Human cell mutagenicity of chlorinated and unchlorinated water and the disinfection byproduct 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX).

Extracts of three water samples--humic acid-enriched water-both peatland water and drinking water, both with and without chlorination were tested for mutagenicity at the tk locus in MCL-5 cells, a line of human B-lymphoblastoid cells that express cytochrome P450 enzymes and microsomal epoxide hydrolase. Our results show that chlorination caused a 5.5-fold increase (P<0.0001) in the mutagenicity of the humic acid-enriched water. The unchlorinated peatland water was mutagenic at the two highest doses (240 and 480 microg equivalent total organic carbon (TOC)/ml), possibly due to polycyclic aromatic hydrocarbons (PAH) that were measured in the peat. In contrast, the chlorinated peatland water was non-mutagenic at low doses, while at the highest dose (240 microg equivalent TOC/ml) the sample was so toxic that an insufficient number of cells survived treatment to allow plating. The chlorinated and unchlorinated drinking water were both non-mutagenic. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent bacterial mutagen and chlorine-disinfection byproduct, was also tested in MCL-5 cells as well as in two other human B-lymphoblastoid cell-lines, AHH-1 TK+/- and h1A1v2 cells, which differ from each other and from MCL-5 cells in the amounts of cytochrome P450 enzymes they can express. MX was mutagenic to all three cell-lines, but there was no apparent correlation between cytochrome P450 enzyme expression and the mutagenicity of MX. Overall, our results show that samples of chlorinated humic acid-enriched water and MX, a model chlorine-disinfection byproduct, are moderately mutagenic to human cells.

Mutagenicity of C24H14 PAH in human cells expressing CYP1A1.

Relatively little is known about the mutagenicity of C24H14 PAH, a diverse group of five- and six-ring PAH, some of which are present at trace levels in the environment. To better understand the mutagenicity of this class of compounds, 11 C24H14 PAH, including benzo[a]perylene, benzo[b]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,f]fluoranthene, dibenzo[j,l]fluoranthene, dibenzo[a,h]pyrene, dibenzo[a,i]pyrene, dibenzo[e,l]pyrene, naphtho[1,2-b]fluoranthene, naphtho[2,3-a]pyrene, and naphtho[2,3-e]pyrene, were tested in a mutagenicity assay based on human h1A1v2 cells. h1A1v2 cells are a line of human B-lymphoblastoid cells that have been engineered to express cytochrome P4501A1 (CYP1A1), an enzyme capable of metabolizing promutagenic PAH. Mutagenicity was measured at the thymidine kinase (tk) locus following a 72-h exposure period. Our results show that nine of the compounds were mutagenic. Benzo[a]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,i]pyrene, and naphtho[2,3-a]pyrene were the most potent mutagens, having minimum mutagenic concentrations (MMC) (i.e., the dose at which the induced response was twice that of the negative controls) in the 1-5 ng/ml range. Benzo[b]perylene, dibenzo[a,h]pyrene, dibenzo[a,f]fluoranthene, and naphtho[2,3-e]pyrene were somewhat less potent mutagens, having MMC in the 10-30 ng/ml range. Dibenzo[e,l]pyrene, which had an MMC of 280 ng/ml, was the least potent mutagen. Dibenzo[j,l]fluoranthene and naphtho[1,2-b]fluoranthene were not mutagenic at the doses tested (1-3000 ng/ml). The most mutagenic compounds were also quite toxic. At the highest doses tested, benzo[a]perylene, dibenzo[a,e]fluoranthene, dibenzo[a,i]pyrene, dibenzo[a,h]pyrene, and dibenzo[a,f]fluoranthene induced > 60% killing, and naphtho[2,3-a]pyrene and naphtho[2,3-e]pyrene induced > 50% killing. Benzo[b]perylene, dibenzo[e,l]pyrene, dibenzo[j,l]fluoranthene, and naphtho[1,2-b]fluoranthene induced < 50% killing at the highest doses tested. Comparing these results to a previous study in which nine other C24H14 PAH were tested for mutagenicity in this same assay, it was found that dibenzo[a]pyrene isomers were generally more mutagenic than the other groups of C24H14 PAH tested. These observations are discussed with emphasis given to identifying C24H14 PAH that may be important environmental mutagens.

Transport and metabolic characterization of Caco-2 cells expressing CYP3A4 and CYP3A4 plus oxidoreductase.

PURPOSE: To further characterize CYP3A4-transfected Caco-2 cells with regard to morphological, transport, and metabolic properties, and to evaluate a different Caco-2 cell strain transfected with both CYP3A4 and oxidoreductase (OR). METHODS: Transfected Caco-2 cells, Caco-2 TC7 cells, and wild-type Caco-2 cells grown onto Millicell were used. We determined the morphological characteristics of transfected cell monolayers using light and transmission electron microscope. We determined the transport and metabolic capabilities of the transfected cells, TC7 cells, and wild-type cells with a variety of drugs, nutrients, and marker compounds. RESULTS: The transfected Caco-2 cells formed a tight monolayer with TEER values and mannitol transport similar to the untransfected parent cell strain (wild type). However, the transfected cells (grown onto Millicell) reached maturity approximately 33% faster than the wild-type cells. Permeabilities of propranolol, nifedipine, testosterone, linopirdine, mannitol, and cephalexin were similar in transfected and wild-type Caco-2 cells. On the other hand, the transfected cells of early passages were much more metabolically active, and metabolized standard CYP3A4 substrates (e.g., testosterone and nifedipine) as much as 100 times faster than untransfected cells. In addition, metabolism of standard substrates was inhibitable by ketoconazole and TAO. Using comparable data, the transfected cells metabolized testosterone the fastest, followed by linopirdine and nifedipine (approximate ratio: 10:6:2). The metabolites of standard substrates were generally preferably excreted to the apical membrane. CONCLUSION: The monolayers of newly transfected cells (CYP3A4 + OR) have a significantly increased level of CYP3A4 activities compared to untransfected cells. These cell monolayers also have desirable morphological and transport characteristics that are similar to untransfected cells.

Human cytochrome P4502B6: interindividual hepatic expression, substrate specificity, and role in procarcinogen activation.

The level of expression and interindividual variation in human hepatic microsomal cytochrome P450 (CYP) 2B6 was characterized using a polyclonal antibody (WB-2B6) raised against rat CYP2B1. Immunoblot analysis using cDNA-expressed human CYPs revealed strong cross-reactivity of this antibody with CYP2B6 (limit of detection < 0.05 pmol) and only minor cross-reactivities with human CYP2A6, CYP2D6, and CYP2E1, all of which could be resolved from CYP2B6 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of human liver microsomes using this antibody revealed immunodetectable CYP2B6 protein in a majority of individual liver samples, with levels up to 74 pmol/mg protein in the CYP2B6-positive samples. Kinetic analysis of cDNA-expressed CYPs identified many of these enzymes as catalysts of 7-ethoxy-4-trifluoromethylcoumarin (7EFC) O-deethylation, but with significantly different apparent K(M) values (CYP1A2 < CYP2B6 approximately CYP1A1 < CYP2C19 < CYP2C9 < CYP2E1 < CYP2A6). By assaying liver microsomal 7EFC O-deethylase activity at a low 7EFC concentration (5 microM) and preincubating human liver microsomes with anti-CYP1A, anti-CYP2C, and anti-CYP2E1 antibodies, we were able to monitor CYP2B6-dependent 7EFC O-deethylase activity in a panel of 17 human liver microsomes and observe a significant correlation (r2 = 0.80) between this activity and CYP2B6 protein content. The ability of CYP2B6 to activate prodrugs and procarcinogens was examined using gene locus mutation assays in CYP2B6-expressing human lymphoblast cells. CYP2B6-expressing cells were found to be more sensitive than control cells to the cytotoxicity and mutagenicity of cyclophosphamide, aflatoxin B1, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. CYP2B6 is thus a widely expressed human liver microsomal CYP that can contribute to a broad range of drug metabolism and procarcinogen activation reactions.

Coexpression of cytochrome P4502A6 and human NADPH-P450 oxidoreductase in the baculovirus system.

Heterologous expression using baculovirus vectors has become a popular method for the production of catalytically active cytochrome P450s (CYPs). We have systematically optimized the multiplicity of infection (MOI) for a coinfection approach for the coexpression of CYP2A6 (viral vector designated v2A6) and NADPH-P450 oxidoreductase (OR; viral vector designated vOR) using Sf9 insect cells. A 3000-fold range of MOI was examined in stationary culture and stirred suspension culture. Surprisingly, our results indicate that the best CYP2A6 catalytic activity (850-1300 pmol/ min/mg total lysate protein as measured by coumarin 7-hydroxylase activity) was obtained only when using a low MOI of v2A6 (1.5-3 x 10(-2)) and a vOR of 10- to 20-fold less. This activity was approximately 7- to 11-fold higher than the best activity obtained when infecting cells with v2A6 alone. At this level of coinfection, the P450 content ranged from 180 to 250 pmol/mg total lysate protein, and the NADPH cytochrome c reductase activity ranged from 350 to 520 nmol/min/mg total lysate protein. Increasing the MOI of both viruses to 50-fold higher resulted in lower overall activity with the optimum (250 pmol/min/mg total lysate protein) being seen earlier postinfection (60 vs. 72 hr). Increasing the MOI of vOR to levels comparable with those of v2A6, decreased coumarin 7-hydroxylase activity 14-fold. These results suggest that the best CYP2A6 catalytic activity depends on properly posttranslationally modified proteins accumulating in a right ratio as a result of primary, secondary, and possibly tertiary infection of both viruses. These results also suggest that high OR expression results in degradation of P450.

Development of a human lymphoblastoid cell line constitutively expressing human CYP1B1 cDNA: substrate specificity with model substrates and promutagens.

An AHH-1 TK+/- cell derivative was developed that stably expresses human cytochrome P4501B1 (CYP1B1) cDNA in an extrachromosomal vector which confers resistance to 1-histidinol and co-expresses NADPH cytochrome P450 oxidoreductase (OR). The CYP1B1-expressing cell line was designated h1B1/OR. Microsomes prepared from CYP1B1 cDNA expressing cells exhibit elevated levels of 7-ethoxy-resorufin deethylase (EROD), 7-ethoxy-4-trifluoromethyl-coumarin deethylase (EFCD), benzo(alpha)pyrene hydroxylase (BPH), bufuralol 1'-hydroxylase, testosterone hydroxylase activities and spectrally quantifiable cytochrome P450. CYP1B1-containing microsomes did not contain detectable coumarin 7-hydroxylase, p-nitrophenol hydroxylase, lauric acid hydroxylase, (S)-mephenytoin 4'-hydroxylase or diclofenac 4'-hydroxylase activities. Kinetic parameters for selected substrates were compared among CYP1B1 and the two additional members of the CYP1 family, CYP1A1 and CYP1A2. For BPH and EFCD, the rank order of rates of substrate metabolism were CYP1A1 > CYP1B1 > CYP1A2. For EROD, the rank order of substrate metabolism was CYP1A1 > CYP1A2 > CYP1B1. For both EROD and EFCD the apparent K(m) values for CYP1B1 were more similar to CYP1A1 than to CYP1A2. In order to begin to characterize the promutagen activating ability of CYP1B1, the mutagenicity of selected chemicals was examined in h1B1/OR cells; there was increased sensitivity (CYP1B1-expressing relative to control cells) to the mutagenicity of benzo(a)pyrene, cyclopenta(c,d)pyrene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and aflatoxin B1 (AFB). CYP1B1, expressed in this system, appears to be particularly efficient at activating AFB.

Mutagenicity of the atmospheric transformation products 2-nitrofluoranthene and 2-nitrodibenzopyranone in Salmonella and human cell forward mutation assays.

The mutagenicity of the atmospheric transformation products 2-nitrofluoranthene (2-NF) and 2-nitrodibenzopyranone (2-NDBP), as well as a related isomer 3-nitrodibenzopyranone (3-NDBP), was measured in quantitative forward mutation assays with bacteria (Salmonella typhimurium TM677) and in two metabolically competent human cell lines (MCL-5 and h1A1v2) that differ in their complement of cytochrome P450s and microsomal epoxide hydrolase. 2-NF was a potent mutagen in Salmonella TM677 both in the absence and presence of rat liver postmitochondrial supernatant (PMS). 2-NDBP was non-mutagenic in the absence of PMS, but was mutagenic in its presence. The converse result was obtained for 3-NDBP. The mutagenic potency series in Salmonella in the absence of PMS, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was: 2-NF, 2.5; 3-NDBP, 16.9; and 2-NDBP, > 415. With PMS, the potency series was: 2-NF, 1.2; 2-NDBP, 15.1; 3-NDBP, 208. Neither 2-NDBP nor 3-NDBP were mutagenic at the tk locus in MCL-5 or h1A1v2 cells at up to 200 nmol/ml. 2-NF was also inactive in MCL-5 cells, but was a potent mutagen in h1A1v2 cells with an MDMC of 0.02 nmol/ml. Cytochrome P450 CYP1A1, present constitutively only in h1A1v2 cells, was implicated in 2-NF activation because mutagenicity was reduced by 55-80% when alpha-naphthoflavone (ANF) was present during incubation. The lack of mutagenicity in MCL-5 cells was attributed to the inability of 2-NF to induce CYP1A1 activity in this cell line. These data indicate a primary role for ring oxidation in 2-NF activation. Previous emphasis placed upon 2-NDBP as a major mutagen in ambient air may need to be modified in view of the negative results for this compound in the human cell assays and in the absence of PMS in Salmonella TM677. However, these findings support the concern that 2-NF may be a risk to human health.

Use of cDNA-expressed human cytochrome P450 enzymes to study potential drug-drug interactions.

Complementary DNA (cDNA)-expressed human cytochrome P450 enzymes provide a reproducible, consistent source of single enzymes for many types of studies. The use of single enzymes systems, relative to multienzyme systems, has distinct advantages and disadvantages depending on the specific application. cDNA-expressed materials have advantages in the analysis of cytochrome P450 form-selective metabolism of a drug or drug candidate. This analysis can be accomplished by direct incubation of the drug with microsomes prepared from cells expressing a single cytochrome P450 form coupled with analysis of either metabolite formation or loss of parent compound. This approach allows the unambiguous assignment of specific biotransformations to specific enzymes. However, extending these data to the balance of enzymes present in human liver microsomes can be problematic. New approaches for relating rates of metabolism for cDNA-expressed enzymes to human liver microsomes metabolism are being developed (Crespi, 1995). In addition, cDNA-expressed enzymes can be used to study the cytochrome P450 form-selective inhibition by drugs or drug candidates. This analysis is accomplished through the study of the inhibition of the metabolism of a model substrate by the drug or drug candidate. Through these analyses, apparent Ki values can be obtained and compared to Ki values for known, clinically significant inhibitors of the same enzyme. For this application, cDNA-expressed, single enzyme systems have distinct advantages because of greater flexibility in the choice of model substrates and the lack of competing pathways of metabolism. Specific data for the use of cDNA-expressed CYP2C9, CYP2D6, and CYP3A4 are presented.

Human cell mutagenicity of oxygenated, nitrated and unsubstituted polycyclic aromatic hydrocarbons associated with urban aerosols.

Polycyclic aromatic compounds (PAC) are ubiquitous pollutants in urban air that may pose risks to human health. In order to better assess the health risks associated with this class of compounds, a total of 67 PAC that either have been identified (55) or are suspected to be present (12) in urban aerosol samples were tested for mutagenicity in a forward mutation assay based on human B-lymphoblastoid cells. The cell line used (designated h1A1v2) constitutively expresses the cytochrome P4501A1, which is known to be necessary for the metabolism of many promutagens. The PAC tested included 39 polycyclic aromatic hydrocarbons (PAH). 19 oxygen-containing PAH (oxy-PAH) and nine NO2-substituted PAH (nitro-PAH). A total of 26 PAH were mutagenic. In comparing the minimum mutagenic concentrations of the mutagenic PAH with that of benzo[a]pyrene (B[a]P) it was found that dibenzo[a,l]pyrene (DB[al]P), cyclopenta[c,d]pyrene (CPP), naphtho[2,1-a]pyrene, dibenzo[a,e]pyrene (B[a]P) and 1-methylbenzo[a]pyrene were 24 +/- 21, 6.9 +/- 4.2, 3.2 + 3.0, 2.9 +/- 2.9 and 1.6+/- 1.4 times, respectively, more mutagenic than B[a]P, and that dibenzo[a,k]fluoranthene and B[a]P were approximately equally mutagenic. The 19 other mutagenic PAH were between approximately 2 and approximately 1800 times less mutagenic than B[a]P. Of the oxy-PAH tested only phenalenone, 7H-benz[d,e]anthracen-7-one, 3-nitro-6H-dibenzo[b,d]pyran-6-one, cyclopenta[c,d]pyren-3(4H)-one, 6H-benzo[c,d]pyren-6-one (BPK) and anthanthrenequinone were mutagenic; however, with the exception of BPK, these were over 50 times less active than B[a]P, BPK was approximately 3 times less active than B[a]P. Seven of the nitro-PAH were mutagenic including 9-nitroanthracene, 1-nitropyrene, 2-nitrofluoranthene, 3-nitrofluoranthene, 1,3-dinitropyrene, 1,6-dinitropyrene (1,6-DNP) and 1,8-dinitropyrene. 1,6-DNP was approximately 4 times less active than B[a]P; the six other mutagenic nitro-PAH were between 20 and 380 times less active than B[a]P. These results are discussed in terms of their relevance for determining the most important mutagens in ambient air. Based on reported concentrations of PAC in ambient aerosols, it is possible that CPP, DB[ae]P, DB[al]P and BPK could account for a greater proportion of the mutagenicity than B[a]P in some aerosols.

Development of Caco-2 cells expressing high levels of cDNA-derived cytochrome P4503A4.

PURPOSE: To develop Caco-2 cell derivatives expressing high levels of human cytochrome P450 drug metabolizing enzymes. METHODS: The cDNAs for two cytochrome P450 forms, CYP2A6 and CYP3A4, were introduced into an extrachromosomal vector under control of the cytomegalovirus early intermediate promoter. Vector-bearing cells were selected via resistance to hygromycin B. RESULTS: Transfected cells exhibited high levels of cDNA-derived protein as measured by Western blot, spectrophotometric P450 determination and/or cytochrome P450 form-selective enzyme assay. CYP3A4 and CYP2A6 catalytic activities were about 100 fold higher than in control cells. cDNA-expressing cells were found to form tight monolayers and were suitable for study of xenobiotic transport and metabolism. The permeabilities of cephalexin, phenylalanine, mannitol and propranolol across transfected monolayers were found to be similar to those across untransfected monolayers. The appropriate transfected monolayers metabolized the CYP2A6 substrate coumarin and the CYP3A4 substrates testosterone and nifedipine. CONCLUSIONS: A Caco-2 cell system to simultaneously study drug transport and metabolism has been developed.

Comparison of substrate metabolism by wild type CYP2D6 protein and a variant containing methionine, not valine, at position 374.

We have analysed kinetic parameters of cDNA-derived CYP2D6 proteins derived from the original CYP2D6 cDNA isolate (Gonzalez FJ et al. Nature 1988: 331, 442-446) which contains methionine at position 374 (CYP2D6-Met) and a modified cDNA which contains valine at position 374 (CYP2D6-Val). This latter protein is predicted from the CYP2D6 genomic sequence. Several quantitative differences, but no qualitative differences in metabolism were observed. CYP2D6-Met was found to have a two-fold lower Km and a three-fold lower turnover rate for (R)(+)-bufuralol 1'-hydroxylation as compared to CYP2D6-Val. In contrast, CYP2D6-Met and CYP2D6-Val had a similar Km for debrisoquine 4-hydroxylation while CYP2D6-Val had an 18-fold higher turnover rate. CYP2D6-Val and CYP2D6-Met had similar Kms for metoprolol but CYP2D6-Val showed a three-fold higher capacity for the O-demethylation reaction compared to alpha-hydroxylation which is more similar to that seen in human liver. In the case of sparteine, CYP2D6-Val and CYP2D6-Met showed similar capacities for formation of the 2-dehydrosparteine metabolite but the Km value for CYP2D6-Met was six-fold higher than that for CYP2D6-Val. Kinetic differences between CYP2D6-Met and CYP2D6-Val were further probed by examination of apparent Ki for inhibition of (R,S)(+/-)-bufuralol 1'-hydroxylation. Similar Ki values (within a factor of three) were observed for perhexiline and (R,S)-propranolol while quinidine and dextromethorphan were 8.5-fold and 21-fold more effective inhibitors of CYP2D6-Val relative to CYP2D6-Met. An allele specific polymerase chain reaction assay was developed for the CYP2D6-Met allele. The CYP2D6-Met allele was not found among 83 individuals. In the aggregate, these data indicated that the CYP2D6-Val allele is the more common allele in human populations. The quantitative kinetic differences between these two enzymes appears most pronounced for substrates/inhibitors with rigid structures. CYP2D6-Val more often has a substantially lower Km and/or a substantially higher capacity to metabolize those substrates.

Mutagenicity of benzo[a]pyrene and dibenzopyrenes in the Salmonella typhimurium TM677 and the MCL-5 human cell forward mutation assays.

The mutagenicity of benzo[a]pyrene (B[a]P), dibenzo[ae]pyrene (DB[ae]P), dibenzo[ah]pyrene (DB[ah]P), dibenzo[ai]pyrene (DB[ai]P), and dibenzo[al]pyrene (DB[al]P) was measured in quantitative forward mutation assays with bacteria (Salmonella typhimurium TM677) and a metabolically competent cell line derived from human B-lymphoblastoid cells (MCL-5) that contained activity for five cytochrome P450s and microsomal epoxide hydrolase found in human liver. DB[al]P and B[a]P, both potent animal carcinogens, were the most mutagenic substances in both assays. DB[al]P was nearly 50-fold more potent than B[a]P in human cells, but only 60% more mutagenic in Salmonella. The carcinogenic isomer DB[ah]P, though nonmutagenic in bacteria, was active in human cells. The following mutagenic potency series, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was obtained with Salmonella in the presence of rat liver postmitochondrial supernatant (PMS): DB[al]P (3.7), B[a]P (5.8), DB[ae]P (6.9), DB[ai]P (14.9), DB[ah]P (> 100). None of the compounds were mutagenic in the absence of PMS. In human MCL-5 cells the potency series was: DB[al]P (3.1 x 10(-4)), B[a]P (1.5 x 10(-2)), DB[ae]P (2.5 x 10(-2)), DB[ah]P (0.5), DB[ai]P (3.2). The human cell assay thus exhibited over a 10,000-fold range between the most mutagenic and least mutagenic compound, whereas in the bacterial assay there was only a corresponding four-fold difference if the nonmutagenic DB[ah]P was excluded. The results were discussed in terms of their concordance with animal carcinogenicity studies.

Human cell mutagenicity of mono- and dinitropyrenes in metabolically competent MCL-5 cells.

Nitropyrenes are ubiquitous environmental pollutants that may pose a human health hazard because some are highly potent mutagens and carcinogens. The mutagenicity (trifluorothymidine resistance at the thymidine kinase locus) of 1-, 2-, and 4-nitropyrene (1-, 2-, and 4-NP), 1,3-, 1,6-, and 1,8-dinitropyrene (1,3-, 1,6-, and 1,8-DNP), and pyrene was assessed in a quantitative forward mutation assay using a metabolically competent line (MCL-5) of human B-lymphoblastoid cells. These cells contain endogenous cytochrome P450 activity (CYP1A1) and two plasmids that express cDNAs for four additional P450s (CYP1A2, CYP2A6, CYP2E1, CYP3A4) and microsomal epoxide hydrolase found in human liver. The major finding is that 2-NP and 1,3-DNP, both potent bacterial mutagens, were nonmutagenic in this assay. The following mutagenic potency series, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was obtained: 1,6-DNP (0.8), 1,8-DNP (1.5), 4-NP (3.1), 1-NP (9.1), 2-NP (> 81), 1,3-DNP (> 86), pyrene (> 494). There was over an 11-fold difference between the most potent (1.6-DNP) and the least potent (1-NP) mutagen. 1,6-DNP was approximately twice as mutagenic as 1,8-DNP, which was almost twice as mutagenic as 4-NP, which, in turn was nearly three times as potent as 1-NP. This is the first report on the testing of 2-NP and 4-NP for mutagenicity in mammalian cell cultures. The human cell mutagenicity of these compounds was discussed in terms of potency series of nitropyrenes obtained from animal carcinogenicity experiments and other mammalian cell mutagenicity assays.

Development of a human lymphoblastoid cell line constitutively expressing human CYP1A1 cDNA: substrate specificity with model substrates and promutagens.

AHH-1 TK+/- cell derivatives were developed that stably express human CYP1A1 cDNA, and an AHH-1 TK+/- derivative expressing higher levels of CYP1A2 cDNA in extrachromosomal vectors which confer resistance to 1-histidinol. The CYP1A1-expressing cell lines, designated h1A1 and h1A1v2, differ by containing one and two CYP1A1 cDNA expression units per vector. The CYP1A2-expressing cell line, designated h1A2v2, also has two CYP1A2 cDNA expression units per vector. Microsomes prepared from CYP1A1 cDNA expressing cells exhibit high, constitutive levels of 7-ethoxyresorufin deethylase (EROD), 7-ethoxycoumarin deethylase (ECD), 7-ethoxy-4-trifluoromethylcoumarin deethylase (EFCD), benzo[a]-pyrene hydroxylase (BPH) activities and spectrally quantifiable cytochrome P450. Kinetic comparisons between cDNA-expressed CYP1A1 and CYP1A2 indicate that CYP1A1 is more active than CYP1A2 for EROD, ECD, EFCD and BPH. CYP1A2 was more active than CYP1A1 for acetanilide hydroxylation and activation of aflatoxin B1 (AFB1). The mutagenicity of selected promutagens were examined in h1A1 cells and control cells. Relative to control cells, the h1A1 cell line exhibits increased sensitivity to the mutagenicity of benzo[a]pyrene, cyclopenta[c,d]pyrene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and AFB1.

Human cell lines, derived from AHH-1 TK+/- human lymphoblasts, genetically engineered for expression of cytochromes P450.

We are developing a panel of human B lymphoblastoid cells which have been engineered to express specific human cDNAs for cytochrome P450 and other xenobiotic metabolizing enzymes. The recipient cells are of a human B lymphoblastoid cell line, designated AHH-1 TK+/-. These cells are transfected using two extrachromosomal vectors both containing OriP sequences derived from Epstein Barr virus but containing independent means of selection in mammalian cells. Using this system, the level of cDNA expression is nearly always stable and consistent from one transfection to another. Thus, once the level of expression has been characterized, cell lines with potentially interesting combinations of xenobiotic-metabolizing enzymes can be predictably developed. cDNAs encoding the following human enzymes have been expressed in this system: CYP1A1, CYP1A2, CYP2A6, CYP2B8, CYP2C6, CYP2C9, CYP2D6, CYP2E1, CYP3A4 and microsomal epoxide hydrolase. We have expressed all of these enzymes individually and have developed cell lines which express combinations of the xenobiotic metabolizing enzymes. The expression of multiple enzymes is important for generalized use of engineered cells as toxicology screening tools. We have primarily used the cell lines in applications to toxicology focusing on procarcinogen activation as detected in assays for the induction of gene locus mutations. In this chapter we discuss the general properties of the system and applications to toxicology testing.

Characterization of a human cell line expressing high levels of cDNA-derived CYP2D6.

We have developed a human B-lymphoblastoid cell, designated h2D6v2, which expresses high levels of CYP2D6 cDNA. Microsomal P450 contents of 160 pmol mg-1 protein were observed. NADPH-fortified microsomes exhibited a substantial capacity to hydroxylate the prototype CYP2D6 substrates bufuralol and debrisoquine. Kinetic parameters, apparent Km, turnover number, Ki for quinidine inhibition and stereospecificity of bufuralol hydroxylation, observed with the human lymphoblast expressed enzyme were similar to those observed in human liver microsomes or purified liver CYP2D6 proteins. Therefore, the human lymphoblast expressed material appears to faithfully reflect the authentic protein. Relative to control cells, h2D6v2 cells were more sensitive to the cytotoxicity and mutagenicity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), supporting our previous observation with a cell line expressing lower levels of CYP2D6. h2D6v2 microsomes were capable of metabolizing NNK and NNK metabolism and mutagenicity were markedly inhibited by the addition of quinidine, a CYP2D6 inhibitor. h2D6v2 cells coupled with control cells, represent a useful in vitro system for studying xenobiotic metabolism by the clinically important, polymorphic CYP2D6. The human lymphoblast system offers the desirable ability to couple metabolic transformation studies with toxicological endpoints such as cytotoxicity and mutagenicity.

DNA damage and mutation in human cells exposed to nitric oxide in vitro.

Nitric oxide (NO.) is a physiological messenger formed by several cell types. Reaction with O2 forms oxides that nitrosate amines at pH values near 7. We now report experiments in which NO. was added to intact human cells and to aerobic solutions of DNA, RNA, guanine, or adenine. TK6 human lymphoblastoid cells were mutated 15- to 18-fold above background levels at both the HPRT and TK gene loci. Xanthine and hypoxanthine, from deamination of guanine and adenine, respectively, were formed in all cases. NO. induced dose-responsive DNA strand breakage. Yields of xanthine ranged from nearly equal to about 80-fold higher than those of hypoxanthine. Yields of xanthine and hypoxanthine from nucleic acids were higher than those from free guanine and adenine. This was most pronounced for xanthine; 0.3 nmol/mg was formed from free guanine vs. 550 nmol/mg from calf thymus RNA. Nitric oxide added to TK6 cells produced a 40- to 50-fold increase in hypoxanthine and xanthine in cellular DNA. We believe that these results, plus the expected deaminations of cytosine to uracil and 5-methylcytosine to thymine, account for the mutagenicity of nitric oxide toward bacteria and mammalian cells.

A metabolically competent human cell line expressing five cDNAs encoding procarcinogen-activating enzymes: application to mutagenicity testing.

A human B-lymphoblastoid cell line, designated MCL-5, constitutively expressing human cytochrome P-450 CYP1A1 and also expressing five transfected human cDNAs encoding drug-metabolizing enzymes, has been developed. cDNAs encoding CYP1A2, CYP2A6, and microsomal epoxide hydrolase (mEH) were introduced by using a vector conferring hygromycin B resistance, and cDNAs encoding CYP2E1 and CYP3A4 were introduced by using a vector conferring resistance to 1-histidinol. MCL-5 cells stably expressed all five cDNAs and the native CYP1A1 as determined by measurement of form-specific enzyme activity levels. The mutagenicity of seven model procarcinogens to MCL-5 cells was examined at the hypoxanthine guanine phosphoribosyltransferase (hprt) and thymidine kinase (tk) loci. Exposure to benzo[a]pyrene (BP), 3-methylcholanthrene (3MC), N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), aflatoxin B1, (AFB1), 2-(acetylamino)fluorene (AAF), or benzidine (BZD) induced a statistically significant increase in mutant frequency. Linear interpolation of the concentration of procarcinogen necessary to produce a doubling of the mutant fraction at the hprt locus in MCL-5 cells and the parent AHH-1 cell line revealed that, for each of the chemicals examined, except BZD, MCL-5 cells were significantly more sensitive than the parent AHH-1 cells. The increase in sensitivity to mutagenicity ranged from 3-fold for AAF to greater than 40,000-fold for NDMA. MCL-5 cells have great potential as a screening system for the analysis of human procarcinogen/promutagen activation.