Syntheses of [3H2]T0901317 and a labeled structural isomer,and characterization of the dispersed labeled compounds via 19F NMR

The synthesis and characterization of [³H₂]T0901317 and a structural isomer are described. The structural assignments of the closely related labeled compounds were primarily accomplished via ¹⁹F NMR analyses of the corresponding ethanolic compound dispersions.     

Liver tumor formation in female rat induced by fluopyram is mediated by CAR/PXR nuclear receptor activation

Fluopyram is a broad spectrum fungicide targeting plant pathogenic fungi (eg. white dot, black mold, botrytis). During the general toxicity evaluation of fluopyram in rodents, the liver was identified as a target organ (hepatomegaly and liver hypertrophy were observed in all studies). At the end of the guideline carcinogenicity study, an increased incidence of hepatocellular adenomas and carcinomas was observed in female Wistar rats following exposure to the highest fluopyram dose evaluated (1500 ppm). Short-term mechanistic studies (3, 7 or 28 days of exposure) were conducted in the female rat to identify the initial key events responsible for the tumor formation and to establish thresholds for each of the early hepatic changes. Increased expression of constitutive androstane receptor (CAR) and pregnane X receptor (PXR) inducible genes was recorded after each exposure period. Further confirmation of CAR/PXR activation was provided by increased activity of specific Phase I enzymes (PROD/BROD respectively). Increased hepatocellular proliferation (measured by Ki67) was observed after each exposure period with the greatest proliferative response occurring after 3 days of treatment. In these studies, dose responses and clear thresholds were established for gene expression, enzyme activity and cell proliferation. Furthermore, these early hepatic changes were shown to be reversible following compound withdrawal. Other modes of action for liver tumor formation such as DNA damage, cytotoxicity and peroxisome proliferation were excluded during the investigations. In conclusion, fluopyram is a threshold carcinogen and the resultant hepatocellular carcinomas in the female rat are due to hepatocellular proliferation mediated by CAR/PXR activation.     

Toxicogenomic effects common to triazole antifungals and conserved between rats and humans

The triazole antifungals myclobutanil, propiconazole and triadimefon cause varying degrees of hepatic toxicity and disrupt steroid hormone homeostasis in rodent in vivo models. To identify biological pathways consistently modulated across multiple timepoints and various study designs, gene expression profiling was conducted on rat livers from three separate studies with triazole treatment groups ranging from 6 h after a single oral gavage exposure, to prenatal to adult exposures via feed. To explore conservation of responses across species, gene expression from the rat liver studies were compared to in vitro data from rat and human primary hepatocytes exposed to the triazoles. Toxicogenomic data on triazoles from 33 different treatment groups and 135 samples (microarrays) identified thousands of probe sets and dozens of pathways differentially expressed across time, dose, and species — many of these were common to all three triazoles, or conserved between rodents and humans. Common and conserved pathways included androgen and estrogen metabolism, xenobiotic metabolism signaling through CAR and PXR, and CYP mediated metabolism. Differentially expressed genes included the Phase I xenobiotic, fatty acid, sterol and steroid metabolism genes Cyp2b2 and CYP2B6, Cyp3a1 and CYP3A4, and Cyp4a22 and CYP4A11; Phase II conjugation enzyme genes Ugt1a1 and UGT1A1; and Phase III ABC transporter genes Abcb1 and ABCB1. Gene expression changes caused by all three triazoles in liver and hepatocytes were concentrated in biological pathways regulating lipid, sterol and steroid homeostasis, identifying a potential common mode of action conserved between rodents and humans. Modulation of hepatic sterol and steroid metabolism is a plausible mode of action for changes in serum testosterone and adverse reproductive outcomes observed in rat studies, and may be relevant to human risk assessment.     

Farnesoid X receptor agonists suppress hepatic apolipoprotein CIII expression

BACKGROUND & AIMS: Increased serum triglyceride levels constitute a risk factor for coronary heart disease. Apolipoprotein CIII (Apo CIII) is a determinant of serum triglyceride metabolism. In this study, we investigated whether activators of the nuclear farnesoid X receptor (FXR) modulate Apo CIII gene expression. METHODS: The influence of bile acids and synthetic FXR activators on Apo CIII and triglyceride metabolism was studied in vivo by using FXR wild-type and FXR-deficient mice and in vitro by using human primary hepatocytes and HepG2 cells. RESULTS: In mice, treatment with the FXR agonist taurocholic acid strongly decreased serum triglyceride levels, an effect associated with reduced Apo CIII serum and liver messenger RNA levels. By contrast, no change was observed in FXR-deficient mice. Incubation of human primary hepatocytes and HepG2 cells with bile acids or the nonsteroidal synthetic FXR agonist GW4064 resulted in a dose-dependent down-regulation of Apo CIII gene expression. Promoter transfection experiments and mutation analysis showed that bile acid-activated FXR decrease human Apo CIII promoter activity via a negative FXR response element located in the I(4) footprint between nucleotides -739 and -704. Chromatin immunoprecipitation experiments showed that bile acid treatment led to binding of FXR/retinoid X receptor heterodimers to and displacement of HNF4alpha from this site. Bile acid treatment still repressed liver Apo CIII gene expression in hepatic HNF4alpha-deficient mice, suggesting an active rather than a competitive mechanism of Apo CIII repression by the FXR. CONCLUSIONS: We identified bile acid and synthetic activators of the nuclear FXR as negative regulators of Apo CIII expression, an effect that may contribute to the triglyceride-decreasing action of FXR agonists.     

A tea catechin, epigallocatechin-3-gallate, is a unique modulator of the farnesoid X receptor

Farnesoid X receptor (FXR) is a ligand-activated nuclear receptor and serves as a key regulator to maintain health of the liver and intestine. Bile acids are endogenous ligands of FXR, and there are increasing efforts to identify FXR modulators to serve as biological probes and/or pharmaceutical agents. Natural FXR ligands isolated from plants may serve as models to synthesize novel FXR modulators. In this study, we demonstrated that epigallocatechin-3-gallate (EGCG), a major tea catechin, specifically and dose-dependently activates FXR. In addition, EGCG induced FXR target gene expression in vitro. Surprisingly, in a co-activator (SRC2) recruitment assay, we found that EGCG does not recruit SRC2 to FXR, but it dose-dependently inhibits recruitment of SRC2 to FXR (IC₅₀, 1 μM) by GW6064, which is a potent FXR synthetic ligand. In addition, EGCG suppressed FXR target gene expression induced by either GW4064 or chenodeoxycholic acid in vitro. Furthermore, wild-type and FXR knockout mice treated with an acute dose of EGCG had induced mRNA expression in a subset of FXR target genes in the intestine but not in the liver. In conclusion, EGCG is a unique modulator of FXR in the intestine and may serve as an important model for future development of FXR modulators.     

Use of comparative genomics approaches to characterize interspecies differences in response to environmental chemicals: Challenges,opportunities, and research needs

A critical challenge for environmental chemical risk assessment is the characterization and reduction of uncertainties introduced when extrapolating inferences from one species to another. The purpose of this article is to explore the challenges, opportunities, and research needs surrounding the issue of how genomics data and computational and systems level approaches can be applied to inform differences in response to environmental chemical exposure across species. We propose that the data, tools, and evolutionary framework of comparative genomics be adapted to inform interspecies differences in chemical mechanisms of action. We compare and contrast existing approaches, from disciplines as varied as evolutionary biology, systems biology, mathematics, and computer science, that can be used, modified, and combined in new ways to discover and characterize interspecies differences in chemical mechanism of action which, in turn, can be explored for application to risk assessment. We consider how genetic, protein, pathway, and network information can be interrogated from an evolutionary biology perspective to effectively characterize variations in biological processes of toxicological relevance among organisms. We conclude that comparative genomics approaches show promise for characterizing interspecies differences in mechanisms of action, and further, for improving our understanding of the uncertainties inherent in extrapolating inferences across species in both ecological and human health risk assessment. To achieve long-term relevance and consistent use in environmental chemical risk assessment, improved bioinformatics tools, computational methods robust to data gaps, and quantitative approaches for conducting extrapolations across species are critically needed. Specific areas ripe for research to address these needs are recommended.     

Role of the nuclear xenobiotic receptors CAR and PXR in induction of cytochromes P450 by non-dioxinlike polychlorinated biphenyls in cultured rat hepatocytes

Polychlorinated biphenyls (PCBs) are among the most ubiquitously detectable ‘persistent organic pollutants’. In contrast to ‘dioxinlike’ (DL) PCBs, less is known about the molecular mode of action of the larger group of the ‘non-dioxinlike’ (NDL) PCBs. Owing to the life-long exposure of the human population, a carcinogenic, i.e., tumor-promoting potency of NDL-PCBs has to be considered in human risk assessment. A major problem in risk assessment of NDL-PCBs is dioxin-like impurities that can occur in commercially available NDL-PCB standards. In the present study, we analyzed the induction of CYP2B1 and CYP3A1 in primary rat hepatocytes using a number of highly purified NDL-PCBs with various degrees of chlorination and substitution patterns. Induction of these enzymes is mediated by the nuclear xenobiotic receptors CAR (Constitutive androstane receptor) and PXR (Pregnane X receptor). For CYP2B1 induction, concentration–response analysis revealed a very narrow window of EC₅₀ estimates, being in the range of 1–4 μM for PCBs 28 and 52, and between 0.4 and 1 μM for PCBs 101, 138, 153 and 180. CYP3A1 induction was less sensitive to NDL-PCBs, the most pronounced induction being achieved at 100 μM with the higher chlorinated congeners. Using okadaic acid and small interfering RNAs targeting CAR and PXR, we could demonstrate that CAR plays a major role and PXR a minor role in NDL-PCB-driven induction of CYPs, both effects showing no stringent structure–activity relationship. As the only obvious relevant determinant, the degree of chlorination was found to be positively correlated with the inducing potency of the congeners.     

Intestinal flora induces the expression of Cyp3a in the mouse liver

1. In order to determine the effects of intestinal flora on the expression of cytochrome P450 (CYP), the mRNA expression of CYP was compared between specific pathogen-free (SPF) and germ-free (GF) mice.

2. Most of the major CYP isozymes showed higher expression in the livers of SPF mice compared with GF mice.

3. Nuclear factors such as pregnane ? receptor (PXR) and constitutive androstane receptor (CAR), as well as transporters and conjugation enzymes involved in the detoxification of lithocholic acid (LCA), also showed higher expression in SPF mice.

4. The findings suggest that in the livers of SPF mice, LCA produced by intestinal flora increases the expression of CYPs via activation of PXR and CAR.

5. Drugs such as antibiotics, some diseases and ageing, etc. are known to alter intestinal flora. The present findings suggest that such changes also affect CYP and are one of the factors responsible for individual differences in pharmacokinetics.