Hydroxytyrosol Attenuates High-Fat-Diet-Induced Oxidative Stress, Apoptosis and Inflammation of Blunt Snout Bream (Megalobrama amblycephala) through Its Regulation of Mitochondrial Homeostasis

m6A Methylation Mediated Autophagy and Nucleotide-Binding Oligomerization Domain-like Receptors Signaling Pathway Provides New Insight into the Mitigation of Oxidative Damage by Mulberry Leaf Polysaccharides

m6A methylation modification is an important genetic modification involved in biological processes such as sexual maturation, antibacterial, and antiviral in aquatic animals. However, few studies have been conducted in aquatic animals on the relationship between m6A methylation modification and autophagy-inflammation induced by lipid metabolism disorders. In the present study, a high-fat (HF) group and HF-MLP group (1 g mulberry leaf polysaccharides (MLPs)/1 kg HF diet) were set up. The mid-hind intestines of Megalobrama amblycephala juveniles from the two groups were collected for MeRIP-seq and RNA-seq after an 8-week feeding trial. The m6A peaks in the HF and HF-MLP groups were mainly enriched in the 3' Untranslated Region (3'UTR), Stop codon, and coding sequence (CDS) region. Compared with the HF group, the m6A peaks in the HF-MLP group were shifted toward the 5'UTR region. 'RRACH' was the common m6A methylation motif in the HF and HF-MLP groups. Methyltransferase mettl14 and wtap expression in the intestines of the HF-MLP group were significantly higher compared with the HF group (p < 0.05). A total of 21 differentially expressed genes(DEGs) with different peaks were screened by the combined MeRIP-seq and RNA-seq analysis. Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis enriched BCL2 interacting protein 3 (bnip3) to autophagy-animal and mitophagy-animal signaling pathways, etc., and nucleotide-binding domain leucine-rich repeat protein 1 (nlrp1) was enriched to the Nucleotide-binding oligomerization domain (NOD)-like receptor signaling pathway. Combined MeRIP-seq and RNA-seq analysis indicated that the expression pattern of bnip3 was hyper-up and that of nlrp1 was hyper-down. Gene Set Enrichment Analysis (GSEA) analysis confirmed that the intestinal genes of HF-MLP group positively regulate lysosomal and autophagy-animal signaling pathways. In the present study, we demonstrated that m6A methylation modification plays a role in regulating autophagy-inflammatory responses induced by HF diets by MLPs, and further explored the molecular mechanisms by which MLPs work from the epigenetic perspective.

Indispensable role of PGC1α signaling in lipid and carbohydrate metabolism of fish PPARα activation

Unlike predictability in mammals, the responses of PPARα activation treatment in fish varies between different fish species. Existing results indicated that the varied effects may result from fish feeding habits difference within their varied dietary protein content, while the exact mechanism remains obscure. In this study, we conducted an in vivo 8-week trial of low protein 30 % (LP) or high protein 38 % (HP) diet fed tilapia (8.83 ± 0.02 g) supplemented with or without fenofibrate and in vitro 24-h of BCAA (leucine, valine and isoleucine) treated primary cell trial, aiming to reveal the unknown mechanism. The in vivo results showed that, in LP group, fenofibrate significantly increased serum levels of TG, glucose, AST, ALT, MDA and liver TG (P < 0.05), while decreased liver, muscle and serum BCAA, liver mitochondria quantity, glucose tolerance (P < 0.05). Moreover, fenofibrate notably lifted the hepatic bckdha, bckdhb, mff, fis, fas and pparγ expression (P < 0.05), while decreased genes expression of pgc1α, tfam, parkin, pink, foxo3a, sod2, cat, ir and irs (P < 0.05). No great impact was found in aco, cyp4a3 and cytb (P > 0.05). In HP group, fenofibrate improved lipid and glucose metabolism along with unaltered BCAA, increased expression of aco, cytb, cpt1a, pparβ, foxo3a, sod2, nrf1, mfn1 and fis (P < 0.05). In vitro findings revealed that the PGC1α and PPARα could be activated by BCAA, particular for leucine (P < 0.05). Taken together, we conclude that: 1) increased lipid deposition, lipid peroxidation, liver damage and glucose intolerance in fenofibrate-LP group could be ascribed to decreased BCAA and done through PGC1α/FOXO3, PGC1α/MQC regulation; 2) improved lipid-lowering and glucose tolerance in fenofibrate-HP group would be attributed to the enhanced PGC1α/MQC, organelles proliferation and fatty acid β-oxidation.

Docosahexaenoic acid-enriched diet improves the flesh quality of freshwater fish (Megalobrama amblycephala): Evaluation based on nutritional value, texture and flavor

Docosahexaenoic acid (DHA) is a potential regulatory substance for flesh quality of fish, while the related evaluation is still barely. In this study, the effects of DHA-enriched diets on the flesh quality of freshwater fish (Megalobrama amblycephala) were investigated systematically. The sub-adult M. amblycephala were randomly fed with control diet (CON), 0.2% DHA diet (DL) or 0.8% DHA diet (DH). After 12-week feeding trial, the DH group flesh had higher concentrations of essential amino acids and polyunsaturated fatty acids compared to the CON group. Meanwhile, the hardness, springiness, shear force and moisture-holding capacity, as well as the values of umami, richness and sweetness were also improved by DH. The non-targeted metabolomics analysis revealed the key metabolites that may have significantly positive influence on flavor. Collectively, the diet supplementation with 0.8% DHA could achieve the improvement of the flesh quality in terms of nutritional value, texture and flavor in freshwater fish.

Uncoordinated 51-like kinase 1a/b and 2 in fish Megalobrama amblycephala: Molecular cloning, functional characterization, and their potential roles in glucose metabolism

Uncoordinated (Unc) 51-like kinase (ulk1) and ulk2 are closely involved in autophagy activation, but little is known about their roles in regulating glucose homeostasis. In this study, the genes of ulk1a, ulk1b and ulk2 were cloned and characterized in fish Megalobrama amblycephala. All the three genes shared the approximate N-terminal kinase domain and the C-terminal Atg1-like_tMIT domain structure, while the amino acid sequence identity of them are different between M. amblycephala and other vertebrates. Their transcripts were widely observed in various tissues (brain, muscle, gill, heart, spleen, eye, liver, intestine, abdominal adipose and kidney), but differed in tissue expression patterns. During the glucose tolerance test and the insulin tolerance test, the up-regulated transcriptions of ulk1a, ulk1b and ulk2 were all found despite some differences in the temporal patterns. At the same time, the activities of glycolytic enzymes like hexokinase and phosphofructokinase both showed parallel increases. Furthermore, the feeding of a high-carbohydrate diet decreased the transcriptions of ulk1a, ulk1b and ulk2. Collectively, this study demonstrated that ulk1a, ulk1b and ulk2 in M. amblycephala had similar molecular characterizations, but with different conservation and tissue expression patterns. In addition, ulk1/2 might play important roles in maintaining the glucose homeostasis in fish through regulating the glycolytic pathway.

Impact of Hydroxytyrosol-Rich Extract Supplementation in a High-Fat Diet on Gilthead Sea Bream (Sparus aurata) Lipid Metabolism

High-fat diets (HFDs) enhance fish growth by optimizing nutrient utilization (i.e., protein-sparing effect); however, their potential negative effects have also encouraged the search for feed additives. This work has investigated the effects of an extract rich in a polyphenolic antioxidant, hydroxytyrosol (HT), supplemented (0.52 g HT/kg feed) in a HFD (24% lipid) in gilthead sea bream (Sparus aurata). Fish received the diet at two ration levels, standard (3% of total fish weight) or restricted (40% reduction) for 8 weeks. Animals fed the supplemented diet at a standard ration had the lowest levels of plasma free fatty acids (4.28 ± 0.23 mg/dL versus 6.42 ± 0.47 in the non-supplemented group) and downregulated hepatic mRNA levels of lipid metabolism markers (ppara, pparb, lpl, fatp1, fabp1, acox1, lipe and lipa), supporting potential fat-lowering properties of this compound in the liver. Moreover, the same animals showed increased muscle lipid content and peroxidation (1.58- and 1.22-fold, respectively, compared to the fish without HT), suggesting the modulation of body adiposity distribution and an enhanced lipid oxidation rate in that tissue. Our findings emphasize the importance of considering this phytocompound as an optimal additive in HFDs for gilthead sea bream to improve overall fish health and condition.

Oxidative Stress Can Be Attenuated by 4-PBA Caused by High-Fat or Ammonia Nitrogen in Cultured Spotted Seabass: The Mechanism Is Related to Endoplasmic Reticulum Stress

Oxidative stress is a common phenomenon in aquaculture, which can be induced by nutritional or environmental factors. Generally, oxidative stress causes poor growth performance, metabolic dysregulation, and even the death of aquatic animals. To identify a nutritional intervention strategy, high-fat diet (HFD) feeding (Experiment I) and acute ammonia nitrogen challenge (Experiment II) tests were carried out. In Experiment I, HFD feeding significantly decreased the growth performance concomitantly with excessive fat deposition in the liver and abdomen. The addition of 4-PBA in the diet improved the excessive fat accumulation. The activities of antioxidative enzymes were suppressed, and the levels of lipid and protein peroxidation were increased, indicating that HFD feeding induced oxidative stress. The endoplasmic reticulum stress (ERs) related genes were downregulated in the HFD group. Under a transmission electron microscope (TEM), more swollen and dilated ER lumen could be observed. These results indicated that the HFD induced ERs activation. Although 4-PBA acted as a potent ERs inhibitor, as evidenced by the alleviated alterations of ERs molecules and the ER ultrastructure, the oxidative stress was also attenuated by 4-PBA. In Experiment II, dietary 4-PBA improved the tolerance to the acute ammonia nitrogen challenge, as lower mortality and serum aminotransferase activity was found. Further results showed that 4-PBA decreased the peroxidation content and attenuated ERs, thus confirming the correlation between oxidative stress and ERs. Our findings showed that dietary 4-PBA supplementation can attenuate oxidative stress induced by a HFD or acute ammonia challenge; the mechanism is related to its potent inhibition effect for ERs.

Hydroxytyrosol Promotes the Mitochondrial Function through Activating Mitophagy

Emerging evidence suggests that mitochondrial dysfunction mediates the pathogenesis for non-alcoholic fatty liver disease (NAFLD). Hydroxytyrosol (HT) is a key component of extra virgin olive oil which can exert beneficial effects on NAFLD through modulating mitochondria. However, the mechanism of the impacts of HT still remains elusive. Thus, an in vivo and a series of in vitro experiments were carried out to examine the impacts of hydroxytyrosol (HT) on lipid metabolism and mitochondrial function in fish. For the in vivo experiment, two diets were produced to contain 10% and 16% fat as normal-fat and high-fat diets (NFD and HFD) and two additional diets were prepared by supplementing 200 mg/kg of HT to the NFD and HFD. The test diets were fed to triplicate groups of spotted seabass (Lateolabrax maculatus) juveniles for 8 weeks. The results showed that feeding HFD leads to increased fat deposition in the liver and induces oxidative stress, both of which were ameliorated by HT application. Furthermore, transmission electron microscopy revealed that HFD destroyed mitochondrial cristae and matrix and induced severe hydropic phenotype, while HT administration relieved these alterations. The results of in vitro studies using zebrafish liver cell line (ZFL) showed that HT promotes mitochondrial function and activates PINK1-mediated mitophagy. These beneficial effects of HT disappeared when the cells were treated with cyclosporin A (Csa) as a mitophagy inhibitor. Moreover, the PINK1-mediated mitophagy activation by HT was blocked when compound C (CC) was used as an AMPK inhibitor. In conclusion, our findings demonstrated that HT alleviates fat accumulation, oxidative stress and mitochondrial dysfunction, and its effects are deemed to be mediated via activating mitophagy through the AMPK/PINK1 pathway.