Secretion of leptin from adipocytes communicates body energy status to the brain by activating the leptin receptor long form (LRb). LRb regulates energy homeostasis and neuroendocrine function; the absence of LRb in db/db mice results in obesity, impaired growth, infertility and diabetes. Tyr 1138 of LRb mediates activation of the transcription factor STAT3 during leptin action. To investigate the contribution of STAT3 signalling to leptin action in vivo, we replaced the gene encoding the leptin receptor (lepr) in mice with an allele coding for a replacement of Tyr 1138 in LRb with a serine residue (lepr(S1138)) that specifically disrupts the LRb-STAT3 signal. Here we show that, like db/db mice, lepr(S1138) homozygotes (s/s) are hyperphagic and obese. However, whereas db/db mice are infertile, short and diabetic, s/s mice are fertile, long and less hyperglycaemic. Furthermore, hypothalamic expression of neuropeptide Y (NPY) is elevated in db/db mice but not s/s mice, whereas the hypothalamic melanocortin system is suppressed in both db/db and s/s mice. LRb-STAT3 signalling thus mediates the effects of leptin on melanocortin production and body energy homeostasis, whereas distinct LRb signals regulate NPY and the control of fertility, growth and glucose homeostasis.

Leptin, a hormone secreted by fat cells (1-3), mediates central hormonal and metabolic responses to nutritional status by binding and activating the long form of the leptin receptor (LRb) 1 in the hypothalamus (4). Alternate splicing of the LR primary transcript results in the production of multiple isoforms (LRa to -e) that contain a common extracellular domain (5-7). While most LR isoforms possess a short 32-40-amino acid intracellular tail, LRb contains a unique approximately 300-amino acid intracellular tail that is required for the normal regulation of energy balance and endocrine function (5-14).LRb is a member of the interleukin-6 receptor family of class I cytokine receptors (9,(15)(16)(17). Ligand binding to LRb results in the activation and tyrosine phosphorylation of Jak2 and the subsequent tyrosine phosphorylation of Tyr 985 and Tyr 1138 of LRb (18 -21). Tyrosine phosphorylation of cytokine and growth factor receptors activates intracellular signals by recruiting specific signaling proteins with specialized phosphotyrosinebinding domains, such as Src homology 2 (SH2) domains (22). The SH2 domain isoforms of different signaling proteins bind phosphotyrosine in the context of unique amino acid motifs; thus, each tyrosine phosphorylation site recruits specific downstream signaling proteins based on its surrounding amino acids. Phosphorylated Tyr 1138 of LRb recruits the SH2 domaincontaining transcription factor STAT3, resulting in the tyrosine phosphorylation of STAT3 and its translocation to the nucleus (17, 21,(23)(24)(25). Once in the nucleus, STAT3 mediates gene transcription, including transcription of the suppressor of cytokine signaling, SOCS3 (21, 26). Phosphorylated Tyr 985 of LRb recruits the SH2 domain-containing protein-tyrosine phosphatase SHP-2 (21, 27, 28). We have previously shown that Tyr 985 regulates ERK activation and c-fos transcription in cultured cells but does not alter phosphorylation of Jak2 or STAT3 during brief ligand stimulation (21). Others have suggested that Tyr 985 , by recruiting the tyrosine phosphatase SHP-2, may mediate dephosphorylation of LRb and Jak2, thereby attenuating LRb signaling (27,28).Mutations that disrupt leptin in rodents and humans result in morbid obesity and endocrine dysfunction (4, 29). Leptin levels are very high in obese humans, suggesting the presence of leptin resistance (4, 30). Human obesity/leptin resistance rarely results from genetic disruption of LRb (31), suggesting that other factors must mediate leptin resistance. A number of potential mechanisms for physiologic leptin resistance have been proposed, including saturation of a transport mechanism across the blood-brain barrier and the presence of biochemical inhibitors of LRb signaling, including SHP-2 and SOCS3 (4, 26 -28, 32-35).SOCS3 (also known as CIS3) is an SH2 domain-containing protein that inhibits signaling by certain cytokine receptor-Jak kinase complexes, either by directly inhibiting Jak2 activity or by targeting the complex to the proteosome (36). Leptin treatment induces...

Leptin signals the status of body energy stores via the leptin receptor (LR), a member of the Type I cytokine receptor family. Type I cytokine receptors mediate intracellular signaling via the activation of associated Jak family tyrosine kinases. Although their COOH-terminal sequences vary, alternatively spliced LR isoforms (LRaLRd) share common NH 2 -terminal sequences, including the first 29 intracellular amino acids. The so-called long form LR (LRb) activates Jak-dependent signaling and is required for the physiologic actions of leptin. In this study, we have analyzed Jak activation by intracellular LR sequences under the control of the extracellular erythropoeitin (Epo) (Epo receptor/LRb chimeras). We show that Jak2 is the requisite Jak kinase for signaling by the LRb intracellular domain and confirm the requirement for the Box 1 motif for Jak2 activation. A minimal LRb intracellular domain for Jak2 activation includes intracellular amino acids 31-48. Although the sequence requirements for intracellular amino acids 37-48 are flexible, intracellular amino acids 31-36 of LRb play a critical role in Jak2 activation and contain a loose homology motif found in other Jak2-activating cytokine receptors. The failure of short form sequences to function in Jak2 activation reflects the absence of this motif.Leptin is a 16-kDa adipocyte-derived hormone that communicates the status of body energy stores to the central nervous system, regulating appetite, metabolic rate, and neuroendocrine function (1, 2). Leptin mediates these effects by binding and activating a cell surface leptin receptor (LR) 1 ; the structure of leptin is homologous to that of the IL-6 family of cytokines, and the LR is a member of the IL-6 receptor family of class I cytokine receptors (3). Alternative splicing of RNA from a single LR gene produces multiple LR isoforms that share a common ligand-binding extracellular domain (4, 5). LRe lacks a transmembrane domain and is secreted. LRa-d each contain the same transmembrane domain and 29 membrane-proximal amino acids including the highly conserved, proline-rich Box 1 sequence that is required for Jak kinase activation by cytokine receptors. The number and identity of the subsequent amino acids varies among murine LRa-d, as well as the three human LR isoforms. LRb, which is highly conserved across species, contains a 282-amino acid extension (total 301-amino acid intracellular tail), robustly activates intracellular signaling, and is required to mediate the physiologic actions of leptin. Murine LRa, LRc, and LRd are the "short forms" of the leptin receptor with unclear physiological roles; these receptors contain 5, 3, and 11 amino acid extensions for 34-, 32-, and 40-amino acid intracellular tails, respectively.