Investigation into the Role of Glycogen Synthase Kinase-3 in Hyperglycemia-Induced Atherosclerosis

Abstract

<p> Diabetes mellitus is a major independent risk factor for cardiovascular disease and stroke. However, the molecular and cellular mechanisms by which diabetes contributes to the development of vascular disease are not fully understood. We have shown that conditions of hyperglycemia are associated with accumulation of intracellular glucosamine, a downstream metabolite of glucose. Our findings indicate that elevated levels of intracellular glucosamine can promote inflammation and lipid accumulation - the hallmark features of atherosclerosis - in vascular cells and HepG2 cells.</p> <p> Here I demonstrate that exposure of HepG2 cells to the branched chain fatty acid, valproic acid, increases cellular resistance to glucosamine-induced lipid accumulation and nuclear factor-KB activation. In vivo I show that hyperglycemic apolipoprotein E-deficient (ApoE-/-) mice fed a diet supplemented with 625 mg/kg valproic acid have significantly reduced lesion volumes relative to non-supplemented controls. Valproate supplementation has no apparent effect on the plasma levels of glucose, or lipids, nor does it affect the expression of ER chaperones. Significant reductions were observed in total hepatic lipids(> 50.4%) and hepatic glycogen synthase kinases (GSK)-3β activity (> 55.8%) in mice fed the valproate supplemented diet.</p> <p> In vitro I demonstrate that valproic acid directly inhibits GSK-3α/β. Also pretreatment with novel GSK-3 inhibitors protects primary mouse hepatocytes from glucosamine-induced unesterified cholesterol accumulation. I further establish the role of GSK-3 by showing that GSK-3-deficient mouse embryonic fibroblasts do not accumulate unesterified cholesterol after glucosamine treatment. Dietary supplementation with 2-ethylbutyric acid, a novel and potent GSK-3 inhibitor in vitro, did not reduce lesion development in hyperglycemic ApoE-/- mice and significantly increased atherosclerosis in normoglycemic mice. This may be a side effect attributed to multiple cellular pathways controlled by GSK-3 .</p> <p> In conclusion, I have identified a pathway involving glucosamine-induced cellular dysfunction that leads to accelerated hyperglycemia-associated atherosclerosis. This pathway involves GSK-3, which regulates glucosamine-induced unesterified cholesterol accumulation.</p>