Role of Immune Cells in Biliary Repair

Angiocrine signaling in sinusoidal homeostasis and liver diseases

The hepatic sinusoids are composed of liver sinusoidal endothelial cells (LSECs), which are surrounded by hepatic stellate cells (HSCs) and contain liver-resident macrophages called Kupffer cells, and other patrolling immune cells. All these cells communicate with each other and with hepatocytes to maintain sinusoidal homeostasis and a spectrum of hepatic functions under healthy conditions. Sinusoidal homeostasis is disrupted by metabolites, toxins, viruses, and other pathological factors, leading to liver injury, chronic liver diseases, and cirrhosis. Alterations in hepatic sinusoids are linked to fibrosis progression and portal hypertension. LSECs are crucial regulators of cellular crosstalk within their microenvironment via angiocrine signaling. This review discusses the mechanisms by which angiocrine signaling orchestrates sinusoidal homeostasis, as well as the development of liver diseases. Here, we summarise the crosstalk between LSECs, HSCs, hepatocytes, cholangiocytes, and immune cells in health and disease and comment on potential novel therapeutic methods for treating liver diseases.

The transcriptomic profile shows the protective effects of celecoxib on cirrhotic splenomegaly

BACKGROUND

Splenomegaly can exacerbate liver cirrhosis and portal hypertension. We have previously demonstrated that cyclooxygenase-2 (COX-2) inhibitor can attenuate cirrhotic splenomegaly. However, the mechanism of cirrhotic splenomegaly remains unclear, thus becoming the focus of the present study.

MATERIALS AND METHODS

Thioacetamide (TAA) intraperitoneal injection was used to induce cirrhotic splenomegaly. Rats were randomized into the control, TAA and TAA + celecoxib groups. Histological analysis and high-throughput RNA sequencing of the spleen were conducted. Splenic collagen III, α-SMA, Ki-67, and VEGF were quantified.

RESULTS

A total of 1461 differentially expressed genes (DEGs) were identified in the spleens of the TAA group compared to the control group. The immune response and immune cell activation might be the major signaling pathways involved in the pathogenesis of cirrhotic splenomegaly. With its immunoregulatory effect, celecoxib presents to ameliorate cirrhotic splenomegaly and liver cirrhosis. Furthermore, 304 coexisting DEGs were obtained between TAA vs. control and TAA + celecoxib vs. TAA. Gene ontology (GO) and KEGG analyses collectively indicated that celecoxib may attenuate cirrhotic splenomegaly through the suppression of splenic immune cell proliferation, inflammation, immune regulation, and fibrogenesis. The impacts on these factors were subsequently validated by the decreased splenic Ki-67-positive cells, macrophages, fibrotic areas, and mRNA levels of collagen III and α-SMA.

CONCLUSIONS

Celecoxib attenuates cirrhotic splenomegaly by inhibiting splenic immune cell proliferation, inflammation, and fibrogenesis. The current study sheds light on the therapeutic strategy of liver cirrhosis by targeting splenic abnormalities and provides COX-2 inhibitors as a novel medical treatment for cirrhotic splenomegaly.

Bile proteome reveals biliary regeneration during normothermic preservation of human donor livers

Normothermic machine perfusion (NMP) after static cold storage is increasingly used for preservation and assessment of human donor livers prior to transplantation. Biliary viability assessment during NMP reduces the risk of post-transplant biliary complications. However, understanding of molecular changes in the biliary system during NMP remains incomplete. We performed an in-depth, unbiased proteomics analysis of bile collected during sequential hypothermic machine perfusion, rewarming and NMP of 55 human donor livers. Longitudinal analysis during NMP reveals proteins reflective of cellular damage at early stages, followed by upregulation of secretory and immune response processes. Livers with bile chemistry acceptable for transplantation reveal protein patterns implicated in regenerative processes, including cellular proliferation, compared to livers with inadequate bile chemistry. These findings are reinforced by detection of regenerative gene transcripts in liver tissue before machine perfusion. Our comprehensive bile proteomics and liver transcriptomics data sets provide the potential to further evaluate molecular mechanisms during NMP and refine viability assessment criteria.

Atypical cholangiocytes derived from hepatocyte-cholangiocyte transdifferentiation mediated by COX-2: a kind of misguided liver regeneration

BACKGROUND

Hepatocyte-cholangiocyte transdifferentiation (HCT) is a potential origin of proliferating cholangiocytes in liver regeneration after chronic injury. This study aimed to determine HCT after chronic liver injury, verify the impacts of HCT on liver repair, and avoid harmful regeneration by understanding the mechanism.

METHODS

A thioacetamide (TAA)-induced liver injury model was established in wild-type (WT-TAA group) and COX-2 panknockout (KO-TAA group) mice. HCT was identified by costaining of hepatocyte and cholangiocyte markers in vivo and in isolated mouse hepatocytes in vitro. The biliary tract was injected with ink and visualized by whole liver optical clearing. Serum and liver bile acid (BA) concentrations were measured. Either a COX-2 selective inhibitor or a β-catenin pathway inhibitor was administered in vitro.

RESULTS

Intrahepatic ductular reaction was associated with COX-2 upregulation in chronic liver injury. Immunofluorescence and RNA sequencing indicated that atypical cholangiocytes were characterized by an intermediate genetic phenotype between hepatocytes and cholangiocytes and might be derived from hepatocytes. The structure of the biliary system was impaired, and BA metabolism was dysregulated by HCT, which was mediated by the TGF-β/β-catenin signaling pathway. Genetic deletion or pharmaceutical inhibition of COX-2 significantly reduced HCT in vivo. The COX-2 selective inhibitor etoricoxib suppressed HCT through the TGF-β-TGFBR1-β-catenin pathway in vitro.

CONCLUSIONS

Atypical cholangiocytes can be derived from HCT, which forms a secondary strike by maldevelopment of the bile drainage system and BA homeostasis disequilibrium during chronic liver injury. Inhibition of COX-2 could ameliorate HCT through the COX-2-TGF-β-TGFBR1-β-catenin pathway and improve liver function.

C-C motif chemokine receptor 2 inhibition reduces liver fibrosis by restoring the immune cell landscape

The accumulation of extracellular matrix (ECM) proteins in the liver leads to liver fibrosis and end-stage liver cirrhosis. C-C motif chemokine receptor 2 (CCR2) is an attractive target for treating liver fibrosis. However, limited investigations have been conducted to explore the mechanism by which CCR2 inhibition reduces ECM accumulation and liver fibrosis, which is the focus of this study. Liver injury and liver fibrosis were induced by carbon tetrachloride (CCl₄) in wild-type mice and Ccr2 knockout (Ccr2^-/-) mice. CCR2 was upregulated in murine and human fibrotic livers. Pharmacological CCR2 inhibition with cenicriviroc (CVC) reduced ECM accumulation and liver fibrosis in prevention and treatment administration. In single-cell RNA sequencing (scRNA-seq), CVC was demonstrated to alleviate liver fibrosis by restoring the macrophage and neutrophil landscape. CVC administration and CCR2 deletion can also inhibit the hepatic accumulation of inflammatory FSCN1⁺ macrophages and HERC6⁺ neutrophils. Pathway analysis indicated that the STAT1, NFκB, and ERK signaling pathways might be involved in the antifibrotic effects of CVC. Consistently, Ccr2 knockout decreased phosphorylated STAT1, NFκB, and ERK in the liver. In vitro, CVC could transcriptionally suppress crucial profibrotic genes (Xaf1, Slfn4, Slfn8, Ifi213, and Il1β) in macrophages by inactivating the STAT1/NFκB/ERK signaling pathways. In conclusion, this study depicts a novel mechanism by which CVC alleviates ECM accumulation in liver fibrosis by restoring the immune cell landscape. CVC can inhibit profibrotic gene transcription via inactivating the CCR2-STAT1/NFκB/ERK signaling pathways.

Liver-Targeted Delivery of Small Interfering RNA of C-C Chemokine Receptor 2 with Tetrahedral Framework Nucleic Acid Attenuates Liver Cirrhosis

Liver cirrhosis is the end stage of chronic liver diseases without approved clinical drugs. In this study, a new strategy that uses a C-C chemokine receptor 2 (CCR2) small interfering RNA silencing (siCcr2)-based therapy by loading multivalent siCcr2 with tetrahedron framework DNA nanostructure (tFNA) vehicle (tFNA-siCcr2) was established to attenuate liver fibrosis. tFNA-siCcr2 was successfully synthesized without changing the physiochemical properties of tFNA. Compared to the naked siCcr2 molecule, the tFNA-siCcr2 complex altered the accumulation from the kidney to the liver after the intraperitoneal injection. The tFNA-siCcr2 complex also prolonged hepatic retention and mainly colocalized within macrophages and endothelial cells. tFNA-siCcr2 efficiently silenced CCR2 and significantly ameliorated liver fibrosis in prevention and treatment interventions. Single-cell RNA sequencing followed by experimental validation suggested that tFNA-siCcr2 can restore the immune cell landscape and construct an antifibrotic niche by inhibiting profibrotic macrophage and neutrophil accumulation in the murine fibrotic liver. Molecularly, the tFNA-siCcr2 complex reduced inflammatory mediator production by inactivating the NF-κB signaling pathway. In conclusion, the tFNA-based liver-targeted tFNA-siCcr2 delivery complex efficiently ameliorated liver fibrosis by restoring the immune cell landscape and constructing an antifibrotic niche, which makes the tFNA-siCcr2 complex a potential therapeutic candidate for the clinical treatment of liver cirrhosis.

Proangiogenic role of circRNA-007371 in liver fibrosis

Circular RNAs (circRNAs) are crucially involved in cancers as competing endogenous RNA (ceRNA) or microRNA (miRNA) sponges. However, the function and mechanism of circRNAs in liver fibrosis remain unknown and are the focus of this study. Murine fibrotic models were induced by thioacetamide (TAA) or carbon tetrachloride (CCl₄ ). Increased angiogenesis is accompanied by liver fibrosis in TAA- and CCl₄ -induced murine fibrotic livers. circRNA microarray and argonaute 2 (AGO2)-RNA immunoprecipitation (RIP) sequencing (AGO2-RIP sequencing) were performed in murine livers to screen for functional circRNAs. Compared to control livers, 86 differentially expressed circRNAs were obtained in TAA-induced murine fibrotic livers using circRNA microarray. In addition, 551 circRNAs were explored by AGO2-RIP sequencing of murine fibrotic livers. The circRNA-007371 was then selected and verified for back-spliced junction, resistance to RNase R, and loop formation. In vitro, murine hemangioendothelioma endothelial (EOMA) cells were transfected with circRNA-007371 overexpressing plasmid or empty plasmid. circRNA-007371 overexpression promoted tube formation, migration, and cell proliferation of EOMA cells. RNA sequencing and miRNA sequencing were then performed to explore the mechanism of the proangiogenic effects of circRNA-007371. circRNA-007371 promotes liver fibrosis via miRNA sponges or ceRNA mechanisms. Stag1, the parent gene of circRNA-007371, may play a significant role in proangiogenic progression. In conclusion, circRNA-007371 enhances angiogenesis via a miRNA sponge mechanism in liver fibrosis. The antiangiogenic effect of circRNA-007371 inhibition may provide a new strategy for treating patients with liver cirrhosis.

Peroxisome Proliferator-Activated Receptors Regulate Hepatic Immunity and Assist in the Treatment of Primary Biliary Cholangitis

Fibrates, which are agonists of peroxisome proliferator-activated receptor alpha, have received increasing attention in the treatment of primary biliary cholangitis. Reduced alkaline phosphatase levels and improved clinical outcomes were observed in patients with primary biliary cholangitis with an inadequate response to ursodeoxycholic acid (UDCA) monotherapy4 when treated with bezafibrate or fenofibrate combined with UDCA. In contrast to obeticholic acid, which exacerbates pruritus in patients, fibrates have been shown to relieve pruritus. Clinical trial outcomes show potential for the treatment of primary biliary cholangitis by targeting peroxisome proliferator-activated receptors. It is currently agreed that primary biliary cholangitis is an autoimmune-mediated cholestatic liver disease, and peroxisome proliferator-activated receptor is a nuclear receptor that regulates the functions of multiple immune cells, thus playing an important role in regulating innate and adaptive immunity. Therefore, this review focuses on the immune disorder of primary biliary cholangitis and summarizes the regulation of hepatic immunity when peroxisome proliferator-activated receptors are targeted for treating primary biliary cholangitis.

The Hepatoprotective and Hepatotoxic Roles of Sex and Sex-Related Hormones

Most liver diseases, including acute liver injury, drug-induced liver injury, viral hepatitis, metabolic liver diseases, and end-stage liver diseases, are strongly linked with hormonal influences. Thus, delineating the clinical manifestation and underlying mechanisms of the “sexual dimorphism” is critical for providing hints for the prevention, management, and treatment of those diseases. Whether the sex hormones (androgen, estrogen, and progesterone) and sex-related hormones (gonadotrophin-releasing hormone, luteinizing hormone, follicle-stimulating hormone, and prolactin) play protective or toxic roles in the liver depends on the biological sex, disease stage, precipitating factor, and even the psychiatric status. Lifestyle factors, such as obesity, alcohol drinking, and smoking, also drastically affect the involving mechanisms of those hormones in liver diseases. Hormones deliver their hepatic regulatory signals primarily via classical and non-classical receptors in different liver cell types. Exogenous sex/sex-related hormone therapy may serve as a novel strategy for metabolic liver disease, cirrhosis, and liver cancer. However, the undesired hormone-induced liver injury should be carefully studied in pre-clinical models and monitored in clinical applications. This issue is particularly important for menopause females with hormone replacement therapy (HRT) and transgender populations who want to receive gender-affirming hormone therapy (GAHT). In conclusion, basic and clinical studies are warranted to depict the detailed hepatoprotective and hepatotoxic mechanisms of sex/sex-related hormones in liver disease. Prolactin holds a promising perspective in treating metabolic and advanced liver diseases.

Inflammasomes and Pyroptosis of Liver Cells in Liver Fibrosis

Inflammasomes are multiprotein complexes that can sense danger signals and activate caspase-1 to mediate pro-inflammatory cytokines release and pyroptotic cell death. There are two main canonical and non-canonical signaling pathways that trigger inflammasome activation. Inflammasomes are expressed and assembled in parenchymal and nonparenchymal cells in response to liver injury in the liver. Additionally, the hepatocytes, biliary epithelial cells (cholangiocytes), hepatic stellate cells (HSCs), hepatic macrophages, and liver sinusoidal endothelial cells (LSECs) contribute to liver fibrosis via different mechanisms. However, the underlying mechanism of the inflammasome and pyroptosis in these liver cells in liver fibrosis remains elusive. This review summarizes the activation and function of inflammasome complexes and then discusses the association between inflammasomes, pyroptosis, and liver fibrosis. Unlike other similar reviewers, we will focus on the effect of inflammasome activation and pyroptosis in the various liver cells during the development of liver fibrosis. We will also highlight the latest progress of pharmacological intervention in inflammasome-mediated liver fibrosis.