Elucidating the Role of Very-long-chain Polyunsaturated Fatty Acids in Retinal Health and Disease

University Of Utah, Salt Lake City UT

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Abstract

Abstract: The effect of a novel fish oil supplement enriched in very-long-chain (C≥24) polyunsaturated fatty acids (VLCPUFAs) on cardiometabolic risk factors and visual function will be assessed in this bench-to-bedside proposal. VLCPUFAs play important tissue specific roles that cannot be fulfilled by more abundant shorter-chain FAs. The ELOVL2 enzyme in the liver is responsible for converting docosapentaenoic acid (DPA: 22:5 n-3) to tetracosapentaenoic acid (TPA: 24:5 n-3), which is further extended in 2-carbon steps to chain lengths greater than 36 carbons by the ELOVL4 enzyme in the retina, brain and testis. Heterozygous ELOVL4 mutations lead to Stargardt-like macular dystrophy, a progressive form of macular degeneration that causes blindness in young adults. Some ELOVL4 mutations lead to spinocerebellar ataxia. VLCPUFAs are also important in spermatogenesis and have been proposed to be involved in male infertility. Fish oil-derived PUFAs (EPA: 20:5 n-3, DHA: 22:6 n-3) exert potent anti-inflammatory and cardiometabolic benefits, but in clinical trials like AREDS2 they have not consistently slowed the progression of age-related macular degeneration (AMD). Standard fish oils, however, contain less than 1% (w:w) VLCPUFAs. Retinal levels of VLCPUFAs decrease with age and are especially low in AMD patients. Because ELOVL2 expression markedly decreases with aging due to promoter methylation, we hypothesized that the age-related declined in VLCPUFAs could be ameliorated with a dietary supplement containing TPA. We recently showed in aged mice that dietary supplementation with a novel fish oil enriched in VLCPUFAs (C24-C28) not only improved retinal function, but also had beneficial effects on cardiometabolic risk factors (decreased plasma lipids and improved insulin-sensitivity). Some of these beneficial effects are possibly due to the ability of VLCPUFAs to act as PPAR-agonists. We found that TPA was a more potent PPAR (α and γ) agonist than EPA and DHA. Our results, therefore, suggest that VLCPUFA-rich fish oil could be more effective than EPA, a currently approved treatment for cardiovascular disease prevention and could also be a new nutritional therapy for AMD. For our basic science aims, we plan to examine in more detail the mechanism of action of VLCPUFAs. EPA/DHA are known to exert their anti-inflammatory and insulin-sensitizing effects by G-protein-coupled receptors (GPRs). Drs. Danner and Wang, NHLBI and CC associate investigators, will identify possible GPR targets for chemically synthesized VLCPUFA and elucidate their signaling pathways, using specific GPR knockdown and knockout cells expressing reporter genes. Dr. Bernstein, an extramural co-investigator, will compare chemically synthesized VLCPUFA versus EPA/DHA, as well as VLCPUFA-rich fish oil versus regular EPA/DHA-rich fish oil, for their ability to slow retinal degeneration in Elovl4 conditional knockout mice. Dr. Remaley’s lab at NHLBI will perform a similar study in ApoE-KO mice comparing the ability of the various fatty acid supplements for reducing plasma lipids and atherosclerosis. Glucose and insulin clamp studies, as well as hepatic and muscle RNAseq analysis, will also be performed. Dr. Fliesler, an extramural co-investigator, will examine in cell culture systems the impact of VLCPUFA on lipid oxidation, a key process in the pathogenesis of AMD and cardiovascular disease. Our first clinical aim will involve the use of human samples provided by Dr. Chew, a NEI co-investigator and PI on the AREDS2 study. She will design a small pilot case-control study from AREDS2 by identifying patients that appeared to show rapid progression vs. those who did not progress to late AMD, after adjusting for random assignment to treatment with or without fish oil supplementation, as well as for fish consumption. Plasma from the two cohorts will then by analyzed by gas chromatography to test the hypothesis that patients who did not progress while on a standard fish oil or a high dietary intake of fish may have increased hepatic production of TPA. In our second clinical aim, Dr. Remaley’s lab will determine the bioavailability and tolerability of VLCPUFA-rich fish oil produced by Epax, a fish oil manufacturer in Norway. They have almost completed the necessary studies to obtain Generally Recognized as Safe (GRAS) status for their supplement and have agreed to supply it to us for our study. A single-dose escalation study in healthy volunteers (50, 500, 1000, 2000-mg/day) will be performed. Besides measuring plasma FAs levels, we will also monitor clinical chemistry and hematology tests to confirm the safety of the supplement and to determine if there are any acute effects on plasma glucose and lipids. Results from this study will then be used for planning for future long-term intervention type studies in patients with AMD and cardiovascular disease.