Abstract 3954: Mechanisms Mediating The Proangiogenic Effects Of Leptin On Endothelial Progenitor Cells From Lean And Obese Individuals
Endothelial progenitor cells (EPC) contribute to neovascularization, and we have previously shown that the adipokine leptin enhanced their potency to promote angiogenesis in the murine hindlimb ischemia model and the chicken chorion allantoic membrane assay. We began to dissect the mechanisms mediating the proangiogenic effects of leptin on EPC. Affymetrix gene chip analysis of EPC mRNA expression following stimulation with leptin revealed differential (>1.5-fold) regulation of several genes involved in angiogenesis including VEGF, SDF1α, eNOS or TIMP1, and these findings were confirmed using RT-PCR. At the intracellular, postreceptor level, use of specific signal transduction inhibitors revealed that the effects of leptin on EPC outgrowth in the spheroid assay were mediated by PI3K and p42/44, but not p38 MAP kinase. Furthermore, spleen-derived EPC isolated from mice with a targeted mutation of Tyr1138 within the leptin receptor (leprS1138) did not respond to leptin (P<0.01 vs. wild-type EPC), suggesting the involvement of STAT3-mediated gene transcription. When EPC from morbidly obese (BMI>35 kg/m2; plasma leptin, 63.8±39.8 ng/mL) subjects were compared to those from lean (BMI<25 kg/m2; 7.4±4.2 ng/mL) individuals, they exhibited reduced angiogenic properties in the ex vivo Matrigel assay, and they failed to respond to leptin stimulation. Conditioned medium (CM) from obese EPC contained elevated levels of proinflammatory cytokines (IL1, MIP1β, MCP1, IGF1), but its ability to enhance the sprouting of mature EC was significantly reduced (P<0.05 vs. CM from lean EPC). In vivo, ischemic hindlimbs of mice treated with EPC from obese persons were characterized by enhanced muscle necrosis (P<0.05) and a higher infiltration with inflammatory cells (P<0.001), along with a reduced angiogenic response (P<0.05). At the intracellular level, significantly higher baseline phosphorylation levels of signal transduction proteins were detected in EPC from obese compared to lean subjects. Thus, leptin appears to promote EPC-mediated angiogenesis by mechanisms involving gene transcription and paracrine effects, and hyperphosphorylation of signaling molecules may contribute to the reduced angiogenic capacity of EPC in obesity and hyperleptinemia.