INHIBITION BY INTERFERON-GAMMA OF HUMAN MONONUCLEAR CELL-MEDIATED LOW-DENSITY-LIPOPROTEIN OXIDATION - PARTICIPATION OF TRYPTOPHAN-METABOLISM ALONG THE KYNURENINE PATHWAY

In this study we examined the potential inhibition by interferon-gamma (IFN gamma) of the early stages of low density lipoprotein (LDL) oxid ation mediated by human peripheral blood mononuclear cells (PBMC) and monocyte-derived macrophages (MDM) in Ham's F-10 medium supplemented w ith physiological amounts of L-tryptophan (Trp). We assessed LDL oxida tion by measuring the consumption of LDL's major antioxidant (i.e., al pha-tocopherol) and targets for oxidation (cholesteryllinoleate and ch olesterylarachidonate), together with the accumulation of cholesteryle ster hydroperoxides and the increase in relative electrophoretic mobil ity of the lipoprotein particle. Exposure of PBMC or MDM to IFN gamma induced the degradation of extracellular Trp with concomitant accumula tion of kynurenine, anthranilic and 3-hydroxyanthranilic acid (3HAA) i n the culture medium. Formation of 3HAA, but neither Trp degradation n or formation of kynurenine and anthranilic acid, was inhibited by low amounts of diphenylene iodonium (DPI) in a concentration-dependent man ner. In contrast to oxidative Trp metabolism, exposure of human PBMC o r MDM to IFN gamma failed to induce degradation of arginine, and nitri te was not detected in the cell supernatant, indicating that nitric ox ide synthase was not induced under these conditions. Incubation of LDL in Trp-supplemented F-10 medium resulted in a time-dependent oxidatio n of the lipoprotein that was accelerated in the presence of PBMC or M DM but inhibited strongly in the presence of both cells and IFN gamma, i.e., when Trp degradation and formation of 3HAA were induced. In con trast, when IFN gamma was added to PBMC or MDM in F-10 medium that was virtually devoid of Trp, inhibition of cell-accelerated LDL oxidation was not observed. Exogenous 3HAA added to PBMC or purified monocytes in the absence of IFN gamma also strongly and in a concentration-depen dent manner inhibited LDL oxidation. Selective inhibition of IFN gamma -induced formation of 3HAA by DPI caused reversion of the inhibitory a ction of this cytokine on both PBMC- and MDM-mediated LDL oxidation. T hese results show that IFN gamma treatment of human PBMC or MDM in vit ro attenuates the extent of LDL oxidation caused by these cells, and i ndicate that Trp degradation with formation of 3HAA is a major contrib uting factor to this inhibitory activity.