Distinct populations of hepatic stellate cells in the mouse liver have different capacities for retinoid and lipid storage

Retinoic acid generates a beneficial microenvironment for liver progenitor cell activation in acute liver failure

BACKGROUND

When massive necrosis occurs in acute liver failure (ALF), rapid expansion of HSCs called liver progenitor cells (LPCs) in a process called ductular reaction is required for survival. The underlying mechanisms governing this process are not entirely known to date. In ALF, high levels of retinoic acid (RA), a molecule known for its pleiotropic roles in embryonic development, are secreted by activated HSCs. We hypothesized that RA plays a key role in ductular reaction during ALF.

METHODS

RNAseq was performed to identify molecular signaling pathways affected by all-trans retinoid acid (atRA) treatment in HepaRG LPCs. Functional assays were performed in HepaRG cells treated with atRA or cocultured with LX-2 cells and in the liver tissue of patients suffering from ALF.

RESULTS

Under ALF conditions, activated HSCs secreted RA, inducing RARα nuclear translocation in LPCs. RNAseq data and investigations in HepaRG cells revealed that atRA treatment activated the WNT-β-Catenin pathway, enhanced stemness genes (SOX9, AFP, and others), increased energy storage, and elevated the expression of ATP-binding cassette transporters in a RARα nuclear translocation-dependent manner. Further, atRA treatment-induced pathways were confirmed in a coculture system of HepaRG with LX-2 cells. Patients suffering from ALF who displayed RARα nuclear translocation in the LPCs had significantly better MELD scores than those without.

CONCLUSIONS

During ALF, RA secreted by activated HSCs promotes LPC activation, a prerequisite for subsequent LPC-mediated liver regeneration.

LX-2 Stellate Cells Are a Model System for Investigating the Regulation of Hepatic Vitamin A Metabolism and Respond to Tumor Necrosis Factor α and Interleukin 1β

Hepatic stellate cells (HSCs) are the major site of vitamin A (retinol) esterification and subsequent storage as retinyl esters within lipid droplets. However, retinyl esters become depleted in many pathophysiological states, including acute and chronic liver injuries. Recently, using a liver slice culture system as a model of acute liver injury and fibrogenesis, a time-dependent increase and decrease in the apparent formation of the bioactive retinoid all-trans-retinoic acid (atRA) and retinyl palmitate was measured, respectively. This coincided with temporal changes in the gene expression of retinoid-metabolizing enzymes and binding proteins, that preceded HSC activation. However, the underlying mechanisms that promote early changes in retinoid metabolism remain unresolved. We hypothesized that LX-2 cells could be applied to investigate differences in quiescent and activated HSC retinoid metabolism. We demonstrate that the hypermetabolic state of activated stellate cells relative to quiescent stellate cells may be attributed to induction of STRA6, RBP4, and CYP26A1, thereby reducing intracellular concentrations of atRA. We further hypothesized that paracrine and autocrine cytokine signaling regulates HSC vitamin A metabolism in both quiescent and activated cells. In quiescent cells, tumor necrosis factor α dose-dependently downregulated LRAT and CRBP1 mRNA, with EC50 values of 30-50 pg/mL. Likewise, interleukin-1β decreased LRAT and CRBP1 gene expression but with less potency. In activated stellate cells, multiple enzymes were downregulated, suggesting that the full effects of altered hepatic vitamin A metabolism in chronic conditions require both paracrine and autocrine signaling events. Further, this study suggests the potential for cell type-specific autocrine effects in hepatic retinoid signaling. SIGNIFICANCE STATEMENT: HSCs are the major site of vitamin A storage and important determinants of retinol metabolism during liver fibrogenesis. Here, two LX-2 culture methods were applied as models of hepatic retinoid metabolism to demonstrate the effects of activation status and dose-dependent cytokine exposure on the expression of genes involved in retinoid metabolism. This study suggests that compared to quiescent cells, activated HSCs are hypermetabolic and have reduced apparent formation of retinoic acid, which may alter downstream retinoic acid signaling.

Netrin-1 feedforward mechanism promotes pancreatic cancer liver metastasis via hepatic stellate cell activation, retinoid, and ELF3 signaling

The biology of metastatic pancreatic ductal adenocarcinoma (PDAC) is distinct from that of the primary tumor due to changes in cell plasticity governed by a distinct transcriptome. Therapeutic strategies that target this distinct biology are needed. We detect an upregulation of the neuronal axon guidance molecule Netrin-1 in PDAC liver metastases that signals through its dependence receptor (DR), uncoordinated-5b (Unc5b), to facilitate metastasis in vitro and in vivo. The mechanism of Netrin-1 induction involves a feedforward loop whereby Netrin-1 on the surface of PDAC-secreted extracellular vesicles prepares the metastatic niche by inducing hepatic stellate cell activation and retinoic acid secretion that in turn upregulates Netrin-1 in disseminated tumor cells via RAR/RXR and Elf3 signaling. While this mechanism promotes PDAC liver metastasis, it also identifies a therapeutic vulnerability, as it can be targeted using anti-Netrin-1 therapy to inhibit metastasis using the Unc5b DR cell death mechanism.

Dietary Vitamin A Affects the Function of Incretin-Producing Enteroendocrine Cells in Male Mice Fed a High-Fat Diet

BACKGROUND

Retinol-binding protein 2 (RBP2) is an intracellular carrier for vitamin A in the absorptive enterocytes. Mice lacking RBP2 (Rbp2-/-) display an unexpected phenotype of obesity, glucose intolerance, and elevated glucose-dependent insulinotropic polypeptide (GIP) levels. GIP and glucagon-like peptide 1 (GLP-1) are incretin hormones secreted by enteroendocrine cells (EECs). We recently demonstrated the presence of RBP2 and other retinoid-related proteins in EECs.

OBJECTIVES

Given RBP2's role in intracellular retinoid trafficking, we aimed to evaluate whether dietary vitamin A affects incretin-secreting cell function and gene expression.

METHODS

Male Rbp2-/- mice and sex- and age-matched controls (n = 6-9) were fed a high-fat diet (HFD) for 18 wk containing normal (VAN, 4000 IU/kg of diet) or low (VAL, 25% of normal) vitamin A concentrations. Body weight was recorded biweekly. Plasma GIP and GLP-1 levels were obtained fasting and 30 min after an oral fat gavage at week 16. Glucose tolerance tests were also performed. Mice were killed at week 18, and blood and tissue samples were obtained.

RESULTS

Rbp2-/- mice displayed greater weight gain on the VAN compared with the VAL diet from week 7 of the intervention (P ≤ 0.01). Stimulated GIP levels were elevated in Rbp2-/- mice compared with their controls fed the VAN diet (P = 0.02), whereas their GIP response was lower when fed the VAL diet (P = 0.03). Although no differences in GLP-1 levels were observed in the VAN diet group, a lower GLP-1 response was seen in Rbp2-/- mice fed the VAL diet (P = 0.02). Changes in incretin gene expression and that of other genes associated with EEC lineage and function were consistent with these observations. Circulating and hepatic retinoid levels revealed no systemic vitamin A deficiency across dietary groups.

CONCLUSIONS

Our data support a role for RBP2 and dietary vitamin A in incretin secretion and gene expression in mice fed a HFD.

Cell lipid droplet heterogeneity and altered biophysical properties induced by cell stress and metabolic imbalance

Lipid droplets (LD) are important regulators of lipid metabolism and are implicated in several diseases. However, the mechanisms underlying the roles of LD in cell pathophysiology remain elusive. Hence, new approaches that enable better characterization of LD are essential. This study establishes that Laurdan, a widely used fluorescent probe, can be used to label, quantify, and characterize changes in cell LD properties. Using lipid mixtures containing artificial LD we show that Laurdan GP depends on LD composition. Accordingly, enrichment in cholesterol esters (CE) shifts Laurdan GP from ~0.60 to ~0.70. Moreover, live-cell confocal microscopy shows that cells present multiple LD populations with distinctive biophysical features. The hydrophobicity and fraction of each LD population are cell type dependent and change differently in response to nutrient imbalance, cell density, and upon inhibition of LD biogenesis. The results show that cellular stress caused by increased cell density and nutrient overload increased the number of LD and their hydrophobicity and contributed to the formation of LD with very high GP values, likely enriched in CE. In contrast, nutrient deprivation was accompanied by decreased LD hydrophobicity and alterations in cell plasma membrane properties. In addition, we show that cancer cells present highly hydrophobic LD, compatible with a CE enrichment of these organelles. The distinct biophysical properties of LD contribute to the diversity of these organelles, suggesting that the specific alterations in their properties might be one of the mechanisms triggering LD pathophysiological actions and/or be related to the different mechanisms underlying LD metabolism.

Retinoids stored locally in the lung are required to attenuate the severity of acute lung injury in male mice

Retinoids are potent transcriptional regulators that act in regulating cell proliferation, differentiation, and other cellular processes. We carried out studies in male mice to establish the importance of local cellular retinoid stores within the lung alveolus for maintaining its health in the face of an acute inflammatory challenge induced by intranasal instillation of lipopolysaccharide. We also undertook single cell RNA sequencing and bioinformatic analyses to identify roles for different alveolar cell populations involved in mediating these retinoid-dependent responses. Here we show that local retinoid stores and uncompromised metabolism and signaling within the lung are required to lessen the severity of an acute inflammatory challenge. Unexpectedly, our data also establish that alveolar cells other than lipofibroblasts, specifically microvascular endothelial and alveolar epithelial cells, are able to take up lipoprotein-transported retinoid and to accumulate cellular retinoid stores that are directly used to respond to an acute inflammatory challenge.

Isolation, Purification, and Culture of Primary Murine Hepatic Stellate Cells: An Update

In the healthy liver, quiescent hepatic stellate cells (HSCs) are found in the perisinusoidal space (i.e., the space of Dissé) in close proximity to endothelial cells and hepatocytes. HSCs represent 5-8% of the total number of liver cells and are characterized by numerous fat vacuoles that store vitamin A in the form of retinyl esters. Upon liver injury caused by different etiologies, HSCs become activated and acquire a myofibroblast (MFB) phenotype in a process called transdifferentiation. In contrast to quiescent HSC, MFB become highly proliferative and are characterized by an imbalance in extracellular matrix (ECM) homeostasis, by producing an excess of collagen and blocking its turnover by synthesis of protease inhibitors. This leads to a net accumulation of ECM during fibrosis. In addition to HSC, there are fibroblasts in the portal fields (pF), which also have the potency to acquire a myofibroblastic phenotype (pMF). The contributions of these two fibrogenic cell types (i.e., MFB and pMF) vary based on the etiology of liver damage (parenchymal vs. cholestatic). Based on their importance to hepatic fibrosis, the isolation and purification protocols of these primary cells are in great demand. Moreover, established cell lines may offer only limited information about the in vivo behavior of HSC/MFB and pF/pMF.Here we describe a method for high-purity isolation of HSC from mice. In the first step, the liver is digested with pronase and collagenase, and the cells are dissociated from the tissue. In the second step, HSCs are enriched by density gradient centrifugation of the crude cell suspension using a Nycodenz gradient. The resulting cell fraction can be further optionally purified by flow cytometric enrichment to generate ultrapure HSC.

Intercellular communication among liver cells in the perisinusoidal space of the injured liver: Pathophysiology and therapeutic directions

Hepatic stellate cells (HSCs) in the perisinusoidal space are surrounded by hepatocytes, liver sinusoidal endothelial cells, Kupffer cells, and other resident immune cells. In the normal liver, HSCs communicate with these cells to maintain normal liver functions. However, after chronic liver injury, injured hepatocytes release several proinflammatory mediators, reactive oxygen species, and damage-associated molecular patterns into the perisinusoidal space. Consequently, such alteration activates quiescent HSCs to acquire a myofibroblast-like phenotype and express high amounts of transforming growth factor-β1, angiopoietins, vascular endothelial growth factors, interleukins 6 and 8, fibril forming collagens, laminin, and E-cadherin. These phenotypic and functional transdifferentiation lead to hepatic fibrosis with a typical abnormal extracellular matrix synthesis and disorganization of the perisinusoidal space of the injured liver. Those changes provide a favorable environment that regulates tumor cell proliferation, migration, adhesion, and survival in the perisinusoidal space. Such tumor cells by releasing transforming growth factor-β1 and other cytokines, will, in turn, activate and deeply interact with HSCs via a bidirectional loop. Furthermore, hepatocellular carcinoma-derived mediators convert HSCs and macrophages into protumorigenic cell populations. Thus, the perisinusoidal space serves as a critical hub for activating HSCs and their interactions with other cell types, which cause a variety of liver diseases such as hepatic inflammation, fibrosis, cirrhosis, and their complications, such as portal hypertension and hepatocellular carcinoma. Therefore, targeting the crosstalk between activated HSCs and tumor cells/immune cells in the tumor microenvironment may also support a promising therapeutic strategy.

MiRNA/mRNA network topology in hepatitis virus B-related liver cirrhosis reveals miR-20a-5p/340-5p as hubs initiating fibrosis

Background

The pathophysiology of hepatitis B-related liver cirrhosis (HBV-LC) remains unclear. This study aimed to explore the disease mechanisms using topological analysis of the miRNA/mRNA network.

Methods

Paired miRNA/mRNA sequencing was performed with thirty-three peripheral blood mononuclear cell samples (LC, n = 9; chronic hepatitis B, n = 12; normal controls, n = 12) collected from a prospective cohort to identify the miRNA/mRNA network. Topological features and functional implications of the network were analyzed to capture pathophysiologically important miRNAs/mRNAs, whose expression patterns were confirmed in the validation group (LC, n = 15; chronic hepatitis B, n = 15; normal controls, n = 10), and functional potentials initiating fibrogenesis were demonstrated in vitro.

Results

The miRNA/mRNA network contained 3121 interactions between 158 differentially expressed (DE) miRNAs and 442 DE-mRNAs. The topological analysis identified a core module containing 99 miRNA/mRNA interactions and two hub nodes (miR-20a-5p/miR-340-5p), which connected to 75 DE-mRNAs. The expression pattern along the disease progression of the core module was found associated with a continuous increase in wound healing, inflammation, and leukocyte migration but an inflection of immune response and lipid metabolic regulation, consistent with the pathophysiology of HBV-LC. MiR-20a-5p/miR-340-5p were found involved in macrophage polarization and hepatic stellate cell (HSC) activation in vitro (THP-1, LX-2 cell lines), and their expression levels were confirmed in the validation group independently.

Conclusion

Topological analysis of the miRNA/mRNA network in HBV-LC revealed the association between fibrosis and miR-20a-5p/miR-340-5p involving initiating activations of macrophage and HSC. Further validations should be performed to confirm the HSC/macrophage activations and the interactions between miR-20a-5p/miR-340-5p and their potential targets, which may help to develop non-invasive prognostic markers or intervention targets for HBV-LC.

Liver tissue remodeling following ablation with irreversible electroporation in a porcine model

Irreversible electroporation (IRE) is a method of non-thermal focal tissue ablation characterized by irreversibly permeabilizing the cell membranes while preserving the extracellular matrix. This study aimed to investigate tissue remodeling after IRE in a porcine model, especially focusing on the extracellular matrix and hepatic stellate cells. IRE ablation was performed on 11 female pigs at 2,000 V/cm electric field strength using a versatile high-voltage generator and 3 cm diameter parallel-plate electrodes. The treated lobes were removed during surgery at 1, 3, 7, 14, and 21 days after IRE. Tissue remodeling and regeneration were assessed by histopathology and immunohistochemistry. Throughout the treated area, IRE led to extensive necrosis with intact collagenous structures evident until day 1. From then on, the necrosis progressively diminished while reparative tissue gradually increased. During this process, the reticulin framework and the septal fibrillar collagen remained in the necrotic foci until they were invaded by the reparative tissue. The reparative tissue was characterized by a massive proliferation of myofibroblast-like cells accompanied by a complete disorganization of the extracellular matrix with the disappearance of hepatic architecture. Hepatic stellate cell markers were associated with the proliferation of myofibroblast-like cells and the reorganization of the extracellular matrix. Between 2 and 3 weeks after IRE, the lobular architecture was almost completely regenerated. The events described in the present study show that IRE may be a valid model to study the mechanisms underlying liver regeneration after extensive acute injury.

Differential recruitment of monocyte-derived macrophages in control and stellate cell-depleted mice during recurrent carbon tetrachloride-induced acute liver injury

Liver depleted of hepatic stellate cells (HSCs) is resistant to ischemia/reperfusion-, concanavalin A-, and acetaminophen-induced acute injury. Whether HSCs regulate carbon tetrachloride (CCl4 )-induced acute liver injury is not known. CCl4 treatment damages pericentral hepatocytes that express CCl4 -metabolizing Cyp2E1 and activates HSCs. We investigated whether HSC-depletion in mice transgenic for thymidine kinase under the glial fibrillary acidic protein promoter (GFAP-TK-Tg) confers resistance to injury and inflammation due to CCl4 rechallenge. GFAP-TK-Tg or wild type (WT) mice were administered 0.16 ml/kg CCl4 (3× at 3 days intervals), then 40 μg/g/day ganciclovir for 10 days. The treatment depletes ~70%-75% HSCs from GFAP-TK-Tg but not WT mice while the liver recovers from earlier CCl4 -induced injury. Mice were then administered CCl4 , and liver injury and inflammation were determined at 24 h. HSC-depleted and HSC-sufficient mice showed similar CCl4 -induced hepatocyte necrosis and oxidative stress. However, increase in F4/80+ macrophages, but not CD68+ cells, was greater in CCl4 rechallenged HSC-depleted compared to HSC-sufficient mice. Expression of tumor necrosis factor-α (TNF-α), CCL2, and CXCL1 increased similarly, whereas increase in interleukin-6 (IL6), IL1β, and IL10 expression was higher in CCl4 rechallenged HSC-depleted compared to HSC-sufficient mice. CCl4 rechallenge of HSC-sufficient mice rapidly activated HSCs causing significant fibrosis with increased expression of Col1a1, transforming growth factor β1 (TGFβ1), tissue inhibitors of metalloproteinases 1 (TIMP1); increase in TIPM1 was much lower and metalloproteinases 13 (MMP13) greater in CCl4 rechallenged HSC-depleted mice. Interestingly, hepatic recruitment of both profibrogenic (Ly6Chi ) and antifibrogenic restorative (Ly6Clo ) macrophages, and neutrophils was significantly greater in CCl4 rechallenged HSC-depleted mice. These data suggest that CCl4 directly damages hepatocytes but HSCs regulate inflammation. Rapid fibrogenesis in CCl4 rechallenged HSC-sufficient mice recovered from earlier injury indicates that even transiently activated HSCs that had reverted to the quiescent phenotype remain primed to become reactivated.

Acute retinol mobilization by retinol-binding protein 4 in mouse liver induces fibroblast growth factor 21 expression

Hepatocytes secrete retinol-binding protein 4 (RBP4) into circulation, thereby mobilizing vitamin A from the liver to provide retinol for extrahepatic tissues. Obesity and insulin resistance are associated with elevated RBP4 levels in the blood. However, in a previous study, we observed that chronically increased RBP4 by forced Rbp4 expression in the liver does not impair glucose homeostasis in mice. Here, we investigated the effects of an acute mobilization of hepatic vitamin A stores by hepatic overexpression of RBP4 in mice. We show that hepatic retinol mobilization decreases body fat content and enhances fat turnover. Mechanistically, we found that acute retinol mobilization increases hepatic expression and serum levels of fibroblast growth factor 21 (FGF21), which is regulated by retinol mobilization and retinoic acid in primary hepatocytes. Moreover, we provide evidence that the insulin-sensitizing effect of FGF21 is associated with organ-specific adaptations in retinoid homeostasis. Taken together, our findings identify a novel crosstalk between retinoid homeostasis and FGF21 in mice with acute RBP4-mediated retinol mobilization from the liver.

Adipose-derived stem cells in the treatment of hepatobiliary diseases and sepsis

Determination of the mesenchymal stem cells is one of the greatest and most exciting achievements that tissue engineering and regenerative medicine have achieved. Adipose-derived mesenchymal stem cells (AD-MSC) are easily isolated and cultured for a long time before losing their stem cell characteristics, which are self-renewal and pluripotency. AD-MSC are mesenchymal stem cells that have pluripotent lineage characteristics. They are easily accessible, and the fraction of stem cells in the adipose tissue lysates is highest among all other sources of mesenchymal stem cells. It is also HLA-DR negative and can be transplanted allogenically without the need for immunosuppression. These advantages have popularized its use in many fields including plastic reconstructive surgery. However, in the field of hepatology and liver transplantation, the progress is slower. AD-MSC have the potential to modulate inflammation, ameliorate ischemia-reperfusion injury, and support liver and biliary tract regeneration. These are very important for the treatment of various hepatobiliary diseases. Furthermore, the anti-inflammatory potential of these cells has paramount importance in the treatment of sepsis. We need alternative therapeutic approaches to treat end-stage liver failure. AD-MSC can provide a means of therapy to bridge to definitive therapeutic alternatives such as liver transplantation. Here we propose to review theoretic applications of AD-MSC in the treatment of hepatobiliary diseases and sepsis.

Optimized Isolation and Characterization of C57BL/6 Mouse Hepatic Stellate Cells

To obtain meaningful results of hepatic stellate cell (HSC) function, it is crucial to use highly pure HSC populations. Our aim was to optimize HSC isolation from mice livers without exploiting the characteristically transient vitamin A autofluorescence of HSC. HSCs were isolated from C57BL/6 mice using a two-step collagenase digestion and Nycodenz gradient separation followed by CD11b-negative sorting step in order to remove contaminating macrophages and dendritic cells. Isolated cells were analyzed for yield, viability, purity, and potential new markers using immunofluorescence and flow cytometry. We obtained a yield of 350,595 ± 100,773 HSC per mouse liver and a viability of isolated cells of 92.4 ± 3.1%. We observed a low macrophage/dendritic cell contamination of 1.22 ± 0.54%. Using flow cytometry, we demonstrated that CD38 was expressed at the surface of HSC subpopulations and that all expressed intracellular markers specific for HSC in the liver. This isolation method, avoiding fluorescent activated cell sorting (FACS), allowed isolation of HSCs with high purity. Further, flow cytometry analysis suggests that CD38 may be a reliable marker of HSCs and may include subpopulations of HSCs without retinoid droplets.

Caffeine mitigates experimental nonalcoholic steatohepatitis and the progression of thioacetamide-induced liver fibrosis by blocking the MAPK and TGF-β/Smad3 signaling pathways

INTRODUCTION AND OBJECTIVES

Caffeine consumption is associated with beneficial effects on hepatic disorders. The objectives of this study were to evaluate the antifibrotic effects of caffeine on experimental nonalcoholic steatohepatitis (NASH) induced with a high-fat, high-sucrose, high-cholesterol diet (HFSCD), as well as to evaluate the ability of caffeine to prevent the progression of experimental liver fibrosis induced by the administration of thioacetamide (TAA) in rats and explore the mechanisms of action.

METHODS

NASH and fibrosis were induced in rats by the administration of an HFSCD for 15 weeks, and liver fibrosis was induced by intraperitoneal administration of 200 mg/kg TAA 3 times per week, for 6 weeks. Caffeine was administered at a dose of 50 mg/kg body weight. The effects of diet, TAA, and caffeine on fibrosis were evaluated by biochemical and histological examinations. The profibrotic pathways were analyzed by western blotting and immunohistochemistry.

RESULTS

Rats exhibited liver fibrosis after HFSCD feeding and the administration of TAA. Caffeine could reduce the hepatic level of collagen and the fibrotic area in the liver. Caffeine prevented the progression of liver fibrosis by decreasing transforming growth factor-beta (TGF-β), connective tissue growth factor (CTGF), and alpha-smooth muscle actin (α-SMA) expression and by inhibiting the activation of mitogen-activated protein kinases (MAPKs) and Smad3 phosphorylation.

CONCLUSIONS

Caffeine attenuates NASH and the progression of liver fibrosis due to its antifibrotic effects and modulating the MAPK and TGF-β pathways. Therefore, caffeine could be a suitable candidate for treating liver diseases associated with fibrosis.

Response of Human Liver Tissue to Innate Immune Stimuli

Precision-cut human liver slice cultures (PCLS) have become an important alternative immunological platform in preclinical testing. To further evaluate the capacity of PCLS, we investigated the innate immune response to TLR3 agonist (poly-I:C) and TLR4 agonist (LPS) using normal and diseased liver tissue. Pathological liver tissue was obtained from patients with active chronic HCV infection, and patients with former chronic HCV infection cured by recent Direct-Acting Antiviral (DAA) drug therapy. We found that hepatic innate immunity in response to TLR3 and TLR4 agonists was not suppressed but enhanced in the HCV-infected tissue, compared with the healthy controls. Furthermore, despite recent HCV elimination, DAA-cured liver tissue manifested ongoing abnormalities in liver immunity: sustained abnormal immune gene expression in DAA-cured samples was identified in direct ex vivo measurements and in TLR3 and TLR4 stimulation assays. Genes that were up-regulated in chronic HCV-infected liver tissue were mostly characteristic of the non-parenchymal cell compartment. These results demonstrated the utility of PCLS in studying both liver pathology and innate immunity.

Alcohol induced hepatic retinoid depletion is associated with the induction of multiple retinoid catabolizing cytochrome P450 enzymes

Chronic alcohol consumption leads to a spectrum of liver disease that is associated with significant global mortality and morbidity. Alcohol is known to deplete hepatic vitamin A content, which has been linked to the pathogenesis of alcoholic liver disease. It has been suggested that induction of Cytochrome P450 2E1 (CYP2E1) contributes to alcohol-induced hepatic vitamin A depletion, but the possible contributions of other retinoid-catabolizing CYPs have not been well studied. The main objective of this study was to better understand alcohol-induced hepatic vitamin A depletion and test the hypothesis that alcohol-induced depletion of hepatic vitamin A is due to CYP-mediated oxidative catabolism. This hypothesis was tested in a mouse model of chronic alcohol consumption, including wild type and Cyp2e1 -/- mice. Our results show that chronic alcohol consumption is associated with decreased levels of hepatic retinol, retinyl esters, and retinoic acid. Moreover, the depletion of hepatic retinoid is associated with the induction of multiple retinoid catabolizing CYPs, including CYP26A1, and CYP26B1 in alcohol fed wild type mice. In Cyp2e1 -/- mice, alcohol-induced retinol decline is blunted but retinyl esters undergo a change in their acyl composition and decline upon alcohol exposure like WT mice. In conclusion, the alcohol induced decline in hepatic vitamin A content is associated with increased expression of multiple retinoid-catabolizing CYPs, including the retinoic acid specific hydroxylases CYP26A1 and CYP26B1.

Autofluorescence in freshly isolated adult human liver sinusoidal cells

Autofluorescent granules of various sizes were observed in primary human liver endothelial cells (LSECs) upon laser irradiation using a wide range of wavelengths. Autofluorescence was detected in LAMP-1 positive vesicles, suggesting lysosomal location. Confocal imaging of freshly prepared cultures and imaging flow cytometry of non-cultured cells revealed fluorescence in all channels used. Treatment with a lipofuscin autofluorescence quencher reduced autofluorescence, most efficiently in the near UV-area. These results, combined with the knowledge of the very active blood clearance function of LSECs support the notion that lysosomally located autofluorescent material reflected accumulation of lipofuscin in the intact liver. These results illustrate the importance of careful selection of fluorophores, especially when labelling of live cells where the quencher is not compatible.

The Liver and the Hepatic Immune Response in Trypanosoma cruzi Infection, a Historical and Updated View

Chagas disease was described more than a century ago and, despite great efforts to understand the underlying mechanisms that lead to cardiac and digestive manifestations in chronic patients, much remains to be clarified. The disease is found beyond Latin America, including Japan, the USA, France, Spain, and Australia, and is caused by the protozoan Trypanosoma cruzi. Dr. Carlos Chagas described Chagas disease in 1909 in Brazil, and hepatomegaly was among the clinical signs observed. Currently, hepatomegaly is cited in most papers published which either study acutely infected patients or experimental models, and we know that the parasite can infect multiple cell types in the liver, especially Kupffer cells and dendritic cells. Moreover, liver damage is more pronounced in cases of oral infection, which is mainly found in the Amazon region. However, the importance of liver involvement, including the hepatic immune response, in disease progression does not receive much attention. In this review, we present the very first paper published approaching the liver's participation in the infection, as well as subsequent papers published in the last century, up to and including our recently published results. We propose that, after infection, activated peripheral T lymphocytes reach the liver and induce a shift to a pro-inflammatory ambient environment. Thus, there is an immunological integration and cooperation between peripheral and hepatic immunity, contributing to disease control.

The miR-455-3p/HDAC2 axis plays a pivotal role in the progression and reversal of liver fibrosis and is regulated by epigenetics

Histone deacetylases (HDACs), especially HDAC2, play a role in alleviating liver fibrosis; however, the specific upstream regulation mechanism is unknown. Herein, TargetScan was used to predict the potential upstream targets of HDAC2, and the role of miR-455-3p was explored. The dual luciferase reporter assay showed that miR-455-3p binds to the 3' UTR of HDAC2 mRNA. Additionally, miR-455-3p was downregulated in the liver tissues of patients with cirrhosis and mice with liver fibrosis, as well as in primary HSCs isolated from fibrotic mouse livers and TGF-β-treated LX-2 cells. In contrast, it is highly expressed in the reversal stage of hepatic fibrosis and MDI-cultured LX-2 cells. Our functional analyses showed that miR-455-3p overexpression facilitated apoptosis and reduced the expression of pro-fibrotic markers and the proliferation of activated LX-2 cells. On the contrary, miR-455-3p inhibition converted inactivated LX-2 cells into activated, proliferative, fibrogenic cells. Interestingly, restoration of HDAC2 expression partially blocked the function of miR-455-3p. Downregulated miR-455-3p expression can be restored by DNA methyltransferases in activated LX-2 cells. Methylation-specific PCR, bisulfite sequencing PCR, and chromatin immunoprecipitation assays indicated that the methylation level of miR-455-3p promoter CpG islands was elevated in TGF-β-treated LX-2 cells and that miR-455-3p was downregulated in activated LX-2 cells by DNA hypermethylation, which is mediated by DNMT3b and DNMT1. In conclusion, miR-455-3p acts as a liver fibrosis suppressor by targeting HDAC2, and its deficiency further aggravates the reversal phase of fibrosis. Thus, the epigenetics mediated miR-455-3p/HDAC2 axis may serve as a novel potential therapeutic target for clinical treatment of hepatic fibrosis.

Cellular Mechanisms of Liver Fibrosis

The liver is a central organ in the human body, coordinating several key metabolic roles. The structure of the liver which consists of the distinctive arrangement of hepatocytes, hepatic sinusoids, the hepatic artery, portal vein and the central vein, is critical for its function. Due to its unique position in the human body, the liver interacts with components of circulation targeted for the rest of the body and in the process, it is exposed to a vast array of external agents such as dietary metabolites and compounds absorbed through the intestine, including alcohol and drugs, as well as pathogens. Some of these agents may result in injury to the cellular components of liver leading to the activation of the natural wound healing response of the body or fibrogenesis. Long-term injury to liver cells and consistent activation of the fibrogenic response can lead to liver fibrosis such as that seen in chronic alcoholics or clinically obese individuals. Unidentified fibrosis can evolve into more severe consequences over a period of time such as cirrhosis and hepatocellular carcinoma. It is well recognized now that in addition to external agents, genetic predisposition also plays a role in the development of liver fibrosis. An improved understanding of the cellular pathways of fibrosis can illuminate our understanding of this process, and uncover potential therapeutic targets. Here we summarized recent aspects in the understanding of relevant pathways, cellular and molecular drivers of hepatic fibrosis and discuss how this knowledge impact the therapy of respective disease.

Single-cell RNA sequencing of human liver reveals hepatic stellate cell heterogeneity

Background & Aims

The multiple vital functions of the human liver are performed by highly specialised parenchymal and non-parenchymal cells organised in complex collaborative sinusoidal units. Although crucial for homeostasis, the cellular make-up of the human liver remains to be fully elucidated. Here, single-cell RNA-sequencing was used to unravel the heterogeneity of human liver cells, in particular of hepatocytes (HEPs) and hepatic stellate cells (HSCs).

Method

The transcriptome of ~25,000 freshly isolated human liver cells was profiled using droplet-based RNA-sequencing. Recently published data sets and RNA in situ hybridisation were integrated to validate and locate newly identified cell populations.

Results

In total, 22 cell populations were annotated that reflected the heterogeneity of human parenchymal and non-parenchymal liver cells. More than 20,000 HEPs were ordered along the portocentral axis to confirm known, and reveal previously undescribed, zonated liver functions. The existence of 2 subpopulations of human HSCs with unique gene expression signatures and distinct intralobular localisation was revealed (i.e. portal and central vein-concentrated GPC3⁺ HSCs and perisinusoidally located DBH⁺ HSCs). In particular, these data suggest that, although both subpopulations collaborate in the production and organisation of extracellular matrix, GPC3⁺ HSCs specifically express genes involved in the metabolism of glycosaminoglycans, whereas DBH⁺ HSCs display a gene signature that is reminiscent of antigen-presenting cells.

Conclusions

This study highlights metabolic zonation as a key determinant of HEP transcriptomic heterogeneity and, for the first time, outlines the existence of heterogeneous HSC subpopulations in the human liver. These findings call for further research on the functional implications of liver cell heterogeneity in health and disease.

Lay summary

This study resolves the cellular landscape of the human liver in an unbiased manner and at high resolution to provide new insights into human liver cell biology. The results highlight the physiological heterogeneity of human hepatic stellate cells.

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A cell atlas from the near-native transcriptome of >25,000 human liver cells is presented.

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Hepatocytes were ordered along the portocentral axis to reveal previously undescribed gene expression patterns and zonated liver functions.

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Two subpopulations of human hepatic stellate cells (HSCs) are reported, characterised by different spatial distribution in the native tissue.

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Characteristic gene signatures of HSC subpopulations are suggestive of far-reaching functional differences.

Hepatic Stellate Cell Regulation of Liver Regeneration and Repair

The hepatic mesenchyme has been studied extensively in the context of liver fibrosis; however, much less is known regarding the role of mesenchymal cells during liver regeneration. As our knowledge of the cellular and molecular mechanisms driving hepatic regeneration deepens, the key role of the mesenchymal compartment during the regenerative response has been increasingly appreciated. Single-cell genomics approaches have recently uncovered both spatial and functional zonation of the hepatic mesenchyme in homeostasis and following liver injury. Here we discuss how the use of preclinical models, from in vivo mouse models to organoid-based systems, are helping to shape our understanding of the role of the mesenchyme during liver regeneration, and how these approaches should facilitate the precise identification of highly targeted, pro-regenerative therapies for patients with liver disease.

Albumin inhibits the nuclear translocation of Smad3 via interleukin-1beta signaling in hepatic stellate cells

Activation of quiescent hepatic stellate cells (HSCs) to myofibroblasts plays a key role in liver fibrosis. We had previously shown that albumin and its derivative, R-III (a retinol-binding protein—albumin domain III fusion protein), inhibited HSC activation by sequestering retinoic acid (RA) and that R-III administration reduced carbon tetrachloride (CCl₄)-induced liver fibrosis. In this study, we aimed to elucidate the mechanism of action of albumin downstream of RA sequestration. Nuclear factor-κB p65 was evenly distributed in the cytoplasm in activated mouse HSCs, whereas albumin expression or R-III treatment (albumin/R-III) caused the nuclear translocation of p65, probably via RA sequestration, resulting in a dramatic increase in interleukin-1beta (IL-1β) expression. Albumin/R-III in turn induced the phosphorylation of Smad3 at the linker region, inhibiting its nuclear import in an IL-1β-dependent manner. Consistent with the in vitro results, the level of IL-1β mRNA expression was higher in CCl₄/R-III-treated livers than in CCl₄-treated livers. These findings reveal that albumin/R-III inhibits the transforming growth factor-β-Smad3 signaling as well as the retinoic acid receptor-mediated pathway, which probably contributes to the inhibition of HSC activation, and suggest that R-III may be an anti-fibrotic drug candidate.

The Power of Plasticity-Metabolic Regulation of Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are resident non-parenchymal liver pericytes whose plasticity enables them to regulate a remarkable range of physiologic and pathologic responses. To support their functions in health and disease, HSCs engage pathways regulating carbohydrate, mitochondrial, lipid, and retinoid homeostasis. In chronic liver injury, HSCs drive hepatic fibrosis and are implicated in inflammation and cancer. To do so, the cells activate, or transdifferentiate, from a quiescent state into proliferative, motile myofibroblasts that secrete extracellular matrix, which demands rapid adaptation to meet a heightened energy need. Adaptations include reprogramming of central carbon metabolism, enhanced mitochondrial number and activity, endoplasmic reticulum stress, and liberation of free fatty acids through autophagy-dependent hydrolysis of retinyl esters that are stored in cytoplasmic droplets. As an archetype for pericytes in other tissues, recognition of the HSC's metabolic drivers and vulnerabilities offer the potential to target these pathways therapeutically to enhance parenchymal growth and modulate repair.

Implementation of Pericytes in Vascular Regeneration Strategies

For the survival and integration of complex large-sized tissue-engineered (TE) organ constructs that exceed the maximal nutrients and oxygen diffusion distance required for cell survival, graft (pre)vascularization to ensure medium or blood supply is crucial. To achieve this, the morphology and functionality of the microcapillary bed should be mimicked by incorporating vascular cell populations, including endothelium and mural cells. Pericytes play a crucial role in microvascular function, blood vessel stability, angiogenesis, and blood pressure regulation. In addition, tissue-specific pericytes are important in maintaining specific functions in different organs, including vitamin A storage in the liver, renin production in the kidneys and maintenance of the blood-brain-barrier. Together with their multipotential differentiation capacity, this makes pericytes the preferred cell type for application in TE grafts. The use of a tissue-specific pericyte cell population that matches the TE organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)-vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts. Impact statement The use of a tissue-specific pericyte cell population that matches the tissue-engineered (TE) organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts.

Playing Jekyll and Hyde-The Dual Role of Lipids in Fatty Liver Disease

Lipids play Jekyll and Hyde in the liver. On the one hand, the lipid-laden status of hepatic stellate cells is a hallmark of healthy liver. On the other hand, the opposite is true for lipid-laden hepatocytes—they obstruct liver function. Neglected lipid accumulation in hepatocytes can progress into hepatic fibrosis, a condition induced by the activation of stellate cells. In their resting state, these cells store substantial quantities of fat-soluble vitamin A (retinyl esters) in large lipid droplets. During activation, these lipid organelles are gradually degraded. Hence, treatment of fatty liver disease is treading a tightrope—unsophisticated targeting of hepatic lipid accumulation might trigger problematic side effects on stellate cells. Therefore, it is of great importance to gain more insight into the highly dynamic lipid metabolism of hepatocytes and stellate cells in both quiescent and activated states. In this review, part of the special issue entitled “Cellular and Molecular Mechanisms underlying the Pathogenesis of Hepatic Fibrosis 2020”, we discuss current and highly versatile aspects of neutral lipid metabolism in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).

Hepatic stellate cell hypertrophy is associated with metabolic liver fibrosis

Hepatic fibrosis is a major consequence of chronic liver disease such as non-alcoholic steatohepatitis which is undergoing a dramatic evolution given the obesity progression worldwide, and has no treatment to date. Hepatic stellate cells (HSCs) play a key role in the fibrosis process, because in chronic liver damage, they transdifferentiate from a “quiescent” to an “activated” phenotype responsible for most the collagen deposition in liver tissue. Here, using a diet-induced liver fibrosis murine model (choline-deficient amino acid-defined, high fat diet), we characterized a specific population of HSCs organized as clusters presenting simultaneously hypertrophy of retinoid droplets, quiescent and activated HSC markers. We showed that hypertrophied HSCs co-localized with fibrosis areas in space and time. Importantly, we reported the existence of this phenotype and its association with collagen deposition in three other mouse fibrosis models, including CCl₄-induced fibrosis model. Moreover, we have also shown its relevance in human liver fibrosis associated with different etiologies (obesity, non-alcoholic steatohepatitis, viral hepatitis C and alcoholism). In particular, we have demonstrated a significant positive correlation between the stage of liver fibrosis and HSC hypertrophy in a cohort of obese patients with hepatic fibrosis. These results lead us to conclude that hypertrophied HSCs are closely associated with hepatic fibrosis in a metabolic disease context and may represent a new marker of metabolic liver disease progression.

Cyp1b1 directs Srebp-mediated cholesterol and retinoid synthesis in perinatal liver; Association with retinoic acid activity during fetal development

Background

Cytochrome P450 1b1 (Cyp1b1) deletion and dietary retinol deficiency during pregnancy (GVAD) affect perinatal liver functions regulated by Srebp. Cyp1b1 is not expressed in perinatal liver but appears in the E9.5 embryo, close to sites of retinoic acid (RA) signaling.

Hypothesis

Parallel effects of Cyp1b1 and retinol on postnatal Srebp derive from effects in the developing liver or systemic signaling.

Approach

Cluster postnatal increases in hepatic genes in relation to effects of GVAD or Cyp1b1 deletion. Sort expression changes in relation to genes regulated by Srebp1 and Srebp2.Test these treatments on embryos at E9.5, examining changes at the site of liver initiation. Use in situ hybridization to resolve effects on mRNA distributions of Aldh1a2 and Cyp26a1 (RA homeostasis); Hoxb1 and Pax6 (RA targets). Assess mice lacking Lrat and Rbp4 (DKO mice) that severely limits retinol supply to embryos.

Results

At birth, GVAD and Cyp1b1 deletion stimulate gene markers of hepatic stellate cell (HSC) activation but also suppress Hamp. These treatments then selectively prevent the postnatal onset of genes that synthesize cholesterol (Hmgcr, Sqle) and fatty acids (Fasn, Scd1), but also direct cholesterol transport (Ldlr, Pcsk9, Stard4) and retinoid synthesis (Aldh1a1, Rdh11). Extensive support by Cyp1b1 is implicated, but with distinct GVAD interventions for Srebp1 and Srebp2. At E9.5, Cyp1b1 is expressed in the septum transversum mesenchyme (STM) with β-carotene oxygenase (Bco1) that generates retinaldehyde. STM provides progenitors for the HSC and supports liver expansion. GVAD and Cyp1b1-/- do not affect RA-dependent Hoxb1 and Pax6. In DKO embryos, RA-dependent Cyp26a1 is lost but Hoxb1 is sustained with Cyp1b1 at multiple sites.

Conclusion

Cyp1b1-/- suppresses genes supported by Srebp. GVAD effects distinguish Srebp1 and Srebp2 mediation. Srebp regulation overlaps appreciably in cholesterol and retinoid homeostasis. Bco1/Cyp1b1 partnership in the STM may contribute to this later liver regulation.

Bayesian correlation is a robust gene similarity measure for single-cell RNA-seq data

Assessing similarity is highly important for bioinformatics algorithms to determine correlations between biological information. A common problem is that similarity can appear by chance, particularly for low expressed entities. This is especially relevant in single-cell RNA-seq (scRNA-seq) data because read counts are much lower compared to bulk RNA-seq. Recently, a Bayesian correlation scheme that assigns low similarity to genes that have low confidence expression estimates has been proposed to assess similarity for bulk RNA-seq. Our goal is to extend the properties of the Bayesian correlation in scRNA-seq data by considering three ways to compute similarity. First, we compute the similarity of pairs of genes over all cells. Second, we identify specific cell populations and compute the correlation in those populations. Third, we compute the similarity of pairs of genes over all clusters, by considering the total mRNA expression. We demonstrate that Bayesian correlations are more reproducible than Pearson correlations. Compared to Pearson correlations, Bayesian correlations have a smaller dependence on the number of input cells. We show that the Bayesian correlation algorithm assigns high similarity values to genes with a biological relevance in a specific population. We conclude that Bayesian correlation is a robust similarity measure in scRNA-seq data.

Understanding the Heterogeneity of Resident Liver Macrophages

Resident tissue macrophages (RTMs) are cells with a high functional plasticity assuming pleiotropic roles in their tissue of residence, from clearance of dead cells and metabolic sensing in steady state to cytokine production and tissue repair during inflammation. The liver has long been considered as only populated by Kupffer cells (KCs), a macrophage population assumed to be in charge of all of these functions. However, we know now that KCs are not the only macrophage population in the liver, that recently was shown to contain also capsular macrophages, monocyte-derived macrophages as well as recruited peritoneal macrophages inherited from previous inflammatory events. These macrophages exhibit different origins, time of establishing residence and locations in the liver, with both ontogenical and environmental factors shaping their identity and functions. Furthermore, liver macrophages reside in a complex environment with a pronounced metabolic zonation. Here, we briefly discuss how these intrinsic and extrinsic factors influence macrophage biology and liver physiology in general. We notably focus on how the recent advances of single cell transcriptomic approaches are changing our understanding of liver macrophages and diseases.

"Let my liver rather heat with wine" - a review of hepatic fibrosis pathophysiology and emerging therapeutics

Cirrhosis is characterized by extensive hepatic fibrosis, and it is the 14th leading cause of death worldwide. Numerous contributing conditions have been implicated in its development, including infectious etiologies, medication overdose or adverse effects, ingestible toxins, autoimmunity, hemochromatosis, Wilson’s disease and primary biliary cholangitis to list a few. It is associated with portal hypertension and its stigmata (varices, ascites, hepatic encephalopathy, combined coagulopathy and thrombophilia), and it is a major risk factor for hepatocellular carcinoma. Currently, orthotopic liver transplantation has been the only curative modality to treat cirrhosis, and the scarcity of donors results in many people waiting years for a transplant. Identification of novel targets for pharmacologic therapy through elucidation of key mechanistic components to induce fibrosis reversal is the subject of intense research. Development of robust models of hepatic fibrosis to faithfully characterize the interplay between activated hepatic stellate cells (the principal fibrogenic contributor to fibrosis initiation and perpetuation), hepatocytes and extracellular matrix components has the potential to identify critical components and mechanisms that can be exploited for targeted treatment. In this review, we will highlight key cellular pathways involved in the pathophysiology of fibrosis from extracellular ligands, effectors and receptors, to nuclear receptors, epigenetic mechanisms, energy homeostasis and cytokines. Further, molecular pathways of hepatic stellate cell deactivation are discussed, including apoptosis, senescence and reversal or transdifferentiation to an inactivated state resembling quiescence. Lastly, clinical evidence of fibrosis reversal induced by biologics and small molecules is summarized, current compounds under clinical trials are described and efforts for treatment of hepatic fibrosis with mesenchymal stem cells are highlighted. An enhanced understanding of the rich tapestry of cellular processes identified in the initiation, perpetuation and resolution of hepatic fibrosis, driven principally through phenotypic switching of hepatic stellate cells, should lead to a breakthrough in potential therapeutic modalities.

Current understanding of adipose-derived mesenchymal stem cell-based therapies in liver diseases

The liver, the largest organ with multiple synthetic and secretory functions in mammals, consists of hepatocytes, cholangiocytes, hepatic stellate cells (HSCs), sinusoidal endothelial cells, Kupffer cells (KCs), and immune cells, among others. Various causative factors, including viral infection, toxins, autoimmune defects, and genetic disorders, can impair liver function and result in chronic liver disease or acute liver failure. Mesenchymal stem cells (MSCs) from various tissues have emerged as a potential candidate for cell transplantation to promote liver regeneration. Adipose-derived MSCs (ADMSCs) with high multi-lineage potential and self-renewal capacity have attracted great attention as a promising means of liver regeneration. The abundance source and minimally invasive procedure required to obtain ADMSCs makes them superior to bone marrow-derived MSCs (BMMSCs). In this review, we comprehensively analyze landmark studies that address the isolation, proliferation, and hepatogenic differentiation of ADMSCs and summarize the therapeutic effects of ADMSCs in animal models of liver diseases. We also discuss key points related to improving the hepatic differentiation of ADMSCs via exposure of the cells to cytokines and growth factors (GFs), extracellular matrix (ECM), and various physical parameters in in vitro culture. The optimization of culturing methods and of the transplantation route will contribute to the further application of ADMSCs in liver regeneration and help improve the survival rate of patients with liver diseases. To this end, ADMSCs provide a potential strategy in the field of liver regeneration for treating acute or chronic liver injury, thus ensuring the availability of ADMSCs for research, trial, and clinical applications in various liver diseases in the future.

Hepatic stellate cell activation: A source for bioactive lipids

Hepatic stellate cells (HSCs) are non-parenchymal liver cells that characteristically contain multiple retinoid (vitamin A)-containing lipid droplets. In this study, we addressed the metabolic fate of non-retinoid lipids originating from lipid droplet loss during HSCs activation. UPLC/MS/MS and qRT-PCR were used to monitor the lipid composition and mRNA expression of selected genes regulating lipid metabolism in freshly isolated, overnight-, 3- and 7-day cultures or primary mouse HSCs. A preferential accumulation of specific C20-C24 fatty acid species, especially arachidonic (C20:4) and docosahexaenoic acids (C22:6), was revealed in culture-activated HSCs along with an upregulation of transcription of fatty acid desaturases (Scd1, Scd2) and elongases (Elovl5, Elovl6). This was accompanied with an enrichment of activated HSCs with 36:4 and 38:4 phosphatidylcholine species containing polyunsaturated fatty acids and associated accumulation of selective lipid mediators, including endocannabinoids and related N-acylethanolamides, as well as ceramides. An increase in 2-arachidonoylglycerol and N-arachydonoylethanolamide concentrations was observed along with an upregulation of Daglα mRNA expression in HSCs during culture activation. N-palmitoylethanolamide was identified as the most abundant endocannabinoid-like species in activated HSCs. An increase in total ceramide levels and enrichment with N-palmitoyl (C16:0), N-tetracosenoyl (C24:1), N-tetracosanoyl (C24:0) and N-docosanoyl (C22:0) ceramides was detected in activated HSC cultures and was preceded by increased mRNA expression of ceramide synthesizing enzymes (CerS2, CerS5 and Smpd1). Our data suggest an active redistribution of non-retinoid lipids in HSCs underlying the formation of low abundance, highly bioactive lipid species that may affect signaling during HSC activation, as well as extracellularly within the liver.

Hepatic stellate cells retain retinoid-laden lipid droplets after cellular transdifferentiation into activated myofibroblasts

Loss of retinyl ester (RE)-rich lipid droplets (LDs) from hepatic stellate cells (HSCs) is cited as a key event in their cellular transdifferentiation to activated, pro-fibrotic myofibroblasts; however, it remains unclear if changes in LD morphology or RE content are causal for transdifferentiation. To better understand LD dynamics in vitro within a common model of HSC activation, we used novel approaches preserving LD morphology and allowing for quantitation of RE. The size and quantity of LDs within in vitro and in vivo bile duct ligation (BDL)-activated HSCs were quantitated using adipocyte differentiation-related protein (ADRP) labeling and oil red o (ORO) staining (gold standard), and RE content was determined using fluorescence microscopy. We found during HSC activation in vitro that LD number differed significantly when measured by ADRP and ORO, respectively ( day 1: 56 vs. 5, P = 0.03; day 4: 101 vs. 39, P = 0.03; day 14: 241 vs. 12, P = 0.02). Ex vivo HSCs activated in vivo contained the same number of LDs as day 4 in vitro activated HSCs (118 vs. 101, P = 0.54). Decline in LD RE occurred beyond day 4 in vitro and day 1 ex vivo , after HSC transdifferentiation was underway. Lastly, in situ HSCs examined using electron microscopy show LDs tend to be smaller but are ultimately retained in BDL injured livers. Therefore, we conclude that during HSC transdifferentiation, LDs are not lost but are retained, decreasing in size. Additionally, RE content declines after transdifferentiation is underway. These data suggest that these LD changes are not causal for HSC transdifferentiation. NEW & NOTEWORTHY Loss of retinoid-laden lipid droplets from hepatic stellate cells has long been held as a hallmark of their transdifferentiation into activated myofibroblasts, the dominant cells that drive hepatic fibrosis. This study demonstrates that stellate cells activated in culture and after liver injury in vivo retain their lipid droplets and that these droplets become smaller and more numerous, with decreases in droplet retinoid concentration occurring only after cellular transdifferentiation is underway.

Retinoids Issued from Hepatic Stellate Cell Lipid Droplet Loss as Potential Signaling Molecules Orchestrating a Multicellular Liver Injury Response

Hepatic stellate cells (HSCs) serve as the main body storage compartment for vitamin A through retinyl ester (RE)-filled lipid droplets (LDs). Upon liver injury, HSCs adopt a myofibroblastic phenotype characterized by an elevated expression of extracellular matrix proteins and a concomitant loss of LDs. On the one hand, LD breakdown has been suggested to provide the energy required for HSC activation into myofibroblast-like cells. On the other hand, this process could mitigate HSC activation following the transformation of released REs into retinoic acids (RAs), ligands for nuclear receptors exerting antifibrotic transcriptional regulatory activities in HSCs. Importantly, RAs may also constitute a means for HSCs to orchestrate the liver response to injury by triggering transcriptional effects in multiple additional surrounding liver cell populations. We envision that new approaches, such as single-cell technologies, will allow to better define how RAs are issued from LD loss in HSCs exert a multicellular control of the liver (patho)physiology.

Isolation and Culture of Primary Murine Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are found in the perisinusoidal space of the liver (i.e., the space of Dissé). They represent 5-8% of the total number of liver cells. In normal liver, these cells have a quiescent phenotype and are characterized by numerous fat vacuoles that store vitamin A in a form of retinyl ester. In injured liver, these cells transdifferentiate into a myofibroblast phenotype, become highly proliferative and are responsible for excess collagen synthesis and deposition during fibrosis. Due to their exceptional pathophysiological relevance, several isolation and purification protocols of primary HSCs have been established that provide the basis for studying HSC biology in vitro. We here describe a method for high-purity isolation of HSCs from mice. This protocol includes the enzymatic digestion of the liver tissue by pronase and collagenase, cellular enrichment by centrifugation of the crude cell suspension through a Nycodenz density gradient, and a final (optional) flow cytometric enrichment that allows generating ultrapure HSC fractions.

Precision-cut human liver slice cultures as an immunological platform

The liver is the central metabolic organ in the human body, and also plays an essential role in innate and adaptive immunity. While mouse models offer significant insights into immune-inflammatory liver disease, human immunology differs in important respects. It is not easy to address those differences experimentally. Therefore, to improve the understanding of human liver immunobiology and pathology, we have established precision-cut human liver slices to study innate immunity in human tissue. Human liver slices collected from resected livers could be maintained in ex vivo culture over a two-week period. Although an acute inflammatory response accompanied by signs of tissue repair was observed in liver tissue following slicing, the expression of many immune genes stabilized after day 4 and remained stable until day 15. Remarkably, histological evidence of pre-existing liver diseases was preserved in the slices for up to 7 days. Following 7 days of culture, exposure of liver slices to the toll-like receptor (TLR) ligands, TLR3 ligand Poly-I:C and TLR4 ligand LPS, resulted in a robust activation of acute inflammation and cytokine genes. Moreover, Poly-I:C treatment induced a marked antiviral response including increases of interferons IFNB, IL-28B and a group of interferon-stimulated genes. Therefore, precision-cut liver slices emerge as a valuable tool to study human innate immunity.

Retinol dehydrogenase 11 is essential for the maintenance of retinol homeostasis in liver and testis in mice

Retinol dehydrogenase 11 (RDH11) is a microsomal short-chain dehydrogenase/reductase that recognizes all-trans- and cis-retinoids as substrates and prefers NADPH as a cofactor. Previous work has suggested that RDH11 contributes to the oxidation of 11-cis-retinol to 11-cis-retinaldehyde during the visual cycle in the eye's retinal pigment epithelium. However, the role of RDH11 in metabolism of all-trans-retinoids remains obscure. Here, we report that microsomes isolated from the testes and livers of Rdh11^-/- mice fed a regular diet exhibited a 3- and 1.7-fold lower rate of all-trans-retinaldehyde conversion to all-trans-retinol, respectively, than the microsomes of WT littermates. Testes and livers of Rdh11^-/- mice fed a vitamin A-deficient diet had ∼35% lower levels of all-trans-retinol than those of WT mice. Furthermore, the conversion of β-carotene to retinol via retinaldehyde as an intermediate appeared to be impaired in the testes of Rdh11^-/-/retinol-binding protein 4^-/-(Rbp4^-/-) mice, which lack circulating holo RBP4 and rely on dietary supplementation with β-carotene for maintenance of their retinoid stores. Together, these results indicate that in mouse testis and liver, RDH11 functions as an all-trans-retinaldehyde reductase essential for the maintenance of physiological levels of all-trans-retinol under reduced vitamin A availability.

Perilipin 5 and liver fatty acid binding protein function to restore quiescence in mouse hepatic stellate cells

Hepatic stellate cell (HSC) activation occurs along with decreased Perilipin5 (Plin5) and liver fatty acid-binding protein (L-Fabp) expression and coincident lipid droplet (LD) depletion. Conversely, the activated phenotype is reversible in WT HSCs upon forced expression of Plin5. Here, we asked if L-Fabp expression is required for Plin5-mediated rescue of the quiescent phenotype. Lentiviral Plin5 transduction of passaged L-Fabp^-/- HSCs failed to reverse activation markers or restore lipogenic gene expression and LD formation. However, adenoviral L-Fabp infection of lentiviral Plin5 transduced L-Fabp^-/- HSCs restored both the quiescent phenotype and LD formation, an effect also mediated by adenoviral intestine-Fabp or adipocyte-Fabp. Expression of exogenous Plin5 in activated WT HSCs induced a transcriptional program of lipogenic gene expression including endogenous L-Fabp, but none of the other FABPs. We further demonstrated that selective, small molecule inhibition of endogenous L-Fabp also eliminated the ability of exogenous Plin5 to rescue LD formation and reverse activation of WT HSCs. This functional coordination of L-Fabp with Plin5 was 5'-AMP-activated protein kinase (AMPK)-dependent and was eliminated by AMPK inhibition. Taken together, our results indicate that L-Fabp is required for Plin5 to activate a transcriptional program that restores LD formation and reverses HSC activation.

Lung and liver growth and retinoic acid status in human fetuses with congenital diaphragmatic hernia

BACKGROUND

Abnormal retinoic acid (RA) signalling is considered a major cause of congenital diaphragmatic hernia (CDH). Pulmonary hypoplasia and pulmonary hypertension are the major causes of morbidity and mortality in infants born with CDH. Experimental studies in animals have found that RA signalling is involved in lung and liver development, but animal models of CDH do not directly correlate with CDH in human fetuses. This study investigated if RA status is also linked to lung and liver growth in human fetuses with CDH.

STUDY DESIGN AND PATIENTS

Hepatic stellate cells (HSC) in autopsy human fetal liver tissue were identified using cRBP-1 immunohistochemistry and the numbers of HSC manually counted. In mammals, RA is principally stored in HSC complexed to cRBP-1 and therefore cRBP-1⁺ HSC numbers were used as an indicator of fetal RA status. The number of HSCs was correlated with liver and lung weights, calculated relative to either normal biometric values or fetal body weight.

RESULTS

The number of cRBP-1⁺ HSCs correlated with lung weight contralateral to the side of the diaphragmatic hernia (r=0.82, p=0.025) and combined lung weight (r=0.78, p=0.039) but not with ipsilateral lung weight (r=0.43, p=0.33), in fetuses with right and left CDH and a case of giant omphalocoele. Liver growth was influenced by contact with diaphragm but not significantly correlated with cRBP-1 expression (r=0.52, p=0.056).

CONCLUSION

Fetal RA stores, reflected in the number of cRBP-1⁺ HSCs, influence lung growth as well as diaphragm development in human fetuses with CDH. Contact with diaphragm influenced liver growth.

Hepatic stellate cells as key target in liver fibrosis

Progressive liver fibrosis, induced by chronic viral and metabolic disorders, leads to more than one million deaths annually via development of cirrhosis, although no antifibrotic therapy has been approved to date. Transdifferentiation (or "activation") of hepatic stellate cells is the major cellular source of matrix protein-secreting myofibroblasts, the major driver of liver fibrogenesis. Paracrine signals from injured epithelial cells, fibrotic tissue microenvironment, immune and systemic metabolic dysregulation, enteric dysbiosis, and hepatitis viral products can directly or indirectly induce stellate cell activation. Dysregulated intracellular signaling, epigenetic changes, and cellular stress response represent candidate targets to deactivate stellate cells by inducing reversion to inactivated state, cellular senescence, apoptosis, and/or clearance by immune cells. Cell type- and target-specific pharmacological intervention to therapeutically induce the deactivation will enable more effective and less toxic precision antifibrotic therapies.

Cyp1b1 deletion and retinol deficiency coordinately suppress mouse liver lipogenic genes and hepcidin expression during post-natal development

Cyp1b1 deletion and gestational vitamin A deficiency (GVAD) redirect adult liver gene expression. A matched sufficient pre- and post-natal diet, which has high carbohydrate and normal iron content (LF12), increased inflammatory gene expression markers in adult livers that were suppressed by GVAD and Cyp1b1 deletion. At birth on the LF12 diet, Cyp1b1 deletion and GVAD each suppress liver expression of the iron suppressor, hepcidin (Hepc), while increasing stellate cell activation markers and suppressing post-natal increases in lipogenesis. Hepc was less suppressed in Cyp1b1-/- pups with a standard breeder diet, but was restored by iron supplementation of the LF12 diet.

CONCLUSIONS

The LF12 diet delivered low post-natal iron and attenuated Hepc. Hepc decreases in Cyp1b1-/- and GVAD mice resulted in stellate activation and lipogenesis suppression. Endothelial BMP6, a Hepc stimulant, is a potential coordinator and Cyp1b1 target. These neonatal changes in Cyp1b1-/- mice link to diminished adult obesity and liver inflammation.

Pancreatic Stellate Cells Have Distinct Characteristics From Hepatic Stellate Cells and Are Not the Unique Origin of Collagen-Producing Cells in the Pancreas

OBJECTIVES

The origin of collagen-producing myofibroblasts in pancreatic fibrosis is still controversial. Pancreatic stellate cells (PSCs), which have been recognized as the pancreatic counterparts of hepatic stellate cells (HSCs), are thought to play an important role in the development of pancreatic fibrosis. However, sources of myofibroblasts other than PSCs may exist because extensive studies of liver fibrosis have uncovered myofibroblasts that did not originate from HSCs. This study aimed to characterize myofibroblasts in an experimental pancreatic fibrosis model in mice.

METHODS

We used transgenic mice expressing green fluorescent protein via the collagen type I α1 promoter and induced pancreatic fibrosis with repetitive injections of cerulein.

RESULTS

Collagen-producing cells that are negative for glial fibrillary acidic protein (ie, not derived from PSCs) exist in the pancreas. Pancreatic stellate cells had different characteristics from those of HSCs in a very small possession of vitamin A using mass spectrometry and a low expression of lecithin retinol acyltransferase. The microstructure of PSCs was entirely different from that of HSCs using flow cytometry and electron microscopy.

CONCLUSIONS

Our study showed that characteristics of PSCs are different from those of HSCs, and myofibroblasts in the pancreas might be derived not only from PSCs but also from other fibrogenic cells.

MHC II-, but not MHC II+, hepatic Stellate cells contribute to liver fibrosis of mice in infection with Schistosoma japonicum

Hepatic stellate cells (HSCs) are considered as the main effector cells in vitamin A metabolism and liver fibrosis, as well as in hepatic immune regulation. Recently, researches have revealed that HSCs have plasticity and heterogeneity, which depend on their lobular location and whether liver is normal or injured. This research aimed to explore the biological characteristics and heterogeneity of HSCs in mice with Schistosoma japonicum (S. japonicum) infection, and determine the subpopulation of HSCs in pathogenesis of hepatic fibrosis caused by S. japonicum infection. Results revealed that HSCs significantly increased the expressions of MHC II and fibrogenic genes after S. japonicum infection, and could be classified into MHC II⁺ HSCs and MHC II^- HSCs subsets. Both two HSCs populations suppressed the proliferation of activated CD4⁺T cells, whereas only MHC II^- HSCs displayed a myofibroblast-like phenotype. In response to IFN-γ, HSCs up-regulated the expressions of MHC II and CIITA, while down-regulated the expression of fibrogenic gene Col1. In addition, praziquantel treatment decreased the expressions of fibrogenic genes in MHC II^- HSCs. These results confirmed that HSCs from S. japonicum-infected mice have heterogeneity. The MHC II^- α-SMA⁺ HSCs were major subsets of HSCs contributing to liver fibrosis and could be considered as a potential target of praziquantel anti-fibrosis treatment.

Diet-dependent retinoid effects on liver gene expression include stellate and inflammation markers and parallel effects of the nuclear repressor Shp

For mice, a maternal vitamin A (VA)-deficient diet initiated from midgestation (GVAD) produces serum retinol deficiency in mature offspring. We hypothesize that the effects of GVAD arise from preweaning developmental changes. We compare the effect of this GVAD protocol in combination with a postweaning high-fat diet (HFD) or high-carbohydrate diet (LF12). Each is compared to an equivalent VA-sufficient combination. GVAD extensively decreased serum retinol and liver retinol, retinyl esters, and retinoid homeostasis genes (Lrat, Cyp26b1 and Cyp26a1). These suppressions were each more effective with LF12 than with HFD. Postweaning initiation of VA deficiency with LF12 depleted liver retinoids, but serum retinol was unaffected. Liver retinoid depletion, therefore, precedes serum attenuation. Maternal LF12 decreased the obesity response to the HFD, which was further decreased by GVAD. LF12 fed to the mother and offspring extensively stimulated genes marking stellate activation (Col1a1, Timp2 and Cyp1b1) and novel inflammation markers (Ly6d, Trem2 and Nupr1). The GVAD with LF12 diet combination suppressed these responses. GVAD in combination with the HFD increased these same clusters. A further set of expression differences on the HFD when compared to a high-carbohydrate diet was prevented when GVAD was combined with HFD. Most of these GVAD gene changes match published effects from deletion of Nr0b2/Shp, a retinoid-responsive, nuclear co-repressor that modulates metabolic homeostasis. The stellate and inflammatory increases seen with the high-carbohydrate LF12 diet may represent postprandial responses. They depend on retinol and Shp, but the regulation reverses with an HFD.

Hepatocyte autotaxin expression promotes liver fibrosis and cancer

Autotaxin (ATX) is a secreted lysophospholipase D that catalyzes the production of lysophosphatidic acid (LPA), a pleiotropic growth-factor-like lysophospholipid. Increased ATX expression has been detected in various chronic inflammatory disorders and different types of cancer; however, little is known about its role and mode of action in liver fibrosis and cancer. Here, increased ATX expression was detected in chronic liver disease (CLD) patients of different etiologies, associated with shorter overall survival. In mice, different hepatotoxic stimuli linked with the development of different forms of CLDs were shown to stimulate hepatocyte ATX expression, leading to increased LPA levels, activation of hepatic stellate cells (HSCs), and amplification of profibrotic signals. Hepatocyte-specific, conditional genetic deletion and/or transgenic overexpression of ATX established a liver profibrotic role for ATX/LPA, whereas pharmacological ATX inhibition studies suggested ATX as a possible therapeutic target in CLDs. In addition, hepatocyte ATX ablation and the consequent deregulation of lipid homeostasis was also shown to attenuate hepatocellular carcinoma (HCC) development, thus implicating ATX/LPA in the causative link of cirrhosis and HCC.

CONCLUSION

ATX is a novel player in the pathogenesis of liver fibrosis and cancer and a promising therapeutic target. (Hepatology 2017;65:1369-1383).

The why, when and how of lipid droplet diversity

Lipid droplets are the universal cellular organelles for the transient or long-term storage of lipids. The number, size and composition of lipid droplets vary greatly within cells in a homogenous population as well as in different cell types. The variability of intracellular lipid-storage organelles reflects the diversification of lipid droplet composition and function. Lipid droplet diversification results, for example, in two cellular lipid droplet populations that are prone to diminish and grow, respectively. The aberrant accumulation or depletion of lipids are hallmarks or causes of various human pathologies. Thus, a better understanding of the origins of lipid droplet diversification is not only a fascinating cell biology question but also potentially serves to improve comprehension of pathologies that entail the accumulation of lipids. This Commentary covers the lipid droplet life cycle and highlights the early steps during lipid droplet biogenesis, which we propose to be the potential driving forces of lipid droplet diversification.