Optimal dietary lipid levels alleviated adverse effects of high temperature on growth, lipid metabolism, antioxidant and immune responses in juvenile turbot (Scophthalmus maximus L.)

Fish physiological health is often negatively impacted by high-temperature environments and there are few studies on how dietary lipids affect fish growth and physiology when exposed to heat stress. The main objective of this research was to examine the impact of dietary lipid levels on growth and physiological status of juvenile turbot (Scophthalmus maximus L.) and determine if dietary lipid concentration could alleviate the possible adverse effects of heat stress. Five diets containing 6.81%, 9.35%, 12.03%, 14.74%, and 17.08% lipid, respectively, were formulated and fed to turbot (initial weight 5.13 ± 0.02 g) under high-temperature conditions (24.0-25.0 °C). Meanwhile, the diet with 12.03% lipid (considered by prior work to be an optimal dietary lipid level) was fed to turbot of the same size at normal temperature. Results suggested that, among the different dietary lipid levels under high-temperature conditions, fish fed the optimal lipid (12.03%) exhibited better growth compared to non-optimal lipid groups, as evidenced by higher weight gain and specific growth rate. Simultaneously, the optimal lipid diet may better maintain lipid homeostasis, as attested by lower liver and serum lipid, along with higher liver mRNA levels of lipolysis-related genes (pgc1α, lipin1, pparα, lpl and hl) and lower levels of synthesis-related genes (lxr, fas, scd1, pparγ, dgat1 and dgat2). Also, the optimal lipid diet might mitigate oxidative damage by improving antioxidant enzyme activity, decreasing malondialdehyde levels, and up-regulating oxidation-related genes (sod1, sod2, cat, gpx and ho-1). Furthermore, the optimal lipid may enhance fish immunity, as suggested by the decrease in serum glutamic-oxalacetic/pyruvic transaminase activities, down-regulation of pro-inflammatory genes and up-regulation of anti-inflammation genes. Correspondingly, the optimal lipid level suppressed MAPK signaling pathway via decreased phosphorylation levels of p38, JNK and ERK proteins in liver. In summary, the optimal dietary lipid level facilitated better growth and physiological status in turbot under thermal stress.

Dietary methanotroph bacteria meal alleviates soybean meal-induced enteritis by improving immune tolerance and intestinal flora profile of juvenile turbot (Scophthalmus maximus L.)

An 8-week growth trial was performed to investigate the protective effects of methanotroph bacteria meal (MBM) produced from methane against soybean meal-induced enteritis (SBMIE) in juvenile turbot (Scophthalmus maximus L.). Five isonitrogenous and isolipidic diets were formulated: fishmeal-based diet (FM, the control group); FM with approximate 50% of fishmeal substituted by 399.4 g/kg soybean meal (SBM); SBM supplemented with 63.6, 127.2 and 190.8 g/kg MBM (named MBM1, MBM2 and MBM3), each diet was randomly assigned to triplicate fibreglass tanks. Results showed that fish fed with SBM exhibited enteritis, identified by reduced relative weight of intestine (RWI), as well as expanded lamina propria width and up-regulated gene expression of pro-inflammatory cytokines (tnf-α, il-6 and il-8) in intestine. While the above symptoms were reversed when diet SBM supplemented with MBM at the levels of 63.6 and 127.2 g/kg, as well as characterized by up-regulated gene expression of anti-inflammatory cytokines (tgf-β and il-10) and tight junction protein (claudin3, claudin4 and claudin7) in intestine. Intestinal transcriptome analysis showed that the differentially expressed genes between groups FM and SBM predominantly enriched in the JAK-STAT signaling pathway, and the enrichment of differentially expressed genes between groups SBM and SBM supplemented with 63.6 g/kg MBM was in the inflammatory bowel disease (IBD) and JAK-STAT signaling pathway. To be specific, the expression of jak1, jak2b, stat1 and stat5a was significantly up-regulated when fish fed with SBM, suggested the activation of JAK-STAT signaling pathway, while the expression of these above genes was depressed by providing MBM to diet SBM, and the gene expression of toll-like receptors tlr2 and tlr5b showed a similar pattern. Moreover, intestinal flora analysis showed that community richness and abundance of beneficial bacteria (Cetobacterium and acillus_coagulans) were improved when fish fed with SBM supplemented with 63.6 g/kg MBM. Overall, methanotroph bacteria meal may alleviate SBMIE by regulating the expression of tight junction protein, toll-like receptors and JAK-STAT signaling pathway, as well as improving intestinal flora profile, which would be beneficial for enhancing the immune tolerance and utilization efficiency of turbot to dietary soybean meal.

Vitamin D3 Regulates Energy Homeostasis under Short-Term Fasting Condition in Zebrafish (Danio Rerio)

Vitamin D3 (VD3) is a steroid hormone that plays pivotal roles in pathophysiology, and 1,25(OH)2D3 is the most active form of VD3. In the current study, the crucial role of VD3 in maintaining energy homeostasis under short-term fasting conditions was investigated. Our results confirmed that glucose-depriving pathways were inhibited while glucose-producing pathways were strengthened in zebrafish after fasting for 24 or 48 h. Moreover, VD3 anabolism in zebrafish was significantly suppressed in a time-dependent manner under short-fasting conditions. After fasting for 24 or 48 h, zebrafish fed with VD3 displayed a higher gluconeogenesis level and lower glycolysis level in the liver, and the serum glucose was maintained at higher levels, compared to those fed without VD3. Additionally, VD3 augmented the expression of fatty acids (FAs) transporter cd36 and lipogenesis in the liver, while enhancing lipolysis in the dorsal muscle. Similar results were obtained in cyp2r1-/- zebrafish, in which VD3 metabolism is obstructed. Importantly, it was observed that VD3 induced the production of gut GLP-1, which is considered to possess a potent gluconeogenic function in zebrafish. Meanwhile, the gene expression of proprotein convertase subtilisin/kexin type 1 (pcsk1), a GLP-1 processing enzyme, was also induced in the intestine of short-term fasted zebrafish. Notably, gut microbiota and its metabolite acetate were involved in VD3-regulated pcsk1 expression and GLP-1 production under short-term fasting conditions. In summary, our study demonstrated that VD3 regulated GLP-1 production in zebrafish by influencing gut microbiota and its metabolite, contributing to energy homeostasis and ameliorating hypoglycemia under short-term fasting conditions.

Differences in diacylglycerol acyltransferases expression patterns and regulation cause distinct hepatic triglyceride deposition in fish

Triglyceride (TAG) deposition in the liver is associated with metabolic disorders. In lower vertebrate, the propensity to accumulate hepatic TAG varies widely among fish species. Diacylglycerol acyltransferases (DGAT1 and DGAT2) are major enzymes for TAG synthesis. Here we show that large yellow croaker (Larimichthys crocea) has significantly higher hepatic TAG level than that in rainbow trout (Oncorhynchus mykiss) fed with same diet. Hepatic expression of DGATs genes in croaker is markedly higher compared with trout under physiological condition. Meanwhile, DGAT1 and DGAT2 in both croaker and trout are required for TAG synthesis and lipid droplet formation in vitro. Furthermore, oleic acid treatment increases DGAT1 expression in croaker hepatocytes rather than in trout and has no significant difference in DGAT2 expression in two fish species. Finally, effects of various transcription factors on croaker and trout DGAT1 promoter are studied. We find that DGAT1 is a target gene of the transcription factor CREBH in croaker rather than in trout. Overall, hepatic expression and transcriptional regulation of DGATs display significant species differences between croaker and trout with distinct hepatic triglyceride deposition, which bring new perspectives on the use of fish models for studying hepatic TAG deposition.

Vitamin D promotes the folate transport and metabolism in zebrafish (Danio rerio)

Vitamin D (VD) is a fat-soluble sterol that possesses a wide range of physiological functions. The present study aimed to evaluate the effects of VD on folate metabolism in zebrafish and further investigated the underlying mechanism. Wild-type (WT) zebrafish were fed with a diet containing 0 IU/kg VD3 or 800 IU/kg VD3 for 3 wk. Meanwhile, cyp2r1 mutant zebrafish with impaired VD metabolism was used as another model of VD deficiency. Our results showed that VD deficiency in zebrafish suppressed the gene expression of folate transporters, including reduced folate carrier (RFC) and proton-coupled folate transporter (PCFT) in the intestine. Moreover, VD influenced the gene expression of several enzymes related to cellular folate metabolism in the intestine and liver of zebrafish. Importantly, VD-deficient zebrafish contained a remarkably lower level of folate content in the liver. Notably, VD was incapable of altering folate metabolism in zebrafish when gut microbiota was depleted by antibiotic treatment. Further studies proved that gut commensals from VD-deficient fish displayed a lower capacity to produce folate than those from WT fish. Our study revealed the potential correlation between VD and folate metabolism in zebrafish, and gut microbiota played a key role in VD-regulated folate metabolism in zebrafish.NEW & NOTEWORTHY Our study has identified that VD influences intestinal uptake and transport of folate in zebrafish while also altering hepatic folate metabolism and storage. Interestingly, the regulatory effects of VD on folate transport and metabolism diminished after the gut flora was interrupted by antibiotic treatment, suggesting that the regulatory effects of VD on folate metabolism in zebrafish are most likely dependent on the intestinal flora.

Sterol Regulatory Element Binding Protein 1: A Mediator for High-Fat Diet-Induced Hepatic Gluconeogenesis and Glucose Intolerance in Fish

BACKGROUND

Sterol regulatory element binding protein (SREBP) 1 is considered to be a crucial regulator for lipid synthesis in vertebrates. However, whether SREBP1 could regulate hepatic gluconeogenesis under high-fat diet (HFD) condition is still unknown, and the underlying mechanism is also unclear.

OBJECTIVES

This study aimed to determine gluconeogenesis-related gene and protein expressions in response to HFD in large yellow croaker and explore the role and mechanism of SREBP1 in regulating the related transcription and signaling.

METHODS

Croakers (mean weight, 15.61 ± 0.10 g) were fed with diets containing 12% crude lipid [control diet (ND)] or 18% crude lipid (HFD) for 10 weeks. The glucose tolerance, insulin tolerance, hepatic gluconeogenesis-related genes, and proteins expressions were determined. To explore the role of SREBP1 in HFD-induced gluconeogenesis, SREBP1 was inhibited by pharmacologic inhibitor (fatostatin) or genetic knockdown in croaker hepatocytes under palmitic acid (PA) condition. To explore the underlying mechanism, luciferase reporter and chromatin immunoprecipitation assays were conducted in HEK293T cells. Data were analyzed using analysis of variance or Student t test.

RESULTS

Compared with ND, HFD increased the mRNA expressions of gluconeogenesis genes (2.40-fold to 2.60-fold) (P < 0.05) and reduced protein kinase B (AKT) phosphorylation levels (0.28-fold to 0.34-fold) (P < 0.05) in croakers. However, inhibition of SREBP1 by fatostatin addition or SREBP1 knockdown reduced the mRNA expressions of gluconeogenesis genes (P < 0.05) and increased AKT phosphorylation levels (P < 0.05) in hepatocytes, compared with that by PA treatment. Moreover, fatostatin addition or SREBP1 knockdown also increased the mRNA expressions of irs1 (P < 0.05) and reduced serine phosphorylation of IRS1 (P < 0.05). Furthermore, SREBP1 inhibited IRS1 transcriptions by binding to its promoter and induced IRS1 serine phosphorylation by activating diacylglycerol-protein kinase Cε signaling.

CONCLUSIONS

This study reveals the role of SREBP1 in hepatic gluconeogenesis under HFD condition in croakers, which may provide a potential strategy for improving HFD-induced glucose intolerance.

[Respiratory virus infection and its influence on outcome in children with septic shock]

Objective: To investigate respiratory virus infection in children with septic shock in pediatric care units (PICU) in China and its influence on clinical outcomes. Methods: The clinical data of children with septic shock in children's PICU from January 2018 to December 2019 in 10 Chinese hospitals were retrospectively collected. They were divided into the pre-COVID-19 and post-COVID-19 groups according to the onset of disease, and the characteristics and composition of respiratory virus in the 2 groups were compared. Matching age, malignant underlying diseases, bacteria, fungi and other viruses, a new database was generated using 1∶1 propensity score matching method. The children were divided into the respiratory virus group and non-respiratory virus group according to the presence or absence of respiratory virus infection; their clinical characteristics, diagnosis, and treatment were compared by t-test, rank sum test and Chi-square test. The correlation between respiratory virus infection and the clinical outcomes was analyzed by logistic regression. Results: A total of 1 247 children with septic shock were included in the study, of them 748 were male; the age was 37 (11, 105) months. In the pre-and post-COVID-19 groups, there were 530 and 717 cases of septic shock, respectively; the positive rate of respiratory virus was 14.9% (79 cases) and 9.8% (70 cases); the seasonal distribution of septic shock was 28.9% (153/530) and 25.9% (185/717) in autumn, and 30.3% (161/530) and 28.3% (203/717) in winter, respectively, and the corresponding positive rates of respiratory viruses were 19.6% (30/153) and 15.7% (29/185) in autumn, and 21.1% (34/161) and 15.3% (31/203) in winter, respectively. The positive rates of influenza virus and adenovirus in the post-COVID-19 group were lower than those in the pre-COVID-19 group (2.1% (15/717) vs. 7.5% (40/530), and 0.7% (5/717) vs. 3.2% (17/530), χ2=21.51 and 11.08, respectively; all P<0.05). Rhinovirus virus were higher than those in the pre-Covid-19 group (1.7% (12/717) vs. 0.2% (1/530), χ2=6.51, P=0.011). After propensity score matching, there were 147 cases in both the respiratory virus group and the non-respiratory virus group. Rate of respiratory failure, acute respiratory distress, rate of disseminated coagulation dysfunction, and immunoglobulin usage of the respiratory virus group were higher than those of non-respiratory virus group (77.6% (114/147) vs. 59.2% (87/147), 17.7% (26/147) vs. 4.1% (6/147), 15.6% (25/147) vs. 4.1% (7/147), and 35.4% (52/147) vs. 21.4% (32/147); χ2=11.07, 14.02, 11.06 and 6.67, all P<0.05); and PICU hospitalization of the former was longer than that of the later (7 (3, 16) vs. 3 (1, 7)d, Z=5.01, P<0.001). Univariate logistic regression analysis showed that the presence of respiratory viral infection was associated with respiratory failure, disseminated coagulation dysfunction, the use of mechanical ventilation, and the use of immunoglobulin and anti-respiratory viral drugs (OR=2.42, 0.22, 0.25, 0.56 and 1.12, all P<0.05). Conclusions: The composition of respiratory virus infection in children with septic shock is different between pre and post-COVID-19. Respiratory viral infection is associated with organ dysfunction in children with septic shock. Decreasing respiratory viral infection through respiratory protection may improve the clinical outcome of these children.

Palmitic acid activates NLRP3 inflammasome through NF-κB and AMPK-mitophagy-ROS pathways to induce IL-1β production in large yellow croaker (Larimichthys crocea)

Studies on marine fish showed that vegetable oils substituted for excessive fish oil increased interleukin-1β (IL-1β) production. However, whether the nucleotide-binding oligomerization domain, leucine-rich repeat-containing family, pyrin domain-containing-3 (NLRP3) inflammasome has a substantial role in fatty acid-induced IL-1β production in fish remains unclear. The associated specific mechanism is also unknown. In this study, nlrp3, caspase-1 and apoptosis-associated speck-like protein containing a CARD (asc) were successfully cloned, and NLRP3 inflammasome consisted of NLRP3, caspase-1 and ASC in large yellow croaker. Primary hepatocytes of fish incubated with palmitic acid (PA) exhibited the highest expression of pro-inflammatory genes (il-1β and tnfα) and NLRP3 inflammasome related genes (nlrp3, caspase-1 and asc), caspase-1 activity and IL-1β production among different treatments. Furthermore, PA-induced NLRP3 inflammasome activation was confirmed to require two signals: the first signal was that PA promoted the NF-κB (P65) protein into the nucleus, and NF-κB increased NLRP3 promoter activity and nlrp3 transcription. The second signal was that PA inhibited AMPK phosphorylation and decreased mitophagy by inhibiting the expression of PINK and parkin proteins, thereby damaging the mitochondria that could not be effectively cleared. Mitochondrial damage generated excessive amounts of reactive oxygen species, which activated the NLRP3 inflammasome and then induced caspase-1 activity and IL-1β production. Therefore, excessive dietary PA activated NLRP3 inflammasome through NF-κB and AMPK-mitophagy-ROS pathways to induce IL-1β production, thereby leading to inflammation in fish.

Dietary palm oil enhances Sterol regulatory element-binding protein 2-mediated cholesterol biosynthesis through inducing endoplasmic reticulum stress in muscle of large yellow croaker (Larimichthys crocea)

Sterol regulatory element-binding protein 2 (SREBP2) is considered to be a major regulator to control cholesterol homoeostasis in mammals. However, the role of SREBP2 in teleost remains poorly understand. Here, we explored the molecular characterisation of SREBP2 and identified SREBP2 as a key modulator for 3-hydroxy-3-methylglutaryl-coenzyme A reductase and 7-dehydrocholesterol reductase, which were rate-limiting enzymes of cholesterol biosynthesis. Moreover, dietary palm oil in vivo or palmitic acid (PA) treatment in vitro elevated cholesterol content through triggering SREBP2-mediated cholesterol biosynthesis in large yellow croaker. Furthermore, our results also found that PA-induced activation of SREBP2 was dependent on the stimulating of endoplasmic reticulum stress (ERS) in croaker myocytes and inhibition of ERS by 4-Phenylbutyric acid alleviated PA-induced SREBP2 activation and cholesterol biosynthesis. In summary, our findings reveal a novel insight for understanding the role of SREBP2 in the regulation of cholesterol metabolism in fish and may deepen the link between dietary fatty acid and cholesterol biosynthesis.

Molecular cloning, tissue expression pattern, responses to different fatty acids and potential functions of lysophosphatidylcholine acyltransferase 1 (LPCAT1) in large yellow croaker (Larimichthys crocea)

In farmed fish, diets rich in palm oil have been observed to promote abnormal lipid build-up in the liver, subsequently leading to physiological harm and disease onset. Emerging research suggests that integrating phospholipids into the feed could serve as a potent countermeasure against hepatic impairments induced by vegetable oil consumption. Phosphatidylcholine is the most abundant type among phospholipids. In the metabolic processes of mammal, lysophosphatidylcholine acyltransferase 1 (LPCAT1), crucial for phosphatidylcholine remodeling, demonstrates a marked affinity towards palmitic acid (PA). Nonetheless, aspects concerning the cloning, tissue-specific distribution, and affinity of the LPCAT1 gene to diverse oil sources have yet to be elucidated in the large yellow croaker (Larimichthys crocea). Within the scope of this study, we successfully isolated and cloned the cDNA of the LPCAT1 gene from the large yellow croaker. Subsequent analysis revealed distinct gene expression patterns of LPCAT1 across ten different tissues of the species. The fully sequenced coding DNA sequence (CDS) of LPCAT1 spans 1503 bp and encodes a sequence of 500 amino acids. Comparative sequence alignment indicates that LPCAT1 shares a 69.75 % amino acid similarity with its counterparts in other species. Although LPCAT1 manifests across various tissues of the large yellow croaker, its predominance is markedly evident in the liver and gills. Furthermore, post exposure of the large yellow croaker's hepatocytes to varied fatty acids, PA has a strong response to LPCAT1. Upon the addition of appropriate lysolecithin to palm oil feed, the mRNA expression of LPCAT1 in the liver cells of the large yellow croaker showed significant variations compared to other subtypes. Concurrently, the mRNA expression of pro-inflammatory genes il-1β, il-6, il-8, tnf-α and ifn-γ in the liver tissue of the large yellow croaker decreased. Interestingly, they exhibit the same trend of change. In conclusion, we have cloned the LPCAT1 gene on fish successfully and find the augmented gene response of LPCAT1 in hepatocytes under PA treatment first. The results of this study suggest that LPCAT1 may be associated with liver inflammation in fish and offer new insights into mitigating liver diseases in fish caused by palm oil feed.

Vitamin D regulates glucose metabolism in zebrafish (Danio rerio) by maintaining intestinal homeostasis

Vitamin D (VD) is a steroid hormone that is widely known to play an important role in maintaining mineral homeostasis, and regulating various physiological functions. Our previous results demonstrated that the interruption of VD metabolism caused hyperglycemia in zebrafish. In the present study we further explored the mechanism that VD regulates glucose metabolism by maintaining intestinal homeostasis in zebrafish. Our results showed that the expression of several peptide hormones including gastric inhibitory peptide, peptide YY, and fibroblast growth factor 19 in the intestine decreased, while the expression of sodium glucose cotransporter-1 and gcg was increased in the intestine of the zebrafish fed with the VD3-deficient diet. Consistently, similar results were obtained in cyp2r1-/- zebrafish, in which endogenous VD metabolism is blocked. Furthermore, the results obtained from germ-free zebrafish exhibited that VD-regulated glucose metabolism was partly dependent on the microbiota in zebrafish. Importantly, the transplantation of gut microbiota collected from cyp2r1-/- zebrafish to germ-free zebrafish led to hyperglycemic symptoms in the fish, which were associated with the altered structure and functions of the microbiota in cyp2r1-/- zebrafish. Interestingly, the treatments with acetate or Cetobacterium somerae, a potent acetate producer, lowered the glucose contents whereas augmented insulin expression in zebrafish larvae. Notably, acetate supplementation alleviated hyperglycemia in cyp2r1-/- zebrafish and other diabetic zebrafish. In conclusion, our study has demonstrated that VD modulates the gut microbiota-SCFAs-gastrointestinal hormone axis, contributing to the maintenance of glucose homeostasis.

Regulation of low-density lipoprotein on lipid metabolism in macrophages of large yellow croaker (Larimichthys crocea)

Low-density lipoprotein (LDL) is the main carrier of cholesterol transport in plasma, which participates in regulating lipid homeostasis. Studies in mammals have shown that high levels of LDL in plasma absorbed by macrophages trigger the formation of lipid-rich foam cells, leading to the development of atherosclerotic plaques. Although lipid-rich atherosclerosis-like lesions have been discovered in the aorta of several fish species, the physiological function of LDL in fish macrophages remains poorly understood. In the present study, LDL was isolated from the plasma of large yellow croaker (Larimichthys crocea), and mass spectrometry analysis identified two truncated forms of apolipoprotein B100 in the LDL protein profile. Transcriptomic analysis of LDL-stimulated macrophages revealed that differentially expressed genes (DEGs) were enriched in various pathways related to lipid metabolism, as confirmed by the fact that LDL increased total cholesterol and cholesteryl esters content. Meanwhile, the gene and protein expression levels of perilipin2 (PLIN2), a DEG enriched in the PPAR signaling pathway, were upregulated in response to LDL stimulation. Importantly, knocking down plin2 significantly attenuates LDL-induced cholesterol accumulation and promotes cholesterol efflux. Furthermore, the transcription factor PPARγ, which is upregulated in response to LDL stimulation, can enhance the promoter activity of plin2. In conclusion, this study suggests that LDL may upregulate plin2 expression through PPARγ, resulting in cholesterol accumulation in fish macrophages. This study will facilitate the investigation of the function of LDL in regulating lipid homeostasis in macrophages and shed light on the evolutionary origin of LDL metabolism in vertebrates.

Butyrate induces STAT3/HIF-1α/IL-22 signaling via GPCR and HDAC3 inhibition to activate autophagy in head kidney macrophages from turbot (Scophthalmus maximus L.)

As one of short-chain fatty acids, butyrate is an important metabolite of dietary fiber by the fermentation of gut commensals. Our recent study uncovered that butyrate promoted IL-22 production in fish macrophages to augment the host defense. In the current study, we further explored the underlying signaling pathways in butyrate-induced IL-22 production in fish macrophages. Our results showed that butyrate augmented the IL-22 expression in head kidney macrophages (HKMs) of turbot through binding to G-protein receptor 41 (GPR41) and GPR43. Moreover, histone deacetylase 3 (HDAC3) inhibition apparently up-regulated the butyrate-enhanced IL-22 generation, indicating HDACs were engaged in butyrate-regulated IL-22 secretion. In addition, butyrate triggered the STAT3/HIF-1α signaling to elevate the IL-22 expression in HKMs. Importantly, the evidence in vitro and in vivo was provided that butyrate activated autophagy in fish macrophages via IL-22 signaling, which contributing to the elimination of invading bacteria. In conclusion, we clarified in the current study that butyrate induced STAT3/HIF-1α/IL-22 signaling pathway via GPCR binding and HDAC3 inhibition in fish macrophages to activate autophagy that was involved in pathogen clearance in fish macrophages.

Exploring the role of SWI/SNF complex subunit BAF60c in lipid metabolism and inflammation in fish

Chromatin remodeling plays an important role in regulating gene transcription, in which chromatin remodeling complex is a crucial aspect. Brg1/Brm-associated factor 60c (BAF60c) subunit forms a bridge between chromatin remodeling complexes and transcription factors in mammals; hence, it has received extensive attention. However, the roles of BAF60c in fish remain largely unexplored. In this study, we identified BAF60c-interacting proteins by using HIS-pull-down and LC-MS/MS analysis in fish. Subsequently, the RNA-seq analysis was performed to identify the overall effects of BAF60c. Then, the function of BAF60c was verified through BAF60c knockdown and overexpression experiments. We demonstrated for the first time that BAF60c interacts with glucose-regulated protein 78 (GRP78) and regulates lipid metabolism, endoplasmic reticulum (ER) stress, and inflammation. Knockdown of BAF60c reduces fatty acid biosynthesis, ER stress, and inflammation. In conclusion, the results enriched BAF60c-interacting protein network and explored the function of BAF60c in lipid metabolism and inflammation in fish.

Phosphatidylethanolamine alleviates OX-LDL-induced macrophage inflammation by upregulating autophagy and inhibiting NLRP1 inflammasome activation

Oxidized low-density lipoprotein (OX-LDL)-induced inflammation and autophagy dysregulation are important events in the progression of atherosclerosis. Phosphatidylethanolamine (PE), a multifunctional phospholipid that is enriched in cells, has been proven to be directly involved in autophagy which is closely associated with inflammation. However, whether PE can influence OX-LDL-induced autophagy dysregulation and inflammation has not been reported. In the present study, we revealed that OX-LDL significantly induced macrophage inflammation through the CD36-NLRP1-caspase-1 signaling pathway in fish. Meanwhile, cellular PE levels were significantly decreased in response to OX-LDL induction. Based on the relationship between PE and autophagy, we then examined the effect of PE supplementation on OX-LDL-mediated autophagy impairment and inflammation induction in macrophages. As expected, exogenous PE restored impaired autophagy and alleviated inflammation in OX-LDL-stimulated cells. Notably, autophagy inhibitors reversed the inhibitory effect of PE on OX-LDL-induced maturation of IL-1β, indicating that the regulation of PE on OX-LDL-induced inflammation is dependent on autophagy. Furthermore, the positive effect of PE on OX-LDL-induced inflammation was relatively conserved in mouse and fish macrophages. In conclusion, we elucidated the role of the CD36-NLRP1-caspase-1 signaling pathway in OX-LDL-induced inflammation in fish and revealed for the first time that altering PE abundance in OX-LDL-treated cells could alleviate inflammasome-mediated inflammation by inducing autophagy. Given the relationship between OX-LDL-induced inflammation and atherosclerosis, this study prompts that the use of PE-rich foods promises to be a new strategy for atherosclerosis treatment in vertebrates.

Glycerol monolaurate improved intestinal barrier, antioxidant capacity, inflammatory response and microbiota dysbiosis in large yellow croaker (Larimichthys crocea) fed with high soybean oil diets

Glycerol monolaurate (GML) is a potential candidate for regulating metabolic syndrome and inflammatory response. However, the role of GML in modulating intestinal health in fish has not been well determined. In this study, a 70-d feeding trial was conducted to evaluate the effect of GML on intestinal barrier, antioxidant capacity, inflammatory response and microbiota community of large yellow croaker (13.05 ± 0.09 g) fed with high level soybean oil (SO) diets. Two basic diets with fish oil (FO) or SO were formulated. Based on the SO group diet, three different levels of GML 0.02% (SO0.02), 0.04% (SO0.04) and 0.08% (SO0.08) were supplemented respectively. Results showed that intestinal villus height and perimeter ratio were increased in SO0.04 treatment compared with the SO group. The mRNA expressions of intestinal physical barrier-related gene odc and claudin-11 were significantly up-regulated in different addition of GML treatments compared with the SO group. Fish fed SO diet with 0.04% GML addition showed higher activities of acid phosphatase and lysozyme compared with the SO group. The content of malonaldehyde was significantly decreased and activities of catalase and superoxide dismutase were significantly increased in 0.02% and 0.04% GML groups compared with those in the SO group. The mRNA transcriptional levels of inflammatory response-related genes (il-1β, il-6, tnf-α and cox-2) in 0.04% GML treatment were notably lower than those in the SO group. Meanwhile, sequencing analysis of bacterial 16S rRNA V4-V5 region showed that GML addition changed gut microbiota structure and increased alpha diversity of large yellow croaker fed diets with a high level of SO. The correlation analysis results indicated that the change of intestinal microbiota relative abundance strongly correlated with intestinal health indexes. In conclusion, these results demonstrated that 0.02%-0.04% GML addition could improve intestinal morphology, physical barrier, antioxidant capacity, inflammatory response and microbiota dysbiosis of large yellow croaker fed diets with a high percentage of SO.

Discovery, characterization, and adipocyte differentiation regulation in perirenal adipose tissue of large yellow croaker (Larimichthys crocea)

Adipose tissue is an essential tissue for lipid deposition in fish and is associated with excess lipid accumulation in aquaculture. However, the knowledge of the distribution and characterization of adipose tissue in fish still needs further investigation. This study for the first time discovered perirenal adipose tissue (PAT) in large yellow croaker by MRI and CT technologies. Then, the morphological and cytological characteristics of PAT were observed, showing a typical characteristic of white adipose tissue. Meanwhile, the mRNA expression of marker genes of white adipose tissue was highly expressed in PAT compared with the liver and muscle in large yellow croaker. Moreover, based on the discovery of PAT, preadipocytes from PAT were isolated, and the differentiation system of preadipocytes was established. The lipid droplet and TG content of cell were gradually increased during adipocyte differentiation. In addition, mRNA expressions of lipoprotein lipase, adipose triglyceride lipase, and transcription factors related to adipogenesis (cebpα, srebp1, pparα, and pparγ) were quantified to explain the regulation mechanism during the differentiation process. In summary, the present study first discovered perirenal adipose tissue in fish, then explored the characterization of PAT, and revealed the regulation of adipocyte differentiation. These results could advance the understanding of adipose tissue in fish and provide a novel idea for the study of the mechanism of lipid accumulation.

Effects of supplemental octanoate on hepatic lipid metabolism, serum biochemical indexes, antioxidant capacity and inflammation-related genes expression of large yellow croaker (Larimichthys crocea) fed with high soybean oil diet

Dietary high soybean oil (SO) levels might cause hepatic lipid deposition, induce oxidative stress and inflammatory response in aquatic animals, while octanoate (OCT) is beneficial to metabolism and health in mammals. However, the effect of OCT has been studied rarely in aquatic animals. In this study, a 10-week feeding trial was conducted to investigate the effect of supplemental OCT on hepatic lipid metabolism, serum biochemical indexes, antioxidant capacity and inflammatory response of large yellow croaker (Larimichthys crocea) fed with high SO levels diet. The negative control diet contained 7% fish oil (FO), while the positive control diet contained 7% SO. The other four experimental diets were supplemented with 0.7, 2.1, 6.3 and 18.9 g/kg sodium octanoate (OCT) based on the positive control diet. Results showed that OCT supplementation effectively reduced the hepatic crude lipid, triglyceride (TG), total cholesterol (TC) and non-esterified free fatty acids contents, and alleviated lipid accumulation caused by the SO diet. Meanwhile, OCT supplementation decreased the serum TG, TC, alanine transaminase, aspartate transaminase and low-density lipoprotein cholesterol levels, increased the serum high-density lipoprotein cholesterol level, improved the serum lipid profiles and alleviated hepatic injury. Furthermore, with the supplementation of OCT, the mRNA expression of genes related to lipogenesis (acc1, scd1, fas, srebp1, dgat1 and cebpα) and fatty acid (FA) transport (fabp3, fatp and cd36) were down-regulated, while the mRNA expression of genes related to lipolysis (atgl, hsl and lpl) and FA β-oxidation (cpt1 and mcad) were up-regulated. Besides that, dietary OCT increased the total antioxidant capacity, activities of peroxidase, catalase and superoxide dismutase and the content of reduced glutathione, decreased the content of 8-hydroxy-deoxyguanosine and malondialdehyde and relieved hepatic oxidative stress. Supplementation of 0.7 and 2.1 g/kg OCT down-regulated the mRNA expression of genes related to pro-inflammatory cytokines (tnfα, il1β and ifnγ), and suppressed hepatic inflammatory response. In conclusion, supplementation with 0.7-2.1 g/kg OCT could reduce hepatic lipid accumulation, relieve oxidative stress and regulate inflammatory response in large yellow croaker fed the diet with high SO levels, providing a new way to alleviate the hepatic fat deposition in aquatic animals.

Effects of supplemental fulvic acid on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae

A 30-day feeding trial was designed to evaluate the effect of supplemental fulvic acid (FA) on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae (initial body weight 11.33 ± 0.57 mg). Four isonitrogenous and isolipids diets containing 0.00%, 0.01%, 0.02% and 0.04% FA were formulated, respectively. Results showed that the supplementation of 0.04% FA significantly improved survival rate of large yellow croaker larvae. Meanwhile, supplemental FA significantly increased final body weight and specific growth rate. Based on the specific growth rate, the optimal supplementation was 0.0135% FA. Larvae fed the diet with 0.01% FA had significantly higher villus height than the control. The supplementation of 0.01%–0.02% FA significantly increased the muscular thickness of intestine. Moreover, supplementation of FA significantly increased mRNA expression of intestinal epithelial proliferation and barrier genes (pcna, zo-1 and zo-2). Diets supplemented with 0.02%–0.04% FA significantly increased the activity of trypsin in the intestinal segment, while 0.01%–0.02% FA significantly increased the activity of trypsin in the pancreatic segment. Compared with the control, supplementation of FA remarkably increased activities of alkaline phosphatase and leucine aminopeptidase in the brush border membrane of intestine. Larvae fed the diet with 0.01% FA significantly increased activities of lysozyme and total nitric oxide synthase. Furthermore, the supplementation of 0.01% to 0.02% FA significantly decreased the mRNA expression of pro-inflammatory cytokines (tnf-α and il-6). Concurrently, supplemental FA significantly increased anti-inflammatory cytokine (il-10) mRNA expression level. In conclusion, this study indicated that the supplementation of FA could improve the survival rate and growth performance of larvae by promoting intestinal development, digestive enzymes activities and innate immunity.

Effects of GRP78 on Endoplasmic Reticulum Stress and Inflammatory Response in Macrophages of Large Yellow Croaker (Larimichthys crocea)

Endoplasmic reticulum (ER) homeostasis plays a vital role in cell physiological functions. Various factors can destroy the homeostasis of the ER and cause ER stress. Moreover, ER stress is often related to inflammation. Glucose-regulated protein 78 (GRP78) is an ER chaperone, which plays a vital role in maintaining cellular homeostasis. Nevertheless, the potential effects of GRP78 on ER stress and inflammation is still not fully elucidated in fish. In the present study, ER stress and inflammation was induced by tunicamycin (TM) or palmitic acid (PA) in the macrophages of large yellow croakers. GRP78 was treated with an agonist/inhibitor before or after the TM/PA treatment. The results showed that the TM/PA treatment could significantly induce ER stress and an inflammatory response in the macrophages of large yellow croakers whereas the incubation of the GRP78 agonist could reduce TM/PA-induced ER stress and an inflammatory response. Moreover, the incubation of the GRP78 inhibitor could further induce TM/PA-induced ER stress and an inflammatory response. These results provide an innovative idea to explain the relationship between GRP78 and TM/PA-induced ER stress or inflammation in large yellow croakers.

Comparative analysis of nutritional and transcriptional regulation of hacd1 in large yellow croaker (Larimichthys crocea) and rainbow trout (Oncorhynchus mykiss)

3-hydroxyacyl-CoA dehydratases 1 (Hacd1) is a critical enzyme in long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis. The difference in expression of hacd1 might account for the stronger capacity of LC-PUFA biosynthesis in freshwater fish than in marine fish, but little is known about fish hacd1. Therefore, this study compared the responses of large yellow croaker and rainbow trout hacd1 to different oil sources or fatty acids, and also examined transcriptional regulation of this gene. In this study, hacd1 was highly expressed in the liver of large yellow croaker and rainbow trout, which is the main organ for LC-PUFA biosynthesis. Therefore, we cloned the hacd1 coding sequence, with a phylogenetic analysis showing that this gene is evolutionarily conserved. Its localization to the endoplasmic reticulum (ER), likely also indicates a conserved structure and function. The expression of hacd1 in the liver was significantly decreased after the substitution of soybean oil (SO) for fish oil but was not significantly affected after palm oil (PO) substitution. Linoleic acid (LA) incubation significantly promoted hacd1 expression in primary hepatocytes of large yellow croaker and eicosapentaenoic acid (EPA) incubation significantly promoted hacd1 expression in primary hepatocytes of rainbow trout. Transcription factors STAT4, C/EBPα, C/EBPβ, HNF1, HSF3 and FOXP3 were identified in both large yellow croaker and rainbow trout. HNF1 had a stronger activation effect in rainbow trout than in large yellow croaker. FOXP3 inhibited hacd1 promoter activity in large yellow croaker but had no effect in rainbow trout. Therefore, the differences between HNF1 and FOXP3 affected the expression of hacd1 in the liver thus being responsible for the high capacity of LC-PUFA biosynthesis in rainbow trout.

Transcription factor EB (TFEB) participates in antiviral immune responses independent of mTORC1 in macrophage of large yellow croaker (Larimichthys crocea)

Transcription factor EB (TFEB) plays an integral role in the production of proinflammatory cytokines and chemokines in response to pathogen stimulation in mammals. However, the role of TFEB in antiviral immune responses and the potential regulatory mechanisms in fish remain poorly understood. Here, we cloned and characterized Larimichthys crocea TFEB (LcTFEB) with 524 amino acids and a typical basic helix-loop-helix-leucine zipper domain. LcTFEB could translocate into the nucleus upon starvation and had a comparatively high expression in immune tissues. Similar to the expression of antiviral immune genes, the transcriptional expression and activity of LcTFEB showed a trend of increasing and then decreasing with the prolongation of stimulation. Inhibition of LcTFEB using siRNA dramatically increased the polyinosinic-polycytidylic acid (poly (I:C))-induced interferon response and pro-inflammatory cytokines mRNA expression levels, whereas pharmacological activation and overexpression of LcTFEB exhibited the reverse effects. Mechanically, LcTFEB might promote the expression of IFNh as negative feedback to limit the virus-induced inflammatory responses. Notably, although inhibition of mTORC1 exacerbated poly (I:C)-triggered inflammatory responses, the effects of LcTFEB were independent of mTORC1. Overall, this study revealed an unidentified critical role of LcTFEB in the regulation of antiviral immune responses and promoted the understanding of TFEB in the antiviral immunity of fish macrophages.

Effects of Tributyrin Supplementation on Growth Performance, Intestinal Digestive Enzyme Activity, Antioxidant Capacity, and Inflammation-Related Gene Expression of Large Yellow Croaker (Larimichthys crocea) Fed with a High Level of Clostridium autoethanogenum Protein

An 8-week growth experiment was conducted to investigate effects of tributyrin (TB) supplementation on growth performance, intestinal digestive enzyme activity, antioxidant capacity, and inflammation-related gene expression of juvenile large yellow croaker (Larimichthys crocea) (initial weight of 12.90 ± 0.02 g) fed diets with high level of Clostridium autoethanogenum protein (CAP). In the negative control diet, 40% fish meal was used as the major source of protein (named as FM), while 45% fish meal protein of FM was substituted with CAP (named as FC) to form a positive control diet. Based on the FC diet, grade levels of 0.05%, 0.1%, 0.2%, 0.4%, and 0.8% tributyrin were added to formulate other five experimental diets. Results showed that fish fed diets with high levels of CAP significantly decreased the weight gain rate (WGR) and specific growth rate (SGR) compared with fish fed the FM diet (P < 0.05). WGR and SGR were significantly higher than in fish fed diets with 0.05% and 0.1% tributyrin that fed the FC diet (P < 0.05). Supplementation of 0.1% tributyrin significantly elevated fish intestinal lipase and protease activities compared to FM and FC diets (P < 0.05). Meanwhile, compared to fish fed the FC diet, fish fed diets with 0.05% and 0.1% tributyrin showed remarkably higher intestinal total antioxidant capacity (T-AOC). Malondialdehyde (MDA) content in the intestine of fish fed diets with 0.05%-0.4% tributyrin was remarkably lower than those in the fish fed the FC diet (P < 0.05). The mRNA expressions of tumor necrosis factor α (tnfα), interleukin-1β (il-1β), interleukin-6 (il-6), and interferon γ (ifnγ) were significantly downregulated in fish fed diets with 0.05%-0.2% tributyrin, and the mRNA expression of il-10 was significantly upregulated in fish fed the 0.2% tributyrin diet (P < 0.05). In regard to antioxidant genes, as the supplementation of tributyrin increased from 0.05% to 0.8%, the mRNA expression of nuclear factor erythroid 2-related factor 2 (nrf2) demonstrated a trend of first rising and then decreasing. However, the mRNA expression of Kelch-like ECH-associated protein 1 (keap1) was remarkably lower in fish fed the FC diet than that fed diets with tributyrin supplementation (P < 0.05). Overall, fish fed tributyrin supplementation diets can ameliorate the negative effects induced by high proportion of CAP in diets, with an appropriate supplementation of 0.1%.

Butyrate-induced IL-22 expression in fish macrophages contributes to bacterial clearance

IL-22 has been characterized as a critical cytokine in maintaining barrier integrity and host immunity. So far, it has been known that IL-22 is mainly produced by lymphoid lineage cells. In the present study, we have thoroughly investigated butyrate-induced production and function of IL-22 in fish macrophages. Our results demonstrated that short-chain fatty acids (SCFAs), major microbiota-derived metabolites, promoted the expression of IL-22 in head kidney macrophages (HKMs) of turbot (Scophthalmus maximus L.). Interestingly, butyrate-mediated intracellular bacterial killing in HKMs diminished when IL-22 expression was interfered. Furthermore, the turbot fed the diet containing sodium butyrate (NaB) exhibited significantly lower mortality after bacterial infection, compared to the fish fed a basal diet. At the meantime, a higher level of IL-22 expression and bactericidal activity was detected in HKMs from the turbot fed NaB-supplemented diet. In addition, NaB treatment promoted the expression of antimicrobial peptides (AMPs) β-defensins in zebrafish (Danio rerio). However, butyrate-induced expression of AMPs was reduced in IL-22 mutant zebrafish compared to wild-type (WT) fish. Meanwhile, NaB treatment was incapable to protect IL-22 mutant fish from bacterial infection as it did in WT zebrafish. Importantly, our results demonstrated that IL-22 expression was remarkably suppressed in macrophage-depleted zebrafish, indicating that macrophage might be a cell source of IL-22 production in vivo. In conclusion, all these findings collectively revealed that SCFAs regulated the production and function of IL-22 in fish macrophages, which facilitated host resistance to bacterial invasion.

Dietary l-carnitine regulates liver lipid metabolism via simultaneously activating fatty acid β-oxidation and suppressing endoplasmic reticulum stress in large yellow croaker fed with high-fat diets

Dietary l-carnitine (LC) is a nutritional factor that reduces liver lipid content. However, whether dietary LC can improve lipid metabolism via simultaneous activation of mitochondrial fatty acid (FA) β-oxidation and suppression of endoplasmic reticulum (ER) stress is still unknown. Large yellow croaker were fed with a high-fat diet (HFD) supplemented with dietary LC at 0, 1·2 or 2·4 ‰ for 10 weeks. The results indicated that a HFD supplemented with LC reduced the liver total lipid and TAG content and improved serum lipid profiles. LC supplementation administered to this fish increased the liver antioxidant capacity by decreasing serum and liver malondialdehyde levels and enhancing the liver antioxidant capacity, which then relieved the liver damage. Dietary LC increased the ATP dynamic process and mitochondrial number, decreased mitochondrial DNA damage and enhanced the protein expression of mitochondrial β-oxidation, biogenesis and mitophagy. Furthermore, dietary LC supplementation increased the expression of genes and proteins related to peroxisomal β-oxidation and biogenesis. Interestingly, feeding fish with LC-enriched diets decreased the protein levels indicative of ER stress, such as glucose-regulated protein 78, p-eukaryotic translational initiation factor 2a and activating transcription factor 6. Dietary LC supplementation downregulated mRNA expression relative to FA synthesis, reduced liver lipid and relieved liver damage through regulating β-oxidation and biogenesis of mitochondria and peroxisomes, as well as the ER stress pathway in fish fed with HFD. The present study provides the first evidence that dietary LC can improve lipid metabolism via simultaneously promoting FA β-oxidation capability and suppressing the ER stress pathway in fish.

Long noncoding RNA lincsc5d regulates hepatic cholesterol synthesis by modulating sterol C5 desaturase in large yellow croaker

Although long noncoding RNA (lncRNA) plays a vital role in cholesterol metabolism, very little information is available in fish. Thus, a 10-week feeding experiment was performed to estimate the effects of lncRNA on cholesterol metabolism in large yellow croaker fed with fish oil (FO), soybean oil (SO), olive oil (OO), and palm oil (PO) diets. Results showed that fish fed with OO and PO diets had higher liver total cholesterol (TC) and cholesterol ester (CE) contents compared with fish fed with FO diets. Analysis of the KEGG pathway showed that the steroid biosynthesis pathway was enriched in comparisons FO vs SO, FO vs OO, and FO vs PO. Meanwhile, sterol C5 desaturase (SC5D), a cholesterol synthase, was up-regulated in the steroid biosynthesis pathway. SC5D was widely expressed in all tissues examined, and the highest expression of SC5D was detected in brain. More importantly, a novel lncRNA associated with sc5d gene was identified by RNA sequencing and named as lincsc5d. The tissue distribution of lincsc5d was similar to that of sc5d. A nuclear/cytoplasmic RNA separation assay showed that lincsc5d was a nucleus-enriched lncRNA. qRT-PCR results demonstrated that lincsc5d was markedly up-regulated in the SO, OO, and PO groups. Furthermore, the results of TC content and the lincsc5d and sc5d expression in hepatocytes agreed with in vivo results. In conclusion, this study indicated that vegetable oils, especially OO and PO, increased hepatic cholesterol levels by promoting cholesterol synthesis, and lncRNA lincsc5d and sc5d might be involved in cholesterol synthesis.

Vitamin D influences gut microbiota and acetate production in zebrafish (Danio rerio) to promote intestinal immunity against invading pathogens

Although evidence has shown that vitamin D (VD) influences gut homeostasis, limited knowledge is available how VD regulates intestinal immunity against bacterial infection. In the present study, cyp2r1 mutant zebrafish, lacking the capacity to metabolize VD, and zebrafish fed a diet devoid of VD, were utilized as VD-deficient animal models. Our results confirmed that the expression of antimicrobial peptides (AMPs) and IL-22 was restrained and the susceptibility to bacterial infection was increased in VD-deficient zebrafish. Furthermore, VD induced AMP expression in zebrafish intestine by activating IL-22 signaling, which was dependent on the microbiota. Further analysis uncovered that the abundance of the acetate-producer Cetobacterium in VD-deficient zebrafish was reduced compared to WT fish. Unexpectedly, VD promoted the growth and acetate production of Cetobacterium somerae under culture in vitro. Importantly, acetate treatment rescued the suppressed expression of β-defensins in VD-deficient zebrafish. Finally, neutrophils contributed to VD-induced AMP expression in zebrafish. In conclusion, our study elucidated that VD modulated gut microbiota composition and production of short-chain fatty acids (SCFAs) in zebrafish intestine, leading to enhanced immunity.

Vitamin D3 enhances the antibacterial ability in head-kidney macrophages of turbot (Scophthalmus maximus L.) through C-type lectin receptors

It has been known that vitamin D₃ (VD₃) not only plays an important role in regulating calcium and phosphorus metabolism in animals, but also has extensive effects on immune functions. In this study, the mechanism how VD₃ influences bactericidal ability in turbot was explored. The transcriptomic analysis identified that dietary VD₃ significantly upregulated the gene expression of C-type lectin receptors (CLRs), including mannose receptors (mrc1, mrc2, pla2r1) and collectins (collectin 11 and collectin 12) in turbot intestine. Further results obtained from in vitro experiments confirmed that the gene expression of mannose receptors and collectins in head-kidney macrophages (HKMs) of turbot was induced after the cells were incubated with different concentrations of VD₃ (0, 1, 10 nM) or 1,25(OH)₂D₃ (0, 10, 100 pM). Meanwhile, both phagocytosis and bactericidal functions of HKMs were significantly improved in VD₃ or 1,25(OH)₂D₃-incubated HKMs. Furthermore, phagocytosis and bacterial killing of HKMs decreased after collectin 11 was knocked down. Moreover, VD₃-enhanced antibacterial activities diminished in collectin 11-interfered cells. Interestingly, the evidence was provided in the present study that inactive VD₃ could be metabolized into active 1,25(OH)₂D₃ via hydroxylases encoded by cyp27a1 and cyp27b1 in fish macrophages. In conclusion, VD₃ could be metabolized to 1,25(OH)₂D₃ in HKMs, which promoted the expression of CLRs in macrophages, leading to enhanced bacterial clearance.

Effects of Glycyrrhizin (GL) Supplementation on Survival, Growth Performance, Expression of Feeding-Related Genes, Activities of Digestive Enzymes, Antioxidant Capacity, and Expression of Inflammatory Factors in Large Yellow Croaker (Larimichthys crocea) Larvae

A 30-day feeding trial was conducted to determine the effects of dietary glycyrrhizin (GL) on survival, growth performance, expression of feeding-related genes, activities of digestive enzymes, antioxidant capacity, and expression of inflammatory factors of large yellow croaker larvae with an initial weight of 3.78 ± 0.27 mg. Four 53.80% crude protein and 16.40% crude lipid diets were formulated with supplementation of 0%, 0.005%, 0.01%, and 0.02% GL, respectively. Results indicated that larvae fed diets with GL had higher survival rate and specific growth rate than the control (P < 0.05). Compared with the control, the mRNA expression of orexigenic factor genes including neuropeptide Y (npy) and agouti-related protein (agrp) were significantly increased in larvae fed the diet with 0.005% GL, while the mRNA expression of anorexigenic factor genes including thyrotropin-releasing hormone (trh), cocaine and amphetamine regulated transcript (cart), and leptin receptor (lepr) were significantly decreased in larvae fed the diet with 0.005% GL (P < 0.05). The trypsin activity in larvae fed the diet with 0.005% GL was significantly higher than the control (P < 0.05). The alkaline phosphatase (AKP) activity in larvae fed the diet with 0.01% GL was significantly higher than the control (P < 0.05). A clear increase of total glutathione (T-GSH) content, activities of superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) was observed in larvae fed the diet with 0.01% GL compared with the control (P < 0.05). Moreover, the mRNA expression of interleukin-1β (il-1β) and interleukin-6 (il-6) (proinflammatory genes) in larvae fed the diet with 0.02% GL were significantly lower than the control (P < 0.05). In conclusion, the supplementation of 0.005% -0.01% GL could enhance the expression of orexigenic factor genes, activities of digestive enzymes and antioxidant capacity, ultimately improving the survival, and growth performance of large yellow croaker larvae.

Effects of supplemental ferulic acid (FA) on survival, growth performance, digestive enzyme activities, antioxidant capacity and lipid metabolism of large yellow croaker (Larimichthys crocea) larvae

A 30-day feeding trial was conducted to investigate the effects of supplemental ferulic acid (FA) on survival, growth performance, digestive enzyme activities, antioxidant capacity and lipid metabolism of the  large yellow croaker larvae (initial weight: 2.58 ± 0.30 mg). Four isonitrogenous and isolipidic micro-diets were formulated with graded levels of FA (0, 20, 40, and 80 mg/kg) and fed to the experimental larvae seven times daily. Results showed that larvae fed the diet with 40 mg/kg FA had significantly higher survival rate, while the specific growth rate was higher in larvae fed diets with 40 and 80 mg/kg FA than the control group (P < 0.05). Activities of trypsin in pancreatic segments (PS) and intestinal segments, lipase in PS and alkaline phosphatase in brush border membrane were significantly increased by supplementation of FA compared to the control group (P < 0.05). Supplementation of FA significantly increased activities of total superoxide dismutase and catalase, and reduced the malondialdehyde content compared to the control group (P < 0.05). Meanwhile, activities of lysozyme, total nitric oxide synthase and nitric oxide content were significantly improved by supplemental FA in diets. Furthermore, supplementation of 40 mg/kg FA reduced the triglyceride content in larval visceral mass probably through down-regulating expression of lipogenesis-related genes (scd1, fas and dgat2) and up-regulating expression of lipid catabolism-related genes (aco, cpt-1 and hl). In conclusion, appropriate supplementation of 40 mg/kg FA could improve the survival and growth performance of large yellow croaker larvae through increasing digestive function, antioxidant capacity and promoting lipid metabolism.

Neuroprotective Effect of Stearidonic Acid on Amyloid β-Induced Neurotoxicity in Rat Hippocampal Cells

Dietary intake of omega-3 fatty acids found in fish has been reported to reduce the risk of Alzheimer's Disease (AD). Stearidonic acid (SDA), a plant-based omega-3 fatty acid, has been targeted as a potential surrogate for fish-based fatty acids. However, its role in neuronal degeneration is unknown. This study was designed to evaluate effects of SDA on Amyloid-β(A-β)-induced neurotoxicity in rat hippocampal cells. Results showed that SDA effectively converted to eicosapentaenoic acid (EPA) in hippocampal cells. Aβ-induced apoptosis in H19-7 cells was protected by SDA pretreatment as evidenced by its regulation on the expression of relevant pro- and anti-apoptotic genes, as well as the inhibition on caspase activation. SDA also protected H19-7 cells from Aβ-induced oxidative stress by regulating the expression of relevant pro- and anti-oxidative genes, as well as the improvement in activity of catalase. As for Aβ/LPS-induced neuronal inflammation, SDA pretreatment reduced the release of IL-1β and TNFα. Further, we found that the anti-Aβ effect of SDA involves its inhibition on the expression of amyloid precursor protein and the regulation on MAPK signaling. These results demonstrated that SDAs have neuroprotective effect in Aβ-induced H19-7 hippocampal cells. This beneficial effect of SDA was attributed to its antiapoptotic, antioxidant, and anti-inflammatory properties.

Activation of farnesoid X receptor suppresses ER stress and inflammation via the YY1/NCK1/PERK pathway in large yellow croaker (Larimichthys crocea)

Unfolded protein responses from endoplasmic reticulum (ER) stress have been implicated in inflammatory signaling. The vicious cycle of ER stress and inflammation makes regulation even more difficult. This study examined effects of farnesoid X receptor (FXR) in ER-stress regulation in large yellow croakers. The soybean-oil-diet-induced expression of ER stress markers was decreased in fish with FXR activated. In croaker macrophages, FXR activation or overexpression significantly reduced inflammation and ER stress caused by tunicamycin (TM), which was exacerbated by FXR knockdown. Further investigation showed that the TM-induced phosphorylation of PERK and EIF2α was inhibited by the overexpression of croaker FXR, and it was increased by FXR knockdown. Croaker NCK1 was then confirmed to be a regulator of PERK, and its expression in macrophages is increased by FXR overexpression and decreased by FXR knockdown. The promoter activity of croaker NCK1 was inhibited by yin-yang 1 (YY1). Furthermore, the results show that croaker FXR overexpression could suppress the P65-induced promoter activity of YY1 in HEK293t cells and decrease the TM-induced expression of yy1 in macrophages. These results indicate that FXR could suppress P65-induced yy1 expression and then increase NCK1 expression, thereby inhibiting the PERK pathway. This study may benefit the understanding of ER stress regulation in fish, demonstrating that FXR can be used in large yellow croakers as an effective target for regulating ER stress and inflammation.

Role of acyl-coenzyme A oxidase 1 (ACOX1) on palmitate-induced inflammation and ROS production of macrophages in large yellow croaker (Larimichthys crocea)

Acyl-coenzyme A oxidase 1 (ACOX1) is the rate-limiting enzyme in peroxisomal β-oxidation, and it plays an essential role in mediating the inflammatory response and reactive oxygen species (ROS) metabolism in mammals. However, the role of ACOX1 in fish has not been completely elucidated. Herein, this study was conducted to investigate the role of large yellow croaker (Larimichthys crocea) ACOX1 (Lc-ACOX1) on palmitate (PA)-induced inflammation and ROS production. In this study, Lc-ACOX1 was cloned and characterized. The full-length CDS of Lc-acox1 was 1986 bp, encoding 661 amino acids. Tissue distribution results showed that the gene expression of Lc-acox1 was the highest in the intestine and the lowest in the spleen. Moreover, results showed that the mRNA expression of Lc-acox1 was upregulated by PA, with elevated pro-inflammatory gene expression, including il-1β, il-6, il-8, tnf-α, cox2 and ifn-γ, as well as ROS content in macrophages of large yellow croaker. Furthermore, the role of Lc-ACOX1 in inflammation induced by PA was investigated by using the ACOX1 inhibitor TDYA. Treatment of macrophages with TDYA reduced the mRNA expression of pro-inflammatory genes induced by PA. Moreover, inhibition of ACOX1 reduced the elevated level of ROS caused by PA and increased the mRNA expression of antioxidant genes. In conclusion, this study first identified that fish ACOX1 was involved in the PA-induced inflammatory response and ROS production.

Replacement of Dietary Fishmeal Protein with Degossypolized Cottonseed Protein on Growth Performance, Nonspecific Immune Response, Antioxidant Capacity, and Target of Rapamycin Pathway of Juvenile Large Yellow Croaker (Larimichthys crocea)

A 70-day feeding experiment was carried out to assess the replacement of dietary fishmeal (FM) protein with degossypolized cottonseed protein (DCP) on large yellow croaker (Larimichthys crocea) with initial body weight (13.09 ± 0.50 g). Five isonitrogenous and isolipidic diets replaced fishmeal protein with 0%, 20%, 40%, 60%, and 80% DCP were formulated and named as FM (the control group), DCP20, DCP40, DCP60, and DCP80, respectively. Results displayed that weight gain rate (WGR) and specific growth rate (SGR) in the DCP20 group (263.91% and 1.85% d^-1) were significantly increased compared with the control group (194.79% and 1.54% d^-1) (P < 0.05). Furthermore, fish fed the diet with 20% DCP significantly increased the activity of hepatic superoxide dismutase (SOD) compared with the control group (P < 0.05). Meanwhile, the content of hepatic malondialdehyde (MDA) in the DCP20, DCP40, and DCP80 groups was significantly lower than that in the control group (P < 0.05). The activity of intestinal trypsin in the DCP20 group was significantly degraded compared with that in the control group (P < 0.05). The transcription of hepatic proinflammatory cytokine genes (interleukin-6 (il-6); tumor necrosis factor-α (tnf-α); and interferon-γ (ifn-γ)) in the DCP20 and DCP40 groups was significantly upregulated compared with that in the control group (P < 0.05). As to the target of rapamycin (TOR) pathway, the transcription of hepatic target of rapamycin (tor) and ribosomal protein (s6) was significantly up-regulated, while the transcription of hepatic eukaryotic translation initiation factor 4E binding protein 1 (4e-bp1) gene was significantly downregulated in the DCP group compared with the control group (P < 0.05). In summary, based on the broken line regression model analysis of WGR and SGR against dietary DCP replacement levels, the optimal replacement level was recommended to be 8.12% and 9.37% for large yellow croaker, respectively. These results revealed that FM protein replaced with 20% DCP could promote digestive enzyme activities and antioxidant capacity and further activate immune response and the TOR pathway so that growth performance of juvenile large yellow croaker was improved.

Vitamin D3 deficiency induced intestinal inflammatory response of turbot through nuclear factor-κB/inflammasome pathway, accompanied by the mutually exclusive apoptosis and autophagy

Vitamin D₃ (VD₃) participated widely in the nuclear factor-κB (NF-κB)-mediated inflammation, apoptosis, and autophagy through the vitamin D receptor (VDR). However, the molecular mechanisms remain not understood in teleost. The present study investigated the functions of VD₃/VDR on intestinal inflammation, autophagy, and apoptosis of turbot in vivo and in vitro. Triple replicates of 30 fish were fed with each of three diets with graded levels of 32.0 (D₀), 1012.6 (D₁), and 3978.2 (D₂) IU/kg VD₃. Obvious intestinal enteritis was observed in the D₀ group and followed with dysfunction of intestinal mucosal barriers. The intestinal inflammatory response induced by VD₃ deficiency was regulated by the NF-κB/inflammasome signalling. The promotion of intestinal apoptosis and suppression of intestinal autophagy were also observed in the D₀ group. Similarly, VD₃ deficiency in vitro induced more intense inflammation regulated by NF-κB/inflammasome signalling. The mutually exclusive apoptosis and autophagy were also observed in the group without 1,25(OH)₂D₃in vitro, accompanied by similar changes in apoptosis and autophagy increased apoptosis. The gene expression of VDRs was significantly increased with the increasing VD₃ supplementation both in vivo and in vitro. Moreover, VDR knockdown in turbot resulted in intestinal inflammation, and this process relied on the activation of inflammasome mediated by NF-κB signalling. Simultaneously, intestinal apoptosis was promoted, whereas intestinal autophagy was inhibited. In conclusion, VD₃ deficiency could induce intestinal inflammation via activation of the NF-κB/inflammasome pathway, intestinal apoptosis, and autophagy formed a mutually exclusive relation in teleost. And VDR is the critical molecule in those processes.

Effects of dietary lysolecithin on growth performance, serum biochemical indexes, antioxidant capacity, lipid metabolism and inflammation-related genes expression of juvenile large yellow croaker (Larimichthys crocea)

A 70-day feeding trial was conducted to investigate effects of dietary lysolecithin on growth performance, serum biochemical indexes, antioxidant capacity, lipid metabolism and inflammation-related genes expression of juvenile large yellow croaker (Larimichthys crocea) with initial weight of 6.04 ± 0.08 g. A formulated diet containing approximately 42% crude protein and 12.5% crude lipid was used as the control diet (CON). The other three experimental diets were formulated with supplementation of 0.2%, 0.4% and 0.6% lysolecithin based on the control diet, respectively. Results showed that weight gain rate (WGR) and specific growth rate (SGR) significantly increased in fish fed diets with lysolecithin compared with those in the control diet (P < 0.05). Fish fed diets with 0.4% and 0.6% lysolecithin had notably higher lipid content in muscle than that in the control diet (P < 0.05). When fish were fed diets with lysolecithin, serum high-density lipoprotein cholesterol (HDL-c) content was notably higher than that in the control diet (P < 0.05), while fish fed the diet with 0.6% lysolecithin had a significant lower serum low-density lipoprotein cholesterol (LDL-c) content than that in the control diet (P < 0.05). Meanwhile, serum aspartate transaminase (AST) and alanine transaminase (ALT) activities in fish fed diets with lysolecithin were remarkably lower than those in the control diet (P < 0.05). With the increase of dietary lysolecithin from 0.2% to 0.6%, mRNA expression of stearoyl-coenzyme A desaturase 1 (scd1), diacylglycerol acyltransferase 2 (dgat2) and sterol-regulatory element binding protein 1 (srebp1) showed decreasing trends. Furthermore, mRNA expression of carnitine palmitoyl transferase 1 (cpt1) and lipoprotein lipase (lpl) among each dietary lysolecithin treatment were significantly higher than those in the control diet (P < 0.05). In terms of inflammation, mRNA expression of tumor necrosis factor α (tnf-α) and interleukin-1 β (il-1β) were significantly down-regulated in fish fed diets with lysolecithin compared with those in the control diet (P < 0.05), while the mRNA expression of interleukin-10 (il-10) was significantly higher than that in the control diet (P < 0.05). In conclusion, dietary lysolecithin could promote the growth performance, improve hepatic lipid metabolism and regulate inflammation response in juvenile large yellow croaker, and the optimal supplement level of lysolecithin was approximately 0.4% in this study.

The Effects of Sodium Propionate Supplementation in the Diet with High Soybean Meal on Growth Performance, Intestinal Health, and Immune Resistance to Bacterial Infection in Turbot (Scophthalmus maximus L.)

Short-chain fatty acids (SCFAs) are the products of the microbial fermentation of dietary fiber in the intestine. Acetate, propionate, and butyrate are the most abundant SCFA metabolites and play an important role in maintaining host health. This study was aimed at investigating the effects of sodium propionate (NaP) supplementation in the diet with a high proportion of soybean meal (SBM) on the growth, inflammatory status, and anti-infectious ability in juvenile turbot. Four experimental diets were designed: (1) fish meal- (FM-) based diet (control group), (2) SBM protein replacing 45% FM protein in the diet (high SBM group), (3) 0.5% NaP supplementation in the high SBM diet (high SBM+0.5% NaP group), and (4) 1.0% NaP supplementation in the high SBM diet (high SBM+1.0% NaP group). The results confirmed that the fish fed the high SBM diet for 8 weeks showed the decreased growth performance, the typical enteritis symptoms, and the increased mortality responding to Edwardsiella tarda (E. tarda) infection. However, 0.5% NaP supplementation in the high SBM diet promoted the growth performance of turbot and restored the activities of digestive enzymes in the intestine. Moreover, dietary NaP ameliorated the intestinal morphology, enhanced the expression of intestinal tight junction proteins, improved the antioxidant capacity, and suppressed the inflammatory status in turbot. Finally, the expression of antibacterial components and the resistance to bacterial infection were increased in NaP-fed turbot, especially in high SBM+1.0% NaP group. In conclusion, the supplementation of NaP in high SBM diet promotes the growth and health in turbot and provides a theoretical basis for the development of NaP as a functional additive in fish feed.

Replacement of dietary fish meal with Clostridium autoethanogenum meal on growth performance, intestinal amino acids transporters, protein metabolism and hepatic lipid metabolism of juvenile turbot (Scophthalmus maximus L.)

Clostridium autoethanogenum meal (CAM) is a novel single-cell protein, which is produced from bacteria using carbon monoxide (CO) as sole carbon source. To evaluate the efficiency of CAM as an alternative for dietary fish meal, a 56-days growth experiment was performed on juvenile turbot (Scophthalmus maximus L.) with initial average weight of 9.13 ± 0.02 g. Six iso-nitrogenous (crude protein, 51.0%) and iso-lipidic (crude lipid, 11.5%) diets were formulated with 0%, 15%, 30%, 45%, 60% and 80% dietary fish meal protein substituted by CAM protein, which were designated as CAM0 (the control group), CAM15, CAM30, CAM45, CAM60 and CAM80, respectively. Results showed that no significant differences were observed in survival rate (over 97.50%) among different dietary treatments (p > 0.05). The specific growth rate (SGR) was not significantly affected when replacement levels of dietary fish meal with CAM were less than 45% (p > 0.05). The feed intake (FI) was significantly linear reduced with increasing dietary CAM (p < 0.05), whereas no significant differences were observed in feed efficiency ratio (FER), protein efficiency ratio (PER) and protein retention (PR) among different dietary treatments (p > 0.05). With increasing dietary CAM, lipid retention (LR) and carcass lipid tended to be increased in both significantly linear and quadratic patterns (p < 0.05). The apparent digestibility coefficient (ADC) of crude protein and some essential amino acids, including threonine, valine, lysine, histidine and arginine, showed significantly linear increase with increasing dietary CAM (p < 0.05). Furthermore, with the increase of dietary CAM, the gene expression of intestinal peptide and amino acids transporters was first up-regulated and then down-regulated with significantly quadratic pattern (p < 0.05), peaking in fish fed with diets CAM30 or CAM45, which was similar to the expression of genes related protein degradation in muscle. For genes related to protein metabolism in liver and muscle, the expression of mammalian target of rapamycin (mtor) was not significantly affected by dietary CAM, while the general control nonderepressible 2 (gcn2) tended to be first up-regulated and then down-regulated with significantly quadratic pattern (p < 0.05). Apart from that, the lipid metabolism of turbot was also affected by high dietary CAM, evidenced by increased expression of hepatic genes related to lipogenesis as well as reduced expression of genes related to lipid oxidation and lipid transport. In conclusion, CAM can replace up to 45% fish meal protein in diet for juvenile turbot without significantly adverse effects on growth performance. But excessive dietary CAM would result in significant growth reduction, and excessive lipid deposition may also occur in fish fed diets with high levels of CAM.

Palmitic acid induces intestinal lipid metabolism disorder, endoplasmic reticulum stress and inflammation by affecting phosphatidylethanolamine content in large yellow croaker Larimichthys crocea

In the 21^st century, intestinal homeostatic imbalance has emerged as a growing health challenge worldwide. Accumulating evidence reveals that excessive intake of saturated fatty acid (SFA) induces intestinal homeostatic imbalance. However, the potential molecular mechanism is still unclear. In the present study, we found that palm oil or palmitic acid (PA) treatment disturbed lipid metabolism homeostasis and triggered endoplasmic reticulum (ER) stress and inflammation in the intestine or intestinal cells of large yellow croaker (Larimichthys crocea). Interestingly, PA treatment significantly decreased phosphatidylethanolamine (PE) content in the intestinal cells. PE supplementation decreased triglyceride content in the intestinal cells induced by PA treatment by inhibiting fatty acid uptake and lipogenesis. PE supplementation suppressed ER stress. Meanwhile, PE supplementation alleviated inflammatory response through p38 MAPK-p65 pathway, reducing the damage of intestinal cells caused by PA treatment to some extent. Our work revealed that intestinal homeostatic imbalance caused by PA treatment was partly due to the decrease of PE content. PE consumption might be a nutritional strategy to regulate intestinal homeostasis in fish and even human beings.

Docosahexaenoic Acid Ameliorates the Toll-Like Receptor 22-Triggered Inflammation in Fish by Disrupting Lipid Raft Formation

BACKGROUND

Although dietary DHA alleviates Toll-like receptor (TLR)-associated chronic inflammation in fish, the underlying mechanism is not well understood.

OBJECTIVES

This study aimed to explore the role of Tlr22 in the innate immunity of large yellow croaker and investigate the anti-inflammatory effects of DHA on Tlr22-triggered inflammation.

METHODS

Head kidney-derived macrophages of croaker and HEK293T cells were or were not pretreated with 100 μM DHA for 10 h prior to polyinosinic-polycytidylic acid (poly I:C) stimulation. We executed qRT-PCR, immunoblotting, and lipidomic analysis to examine the impact of DHA on Tlr22-triggered inflammation and membrane lipid composition. In vivo, croakers (12.03 ± 0.05 g) were fed diets containing 0.2% [control (Ctrl)], 0.8%, and 1.6% DHA for 8 wk before injection with poly I:C. Inflammatory genes expression and rafts-related lipids and protein expression were measured in the head kidney. Data were analyzed by ANOVA or Student t test.

RESULTS

The activation of Tlr22 by poly I:C induced inflammation, and DHA diminished Tlr22-targeted inflammatory gene expression by 56-73% (P ≤ 0.05). DHA reduced membrane sphingomyelin (SM) and SFA-containing phosphatidylcholine (SFA-PC) contents, as well as lipid raft marker caveolin 1 amounts. Furthermore, lipid raft disruption suppressed Tlr22-induced Nf-κb and interferon h activation and p65 nuclear translocation. In vivo, expression of Tlr22 target inflammatory genes was 32-64% lower in the 1.6% DHA group than in the Ctrl group upon poly I:C injection (P ≤ 0.05). Also, the 1.6% DHA group showed a reduction in membrane SM and SFA-PC contents, accompanied by a decrease in caveolin 1 amounts, compared with the Ctrl group.

CONCLUSIONS

The activation of Tlr22 signaling depends on lipid rafts, and DHA ameliorates the Tlr22-triggered inflammation in both head kidney and head kidney-derived macrophages of croaker partially by altering membrane SMs and SFA-PCs that are required for lipid raft organization.

Nrf2 pathway in vegetable oil-induced inflammation of large yellow croaker (Larimichthys crocea)

This study was conducted to investigate the effects and regulation of dietary vegetable oil (VO, enriched with α-linolenic acid [ALA] and linoleic acid [LNA]) on the nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor-κB (NF-κB) pathways in large yellow croaker. In vivo study showed that the VO diet significantly decreased the activity of antioxidant enzymes and antioxidant enzyme-related mRNA expression in the liver tissue, in comparison with the fish oil (FO) diet (P < 0.05). The suppression of antioxidant capacity might be due to the decrease of nuclear Nrf2 protein translocation, Nrf2 binding to antioxidant response element (ARE) sequences, and subsequently, antioxidant genes transcription as electrophoretic mobility shift assay (EMSA) and luciferase assay showed. VO-derivated ALA and LNA exerted a lower antioxidant capacity than FO-derivated DHA and EPA, characterized by significantly lower nucleus Nfr2 protein expression but significantly higher ROS production values in primary hepatocytes (P < 0.05). The pro-inflammatory genes (tumor necrosis factor α [TNFα] and interleukin 1β [IL1β]) expression was significantly higher in the liver tissue of fish fed the VO diet which might be due to the activation of the NF-κB pathway (P < 0.05). Knockdown of the Nrf2 gene negatively affected the anti-inflammatory effect of fatty acids by increasing the expression of TNFα and the IL1β gene and nuclear p65 protein (P < 0.05). In general, the results indicated that dietary vegetable oil decreased antioxidant capacity but induced inflammatory responses through the Nrf2/NF-κB pathway.

Octanoate Alleviates Dietary Soybean Oil-Induced Intestinal Physical Barrier Damage, Oxidative Stress, Inflammatory Response and Microbial Dysbiosis in Large Yellow Croaker (Larimichthys Crocea)

Octanoate is a type of classical medium-chain fatty acids, which is widely used to treat neurological and metabolic syndrome. However, the specific role of octanoate in repairing intestinal health impairment is currently unknown. Therefore, we investigated whether dietary octanoate repaired the intestinal damage induced by surplus soybean oil in Larimichthys crocea. In this study, dietary octanoate alleviated abnormal morphology of the intestine and enhanced expression of ZO-1 and ZO-2 to improve intestinal physical barrier. Further, dietary octanoate increased antioxidant enzymic activities and decreased the level of ROS to alleviate the intestinal oxidative stress. Dietary octanoate also attenuated the expression of proinflammatory cytokines and the polarity of macrophage to reduce the intestinal inflammatory response. Moreover, the result of intestinal microbial 16S rRNA sequence showed that dietary octanoate repaired the intestinal mucosal microbial dysbiosis, and increased the relative abundance of Lactobacillus. Dietary octanoate supplementation also increased the level of acetic acid in intestinal content and serum through increasing the abundance of acetate-producing strains. Overall, in Larimichthys crocea, dietary octanoate might alleviated oxidative stress, inflammatory response and microbial dysbiosis to repair the intestinal damage induced by surplus soybean oil. This work provides vital insights into the underlying mechanisms and treatment strategies for intestinal damage in vertebrates.

Nutritional programming of large yellow croaker (Larimichthys crocea) larvae by dietary vegetable oil: effects on growth performance, lipid metabolism and antioxidant capacity

The nutritional status experienced in the early development of life plays a vital role in the long-term metabolic state of the individual, which is known as nutritional programming. The present study investigated the long-term effects of vegetable oil (VO) nutritional programming during the early life of large yellow croaker. First, larvae were fed either a fish oil (FO) diet or a VO diet for 30 d. Subsequently, under the same conditions, all fish were fed a commercial diet for 90 d and thereafter challenged with an FO or VO diet for 30 d. The results showed that growth performance was significantly lower in larvae fed the VO diet than in those in fed the FO diet in the stimulus phase. Notably, VO nutritional history fish showed lower levels of liver lipids liver total triglycerides and serum nonesterified free fatty acids than the FO nutritional history fish when juveniles were challenged with the VO diet, which was consistent with the expression of lipogenesis-related genes and proteins. Moreover, the VO nutritional history fish showed lower liver damage and higher antioxidant capacity than FO nutritional history fish when challenged with the VO diet. In summary, this study showed that a short VO stimulus during the early life stage of large yellow croaker, had a long-term effect on lipid metabolism and the antioxidant system. Specifically, VO nutritional programming had a positive effect on alleviating abnormal lipid deposition on the liver, liver damage, and the reduction of hepatic antioxidant capacity caused by a VO diet.

Lysophosphatidylcholine acyltransferase 3 (LPCAT3) mediates palmitate-induced inflammation in macrophages of large yellow croaker (Larimichthys crocea)

LPCAT3, a subtype of lysophosphatidylcholine acyltransferases, is a key enzyme in phosphatidylcholine remodeling pathway and plays a significant role in mediating inflammatory response in mammals. However, its inflammatory function in fish has yet to be discovered. Herein, this study aimed to investigate its role in inflammation in Larimichthys crocea. We analyzed the coding sequence of Larimichthys crocea LPCAT3 (Lc-LPCAT3) and explored the effect of Lc-LPCAT3 on palmitate (PA)-induced inflammation. We found that in macrophage cell line of Larimichthys crocea, the mRNA expression of Lc-lpcat3 was upregulated by PA with the elevated pro-inflammatory genes expression, including il1β, il6, il8, tnfα and ifnγ. Next, the role of Lc-LPCAT3 in inflammation induced by PA was further investigated. Results showed that knockdown of Lc-LPCAT3 mitigated PA-induced pro-inflammatory genes mRNA expression, including il1β, il8, tnfα and ifnγ, in which JNK signaling pathway was involved. In contrast, overexpression of Lc-LPCAT3 induced pro-inflammatory genes expression including il1β, tnfα and ifnγ. Furthermore, several transcription factors with negative regulation of Lc-LPCAT3 promoter activity were discovered including LXRα, RXRα, PPARα, PPARγ, CEBPα, CEBPβ, CEBPδ, SREBP1 and SREBP2, and SREBP1 had the strongest regulatory effect. In conclusion, we first discovered that fish LPCAT3 participated in PA-induced inflammation, and targeting SREBP1 might be an effective coping strategy.

LPS stimulation stabilizes HIF-1α by enhancing HIF-1α acetylation via the PARP1-SIRT1 and ACLY-Tip60 pathways in macrophages

Hypoxia and inflammatory mediators stabilize hypoxia-inducible factor (HIF)-1α through posttranslational modifications, such as phosphorylation and succinylation. Here, we identified sirtuin 1 (SIRT1) and 60 kDa Tat-interactive protein (Tip60)-mediated acetylation as another critical posttranslational modification that regulates HIF-1α protein stability under lipopolysaccharide (LPS) stimulation. Mechanistically, DNA damage induced by excessive reactive oxygen species (ROS) activated poly (ADP-ribose) polymerase 1 (PARP1) to consume oxidized nicotinamide adenine dinucleotide (NAD⁺ ). Correspondingly, SIRT1 activity was decreased with the decline in NAD⁺ levels, resulting in increased HIF-1α acetylation. LPS also activated the ATP-citrate lyase (ACLY)-Tip60 pathway to further enhance HIF-1α acetylation. Acetylation contributed to HIF-1α stability and exacerbated LPS-induced inflammation. Thus, inhibiting HIF-1α stability by decreasing its acetylation could partly alleviate LPS-induced inflammation. In conclusion, we revealed the mechanism by which LPS stabilized HIF-1α by increasing its acetylation via the PARP1-SIRT1 and ACLY-Tip60 pathways in fish macrophages. This study may provide novel insights for manipulation of HIF-1α acetylation as a therapeutic strategy against inflammation from the perspective of acetylation in vertebrates.

Increased LDL receptor by SREBP2 or SREBP2-induced lncRNA LDLR-AS promotes triglyceride accumulation in fish

LDLR, as the uptake receptor of low-density lipoprotein, plays a crucial role in lipid metabolism. However, the detailed mechanism by which LDLR affects hepatic triglyceride (TG) accumulation has rarely been reported. Here, we found that knockdown of LDLR effectively mitigated PA-induced TG accumulation. Further analysis revealed that the expression of LDLR was controlled by SREBP2 directly and indirectly. On one hand, transcription factor SREBP2 activated the transcription of LDLR directly. On the other hand, SREBP2 indirectly regulated LDLR by increasing the transcription of lncRNA LDLR-AS in fish. Mechanism analysis found that LDLR-AS functioned as an RNA scaffold to recruit heterogeneous nuclear ribonucleoprotein R (hnRNPR) to the 5′ UTR region of LDLR mRNA, which stabilized LDLR mRNA at the post-transcription level. In conclusion, our study demonstrates that increased LDLR transcription and mRNA stability is regulated by SREBP2 directly or indirectly, and promotes hepatic TG accumulation by endocytosing LDL in fish.

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PA-mediated LDLR increases triglyceride accumulation via the uptake of LDL in fish

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SREBP2 activated by TNFα promotes LDLR transcription in fish

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LncRNA LDLR-AS increases LDLR mRNA stability by recruiting hnRNPR in fish

Suppression of cideb under endoplasmic reticulum stress exacerbated hepatic inflammation by inducing hepatic steatosis and oxidative stress

Previous studies have shown that endoplasmic reticulum (ER) stress contributes to inflammation in several manners. However, whether cell death inducing DFF45-like effector b (Cideb), a lipid droplet (LD) associated protein that plays an important role in hepatic lipid metabolism, participates in this process has not been reported. In the present study, we demonstrated that deficiency of cideb alone did not trigger violent inflammation in the liver. However, the expression of cideb was suppressed by Chop (C/EBP homologous protein) under ER stress, which inhibited the transport of lipoproteins in the liver and led to the exacerbation of hepatic steatosis and oxidative stress, and ultimately exacerbated inflammation. Our results might provide a novel mechanism explaining inflammation triggered by ER stress.

Conventional Soybean Meal as Fishmeal Alternative in Diets of Japanese Seabass (Lateolabrax japonicus): Effects of Functional Additives on Growth, Immunity, Antioxidant Capacity and Disease Resistance

Aiming to optimize soymeal-based diets for Japanese seabass (Lateolabrax japonicas), a 105-day feeding trial was conducted to evaluate the effects of functional additives, including antioxidants (ethoxyquin, thymol and carvacrol) and chelated trace elements (Cu, Mn and Zn), on the growth, immunity, antioxidant capacity and disease resistance of fish fed diets with conventional soybean meal replacing 50% of fishmeal. Three isonitrogenous (45%) and isolipidic (11%) diets were formulated: (1) standard reference diet (FM, 42% fishmeal); (2) soymeal-based diet (SBM, 21% fishmeal and 30% conventional soybean meal); (3) SBM diet supplemented 0.0665% functional additives (FAS). Each experimental diet was randomly fed to quadruplicate groups of forty feed-trained Japanese seabass (initial average body weight = 125.6 ± 0.6 g) stocked in a saltwater floating cage. Upon the conclusion of the feeding trial, lower feed intake was observed in fish fed SBM compared to those fed FM and FAS. Fish fed FM showed the highest growth performance, estimated as the weight gain rate. Notably, FAS supported faster growth of fish than those fed SBM, indicating the optimal growth performance of dietary functional additives. The feed conversion rate showed the opposite trend among dietary treatments, with the highest value in fish fed SBM. Regarding immunity, fish fed soymeal-based diets suppressed the serum alternative complement pathway activities compared to FM, whereas the respiratory burst activity in macrophages of head kidneys showed a similar picture, but no statistical differences were observed. Further, fish fed soymeal-based diets had lower serum Cu-Zn SOD, CAT and GPx activities as well as liver vitamin E levels and scavenging rates of hydroxyl radical but higher liver MDA contents compared to the FM-fed group. Fish fed FAS had higher serum Cu-Zn SOD and GPx activities and liver vitamin E levels than those fed SBM, suggesting the enhancement of antioxidant capacity of dietary functional additives. For the disease resistance against Vibrio harveyi infection, fish fed SBM had the highest cumulative mortality, followed by the FAS and FM groups. Additionally, the biomarkers related to the immune and antioxidant capacities had a positive correlation with the relative abundance of Paracoccus and Pseudomonas, while liver MDA levels had a negative correlation with the relative abundance of Pseudomonas and Psychrobacter. Collectively, soymeal replacing 50% of fishmeal suppressed the growth, immunity, antioxidant capacity and disease resistance of Japanese seabass, while dietary supplementation of antioxidants and chelated trace elements could mitigate soymeal-induced adverse effects on growth and disease resistance through the improvement in antioxidant capacity and regulation of gut microbiota.