3-Methylindole (3MI), melatonin (Mel), serotonin (Ser), and tryptamine (Tryp) were evaluated in vitro for their potential to induce DNA adducts, DNA strand breaks, chromosomal aberrations (Abs), inhibition of DNA synthesis, and mutations. All compounds produced DNA adducts in calf thymus DNA in the presence of rat liver S9. In cultured rat hepatocytes, all produced DNA adducts but none induced DNA strand breaks. In Chinese hamster ovary cells, 3MI and Mel produced DNA adducts, Abs, and inhibition of DNA synthesis with and without S9, except that Mel without S9 did not form adducts. Ser formed DNA adducts, was an equivocal Abs inducer, and suppressed DNA synthesis. Tryp induced neither adducts nor Abs, but did suppress DNA synthesis with S9. Ser and Tryp were less cytotoxic than 3MI and Mel. Mel, Ser, and Tryp failed to induce mutations in Salmonella and E. coli strains with or without S9. 3MI and Mel produced DNA adducts but not mutations in Salmonella TA100 with S9. 3MI and its metabolite indole 3-carbinol also did not induce mutations in a shuttle vector system in human cells. The lack of correlation between DNA adducts and other genotoxicity endpoints for these indole compounds may be due to the higher sensitivity of the (32)P-postlabeling adduct assay or it may indicate that the indole-DNA adducts per se are not mutagenic and are not able to induce strand breaks or alkali-labile lesions. The indole-induced Abs may result from cytotoxicity and suppression of DNA synthesis with minimal if any contribution from DNA adducts.

The detection of DNA adducts is an important component in assessing the mutagenic potential of exogenous and endogenous compounds. Here, we report an in vitro quantitative long PCR (XL-PCR) assay to measure DNA adducts in human genomic DNA based on their ability to block and inhibit PCR amplification. Human genomic DNA was exposed to test compounds and then a target sequence was amplified by XL-PCR. The amplified sequence was then quantified using fluorogenic 5′ nuclease PCR (TaqMan®) and normalized to a solvent-treated control. The extent of DNA adduction was determined based on the reduction in amplification of the target sequence in the treated sample. A 17.7 kb β-globin fragment was chosen as the target sequence for these studies, since preliminary experiments revealed a two-fold increased sensitivity of this target compared to a 10.4 kb HPRT fragment for detecting hydrogen peroxide-induced DNA damage. Validation of the XL-PCR assay with various compounds demonstrated the versatility of the assay for detecting a wide range of adducts formed by direct acting or S9-activated mutagens. The same DNA samples were also analyzed using 32P-postlabeling techniques (thin-layer chromatography or high-performance liquid chromatography) to confirm the presence of DNA adducts and estimate their levels. Whereas 32P-postlabeling with nuclease P1 enrichment was more sensitive for detecting bulky adducts induced by the compounds benzo[a]pyrene, dimethylbenzanthracene, 3-methylindole, indole 3-carbinol, or 2-acetylaminofluorene, the XL-PCR procedure was more sensitive for detecting smaller or labile DNA adducts formed by the compounds methyl methanesulfonate, diethyl nitrosamine, ethylnitrosourea, diepoxybutane, ICR-191, styrene oxide, or aflatoxin B1. Compounds not expected to form adducts in DNA, such as clofibrate, phenobarbital, chloroform or acetone, did not produce a positive response in the XL-PCR assay. Thus, quantitative XL-PCR provides a rapid, high-throughput assay for detecting DNA damage that complements the existing 32P-postlabeling assay with nuclease P1 enrichment.

Levels of apoptosis induction, nuclear p53 expression, and cytoplasmic Bax protein expression were evaluated in various tissues before and after exposure of 2 neonatal and 6 adult rats to gamma irradiation. These studies served to provide positive control material for an evaluation of the effects of aflatoxin B1 administered to adult rats at several doses followed by sacrifice at several timepoints. The irradiation work also was run to determine any differences between neonate and adult expression of apoptosis, p53 and Fax. For the aflatoxin B1 study, only dUTP-biotin nick end labeling (TUNEL) staining to demonstrate apoptotic bodies and immunohistochemical staining to localize p53 nuclear protein were performed. In neonates, untreated control liver showed slight apoptosis. Following irradiation, apoptosis increased markedly in liver; other neonate tissues showed no apoptosis before or after irradiation. Results in untreated adult rats revealed slight apoptosis in some tissues with high cell turnover rates (thymus, spleen, intestine, and lymph node). Little apoptosis was evident in liver or kidney. Following irradiation, apoptosis increased in all tissues examined, especially at the 6 hr timepoint. In aflatoxin B1-treated adults, apoptosis evaluated by TUNEL appeared to increase slightly 24 hr post-dose, although morphological assessment showed increases at 3, 6, and 24 hr post dose sacrifices. The degree of apoptosis was directly proportional to increasing aflatoxin B1 dose. In neonates, p53 showed a significant increase only in thymus following irradiation, especially at the 4 hr timepoint. In adults, irradiation produced significant increases of p53 expression in all tissues except kidney. Following aflatoxin administration, p53 was present only in rat liver at 24 hr post-dose; and 1, 3, and 10 mg/kg doses showed slight dose-related increases in the number of positive cells. Bax protein staining was ubiquitous in adults and was present in all tissues; its detection was not affected by irradiation.

The p53 tumor suppressor protein plays an important role in regulating the cellular response to DNA damage, including cell cycle arrest and apoptosis induction. Normal p53 function is critical for the maintenance of genomic stability. The mouse lymphoma L5178Y/TK+/−-3.7.2C cell line is widely used in genetic toxicology for mutagenesis and clastogenesis testing. A related line L5178Y-R, has previously been shown to react with antibodies specific for mutant as well as wild-type p53 protein and to exhibit delayed cell death after radiation. For this reason, as well as the mouse lymphoma assay's reputation for high sensitivity of detection for genotoxic agents but low specificity, we examined several clones of L5178Y cells for mutations in the conserved core domain (exons 5–8) of the p53 gene. Using single-strand conformational polymorphism analysis, we found evidence for the same mutation in exon 5 of p53 in L5178Y-R, L5178Y-S and L5178Y/TK+/+-3.7.2C cells. The mutation was identified by sequencing of exon 5 as a TGC (Cys) to CGC (Arg) transition in codon 170 (=codon 176 in humans). Sequencing showed approximately equivalent signals for the mutant and normal alleles for all 3 lines. The mutation in codon 170 is adjacent to a mutation hotspot of the human p53 gene (codon 175) and eliminates a critical zinc-coordinating cysteine residue such that the mutant protein is likely to be denatured and have a dominant negative effect on normal p53 function. Western blots showed approximately 100-fold higher levels of p53 protein in unirradiated L5178Y cells as compared to induced levels of p53 in normal mouse splenocytes 4 h after 5 Gy of gamma radiation. The high levels of p53 protein in L5178Y cells were not further inducible by radiation, whereas an 11-fold induction was seen in the irradiated splenocytes. These results indicate that p53 protein in L5178Y cells is dysfunctional and suggest that this line may therefore be abnormally susceptible to the induction of genetic alterations.

The in vitro alkaline elution/rat hepatocyte assay is a sensitive assay for genotoxicity, measured as DNA strand breaks induced in primary cultures of rat hepatocytes after 3-h treatments with test compounds. Since DNA degradation can be rapid and extensive in dead and/or dying cells, the original criteria for a positive result in the assay were that a compound induce a 3.0-fold or greater increase in the elution slope (for the terminal phase of alkaline elution from 3 to 9 h) in the absence of significant cytotoxicity (defined as relative cell viability of less than 70% by trypan blue dye exclusion; TBDE). Recently we have shown that false-positive results can still be obtained due to cytotoxicity when loss of membrane integrity is a late event in toxic cell death relative to the induction of endonucleolytic DNA degradation. To improve the ability of the assay to discriminate between genotoxic vs. cytotoxic effects of chemicals, we have evaluated additional assays of cytotoxicity including cell adenosine triphosphate (ATP) and potassium (K+) content, tetrazolium dye reduction (MTT), TBDE after a further 3-h recovery incubation without test chemicals (delayed toxicity), cell blebbing and endonucleolytic DNA degradation (double-strand breaks; DSBs) assessed by pulsed-field gel electrophoresis (PFGE). We have also evaluated 2 parameters derived from the elution data which can indicate extensive, cytotoxicity-induced DNA degradation: the fraction of the DNA recovered in the neutral lysis/rinse fraction and the gamma-intercept of the extrapolation of the 3-9-h segment of the elution curve. Twenty-eight rodent non-carcinogens that are negative (or inconclusive) in the Ames assay with no, or limited, other evidence of genotoxicity, and 33 genotoxins, most of which are also carcinogens, were evaluated. The results showed that DNA degradation as measured by a 1-h PACE (Programmed Autonomously Controlled Electrodes)/PFGE assay was a sensitive indicator of cytotoxicity which correlated well with results of the other cytotoxicity indicators. The delayed TBDE (after a 3-h recovery), intracellular potassium and ATP assays as well as the gamma-intercept parameter were also shown to be sensitive and in some cases complementary measures of cytotoxicity. Using new criteria based on these data of an induced slope (treatment slope-negative control slope) of 0.020 for the 3- to 9-h elution period and cytotoxicity limits of 70% relative viability for the delayed TBDE assay and 50% for intracellular ATP content, the assay scores the genotoxicity of these 61 reference compounds with an overall accuracy of 92%. Test results using these new criteria are provided for an additional 20 compounds (5 non-genotoxic carcinogens and 15 compounds whose genotoxic and carcinogenic potential are unknown or equivocal).

Positive outcomes of in vitro genotoxicity tests may not always occur as a consequence of direct reaction of a compound or a metabolite with DNA. To follow-up positive responses in in vitro test, we developed two supplemental, cell-free assays to examine the potential of compounds and metabolites to directly damage DNA. Calf thymus DNA was used as the target for the direct detection of adducts by 32P-postlabeling/TLC and electrochemical detection, and alkaline gel electrophoresis was used to detect single-strand breakage of bacteriophage λ DNA. To show that these assays would detect damage from relevant compounds, we examined nine human carcinogens (aflatoxin B1, busulfan, chlorambucil, cyclophosphamide, diethylstilbestrol, melphalan, 2-naphthylamine, phenacetin and potassium chromate). Each of the nine compounds produced a positive result for one or both endpoints. Using multifraction contact-transfer TLC, we detected 32P-labeled DNA adducts produced by aflatoxin B1, chlorambucil, diethylstilbestrol, melphalan, 2-naphthylamine, and potassium chromate (plus hydrogen peroxide). Aflatoxin B1, diethylstilbestrol and 2-naphthylamine required metabolic activation (induced rat liver S9) to generate DNA adducts. Although potassium chromate alone induced a slight increase in the content of 8-hydroxydeoxyguanosine (a promutagenic adduct produced by reactive oxygen species), addition of hydrogen peroxide greatly increased 8-hydroxydeoxyguanosine levels. The damage to λ DNA by each human carcinogen (or metabolites), except diethylstilbestrol, was sufficient to generate single-strand breaks after neutral thermal hydrolysis at 70°C. Chromate was a weak inducer of DNA fragmentation, but adding hydrogen peroxide to the reaction mixtures dramatically increased the DNA strand breakage. Our data suggest that these non-routine, acellular tests for determining direct DNA damage may provide valuable mechanistic insight for positive responses in cell-based genetic toxicology tests.

The extent of DNA damage and cellular proliferation induced in rat kidneys by single doses of the diabetogenic alkylating agent streptozotocin (STZ) and the time course of repair of that damage were evaluated using an in vivo alkaline elution assay for DNA strand breaks and a bromodeoxyuridine (BrdUrd) labeling assay for cell replication. Male Sprague-Dawley rats were given iv injections of 0.25 to 60 mg/kg STZ and kidneys were harvested 3 hr later for alkaline elution. A dose of 2.5 mg/kg STZ was the lowest dose to induce detectable DNA strand breaks and extensive damage was produced by the commonly used diabetogenic dose of 60 mg/kg. To characterize the repair of the drug-induced DNA damage, kidneys were harvested from a 60 mg/kg group of animals 3 hr to 27 days after dosing. BrdUrd-labeled kidney sections were also evaluated to assess any cellular proliferative response associated with STZ administration. Significant DNA damage was detected up to 14 days after dosing with return to near background levels by 20 days. Similarly, treatment with 60 mg/kg STZ was associated with increases in BrdUrd labeling indices 4 and 9 days after treatment with resolution by 27 days. These results indicate that the cellular and molecular repair responses to a single diabetogenic dose of STZ are prolonged, requiring up to 3 weeks to complete. Thus, to avoid potential additive or synergistic effects on STZ-induced nephrotoxicity and/or genotoxicity, a delay in the start of experimental therapies in this model (other than insulin) should be considered.

E mu-pim-1 transgenic mice, which overexpress the pim-1 oncogene in lymphoid tissues, have shown increased susceptibility to induction of T cell lymphomas by N-ethyl-N-nitrosourea, a direct-acting chemical carcinogen (Nature, 340, 61-63, 1989). We sought to further evaluate E mu-pim-1 transgenic mice as a potential test animal for a short-term carcinogenesis bioassay. We chose to test four genotoxic procarcinogens; 2-acetylaminofluorene (2-AAF), N-nitro-sodiethylamine (NDEA), 1,2-dichloroethane (1,2-DCE) and benzene (BEN). These compounds require metabolic activation and, with the exception of benzene, are not mouse lymphomagens. Compounds were administered by gavage daily for 38 (NDEA and 2-AAF) or 40 (BEN and 1,2-DCE) weeks to groups of 25-29 male and female PIM mice at 1 and 3 mg/kg for NDEA, 50 and 100 mg/kg for BEN, 25-100 mg/kg for 2-AAF and 100-300 mg/kg for 1,2-DCE. Small but statistically significant increases in the incidence of malignant lymphoma were seen for three of the four carcinogens tested; in high dose males treated with 2-AAF, high and low dose females treated with NDEA and high dose females treated with 1,2-DCE. Results for BEN, the only mouse lymphomagen tested, did not show a statistically significant increase in the incidence of malignant lymphomas in transgenic mice within the 40 week duration of the study. NDEA also produced a high incidence (> 70%) of hepatic hemangiosarcomas in both sexes at the low and high dose levels. These results demonstrate that over-expression of the pim-1 oncogene in lymphoid tissue can confer susceptibility of this tissue to chemical carcinogenesis by genotoxic procarcinogens. However, whereas potent genotoxic carcinogens produced only small increases in the incidence of lymphoma and since BEN, a mouse lymphomagen, was negative, PIM transgenic mice may lack sufficient sensitivity to established carcinogens to justify their routine use in a short-term carcinogenesis screening assay.

Recently, Phillips et al. reported that small amounts of radioactivity derived from [14C]omeprazole were 'associated' with DNA purified from gastrointestinal tissues of treated rats (Mutagenesis 7, 277-283, 1992). We hypothesized that this radioactivity arose from omeprazole bound to contaminating protein in the DNA fraction (Mutagenesis 7, 395-396, 1992). Using rats injected with 35S-labeled amino acids, we found significant protein contamination (0.06 microgram of protein per microgram of DNA) in DNA purified from gastrointestinal tissues. Gastric mucous proteins represent likely candidates for binding of omeprazole in the rat model used by Phillips et al. To investigate this, we partially purified proteins from gastric mucus, incubated them with [14C]omeprazole, and then added these radiolabeled mucoproteins to homogenates of rat colon and duodenum before starting the DNA purification. Detectable amounts of the added mucoproteins remained in the DNA fraction, but none of the control protein, bovine serum albumin, remained with the DNA. Further characterization of the mucoproteins by hydroxyapatite chromatography indicated that a certain population of these proteins survived the DNA purification procedures. These data indicate that the association of omeprazole with DNA reported by Phillips et al. most probably is explained by binding of omeprazole to mucous glycoproteins (or other proteins present in the GI tract) that selectively survive DNA purification protocols.

Treatment of B6C3F(1) mice with concentrations of 62.5-625 p.p.m. 1,3-butadiene by inhalation for up to 2 years causes a significantly increased incidence of Harderian gland (HG) neoplasms over untreated controls (Melnick,R., Huff, J., Chou, B.J. and Miller, R.A. Cancer Res., 50, 6592-6599, 1990). Since a specific K-r as mutation (codon 13 GGC-->CGC) had previously been described in lung and liver tumors from 1,3-butadiene-treated B6C3F(1) mice, we analyzed 23 adenomas and six adenocarcinomas of the HG from mice exposed to 1,3-butadiene for this mutation and mutations in the H-ms gene. We also examined ras activation in 16 spontaneously occurring HG adenomas and one adenocarcinoma. DNA samples were prepared from paraffin-embedded tissues and analyzed by PCR followed by direct sequencing methods. Only one 1,3-butadiene-induced HG tumor contained the K-ras codon 13 mutation previously detected in lung and liver tumors. However, 16/29 HG tumors from the treated B6C3F(1) mice contained H-ms codon 61 mutations. The mutations detected were: 12 CAA-->CGA transitions, two CAA-->CTA and two CAA-->AAA transversions. Eleven of 17 spontaneous HG tumors contained mutations in H-ras codon 61: five CAA-->CGA transitions, two CAA-->CTA transversions and four CAA-->AAA transversions. While the spectrum of ras mutations did not differ between the spontaneously occurring and chemically induced tumors, these data indicate that activation of H-ras contributes to the process of HG tumorigenesis in both groups of these neoplasms.

Journal Article Comments on the report of ‘association’ of omeprazole with DNA by Phillips et al Get access Stephen P. Adams, Stephen P. Adams Genetic and Cellular Toxicology, Merck Research LaboratoriesWest Point, PA 19486, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Richard D. Storer, Richard D. Storer Genetic and Cellular Toxicology, Merck Research LaboratoriesWest Point, PA 19486, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar John G. Deluca, John G. Deluca Genetic and Cellular Toxicology, Merck Research LaboratoriesWest Point, PA 19486, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Sheila M. Galloway, Sheila M. Galloway Genetic and Cellular Toxicology, Merck Research LaboratoriesWest Point, PA 19486, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Warren W. Nichols Warren W. Nichols Genetic and Cellular Toxicology, Merck Research LaboratoriesWest Point, PA 19486, USA Search for other works by this author on: Oxford Academic PubMed Google Scholar Mutagenesis, Volume 7, Issue 5, September 1992, Pages 395–396, https://doi.org/10.1093/mutage/7.5.395 Published: 01 September 1992

As part of an evaluation of the effectiveness of using ras mutation analysis for distinguishing carcinogen‐induced from spontaneous tumors, we examined the profile of ras gene point mutations in spontaneous, 7,12‐dimethylbenz[ a ]anthracene (DMBA)‐induced, and N‐nitrosodiethylamine (DEN)‐induced lung tumors from Crl:CD‐1(ICR)BR (CD‐1) mice. Although all of the lung tumors were assayed for mutations in the Ha‐ ras , Ki‐ ras , and N‐ ras genes (codons 12, 13, and 61), only Ki‐ ras mutations were found, which is consistent with other studies that have noted a strong preference for Ki‐ ras gene activation in mouse, rat, and human lung tumors. We found that spontaneous CD‐1 mouse lung tumors had a very high frequency of Ki‐ ras gene activation (17 of 20 tumors; 85%), distributed among codons 12 (5 of 20), 13 (1 of 20), and 61(11 of 20). DMBA‐induced lung tumors had a slightly higher frequency of Ki‐ ras gene mutations (16 of 16; 100%), again distributed among codons 12(5 of 16), 13 (2 of 16), and 61(9 of 16). However, seven of the DMBA tumors had mutations qualitatively different from those found in spontaneous tumors. In contrast to DMBA‐induced tumors, DEN‐induced tumors had a lower frequency of Ki‐ ras mutations (36%) when compared with spontaneous lung tumors, suggesting that DEN primarily induces lung carcinogenesis by a mechanism other than ras gene activation. Thus, although spontaneous and induced CD‐1 mouse lung tumors have a strong tissue‐specific preference for carrying an activated Ki‐ ras gene, the nature of the initiating carcinogen can influence the frequency or profile of Ki‐ ras mutations. © 1992 wiley‐Liss, inc.

Inactivating point mutations and small deletions in the p53 tumor suppressor gene have been found in human liver and lung tumor--derived cell lines and tumors. However, little evidence has been reported concerning inactivation or mutation of the p53 gene in mouse primary tumors. To examine CD-1 mouse liver and lung tumors for mutations in the p53 gene, we first sequenced p53 introns 5-8 so that polymerase chain reaction amplification and sequencing primers located within the introns could be prepared. Use of these primers prevented amplification of the mouse p53 pseudogene and allowed sequencing of exons 5-8 in their entirety as well as their intron-exon junctions. DNA isolated from CD-1 mouse tumors was amplified and directly sequenced using nested primers. Nine spontaneous hepatocellular carcinomas (HCCs) and 34 chemically induced HCCs (induced by single intraperitoneal injections of N-nitrosodiethylamine [DEN] [8 HCCs], 7,12-dimethylbenz[a]anthracene [DMBA] [8 HCCs], 4-aminoazobenzene [8 HCCs], and N-OH-2-acetylaminofluorene [10 HCCs]) were examined for mutations in exons 5-8 of the p53 gene. In addition, 12 spontaneous, 10 DMBA-induced, and 13 DEN-induced lung adenocarcinomas or adenomas were analyzed for mutations. No mutations were found in any of the tumors examined. However, a mutation was demonstrated at codon 135 in the positive-control plasmid LTRp53cG(val). The results of this study suggest that inactivation of p53 is unlikely to play a major role in murine lung or liver carcinogenesis. However, inactivation of p53 may occur at a very low frequency, or it may occur as a late event and therefore be present in only a very small number of the tumor cells, rendering it undetectable by this method. Lastly, although few p53-inactivating mutations are found outside of exons 5-8 in human tumors, it is possible that these murine tumors contained mutations outside of this region and were therefore missed by our approach.

The efficiency of detection of H- and K-ras mutations in 27 CD-1 mouse liver tumors by direct sequencing of polymerase chain reaction (PCR)-amplified DNA isolated from formalin-fixed and paraffin-embedded tissues was compared with that after assay by both NIH 3T3 transfection (followed by sequencing of amplified transformant DNA) and direct sequencing of PCR-amplified DNA isolated from frozen tumors. Some tumor samples were chosen for comparison because they contained ras mutations that were detected by either NIH 3T3 transfection or sequencing of PCR-amplified DNA derived from frozen tumors, but were not detected by both techniques. The efficiency of detecting K-ras mutations was similar for sequencing of amplified fragments derived from both paraffin-embedded tissues and from frozen tumors. However, these two techniques differed in their efficacy for detection of H-ras codon 61 mutations. Furthermore, this difference appeared to be mutation-specific: the sequencing of amplified products from paraffin-embedded tumor tissues allowed increased detection of CAA to AAA mutations but decreased detection of CAA to CTA mutations relative to sequencing of amplified fragments derived from frozen tumor DNA. Direct sequencing of PCR products from paraffin-embedded sections was more sensitive than NIH 3T3 transfection for detection of activated K-ras genes containing codon 13 mutations but less sensitive for detection of activated H-ras genes containing codon 61 mutations. In summary, direct sequencing of amplified DNA from either frozen tumors or formalin-fixed, paraffin-embedded tissues can be more sensitive than NIH 3T3 transfection for detection of codon 13-activated K-ras genes. However, it appears to be less sensitive than NIH 3T3 transfection for detection of certain activating H-ras mutations. Depending upon the questions being asked of the data, each of the methods can provide useful information about ras gene mutations in tumor samples. The apparent differences in sensitivities between the methods is not yet understood, but such differences should be considered in the analysis of data obtained when only one method is used.

We compared the profile of ras gene mutations in spontaneous CD-1 mouse liver tumors with that found in liver tumors that were induced by a single i.p. injection of either 7,12-dimethylbenz(a)anthracene (DMBA), 4-aminoazobenzene, N-hydroxy-2-acetylaminofluorene, or N-nitrosodiethylamine. By direct sequencing of polymerase chain reaction-amplified tumor DNA, the carcinogen-induced tumors were found to have much higher frequencies of ras gene activation than spontaneous tumors. Furthermore, each carcinogen caused specific types of ras mutations not detected in spontaneous tumors, including several novel mutations not previously associated with either the carcinogen or mouse hepatocarcinogenesis. For example, the model compound DMBA is known to cause predominantly A to T transversions in Ha-ras codon 61 in mouse skin and mammary tumors, consistent with the ability of DMBA to form bulky adducts with adenosine. Our results demonstrate that the predominant mutation caused by DMBA in mouse liver tumors is a G to C transversion in Ki-ras codon 13 (DMBA is also known to form guanosine adducts), illustrating the influence of both chemical- and tissue-specific factors in determining the type of ras gene mutations in a tumor. 4-Aminoazobenzene and N-hydroxy-2-acetylaminofluorene also caused the Ki-ras codon 13 mutation. In addition, we found that N-nitrosodiethylamine, 4-aminoazobenzene, and N-hydroxy-2-acetylaminofluorene all caused G to T transversions in the N-ras gene (codons 12 or 13). This is the first demonstration of N-ras mutations in mouse liver tumors, establishing a role for the N-ras gene in mouse liver carcinogenesis. Finally, comparison of the ras mutations detected in the direct tumor analysis with those detected after NIH3T3 cell transfection indicates that spontaneous ras mutations (in Ha-ras codon 61) are often present in only a small fraction of the tumor cells, raising the possibility that they may sometimes occur as a late event in CD-1 mouse hepatocarcinogenesis.

Deuterated and non-deuterated N-nitrosodimethylamine, epichlorohydrin and dimethyl sulfate were evaluated for the ability to induce DNA single-strand breaks in rat hepatocytes as measured by alkaline elution. Non-deuterated nitrosodimethylamine induced twice the amount of DNA-strand breaks as the deuterated form. No evidence of a deuterium isotope effect was seen for the direct-acting alkylating agents epichlorohydrin and dimethyl sulfate.

Determining to what degree chemicals and environmental agents contribute to the development of cancer would be materially enhanced by the ability to distinguish chemically induced tumors from those that arise spontaneously. Using DNA fingerprinting as an assay, we investigated whether somatic DNA rearrangements are more frequent in chemically induced mouse liver tumors than they are in spontaneous mouse liver tumors. Tumors were induced by a single i.p. injection of 12-day old male Crl:CD-1(ICR)BR (CD-1) mice with 20 nmol/g 7,12-dimethylbenz[a]-anthracene and were harvested 9 to 12 months after injection. Spontaneous tumors were obtained from 94- to 98-week old male CD-1 mice. We detected 8 rearrangements in 14 7,12-dimethylbenz[a]anthracene-induced tumors, which corresponds to a high rearrangement frequency of about 2% (of the minisatellite bands examined). Furthermore, 6 of these rearrangements included complete band losses which must have occurred early in tumor development. However, only 2 band changes were observed in 15 spontaneous tumors, and both changes were intensity shifts which may represent rearrangements that occurred later during tumor progression. Histological examination showed that the higher frequency of rearrangements in 7,12-dimethylbenz[a]anthracene-induced tumors versus spontaneous tumors was not related to differences in the degree of tumor progression or malignancy. Our results suggest that DNA fingerprinting may be a valuable assay for differentiating certain chemically induced tumors from spontaneous tumors.

First passage rat embryo cells were transfected with plasmids carrying a mutated EJ c-Ha-ras oncogene alone or in combination with the c-myc oncogene. Three days later, unselected cultures were harvested and injected into nude mice either subcutaneously to assay for tumorigenicity or intravenously to assay for metastasis to the lung. The results indicate that a transcriptionally-enhanced EJ-c-Ha-ras oncogene alone can convert normal rat cells to a tumorigenic but not metastatic phenotype. Co-transfection of a c-myc oncogene with the EJ c-Ha-ras oncogene was necessary to produce the rapid, phenotypic conversion of normal cells to transformed cells with both tumorigenic and metastatic potential. No tumors were observed in animals injected with c-myc-transfected cells. Cell lines established from EJ c-Ha-ras-induced shoulder tumors were metastatic when reinjected intravenously into nude mice. These results support the hypothesis that the cooperative action of c-Ha-ras and c-myc oncogenes is more potent in inducing malignant transformation than either oncogene acting alone. Our results also suggest that the phenotypic conversion of normal cells to tumorigenic cells with experimental metastatic potential by ras and myc oncogenes can be completed within 3-4 cell divisions after transfection.