Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIVrelated lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics. adipokines; adipose tissue; leptin resistance; leptin deficiency; hypoleptinemia STUDIES OF GENETICALLY OBESE MICE serendipitously found in the Jackson Laboratories revealed that their phenotypes derive from homozygous mutations of either the obese (ob) or diabetic (db) genes that result in obesity and insulin resistance or diabetes as well as endocrine and immune dysfunction (53, 54,115,117,183,261). The gene mutation in the ob/ob mouse results in a complete deficiency of or a truncated and biologically inactive ob gene product (287); the latter subsequently was given the name leptin (95), from the Greek word "leptos" (meaning "thin"), because when this protein was given to the obese ob/ob mice they lost significant amounts of body weight. It was then recognized that the db gene codes for the leptin receptor (140). Consequently, exogenously administered leptin reduces body weight and resolves the metabolic, endocrine, and immune disturbances in ob/ob mice but has no effects in db/db mice (100, 111, 28...

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Objective Animal and in vitro studies indicate that leptin alleviates starvation-induced reduction in circulating vaspin and stimulates the production of visfatin. We thus examined whether vaspin and visfatin are affected by short- and long-term energy deprivation and leptin administration in human subjects in vivo. Design and Methods We measured circulating levels of vaspin and visfatin 1) before and after 72-h of starvation (leading to severe hypoleptinemia) with or without leptin administration in replacement doses in 13 normal-weight subjects, 2) before and after 72-h of starvation with leptin administration in pharmacological doses in 13 lean and obese subjects, 3) during chronic energy deficiency in 8 women with hypothalamic amenorrhea on leptin replacement for 3 months, and 4) during chronic energy deficiency in 18 women with hypothalamic amenorrhea on leptin replacement or placebo for 3 months. Results Acute starvation decreased serum leptin (to 21% of baseline values, P=0.002) but had no significant effect on vaspin and visfatin concentrations (P>0.05). Nor did normalization of leptin levels affect the concentrations of these two adipokines (P>0.9). Leptin replacement in women with hypothalamic amenorrhea did not significantly alter vaspin and visfatin concentrations, whether relative to baseline (P>0.15) or relative to placebo administration (P>0.9). Pharmacological doses of leptin did not affect circulating vaspin and visfatin concentrations (P>0.9). Conclusions Circulating vaspin and visfatin are not affected by acute or chronic energy deficiency leading to hypoleptinemia and are not regulated by leptin in human subjects, indicating that these adipocyte-secreted hormonal regulators of metabolism are independently regulated in humans.