Transcriptional control mechanisms of genes of lipid and fatty acid metabolism in the Atlantic salmon (Salmo salar L.) established cell line, SHK-1

The regulatory control mechanisms of lipid and fatty acid metabolism were investigated in Atlantic salmon. We identified sterol regulatory element binding protein (SREBP) genes in salmon and characterised their response, and the response of potential target and other regulatory genes including liver X receptor (LXR), to cholesterol and long-chain polyunsaturated fatty acids (LC-PUFA) in the salmon established cell line, SHK-1. Two cDNAs for SREBPs homologous to mammalian SREBP-1 and SREBP-2 were characterised. We identified three groups of genes whose expression responded differently to the treatments. One group of genes, including cholesterol biosynthetic genes, showed increased expression in response to lipid depletion but supplementary cholesterol or LC-PUFA had no further effect. The expression of a second group of genes belonging to fatty acid biosynthetic pathways, included fatty acid synthase, Δ6 and Δ5 fatty acyl desaturases, also increased after lipid depletion but this was negated by cholesterol or by LC-PUFA supplementation. The expression of a third group of genes including acyl-CoA oxidase, HMG-CoA reductase and Elovl5 elongase was increased by cholesterol treatment but was not affected by lipid depletion or by LC-PUFA. This same pattern of expression was also shown by liver X receptor (LXR), indicating that acyl-CoA oxidase, HMG-CoA reductase and Elovl5 are possible direct targets of LXR. This suggests that salmon Elovl5 may be regulated differently from mammalian Elovl5, which is an indirect target of LXR, responding to LXR-dependent increases in SREBP-1.     

Assessment of a land-locked Atlantic salmon (Salmo salar L.) population as a potential genetic resource with a focus on long-chain polyunsaturated fatty acid biosynthesis

The natural food for Atlantic salmon (Salmo salar) in freshwater has relatively lower levels of omega − 3 (n − 3) long-chain polyunsaturated fatty acids (LC-PUFA) than found in prey for post-smolt salmon in seawater. Land-locked salmon such as the Gullspång population feed exclusively on freshwater type lipids during its entire life cycle, a successful adaptation derived from divergent evolution. Studying land-locked populations may provide insights into the molecular and genetic control mechanisms that determine and regulate n − 3 LC-PUFA biosynthesis and retention in Atlantic salmon. A two factorial study was performed comparing land-locked and farmed salmon parr fed diets formulated with fish or rapeseed oil for 8 weeks. The land-locked parr had higher capacity to synthesise n − 3 LC-PUFA as indicated by higher expression and activity of desaturase and elongase enzymes. The data suggested that the land-locked salmon had reduced sensitivity to dietary fatty acid composition and that dietary docosahexaenoic acid (DHA) did not appear to suppress expression of LC-PUFA biosynthetic genes or activity of the biosynthesis pathway, probably an evolutionary adaptation to a natural diet lower in DHA. Increased biosynthetic activity did not translate to enhanced n − 3 LC-PUFA contents in the flesh and diet was the only factor affecting this parameter. Additionally, high lipogenic and glycolytic potentials were found in land-locked salmon, together with decreased lipolysis which in turn could indicate increased use of carbohydrates as an energy source and a sparing of lipid.     

Influence of dietary conjugated linoleic acid (CLA) on lipid and fatty acid composition in liver and flesh of Atlantic salmon (Salmo salar)

The aim of the present study was to determine the effects of conjugated linoleic acid (CLA) on lipid and fatty acid metabolism in Atlantic salmon. The overall objective being to test the hypotheses that CLA has beneficial effects in salmon including growth enhancement, improved flesh quality through decreased adiposity and lipid deposition thereby minimising detrimental effects of feeding high fat diets, and increased nutritional quality through increased levels of beneficial fatty acids including n-3 highly unsaturated fatty acids (HUFA) and CLA itself. Salmon smolts were fed diets containing two levels of fish oil (low, approximately 18% and high, approximately 34%) containing three levels of CLA (a 1:1 mixture of 9-cis,trans-11 and trans-10,cis-12. at 0, 1 and 2% of diet) for 3 months and the effects on growth performance, liver and muscle (flesh) lipid contents and class compositions, and fatty acid compositions determined. The diets were also specifically formulated to investigate whether the effects of CLA, if any, were more dependent upon absolute content of CLA in the diet (as percentage of total diet) or the relative level of CLA to other fatty acids. Dietary CLA in salmon smolts had no effect on growth parameters or biometric parameters. However, there was a clear trend of increased total lipid and triacylglycerol contents in both liver and flesh in fish fed CLA, particularly in fish fed the high oil diets. Finally, CLA was incorporated into tissue lipids, with levels in flesh being 2-fold higher than in liver, but importantly, incorporation in liver was at the expense of saturated and monounsaturated fatty acids whereas in flesh it was at the expense of n-3HUFA.     

Determination of n-3 HUFA content in Atlantic salmon flesh based on the lipid content, morphometric measurements and blood fatty acid composition: A modeling approach

Fish are the most important source of n-3 highly unsaturated fatty acids (HUFA) in the human diet and, with declining wild stocks, an increasing proportion is being provided by aquaculture. Paradoxically, aquaculture fish diets have traditionally incorporated fish oil and meal derived from wild fisheries. Continued aquaculture development requires fish oil to be replaced with vegetable oils, the only sustainable alternative. However, vegetable oils lack n-3 HUFA and so flesh from fish reared on these diets can also have reduced n-3 HUFA and thus reduced nutritional quality. This accepted, the flesh n-3 HUFA content should be an economically important trait, however to be included in the breeding goal the trait must be measurable. In the present study, we investigated whether the flesh n-3 HUFA content of salmon can be estimated in a non-fatal way. We showed that a general regression model based on flesh lipid content, morphometric and blood fatty acid measurements could estimate and predict flesh n-3 HUFA content. This would allow a choice from a range of selection methods, including mass selection or within-family selection, if this important flesh quality trait is to be included in future salmon breeding programmes.     

Effect of functional feeds on fatty acid and eicosanoid metabolism in liver and head kidney of Atlantic salmon (Salmo salar L.) with experimentally induced Heart and Skeletal Muscle Inflammation

Heart and Skeletal Muscle Inflammation (HSMI) is an emerging viral disease caused by a novel Atlantic salmon reovirus (ASRV) affecting farmed fish. Primary symptoms associated with HSMI include myocardial and skeletal muscle necrosis indicating a severe inflammatory process. Recently, we applied the concept of clinical nutrition to moderate the long-term inflammatory process associated with HSMI in salmon subjected to experimental ASRV challenge. The use of functional feeds with lower lipid (hence energy) content reduced the inflammatory response to ASRV infection and the severity of associated heart lesions. The aim of the present study was to elucidate possible mechanisms underpinning the observed effects of the functional feeds, focussing on eicosanoid and fatty acid metabolism in liver and head kidney. Here we show that liver was also a site for histopathological lesions in HSMI showing steatosis reflecting impaired lipid metabolism. This study is also the first to evaluate the expression of a suite of key genes involved in pathways relating diet and membrane phospholipid fatty acid compositions, and the inflammatory response after ASRV infection. The expression of hepatic Δ6 and Δ5 desaturases was higher in fish fed the functional feeds, potentially increasing their capacity for endogenous production and availability of anti-inflammatory EPA. Effects on mobilization of lipids and changes in the LC-PUFA composition of membrane phospholipids, along with significant changes in the expression of the genes related to eicosanoid pathways, showed the important role of the head kidney in inflammatory diseases caused by viral infections. The results from the present study suggest that clinical nutrition through functional feeding could be an effective complementary therapy for emerging salmon viral diseases associated with long-term inflammation.     

Effects of salmon oil and corn oil on plasma lipid level and hepato-biliary cholesterol metabolism in rats

The aim of this work was to compare the effects of n-3 and n-6 fatty acids on plasma lipid level and hepato-biliary cholesterol metabolism by studying rats fed semi-synthetic diets enriched with either 10% salmon oil, 10% corn oil, or a blend of 6% corn oil and 4% salmon oil. After 4 weeks of feeding, a drop in plasma lipid level was noted in the salmon oil group in comparison to the control group, whereas no change was observed in the corn oil group. An increase in production of cholesterol ester by the liver was recorded in the salmon oil group with a marked enhancement in acyl-CoA:cholesterol acyltransferase (ACAT: EC 2.3.1.26) activity and hepatic cholesterol concentration. Corn oil did not affect either ACAT activity or hepatic cholesterol storage. All bile parameters (flow, bile salts, phospholipids, cholesterol) increased in the salmon oil group, but the molar ratio of cholesterol participation in the bile secretion decreased. These changes in bile composition, as well as in hepatic metabolism of cholesterol, may help to explain the hypolipidemia following the intake of fish oil.     

Dietary LC-PUFA and environmental salinity modulate the fatty acid biosynthesis capacity of the euryhaline teleost thicklip grey mullet (Chelon labrosus)

The capacity to biosynthesise long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) depends upon the complement and function of key enzymes commonly known as fatty acyl desaturases and elongases. The presence of a Δ5/Δ6 desaturase enabling the biosynthesis of docosahexaenoic acid (22:6n-3, DHA) through the “Sprecher pathway” has been reported in Chelon labrosus. Research in other teleosts have demonstrated that LC-PUFA biosynthesis can be modulated by diet and ambient salinity. The present study aimed to assess the combined effects of partial dietary replacement of fish oil (FO) by vegetable oil (VO) and reduced ambient salinity (35 ppt vs 20 ppt) on the fatty acid composition of muscle, enterocytes and hepatocytes of C. labrosus juveniles. Moreover, the enzymatic activity over radiolabelled [1-¹⁴C] 18:3n-3 (α-linolenic acid, ALA) and [1-¹⁴C] 20:5n-3 (eicosapentaenoic acid, EPA) to biosynthesise n-3 LC-PUFA in hepatocytes and enterocytes, and the gene regulation of the C. labrosus fatty acid desaturase-2 (fads2) and elongation of very long chain fatty acids protein 5 (elovl5) in liver and intestine was also investigated. Recovery of radiolabelled products including stearidonic acid (18:4n-3, SDA), 20:5n-3, tetracosahexaenoic acid (24:6n-3, THA) and 22:6n-3 in all treatments except FO35-fish, provided compelling evidence that a complete pathway enabling the biosynthesis of EPA and DHA from ALA is present and active in C. labrosus. Low salinity conditions upregulated fads2 in hepatocytes and elovl5 in both cell types, regardless of dietary composition. Interestingly, FO20-fish showed the highest amount of n-3 LC-PUFA in muscle, while no differences in VO-fish reared at both salinities were found. These results demonstrate a compensatory capacity of C. labrosus to biosynthesise n-3 LC-PUFA under reduced dietary supply, and emphasise the potential of low salinity conditions to stimulate this pathway in euryhaline fish.     

Effect of diets enriched in Delta6 desaturated fatty acids (18:3n-6 and 18:4n-3), on growth, fatty acid composition and highly unsaturated fatty acid synthesis in two populations of Arctic charr (Salvelinus alpinus L.)

This study aimed to test the hypothesis that diets containing relatively high amounts of the Delta6 desaturated fatty acids stearidonic acid (STA, 18:4n-3) and gamma-linolenic acid (GLA, 18:3n-6), may be beneficial in salmonid culture. The rationale being that STA and GLA would be better substrates for highly unsaturated fatty acid (HUFA) synthesis as their conversion does not require the activity of the reputed rate-limiting enzyme, fatty acid Delta6 desaturase. Duplicate groups of two Arctic charr (Salvelinus alpinus L.) populations with different feeding habits, that had been reported previously to show differences in HUFA biosynthetic capacity, were fed for 16 weeks on two fish meal based diets containing 47% protein and 21% lipid differing only in the added lipid component, which was either fish oil (FO) or echium oil (EO). Dietary EO had no detrimental effect on growth performance and feed efficiency, mortalities, or liver and flesh lipid contents in either population. The proportions of 18:2n-6, 18:3n-3, 18:3n-6, 18:4n-3, 20:3n-6 and 20:4n-3 in total lipid in both liver and flesh were increased by dietary EO in both populations. However, the percentages of 20:5n-3 and 22:6n-3 were reduced by EO in both liver and flesh in both strains, whereas 20:4n-6 was only significantly reduced in flesh. In fish fed FO, HUFA synthesis from both [1-(14)C]18:3n-3 and [1-(14)C]20:5n-3 was significantly higher in the planktonivorous Coulin charr compared to the demersal, piscivorous Rannoch charr morph. However, HUFA synthesis was increased by EO in Rannoch charr, but not in Coulin charr. In conclusion, dietary EO had differential effects in the two populations of charr, with HUFA synthesis only stimulated by EO in the piscivorous Rannoch morph, which showed lower activities in fish fed FO. However, the hypothesis was not proved as, irrespective of the activity of the HUFA synthesis pathway in either population, feeding EO resulted in decreased tissue levels of n-3HUFA and 20:4n-6. This has been observed previously in salmonids fed vegetable oils, and thus the increased levels of Delta6 desaturated fatty acids in EO did not effectively compensate for the lack of dietary HUFA.     

Maternal supplementation with n-3 long chain polyunsaturated fatty acids during perinatal period alleviates the metabolic syndrome disturbances in adult hamster pups fed a high-fat diet after weaning

Perinatal nutrition is thought to affect the long-term risk of the adult to develop metabolic syndrome. We hypothesized that maternal supplementation with eicosapentaenoic acid and docosahexaenoic acid during pregnancy and lactation would protect offspring fed a high-fat diet from developing metabolic disturbances. Thus, two groups of female hamsters were fed a low-fat control diet, either alone (LC) or enriched with n-3 long chain polyunsaturated fatty acids (LC-PUFA) (LO), through the gestational and lactation periods. After weaning, male pups were randomized to separate groups that received either a control low-fat diet (LC) or a high-fat diet (HC) for 16 weeks. Four groups of pups were defined (LC-LC, LC-HC, LO-LC and LO-HC), based on the combinations of maternal and weaned diets. Maternal n-3 LC-PUFA supplementation was associated with reduced levels of basal plasma glucose, hepatic triglycerides secretion and postprandial lipemia in the LO-HC group compared to the LC-HC group. Respiratory parameters were not affected by maternal supplementation. In contrast, n-3 LC-PUFA supplementation significantly enhanced the activities of citrate synthase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase compared to the offspring of unsupplemented mothers. Sterol regulatory element binding protein-1c, diacylglycerol O-acyltransferase 2, fatty acid synthase, stearoyl CoA desaturase 1 and tumor necrosis factor α expression levels were not affected by n-3 LC-PUFA supplementation. These results provide evidence for a beneficial effect of n-3 LC-PUFA maternal supplementation in hamsters on the subsequent risk of metabolic syndrome. Underlying mechanisms may include improved lipid metabolism and activation of the mitochondrial oxidative pathway.