Oxidized omega-3 fatty acids in fish oil inhibit leukocyte-endothelial interactions through activation of PPARα

Abstract

Omega-3 fatty acids, which are abundant in fish oil, improve the prognosis of several chronic inflammatory diseases although the mechanism for such effects remains unclear. These fatty acids, such as eicosapentaenoic acid (EPA), are highly polyunsaturated and readily undergo oxidation. We show that oxidized, but not native unoxidized, EPA significantly inhibited human neutrophil and monocyte adhesion to endothelial cells in vitro by inhibiting endothelial adhesion receptor expression. In transcriptional coactivation assays, oxidized EPA potently activated the peroxisome proliferator-activated receptor ␣ (PPAR␣), a member of the nuclear receptor family. In vivo, oxidized, but not native, EPA markedly reduced leukocyte rolling and adhesion to venular endothelium of lipopolysaccharide (LPS)-treated mice. This occurred via a PPAR␣-dependent mechanism because oxidized EPA had no such effect in LPS-treated PPAR␣-deficient mice. Therefore, the beneficial effects of omega-3 fatty acids may be explained by a PPAR␣-mediated anti-inflammatory effect of oxidized EPA. IntroductionConsumption of omega-3 fatty acids in fish oil has been reported to improve the prognosis of several chronic inflammatory diseases characterized by leukocyte accumulation, including atherosclerosis, systemic lupus erythematosus, psoriasis, inflammatory bowel disease, and rheumatoid arthritis. [1][2][3][4] Fish oil is also recommended for treatment of IgA nephropathy, the most common form of primary renal disease worldwide. 5,6 Several studies suggest that omega-3 fatty acid supplementation may reduce the inflammatory response by attenuating leukocyte adhesion to the vessel wall. [7][8][9] However, the primary mechanism for the anti-inflammatory effects of fish oil remains unclear. 10 Omega-3 fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are highly polyunsaturated and readily undergo oxidation. 11 This suggests the possibility that oxidized omega-3 fatty acids may be an important component of the observed anti-inflammatory effects of fish oil. Indeed, Sethi and colleagues showed that oxidized EPA and DHA are more potent than native fatty acids in reducing RNA levels of leukocyte adhesion receptors and the adhesion of leukocyte cell lines to endothelial cells in vitro. 12 Polyunsaturated fatty acids may exert their effects in cells by activation of peroxisome proliferatoractivated receptors (PPARs). PPARs are ligand-activated transcription factors that regulate genes important in cell differentiation and various metabolic processes. Known PPAR isoforms include PPAR␥, important in lipoprotein metabolism, adipogenesis, and insulin sensitivity; PPAR␣, implicated in fatty acid metabolism; and PPAR␦, about which the least is known. 13,14 Synthetic PPAR␣ ligands such as fibrates (eg, fenofibric acid) are used in patients to lower triglyceride-rich lipoproteins. Synthetic PPAR␥ ligands such as thiazolidinediones (eg, rosiglitazone) are insulin-sensitizing drugs used in the treatment of diabetes. Polyunsaturated fatty acids...

Oxidized Omega-3 Fatty Acids Inhibit NF-κB Activation Via a PPARα-Dependent Pathway

“…In our previous studies, we showed that oxidized EPA and not unoxidized EPA pretreatment of HUVEC inhibits leukocyte adhesion to cytokine-stimulated HUVEC, and this effect is mediated through a PPAR␣-dependent pathway. 11,12 In our present studies, we extend these observations and show that oxidized EPA is also effective in inhibiting cytokine-induced endothelial chemokine expression, particularly MCP-1, and propose a mechanism for these antiinflammatory effects.…”

“…In our previous studies, we showed that oxidized EPA and not unoxidized EPA pretreatment of HUVEC inhibits leukocyte adhesion to cytokine-stimulated HUVEC, and this effect is mediated through a PPAR␣-dependent pathway. 11,12 In our present studies, we extend these observations and show that oxidized EPA is also effective in inhibiting cytokine-induced endothelial chemokine expression, particularly MCP-1, and propose a mechanism for these antiinflammatory effects.The fact that oxidation of the omega-3 fatty acids is required for the aforementioned antiinflammatory effects is also pointed out by other studies. Nohe et al and De Caterina et al have shown that prolonged incubation of endothelial cells with native omega-3 fatty acids (26 hours of total exposure, 6 hours before and 20 hours after TNF␣ stimulation) results in inhibition of cytokine and adhesion molecule The nuclear extracts were coincubated with 32 P-labeled NF-B oligonucleotide, and electrophoresis mobility shift assays were performed (nϭ3).…”

“…Delerive et al (1999) have shown, using glutathione S-transferase pulldown experiments, that after fibrate (PPAR␣ agonist) treatment, PPAR␣ physically interacts with p65 in vitro. 33 Taken together with our previous studies, 11,12 we show that auto-oxidation of omega-3 fatty acids results in the generation of oxidized compounds with potent antiinflammatory properties that inhibit proinflammatory responses such as leukocyte adhesion receptor and chemokine expression. The central theme for the antiinflammatory effects of oxidized omega-3 fatty acids is likely through inhibition of NF-B via a PPAR␣-dependent pathway.…”

“…Also, oxidized EPA did not have any effect on the constitutively expressed surface proteins such as von Willebrand factor, endoglin, and human leukocyte antigen class I molecules. 11,12 The expression of MCP-1 and IL-8 is regulated through activation of NF-B. NF-B activation appears to be necessary for the induction of chemokine genes, and deletion of NF-B binding sites results in an inability to induce these genes.…”

“…Indeed, our previous studies have shown that oxidized EPA, and not unoxidized EPA, potently inhibits leukocyte-endothelial interactions, both in vitro and in vivo, through a peroxisome proliferator-activated receptor (PPAR)␣-dependent mechanism. 11,12 One of the early events in inflammation is the upregulation of endothelial chemokines, monocyte chemoattractant protein-1 (MCP-1) and IL-8, in response to proinflammatory cytokines such as tumor necrosis factor (TNF)-␣ and IL-1. MCP-1 and IL-8 in turn promote leukocyte chemotaxis, adhesion, and transendothelial migration, 13,14 and neutralization of MCP-1 has been shown to attenuate in vivo injury arising from inflammatory mechanisms.…”