Comparative induction of unscheduled DNA synthesis by physical and chemical agents in non-proliferating primary cultures of rat hepatocytes

Development and utilization of primary hepatocyte culture systems to evaluate metabolism, DNA binding, and DNA repair of xenobiotics

The use of isolated hepatocytes as an approach to evaluate hepatotoxic and hepatocarcinogenic compounds and investigate mechanisms by which chemicals induce liver lesions is well established. This review discusses techniques developed in the author's laboratory describing (1) isolation and primary culture of rodent hepatocytes detailing methods which are optimal for obtaining large numbers of viable cells, (2) DNA damage induced by physical and chemical agents in rodent hepatocytes measured as unscheduled DNA synthesis, and (3) metabolic activation of model hepatocarcinogens, their binding to DNA, and identification of individual adducts thought to be responsible for induction of DNA repair.

Extensive metabolism of the flavonoid chrysin by human Caco-2 and Hep G2 cells

1. Chrysin is one of many bioflavonoids with chemopreventive properties in cardiovascular disease and cancer. In an effort better to understand factors that may affect the oral bioavailability of the bioflavonoids from dietary sources, the metabolism of chrysin by cultured intestinal Caco-2 cells and hepatic Hep G2 cells was studied, together modelling human presystemic metabolism. 2. At concentrations that may be achieved in the diet, chrysin was extensively metabolized to two conjugated metabolites, M1 and M2, with no CYP-mediated oxidation. M1 was identified as a glucuronide, and M2 as a sulphate conjugate by LC/MS and other spectroscopic and biochemical techniques. Sulphate conjugation occurred at a rate twice that of glucuronic acid conjugation in both cell types. 3. M1 was catalyzed by UGT1A6 with a Km = 12 microM. M2 was catalyzed both by M- and P-form phenolsulphotransferases (SULT 1A3 and SULT 1A1) with very low Km of 3.1 and 0.05 microM respectively. 4. Pretreatment with 3-methylcholanthrene, interestingly, did not result in oxidation of chrysin but rather in increased glucuronidation. 5. Also, M1 and M2 were the only metabolites formed from chrysin in fresh rat hepatocytes. The metabolism of another flavonoid, apigenin, was very similar to that of chrysin. 6. These observations suggest that both sulphation and glucuronidation are critical determinants of the oral bioavailability of bioflavonoids in humans, although a contribution from CYP-mediated oxidation can not be excluded.

Modulation of chemical carcinogen-induced unscheduled DNA synthesis by dehydroepiandrosterone (DHEA) in the primary rat hepatocytes

Modulation of unscheduled DNA synthesis by dehydroepiandrosterone (DHEA) after exposure to various chemical carcinogens was investigated in the primary rat hepatocytes. Unscheduled DNA synthesis was induced by treatment of such direct acting carcinogens as methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) or procarcinogens including benzo(a)pyrene (BaP) and 7,12-dimethylbenz(a)anthracene (DMBA). Unscheduled DNA synthesis was determined by measuring [methyl-3H]thymidine radioactivity incorporated into nuclear DNA of hepatocytes treated with carcinogens in the presence or absence of DHEA. Hydroxyurea (5x10(-3) M) was added to growth medium to selectively suppress normal replication. DHEA at concentrations ranging from 1x10(-6) M to 5x10(-4) M did not significantly inhibit unscheduled DNA synthesis induced by either MMS (1x10(-4) M) or EMS (1x10(-2) M). In contrast, DHEA significantly inhibited unscheduled DNA synthesis induced by BaP (6.5x10(-5) M) and DMBA (2x10(-5) M). DHEA-induced hepatotoxicity in rats was examined using lactate dehydrogenase (LDH) release as an indicator of cytotoxicity. DHEA exhibit no significant increase in LDH release compared with the solvent control at 18 h. These data suggest that nontoxic concentration of DHEA does not affect the DNA excision repair process, but it probably influence the enzymatic system responsible for the metabolic activation of procarcinogens and thereby decreases the amount of the effective DNA adducts formed by the ultimate reactive carcinogenic species.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Taxol metabolism in rat hepatocytes

Metabolism of the anticancer drug taxol was investigated in freshly isolated rat hepatocytes. Two main metabolites were separated by reversed-phase HPLC and shown by tandem mass spectrometry to be monohydroxylated metabolites. Kinetic studies revealed apparent Km values of 68 and 61 microM with identical Vmax values for the two metabolites. Verapamil and midazolam, but not phenacetin, showed concentration-dependent inhibition of taxol metabolism with both metabolites being affected equally. The IC50 was about 100 microM for verapamil and 25 microM for midazolam. These observations demonstrate for the first time in vitro metabolism of taxol and suggest that the metabolism may be subject to potentially important interactions with numerous other drugs.

Galactosamine hepatotoxicity: effect of galactosamine on glutathione resynthesis in rat primary hepatocyte cultures

The effect of galactosamine on the resynthesis of glutathione in rat primary hepatocyte cultures was investigated. Cultured rat hepatocytes were treated with galactosamine (4 mM) 1.5 hr prior to concurrent with, or 1.5 hr after cell attachment; total cellular glutathione was then measured over time. Addition of galactosamine at any of these times suppressed methionine-enhanced glutathione resynthesis in the cultures after a lag period of about 120 min. The lag period was not due to slow uptake of galactosamine by the cultured cells, since cellular UTP levels fell to less than 10% of controls within 60 min, a time frame comparable to that observed in vivo. Neither was the lag period a result of interference with cellular uptake of methionine or with conversion of methionine to cysteine, since the phenomenon was observed regardless of whether methionine or cysteine was used to promote glutathione resynthesis. Addition of uridine, which protects against galactosamine hepatotoxicity in vivo by replenishing hepatic UTP levels, did not prevent the suppression of glutathione resynthesis. The data indicate that (a) galactosamine inhibits the time-dependent resynthesis of glutathione in primary hepatocyte cultures, (b) a lag period exists for this response, and (c) this effect is not directly related to depletion of cellular UTP stores.

Evaluation of four methods for scoring cytoplasmic grains in the in vitro unscheduled DNA synthesis (UDS) assay

The in vitro unscheduled DNA synthesis (UDS) assay measures DNA repair (incorporation of [3H]thymidine) following in vitro treatment of rat primary hepatocytes. The autoradiographic method was used to detect UDS by counting developed silver grains in the photographic emulsion overlaying nuclei and cytoplasmic areas of the hepatocytes. In this communication we report results using 4 scoring methods: (1) the 2 most heavily labeled cytoplasmic areas adjacent to the nucleus (our standard method), (2) the cytoplasmic area left of the nucleus, (3) the cytoplasmic areas left and right of the nucleus, and (4) 2 cytoplasmic areas whose positions were selected at random. Rat primary hepatocyte cultures treated with a medium control, a solvent control (dimethyl sulfoxide) and 5 known genotoxic chemicals (2-acetylaminofluorene, dimethylnitrosamine, diethylnitrosamine, methyl methanesulfonate and ethyl methanesulfonate) were scored using these 4 methods. The average or maximum cytoplasmic grain count was subtracted from the nuclear grain count to yield net grains/nucleus (NG). In general, NG counts for Methods 2, 3 and 4 were similar, although shifted about 3-10 grains higher than Method 1 for controls and most treated groups. Methods 2, 3 and 4 showed more experiment-to-experiment variability in sensitivity for detecting statistically significant increases in treated groups than did our standard method. Thus, the alternative methods afforded no consistent improvements in sensitivity or reduction of variability for this assay. Subtraction of the average or the highest cytoplasmic count had virtually no effect on the sensitivity of the assay, but simply requires an appropriate adjustment of the criteria for a positive response.

Recommended protocols based on a survey of current practice in genotoxicity testing laboratories: I. Unscheduled DNA synthesis assay in rat hepatocyte cultures

A protocol based primarily on current laboratory practices in the performance of the unscheduled DNA synthesis (UDS) assay with primary rat hepatocyte cultures has been developed. These guidelines were developed using tabulated responses to a detailed questionnaire completed by North American and European governmental, university and contract laboratories involved with the UDS test. This report identifies those modifications to previously described methodologies which are used on a regular basis and also serves to clarify confusing or inconsistent practices. Although this protocol pertains specifically to the use of primary rat hepatocyte cultures, it can be modified to incorporate other types of cells in which certain aspects remain the same.

Structure-activity relationships in the rat hepatocyte DNA-repair test for 300 chemicals

312 chemicals/mixtures were tested for genotoxicity in the rat hepatocyte/DNA-repair test. A variety of structure-activity relationships was evident. Of the 309 pure chemicals, 142 were positive. Of these, 43 were judged by IARC to have sufficient or limited evidence of carcinogenicity and none of the remainder was a proven noncarcinogen. Among the 167 negative chemicals, 44 were carcinogens. Some of these are known to be genotoxic in other systems, but based on several lines of evidence, many are considered to be epigenetic carcinogens that lack the ability to react with DNA and rather lead to neoplasia by nongenotoxic mechanisms.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes. Validation of improved methods for primary culture including data on the lack of effect of ionizing radiation

The in vitro unscheduled DNA synthesis (UDS) assay was evaluated for inclusion in a battery of assays used at The Upjohn Company for evaluation of lead compounds in the development of new and existing drug entities. This evaluation process encompassed aspects of the isolation of hepatocytes and tests of reference mutagens and genotoxins. The flow rate of perfusion solutions and their temperatures were critical in the isolation of high viability hepatocytes in good yield. The attachment of freshly isolated hepatocytes to coverslips was greatly enhanced by coating the coverslips with type III collagen. Results of testing 12 known genotoxic agents (UV light, cyclophosphamide, 7,12-dimethylbenzanthracene, dimethylnitrosamine, diethylnitrosamine, 2-acetylaminofluorene, benzo[a]pyrene, methyl methanesulfonate, ethyl methanesulfonate, N-propyl-N'-nitro-N-nitrosoguanidine, benzidine and 4-aminobiphenyl) were in agreement with the literature. The use of X-ray did not induce unscheduled DNA synthesis in hepatocytes. This latter finding draws attention to the inability of this assay to detect agents which result in 'short-patch' repair of damage.

Genetic toxicity evaluation of McN-5195: a novel analgesic

McN-5195, (+/-)-trans-3-(2-bromophenyl)octahydroindolizine, a novel analgesic, was tested for genotoxic potential in a battery of tests with endpoints of mutagenicity, chromosomal alterations and DNA damage/repair. McN-5195 was not mutagenic when tested in the Ames test using strains TA98, TA100, TA1535, TA1537 and TA1538, in the absence of metabolic activation and in the presence of Aroclor 1254-induced rat or hamster S-9. Negative results were also obtained in the mouse lymphoma assay in the absence of activation, but reproducible mutagenic responses were seen in this mammalian cell assay in the presence of rat S-9 at high levels of induced toxicity (reduced cell growth). Testing of the enantiomers of McN-5195 in this assay supported these findings. A predominance of small mutant colonies in the mouse lymphoma assay suggested a potential chromosomal effect of McN-5195. This was confirmed with positive findings in an in vitro cytogenetics assay using CHO cells, again at toxic exposure levels and only in the presence of S-9. McN-5195 did not induce DNA repair in the primary rat hepatocyte/DNA repair assay, nor did it induce alterations in vivo of chromosome structure or number when tested in a rat bone marrow cytogenetics assay. The findings from this battery of tests indicate that McN-5195 has modest genotoxic activity when tested in the presence of rat liver S-9 in in vitro systems sensitive to cytogenetic change. The absence of genotoxicity in vitro in Salmonella and intact liver cells and in vivo in rat bone marrow suggests that McN-5195 is unlikely to present a genotoxic risk to whole animals.

Use of scintillometric quantitation of unscheduled DNA synthesis in isolated rat hepatocytes for the screening of genotoxic agents

The induction of unscheduled DNA synthesis has been considered as a suitable endpoint for the screening of genotoxic agents. Experimentally, unscheduled DNA synthesis is most frequently measured by autoradiography. The purpose of this report was to examine the usefulness of the liquid scintillation counting technique in measuring unscheduled DNA synthesis response in isolated rat hepatocytes. The various liquid scintillation counting-based unscheduled DNA synthesis assay procedures were examined according to the following groupings: (1) procedures based on the acid precipitation of cellular macromolecules, (2) procedures based on isopycnic gradient centrifugation of solubilized cells, (3) procedures based on nuclei isolation in conjunction with other DNA purification methods, and (4) procedures based on the selective retention of hepatocellular DNA. Limited cases in which test chemicals gave positive unscheduled DNA synthesis response in liquid scintillation counting-based assays and negative unscheduled DNA synthesis response in autoradiography-based assays are presented. It is concluded that liquid scintillation counting-based unscheduled DNA synthesis assays represent an appropriate system for inclusion in carcinogenicity and mutagenicity testing programs.

Comparison of multiple parameters of rodent carcinogenicity and in vitro genetic toxicity

The relationship between chemically induced patterns of tumorigenesis in rodents and of in vitro genetic toxicity was evaluated for 73 substances. tumorigenicity patterns were defined according to sex and species effects, the induction of common or uncommon tumors, and benign or malignant tumors. These results and the genetic toxicity results derived from the testing of chemicals under code were compared. Chemicals that induced tumors in both sexes of both rodent species (trans-sex/species carcinogens) were divided into those that showed multiple responses in genetic toxicity assays and those that showed little or no response. Some of the nongenotoxic trans-sex/species carcinogens exhibit properties that do not necessarily fit classification as only tumor promoters and may involve some other mode(s) of action. Those chemicals showing tumorigenicity in only one of the four groups exposed (uni-sex/species carcinogens) generally showed little or no response in genetic toxicity assays. Uni-sex/species carcinogens may be difficult to identify by in vitro assays because of their high tissue specificity. Chemicals that are tumorigenic in both sexes of both species are logically more likely to be tumorigenic in a third species than are those that are tumorigenic in only one sex of the exposed species. Therefore, while positive genetic toxicity test results are not predictive of all carcinogens, a consistent positive response among the multiple endpoints in these assays is more likely to identify chemicals with the potential for trans-sex/species carcinogenesis. Such trans-sex/species carcinogens may have the most direct implication for human health effects.

A study of the potential genotoxicity of methapyrilene and related antihistamines using the hepatocyte/DNA repair assay

Methapyrilene and four related antihistamines were evaluated for their ability to cause DNA repair measured autoradiographically as unscheduled DNA synthesis (UDS) in primary cultures of Fischer-344 rat hepatocytes. Methapyrilene failed to induce UDS at all doses tested while pyrilamine and tripelennamine induced a concentration-dependent increase in DNA repair. Doxylamine and thenyldiamine, previously untested in this system, induced a weak response at the highest non-toxic doses tested. Methapyrilene was clearly cytotoxic at doses of 100 microM and higher, as judged by morphology, and precursor incorporation into RNA and protein. Precursor incorporation into RNA was irreversibly inhibited 90% and 55% at 1000 microM and 100 microM methapyrilene, respectively, while precursor incorporation into protein was inhibited 80% and 60%. These data verify the genotoxicity of pyrilamine and tripelennamine and the failure of methapyrilene to elicit DNA repair, and suggest that doxylamine and thenyldiamine may be weak DNA-damaging agents.

Unscheduled DNA synthesis tests. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The utility of unscheduled DNA synthesis (UDS) testing for screening potentially hazardous chemicals was evaluated using the published papers and technical reports available to the UDS Work Group. A total of 244 documents were reviewed. Based on criteria defined in advance for evaluation of the results, 169 were rejected. From the 75 documents accepted, results were reviewed for 136 chemicals tested using autoradiographic approaches and for 147 chemicals tested using liquid scintillation counting (LSC) procedures; 38 chemicals were tested by both approaches to measure UDS. Since there were no documents available that provided detailed recommendations of UDS screening protocols or criteria for evaluating the results, the UDS Work Group presents suggested protocols and evaluation criteria suitable for measuring and evaluating UDS by autoradiography in primary rat hepatocytes and diploid human fibroblasts and by the LSC approach in diploid human fibroblasts. UDS detection is an appropriate system for inclusion in carcinogenicity and mutagenicity testing programs, because it measures the repair of DNA damage induced by many classes of chemicals over the entire mammalian genome. However, for this system to be utilized effectively, appropriate metabolic activation systems for autoradiographic measurements of UDS in human diploid fibroblasts must be developed, the nature of hepatocyte-to-hepatocyte variability in UDS responses must be determined, and the three suggested protocols must be thoroughly evaluated by using them to test a large number of coded chemicals of known in vivo mutagenicity and carcinogenicity.

A rapid and simple method to quantitate chemically induced unscheduled DNA synthesis in freshly isolated rat hepatocytes facilitated by DNA retention of membrane filters

The filter elution method used for the detection of DNA strand breaks has been modified to quantitate chemically induced DNA repair which is measured as unscheduled DNA synthesis (UDS) in suspension of freshly isolated rat hepatocytes. Our method is based on DNA purification by retention on polyvinyl chloride filters, and is capable of handling a large number of samples simultaneously. By using the present assay system, positive dose-dependent UDS data was obtained on the following carcinogens: aflatoxin B1, 2-acetylaminofluorene, 4-aminobiphenyl, 2-aminofluorene, methyl methanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, and 4-nitroquinoline-1-oxide. In contrast, non-carcinogenic biphenyl, fluorene, and sodium ascorbate did not elicit any detectable levels of UDS at all concentrations tested. Thus, UDS as measured by the present filter retention method may serve as an efficient and reliable means of screening chemical mutagens/carcinogens.

A method for rapid, sensitive quantitation of short-patch DNA repair in cultured rat hepatocytes

The sensitivity of DNA-repair detection after incubation of cultured rat hepatocytes with dimethylnitrosamine (DMN) and [Me-1',2'-3H]thymidine ([3H]TdR) was enhanced by the preparation of isolated nuclei. Nuclear isolation-liquid scintillation counting (NI-LSC) provided a rapid method for the determination of unscheduled DNA synthesis (UDS). DMN-induced UDS detected by autoradiography and NI-LSC correlated well and provided similar dose-response curves, indicating the utility of the NI-LSC method for the quantitation of short-patch DNA repair.

Inhibition of 4-nitroquinoline 1-oxide induced unscheduled DNA synthesis in primary cultures of rat urothelial cells by dicumarol and pyrophosphate

Primary cultures of rat urothelial cells were exposed to hydroxyurea, [3H]thymidine, and 4-nitroquinoline 1-oxide (NQO) or N-hydroxy-4-aminoquinoline 1-oxide (HAQO) in a serum-free media for 2 h; unscheduled DNA synthesis (UDS) was measured by autoradiography. Both NQO and HAQO produced unscheduled DNA synthesis. Dicumarol, an inhibitor of NQO nitroreductase, inhibited the activity of NQO and, to a lesser extent, HAQO. Pyrophosphate, an inhibitor of seryl-AMP synthetase, inhibited the activity of both compounds. Neither dicumarol nor pyrophosphate, under similar experimental conditions, inhibited the activity of N-hydroxy-N-2-acetylaminofluorene (N-OH-AAF). These results support the idea that nitro-reductase and seryl-AMP synthetase may be involved in the activation of NQO.

Isolation and culture of adult hepatocytes from liver biopsies

Hepatocytes were isolated from liver biopsies of rats, guinea pigs, rabbits, dogs, and humans. The procedure is based on cannulation of large veins in the cut face of the biopsy, followed by collagenase perfusion. Yields averaged 19 x 10(6) viable hepatocytes/g liver. Viability averaged 84%, as determined by trypan blue dye exclusion. Cultures were prepared from the isolated hepatocytes and were found to be comparable in morphology and N-demethylase activity to hepatocyte cultures prepared by the in situ perfusion of the liver. The development of this method should facilitate comparative studies of the cytotoxicity, genotoxicity, and metabolism of foreign chemicals in primary hepatocyte cultures.

Characterization of DNA repair elicited by carcinogens and drugs in the hepatocyte primary culture/DNA repair test

The autoradiographic unscheduled DNA synthesis measured in the hepatocyte primary culture/DNA repair test after exposure to chemical carcinogens was characterized. In order to document that this synthesis was occurring in nonreplicated DNA, density labeled, replicated DNA was separated from nonreplicated DNA by cesium chloride gradient centrifugation. Incorporation of [3H] thymidine into nonreplicated DNA was detected after exposure of hepatocytes to methyl methanesulfonate, an activation independent carcinogen, or N-2-acetylaminofluorene, an activation-dependent carcinogen, but not with fluorene, a noncarcinogen. N-4-Acetylaminofluorene, a mutagenic compound of uncertain carcinogenicity, also caused DNA repair. Both the autoradiographic assay and density gradient centrifugation detected incorporation of [3H] thymidine into DNA from the same hepatocyte preparation. By using density gradient centrifugation, DNA repair was detected in hepatocytes following exposure to the antihistamines pyrilamine maleate and tripelennamine HCl. Methapyrilene failed to elicit repair. Thus this study (1) verifies that the unscheduled DNA synthesis seen in the hepatocyte primary culture/DNA repair test after carcinogen exposure in DNA repair And (2) confirms the induction of DNA repair by pyrilamine maleate and tripelennamine HCl but not by methapyrilene.

Chemically-induced unscheduled DNA synthesis in primary rat hepatocyte cultures: a comparison with bacterial mutagenicity using 218 compounds

The autoradiographic identification of unscheduled DNA synthesis (UDS) in primary cultures of adult rat hepatocytes (HPC) has been proposed as a predictive test for mutagens/carcinogens. To assess the predictive value of this test, results in the hepatocyte UDS assay were compared with data for bacterial mutagenicity using a modified Ames test. Over 200 compounds representing a variety of chemical classes consisting of procarcinogens, ultimate carcinogens, and noncarcinogens were tested in each system. The accurate discrimination of many carcinogens/noncarcinogens was demonstrated by both systems. The induction of UDS in hepatocytes showed an excellent correlation with bacterial mutagenesis in response to polycyclic aromatic hydrocarbons, aromatic amines, biphenyls, nitrosamines, carbamates, azo-compounds, acridines, halogenated compounds, nitrosureas, quinolines, pyridines, purines, pyrimidines, esters and carbamates. Nitrocompounds, although active in bacteria, were poor inducers of UDS. The results support the complementary and confirmatory nature of these tests for genotoxic chemicals and indicate the usefulness of the hepatocyte UDS system as a component in a battery of short-term predictive tests for mutagens/carcinogens.