Influence of a Dietary Vegetable Oil Blend on Serum Lipid Profiles in Large Yellow Croaker ( Larimichthys crocea).

Serum lipid metabolic responses are associated with certain metabolic disorders induced by dietary habits in mammals. However, such associations have not been reported in fish. Lipidomic analyses were performed to investigate fish lipid metabolic responses to a dietary vegetable oil (VO) blend and to elucidate the mechanism of how the dietary VO blend affects serum lipid profiles. Results showed that the dietary VO blend strongly affects serum lipid profiles, especially the ratio of triglyceride/phosphatidylcholine (TAG/PC), via inhibiting hepatic PC biosynthesis and facilitating hepatic and intestinal lipoprotein assembly. Studies in vitro suggested that changes of serum TAG/PC ratio may be partially attributed to altered fatty acid composition in diets. Additionally, the reduction of 16:0/18:1-PC induced by the dietary VO blend may play a role in abnormal lipid deposition through inhibiting PPARA-mediated activation of ß-oxidation. These findings suggested that the serum TAG/PC ratio might be a predictive parameter for abnormal lipid metabolism induced by dietary nutrition in fish.

Regulation of hepatic lipid deposition by phospholipid in large yellow croaker.

Dietary phospholipid (PL) supplementation has been shown to reduce lipid accumulation in the tissues of farmed fish; however, the mechanisms underlying this effect are largely unknown. Thus, the present study was conducted to evaluate the potential impacts of PL on hepatic lipid metabolism both in vivo and in vitro. For in vivo study, four experimental diets - low lipid and low PL diet, as control diet (LL-LP diet, containing 12 % lipid and 1·5 % PL), low-lipid and high-PL diet (containing 12 % lipid and 8 % PL), high-lipid and low-PL diet (HL-LP diet, containing 20 % lipid and 1·5 % PL) and high-lipid and high-PL diet (HL-HP diet, containing 20 % lipid and 8 % PL) - were randomly allocated to four groups of large yellow croaker (Larimichthys crocea) (three cages per group) with similar initial body weight (approximately 8 g). For in vitro study, primary hepatocytes isolated from large yellow croaker were incubated either with graded levels of phosphatidylcholine (PC) (0-250 µm) or small interfering RNA (siRNA) for CTP: choline phosphate cytidylyltranferase α (CCTα) (siRNA-CCTα). Results showed that survival was independent of dietary treatments (P>0·05). Weight gain and feed efficiency in the HL-HP group were significantly higher than in the LL-LP and HL-LP groups (P<0·05). High level of dietary PL could markedly reduce abnormal hepatic lipid accumulation induced by the HL-LP diet (P<0·05). Similarly, compared with the corresponding controls, a significant decrease/increase in lipid content was observed in primary hepatocytes incubated with PC/siRNA-CCTα (P<0·05). High level of dietary PL reversed the HL-LP diet-induced increased levels of mRNA of fatty acid uptake and lipid synthesis related genes (P<0·05). In addition, High level of dietary PL markedly down-regulated the transcript levels of fatty acid oxidation-related genes and enhanced the transcript levels of VLDL assembly-related genes regardless of dietary lipid levels (P<0·05). Compared with corresponding controls, primary hepatocytes treated with PC showed significantly higher mRNA expression of lipid synthesis and VLDL assembly-related genes and lower mRNA expression of fatty acid oxidation-related genes, with hepatocytes treated with siRNA-CCTα exhibiting the opposite trend (P<0·05). In summary, these results demonstrated that high level of dietary PL might reverse the HL-LP diet-induced abnormal lipid accumulation in the liver through inhibiting fatty acid uptake and lipid synthesis, together with promoting the lipid export at the transcriptional level. Lipid export-promoting effect of PC was confirmed by in vitro studies. The present study showed for the first time that PL or PC could influence various metabolic pathways to regulate hepatic lipid deposition in fish at least at the transcriptional level.

Regulation of FADS2 transcription by SREBP-1 and PPAR-α influences LC-PUFA biosynthesis in fish.

The present study was conducted to explore the mechanisms leading to differences among fishes in the ability to biosynthesize long-chain polyunsaturated fatty acids (LC-PUFAs). Replacement of fish oil with vegetable oil caused varied degrees of increase in 18-carbon fatty acid content and decrease in n-3 LC-PUFA content in the muscle and liver of rainbow trout (Oncorhynchus mykiss), Japanese seabass (Lateolabrax japonicus) and large yellow croaker (Larimichthys crocea), suggesting that these fishes have differing abilities to biosynthesize LC-PUFAs. Fish oil replacement also led to significantly up-regulated expression of FADS2 and SREBP-1 but different responses of the two PPAR-α homologues in the livers of these three fishes. An in vitro experiment indicated that the basic transcription activity of the FADS2 promoter was significantly higher in rainbow trout than in Japanese seabass or large yellow croaker, which was consistent with their LC-PUFA biosynthetic abilities. In addition, SREBP-1 and PPAR-α up-regulated FADS2 promoter activity. These regulatory effects varied considerably between SREBP-1 and PPAR-α, as well as among the three fishes. Taken together, the differences in regulatory activities of the two transcription factors targeting FADS2 may be responsible for the different LC-PUFA biosynthetic abilities in these three fishes that have adapted to different ambient salinity.

Effects of dietary phospholipid on lipase activity, antioxidant capacity and lipid metabolism-related gene expression in large yellow croaker larvae (Larimichthys crocea).

It was previously shown that lipid content of the whole larvae body fed the phospholipid (PL)-devoid diet was significantly lower than that of larvae fed PL-supplemented diets (P<0.05) (Feng et al., unpublished results). The mechanisms involved remain unclear and were explored from the perspective of fatty acids delivery, uptake, synthesis and oxidation in the present study. Besides, this study was also designed to investigate the effect of dietary PL on antioxidant capacity in large yellow croaker larvae (Larimichthys crocea). Triplicate groups of larvae (initial body weight: 3.86±0.24mg) were fed three isonitrogenous and isolipidic diets with increasing levels of PL (2.53%, 6.32% and 12.7%) eight times daily for 30days. Results showed that the specific activities of superoxide dismutase and catalase were significantly higher in 12.7% PL group compared to those in 2.53% PL group (P<0.05), while an opposite trend was observed for MDA content (P<0.05). The specific activity of lipase and the mRNA abundance of fatty acids delivery and uptake-related genes, including lipoprotein lipase, hepatic lipase and fatty acids translocase (cluster of differentiation) were significantly higher in 12.7% PL group than those in 2.52% group (P<0.05). Compared to 2.53% PL group, the transcript levels of fatty acid synthase and stearoyl-CoA desaturase I were significantly lower in 6.32% PL group (P<0.05), while peroxisome proliferators-activated receptor α, carnitine palmitoyl transferase-I and acyl CoA oxidase mRNA expression levels were significantly higher in 12.7% PL group (P<0.05). These results indicated that dietary PL could enhance antioxidant capacity of larvae. Dietary PL might regulate lipid metabolism in large yellow croaker larvae by modulating fatty acids delivery, uptake, synthesis and oxidation at transcriptional level and improved fatty acids delivery and uptake might be responsible for higher body lipid content in 6.32% and 12.7% groups.