The induction of hepatic and extrahepatic xenobiotic metabolism in the rat and ferret by a polychlorinated biphenyl mixture (Aroclor 1254)

Metabolic Stability and Metabolite Identification of CYP450 Probe Substrates in Ferret Hepatocytes

BACKGROUND

Ferrets exhibit similar lung physiology to humans and display similar clinical signs following influenza infection, making them a valuable model for studying high susceptibility and infection patterns. However, the metabolic fate of several common human CYP450 probe substrates in ferrets is still unknown and has not been studied.

OBJECTIVE

The purpose of this study was to investigate the metabolism of nine human CYP450 probe substrates in ferret hepatocytes and explore their metabolic rate differences between ferrets and other species.

METHOD

Nine substrates were individually incubated in ferret hepatocytes for up to 120 min. At each time point, 30 μL mixtures were extracted for stability analysis using LC-MS/MS methods. After a 120-minute incubation period, 400 μL of the mixtures were extracted for metabolite identification using UHPLC-QExactive Plus.

RESULTS

The metabolic clearance was determined as follows: testosterone > phenacetin > bupropion > omeprazole > midazolam > dextromethorphan > chlorzoxazone > taxol > diclofenac. Seven metabolites were identified from phenacetin. Deethylation was found to be the major pathway, and the major metabolite was matched with acetaminophen as probed with the CYP1A2 enzyme. Six metabolites were identified from diclofenac. Glucuronidation was the primary pathway, and a metabolite was found to match 4-OH-diclofenac as probed with the CYP2C9 enzyme. Twenty-two metabolites were identified from omeprazole. The major metabolic pathways included mono-oxygenation and sulfoxide to thioether conversion. No metabolite was found to match with 5-OH-omeprazole as probed with the CYP2C19 enzyme. Twenty-two metabolites were identified from dextromethorphan. Demethylation was found to be the major metabolic pathway, and one demethylation metabolite was matched with dextrorphan as probed with the CYP2D6 enzyme. Fourteen metabolites were identified from midazolam. Mono-oxygenation was found to be the primary metabolic pathway, and one of the mono-oxygenation metabolites was matched with 1-OH-midazolam as probed with the CYP3A4 enzyme. Eight metabolites were identified from testosterone. Mono-oxygenation and glucuronidation were identified as the major metabolic pathways. One mono-oxygenation was matched with 6-.-testosterone as probed with the CYP3A4 enzyme. Six metabolites were identified from taxol. Hydrolysis and mono-oxygenation were the top two metabolic pathways. No metabolite was matched with 6-.-OH-taxol as probed with the CYP2C8 enzyme. Ten metabolites were identified from bupropion. Mono-oxygenation and hydrogenation were identified as the top two metabolic pathways. No mono-oxygenation metabolite was matched with hydroxy-bupropion as probed with the CYP2B6 enzyme. Nine metabolites were identified from chlorzoxazone. Monooxygenation and sulfation were the top two metabolic pathways. One mono-oxygenation metabolite was matched with 6-OH-chlorzoxazone as probed with the CYP2E1 enzyme.

CONCLUSION

Nine human CYP probe substrates were clearly metabolized in ferret hepatocytes, demonstrating substrate-dependent metabolic rates in ferret hepatocytes and species-dependent metabolic rates in mouse, rat, dog, monkey, and human hepatocytes. Except for 6-a-5-OH-omeprazole, 6-.-OH-taxol, and hydroxy-bupropion, specific metabolites of other six probe substrates in ferret hepatocytes were detected and identified as probed with six human CYP enzymes, respectively.

Examples of Alternative Use in Toxicology for Common Species

The principle of refinement of animal usage in toxicology dictates that the appropriateness of a particular animal species for a particular protocol or experiment be thoroughly explored. Species are selected all too often on the basis of convenience or tradition. Rats are traditionally used for acute lethality testing and carcinogenicity testing. Dogs are traditionally used as a “nonrodent” species for general toxicity assessments. This review seeks to make the case that, for both scientific and economic reasons, other species can be appropriately substituted for rats or dogs for general toxicity assessment studies. These alternative species need not be totally exotic, but can, in fact, be species used in other areas of toxicology. Earthworms and fish are nonvertebrate animals used in environmental assessment studies. Earthworms could be used for lethality assessment in place of rodents, particularly for “QC batch” release or toxicity rating purposes. Fish could be used to further define hepatic carcinogenicity. Guinea pigs are frequently used for dermatologic studies, but rarely for other purposes. While a rodent, the guinea pig possesses many physiologic and metabolic characteristics that may make it more appropriate than rats for the chronic testing of certain classes of chemicals (NSAIDs, peroxisomal proliferators). Ferrets have been well studied in teratologic assessments, but have not gained wide acceptance as a “nonrodent” model. This review discusses in detail the available technology and published data that justifies the expanded and appropriate use of these “alternative” species. Special emphasis is given to xenobiotic metabolism, which is a major determinant in speciesrelated differences in toxicity.

Pigs and Ferrets as Models in Toxicology and Biological Safety Assessment

Both the pig and the ferret are used as animal models in toxicology and drug development, with the pig having been subject to a significant increase in usage (particularly in Europe) over the last 5 years. These two species are quite different from primates and dogs, the “standard” nonrodent models. This article seeks to both survey the rationales for and extent of use of the pig and ferret, and to highlight consideration and key factors in their use in studies.

Polychlorinated biphenyls induce arachidonic acid release in human platelets in a tamoxifen sensitive manner via activation of group IVA cytosolic phospholipase A2-alpha

Polychlorinated biphenyls (PCBs) are stable compounds commonly found in nature as environmental pollutants. PCBs can affect the endocrine function of hormones such as steroid-hormones. Also, PCBs are known to be inducers of arachidonic acid release in various cells. We report, here, the effects of PCBs on eicosanoid formation, arachidonic acid release and cytosolic phospholipase A2-alpha (cPLA2-alpha) activation in human platelets. Ortho-substituted PCBs induced a time and dose-dependent release of arachidonic acid and the concomitant formation of 12(S)-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid (12-HETE) and 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid (12-HHT) in human platelets. The release of arachidonic acid and the formation of 12-HETE was completely blocked by the cPLA2-alpha inhibitors AACOCF3 or pyrrolidine-1. PCB-treatment of platelets demonstrated that the cPLA2-alpha protein as well as PLA2 activity translocated to the membrane fraction, independent of a rise in intracellular Ca2+. Furthermore, electrophoretic gel mobility shift analysis of cPLA2-alpha on SDS-PAGE demonstrated a PCB-dependent phosphorylation of cPLA2-alpha. The effects of 17beta-estradiol and two structurally unrelated anti-estrogens, nafoxidin and tamoxifen on PCB-induced arachidonic acid release in platelets were also investigated. Both nafoxidin and tamoxifen inhibited PCB-induced arachidonic acid release as well as 12-HETE and 12-HHT formation. Interestingly, platelets incubated with PCBs did not aggregate despite the fact that robust release of arachidonic acid was observed. In summary, these results demonstrate that certain PCBs induce activation of cPLA2-alpha independent of a rise in intracellular calcium and a robust release of arachidonic acid release with resulting eicosanoid formation in human platelets.

Some dietary fibers increase elimination of orally administered polychlorinated biphenyls but not that of retinol in mice

Dietary fiber supplementation can increase the size and nutrient absorption capacities of the small intestine in some mammals, but does this increase the risk of accumulating environmental contaminants? This study addressed this question by feeding mice diets containing various types of fiber at 0 or 100 g/kg (cellulose, lactosucrose, polydextrose, indigestible dextrin, soy polysaccharide, rice bran and chitosan) for 10 wk. During the final 2 wk, the mice were fed retinol and a dose of Arochlor 1254 [polychlorinated biphenyls (PCB)] estimated to be 5% of the median lethal dose. Accumulation was determined using whole blood samples collected on days 1, 3 and 7 as well as eight tissues (whole blood, small and large intestine, liver, gall bladder, mesentery, kidney and brain). Elimination of Arochlor 1254 and retinol was determined using daily collections of feces and urine. The patterns of accumulation and elimination differed between Arochlor 1254 and retinol, among tissues, and among mice fed diets with various amounts and types of fiber. Dietary fiber supplementation did not decrease accumulation of PCB. However, the diet with chitosan increased fecal excretion of Arochlor 1254 compared to the fiber-free diet (P<0.05). The diets with fermentable fiber (polydextrose, indigestible dextrin and soy polysaccharides) increased urinary excretion of PCB compared to the diets with water-insoluble fiber (cellulose, rice bran and chitosan; P<0.05). The most efficacious diets for minimizing accumulation of environmental contaminants and accelerating elimination likely include a combination of soluble and insoluble fiber, but the specific types, proportions and amounts remain to be determined.

Acetaminophen UDP-glucuronosyltransferase in ferrets: species and gender differences, and sequence analysis of ferret UGT1A6

The principal objective of this study was to determine whether ferrets glucuronidate acetaminophen more slowly compared with other species, and if so investigate the molecular basis for the difference. Acetaminophen-UDP-glucuronosyltransferase (UGT) activities were measured using hepatic microsomes from eight ferrets, four humans, four cats, four dogs, rat, mouse, cow, horse, monkey, pig and rabbit. Gender differences between male and female ferret livers were explored using enzyme kinetic analysis. Immunoblotting of microsomal proteins was also performed using UGT-specific antibodies. Finally, the exon 1 region of UGT1A6, a major acetaminophen-UGT, was sequenced. Glucuronidation of acetaminophen was relatively slow in ferret livers compared with livers from all other species except cat. Gender differences were also apparent, with intrinsic clearance (Vmax/Km) values significantly higher in male compared with female ferret livers. Furthermore, Vmax values correlated with densitometric measurements of two protein bands identified with a UGT1A subfamily-specific antibody. No deleterious mutations were identified in the exon 1 or flanking regions of the ferret UGT1A6 gene. In conclusion, like cats, ferret livers glucuronidate acetaminophen relatively slowly. However, unlike cats, in which UGT1A6 is encoded by a pseudogene and dysfunctional, there are no defects in the ferret UGT1A6 gene which could account for the low activity.

Purification and properties of a new beta-naphthoflavone inducible cytochrome P-450, aryl hydrocarbon hydroxylase from rat kidney

In rat kidney, beta-naphthoflavone induced 53 kDa and 55 kDa proteins, which were both recognized by the antibodies against rat liver cytochrome P-450 1A1 (55kDa). The major inducible 53 kDa protein was purified from the beta naphthoflavone-treated rat kidney and shown to be a new cytochrome P-450 having a high aryl hydrocarbon hydroxylase activity. Purified cytochrome P-450, named P-450KAh, was homogeneous on SDS-polyacrylamide gel electrophoresis, and the apparent molecular weight was estimated to be 53 kDa. The absorption spectra of the oxidized form of P-450KAh showed a Soret peak at 416 nm, a characteristic of low-spin hemoprotein, and the Soret peak of the reduced cytochrome P-450-CO complex was at 446 nm. In the reconstituted system, purified P-450KAh showed high catalytic activity for benzo[a]pyrene hydroxylation and 7-ethoxycoumarin O-deethylation. P-450KAh could activate genotoxicities of not only B[a]P, but also 2-acetylaminofluorene and aflatoxin B1 on the umu test. These catalytic properties of P-450KAh were almost the same as those of P-4501A1, a major P-450 form having arylhydrocarbon hydroxylase in liver microsomes of 3-methylcholanthrene-treated rats, and P-450KAh could not be distinguished from P-4501A1 even by immunochemical analysis. However, the electrophoretic peptide patterns after alpha-chymotrypsin or trypsin treatment of P-450KAh were different from those of P-4501A1, and the NH2-terminal 11 amino acid sequence of the P-450 was also different from that of P-4501A1 and any other P-450s of rat.

Comparison of the metabolism of 7-ethoxycoumarin and coumarin in precision-cut rat liver and lung slices

The metabolism of 7-ethoxycoumarin and [3-(14)C]coumarin was compared in precision-cut rat liver and lung slices. The lung slices were prepared using an agarose gel instilling technique enabling the production of tissue cylinders followed by lung slices employing a Krumdieck tissue slicer. Both 50 microM 7-ethoxycoumarin and 50 microM [3-(14)C]coumarin were metabolized by rat liver and lung slices. 7-Ethoxycoumarin was converted to 7-hydroxycoumarin (7-HC) which was conjugated with both D-glucuronic acid and sulfate. 7-HC sulfate was the major metabolite formed by both liver and lung slices. [3-(14)C]Coumarin was metabolized by rat liver and lung slices to both polar products and to metabolite(s) that bound covalently to tissue slice proteins. The polar products included unidentified metabolites and 3-hydroxylation pathway products, with only very small quantities of 7-HC being formed. These results demonstrate that precision-cut lung slices are a useful model in vitro system for studying the pulmonary metabolism of xenobiotics. Moreover, the precision-cut tissue slice technique may be employed for comparisons of hepatic and extrahepatic xenobiotic metabolism.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Persistent effects of a single dose of Aroclor 1254 on cytochromes P450IA1 and IIB1 in mouse lung

The polychlorinated biphenyl mixture Aroclor 1254 has been shown to elicit prolonged biochemical responses in several rodent species, particularly induction of mixed function oxygenases in hepatic tissue. Lung is also of interest since a single dose of Aroclor 1254 has been demonstrated to have a tumor promoting effect, increasing the numbers of lung tumors in Swiss mice initiated with N-nitrosodimethylamine. To investigate the enzyme induction response in lung, male Swiss mice were given a single 100 or 500 mg/kg dose of Aroclor 1254 and euthanized at time intervals ranging from 48 hr to 30 weeks. Both cytochromes P450IA1 and IIB1 were followed by use of specific enzyme activities and Western immunoblotting. The IA1 isoform, as quantified by ethoxyresorufin-O-deethylase activity and immunoblotting with monoclonal antibody 1-7-1, was significantly elevated for 30 weeks after both doses. In contrast, benzyloxy-resorufin-O-dealkylase activity (P450IIB1 specific), which is constitutively expressed in rodent lung, was unaffected by Aroclor treatment at the lower dose at early time points, but induced twofold at 30 weeks. At the higher dose, however, enzymatic activity was decreased to 50% of control values, an effect which persisted for 4 weeks postexposure. These changes were confirmed by Western immunoblotting utilizing monoclonal antibody 2-66-3. Concomitantly, content of individual PCB congeners in lungs and carcass was quantified by gas chromatography with electron capture detection. One congener, 2,3,3',4,4'-pentachlorobiphenyl, was selectively retained in lung compared to carcass. Lack of correlation between changes in lung content of PCBs and levels of the P450 isoforms suggested interactions between congeners in control of P450 induction and repression. These data confirm a prolonged P450 induction response in nonhepatic tissue following Aroclor exposure, and further suggest a bidirectional role for certain PCB congeners in the regulation of P450IA1 and P450IIB1 expression in lung tissue.

Chlorinated paraffins: effect of some microsomal enzyme inducers and inhibitors on the degradation of 1-14C-chlorododecanes to 14CO2 in mice

The degradation of 1-14C-chlorododecanes to 14CO2 in C57BL mice was studied. 1-14C-Chlorododecane-injected mice were transferred to an all-glass metabolism cage and the exhaled air was monitored for 14CO2. Pretreatment with cytochrome P-450 inhibitors resulted in a marked decrease in the rate of 14CO2-formation, when measured as peak 14CO2-exhalation rate (PER): After piperonyl butoxide pretreatment the degradation rate of a high-chlorinated 14C-dodecane (PCDD II; 68% Cl w/w) to 14CO2 was 16% of control, and after metyrapone pretreatment 40%. It was also shown that piperonyl butoxide pretreatment decreased the rate of 14CO2-formation, and the amount of 14CO2 formed, in proportion to the chlorine content of four differently chlorinated dodecanes. The cytochrome P-450 inducer phenobarbital moderately (PER 152%) increased the rate of 14CO2-formation from PCDD II, whereas 3-methylcholanthrene and several technical grade chlorinated paraffins generally gave less or no inductive effects. Also the cumulative 14CO2-exhalation, measured during six hours (CE-6), was inhibited and induced after the above pretreatments. The results indicate a cytochrome P-450-dependent degradation of C12-chloroalkanes to 14CO2 in vivo. The degradation via cytochrome P-450 seems to be relatively more important for higher chlorinated alkanes.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.