Thyroid Follicular Cell Tumors Research Articles

Risk Assessment of the Fungicide Luna Privilege with the Active Substance Fluopyram

Luna Privilege is a new fungicide for use in apples, pears, outdoor and indoor strawberries, outdoor and indoor lettuces, peas, beans and indoor tomatoes, and contains the new active substance fluopyram.
 VKM was requested by the Norwegian Food Safety Authority to consider the possible health risk for operators related to the properties of Luna Privilege and fluopyram; in particular to evaluate the potential for bioaccumulation, reproductive and neurotoxic effects, and discuss possible mechanisms involved in liver and thyroid tumor formation observed in rats, and the establishment of NOAELs and reference values.
 VKM was also asked to evaluate the fate and behaviour of fluopyram in the environment, and the ecotoxicological effects and risks related to its use, in particular the potential for groundwater contamination, safety margins and possible effects on birds and aquatic organisms.
 The assessment was finalized in a meeting on December 13. 2013 by VKM’s Panel on Plant Protection Products.
 VKM’s conclusions are as follows:
 Health:
 It is the opinion of VKM that:
 
 The active ingredient fluopyram has low potential for bioaccumulation, and the data do not suggest a sex-specific excretion.
 
 
 
 An in vivo Comet Assay in rat liver could help to further elucidate the genotoxic potential of fluopyram.
 
 
 
 It can not be excluded that the reported incidence of “gall bladder absent” is treatment related.
 
 
 
 NOAEL for the 90-day feeding study in rats should be set to 3.06 mg/kg bw/day, resulting in an AOEL of 0.03 mg/kg bw/day.
 
 
 
 The “dumb-bell or incomplete ossification and/or bipartite/normal cartilage” should be considered as a malformation and regarded as treatment related.
 
 
 
 The time points used for neurotoxic measurements are not optimal to detect neurotoxic effects from acute exposure, since the time window between the first and second measurements is too long.
 
 
 
 The studies where effects of fluopyram and phenobarbital are compared can not be used to exclude human relevance of the tumor-inducing effect of fluopyram in the liver of female rats.
 
 
 
 The results from the mechanistic studies are insufficient to support the proposed mode of action for the induction of thyroid follicular cell tumors in male mice, and thus the induction of thyroid tumors in male mice could be relevant for humans.
 
 VKM proposes:
 
 
 NOAEL of 1.2 mg/kg bw/day based on a 2 year feeding study in rats.
 
 
 
 ADI: 0.012 mg/kg bw/day
 
 
 
 AOEL: 0.03 mg/kg bw/day
 
 
 
 ARfD: 0.25 mg/kg bw/day
 
 
 Risk calculations for field use of Luna Privilege show minimal risk if personal protective equipment is used. The AOEL of fluopyram for greenhouse use is not exceeded, even without protective equipment. Re-entry and bystander exposure is calculated to be well below the AOEL.
 Environment:
 It is the opinion of VKM that:
 
 Worst case degradation rates from laboratory studies should preferably be used to calculate PECsoil values for fluopyram.
 Both fluopyram and fluopyram-7-hydroxy have a high potential for groundwater contamination.
 
 
 
 The efficacy of buffer zones needs to be considered on a case-by-case basis, and further validation of values for efficacy from the model simulation is necessary.
 
 
 
 It is further the opinion of VKM that all the refinements used in the risk assessment for birds are relevant. Since the TER values estimated for all crops except for orchards are below the trigger following refinements, it is the view of VKM that the data indicate medium risk for strawberries and pulses, and high risk for lettuce in open field.
 
 
 
 The use of LC50 values for fluopyram in the TER calculations with a TER trigger value of 100 is overly conservative, and a reduction of the acute trigger for both invertebrates and fish from 100 to 10 for such calculations are suggested.
 
 
 
 The trigger for acute toxicity is not exceeded for any of the crops. For chronic toxicity, VKM concludes that there is a moderate risk for effects on fish when Luna Privilege is applied to beans/strawberries without the use of a vegetated mitigation buffer-strip.

Thyroid follicular cell tumors: cytohistological comparisons in the context of the new international classification

To carry out a correlation retrospective analysis of the histological and cytological findings of follicular cell neoplasms over a two-year period after the publication of the new WHO classification of thyroid tumors (2017), which identifies a category of follicular tumors of uncertain malignant potential (T-UMP). The investigators of the Clinic, National Medical Research Center for Endocrinology, Ministry of Health of Russia, made a cytohistological comparison of morphological findings, by using the material of surgically removed follicular cell neoplasms. The investigation showed the lack of unreasonably common usage of the histological classification category of T-UMP (3.08%). Non-invasive follicular neoplasm with papillary-like nuclei predominantly diagnosed in the T-UMP category was more frequently (83.3) assigned to Diagnostic Category IV of the 2017 Bethesda system for the cytological diagnosis of thyroid tumors. Cytology showed the highest (98.1%) sensitivity, including classic (97.8%), encapsulated (100%), and follicular (97.7%) options in diagnosing papillary cancer, being substantially inferior in diagnosing follicular and Hürthle cell neoplasms, in which diagnostic category IV category was more frequently established for adenomas (81.4%) and carcinomas (69.5%). The morphological differential diagnosis of follicular cell neoplasms seems to be a relevant problem.

Mosapride

The Food Safety Commission of Japan (FSCJ) conducted a risk assessment of mosapride citrate (CAS No. 112885–42-4), a gastrointestinal agent, based on documents including a written application for marketing approval of a new veterinary medicinal product. Although hepatocellular and thyroid follicular cell tumors were induced in carcinogenicity studies in rats and mice, no genotoxicities were suggested in all the studies of mosapride citrate. The involvement of genotoxic mechanism is thus unlikely. Therefore, an acceptable daily intake (ADI) for mosapride citrate is able to be specified. In a 26-week subacute toxicity study using both sexes of rats, the no-observed-adverse-effect level (NOAEL) and the lowest-observed-adverse-effect level (LOAEL) values were obtained, respectively, to be 2 mg/kg bw/day and 10 mg/kg bw/day for the female, and to be 10 mg/kg bw/day and 50 mg/kg bw/day for the male. However, hepatocellular swelling observed in the female in the 26-week subacute toxicity study was also observed in the 104-week carcinogenicity study in the male given 10 mg/kg bw/day (lowest dose) and in the female given 30 mg/kg bw/day. Liver altered foci were observed in the female given 10 mg/kg bw/day (LOAEL), and the NOAEL value for the female was 3 mg/kg bw/day. Lowest values for all the hepatic lesion was 10 mg/kg bw/day for the female on both the 26-week and 104-week studies. A clear sex-related difference was observed in the plasma levels of mosapride in rats, in which the higher levels were observed in the female. The result, together with the data on the 26-week study, suggests that the NOAEL value in the male rats is unlikely to be lower than 3 mg/kg bw/day. FSCJ judged it appropriate to adopt this value as the NOAEL for the ADI of mosapride citrate, based on the following reasons: 1) The clear sex-related difference was observed in the plasma levels of mosapride in rats, in which the higher levels were observed in the female; and 2) The common ratio of the dose was smaller in the 104-week carcinogenicity study than that in the 26-week subacute toxicity study. Consequently, FSCJ established an ADI of 0.03 mg/kg bw/day, applying a safety factor of 100 (10 for species difference, 10 for individual difference) to the NOAEL obtained in the 104-week carcinogenicity study in rats.

Two-year carcinogenicity study of acrylamide in Wistar Han rats with in utero exposure

Acrylamide is an important chemical with widespread industrial and other uses in addition to generalized population exposure from certain cooked foods. Previous rat studies to assess the carcinogenic potential of acrylamide have been carried out exclusively in the Fischer 344 rat with identification of a number of tumors amongst which mesotheliomas of the tunica vaginalis is an important tumor endpoint in the classification of acrylamide as a ‘probably human carcinogen. In a rat carcinogenicity study to determine the human relevance of mesotheliomas Wistar Han rats were exposed to 0, 0.5, 1.5, or 3.0mg acrylamide/kg body weight/day in drinking water starting at gestation day 6. At the end of two years, mammary gland fibroadenomas in females and thyroid follicular cell tumors in both sexes were the only tumors increased in acrylamide treated rats. These tumor endpoints have rat-specific modes of action suggesting less likelihood of human cancer risk than previously estimated. This study demonstrates that tunica vaginalis mesotheliomas are strain specific and not likely of genotoxic origin.

Prognostic classification of thyroid follicular cell tumors using Ki-67 labeling index: risk stratification of thyroid follicular cell carcinomas.

This review emphasizes that the so-called high-risk thyroid carcinoma is not a distinct tumor entity, but a group of tumors with different histologies. High-grade histological features, such as tumor necrosis, increased mitoses, and nuclear pleomorphism, together with high Ki-67 labeling index (more than 10%), are good indicators of high-risk thyroid carcinoma and suggest a possible risk for anaplastic transformation. This review proposes the stratification of patients with thyroid carcinoma into low-, moderate-, and high-risk groups based on Ki-67 labeling index, which should be useful for the clinical management of patients, even after initial surgery. Currently, both the aggressive variant of papillary carcinoma and poorly differentiated carcinoma are aggressively treated by a completion of total thyroidectomy with prophylactic lymph node dissection followed by radioactive iodine treatment. Therefore, patients with moderate-risk or high-risk thyroid carcinoma based on Ki-67 labeling index should also be considered candidates for this treatment strategy.

Classification of thyroid follicular cell tumors: with special reference to borderline lesions [Review

We propose a new classification of thyroid follicular cell tumors which is correlated with patient's prognosis. It is unique as to two new categories: borderline malignancy between benign and malignant, and moderately differentiated adenocarcinoma (MDA) as a differentiation classification to stratify tumor aggressiveness. As to diagnostic criteria, we recommend invasiveness (capsular and vascular invasion) to separate benign and malignant and it should not be based on presence or absence of papillary thyroid carcinoma (PTC) type nuclear features (PTC-N). Thus borderline malignancy in our new classification includes some of the formerly malignant tumors and they are 1) papillary microcarcinoma, 2) encapsulated conventional PTC (EncPTC), 3) encapsulated follicular variant PTC (EnFVPTC), 4) well differentiated tumor of uncertain malignant potential (WDT-UMP), 5) follicular tumors of uncertain malignant potential (FT-UMP), and 6) capsular invasion only follicular thyroid carcinoma (FTC). Review of the literature revealed that those thyroid tumors have consistently excellent outcome. Well differentiated follicular cell adenocarcinoma (WDA) in our classification includes common type PTC and low-risk follicular carcinoma (FTC). They are invasive (diffuse infiltrative) common type PTC and minimally invasive type FTC with less than 4 foci of angioinvasion. Moderately differentiated follicular cell adenocarcinoma (MDA) includes FTC with angioinvasion (more than 4), aggressive variants of PTC, such as tall cell, columnar cell, solid, loss of cellular polarity/cohesiveness (hobnail) variants and encapsulated carcinoma with high grade histology. Poorly differentiated carcinoma (PDC) includes PDC of WHO definition, insular carcinoma, tumors with minor anaplastic transformation and tumors with distant metastasis at presentation.

Cancer Risk Assessment: Chemical Carcinogenesis, Hazard Evaluation, and Risk QuantificationHsuChing-HungEdited by StedefordTodd Hoboken, NJ. : John Wiley & Sons. , 2010. . 824 pp. ISBN: ISBN: 978-0-470-23822-6. , $185.

Cancer Risk Assessment: Chemical Carcinogenesis, Hazard Evaluation, and Risk QuantificationHsuChing-HungEdited by StedefordTodd Hoboken, NJ. : John Wiley & Sons. , 2010. . 824 pp. ISBN: ISBN: 978-0-470-23822-6. , $185.

PAX 8 expression in non-neoplastic tissues, primary tumors, and metastatic tumors: a comprehensive immunohistochemical study

PAX 8 is a transcription factor that is essential for embryonic development of the kidney, Müllerian organs, and thyroid. It may also have a role in tumor development in these organs. The diagnostic utility of PAX 8 has not been comprehensively studied. Formalin-fixed, paraffin-embedded tissue samples for non-neoplastic tissues (n=1601), primary neoplasms (n=933), and metastatic neoplasms (n=496) were subjected to PAX 8 immunostain. In non-neoplastic tissues, PAX 8 was consistently noted in glomerular parietal epithelial cells, renal collecting ductal cells, atrophic renal tubular epithelial cells regardless of nephronic segments, and epithelial cells of the endocervix, endometrium, fallopian tube, seminal vesicle, epidydimis, thyroid, pancreatic islet cells, and lymphoid cells. PAX 8 was not seen in the rest of the tissue samples. In primary neoplasms, PAX 8 was expressed by 194 of 240 (89%) renal cell neoplasms, by 238 of 267 (89%) Müllerian-type neoplasms, by 65 of 65 (100%) thyroid follicular cell neoplasms, by 8 of 8 (100%) nephrogenic adenomas, and by 17 of 17 (100%) lymphomas. Weak focal staining was noted in 5 of 12 (42%) cases of parathyroid hyperplasia/adenoma and in 6 of 17 (35%) well-differentiated neuroendocrine tumors of the pancreas. PAX 8 was not seen in other neoplasms. In metastatic neoplasms, PAX 8 was expressed by 90 of 102 (88%) metastatic renal cell carcinomas, by 57 of 63 metastatic Müllerian tumors (90%), and by 6 of 6 metastatic papillary thyroid carcinomas (100%). There was also weak focal staining for 1 of 15 metastatic small cell carcinomas and for 1 of 9 metastatic well-differentiated neuroendocrine carcinomas. PAX 8 was not seen in other metastatic neoplasms. It can be successfully identified in routinely processed tissue samples, and its expression is mostly nuclear. PAX 8 expression in non-neoplastic mature tissues is limited to the organs, the embryonic development of which depends on this transcription factor. This tissue/cell-specific expression is maintained during both neoplastic transformation and metastasis. PAX 8 is a sensitive and specific marker for tumors of renal, Müllerian, or thyroid origin in both primary and metastatic sites.

Iodomethane human health risk characterization

Iodomethane is a new pre-plant soil fumigant approved in the United States. Human exposure may occur via inhalation due to the high vapor pressure of iodomethane. A quantitative human health risk assessment was conducted for inhalation exposure. The critical effects of acute duration iodomethane exposure are: (1) fetal losses in rabbits, (2) lesions in rat nasal epithelium, and (3) transient neurotoxicity in rats. Chronic exposure of rats resulted in increased thyroid follicular cell tumors from sustained perturbation of thyroid hormone homeostasis. A physiologically based pharmacokinetic (PBPK) model for iodomethane was developed to characterize potential human health effects from iodomethane exposure. The model enabled calculation of human equivalent concentrations (HECs) to the animal no-observed-adverse-effect levels (NOAELs) using chemical-specific parameters to determine the internal dose instead of default assumptions. Iodomethane HECs for workers and bystanders were derived using the PBPK model and NOAELs for acute exposure endpoints of concern. The developmental endpoint NOAEL was 10 ppm and corresponding bystander HEC was 7.4 ppm. The nasal endpoint NOAEL was 21 ppm and the HEC was 4.5 ppm. The transient neurotoxicity endpoint NOAEL was 27 ppm and the HEC was10 ppm. Data demonstrated that humans are less sensitive to the effect that causes developmental toxicity in rabbits and the PBPK model incorporated this information, resulting in a higher HEC for the developmental endpoint than for the nasal endpoint. Nasal olfactory degeneration is the primary endpoint for risk assessment of acute exposure to iodomethane.

Mechanism of trifluralin-induced thyroid tumors in rats

Trifluralin, an herbicide, has been reported to cause a significant increase in thyroid follicular cell tumors in male Fischer 344 rats. This study was designed to determine the mechanism of thyroid hyperactivity after trifluralin exposure. A group of 15 male Fischer 344 rats were exposed to trifluralin-fortified (6500 ppm) diet for 2 weeks. The time weighted average daily intake of trifluralin was 441 ± 77 mg/kg/day. Ten rats of the group were sacrificed and the sera analyzed for T3, T4, and TSH levels. The livers were also analyzed for selected T4-specific UGT gene expression and total UGT enzyme activity. In the trifluralin treated rats, the serum T3 and T4 levels decreased by 17% and 90%, respectively and TSH increased by 37% more than the control rats. Trifluralin-induced total hepatic UGT enzymes (2.4-fold) and mRNA expression of selected hepatic UGT isozymes (UGT1A1, 1.4-fold; UGT1A6, 6.4-fold; UGT2B1, 3.7-fold). For the remaining 5 rats in the group, bile was collected for 2 h and analyzed for free and conjugated T3 and T4. The total amount of T4 in bile more than doubled in trifluralin treated rats. Trifluralin treatment increased bile flow, caused a 3.2-fold increase in biliary elimination of conjugated T4 and 63% increase in conjugated T3. Based on these data, the decrease in total serum T3 and T4 levels in the trifluralin treated rats was due to enhanced peripheral metabolism and an increase in bile flow that results in a compensatory increase in TSH synthesis and secretion. The increased levels of TSH with chronic exposure to trifluralin would exert a continuous stimulation of the thyroid gland leading to cellular hypertrophy and proliferation predisposing to the development of follicular cell tumors in rats.

Toxicity Profiles in Rats Treated with Tumorigenic and Nontumorigenic Triazole Conazole Fungicides: Propiconazole, Triadimefon, and Myclobutanil

Conazoles are a class of azole based fungicides used in agriculture and as pharmaceutical products. They have a common mode of antifungal action through inhibition of ergosterol biosynthesis. Some members of this class have been shown to be hepatotoxic and will induce mouse hepatocellular tumors and/or rat thyroid follicular cell tumors. The particular mode of toxic and tumorigenic action for these compounds is not known, however it has been proposed that triadimefon-induced rat thyroid tumors arise through the specific mechanism of increased TSH. The present study was designed to identify commonalities of effects across the different conazoles and to determine unique features of the tissue responses that suggest a toxicity pathway and a mode of action for the observed thyroid response for triadimefon. Male Wistar/Han rats were treated with triadimefon (100, 500, 1800 ppm), propiconazole (100, 500, 2500 ppm), or myclobutanil (100, 500, 2000 ppm) in feed for 4, 30, or 90 days. The rats were evaluated for clinical signs, body and liver weight, histopathology of thyroid and liver, hepatic metabolizing enzyme activity, and serum T3, T4, TSH, and cholesterol levels. There was a dose-dependent increase in liver weight but not body weight for all treatments. The indication of cytochrome induction, pentoxyresorufin O-dealkylation (PROD) activity, had a dose-related increase at all time points for all conazoles. Uridine diphopho-glucuronosyl transferase (UDPGT), the T4 metabolizing enzyme measured as glucuronidation of 1-naphthol, was induced to the same extent after 30 and 90 days for all three conazoles. Livers from all high dose treated rats had centrilobular hepatocyte hypertrophy after 4 days, while only triadimefon and propiconazole treated rats had hepatocyte hypertrophy after 30 days, and only triadimefon treated rats had hepatocyte hypertrophy after 90 days. Thyroid follicular cell hypertrophy, increased follicular cell proliferation, and colloid depletion were present only after 30 days in rats treated with the high dose of triadimefon. A dose-dependent decrease in T4 was present after 4 days with all 3 compounds but only the high doses of propiconazole and triadimefon produced decreased T4 after 30 days. T3 was decreased after high-dose triadimefon after 4 days and in a dose-dependent manner for all compounds after 30 days. Thyroid hormone levels did not differ from control values after 90 days and TSH was not increased in any exposure group. A unique pattern of toxic responses was not identified for each conazole and the hypothesized mode of action for triadimefon-induced thyroid gland tumors was not supported by the data.

Increase in thyroid follicular cell tumors in nelfinavir-treated rats observed in a 2-year carcinogenicity study is consistent with a rat-specific mechanism of thyroid neoplasia

The carcinogenic potential of nelfinavir mesylate (nelfinavir) was evaluated in a 2-year oral (gavage) study on Sprague-Dawley rats at dose levels of 0 (control), 0 (vehicle control), 100, 300 and 1000 mg/kg per day. At the end of the treatment, increased incidences of thyroid follicular cell hyperplasia and neoplasms were observed at 300 (males) and 1000 mg/kg per day (both sexes). There were no other treatment-related effects and no tumors at other sites. Results from previous studies indicated a number of effects in the liver and thyroid, as well as metabolic profiles that suggested nelfinavir might cause thyroid hyperplasia/neoplasia secondary to hormone imbalance by altering thyroid hormone disposition. To investigate this hypothesis, the effects of nelfinavir on gene expression in rat hepatocytes and liver slices (in vitro), thyroxine plasma clearance, and thyroid gland function were evaluated. Compared to controls, gene expression analyses demonstrated an increased expression of glucuronyltransferase (UDPGT) and CYP450 3A1 in nelfinavir-treated rat hepatocytes and liver slices. In rats treated with nelfinavir (1000 mg/kg per day) for 4 weeks, liver weights and centrilobular hepatocellular hypertrophy were increased and minimal to mild diffuse thyroid follicular cell hypertrophy and follicular cell hyperplasia were evident in the thyroid gland. Thyroid-stimulating hormone (TSH) levels were significantly increased (three-fold), while tri-iodothyronine (T3)/tetra-iodothyronine (T4) and reverse T3(rT3) levels were unchanged, indicating that a compensated state to maintain homeostasis of T3/T4 had been achieved. Plasma 125I-thyroxine clearance was increased and the plasma thyroxine AUC0-48 was decreased (24%) compared to control. In conclusion, these data indicate that thyroid neoplasms observed in the nelfinavir-treated rats were secondary to thyroid hormone imbalance. Increased thyroxine clearance contributes to the effects of nelfinavir on thyroid gland function and is probably a result of UDPGT induction that leads to elevated TSH levels in the rat and eventual thyroid neoplasia. These results are consistent with a well-recognized rat-specific mechanism for thyroid neoplasms.

Mechanism of clobazam-induced thyroidal oncogenesis in male rats

In order to elucidate the mechanisms by which long-term treatment with clobazam (CLB), 1,5-benzodiazepine, induces thyroid follicular cell tumors in male rats, male Sprague-Dawley (SD) rats were treated orally with 400 mg/kg of CLB for up to 4 weeks, and the contribution of feedback through elevated thyroid stimulating hormone (TSH) was investigated. Measurements taken after 1, 2, and 4 weeks of treatment revealed that thyroxine (T 4)-UDP-glucuronosyltransferase (T 4-UDPGT) activity was higher than that of untreated animals. This change was accompanied by increase in liver weights and centrilobular hepatocyte hypertrophy. In addition, plasma total triiodothyronine (T 3) and T 4 levels were lower than in the untreated rats when measured after 1 week of treatment. However, a high plasma TSH level was sustained throughout the 4-week treatment. Thyroid follicular cell hypertrophy began after 1 week of treatment, followed by increased thyroid weight after 2 weeks. Clearance of exogenous [ 125 I ] T 4 from the blood of treated rats, determined after 4 weeks of treatment, was significantly faster than that in untreated rats, whereas iodine uptake and organification in the thyroid glands were not affected. These results suggest that CLB increases hepatic T 4-UDPGT activity leading to acceleration of T 4-clearance, which results in decreased plasma thyroidal hormones followed by compensatory increase of TSH biosynthesis and secretion. Chronic high levels of TSH would exert a continuous growth pressure on the thyroid, under which hypertrophic follicular cells can ultimately progress to frank neoplasms.

Expression of galectin-3 in thyroid gland and follicular cell tumors of the thyroid. A critical study of its possible role in preoperative differential diagnosis

Galectin-3 belongs to a group of endogenous lectins with an affinity to glycoconjugates containing beta-galactoside residues. It has been detected in numerous tissues and studied in connection with tumor growth, dedifferentiation, and metastasis. Only few studies have dealt with galectin-3 detection in tumors of the thyroid gland and with its possible role for differential diagnosis. We studied 118 cases of thyroid gland tumors with a monoclonal antibody against galectin-3; we compared the preparations by a semiquantitative score to determine differences in expression. Normal thyroid gland tissue, goiter tissue, and tissue with functional enhancement were largely negative for galectin-3. Adenomas with a typical cytological pattern were predominantly negative, but a focal positive reaction in single cells and cell groups or follicles was possible. Almost all papillary carcinomas showed a distinct galectin-3 expression. While findings in follicular carcinomas and oxyphilic adenomas and carcinomas were very uneven, with both positive and negative tumors, the galectin-3 reaction can be helpful in recognizing follicular variants of the papillary carcinoma. Investigation of the biological significance of tumors should always be cautious and consider known histological criteria for malignancy.

3-Cloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a rat thyroid gland carcinogen, does not affect serum levels of TSH and thyroid hormones

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a chlorine disinfection by-product in drinking water, causes follicular adenomas and carcinomas in thyroid glands of Wistar rats with an unknown mechanism. We evaluated effects of MX on blood thyroid stimulating hormone (TSH), thyroxine (T 4), triiodothyronine (T 3), prolactin (PRL) and growth hormone (GH) levels in male and female Wistar rats to assess their role in the tumorigenesis. The levels of TSH, PRL and GH in serum of male rats were not significantly affected by a single dose of 1, 10 or 60 mg/kg of MX administered by gavage 2 h before sampling. In repeated dose experiments MX was administered at dose levels of 1, 10 or 60 mg/kg of MX (40 mg/kg for females) in water by gavage daily for 1 or 3 weeks. Thyroid glands, adrenal glands and the liver were evaluated for morphological changes and cell proliferation activity after staining with proliferating cell nuclear antigen (PCNA). The dose of 60 mg/kg MX was toxic upon repeated administration. Nevertheless, MX did not affect blood TSH and T 4 levels at any time point in either sex. T 3 concentration increased transiently in males (by 37% after week 1) at the highest MX dose but not in females. MX did not change the weights of thyroid glands, their morphology and cell proliferation activity by the end of the week 3. MX did not affect blood PRL levels but decreased GH levels in males at all doses after the first week of MX treatment. The results indicate that MX does not alter blood TSH and thyroid hormone levels in rats, and imply that MX may not cause thyroid follicular cell tumors by TSH-mediated hormonal promotion.

Preconception Urethane or Chromium(III) Treatment of Male Mice: Multiple Neoplastic and Non-neoplastic Changes in Offspring

Increase in neoplasia in offspring after preconception exposure of parents presents puzzling features such as high frequency of effects and lack of Mendelian inheritance. The present study examined the hypothesis that preconception carcinogenesis involves an increase in the rate of occurrence of neoplasms with a spontaneous incidence. Male NIH Swiss mice (12 per group) were exposed 2 weeks before mating (once, ip) to urethane (1.5 g/kg) or chromium(III) chloride (1 mmol/kg). Offspring (48–78/sex/group) were examined for all grossly apparent changes when moribund or at natural death, followed by histopathological diagnosis and statistical analysis. Significant exposure-related changes occurred in multiple organs. Ten to 20 percent of offspring showed changes related to paternal exposure, including at least one sired by most treated males. Pheochromocytomas occurred in both male and female offspring after both treatments, with none in controls. These neoplasms are rare in mice and suggest endocrine dysfunction as a component of preconception carcinogenesis. This was supported by increases in thyroid follicular cell and Harderian gland tumors, ovarian cysts, and uterine abnormalities. Lung tumors were increased in female offspring only. Effects seen in offspring only after paternal urethane exposure were an increase in preneoplasia/neoplasia in the glandular stomach (males) and in females, increased lymphoma but decreased incidence of histiocytic sarcoma. Increases in incidence of male reproductive gland tumors and of renal non-neoplastic lesions occurred only after chromium exposure. Thus, preconception exposure of fathers to toxicants had a significant impact on both neoplastic and non-neoplastic changes in almost all tissues in which these lesions often occur naturally during the aging process.

Mechanism for inhibition of thyroid peroxidase by leucomalachite green.

The triphenylmethane dye, malachite green (MG), is used to treat and prevent fungal and parasitic infections in the aquaculture industry. It has been reported that the reduced metabolite of MG, leucomalachite green (LMG), accumulates in the tissues of fish treated with MG. MG is structurally related to other triphenylmethane dyes (e.g., gentian violet and pararosaniline) that are carcinogenic in the liver, thyroid, and other organs of experimental animals. The ability of LMG to inhibit thyroid peroxidase (TPO), the enzyme that catalyzes the iodination and coupling reactions required for thyroid hormone synthesis, was determined in this study. LMG inhibited TPO-catalyzed tyrosine iodination (half-maximal inhibition at ca. 10 microM). LMG also inhibited the TPO-catalyzed formation of thyroxine in low-iodine human goiter thyroglobulin (half-maximal inhibition at ca. 10 microM) using a model system that measures simultaneous iodination and coupling. Direct inhibition of the coupling reaction by LMG was shown using a coupling-only system containing chemically preiodinated thyroglobulin as the substrate. Incubation of LMG with TPO, iodide, and tyrosine in the presence of a H2O2-generating system yielded oxidation products that were identified by using on-line LC/APCI-MS as desmethyl LMG, 2desmethyl LMG, 3desmethyl LMG, MG, and MG N-oxide. Similar products from LMG were observed in incubations with TPO and H2O2 alone. These findings suggest that the anti-thyroid effects (increased serum thyroid-stimulating hormone and decreased serum thyroxine) observed in rats treated with LMG result from blockade of hormone synthesis through alternate substrate inhibition and that chronic exposure could cause thyroid follicular cell tumors through a hormonal mechanism. The observed TPO-catalyzed oxidative demethylation of LMG to a primary arylamine also suggests a genotoxic mechanism for tumor formation is possible.

Risk Assessment of Thyroid Follicular Cell Tumors

Thyroid follicular cell tumors arise in rodents from mutations, perturbations of thyroid and pituitary hormone status with increased stimulation of thyroid cell growth by thyroid-stimulating hormone (TSH), or a combination of the two. The only known human thyroid carcinogen is ionizing radiation. It is not known for certain whether chemicals that affect thyroid cell growth lead to human thyroid cancer. The U.S. Environmental Protection Agency applies the following science policy positions: 1) chemically induced rodent thyroid tumors are presumed to be relevant to humans; 2) when interspecies information is lacking, the default is to assume comparable carcinogenic sensitivity in rodents and humans; 3) adverse rodent noncancer thyroid effects due to chemically induced thyroid-pituitary disruption are presumed to be relevant to humans; 4) linear dose-response considerations are applied to thyroid cancer induced by chemical substances that either do not disrupt thyroid functioning or lack mode of action information; 5) nonlinear thyroid cancer dose-response considerations are applied to chemicals that reduce thyroid hormone levels, increase TSH and thyroid cell division, and are judged to lack mutagenic activity; and 6) nonlinear considerations may be applied in thyroid cancer dose-response assessments on a case-by-case basis for chemicals that disrupt thyroid-pituitary functioning and demonstrate some mutagenic activity. Required data for risk assessment purposes is mode of action information on mutagenicity, increases in follicular cell growth (cell size and number) and thyroid gland weight, thyroid-pituitary hormones, site of action, correlations between doses producing thyroid effects and cancer, and reversibility of effects when dosing ceases.

Mode of Carcinogenic Action of Pesticides Inducing Thyroid Follicular Cell Tumors in Rodents

Mode of Carcinogenic Action of Pesticides Inducing Thyroid Follicular Cell Tumors in Rodents

Mechanism of Thiazopyr-Induced Effects on Thyroid Hormone Homeostasis in Male Sprague–Dawley Rats

Chronic administration of thiazopyr in the diet at dose levels of 1000 and 3000 ppm, but not 100 ppm, has demonstrated an increase in thyroid follicular cell tumors in male Sprague–Dawley rats. In the studies reported here we have evaluated the mechanism of thiazopyr-induced thyroid tumors by studying the effect of thiazopyr on a number of endpoints that indicate hypothalamic–pituitary–thyroid homeostasis. At a dose level of 3000 ppm, thiazopyr caused a marked depression in circulating levels of T4as soon as 7 days after commencement of treatment. Concurrent with this decrease in T4was an increase in TSH levels, an increase in thyroid and liver weights, a three- to sixfold increase in hepatic T4-uridine diphosphate glucuronosyl transferase (UDPGT) activity, and increases in thyroid follicular cell hypertrophy and hyperplasia. Dose-related changes associated with thiazopyr treatment were significant increases in liver weight, thyroid weight, and hepatic T4-UDPGT activity at high doses. Increased levels of serum TSH, T3, and rT3, decreased levels of T4, and an increased incidence of thyroid follicular cell hypertrophy and hyperplasia were observed 56 days after the initiation of 3000 ppm thiazopyr. All the changes, except thyroid weight, were partially or completely reversible upon removal of thiazopyr from the diet. Increased thyroid T4elimination, primarily via increased hepatic conjugation by T4-UDPGT, resulting in decreased serum T4, appeared to be responsible for the increased TSH levels. The sustained increase in TSH by thiazopyr appears responsible for the stimulation of the thyroid follicular cells resulting in follicular cell hypertrophy, hyperplasia, and ultimately neoplasia. In summary, evidence is presented for a hormonally mediated, threshold-dependent process for the development of thyroid follicular cell tumors from high-dose thiazopyr administration in male rats. This mechanism is not considered to be relevant to humans, since the thyroid of humans is much less sensitive to this pathogenic phenomenon than rodents.