The Role of Endothelial Cell Metabolic Reprogramming in Diabetes-impaired Angiogenesis

Abstract

An impairment to ischaemia-driven angiogenesis underpins the development of many vascular complications in patients with diabetes. Angiogenesis is driven by endothelial cells, and to develop new therapies that can effectively support endothelial cell function and stimulate angiogenesis in the diabetic context, we need to understand more about how diabetes affects these cells and identify new essential pathways that can be therapeutically targeted. The role of mitochondrial respiration in diabetes-impaired angiogenesis has not previously been examined, and may play an important role in endothelial cell function. The PDK4/PDC axis represents one critical point at which mitochondrial respiration can be regulated, but little is known about its role in angiogenesis. The overarching aim of this thesis is to characterise the role of the PDK4/PDC axis in endothelial cell angiogenesis, determine the effect of diabetes upon the axis, and assess the efficacy of targeting the axis as a potential therapeutic strategy for diabetic vascular complications. The studies conducted in this thesis have characterised, for the first time, a highly context-specific and essential role for the PDK4/PDC axis and mitochondrial respiration in diabetes-impaired angiogenesis. Disruption of the axis by siRNA-mediated PDK4 knockdown impaired endothelial cell function including angiogenesis and migration, and high glucose exposure in vitro and hyperglycaemia in vivo impaired the induction of the PDK4/PDC axis in response to hypoxia. Overexpression of PDK4 and suppression of mitochondrial respiration rescued high glucose-impaired endothelial cell angiogenesis. rHDL also rescued diabetes-impaired angiogenesis in parallel with correction of the PDK4 response to hypoxia, and using siRNA knockdown the PDK4/PDC axis was found to be essential for these pro-angiogenic effects. Consistent with the effects of PDK4 overexpression in vitro, endothelial-specific, inducible overexpression of PDK4 by lentiviral gene transfer in vivo significantly enhanced wound angiogenesis and healing. Collectively, these data support our hypothesis that the PDK4/PDC axis plays a critical role in endothelial cell angiogenesis, and that diabetes impairs the integrity of the axis. Restoration of the PDK4/PDC axis, using either rHDL treatment or PDK4 overexpression, improved endothelial cell angiogenesis under high glucose conditions in vitro, and improved wound angiogenesis and healing in diabetic mice. In conclusion, endothelial cell metabolic reprogramming through the PDK4/PDC axis plays a critical role in endothelial cell angiogenic function, and shows promise as a novel therapeutic target in the treatment of diabetic vascular complications.