Eicosapentaenoic Acid (EPA), but not other Triglyceride-Lowering Agents, Inhibits Glucose-Induced Changes in Membrane Width and Cholesterol Domain Formation through a Potent Antioxidant Mechanism

Abstract

Elevated glucose levels are causally related to lipid oxidation and membrane structural changes, including the formation of cholesterol crystalline domains. In this study, we tested the comparative effects of eicosapentaenoic acid (EPA) and other triglyceride-lowering agents (fenofibrate, niacin, and gemfibrozil) on glucose-induced changes in membrane structure. Iodometric approaches were used to measure lipid hydroperoxide (LOOH) formation in vesicles prepared as binary mixtures of dilinoleoylphosphatidylcholine and cholesterol at a cholesterol-to-phospholipid (C/P) mole ratio of 0.6:1. Membrane samples were treated with glucose at 200 mg/dL and subjected to autoxidation at 37°C for 72-96 hr. Small angle x-ray diffraction approaches were used to measure changes in membrane lipid structural organization, including bilayer width (d-space) and cholesterol crystalline domain formation. We found that glucose treatment significantly increased LOOH formation concomitant with a reduction in membrane bilayer width and an increase in cholesterol domain formation. EPA reduced LOOH formation by 90% as compared to glucose treatment alone (p<0.001) and preserved membrane width at 52.6 ± 0.3 A while inhibiting cholesterol domain formation. By contrast, there were pronounced reductions in membrane d-space with fenofibrate (45.4 ± 0.7 A), niacin (46.8 ± 0.9 A) and gemfibrozil (45.0 ± 1.3 A) as compared to vehicle (50.6 ± 1.9 A), all of which were highly significant (p<0.001) and associated with increased lipid oxidation and cholesterol domain formation. These data suggest that EPA inhibits glucose-induced changes in membrane width and cholesterol crystalline domain formation through a potent antioxidant mechanism. The other triglyceride-lowering agents enhanced glucose-induced reductions in membrane width and failed to interfere with cholesterol domain formation or lipid peroxidation. These findings provide insight into novel potentially atheroprotective benefits of EPA that are independent of lipid changes.