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

Wnt/β-catenin and NFκB signaling synergize to trigger growth factor-free regeneration of adult primary human hepatocytes

BACKGROUND AND AIMS

The liver has a remarkable capacity to regenerate, which is sustained by the ability of hepatocytes to act as facultative stem cells that, while normally quiescent, re-enter the cell cycle after injury. Growth factor signaling is indispensable in rodents, whereas Wnt/β-catenin is not required for effective tissue repair. However, the molecular networks that control human liver regeneration remain unclear.

METHODS

Organotypic 3D spheroid cultures of primary human or murine hepatocytes were used to identify the signaling network underlying cell cycle re-entry. Furthermore, we performed chemogenomic screening of a library enriched for epigenetic regulators and modulators of immune function to determine the importance of epigenomic control for human hepatocyte regeneration.

RESULTS

Our results showed that, unlike in rodents, activation of Wnt/β-catenin signaling is the major mitogenic cue for adult primary human hepatocytes. Furthermore, we identified TGFβ inhibition and inflammatory signaling through NF-κB as essential steps for the quiescent-to-regenerative switch that allows Wnt/β-catenin-induced proliferation of human cells. In contrast, growth factors, but not Wnt/β-catenin signaling, triggered hyperplasia in murine hepatocytes. High-throughput screening in a human model confirmed the relevance of NFκB and revealed the critical roles of polycomb repressive complex 2, as well as of the bromodomain families I, II, and IV.

CONCLUSIONS

This study revealed a network of NFκB, TGFβ, and Wnt/β-catenin that controls human hepatocyte regeneration in the absence of exogenous growth factors, identified novel regulators of hepatocyte proliferation, and highlighted the potential of organotypic culture systems for chemogenomic interrogation of complex physiological processes.

Wnt/β-catenin signaling pathway in liver biology and tumorigenesis

The Wnt/β-catenin pathway is an evolutionarily conserved signaling pathway that controls fundamental physiological and pathological processes by regulating cell proliferation and differentiation. The Wnt/β-catenin pathway enables liver homeostasis by inducing differentiation and contributes to liver-specific features such as metabolic zonation and regeneration. In contrast, abnormalities in the Wnt/β-catenin pathway promote the development and progression of hepatocellular carcinoma (HCC). Similarly, hepatoblastoma, the most common childhood liver cancer, is frequently associated with β-catenin mutations, which activate Wnt/β-catenin signaling. HCCs with activation of the Wnt/β-catenin pathway have unique gene expression patterns and pathological and clinical features. Accordingly, they are highly differentiated with retaining hepatocyte-like characteristics and tumorigenic. Activation of the Wnt/β-catenin pathway in HCC also alters the state of immune cells, causing "immune evasion" with inducing resistance to immune checkpoint inhibitors, which have recently become widely used to treat HCC. Activated Wnt/β-catenin signaling exhibits these phenomena in liver tumorigenesis through the expression of downstream target genes, and the molecular basis is still poorly understood. In this review, we describe the physiological roles of Wnt/b-catenin signaling and then discuss their characteristic changes by the abnormal activation of Wnt/b-catenin signaling. Clarification of the mechanism would contribute to the development of therapeutic agents in the future.

Unveiling the power of microenvironment in liver regeneration: an in-depth overview

The liver serves as a vital regulatory hub for various physiological processes, including sugar, protein, and fat metabolism, coagulation regulation, immune system maintenance, hormone inactivation, urea metabolism, and water-electrolyte acid-base balance control. These functions rely on coordinated communication among different liver cell types, particularly within the liver's fundamental hepatic lobular structure. In the early stages of liver development, diverse liver cells differentiate from stem cells in a carefully orchestrated manner. Despite its susceptibility to damage, the liver possesses a remarkable regenerative capacity, with the hepatic lobule serving as a secure environment for cell division and proliferation during liver regeneration. This regenerative process depends on a complex microenvironment, involving liver resident cells, circulating cells, secreted cytokines, extracellular matrix, and biological forces. While hepatocytes proliferate under varying injury conditions, their sources may vary. It is well-established that hepatocytes with regenerative potential are distributed throughout the hepatic lobules. However, a comprehensive spatiotemporal model of liver regeneration remains elusive, despite recent advancements in genomics, lineage tracing, and microscopic imaging. This review summarizes the spatial distribution of cell gene expression within the regenerative microenvironment and its impact on liver regeneration patterns. It offers valuable insights into understanding the complex process of liver regeneration.

Wnt-β-catenin in hepatobiliary homeostasis, injury, and repair

Wnt-β-catenin signaling has emerged as an important regulatory pathway in the liver, playing key roles in zonation and mediating contextual hepatobiliary repair after injuries. In this review, we will address the major advances in understanding the role of Wnt signaling in hepatic zonation, regeneration, and cholestasis-induced injury. We will also touch on some important unanswered questions and discuss the relevance of modulating the pathway to provide therapies for complex liver pathologies that remain a continued unmet clinical need.

A comprehensive appraisal of HER2 heterogeneity in HER2-amplified and HER2-low colorectal cancer

BACKGROUND

This study aimed to elucidate the clinicopathological and molecular features of HER2-amplified and HER2-low colorectal cancers (CRCs). We also characterised HER2 expression statuses in CRCs focusing on their intratumoral heterogeneity and alterations in metastatic lesions to establish practical HER2 status assessment.

METHODS

We evaluated 1009 CRCs for HER2 expression and HER2 amplification by immunohistochemistry and FISH, respectively, and correlated the results to clinicopathological and molecular data. For HER2-positive tumours, HER2 expression in metastatic lesions was also assessed.

RESULTS

Twenty-five HER2-amplified (2.5%) and 46 HER2-low tumours (4.6%) were identified. HER2-amplified tumours consistently lacked a mucinous component and HER2-low tumours tended to be in the right colon, but no other clinicopathological features were noted. KRAS, NRAS or BRAF mutations were detected in only two HER2-amplified tumours (8%), whereas 23 HER2-low tumours (50%) had one of these mutations. Most HER2-amplified and HER2-low tumours showed a homogeneous or mosaic HER2 expression pattern and a clustered heterogeneous expression pattern was rather rare. HER2 expression was maintained in most metastatic lesions in both HER2-amplified (93%) and HER2-low tumours (81%).

CONCLUSIONS

These results suggest that biopsy-based assessment of primary lesions is appropriate for the identification of CRC patients eligible for systemic HER2-targeted therapy.

MYC-an emerging player in mitochondrial diseases

The mitochondrion is a major hub of cellular metabolism and involved directly or indirectly in almost all biological processes of the cell. In mitochondrial diseases, compromised respiratory electron transfer and oxidative phosphorylation (OXPHOS) lead to compensatory rewiring of metabolism with resemblance to the Warburg-like metabolic state of cancer cells. The transcription factor MYC (or c-MYC) is a major regulator of metabolic rewiring in cancer, stimulating glycolysis, nucleotide biosynthesis, and glutamine utilization, which are known or predicted to be affected also in mitochondrial diseases. Albeit not widely acknowledged thus far, several cell and mouse models of mitochondrial disease show upregulation of MYC and/or its typical transcriptional signatures. Moreover, gene expression and metabolite-level changes associated with mitochondrial integrated stress response (mt-ISR) show remarkable overlap with those of MYC overexpression. In addition to being a metabolic regulator, MYC promotes cellular proliferation and modifies the cell cycle kinetics and, especially at high expression levels, promotes replication stress and genomic instability, and sensitizes cells to apoptosis. Because cell proliferation requires energy and doubling of the cellular biomass, replicating cells should be particularly sensitive to defective OXPHOS. On the other hand, OXPHOS-defective replicating cells are predicted to be especially vulnerable to high levels of MYC as it facilitates evasion of metabolic checkpoints and accelerates cell cycle progression. Indeed, a few recent studies demonstrate cell cycle defects and nuclear DNA damage in OXPHOS deficiency. Here, we give an overview of key mitochondria-dependent metabolic pathways known to be regulated by MYC, review the current literature on MYC expression in mitochondrial diseases, and speculate how its upregulation may be triggered by OXPHOS deficiency and what implications this has for the pathogenesis of these diseases.

The Wnt/β-catenin signaling pathway plays a role in drug-induced liver injury by regulating cytochrome P450 2E1 expression

Drug-induced liver injury (DILI) is a major cause of acute liver failure and drug withdrawal. Cytochrome P450 (CYP) 2E1 is involved in the metabolism of several drugs, and can induce liver injury through the production of toxic metabolites and the generation of reactive oxygen species. This study aimed to elucidate the role of Wnt/β-catenin signaling in CYP2E1 regulation for drug-induced hepatotoxicity. To achieve this, mice were administered cisplatin or acetaminophen (APAP) 1 h after treatment with the CYP2E1 inhibitor dimethyl sulfoxide (DMSO), and histopathological and serum biochemical analyses were performed. APAP treatment induced hepatotoxicity, as evidenced by an increase in liver weight and serum ALT levels. Moreover, histological analysis indicated severe injury, including apoptosis, in the liver tissue of APAP-treated mice, which was confirmed by TUNEL assay. Additionally, APAP treatment suppressed the antioxidant capacity of the mice and increased the expression of the DNA damage markers γ-H2AX and p53. However, these effects of APAP on hepatotoxicity were significantly attenuated by DMSO treatment. Furthermore, the activation of Wnt/β-catenin signaling using the Wnt agonist CHIR99021 (CHIR) increased CYP2E1 expression in rat liver epithelial cells (WB-F344), whereas treatment with the Wnt/β-catenin antagonist IWP-2 inhibited nuclear β-catenin and CYP2E1 expression. Interestingly, APAP-induced cytotoxicity in WB-F344 cells was exacerbated by CHIR treatment and suppressed by IWP-2 treatment. Overall, these results showed that the Wnt/β-catenin signaling is involved in DILI through the upregulation of CYP2E1 expression by directly binding the transcription factor β-cat/TCF to the Cyp2e1 promoter, thus exacerbating DILI.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-023-00180-6.

Hepatic stellate cells maintain liver homeostasis through paracrine neurotrophin-3 signaling that induces hepatocyte proliferation

Organ size is maintained by the controlled proliferation of distinct cell populations. In the mouse liver, hepatocytes in the midlobular zone that are positive for cyclin D1 (CCND1) repopulate the parenchyma at a constant rate to preserve liver mass. Here, we investigated how hepatocyte proliferation is supported by hepatic stellate cells (HSCs), pericytes that are in close proximity to hepatocytes. We used T cells to ablate nearly all HSCs in the murine liver, enabling the unbiased characterization of HSC functions. In the normal liver, complete loss of HSCs persisted for up to 10 weeks and caused a gradual reduction in liver mass and in the number of CCND1+ hepatocytes. We identified neurotrophin-3 (Ntf-3) as an HSC-produced factor that induced the proliferation of midlobular hepatocytes through the activation of tropomyosin receptor kinase B (TrkB). Treating HSC-depleted mice with Ntf-3 restored CCND1+ hepatocytes in the midlobular region and increased liver mass. These findings establish that HSCs form the mitogenic niche for midlobular hepatocytes and identify Ntf-3 as a hepatocyte growth factor.

AhR and Wnt/β-Catenin Signaling Pathways and Their Interplay

As evolutionarily conserved signaling cascades, AhR and Wnt signaling pathways play a critical role in the control over numerous vital embryonic and somatic processes. AhR performs many endogenous functions by integrating its signaling pathway into organ homeostasis and into the maintenance of crucial cellular functions and biological processes. The Wnt signaling pathway regulates cell proliferation, differentiation, and many other phenomena, and this regulation is important for embryonic development and the dynamic balance of adult tissues. AhR and Wnt are the main signaling pathways participating in the control of cell fate and function. They occupy a central position in a variety of processes linked with development and various pathological conditions. Given the importance of these two signaling cascades, it would be interesting to elucidate the biological implications of their interaction. Functional connections between AhR and Wnt signals take place in cases of crosstalk or interplay, about which quite a lot of information has been accumulated in recent years. This review is focused on recent studies about the mutual interactions of key mediators of AhR and Wnt/β-catenin signaling pathways and on the assessment of the complexity of the crosstalk between the AhR signaling cascade and the canonical Wnt pathway.

Hepatocyte β-catenin loss is compensated by Insulin-mTORC1 activation to promote liver regeneration

BACKGROUND AND AIMS

Liver regeneration (LR) following partial hepatectomy (PH) occurs via activation of various signaling pathways. Disruption of a single pathway can be compensated by activation of another pathway to continue LR. The Wnt-β-catenin pathway is activated early during LR and conditional hepatocyte loss of β-catenin delays LR. Here, we study mechanism of LR in the absence of hepatocyte-β-catenin.

APPROACH AND RESULTS

Eight-week-old hepatocyte-specific Ctnnb1 knockout mice (β-catenin ΔHC ) were subjected to PH. These animals exhibited decreased hepatocyte proliferation at 40-120 h and decreased cumulative 14-day BrdU labeling of <40%, but all mice survived, suggesting compensation. Insulin-mediated mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) activation was uniquely identified in the β-catenin ΔHC mice at 72-96 h after PH. Deletion of hepatocyte regulatory-associated protein of mTOR (Raptor), a critical mTORC1 partner, in the β-catenin ΔHC mice led to progressive hepatic injury and mortality by 30 dys. PH on early stage nonmorbid Raptor ΔHC -β-catenin ΔHC mice led to lethality by 12 h. Raptor ΔHC mice showed progressive hepatic injury and spontaneous LR with β-catenin activation but died by 40 days. PH on early stage nonmorbid Raptor ΔHC mice was lethal by 48 h. Temporal inhibition of insulin receptor and mTORC1 in β-catenin ΔHC or controls after PH was achieved by administration of linsitinib at 48 h or rapamycin at 60 h post-PH and completely prevented LR leading to lethality by 12-14 days.

CONCLUSIONS

Insulin-mTORC1 activation compensates for β-catenin loss to enable LR after PH. mTORC1 signaling in hepatocytes itself is critical to both homeostasis and LR and is only partially compensated by β-catenin activation. Dual inhibition of β-catenin and mTOR may have notable untoward hepatotoxic side effects.

A Spatial Atlas of Wnt Receptors in Adult Mouse Liver

Hepatic zonation is critical for most metabolic functions in liver. Wnt signaling plays an important role in establishing and maintaining liver zonation. Yet, the anatomic expression of Wnt signaling components, especially all 10 Frizzled (Fzd) receptors, has not been characterized in adult liver. To address this, the spatial expression of Fzd receptors was quantitatively mapped in adult mouse liver via multiplex fluorescent in situ hybridization. Although all 10 Fzd receptors were expressed within a metabolic unit, Fzd receptors 1, 4, and 6 were the highest expressed. Although most Wnt signaling occurs in zone 3, expression of most Fzd receptors was not zonated. In contrast, Fzd receptor 6 was preferentially expressed in zone 1. Wnt2 and Wnt9b expression was highly zonated and primarily found in zone 3. Therefore, the current results suggest that zonated Wnt/β-catenin signaling at baseline occurs primarily due to Wnt2 and Wnt9b rather than zonation of Fzd mRNA expression. Finally, the study showed that Fzd receptors and Wnts are not uniformly expressed by all hepatic cell types. Instead, there is broad distribution among both hepatocytes and nonparenchymal cells, including endothelial cells. Overall, this establishment of a definitive mRNA expression atlas, especially of Fzd receptors, opens the door to future functional characterization in healthy and diseased liver states.

Mitochondrial complex III deficiency drives c-MYC overexpression and illicit cell cycle entry leading to senescence and segmental progeria

Accumulating evidence suggests mitochondria as key modulators of normal and premature aging, yet whether primary oxidative phosphorylation (OXPHOS) deficiency can cause progeroid disease remains unclear. Here, we show that mice with severe isolated respiratory complex III (CIII) deficiency display nuclear DNA damage, cell cycle arrest, aberrant mitoses, and cellular senescence in the affected organs such as liver and kidney, and a systemic phenotype resembling juvenile-onset progeroid syndromes. Mechanistically, CIII deficiency triggers presymptomatic cancer-like c-MYC upregulation followed by excessive anabolic metabolism and illicit cell proliferation against lack of energy and biosynthetic precursors. Transgenic alternative oxidase dampens mitochondrial integrated stress response and the c-MYC induction, suppresses the illicit proliferation, and prevents juvenile lethality despite that canonical OXPHOS-linked functions remain uncorrected. Inhibition of c-MYC with the dominant-negative Omomyc protein relieves the DNA damage in CIII-deficient hepatocytes in vivo. Our results connect primary OXPHOS deficiency to genomic instability and progeroid pathogenesis and suggest that targeting c-MYC and aberrant cell proliferation may be therapeutic in mitochondrial diseases.

Wnt signaling in liver regeneration, disease, and cancer

The liver exhibits the highest recovery rate from acute injuries. However, in chronic liver disease, the long-term loss of hepatocytes often leads to adverse consequences such as fibrosis, cirrhosis, and liver cancer. The Wnt signaling plays a pivotal role in both liver regeneration and tumorigenesis. Therefore, manipulating the Wnt signaling has become an attractive approach to treating liver disease, including cancer. Nonetheless, given the crucial roles of Wnt signaling in physiological processes, blocking Wnt signaling can also cause several adverse effects. Recent studies have identified cancer-specific regulators of Wnt signaling, which would overcome the limitation of Wnt signaling target approaches. In this review, we discussed the role of Wnt signaling in liver regeneration, precancerous lesion, and liver cancer. Furthermore, we summarized the basic and clinical approaches of Wnt signaling blockade and proposed the therapeutic prospects of cancer-specific Wnt signaling blockade for liver cancer treatment.

Single-cell spatial transcriptomics reveals a dynamic control of metabolic zonation and liver regeneration by endothelial cell Wnt2 and Wnt9b

The conclusive identity of Wnts regulating liver zonation (LZ) and regeneration (LR) remains unclear despite an undisputed role of β-catenin. Using single-cell analysis, we identified a conserved Wnt2 and Wnt9b expression in endothelial cells (ECs) in zone 3. EC-elimination of Wnt2 and Wnt9b led to both loss of β-catenin targets in zone 3, and re-appearance of zone 1 genes in zone 3, unraveling dynamicity in the LZ process. Impaired LR observed in the knockouts phenocopied models of defective hepatic Wnt signaling. Administration of a tetravalent antibody to activate Wnt signaling rescued LZ and LR in the knockouts and induced zone 3 gene expression and LR in controls. Administration of the agonist also promoted LR in acetaminophen overdose acute liver failure (ALF) fulfilling an unmet clinical need. Overall, we report an unequivocal role of EC-Wnt2 and Wnt9b in LZ and LR and show the role of Wnt activators as regenerative therapy for ALF.

Proposed hypothesis of GSK-3 β inhibition for stimulating Wnt/β-catenin signaling pathway which triggers liver regeneration process

Almost every human organ has a poor ability to regenerate, notable exceptions are liver, skin, gut, etc. Molecular and cellular underpinnings of liver regeneration might pave the way for novel treatments concerned with chronic liver disorder. Such treatments would eliminate the disadvantages of liver transplantation, such as a scarcity of donor organs, a lengthy waitlist, significant medical expenses, surgical complications, and the necessity for lifelong immunosuppressive medications. Advancement in the development of regenerative therapy is giving hope to those suffering from end-stage liver disorder. The regeneration process is unique, intricate, and well coordinated, which involve the interaction of numerous signaling pathways, cytokines, and growth factor. Various signaling pathways for liver regeneration are HO-1/BER pathway, Tweak/Fn14 signaling pathway, Hippo pathway, Wnt/beta-catenin pathway, Hedgehog signaling pathway, bile acids repairing pathway, serotonin (5HT) pathway, estrogen pathway, thyrotropin-releasing hormone (TRH) pathway, insulin repairing pathway, etc. The in vitro scientific literature revealed that numerous GSK-3 β inhibitors (LY 2090314, AR-A014418, Tideglusib, Solasodine, CHIR99021, 9-ING-41, SB-216763) play an important role in stimulating the liver regeneration process. Similarly, from the above discussion, the direction is highlighted to emphasize the proposed molecular Wnt/β-catenin signaling pathway which is associated with GSK-3 β inhibition for the induction of the repairing and regeneration process.

Canonical Thyroid Hormone Receptor β Action Stimulates Hepatocyte Proliferation in Male Mice

CONTEXT

3,5,3'-L-triiodothyronine (T3) is a potent inducer of hepatocyte proliferation via the Wnt/β-catenin signaling pathway. Previous studies suggested the involvement of rapid noncanonical thyroid hormone receptor (TR) β signaling, directly activating hepatic Wnt/β-catenin signaling independent from TRβ DNA binding. However, the mechanism by which T3 increases Wnt/β-catenin signaling in hepatocytes has not yet been determined.

OBJECTIVE

We aimed to determine whether DNA binding of TRβ is required for stimulation of hepatocyte proliferation by T3.

METHODS

Wild-type (WT) mice, TRβ knockout mice (TRβ KO), and TRβ mutant mice with either specifically abrogated DNA binding (TRβ GS) or abrogated direct phosphatidylinositol 3 kinase activation (TRβ 147F) were treated with T3 for 6 hours or 7 days. Hepatocyte proliferation was assessed by Kiel-67 (Ki67) staining and apoptosis by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. Activation of β-catenin signaling was measured in primary murine hepatocytes. Gene expression was analyzed by microarray, gene set enrichment analysis (GSEA), and quantitative reverse transcription polymerase chain reaction.

RESULTS

T3 induced hepatocyte proliferation with an increased number of Ki67-positive cells in WT and TRβ 147F mice (9.2% ± 6.5% and 10.1% ± 2.9%, respectively) compared to TRβ KO and TRβ GS mice (1.2% ± 1.1% and 1.5% ± 0.9%, respectively). Microarray analysis and GSEA showed that genes of the Wnt/β-catenin pathway-among them, Fzd8 (frizzled receptor 8) and Ctnnb1 (β-catenin)-were positively enriched only in T3-treated WT and TRβ 147F mice while B-cell translocation gene anti-proliferation factor 2 was repressed. Consequently, expression of Ccnd1 (CyclinD1) was induced.

CONCLUSIONS

Instead of directly activating Wnt signaling, T3 and TRβ induce key genes of the Wnt/β-catenin pathway, ultimately stimulating hepatocyte proliferation via CyclinD1. Thus, canonical transcriptional TRβ action is necessary for T3-mediated stimulation of hepatocyte proliferation.

Loss of hepatic miR-194 promotes liver regeneration and protects from acetaminophen-induced acute liver injury

The two microRNAs miR-192 and miR-194 are abundantly expressed in the liver and are considered serum biomarkers of liver injury. However, their role in the development of liver injury has not yet been determined. In this study, we generated miR-192/194 mutant mice and determined the effect of miR-192/194 loss on acetaminophen (APAP)-induced acute liver injury. With genetic depletion of miR-192/194, mutant mice were fertile and normally developed. No spontaneous liver injuries were observed in mutant mice. After APAP administration, mutant mice developed less severe liver damage than control mice. Specifically, mutant mice exhibited significantly lower serum alanine transaminase (ALT) levels and pericentral necrosis/apoptosis than control mice receiving APAP. β-catenin signaling was activated during the early phase of liver injury. Activated β-catenin signaling led to faster cellular proliferation and higher expression of AXIN2 and glutamine synthetases. After partial hepatectomy, the miR-192/194 mutant hepatocytes were more regenerative than control hepatocytes (as shown by BrdU incorporation). Moreover, in vitro experiments indicated that miR-194, but not miR-192, specifically repressed β-catenin signaling, while animal experiments revealed that chemical-mediated knockdown of β-catenin signaling compromised APAP resistance that liver protected from miR-192/194 genetic depletion. Collectively, our data indicated that the loss of miR-194 promoted liver regeneration and protected the liver from APAP-induced injury.

Wnt ligands 3a and 5a regulate proliferation and migration in human fetal liver progenitor cells

Background

Since human fetal liver progenitor cells (hFLPC) can differentiate into multiple liver cell types in vitro and in vivo, hFLPC may be a suitable source for cell therapy and regeneration strategies. Imperative for effective clinical applications of hFLPC is the enhanced knowledge of growth factors that mediate and improve migration and proliferation. The canonical wingless/int-1 (Wnt) signal transduction pathway is known to play a key role in proliferation and migration of stem cells. So, we investigated a role for Wnt3a and Wnt5a ligands in regulating the proliferation and migration of hFLPC.

Methods

We used alamarBlue assay and transwell migration assay and examined proliferation and migration of hFLPC to Wnt3a and Wnt5a. In addition, the target genes of Wnt signal transduction pathway was identified using microarray analysis and validated by quantitative real-time polymerase chain reaction (qPCR).

Results

We found that Wnt3a or Wnt5a independently significantly increased migration and proliferation in a dose-dependent manner which was significantly inhibited by Wnt inhibitors Wnt-C59 or KN-62. Addition of Wnt3a to hFLPC resulted in increased mRNA expression of the known Wnt target genes Axin-2, DKK2, while Wnt5a increased CXCR7, all of which are closely associated with an enhanced proliferation capacity of stem cells.

Conclusions

Thus, we report that Wnt3a and Wnt5a may play an important role in the proliferation and migration of hFLPC by possibly regulating key target genes-involved in these processes. Incorporating recombinant human Wnt3a and Wnt5a in regenerative strategies using liver stem/progenitor cells might improve the process of liver regeneration.

Genetic and Cellular Contributions to Liver Regeneration

The regenerative capabilities of the liver represent a paradigm for understanding tissue repair in solid organs. Regeneration after partial hepatectomy in rodent models is well understood, while regeneration in the context of clinically relevant chronic injuries is less studied. Given the growing incidence of fatty liver disease, cirrhosis, and liver cancer, interest in liver regeneration is increasing. Here, we will review the principles, genetics, and cell biology underlying liver regeneration, as well as new approaches being used to study heterogeneity in liver tissue maintenance and repair.

CoQ10 promotes resolution of necrosis and liver regeneration after acetaminophen-induced liver injury

Coenzyme Q10 (CoQ10) which acts as an electron transporter in the mitochondrial respiratory chain has many beneficial effects on liver diseases. In our previous research, CoQ10 has been found to attenuate acetaminophen (APAP) induced acute liver injury (ALI). However, whether CoQ10 administration is still effective at the late stage of APAP overdose is still unknown. In this study, we aimed to test CoQ10 efficacy at the late stage of APAP overdose. C57BL/6J mice were intraperitoneally treated with APAP to induce liver injury. CoQ10 (5 mg/kg) was given to mice at 16 hours after APAP treatment. The results showed that while CoQ10 treatment at 16 hours post-APAP overdose had no effects on the expression of ROS generated genes or scavenged genes, it still significantly decreased necrosis of hepatocytes following APAP-induced ALI. Moreover, CoQ10 increased MerTK+ macrophages accumulation in the APAP-overdose liver and inhibition of MerTK signaling partly abrogated the protective role of CoQ10 treatment on the hepatic necrosis. CoQ10 treatment also significantly enhanced hepatocytes proliferation as shown in the increased BrdU incorporation in the APAP-intoxicated mice liver section. In addition, CoQ10 treatment increased hepatic PCNA and Cyclin D1 expression and promoted activation of the β-catenin signaling in APAP-overdose mice. To conclude, these data provide evidence that CoQ10 treatment is still effective at the late stage of APAP-induced ALI and promotes resolution of necrosis and liver regeneration following ALI.

The Aging Liver: Redox Biology and Liver Regeneration

Significance: During aging, excessive production of reactive species in the liver leads to redox imbalance with consequent oxidative damage and impaired organ homeostasis. Nevertheless, slight amounts of reactive species may modulate several transcription factors, acting as second messengers and regulating specific signaling pathways. These redox-dependent alterations may impact the age-associated decline in liver regeneration. Recent Advances: In the last few decades, relevant findings related to redox alterations in the aging liver were investigated. Consistently, recent research broadened understanding of redox modifications and signaling related to liver regeneration. Other than reporting the effect of oxidative stress, epigenetic and post-translational modifications, as well as modulation of specific redox-sensitive cellular signaling, were described. Among them, the present review focuses on Wnt/β-catenin, the nuclear factor (erythroid-derived 2)-like 2 (NRF2), members of the Forkhead box O (FoxO) family, and the p53 tumor suppressor. Critical Issues: Even though alteration in redox homeostasis occurs both in aging and in impaired liver regeneration, the associative mechanisms are not clearly defined. Of note, antioxidants are not effective in slowing hepatic senescence, and do not clearly improve liver repopulation after hepatectomy or transplant in humans. Future Directions: Further investigations are needed to define mutual redox-dependent molecular pathways involved both in aging and in the decline of liver regeneration. Preclinical studies aimed at the characterization of these pathways would define possible therapeutic targets for human trials. Antioxid. Redox Signal. 35, 832-847.

Glutamine synthetase as a central element in hepatic glutamine and ammonia metabolism: novel aspects

Glutamine synthetase (GS) in the liver is expressed in a small perivenous, highly specialized hepatocyte population and is essential for the maintenance of low, non-toxic ammonia levels in the organism. However, GS activity can be impaired by tyrosine nitration of the enzyme in response to oxidative/nitrosative stress in a pH-sensitive way. The underlying molecular mechanism as investigated by combined molecular simulations and in vitro experiments indicates that tyrosine nitration can lead to a fully reversible and pH-sensitive regulation of protein function. This approach was also used to understand the functional consequences of several recently described point mutations of human GS with clinical relevance and to suggest an approach to restore impaired GS activity.

Wnt/-Catenin Signaling and Liver Regeneration: Circuit, Biology, and Opportunities

The liver is uniquely bestowed with an ability to regenerate following a surgical or toxicant insult. One of the most researched models to demonstrate the regenerative potential of this organ is the partial hepatectomy model, where two thirds of the liver is surgically resected. The remnant liver replenishes the lost mass within 1014 days in mice. The distinctive ability of the liver to regenerate has allowed living donor and split liver transplantation. One signaling pathway shown to be activated during the process of regeneration to contribute toward the mass and functional recovery of the liver is the Wnt/-catenin pathway. Very early after any insult to the liver, the cellmolecule circuitry of the Wnt/-catenin pathway is set into motion with the release of specific Wnt ligands from sinusoidal endothelial cells and macrophages, which, in a paracrine manner, engage Frizzled and LDL-related protein-5/6 coreceptors on hepatocytes to stabilize -catenin inducing its nuclear translocation. Nuclear -catenin interacts with T-cell factor family of transcription factors to induce target genes including cyclin D1 for proliferation, and others for regulating hepatocyte function. Working in collaboration with other signaling pathways, Wnt/-catenin signaling contributes to the restoration process without any compromise of function at any stage. Also, stimulation of this pathway through innovative means induces liver regeneration when this process is exhausted or compromised and thus has applications in the treatment of end-stage liver disease and in the field of liver transplantation. Thus, Wnt/-catenin signaling pathway is highly relevant in the discipline of hepatic regenerative medicine.

β-Catenin Activation in Hepatocellular Cancer: Implications in Biology and Therapy

Simple Summary

Liver cancer is a dreadful tumor which has gradually increased in incidence all around the world. One major driver of liver cancer is the Wnt–β-catenin pathway which is active in a subset of these tumors. While this pathway is normally important in liver development, regeneration and homeostasis, it’s excessive activation due to mutations, is detrimental and leads to tumor cell growth, making it an important therapeutic target. There are also some unique characteristics of this pathway activation in liver cancer. It makes the tumor addicted to specific amino acids and in turn to mTOR signaling, which can be treated by certain existing therapies. In addition, activation of the Wnt–β-catenin in liver cancer appears to alter the immune cell landscape making it less likely to respond to the new immuno-oncology treatments. Thus, Wnt–β-catenin active tumors may need to be treated differently than non-Wnt–β-catenin active tumors.

Abstract

Hepatocellular cancer (HCC), the most common primary liver tumor, has been gradually growing in incidence globally. The whole-genome and whole-exome sequencing of HCC has led to an improved understanding of the molecular drivers of this tumor type. Activation of the Wnt signaling pathway, mostly due to stabilizing missense mutations in its downstream effector β-catenin (encoded by CTNNB1) or loss-of-function mutations in AXIN1 (the gene which encodes for Axin-1, an essential protein for β-catenin degradation), are seen in a major subset of HCC. Because of the important role of β-catenin in liver pathobiology, its role in HCC has been extensively investigated. In fact, CTNNB1 mutations have been shown to have a trunk role. β-Catenin has been shown to play an important role in regulating tumor cell proliferation and survival and in tumor angiogenesis, due to a host of target genes regulated by the β-catenin transactivation of its transcriptional factor TCF. Proof-of-concept preclinical studies have shown β-catenin to be a highly relevant therapeutic target in CTNNB1-mutated HCCs. More recently, studies have revealed a unique role of β-catenin activation in regulating both tumor metabolism as well as the tumor immune microenvironment. Both these roles have notable implications for the development of novel therapies for HCC. Thus, β-catenin has a pertinent role in driving HCC development and maintenance of this tumor-type, and could be a highly relevant therapeutic target in a subset of HCC cases.

TMEM9-v-ATPase Activates Wnt/β-Catenin Signaling Via APC Lysosomal Degradation for Liver Regeneration and Tumorigenesis

BACKGROUND AND AIMS

How Wnt signaling is orchestrated in liver regeneration and tumorigenesis remains elusive. Recently, we identified transmembrane protein 9 (TMEM9) as a Wnt signaling amplifier.

APPROACH AND RESULTS

TMEM9 facilitates v-ATPase assembly for vesicular acidification and lysosomal protein degradation. TMEM9 is highly expressed in regenerating liver and hepatocellular carcinoma (HCC) cells. TMEM9 expression is enriched in the hepatocytes around the central vein and acutely induced by injury. In mice, Tmem9 knockout impairs hepatic regeneration with aberrantly increased adenomatosis polyposis coli (Apc) and reduced Wnt signaling. Mechanistically, TMEM9 down-regulates APC through lysosomal protein degradation through v-ATPase. In HCC, TMEM9 is overexpressed and necessary to maintain β-catenin hyperactivation. TMEM9-up-regulated APC binds to and inhibits nuclear translocation of β-catenin, independent of HCC-associated β-catenin mutations. Pharmacological blockade of TMEM9-v-ATPase or lysosomal degradation suppresses Wnt/β-catenin through APC stabilization and β-catenin cytosolic retention.

CONCLUSIONS

Our results reveal that TMEM9 hyperactivates Wnt signaling for liver regeneration and tumorigenesis through lysosomal degradation of APC.

Establishment of human fetal hepatocyte organoids and CRISPR-Cas9-based gene knockin and knockout in organoid cultures from human liver

The liver is composed of two epithelial cell types: hepatocytes and liver ductal cells. Culture conditions for expansion of human liver ductal cells in vitro as organoids were previously described in a protocol; however, primary human hepatocytes remained hard to expand, until recently. In this protocol, we provide full details of how we overcame this limitation, establishing culture conditions that facilitate long-term expansion of human fetal hepatocytes as organoids. In addition, we describe how to generate (multi) gene knockouts using CRISPR-Cas9 in both human fetal hepatocyte and adult liver ductal organoid systems. Using a CRISPR-Cas9 and homology-independent organoid transgenesis (CRISPR-HOT) approach, efficient gene knockin can be achieved in these systems. These gene knockin and knockout approaches, and their multiplexing, should be useful for a variety of applications, such as disease modeling, investigating gene functions and studying processes, such as cellular differentiation and cell division. The protocol to establish human fetal hepatocyte organoid cultures takes ~1-2 months. The protocols to genome engineer human liver ductal organoids and human fetal hepatocyte organoids take 2-3 months.

Functional compensation precedes recovery of tissue mass following acute liver injury

The liver plays a central role in metabolism, protein synthesis and detoxification. It possesses unique regenerative capacity upon injury. While many factors regulating cellular proliferation during liver repair have been identified, the mechanisms by which the injured liver maintains vital functions prior to tissue recovery are unknown. Here, we identify a new phase of functional compensation following acute liver injury that occurs prior to cellular proliferation. By coupling single-cell RNA-seq with in situ transcriptional analyses in two independent murine liver injury models, we discover adaptive reprogramming to ensure expression of both injury response and core liver function genes dependent on macrophage-derived WNT/β-catenin signaling. Interestingly, transcriptional compensation is most prominent in non-proliferating cells, clearly delineating two temporally distinct phases of liver recovery. Overall, our work describes a mechanism by which the liver maintains essential physiological functions prior to cellular reconstitution and characterizes macrophage-derived WNT signals required for this compensation.

The liver possesses the ability to regenerate following sudden injury. Here, the authors use single-cell RNA-sequencing and in situ transcriptional analyses to identify a new phase of liver regeneration in mice aimed at maintaining essential functions throughout the regenerative process.

Hepatic sinusoids versus central veins: Structures, markers, angiocrines, and roles in liver regeneration and homeostasis

The blood circulates through the hepatic sinusoids delivering nutrients and oxygen to the liver parenchyma and drains into the hepatic central vein, yet the structures and phenotypes of these vessels are distinctively different. Sinusoidal endothelial cells are uniquely fenestrated, lack basal lamina and possess organelles involved in endocytosis, pinocytosis, degradation, synthesis and secretion. Hepatic central veins are nonfenestrated but are also active in synthesis and secretion. Endothelial cells of sinusoids and central veins secrete angiocrines that play respective roles in hepatic regeneration and metabolic homeostasis. The list of markers for identifying sinusoidal endothelial cells is long and their terminologies are complex. Further, their uses vary in different investigations and, in some instances, could be confusing. Central vein markers are fewer but more distinctive. Here we analyze and categorize the molecular pathways/modules associated with the sinusoid-mediated liver regeneration in response to partial hepatectomy and chemical-induced acute or chronic injury. Similarly, we highlight the findings that central vein-derived angiocrines interact with Wnt/β-catenin in perivenous hepatocytes to direct gene expression and maintain pericentral metabolic zonation. The proposal that perivenous hepatocytes behave as stem/progenitor cells to provoke hepatic homeostatic cell renewal is reevaluated and newer concepts of broad zonal distribution of hepatocyte proliferation in liver homeostasis and regeneration are updated. Thus, this review integrates the structures, biology and physiology of liver sinusoids and central veins in mediating hepatic regeneration and metabolic homeostasis.

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.

Dependence and Guidance Receptors-DCC and Neogenin-In Partial EMT and the Actions of Serine Proteases

The Epithelial-Mesenchymal Transition (EMT) is an important concept in understanding the processes of oncogenesis, especially with respect to the relationship between cell proliferation and metastatic properties such as spontaneous cell motility, chemotaxic migration and tissue invasion. EMT is now recognized as a more complex phenomenon than an all-or-nothing event, in which different components of the EMT may have distinct roles in the physio-pathological regulation of cell function and which may in turn depend on differential interactions with cell constituents and metabolic products. This mini-review summarizes recent work on the induction of cancer properties in parallel with the presence of EMT activities in the presence of serine proteases, with the focus on those tumor suppressors known as "dependence" receptors such as neogenin and Deleted in Colorectal Cancer (DCC). It is concluded that various forms of partial EMT should be given more detailed investigation and consideration as the results could have valuable implications for the development of disease-specific and patient-specific therapies.

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.

The Hepatic Central Vein: Structure, Fibrosis, and Role in Liver Biology

The hepatic central vein is a primary source of Wnt2, Wnt9b, and R-spondin3. These angiocrines activate ß-catenin signaling to regulate hepatic metabolic zonation and perivenous gene expression in mice. Little is known about the central vein ultrastructure. Here, we describe the morphological-functional correlates of the central vein and its draining and branching patterns. Central vein fibrosis occurs in liver disease and is often accompanied by perivenous perisinusoidal fibrosis, which may affect perivenous gene expression. We review the biological properties of perivenous hepatocytes and glutamine synthetase that serve as a biomarker of perivenous hepatocytes. Glutamine synthetase and P4502E1 are indicators of ß-catenin activity in centrilobular liver injury and regeneration. The Wnt/ß-catenin pathway is the master regulator of hepatic metabolic zonation and perivenous gene expression and is modulated by the R-spondin-LGR4/5-ZNRF3/RNF43 module. We examined the structures of the molecules of these pathways and their involvements in liver biology. Central vein-derived Wnts and R-spondin3 participate in the cellular-molecular circuitry of the Wnt/ß-catenin and R-spondin-LGR4/5-ZNRF3/RNF43 module. The transport and secretion of lipidated Wnts in Wnt-producing cells require Wntless protein. Secreted Wnts are carried on exosomes in the extracellular matrix to responder cells. The modes of release of Wnts and R-spondin3 from central veins and their transit in the venular wall toward perivenous hepatocytes are unknown. We hypothesize that central vein fibrosis may impact perivenous gene expression. The proposal that the central vein constitutes an anatomical niche of perivenous stem cells that subserve homeostatic hepatic renewal still needs studies using additional mouse models for validation. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1747-1767, 2020. © 2019 American Association for Anatomy.

Tissue Repair in the Mouse Liver Following Acute Carbon Tetrachloride Depends on Injury-Induced Wnt/β-Catenin Signaling

In the liver, Wnt/β-catenin signaling is involved in regulating zonation and hepatocyte proliferation during homeostasis. We examined Wnt gene expression and signaling after injury, and we show by in situ hybridization that Wnts are activated by acute carbon tetrachloride (CCl₄ ) toxicity. Following injury, peri-injury hepatocytes become Wnt-responsive, expressing the Wnt target gene axis inhibition protein 2 (Axin2). Lineage tracing of peri-injury Axin2⁺ hepatocytes shows that during recovery the injured parenchyma becomes repopulated and repaired by Axin2⁺ descendants. Using single-cell RNA sequencing, we show that endothelial cells are the major source of Wnts following acute CCl₄ toxicity. Induced loss of β-catenin in peri-injury hepatocytes results in delayed repair and ultimately injury-induced lethality, while loss of Wnt production from endothelial cells leads to a delay in the proliferative response after injury. Conclusion: Our findings highlight the importance of the Wnt/β-catenin signaling pathway in restoring tissue integrity following acute liver toxicity and establish a role of endothelial cells as an important Wnt-producing regulator of liver tissue repair following localized liver injury.

Serine protease modulation of Dependence Receptors and EMT protein expression

Expression of the tumour suppressor Deleted in Colorectal Cancer (DCC) and the related protein neogenin is reduced by the mammalian serine protease chymotrypsin or the bacterial serine protease subtilisin, with increased cell migration. The present work examines whether these actions are associated with changes in the expression of cadherins, β-catenin and vimentin, established markers of the Epithelial-Mesenchymal Transition (EMT) which has been linked with cell migration and tumour metastasis. The results confirm the depletion of DCC and neogenin and show that chymotrypsin and subtilisin also reduce expression of β-catenin in acutely prepared tissue sections but not in human mammary adenocarcinoma MCF-7 or MDA-MB-231 cells cultured in normal media, or primary normal human breast cells. A loss of β-catenin was also seen in low serum media but transfecting cells with a dcc-containing plasmid induced resistance. E-cadherin was not consistently affected but vimentin was induced by low serum-containing media and was increased by serine proteases in MCF-7 and MDA-MB-231 cells in parallel with increased wound closure. Vimentin might contribute to the promotion of cell migration. The results suggest that changes in EMT proteins depend on the cells or tissues concerned and do not parallel the expression of DCC and neogenin. The increased cell migration induced by serine proteases is not consistently associated with the expression of the EMT proteins implying either that the increased migration may be independent of EMT or supporting the view that EMT is not itself consistently related to migration. (241).

Endothelial Wnts regulate β-catenin signaling in murine liver zonation and regeneration: A sequel to the Wnt-Wnt situation

β-Catenin in hepatocytes, under the control of Wnts, regulates pericentral gene expression. It also contributes to liver regeneration (LR) after partial hepatectomy (PH) by regulating cyclin-D1 gene expression as shown in the β-catenin and Wnt coreceptors low-density lipoprotein receptor-related protein 5/6 conditional knockouts (KO). However, conditional deletion of Wntless (Wls), required for Wnt secretion, in hepatocytes, cholangiocytes, or macrophages lacked any impact on zonation, while Wls deletion in macrophages only marginally affected LR. Here, we address the contribution of hepatic endothelial cells (ECs) in zonation and LR by characterizing EC-Wls-KO generated by interbreeding Wls-floxed and lymphatic vessel endothelial hyaluronan receptor (Lyve1)-cre mice. These mice were also used to study LR after PH. While Lyve1 expression in adult liver is limited to sinusoidal ECs only, Lyve1-cre mice bred to ROSA26-Stopflox/flox-enhanced yellow fluorescent protein (EYFP) mice showed EYFP labeling in sinusoidal and central vein ECs. EC-Wls-KO mice showed decreased liver weights; lacked glutamine synthetase, cytochrome P450 2e1, and cytochrome P450 1a2; and were resistant to acetaminophen-induced liver injury. After PH, EC-Wls-KO showed quantitative and qualitative differences in cyclin-D1 expression at 24-72 hours, which led to a lower hepatocyte proliferation at 40 hours but a rebound increase by 72 hours. ECs and macrophages isolated from regenerating livers at 12 hours showed significant up-regulation of Wnt2 and Wnt9b messenger RNA; these are the same two Wnts involved in baseline β-catenin activity in pericentral hepatocytes. Conclusion: At baseline, ECs secrete Wnt proteins essential for β-catenin activation in pericentral hepatocytes. During LR, sinusoidal and central vein ECs and secondarily macrophages secrete Wnt2, while predominantly central vein ECs and secondarily macrophages are the likely source of Wnt9b. This process spatiotemporally regulates β-catenin activation in hepatocytes to induce cell proliferation. (Hepatology Communications 2018;2:845-860).

Autonomic nervous system network and liver regeneration

To date, various signal transducers, cytokines, growth factors, and hormones have been reported to play an important role in homeostasis of various organs. Various cells and organs are involved in the hepatic regeneration process, which proceeds as a result of the coordination of many factors. While these factors are well known to be involved in the liver regeneration after the liver injury, however, as the details of such mechanisms have not been sufficiently elucidated, the practical applicability of hepatic regeneration based on the action of these and cytokines growth factors is still unclear. In terms of the involvement of the autonomic nervous system in hepatic regeneration, cell proliferation resulting from direct signal transduction to the liver has also been reported and recent studies focusing on the inter-organ communication via neural network opened a novel aspect of this field for therapeutic applicability. Therefore, the appropriate understanding of the relationship between autonomic neural network and liver regeneration through various organs including brain, afferent nerve, efferent nerve, etc. is essential. This mini-review explains the principle of neural system involved in the inter-organ communication and its contribution on the liver regeneration upon the liver injury reviewing recent progress in this field.

Hepatocyte specific expression of an oncogenic variant of β-catenin results in cholestatic liver disease

BACKGROUND

The Wnt/β-catenin signaling pathway plays a crucial role in embryonic development, tissue homeostasis, wound healing and malignant transformation in different organs including the liver. The consequences of continuous β-catenin signaling in hepatocytes remain elusive.

RESULTS

Livers of Ctnnb1CA hep mice were characterized by disturbed liver architecture, proliferating cholangiocytes and biliary type of fibrosis. Serum ALT and bile acid levels were significantly increased in Ctnnb1CA hep mice. The primary bile acid synthesis enzyme Cyp7a1 was increased whereas Cyp27 and Cyp8b1 were reduced in Ctnnb1CA hep mice. Expression of compensatory bile acid transporters including Abcb1, Abcb4, Abcc2 and Abcc4 were significantly increased in Ctnnb1CA hep mice while Ntcp was reduced. Accompanying changes of bile acid transporters favoring excretion of bile acids were observed in intestine and kidneys of Ctnnb1CA hep mice. Additionally, disturbed bile acid regulation through the FXR-FGF15-FGFR4 pathway was observed in mice with activated β-catenin.

MATERIALS AND METHODS

Mice with a loxP-flanked exon 3 of the Ctnnb1 gene were crossed to Albumin-Cre mice to obtain mice with hepatocyte-specific expression of a dominant stable form of β-catenin (Ctnnb1CA hep mice). Ctnnb1CA hep mice were analyzed by histology, serum biochemistry and mRNA profiling.

CONCLUSIONS

Expression of a dominant stable form of β-catenin in hepatocytes results in severe cholestasis and biliary type fibrosis.

Effect of a neural relay on liver regeneration in mice: activation of serotonin release from the gastrointestinal tract

The development of therapeutic options to promote hepatic regeneration following severe liver injury is essential. While humoral factors have been reported as mechanisms of liver regeneration, the contributions of interorgan communication to liver regeneration have not been reported. In this study, we examined the effect of a neural relay on liver regeneration via activation of serotonin release from the gastrointestinal (GI) tract. Our results demonstrated that the afferent visceral nerve from the liver activates the efferent vagus nerve from the brain, leading to activation of serotonin release from the GI tract and contributing to liver regeneration. While it is difficult to apply these results directly to human health, we believe that this study may represent a step toward developing essential therapeutics to promote liver regeneration.

Deregulation of Frizzled Receptors in Hepatocellular Carcinoma

G protein-coupled receptors (GPCRs) have a substantial role in tumorigenesis and are described as a “cancer driver”. Aberrant expression or activation of GPCRs leads to the deregulation of downstream signaling pathways, thereby promoting cancer progression. In hepatocellular carcinoma (HCC), the Wnt signaling pathway is frequently activated and it is associated with an aggressive HCC phenotype. Frizzled (FZD) receptors, a family member of GPCRs, are known to mediate Wnt signaling. Accumulating findings have revealed the deregulation of FZD receptors in HCC and their functional roles have been implicated in HCC progression. Given the important role of FZD receptors in HCC, we summarize here the expression pattern of FZD receptors in HCC and their corresponding functional roles during HCC progression. We also further review and highlight the potential targeting of FZD receptors as an alternative therapeutic strategy in HCC.

Heterogeneity of Stem Cells: A Brief Overview

Stem cells possess the extraordinary capacity of self-renewal and differentiation to various cell types, thus to form original tissues and organs. Stem cell heterogeneity including genetic and nongenetic mechanisms refers to biological differences amongst normal and stem cells originated within the same tissue. Cell differentiation hierarchy and stochasticity in gene expression and signaling pathways may result in phenotypic differences of stem cells. The maintenance of stemness and activation of differentiation potential are fundamentally orchestrated by microenvironmental stem cell niche-related cellular and humoral signals.

Wnt Signaling in Adult Epithelial Stem Cells and Cancer

Wnt/β-catenin signaling is integral to the homeostasis and regeneration of many epithelial tissues due to its critical role in adult stem cell regulation. It is also implicated in many epithelial cancers, with mutations in core pathway components frequently present in patient tumors. In this chapter, we discuss the roles of Wnt/β-catenin signaling and Wnt-regulated stem cells in homeostatic, regenerative and cancer contexts of the intestines, stomach, skin, and liver. We also examine the sources of Wnt ligands that form part of the stem cell niche. Despite the diversity in characteristics of various tissue stem cells, the role(s) of Wnt/β-catenin signaling is generally coherent in maintaining stem cell fate and/or promoting proliferation. It is also likely to play similar roles in cancer stem cells, making the pathway a salient therapeutic target for cancer. While promising progress is being made in the field, deeper understanding of the functions and signaling mechanisms of the pathway in individual epithelial tissues will expedite efforts to modulate Wnt/β-catenin signaling in cancer treatment and tissue regeneration.

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.

Wnt ligand presentation and reception: from the stem cell niche to tissue engineering

Stem cells reside in niches where spatially restricted signals maintain a delicate balance between stem cell self-renewal and differentiation. Wnt family proteins are particularly suited for this role as they are modified by lipids, which constrain and spatially regulate their signalling range. In recent years, Wnt/β-catenin signalling has been shown to be essential for the self-renewal of a variety of mammalian stem cells. In this review, we discuss Wnt-responsive stem cells in their niche, and mechanisms by which Wnt ligands are presented to responsive cells. We also highlight recent progress in molecular visualization that has allowed for the monitoring of Wnt signalling within the stem cell compartment and new approaches to recapitulate this niche signalling in vitro Indeed, new technologies that present Wnt in a localized manner and mimic the three-dimensional microenvironment of stem cells will advance our understanding of Wnt signalling in the stem cell niche. These advances will expand current horizons to exploit Wnt ligands in the rapidly evolving fields of tissue engineering and regenerative medicine.

Role and Regulation of p65/β-Catenin Association During Liver Injury and Regeneration: A "Complex" Relationship

An important role for β-catenin in regulating p65 (a subunit of NF-κB) during acute liver injury has recently been elucidated through use of conditional β-catenin knockout mice, which show protection from apoptosis through increased activation of p65. Thus, we hypothesized that the p65/β-catenin complex may play a role in regulating processes such as cell proliferation during liver regeneration. We show through in vitro and in vivo studies that the p65/β-catenin complex is regulated through the TNF-α pathway and not through Wnt signaling. However, this complex is unchanged after partial hepatectomy (PH), despite increased p65 and β-catenin nuclear translocation as well as cyclin D1 activation. We demonstrate through both in vitro silencing experiments and chromatin immunoprecipitation after PH that β-catenin, and not p65, regulates cyclin D1 expression. Conversely, using reporter mice we show p65 is activated exclusively in the nonparenchymal (NPC) compartment during liver regeneration. Furthermore, stimulation of macrophages by TNF-α induces activation of NF-κB and subsequent secretion of Wnts essential for β-catenin activation in hepatocytes. Thus, we show that β-catenin and p65 are activated in separate cellular compartments during liver regeneration, with p65 activity in NPCs contributing to the activation of hepatocyte β-catenin, cyclin D1 expression, and subsequent proliferation.

Inhibition of Glycogen Synthase Kinase 3 Accelerated Liver Regeneration after Acetaminophen-Induced Hepatotoxicity in Mice

Overdose of acetaminophen (APAP) is the leading cause of acute liver failure (ALF) in the United States. Timely initiation of compensatory liver regeneration after APAP hepatotoxicity is critical for final recovery, but the mechanisms of liver regeneration after APAP-induced ALF have not been extensively explored yet. Previous studies from our laboratory have demonstrated that activation of β-catenin signaling after APAP overdose is associated with timely liver regeneration. Herein, we investigated the role of glycogen synthase kinase 3 (GSK3) in liver regeneration after APAP hepatotoxicity using a pharmacological inhibition strategy in mice. Treatment with specific GSK3 inhibitor (L803-mts), starting from 4 hours after 600 mg/kg dose of APAP, resulted in early initiation of liver regeneration in a dose-dependent manner, without modifying the peak regenerative response. Acceleration of liver regeneration was not secondary to alteration of APAP-induced hepatotoxicity, which remained unchanged after GSK3 inhibition. Early cell cycle initiation in hepatocytes after GSK3 inhibition was because of rapid induction of cyclin D1 and phosphorylation of retinoblastoma protein. This was associated with increased activation of β-catenin signaling after GSK3 inhibition. Taken together, our study has revealed a novel role of GSK3 in liver regeneration after APAP overdose and identified GSK3 as a potential therapeutic target to improve liver regeneration after APAP-induced ALF.

The RSPO-LGR4/5-ZNRF3/RNF43 module controls liver zonation and size

LGR4/5 receptors and their cognate RSPO ligands potentiate Wnt/β-catenin signalling and promote proliferation and tissue homeostasis in epithelial stem cell compartments. In the liver, metabolic zonation requires a Wnt/β-catenin signalling gradient, but the instructive mechanism controlling its spatiotemporal regulation is not known. We have now identified the RSPO-LGR4/5-ZNRF3/RNF43 module as a master regulator of Wnt/β-catenin-mediated metabolic liver zonation. Liver-specific LGR4/5 loss of function (LOF) or RSPO blockade disrupted hepatic Wnt/β-catenin signalling and zonation. Conversely, pathway activation in ZNRF3/RNF43 LOF mice or with recombinant RSPO1 protein expanded the hepatic Wnt/β-catenin signalling gradient in a reversible and LGR4/5-dependent manner. Recombinant RSPO1 protein increased liver size and improved liver regeneration, whereas LGR4/5 LOF caused the opposite effects, resulting in hypoplastic livers. Furthermore, we show that LGR4(+) hepatocytes throughout the lobule contribute to liver homeostasis without zonal dominance. Taken together, our results indicate that the RSPO-LGR4/5-ZNRF3/RNF43 module controls metabolic liver zonation and is a hepatic growth/size rheostat during development, homeostasis and regeneration.

Wnt agonist stimulates liver regeneration after small-for-size liver transplantation in rats

AIM

Liver regeneration is inhibited in small-for-size grafts, which plays a role in the failure of partial liver grafts after transplantation. The Wnt/β-catenin signaling pathway plays a critical role in liver development, regeneration and homeostasis. In this study, we investigated whether pharmacological activation of Wnt signaling improves liver regeneration after small-for-size liver transplantation.

METHODS

The livers of male Sprague-Dawley rats were reduced to approximately 50% and 30% of their original sizes and transplanted. A Wnt agonist (2-amino-4-[3,4-[methylenedioxy]benzylamino]-6-[3-methoxyphenyl] pyrimidine], 5 mg/kg bodyweight) or an equal volume of vehicle was administrated i.p. into the donor 1 h before the transplantation. Tissue and blood samples were collected at various times after transplantation, and a survival study was performed.

RESULTS

Hepatic expression of active β-catenin and its downstream target gene Axin2 were decreased in 30% of liver grafts after transplantation while the Wnt agonist increased their expression similar to the 50% liver grafts. The Wnt agonist reversed inhibition of cyclin D1 expression and adenosine triphosphate production in the 30% liver grafts compared with the 50% grafts. The Wnt agonist also attenuated hepatocellular injury and increased the hepatocyte proliferation response, liver regeneration rate and survival after transplantation of the 30% liver graft.

CONCLUSION

Activation of Wnt/β-catenin signaling in liver grafts by pharmacological pretreatment can accelerate regeneration in a partial liver transplant model.

Xenosensor CAR mediates down-regulation of miR-122 and up-regulation of miR-122 targets in the liver

MiR-122 is a major hepatic microRNA, accounting for more than 70% of the total liver miRNA population. It has been shown that miR-122 is associated with liver diseases, including hepatocellular carcinoma. Mir-122 is an intergenic miRNA with its own promoter. Pri-miR-122 expression is regulated by liver-enriched transcription factors, mainly by HNF4α, which mediates the expression via the interaction with a specific DR1 site. It has been shown that phenobarbital-mediated activation of constitutive androstane receptor (CAR), xenobiotic nuclear receptor, is associated with a decrease in miR-122 in the liver. In the present study, we investigated HNF4α-CAR cross-talk in the regulation of miR-122 levels and promitogenic signalling in mouse livers. The level of miR-122 was significantly repressed by treatment with 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), which is an agonist of mouse CAR. ChIP assays demonstrated that TCPOBOP-activated CAR inhibited HNF4α transactivation by competing with HNF4α for binding to the DR1 site in the pri-miR-122 promoter. Such transcription factor replacement was strongly correlated with miR-122 down-regulation. Additionally, the decrease in miR-122 levels produced by CAR activation is accompanied by an increase in mRNA and cellular protein levels of E2f1 and its accumulation on the target cMyc gene promoter. The increase in accumulation of E2f1 on the target cMyc gene promoter is accompanied by an increase in cMyc levels and transcriptional activity. Thus, our results provide evidence to support the conclusion that CAR activation decreases miR-122 levels through suppression of HNF4α transcriptional activity and indirectly regulates the promitogenic protein cMyc. HNF4α-CAR cross-talk may provide new opportunities for understanding liver diseases and developing more effective therapeutic approaches to better drug treatments.

Restoration of Wnt/β-catenin signaling attenuates alcoholic liver disease progression in a rat model

BACKGROUND & AIMS

Alcoholic liver disease (ALD) is characterized by the development of fatty liver, alcoholic hepatitis, fibrosis and cirrhosis. However, the underlying mechanism(s) associated with progression remains elusive. Pro-inflammatory cytokines have been implicated in ALD progression due to pro-apoptotic effects on hepatocytes. Wnt/β-catenin signaling recently has been shown to promote inflammation and apoptosis, suggesting that activation of this signaling pathway may modulate ALD progression. The current study was designed to test whether pharmacological activation of Wnt/β-catenin signaling altered ALD development and progression in a rat model.

METHODS

Adult male Long Evans rats were fed with isocaloric liquid diets containing 0% or 37% ethanol for 8 weeks, and also treated with Wnt agonist during the last 3 weeks of the feeding regimen. Liver and blood samples were subjected to histology, TUNEL assay, immunoblot analysis, real-time quantitative PCR, and alanine transaminase (ALT) assay.

RESULTS

Wnt/β-catenin signaling was negatively correlated with Foxo3A expression and reduced steatosis, cellular injury and apoptosis in ALD rats. Mutation experiments demonstrated that Foxo3A was critical for modulating these effects. Activation of Wnt/β-catenin signaling suppressed Foxo3A-induced apoptosis through upregulation of serum/glucocorticoid regulated kinase 1 (SGK1). Moreover, pharmacological restoration of Wnt/β-catenin signaling reduced ALD progression in vivo.

CONCLUSIONS

Wnt/β-catenin signaling plays a protective role in ALD progression via antagonizing Foxo3A-induced apoptosis, and activation of the Wnt/β-catenin signaling cascade attenuates ALD progression.