Endogenous and exogenous antioxidants and the generation of antigenic epitopes in oxidatively-modified LDL

DISTRIBUTION OF LIPID-SOLUBLE ANTIOXIDANTS IN LIPOPROTEINS FROM HEALTHY SUBJECTS. I. CORRELATION WITH PLASMA ANTIOXIDANT LEVELS AND COMPOSITION OF LIPOPROTEINS

The concentration of five lipid-soluble antioxidants (gamma- and alpha-tocopherol, lycopene, beta-carotene and ubiquinol-10) was measured in plasma and very low-density, low-density and high-density lipoproteins (VLDL, LDL and HDL) isolated from young healthy normo- cholesterolemic subjects. Alpha-tocopherol was the exclusive antioxidant whose plasma concentration significantly correlated with the absolute concentration of total cholesterol (r =0.541, P<0.001). No correlation was found between plasma concentration and lipoprotein content of alpha-tocopherol and ubiquinol-10, whereas it reached statistically significant values for gamma-tocopherol, lycopene and beta-carotene. The alpha-tocopherol content in VLDL and HDL, but not in LDL, was strictly associated with the relative abundance of cholesterol and phospholipids in the lipoprotein particles. Moreover, the difference between alpha-tocopherol concentration in VLDL and LDL appeared to be strictly related to the differences in cholesterol, phospholipids and triglycerides. The percent distribution of the total plasma pool of antioxidant in each lipoprotein class revealed that gamma- and alpha-tocopherol were roughly equally distributed in LDL and HDL. On the other hand, lycopene, beta-carotene and ubiquinol-10 were preferentially sequestered in LDL. Finally, the absolute and relative concentration of alpha-tocopherol, but not that of other antioxidants, in HDL exhibited a statistically significant correlation with plasma HDL/LDL cholesterol ratio. These findings indicate that: (i) plasma concentrations of major lipid-soluble antioxidants are not always predictive of their levels in lipoproteins and that, within individual lipoprotein classes, (ii) the lipid composition, metabolism and relative plasma concentration may significantly affect their abundance. 2000 Academic Press@p$hr Copyright 2000 Academic Press.

Tissue-specific functions of individual glutathione peroxidases

The family of glutathione peroxidases comprises four distinct mammalian selenoproteins. The classical enzyme (cGPx) is ubiquitously distributed. According to animal, cell culture and inverse genetic studies, its primary function is to counteract oxidative attack. It is dispensible in unstressed animals, and accordingly ranks low in the hierarchy of glutathione peroxidases. The gastrointestinal isoenzyme (GI-GPx) is most related to cGPx and is exclusively expressed in the gastrointestinal tract. It might provide a barrier against hydroperoxides derived from the diet or from metabolism of ingested xenobiotics. The extreme stability in selenium deficiency ranks this glutathione peroxidase highest in the hierarchy of selenoproteins and points to a more vital function than that of cGPx. Plasma GPx (pGPx) behaves similar to cGPx in selenium deficiency. It is directed to extracellular compartments and is expressed in various tissues in contact with body fluids, e.g., kidney, ciliary body, and maternal/fetal interfaces. It has to be rated as an efficient extracellular antioxidant device, though with low capacity because of the limited extracellular content of potential thiol substrates. Phospholipid hydroperoxide glutathione peroxidase (PHGPx), originally presumed to be a universal antioxidant enzyme protecting membrane lipids, appears to have adopted a variety of specific roles like silencing lipoxygenases and becoming an enzymatically inactive structural component of the mitochondrial capsule during sperm maturation. Thus, all individual isoenzymes are efficient peroxidases in principle, but beyond their mere antioxidant potential may exert cell- and tissue-specific roles in metabolic regulation, as is evident for PHGPx and may be expected for others.

13-HPODE and 13-HODE modulate cytokine-induced expression of endothelial cell adhesion molecules differently

Expression of cellular adhesion molecules (CAMs) at endothelial surfaces represents a physiological response to vascular damage and mediates the initiation of inflammation and possibly of atherogenesis. The cytokines TNF alpha and IL-1 are potent inducers of CAMs in endothelial cells. Reactive oxygen species comprising lipid oxidation products have been implicated in the signaling pathways of both TNF alpha and IL-1 and accordingly could modulate atherogenic events. We, therefore, investigated the potential role of the lipoxygenase product, 13-hydroperoxyoctadecadienoic acid (13-HPODE), which has also been identified in oxidized low density lipoproteins on CAM expression in HUVEC. 13-HPODE induced the expression of ICAM-1 in a concentration dependent manner up to 75 microM. Higher concentrations were toxic. Similar effects were observed with H2O2 and phosphatidylcholine hydroperoxide. VCAM-1 and E-selectin were not induced by 13-HPODE. 13-HPODE administered simultaneously with IL-1 or TNF alpha induced ICAM-1 additively, suggesting that hydroperoxides and cytokines act on the same signaling pathways. In contrast, pretreatment of cells with 50 microM 13-HPODE for 1 hour rather inhibited subsequent cytokine-induced ICAM-1 and E-selectin expression. Surprisingly, the reduction product of 13-HPODE, 13-hydroxyoctadecadienoic acid (13-HODE) proved to be an even better inducer of ICAM-1 than 13-HPODE. Pretreatment with 13-HODE did not show any inhibitory effect on ICAM-1 expression. Our data show that lipoxygenase products differentially affect CAM expression. 13-HPODE is stimulatory by itself and can positively or negatively affect cytokine signaling depending on time of exposure. 13-HODE induces CAM expression by itself but does not inhibit cytokine signaling. Thus, the interplay of lipoxygenase products with proinflammatory cytokines can not simply be explained by an oxidant-mediated facilitation of cytokine signaling.