Janina E.E. Tirnitz-Parker

TWEAK and LTβ signaling during chronic liver disease

Chronic liver diseases (CLD) such as hepatitis B and C virus infection, alcoholic liver disease, and non-alcoholic steatohepatitis are associated with hepatocellular necrosis, continual inflammation, and hepatic fibrosis. The induced microenvironment triggers the activation of liver-resident progenitor cells (LPCs) while hepatocyte replication is inhibited. In the early injury stages, LPCs regenerate the liver by proliferation, migration to sites of injury, and differentiation into functional biliary epithelial cells or hepatocytes. However, when this process becomes dysregulated, wound healing can progress to pathological fibrosis, cirrhosis, and eventually hepatocellular carcinoma. The other key mediators in the pathogenesis of progressive CLD are fibrosis-driving, activated hepatic stellate cells (HSCs) that usually proliferate in very close spatial association with LPCs. Recent studies from our group and others have suggested the potential for cytokine and chemokine cross-talk between LPCs and HSCs, which is mainly driven by the tumor necrosis factor (TNF) family members, TNF-like weak inducer of apoptosis (TWEAK) and lymphotoxin-β, potentially dictating the pathological outcomes of chronic liver injury.

Tumor necrosis factor-like weak inducer of apoptosis is a mitogen for liver progenitor cells

Liver progenitor cells (LPCs) represent the cell compartment facilitating hepatic regeneration during chronic injury while hepatocyte‐mediated repair mechanisms are compromised. LPC proliferation is frequently observed in human chronic liver diseases such as hereditary hemochromatosis, fatty liver disease, and chronic hepatitis. In vivo studies have suggested that a tumor necrosis factor family member, tumor necrosis factor–like weak inducer of apoptosis (TWEAK), is promitotic for LPCs; whether it acts directly is not known. In our murine choline‐deficient, ethionine‐supplemented (CDE) model of chronic liver injury, TWEAK receptor [fibroblast growth factor‐inducible 14 (Fn14)] expression in the whole liver is massively upregulated. We therefore set out to investigate whether TWEAK/Fn14 signaling promotes the regenerative response in CDE‐induced chronic liver injury by mitotic stimulation of LPCs. Fn14 knockout (KO) mice showed significantly reduced LPC numbers and attenuated inflammation and cytokine production after 2 weeks of CDE feeding. The close association between LPC proliferation and activation of hepatic stellate cells in chronic liver injury prompted us to investigate whether fibrogenesis was also modulated in Fn14 KO animals. Collagen deposition and expression of key fibrogenesis mediators were reduced after 2 weeks of injury, and this correlated with LPC numbers. Furthermore, the injection of 2‐week‐CDE‐treated wildtype animals with TWEAK led to increased proliferation of nonparenchymal pan cytokeratin–positive cells. Stimulation of an Fn14‐positive LPC line with TWEAK led to nuclear factor kappa light chain enhancer of activated B cells (NFκB) activation and dose‐dependent proliferation, which was diminished after targeting of the p50 NFκB subunit by RNA interference. Conclusion: TWEAK acts directly and stimulates LPC mitosis in an Fn14‐dependent and NFκB‐dependent fashion, and signaling via this pathway mediates the LPC response to CDE‐induced injury and regeneration. (HEPATOLOGY 2010)

Lymphotoxin‐β receptor signaling regulates hepatic stellate cell function and wound healing in a murine model of chronic liver injury

Lymphotoxin‐beta (LTβ) is a proinflammatory cytokine and a member of the tumor necrosis factor (TNF) superfamily known for its role in mediating lymph node development and homeostasis. Our recent studies suggest a role for LTβ in mediating the pathogenesis of human chronic liver disease. We hypothesize that LTβ co‐ordinates the wound healing response in liver injury via direct effects on hepatic stellate cells. This study used the choline‐deficient, ethionine‐supplemented (CDE) dietary model of chronic liver injury, which induces inflammation, liver progenitor cell proliferation, and portal fibrosis, to assess (1) the cellular expression of LTβ, and (2) the role of LTβ receptor (LTβR) in mediating wound healing, in LTβR−/− versus wild‐type mice. In addition, primary isolates of hepatic stellate cells were treated with LTβR‐ligands LTβ and LTβ‐related inducible ligand competing for glycoprotein D binding to herpesvirus entry mediator on T cells (LIGHT), and mediators of hepatic stellate cell function and fibrogenesis were assessed. LTβ was localized to progenitor cells immediately adjacent to activated hepatic stellate cells in the periportal region of the liver in wild‐type mice fed the CDE diet. LTβR−/− mice fed the CDE diet showed significantly reduced fibrosis and a dysregulated immune response. LTβR was demonstrated on isolated hepatic stellate cells, which when stimulated by LTβ and LIGHT, activated the nuclear factor kappa B (NF‐κB) signaling pathway. Neither LTβ nor LIGHT had any effect on alpha‐smooth muscle actin, tissue inhibitor of metalloproteinase 1, transforming growth factor beta, or procollagen α1(I) expression; however, leukocyte recruitment‐associated factors intercellular adhesion molecule 1 and regulated upon activation T cells expressed and secreted (RANTES) were markedly up‐regulated. RANTES caused the chemotaxis of a liver progenitor cell line expressing CCR5. Conclusion: This study suggests that LTβR on hepatic stellate cells may be involved in paracrine signaling with nearby LTβ‐expressing liver progenitor cells mediating recruitment of progenitor cells, hepatic stellate cells, and leukocytes required for wound healing and regeneration during chronic liver injury. (HEPATOLOGY 2009;49:227–239.)

The intrahepatic signalling niche of hedgehog is defined by primary cilia positive cells during chronic liver injury

BACKGROUND & AIMS In vertebrates, canonical Hedgehog (Hh) pathway activation requires Smoothened (SMO) translocation to the primary cilium (Pc), followed by a GLI-mediated transcriptional response. In addition, a similar gene regulation occurs in response to growth factors/cytokines, although independently of SMO signalling. The Hh pathway plays a critical role in liver fibrosis/regeneration, however, the mechanism of activation in chronic liver injury is poorly understood. This study aimed to characterise Hh pathway activation upon thioacetamide (TAA)-induced chronic liver injury in vivo by defining Hh-responsive cells, namely cells harbouring Pc and Pc-localised SMO. METHODS C57BL/6 mice (wild-type or Ptc1(+/-)) were TAA-treated. Liver injury and Hh ligand/pathway mRNA and protein expression were assessed in vivo. SMO/GLI manipulation and SMO-dependent/independent activation of GLI-mediated transcriptional response in Pc-positive (Pc(+)) cells were studied in vitro. RESULTS In vivo, Hh activation was progressively induced following TAA. At the epithelial-mesenchymal interface, injured hepatocytes produced Hh ligands. Progenitors, myofibroblasts, leukocytes and hepatocytes were GLI2(+). Pc(+) cells increased following TAA, but only EpCAM(+)/GLI2(+) progenitors were Pc(+)/SMO(+). In vitro, SMO knockdown/hGli3-R overexpression reduced proliferation/viability in Pc(+) progenitors, whilst increased proliferation occurred with hGli1 overexpression. HGF induced GLI transcriptional activity independently of Pc/SMO. Ptc1(+/-) mice exhibited increased progenitor, myofibroblast and fibrosis responses. CONCLUSIONS In chronic liver injury, Pc(+) progenitors receive Hh ligand signals and process it through Pc/SMO-dependent activation of GLI-mediated transcriptional response. Pc/SMO-independent GLI activation likely occurs in Pc(-)/GLI2(+) cells. Increased fibrosis in Hh gain-of-function mice likely occurs by primary progenitor expansion/proliferation and secondary fibrotic myofibroblast expansion, in close contact with progenitors.

Disruption of hemochromatosis protein and transferrin receptor 2 causes iron-induced liver injury in mice†

Mutations in hemochromatosis protein (HFE) or transferrin receptor 2 (TFR2) cause hereditary hemochromatosis (HH) by impeding production of the liver iron‐regulatory hormone, hepcidin (HAMP). This study examined the effects of disruption of Hfe or Tfr2, either alone or together, on liver iron loading and injury in mouse models of HH. Iron status was determined in Hfe knockout (Hfe−/−), Tfr2 Y245X mutant (Tfr2mut), and double‐mutant (Hfe−/−×Tfr2mut) mice by measuring plasma and liver iron levels. Plasma alanine transaminase (ALT) activity, liver histology, and collagen deposition were evaluated to assess liver injury. Hepatic oxidative stress was assessed by measuring superoxide dismutase (SOD) activity and F2‐isoprostane levels. Gene expression was measured by real‐time polymerase chain reaction. Hfe−/−×Tfr2mut mice had elevated hepatic iron with a periportal distribution and increased plasma iron, transferrin saturation, and non‐transferrin‐bound iron, compared with Hfe−/−, Tfr2mut, and wild‐type (WT) mice. Hamp1 expression was reduced to 40% (Hfe−/− and Tfr2mut) and 1% (Hfe−/−×Tfr2mut) of WT values. Hfe−/− ×Tfr2mut mice had elevated plasma ALT activity and mild hepatic inflammation with scattered aggregates of infiltrating inflammatory cluster of differentiation 45 (CD45)–positive cells. Increased hepatic hydoxyproline levels as well as Sirius red and Massons Trichrome staining demonstrated advanced portal collagen deposition. Hfe−/− and Tfr2mut mice had less hepatic inflammation and collagen deposition. Liver F2‐isoprostane levels were elevated, and copper/zinc and manganese SOD activities decreased in Hfe−/−×Tfr2mut, Tfr2mut, and Hfe−/− mice, compared with WT mice. Conclusion: Disruption of both Hfe and Tfr2 caused more severe hepatic iron overload with more advanced lipid peroxidation, inflammation, and portal fibrosis than was observed with the disruption of either gene alone. The Hfe−/−×Tfr2mut mouse model of iron‐induced liver injury reflects the liver injury phenotype observed in human HH. (HEPATOLOGY 2012)

Iron uptake from plasma transferrin by a transferrin receptor 2 mutant mouse model of haemochromatosis

BACKGROUND & AIMS Hereditary haemochromatosis type 3 is caused by mutations in transferrin receptor (TFR) 2. TFR2 has been shown to mediate iron transport in vitro and regulate iron homeostasis. The aim of this study was to determine the role of Tfr2 in iron transport in vivo using a Tfr2 mutant mouse. METHODS Tfr2 mutant and wild-type mice were injected intravenously with (59)Fe-transferrin and tissue (59)Fe uptake was measured. Tfr1, Tfr2 and ferroportin expression was measured by real-time PCR and Western blot. Cellular localisation of ferroportin was determined by immunohistochemistry. RESULTS Transferrin-bound iron uptake by the liver and spleen in Tfr2 mutant mice was reduced by 20% and 65%, respectively, whilst duodenal and renal uptake was unchanged compared with iron-loaded wild-type mice. In Tfr2 mutant mice, liver Tfr2 protein was absent, whilst ferroportin protein was increased in non-parenchymal cells and there was a low level of expression in hepatocytes. Tfr1 expression was unchanged compared with iron-loaded wild-type mice. Splenic Tfr2 protein expression was absent whilst Tfr1 and ferroportin protein expression was increased in Tfr2 mutant mice compared with iron-loaded wild-type mice. CONCLUSIONS A small reduction in hepatic transferrin-bound iron uptake in Tfr2 mutant mice suggests that Tfr2 plays a minor role in liver iron transport and its primary role is to regulate iron metabolism. Increased ferroportin expression due to decreased hepcidin mRNA levels is likely to be responsible for impaired splenic iron uptake in Tfr2 mutant mice.

A Transcriptomic Signature of Mouse Liver Progenitor Cells

Liver progenitor cells (LPCs) can proliferate extensively, are able to differentiate into hepatocytes and cholangiocytes, and contribute to liver regeneration. The presence of LPCs, however, often accompanies liver disease and hepatocellular carcinoma (HCC), indicating that they may be a cancer stem cell. Understanding LPC biology and establishing a sensitive, rapid, and reliable method to detect their presence in the liver will assist diagnosis and facilitate monitoring of treatment outcomes in patients with liver pathologies. A transcriptomic meta-analysis of over 400 microarrays was undertaken to compare LPC lines against datasets of muscle and embryonic stem cell lines, embryonic and developed liver (DL), and HCC. Three gene clusters distinguishing LPCs from other liver cell types were identified. Pathways overrepresented in these clusters denote the proliferative nature of LPCs and their association with HCC. Our analysis also revealed 26 novel markers, LPC markers, including Mcm2 and Ltbp3, and eight known LPC markers, including M2pk and Ncam. These markers specified the presence of LPCs in pathological liver tissue by qPCR and correlated with LPC abundance determined using immunohistochemistry. These results showcase the value of global transcript profiling to identify pathways and markers that may be used to detect LPCs in injured or diseased liver.

The role of liver progenitor cells during liver regeneration, fibrogenesis, and carcinogenesis.

The growing worldwide challenge of cirrhosis and hepatocellular carcinoma due to increasing prevalence of excessive alcohol consumption, viral hepatitis, obesity, and the metabolic syndrome has sparked interest in stem cell-like liver progenitor cells (LPCs) as potential candidates for cell therapy and tissue engineering, as an alternative approach to whole organ transplantation. However, LPCs always proliferate in chronic liver diseases with a predisposition to cancer; they have been suggested to play major roles in driving fibrosis, disease progression, and may even represent tumor-initiating cells. Hence, a greater understanding of the factors that govern their activation, communication with other hepatic cell types, and bipotential differentiation as opposed to their potential transformation is needed before their therapeutic potential can be harnessed.

Immune checkpoint inhibition: prospects for prevention and therapy of hepatocellular carcinoma

The global prevalence of liver cancer is rapidly rising, mostly as a result of the amplified incidence rates of viral hepatitis, alcohol abuse and obesity in recent decades. Treatment options for liver cancer are remarkably limited with sorafenib being the gold standard for advanced, unresectable hepatocellular carcinoma but offering extremely limited improvement of survival time. The immune system is now recognised as a key regulator of cancer development through its ability to protect against infection and chronic inflammation, which promote cancer development, and eliminate tumour cells when present. However, the tolerogenic nature of the liver means that the immune response to infection, chronic inflammation and tumour cells within the hepatic environment is usually ineffective. Here we review the roles that immune cells and cytokines have in the development of the most common primary liver cancer, hepatocellular carcinoma (HCC). We then examine how the immune system may be subverted throughout the stages of HCC development, particularly with respect to immune inhibitory molecules, also known as immune checkpoints, such as programmed cell death protein‐1, programmed cell death 1 ligand 1 and cytotoxic T lymphocyte antigen 4, which have become therapeutic targets. Finally, we assess preclinical and clinical studies where immune checkpoint inhibitors have been used to modify disease during the carcinogenic process. In conclusion, inhibitory molecule‐based immunotherapy for HCC is in its infancy and further detailed research in relevant in vivo models is required before its full potential can be realised.

Widespread GLI expression but limited canonical hedgehog signaling restricted to the ductular reaction in human chronic liver disease

Canonical Hedgehog (Hh) signaling in vertebrate cells occurs following Smoothened activation/translocation into the primary cilia (Pc), followed by a GLI transcriptional response. Nonetheless, GLI activation can occur independently of the canonical Hh pathway. Using a murine model of liver injury, we previously identified the importance of canonical Hh signaling within the Pc+ liver progenitor cell (LPC) population and noted that SMO-independent, GLI-mediated signals were important in multiple Pc-ve GLI2+ intrahepatic populations. This study extends these observations to human liver tissue, and analyses the effect of GLI inhibition on LPC viability/gene expression. Human donor and cirrhotic liver tissue specimens were evaluated for SHH, GLI2 and Pc expression using immunofluorescence and qRT-PCR. Changes to viability and gene expression in LPCs in vitro were assessed following GLI inhibition. Identification of Pc (as a marker of canonical Hh signaling) in human cirrhosis was predominantly confined to the ductular reaction and LPCs. In contrast, GLI2 was expressed in multiple cell populations including Pc-ve endothelium, hepatocytes, and leukocytes. HSCs/myofibroblasts (>99%) expressed GLI2, with only 1.92% displaying Pc. In vitro GLI signals maintained proliferation/viability within LPCs and GLI inhibition affected the expression of genes related to stemness, hepatocyte/biliary differentiation and Hh/Wnt signaling. At least two mechanisms of GLI signaling (Pc/SMO-dependent and Pc/SMO-independent) mediate chronic liver disease pathogenesis. This may have significant ramifications for the choice of Hh inhibitor (anti-SMO or anti-GLI) suitable for clinical trials. We also postulate GLI delivers a pro-survival signal to LPCs whilst maintaining stemness.