RNA interference against hepatic epidermal growth factor receptor has suppressive effects on liver regeneration in rats

Maladaptive regeneration and metabolic dysfunction associated steatotic liver disease: Common mechanisms and potential therapeutic targets

The normal liver has an extraordinary capacity of regeneration. However, this capacity is significantly impaired in steatotic livers. Emerging evidence indicates that metabolic dysfunction associated steatotic liver disease (MASLD) and liver regeneration share several key mechanisms. Some classical liver regeneration pathways, such as HGF/c-Met, EGFR, Wnt/β-catenin and Hippo/YAP-TAZ are affected in MASLD. Some recently established therapeutic targets for MASH such as the Thyroid Hormone (TH) receptors, Glucagon-like protein 1 (GLP1), Farnesoid X receptor (FXR), Peroxisome Proliferator-Activated Receptors (PPARs) as well as Fibroblast Growth Factor 21 (FGF21) are also reported to affect hepatocyte proliferation. With this review we aim to provide insight into common molecular pathways, that may ultimately enable therapeutic strategies that synergistically ameliorate steatohepatitis and improve the regenerating capacity of steatotic livers. With the recent rise of prolonged ex-vivo normothermic liver perfusion prior to organ transplantation such treatment is no longer restricted to patients undergoing major liver resection or transplantation, but may eventually include perfused (steatotic) donor livers or even liver segments, opening hitherto unexplored therapeutic avenues.

Three Dimensional Bioprinting for Hepatic Tissue Engineering: From In Vitro Models to Clinical Applications

Fabrication of functional organs is the holy grail of tissue engineering and the possibilities of repairing a partial or complete liver to treat chronic liver disorders are discussed in this review. Liver is the largest gland in the human body and plays a responsible role in majority of metabolic function and processes. Chronic liver disease is one of the leading causes of death globally and the current treatment strategy of organ transplantation holds its own demerits. Hence there is a need to develop an in vitro liver model that mimics the native microenvironment. The developed model should be a reliable to understand the pathogenesis, screen drugs and assist to repair and replace the damaged liver. The three-dimensional bioprinting is a promising technology that recreates in vivo alike in vitro model for transplantation, which is the goal of tissue engineers. The technology has great potential due to its precise control and its ability to homogeneously distribute cells on all layers in a complex structure. This review gives an overview of liver tissue engineering with a special focus on 3D bioprinting and bioinks for liver disease modelling and drug screening.

Liver Regeneration in Acute on Chronic Liver Failure

Liver regeneration is a multifaceted process by which the organ regains its original size and histologic organization. In recent decades, substantial advances have been made in our understanding of the mechanisms underlying regeneration following loss of hepatic mass. Liver regeneration in acute liver failure possesses several classic pathways, while also exhibiting unique differences in key processes such as the roles of differentiated cells and stem cell analogs. Here we summarize these unique differences and new molecular mechanisms involving the gut-liver axis, immunomodulation, and microRNAs with an emphasis on applications to the patient population through stem cell therapies and prognostication.

EGF-receptor phosphorylation and downstream signaling are activated by genistein during subacute liver damage

The epidermal growth factor receptor (EGFR) plays an important role on hepatic protection in acute and chronic liver injury. The aim of this study was to investigate the role of genistein on EGFR expression, phosphorylation and signaling pathways in experimental subacute liver damage induced by carbon tetrachloride (CCl₄). We used male Wistar rats that were randomly divided into four groups: (1) Control; (2) Genistein 5 mg/kg per oral; (3) Subacute liver damage induced by CCl₄ 4 mg/kg subcutaneously; and (4) Animals received CCl₄ and genistein at the dosage indicated. The effect of genistein on EGFR expression, phosphorylation and signaling pathways were investigated by western blot and densitometric analyses. Histological changes were evaluated on slices stained with Hematoxylin-Eosin and Masson´s trichromic, as well as an immunohistochemical analysis for proliferating cell nuclear antigen (PCNA). Additionally, pro-inflammatory cytokines and liver enzymes were quantified. Our study showed that genistein increased EGFR expression, EGFR-specific tyrosine residues phosphorylation (pY1068-EGFR and pY84-EGFR), signal transducer and activator of transcription phosphorylation (pSTAT5), protein kinase B phosphorylation (pAKT) and PCNA in animals with CCl₄-induced subacute liver damage. It was found a significant reduction of pro-inflammatory cytokines in serum from animals with subacute liver damage treated with genistein. Those effects were reflected in an improvement in the architecture and liver function. In conclusion, genistein can induce a transactivation of EGFR leading to downstream cell signaling pathways as early events associated with regeneration and hepatoprotection following subacute liver damage.

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.

Liver regeneration biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

Liver regeneration: biological and pathological mechanisms and implications

The liver is the only solid organ that uses regenerative mechanisms to ensure that the liver-to-bodyweight ratio is always at 100% of what is required for body homeostasis. Other solid organs (such as the lungs, kidneys and pancreas) adjust to tissue loss but do not return to 100% of normal. The current state of knowledge of the regenerative pathways that underlie this 'hepatostat' will be presented in this Review. Liver regeneration from acute injury is always beneficial and has been extensively studied. Experimental models that involve partial hepatectomy or chemical injury have revealed extracellular and intracellular signalling pathways that are used to return the liver to equivalent size and weight to those prior to injury. On the other hand, chronic loss of hepatocytes, which can occur in chronic liver disease of any aetiology, often has adverse consequences, including fibrosis, cirrhosis and liver neoplasia. The regenerative activities of hepatocytes and cholangiocytes are typically characterized by phenotypic fidelity. However, when regeneration of one of the two cell types fails, hepatocytes and cholangiocytes function as facultative stem cells and transdifferentiate into each other to restore normal liver structure. Liver recolonization models have demonstrated that hepatocytes have an unlimited regenerative capacity. However, in normal liver, cell turnover is very slow. All zones of the resting liver lobules have been equally implicated in the maintenance of hepatocyte and cholangiocyte populations in normal liver.

TLR3 promotes hepatocyte proliferation after partial hepatectomy by stimulating uPA expression and the release of tissue-bound HGF

TLR3 is implicated in anti-viral immune responses, but may also act as a sensor of tissue damage in the absence of infection. Here, we provide evidence for an essential role of TLR3 in liver regeneration after an acute loss of tissue due to partial hepatectomy. Mice lacking TLR3 had a severe and sustained defect in the restoration of liver tissue with reduced liver-to-body weight ratios even after an extended recovery period of 2 weeks. Hepatocyte cell cycle progression into S phase was impaired in TLR3-deficient mice. Mechanistic analyses revealed that TLR3-deficient mice had markedly reduced systemic levels of active HGF, but had increased amounts of inactive tissue-bound HGF. Importantly, expression of uPA, which orchestrates the processing and release of HGF from the hepatic extracellular matrix, was reduced in regenerating livers of TLR3-deficient mice. In addition, expression of the HGF maturation factor HGFAC was transiently diminished in TLR3-deficient mice. In vitro, engagement of TLR3 directly stimulated expression of uPA by hepatic stellate cells. Thus, TLR3 supports liver regeneration through upregulation of uPA, which promotes the release of preformed HGF from extracellular matrix stores.

Overexpression of Platelet-Derived Growth Factor Receptor Α D842V Mutants Prevents Liver Regeneration and Chemically Induced Hepatocarcinogenesis via Inhibition of MET and EGFR

Platelet-derived growth receptor α (PDGFRα) is a key factor in many pathophysiological processes. The expression level of PDGFRα is significantly elevated in the early stage of liver development and maintained at a lower level in adult normal livers. In this study, we constructed a liver-specific PDGFRαD842 mutant transgenic (TG) mice model to explore the effect of continuous activation of PDGFRα on liver regeneration and hepatocarcinogenesis. 14-day-old TG and wild-type (WT) mice were intraperitoneally injected with diethylnitrosamine (DEN) at a dose of 25 μg/g body weight. Two-month-old male TG and WT mice were subjected to partial hepatectomy (PH). The liver tissues were collected for further analysis at different time points. Overexpression of PDGFRα ^D842V and its target genes, Akt, c-myc and cyclin D1 in hepatocytes with no overt phenotype versus WT mice were compared. Unexpectedly, a dramatic decrease in hepatocyte proliferation was noted after PH in TG versus WT mice, possibly due to the downregulation of hepatocyte growth factor receptor (MET) and epidermal growth factor receptor (EGFR). No TG mice developed HCC spontaneously after 14 months follow-up. However, TG mice were more resistant to DEN-induced hapatocarcinogenesis at 6, 10, and 12 months of age, showing delayed hepatocyte proliferation and apoptosis, lower tumor incidence, smaller size and fewer number, compared with age-matched WTs, partially through downregulation of MET and EGFR. In conclusion, continuous activation of PDGFRα signaling by expression of PDGFRα ^D842V does not promote, but inhibit hepatic regeneration and hepatocarcinogenesis, possibly through compensatory downregulation of MET and EGFR.

Toxicological Profile of Nanostructured Bone Substitute Based on Hydroxyapatite and Poly(lactide-co-glycolide) after Subchronic Oral Exposure of Rats

Novel three-dimensional (3D) nanohydroxyapatite-PLGA scaffolds with high porosity was developed to better mimic mineral component and microstructure of natural bone. To perform a final assessment of this nanomaterial as a potential bone substitute, its toxicological profile was particularly investigated. Therefore, we performed a comet assay on human monocytes for in vitro genotoxicity investigation, and the systemic subchronic toxicity investigation on rats being per oral feed with exactly administrated extract quantities of the nano calcium hydroxyapatite covered with tiny layers of PLGA (ALBO-OS) for 120 days. Histological and stereological parameters of the liver, kidney, and spleen tissue were analyzed. Comet assay revealed low genotoxic potential, while histological analysis and stereological investigation revealed no significant changes in exposed animals when compared to controls, although the volume density of blood sinusoids and connective tissue, as well as numerical density and number of mitosis were slightly increased. Additionally, despite the significantly increased average number of the Ki67 and slightly increased number of CD68 positive cells in the presence of ALBO-OS, immunoreactive cells proliferation was almost neglected. Blood analyses showed that all of the blood parameters in rats fed with extract nanomaterial are comparable with corresponding parameters of no feed rats, taken as blind probe. This study contributes to the toxicological profiling of ALBO-OS scaffold for potential future application in bone tissue engineering.

Role of epidermal growth factor receptor in liver injury and lipid metabolism: Emerging new roles for an old receptor

Epidermal growth factor receptor (EGFR) is conventionally known to play a crucial role in hepatocyte proliferation, liver regeneration and is also associated with hepatocellular carcinogenesis. In addition to these proliferative roles, EGFR has also implicated in apoptotic cell death signaling in various hepatic cells, mitochondrial dysfunction and acute liver necrosis in a clinically relevant murine model of acetaminophen overdose, warranting further comprehensive exploration of this paradoxical role of EGFR in hepatotoxicity. Apart from ligand dependent activation, EGFR can also be activated in ligand-independent manner, which is mainly associated to liver injury. Recent evidence has also emerged demonstrating important role of EGFR in lipid and fatty acid metabolism in quiescent and regenerating liver. Based on these findings, EGFR has also been shown to play an important role in steatosis in clinically relevant murine NAFLD models via regulating master transcription factors governing fatty acid synthesis and lipolysis. Moreover, several lines of evidences indicate that EGFR is also involved in hepatocellular injury, oxidative stress, inflammation, direct stellate cell activation and fibrosis in chronic liver injury models, including repeated CCl₄ exposure, high-fat diet and fast-food diet models. In addition to briefly summarizing role of EGFR in liver regeneration, this review comprehensively discusses all these non-conventional emerging roles of EGFR. Considering evidences of multi-facet role of EGFR at various levels in these pathophysiological process, EGFR can be a promising therapeutic target for various liver diseases, including acute liver failure and NAFLD, requiring further exploration. These roles of EGFR are relevant for alcoholic liver diseases (ALD) as well, thus providing a valid rationale for future investigations exploring a role of EGFR in ALD.

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.

The roles of signaling pathways in liver repair and regeneration

The liver has remarkable regeneration potency that restores liver mass and sustains body hemostasis. Liver regeneration through signaling pathways following resection or moderate damages are well studied. Various cell signaling, growth factors, cytokines, receptors, and cell types implicated in liver regeneration undergo controlled hypertrophy and proliferation. Some aspects of liver regeneration have been discovered and many investigations have been carried out to identify its mechanisms. However, for optimizing liver regeneration more should be understood about mechanisms that control the growth of hepatocytes and other liver cell types in adults. The current paper deals with the possible applicability of liver regeneration signaling pathways as a target for therapeutic approaches and preventing various liver damages. Furthermore, the latest findings of spectrum-specific signaling pathway mechanisms that underlie liver regeneration are briefly described.

Hepatocyte growth control by SOCS1 and SOCS3

The extraordinary capacity of the liver to regenerate following injury is dependent on coordinated and regulated actions of cytokines and growth factors. Whereas hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are direct mitogens to hepatocytes, inflammatory cytokines such as TNFα and IL-6 also play essential roles in the liver regeneration process. These cytokines and growth factors activate different signaling pathways in a sequential manner to elicit hepatocyte proliferation. The kinetics and magnitude of these hepatocyte-activating stimuli are tightly regulated to ensure restoration of a functional liver mass without causing uncontrolled cell proliferation. Hepatocyte proliferation can become deregulated under conditions of chronic inflammation, leading to accumulation of genetic aberrations and eventual neoplastic transformation. Among the control mechanisms that regulate hepatocyte proliferation, negative feedback inhibition by the 'suppressor of cytokine signaling (SOCS)' family proteins SOCS1 and SOCS3 play crucial roles in attenuating cytokine and growth factor signaling. Loss of SOCS1 or SOCS3 in the mouse liver increases the rate of liver regeneration and renders hepatocytes susceptible to neoplastic transformation. The frequent epigenetic repression of the SOCS1 and SOCS3 genes in hepatocellular carcinoma has stimulated research in understanding the growth regulatory mechanisms of SOCS1 and SOCS3 in hepatocytes. Whereas SOCS3 is implicated in regulating JAK-STAT signaling induced by IL-6 and attenuating EGFR signaling, SOCS1 is crucial for the regulation of HGF signaling. These two proteins also module the functions of certain key proteins that control the cell cycle. In this review, we discuss the current understanding of the functions of SOCS1 and SOCS3 in controlling hepatocyte proliferation, and its implications to liver health and disease.

Influence of nanostructured calcium aluminate and calcium silicate on the liver: histological and unbiased stereological analysis

AIM

To examine the potential systemic toxicity of nanostructured materials based on calcium silicate and calcium aluminate, for potential application in Dentistry.

METHODOLOGY

Twenty-four Albino Wistar rats aged 2 months were used as an in vivo animal model for subcutaneous implantation of the investigated materials, placed in polyethylene tubes. Thirty days after implantation, the livers of the rats were analysed and following histological and stereological parameters were evaluated for volume density of hepatocytes and blood sinusoids, number and numerical density of hepatocytes, surface of hepatocytes and their nucleuses, nucleocytoplasmic ratio and mitotic index of hepatocytes. Stereological measurements were achieved using Cavalieri's principle, with grid P2 and unbiased analysis. Additionally, immunohistochemistry studies were performed to further analyse changes in liver tissue. Several haematological and biochemical parameters of blood of experimental animals were also analysed, as well as local tissue reactions around the implants. Statistical analysis was performed using parametric (anova and t-test) and nonparametric tests (Kruskal-Wallis and Mann-Whitney U-test) depending on data distribution.

RESULTS

Implanted dental cements led to an increase in stereological and histological parameters in liver tissue compared to control rats. Although the investigated parameters mostly showed significant differences between control and experimental animals, the liver tissue of the experimental animals did not have visible signs of pathological changes. This was supported by the analysis of blood parameters which were not significantly different between control and experimental animals. Also, the subcutaneous tissues had minimal inflammatory reactions. Immunohistochemistry studies revealed that nanostructured materials induced proliferation of hepatocytes, but that the immunological response to the materials was not strong enough to induce proliferation of immunoreactive cells in liver in the observed time period.

CONCLUSIONS

This study was performed as a contribution to the attestation of the biocompatibility of dental cements based on calcium silicate and calcium aluminate. Although these materials induced several changes in the liver structure, they were not clinically relevant and represent a normal and reversible response of the liver to the presence of biocompatible materials in the body. Blood and immunohistochemistry analyses and local tissue reactions further confirmed that these materials possess good biocompatible potential.

Combined Systemic Disruption of MET and Epidermal Growth Factor Receptor Signaling Causes Liver Failure in Normal Mice

MET and epidermal growth factor receptor (EGFR) tyrosine kinases are crucial for liver regeneration and normal hepatocyte function. Recently, we demonstrated that in mice, combined inhibition of these two signaling pathways abolished liver regeneration after hepatectomy, with subsequent hepatic failure and death at 15 to 18 days after resection. Morbidity was associated with distinct and specific alterations in important downstream signaling pathways that led to decreased hepatocyte volume, reduced proliferation, and shutdown of many essential hepatocyte functions, such as fatty acid synthesis, urea cycle, and mitochondrial functions. Herein, we explore the role of MET and EGFR signaling in resting mouse livers that are not subjected to hepatectomy. Mice with combined disruption of MET and EGFR signaling were noticeably sick by 10 days and died at 12 to 14 days. Mice with combined disruption of MET and EGFR signaling mice showed decreased liver/body weight ratios, increased apoptosis in nonparenchymal cells, impaired liver metabolic functions, and activation of distinct downstream signaling pathways related to inflammation, cell death, and survival. The present study demonstrates that, in addition to controlling the regenerative response, MET and EGFR synergistically control baseline liver homeostasis in normal mice in such a way that their combined disruption leads to liver failure and death.

Calcium Signaling in Liver Injury and Regeneration

The liver fulfills central roles in metabolic control and detoxification and, as such, is continuously exposed to a plethora of insults. Importantly, the liver has a unique ability to regenerate and can completely recoup from most acute, non-iterative insults. However, multiple conditions, including viral hepatitis, non-alcoholic fatty liver disease (NAFLD), long-term alcohol abuse and chronic use of certain medications, can cause persistent injury in which the regenerative capacity eventually becomes dysfunctional, resulting in hepatic scaring and cirrhosis. Calcium is a versatile secondary messenger that regulates multiple hepatic functions, including lipid and carbohydrate metabolism, as well as bile secretion and choleresis. Accordingly, dysregulation of calcium signaling is a hallmark of both acute and chronic liver diseases. In addition, recent research implicates calcium transients as essential components of liver regeneration. In this review, we provide a comprehensive overview of the role of calcium signaling in liver health and disease and discuss the importance of calcium in the orchestration of the ensuing regenerative response. Furthermore, we highlight similarities and differences in spatiotemporal calcium regulation between liver insults of different etiologies. Finally, we discuss intracellular calcium control as an emerging therapeutic target for liver injury and summarize recent clinical findings of calcium modulation for the treatment of ischemic-reperfusion injury, cholestasis and NAFLD.

Liver Regeneration in the Acute Liver Failure Patient

Liver regeneration after simple resection represents a unique process in which the organ returns to its original size and histologic structure. Over the past 30 years, there has been significant progress in elucidating the mechanisms associated with regeneration after loss of hepatic mass. Liver regeneration after acute liver failure shares several of these classical pathways. It differs, however, in key processes, including the role of both differentiated and stemlike cells. This article outlines these differences in addition to new molecular mechanisms, including immunomodulation, microRNAs, and the gut-liver axis. In addition, applications to the patient population, including prognostication and stem cell therapies, are explored.

Liver Regeneration: Analysis of the Main Relevant Signaling Molecules

Liver regeneration is a highly organized tissue regrowth process and is the most important reaction of the liver to injury. The overall process of liver regeneration includes three phases: priming stage, proliferative phase, and termination phase. The initial step aims to induce hepatocytes to be sensitive to growth factors with the aid of some cytokines, including TNF-α and IL-6. The proliferation phase promotes hepatocytes to re-enter G1 with the stimulation of growth factors. While during the termination stage, hepatocytes will discontinue to proliferate to maintain normal liver mass and function. Except for cytokine- and growth factor-mediated pathways involved in regulating liver regeneration, new substances and technologies emerge to influence the regenerative process. Here, we reviewed novel and important signaling molecules involved in the process of liver regeneration to provide a cue for further research.

Combined systemic elimination of MET and epidermal growth factor receptor signaling completely abolishes liver regeneration and leads to liver decompensation

Receptor tyrosine kinases MET and epidermal growth factor receptor (EGFR) are critically involved in initiation of liver regeneration. Other cytokines and signaling molecules also participate in the early part of the process. Regeneration employs effective redundancy schemes to compensate for the missing signals. Elimination of any single extracellular signaling pathway only delays but does not abolish the process. Our present study, however, shows that combined systemic elimination of MET and EGFR signaling (MET knockout + EGFR-inhibited mice) abolishes liver regeneration, prevents restoration of liver mass, and leads to liver decompensation. MET knockout or simply EGFR-inhibited mice had distinct and signaling-specific alterations in Ser/Thr phosphorylation of mammalian target of rapamycin, AKT, extracellular signal-regulated kinases 1/2, phosphatase and tensin homolog, adenosine monophosphate-activated protein kinase α, etc. In the combined MET and EGFR signaling elimination of MET knockout + EGFR-inhibited mice, however, alterations dependent on either MET or EGFR combined to create shutdown of many programs vital to hepatocytes. These included decrease in expression of enzymes related to fatty acid metabolism, urea cycle, cell replication, and mitochondrial functions and increase in expression of glycolysis enzymes. There was, however, increased expression of genes of plasma proteins. Hepatocyte average volume decreased to 35% of control, with a proportional decrease in the dimensions of the hepatic lobules. Mice died at 15-18 days after hepatectomy with ascites, increased plasma ammonia, and very small livers.

CONCLUSION

MET and EGFR separately control many nonoverlapping signaling endpoints, allowing for compensation when only one of the signals is blocked, though the combined elimination of the signals is not tolerated; the results provide critical new information on interactive MET and EGFR signaling and the contribution of their combined absence to regeneration arrest and liver decompensation. (Hepatology 2016;64:1711-1724).

Dual Role of Epidermal Growth Factor Receptor in Liver Injury and Regeneration after Acetaminophen Overdose in Mice

Epidermal growth factor receptor (EGFR) plays a crucial role in hepatocyte proliferation. Its role in acetaminophen (APAP)-mediated hepatotoxicity and subsequent liver regeneration is completely unknown. Role of EGFR after APAP-overdose in mice was studied using pharmacological inhibition strategy. Rapid, sustained and dose-dependent activation of EGFR was noted after APAP-treatment in mice, which was triggered by glutathione depletion. EGFR-activation was also observed in primary human hepatocytes after APAP-treatment, preceding elevation of toxicity markers. Treatment of mice with an EGFR-inhibitor (EGFRi), Canertinib, 1h post-APAP resulted in robust inhibition of EGFR-activation and a striking reduction in APAP-induced liver injury. Metabolic activation of APAP, formation of APAP-protein adducts, APAP-mediated JNK-activation and its mitochondrial translocation were not altered by EGFRi. Interestingly, EGFR rapidly translocated to mitochondria after APAP-treatment. EGFRi-treatment abolished mitochondrial EGFR activity, prevented APAP-mediated mitochondrial dysfunction/oxidative-stress and release of endonucleases from mitochondria, which are responsible for DNA-damage/necrosis. Treatment with N-acetylcysteine (NAC), 4h post-APAP in mice did not show any protection but treatment of EGFRi in combination with NAC showed decrease in liver injury. Finally, delayed treatment with EGFRi, 12-h post-APAP, did not alter peak injury but caused impairment of liver regeneration resulting in sustained injury and decreased survival after APAP overdose in mice. Impairment of regeneration was due to inhibition of cyclinD1 induction and cell cycle arrest. Our study has revealed a new dual role of EGFR both in initiation of APAP-injury and in stimulation of subsequent compensatory regeneration after APAP-overdose.

Expression and Function of the Epidermal Growth Factor Receptor in Physiology and Disease

The epidermal growth factor receptor (EGFR) is the prototypical member of a family of membrane-associated intrinsic tyrosine kinase receptors, the ErbB family. EGFR is activated by multiple ligands, including EGF, transforming growth factor (TGF)-α, HB-EGF, betacellulin, amphiregulin, epiregulin, and epigen. EGFR is expressed in multiple organs and plays important roles in proliferation, survival, and differentiation in both development and normal physiology, as well as in pathophysiological conditions. In addition, EGFR transactivation underlies some important biologic consequences in response to many G protein-coupled receptor (GPCR) agonists. Aberrant EGFR activation is a significant factor in development and progression of multiple cancers, which has led to development of mechanism-based therapies with specific receptor antibodies and tyrosine kinase inhibitors. This review highlights the current knowledge about mechanisms and roles of EGFR in physiology and disease.

Dissecting the role of epidermal growth factor receptor catalytic activity during liver regeneration and hepatocarcinogenesis

Different data support a role for the epidermal growth factor receptor (EGFR) pathway during liver regeneration and hepatocarcinogenesis. However, important issues, such as the precise mechanisms mediating its actions and the unique versus redundant functions, have not been fully defined. Here, we present a novel transgenic mouse model expressing a hepatocyte-specific truncated form of human EGFR, which acts as negative dominant mutant (ΔEGFR) and allows definition of its tyrosine kinase-dependent functions. Results indicate a critical role for EGFR catalytic activity during the early stages of liver regeneration. Thus, after two-thirds partial hepatectomy, ΔEGFR livers displayed lower and delayed proliferation and lower activation of proliferative signals, which correlated with overactivation of the transforming growth factor-β pathway. Altered regenerative response was associated with amplification of cytostatic effects of transforming growth factor-β through induction of cell cycle negative regulators. Interestingly, lipid synthesis was severely inhibited in ΔEGFR livers after partial hepatectomy, revealing a new function for EGFR kinase activity as a lipid metabolism regulator in regenerating hepatocytes. In spite of these profound alterations, ΔEGFR livers were able to recover liver mass by overactivating compensatory signals, such as c-Met. Our results also indicate that EGFR catalytic activity is critical in the early preneoplastic stages of the liver because ΔEGFR mice showed a delay in the appearance of diethyl-nitrosamine-induced tumors, which correlated with decreased proliferation and delay in the diethyl-nitrosamine-induced inflammatory process.

CONCLUSION

These studies demonstrate that EGFR catalytic activity is critical during the initial phases of both liver regeneration and carcinogenesis and provide key mechanistic insights into how this kinase acts to regulate liver pathophysiology. (Hepatology 2016;63:604-619).

EGFR Signaling in Liver Diseases

The epidermal growth factor receptor (EGFR) is a transmembrane receptor tyrosine kinase that is activated by several ligands leading to the activation of diverse signaling pathways controlling mainly proliferation, differentiation, and survival. The EGFR signaling axis has been shown to play a key role during liver regeneration following acute and chronic liver damage, as well as in cirrhosis and hepatocellular carcinoma (HCC) highlighting the importance of the EGFR in the development of liver diseases. Despite the frequent overexpression of EGFR in human HCC, clinical studies with EGFR inhibitors have so far shown only modest results. Interestingly, a recent study has shown that in human HCC and in mouse HCC models the EGFR is upregulated in liver macrophages where it plays a tumor-promoting function. Thus, the role of EGFR in liver diseases appears to be more complex than what anticipated. Further studies are needed to improve the molecular understanding of the cell-specific signaling pathways that control disease development and progression to be able to develop better therapies targeting major components of the EGFR signaling network in selected cell types. In this review, we compiled the current knowledge of EGFR signaling in different models of liver damage and diseases, mainly derived from the analysis of HCC cell lines and genetically engineered mouse models (GEMMs).

Identification of an epigenetic signature of early mouse liver regeneration that is disrupted by Zn-HDAC inhibition

Liver regeneration has been well studied with hope of discovering strategies to improve liver disease outcomes. Nevertheless, the signals that initiate such regeneration remain incompletely defined, and translation of mechanism-based pro-regenerative interventions into new treatments for hepatic diseases has not yet been achieved. We previously reported the isoform-specific regulation and essential function of zinc-dependent histone deacetylases (Zn-HDACs) during mouse liver regeneration. Those data suggest that epigenetically regulated anti-proliferative genes are deacetylated and transcriptionally suppressed by Zn-HDAC activity or that pro-regenerative factors are acetylated and induced by such activity in response to partial hepatectomy (PH). To investigate these possibilities, we conducted genome-wide interrogation of the liver histone acetylome during early PH-induced liver regeneration in mice using acetyL-histone chromatin immunoprecipitation and next generation DNA sequencing. We also compared the findings of that study to those seen during the impaired regenerative response that occurs with Zn-HDAC inhibition. The results reveal an epigenetic signature of early liver regeneration that includes both hyperacetylation of pro-regenerative factors and deacetylation of anti-proliferative and pro-apoptotic genes. Our data also show that administration of an anti-regenerative regimen of the Zn-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) not only disrupts gene-specific pro-regenerative changes in liver histone deacetylation but also reverses PH-induced effects on histone hyperacetylation. Taken together, these studies offer new insight into and suggest novel hypotheses about the epigenetic mechanisms that regulate liver regeneration.

Loss of hepatocyte EGFR has no effect alone but exacerbates carbon tetrachloride-induced liver injury and impairs regeneration in hepatocyte Met-deficient mice

The role(s) of the epidermal growth factor receptor (EGFR) in hepatocytes is unknown. We generated a murine hepatocyte specific-EGFR knockout (KO) model to evaluate how loss of hepatocellular EGFR expression affects processes such as EGF clearance, circulating EGF concentrations, and liver regeneration following 70% resection or CCl4-induced centrilobular injury. We were able to disrupt EGFR expression effectively in hepatocytes and showed that the ability of EGF and heregulin (HRG) to phosphorylate EGFR and ERBB3, respectively, required EGFR. Loss of hepatocellular EGFR impaired clearance of exogenous EGF from the portal circulation but paradoxically resulted in reduced circulating levels of endogenous EGF. This was associated with decreased submandibular salivary gland production of EGF. EGFR disruption did not result in increased expression of other ERBB proteins or Met, except in neonatal mice. Liver regeneration following 70% hepatectomy revealed a mild phenotype, with no change in cyclin D1 expression and slight differences in cyclin A expression compared with controls. Peak 5-bromo-2'-deoxyuridine labeling was shifted from 36 to 48 h. Centrilobular damage and regenerative response induced by carbon tetrachloride (CCl4) were identical in the KO and wild-type mice. In contrast, loss of Met increased CCl4-induced necrosis and delayed regeneration. Although loss of hepatocellular EGFR alone did not have an effect in this model, EGFR-Met double KOs displayed enhanced necrosis and delayed liver regeneration compared with Met KOs alone. This suggests that EGFR and Met may partially compensate for the loss of the other, although other compensatory mechanisms can be envisioned.

PDGFRα in liver pathophysiology: emerging roles in development, regeneration, fibrosis, and cancer

Platelet-derived growth factor receptor α (PDGFRα) is an isoform of the PDGFR family of tyrosine kinase receptors involved in cell proliferation, survival, differentiation, and growth. In this review, we highlight the role of PDGFRα and the current evidence of its expression and activities in liver development, regeneration, and pathology-including fibrosis, cirrhosis, and liver cancer. Studies elucidating PDGFRα signaling in processes ranging from profibrotic signaling, angiogenesis, and oxidative stress to epithelial-to-mesenchymal transition point toward PDGFRα as a potential therapeutic target in various hepatic pathologies, including hepatic fibrosis and liver cancer. Furthermore, PDGFRα localization and modulation during liver development and regeneration may lend insight into its potential roles in various pathologic states. We will also briefly discuss some of the current targeted treatments for PDGFRα, including multi receptor tyrosine kinase inhibitors and PDGFRα-specific inhibitors.

Advances in liver regeneration

Liver regeneration after partial hepatectomy is the only example of a regenerative process in mammals in which the organ/body weight ratio returns to 100% of the original when the process is complete. The adjustment of liver weight to the needs of the body suggests a complicated set of control points, a 'hepatostat'. There has been much progress in elucidation of mechanisms involved in initiation of liver regeneration. More recent studies have focused on termination pathways, because these may be the underlying controls of the hepatostat and their elimination may be relevant to hepatic neoplasia. When the standard regenerative process is thwarted due to failure of either hepatocytes or biliary epithelial cells to proliferate, each of the two epithelial compartments can function as a source of facultative stem cells for the other.

GH administration rescues fatty liver regeneration impairment by restoring GH/EGFR pathway deficiency

GH pathway has been shown to play a major role in liver regeneration through the control of epidermal growth factor receptor (EGFR) activation. This pathway is down-regulated in nonalcoholic fatty liver disease. Because regeneration is known to be impaired in fatty livers, we wondered whether a deregulation of the GH/EGFR pathway could explain this deficiency. Hepatic EGFR expression and triglyceride levels were quantified in liver biopsies of 32 obese patients with different degrees of steatosis. We showed a significant inverse correlation between liver EGFR expression and the level of hepatic steatosis. GH/EGFR down-regulation was also demonstrated in 2 steatosis mouse models, a genetic (ob/ob) and a methionine and choline-deficient diet mouse model, in correlation with liver regeneration defect. ob/ob mice exhibited a more severe liver regeneration defect after partial hepatectomy (PH) than methionine and choline-deficient diet-fed mice, a difference that could be explained by a decrease in signal transducer and activator of transcription 3 phosphorylation 32 hours after PH. Having checked that GH deficiency accounted for the GH signaling pathway down-regulation in the liver of ob/ob mice, we showed that GH administration in these mice led to a partial rescue in hepatocyte proliferation after PH associated with a concomitant restoration of liver EGFR expression and signal transducer and activator of trnascription 3 activation. In conclusion, we propose that the GH/EGFR pathway down-regulation is a general mechanism responsible for liver regeneration deficiency associated with steatosis, which could be partially rescued by GH administration.

Knockdown and knockout of β1-integrin in hepatocytes impairs liver regeneration through inhibition of growth factor signalling

The liver has a unique regenerative capability, which involves extensive remodelling of cell-cell and cell-matrix contacts. Here we study the role of integrins in mouse liver regeneration using Cre/loxP-mediated gene deletion or intravenous delivery of β1-integrin siRNA formulated into nanoparticles that predominantly target hepatocytes. We show that although short-term loss of β1-integrin has no obvious consequences for normal livers, partial hepatectomy leads to severe liver necrosis and reduced hepatocyte proliferation. Mechanistically, loss of β1-integrin in hepatocytes impairs ligand-induced phosphorylation of the epidermal growth factor and hepatocyte growth factor receptors, thereby attenuating downstream receptor signalling in vitro and in vivo. These results identify a crucial role and novel mechanism of action of β1-integrins in liver regeneration and demonstrate that protein depletion by nanoparticle-based delivery of specific siRNA is a powerful strategy to study gene function in the regenerating liver.

Control of growth during regeneration

Regeneration is a process by which organisms replace damaged or amputated organs to restore normal body parts. Regeneration of many tissues or organs requires proliferation of stem cells or stem cell-like blastema cells. This regenerative growth is often initiated by cell death pathways induced by damage. The executors of regenerative growth are a group of growth-promoting signaling pathways, including JAK/STAT, EGFR, Hippo/YAP, and Wnt/β-catenin. These pathways are also essential to developmental growth, but in regeneration, they are activated in distinct ways and often at higher strengths, under the regulation by certain stress-responsive signaling pathways, including JNK signaling. Growth suppressors are important in termination of regeneration to prevent unlimited growth and also contribute to the loss of regenerative capacity in nonregenerative organs. Here, we review cellular and molecular growth regulation mechanisms induced by organ damage in several models with different regenerative capacities.

The EGFR signalling system in the liver: from hepatoprotection to hepatocarcinogenesis

The liver displays an outstanding wound healing and regenerative capacity unmatched by any other organ. This reparative response is governed by a complex network of inflammatory mediators, growth factors and metabolites that are set in motion in response to hepatocellular injury. However, when liver injury is chronic, these regenerative mechanisms become dysregulated, facilitating the accumulation of genetic alterations leading to unrestrained cell proliferation and the development of hepatocellular carcinoma (HCC). The epidermal growth factor receptor (EGFR or ErbB1) signaling system has been identified as a key player in all stages of the liver response to injury, from early inflammation and hepatocellular proliferation to fibrogenesis and neoplastic transformation. The EGFR system engages in extensive crosstalk with other signaling pathways, acting as a true signaling hub for other growth factors, cytokines and inflammatory mediators. Here, we briefly review essential aspects of the biology of the EGFR, the other ErbB receptors, and their ligands in liver injury, regeneration and HCC development. Some aspects of the preclinical and clinical experience with EGFR therapeutic targeting in HCC are also discussed.

Principles of liver regeneration and growth homeostasis

Liver regeneration is perhaps the most studied example of compensatory growth aimed to replace loss of tissue in an organ. Hepatocytes, the main functional cells of the liver, manage to proliferate to restore mass and to simultaneously deliver all functions hepatic functions necessary to maintain body homeostasis. They are the first cells to respond to regenerative stimuli triggered by mitogenic growth factor receptors MET (the hepatocyte growth factor receptor] and epidermal growth factor receptor and complemented by auxiliary mitogenic signals induced by other cytokines. Termination of liver regeneration is a complex process affected by integrin mediated signaling and it restores the organ to its original mass as determined by the needs of the body (hepatostat function). When hepatocytes cannot proliferate, progenitor cells derived from the biliary epithelium transdifferentiate to restore the hepatocyte compartment. In a reverse situation, hepatocytes can also transdifferentiate to restore the biliary compartment. Several hormones and xenobiotics alter the hepatostat directly and induce an increase in liver to body weight ratio (augmentative hepatomegaly). The complex challenges of the liver toward body homeostasis are thus always preserved by complex but unfailing responses involving orchestrated signaling and affecting growth and differentiation of all hepatic cell types.

The involvement of the docking protein Gab1 in mitogenic signalling induced by EGF and HGF in rat hepatocytes

Grb2-associated binder (Gab) family proteins are docking molecules that can interact with receptor tyrosine kinases (RTKs) and cytokine receptors and bind several downstream signalling proteins. Studies in several cell types have shown that Gab1 may have a role in signalling mediated by the two RTKs epidermal growth factor (EGF) receptor (EGFR) and Met, the receptor for hepatocyte growth factor (HGF), but the involvement of Gab1 in EGFR and Met signalling has not been directly compared in the same cell. We have studied mechanisms of activation and role in mitogenic signalling of Gab1 in response to EGF and HGF in cultured rat hepatocytes. Gab1, but not Gab2, was expressed in the hepatocytes and was phosphorylated upon stimulation with EGF or HGF. Depletion of Gab1, using siRNA, decreased the ERK and Akt activation, cyclin D1 expression, and DNA synthesis in response to both EGF and HGF. Studies of mechanisms of recruitment to the receptors showed that HGF induced co-precipitation of Gab1 and Met while EGF induced binding of Gab1 to Grb2 but not to EGFR. Gab1 activation in response to both EGF and HGF was dependent on PI3K. While EGF activated Gab1 and Shc equally, within the same concentration range, HGF very potently and almost exclusively activated Gab1, having only a minimal effect on Shc. Collectively, our results strongly suggest that although Gab1 interacts differently with EGFR and Met, it is involved in mitogenic signalling mediated by both these growth factor receptors in hepatocytes.

Novel targets of miR-30, a microRNA required for biliary development

MicroRNAs have been found to play a profound role in embryonic and post-natal development through their regulation of processes such as cell proliferation, differentiation, and morphogenesis. The microRNA-30 (miR-30) family is necessary for vertebrate hepatobiliary development; however, the mechanism through which miR-30 regulates these processes is not fully understood. Here, we identify genes directly regulated by miR-30 that have been characterized as key developmental factors. The targets were confirmed via a luciferase reporter assay, following exogenous over-expression of miR-30a and miR-30c2 in cultured cells. Five novel miR-30ac2 targets were identified using this approach, all of which play crucial roles in hepatobiliary development or are involved in hepatocellular carcinoma and cholangiocarcinoma.

A disintegrin and metalloproteinase 17 regulates TNF and TNFR1 levels in inflammation and liver regeneration in mice

A disintegrin and metalloproteinase 17 (ADAM17), or tumor necrosis factor (TNF)-α-converting enzyme, is a key metalloproteinase and physiological convertase for a number of putative targets that play critical roles in cytokine and growth factor signaling. These interdependent pathways are essential components of the signaling network that links liver function with the compensatory growth that occurs during liver regeneration following 2/3 partial hepatectomy (PH) or chemically induced hepatotoxicity. Despite identification of many soluble factors needed for efficient liver regeneration, very little is known about how such ligands are regulated in the liver. To directly study the role of ADAM17 in the liver, we employed two cell-specific ADAM17 knockout (KO) mouse models. Using lipopolysaccharide (LPS) as a robust stimulus for TNF release, we found attenuated levels of circulating TNF in myeloid-specific ADAM17 KO mice (ADAM17 m-KO) and, unexpectedly, in mice with hepatocyte-specific ADAM17 deletion (ADAM17 h-KO), indicating that ADAM17 expression in both cell types plays a role in TNF shedding. After 2/3 PH, induction of TNF, TNFR1, and amphiregulin (AR) was significantly attenuated in ADAM17 h-KO mice, implicating ADAM17 as the primary sheddase for these factors in the liver. Surprisingly, the extent and timing of hepatocyte proliferation were not affected after PH or carbon tetrachloride injection in ADAM17 h-KO or ADAM17 m-KO mice. We conclude that ADAM17 regulates TNF, TNFR1, and AR in the liver, and its expression in both hepatocytes and myeloid cells is important for TNF regulation after LPS injury or 2/3 PH, but is not required for liver regeneration.

Role and regulation of PDGFRα signaling in liver development and regeneration

Aberrant platelet-derived growth factor receptor-α (PDGFRα) signaling is evident in a subset of hepatocellular cancers (HCCs). However, its role and regulation in hepatic physiology remains elusive. In the current study, we examined PDGFRα signaling in liver development and regeneration. We identified notable PDGFRα activation in hepatic morphogenesis that, when interrupted by PDGFRα-blocking antibody, led to decreased hepatoblast proliferation and survival in embryonic liver cultures. We also identified temporal PDGFRα overexpression, which is regulated by epidermal growth factor (EGF) and tumor necrosis factor α, and its activation at 3 to 24 hours after partial hepatectomy. Through generation of hepatocyte-specific PDGFRA knockout (KO) mice that lack an overt phenotype, we show that absent PDGFRα compromises extracelluar signal-regulated kinases and AKT activation 3 hours after partial hepatectomy, which, however, is alleviated by temporal compensatory increases in the EGF receptor (EGFR) and the hepatocyte growth factor receptor (Met) expression and activation along with rebound activation of extracellular signal-regulated kinases and AKT at 24 hours. These untimely increases in EGFR and Met allow for normal hepatocyte proliferation at 48 hours in KO, which, however, are aberrantly prolonged up to 72 hours. Intriguingly, such compensation also was visible in primary KO hepatocyte cultures but not in HCC cells after siRNA-mediated PDGFRα knockdown. Thus, temporal activation of PDGFRα in liver development is important in hepatic morphogenesis. In liver regeneration, despite increased signaling, PDGFRα is dispensable owing to EGFR and Met compensation, which is unique to normal hepatocytes but not HCC cells.

Signals and cells involved in regulating liver regeneration

Liver regeneration is a complex phenomenon aimed at maintaining a constant liver mass in the event of injury resulting in loss of hepatic parenchyma. Partial hepatectomy is followed by a series of events involving multiple signaling pathways controlled by mitogenic growth factors (HGF, EGF) and their receptors (MET and EGFR). In addition multiple cytokines and other signaling molecules contribute to the orchestration of a signal which drives hepatocytes into DNA synthesis. The other cell types of the liver receive and transmit to hepatocytes complex signals so that, in the end of the regenerative process, complete hepatic tissue is assembled and regeneration is terminated at the proper time and at the right liver size. If hepatocytes fail to participate in this process, the biliary compartment is mobilized to generate populations of progenitor cells which transdifferentiate into hepatocytes and restore liver size.

Expression of hepatocyte epidermal growth factor receptor, FAS and glypican 3 in EpCAM-positive regenerative clusters of hepatocytes, cholangiocytes, and progenitor cells in human liver failure

Liver regeneration under normal circumstances proceeds through proliferation of all cellular elements of the liver. Studies with rodent models have shown that when proliferation of hepatocytes is inhibited, progenitor cells arising from the biliary compartment transdifferentiate into "oval/progenitor" cells, which proceed to differentiate into hepatocytes. Recent studies have shown that the same pathways may operate in human liver failure. The growth factor receptors (HGF [hepatocyte growth factor] receptor) and epidermal growth factor receptor are key mitogenic receptors for both hepatocytes and progenitor cells. Our current study used the biliary and progenitor marker EpCAM (epithelial cell adhesion molecule) to detect "regenerative clusters" of mixed cholangiocyte-hepatocyte differentiation. We determined that expression of metabolic equivalent and epidermal growth factor receptor occurs in biliary cells, progenitor cells, and hepatocytes, whereas activation of metabolic equivalent and epidermal growth factor receptor is limited to regenerative cluster hepatocytes. These histologic events are associated with expression of apoptosis-inducing FAS and mitoinhibitory protein glypican 3. Cell proliferation was overall suppressed in regenerative clusters. Transdifferentiation of biliary and progenitor cells appears to be regulated by a complex interaction of signals promoting and arresting growth.

Expression of SOCS3 throughout liver regeneration is not regulated by DNA methylation

BACKGROUND

While suppressor of cytokine signaling 3 (SOCS3) plays a crucial role in suppressing dysplasia and tumorigenesis, it also offers a typical instance of DNA methylation in the regulation of gene transcription, since the promoter region of the SOCS3 gene is rich in CpG islands (CGIs). During liver regeneration initiated by partial hepatectomy, SOCS3 acts as a suppressor to balance the acute-phase response and terminate the regeneration. This study aimed to determine whether the variation of SOCS3 expression throughout liver regeneration is also regulated by its DNA methylation.

METHODS

We established a 70% partial hepatectomy mouse model and the animals were sacrificed at indicated times to assess the SOCS3 expression. We performed bisulfite sequencing PCR and DNA sequencing to investigate the detailed cytosine methylation in the SOCS3 gene.

RESULTS

Within the promoter sequence, 58 CGIs were identified and 30 were found variously methylated before or after operation; however, methylation remained at a very low level. No evidence indicated that the total methylation level or the methylation of any CpG site regularly changed throughout liver regeneration.

CONCLUSION

DNA methylation or demethylation seems to be a relatively stable modification of cytosine, but not a dynamic and reversible process to regulate gene transcription in daily and acute pathophysiological events.

Amphiregulin stimulates liver regeneration after small-for-size mouse liver transplantation

This study investigated whether amphiregulin (AR), a ligand of the epidermal growth factor receptor (EGFR), improves liver regeneration after small-for-size liver transplantation. Livers of male C57BL/6 mice were reduced to ~50% and ~30% of original sizes and transplanted. After transplantation, AR and AR mRNA increased in 50% but not in 30% grafts. 5-Bromodeoxyuridine (BrdU) labeling, proliferating cell nuclear antigen (PCNA) expression and mitotic index increased substantially in 50% but not 30% grafts. Hyperbilirubinemia and hypoalbuminemia occurred and survival decreased after transplantation of 30% but not 50% grafts. AR neutralizing antibody blunted regeneration in 50% grafts whereas AR injection (5 μg/mouse, iv) stimulated liver regeneration, improved liver function and increased survival after transplantation of 30% grafts. Phosphorylation of EGFR and its downstream signaling molecules Akt, mTOR, p70S6K, ERK and JNK increased markedly in 50% but not 30% grafts. AR stimulated EGFR phosphorylation and its downstream signaling pathways. EGFR inhibitor PD153035 suppressed regeneration of 50% grafts and largely abrogated stimulation of regeneration of 30% grafts by AR. AR also increased cyclin D1 and cyclin E expression in 30% grafts. Together, liver regeneration is suppressed in small-for-size grafts, as least in part, due to decreased AR formation. AR supplementation could be a promising therapy to stimulate regeneration of partial liver grafts.

Role of ErbB2 in the prostaglandin E₂-induced enhancement of the mitogenic response to epidermal growth factor in cultured hepatocytes

Prostaglandin E(2) (PGE(2)) enhances the mitogenic response to epidermal growth factor (EGF) in hepatocytes, but the underlying mechanisms are not clear. We previously observed that PGE(2) upregulates EGF-induced signalling in the MEK/ERK and PI3K/Akt pathways in hepatocytes. Other investigations have indicated that ErbB2 enhances the mitogenic effect of EGF in these cells. In the present study we found that treatment with PGE(2) increased ErbB2 and decreased ErbB3 expression at both the mRNA and protein level in cultured rat hepatocytes. Silencing of the ErbB2 expression with specific siRNA blocked the stimulation by PGE(2) and EGF of cyclin D1 expression and DNA synthesis. Both EGF and PGE(2) increased the expression of ERK and Akt, but while the effect of EGF was inhibited by ErbB2-directed siRNA, this did not affect the PGE(2)-induced upregulation of ERK and Akt. These data suggest that PGE(2) can enhance the mitogenic effect of EGF both by increasing ErbB2 expression and by ErbB2-independent mechanisms.

EGFR: A Master Piece in G1/S Phase Transition of Liver Regeneration

Unraveling the molecular clues of liver proliferation has become conceivable thanks to the model of two-third hepatectomy. The synchronicity and the well-scheduled aspect of this process allow scientists to slowly decipher this mystery. During this phenomenon, quiescent hepatocytes of the remnant lobes are able to reenter into the cell cycle initiating the G1-S progression synchronously before completing the cell cycle. The major role played by this step of the cell cycle has been emphasized by loss-of-function studies showing a delay or a lack of coordination in the hepatocytes G1-S progression. Two growth factor receptors, c-Met and EGFR, tightly drive this transition. Due to the level of complexity surrounding EGFR signaling, involving numerous ligands, highly controlled regulations and multiple downstream pathways, we chose to focus on the EGFR pathway for this paper. We will first describe the EGFR pathway in its integrity and then address its essential role in the G1/S phase transition for hepatocyte proliferation. Recently, other levels of control have been discovered to monitor this pathway, which will lead us to discuss regulations of the EGFR pathway and highlight the potential effect of misregulations in pathologies.

Protection against Fas-induced fulminant hepatic failure in liver specific integrin linked kinase knockout mice

Background

Programmed cell death or apoptosis is an essential process for tissue homeostasis. Hepatocyte apoptosis is a common mechanism to many forms of liver disease. This study was undertaken to test the role of ILK in hepatocyte survival and response to injury using a Jo-2-induced apoptosis model.

Methods

For survival experiments, ILK KO and WT mice received a single intraperitoneal injection of the agonistic anti-Fas monoclonal antibody Jo-2 at the lethal dose (0.4 μg/g body weight) or sublethal dose (0.16 μg/g body weight). For further mechanistic studies sublethal dose of Fas monoclonal antibody was chosen.

Results

There was 100% mortality in the WT mice as compared to 50% in the KO mice. We also found that hepatocyte specific ILK KO mice (integrin linked kinase) died much later than WT mice after challenge with a lethal dose of Fas agonist Jo-2. At sublethal dose of Jo-2, there was 20% mortality in KO mice with minimal apoptosis whereas WT mice developed extensive apoptosis and liver injury leading to 70% mortality due to liver failure at 12 h. Proteins known to be associated with cell survival/death were differentially expressed in the 2 groups. In ILK KO mice there was downregulation of proapoptotic genes and upregulation of antiapoptotic genes.

Conclusions

Mechanistic insights revealed that pro-survival pathways such as Akt, ERK1/2, and NFkB signaling were upregulated in the ILK KO mice. Inhibition of only NFkB and ERK1/2 signaling led to an increase in the susceptibility of ILK KO hepatocytes to Jo-2-induced apoptosis. These studies suggest that ILK elimination from hepatocytes protects against Jo-2 induced apoptosis by upregulating survival pathways. FAK decrease may also play a role in this process. The results presented show that the signaling effects of ILK related to these functions are mediated in part mediated through NFkB and ERK1/2 signaling.

New insights into liver regeneration

Even if the Greeks probably anticipated rather than discovered the extraordinary regenerative capacity of the liver with the Prometheus myth, this phenomenon still fascinates scientists nowadays with the same enthusiasm. There are good reasons to decipher this process other than to find an answer to our fantasy of immortality: it could indeed help patients needing large liver resections or living-donor liver transplantation, it could increase our understanding of liver pathology and finally it could enable novel cell-therapy approaches. For decades, most of our knowledge about the mechanisms involved in liver regeneration came from the classic two-thirds partial hepatectomy (PH) model. In this scenario, hepatocytes play the leading role, which raises the question of the simple existence of a stem cell population. Recently however, hepatic progenitor cells come again under the limelight, seeming to play a role in liver physiology and in various liver diseases such as steatosis or cirrhosis. Excellent reviews have recently addressed liver regeneration. Our goal is therefore to focus on recent improvements in the field, highlighting data mostly published in the last two years in order to draw a putative picture of what the future research axes on liver regeneration might look like.

Activation of inactive hepatocytes through histone acetylation: a mechanism for functional compensation after massive loss of hepatocytes

The mechanisms by which hepatic function is maintained after extensive parenchymal loss are unclear. In this study, we propose a novel concept of "functional heterogeneity" of hepatocytes based on their different expression of acetylated histones, the markers of active gene transcription, to explain the powerful compensatory capability of the liver. In the healthy human liver, only a fraction of the hepatocytes were marked by acetylated histones (ac-H2AK5, ac-H2BK5, ac-H3K9, ac-H3K14, ac-H3K27, and ac-H3K9/14). With the progression of cirrhosis, the ratio of the positive cells was gradually elevated, accompanied by the gradual exhaustion of the negative cells. By examining the global transcriptome of the mouse hepatocytes, we observed that the primed genes in the positive cells were much more numerous than those in negative cells. In a 70% hepatectomized mouse, the remnant hepatocytes were extensively activated, and the liver function was well maintained even when regeneration was severely inhibited. The functional compensation was absolutely dependent on the elevated expression of acetyl-histones. Additionally, when liver regeneration was blocked, the metabolism-related genes seemed to be preferentially transcribed. In conclusion, we demonstrate that normally, part of the active hepatocytes are competent for routine physiological requirements. The inactive hepatocytes, delicately regulated by acetyl-histones, act as a functional reservoir for future activation to restore the liver function after massive parenchymal loss.