In vitro metabolic activation of chemical mutagens. II. The relationships among mutagen formation, metabolism and carcinogenicity for dimethylnitrosamine and diethylnitrosamine in the livers, kidneys and lungs of BALB/cJ, C57BL/6J and RF/J mice

Evaluation of the DNA-repair host-mediated assay. III. Relationship between metabolic activation of dimethylnitrosamine and organ-specific differential lethality induced in E. coli indicator strains

In the present study the sensitivity of differential lethality as an endpoint for monitoring the presence of organ-specific genotoxic factors within the DNA-repair host-mediated assay (HMA) was determined. The induction of differential lethality in chemically exposed animals was assessed by measuring the recovery ratio Q, i.e., the relative survival of a repair-deficient E. coli K-12 derivative in comparison with its repair-proficient counterpart. Using untreated animals the interindividual fluctuation of the recovery ratio Q was first quantified and then used to determine the level below which it could be considered indicative of chemically induced differential lethality. This Q value was found to be 0.65 or lower. Using this criterion, a significant decrease of the Q value was observed in mice exposed to DMNA at a dose level as low as 15-30 mumole/kg, i.p. Inter-organ transport (liver----extrahepatic organs) of indicator bacteria was studied in reconstruction experiments using the direct-acting methylating agent MNU. These studies showed that inter-organ transport of indicator bacteria did not interfere with MNU-induced differential lethality. Time-related experiments were used to study the effects of inter-organ transport of genotoxic DMNA metabolites. In these studies significant, time-related differences were found in the induction of differential lethality in various organs of mice treated with DMNA. At a dose level of 200 mumole/kg (i.p.) genotoxic factors appeared within 25 min after administration in the liver. In the lungs and kidneys such factors appeared at a substantially slower rate, e.g., 20-120 min after DMNA administration. In persistence experiments differential lethality reached a maximum 30 min after DMNA treatment. No residual effects were detected 60 min after the injection of the carcinogen. These experiments showed that DMNA-derived genotoxic factors diffused from the liver into the bloodstream. The diffusion of these reactive species followed by their transport via the bloodstream to the lungs accounted for maximally 50% of differential lethality observed in bacteria recovered from the latter organ. In contrast, no indications were found for the transport of genotoxic DMNA metabolites from the liver via the bloodstream to the spleen and the kidneys. These results show that organ-specific effects observed in the DNA-repair HMA procedure after DMNA exposure can be primarily attributed to in situ metabolism, rather than diffusion of genotoxic metabolites from the liver to extrahepatic organs.

N-nitrosamines: bacterial mutagenesis and in vitro metabolism

Many nitrosamines are potent mutagens. The rate-limiting step in their in vitro metabolism to mutagens is usually a single enzymatic reaction catalyzed by one or more of the many cytochrome P-450-dependent mixed-function oxidases present in the microsomal cell fraction. Current evidence indicates that this reaction activates nitrosamines to alpha-hydroxynitrosamines, which have half-lives on the order of seconds. This product decomposes to an aldehyde and a much shorter-lived ultimate metabolite which is probably an alkyl diazonium ion or an alkyl carbocation. This may react with DNA leading to premutagenic adducts. Such adducts represent a very small fraction of the ultimate mutagen, with the rest reacting with water to yield the corresponding alcohol. Evidence for this pathway includes (1) the observation of deuterium isotope effects in metabolism and mutagenesis, (2) products (aldehydes, alcohols, and N2) consistent with this pathway, (3) studies on metabolism of nitrosamines using purified cytochrome P-450, (4) formation of DNA adducts such as O6-alkylguanines which are consistent with those expected from the ultimate mutagen, (5) expected products and genotoxic effects of other sources of activated nitrosamines, e.g., alpha-acetoxynitrosamines, alkanediazotates and related compounds. Hydroxylation of nitrosamines at other positions also occurs in vitro (usually to a lesser extent), but these products are generally stable and must be further metabolized to exert mutagenic effects (with the exception of N-nitrosoalkyl(formylmethyl)amines, which are direct-acting mutagens). Because only low percentages of nitrosamines are metabolized in vitro, the contribution to mutagenesis by secondary metabolism is small. In this respect, in vitro metabolism can differ significantly from in vivo metabolism. Bacterial mutagenesis by nitrosamines has most often been studied in Salmonella typhimurium and to a lesser extent E. coli. Mutagenesis by nitrosamines generally requires a source of microsomes (a 9000 X g supernatant fraction is often used), and NADPH. Liver fractions from Aroclor-1254- or PB-induced rodents have been most frequently employed but liver fractions from untreated animals, and homogenates of other organs (lung, kidney, nasal mucosa, and pancreas) have also been utilized. Liver homogenates from humans are generally similar to those from untreated rats in metabolizing nitrosamines to mutagens but large interindividual variations are observed. Mutagenesis is often most effective using a liquid preincubation, a slightly acidic incubation mixture and hamster liver fractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Dimethylnitrosamine metabolism: II. In vitro activation of dimethylnitrosamine by hepatic and renal tissues from crosses among BALB/c, DBA and C57BL mice

Crosses among BALB/c, C57BL and DBA mice were performed to investigate the genetic mechanisms involved in metabolism of DMN by renal and hepatic tissues. Liver S-9 fractions from parental strain DBA had the greatest potential to activate DMN and liver fractions from parental strain BALB/c had the lowest. No age or sex-related differences were observed within strain. Crossing of either C57BL or DBA to BALB/c mice resulted in F1 hybrids with liver microsomal enzymes that gave results similar to the BALB/c parental strain. There were no sex or age differences within crossbred strains in the potential of liver to activate DMN. In contrast male DBA and C57BL parental mice renal S-9 fractions did not differ significantly from each other but did differ significantly from male BALB/c renal fractions and from female and immature animals of all strains. Crossing of either DBA or C57BL mice with BALB/c mice resulted in male F1 hybrids whose renal S-9 fractions did not differ significantly from males of the parental BALB/c strain. In all instances, male renal S-9 fractions had a significantly greater potential to activate DMN than female or immature animals. F1 DBA X C57BL hybrids had renal S-9 fractions that did not differ significantly from the parental strains. These data suggest that the gene(s) for low DMN metabolism of BALB/c mice are apparently dominant over the genes from both DBA and C57BL. The exact genetic or physiological mechanism needs further elucidation.

Dimethylnitrosamine metabolism: I. In vitro activation of dimethylnitrosamine to mutagenic substance(s) by hepatic and renal tissues from three inbred strains of mice

The potential of hepatic and renal homogenates from three inbred strains of mice (BALB/c, C57BL and DBA) to activate dimethylnitrosamine (DMN) was investigated. Microsomal enzyme (S-9) preparations of liver and kidney from mature and immature mice were used in the Ames Salmonella mutagenicity assay. No age or sex-related differences in the formation of active mutagenic DMN Metabolites by liver microsomal enzymes were observed within any of the three inbred strains. In contrast, mature male kidney S-9 fractions from all three strains had a significantly greater potential to activate DMN than mature female and immature animals. Testosterone treatment resulted in no apparent changes in the ability of hepatic tissue to biotransform DMN to its mutagenic metabolites among age and sex classes. However, after testosterone treatment, renal microsomal fractions from mature female mice of all three strains did not differ significantly from their male counterparts in their ability to transform DMN to mutagenic metabolites.

Effect of sex hormone status on chloroform nephrotoxicity and renal mixed function oxidases in mice

In mice, only males are susceptible to chloroform (CHCl3) nephrotoxicity and the susceptibility appears to be related to renal mixed function oxidase activity. There were sex-related differences of renal cytochrome P-450 and b5 concentrations and of ethoxycoumarin O-deethylase activity in mouse kidneys; in all cases activity was higher in males. Castration of male mice eliminated susceptibility to CHCl3 nephrotoxicity and reduced renal mixed function oxidases to concentrations observed in female mice. Treatment of male and female mice with testosterone increased the susceptibility to CHCl3 nephrotoxicity and increased renal mixed function oxidases to similar activities in both sexes. Previous data have suggested that CHCl3 is metabolized in situ by the kidney, possibly by a mechanism similar to that occurring in the liver. The data from this investigation are consistent with the concept that CHCl3 is metabolized by a cytochrome P-450-dependent mechanism in the kidney.

Genetic differences in dimethylnitrosamine mutagenicity in vitro associated with mouse hepatic aryl hydrocarbon hydroxylase activity induced by 3-methylcholanthrene

The effects of 3-methylcholanthrene (MC) pretreatment on metabolism and mutagenic activation of dimethylnitrosamine (DMN) were studied with liver subfractions from two strains of mice differing genetically with respect to aromatic hydrocarbon responsiveness. Both mutagenic activation and DMN N-demethylase activity segregated with aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity as a dominant trait in appropriate crosses between C57BL/6J (Ahb Ahb) and DBA/2J (Ahd Ahd) mice. DMN metabolism and mutagenicity were increased by MC-pretreatment in responsive Ahb Ahb and Ahb Ahd mice, but not in non-responsive Ahd Ahd mice. This indicates the involvement of the Ah locus in the genetic regulation of these activities in mice. Deuteration of DMN reduced mutagenicity and DMN N-demethylase activity by approximately 90 and 50 percent, respectively.

Bioactivation of dimethylnitrosamine in intrasanguinous host-mediated assay and its association with in vitro mutagenesis assays

These studies have revealed the usefulness of in vivo intrasanguine host-mediated assay (HMA) to detect point mutations. Mutations were found to occur at a significant rate in Salmonella typhimurium G-46 employed as indicator organisms recovered from liver, lung, kidney and spleen of DMN-treated animals compared to negative control animals. These differences were true for both male and female animals. The number of Salmonella typhimurium G-46 recovered from the testes was not large enough to make a valid judgment about mutations occurring in testes. The results from in vitro studies do not match with the in vivo host-mediated assay results for mutants occurring in spleen from the male and the female mice. The results also do not correlate for in vitro and in vivo studies involving female kidneys. These results suggest there may be no one-to-one correlation between the organ bioactivation in vitro and in vivo, and predictions of in vivo target organ cannot always be made from in vitro studies with isolated microsomal enzymes.

Regulation of dimethylnitrosamine metabolism by androgenic hormones

Hormonal regulation of dimethylnitrosamine (DMN) metabolism by mouse kidney microsomes was investigated using an in vitro mutagenesis system that detects bioactive metabolites of this procarcinogen by measuring reverse mutation in Salmonella typhimurium his- auxotrophs. Induction of microsomal DMN metabolizing enzymes was androgen-specific. Testosterone and medroxyprogesterone acetate were active inducers, d/1 norgestrel was less active, while epitestosterone, estradiol, and progesterone were ineffective. THe response to testosterone and medroxyprogesterone acetate was time-and dose-dependent. Eleven strains of inbred mice were screened for their response to exogenous testosterone. DMN metabolism was stimulated in all mouse strains except for RF/J mice. Other androgenic end points were responsive in the latter strain, however. These observations suggest that induction of renal microsomal DMN metabolising enzymes is androgen-specific and probably mediated via the androgen receptor. The insensitivity of RF/J mice may be due to a mutation affecting a key step in the enzymatic activation of DMN.

Evaluation of three metabolic activation systems by a forward mutation assay in Salmonella

The strain SV3 of Salmonella typhimurium was used as the indicator bacterium in the intrasanguineous host-mediated mutagenicity assay. Bacterial distribution and spontaneous mutation frequency were determined after intravenous injection of SV3 into CD1 male mice. Bacteria were cleared at an exponential rate from the blood stream and recovered mainly from the liver and in smaller quantities from the lungs and kidneys. No bactericidal effect was observed during incubation within the animal, and bacterial division occurred in the liver and probably in the kidneys. The significance of an increased mutation frequency of bacteria recovered from untreated animals is discussed. Mutation induction was measured in bacteria recovered from liver, lungs and kidneys of CD1 mice and CD rats treated with dimethylnitrosamine (DMN). The sensitivity of the intrasanguineous host-mediated technique was compared with the sensitivity of the assay in vitro with microsomal preparations from each tissue and host. Activation by isolated perfused liver and lungs from CD rats was included for comparison with the results from experiments in vivo and in vitro.

Mutagenic and carcinogenic properties of polycyclic aromatic hydrocarbons

The rapid development of the chemical industry, combustion of fossil fuels, and smoking of tobacco have resulted in contact of the general population with benzo(a)pyrene and other carcinogenic aromatic hydrocarbons. Persons especially at risk occupationally are those engaged in thermal processing of oil shale, coal, and heavy residual petroleum. It has been shown that polycyclic aromatic hydrocarbons require metabolic activation before they can act as mutagens or carcinogens. This metabolic activation results from interaction with microsomal enzymes present in many body cells, yielding reactive epoxides which react with DNA and produce mutations in the count frame shift or participate in covalent bounding. While opinions differ regarding the relative role of these processes in mutagenesis, considerable evidence exists which links mutagenesis and carcinogenesis. Metabolites of the polycyclic aromatic hydrocarbons which are carcinogenic are usually mutagenic, which supports the hypothesis that damage to chromosomes plays an important role in carcinogenesis. These facts open the possibility to monitoring the spread of carcinogenic substances in the biosphere by relatively simple tests whose endpoint is mutagenesis.

In vitro mutagenesis assays as predictors of chemical carcinogenesis in mammals

In vitro microbial mutagenesis assays coupled with mammalian activation systems offer promising technique to screen chemicals for their potential carcinogenic activity. The correlation between mutagenic and carcinogenic properties for a large array of chemicals is approximately 0.9. The best correlation exists for those carcinogens which are themselves highly electrophilic or produce electrophilic metabolites. Correlation between mutagenicity and carcinogenicity for hormonal, metallic, or physical carcinogens has been disappointing but not unexpected based on their proposed mechanisms of action. In addition to the application of in vitro mutagenesis techniques to screening chemicals for the identification of potential carcinogens, they are useful tools for investigating genetic, biochemical, and pharmacologic properties of different animal species. Studies with the chemical carcinogen dimethylnitrosamine have been conducted and show a functional relationship between mutagenesis and carcinogenesis. The assays can also be conducted using activation systems prepared from the tissues of any mammalian species. This permits a direct assessment of phylogenic extrapolation by comparing the metabolic activation capabilities of tissues from several mammalian species, including human samples. The advantages of mutagenicity testing are the short period of time required for results, the high sensitivity of the assay (microgram of nanogram quantities of chemicals can be used), and the fact that the ultimate agent can be detected biologically without first necessitating chemical identification and isolation. It appears from current studies that in vitro mutagenesis techniques may well open new avenues of investigation into some old toxicologic problems.

The utilization of in vitro mutagenesis techniques to explain strain, age and sex related differences in dimethylnitrosamine tumor susceptibilities in mice

The carcinogen dimethylnitrosamine (DMNA) is known to exhibit a high degree of strain, organ, age, and sex related tumor specificity in mice. Using microbial mutagenesis assay coupled with mouse tissue microsomal enzyme activation systems, evidence has been obtained that demonstrated a close relationship between the level of in vitro DMNA activation to a mutagen and in vivo tumor susceptibility. DMNA activation by liver, lung, and kidney microsomes from several mouse strains was compared by measuring the rate of mutagenic metabolites formed during incubation of the carcinogen in mutation assays using Salmonella typhimurium G-46 as the indicator microorganism.