glucose homeostasis, lipid metabolism, inflammation, vascular tone, and angiogenesis. Because they affect such diverse and important processes, regulation of adipokine secretion from AT is critically important to regulating systemic metabolism. Notably, increased AT mass (as in obesity) induces characteristic qualitative and quantitative changes in adipose tissue metabolism and adipokine secretion. These changes are now implicated in the development of metabolic syndrome and its progression to more severe obesity-associated pathologies, including type 2 diabetes and cardiovascular disease.