Gender-specific interplay of signaling through β-catenin and CAR in the regulation of xenobiotic-induced hepatocyte proliferation

Current view of liver cancer cell-of-origin and proposed mechanisms precluding its proper determination

Hepatocellular carcinoma and intrahepatic cholangiocarcinoma are devastating primary liver cancers with increasing prevalence in many parts of the world. Despite intense investigation, many aspects of their biology are still largely obscure. For example, numerous studies have tackled the question of the cell-of-origin of primary liver cancers using different experimental approaches; they have not, however, provided a clear and undisputed answer. Here, we will review the evidence from animal models supporting the role of all major types of liver epithelial cells: hepatocytes, cholangiocytes, and their common progenitor as liver cancer cell-of-origin. Moreover, we will also propose mechanisms that promote liver cancer cell plasticity (dedifferentiation, transdifferentiation, and epithelial-to-mesenchymal transition) which may contribute to misinterpretation of the results and which make the issue of liver cancer cell-of-origin particularly complex.

Proteomic analysis of hepatic effects of phenobarbital in mice with humanized liver

Activation of the constitutive androstane receptor (CAR) may induce adaptive but also adverse effects in rodent liver, including the induction of drug-metabolizing enzymes, transient hepatocellular proliferation, and promotion of liver tumor growth. Human relevance of CAR-related adverse hepatic effects is controversially debated. Here, we used the chimeric FRG-KO mouse model with livers largely repopulated by human hepatocytes, in order to study human hepatocytes and their response to treatment with the model CAR activator phenobarbital (PB) in vivo. Mice received an intraperitoneal injection with 50 mg/kg body weight PB or saline, and were sacrificed after 72–144 h. Non-repopulated FRG-KO mice were used as additional control. Comprehensive proteomics datasets were generated by merging data obtained by targeted as well as non-targeted proteomics approaches. For the first time, a novel proteomics workflow was established to comparatively analyze the effects of PB on human and murine proteins within one sample. Analysis of merged proteome data sets and bioinformatics data mining revealed comparable responses in murine and human hepatocytes with respect to nuclear receptor activation and induction of xenobiotic metabolism. By contrast, activation of MYC, a key regulator of proliferation, was predicted only for mouse but not human hepatocytes. Analyses of 5-bromo-2′-deoxyuridine incorporation confirmed this finding. In summary, this study for the first time presents a comprehensive proteomic analysis of CAR-dependent effects in human and mouse hepatocytes from humanized FRG-KO mice. The data support the hypothesis that PB does induce adaptive metabolic responses, but not hepatocellular proliferation in human hepatocytes in vivo.

Clinical Relevance of the Constitutive Androstane Receptor

Constitutive androstane receptor (CAR) (NR1I3), a xenobiotic receptor, has long been considered a master mediator of drug disposition and detoxification. Accumulating evidence indicates that CAR also participates in various physiologic and pathophysiological pathways regulating the homeostasis of glucose, lipid, and bile acids, and contributing to cell proliferation, tissue regeneration and repair, as well as cancer development. The expression and activity of CAR can be regulated by various factors, including small molecular modulators, CAR interaction with other transcription factors, and naturally occurring genetic variants. Given that the influence of CAR has extended beyond the realm of drug metabolism and disposition and has expanded into a potential modulator of human diseases, growing efforts have centered on understanding its clinical relevance and impact on human pathophysiology. This review highlights the current information available regarding the contribution of CAR to various metabolic disorders and cancers and ponders the possible challenges that might arise from pursuing CAR as a potential therapeutic target for these diseases. SIGNIFICANCE STATEMENT: The growing importance of the constitutive androstane receptor (CAR) in glucose and lipid metabolism as well as its potential implication in cell proliferation emphasizes a need to keenly understand the biological function and clinical impact of CAR. This minireview captures the clinical relevance of CAR by highlighting its role in metabolic disorders and cancer development.

In Vivo and In Vitro Models of Hepatocellular Carcinoma: Current Strategies for Translational Modeling

Hepatocellular carcinoma (HCC) is the sixth most common cancer worldwide and the third leading cause of cancer-related death globally. HCC is a complex multistep disease and usually emerges in the setting of chronic liver diseases. The molecular pathogenesis of HCC varies according to the etiology, mainly caused by chronic hepatitis B and C virus infections, chronic alcohol consumption, aflatoxin-contaminated food, and non-alcoholic fatty liver disease associated with metabolic syndrome or diabetes mellitus. The establishment of HCC models has become essential for both basic and translational research to improve our understanding of the pathophysiology and unravel new molecular drivers of this disease. The ideal model should recapitulate key events observed during hepatocarcinogenesis and HCC progression in view of establishing effective diagnostic and therapeutic strategies to be translated into clinical practice. Despite considerable efforts currently devoted to liver cancer research, only a few anti-HCC drugs are available, and patient prognosis and survival are still poor. The present paper provides a state-of-the-art overview of in vivo and in vitro models used for translational modeling of HCC with a specific focus on their key molecular hallmarks.

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.

β-catenin signaling, the constitutive androstane receptor and their mutual interactions

Aberrant signaling through β-catenin is an important determinant of tumorigenesis in rodents as well as in humans. In mice, xenobiotic activators of the constitutive androstane receptor (CAR), a chemo-sensing nuclear receptor, promote liver tumor growth by means of a non-genotoxic mechanism and, under certain conditions, select for hepatocellular tumors which contain activated β-catenin. In normal hepatocytes, interactions of β-catenin and CAR have been demonstrated with respect to the induction of proliferation and drug metabolism-related gene expression. The molecular details of these interactions are still not well understood. Recently it has been hypothesized that CAR might activate β-catenin signaling, thus providing a possible explanation for some of the observed phenomena. Nonetheless, many aspects of the molecular interplay of the two regulators have still not been elucidated. This review briefly summarizes our current knowledge about the interplay of CAR and β-catenin. By taking into account data and observations obtained with different mouse models and employing different experimental approaches, it is shown that published data also contain substantial evidence that xenobiotic activators of CAR do not activate, or do even inhibit signaling through the β-catenin pathway. The review highlights new aspects of possible ways of interaction between the two signaling cascades and will help to stimulate scientific discussion about the crosstalk of β-catenin signaling and the nuclear receptor CAR.

The Connection of Azole Fungicides with Xeno-Sensing Nuclear Receptors, Drug Metabolism and Hepatotoxicity

Azole fungicides, especially triazole compounds, are widely used in agriculture and as pharmaceuticals. For a considerable number of agricultural azole fungicides, the liver has been identified as the main target organ of toxicity. A number of previous studies points towards an important role of nuclear receptors such as the constitutive androstane receptor (CAR), the pregnane-X-receptor (PXR), or the aryl hydrocarbon receptor (AHR), within the molecular pathways leading to hepatotoxicity of these compounds. Nuclear receptor-mediated hepatic effects may comprise rather adaptive changes such as the induction of drug-metabolizing enzymes, to hepatocellular hypertrophy, histopathologically detectable fatty acid changes, proliferation of hepatocytes, and the promotion of liver tumors. Here, we present a comprehensive review of the current knowledge of the interaction of major agricultural azole-class fungicides with the three nuclear receptors CAR, PXR, and AHR in vivo and in vitro. Nuclear receptor activation profiles of the azoles are presented and related to histopathological findings from classic toxicity studies. Important issues such as species differences and multi-receptor agonism and the consequences for data interpretation and risk assessment are discussed.

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.

Regulation of expression of drug-metabolizing enzymes by oncogenic signaling pathways in liver tumors: a review

Mutations in genes encoding key players in oncogenic signaling pathways trigger specific downstream gene expression profiles in the respective tumor cell populations. While regulation of genes related to cell growth, survival, and death has been extensively studied, much less is known on the regulation of drug-metabolizing enzymes (DMEs) by oncogenic signaling. Here, a comprehensive review of the available literature is presented summarizing the impact of the most relevant genetic alterations in human and rodent liver tumors on the expression of DMEs with a focus on phases I and II of xenobiotic metabolism. Comparably few data are available with respect to DME regulation by p53-dependent signaling, telomerase expression or altered chromatin remodeling. By contrast, DME regulation by constitutive activation of oncogenic signaling via the RAS/RAF/mitogen-activated protein kinase (MAPK) cascade or via the canonical WNT/β-catenin signaling pathway has been analyzed in greater depth, demonstrating mostly positive-regulatory effects of WNT/β-catenin signaling and negative-regulatory effects of MAPK signaling. Mechanistic studies have revealed molecular interactions between oncogenic signaling and nuclear xeno-sensing receptors which underlie the observed alterations in DME expression in liver tumors. Observations of altered DME expression and inducibility in liver tumors with a specific gene expression profile may impact pharmacological treatment options.

Stilbene compound trans-3,4,5,4´-tetramethoxystilbene, a potential anticancer drug, regulates constitutive androstane receptor (Car) target genes, but does not possess proliferative activity in mouse liver

The constitutive androstane receptor(CAR) activation is connected with mitogenic effects leading to liver hyperplasia and tumorigenesis in rodents. CAR activators, including phenobarbital, are considered rodent non-genotoxic carcinogens. Recently, trans-3,4,5,4´-tetramethoxystilbene(TMS), a potential anticancer drug (DMU-212), have been shown to alleviate N-nitrosodiethylamine/phenobarbital-induced liver carcinogenesis. We studied whether TMS inhibits mouse Car to protect from the PB-induced tumorigenesis. Unexpectedly, we identified TMS as a murine CAR agonist in reporter gene experiments, in mouse hepatocytes, and in C57BL/6 mice in vivo. TMS up-regulated Car target genes Cyp2b10, Cyp2c29 and Cyp2c55 mRNAs, but down-regulated expression of genes involved in gluconeogenesis and lipogenesis. TMS did not change or down-regulate genes involved in liver proliferation or apoptosis such as Mki67, Foxm1, Myc, Mcl1, Pcna, Bcl2, or Mdm2, which were up-regulated by another Car ligand TCPOBOP. TMS did not increase liver weight and had no significant effect on Ki67 and Pcna labeling indices in mouse liver in vivo. In murine hepatic AML12 cells, we confirmed a Car-independent proapoptotic effect of TMS. We conclude that TMS is a Car ligand with limited effects on hepatocyte proliferation, likely due to promoting apoptosis in mouse hepatic cells, while controlling Car target genes involved in xenobiotic and endobiotic metabolism.

TCPOBOP-Induced Hepatomegaly and Hepatocyte Proliferation are Attenuated by Combined Disruption of MET and EGFR Signaling

TCPOBOP (1,4-Bis [2-(3,5-Dichloropyridyloxy)] benzene) is a constitutive androstane receptor (CAR) agonist that induces robust hepatocyte proliferation and hepatomegaly without any liver injury or tissue loss. TCPOBOP-induced direct hyperplasia has been considered to be CAR-dependent with no evidence of involvement of cytokines or growth factor signaling. Receptor tyrosine kinases (RTKs), MET and epidermal growth factor receptor (EGFR), are known to play a critical role in liver regeneration after partial hepatectomy, but their role in TCPOBOP-induced direct hyperplasia, not yet explored, is investigated in the current study. Disruption of the RTK-mediated signaling was achieved using MET knockout (KO) mice along with Canertinib treatment for EGFR inhibition. Combined elimination of MET and EGFR signaling [MET KO + EGFR inhibitor (EGFRi)], but not individual disruption, dramatically reduced TCPOBOP-induced hepatomegaly and hepatocyte proliferation. TCPOBOP-driven CAR activation was not altered in [MET KO + EGFRi] mice, as measured by nuclear CAR translocation and analysis of typical CAR target genes. However, TCPOBOP-induced cell cycle activation was impaired in [MET KO + EGFRi] mice due to defective induction of cyclins, which regulate cell cycle initiation and progression. TCPOBOP-driven induction of FOXM1, a key transcriptional regulator of cell cycle progression during TCPOBOP-mediated hepatocyte proliferation, was greatly attenuated in [MET KO + EGFRi] mice. Interestingly, TCPOBOP treatment caused transient decline in hepatocyte nuclear factor 4 alpha expression concomitant to proliferative response; this was not seen in [MET KO + EGFRi] mice. Transcriptomic profiling revealed the vast majority (~40%) of TCPOBOP-dependent genes primarily related to proliferative response, but not to drug metabolism, were differentially expressed in [MET KO + EGFRi] mice. Conclusion: Taken together, combined disruption of EGFR and MET signaling lead to dramatic impairment of TCPOBOP-induced proliferative response without altering CAR activation.

Loss of hepatocyte β-catenin protects mice from experimental porphyria-associated liver injury

BACKGROUND & AIMS

Porphyrias result from anomalies of heme biosynthetic enzymes and can lead to cirrhosis and hepatocellular cancer. In mice, these diseases can be modeled by administration of a diet containing 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), which causes accumulation of porphyrin intermediates, resulting in hepatobiliary injury. Wnt/β-catenin signaling has been shown to be a modulatable target in models of biliary injury; thus, we investigated its role in DDC-driven injury.

METHODS

β-Catenin (Ctnnb1) knockout (KO) mice, Wnt co-receptor KO mice, and littermate controls were fed a DDC diet for 2 weeks. β-Catenin was exogenously inhibited in hepatocytes by administering β-catenin dicer-substrate RNA (DsiRNA), conjugated to a lipid nanoparticle, to mice after DDC diet and then weekly for 4 weeks. In all experiments, serum and livers were collected; livers were analyzed by histology, western blotting, and real-time PCR. Porphyrin was measured by fluorescence, quantification of polarized light images, and liquid chromatography-mass spectrometry.

RESULTS

DDC-fed mice lacking β-catenin or Wnt signaling had decreased liver injury compared to controls. Exogenous mice that underwent β-catenin suppression by DsiRNA during DDC feeding also showed less injury compared to control mice receiving lipid nanoparticles. Control livers contained extensive porphyrin deposits which were largely absent in mice lacking β-catenin signaling. Notably, we identified a network of key heme biosynthesis enzymes that are suppressed in the absence of β-catenin, preventing accumulation of toxic protoporphyrins. Additionally, mice lacking β-catenin exhibited fewer protein aggregates, improved proteasomal activity, and reduced induction of autophagy, all contributing to protection from injury.

CONCLUSIONS

β-Catenin inhibition, through its pleiotropic effects on metabolism, cell stress, and autophagy, represents a novel therapeutic approach for patients with porphyria.

LAY SUMMARY

Porphyrias are disorders resulting from abnormalities in the steps that lead to heme production, which cause build-up of toxic by-products called porphyrins. Liver is commonly either a source or a target of excess porphyrins, and complications can range from minor abnormalities to liver failure. In this report, we inhibited Wnt/β-catenin signaling in an experimental model of porphyria, which resulted in decreased liver injury. Targeting β-catenin affected multiple components of the heme biosynthesis pathway, thus preventing build-up of porphyrin intermediates. Our study suggests that drugs inhibiting β-catenin activity could reduce the amount of porphyrin accumulation and help alleviate symptoms in patients with porphyria.

Mouse Hepatomas with Ha-ras and B-raf Mutations Differ in Mitogen-Activated Protein Kinase Signaling and Response to Constitutive Androstane Receptor Activation

Nuclear receptors mediate the hepatic induction of drug-metabolizing enzymes by xenobiotics. Not much is known about enzyme induction in liver tumors. Here, we treated tumor-bearing mice with phenobarbital, an activator of the constitutive androstane receptor (CAR), to analyze the response of chemically induced Ha-ras- and B-raf-mutated mouse liver adenoma to CAR activation in vivo. Both tumor subpopulations possess almost identical gene expression profiles. CAR target gene induction in the tumors was studied at the mRNA and protein levels, and a reverse-phase protein microarray approach was chosen to characterize important signaling cascades. CAR target gene induction was pronounced in B-raf-mutated but not in Ha-ras-mutated tumors. Phosphoproteomic profiling revealed that phosphorylation-activated extracellular signal-regulated kinase (ERK) 1/2 was more abundant in Ha-ras-mutated than in B-raf-mutated tumors. ERK activation in tumor tissue was negatively correlated with CAR target induction. ERK activation is known to inhibit CAR-dependent transcription. In summary, profound differences exist between the two closely related tumor subpopulations with respect to the activation of mitogenic signaling cascades, and these dissimilarities might explain the differences in xenobiotic induction of CAR target genes.

Nuclear Receptor CAR Suppresses GADD45B-p38 MAPK Signaling to Promote Phenobarbital-induced Proliferation in Mouse Liver

Phenobarbital, a nongenotoxic hepatocarcinogen, induces hepatic proliferation and promotes development of hepatocellular carcinoma (HCC) in rodents. Nuclear receptor constitutive active/androstane receptor (NR1I3/CAR) regulates the induction and promotion activities of phenobarbital. Here, it is demonstrated that phenobarbital treatment results in dephosphorylation of a tumor suppressor p38 MAPK in the liver of C57BL/6 and C3H/HeNCrlBR mice. The molecular mechanism entails CAR binding and inhibition of the growth arrest and DNA-damage-inducible 45 beta (GADD45B)-MAPK kinase 6 (MKK6) scaffold to repress phosphorylation of p38 MAPK. Phenobarbital-induced hepatocyte proliferation, as determined by BrdUrd incorporation, was significantly reduced in both male and female livers of GADD45B knockout (KO) mice compared with the wild-type mice. The phenobarbital-induced proliferation continued until 48 hours after phenobarbital injection in only the C57BL/6 males, but neither in males of GADD45B KO mice nor in females of C57BL/6 and GADD45B KO mice. Thus, these data reveal nuclear receptor CAR interacts with GADD45B to repress p38 MAPK signaling and elicit hepatocyte proliferation in male mice.Implications: This GADD45B-regulated male-predominant proliferation can be expanded as a phenobarbital promotion signal of HCC development in future studies.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/16/8/1309/F1.large.jpg Mol Cancer Res; 16(8); 1309-18. ©2018 AACR.

Role of CYP2B in Phenobarbital-Induced Hepatocyte Proliferation in Mice

Phenobarbital (PB) promotes liver tumorigenesis in rodents, in part through activation of the constitutive androstane receptor (CAR) and the consequent changes in hepatic gene expression and increases in hepatocyte proliferation. A typical effect of CAR activation by PB is a marked induction of Cyp2b10 expression in the liver; the latter has been suspected to be vital for PB-induced hepatocellular proliferation. This hypothesis was tested here by using a Cyp2a(4/5)bgs-null (null) mouse model in which all Cyp2b genes are deleted. Adult male and female wild-type (WT) and null mice were treated intraperitoneally with PB at 50 mg/kg once daily for 5 successive days and tested on day 6. The liver-to-body weight ratio, an indicator of liver hypertrophy, was increased by 47% in male WT mice, but by only 22% in male Cyp2a(4/5)bgs-null mice, by the PB treatment. The fractions of bromodeoxyuridine-positive hepatocyte nuclei, assessed as a measure of the rate of hepatocyte proliferation, were also significantly lower in PB-treated male null mice compared with PB-treated male WT mice. However, whereas few proliferating hepatocytes were detected in saline-treated mice, many proliferating hepatocytes were still detected in PB-treated male null mice. In contrast, female WT mice were much less sensitive than male WT mice to PB-induced hepatocyte proliferation, and PB-treated female WT and PB-treated female null mice did not show significant difference in rates of hepatocyte proliferation. These results indicate that CYP2B induction plays a significant, but partial, role in PB-induced hepatocyte proliferation in male mice.

Genetic lineage tracing analysis of the cell of origin of hepatotoxin-induced liver tumors in mice

The expression of biliary/progenitor markers by hepatocellular carcinoma (HCC) is often associated with poor prognosis and stem cell-like behaviors of tumor cells. Hepatocellular adenomas (HCAs) also often express biliary/progenitor markers and frequently act as precursor lesions for HCC. However, the cell of origin of HCA and HCC that expresses these markers remains unclear. Therefore, to evaluate if mature hepatocytes give rise to HCA and HCC tumors and to understand the molecular pathways involved in tumorigenesis, we lineage-labeled hepatocytes by injecting adeno-associated virus containing thyroxine-binding globulin promoter-driven causes recombination (AAV-TBG-Cre) into Rosa(YFP) mice. Yellow fluorescent protein (YFP) was present in >96% of hepatocytes before exposure to carcinogens. We treated AAV-TBG-Cre; Rosa(YFP) mice with diethylnitrosamine (DEN), followed by multiple injections of carbon tetrachloride to induce carcinogenesis and fibrosis and found that HCA and HCC nodules were YFP(+) lineage-labeled; positive for osteopontin, SRY (sex determining region Y)-box 9, and epithelial cell adhesion molecule; and enriched for transcripts of biliary/progenitor markers such as prominin 1, Cd44, and delta-like 1 homolog. Next, we performed the converse experiment and lineage-labeled forkhead box protein L1(Foxl1)-positive hepatic progenitor cells simultaneously with exposure to carcinogens. None of the tumor nodules expressed YFP, indicating that Foxl1-expressing cells are not the origin for hepatotoxin-induced liver tumors. We confirmed that HCA and HCC cells are derived from mature hepatocytes and not from Foxl1-Cre-marked cells in a second model of toxin-induced hepatic neoplasia, using DEN and 3,3',5,5'-tetrachloro-1,4-bis(pyridyloxy)benzene (TCPOBOP).

CONCLUSION

Hepatocytes are the cell of origin of HCA and HCC in DEN/carbon tetrachloride and DEN/TCPOBOP induced liver tumors. (Hepatology 2016;64:1163-1177).

Coordinating Role of RXRα in Downregulating Hepatic Detoxification during Inflammation Revealed by Fuzzy-Logic Modeling

During various inflammatory processes circulating cytokines including IL-6, IL-1β, and TNFα elicit a broad and clinically relevant impairment of hepatic detoxification that is based on the simultaneous downregulation of many drug metabolizing enzymes and transporter genes. To address the question whether a common mechanism is involved we treated human primary hepatocytes with IL-6, the major mediator of the acute phase response in liver, and characterized acute phase and detoxification responses in quantitative gene expression and (phospho-)proteomics data sets. Selective inhibitors were used to disentangle the roles of JAK/STAT, MAPK, and PI3K signaling pathways. A prior knowledge-based fuzzy logic model comprising signal transduction and gene regulation was established and trained with perturbation-derived gene expression data from five hepatocyte donors. Our model suggests a greater role of MAPK/PI3K compared to JAK/STAT with the orphan nuclear receptor RXRα playing a central role in mediating transcriptional downregulation. Validation experiments revealed a striking similarity of RXRα gene silencing versus IL-6 induced negative gene regulation (r_s = 0.79; P<0.0001). These results concur with RXRα functioning as obligatory heterodimerization partner for several nuclear receptors that regulate drug and lipid metabolism.

During inflammation, circulating proinflammatory cytokines such as TNFα, IL-1ß, and IL-6, which are produced by, e.g., Kupffer cells, macrophages, or tumor cells, play important roles in hepatocellular signaling pathways and in the regulation of cellular homeostasis. In particular, these cytokines are responsible for the acute phase response (APR) but also for a dramatic reduction of drug detoxification capacity due to impaired expression of numerous genes coding for drug metabolic enzymes and transporters. Here we used high-throughput (phospho-)proteomic and gene expression data to investigate the impact of canonical signaling pathways in mediating IL-6-induced downregulation of drug metabolism related genes. We performed chemical inhibition perturbations to show that most of the IL-6 effects on gene expression are mediated through the MAPK and PI3K/AKT pathways. We constructed a prior knowledge network as basis for a fuzzy logic model that was trained with extensive gene expression data to identify critical regulatory nodes. Our results suggest that the nuclear receptor RXRα plays a central role, which was convincingly validated by RXRα gene silencing experiments. This work shows how computational modeling can support identifying decisive regulatory events from large-scale experimental data.

Activating and Inhibitory Functions of WNT/β-Catenin in the Induction of Cytochromes P450 by Nuclear Receptors in HepaRG Cells

The WNT/β-catenin signaling pathway has been identified as an important endogenous regulator of hepatic cytochrome P450 (P450) expression in mouse liver. In particular, it is involved in the regulation of P450 expression in response to exposure to xenobiotic agonists of the nuclear receptors constitutive androstane receptor (CAR), aryl hydrocarbon receptor (AhR), and Nrf2. To systematically elucidate the effect of the WNT/β-catenin pathway on the regulation and inducibility of major human P450 enzymes, HepaRG cells were treated with either the WNT/β-catenin signaling pathway agonist, WNT3a, or with small interfering RNA directed against β-catenin, alone or in combination with a panel of activating ligands for AhR [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)], CAR [6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO)], pregnane X receptor (PXR) [rifampicin], and peroxisome proliferator-activated receptor (PPAR) α [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY14,643)]. Assessment of P450 gene expression and enzymatic activity after downregulation or activation of the WNT/β-catenin pathway revealed a requirement of β-catenin in the AhR-, CAR-, and PXR-mediated induction of CYP1A, CYP2B6 and CYP3A4 (for CAR and PXR), and CYP2C8 (for PXR) gene expression. By contrast, activation of the WNT/β-catenin pathway prevented PPARα-mediated induction of CYP1A, CYP2C8, CYP3A4, and CYP4A11 genes, suggesting a dominant-negative role of β-catenin in PPARα-mediated regulation of these genes. Our data indicate a significant effect of the WNT/β-catenin pathway on the regulation of P450 enzymes in human hepatocytes and reveal a novel crosstalk between β-catenin and PPARα signaling pathways in the regulation of P450 expression.

Activating CAR and β-catenin induces uncontrolled liver growth and tumorigenesis

Aberrant β-catenin activation contributes to a third or more of human hepatocellular carcinoma (HCC), but β-catenin activation alone is not sufficient to induce liver cancer in mice. Differentiated hepatocytes proliferate upon acute activation of either β-catenin or the nuclear xenobiotic receptor CAR. These responses are strictly limited and are tightly linked, since β-catenin is activated in nearly all of the CAR-dependent tumors generated by the tumor promoter phenobarbital. Here we show that full activation of β-catenin in the liver induces senescence and growth arrest, which is overcome by combined CAR activation, resulting in uncontrolled hepatocyte proliferation, hepatomegaly, and rapid lethality despite maintenance of normal liver function. Combining CAR activation with limited β-catenin activation induces tumorigenesis, and the tumors share a conserved gene expression signature with β-catenin positive human HCC. These results reveal an unexpected route for hepatocyte proliferation and define a murine model of hepatocarcinogenesis with direct relevance to human HCC.

Complete response of Ctnnb1-mutated tumours to β-catenin suppression by locked nucleic acid antisense in a mouse hepatocarcinogenesis model

BACKGROUND & AIMS

Hepatocellular cancer (HCC) remains a disease of poor prognosis, highlighting the relevance of elucidating key molecular aberrations that may be targeted for novel therapies. Wnt signalling activation, chiefly due to mutations in CTNNB1, have been identified in a major subset of HCC patients. While several in vitro proof of concept studies show the relevance of suppressing Wnt/β-catenin signalling in HCC cells or tumour xenograft models, no study has addressed the impact of β-catenin inhibition in a relevant murine HCC model driven by Ctnnb1 mutations.

METHODS

We studied the in vivo impact of β-catenin suppression by locked nucleic acid (LNA) antisense treatment, after establishing Ctnnb1 mutation-driven HCC by diethylnitrosamine and phenobarbital (DEN/PB) administration.

RESULTS

The efficacy of LNA directed against β-catenin vs. scrambled on Wnt signalling was demonstrated in vitro in HCC cells and in vivo in normal mice. The DEN/PB model leads to HCC with Ctnnb1 mutations. A complete therapeutic response in the form of abrogation of HCC was observed after ten treatments of tumour-bearing mice with β-catenin LNA every 48h as compared to the scrambled control. A decrease in β-catenin activity, cell proliferation and increased cell death was evident after β-catenin suppression. No effect of β-catenin suppression was evident in non-Ctnnb1 mutated HCC, observed after DEN-only administration.

CONCLUSIONS

Thus, we provide the in vivo proof of concept that β-catenin suppression in HCC will be of significant therapeutic benefit, provided the tumours display Wnt activation via mechanisms like CTNNB1 mutations.

Liver zonation: Novel aspects of its regulation and its impact on homeostasis

Liver zonation, the spatial separation of the immense spectrum of different metabolic pathways along the liver sinusoids, is fundamental for proper functioning of this organ. Recent progress in elucidating localization and interactions of different metabolic pathways by using “omics” techniques and novel approaches to couple them with refined spatial resolution and in characterizing novel master regulators of zonation by using transgenic mice has created the basis for a deeper understanding of core mechanisms of zonation and their impact on liver physiology, pathology and metabolic diseases. This review summarizes the fascinating technical achievements for investigating liver zonation and the elucidation of an emerging network of master regulators of zonation that keep the plethora of interrelated and sometimes opposing functions of the liver in balance with nutritional supply and specific requirements of the entire body. In addition, a brief overview is given on newly described zonated functions and novel details on how diverse the segmentation of metabolic zonation may be. From these facts and developments a few fundamental principles are inferred which seem to rule zonation of liver parenchyma. In addition, we identify important questions that still need to be answered as well as interesting fields of research such as the connection of zonation with circadian rhythm and gender dimorphism which need to be pushed further, in order to improve our understanding of metabolic zonation. Finally, an outlook is given on how disturbance of liver zonation and its regulation may impact on liver pathology and the development of metabolic diseases.

Phenobarbital-mediated tumor promotion in transgenic mice with humanized CAR and PXR

The nuclear receptors CAR (constitutive androstane receptor) and possibly PXR (pregnane X receptor) mediate the hepatic effects of phenobarbital (PB) and similar-acting compounds. Although PB is a potent nongenotoxic tumor promoter in rodent liver, epidemiological data from epilepsy patients treated with phenobarbital do not show a specific role of PB in human liver cancer risk. That points to species differences in the susceptibility to tumor promotion by PB, which might be attributed to divergent functions of the PB receptors CAR and PXR in mice and humans. In the present study, male transgenic mice expressing human CAR and PXR were used to detect possible differences between wild-type (WT) and humanized mice in their response to CAR activation in a tumor initiation/promotion experiment with a single injection of the tumor initiator N-nitrosodiethylamine preceding chronic PB treatment for 10 months. Analysis of liver tumor burden revealed that PB strongly promoted the outgrowth of hepatocellular adenoma driven by activated β-catenin in WT mice, whereas the tumor-promoting effect of PB was much less pronounced in the humanized group. In conclusion, the present findings demonstrate that human CAR and PXR support tumor promotion by PB in mouse liver, but to a significantly lesser extent than the WT murine receptors.

Ha-ras and β-catenin oncoproteins orchestrate metabolic programs in mouse liver tumors

The process of hepatocarcinogenesis in the diethylnitrosamine (DEN) initiation/phenobarbital (PB) promotion mouse model involves the selective clonal outgrowth of cells harboring oncogene mutations in Ctnnb1, while spontaneous or DEN-only-induced tumors are often Ha-ras- or B-raf-mutated. The molecular mechanisms and pathways underlying these different tumor sub-types are not well characterized. Their identification may help identify markers for xenobiotic promoted versus spontaneously occurring liver tumors. Here, we have characterized mouse liver tumors harboring either Ctnnb1 or Ha-ras mutations via integrated molecular profiling at the transcriptional, translational and post-translational levels. In addition, metabolites of the intermediary metabolism were quantified by high resolution (1)H magic angle nuclear magnetic resonance. We have identified tumor genotype-specific differences in mRNA and miRNA expression, protein levels, post-translational modifications, and metabolite levels that facilitate the molecular and biochemical stratification of tumor phenotypes. Bioinformatic integration of these data at the pathway level led to novel insights into tumor genotype-specific aberrant cell signaling and in particular to a better understanding of alterations in pathways of the cell intermediary metabolism, which are driven by the constitutive activation of the β-Catenin and Ha-ras oncoproteins in tumors of the two genotypes.

Bovine NR1I3 gene polymorphisms and its association with feed efficiency traits in Nellore cattle

The Nuclear receptor 1 family I member 3 (NR1I3), also known as the Constitutive Androstane Receptor (CAR), was initially characterized as a key regulator of xenobiotic metabolism. However, recent biochemical and structural data suggest that NR1I3 is activated in response to metabolic and nutritional stress in a ligand-independent manner. Thus, we prospected the Bovine NR1I3 gene for polymorphisms and studied their association with feed efficiency traits in Nellore cattle. First, 155 purebred Nellore bulls were individually measured for Residual Feed Intake (RFI) and the 25 best (High Feed Efficiency group, HFE) and the 25 worst animals (Low Feed Efficiency group, LFE) were selected for DNA extraction. The entire Bovine NR1I3 gene was amplified and polymorphisms were identified by sequencing. Then, one SNP different between HFE and LFE groups was genotyped in all the 155 animals and in another 288 animals totalizing 443 Nellore bulls genotyped for association of NR1I3 SNPs with feed efficiency traits. We found 24 SNPs in the NR1I3 gene and choose a statistically different SNP between HFE and LFE groups for further analysis. Genotyping of the 155 animals showed a significant association within SNP and RFI (p = 0.04), Residual Intake and BW Gain (p = 0.04) and Dry Matter Intake (p = 0.01). This SNP is located in the 5′flanking promoter region of NR1I3 gene and different alleles alter the binding site for predicted transcriptional factors as HNF4alpha, CREM and c-MYB, leading us to conclude that NR1I3 expression and regulation might be important to feed efficiency.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Mechanisms of RAS/β-catenin interactions

Signaling through the WNT/β-catenin and the RAS (rat sarcoma)/MAPK (mitogen-activated protein kinase) pathways plays a key role in the regulation of various physiological cellular processes including proliferation, differentiation, and cell death. Aberrant mutational activation of these signaling pathways is closely linked to the development of cancer in many organs, in humans as well as in laboratory animals. Over the past years, more and more evidence for a close linkage of the two oncogenic signaling cascades has accumulated. Using different experimental approaches, model systems, and experimental conditions, a variety of molecular mechanisms have been identified by which signal transduction through WNT/β-catenin and RAS interact, either in a synergistic or an antagonistic manner. Mechanisms of interaction comprise an upstream crosstalk at the level of pathway-activating ligands and their receptors, interrelations of cytosolic kinases involved in either pathways, as well as interaction in the nucleus related to the joint regulation of target gene transcription. Here, we present a comprehensive review of the current knowledge on the interaction of RAS/MAPK- and WNT/β-catenin-driven signal transduction in mammalian cells.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.