Endoplasmic Reticulum Stress-Mediated Autophagy and Apoptosis Alleviate Dietary Fat-Induced Triglyceride Accumulation in the Intestine and in Isolated Intestinal Epithelial Cells of Yellow Catfish

Regulation of lipid metabolism in grass carp primary hepatocytes by exosomes derived from fatty hepatocytes though GRP78

Exosomes regulate lipid metabolism by carrying miRNAs, nucleic acids, and proteins, thereby influencing the function of receptor cells. Glucose-regulated protein 78 (GRP78) is also involved in the regulation of lipid metabolism. However, it remains unclear whether exosomes derived from fatty hepatocytes (OA-Exo) regulate lipid metabolism through the enrichment of GRP78. In this study, we observed the expression of GRP78 was significantly increased in fatty hepatocytes (incubating hepatocytes with oleic acid (OA) for 24 h) and OA-Exo (P < 0.05). In addition, OA-Exo (50 μg/mL) and GRP78 protein (1 μg/mL) significant increased the content of triacylglycerol (TG) and total cholesterol (TC), as well as up-regulated the expression of GRP78 and inositol-requiring enzyme-1alpha (IRE1α) protein (P < 0.05). We further used YUM70 (an inhibitor of GRP78) to inhibit endogenous GRP78, and compared with the YUM70 group, OA-Exo reversed the effect of YUM70 and increased the content of TG, TC, and the expression of GRP78 protein in hepatocytes (P < 0.05). Furthermore, the inhibition of the IRE1α pathway with 4μ8C resulted in a significant decrease in TG content compared to the control group (P < 0.05). However, when compared with the 4μ8C group, OA-Exo and GRP78 reversed the effect of 4μ8C and significantly increased TG content (P < 0.05). Taken together, these results indicated that OA-Exo activated IRE1α to promote lipid accumulation in hepatocytes through the enrichment of GRP78. This study provided a new perspective for further exploration of exosomal lipid metabolism in fish.

Nobiletin restores HFD-induced enteric nerve injury by regulating enteric glial activation and the GDNF/AKT/FOXO3a/P21 pathway

BACKGROUND

To explore whether nobiletin has a protective effect on high-fat diet (HFD)-induced enteric nerve injury and its underlying mechanism.

METHODS

An obesity model was induced by a HFD. Nobiletin (100 mg/kg and 200 mg/kg) and vehicle were administered by gastric gavage for 4 weeks. Lee's index, body weight, OGTT and intestinal propulsion assays were performed before sacrifice. After sampling, lipids were detected using Bodipy 493/503; lipid peroxidation was detected using MDA and SOD kits and the expression of PGP 9.5, Trem2, GFAP, β-tubulin 3, Bax, Bcl2, Nestin, P75 NTR, SOX10 and EDU was detected using immunofluorescence. The GDNF, p-AKT, AKT, p-FOXO3a, FOXO3a and P21 proteins were detected using western blotting. The relative mRNA expression levels of NOS2 were detected via qPCR. Primary enteric neural stem cells (ENSCs) were cultured. After ENSCs were treated with palmitic acid (PA) and nobiletin, CCK-8 and caspase-3/7 activity assays were performed to evaluate proliferation and apoptosis.

RESULTS

HFD consumption caused colon lipid accumulation and peroxidation, induced enteric nerve damage and caused intestinal motor dysfunction. However, nobiletin reduced lipid accumulation and peroxidation in the colon; promoted Trem2, β-tubulin 3, Nestin, P75NTR, SOX10 and Bcl2 expression; inhibited Bax and GFAP expression; reduced NOS2 mRNA transcription; and regulated the GDNF/AKT/FOXO3a/P21 pathway. Nobiletin also promoted PA-induced impairment of ENSCs.

CONCLUSIONS

Nobiletin restored HFD-induced enteric nerve injury, which may be associated with inhibiting enteric nerve apoptosis, promoting enteric nerve survival and regulating the GDNF/AKT/FOXO3a/P21 pathway.

Dietary lipid sensing through fatty acid oxidation and chylomicron formation in the gastrointestinal tract of rainbow trout

In mammals, physiological processes related to lipid metabolism, such as chylomicron synthesis or fatty acid oxidation (FAO), modulate eating, highlighting the importance of energostatic mechanisms in feeding control. This study, using rainbow trout (Oncorhynchus mykiss) as model, aimed to characterize the role of FAO and chylomicron formation as peripheral lipid sensors potentially able to modulate feeding in fish. Fish fed with either a normal- (24%) or high- (32%) fat diet were intraperitoneally injected with water alone or containing etomoxir (inhibitor of FAO rate-limiting enzyme carnitine palmitoyl-transferase 1). First, feed intake levels were recorded. We observed an etomoxir-derived decrease in feeding at short times, but a significant increase at 48 h after treatment in fish fed normal-fat diet. Then, we evaluated putative etomoxir effects on the mRNA abundance of genes related to lipid metabolism, chylomicron synthesis and appetite-regulating peptides. Etomoxir treatment upregulated mRNA levels of genes related to chylomicron assembly in proximal intestine, while opposite effects occurred in distal intestine, indicating a clear regionalization in response. Etomoxir also modulated gastrointestinal hormone mRNAs in proximal intestine, upregulating ghrl in fish fed normal-fat diet and pyy and gcg in fish fed high-fat diet. These results provide evidence for an energostatic control of feeding related to FAO and chylomicron formation at the peripheral level in fish.

Dietary cholesterol intervention could alleviate the intestinal injury of Oreochromis niloticus induced by plant-based diet via the intestinal barriers

This study aims to explore the effects of supplementing cholesterol in plant-based feed on intestinal barriers (including physical barrier, chemical barrier, immune barrier, biological barrier) of GIFT strain tilapia (Oreochromis niloticus). Four isonitrogenous and isolipidic diets were prepared as follows: plant-based protein diet (Con group) containing corn protein powder, soybean meal, cottonseed meal, and rapeseed meal, with the addition of cholesterol at a level of 0.6 % (C0.6 % group), 1.2 % (C1.2 % group), and 1.8 % (C1.8 % group), respectively. A total of 360 fish (mean initial weight of (6.08 ± 0.12) g) were divided into 12 tanks with 30 fish per tank, each treatment was set with three tanks and the feeding period lasted 9 weeks. Histological analysis revealed that both the C0.6 % and C1.2 % groups exhibited a more organized intestinal structure, with significantly increased muscle layer thickness compared to the Con group (P < 0.05). Furthermore, in the C1.2 % group, there was a significant up-regulation of tight junction-related genes (claudin-14, occludin, zo-1) compared to the Con group (P < 0.05). 5-ethynyl-2'-deoxyuridine staining results also demonstrated a notable enhancement in intestinal cell proliferation within the C1.2 % group (P < 0.05). Regarding the intestinal chemical barrier, trypsin and lipase activities were significantly elevated in the C1.2 % group (P < 0.05), while hepcidin gene expression was considerably down-regulated in this group but up-regulated in the C1.8 % group (P < 0.05). In terms of the intestinal immune barrier, inflammation-related gene expression levels (tnf-α, il-1β, caspase 9, ire1, perk, atf6) were markedly reduced in the C1.2 % group (P < 0.05). Regarding the intestinal biological barrier, the composition of the intestinal microbiota indicated that compared to the Con group, both the 0.6 % and 1.2 % groups showed a significant increase in Shannon index (P < 0.05). Additionally, there was a significant increase in the abundance of Firmicutes and Clostridium in the C1.2 % group (P < 0.05). In summary, supplementation of 1.2 % cholesterol in the plant-based diet exhibits the potential to enhance intestinal tight junction function and improve the composition of intestinal microbiota, thereby significantly promoting tilapia's intestinal health.

Dietary chenodeoxycholic acid attenuates high-fat diet-induced growth retardation, lipid accumulation and bile acid metabolism disorder in the liver of yellow catfish Pelteobagrus fulvidraco

This experiment was conducted to investigate whether dietary chenodeoxycholic acid (CDCA) could attenuate high-fat (HF) diet-induced growth retardation, lipid accumulation and bile acid (BA) metabolism disorder in the liver of yellow catfish Pelteobagrus fulvidraco. Yellow catfish (initial weight: 4·40 (sem 0·08) g) were fed four diets: the control (105·8 g/kg lipid), HF diet (HF group, 159·6 g/kg lipid), the control supplemented with 0·9 g/kg CDCA (CDCA group) and HF diet supplemented with 0·9 g/kg CDCA (HF + CDCA group). CDCA supplemented in the HF diet significantly improved growth performance and feed utilisation of yellow catfish (P < 0·05). CDCA alleviated HF-induced increment of hepatic lipid and cholesterol contents by down-regulating the expressions of lipogenesis-related genes and proteins and up-regulating the expressions of lipololysis-related genes and proteins. Compared with the control group, CDCA group significantly reduced cholesterol level (P < 0·05). CDCA significantly inhibited BA biosynthesis and changed BA profile by activating farnesoid X receptor (P < 0·05). The contents of CDCA, taurochenodeoxycholic acid and glycochenodeoxycholic acid were significantly increased with the supplementation of CDCA (P < 0·05). HF-induced elevation of cholic acid content was significantly attenuated by the supplementation of CDCA (P < 0·05). Supplementation of CDCA in the control and HF groups could improve the liver antioxidant capacity. This study proved that CDCA could improve growth retardation, lipid accumulation and BA metabolism disorder induced by HF diet, which provided new insight into understanding the physiological functions of BA in fish.

High-fat diet alters intestinal microbiota and induces endoplasmic reticulum stress via the activation of apoptosis and inflammation in blunt snout bream

The primary organ for absorbing dietary fat is the gut. High dietary lipid intake negatively affects health and absorption by causing fat deposition in the intestine. This research explores the effect of a high-fat diet (HFD) on intestinal microbiota and its connections with endoplasmic reticulum stress and inflammation. 60 fish (average weight: 45.84 ± 0.07 g) were randomly fed a control diet (6% fat) and a high-fat diet (12 % fat) in four replicates for 12 weeks. From the result, hepatosomatic index (HSI), Visceralsomatic index (VSI), abdominal fat (ADF), Intestosomatic index (ISI), mesenteric fat (MFI), Triglycerides (TG), total cholesterol (TC), non-esterified fatty acid (NEFA) content were substantially greater on HFD compared to the control diet. Moreover, fish provided the HFD significantly obtained lower superoxide dismutase (SOD) and glutathione peroxidase (GPX) activities. In contrast, an opposite result was seen in malondialdehyde (MDA) content in comparison to the control. HFD significantly altered intestinal microbiota in blunt snout bream, characterized by an increased abundance of Aeromonas, Plesiomonas proteobacteria, and firmicutes with a reduced abundance of Cetobacterium and ZOR0006. The transcriptional levels of glucose-regulated protein 78 (grp78), inositol requiring enzyme 1 (ire1), spliced X box-binding protein 1 (xbp1), DnaJ heat shock protein family (Hsp40) member B9 (dnajb9), tumor necrosis factor alpha (tnf-α), nuclear factor-kappa B (nf-κb), monocyte chemoattractant protein-1 (mcp-1), and interleukin-6 (il-6) in the intestine were markedly upregulated in fish fed HFD than the control group. Also, the outcome was similar in bax, caspases-3, and caspases-9, ZO-1, Occludin-1, and Occludin-2 expressions. In conclusion, HFD could alter microbiota and facilitate chronic inflammatory signals via activating endoplasmic reticulum stress.

Hemato-biochemical alteration in the bronze featherback Notopterus notopterus (Pallas, 1769) as a biomonitoring tool to assess riverine pollution and ecology: a case study from the middle and lower stretch of river Ganga

Fishes are poikilothermic animals and are rapid responders to any sort of ecological alteration. The responses in the fish can be easily assessed from their hematological and biochemical responses. To study the variation in the hemato-biochemical parameters in retort to ecological alteration and ecological regime, a study was conducted at six different sampling stations of the middle and lower stretches of river Ganga. Various hematological and biochemical responses of fishes were also monitored in response to multiple ecological alterations. For the assessment of ecological alteration, various indices were calculated such as the water pollution index (WPI), National Sanitation Foundation-water quality index (NSF-WQI), and Nemerow's pollution index (NPI) has been calculated based on various water quality parameters such as dissolved oxygen (DO), pH, total dissolved solids (TDS), total alkalinity (TA), total hardness (TH), electrical conductivity (EC), biochemical oxygen demand (BOD), chlorinity (CL), total nitrogen (TN), and total phosphorus (TP). The hematological parameters such as WBC, RBC, platelet, hemoglobin, and hematocrit were monitored. The serum biochemical parameters such as SGPT, SGOT, ALP, amylase, bilirubin, glucose, triglyceride (TRIG), and cholesterol (CHOL) were investigated. The study revealed that NSF-WQI varied from 45.08 at Buxar to 110.63 at Rejinagar and showed a significantly positive correlation with SGPT, SGOT, ALP, TRIG, CHOL, and WBC, whereas a significantly negative correlation was observed between TRIG and RBC. WPI varied from 19 to 23 and showed a significant positive correlation with SGOT and a negative correlation was observed with total nitrogen. The PCA analysis illustrated the significance of both natural as well as anthropogenic factors on riverine ecology. Strong positive loading was observed with SGPT, SGOT, ALP, and platelet. The study signified the need for monitoring the hemato-biochemical responses of fishes in response to alterations in the ecological regime.

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.

Klf4-Sirt3/Pparα-Lcad pathway contributes to high phosphate-induced lipid degradation

BACKGROUND

Phosphorus commonly reduces lipid deposition in the vertebrates. However, the underlying mechanisms involved in the process remain unclear.

METHODS

Yellow catfish were given three experimental diets with dietary phosphate levels of 3.22, 6.47 and 7.99 g Pi kg^- 1, respectively, for 8 weeks. The contents of triglyceride, non-esterified free fatty acids, adenosine triphosphate, nicotinamide adenine dinucleotide, nicotinamide adenine dinucleotide, enzymatic activities, mRNA and protein expression were determined in the intestinal tissues. Hematoxylin and eosin, Oil Red O staining, and transmission electron microscope were performed for intestinal tissues. Primary intestinal epithelial cells were isolated from yellow catfish intestine. Western blot analysis, Immunoprecipitation assays, Immunofluorescence staining, and RNA extraction and quantitative real-time PCR were decided. Luciferase reporter assays and electrophoretic mobility shift assay were used to evaluate the function of Sirt3, PPARα and Lcad promoters.

RESULTS

High dietary phosphate intake activated intestinal phosphate absorption and excretion, and reduced lipid deposition through increasing lipolysis in the intestine. Moreover, phosphate incubation increased the mRNA and protein expression of krüppel like factor 4 (klf4), silent mating-type information regulation 2 homolog 3 (sirt3), peroxisome proliferator activated receptor alpha (pparα) and long chain acyl-CoA dehydrogenase (lcad) in the intestinal epithelial cells (IECs), and klf4 knockdown attenuated the phosphate-induced increase of protein levels of Sirt3, Pparα and Lcad. Further investigation found that Klf4 overexpression increased the activity of sirt3 and pparα promoters, which in turn reduced the acetylation and protein level of Lcad.

CONCLUSION

Dietary Pi excess induced lipid degradation by the activation of the Klf4-Sirt3/Pparα-Lcad pathway in the intestine and primary IECs. Video Abstract.

DHA induces adipocyte lipolysis through endoplasmic reticulum stress and the cAMP/PKA signaling pathway in grass carp (Ctenopharyngodon idella)

Docosahexaenoic acid (DHA) is a biologically active fatty acid that reduces the accumulation of lipids. However, the molecular mechanism underlying this process, particularly in fish, is not well understood. Recent studies show that endoplasmic reticulum (ER) stress triggers the activation of the unfolded protein response, which has been revealed to play an essential role in lipid metabolism. In this study, we explored the effect of DHA on ER stress and investigated the potential molecular mechanisms underlying DHA-induced adipocyte lipolysis in grass carp (Ctenopharyngodon idella) both in vivo and in vitro. We found that DHA remarkably reduced the triglyceride content, increased the secretion of glycerol, promoted lipolysis in adipocytes and evoked ER stress, whereas inhibiting ER stress using 4-phenyl butyric acid (4-PBA) inhibited the effects of DHA (P < 0.05). These results implied that ER stress potentially participates in DHA-induced adipocyte lipolysis. Additionally, STF-083010, a specific inositol-requiring enzyme 1α (IRE1α)-inhibitor, attenuated the effects of DHA on lipolysis, demonstrating that IRE1α and X-box binding protein 1 potentially participate in DHA-induced lipolysis. DHA also activated the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) pathway by increasing the level of cAMP and activating the PKA enzyme (P < 0.05). Nevertheless, H89, a PKA inhibitor, weakened DHA-induced lipolysis by inhibiting the cAMP/PKA signaling pathway. Furthermore, inhibiting ER stress using 4-PBA also inhibited lipolysis and alleviated DHA-induced activation of the cAMP/PKA signaling pathway, suggesting that ER stress may participate in DHA-induced lipolysis through the activation of the cAMP/PKA signaling pathway. Our data illustrate that DHA supplementation can be a promising nutritional strategy for ameliorating lipid accumulation in grass carp. The present study elucidated the molecular mechanism for DHA-induced lipolysis in grass carp adipocytes and emphasized the importance of ER stress and the cAMP/PKA pathway in DHA-induced lipolysis. These results deepen our understanding of ameliorating lipids deposition in freshwater fish by targeting DHA.

Dietary Choline Mitigates High-Fat Diet-Impaired Chylomicrons Assembly via UPRer Modulated by perk DNA Methylation

High-fat diets (HFD) lead to impairment of chylomicrons (CMs) assembly and adversely influence intestinal lipid homeostasis. However, the mechanisms of HFD impairing CMs assembly have yet to be fully understood. Additionally, although choline, as a lipid-lowering agent, has been widely used and its deficiency has been closely linked to non-alcoholic steatohepatitis (NASH), the contribution of choline by functioning as a methyl donor in alleviating HFD-induced intestinal lipid deposition is unknown. Thus, this study was conducted to determine the mechanism of HFD impairing CMs assembly and also tested the effect of choline acting as a methyl donor in this process. To this end, in this study, four diets (control, HFD, choline and HFD + choline diet) were fed to yellow catfish for 10 weeks in vivo and their intestinal epithelial cells were isolated and incubated for 36 h in fatty acids (FA) with or without choline solution combining si-perk transfection in vitro. The key findings from this study as follows: (1) HFD caused impairment of CMs assembly main by unfolded protein response (UPRer). HFD activated perk and then induced UPRer, which led to endoplasmic reticulum dysfunction and further impaired CMs assembly via protein-protein interactions between Perk and Apob48. (2) Choline inhibited the transcriptional expression level of perk via activating the -211 CpG methylation site, which initiated the subsequent ameliorating effect on HFD-impaired CMs assembly and intestinal lipid dysfunction. These results provide a new insight into direct crosstalk between UPRer and CMs assembly, and also emphasize the critical contribution of choline acting as a methyl donor and shed new light on choline-deficient diet-induced NASH.

Dietary choline prevents high fat-induced disorder of hepatic cholesterol metabolism through SREBP-2/HNF-4α/CYP7A1 pathway in a freshwater teleost yellow catfish Pelteobagrus fulvidraco

Lipid overload-induced hepatic cholesterol accumulation is a major public health problem worldwide, and choline has been reported to ameliorate cholesterol accumulation, but its mechanism remains unclear. Our study found that choline prevented high-fat diet (HFD)-induced cholesterol metabolism disorder and enhanced choline uptake and phosphatidylcholine synthesis in the liver tissues; choline incubation prevented fatty acid (FA)-induced cholesterol accumulation and FA-induced inhibition of bile acid synthesis. Moreover, compared to single FA incubation, choline incubation or FA + choline co-incubation increased the mRNA abundances and protein levels of HNF4α and up-regulated the degradation of cholesterol into bile acids. Mechanistically, choline prevented the FA-induced accumulation of SREBP2 protein and the interaction between SREBP2 and HNF4α, thereby enhancing the DNA binding capacity of the HNF4α to the CYP7A1 promoter, and promoting the degradation of cholesterol into bile acids. Our study elucidated the novel regulatory mechanisms of choline preventing HFD-induced cholesterol accumulation and increasing bile acid synthesis by SREBP-2/HNF-4α/CYP7A1 pathway.

Dietary berberine alleviates high carbohydrate diet-induced intestinal damages and improves lipid metabolism in largemouth bass (Micropterus salmoides)

High carbohydrate diet (HCD) causes metabolism disorder and intestinal damages in aquaculture fish. Berberine has been applied to improve obesity, diabetes and NAFLD. However, whether berberine contributes to the alleviation of HCD-induced intestinal damages in aquaculture fish is still unclear. Here we investigated the effects and mechanism of berberine on HCD-induced intestinal damages in largemouth bass (Micropterus salmoides). We found dietary berberine (50 mg/kg) improved the physical indexes (VSI and HSI) without affecting the growth performance and survival rate of largemouth bass. Importantly, the results showed that dietary berberine reduced the HCD-induced tissue damages and repaired the barrier in the intestine of largemouth bass. We observed dietary berberine significantly suppressed HCD-induced intestinal apoptosis rate (from 31.21 to 8.35%) and the activity level of Caspase3/9 (P < 0.05) by alleviating the inflammation (il1β, il8, tgfβ, and IL-6, P < 0.05) and ER stress (atf6, xbp1, perk, eif2α, chopa, chopb, and BIP, P < 0.05) in largemouth bass. Further results showed that dietary berberine declined the HCD-induced excessive lipogenesis (oil red O area, TG content, acaca, fasn, scd, pparγ, and srebp1, P < 0.05) and promoted the lipolysis (hsl, lpl, cpt1a, and cpt2, P < 0.05) via activating adenosine monophosphate-activated protein kinase (AMPK, P < 0.05) and inhibiting sterol regulatory element-binding protein 1 (SREBP1, P < 0.05) in the intestine of largemouth bass. Besides, we also found that dietary berberine significantly promoted the hepatic lipid catabolism (hsl, lpl, cpt1a, and cpt2, P < 0.05) and glycolysis (pk and ira, P < 0.05) to reduce the systematic lipid deposition in largemouth bass fed with HCD. Therefore, we elucidated that 50 mg/kg dietary berberine alleviated HCD-induced intestinal damages and improved AMPK/SREBP1-mediated lipid metabolism in largemouth bass, and evaluated the feasibility for berberine as an aquafeed additive to enhance the intestinal function of aquaculture species.

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.

Iron increases lipid deposition via oxidative stress-mediated mitochondrial dysfunction and the HIF1α-PPARγ pathway

Iron is an essential micro-element, involved in multiple biological activities in vertebrates. Excess iron accumulation has been identified as an important mediator of lipid deposition. However, the underlying mechanisms remain unknown. In the present study, we found that a high-iron diet significantly increased intestinal iron content and upregulated the mRNA expression of two iron transporters (zip14 and fpn1). Intestinal iron overload increased lipogenesis, reduced lipolysis and promoted oxidative stress and mitochondrial dysfunction. Iron-induced lipid accumulation was mediated by hypoxia-inducible factor-1 α (HIF1α), which was induced in response to mitochondrial oxidative stress following inhibition of prolyl hydroxylase 2 (PHD2). Mechanistically, iron promoted lipid deposition by enhancing the DNA binding capacity of HIF1α to the pparγ and fas promoters. Our results provide experimental evidence that oxidative stress, mitochondrial dysfunction and the HIF1α-PPARγ pathway are critical mediators of iron-induced lipid deposition.

Dietary Choline Alleviates High-Fat Diet-Induced Hepatic Lipid Dysregulation via UPRmt Modulated by SIRT3-Mediated mtHSP70 Deacetylation

The mitochondrial unfolded protein response (UPRmt) is known as a conservative mechanism in response to mitochondrial dysfunction. Thus, based on UPRmt, this study was conducted to determine the mechanism of a high-fat diet (HFD) inducing mitochondrial dysfunction and its role in stimulating hepatic lipid dysregulation. The choline-activated alleviating effect was also evaluated. In vivo, yellow catfish were fed three diets (control, HFD, and HFD + choline diet) for 10 weeks. In vitro, hepatocytes isolated from yellow catfish and the HepG2 cell line were cultured and incubated with fatty acid (FA) for 48 h. (1) HFD-induced mitochondrial dysfunction via SIRT3/mtHSP70-mediated UPRmt. HFD inhibited the subcellular localization of SIRT3 into the mitochondrion, resulting in the up-regulating of mtHSP70 acetylation via lysine residues 493 and 507. The mtHSP70 acetylation promoted the stability of mtHSP70, which then led to the UPRmt and further mitochondrial dysfunction. (2) SIRT3/mtHSP70-mediated UPRmt regulated HFD/FA-induced hepatic lipid dysregulation. SIRT3/mtHSP70-mediated UPRmt reduced FA ß-oxidation via mitochondrial dysfunction and then led to lipid dysregulation. Additionally, the mtHSP70-ACOX1 interaction was confirmed. (3) Choline alleviated HFD-induced UPRmt via up-regulating the localization of SIRT3 into the mitochondrion, which in turn led to the subsequent ameliorating effect on HFD-induced hepatic lipid dysregulation. Through SIRT3-mediated mtHSP70 deacetylation, dietary choline alleviates HFD-induced hepatic lipid dysregulation via UPRmt. This provides the first proof of acetylation regulating UPRmt and the crosstalk between UPRmt and FA ß-oxidation.

Docosahexaenoic acid lessens hepatic lipid accumulation and inflammation via the AMP-activated protein kinase and endoplasmic reticulum stress signaling pathways in grass carp (Ctenopharyngodon idella)

The liver is the primary organ for frontline immune defense and lipid metabolism. Excessive lipid accumulation in the liver severely affects its metabolic homeostasis and causes metabolic diseases. Docosahexaenoic acid (DHA) is known for its beneficial effects on lipid metabolism and anti-inflammation, but its molecular mechanism remains unknown, especially in fish. In this study, we evaluated the protective effects of DHA on hepatic steatosis of grass carp (Ctenopharyngodon idella) in vivo and in vitro and mainly focused on the AMP-activated protein kinase (AMPK) and endoplasmic reticulum stress (ER stress) signaling pathway analysis. Grass carp were fed with purified diets supplemented with 0%, 0.5% and 1% DHA for 8 weeks in vivo. 1% DHA supplementation significantly decreased the liver triglyceride (TG), malondialdehyde (MDA), serum tumor necrosis factor α (TNFα) and nuclear factor kappa B (NFκB) contents. DHA administration suppressed ER stress and decreased the mRNA expressions related to hepatic inflammation and lipogenesis, accompanied by the activation of AMPK. Correspondingly, DHA activated the AMPK signaling pathway, and inhibited palmitic acid (PA)-evoked ER stress and lipid accumulation and inflammation of grass carp hepatocytes in vitro. In contrast, the inhibitor of AMPK (compound C, CC) abrogated the effects of DHA to improve PA-induced liver injury and ER stress. In conclusion, DHA inhibits ER stress in hepatocytes by the activation of AMPK and exerts protective effects on hepatic steatosis in terms of improving antioxidant ability, relieving hepatic inflammation and inhibiting hepatic lipogenesis. Our findings give a theoretical foundation for further elucidation of the beneficial role of DHA in vertebrates.

Endoplasmic reticulum stress is involved in lipid accumulation induced by oleic acid in adipocytes of grass carp (Ctenopharyngodon idella): focusing on the transcriptional level

It has been extensively claimed that endoplasmic reticulum stress (ER stress) is related to lipid accumulation in mammals, but little is known in fish. This study aims at elucidating the role of ER stress in mediating lipid accumulation induced by monounsaturated oleic acid (OA) with a focus on the transcriptional level. We treated the adipocytes of grass carp with 200 μM and 400 μM OA, respectively, while the control group was treated with 2% bovine serum albumin (BSA). The results showed that cell viability was significantly improved, while 400 μM OA treatment promoted neutral lipid accumulation along with stimulating ER stress more obviously. Although lipolysis and fatty acid β-oxidation were activated simultaneously, the primary effect of OA seems to be promotion of lipid accumulation. To further explore whether ER stress affects lipid accumulation, 4-phenyl butyric acid (4-PBA), an effective inhibitor of ER stress, was used to pretreat the cells for 4 h. Unsurprisingly, it was found that the mRNA expressions of genes linked with ER stress were decreased. Intracellular triglyceride (TG) content was also decreased, which was in accordance with the mRNA expressions of adipogenic and lipogenic transcription factors as well as their target genes. Collectively, our data shows that ER stress may take part in OA-induced lipid accumulation in adipocytes via activating adipogenesis and lipogenesis. Based on this, strategies for protecting ER could be used to alleviate excessive accumulation of lipid in grass carp adipose tissue.

High-Fat-Diet-Induced Oxidative Stress in Giant Freshwater Prawn (Macrobrachium rosenbergii) via NF-κB/NO Signal Pathway and the Amelioration of Vitamin E

Lipids work as essential energy sources for organisms. However, prawns fed on high-fat diets suffer from oxidative stress, whose potential mechanisms are poorly understood. The present study aimed to explore the regulation mechanism of oxidative stress induced by high fat and the amelioration by vitamin E (VE) of oxidative stress. Macrobrachium rosenbergii were fed with two dietary fat levels (LF 9% and HF 13%) and two VE levels (200 mg/kg and 600 mg/kg) for 8 weeks. The results showed that the HF diet decreased the growth performance, survival rate and antioxidant capacity of M. rosenbergii, as well as inducing hypertrophied lipid droplets, lipophagy and apoptosis. A total of 600 mg/kg of VE in the HF diet alleviated the negative effects induced by HF. In addition, the HF diet suppressed the expression of toll-dorsal and imd-relish signal pathways. After the relish and dorsal pathways were knocked down, the downstream iNOS and NO levels decreased and the MDA level increased. The results indicated that M. rosenbergii fed with a high-fat diet could cause oxidative damage. Its molecular mechanism may be attributed to the fact that high fat suppresses the NF-κB/NO signaling pathway mediating pro-oxidant and antioxidant targets for regulation of oxidative stress. Dietary VE in an HF diet alleviated hepatopancreas oxidative stress and apoptosis.

Dietary Glutamine Inclusion Regulates Immune and Antioxidant System, as Well as Programmed Cell Death in Fish to Protect against Flavobacterium columnare Infection

The susceptibility of animals to pathogenic infection is significantly affected by nutritional status. The present study took yellow catfish (Pelteobagrus fulvidraco) as a model to test the hypothesis that the protective roles of glutamine during bacterial infection are largely related to its regulation on the immune and antioxidant system, apoptosis and autophagy. Dietary glutamine supplementation significantly improved fish growth performance and feed utilization. After a challenge with Flavobacterium columnare, glutamine supplementation promoted il-8 and il-1β expression via NF-κB signaling in the head kidney and spleen, but inhibited the over-inflammation in the gut and gills. Additionally, dietary glutamine inclusion also enhanced the systematic antioxidant capacity. Histological analysis showed the protective role of glutamine in gill structures. Further study indicated that glutamine alleviated apoptosis during bacterial infection, along with the reduced protein levels of caspase-3 and the reduced expression of apoptosis-related genes. Moreover, glutamine also showed an inhibitory role in autophagy which was due to the increased activation of the mTOR signaling pathway. Thus, our study for the first time illustrated the regulatory roles of glutamine in the fish immune and antioxidant system, and reported its inhibitory effects on fish apoptosis and autophagy during bacterial infection.

Phospholipase C signal mediated the glucose-induced changes of glucose absorption and lipid accumulation in the intestinal epithelial cells of yellow catfish Pelteobagrus fulvidraco

In present study, we explored the effects and the underlying mechanisms of phospholipase C (PLC) mediating glucose-induced changes in intestinal glucose transport and lipid metabolism by using U-73122 (a PLC inhibitor). We found that glucose incubation activated the PLC signal and U-73122 pre-incubation alleviated the glucose-induced increase in plcb2, plce1 and plcg1 mRNA expression. Meanwhile, U-73122 pre-treatment blunted the glucose-induced increase in sodium/glucose co-transporters 1/2 mRNA and protein expressions. U-73122 pre-treatment alleviated the glucose-induced increase in TAG content, BODIPY 493/503 fluorescence intensity, lipogenic enzymes (glucose 6-phospate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGD), malic enzyme and fatty acid synthase (FAS)) activity and the mRNA expressions of lipogenic genes and related transcription factors (6pgd, g6pd, fas, acca, srebp1 and carbohydrate response element-binding protein (chrebp)) in intestinal epithelial cells of yellow catfish. Further research found that U-73122 pre-incubation mitigated the glucose-induced increase in the ChREBP protein expression and the acetylation level of ChREBP in HEK293T cells. Taken together, these data demonstrated that the PLC played a major role in the glucose-induced changes of glucose transport and lipid metabolism and provide a new perspective for revealing the molecular mechanism of glucose-induced changes of intestinal glucose absorption, lipid deposition and metabolism.

High carbohydrate diet induced endoplasmic reticulum stress and oxidative stress, promoted inflammation and apoptosis, impaired intestinal barrier of juvenile largemouth bass (Micropterus salmoides)

This study assessed the effects of feed carbohydrate content on intestinal physical barrier and immunity in juvenile largemouth bass (Micropterus salmoides). Triplicate groups of juvenile fish (4.1 ± 0.2 g) were fed low (LCD, 7%), medium (MCD, 12%) and high (HCD, 17%) carbohydrate diets for eight weeks. Gut histology revealed the slight infiltration of inflammatory cells and moderate loss of mucous membrane layer in HCD group. Expression of ZO1, occluding, and claudin7 genes and epidermal growth factor receptor (EGFR) gene were significantly decreased in HCD group indicating the impairment of tight junction and epithelial cell regeneration. The results showed the significant (P < 0.05) reduction of antioxidant capacity in HCD group compared to LCD. Furthermore, expression of intestinal ERS-related genes such as IRE1, Eif2α, GRP78, CHOPα and CHOPβ in HCD group was significantly higher than the LCD group. In addition, HCD induced the up-regulated expression of inflammatory (IL-8, IL-1β, TNFα and COX2) and apoptosis (TRAF2, bax, casepase3, caspase8 and casepase9) related genes in fish intestine. The data generated in this study clearly demonstrated that HCD induced ERS and oxidative stress, which promoted intestinal inflammation and apoptosis in juvenile largemouth bass.

Lipophagy mediated glucose-induced changes of lipid deposition and metabolism via ROS dependent AKT-Beclin1 activation

High dietary carbohydrate intake leads to lipid accumulation in the intestinal tract, but the molecular mechanism remains unknown. In the present study, using yellow catfish (Pelteobagrus fulvidraco) as a model, we found that (1) high carbohydrate diets (HCD) and high glucose (HG) increased lipid deposition, up-regulated lipogenesis and fatty acid β-oxidation, activated autophagy and induced oxidative stress in the intestinal tissues and intestinal epithelial cells (IECs); (2) lipophagy alleviated HG-induced lipid accumulation via the up-regulation of fatty acid β-oxidation; (3) Akt interacted directly with Beclin1; (4) HG suppressed Akt1 phosphorylation, downregulated Akt1-mediated phosphorylation of Beclin1, activated lipophagy and alleviated the increment of TG deposition induced by HG with S87 and S292 being the key phosphorylation residues of Beclin1 in response to HG; (5) ROS generation mediated HG-induced activation of lipophagy and HG-induced suppression of AKT phosphorylation, activated AMPK and alleviated HG-induced increase of TG deposition. Our study provides mechanistic evidence that high carbohydrate- and glucose-induced lipophagy in intestine and IECs is associated with ROS-AKT-Beclin1-dependent activation of autophagy, which alleviates glucose-induced lipid accumulation. Our findings are important since the regulation of autophagy can be used as potential molecular targets for the prevention and treatment of lipotoxicity in the intestine of vertebrates, including humans.

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

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

Endoplasmic Reticulum Stress Disturbs Lipid Homeostasis and Augments Inflammation in the Intestine and Isolated Intestinal Cells of Large Yellow Croaker (Larimichthys crocea)

The small intestine is crucial for lipid homeostasis and immune regulation of the whole body. Endoplasmic reticulum (ER) stress may affect lipid metabolism and inflammation in the intestine, but the potential mechanism is not completely understood. In the present study, intraperitoneal injection of tunicamycin (TM) induced ER stress in the intestine of large yellow croaker (Larimichthys crocea). ER stress induced excessive accumulation of triglyceride (TG) in the intestine by promoting lipid synthesis. However, it also enhanced lipid secretion and fatty acid β-oxidation. In addition, ER stress augmented inflammation in the intestine by promoting p65 into the nucleus and increasing proinflammatory genes expression. In the isolated intestinal cells, the obtained results showed that TM treatment significantly upregulated the mRNA expression of lipid synthesis and inflammatory response genes, which were consistent with those in vivo. Moreover, overexpression of unfolded protein response (UPR) sensors significantly upregulated promoter activities of lipid synthesis and proinflammatory genes. In conclusion, the results suggested that ER stress disturbed lipid metabolism and augmented inflammation in the intestine and isolated intestinal cells of large yellow croaker, which may contribute to finding novel therapies to tackle lipid dysregulation and inflammation in the intestine of fish and human beings.

Low expression of miR-19a-5p is associated with high mRNA expression of diacylglycerol O-acyltransferase 2 (DGAT2) in hybrid tilapia

DGAT2 (acyl CoA:diacylglycerol acyltransferase 2) is a key and rate-limiting enzyme that catalyzes the final step of triglyceride (TG) synthesis. In this study, hybrid tilapia were generated from Nile tilapia (♀) and blue tilapia (♂) crossing. The TG content levels in the liver of these tilapia were measured. The results showed that the TG content was higher in the hybrid tilapia. In addition, protein and mRNA expression levels in the tilapia livers were determined. Higher hepatic mRNA and protein expression of DGAT2 in the hybrid fish was found. A luciferase reporter assay with HEK293T cells revealed that miRNA-19a-5p targeted the 3'UTR of DGAT2, suggesting a direct regulatory mechanism. Using qRT-PCR, we found that DGAT2 mRNA levels had a negative correlation with miRNA-19a-5p expression in Nile tilapia and hybrid. Taken together, these findings provide evidence that miRNA-19a-5p is involved in TG synthesis in the regulation of lipid metabolism in tilapia.

Oxidized fish oils increased lipid deposition via oxidative stress-mediated mitochondrial dysfunction and the CREB1-Bcl2-Beclin1 pathway in the liver tissues and hepatocytes of yellow catfish

At present, the harmful effects and relevant mechanism of oxidized fish oils on fish and fish cells remain unknown. Our study found that oxidized fish oils increased lipogenesis, and reduced lipolysis, activated oxidative stress by decreasing glutathione peroxidase (GPX) activity, increasing malondialdhyde (MDA) content and damaging mitochondrial structure, and activated autophagy in the liver of yellow catfish; oxidized eicosapentaenoic acid (oxEPA) induced oxidative stress in yellow catfish hepatocytes. Oxidative stress, mitochondrial dysfunction and lipophagy mediated oxEPA induced-variations in lipid metabolism. Our further investigation indicated that oxEPA-activated lipophagy was via inhibiting the DNA binding capacity of the cAMP-response element binding protein (CREB)-1 to the region of Bcl-2 promoter, which in turn suppressed the binding activity of Bcl-2 to Beclin1 and promoted autophagosome formation. For the first time, our study elucidated the mechanisms of oxidized fish oils-induced lipid deposition by the oxidative stress, mitochondrial dysfunction and CREB1-Bcl-2-Beclin1 pathway in fish.

Methionine-chelated Zn promotes anabolism by integrating mTOR signal and autophagy pathway in juvenile yellow catfish

BACKGROUND

Amino acid-chelated zinc (Zn) can increase anabolism of animals. However, the underlying mechanisms are unclear. We aimed to examine how autophagy impact anabolism following a diet containing methionine-chelated Zn (ZnMet) compared with inorganic Zn (ZnSO₄).

METHODS

Yellow catfish (weight: 4.02 ± 0.08 g) were fed two diets containing ZnSO₄ or ZnMet for 8 wk. The differences in transcriptional responses and corresponding biological profiles were compared between the livers of fish fed the two Zn sources of diets. Hepatocytes of yellow catfish were incubated for 48 h in medium containing ZnSO₄ (10 μM ZnSO₄) or ZnMet (10 μM ZnMet) after 2 h pretreated with or without pathway inhibitors. Intracellular Zn, TG and protein contents, lipid droplet and autophagic vesicles were detected. Ultrastructural observation, enzymatic activities, qPCR assays, western blot and immunofluorescence analysis were conducted.

RESULTS

ZnMet up-regulated the expression of genes associated with anabolism and autophagy. The differentially expressed genes (DEG) analysis indicated that both mTOR and autophagy pathways were activated. ZnMet-induced activation of autophagy was mTOR-independent. In this process, forkhead box class O was deacetylated and activated, and induced autophagy, which provided substrates for energy generation.

CONCLUSIONS

ZnMet increased anabolism through integrating mTOR signal and autophagy pathway in yellow catfish. The present study unravels a novel mechanism for amino acid-chelated minerals improving anabolism.

Myo-inositol improves growth performance and regulates lipid metabolism of juvenile Chinese mitten crab (Eriocheir sinensis) fed different percentage of lipid

This study evaluated the effects of dietary myo-inositol (MI) on growth performance, antioxidant status and lipid metabolism of juvenile Chinese mitten crab (Eriocheir sinensis) fed different percentage of lipid. Crabs (4·58 (sem 0·05) g) were fed four diets including a normal lipid diet (N, containing 7 % lipid and 0 mg/kg MI), N with MI supplementation (N + MI, containing 7 % lipid and 1600 mg/kg MI), a high lipid diet (H, containing 13 % lipid and 0 mg/kg MI) and H with MI supplementation (H + MI, containing 13 % lipid and 1600 mg/kg MI) for 8 weeks. The H + MI group showed higher weight gain and specific growth rate than those in the H group. The dietary MI could improve the lipid accumulations in the whole body, hepatopancreas and muscle as a result of feeding on the high dietary lipid (13 %) in crabs. Besides, the crabs fed the H + MI diets increased the activities of antioxidant enzymes but reduced the malondialdehyde content in hepatopancreas compared with those fed the H diets. Moreover, dietary MI enhanced the expression of genes involved in lipid oxidation and exportation, yet reduced lipid absorption and synthesis genes expression in the hepatopancreas of crabs fed the H diet, which might be related to the activation of inositol 1,4,5-trisphosphate receptor (IP3R)/calmodulin-dependent protein kinase kinase-β (CaMKKβ)/adenosine 5'-monophosphate-activated protein kinase (AMPK) signalling pathway. This study demonstrates that MI could increase lipid utilisation and reduce lipid deposition in the hepatopancreas of E. sinensis fed a high lipid diet through IP3R/CaMKKβ/AMPK activation. This work provides new insights into the function of MI in the diet of crustaceans.

Dietary Marginal and Excess Selenium Increased Triglycerides Deposition, Induced Endoplasmic Reticulum Stress and Differentially Influenced Selenoproteins Expression in the Anterior and Middle Intestines of Yellow Catfish Pelteobagrus fulvidraco

Selenium (Se) is an essential micro-mineral and plays important roles in antioxidant responses, and also influences lipid metabolism and selenoprotein expression in vertebrates, but the effects and mechanism remain unknown. The study was undertaken to decipher the insights into dietary Se influencing lipid metabolism and selenoprotein expression in the anterior and middle intestine (AI and MI) of yellow catfish Pelteobagrus fulvidraco. Yellow catfish (weight: 8.27 ± 0.03 g) were fed a 0.03- (M-Se), 0.25- (A-Se), or 6.39- (E-Se) mg Se/kg diet for 12 wk. AI and MI were analyzed for triglycerides (TGs) and Se concentrations, histochemistry and immunofluorescence, enzyme activities, and gene and protein levelsassociated with antioxidant responses, lipid metabolism, endoplasmic reticulum (ER) stress, and selenoproteome. Compared to the A-Se group, M-Se and E-Se diets significantly decreased weight gain (WG) and increased TGs concentration in the AI and MI. In the AI, compared with A-Se group, M-Se and E-Se diets significantly increased activities of fatty acid synthase, expression of lipogenic genes, and suppressed lipolysis. In the MI, compared to the A-Se group, M-Se and E-Se diets significantly increased activities of lipogenesis and expression of lipogenic genes. Compared with A-Se group, E-Se diet significantly increased glutathione peroxidase (GPX) activities in the AI and MI, and M-Se diet did not significantly reduce GPX activities in the AI and MI. Compared with the A- Se group, E-Se diet significantly increased glutathione peroxidase (GPX) activities in the plasma and liver, and M-Se diet significantly reduced GPX activities in the plasma and liver. Compared with the A-Se group, M-Se and E-Se groups also increased glucose-regulated protein 78 (GRP78, ER stress marker) protein expression of the intestine. Dietary Se supplementation also differentially influenced the expression of the 28 selenoproteins in the AI and MI, many of which possessed antioxidant characteristics. Compared with the A-Se group, the M-Se group significantly decreased mRNA levels of txnrd2 and txnrd3, but made no difference on mRNA levels of these seven GPX proteins in the MI. Moreover, we characterized sterol regulatory element binding protein 1c (SREBP1c) binding sites of three ER-resident proteins (selenom, selenon, and selenos) promoters, and found that Se positively controlled selenom, selenon, and selenos expression via SREBP1c binding to the selenom, selenon, and selenos promoter. Thus, dietary marginal and excess Se increased TGs deposition of yellow catfish P. fulvidraco, which might be mediated by ER-resident selenoproteins expression and ER stress.

Waterborne copper exposure up-regulated lipid deposition through the methylation of GRP78 and PGC1α of grass carp Ctenopharyngodon idella

Early molecular events after the exposure of heavy metals, such as aberrant DNA methylation, suggest that DNA methylation was important in regulating physiological processes for animals and accordingly could be used as environmental biomarkers. In the present study, we found that copper (Cu) exposure increased lipid content and induced the DNA hypermethylation at the whole genome level. Especially, Cu induced hypermethylation of glucose-regulated protein 78 (grp78) and peroxisome proliferator-activated receptor gamma coactivator-1α (pgc1α). CCAAT/enhancer binding protein α (C/EBPα) could bind to the methylated sequence of grp78, whereas C/EBPβ could not bind to the methylated sequence of grp78. These synergistically influenced grp78 expression and increased lipogenesis. In contrast, DNA methylation of PGC1α blocked the specific protein 1 (SP1) binding and interfered mitochondrial function. Moreover, Cu increased reactive oxygen species (ROS) production, activated endoplasmic reticulum (ER) stress and damaged mitochondrial function, and accordingly increased lipid deposition. Notably, we found a new toxicological mechanism for Cu-induced lipid deposition at DNA methylation level. The measurement of DNA methylation facilitated the use of these epigenetic biomarkers for the evaluation of environmental risk.

Dietary Glucose Increases Glucose Absorption and Lipid Deposition via SGLT1/2 Signaling and Acetylated ChREBP in the Intestine and Isolated Intestinal Epithelial Cells of Yellow Catfish

BACKGROUND

Dietary carbohydrate affects intestinal glucose absorption and lipid deposition, but the underlying mechanisms are unknown.

OBJECTIVES

We used yellow catfish and their isolated intestinal epithelial cells (IECs) to test the hypothesis that sodium/glucose cotransporters (SGLTs) 1/2 and acetylated carbohydrate response element binding protein (ChREBP) mediated glucose-induced changes in glucose absorption and lipid metabolism.

METHODS

Yellow catfish (mean ± SEM weight: 4.68 ± 0.02 g, 3 mo old, mixed sex) were fed diets containing 250 g carbohydrates/kg from glucose (G, control), corn starch (CS), sucrose (S), potato starch (PS), or dextrin (D) for 10 wk. IECs were isolated from different yellow catfish and incubated for 24 h in a control or glucose (15 mM) solution with or without a 2-h pretreatment with an inhibitor [sotagliflozin (LX-4211) or tubastatin A (TBSA)]. Human embryonic kidney cells (HEK293T cells) were transfected with a Flag-ChREBP plasmid to explore ChREBP acetylation. Triglyceride (TG) and glucose concentrations and enzymatic activities were measured in the intestine and IECs of yellow catfish. They also were subjected to immunofluorescence, immunoprecipitation, qPCR, and immunoblotting. Immunoblotting and immunoprecipitation were performed with HEK293T cells.

RESULTS

The G group had greater intestine TGs (0.99- to 2.30-fold); activities of glucose 6-phospate dehydrogenase, 6-phosphogluconate dehydrogenase, and isocitrate dehydrogenase (0.12- to 2.10-fold); and expression of lipogenic genes (0.32- to 2.34-fold) than the CS, PS, and D groups. The G group had greater intestine sglt1/2 mRNA and protein expression than the CS, S and D groups (0.35- to 1.12-fold and 0.40- to 4.67-fold, respectively), but lower mRNA amounts of lipolytic genes (48.6%-65.8%) than the CS and PS groups. LX-4211 alleviated the glucose-induced increase in sglt1/2 mRNA (38.2%-47.4%) and SGLT1 protein (48.0%) expression, TGs (29.3%), and lipogenic enzyme activities (27.7%-42.1%) and gene expression (38.0%-55.5%) in the IECs. TBSA promoted the glucose-induced increase in TGs (11.3%), fatty acid synthase activity (32.6%), and lipogenic gene expression (21.6%-34.4%) in the IECs and acetylated ChREBP (10.5%) in HEK293T cells.

CONCLUSIONS

SGLT1/2 signaling and acetylated ChREBP mediated glucose-induced changes in glucose absorption and lipid metabolism in the intestine and IECs of yellow catfish.

Interplay Between Lipid Metabolism and Autophagy

Autophagy is a self-eating process of using lysosomes to degrade macromolecular substances (e.g., proteins and organelles) that are damaged, degenerated, or aging. Lipid metabolism is the synthesis and degradation of lipids (e.g., triglycerides, steroids, and phospholipids) to generate energy or produce the structural components of cell membranes. There is a complex interplay between lipid metabolism (e.g., digestion, absorption, catabolism, biosynthesis, and peroxidation) and autophagy machinery, leading to the modulation of cell homeostasis, including cell survival and death. In particular, lipid metabolism is involved in the formation of autophagic membrane structures (e.g., phagophores and autophagosomes) during stress. Moreover, autophagy, especially selective autophagy (e.g., lipophagy, ferritinophagy, clockophagy, and mitophagy), promotes lipid catabolism or lipid peroxidation-induced ferroptosis through the degradation of various substances within the cell. A better understanding of the mechanisms of autophagy and possible links to lipid metabolism will undoubtedly promote potential treatments for a variety of diseases.

miR-144 Mediates High Fat-Induced Changes of Cholesterol Metabolism via Direct Regulation of C/EBPα in the Liver and Isolated Hepatocytes of Yellow Catfish

BACKGROUND

microRNAs (miRNAs) post-transcriptionally regulate gene expression and act as important modulators of cholesterol homeostasis.

OBJECTIVE

The study explores the mechanism by which miRNAs mediate high fat-induced changes of cholesterol metabolism in yellow catfish.

METHODS

Yellow catfish (weight: 3.79 ± 0.16 g, 3 mo old, mixed sex) were fed 2 diets containing lipids at 11.3% [control (CON)] or 15.4% [high-fat diet (HFD)] (by weight) for 8 wk. Cholesterol content was measured; hematoxylin-eosin (H&E) staining, qPCR assays, and small RNA sequencing were conducted in the liver. Hepatocytes were isolated from separate, untreated fish and incubated for 24 h in control solution or palmitic acid (PA; 0.25 mM)/oleic acid (OA; 0.5 mM) after 4 h pretreatment with or without miR-144 inhibitor/mimic (40 nM). Cholesterol content was measured; qPCR assays and Western blotting were conducted in the hepatocytes. HEK293T cells were co-transfected with plasmids to validate miR-144 target genes. The promoter activities of miR-144 were analyzed in HEK293T cells with PA (0.25 mM) or OA (0.25 or 0.5 mM) treatment for 24 h. Luciferase activity assays, electrophoretic mobility shift assay, and Western blotting were conducted in HEK293T cells.

RESULTS

Compared with CON, HFD induced hepatic cholesterol accumulation (31.5%), and upregulated miR-144 expression (8.40-fold, P < 0.05). miR-144 directly targeted hydroxymethylglutaryl-CoA reductase (hmgcr), cholesterol 7α-monooxygenase A1 (cyp7a1), and adenosine triphosphate binding cassette transporter A1 (abca1) in HEK293T cells. In the hepatocytes of yellow catfish, miR-144 inversely regulated the expression of hmgcr, cyp7a1, and abca1 (30.3-78.5%, P < 0.05); loss of miR-144 function alleviated PA- or OA-induced cholesterol accumulation (19.5-61.1%, P < 0.05). We also characterized the C/EBPα binding site in the miR-144 promoter, and found that C/EBPα positively regulated miR-144 expression through binding to the miR-144 promoter.

CONCLUSIONS

miR-144 mediated HFD-induced changes in the liver and hepatocytes of yellow catfish, suggesting a possible mechanism for HFD-induced dysfunction in cholesterol metabolism.

Creb-Pgc1α pathway modulates the interaction between lipid droplets and mitochondria and influences high fat diet-induced changes of lipid metabolism in the liver and isolated hepatocytes of yellow catfish

Although the crucial role of lipid droplets (LDs), mitochondria (MT) and their interactions in regulating lipid metabolism are well accepted, the mechanism of LDs-MT interactions in high fat diet (HFD)-induced changes of lipid metabolism remains unknown. Thus, this study was conducted to determine the mechanism of LDs-MT interactions in HFD-induced changes of lipid accumulation. We found that HFD not only up-regulated the expression of key proteins linked with TAG biosynthesis, but also increased the expression of proteins involved in lipolysis and fatty acid (FA) oxidation in LDs, including Rab32 (the only Rab protein associated with the MT). FA-induced LDs accumulation coincided with increased mitochondrial biogenesis, suggesting the potential LDs-MT interaction in hepatocytes after FA incubation. Also, FA incubation markedly increased the localization of Rab32 into LDs and MT, which confirmed the LDs-MT interaction and indicated the involvement of Rab32 in LDs-MT interaction following FA incubation. Inhibitors of Creb-Pgc1α pathway significantly blocked the localization of Rab32 into LDs and MT, and significantly reduced FA-induced LDs lipolysis by targeting Atgl and Plin5. Meanwhile, the FA-enhanced LDs accumulation, and mitochondrial biogenesis, fusion and oxidation were also significantly repressed. These indicated the regulatory role of Creb-Pgc1α in Rab32-mediated LDs-MT interactions and lipolysis after FA incubation. Taken together, these results revealed a novel mechanism of HFD- and FA-induced LDs-MT interactions in regulating hepatic LDs lipolysis, which provided new insight into the crosstalk between LDs-MT interaction and their potential role in HFD-induced hepatic steatosis.

Effects of Fat and Fatty Acids on the Formation of Autolysosomes in the Livers from Yellow Catfish Pelteobagrus Fulvidraco

The autophagy-lysosome pathway, which involves many crucial genes and proteins, plays crucial roles in the maintenance of intracellular homeostasis by the degradation of damaged components. At present, some of these genes and proteins have been identified but their specific functions are largely unknown. This study was performed to clone and characterize the full-length cDNA sequences of nine key autolysosome-related genes (vps11, vps16, vps18, vps33b, vps41, lamp1, mcoln1, ctsd1 and tfeb) from yellow catfish Pelteobagrus fulvidraco. The expression of these genes and the transcriptional responses to a high-fat diet and fatty acids (FAs) (palmitic acid (PA) and oleic acid (OA)) were investigated. The mRNAs of these genes could be detected in heart, liver, muscle, spleen, brain, mesenteric adipose tissue, intestine, kidney and ovary, but varied with the tissues. In the liver, the mRNA levels of the nine autolysosome-related genes were lower in fish fed a high-fat diet than those fed the control, indicating that a high-fat diet inhibited formation of autolysosomes. Palmitic acid (a saturated FA) significantly inhibited the formation of autolysosomes at 12 h, 24 h and 48 h incubation. In contrast, oleic acid (an unsaturated FA) significantly induced the formation of autolysosomes at 12 h, but inhibited them at 24 h. At 48 h, the effects of OA incubation on autolysosomes were OA concentration-dependent in primary hepatocytes of P. fulvidraco. The results of flow cytometry and laser confocal observations confirmed these results. PA and OA incubation also increased intracellular non-esterified fatty acid (NEFA) concentration at 12 h, 24 h and 48 h, and influenced mRNA levels of fatty acid binding protein (fabp) and fatty acid transport protein 4 (fatp4) which facilitate FA transport in primary hepatocytes of P. fulvidraco. The present study demonstrated the molecular characterization of the nine autolysosome-related genes and their transcriptional responses to fat and FAs in fish, which provides the basis for further exploring their regulatory mechanism in vertebrates.