Cyp2e1 and ALD

Xenobiotic and Endobiotic Mediated Interactions Between the Cytochrome P450 System and the Inflammatory Response in the Liver

The liver is a unique organ in the body as it has significant roles in both metabolism and innate immune clearance. Hepatocytes in the liver carry a nearly complete complement of drug metabolizing enzymes, including numerous cytochrome P450s. While a majority of these enzymes effectively detoxify xenobiotics, or metabolize endobiotics, a subportion of these reactions result in accumulation of metabolites that can cause either direct liver injury or indirect liver injury through activation of inflammation. The liver also contains multiple populations of innate immune cells including the resident macrophages (Kupffer cells), a relatively large number of natural killer cells, and blood-derived neutrophils. While these cells are primarily responsible for clearance of pathogens, activation of these immune cells can result in significant tissue injury during periods of inflammation. When activated chronically, these inflammatory bouts can lead to fibrosis, cirrhosis, cancer, or death. This chapter will focus on interactions between how the liver processes xenobiotic and endobiotic compounds through the cytochrome P450 system, and how these processes can result in a response from the innate immune cells of the liver. A number of different clinically relevant diseases, as well as experimental models, are currently available to study mechanisms related to the interplay of innate immunity and cytochrome P450-mediated metabolism. A major focus of the chapter will be to evaluate currently understood mechanisms in the context of these diseases, as a way of outlining mechanisms that dictate the interactions between the P450 system and innate immunity.

Role of CYP2E1 in ethanol-induced oxidant stress, fatty liver and hepatotoxicity

BACKGROUND/AIMS

Several pathways contribute to mechanisms by which ethanol induces oxidant stress. While some studies support a role for cytochrome P450 2E1 (CYP2E1), others do not. There is a need to develop oral models of significant ethanol-induced liver injury and to evaluate the possible role of CYP2E1 in ethanol actions in such models.

METHODS

We evaluated chronic ethanol-induced liver injury, steatosis and oxidant stress in wild-type (WT) mice, CYP2E1 knockout (KO) mice and in humanized CYP2E1 knockin (KI) mice, where the human 2E1 was added back to mice deficient in the mouse 2E1. WT mice and CYP2E1 KO and KI mice (both provided by Dr. F. Gonzalez, NCI) were fed a high-fat Lieber-DeCarli liquid diet for 3 weeks; pair-fed controls received dextrose.

RESULTS

Ethanol produced fatty liver and oxidant stress in WT mice, but liver injury (transaminases, histopathology) was minimal. Ethanol-induced steatosis and oxidant stress were blunted in the KO mice (no liver injury) but restored in the KI mice. Significant liver injury was produced in the ethanol-fed KI mice with elevated transaminases and necrosis. This liver injury in the KI mice was associated with elevated oxidant stress and elevated levels of the human CYP2E1 compared to levels of the mouse 2E1 in WT mice. Activation of JNK was observed in the ethanol-fed KI mice compared to the other groups. Fatty liver in WT and KI mice was associated with lower levels of lipolytic PPAR-α. No such changes were found in the ethanol-fed KO mice.

CONCLUSIONS

These results show that CYP2E1 plays a major role in ethanol-induced fatty liver and oxidant stress. Restoring CYP2E1 in the CYP2E1 KO mice restores ethanol-induced fatty liver and oxidant stress.

Biochemical mechanism of Wu-Zi-Yan-Zong-Wan, a traditional Chinese herbal formula, against alcohol-induced oxidative damage in CYP2E1 cDNA-transfected HepG2 (E47) cells

AIM OF THE STUDY

Wu-Zi-Yan-Zong-Wan (WZ) is a traditional Chinese herbal formula which is commonly used for treating patients with "Yang deficiency". In the present study, the effect of WZ on ethanol-induced toxicity in CYP2E1 cDNA-transfected HepG2 (E47) cells was investigated.

MATERIALS AND METHODS

WZ extract was obtained by extracting the herbal powder with 50% ethanol (v/v, in water) and the effect of the extract on ethanol-induced toxicity was investigated in cultured cells.

RESULTS

The treatment with WZ extract (12.5-200 microg/mL) for 24h dose-dependently protected against ethanol-induced toxicity in E47 cells, as evidenced by the enhanced cell viability and decreased extent of lactate dehydrogeanse leakage. The cytoprotection against ethanol-induced toxicity was associated with decreases in the extents of reactive oxygen species production and lipid peroxidation, as well as increases in mitochondrial reduced glutathione and membrane potential. In addition, WZ extract treatment also suppressed the formation of DNA fragments in ethanol-intoxicated E47 cells.

CONCLUSIONS

WZ extract was found to protect against the ethanol-induced toxicity in E47 cells, possibly by virtues of its antioxidant activity.

CYP2E1 potentiation of LPS and TNFα-induced hepatotoxicity by mechanisms involving enhanced oxidative and nitrosative stress, activation of MAP kinases, and mitochondrial dysfunction

The mechanisms by which alcohol causes cell injury are not clear. A major mechanism that is the focus of considerable research is the role of lipid peroxidation and oxidative stress in alcohol toxicity. Many pathways have been suggested to play a role in how alcohol induces oxidative stress. Considerable attention has been given to alcohol-elevated production of lipopolysaccharide (LPS) and TNFα and to alcohol induction of CYP2E1. These two pathways are not exclusive of each other, however, associations and interactions between them, especially in vivo, have not been extensively evaluated. We have shown that increased oxidative stress from induction of CYP2E1 in vivo sensitizes hepatocytes to LPS and TNF toxicity and that oxidants, such as peroxynitrite, activation of p38 and JNK MAP kinases, inactivation of NF-kB protective pathways and mitochondrial dysfunction are downstream mediators of this CYP2E1-LPS/TNF potentiated hepatotoxicity. This review will summarize studies showing potentiated interactions between these two risk factors in promoting liver injury and the mechanisms involved.

Role of oxidative stress in alcohol-induced liver injury

Reactive oxygen species (ROS) are highly reactive molecules that are naturally generated in small amounts during the body's metabolic reactions and can react with and damage complex cellular molecules such as lipids, proteins, or DNA. Acute and chronic ethanol treatments increase the production of ROS, lower cellular antioxidant levels, and enhance oxidative stress in many tissues, especially the liver. Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol produces liver injury. Many pathways play a key role in how ethanol induces oxidative stress. This review summarizes some of the leading pathways and discusses the evidence for their contribution to alcohol-induced liver injury. Special emphasis is placed on CYP2E1, which is induced by alcohol and is reactive in metabolizing and activating many hepatotoxins, including ethanol, to reactive products, and in generating ROS.

Negundoside, an iridiod glycoside from leaves of Vitex negundo, protects human liver cells against calcium-mediated toxicity induced by carbon tetrachloride

AIM: To evaluate the protective effect of 2'-p-hydroxybenzoylmussaenosidic acid [negundoside (NG), against carbon tetrachloride (CCl₄)-induced toxicity in HuH-7 cells.

METHODS: CCl₄ is a well characterized hepatotoxin, and inducer of cytochrome P450 2E1 (CYP2E1)-mediated oxidative stress. In addition, lipid peroxidation and accumulation of intracellular calcium are important steps in the pathway involved in CCl₄ toxicity. Liver cells (HuH-7) were treated with CCl₄, and the mechanism of the cytoprotective effect of NG was assessed. Silymarin, a known hepatoprotective drug, was used as control.

RESULTS: NG protected HuH-7 cells against CCl₄ toxicity and loss of viability without modulating CYP2E1 activity. Prevention of CCl₄ toxicity was associated with a reduction in oxidative damage as reflected by decreased generation of reactive oxygen species (ROS), a decrease in lipid peroxidation and accumulation of intracellular Ca²⁺ levels and maintenance of intracellular glutathione homeostasis. Decreased mitochondrial membrane potential (MMP), induction of caspases mediated DNA fragmentation and cell cycle arrest, as a result of CCl₄ treatment, were also blocked by NG. The protection afforded by NG seemed to be mediated by activation of cyclic adenosine monophosphate (cAMP) synthesis and inhibition of phospholipases (cPLA2).

CONCLUSION: NG exerts a protective effect on CYP2E1-dependent CCl₄ toxicity via inhibition of lipid peroxidation, followed by an improved intracellular calcium homeostasis and inhibition of Ca²⁺-dependent proteases.

Effect of dietary epigallocatechin-3-gallate on cytochrome P450 2E1-dependent alcoholic liver damage: enhancement of fatty acid oxidation

This study was designed to determine whether dietary epigallocatechin-3-gallate (EGCG), the most abundant catechin polyphenol in green tea, can protect the liver from cytochrome P450 2E1 (CYP2E1)-dependent alcoholic liver damage. Compared with an ethanol group, when EGCG was present in the ethanol diet, the formation of a fatty liver was significantly reduced and the serum aspartate transaminase (AST) and alanine transaminase (ALT) levels were much lower. Ethanol treatment significantly elevated hepatic CYP2E1 expression while simultaneously reducing hepatic phospho-acetyl CoA carboxylase (p-ACC) and carnitine palmitoyl-transferase 1 (CPT-1) levels. While EGCG markedly reversed the effect of ethanol on hepatic p-ACC and CPT-1 levels, it had no effect on the ethanol-induced elevation in CYP2E1 expression. EGCG prevents ethanol-induced hepatotoxicity and inhibits the development of a fatty liver. These effects were associated with improvements in p-ACC and CPT-1 levels. The use of EGCG might be useful in treating patients with an alcoholic fatty liver.

CYP2E1-dependent oxidative stress and toxicity: role in ethanol-induced liver injury

Ethanol-induced oxidative stress plays a major role in the mechanisms by which ethanol causes liver injury. Many pathways contribute to how ethanol induces a state of oxidative stress. One central pathway appears to be the induction, by ethanol, of the CYP2E1 form of cytochrome P450 enzymes. CYP2E1 is of interest because it metabolises and activates many toxicological substrates, including ethanol, to more reactive products. Levels of CYP2E1 are elevated under a variety of physiological and pathophysiological conditions. CYP2E1 is an effective generator of reactive oxygen species. This review summarises some of the biochemical and toxicological properties of CYP2E1, and briefly describes the use of HepG2 cell lines in assessing the actions of CYP2E1. Future directions, which may help to better understand the actions of CYP2E1 and its role in alcoholic liver injury, are suggested.

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.

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.

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.

Ca2+-dependent and independent mitochondrial damage in HepG2 cells that overexpress CYP2E1

CYP2E1-dependent mitochondrial damage, in the presence or absence of extracellular calcium, was investigated. HepG2 cells expressing CYP2E1 (E47 cells) were preloaded with arachidonic acid (AA), washed, and incubated with iron-nitrilotriacetate 1:3 complex (Fe-NTA) in minimum essential medium (MEM) (1.8mM Ca(2+)) or Ca(2+)-free MEM (SMEM). Toxicity in SMEM was CYP2E1-dependent, necrotic, and lipid peroxidation-dependent. Intracellular calcium did not significantly change during the incubation in SMEM. Mitochondrial damage preceded the loss of plasma membrane integrity and was significant at 12h of incubation, in coincidence with the toxicity. E47 cells treated with AA+Fe in MEM also showed a decline of mitochondrial membrane potential (Delta(Psi)(m)) that preceded the loss of plasma membrane integrity, but starting at earlier times, e.g., 3h than in SMEM. The decline in Delta(Psi)(m) and the toxicity in both MEM and SMEM were inhibited by alpha-tocopherol and cyclosporin A, while the calpain inhibitor calpeptin was only effective in MEM. In conclusion, oxidative damage to mitochondria and the permeability transition plays a role in the CYP2E1-dependent toxicity of Fe+AA in HepG2 cells, both in MEM and SMEM. Ca(2+) mobilization and activation of calpain contributes to the more rapid onset of mitochondrial damage in MEM, while oxidative damage and lipid peroxidation are involved in the Ca(2+)-independent later onset of mitochondrial damage.

Role of calcium and calcium-activated proteases in CYP2E1-dependent toxicity in HEPG2 cells

The objective of this work was to investigate whether CYP2E1- and oxidative stress-dependent toxicity in HepG2 cells is mediated by an increase of cytosolic Ca2+ and activation of Ca2+-modulated processes. HepG2 cells expressing CYP2E1 (E47 cells) or control cells not expressing CYP2E1 (C34 cells) were preloaded with arachidonic acid (AA, up to 10 microm) and, after washing, incubated with iron-nitrilotriacetic acid (up to 100 microm) for variable periods (up to 12 h). Toxicity was greater in E47 cells than in C34 cells at all times and combinations of iron/AA tested. Cytosolic calcium increased with incubation time in both cell lines, but the increase was higher in E47 cells than in C34 cells. The rise in calcium was an early event and preceded the developing toxicity. Toxicity in E47 cells and the increase in Ca2+ were inhibited by omission of Ca2+ from the extracellular medium, and toxicity was restored by reincorporation of Ca2+. An inhibitor of Ca2+ release from intracellular stores did not prevent the toxicity or the increase in Ca2+, reflecting a role for the influx of extracellular Ca2+ in the toxicity. Reactive oxygen production was similar in media with or without calcium, indicating that calcium was not modulating CYP2E1-dependent oxidative stress. Toxicity, lipid peroxidation, and the increase of Ca2+ in E47 cells exposed to iron-AA were inhibited by alpha-tocopherol. E47 cells (but not C34 cells) exposed to iron-AA showed increased calpain activity in situ (40-fold). The toxicity in E47 cells mirrored calpain activation and was inhibited by calpeptin, suggesting that calpain activation plays a causal role in toxicity. These results suggest that CYP2E1-dependent toxicity in this model depends on the activation of lipid peroxidation, followed by an increased influx of extracellular Ca2+ and activation of Ca2+-dependent proteases.