Fatty Acid Profile Is Modulated by Dietary Resveratrol in Rainbow Trout (Oncorhynchus mykiss)

Abstract

To produce fish of a high quality that are rich in omega-3 fatty acids (n-3 FA) and simultaneously generate more sustainable aquaculture, the combined use of phytochemicals and vegetable oils in fish feed seems to be a promising approach. Resveratrol (RV) potentially induces endogenous fatty acid synthesis, resulting in elevated n-3 FA levels in fish. RV putatively influences ∆6-desaturase, the key enzyme in FA metabolism, and serves as a ligand for PPARα, a transcription factor regulating β-oxidation. Rainbow trout (36.35 ± 0.03 g) were randomly allocated into six groups and fed diets with reduced fish oil levels (F4 = 4%, F2 = 2% and F0 = 0% of dry matter) supplemented with 0.3% (w/w) RV (F4 + RV, F2 + RV and F0 + RV). RV significantly affected FA composition in liver tissue and whole fish homogenates. 20:5n-3 (EPA) and 22:6n-3 (DHA) were significantly increased whereas precursor FA were diminished in fish fed the F2 + RV and F0 + RV diets when compared to F4 + RV and F0. RV significantly elevated ∆6-desaturase protein levels in the livers of F0 + RV fed animals. Hepatic mRNA expression of ∆6-desaturase, PPARα, and its target genes were affected by the dietary fish oil level and not by dietary RV. The results of this study indicated a potential benefit of supplementing RV in fish oil deprived diets elevating n-3 FA levels in rainbow trout.

Keywords: CPT1; PPARα; fish oil replacement; hepatic fatty acid synthesis; long chain polyunsaturated fatty acids; mRNA expression; phytochemical.

Figure 1

Molecular pathways of the hepatic endogenous fatty acid synthesis in the endoplasmic reticulum and peroxisomes and putative effects of resveratrol (RV) on the expression level and activity of involved enzymes and transcription factors. Alpha-linolenic acid (ALA) is converted to eicosapentaenoic acid (EPA) and further to 24:6n-3 via elongation and desaturation (∆6-desaturase (∆6-D) and ∆5-desaturase (∆5-D)) steps [26]. The partial β-oxidation to form docosahexaenoic acid (DHA) takes place in peroxisomes. RV possibly affects ∆6-D and ∆5-D activity, or interacts with ∆6-D via transcriptional control of gene expression [48]. Further, RV putatively enhances the peroxisome proliferator-activated receptor α (PPARα) gene expression [46,47]. PPARα might induce the expression of target genes, for example carnitine palmitoyl transferase 1 (CPT1), sterol regulatory element binding protein 1 (SREBP-1) and acyl-CoA oxidase 1 (acox1) [37,38], putatively increasing mitochondrial β-oxidation (CPT1), peroxisomal β-oxidation (acox1) and ∆6-D gene expression (SREBP-1). Graph made according to Sargent et al. [26] and Burdge [28] and modified according to results from previous studies [37,38,46,47,48] and own data.

Figure 2

Hepatic mRNA expression levels in rainbow trout liver following dietary treatment with varying levels of fish oil and resveratrol supplementation for eight weeks. (a) ∆6-desaturase; (b) PPARα; (c) CPT1a and (d) CPT1c were measured in the liver of fish using qRT-PCR and were normalized to the housekeeping gene EF1α. Feeding groups F4, F2 and F0 were fed basal diets containing 4%, 2% and 0% DM fish oil, groups with +RV were fed respective diets supplemented with resveratrol. Boxes represent values (n = 15) between the 25 and 75 percentiles; whiskers indicate 1.5 SD; the solid line indicates the median; circles represent values above and below SD. Significant differences (p < 0.05) were analyzed using multiple contrast tests for relative effects. Tests were based on comparisons of fish oil inclusion level within one supplement group (indicated by *) or supplement type within one fish oil inclusion level (no significant differences).

Figure 3

∆6-desaturase (∆6-D) protein levels in the livers of rainbow trout following dietary treatment with varying levels of fish oil and resveratrol supplementation for eight weeks. The ∆6-D levels were measured using ELISA and were normalized to the total protein level (ng/mg protein). Feeding groups F4, F2 and F0 were fed basal diets containing 4%, 2% and 0% DM fish oil and groups +RV were fed respective diets supplemented with resveratrol. The dashed line indicates the initial expression value (n = 1), bars indicate the final expression values (mean + SD, n = 3). Statistically significant differences (p < 0.05) were analyzed using multiple contrast tests based on comparisons of fish oil inclusion level within one supplement group (*) or supplement type within one fish oil inclusion group (a, b: F0 diets; x, y: F4 diets).