Fibroblasts in liver cancer: functions and therapeutic translation

Accumulation of fibroblasts in the premalignant or malignant liver is a characteristic feature of liver cancer, but has not been therapeutically leveraged despite evidence for pathophysiologically relevant roles in tumour growth. Hepatocellular carcinoma is a largely non-desmoplastic tumour, in which fibroblasts accumulate predominantly in the pre-neoplastic fibrotic liver and regulate the risk for hepatocellular carcinoma development through a balance of tumour-suppressive and tumour-promoting mediators. By contrast, cholangiocarcinoma is desmoplastic, with cancer-associated fibroblasts contributing to tumour growth. Accordingly, restoring the balance from tumour-promoting to tumour-suppressive fibroblasts and mediators might represent a strategy for hepatocellular carcinoma prevention, whereas in cholangiocarcinoma, fibroblasts and their mediators could be leveraged for tumour treatment. Importantly, fibroblast mediators regulating hepatocellular carcinoma development might exert opposite effects on cholangiocarcinoma growth. This Review translates the improved understanding of tumour-specific, location-specific, and stage-specific roles of fibroblasts and their mediators in liver cancer into novel and rational therapeutic concepts.

A Therapeutically Targetable TAZ-TEAD2 Pathway Drives the Growth of Hepatocellular Carcinoma via ANLN and KIF23

BACKGROUND & AIMS

Despite recent progress, long-term survival remains low for hepatocellular carcinoma (HCC). The most effective HCC therapies target the tumor immune microenvironment (TIME), and there are almost no therapies that directly target tumor cells. Here, we investigated the regulation and function of tumor cell-expressed Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) in HCC.

METHODS

HCC was induced in mice by Sleeping Beauty-mediated expression of MET, CTNNB1-S45Y, or TAZ-S89A, or by diethylnitrosamine plus CCl4. Hepatocellular TAZ and YAP were deleted in floxed mice via adeno-associated virus serotype 8-mediated expression of Cre. TAZ target genes were identified from RNA sequencing, confirmed by chromatin immunoprecipitation, and evaluated in a clustered regularly interspaced short palindromic repeats interference (CRISPRi) screen. TEA domain transcription factors (TEADs), anillin (ANLN), Kif23, and programmed cell death protein ligand 1 were knocked down by guide RNAs in dead clustered regularly interspaced short palindromic repeats-associated protein 9 (dCas9) knock-in mice.

RESULTS

YAP and TAZ were up-regulated in murine and human HCC, but only deletion of TAZ consistently decreased HCC growth and mortality. Conversely, overexpression of activated TAZ was sufficient to trigger HCC. TAZ expression in HCC was regulated by cholesterol synthesis, as demonstrated by pharmacologic or genetic inhibition of 3-hydroxy-3-methylglutaryl- coenzyme A reductase (HMGCR), farnesyl pyrophosphate synthase, farnesyl-diphosphate farnesyltransferase 1 (FDFT1), or sterol regulatory element-binding protein 2 (SREBP2). TAZ- and MET/CTNNB1-S45Y-driven HCC required the expression of TEAD2 and, to a lesser extent, TEAD4. Accordingly, TEAD2 displayed the most profound effect on survival in patients with HCC. TAZ and TEAD2 promoted HCC via increased tumor cell proliferation, mediated by TAZ target genes ANLN and kinesin family member 23 (KIF23). Therapeutic targeting of HCC, using pan-TEAD inhibitors or the combination of a statin with sorafenib or anti-programmed cell death protein 1, decreased tumor growth.

CONCLUSIONS

Our results suggest the cholesterol-TAZ-TEAD2-ANLN/KIF23 pathway as a mediator of HCC proliferation and tumor cell-intrinsic therapeutic target that could be synergistically combined with TIME-targeted therapies.

Collagenolysis-dependent DDR1 signalling dictates pancreatic cancer outcome

Pancreatic ductal adenocarcinoma (PDAC) is a highly desmoplastic, aggressive cancer that frequently progresses and spreads by metastasis to the liver1. Cancer-associated fibroblasts, the extracellular matrix and type I collagen (Col I) support2,3 or restrain the progression of PDAC and may impede blood supply and nutrient availability4. The dichotomous role of the stroma in PDAC, and the mechanisms through which it influences patient survival and enables desmoplastic cancers to escape nutrient limitation, remain poorly understood. Here we show that matrix-metalloprotease-cleaved Col I (cCol I) and intact Col I (iCol I) exert opposing effects on PDAC bioenergetics, macropinocytosis, tumour growth and metastasis. Whereas cCol I activates discoidin domain receptor 1 (DDR1)-NF-κB-p62-NRF2 signalling to promote the growth of PDAC, iCol I triggers the degradation of DDR1 and restrains the growth of PDAC. Patients whose tumours are enriched for iCol I and express low levels of DDR1 and NRF2 have improved median survival compared to those whose tumours have high levels of cCol I, DDR1 and NRF2. Inhibition of the DDR1-stimulated expression of NF-κB or mitochondrial biogenesis blocks tumorigenesis in wild-type mice, but not in mice that express MMP-resistant Col I. The diverse effects of the tumour stroma on the growth and metastasis of PDAC and on the survival of patients are mediated through the Col I-DDR1-NF-κB-NRF2 mitochondrial biogenesis pathway, and targeting components of this pathway could provide therapeutic opportunities.

Opposing roles of hepatic stellate cell subpopulations in hepatocarcinogenesis

Hepatocellular carcinoma (HCC), the fourth leading cause of cancer mortality worldwide, develops almost exclusively in patients with chronic liver disease and advanced fibrosis1,2. Here we interrogated functions of hepatic stellate cells (HSCs), the main source of liver fibroblasts3, during hepatocarcinogenesis. Genetic depletion, activation or inhibition of HSCs in mouse models of HCC revealed their overall tumour-promoting role. HSCs were enriched in the preneoplastic environment, where they closely interacted with hepatocytes and modulated hepatocarcinogenesis by regulating hepatocyte proliferation and death. Analyses of mouse and human HSC subpopulations by single-cell RNA sequencing together with genetic ablation of subpopulation-enriched mediators revealed dual functions of HSCs in hepatocarcinogenesis. Hepatocyte growth factor, enriched in quiescent and cytokine-producing HSCs, protected against hepatocyte death and HCC development. By contrast, type I collagen, enriched in activated myofibroblastic HSCs, promoted proliferation and tumour development through increased stiffness and TAZ activation in pretumoural hepatocytes and through activation of discoidin domain receptor 1 in established tumours. An increased HSC imbalance between cytokine-producing HSCs and myofibroblastic HSCs during liver disease progression was associated with increased HCC risk in patients. In summary, the dynamic shift in HSC subpopulations and their mediators during chronic liver disease is associated with a switch from HCC protection to HCC promotion.

The purinergic P2Y14 receptor links hepatocyte death to hepatic stellate cell activation and fibrogenesis in the liver

Fibrosis contributes to ~45% of deaths in western countries. In chronic liver disease, fibrosis is a major factor determining outcomes, but efficient antifibrotic therapies are lacking. Although platelet-derived growth factor and transforming growth factor-β constitute key fibrogenic mediators, they do not account for the well-established link between cell death and fibrosis in the liver. Here, we hypothesized that damage-associated molecular patterns (DAMPs) may link epithelial cell death to fibrogenesis in the injured liver. DAMP receptor screening identified purinergic receptor P2Y14 among several candidates as highly enriched in hepatic stellate cells (HSCs), the main fibrogenic cell type of the liver. Conversely, P2Y14 ligands uridine 5'-diphosphate (UDP)-glucose and UDP-galactose were enriched in hepatocytes and were released upon different modes of cell death. Accordingly, ligand-receptor interaction analysis that combined proteomic and single-cell RNA sequencing data revealed P2Y14 ligands and P2Y14 receptor as a link between dying cells and HSCs, respectively. Treatment with P2Y14 ligands or coculture with dying hepatocytes promoted HSC activation in a P2Y14-dependent manner. P2Y14 ligands activated extracellular signal-regulated kinase (ERK) and Yes-associated protein (YAP) signaling in HSCs, resulting in ERK-dependent HSC activation. Global and HSC-selective P2Y14 deficiency attenuated liver fibrosis in multiple mouse models of liver injury. Functional expression of P2Y14 was confirmed in healthy and diseased human liver and human HSCs. In conclusion, P2Y14 ligands and their receptor constitute a profibrogenic DAMP pathway that directly links cell death to fibrogenesis.

A molecular single-cell lung atlas of lethal COVID-19

Respiratory failure is the leading cause of death in patients with severe SARS-CoV-2 infection^1,2, but the host response at the lung tissue level is poorly understood. Here we performed single-nucleus RNA sequencing of about 116,000 nuclei from the lungs of nineteen individuals who died of COVID-19 and underwent rapid autopsy and seven control individuals. Integrated analyses identified substantial alterations in cellular composition, transcriptional cell states, and cell-to-cell interactions, thereby providing insight into the biology of lethal COVID-19. The lungs from individuals with COVID-19 were highly inflamed, with dense infiltration of aberrantly activated monocyte-derived macrophages and alveolar macrophages, but had impaired T cell responses. Monocyte/macrophage-derived interleukin-1β and epithelial cell-derived interleukin-6 were unique features of SARS-CoV-2 infection compared to other viral and bacterial causes of pneumonia. Alveolar type 2 cells adopted an inflammation-associated transient progenitor cell state and failed to undergo full transition into alveolar type 1 cells, resulting in impaired lung regeneration. Furthermore, we identified expansion of recently described CTHRC1⁺ pathological fibroblasts³ contributing to rapidly ensuing pulmonary fibrosis in COVID-19. Inference of protein activity and ligand-receptor interactions identified putative drug targets to disrupt deleterious circuits. This atlas enables the dissection of lethal COVID-19, may inform our understanding of long-term complications of COVID-19 survivors, and provides an important resource for therapeutic development.

Promotion of cholangiocarcinoma growth by diverse cancer-associated fibroblast subpopulations

Cancer-associated fibroblasts (CAF) are a poorly characterized cell population in the context of liver cancer. Our study investigates CAF functions in intrahepatic cholangiocarcinoma (ICC), a highly desmoplastic liver tumor. Genetic tracing, single-cell RNA sequencing, and ligand-receptor analyses uncovered hepatic stellate cells (HSC) as the main source of CAF and HSC-derived CAF as the dominant population interacting with tumor cells. In mice, CAF promotes ICC progression, as revealed by HSC-selective CAF depletion. In patients, a high panCAF signature is associated with decreased survival and increased recurrence. Single-cell RNA sequencing segregates CAF into inflammatory and growth factor-enriched (iCAF) and myofibroblastic (myCAF) subpopulations, displaying distinct ligand-receptor interactions. myCAF-expressed hyaluronan synthase 2, but not type I collagen, promotes ICC. iCAF-expressed hepatocyte growth factor enhances ICC growth via tumor-expressed MET, thus directly linking CAF to tumor cells. In summary, our data demonstrate promotion of desmoplastic ICC growth by therapeutically targetable CAF subtype-specific mediators, but not by type I collagen.

Tumor restriction by type I collagen opposes tumor-promoting effects of cancer-associated fibroblasts

Cancer-associated fibroblasts (CAF) may exert tumor-promoting and tumor-suppressive functions, but the mechanisms underlying these opposing effects remain elusive. Here, we sought to understand these potentially opposing functions by interrogating functional relationships among CAF subtypes, their mediators, desmoplasia, and tumor growth in a wide range of tumor types metastasizing to the liver, the most common organ site for metastasis. Depletion of hepatic stellate cells (HSC), which represented the main source of CAF in mice and patients in our study, or depletion of all CAF decreased tumor growth and mortality in desmoplastic colorectal and pancreatic metastasis but not in nondesmoplastic metastatic tumors. Single-cell RNA-Seq in conjunction with CellPhoneDB ligand-receptor analysis, as well as studies in immune cell-depleted and HSC-selective knockout mice, uncovered direct CAF-tumor interactions as a tumor-promoting mechanism, mediated by myofibroblastic CAF-secreted (myCAF-secreted) hyaluronan and inflammatory CAF-secreted (iCAF-secreted) HGF. These effects were opposed by myCAF-expressed type I collagen, which suppressed tumor growth by mechanically restraining tumor spread, overriding its own stiffness-induced mechanosignals. In summary, mechanical restriction by type I collagen opposes the overall tumor-promoting effects of CAF, thus providing a mechanistic explanation for their dual functions in cancer. Therapeutic targeting of tumor-promoting CAF mediators while preserving type I collagen may convert CAF from tumor promoting to tumor restricting.

A Therapeutic Silencing RNA Targeting Hepatocyte TAZ Prevents and Reverses Fibrosis in Nonalcoholic Steatohepatitis in Mice

Nonalcoholic steatohepatitis (NASH) is emerging as a major public health issue and is associated with significant liver-related morbidity and mortality. At present, there are no approved drug therapies for NASH. The transcriptional coactivator with PDZ-binding motif (TAZ; encoded by WW domain-containing transcription regulator 1 [WWTR1]) is up-regulated in hepatocytes in NASH liver from humans and has been shown to causally promote inflammation and fibrosis in mouse models of NASH. As a preclinical test of targeting hepatocyte TAZ to treat NASH, we injected stabilized TAZ small interfering RNA (siRNA) bearing the hepatocyte-specific ligand N-acetylgalactosamine (GalNAc-siTAZ) into mice with dietary-induced NASH. As a preventative regimen, GalNAc-siTAZ inhibited inflammation, hepatocellular injury, and the expression of profibrogenic mediators, accompanied by decreased progression from steatosis to NASH. When administered to mice with established NASH, GalNAc-siTAZ partially reversed hepatic inflammation, injury, and fibrosis. Conclusion: Hepatocyte-targeted siTAZ is potentially a novel and clinically feasible treatment for NASH.

Contributions of Fibroblasts, Extracellular Matrix, Stiffness, and Mechanosensing to Hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the third leading cause of cancer mortality worldwide. A unique feature of liver cancer is its close association with liver fibrosis. About 90% of HCCs develop in advanced liver fibrosis or cirrhosis, suggesting an important role for the fibrotic microenvironment in driving HCC development. Here, the authors will discuss functional contributions of liver fibrosis to the development of HCC, focusing on mechanisms through which fibrosis may promote HCC development such as hepatic stellate cell-derived extracellular matrix, growth factors, and cytokines, stiffness-induced signaling pathways, and immunosuppression. Better understanding of these factors in HCC development and progression may provide the basis for novel stromal-based therapies for tumor prevention or therapy.

Macrophage-derived HMGB1 is dispensable for tissue fibrogenesis

Alarmins and damage-associated molecular patterns (DAMPs) are powerful inflammatory mediators, capable of initiating and maintaining sterile inflammation during acute or chronic tissue injury. Recent evidence suggests that alarmins/DAMPs may also trigger tissue regeneration and repair, suggesting a potential contribution to tissue fibrogenesis. High mobility group B1 (HMGB1), a bona fide alarmin/DAMP, may be released passively by necrotic cells or actively secreted by innate immune cells. Macrophages can release large amounts of HMGB1 and play a key role in wound healing and regeneration processes. Here, we hypothesized that macrophages may be a key source of HMGB1 and thereby contribute to wound healing and fibrogenesis. Surprisingly, cell-specific deletion approaches, demonstrated that macrophage-derived HMGB1 is not involved in tissue fibrogenesis in multiple organs with different underlying pathologies. Compared to control HMGB1Flox mice, mice with macrophage-specific HMGB1 deletion (HMGB1ΔMac) do not display any modification of fibrogenesis in the liver after CCL4 or thioacetamide treatment and bile duct ligation; in the kidney following unilateral ureter obstruction; and in the heart after transverse aortic constriction. Of note, even under thermoneutral housing, known to exacerbate inflammation and fibrosis features, HMGB1ΔMac mice do not show impairment of fibrogenesis. In conclusion, our study clearly establishes that macrophage-derived HMGB1 does not contribute to tissue repair and fibrogenesis.

Endogenous IL-33 has no effect on the progression of fibrosis during experimental steatohepatitis

Interleukin (IL)-33 has been recently reported to be strongly pro-fibrogenic in various models of liver disease. Our aim was to study the role of endogenous IL-33 in a diet-induced model of steatohepatitis. IL-33 deficient mice and wild type (WT) littermates received a high-fat diet (HFD), or a standard diet for 12 weeks. The HFD-induced steatohepatitis was associated with an upregulation of IL-33 transcripts and protein. An insulin tolerance test revealed lower systemic insulin sensitivity in IL-33-/—HFD mice than in WT-HFD mice. Nevertheless, IL-33 deficiency did not affect the severity of liver inflammation by histological and transcriptomic analyses, nor the quantity of liver fibrosis. Livers from HFD mice had more myeloid populations, markedly fewer NKT cells and higher proportion of ST2+ Treg cells and ST2+ type 2 innate lymphoid cells (ILC2), all unaffected by IL-33 deficiency. In conclusion, deficiency of endogenous IL-33 does not affect the evolution of experimental diet-induced steatohepatitis towards liver fibrosis.

PARP2 deficiency affects invariant-NKT-cell maturation and protects mice from concanavalin A-induced liver injury

Excessive or persistent inflammation and hepatocyte death are the key triggers of liver diseases. The poly(ADP-ribose) polymerase (PARP) proteins induce cell death and inflammation. Chemical inhibition of PARP activity protects against liver injury during concanavalin A (ConA)-induced hepatitis. In this mice model, ConA activates immune cells, which promote inflammation and induce hepatocyte death, mediated by the activated invariant natural killer T (iNKT) lymphocyte population. We analyzed immune cell populations in the liver and several lymphoid organs, such as the spleen, thymus, and bone marrow in Parp2-deficient mice to better define the role of PARP proteins in liver immunity and inflammation at steady state and during ConA-induced hepatitis. We show that 1) the genetic inactivation of Parp2, but not Parp1, protected mice from ConA hepatitis without deregulating cytokine expression and leucocyte recruitment; 2) cellularity was lower in the thymus, but not in spleen, liver, or bone marrow of Parp2^-/- mice; 3) spleen and liver iNKT lymphocytes, as well as thymic T and NKT lymphocytes were reduced in Parp2 knockout mice. In conclusion, our results suggest that the defect of T-lymphocyte maturation in Parp2 knockout mice leads to a systemic reduction of iNKT cells, reducing hepatocyte death during ConA-mediated liver damage, thus protecting the mice from hepatitis.NEW & NOTEWORTHY The genetic inactivation of Parp2, but not Parp1, protects mice from concanavalin A hepatitis. Immune cell populations are lower in the thymus, but not in the spleen, liver, or bone marrow of Parp2-deficient mice compared with wild-type mice. Spleen and liver invariant natural killer T (NKT) lymphocytes, as well as thymic T and NKT lymphocytes, are reduced in Parp2-deficient mice.

RIPK1 protects hepatocytes from death in Fas-induced hepatitis

Hepatocyte death is a central event during liver disease progression, in which immune cells play key roles by activating members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF), including TNFR1 (TNFRSF1A), Fas (TNFRSF6) and TRAIL-R2 (TNFRSF10B). Receptor Interacting Protein Kinase 1 (RIPK1) emerged as a signaling node downstream of these receptors. In the case of TNFR1, RIPK1 has been demonstrated to paradoxically serve as a scaffold to promote the survival of hepatocytes and as a kinase to kill them. To evaluate whether RIPK1 also protects hepatocytes from death in response to FasL or TRAIL, we took advantage of liver parenchymal cell-specific Ripk1 knockout mice (Ripk1^LPC-KO). We found that Ripk1^LPC-KO mice, as well as primary hepatocytes derived from them, were more susceptible to Fas-mediated apoptosis than their respective WT counterparts. Fas-induced hepatocyte death was independent of TNF-α signaling. Interestingly, while TRAIL administration did not induce hepatitis in Ripk1^LPC-KO mice or in their WT counterparts, its combination with IFN-γ only induced TNF-α dependent apoptosis in the Ripk1^LPC-KO mice. Together, our data demonstrate the protective role of RIPK1 downstream of Fas and highlight the general protective function of RIPK1 in hepatocytes exposed to inflammatory conditions, where TNF-α, FasL and/or TRAIL are present.

Sibiriline, a new small chemical inhibitor of receptor-interacting protein kinase 1, prevents immune-dependent hepatitis

Necroptosis is a regulated form of cell death involved in several disease models including in particular liver diseases. Receptor-interacting protein kinases, RIPK1 and RIPK3, are the main serine/threonine kinases driving this cell death pathway. We screened a noncommercial, kinase-focused chemical library which allowed us to identify Sibiriline as a new inhibitor of necroptosis induced by tumor necrosis factor (TNF) in Fas-associated protein with death domain (FADD)-deficient Jurkat cells. Moreover, Sib inhibits necroptotic cell death induced by various death ligands in human or mouse cells while not protecting from caspase-dependent apoptosis. By using competition binding assay and recombinant kinase assays, we demonstrated that Sib is a rather specific competitive RIPK1 inhibitor. Molecular docking analysis shows that Sib is trapped closed to human RIPK1 adenosine triphosphate-binding site in a relatively hydrophobic pocket locking RIPK1 in an inactive conformation. In agreement with its RIPK1 inhibitory property, Sib inhibits both TNF-induced RIPK1-dependent necroptosis and RIPK1-dependent apoptosis. Finally, Sib protects mice from concanavalin A-induced hepatitis. These results reveal the small-molecule Sib as a new RIPK1 inhibitor potentially of interest for the treatment of immune-dependent hepatitis.

RIPK1 protects hepatocytes from Kupffer cells-mediated TNF-induced apoptosis in mouse models of PAMP-induced hepatitis

BACKGROUND & AIMS

The severity of liver diseases is exacerbated by the death of hepatocytes, which can be induced by the sensing of pathogen associated molecular patterns (PAMPs) derived from the gut microbiota. The molecular mechanisms regulating these cell death pathways are poorly documented. In this study, we investigated the role of the receptor interacting protein kinase 1 (RIPK1), a protein known to regulate cell fate decisions, in the death of hepatocytes using two in vivo models of PAMP-induced hepatitis.

METHODS

Hepatitis was induced in mice by independent injections of two different bacterial PAMPs: lipopolysaccharide (LPS) and unmethylated CpG oligodeoxynucleotide (CpG-DNA) motifs. The role of RIPK1 was evaluated by using mice specifically lacking RIPK1 in liver parenchymal cells (Ripk1^LPC-KO). Administration of liposome-encapsulated clodronate served to investigate the role of Kupffer cells in the establishment of the disease. Etanercept, a tumor necrosis factor (TNF)-decoy receptor, was used to study the contribution of TNF-α during LPS-mediated liver injury.

RESULTS

Whereas RIPK1 deficiency in liver parenchymal cells did not trigger basal hepatolysis, it greatly sensitized hepatocytes to apoptosis and liver damage following a single injection of LPS or CpG-DNA. Importantly, hepatocyte death was prevented by previous macrophage depletion or by TNF inhibition.

CONCLUSIONS

Our data highlight the pivotal function of RIPK1 in maintaining liver homeostasis in conditions of macrophage-induced TNF burst in response to PAMPs sensing.

LAY SUMMARY

Excessive death of hepatocytes is a characteristic of liver injury. A new programmed cell death pathway has been described involving upstream death ligands such as TNF and downstream kinases such as RIPK1. Here, we show that in the presence of LPS liver induced hepatic injury was due to secretion of TNF by liver macrophages, and that RIPK1 acts as a powerful protector of hepatocyte death. This newly identified pathway in the liver may be helpful in the management of patients to predict their risk of developing acute liver failure.

Toll-like receptor-2 exacerbates murine acute viral hepatitis

Viral replication in the liver is generally detected by cellular endosomal Toll‐like receptors (TLRs) and cytosolic helicase sensors that trigger antiviral inflammatory responses. Recent evidence suggests that surface TLR2 may also contribute to viral detection through recognition of viral coat proteins but its role in the outcome of acute viral infection remains elusive. In this study, we examined in vivo the role of TLR2 in acute infections induced by the highly hepatotrophic mouse hepatitis virus (MHV) type 3 and weakly hepatotrophic MHV‐A59 serotype. To address this, C57BL/6 (wild‐type; WT) and TLR2 knockout (KO) groups of mice were intraperitoneally infected with MHV3 or MHV‐A59. MHV3 infection provoked a fulminant hepatitis in WT mice, characterized by early mortality and high alanine and aspartate transaminase levels, histopathological lesions and viral replication whereas infection of TLR2 KO mice was markedly less severe. MHV‐A59 provoked a comparable mild and subclinical hepatitis in WT and TLR2 KO mice. MHV3‐induced fulminant hepatitis in WT mice correlated with higher hepatic expression of interferon‐β, interleukin‐6, tumour necrosis factor‐α, CXCL1, CCL2, CXCL10 and alarmin (interleukin‐33) than in MHV‐A59‐infected WT mice and in MHV3‐infected TLR2 KO mice. Intrahepatic recruited neutrophils, natural killer cells, natural killer T cells or macrophages rapidly decreased in MHV3‐infected WT mice whereas they were sustained in MHV‐A59‐infected WT mice and MHV3‐infected TLR2 KO. MHV3 in vitro infection of macrophagic cells induced rapid and higher viral replication and/or interleukin‐6 induction in comparison to MHV‐A59, and depended on viral activation of TLR2 and p38 mitogen‐activated protein kinase. Taken together, these results support a new aggravating inflammatory role for TLR2 in MHV3‐induced acute fulminant hepatitis.

Ablation of interaction between IL-33 and ST2+ regulatory T cells increases immune cell-mediated hepatitis and activated NK cell liver infiltration

The IL-33/ST2 axis plays a protective role in T-cell-mediated hepatitis, but little is known about the functional impact of endogenous IL-33 on liver immunopathology. We used IL-33-deficient mice to investigate the functional effect of endogenous IL-33 in concanavalin A (Con A)-hepatitis. IL-33(-/-) mice displayed more severe Con A liver injury than wild-type (WT) mice, consistent with a hepatoprotective effect of IL-33. The more severe hepatic injury in IL-33(-/-) mice was associated with significantly higher levels of TNF-α and IL-1β and a larger number of NK cells infiltrating the liver. The expression of Th2 cytokines (IL-4, IL-10) and IL-17 was not significantly varied between WT and IL-33(-/-) mice following Con A-hepatitis. The percentage of CD25(+) NK cells was significantly higher in the livers of IL-33(-/-) mice than in WT mice in association with upregulated expression of CXCR3 in the liver. Regulatory T cells (Treg cells) strongly infiltrated the liver in both WT and IL-33(-/-) mice, but Con A treatment increased their membrane expression of ST2 and CD25 only in WT mice. In vitro, IL-33 had a significant survival effect, increasing the total number of splenocytes, including B cells, CD4(+) and CD8(+) T cells, and the frequency of ST2(+) Treg cells. In conclusion, IL-33 acts as a potent immune modulator protecting the liver through activation of ST2(+) Treg cells and control of NK cells.

The chemical inhibitors of cellular death, PJ34 and Necrostatin-1, down-regulate IL-33 expression in liver

Interleukin-33 (IL-33), a cytokine belonging to the IL-1 family, is crucially involved in inflammatory pathologies including liver injury and linked to various modes of cell death. However, a link between IL-33 and necroptosis or programmed necrosis in liver pathology remains elusive. We aimed to investigate the regulation of IL-33 during necroptosis-associated liver injury. The possible regulation of IL-33 during liver injury by receptor-interacting protein kinase 1 (RIPK1) and poly(ADP-ribose) polymerase 1 (PARP-1) was investigated in mice in vivo and in hepatic stellate cells in vitro. The liver immunohistopathology, flow cytometry, serum transaminase measurement, ELISA, and qPCR-based cytokine measurement were carried out. By using a chemical approach, we showed that pretreatment of mice with Necrostatin-1 (Nec-1) (inhibitor of RIPK1) and/or PJ34 (inhibitor of PARP-1) significantly protected mice against concanavalin A (ConA) liver injury (aspartate amino-transferase (AST)/alanine amino-transferase (ALT)) associated with down-regulated hepatocyte-specific IL-33 expression. In contrast, the expression level of most systemic cytokines (except for IL-6) or activation of liver immune cells was not altered by chemical inhibitors rather an increased infiltration of neutrophils in the liver. During polyinosine-polycytidylic acid (Poly(I:C))-induced acute hepatitis, liver injury and hepatocyte-specific IL-33 expression was also inhibited by PJ34 without any protective effect of PJ34 in CCl4-induced liver injury. Moreover, PJ34 down-regulated the protein expression of IL-33 in activated hepatic stellate cells by cocktail of cytokines or staurosporine in vitro. In conclusion, we evidenced that the Nec-1/PJ34 is a potent inhibitor of liver injury and Nec-1/PJ34 down-regulated hepatocyte-specific IL-33 expression in the liver in vivo or in hepatic stellate cells in vitro, suggesting IL-33 as a possible readout of necroptosis-involved liver pathologies.

KEY MESSAGE

Necroptosis inhibitors can protect mice against liver injury induced by ConA or Poly(I:C). IL-33 expression in liver injury in vivo is inhibited by PJ34. IL-33 expression in hepatic stellate cells in vitro is inhibited by PJ34. Hepatocyte-specific IL-33 expression is down-regulated by Nec-1/PJ34 during hepatitis. IL-33 is a new marker of necroptosis-associated liver injuries.