Comparison of urethane-induced sister-chromatid exchanges in various murine strains, and the effect of enzyme inducers

Genetic variations in baseline and ultraviolet light-induced sister chromatid exchanges in peritoneal lymphocytes among different mouse strains

The variation in the induction of sister chromatid exchanges (SCEs) in the peritoneal lymphocytes of different mouse strains was investigated. For the baseline SCEs, BALB/c and outbred ICR showed the lowest frequency and DBA/2 and C57BL/6 the highest. BDF1 (C57BL/6 x DBA/2) was ranked among the highest, while CDF1 (BALB/c x DBA/2) was intermediate between the parental strains. Regarding UV-induced SCEs, BALB/c was less susceptible as compared to DBA/2 and C57BL/6. Both BDF1 and CDF1 showed values significantly higher than BALB/c, but not significantly different from DBA/2 or C57BL/6. ICR was ranked in the susceptible group. For the baseline SCEs of bone marrow cells, the overall ranking among strains was essentially the same as that for the baseline, but different from that for the UV-induced, SCEs in peritoneal lymphocytes. The present results can be explained by assuming that the major genetic factor contributing to the strain-dependent difference in the baseline SCEs is due to a codominant trait of a single allele, but that the UV-induced SCEs are complicated by other genetic factor(s).

Mouse strain differences in induction of micronuclei by base analogues and nucleosides

The frequency of induced micronucleated polychromatic erythrocytes (MNPCEs) was compared in BALB/c, C57BL/6, and DBA/2 mice after intraperitoneal (i.p.) injection of 5-bromodeoxyuridine (BUdR), 5-fluorodeoxyuridine (FUdR), cytosine arabinoside (Ara-C), 6-mercaptopurine (6-MP), 5-bromouracil (5-BU), thymidine (TdR), uridine (UdR), adenosine (AdR) and guanosine (GdR). The experimental procedure was a single i.p. injection followed by harvest at 30 h. The frequency of MNPCEs was significantly increased in all strains by treatment with BUdR, FUdR, Ara-C and 6-MP compared to vehicle control. TdR and UdR induced MNPCEs slightly in BALB/c mice but showed no effect on C57BL/6 and DBA/2 mice. 5-BU, AdR, and GdR did not increase the frequency of MNPCEs in any mouse strain used. These results suggest that BALB/c mice are more susceptible to induction of MNPCEs by clastogenic base analogues and nucleosides than are C57BL/6 or DBA/2 mice.

Activity of urethane and N,N-dimethylurethane in the mouse bone-marrow micronucleus assay: equivalence of oral and intraperitoneal routes of exposure

Urethane is shown to be active in the mouse bone-marrow micronucleus assay when administered as a single dose by either gavage or intraperitoneal injection. The magnitude of the response using the two routes was not statistically significantly different. N,N-Dimethylurethane (DMU) is shown to be mutagenic to Salmonella and active in the bone-marrow micronucleus assay by both routes of administration. The activity of DMU in the bone marrow precludes elimination of ethanol, yielding cyanate ion, as an explanation for the micronucleus-inducing activity of urethane.

A comparison of micronucleus induction in 3 mouse strains with representative clastogens

The frequency of micronucleated polychromatic erythrocytes (MNPCEs) was compared in BALB/c, C57BL/6 and DBA/2 mice after the intraperitoneal injection (i.p.) of methyl methanesulfonate (MMS), potassium chromate (K2CrO4), colchicine (COL), cycloheximide (CYH), 4-nitroquinoline-1-oxide (4-NQO) and 5-fluorouracil (5-FU). In BALB/c mice, the frequency of MNPCEs was significantly increased by all the chemicals tested, as compared to the vehicle control. In both C57BL/6 and DBA/2 mice, MMS, K2CrO4, COL and 4-NQO significantly increased MNPCEs, but CYH and 5-FU only slightly increased the MNPCEs in C57BL/6 and had no effect on DBA/2 mice. These results suggest that BALB/c mice are more sensitive to the induction of MNPCEs with a wider spectrum of chemicals than C57BL/6 or DBA/2 mice.

Induction of micronuclei in bone marrow of mice exposed to 1, 2 or 3 daily doses of urethane

Urethane was studied for its potential to induce micronuclei in bone marrow of CD-1 mice following various dosing and sampling schedules. It was found that: (1) mice dosed for 3 days by gavage with urethane at daily doses of 300, 600 and 1200 mg.kg-1 showed no increases in micronuclei compared to normal control values; (2) mice injected with 3 daily intraperitoneal (i.p.) doses of urethane at 400 and 1200 mg.kg-1 demonstrated substantial and dose-related increases in micronuclei; (3) mice given urethane at i.p. doses of 400 mg.kg-1 for 1, 2 and 3 days showed protocol related micronucleus yields with triple greater than single greater than double dosing, while mice given urethane at i.p. doses of 1200 mg.kg-1 showed a different trend with double greater than single dosing, the triple-dosing regimen proving cytotoxic to the bone marrow; (4) mice exposed i.p. to 400 mg.kg-1 of urethane had more micronuclei when sampled at 24 h than at 48 h after 1 or 2 daily treatments, while mice dosed i.p. with 1200 mg.kg-1 of urethane showed more micronuclei at 48 h than at 24 h after a single treatment, the double-dosing protocol being toxic to the marrow; (5) female mice were more susceptible to bone-marrow micronucleus induction than males with both quantitative and qualitative sex differences noted, depending on dose, regimen and sample.

Comparative in vivo sister chromatid exchange induction by ethyl carbamate in maternal and fetal tissues of tumor-susceptible and -resistant murine strains

Murine susceptibility to ethyl carbamate-induced carcinogenesis is strain dependent. In vivo sister chromatid exchange (SCE) responses to ethyl carbamate were evaluated in bone marrow cells of gravid adenoma-susceptible (ICR/Jcl), and resistant (C57Bl/6J) and (DBA/2J) murine dams, as well as in liver cells of their respective ICR/Jcl, C57Bl/6J X DBA/2J (BDF1), and DBA/2J X C57Bl/6J (BDF), fetuses following a single intravenous injection of 1.1, 2.2, or 3.3 mmol/kg of ethyl carbamate on gestation day 13/14. Bone marrow tissues of C57Bl/6J and DBA/2J, but not ICR/Jcl dams, demonstrated greater sensitivity to SCE induction than liver cells of their respective fetuses. Furthermore, relative SCE responses in bone marrow among dams indicated greater sensitivity of the more tumor-susceptible ICR/Jcl and C57Bl/6J strains to SCE induction by ethyl carbamate relative to the more tumor-resistant DBA/2J strain. In addition, concurrent alterations (stimulation or inhibition) of bone marrow cell cycle kinetics by ethyl carbamate were consistent with hormone-related, strain-dependent hematopoietic stress during pregnancy.

Mutagenicity, metabolism, and DNA interactions of urethane

Urethane, a known animal carcinogen, has been the subject of intensive research efforts spanning 40 years. Recent concerns have focused on the presence of urethane in a variety of fermented foods and alcoholic beverages, although no epidemiological studies or human case reports have been published. Much information is available about the mutagenesis, metabolism, and DNA interactions of urethane in experimental systems. Urethane is generally not mutagenic in bacteria although in some instances it acts as a weak mutagen. Urethane is not mutagenic in Nuerospora but is weakly mutagenic in Saccharomyces. Drosophila appear to be the only organisms that consistently give positive mutagenic results with urethane, but its mutagenicity is weak and in many cases shows no clear dose dependence. Urethane is a good clastogen in mammalian somatic cells in vivo, but it shows variable results with cells in vitro. It efficiently induces sister chromatid exchanges in a variety of cells. Mammalian spermatogenic cells are insensitive to the induction of specific locus and dominant lethal mutations by urethane. Mutational synergism has been reported to occur between ethyl methanesulfonate and urethane when administered two generations apart, and some investigators have suggested possible synergism for cancer-causing mutations in mice exposed to X-rays and urethane one generation apart. These studies are controversial and have not been confirmed. Studies on the induction of cancer-causing dominant mutations by urethane are at variance with results from extensive studies with the specific locus test in mice. Urethane studies with the unscheduled DNA synthesis assay in mouse spermatogenic cells and with the sperm abnormality test have given negative results. Urethane is rapidly and evenly distributed in the body. The rate of elimination of urethane from plasma is a saturable process and varies according to the strain and age of the animal. Recent studies have concentrations similar to those in wine, ethanol inhibits the tissue distribution of urethane in mice. These results are important because they suggest a lower carcinogenic/mutagenic risk than expected from exposure to urethane in alcoholic beverages. Although research on the metabolic activation of urethane has been extensive, no conclusive results have been obtained about its active metabolite, at one time thought to be N-hydroxyurethane. More recently, it has been postulated that urethane is activated to vinyl carbamate and that this metabolite is capable of reacting with DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of sister chromatid exchange in multiple murine tissues in vivo by various methylating agents

In order to study the reliability of in vivo sister chromatid exchange (SCE) assays for predicting carcinogenicity, several known animal carcinogens were tested in a multicellular in vivo SCE assay and an in vivo/in vitro murine lymphocyte assay. The methylating agents 1,2-dimethylhydrazine.2 HCl (DMH), dimethylnitrosamine (DMN), methylnitrosourea (MNU), methyl methane-sulphonate (MMS), and methylazoxymethanol acetate (MAM) were tested for SCE induction in several murine tissues in vivo, including bone marrow, alveolar macrophages, regenerating and intact liver, and kidney from B6D2F1 mice. In all cell types, clear dose-responses were observed following exposure of mice to subcytotoxic fractions of the LD50 dose of DMH, MNU, or MMS. DMN (0.03-0.27 mmol/kg) produced small, although not dose-related, increases in SCE in all cell types. At the doses tested (0.06 and 0.08 mmol/kg), MAM did not induce elevated SCE in the various cell types. Following a series of multiple i.p. injections of low, non-toxic doses of DMH (0.15 mmol/kg, once a week, for 10 weeks), significant differences were observed in intact vs. regenerating liver and in single vs. multiple injections in regenerating liver. Following exposure of B6D2F1 mice to a single i.p. injection of 0.25 mmol/kg DMN, DMH, or MMS or 0.19 mmol/kg MNU, SCE responses were evaluated in Concanavalin A (Con A)- and LPS-stimulated blood and spleen lymphocytes. Considerable cytotoxicity was observed in blood lymphocytes. In Con A- and LPS-stimulated spleen lymphocytes, DMH-, and DMN- and MMS-induced SCE frequencies were approximately 1.5-2 x baseline levels and MNU-induced SCE were approximately three- to fourfold higher than baseline values in cultures initiated at 1 and 24 h postexposure. At 48 and 72 h after an i.p. injection of 0.131 mmol/kg MNU, SCE responses in lymphocytes were approximately 2 x baseline levels. At 24 h following one, two, or four injections (one/week) of 0.075 mmol/kg MNU dose-related increases in SCE were observed in spleen lymphocytes. These studies illustrate that carefully designed in vivo SCE assays may have the capacity to predict the tumorigenic potential of chemical agents.

Individual and strain differences in patterns of long-term persistence of urethane-induced sister chromatid exchanges (SCEs) in mouse lymphocytes and their relation to carcinogen susceptibility

Using an improved microculture method, we investigated how the patterns of persistence of urethane-induced SCEs in lymphocytes differ among individuals and strains of mice (the ddY and C57BL strains), and we attempted to speculate on their relationship to carcinogen susceptibility. After a single intraperitoneal (i.p.) injection of 900 mg/kg of body weight of urethane into ten female mice each for the two strains, blood samples for the SCE analysis were collected from the tail vein at ten times during the 180 posttreatment days for each individual. Immediately after the treatment, SCE values increased to about three to four times the spontaneous values in all of the animals tested and then fell gradually. (The difference from spontaneous values was statistically significant until 120 days after the treatment.) Even after 180 days, however, some "outlier" cells with exceptionally high SCEs (greater than 20) persisted. Although there was some difference in average SCEs between the ddY and C57BL strains, the magnitude of the difference was too small to account for the difference between the strains in the incidence of urethane-induced malignancy. Also, when autopsy data at the 200th posttreatment day were matched individually with the data of SCE values within each strain, it was difficult to predict the individual risk of the occurrence of lung adenoma or other tumors from the relative difference in SCE values.

Effects of H-2 complex and non-H-2 background on urethane-induced chromosomal aberrations in mice

The incidence of in vivo urethane-induced chromosomal aberrations was examined in H-2 congenic strains of mice with B10 and A backgrounds. Chromosome analysis of bone-marrow cells could divide 7 lines of A.H-2 congenic strains into 2 groups: one with a higher frequency of chromosomal aberrations such as in A/Wy (haplotype H-2a), A/J (H-2a), A.AL (H-2al) and A.TL (H-2tl), and the other consisting of A.TH (H-2t2), A.CA (H-2f), A.BY (H-2b) and A.SW (H-2s). The same tendency was also observed in the spleen cells. Among B10.H-2 congenic mice, B10.A (H-2a), B10.BR (H-2k), B10.A(3R) (H-2i3), B10.A(5R) (H-2i5) and B10.S(9R) (H-2t4) exhibited significantly higher rates of induced chromosomal aberrations than those in B10 (H-2b), B10.S (H-2s), B10.A(2R) (H-2h2), B10.A(4R) (H-2h4) and B10.S(7R) (H-2t2). To determine the effect on non-H-2 genetic backgrounds on urethane-induced chromosomal aberrations, 4 pairs of strains which have the same H-2 haplotypes, such as in B10 vs. A.BY (H-2b), B10.A vs. A/Wy (H-2a), B10.S vs. A.SW (H-2s), and B10.S(7R) vs. A.TH (H-2t2), were compared. The strains with a B10 background exhibited significantly higher frequencies of deletions and lower frequencies of exchanges than the strains with an A background. These data suggested that at least two genes are involved in the regulation of urethane-induced chromosomal aberrations in mice, one of which is mapped between the S and D regions in the H-2 complex, and another not belonging to H-2.

Kinetics of sister-chromatid exchange induction by different carcinogens in C57BL/6J and DBA/2 mice

Sister-chromatid exchange (SCE) levels were determined in bone marrow cells of DBA/2 and C57BL/6J mice at 18, 24, 30 and 48 h after treatment with 10 mg/kg cyclophosphamide (CP), 300 mg/kg urethane or 25 mg/kg N-nitroso-N-methylurea (NMU). DBA/2 mice showed higher SCE frequencies than C57BL/6J mice at all time points examined after CP and urethane treatment, whereas NMU administration resulted in similar SCE levels in both strains. After CP and urethane treatments, SCE values reached the highest level at 18 h, were similarly high at 24 and 30 h, and returned to base-line level at 48 h. In NMU-treated mice, the SCE values had the same level at 18, 24 and 30 h and returned to normal at 48 h. The results are interpreted as indicating that different metabolic capabilities are responsible for the difference in SCE response in the two strains.