Liver zonation occurs through a beta-catenin-dependent, c-Myc-independent mechanism

Binucleated human hepatocytes arise through late cytokinetic regression during endomitosis M phase

Binucleated polyploid cells are common in many animal tissues, where they arise by endomitosis, a non-canonical cell cycle in which cells enter M phase but do not undergo cytokinesis. Different steps of cytokinesis have been shown to be inhibited during endomitosis M phase in rodents, but it is currently unknown how human cells undergo endomitosis. In this study, we use fetal-derived human hepatocyte organoids (Hep-Orgs) to investigate how human hepatocytes initiate and execute endomitosis. We find that cells in endomitosis M phase have normal mitotic timings, but lose membrane anchorage to the midbody during cytokinesis, which is associated with the loss of four cortical anchoring proteins, RacGAP1, Anillin, SEPT9, and citron kinase (CIT-K). Moreover, reduction of WNT activity increases the percentage of binucleated cells in Hep-Orgs, an effect that is dependent on the atypical E2F proteins, E2F7 and E2F8. Together, we have elucidated how hepatocytes undergo endomitosis in human Hep-Orgs, providing new insights into the mechanisms of endomitosis in mammals.

Neutrophil and macrophage crosstalk might be a potential target for liver regeneration

The regenerative capability of the liver is remarkable, but further research is required to understand the role that neutrophils play in this process. In the present study, we reanalyzed single-cell RNA sequencing data from a mouse partial hepatectomy (PH) model to track the transcriptional changes in hepatocytes and non-parenchymal cells. Notably, we unraveled the regenerative capacity of hepatocytes at diverse temporal points after PH, unveiling the contributions of three distinct zones in the liver regeneration process. In addition, we observed that the depletion of neutrophils reduced the survival and liver volume after PH, confirming the important role of neutrophils in liver regeneration. CellChat analysis revealed an intricate crosstalk between neutrophils and macrophages promoting liver regeneration and, using weighted gene correlation network analysis, we identified the most significant genetic module associated with liver regeneration. Our study found that hepatocytes in the periportal zone of the liver are more active than in other zones, suggesting that the crosstalk between neutrophils and macrophages might be a potential target for liver regeneration treatment.

Emerging Roles of Spatial Transcriptomics in Liver Research

Spatial transcriptomics, leveraging sequencing- and imaging-based techniques, has emerged as a groundbreaking technology for mapping gene expression within the complex architectures of tissues. This approach provides an in-depth understanding of cellular and molecular dynamics across various states of healthy and diseased livers. Through the integration of sophisticated bioinformatics strategies, it enables detailed exploration of cellular heterogeneity, transitions in cell states, and intricate cell-cell interactions with remarkable precision. In liver research, spatial transcriptomics has been particularly revelatory, identifying distinct zonated functions of hepatocytes that are crucial for understanding the metabolic and detoxification processes of the liver. Moreover, this technology has unveiled new insights into the pathogenesis of liver diseases, such as the role of lipid-associated macrophages in steatosis and endothelial cell signals in liver regeneration and repair. In the domain of liver cancer, spatial transcriptomics has proven instrumental in delineating intratumor heterogeneity, identifying supportive microenvironmental niches and revealing the complex interplay between tumor cells and the immune system as well as susceptibility to immune checkpoint inhibitors. In conclusion, spatial transcriptomics represents a significant advance in hepatology, promising to enhance our understanding and treatment of liver diseases.

Strategic Replication of the Hepatic Zonation In Vitro Employing a Biomimetic Approach

The varied functions of the liver are dependent on the metabolic heterogeneity exhibited by the hepatocytes within the liver lobule spanning the porto-central axis. This complex phenomenon plays an important role in maintaining the physiological homeostasis of the liver. Standard in vitro culture models fail to mimic this spatial heterogeneity of hepatocytes, assuming a homogeneous population of cells, which leads to inaccurate translation of results. Here, we demonstrate the development of an in vitro model of hepatic zonation by mimicking the microarchitecture of the liver using a 3D printed mini bioreactor and decellularized liver matrix to provide the native microenvironmental cues. There was a differential expression of hypoxic and metabolic markers across the developed mini bioreactor, showing the establishment of gradients of oxygen, Wnt/β-catenin pathway, and other metabolic pathways. The model also showed the establishment of zone-dependent toxicity on treatment with acetaminophen. The developed model would thus be a promising avenue in the field of tissue engineering for understanding the liver physiology and pathophysiology and for drug screening to evaluate the potential of new pharmaceutical interventions.

Extracellular matrix modulates the spatial hepatic features in hepatocyte-like cells derived from human embryonic stem cells

BACKGROUND

Human pluripotent stem cell (hPSC)-derived hepatocyte-like cells (HLCs) can provide a valuable in vitro model for disease modelling and drug development. However, generating HLCs with characteristics comparable to hepatocytes in vivo is challenging. Extracellular matrix (ECM) plays an important role in supporting liver development and hepatocyte functions, but their impact on hepatocyte differentiation and maturation during hPSC differentiation remains unclear. Here, we investigate the effects of two ECM components-Matrigel and type I collagen on hepatic differentiation of human embryonic stem cells (hESCs).

METHODS

hESC-derived HLCs were generated through multistage differentiation in two-dimensional (2D) and three-dimensional (3D) cultures, incorporating either type I collagen or Matrigel during hepatic specification and maturation. The resulting HLCs was characterized for their gene expression and functionality using various molecular and cellular techniques.

RESULTS

Our results showed that HLCs cultured with collagen exhibited a significant increase in albumin and alpha-1 anti-trypsin expression with reduced AFP compared to HLCs cultured with Matrigel. They also secreted more urea than Matrigel cultures. However, these HLCs exhibited lower CYP3A4 activity and glycogen storage than those cultured with Matrigel. These functional differences in HLCs between collagen and Matrigel cultures closely resembled the hepatocytes of periportal and pericentral zones, respectively.

CONCLUSION

Our study demonstrates that Matrigel and collagen have differential effects on the differentiation and functionality of HLCs, which resemble, to an extent, hepatic zonation in the liver lobules. Our finding has an important impact on the generation of hPSC-HLCs for biomedical and medical applications.

A Mouse Model for the Rapid and Binomial Assessment of Putative WNT/β-Catenin Signalling Inhibitors

Specific signalling thresholds of the WNT/β-catenin pathway affect embryogenesis and tissue homeostasis in the adult, with mutations in this pathway frequently occurring in cancer. Excessive WNT/β-catenin activity inhibits murine anterior development associated with embryonic lethality and accounts for the driver event in 80% of human colorectal cancers. Uncontrolled WNT/β-catenin signalling arises primarily from impairment mutation in the tumour suppressor gene APC that otherwise prevents prolonged stabilisation of β-catenin. Surprisingly, no inhibitor compounds for WNT/β-catenin signalling have reached clinical use in part owing to the lack of specific in vivo assays that discriminate between on-target activities and dose-limiting toxicities. Here, we present a simple in vivo assay with a binary outcome whereby the administration of candidate compounds to pregnant and phenotypically normal Apcflox/flox mice can rescue in utero death of Apcmin/flox mutant conceptus without subsequent post-mortem assessment of WNT/β-catenin signalling. Indeed, the phenotypic plasticity of born Apcmin/flox conceptus enables future refinement of our assay to potentially enable dosage finding and cross-compound comparisons. Thus, we show for the first time the suitability of endogenous WNT/β-catenin signalling during embryonic development to provide an unambiguous and sensitive mammalian in vivo model to assess the efficacy and bioavailability of potential WNT/β-catenin antagonists.

Wnt Signaling Stimulates Cooperation between GREB1 and HNF4α to Promote Proliferation in Hepatocellular Carcinoma

Wnt signaling is known to maintain two cell states, hepatocyte differentiation and proliferation, in hepatocellular carcinoma (HCC). On the other hand, activation of Wnt signaling in colon cancer promotes uncontrollable stereotypic proliferation, whereas cells remain undifferentiated. To elucidate the unique mode of Wnt signaling in HCC, we comprehensively investigated HCC-specific Wnt pathway target genes and identified GREB1. Wnt signaling induced expression of GREB1 coupled with HNF4α and FOXA2, master transcription factors that maintain hepatic differentiation. Moreover, GREB1 was enriched at the regulatory region of atypical HNF4α target genes, including progrowth genes, thereby stimulating HCC proliferation. Therefore, GREB1 acts as a unique mediator of versatile Wnt signaling in HCC progression, bridging the roles of the Wnt pathway in differentiation and proliferation.

SIGNIFICANCE

GREB1 is a liver cancer-specific Wnt signaling target gene that induces an oncogenic shift of HNF4α, a putative tumor suppressor, and may represent a therapeutic target in Wnt-activated hepatocellular carcinoma.

Temporal Dynamics of Metabolic Acquisition in Grafted Engineered Human Liver Tissue

Liver disease affects millions globally, and end-stage liver failure is only cured by organ transplant. Unfortunately, there is a growing shortage of donor organs as well as inequitable access to transplants across populations. Engineered liver tissue grafts that supplement or replace native organ function can address this challenge. While engineered liver tissues have been successfully engrafted previously, the extent to which these tissues express human liver metabolic genes and proteins remains unknown. Here, it is built engineered human liver tissues and characterized their engraftment, expansion, and metabolic phenotype at sequential stages post-implantation by RNA sequencing, histology, and host serology. Expression of metabolic genes is observed at weeks 1-2, followed by the cellular organization into hepatic cords by weeks 4-9.5. Furthermore, grafted engineered tissues exhibited progressive spatially restricted expression of critical functional proteins known to be zonated in the native human liver. This is the first report of engineered human liver tissue zonation after implantation in vivo, which can have important translational implications for this field.

Single-cell transcriptomics shows dose-dependent disruption of hepatic zonation by TCDD in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dose-dependently induces the development of hepatic fat accumulation and inflammation with fibrosis in mice initially in the portal region. Conversely, differential gene and protein expression is first detected in the central region. To further investigate cell-specific and spatially resolved dose-dependent changes in gene expression elicited by TCDD, single-nuclei RNA sequencing and spatial transcriptomics were used for livers of male mice gavaged with TCDD every 4 days for 28 days. The proportion of 11 cell (sub)types across 131 613 nuclei dose-dependently changed with 68% of all portal and central hepatocyte nuclei in control mice being overtaken by macrophages following TCDD treatment. We identified 368 (portal fibroblasts) to 1339 (macrophages) differentially expressed genes. Spatial analyses revealed initial loss of portal identity that eventually spanned the entire liver lobule with increasing dose. Induction of R-spondin 3 (Rspo3) and pericentral Apc, suggested dysregulation of the Wnt/β-catenin signaling cascade in zonally resolved steatosis. Collectively, the integrated results suggest disruption of zonation contributes to the pattern of TCDD-elicited NAFLD pathologies.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

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.

The RSPO-LGR4/5-ZNRF3/RNF43 module in liver homeostasis, regeneration, and disease

WNT/β-catenin signaling plays pivotal roles during liver development, homeostasis, and regeneration. Likewise, its deregulation disturbs metabolic liver zonation and is responsible for the development of a large number of hepatic tumors. Liver fibrosis, which has become a major health burden for society and a hallmark of NASH, can also be promoted by WNT/β-catenin signaling. Upstream regulatory mechanisms controlling hepatic WNT/β-catenin activity may constitute targets for the development of novel therapies addressing these life-threatening conditions. The R-spondin (RSPO)-leucine-rich repeat-containing G protein-coupled receptor (LGR) 4/5-zinc and ring finger (ZNRF) 3/ring finger 43 (RNF43) module is fine-tuning WNT/β-catenin signaling in several tissues and is essential for hepatic WNT/β-catenin activity. In this review article, we recapitulate the role of the RSPO-LGR4/5-ZNRF3/RNF43 module during liver development, homeostasis, metabolic zonation, regeneration, and disease. We further discuss the controversy around LGR5 as a liver stem cell marker.

Transcriptional switch of hepatocytes initiates macrophage recruitment and T-cell suppression in endotoxemia

BACKGROUND & AIMS

The liver plays crucial roles in the regulation of immune defense during acute systemic infections. However, the roles of liver cellular clusters and intercellular communication in the progression of endotoxemia have not been well-characterized.

METHODS

Single-cell RNA sequencing analysis was performed, and the transcriptomes of 19,795 single liver cells from healthy and endotoxic mice were profiled. The spatial and temporal changes in hepatocytes and non-parenchymal cell types were validated by multiplex immunofluorescence staining, bulk transcriptomic sequencing, or flow cytometry. Furthermore, we used an adeno-associated virus delivery system to confirm the major mechanisms mediating myeloid cell infiltration and T-cell suppression in septic murine liver.

RESULTS

We identified a proinflammatory hepatocyte (PIH) subpopulation that developed primarily from periportal hepatocytes and to a lesser extent from pericentral hepatocytes and played key immunoregulatory roles in endotoxemia. Multicellular cluster modeling of ligand-receptor interactions revealed that PIHs play a crucial role in the recruitment of macrophages via the CCL2-CCR2 interaction. Recruited macrophages (RMs) released cytokines (e.g., IL6, TNFα, and IL17) to induce the expression of inhibitory ligands, such as PD-L1, on hepatocytes. Subsequently, RM-stimulated hepatocytes led to the suppression of CD4⁺ and memory T-cell subsets partly via the PD-1/PD-L1 interaction in endotoxemia. Furthermore, sinusoidal endothelial cells expressed the highest levels of proapoptotic and inflammatory genes around the periportal zone. This pattern of gene expression facilitated increases in the number of fenestrations and infiltration of immune cells in the periportal zone.

CONCLUSIONS

Our study elucidates unanticipated aspects of the cellular and molecular effects of endotoxemia on liver cells at the single-cell level and provides a conceptual framework for the development of novel therapeutic approaches for acute infection.

LAY SUMMARY

The liver plays a crucial role in the regulation of immune defense during acute systemic infections. We identified a proinflammatory hepatocyte subpopulation and demonstrated that the interactions of this subpopulation with recruited macrophages are pivotal in the immune response during endotoxemia. These novel findings provide a conceptual framework for the discovery of rational therapeutic targets in acute infection.

The Role of Liver Zonation in Physiology, Regeneration, and Disease

As blood flows from the portal triad to the central vein, cell-mediated depletion establishes gradients of soluble factors such as oxygen, nutrients, and hormones, which act through molecular pathways (e.g., Wnt/β-catenin, hedgehog) to spatially regulate hepatocyte functions along the sinusoid. Such "zonation" can lead to the compartmentalized initiation of several liver diseases, including alcoholic/non-alcoholic fatty liver diseases, chemical/drug-induced toxicity, and hepatocellular carcinoma, and can also modulate liver regeneration. Transgenic rodent models provide valuable information on the key molecular regulators of zonation, while in vitro models allow for subjecting cells to precisely controlled factor gradients and elucidating species-specific differences in zonation. Here, we discuss the latest advances in both in vivo and in vitro models of liver zonation and pending questions to be addressed moving forward. Ultimately, obtaining a deeper understanding of zonation can lead to the development of more effective therapeutics for liver diseases, microphysiological systems, and scalable cell-based therapies.

Hepatocyte generation in liver homeostasis, repair, and regeneration

The liver has remarkable capability to regenerate, employing mechanism to ensure the stable liver-to-bodyweight ratio for body homeostasis. The source of this regenerative capacity has received great attention over the past decade yet still remained controversial currently. Deciphering the sources for hepatocytes provides the basis for understanding tissue regeneration and repair, and also illustrates new potential therapeutic targets for treating liver diseases. In this review, we describe recent advances in genetic lineage tracing studies over liver stem cells, hepatocyte proliferation, and cell lineage conversions or cellular reprogramming. This review will also evaluate the technical strengths and limitations of methods used for studies on hepatocyte generation and cell fate plasticity in liver homeostasis, repair and regeneration.

Role and Regulation of Wnt/β-Catenin in Hepatic Perivenous Zonation and Physiological Homeostasis

Metabolic heterogeneity or functional zonation is a key characteristic of the liver that allows different metabolic pathways to be spatially regulated within the hepatic system and together contribute to whole body homeostasis. These metabolic pathways are segregated along the portocentral axis of the liver lobule into three hepatic zones: periportal, intermediate or midzonal, and perivenous. The liver performs complementary or opposing metabolic functions within different hepatic zones while synergistic functions are regulated by overlapping zones, thereby maintaining the overall physiological stability. The Wnt/β-catenin signaling pathway is well known for its role in liver growth, development, and regeneration. In addition, the Wnt/β-catenin pathway plays a fundamental and dominant role in hepatic zonation and signals to orchestrate various functions of liver metabolism and pathophysiology. The β-catenin protein is the central player in the Wnt/β-catenin signaling cascade, and its activation is crucial for metabolic patterning of the liver. However, dysregulation of Wnt/β-catenin signaling is also implicated in different liver pathologies, including those associated with metabolic syndrome. β-Catenin is preferentially localized in the central region of the hepatic lobule surrounding the central vein and regulates multiple functions of this region. This review outlines the role of Wnt/β-catenin signaling pathway in controlling the different metabolic processes surrounding the central vein and its relation to liver homeostasis and dysfunction.

Single-cell transcriptomics reveals opposing roles of Shp2 in Myc-driven liver tumor cells and microenvironment

The mechanisms of Myc-driven liver tumorigenesis are inadequately understood. Herein we show that Myc-driven hepatocellular carcinoma (HCC) is dramatically aggravated in mice with hepatocyte-specific Ptpn11/Shp2 deletion. However, Myc-induced tumors develop selectively from the rare Shp2-positive hepatocytes in Shp2-deficent liver, and Myc-driven oncogenesis depends on an intact Ras-Erk signaling promoted by Shp2 to sustain Myc stability. Despite a stringent requirement of Shp2 cell autonomously, Shp2 deletion induces an immunosuppressive environment, resulting in defective clearance of tumor-initiating cells and aggressive tumor progression. The basal Wnt/β-catenin signaling is upregulated in Shp2-deficient liver, which is further augmented by Myc transfection. Ablating Ctnnb1 suppresses Myc-induced HCC in Shp2-deficient livers, revealing an essential role of β-catenin. Consistently, Myc overexpression and CTNNB1 mutations are frequently co-detected in HCC patients with poor prognosis. These data elucidate complex mechanisms of liver tumorigenesis driven by cell-intrinsic oncogenic signaling in cooperation with a tumor-promoting microenvironment generated by disrupting the specific oncogenic pathway.

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.

A Negative Feedback Loop and Transcription Factor Cooperation Regulate Zonal Gene Induction by 2, 3, 7, 8-Tetrachlorodibenzo-p-Dioxin in the Mouse Liver

The cytochrome P450 (Cyp) proteins Cyp1A1 and Cyp1A2 are strongly induced in the mouse liver by the potent environmental toxicant 2, 3, 7, 8‐tetrachlorodibenzo‐p‐dioxin (TCDD), acting through the aryl hydrocarbon receptor (AHR). The induction of Cyp1A1 is localized within the centrilobular regions of the mouse liver at low doses of TCDD, progressing to pan‐lobular induction at higher doses. Even without chemical perturbation, metabolic functions and associated genes are basally zonated in the liver lobule along the central‐to‐portal axis. To investigate the mechanistic basis of spatially restricted gene induction by TCDD, we have developed a multiscale computational model of the mouse liver lobule with single‐cell resolution. The spatial location of individual hepatocytes in the model was calibrated from previously published high‐resolution images. A systems biology model of the network of biochemical signaling pathways underlying Cyp1A1 and Cyp1A2 induction was then incorporated into each hepatocyte in the model. Model simulations showed that a negative feedback loop formed by binding of the induced Cyp1A2 protein to TCDD, together with cooperative gene induction by the β‐catenin/AHR/TCDD transcription factor complex and β‐catenin, help produce the spatially localized induction pattern of Cyp1A1. Although endogenous WNT regulates the metabolic zonation of many genes, it was not a driver of zonal Cyp1A1 induction in our model. Conclusion: In this work, we used data‐driven computational modeling to identify the mechanistic basis of zonally restricted gene expression induced by the potent and persistent environmental pollutant TCDD. The multiscale model and derived results clarify the mechanisms of dose‐dependent hepatic gene induction responses to TCDD. Additionally, this work contributes to our broader understanding of spatial gene regulation along the liver lobule.

ZNRF3 and RNF43 cooperate to safeguard metabolic liver zonation and hepatocyte proliferation

AXIN2 and LGR5 mark intestinal stem cells (ISCs) that require WNT/β-Catenin signaling for constant homeostatic proliferation. In contrast, AXIN2/LGR5+ pericentral hepatocytes show low proliferation rates despite a WNT/β-Catenin activity gradient required for metabolic liver zonation. The mechanisms restricting proliferation in AXIN2+ hepatocytes and metabolic gene expression in AXIN2+ ISCs remained elusive. We now show that restricted chromatin accessibility in ISCs prevents the expression of β-Catenin-regulated metabolic enzymes, whereas fine-tuning of WNT/β-Catenin activity by ZNRF3 and RNF43 restricts proliferation in chromatin-permissive AXIN2+ hepatocytes, while preserving metabolic function. ZNRF3 deletion promotes hepatocyte proliferation, which in turn becomes limited by RNF43 upregulation. Concomitant deletion of RNF43 in ZNRF3 mutant mice results in metabolic reprogramming of periportal hepatocytes and induces clonal expansion in a subset of hepatocytes, ultimately promoting liver tumors. Together, ZNRF3 and RNF43 cooperate to safeguard liver homeostasis by spatially and temporally restricting WNT/β-Catenin activity, balancing metabolic function and hepatocyte proliferation.

Functional Consequences of Metabolic Zonation in Murine Livers: Insights for an Old Story

BACKGROUND AND AIMS

Zone-dependent differences in expression of metabolic enzymes along the portocentral axis of the acinus are a long-known feature of liver metabolism. A prominent example is the preferential localization of the enzyme, glutamine synthetase, in pericentral hepatocytes, where it converts potentially toxic ammonia to the valuable amino acid, glutamine. However, with the exception of a few key regulatory enzymes, a comprehensive and quantitative assessment of zonal differences in the abundance of metabolic enzymes and, much more important, an estimation of the associated functional differences between portal and central hepatocytes is missing thus far.

APPROACH AND RESULTS

We addressed this problem by establishing a method for the separation of periportal and pericentral hepatocytes that yields sufficiently pure fractions of both cell populations. Quantitative shotgun proteomics identified hundreds of differentially expressed enzymes in the two cell populations. We used zone-specific proteomics data for scaling of the maximal activities to generate portal and central instantiations of a comprehensive kinetic model of central hepatic metabolism (Hepatokin1).

CONCLUSIONS

The model simulations revealed significant portal-to-central differences in almost all metabolic pathways involving carbohydrates, fatty acids, amino acids, and detoxification.

Commensal-driven immune zonation of the liver promotes host defence

The liver connects the intestinal portal vasculature with the general circulation, using a diverse array of immune cells to protect from pathogens that translocate from the gut¹. In liver lobules, blood flows from portal triads that are situated in periportal lobular regions to the central vein via a polarized sinusoidal network. Despite this asymmetry, resident immune cells in the liver are considered to be broadly dispersed across the lobule. This differs from lymphoid organs, in which immune cells adopt spatially biased positions to promote effective host defence^2,3. Here we used quantitative multiplex imaging, genetic perturbations, transcriptomics, infection-based assays and mathematical modelling to reassess the relationship between the localization of immune cells in the liver and host protection. We found that myeloid and lymphoid resident immune cells concentrate around periportal regions. This asymmetric localization was not developmentally controlled, but resulted from sustained MYD88-dependent signalling induced by commensal bacteria in liver sinusoidal endothelial cells, which in turn regulated the composition of the pericellular matrix involved in the formation of chemokine gradients. In vivo experiments and modelling showed that this immune spatial polarization was more efficient than a uniform distribution in protecting against systemic bacterial dissemination. Together, these data reveal that liver sinusoidal endothelial cells sense the microbiome, actively orchestrating the localization of immune cells, to optimize host defence.

The Conundrum of the Pericentral Hepatic Niche: WNT/-Catenin Signaling, Metabolic Zonation, and Many Open Questions

WNT/-catenin signaling promotes stemness, proliferation, and cell fate decisions in various tissue stem cell compartments, which maintain organs with a high turnover of cells (e.g., skin, stomach, and gut). Thus, the -catenin target genes AXIN2 and LGR5 are widely considered as tissue stem cell markers. In contrast, AXIN2 and LGR5 are expressed in pericentral hepatocytes, which do not show overt proliferation during liver homeostasis. Given the low hepatocyte turnover, the liver does not require constant high rates of proliferation, whereas WNT/-catenin signaling is critical for metabolic zonation. Yet, WNT/-catenin pathway upregulation, including AXIN2 and LGR5 induction in hepatocytes throughout the liver, enables hepatocyte regeneration in response to various injuries. In this brief review, I discuss the role of WNT/-catenin signaling in controlling metabolic zonation and the conundrum around pericentral hepatocytes that have been proposed as liver stem cells.

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

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

Generation of metabolically functional hepatocyte-like cells from dedifferentiated fat cells by Foxa2, Hnf4a and Sall1 transduction

Mature adipocyte-derived dedifferentiated fat (DFAT) cells have been identified to possess similar multipotency to mesenchymal stem cells, but a method for converting DFAT cells into hepatocytes was previously unknown. Here, using comprehensive analysis of gene expression profiles, we have extracted three transcription factors, namely Foxa2, Hnf4a and Sall1 (FHS), that can convert DFAT cells into hepatocytes. Hepatogenic induction has converted FHS-infected DFAT cells into an epithelial-like morphological state and promoted the expression of hepatocyte-specific features. Furthermore, the DFAT-derived hepatocyte-like (D-Hep) cells catalyzed the detoxification of several compounds. These results indicate that the transduction of DFAT cells with three genes, which were extracted by comprehensive gene expression analysis, efficiently generated D-Hep cells with detoxification abilities similar to those of primary hepatocytes. Thus, D-Hep cells may be useful as a new cell source for surrogate hepatocytes and may be applied to drug discovery studies, such as hepatotoxicity screening and drug metabolism tests.

Cooperation Between MYC and β-Catenin in Liver Tumorigenesis Requires Yap/Taz

BACKGROUND AND AIMS

Activation of MYC and catenin beta-1 (CTNNB1, encoding β-catenin) can co-occur in liver cancer, but how these oncogenes cooperate in tumorigenesis remains unclear.

APPROACH AND RESULTS

We generated a mouse model allowing conditional activation of MYC and WNT/β-catenin signaling (through either β-catenin activation or loss of APC - adenomatous polyposis coli) upon expression of CRE recombinase in the liver and monitored their effects on hepatocyte proliferation, apoptosis, gene expression profiles, and tumorigenesis. Activation of WNT/β-catenin signaling strongly accelerated MYC-driven carcinogenesis in the liver. Both pathways also cooperated in promoting cellular transformation in vitro, demonstrating their cell-autonomous action. Short-term induction of MYC and β-catenin in hepatocytes, followed by RNA-sequencing profiling, allowed the identification of a "Myc/β-catenin signature," composed of a discrete set of Myc-activated genes whose expression increased in the presence of active β-catenin. Notably, this signature enriched for targets of Yes-associated protein (Yap) and transcriptional coactivator with PDZ-binding motif (Taz), two transcriptional coactivators known to be activated by WNT/β-catenin signaling and to cooperate with MYC in mitogenic activation and liver transformation. Consistent with these regulatory connections, Yap/Taz accumulated upon Myc/β-catenin activation and were required not only for the ensuing proliferative response, but also for tumor cell growth and survival. Finally, the Myc/β-catenin signature was enriched in a subset of human hepatocellular carcinomas characterized by comparatively poor prognosis.

CONCLUSIONS

Myc and β-catenin show a strong cooperative action in liver carcinogenesis, with Yap and Taz serving as mediators of this effect. These findings warrant efforts toward therapeutic targeting of Yap/Taz in aggressive liver tumors marked by elevated Myc/β-catenin activity.

The Canonical Wnt Pathway as a Key Regulator in Liver Development, Differentiation and Homeostatic Renewal

The canonical Wnt (Wnt/β-catenin) signalling pathway is highly conserved and plays a critical role in regulating cellular processes both during development and in adult tissue homeostasis. The Wnt/β-catenin signalling pathway is vital for correct body patterning and is involved in fate specification of the gut tube, the primitive precursor of liver. In adults, the Wnt/β-catenin pathway is increasingly recognised as an important regulator of metabolic zonation, homeostatic renewal and regeneration in response to injury throughout the liver. Herein, we review recent developments relating to the key role of the pathway in the patterning and fate specification of the liver, in the directed differentiation of pluripotent stem cells into hepatocytes and in governing proliferation and zonation in the adult liver. We pay particular attention to recent contributions to the controversy surrounding homeostatic renewal and proliferation in response to injury. Furthermore, we discuss how crosstalk between the Wnt/β-catenin and Hedgehog (Hh) and hypoxia inducible factor (HIF) pathways works to maintain liver homeostasis. Advancing our understanding of this pathway will benefit our ability to model disease, screen drugs and generate tissue and organ replacements for regenerative medicine.

Visualization and isolation of zone-specific murine hepatocytes that maintain distinct cytochrome P450 oxidase expression in primary culture

Parenchymal hepatocytes are responsible for most of the metabolic functions of the liver, but exhibit distinct functional properties depending on their localization within the hepatic lobule. Cytochrome P450 oxidases represent a family of drug-metabolizing enzymes, which are expressed predominantly in hepatocytes localized in the centrilobular area (zone 3). The present study describes a unique transgenic mouse strain that distinguishes zone 3 hepatocytes from periportal zone 1 hepatocytes by the intensity of EGFP fluorescence. Both zone 1 and zone 3 hepatocytes isolated from these mice showed the same zone-specific gene expression patterns as in liver tissue in vivo. Experiments using primary cultures of hepatocytes indicated that a combination of low oxygen concentration and activation of Wnt/β-catenin signaling maintained the expression of zone 3-specific P450 drug-metabolizing enzymes, which was characterized by their susceptibility to acetaminophen-induced mitochondrial dysfunction. These zone-specific hepatocytes provide a useful system in the research area of liver pathophysiology and drug development.

The Hepatitis B Virus Pre-Core Protein p22 Activates Wnt Signaling

An emerging theme for Wnt-addicted cancers is that the pathway is regulated at multiple steps via various mechanisms. Infection with hepatitis B virus (HBV) is a major risk factor for liver cancer, as is deregulated Wnt signaling, however, the interaction between these two causes is poorly understood. To investigate this interaction, we screened the effect of the various HBV proteins for their effect on Wnt/β-catenin signaling and identified the pre-core protein p22 as a novel and potent activator of TCF/β-catenin transcription. The effect of p22 on TCF/β-catenin transcription was dose dependent and inhibited by dominant-negative TCF4. HBV p22 activated synthetic and native Wnt target gene promoter reporters, and TCF/β-catenin target gene expression in vivo. Importantly, HBV p22 activated Wnt signaling on its own and in addition to Wnt or β-catenin induced Wnt signaling. Furthermore, HBV p22 elevated TCF/β-catenin transcription above constitutive activation in colon cancer cells due to mutations in downstream genes of the Wnt pathway, namely APC and CTNNB1. Collectively, our data identifies a previously unappreciated role for the HBV pre-core protein p22 in elevating Wnt signaling. Understanding the molecular mechanisms of p22 activity will provide insight into how Wnt signaling is fine-tuned in cancer.

Metabolic and non-metabolic liver zonation is established non-synchronously and requires sinusoidal Wnts

The distribution of complementary metabolic functions in hepatocytes along a portocentral axis is called liver zonation. Endothelial secreted Wnt ligands maintain metabolic zonation in the adult murine liver but whether those ligands are necessary to initiate zonation in the immature liver has been only partially explored. Also, numerous non-metabolic proteins display zonated expression in the adult liver but it is not entirely clear if their localization requires endothelial Wnts. Here we used a novel transgenic mouse model to compare the spatial distribution of zonated non-metabolic proteins with that of typical zonated metabolic enzymes during liver maturation and after acute injury induced by carbon tetrachloride (CCl₄). We also investigated how preventing Wnt ligand secretion from endothelial cells affects zonation patterns under homeostasis and after acute injury. Our study demonstrates that metabolic and non-metabolic zonation are established non-synchronously during maturation and regeneration and require multiple endothelial Wnt sources.

Cellular and molecular basis of liver regeneration

Recent advances in genetics and genomics have reinvigorated the field of liver regeneration. It is now possible to combine lineage-tracing with genome-wide studies to genetically mark individual liver cells and their progenies and detect precise changes in their genome, transcriptome, and proteome under normal versus regenerative settings. The recent use of single-cell RNA sequencing methodologies in model organisms has, in some ways, transformed our understanding of the cellular and molecular biology of liver regeneration. Here, we review the latest strides in our knowledge of general principles that coordinate regeneration of the liver and reflect on some conflicting evidence and controversies surrounding this topic. We consider the prominent mechanisms that stimulate homeostasis-related vis-à-vis injury-driven regenerative responses, highlight the likely cellular sources/depots that reconstitute the liver following various injuries and discuss the extrinsic and intrinsic signals that direct liver cells to proliferate, de-differentiate, or trans-differentiate while the tissue recovers from acute or chronic damage.

Precision pathology analysis of the development and progression of hepatocellular carcinoma: Implication for precision diagnosis of hepatocellular carcinoma

Outcomes for patients with hepatocellular carcinoma (HCC) remain poor because the condition is often unresponsive to the available treatments. Consequently, the early and precise diagnosis of HCC is crucial to achieve improvements in prognosis. For patients with chronic liver disease, the assessment of liver fibrosis is also important to ascertain both the staging of fibrosis and the risk of HCC occurrence. Early HCC was first described in 1991 in Japan and was defined internationally in 2009. As the concept of early HCC spread, the multistage hepatocarcinogenesis process became accepted. Consequently, improvements in imaging technology made the early diagnosis of HCC possible. At present, the most appropriate therapeutic strategy for HCC is determined using an integrated staging system that assesses the tumor burden, the degree of liver dysfunction and the patient performance status; however, pathological and molecular features are not taken into account. The recent introduction of several new therapeutic agents will change the treatment strategy for HCC. Against this background, HCC subclassification based on tumor cellular and microenvironmental characteristics will become increasingly important. In this review, we give an overview of how pathological analysis contributes to understanding the development and progression of HCC and establishing a precision diagnosis of HCC.

[Generation of Zone-specific Hepatocyte-like Cells from Human Induced Pluripotent Stem Cells for Accurate Prediction of Drug-induced Hepatotoxicity]

Human induced pluripotent stem (iPS) cell-derived hepatocyte-like cells (iPS-HLCs) are expected to be applicable to large-scale in vitro hepatotoxicity screening systems. Accordingly, methods for generating HLCs from human iPS cells have been improved over the past decade. However, although human hepatocytes have zone-specific characteristics in vivo, there is currently no technique to generate zone-specific HLCs from human iPS cells. Therefore, to generate HLCs with zone-specific properties from human iPS cells, we cultured iPS-HLCs using a parenchymal or nonparenchymal cell-conditioned medium (CM). The results showed that urea production and gluconeogenesis capacity in iPS-HLCs were increased by culturing with cholangiocyte-CM, and glutamine production and drug metabolism capacity in iPS-HLCs were increased by culturing with hepatocyte-CM. It was thus clarified that iPS-HLCs acquire zone 1 hepatocyte-like properties by culturing with cholangiocyte-CM and that iPS-HLCs acquire zone 3 hepatocyte-like properties by culturing with hepatocyte-CM. In addition, we found that WNT inhibitory factor-1 secreted from cholangiocytes, and WNT7B and WNT8B secreted from hepatocytes play important roles in the zone-specific conversion of iPS-HLCs. We hope that our findings will facilitate the application of iPS-HLCs to drug discovery research.

Single-Cell Transcriptomics Uncovers Zonation of Function in the Mesenchyme during Liver Fibrosis

Iterative liver injury results in progressive fibrosis disrupting hepatic architecture, regeneration potential, and liver function. Hepatic stellate cells (HSCs) are a major source of pathological matrix during fibrosis and are thought to be a functionally homogeneous population. Here, we use single-cell RNA sequencing to deconvolve the hepatic mesenchyme in healthy and fibrotic mouse liver, revealing spatial zonation of HSCs across the hepatic lobule. Furthermore, we show that HSCs partition into topographically diametric lobule regions, designated portal vein-associated HSCs (PaHSCs) and central vein-associated HSCs (CaHSCs). Importantly we uncover functional zonation, identifying CaHSCs as the dominant pathogenic collagen-producing cells in a mouse model of centrilobular fibrosis. Finally, we identify LPAR1 as a therapeutic target on collagen-producing CaHSCs, demonstrating that blockade of LPAR1 inhibits liver fibrosis in a rodent NASH model. Taken together, our work illustrates the power of single-cell transcriptomics to resolve the key collagen-producing cells driving liver fibrosis with high precision.

Parkin deficiency prevents chronic ethanol-induced hepatic lipid accumulation through β-catenin accumulation

Background

Alcohol abuse and alcoholism lead to alcohol liver disease such as alcoholic fatty liver. Parkin is a component of the multiprotein E3 ubiquitin ligase complex and is associated with hepatic lipid accumulation. However, the role of parkin in ethanol-induced liver disease has not been reported. Here, we tested the effect of parkin on ethanol-induced fatty liver in parkin knockout (KO) mice with chronic ethanol feeding.

Methods

Male wild type (WT) and parkin KO mice (10–12 weeks old, n = 10) were fed on a Lieber-DeCarli diet containing 6.6% ethanol for 10 days. Liver histological, biochemical, and gene-expression studies were performed.

Results

Parkin KO mice exhibited lower hepatosteatosis after ethanol consumption. Because several studies reported that β-catenin is a critical factor in ethanol metabolism and protects against alcohol-induced hepatosteatosis, we investigated whether parkin changes β-catenin accumulation in the liver of ethanol-fed mice. Our results show that β-catenin was greatly accumulated in the livers of ethanol-fed parkin KO mice compared to ethanol-fed WT mice, and that parkin binds to β-catenin and promotes its degradation it by ubiquitination. Moreover, the β-catenin inhibitor IWR-1 abrogated the attenuation of ethanol-induced hepatic lipid accumulation by parkin deficiency in the livers of parkin KO mice and parkin siRNA-transfected human hepatic cell line.

Conclusions

Parkin deficiency prevents ethanol-induced hepatic lipid accumulation through promotion of β-catenin signaling by failure of β-catenin degradation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0424-5) contains supplementary material, which is available to authorized users.

Negative regulators of Wnt signaling in non-small cell lung cancer: Theoretical basis and therapeutic potency

Significant advances in the treatment of non-small cell lung cancer (NSCLC) have been made over the past decade, and they predominantly involve molecular targets such as epidermal growth factor receptor (EGFR) mutations and anaplastic lymphoma kinase (ALK) rearrangements. However, despite the initial good response, drug resistance eventually develops. The Wnt signaling pathway has recently been considered important in embryonic development and tumorigenesis in many cancers, particularly NSCLC. Moreover, the aberrant Wnt pathway plays a significant role in NSCLC and is associated with cancer cell proliferation, metastasis, invasion and drug resistance, and the suppression of canonical or noncanonical Wnt signaling through various biological or pharmacological negative regulators has been proven to produce specific anticancer effects. Thus, blocking the Wnt pathway via its negative regulators may overcome the resistance of current treatment methods and lead to new treatment strategies for NSCLC. Therefore, in this review, we summarize recent studies on the role of negative regulators in Wnt signaling in NSCLC and the therapeutic potency of these molecules as agents and targets for NSCLC treatments.

Liver to Pancreas Transdifferentiation

PURPOSE OF THE REVIEW

Here, we review recent findings in the field of generating insulin-producing cells by pancreatic transcription factor (pTF)-induced liver transdifferentiation (TD). TD is the direct conversion of functional cell types from one lineage to another without passing through an intermediate stage of pluripotency. We address potential reasons for the restricted efficiency of TD and suggest modalities to overcome these challenges, to bring TD closer to its clinical implementation in autologous cell replacement therapy for insulin-dependent diabetes.

RECENT FINDINGS

Liver to pancreas TD is restricted to cells that are a priori predisposed to undergo the developmental process. In vivo, the predisposition of liver cells is affected by liver zonation and hepatic regeneration. The TD propensity of liver cells is related to permissive epigenome which could be extended to TD-resistant cells by specific soluble factors. An obligatory role for active Wnt signaling in continuously maintaining a "permissive" epigenome is suggested. Moreover, the restoration of the pancreatic niche and vasculature promotes the maturation of TD cells along the β cell function. Future studies on liver to pancreas TD should include the maturation of TD cells by 3D culture, the restoration of vasculature and the pancreatic niche, and the extension of TD propensity to TD-resistant cells by epigenetic modifications. Liver to pancreas TD is expected to result in the generation of custom-made "self" surrogate β cells for curing diabetes.

The Function of Lgr5+ Cells in the Gastric Antrum Does Not Require Fzd7 or Myc In Vivo

The extreme chemical and mechanical forces endured by the gastrointestinal tract drive a constant renewal of the epithelial lining. Stem cells of the intestine and stomach, marked by the cell surface receptor Lgr5, preserve the cellular status-quo of their respective tissues through receipt and integration of multiple cues from the surrounding niche. Wnt signalling is a critical niche component for gastrointestinal stem cells and we have previously shown that the Wnt receptor, Frizzled-7 (Fzd7), is required for gastric homeostasis and the function of Lgr5⁺ intestinal stem cells. Additionally, we have previously shown a requirement for the Wnt target gene Myc in intestinal homeostasis, regeneration and tumourigenesis. However, it is unknown whether Fzd7 or Myc have conserved functions in gastric Lgr5⁺ stem cells. Here we show that gastric Lgr5⁺ stem cells do not require Fzd7 or Myc and are able to maintain epithelial homeostasis, highlighting key differences in the way Wnt regulates homeostasis and Lgr5⁺ stem cells in the stomach compared to the intestinal epithelium. Furthermore, deletion of Myc throughout the epithelium of the gastric antrum has no deleterious effects suggesting therapeutic targeting of Myc in gastric cancer patients will be well tolerated by the surrounding normal tissue.

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

Background

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

Methods

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

Results

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

Conclusion

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

Dynamic Metabolic Zonation of the Hepatic Glucose Metabolism Is Accomplished by Sinusoidal Plasma Gradients of Nutrients and Hormones

Being the central metabolic organ of vertebrates, the liver possesses the largest repertoire of metabolic enzymes among all tissues and organs. Almost all metabolic pathways are resident in the parenchymal cell, hepatocyte, but the pathway capacities may largely differ depending on the localization of hepatocytes within the liver acinus-a phenomenon that is commonly referred to as metabolic zonation. Metabolic zonation is rather dynamic since gene expression patterns of metabolic enzymes may change in response to nutrition, drugs, hormones and pathological states of the liver (e.g., fibrosis and inflammation). This fact has to be ultimately taken into account in mathematical models aiming at the prediction of metabolic liver functions in different physiological and pathological settings. Here we present a spatially resolved kinetic tissue model of hepatic glucose metabolism which includes zone-specific temporal changes of enzyme abundances which are driven by concentration gradients of nutrients, hormones and oxygen along the hepatic sinusoids. As key modulators of enzyme expression we included oxygen, glucose and the hormones insulin and glucagon which also control enzyme activities by cAMP-dependent reversible phosphorylation. Starting with an initially non-zonated model using plasma profiles under fed, fasted and diabetic conditions, zonal patterns of glycolytic and gluconeogenetic enzymes as well as glucose uptake and release rates are created as an emergent property. We show that mechanisms controlling the adaptation of enzyme abundances to varying external conditions necessarily lead to the zonation of hepatic carbohydrate metabolism. To the best of our knowledge, this is the first kinetic tissue model which takes into account in a semi-mechanistic way all relevant levels of enzyme regulation.

The Wnt/β-catenin pathway determines the predisposition and efficiency of liver-to-pancreas reprogramming

Transdifferentiation (TD) is the direct reprogramming of adult cells into cells of alternate fate and function. We have previously shown that liver cells can be transdifferentiated into beta-like, insulin-producing cells through ectopic expression of pancreatic transcription factors (pTFs). However, the efficiency of the process was consistently limited to <15% of the human liver cells treated in culture. The data in the current study suggest that liver-to-pancreas TD is restricted to a specific population of liver cells that is predisposed to undergo reprogramming. We isolated TD-predisposed subpopulation of liver cells from >15 human donors using a lineage tracing system based on the Wnt response element, part of the pericentral-specific promoter of glutamine synthetase. The cells, that were propagated separately, consistently exhibited efficient fate switch and insulin production and secretion in >60% of the cells upon pTF expression. The rest of the cells, which originated from 85% of the culture, resisted TD. Both populations expressed the ectopic pTFs with similar efficiencies, followed by similar repression of hepatic genes. Our data suggest that the TD-predisposed cells originate from a distinct population of liver cells that are enriched for Wnt signaling, which is obligatory for efficient TD. In TD-resistant populations, Wnt induction is insufficient to induce TD. An additional step of chromatin opening enables TD of these cells.

CONCLUSION

Liver-to-pancreas TD occurs in defined predisposed cells. These cells' predisposition is maintained by Wnt signaling that endows the cells with the plasticity needed to alter their transcriptional program and developmental fate when triggered by ectopic pTFs. These results may have clinical implications by drastically increasing the efficacy of TD in future clinical uses. (Hepatology 2018).

Aryl Hydrocarbon Receptor Promotes Liver Polyploidization and Inhibits PI3K, ERK, and Wnt/β-Catenin Signaling

Aryl hydrocarbon receptor (AhR) deficiency alters tissue homeostasis. However, how AhR regulates organ maturation and differentiation remains mostly unknown. Liver differentiation entails a polyploidization process fundamental for cell growth, metabolism, and stress responses. Here, we report that AhR regulates polyploidization during the preweaning-to-adult mouse liver maturation. Preweaning AhR-null (AhR−/−) livers had smaller hepatocytes, hypercellularity, altered cell cycle regulation, and enhanced proliferation. Those phenotypes persisted in adult AhR−/− mice and correlated with compromised polyploidy, predominance of diploid hepatocytes, and enlarged centrosomes. Phosphatidylinositol-3-phosphate kinase (PI3K), extracellular signal-regulated kinase (ERK), and Wnt/β-catenin signaling remained upregulated from preweaning to adult AhR-null liver, likely increasing mammalian target of rapamycin (mTOR) activation. Metabolomics revealed the deregulation of mitochondrial oxidative phosphorylation intermediates succinate and fumarate in AhR−/− liver. Consistently, PI3K, ERK, and Wnt/β-catenin inhibition partially rescued polyploidy in AhR−/− mice. Thus, AhR may integrate survival, proliferation, and metabolism for liver polyploidization. Since tumor cells tend to be polyploid, AhR modulation could have therapeutic value in the liver.

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AhR is required for liver polyploidization during preweaning-to-adult transition

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INS-R/PI3K/AKT, ERK, Wnt/β-Cat and mTOR are downregulated during liver polyploidization

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Reduced polyploidy relates with enhanced mitochondrial metabolism in AhR-null liver

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Understanding how AhR modulates polyploidy may provide strategies against cancer

Salvianolic acid A alleviates chronic ethanol-induced liver injury via promotion of β-catenin nuclear accumulation by restoring SIRT1 in rats

In recent years, alcoholic liver disease (ALD) has emerged as a growing public health problem worldwide. β-catenin plays an important role in the growth, development, regeneration and metabolic activity of the liver. Salvianolic acid A (SalA) is a water-soluble component from the root extract of Salvia miltiorrhiza Bunge, and its effect on ALD has not yet been investigated. This study aimed to investigate the effect of SalA on chronic alcohol-induced liver injury and to explore the role of SIRT1-mediated β-catenin deacetylation in such an effect. In this study, SalA treatment significantly alleviated the accumulation of lipid droplets and reduced the plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), total cholesterol (TC), triglyceride (TG), alcohol and ammonia levels in rats. SalA enhanced ethanol and ammonia metabolism and maintained mitochondrial homeostasis. Moreover, SalA restored the activity of the major ethanol-metabolizing enzymes and oxidative stress functions in the liver. Importantly, we found that SalA treatment effectively inhibited the ethanol-mediated decrease in nuclear β-catenin by upregulating SIRT1 in the liver. SIRT1 then deacetylated β-catenin to promote its accumulation in the nucleus, thereby preventing alcohol-induced liver injury. The results demonstrate that the SIRT1/β-catenin pathway is a key therapeutic target in liver injury caused by chronic alcohol exposure and that SalA protects against alcohol-induced liver injury via the SIRT1-mediated deacetylation of β-catenin.

Wnt Signalling in Gastrointestinal Epithelial Stem Cells

Wnt signalling regulates several cellular functions including proliferation, differentiation, apoptosis and migration, and is critical for embryonic development. Stem cells are defined by their ability for self-renewal and the ability to be able to give rise to differentiated progeny. Consequently, they are essential for the homeostasis of many organs including the gastrointestinal tract. This review will describe the huge advances in our understanding of how stem cell functions in the gastrointestinal tract are regulated by Wnt signalling, including how deregulated Wnt signalling can hijack these functions to transform cells and lead to cancer.

Glucagon contributes to liver zonation

The lobules are the functional units of the liver. They consist of 15–25 layers of hepatocytes with specialized metabolic functions and gene expression patterns relative to their position along the lobule, a phenomenon referred to as metabolic zonation. The Wnt/β-catenin pathway regulates hepatocyte function but how the zonation is controlled to meet the metabolic demands of the liver is unclear. Glucagon regulates hepatic function. We now demonstrate that glucagon contributes to liver zonation by interacting and opposing the actions of the Wnt/β-catenin pathway.

Liver zonation characterizes the separation of metabolic pathways along the lobules and is required for optimal function. Wnt/β-catenin signaling controls metabolic zonation by activating genes in the perivenous hepatocytes, while suppressing genes in the periportal counterparts. We now demonstrate that glucagon opposes the actions of Wnt/β-catenin signaling on gene expression and metabolic zonation pattern. The effects were more pronounced in the periportal hepatocytes where 28% of all genes were activated by glucagon and inhibited by Wnt/β-catenin. The glucagon and Wnt/β-catenin receptors and their signaling pathways are uniformly distributed in periportal and perivenous hepatocytes and the expression is not regulated by the opposing signal. Collectively, our results show that glucagon controls gene expression and metabolic zonation in the liver through a counterplay with the Wnt/β-catenin signaling pathway.

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.

Spatiotemporal regulation of liver development by the Wnt/β-catenin pathway

While the Wnt/β-catenin pathway plays a critical role in the maintenance of the zonation of ammonia metabolizing enzymes in the adult liver, the mechanisms responsible for inducing zonation in the embryo are not well understood. Herein we address the spatiotemporal role of the Wnt/β-catenin pathway in the development of zonation in embryonic mouse liver by conditional deletion of Apc and β-catenin at different stages of mouse liver development. In normal development, the ammonia metabolising enzymes carbamoylphosphate synthetase I (CPSI) and Glutamine synthetase (GS) begin to be expressed in separate hepatoblasts from E13.5 and E15.5 respectively and gradually increase in number thereafter. Restriction of GS expression occurs at E18 and becomes increasingly limited to the terminal perivenous hepatocytes postnatally. Expression of nuclear β-catenin coincides with the restriction of GS expression to the terminal perivenous hepatocytes. Conditional loss of Apc resulted in the expression of nuclear β-catenin throughout the developing liver and increased number of cells expressing GS. Conversely, conditional loss of β-catenin resulted in loss of GS expression. These data suggest that the Wnt pathway is critical to the development of zonation as well as maintaining the zonation in the adult liver.

Hepatocyte specific expression of an oncogenic variant of β-catenin results in lethal metabolic dysfunction in mice

Background

Wnt/β-catenin signaling plays a crucial role in embryogenesis, tissue homeostasis, metabolism and malignant transformation of different organs including the liver. Continuous β-catenin signaling due to somatic mutations in exon 3 of the Ctnnb1 gene is associated with different liver diseases including cancer and cholestasis.

Results

Expression of a degradation resistant form of β-catenin in hepatocytes resulted in 100% mortality within 31 days after birth. Ctnnb1^CAhep mice were characterized by reduced body weight, significantly enlarged livers with hepatocellular fat accumulation around central veins and increased hepatic triglyceride content. Proteomics analysis using whole liver tissue revealed significant deregulation of proteins involved in fat, glucose and mitochondrial energy metabolism, which was also reflected in morphological anomalies of hepatocellular mitochondria. Key enzymes involved in transport and synthesis of fatty acids and cholesterol were significantly deregulated in livers of Ctnnb1^CAhep mice. Furthermore, carbohydrate metabolism was substantially disturbed in mutant mice.

Conclusion

Continuous β-catenin signaling in hepatocytes results in premature death due to severe disturbances of liver associated metabolic pathways and mitochondrial dysfunction.

Methods

To investigate the influence of permanent β-catenin signaling on liver biology we analyzed mice with hepatocyte specific expression of a dominant stable form of β-catenin (Ctnnb1^CAhep) and their WT littermates by serum biochemistry, histology, electron microscopy, mRNA profiling and proteomic analysis of the liver.