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.

Expression and alternative splicing of the cytochrome P-450 CYP2A7

In order to investigate the relative levels of expression of human cytochrome P-450 (P-450) CYP2A genes and determine how this relates to polymorphism in coumarin hydroxylase activity, cDNA clones for members of the CYP2A gene family were isolated. These clones were CYP2A6, CYP2A7 and an alternatively spliced version of CYP2A7 (CYP2A7AS). The latter clone was missing exon 2, but contained a 10 bp segment of intron 1. Translation of CYP2A7AS resulted in an in-frame deletion of 51 amino acids. The expression of these cDNAs in COS-7 cells showed that both CYP2A6 and CYP2A7 generated a protein of molecular mass 49 kDa, whereas the protein product of CYP2A7AS was about 44 kDa. Only the CYP2A6 had coumarin hydroxylase activity. The relative level of CYP2A7 and CYP2A7AS mRNA was investigated by reverse transcription followed by PCR (RT-PCR) using human liver RNAs and an RNA sample from a human skin fibroblast cell line. In one of five liver RNAs studied, the aberrantly spliced CYP2A7 mRNA was 3-4-fold more abundant than the normal mRNA. The other samples contained very low levels of this mRNA species. Interestingly, CYP2A7AS mRNA was the major CYP2A7 mRNA detected in the fibroblast cell line. In this case only a protein band of 44 kDa was observed by Western-blot analysis. The relative of mRNA encoding CYP2A6 and CYP2A7 was established in seven human liver samples by RT-PCR and found to range between 1:0.5 and 1:3. These data strength the previous findings that alternative splicing is an important factor in determining the levels of many human P-450s and that this may be subject to tissue-specific effects. Whether in this case the protein product has some function remains to be determined.

Metabolism of coumarin by precision-cut calf liver slices and calf liver microsomes

1. The metabolism of 50 microM [3-14C]coumarin has been studied in precision-cut-calf liver slices. 2. The metabolism of 50 microM coumarin to 7-hydroxycoumarin has also been examined in calf, rat, Cynomolgus monkey and human liver microsomal preparations. 3. In precision-cut calf liver slices, [3-14C]coumarin was metabolized to various polar products and to metabolite(s) that bound covalently to calf liver slice proteins. The polar products included 7-hydroxycoumarin (which was extensively conjugated with D-glucuronic acid and/or sulphate), metabolites of the 3-hydroxylation pathway (mainly o-hydroxyphenylethanol and o-hydroxyphenylacetic acid), and unknown metabolites. 4. Coumarin 7-hydroxylase activity was readily detectable in calf, Cynomolgus monkey and human liver microsomes, but only barely detectable in rat liver microsomes. Enzyme activity in calf, Cynomolgus monkey and human liver microsomes was inhibited by 8-methoxypsoralen (methoxsalen) with IC50's (concentration required to produce a 50% inhibition of enzyme activity) ranging from 0.3 to 2.8 microM. 5. These results and those of other studies demonstrate that precision-cut liver slices are a valuable in vitro model system for investigating species differences in xenobiotic metabolism. Coumarin is metabolized in calf liver by various pathways including both 3- and 7-hydroxylation. The inhibition of coumarin 7-hydroxylase activity by 8-methoxypsoralen suggests that calf liver microsomes contain P450A isoenzyme(s) similar to mouse 2A5 and human 2A6.

Use of precision-cut liver slices for studies of unscheduled DNA synthesis

Precision-cut liver slices were prepared from untreated and Aroclor 1254 (ARO)-treated male Sprague-Dawley rats with a Krumdieck tissue slicer. Liver slices were cultured for 24 hr in medium containing [3H]thymidine and 0-0.1 mM 2-acetylaminofluorene (2-AAF) using a dynamic organ culture system and processed for autoradiographic evaluation of unscheduled DNA synthesis (UDS). Compared with control (i.e. 0 mM 2-AAF) liver slice cultures, 2-AAF produced a concentration-dependent increase in UDS, the effect being more marked in liver slices from ARO-treated than from untreated rats. With liver slices from untreated rats, 2-AAF produced the greatest increase in UDS in centrilobular hepatocytes. 2-AAF-induced UDS in liver slices from ARO-treated rats was most marked in centrilobular hepatocytes but the effect also extended to other areas of the liver lobule. These results demonstrate that precision-cut liver slices may be a valuable alternative in vitro system to hepatocyte cultures for screening chemicals for potential genotoxicity. Unlike hepatocyte cultures, liver slices permit the study of zonal differences in UDS. Moreover, this technique could be applied to other tissues and the study of species differences in response.

Inhibition of rat hepatic aryl hydrocarbon hydroxylase activity by a series of 7-hydroxy coumarins: QSAR studies

1. Molecular orbital calculations, by the Modified Intermediate Neglect of Differential Overlap (MINDO/3) method, of a series of twenty-five 8-acyl-7-hydroxy coumarins show that the inhibition of aryl hydrocarbon hydroxylase (AHH) activities (cytochrome P4501, CYP1 activity, primarily CYP1A1) for 23 of these compounds is related to their structural parameters. The two remaining compounds are the only chlorinated derivatives; these are inactive towards the AHH system and were excluded from the quantitative structure-activity relationship (QSAR) analysis. 2. The results of multiple regression analyses show that AHH activity is dependent on the energy of the highest occupied molecular orbital, E(HOMO), in a single variable expression for the 23 compounds. However, a three-variable expression involving superdelocalizabilities provides a more significant correlation with biological activity. 3. The inactivity of the two chlorinated derivatives can be rationalized in terms of their low degree of molecular planarity, as estimated by the area/depth2 parameter, which presumably precludes them from interaction with CYP1.

Metabolism of coumarin and 7-ethoxycoumarin by rat, mouse, guinea pig, cynomolgus monkey and human precision-cut liver slices

1. The metabolism of 50 microM 7-ethoxycoumarin and 50 microM [3-14C]coumarin has been studied in precision-cut liver slices from the male Sprague-Dawley rat, female DBA/2 mouse, male Dunkin-Hartley guinea pig, male Cynomolgus monkey and man. 2. In liver slices from all five species 7-ethoxycoumarin was metabolized to 7-hydroxycoumarin (7-HC), which was extensively conjugated with D-glucuronic acid and sulphate. In rat and mouse, 7-HC was preferentially conjugated with sulphate, whereas rates of glucuronidation and sulphation were similar in the other three species. 3. [3-14C]coumarin was metabolized by liver slices from all five species to various polar products and to metabolite(s) that bound covalently to liver slice proteins. In Cynomolgus monkey and both human subjects studied, 7-HC was the major metabolite that was conjugated with D-glucuronic acid and sulphate, whereas in rat the major metabolites were products of the 3-hydroxylation pathway and unknown metabolites. Major metabolites in mouse liver slices were 7-HC, 3-hydroxylation pathway products and unknown metabolites, and in guinea pig liver slices, 7-HC and unknown metabolites. 4. The metabolism of 7-ethoxycoumarin to free and conjugated 7-HC and [3-14C]coumarin to total polar products was greater in liver slices from mouse and Cynomolgus monkey than the other three species. 5. With liver slices from all five species there appeared to be little difference in the extent of metabolism of 7-ethoxycoumarin and [3-14C]coumarin to various products in either a complex tissue culture medium (RPMI 1640 plus foetal calf serum) or a simple balanced salt solution (Earle's balanced salt solution). 6. These results demonstrate that precision-cut liver slices are a valuable in vitro model system for investigating species differences in xenobiotic metabolism. Generally, the observed species differences in coumarin metabolism in vitro agree well with available in vivo data.

Metabolism of [3-14C] coumarin by human liver microsomes

1. The metabolism of 50 microM [3-14C] coumarin has been studied in a panel of 12 human liver microsomal samples of known P450 isoenzyme profile. 2. [3-14C] coumarin was metabolized by human liver microsomes to various polar products including 3-, 4- and 7-hydroxycoumarins (3-HC, 4-HC and 7-HC) 6,7-dihydroxycoumarin (6,7-DiHC), o-coumaric acid (o-CA), o-hydroxyphenyl-acetaldehyde (o-HPA), o-hydroxyphenylethanol (o-HPE), o-hydroxyphenylacetic acid (o-HPAA) and o-hydroxyphenylpropionic acid (o-HPPA) and to product(s) that bind covalently to microsomal proteins. 3. For all 12 subjects, mean rates of [3-14C] coumarin metabolism to total polar products (metabolism to all products except product(s) covalently bound to microsomal proteins), 7-HC, the 3-hydroxylation pathway (sum of 3-HC, o-HPA, o-HPE and o-HPAA), o-HPPA, 6,7-DiHC and covalent binding were 1420, 1230, 73.8, 52.5, 9.5 and 4.8 pmol/min/mg protein respectively. 4. Marked interindividual differences in [3-14C] coumarin metabolism to total polar products (30-fold variation) and 7-HC (2250-fold variation) were observed. 5. Good correlations were observed between [3-14C] coumarin metabolism and total polar products, 7-HC, o-HPPA and 6,7-DiHC, but not to 3-hydroxylation pathway products and levels of 2A6 and 2B6 in human liver microsomes. 6. [3-14C] coumarin metabolism to any polar products did not correlate with levels of 1A2, 2C8, 2C9, 2E1, 3A3/4 and 4A1 in human liver microsomes.

Comparison of the hepatic effects of coumarin, 3,4-dimethylcoumarin, dihydrocoumarin and 6-methylcoumarin in the rat

The mechanism of coumarin-induced hepatotoxicity in the rat has been investigated by comparing the effects of coumarin with those of three coumarin derivatives, namely 3,4-dihydrocoumarin (DHC), 3,4-dimethylcoumarin (3,4-DMC) and 6-methylcoumarin (6-MC). Male Sprague-Dawley rats were fed either control diet or diets containing 0.5 or 0.75% coumarin, 0.76% DHC, 0.6 or 0.9% 3,4-DMC or 0.82% 6-MC for 13 wk. The dietary levels of 0.5% coumarin and 0.6% 3,4-DMC, were equimolar (3.43 mmol/100 g diet), as were the dietary levels of 0.75% coumarin, 0.76% DHC, 0.9% 3,4-DMC and 0.82% 6-MC (5.14 mmol/100 g diet). All treatments resulted in an increase in relative liver weight, but only coumarin increased plasma alanine aminotransferase and aspartate aminotransferase activities. Morphological examination of liver sections from coumarin treated rats revealed vacuolation of centrilobular hepatocytes and bile duct hyperplasia. Cholangiofibrosis was also observed, particularly in rats given 0.75% coumarin. Treatment with DHC produced no abnormalities, whereas a slight hypertrophy of centrilobular hepatocytes was observed in some 3,4-DMC treated animals and a slight vacuolation of individual hepatocytes was noted in some 6-MC treated rats. DHC, 6-MC and particularly 3,4-DMC treatment resulted in an induction of cytochrome P-450 dependent mixed function oxidase enzyme activities. All treatments induced hepatic GSHS-transferase and gamma-glutamyltransferase activities, induction being most marked in rats given coumarin and 6-MC. These results provide further evidence that coumarin-induced hepatotoxicity in the rat is due to the formation of a 3,4-epoxide intermediate.

Sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity

Hepatic microsomal coumarin 7-hydroxylase activity has been determined in male and female mice of strains A/J, AKR, BALB/c, CBA/Ca, C3H/He, C57BL/6J, DBA/2 and 129. In males, coumarin 7-hydroxylase activity was highest in liver microsomes from DBA/2 mice and lowest in BALB/c mice. With female mice enzyme activity was highest in DBA/2 and 129 strains, intermediate in the CBA/Ca strain and comparatively low in the other five strains. Marked sex differences were observed in coumarin 7-hydroxylase activity with enzyme activity in female animals from strains DBA/2, 129 and CBA/Ca being 4.8-, 6.2- and 4.8-fold higher, respectively, than in male mice. In contrast, only minor sex and strain differences in levels of total microsomal cytochrome P-450 were observed. These results demonstrate marked sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity. Such differences may be due to variations in particular cytochrome P-450 isoenzymes such as CYP2A5, not all of which can be explained by the known allelic difference in the Cyp2a-5 locus.

Studies on the acute effects of coumarin and some coumarin derivatives in the rat

The mechanism of acute coumarin-induced hepatotoxicity in the rat has been investigated by comparing the effects of coumarin with those of a number of methyl-substituted coumarin derivatives. Male Sprague-Dawley rats were given single ip doses of corn oil (control), coumarin (0.86 and 1.71 mmol/kg body weight), 3,4-dimethylcoumarin (3,4-DMC, 1.71 and 2.57 mmol/kg), 3-, 4- and 6-methylcoumarins (3-MC, 4-MC and 6-MC, 1.71 mmol/kg) and 3- and 4-methyloctahydrocoumarins (3-MOHC and 4-MOHC, 2.57 mmol/kg) and hepatotoxicity assessed after 24 hr. Coumarin administration produced dose-related hepatic necrosis and a marked elevation of plasma alanine aminotransferase and aspartate aminotransferase activities. In contrast, none of the coumarin derivatives examined produced either hepatic necrosis or elevated plasma transaminase activities. Treatment with coumarin reduced hepatic microsomal ethylmorphine N-demethylase and 7-ethoxycoumarin O-deethylase activities, whereas one or both mixed-function oxidases appeared to be induced by treatment with 3,4-DMC, 4-MC, 3-MOHC and 4-MOHC. These results provide further evidence that acute coumarin-induced hepatotoxicity in the rat is due to the formation of a coumarin 3,4-epoxide intermediate. That 3- and/or 4-methyl substitution (i.e. 3-MC, 4-MC and 3,4-DMC) leads to a reduction in coumarin-induced hepatotoxicity, due to diminished formation of 3,4-epoxide intermediates, was confirmed by the results of molecular orbital calculations.

Effect of some peroxisome proliferators on transforming growth factor-beta 1 gene expression and insulin-like growth factor II/mannose-6-phosphate receptor gene expression in rat liver

Male Sprague-Dawley rats were given daily oral doses of either corn oil (control), 80 mg/kg nafenopin (NAF), 50 mg/kg methylclofenapate (MCP), 50 mg/kg Wy-14,643 (WY) or 250 mg/kg clofibric acid (CA) for 7 days. All four compounds increased relative liver weight and produced hepatic peroxisome proliferation as assessed by induction of both peroxisomal (palmitoyl-CoA oxidation) and microsomal (lauric acid 12-hydroxylase) fatty acid oxidising enzyme activities. RNA was extracted from liver samples and analysed for expression of transforming growth factor-beta 1 (TGF-beta 1) and the insulin-like growth factor II/mannose-6-phosphate (IGFII/Man6P) receptor (which may be involved in transporting latent TGF-beta 1 into hepatocytes). TGF-beta 1 mRNA levels were increased to 151-178% of control by all four compounds, whereas NAF, MCP and WY, but not CA, increased IGFII/Man6P receptor mRNA levels to 195-209% of control. The induction of TGF-beta 1 and IGFII/Man6P receptor expression by short term treatment with peroxisome proliferators may represent an adaptive response to limit the initial hyperplastic effects of such compounds.

Effect of pretreatment with some mixed-function oxidase enzyme inducers on the acute hepatotoxicity of coumarin in the rat

Male Sprague-Dawley rats were pretreated with saline, corn oil, sodium phenobarbitone (PB) (100 mg/kg body weight/day), 20-methylcholanthrene (20 MC) (20 mg/kg body weight/day) or Aroclor 1254 (ARO) (100 mg/kg body weight/day) by daily ip injections for 5 days. Animals were then given single oral doses of either 250 or 500 mg coumarin/kg body weight and hepatotoxicity was assessed after 24 hr. Coumarin produced hepatotoxicity, which comprised hepatocyte necrosis and elevation of plasma alanine aminotransferase and aspartate aminotransferase activities, in all pretreated groups. Hepatic microsomal cytochrome P-450 levels were reduced after coumarin administration. In rats pretreated with saline, corn oil or PB, coumarin produced centrilobular hepatic necrosis, whereas in rats pretreated with 20 MC or ARO, coumarin produced periportal hepatic necrosis. These results demonstrate that mixed-function oxidase enzyme inducers can modulate acute coumarin-induced hepatotoxicity in the rat. As coumarin is known to be bioactivated by cytochrome P-450-dependent enzymes, the change in the lobular distribution of toxicity after pretreatment with 20 MC or ARO is presumably due to the induction of particular cytochrome P-450 isoenzymes in periportal hepatocytes.

Comparison of the hepatic effects of nafenopin and WY-14,643 on peroxisome proliferation and cell replication in the rat and Syrian hamster

Male Sprague-Dawley rats were fed control diet or diet containing 0.05% nafenopin (NAF) or 0.025% WY-14,643 (WY) and male Syrian hamsters were fed control diet or diet containing 0.25% NAF or 0.025% WY for periods of 1, 15, 40, and 60 weeks. Both NAF and WY produced a sustained increase in liver weight and induction of peroxisomal fatty acid beta-oxidation in the rat and Syrian hamster. Replicative DNA synthesis was studied by implanting osmotic pumps containing [3H] thymidine during weeks 0-1, 14-15, 39-40, and 59-60. Cell replication, determined either as the hepatocyte labelling index or by incorporation of radioactivity into liver whole homogenate DNA, was increased in rats given NAF and WY for 1 week. However, only WY produced a sustained increased in cell replication after 15-60 weeks. After 40 weeks, liver nodules and tumors were present in WY-treated rats, and these lesions were observed in all WY-treated and some NAF-treated rats after 60 weeks. In contrast to the rat, no marked effect on replicative DNA synthesis and no liver nodules and tumors were observed in Syrian hamsters given NAF and WY for up to 60 weeks. The rat study demonstrates that liver tumors are produced more rapidly by doses of peroxisome proliferators that produce a sustained stimulation of cell replication, whereas the hamster study suggests that species differences may exist in both peroxisome proliferator-induced cell replication and liver tumor formation.

Comparison of the acute and chronic mitogenic effects of the peroxisome proliferators methylclofenapate and clofibric acid in rat liver

Peroxisome proliferators are well known to cause liver enlargement in rodents. In this investigation, we have examined the effect of acute (1 week) and chronic (26 week) exposure to the peroxisome proliferators methylclofenapate (MCP) and clofibric acid (CA), at 0.05 and 0.5% in the diet respectively, on hepatocyte replication in the Sprague-Dawley rat. Both compounds induced an early increase in hepatocyte replication, with a concomitant increase in peroxisome proliferation as assessed by induction of palmitoyl CoA oxidation. However, after 26 weeks of treatment, there was no difference in the labelling index (LI) of control and CA-treated rat livers, while in MCP-treated rats the LI was 5- to 6-fold above control. Palmitoyl CoA oxidation remained elevated in both treated groups at 26 weeks. Analysis of the slides by a 'zonal' scoring procedure demonstrated that the induced replication was predominantly periportal after 1 week of treatment with either compound. The number of 5-bromo-2'-deoxyuridine (BrdU)-positive hepatocyte nuclei per field in the periportal region increased approximately 4-fold after CA treatment and 7-fold after MCP treatment. There was no significant difference in the number of BrdU-positive nuclei per field in the centrilobular areas of control and treated rats after 1 week. After 26 weeks of treatment, periportal replication was still elevated in the MCP-treated animals (approximately 10-fold above control), but there was no difference in periportal replication between control and CA-treated rats. CA induced a significant reduction in the replication of centrilobular areas at 26 weeks, while there was no effect of MCP. In summary, these results demonstrate that the acute mitogenic effects of MCP and CA are predominantly periportal, and, in the case of MCP, the mitogenicity is sustained up to 26 weeks of treatment.

Induction of cytochrome P-450-dependent enzyme activities in cultured rat liver slices

Precision-cut liver slices were prepared from male Sprague-Dawley rats with a Krumdieck tissue slicer and cultured in RPMI 1640 medium for up to 72 hr. After 48 hr, cytochrome P-450 content in the slices declined to 36% of levels present in freshly cut rat liver slices. The addition of either beta-naphthoflavone (BNF) or Aroclor 1254 (ARO) partially prevented the loss of cytochrome P-450. Culture of liver slices with phenobarbitone (PB), BNF and ARO resulted in the induction of 7-ethoxycoumarin O-deethylase, 7-benzoxyresorufin O-debenzylase and 7-ethoxyresorufin O-deethylase activities. Generally, the induction of mixed-function oxidase enzymes was greater in 72- than in 48-hr cultured slices, and at the concentrations examined ARO produced a greater stimulation of enzyme activities than did either PB or BNF. These results demonstrate that rat liver slices may be maintained in culture for up to 72 hr, and that they respond in a similar manner to rat primary hepatocyte cultures to some inducers of xenobiotic metabolism. Precision-cut liver slices may therefore be a useful alternative in vitro system to hepatocyte cultures for screening compounds for effects on mixed-function oxidases and for assessing species differences in response.

Effect of treatment with pyrazine and some derivatives on cytochrome P450 and some enzyme activities in rat liver

1. The effect of pyrazine and three pyrazine derivatives, namely (methylthio) methylpyrazine (MTMP), 5, 6, 7, 8-tetrahydroquinoxaline (CHP) and 5-methyl-6, 7-dihydro-5'-cyclopentapyrazine (CPP), on hepatic peroxisomal and microsomal enzyme activities have been studied in male Sprague-Dawley rats. MTMP (0.25-2 mmol/kg per day) and the other compounds (1 mmol/kg/day) were administered by i.p. injections for 3 days. 2. None of the test compounds appeared to be peroxisome proliferators as there was no marked effect on hepatic palmitoyl-CoA oxidation, and neither pyrazine nor MTMP induced microsomal lauric acid 12-hydroxylase. 3. In contrast, all four compounds induced hepatic microsomal cytochrome P450-dependent enzyme activities. MTMP induced the metabolism of several mixed-function oxidase substrates including, 7-pentoxyresorufin, 7-benzoxyresorufin, benzphetamine, 4-nitrophenol and aniline, whereas pyrazine induced the metabolism of fewer substrates but including 4-nitrophenol and aniline. 4. By Western immunoblotting MTMP was found to increase levels of CYP2B1 and CYP3A isoenzymes, whereas pyrazine increased CYP2E1. 5. Thus, while pyrazine appears to be mainly a CYP2E inducer, MTMP is a mixed inducer of cytochrome P450 isoenzymes in the CYP2B, CYP3A and CYP2E subfamilies. CPP is probably a CYP2E inducer in rat liver, whereas CHP appears to be a mixed inducer of cytochrome P450 isoenzymes in the CYP2B, CYP3A and CYP2E subfamilies.

Culture of precision-cut liver slices: effect of some peroxisome proliferators

Precision-cut rat liver slices were prepared with a Krumdieck tissue slicer and cultured in three standard hepatocyte culture media. Rat liver slices cultured in either RPMI 1640 medium or Williams Medium E could be maintained in culture for up to 72 hr. In contrast, Leibovitz's L-15 medium was unsatisfactory in that slice viability, assessed either by morphological examination or by measurement of enzyme activities, could not be maintained for periods greater than 24 hr. As a measure of functional viability liver slices were cultured with some known rodent peroxisome proliferators, namely clofibric acid, nafenopin, ciprofibrate and Wy-14,643. The peroxisome proliferators induced both palmitoyl CoA oxidation and carnitine acetyltransferase activities in 48- and 72-hr slice cultures. Ultrastructural examination of liver slices cultured with either ciprofibrate or Wy-14,643 for 72 hr revealed an increase in the number of peroxisomes. These results demonstrate that rat liver slices may be maintained in culture for up to 72 hr, and that they respond in a similar manner to rat primary hepatocyte cultures to some peroxisome proliferators. Precision-cut liver slices may therefore be a useful alternative in vitro system to hepatocyte cultures for screening compounds for effects on enzyme activities and for assessing species differences in response.

Interaction of some peroxisome proliferators with the mouse liver peroxisome proliferator-activated receptor (PPAR): a molecular modelling and quantitative structure-activity relationship (QSAR) study

1. The three-dimensional structure of a portion of the ligand-binding domain of the mouse liver peroxisome proliferator-activated receptor (PPAR) described by Issemann and Green (1990) has been modelled from amino acid sequence data. 2. By inspection of the three-dimensional structure of the portion of the PPAR ligand-binding domain, a putative binding site for peroxisome proliferators, consisting of one isoleucine, one lysine and two phenylalanine moieties (residues 354, 358, 359 and 361, respectively), has been identified. 3. The interaction of 12 peroxisome proliferators with the putative PPAR binding site has been investigated and energetics of binding calculated from ligand-bound and ligand-free receptor geometries. 4. The interaction data have been used to establish quantitative structure-activity relationships (QSARs) between peroxisome proliferator binding and either PPAR activation in COS1 cells or induction of palmitoyl-CoA oxidation in rat hepatocyte cultures. 5. The results are discussed in terms of the role of PPAR in the mechanism of initiation of peroxisome proliferation in rodent liver.

Comparison of the effects of nafenopin on hepatic peroxisome proliferation and replicative DNA synthesis in the rat and Syrian hamster

Male Sprague-Dawley rats were fed control or 0.1% nafenopin diet and male Syrian hamsters were fed control or 0.25% nafenopin diet for periods of 7 and 54 days. Nafenopin treatment produced a sustained increase in liver weight and induction of hepatic peroxisomal and microsomal fatty acid-oxidizing enzyme activities, with a greater effect being observed in the rat. Replicative DNA synthesis was studied by implanting osmotic pumps containing [3H]thymidine during study days 0-7 and 47-54. Cell replication, determined either as the hepatocyte labelling index or by incorporation of radioactivity into liver whole homogenate DNA, was increased in rats given nafenopin for 7 and 54 days. In contrast to the rat, no significant effect on replicative DNA synthesis was observed in the Syrian hamster. These results provide further evidence for species differences in hepatic peroxisome proliferation, with the Syrian hamster being less responsive than the rat. Furthermore, while peroxisome proliferators produce hyperplasia in rat and mouse liver, these data suggest that they may not have any marked effect on hepatic replicative DNA synthesis in the Syrian hamster.

The histology and development of hepatic nodules and carcinoma in C3H/He and C57BL/6 mice following chronic phenobarbitone administration

Male C3H/He and C57BL/6 mice were given diets containing sodium phenobarbitone (PB) to allow a daily intake of 85 mg/kg. Control and treated animals were killed at 5, 30, 40, 60, and 80 wk. Other mice were killed in extremis or at the end of the respective experiments: 91 wk for C3H/He and 100 wk for the C57BL/6 animals. A basophilic nodule was found in 1/5 control C3H/He mice at 30 wk; these nodules increased in number with time so that nodules of this type were found in approximately 70% of animals by 91 wk. Nodules were not found in control C57BL/6 mice until 80 wk, when they were found in 4% of mice. PB treatment markedly increased the number of hepatic nodules in both strains of mice. The additional nodule burden was due to the development of a second nodule type formed of large cells with a predominantly eosinophilic cytoplasm. C3H/He animals given PB for 60 wk and then returned to a control diet bore fewer nodules at 91 wk than treated mice killed at 60 or 91 wk. The cumulative incidence of carcinoma in control C3H/He and C57BL/6 mice was 28 and 4%, respectively. The incidence of carcinoma was not increased by PB treatment in either strain. It is concluded that both strains of mice behave in a qualitively similar way to PB administration, although they show considerable quantitative differences in terms of the time and number of nodules that develop. Furthermore, the increased nodule numbers associated with PB treatment were not accompanied by an increase in the number of carcinomas.

Effect of hepatic cytochrome P-450 inducing agents on mutagen activity in the host-mediated assay

Intraperitoneal treatment of female BALB/c mice with either phenobarbitone or beta-naphthoflavone led to the induction of various hepatic enzymes associated with xenobiotic metabolism and to increased abilities of hepatic S9 fractions to convert the dietary carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) to an active bacterial mutagen. In the case of another carcinogen, aflatoxin B1 an increase in in vitro hepatic activation was seen only in mice treated with phenobarbitone. In contrast, pretreatment with either phenobarbitone or beta-naphthoflavone reduced the in vivo activity of both aflatoxin B1 and MeIQx in the host mediated bacterial mutation assay. These data indicate that, for some carcinogens at least, the host-mediated assay may be used to predict the carcinogenic consequences of hepatic enzyme induction.

Metabolism of [3-14C]coumarin to polar and covalently bound products by hepatic microsomes from the rat, Syrian hamster, gerbil and humans

The metabolism of 0.19 and 2.0 mM-[3-14C]coumarin to polar products and covalently bound metabolites has been studied with hepatic microsomes from the rat, Syrian hamster, Mongolian gerbil and humans. [3-14C]Coumarin was metabolized by liver microsomes from all species to a number of polar products and to metabolite(s) that became covalently bound to microsomal proteins. The polar products included 3-, 5- and 7-hydroxycoumarins, o-hydroxyphenylacetaldehyde and o-hydroxyphenylacetic acid. Coumarin 7-hydroxylation was observed in all species except the rat. With 0.19 mM-[3-14C]coumarin, 7-hydroxycoumarin was the major metabolite in human liver microsomes, whereas in the other species with 0.19 mM substrate and in all species with 2.0 mM substrate o-hydroxyphenylacetaldehyde was the major metabolite. Of the three animal species studied the gerbil most resembled humans as this species also had a high coumarin 7-hydroxylase activity. The administration of Aroclor 1254 to the rat and Syrian hamster induced both microsomal cytochrome P-450 content and [3-14C]coumarin metabolism. With liver microsomes from all species a good correlation between rates of [3-14C]coumarin metabolism and covalent binding was observed at both substrate concentrations. However, in view of the known species difference between the rat and Syrian hamster in coumarin-induced hepatotoxicity, the present data are not consistent with microsomal coumarin metabolite covalent binding being an indicator of potential liver damage.

Identification of o-hydroxyphenylacetaldehyde as a major metabolite of coumarin in rat hepatic microsomes

The metabolism of [3-14C]coumarin has been studied in hepatic microsomes from control (corn-oil treated) and Aroclor 1254-treated (100 mg/kg body weight/day, 5 days, ip) rats. [3-14C]Coumarin metabolites in incubate extracts were separated by HPLC and identified by comparison with the retention times of known coumarin metabolites. The major product produced by incubation of 0.25-2.5 mM-[3-14C]coumarin with both control and Aroclor 1254-induced hepatic microsomes was a novel coumarin metabolite. This novel metabolite was extracted from pooled microsomal incubations, purified by semi-preparative HPLC and identified by mass spectrometry as o-hydroxyphenylacetaldehyde (o-HPA). Some possible pathways for the formation of o-HPA from coumarin are proposed.

Effect of inducers of cytochrome P-450 on the metabolism of [3-14C]coumarin by rat hepatic microsomes

1. The metabolism of [3-14C]coumarin has been studied in rat hepatic microsomes and with two purified cytochrome P-450 isoenzymes. 2. [3-14C]Coumarin was converted by liver microsomes to several polar products including 3- and/or 5-hydroxycoumarin, omicron-hydroxyphenylacetic acid and a major unidentified novel coumarin metabolite. 3. [3-14C]Coumarin was also converted to reactive metabolite(s) as indicated by covalent binding to proteins, and by the depletion of reduced glutathione added to the microsomal incubations. 4. [3-14C]Coumarin metabolism to polar and covalently bound metabolites by rat liver microsomes was induced by pretreatment with phenobarbitone, 3-methylcholanthrene, beta-naphthoflavone, Aroclor 1254 and isosafrole; but not by dexamethasone or nafenopin. 5. The profile of [3-14C]coumarin metabolism to polar products was similar in control and pretreated liver microsomes and in incubations with purified cytochrome P450 IA1 and P450 IIB1 isoenzymes. 6. The results indicate that coumarin is a substrate for isoenzymes of the cytochrome P450 IA and P450 IIB subfamilies. The bioactivation of coumarin by rat hepatic microsomes is postulated to result in the formation of a coumarin 3,4-epoxide intermediate which may rearrange to 3-hydroxycoumarin, be further metabolized to a coumarin 3,4-dihydrodiol, or form a glutathione conjugate.