Differential inhibition of murine prostaglandin synthase-1 and -2 by nonsteroidal anti-inflammatory drugs using exogenous and endogenous sources of arachidonic acid

Mouse embryonic fibroblasts (10T1/2) and Chinese hamster ovary (AS52) cell lines that stably express murine prostaglandin G/H synthase (PGHS)-1 or -2 were used to compare the effects of exogenous and endogenous arachidonic acid (AA) on isozyme-selective inhibition by acetylsalicylic acid, indomethacin, and N-[2-cyclohexyloxyl-4-nitrophenyl] methanesulfonamide (NS-398). The rationale for developing in vitro systems that identify PGHS-2-selective inhibitors is the belief that inhibition of this isoform accounts for the therapeutic benefits of nonsteroidal anti-inflammatory drugs (NSAIDs). Conversely, inhibition of PGHS-1 is believed to cause the toxic effects of NSAIDs, such as gastric and renal damage. When exogenous AA was used, acetylsalicylic acid was a 5- to 10-fold more potent inhibitor of PGHS-1, whereas indomethacin was a 4- to 5-fold more potent inhibitor of PGHS-2. Within the dose range tested (1 x 10(-6) microM to 100 microM), NS-398 was highly selective for PGHS-2. When calcium ionophore A23187 was used to mobilize endogenous AA, acetylsalicylic acid and indomethacin equipotently inhibited both PGHS-1 and PGHS-2 isozymes. NS-398 remained highly selective for PGHS-2 in 10T1/2 and AS52 cells but also effectively (100%) inhibited PGHS-1 in AS52 cells. Pharmacological data derived using endogenous AA correlated better with the anti-inflammatory efficacy of these NSAIDs in laboratory animals and with the therapeutic/toxic activities of these NSAIDs in rheumatoid arthritic patients. Therefore, screening for PGHS-selective NSAIDs may best be conducted in intact cells that express high levels of each isozyme using endogenous sources of AA.

Leukocyte lipid body formation and eicosanoid generation: cyclooxygenase-independent inhibition by aspirin

Lipid bodies, cytoplasmic inclusions that develop in cells associated with inflammation, are inducible structures that might participate in generating inflammatory eicosanoids. Cis-unsaturated fatty acids (arachidonic and oleic acids) rapidly induced lipid body formation in leukocytes, and this lipid body induction was inhibited by aspirin and nonsteroidal antiinflammatory drugs (NSAIDs). Several findings indicates that the inhibitory effect of aspirin and NSAIDs on lipid body formation was independent of cyclooxygenase (COX) inhibition. First, the non-COX inhibitor, sodium salicylate, was as potent as aspirin in inhibiting lipid body formation elicited by cis-fatty acids. Second, cis-fatty acid-induced lipid body formation was not impaired in macrophages from COX-1 or COX-2 genetically deficient mice. Finally, NSAIDs inhibited arachidonic acid-induced lipid body formation likewise in macrophages from wild-type and COX-1- and COX-2-deficient mice. An enhanced capacity to generate eicosanoids developed after 1 hr concordantly with cis-fatty acid-induced lipid body formation. Arachidonic and oleic acid-induced lipid body numbers correlated with the enhanced levels of leukotrienes B4 and C4 and prostaglandin E2 produced after submaximal calcium ionophore stimulation. Aspirin and NSAIDs inhibited both induced lipid body formation and the enhanced capacity for forming leukotrienes as well as prostaglandins. Our studies indicate that lipid body formation is an inducible early response in leukocytes that correlates with enhanced eicosanoid synthesis. Aspirin and NSAIDs, independent of COX inhibition, inhibit cis-fatty acid-induced lipid body formation in leukocytes and in concert inhibit the enhanced synthesis of leukotrienes and prostaglandins.

Relative activities of retrovirally expressed murine prostaglandin synthase-1 and -2 depend on source of arachidonic acid

We have developed derivatives of mouse embryonic fibroblasts (10T1/2) and Chinese hamster ovary (AS52) cells that stably express high levels of murine prostaglandin synthase-1 or -2 (PGHS-1 or -2). The cDNAs were transferred using retroviral vectors and the resulting G418-resistant clones were analyzed for prostaglandin E2 (PGE2) production. Specific expression was confirmed by Western and Northern analyses. Enzyme activities, protein, and message levels peaked 1 (10T1/2) or 2 (AS52) days after seeding but decreased as cells became density arrested. Upon subculturing, enzyme activities returned to their initial high levels. With 10 microM exogenous arachidonic acid (AA) as the substrate, PGHS-1 activities were approximately 3- to 5-fold higher than PGHS-2 activities. Conversely, when exogenous AA was left out of the medium and only endogenous AA was available as substrate, enzyme activities were lower; but PGHS-2 activities were 5-fold (10T1/2) or 1.5-fold (AS52) higher than PGHS-1 activities. Following phorbol ester treatment to stimulate endogenous AA release, PGHS-2 activities increased over time and by 6 hours, were 4-fold (10T1/2) or 2-fold (AS52) higher than PGHS-1 activities. However, when calcium ionophore A23187 was used to stimulate endogenous AA release, maximum PGHS activities occurred within 30 min of treatment; PGHS-1 activities were equal to (10T1/2) or 2-fold higher (AS52) than PGHS-2 activities. Because these cell lines allow us to measure specific PGHS activity in intact cells, we were able to demonstrate that the relative activities of the two PGHS isozymes depend on the source of AA (exogenous versus endogenous) or biochemical stimulus used to mobilize endogenous AA (A23187 versus phorbol ester). These data suggest that PGHS-1 and PGHS-2 preferentially utilize different pools of AA and may be modulated through different stimulus-initiated pathways.

4-Ipomeanol and 2-aminoanthracene cytotoxicity in C3H/10T1/2 cells expressing rabbit cytochrome P450 4B1

In the present study, retroviral vectors were used to stably transfer and express the cDNA encoding rabbit CYP4B1 in mouse C3H/10T1/2 cells. The replication defective retroviral vector was packaged in the ecotropic packaging cell line, GP+E-86, with infectious titer of approximately 1 x 10(6) cfu/mL. Infection, followed by selection with G418, showed an infection efficiency of approximately 70% for the recipient C3H/10T1/2 cells. Analysis of ten G418 resistant clones showed that the number of vector inserts ranged from 4 to 13 copies per cell genome. Each clone was positive for microsomal CYP4B1 protein as determined by immunoblotting. Cytochrome P450 4B1 activity was assessed by the cytotoxicity of 4-ipomeanol, a known substrate for P450 4B1 and a model compound for chemical-induced injury to the lung. The initial clonigenic assays showed that 100% toxicity occurred in all the clones after a 96-hr exposure to 250 microM 4-ipomeanol. Parental C3H/10T1/2 cells were resistant to 4-ipomeanol at concentrations as high as 1 mM. Two clones, designated No. 2 and No. 19, differing in levels of P450 4B1 protein, were characterized further for 4-ipomeanol and other chemical toxicities. A concentration-response study indicated 50% cytotoxicity at 4-ipomeanol concentrations of 1.5 micrograms/mL for clone No. 2 and 2.5 micrograms/mL for clone No. 19. A panel of agents representing the aromatic amines, some of which are known or suspected P450 4B1 substrates, were tested for cytotoxicity in clone No. 2. These agents included 2-aminoanthracene, 2-aminonaphthalene, 2-aminofluorene, 2-acetylaminofluorene and 4-aminobiphenyl. Only 2-aminoanthracene gave a clear cytotoxic response reducing the survival fraction of clone No. 2 to 50% at 0.2 micrograms/mL while affecting parental cells minimally. In vitro expression of CYP4B1 provides a new experimental system for further elucidating the cytotoxic and mutagenic effects of P450 4B1 substrates.

Prostaglandin synthase 2 gene disruption causes severe renal pathology in the mouse

The prostaglandin endoperoxide H synthase isoform 2, cyclooxygenase 2 (COX-2), is induced at high levels in migratory and other responding cells by pro-inflammatory stimuli. COX-2 is generally considered to be a mediator of inflammation. Its isoform, COX-1, is constitutively expressed in most tissues and is thought to mediate "housekeeping" functions. These two enzymes are therapeutic targets of the widely used nonsteroidal anti-inflammatory drugs (NSAIDs). To investigate further the different physiologic roles of these isoforms, we have used homologous recombination to disrupt the mouse gene encoding COX-2 (Ptgs2). Mice lacking COX-2 have normal inflammatory responses to treatments with tetradecanoyl phorbol acetate or with arachidonic acid. However, they develop severe nephropathy and are susceptible to peritonitis.

Prostaglandin synthase 1 gene disruption in mice reduces arachidonic acid-induced inflammation and indomethacin-induced gastric ulceration

Cyclooxygenases 1 and 2 (COX-1 and COX-2) are key enzymes in prostaglandin biosynthesis and the target enzymes for the widely used nonsteroidal anti-inflammatory drugs. To study the physiological roles of the individual isoforms, we have disrupted the mouse Ptgs1 gene encoding COX-1. Homozygous Ptgs1 mutant mice survive well, have no gastric pathology, and show less indomethacin-induced gastric ulceration than wild-type mice, even though their gastric prostaglandin E2 levels are about 1% of wild type. The homozygous mutant mice have reduced platelet aggregation and a decreased inflammatory response to arachidonic acid, but not to tetradecanoyl phorbol acetate. Ptgs1 homozygous mutant females mated to homozygous mutant males produce few live offspring. COX-1-deficient mice provide a useful model to distinguish the physiological roles of COX-1 and COX-2.

Human CYP2A6 activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK): mutational specificity in the gpt gene of AS52 cells

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK) is a potential human carcinogen that is known to be metabolized to DNA-reactive intermediates by the cytochromes P450. We have examined the nature of NNK's DNA damaging effects in a mammalian cell system expressing a specific human cytochrome P450 (2A6) and containing a target gene for mutagenesis. Human CYP2A6, which is known to activate NNK to a mutagen, was lipofected via a retroviral vector into the Chinese hamster ovary AS52 cell line, which contains the bacterial gpt gene and can be mutated to 6-thioguanine resistance. AS52 cells expressed negligible CYP2A6-specific coumarin 7-hydroxylase activity (0.7 pmol/mg protein/min), while a CYP2A6 transfected clone (AS52-E8) expressed 30 pmol/mg protein/min. Both cell lines were equally sensitive to the cytotoxic and mutagenic effects of the direct-acting mutagen ethylmethanesulfonate; however, only the AS52-E8 cells exhibited a dose-dependent increase in cytotoxicity and mutant frequency upon treatment with NNK. At the highest NNK dose (1200 micrograms/ml), the mutant frequency in AS52-E8 cells was 14-fold (339 x 10(-6)) greater than the spontaneous frequency of 24 x 10(-6). Ninty-eight mutant clones were isolated following NNK treatment. Based on PCR analysis, 21 clones contained deletions/rearrangements and 77 were putative point mutants. Sequencing potential point mutants showed that 81% contained G:C to A:T transitions. Four of six G:C to A:T hotspots were at the second G of the GGT motif, which is the motif and major mutation found in codon 12 of Ki-ras from NNK-induced lung tumors in strain A mice. Since NNK may be metabolized via different pathways to pyridyloxobutylate or methylate DNA, the data suggest that methylation damage causes the major mutagenic events in AS52-E8 cells when NNK is activated by human CYP2A6.

Deletion and replacement of the mouse adult beta-globin genes by a "plug and socket" repeated targeting strategy

We describe a two-step strategy to alter any mouse locus repeatedly and efficiently by direct positive selection. Using conventional targeting for the first step, a functional neo gene and a nonfunctional HPRT minigene (the "socket") are introduced into the genome of HPRT- embryonic stem (ES) cells close to the chosen locus, in this case the beta-globin locus. For the second step, a targeting construct (the "plug") that recombines homologously with the integrated socket and supplies the remaining portion of the HPRT minigene is used; this homologous recombination generates a functional HPRT gene and makes the ES cells hypoxanthine-aminopterin-thymidine resistant. At the same time, the plug provides DNA sequences that recombine homologously with sequences in the target locus and modifies them in the desired manner; the plug is designed so that correctly targeted cells also lose the neo gene and become G418 sensitive. We have used two different plugs to make alterations in the mouse beta-globin locus starting with the same socket-containing ES cell line. One plug deleted 20 kb of DNA containing the two adult beta-globin genes. The other replaced the same region with the human beta-globin gene containing the mutation responsible for sickle cell anemia.

N-nitrosodiethylamine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone induced morphological transformation of C3H/10T1/2CL8 cells expressing human cytochrome P450 2A6

Transfection of specific genes into cells capable of expressing chemically induced morphological cell transformation provides a valuable approach to study the mechanisms of action of carcinogens. A human cytochrome P450 isozyme, CYP2A6, has been successfully expressed from a retroviral vector in transformable C3H/10T1/2 (10T1/2) mouse embryo fibroblasts and these resulting 10T1/2 clones were evaluated for the cytotoxic and transforming activities of two nitrosamines, 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosodiethylamine (DEN). 10T1/2 clone 29 cells, which expressed high levels of CYP2A6 activity, were responsive to the cytotoxic and morphological transforming effects of DEN or NNK on a concentration-related basis. In 10T1/2 clone 29 cells, DEN at 600 micrograms/ml decreased cell survival to 67%, and induced 0.5 type II&III foci/dish. NNK at 400 micrograms/ml administered to 10T1/2 clone 29 cells decreased survival to 57% and induced 0.43 type II&III foci/dish. Wild-type 10T1/2 cells and 10T1/2 clone 4 cells (infected with the vector but not expressing the CYP2A6 activity) were unresponsive. These results indicate that expression of a cDNA coding for cytochrome P450 in 10T1/2 cells can provide information about the role of the enzyme in the activities of chemical carcinogens and also increase the sensitivity of 10T1/2 cells to a larger number of classes of chemical carcinogens.

Localization of oxidative damage by a glutathione-gamma-glutamyl transpeptidase system in preneoplastic lesions in sections of livers from carcinogen-treated rats

Previous studies from our laboratories have shown that catabolism of glutathione (GSH) by gamma-glutamyl transpeptidase (GGT) in the presence of transition metals leads to oxidative damage (OD). This damage is exemplified in vitro by GGT-dependent GSH mutagenesis which involves reactive oxygen species and by GGT-dependent accumulation of lipid peroxidation (LPO) products in systems containing polyunsaturated fatty acid and GSH. In order to test whether catabolism of GSH by membranal GGT in enzyme-altered preneoplastic hepatic lesions can induce oxidative damage in situ, and to test whether the OD is localized in these lesions, 21 day old Fischer rats were treated with 12 mg/kg diethylnitrosamine (DEN) followed by 0.1% or 0.25% phenobarbital (PB) in the diet. Cryostat sections were examined histochemically for GGT-rich hepatic lesions. Adjacent sections were incubated with GSH and iron and examined for areas staining for lipid peroxidation. Distinct LPO-positive areas were shown to correspond well with the GGT-positive hepatic lesions. Promotion with 0.25% PB led to increasing proportions of LPO-positive lesions with time among GGT-positive lesions. The visualization of LPO in GGT-rich hepatic lesions depended on the presence of GSH and iron, and was not observed following chelation of iron by diethyl triaminopentaacetic acid (DTPA), in the presence of acivicin, an inhibitor of GGT, or in the presence of the radical scavenger butylated hydroxytoluene (BHT). The factors affecting GSH-GGT-dependent LPO in the GGT-rich foci were identical to those affecting GSH-GGT-driven LPO in vitro, and were similar to those affecting oxidative GSH-mutagenesis catalyzed by GGT. The results indicate that metabolism of GSH by GGT in preneoplastic liver foci can initiate an oxidative process leading to a radical-rich environment and to oxidative damage. Such damage may contribute to the processes by which cells within such foci progress to malignancy.

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.

Retroviral mediated expression of human cytochrome P450 2A6 in C3H/10T1/2 cells confers transformability by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

In order to develop more efficient in vitro systems for the study of pro-mutagenic or pro-carcinogenic chemicals, we have produced transgenic C3H/10T1/2 cell lines expressing human cytochrome P450 (CYP) 2A6. A retroviral vector containing the cDNA was packaged in psi-2 cells, and used to infect C3H/10T1/2 cells. From 100 G418-resistant clones initially isolated, three cell lines were chosen for further study based upon their morphologies, growth rates and CYP2A6-dependent coumarin 7-hydroxylase activities. Infected clone 10T1/2-04, like the 10T1/2 cells, had no detectable CYP2A6 enzyme activity, while clones 10T1/2-10 and 10T1/2-29 had microsomal CYP2A6 enzyme activities within the range found in human liver microsomes. CYP2A6 protein levels were in agreement with the observed enzyme activities. Southern blots revealed that cells from clone 10T1/2-04 contained a vector lacking the CYP2A6 cDNA, while cells from clones 10T1/2-10 and 10T1/2-29 contained multiple full-length inserts. Southern analysis also indicated the presence of an endogenous CYP2A6 ortholog in the four cell lines. All cell lines exhibited about equal sensitivity to induction of cytotoxicity and conversion to ouabain resistance by the direct acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine. The four lines were also about equally sensitive to transformation by benzo[a]pyrene, a chemical requiring metabolic activation. However, only clones 10T1/2-10 and 10T1/2-29, which express CYP2A6 activity, were mutated and morphologically transformed by the tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone.

Recombinant DNA approaches for the development of metabolic systems used in in vitro toxicology

In the past few years there has been considerable progress in the development of mammalian cell systems for use in genetic toxicology by the stable transfer of genes/cDNAs coding for drug metabolizing enzymes directly into the target cell. Alternative approaches have also been developed in which mammalian cells are transiently transfected with cDNAs coding for drug-metabolizing enzymes and S9 preparations expressing a single metabolizing enzyme isolated and used for metabolic activation. Progress in these areas is reviewed here and the relative merits of the different approaches are discussed. Work to date has focused primarily on the cytochrome P450 family of enzymes, although other enzyme systems involved in xenobiotic metabolism have been used. The central theme of this review is the transfer of genetic information to improve the metabolic capability of cell systems used in genetic toxicology. However, a basic philosophy of the review is that genetic manipulation of cultured mammalian cells has the potential for developing systems to be used to better understand chemically induced toxicological effects.

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.

Isolation of the mutagenic and DNA adduct-inducing components from a commercial preparation of HC blue 1 using Salmonella (TA98) bioassay-directed HPLC fractionation

In the present study we report the separation of the mutagenic impurities from the nitrophenylenediamine hair dye HC Blue 1. This was accomplished by bioassay-directed HPLC fractionation, using Salmonella strain TA98 and reverse phase HPLC analysis. The mutagenic fraction eluted between 80 and 90% methanol, whereas the HPLC fraction containing the parent compound HC Blue 1 eluted with 30% methanol and was non-mutagenic. 100% of the mutagenic activity applied to the column was recovered in fractions that did not possess the blue color of HC Blue 1. Also, HPLC-purified HC Blue 1 did not form DNA adducts (32P-postlabeling) in Salmonella strain TA98. On the other hand, commercial HC Blue 1 and the mutagenic fraction derived from commercial HC Blue 1 (HPLC-isolated) gave similar DNA-adduct profiles that consisted of 7 adducts. DNA adduction was examined concomitantly with mutagenicity and toxicity studies on the HC Blue 1 samples in TA98. The data indicated that, in Salmonella, both the mutagenicity and DNA adduction of commercial HC Blue 1 are due to impurities and not the parent compound.

The chemotherapeutic potential of glycol alkyl ethers: structure-activity studies of nine compounds in a Fischer-rat leukemia transplant model

Structure-activity studies with nine glycol alkyl ethers were conducted with a cellular leukemia transplant model in male Fischer rats. This in vivo assay measures the effects of chemical treatment on neoplastic progression in transplant recipients. Chemicals were given ad libitum in the drinking water simultaneously with the transplants and continued throughout the study. In all, 20 million leukemic cells were injected s.c. into syngeneic rats, which after 60 days resulted in a 10-fold increase in relative spleen weights, a 100-fold increase in white blood cell counts, and a 50% reduction in red blood cell (RBC) indices and platelet counts. At this interval, ethylene glycol monomethyl ether (2-ME) given at a dose of 2.5 mg/ml in the drinking water completely eliminated all clinical, morphological, and histopathological evidence of leukemia, whereas the same dose of ethylene glycol monoethyl ether (2-EE) reduced these responses by about 50%. Seven of the glycol ethers were ineffective as anti-leukemic agents, including ethylene glycol, the monopropyl, monobutyl, and monophenyl ethylene glycol ethers, diethylene glycol, and the monomethyl and monoethyl diethylene glycol ethers. 2-ME more than doubled the latency period of leukemia expression and extended survival for at least 210 days. A minimal effective dose for a 50% reduction in the leukemic responses was 0.25 mg/ml 2-ME in the drinking water (15 mg/kg body weight), whereas a 10-fold higher dose of 2-EE was required for equivalent antileukemic activity. In addition, the in vitro exposure of a leukemic spleen mononuclear cell culture to 2-ME caused a dose- and time-dependent reduction in the number of leukemia cells after a single exposure to 1-100 microM concentrations, whereas the 2-ME metabolite, 2-methoxyacetic acid, was only half as effective. The two glycol alkyl ethers with demonstrable anti-leukemic activity, 2-ME and 2-EE, also exhibited a favorable efficacy-to-toxicity ratio and should be considered for further development as chemotherapeutic agents.

Comparative metabolism and genotoxicity of the structurally similar nitrophenylenediamine dyes, HC Blue 1 and HC Blue 2, in mouse hepatocytes

Previous studies indicated that HC Blue 1 induced heptocellular carcinomas in B6C3F1 mice whereas the structurally similar nitroaromatic amine HC Blue 2 did not. In an attempt to elucidate the biochemical mechanisms responsible for their different carcinogenic potencies, comparative metabolism and genetic toxicity studies were undertaken. Eighteen-hour urinary recovery of administered radioactivity was equivalent for both compounds following oral gavage (100 mg/kg) in female B6C3F1 mice. By HPLC analysis, HC Blue 1 yielded 3 major polar metabolite peaks, one of which was susceptible to glucuronidase. In vivo metabolism of HC Blue 2 yielded a single major metabolite peak which was not hydrolyzed by glucuronidase. Metabolism by B6C3F1 mouse hepatocytes yielded metabolite profiles which were qualitatively similar to the profiles observed after in vivo metabolism. HC Blue 1 was metabolized by hepatocytes at approximately twice the rate of HC Blue 2. Cytogenetic evaluations of mouse hepatocytes after in vitro treatment indicated HC Blue 1 was more potent than HC Blue 2 in inducing chromosomal aberrations while both chemicals showed weak activity for inducing sister-chromatid exchanges. Furthermore, in the V79 cell metabolic cooperation assay, HC Blue 1, but not HC Blue 2, inhibited cell-to-cell communication suggesting a non-genotoxic activity may be present for HC Blue 1. It is concluded that qualitative and quantitative differences exist in the metabolism of these compounds and that genotoxic as well as nongenotoxic effects may contribute to their different carcinogenic potencies.

Cationic liposomes (Lipofectin) mediate retroviral infection in the absence of specific receptors

We have used cationic liposomes (Lipofectin) to facilitate retrovirus infection of cells lacking the homologous viral receptor. Ecotropic murine leukemia virus and packaged retroviral vectors were shown to infect mink cells, and amphotropic packaged retroviral vectors were shown to infect hamster cells in the presence of Lipofectin but not in the presence of Polybrene. Lipofectin-mediated infection of cells lacking the homologous receptor results in a titer approximately 0.1% of the titer in cells with the homologous receptor, using the standard Polybrene protocol. The use of Lipofectin may provide a simple means to experimentally infect a wide variety of cells with viruses not normally infectious for the species, tissue, or cell type of interest.

Development of a human cell line by selection and drug-metabolizing gene transfection with increased capacity to activate promutagens

We have isolated a human lymphoblastoid cell line with higher levels of native cytochrome P450IA1 activity and by DNA transfection introduced human cDNAs for a putative cytochrome P450IIA2 and epoxide hydrolase (E.C. 3.3.2.3). The resultant cell line, designated MCL-1, was substantially more sensitive to the mutagenicity of dimethylnitrosamine and benzo[a]pyrene than the AHH-1 cell line and was found to have increased metabolism of benzo[a]pyrene to dihydrodiols. The increase in native cytochrome P450IA1 activity was achieved by mutation and selection based on resistance to the phototoxicity of benzo[ghi]perylene. One resistant clone, designated L3, was used for subsequent studies. Two complete cDNAs, one encoding a putative cytochrome P450IIA2 and the other a microsomal epoxide hydrolase, were isolated from a human liver cDNA library. After introduction of the cDNAs into an expression vector and transfection into AHH-1 cells, gene expression was detected at the level of enzyme activity (epoxide hydrolase) or by increased sensitivity to dimethylnitrosamine cytotoxicity/mutagenicity (putative P450IIA2). A vector containing both cDNAs was then constructed and transfected into L3 cells to produce MCL-1 cells. The potential usefulness of drug-metabolizing gene transfection and of the MCL-1 cell line, in particular, for genetic toxicity testing is discussed.

Human and rat kidney cell metabolism of 2-acetylaminofluorene and benzo(a)pyrene

The metabolism and mutagenic activation of the model carcinogens benzo(a)pyrene [B(a)P] and 2-acetylaminofluorene (AAF) by human and rat kidney cells were measured. A slicing technique followed by enzyme digestion was utilized to obtain the kidney cells. Although levels of total metabolism of B(a)P by rat and human kidney cells were similar, analysis of specific metabolites of B(a)P indicated that species differences existed. Human kidney cells produced the organic-soluble metabolites B(a)P-9,10-diol, B(a)P-4,5-diol, B(a)P-7,8-diol, B(a)P-3,6-quinone, and B(a)P-9-phenol. Rat kidney cells produced organic-soluble B(a)P-pre-9,10-diols, B(a)P-9,10-diol, B(a)P-4,5-diol, and B(a)P-6,12-quinone. Both species produced sulfate and glucuronide conjugates of all products. For AAF, kidney cells from some human tissues produced up to four times the level of total metabolites compared to rat kidney cells. Organic-soluble metabolites were qualitatively similar between the species and consisted of 2-aminofluorene (AF), N-hydroxy-AAF and ring-hydroxylated products at the 1, 3, 5/9, 7, and 8 positions. Sulfate and glucuronide conjugates of these metabolites were also detected. Human interindividual variation with kidney cells was about 2.5-fold for total AAF metabolism and up to 6-fold for individual AAF metabolites. For B(a)P metabolism, human interindividual variation in total metabolism was low while for specific metabolites there was up to a 4-fold variation. Levels of AAF and AF cell-mediated Salmonella typhimurium mutagenesis were significantly higher with human cells as compared to rat kidney cells. It appears that the differences between human and rodent kidney cell metabolism of chemical carcinogens vary with the chemical class and understanding these differences will be necessary in the extrapolation of rodent carcinogenesis data to humans.

Transfection of a human cytochrome P-450 gene into the human lymphoblastoid cell line, AHH-1, and use of the recombinant cell line in gene mutation assays

We have demonstrated that the human cytochrome P1-450 gene can be transfected into the AHH-1 human lymphoblastoid cell line using the pHEBo vector and hygromycin selection. The transfected gene was expressed when regulatory sequences derived from the herpes simplex virus thymidine kinase gene were incorporated in appropriate orientations. Gene expression was monitored at the enzyme level using assays for 7-ethoxyresorufin deethylase, 7-ethoxycoumarin deethylase and benzo[a]pyrene hydroxylase activities. Bulk transformed cell populations had 2- to 3-fold more of these enzyme activities compared with control populations. Subclones of the bulk population expressing still higher levels of 7-ethoxyresorufin deethylase activity were also obtained. Expression of the transfected cytochrome P1-450 gene was stable for 20-30 days in the presence of hygromycin B. The transformed cell populations were found to be suitable for use in gene locus mutation assays and the mutagenicity of aflatoxin-B1 and 2-acetylaminofluorene (AAF) were examined. Aflatoxin-B1 was found to be 2-3 times more mutagenic to cells bearing the transfected cytochrome P1-450 activity as compared with control cells. In contrast, no difference in AAF mutagenicity was observed. Analysis of the AAF metabolite profile indicated that cells expressing the transfected cytochrome P1-450 gene produced 8-fold more N- and 7-hydroxy-AAF than control cells. The similarity in mutagenic responses between control cells and cells bearing the transfected cytochrome P1-450 gene may be due to the low deacetylase activity of AHH-1 cells. These observations indicate that this vector and expression system are suitable for introducing novel metabolic activities into the AHH-1 cell line.