Evaluation of the carcinogenic potential of clofibrate in the rasH2 mouse

The Cell Transformation Assay: A Historical Assessment of Current Knowledge of Applications in an Integrated Approach to Testing and Assessment for Non-Genotoxic Carcinogens

The history of the development of the cell transformation assays (CTAs) is described, providing an overview of in vitro cell transformation from its origin to the new transcriptomic-based CTAs. Application of this knowledge is utilized to address how the different types of CTAs, variously addressing initiation and promotion, can be included on a mechanistic basis within the integrated approach to testing and assessment (IATA) for non-genotoxic carcinogens. Building upon assay assessments targeting the key events in the IATA, we identify how the different CTA models can appropriately fit, following preceding steps in the IATA. The preceding steps are the prescreening transcriptomic approaches, and assessment within the earlier key events of inflammation, immune disruption, mitotic signaling and cell injury. The CTA models address the later key events of (sustained) proliferation and change in morphology leading to tumor formation. The complementary key biomarkers with respect to the precursor key events and respective CTAs are mapped, providing a structured mechanistic approach to represent the complexity of the (non-genotoxic) carcinogenesis process, and specifically their capacity to identify non-genotoxic carcinogenic chemicals in a human relevant IATA.

Survey of tumorigenic sensitivity in 6-month rasH2-Tg mice studies compared with 2-year rodent assays

The pharmacokinetic endpoint of a 25-fold increase in human exposure is one of the specified criteria for high-dose selection for 2-year carcinogenicity studies in rodents according to ICH S1C(R2). However, this criterion is not universally accepted for 6-month carcinogenicity tests in rasH2-Tg mice. To evaluate an appropriate multiple for rasH2-Tg mice, we evaluated data for 53 compounds across five categories of rasH2-Tg mouse-positive [(1) genotoxic and (2) non-genotoxic] carcinogens and rasH2-Tg mouse-negative [(3) non-genotoxic carcinogens with clear or uncertain human relevance; (4) non-genotoxic rodent-specific carcinogens; and (5) non-carcinogens], and surveyed their tumorigenic activities and high doses in rasH2-Tg mice and 2-year rodent models. Our survey indicated that area under the curve (AUC) margins (AMs) or body surface area-adjusted dose ratios (DRs) of tumorigenesis in rasH2-Tg mice to the maximum recommended human dose (MRHD) were 0.05- to 5.2-fold in 6 category (1) compounds with small differences between models and 0.2- to 47-fold in 7 category (2) including three 2-year rat study-negative compounds. Among all 53 compounds, including 40 compounds of the rasH2-Tg mouse-negative category (3), (4), and (5), no histopathologic risk factors for rodent neoplasia were induced only at doses above 50-fold AM or DR in rasH2-Tg mice except for two compounds, which induced hyperplasia and had no relationship with the tumors observed in the rasH2-Tg mouse or 2-year rodent studies. From the results of these surveys, we confirmed that exceeding a high dose level of 50-fold AM in rasH2-Tg mouse carcinogenicity studies does not appear to be of value.

Gemfibrozil Induces Anemia, Leukopenia and Reduces Hematopoietic Stem Cells via PPAR-α in Mice

Hypercholesterolemia, also called high cholesterol, is a form of hyperlipidemia, which may be a consequence of diet, obesity or diabetes. In addition, increased levels of low-density lipoprotein (LDL) and reduced levels of high-density lipoprotein (HDL) cholesterol are associated with a higher risk of atherosclerosis and coronary heart disease. Thus, controlling cholesterol levels is commonly necessary, and fibrates have been used as lipid-lowering drugs. Gemfibrozil is a fibrate that acts via peroxisome proliferator-activated receptor alpha to promote changes in lipid metabolism and decrease serum triglyceride levels. However, anemia and leukopenia are known side effects of gemfibrozil. Considering that gemfibrozil may lead to anemia and that gemfibrozil acts via peroxisome proliferator-activated receptor alpha, we treated wild-type and peroxisome proliferator-activated receptor alpha-knockout mice with gemfibrozil for four consecutive days. Gemfibrozil treatment led to anemia seven days after the first administration of the drug; we found reduced levels of hemoglobin, as well as red blood cells, white blood cells and a reduced percentage of hematocrits. PPAR-alpha-knockout mice were capable of reversing all of those reduced parameters induced by gemfibrozil treatment. Erythropoietin levels were increased in the serum of gemfibrozil-treated animals, and we also observed an increased expression of hypoxia-inducible factor-2 alpha (HIF-2α) and erythropoietin in renal tissue, while PPAR-alpha knockout mice treated with gemfibrozil did not present increased levels of serum erythropoietin or tissue HIF-2α and erythropoietin mRNA levels in the kidneys. We analyzed bone marrow and found that gemfibrozil reduced erythrocytes and hematopoietic stem cells in wild-type mice but not in PPAR-alpha-knockout mice, while increased colony-forming units were observed only in wild-type mice treated with gemfibrozil. Here, we show for the first time that gemfibrozil treatment leads to anemia and leukopenia via peroxisome proliferator-activated receptor alpha in mice.

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

Evaluation of the Carcinogenic Potential of Clofibrate in the Neonatal Mouse

This study was conducted in support of the International Life Sciences Institute (ILSI) alternative carcinogenicity models initiative to evaluate the carcinogenic potential of clofibrate, a nongenotoxic peroxisome proliferator-activated receptor (PPAR) α agonist, following oral administration to neonatal mice. Male and female neonatal CD-1 mice were dosed with clofibrate at doses of 100, 250, and 500 mg/kg or with the positive control, diethyl-nitrosamine (DEN), at 2 mg/kg by oral gavage on days 9 and 16 post birth and observed for approximately 1 year for the development of tumors. Plasma levels of clofibric acid after the second administration increased with dose, but were not dose proportional. Clofibrate administered by gavage on litter days 9 and 16 to neonatal mice at doses of 100, 250, or 500 mg/kg did not produce a carcinogenic effect. The positive control DEN did produce tumors in the liver and lung (single and multiple adenomas and carcinomas) and harderian gland (adenoma) of both sexes. Non-neoplastic lesions related to DEN treatment were confined to myocardial degeneration/fibrosis and testicular interstitial hyperplasia in males, and to glomerulonephrosis and gastritis in both sexes.

Evaluation of the Carcinogenic Potential of Clofibrate in the p53+/− Mouse

This study was conducted as part of International Life Sciences Institute (ILSI) program to evaluate the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) α agonist, following oral administration to p53+ /− heterozygous mice for a minimum of 26 weeks. p-Cresidine, a urinary bladder carcinogen, was given orally at 400 mg/kg/day as a positive control. Initial clofibrate doses were 50, 250, and 400 mg/kg/day for males and 50, 200, and 500 mg/kg/day for females. Due to unexpected mortality during the first week of dosing, clofibrate doses were lowered to 25, 75, and 100 mg/kg/day for males and 25, 75, and 125 mg/kg/day for females. Clinical signs and mortality were greater in p53+ /− than wild-type (WT) mice. With the exception of liver weights, no marked differences in any other parameters either between the sexes or between WT and p53+ /− mice were noted. Moderate increases in liver weights noted in WT males given 100 mg/kg/day clofibrate were not associated with any microscopic changes. No neoplastic response was observed in p53+ /−mice after 6 months of exposure to clofibrate at doses up to 100 mg/kg/day for males and 125 mg/kg/day for females. Transitional-cell hyperplasia and carcinoma of the urinary bladder were noted in both sexes given p-cresidine, demonstrating that the p53+ /− mouse responded to a known mouse carcinogen as expected. Clofibrate produced non-neoplastic findings in the adrenals, pancreas, and prostate, whereas p-cresidine affected the kidney, liver, pancreas, and spleen.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Peroxisome proliferator-activated receptor agonists and bladder cancer: lessons from animal studies

This article reviews available animal studies on the possible link between the use of peroxisome proliferator-activated receptor (PPAR) agonists and bladder cancer, with further discussion on the possible implications to humans. Carcinogenicity studies suggest that the PPARγ agonist pioglitazone and dual PPARα/γ agonists such as ragaglitazar, muraglitazar, and naveglitazar may increase the risk of bladder cancer in a dose-responsive pattern in rats. It is interesting that bladder cancer related to PPAR agonists shows remarkable species- and sex-specificity and has a predilection to occur in the ventral dome of bladder in rodents. While male rats treated with pioglitazone or muraglitazar have a higher propensity to develop bladder cancer than female rats, mice of both sexes do not develop bladder cancer even when exposed to very high doses. Direct genotoxicity or cytotoxicity of PPAR agonists is unlikely to be the mode of action because most of the parent compounds or their metabolites of the PPAR agonists are neither mutagenic nor genotoxic, and they are rarely excreted in the urine; but a receptor-mediated PPAR effect cannot be excluded. Some suggest a "urolithiasis hypothesis" referring to the formation of urinary solids and calculi, which subsequently causes bladder necrosis, regenerative proliferation, hypertrophy, and cancer. However, whether these animal findings could have human relevance is not yet fully understood. Some argue that the urolithiasis-induced bladder cancer might be rat-specific and would probably not be applicable to humans. An effect of increased urinary growth factors induced by PPAR agonists has also been proposed, but this requires more investigations. Before fully clarified, a balance between the risks and benefits of the use of pioglitazone, an approved oral antidiabetic agent that has recently been linked to an increased but not yet confirmed risk of bladder cancer in humans, should be justified for individual use.

Hepatocarcinogenic Susceptibility of Fenofibrate and Its Possible Mechanism of Carcinogenicity in a Two-Stage Hepatocarcinogenesis Model of rasH2 Mice

Fenofibrate (FF) has previously been shown to induce hepatocellular neoplasia in a conventional mouse bioassay (NDA 1993), but there has been no report to examine the carcinogenic susceptibility of rasH2 mice to this chemical. In the present study, male rasH2 mice were subjected to a two-thirds partial hepatectomy (PH), followed by an N-diethylnitrosamine (DEN) initiation twenty-four hours after PH, and given a diet containing 0, 1200, or 2400 ppm FF for seven weeks. The incidences of preneoplastic foci were significantly increased in mice from the FF-treated groups. Immunohistochemistry revealed that significant increases in proliferating cell nuclear antigen (PCNA)-positive cells and cytokeratin 8/18 positive foci were observed in FF-treated groups. In addition, the transgene and several downstream molecules such as c- myc, c- jun, activating transcription factor 3 (ATF3), and cyclin D1 were overexpressed in these groups. These results suggest that the hepatocarcinogenic activity of rasH2 mice to FF can be detected in this hepatocarcinogenesis model and that up-regulation of genes for the ras/MAPK pathway and cell cycle was probably involved in the hepatocarcinogenic mechanism of rasH2 mice.

Hepatocarcinogenic susceptibility of rasH2 mice to troglitazone in a two-stage hepatocarcinogenesis model

Six-week-old rasH2 mice were injected intraperitoneally with N-diethylnitrosamine (DEN) after partial hepatectomy and administrated 0 or 6,000 ppm troglitazone (TRG) for 10 weeks. Relative liver weight of females increased significantly in the DEN + TRG group compared to the DEN-alone group. The numbers of gamma-glutamyltranspeptidase- and proliferating cell nuclear antigen (PCNA)-positive cells tended to increase in both the sexes in the DEN + TRG group; however, these changes were not significantly different from those in the DEN-alone group. Levels of gene expressions for vascular endothelial growth factor (VEGF) and VEGFB (related to angiogenesis), tropomyosin 1 (Tpm1) and transforming growth factor-beta (related to ras/MAPK cascade activation), and PCNA (related to cell proliferation) in females were significantly higher in the DEN + TRG than in the untreated control group but not in the DEN-alone group. Only Tpm1 gene had significantly higher expression in the DEN + TRG group than in the DEN-alone group. These results suggest that rasH2 mice are not susceptible to TRG in a two-stage hepatocarcinogenesis model.

Extremely weak tumor-promoting effect of troglitazone on splenic hemangiosarcomas in rasH2 mice induced by urethane

To examine the tumor-promoting effect of troglitazone (TRG), a novel thiazolidinedione insulin-sensitizing agent, on splenic hemangiosarcomas in rasH2 mice, histopathological and molecular analyses were performed in the spleen of female rasH2 mice fed a diet containing 6,000 or 0 ppm TRG for 16 weeks after 1,000 or 0 mg/kg urethane (UR) initiation. Histopathologically, splenic hemangiosarcomas were observed in the UR-alone and UR + TRG groups, but there was no significant difference in the incidence of splenic hemangiosarcomas between the UR-alone and UR+TRG groups. There were increasing tendencies in the number of positive cells for anti-PCNA antibody and gene expression in the UR + TRG group, but such a change was not statistically significant as compared to that in the UR-alone group. The gene expressions of VEGF, VEGFR1, VEGFC, VEGFR2 and Tie2 related to angiogenesis; c-fos related to MAPK cascade activation; and cyclin D1 related to cell cycle in the UR-alone and UR + TRG groups were significantly higher than those in the untreated control group. However, only the Tie2 gene in the UR + TRG group was significantly increased as compared to that in the UR-alone group. These results suggest that the vascular tumor-promoting activity of TRG in rasH2 mice is extremely low in the present experimental condition and a part of the gene related to angiogenesis probably contributes to the promotion of splenic hemangiosarcomas in rasH2 mice given TRG.

Carcinogenic susceptibility of rasH2 mice to troglitazone

To evaluate the carcinogenicity of troglitazone in rasH2 mice, 7-week-old male and female rasH2 mice were fed a diet containing 0, 3,000 or 6,000 ppm troglitazone for 26 weeks. An increased tendency in the incidence of vascular tumors was observed in females of the 6,000 ppm group. The preliminary analysis using a high-density oligonucleotide microarray on a splenic hemangiosarcoma of a high dose female that could be obtained as a fresh sample showed that several genes related to the ras/MAPK pathway activation, angiogenesis, cell cycle and cell multiplication were up-regulated. In addition, most of the genes up-regulated were confirmed by the reverse transcriptase-polymerase chain reaction (RT-PCR). These results may suggest that the carcinogenic susceptibility of rasH2 mice to troglitazone is relatively low and up-regulations of the ras/MAPK pathway and angiogenesis-related genes are probably involved in the production of splenic hemangiosarcomas in rasH2 mice given troglitazone.

Peroxisome proliferator-activated receptor alpha regulates a microRNA-mediated signaling cascade responsible for hepatocellular proliferation

Activation of peroxisome proliferator-activated receptor alpha (PPARalpha) leads to hepatocellular proliferation and liver carcinomas. The early events mediating these effects are unknown. A novel mechanism by which PPARalpha regulates gene expression and hepatocellular proliferation was uncovered. MicroRNA (miRNA) expression profiling demonstrated that activated PPARalpha was a major regulator of hepatic miRNA expression. Of particular interest, let-7C, an miRNA important in cell growth, was inhibited following 4-h treatment and 2-week and 11-month sustained treatment with the potent PPARalpha agonist Wy-14,643 in wild-type mice. let-7C was shown to target c-myc via direct interaction with the 3' untranslated region of c-myc. The PPARalpha-mediated induction of c-myc via let-7C subsequently increased expression of the oncogenic mir-17-92 cluster; these events did not occur in Pparalpha-null mice. Overexpression of let-7C decreased c-myc and mir-17 and suppressed the growth of Hepa-1 cells. Furthermore, using the human PPARalpha-expressing mouse model, which is responsive to Wy-14,643 effects on beta-oxidation and serum triglycerides but resistant to hepatocellular proliferation and tumorigenesis, we demonstrated a critical role for let-7C in liver oncogenesis. Wy-14,643 treatment did not inhibit let-7C or induce c-myc and mir-17 expression. These observations reveal a let-7C signaling cascade critical for PPARalpha agonist-induced liver proliferation and tumorigenesis.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after dermal application--part II

This study was conducted as part of the International Life Sciences Institute (ILSI) Alternatives to Carcinogenicity Testing program and evaluated the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following dermal application to transgenic Tg.AC and nontransgenic FVB mice for a minimum of 26 weeks. Clofibrate doses of 12, 28, or 36 mg/200 microl/day were used. Positive controls for papilloma formation were benzene (174.8 mg/200 microl), and 12-o-tetradecanoylphorbol-13-acetate (TPA [0.00250 mg/200 microl]). Clofibrate was tolerated at doses up to 36 mg/200 microl. In Tg.AC mice, clofibrate produced a dose-related increase in the incidence of mice with cutaneous papillomas; and dose-related decreases in mean time to first tumor, mean multiplicity of tumors per mouse, and mean weeks to maximal yield, as well as numerous nonneoplastic microscopic lesions in the liver, kidney, spleen, and skin. Benzene and TPA induced both neoplastic and/or non-neoplastic proliferative lesions in Tg.AC mice. Clofibrate did not increase the incidence or multiplicity of papillomas, or any other tumors in FVB mice. These data show that the Tg.AC dermal model has increased sensitivity in detecting skin papillomas caused by the nongenotoxic rodent carcinogen, clofibrate, compared to wild type FVB mice, at systemic exposures that are 3x higher than the systemic exposure observed in humans taking clofibrate (AUC = 1100 microg.h/ml) at the recommended maximum therapeutic dose of 500 mg. In addition, this study supports the proposed concept that Tg.AC model may detect compounds with nongenotoxic carcinogenic potential in a shorter timeframe than conventional mouse carcinogenicity bioassays.

Evaluation of the carcinogenic potential of clofibrate in the FVB/Tg.AC mouse after oral administration--part I

This study was conducted as part of the International Life Sciences Institute (ILSI) program to evaluate the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) alpha agonist following oral administration to Tg.AC (transgenic) and wild-type FVB (nontransgenic) mice for a minimum for 6 months. Clofibrate was well tolerated at doses up to 500 (males) and 650 (females) mg/kg/day. Oral administration of clofibrate to Tg.AC or FVB (wild-type) male and female mice for 6 months did not result in the increased formation of neoplastic lesions. Epithelial hyperplasia in the urinary bladder (Tg.AC and FVB) and prostate gland (Tg.AC only), and interstitial-cell hyperplasia in the testes (Tg.AC) were noted at 500 mg/kg/day. Non-neoplastic nonproliferative findings included hepatic hypertrophy and hematopoietic changes (myeloid hyperplasia, myelodysplasia, lymphoid depletion, and erythropoiesis) in Tg.AC and FVB mice of both sexes; reproductive (cystic degeneration and dilatation, hypospermia, spermatocele, dilated inspissated protein) and urogenital (tubular-cell hypertrophy, degenerative/regenerative nephropathy, necrosis/fibrosis) changes in Tg.AC and FVB male mice; congestion in the lung in male Tg.AC mice; gall bladder dilatation in female Tg.AC mice; and adrenal (intracellular lipofuscinosis and atrophy) and heart (eosinophillic myofibers) findings in Tg.AC mice of both sexes and in female FVB mice. The results of this study indicate that the clofibrate is not carcinogenic when administered to Tg.AC mice by oral gavage for 6 months at doses up to 500 (males) and 650 (females) mg/kg/day, which did produce liver hypertrophy.