Development of an in vitro high content imaging assay for quantitative assessment of CAR-dependent mouse, rat, and human primary hepatocyte proliferation

Marked differences in thyroxine (T4) metabolism following in vitro exposure of Wistar rat and human hepatocytes to several reference CAR/PXR nuclear receptor activators

Our study builds upon previous findings (Baze et al., 2024) by investigating species differences in thyroxine (T4) metabolism regulation by CAR/PXR activators using cryopreserved primary Wistar rat hepatocytes (PRH) and human hepatocytes (PHH) in 2D-sandwich over a 7-day treatment period. Daily exposure of PRH to phenobarbital, 5-Pregnen-3β-ol-20-one-16α‑carbonitrile (PCN) or dexamethasone increased T4 clearance over the last 24 h exposure (up to 60 %, 79 % and 67 % over control, respectively) and secretion of T4-glucuronide (T4-G; up to 463, 661 and 545 pmol/106 cells over control, respectively). Effects were concentration-dependent for phenobarbital and PCN and highest at the lowest concentration for dexamethasone, while rifampicin barely affected T4 clearance and T4-G secretion. None of the compounds, at any tested concentration, affected these parameters in PHH. Additionally, mRNA expression data were consistent with the species-specific and concentration-dependent regulation of phase I Cyp/CYP, phase II Ugt/UGT and phase III Mrp2/MRP2 pathways occurring in rat and human liver following CAR/PXR activation. T4-UGT relative activity increased in PRH only, specifically by PCN, dexamethasone and phenobarbital. The comparison of PRH and PHH responses to compounds represents an important step towards using in vitro methods to reduce animal testing. We recommend using relative T4-UGT activity thresholds observed in PRH as benchmarks for defining compound-related effects across species, helping determine the human relevance of thyroid effects in rodents.

Constitutive androstane receptor (CAR) functions as a tumor suppressor via regulating stemness in liver cancer

Constitutive androstane receptor (CAR) is a xenosensor that is almost exclusively expressed in the liver. Studies in rodents suggest an oncogenic role for CAR in liver cancer, but its role in human liver cancer is unclear. We aimed to investigate the functional roles of CAR in human liver cancer with a focus on the liver cancer stem cells. We used bioinformatics to increase our understanding of CAR in human liver cancer and associated stem cell markers. We studied the functional roles of CAR in human liver cancer with a focus on the liver cancer stem cell using siRNA, modulation of CAR activity, and tumorsphere formation assays. We have revealed significant associations between CAR and a wide variety of signalling pathways including stemness signalling. Further in vitro studies have shown that activation of CAR significantly reduces cancer cell stemness and represses proliferation, migration, invasion, and the tumorsphere-forming abilities of liver cancer cells (p < 0.05). Our data demonstrates the unequivocal tumor-suppressive role of CAR in liver cancer. While more detailed mechanistic studies are warranted, the efficacy of CAR xeno-activators in the treatment of advanced hepatocellular carcinoma (HCC) may potentially open a new avenue for liver cancer therapy.

Mode of action analysis for fluxapyroxad-induced rat liver tumour formation: evidence for activation of the constitutive androstane receptor and assessment of human relevance

The fungicide fluxapyroxad (BAS 700 F) has been shown to significantly increase the incidence of liver tumours in male Wistar rats at dietary levels of 1500 and 3000 ppm and in female rats at a dietary level of 3000 ppm via a non-genotoxic mechanism. In order to elucidate the mode of action (MOA) for fluxapyroxad-induced rat liver tumour formation a series of in vivo and in vitro investigative studies were undertaken. The treatment of male and female Wistar rats with diets containing 0 (control), 50, 250, 1500 and 3000 ppm fluxapyroxad for 1, 3, 7 and 14 days resulted in a dose-dependent increases in relative weight at 1500 and 3000 ppm from day 3 onwards in both sexes, with an increase in relative liver weight being also observed in male rats given 250 ppm fluxapyroxad for 14 days. Examination of liver sections revealed a centrilobular hepatocyte hypertrophy in some fluxapyroxad treated male and female rats. Hepatocyte replicative DNA synthesis (RDS) was significantly increased in male rats given 1500 and 3000 ppm fluxapyroxad for 3 and 7 days and in female rats given 50-3000 ppm fluxapyroxad for 7 days and 250-3000 ppm fluxapyroxad for 3 and 14 days; the maximal increases in RDS in both sexes being observed after 7 days treatment. The treatment of male and female Wistar rats with 250-3000 ppm fluxapyroxad for 14 days resulted in significant increases in hepatic microsomal total cytochrome P450 (CYP) content and CYP2B subfamily-dependent enzyme activities. Male Wistar rat hepatocytes were treated with control medium and medium containing 1-100 μM fluxapyroxad or 500 μM sodium phenobarbital (NaPB) for 4 days. Treatment with fluxapyroxad and NaPB increased CYP2B and CYP3A enzyme activities and mRNA levels but had little effect on markers of CYP1A and CYP4A subfamily enzymes and of the peroxisomal fatty acid β-oxidation cycle. Hepatocyte RDS was significantly increased by treatment with fluxapyroxad, NaPB and 25 ng/ml epidermal growth factor (EGF). The treatment of hepatocytes from two male human donors with 1-100 μM fluxapyroxad or 500 μM NaPB for 4 days resulted in some increases in CYP2B and CYP3A enzyme activities and CYP mRNA levels but had no effect on hepatocyte RDS, whereas treatment with EGF resulted in significant increase in RDS in both human hepatocyte preparations. Hepatocytes from male Sprague-Dawley wild type (WT) and constitutive androstane receptor (CAR) knockout (CAR KO) rats were treated with control medium and medium containing 1-16 μM fluxapyroxad or 500 μM NaPB for 4 days. While both fluxapyroxad and NaPB increased CYP2B enzyme activities and mRNA levels in WT hepatocytes, only minor effects were observed in CAR KO rat hepatocytes. Treatment with both fluxapyroxad and NaPB only increased RDS in WT and not in CAR KO rat hepatocytes, whereas treatment with EGF increased RDS in both WT and CAR KO rat hepatocytes. In conclusion, a series of in vivo and in vitro investigative studies have demonstrated that fluxapyroxad is a CAR activator in rat liver, with similar properties to the prototypical CAR activator phenobarbital. A robust MOA for fluxapyroxad-induced rat liver tumour formation has been established. Based on the lack of effect of fluxapyroxad on RDS in human hepatocytes, it is considered that the MOA for fluxapyroxad-induced liver tumour formation is qualitatively not plausible for humans.

Suppression of the Epithelial-Mesenchymal Transition and Maintenance of the Liver Functions in Primary Hepatocytes through Dispersion Culture within a Dome-Shaped Collagen Matrix

Primary hepatocytes are valuable for studying liver diseases, drug-induced liver injury, and drug metabolism. However, when cultured in a two-dimensional (2D) environment, primary hepatocytes undergo rapid dedifferentiation via an epithelial-mesenchymal transition (EMT) and lose their liver-specific functions. On the other hand, a three-dimensional (3D) culture of primary hepatocyte organoids presents challenges for analyzing cellular functions and molecular behaviors due to strong cell-cell adhesion among heterogeneous cells. In this study, we developed a novel dispersion culture method of hepatocytes within a dome-shaped collagen matrix, overcoming conventional limitations. The expression levels of EMT-related genes were lower in rat primary hepatocytes cultured using this method for 4 d than in cells cultured using the 2D method. Furthermore, albumin production, a marker of liver function, declined sharply in rat primary hepatocytes cultured in two dimensions from 6.40 µg/mL/48 h on day 4 to 1.35 µg/mL/48 h on day 8, and declined gradually from 4.92 µg/mL/48 h on day 8 to 3.89 µg/mL/48 h on day 14 in rat primary hepatocytes cultured using our new method. These findings indicate that the newly developed culture method can suppress EMT and maintain liver functions for 14 d in rat primary hepatocytes, potentially expanding the utility of primary hepatocyte cultured by using conventional 3D methods.

Increased Cell Proliferation as a Key Event in Chemical Carcinogenesis: Application in an Integrated Approach for the Testing and Assessment of Non-Genotoxic Carcinogenesis

In contrast to genotoxic carcinogens, there are currently no internationally agreed upon regulatory tools for identifying non-genotoxic carcinogens of human relevance. The rodent cancer bioassay is only used in certain regulatory sectors and is criticized for its limited predictive power for human cancer risk. Cancer is due to genetic errors occurring in single cells. The risk of cancer is higher when there is an increase in the number of errors per replication (genotoxic agents) or in the number of replications (cell proliferation-inducing agents). The default regulatory approach for genotoxic agents whereby no threshold is set is reasonably conservative. However, non-genotoxic carcinogens cannot be regulated in the same way since increased cell proliferation has a clear threshold. An integrated approach for the testing and assessment (IATA) of non-genotoxic carcinogens is under development at the OECD, considering learnings from the regulatory assessment of data-rich substances such as agrochemicals. The aim is to achieve an endorsed IATA that predicts human cancer better than the rodent cancer bioassay, using methodologies that equally or better protect human health and are superior from the view of animal welfare/efficiency. This paper describes the technical opportunities available to assess cell proliferation as the central gateway of an IATA for non-genotoxic carcinogenicity.

Nuclear Receptor-Mediated Hepatomegaly and Liver Regeneration: An Update

Nuclear receptors (NRs), a superfamily of ligand-activated transcription factors, are critical in cell growth, proliferation, differentiation, metabolism, and numerous biologic events. NRs have been reported to play important roles in hepatomegaly (liver enlargement) and liver regeneration by regulating target genes or interacting with other signals. In this review, the roles and involved molecular mechanisms of NRs in hepatomegaly and liver regeneration are summarized and the future perspectives of NRs in the treatment of liver diseases are discussed. SIGNIFICANCE STATEMENT: NRs play critical roles in hepatomegaly and liver regeneration, indicating the potential of NRs as targets to promote liver repair after liver injury. This paper reviews the characteristics and molecular mechanisms of NRs in regulating hepatomegaly and liver regeneration, providing more evidence for NRs in the treatment of related liver diseases.

Application of humanized mice to toxicology studies: Evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR)-mediated mode of action (MOA) for phenobarbital (PB)-induced rodent liver tumor formation has been established, with increased hepatocyte proliferation, which is a key event in tumor formation. Previous studies have demonstrated that PB and other CAR-activators stimulate proliferation in cultured rodent hepatocytes, but not in cultured human hepatocytes. However, in the genetically humanized CAR and pregnane X receptor (PXR) mouse (hCAR/hPXR mouse, downstream genes are still mouse), PB increased hepatocyte proliferation and tumor production in vivo. In contrast to the hCAR/hPXR mouse, studies with chimeric mice with human hepatocytes (PXB-mouse, both receptor and downstream genes are human) demonstrated that PB did not increase human hepatocyte proliferation in vivo. PB increased hepatocyte proliferation in a chimeric mouse model with rat hepatocytes, indicating that the lack of human hepatocyte proliferation is not due to any functional defect in the chimeric mouse liver environment. Gene expression analysis demonstrated that the downstream genes of CAR/PXR activation were similar in hCAR/hPXR and CD-1 mice, but differed from those observed in chimeric mice with human hepatocytes. These findings strongly support the conclusion that the MOA for CAR-mediated rodent liver tumor formation is qualitatively implausible for humans. Indeed, epidemiological studies have found no causal link between PB and human liver tumors. There are many similarities with respect to hepatic effects and species differences between rodent CAR and peroxisome proliferator-activated receptor α activators. Based on our research, the chimeric mouse with human hepatocytes (PXB-mouse) is reliable for human cancer risk assessment of test chemicals.

Comparison of the Hepatic Effects of Phenobarbital in Chimeric Mice Containing Either Rat or Human Hepatocytes With Humanized Constitutive Androstane Receptor and Pregnane X Receptor Mice

Using a chimeric mouse humanized liver model, we provided evidence that human hepatocytes are refractory to the mitogenic effects of rodent constitutive androstane receptor (CAR) activators. To evaluate the functional reliability of this model, the present study examined mitogenic responses to phenobarbital (PB) in chimeric mice transplanted with rat hepatocytes, because rats are responsive to CAR activators. Treatment with 1000 ppm PB for 7 days significantly increased replicative DNA synthesis (RDS) in rat hepatocytes of the chimeric mice, demonstrating that the transplanted hepatocyte model is functionally reliable for cell proliferation analysis. Treatment of humanized CAR and pregnane X receptor (PXR) mice (hCAR/hPXR mice) with 1000 ppm PB for 7 days significantly increased hepatocyte RDS together with increases in several mitogenic genes. Global gene expression analysis was performed with liver samples from this and from previous studies focusing on PB-induced Wnt/β-catenin signaling and showed that altered genes in hCAR/hPXR mice clustered most closely with liver tumor samples from a diethylnitrosamine/PB initiation/promotion study than with wild-type mice. However, different gene clusters were observed for chimeric mice with human hepatocytes for Wnt/β-catenin signaling when compared with those of hCAR/hPXR mice, wild-type mice, and liver tumor samples. The results of this study demonstrate clear differences in the effects of PB on hepatocyte RDS and global gene expression between human hepatocytes of chimeric mice and hCAR/hPXR mice, suggesting that the chimeric mouse model is relevant to humans for studies on the hepatic effects of rodent CAR activators whereas the hCAR/hPXR mouse is not.

Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARα), with PPARα activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor formation by PB and other CAR activators is qualitatively not plausible for humans. This conclusion is supported by data from several human epidemiology studies.

Role of the constitutive androstane receptor (CAR) in human liver cancer

The constitutive androstane receptor (CAR) is a member of the nuclear receptor superfamily (subfamily 1, group I, member 3, also known as NR1I3) that is almost exclusively expressed in the liver. CAR interacts with key signalling pathways such as those involved in drug, energy and bilirubin metabolism. In mouse models, activation of CAR leads to tumorigenesis by inducing pro-proliferative and anti-apoptotic signalling. However, many previous reports have shown species differences between CAR activity in animal models and humans. Recent studies have demonstrated that the mode of action of CAR in rodent liver tumorigenesis is not applicable to humans. Despite this, many studies still continue to study the role of CAR in animal models, hence, there is a need to further explore the role of CAR in human diseases particularly cancers. While there is limited evidence for a role of CAR in human cancers, some studies have proposed a tumour-suppressive role of CAR in liver cancer. In addition, recent studies exploring CAR in human livers demonstrated a hepato-protective role for CAR in and more specifically, its ability to drive differentiation and liver regeneration. This review will discuss the role of CAR in liver cancer, with a focus on species differences and its emerging, tumour-suppressive role in liver cancer and its role in the regulation of liver cancer stem cells.

The xenobiotic receptors PXR and CAR in liver physiology, an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.

Species differences in phenobarbital-mediated UGT gene induction in rat and human liver microtissues

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Species differences in UGT induction could mediate thyroid cancer susceptibility.

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The effect of CAR activators on rat thyroid carcinogenesis could be partly explained by differential induction of Ugt 2b17.

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Human UGT changes would likely contribute less to species differences in T4 metabolism than rat UGT changes.

Species differences in hepatic metabolism of thyroxine (T4) by uridine diphosphate glucuronosyl transferase (UGT) and susceptibility to thyroid hormone imbalance could underlie differences in thyroid carcinogenesis caused by hepatic enzyme inducers in rats and humans. To investigate this hypothesis we examined profiles of hepatic UGT induction by the prototypical CAR activator phenobarbital (PB) in rat and human liver 3D microtissues. The rationale for this approach was that 3D microtissues would generate data more relevant to humans. Rat and human liver 3D microtissues were exposed to PB over a range of concentrations (500 u M - 2000 u M) and times (24−96 hr). Microarray and proteomics analyses were performed on parallel samples to generate integrated differentially expressed gene (DEG) datasets. Bioinformatics analysis of DEG data, including CAR response element (CRE) sequence analysis of UGT promoters, was used to assess species differences in UGT induction relative to CAR-mediated transactivation potential. A higher proportion of human UGT promoters were found to contain consensus CREs compared to the rat homologs. UGTs 1a6, 2b17 and 2b37 were upregulated by PB in rat liver 3D microtissues, but unaltered in human liver 3D microtissues. By contrast, human UGTs 1A8, 1A10 and 2B10 showed higher levels of induction (RNA and /or protein) compared to the rat homologs. There was general concordance between the presence of CREs and the induction of UGT RNA. As UGT1A and 2B isoforms metabolise T4, these results suggest that differences in UGT induction could contribute to differential susceptibility to CAR-mediated thyroid carcinogenesis in rats and humans.

MicroRNA-125b-5p Regulates Hepatocyte Proliferation During the Termination Phase of Liver Regeneration

The ability of the liver to regenerate and restore mass limits the increasing mortality rate due to life-threatening liver diseases. Successful liver regeneration is accomplished in multiple stages, of which the priming and proliferation phases are well studied. However, the regulatory pathways, specifically microRNA (miRNA)-mediated posttranscriptional regulation, which prevent uncontrolled proliferation and mediate the termination of liver regeneration, are not well understood. We identified differentially regulated miRNAs during the termination phase after 2/3 partial hepatectomy (PH) in mice, which is a well-established mouse model of liver regeneration. We further evaluated the function of differentially regulated miRNAs in primary mouse hepatocytes by using mimics and inhibitors and in vivo by using adeno-associated virus (AAV) serotype 8. A candidate miRNA target was identified by messenger RNA array in silico analyses and validated in primary mouse and human hepatocytes. Using miRNA profiling, we discovered miR-125b-5p as a novel regulator of hepatocyte proliferation in the late phase of liver regeneration. AAV-mediated miR-125b-5p delivery in mice enhanced the endogenous regenerative capacity and resulted in improved restoration of liver mass after 2/3 PH. Further, we found that ankyrin repeat and BTB/POZ domain containing protein 1 (Abtb1) is a direct target of miR-125b-5p in primary mouse and human hepatocytes and contributes to the pro-proliferative activity of miR-125b-5p by forkhead box G1 (FOXG1) and the cyclin-dependent kinase inhibitor 1A (p21) pathway. Conclusion: miR-125b-5p has an important role in regulating hepatocyte proliferation in the termination phase of liver regeneration and may serve as a potential therapeutic target in various liver diseases that often exhibit deregulated hepatocyte proliferation.

Piperonyl butoxide: Mode of action analysis for mouse liver tumour formation and human relevance

In a 79 week bioassay the pesticide synergist piperonyl butoxide (PBO) was shown to significantly increase the incidence of hepatocellular adenoma (but not hepatocellular carcinoma) in male CD-1 mice at dietary levels of 100 and 300 mg/kg/day PBO and in female mice at a dietary level of 300 mg/kg/day. As PBO is not a genotoxic agent, a series of investigative studies were undertaken to elucidate the mode of action (MOA) for PBO-induced mouse liver tumour formation. Male CD-1 mice were fed diets to provide intakes of 0 (control), 30, 100 and 300 mg/kg/day PBO and for purposes of comparison 500 ppm sodium phenobarbital (NaPB), a known constitutive androstane receptor (CAR) activator, for 7 and 14 days. Treatment with 100 and 300 mg/kg/day PBO and 500 ppm NaPB increased relative liver weight which was associated with hepatocyte hypertrophy, with hepatocyte replicative DNA synthesis (RDS) being increased after 7 days treatment. The treatment of CD-1 mice with 30-300 mg/kg/day PBO for 14 days resulted in significant dose-dependent increases in hepatic microsomal cytochrome P450 (CYP) content and 7-pentoxyresorufin O-depentylase (PROD) activity and in hepatic Cyp2b10 mRNA levels. In contrast, PBO produced a biphasic effect on markers of activation of the peroxisome proliferator-activated receptor alpha (PPARα), with small increases in microsomal lauric acid 12-hydroxylase activity and hepatic Cyp4a10 mRNA levels being observed in mice given 100 mg/kg/day with PBO, with either no increase or a significant inhibition being observed in mice given 300 mg/kg/day PBO. The hepatic effects of PBO in male CD-1 mice were generally similar to those produced by NaPB and were reversible after the cessation of treatment for 28 days. Studies were also performed in male C57BL/6J (wild type) mice and in hepatic CAR and pregnane X receptor (PXR) knockout mice (CAR KO/PXR KO mice), where in the CAR KO/PXR KO mice PBO had little effect on markers of CAR activation, but produced some increases in markers of PPARα activation. The treatment of male CD-1 mouse hepatocytes for 4 days with 5-50 μM PBO, 10-1000 μM NaPB and 25 ng/mL epidermal growth factor (EGF) resulted in significant increases in hepatocyte RDS. While treatment of hepatocytes from one male and one female human donor with 5-500 μM PBO and 10-1000 μM NaPB for 4 days had no effect on hepatocyte RDS, treatment with EGF resulted in significant increases in RDS in both human hepatocyte preparations. In summary, PBO is predominantly a hepatic CAR activator at carcinogenic dose levels in CD-1 mice, with activation of hepatic CAR resulting in a suppression of the effect of PBO on hepatic PPARα. A robust MOA for PBO-induced mouse liver tumour formation has been established, this MOA being similar to that previously identified for NaPB and some other rodent liver CAR activators. Based on the lack of effect of PBO on RDS in human hepatocytes, it is considered that the MOA for PBO-induced mouse liver tumour formation is qualitatively not plausible for humans.

Nuclear receptor CAR-mediated liver cancer and its species differences

Introduction: The nuclear receptor CAR plays an important role in the regulation of hepatic responses to xenobiotic exposure, including the induction of hepatocyte proliferation and chemical carcinogenesis. Phenobarbital, a well-known liver cancer promoter, has been found to promote hepatocyte proliferation via CAR activation. However, the molecular mechanisms by which CAR induces liver carcinogenesis remain unknown. In addition, it is believed that CAR-mediated liver carcinogenesis shows a species difference; phenobarbital treatment induces hepatocyte proliferation and liver cancer in rodents but not in humans. However, the mechanisms are also unknown.Areas covered: Several reports indicate that the key oncogenic signaling pathways Wnt/β-catenin and Hippo/YAP are involved in CAR-mediated liver carcinogenesis. We introduce current data about the possible molecular mechanisms involved in CAR-mediated liver carcinogenesis and species differences by focusing on these two signaling pathways.Expert opinion: CAR may activate both the Wnt/β-catenin and Hippo/YAP signaling pathways. The synergistic activation of both signaling pathways seems to be important for CAR-mediated liver cancer development. Low homology between the ligand binding domains of human CAR and rodent CAR might cause species differences in the interactions with proteins that control the Wnt/β-catenin and Hippo/YAP pathways as well as liver cancer induction.

Comparative studies on the effects of sodium phenobarbital and two other constitutive androstane receptor (CAR) activators on induction of cytochrome P450 enzymes and replicative DNA synthesis in cultured hepatocytes from wild type and CAR knockout rats

Nongenotoxic chemicals can produce liver tumours in rats and mice by a mitogenic mode of action involving activation of the constitutive androstane receptor (CAR). The aim of this study was to evaluate the usefulness of cultured hepatocytes from normal (wild type; WT) and CAR knockout (KO) rats to screen compounds as potential activators of rat CAR and to validate this test system. Cultured hepatocytes from male Sprague-Dawley WT and CAR KO rats were treated with either 100 and 1000 μM sodium phenobarbital (NaPB), 3-100 μM fluquinconazole (FQZ), or 3-300 μM 3-(difluoromethyl)-1-methyl-N-(3´,4´,6-trifluoro[1,1´-biphenyl]-2-yl)-1H-pyrazole-4-carboxamide (TI1) for 96 h. Induction of cytochrome P450 (CYP) enzymes was monitored by measurement of 7-pentoxyresorufin O-depentylase (PROD), 7-benzyloxyresorufin O-debenzylase (BROD) and 7-benzyloxyquinoline O-debenzylase (BQ) activities. Hepatocytes undergoing replicative DNA synthesis (RDS) were labelled by adding 10 μM 5-bromo-2´-deoxyuridine to the culture medium for determination of the hepatocyte labelling index. The treatment of WT, but not of CAR KO, rat hepatocytes with NaPB, FQZ and TI1 increased hepatocyte RDS and induced CYP2B-dependent PROD activity. In contrast, all three compounds increased CYP2B/3A-dependent BROD and CYP3A-dependent BQ activities in both WT and CAR KO rat hepatocytes. Hepatocyte RDS was increased in both WT and CAR KO rat hepatocytes by treatment with 25 ng/ml epidermal growth factor as a positive control. Overall, these results demonstrate that the effects of three CAR activators on RDS and CYP2B enzyme induction are abolished in cultured CAR KO rat hepatocytes. As demonstrated by this validation study, the CAR KO hepatocyte model is a useful in vitro mechanistic tool for the rapid screening of chemicals as potential activators of rat CAR.

The Roles of Xenobiotic Receptors: Beyond Chemical Disposition

Over the past 20 years, the ability of the xenobiotic receptors to coordinate an array of drug-metabolizing enzymes and transporters in response to endogenous and exogenous stimuli has been extensively characterized and well documented. The constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) are the xenobiotic receptors that have received the most attention since they regulate the expression of numerous proteins important to drug metabolism and clearance and formulate a central defensive mechanism to protect the body against xenobiotic challenges. However, accumulating evidence has shown that these xenobiotic sensors also control many cellular processes outside of their traditional realms of xenobiotic metabolism and disposition, including physiologic and/or pathophysiologic responses in energy homeostasis, cell proliferation, inflammation, tissue injury and repair, immune response, and cancer development. This review will highlight recent advances in studying the noncanonical functions of xenobiotic receptors with a particular focus placed on the roles of CAR and PXR in energy homeostasis and cancer development.

Case examples of an evaluation of the human relevance of the pyrethroids/pyrethrins-induced liver tumours in rodents based on the mode of action

Rodent carcinogenicity studies are useful for screening for human carcinogens but they are not perfect. Some modes of action (MOAs) lead to cancers in both experimental rodents and humans, but others that lead to cancers in rodents do not do so in humans. Therefore, analysing the MOAs by which chemicals produce tumours in rodents and determining the relevance of such tumour data for human risk are critical. Recently, experimental data were obtained as case examples of an evaluation of the human relevance of pyrethroid (metofluthrin and momfluorothrin)- and pyrethrins-induced liver tumours in rats based on MOA. The MOA analysis, based on the International Programme on Chemical Safety (IPCS) framework, concluded that experimental data strongly support that the postulated MOA for metofluthrin-, momfluorothrin- and pyrethrins-produced rat hepatocellular tumours is mediated by constitutive androstane receptor (CAR) activation. Since metofluthrin and momfluorothrin are close structural analogues, reproducible outcomes for both chemicals provide confidence in the MOA findings. Furthermore, cultured human hepatocyte studies and humanized chimeric mouse liver studies demonstrated species difference between human hepatocytes (refractory to the mitogenic effects of these compounds) and rat hepatocytes (sensitive to their mitogenic effects). These data strongly support the hypothesis that the CAR-mediated MOA for liver tumorigenesis is of low carcinogenic risk for humans. In this research, in addition to cultured human hepatocyte studies, the usefulness of the humanized chimeric liver mouse models was clearly demonstrated. These data substantially influenced decisions in regulatory toxicology. In this review I comprehensively discuss the human relevance of the CAR-mediated MOA for rodent liver tumorigenesis based on published information, including our recent molecular research on CAR-mediated MOA.

Human relevance of rodent liver tumour formation by constitutive androstane receptor (CAR) activators

A large number of nongenotoxic chemicals have been shown to increase the incidence of liver tumours in rats and/or mice by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with the model CAR activator phenobarbital (PB) and its sodium salt (sodium phenobarbital; NaPB) have demonstrated that the key and associative events for rat and mouse liver tumour formation include CAR activation, increased hepatocyte replicative DNA synthesis (RDS), induction of cytochrome P450 CYP2B subfamily enzymes, liver hypertrophy, increased altered hepatic foci and hepatocellular adenomas/carcinomas. The key species difference between the rat and mouse compared to humans, is that human hepatocytes are refractory to the mitogenic effects of PB/NaPB and other CAR activators. While PB/NaPB and other CAR activators stimulate RDS in rat and mouse hepatocytes in both in vitro and in vivo studies, such compounds do not stimulate RDS in cultured human hepatocytes and in in vivo studies performed in chimeric mice with humanised livers. In terms of species differences in RDS, unlike the rat and mouse, humans are similar to other species such as the Syrian hamster and guinea pig in being nonresponsive to the mitogenic effects of CAR activators. Overall, the MOA for rat and mouse liver tumour formation by PB/NaPB and other CAR activators is considered qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies, which demonstrate that chronic treatment with PB does not increase the incidence of liver cancer in humans.

Comparison of the effects of sodium phenobarbital in wild type and humanized constitutive androstane receptor (CAR)/pregnane X receptor (PXR) mice and in cultured mouse, rat and human hepatocytes

Phenobarbital (PB), a constitutive androstane receptor (CAR) activator, produces liver tumours in rodents by a mitogenic mode of action involving CAR activation. In this study, the hepatic effects of sodium phenobarbital (NaPB) were compared in male C57BL/6J wild type (WT) mice and in humanized mice, where both the mouse CAR and pregnane X receptor (PXR) have been replaced by their human counterparts (hCAR/hPXR mice). Investigations were also performed in cultured male C57BL/6J and CD-1 mouse, male Sprague-Dawley rat and male and female human hepatocytes. The treatment of WT and hCAR/hPXR mice with 186-984 ppm NaPB in the diet for 7 days resulted in increased relative liver weight, hypertrophy and induction of cytochrome P450 (CYP) enzyme activities. Treatment with NaPB also produced dose-dependent increases in hepatocyte replicative DNA synthesis (RDS), with the effect being more marked in WT than in hCAR/hPXR mice. While the treatment of cultured C57BL/6J and CD-1 mouse, Sprague-Dawley rat and human hepatocytes with 100 and/or 1000 μM NaPB for 4 days induced CYP enzyme activities, increased RDS was only observed in mouse and rat hepatocytes. However, as a positive control, epidermal growth factor increased RDS in hepatocytes from all three species. In summary, although human hepatocytes are refractory to the mitogenic effects of NaPB, treatment with NaPB induced RDS in vivo in hCAR/hPXR mice, which is presumably due to the human CAR and PXR receptors operating in a mouse hepatocyte regulatory environment. As the response of the hCAR/hPXR mouse to the CAR activator NaPB differs markedly from that of human hepatocytes, the hCAR/hPXR mouse is thus not a suitable animal model for studies on the hepatic effects of nongenotoxic rodent CAR activators.

Human relevance of rodent liver tumors: Key insights from a Toxicology Forum workshop on nongenotoxic modes of action

The Toxicology Forum sponsored a workshop in October 2016, on the human relevance of rodent liver tumors occurring via nongenotoxic modes of action (MOAs). The workshop focused on two nuclear receptor-mediated MOAs (Constitutive Androstane Receptor (CAR) and Peroxisome Proliferator Activated Receptor-alpha (PPARα), and on cytotoxicity. The goal of the meeting was to review the state of the science to (1) identify areas of consensus and differences, data gaps and research needs; (2) identify reasons for inconsistencies in current regulatory positions; and (3) consider what data are needed to demonstrate a specific MOA, and when additional research is needed to rule out alternative possibilities. Implications for quantitative risk assessment approaches were discussed, as were implications of not considering MOA and dose in hazard characterization and labeling schemes. Most, but not all, participants considered the CAR and PPARα MOAs as not relevant to humans based on quantitative and qualitative differences. In contrast, cytotoxicity is clearly relevant to humans, but a threshold applies. Questions remain for all three MOAs concerning what data are necessary to determine the MOA and to what extent it is necessary to exclude other MOAs.