Fatty acid utilisation and metabolism in caecal enterocytes of rainbow trout (Oncorhynchus mykiss) fed dietary fish or copepod oil

Dietary lipid sensing through fatty acid oxidation and chylomicron formation in the gastrointestinal tract of rainbow trout

In mammals, physiological processes related to lipid metabolism, such as chylomicron synthesis or fatty acid oxidation (FAO), modulate eating, highlighting the importance of energostatic mechanisms in feeding control. This study, using rainbow trout (Oncorhynchus mykiss) as model, aimed to characterize the role of FAO and chylomicron formation as peripheral lipid sensors potentially able to modulate feeding in fish. Fish fed with either a normal- (24%) or high- (32%) fat diet were intraperitoneally injected with water alone or containing etomoxir (inhibitor of FAO rate-limiting enzyme carnitine palmitoyl-transferase 1). First, feed intake levels were recorded. We observed an etomoxir-derived decrease in feeding at short times, but a significant increase at 48 h after treatment in fish fed normal-fat diet. Then, we evaluated putative etomoxir effects on the mRNA abundance of genes related to lipid metabolism, chylomicron synthesis and appetite-regulating peptides. Etomoxir treatment upregulated mRNA levels of genes related to chylomicron assembly in proximal intestine, while opposite effects occurred in distal intestine, indicating a clear regionalization in response. Etomoxir also modulated gastrointestinal hormone mRNAs in proximal intestine, upregulating ghrl in fish fed normal-fat diet and pyy and gcg in fish fed high-fat diet. These results provide evidence for an energostatic control of feeding related to FAO and chylomicron formation at the peripheral level in fish.

Enhanced micronutrient supplementation in low marine diets reduced vertebral malformation in diploid and triploid Atlantic salmon (Salmo salar) parr, and increased vertebral expression of bone biomarker genes in diploids

Previously we showed that, for optimum growth, micronutrient levels should be supplemented above current National Research Council (2011) recommendations for Atlantic salmon when they are fed diets formulated with low levels of marine ingredients. In the present study, the impact of graded levels (100, 200, 400%) of a micronutrient package (NP) on vertebral deformities and bone gene expression were determined in diploid and triploid salmon parr fed low marine diets. The prevalence of radiologically detectable spinal deformities decreased with increasing micronutrient supplementation in both ploidy. On average, triploids had a higher incidence of spinal deformity than diploids within a given diet. Micronutrient supplementation particularly reduced prevalence of fusion deformities in diploids and compression and reduced spacing deformities in triploids. Prevalence of affected vertebrae within each spinal region (cranial, caudal, tail and tail fin) varied significantly between diet and ploidy, and there was interaction. Prevalence of deformities was greatest in the caudal region of triploids and the impact of graded micronutrient supplementation in reducing deformities also greatest in triploids. Diet affected vertebral morphology with length:height (L:H) ratio generally increasing with level of micronutrient supplementation in both ploidy with no difference between ploidy. Increased dietary micronutrients level in diploid salmon increased the vertebral expression of several bone biomarker genes including bone morphogenetic protein 2 (bmp2), osteocalcin (ostcn), alkaline phosphatase (alp), matrix metallopeptidase 13 (mmp13), osteopontin (opn) and insulin-like growth factor 1 receptor (igf1r). In contrast, although some genes showed similar trends in triploids, vertebral gene expression was not significantly affected by dietary micronutrients level. The study confirmed earlier indications that dietary micronutrient levels should be increased in salmon fed diets with low marine ingredients and that there are differences in nutritional requirements between ploidies.

Transepithelial transfer of phenanthrene, but not of benzo[a]pyrene, is inhibited by fatty acids in the proximal intestine of rainbow trout (Oncorhynchus mykiss)

The inclusion of vegetable oils in aquafeeds introduces contaminating polycyclic aromatic hydrocarbons (PAHs) in salmonids. Since lipophilic PAHs solubilize in micelles composed of lipids, bile salts and fatty acids, dietary lipid composition can alter intestinal transepithelial PAH transfer. We studied the uptake of two PAHs, viz. benzo[a]pyrene (BaP) and phenanthrene (PHE), in rainbow trout (Oncorhynchus mykiss) intestine. We also investigated the effects of two fatty acids, viz. fish oil-derived eicosapentaenoic acid (EPA, 20:5n-3) and vegetable oil-derived oleic acid (18:1n-9) on intestinal uptake. Radiolabeled PAHs were solubilized in micelles composed of tritiated EPA and oleic acid, respectively, and administrated to intestinal segments mounted in Ussing chambers. In the absence of micelles, PHE accumulation was two times higher than BaP in the mucosal and serosal layers of proximal and distal intestine. Administration of PHE in micelles composed of oleic acid resulted in a 50% lower accumulation of PHE in the mucosal layers of the proximal intestine compared to EPA-composed micelles. Accumulation of EPA and oleic acid in the proximal intestinal mucosa correlated negatively with the transepithelial transfer of these fatty acids across the proximal intestinal epithelium. Transepithelial PHE transfer across the proximal intestine was reduced by 30% in co-exposure with EPA-composed micelles compared to 80% with oleic acid micelles. BaP was not transferred across the intestine. We conclude that the lipid composition of an aquafeed is an important determinant of PAH bioavailability. Therefore, lipid composition should be an important consideration in choosing vegetable oils as alternatives for fish oil in aquafeeds.

Influence of the dietary protein:lipid ratio and fish oil substitution on fatty acid composition and metabolism of Atlantic salmon (Salmo salar) reared at high water temperatures

A factorial, two-way, experimental design was used for this 10-week nutritional trial, aiming to elucidate the interactive effects of decreasing dietary protein:lipid level and substitution of fish oil (FO) with rapeseed oil (RO) on tissue fatty acid (FA) composition and metabolism of large Atlantic salmon (Salmo salar L.) reared at high water temperatures (sub-optimal, summer temperatures: 11·6°C). The six experimental diets were isoenergetic and formulated to include either FO or RO (60 % of the added oil) at three dietary protein:lipid levels, specifically (1) 350 g/kg protein and 350 g/kg lipid, (2) 330 g/kg protein and 360 g/kg lipid, (3) 290 g/kg protein and 380 g/kg lipid. Final weight, specific growth rate and thermal growth coefficient were positively affected by the dietary RO inclusion at the expense of FO, while no significant effects were seen on growth due to the decreasing protein level. The oil source had a significant effect on muscle and liver FA composition. However, the changes in muscle and liver FA indicate selective utilisation or retention of individual FA and moderate reductions in tissue EPA and DHA. Pyloric caeca phospholipid FA composition was significantly affected by the two factors and, in some cases, significant interactions were also revealed. Liver and red muscle β-oxidation capacities were significantly increased due to RO inclusion, while an interactive effect of protein level and oil source was shown for white muscle β-oxidation capacity. The results could explain, at least partially, the better performance that was shown for the RO groups and the enhanced protein-sparing effect.

Endocrine control of oleic acid and glucose metabolism in rainbow trout (Oncorhynchus mykiss) muscle cells in culture

The effects of insulin and IGF-I on fatty acid (FA) and glucose metabolism were examined using oleic acid or glucose as tracers in differentiated rainbow trout (Oncorhynchus mykiss) myotubes. Insulin and IGF-I significantly reduced the production of CO(2) from oleic acid with respect to the control values. IGF-I also significantly reduced the production of acid-soluble products (ASP) and the concentration of FA in the medium, while cellular triacylglycerols (TAG) tended to increase. Only insulin produced a significant accumulation of glycogen inside the cells in glucose distribution experiments. Incubation with catecholamines did not affect oleic acid metabolism. Cells treated with rapamycin [a target of rapamycin (TOR) inhibitor] significantly increased the oxidation of oleic acid to CO(2) and ASP, while the accumulation of TAG diminished. Rosiglitazone (a peroxisome proliferator-activated receptor gamma agonist) and etomoxir (a CPT-1 inhibitor) produced a severe and significant reduction in the production of CO(2) and ASP. Rosiglitazone and etomoxir also produced a significant accumulation of FA outside and inside the cells, respectively. No significant effects of these drugs on glucose distribution were observed. These data indicate that insulin and IGF-I act as anabolic hormones in trout myotubes in both oleic acid and glucose metabolism, although glucose oxidation appears to be less sensitive than FA oxidation to insulin and IGF-I. The use of rapamycin, etomoxir, and rosiglitazone may help us to understand the mechanisms of regulation of lipid metabolism in fish.

A vegetable oil feeding history affects digestibility and intestinal fatty acid uptake in juvenile rainbow trout Oncorhynchus mykiss

Future expansion of aquaculture relies on the use of alternatives to fish oil in fish feed. This study examined to what extent the nature of the feed oil affects intestinal lipid uptake properties in rainbow trout. The fish were fed a diet containing fish (FO), rapeseed (RO) or linseed (LO) oil for 8 weeks after which absorptive properties were assessed. Differences in digestibility due to feed oil history were measured using diet FO with an indigestible marker. Intestinal integrity, paracellular permeability, in vitro transepithelial fatty acid transport (3H-18:3n-3 and 14C-16:0) and their incorporation into intestinal epithelia were compared using Ussing chambers. Feed oil history did not affect the triacylglycerol/phosphatidylcholine ratio (TAG/PC) of the newly synthesized lipids in the segments. The lower TAG/PC ratio with 16:0 (2:1) than with 18:3 (10:1) showed the preferential incorporation of 16:0 into polar lipids. The FO-feeding history decreased permeability and increased transepithelial resistance of the intestinal segments. Transepithelial passage rates of 18:3n-3 were higher when pre-fed LO compared to RO or FO. Similarly, pre-feeding LO increased apparent lipid and fatty acid digestibilities compared to RO or FO. These results demonstrate that the absorptive intestinal functions in fish can be altered by the feed oil history and that the effect remains after a return to a standard fish oil diet.

Localization of Fatty Acids with Selective Chain Length by Imaging Time-of-Flight Secondary Ion Mass Spectrometry

Localization of fatty acids in biological tissues was made by using TOF-SIMS (time-of-flight secondary ion mass spectrometry). Two cell-types with a specific fatty acid distribution are shown. In rat cerebellum, different distribution patterns of stearic acid (C18:0), palmitic acid (C16:0), and oleic acid (C18:1) were found. Stearic acid signals were observed accumulated in Purkinje cells with high intensities inside the cell, but not in the nucleus region. The signals colocalized with high intensity signals of the phosphocholine head group, indicating origin from phosphatidylcholine or sphingomyelin. In mouse intestine, high palmitic acid signals were found in the secretory crypt cells together with high levels of phosphorylinositol colocalized in the crypt region. Palmitic acid was also seen in the intestinal lumen that contains high amounts of mucine, which is known to be produced in the crypt cells. Linoleic acid signals (C18:2) were low in the crypt region and high in the villus region. Oleic acid signals were seen in the villi and stearic acid signals were ubiquitous with no specific localization in the intestine. We conclude that the results obtained by using imaging TOF-SIMS are consistent with known brain and intestine biochemistry and that the localization of fatty acids is specific in differentiated cells.

Sn-2-monoacylglycerol, not glycerol, is preferentially utilised for triacylglycerol and phosphatidylcholine biosynthesis in Atlantic salmon (Salmo salar L.) intestine

Pathways of lipid resynthesis in the intestine of fish are relatively unknown. Various reports have suggested the existence of both sn-1,3-specific (pancreatic) and non-specific (bile salt-activated) lipase activity operating on dietary triacylglycerol (TAG) in the intestinal lumen of fish during digestion. Thus, sn-2-monoacylglycerol (2-MAG) and glycerol, respective hydrolytic products of each lipase, are absorbed and utilised for glycerolipid synthesis in enterocytes via two alternative routes: monoacylglycerol (MAG) and glycerol-3-phosphate (G3P) pathways. Despite different precursors, both pathways converge at the production of sn-1,2-diacylglycerol (1,2-DAG) where TAG or phosphatidylcholine (PC) synthesis can occur. To elucidate the relative activities of MAG and G3P pathways in Atlantic salmon enterocytes, intestinal segments were mounted in Ussing chambers where equimolar mixtures of sn-2-oleoyl-[1,2,3-(3)H]glycerol (2-MAG) and [(14)C(U)]glycerol, plus unlabelled 16:0 and 18:2n-6 as exogenous fatty acid sources, were delivered in bile salt-containing Ringer solution to the mucosa. The MAG pathway predominated, over the G3P pathway, synthesizing ca. 95% of total TAG and ca. 80% of total PC after a 3 h incubation period at 10 degrees C. Further, the 1,2-DAG branch point into TAG or PC was polarised towards TAG synthesis (6:1) via the MAG pathway but more evenly distributed between TAG and PC (1:1) via the G3P pathway. Effect of long-chain saturated, monounsaturated and polyunsaturated fatty acids on the synthesized TAG/PC ratio was assessed by individually exchanging 16:0, 18:1n-9 or 18:2n-6, for 16:0+18:2n-6, in mucosal solutions. TAG synthesis was influenced considerably more than PC synthesis, via either pathway, by exogenous fatty acids utilised. 18:1n-9 significantly stimulated TAG synthesis via the MAG pathway yielding a TAG/PC ratio of 12:1. Alternatively, 18:2n-6 stimulated TAG synthesis the most via the G3P pathway (TAG/PC=4:1). 16:0 significantly attenuated TAG synthesis via either pathway. Micellar fatty acid species also significantly affected intestinal active transport mechanisms as shown by decreasing transepithelial potential (TEP) and short-circuit current (SSC) with increasing fatty acid unsaturation. The epithelial integrity was, however, not compromised after 3 h of exposure to any of the fatty acids. The implications of these findings on dietary fatty acid composition and enterocytic lipid droplet accumulation are discussed.