Hepatocyte Nuclear Factor 4α (HNF4α) Is a Transcription Factor of Vertebrate Fatty Acyl Desaturase Gene as Identified in Marine Teleost Siganus canaliculatus

Dominant Elongase Activity of Elovl5a but Higher Expression of Elovl5b in Common Carp (Cyprinus carpio)

Most diploid freshwater and marine fish encode one elovl5 elongase, having substrate specificity and activities towards C18, C20 and C22 polyunsaturated fatty acids (PUFAs). The allo-tetraploid common carp is hypothesized to encode two duplicated elovl5 genes. How these two elovl5 genes adapt to coordinate the PUFA biosynthesis through elongase function and expression divergence requires elucidation. In this study, we obtained the full-length cDNA sequences of two elovl5 genes in common carp, named as elovl5a and elovl5b. Functional characterization showed that both enzymes had elongase activity towards C18, C20 and C22 PUFAs. Especially, the activities of these two enzymes towards C22 PUFAs ranged from 3.87% to 8.24%, higher than those in most freshwater and marine fish. The Elovl5a had higher elongase activities than Elovl5b towards seven substrates. The spatial-temporal expression showed that both genes co-transcribed in all tissues and development stages. However, the expression levels of elovl5b were significantly higher than those of elovl5a in all examined conditions, suggesting that elovl5b would be the dominantly expressed gene. These two genes had different potential transcriptional binding sites. These results revealed the complicated roles of elovl5 on PUFA synthesis in common carp. The data also increased the knowledge of co-ordination between two homoeologs of the polyploid fish through function and expression divergence.

Contrasting population differentiation in two sympatric Triplophysa loaches on the Qinghai–Tibet Plateau

Genetic differentiation in aquatic organisms is usually shaped by drainage connectivity. Sympatric aquatic species are thus expected to show similar population differentiation patterns and similar genetic responses to their habitats. Water bodies on the Qinghai–Tibet Plateau (QTP) have recently experienced dramatic physicochemical changes, threatening the biodiversity of aquatic organisms on the “roof of the world.” To uncover ecological genetics in Tibetan loaches (Triplophysa)—the largest component of the QTP ichthyofauna—we characterized population differentiation patterns and adaptive mechanisms to salinity change in two sympatric and phylogenetically closely related Tibetan loaches, T. stewarti and T. stenura, by integrating population genomic, transcriptomic, and electron probe microanalysis approaches. Based on millions of genome-wide SNPs, the two Tibetan loach species show contrasting population differentiation patterns, with highly geographically structured and clear genetic differentiation among T. stewarti populations, whereas there is no such observation in T. stenura, which is also supported by otolith microchemistry mapping. While limited genetic signals of parallel adaption to salinity changes between the two species are found from either genetic or gene expression variation perspective, a catalog of genes involved in ion transport, energy metabolism, structural reorganization, immune response, detoxification, and signal transduction is identified to be related to adaptation to salinity change in Triplophysa loaches. Together, our findings broaden our understanding of the population characteristics and adaptive mechanisms in sympatric Tibetan loach species and would contribute to biodiversity conservation and management of aquatic organisms on the QTP.

Insulin can up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and stimulatory protein 1 (Sp1) in marine teleost Siganus canaliculatus

The rabbitfish Siganus canaliculatus is the first marine teleost found to have the biosynthetic ability of long-chain polyunsaturated fatty acids (LC-PUFA) from C₁₈ precursors catalyzed by fatty acyl desaturases (Δ6/Δ5 Fads, Δ4 Fads) and elongases of very long chain fatty acids (Elovls). Previously, we predicted the existence of insulin (INS) response elements (IREs) including nuclear factor Y (NF-Y) and sterol regulatory element (SRE) in the core promoter region of rabbitfish Δ6/Δ5 fads and Δ4 fads. To clarify the potential regulatory effect and mechanism of INS in LC-PUFA biosynthesis, INS responding region was identified at -456 bp to + 51 bp of Δ6/Δ5 fads core promoter, but not in Δ4 fads promoter. Moreover, a unique stimulatory protein 1 (Sp1) element was predicted in the INS responding region of Δ6/Δ5 fads. Subsequently, SRE, NF-Y and Sp1 elements were proved as IREs in Δ6/Δ5 fads promoter. The up-regulation of INS on gene expression of Srebp-1c, Sp1, Δ6/Δ5 fads and elovl5 as well as the LC-PUFA biosynthesis was further demonstrated in S. canaliculatus hepatocyte line (SCHL) cells, but no influence was detected on Δ4 fads. Besides, inhibitors of transcription factors Srebp-1c (Fatostatin, PF-429242) and Sp1 (Mithramycin) could inhibit the gene expression of Srebp-1c, Δ6/Δ5 fads and elovl5, and abolish the up-regulation of INS on these genes' expression and LC-PUFA biosynthesis. These results indicated that INS could up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and Sp1 in rabbitfish S. canaliculatus, which is the first report in teleost.

Insulin activates LC-PUFA biosynthesis of hepatocytes by regulating the PI3K/Akt/mTOR/Srebp1 pathway in teleost Siganus canaliculatus

Insulin is well known an important metabolic regulator in glucose and lipid metabolism. It has been proved to activate long-chain (≥ C₂₀) polyunsaturated fatty acids (LC-PUFA) biosynthesis in mammals, but little is known about such a role in fish. To explore the effects and molecular mechanisms of insulin in fish LC-PUFA biosynthesis, we treated the rabbitfish S. canaliculatus hepatocyte line (SCHL) cells with 65 nM insulin for 12 h, and the results showed that the mRNA levels of genes encoding the key enzymes and transcription factor involved in rabbitfish LC-PUFA biosynthesis such as Δ6Δ5 fads2, elovl5 and srebp1, as well as those of PI3K pathway genes including pdk1, akt2 and mtor increased significantly. Moreover, SCHL cells treated with different PI3K/Akt pathway inhibitors (LY294002, Wortmannin, AKTi-1/2) alone or combined with insulin decreased the mRNA levels of PI3K/Akt/mTOR downstream signaling genes, including Δ6Δ5 fads2, Δ4 fads2, elovl5, elovl4 and srebp1. While PI3K/Akt agonists (740 Y-P, IGF-1, SC-79) had the opposite results. The results of fatty acid composition analysis of hepatocytes showed that insulin stimulation increased the Δ6Δ5 Fads2-dependent PUFA desaturation indexes, while Elovl5-dependent PUFA elongation indexes had upward trends, and consequently LC-PUFA contents increased. Taken together, these results indicated that insulin activated LC-PUFA biosynthesis probably through PI3K/Akt/mTOR/Srebp1 pathway in S. canaliculatus hepatocytes.

Regulation of Δ6Fads2 Gene Involved in LC-PUFA Biosynthesis Subjected to Fatty Acid in Large Yellow Croaker (Larimichthys crocea) and Rainbow Trout (Oncorhynchus mykiss)

Δ6 fatty acyl desaturase (Δ6Fads2) is regarded as the first rate-limiting desaturase that catalyzes the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA) from 18-carbon fatty acid in vertebrates, but the underlying regulatory mechanism of fads2 has not been comprehensively understood. This study aimed to investigate the regulation role of fads2 subjected to fatty acid in large yellow croaker and rainbow trout. In vivo, large yellow croaker and rainbow trout were fed a fish oil (FO) diet, a soybean oil (SO) diet or a linseed oil (LO) diet for 10 weeks. The results show that LO and SO can significantly increase fads2 expression (p < 0.05). In vitro experiments were conducted in HEK293T cells or primary hepatocytes to determine the transcriptional regulation of fads2. The results show that CCAAT/enhancer-binding protein α (C/EBPα) can up-regulate fads2 expression. GATA binding protein 3 (GATA3) can up-regulate fads2 expression in rainbow trout but showed opposite effect in large yellow croaker. Furthermore, C/EBPα protein levels were significantly increased by LO and SO (p < 0.05), gata3 expression was increased in rainbow trout by LO but decreased in large yellow croaker by LO and SO. In conclusion, we revealed that FO replaced by LO and SO increased fads2 expression through a C/EBPα and GATA3 dependent mechanism in large yellow croaker and rainbow trout. This study might provide critical insights into the regulatory mechanisms of fads2 expression and LC-PUFA biosynthesis.

Genetic Analysis of the Fatty Acid Profile in Gilthead Seabream (Sparus aurata L.)

Simple Summary

Humans require essential fatty acids in their diet and marine fish are a source of them, especially omega3 fatty acids that present high benefits on diverse vascular diseases and the immune system. Breeding programs in gilthead seabream usually include growth as the first criterion in the selection process of the fish. However, that could lead to fish with a higher fillet fat content and a fatty acid profile with a lower polyunsaturated fatty acids percentage. Fillet fat content and its fatty acids profile have been revealed as heritable traits. Therefore, further studies to go deeper in the selection process are advisable.

Abstract

The gilthead seabream is one of the most valuable species in the Mediterranean basin both for fisheries and aquaculture. Marine fish, such as gilthead seabream, are a source of n3 polyunsaturated fatty acids, highly appreciated for human food owing to their benefits on the cardiovascular and immune systems. The aim of the present study was to estimate heritability for fatty acid (FA) profile in fillet gilthead seabream to be considered as a strategy of a selective breeding program. Total of 399 fish, from a broodstock Mediterranean Sea, were analysed for growth, flesh composition and FA profile. Heritabilities for growth traits, and flesh composition (fat, protein, and moisture content) were medium. Heritability was moderate for 14:0, 16:0 and 18:1n9 and for sum of monounsaturated FA and n6/n3 ratio, and it was low for 20:1n11 and 22:6n3 and the ratio unsaturated/saturated FA. Breeding programs in gilthead seabream usually include growth as the first criterion in the selection process of the fish. However, other quality traits, such as fillet fat content and its fatty acids profile should be considered, since they are very important traits for the consumer, from a nutritional point of view and the benefits for the health.

Regulation of long-chain polyunsaturated fatty acid biosynthesis in teleost fish

Omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA, C_20-24), including eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), are involved in numerous biological processes and have a range of health benefits. Fish have long been considered as the main source of n-3 LC-PUFA in human diets. However, the capacity for endogenous biosynthesis of LC-PUFA from C₁₈ PUFA varies in fish species based on the presence, expression and activity of key enzymes including fatty acyl desaturases (Fads) and elongation of very long-chain fatty acids (Elovl) proteins. In this article, we review progress on the identified Fads and Elovl, as well as the regulatory mechanisms of LC-PUFA biosynthesis both at transcriptional and post-transcriptional levels in teleosts. The most comprehensive advances have been obtained in rabbitfish Siganus canaliculatus, a marine teleost demonstrated to have the entire pathway for LC-PUFA biosynthesis, including the roles of transcription factors hepatocyte nuclear factor 4α (Hnf4α), liver X receptor alpha (Lxrα), sterol regulatory element-binding protein 1 (Srebp-1), peroxisome proliferator-activated receptor gamma (Pparγ) and stimulatory protein 1 (Sp1), as well as post-transcriptional regulation by individual microRNA (miRNA) or clusters. This research has, for the first time, demonstrated the involvement of Hnf4α, Pparγ and miRNA in the regulation of LC-PUFA biosynthesis in vertebrates. The present review provides readers with a relatively comprehensive overview of the progress made into understanding LC-PUFA biosynthetic systems in teleosts, and some insights into improving endogenous LC-PUFA biosynthesis capacity aimed at reducing the dependence of aquafeeds on fish oil while maintaining or increasing flesh LC-PUFA content and the nutritional quality of farmed fish.

PPARγ regulates fabp4 expression to increase DHA content in golden pompano (Trachinotus ovatus) hepatocytes

N-3 long-chain (≥C20) PUFA (LC-PUFA) are vital fatty acids for fish and humans. As a main source of n-3 LC-PUFA for human consumers, the n-3 LC-PUFA content of farmed fish is important. Previously, we identified fatty acid-binding protein (fabp)-4 as a candidate gene for regulating the n-3 LC-PUFA content. Herein, we further assessed the role of fabp4 in this process. First, a 2059 bp promoter sequence of fabp4 in Trachinotus ovatus was cloned and, using progressive deletion, determined -2006 bp to -1521 bp to be the core promoter sequence. The PPAR-γ binding sites were predicted to occur in this region. A luciferase reporter assay showed that the promoter activity of fabp4 decreased following mutation of the PPARγ binding site and that PPARγ increased the fabp4 promoter activity in a dose-dependent manner, implying that T. ovatus fabp4 is a target of PPARγ. The overexpression of fabp4 or PPARγ increased the DHA content in hepatocytes, whereas suppression of their expression diminished this effect, suggesting that both fabp4 and PPARγ play an active role in regulating DHA content. Moreover, the inhibition of fabp4 attenuated the increase in PPARγ-mediated DHA content, and the overexpression of fabp4 alleviated this effect. Collectively, our findings indicated that fabp4, which is controlled by PPARγ, plays an important role in DHA content regulation. The new regulation axis can be considered a promising novel target for increasing the n-3 LC-PUFA content in T. ovatus.

Identification of miR-145 as a Key Regulator Involved in LC-PUFA Biosynthesis by Targeting hnf4α in the Marine Teleost Siganus canaliculatus

Fish, particularly marine species, are considered as the major source of long-chain polyunsaturated fatty acids (LC-PUFA) in the human diet. The extent to which fish can synthesize LC-PUFA varies with species and is regulated by dietary fatty acids and ambient salinity. Therefore, to enable fish to produce more LC-PUFA, comprehending the mechanisms underlying the regulation of LC-PUFA biosynthesis is necessary. Here, the regulatory roles of miR-145 were investigated in the marine teleost rabbitfish Siganus canaliculatus. The hepatic abundance of miR-145 was lower in rabbitfish reared in low salinity (10 ppt) in comparison with that of those cultured in seawater (32 ppt), while the opposite pattern was observed for the transcripts of the transcription factor hepatocyte nuclear factor 4 alpha (Hnf4α), known to affect rabbitfish LC-PUFA biosynthesis. Rabbitfish hnf4α was identified as a target of miR-145 by luciferase reporter assays, and overexpression of miR-145 in the S. canaliculatus hepatocyte line (SCHL) markedly reduced the expression of Hnf4α and its target genes involved in LC-PUFA biosynthesis, namely, Δ4 fads2, Δ6Δ5 fads2, and elovl5. The opposite pattern was observed when miR-145 was knocked down in SCHL cells, with these effects being attenuated by subsequent hnf4α knockdown. Moreover, increasing endogenous Hnf4α by the knockdown of miR-145 increased the expression of LC-PUFA biosynthesis genes and enhanced the synthesis of LC-PUFA in both SCHL cells and rabbitfish in vivo. This is the first report to identify miR-145 as a key effector of LC-PUFA biosynthesis by targeting hnf4α, providing a novel insight into the mechanisms of the regulation of LC-PUFA biosynthesis in vertebrates.

miR-26a mediates LC-PUFA biosynthesis by targeting the Lxrα-Srebp1 pathway in the marine teleost Siganus canaliculatus

MicroRNAs have been recently shown to be important regulators of lipid metabolism. However, the mechanisms of microRNA-mediated regulation of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis in vertebrates remain largely unknown. Herein, we for the first time addressed the role of miR-26a in LC-PUFA biosynthesis in the marine rabbitfish Siganus canaliculatus The results showed that miR-26a was significantly down-regulated in liver of rabbitfish reared in brackish water and in S. canaliculatus hepatocyte line (SCHL) incubated with the LC-PUFA precursor α-linolenic acid, suggesting that miR-26a may be involved in LC-PUFA biosynthesis because of its abundance being regulated by factors affecting LC-PUFA biosynthesis. Opposite patterns were observed in the expression of liver X receptor α (lxrα) and sterol regulatory element-binding protein-1 (srebp1), as well as the LC-PUFA biosynthesis-related genes (Δ4 fads2, Δ6Δ5 fads2, and elovl5) in SCHL cells incubated with α-linolenic acid. Luciferase reporter assays revealed rabbitfish lxrα as a target of miR-26a, and overexpression of miR-26a in SCHL cells markedly reduced protein levels of Lxrα, Srebp1, and Δ6Δ5 Fads2 induced by the agonist T0901317. Moreover, increasing endogenous Lxrα by knockdown of miR-26a facilitated Srebp1 activation and concomitant increased expression of genes involved in LC-PUFA biosynthesis and consequently promoted LC-PUFA biosynthesis both in vitro and in vivo These results indicate a critical role of miR-26a in regulating LC-PUFA biosynthesis through targeting the Lxrα-Srebp1 pathway and provide new insights into the regulatory network controlling LC-PUFA biosynthesis and accumulation in vertebrates.

Functional analysis of two MyoDs revealed their role in the activation of myomixer expression in yellowfin seabream (Acanthopagrus latus) (Hottuyn, 1782)

Myoblast determination protein (MyoD), a muscle-specific basic helix-loop-helix (bHLH) transcription factor, plays a pivotal role in regulating skeletal muscle growth and development. However, the regulation mechanism of MyoD has not been determined in marine fishes. In the present study, we isolated the MyoD1 (AlMyoD1) and MyoD2 (AlMyoD2) genomic sequences and analyzed the expression patterns in different tissues of yellowfin seabream (Acanthopagrus latus). The open reading frame (ORF) sequences of AlMyoD1 and AlMyoD2 encoded 297 and 271 amino acids possessing three common characteristic domains, respectively, containing a myogenic basic domain, a bHLH domain, and a ser-rich region (helix III). Phylogenetic and genome structure analyses exhibited classic phylogeny and highly conserved exon/intron architecture. Furthermore, the AlMyoD1 and AlMyoD2 transcription levels were higher in white muscle than in the other tissues. In order to further study AlMyoD function in muscle, promoter sequence analysis found that several E-box binding sites were present. Additionally, binding sites of Almyomixer involved in mammal myoblast fusion, which expression was also the highest in white muscle, were found in the promoter of AlMyoD. Pomoter activity assays further confirmed that both AlMyoD1 and AlMyoD2 can dramatically activate Almyomixer expression, and the AlMyoD1 M2 and AlMyoD2 M5 E-box binding sites were functionally important for Almyomixer transcription based on mutation analysis and electrophoretic mobile shift assays (EMSA). In summary, two MyoDs play a core role in Almyomixer regulation and may promote myofibre formation during muscle development and growth by regulating Almyomixer expression.

The miR-15/16 Cluster Is Involved in the Regulation of Vertebrate LC-PUFA Biosynthesis by Targeting pparγ as Demonostrated in Rabbitfish Siganus canaliculatus

Post-transcriptional regulatory mechanisms play important roles in the regulation of long-chain (≥ C₂₀) polyunsaturated fatty acid (LC-PUFA) biosynthesis. Here, we address a potentially important role of the miR-15/16 cluster in the regulation of LC-PUFA biosynthesis in rabbitfish Siganus canaliculatus. In rabbitfish, miR-15 and miR-16 were both highly responsive to fatty acids affecting LC-PUFA biosynthesis and displayed a similar expression pattern in a range of rabbitfish tissues. A common potential binding site for miR-15 and miR-16 was predicted in the 3'UTR of peroxisome proliferator-activated receptor gamma (pparγ), an inhibitor of LC-PUFA biosynthesis in rabbitfish, and luciferase reporter assays revealed that pparγ was a potential target of miR-15/16 cluster. In vitro individual or co-overexpression of miR-15 and miR-16 in rabbitfish hepatocyte line (SCHL) inhibited both mRNA and protein levels of Pparγ, and increased the mRNA levels of Δ6Δ5 fads2, Δ4 fads2, and elovl5, key enzymes of LC-PUFA biosynthesis. Inhibition of pparγ was more pronounced with co-overexpression of miR-15 and miR-16 than with individual overexpression in SCHL. Knockdown of miR-15/16 cluster gave opposite results, and increased mRNA levels of LC-PUFA biosynthesis enzymes were observed after knockdown of pparγ. Furthermore, miR-15/16 cluster overexpression significantly increased the contents of 22:6n-3, 20:4n-6 and total LC-PUFA in SCHL with higher 18:4n-3/18:3n-3 and 22:6n-3/22:5n-3 ratio. These suggested that miR-15 and miR-16 as a miRNA cluster together enhanced LC-PUFA biosynthesis by targeting pparγ in rabbitfish. This is the first report of the participation of miR-15/16 cluster in LC-PUFA biosynthesis in vertebrates.

Molecular Cloning, Characterization, and Expression Regulation of Acyl-CoA Synthetase 6 Gene and Promoter in Common Carp Cyprinus carpio

Omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA), particularly docosahexaenoic acids (22:6n-3, DHA), have positive effects on multiple biologic and pathologic processes. Fish are the major dietary source of n-3 LC-PUFA for humans. Growing evidence supports acyl-coenzyme A (acyl-CoA) synthetase 6 (acsl6) being involved in cellular DHA uptake and lipogenesis in mammals, while its molecular function and regulatory mechanism remain unknown in fish. The present study focused on investigating the molecular characterization and transcription regulation of the acsl6 gene in the freshwater teleost common carp (Cyprinus carpio). First, the full length of acsl6 cDNA contained a coding region of 2148 bp for 715 amino acids, which possessed all characteristic features of the acyl-CoA synthetase (ACSL) family. Its mRNA expression was the highest in the brain, followed by in the heart, liver, kidney, muscle, and eyes, but little expression was detected in the ovary and gills. Additionally, a candidate acsl6 promoter region of 2058 bp was cloned, and the sequence from -758 bp to -198 bp was determined as core a promoter by equal progressive deletion and electrophoretic mobility shift assay. The binding sites for important transcription factors (TFs), including stimulatory protein 1 (SP1), CCAAT enhancer-binding protein (C/EBPα), sterol-regulatory element binding protein 1c (SREBP1c), peroxisome proliferator activated receptor α (PPARα), and PPARγ were identified in the core promoter by site-directed mutation and functional assays. Furthermore, the intraperitoneal injection of PPARγ agonists (balaglitazone) increased the expression of acsl6 mRNA, coupling with an increased proportion of DHA in the muscle, while opposite results were obtained in the injection of the SREBP1c antagonist (betulin). However, the expression of acsl6 and DHA content in muscle were largely unchanged by PPARα agonist (fenofibrate) treatment. These results indicated that acsl6 may play an important role for the muscular DHA uptake and deposition in common carp, and PPARγ and SREBP-1c are the potential TFs involved in the transcriptional regulation of acsl6 gene. To our knowledge, this is the first report of the characterization of acsl6 gene and its promoter in teleosts.

Structural and expression analysis of golden pompano Trachinotus ovatus IRF5 and its role in regulation of type I IFN

The interferon regulatory factor 5 (IRF5) is a mediator of the type I IFN signalling pathways, thereby playing a key role in innate immunity. However, the detailed mechanism through which IRF5 regulates type I IFN in fish remains unclearly. In the present study, we first describe the identification of IRF5 (ToIRF5) from golden pompano (Trachinotus ovatus) and its features at the genomic sequence and expression level. The genomic DNA sequence consists of eight exons and seven introns. The full-length ToIRF5 cDNA is composed of 2, 059 bp and encodes for 499 amino acid polypeptides. The putative protein sequence shares 66.3%-82.9% identity to fish IRF5 and possesses three representative conserved domains (a DNA-binding domain (DBD) at the N-terminus, an IRF-associated domain (IAD), and a virus-activated domain (VAD) at the C-terminus) and one highly variable domain (middle region (MR)). Furthermore, the ToIRF5 transcript is constitutively expressed in all examined tissues, with higher levels observed in the immune relevant tissues. The mRNA levels of ToIRF5 are increased by polyinosinic: polycytidylic acid [poly (I: C)], lipopolysaccharide (LPS) and flagellin stimulation in the immune- and nonimmune-related tissues. The subcellular localization indicates that ToIRF5 is mainly localized in the cytoplasm with or without poly (I: C) induction. In addition, to explore whether ToIRF5 is a modulator of ToIFNa3, promoter analysis is performed. The region from -200 bp to +1 bp is identified as the core promoter by different truncated mutants of ToIFNa3. Mutation analyse declares that the activity of the ToIFNa3-5 promoter significantly decreases after targeted mutation of M2 binding sites. Moreover, overexpression of ToIRF5 in vitro memorably aggrandizes the expression of some IFN/IRF-based signalling pathway genes. These results provide new insights into the roles of teleost IRF5 in transcriptional mechanisms of type I IFN in the immunity process.

Interferon regulatory factor 2 plays a positive role in interferon gamma expression in golden pompano, Trachinotus ovatus (Linnaeus 1758)

In fish, interferon (IFN) regulatory factor 2 (IRF2) is a regulator of the type I IFN-dependent immune response, thereby playing a crucial role in innate immunity. However, the specific mechanism by which IRF2 regulates type II IFN in fish remains unclear. In the present study, first, to analyse the potential role of golden pompano (Trachinotus ovatus) IRF2 (ToIRF2) in the immune response, the mRNA level of ToIRF2 was detected by quantitative real-time polymerase chain reaction (qRT-PCR) after parasite infection. ToIRF2 was upregulated at early time points in both local infection sites (skin and gill) and system immune tissues (liver, spleen, and head-kidney) after stimulation with Cryptocaryon irritans. Second, to investigate the modulation effect of ToIRF2 on type II IFN (interferon gamma, IFNγ) expression, a promoter analysis was performed using progressive deletion mutations of ToIFNγ. The expression level of IFNγ-5 was highest among the five truncated mutants in response to ToIRF2, indicating that the core promoter region was located from -189 bp to +120 bp, which included the IRF2 binding sites. Mutation analyses showed that the activity of the ToIFNγ promoter dramatically decreased after the targeted mutation of the M1, M2 or M3 binding sites. Additionally, electrophoretic mobile shift assay (EMSA) confirmed that IRF2 interacted with the M1 binding site in the ToIFNγ promoter region to dominate ToIFNγ expression. Finally, overexpressing ToIRF2 in vitro notably increased ToIFNγ and the transcription of several type II IFN/IRF-based signalling pathway genes. These results suggested that ToIRF2 might be involved in the host defence against C. irritans infection and contribute to a better understanding of the transcriptional mechanisms by which ToIRF2 regulates type II IFN in fish.

Functional characterization of IRF8 regulation of type II IFN in golden pompano (Trachinotus ovatus)

Interferon regulatory factor 8 (IRF8) increases type I IFN transcription levels by binding to IFN promoters, thereby playing a role in innate immunity. Nevertheless, the detailed mechanism through which IRF8 regulates type II IFN in fish remains ambiguous. In the present study, two genes from the golden pompano (Trachinotus ovatus), IRF8 (ToIRF8) and IFN gamma (ToIFNγ), were identified in the IFN/IRF-based signalling pathway. The full-length ToIRF8 cDNA was composed of 2,141 bp and encoded a 421 amino acid polypeptide; the genomic DNA was 2,917 bp in length and consisted of 8 exons and 7 introns. The putative protein showed the highest sequence identity (90-92%) with fish IRF8 and possessed a DNA-binding domain (DBD), an IRF-association domain (IAD) and a nuclear localization signal (NLS) motif consistent with those of IRF8 in other vertebrates. Furthermore, the ToIRF8 transcripts were expressed in all examined tissues of healthy fish, with higher levels observed in the central nervous and immune relevant tissues. They were upregulated by polyinosinic acid: polycytidylic acid [poly (I: C)], lipopolysaccharide (LPS) and flagellin treatments in the blood, liver, intestine and kidney. The results from assays of subcellular localization showed that ToIRF8 was localized to the cytoplasm. Moreover, to investigate whether ToIRF8 was a regulator of ToIFNγ, a promoter analysis was performed using progressive deletion mutations of ToIFNγ. The results indicated that the region from -601 bp to -468 bp includes the core promoter. Mutation analyses indicated that the activity of the ToIFNγ promoter significantly decreased after the targeted mutation of the M1-M3 binding sites. Additionally, overexpressed ToIRF8 in vitro notably increased the expression of several IFN/IRF-based signalling pathway genes. These results suggest that IRF8 is vital in the defence of T. ovatus against bacterial infection and contributes to a better understanding of the transcriptional mechanisms of ToIRF8 on type II IFN in fish.

Sp1 is Involved in Vertebrate LC-PUFA Biosynthesis by Upregulating the Expression of Liver Desaturase and Elongase Genes

The rabbitfish Siganus canaliculatus was the first marine teleost demonstrated to have the ability for the biosynthesis of long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) from C18 PUFA precursors, and all the catalytic enzymes including two fatty acyl desaturase 2 (Δ4 Fads2 and Δ6/Δ5 Fads2) and two elongases (Elovl4 and Elovl5) have been identified, providing a good model for studying the regulatory mechanisms of LC-PUFA biosynthesis in fish. Stimulatory protein 1 (Sp1) has been speculated to be a vital transcription factor in determining the promoter activity of Fads-like genes in fish, however its regulatory effects on gene expression and LC-PUFA biosynthesis have not been demonstrated. Bioinformatic analysis predicted potential Sp1 binding sites in the promoters of the rabbitfish Δ6/Δ5 fads2 and elovl5, but not in Δ4 fads2 promoter. Here we cloned full-length cDNA of the rabbitfish sp1 gene, which encoded a putative protein of 701 amino acids, and was expressed in all tissues studied with highest levels in gill and eyes. The dual luciferase reporter assay in HepG2 line cells demonstrated the importance of the Sp1 binding site for the promoter activities of both Δ6/Δ5 fads2 and elovl5. Moreover, the electrophoretic mobility shift assay confirmed the direct interaction of Sp1 with the two promoters. Insertion of the Sp1 binding site of Δ6/Δ5 fads2 promoter into the corresponding region of the Δ4 fads2 promoter significantly increased activity of the latter. In the Siganus canaliculatus hepatocyte line (SCHL) cells, mRNA levels of Δ6/Δ5 fads2 and elovl5 were positively correlated with the expression of sp1 when sp1 was overexpressed or knocked-down by RNAi or antagonist (mithramycin) treatment. Moreover, overexpression of sp1 also led to a higher conversion of 18:2n−6 to 18:3n−6, 18:2n−6 to 20:2n−6, and 18:3n−3 to 20:3n−3, which related to the functions of Δ6/Δ5 Fads2 and Elovl5, respectively. These results indicated that Sp1 is involved in the transcriptional regulation of LC-PUFA biosynthesis by directly targeting Δ6/Δ5 fads2 and elovl5 in rabbitfish, which is the first report of Sp1 involvement in the regulation of LC-PUFA biosynthesis in vertebrates.

Functional characterization of interferon regulatory factor 2 and its role in the transcription of interferon a3 in golden pompano Trachinotus ovatus (Linnaeus 1758)

Similar to mammals, fish possess interferon (IFN) regulatory factor 2 (IRF2)-dependent type I IFN responses. Nevertheless, the detailed mechanism through which IRF2 regulates type I IFNa3 remains largely unknown. In the present study, we first identified two genes from golden pompano (Trachinotus ovatus), IRF2 (ToIRF2) and IFNa3 (ToIFNa3), in the IFN/IRF-based signalling pathway. The open reading frame (ORF) sequence of ToIRF2 encoded 335 amino acids possessing four typical characteristic domains, including a conserved DNA-binding domain (DBD), an interferon association domain 2 (IAD2), a transcriptional activation domain (TAD), and a transcriptional repression domain (TRD). Furthermore, transcripts of ToIRF2 were significantly upregulated after stimulation by polyinosinic: polycytidylic acid [poly (I:C)], lipopolysaccharide (LPS) and flagellin in immune-related tissues (blood, liver, and head-kidney). Moreover, to investigate whether ToIRF2 was a regulator of ToIFNa3, promoter analysis was performed. The results showed that the region from -896 bp to -200 bp is defined as the core promoter using progressive deletion mutations of IFNa3. Additionally, ToIRF2 overexpression led to a clear time-dependent enhancement of ToIFNa3 promoter expression in HEK293T cells. Mutation analyses indicated that the activity of the ToIFNa3 promoter significantly decreased after targeted mutation of M4/5 binding sites. Electrophoretic mobile shift assays (EMSAs) verified that IRF2 interacted with the binding site of the ToIFNa3 promoter region to regulate ToIFNa3 transcription. Last, the promoter activity of ToIFNa3-2 was more responsive to treatment with poly (I:C) than LPS and flagellin. Furthermore, overexpression of ToIRF2 in vitro obviously increased the expression of several IFN/IRF-based signalling pathway genes after poly (I:C) abduction. In conclusion, the present study provides the first evidence of the positive regulation of ToIFNa3 transcription by ToIRF2 and contributes to a better understanding of the transcriptional mechanisms of ToIRF2 in fish.

Elovl4a participates in LC-PUFA biosynthesis and is regulated by PPARαβ in golden pompano Trachinotus ovatus (Linnaeus 1758)

The elongases of very long-chain fatty acids (Elovls) are responsible for the rate-limiting elongation process in long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis. The transcription factor, PPARα, regulates lipid metabolism in mammals; however, the detailed mechanism whereby PPARαb regulates Elovls remains largely unknown in fish. In the present study, we report the full length cDNA sequence of Trachinotus ovatus Elovl4a (ToElovl4a), which encodes a 320 amino acid polypeptide that possesses five putative membrane-spanning domains, a conserved HXXHH histidine motif and an ER retrieval signal. Phylogenetic analysis revealed that the deduced protein of ToElovl4a is highly conserved with the Oreochromis niloticus corresponding homologue. Moreover, functional characterization by heterologous expression in yeast indicated that ToElovl4a can elongate C18 up to C20 polyunsaturated fatty acids. A nutritional study showed that the protein expressions of ToElovl4a in the brain and liver were not significantly affected among the different treatments. The region from PGL3-basic-Elovl4a-5 (−148 bp to +258 bp) is defined as the core promoter via a progressive deletion mutation of ToElovl4a. The results from promoter activity assays suggest that ToElovl4a transcription is positively regulated by PPARαb. Mutation analyses indicated that the M2 binding site of PPARαb is functionally important for protein binding, and transcriptional activity of the ToElovl4a promoter significantly decreased after targeted mutation. Furthermore, PPARαb RNA interference reduced ToPPARαb and ToElovl4a expression at the protein levels in a time-dependent manner. In summary, PPARαb may promote the biosynthesis of LC-PUFA by regulating ToElovl4a expression in fish.

Pparγ Is Involved in the Transcriptional Regulation of Liver LC-PUFA Biosynthesis by Targeting the Δ6Δ5 Fatty Acyl Desaturase Gene in the Marine Teleost Siganus canaliculatus

As the first marine teleost demonstrated to have the ability of long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis from C₁₈ PUFA precursors, the rabbitfish Siganus canaliculatus provides us a unique model for clarifying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts aiming at the replacement of dietary fish oil (rich in LC-PUFA) with vegetable oils (rich in C₁₈ PUFA precursors but devoid of LC-PUFA). In the study of transcription regulation of gene encoding the Δ6Δ5 fatty acyl desaturase (Δ6Δ5 Fads), a rate-limiting enzyme catalyzing the first step of LC-PUFA biosynthesis in rabbitfish, a binding site for the transcription factor (TF), peroxisome proliferator-activated receptor γ (Pparγ), was predicted in Δ6Δ5 fads2 promoter by bioinformatics analysis, and thus the present study focused on the regulatory roles of Pparγ on Δ6Δ5 fads2. First, the activity of the Δ6Δ5 fads2 promoter was proved to be downregulated by pparγ overexpression and upregulated by treatment of Pparγ antagonist (GW9662), respectively, in HEK 293T cells with the dual luciferase reporter assay. Pparγ was further confirmed to interact with the promoter by electrophoretic mobility shift assay. Moreover, in S. canaliculatus hepatocyte line (SCHL) cells, GW9662 decreased the expression of pparγ together with increase of Δ6Δ5 fads2 mRNA. Besides, Δ6Δ5 fads2 expression was increased by pparγ RNAi knockdown and reduced by its mRNA overexpression. Furthermore, knockdown of pparγ induced a high conversion of 18:3n-3 to 18:4n-3 and 18:2n-6 to 18:3n-6, while pparγ mRNA overexpression led to a lower conversion of that, and finally a significant decrease of 20:4n-6(ARA), 20:5n-3(EPA), and 22:6n-3(DHA) production. The results indicate that Pparγ is involved in the transcriptional regulation of liver LC-PUFA biosynthesis by targeting Δ6Δ5 fads2 in rabbitfish, which is the first report of Pparγ involvement in the regulation of LC-PUFA biosynthesis in teleosts.

Comparison of the growth performance and long-chain PUFA biosynthetic ability of the genetically improved farmed tilapia (Oreochromis niloticus) reared in different salinities

To compare the growth and biosynthetic ability of long-chain PUFA (LC-PUFA) of the genetically improved farmed tilapia (GIFT) (Oreochromis niloticus) in different water salinities, an 8-week feeding trial was conducted on the GIFT juveniles at 0, 12 and 24 ‰ (parts per thousand; ppt), respectively, with three isonitrogenous (32 %) and isolipidic (8 %) diets (D1-D3). Diet D1 with fish oils (rich in LC-PUFA) as lipid source was used as the control, while D2 and D3 with vegetable oil (free LC-PUFA) blends as lipid source contained different ratios of linoleic acid (LA, 18 : 2n-6) and α-linolenic acid (ALA, 18 : 3n-3) at 4·04 (D2) and 0·54 (D3), respectively. At the end of feeding trial, the growth performance of D2 and D3 groups under all salinity treatments was as good as that of D1 group, which indicates that the GIFT juveniles may convert dietary LA and ALA into LC-PUFA to meet the requirement of essential fatty acids for normal growth and physiology. When fed the same diets, GIFT at 12 ppt had a better growth performance coupled with a higher liver and muscle arachidonic acid content than those in freshwater. Furthermore, brackish water (24 ppt) significantly promoted the mRNA levels of elongase 5 of very long-chain fatty acids (elovl5) and peroxisome proliferator-activated receptor α (pparα) in liver, when compared with freshwater. These results suggest that the GIFT may display better growth performance together with a relatively higher endogenous LC-PUFA biosynthetic ability under brackish water (12 and 24 ppt), probably through improving the expression of elovl5 and pparα in liver.

Identification of Fatty Acid Desaturase 6 in Golden Pompano Trachinotus Ovatus (Linnaeus 1758) and Its Regulation by the PPARαb Transcription Factor

Fatty acid desaturases are rate-limiting enzymes in long-chain polyunsaturated fatty acid biosynthesis. The transcription factor peroxisome proliferator-activated receptor alpha b (PPARαb) regulates lipid metabolism in mammals, however, the mechanism whereby PPARαb regulates fatty acid desaturases is largely unknown in fish. In this study, we report the full length cDNA sequence of Trachinotus ovatus fatty acid desaturase, which encodes a 380 amino acid polypeptide, possessing three characteristic histidine domains. Phylogenetic and gene exon/intron structure analyses showed typical phylogeny: the T. ovatus fatty acid desaturase contained a highly conserved exon/intron architecture. Moreover, functional characterization by heterologous expression in yeast indicated that T. ovatus desaturase was a fatty acid desaturase, with Δ4/Δ5/Δ8 Fad activity. Promoter activity assays indicated that ToFads6 desaturase transcription was positively regulated by PPARαb. Similarly, PPARαb RNA interference decreased ToPPARαb and ToFads6 expression at the mRNA and protein levels in a time-dependent manner. Mutation analyses showed that the M2 binding site of PPARαb was functionally important for protein binding, and transcriptional activity of the ToFads6 promoter was significantly decreased after targeted mutation of M2. Electrophoretic mobile shift assays confirmed that PPARαb interacted with the binding site of the ToFads6 promoter region, to regulate ToFads6 transcription. In summary, PPARαb played a vital role in ToFads6 regulation and may promote the biosynthesis of long-chain polyunsaturated fatty acids by regulating ToFads6 expression.

Hnf4α Is Involved in LC-PUFA Biosynthesis by Up-Regulating Gene Transcription of Elongase in Marine Teleost Siganus canaliculatus

The rabbitfish Siganus canaliculatus is the first marine teleost shown to be able to biosynthesize long-chain polyunsaturated fatty acids (LC-PUFA) from C18 PUFA precursors catalyzed by two fatty acyl desaturases (fad) including Δ4 Fad and Δ6/Δ5 Fad as well as two elongases (Elovl4 and Elovl5). Previously, hepatocyte nuclear factor 4α (Hnf4α) was demonstrated to be predominant in the transcriptional regulation of two fads. To clarify the regulatory mechanisms involved in rabbitfish lipogenesis, the present study focused on the regulatory role of Hnf4α to elovl5 expression and LC-PUFA biosynthesis. Bioinformatics analysis predicted two potential Hnf4α elements in elovl5 promoter, one binding site was confirmed to interact with Hnf4α by gel shift assays. Moreover, overexpression of hnf4α caused a remarkable increase both in elovl5 promoter activity and mRNA contents, while knock-down of hnf4α in S. canaliculatus hepatocyte line (SCHL) resulted in a significant decrease of elovl5 gene expression. Meanwhile, hnf4α overexpression enhanced LC-PUFA biosynthesis in SCHL cell, and intraperitoneal injection to rabbitfish juveniles with Hnf4α agonists (Alverine and Benfluorex) increased the expression of hnf4α, elvol5 and Δ4 fad, coupled with an increased proportion of total LC-PUFA in liver. The results demonstrated that Hnf4α is involved in LC-PUFA biosynthesis by up-regulating the transcription of the elovl5 gene in rabbitfish, which is the first report of Hnf4α as a transcription factor of the elovl5 gene in vertebrates.

Hnf4α is involved in the regulation of vertebrate LC-PUFA biosynthesis: insights into the regulatory role of Hnf4α on expression of liver fatty acyl desaturases in the marine teleost Siganus canaliculatus

Long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis is an important metabolic pathway in vertebrates, especially fish, considering they are the major source of n-3 LC-PUFA in the human diet. However, most fish have only limited capability for biosynthesis of LC-PUFA. The rabbitfish (Siganus canaliculatus) is able to synthesize LC-PUFA as it has all the key enzyme activities required including Δ6Δ5 Fads2, Δ4 Fads2, Elovl5, and Elovl4. We previously reported a direct interaction between the transcription factor Hnf4α and the promoter regions of Δ4 and Δ6Δ5 Fads2, which suggested that Hnf4α was involved in the transcriptional regulation of fads2 in rabbitfish. For functionally investigating it further, a full-length cDNA of 1736-bp-encoding rabbitfish Hnf4α with 454 amino acids was cloned, which was highly expressed in intestine, followed by liver and eyes. Similar to the expression characteristics of its target genes Δ4 and Δ6Δ5 fads2, levels of hnf4α mRNA in liver and eyes were higher in fish reared at low salinity than those reared in high salinity. After the rabbitfish primary hepatocytes were, respectively, incubated with alverine, benfluorex or BI6015, which were anticipated agonists or antagonist for Hnf4α, the mRNA level of Δ6Δ5 and Δ4 fads2 displayed a similar change tendency with that of hnf4α mRNA. Furthermore, when the mRNA level of hhf4α was knocked down using siRNA, the expression of Δ6Δ5 and Δ4 fads2 also decreased. Together, these data suggest that Hnf4α is involved in the transcriptional regulation of LC-PUFA biosynthesis, specifically, by targeting Δ4 and Δ6Δ5 fads2 in rabbitfish.

Characteristics of the fads2 gene promoter in marine teleost Epinephelus coioides and role of Sp1-binding site in determining promoter activity

Δ6 fatty acyl desaturase (Fads2) is a rate-limiting enzyme in long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis. Comparative analysis of gene promoters of Fads2 between salmonids and carnivorous marine fish suggested that the lack of binding site for stimulatory protein 1 (Sp1) was responsible for the low expression of fads2 gene of carnivorous marine species. To confirm this speculation, the fads2 candidate promoter (2646 bp) was cloned from carnivorous marine teleost Epinephelus coioides, and 330 bp core regulatory region was identified. Several binding sites for transcriptional factors such as nuclear factor 1, nuclear factor Y, sterol regulatory element and hepatocyte nuclear factor 4γ were identified, while that for Sp1 was shown to be absent in the promoter by both bioinformatic analysis and site-directed mutation. Moreover, after the Sp1-binding site from the fads2 promoter of herbivorous Siganus canaliculatus, the first marine teleost demonstrated to have LC-PUFA biosynthetic ability, was inserted into the corresponding region of E. coioides fads2 promoter, activity was significantly increased. The results provided direct data for the importance of the Sp1-binding site in determining fads2 promoter activity, and indicated that its lack may be a reason for low expression of fads2 and poor LC-PUFA biosynthetic ability in E. coioides.