[Adipose tissue and obesity]

Adipose tissue is not simply a storage depot. Adipocytes secrete hormones, growth factors and cytokines, such as leptin and TNF-alpha, as well as proteins that are related to the immune system and vascular functions. Through this network of endocrine, paracrine, and autocrine signals fat cells participate in the regulation of energy homeostasis, host defense and reproduction, and may also contribute to the development of pathological states, such as insulin resistance. Adipose tissue is confined to distinct depots. In Cushing's disease or following treatment of AIDS, certain adipose depots enlarge whereas others shrink, suggesting the existence of site-specific differences in fat cell function. Increases in adipocyte number occur via replication of preadipocytes, a process that is not restricted to infancy but occurs throughout life. In contrast to still widely-held beliefs, mature fat cells can be eliminated by dedifferentiation or apoptosis. PPAR-gamma, a transcription factor that is activated by fatty acids and prostaglandins, plays a central role in adipose conversion of preadipocytes and appears to participate in controlling the size of mature fat cells as well.