Docosahexaenoic acid suppresses the generation of reactive oxygen species in endothelial cells: a role for retinoid X receptor RXR

Background and objectives: Docosahexaenoic acid (DHA) is known to play an important role in several physiological functions. However, clinical evaluations of DHA for arteriosclerosis prevention have not been conducted conclusive and molecular mechanisms underlying DHA's effect remain unclear. To fill this research gap, we investigated molecular mechanisms for DHA's effect on endothelial cells using human umbilical vein endothelial cells (HUVEC). Methods and results: High levels of reactive oxygen species (ROS) were generated following oxidative stress generated by irradiation of cells with light and the ROS thus produced diffused from the nucleus to the cytoplasm. We found that DHA (as low as 10 nM) strongly suppressed endothelial ROS generation, more so than eicosapentaenoic acid (EPA). We also noted that HUVEC showed a large variation in the reactivity of DHA depending on individual differences of donors. The effect of DHA was canceled by HX531, a retinoid X receptor (RXR)-selective antagonist. Additionally, bexarotene as an RXR-selective agonist similarly suppressed endothelial ROS. We also found some DHA-insensitive HUVEC, from different donors, in which DHA does not suppress ROS generation even with high doses of DHA. However, when these cells were pretreated with LE 540 as a retinoid A receptor (RAR)-selective antagonist, then DHA suppressed ROS generation. When the DHA-insensitive HUVEC strains were cultured in a medium without retinol acetate, vitamin D2 and cholesterol, the ROS inhibitory effect of DHA was restored. These factors are ligands of nuclear receptors that dimerize with RXR. Conclusions: These results indicate that DHA suppresses the generation of endothelial ROS through interaction with RXR. Thus, the difference in reactivity to DHA may be determined by the amount of RXR available to bind to DHA.