Effects of stones and other physical factors on the induction of rodent bladder cancer

Animal Models in Bladder Cancer

BACKGROUND

Bladder cancer (urothelial cancer of the bladder) is the most common malignancy affecting the urinary system with an increasing incidence and mortality. Mouse models of bladder cancer should possess a high value of reproducibility, predictability, and translatability to allow mechanistic, chemo-preventive, and therapeutic studies that can be furthered into human clinical trials.

OBJECTIVES

To provide an overview and resources on the origin, molecular and pathological characteristics of commonly used animal models in bladder cancer.

METHODS

A PubMed and Web of Science search was performed for relevant articles published between 1980 and 2021 using words such as: "bladder" and/or "urothelial carcinoma" and animal models. Animal models of bladder cancer can be categorized as autochthonous (spontaneous) and non-autochthonous (transplantable). The first are either chemically induced models or genetically engineered models. The transplantable models can be further subclassified as syngeneic (murine bladder cancer cells implanted into immunocompetent or transgenic mice) and xenografts (human bladder cancer cells implanted into immune-deficient mice). These models can be further divided-based on the site of the tumor-as orthotopic (tumor growth occurs within the bladder) and heterotopic (tumor growth occurs outside of the bladder).

Urinary crystal formation and urothelial effects of pyroxasulfone administered to male rats

Pyroxasulfone induced a low incidence of urinary bladder tumors in male rats in a 2-year bioassay at 1000 and 2000 ppm, with occasional urinary calculi. No increased incidence of tumors of any tissue occurred in female rats or in mice of either gender. We performed three short-term studies to evaluate early development of pyroxasulfone-induced urinary crystals and urothelial cytotoxicity with consequent regenerative proliferation. First, male rats were treated with dietary 50, 1000 or 2000 ppm pyroxasulfone for 1, 3 or 7 days. The urothelium was examined by light and scanning electron microscopy (LM, SEM) and bromodeoxyuridine labeling index (BrdU LI). In two other studies, male rats were treated with dietary 20 000 ppm pyroxasulfone for 1 week. Urine collected at various times of day was examined by SEM and energy dispersive spectroscopy (EDS) or by LM, SEM, EDS, and infrared spectroscopy (IFS). Urinary crystals were present at various time points. EDS and IFS showed some contained calcium; others contained organic matter. Cytotoxicity was detected by SEM as cellular swelling, craters, and necrosis and by LM as cellular hypertrophy. Increased cell proliferation was detected by LM (hyperplasia), SEM (piling up of round cells), and by increased BrdU LI. There was no evidence of increased apoptosis. These findings support a mode of action for pyroxasulfone-associated bladder tumors in male rats involving formation of urinary crystals leading to urothelial cytotoxicity and regenerative proliferation. This is a high dose phenomenon, therefore, pyroxasulfone is not likely to be carcinogenic to humans at exposure levels that do not cause crystals with subsequent calculi formation in the urinary tract.

Crystalluria and Chronic Kidney Disease

Crystalluria can involve the kidney and lower urinary tract, can produce acute and chronic effects, and occurs in all mammalian species. Most commonly urinary crystals contain calcium. Numerous other endogenous and exogenous substances can produce crystalluria. Crystals are identified in kidneys of many species, up to 100% in certain rat strains. More severe renal disease (acute tubular necrosis and chronic renal disease) can be secondary to crystal accumulation, such as observed with melamine-cyanuric acid in cats and dogs. Aggregation of crystals leads to calculi that act as urothelial abrasives with consequent regenerative proliferation. Accumulation in the kidney pelvis or bladder can lead to partial or complete obstruction and hydronephrosis. Long-term presence of urinary tract calculi in rodents leads to increased risk of urothelial tumors, but not in humans. Crystals in the lower urinary tract can act as irritants in rodents, but not in humans. It is critical that specific procedures are followed to optimize the presence of crystals in urine for diagnosis, including not fasting the animals. Numerous factors have been identified which can enhance or inhibit crystal formation. Extrapolation from animals for the threshold toxicity of crystals/calculi is appropriate but is not relevant for cancer risk assessment.

Screening for human urinary bladder carcinogens: two-year bioassay is unnecessary

Screening for carcinogens in general, and for the urinary bladder specifically, traditionally involves a two-year bioassay in rodents, the results of which often do not have direct relevance to humans with respect to mode of action (MOA) and/or dose response. My proposal describes a multi-step short-term (90 day) screening process that characterizes known human urinary bladder carcinogens, and identifies those reported in rodent two-year bioassays. The initial step is screening for urothelial proliferation, by microscopy or by increased Ki-67 labeling index. If these are negative, the agent is not a urinary bladder carcinogen. If either of these is positive, an MOA and dose response analysis are performed. DNA reactivity is evaluated. If the chemical is non-DNA reactive, evaluation for cytotoxicity is performed. This involves examination of the urothelium and urine, the latter to identify the generation of urinary solids (e.g. calculi). If urinary solids are the cause of cytotoxicity, the MOA is not relevant to human cancer, but dose response becomes essential for evaluating potential toxicity to humans. If cytotoxicity occurs but no urinary solids are detected, urinary concentrations of the chemical and its metabolites are evaluated, and compared to in vitro cytotoxicity against rodent and human immortalized urothelial cell lines. Based on this process, a screen for urinary bladder carcinogenicity is reliable, and more importantly, can be based on MOA and dose response analyses useful in the overall risk assessment for possible human bladder cancer. The proposed procedure is shorter, less expensive and more relevant than the two-year bioassay.

Insights from animal models of bladder cancer: recent advances, challenges, and opportunities

Bladder cancer (urothelial cancer of the bladder) is the most common malignancy affecting the urinary system with increasing incidence and mortality. Treatment of bladder cancer has not advanced in the past 30 years. Therefore, there is a crucial unmet need for novel therapies, especially for high grade/stage disease that can only be achieved by preclinical model systems that faithfully recapitulate the human disease. Animal models are essential elements in bladder cancer research to comprehensively study the multistep cascades of carcinogenesis, progression and metastasis. They allow for the investigation of premalignant phases of the disease that are not clinically encountered. They can be useful for identification of diagnostic and prognostic biomarkers for disease progression and for preclinical identification and validation of therapeutic targets/candidates, advancing translation of basic research to clinic. This review summarizes the latest advances in the currently available bladder cancer animal models, their translational potential, merits and demerits, and the prevalent tumor evaluation modalities. Thereby, findings from these model systems would provide valuable information that can help researchers and clinicians utilize the model that best answers their research questions.

Risk Assessment in the Genomic Era

The sequencing of the human and mouse genomes, and soon that of the rat, offers a foundation to evaluate biological phenomena, including toxicologic effects. Numerous tools are being developed to evaluate aspects of biology based on the DNA sequence. These tools can be utilized to evaluate absorption, distribution, metabolism and excretion, and effects of the toxicologically active product on the target organ. The genes involved can be broadly categorized as those affecting susceptibility to a toxicologic effect and those that are involved in the biologic response. For risk assessment to be performed in a rational manner, fundamental mechanisms of toxicologic processes must be ascertained. Based on successes already achieved, such as development of transgenic and knockout mouse strains, the application of aspects of the genomics revolution could be useful in developing a better understanding of mechanisms, and possibly in identifying specific markers of responses. In addition, genomics are likely to be useful in translating effects between species. However, genomics are being portrayed as the ultimate solution to all of toxicology. This is hardly the case. Basic chemistry, biochemistry, toxicokinetics, pharmacology, and pathology will continue to be needed in the overall weight of evidence approach to risk assessment. Genomics are likely to be of limited usefulness in predicting individual, in contrast to population susceptibility to various toxicological responses. Concordance of various diseases in identical twins, for example, different cancers, is rarely greater than 20% over the lifetime of these individuals. Similarly, genomics are likely to be of limited usefulness in screening for toxicologic end points. As with other tools of biology, those to be developed based on the genome are likely to provide greater usefulness in dissecting the mechanistic processes involved and defining the basis for susceptibility.

Selection of appropriate tumour data sets for Benchmark Dose Modelling (BMD) and derivation of a Margin of Exposure (MoE) for substances that are genotoxic and carcinogenic: considerations of biological relevance of tumour type, data quality and uncertainty assessment

This article addresses a number of concepts related to the selection and modelling of carcinogenicity data for the calculation of a Margin of Exposure. It follows up on the recommendations put forward by the International Life Sciences Institute - European branch in 2010 on the application of the Margin of Exposure (MoE) approach to substances in food that are genotoxic and carcinogenic. The aims are to provide practical guidance on the relevance of animal tumour data for human carcinogenic hazard assessment, appropriate selection of tumour data for Benchmark Dose Modelling, and approaches for dealing with the uncertainty associated with the selection of data for modelling and, consequently, the derived Point of Departure (PoD) used to calculate the MoE. Although the concepts outlined in this article are interrelated, the background expertise needed to address each topic varies. For instance, the expertise needed to make a judgement on biological relevance of a specific tumour type is clearly different to that needed to determine the statistical uncertainty around the data used for modelling a benchmark dose. As such, each topic is dealt with separately to allow those with specialised knowledge to target key areas of guidance and provide a more in-depth discussion on each subject for those new to the concept of the Margin of Exposure approach.

Long-term treatment with L-isoleucine or L-leucine in AIN-93G diet has promoting effects on rat bladder carcinogenesis

In the present study, effects of L-leucine and L-isoleucine on rat bladder carcinogenesis were investigated using AIN-93G and MF basal diet. In Experiment 1, N-butyl-N-(4-hydroxybutyl)-nitrosamine was used as an initiator of bladder carcinogenesis. In the AIN-93G diet groups, a significantly higher incidence and multiplicity of bladder tumors, accompanied by decreased final body weight, was observed in the L-leucine-supplemented group and a significantly higher incidence of papillomas and total tumors was observed in the L-isoleucine-supplemented group. In the MF diet groups, the multiplicity of papillary and nodular hyperplasia was significantly increased in the L-isoleucine-supplemented group. Urinary pH values were not affected by supplementing either type of diet with L-leucine or L-isoleucine. In Experiment 2, the amino acid was administered in the basal diets for 2 weeks without initiator. No pathological lesions were observed in the bladder urothelium in any of the groups, and no significant differences in urinary pH values, microcrystals or aggregates were observed between the amino acid-supplemented groups and their respective control groups. In conclusion, long-term treatment with L-leucine or L-isoleucine has a promoting effect on rat bladder carcinogenesis; therefore, their long-term use as a dietary supplement for bladder cancer patients should be avoided until more is known.

Toxicological review and oral risk assessment of terephthalic acid (TPA) and its esters: A category approach

Polyethylene terephthalate, a copolymer of terephthalic acid (TPA) or dimethyl terephthalate (DMT) with ethylene glycol, has food, beverage, and drinking water contact applications. Di-2-ethylhexyl terephthalate (DEHT) is a plasticizer in food and drinking water contact materials. Oral reference doses (RfDs) and total allowable concentrations (TACs) in drinking water were derived for TPA, DMT, and DEHT. Category RfD and TAC levels were also established for nine C(1)-C(8) terephthalate esters. The mode of action of TPA, and of DMT, which is metabolized to TPA, involves urinary acidosis, altered electrolyte elimination and hypercalciuria, urinary supersaturation with calcium terephthalate or calcium hydrogen terephthalate, and crystallization into bladder calculi. Weanling rats were more sensitive to calculus formation than dams. Calculi-induced irritation led to bladder hyperplasia and tumors in rats fed 1000 mg/kg-day TPA. The lack of effects at 142 mg/kg-day supports a threshold for urine saturation with calcium terephthalate, a key event for calculus formation. Chronic dietary DMT exposure in rodents caused kidney inflammation, but not calculi. Chronic dietary DEHT exposure caused general toxicity unrelated to calculi, although urine pH was reduced suggesting the TPA metabolite was biologically-active, but of insufficient concentration to induce calculi. Respective oral reference doses of 0.5, 0.5, and 0.2 mg/kg-day and total allowable drinking water concentrations of 3, 3, and 1 mg/L were derived for TPA, DMT, and DEHT. An oral RfD of 0.2 mg/kg-day for the terephthalate category chemicals corresponded to a drinking water TAC of 1 mg/L.

Guidance for the classification of carcinogens under the Globally Harmonised System of Classification and Labelling of Chemicals (GHS)

The United Nations Conference on Environment and Development (UNCED) has developed criteria for a globally harmonised system of classification and labelling of chemicals (GHS). With regard to carcinogenicity, GHS distinguishes between Category 1 ('known or presumed human carcinogens') and Category 2 ('suspected human carcinogens'). Category 1 carcinogens are divided into Category 1A ('known to have carcinogenic potential for humans'), based largely on human evidence, and 1B ('presumed to have carcinogenic potential for humans'), based largely on experimental animal data. Concerns have been raised that the criteria for applying these carcinogenicity classifications are not sufficiently well defined and potentially allow different conclusions to be drawn. The current document describes an attempt to reduce the potential for diverse conclusions resulting from the GHS classification system through the application of a series of questions during the evaluation of data from experiments with rodents; epidemiological data, which could lead to Category 1A, have not been considered. Answers to each question can lead either to a classification decision or to the next question, but this process should only be implemented in an environment of informed scientific opinion. The scheme is illustrated with five case studies. These questions are: (1) Has a relevant form of the substance been tested? (2) Is the study design relevant to human exposure? (3) Is there a substance-related response? (4) Is the target tissue exposure relevant to humans? (5) Can a mode of action be established? (6) Is the mode of action relevant to humans? (7) What is the potency?

Derivation of a melamine oral reference dose (RfD) and drinking-water total allowable concentration

Due to its high nitrogen content, melamine has been used to adulterate food to increase apparent protein content. In 2008, thousands of Chinese infants consumed reconstituted formula derived from melamine-adulterated milk. Urinary-tract stones (comprised of melamine and uric acid) accumulated in some victims and lead to acute renal failure or death. Premature infants and children (<2 yr) have an increased susceptibility to ingested melamine. Due to incomplete reporting, the human data were inadequate to identify a no-observed-adverse-effect level (NOAEL) for melamine-induced pediatric urolithiasis. Urolithiasis, urinary bladder cystitis, and ulcerations were observed in F344 rats after subchronic or chronic ingestion of melamine at > or =72 mg/kg-d. Bladder epithelial damage was followed by epithelial hyperplasia that progressed to bladder papillomas and carcinomas in male but not female F344 rats or male or female B6C3F1 mice. Short-term assays suggest, at best, weak genotoxic activity, and kinetic data show that melamine is not metabolized. Since reliable exposure information was lacking from the clinical reports, an oral reference dose (RfD) based on urolithiasis in male rats after 13 wk of continuous melamine ingestion was calculated as a 10% benchmark dose (38 mg/kg-d). Incorporation of 10-fold interspecies and intraspecies (for the increased susceptibility of infants) uncertainty factors and a threefold database uncertainty factor (for the lack of immunological, neurological and reproduction toxicity data) yields an oral RfD of 0.13 mg/kg-d. Assuming the 70-kg adult consumes 2 L of drinking water daily, a total allowable concentration of 0.9 mg/L (900 microg/L) was calculated for melamine in drinking water.

Neoplastic and Non-neoplastic Changes in F-344 Rats Treated with Naveglitazar, a γ-Dominant PPAR α/γ Agonist

The carcinogenic potential of naveglitazar, a γ-dominant peroxisome proliferator-activated receptor (PPAR) α/γ dual agonist, was evaluated in a two-year study in F344 rats (0, 0.3, 1.0, or 3.0 mg/kg, males; 0, 0.1, 0.3, or 1.0 mg/kg, females). Increased mortality in male rats of the high-dose group was related to cardiac-associated lesions, neoplasms, and undetermined causes. Degeneration and hypertrophy of the myocardium occurred with dose-responsive increased incidence and severity. Neoplasms with increased incidence included sarcomas in male rats and urinary bladder neoplasms in female rats. Most sarcomas in male rats occurred in the adipose tissue of the subcutis and were diagnosed as fibrosarcomas, with fewer liposarcomas and other histologic types. Non-neoplastic changes in adipose tissue included expansion of adipose tissue in multiple sites, alterations in cytoplasmic vesicular pattern in brown and white fat, increases in stroma and mesenchymal cells, and fibrosis. The severity of chronic progressive nephropathy was decreased in a dose-responsive manner in males, and hyperplasia and neoplasia of the mammary gland were decreased in incidence in females. The adverse effects of cardiotoxicity and increased incidence of neoplasms occurred with dose-responsive incidence and/or severity, and a no-effect level for these effects was not achieved in this study.

A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

The human relevance framework (HRF) outlines a four-part process, beginning with data on the mode of action (MOA) in laboratory animals, for evaluating the human relevance of animal tumors. Drawing on U.S. EPA and IPCS proposals for animal MOA analysis, the HRF expands those analyses to include a systematic evaluation of comparability, or lack of comparability, between the postulated animal MOA and related information from human data sources. The HRF evolved through a series of case studies representing several different MOAs. HRF analyses produced divergent outcomes, some leading to complete risk assessment and others discontinuing the process, according to the data available from animal and human sources. Two case examples call for complete risk assessments. One is the default: When data are insufficient to confidently postulate a MOA for test animals, the animal tumor data are presumed to be relevant for risk assessment and a complete risk assessment is necessary. The other is the product of a data-based finding that the animal MOA is relevant to humans. For the specific MOA and endpoint combinations studied for this article, full risk assessments are necessary for potentially relevant MOAs involving cytotoxicity and cell proliferation in animals and humans (Case Study 6, chloroform) and formation of urinary-tract calculi (Case Study 7, melamine). In other circumstances, when data-based findings for the chemical and endpoint combination studied indicate that the tumor-related animal MOA is unlikely to have a human counterpart, there is little reason to continue the risk assessment for that combination. Similarly, when qualitative considerations identify MOAs specific to the test species or quantitative considerations indicate that the animal MOA is unlikely to occur in humans, such hazard findings are generally conclusive and further risk assessment is not necessary for the endpoint-MOA combination under study. Case examples include a tumor-related protein specific to test animals (Case Study 3, d-limonene), the tumor consequences of hormone suppression typical of laboratory animals but not humans (Case Study 4, atrazine), and chemical-related enhanced hormone clearance rates in animals relative to humans (Case Study 5, phenobarbital). The human relevance analysis is highly specific for the chemical-MOA-tissue-endpoint combination under analysis in any particular case: different tissues, different endpoints, or alternative MOAs for a given chemical may result in different human relevance findings. By providing a systematic approach to using MOA data, the HRF offers a new tool for the scientific community's overall effort to enhance the predictive power, reliability and transparency of cancer risk assessment.

Urothelial carcinogenesis in the urinary bladder of rats treated with naveglitazar, a gamma-dominant PPAR alpha/gamma agonist: lack of evidence for urolithiasis as an inciting event

Naveglitazar, a gamma-dominant peroxisome proliferator-activated receptor (PPAR) alpha/gamma dual agonist, was tested for carcinogenicity in F344 rats in a 2-year study. Changes in urine composition and urothelial morphology were characterized in a companion 18-month investigative study. A significant increase in neoplasms of the bladder occurred only in females of the high-dose group (14/60) in the carcinogenicity study. Trends toward increased cell proliferation in the urothelium were noted in both sexes at all time points evaluated in the 18-month study. Group means for urothelial mitogenesis were increased statistically significantly only in high-dose females at 12 and 18 months. Urothelial hyperplasia occurred in high-dose females at 18 months. Morphologic changes in the urothelium at earlier time points were limited to hypertrophy and decreased immunolabeling of the superficial cells for cytokeratin 20 (a marker of terminal differentiation in urothelial cells) in both males and females. No treatment-related changes in urinary parameters, including urinary sediments, were associated with the occurrence of urothelial proliferation. Urinary pH was unaffected by treatment in both males and females, but expected diurnal changes were demonstrated. Collectively, these data indicate that naveglitazar was associated with hypertrophic and proliferative effects on the urothelium, but a link with changes in urinary parameters was not demonstrated.

Urothelial carcinogenesis in the urinary bladder of male rats treated with muraglitazar, a PPAR alpha/gamma agonist: Evidence for urolithiasis as the inciting event in the mode of action

Muraglitazar, a PPARalpha/gamma agonist, dose-dependently increased urinary bladder tumors in male Harlan Sprague-Dawley (HSD) rats administered 5, 30, or 50 mg/kg/day for up to 2 years. To determine the mode of tumor development, male HSD rats were treated daily for up to 21 months at doses of 0, 1, or 50 mg/kg while being fed either a normal or 1% NH4Cl-acidified diet. Muraglitazar-associated, time-dependent changes in urine composition, urothelial mitogenesis and apoptosis, and urothelial morphology were assessed. In control and treated rats fed a normal diet, urine pH was generally > or = 6.5, which facilitates formation of calcium-and magnesium-containing solids, particularly in the presence of other prolithogenic changes in rat urine. Urinary citrate, an inhibitor of lithogenesis, and soluble calcium concentrations were dose dependently decreased in association with increased calcium phosphate precipitate, crystals and/or microcalculi; magnesium ammonium phosphate crystals and aggregates; and calcium oxalate-containing thin, rod-like crystals. Morphologically, sustained urothelial cytotoxicity and proliferation with a ventral bladder predilection were noted in treated rats by month 1 and urinary carcinomas with a similar distribution occurred by month 9. Urothelial apoptotic rates were unaffected by muraglitazar treatment or diet. In muraglitazar-treated rats fed an acidified diet, urine pH was invariably < 6.5, which inhibited formation of calcium-and magnesium-containing solids. Moreover, dietary acidification prevented the urothelial cytotoxic, proliferative, and tumorigenic responses. Collectively, these data support an indirect pharmacologic mode of urinary bladder tumor development involving alterations in urine composition that predispose to urolithiasis and associated decreases in urine-soluble calcium concentrations.

Deregulation of the p16-cyclin D1/cyclin-dependent kinase 4–retinoblastoma pathway involved in the rat bladder carcinogenesis induced by terephthalic acid-calculi

Prolonged cell proliferation in response to irritation by calculi may itself evoke malignant transformation of the urothelium. However, the molecular mechanisms underlying this process are still unknown. The aim of the present study was to investigate cell cycle regulatory mechanisms in bladder carcinogenesis induced by bladder calculi. Six-week-old Wistar rats were consecutively fed a diet containing 5% terephthalic acid (TPA), 5% TPA plus 4% sodium bicarbonate (NaHCO(3)), 4% NaHCO(3), or basal diet for 48 weeks. Animals were killed at weeks 12, 24, and 48. Treatment with 5% TPA caused high incidences of bladder calculi, preneoplastic lesions, and neoplastic lesions. Immunohistochemical examination revealed overexpression of cyclin D1, cyclin-dependent kinase 4 (Cdk4), retinoblastoma (Rb), and proliferating cell nuclear antigen (PCNA) in bladder preneoplastic and neoplastic lesions. In contrast, p16 expression was reduced or absent. These results were confirmed by immunoblotting analysis. Quantitation of mRNA by real-time reverse transcription-polymerase chain reaction (RT-PCR) showed a significant increase in cyclin D1 and PCNA mRNA in tumor cells. None of the 16 transitional cell carcinomas (TCCs) had ras mutations as examined by PCR-single strand conformational polymorphism (PCR-SSCP) analysis. These results suggested that deregulation of p16-cyclin D1/Cdk4-Rb pathway, but not oncogenic activation of ras, plays a crucial role in bladder tumorigenesis induced by bladder calculi.

Bladder epithelial cell proliferation of rats induced by terephthalic acid-calculi

OBJECTIVE

Urinary bladder hyperplasia associated with terephthalic acid (TPA) treatment was examined with concomitant use of sodium bicarbonate (NaHCO3) or hydrochlorothiazide to allow assessment of the relationship among bladder stones, epithelial hyperplasia, and corresponding cell cycle checkpoint gene expression in Sprague-Dawley (SD) rat.

METHODS

A total of 112 weanling male SD rats that divided between six groups were given basal diet (control), diets containing 5% TPA or in combination with either 4% sodium NaHCO3 or 0.02% hydrochlorothiazide. After 90-day feeding, bladder samples were collected for histopathological diagnoses, and immunohistochemical method was used to characterize the expression of p16Ink4a cyclin D1, CDK4, EGFr and cyclin E in relation to that of proliferating cell nuclear antigen (PCNA).

RESULTS

In TPA treatment groups, bladder stone incidence was 40% (21/52) with 14 cases of proliferative bladder. In control and other groups, neither stone nor epithelial cell proliferation was diagnosed. PCNA-positive focal hyperplasic lesions involved all epithelial layers. Overexpressions of cyclin D1, CDK4, EGFr are found in the corresponding lesion. p16Ink4a nuclear staining reduced in proliferative bladders especially with a great quantity of stone. In addition, no positive expression was detected on cyclin E.

CONCLUSION

The present study provides a strong evidence of a link between induction of bladder hyperplasia, deregulation of the p16Ink4a-cyclin D1/CDK4 pathway, and abnormal EGFr mediated signal transduction pathway.

Mammalian toxicology overview and human risk assessment for sulfosulfuron

Sulfosulfuron is a low-use rate sulfonylurea herbicide. A review of the toxicity database for sulfosulfuron indicates that the molecule has a low order of acute toxicity. It is not genotoxic and is not a reproductive, developmental, or nervous system toxicant. There were no indications of endocrine disruption in any study performed with the molecule. The only findings considered to be an adverse effect in mammalian laboratory animals following prolonged subchronic or chronic exposure to sulfosulfuron were isolated to the urinary tract. These findings occurred in conjunction with findings of urolith formation following high-level chemical dosing, resulting in epithelial hyperplasia that, in a few cases, progressed to tumor formation. Mode-of-action information supports the conclusion that these tumors result from a non-genotoxic, threshold-based process that is well established and widely considered to be not relevant to humans. Based on its short-term, infrequent application pattern and very low use rate and crop residues, aggregate and cumulative risk assessments indicate that sulfosulfuron has substantial margins of exposure and does not represent a significant risk to human health.

Toxicity and carcinogenicity of acidogenic or alkalogenic diets in rats; effects of feeding NH 4 Cl, KHCO 3 or KCl

The effects of diet-induced acid-base disturbances were examined in 4-week, 13-week and 18-month toxicity studies, and in a 30-month carcinogenicity study. Rats were fed a natural ingredient diet (controls), supplemented with 2% or 4% KHCO(3) (base-forming diets), or with 1% or 2.1% NH(4)Cl (acid-forming diets). Additional controls were fed 3% KCl (neutral diet providing K(+) and Cl(-) in amounts equimolar to those in the 4% KHCO(3) diet and the 2.1% NH(4)Cl diet, respectively). NH(4)Cl induced the expected metabolic acidosis, as shown by decreased base excess in blood, decreased urinary pH and increased urinary net acid excretion. KHCO(3) induced the opposite effects. KCl did not affect the acid-base balance. Clinical condition and death rate were not affected. The feeding of high levels of each salt resulted in growth retardation and increased water intake and urinary volume. Plasma potassium and urinary potassium excretion were increased with KHCO(3) and KCl. Plasma chloride was increased with NH(4)Cl, but not with KCl. Urinary calcium and phosphate excretion were increased with NH(4)Cl, but there were no indications that bone minerals were involved (weight, calcium content and fat free solid of the femur were not affected). Standard haematological and clinical chemistry parameters were not affected. Kidney weights were increased with 2.1% NH(4)Cl. Hypertrophy of the adrenal zona glomerulosa occurred with KHCO(3), KCl and NH(4)Cl, due to chronic stimulation of the adrenal cortex by either K(+) or by NH(4)Cl-induced acidosis. An early onset (from week 13) of oncocytic tubules was noted in the kidneys of rats fed KHCO(3) and, after 30 months, the incidence of this lesion was much higher than the background incidence in ageing controls. No progression to oncocytomas was noted. KCl showed only slight effects on the early onset of oncocytic tubules (from 18 months). In contrast, the severity of nephrosis and the incidence of oncocytic tubules were decreased with 2.1% NH(4)Cl, suggesting a protective effect of acidosis. The feeding of KHCO(3) resulted in hyperplasia, papillomas and carcinomas of the urinary bladder. With KCl only a slight increase in proliferative urothelial lesions was noted. Apart from these (pre-)neoplastic lesions in the urinary bladder there were no treatment-related differences in tumour response among the groups. We concluded that most of the observed changes represent physiological adaptations to the feeding of acid- or base-forming salts. Remarkable effects noted with KHCO(3), and to a far lesser extent with KCl, consisted of renal oncocytic tubules and (pre-)neoplastic lesions of the urinary bladder epithelium. NH(4)Cl-induced chronic metabolic acidosis was not associated with dissolution of alkaline bone salts in rats. Finally, a protective effect of chronic acidosis on tumour development was not found.

Comparative pathology of proliferative lesions of the urinary bladder

Bladder neoplasia in humans consists of 2 diseases, a low-grade papillary tumor that does not invade or metastasize, and a high-grade lesion that usually invades and metastasizes. Bladder carcinogenesis in rats is most like the low-grade, papillary tumor, although it eventually does progress and invade. In the mouse, models are available that mimic each of these disease processes. Preneoplastic lesions in humans and rodents include various types of hyperplasia, proliferative cystitis, and dysplasia. These preneoplastic and neoplastic lesions arise throughout the urothelium, from the renal pelvis to the urethra, although most commonly in the bladder. Rarely, benign and malignant mesenchymal lesions occur in rats and mice, with a unique submucosal mesenchymal lesion present in some strains of mice. In addition, eosinophilic and clear inclusions in the superficial layer of urothelium in mice, which do not appear to be associated with toxicity or carcinogenesis, have been reported. An approach to evaluation of carcinogenic mechanisms involved in the urothelium is presented. It focuses on distinguishing between DNA reactive carcinogens vs those that act by increasing cell proliferation. Although rodent models do not precisely mimic the human disease, they have provided useful models for furthering our understanding of the carcinogenic process in the urothelium as it pertains to human diseases.

Urinary tract calculi and thresholds in carcinogenesis

Numerous chemicals administered to rodents at relatively high doses produce urinary tract calculi, resulting in erosions or ulcerations of the urothelium, consequent regenerative hyperplasia, and ultimately tumors. This is a high-dose (threshold) phenomenon, which appears to occur more readily in rodents than in primates, including humans. Several anatomic and urinary physiologic differences between rodents and humans affect the quantitative extrapolation from results in rodent bioassays to human risk assessment. For most chemicals producing tumors by this mode of action, human exposures are significantly lower than would be expected to be required for production of calculi, and therefore pose no carcinogenic hazard to humans.

Chronic toxicity of thiabendazole (TBZ) in CD-1 mice

Male and female CD-1 mice (50 mice per group) were administered thiabendazole (TBZ) in diet at levels of 0 (control), 0.031, 0.125 and 0.5% for 78 weeks. A life time study was terminated after 78 weeks due to enhanced strain specific mortality. There were no significant differences in mortality between the control and treated groups. Mean body weights of high-dose groups showed significant decreases compared with the controls. The bladder weights of male and female mice of the 0.5% group were significantly higher than those of the control mice. Gross findings in treated mice included the renal atrophy, hydronephrosis, calculi in renal pelvis and/or bladder and ovarian atrophy. Microscopic findings in the kidneys of treated mice included the nephrosis, hydronephrosis or hyperplasia of transitional epithelium of renal pelvis or papilla. In the bladder of treated mice, hyperplasia or squamous metaplasia of transitional epithelium and one transitional cell papilloma were observed. Dose-dependent decreases in the incidence of spontaneous lesion in the male or female reproductive system were recognized. It is concluded that TBZ is not carcinogenic to CD-1 mice of both sexes. However, caution should be exercised in the long-term application of high TBZ doses.

Effect of sulfosulfuron on the urine and urothelium of male rats

Sulfosulfuron, developed as a herbicide, caused increased microcrystalluria and the formation of urinary tract calculi when fed to male and female rats in a chronic 2-year study at doses of 5,000 ppm and 20,000 ppm. Hyperplasia was also seen in urinary bladders at 5,000 ppm and 20,000 ppm, almost exclusively in the presence of observable calculi/microcalculi. Urinary bladder tumors were found in 2 females in the 5000 ppm group, both in the presence of calculi. No increased microcrystalluria, calculi, or tumors were found at doses of 500 ppm and lower. In the current study, 5 groups of male Sprague-Dawley rats were fed sulfosulfuron at doses of 50, 500, 5,000, and 20,000 ppm for 10 weeks. Ten animals were co-administered 5,000 ppm sulfosulfuron with 12,300 ppm NH4Cl to determine if inhibition of the formation of calculi would prevent any urothelial effects of treatment with sulfosulfuron. Ten animals in the control group and in the high-dose sulfosulfuron group were fed only basal diet for an additional 10 weeks to determine if the effects of sulfosulfuron on the bladder epithelium were reversible. There was an increased incidence of microcrystalluria observed at 5,000 and 20,000 ppm. There was no increase in microcrystalluria observed in the urine of rats co-administered sulfosulfuron and NH4Cl. Urinary bladder calculi were found in the bladder of 1 animal fed 20,000 ppm. Examination by light microscopy showed diffuse papillary/nodular hyperplasia of the bladder epithelium in this animal. No increased microcrystalluria was observed after withdrawal of the chemical from the diet and the bladder epithelium was normal by light microscopy. The hyperplastic effects associated with the feeding of high doses of sulfosulfuron occur only with the appearance of urinary tract calculi. Based on these results and anatomical differences between rats and humans, it may be concluded that the hyperplastic and carcinogenic effects of sulfosulfuron in rats are high-dose, threshold phenomena that are not likely to occur in humans under environmentally relevant exposures.

Calcium phosphate-containing precipitate and the carcinogenicity of sodium salts in rats

Sodium saccharin, ascorbate and other sodium salts fed at high doses to rats produce urinary bladder urothelial cytotoxicity with consequent regenerative hyperplasia. For sodium salts that have been tested, tumor activity is enhanced when administered either alone or after a brief exposure to a known genotoxic bladder carcinogen. These sodium salts alter urinary composition of rats resulting in formation of an amorphous precipitate. We examined the precipitate to ascertain its composition and further delineate the basis for its formation in rat urine. Using scanning electron microscopy with attached X-ray energy dispersive spectroscopy, the principal elements present were calcium, phosphorus, minor amounts of silicon and sulfur. Smaller elements are not detectable by this method. Infrared analyses demonstrated that calcium phosphate was in the tribasic form and silicon was most likely in the form of silica. Small amounts of saccharin were present in the precipitate from rats fed sodium saccharin (<5%), but ascorbate was not detectable in the precipitate from rats fed similar doses of sodium ascorbate. Large amounts of urea and mucopolysaccharide, apparently chondroitin sulfate, were detected in the precipitate by infrared analysis. Chemical analyses confirmed the presence of large amounts of calcium phosphate with variably small amounts of magnesium, possibly present as magnesium ammonium phosphate crystals, present in urine even in controls. Small amounts of protein, including albumin and alpha(2u)-globulin, were also detected (<5% of the precipitate). Calcium phosphate is an essential ingredient of the medium for tissue culture of epithelial cells, but when present at high concentrations (>5 mM) it precipitates and becomes cytotoxic. The nature of the precipitate reflects the unique composition of rat urine and helps to explain the basis for the species specificity of the cytotoxic and proliferative effects of high doses of these sodium salts.

Effects of dietary dimethylarsinic acid on the urine and urothelium of rats

Dimethylarsinic acid (DMA), fed to rats for 2 years, produced bladder hyperplasia and tumors at doses of 40 and 100 p.p.m., more in females than males. No urothelial proliferation was seen in mice. Our objectives were to investigate the mode of action of bladder tumor formation, evaluate the dose-response and the role of diet and to determine if the urothelial effects were reversible. The study included groups of female F344 rats fed DMA in Purina 5002 diet at doses of 0, 2, 10, 40 or 100 p.p.m. for 10 weeks; two groups of females fed DMA (0 and 100 p.p.m.) in Altromin 1321 for 10 weeks; two groups of males fed DMA (0 and 100 p.p.m.) in Purina 5002 for 10 weeks; a female high-dose recovery group (100 p.p.m. in Purina 5002 diet for 10 weeks followed by control diet for 10 weeks); and two female groups (0 and 100 p.p.m.) in Purina diet for 20 weeks. Urothelial toxicity and hyperplasia were detected by light and scanning electron microscopy (SEM), and the bromodeoxyuridine labeling index was increased in the female 40 and 100 p.p.m. groups. The effects were less in males, but were similar in females fed DMA in Altromin 1321. SEM detected no abnormal urinary solids related to treatment in any group. Urinary calcium was increased in the females fed 40 and 100 p.p.m. in Purina diet, despite overall urinary dilution. Calcification was increased in kidneys of female rats fed Purina diet. The urothelial effects of DMA were reversible. The findings support a non-DNA reactive mechanism for DMA rat bladder carcinogenicity related to urothelial toxicity and regeneration. The toxicity is probably not due to urinary solids. The toxicity and regeneration are produced in a dose-responsive manner in female rats, are greater in female than in male rats, and are reversible.

A dietary toxicity/oncogenicity study of tributyl phosphate in the rat

Tri-n-butyl phosphate (TBP, CAS No. 126-73-8), an industrial chemical, was administered in the diet at concentrations of 0, 200, 700 and 3000 ppm to groups of 50 male and 50 female Sprague-Dawley rats for 2 years. Body weights and food consumption were measured weekly for the first 13 weeks and monthly thereafter. Hematology was performed at 12, 18 and 24 months; urinalyses were performed at 3 weeks and 3, 6, 12 and 18 months. All surviving animals were euthanized after 24 months of treatment. Macroscopic postmortem examinations were performed on all animals; complete histopathological evaluation was performed on control and high dose animals; target organs were examined in all dose groups. Significant decreases in body weight gain occurred in males and females receiving the 3000 ppm concentration and a slight decrease in weight gain occurred in females receiving the 700 ppm concentration. The only clinical sign attributed to TBP was an increased incidence of red discoloration of the urine in some high-dose males. Survival, hematology and urinalysis parameters were unaffected by treatment at any concentration. A dose-related increase in the incidence and severity of urinary bladder hyperplasia and the incidence of urinary bladder papillomas was evident in male and female rats receiving the 700 and 3000 ppm concentrations. Transitional cell carcinomas were present in six of 49 males and two of 50 females and a squamous cell carcinoma was present in one of 49 males in the group which received 3000 ppm. The oncogenic effects showed a clear threshold of 700 ppm in the diet. The NOEL (no observable effect level) for chronic toxicity was 200 ppm. Mean intake of TBP was 9 and 12 mg/kg/day for males and females, respectively, receiving 200 ppm; 33 and 42 mg/kg/day for males and females, respectively, receiving 700 ppm, and 143 and 182 mg/kg/day for males and females, respectively, receiving 3000 ppm. TBP was negative in genotoxicity tests, suggesting that the tumors are induced by nongenotoxic mechanisms.

Urinary physiologic and chemical metabolic effects on the urothelial cytotoxicity and potential DNA adducts of o-phenylphenol in male rats

ortho-Phenylphenol (OPP), a fungicide and antibacterial agent with food residues, is carcinogenic to rat bladder. The present studies provide information on changes in urinary composition and urinary metabolites, urothelial cytotoxicity and regenerative hyperplasia, and DNA adducts in male F344 rats fed OPP. An initial experiment evaluated dietary doses of 0, 1,000, 4,000, and 12,500 ppm OPP fed for 13 weeks. There was no evidence of urinary calculi, microcrystalluria, or calcium phosphate-containing precipitate, but urothelial cytotoxicity and hyperplasia occurred at the highest dose only. In a second experiment, rats were fed dietary OPP levels of 0, 800, 4,000, 8,000, and 12,500 ppm. Urinary pH was > 7 in all groups. Urinary volume was increased at the 2 highest doses with consequent decreases in osmolality, creatinine, and other solutes. Total urinary OPP metabolite excretions were increased, mostly excreted as conjugates of OPP and of phenylhydroquinone. Free OPP or free metabolites accounted for less than 2% excreted in the urine without a dose response. Urothelial toxicity and hyperplasia occurred only at doses of 8,000 and 12,500 ppm. OPP-DNA adducts were not detected in the urothelium at any dose. In summary, OPP produces cytotoxicity and proliferation of the urothelium at dietary doses > or = 8,000 ppm without formation of urinary solids. The paucity of unconjugated metabolites and the lack of OPP-DNA adducts suggests that OPP is acting as a bladder carcinogen in male rats by inducing cytotoxicity and hyperplasia without it or its metabolites directly binding to DNA.

Urinary bladder carcinogenesis

Urinary bladder carcinogenesis in rodents bears numerous similarities to the diseases in humans. In rats, the process progresses through the morphologic stages of simple hyperplasia, papillary and nodular hyperplasia, papilloma, noninvasive, and invasive carcinoma. In mice, the pathogenesis can be similar or can follow a sequence of marked dysplasia with or without hyperplasia, leading to carcinoma in situ and ultimately to high-grade invasive carcinoma. Although the papillary and nonpapillary diseases appear to be related in rodents and in humans, they are distinct morphologically, biologically, and molecularly. Numerous classes of genotoxic chemicals have been identified as bladder carcinogens in rodents, and some of these have also been identified as carcinogenic in humans, most notably, aromatic amines, nitrosamines, and cyclophosphamide. In contrast, nongenotoxic chemicals appear to be highly specific with respect to species, strain, diet, agent, dose, and mechanism. For some, it is unclear whether the results at high doses in rodents can be extrapolated to low doses or to humans, e.g., chemicals that cause bladder cancer only at high doses related to the formation of calculi. Numerous observations in rodents can assist in identifying possible mechanisms involved for these nongenotoxic chemicals and therefore can be important for a rational evaluation of human risk.

Carcinogen risk assessment: a necessary dilemma?

Carcinogen risk assessment is the process by which an attempt is made to estimate human risk due to carcinogens, from the results of animal studies. It is based upon a number of prudent default assumptions, that is, assumptions that cannot be proved scientifically because either the basic concept is philosophical in nature or because the amount of scientific evidence required is too costly to obtain even on a world-wide basis. Recently, scientific effort has shown that more and more examples have been described suggesting these examples do not behave in the way indicated by the default assumptions. Since carcinogen risk assessment processes were initiated, it has been demonstrated that cancer may arise by four or more different mechanisms. It is the purpose of this paper to enquire whether consideration of these basically different mechanisms may facilitate carcinogen risk assessment.

Effects of diet on urinary bladder carcinogenesis and cancer prevention

Urine plays a major role in bladder carcinogenesis, acting as a transport mechanism for carcinogens, containing several growth factors stimulating cell proliferation, and indirectly affecting chemicals by alterations in concentrations of normal urinary components such as electrolytes, water and proteins. These latter effects are greatly modified by diet composition and consumption and also by water consumption. Several examples of these effects are presented.

Role of cell proliferation in regenerative and neoplastic disease

DNA replication does not have 100% fidelity. Consequently, a chemical can increase the risk of cancer either by directly damaging DNA (genotoxic) or by increasing the number of cell replications, or both. Increased cell proliferation can be produced by increasing cell births (by direct mitogenesis or regeneration following toxicity), or decreasing cell deaths (by inhibiting apoptosis or differentiation). Cell proliferation can affect the dose-response curve for genotoxic carcinogens and is the basis for carcinogenicity by nongenotoxic agents. Bladder carcinogens will be used to illustrate these mechanisms, and their implications with respect to human risk assessment will be presented.

Human bladder cancer: evidence for a potential irritation-induced mechanism

Bladder cancer is one of the most common human cancers, constituting about 6% and 2% of all cancers among males and females, respectively. Over 90% of all bladder cancers are transitional cell carcinomas, with most of the remainder being squamous cell carcinomas. Smoking and occupational exposure to aromatic amines and other agents are most prominent among the risk factors identified. Inflammation of the bladder, largely by infection but also by stones or a combination of the two, may play some role in human bladder cancer development. The association between inflammation and cancer appears to be stronger for squamous cell than for transitional cell carcinoma. Stones and infection can be important factors in the development of bladder tumours in rodents, but the tumours are predominantly transitional cell rather than squamous cell carcinomas.

Role of urinary physiology and chemistry in bladder carcinogenesis

Urine is a complex mixture of numerous substances, only some of which are described above. Literally thousands of substances have been identified in normal urine, including a variety of ions, non-ionic substances and macromolecules. Their presence and concentrations are highly variable, dependent on fluid intake and on nutritional, physiological and biochemical influences. Marked diurnal variations exist. Methodologies involved in the collection and analysis of these components can greatly influence the interpretation of the results. The influence of these various parameters in the urine on bladder carcinogenicity can be either direct or indirect. A major difficulty in studying this aspect of urothelial carcinogenesis is that it is essentially impossible to alter only one variable in the urine at a time. Alteration of any one variable results in physiological alteration of several other of the constituents in the urine. In addition, the processes involved in urothelial carcinogenesis frequently involve a complex interaction of multiple variables, such as volume, osmolality, cationic concentration, anionic concentration, quantitative and qualitative differences in protein, and generation of precipitate, crystals or calculi. Thus, it is likely that the actual mechanisms involved in the carcinogenic process with many of these chemicals, particularly those that are non-genotoxic, will involve a complex interaction of several constituents of the urine. Although this poses a formidable obstacle to our understanding in experimental situations as well as in extrapolating to humans, the role of specific factors appears to be discernible and should offer insight into the risk assessment process (Cohen and Ellwein, 1991 a,b and 1992).

Rodent bladder tumors do not always predict for humans

Dr. David Clayson, 20 years ago, suggested that chemicals which lead to the formation of calculi in rodents might pose an artifact with respect to extrapolation to potential carcinogenic risk to humans. We reviewed what has been learned about the role of calculi in urinary bladder carcinogenesis in the ensuing 20 years, along with several examples. Formation of microcrystalluria and amorphous precipitate also poses problems in interpretation and examples are described. The chemicals producing these solid urinary materials are non-genotoxic, with marked increase in cell proliferation being the mode of action by which they are able to produce cancer in long-term rodent bioassays.