Promoting effect of the peroxisome proliferator, clofibrate, but not di(2-ethylhexyl)phthalate, on urinary bladder carcinogenesis in F344 rats initiated by N-butyl-N-(4-hydroxybutyl)nitrosamine

In Utero Exposure to 3,3',4,4', 5-Pentachlorobiphenyl Dose-Dependently Induces N-butyl-4-(hydroxybutyl) Nitrosamine in Rats With Urinary Bladder Carcinoma

Polychlorinated biphenyls (PCBs) are fat-soluble environmental pollutants that can accumulate in adipose tissue or be secreted in milk. N-butyl-4-(hydroxy butyl) (BBN), a rat bladder carcinogen, recruits the host metabolism to yield its ultimate carcinogenic form via CYP1s. Since estrogen receptors (ERs) mediate biological responses important for the growth of bladder carcinoma, we investigated PCNA, Cyclin D1, ERs, CYP1s, and AhR expression in BBN rat bladder carcinomas with prenatal PCB exposure. Female SD rats were treated with 7.5 μg, 250 ng, and 2.5 ng of 3,3',4,4',5-pentachlorobiphenyl (PCB126)/kg or vehicle on days 13 to 19 post-pregnancy. Six-week-old male offspring were treated with 0.05% BBN for 10 weeks before being anesthetized and the urinary bladder wall incised to expose the bladder carcinomas. N-butyl-4-(hydroxybutyl) bladder carcinoma incidence increased with prenatal PCB exposure dose-dependently. In bladder carcinoma, PCB126 exposure significantly increased PCNA, D1, ERα, CYPIA1, CYP1B1, and AhR expression dose-dependently, and increased ERα expression was particularly prominent. However, the expression of ERβ was low, independent of the volume of PCB126 given, indicating similarity to the Vehicle group. We conclude that prenatal PCB126 exposure in rats can induce PCB126 to dose-dependently metabolize BBN via CYP1A1, and contribute to bladder carcinogenesis with upregulation of ERα expression.

Deciphering the Roles of Thiazolidinediones and PPAR in Bladder Cancer

The use of thiazolidinedione (TZD) therapy in type II diabetic patients has proven useful in the lowering of blood glucose levels. However, recent investigations have shown that there may be potential health concerns associated, including the risk of developing bladder cancer as well as complications in the cardiovasculature. TZDs are ligands for the nuclear receptor PPARγ, and activation causes lipid uptake and insulin sensitization, both of which are critical processes for diabetic patients whose bodies are unable to utilize insulin effectively. Several studies have shown that PPARγ/TZDs decrease IGF-1 levels and, thus, reduce cancer growth in carcinomas such as the pancreas, colon, liver, and prostate. However, other studies have shed light on the potential of the receptor as a biomarker for uroepithelial carcinomas, particularly due to its stimulatory effect on migration of bladder cancer cells. Furthermore, PPARγ may provide the tumor-promoting microenvironment by de novo synthesis of nutrients that are needed for bladder cancer development. In this review, we closely examine the TZD class of drugs and their effects on PPARγ in patient studies along with additional molecular factors that are positive modulators, such as protein phosphatase 5 (PP5), which may have considerable implications for bladder cancer therapy.

Rat Urinary Bladder Carcinogenesis by Dual-Acting PPARalpha + gamma Agonists

Despite clinical promise, dual-acting activators of PPARalpha and gamma (here termed PPARalpha+gamma agonists) have experienced high attrition rates in preclinical and early clinical development, due to toxicity. In some cases, discontinuation was due to carcinogenic effect in the rat urothelium, the epithelial layer lining the urinary bladder, ureters, and kidney pelvis. Chronic pharmacological activation of PPARalpha is invariably associated with cancer in rats and mice. Chronic pharmacological activation of PPARgamma can in some cases also cause cancer in rats and mice. Urothelial cells coexpress PPARalpha as well as PPARgamma, making it plausible that the urothelial carcinogenicity of PPARalpha+gamma agonists may be caused by receptor-mediated effects (exaggerated pharmacology). Based on previously published mode of action data for the PPARalpha+gamma agonist ragaglitazar, and the available literature about the role of PPARalpha and gamma in rodent carcinogenesis, we propose a mode of action hypothesis for the carcinogenic effect of PPARalpha+gamma agonists in the rat urothelium, which combines receptor-mediated and off-target cytotoxic effects. The proposed mode of action hypothesis is being explored in our laboratories, towards understanding the human relevance of the rat cancer findings, and developing rapid in vitro or short-term in vivo screening approaches to faciliate development of new dual-acting PPAR agonist compounds.

Morphological transformation and effect on gap junction intercellular communication in Syrian hamster embryo cells as screening tests for carcinogens devoid of mutagenic activity

A large fraction of chemicals observed to cause cancer in experimental animals is devoid of mutagenic activity. It is therefore of importance to develop methods that can be used to detect and study environmental carcinogenic agents that do not interact directly with DNA. Previous studies have indicated that induction of in vitro cell transformation and inhibition of gap junction intercellular communication are endpoints that could be useful for the detection of non-genotoxic carcinogens. In the present work, 13 compounds [chlordane, Arochlor 1260, di(2-ethylhexyl)phthalate, 1,1,1-trichloro-2, 2-bis(4-chlorophenyl)ethane, limonene, sodium fluoride, ethionine, o-anisidine, benzoyl peroxide, o-vanadate, phenobarbital, 12-O-tetradecanoylphorbol 13-acetate and clofibrate] have been tested for their ability to induce morphological transformation and affect intercellular communication in Syrian hamster embryo cells. The substances were selected on the basis of being proven or suspected non-genotoxic carcinogens, and thus difficult to detect in short-term tests. The data show that nine of the 13 compounds induced morphological transformation, and seven of the 13 inhibited intercellular communication in hamster embryo cells. Taken together, 12 of the 13 substances either induced transformation or caused inhibition of communication. The data suggest that the combined use of morphological transformation and gap junction intercellular communication in Syrian hamster embryo cells may be beneficial when screening for non-genotoxic carcinogens.

Hepatocarcinogenic potential of di(2-ethylhexyl)phthalate in rodents and its implications on human risk

The plasticizer di(2-ethylhexyl) phthalate (DEHP), to which humans are extensively exposed, was found to be hepatocarcinogenic in rats and mice. DEHP is potentially set free from objects made of synthetic materials (e.g., those used in medicine). Chronically, the greatest amounts are transferred to persons undergoing hemodialysis (up to 3.1 mg/kg b.w. per day) who would thus be considered the individuals most endangered by tumorigenesis. Although toxicokinetics seem to play a certain unclear role in the course of DEHP-related toxicity, toxicodynamic factors appear more decisive. DEHP is a representative of "peroxisome proliferators" (PP), a distinct group of substances that, in rodents, do not only induce peroxisomes but also specific enzymes in other organelles, organ growth, and DNA synthesis. The cluster of the characteristic effects of PP is generally, although perhaps not quite appropriately summarized as "peroxisome proliferation," and is strongest in the liver. The lowest observed effect level (LOEL) and the no observed effect level (NOEL) of peroxisome proliferation in the rat, as determined by the induction of specific enzymes (peroxisomal beta-oxidation, carnitine-acetyl-transferase, cytochrome P-452), DNA synthesis, and hepatomegaly, may be assumed as 50 and 25 mg/kg b.w. per day, respectively. DEHP and other carcinogenic PP are neither genotoxic nor tumor initiators, but they appear to be tumor promoters, also implicating a threshold level for the carcinogenic effect. Although a causal relationship between a particular effect of peroxisome proliferation and hepatocarcinogenesis is as yet unknown, peroxisome proliferation as a whole phenomenon appears to be associated with the potential of tumor induction, as shown by comparison of the relative strength of individual PP and by comparison of species and organ specificities. Likewise, LOEL and NOEL of rodent carcinogenesis, that is, 300 and 50 to 100 mg/kg b.w. per day, respectively, are above but not too far from the corresponding values for the investigated parameters of peroxisome proliferation. Thus, with respect to dose alone, worst-case exposure in hemodialysis patients is at least 16-fold below the LOEL of any characterized PP-specific effect of DEHP and approximately 100-fold below that of DEHP-related tumorigenesis. Also, primates are less responsive to PP than rats with respect to the investigated biochemical and morphological parameters. If this lower primate responsiveness is extrapolated to estimate carcinogenicity in humans, we might thus arrive at an even larger safety margin than when based on exposure alone. Doses of PP hypolipidemics that had clearly induced several indicators of peroxisome proliferation in rats did not cause any clear-cut enhancements in the peroxisomes of patients, even though most of these hypolipidemics were considerably stronger PP than DEHP. Thus, an actual threat to humans by DEHP seems rather unlikely. Accordingly, hepatocarcinogenesis was neither enhanced in workers exposed to DEHP nor in patients treated with hypolipidemics.

Modifying influence of prior treatment with toxic agents on induction of preneoplastic and neoplastic lesions in a medium-term multi-organ carcinogenesis bioassay

The modifying potential of prior administration of toxic agents was investigated in our multi-organ carcinogenesis model using male F344/DuCrj rats with the aim of assessing the link between tissue damage and initiation. Animals were administered one of four toxic agents for 8 wk, and then treated with N-diethylnitrosamine (DEN, 100 mg/kg body weight (b.w.), intraperitoneally (i.p.), single injection), N-methylnitrosourea (MNU, 20 mg/kg b.w., i.p., four times during wk 9 and 10), and dihydroxy-di-N-propylnitrosamine (DHPN, 0.1% in drinking water, during wk 11 and 12) for multi-organ carcinogenesis. All surviving rats were killed at the end of wk 36, and the major organs carefully examined for preneoplastic and neoplastic lesion development. Immunohistochemical demonstration of glutathione S-transferase placental form (GST-P) positive foci was also performed to facilitate quantitative assessment of liver lesion development. D-galactosamine (300 mg/kg b.w., i.p., once a week), a hepatotoxin, significantly inhibited the induction of GST-P positive foci, while 4,4'-diaminodiphenylmethane (DDPM, 0.1% in diet), a bile duct proliferator which is itself a hepatocarcinogen, possessed enhancing activity. DDPM, also a goitrogen, clearly inhibited the development of follicular cell tumors in the thyroid. Uracil (3.0% in diet), which is an inducer of papillomatosis in the urinary bladder, did not exert any enhancing potential on bladder carcinogenesis. Bleomycin (2 mg/kg b.w., i.p., twice a week), which is an alveolar epithelium injuring agent, also did not modify the induction of alveolar epithelium proliferative lesions. These results indicate that prior organ injury by toxic agents does not always act to enhance sensitivity to carcinogenesis.

Enhancing effects of harman and norharman on induction of preneoplastic and neoplastic kidney lesions in rats initiated with N-ethyl-N-hydroxyethylnitrosamine

The modifying potential of two nephrotoxic agents, harman and norharman, on N-ethyl-N-hydroxyethylnitrosamine (EHEN)-induced renal and hepatic carcinogenesis was investigated in male F344/DuCrj rats. Animals were given 0.1% EHEN in their drinking water for the first 2 weeks as an initiator. Subsequently, starting 3 weeks from the commencement, they were fed diet containing these compounds at concentrations of 1000, 500 or 0 ppm until week 26, and then killed for light microscopic examination. The mean numbers of renal tubular cell hyperplasias/cm2 and those of tumors/cm2 in rats given harman and norharman at 1000 ppm after initiation, but not at 500 ppm, were significantly increased as compared to the control values. However, neither compound modified liver carcinogenesis. It is concluded that harman and norharman show enhancing effects on rat kidney carcinogenesis, when ingested at dose levels which cause renal tubular damage.

Dose-dependent renal tubular toxicity of harman and norharman in male F344 rats

The renal toxicity of harman and norharman, administered for 2 or 4 weeks at dietary levels of 1,000, 500, or 0 parts per million (ppm), was investigated in 6-week-old male F344/DuCrj rats. Although rats fed 1,000 ppm harman or norharman, but not the 500 ppm level, demonstrated marked body weight retardation from 1 week to termination, no mortalities occurred. Marked elevation of water consumption was evident in rats given harman or norharman at 1,000 ppm, but not at 500 ppm, together with large increases in urine of low specific gravity. Urinary lysosomal enzymes (N-acetyl-beta-D-glucosaminidase, NAG, and lactate dehydrogenase, LDH) and sugar levels were increased, and the brush border enzymes (gamma-glutamyl transpeptidase, GGT, and alkaline phosphatase, ALP) decreased. Furthermore, serum biochemistry revealed clear elevation of parameters indicating renal toxicity in these rats. Histopathologically, rats fed 1,000 ppm harman or norharman, but not 500 ppm, demonstrated focal toxic renal degenerative/necrotic and regenerative lesions in proximal, distal, and collecting tubules. These changes were associated with a clearly increased labeling index (LI) of the nuclei of renal tubular epithelial cells on immunohistochemical staining for 5-bromo-2'-deoxyuridine (BrdU). Chemical specific crystal formation within tubular lumina was evident in rats fed 1,000 ppm, but not 500 ppm, this being considered the cause of the renal tubular lesions. It was concluded that harman and norharman exert renal toxicity at the dietary level of 1,000 ppm, but not 500 ppm, in male F344 rats.

Summation effects of uracil and other promoters on epithelial lesion development in the F344 rat urinary bladder initiated by N-butyl-N-(4-hydroxybutyl)nitrosamine

Five non-genotoxic chemicals previously demonstrated to be bladder cancer promoters in 36-week in vivo assays for carcinogenesis were reevaluated in a 20-week experiment in order to assess the summation influence of dietary uracil, a component of RNA, on the development of (pre)neoplastic lesions. The test chemicals, sodium bicarbonate, sodium L-ascorbate, sodium citrate, butylated hydroxytoluene and ethoxyquin, were mixed into the diet at concentrations of 3%, 5%, 5%, 1% and 0.8%, respectively, and administered to male F344 rats after initiation with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in their drinking water for 4 weeks. The test chemicals were given from the 4th to the 8th and the 11th to 20th experimental weeks, uracil being administered at the level of 3% in the diet during the intervening period. Rats in the control group received only BBN and uracil. All animals were killed at week 20 and the bladders were evaluated for the occurrence of putative preneoplastic papillary or nodular (PN) hyperplasia and tumors. Significant increase in the occurrence of PN hyperplasia was observed in all groups initiated with BBN and fed uracil and test chemicals. Quantitative values for papillomas were also significantly increased except in the ethoxyquin-treated group. The results confirm that uracil given in the middle of the post-initiation stage enhances the promoting activity of chemicals and suggest that the use of this chemical might be useful to reduce the duration of current bioassays for bladder chemical carcinogens.

Oral toxicity study of tragacanth gum in B6C3F1 mice: development of squamous-cell hyperplasia in the forestomach and its reversibility

Tragacanth gum was administered at dietary levels of 0 (control), 0.625, 1.25, 2.5, and 5.0% to groups of 10 male and 10 female B6C3F1 mice for 13 wk. There were no treatment-associated effects regarding clinical signs, body or organ weights, and urinalysis or hematology data. Significant dose-related, but slight, elevations of plasma gamma-glutamyl transpeptidase (GGT) level were observed in all treated animals except the 0.625% females. Single or small numbers of tiny nodules were observed on the luminal surface of the forestomach in 4 males of the 5.0% group, 2 males of the 2.5% group, and 1 male each from the 1.25 and 0.625% groups. Histopathologically, they were diagnosed as squamous-cell hyperplasia. To investigate the nature of these gross lesions, tragacanth gum was fed to groups of 30 male mice at the dietary level of 5.0% for periods of up to 48 wk; 20 males served as controls. There were no treatment-related increases of plasma GGT levels at wk 24 and 48. Although squamous-cell hyperplasias were seen in 2 out of 10 mice at wk 24, none of these proliferative lesions were apparent at wk 48, after either chronic exposure or 24 wk on basal diet. Furthermore, the levels of DNA synthesis in forestomach epithelium as measured by 5-bromo-2-deoxyuridine (BrdU) immunohistochemistry were comparable to control values at wk 24 and 48. Thus, the oral toxicity of tragacanth gum to B6C3F1 mice was concluded to be negligible.