The Impact of Omega-3 Fatty Acids on Inflammatory Pathways in the Vessel Wall that Promote Atherosclerosis

Abstract

Despite the advances that have occurred in the prevention and management of coronary heart disease, there remains a substantial residual risk in the general population. Inflammation has received considerable attention as a target for therapies, given the inflammatory nature of atherosclerosis. Omega-3 fatty acids have anti-inflammatory properties, and recent clinical trial evidence has demonstrated a reduction in major adverse cardiovascular events with fish oil. The studies presented in this thesis investigated the effects of omega-3 fatty acids on vascular inflammation as a possible mechanism of atheroprotection. A review of the literature was performed, focusing on the inflammatory mechanisms of atherosclerosis, the effects of omega-3 fatty acids on inflammation, particularly vascular inflammation, and the results of cardiovascular outcome trials of fish and fish oil. This provided a theoretical basis for the studies presented. A systematic review of high-quality randomised controlled trials catalogued in the Cochrane Library was performed to evaluate the impact of omega-3 fatty acids on the circulating mediators of atherosclerosis. These are among the earliest inducers of endothelial injury and vascular inflammation. Omega-3 fatty acids reduced levels of atherogenic mediators in all four categories. A combined randomised controlled trial and cell culture study was performed to evaluate the effects of omega-3 fatty acids on the gene expression of markers of acute vascular inflammation (AVI). Healthy volunteers were supplemented with 4 grams daily of either EPA, DHA, fish oil with a 2:1 EPA:DHA ratio, or placebo for 30 days. Serum before and after supplementation was added to TNF-stimulated HUVECs in culture. The serum from those supplemented with high-dose EPA reduced the gene expression of MCP-1. The gene expression of VCAM-1 and MCP-1 correlated positively with HDL-C levels, suggesting a pro-inflammatory effect at high HDL-C levels. To study the impact of omega-3 fatty acids on the protein expression of markers of AVI, C57Bl/6 mice had non-occlusive collars applied surgically to their right carotid arteries after receiving 30 days of pre-treatment with either EPA, DHA, an oil control, or no treatment, by oral gavage. The intense inflammatory response was reduced by EPA, manifested by reduced expression of VCAM-1 and MCP-1 in the artery wall. ApoE-deficient mice were fed an atherogenic diet for 16 weeks to induce advanced atherosclerotic lesions and chronic vascular inflammation. In the last 8 weeks they were randomised to either EPA, DHA, olive oil, or no treatment by oral gavage. EPA reduced gene expression of markers of chronic inflammation in the aorta, however no treatment altered the burden, characteristics or lipid content of plaque. EPA and DHA stabilised cholesterol levels and reduced triglyceride levels. In all experimental studies, blood levels of omega-3 fatty acids, especially EPA, correlated inversely with markers of vascular inflammation. The findings of this body of work demonstrate the suppressive effects of omega-3 fatty acids on acute and chronic vascular inflammation, with EPA being superior to DHA. These findings provide a rationale for the reduction in major adverse cardiovascular events seen recently with EPA therapy, and provide a mechanistic basis to guide future clinical trials