Phenotype and growth behavior of residual β-catenin-positive hepatocytes in livers of β-catenin-deficient mice

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.

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.

Wnt signaling in liver disease: emerging trends from a bibliometric perspective

Background

The Wnt signaling pathway, an evolutionarily conserved molecular transduction cascade, has been identified as playing a pivotal role in various physiological and pathological processes of the liver, including homeostasis, regeneration, cirrhosis, and hepatocellular carcinoma (HCC). In this study, we aimed to use a bibliometric method to evaluate the emerging trends on Wnt signaling in liver diseases.

Methods

Articles were retrieved from the Web of Science Core Collection. We used a bibliometric software, CiteSpace V 5.3.R4, to analyze the active countries or institutions in the research field, the landmark manuscripts, important subtopics, and evolution of scientific ideas.

Results

In total, 1,768 manuscripts were published, and each was cited 33.12 times on average. The U.S. published most of the articles, and the most active center was the University of Pittsburgh. The top 5 landmark papers were identified by four bibliometric indexes including citation, burstness, centrality, and usage 2013. The clustering process divided the whole area into nine research subtopics, and the two major important subtopics were "liver zonation" and "HCC." Using the "Part-of-Speech" technique, 1,743 terms representing scientific ideas were identified. After 2008, the bursting phrases were "liver development," "progenitor cells," "hepatic stellate cells," "liver regeneration," "liver fibrosis," "epithelial-mesenchymal transition," and etc.

Conclusion

Using bibliometric methods, we quantitatively summarized the advancements and emerging trends in Wnt signaling in liver diseases. These bibliometric findings may pioneer the future direction of this field in the next few years, and further studies are needed.

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.

Creation of Three-Dimensional Liver Tissue Models from Experimental Images for Systems Medicine

In this chapter, we illustrate how three-dimensional liver tissue models can be created from experimental image modalities by utilizing a well-established processing chain of experiments, microscopic imaging, image processing, image analysis and model construction. We describe how key features of liver tissue architecture are quantified and translated into model parameterizations, and show how a systematic iteration of experiments and model simulations often leads to a better understanding of biological phenomena in systems biology and systems medicine.

NH4Cl affects the expression of Wnt/β-catenin pathway in hepatocytes

We intended to explore whether NH₄Cl influences the viability and regulates the expression of Wnt/β-catenin pathway in hepatocytes. The Chang liver cell line was used and cultured with different concentrations of NH₄Cl (2.5, 5, 10, 20, 40, and 50 mmol/L) for 12, 24, and 48 h. The viability of hepatocytes was detected by MTT assay. The mRNA and protein expression level was analyzed with qRT-PCR and Western blotting, respectively. NH₄Cl concentration significantly affects the viability of hepatocytes. With the increase of NH₄Cl concentration, the viability of hepatocytes was decreased, accordingly. The mRNA and protein expression of Wnt1, β-catenin, and cyclin D was significantly increased after treatment with low concentrations of NH₄Cl as compared with the control group, whereas their expression levels were decreased after treatment with high concentrations of NH₄Cl. The mRNA and protein expression of Wnt1, β-catenin, and cyclin D was also significantly increased after treatment with NH₄Cl for a short period as compared with the control group, whereas their expression levels were decreased after treatment with NH₄Cl for a long period. In addition, we found NH₄Cl treatment significantly reversed the results after RNA silencing of Wnt1 in hepatocytes. NH₄Cl influences the viability of hepatocytes and affects the expression of Wnt/β-catenin pathway in hepatocytes.

A frequent misinterpretation in current research on liver fibrosis: the vessel in the center of CCl4-induced pseudolobules is a portal vein

Carbon tetrachloride-induced liver injury is a thoroughly studied model for regeneration and fibrosis in rodents. Nevertheless, its pattern of liver fibrosis is frequently misinterpreted as portal type. To clarify this, we show that collagen type IV⁺ "streets" and α-SMA⁺ cells accumulate pericentrally and extend to neighbouring central areas of the liver lobule, forming a 'pseudolobule'. Blood vessels in the center of such pseudolobules are portal veins as indicated by the presence of bile duct cells (CK19⁺) and the absence of pericentral hepatocytes (glutamine synthetase⁺). It is critical to correctly describe this pattern of fibrosis, particulary for metabolic zonation studies.

Coordinate regulation of Cyp2e1 by β-catenin- and hepatocyte nuclear factor 1α-dependent signaling

Depending on their position within the liver lobule, hepatocytes fulfill different metabolic functions. Cytochrome P450 (CYP) 2E1 is a drug-metabolizing enzyme which is exclusively expressed in hepatocytes surrounding branches of the hepatic central vein. Previous publications have shown that signaling through the Wnt/β-catenin pathway, a major determinant of liver zonation, and the hepatocyte-enriched transcription factor HNF (hepatocyte nuclear factor) 1α participate in the regulation of the gene. This study was aimed to decipher the molecular mechanisms by which the two transcription factors, β-catenin and HNF1α, jointly regulate CYP2E1 at the gene promoter level. Chromatin immunoprecipitation identified a conserved Wnt/β-catenin-responsive site (WRE) in the murine Cyp2e1 promoter adjacent to a known HNF1α response element (HNF1-RE). In vitro analyses demonstrated that both, activated β-catenin and HNF1α, are needed for the full response of the promoter. The WRE was dispensable for β-catenin-mediated effects on the Cyp2e1 promoter, while activity of β-catenin was integrated into the promoter response via the HNF1-RE. Physical interaction of β-catenin and HNF1α was demonstrated by co-immunoprecipitation. In conclusion, present data the first time identify and characterize the interplay of HNF1α and β-catenin and elucidate molecular determinants of CYP2E1 expression in the liver.

Signal integration by the CYP1A1 promoter--a quantitative study

Genes involved in detoxification of foreign compounds exhibit complex spatiotemporal expression patterns in liver. Cytochrome P450 1A1 (CYP1A1), for example, is restricted to the pericentral region of liver lobules in response to the interplay between aryl hydrocarbon receptor (AhR) and Wnt/β-catenin signaling pathways. However, the mechanisms by which the two pathways orchestrate gene expression are still poorly understood. With the help of 29 mutant constructs of the human CYP1A1 promoter and a mathematical model that combines Wnt/β-catenin and AhR signaling with the statistical mechanics of the promoter, we systematically quantified the regulatory influence of different transcription factor binding sites on gene induction within the promoter. The model unveils how different binding sites cooperate and how they establish the promoter logic; it quantitatively predicts two-dimensional stimulus-response curves. Furthermore, it shows that crosstalk between Wnt/β-catenin and AhR signaling is crucial to understand the complex zonated expression patterns found in liver lobules. This study exemplifies how statistical mechanical modeling together with combinatorial reporter assays has the capacity to disentangle the promoter logic that establishes physiological gene expression patterns.

Genetic ablation of β-catenin inhibits the proliferative phenotype of mouse liver adenomas

BACKGROUND

Aberrant activation of Wnt/β-catenin has been implicated in various cancer-related processes, for example, proliferation or tumour cell survival. However, the exact mechanism by which β-catenin provides liver tumour cells with a selective advantage is still unclear. This study was aimed to analyse growth behaviour and survival of β-catenin-driven mouse liver tumours after β-catenin ablation.

METHODS

Transgenic mice with a controllable hepatocyte-specific knockout of Ctnnb1 (encoding β-catenin) were generated and liver tumours were induced by means of a N-nitrosodiethylamine/phenobarbital tumour initiation/promotion protocol, which leads to the outgrowth of hepatocellular tumours with activated β-catenin. Cre recombinase was activated and the effects of the knockout in the tumours were studied.

RESULTS

Activation of Cre recombinase led to the knockout of Ctnnb1 in a fraction of tumour cells, thus resulting in the formation of two different tumour cell subpopulations, with or without β-catenin. Comparative analysis of the two subpopulations revealed that cell proliferation was significantly decreased in Ctnnb1-deleted hepatoma cells, compared with the corresponding non-deleted cell population, whereas no increased rate of apoptosis after knockout of Ctnnb1 was observed.

CONCLUSIONS

β-catenin-dependent signalling is an important regulator of hepatoma cell growth in mice, but not a crucial factor in the regulation of tumour survival.

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.

Protocols for staining of bile canalicular and sinusoidal networks of human, mouse and pig livers, three-dimensional reconstruction and quantification of tissue microarchitecture by image processing and analysis

Histological alterations often constitute a fingerprint of toxicity and diseases. The extent to which these alterations are cause or consequence of compromised organ function, and the underlying mechanisms involved is a matter of intensive research. In particular, liver disease is often associated with altered tissue microarchitecture, which in turn may compromise perfusion and functionality. Research in this field requires the development and orchestration of new techniques into standardized processing pipelines that can be used to reproducibly quantify tissue architecture. Major bottlenecks include the lack of robust staining, and adequate reconstruction and quantification techniques. To bridge this gap, we established protocols employing specific antibody combinations for immunostaining, confocal imaging, three-dimensional reconstruction of approximately 100-μm-thick tissue blocks and quantification of key architectural features. We describe a standard procedure termed ‘liver architectural staining’ for the simultaneous visualization of bile canaliculi, sinusoidal endothelial cells, glutamine synthetase (GS) for the identification of central veins, and DAPI as a nuclear marker. Additionally, we present a second standard procedure entitled ‘S-phase staining’, where S-phase-positive and S-phase-negative nuclei (stained with BrdU and DAPI, respectively), sinusoidal endothelial cells and GS are stained. The techniques include three-dimensional reconstruction of the sinusoidal and bile canalicular networks from the same tissue block, and robust capture of position, size and shape of individual hepatocytes, as well as entire lobules from the same tissue specimen. In addition to the protocols, we have also established image analysis software that allows relational and hierarchical quantifications of different liver substructures (e.g. cells and vascular branches) and events (e.g. cell proliferation and death). Typical results acquired for routinely quantified parameters in adult mice (C57Bl6/N) include the hepatocyte volume (5,128.3 ± 837.8 μm³) and the fraction of the hepatocyte surface in contact with the neighbouring hepatocytes (67.4 ± 6.7 %), sinusoids (22.1 ± 4.8 %) and bile canaliculi (9.9 ± 3.8 %). Parameters of the sinusoidal network that we also routinely quantify include the radius of the sinusoids (4.8 ± 2.25 μm), the branching angle (32.5 ± 11.2°), the length of intersection branches (23.93 ± 5.9 μm), the number of intersection nodes per mm³ (120.3 × 103 ± 42.1 × 10³), the average length of sinusoidal vessel per mm³ (5.4 × 10³ ± 1.4 × 10³mm) and the percentage of vessel volume in relation to the whole liver volume (15.3 ± 3.9) (mean ± standard deviation). Moreover, the provided parameters of the bile canalicular network are: length of the first-order branches (7.5 ± 0.6 μm), length of the second-order branches (10.9 ± 1.8 μm), length of the dead-end branches (5.9 ± 0.7 μm), the number of intersection nodes per mm³ (819.1 × 10³ ± 180.7 × 10³), the number of dead-end branches per mm³ (409.9 × 10³ ± 95.6 × 10³), the length of the bile canalicular network per mm³ (9.4 × 10³ ± 0.7 × 10³ mm) and the percentage of the bile canalicular volume with respect to the total liver volume (3.4 ± 0.005). A particular strength of our technique is that quantitative parameters of hepatocytes and bile canalicular as well as sinusoidal networks can be extracted from the same tissue block. Reconstructions and quantifications performed as described in the current protocols can be used for quantitative mathematical modelling of the underlying mechanisms. Furthermore, protocols are presented for both human and pig livers. The technique is also applicable for both vibratome blocks and conventional paraffin slices.

A novel role of nucleostemin in maintaining the genome integrity of dividing hepatocytes during mouse liver development and regeneration

During liver development and regeneration, hepatocytes undergo rapid cell division and face an increased risk of DNA damage associated with active DNA replication. The mechanism that protects proliferating hepatocytes from replication-induced DNA damage remains unclear. Nucleostemin (NS) is known to be up-regulated during liver regeneration, and loss of NS is associated with increased DNA damage in cancer cells. To determine whether NS is involved in protecting the genome integrity of proliferating hepatocytes, we created an albumin promoter-driven NS conditional-null (albNS(cko) ) mouse model. Livers of albNS(cko) mice begin to show loss of NS in developing hepatocytes from the first postnatal week and increased DNA damage and hepatocellular injury at 1-2 weeks of age. At 3-4 weeks, albNS(cko) livers develop bile duct hyperplasia and show increased apoptotic cells, necrosis, regenerative nodules, and evidence suggestive of hepatic stem/progenitor cell activation. CCl4 treatment enhances degeneration and DNA damage in NS-deleted hepatocytes and increases biliary hyperplasia and A6(+) cells in albNS(cko) livers. After 70% partial hepatectomy, albNS(cko) livers show increased DNA damage in parallel with a blunted and prolonged regenerative response. The DNA damage in NS-depleted hepatocytes is explained by the impaired recruitment of a core DNA repair enzyme, RAD51, to replication-induced DNA damage foci.

CONCLUSION

This work reveals a novel genome-protective role of NS in developing and regenerating hepatocytes.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

Pericentral activity of alpha-fetoprotein enhancer 3 and glutamine synthetase upstream enhancer in the adult liver are regulated by β-catenin in mice

We previously showed that mouse alpha-fetoprotein (AFP) enhancer 3 activity is highly restricted to pericentral hepatocytes in the adult liver. Here, using transgenic mice, we show that the upstream enhancer of the rat glutamine synthetase gene is also active, specifically in pericentral regions. Activity of both enhancers is lost in the absence of β-catenin, a key regulator of zonal gene expression in the adult liver. Both enhancers contain a single, highly conserved T-cell factor/lymphoid enhancer factor binding site that is required for responsiveness to β-catenin. We also show that endogenous AFP messenger RNA levels in the perinatal liver are lower when β-catenin is reduced.

CONCLUSION

These data identify the first distinct zonally active regulatory regions required for β-catenin responsiveness in the adult liver, and suggest that postnatal AFP repression and the establishment of zonal regulation are controlled, at least in part, by the same factors.

Hepatocyte γ-catenin compensates for conditionally deleted β-catenin at adherens junctions

BACKGROUND & AIMS

Wnt/β-catenin signaling is important in liver physiology. Moreover, β-catenin is also pivotal in adherens junctions (AJ). Here, we investigate hepatocyte-specific β-catenin conditional null mice (KO) for any alterations in AJ and related tight junctions (TJ).

METHODS

Using gene array, PCR, Western blot, immunohistochemistry, immunofluorescence, and co-immunoprecipitation, we compare and contrast the composition of AJ and TJ in KO and littermate wild-type (WT) control livers.

RESULTS

We show association of E-cadherin with β-catenin in epithelial cells of WT livers, which is lost in the KOs. While total levels of α-catenin, E-cadherin, and F-actin were modestly decreased, KO livers show increased γ-catenin/plakoglobin. By co-immunoprecipitation, E-cadherin/β-catenin/F-actin association was observed in WT livers, while the association of E-cadherin/γ-catenin/F-actin was evident in KO livers. γ-Catenin was localized at the hepatocyte membrane at baseline in the KO liver. While γ-catenin gene expression remained unaltered, an increase in serine- and threonine-phosphorylated, but not tyrosine-phosphorylated γ-catenin was observed in KO livers. A continued presence of γ-catenin at the hepatocyte membrane, without any nuclear localization, was observed in liver regeneration after partial hepatectomy at 40 and 72 h, in both KO and WT. Analysis of TJ revealed lack of claudin-2 and increased levels of JAM-A and claudin-1 in KO livers.

CONCLUSIONS

β-Catenin adequately maintains AJ in the absence of β-catenin in hepatocytes; however, it lacks nuclear localization. Moreover, β-catenin/claudin-2 may be an important mechanism of crosstalk between the AJ and TJ.

Depletion of β-catenin from mature hepatocytes of mice promotes expansion of hepatic progenitor cells and tumor development

Depletion of β-catenin impairs regeneration of the rapid turn-over gut epithelial cells, but appears dispensable for that of the slow turn-over mature hepatocytes in mice until 1 y of age. As the life span of mature murine hepatocytes is about 400 d, we studied conditional β-catenin knockout mice (Alb-Cre;Ctnnb1(flx/flx)) until 20 mo of age to determine the function of β-catenin in the postnatal liver. β-catenin was absent from the hepatocytes of β-catenin knockout mice 4 wk after delivery. From 9 mo of age, hepatocytes were gradually replaced by newly formed β-catenin-positive hepatocytes, which constituted about 90% of hepatocytes at 18-20 mo of age. This process was accompanied by active proliferation of bile duct/ductule cells. β-catenin-positive hepatocytes exhibited elevated proliferation activity and expression of progenitor cell markers, but lower albumin and Cre. This might explain their intact β-catenin protein, and suggest their origins from hepatic progenitor cells. Liver tumors arose spontaneously from β-catenin-positive cells, and tumorigenesis was accelerated by hepatitis B X protein. These results indicate β-catenin critical for the regeneration of mature hepatocytes. Failure to regenerate mature hepatocytes results in proliferation of hepatic progenitor cells that are able to maintain liver function but are predisposed to form liver tumors.

Spontaneous repopulation of β-catenin null livers with β-catenin-positive hepatocytes after chronic murine liver injury

Prolonged exposure of mice to diet containing 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) results in hepatobiliary injury, atypical ductular proliferation, oval cell appearance, and limited fibrosis. Previously, we reported that short-term ingestion of DDC diet by hepatocyte-specific β-catenin conditional knockout (KO) mice led to fewer A6-positive oval cells than wildtype (WT) littermates. To examine the role of β-catenin in chronic hepatic injury and repair, we exposed WT and KO mice to DDC for 80 and 150 days. Paradoxically, long-term DDC exposure led to significantly more A6-positive cells, indicating greater atypical ductular proliferation in KO, which coincided with increased fibrosis and cholestasis. Surprisingly, at 80 and 150 days in KO we observed a significant amelioration of hepatocyte injury. This coincided with extensive repopulation of β-catenin null livers with β-catenin-positive hepatocytes at 150 days, which was preceded by appearance of β-catenin-positive hepatocyte clusters at 80 days and a few β-catenin-positive hepatocytes at earlier times. Intriguingly, occasional β-catenin-positive hepatocytes that were negative for progenitor markers were also observed at baseline in the KO livers, suggesting spontaneous escape from cre-mediated recombination. These cells with hepatocyte morphology expressed mature hepatocyte markers but lacked markers of hepatic progenitors. The gradual repopulation of KO livers with β-catenin-positive hepatocytes occurred only following DDC injury and coincided with a progressive loss of hepatic cre-recombinase expression. A few β-catenin-positive cholangiocytes were observed albeit only after long-term DDC exposure and trailed the appearance of β-catenin-positive hepatocytes.

CONCLUSION

In a chronic liver injury model, β-catenin-positive hepatocytes exhibit growth and survival advantages and repopulate KO livers, eventually limiting hepatic injury and dysfunction despite increased fibrosis and intrahepatic cholestasis.

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

Aberrant signaling through the Wnt/β-catenin pathway is a critical determinant in human and rodent liver carcinogenesis and generally accepted to be a potent driver of proliferation. Xenobiotic agonists of the constitutive androstane receptor (CAR) induce massive acute hyperplasia of mouse liver and facilitate the outgrowth of hepatocellular carcinomas with activated β-catenin. In the present study, the interplay of β-catenin-dependent and CAR-dependent signaling in the liver and its effect on hepatocyte proliferation were analyzed in transgenic mice with hepatocyte-specific knockout of Ctnnb1 (encoding β-catenin) following treatment with two CAR agonists, 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) and phenobarbital. Hepatocyte-specific knockout of β-catenin inhibited CAR agonists-induced hepatocyte proliferation in male mice. By contrast, the proliferative effect of CAR agonists was strongly augmented in female β-catenin knockout animals. This was due to prolonged proliferation of the knockout hepatocytes. CAR-mediated hepatocyte proliferation was, at least in part, dependent on estrogen signaling and was associated with enhanced expression of FoxM1 and elevated activity of the PDK1/p90RSK pathway. In conclusion, our study shows that gender-specific factors determine whether β-catenin signaling plays a pro- or an antiproliferative role in the regulation of mouse hepatocyte proliferation induced by CAR agonists.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Hepatomas with activating Ctnnb1 mutations in 'Ctnnb1-deficient' livers: a tricky aspect of a conditional knockout mouse model

Conditional knockout mice, based on the Cre-loxP system, are a widely used model for examining organ-specific gene functions. To date, efficient hepatocyte-specific knockout has been reported in many different models, but little attention has been paid to the long-term stability of the recombination efficiency. In the present study, we characterized Alb-Cre;Ctnnb1flox/flox 'hepatocyte-specific Ctnnb1 knockout' mice of different ages to test whether efficient recombination is maintained over time. At 2 months of age, the knockout mouse livers achieved efficient deletions of β-catenin in hepatocytes. However, as the mice aged, the reappearance and expansion of β-catenin-expressing hepatocytes were observed. In 1-year-old mice, a significant proportion of the pericentral hepatocytes in the knockout mouse livers were replaced with β-catenin-positive hepatocytes, whereas the periportal hepatocytes mostly remained β-catenin-negative. Furthermore, most of the 1-year-old mice spontaneously developed hepatocellular adenomas and carcinomas that were positive for β-catenin and overexpressed glutamine synthetase and Slc1a2, both of which are hallmarks of active β-catenin signaling. Sequencing analysis revealed that the Ctnnb1 alleles were not inactivated but had activating mutations in these tumors. The present study suggests that recombination efficiency should be carefully examined when hepatocyte-specific knockout mice of different ages are analyzed. In addition, illegitimate deletion mutations should be recognized as potential adverse effects of the Cre-loxP system.

Tumor formation in liver of conditional β-catenin-deficient mice exposed to a diethylnitrosamine/phenobarbital tumor promotion regimen

The antiepileptic drug phenobarbital (PB) is a potent tumor promoter in mouse liver, where it stimulates the selective outgrowth of tumor populations harboring activating mutations in Ctnnb1, encoding β-catenin. A tumor initiation-promotion study was conducted in mice with conditional hepatocyte-specific knockout (KO) of Ctnnb1 and in Ctnnb1 wild-type controls. Mice received a single injection of N-nitrosodiethylamine (DEN) at the age of 6 weeks followed by continuous administration of PB given in the diet (0.05%) for 27 weeks. Metabolic activation of DEN in hepatocytes from both Ctnnb1 wild-type and KO mice was demonstrated. PB strongly enhanced liver tumor formation in Ctnnb1 wild-type mice, and 90% of the PB-promoted tumors were Ctnnb1-mutated. A similar increase in carcinogenic response was seen when using glucose-6-phosphatase and glutamine synthetase as tumor markers. The prevalence of tumors in Ctnnb1 KO mice was ∼7-fold higher than in wild-type mice, suggesting an enhancing effect of the gene KO on liver tumor development. However, in strong contrast to wild-type mice, PB did not promote tumor formation in the Ctnnb1 KO mice. Livers of KO mice, particularly from the PB treatment group, demonstrated fibrosis and massive infiltration of immune cells, an effect not seen in wild-type mice. In summary, our data demonstrate that (i) liver tumor promotion by PB requires functional β-catenin signaling and (ii) absence of β-catenin enhances carcinogen-induced hepatocarcinogenesis and induces a pre-cirrhotic phenotype in mouse liver.