Leptin is a hormone that regulates food intake, and its receptor (OB-Rb) is expressed primarily in the hypothalamus. Here, it is shown that OB-Rb is also expressed in human vasculature and in primary cultures of human endothelial cells. In vitro and in vivo assays revealed that leptin has angiogenic activity. In vivo, leptin induced neovascularization in corneas from normal rats but not in corneas from fa/fa Zucker rats, which lack functional leptin receptors. These observations indicate that the vascular endothelium is a target for leptin and suggest a physiological mechanism whereby leptin-induced angiogenesis may facilitate increased energy expenditure.

In addition to having a major role in energy homeostasis, leptin is emerging as a pleiotropic cytokine with multiple physiological effector functions. The recently discovered proangiogenic activity of leptin suggested the hypothesis that its production might be regulated by hypoxia, as are other angiogenic factors. To examine this proposal, the expression of leptin protein and mRNA was measured and found to be markedly up-regulated in response to ambient or chemical hypoxia (upon exposure to desferrioxamine or cobalt chloride), an effect that requires intact RNA synthesis, suggesting a transcriptional mechanism. Transient transfection of cultured cells with deletion constructs of the leptin gene promoter linked to a reporter gene revealed a functional hypoxia response element (HRE) located at position -116 within the proximal upstream region. This putative HRE harbors a characteristic 5-RCGTG-3 core motif, a hallmark of hypoxia-sensitive genes and recognized by the hypoxia-inducible factor 1 (HIF1), which consists of a HIF1␣/HIF␤ heterodimer. Constructs harboring this -116/HRE supported reporter gene expression in response to hypoxia but not when mutated. Expression of HIF1␣ cDNA in normoxic cells mimicked hypoxia-induced reporter gene expression in cells cotransfected with the wild type leptin -116/HRE construct but not with the mutant. Gel shift assays with a 32 P-labeled leptin promoter -116/HRE probe and nuclear extracts from hypoxia-treated cells indicated binding of the HIF1␣/␤ heterodimer, which was blocked with an excess of unlabeled -116/HRE probe or a HIF1-binding probe from the erythropoietin gene enhancer. Taken together, these observations demonstrate that the leptin gene is actively engaged by hypoxia through a transcriptional pathway commonly utilized by hypoxia-sensitive genes.Physiological mechanisms that ensure an appropriate level of oxygen (O 2 ) delivery to tissues have evolved in complex multicellular organisms. Virtually all cells are capable of sensing changes in O 2 tension (pO 2 ) and respond adaptively when the O 2 demand exceeds supply, a condition referred to as hypoxia (1). Hypoxia can develop as a result of ischemia resulting from hypoperfusion, either as a pathological condition or as a transient physiological event (1). Under chronic conditions of hypoxia, typical adaptation responses generally include changes in the expression of genes encoding molecules that facilitate O 2 delivery or by activating metabolic pathways that do not require O 2 , thus maintaining energy homeostasis when O 2 availability is limited (1, 2). For example, hypobaric hypoxia leads to a classical response characterized by increased red blood cell mass formation after induction of the erythropoietin (Epo) 1 gene, whose expression is elevated markedly under these conditions (3, 4). In addition, the vasodilators nitric oxide and carbon monoxide are generated by the catalytic activity of inducible nitric oxide and heme oxygenase-1, respectively; expression of the genes encoding these enzymes is induced readil...

Leptin, a 16 kDa pleiotropic cytokine primarily expressed in adipose tissue, has been shown to cause multiple systemic biological actions. Recently, leptin has also been documented as an important component of the wound healing process and its receptor appears to be expressed in wound tissue. We have previously demonstrated that leptin is a potent angiogenic factor exerting direct effects on endothelial cells and that transcription of its encoding gene is regulated by hypoxia. Here, we hypothesize that leptin expression is acutely up-regulated in the ischemic tissue of experimental wounds. Using a combination of in situ hybridization and quantitative RT-PCR experiments, we show that leptin expression is rapidly and steadily up-regulated in skin tissue from incisional and excisional wounds. By immunohistochemistry, we demonstrate increased and sustained leptin protein levels in basal keratinocytes, blood vessel walls, and fibroblasts. To determine whether leptin is required for normal healing, excisional wounds were treated with neutralizing anti-leptin antibodies. This treatment markedly hampered healing progression and prevented wound closure and contraction. Finally, a transient rise in circulating blood leptin levels was detected within the first 24 h after inflicting the injury; we present evidence suggesting that this elevation is due to increased leptin production at the ischemic wound site. We conclude that leptin is acutely up-regulated in the injured skin and propose that this local production of leptin serves a critical functional role as an autocrine/paracrine regulator of normal wound healing.