Adipose tissue

White adipose tissue was long seen as a passive reservoir for the storage of fat derived from the diet or from endogenous synthesis. Meanwhile it is clear that white adipose tissue is also an important endocrine organ [for review see Kershaw and Flier (2004)]. The proteins that are produced and released by adipose tissue are called adipokines. It is important to note that white adipose tissue is not a homogeneous organ. The two best-described adipose tissue depots are subcutaneous and visceral adipose tissues. FFA, glycerol and hormones from visceral adipose tissue are directly released into the hepatic portal vein and thus have direct access to the liver, whereas the subcutaneous fat depots release their adipokines and metabolites into the systemic circulation. Therefore it is clear that visceral adipose tissue has a greater effect on hepatic metabolism than subcutaneous adipose tissue. Finally, the two adipose tissues have different adipokine secretion patterns, with visceral adipose tissue secreting mainly interleukin-6 (IL-6) and plas-minogen activator inhibitor (PAI), whereas the secretion of leptin and adiponectin is relatively greater from subcutaneous than from visceral adipose tissue. Furthermore, visceral and subcutaneous adipocytes also carry different receptors on their surfaces and, hence, respond differently to signals. For example, expression of p3-adrenergic, glucocorticoid, and androgen receptors is greater in visceral than in subcutaneous adipose tissue. All these differences might contribute to the fact that enlarged visceral but not subcutaneous adipose tissue is associated with increased risk for several diseases and in particular the metabolic syndrome (Kershaw and Flier 2004). In the following section we will discuss the most important proteins secreted by white adipose tissue.

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