Stress and Liver Fibrogenesis: Understanding the Role and Regulation of Stress Response Pathways in Hepatic Stellate Cells

Stress response pathways are crucial for cells to adapt to physiological and pathologic conditions. Increased transcription and translation in response to stimuli place a strain on the cell, necessitating increased amino acid supply, protein production and folding, and disposal of misfolded proteins. Stress response pathways, such as the unfolded protein response (UPR) and the integrated stress response (ISR), allow cells to adapt to stress and restore homeostasis; however, their role and regulation in pathologic conditions, such as hepatic fibrogenesis, are unclear. Liver injury promotes fibrogenesis through activation of hepatic stellate cells (HSCs), which produce and secrete fibrogenic proteins to promote tissue repair. This process is exacerbated in chronic liver disease, leading to fibrosis and, if unchecked, cirrhosis. Fibrogenic HSCs exhibit activation of both the UPR and ISR, due in part to increased transcriptional and translational demands, and these stress responses play important roles in fibrogenesis. Targeting these pathways to limit fibrogenesis or promote HSC apoptosis is a potential antifibrotic strategy, but it is limited by our lack of mechanistic understanding of how the UPR and ISR regulate HSC activation and fibrogenesis. This article explores the role of the UPR and ISR in the progression of fibrogenesis, and highlights areas that require further investigation to better understand how the UPR and ISR can be targeted to limit hepatic fibrosis progression.

The endoplasmic reticulum stress sensor IRE1 regulates collagen secretion through the enforcement of the proteostasis factor P4HB/PDIA1 contributing to liver damage and fibrosis

Collagen is one the most abundant proteins and the main cargo of the secretory pathway, contributing to hepatic fibrosis and cirrhosis due to excessive deposition of extracellular matrix. Here we investigated the possible contribution of the unfolded protein response, the main adaptive pathway that monitors and adjusts the protein production capacity at the endoplasmic reticulum, to collagen biogenesis and liver disease. Genetic ablation of the ER stress sensor IRE1 reduced liver damage and diminished collagen deposition in models of liver fibrosis triggered by carbon tetrachloride (CCl ₄ ) administration or by high fat diet. Proteomic and transcriptomic profiling identified the prolyl 4-hydroxylase (P4HB, also known as PDIA1), which is known to be critical for collagen maturation, as a major IRE1-induced gene. Cell culture studies demonstrated that IRE1 deficiency results in collagen retention at the ER and altered secretion, a phenotype rescued by P4HB overexpression. Taken together, our results collectively establish a role of the IRE1/P4HB axis in the regulation of collagen production and its significance in the pathogenesis of various disease states.

Transcriptomic Analysis Reveals the Messenger RNAs Responsible for the Progression of Alcoholic Cirrhosis

Alcohol-associated liver disease is the leading cause of chronic liver disease. We hypothesized that the expression of specific coding genes is critical for the progression of alcoholic cirrhosis (AC) from compensated to decompensated states. For the discovery phase, we performed RNA sequencing analysis of 16 peripheral blood RNA samples, 4 healthy controls (HCs) and 12 patients with AC. The DEGs from the discovery cohort were validated by quantitative polymerase chain reaction in a separate cohort of 17 HCs and 48 patients with AC (17 Child-Pugh A, 16 Child-Pugh B, and 15 Child-Pugh C). We observed that the numbers of differentially expressed messenger RNAs (mRNAs) were more pronounced with worsening disease severity. Pathway analysis for differentially expressed genes for patients with Child-Pugh A demonstrated genes involved innate immune responses; those in Child-Pugh B belonged to genes related to oxidation and alternative splicing; those in Child-Pugh C related to methylation, acetylation, and alternative splicing. We found significant differences in the expression of heme oxygenase 1 (HMOX1) and ribonucleoprotein, PTB binding 1 (RAVER1) in peripheral blood of those who died during the follow-up when compared to those who survived. Conclusion: Unique mRNAs that may implicate disease progression in patients with AC were identified by using a transcriptomic approach. Future studies to confirm our results are needed, and comprehensive mechanistic studies on the implications of these genes in AC pathogenesis and progression should be further explored.

Traf2 and NCK Interacting Kinase Is a Critical Regulator of Procollagen I Trafficking and Hepatic Fibrogenesis in Mice

Hepatic fibrosis is driven by deposition of matrix proteins following liver injury. Hepatic stellate cells (HSCs) drive fibrogenesis, producing matrix proteins, including procollagen I, which matures into collagen I following secretion. Disrupting intracellular procollagen processing and trafficking causes endoplasmic reticulum stress and stress-induced HSC apoptosis and thus is an attractive antifibrotic strategy. We designed an immunofluorescence-based small interfering RNA (siRNA) screen to identify procollagen I trafficking regulators, hypothesizing that these proteins could serve as antifibrotic targets. A targeted siRNA screen was performed using immunofluorescence to detect changes in intracellular procollagen I. Tumor necrosis factor receptor associated factor 2 and noncatalytic region of tyrosine kinase-interacting kinase (TNIK) was identified and interrogated in vitro and in vivo using the TNIK kinase inhibitor NCB-0846 or RNA interference-mediated knockdown. Our siRNA screen identified nine genes whose knockdown promoted procollagen I retention, including the serine/threonine kinase TNIK. Genetic deletion or pharmacologic inhibition of TNIK through the small molecule inhibitor NCB-0846 disrupted procollagen I trafficking and secretion without impacting procollagen I expression. To investigate the role of TNIK in liver fibrogenesis, we analyzed human and murine livers, finding elevated TNIK expression in human cirrhotic livers and increased TNIK expression and kinase activity in both fibrotic mouse livers and activated primary human HSCs. Finally, we tested whether inhibition of TNIK kinase activity could limit fibrogenesis in vivo. Mice receiving NCB-0846 displayed reduced CCl₄ -induced fibrogenesis compared to CCl₄ alone, although α-smooth muscle actin levels were unaltered. Conclusions: Our siRNA screen effectively identified TNIK as a key kinase involved in procollagen I trafficking in vitro and hepatic fibrogenesis in vivo.

Coordinated signaling of activating transcription factor 6α and inositol-requiring enzyme 1α regulates hepatic stellate cell-mediated fibrogenesis in mice

Liver injury and the unfolded protein response (UPR) are tightly linked, but their relationship differs with cell type and injurious stimuli. UPR initiation promotes hepatic stellate cell (HSC) activation and fibrogenesis, but the underlying mechanisms are unclear. Despite the complexity and overlap downstream of UPR transducers inositol-requiring protein 1α (IRE1α), activating transcription factor 6α (ATF6α), and protein kinase RNA-like ER kinase (PERK), previous research in HSCs primarily focused on IRE1α. Here, we investigated the fibrogenic role of ATF6α or PERK in vitro and HSC-specific UPR signaling in vivo. Overexpression of ATF6α, but not the PERK effector activating transcription factor 4 (ATF4), promoted HSC activation and fibrogenic gene transcription in immortalized HSCs. Furthermore, ATF6α inhibition through Ceapin-A7, or Atf6a deletion, disrupted transforming growth factor β (TGFβ)-mediated activation of primary human hepatic stellate cells (hHSCs) or murine hepatic stellate cells (mHSCs), respectively. We investigated the fibrogenic role of ATF6α in vivo through conditional HSC-specific Atf6a deletion. Atf6a^HSCΔ/Δ mice displayed reduced fibrosis and HSC activation following bile duct ligation (BDL) or carbon tetrachloride (CCl₄)-induced injury. The Atf6a^HSCΔ/Δ phenotype differed from HSC-specific Ire1a deletion, as Ire1a^HSCΔ/Δ mice showed reduced fibrogenic gene transcription but no changes in fibrosis compared with Ire1a^fl/fl mice following BDL. Interestingly, ATF6α signaling increased in Ire1a^Δ/Δ HSCs, whereas IRE1α signaling was upregulated in Atf6a^Δ/Δ HSCs. Finally, we asked whether co-deletion of Atf6a and Ire1a additively limits fibrosis. Unexpectedly, fibrosis worsened in Atf6a^HSCΔ/ΔIre1a^HSCΔ/Δ mice following BDL, and Atf6a^Δ/ΔIre1a^Δ/Δ mHSCs showed increased fibrogenic gene transcription. ATF6α and IRE1α individually promote fibrogenic transcription in HSCs, and ATF6α drives fibrogenesis in vivo. Unexpectedly, disruption of both pathways sensitizes the liver to fibrogenesis, suggesting that fine-tuned UPR signaling is critical for regulating HSC activation and fibrogenesis.NEW & NOTEWORTHY ATF6α is a critical driver of hepatic stellate cell (HSC) activation in vitro. HSC-specific deletion of Atf6a limits fibrogenesis in vivo despite increased IRE1α signaling. Conditional deletion of Ire1α from HSCs limits fibrogenic gene transcription without impacting overall fibrosis. This could be due in part to observed upregulation of the ATF6α pathway. Dual loss of Atf6a and Ire1a from HSCs worsens fibrosis in vivo through enhanced HSC activation.

ER Disposal Pathways in Chronic Liver Disease: Protective, Pathogenic, and Potential Therapeutic Targets

The endoplasmic reticulum is a central player in liver pathophysiology. Chronic injury to the ER through increased lipid content, alcohol metabolism, or accumulation of misfolded proteins causes ER stress, dysregulated hepatocyte function, inflammation, and worsened disease pathogenesis. A key adaptation of the ER to resolve stress is the removal of excess or misfolded proteins. Degradation of intra-luminal or ER membrane proteins occurs through distinct mechanisms that include ER-associated Degradation (ERAD) and ER-to-lysosome-associated degradation (ERLAD), which includes macro-ER-phagy, micro-ER-phagy, and Atg8/LC-3-dependent vesicular delivery. All three of these processes are critical for removing misfolded or unfolded protein aggregates, and re-establishing ER homeostasis following expansion/stress, which is critical for liver function and adaptation to injury. Despite playing a key role in resolving ER stress, the contribution of these degradative processes to liver physiology and pathophysiology is understudied. Analysis of publicly available datasets from diseased livers revealed that numerous genes involved in ER-related degradative pathways are dysregulated; however, their roles and regulation in disease progression are not well defined. Here we discuss the dynamic regulation of ER-related protein disposal pathways in chronic liver disease and cell-type specific roles, as well as potentially targetable mechanisms for treatment of chronic liver disease.

Apolipoprotein H drives hepatitis B surface antigen retention and endoplasmic reticulum stress during hepatitis B virus infection

BACKGROUND

Apolipoprotein H (APOH), also known as beta2-glycoprotein I (beta2-GPI), is an acute phase protein in hepatitis B virus (HBV) infection and binds to hepatitis B surface antigen (HBsAg) with high-affinity. APOH expression is upregulated by HBV and the large surface protein (LHBs), but also elevated in HBV-related hepatoma cells. Previous studies show that intracellular retention of HBsAg induces endoplasmic reticulum (ER) stress, a key driver of hepatocyte damage during chronic liver injury, but the mechanisms are unclear. We hypothesize that APOH mediates HBV-induced ER stress through increased retention of HBsAg.

METHODS

VR-APOH-myc and VR-LHBs-flag plasmids were constructed by PCR using pcDNA3.1(-)-APOH or an HBV expression vector, respectively. APOH and ER stress markers were examined at protein and mRNA levels by Western Blot or RT-qPCR. HBsAg titer was assayed by ELISA. RNA-seq was performed to elucidate the transcriptional impact of APOH manipulation in HBV-producing cells (HepG2.2.15 cells).

RESULTS

We found that HBV upregulates APOH expression in 293 T cells, and APOH overexpression subsequently inhibits secretion of HBsAg. Next, we show that LHBs overexpression in conjunction with APOH leads to ER stress in 293 T cells, as evidenced by production of the binding immunoglobulin protein (BiP) and C/EBP homologous protein (CHOP), as well as increased splicing of X-box binding protein 1 (XBP1). We further observed that loss of beta2-GPI reduced CHOP expression in HepG2.2.15 cells, while beta2-GPI overexpression enhanced CHOP production.

CONCLUSION

The interaction of beta2-GPI and HBV initiates ER stress through driving intracellular retention of HBsAg and activates the UPR.

IRE1A Stimulates Hepatocyte-Derived Extracellular Vesicles That Promote Inflammation in Mice With Steatohepatitis

BACKGROUND & AIMS

Endoplasmic reticulum to nucleus signaling 1 (ERN1, also called IRE1A) is a sensor of the unfolded protein response that is activated in the livers of patients with nonalcoholic steatohepatitis (NASH). Hepatocytes release ceramide-enriched inflammatory extracellular vesicles (EVs) after activation of IRE1A. We studied the effects of inhibiting IRE1A on release of inflammatory EVs in mice with diet-induced steatohepatitis.

METHODS

C57BL/6J mice and mice with hepatocyte-specific disruption of Ire1a (IRE1α^Δhep) were fed a diet high in fat, fructose, and cholesterol to induce development of steatohepatitis or a standard chow diet (controls). Some mice were given intraperitoneal injections of the IRE1A inhibitor 4μ8C. Mouse liver and primary hepatocytes were transduced with adenovirus or adeno-associated virus that expressed IRE1A. Livers were collected from mice and analyzed by quantitative polymerase chain reaction and chromatin immunoprecipitation assays; plasma samples were analyzed by enzyme-linked immunosorbent assay. EVs were derived from hepatocytes and injected intravenously into mice. Plasma EVs were characterized by nanoparticle-tracking analysis, electron microscopy, immunoblots, and nanoscale flow cytometry; we used a membrane-tagged reporter mouse to detect hepatocyte-derived EVs. Plasma and liver tissues from patients with NASH and without NASH (controls) were analyzed for EV concentration and by RNAscope and gene expression analyses.

RESULTS

Disruption of Ire1a in hepatocytes or inhibition of IRE1A reduced the release of EVs and liver injury, inflammation, and accumulation of macrophages in mice on the diet high in fat, fructose, and cholesterol. Activation of IRE1A, in the livers of mice, stimulated release of hepatocyte-derived EVs, and also from cultured primary hepatocytes. Mice given intravenous injections of IRE1A-stimulated, hepatocyte-derived EVs accumulated monocyte-derived macrophages in the liver. IRE1A-stimulated EVs were enriched in ceramides. Chromatin immunoprecipitation showed that IRE1A activated X-box binding protein 1 (XBP1) to increase transcription of serine palmitoyltransferase genes, which encode the rate-limiting enzyme for ceramide biosynthesis. Administration of a pharmacologic inhibitor of serine palmitoyltransferase to mice reduced the release of EVs. Levels of XBP1 and serine palmitoyltransferase were increased in liver tissues, and numbers of EVs were increased in plasma, from patients with NASH compared with control samples and correlated with the histologic features of inflammation.

CONCLUSIONS

In mouse hepatocytes, activated IRE1A promotes transcription of serine palmitoyltransferase genes via XBP1, resulting in ceramide biosynthesis and release of EVs. The EVs recruit monocyte-derived macrophages to the liver, resulting in inflammation and injury in mice with diet-induced steatohepatitis. Levels of XBP1, serine palmitoyltransferase, and EVs are all increased in liver tissues from patients with NASH. Strategies to block this pathway might be developed to reduce liver inflammation in patients with NASH.

Selective YAP/TAZ inhibition in fibroblasts via dopamine receptor D1 agonism reverses fibrosis

Tissue fibrosis is characterized by uncontrolled deposition and diminished clearance of fibrous connective tissue proteins, ultimately leading to organ scarring. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) have recently emerged as pivotal drivers of mesenchymal cell activation in human fibrosis. Therapeutic strategies inhibiting YAP and TAZ have been hindered by the critical role that these proteins play in regeneration and homeostasis in different cell types. Here, we find that the Gα_s-coupled dopamine receptor D1 (DRD1) is preferentially expressed in lung and liver mesenchymal cells relative to other resident cells of these organs. Agonism of DRD1 selectively inhibits YAP/TAZ function in mesenchymal cells and shifts their phenotype from profibrotic to fibrosis resolving, reversing in vitro extracellular matrix stiffening and in vivo tissue fibrosis in mouse models. Aromatic l-amino acid decarboxylase [DOPA decarboxylase (DDC)], the enzyme responsible for the final step in biosynthesis of dopamine, is decreased in the lungs of subjects with idiopathic pulmonary fibrosis, and its expression inversely correlates with disease severity, consistent with an endogenous protective role for dopamine signaling that is lost in pulmonary fibrosis. Together, these findings establish a pharmacologically tractable and cell-selective approach to targeting YAP/TAZ via DRD1 that reverses fibrosis in mice.

p300 Acetyltransferase Is a Cytoplasm-to-Nucleus Shuttle for SMAD2/3 and TAZ Nuclear Transport in Transforming Growth Factor β-Stimulated Hepatic Stellate Cells

Nuclear translocation of mothers against decapentaplegic homolog 2/3 (SMAD2/3), core transcription factors of transforming growth factor β (TGF-β) signaling, is critical for hepatic stellate cell (HSC) differentiation into metastasis-promoting myofibroblasts. SMAD2/3 have multiple coactivators, including WW domain-containing transcription regulator protein 1 (WWTR1 or TAZ) and p300 acetyltransferase. In the nucleus, TAZ binds to SMAD2/3 to prevent SMAD2/3 nuclear export. However, how TAZ and SMAD2/3 enter the nucleus remains poorly understood because neither contains a nuclear localization signal (NLS), an amino acid sequence tagging proteins for nuclear transport. p300 is an NLS-containing large scaffold protein, so we hypothesized that SMAD2/3 and TAZ may undergo nuclear import through complexing with p300. Coimmunoprecipitation, immunofluorescence, and nuclear fractionation assays revealed that TGF-β1 promoted binding of SMAD2/3 and TAZ to p300 and that p300 inactivation disrupted TGF-β1-mediated SMAD2/3 and TAZ nuclear accumulation. Deleting the p300 NLS blocked TGF-β1-induced SMAD2/3 and TAZ nuclear transport. Consistently, p300 inactivation suppressed TGF-β1-mediated HSC activation and transcription of genes encoding tumor-promoting factors, such as connective tissue growth factor, Tenascin C, Periostin, platelet-derived growth factor C, and fibroblast growth factor 2, as revealed by microarray analysis. Chromatin immunoprecipitation-real-time quantitative PCR showed that canonical p300-mediated acetylation of histones also facilitated transcription in response to TGF-β1 stimulation. Interestingly, although both TGF-β1-mediated and stiffness-mediated HSC activation require p300, comparison of gene expression data sets revealed that transcriptional targets of TGF-β1 were distinct from those of stiffness-p300 mechanosignaling. Lastly, in tumor/HSC coinjection and intrasplenic tumor injection models, targeting p300 of activated-HSC/myofibroblasts by C646, short hairpin RNA, or cre-mediated gene disruption reduced tumor and liver metastatic growth in mice. Conclusion: p300 facilitates TGF-β1-stimulated HSC activation by both noncanonical (cytoplasm-to-nucleus shuttle for SMAD2/3 and TAZ) and canonical (histone acetylation) mechanisms. p300 is an attractive target for inhibiting HSC activation and the prometastatic liver microenvironment.

Endoplasmic Reticulum Stress in Metabolic Liver Diseases and Hepatic Fibrosis

Endoplasmic reticulum (ER) stress is a major contributor to liver disease and hepatic fibrosis, but the role it plays varies depending on the cause and progression of the disease. Furthermore, ER stress plays a distinct role in hepatocytes versus hepatic stellate cells (HSCs), which adds to the complexity of understanding ER stress and its downstream signaling through the unfolded protein response (UPR) in liver disease. Here, the authors focus on the current literature of ER stress in nonalcoholic and alcoholic fatty liver diseases, how ER stress impacts hepatocyte injury, and the role of ER stress in HSC activation and hepatic fibrosis. This review provides insight into the complex signaling and regulation of the UPR, parallels and distinctions between different liver diseases, and how ER stress may be targeted as an antisteatotic or antifibrotic therapy to limit the progression of liver disease.

Transforming growth factor β (TGFβ) cross-talk with the unfolded protein response is critical for hepatic stellate cell activation

Transforming growth factor β (TGFβ) potently activates hepatic stellate cells (HSCs), which promotes production and secretion of extracellular matrix (ECM) proteins and hepatic fibrogenesis. Increased ECM synthesis and secretion in response to TGFβ is associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). TGFβ and UPR signaling pathways are tightly intertwined during HSC activation, but the regulatory mechanism that connects these two pathways is poorly understood. Here, we found that TGFβ treatment of immortalized HSCs (i.e. LX-2 cells) induces phosphorylation of the UPR sensor inositol-requiring enzyme 1α (IRE1α) in a SMAD2/3-procollagen I-dependent manner. We further show that IRE1α mediates HSC activation downstream of TGFβ and that its role depends on activation of a signaling cascade involving apoptosis signaling kinase 1 (ASK1) and c-Jun N-terminal kinase (JNK). ASK1-JNK signaling promoted phosphorylation of the UPR-associated transcription factor CCAAT/enhancer binding protein β (C/EBPβ), which is crucial for TGFβ- or IRE1α-mediated LX-2 activation. Pharmacological inhibition of C/EBPβ expression with the antiviral drug adefovir dipivoxil attenuated TGFβ-mediated activation of LX-2 or primary rat HSCs in vitro and hepatic fibrogenesis in vivo Finally, we identified a critical relationship between C/EBPβ and the transcriptional regulator p300 during HSC activation. p300 knockdown disrupted TGFβ- or UPR-induced HSC activation, and pharmacological inhibition of the C/EBPβ-p300 complex decreased TGFβ-induced HSC activation. These results indicate that TGFβ-induced IRE1α signaling is critical for HSC activation through a C/EBPβ-p300-dependent mechanism and suggest C/EBPβ as a druggable target for managing fibrosis.

Interplay between tight junctions & adherens junctions

Cell-cell adhesions are critical for the development and maintenance of tissues. Present at sites of cell-cell contact are the adherens junctions and tight junctions. The adherens junctions mediate cell-cell adhesion via the actions of nectins and cadherins. The tight junctions regulate passage of ions and small molecules between cells and establish cell polarity. Historically, the adherens and tight junctions have been thought of as discrete complexes. However, it is now clear that a high level of interdependency exists between the two junctional complexes. The adherens junctions and tight junctions are physically linked, by the zonula occludens proteins, and linked via signaling molecules including several polarity complexes and actin cytoskeletal modifiers. This review will first describe the individual components of both the adherens and tight junctions and then discuss the coupling of the two complexes with an emphasis on the signaling links and physical interactions between the two junctional complexes.

The unfolded protein response mediates fibrogenesis and collagen I secretion through regulating TANGO1 in mice

Fibrogenesis encompasses the deposition of matrix proteins, such as collagen I, by hepatic stellate cells (HSCs) that culminates in cirrhosis. Fibrogenic signals drive transcription of procollagen I, which enters the endoplasmic reticulum (ER), is trafficked through the secretory pathway, and released to generate extracellular matrix. Alternatively, disruption of procollagen I ER export could activate the unfolded protein response (UPR) and drive HSC apoptosis. Using a small interfering RNA screen, we identified Transport and Golgi organization 1 (TANGO1) as a potential participant in collagen I secretion. We investigated the role of TANGO1 in procollagen I secretion in HSCs and liver fibrogenesis. Depletion of TANGO1 in HSCs blocked collagen I secretion without affecting other matrix proteins. Disruption of secretion led to procollagen I retention within the ER, induction of the UPR, and HSC apoptosis. In wild-type (WT) HSCs, both TANGO1 and the UPR were induced by transforming growth factor β (TGFβ). As the UPR up-regulates proteins involved in secretion, we studied whether TANGO1 was a target of the UPR. We found that UPR signaling is responsible for up-regulating TANGO1 in response to TGFβ, and this mechanism is mediated by the transcription factor X-box binding protein 1 (XBP1). In vivo, murine and human cirrhotic tissue displayed increased TANGO1 messenger RNA levels. Finally, TANGO1^+/- mice displayed less hepatic fibrosis compared to WT mice in two separate murine models: CCl₄ and bile duct ligation.

CONCLUSION

Loss of TANGO1 leads to procollagen I retention in the ER, which promotes UPR-mediated HSC apoptosis. TANGO1 regulation during HSC activation occurs through a UPR-dependent mechanism that requires the transcription factor, XBP1. Finally, TANGO1 is critical for fibrogenesis through mediating HSC homeostasis. The work reveals a unique role for TANGO1 and the UPR in facilitating collagen I secretion and fibrogenesis. (Hepatology 2017;65:983-998).

Inhibition of sphingosine 1-phosphate signaling ameliorates murine nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a lipotoxic disorder, wherein proinflammatory lipids, such as ceramide and its derivative sphingosine 1-phosphate (S1P), contribute to macrophage-associated liver inflammation. For example, we have previously demonstrated a role for S1P in steatotic hepatocyte-derived S1P-enriched extracellular vesicles in macrophage chemotaxis in vitro. Therefore, we hypothesized that FTY720, an S1P antagonist, would ameliorate NASH by inhibiting proinflammatory monocyte chemotaxis. To test our hypothesis, NASH was established in C57BL/6 male mice by feeding a diet high in fructose, saturated fat, and cholesterol for 22 wk. Then mice received daily intraperitoneal injections of FTY720 for 2 wk before analysis of liver injury, inflammation, and fibrosis. FTY720-treated mice with NASH demonstrated improved liver histology with a significant reduction in hepatocyte ballooning and inflammatory foci. Hepatomegaly was reversed, and liver triglycerides were reduced following FTY720 administration to mice with NASH. Correspondingly, serum ALT levels, hepatic inflammatory macrophage accumulation, and the expression of Ly6C in recruited myeloid cells was reduced in FTY720-treated mice. Hepatic collagen accumulation and expression of α-smooth muscle actin were significantly lowered as well. Body composition, energy consumption and utilization, and hepatic sphingolipid composition remained unchanged following FTY720 administration. FTY720 ameliorates murine nonalcoholic steatohepatitis. Reduction in liver injury and inflammation is associated with a reduction in hepatic macrophage accumulation, likely due to dampened recruitment of circulating myeloid cells into the liver. Nonalcoholic steatohepatitis may be a novel indication for the therapeutic use of FTY720.NEW & NOTEWORTHY There are no approved pharmacologic therapies for nonalcoholic steatohepatitis (NASH), the leading cause of chronic liver disease worldwide. This study describes the use of FTY720, a novel small molecule, for the amelioration of NASH in a mouse model. We demonstrate that 2-wk administration of FTY720 to mice with NASH led to a reduction in liver injury, inflammation, and fibrosis. These data provide a preclinical rationale for studying this drug in human NASH.

Hepatic Stellate Cell Selective Disruption of Dynamin-2 GTPase Increases Murine Fibrogenesis through Up-Regulation of Sphingosine-1 Phosphate-Induced Cell Migration

Dynamin-2 (Dyn2) is implicated in endocytosis of receptor tyrosine kinases, which contribute to hepatic stellate cell (HSC) activation and liver fibrosis. A point mutation converting lysine 44 of Dyn2 to alanine (Dyn2K44A) disrupts its GTPase activity. We hypothesized that Dyn2K44A expression in HSCs would decrease HSC activation and fibrogenesis in vivo by disrupting receptor tyrosine kinase endocytosis and signaling. Dyn2K44A^fl/fl mice were crossed with Collagen1-Cre (Col1^Cre) mice to generate offspring with HSC selective expression of Dyn2K44A (Col1^Cre/Dyn2K44A^fl/fl). Contrary to our hypothesis, Col1^Cre/Dyn2K44A^fl/fl mice showed increased hepatic fibrosis in response to liver injury. To elucidate mechanisms, we conducted in vitro experiments with HSCs infected with adenoviral vectors encoding LacZ, Dyn2K44A, or Dyn2WT. HSC-expressing Dyn2K44A displayed increased mRNA and protein levels of sphingosine kinase-1 (SK1), an enzyme previously implicated in the pathogenesis of fibrosis. To study the functional effects of Dyn2K44A regulation of SK1, we examined effects of AKT signaling and migration in HSCs. Dyn2K44A promoted both AKT phosphorylation and HSC migration in an SK1-dependent manner. Genetic disruption of Dyn2 GTPase activity selectively in HSC enhances fibrogenesis, driven at least in part through up-regulation of the SK1 pathway and cell migration in HSCs.

ZO-1 recruitment to α-catenin--a novel mechanism for coupling the assembly of tight junctions to adherens junctions

The formation of a barrier between epithelial cells is a fundamental determinant of cellular homeostasis, protecting underlying cells against pathogens, dehydration and damage. Assembly of the tight junction barrier is dependent upon neighboring epithelial cells binding to one another and forming adherens junctions, but the mechanism for how these processes are linked is poorly understood. Using a knockdown and substitution system, we studied whether ZO-1 binding to α-catenin is required for coupling tight junction assembly to the formation of adherens junctions. We found that preventing ZO-1 binding to α-catenin did not appear to affect adherens junctions. Rather the assembly and maintenance of the epithelial barrier were disrupted. This disruption was accompanied by alterations in the mobility of ZO-1 and the organization of the actin cytoskeleton. Thus, our study identifies α-catenin binding to ZO-1 as a new mechanism for coupling the assembly of the epithelial barrier to cell-to-cell adhesion.

Functional analysis of a de novo ACTB mutation in a patient with atypical Baraitser-Winter syndrome

Exome sequence analysis can be instrumental in identifying the genetic etiology behind atypical disease. We report a patient presenting with microcephaly, dysmorphic features, and intellectual disability with a tentative diagnosis of Dubowitz syndrome. Exome analysis was performed on the patient and both parents. A de novo missense variant was identified in ACTB, c.349G>A, p.E117K. Recent work in Baraitser-Winter syndrome has identified ACTB and ACTG1 mutations in a cohort of individuals, and we rediagnosed the patient with atypical Baraitser-Winter syndrome. We performed functional characterization of the variant actin and show that it alters cell adhesion and polymer formation supporting its role in disease. We present the clinical findings in the patient, comparison of this patient to other patients with ACTB/ACTG1 mutations, and results from actin functional studies that demonstrate novel functional attributes of this mutant protein.

α-Catenin uses a novel mechanism to activate vinculin

Vinculin, an actin-binding protein, is emerging as an important regulator of adherens junctions. In focal-adhesions, vinculin is activated by simultaneous binding of talin to its head domain and actin filaments to its tail domain. Talin is not present in adherens junctions. Consequently, the identity of the ligand that activates vinculin in cell-cell junctions is not known. Here we show that in the presence of F-actin, α-catenin, a cytoplasmic component of the cadherin adhesion complex, activates vinculin. Direct binding of α-catenin to vinculin is critical for this event because a point mutant (α-catenin L344P) lacking high affinity binding does not activate vinculin. Furthermore, unlike all known vinculin activators, α-catenin binds to and activates vinculin independently of an A50I substitution in the vinculin head, a mutation that inhibits vinculin binding to talin and IpaA. Collectively, these data suggest that α-catenin employs a novel mechanism to activate vinculin and may explain how vinculin is differentially recruited and/or activated in cell-cell and cell-matrix adhesions.