Protective effect of docosahexaenoic acid against hydrogen peroxide-induced oxidative stress in human lymphocytes

Abstract

Oxidatively stressed lymphocytes exhibit decreased proliferative response to mitogenic stimulation. Although several sensitive targets involved in lymphocyte suppression have already been identified, little is known about the influence of oxidative stress on cyclic nucleotide phosphodiesterases (PDE) (EC 3.1.4.17), thought to play a major role in the control of cyclic AMP (cAMP) level, a well-recognized negative effector of lymphoproliferation. Although the polyunsaturated fatty acid content of membrane phospholipids is thought to be directly related to the extent of oxidant-induced lipid peroxidation, some n-3 fatty acids also seem to have antioxidant effects, depending on the concentration used and the overall redox status of the cells in question. Results of the present study showed that human peripheral blood mononuclear cells (PBMC) as well as rat thymocytes were relatively resistant to a short-term exposure (10 min) to hydrogen peroxide (H 2O 2). Indeed, H 2O 2-induced lipid peroxidation, estimated by malondialdehyde (MDA) production, was only 2-fold increased by H 2O 2 concentrations lower than 2 mM, whereas a larger increase (10-fold) could be observed in PBMC at the highest dose (5 mM). Previous enrichment of PBMC with 5 μM docosahexaenoic acid (22:6n-3), brought to the cells as a fatty acid–albumin complex (ratio 1), significantly reduced MDA production induced by low doses of H 2O 2, the protective effect no longer being observed at the highest doses. In contrast, eicosapentaenoic acid (20:5n-3) did not have any protective effect. Cytosolic PDE activities of both human PBMC and rat thymocytes were significantly inhibited (40–50%) after H 2O 2 treatment of the cells, whereas particulate PDE activities were not modified. Different responses of PDE activities to H 2O 2 treatment were observed when PBMC were first enriched with 22:6n-3 prior to H 2O 2 addition. In 22:6n-3-treated cells, the H 2O 2-induced inhibition of both cAMP- and cGMP-PDE cytosolic activities was abolished, whereas the particulate activities were increased by the highest H 2O 2 concentration used (5 mM). At the same time, the glutathione peroxidase (glutathione: oxidoreductase, EC 1.11.1.9) (GSH-Px) activity of PBMC and thymocytes was only marginally inhibited by H 2O 2 addition (20%), and pretreatment of the cells with 22:6n-3 did not modify the slight inhibitory effect of H 2O 2. Collectively, these results suggest that lymphocytes are relatively resistant to H 2O 2-induced lipid peroxidation due to their high GSH-Px content, and that low doses of 22:6n-3 are able to prevent some of the H 2O 2-induced alterations such as lipid peroxidation and PDE inhibition. Docosahexaenoic acid might thus offer some protection against oxidant-induced lymphocyte suppression.