Leptin Signaling, Adiposity, and Energy Balance

The adipocyte-specific hormone leptin, the product of the obese (ob) gene, regulates adipose-tissue mass through hypothalamic effects on satiety and energy expenditure. Leptin acts through the leptin receptor, a single-transmembrane-domain receptor of the cytokine-receptor family. In rodents, homozygous mutations in genes encoding leptin or the leptin receptor cause early-onset morbid obesity, hyperphagia and reduced energy expenditure. These rodents also show hypercortisolaemia, alterations in glucose homeostasis, dyslipidaemia, and infertility due to hypogonadotropic hypogonadisms. In humans, leptin deficiency due to a mutation in the leptin gene is associated with early-onset obesity. Here we describe a homozygous mutation in the human leptin receptor gene that results in a truncated leptin receptor lacking both the transmembrane and the intracellular domains. In addition to their early-onset morbid obesity, patients homozygous for this mutation have no pubertal development and their secretion of growth hormone and thyrotropin is reduced. These results indicate that leptin is an important physiological regulator of several endocrine functions in humans.

Mutations in the mouse diabetes (db) gene result in obesity and diabetes in a syndrome resembling morbid human obesity. Previous data suggest that the db gene encodes the receptor for the obese (ob) gene product, leptin. A leptin receptor was recently cloned from choroid plexus and shown to map to the same 6-cM interval on mouse chromosome 4 as db. This receptor maps to the same 300-kilobase interval as db, and has at least six alternatively spliced forms. One of these splice variants is expressed at a high level in the hypothalamus, and is abnormally spliced in C57BL/Ks db/db mice. The mutant protein is missing the cytoplasmic region, and is likely to be defective in signal transduction. This suggests that the weight-reducing effects of leptin may be mediated by signal transduction through a leptin receptor in the hypothalamus.

Obesity in humans and in rodents is usually associated with high circulating leptin levels and leptin resistance. To examine the molecular basis for leptin resistance, we determined the ability of leptin to induce hypothalamic STAT3 (signal transducer and activator of transcription) signaling in C57BL/6J mice fed either low-fat or high-fat diets. In mice fed the low-fat diet, leptin activated STAT3 signaling when administered via the intraperitoneal (ip) or the intracerebroventricular (icv) route, with the half-maximal dose being 30-fold less when given by the icv route. The high-fat diet increased body-weight gain and plasma leptin levels. After 4 weeks on the diet, hypothalamic STAT3 signaling after ip leptin administration was equivalent in both diet groups. In contrast, peripherally administered leptin was completely unable to activate hypothalamic STAT3 signaling, as measured by gel shift assay after 15 weeks of high-fat diet. Despite the absence of detectable signaling after peripheral leptin at 15 weeks, the mice fed the high-fat diet retained the capacity to respond to icv leptin, although the magnitude of STAT3 activation was substantially reduced. These results suggest that leptin resistance induced by a high-fat diet evolves during the course of the diet and has at least two independent causes: an apparent defect in access to sites of action in the hypothalamus that markedly limits the ability of peripheral leptin to activate hypothalamic STAT signaling, and an intracellular signaling defect in leptin-responsive hypothalamic neurons that lies upstream of STAT3 activation.