Liver-specific loss of beta-catenin results in delayed hepatocyte proliferation after partial hepatectomy

Proteomic analysis of beta-catenin activation in mouse liver by DIGE analysis identifies glucose metabolism as a new target of the Wnt pathway

The Wnt/beta-catenin signaling pathway has been increasingly implicated in liver development and physiology. Aberrant activation of this pathway is one of the major genetic events observed during the process of human HCC development. To gain insight into the mechanism underlying beta-catenin action in the liver, we conducted a quantitative differential proteomic analysis using 2-D DIGE combined with MS, in mice with liver-specific deletion of Apc resulting in acute activation of beta-catenin signaling (Apc(KOliv) mice). We identified 94 protein spots showing differential expression between mutant Apc(KOliv) and control mice, corresponding to 56 individual proteins. Most of the proteins identified were associated with metabolic pathways, such as ammonia and glucose metabolism. Our analysis showed an increase in lactate dehydrogenase activity together with a downregulation of two mitochondrial ATPase subunits (ATP5a1 and ATP5b). These observations indicate that beta-catenin signaling may induce a shift in the glucose metabolism from oxidative phosphorylation to glycolysis, known as the "Warburg effect". Imaging with (18)F-fluoro-2-deoxy-D-glucose-positron emission tomography suggests that the specific metabolic reprogramming induced by beta-catenin in the liver does not imply the first step of glycolysis. This observation may explain why some HCCs are difficult to assess by fluoro-2-deoxy-D-glucose-positron emission tomography imaging.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.

Beta-catenin activation promotes liver regeneration after acetaminophen-induced injury

Acute liver failure (ALF) remains a disease with poor patient outcome. Improved prognosis is associated with spontaneous liver regeneration, which supports the relevance of exploring 'regenerative' therapies. Therefore, the role of the Wnt/beta-catenin pathway in liver regeneration following ALF was investigated. ALF was induced in mice by acetaminophen overdose, which is also a leading cause of liver failure in patients. beta-catenin distribution was also studied in liver sections from acetaminophen-induced ALF patients. A nonlethal dose of acetaminophen, which induces liver regeneration, led to stabilization and activation of beta-catenin for 1 to 12 hours. These data were also verified by increased expression of the beta-catenin surrogate target glutamine synthetase. Beta-catenin activation occurred secondary to the inactivation of glycogen synthase kinase-3beta and an increase in levels of casein kinase 2alpha, and led to increased cyclin-D1, another known beta-catenin target. These observations were next substantiated in beta-catenin conditional-null mice (beta-catenin-null), which show dampened regeneration after acetaminophen injury following induction of CYP2e1/1a2 expression. In light of decreased acetaminophen injury in beta-catenin-null mice despite CYP induction, equitoxic studies in control mice were performed. Significant differences in regeneration persisted following comparable injury in beta-catenin-null and control animals. Retrospective analysis of liver samples from acetaminophen-overdose patients demonstrated a positive correlation between nuclear beta-catenin, proliferation, and spontaneous liver regeneration. Thus, our studies demonstrate early activation of beta-catenin signaling during acetaminophen-induced injury, which contributes to hepatic regeneration.

p21 is required for dextrose-mediated inhibition of mouse liver regeneration

The inhibitory effect of dextrose supplementation on liver regeneration was first described more than 4 decades ago. Nevertheless, the molecular mechanisms responsible for this observation have not been elucidated. We investigated these mechanisms using the partial hepatectomy model in mice given standard or 10% dextrose (D10)-supplemented drinking water. The results showed that D10-treated mice exhibited significantly reduced hepatic regeneration compared with controls, as assessed by hepatocellular bromodeoxyuridine (BrdU) incorporation and mitotic frequency. D10 supplementation did not suppress activation of hepatocyte growth factor (HGF), induction of transforming growth factor alpha (TGF-alpha) expression, or tumor necrosis factor alpha-interleukin-6 cytokine signaling, p42/44 extracellular signal-regulated kinase (ERK) activation, immediate early gene expression, or expression of CCAAT/enhancer binding protein beta (C/EBPbeta), but did augment expression of the mito-inhibitory factors C/EBPalpha, p21(Waf1/Cip1), and p27(Kip1). In addition, forkhead box M1 (FoxM1) expression, which is required for normal liver regeneration, was suppressed by D10 treatment. Finally, D10 did not suppress either FoxM1 expression or hepatocellular proliferation in p21 null mice subjected to partial hepatectomy, establishing the functional significance of these events in mediating the effects of D10 on liver regeneration.

CONCLUSION

These data show that the inhibitory effect of dextrose supplementation on liver regeneration is associated with increased expression of C/EBPalpha, p21, and p27, and decreased expression of FoxM1, and that D10-mediated inhibition of liver regeneration is abrogated in p21-deficient animals. Our observations are consistent with a model in which hepatic sufficiency is defined by homeostasis between the energy-generating capacity of the liver and the energy demands of the body mass, with liver regeneration initiated when the functional liver mass is no longer sufficient to meet such demand.

Disruption of Dicer1 induces dysregulated fetal gene expression and promotes hepatocarcinogenesis

BACKGROUND & AIMS

Growing evidence suggests that microRNAs coordinate various biological processes in the liver. We describe experiments to address the physiologic roles of these new regulators of gene expression in the liver that are as of yet largely undefined.

METHODS

We disrupted Dicer, an enzyme essential for the processing of microRNAs, in hepatocytes using a conditional knockout mouse model to elucidate the consequences of loss of microRNAs.

RESULTS

The conditional knockout mouse livers showed the efficient disruption of Dicer1 at 3 weeks after birth. This resulted in prominent steatosis and the depletion of glycogen storage. Dicer1-deficient liver exhibited increased growth-promoting gene expression and the robust expression of fetal stage-specific genes. The consequence of Dicer elimination included both increased hepatocyte proliferation and overwhelming apoptosis. Over time, Dicer1-expressing wild-type hepatocytes that had escaped Cre-mediated recombination progressively repopulated the entire liver. Unexpectedly, however, two thirds of the mutant mice spontaneously developed hepatocellular carcinomas derived from residual Dicer1-deficient hepatocytes at 1 year of age.

CONCLUSIONS

Dicer and microRNAs have critical roles in hepatocyte survival, metabolism, developmental gene regulation, and tumor suppression in the liver. Loss of Dicer primarily impairs hepatocyte survival but can promote hepatocarcinogenesis in cooperation with additional oncogenic stimuli.

Unique phenotype of hepatocellular cancers with exon-3 mutations in beta-catenin gene

Wnt/beta-catenin signaling plays an important role in liver development and regeneration. Its aberrant activation, however, is observed in a subset of primary hepatocellular cancers (HCCs). In the current study, we compare and contrast the tumor characteristics of HCC in the presence or absence of mutations in the beta-catenin gene (CTNNB1). Frozen HCCs (n = 32), including five fibrolamellar (FL) variants, and control livers (n = 3) from Health Sciences Tissue Bank and Department of Surgery at the University of Pittsburgh Medical Center, were examined for mutations in CTNNB1, protein levels of beta-catenin, tyrosine-654-phosphorylated-beta-catenin (Y654-beta-catenin), and glutamine synthetase (GS). Missense mutations in the exon-3 of CTNNB1were identified in 9/32 HCCs. Total beta-catenin levels were higher than controls in most tumors; however, GS was exclusively increased in HCCs with mutations. Phenotypically, greater percentages of mutated HCCs showed macrovascular and microvascular invasion. Also, the tumor size was greater than double in mutated HCCs. High levels of total beta-catenin protein were observed in multinodular tumors independent of beta-catenin mutations. In addition, significant cases with mutations showed absence of cirrhosis. Finally, the highest levels of Y654-beta-catenin were exclusively observed in fibrolamellar (FL)-HCC cases.

CONCLUSION

Thus, HCCs that harbor missense mutations in exon-3 of CTNNB1 exhibit, histologically, a more aggressive phenotype. Also, CTNNB1 mutations might lead to HCC in the absence of cirrhosis. Finally, FL-HCC cases display a unique up-regulation of tyrosine-phosphorylated-beta-catenin, suggesting robust receptor tyrosine kinase signaling in this tumor type.

Inducibility of drug-metabolizing enzymes by xenobiotics in mice with liver-specific knockout of Ctnnb1

Basal as well as xenobiotic-induced expression of the main enzymes from phase I and phase II of drug metabolism is confined to the perivenous areas of the mammalian liver lobule. Whereas signal transduction pathways that govern xenobiotic-induced expression of these enzymes via ligand-activated transcription factors such as constitutive androstane receptor (CAR) or the aryl hydrocarbon receptor (AhR) have been intensively studied, the mechanisms regulating zone-specific basal expression of genes related to drug metabolism and preferential response of perivenous hepatocytes to xenobiotic inducers are still largely unknown. Recent publications by our and other groups point to an important role for the Wnt/beta-catenin pathway in the maintenance of the perivenous hepatocyte gene expression profile including the main hepatic detoxification enzymes, and beta-catenin signaling was recently implicated in the expression of several cytochrome P450 isoenzymes. To analyze, whether the beta-catenin pathway would also affect inducible expression of drug-metabolizing enzymes, mice with liver-specific knockout of the Ctnnb1 gene (encoding beta-catenin) were treated with different model inducers of xenobiotic metabolism. Knockout of beta-catenin led to alterations in basal expression of most drug metabolism-related genes analyzed and resulted in strongly diminished responses to agonists of CAR-, AhR-, and nuclear factor erythroid-related factor 2-dependent transcription. Taken together, the data presented in this study indicate that beta-catenin not only regulates basal expression of drug-metabolizing enzymes but also determines the magnitude and hepatic localization of response to xenobiotic inducers in vivo.

Wnt/beta-catenin pathway in hepatocellular carcinoma pathogenesis and liver physiology

The Wnt/beta-catenin pathway is a key developmental pathway for which alterations have been described in various human cancers. The aberrant activation of this pathway is a major event in human hepatocellular carcinoma. Several laboratories have shown that the Wnt/beta-catenin pathway plays an essential role in all phases of liver development and maturation, and is required for the metabolic function of this organ. In this review, we summarize current knowledge regarding the role of the Wnt/beta-catenin pathway in hepatocellular carcinoma pathogenesis and liver biology, and the possibilities for developing new therapeutic interventions based on this knowledge.

Unique phenotype of hepatocellular cancers with exon-3 mutations in beta-catenin gene

Wnt/beta-catenin signaling plays an important role in liver development and regeneration. Its aberrant activation, however, is observed in a subset of primary hepatocellular cancers (HCCs). In the current study, we compare and contrast the tumor characteristics of HCC in the presence or absence of mutations in the beta-catenin gene (CTNNB1). Frozen HCCs (n = 32), including five fibrolamellar (FL) variants, and control livers (n = 3) from Health Sciences Tissue Bank and Department of Surgery at the University of Pittsburgh Medical Center, were examined for mutations in CTNNB1, protein levels of beta-catenin, tyrosine-654-phosphorylated-beta-catenin (Y654-beta-catenin), and glutamine synthetase (GS). Missense mutations in the exon-3 of CTNNB1were identified in 9/32 HCCs. Total beta-catenin levels were higher than controls in most tumors; however, GS was exclusively increased in HCCs with mutations. Phenotypically, greater percentages of mutated HCCs showed macrovascular and microvascular invasion. Also, the tumor size was greater than double in mutated HCCs. High levels of total beta-catenin protein were observed in multinodular tumors independent of beta-catenin mutations. In addition, significant cases with mutations showed absence of cirrhosis. Finally, the highest levels of Y654-beta-catenin were exclusively observed in fibrolamellar (FL)-HCC cases.

CONCLUSION

Thus, HCCs that harbor missense mutations in exon-3 of CTNNB1 exhibit, histologically, a more aggressive phenotype. Also, CTNNB1 mutations might lead to HCC in the absence of cirrhosis. Finally, FL-HCC cases display a unique up-regulation of tyrosine-phosphorylated-beta-catenin, suggesting robust receptor tyrosine kinase signaling in this tumor type.

Wnt5a is essential for intestinal elongation in mice

Morphogenesis of the mammalian small intestine entails extensive elongation and folding of the primitive gut into a tightly coiled digestive tube. Surprisingly, little is known about the cellular and molecular mechanisms that mediate the morphological aspects of small intestine formation. Here, we demonstrate that Wnt5a, a member of the Wnt family of secreted proteins, is essential for the development and elongation of the small intestine from the midgut region. We found that the small intestine in mice lacking Wnt5a was dramatically shortened and duplicated, forming a bifurcated lumen instead of a single tube. In addition, cell proliferation was reduced and re-intercalation of post-mitotic cells into the elongating gut tube epithelium was disrupted. Thus, our study demonstrates that Wnt5a functions as a critical regulator of midgut formation and morphogenesis in mammals.

Activation of canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cell line WB-F344 in vitro

AIM: To investigate the effect of activation of canonical Wnt signaling pathway on the proliferation and differentiation of hepatic oval cells in vitro.

METHODS: WB-F344 cells were treated with recombinant Wnt3a (20, 40, 80, 160, 200 ng/mL) in serum-free medium for 24 h. Cell proliferation was measured by Brdu incorporation analysis; untreated WB-F344 cells were taken as controls. After treatment with Wnt3a (160 ng/mL) for 24 h, subcellular localization and protein expression of β-catenin in WB-F344 cells treated and untreated with Wnt3a were examined by immunofluorescence staining and Western blot analysis. CyclinD1 mRNA expression was determined by semi-quantitative reverse-transcript polymerase chain reaction (RT-PCR). The mRNA levels of some phenotypic markers (AFP, CK-19, ALB) and two hepatic nuclear factors (HNF-4, HNF-6) were measured by RT-PCR. Expressions of CK-19 and AFP protein were detected by Western blot analysis.

RESULTS: Wnt3a promoted proliferation of WB-F344 cells. Stimulation of WB-F344 cells with recombinant Wnt3a resulted in accumulation of the transcriptional activator β-catenin, together with its translocation into the nuclei, and up-regulated typical Wnt target gene CyclinD1. After 3 d of Wnt3a treatment in the absence of serum, WB-F344 cells retained their bipotential to express several specific phenotypic markers of hepatocytes and cholangiocytes, such as AFP and CK-19, following activation of the canonical Wnt signaling pathway.

CONCLUSION: The canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cells.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

APC mutant zebrafish uncover a changing temporal requirement for wnt signaling in liver development

Developmental signaling pathways hold the keys to unlocking the promise of adult tissue regeneration, and to inhibiting carcinogenesis. Patients with mutations in the Adenomatous Polyposis Coli (APC) gene are at increased risk of developing hepatoblastoma, an embryonal form of liver cancer, suggesting that Wnt affects hepatic progenitor cells. To elucidate the role of APC loss and enhanced Wnt activity in liver development, we examined APC mutant and wnt inducible transgenic zebrafish. APC(+/-) embryos developed enlarged livers through biased induction of hepatic gene programs and increased proliferation. Conversely, APC(-/-) embryos formed no livers. Blastula transplantations determined that the effects of APC loss were cell autonomous. Induction of wnt modulators confirmed biphasic consequences of wnt activation: endodermal pattern formation and gene expression required suppression of wnt signaling in early somitogenesis; later, increased wnt activity altered endodermal fate by enhancing liver growth at the expense of pancreas formation; these effects persisted into the larval stage. In adult APC(+/-) zebrafish, increased wnt activity significantly accelerated liver regeneration after partial hepatectomy. Similarly, liver regeneration was significantly enhanced in APC(Min/+) mice, indicating the conserved effect of Wnt pathway activation in liver regeneration across vertebrate species. These studies reveal an important and time-dependent role for wnt signaling during liver development and regeneration.

Beta-catenin deletion in hepatoblasts disrupts hepatic morphogenesis and survival during mouse development

Beta-catenin, the central component of the canonical Wnt pathway, plays important roles in the processes of liver regeneration, growth, and cancer. Previously, we identified temporal expression of beta-catenin during liver development. Here, we characterize the hepatic phenotype, resulting from the successful deletion of beta-catenin in the developing hepatoblasts utilizing Foxa3-cyclization recombination and floxed-beta-catenin (exons 2 through 6) transgenic mice. Beta-catenin loss in developing livers resulted in significantly underdeveloped livers after embryonic day 12 (E12) with lethality occurring at around E17 stages. Histology revealed an overall deficient hepatocyte compartment due to (1) increased cell death due to oxidative stress and apoptosis, and (2) diminished expansion secondary to decreased cyclin-D1 and impaired proliferation. Also, the remnant hepatocytes demonstrated an immature phenotype as indicated by high nuclear to cytoplasmic ratio, poor cell polarity, absent glycogen, and decreased expression of key liver-enriched transcription factors: CCAAT-enhancer binding protein-alpha and hepatocyte nuclear factor-4alpha. A paucity of primitive bile ducts was also observed. While the stem cell assays demonstrated no intrinsic defect in hematopoiesis, distorted hepatic architecture and deficient hepatocyte compartments resulted in defective endothelial cell organization leading to overall fetal pallor.

CONCLUSION

Beta-catenin regulates multiple, critical events during the process of hepatic morphogenesis, including hepatoblast maturation, expansion, and survival, making it indispensable to survival.

Stabilization of beta-catenin affects mouse embryonic liver growth and hepatoblast fate

During hepatogenesis, after the liver has budded out of the endoderm, the hepatoblasts quickly expand and differentiate into either hepatocytes or biliary cells, the latter of which arise only within the ductal plate surrounding the portal vein. Because the Wnt/beta-catenin pathway is involved in liver homeostasis and regeneration and in liver carcinogenesis, we investigated here a role for Wnt/beta-catenin signaling in the embryonic liver. A cyclization recombination (Cre)/locus of X-over P1 (loxP) strategy was chosen to perform adenomatous polyposis coli (Apc) invalidation in order to activate ectopic beta-catenin signaling in hepatoblasts; an appropriate transgenic model expressing the Cre recombinase was used. Phenotypic and immunolocalization studies, together with messenger RNA analyses, by microarray and real-time quantitative polymerase chain reaction approaches were performed on this model during normal hepatogenesis. The loss of Apc allowed beta-catenin activation in the hepatoblasts after the formation of the liver bud and led to embryonic lethality. In this model, the liver became hypoplastic, and hepatocyte differentiation failed, whereas beta-catenin-activated ducts developed and gave rise to fully differentiated bile ducts when transplanted into adult recipient livers. Microarray analyses suggested that beta-catenin plays a role in repressing the hepatocyte genetic program and remodeling the ductal plate. According to these data, in normal embryonic livers, beta-catenin was transiently activated in the nascent bile ducts.

CONCLUSION

We demonstrated a key role for the Wnt/beta-catenin pathway in liver embryonic growth and in controlling the fate of hepatoblasts, preventing them from differentiating toward the hepatocyte lineage, and guiding them to biliary ductal morphogenesis.

Wnt/beta-catenin signaling in murine hepatic transit amplifying progenitor cells

BACKGROUND & AIMS

Oval cells are postnatal hepatic progenitors with high proliferative potential and bipotent differentiation ability to become hepatocytes and cholangiocytes. Because Wnt/beta-catenin signaling is a known regulatory pathway for liver development and regeneration, we studied the role of Wnt signaling in oval cells using a mouse model of chronic liver injury.

METHODS

A 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-enriched diet was used to stimulate oval cell proliferation. Livers were harvested for histologic analysis and determination of Wnt family gene expression by quantitative reverse transcription-polymerase chain reaction and in situ hybridization. The transgenic beta-catenin reporter mouse (TOPGAL) was use to confirm canonical Wnt/beta-catenin signal transduction in proliferating oval cells within atypical ductal proliferations (ADPs). Confocal fluorescence microscopy and immunohistochemistry was used to confirm colocalization of beta-catenin with the oval cell antigen A-6.

RESULTS

Several Wnt ligands were significantly induced in the liver of DDC-fed mice and localized to proliferating cells in and adjacent to the ADPs. Oval cells isolated from DDC-fed mouse livers showed the presence of active beta-catenin in the nucleus along with cell-cycle entry in response to purified Wnt3a in vitro. Moreover, Wnt3a-induced beta-catenin/T-cell factor/lymphoid enhancer factor (TCF/LEF) transcriptional activation was quantified by TCF/LEF luciferase reporter assays.

CONCLUSIONS

From these data, we conclude that oval cells respond to Wnt ligands (Wnt3a) in vitro with an increase in amino-terminus dephosphorylated beta-catenin and cell-cycle entry and that canonical Wnt/beta-catenin/TCF signaling is active in proliferating facultative hepatic progenitor cells in vivo. These findings may lend insight to the consequences of increased canonical Wnt signaling during periods of chronic liver injury.