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The unfolded protein response (UPR) is a mechanism to cope with perturbed endoplasmic reticulum (ER) functions or accumulation of unfolded protein in the ER in eukaryotic cells. Furthermore, the UPR also participates in a number of physiological and pathological processes, such as nutrient sensing, lipid synthesis, and inflammatory response. In this study, four UPR-related genes (GRP78/BiP, ATF6α, XBP1 and CHO) were isolated characterized from large yellow croaker (Larimichthys crocea), and their expression in response to dietary lipid sources (various fatty acids) such as fish oil (FO), palmic acid (PA), olive oil (OO), sunflower oil (SO), and perilla oil (PO), were examined following feeding. The results showed that the four UPR-related proteins contained highly conserved functional domains and had the closest phylogenetic relationships with other fishes. Additionally, these genes were ubiquitously expressed in large yellow croaker, as in zebrafish and medaka. Moreover, GRP78, ATF6α and spliced XBP1 (XBP1s) mRNA levels in the liver, not in adipose tissue, were significantly increased in the SO group compared to the other groups (P<0.05). These results indicated that dietary SO activated UPR, and the activation of UPR might provide a mechanism to improve ER function, but probably stimulated lipid synthesis and caused inflammatory response in the liver of large yellow croaker., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Ectopic lipid accumulation has been observed in fish fed a high-lipid diet. However, no information is available on the mechanism by which dietary lipid levels comprehensively regulate lipid transport, uptake, synthesis and catabolism in fish. Therefore, the present study aimed to gain further insight into how dietary lipids affect lipid deposition in the liver of large yellow croaker(Larimichthys crocea). Fish (150.00±4.95 g) were fed a diet with a low (6%), moderate (12%, the control diet) or high (18%) crude lipid content for 10 weeks. Growth performance, plasma biochemical indexes, lipid contents and gene expression related to lipid deposition, including lipoprotein assembly and clearance, fatty acid uptake and triacylglycerol synthesis and catabolism, were assessed. Growth performance was not significantly affected. However, the hepato-somatic and viscera-somatic indexes as well as plasma triacylglycerol, non-esterified fatty acids and LDL-cholesterol levels were significantly increased in fish fed the high-lipid diet. In the livers of fish fed the high-lipid diet, the expression of genes related to lipoprotein clearance (LDLR) and fatty acid uptake (FABP11) was significantly up-regulated, whereas the expression of genes involved in lipoprotein assembly (apoB100), triacylglycerol synthesis and catabolism (DGAT2, CPT I) was significantly down-regulated compared with fish fed the control diet, and hepatic lipid deposition increased. In fish fed the low-lipid diet, the expression of genes associated with lipoprotein assembly and clearance (apoB100, LDLR, LRP-1), fatty acid uptake (CD36, FATP1, FABP3) and triacylglycerol synthesis (FAS) was significantly increased, whereas the expression of triacylglycerol catabolism related genes (ATGL, CPT I) was reduced compared with fish fed the control diet. However, hepatic lipid content in fish fed the low-lipid diet decreased mainly due to low dietary lipid intake. In summary, findings of this study provide molecular insight into the role of lipid deposition in the liver in response to different dietary lipid contents.

The enzymes involved in the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs) are widely studied in fish species, as fish are the main source of n-3 LC-PUFAs for human beings. In the present study, a putative gene for elovl5, which encodes a key enzyme involved in LC-PUFA synthesis, was cloned and functionally characterized, and its transcription in response to dietary n-3 LC-PUFA exposure was investigated. Moreover, cell transfection and luciferase assays were used to explore the mechanism underlying the regulation of elovl5. The full-length cDNA of elovl5 was 1242 bp (excluding the polyA tail), including an 885 bp coding region encoding a 295 amino acid protein that possesses all of the characteristic features of elovl proteins. Functional characterization of heterologously expressed grouper Elovl5 indicated that it effectively elongates both C18 (18:2n-6, 18:3n-3, 18:3n-6 and 18:4n-3) and C20 (20:4n-6 and C20:5n-3) PUFAs, but not the C22 substrates. The expression of elovl5 was significantly affected by dietary n-3 LC-PUFA exposure: a high n-3 LC-PUFA level repressed the expression of elovl5 by slightly down-regulating the expression of sterol regulatory element-binding protein (SREBP)-1 and liver X receptor (LXR) α, which are major regulators of hepatic lipid metabolism. Promoter studies showed that grouper elovl5 reporter activity was induced by over-expression of LXRα but not SREBP-1. This finding suggests that elovl5 is a direct target of LXRα, which is involved in the biosynthesis of PUFAs via transcriptional regulation of elovl5. These findings may contribute to a further understanding of the mechanism underlying the regulation of LC-PUFA biosynthesis in marine fish species. [ABSTRACT FROM AUTHOR]