Physiological role of adipose tissue: white adipose tissue as an endocrineand secretory organ

The traditional role attributed to white adipose tissue is energy storage, fatty acids being released when fuel is required. The metabolic role of whi te fat is, however, complex. For example, the tissue is needed for normal g lucose homeostasis and a role in inflammatory processes has been proposed. A radical change in perspective followed the discovery of leptin; this crit ical hormone in energy balance is produced principally by white fat, giving the tissue an endocrine function. Leptin is one of a number of proteins se creted from white adipocytes, which include angiotensinogen, adipsin, acyla tion-stimulating protein, adiponectin, retinol-binding protein, tumour neor osis factor alpha, interleukin 6, plasminogen activator inhibitor-1 and tis sue factor. Some of these proteins are inflammatory cytokines, some play a role in lipid metabolism, while others are involved in vascular haemostasis or the complement system. The effects of specific proteins may be autocrin e or paracrine, or the site of action may be distant from adipose tissue. T he most recently described adipocyte secretory proteins are fasting-induced adipose factor, a fibrinogen-angiopoietin-related protein, metallothionein and resistin. Resistin is an adipose tissue-specific factor which is repor ted to induce insulin resistance, linking diabetes to obesity. Metallothion ein is a metal-binding and stress-response protein which may have an antiox idant role. The key challenges in establishing the secretory functions of w hite fat are to identify the complement of secreted proteins, to establish the role of each secreted protein, and to assess the pathophysiological con sequences of changes in adipocyte protein production with alterations in ad iposity (obesity, fasting, cachexia). There is already considerable evidenc e of links between increased production of some adipocyte factors and the m etabolic and cardiovascular complications of obesity. In essence, white adi pose tissue is a major secretory and endocrine organ involved in a range of functions beyond simple fat storage.