Peroxisome proliferator-activated receptor gamma (PPARγ) in yellow catfish Pelteobagrus fulvidraco: molecular characterization, mRNA expression and transcriptional regulation by insulin in vivo and in vitro

The Peroxisome Proliferator-Activated Receptors of Ray-Finned Fish: Unique Structures, Elusive Functions

The Actinopterygian and specifically the Teleostean peroxisome proliferator-activated receptors (PPARs) present an impressive variability and complexity in their structures, both at the gene and protein levels. These structural differences may also reflect functional divergence from their mammalian homologs, or even between fish species. This review, taking advantage of the data generated from the whole-genome sequencing of several fish species, highlights the differences in the primary structure of the receptors, while discussing results from the literature pertaining to the functions of fish PPARs and their activation by natural and synthetic compounds.

Hepatic transcriptome profiling of largemouth bass (Micropterus salmoides Lacépède) injected with Flavobacterium covae or lipopolysaccharide

Flavobacterium covae (columnaris) is the most detrimental bacterial disease affecting the largemouth bass (Micropterus salmoides Lacépède) aquaculture industry. In the current study, fish received an intraperitoneal injection of either 1× PBS (100 μL), LPS in PBS (100 μL, 10 μg/mL), or F. covae (100 μL, 2.85 × 1011 CFU/mL) to simulate immunological challenges. After 24 h post-injection, liver tissue from the control and treated groups were then collected for transcriptome analysis. Results of the Gene Ontology (GO) and KEGG pathway analyses for the F. covae and LPS-injected groups found differentially expressed genes (DEGs) enriched primarily in toll-like receptors (TLRs), cytokine-cytokine receptors, complement and coagulation cascades, and the PPAR signalling pathways. This suggests that the liver immune system is enhanced by these five combined pathways. Additionally, the DEGs TLR5, MYD88, and IL-1 were significantly upregulated in F. covae and LPS-injected fish compared to the controls, whereas IL-8 was downregulated. The upregulation of TLR5 was unexpected as F. covae lacks flagellin, the protein that binds to TLR5. Additionally, it is unknown whether the TLR5 is upregulated by LPS. Further research into the upregulation of TLR5 is warranted. These results provide insight into immune responses and associated pathways contributing to the immune system in the liver during columnaris infection and induced response to LPS in largemouth bass.

Stress response and tolerance mechanisms of NaHCO3 exposure based on biochemical assays and multi-omics approach in the liver of crucian carp (Carassius auratus)

The development and utilization of saline-alkaline water, an important backup resource, has received widespread attention. However, the underuse of saline-alkaline water, threatened by the single species of saline-alkaline aquaculture, seriously affects the development of the fishery economy. In this work, a 30-day NaHCO₃ stress experimental study combined with analyses of untargeted metabolomics, transcriptome, and biochemical approaches was conducted on crucian carp to provide a better understanding of the saline-alkaline stress response mechanism in freshwater fish. This work revealed the relationships among the biochemical parameters, endogenous differentially expressed metabolites (DEMs), and differentially expressed genes (DEGs) in the crucian carp livers. The biochemical analysis showed that NaHCO₃ exposure changed the levels of several physiological parameters associated with the liver, including antioxidant enzymes (SOD, CAT, GSH-Px), MDA, AKP, and CPS. According to the metabolomics study, 90 DEMs are involved in various metabolic pathways such as ketone synthesis and degradation metabolism, glycerophospholipid metabolism, arachidonic acid metabolism, and linoleic acid metabolism. In addition, transcriptomics data analysis showed that a total of 301 DEGs were screened between the control group and the high NaHCO₃ concentration group, of which 129 up-regulated genes and 172 down-regulated genes. Overall, NaHCO₃ exposure could cause lipid metabolism disorders and induce energy metabolism imbalance in the crucian carp liver. Simultaneously, crucian carp might regulate its saline-alkaline resistance mechanism by enhancing the synthesis of glycerophospholipid metabolism, ketone bodies, and degradation metabolism, at the same time increasing the vitality of antioxidant enzymes (SOD, CAT, GSH-Px) and nonspecific immune enzyme (AKP). Herein, all results will provide new insights into the molecular mechanisms underlying the stress responses and tolerance to saline-alkaline exposure in crucian carp.

Effect of Siberian Ginseng Water Extract as a Dietary Additive on Growth Performance, Blood Biochemical Indexes, Lipid Metabolism, and Expression of PPARs Pathway-Related Genes in Genetically Improved Farmed Tilapia (Oreochromis niloticus)

Overnutrition in high-density aquaculture can negatively affect the health of farmed fish. The Chinese herbal medicine Siberian ginseng (Acanthopanax senticosus, AS) can promote animal growth and immunity, and regulate lipid metabolism. Therefore, we conducted an 8-week experiment, in which Oreochromis niloticus was fed with a diet supplemented with different concentrations of AS water extract (ASW) (0‰, 0.1‰, 0.2‰, 0.4‰, 0.8‰, and 1.6‰). The ASW improved the growth performance and increased the specific growth rate (SGR). Linear regression analysis based on the SGR estimated that the optimal ASW amount was 0.74‰. Dietary supplementation with 0.4–0.8‰ ASW reduced the triglyceride and total cholesterol levels in the serum and liver, and regulated lipid transport by increasing the high-density lipoprotein cholesterol concentration and lowering the low-density lipoprotein cholesterol concentration. Dietary supplementation with ASW increased the activities of superoxide dismutase and catalase in the liver, thereby improving the antioxidant capacity. Moreover, ASW modulated the transcription of genes in the peroxisome proliferator-activated receptor signaling pathway in the liver (upregulation of PPARα, APOA1b, and FABP10a and downregulation of PPARγ), thereby regulating fatty acid synthesis and metabolism and slowing fat deposition. These results showed that 0.4–0.8‰ ASW can slow fat deposition and protected the liver from cell damage and abnormal lipid metabolism.

Novel insights for PI3KC3 in mediating lipid accumulation in yellow catfish Pelteobagrus fulvidraco

In this study, the transcriptional regulation of PI3KC3 by three transcription factors (PPARγ, PPARα, and STAT3) and the potential role of PI3KC3 in mediating lipid accumulation were determined in yellow catfish Pelteobagrus fulvidraco. The 5'-deletion assay, overexpression assay, site-mutation assay, and electrophoretic mobility shift assay suggested that PPARα, PPARγ, and STAT3 negatively regulated the promoter activity of pi3kc3. Moreover, the transcriptional inactivation of pi3kc3 was directly mediated by PPARα and PPARγ under fatty acid (FA) treatment. Using primary hepatocytes from yellow catfish, FA incubation significantly increased triacylglyceride (TG) content, non-esterified fatty acid (NEFA) content, and lipid drops (LDs) content, the mRNA level of pparα, pparγ, stat3, and dnmt3b, the protein level of PPARα, PPARγ, and STAT3, and the methylation level of pi3kc3, but significantly reduced the mRNA and protein level of PI3KC3. Our findings offer new insights into the mechanisms for transcriptional regulation of PI3KC3 and for PI3KC3-mediated lipid accumulation in fish.

Agonistic and potentiating effects of perfluoroalkyl substances (PFAS) on the Atlantic cod (Gadus morhua) peroxisome proliferator-activated receptors (Ppars)

Toxicity mediated by per- and polyfluoroalkyl substances (PFAS), and especially perfluoroalkyl acids (PFAAs), has been linked to activation of peroxisome proliferator-activated receptors (Ppar) in many vertebrates. Here, we present the primary structures, phylogeny, and tissue-specific distributions of the Atlantic cod (Gadus morhua) gmPpara1, gmPpara2, gmPparb, and gmPparg, and demonstrate that the carboxylic acids PFHxA, PFOA, PFNA, as well as the sulfonic acid PFHxS, activate gmPpara1 in vitro, which was also supported by in silico analyses. Intriguingly, a binary mixture of PFOA and the non-activating PFOS produced a higher activation of gmPpara1 compared to PFOA alone, suggesting that PFOS has a potentiating effect on receptor activation. Supporting the experimental data, docking and molecular dynamics simulations of single and double-ligand complexes led to the identification of a putative allosteric binding site, which upon binding of PFOS stabilizes an active conformation of gmPpara1. Notably, binary exposures of gmPpara1, gmPpara2, and gmPparb to model-agonists and PFAAs produced similar potentiating effects. This study provides novel mechanistic insights into how PFAAs may modulate the Ppar signaling pathway by either binding the canonical ligand-binding pocket or by interacting with an allosteric binding site. Thus, individual PFAAs, or mixtures, could potentially modulate the Ppar-signaling pathway in Atlantic cod by interfering with at least one gmPpar subtype.

Molecular characterization, expression and functional analysis of large yellow croaker (Larimichthys crocea) peroxisome proliferator-activated receptor gamma

The peroxisome proliferator-activated receptor gamma (PPARγ) are nuclear receptors with distinct roles in energy metabolism and immunity. Although extensively studied in mammals, immunomodulatory roles of this molecule in teleost fish remain to be investigated. In this study, large yellow croaker (Larimichthys crocea) PPARγ (LcPPARγ) sequence was cloned, which encodes a polypeptide of 541 amino acids that include signature domains belonging to the nuclear receptor superfamily. Phylogenetically, LcPPARγ was most closely related to PPARγ derived from European sea bass (Dicentrarchus labrax). Quantitative analysis shown a ubiquitous expression of this molecule, with highest expression level detected in the intestine. The expression of LcPPARγ was decreased in the intestine, muscle, body kidney, spleen and head kidney-derived monocytes/macrophages (MO/MФs) over the course of Vibrio alginolyticus (V. alginolyticus) infection. In contrast, an up-regulation of LcPPARγ was observed in head kidney-derived MO/MФs following docosahexaenoic acid (DHA) treatment. This increase in LcPPARγ leads to an up-regulation of LcCD11b and LcCD18 and an enhancement of complement-mediated phagocytosis. Furthermore, cytokine secretions of V. alginolyticus-stimulated MO/MФs were modulated following LcPPARγ activations that up-regulated the expression of LcIL-10, while decreased the expression of LcIL-1β, LcTNF-α and LcTGF-β1. Overall, our results indicated that LcPPARγ plays a role in regulating functions of MO/MФs and likely contribute to MO/MФs polarization.

Copper (Cu) induced changes of lipid metabolism through oxidative stress-mediated autophagy and Nrf2/PPARγ pathways

Oxidative stress can induce occurrence of non-alcoholic fatty liver disease (NAFLD). Nrf2 is a central regulator of cellular oxidative stress and also participates in the control of lipid deposition and metabolism. Here, we hypothesize that oxidative stress-mediated Nrf2 activation participates in the regulation of the Cu-induced lipid deposition. We found that Cu excess activated oxidative stress and autophagy, up-regulated lipogenesis and lipid metabolism, suppressed Keap1 expression and activated Nrf2 signaling. Moreover, Cu induced lipid deposition via oxidative stress and the mitochondrial dysfunction. Oxidative stress mediated Cu-induced activation of Nrf2 and autophagy. The activation of autophagy helps to alleviate Cu-induced lipid deposition and accordingly provided a protective role against Cu-induced NAFLD. Meantime, Cu-induced oxidative stress promoted Nrf2 recruitment to the PPARγ promoter, inducing target gene transcription, and subsequent lipogenesis. Our findings, for the first time, provide direct evidences for Nrf2 function in the modulation of lipogenic metabolism via the transcriptional activation of PPARγ, and elucidate the mechanisms by which Nrf2 functions as the central regulator of lipogenic genes and highlights the significance of Nrf2 as potential therapeutic targets for oxidative stress-associated obesity and NAFLD for fish and human beings.

Lipophagy mediated carbohydrate-induced changes of lipid metabolism via oxidative stress, endoplasmic reticulum (ER) stress and ChREBP/PPARγ pathways

High-carbohydrate diets (HCD) can induce the occurrence of nonalcoholic fatty liver disease (NAFLD), characterized by dramatic accumulation of hepatic lipid droplets (LDs). However, the potential molecular mechanisms are still largely unknown. In this study, we investigated the role of autophagy in the process of HCD-induced changes of hepatic lipid metabolism, and to examine the process of underlying mechanisms during these molecular contexts. We found that HCD significantly increased hepatic lipid accumulation and activated autophagy. Using primary hepatocytes, we found that HG increased lipid accumulation and stimulated the release of NEFA by autophagy-mediated lipophagy, and that lipophagy significantly alleviated high glucose (HG)-induced lipid accumulation. Oxidative and endoplasmic reticulum (ER) stress pathways played crucial regulatory roles in HG-induced lipophagy activation and HG-induced changes of lipid metabolism. Further investigation found that HG-activated lipophagy and HG-induced changes of lipid metabolism were via enhancing carbohydrate response element-binding protein (ChREBP) DNA binding capacity at PPARγ promoter region, which in turn induced transcriptional activation of the key genes related to lipogenesis and autophagy. The present study, for the first time, revealed the novel mechanism for lipophagy mediating HCD-induced changes of lipid metabolism by oxidative stress and ER stress, and ChREBP/PPARγ pathways. Our study provided innovative evidence for the direct relationship between carbohydrate and lipid metabolism via ChREBP/PPARγ pathway.

Functional Analysis of Promoters of Genes in Lipid Metabolism and Their Transcriptional Response to STAT3 under Leptin Signals

We characterized the promoters of target genes of the signal transducer and activator of transcription 3, STAT3 (carnitine palmitoyltransferase I, CPT Iα1b, acetyl-CoA carboxylase alpha, ACCα; fatty acid synthase, FAS; and peroxisome proliferator-activated receptor gamma, PPARγ) in a teleost Pelteobagrus fulvidraco. Binding sites of STAT3 were predicted on these promoters, indicating that STAT3 probably mediated their transcriptional activities. Leptin had no effect on the activity of ACCα and PPARγ promoters, but increased CPT Iα1b promoter activity and decreased FAS promoter activity. The −979/−997 STAT3 binding site of CPT Iα1b and the −794/−812 STAT3 binding site of FAS were functional binding loci responsible for leptin-induced transcriptional activation. The study provided direct evidence that STAT3 regulated the expression of CPT Iα1b and FAS at the transcription level, and determined the STAT3 response element on promoters of CPT Iα1b and FAS under leptin signal.

Waterborne Zn influenced Zn uptake and lipid metabolism in two intestinal regions of juvenile goby Synechogobius hasta

The present study explored the influence of Zn addition in the water on Zn transport and lipid metabolism of two intestinal regions in goby Synechogobius hasta. Zn contents in water were 0.004 (control), 0.181 and 0.361mg Zn L^-1, respectively. The experiment lasted for 28 days. TG and Zn contents, mRNA contents of genes of Zn transport and lipid metabolism, and enzyme activity from anterior and mid-intestine tissues were analyzed. In anterior intestine, Zn addition in the water increased Zn contents, and mRNA concentrations of ZIP4, ZIP5, ATGL, PPARα, ZNF202 and KLF7, decreased TG contents, 6PGD and G6PD activities, and mRNA contents of 6PGD, G6PD, FAS, PPARγ, ICDH and KLF4. In mid-intestine tissue, the highest Zn and TG contents were observed for 0.18mg Zn/l group, in parallel with the highest expressions of ZnT1, ZIP4, ZIP5, 6PGD, FAS, ICDH, PPARγ, PPARα, ZNF202, KLF4 and KLF7, and with the highest FAS, 6PGD and G6PD activities. Thus, in the anterior intestine, Zn addition increased lipolysis and decreased lipogenesis, and accordingly reduced TG content. However, the highest mid-intestinal TG content in 0.18mg Zn/l group was due to the up-regulated lipogenesis. Although lipolysis was also increased, the incremental lipid synthesis was enough to compensate for lipid degradation, which led TG accumulation. Our results, for the first time, show an anterior/mid functional regionalization of the intestine in lipid metabolism and Zn transport of S. hasta following Zn exposure.

Functional Analysis of Promoters from Three Subtypes of the PI3K Family and Their Roles in the Regulation of Lipid Metabolism by Insulin in Yellow Catfish Pelteobagrus fulvidraco

In the present study, the length of 360, 1848 and 367 bp sequences of promoters from three subtypes of PI3K family (PI3KCa, PI3KC2b and PI3KC3) of yellow catfish Pelteobagrus fulvidraco were cloned and characterized. Bioinformatics analysis revealed that PI3KCa, PI3KC2b and PI3KC3 had different structures in their core promoter regions. The promoter regions of PI3KCa and PI3KC2b had CpG islands but no CAAT and TATA box. In contrast, the promoter of PI3KC3 had the canonical TATA and CAAT box but no CpG island. The binding sites of several transcription factors, such as HNF1, STAT and NF-κB, were predicted on PI3KCa promoter. The binding sites of transcription factors, such as FOXO1, PPAR-RXR, STAT, IK1, HNF6 and HNF3, were predicted on PI3KC2b promoter and the binding sites of FOXO1 and STAT transcription factors were predicted on PI3KC3 promoter. Deletion analysis indicated that these transcriptional factors were the potential regulators to mediate the activities of their promoters. Subsequent mutation analysis and electrophoretic mobility-shift assay (EMSA) demonstrated that HNF1 and IK1 directly bound with PI3KCa and PI3KC2b promoters and negatively regulated the activities of PI3KCa and PI3KC2b promoters, respectively. Conversely, FOXO1 directly bound with the PI3KC2b and PI3KC3 promoters and positively regulated their promoter activities. In addition, AS1842856 (AS, a potential FOXO1 inhibitor) incubation significantly reduced the relative luciferase activities of several plasmids of PI3KC2b and PI3KC3 but did not significantly influence the relative luciferase activities of the PI3KCa plasmids. Moreover, by using primary hepatocytes from yellow catfish, AS incubation significantly down-regulated the mRNA levels of PI3KCa, PI3KC2b and PI3KC3 and reduced triacylglyceride (TG) accumulation and insulin-induced TG accumulation, as well as the activities and the mRNA levels of several genes involved in lipid metabolism. Thus, the present study offers new insights into the mechanisms for transcriptional regulation of PI3Ks and for PI3Ks-mediated regulation of lipid metabolism by insulin in fish.

Structure and Functional Analysis of Promoters from Two Liver Isoforms of CPT I in Grass Carp Ctenopharyngodon idella

Carnitine palmitoyltransferase I (CPT I) is a key enzyme involved in the regulation of lipid metabolism and fatty acid β-oxidation. To understand the transcriptional mechanism of CPT Iα1b and CPT Iα2a genes, we cloned the 2695-bp and 2631-bp regions of CPT Iα1b and CPT Iα2a promoters of grass carp (Ctenopharyngodon idella), respectively, and explored the structure and functional characteristics of these promoters. CPT Iα1b had two transcription start sites (TSSs), while CPT Iα2a had only one TSS. DNase I foot printing showed that the CPT Iα1b promoter was AT-rich and TATA-less, and mediated basal transcription through an initiator (INR)-independent mechanism. Bioinformatics analysis indicated that specificity protein 1 (Sp1) and nuclear factor Y (NF-Y) played potential important roles in driving basal expression of CPT Iα2a gene. In HepG2 and HEK293 cells, progressive deletion analysis indicated that several regions contained cis-elements controlling the transcription of the CPT Iα1b and CPT Iα2a genes. Moreover, some transcription factors, such as thyroid hormone receptor (TR), hepatocyte nuclear factor 4 (HNF4) and peroxisome proliferator-activated receptor (PPAR) family, were all identified on the CPT Iα1b and CPT Iα2a promoters. The TRα binding sites were only identified on CPT Iα1b promoter, while TRβ binding sites were only identified on CPT Iα2a promoter, suggesting that the transcription of CPT Iα1b and CPT Iα2a was regulated by a different mechanism. Site-mutation and electrophoretic mobility-shift assay (EMSA) revealed that fenofibrate-induced PPARα activation did not bind with predicted PPARα binding sites of CPT I promoters. Additionally, PPARα was not the only member of PPAR family regulating CPT I expression, and PPARγ also regulated the CPT I expression. All of these results provided new insights into the mechanisms for transcriptional regulation of CPT I genes in fish.

Lipid metabolism-related gene expression pattern of Atlantic bluefin tuna (Thunnus thynnus L.) larvae fed on live prey

The present study is the first to evaluate lipid metabolism in first-feeding Atlantic bluefin tuna (ABT; Thunnus thynnus L.) larvae fed different live prey including enriched rotifers Brachionus plicatilis and Acartia sp. copepod nauplii from 2 days after hatch. Understanding the molecular basis of lipid metabolism and regulation in ABT will provide insights to optimize diet formulations for this high-value species new to aquaculture. To this end, we investigated the effect of dietary lipid on whole larvae lipid class and fatty acid compositions and the expression of key genes involved in lipid metabolism in first feeding ABT larvae fed different live prey. Additionally, the expression of lipid metabolism genes in tissues of adult broodstock ABT was evaluated. Growth and survival data indicated that copepods were the best live prey for first feeding ABT and that differences in growth performance and lipid metabolism observed between larvae from different year classes could be a consequence of broodstock nutrition. In addition, expression patterns of lipid metabolic genes observed in ABT larvae in the trials could reflect differences in lipid class and fatty acid compositions of the live prey. The lipid nutritional requirements, including essential fatty acid requirements of larval ABT during the early feeding stages, are unknown, and the present study represents a first step in addressing these highly relevant issues. However, further studies are required to determine nutritional requirements and understand lipid metabolism during development of ABT larvae and to apply the knowledge to the commercial culture of this iconic species.

Peroxisome proliferator-activated receptor gamma (PPARγ) in brown trout: Interference of estrogenic and androgenic inputs in primary hepatocytes

Peroxisome proliferator-activated receptor gamma (PPARγ) is a pivotal regulator of lipid and glucose metabolism in vertebrates. Here, we isolated and characterized for the first time the PPARγ gene from brown trout (Salmo trutta f. fario). Hormones have been reported to interfere with the regulatory function of PPARγ in various organisms, albeit with little focus on fish. Thus, primary hepatocytes isolated from juveniles of brown trout were exposed to 1, 10 and 50μM of ethinylestradiol (EE2) or testosterone (T). A significant (3 fold) decrease was obtained in response to 50μM of EE2 and to 10 and 50μM of T (13 and 14 folds), while a 3 fold increase was observed at 1μM of EE2. Therefore, trout PPARγ seems a target for natural/synthetic compounds with estrogenic or androgenic properties and so, we advocate considering PPARγ as another alert sensor gene when assessing the effects of sex-steroid endocrine disruptors.

Cloning, mRNA expression and transcriptional regulation of five retinoid X receptor subtypes in yellow catfish Pelteobagrus fulvidraco by insulin

Retinoid X receptors (RXRs) are members of the nuclear receptor superfamily and mediate development, reproduction, homeostasis and cell differentiation processes in vertebrates. In this study, full-length cDNA sequences of five rxr subtypes from yellow catfish Pelteobagrus fulvidraco were cloned. Their mRNA expression patterns in different tissues and transcriptional regulation by insulin were determined. Five P. fulvidraco rxr (Pf-rxr) subtypes differed in the length of cDNA sequence and the open reading frame, but shared the similar domain structures as in typical nuclear receptors. Phylogenetic analysis revealed that the five Pf-rxr subtypes were paralogous genes, and that Pf-rxrβa and Pf-rxrβb had arisen during a teleost-specific genome duplication event. Five subtypes of Pf-rxr were detected in all the tested tissues. Overlapping and distinct expression patterns were found for different Pf-rxr subtypes, suggesting functional redundancy and divergence of these duplicates. Intraperitoneal insulin injection and incubation reduced the mRNA expression of Pf-rxrgb, but not other subtypes, in the liver and hepatocytes of P. fulvidraco, respectively, suggesting that Pf-rxrgb is the dominant rxr subtype involved in the insulin signaling pathway in P. fulvidraco.

Regulation and mechanism of leptin on lipid metabolism in ovarian follicle cells from yellow catfish Pelteobagrus fulvidraco

The present study was conducted to determine the effect of leptin on lipid metabolism in ovarian follicle cells of yellow catfish Pelteobagrus fulvidraco. For that purpose, primary ovarian follicle cells were isolated from yellow catfish, cultured and subjected to different treatments (control, 0.1% DMSO, 500ng/ml leptin, 500ng/ml leptin plus 100μM wortmannin, 500ng/ml leptin plus 50nM AG490, respectively) for 48h. Intracellular triglyceride (TG) content, the activities (CPT I, FAS, G6PD, and 6PGD) and/or expression level of several enzymes (CPT I, FAS, G6PD, 6PGD, ACCa and ACCb), as well as the mRNA expression of transcription factors (PPARα, PPARγ and SREBP-1) involved in lipid metabolism were determined. Recombinant human leptin (rt-hLEP) incubation significantly reduced intracellular TG content, activities and mRNA levels of FAS, G6PD and 6PGD, SREBP-1 and PPARγ, but enhanced activity and mRNA level of CPT I, PPARα and ACCa. Specific inhibitors AG490 and wortmannin of JAK-STAT and IRS-PI3K signaling pathways prevented leptin-induced changes, indicating that JAK-STAT and IRS-PI3K signaling pathways were involved in the process of leptin-induced changes of lipid metabolism. Based on these observations above, for the first time, our study indicated that leptin reduced lipid deposition by activating lipolysis and suppressing lipogenesis in ovarian follicles of yellow catfish, and both JAK-STAT and IRS-PI3K signaling pathways were involved in the changes of leptin-induced lipid metabolism.

Effects of insulin and its related signaling pathways on lipid metabolism in the yellow catfish Pelteobagrus fulvidraco

The influence of insulin on hepatic metabolism in fish is not well understood. The present study was therefore conducted to investigate the effects of insulin on lipid metabolism, and the related signaling pathways, in the yellow catfish Pelteobagrus fulvidraco. Hepatic lipid and intracellular triglyceride (TG) content, the activity and expression levels of several enzymes and the mRNA expression of transcription factors (PPARα and PPARγ) involved in lipid metabolism were determined. Troglitazone, GW6471, fenofibrate and wortmannin were used to explore the signaling pathways by which insulin influences lipid metabolism. Insulin tended to increase hepatic lipid accumulation, the activity of lipogenic enzymes (6PGD, G6PD, ME, ICDH and FAS) and mRNA levels of FAS, G6PD, 6PGD, CPT IA and PPARγ, but down-regulated PPARα mRNA level. The insulin-induced effect could be stimulated by the specific PPARγ activator troglitazone or reversed by the PI3 kinase/Akt inhibitor wortmannin, demonstrating that signaling pathways of PPARγ and PI3 kinase/Akt were involved in the insulin-induced alteration of lipid metabolism. The specific PPARα pathway activator fenofibrate reduced insulin-induced TG accumulation, down-regulated the mRNA levels of FAS, G6PD and 6PGD, and up-regulated mRNA levels of CPT IA, PPARα and PPARγ. The specific PPARα pathway inhibitor GW6471 reduced insulin-induced changes in the expression of all the tested genes, indicating that PPARα mediated the insulin-induced changes of lipid metabolism. The present results contribute new knowledge on the regulatory role of insulin in hepatic metabolism in fish.