Effects of 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene on the cellular levels of epoxide hydratase, cytochrome P-450b, and NADPH-cytochrome c (P-450) oxidoreductase messenger ribonucleic acids

Polymorphisms in the Microsomal Epoxide Hydrolase Gene: Role in Lung Cancer Susceptibility and Prognosis

Aims and background

The aim of this study was to investigate the relationship between EPHXI exon 3 Tyr113His and exon 4 His139Arg polymorphisms, predicted microsomal epoxide hydrolase (mEH) activity, and lung cancer development. mEH is a protective enzyme involved in oxidative defences against a number of environmental chemicals and pollutants, but it is also responsible for the xenobiotic activation of carcinogens.

Methods

We investigated the two polymorphisms of the mEH gene (EPHX1) in 58 lung cancer patients and 41 controls using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).

Results

The exon 3 Tyr113His polymorphism was associated with lung cancer (P <0.001). The frequency of the His113His homozygote genotype in exon 3 was significantly increased in patients compared with controls (P <0.001). In contrast, there was no significant difference in exon 4 polymorphisms between patients and controls. When the exon 3 and 4 polymorphisms were considered together, the combined EPHX1 His113His113/His139His139 genotype (very low predicted enzyme activity) was found to be associated with an increased risk of lung cancer (P = 0.044, OR = 3.063, CI = 0.932–10.069). We observed that patients with T3 + T4 tumors had an approximately 3-fold higher risk of the Tyr113/His113 genotype than patients with T1 + T2 tumors. Lung cancer patients carrying aheterozygote Tyr113/His113 genotype had a 2-fold increased risk of lymph node metastases (P = 0.051).

Conclusion

These findings suggest that the exon 3 Tyr113His and exon 4 His139Arg polymorphisms of EPHXI may be associated with a increased risk of lung cancer and a worse prognosis. Free full text available at www.tumorionline.it

Antioxidant and hepatoprotective potential of Lawsonia inermis L. leaves against 2-acetylaminofluorene induced hepatic damage in male Wistar rats

Background

Lawsonia inermis (Lythraceae) is an ethnomedicinal plant, traditionally known for curing several ailments such as skin diseases, bacterial infections, jaundice, renal lithiases and inflammation etc. The present work deals with assessment of in vitro antioxidant and in vivo hepatoprotective potential of butanolic fraction (But-LI) of Lawsonia inermis L. leaves.

Methods

Antioxidant activity was evaluated using deoxyribose degradation, lipid peroxidation inhibition and ferric reducing antioxidant power (FRAP) assay. In vivo protective potential of But-LI was assessed at 3 doses [100, 200 & 400 mg/kg body weight (bw)] against 2-acetylaminofluorene (2-AAF) induced hepatic damage in male Wistar rats.

Results

But-LI effectively scavenged hydroxyl radicals in deoxyribose degradation assay (IC₅₀ 149.12 μg/ml). Fraction also inhibited lipid peroxidation and demonstrated appreciable reducing potential in FRAP assay. Treatment of animals with 2-AAF resulted in increased hepatic parameters such as SGOT (2.22 fold), SGPT (1.72 fold), ALP (5.68 fold) and lipid peroxidation (2.94 fold). Different concentration of But-LI demonstrated pronounced protective effects via decreasing levels of SGOT, SGPT, ALP and lipid peroxidation altered by 2-AAF treatment. But-LI administration also restored the normal liver architecture as evident from histopathological studies.

Conclusions

The present experimental findings revealed that phytoconstituents of Lawsonia inermis L. possess potential to effectively protect rats from the 2-AAF induced hepatic damage in vivo possibly by inhibition of reactive oxygen species and lipid peroxidation.

Genetic enhancement of microsomal epoxide hydrolase improves metabolic detoxification but impairs cerebral blood flow regulation

Microsomal epoxide hydrolase (mEH) is a detoxifying enzyme for xenobiotic compounds. Enzymatic activity of mEH can be greatly increased by a point mutation, leading to an E404D amino acid exchange in its catalytic triad. Surprisingly, this variant is not found in any vertebrate species, despite the obvious advantage of accelerated detoxification. We hypothesized that this evolutionary avoidance is due to the fact that the mEH plays a dualistic role in detoxification and control of endogenous vascular signaling molecules. To test this, we generated mEH E404D mice and assessed them for detoxification capacity and vascular dynamics. In liver microsomes from these mice, turnover of the xenobiotic compound phenanthrene-9,10-oxide was four times faster compared to WT liver microsomes, confirming accelerated detoxification. mEH E404D animals also showed faster metabolization of a specific class of endogenous eicosanoids, arachidonic acid-derived epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs). Significantly higher DHETs/EETs ratios were found in mEH E404D liver, urine, plasma, brain and cerebral endothelial cells compared to WT controls, suggesting a broad impact of the mEH mutant on endogenous EETs metabolism. Because EETs are strong vasodilators in cerebral vasculature, hemodynamics were assessed in mEH E404D and WT cerebral cortex and hippocampus using cerebral blood volume (CBV)-based functional magnetic resonance imaging (fMRI). Basal CBV₀ levels were similar between mEH E404D and control mice in both brain areas. But vascular reactivity and vasodilation in response to the vasodilatory drug acetazolamide were reduced in mEH E404D forebrain compared to WT controls by factor 3 and 2.6, respectively. These results demonstrate a critical role for mEH E404D in vasodynamics and suggest that deregulation of endogenous signaling pathways is the undesirable gain of function associated with the E404D variant.

Cytochrome P450 enzyme systems in fungi

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

The role of individual susceptibility in cancer burden related to environmental exposure

Individual susceptibility to cancer may result from host factors including differences n metabolism, DNA repair, altered expression of protooncogenes and tumor suppressor genes, and nutritional status. Since most carcinogens require metabolic activation before binding to DNA, variations in an individual's metabolic phenotype that have detected in enzymes involved in activation and detoxification should play an essential role in the development of environmental cancer. This phenotypic metabolic variation has now been related to genetic polymorphisms, and many genes encoding carcinogen-metabolizing enzymes have been identified and cloned. Consequently, allelic variants or genetic defects that give rise to the observed variation and new polymorphisms have been recognized. Development of simple polymerase chain reaction (PCR)-based assays has enabled identification of an individual's genotype for a variety of metabolic polymorphisms. Thus, recent knowledge of the genetic basis for individual metabolic variation has opened new possibilities of studies focusing on increased individual susceptibility to environmentally induced cancer, which are reviewed with special reference to smoking-induced lung cancer. Cancer susceptibility due to chemical exposure is likely to be determined by an individual's phenotype for a number of enzymes (both activating and detoxifying) relevant to that of a single carcinogen or mixtures of carcinogens. Given the number and variability in expression of carcinogen-metabolizing enzymes and the complexity of chemical exposures, assessment of a single polymorphic enzyme (genotype) may not be sufficient. Mutations in the p53 gene are among the most common genetic changes in human cancer. The frequency and type p53 mutations can act as a fingerprint of carcinogen exposure and may therefore provide information about external etiological agents, intensity of exposure, and host factors affecting the tumorigenesis process. In human lung cancer, p53 mutations (both the mutation pattern and frequency) have been linked with tobacco smoking; the type of mutation most frequently observed is G:C to T:A transversion, a mutation preferentially induced by benzo[a]pyrene diol epoxide. An association between the presence of this transversion and the genotype deficient in glutathione S-transferase M1-mediated detoxification has been observed in lung cancer. Taken together, these findings suggest that determination of metabolic at risk genotypes in combination with levels of DNA adducts in target (surrogate) tissues and the p53 mutation pattern should allow the identification of susceptible individuals and subgroups in carcinogen-exposed populations.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Hepatic transcriptional up-regulator of the rat microsomal epoxide hydrolase gene

Rat microsomal epoxide hydrolase (mEH) is one of the detoxification enzymes and selectively expressed in liver. A 350-bp DNA fragment of the proximal promoter was found to contain information sufficient to express the mEH gene in hepatoma cells, however not in nonhepatoma cells. We identified two cis-acting elements, epoxide hydrolase proximal element 1 (EHP1) and 2 (EHP2), in this promoter region by using transient transfection assays. Each element is a new cell-type-specific transcriptional up-regulator. The cell-type-specific activity of EHP1 correlates to the limited cell distribution of its cognate transacting factor(s). In the case of EHP2, a similar or possibly the same cognate factor(s) binding to EHP2 was detected by DNase I footprinting and gel retardation assays in both hepatoma and nonhepatoma cells. However, EHP2 functions as an up-regulator only in hepatoma cells. Our finding adds repertoire to a battery of cis-regulatory elements that are required for liver-specific transcription.

Induction of microsomal epoxide hydrolase by nitrosamines in rat liver. Effect on messenger ribonucleic acids

Nitrosomethylethylamine and nitrosomethylpropylamine were found to be more potent inducers of rat liver microsomal epoxide hydrolase (styrene oxide hydrolase) than nitrosodiethylamine or nitrosodimethylamine. The time course of induction following a single administration of nitrosodimethylethylamine, nitrosomethylpropylamine or nitrosodiethylamine each showed a delay of 24 hr during which enzyme activity was unaltered. After that time activity increased and reached a maximum at between 72 and 120 hr. Increased enzyme activity following NDEA was paralleled by changes in the content of epoxide hydrolase in microsomes as measured by Western blots. Nitrosamines caused an increase of mRNA for epoxide hydrolase which was detected by probing Northern blots with a [32]-P labelled epoxide hydrolase cDNA and by in vitro translation of polyadenylated mRNA. Both methods showed a maximal increase at 72 hr after nitrosodiethylamine treatment but a significant increase was also observed at 24 hr although at this time no increase in enzyme activity was apparent.

Complementary DNA and amino acid sequence of rat liver microsomal, xenobiotic epoxide hydrolase

The coding nucleotide sequence for rat liver microsomal, xenobiotic epoxide hydrolase was determined from two overlapping cDNA clones, which together contain 1750 nucleotides complementary to epoxide hydrolase mRNA. The single open reading frame of 1365 nucleotides codes for a 455 amino acid polypeptide with a molecular weight of 52,581. The deduced amino acid composition agrees well with those determined by direct amino acid analysis of the rat protein, and the amino acid sequence is 81% identical to that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and comparison to codon usage for NADPH-cytochrome P-450 oxidoreductase and cytochromes P-450b, P-450d, and P-450PCN, suggest that epoxide hydrolase is more conserved than cytochromes P-450b and P-450PCN; comparison of the extent of sequence conservation for 12 homologous proteins between the rat and rabbit, including cytochrome P-450b, supports this hypothesis, and indicates that much of epoxide hydrolase is constrained to maintain its hydrophobic character, consistent with its intramembranous location. The predicted membrane topology of epoxide hydrolase delineates 6 membrane-spanning segments, less than the 8 or 10 predicted for two cytochrome P-450 isozymes; the lower number of membrane-spanning segments predicted for epoxide hydrolase correlates with its lesser dependence on the membrane for maintenance of its tertiary structure and catalytic activity.

Induction of different isozymes of cytochrome P-450 and of microsomal epoxide hydrolase in rat liver by 2-acetylaminofluorene and structurally related compounds

The amounts of five different forms of cytochrome P-450 and of microsomal epoxide hydrolase were determined immunochemically in rat liver microsomes before and after treatment of the animals with 2-acetylaminofluorene and 15 structurally related compounds. The amount of cytochrome P-450c was found to be increased about 60-fold after treatment with 2-aminofluorene and 3-aminofluorene. Administration of 1-aminofluorene, 4-aminofluorene, 2-acetylaminofluorene and nitrofluorene increased this isozyme about 15-19 times. 2-Aminofluorene was found to inhibit the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to a cytoplasmic receptor 50% at a concentration of 3.12 microM, while no such inhibition could be detected with 2-acetylaminofluorene. Induction of ethoxyresorufin O-deethylase activity was found to be highly correlated (+0.95) with the induction of cytochrome P-450c. Also correlated with the induction of this form was the amount of cytochrome P-450d (+0.84), which could be maximally increased about fourfold. Cytochromes P-450b + e were induced by 2-acetylaminofluorene, 4-acetylaminofluorene and fluorene (about tenfold), while 4-aminofluorene and 4-acetylaminofluorene were found to elevate cytochrome P-450PB/PCN-E about threefold. Microsomal epoxide hydrolase was induced by many of the compounds tested, with 2,7-diaminofluorene, 2,7-diacetylaminofluorene, 2-acetylaminofluorene and 2-(N-hydroxy)acetylaminofluorene being the most potent. No correlation of the induction of this enzyme with the induction of any isozyme of cytochrome P-450 was observed.

Induction of epoxide hydrolase in cultured rat hepatocytes and hepatoma cell lines

The expression of epoxide hydrolases was studied in cultured rat hepatocytes and hepatoma cell lines. Styrene 7,8-oxide and benzo[a]pyrene 4,5-oxide were used as substrates for microsomal epoxide hydrolase and trans-stilbene oxide for the cytosolic form of this enzyme. In freshly isolated hepatocytes from control rats, microsomal epoxide hydrolase activity was 7.7 and 10.8 nmoles/mg cellular protein/min with benzo[a]pyrene 4,5-oxide and styrene 7,8-oxide as substrates respectively. This enzyme activity increased by more than 2-fold in hepatocytes after 24 hr in culture and remained elevated throughout 96 hr using both substrates. In cultured hepatocytes from rats pretreated in vivo with phenobarbital, trans-stilbene oxide, 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene, both benzo[a]pyrene 4,5-oxide and styrene 7,8-oxide hydrolase activities were increased greater than 1.8 relative to controls. Hepatocytes from 2-acetylaminofluorene-pretreated animals at 24 hr in culture had approximately 9-fold higher activities than control hepatocytes. In marked contrast to microsomal epoxide hydrolase activity, the cytosolic enzyme showed an initial activity of 191 pmoles/mg cellular protein/min in freshly isolated hepatocytes, decreased by 75% after 24 hr in culture, and was barely detectable at 96 hr. A similar trend was apparent in hepatocytes from the pretreated animals. In vitro treatment of hepatocytes with trans-stilbene oxide and phenobarbital increased microsomal epoxide hydrolase, while this activity was refractory to 2-acetylaminofluorene treatment. Styrene 7,8-oxide hydrolase activity was increased in the McA-RH-7777 rat hepatoma cell line by phenobarbital, trans-stilbene oxide and 2-acetylaminofluorene treatment. Similarly, benzo[a]pyrene 4,5-oxide hydrolase activity was also increased in this cell line by treatment with phenobarbital and trans-stilbene oxide but not by 2-acetylaminofluorene. Microsomal epoxide hydrolase activity in rat H4-II-E hepatoma cells was refractory to induction, except by trans-stilbene oxide treatment, which caused a 70% increase in benzo[a]pyrene 4,5-oxide hydrolase activity.

Identification of the forms of cytochrome P-450 induced in rat liver by 2-acetylaminofluorene using immunoblotting and partial purification

The amounts of 5 different forms of cytochrome P-450 in liver microsomes from rats treated with 2-acetylaminofluorene were determined and compared with the corresponding patterns in microsomes from control, 3-methylcholanthrene- and phenobarbital-treated animals. 2-Acetylaminofluorene was found to increase the amount of cytochromes P-450b + e 10-fold and of cytochrome P-450d 3-fold, while there was a 54% increase in the level of cytochrome P-450 PB/PCN-E. Cytochrome P-450c was increased from a level too low to detect (less than 0.001 pmol/mg protein) to 0.019 pmol/mg protein. These findings were also confirmed by partial purification of cytochromes P-450b + e and c after 2-acetylaminofluorene treatment.