Assessment of rat liver microsomal epoxide hydrolase as a marker of hepatocarcinogenesis

Pistacia Terebinthus Coffee Protects against Thioacetamide-Induced Liver Injury in Rats

Aim/background: Pistacia terebinthus is used as a coffee substitute in the East and Southern Anatolia regions of Turkey. It contains unsaturated fatty acids, tocopherols, polyphenols and carotenoids. P. terebinthus has anti-inflammatory and potential antioxidant activity. In this study we evaluated the protective effects of P. terebinthus coffee (PTC) on thioacetamide (TAA)-induced liver injury in rats. Materials and methods: Twenty-eight male Sprague-Dawley rats were equally randomized into four groups. Chronic liver injury was induced with TAA (100 mg/kg i.p. three times weekly). The first group of rats served as control and received only tap water (G1), and the remaining groups of rats received PTC, p.o (G2); TAA (G3); TAA plus PTC, p.o (G4), respectively. Results: After 8 weeks, PTC intake significantly reduced fibrosis/inflammation scores (p < 0.05) in the livers of TAA-treated group. Compared to control group, PTC intake reduced transforming growth factor beta (TGF-β) concentrations in the liver (p < 0.05). Compared to the TAA group, TGF-β, nuclear factor kappa B (NF)-κB (p < 0.05), tumor necrosis factor alpha (TNF-α) concentrations in the liver tissue were reduced by PTC intake. Discussion and conclusion: PTC intake provided beneficial effects against TAA-induced liver injury in rats. PTC probably suppresses the proinflammatory cytokines through NF-κB signaling pathway.

Rat HMGCoA reductase activation in thioacetamide-induced liver injury is related to an increased reactive oxygen species content

BACKGROUND/AIMS

In thioacetamide-induced liver injury a modification of isoprenoid content and an increase of reactive oxygen species has been described. We have examined how reactive oxygen species influence the 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate limiting enzyme of the isoprenoid biosynthetic pathway, to verify if changes of that enzyme activity are involved in the changed lipid composition of the liver.

METHODS

In chronic and acute thioacetamide-treated rat liver we measured the reactive oxygen species content, the activation state and K(M), the level and degradation rate of the hepatic reductase, its short term regulatory enzymes and the liver lipid profile.

RESULTS

In thioacetamide-treated rat liver, the reactive oxygen species content is high and the reductase is fully activated with no modifications in its K(M) and its short term regulatory enzymes. The reductase level is reduced in chronic thioacetamide treated rats and its degradation rate is altered.

CONCLUSIONS

The data show a relationship between reactive oxygen species production and altered 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. It is suggested that reducing the levels of reactive oxygen species may improve the altered lipid profile found in liver injury.

A proteomic analysis of thioacetamide-induced hepatotoxicity and cirrhosis in rat livers

Thioacetamide (TAA) administration is an established technique for generating rat models of liver fibrosis and cirrhosis. Oxidative stress is believed to be involved as TAA-induced liver fibrosis is initiated by thioacetamide S-oxide, which is derived from the biotransformation of TAA by the microsomal flavine-adenine dinucleotide (FAD)-containing monooxygense (FMO) and cytochrome P450 systems. A two-dimensional gel electrophoresis-mass spectrometry approach was applied to analyze the protein profiles of livers of rats administered with sublethal doses of TAA for 3, 6 and 10 weeks respectively. With this approach, 59 protein spots whose expression levels changed significantly upon TAA administration were identified, including three novel proteins. These proteins were then sorted according to their common biochemical properties and functions, so that pathways involved in the pathogenesis of rat liver fibrosis due to TAA-induced toxicity could be elucidated. As a result, it was found that TAA-administration down-regulated the enzymes of the primary metabolic pathways such as fatty acid beta-oxidation, branched chain amino acids and methionine breakdown. This phenomenon is suggestive of the depletion of succinyl-CoA which affects heme and iron metabolism. Up-regulated proteins, on the other hand, are related to oxidative stress and lipid peroxidation. Finally, these proteomics data and the data obtained from the scientific literature were integrated into an "overview model" for TAA-induced liver cirrhosis. This model could now serve as a useful resource for researchers working in the same area.

Effects of environmentally encountered epoxides on mouse liver epoxide-metabolizing enzymes

Male mice were treated (i.p.) for 3 days with 15 different environmentally encountered epoxides, and the effects of these compounds on liver microsomal and cytosolic epoxide hydrolase (mEH and cEH), glutathione S-transferase (mGST and cGST) and carboxylesterase (mCE) activities were determined. The epoxides included the pesticides: heptachlor epoxide, dieldrin, tridiphane, and juvenoid R-20458; the natural products: disparlure, limonin, nomilin, and epoxymethyloleate; the endogenous steroids: lanosterol epoxide, cholesterol-alpha-epoxide, and progesterone epoxide; and the industrial or synthetic epoxides: epichlorohydrin, araldite, trans-stilbene oxide, and 4'-phenylchalcone oxide. The pesticide epoxides were the most effective inducers of liver weight, microsomal protein, and the enzyme activities measured, with mEH and cEH activities towards cis-stilbene oxide (mEHcso and cEHcso), cGST activities towards four of five substrates, and mCE towards clofibrate (mCEclof) and p-nitrophenylacetate (mCEpna) increased following treatment with most of the pesticides. The synthetic epoxides increased some of the same activities, while the natural products, except for increases in cGST activities, and endogenous steroid epoxides were generally not inductive. cEH activity towards trans-stilbene oxide (cEHtso) was increased only following treatment with the peroxisome proliferator, tridiphane, but decreased following treatment with several of the epoxides, while microsomal cholesterol epoxide hydrolase (mEHchol) was increased only moderately by disparlure. Microsomes could effectively conjugate glutathione to chlorodinitrobenzene (mGSTcdnb) and cis-stilbene oxide (mGSTcso). These two activities were differentially induced by a few of the epoxides, suggesting that they may be selective substrates for different isozymes of mGST. Correlation coefficients were determined for the relative response of liver weight, subfraction protein, and enzyme activities. A relatively high correlation was found between the response of liver weight and cytosolic hydrolysis of trans-stilbene oxide (r = 0.73) and cis-stilbene oxide (r = 0.62), and cytosolic glutathione conjugation of dichloronitrobenzene (r = 0.66) and trans-stilbene oxide (r = 0.75). In addition, relatively high correlations were found between the different cGST activities, in particular for dichloronitrobenzene with trans-stilbene oxide (r = 0.89). These studies show that there exists a wide variation in the response of xenobiotic-metabolizing enzymes to environmentally encountered epoxides and that a fairly strong correlation exists between the increases in liver size and increases in certain cytosolic enzyme activities; they also suggest further studies concerning the possibility of an additional isozyme of mGST.

Cytosolic epoxide hydrolase

Epoxide hydrolase activity is recovered in the high-speed supernatant fraction from the liver of all mammals so far examined, including man. For some as yet unexplained reason, the rat has a very low level of this activity, so that cytosolic epoxide hydrolase is generally studied in mice. This enzyme selectively hydrolyzes trans epoxides, thereby complementing the activity of microsomal epoxide hydrolase, for which cis epoxides are better substrates. Cytosolic epoxide hydrolase has been purified to homogeneity from the livers of mice, rabbits and humans. Certain of the physicochemical and enzymatic properties of the mouse enzyme have been thoroughly characterized. Neither the primary amino acid, cDNA nor gene sequences for this protein are yet known, but such characterization is presently in progress. Unlike microsomal epoxide hydrolase and most other enzymes involved in xenobiotic metabolism, cytosolic epoxide hydrolase is not induced by treatment of rodents with substances such as phenobarbital, 2-acetylaminofluorene, trans-stilbene oxide, or butylated hydroxyanisole. The only xenobiotics presently known to induce cytosolic epoxide hydrolase are substances which also cause peroxisome proliferation, e.g., clofibrate, nafenopin and phthalate esters. These and other observations indicate that this enzyme may actually be localized in peroxisomes in vivo and is recovered in the high-speed supernatant because of fragmentation of these fragile organelles during homogenization, i.e., recovery of this enzyme in the cytosolic fraction is an artefact. The functional significance of cytosolic epoxide hydrolase is still largely unknown. In addition to deactivating xenobiotic epoxides to which the organism is exposed directly or which are produced during xenobiotic metabolism, primarily by the cytochrome P-450 system, this enzyme may be involved in cellular defenses against oxidative stress.

Detection of epoxide hydrolase in rat hepatoma cell lines and primary rat liver cells by immunoblotting

Rat epoxide hydrolase (EH) (EC 3.3.2.3) is elevated in cells of premalignant liver lesions, and variable EH activity has been reported for hepatocellular carcinomas. To facilitate detection of altered EH levels in liver cells, an immunoblotting method was devised. Rabbit antiserum specific for rat EH was prepared and used to detect EH extracted from suspensions of normal liver cells and from hepatoma cell lines. Compared with normal liver cells, 3 rat hepatoma cell lines, 7777, HTC and 17X, showed virtually undetectable EH levels by immunoblotting. The immunoblotting results rule out the possibility that very low EH enzymatic activity in the hepatoma cells results from production of normal amounts of non-functional enzyme protein.

Immunochemical characterization of human lung epoxide hydrolases

Immunochemical techniques were used to investigate the biochemical properties of human lung epoxide hydrolases. Two epoxide hydrolases with different immunoreactive properties were identified. These two epoxide hydrolases were found in both cytosolic and microsomal cell fractions. Immunotitration of enzyme activity showed that enzymes that catalyze the hydration of benzo(a)pyrene 4,5-oxide react with antiserum to rat microsomal epoxide hydrolase; those that hydrate trans-stilbene oxide do not. Immunotitration and Western blot experiments showed that microsomal and cytosolic benzo(a)pyrene 4,5-oxide hydrolases have significant structural homology. Immunohistochemical staining of human lung benzo(a)pyrene 4,5-oxide hydrolase showed that the enzyme is localized primarily in the bronchial epithelium. No cell type-specific localization was observed. An enzyme-linked immunosorbent assay was developed which allows direct quantitation of benzo(a)pyrene 4,5-oxide hydrolase protein. Levels of enzyme protein detected by this assay correlated well with enzyme levels determined by substrate conversion assays.

Characterization of multiple epoxide hydrolase activities in mouse liver nuclear envelope

A nuclear envelope-associated epoxide hydrolase in mouse liver that hydrates trans-stilbene oxide has been identified and characterized. This epoxide hydrolase is distinct from the enzyme in nuclear envelopes that hydrates benzo[a]pyrene 4,5-oxide and other arene oxides. This distinction was demonstrated by the criteria of pH optima, response to specific inhibitors in vitro, and precipitation by specific antibodies. The new epoxide hydrolase had a pH optimum of 6.8, was poorly inhibited by trichloropropene oxide, was potently inhibited by 4-phenylchalcone oxide, and did not bind to antiserum against benzo[a]pyrene 4,5-oxide hydrolase. This nuclear enzyme is similar in many of its properties to cytosolic and microsomal trans-stilbene oxide hydrolases and may be nuclear envelope-bound form of these other epoxide hydrolases. It differed from these other trans-stilbene oxide hydrolases in that its affinities for both trans-stilbene oxide (measured as apparent Km) and 4-phenylchalcone oxide (measured as I50) were 4- to 20-fold lower than those of either the cytosolic or microsomal forms.