SREBP-1 and LXRα pathways mediated Cu-induced hepatic lipid metabolism in zebrafish Danio rerio.

The present study was performed to explore the underlying molecular mechanism of Cu-induced disorder of lipid metabolism in fish. To this end, adult zebrafish were exposed to three waterborne Cu concentrations (0 (control), 8 and 16 µg Cu/L, respectively) for 60 days. Hepatic Cu content and hepatosomatic index increased after waterborne Cu exposure. H&E and oil red O stainings showed extensive steatosis in the liver of Cu-exposed fish. Cu exposure up-regulated lipogenic enzymes activities of ME, ICDH, 6PGD, G6PD and FAS, but down-regulated CPTI activities. Transcriptomic analysis indicated that lipid metabolism related pathways were significantly enriched in both low-dose and high-dose Cu exposure group. Genes involved in lipogenic process from fatty acid biosynthesis, fatty acid elongation, fatty acid desaturation to glycerolipid biosynthesis were up-regulated by Cu. To elucidate the mechanism, LXRα inhibitor SR9243 and SREBP1 inhibitor fatostatin were used to verify the role of LXRα and SREBP1 in Cu-induced disorder of lipid metabolism. Both SR9243 and fatostatin significantly attenuated the Cu-induced increase of TG accumulation of hepatocytes. Meanwhile, SR9243 significantly attenuated the Cu-induced up-regulation of expression of lipogenic genes (acaca, fas, icdh, dgat1, moat2 and moat3), and fatostatin significantly attenuated the up-regulation of expression of acaca, fas, g6pd, dgat1 and moat2. Enzymes analysis showed both SR9243 and fatostatin blocked the Cu-induced increase of lipogenic enzymes activities. Taken together, our findings highlight the importance of LXRα and SREBP1 in Cu-induced hepatic lipid deposition, which proposed a novel mechanism for elucidating metal element exposure inducing the disorder of lipid metabolism in aquatic vertebrates.

CREB element is essential for unfolded protein response (UPR) mediating the Cu-induced changes of hepatic lipogenic metabolism in Chinese yellow catfish (Pelteobagrus fulvidraco).

The present study was conducted to explore the underlying mechanism of unfolded protein response (UPR) mediating the Cu-induced changes of hepatic lipogenic metabolism in a low vertebrate, freshwater teleost yellow catfish Pelteobagrus fulvidraco. To this end, three experiments were conducted. In Exp. 1, we cloned the regions of grp78, perk, ire-1α and atf-6α promoters, and found that multiple cAMP-response element binding protein (CREB) binding sites were identified in their promoter regions. Furthermore, these CREB binding sites played crucial role in transcriptional regulation of UPR. In Exp. 2, the involvement of perk, ire-1α and atf-6α in Cu-induced changes of hepatic lipid metabolism was confirmed by specific miRNA. In Exp. 3, the regulatory mechanism of CREB underlying UPR mediating Cu-induced hepatic lipogenic metabolism were investigated. Cu induced UPR via the activation of CREB binding sites in the promoter regions of grp78, perk, ire-1α and atf-6α. In addition, the inhibition of CREB markedly attenuated the Cu-induced up-regulation of hepatic lipogenic metabolism in hepatocytes. This conclusion was further supported by the results from the trial of CREB over-expression. Taken together, the present study indicated that CREB was essential for UPR mediating Cu-induced lipogenic metabolism, supporting a mechanistic link among CREB, UPR and Cu-induced changes of lipid metabolism.

Oxidative stress and mitochondrial dysfunction mediated Cd-induced hepatic lipid accumulation in zebrafish Danio rerio.

The present study was performed to determine the effect of waterborne CdCl2 exposure influencing lipid deposition and metabolism, oxidative stress and mitochondrial dysfunction, and explore the underlying molecular mechanism of cadmium (Cd)-induced disorder of hepatic lipid metabolism in fish. To this end, adult zebrafish were exposed to three waterborne CdCl2 concentrations (0(control), 5 and 25 µg Cd/l, respectively) for 30 days. Lipid accumulation, the activities of enzymes related to lipid metabolism and oxidative stress, as well as the expression level of genes involved in lipid metabolism and mitophagy were determined in the liver of zebrafish. Waterborne CdCl2 exposure increased hepatic triglyceride (TG) and Cd accumulation, the activities of fatty acid synthase (FAS), 6-phosphogluconate dehydrogenase (6PGD), glucose 6-phosphate dehydrogenase (G6PD) and malic enzyme (ME), and the mRNA level of fatty acid synthase (fas), acetyl-CoA carboxylase alpha (acaca), glucose 6-phosphate dehydrogenase (g6pd) and malic enzyme (me), but reduced the mRNA level of carnitine palmitoyl transferase 1 (cpt1), hormone-sensitive lipase alpha (hsla), and adipose triacylglyceride lipase (atgl). The activities of superoxide dismutase (SOD), glutathoinine peroxidase (GPx) and cytochrome c oxidase (COX) and the ATP level were significantly reduced after CdCl2 exposure. CdCl2 exposure significantly increased the mRNA level of genes (microtubule-associated protein light chain 3 alpha (lc3a), PTEN-induced putative kinase 1 (pink1), NIP3-like protein X (nix) and PARKIN (parkin)) related to mitophagy. To elucidate the mechanism, reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) and the mitochondrial permeability transition (MPT) inhibitor cyclosporine A (CsA) were used to verify the role of ROS and mitochondrial dysfunction in Cd-induced disorder of lipid metabolism. NAC pretreatment reversed the Cd-induced up-regulation of TG accumulation and activities of lipogenic enzymes, and the Cd-induced down-regulation of mRNA levels of lipolytic genes. Meanwhile, NAC pretreatment also blocked the mitochondrial membrane potential (MMP) collapse and decreased the ATP level, suggesting that ROS played a crucial role in regulating the Cd-induced mitochondrial dysfunction. Taken together, our findings, for the first time, highlight the importance of the oxidative stress and mitochondrial dysfunction in Cd-induced disorder of hepatic lipid metabolism, which proposed a novel mechanism for elucidating metal element exposure inducing the disorder of lipid metabolism in vertebrates.

Dietary fenofibrate reduces hepatic lipid deposition by regulating lipid metabolism in yellow catfish Pelteobagrus fulvidraco exposed to waterborne Zn.

Fenofibrate is known to possess lipid-lowering effects by regulation of gene transcription involved in lipid metabolism. Waterborne Zn exposure induces lipid deposition in yellow catfish Pelteobagrus fulvidraco. Thus, the present working hypothesis is that dietary fenofibrate addition will reduce hepatic lipids in yellow catfish exposed to waterborne Zn. To this end, juvenile yellow catfish were exposed to 0.04 (control), 0.35 mg/L waterborne Zn, 0.15% dietary fenofibrate, and 0.35 mg Zn/l + 0.15% dietary fenofibrate for 8 weeks. Growth performance, lipid deposition and metabolism in the liver were determined. Dietary fenofibrate promoted growth performance and reduced hepatic lipid content of yellow catfish exposed to waterborne Zn. However, these effects did not appear in fish in normal water. The lipid-lowering effect of fenofibrate on fish exposed to waterborne Zn was associated with increased lipolysis, as indicated by increased CPT I activities and expression of lipolytic genes PPARα, CPT IA, ATGL and HSL, and with reduced lipogenesis as indicated by reduced activities of G6PD, 6PGD, ME and ICDH. Dietary fenofibrate significantly increased mRNA levels of FAS, LPL and ACCα, but reduced mRNA levels of ACCß and PPARγ in fish exposed to waterborne Zn. Pearson correlations between transcriptional factors expression, and activities and expression of several enzymes were observed, indicating that changes at the molecular and enzymatic levels may underlie the patterns of lipid metabolism and accordingly affect hepatic fat storage. Taken together, our results suggest that the lipid-lowering effect of fenofibrate was attributed, in part, to the down-regulation of lipogenesis and up-regulation of fatty acid oxidation.

Different effect of dietborne and waterborne Zn exposure on lipid deposition and metabolism in juvenile yellow catfish Pelteobagrus fulvidraco.

Juvenile yellow catfish Pelteobagrus fulvidraco were exposed to 0.04 or 0.35 mg l(-1) waterborne Zn, 27.25 or 213.84 mg kg(-1) dietary Zn, singly or in combination for 42 days. Growth and lipid metabolism in juvenile yellow catfish were investigated. Growth and survival were significantly inhibited by single waterborne Zn exposure but not by dietary Zn exposure. Dietary Zn addition reduced but waterborne Zn exposure increased hepatic lipid content. In contrast, muscle lipid content was reduced by waterborne Zn exposure but not by dietborne Zn exposure. The single exposure also affected several lipogenic enzymatic activities and expression of genes (in this article gene expression is taken synonymous to mRNA expression) related to lipogenesis and lipolysis. Pearson correlations among lipid content, enzymatic activities and mRNA expression levels were also observed, suggesting that changes at molecular and enzymatic levels may underlie the patterns of lipid metabolism and accordingly affect lipid deposition. For the first time, our study demonstrates the differential effect of different Zn exposure pathways on lipid metabolism at the molecular level in fish, indicating that the exposure route is critical to lipid deposition and metabolism.

Dietary L-carnitine supplementation increases lipid deposition in the liver and muscle of yellow catfish (Pelteobagrus fulvidraco) through changes in lipid metabolism.

Carnitine has been reported to improve growth performance and reduce body lipid content in fish. Thus, we hypothesised that carnitine supplementation can improve growth performance and reduce lipid content in the liver and muscle of yellow catfish (Pelteobagrus fulvidraco), a commonly cultured freshwater fish in inland China, and tested this hypothesis in the present study. Diets containing l-carnitine at three different concentrations of 47 mg/kg (control, without extra carnitine addition), 331 mg/kg (low carnitine) and 3495 mg/kg (high carnitine) diet were fed to yellow catfish for 8 weeks. The low-carnitine diet significantly improved weight gain (WG) and reduced the feed conversion ratio (FCR). In contrast, the high-carnitine diet did not affect WG and FCR. Compared with the control diet, the low-carnitine and high-carnitine diets increased lipid and carnitine contents in the liver and muscle. The increased lipid content in the liver could be attributed to the up-regulation of the mRNA levels of SREBP, PPARγ, fatty acid synthase (FAS) and ACCa and the increased activities of lipogenic enzymes (such as FAS, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme) and to the down-regulation of the mRNA levels of the lipolytic gene CPT1A. The increased lipid content in muscle could be attributed to the down-regulation of the mRNA levels of the lipolytic genes CPT1A and ATGL and the increased activity of lipoprotein lipase. In conclusion, in contrast to our hypothesis, dietary carnitine supplementation increased body lipid content in yellow catfish.