Docosahexaenoic Acid (DHA), but not Eicosapentaenoic Acid (EPA), Increases Both Membrane Fluidity and Cholesterol Crystalline Domain Formation in Lipid Vesicles

Abstract

Elevated free cholesterol levels lead to atherosclerotic-like changes in membrane structure, including cholesterol crystalline domain formation. Omega-3 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) may differentially affect cholesterol domain formation as a result of their distinct biophysical properties. In this study, we tested the comparative effects of EPA and DHA on lipid oxidation, membrane fluidity, and lipid structural organization. The lipid oxidation effects of each agent (10 μM) were measured in vesicles prepared as binary mixtures of dilinoleoylphosphatidylcholine and cholesterol and exposed to autoxidative conditions for 72 hr. Changes in membrane fluidity and lipid structural organization were measured using diphenylhexatriene (DPH) fluorescence anisotropy and small angle x-ray scattering approaches, respectively, in palmitoyloleoylphosphatidylcholine-enriched membrane vesicles prepared at 50 mol% cholesterol in the absence or presence of each agent at 5-10 mol% and tested over a broad range of temperatures. EPA and DHA were found to reduce lipid oxidation to a similar extent. However, only DHA affected membrane lipid fluidity, reducing DPH apparent rotational correlation values by 20% (19.4 to 15.6 nsec). Notably, DHA also induced cholesterol domain formation and reduced the lipid bilayer unit cell periodicity (d-space) from 57A to 55A with increasing temperature (15-30°C). By contrast, EPA had no effect on cholesterol domain formation and produced larger d-space values (60A to 57A, with increasing temperature) as compared to DHA (p<0.05). These data indicate that EPA and DHA, despite similar antioxidant benefits, had different effects on membrane bilayer width, lipid fluidity, and cholesterol crystalline domain formation. These results suggest that omega-3 fatty acids may differentially affect membrane lipid dynamics and structural organization as a result of their unique phospholipid/sterol interactions.