ROLE OF LEPTIN IN WOUND ANGIOGENESIS

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DESCRIPTION (provided by applicant): The healing of a wound is a complex, orchestrated process that requires rapid and effective restoration of the severed nicrocirculation through neovascularization. Accordingly, a variety of angiogenic factors and their cellular receptors are actively upregulated in response to conditions that typically develop in the wound environment, most notably hypoxia. Among these angiogenic factors, the P.I. has identified the pleiotropic cytokine leptin as an important member of this repertoire and subsequently established that expression of the leptin gene is acutely induced by hypoxia. Although leptin plays a central role in body weight regulation, it is also a potent angiogenic factor that acts upon target endothelial cells, which express fully functional, signaling-competent leptin receptors. Given the crucial role of neovascularization for normal wound healing progression, it is hypothesized here that the angiogenic activity of leptin is important for wound resolution. Consistent with this idea, it is well known that leptin- and leptin receptor-null mouse models exhibit severe impairment of wound healing. Studies conducted in the P.I. laboratory indicate that leptin is upregulated in wounds, and that treatment of wounds with leptin enhances healing in normal mice and restores healing in leptin-null mice, but not in leptin receptor-null mice. Furthermore, leptin treatment accelerates formation of small caliber vessels within the wound, achieving earlier the vascular density endpoint typical of uncomplicated wound healing. In addition, preliminary results shown in this proposal indicate that leptin rapidly increases expression of various hypoxia-inducible genes, including hypoxia inducible factor-1 (HIF-1). Based on these studies, it is hypothesized that the mechanism of angiogenesis promoted by leptin reflects a hypoxia-driven, cooperative interaction with HIF-1, whereby a coordinated response of a group of angiogenic genes ensues. Thus, the discovery that leptin promotes angiogenesis, coupled with more recent observations documenting the role of leptin in wound healing and the regulation of leptin synthesis under hypoxic conditions, suggest that hypoxia and leptin-induced neovascularization in the wound are two crucial, mutually linked events that participate in the normal tissue repair process. Therefore, two ensuing central hypotheses in this proposal are: 1) The angiogenic function of leptin is a key requirement for its wound healing-promoting activity; and 2) Hypoxia- and leptin-induced neovascularization in the wound are two crucial, linked events that are necessary for the normal tissue repair process. These hypotheses are explored in the research plan by the following specific aims: 1) to characterize and compare the wound angiogenic response in normal, leptin-null, and leptin receptor-null mice; 2) to determine the ability of dermal microvascular endothelial cells isolated from normal, leptin-null, and leptin receptor-null mice to form vascular structures in response to leptin in vitro; and 3) to identify the mechanism(s) by which leptin and HIF-1 coordinately induce expression of early hypoxia-sensitive genes in wounds.. The proposed research will help explain the physiological mechanisms by which leptin promotes wound neovascularization and will contribute to the understanding of the mechanisms of normal and impaired wound healing.