In a 2-year NTP bioassay, Bromoethane (BE) was found to induce endometrial neoplasms in the uterus of B6C3F1 mice [National Toxicology Program, 1989. Toxicology and Carcinogenesis Studies of Bromoethane (Ethyl Bromide) in F344/N Rats and B6C3F1 Mice. TR No. 363, U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, ResearchTriangle Park, NC; Picut, C.A., Aoyama, H., Holder, J.W., Gold, L.S., Maronpot, R.R., Dixon, D., 2003. Bromoethane, chloroethane and ethylene oxide induced uterine neoplasms in B6C3F1 mice from 2-year NTP inhalation bioassays: pathology and incidence data revisited. Exp. Toxicol. Pathol. 55, 1–9]. In women, hormonal influences, such as “unopposed” estrogenic stimulus, have been implicated as important etiologic factors in uterine cancer. BE, however, does not affect the serum concentrations of sex hormones in female B6C3F1 mice [Bucher, J.R., Morgan, D.L., Adkins Jr., B., Travlos, G.S., Davis, B.J., Morris, R., Elwell, M.R., 1995. Early changes in sex hormones are not evident in mice exposed to the uterine carcinogens chloroethane or bromoethane. Toxicol. Appl. Pharmacol. 130, 169–173] and the mechanism of BE-induced uterine carcinogenesis still remains unclear. In the present study, we examined the estrogenic effects of BE on the uterus of ovariectomized B6C3F1 mice and on Ishikawa cells. Groups of 6 mice were given daily s.c. injections of 0, 100, 500 or 1000 mg BE/kg for 3 consecutive days. Mice treated with 17β-estradiol served as positive controls. Mice were necropsied 24 h after the final injection, and uteri were weighed and examined histologically and immunohistochemically along with the vagina. Changes observed in the estrogen-treated mice included increased uterine weights, edema and inflammation of the endometrium, increased epithelial layers of the uterine and vaginal lumens and keratinization of the vaginal epithelium. In the BE-treated mice, no such changes occurred; however, immunohistochemical staining of the uterus revealed a significant increase in immunoexpression of the estrogen receptor alpha (ERα) in the two higher dose groups. Analysis of mRNA also showed slightly increased uterine ERα expression in these groups. Upregulated expression of ERα was confirmed in BE-treated Ishikawa cells, in which Western blotting analyses identified an intense signal at approximately 66 kDa, which is consistent with ERα. These data suggest that upregulated expression of ERα may be important in the induction of endometrial neoplasms in BE-treated mice.

The NTP has a long history of using Fischer rats and has compiled a large database of incidences of lesions seen in control animals. Such a database is lacking for Harlan Sprague–Dawley (SD) rats. The intention of this paper is to report spontaneous lesions observed in female vehicle control Harlan SD rats, and to compare the incidence in 2 strains of rats (Fischer and Harlan SD) used in NTP studies. Female Harlan SD rats served as the test animals for a special series of 2-year studies. Male rats were not used in these studies. Complete histopathology was performed on all animals, and the pathology results underwent comprehensive NTP pathology peer review. The most commonly observed neoplasms in these female control Harlan SD rats were mammary gland fibroadenoma (71%), tumors of the pars distalis of the pituitary (41%) and thyroid gland C-cell tumors (30%). Female Fischer rats had incidences of 44% for mammary gland fibroadenomas, 34% for tumors of the pars distalis, and 16% for thyroid gland C-cell tumors. Fischer rats had a 15% incidence of clitoral gland tumors, while the Harlan SD rats had an incidence of < 1%. In contrast to Fischer F344 rats, the Harlan SD rats had a high incidence of squamous metaplasia of the uterus (44.2%). Squamous metaplasia is not a lesion commonly observed in NTP control Fischer rats. The Harlan SD rats had a very low incidence of mononuclear cell leukemia (0.5%), compared with an incidence of 24% in female Fischer rats.

We evaluated gingival toxicities induced by chronic exposure of female Harlan Sprague-Dawley rats to dioxin and dioxin-like compounds (DLCs) and compared them to similarly induced oral lesions reported in the literature. This investigation represents part of an ongoing initiative of the National Toxicology Program to determine the relative potency of chronic toxicity and carcinogenicity of polychlorinated dioxins, furans, and biphenyls. For two years, animals were administered by gavage 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); 3,3',4,4',5-pentachlorobiphenyl (PCB126); 2,3,4,7,8-pentachlorodibenzofuran (PeCDF); 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153); a tertiary mixture of TCDD, PCB126, and PeCDF; a binary mixture of PCB126 and 153; or a binary mixture of PCB126 and 2,3',4,4',5-pentachlorobiphenyl (PCB118); control animals received corn oil-acetone vehicle (99:1) alone. A full complement of tissues, including the palate with teeth, was examined microscopically. In the groups treated with TCDD and the mixtures of TCDD, PCB126, and PeCDF; PCB126 and 153; and PCB126 and 118, the incidences of gingival squamous hyperplasia increased significantly. Moreover, in the groups treated with TCDD, PCB126, and the mixture of PCB126 and 153, squamous cell carcinoma (SCC) in the oral cavity increased significantly. This investigation constitutes the first report documenting that chronic administration of dioxin-like PCBs can induce gingival SCC in rats. These results indicate that dioxin and DLCs target the gingiva of the oral cavity, in particular the junctional epithelium of molars.

Use of the dioxin toxic equivalency factor (TEF) approach in human risk assessments assumes that the combined effects of dioxin-like compounds in a mixture can be predicted based on a potency-adjusted dose-additive combination of constituents of the mixture. In this study, we evaluated the TEF approach in experimental 2-year rodent cancer bioassays with female Harlan Sprague-Dawley rats receiving 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3 ,4,4 ,5-pentachlorobiphenyl (PCB-126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), or a mixture of the three compounds. Statistically based dose-response modeling indicated that the shape of the dose-response curves for hepatic, lung, and oral mucosal neoplasms was the same in studies of the three individual chemicals and the mixture. In addition, the dose response for the mixture could be predicted from a combination of the potency-adjusted doses of the individual compounds. Finally, we showed that use of the current World Health Organization dioxin TEF values adequately predicted the increased incidence of liver tumors (hepatocellular adenoma and cholangiocarcinoma) induced by exposure to the mixture. These data support the use of the TEF approach for dioxin cancer risk assessments.

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) affects the thyroid morphologically and/or functionally in adult animals. Recently, the National Toxicology Program conducted a 2-year gavage study of TCDD in female Harlan Sprague—Dawley rats. The only treatment-related alterations found in thyroid follicles were decreased luminal size and increased height of the follicular epithelial cells, without prominent protrusion into the lumen. The present study elucidated the nature of these follicular lesions. Thyroid glands of 10 rats each from the control, high (100 ng/kg/day)-dose, and stop-study (100 ng/kg/day, 30 weeks; vehicle to study termination) groups in the 2-year study were evaluated microscopically. Twenty randomly selected follicles were measured morphometrically in each animal. TCDD treatment significantly decreased the mean ratio of luminal/epithelial areas and increased the mean sectional epithelial height of the high-dose group compared to controls. Thyroid sections were immunostained with antibody against minichromosome maintenance (MCM) proteins, a novel cell-cycle biomarker. The MCM labeling index of the high-dose group was significantly higher than that of the control; however, the TUNEL labeling index was also higher in the high-dose group than the control. All data from the stop group were comparable to those from controls. These results indicate that the follicular cell response was hypertrophic and reversible. This information should contribute to diagnosis of nonneoplastic thyroid follicular lesions in rats.

Dipropylene glycol (DPG) is a component of many commercial products such as antifreeze, air fresheners, cosmetic products, solvents, and plastics. Male and female F344/N rats and B6C3F1 mice were exposed to DPG in the drinking water for 2 weeks, 3 months, or 2 years. In the 2-week and 3-month studies, rats and mice were exposed to 0, 5000, 10,000, 20,000, 40,000, or 80,000 ppm DPG. There was no mortality in the 2-week studies. In the 3-month rat study, all animals survived to the end of the study. Liver weights of rats exposed to 10,000 ppm or greater and kidney weights of rats exposed to 40,000 and 80,000 ppm were greater than those of the controls. The incidences of liver and kidney lesions were significantly increased in males exposed to 20,000 ppm or greater and females exposed to 80,000 ppm. Focal olfactory epithelial degeneration was present in all rats exposed to 80,000 ppm. In males, the incidences of testicular atrophy, epididymal hypospermia, and preputial gland atrophy were significantly increased in the 80,000 ppm group. In the 3-month mouse study, three males and one female exposed to 80,000 ppm died. Liver weights were increased, as was the incidence of centrilobular hypertrophy in males exposed to 40,000 ppm and males and females exposed to 80,000 ppm. In the 2-year studies, exposure groups were 0, 2500 (rats only), 10,000, 20,000 (mice only) or 40,000 ppm DPG. Survival of male rats exposed to 40,000 ppm and mean body weights of males and females exposed to 40,000 ppm were significantly less than controls. In male rats, exposure to DPG resulted in increased incidences and severities of nephropathy and secondary lesions in the parathyroid and forestomach. Increased incidences of focal histiocytic and focal granulomatous inflammation of the liver were also observed. In male and female rats, there were increased incidences of bile duct hyperplasia and changes in the olfactory epithelium of the nose. In mice, survival of males and females was similar to controls. Mean body weights and water consumption of males exposed to 40,000 ppm were less than that of the controls. Treatment-related nonneoplastic lesions did not occur in mice. Treatment-related neoplastic lesions did not occur in rats or mice.

We evaluated the effect of chronic exposure to dioxin and dioxin-like compounds on the pancreas in female Harlan Sprague-Dawley rats. This investigation represents part of an ongoing National Toxicology Program initiative to determine the relative potency of chronic toxicity and carcinogenicity of polychlorinated dioxins, furans, and biphenyls. Animals were treated by gavage for up to 2 years with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3,4,4,5-pentachlorobiphenyl (PCB-126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), or a toxic-equivalency-factor (TEF) mixture of these agents; control animals received corn oil-acetone vehicle alone. A complete necropsy was performed on all animals, and a full complement of tissues was collected and examined microscopically. Administration of each of the four compounds was associated with increased incidences of several nonneoplastic changes in the exocrine pancreas, including cytoplasmic vacuolation, chronic active inflammation, atrophy, and arteritis. Low incidences, but higher than those in the historical database, of pancreatic acinar adenoma and carcinoma were seen in the TCDD, PeCDF, and TEF-mixture groups. These results indicate that the pancreatic acini are target tissues for dioxin and certain dioxin-like compounds. Key words: carcinogenesis, dioxin, furans, inflammation, pancreas, polychlorinated biphenyls.

Summary The paper presents a case-study of skin fibromas among male rats in the 2-year cancer bioassay of methyleugenol that was conducted by the US National Toxicology Program (NTP). In animal carcinogenicity experiments such as this one, tumour rates are often compared with the Cochran–Armitage (CA) trend test. The operating characteristics of the CA test, however, can be adversely affected by survival differences across groups and by the assumed dose metric. Survival-adjusted generalizations of the CA test have been proposed, but they are still sensitive to the choice of scores that are assigned to the dose groups. We present an alternative test, which outperforms the survival-adjusted CA test which is currently used by the NTP to compare incidence rates. Simulated data from a wide range of realistic situations show that the operating characteristics of the test proposed are superior to those of the NTP’s survival-adjusted CA test, especially for rare tumours and wide logarithmic spacings of the dose metric.

Propylene glycol mono-t-butyl ether (PGMBE) is used as a solvent in a variety of commercial applications. Male and female F344/N rats and B6C3F1 mice were exposed to PGMBE by whole-body inhalation for 2 or 14 weeks (0, 75, 150, 300, 600, or 1200 ppm) or 2 years (0, 75, 300, or 1200 ppm); male NBR rats were exposed for 2 weeks. The kidney and the liver were targets of PGMBE toxicity in rats. Renal lesions suggestive of α2u-globulin nephropathy were observed in male F344/N, in the 2 and 14-week studies, no kidney lesions were seen in NBR rats. In the 2-year study, male rats displayed exposure-related nonneoplastic lesions in the kidney, and may have shown marginal increases in tubular neoplasms. In the liver, the incidences of hepatocellular adenomas occurred with a positive trend in male rats, and may have been related to PGMBE exposure. In mice of both sexes, the major target of PGMBE toxicity was the liver. In the 2-week study, liver weights and in the 14-week study, liver weights and the incidences of centrilobular hypertrophy were increased. In the 2-year study, the incidences of exposure-related hepatocellular adenoma, adenoma or carcinoma combined, and hepatoblastoma occurred with a positive trend, and were significantly increased in 1200 ppm groups. In summary, exposure to PGMBE resulted in nonneoplastic lesions of the kidney characteristic of α2u-globulin nephropathy, and may have increased renal tubular neoplasms in male rats. Exposure to PGMBE also produced increases in hepatic tumors in male and female mice.

The literature evidencing the role of iron in promoting a range of neoplasms in humans and animals prompted us to search for a possible association between chemically induced hemosiderosis and hemangiosarcomas in the liver of mice in selected studies conducted by the National Toxicology Program (NTP). Its historical control database was examined for studies in which treatment-related liver hemangiosarcoma was noted; 130 consecutive NTP studies in B6C3F1 mice from Technical Report (TR)-340 to TR-493 were evaluated. Three compounds (2-butoxyethanol, p-nitroaniline, and para-chloroaniline) were associated with a relatively high incidence of Kupffer cell pigmentation consisting of hemosiderin in both sexes; only the male mice developed a relatively low incidence of treatment-related hemangiosarcoma. With a fourth compound (o-nitroanisole), a relatively low incidence (16/50, high-dose males) of chemical-related hemosiderosis was noted, with no associated increase of hemangiosarcoma. Two chemicals (pentachlorophenol and tetrafluoroethylene) increased the incidence of liver hemangiosarcoma in male and female mice, with no increase in Kupffer cell pigmentation. The overall association between liver hemangiosarcoma and Kupffer cell pigmentation was highly significant (p < 0.001). The cause for hemosiderosis in all cases was the erythrocytic hemolytic effect of the compounds. The reason for the sex-increased susceptibility for development of hemangiosarcoma is unknown but may be due to a hormone-related, reduced antioxidative defensive capacity through modulation of the activities of antioxidative enzymes.

Diet is an important environmental factor affecting body weight, survival, and age-related diseases of rodents. The NIH-07 open formula diet was the diet used in the National Toxicology Program's (NTPs) rodent carcinogenicity studies from 1980 to 1994. In 1994 the NTP began using a new diet designated the NTP-2000 diet. This paper compares body weight, survival, tumor incidence, and nephropathy severity in untreated control groups of Fischer 344 (F344) rats fed the NTP-2000 or NIH-07 diets, using data from 22 separate 2-year feed and inhalation studies. The feed studies were conducted in 3 different facilities, and all the inhalation studies were conducted in a single facility. During feed studies, rats were group housed in polycarbonate cages and fed diets in powder (mash) form, while in inhalation studies, rats were housed individually in wire mesh cages, and fed diets in pelleted form. Survival was significantly ( p < 0 .05) higher in groups fed NTP-2000 diet compared to the corresponding groups fed NIH-07 diet, irrespective of sex or housing conditions. Use of the NTP-2000 diet was also associated with a decreased incidence of pituitary gland tumors in both sexes and decreased incidences of adrenal pheochromocytoma and preputial gland tumors in males. The incidence and severity of nephropathy was also decreased in animals receiving the NTP-2000 diet, especially males. The decreased nephropathy severity and the decreased incidence of pituitary gland tumors are likely the major factors contributing to the improved survival of rats receiving the NTP-2000 diet relative to those given the NIH-07 diet. These data also support earlier findings that decreased incidences of adrenal pheochromocytoma are associated with reduced nephropathy severity in male F344 rats. Throughout the two-year study female rats receiving the NTP-2000 diet were significantly ( p < 0 .05) lighter than those receiving the NIH-07 diet. However, it is uncertain if this difference can be attributed to the NTP-2000 diet, since implementation of this diet by the NTP approximately coincided with changes in the F344 rat production colony that resulted in somewhat lighter animals being provided to the NTP. Controls from inhalation studies and feed studies differed significantly ( p < 0 .01) in the incidence of a variety of tumors, irrespective of diet. This suggests that differences in animal care and housing protocols may impact tumor incidence in F344 rats, most notably pituitary gland and testis tumors.

The present study characterized the immunohistochemical localization of β-catenin protein in hepatocellular neoplasms and hepatoblastomas in B6C3F1 mice exposed to diethanolamine (DEA) for 2 years and evaluated genetic alterations in the Catnb and H-ras genes which are known to play important roles in the pathogenesis of liver malignancies. Genomic DNA was isolated from paraffin sections of each liver tumor. Catnb exon 2 (corresponds to exon 3 in human) genetic alterations were identified in 18/18 (100%) hepatoblastomas from DEA exposed mice. Deletion mutations (15/18, 83%) were identified more frequently than point mutations (6/18, 33%) in hepatoblastomas. Eleven of 34 (32%) hepatocellular adenomas and carcinomas from DEA treated mice had mutations in exon 2 of the β-catenin gene, while only 1 of 10 spontaneous neoplasms had a deletion mutation of codon 5–6. Common to all liver neoplasms (hepatocellular adenomas, carcinomas and hepatoblastomas) was membrane staining for the β-catenin protein, while cytoplasmic and nuclear staining was observed only in hepatoblastomas. The lack of H-ras mutations in hepatocellular neoplasms and hepatoblastomas suggests that the ras signal transduction pathway is not involved in the development of liver tumors following DEA exposure which is different from that of spontaneous liver tumors that often contain H-ras mutations.

Vanadium pentoxide (V2O5) is a slightly soluble compound found in airborne particle emissions from metallurgical works and oil and coal burning. Because the carcinogenic potential of V2O5 was not known, F344/N rats and B6C3F1 mice (N=50/sex/species) were exposed to V2O5 at concentrations of 0, 0.5 (rats only), 1, 2, or 4 (mice only) mg/m3, by whole-body inhalation for 2 years. The survival and body weights of rats were minimally affected by exposure to V2O5. The survival and body weights of male mice exposed to 4 mg/m3 and body weights of all exposed groups of female mice were lower than the controls. Alveolar/bronchiolar (A/B) neoplasms occurred in male rats exposed to 0.5 and 2 mg/m3 at incidences exceeding the National Toxicology Program (NTP) historical control ranges. A marginal increase in A/B neoplasms was also observed in female rats exposed to 0.5 mg/m3. Increases in chronic inflammation, interstitial fibrosis, and alveolar and bronchiolar hyperplasia/metaplasia and squamous metaplasia were observed in exposed male and female rats. A/B neoplasms were significantly increased in all groups of exposed mice. As with rats, increases in chronic inflammation, interstitial fibrosis, and alveolar and bronchiolar epithelial hyperplasia were observed in mice exposed to V2O5. Thus, V2O5 exposure was a pulmonary carcinogen in male rats and male and female mice. The marginal tumor response in the lungs of female rats could not be attributed conclusively to exposure to V2O5. These responses were noted at and slightly above the OSHA permissible occupational exposure limit of 0.5 mg/m3 (dust) (National Institute for Occupational Safety and Health, NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, Washington, DC, 1997, p. 328).

The TRAMP model and human prostatic cancer (PCA) cell lines DU145 and PC3 are useful for chemopreventive studies. We compared the efficacy of 3 anti-oxidants [a water-soluble natural anti-oxidant, NAO (200 mg/kg), found in spinach leaves; epigallocatechin-3 gallate, EGCG (200 mg/kg), a major green tea polyphenol; and N-acetylcysteine, NAC (125 mg/kg)] plus vehicle in slowing spontaneous tumorigenic progression in TRAMP and wild-type male mice. Sacrifices occurred on weeks 5, 9, and 13. Prostatic histopathology and oxidative-stress blood markers were evaluated. Hyperplasias were ranked by a combination of severity grade and distribution (focal, multifocal, and diffuse). The effectivity of each tested compound in reducing the severity/focalness of hyperplasia varied from lobe to lobe. NAO exerted a significant effect on the dorsal and lateral lobes; NAC, on the anterior and ventral lobes, and EGCG, on the ventral lobe. When the most severe hyperplasia in all 4 lobes of TRAMPs was evaluated, only NAO reduced hyperplasia at weeks 9 and 13. Plasma peroxide levels in TRAMPs were reduced following oral administration of NAO or NAC for 13 weeks; EGCG only slightly reduced these levels. In NAO-treated DU145 and PC3 PCA cells, inhibition of cellular proliferation occurred in a dose-dependent manner, increasing numbers of G1 cells and reducing ROS levels. The anti-oxidative and antiproliferative properties of NAO may explain its efficacy in slowing the spontaneous prostatic carcinogenic process in the TRAMP and its effects in the cell lines.

To improve the precision and consistency of experimental results, we have developed a scoring system for proliferative epithelial lesions in the mouse prostate based on histological growth patterns observed in individual lobes. Severity of proliferative lesions was divided into 6 categories; the grade of the most advanced lesion was identified for each lobe and its distribution estimated semiquantitatively. A numerical score combining grade and distribution of the most advanced lesion in each lobe was assigned and termed the “distribution-adjusted lesion grade”; the mean of these scores was calculated for each treatment group. Using this grading scheme, we assessed lesion development in ad libitum-fed and 20%-diet-restricted groups of TRAMP (Transgenic Adenocarcinoma of Mouse Prostate) mice that were started on study at 7 weeks of age and sacrificed when 11 and 20 weeks old. The anterior, dorsal, lateral, and ventral prostate lobes showed clear reductions in lesion severity in diet-restricted TRAMPs at 11 and 20 weeks. This method for scoring the epithelial pathology of the prostate in the TRAMP model with minimal to severe proliferative lesions utilizes the natural history of lesion development for assessing the effects of chemical and dietary interventions.

These investigations of riddelliine analyzed potential carcinogenesis and the utility of the female-rat/male-mouse design in bioassays and dose-response. Groups of 50 Fischer rats and B6C3F1 mice were gavage-administered riddelliine 5 days per week for 105 weeks. The dose levels for male rats were 0 or 1.0 mg/kg body weight; female rats 0, 0.01, 0.033, 0.1, 0.33, or 1.0 mg/kg; male mice 0, 0.1, 0.3, or 1.0, 3.0 mg/kg; and female mice 0 or 3.0 mg/kg. The dose groups were purposely designed to evaluate the dose-response relationship only in female rats and male mice. In rats, liver hemangiosarcoma, hepatocellular adenoma, and mononuclear cell leukemia were significantly increased in the 1.0 mg/kg male and female dose groups. Non-neoplastic lesions occurred in the liver and kidney of male and female rats. In mice, hemangiosarcomas increased significantly in the liver of males in the 3.0 mg/kg dose group. Alveolar/bronchiolar neoplasms in the 3.0 mg/kg dose group of female mice were significantly increased. Hepatocellular neoplasms were significantly decreased in the 1.0 mg/kg dose group of male and 3.0 mg/kg dose groups of male and female mice. Non-neoplastic lesions occurred in the liver and kidney of male and female, and lung and arteries of female mice. These studies demonstrate toxicity and carcinogenicity of riddelliine in rats and mice, and a dose-response relationship in female rats and male mice under the experimental conditions employed.

Chronic progressive nephropathy (CPN) is the most frequently diagnosed lesion in the rat kidney. It has many component s including degeneration and regeneration of renal tubule (RT) epithelium, glomerular lesions and interstitial inflammation and fi brosis. The incidence and severity of CPN is strain, age, and sex dependent and may be altered by a number of factors including exposure to xenobiotics. In National Toxicology Program (NTP) 2-year bioassays, xenobiotic-associated increased severity (exacerbation) of CPN often occurs in association with a marginal increased incidence of renal tubule cell neoplasms (RTCN). The relationship between CPN and RTCN development has not been defi nitively determined. The present study evaluated the association between severity of CPN and the occurrence of RTCN in control male F344 rats. A slight but statistically signifi cant increase in CPN severity was present in those animals with RTCN compared to aged-matched controls without RTCN. Although these data suggest there is a positive correlation between CPN and RTCN, cause and effect were not determined. This marginal association suggests that the number of RTCNs that may develop secondary to chemically exacerbated nephropathy would be few.