Reconstituted High-Density Lipoproteins Rescue Diabetes-Impaired Endothelial Cell Metabolic Reprograming and Angiogenic Responses to Hypoxia

BACKGROUND: Impaired angiogenic responses to ischemia underlie diabetic vascular complications. Reconstituted high-density lipoproteins (rHDLs) have proangiogenic effects in diabetes. The PDK4 (pyruvate dehydrogenase kinase 4)/PDC (pyruvate dehydrogenase complex) axis is an oxygen-conserving mechanism that preserves endothelial cell function in hypoxia. We aimed to determine the role of the PDK4/PDC axis in angiogenesis, the effect of diabetes on its regulation in response to ischemia, and the proangiogenic properties of rHDL. METHODS: Expression of PDK4 and phosphorylated PDC (pPDC) were measured in PBS- or rHDL-treated wounds of nondiabetic and streptozotocin-induced diabetic mice and PBS- or rHDL-treated endothelial cells exposed to glucose and hypoxia. The importance of PDK4 in the action of rHDL was determined by siRNA knockdown in vitro and PDK4 inhibitor in vivo. Chromatin immunoprecipitation assay was performed to identify the mechanism for PDK4 induction by rHDL. RESULTS: PDK4 and pPDC were elevated early (24 hours) post-induction of wound ischemia in nondiabetic wounds, which did not occur in diabetic mice. Topical rHDL rescued this impairment, enhancing PDK4 (68%; P =0.0041) and pPDC (165%; P =0.029) in diabetic wounds. Wound neovascularization (62%; P <0.05) and closure (154%; P <0.001) were increased in diabetic rHDL-treated wounds. In vitro, PDK4 and pPDC levels were increased with hypoxia (65%, P =0.043 and 64%, P =0.026, respectively). High glucose did not elicit a further stepwise induction in PDK4/pPDC, with aberrant increases in mitochondrial respiration (19%; P =0.026), and impaired angiogenic functions. Importantly, rHDL increased PDK4 and pPDC 2-fold, returning mitochondrial respiration and angiogenic functions to normal glucose levels. PDK4 inhibition ameliorated the proangiogenic effects of rHDL. rHDL increased FOXO1 (forkhead box O1) binding to the PDK4 promoter and suppressed FOXO1 phosphorylation, presenting FOXO1 as a mechanism for rHDL-mediated induction of PDK4. CONCLUSIONS: The PDK4/PDC axis response to ischemia is impaired in diabetes and is important for the proangiogenic effects of rHDL.