Resistance of HepG2 cells against the adverse effects of ethanol related to neutral lipid and phospholipid metabolism

14-Deoxyandrographolide targets adenylate cyclase and prevents ethanol-induced liver injury through constitutive NOS dependent reduced redox signaling in rats

Chronic alcoholism is one of the most common causes of liver diseases worldwide. Nitric oxide (NO) has been proposed to have potential for clinical application against chronic hepatocellular injuries. However, mechanisms underlying hepatoprotective functions of NO in ethanol-induced apoptosis are largely unknown. Sprauge-Dawley rats were exposed to ethanol for 8 weeks. Half of the ethanol-fed animals received 14-deoxyandrographolide (14-DAG) treatment for the last 4 weeks of study. Preventive effect of 14-DAG against ethanol-induced hepatotoxicity involved constitutive nitric oxide synthase (cNOS) activation followed by up-regulation of γ-glutamylcysteine synthetase activity and reduced oxidative stress. Enhanced interaction of cNOS with caveolin-1 caused down-regulation of enzyme activity and led to depletion of NO in the hepatocytes of ethanol-fed animals. 14-DAG acted as activator of adenylate cyclase and modulated cyclic AMP (cAMP) mediated expression of caveolin-1 and calmodulin. This eventually favored activation of cNOS through inhibition of cNOS-caveolin-1 interaction. Our results suggest that, protective effect of 14-DAG against ethanol-induced hepatic injury is based on its ability to reduce oxidative stress through cNOS dependent improvement of redox status. 14-DAG mediated activation of adenylate cyclase-cAMP signaling leading to up-regulation of cNOS may provide a promising approach in the prevention of liver diseases during chronic alcoholism.

Protective effects of Antrodia cinnamomea against liver injury

Chinese herbal medicine () attracts much attention in the treatment of liver injuries. Numerous studies have revealed various biological activities of medicinal mushrooms such as Antrodia Cinnamomea (). Although A. cinnamomea is rare in the wild, recent developments in fermentation and cultivation technologies make the mycelia and fruiting bodies of this valuable medicinal mushroom readily available. Liver diseases such as fatty liver, hepatitis, hepatic fibrosis, and liver cancer are complicated processes of liver injuries that have tremendous impact on human society. In this article, we reviewed studies about the hepatoprotective effects of the fruiting bodies and mycelia of A. cinnamomea performed in different experimental models. The results of those studies suggest the potential application of A. cinnamomea in preventing and treating liver diseases and its potential to be developed into health foods or new drugs.

Alcohol depletes coenzyme-Q(10) associated with increased TNF-alpha secretion to induce cytotoxicity in HepG2 cells

Alcohol consumption has been implicated to cause severe hepatic steatosis which is mediated by alcohol dehydrogenase (ADH) activity and CYP(450) 2E1 expression. In this context, the effect of ethanol was studied for its influence on lipogenesis in HepG2 cell which is deficient of ADH and does not express CYP(450) 2E1. The results showed that ethanol at 100mM concentration caused 40% cytotoxicity at 72h as determined by MTT assay. The incorporation of labeled [2-(14)C] acetate into triacylglycerol and phospholipid was increased by 40% and 26% respectively upon 24h incubation, whereas incorporation of labeled [2-(14)C] acetate into cholesterol was not significantly increased. Further, ethanol inhibited HMG-CoA reductase which is a rate-limiting enzyme in the cholesterol biosynthesis. It was observed that, HMG-CoA reductase inhibition was brought about by ethanol as a consequence of decreased cell viability, since incubation of HepG2 cells with mevalonate could not increase the cholesterol content and increase the cell viability. Addition of ethanol significantly increased TNF-alpha secretion and depleted mitochondrial coenzyme-Q(10) which is detrimental for cell viability. But vitamin E (10mM) could partially restore coenzyme-Q(10) and glutathione content with decreased TNF-alpha secretion in ethanol treated cells. Further, lipid peroxidation, glutathione peroxidase and superoxide dismutase enzyme activities remained unaffected. Ethanol decreased glutathione content while, GSH/GSSG ratio was significantly higher compared to other groups showing cellular pro-oxidant and antioxidant balance remained intact. Alanine amino transferase activity was increased by 4.85 folds in cells treated with ethanol confirming hepatocyte damage. Hence, it is inferred that ethanol induced cytotoxicity in HepG2 cells due to coenzyme-Q(10) depletion and increased TNF-alpha secretion.

Concerted action of sulfiredoxin and peroxiredoxin I protects against alcohol-induced oxidative injury in mouse liver

Peroxiredoxins (Prxs) are peroxidases that catalyze the reduction of reactive oxygen species (ROS). The active site cysteine residue of members of the 2-Cys Prx subgroup (Prx I to IV) of Prxs is hyperoxidized to cysteine sulfinic acid (Cys-SO(2) ) during catalysis with concomitant loss of peroxidase activity. Reactivation of the hyperoxidized Prx is catalyzed by sulfiredoxin (Srx). Ethanol consumption induces the accumulation of cytochrome P450 2E1 (CYP2E1), a major contributor to ethanol-induced ROS production in the liver. We now show that chronic ethanol feeding markedly increased the expression of Srx in the liver of mice in a largely Nrf2-dependent manner. Among Prx I to IV, only Prx I was found to be hyperoxidized in the liver of ethanol-fed wildtype mice, and the level of Prx I-SO(2) increased to ≈30% to 50% of total Prx I in the liver of ethanol-fed Srx(-/-) mice. This result suggests that Prx I is the most active 2-Cys Prx in elimination of ROS from the liver of ethanol-fed mice and that, despite the up-regulation of Srx expression by ethanol, the capacity of Srx is not sufficient to counteract the hyperoxidation of Prx I that occurs during ROS reduction. A protease protection assay revealed that a large fraction of Prx I is located together with CYP2E1 at the cytosolic side of the endoplasmic reticulum membrane. The selective role of Prx I in ROS removal is thus likely attributable to the proximity of Prx I and CYP2E1.

CONCLUSION

The pivotal functions of Srx and Prx I in protection of the liver in ethanol-fed mice was evident from the severe oxidative damage observed in mice lacking either Srx or Prx I.

Nitric oxide and thioredoxin type 1 modulate the activity of caspase 8 in HepG2 cells

Herein, we report that nitric oxide (NO) and the thioredoxin/thioredoxin reductase system affect the activity of caspase 8 in HepG2 cells. Exposure of cells to NO resulted in inhibition of caspase 8, while a subsequent incubation of the cells in NO-free medium resulted in spontaneous reactivation of the protease. The latter process was inhibited in thioredoxin reductase-deficient HepG2 cells, in which, however, lipoic acid markedly reactivated caspase 8. The data obtained suggest that extrinsic apoptosis can be subjected to redox regulation before induction of proteolytic damage by caspase 3.

Nitric oxide and dihydrolipoic acid modulate the activity of caspase 3 in HepG2 cells

Herein, we report that dihydrolipoic acid and lipoic acid (LA) plus lipoamide dehydrogenase and NADH denitrosate S-nitrosocaspase 3 (CASP-SNO). In HepG2 cells, S-nitroso-L-cysteine ethyl ester (SNCEE) impeded the activity of caspase 3 (CASP-SH), while a subsequent incubation of the cells in SNCEE-free medium resulted in endogenous denitrosation and reactivation of CASP-SH. The latter process was inhibited in thioredoxin reductase-deficient HepG2 cells, in which, however, LA markedly reactivated CASP-SH. The data obtained are discussed with focus on low molecular mass dithiols that mimic the activity of thioredoxin in reactions of protein S-denitrosation.

Niuchangchih (Antrodia camphorata) and its potential in treating liver diseases

Niuchangchih (Antrodia camphorata (M. Zang & C.H. Su) Sheng H. Wu, Ryvarden & T.T. Chang) is a basidiomycete endemic to Taiwan. It is well known as a Traditional Chinese Medicine (TCM), and Taiwanese aborigines used this species to treat liver diseases and food and drug intoxication. The compounds identified in Niuchangchih are predominantly polysaccharides, triterpenoids, steroids, benzenoids and maleic/succinic acid derivatives. Recent research has revealed that Niuchangchih possesses extensive biological activity, such as hepatoprotective, antihypertensive, anti-hyperlipidemic, immuno-modulatory, anticancer, anti-inflammatory and antioxidant activities. The fruiting bodies and fermented products of Niuchangchih have been reported to exhibit activity when treating liver diseases, such as preventing ethanol-, CCl(4)- and cytokine-induced liver injury, inhibiting the hepatitis B virus, ameliorating fatty liver and liver fibrosis, and inhibiting liver cancer cells. This review will address the protective effects of Niuchangchih on the pathological development of liver diseases, and the underlying mechanisms of action are also discussed.

Liv.52 regulates ethanol induced PPARgamma and TNF alpha expression in HepG2 cells

Liver is a prime target of alcohol-induced damage by inducing inflammatory cytokines especially tumor necrosis factor alpha (TNFalpha). Activator of peroxisome proliferator activator receptor gamma (PPARgamma) is protective against alcohol-induced liver injury in animals. Liv.52, one of the major herbal hepatoprotective drugs, is shown to protect the liver from toxicity and is considered to be an effective hepatoprotective agent. However, the signal pathway involved in the Liv.52-induced hepatoprotection is not understood well especially in the case of cultured liver cells treated with ethanol. Hence, the study was aimed at determining whether ethanol and Liv.52 could modulate PPARgamma and TNFalpha induction in human hepatoma cells, HepG2. The present study with RT-PCR and confocal microscopy experiments showed that ethanol (100 mM) induced suppression of PPARgamma expression in HepG2 cells. The ethanol-induced PPARgamma suppression was abrogated by Liv.52. Moreover, Liv.52 also induced upregulation of PPARgamma mRNA in liver cells as compared to the untreated cells. Further, 100 mM ethanol has also induced TNFalpha gene expression in HepG2 cells and interestingly Liv.52 abolished ethanol-induced TNFalpha. The study also shows that Liv.52 alone downregulated TNFalpha expression in HepG2 cells. Taken together, these findings suggest that Liv.52 is capable of attenuating ethanol-induced expression of TNFalpha and abrogating ethanol-induced suppression of PPARgamma in liver cells. These results indicate that Liv.52-induced PPARgamma expression and concomitant suppression of ethanol-induced elevation of TNFalpha in HepG2 cells suggest the immunomodulatory and hepatoprotective nature of Liv.52.

Metabolic basis of ethanol-induced cytotoxicity in recombinant HepG2 cells: role of nonoxidative metabolism

Chronic alcohol abuse, a major health problem, causes liver and pancreatic diseases and is known to impair hepatic alcohol dehydrogenase (ADH). Hepatic ADH-catalyzed oxidation of ethanol is a major pathway for the ethanol disposition in the body. Hepatic microsomal cytochrome P450 (CYP2E1), induced in chronic alcohol abuse, is also reported to oxidize ethanol. However, impaired hepatic ADH activity in a rat model is known to facilitate a nonoxidative metabolism resulting in formation of nonoxidative metabolites of ethanol such as fatty acid ethyl esters (FAEEs) via a nonoxidative pathway catalyzed by FAEE synthase. Therefore, the metabolic basis of ethanol-induced cytotoxicity was determined in HepG2 cells and recombinant HepG2 cells transfected with ADH (VA-13), CYP2E1 (E47) or ADH + CYP2E1 (VL-17A). Western blot analysis shows ADH deficiency in HepG2 and E47 cells, compared to ADH-overexpressed VA-13 and VL-17A cells. Attached HepG2 cells and the recombinant cells were incubated with ethanol, and nonoxidative metabolism of ethanol was determined by measuring the formation of FAEEs. Significantly higher levels of FAEEs were synthesized in HepG2 and E47 cells than in VA-13 and VL-17A cells at all concentrations of ethanol (100-800 mg%) incubated for 6 h (optimal time for the synthesis of FAEEs) in cell culture. These results suggest that ADH-catalyzed oxidative metabolism of ethanol is the major mechanism of its disposition, regardless of CYP2E1 overexpression. On the other hand, diminished ADH activity facilitates nonoxidative metabolism of ethanol to FAEEs as found in E47 cells, regardless of CYP2E1 overexpression. Therefore, CYP2E1-mediated oxidation of ethanol could be a minor mechanism of ethanol disposition. Further studies conducted only in HepG2 and VA-13 cells showed lower ethanol disposition and ATP concentration and higher accumulation of neutral lipids and cytotoxicity (apoptosis) in HepG2 cells than in VA-13 cells. The apoptosis observed in HepG2 vs. VA-13 cells incubated with ethanol appears to be mediated by release of mitochondrial cytochrome c via activation of caspase-9 and caspase-3. These results strongly support our hypothesis that diminished hepatic ADH activity facilitates nonoxidative metabolism of ethanol and the products of ethanol nonoxidative metabolism cause apoptosis in HepG2 cells via intrinsic pathway.

Comparison of basal gene expression profiles and effects of hepatocarcinogens on gene expression in cultured primary human hepatocytes and HepG2 cells

Toxicogenomics is a relatively new discipline of toxicology. Microarrays and bioinformatics tools are being used successfully to understand the effects of toxicants on in vivo and in vitro model systems, and to gain a better understanding of the relevance of in vitro models commonly used in toxicological studies. In this study, cDNA filter arrays were used to determine the basal expression patterns of human cultured primary hepatocytes from different male donors; compare the gene expression profile of HepG2 to that of primary hepatocytes; and analyze the effects of three genotoxic hepatocarcinogens; aflatoxin B(1) (AFB(1)), 2-acetylaminofluorene (2AAF), and dimethylnitrosamine (DMN), as well as one non-gentoxic hepatotoxin, acetaminophen (APAP) on gene expression in both in vitro systems. Real-time PCR was used to verify differential gene expression for selected genes. Of the approximately 31,000 genes screened, 3-6% were expressed in primary hepatocytes cultured on matrigel for 16 h. Of these genes, 867 were expressed in cultured hepatocytes from all donors. HepG2 cells expressed about 98% of the genes detectable in cultured primary hepatocytes, however, 31% of the HepG2 transcriptome was unique to the cell line. A number of these genes are expressed in human liver but expression is apparently lost during culture. There was considerable variability in the response to chemical carcinogen exposure in primary hepatocytes from different donors. The transcription factors, E2F1 and ID1 mRNA were increased three-fold and six-fold (P < 0.05, P < 0.01), respectively, in AFB(1) treated primary human hepatocytes but were not altered in HepG2. ID1 expression was also increased by dimethylnitrosamine, acetylaminofluorene and acetaminophen in both primary hepatocytes and HepG2. Identification of genes that are expressed in primary hepatocytes from most donors, as well as those genes with variable expression, will aid in understanding the variability in human reactions to drugs and chemicals. This study suggests that identification of biomarkers of exposure to some chemicals may be possible in the human through microarray analysis, despite the variability in responses.

Hexadecylphosphocholine inhibits phosphatidylcholine synthesis via both the methylation of phosphatidylethanolamine and CDP-choline pathways in HepG2 cells

We reported in a recent publication that hexadecylphosphocholine (HePC), a lysophospholipid analogue, reduces cell proliferation in HepG2 cells and at the same time inhibits the biosynthesis of phosphatidylcholine (PC) via CDP-choline by acting upon CTP:phosphocholine cytidylyltransferase (CT). We describe here the results of our study into the influence of HePC on other biosynthetic pathways of glycerolipids. HePC clearly decreased the incorporation of the exogenous precursor [1,2,3-3H]glycerol into PC and phosphatidylserine (PS) whilst increasing that of the neutral lipids diacylglycerol (DAG) and triacylglycerol (TAG). Interestingly, the uptake of L-[3-3H]serine into PS and other phospholipids remained unchanged by HePC and neither was the activity of either PS synthase or PS decarboxylase altered, demonstrating that the biosynthesis of PS is unaffected by HePC. We also analyzed the water-soluble intermediates and final product of the CDP-ethanolamine pathway and found that HePC caused an increase in the incorporation of [1,2-14C]ethanolamine into CDP-ethanolamine and phosphatidylethanolamine (PE) and a decrease in ethanolamine phosphate, which might be interpreted in terms of a stimulation of CTP:phosphoethanolamine cytidylyltransferase activity. Since PE can be methylated to give PC, we studied this process further and observed that HePC decreased the synthesis of PC from PE by inhibiting the PE N-methyltransferase activity. These results constitute the first experimental evidence that the inhibition of the synthesis of PC via CDP-choline by HePC is not counterbalanced by any increase in its formation via methylation. On the contrary, in the presence of HePC both pathways seem to contribute jointly to a decrease in the overall synthesis of PC in HepG2 cells.

Hexadecylphosphocholine inhibits phosphatidylcholine biosynthesis and the proliferation of HepG2 cells

Hexadecylphosphocholine (HePC) is a synthetic lipid representative of a new group of antiproliferative agents, alkylphosphocholines (APC), which are promising candidates in anticancer therapy. Thus we have studied the action of HePC on the human hepatoblastoma cell line HepG2, which is frequently used as a model for studies into hepatic lipid metabolism. Non-toxic, micromolar concentrations of HePC exerted an antiproliferative effect on this hepatoma cell line. The incorporation into phosphatidylcholine (PC) of the exogenous precursor [methyl-14C]choline was substantially reduced by HePC. This effect was not due to any alteration in choline uptake by the cells, the degradation rate of PC or the release of PC into the culture medium. As anaccumulation of soluble choline derivatives points to CTP:phosphocholine cytidylyltransferase (CT) as the target of HePC activity we examined its effects on the different enzymes involved in the biosynthesis of PC via CDP-choline. Treatment with HePC altered neither the activity of choline kinase (CK) nor that of diacylglycerol cholinephosphotransferase (CPT), but it did inhibit CT activity in HepG2 cells. In vitro HePC also inhibited the activity of cytosolic but not membrane-bound CT. Taken together our results suggest that HePC interferes specifically with the biosynthesis of PC in HepG2 cells by depressing CT translocation to the membrane, which may well impair their proliferation.