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

Oxidatively stressed lymphocytes exhibit decreased proliferative response t o mitogenic stimulation. Although several sensitive targets involved in lym phocyte 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. A lthough the polyunsaturated fatty acid content of membrane phospholipids is thought to be directly related to the extent of oxidant-induced lipid pero xidation, some n-3 fatty acids also seem to have antioxidant effects, depen ding 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 resistan t to a short-term exposure (10 min) to hydrogen peroxide (H2O2). Indeed, H2 O2-induced lipid peroxidation, estimated by malondialdehyde (MDA) productio n, was only 2-fold increased by H2O2 concentrations lower than 2 mM, wherea s a larger increase (10-fold) could be observed in PBMC at the highest dose (5 mM). Previous enrichment of PBMC with 5 mu M docosahexaenoic acid (22:6 n-3), brought to the cells as a fatty acid-albumin complex (ratio 1), signi ficantly reduced MDA production induced by low doses of H2O2, the protectiv e effect no longer being observed at the highest doses. In contrast, eicosa pentaenoic acid (20:5n-3) did not have any protective effect. Cytosolic PDE activities of both human PBMC and rat thymocytes were significantly inhibi ted (40-50%) after H2O2 treatment of the cells, whereas particulate PDE act ivities were not modified. Different responses of PDE activities to H2O2 tr eatment were observed when PBMC were first enriched with 22:6n-3 prior to H 2O2 addition. In 22:6n-3-treated cells, the H2O2-induced inhibition of both cAMP- and cGMP-PDE cytosolic activities was abolished, whereas the particu late activities were increased by the highest H2O2 concentration used (5 mM ). At the same time, the glutathione peroxidase (glutathione: oxidoreductas e, EC 1.11.1.9) (GSH-Px) activity of PBMC and thymocytes was only marginall y inhibited by H2O2 addition (20%), and pretreatment of the cells with 22:6 n-3 did not modify the slight inhibitory effect of H2O2. Collectively, thes e results suggest that lymphocytes are relatively resistant to H2O2-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 H2O2-induced alterations such as l ipid peroxidation and PDE inhibition. Docosahexaenoic acid might thus offer some protection against oxidant-induced lymphocyte suppression. (C) 1999 E lsevier Science Inc.