Use of precision-cut rat liver slices for studies of xenobiotic metabolism and toxicity: comparison of the Krumdieck and Brendel tissue slicers

1. In this study we have compared freshly cut and cultured precision-cut rat liver slices produced by the Krumdieck and Brendel-Vitron tissue slicers. 2. No significant differences were observed in levels of protein, potassium, total glutathione (i.e. GSH and GSSG), reduced glutathione (GSH) and cytochrome P450 and activities of 7-ethoxyresorufin O-deethylase and 7-benzoxyresorufin O-debenzylase in freshly cut rat liver slices produced by the two tissue slicers. However, levels of oxidized glutathione (GSSG) were significantly greater in liver slices produced with the Brendel-Vitron tissue slicer. 3. Precision-cut rat liver slices produced with both tissue slicers were cultured for 0 (i.e. a 1-h preincubation), 24 and 72 h in a dynamic organ culture system in an atmosphere of either 95% 02/5% CO2 or 95% air/5% CO2. 4. Apart from small differences in glutathione levels in 0 and 24 h cultured liver slices, no significant differences were observed in the parameters measured between liver slices prepared with both tissue slicers and cultured in both gas phases. 5. With liver slices produced by both tissue slicers 50 microM sodium arsenite produced a greater induction of heat shock protein 70 levels in slices cultured for 24 h in a high oxygen than in an air atmosphere. 6. These results suggest that both tissue slicers can readily produce precision-cut liver slices for studies of xenobiotic metabolism and toxicity. However, the data suggest that for any given application of precision-cut tissue slices it is desirable to establish optimal culture conditions

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response. Copyright 1998 Academic Press.

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response.

Molecular modelling and quantitative structure-activity relationship studies on the interaction of omeprazole with cytochrome P450 isozymes

Molecular modelling of the anti-ulcerative agent, omeprazole, with the putative active sites of cytochromes P4503A4 and P4502C19, enzymes which are the major catalysts of omeprazole metabolism in man, are reported. Interactive docking of omeprazole in both CYP3A4 and CYP2C19 gives rise to binding orientations which are consistent with both the known sites of metabolism reported for these isoforms and with evidence from site-directed mutagenesis experiments on CYP2C19, a P450 associated with genetic polymorphism in human drug metabolism. The potential P450 enzymic interactions, inhibition and induction of omeprazole are discussed in the light of molecular modelling and QSAR (quantitative structure-activity relationship) studies on related compounds.

Comparison of the effects of di-(2-ethylhexyl)adipate on hepatic peroxisome proliferation and cell replication in the rat and mouse

The effects of di-(2-ethylhexyl)adipate (DEHA) have been compared in female F344 rats and female B6C3F1 mice fed diets containing 0-4.0% DEHA and 0-2.5% DEHA, respectively, for periods of 1, 4 and 13 weeks. In both the rat and mouse treatment with DEHA at all time points produced a dose-dependent increase in relative liver weight and hepatic peroxisome proliferation as demonstrated by the induction of peroxisomal (cyanide-insensitive palmitoyl-CoA oxidation) and microsomal (lauric acid 12-hydroxylase) fatty acid oxidising enzyme activities. The magnitude of induction of peroxisome proliferation was similar in both species. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during study weeks 0-1, 3-4 and 12-13. After 1 week DEHA treatment hepatocyte labelling index values were increased in rats given 2.5 and 4.0% DEHA and mice given 0.6-2.5% DEHA. While DEHA treatment for 4 and 13 weeks did not increase labelling index values in the rat, a sustained stimulation of replicative DNA synthesis was observed in mice given 1.2 and 2.5% DEHA. The results of this study demonstrate a species difference in the hepatic effects of DEHA, in that at some dose levels DEHA can produce a sustained stimulation of replicative DNA synthesis in mouse but not in rat liver. Sustained cell replication provides a better correlation with the observed formation of liver tumours in chronic studies with DEHA in female mice, but not in female rats, than the magnitude of stimulation of hepatic peroxisome proliferation.

Induction of CYP3A isoforms in cultured precision-cut human liver slices

1. The effect of rifampicin on cytochrome P450 isoforms in the CYP1A and CYP3A subfamilies has been studied in 72-h cultured precision-cut human liver slices. 2. In cultured human liver slices 50 microM rifampicin induced testosterone 6 beta-hydroxylase activity, but had no effect on 7-ethoxyresorufin O-deethylase and 7-methoxyresorufin O-demethylase activities. 3. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2 and human CYP3A4. Compared with control (dimethyl sulphoxide only treated) liver slice microsomes, rifampicin increased levels of CYP3A4 but had no effect on CYP1A2. 4. These results demonstrate that rifampicin induces CYP3A isoforms, but not CYP1A2, in cultured human liver slices. Some variability in the magnitude of induction by rifampicin was observed in the six human liver samples examined. 5. These results demonstrate that cultured human liver slices may be used to evaluate the effects of xenobiotics on CYP3A isoforms.

Comparison of the hepatic and renal effects of 1,4-dichlorobenzene in the rat and mouse

The effects of 1,4-dichlorobenzene (DCB) have been compared in male F344 rats given 0 (corn oil control), 25, 75, 150, and 300 mg/kg DCB and male B6C3F1 mice given 0 (corn oil control), 300, and 600 mg/kg DCB by daily oral gavage five days per week for 1, 4, and 13 weeks. The two highest rat and both mouse dose levels were the same as those employed in a NTP bioassay, where DCB produced kidney tumors in male rats and liver tumors in mice. DCB produced significant dose-related increases in relative liver weight in both the rat and the mouse which was associated with, respectively, mild and marked centrilobular hypertrophy. Administration of DCB also produced a sustained induction of microsomal cytochrome P450 content and 7-pentoxyresorufin O-depentylase activity in both species. Western immunoblotting studies demonstrated that DCB induced CYP2B isoenzyme(s) in both rat and mouse liver microsomes. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine in study Weeks 0-1, 3-4, and 12-13. In the rat hepatocyte labeling index values were only increased in animals given 300 mg/kg DCB for 1 week, whereas hepatocyte labeling index values were significantly increased in mice given 300 and 600 mg/kg DCB for 1 and 4 weeks. DCB treatment produced significant increases in rat renal P1/P2 proximal tubule cell labeling index values at all time points, whereas little effect was observed in mouse kidney. The observed species difference in DCB-induced liver tumor formation may reflect the greater sensitivity of the mouse to tumor promotion by a CYP2B inducer. For the kidney, the present data provides further evidence that while DCB-induced alpha2U-globulin nephropathy is associated with a sustained stimulation of cell replication in male rat renal proximal tubule cells, this effect is not observed in the male mouse.

Comparison of the effects of cinnamyl anthranilate on hepatic peroxisome proliferation and cell replication in the rat and mouse

The effects of cinnamyl anthranilate (CA) have been compared in female B6C3F1 mice and female F344 rats fed diets containing 0-3.0% CA for periods of 1, 4, and 13 weeks. In the mouse, treatment with CA at all time points produced a marked dose-dependent increase in relative liver weight and hepatic peroxisome proliferation as demonstrated by the induction of peroxisomal (cyanide-insensitive palmitoyl-CoA oxidation) and microsomal (lauric acid 12-hydroxylase) fatty acid oxidizing enzyme activities. CA produced only small increases in relative liver weight and palmitoyl-CoA oxidation in the rat and did not induce lauric acid 12-hydroxylase activity. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during Study Weeks 0-1, 3-4, and 12-13. After 1 week of CA treatment, labeling index values were increased in rat and to a greater extent in mouse hepatocytes. While CA treatment for 4 and 13 weeks did not increase hepatocyte-labeling index values in the rat, a sustained stimulation of replicative DNA synthesis was observed at some dietary levels in the mouse. These results demonstrate a marked species difference between the hepatic effects of CA in female B6C3F1 mice and female F344 rats. While CA is a potent peroxisome proliferator in the mouse, it is only a very weak agent in the rat. The formation of liver tumors in long-term studies, at high doses of CA, appears to be attributable to a sustained stimulation of both peroxisome proliferation and cell replication in mouse hepatocytes.

Effect of piperonyl butoxide on cell replication and xenobiotic metabolism in the livers of CD-1 mice and F344 rats

Male CD- 1 mice were fed diets containing 0 (control), 10, 30, 100, and 300 mg/kg/day piperonyl butoxide (PBO) and 0.05% sodium phenobarbital (NaPB) and male F344 rats were fed diets containing 0 (control), 100, 550, 1050, and 1850 mg/kg/day PBO and 0.5% NaPB for periods of 7 and 42 days. In both species PBO and NaPB increased relative liver weight and whereas PBO produced a midzonal (mouse) or periportal/midzonal (rat) hypertrophy, NaPB produced a centrilobular hypertrophy. In the rat, individual cell necrosis was also observed at 42 days after high doses of PBO. Replicative DNA synthesis, assessed as the hepatocyte labeling index following implantation of 7-day osmotic pumps containing 5-bromo-2'-deoxyuridine during Study Days 0-7 and 35-42, was increased in mice given 300 mg/kg/day PBO and NaPB for 7 days and in rats given 550 and 1050 mg/kg/day PBO and NaPB for 7 days and 1050 mg/kg/day PBO for 42 days. While PBO had no effect on body weights in mice, the body weights of rats given 550, 1050, and 1850 mg/kg/day PBO for 42 days were reduced to 92, 89, and 70% of control, respectively. PBO induced microsomal cytochrome P450 content and mixed function oxidase activities in the mouse and rat, although the effects were less marked than those produced by NaPB. In summary, this data demonstrates that PBO can produce liver enlargement in the mouse and the rat which is associated with induction of xenobiotic metabolism, hypertrophy, and hyperplasia. The hepatic effects of PBO in the mouse were similar to but less marked than those produced by NaPB. In the rat high doses of PBO were hepatotoxic and resulted in a marked reduction in body weight. Thus while the reported formation of eosinophilic nodules in mouse liver by PBO may occur by a mechanism(s) similar to that of NaPB and other nongenotoxic enzyme inducers, the reported tumor formation in rats at greater than the maximum tolerated dose is most likely associated with marked enzyme induction in conjunction with a regenerative hyperplasia resulting from PBO-induced hepatotoxicity.

Molecular modelling of mammalian CYP2B isoforms and their interaction with substrates, inhibitors and redox partners

1. The construction of three-dimensional models of CYP2B isozymes from rat (CYP2B1), rabbit (CYP2B4) and man (CYP2B6), based on a multiple sequence alignment with CYP102, a unique eukaryotic-like bacterial P450 (in terms of possessing an NADPH-dependent FAD- and FMN-containing oxidoreductase redox partner) of known crystal structure, is reported. 2. The enzyme models described are shown to be consistent with experimental evidence from site-directed mutagenesis studies, antibody recognition sites and amino acid residues identified as being associated with redox partner interactions, together with the location of a key serine residue (Ser-128) likely to be involved in protein kinaseA-mediated phosphorylation. 3. A substantial number of known substrates and inhibitors of CYP2B isozymes are shown to fit the putative active sites of the enzyme models in agreement with their reported position of metabolism or mode of inhibition respectively. In particular, there is complementarity between the characteristic non-planar geometries of CYP2B substrates and key groups in the enzymes' active sites. 4. Molecular modelling of CYP2B isozymes appears to rationalize a number of the reported findings from quantitative structure-activity relationship investigations on series of CYP2B substrates and inhibitors.

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.

Comparison of the hepatotoxicity of coumarin in the rat, mouse, and Syrian hamster: a dose and time response study

The effects of coumarin treatment have been compared in male Sprague-Dawley CD rats, male CD-1 mice, and male Syrian hamsters. Rats were fed 0-0.75% coumarin for 1 and 4 weeks and 0-0.5% coumarin for 13 weeks, whereas mice and Syrian hamsters were fed 0-0.5 and 0-1.0% coumarin, respectively, for periods of 1, 4, and 13 weeks. In the rat, coumarin produced dose-related hepatotoxic effects which included vacuolar degeneration, apoptosis, and bile duct proliferation. These effects were particularly marked at dose levels of 0.3 and 0.5%, where liver tumors have been observed in a chronic study. Coumarin administration to rats also increased serum bilirubin content and both serum and hepatic gamma-glutamyltransferase activity. While levels of hepatic total glutathione were increased by coumarin administration, microsomal cytochrome P450 content and ethylmorphine N-demethylase activity were reduced. Such effects were either less marked or absent in the mouse and Syrian hamster. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during Study Weeks 0-1, 3-4, and 12-13. In the rat, coumarin administration for 4 and 13 weeks at dose levels of 0.3 and 0.5% produced a sustained stimulation of hepatocyte replicative DNA synthesis. No such effects were observed in the mouse and Syrian hamster. These results demonstrate marked species differences in coumarin-induced hepatotoxicity. While tumor formation in the rat appears due to chronic hepatotoxicity associated with a sustained regenerative hyperplasia, such effects were not observed in the CD-1 mouse and Syrian hamster. In assessing the hazard of coumarin to humans, account needs to be taken of both levels of exposure and species differences in response.

Strain-specific enhancement or inhibition of coumarin hepatotoxicity in mice following pretreatment with two different liver enzyme-inducing agents

Human exposure to coumarin continues despite controversy over its hepatotoxic potential. Greater understanding of human reactions to coumarin may be achieved by studying murine interstrain differences. The metabolic basis of coumarin hepatotoxicity and its modulation by liver enzyme inducers, beta-naphthoflavone (beta NF) and aroclor 1254 (ARO), were investigated in C3H/He and DBA/2 mice. Coumarin (200 mg/kg) was hepatotoxic to both strains, resulting in 2- to 15-fold plasma aminotransferase elevations, mild subcapsular linear hepatocyte necrosis after 24 hr, and, in some C3H/He mice, centrilobular necrosis. In this strain, beta NF pretreatment caused a 2- to 3-fold further increase in plasma aminotransferases and produced periportal necrosis. In contrast, ARO-pretreated C3H/He mice tended to exhibit lower plasma aminotransferases and occasional midzonal damage. Neither pretreatment significantly altered coumarin hepatotoxicity in DBA/2 mice. In C3H/He mice, hepatic microsomal metabolism of [3-14C]-coumarin via the 3-hydroxylation pathway doubled following both beta NF and ARO treatment. The contrasting nonresponsiveness of DBA/2 mice suggested that this pathway is linked to the Ah locus, which is defective in this strain. ARO treatment caused a maximal 5-fold increase in coumarin 7-hydroxylation in C3H/He mice, whereas DBA/2 mice were 30% less responsive. Potentiation of coumarin hepatotoxicity corresponded to an increase in the 3-:7-coumarin hydroxylation ratio. Pretreatment-dependent shifts in the location of hepatocyte damage may be related to changes in the translobular ratio of enzymes involved in activation and detoxication of coumarin. These data highlight how genetic background, individual variation, and xenobiotic-induced alterations in enzyme profiles, factors all relevant to human risk assessment, can influence the consequence of coumarin exposure.

Molecular modelling of CYP1A subfamily members based on an alignment with CYP102: rationalization of CYP1A substrate specificity in terms of active site amino acid residues

1. Using a novel amino acid sequence alignment, proteins of the CYP1A subfamily have been produced from the CYP102 crystal structure template via residue replacement and energy minimization procedures. 2. Known substrates and inhibitors of CYP1A1 and CYP1A2 are shown to fit their respective active sites via key interactions with complementary amino acid residues. Substrates used in the modelling studies include: caffeine, PhIP, oestradiol, 2,4- and 2,5-diaminotoluenes, Glu-P-1, phenacetin, acetanilide, 7-methoxy and 7-ethoxyresorufins, 11-methyl cyclopenta[a]phenanthren-17-one, 7-ethoxycoumarin, aflatoxin B1, benzo[a]pyrene, benzo[a]pyrene-7,8-diol and 1'-hydroxy 3-methylcholanthrene. 3. A number of aspects relating to CYP1A substrate specificity and metabolism can be explained in terms of the enzyme models, as it is found that key interactions with active site amino acid residues direct CYP1A-mediated metabolism in the known positions.

Metabolism of [ring-U-14C] agaritine by precision-cut rat, mouse and human liver and lung slices

Agaritine [(beta-N-[gamma-L(+)glutamyl]-4-hydroxymethylphenylhydrazine] is present in the common cultivated mushroom Agaricus bisporus and several agaritine derivatives have been shown to produce tumours in experimental animals. In this investigation the metabolism of [ring-U-14C]agaritine has been studied in precision-cut rat, mouse and human liver slices and in precision-cut rat and mouse lung slices. To confirm the functional viability of the tissue slice preparations, the metabolism of 7-ethoxycoumarin was also studied. Liver and lung slices from all species metabolized 50 microM 7-ethoxycoumarin to 7-hydroxycoumarin, which was conjugated with D-glucuronic acid and sulfate. Incubation of rat, mouse and human liver slices, and rat and mouse lung slices with 25 microM [14C]agaritine resulted in a time-dependent formation of metabolite(s), which bound covalently to tissue slice proteins. Agaritine metabolite covalent binding was greater in mouse liver than in rat and human liver slices and was greater in mouse lung than in rat lung slices. No correlation was observed between agaritine metabolite covalent binding and tissue slice gamma-glutamyltransferase activity. Additional studies with mouse liver slices showed that [14C]agaritine was also metabolized to a number of unknown polar metabolites. These results demonstrate that agaritine can be metabolized by enzymes present in mammalian liver and lung.

Use of precision-cut liver slices to evaluate species differences in 2-acetylaminofluorene-induced unscheduled DNA synthesis

Precision-cut liver slices were prepared from male Sprague-Dawley rats (pretreated with or without Aroclor 1254), male Dunkin-Hartley guinea pigs, male cynomolgus monkeys, and humans. Liver slices were cultured for 24 hr using a dynamic organ culture system in medium containing [3H]thymidine and 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1), or 6-aminochrysene (6-AC). The liver slices were then harvested and processed for autoradiographic evaluation of unscheduled DNA synthesis (UDS). All three genotoxins induced UDS in liver slices from untreated and Aroclor 1254-treated rats. In human liver slices 2-AAF produced a concentration-dependent induction of UDS and at the highest concentration examined 2-AAF also induced UDS in guinea pig liver slices. However, 2-AAF did not induce UDS in cynomolgus monkey liver slices, although both AFB1 and 6-AC induced UDS in liver slices from this species as well as from guinea pigs and humans. The inability of 2-AAF to induce UDS in cynomolgus monkey liver slices appears to be at least partially due to the absence of hepatic CYP1A2 in this species. Precision-cut liver slices appear to be a useful alternative to primary hepatocyte cultures for studies of xenobiotic-induced genotoxicity employing the UDS technique. As shown by this study they may also be employed to evaluate species differences in xenobiotic-induced genotoxicity.

Induction of cytochrome P450 isoenzymes in cultured precision-cut rat and human liver slices

1. The effect of some xenobiotics on levels of selected cytochrome P450 (CYP) isoenzymes determined by Western immunoblotting and associated enzyme activities has been studied in 72-h cultured rat and human precision-cut liver slices. 2. In cultured rat liver slices, 0.5 mM sodium phenobarbitone (PB), 25 microM beta-naphthoflavone (BNF), and 20 micrograms/ml Aroclor 1254 (ARO) induced mixed-function oxidase enzyme activities. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2, 2B1/2 and 3A. Compared with 72-h control (dimethyl sulphoxide only treated) rat liver slice microsomes, PB induced CYP2B1/2 and 3A, BNF induced CYP1A2, and ARO induced CYP1A2, 2B1/2, and 3A. 3. The peroxisome proliferators methylclofenapate (MCP), ciprofibrate (CIP) and Wy-14,643 (WY) induced palmitoyl-CoA oxidation in 72-h cultured rat liver slices. Compared with 72-h control rat liver slice microsomes, MCP, CIP, and WY all induced levels of CYP4A. 4. In cultured human liver slices, 20 micrograms/ml ARO, but not 0.5 mM MCP, induced 7-ethoxyresorufin O-deethylase activity. Neither ARO nor MCP had any effect on homogenate palmitoyl-CoA oxidation and microsomal lauric acid 11- and 12-hydroxylase activities. Compared with 72-h control human liver slice microsomes, ARO induced CYP1A2, and MCP appeared to induce CYP4A. Further studies would be required to confirm that CYP4A isoenzymes could be induced by xenobiotics in human liver slices. 5. These results demonstrate that cultured liver slices may be used in evaluating the effect of xenobiotics on both rat and human CYP isoenzymes.

Comparison of the toxicity of allyl alcohol, coumarin and menadione in precision-cut rat, guinea-pig, cynomolgus monkey and human liver slices

The toxicity of allyl alcohol, coumarin and menadione has been studied in precision-cut liver slice cultures. Liver slices were prepared from male Sprague-Dawley rats, male Dunkin-Hartley guinea-pigs and from samples of Cynomolgus monkey and human liver using a Krumdieck tissue slicer. The liver slices were cultured with the test compounds for 24 h in a dynamic organ culture system. Toxicity was assessed by measurement of protein synthesis, potassium content and the MTT assay. At the concentrations examined, menadione produced marked toxicity in liver slices from all four species, whereas rat liver slices were less susceptible to allyl alcohol toxicity. Coumarin produced concentration-dependent toxic effects in rat and guinea-pig liver slices, whereas Cynomolgus monkey and human liver slices were relatively resistant, especially at low coumarin concentrations. At some concentrations of the test compounds examined, the MTT assay appeared to be a less sensitive indicator of toxicity than either protein synthesis or potassium content. These results demonstrate the usefulness of precision-cut liver slices for assessing species differences in xenobiotic-induced toxicity.

Molecular orbital-generated QSARs in a homologous series of alkoxyresorufins and studies of their interactive docking with P450s

1. Molecular and electronic structural parameters have been determined, by molecular orbital (MO) calculations, for a homologous series of 8 alkoxyresorufins (methoxy- to octoxy-). 2. Quantitative structure-activity relationships (QSARs) between these structural parameters and the rates of metabolism of the alkoxyresorufins in hepatic microsomes from the 3-methylcholanthrene (MC)-, and phenobarbital (PB)-pretreated mouse, and the beta-naphthoflavone (beta NF)-pretreated rat have been established. 3. The most significant single relationship is between beta NF-induction of cytochrome P4501 (CYP1A) and the total nucleophilic superdelocalizability (sigma SN) for the eight compounds in the series. 4. For double regressions, the electronic charge on the alkoxy oxygen, Q(O), or alpha-carbon Q(C), is important when combined with the hydrophobic substituent constant (pi). 5. These findings indicate that the rates of metabolism of these alkoxyresorufins are dependent upon their ability to cross cellular membranes, to fit the relevant CYP1A binding site, and on their ability to accept electrons from a donor nucleophilic species. 6. A different set of parameters correlated with CYP2B activity, namely, parameters of overall shape, which indicates that the way in which the alkoxyresorufins fit the CYP2B site, determines their differences in specificity. 7. Computer graphic interactive docking studies of the alkoxyresorufins with their affinity-specific cytochromes P450, namely, methoxy- with CYP1A2; ethoxy- with CYP1A1; pentoxy- with CYP2B1; and benzyloxy- with CYP3A, have also been undertaken to show the specific interactions of the alkoxyresorufins with the binding sites of the individual P450s.

Peroxisome proliferation: current mechanisms relating to nongenotoxic carcinogenesis

A wide variety of chemicals have been shown to produce liver enlargement, peroxisome proliferation and induction of peroxisomal and microsomal fatty acid-oxidising enzyme activities in rats and mice. Some peroxisome proliferators have also been shown to increase the incidence of liver tumours in these species. Rodent peroxisome proliferators are not considered to be genotoxic agents. Proposed mechanisms of liver tumour formation include induction of sustained oxidative stress, enhanced cell replication, promotion of spontaneous preneoplastic lesions and inhibition of apoptosis. Marked species differences in the effects of peroxisome proliferators have been observed in both in vitro and in vivo studies. Key issues concerning the risk assessment to humans of exposure to rodent peroxisome proliferators are discussed.

Molecular modelling of members of the P4502A subfamily: application to studies of enzyme specificity

1. Using the recently published crystal structure of a bacterial P450, namely 102 (also termed P450bm3), as a template molecular models of mammalian 2A1, 2A4, 2A5 and 2A6 were constructed. 2. Substrate interaction studies demonstrated that in keeping with known catalytic activities the putative binding sites of mouse hepatic P4502A4 and 2A5 oriented testosterone for 15 alpha-hydroxylation and coumarin for 7-hydroxylation respectively. 3. Substrate interaction studies with the putative binding site of human liver P4502A6 demonstrated that coumarin was oriented for 7-hydroxylation. However, in keeping with previous site-directed mutagenesis studies with P4502A4 and 2A5, changing a single phenylalanine residue to leucine in 2A6 gave rise to a mutant enzyme, which could bind testosterone as a substrate for 15 alpha-hydroxylation rather than coumarin. 4. Substrate interaction studies with the putative binding site of rat hepatic P4502A1 suggested that this isoenzyme would hydroxylate coumarin at the 3- rather than at the 7-position. 5. The results of these molecular modelling studies demonstrate that apparently minor modifications to P4502A subfamily amino acid sequences can result in major alterations in enzyme specificity. 6. Molecular modelling is thus a useful technique that can aid in elucidating substrate specificities of P450 isoenzymes and species differences in xenobiotic metabolism. The technique can also be utilized to complement site-directed mutagenesis studies in order to identify critical structural features of P450s and other enzymes.

Toxicity of 3-methylindole, 1-nitronaphthalene and paraquat in precision-cut rat lung slices

The toxicity of 3-methylindole, 1-nitronaphthalene and paraquat has been studied in precision-cut rat lung slice cultures. Lung slices were prepared from male Sprague-Dawley rats using an agarose gel instilling technique with a Krumdieck tissue slicer and cultured for 24 h in a dynamic organ culture system. Treatment of rat lung slices with 3-methylindole, 1-nitronaphthalene or paraquat produced concentration dependent decreases in lung slice protein synthesis and potassium content. EC50 values (concentration to produce a 50% inhibition) for protein synthesis were 0.024, 0.27 and 0.57 mM for paraquat, 1-nitronaphthalene and 3-methylindole, respectively. These results demonstrate that precision-cut lung slices are a useful in vitro model system for studying the pulmonary toxicity of xenobiotics. Lung slices offer the potential as a rapid in vitro screen for identifying pulmonary toxicants and to evaluate species differences in response.

Mechanisms of hepatocarcinogenicity of peroxisome-proliferating drugs and chemicals

A wide variety of chemicals have been shown to produce liver enlargement, peroxisome proliferation, and induction of peroxisomal and microsomal fatty acid-oxidizing enzyme activities in rats and mice. Moreover, certain peroxisome proliferators have been shown to increase the incidence of liver tumors in these two species. This review describes the characteristics of peroxisome proliferation in rodent liver and in vitro in primary hepatocyte cultures and gives examples of the range of different classes of chemicals that produce this effect. Mechanisms of initiation of peroxisome proliferation in rodent hepatocytes, including peroxisome proliferator-activated receptors, are also described. Peroxisome proliferators are not considered to be genotoxic carcinogens, and proposed mechanisms of liver tumor formation include induction of sustained oxidative stress, a role for enhanced cell replication, and the promotion of spontaneous preneoplastic lesions. Data are also presented on species differences in response and key issues concerning the risk assessment to humans of rodent liver peroxisome proliferators.

Imidocarb residues in edible bovine tissues and in vitro assessment of imidocarb metabolism and cytotoxicity

Imidocarb residues in liver and muscle were measured by HPLC after a single therapeutic dose to cattle (3 mg imidocarb dipropionate kg-1). Imidocarb and 7-ethoxycoumarin metabolism were compared in three different in vitro systems prepared from bovine liver: cultures of hepatocyte monolayers, precision-cut liver slices, and microsomes. The potential hepatotoxicity of imidocarb residues was tested on hepatocyte monolayers and assessed using the neutral red and lactate dehydrogenase leakage assays. The concentration of imidocarb (mean +/- SD) decreased between days 14 and 224 after treatment from 5.40 +/- 0.61 to 0.12 +/- 0.01 and from 1.05 +/- 0.31 to 0.06 +/- 0.02 microgram g-1 in liver and muscle, respectively. The depletion kinetics of imidocarb fitted a two-compartment model with alpha- and beta-phase half-lives of 31.7 and 48.5 days in liver and 34.9 and 120.7 days in muscle, respectively. Imidocarb metabolites were not detected in any in vitro system. 7-Ethoxycoumarin metabolism was found in all in vitro systems; the predominant metabolite produced by hepatocyte and liver slice cultures was umbelliferone glucuronide. Cytotoxicity of imidocarb (100 microM) to hepatocyte monolayers was maximal after 72 hr treatment and dose-dependent above 10 microM imidocarb. It is most likely that the hepatotoxicity of imidocarb is caused by the parent compound, because no evidence for imidocarb metabolism was found.