Human leptin: from an adipocyte hormone to an endocrine mediator

Leptin is a mainly adipocyte-secreted protein that was discovered 5 years a go. Most of the research following this discovery focused on the role of le ptin in body weight regulation, aiming to illuminate the pathophysiology of human obesity. However, more and more data are emerging that leptin is not only important in the regulation of food intake and energy balance, but th at it also has a function as a metabolic and neuroendocrine hormone. It is now clear that it is especially involved in glucose metabolism, as well as in normal sexual maturation and reproduction. Besides this, interactions wi th the hypothalamic-pituitary-adrenal, thyroid and GH axes and even with ha ematopoiesis and the immune system have also been described. It has been sh own that leptin secretion by the adipocyte is partly regulated by other hor mones, such as insulin, cortisol, and sex steroids, mainly testosterone. Al so, other hormones like thyroid hormone and GH are possibly involved in lep tin synthesis. Leptin itself exerts effects on different endocrine axes, ma inly on the hypothalamic-pituitary-gonadal axis and on insulin metabolism, but also on the hypothalamic-pituitary-adrenal, thyroid and GH axes. Leptin map thus be considered a new endocrine mediator, besides its obvious role in body weight regulation.