Mutagenesis by chemical agents in V79 chinese hamster cells: a review and analysis of the literature. A report of the Gene-Tox Program

A role of oxidative DNA sugar damage in mutagenesis by neocarzinostatin and bleomycin

The anti-tumor antibiotics neocarzinostatin and bleomycin specifically oxidize deoxyribose in DNA at the C-5' and C-4' positions, respectively. The resulting DNA lesions include strand breaks and apyrimidinic sites. Both agents are broad specificity mutagens, inducing, in various systems, base substitutions, frameshifts and deletions. Sequencing studies in bacterial systems have suggested that the base substitutions may result primarily from replicative bypass of the oxidized apyrimidinic sites.

Mutagenicity and clastogenicity of teniposide (VM-26) in L5178Y/TK +/- -3.7.2C mouse lymphoma cells

The antitumor drug teniposide (VM-26) is a potent inducer of DNA breaks (Long et al., Cancer Res., (1985) 45, 3106), but it is only weakly mutagenic at the hprt locus in CHO cells (Singh and Gupta, Cancer Res., (1983) 43, 577). In the present study, the mutagenic and clastogenic activities of teniposide were evaluated in L5178Y/TK +/- -3.7.2C mouse lymphoma cells. Although teniposide is a weak mutagen at the hprt locus, it is a potent mutagen at the tk locus, with as little as 0.5 ng/ml producing 220 TK mutants/10(6) survivors at 96% survival (background = 100/10(6) survivors). This same dose of teniposide induced 38 aberrations per 100 metaphases (background = 7/100 cells). At 7 ng/ml, teniposide induced approximately 2700 TK mutants/10(6) survivors at approximately 10% survival. At the highest dose sampled for aberration analysis (5 ng/ml), teniposide induced 44 aberrations/100 cells. Most of the aberrations were chromosomal rather than chromatid events. As expected for a compound acting primarily by a clastogenic mechanism, most of the TK mutants were small colonies. Thus, teniposide is a potent clastogen, and it is a potent mutagen at the tk locus but not at the hprt locus. These results support the hypothesis that the location of the target gene affects the ability of the assay to detect both intragenic events and events causing functional multilocus effects. Thus, a heterozygous locus (like tk) but not a functionally hemizygous locus (like hprt) may permit the detection of mutagens that act primarily by a clastogenic mechanism. Because teniposide induces topoisomerase II-associated DNA breaks, and because there is evidence that teniposide may not interact directly with DNA, we discuss the possibility that the potent clastogenic/mutagenic activity of teniposide may be mediated by topoisomerase II.

Mutagenicity of 4-hydroxynonenal in V79 Chinese hamster cells

4-Hydroxynonenal (HNE), a major product of the peroxidation of liver microsomal lipids, was examined for mutagenic activity at the hypoxanthine-guanine phosphoribosyltransferase locus in V79 Chinese hamster lung cells. At concentrations ranging from 10 to 45 microM, HNE induced a dose-dependent increase in the number of mutations to 6-thioguanine resistance, which reached the level of 4.7X baseline at the highest concentration tested.

Study of mutagenic activity of troxerutin, a flavonoid derivative

Troxerutin, a flavonoid derivative, used in vascular diseases was studied in 4 mutagenicity tests: the Ames test, the point mutation test (V79/HPRT), the in vitro metaphase analysis in human lymphocytes and the micronucleus test in mice. The aglycone trihydroxyethylquercetin (THEQ) and quercetin were studied too. Troxerutin was not mutagenic, whereas quercetin was positive in the Ames test, V79 cells and in vitro metaphase analysis. THEQ was negative in the Ames test. The substitution of quercetin with hydroxyethyl groups in 7,3' and 4' positions abolished mutagenic activity of quercetin.

Factors that affect the frequency of thioguanine-resistant lymphocytes in mice following exposure to ethylnitrosourea

The frequency of thioguanine(TG)-resistant lymphocytes in mice treated with ethylnitrosourea (ENU) was followed for a period of 51 wk using our clonogenic assay [Jones et al, 1985a,b]. The effects of dose (0-58 mg/kg), time since treatment (2-51 wk), dose rate (5 weekly X 11.7 mg/kg versus 1 X 58 mg/kg), and age at time of treatment (3 vs 15 mo) on the frequency of TG-resistant, concanavalin A-responsive spleen cells were evaluated. The frequencies of TG-resistant spleen cells were generally dose responsive for 51 wk after exposure to ENU. They also were dependent upon the time that had elapsed since treatment with ENU, increasing to maximal values at 10 wk as previously reported [Jones et al, 1985a], and holding essentially stable at values of approximately 20% of the maximum frequency from week 15 until at least week 40 for the 3-month-old mice. Fractionation of 58 mg ENU/kg into 5 weekly doses did not affect the frequency of ENU-induced TG-resistant cells detected in the spleen but did increase the rate of appearance in the spleen, and the efficiency of induction by the unit dose, of TG-resistant cells. The mice exposed to ENU at 15 mo of age appeared to have a 4-fold reduction in the rate of increase in frequency of ENU-induced TG-resistant spleen cells. One set of control mice was found to have a 10-fold elevated frequency of TG-resistant cells in both the spleen and thymus, indicating that mutations can occur in stem cells of untreated animals.

Chemical carcinogens. A review and analysis of the literature of selected chemicals and the establishment of the Gene-Tox Carcinogen Data Base. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on 506 selected chemicals has been evaluated for evidence that these chemicals induce tumors in experimental animals and this assessment comprises the Gene-Tox Carcinogen Data Base. Three major sources of information were used to create this evaluated data base: all 185 chemicals determined by the International Agency for Research on Cancer to have Sufficient evidence of carcinogenic activity in experimental animals, 28 selected chemicals bioassayed for carcinogenic activity by the National Toxicology Program/National Cancer Institute and found to induce tumors in mice and rats, and 293 selected chemicals which had been evaluated in genetic toxicology and related bioassays as determined from previous Gene-Tox reports. The literature data on the 239 chemicals were analyzed by the Gene-Tox Carcinogenesis Panel in an organized, rational and consistent manner. Criteria were established to assess individual studies employing single chemicals and 4 categories of response were developed: Positive, Negative, Inconclusive (Equivocal) and Inconclusive. After evaluating each of the individual studies on the 293 chemicals, the Panel placed each of the 506 chemicals in an overall classification category based on the strength of the evidence indicating the presence or absence of carcinogenic effects. An 8-category decision scheme was established using a modified version of the International Agency for Research on Cancer approach. This scheme included two categories of Positive (Sufficient and Limited), two categories of Negative (Sufficient and Limited), a category of Equivocal (the evidence of carcinogenicity from well-conducted and well-reported lifetime studies had uncertain significance and was neither clearly positive nor negative), and three categories of Inadequate (the evidence of carcinogenicity was insufficient to make a decision, however, the data suggested a positive or negative indication). Of the 506 chemicals in the Gene-Tox Carcinogen Data Base, 252 were evaluated as Sufficient Positive, 99 as Limited Positive, 40 as Sufficient Negative, 21 as Limited Negative, 1 as Equivocal, 13 as Inadequate with the data suggesting a positive indication, 32 as Inadequate with the data suggesting a negative indication, and 48 Inadequate with the data not suggesting any indication of activity. This data base was analyzed and examined according to chemical class, using a 29 chemical class scheme.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutagenesis of L5178Y/TK(+/-)-3.7.2C mouse lymphoma cells by the clastogen ellipticine

Ellipticine is a potent clastogen in CHO cells (Bhuyan et al: Cancer Res 32:2538-2544, 1972). The reported mutant frequencies produced by ellipticine at the hprt locus in CHO cells are less than or equal to 50/10(6) survivors (background approximately 2/10(6); survival = 10%) (DeMarini et al: Cancer Res 43:3544-3552, 1983; Singh and Gupta: Cancer Res 43:577-584, 1983; Environ Mutagen 5:871-880, 1983). In the present study, the mutagenic and clastogenic activities of ellipticine were evaluated in L5178Y/TK(+/-)-3.7.2C mouse lymphoma cells. Unlike the results at the hprt locus, ellipticine is a potent mutagen at the tk locus, with as little as 50 ng/ml producing an induced mutant frequency of 142/10(6) survivors (background = 56/10(6); survival = 61%) and 198/10(6) survivors (background = 72/10(6); survival = 50%) in two separate experiments. This same dose of ellipticine induced 44 aberrations per 100 metaphases (background = 5/100 cells). At 400 ng/ml, ellipticine induced over 1,000 mutants/10(6) survivors at approximately 10% survival and produced 242 aberrations/100 cells. Under the test conditions, most of the aberrations were chromosome rather than chromatid events. As expected for a compound acting primarily by a clastogenic mechanism, almost all of the TK-deficient mutants were small colonies. Thus, ellipticine is a potent clastogen in both Chinese hamster cells and in mouse lymphoma cells; however, it is a potent mutagen at only the tk locus and not at the hprt locus. These results support the hypothesis that the location of the target gene affects the ability of the assay to detect both intragenic events and events causing the loss of multiple loci. Thus, a heterozygous locus (like tk) but not a functionally hemizygous locus (like hprt) may permit the more efficient detection of mutagens that act primarily by a clastogenic mechanism.

Dioxin-inducible enhancer region upstream from the mouse P(1)450 gene and interaction with a heterologous SV40 promoter

In mouse hepatoma Hepa-1 cells, polycyclic aromatic compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activate transcription of the mouse P(1)450 gene via trans-acting regulatory factors that include the TCDD X receptor complex. The positive control element in the P(1)450 5'-flanking region was examined in control and TCDD-treated Hepa-1 stable transformants that had been transfected with either of two expression vectors containing the chloramphenicol acetyltransferase (CAT) gene: pA10-cat, which has the simian virus 40 (SV40) early core promoter (without enhancers) immediately upstream from the CAT gene; and pSV0-cat, which has no promoter or enhancer. When the 1-kb DNA fragment from -1,647 to -611 upstream from the P(1)450 gene is inserted in either orientation--immediately upstream or almost 2 kb further upstream--from the SV40 promoter in pA10-cat, there is enhancement of CAT activity that can be further induced three- to fourfold by TCDD. When the same experiment is carried out with the -1,247 to -823 fragment or the -1,051 to -823 fragment, but not the -1,247 to -1,052 fragment, TCDD responsiveness is lost, or at least masked, because of a large increase in constitutive CAT activity. pSV0-cat mutants containing internal deletions in the upstream flanking sequences of P(1)450 were constructed. A region of 300 bases (-1,218 to -918) is shown to be required for TCDD responsiveness, and one TCDD-inducible element can be dissociated from an enhancer of constitutive gene expression, whereas one or more other TCDD-inducible elements cannot.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous and induced chromosomal aberrations and gene mutations in human lymphoblasts: mitomycin C, methylnitrosourea, and ethylnitrosourea

The concentration-dependent mutagenic, clastogenic, and cytocidal activities of mitomycin C (MC), methylnitrosourea (MNU), and ethylnitrosourea (ENU) were measured in the human lymphoblast cell line TK6. For treatments resulting in fewer than 2 lethal hits, MNU, ENU, and MC gave rise to apparently linear dose-response curves for gene mutations (hgprt and tk genes) as well as for chromosomal aberrations. The numbers of induced mutants at the tk and hgprt loci were similar between the two loci for each compound. However, the ratio of mutagenic activity relative to the clastogenic activity (aberrations/cell) was lowest for mitomycin C, intermediate for methylnitrosourea, and highest for ethylnitrosourea. These results confirm in human cells the general observation that the processes of mutagenesis and clastogenesis are nonidentical: compounds vary independently in their mutagenic and clastogenic potentials.

Mutagenicity of N-substituted phenanthrene 9,10-imines in Salmonella typhimurium and Chinese hamster V79 cells

We previously showed that some (nonsubstituted) aziridines derived from polycyclic aromatic hydrocarbons (arene imines) elicit various mutagenic and genotoxic effects in bacteria and mammalian cells and that these arene imines are active at much lower concentrations than the corresponding epoxide analogues. In the present study, N-substituted derivatives of phenanthrene 9,10-imine were investigated. All 10 derivatives studied showed direct mutagenicity in Salmonella typhimurium TA100. Some of the compounds additionally exhibited weak effects in the strains TA98 and TA1537. Most N-substituted derivatives were weaker mutagens than unsubstituted phenanthrene 9,10-imine but stronger mutagens than phenanthrene 9,10-oxide. Bulky substituents reduced the mutagenicity more than did small substituents. In addition, the derivatives with electron-withdrawing substituents (with the exception of N-chlorophenanthrene 9,10-imine) were weaker mutagens than those with electron-donating substituents. Phenanthrene 9,10-imine and five N-substituted derivatives were investigated to determine whether they induce gene mutations at the hgprt locus in V79 cells. Four compounds, including the parent aziridine, were positive in the V79 test. The other two compounds were negative. The mutagenic potencies in the V79 cell system did not correlate well with those obtained with the Salmonella system. Overall, the study shows that in addition to unsubstituted arene imines, N-substituted derivatives are mutagenic. This finding is of interest, as metabolic pathways leading from aromatic compounds to N-substituted arene imines are conceivable.

Inhibitory effects of the phorbolester TPA and cigarette smoke condensate on the mutagenicity of benzo[a]pyrene in a co-cultivation system

The transport of reactive intermediates was studied in a co-cultivation system of primary chick embryo hepatocytes and V79 Chinese hamster cells. Two test systems with different genetic endpoints--sister-chromatid exchange (SCE) and gene mutation at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus--were used. Benzo[a]pyrene (B[a]P) was positive in both test systems. When the V79 cells were co-cultivated with the hepatocytes at a distance of 1 mm, only a slight increase in the number of SCEs was observed after exposure to benzo[a]pyrene. When the two cell types were in direct contact, addition of the phorbolester TPA or cigarette smoke condensate inhibited the mutagenic effects of B[a]P in both assays by 50%. No influence of TPA on the number of SCEs induced by B[a]P was observed in a preincubation assay using Aroclor-1254-induced rat liver homogenate. The results indicate that metabolic co-operation may play a role in the transport of reactive intermediates in this co-cultivation system. The mutagenic potential of compounds may be underestimated in systems using intact cells for metabolic activation if the compounds or their metabolites are capable of inhibiting metabolic co-operation.

Mutation and enhanced virus transformation of cultured hamster cells by exposure to gaseous ethylene oxide

Ethylene oxide is a classical mutagen and a carcinogen based on evidence from studies in experimental animals. It is widely distributed in industrial, research, hospital, and food environments. In an effort to explore the use of newly developed methods for exposing mammalian cells to gaseous or volatile mutagens/carcinogens, Chinese hamster V79 cells were treated for 2 hr with gaseous ethylene oxide, in sealed treatment chambers, and assayed for survival and mutagenic response by analysis of induced resistance to 6-thioguanine or ouabain. Significant numbers of mutants were produced at both genetic markers by 1,250-7,500 ppm ethylene oxide. Similarly, primary Syrian hamster embryo cells were treated for 2 or 20 hr with gaseous ethylene oxide in sealed treatment chambers and subsequently assayed for survival and increased sensitivity to SA7 virus transformation. Treatment concentrations extended from toxic to several nontoxic concentrations. After 2-hr ethylene oxide treatment at 625-2,500 ppm a significant enhancement of virus transformation was observed. At 20 hr after treatment no enhancement was observed. Treatment of hamster cells with ethylene oxide in both bioassay systems yielded concentration-related, quantitative results.

Microsome- and hepatocyte-mediated mutagenicity of hydroxyurea and related aliphatic hydroxamic acids in V79 Chinese hamster cells

The potential of N-hydroxyurea to induce gene mutations in V79 Chinese hamster cells was investigated. Upon metabolic activation by liver microsomes from phenobarbital-treated rats or by isolated rat hepatocytes co-cultured with the V79 cells, hydroxyurea caused a concentration-dependent increase in the frequency of HGPRT-deficient mutants. Hydroxyurea was not mutagenic in the absence of metabolic activation. Addition of catalase inhibited microsome-mediated mutagenicity, indicating that hydrogen peroxide was involved in the formation of the mutagenic DNA lesion. Acetohydroxamic acid and N-hydroxyurethane also induced hepatocyte-mediated mutagenicity, suggesting that the potential to elicit metabolism-dependent mutagenicity may be a common property of aliphatic hydroxamic acids.

Autoregulation plus upstream positive and negative control regions associated with transcriptional activation of the mouse P1(450) gene

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to interact with a cytosolic receptor and, in turn, activate transcription of the mouse P1(450) gene. Various lengths of DNA upstream of the P1(450) gene were inserted into the pSV0-cat expression vector, with and without addition of the Harvey murine sarcoma virus (Ha-MSV) 72-bp repeat enhancer element. The constructs were cotransfected with pSV2-neo into mouse hepatoma wild-type cells and two variant cell lines. One variant is believed to result from a mutation in the P1(450) structural gene and expresses high levels of P1(450) mRNA constitutively; the other variant has a defect in nuclear translocation of the inducer-receptor complex. After selection in G418, the cells were treated with control medium, TCDD, cycloheximide, or TCDD plus cycloheximide and then assayed for chloramphenicol acetyltransferase (CAT) activity. The data are consistent with the presence of several functional regions within the upstream sequence: a promoter region, a region that is negatively autoregulated, possible repressor-binding and inducer-receptor complex-binding sites, and an upstream activation element that is required for transcriptional activation by TCDD. The Ha-MSV enhancer can substitute for this upstream activation element.

Nuclear proteins damage by alkylating agents with different degrees of carcinogenicity

We have tried to establish a correlation between the carcinogenic potency of two alkylating compounds and specific target sites in chromatin. We have therefore compared the nuclear metabolism of radioactively-labelled methylmethanesulfonate (MMS), a relatively weak carcinogen and N-methylnitrosourea (MNU), a highly potent carcinogen in cultured primary hepatocytes which have, high microsomal drug-metabolizing activity and in V79 Chinese hamster cells which have low microsomal enzymatic activity. The modification of specific amino acid residues in acid-soluble nuclear proteins (H) and non-histone nuclear proteins (NH) was studied after exposing the cells to various doses of alkylating agents overnight. We found that at all doses, mainly the cysteine (Cys), but also to a lower extent the histidine (His) residues are methylated in both H and NH protein fractions by MMS. At high doses of MMS, traces of methylarginine and methylated lysines could be detected. MNU predominantly methylates lysine and arginine residues, the former being found mostly in H, the latter in NH. Although both hepatocytes and V79 cells metabolized radioactively-labelled carcinogen, a higher percentage of counts were incorporated by the hepatocytes; 'unusually' methylated amino acids were detectable in the hepatocyte proteins with relatively low doses of the alkylating agents but not in V79 cells. In the presence of exogenous microsomes, during exposure of V79 cells to the alkylating agents, the amount of amino acid methylation is qualitatively and quantitatively similar to that found in hepatocytes. Our data suggest a specific mechanism of protein methylation, at the level of target amino acids, for carcinogens with different potencies similar to what has been found for DNA bases. A component of the microsomal fraction (S9) may be able to enhance this effect.

Effects of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation between Chinese hamster V79 lung fibroblasts

The effect of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation were assessed as a function of mutant cell recovery from populations of cocultivated hypoxanthine-guanine phosphoribosyl transferase-deficient mutant (HGPRT-) and wild-type (HGPRT+) Chinese hamster V79 lung fibroblasts. Phorbol myristate acetate and phorbol dibutyrate, two established tumor promoters, were potent inhibitors of metabolic cooperation. Ethanol and dimethylsulfoxide, solvents commonly used to prepare chemicals for testing, weakly inhibited metabolic cooperation. Phenol and phenylglucuronide had no effect on metabolic cooperation. Four oxidative metabolites (1,4-benzoquinone, catechol, hydroxyquinol and quinol) inhibited metabolic cooperation. Phenylsulfate weakly inhibited metabolic cooperation. Conversely, 2-methoxyphenol, a methylated derivative of catechol, appeared to enhance metabolic cooperation. These results generally support the hypothesis that tumor promoters inhibit metabolic cooperation and illustrate the importance of considering metabolites when testing this hypothesis. The weak capacity of five metabolites of phenol to inhibit metabolic cooperation correlates with the weakness of phenol as a tumor promoter. Interpretation of these results is complicated because two metabolic cooperation-inhibiting metabolites (catechol and quinol) are nonpromoting when tested individually in the same assay where phenol shows promoting activity. Such metabolites may be incomplete (stage) promoters, and exposure to two or more may be required for a promoting effect. The significance of enhanced metabolic cooperation requires further investigation, particularly in relation to antipromoting effects.

Induced reversion of a Chinese hamster ovary triple auxotroph. Validation of the system with several mutagens

A Chinese hamster ovary triple auxotroph (CHO AUXB1) requires glycine, adenosine, and thymidine (GAT) for growth and survival due to a defect in the structural gene for folylpolyglutamate synthetase (FPGS). This auxotroph and others like it contain less than 3% of the parental amounts of FPGS activity. In order to develop a reverse mutation assay with CHO AUXB1, we determined the optimal conditions for measuring reversion and characterized some of the revertants. We also obtained quantitative mutagenicity data for several direct-acting mutagens for comparison to the parental CHO-S/HGPRT locus. Induced revertants appear in the culture immediately following 20-22 h exposures in +GAT complete medium, indicative of dominant genetic changes. They are maximally expressed after 2 population doublings and can be conveniently selected after 44-48 h of expression growth by plating 1 X 10(6) cells/100-mm dish into -GAT-deficient medium and incubating 12-13 days. Plating reconstruction experiments show that the cloning efficiencies of revertants in -GAT medium are not influenced by the presence of up to 1 X 10(6) CHO AUXB1 cells. Dose-dependent increases above the spontaneous revertant frequency (average = 5 X 10(7)) are induced with cis-Pt(NH3)2Cl2 (14-fold) (but not trans-Pt(NH3)2Cl2), PtCl4(10-fold), Pt(SO4)2 (14-fold), K2CrO4 (8-fold), EMS (10-fold), 4-NQO (53-fold), ICR-191 (60-fold), and ICR-170 (30-fold). All of the revertants that have been isolated are stable to repeated subculturing in -GAT medium; 40 out of 42 that have been analyzed are characterized by an increased 72-h growth incorporation of labeled folate and their extracts contain 5-94% as much FPGS as the original, parental CHO-S line. Spontaneous and induced reversion to the GAT+ phenotype primarily reflects mutations involving the FPGS gene locus. But the re-acquisition by most of the revertants of much less than normal amounts of FPGS activity suggests that they arise from compensatory second-site mutations within this gene. Comparison of the mutagenicity patterns of the foregoing compounds as a function of the applied concentration and the relative percent survival reveals some interesting similarities, as well as differences, between the CHO AUXB1/FPGS and CHO-S/HGPRT loci. In particular, the FPGS locus is rather insensitive to EMS (or other simple alkylating agents). However, it seems to be quite susceptible to reversion by other chemicals that are known to react selectively with guanine bases in DNA. CHO AUXBI is a useful supplemental mammalian assay system for assessing quantitatively the generally weak mutagenic activities of metal compounds.

Genotoxicity testing of arsenobetaine, the predominant form of arsenic in marine fishery products

Marine fishery products may contain high levels of arsenic, mainly in the form of organic arsenic compounds. Arsenobetaine has been identified as the predominant form occurring in marine fishery products. The potential initiating and promoting capacities of this compound were therefore investigated in vitro. In the Salmonella typhimurium assay, no mutagenicity was observed in strains TA97, TA98 and TA100 without activation or after addition of a liver-enzyme fraction or gut-flora extract. The compound was also negative in the forward mutation assay of the HGPRT gene and in the test for sister chromatid exchanges in V79 Chinese hamster cells. No inhibition of metabolic co-operation between V79 Chinese hamster cells was observed at arsenobetaine concentrations up to 10 mg/ml. In addition, arsenobetaine had no synergistic or antagonistic effects on the action of the positive controls benzo[a]pyrene and tetradecanoylphorbol-13-acetate.

Genotoxicity testing of cigarette-smoke condensate in the SCE and HGPRT assays with V79 Chinese hamster cells

The genotoxicity of cigarette-smoke condensate (CSC) was investigated using V79 Chinese hamster fibroblasts in the sister-chromatid exchange (SCE) and HGPRT (hypoxanthine-guanine-phosphoribosyl-transferase) tests. CSC was negative in the SCE test both with and without metabolic activation (co-cultivation with chick-embryo primary hepatocytes). In the HGPRT test no direct mutagenicity of CSC was observed but after metabolic activation there was a considerable increase in the number of mutants. Comparison of different metabolizing systems showed that non-induced chick hepatocytes, liver homogenate from non-induced chick embryos and liver homogenate from rats pretreated with Aroclor 1254 generated similar numbers of mutants in cells treated with CSC. In addition the capacity of CSC to inhibit metabolic co-operation between V79 cells was studied. A dose-related increase in the inhibition of metabolic co-operation was observed.

A testing strategy to evaluate the mutagenic activity of industrial chemicals in cultured mammalian cells

Based on the published guidelines of regulatory agencies and the latest scientific findings, an experimental strategy for the routine testing of the mutagenic activity of industrial chemicals in mammalian cells in vitro is designed. The strategy includes three individual experiments: (1) a range-finding experiment for cytotoxicity to determine the appropriate doses to be used in subsequent mutagenicity testing; (2) mutagenicity testing at limited doses with different concentrations of Aroclor 1254-induced rat liver homogenate (S9); and (3) a more extensive dose-response experiment using an S9 concentration determined to be optimum in mutagenicity. Besides satisfying the guidelines of regulatory agencies, this strategy should yield data that allow sound scientific judgment on the mutagenicity of the chemicals tested.

Assembly and preliminary analysis of a genotoxicity data base for predicting carcinogens

With a view to developing methodologies for predicting the carcinogenicity of chemicals on the basis of the results of short-term assays and selecting highly predictive batteries of short-term tests, a data base was assembled. The present is a compilation of data extracted from the reports of Gene-Tox working groups, Salmonella mutagenicity data obtained from the U.S. National Toxicology Program and the Environmental Mutagen Information Center and results from BHK21 transformation assays.

In vitro short-term testing systems as prescreens for potential carcinogens: simultaneous detection of sister chromatid exchanges and mutation response in a cell-mediated assay with V79 cells

Sister chromatid exchange (SCE) is recognized as a sensitive indicator of genetic damage, and this has led numerous investigators to suggest that the analysis of SCE can provide a useful step toward the identification of environmental mutagens and/or carcinogens. To explore this approach, we measured SCE induction in V79 Chinese hamster lung cells and the frequency of mutation to 6-thioguanine resistance at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) locus in a cell-mediated mutation assay. Karyotypic analysis of V79 cells showed a stable modal chromosome number of 22 and an XY chromosome complement. When exposed to the procarcinogen 7,12-dimethylbenz[a]anthracene (DMBA) at marginally cytotoxic dose levels of 1.0, 0.5 and 0.25 microM (2.6, 1.3 and 0.65 micrograms/ml), SCE frequencies were highest within the first 24 h of activation with rat or hamster hepatocytes, showed somewhat lower values after 48 h of activation, and, following withdrawal of the chemical, declined to background levels during the period of expression. While this decline may involve several factors, the possibility is not excluded that DNA repair can contribute to the progressive elimination of SCE. The induction of SCE in V79 cells appeared unrelated to the expression of single-point mutation at the HGPRT locus. These findings demonstrate the advantage of multiple endpoint analysis which enabled cytotoxicity, mutagenicity and conditions optimal for the induction of SCE to be determined concurrently in a hepatocyte-mediated assay with V79 cells.

Significance of the genotoxic activities observed in vitro for 35 of 70 NTP noncarcinogens

A speculative analysis is presented of the in vitro genotoxicity data reported by Shelby and Stasiewicz for 70 chemicals defined as noncarcinogenic to rodents by the National Toxicology Program. It is concluded that the genotoxic activities observed are probably subject to logical explanation. It is suggested that short-term genotoxicity assays conducted in vivo on newly defined in vitro genotoxins may have a useful role to play in discriminating animal carcinogens from noncarcinogens. It is clear from the results reported that genotoxic activities observed in vitro for a new test chemical only provide evidence of its possible animal carcinogenicity; they are not definitive of carcinogenicity--the difference may be negligible in general but might prove unacceptable in the particular.

Comparative genotoxicity and nephrotoxicity studies of the two halogenated flame retardants tris(1,3-dichloro-2-propyl)phosphate and tris(2,3-dibromopropyl)phosphate

Tris(1,3-dichloro-2-propyl)phosphate (Tris-CP) was metabolized to products which were mutagenic for Salmonella typhimurium TA100 in the presence of liver microsomes from phenobarbital (PB)-pretreated rats and NADPH. Effects of various inhibitors and inducers of cytochrome P-450 on Tris-CP mutagenicity were in accordance with PB-inducible forms of this enzyme system being responsible for the formation of mutagenic product(s). A comparison was made between the toxic potential of the two halogenated flame retardants Tris-CP and tris(2,3-dibromopropyl)phosphate (Tris-BP) in 5 in vitro tests. Tris-CP was much less potent than Tris-BP with respect to bacterial (Salmonella/microsome or Salmonella/hepatocyte assay) and mammalian (V79 cells) mutagenicity, as well as DNA repair synthesis in hepatocytes. On the other hand, Tris-CP and Tris-BP were both equally effective in transforming Syrian hamster embryo cells in vitro. Tris-CP was not nephrotoxic to rats after a single dose of 500 mg/kg intraperitoneally, whereas Tris-BP caused extensive tubular necrosis accompanied by elevated levels of plasma urea and creatinine.

Assay of phenacetin genotoxicity using in vitro and in vivo test systems

Phenacetin was assayed in a battery of five short-term tests. (1) In a DNA-repair test using various Escherichia coli strains, the drug was not directly genotoxic nor did it induce nonreparable DNA damage in the presence of rat liver S9 fractions, while it was weakly active following activation with hamster liver S9. (2) In the Ames reversion test (strains TA97, TA98, TA100, and TA102 of Salmonella typhimurium, phenacetin reverted only TA100, and only in the presence of hamster liver S9. Mutagenicity was related to the concentration both of the drug and of the above metabolic system. There was no activation with hamster kidney S9, uninduced chicken liver S9, or with a variety of liver S9 preparations from rats treated with enzyme inducers (Aroclor 1254, phenobarbital, or 3-methylcholanthrene) and/or glutathione depletors (diethyl maleate or buthionine sulfoximine). Hamster liver S9 compared favorably to rat and even more to chicken liver S9 fractions also in activating various promutagens [3-amino-1-methyl-SH-pyrido (4,3-b)-indole, 2-aminofluorene, aflatoxin B1, benzo[a]pyrene, and benzo[a]pyrene-trans-7,8-diol] and in decreasing the mutagenicity of direct-acting compounds (4-nitroquinoline N-oxide and sodium dichromate). (3) Phenacetin was borderline positive in a forward mutation test (6-thioguanine resistance) in V79 cells, only in the presence of hamster liver S9, and gave negative results in the presence of rat liver S9 or without any metabolic system. (4) Following in vivo treatment, the alkaline elution assay did not reveal any DNA fragmentation in bone-marrow cells of ip-treated mice or in liver cells of rats treated by gavage. Apparent DNA damage was instead observed in the kidneys of rats receiving the drug by gavage or in the liver following ip administration. However, the effect was prevented (liver) or reduced (kidney) by preliminary perfusion of the organs, which discards (liver) or makes uncertain (kidney) the hypothesis of a true in vivo DNA damage. (5) Phenacetin ip induced in mouse bone-marrow cells a poor yet statistically significant increase in sister chromatid exchanges.

Deoxyribonucleoside-induced selective modulation of cytotoxicity and mutagenesis

Treatment of Chinese hamster V79 cells with dThd, dCyd, or dThd plus dCyd increased MNNG-induced AGr-, TGr-, and Ouar-mutant frequencies but did not significantly increase background mutant frequencies. All the AGr colonies that were isolated possessed phenotypes characteristic of HGPRT-deficient mutants, and the deoxyribonucleosides did not selectively affect the growth of the mutants, nor the selecting efficiency of AG, and did not significantly enhance background mutagenesis. These data show that both dThd and dCyd facilitated MNNG-induced mutagenesis. This facilitation was maximal when cells were exposed to the deoxyribonucleosides throughout the first doubling time (24 h) after treatment with MNNG and for 4 more doubling times prior to mutant selection with AG. This indicates that one round of DNA replication was sufficient for mispairing of methylated bases in the DNA with the C and T provided by the deoxyribonucleosides, and that 4-6 doublings prior to mutant selection with AG were necessary to deplete pre-existing hypoxanthine: guanine phosphoribosyl transferase in newly mutated cells. The dCyd facilitated mutagenesis by FdUrd, which was not mutagenic without dCyd, indicating that increased dCTP:dTTP ratios were mutagenic. Treatment with FdUrd plus dCyd also induced FdUrdr cells, suggesting that inhibition of dCyd utilization may prevent the development of FdUrd-resistance in cancer chemotherapy. Although dCyd and dThd facilitated mutagenesis in cells treated with monofunctional alkylating agents that methylate DNA oxygens, facilitation of mutagenesis did not occur in cells treated with BCNU, which cross links DNA, nor with benzo(a)pyrene and aflatoxin B1, which are frame shift mutagens, nor with MMS, which produces barely detectable levels of O-methylation in DNA. Virtually non toxic concentrations of dThd potentiated the cytotoxicity of MNNG more than 10-fold but that of MMS was potentiated only about 2-fold showing that O-alkylation of DNA was associated not only with the facilitation of mutagenesis but also with the potentiation of cytotoxicity. The potentiation of MNNG-induced cytotoxicity was maximal in V79 and L1210 cells after only 2 h treatment with dThd, showing that not even one round of DNA replication was necessary for this potentiation. Moreover, dCyd abolished the potentiation, and, at equitoxic concentrations, MNNG induced higher mutant frequencies than did MMS. These data show that the mechanisms by which methylating agents plus dThd induce mutagenesis are fundamentally different from their mechanisms of cytotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutagenicity of alpha-hydroxy N-nitrosamines in V79 Chinese hamster cells

Carcinogenic and mutagenic N-nitrosodialkylamines are metabolically activated through alpha-hydroxylation. The synthesis, chemical properties, and microbial mutagenicity of alpha-hydroxy N-nitrosamines have been reported previously. Potent mutagenicity of four N-nitroso-N-(hydroxymethyl)-alkylamines (alkyl = methyl, ethyl, propyl, and butyl) was demonstrated in the present study in V79 Chinese hamster cells, ouabain resistance being used as an indicator. All the compounds were strong mutagens in the absence of metabolic activation systems. The mutagenic and cytotoxic potencies correlated well with each other, and depended on the alkyl group, decreasing in potency in the following order: methyl greater than ethyl greater than propyl = butyl. Their alkylating reactivity was measured by alkylation of thiophenol, and a good linear relationship was observed between the mutagenic and cytotoxic potencies and their alkylating reactivity. The mutagenic and cytotoxic potencies of the alpha-hydroxy N-nitrosamines in V79 cells were well correlated with those of alpha-acetoxy and alpha-hydroperoxy N-nitrosamines with respect to the effect of alkyl group. The results obtained here supported further that alpha-hydroxy N-nitrosamine is the active species in the metabolic activation of N-nitrosodialkylamine.

Growth control variant cell line having increased serum requirement and decreased response to platelet-derived growth factor: reversion by 5-azacytidine

Variants of the mouse embryo fibroblast X melanoma hybrid clone 100A have been isolated by a procedure that selects against cells that are able to grow in medium containing low concentrations of serum plus insulin. Three variant clones derived from this selection were found to have a much higher serum requirement than the parental clone 100A cells, as evidenced by a very low rate of DNA synthesis and growth in medium containing low concentrations of serum. Two of the variants had approximately double the number of chromosomes as the parental cell line, while one had approximately the same number of chromosomes as the parental cells. One of the variants was very strongly reverted by 5-azacytidine but not by ethyl methanesulfonate, suggesting that it reverted by a nonmutational mechanism such as a stable change in DNA methylation. Analysis of the growth requirements in hormone-supplemented serum-free media of the 100A parent, the INS 471 variant, and revertants of the variant indicated that the variant had a specific deficiency in its growth response to platelet-derived growth factor (PDGF). PDGF dose-response curves obtained with the variant cells were shifted approximately an order of magnitude toward higher PDGF concentrations relative to PDGF dose-response curves obtained with the parental 100A cells. This quantitative increase in PDGF requirement of the INS 471 variant appears to explain the increased serum requirement of this variant. Equilibrium binding experiments performed with 125I-PDGF suggest that the variant does not have a decreased number of PDGF receptors.

The in vitro micronucleus assay for detection of cytogenetic effects induced by mutagen-carcinogens: comparison with the in vitro sister-chromatid exchange assay

The sensitivity of a cytogenetic assay, as expressed by the in vitro induction of micronuclei (MN), was compared to the in vitro induction of sister-chromatid exchanges (SCEs). Chinese hamster lung (V79) cells were exposed to 3 known alkylating agents: methyl methanesulphonate (MMS), ethyl methanesulphonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and to 5 newly synthesized naphthofurans: 2-nitro-7-methoxynaphtho[2,1-b]furan (A), 2-nitro-8-methoxynaphtho[2,1-b]furan (B), 2-nitronaphtho[2,1-b]furan (C), 2-nitro-7-bromonaphtho[2,1-b]furan (D) and 7-methoxynaphtho[2,1-b]furan (E). The induction of MN only was also analysed after exposure of the cells to 4 alcohols: ethanol, methanol, butanol and propanol. The lowest dose at which a significant effect could be observed was determined. In both assays, MNNG, MMS and EMS were equally active with the following order of potency: MNNG greater than MMS greater than EMS, the latter being a very weak inducer of MN and SCE. Compounds A and B were also very effective in both assays. Compound C was a more active inducer of SCE than MN. Compounds D and E were not active in either assay. None of the 4 alcohols induced MN. Our results are compared with the previously published data on in vitro and in vivo induction of SCE and MN. We conclude that the MN in vitro assay which detects clastogens as well as agents affecting the spindle apparatus, is a good indicator of genotoxicity, though slightly less sensitive than the in vitro SCE test. It could provide a rapid, simple and inexpensive complementary short-term test for the evaluation of potentially mutagenic chemicals.

Induction of mutations by chemical agents at the hypoxanthine-guanine phosphoribosyl transferase locus in human epithelial teratoma cells

Induction of 6-thioguanine (TG) resistance by chemical mutagens was examined in a line of cells derived from a human epithelial teratocarcinoma cell clone. The cells, designated as P3 cells, have a stable diploid karyotype with 46(XX) chromosomes, including a translocation between chromosomes 15 and 20. Efficient recovery of TG-resistant mutants induced by the direct-acting mutagens: N-methyl-N'-nitro-N-nitrosoguanidine (MNNG); 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10 -tetrahydrobenzo[a]pyrene (BPDE); and benzo[a]pyrene (B[a]P); activated in a cell-mediated assay, required an expression time of 7 days and a saturation density of 2 X 10(4) cells/60-mm petri dish. The TG-resistant mutant cells induced by MNNG and BPDE maintained their resistant phenotype 4-6 weeks after isolation. This mutant phenotype was associated with a more than 10-fold reduction in hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity relative to that of the parental P3 cell line, which was shown to catalyze the formation of 4.6 pmoles inosine-5'-monophosphate (IMP)/min/microgram protein. Induction of TG resistance was also observed in P3 cells cocultivated in a cell-mediated assay with human breast carcinoma cells, which are capable of polycyclic aromatic hydrocarbon (PAH) metabolism, after treatment with the carcinogenic PAHs: B[a]P, chrysene, 7,12-dimethylbenz[a]anthracene (DMBA), and 3-methylcholanthrene (MCA). The degree of mutant induction in this assay was related to the carcinogenic potency of these PAHs in experimental animals. The most potent mutagen was DMBA, followed in decreasing order by MCA, B[a]P, and chrysene. DMBA, at 0.4 microM, increased the frequency of mutants for TG resistance from 2 for the control to about 200 TG-resistant mutants/10(6) colony-forming cells (CFC). Benzo[e]pyrene (B[e]P) and pyrene, which are not carcinogenic, were not effective in the assay. None of the PAHs was mutagenic in the P3 cells cultivated in the absence of the PAH-metabolizing cells. These results indicate that the P3 cells can be useful for the study of mutagenesis at the HGPRT locus by direct-acting chemical mutagens, as well as by chemicals activated in a cell-mediated assay.

Genotoxicity of quercetin in cultured mammalian cells

The naturally occurring flavonol, quercetin, was investigated concerning its ability to induce SCEs and HGPRT-deficient mutants in V79 Chinese hamster cells, and HGPRT- and TK-deficient mutants in mouse lymphoma L5178Y cells. V79 cells were exposed to quercetin in monolayer, under exponential growing condition, in suspension in the presence of liver homogenate, and in co-cultivation with primary chick embryo hepatocytes. No induction of HGPRT-deficient mutants was observed. Furthermore, under standard conditions, no relevant increase in the number of SCEs could be detected. If, however, the cells were exposed simultaneously to quercetin and BrdUrd, a greater than 3-fold increase in the number of SCEs was observed. This induction was dose-related for both quercetin and BrdUrd. Treatment of L5178Y cells with quercetin did not result in an increase in HGPRT-deficient mutants. At the TK locus a weak increase in the number of TK-deficient mutants was found. Addition of liver homogenate abolished this effect. The inability of quercetin to induce SCEs and point mutations in mammalian cells, and the fact that the clastogenic effect of quercetin, whereby it induces TK-deficient mutants in mouse lymphoma L5178Y cells, is abolished by the addition of liver homogenate, may explain the negative outcome of the majority of carcinogenicity studies on quercetin in mammals.

Recovery of thioguanine-resistant cells from Chinese hamster V79 spheroids

Multicell spheroids may prove useful in evaluating the interactions of mutagens with cells exposed in a tissue-like environment. However, direct comparisons among populations of Chinese hamster V79 spheroids of different sizes or with monolayers are complicated by the observation that as spheroids enlarge, the fraction of mutant cells resistant to 6-thioguanine (TGr) gradually decreases from about 5 in 10(5) to less than 1 in 10(5). There appear to be at least 2 explanations for these observations. First, TGr cells grow less well as spheroids than do 6-thioguanine-sensitive (TGS) cells. Second, the clonal nature of spheroid growth means that small samples of spheroids are likely to contain fewer pre-existing TGr cells.

Rat liver homogenate (S9)-mediated binding of benzo[alpha]pyrene to DNA in V79 cells, V79 cell nuclei and aqueous solution

The role of the target cell in determining the structures and the amounts of hydrocarbon-DNA adducts formed after hydrocarbon activation by an exogenous metabolic activation system was investigated by exposing intact cells of the Chinese hamster lung cell line V79, V79 cell nuclei and calf thymus DNA to benzo[a]pyrene (B[a]P) in the presence of a rat liver homogenate activation system (S9). The DNA was isolated, enzymatically degraded to deoxyribonucleosides and the B[a]P-deoxyribonucleoside adducts analyzed by high-performance liquid chromatography. Two major adducts were present in all samples; one formed by reaction of r-7, t-8-dihydroxy-t-9, 10-epoxy-7, 8, 9, 10-tetrahydro-B[a]P (anti-B[a]PDE) with the 2-amino group of deoxyguanosine, the other formed by reaction of a metabolite of 9-hydroxybenzo[a]pyrene (9-OH-B[a]P) with an unidentified deoxyribonucleoside. The ratios of the anti-B[a]PDE-DNA adduct to the 9-OH-B[a]P-DNA adduct were: calf thymus DNA, 3 to 1: DNA from V79 nuclei, 8 to 1; DNA from intact V79 cells, 11 to 1. Similar several-fold increases in the proportion of anti-B[a]PDE-DNA adducts in V79 cells over those in calf thymus DNA were observed for a dose range of 1-10 micrograms B[a]P per ml. The relative extent of binding of the activated metabolite of 9-OH-B[a]P to DNA was also much lower in intact V79 cells than in calf thymus DNA after exposure to 9-OH-B[a]P in the presence of the S9 activation system. These results demonstrate that the relative abilities of various reactive benzo[a]pyrene metabolites formed by an exogenous activation system to reach cellular DNA differ substantially. Therefore, assessment of the biological activity of hydrocarbons in mutation assays using exogenous activation systems must take into account not only the amounts of different reactive hydrocarbon metabolites formed but also the relative abilities of these metabolites to reach the DNA of the target cell.

Forward mutation assay of V79 cells to 6-thioguanine resistance in a soft agar technique that eliminates effects of metabolic co-operation

A technique involving culture in soft agar was used for the assay of forward mutation of V79 cells to 6-thioguanine (6TG) resistance. The main reason for the use of soft agar was to prevent reduction in recovery of mutants depending on the cell density plated for mutation selection, which is the chief problem in the liquid method, and which results mainly from metabolic co-operation due to cell-to-cell contact. V79 cells grew well in fortified soft agar medium (DMEM + 20% FBS) showing cloning efficiencies (greater than 80%) as high as in liquid culture. Therefore, V79/HGPRT mutagenesis could be assayed quantitatively in soft agar culture. The frequency of 6TG-resistant colonies in agar selective medium increased linearly with increase in concentration of EMS. Toxicity and mutagenic responses were greater in soft agar than in liquid culture. In cultures of untreated and EMS-treated cells, more than 95% of the 6TG-resistant colonies isolated were aminopterin-sensitive. Use of soft agar for selection prevented the reduction in the number of mutants with increase in the size of inocula on plating up to 1-2 X 10(6) cells per 9-cm dish: in liquid culture, even with a lower plating number (2 X 10(5) cells per 9-cm dish), a notable reduction in numbers of mutants was observed. This character was re-examined in a reconstruction experiment. The results show that, when up to 2 X 10(6) cells were plated per 9-cm dish, 6TG-resistant cells were almost completely recovered from the soft agar medium, whereas only 10% were recovered from liquid culture.

Chemicals showing no evidence of carcinogenicity in long-term, two-species rodent studies: the need for short-term test data

Studies to determine the performance of short-term tests (STT) in predicting chemical carcinogenicity are frequently characterized by a poor representation of noncarcinogens among the test chemicals. As a result, the performance of several STT is well characterized with carcinogenic chemicals, but the ability of the tests to discriminate between carcinogens and noncarcinogens is largely unknown because of the shortage of test results for noncarcinogens. A list of 70 chemicals that have been tested by the National Cancer Institute or the National Toxicology Program in long-term rodent carcinogenicity studies and that have yielded no evidence of cancer induction in male and female rats and mice is presented. The list helps document the shortage of STT results of noncarcinogens and provides a potential source of chemicals for consideration in future studies designed to assess the ability of STT to predict chemical carcinogenicity.

Inhibition of metabolic cooperation between Chinese hamster V79 cells by structurally diverse teratogens

Cell-cell communication through chemical messengers is a fundamental event required for the differentiation of embryonal cells. Interference with this process by xenobiotics may disrupt embryogenesis. Chinese hamster cells (V79) which display a specific form of cell-cell communication called metabolic cooperation were cultured in the presence of structurally diverse chemical teratogens. Among them were 12-O-tetradecanoyl-phorbol-13-acetate (TPA), diphenylhydantoin (DPH), warfarin, and a series of monoalkyl ethers of ethylene glycol with alcohol chain lengths from methyl to butyl. Sodium saccharin and ascorbic acid were examined to represent two chemicals which have been thoroughly tested for teratogenic effects in laboratory animals and cause no birth defects. Recovery of 100 6-thioguanine-resistant V79 (6-TGr) cells in coculture with 400,000 6-thio-guanine-sensitive V79 (6-TGs) cells in the presence of 6-thioguanine (6-TG) and the chemical agent was measured. In amounts that neither interfered with colony forming ability nor caused cytostasis when 100 6-TGr cells were plated alone, all of the substances except for saccharin and vitamin C increased the number of surviving 6-TGr cells in a concentration-related manner. The recovery was increased by the presence of TPA (to 100% by 4 ng/ml), DPH (from 26% at 91 microM to 43% at 274 microM), warfarin (from 15.5% at 162 microM to 44.5% at 487 microM) and to variable extents by all five glycol ethers. The most efficacious in the latter group of compounds was the isopropyl ether which raised 6-TGr recovery from 8% at 0.017 M to 66% at 0.087 M. Based on the evidence accumulated by previous studies involving TPA, we postulate that the teratogens employed inhibited metabolic cooperation. These observations suggest that V79 cells may be suitable to study inhibition of cell-cell communication as a mechanism of teratogenesis.

Possible epigenetic mechanisms of tumor progression: induction of high-frequency heritable but phenotypically unstable changes in the tumorigenic and metastatic properties of tumor cell populations by 5-azacytidine treatment

Treatment of a variety of highly tumorigenic mouse lines in vitro with chemical mutagens, such as ethyl methane sulfonate (EMS) or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), can result in extraordinarily high frequencies (sometimes in excess of 90%) of strongly immunogenic clones unable to grow progressively in normal syngeneic hosts. These clones will, however, grow in immunosuppressed hosts and gradually regain tumorigenic ability in normal mice if maintained in long-term (several months-1 year) culture, i.e., they are often phenotypically unstable. These features--phenotypic drift and high frequency--make it unlikely that point mutations are the underlying mechanism involved in the generation of the variants. Results presented here demonstrate that these observations can be reproduced on the same tumor lines using 5-azacytidine--an analogue of cytidine which can be incorporated into DNA causing subsequent extensive hypomethylation of cytosine residues in the absence of any significant mutagenic effects. Furthermore, 5-azacytidine treatment of a nonmetastatic mouse mammary tumor led to the emergence of a small number of heritable but unstable tumor clones capable of spontaneous metastatic spread. Because it is known that DNA hypomethylation can lead to transcriptional activation of normally silent genes, that altered methylation patterns can be somatically replicated with a high but not perfect fidelity, and that mutagens can cause DNA hypomethylation, we propose that DNA hypomethylation followed by de novo methylation represents a plausible mechanism to account not only for the induction of the nontumorigenic variants but for a number of aspects of tumor progression and tumor heterogeneity, as well. In particular, we refer to heritable phenotypic alterations in tumor cell populations which occur at very high frequency but which are not necessarily stable over very long periods of time.

A microtiter plate assay for the selection of 6-thioguanine-resistant mutants in Chinese hamster V79 cells in the presence of phorbol-12-myristate-13-acetate

6-Thioguanine-resistant mutants can be efficiently recovered from Chinese hamster V79 cells incubated at high cell densities in microtiter plates (10(3)-10(4) cells/0.2 ml growth medium/0.4 cm2) when selected with 30 microM 6-thioguanine and 0.1 microgram/ml phorbol-12-myristate-13-acetate, an inhibitor of metabolic cooperation among V79 cells. Mutant frequencies in the microtiter plates were calculated from a direct count of mutant colonies. After treatment of the V79 cells with the carcinogen benzo[a]pyrene in a fibroblast-mediated assay, the mutation frequencies determined with the microtiter assay system were quantitatively similar to those obtained with a conventional procedure in which selection with 6-thioguanine was performed in petri dishes. The mutagenic activities of 3 polycyclic aromatic hydrocarbons (activated in the cell-mediated assay) were assessed with the microtiter plate selection procedure. The active carcinogen benzo[a]pyrene at 1 microgram/ml yielded about 100 mutants per 10(5) colony-forming cells. The same dose of a less active carcinogen, cyclopenta[c,d]pyrene, yielded about 20 mutants per 10(5) colony-forming cells, and benz[a]anthracene, not an active carcinogen, was inactive as a mutagen at all doses tested. Because of the small requirements for growth medium and tissue culture vessels compared with other assays, this microtiter plate assay can serve as an inexpensive system for detecting the mutagenic activity of environmental chemicals in mammalian cells.

Evaluation of the alkaline elution/rat hepatocyte assay as a predictor of carcinogenic/mutagenic potential

We have recently developed an alkaline elution/rat hepatocyte assay to sensitively measure DNA single-strand breaks induced by xenobiotics in non-radiolabeled rat hepatocytes. Here we have evaluated this assay as a predictor of carcinogenic/mutagenic activity by testing 91 compounds (64 carcinogens and 27 non-carcinogens) from more than 25 diverse chemical classes. Hepatocytes were isolated from uninduced rats by collagenase perfusion, exposed to chemicals for 3 h, harvested, and analyzed for DNA single-strand breaks by alkaline elution. DNA determinations were done fluorimetrically. Cytotoxicity was estimated by glutamate-oxaloacetate transaminase release or by trypan blue dye exclusion. The assay correctly predicted the reported carcinogenic/non-carcinogenic potential of 92% of the carcinogens tested and 85% of non-carcinogens tested. The assay detected a number of compounds, including inorganics, certain pesticides, and steroids, which give false-negative results in other short-term tests. Only 2 rat liver carcinogens were incorrectly identified; the other carcinogens incorrectly identified are weakly or questionably carcinogenic (i.e., they cause tumors only in one species, after lifetime exposure, or at high doses). Some chemicals cause DNA damage only at cytotoxic concentrations; of 16 such compounds in this study, 12 are weak carcinogens suggesting a link between DNA damage caused by cytotoxicity and carcinogenesis. Our data indicate that this assay rapidly, reproducibly, sensitively, and accurately detects DNA single-strand breaks in rat hepatocytes and that the production of these breaks correlates well with carcinogenic and mutagenic activity.

Cell-mediated mutagenesis of Chinese hamster V79 cells and Salmonella typhimurium

In the cell-mediated approach, intact cells metabolically activate the chemical and the genetic end points are measured in cocultivated or coincubated target cells. Cell-mediated systems have been used to study fundamental problems in carcinogenesis, such as organ and species specificity of carcinogen activation, and in screening for carcinogenic chemicals. In the studies discussed here, cells from various rat, hamster, or bovine tissues are used to metabolically activate the chemical, and mutation and/or SCE induction in V79 cells and mutation of S. typhimurium are measured as genetic end points. The detection of genetic activity of a chemical depends both on the cell (organ, species, type, etc.) used for metabolic activation and on the genetic end point measured. Hydrocarbons and nitrosamines are two classes of environmentally significant chemicals that are sensitively detected with cell-mediated systems. The cell-mediated approach provides a valuable metabolic activation component for short-term in vitro systems, and further studies are needed to utilize and evaluate its full potential.