Inhibition of CYP2E1 catalytic activity in vitro by S-adenosyl-L-methionine

The objective of this work was to evaluate the possible in vitro interactions of S-adenosyl-l-methionine (SAM) and its metabolites S-(5'-Adenosyl)-l-homocysteine (SAH), 5'-Deoxy-5'-(methylthio)adenosine (MTA) and methionine with cytochrome P450 enzymes, in particular CYP2E1. SAM (but not SAH, MTA or methionine) produced a type II binding spectrum with liver microsomal cytochrome P450 from rats treated with acetone or isoniazid to induce CYP2E1. Binding was less effective for control microsomes. SAM did not alter the carbon monoxide binding spectrum of P450, nor denature P450 to P420, nor inhibit the activity of NADPH-P450 reductase. However, SAM inhibited the catalytic activity of CYP2E1 with typical substrates such as p-nitrophenol, ethanol, and dimethylnitrosamine, with an IC(50) around 1.5-5mM. SAM was a non-competitive inhibitor of CYP2E1 catalytic activity and its inhibitory actions could not be mimicked by methionine, SAH or MTA. However, SAM did not inhibit the oxidation of ethanol to alpha-hydroxyethyl radical, an assay for hydroxyl radical generation. In microsomes engineered to express individual human P450s, SAM produced a type II binding spectrum with CYP2E1-, but not with CYP3A4-expressing microsomes, and SAM was a weaker inhibitor against the metabolism of a specific CYP3A4 substrate than a specific CYP2E1 substrate. SAM also inhibited CYP2E1 catalytic activity in intact HepG2 cells engineered to express CYP2E1. These results suggest that SAM interacts with cytochrome P450s, especially CYP2E1, and inhibits the catalytic activity of CYP2E1 in a reversible and non competitive manner. However, SAM is a weak inhibitor of CYP2E1. Since the K(i) for SAM inhibition of CYP2E1 activity is relatively high, inhibition of CYP2E1 activity is not likely to play a major role in the ability of SAM to protect against the hepatotoxicity produced by toxins requiring metabolic activation by CYP2E1 such as acetaminophen, ethanol, carbon tetrachloride, thioacetamide and carcinogens.

Diallyl sulfide induces heme oxygenase-1 through MAPK pathway

Diallyl sulfide (DAS), is protective against chemically induced heptotoxicity, mutagenesis, and carcinogenesis. The mechanism of its protective effects is not fully understood. In this study, we found that DAS can induce the expression of heme oxygenase-1 (HO-1), which plays a critical role in the cell defense system against oxidative stress. DAS causes a dose- and time-dependent increase of HO-1 protein and mRNA level without toxicity in HepG2 cells. DAS-induced HO-1 protein expression is dependent on newly synthesized mRNA and newly synthesized protein. DAS increases Nrf2 protein expression, nuclear translocation, and DNA-binding activity. The MAP kinase ERK is activated by DAS. Both ERK and p38 pathways play an important role in DAS-induced Nrf2 nuclear translocation and ho-1 gene activation. DAS stimulates a transient increase of reactive oxygen species (ROS). N-Acetyl-cysteine blocked this increase of ROS production as well as DAS-induced ERK activation, Nrf2 protein expression and nuclear translocation, and ho-1 gene activation. The increase in HO-1 produced by DAS protected the HepG2 cells against toxicity by hydrogen peroxide or arachidonic acid. These results suggest that DAS induces ho-1 through production of ROS, and Nrf2 and MAPK (ERK and p38) mediate this induction. Induction of ho-1 may play a role in the protective effects of DAS.

A high-fat diet leads to the progression of non-alcoholic fatty liver disease in obese rats

Fatty livers of obese fa/fa rats are vulnerable to injury when challenged by insults such as endotoxin, ischemia-reperfusion or acute ethanol treatment. The objective of this study was to evaluate whether a high-fat diet can act as a "second hit" and cause progression to liver injury in obese fa/fa rats compared with lean Fa/? rats. Accordingly, obese fa/fa rats and their lean littermates were fed a diet low in fat (12% of total calories) or a diet with 60% calories as lard for 8 weeks. Hyperglycemia and steatohepatitis occurred in the fa/fa rats fed the high-fat diet. This was accompanied by liver injury as assessed by alanine aminotransferase, hematoxilin and eosin staining, increased TNFalpha and stellate cell-derived TGFbeta, collagen deposition, and up-regulation of alpha-smooth muscle actin. Active MMP13 decreased in fa/fa rats independently of the diet, and TIMP1 expression increased with the high-fat diet, especially in fa/fa rats. Although UCP2 expression was higher in fa/fa rats regardless of the diet, minor changes in ATP levels were observed. Oxidative stress occurred in the fa/fa rats fed the high-fat diet as lipid peroxidation and protein carbonyls were elevated, while glutathione and antioxidant enzymes were very low. Expression and activity of cytochrome P450 2E1 and xanthine oxidase activity were down-regulated in fa/fa compared with Fa/? rats, and no effect was seen by the high-fat diet. However, NADPH oxidase activity increased 2.5-fold in fa/fa rats fed with the high-fat diet. In summary, a high-fat diet induces liver injury in fa/fa rats leading to periportal fibrosis. A role for oxidative stress is suggested via increased NADPH oxidase activity, lipid peroxidation, protein carbonyl formation, and low antioxidant defense.

Protein Kinase C Signaling as a Survival Pathway against CYP2E1-Derived Oxidative Stress and Toxicity in HepG2 Cells

Hepatic induction of CYP2E1 is a major pathway involved in oxidative stress and damage caused by chronic ethanol consumption; CYP2E1 also promotes the activation of a variety of hepatotoxins to reactive intermediates. Phorbol esters activate protein kinase C (PKC), thereby blocking cell differentiation and promoting tumor growth. In this study, we examined the possible role of PKC signaling as a survival pathway against CYP2E1-mediated toxicity using transfected HepG2 hepatoma cells stably overexpressing CYP2E1 (E47 cells). Cells were exposed to arachidonic acid (AA) plus Fe, which has been previously reported to cause a synergistic toxicity in E47 cells by a mechanism dependent on CYP2E1 activity and involving oxidative stress and lipid peroxidation. Phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), but not the inactive analog 4-alpha-TPA, prevented lipid peroxidation, glutathione depletion, and loss of viability produced by AA + Fe in E47 cells. TPA also protected against the toxicity caused by AA alone, or by iron alone, in the E47 cells. TPA did not lower but instead induced catalytically active CYP2E1 in these cells. The protective effect of TPA on CYP2E1-dependent AA + Fe toxicity seemed to involve a PKC-related survival mechanism, since PKC inhibitors such as Ro 31-8425 (bisindolylmaleimide X hydrochloride) or staurosporine abolished that protection, and activation of PKC by TPA was an early event that occurs prior to the developing toxicity. In conclusion, PKC activation by TPA prevents CYP2E1-derived acute oxidative stress and toxicity in HepG2 cells, and this appears to involve maintenance of the intracellular redox homeostasis via PKC signal transduction.

Adenovirus mediated overexpression of CYP2E1 increases sensitivity of HepG2 cells to acetaminophen induced cytotoxicity

To study the biochemical and toxicological properties of cytochrome P450 2E1 (CYP2E1), an adenovirus containing human CYP2E1 cDNA (Ad-CYP2E1) was constructed and was shown to successfully mediate the overexpression of CYP2E1 in HepG2 cells. Acetaminophen (APAP) toxicity to HepG2 cells infected with Ad-CYP2E1 was characterized as a preliminary proof of principle experiment to validate the functionality of the CYP2E1 adenovirus. Compared with cells infected with Ad-LacZ, HepG2 cells infected with Ad-CYP2E1 were more sensitive to APAP induced necrosis and apoptosis when the cells were depleted of intracellular reduced glutathione (GSH). The APAP cytotoxicity was dependent on both the concentration of APAP and the multiplicity of infection of the Ad-CYP2E1 virus. Apoptosis induced by APAP in HepG2 cells overexpressing CYP2E1 was caspase dependent and could be inhibited by the pan-caspase inhibitor Z-VAD-fmk. After treatment with APAP, mitochondrial membrane potential was dramatically decreased in the CYP2E1-expressing cells. APAP protein adducts were elevated in HepG2 cells infected with Ad-CYP2E1 compared with that in cells infected with Ad-LacZ; two bands around 90 KD were found only in the CYP2E1-expressing cells. These results demonstrate that adenovirus-mediated overexpression of human CYP2E1 activates APAP to reactive metabolites which damage mitochondria, form protein adducts, and result in toxicity to HepG2 cells. The Ad-CYP2E1 may be useful for studies designed to investigate the role of CYP2E1 in APAP and alcoholic liver injury and to further characterize the actions and effects of CYP2E1.

Antioxidant properties of S-adenosyl-L-methionine in Fe(2+)-initiated oxidations

S-Adenosylmethionine (SAM) is protective against a variety of toxic agents that promote oxidative stress. One mechanism for this protective effect of SAM is increased synthesis of glutathione. We evaluated whether SAM is protective via possible antioxidant-like activities. Aerobic Hepes-buffered solutions of Fe2+ spontaneously oxidize and consume O2 with concomitant production of reactive oxygen species and oxidation of substrates to radical products, e.g., ethanol to hydroxyethyl radical. SAM inhibited this oxidation of ethanol and inhibited aerobic Fe2+ oxidation and consumption of O2. SAM did not regenerate Fe2+ from Fe3+ and was not consumed after incubation with Fe2+. SAM less effectively inhibited aerobic Fe2+ oxidation in the presence of competing chelating agents such as EDTA, citrate, and ADP. The effects of SAM were mimicked by S-adenosylhomocysteine, but not by methionine or methylthioadenosine. SAM did not inhibit Fe2+ oxidation by H2O2 and was a relatively poor inhibitor of the Fenton reaction. Lipid peroxidation initiated by Fe2+ in liposomes was associated with Fe2+ oxidation; these two processes were inhibited by SAM. However, SAM did not show significant peroxyl radical scavenging activity. SAM also inhibited the nonenzymatic lipid peroxidation initiated by Fe2+ + ascorbate in rat liver microsomes. These results suggest that SAM inhibits alcohol and lipid oxidation mainly by Fe2+ chelation and inhibition of Fe2+ autoxidation. This could represent an important mechanism by which SAM exerts cellular protective actions and reduces oxidative stress in biological systems.

Oxidative stress, toxicology, and pharmacology of CYP2E1

This review describes some of the biochemical and toxicological properties of CYP2E1, especially as it relates to alcohol metabolism and toxicity and the establishment of human hepatoma HepG2 cell lines that overexpress human CYP2E1. Ethanol, polyunsaturated fatty acids, and iron were found to be cytotoxic in HepG2 cells that overexpress CYP2E1. GSH appears to be essential in protecting HepG2 cells against the CYP2E1-dependent cytotoxicity, and GSH levels were elevated owing to a twofold increase in activity and expression of glutamate cysteine ligase. We suggest that this up-regulation of GSH synthesis was an adaptive response to attenuate CYP2E1-dependent oxidative stress and toxicity. Induction of a state of oxidative stress appears to play a central role in the CYP2E1-dependent cytotoxicity. Mitochondrial membrane potential decreased in the CYP2E1-expressing HepG2 cells, and this decrease shared similar characteristics with the developing toxicity. Alcohol-dependent liver injury is likely to be a multifactorial process involving several mechanisms. We believe that the linkage between CYP2E1-dependent oxidative stress, mitochondrial injury, and GSH homeostasis contribute to the toxic actions of ethanol on the liver.

Lipid peroxidation and oxidant stress regulate hepatic apolipoprotein B degradation and VLDL production

How omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) lower plasma lipid levels is incompletely understood. We previously showed that marine omega-3 PUFAs (docosahexaenoic acid [DHA] and eicosapentaenoic acid) stimulate a novel pathway, post-ER presecretory proteolysis (PERPP), that degrades apolipoprotein B100 (ApoB100), thereby reducing lipoprotein secretion from liver cells. To identify signals stimulating PERPP, we examined known actions of omega-3 PUFA. In rat hepatoma or primary rodent hepatocytes incubated with omega-3 PUFA, cotreatment with the iron chelator desferrioxamine, an inhibitor of iron-dependent lipid peroxidation, or vitamin E, a lipid antioxidant, suppressed increases in thiobarbituric acid-reactive substances (TBARSs; a measure of lipid peroxidation products) and restored ApoB100 recovery and VLDL secretion. Moreover, omega-6 and nonmarine omega-3 PUFA, also prone to peroxidation, increased ApoB100 degradation via intracellular induction of TBARSs. Even without added fatty acids, degradation of ApoB100 in primary hepatocytes was blocked by desferrioxamine or antioxidant cotreatment. To extend these results in vivo, mice were infused with DHA, which increased hepatic TBARSs and reduced VLDL-ApoB100 secretion. These results establish a novel link between lipid peroxidation and oxidant stress with ApoB100 degradation via PERPP, and may be relevant to the hypolipidemic actions of dietary PUFAs, the basal regulation of ApoB100 secretion, and hyperlipidemias arising from ApoB100 overproduction.

The liver-selective nitric oxide donor O2-vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO) protects HepG2 cells against cytochrome P450 2E1-dependent toxicity

HepG2 cells expressing CYP2E1 (E47 cells) are more susceptible to toxicity by arachidonic acid (AA) or after glutathione depletion with an inhibitor of glutamate-cysteine ligase, l-buthionine-(S,R)-sulfoximine (BSO), compared with control HepG2 cells (C34 cells). The ability of nitric oxide (NO) to protect against CYP2E1-dependent toxicity has not been evaluated. We therefore studied the ability of O2-vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO), a liver-selective NO donor, to protect against CYP2E1-dependent toxicity and compared this with protection by chemical NO donors. E47 cells incubated with V-PYRRO/NO produced NO, whereas C34 cells did not. Incubation of E47 cells with 50 microM AA or 100 microM BSO for 2 days resulted in a 50% loss of cell viability. VPYRRO/NO (1 mM) blocked this toxicity of AA and BSO by a mechanism involving NO release via CYP2E1 metabolism of VPYRRO/NO. NO scavengers hemoglobin and 2-(4-carboxophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide blocked the protective effects of V-PYRRO/NO. V-PYRRO/NO inhibited CYP2E1 activity and production of reactive oxygen species, whereas hemoglobin prevented these events. AA and BSO induced lipid peroxidation and decreased mitochondrial membrane potential; both of these effects were blocked by V-PYRRO/NO. Unlike V-PYRRO/NO, the chemical donors spermine/NO and (S)-nitroso-N-acetylpenicillamine release NO directly when added to the medium; however, they could partially protect against the CYP2E1-dependent toxicity. These results suggest that VPYRRO/NO protects HepG2 cells against CYP2E1-dependent toxicity through inhibition of CYP2E1-derived reactive oxygen species production and lipid peroxidation by the generated NO and that this compound may be valuable in protecting against CYP2E1-dependent toxicity via liver P450-specific generation of NO.

Green tea polyphenol epigallocatechin-3-gallate protects HepG2 cells against CYP2E1-dependent toxicity

Chronic ethanol consumption causes oxidative damage in the liver, and induction of cytochrome P450 2E1 (CYP2E1) is one pathway involved in oxidative stress produced by ethanol. The hepatic accumulation of iron and polyunsaturated fatty acids significantly contributes to ethanol hepatotoxicity in the intragastric infusion model of ethanol treatment. The objective of this study was to analyze the effect of the green tea flavanol epigallocatechin-3-gallate (EGCG), which has been shown to prevent alcohol-induced liver damage, on CYP2E1-mediated toxicity in HepG2 cells overexpressing CYP2E1 (E47 cells). Treatment of E47 cells with arachidonic acid plus iron (AA + Fe) was previously reported to produce synergistic toxicity in E47 cells by a mechanism dependent on CYP2E1 activity and involving oxidative stress and lipid peroxidation. EGCG protected E47 cells against toxicity and loss of viability induced by AA+Fe; EGCG had no effect on CYP2E1 activity. Prevention of this toxicity was associated with a reduction in oxidative damage as reflected by decreased generation of reactive oxygen species, a decrease in lipid peroxidation, and maintenance of intracellular glutathione in cells challenged by AA+Fe in the presence of EGCG. AA+Fe treatment caused a decline in the mitochondrial membrane potential, which was also blocked by EGCG. In conclusion, EGCG exerts a protective action on CYP2E1-dependent oxidative stress and toxicity that may contribute to preventing alcohol-induced liver injury, and may be useful in preventing toxicity by various hepatotoxins activated by CYP2E1 to reactive intermediates.

Heme oxygenase-1 protects HepG2 cells against cytochrome P450 2E1-dependent toxicity

The inducible form of heme oxygenase (HO-1) is increased during oxidative injury and HO-1 is believed to be an important defense mechanism against such injury. Arachidonic acid (AA) and l-buthionine-(S,R)-sulfoximine (BSO), which lowers GSH levels, cause cytochrome P450 2E1 (CYP2E1)-dependent oxidative injuries in HepG2 cells (E47 cells). Treatment of E47 cells with 50 microM AA or 100 microM BSO for 48 h was recently shown to increase HO-1 mRNA, protein, and activity. The possible functional significance of this increase in protecting against CYP2E1-dependent toxicity was evaluated in the current study. The treatment with AA and BSO caused loss of cell viability (40 and 50%, respectively) in E47 cells. Chromium mesoporphyrin (CrMP), an inhibitor of HO activity, significantly potentiated this cytotoxicity. ROS production, lipid peroxidation, and the decline in mitochondrial membrane potential produced by AA and BSO were also enhanced in the presence of CrMP in E47 cells. Infection with an adenovirus expressing rat HO-1 protected E47 cells from AA toxicity, increasing cell viability and reducing LDH release. HO catalyzes formation of CO, bilirubin, and iron from the oxidation of heme. Bilirubin was not protective whereas iron catalyzed the AA toxicity. The carbon monoxide (CO) scavenger hemoglobin enhanced AA toxicity in E47 cells analogous to CrMP, whereas exposure to exogenous CO partially reduced AA toxicity and the enhanced AA toxicity by CrMP. Addition of exogenous CO to the cells inhibited CYP2E1 catalytic activity, as did overexpression of the rat HO-1 adenovirus. These results suggest that induction of HO-1 protects against CYP2E1-dependent toxicity and this protection may be mediated in part via production of CO and CO inhibition of CYP2E1 activity.

CYP2E1: biochemistry, toxicology, regulation and function in ethanol-induced liver injury

Ethanol-induced oxidative stress appears to play a major role in mechanisms by which ethanol causes liver injury. Many pathways have been suggested to contribute to the ability of ethanol to induce a state of oxidative stress. One central pathway appears to be the induction of the CYP2E1 form of cytochrome P450 enzymes by ethanol. CYP2E1 is of interest because of its ability to metabolize and activate many toxicological substrates, including ethanol, to more reactive, toxic products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions, and after acute and chronic alcohol treatment. CYP2E1 is also an effective generator of reactive oxygen species such as the superoxide anion radical and hydrogen peroxide, and in the presence of iron catalysts, produces powerful oxidants such as the hydroxyl radical. This Review Article summarizes some of the biochemical and toxicological properties of CYP2E1, and briefly describes the use of HepG2 cell lines developed to constitutively express the human CYP2E1 in assessing the actions of CYP2E1. Regulation of CYP2E1 is quite complex and will be briefly reviewed. Possible therapeutic implications for treatment of alcoholic liver injury by inhibition of CYP2E1 or CYP2E1-dependent oxidative stress will be discussed, followed by some future directions which may help to understand the actions of CYP2E1 and its role in alcoholic liver injury.

Binge ethanol exposure increases liver injury in obese rats

BACKGROUND & AIMS

The objective of this study was to address the hepatic effects of acute alcohol consumption in obesity by simulating an alcohol binge in genetically obese fa/fa rats compared with lean Fa/? rats.

METHODS

Ethanol 4 g/kg or saline was administered by gavage every 12 hours for 3 days.

RESULTS

Plasma alcohol levels were similar in both groups. Binge ethanol exposure caused liver injury in obese fa/fa but not in lean Fa/? rats, as assessed by alanine aminotransferase and H&E staining. Obesity impaired the antioxidant defense because basal levels of glutathione, glutamate cysteine ligase modulatory subunit, catalase, glutathione reductase, and superoxide dismutase were lower in fa/fa compared with Fa/? rats; the ethanol binge further decreased these antioxidants in fa/fa rats and also decreased glutathione peroxidase activity. Nonesterified fatty acids and lipid peroxidation were increased after ethanol treatment in fa/fa rats. Cytochrome P450 2E1 was down-regulated in fa/fa compared with Fa/? rats; however, the ethanol binge increased cytochrome P450 2E1 in both genotypes. Adenosine triphosphate decreased and uncoupling protein 2 increased in fa/fa rats treated with ethanol. 3-Nitrotyrosine protein adducts were detected only in fa/fa rats treated with ethanol, and this was accompanied by an induction of inducible nitric oxide synthase. Ethanol binge increased caspase-3 and caspase-8 activity, the expression of Fas ligand, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling in fa/fa rats.

CONCLUSIONS

These data indicate that binge drinking increases apoptosis and liver injury in obese rats more than in lean controls and suggest that the injury may involve oxidative and nitrosative damage.

Alcohol-induced liver injury in mice lacking Cu, Zn-superoxide dismutase

Because alcoholic liver disease has been linked to oxidative stress, we investigated the effect of a compromised antioxidant defense system, Cu, Zn-superoxide dismutase (Sod1) deficiency, on alcohol-induced liver injury. C57BL/129SV wild-type (Sod1(+/+)) and Sod1 knockout (Sod1(-/-)) mice were fed dextrose or ethanol (10% of total calories) liquid diets for 3 weeks. Histologic evaluation of liver specimens of Sod1(-/-) mice fed ethanol showed the development of liver injury ranging from mild to extensive centrilobular necrosis and inflammation. Sod1(+/+) mice fed ethanol showed mild steatosis; both Sod1(+/+) and Sod1(-/-) mice fed the dextrose diet had normal histology. Alanine transaminase levels were significantly elevated only in Sod1(-/-) mice fed ethanol. Cytochrome P450 2E1 (CYP2e1) activity was elevated about 2-fold by ethanol in Sod1(+/+) and Sod1(-/-) mice. Ethanol consumption increased levels of protein carbonyls and lipid peroxidation aldehydic products in the liver of Sod1(-/-) mice. Hepatic adenosine triphosphate (ATP) content was reduced dramatically in Sod1(-/-) mice fed ethanol in association with a decrease in the mitochondrial reduced glutathione (GSH) level and activity of MnSOD. Immunohistochemical determination of 3-nitrotyrosine (3NT) residues in liver sections of the Sod1 knockout mice treated with ethanol showed a significant increase of 3NT staining in the centrilobular areas. In conclusion, a rather moderate ethanol consumption promoted oxidative stress in Sod1(-/-) mice, with increased formation of peroxynitrite, protein carbonyls, and lipid peroxidation and decreased mitochondrial GSH and MnSOD. We speculate that the increased oxidative stress causes mitochondrial damage and reduction of ATP content, leading to alcoholic liver injury. This model may be useful in further mechanistic studies on alcohol-induced liver injury.

Adenovirus-mediated expression of Cu/Zn- or Mn-superoxide dismutase protects against CYP2E1-dependent toxicity

CYP2E1 induction by ethanol is one mechanism by which ethanol creates oxidative stress in the liver. The superoxide dismutases (SODs) are an important antioxidant enzyme defense system against reactive oxygen species (ROS). To investigate the protective role of SOD against CYP2E1-dependent toxicity, a transfected HepG2 cell line overexpressing CYP2E1 (E47 cells) was infected with adenoviral vectors containing Cu/Zn-SOD complementary DNA (cDNA) (Ad.SOD1) and Mn-SOD cDNA (Ad.SOD2). Forty-eight hours after infection, intracellular levels and activity of Cu/Zn-SOD and Mn-SOD were increased about 2- and 3-fold, respectively. Localization of the overexpressed Cu/Zn-SOD in the cytosol and Mn-SOD in the mitochondria was confirmed by assaying the levels and activity of SOD in the corresponding isolated fractions. Arachidonic acid (AA) plus iron-induced cell death was partially prevented in both Ad.SOD1- and Ad.SOD2-infected E47 cells. Overexpression of Cu/Zn-SOD and Mn-SOD also partially protected E47 cells from the increase in reactive oxygen production and lipid peroxidation and the loss of mitochondrial membrane potential induced by AA and iron. Infection with Cu/Zn-SOD and Mn-SOD also protected the E47 cells against AA toxicity or buthionine sulfoximine (BSO)-dependent toxicity. CYP2E1 levels and catalytic activity were not altered by overexpression of Cu/Zn-SOD or Mn-SOD. Cu/Zn-SOD in the cytosol and Mn-SOD in mitochondria each are capable of protecting HepG2 cells expressing CYP2E1 against cytotoxicity induced by pro-oxidants. In conclusion, these enzymes may be useful in the prevention or improvement of liver injury produced by agents known to be metabolized by CYP2E1 to reactive intermediates and to cause oxidative stress.

Threshold for antiproliferative and proapoptotic activity of ttaxol in HepG2 cells expressing human CYP3A4: effect of P-glycoprotein transporters

Taxol treatment froze the cell cycle in the G(2)/M phase, induced morphological changes characteristic of apoptotic/necrotic cell death and increased CYP3A4 enzymatic activity, CYP3A4 mRNA and protein levels in HepG2 cells overexpressing CYP3A4. Apoptosis was associated with cytochrome c release to the cytosol; however, at higher Taxol levels, cells became relatively resistant to the drug-induced freezing of the cell cycle and saturation thresholds for both antiproliferative and proapoptotic activity of Taxol were observed. P-Glycoprotein expression was only slightly increased by Taxol, however, P-glycoprotein-mediated pumping efficiency was significantly increased. Preincubation of cells with an anti-MDR1 monoclonal antibody prior to the drug treatment, coincubation of cells with a potent CYP3A4 inhibitor--ketoconazole--or with both compounds increased Taxol toxicity and proapoptotic activity, indicating that the P-glycoprotein system has a major role in Taxol disposition in hepatoblastoma cells.

Role of phospholipase A2 activation and calcium in CYP2E1-dependent toxicity in HepG2 cells

Previous studies suggested a role for calcium in CYP2E1-dependent toxicity. The possible role of phospholipase A2 (PLA2) activation in this toxicity was investigated. HepG2 cells that overexpress CYP2E1 (E47 cells) exposed to arachidonic acid (AA) +Fe-NTA showed higher toxicity than control HepG2 cells not expressing CYP2E1 (C34 cells). This toxicity was inhibited by the PLA2 inhibitors aristolochic acid, quinacrine, and PTK. PLA2 activity assessed by release of preloaded [3H]AA after treatment with AA+Fe was higher in the CYP2E1 expressing HepG2 cells. This [3H]AA release was inhibited by PLA2 inhibitors, alpha-tocopherol, and by depleting Ca2+ from the cells (intracellular + extracellular sources), but not by removal of extracellular calcium alone. Toxicity was preceded by an increase in intracellular calcium caused by influx from the extracellular space, and this was prevented by PLA2 inhibitors. PLA2 inhibitors also blocked mitochondrial damage in the CYP2E1-expressing HepG2 cells exposed to AA+Fe. Ca2+ depletion and removal of extracellular calcium inhibited toxicity at early time periods, although a delayed toxicity was evident at later times in Ca2+-free medium. This later toxicity was also inhibited by PLA2 inhibitors. Analogous to PLA2 activity, Ca2+ depletion but not removal of extracellular calcium alone prevented the activation of calpain activity by AA+Fe. These results suggest that release of stored calcium by AA+Fe, induced by lipid peroxidation, can initially activate calpain and PLA2 activity, that PLA2 activation is critical for a subsequent increased influx of extracellular Ca2+, and that the combination of increased PLA2 and calpain activity, increased calcium and oxidative stress cause mitochondrial damage, that ultimately produces the rapid toxicity of AA+Fe in CYP2E1-expressing HepG2 cells.

Increased expression of cytochrome P450 2E1 induces heme oxygenase-1 through ERK MAPK pathway

The inducible form of heme oxygenase (HO-1) is increased during oxidative injury, and this may be an important defense mechanism against such injury. Cytochrome P450 2E1 (CYP2E1) generates reactive oxygen species and promotes lipid peroxidation. In this study induction of HO-1 by CYP2E1 and the possible role of mitogen-activated protein kinase (MAPK) in this process were evaluated. HO-1 induction was observed in the livers of chronic alcohol-fed mice or pyrazole-treated rats, conditions known to elevate CYP2E1 levels. Increased levels of HO-1 were observed in HepG2 cells overexpressing CYP2E1 (E47 cells) compared with control HepG2 cells or HepG2 cells expressing CYP3A4. Expression of CYP2E1 in HepG2 cells transcriptionally activated the HO-1 gene, increasing HO-1 mRNA and protein expression and activity of a HO-1 reporter construct. CYP2E1 inhibitors and catalase blocked the increased production of reactive oxygen species as well as HO-1 induction. Increasing oxidative stress by the addition of arachidonic acid or depletion of glutathione further increased HO-1 induction. The phosphorylated form of ERK MAPK but not that of p38 or JNK MAPK was increased in E47 cells compared with the control C34 HepG2 cells. PD98059, a specific inhibitor of ERK MAPK, blocked the activity of a HO-1 reporter in E47 cells but not in C34 cells. These results suggest that increased CYP2E1 activity leads to induction of the HO-1 gene, and the ERK MAPK pathway is important in mediating this process. This induction may serve as an adaptive mechanism to protect the E47 cells against the CYP2E1-dependent oxidative stress.

Role of apoptosis in alcoholic liver injury

This article represents the proceedings of a symposium at the 2002 ISBRA/RSA meeting in San Francisco, USA. The organizer and chair was H. Ishii and co-chair was A.A. Nanji. The presentations were (1) INTRODUCTION: Apoptosis in alcoholic liver disease, by A. A. Nanji; (2) Mitochondria, oxidative stress and apoptosis in alcoholic liver disease, by M. Adachi; (3) Regulation of cell death by mitochondrial glutathione, by J.C. Fernández-Checa; (4) Toxicity of ethanol in HepG2 cells that express CYP2E1, by A.I. Cederbaum; (5) Is alcohol-enhanced liver apoptosis a pathogenic factor in alcoholic liver disease? by I.V. Deaciuc.

Proteasome inhibition potentiates CYP2E1-mediated toxicity in HepG2 cells

Chronic ethanol consumption causes increased oxidative damage in the liver. Induction of CYP2E1 is one pathway involved in how ethanol produces oxidative stress. Ethanol can cause protein accumulation, decreased proteolysis, and decreased proteasome activity. The objective of this study was to investigate the effect of inhibition of the proteasome activity on CYP2E1-dependent toxicity. HepG2 cells over-expressing CYP2E1 (E47 cells) were treated with arachidonic acid (AA) plus iron, agents important in development of alcoholic liver injury and which are toxic to E47 cells by a mechanism dependent on CYP2E1, oxidative stress, and lipid peroxidation. Addition of various proteasome inhibitors was associated with significant potentiation of the loss of cell viability caused by AA plus iron. Potentiation of toxicity was associated with increased oxidative damage as reflected by an increase in lipid peroxidation and accumulation of oxidized and nitrated proteins in E47 cells and an enhanced decline in mitochondrial membrane potential. Antioxidants prevented the loss of viability and the potentiation of this loss of viability by proteasome inhibition. CYP2E1 levels were elevated about 3-fold by the proteasome inhibitors. Inhibition of proteasome activity also potentiated toxicity of AA alone and toxicity after treatment to remove glutathione (GSH). Similar results were found in hepatocytes from pyrazole-treated rats with high levels of CYP2E1. In conclusion, proteasome activity plays an important role in modulating CYP2E1-mediated toxicity in HepG2 cells by regulating CYP2E1 levels and by removal of oxidized proteins. Such interactions may be important in CYP2E1-catalyzed toxicity of hepatotoxins and in alcohol-induced liver injury.

Iron and CYP2E1-dependent oxidative stress and toxicity

Iron plays a critical role in catalyzing the formation of potent oxidants. Increases in iron content enhance oxidative stress, whereas removal of iron deceases such stress. An association between iron and alcoholic liver injury has been proposed. The ability of iron to modulate the biochemical and toxicologic actions of cytochrome P450 2E1 (CYP2E1) has been evaluated by using isolated microsomes and intact liver cells. The ability of different iron complexes to stimulate microsomal lipid peroxidation and hydroxyl radical production during reduced form of nicotinamide adenine dinucleotide phosphate (NADPH)- and reduced form of nicotinamide adenine dinucleotide (NADH)-dependent electron transfer has been characterized. Certain iron complexes have been shown to be effective in promoting lipid peroxidation; others are better catalysts of hydroxyl radical production as a complex pattern has been found. Reactive oxygen production, lipid peroxidation, and interaction with iron chelates have been shown to be enhanced with microsomes isolated from ethanol-treated rats with elevated levels of CYP2E1. This increase was prevented by anti-CYP2E1 immunoglobulin (Ig)G or chemical inhibitors of CYP2E1. Thus, in the presence of iron complexes, microsomes enriched in CYP2E1 are especially reactive in generation of reactive oxygen species. To assess the toxicologic significance of this iron-CYP2E1 interaction, iron (ferric-nitrilotriacetate) was added to HepG2 cells, which were engineered to express the human CYP2E1. Ferric-nitrilotriacetate produced a greater toxicity in the CYP2E1-expressing HepG2 cells than that in control HepG2 cells. This enhanced, synergistic toxicity was blocked by antioxidants and inhibitors of CYP2E1. Mitochondrial membrane potential and ATP levels were decreased, and damage to the mitochondria played a critical role in the CYP2E1-plus-iron-dependent toxicity. These results support the suggestion that low concentrations of iron and polyunsaturated fatty acids can act as priming or sensitizing factors for CYP2E1-induced injury in HepG2 cells and hepatocytes. Such interactions may play a role in alcohol-induced liver injury.

Increased Sp1-dependent transactivation of the LAMgamma 1 promoter in hepatic stellate cells co-cultured with HepG2 cells overexpressing cytochrome P450 2E1

Laminin is a basement-membrane protein that increases in liver fibrosis. To study the role of oxidative stress on laminin expression, hepatic stellate cells (HSC) were co-cultured with HepG2 cells that do or do not express (E47 or C34 cells, respectively) CYP2E1, a potent generator of oxygen radicals. Co-incubation of HSC with E47 cells increased laminin beta1 and gamma1 proteins compared with co-incubation with C34 cells; this increase was prevented by antioxidants and CYP2E1 inhibitors. Similar results were observed in co-culture with primary hepatocytes from saline- or pyrazole-treated (with high levels of CYP2E1) rats. Laminin alpha1 chain was not detectable in the HSC in any of the systems; however, laminin alpha2 chain increased in HSC co-cultured with E47 cells. Synthesis but not turnover of laminin beta1 and gamma1 proteins was increased in HSC in the E47 co-culture. Laminin beta1 and gamma1 mRNAs were up-regulated in HSC in the E47 co-culture because of transcriptional activation of both genes. Transfection experiments in HSC with reporter constructs driven by the laminin gamma1 promoter showed maximal responsiveness with the -230/+106 and the -1400/+106 constructs in the E47 system. Gel-shift assays demonstrated an increase in Sp1 binding to the laminin gamma1 promoter in HSC when co-incubated with E47 cells, which was blocked by an anti-Sp1 antibody. Co-transfection of a Sp1 expression vector further increased the responsiveness of the -330LAMgamma1-CAT reporter vector in HSC in the HSC/E47 system. These results show that diffusable CYP2E1-derived oxidative-stress mediators induce synthesis of laminins by a transcriptional mechanism in HSC. Such interactions between hepatocytes and HSC may be important during liver fibrosis.

Metallothionein 2A induction by zinc protects HEPG2 cells against CYP2E1-dependent toxicity

Zinc has been shown to have antioxidant actions, which may be due, in part, to induction of metallothionein (MT). Such induction can protect tissues against various forms of oxidative injury because MT can function as an antioxidant. The objective of this study was to investigate if zinc or MT induction by zinc could afford protection against CYP2E1-dependent toxicity. HepG2 cells overexpressing CYP2E1 (E47cells) were treated with 60 microM arachidonic acid (AA), which is known to be toxic to these cells by a mechanism dependent on CYP2E1, oxidative stress, and lipid peroxidation. E47 cells were preincubated overnight in the absence or presence of metals such as zinc or cadmium that can induce MT. The culture medium containing the metals was removed, AA was added, and cell viability determined after 24 h incubation. Preincubation overnight with 150 microM zinc sulfate or 5 microM cadmium chloride induced a 20- to 30-fold increase of MT2A mRNA; high levels of MT2A mRNA were maintained during the subsequent challenge period with AA, even after the zinc was removed. MT protein levels were increased about 4- to 5-fold during the overnight preincubation with zinc and a 20- to 30-fold increase was observed 24 h after zinc removal during the AA challenge. The treatment with zinc was associated with significant protection against the loss of cell viability caused by AA in E47 cells. The zinc pretreatment protected about 50% against the DNA fragmentation, cell necrosis, the enhanced lipid peroxidation and increased generation of reactive oxygen species, and the loss of mitochondrial membrane potential induced by AA treatment in E47 cells. CYP2E1 catalytic activity and components of the cell antioxidant defense system such as glutathione (GSH), glutathione-S-transferase (GST), glutathione peroxidase (GPX), catalase, Cu,Zn superoxide dismutase (SOD), and MnSOD were not altered under these conditions. Zinc preincubation also protected the E47 cells against BSO-dependent toxicity. When E47 cells were coincubated with zinc plus AA for 24 h (i.e., zinc was not removed, nor was there a preincubation period prior to challenge with AA), AA toxicity was increased. Thus, zinc had a direct pro-oxidant effect in this model and an indirect antioxidant effect, perhaps via induction of MT. MT may have potential clinical utility for the prevention or improvement of liver injury produced by agents known to be metabolized by CYP2E1 to reactive intermediates and to cause oxidative stress.

Catalase protects HepG2 cells from apoptosis induced by DNA-damaging agents by accelerating the degradation of p53

Oxidants such as H(2)O(2) play a role in the toxicity of certain DNA-damaging agents, a process that often involves the tumor suppressor p53. H(2)O(2) is rapidly degraded by catalase, which protects cells against oxidant injury. To study the effect of catalase on apoptosis induced by DNA-damaging agents, HepG2 cells were infected with adenovirus containing the cDNA of catalase (Ad-Cat). Forty-eight hours after infection, catalase protein and activity was increased 7-10-fold compared with control cells infected with Ad-LacZ. After treatment with Vp16 or mitomycin C, control cells underwent apoptosis in a p53-dependent manner; however, overexpression of catalase inhibited this apoptosis. Basal levels as well as Vp16- or mitomycin C-stimulated levels of p53 and p21 protein were decreased in the catalase-overexpressing cells as compared with control cells; however, p53 mRNA levels were not decreased by catalase. There was no difference in p53 protein synthesis between catalase-overexpressing cells and control cells. However, pulse-chase experiments indicated that p53 protein degradation was enhanced in the catalase-overexpressing cells. Proteasome inhibitors but not calpeptin prevented the catalase-mediated decrease of p53 content. Whereas Vp16 increased, catalase overexpression decreased the phosphorylation of p53. The protein phosphatase inhibitor okadaic acid did not prevent the catalase-mediated down-regulation of p53 or phosphorylated p53. These results demonstrate that catalase protects HepG2 cells from apoptosis induced by DNA-damaging agents in association with decreasing p53 phosphorylation; the latter may lead to an acceleration in the degradation of p53 protein by the proteasome complex. This suggests that the level of catalase may play a critical role in cell-induced resistance to the effects of anti-cancer drugs which up-regulate p53.