An initiation-promotion assay in rat liver as a potential complement to the 2-year carcinogenesis bioassay

Several pharmaceutical agents, manufacturing chemicals, and environmental contaminants were found to act primarily as promoting agents in an initiation-promotion paradigm. The phenotypic distribution of four enzyme markers--placental glutathione-S-transferase (PGST), gamma-glutamyl transpeptidase (GGT), canalicular ATPase (ATPase), and glucose-6-phosphatase (G6Pase)--was analyzed in altered hepatic foci (AHF) by quantitative stereology. The number and volume distribution of AHF were determined for each promoter tested. For phenobarbital and 2,3,7,8-tetrachloro-p-dioxin, PGST and GGT together scored 100% of the AHF; for 1-(phenylazo)-2-naphthol (CI solvent yellow 14) and chlorendic acid, PGST alone marked 90% of the AHF; after chronic administration of WY-14,643, ATP and G6Pase were the predominant markers. In rats fed tamoxifen, G6P scored more than half of the AHF. Differences in the number of AHF promoted by each of these agents and in their phenotypic distributions may reflect the differentially responsive nature of individual initiated hepatocytes to the action of specific promoters. Since the chronic bioassay of suspected carcinogens does not allow one to differentiate between weak complete carcinogens and those carcinogenic agents that act in a reversible manner to promote the growth of previously initiated cells, the partial hepatectomy, altered-hepatic-focus model of cancer development is proposed as a supplement to the chronic bioassay for the identification of those carcinogenic agents that are primarily, if not exclusively, promoting agents in rat liver.

Correlation of data from the strain A mouse bioassay with long-term bioassays

Fifty-nine chemicals that had completed National Cancer Institute rat and mouse 2-year carcinogenicity tests were tested in the strain A mouse pulmonary tumor assay. Without knowledge of chemical identity, 53 chemicals were tested in strain A mice in one laboratory and 30 were tested in a second independent laboratory. Strain A tests on 24 of these chemicals were conducted in both laboratories. The strain A results were generally not predictive of the 2-year rat and mouse carcinogenicity test results. Furthermore, there was poor agreement of strain A results between the two laboratories. Although a variety of explanations may be invoked to explain the lack of concordance between the strain A tests and the 2-year rat and mouse tests, no one factor is sufficient to rationalize the poor concordance between strain A and 2-year carcinogenicity bioassay results.

Histomorphology and ultrastructure of spontaneous pulmonary neoplasms in strain A mice

The microscopic and ultrastructural characteristics of spontaneous pulmonary neoplasms in strain A (strA) mice are described. Fifty-one spontaneous lung tumors were identified in 34 out of 57, 11-23-month-old male strA/Hen mice. Grossly, all tumors appeared as yellow-white, discrete nodules ranging in size from 1.0-10 mm. Tumor types were randomly distributed throughout the lung; however, the right lung lobes were most frequently involved. Histologically, tumors were classified as adenoma (34/51) or carcinoma (17/51) as defined by standard histopathologic criteria. Adenomas were usually less than 4 mm in diameter and had solid (16/34), papillary (10/34), or mixed (8/34) histologic growth patterns. Carcinomas were usually greater than 4 mm in diameter and had papillary (13/17) or mixed (4/17) histologic growth patterns. Ultrastructurally, benign tumors consisted of solid or papillary areas of neoplastic type II-like cells. Cells comprising malignant tumors had varying ultrastructural characteristics ranging from well-differentiated alveolar cell types to undifferentiated cells having intracytoplasmic osmiophilic dense bodies, vacuoles, or few specialized organelles commonly observed in mature nonneoplastic pulmonary epithelial cells.

Proliferative lesions of the mouse lung: progression studies in strain A mice

The progression of pulmonary neoplasia was examined in strain A/J male mice treated with a single dose of vinyl carbamate (60 mg/kg, i.p.) 6 weeks after birth. Interim sacrifices were performed at 7, 8, 10, 12, or 14 months. Proliferative lesions of the lung were divided into four categories: hyperplasias, adenomas, carcinomas arising within adenomas, and carcinomas. Grossly visible surface tumor counts, histologic diagnoses, and morphometric measurements of histologic lesions were used to evaluate progression. Vinyl carbamate-treated mice showed increased mean surface tumor counts at all time points. Diagnostic evaluation suggested that as a function of time, the relative frequency of hyperplasias decreased and the relative frequency of adenomas increased. The relative frequency of adenomas subsequently decreased, whereas the relative frequency of carcinomas increased. At all time points, carcinomas arising within adenomas were present. As time progressed, the number of carcinomas arising within adenomas decreased, whereas the number of "pure" carcinomas increased. Morphometric analysis of lesions indicated hyperplasias to be small, that adenomas were larger than hyperplasias, and carcinomas were larger than adenomas and hyperplasias, suggesting that few adenomas or carcinomas arise de novo. Collectively, these data suggest that the majority of pulmonary tumors in A/J mice treated with vinyl carbamate arise as hyperplasias, progress to adenomas, and ultimately result in carcinomas.

Pulmonary carcinogenesis in transgenic mice

Use of genetically engineered mice offers a unique approach to identifying and investigating factors that may influence tumor development. We have used conventional histopathologic and ultrastructural techniques to characterize lung tumors in three lines of transgenic mice bearing an albumin enhancer/promoter linked to a mutated human H-ras gene. Mice in all three lines developed multiple alveolar-bronchiolar (A/B) adenocarcinomas that are eventually lethal. The large diversity in tumor morphological features and differential tumor growth rates suggests that secondary events contribute to tumor phenotype and biological behavior. Two of the transgenic lines developed numerous A/B neoplasms within 6 to 8 weeks and thus may be useful animal models for testing potential anticancer chemotherapeutic agents. The other line lived for approximately 10 months, had fewer A/B tumors, but also developed bronchiogenic tumors. All three transgenic lines may be useful models for studying factors that affect lung tumor development.

Role of Clara cells and type II cells in the development of pulmonary tumors in rats and mice following exposure to a tobacco-specific nitrosamine

The role of the Clara and type II cell in the development of pulmonary tumors in the A/J mouse and Fischer rat was investigated by determining the relationship of DNA methylation and repair in pulmonary cells to oncogene activation and by characterizing the morphology of pulmonary tumors induced by treatment with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Marked differences in the formation of the promutagenic adduct O6-methylguanine (O6MG) were observed in pulmonary cells following treatment of rats with NNK. Concentrations of this adduct in Clara cells greatly exceeded (3- to 30-fold) those detected in type II cells and whole lung with doses of NNK ranging from 0.1 to 50 mg/kg. In addition, very low rates of repair of this adduct were detected in Clara cells, whereas efficient adduct removal occurred in type II cells. The importance of this adduct and the role of cell specificity was suggested by the fact that a strong correlation was observed between the concentration of O6MG in Clara cells and tumor incidence in the Fischer rat with doses of NNK ranging from 0.03-50 mg/kg. In contrast, no differences in adduct concentration between type II and Clara cells from A/J mice were observed under conditions resulting in pulmonary tumor formation. Activation of the K-ras gene was detected in lung tumors from A/J mice. This gene was activated by a mutation in codon 12 involving a GC to AT transition (GGT to GAT) and is consistent with base mispairing produced by the formation of O6MG. Activation of this gene was not associated with lung tumor formation in the Fischer rat. DNA from rat lung tumors did induce tumors in the nude mouse carcinogenicity assay. In addition, rat repetitive sequences were detected in DNA isolated from these nude mouse tumors. In spite of the cell selectivity for DNA methylation in Clara cells from rat and the relationship between O6MG formation and tumorigenicity, early proliferative lesions observed in both mice and rats involved the alveolar areas. Ultrastructural examination of these lesions and adenomas revealed morphologic features characteristic of the type II cell. Thus the lack of agreement between biochemical and morphological findings makes it difficult to hypothesize a cell of origin for the pulmonary neoplasms induced by NNK. However, these studies indicate that the concentration of O6MG in Clara cells is an excellent indicator of the carcinogenic potency of NNK in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of protooncogenes in mouse lung tumors

The detection of activated protooncogenes in mouse lung tumors has led to a major advance in our understanding of carcinogenesis of the lung at the molecular level. A high frequency of activated K-ras protooncogenes has been detected in tetranitromethane (TNM)- and 1,3-butadiene-induced lung tumors in B6C3F1 mice. In the past several years, we have pursued protooncogene activation in spontaneous and chemically induced tumors of strain A mice. The strain A mouse has a high incidence of spontaneous lung tumors and is susceptible to tumor induction by chemical carcinogens. We have detected and characterized the activated protooncogenes in the DNA of both spontaneously occurring and chemically induced lung tumors of strain A mice. Activated K-ras genes were detected using the NIH/3T3 transfection assay, and the activating mutations were identified by utilizing the polymerase chain reaction (PCR) and direct sequence analysis. A strong selectivity of mutations in the K-ras genes were observed in chemically induced lung tumors, as compared to spontaneous tumors, indicating that the carcinogens directly induced point mutations in the K-ras protooncogene. These findings suggest that the strain A mouse lung tumor model appears to be a very sensitive system to identify the mechanism by which chemical carcinogens activate the K-ras gene in lung tissue in vivo.

Correlation of hepatocellular proliferation with hepatocarcinogenicity induced by the mutagenic noncarcinogen:carcinogen pair--2,6- and 2,4-diaminotoluene

2,4-Diaminotoluene (2,4-DAT) and 2,6-diaminotoluene (2,6-DAT) are equally genotoxic in the Ames/Salmonella assay and are both readily absorbed, metabolized, and excreted and metabolites of both compounds are mutagenic with metabolic activation. However, there are marked differences in the results of chronic rodent bioassays with these two compounds. 2,4-DAT is a potent hepatocarcinogen whereas 2,6-DAT failed to produce an increased incidence of tumors in any tissue even when administered at a dose higher than that of 2,4-DAT. In an effort to elucidate the source of these apparently discordant results, the present studies were designed to determine the effects of these two chemicals on cell proliferation in the liver when administered at the dose levels comparable to those used in the original bioassays. This study utilized repeated oral dosing, osmotic minipumps to deliver bromodeoxyuridine (BrDU) for 8 days, and immunohistochemistry to quantitate BrDU incorporation into hepatic DNA, CCl4 (0.4 ml/rat, single ip dose) or vehicle control groups were included as positive and negative controls, respectively. The degree of cell proliferation was quantified by the labeling index from at least 1000 hepatocytes. Results from the control studies indicate that approximately 1.1% of the hepatocytes from vehicle-treated animals replicated during the exposure period whereas approximately 50% replicated in the positive controls. The carcinogen 2,4-DAT produced a dose-dependent increase in cell proliferation of approximately 10% and 20% in livers of animals exposed to 12.5 and 25.0 mg/kg/day, respectively, whereas the noncarcinogen 2,6-DAT produced no increase in cell turnover compared to vehicle control following treatment with 25.0 or 50.0 mg/kg/day. These results indicate a positive correlation between increased cell proliferation and hepatocarcinogenesis induced by these two isomers of diaminetoluene.

Activation of protooncogenes in spontaneously occurring non-liver tumors from C57BL/6 x C3H F1 mice

The C57BL/6 x C3H F1 (hereafter called B6C3F1) mouse is an important animal model for long-term carcinogenesis studies. Maintained under normal laboratory conditions, these mice develop various types of spontaneous tumors during their lifetime. Activated Ha-ras genes have been detected in 66% of spontaneous hepatocellular tumors in the B6C3F1 mouse [Reynolds et al., Science (Washington DC), 237:1309, 1988]. In this study 49 spontaneous non-liver tumors were investigated for oncogene activation by DNA transfection techniques. Of the 49 tumor DNAs analyzed, only 5 yielded multiple foci in the NIH 3T3 focus assay: 2 of 10 pulmonary adenocarcinomas; 0 of 25 lymphomas; 2 of 2 Harderian gland adenomas; 0 of 1 adenocarcinoma of the small intestine; 1 of 6 malignant skin tumors; 0 of 4 hemangiosarcomas; and 0 of 1 lung metastasis of a hepatocellular carcinoma. DNA from six lymphomas which were negative in the NIH 3T3 focus assay were further analyzed for transforming genes by the nude mouse tumorigenicity assay. One of the five lymphomas tested positive with this assay. Southern blot analysis identified five activated ras genes: H-ras in two Harderian gland adenomas; K-ras in one pulmonary adenocarcinoma and in one s.c. adenocarcinoma; and N-ras in one lymphoma. The mutations involved were CG to AT and AT to TA in codon 61 of the Ha-ras genes, GC to AT or TA in codon 12 of the K-ras genes, and a GC to AT mutation in codon 12 of the N-ras gene. Transformant DNA from a pulmonary adenocarcinoma which yielded multiple foci in the transfection assay did not hybridize to DNA probes specific for the K-, H-, and N-ras, raf, neu, and met genes. Thirteen additional tumor DNAs yielded a single focus in the NIH 3T3 transfection assay. The transformant DNAs retransmitted in a second cycle transfection assay. Rearranged and/or amplified raf genes were detected in six of the transformant DNAs. At present we do not know whether these activated raf genes were present in the original tumor DNA. The other seven transformant DNAs did not hybridize with any of the above mentioned specific DNA probes utilized in Southern blot analysis. Unlike liver tumors, the activation of ras protooncogenes is not a frequent event in the development of spontaneous non-liver tumors of the B6C3F1 mouse. The results from this study should aid in understanding the neoplastic development associated with exposure to chemical carcinogens in the B6C3F1 mouse.

Effects of sulfhydryl modulation on ethyl acrylate-induced forestomach toxicity

Acute administration of a single dose of ethyl acrylate (EA) to F344 rats by gavage caused time- and dose-dependent forestomach edema. Evidence from our laboratory and others suggested that EA is hydrolyzed to acrylic acid (AA) and ethanol both in vivo and in vitro. The major metabolites detected in teh urine of rats treated with EA were derivatives of the glutathione conjugates of EA and AA. The current work was undertaken to investigate the effects of sulfhydryl-depleting agents (diethylmaleate and fasting) and sulfhydryl-containing agents (cysteine and cysteamine) on EA-induced forestomach edema. Results presented in this report revealed that pretreatment of rats with sulfhydryl-containing chemicals such as cysteine or cysteamine has potentiated EA-induced forestomach edema. In contrast, depletion of indigenous sulfhydryls by fasting of rats or pretreatment with diethylmaleate (DEM) protected against EA-induced forestomach edema. Furthermore, repetitive daily administration of EA by gavage induced mucosal forestomach hyperplasia. Co-administration of cysteamine and EA resulted in a significant enhancement of the severity of EA-induced forestomach mucosal hyperplasia. In conclusion, current data suggest that modulation of indigenous sulfhydryls play a role in EA-induced forestomach toxicity; however, the exact mechanism underlying this role remains to be characterized.

Correlation of changes in serum analytes and hepatic histopathology in rats exposed to carbon tetrachloride

Clinical pathology data can significantly contribute to the characterization of a disease process if suitable time points for sample collection are chosen and combined with the measurement of biochemical analytes that are sensitive and specific for damage to a potential target organ. Using a well-defined model for hepatotoxicity, we correlated histopathological lesions in the liver with changes in selected serum analytes. Groups of Fischer-344 rats were treated with carbon tetrachloride (280 mg/kg in corn oil) for 1, 2, 4, 6, 8 or 10 days. Subgroups were allowed to recover for 1, 5 or 8 days, at which time blood and liver specimens were collected. Histologically, necrosis was detected in livers from rats treated for 1 and 2 days and allowed to recover for 1 day. This was followed by generalized fatty change in animals treated for longer periods. The maximum severity of fatty change occurred 7-12 days (total experimental time). A sharp rise and fall (48 h) in cytosolic enzyme activities were seen in serum. This preceded gradual increases in all analytes measured which eventually peaked at 9-11 days (total experimental time). The pattern seen in biochemical analytes paralleled the development of marked fatty change. We discuss relationships between the histologic and biochemical findings and conclude that appropriate clinical biochemistry measurements in a toxicology experiment can provide valuable mechanistic information.

Histomorphologic features of spontaneous and chemically-induced pulmonary neoplasms in B6C3F1 mice and Fischer 344 rats

The histomorphologic features of spontaneous and chemically-induced lung neoplasms in male and female B6C3F1 mice and Fischer 344 rats are described. Primary pulmonary neoplasms in mice and rats were classified as alveolar/bronchiolar (A/B) adenoma or carcinoma (including variants with squamous and mucinous cell differentiation), bronchial adenoma or carcinoma, squamous cell carcinoma or mesenchymal tumors. A/B adenomas and carcinomas were the most common spontaneous pulmonary neoplasms observed in both mice and rats, but were observed less frequently in rats. In the National Toxicology Program (NTP) historical control database the incidence of spontaneous A/B adenomas in male (n = 2,084) and female (n = 2,079) mice is 13.8% and 4.9%, respectively; for A/B carcinomas, it is 5.3% and 2.4%, respectively. In male (n = 3,877) and female (n = 3,919) rats, spontaneous pulmonary neoplasms are rare with historical control rates less than 3% for A/B adenomas or carcinomas in either sex. The spontaneous A/B adenomas and carcinomas observed in mice and rats typically had papillary, solid or mixed (papillary and solid) histologic growth patterns. Pulmonary neoplasms from mice and rats treated with chemical carcinogens reviewed from 2-year studies consisted primarily of A/B adenomas and carcinomas. These tumors had papillary, glandular/tubular, solid or mixed (combination of 2 or more) histologic growth patterns. A few of the A/B neoplasms had areas of squamous or mucinous cell differentiation. Other less frequently occurring spontaneous and chemically-induced neoplasms included squamous cell carcinomas, bronchial adenomas and carcinomas, and sarcomas.

Dose-response relationship between O6-methylguanine formation in Clara cells and induction of pulmonary neoplasia in the rat by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The relationship between the formation of O6-methylguanine (O6MG) and the induction of lung, liver, and nasal tumors in the Fisher 344 rat by the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) was examined in a dose-response study. Animals were treated for 20 wk (3 times/wk) with concentrations of NNK ranging from 0.03 to 50 mg/kg to induce tumors. Steady-state concentrations of O6MG were quantitated, and cytotoxicity was assessed in target cells and tissues after 4 wk of treatment with NNK. No cytotoxicity was detected in the lung during treatment with NNK. The formation of O6MG was greatest in Clara cells compared with macrophages, type II cells, small cells, and whole lung at all doses examined. The difference in adduct concentration between the Clara cell and other pulmonary cell types was most pronounced with low doses of carcinogen. The O6MG:dose ratio, an index of alkylation efficiency, increased 29-fold as the dose of NNK was decreased from 50 to 1 mg/kg of carcinogen. In contrast, only a small increase in alkylation efficiency was observed in type II cells and whole lung. A significant number of tumors were induced in the lung at doses of 0.1 to 50 mg/kg with incidences ranging from 10% at the lowest dose up to 87% in the group of animals which received 50 mg/kg of NNK. A linear relationship was observed when the concentration of O6MG in Clara cells as a function of dose was plotted against the corresponding tumor incidence. This relationship was not observed using DNA adduct concentrations in type II cells or whole lung. The development of pulmonary tumors appeared to involve the formation of alveolar hyperplasias which progressed to adenomas and finally to carcinomas. The majority of adenomas were solid, whereas carcinomas were mainly papillary. Examination of the ultrastructure of the hyperplasias, adenomas, and carcinomas revealed morphological structures (e.g., lamellar bodies, tubular myelin) which are associated with type II cells. Thus, these data suggest that the majority of neoplasms in the lung begin as type II cell proliferations with progression to adenomas and carcinomas within the areas of hyperplasia. The lack of agreement between biochemical and morphological findings makes it difficult to hypothesize a cell of origin for the pulmonary neoplasms. In contrast to the lung, tumors were induced in the liver and nasal passages only after exposure to high doses of NNK. Moreover, both the formation of DNA adducts and cytotoxicity appear obligatory for the generation of tumors in these tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

The chemotherapeutic potential of glycol alkyl ethers: structure-activity studies of nine compounds in a Fischer-rat leukemia transplant model

Structure-activity studies with nine glycol alkyl ethers were conducted with a cellular leukemia transplant model in male Fischer rats. This in vivo assay measures the effects of chemical treatment on neoplastic progression in transplant recipients. Chemicals were given ad libitum in the drinking water simultaneously with the transplants and continued throughout the study. In all, 20 million leukemic cells were injected s.c. into syngeneic rats, which after 60 days resulted in a 10-fold increase in relative spleen weights, a 100-fold increase in white blood cell counts, and a 50% reduction in red blood cell (RBC) indices and platelet counts. At this interval, ethylene glycol monomethyl ether (2-ME) given at a dose of 2.5 mg/ml in the drinking water completely eliminated all clinical, morphological, and histopathological evidence of leukemia, whereas the same dose of ethylene glycol monoethyl ether (2-EE) reduced these responses by about 50%. Seven of the glycol ethers were ineffective as anti-leukemic agents, including ethylene glycol, the monopropyl, monobutyl, and monophenyl ethylene glycol ethers, diethylene glycol, and the monomethyl and monoethyl diethylene glycol ethers. 2-ME more than doubled the latency period of leukemia expression and extended survival for at least 210 days. A minimal effective dose for a 50% reduction in the leukemic responses was 0.25 mg/ml 2-ME in the drinking water (15 mg/kg body weight), whereas a 10-fold higher dose of 2-EE was required for equivalent antileukemic activity. In addition, the in vitro exposure of a leukemic spleen mononuclear cell culture to 2-ME caused a dose- and time-dependent reduction in the number of leukemia cells after a single exposure to 1-100 microM concentrations, whereas the 2-ME metabolite, 2-methoxyacetic acid, was only half as effective. The two glycol alkyl ethers with demonstrable anti-leukemic activity, 2-ME and 2-EE, also exhibited a favorable efficacy-to-toxicity ratio and should be considered for further development as chemotherapeutic agents.

Normal histology of the nasal cavity and application of special techniques

There are three major epithelial types in the nasal mucosa, in addition to numerous accessory structures, some of which are species specific. Without careful and consistent processing of the nose tissue, histopathologic assessment of lesions in the nasal cavity may be compromised. While formalin fixation may be used for routine review of the nasal cavity, Bouin's fixation provides better histologic detail and fewer artifacts. Decalcification is not recommended for nasal tissues to be examined by transmission electron microscopy because of the detrimental effect of decalcifying solutions on sensory cells. Three levels of the nasal cavity may be used for routine histologic review of the nasal cavity, but four or five levels may be more appropriate for certain studies.

Effects of mononuclear cell leukemia on altered hepatocellular foci in Fischer 344 rats

Quantitative stereologic analyses were conducted on hematoxylin and eosin-stained liver sections to investigate the potential effects of the presence of mononuclear cell leukemia in Fischer 344 rats on the occurrence of altered hepatocellular foci (AHF). The study consisted of 132 male and 144 female rats taken from control groups at the termination of seven, 2-year National Toxicology Program (NTP) carcinogenicity studies. A minimum of 10 male and 10 female rats were killed at 6, 9, 12, 15, and 18 months into the seven NTP studies to assess progressive development of AHF. At the end of the studies, 43 males and 35 females had histologic evidence of mononuclear cell leukemia in the liver. There were no differences in the morphologic features of AHF between leukemic and non-leukemic rats; however, there was a decreased incidence of clear AHF in both sexes and in basophilic, vacuolated, and mixed-cell AHF in males. Stereologic analysis revealed that there was also a 40 to 73% reduction in the density of basophilic, clear, vacuolated, and mixed-cell AHF in male rats with leukemia and a 31 and 70% reduction in basophilic and clear AHF, respectively, in females with leukemia. The number of eosinophilic AHF was statistically unchanged in both sexes. The eosinophilic AHF, however, were 2.3 times larger and occupied a 4.3-fold greater volume fraction of the liver in leukemic male rats. Changes in the incidence and density of AHF were directly associated with the severity of the leukemia.(ABSTRACT TRUNCATED AT 250 WORDS)

ras gene activation in rat tumors induced by benzidine congeners and derived dyes

Dimethoxybenzidine (DMO) and dimethylbenzidine (DM) are used to synthesize dyes such as C.I. Direct Blue 15 and C.I. Acid Red 114, respectively. These commercially used dyes are metabolically degraded to DMO or DM in the intestinal tract of rodents and subsequently DMO and DM are absorbed into the blood stream. Animals were exposed to DMO, DM, or the dyes in the drinking water. Tumors obtained from control and chemical-treated animals were examined for the presence of activated oncogenes by the NIH 3T3 DNA transfection assay. Activated oncogenes were detected in less than 3% (1/38) of the tumors from control animals whereas 68% (34/50) of the tumors from chemical-treated animals contained detectable oncogenes. Activated oncogenes were detected in both malignant (25/36) and benign (9/14) tumors from the chemically treated animals but only in one of 13 malignant tumors from the control animals. The presence of oncogenes in the chemically induced benign tumors suggests that oncogene activation was an early event in those tumors. Southern blot analysis of transfectant DNA showed that the transforming properties of the chemically induced rat tumor DNAs were due to the transfer of an activated H-ras (31/34) or N-ras (3/34) gene. One spontaneous rat tumor DNA was found to contain an activated H-ras gene. Oligonucleotide hybridization analysis indicated that the H-ras oncogenes from chemical-associated tumors contained mutations at codons 12, 13, or 61 whereas the spontaneously activated H-ras gene contained a point mutation at codon 61. These data suggest that activation of cellular ras genes by point mutation is an important step in the induction of tumors, at least in rats, by this class of benzidine-derived dyes. Moreover, in light of common histogenesis of the normal counterparts of many of the chemically induced neoplasms and histological evidence of varied tissue differentiation in some basal cell neoplasms, it is possible that most or all of the chemically induced neoplasms were derived from a common epidermal progenitor stem cell population.

Relationship between the formation of promutagenic adducts and the activation of the K-ras protooncogene in lung tumors from A/J mice treated with nitrosamines

Lung and liver tumors were induced in female A/J mice after treatment for 7 weeks (3 times/week, i.p.) with either 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) (50 mg/kg) or nitrosodimethylamine (NDMA) (3 mg/kg). Both compounds can be activated via alpha-hydroxylation to methylating agents, while NNK may also undergo hydroxylation at the N-methyl carbon to form a pyridyloxobutylated adduct. The purpose of these studies was to identify and characterize the activated oncogenes present in tumors induced by NDMA and NNK. Following transfection of high molecular weight DNA onto NIH/3T3 mouse fibroblasts, transforming genes were detected in 90% of both NNK- (10 of 11) and NDMA- (9 of 10) induced lung tumors. In contrast, transformation of NIH/3T3 fibroblasts was observed only in 40% (2 of 5) and 13% (1 of 8) of the liver tumors from NNK- and NDMA-treated mice, respectively. Southern blot analysis indicated that the transforming gene present in all lung tumors was an activated K-ras oncogene. Both rearranged bands and amplified signals were detected in the transfectants. The one transformant from the NDMA-induced liver tumor contained an activated K-ras gene. In contrast, the two liver transformants from NNK-induced tumors did not contain an activated ras or raf gene. Hybridization with oligonucleotide probes that were centered around either codon 12 or 61 of the K-ras gene were utilized to localize the mutations. Activation of this gene appeared to occur largely via a mutation in codon 12 (15 of 20 transformants) and was observed with a similar frequency in pulmonary tumors induced by either compound. The remaining mutations were found in codon 61. The specific mutation within these two codons was determined by amplifying the exon containing the base change, followed by direct sequencing. With one exception the mutation observed in codon 12 was a GC to AT transition (GGT to GAT). One transformant contained a GC to TA transversion. The activating mutation detected in codon 61 was always an AT to GC transition of the middle A (CAA to CGA). The GC to AT mutation observed in codon 12 is consistent with the formation of the O6-methylguanine adduct. Similar concentrations (23 to 32 pmol/mumol deoxyguanosine) of this promutagenic adduct were detected in lungs during treatment with either NNK or NDMA.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple-site carcinogenicity of benzene in Fischer 344 rats and B6C3F1 mice

Toxicology and carcinogenesis studies of benzene (CAS No. 71-43-2; greater than 99.7% pure) were conducted in groups of 60 F344/N rats and 60 B6C3F1 mice of each sex for each of three exposure doses and vehicle controls. These composite studies on benzene were designed and conducted because of large production volume and widespread human exposure, because of the epidemiologic association with leukemia, and because previous experiments were considered inadequate or inconclusive for determining carcinogenicity in laboratory animals. Using the results from 17-week studies, doses for the 2-year studies were selected based on clinical observations (tremors in higher dosed mice), on clinical pathologic findings (lymphoid depletion in rats and leukopenia in mice), and on body weight effects. Doses of 0, 50, 100, or 200 mg/kg body weight benzene in corn oil were administered by gavage to male rats, 5 days per week, for 103 weeks. Doses of 0, 25, 50, or 100 mg/kg benzene in corn oil were administered by gavage to female rats and to male and female mice for 103 weeks. Ten animals in each of the 16 groups were killed at 12 months, and necropsies were performed. Hematologic profiles were performed at 3-month intervals. For the 2-year studies, mean body weights of the top dose groups of male rats and of both sexes of mice were lower than those of the controls. Survivals of the top dose group of rats and mice of each sex were reduced; however, at week 92 for rats and week 91 for mice, survival was greater than 60% in all groups; most of the dosed animals that died before week 103 had neoplasia. Compound-related nonneoplastic or neoplastic effects on the hematopoietic system, Zymbal gland, forestomach, and adrenal gland were found both for rats and mice. Further, the oral cavity was affected in rats, and the lung, liver, Harderian gland, preputial gland, ovary, and mammary gland were affected in mice. Under the conditions of these 2-year gavage studies, there was clear evidence of carcinogenicity of benzene in male F344/N rats, female F344/N rats, male B6C3F1 mice, and female B6C3F1 mice. In male rats, benzene caused increased incidences of Zymbal gland carcinomas, squamous cell papillomas and squamous cell carcinomas of the oral cavity, and squamous cell papillomas and squamous cell carcinomas of the skin. In female rats, benzene caused increased incidences of Zymbal gland carcinomas and squamous cell papillomas and squamous cell carcinomas of the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

Magnetic resonance imaging of hepatic neoplasms in the rat

Magnetic resonance imaging (MRI) at microscopic resolution was done on a live rat that had chemically induced hepatic neoplasms. Beginning at the anterior aspect of the liver, 16 contiguous transaxial slices (each 1.25 mm thick) were produced using three-dimensional Fourier transform sequences. The rat had been treated with diethylnitrosamine (200 mg/kg) at 70 days of age, and, subsequently, received periodic implants of 17a-ethynylestradiol for 60 weeks. Carr-Purcell-Meiboom-Gill (CPMG) sequences (repetition time = 2,000 and echo time = 20, 40, 60, 80 ms) were done to give quantitative measures of spin-spin relaxation times (T2). Pixel-by-pixel curve fitting from these multiple images yielded calculated T2 images. Histologic evaluation of three abnormal areas in the liver revealed solid and cystic hepatocellular adenomas. Although lesions were evident in early-echo images of the CPMG sequence, they were more apparent in the late-echo images. This was consistent with longer T2 relaxation times for the lesions. The voxels of dimensions (230 x 230 x 1,250 microns) permitted resolution of volume elements less than 0.07 mm3. This in turn permitted clear delineation of focal lesions less than 3 mm in diameter. The potential for MRI at microscopic resolution in toxicologic research is clearly demonstrated.

Implanted coil MR microscopy of renal pathology

Inductively coupled implanted coils have been shown to provide up to a 10-fold increase in signal-to-noise ratio when compared to whole-body imaging of small animals. The current study was designed to extend the implanted coil imaging technique to a rodent model of renal pathology. Resonant radiofrequency (RF) coils were implanted around the left kidney of four rats and inductively coupled from within a birdcage body coil. All images were acquired at 2 T using a T1-weighted spin-echo sequence with TR = 500 ms and TE = 20 ms. In vivo MR microscopy with voxels of 117 x 117 x 2000 microns demonstrated cortex, inner and outer medulla, and major vascular structures on baseline images. Mercuric chloride-induced nephrotoxic acute tubular necrosis (ATN) diminished cortico-medullary contrast at 24 h after dosing with pathologic evaluation demonstrating nephrotoxic changes in the inner cortex. The kidney regained a baseline MR appearance 360 h after dosing and resolution of the damage was confirmed with histology. T1 data were gathered on excised kidneys as an adjunct to the images to help correlate the loss and return of cortico-medullary contrast with the pathology and pathophysiology of nephrotoxic ATN. With implanted RF coils we were able to demonstrate renal pathology and follow its subsequent resolution. Specifically, loss and return of cortico-medullary contrast as a result of nephrotoxic ATN were serially documented in four rats. Such serial in vivo studies performed on single animals should further the use of MR microscopy by minimizing the number of animals required for adequate biostatistics.

Activation of the Ki-ras protooncogene in spontaneously occurring and chemically induced lung tumors of the strain A mouse

The strain A mouse has a high incidence of spontaneous lung tumors and is susceptible to lung tumor induction by chemical carcinogens. By utilizing transfection assay, Southern blot analysis, and DNA amplification techniques, we have detected an activated Ki-ras gene in the DNAs of both spontaneously occurring and chemically induced lung tumors of strain A mice. The point mutations in the spontaneous lung tumors were in both codon 12 (60%) and codon 61 (30%). In contrast, 100% of the mutations in the Ki-ras gene detected in methylnitrosourea-induced lung tumors and 93% of the mutations in the Ki-ras genes detected in benzo[a]pyrene-induced lung tumors were in codon 12, whereas 90% of the mutations in the Ki-ras genes detected in ethyl carbamate-induced lung tumors were in codon 61. The selectivity of mutations in the Ki-ras oncogene observed in chemically induced tumors, as compared to spontaneous tumors, suggests that these chemicals directly induce point mutations in the Ki-ras protooncogene. These data indicate that the strain A mouse lung tumor model is a very sensitive system to detect the ability of chemicals to activate the Ki-ras protooncogene in lung tissue.

Observations on altered hepatocellular foci in National Toxicology Program two-year carcinogenicity studies in rats

Retrospective characterization of morphological and stereological features of altered hepatocellular foci (AHF) in hematoxylin & eosin (H&E)-stained sections was performed on 6 conventional 2-yr carcinogenicity studies conducted in Fischer 344 (F344) rats by the National Toxicology Program (NTP). In 3 of these studies where there was clear evidence of hepatocarcinogenicity [1-amino-2,4-dibromoanthraquinone (ADBAQ), C.I. Acid Red 114, methyl carbamate], there was greater morphological variability in AHF than in the studies of chemicals that were not hepatocarcinogenic [4-hydroxyacetanilide, epinephrine, dimethoxane]. In addition to having the expected types of AHF, rats treated with ADBAQ, C.I. Acid Red 114, and methyl carbamate had atypical basophilic AHF. In addition, atypical eosinophilic AHF were present in rats treated with ADBAQ. Both types of atypical AHF showed a morphological spectrum and sequential changes suggesting they could develop into hepatocellular neoplasms. For the 3 liver tumor positive studies, there were dose and time-dependent increases in stereological parameters for the atypical as well as commonly occurring clear, vacuolated, and mixed cell AHF. Consistent stereological changes were not found for commonly occurring basophilic and eosinophilic AHF. Aside from some decreases in stereological measurements in some rats treated with 4-hydroxyacetanilide and epinephrine, there were no significant quantitative changes in AHF in the three liver tumor negative studies. These results show that hepatocarcinogens may induce unique types of AHF in conventional 2-yr carcinogenicity/toxicity studies in rats and may cause quantitative increases in commonly occurring clear, vacuolated, and mixed cell AHF. Such qualitative and quantitative changes are potentially useful predictors of hepatic neoplasia.

Morphological and stereological characterization of hepatic foci of cellular alteration in control Fischer 344 rats

Quantitative evaluation of altered hepatocellular foci (AHF), followed by stereological analysis was performed on standard hematoxylin and eosin-stained liver sections from control Fischer 344 (F344) rats of both sexes from seven 2-yr carcinogenicity studies conducted by the National Toxicology Program (NTP). Liver samples were collected at 6, 9, 12, 15, 18, and/or 24 months on study. Although AHF had a broad spectrum of morphological features, they could be classified into the following 5 types using previously published criteria: basophilic, eosinophilic, clear, vacuolated, mixed cell foci. Approximately 50% of the animals had foci at 6 months, and the incidence reached nearly 100% at 15 months in both sexes. The number, size and volume fraction of AHF increased with age in both sexes; these changes were most evident for basophilic and clear cell foci. The number of basophilic foci was significantly greater in females than in males while clear cell foci were more numerous in males. This sex difference was observed at each time point. Mean number of all types of AHF in males and females at 24 months was 547 and 460 per cubic centimeter of liver, respectively. Despite the high incidence of AHF in control rats, the incidence of hepatocellular neoplasms is low. The implication is that most foci do not progress to neoplasia in control F344 rats used in 2-yr studies.

Use of rat liver altered focus models for testing chemicals that have completed two-year carcinogenicity studies

Partial hepatectomy (PH) and neonatal rat short-term liver focus models were used to examine the effects of selected chemicals that had been previously tested in the National Toxicology Program (NTP) 2-yr carcinogenicity studies. C.I. Solvent Yellow 14, monuron, chlorendic acid, and 4-hydroxyacetanilide were tested for initiating and promoting activity in the PH model. Chlorendic acid, 4,4'-oxydianiline, 1-amino-2,4-dibromoanthraquinone (ADBAQ), and 4-hydroxyacetanilide were similarly tested in a neonatal rat liver focus model. With the exception of 4-hydroxyacetanilide which was not carcinogenic in the NTP studies, all chemicals tested showed clear evidence of hepatocarcinogenicity. While none of the chemicals showed initiating activity in either the PH or neonatal models, promoting activity, as indicated by increased number, size, or volume fraction of histochemically detected hepatic foci of cellular alteration, was evident for all chemicals with previously demonstrated hepatocarcinogenicity. Liver tumor incidence was documented at 14 months in the PH model and at 300 days in the neonatal model. On the basis of the results obtained from these few chemicals, it is suggested that the use of short-term rat liver focus models may represent a reliable means for identifying chemicals with hepatocarcinogenic potential.

Magnetic resonance microscopy of chemically-induced liver foci

Magnetic resonance imaging (MRI) is a new imaging technique used in clinical diagnosis. This paper describes extension of the technique to basic research applications--specifically detecting and characterizing chemically-induced liver neoplasms and foci of cellular alteration. Two systems have been built that allow spatial microscopic resolution--more than 100,000 x greater than that of earlier efforts. Use of spin-lattice (T1) and spin-spin (T2) relaxation times permits detailed characterization of the tissue.

Documenting foci of hepatocellular alteration in two-year carcinogenicity studies: current practices of the National Toxicology Program

Altered hepatocellular foci (AHF) can be reliably identified in hematoxylin and eosin (H&E)-stained sections of liver from interim and final sacrifice intervals in 2-yr carcinogenicity studies in rats. While most AHF can be categorized on the basis of a defined set of descriptive terms, viz., basophilic, eosinophilic, clear vacuolated, and mixed foci, exposure to hepatocarcinogenic agents may induce unique types of AHF which should be distinguished from those that occur more commonly. It is proposed that unique treatment-associated AHF be classified as atypical AHF and that they be completely described in the pathology narrative accompanying the study. Since profound changes in the number and size of AHF have been documented in Fischer 344 rats with mononuclear cell leukemia, it is recommended that liver focus data from leukemic animals be censored in assessing potential effects of treatment on AHF. At the present time, there are insufficient data to allow routine use of AHF in regulatory decision-making in the absence of a liver tumor response. However, such data may form part of weight-of-evidence considerations used by regulatory bodies when accompanied by a concomitant liver tumor response.

Transplantation studies of preputial gland and epithelial skin neoplasms derived from benzidine-based dye carcinogenicity assays in Fischer 344 male rats

Neoplasms of preputial gland and skin were obtained from Fischer 344 male rats on lifetime drinking water studies of the benzidine congener 3,3'-dimethoxybenzidine, C.I. Direct Blue 15 or C.I. Acid Red 114, bisazobiphenyl dyes derived from 3,3'-dimethoxybenzidine and 3,3'-dimethylbenzidine. Portions of these well differentiated neoplasms were implanted into the left mammary fat pad of Fischer 344 male recipients. The rate of growth, presence of local invasion and distant metastases, and morphologic features were observed following 4 serial transplantations. All implants appeared early, grew rapidly, and were histomorphologically similar to the original neoplasms. Metastases from transplants were observed with both preputial gland and skin tumor lines in serial passages. The transplantation results confirm the malignant nature of these neoplasms.

Regulation of the expression of some genes for enzymes of glutathione metabolism in hepatotoxicity and hepatocarcinogenesis

The reversible stage of tumor promotion, which follows the stage of initiation and precedes that of progression in multistage carcinogenesis, is a unique example of reversible toxicity in biological systems. In order to study the molecular mechanisms involved in the action of promoting agents during this stage, the regulation of the expression of genes for two enzymes of glutathione metabolism, gamma-glutamyl transpeptidase (GGT) and the placental isozyme of glutathione S-transferase (GST-P), was studied under several different conditions of promotion during multistage hepatocarcinogenesis in the rat. Promotion by phenobarbital caused an increased expression of both of these genes in altered hepatic focal lesions, although this was somewhat more variable in the case of the GGT gene. C.I. Solvent Yellow 14, an industrial dye, served as an effective promoting agent. Feeding this dye resulted in a dramatic increase in the expression of GST-P, but not that of GGT in altered hepatic foci. Factors in crude, cereal-based diets inhibited the stage of promotion by diethylnitrosamine, but enhanced promotion by phenobarbital in a synergistic manner. In contrast, at least one purified diet had the converse effect during this stage. The mRNA levels of GST-P were uniformly elevated dramatically in reversible nodules and neoplasms of rat liver that had been induced by diethylnitrosamine and phenobarbital promotion. In contrast, the level of GGT mRNA was somewhat variable, with an occasional neoplasm exhibiting almost a background level of expression of this gene. Therefore, the altered regulation of multiple genes in hepatocytes during the stage of promotion can vary with the promoting agent itself; this process may be related to the heterogeneous gene expression seen in hepatic neoplasms. A possible role for specific DNA sequences in the 5' flanking regions of such genes is considered. In addition, a cDNA clone to the mRNA of human liver GGT was isolated and sequenced. The homology of the coding sequence of the human liver GGT mRNA to that of rat kidney GGT mRNA was striking.

Cell specific differences in O6-methylguanine-DNA methyltransferase activity and removal of O6-methylguanine in rat pulmonary cells

Previous studies have demonstrated that cell specificity exists for the alkylation of DNA from lung cells following treatment of rats with the tobacco specific carcinogen 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The concentration of the promutagenic adduct O6-methylguanine (O6MG) was found to be greatest in Clara cells followed by macrophages, type II cells and alveolar small cells. The purpose of this study was to measure the activity of the repair protein O6-methylguanine-DNA methyltransferase (O6MGMT) and to determine whether differences exist for the removal of O6MG among pulmonary cell types. Constitutive activity of O6MGMT was 2-fold greater in macrophages and type II cells than alveolar small cells and Clara cells. Treatment for 4 days with NNK (10 mg/kg/day) had no effect on O6MGMT activity in macrophages, but decreased activity in alveolar small cells and type II cells by 57 and 84%, respectively. O6MGMT activity was reduced to below limits of detection in Clara cells following treatment with NNK. The effect of NNK on O6MGMT activity was consistent with rates of removal of O6MG in macrophages and Clara cells. The loss of O6MG from DNA of macrophages followed first order kinetics (t1/2 = 48 h) while very little loss of this adduct was observed in Clara cells over an 8 day period following cessation of carcinogen treatment. Even though O6MGMT activity was reduced in alveolar small cells and type II cells, approximately 90% of the O6MG bound to DNA in these cell types was removed within 8 days after treatment was discontinued. The loss of O6MG from pulmonary cells appears to result largely from the removal of this adduct by O6MGMT since rates of cell turnover were very low (0.5-1.5%/day) in the lung and were not affected by treatment with NNK. This study indicates that the activity of O6MGMT and the rate of resynthesis of this repair enzyme differ considerably among pulmonary cells following the methylation of DNA. The high concentration of O6MG in Clara cells and the low rate of repair of this promutagenic adduct may be critical factors in the potent pulmonary carcinogenicity induced by the tobacco specific carcinogen NNK.

Increases in cytochrome P-450 mediated 17 beta-estradiol 2-hydroxylase activity in rat liver microsomes after both acute administration and subchronic administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin in a two-stage hepatocarcinogenesis model

Administration of single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (10 micrograms/kg) increased estradiol 2-hydroxylase (E2OHase) activity approximately 2-fold in liver microsomes of female rats but had no effect on E2OHase activity in hepatic microsomes of male rats. In contrast, TCDD increased P-450d (an enzyme which has a high turnover number for E2OHase in a reconstituted enzyme system) 10- to 20-fold in livers of both male and female rats. The discrepancy between the increases in P-450d and E2OHase activity in liver microsomes of TCDD-induced rats was abolished by the addition of exogenous purified P-450 reductase to the microsomal assays for E2OHase, suggesting that reductase was limiting in the in vitro assays. When E2OHase activity was assayed in the presence of exogenous reductase, TCDD increased E2OHase 2-fold and 4-fold respectively in liver microsomes of male and female rates. Antibody to P-450d completely inhibited the increase in E2OHase activity in liver microsomes of TCDD-treated male and female rats, but had little effect on E2OHase activity in liver microsomes of untreated male or female rats. These data indicate that P-450d is responsible for the increase in E2OHase activity in TCDD-treated rats, but other P-450 isozymes are responsible for constitutive E2OHase activity. Biweekly administration of 1.4 micrograms/kg of TCDD for 30 weeks as a potential promoter of hepatocarcinogenesis increased the volume of the liver occupied by gamma-glutamyl transpeptidase (GGT)-positive foci in livers of female rats given a necrogenic dose of diethylnitrosamine (DEN) (200 mg/kg) as the initiator. Biweekly doses of 0.14-1.4 micrograms/kg TCDD in this model also increased P-450d (7-fold) and E2OHase activity maximally (4-fold) in DEN-initiated rats. Moreover, initiation with DEN substantially enhanced the effects of the low dose of TCDD on both hepatic microsomal P-450d and E2OHase activity.

Comparative developmental and phenotypic properties of altered hepatocyte foci and hepatic tumors in rats

Previous investigations in this laboratory have provided evidence that histochemically detectable altered hepatocyte foci and hepatic tumors appearing in rats given a single neonatal treatment with a low dose of carcinogen followed by chronic dietary phenobarbital administration are developmentally independent. The present investigation further evaluates developmental relationships among these lesions. Altered hepatocyte foci were divided into two subclasses consisting of foci that were detectable by histochemical as well as by hematoxylin-eosin staining [designated hist(+)/morph(+) foci] and those foci that were detectable solely by histochemical staining [designated hist(+)/morph(-) foci]. The developmental and phenotypic properties of the hist(+)/morph(-) foci, hist(+)/morph(+) foci, and hepatic tumors were compared in rats initiated once neonatally with different doses of diethylnitrosamine and promoted with dietary phenobarbital from weaning. The morph(+) and morph(-) lesion subclasses were distinguishable on the basis of several developmental characteristics. Hist(+)/morph(+) foci were present at low frequency until at least 150 days after initiation. Although the development of hist(+)/morph(-) foci was essentially complete at that point, the rate of appearance of hist(+)/morph(+) increased significantly. The diethylnitrosamine dose response of the hist(+)/morph(+) foci followed the histochemical marker patterns of the tumor lesion class more closely than that of the hist(+)/morph(-) group. The rates of expression of the hist(+)/morph(+) foci increased with the increasing level of histochemical complexity, whereas the rates of expression of the hist(+)/morph(-) foci groups were inversely correlated to their complexity level. Although the average focus size or diameter in the hist(+)/morph(+) groups was greater than that of the hist(+)/morph(-) foci, the focus growth rates of morph(+) and morph(-) subsets matched for histochemical phenotype were comparable. The complexity level and individual marker distribution patterns of the hist(+)/morph(+) focus class were more similar to tumor patterns than to the distribution patterns of the hist(+)/morph(-) lesion class. The results suggest the following. (a) The development of lesion classes with successively greater deviation from normalcy does not occur via lineal progression from less to more deviated forms within a given lesion class. The three lesion classes appear to develop independently, with the developmental characteristics of each lesion class determined at the time of initiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogene activation in spontaneous and chemically induced rodent tumors: implications for risk analysis

The validity of rodent tumor end points in assessing the potential hazards of chemical exposure to humans is a somewhat controversial but very important issue since most chemicals are classified as potentially hazardous to humans on the basis of long-term carcinogenesis studies in rodents. The ability to distinguish between genotoxic, cytotoxic, or receptor-mediated promotion effects of chemical treatment would aid in the interpretation of rodent carcinogenesis data. Activated oncogenes in spontaneously occurring and chemically induced rodent tumors were examined and compared as one approach to determine the mechanism by which chemical treatment caused an increased incidence of rodent tumors. Different patterns of activated oncogenes were found not only in spontaneous versus chemically induced mouse liver tumors but also in a variety of spontaneous rat tumors versus chemically induced rat lung tumors. In the absence of cytotoxic effects, it could be argued that the chemicals in question activated protooncogenes by a direct genotoxic mechanism. These results provided a basis for the analysis of activated oncogenes in spontaneous and chemically induced rodent tumors to provide information at a molecular level to aid in the extrapolation of rodent carcinogenesis data to human risk assessment.

The effects of allyl isovalerate on the hematopoietic and immunologic systems in rodents

Female B6C3F1 mice plus male and female Fischer 344/N rats were gavaged with allyl isovalerate (AIV) in corn oil at 0, 31, 62, or 125 (mice) and 0, 31, 62, 125, or 250 (rats) mg/kg body weight for five daily exposures per week for a 2-week period. Hematologic, immunologic, and histopathologic studies were performed 48 to 72 hr following the final treatment. AIV exposure had no effect on hematology or bone marrow cellularity in mice or rats. AIV exposure at 250 mg/kg was toxic to rats causing reduced weight gain and hepatotoxicity. In vivo and in vitro studies revealed that pluripotent hematopoietic stem cells (CFU-S) and granulocyte-macrophage progenitors (CFU-GM) in the bone marrow were decreased in the treated mice. Hematopoietic suppression was correlated with the reduction in the hexose monophosphate shunt metabolism of bone marrow cells but the Embden-Meyerhof pathway and tricarboxylic acid pathway enzymes did not appear to be affected. Examination of host resistance following Plasmodium and Listeria challenge did not demonstrate significant differences between treated and control mice, nor were there other effects on the immune system. This suggests that the myelotoxic effects were minimal and of a degree that would not alter host resistance.

Transplantation characteristics, morphologic features, and interpretation of preputial gland neoplasia in the Fischer 344 rat

Preputial gland neoplasms in the Fischer 344 rat are relatively uncommon tumors with a prevalence of approximately 3% in the National Toxicology Program data base. They occur late in life, are well differentiated, and rarely metastasize. Based on studies through 4 serial passages, 10 well-differentiated preputial gland neoplasms transplanted into the mammary fat pads of syngeneic recipients grew to 30 mm within 10 weeks. Recipients died or were sacrificed with large transplanted tumors within 6 months. The morphologic features of the transplanted neoplasms were similar to those of the primary neoplasms through the four passages. Proliferative lesions of the preputial glands comprise a morphological continuum and separation of these growths into categories of hyperplasia, adenoma, and carcinomas is based largely on cytological features and the degree of altered growth patterns. Morphologic features to assist in diagnosis of preputial gland neoplasms and recommendations for interpreting treatment-associated increases of these neoplasms are presented.

Magnetic resonance imaging (MRI): a new tool in experimental toxicologic pathology

Magnetic Resonance Imaging (MRI) is a noninvasive imaging technique that provides multidimensional images of the soft tissues of the body. This imaging technique has proven to be an excellent diagnostic and experimental tool for the detection of pathologic alterations in soft tissues, as well as an adjunct screening method for following the genesis, progression, or regression of chemically induced lesions in the same live animal. Future applications of MRI technology in small animals include MRI microscopy, mapping of vascular or circulatory alterations, measurement of perfusion and diffusion rates of body fluids, and acquisition of cell metabolic states in combination with Nuclear Magnetic Resonance (NMR) spectroscopy, all of which will contribute immensely to the advancement of toxicologic and biomolecular research.

Effect of intratracheal gallium arsenide administration on delta-aminolevulinic acid dehydratase in rats: relationship to urinary excretion of aminolevulinic acid

Exposure to gallium arsenide (GaAs) is a potential hazard in the semiconductor industry and there is a need for specific biological indicators of exposure/toxicity for this compound. These studies examined effects of GaAs exposure on the heme biosynthetic pathway enzyme delta-aminolevulinic acid dehydratase (ALAD). Male CD rats received GaAs suspensions at doses of 50, 100, or 200 mg/kg via a single intratracheal instillation. Six days after treatment a dose-dependent inhibition of blood ALAD was observed with activity decreasing to 5% of controls at the highest dose, with a concomitant marked increase in the urinary excretion of aminolevulinic acid (ALA). Inhibition of blood ALAD following administration of GaAs was maximal (30% of control) 3 to 6 days postexposure and returned to approximately control values on day 18. Urinary excretion of ALA was maximal 3 to 6 days postexposure and recovered toward control values at 18 days. Inhibition of kidney and liver ALAD following GaAs exposure was also evident. Intratracheal instillation of silica did not alter the activity of ALAD in blood, liver, or kidney. Marked increases in lung wet weight/body weight ratios were evident in lungs of silica- and GaAs-treated rats. Histopathological changes in the lungs were characterized by multifocal granulomas following silica treatment and Type II pneumocyte hyperplasia following GaAs treatment; mild necrosis was evident in both groups. Rats treated with 100 mg/kg GaAs exhibited swelling of kidney proximal tubule mitochondria 6 days following exposure. Silica and GaAs exposure produced marked decreases in cumulative weight gain. The concentration of gallium required to achieve half-maximal inhibition of ALAD in vitro was 200-fold less in blood and 40-fold less in kidney and liver than that required for arsenite and the inhibition was partially prevented by excess zinc. These data suggest that gallium is the primary inhibitor of ALAD following dissolution of GaAs in vivo and that competition for or displacement of zinc from the enzyme active site may be involved in the mechanism of inhibition. The data also demonstrated the utility of including a particulate control group when assessing the chemical-induced toxicity of compounds administered intratracheally as particulate suspensions. Finally, measurement of heme precursors, e.g., ALA, in urine coupled with assay of red blood cell ALAD activity may be of value as an early biological indicator of GaAs exposure and/or toxicity.

Lack of a relationship between immune function and chemically induced hepatocarcinogenesis in B6C3F1 mice

The relationship between immune function and chemically induced hepatocarcinogenesis was studied employing an in vivo murine model. Neonatal B6C3F1 mice were given a single carcinogenic dose of diethylnitrosamine (DEN) and the time-response kinetics for the early (foci of alteration) and late (adenomas/carcinomas) phases of hepatocellular carcinogenesis were compared to changes in hematopoiesis and immune functions associated with immune surveillance and natural resistance. Increases in hematopoiesis occurred just prior to or concurrent with the appearance of hepatocellular carcinomas, while increased macrophage and natural killer cell cytotoxicity and suppression of cell-mediated immunity occurred following tumor appearance and progressed with increasing tumor burden. Neither immunological nor hematopoietic changes were associated with early phases of hepatocarcinogenesis, as monitored by the appearance of altered hepatocellular foci. Although changes in hematopoiesis may represent an early indicator for hepatocarcinogenesis in the mouse tumor model, the data suggest that altered immune surveillance and natural resistance are not factors in the development of chemically induced hepatocellular tumors, and the changes in immune function are probably secondary to tumor development.

Molecular dosimetry of DNA adduct formation and cell toxicity in rat nasal mucosa following exposure to the tobacco specific nitrosamine 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone and their relationship to induction of neoplasia

The molecular dosimetry of O6-methylguanine (O6MG) formation in DNA and cytotoxicity in respiratory and olfactory mucosa was determined during administration of 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) to male Fischer 344 rats. The dose response for O6MG formation differed considerably between respiratory and olfactory mucosa. The dose response was nonlinear in respiratory mucosa where the slope of the curve was very large for doses of NNK ranging from 0.3 to 3.0 mg/kg but much smaller in the dose range of 10 to 100 mg/kg. In contract, the dose response in the olfactory mucosa did not demonstrate such a large change in slope over the same dose range. The concentration of O6MG formed to dose of NNK ratio, an index of efficiency of alkylation, increased dramatically only in the respiratory mucosa as the dose of NNK was decreased from 100 to 0.3 mg/kg. The concentration of O6MG was four times greater in respiratory than olfactory mucosa after treatment of rats with 1 mg/kg NNK. Alkylation in the two regions of the nose became similar as the dose of NNK was increased. In rats treated for up to 12 days with NNK (10 mg/kg/day), the concentration of O6MG was 60 to 90% greater in respiratory than olfactory mucosa throughout treatment. Regional differences in the amount of O6MG formed may stem from the presence of a low Km pathway for biotransformation of NNK in the cells of the respiratory mucosa. This conclusion is supported by autoradiographic studies. Four h after treatment with 1 mg/kg [3H]NNK, silver grains were more heavily concentrated in respiratory than olfactory epithelium. Histopathological examination of the nasal passages revealed dose related, cell specific differences in toxicity following treatment of rats with 10, 30, or 100 mg/kg NNK for 12 days. No toxicity was observed in the nose when 1 mg/kg NNK was administered. Bowman's glands underlying the olfactory mucosa and Steno's glands were the most sensitive sites for toxicity, exhibiting necrosis after as little as 2 days of treatment with 10 mg/kg NNK. Damage to these glands progressed in a dose- and time-dependent manner. Respiratory epithelium exhibited only mild toxicity while basal cell metaplasia was evident in olfactory epithelium. Rats treated with NNK for 20 weeks (50 mg/kg, three times a week) had a 45% incidence of carcinomas in the olfactory region. These neoplasms appeared to arise from Bowman's glands. In contrast, there was only a 5% incidence of malignant neoplasia and a 29% incidence of benign neoplasia in the respiratory region.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of age on the toxicity and metabolism of ethylene glycol monobutyl ether (2-butoxyethanol) in rats

Heavy production as well as the diversity of 2-butoxyethanol (BE) uses, which include preparation of products intended for household uses, pose a high risk of human exposure to BE. The current studies were designed to investigate the acute toxicity of BE and to evaluate the effect of age on BE-induced toxicity in F344 male rats. Data presented in this report show that BE causes severe acute hemolytic anemia resulting in significant increases in the concentration of free plasma hemoglobin. Secondary to the hemolytic effects, BE also caused hemoglobinuria as well as histopathologic changes in the liver and kidney. These effects of BE were dose- and time-dependent. Further, both the hemolytic effects and the secondary effects of BE were age dependent with older rats being more sensitive than younger rats. The metabolic basis of the greater susceptibility of older rats to BE-induced toxicity was investigated by comparing BE metabolism in adult (9- to 13-week-old) and young (4- to 5-week-old) rats. These studies revealed that there was a significantly higher portion of the administered dose eliminated by young rats as CO2 as compared to that eliminated by older rats. Similarly, a significantly higher portion of the administered dose was excreted in the urine of young rats. HPLC analysis of the urinary metabolites of BE in adult and young rats showed that the ratio of butoxyacetic acid (BAA)/BE-glucuronide + BE - sulfate (previously thought to reflect an activation/detoxification index of BE; see text) was significantly higher in older rats. We currently believe that the increase in the activation/detoxification index in older rats is caused by decreased degradation of BAA to CO2 (as evident by the lower percentage of the dose excreted as CO2 by older rats) and by depressed urinary excretion of BAA (as evident from the lower percentage of the dose excreted in the urine of older rats.

The role of oncogenes in chemical carcinogenesis

Proto-oncogenes are cellular genes that are expressed during normal growth and developmental processes. Altered versions of normal proto-oncogenes have been implicated in the development of human neoplasia. In this report, we show the detection of activated proto-oncogenes in various spontaneous and chemically induced rodent tumors. The majority of activated proto-oncogenes found in these tumors are members of the ras gene family and have been activated by a point mutation. Characterization of the activating mutation may be useful in determining whether this proto-oncogene was activated by direct interaction of the chemical with the DNA. Comparison of activating lesions in spontaneous versus chemically induced tumors should be helpful in determining whether the chemical acts via a genotoxic or a nongenotoxic mechanism. All of this information may be helpful in the assessment of potential carcinogenic hazards of human exposure to chemicals.

A method to quantitate the relative initiating and promoting potencies of hepatocarcinogenic agents in their dose-response relationships to altered hepatic foci

The relative response to various initiating doses of diethylnitrosamine (DEN) and dimethylbenz[a]anthracene of the induction of numbers and size (vol. % of liver) of altered hepatic foci (AHF) in livers of adult female rats of the Sprague-Dawley and Fischer 344 (F-344) strains was studied by methods of quantitative stereology in the presence and absence of the promoting agent, phenobarbital (PB, 0.05% in the diet). In all cases, a relatively linear response with dose, even at the lowest doses employed, was obtained except for the numbers of AHF at the highest dose of DEN (30 mg/kg), which was not significantly different from that at a dose of 10 mg/kg in F-344 female rats. Similar dose-response data were obtained at various doses of two promoting agents effective in hepatocarcinogenesis, PB and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), in livers of F-344 female rats following initiation with DEN (10 mg/kg) 24 h post-70% hepatectomy. The response to these agents exhibited threshold levels below which no increase in number or vol. % of liver of AHF was noted in comparison with that in livers of animals not treated with the promoting agents. At several subthreshold doses of both PB and TCDD an inhibition of AHF formation and growth (measured as vol. % of liver) was observed. Based on quantitative stereologic calculations, parameters for the estimation for the relative potency of chemicals as initiating or promoting agents have been established. These are defined as: initiation index = no. of foci induced X liver-1 X [mmol/kg body wt]-1 and promotion index = Vf/Vc X mmol-1 X weeks-1, where Vf is the total volume fraction (%) occupied by AHF in the livers of rats treated with the test agent and Vc is the total volume of AHF in control animals which have only been initiated. These parameters were calculated for a number of agents based on data published in the literature and from those reported herein. Neither parameter varied significantly with the dose of the initiating agent based on the data in this paper. The range of promotion indices extended over more than eight orders of magnitude, whereas that of initiation indices was much less variable. Such parameters may be useful as quantitative estimates of the potency of hepatocarcinogenic agents, such values having potential application to risk estimations.

Activated oncogenes in B6C3F1 mouse liver tumors: implications for risk assessment

The validity of mouse liver tumor end points in assessing the potential hazards of chemical exposure to humans is a controversial but important issue, since liver neoplasia in mice is the most frequent tumor target tissue end point in 2-year carcinogenicity studies. The ability to distinguish between promotion of background tumors versus a genotoxic mechanism of tumor initiation by chemical treatment would aid in the interpretation of rodent carcinogenesis data. Activated oncogenes in chemically induced and spontaneously occurring mouse liver tumors were examined and compared as one approach to determine the mechanism by which chemical treatment caused an increased incidence of mouse liver tumors. Data suggest that furan and furfural caused an increased incidence in mouse liver tumors at least in part by induction of novel weakly activating point mutations in ras genes even though both chemicals did not induce mutations in Salmonella assays. In addition to ras oncogenes, two activated raf genes and four non-ras transforming genes were detected. The B6C3F1 mouse liver may thus provide a sensitive assay system to detect various classes of proto-oncogenes that are susceptible to activation by carcinogenic insult. As illustrated with mouse liver tumors, analysis of activated oncogenes in spontaneously occurring and chemically induced rodent tumors will provide information at a molecular level to aid in the use of rodent carcinogenesis data for risk assessment.

Ovarian toxicity and carcinogenicity in eight recent National Toxicology Program studies

Ovarian toxicity and/or carcinogenicity has been documented for at least eight chemicals recently tested in National Toxicity Program prechronic and chronic rodent studies. The chemicals that yielded treatment-related ovarian lesions were 1,3-butadiene, 4-vinylcyclohexene, vinylcyclohexene deipoxide, nitrofurantoin, nitrofurazone, benzene, delta-9-tetrahydrocannabinol, and tricresylphosphate. Typical nonneoplastic ovarian changes included hypoplasia, atrophy, follicular necrosis, and tubular hyperplasia. The most commonly observed treatment-related neoplasms were granulosa cell tumors and benign mixed tumors. A relationship between antecedent ovarian hypoplasia, atrophy, and hyperplasia and subsequent ovarian neoplasia is supported by some of these National Toxicology Program studies. Pathologic changes in other tissues such as the adrenal glands and uterus were associated with the treatment-related ovarian changes.

Expression of retroviral sequences and oncogenes in rat liver tumors induced by diethylnitrosamine

The expression of three cellular oncogenes (c-myc, c-Ha-ras, and c-delta-raf), the epidermal growth factor receptor gene, and two endogenous retrovirus-like sequences [rat leukemia virus (RaLV) and 30S] was examined in control rat livers and in 16 liver tumors. The tumors were induced in Sprague-Dawley male and female rats by a single i.p. injection of diethylnitrosamine at 1 or 2 days after birth, followed by dietary exposure to phenobarbital beginning at weaning. Increased expression of c-myc was seen in most of the tumors, but there was no consistent increase or decrease in expression of c-Ha-ras or c-delta-raf. It is of interest that a number of the tumor samples showed a decrease in epidermal growth factor receptor RNA. In all of the tumors, including both hepatocellular adenomas and carcinomas, there was a marked increase in expression of the endogenous RaLV sequence, and over 90% of the tumors displayed increased expression of the 30S endogenous retroviral-like sequence. No or a very low level of expression of the RaLV and 30S sequences was found in the control livers. The extent of expression of the RaLV and 30S sequences in individual tumors did not correlate with the extent of expression of c-myc or c-Ha-ras. Although increased expression of certain endogenous retrovirus-related sequences appears to be a common finding during rat liver carcinogenesis, the significance of this finding remains to be determined.

Activation of the K-ras protooncogene in lung tumors from rats and mice chronically exposed to tetranitromethane

Dominant transforming genes were detected in lung tumors from Fischer 344 rats and C57BL/6 X C3H F1 mice chronically exposed by inhalation to tetranitromethane, a highly volatile compound used in several industrial processes. The rat lung neoplasms were classified as adenocarcinomas, squamous cell carcinomas (epidermoid carcinomas), or adenosquamous carcinomas. The mouse lung tumors were classified as papillary adenocarcinomas or adenomas. In both species, the tumors were morphologically similar to lung tumors in humans. The transfection assay using NIH/3T3 mouse fibroblasts detected transforming genes in 74% (14 of 19) of the rat lung tumors and in 100% (4 of 4) of the mouse lung tumors. Southern blot analysis indicated that transforming gene was an activated K-ras protooncogene in both species. The first exon of the K-ras gene in normal DNA and in DNA from two cell lines transformed by tumor DNA was compared by cloning and sequencing the gene. Experiments showed that there was a GC----AT transition in the second base of the 12th codon of the K-ras oncogene in the two transfectant DNAs. Oligonucleotide hybridization indicated that all of the rat and mouse transfectants had this activating lesion. Additional tumor DNA was then tested for the presence of a mutated allele with the GC----AT transition. All of the rat tumors tested and all of the mouse tumors tested had this mutation present. Hybridization using the normal oligonucleotide sequence around the 12th codon indicated that the normal allele was also present in the majority of the tumors, suggesting that the loss of normal allele is not necessary for the development of neoplasia. One rat lung tumor had no normal allele present, possibly suggesting that this tumor could have been in a more advanced stage than the other tumors. This is the first study to detect activated protooncogenes in rodent tumors induced under conditions which mimic human exposure to a chemical in the workplace. Tetranitromethane may exert its carcinogenic action by both activation of the K-ras oncogene and stimulation of cell proliferation by its irritant properties.

Continued development of hepatic gamma-glutamyltranspeptidase-positive foci upon withdrawal of 17 alpha-ethynylestradiol in diethylnitrosamine-initiated rats

Adult ovariectomized Sprague-Dawley rats were administered a single initiating dose of 200 mg diethylnitrosamine (DEN)/kg, i.p. 17 alpha-ethynylestradiol (EE2) was then chronically administered to the rats by means of s.c. Silastic implants at an estimated dose of 90 micrograms EE2/kg/day. Hepatic gamma-glutamyltranspeptidase-positive foci were evaluated after 20-60 (+20,+30,...,+60) weeks of chronic EE2 treatment and after 20 or 30 weeks of EE2 followed by 20 or 30 weeks with no EE2 [(+20,-20), (+30,-20), (+30,-30)] to determine the effects of withdrawal of the promoting agent on the persistence or reversibility of these focal lesions. Our results show that gamma-glutamyltranspeptidase-positive foci are no longer dependent on exogenous EE2 administration for their continued growth in initiated animals given EE2 chronically for 20 weeks. In DEN-initiated, EE2-promoted animals the number of foci per cc (Nv) seen at 20 weeks increased over the next 20 weeks in the absence of further EE2 treatment, but was not statistically different than Nv for continued EE2 treatment. The proportion of total liver volume occupied by foci (Vv) was 0.0054 (+30), 0.0191 (+30,-20), and 0.0135 (+50). The (+30,-20) Vv was significantly different than that for (+30) (P less than 0.01). Hepatocellular adenomas and carcinomas were detected in DEN-initiated and EE2- promoted animals as early as 30 weeks. Hepatic tumor incidence continued to increase after withdrawal of EE2 in initiated animals which had received only 30 weeks of promotion. Within the framework of our studies, the promoting effects of EE2 do not appear to be reversible by withdrawal of EE2 after 20 weeks of treatment. It may be that events or factors which were estrogen dependent in the early stages of promotion are now constitutive.

Application of microencapsulation for toxicology studies. II. Toxicity of microencapsulated trichloroethylene in Fischer 344 rats

Gelatin-sorbitol microcapsules containing 44.1% trichloroethylene (TCE) were prepared and mixed in NIH-07 rodent meal diet and provided at microcapsule concentrations of 0 (untreated control group), 1.25, 2.5, 5.0, or 10% (equivalent to 0, 0.55, 1.10, 2.21, or 4.41% TCE, respectively) to groups of 10 male F344 rats for 14 days. An additional control group received diets containing 5% empty capsules. For comparisons, TCE dissolved in corn oil was administered by gavage to different groups of 10 male rats for 14 consecutive days at dose levels adjusted to correspond to those in the feed study. Treatment-related deaths occurred only in the highest dose group of the gavage study. Body weight gain and feed consumption were reduced in high-dose groups of both the feed and gavage studies. There was no measurable loss of TCE in feed sampled from the cages during the study. Dose-related increases in organ (liver and kidney) weight/body weight ratios, individual cell necrosis in the liver, and hepatic microsomal NADPH cytochrome c reductase and peroxisomal palmitoyl-CoA oxidase and catalase activities were found in both the dosed-fed and gavage groups. Induction of cytochrome P-450 occurred only in the dosed-feed study. There were no significant compound-related pathologic lesions observed in the kidneys, the only other organ examined microscopically. Differences in lethality, cytochrome P-450 levels, and induction of microsomal or peroxisomal enzyme activities were attributed to differences in the method of dosing (gavage versus dosed-feed). The demonstration of no significant loss of TCE from the feed and of similar toxic effects produced by microencapsulated TCE via feed and TCE in corn oil via gavage indicate that microencapsulation can provide an excellent alternative exposure route for studying the oral toxicological properties of volatile chemicals, such as TCE, in rats.