Leptin--from a signal of adiposity to a hormonal mediator in peripheral tissues.

The biology of leptin has been studied most extensively in the central nervous system for the regulation of food intake and energy balance. In recent years, a growing number of publications have reported several activities of this adipose-secreted protein in different organs. These effects appear to be independent of the regulation of food intake or at least not directly correlated to it, but rather related to the hormonal regulation of these particular tissues. Thus leptin is now also considered to be a hormonal factor that informs several hormonal circuits and biological peripheral functions of the nutrition status of the organism. Different systems are involved in leptin activity, such as the pituitary, male and female reproductive organs, the mammary gland, the immune system, the gut, the kidney and the lung. Functional leptin receptors and/or leptin protein have been shown to be expressed in these tissues. Furthermore, interesting interactions have been reported with classical hormones involved in the regulation of activities in such organs. These observations give more detailed evidence of the relationship between nutrition and tissue differentiation in peripheral sites, possibly mediated by classical hormonal circuits. This work aims to review the most important functional findings on leptin's effects in these peripheral sites, and potential future studies are suggested, based on currently available data.