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Ppara -null and PPARA -humanized mice are refractory to hepatocarcinogenesis caused by the peroxisome proliferator-activated receptor-α (PPARα) agonist Wy-14,643. However, the duration of these earlier studies was limited to approximately 1 year of treatment, and the ligand used has a higher affinity for the mouse PPARα compared to the human PPARα. Thus, the present study examined the effect of long-term administration of a potent, high-affinity human PPARα agonist (GW7647) on hepatocarcinogenesis in wild-type, Ppara -null, or PPARA -humanized mice. In wild-type mice, GW7647 caused hepatic expression of known PPARα target genes, hepatomegaly, hepatic MYC expression, hepatic cytotoxicity, and a high incidence of hepatocarcinogenesis. By contrast, these effects were essentially absent in Ppara -null mice or diminished in PPARA -humanized mice, although hepatocarcinogenesis was observed in both genotypes. Enhanced fatty change (steatosis) was also observed in both Ppara -null and PPARA -humanized mice independent of GW7647. PPARA -humanized mice administered GW7647 also exhibited increased necrosis after 5 weeks of treatment. Results from these studies demonstrate that the mouse PPARα is required for hepatocarcinogenesis induced by GW7647 administered throughout adulthood. Results also indicate that a species difference exists between rodents and human PPARα in the response to ligand activation of PPARα. The hepatocarcinogenesis observed in control and treated Ppara -null mice is likely mediated in part by increased hepatic fatty change, whereas the hepatocarcinogenesis observed in PPARA -humanized mice may also be due to enhanced fatty change and cytotoxicity that could be influenced by the minimal activity of the human PPARα in this mouse line on downstream mouse PPARα target genes. The Ppara -null and PPARA -humanized mouse models are valuable tools for examining the mechanisms of PPARα-induced hepatocarcinogenesis, but the background level of liver cancer must be controlled for in the design and interpretation of studies that use these mice. [ABSTRACT FROM AUTHOR]

Fluensulfone is a nematicide for agricultural use. Chronic dietary exposure led to bronchiolo-alveolar hyperplasia and bronchiolo-alveolar adenomas in CD-1 mice but not in rats. Genotoxicity could be excluded as a mode of action (MOA). An earlier publication (Strupp, C., Banas, D. A., Cohen, S. M., Gordon, E. B., Jaeger, M., and Weber, K. (2012). Relationship of metabolism and cell proliferation to the mode of action of fluensulfone-induced mouse lung tumors: analysis of their human relevance using the IPCS framework. Toxicol. Sci. 128, 284-294.) reported MOA studies identifying the following key events: increased metabolismof fluensulfone by CYP2f2 in mouse lung Club cells, followed by local proliferation, finally leading to adenoma formation. Human lung microsomes were found not to metabolize fluensulfone. The Joint FAO/WHO Meeting on Pesticide Residues has reviewed the previous data and concluded that the MOA is plausible however some areas of uncertainty were identified. This publication provides additional data to address these. New cell proliferation studies in mice showed that the MOA is functionally independent of sex. A threshold of cell proliferation in Club cells correlating with the dose response for adenoma formation was shown. CYP2f2 knockout mice did not react to fluensulfone exposure with cell proliferation like wild-type mice, confirming the key role of this enzyme. The collective data for fluensulfone were evaluated according to the International Programme on Chemical Safety (IPCS) Mode of Action Framework which leads to the conclusion that the mouse-specific lung tumors after fluensulfone are not relevant to humans. [ABSTRACT FROM AUTHOR]

High doses of sodium phenobarbital (NaPB), a constitutive androstane receptor (CAR) activator, have been shown to produce hepatocellular tumors in rodents by a mitogenic mode of action (MOA) involving CAR activation. The effect of 1-week dietary treatment with NaPB on liver weight and histopathology, hepatic CYP2B enzyme activity and CYP2B/3A mRNA expression, replicative DNA synthesis and selected genes related to cell proliferation, and functional transcriptomic and metabolomic analyses was studied in male CD-1 mice, Wistar Hannover (WH) rats, and chimeric mice with human hepatocytes. The treatment of chimeric mice with 1000-1500-ppm NaPB resulted in plasma levels around 3-5-fold higher than those observed in human subjects given therapeutic doses of NaPB. NaPB produced dose-dependent increases in hepatic CYP2B activity and CYP2B/3A mRNA levels in all animal models. Integrated functional metabolomic and transcriptomic analyses demonstrated that the responses to NaPB in the human liver were clearly different from those in rodents. Although NaPB produced a dose-dependent increase in hepatocyte replicative DNA synthesis in CD-1 mice and WH rats, no increase in replicative DNA synthesis was observed in human hepatocyte-originated areas of chimeric mice. In addition, treatment with NaPB had no effect on Ki-67, PCNA, GADD45ß, and MDM2 mRNA expression in chimeric mice, whereas significant increases were observed in CD-1 mice and/or WH rats. However, increases in hepatocyte replicative DNA synthesis were observed in chimeric mice both in vivo and in vitro after treatment epidermal growth factor. Thus, although NaPB could activate CAR in both rodent and human hepatocytes, NaPB did not increase replicative DNA synthesis in human hepatocytes of chimeric mice, whereas it was mitogenic to rat and mouse hepatocytes. As human hepatocytes are refractory to the mitogenic effects of NaPB, the MOA for NaPB-induced rodent liver tumor formation is thus not relevant for humans. [ABSTRACT FROM AUTHOR]

Inorganic arsenic (iAs) is a known human carcinogen at high exposures, increasing the incidences of urinary bladder, skin, and lung cancers. In most mammalian species, ingested iAs is excreted mainly through urine primarily as dimethylarsinic acid (DMAV). In wild-type (WT) mice, iAs, DMAV, and dimethylarsinous acid (DMAIII) exposures induce formation of intramitochondrial urothelial inclusions. Arsenite (iAsIII) also induced intranuclear inclusions in arsenic (+3 oxidation state) methyltransferase knockout (As3mt KO) mice. The arsenic-induced formation of inclusions in the mouse urothelium was dose and time dependent. The inclusions do not occur in iAs-treated rats and do not appear to be related to arsenic-induced urothelial cytotoxicity. Similar inclusions in exfoliated urothelial cells from humans exposed to iAs have been incorrectly identified as micronuclei. We have characterized the urothelial inclusions using transmission electron microscopy (TEM), DNA-specific 4′,6-diamidino-2-phenylindole (DAPI), and non-DNA-specific Giemsa staining and determined the arsenical content. The mouse inclusions stained with Giemsa but not with the DAPI stain. Analysis of urothelial mitochondrial- and nuclear-enriched fractions isolated from WT (C57BL/6) and As3mt KO mice exposed to arsenate (iAsV) for 4 weeks showed higher levels of iAsV in the treated groups. iAsIII was the major arsenical present in the enriched nuclear fraction from iAsV-treated As3mt KO mice. In conclusion, the urothelial cell inclusions induced by arsenicals appear to serve as a detoxifying sequestration mechanism similar to other metals, and they do not represent micronuclei. [ABSTRACT FROM AUTHOR]

Hemangiosarcomas are uncommon aggressive vascular tumors that have recently become the focus of attention because several chemicals and pharmaceuticals increase their incidence in mice. The relevance of these mouse vascular tumors to humans is unclear. In the present study, we semiquantitatively evaluated the expression profiles of hematopoietic stem cell markers (CD117 [c-kit], CD133, CD34, and CD45), endothelial cell markers (vascular endothelial growth factor receptor 2, CD31, and factor VIII-related antigen), and a myeloid lineage cell marker (CD14) in human hemangiosarcoma (n = 12) and hemangioma (n = 10) specimens using immunohistochemistry. CD133 was completely negative in almost all cases of hemangiosarcomas and hemangiomas. Most hemangiosarcomas, but not hemangiomas, stained for CD117 and CD45. Both groups diffusely expressed CD34, vascular endothelial growth factor receptor 2, and factor VIII-related antigen; however, hemangiomas had more intense and diffuse CD34 and factor VIII-related antigen expression compared with hemangiosarcomas, whereas CD31 was positive in all hemangiosarcomas but only half of the hemangiomas. CD14 staining was negative in most hemangiosarcoma and hemangioma cases. Our results indicate that multipotential bone marrow-derived hematopoietic stem cells or early endothelial progenitor cells (EPCs) expressing CD117, CD34, and CD45 are involved in hemangiosarcoma formation, whereas hemangiomas originate from late EPCs or differentiated endothelial cells, which have lost the expression of most hematopoietic stem cell markers. This contrasts with our previous results that demonstrated that both hemangiosarcomas and hemangiomas in mice may be derived from early EPCs that are not completely differentiated. [ABSTRACT FROM AUTHOR]

Intramitochondrial inclusions containing arsenite that occur within urothelial cells have been previously described in mice exposed to high concentrations of arsenic but not in rats. In epidemiology studies, similar urothelial cell inclusions have also been observed in the urine of humans exposed to high concentrations of arsenic in the drinking water; however, these inclusions were mistakenly identified as micronuclei. To further examine the urothelial cell inclusions that occur in inorganic arsenic-exposed humans, we evaluated two patients with a history of acute promyelocytic leukemia treated for disease relapse with a combination of all-trans retinoic acid and arsenic trioxide. Posttreatment examination of the patients’ urine cytology specimens by light and electron microscopy demonstrated cytoplasmic inclusions in exfoliated superficial urothelial cells similar to those seen in mice. The inclusions were present in decreasing quantities at 3 and 7 months after completion of treatment. No comparable inclusions were detected in exfoliated urothelial cells in urine from six individuals not treated with arsenic trioxide. Based on the results of the examination by light and electron microscopy, we have determined that urothelial cell inclusions in the urine of humans previously identified as micronuclei are instead intracytoplasmic inclusions similar to those found in arsenic-treated mice. [ABSTRACT FROM AUTHOR]

It is unclear whether the process of spontaneous and chemically induced hemangiosarcoma and hemangioma formation in mice involves the transformation of differentiated endothelial cells (ECs) or recruitment of multipotential bone marrow–derived hematopoietic stem cells or endothelial progenitor cells (EPCs), which show some degree of endothelial differentiation. In the present study, immunohistochemical staining for hematopoietic stem cell markers (CD45 and CD34), EC markers (vascular endothelial growth factor receptor 2 [VEGFR2], CD31, and factor VIII–related antigen), and a myeloid lineage marker (CD14) was employed to better define the origin of hemangiosarcomas and hemangiomas in mice. Staining was negative for CD45, factor VIII–related antigen, and CD14 and positive for CD34, VEGFR2, and CD31, indicating that mouse hemangiosarcomas and hemangiomas are composed of cells derived from EPCs expressing CD34, VEGFR2, and CD31 but not factor VIII–related antigen. The lack of CD45 expression suggests that mouse vascular tumors may arise from EPCs that are at a stage later than hematopoietic stem cells. Since factor VIII–related antigen expression is known to occur later than CD31 expression in EPCs, our observations may indicate that these tumor cells are arrested at a stage prior to complete differentiation.  In addition, myeloid lineage cells do not appear to contribute to hemangiosarcoma and hemangioma formation in mice. [ABSTRACT FROM PUBLISHER]

Lack of adherence, inaccessibility to viral reservoirs, long-term drug toxicities, and treatment failures are limitations of current antiretroviral therapy (ART). These limitations lead to increased viral loads, medicine resistance, immunocompromise, and comorbid conditions. To this end, we developed long-acting nanoformulated ART (nanoART) through modifications of existing atazanavir, ritonavir, and efavirenz suspensions in order to establish cell and tissue drug depots to achieve sustained antiretroviral responses. NanoART's abilities to affect immune and antiviral responses, before or following human immunodeficiency virus type 1 infection were tested in nonobese severe combined immune-deficient mice reconstituted with human peripheral blood lymphocytes. Weekly subcutaneous injections of drug nanoformulations at doses from 80 mg/kg to 250 mg/kg, 1 day before and/or 1 and 7 days after viral exposure, elicited drug levels that paralleled the human median effective concentration, and with limited toxicities. NanoART treatment attenuated viral replication and preserved CD4+ Tcell numbers beyond that seen with orally administered native drugs. These investigations bring us one step closer toward using long-acting antiretrovirals in humans. [ABSTRACT FROM AUTHOR]

Species-specific lung tumors in the mouse are induced by a number of chemicals. The underlying cause appears to be a high metabolic activity of mouse lung, due to relatively high abundance of Clara cells in mice compared with humans and the mouse-specific cytochrome P450 isoform 2f2 in the Clara cells. The chemicals are activated to reactive intermediates, leading to local cytotoxicity or mitogenicity resulting in increased cell proliferation and tumors. Rats have lower metabolic activity than mice (already below the threshold needed to cause lung tumors upon lifetime exposure) and activity in humans is lower than in rats. The carcinogenic risk for human lung is low for this mode of action (MOA). Fluensulfone has shown an increased incidence of lung adenomas in mice, but not in rats, at high doses. Fluensulfone is not genotoxic. MOA studies were conducted investigating key events of the postulated MOA. Fluensulfone is extensively metabolized by mouse lung microsomes, whereas no metabolic activity is seen with human lung microsomes. Cyp 2f2 is a major contributor in fluensulfone’s metabolism and Cyp 2e1 is not involved. Furthermore, administration of fluensulfone to mice led to an early increase in Clara cell proliferation. The International Programme on Chemical Safety (IPCS) MOA and human relevance framework was used to evaluate the collective data on fluensulfone. We concluded that fluensulfone leads to species-specific mouse lung tumors and that these tumors are likely not relevant to human hazard or risk. [ABSTRACT FROM PUBLISHER]

Folpet, an agricultural fungicide, induces tumors in the mouse gastrointestinal tract, primarily in the duodenum. Bioassays show a threshold for tumors at ~1000 ppm dietary administration. We investigated the early histologic changes to the mouse duodenum in mice fed a diet containing 6000 ppm folpet for 28 days. Reversibility of folpet-induced changes was evaluated after treatment for 28 days and a recovery period of 17 days. Macroscopic changes in the cecum (dilatation) and duodenum (roughening) were evident by day 7, continued through day 28, then returned to normal by recovery day 17. The duodenal mucosa also appeared to be thickened. Macroscopic changes to the forestomach were also evident as a rough surface or depressions; they also decreased in incidence and severity in the recovery animals. Histologic changes of the duodenum (crypt cell hyperplasia, villous hypertrophy, numerous intraepithelial lymphocytes, and elongation of epithelial columnar cells) were evident in all treated mice by day 7 and continued and increased in severity through 28 days of administration. The incidence and severity of these findings was reduced on recovery day 17, indicating reversibility. Histologic changes (epithelial hyperplasia and hyperkeratosis) of the non-glandular squamous epithelium in the forestomach occurred later than the changes to the duodenum. The incidence and severity of these changes also lessened by recovery day 17. These early histologic changes support a non-DNA reactive mode of action for folpet carcinogenicity in mice involving the key events of mucosal cytotoxicity with consequent regenerative proliferation. Exposures that are not sufficient to produce cytotoxicity would also not lead to tumor formation. [ABSTRACT FROM AUTHOR]

Arsenic (+3 oxidation state) methyltransferase (As3mt) catalyzes methylation of inorganic arsenic (iAs) producing a number of methylated arsenic metabolites. Although methylation has been commonly considered a pathway for detoxification of arsenic, some highly reactive methylated arsenicals may contribute to toxicity associated with exposure to inorganic arsenic. Here, adult female wild-type (WT) C57BL/6 mice and female As3mt knockout (KO) mice received drinking water that contained 1, 10, or 25 ppm (mg/l) of arsenite for 33 days and blood, liver, kidney, and lung were taken for arsenic speciation. Genotype markedly affected concentrations of arsenicals in tissues. Summed concentrations of arsenicals in plasma were higher in WT than in KO mice; in red blood cells, summed concentrations of arsenicals were higher in KO than in WT mice. In liver, kidney, and lung, summed concentrations of arsenicals were greater in KO than in WT mice. Although capacity for arsenic methylation is much reduced in KO mice, some mono-, di-, and tri-methylated arsenicals were found in tissues of KO mice, likely reflecting the activity of other tissue methyltransferases or preabsorptive metabolism by the microbiota of the gastrointestinal tract. These results show that the genotype for arsenic methylation determines the phenotypes of arsenic retention and distribution and affects the dose- and organ-dependent toxicity associated with exposure to inorganic arsenic. [ABSTRACT FROM AUTHOR]

The enzyme arsenic (+3 oxidation state) methyltransferase (As3mt) catalyzes reactions converting inorganic arsenic to methylated metabolites, some of which are highly cytotoxic. In a previous study, female As3mt knockout (KO) mice treated with diet containing 100 or 150 ppm arsenic as arsenite showed systemic toxicity and significant effects on the urothelium. In the present study, we showed that the cytotoxic and proliferative effects of arsenite administration on the urothelium are dose dependent. Female wild-type C57BL/6 mice and As3mt KO mice were divided into five groups (n = 7) with free access to drinking water containing 0, 1, 10, 25, or 50 ppm arsenic as arsenite for 4 weeks. At sacrifice, urinary bladders of both As3mt KO and wild-type mice showed hyperplasia by light microscopy; however, the hyperplasia was more severe in the As3mt KO mice. Intracytoplasmic granules were detected in the urothelium of As3mt KO and wild-type mice at arsenic doses ≥ 10 ppm but were more numerous, more extensive, and larger in the KO mice. A no effect level for urothelial effects was identified at 1 ppm arsenic in the wild-type and As3mt KO mice. In As3mt KO mice, livers showed mild acute inflammation and kidneys showed hydronephrosis. The present study shows a dose-response for the effects of orally administered arsenite on the bladder urothelium of wild-type and As3mt KO mice, with greater effects in the KO strain but with a no effect level of 1 ppm for both. [ABSTRACT FROM AUTHOR]

Based on epidemiological data, chronic exposure to high levels of inorganic arsenic in the drinking water is carcinogenic to the urinary bladder of humans. Recently, models have been developed involving transplacental administration of inorganic arsenic and subsequent administration of another substance that produces a low incidence of urogenital neoplasms. Administration of arsenite or arsenate in the diet or drinking water to five-to eight-week-old mice or rats rapidly induces urothelial cytotoxicity and regenerative hyperplasia. In mice administered arsenite, we observed eosinophilic intracytoplasmic granules present in the urothelial cells. These granules were not present in urothelial cells of untreated mice or in treated or untreated rats. By transmission electron microscopy, the granules were located within the mitochondrial matrix, that is, mitochondrial inclusions. Arsenic, primarily as arsenite, was present in partially purified mitochondria containing these granules. Cells containing the granules were not usually associated with degenerative changes. Lack of these granules in rats suggests that they are not necessary for inorganic arsenic-induced urothelial cytotoxicity or hyperplasia. These granules have also been observed with exposures to other metals in other tissues and other species, suggesting that they represent a protective mechanism against metal-induced toxicity. [ABSTRACT FROM AUTHOR]

Monomethylarsonic acid (MMA V ) and dimethylarsinic acid (DMA V ) are active ingredients in pesticidal products used mainly for weed control. MMA V and DMA V are also metabolites of inorganic arsenic, formed intracellularly, primarily in liver cells in a metabolic process of repeated reductions and oxidative methylations. Inorganic arsenic is a known human carcinogen, inducing tumors of the skin, urinary bladder, and lung. However, a good animal model has not yet been found. Although the metabolic process of inorganic arsenic appears to enhance the excretion of arsenic from the body, it also involves formation of methylated compounds of trivalent arsenic as intermediates. Trivalent arsenicals (whether inorganic or organic) are highly reactive compounds that can cause cytotoxicity and indirect genotoxicity in vitro. DMA V was found to be a bladder carcinogen only in rats and only when administered in the diet or drinking water at high doses. It was negative in a two-year bioassay in mice. MMA V was negative in 2-year bioassays in rats and mice. The mode of action for DMA V -induced bladder cancer in rats appears to not involve DNA reactivity, but rather involves cytotoxicity with consequent regenerative proliferation, ultimately leading to the formation of carcinoma. This critical review responds to the question of whether DMA V -induced bladder cancer in rats can be extrapolated to humans, based on detailed comparisons between inorganic and organic arsenicals, including their metabolism and disposition in various animal species. The further metabolism and disposition of MMA V and DMA V formed endogenously during the metabolism of inorganic arsenic is different from the metabolism and disposition of MMA V and DMA V from exogenous exposure. The trivalent arsenicals that are cytotoxic and indirectly genotoxic in vitro are hardly formed in an organism exposed to MMA V or DMA V because of poor cellular uptake and limited metabolism of the ingested compounds. Furthermore, the evidence strongly supports a nonlinear dose-response relationship for the biologic processes involved in the carcinogenicity of arsenicals. Based on an overall review of the evidence, using a margin-of-exposure approach for MMA V and DMA V risk assessment is appropriate. At anticipated environmental exposures to MMA V and DMA V , there is not likely to be a carcinogenic risk to humans. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Lung Tumors in Mice Induced by "Whole-Life" Inorganic Arsenic Exposure at Human- Relevant Doses," by M. P. Waalkes and colleagues, published in a previous issue.

The therapeutic performance of oxaliplatin can be improved by incorporating the central cis-dichloro(1,2- diaminocyclohexane)platinum(II) (DACHPt) motif into the core cross-linked block copolymer micelles. We describe here the preparation, cellular uptake, and in vivo evaluation of core cross-linked micelles loaded with DACHPt. Stable drug-loaded micelles were prepared at high drug loading (~25 w/w%) and displayed a considerably increased in vitro cytotoxicity compared to free oxaliplatin against A2780 ovarian cancer cells. The DACHPt-loaded micelle formulation was well tolerated in mice and exhibited improved antitumor activity than oxaliplatin alone in an ovarian tumor xenograft model. [ABSTRACT FROM AUTHOR]

Abstract: The PPARγ agonist troglitazone (TG) induced an increased incidence of hemangiosarcomas in mice but was not carcinogenic in rats. In contrast, pioglitazone (PIO) did not induce hemangiosarcomas or any other tumors in mice. We previously demonstrated that TG increased the proliferation of endothelial cells (ECs) in liver and adipose tissue in mice, and acted as a mitogenic stimulant and an inhibitor of apoptosis in vitro in mouse, but not human, ECs. In the present study, we investigated whether TG had any effect on the proliferation of ECs in rats. We also evaluated the in vivo and in vitro effects of PIO on ECs in mice. In rats, TG did not increase the Ki-67 labeling index (LI) of ECs in liver or adipose tissue at doses used in the two-year bioassay, and did not increase hepatocyte proliferation. PIO administered to mice did not increase the Ki-67 LI of hepatocytes or ECs in liver or white adipose tissue, but slightly increased the EC proliferation in brown adipose tissue. PIO was slightly mitogenic on cultured mouse ECs after 3 days of treatment but not after 6 days, and there was no inhibition of apoptosis, in contrast to what was seen with TG. The data support the conclusion that sustained EC proliferation in mice is necessary, for the induction of hemangiosarcomas by TG, and these short-term and long-term effects are not seen with TG in the rat or with PIO in mice, treatments that also are not related to the induction of hemangiosarcomas in two-year bioassays. [Copyright &y& Elsevier]

Abstract: Arsenic (+3 oxidation state) methyltransferase (As3mt) catalyzes reactions which convert inorganic arsenic to methylated metabolites. This study determined whether the As3mt null genotype in the mouse modifies cytotoxic and proliferative effects seen in urinary bladders of wild type mice after exposure to inorganic arsenic. Female wild type C57BL/6 mice and As3mt KO mice were divided into 3 groups each (n =8) with free access to a diet containing 0, 100 or 150ppm of arsenic as arsenite (AsIII). During the first week of AsIII exposure, As3mt KO mice exhibited severe and lethal systemic toxicity. At termination, urinary bladders of both As3mt KO and wild type mice showed hyperplasia by light microscopy. As expected, arsenic-containing granules were found in the superficial urothelial layer of wild type mice. In As3mt KO mice these granules were present in all layers of the bladder epithelium and were more abundant and larger than in wild type mice. Scanning electron microscopy of the bladder urothelium of As3mt KO mice treated with 100ppm AsIII showed extensive superficial necrosis and hyperplastic changes. In As3mt KO mice, livers showed severe acute inflammatory changes and spleen size and lymphoid areas were decreased compared with wild type mice. Thus, diminished arsenic methylation in As3mt KO mice exacerbates systemic toxicity and the effects of AsIII on the bladder epithelium, showing that altered kinetic and dynamic behavior of arsenic can affect its toxicity. [Copyright &y& Elsevier]

Abstract: Peroxisome proliferator-activated receptor gamma (PPARγ) agonists and PPARγ/α dual agonists have been or are being developed for clinical use in the treatment of type 2 diabetes mellitus and hyperlipidemias. A common tumor finding in rodent carcinogenicity studies for these agonists is hemangioma/hemangiosarcoma in mice but not in rats. We hypothesized that increased endothelial cell proliferation may be involved in the mechanism of PPAR agonist-induced vascular tumors in mice, and we investigated the effects on endothelial cells utilizing troglitazone, the first clinically used PPARγ agonist, in vivo and in vitro. Troglitazone (400 and 800 mg/kg/day) induced hemangiosarcomas in mice in a 2-year bioassay. We showed that troglitazone increased endothelial cell proliferation in brown and white adipose tissue and liver in mice at sarcomagenic doses after 4 weeks of treatment. Troglitazone was cytotoxic both to human dermal microvascular endothelial cells (HMEC1) and mouse mammary fat pad microvascular endothelial cells (MFP MVEC) at high concentrations. However, MFP MVEC were more resistant to the cytotoxic effects of troglitazone based on the much lower LC50 in HMEC1 (17.4 μM) compared to MFP MVEC (92.2 μM). Troglitazone increased the proliferation and survival of MFP MVEC but not HMEC1 in growth factor reduced conditions. Our data demonstrate that troglitazone may induce hemangiosarcomas in mice, at least in part, through enhancement of survival and proliferation of microvascular endothelial cells. Such an effect does not occur with human cells, suggesting that human may react differently to exposure to PPAR agonists compared with mice. [Copyright &y& Elsevier]