Effects of linoleate on cell viability and lipid metabolic homeostasis in goose primary hepatocytes

Studies have shown linoleate could not only promote cell viability but also affect lipid metabolism in mammals. However, to what degree these effects are mediated by steatosis in goose primary hepatocytes is unknown. In this study, the effect of linoleate on the lipid metabolic homeostasis pathway was determined. We measured the mRNA levels of genes involved in triglyceride synthesis, lipid deposition, β-oxidation, and assembly and secretion of VLDL-TGs in goose (Anser cygnoides) primary hepatocytes. Linoleate significantly increased goose hepatocyte viability, and linoleate at 0.125 mM, 0.25 mM, 0.5 mM and 1.0 mM all showed a significant effect on TG accumulation. However, with increasing linoleate concentrations, the extracellular TG concentration and extracellular VLDL gradually decreased. DGAT1, DGAT2, PPARα, PPARγ, FoxO1, MTP, PLIN and CPT-1 mRNA was detected by real-time PCR. With increasing linoleate concentrations, the changes in DGAT1, DGAT2, PPARα and CPT-1 gene expression, which regulates hepatic TG synthesis and fatty acid oxidation, first increased and then decreased. Additionally, FoxO1 and MTP gene expression was reduced with increasing linoleate concentrations, and the change in PLIN gene expression was increased at all concentrations, similar to the regulation of intracellular TG accumulation. In conclusion, linoleate regulated TG accumulation and increased hepatocyte viability. The data suggest that linoleate does promote goose hepatocyte viability and steatosis, which may up-regulate TG synthesis-relevant gene expression, suppress assembly and secretion of VLDL-TGs, and increase fatty acid oxidation properly to function of goose primary hepatocytes.     

Construction of a recombinant human FGF1 expression vector for mammary gland-specific expression in human breast cancer cells

Studies have shown that not only does palmitic acid promote triglyceride (TG) accumulation, but it also affects cell viability in in vitro steatosis models. However, to what degree these effects are mediated by steatosis in goose primary hepatocytes is unknown. In this study, the effects of palmitic acid on the lipid metabolism homeostasis pathway and on apoptosis were determined. The authors measured the mRNA levels of genes involved in TG synthesis, lipid deposition, fatty acid oxidation and the assembly and secretion of VLDL-TG in goose primary hepatocytes. The results indicated that palmitic acid can significantly reduce the activity of goose hepatocytes, and that palmitic acid had a significant effect on TG accumulation; however, with increasing palmitic acid concentrations, the extracellular TG and extracellular VLDL concentration gradually decreased. With increasing palmitic acid concentrations, the gene expression levels of DGAT1, DGAT2, PPARα, CPT-1, FoxO1 and MTTP (which regulate hepatic TG synthesis, fatty acid oxidation and the assembly and secretion of VLDL-TGs) first increased and then decreased; the change in PLIN gene expression was palmitic acid dose-dependent, similar to the regulatory mode of intracellular TG accumulation. In conclusion, this study clearly shows that palmitic acid can promote TG accumulation and induce apoptosis in goose primary hepatocytes, and this effect may be related to the lipid metabolism pathway.     

Molecular cloning of the goose ACSL3 and ACSL5 coding domain sequences and their expression characteristics during goose fatty liver development

It has been demonstrated that ACSL3 and ACSL5 play important roles in fat metabolism. To investigate the primary functions of ACSL3 and ACSL5 and to evaluate their expression levels during goose fatty liver development, we cloned the ACSL3 and ACSL5 coding domain sequences (CDSs) of geese using RT-PCR and analyzed their expression characteristics under different conditions using qRT-PCR. The results showed that the goose ACSL3 (JX511975) and ACSL5 (JX511976) sequences have high similarities with the chicken sequences both at the nucleotide and amino acid levels. Both ACSL3 and ACSL5 have high expression levels in goose liver. The expression levels of ACSL3 and ACSL5 in goose liver and hepatocytes can be changed by overfeeding geese and by treatment with unsaturated fatty acids, respectively. Together, these results indicate that ACSL3 and ACSL5 play important roles during fatty liver development. The different expression characteristics of goose ACSL3 and ACSL5 suggest that these two genes may be responsible for specific functions.     

A human hepatocellular in vitro model to investigate steatosis

The present study was designed to define an experimental model of hepatocellular steatosis with a fat overaccumulation profile in which the metabolic and cytotoxic/apoptotic effects could be separated. This was accomplished by defining the experimental conditions of lipid exposure that lead to significant intracellular fat accumulation in the absence of overt cytotoxicity, therefore allowing to differentiate between cytotoxic and apoptotic effects. Palmitic (C16:0) and oleic (C18:1) acids are the most abundant fatty acids (FFAs) in liver triglycerides in both normal subjects and patients with nonalcoholic fatty liver disease (NAFLD). Therefore, human hepatocytes and HepG2 cells were incubated with a mixture of different proportions of saturated (palmitate) and unsaturated (oleate) FFAs to induce fat-overloading. Similar intracellular levels of lipid accumulation as in the human steatotic liver were achieved. Individual FFAs have a distinct inherent toxic potential. Fat accumulation, cytotoxicity and apoptosis in cells exposed to the FFA mixtures were investigated. The FFA mixture containing a low proportion of palmitic acid (oleate/palmitate, 2:1 ratio) is associated with minor toxic and apoptotic effects, thus representing a cellular model of steatosis that mimics benign chronic steatosis. On the other hand, a high proportion of palmitic acid (oleate/palmitate, 0:3 ratio) might represent a cellular model of steatosis in which saturated FFAs promote an acute harmful effect of fat overaccumulation in the liver. These hepatic cellular models are apparently suitable to experimentally investigate the impact of fat overaccumulation in the liver excluding other factors that could influence hepatocyte behaviour.     

Bromide alleviates fatty acid‐induced lipid accumulation in mouse primary hepatocytes through the activation of PPARα signals

Increased plasma free fatty acids (FFAs) and liver triglyceride (TG) accumulations have been implicated in the pathogenesis of hepatic steatosis. On the other hand, trace elements function as essential cofactors that are involved in various biochemical processes in mammals, including metabolic homeostasis. Notably, clinical and animal studies suggest that the plasma levels of bromide negatively correlate with those of TG, total cholesterol (TC) and high‐density lipoprotein‐cholesterol (HDL‐C). However, the effect of bromide on lipid accumulation and the direct molecular target responsible for its action remains unknown. Oil red O (ORO) and Nile red staining were used to detect the effect of bromide on lipid accumulation in mouse primary hepatocytes (PHs) treated with different doses of sodium bromide (NaBr) in the presence of FFAs (0.4 mM oleate/palmitic acid 1:1). Spectrophotometric and fluorometric analyses were performed to assess cellular TG concentrations and rates of fatty acid oxidation (FAO), respectively, in mouse PHs. We found that bromide decreased FFA‐induced lipid accumulation and increased FFA‐inhibited oxygen consumptions in mouse PHs in a dose‐dependent manner via activation of PPARα. Mechanical studies demonstrated that bromide decreased the phosphorylation levels of JNK. More importantly, the PPARα‐specific inhibitor GW6471 partially abolished the beneficial effects of bromide on mouse PHs. Bromide alleviates FFA‐induced excessive lipid storage and increases rates of FAO through the activation of PPARα/JNK signals in mouse PHs. Therefore, bromide may serve as a novel drug in the treatment of hepatic steatosis.     

Lipid-induced up-regulation of human acyl-CoA synthetase 5 promotes hepatocellular apoptosis

In the pathogenesis of nonalcoholic fatty liver disease, accumulation of lipids in hepatocytes and hepatocyte apoptosis are strongly implicated in disease progression from the potentially reversible condition of steatosis to severe acute and chronic liver injury. Acyl-CoA synthetase 5, a member of the ACSL gene family that catalyzes the activation of long-chain fatty acids for lipid biosynthesis, is the only ACSL isoform that is both, located on mitochondria and functionally involved in enterocyte apoptosis. In this study, the regulation of human ACSL5 in hepatocellular fatty acid degeneration and its involvement in hepatocyte apoptosis was investigated using models of in vitro and in vivo steatosis as well as plasmid-mediated stable gene transfer and RNAi-mediated gene silencing. ACSL5 mRNA and protein were strongly increased by uptake of dietary derived fatty acids in primary human hepatocytes, HepG2 cells and human steatotic liver. Over-expression of ACSL5 decreased HepG2 cell viability and increased susceptibility to TRAIL- and TNFα-, but not FAS- induced apoptosis, whereas knock down of ACSL5 reduced apoptosis susceptibility. High ACSL5 activity resulted in enhanced caspase-3/7 activity, but was not accompanied by up-regulation of death receptors, DR4, DR5 or TNF-R1. This study gives evidence that hepatocyte steatosis is associated with ACSL5 up-regulation resulting in increased susceptibility to hepatic cell death. We propose that ACSL5 could play a role in promoting fatty acid-induced lipoapoptosis in hepatocytes as important mechanism in fatty liver-related disorders.     

The role of LXRα in goose primary hepatocyte lipogenesis

In this study, we investigate the role of liver X receptor alpha (LXR alpha) in lipogenesis in geese in order to understand the differences in hepatic steatosis mechanisms between mammals and waterfowl. Primary goose hepatocytes were isolated and treated with the LXR alpha agonist T0901317. Triglyceride (TG) accumulation, acetyl-CoA carboxylase alpha (ACC alpha) and fatty acid synthase (FAS) activities, and gene expression levels of LXR alpha, sterol regulatory element-binding proteins-1 (SREBP-1), FAS, ACC alpha and lipoprotein lipase (LPL) were measured in primary hepatocytes. We found a dose-dependent up-regulation of TG accumulation, ACC, and FAS activities and the mRNA levels of LXR alpha, SREBP-1, FAS, ACC alpha, and LPL genes in the presence of To-901317. We also found that binding of nuclear SREBP-1 to ACC alpha SRE sequence was induced by To-901317 (P < 0.05). In conclusion, LXR alpha is involved in the induction of the lipogenic pathway through activation of SREBP-1 and its target genes in goose primary hepatocytes.     

Astaxanthin reduces hepatic lipid accumulations in high-fat-fed C57BL/6J mice via activation of peroxisome proliferator-activated receptor (PPAR) alpha and inhibition of PPAR gamma and Akt

We have previously reported that astaxanthin (AX), a dietary carotenoid, directly interacts with peroxisome proliferator-activated receptors PPARα and PPARγ, activating PPARα while inhibiting PPARγ, and thus reduces lipid accumulation in hepatocytes in vitro. To investigate the effects of AX in vivo, high-fat diet (HFD)-fed C57BL/6J mice were orally administered AX (6 or 30 mg/kg body weight) or vehicle for 8 weeks. AX significantly reduced the levels of triglyceride both in plasma and in liver compared with the control HFD mice. AX significantly improved liver histology and thus reduced both steatosis and inflammation scores of livers with hematoxylin and eosin staining. The number of inflammatory macrophages and Kupffer cells were reduced in livers by AX administration assessed with F4/80 staining. Hepatic PPARα-responsive genes involved in fatty acid uptake and β-oxidation were upregulated, whereas inflammatory genes were downregulated by AX administration. In vitro radiolabeled assays revealed that hepatic fatty acid oxidation was induced by AX administration, whereas fatty acid synthesis was not changed in hepatocytes. In mechanism studies, AX inhibited Akt activity and thus decreased SREBP1 phosphorylation and induced Insig-2a expression, both of which delayed nuclear translocation of SREBP1 and subsequent hepatic lipogenesis. Additionally, inhibition of the Akt-mTORC1 signaling axis by AX stimulated hepatic autophagy that could promote degradation of lipid droplets. These suggest that AX lowers hepatic lipid accumulation in HFD-fed mice via multiple mechanisms. In addition to the previously reported differential regulation of PPARα and PPARγ, inhibition of Akt activity and activation of hepatic autophagy reduced hepatic steatosis in mouse livers.     

Catalposide is a natural agonistic ligand of peroxisome proliferator-activated receptor-α

Peroxisome proliferator-activated receptor-alpha (PPARα) is a nuclear receptor that regulates the expression of genes related to cellular lipid uptake and oxidation. Thus, PPARα agonists may be important in the treatment of hypertriglyceridemia and hepatic steatosis. In this study, we demonstrated that catalposide is a novel natural PPARα agonist, identified from reporter gene assay-based activity screening with approximately 900 natural plant and seaweed extracts. Results of time-resolved fluorescence resonance energy transfer analyses suggested that the compound interacted directly with the ligand-binding domain of PPARα. Cultured hepatocytes stimulated with catalposide exhibited significantly reduced cellular triglyceride concentrations, by 21%, while cellular uptake of fatty acids was increased, by 70% (P<0.05). Quantitative PCR analysis revealed that the increase in cellular fatty acid uptake was due to upregulation of fatty acid transporter protein-4 (+19% vs. the control) in cells stimulated with catalposide. Additionally, expression of genes related to fatty acid oxidation and high-density lipoprotein metabolism were upregulated, while that of genes related to fatty acid synthesis were suppressed. In conclusion, catalposide is hypolipidemic by activation of PPARα via a ligand-mediated mechanism that modulates the expression of in lipid metabolism genes in hepatocytes.     

Trivalent chromium alleviates oleic acid induced steatosis in SMMC-7721 cells by decreasing fatty acid uptake and triglyceride synthesis

Trivalent chromium [Cr(III)] has been shown as an essential trace element for human health. Previous studies depict that Cr(III) plays important roles in maintaining normal glucose and lipid metabolism, whereas its effect on the hepatic lipid metabolism is still unknown. In the present study, we investigated the effects and underlying mechanisms of Cr on hepatic steatosis induced by oleic acid (OA) in human hepatoma SMMC-7721 cells. Hepatic steatosis model was co-administered with Cr. Indexes of lipid accumulation were determined and associated genes expression were analyzed. The data showed that OA could induce lipid accumulation and triglyceride (TG) content in SMMC-7721 cells, and significantly increase the expression of cluster of differentiation 36 (CD36) and diacylglycerol acyltransferase 2 (DGAT2). This steatosis effect of OA was ameliorated by Cr. The TG accumulation and up-regulation of CD36 and DGAT2 genes followed steatosis induction were inhibited by Cr. After the treatment of Cr, excessive intracellular OA content was also attenuated. Furthermore, Cr still performed inhibitory effect of DGAT2 expression at the presence of DGAT2 agonist or inhibitor, which indicated that the inhibitory effect of Cr on lipogenesis is associated with the downregulation of DGAT2 expression. These findings demonstrate that Cr alleviates hepatic steatosis via suppressing CD36 expression to prevent fatty acid uptake, as well as suppressing DGAT2 expression to inhibit TG synthesis. It suggests that CD36 and DGAT2 might become the novel drug targets for their properties in hepatic steatosis. Most importantly, Cr may be a potential anti-steatosis candidate to offer protective effects against liver damage.     

Regulatory role of Sirt1 on the gene expression of fatty acid‐binding protein 3 in cultured porcine adipocytes

To investigate whether Sirt1 could modulate fatty acid‐binding protein 3 (FABP3), we treated porcine adipocytes either with the Sirt1 inhibitor nicotinamide (NAM), with the Sirt1 activator resveratrol (RES), or by knockdown of Sirt1 by Sirt1‐siRNA. NAM or knockdown with Sirt1‐siRNA significantly inhibited Sirt1 mRNA expression, while increasing FABP3 mRNA levels. RES or RES + Sirt1‐siRNA treatments further proved that Sirt1 negatively regulated FABP3 gene expression in adipocytes. We also found a similar Sirt1 regulation pattern for PPARγ to that of FABP3 in adipocytes. Furthermore, NAM/RES + PPARγ‐siRNA treatments showed that Sirt1 may regulate the FABP3 gene expression partly through the PPARγ‐mediated signals. In summary, Sirt1 regulates the expression of FABP3 gene in adipocytes, and PPARγ apparently plays an important role in this process. J. Cell. Biochem. 107: 984–991, 2009. © 2009 Wiley‐Liss, Inc.     

Ghrelin deletion protects against age‐associated hepatic steatosis by downregulating the C/EBPα‐p300/DGAT1 pathway

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. NAFLD usually begins as low‐grade hepatic steatosis which further progresses in an age‐dependent manner to nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma in some patients. Ghrelin is a hormone known to promote adiposity in rodents and humans, but its potential role in hepatic steatosis is unknown. We hypothesized that genetic ghrelin deletion will protect against the development of age‐related hepatic steatosis. To examine this hypothesis, we utilized ghrelin knockout (KO) mice. Although no different in young animals (3 months old), we found that at 20 months of age, ghrelin KO mice have significantly reduced hepatic steatosis compared to aged‐matched wild‐type (WT) mice. Examination of molecular pathways by which deletion of ghrelin reduces steatosis showed that the increase in expression of diacylglycerol O‐acyltransferase‐1 (DGAT1), one of the key enzymes of triglyceride (TG) synthesis, seen with age in WT mice, is not present in KO mice. This was due to the lack of activation of CCAAT/enhancer binding protein‐alpha (C/EBPα) protein and subsequent reduction of C/EBPα‐p300 complexes. These complexes were abundant in livers of old WT mice and were bound to and activated the DGAT1 promoter. However, the C/EBPα‐p300 complexes were not detected on the DGAT1 promoter in livers of old KO mice resulting in lower levels of the enzyme. In conclusion, these studies demonstrate the mechanism by which ghrelin deletion prevents age‐associated hepatic steatosis and suggest that targeting this pathway may offer therapeutic benefit for NAFLD.     

不同来源肝细胞在体外降脂药物评价中药效反应的对比

目的:通过对比不同来源的人肝癌细胞系HepG2和原代大鼠肝细胞在体外降脂药物评价中药效反应,指导两种肝细胞在体外降脂药物评价中的实际应用。方法:用游离脂肪酸(油酸/棕榈酸,2:1)诱导HepG2细胞、原代大鼠肝细胞脂肪变性,并用100μmol·L-1苯扎贝特干预,检测细胞内甘油三酯(TG)、总胆固醇(TC)、活性氧(ROS)含量,细胞内脂滴数目、并检测细胞上清液中丙二醛(MDA)含量和超氧化物歧化酶(SOD)活性。结果:FFA刺激使HepG2细胞和原代大鼠肝细胞脂质沉积(TG、脂滴)和氧化应激(ROS、MDA、SOD)水平上升。苯扎贝特对HepG2细胞1 mmol·L-1FFA造模组和原代大鼠肝细胞0.5 mmol·L-1FFA造模组脂质沉积和氧化应激水平改善显著;而HepG2细胞0.5 mmol·L-1FFA造模组和原代大鼠肝细胞1 mmol·L-1FFA造模组脂质沉积和氧化应激水平在苯扎贝特干预后变化不明显。结论:在相同FFA造模浓度,原代大鼠肝细胞病理特征变化更为明显;苯扎贝特对两种肝细胞在脂质沉积和氧化应激水平的作用也不完全相同。因而HepG2细胞和原代大鼠肝细胞在体外降脂药物评价中药效反应是不完全相同的。