Plasma and cell levels of vitamin C, or ascorbic acid, are decreased about 50% in persons with type II diabetes in sub-optimal glycemic control, despite presumably adequate dietary intakes. This decrease is due to oxidative stress from excess glucose and fatty acid metabolism. This gluco/lipotoxicity is especially damaging to the vascular endothelium, where it impairs endothelial function and increases endothelial permeability to blood constituents. Results from the previous project period showed that ascorbic acid tightens the endothelial barrier to passage of large molecules, at least in part by sparing the vasoactive molecule nitric oxide. More recent results show that intracellular ascorbic acid also prevents the increase in endothelial barrier permeability caused by high glucose concentrations. This leads to the main hypothesis of this proposal that ascorbic acid helps to maintain endothelial barrier function in vivo and especially in the face of diabetes-induced oxidative stress. The mechanism of this barrier protection is proposed to involve ascorbic acid scavenging of reactive oxygen species generated in response to high glucose concentrations, as well as ascorbic acid recycling of tetrahydrobiopterin to prevent uncoupling of endothelial nitric oxide synthase. Uncoupling of the enzyme will cause it to generate superoxide rather than nitric oxide, thus increasing oxidative stress. This hypothesis and mechanism will be tested in endothelial cells cultured at hyperglycemic glucose concentrations and extended to novel mouse models of ascorbic acid deficiency or repletion in the context of streptozotocin-induced diabetes.
The first aim asks how physiologic ascorbic acid concentrations support basal nitric oxide generation in endothelial cells and whether this contributes to the ability of the vitamin to tighten the endothelial barrier to passage of large molecules.
The second aim tests how ascorbic acid prevents endothelial barrier failure caused by high glucose concentrations, again with focus on a mechanism involving ascorbic acid prevention of uncoupling of nitric oxide synthase. Since many of the deleterious effects of high glucose concentrations are caused by activation of the Receptor for Advanced Glycation End-products (RAGE), the third aim will test the mechanism by which ascorbic acid prevents increased endothelial permeability due to RAGE ligands.
The fourth aim will extend the in vitro studies to novel mouse models in which ascorbic concentrations can be manipulated by dietary and genetic means to test effects of increased oxidative stress due to streptozotocin- induced diabetes. Key to his aim is to assess whether diabetes-induced oxidative stress and subsequent endothelial dysfunction is worsened by depletion of ascorbic acid and reversed by its repletion.
Persons with diabetes have increased inflammation that generates oxidative stress. This oxidative stress in turn damages the cells that line vessel walls and increases leak of blood components into tissues and urine. This project proposes to test the hypothesis that diabetes- induced decreases in vitamin C contribute to this loss of barrier function and that full repletion of vitamin C will help to preserve the integrity of the vascular bd.
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