Neoplastic and non-neoplastic lesions in aging Slc: Wistar rats

Spontaneous Primary Pleural Mesothelioma in Fischer 344 (F344) and Other Rat Strains: A Retrospective Review

Spontaneous primary pleural mesotheliomas in Fischer 344 (F344) or other rat strains have rarely been reported. The objectives of this retrospective study were to develop historical incidence data and better characterize the light-microscopic morphology of these naturally occurring neoplasms in a large cohort of rats of several strains. A retrospective review was performed of National Toxicology Program (NTP) studies in rats conducted between 1980 and 2019 and comprising a total of 104,029 rats (51,326 males, 52,703 females), predominantly (90%) of the F344 strain. Of the 94,062 F344 rats surveyed, there were 30 cases of primary pleural mesotheliomas (22 males, 8 females). Of the 2998 Wistar Han rats surveyed, primary pleural mesotheliomas were present in 2 male rats. No primary pleural mesotheliomas were noted in male and female rats of other strains (6669 Sprague Dawley; 300 Osborne-Mendel). All primary pleural mesotheliomas in control and treated F344 and Wistar Han rats were considered spontaneous and unrelated to treatment. Based on light-microscopic evaluation of paraffin-embedded hematoxylin and eosin stained sections, only epithelioid and biphasic histologic subtypes were observed: 18 and 12 in F344 rats, respectively, and one each in Wistar Han rats. No sarcomatoid subtype cases were noted in any strain of rat.

Significance of Leydig Cell Neoplasia in Rats Fed Lactitol or Lactose

Lactitol is a disaccharide sugar alcohol (polyol) which is derived from lactose by catalytic hydrogenation and which may be used as a noncariogenic, reduced calorie sugar substitute in different foods. In the context of the safety evaluation of lactitol, a chronic carcinogenicity/toxicity study was conducted in a Wistar-derived strain of rats. In addition to effects that occur commonly in rats fed high doses of polyols, an increased incidence of Leydig cell tumors was observed in rats fed a diet with 10% lactitol for their lifetime. A comparison group receiving a diet with 20% lactose exhibited the same effect. At the 5% dose level of lactitol, no testicular changes were seen. Although lactitol is not genotoxic in standard in vitro tests and was also not associated with tumor formation in female rats and mice of either sex, it was necessary to assess the relevance of the testicular neoplastic growth for human safety. A comparative evaluation of the spontaneous and chemically induced formation of Leydig cell tumors in rats and humans demonstrates that the spontaneous occurrence is extremely low in humans but rather high in rats. Chemical agents or experimental conditions that in rats are associated with interstitial cell hyperplasia or neoplasia have not been associated with similar effects in humans. This is also true for lactose which, in Western countries, is consumed regularly and in substantial amounts with dairy products. Since lactitol is essentially not hydrolyzed in the small intestine, it gains access to the metabolism only after fermentation by the intestinal flora. It is therefore reasonable to assume that the testicular effects of lactitol and lactose were mediated by changes in the digestive tract such as by the known increase of calcium absorption which occurs in lactitol- and lactose-fed rats but not in humans or by effects of these compounds on the enterohepatic cycling of steroid hormones. Although these mechanisms are not yet elucidated, the available data on Leydig cell tumors indicate that the effects seen in male rats are not relevant to humans. The major lines of evidence supporting the human safety of lactitol and lactose are: (1) the lack of genotoxicity of lactitol, (2) the rat specificity of the testicular effects of lactose and lactitol, (3) the long history of safe consumption of lactose in humans, (4) the insensitivity of the human Leydig cells to agents and conditions that are known to cause neoplastic growth of Leydig cells in rats, (5) the generally very low spontaneous incidence of Leydig cell tumors in the human population, and (6) the absence of any epidemiological evidence establishing a link between nutritional factors and the occurrence of Leydig cell tumors in humans.

Slc:Wistar outbred rats show close genetic similarity with F344 inbred rats

Although Slc:Wistar rats are used widely in biomedical research as outbred rats, close similarities in growth curves, survival rates, and immunological and biochemical phenotypes have been reported between Slc:Wistar and F344 inbred rats. We reported previously that nine genetic variations that were fixed in Slc:Wistar rats had identical genotypes in F344 rats. Here, we examined the genetic characteristics of Slc:Wistar rats using 27 simple-sequence length polymorphism (SSLP) markers and compared them with other Wistar stocks available in Japan and with some F344 strains. Among 27 SSLP loci, 23 (85%) were fixed in the Slc:Wistar rats, which was the highest among the other Wistar stocks. The 23 fixed loci shared identical genotypes with corresponding loci in F344 rats. Further, the predominant allele types in the unfixed loci had allele frequencies as high as 80%, and these alleles were identical in the F344 rats. When the nine genetic variations reported previously are added, a total of 32 (89%) out of the 36 loci examined were fixed and identical in the Slc:Wistar and F344 rat genomes. These findings indicate the low genetic variation in Slc:Wistar rats and the high genetic similarity between the Slc:Wistar and F344 inbred rats. This study demonstrates the importance of characterizing outbred rats and the need to pay ample attention to the genetic characteristics the Slc:Wistar rats for their proper use.

Chronic exposure to a 1.439 GHz electromagnetic field used for cellular phones does not promote N-ethylnitrosourea induced central nervous system tumors in F344 rats

The present study was designed to evaluate whether a 2 year exposure to an electromagnetic field (EMF) equivalent to that generated by cellular phones can accelerate tumor development in the central nervous system (CNS) of rats. Brain tumorigenesis was initiated by an intrauterine exposure to N-ethylnitrosourea (ENU) on gestational day 18. A total of 500 pups were divided into five groups, each composed of 50 males and 50 females: Group 1, untreated control; Group 2, ENU alone; Groups 3-5, ENU + EMF (sham exposure and 2 exposure levels). A 1.439 GHz time division multiple access (TDMA) signal for the Personal Digital Cellular (PDC), Japanese standard cellular system was used for the exposure of the rat head starting from 5 weeks of age, 90 min a day, 5 days a week, for 104 weeks. Brain average specific absorption rate (SAR) was 0.67 and 2.0 W/kg for low and high exposures, respectively: whole body average SAR was less than 0.4 W/kg. There were no inter-group differences in body weights, food consumption, and survival rates. No increase in the incidences or numbers per group of brain and/or spinal cord tumors, either in the males or females, was detected in the EMF exposed groups. In addition, no clear changes in tumor types were evident. Thus, under the present experimental conditions, 1.439 GHz EMF exposure to the heads of rats for a 2 year period was not demonstrated to accelerate or affect ENU initiated brain tumorigenesis.

Iron-induced carcinogenesis: the role of redox regulation

Redox cycling is a characteristic of transition metals such as iron. Iron is hypothesized to have been actively involved in the birth of primitive life on earth through the generation of reducing equivalents in the presence of UV light. Iron is an essential metal in mammals for oxygen transport by hemoglobin and for the function of many enzymes including catalase and cytochromes. However, the "free" or "catalytic" form of iron mediates the production of reactive oxygen species via the Fenton reaction and induces oxidative stress. Serum "free" iron is observed in rare situations such as in severe hemochromatosis in which serum transferrin is saturated. However, it is known that superoxide can release "free" iron from ferritin and hemosiderin in the cell. "Free" iron is quite cytotoxic as well as mutagenic and carcinogenic. Iron compounds were first reported to induce sarcomas in rats by Richmond in 1959. Thereafter, several iron-induced carcinogenesis models were established, including the ferric nitrilotriacetate model by Okada and colleagues. Iron may have a role in the carcinogenic process of other transition metals such as copper and nickel, or other kinds of carcinogens such as nitrosamine and even virus-induced carcinogenesis. In humans, genetic hemochromatosis and asbestosis are two major diseases associated with iron-induced carcinogenesis. There is an increasing number of reports of an association between increased body iron stores and increased risk of cancer. Iron-induced oxidative stress results in two possible consequences: (1) redox regulation failure that leads to lipid peroxidation and oxidative DNA and protein damage; (2) redox regulation that activates a variety of reducing and oxystress-protective mechanisms via signal transduction. Both consequences appear to play a role in iron-induced carcinogenesis.

Intrapleural administration of vitreous high duty ceramic fibres and heated devitrified ceramic fibres does not give rise to pleural mesothelioma in rats

In order to determine whether they are potentially carcinogenic to the pleural mesothelium, three samples of ceramic fibre have been administered to rats by the intrapleural route. These samples were a high-duty grade refractory ceramic fibre (manufactured by Thermal Ceramics Ltd) in the as-manufactured vitreous state and two devitrified samples produced by heating the same fibre for 2 weeks at 1200 degrees C and for two weeks at 1400 degrees C. The mean lifespans of the groups of rats treated with vitrified and devitrified ceramic fibres were not significantly different from that of the control rats. In these studies none of the treated or control rats developed pleural mesothelioma, making it unlikely that ceramic fibres of this type, whether vitreous or devitrified, are potentially carcinogenic to the pleural mesothelium.

Influence of chronic alcohol consumption on the development of altered hepatocellular foci in rats

The effect of chronic alcohol consumption on the development of altered hepatocellular foci was examined in 150 male Slc:Wistar rats fed an ethanol solution (0, 5 or 20%, w/v) ad libitum instead of drinking water. In the 120th week of the treatment period, the liver was examined histopathologically. There was no significant difference in the incidence, number and volume fractions of altered hepatocellular foci between the 0% and the 5% ethanol groups. In the 20% ethanol group, the incidence of altered hepatocellular foci was significantly higher than in the 0% ethanol group, and the number and the volume fraction of altered hepatocellular foci were significantly greater than in the 0% and the 5% ethanol groups. These results indicate that chronic consumption of a large amount of alcohol promotes the development of altered hepatocellular foci. However, there was no statistically significant difference in the incidence of benign liver cell tumours and hepatocellular carcinoma among the three groups.

Effects of high-fat diet on incidence of spontaneous tumors in Wistar rats

In a 2.5-year carcinogenicity study, two groups, both including male and female Wistar rats, were fed two different diets with 4% and 16% fat. In addition to 4% soybean oil, the high-fat diet contained 12% mono- and diglycerides, of which 85% was stearic acid and 13% palmitic acid. There was no difference in food consumption, body weight, weight gain, and longevity between the two groups. A statistically significant increase in the incidence of tumors in the high-fat group was seen in fibroadenoma of the mammae (female, p = 0.05). No statistically significant difference was seen when the incidence of benign mammary tumors (adenomas and fibroadenomas) was combined, just as the overall incidence of mammary tumors (adenomas, fibroadenomas, and adenocarcinomas) was not significantly different between the groups. A statistically significant decrease in the incidence of tumors in the high-fat group was seen in adenoma of the parathyroid gland (male, p = 0.04) and medullary carcinoma of the adrenal gland (male, p = 0.04). Combining the incidence of benign and malignant tumors of the adrenal medulla led to a further increase in the level of significance (p = 0.02). The present study showed that a high-fat diet influenced the tumor incidence in certain organs of rats. However, the overall differences in tumor incidence between rats fed the low- and the high-fat diet are considered marginal. Therefore we were not able to confirm or deny the hypothesis that a high-fat diet promotes the development of cancer. It should be noted that, in our study, fat accounted for about 30% of the total energy in the high-fat diet. This is much below the amount of fat normally found in the western diet but corresponds well to the level recommended for human intake. In addition, the rats fed the high-fat diet did not gain more weight, even though no difference was recorded in food consumption (g/kg body wt) between the groups.

Chronic oral toxicity and carcinogenicity study of thiabendazole in rats

The potential toxicity and carcinogenicity of thiabendazole (TBZ) was investigated in F344 rats. TBZ was given in pelleted diets at different dietary levels (0, 0.05, 0.1, 0.2 or 0.4%) to groups of 30 rats of each sex for 104 wk. Throughout the study, a marked decrease in body-weight gain was observed in males and females fed 0.4% TBZ. Transitional cell hyperplasia of the renal pelvis and/or papilla was observed in dosed groups of each sex with significant positive trends. Low incidence of transitional cell papilloma or carcinoma of the urinary system was recorded in the dosed groups. Preputial gland adenoma occurred in males treated with 0.4% TBZ, and the incidence was significantly higher than in controls. There was no clear-cut evidence of the carcinogenic activity of TBZ in the urinary system of male and female rats.

Carcinogenic effects of bis(2,3-dibromopropyl)-phosphate in Wistar rats

Four groups of 40 Wistar rats of each sex per dose level were fed diets containing 0, 80, 400 or 2000 ppm of the magnesium salt of bis(2,3-dibromopropyl)phosphate (Bis-BP) for 24 months. A high incidence of tumours was induced, in both sexes, in the digestive system. Tumours included papillomas and adenocarcinomas of the tongue, oesophagus and forestomach, adenocarcinomas of the intestine and hepatocellular adenomas (neoplastic nodules) and carcinomas. Pre-terminal mortalities were associated with an increased incidence of forestomach papillomas in both sexes, adenocarcinomas of the small intestine in male rats and increased incidence of hepatocellular carcinomas in females. The data suggest that Bis-BP is a more potent proximate carcinogen than tris-(2,3-dibromopropyl)phosphate (Tris-BP).

Induction of mesothelioma by intraperitoneal injections of ferric saccharate in male Wistar rats

Iron appears to play a major role in catalysing free radical production, leading to lipid peroxidation and DNA damage. We, therefore, investigated the effect of colloidal iron deposited in the peritoneum. Wistar male rats were given either ferric saccharate, ferric saccharate and nitrilotriacetic acid (NTA), NTA or saline. NTA was shown previously to 'free' iron to promote lipid peroxidation and an iron chelate of NTA is known to be carcinogenic to the kidney. Iron at a dose of 5 mg kg-1 day-1, and saline at a dose of 0.5 ml day-1 were injected i.p. for 3 months. NTA at a dose of 83.5 mg kg-1 day-1 was give i.p. for 5 months. All the rats were killed about a year later for histological examination. In nine of the 19 rats treated with ferric saccharate, mesothelial tumors were induced in the serosa of the tunica vaginalis or the length of the spermatic cord. Among rats treated with ferric saccharate and NTA, seven had localised mesotheliomas in the above locations and six had wide-spread peritoneal mesotheliomas. No mesothelial tumors developed in either NTA treated or saline treated rats. No pleural mesotheliomas were found in any group. These findings add to the evidence that iron is involved in some carcinogenic processes.

Initiation activity of endogenously synthesized N-nitrosobis(2-hydroxypropyl)amine in the rat liver

The initiation potential of N-nitrosobis(2-hydroxypropyl)amine (NDHPA) endogenously synthesized from bis(2-hydroxypropyl)amine (DHPA) or tris(2-hydroxypropyl)amine (THPA) in the presence of sodium nitrite (NaNO2) was investigated in the rat liver by quantitation of hepatocellular foci showing phenotypic expression of glutathione S-transferase placental form (GST-P). The investigation consisted of two experiments. In the first, male Wistar rats were divided into six groups as follows: group 1 was non-treated; groups 2 and 3 received 0.15% and 0.3% NaNO2, respectively; group 4 received 1% DHPA; groups 5 and 6 received 1% DHPA together with 0.15% and 0.3% NaNO2, respectively. In experiment 2, the same protocol was used except that 2% THPA was substituted for 1% DHPA. The treatments were continuous until sacrifice at week 94 in experiment 1 and week 104 in experiment 2. As a result putative preneoplastic GST-P-positive foci observed in the liver and increased dose-dependently in rats from groups 5 and 6 which had received DHPA and NaNO2, but not in rats administered THPA and NaNO2. The results indicate that endogenously synthesized NDHPA from DHPA and NaNO2 is capable of initiating neoplastic development in the rat liver.

Chronic toxicity study on formaldehyde administered orally to rats

Groups of 20 male and 20 female Wistar rats were given formaldehyde solution in their drinking water at concentrations of 0.50, 0.10, 0.02 and 0% for 24 months. Significant decreases in body weight and food and water intake were observed in the 0.50% group of both sexes and all rats in this group died by 24 months. Various non-neoplastic lesions were observed in rats, mostly in the 0.50% group. In this group, erosions and/or ulcers were evident in both the forestomach and glandular stomach. In the forestomach, squamous cell hyperplasia with or without hyperkeratosis and downward growth of basal cells were observed. Glandular hyperplasia of the fundic mucosa was noted along the limiting ridge. A few of such changes of the upper GI tract were seen in the 0.10% group. No toxicological abnormalities were found in 0.02% group of both sexes. There were no significant differences in the incidences of any tumors among groups of both sexes. Based on these findings, the no observable effect level of formaldehyde was 0.02% in the drinking water (10 mg/kg body wt./day).

Glandular changes associated with the spontaneous interstitial cell tumor of the rat testes

Testes of untreated F344 and Wistar rats in the control groups of carcinogenicity studies were histologically examined, and the histopathological characteristics and histogenesis of glandular changes in these testes were studied. In 266 testes of 2-year-old F344 rats, 263 had interstitial cell tumors (ICTs) (98.9%) and 39 had glandular changes (14.7%). These glandular changes were also found in 1 out of 38 1-year-old F344 rat testes (2.6%), and 3 in 154 2-year-old Wistar rat testes (1.9%). The changes were observed exclusively in the interstitial cell tumors (ICTs). These glandular changes showed variation in size, shape and number. They were composed of tubules or cysts lined by a layer of cuboidal or columnar epithelial cells, which had terminal bars, occasionally PAS- and alcian blue-positive brush borders, and rarely, alcian blue-positive cytoplasmic vacuoles. Serial sections revealed that the changes were not connected with the rete testes, but with the degenerative seminiferous tubules involved in the ICTs lined by a layer of flat endothelial-like cells. The findings suggest that the lesions constitute metaplastic changes of the Sertoli cells.

Spontaneous mesotheliomas in Fischer rats--a histological and electron microscopic study

Seventeen mesotheliomas from 395 untreated male Fischer 344/DuCrj rats were studied by light and electron microscopy to define the morphological characteristics of the tumors. In 16 out of 17 rats, mesotheliomas were observed in the abdominal and/or scrotal sac, and the other one was localized on the pleura. Grossly, tumors were yellow-brown with various-sized multiple modules growing irregularly over the surface of the serosa. Microscopically, they varied from complex papillary to sessile nodular growths. Tumor cells were cuboidal to polygonal with round to oval nuclei, and were sometimes arranged in tubule-like structures. Occasionally, the cells contained Mowry's colloidal iron positive materials, which were negative following prior incubation with hyaluronidase. Furthermore, intracellular keratins were detected using the peroxidase-antiperoxidase method. Ultrastructural features of tumor cells included numerous microvilli, a basement membrane, junctional complexes, abundant cytofilaments, dilated rough surfaced endoplasmic reticulum cisternae, and a well-developed Golgi apparatus. The morphological characteristics of these tumors in Fischer 344 rats were consistent with those in humans and with experimentally induced counterparts in rats. The histogenesis of these tumors and the variability in their incidence following oral administration of chemical carcinogens is discussed.

Malignant fibrous histiocytomas induced in rats by polymers

Five polymeric materials (3 polyvinyl chlorides, 1 polyhydroxyethyl methacrylate, and 1 dimethyl polysiloxane) were implanted into subcutaneous (SC) tissues of rats. Subcutaneous tumors developed in all experimental groups. The incidences of the tumors differed however, although the experimental conditions were the same for all these materials. This result indicates that chemical characteristics of the materials may influence the incidence of SC tumors. From the histological and electron-microscopic findings many of these tumors were diagnosed as malignant fibrous histiocytomas.