Endothelial inflammation induced by excess glucose is associated with cytosolic glucose 6-phosphate but not increased mitochondrial respiration.

Abstract

Exposure of endothelial cells to high glucose levels suppresses responses to insulin, including induction of endothelial nitric oxide synthase activity, through pro-inflammatory signalling via the inhibitor of nuclear factor kappaB (IkappaB)alpha-nuclear factor kappaB (NF-kappaB) pathway. In the current study, we aimed to identify metabolic responses to glucose excess that mediate endothelial cell inflammation and insulin resistance. Since endothelial cells decrease their oxygen consumption rate (OCR) in response to glucose, we hypothesised that increased mitochondrial function would not mediate these cells' response to excess substrate. The effects of glycolytic and mitochondrial fuels on metabolic intermediates and end-products of glycolytic and oxidative metabolism, including glucose 6-phosphate (G6P), lactate, CO(2), NAD(P)H and OCR, were measured in cultured human microvascular endothelial cells and correlated with IkappaBalpha phosphorylation. In response to increases in glucose concentration from low to physiological levels (0-5 mmol/l), production of G6P, lactate, NAD(P)H and CO(2) each increased as expected, while OCR was sharply reduced. IkappaBalpha activation was detected at glucose concentrations >5 mmol/l, which was associated with parallel increases of G6P levels, whereas downstream metabolic pathways were insensitive to excess substrate. Phosphorylation of IkappaBalpha by excess glucose correlates with increased levels of the glycolytic intermediate G6P, but not with lactate generation or OCR, which are inhibited well below saturation levels at physiological glucose concentrations. These findings suggest that oxidative stress due to increased mitochondrial respiration is unlikely to mediate endothelial inflammation induced by excess glucose and suggests instead the involvement of G6P accumulation in the adverse effects of hyperglycaemia on endothelial cells.