Tricellular tight junction protein LSR/angulin-1 contributes to the epithelial barrier and malignancy in human pancreatic cancer cell line

Tricellulin facilitates colorectal cancer metastasis through activation of the TGFβ/SMAD2/3 signalling pathway

Background

Tricellulin belongs to the TAMP family of proteins and is primarily localized at the tricellular tight junctions. While its role in the progression of cancer has been reported, its importance in the progression of colorectal cancer (CRC) remains unclear.

Objective

This study aimed to determine the function and mechanism of tricellulin in CRC progression.

Methods

The proteins expression in cells and/or tissues was determined by Western blot, immunohistochemistry staining, and/or RT-qPCR analyses. The biological functions of tricellulin were investigated through in vitro assays (CCK-8, Transwell migration, and colony formation assays) and in vivo xenograft models. Tricellulin was significantly upregulated in CRC tissues compared to adjacent normal tissues. The expression of tricellulin was correlated with poor prognosis in patients with CRC.

Results

In vitro assays showed that tricellulin enhanced CRC cell proliferation, migration, and invasion. Mechanistically, tricellulin activated the TGFb1/SMAD2/3 pathway, while TGFb1 reciprocally controlled the expression of tricellulin. Also, tricellulin promotes CRC cell migration/invasion through EMT. In vivo models confirmed that the overexpression of tricellulin facilitated tumor growth and activated the TGFb1/ SMAD2/3 pathway in CRC.

Conclusion

Our findings demonstrate thatTricellulin promotes the metastasis of colorectal cancer by activating the TGF-β/SMAD2/3 signaling pathway, and TGF-β1 can reciprocally regulate the expression of tricellulin.We have revealed a novel mechanism by which tricellulin forms a positive feedback loop to promote the growth and metastasis of CRC. This mechanism provides novel insights into CRC progression and suggests potential therapeutic targets.

The interplay between the epithelial permeability barrier, cell migration and mitochondrial metabolism of growth factors and their inhibitors in a human endometrial carcinoma cell line

It is known that there are abnormalities of tight junction functions, cell migration and mitochondrial metabolism in human endometriosis and endometrial carcinoma. In this study, we investigated the effects of growth factors and their inhibitors on the epithelial permeability barrier, cell migration and mitochondrial metabolism in 2D and 2.5D cultures of human endometrioid endometrial carcinoma Sawano cells. We also investigated the changes of bicellular and tricellular tight junction molecules and ciliogenesis induced by these inhibitors. The growth factors TGF-β and EGF affected the epithelial permeability barrier, cell migration and expression of bicellular and tricellular tight junction molecules in 2D and 2.5D cultures of Sawano cells. EW-7197 (a TGF-β receptor inhibitor), AG1478 (an EGFR inhibitor) and SP600125 (a JNK inhibitor) affected the epithelial permeability barrier, cell migration and mitochondrial metabolism and prevented the changes induced by TGF-β and EGF in 2D and 2.5D cultures. EW-7197 and AG1478 induced ciliogenesis in 2.5D cultures. In conclusion, TGF-β and EGF promoted the malignancy of endometrial cancer via interplay among the epithelial permeability barrier, cell migration and mitochondrial metabolism. EW-7197 and AG1478 may be useful as novel therapeutic treatments options for endometrial cancer.

Cellular Uptake and Cytotoxicity of Clostridium perfringens Iota-Toxin

Clostridium perfringens iota-toxin is composed of two separate proteins: a binding protein (Ib) that recognizes a host cell receptor and promotes the cellular uptake of a catalytic protein and (Ia) possessing ADP-ribosyltransferase activity that induces actin cytoskeleton disorganization. Ib exhibits the overall structure of bacterial pore-forming toxins (PFTs). Lipolysis-stimulated lipoprotein receptor (LSR) is defined as a host cell receptor for Ib. The binding of Ib to LSR causes an oligomer formation of Ib in lipid rafts of plasma membranes, mediating the entry of Ia into the cytoplasm. Ia induces actin cytoskeleton disruption via the ADP-ribosylation of G-actin and causes cell rounding and death. The binding protein alone disrupts the cell membrane and induces cytotoxicity in sensitive cells. Host cells permeabilized by the pore formation of Ib are repaired by a Ca2+-dependent plasma repair pathway. This review shows that the cellular uptake of iota-toxin utilizes a pathway of plasma membrane repair and that Ib alone induces cytotoxicity.

Matrix stiffness regulates the tight junction phenotypes and local barrier properties in tricellular regions in an iPSC-derived BBB model

The blood-brain barrier (BBB) can respond to various mechanical cues such as shear stress and substrate stiffness. In the human brain, the compromised barrier function of the BBB is closely associated with a series of neurological disorders that are often also accompanied by the alteration of brain stiffness. In many types of peripheral vasculature, higher matrix stiffness decreases barrier function of endothelial cells through mechanotransduction pathways that alter cell-cell junction integrity. However, human brain endothelial cells are specialized endothelial cells that largely resist changes in cell morphology and key BBB markers. Therefore, it has remained an open question how matrix stiffness affects barrier integrity in the human BBB. To gain insight into the effects of matrix stiffness on BBB permeability, we differentiated brain microvascular endothelial-like cells from human induced pluripotent stem cells (iBMEC-like cells) and cultured the cells on extracellular matrix-coated hydrogels of varying stiffness. We first detected and quantified the junction presentation of key tight junction (TJ) proteins. Our results show matrix-dependent junction phenotypes in iBMEC-like cells, where cells on softer gels (1 kPa) have significantly lower continuous and total TJ coverages. We also determined that these softer gels also lead to decreased barrier function in a local permeability assay. Furthermore, we found that matrix stiffness regulates the local permeability of iBMEC-like cells through the balance of continuous ZO-1 TJs and no junction regions ZO-1 in tricellular regions. Together, these findings provide valuable insights into the effects of matrix stiffness on TJ phenotypes and local permeability of iBMEC-like cells. STATEMENT OF SIGNIFICANCE: Brain mechanical properties, including stiffness, are particularly sensitive indicators for pathophysiological changes in neural tissue. The compromised function of the blood-brain barrier is closely associated with a series of neurological disorders often accompanied by altered brain stiffness. In this study, we use polymeric biomaterials and provide new evidence that biomaterial stiffness regulates the local permeability in iPSC-derived brain endothelial cells in tricellular regions through the tight junction protein ZO-1. Our findings provide valuable insights into the changes in junction architecture and barrier permeability in response to different substrate stiffnesses. Since BBB dysfunction has been linked to many diseases, understanding the influence of substrate stiffness on junction presentations and barrier permeability could lead to the development of new treatments for diseases associated with BBB dysfunction or drug delivery across BBB systems.

Downregulation of angulin-1/LSR induces malignancy via upregulation of EGF-dependent claudin-2 and TGF-β-dependent cell metabolism in human lung adenocarcinoma A549 cells

Abnormal expression of bicellular tight junction claudins, including claudin-2 are observed during carcinogenesis in human lung adenocarcinoma. However, little is known about the role of tricellular tight junction molecule angulin-1/lipolysis-stimulated lipoprotein receptor (LSR). In the lung adenocarcinoma tissues examined in the present study, expression of claudin-2 was higher than in normal lung tissues, while angulin-1/LSR was poorly or faintly expressed. We investigated how loss of angulin-1/LSR affects the malignancy of lung adenocarcinoma cell line A549 and normal human lung epithelial (HLE) cells. The EGF receptor tyrosine kinase inhibitor AG1478 prevented the increase of claudin-2 expression induced by EGF in A549 cells. Knockdown of LSR induced expression of claudin-2 at the protein and mRNA levels and AG1478 prevented the upregulation of claudin-2 in A549 cells. Knockdown of LSR induced cell proliferation, cell migration and cell metabolism in A549 cells. Knockdown of claudin-2 inhibited the cell proliferation but did not affect the cell migration or cell metabolism of A549 cells. The TGF-β type I receptor inhibitor EW-7197 prevented the decrease of LSR and claudin-2 induced by TGF-β1 in A549 cells and 2D culture of normal HLE cells. EW-7197 prevented the increase of cell migration and cell metabolism induced by TGF-β1 in A549 cells. EW-7197 prevented the increase of epithelial permeability of FITC-4kD dextran induced by TGF-β1 in 2.5D culture of normal HLE cells. In conclusion, downregulation of angulin-1/LSR induces malignancy via EGF-dependent claudin-2 and TGF-β-dependent cell metabolism in human lung adenocarcinoma.

Atypical Macropinocytosis Contributes to Malignant Progression: A Review of Recent Evidence in Endometrioid Endometrial Cancer Cells

Simple Summary

A novel type of macropinocytosis has been identified as a trigger for the malignant progression of endometrial cancer. Transiently reducing epithelial barrier homeostasis leads to macropinocytosis by splitting between adjacent cells in endometrioid endometrial cancer. Macropinocytosis causes morphological changes in well-differentiated to poorly differentiated cancer cells. Inhibition of macropinocytosis promotes a persistent dormant state in the intrinsic KRAS-mutated cancer cell line Sawano. This review focuses on the mechanisms of atypical macropinocytosis and its effects on cellular function, and it describes the physiological processes involved in inducing resting conditions in endometrioid endometrial cancer cells.

Abstract

Macropinocytosis is an essential mechanism for the non-specific uptake of extracellular fluids and solutes. In recent years, additional functions have been identified in macropinocytosis, such as the intracellular introduction pathway of drugs, bacterial and viral infection pathways, and nutritional supplement pathway of cancer cells. However, little is known about the changes in cell function after macropinocytosis. Recently, it has been reported that macropinocytosis is essential for endometrial cancer cells to initiate malignant progression in a dormant state. Macropinocytosis is formed by a temporary split of adjacent bicellular junctions of epithelial sheets, rather than from the apical surface or basal membrane, as a result of the transient reduction of tight junction homeostasis. This novel type of macropinocytosis has been suggested to be associated with the malignant pathology of endometriosis and endometrioid endometrial carcinoma. This review outlines the induction of malignant progression of endometrial cancer cells by macropinocytosis based on a new mechanism and the potential preventive mechanism of its malignant progression.

Effects of Lipolysis-Stimulated Lipoprotein Receptor on Tight Junctions of Pancreatic Ductal Epithelial Cells in Hypertriglyceridemic Acute Pancreatitis

Objective

We investigated the effects of lipolysis-stimulated lipoprotein receptor (LSR) on the tight junctions (TJs) of pancreatic ductal epithelial cells (PDECs) in hypertriglyceridemic acute pancreatitis (HTGAP).

Methods

Sprague-Dawley rats were fed standard rat chow or a high-fat diet and injected with sodium taurocholate to obtain normal and HTGAP rats, respectively. Serum triglyceride (TG) levels, pathological changes, TJ proteins in the pancreas, and TJ ultrastructure of PDECs were assessed. LSR overexpression (OE) and knockdown (KD) HPDE6-C7 models were designed and cultured in a high-fat environment. Protein levels were quantified by Western blotting. Cell monolayer permeability was detected using FITC-Dextran.

Results

Serum TG concentration and pancreatic scores were higher in the HTGAP group than in the normal group. Among the TJ proteins, LSR protein expression was significantly lower in the HTGAP group than in the acute pancreatitis (AP) group. Tricellulin (TRIC) expression in the pancreatic ductal epithelia was higher in the HTGAP group than in the AP group. The HTGAP group had lower TJ protein levels, wider intercellular space, and widespread cellular necrosis with disappearance of cell junction structures. In the cell study, TJ proteins were downregulated and the cellular barrier was impaired by palmitic acid (PA), which was reversed by LSR-OE, whereas LSR-KD downregulated the TJ proteins and aggravated PA-induced cellular barrier impairment.

Conclusions

Hypertriglyceridemia downregulates the TJ proteins in PDECs, which may impair the pancreatic ductal mucosal barrier function. LSR regulation can change the effects of HTG on cellular barrier function by upregulating the TJ proteins.

The Roles of Tricellular Tight Junction Protein Angulin-1/Lipolysis-Stimulated Lipoprotein Receptor (LSR) in Endometriosis and Endometrioid-Endometrial Carcinoma

Tight junction proteins play roles beyond permeability barriers functions and control cell proliferation and differentiation. The relation between tight junctions and the signal transduction pathways affects cell growth, invasion and migration. Abnormality of tight junction proteins closely contributes to epithelial mesenchymal transition (EMT) and malignancy of various cancers. Angulin-1/lipolysis-stimulated lipoprotein receptor (LSR) forms tricellular contacts that has a barrier function. Downregulation of angulin-1/LSR correlates with the malignancy in various cancers, including endometrioid-endometrial carcinoma (EEC). These alterations have been shown to link to not only multiple signaling pathways such as Hippo/YAP, HDAC, AMPK, but also cell metabolism in ECC cell line Sawano. Moreover, loss of angulin-1/LSR upregulates claudin-1, and loss of apoptosis stimulating p53 protein 2 (ASPP2) downregulates angulin-1/LSR. Angulin-1/LSR and ASPP2 concentrate at both midbody and centrosome in cytokinesis. In EEC tissues, angulin-1/LSR and ASPP2 are reduced and claudin-2 is overexpressed during malignancy, while in the tissues of endometriosis changes in localization of angulin-1/LSR and claudin-2 are seen. This review highlights how downregulation of angulin-1/LSR promotes development of endometriosis and EEC and discusses about the roles of angulin-1/LSR and its related proteins, including claudins and ASPP2.

Dysfunction of epithelial permeability barrier induced by HMGB1 in 2.5D cultures of human epithelial cells

Airway and intestinal epithelial permeability barriers are crucial in epithelial homeostasis. High mobility group box 1 (HMGB1), increased by various stimuli, is involved in the induction of airway inflammation, as well as the pathogenesis of inflammatory bowel disease. HMGB1 enhances epithelial hyperpermeability. Two-and-a-half dimensional (2.5D) culture assays are experimentally convenient and induce cells to form a more physiological tissue architecture than 2D culture assays for molecular transfer mechanism analysis. In 2.5D culture, treatment with HMGB1 induced permeability of FITC-dextran into the lumen formed by human lung, nasal and intestinal epithelial cells. The tricellular tight junction molecule angulin-1/LSR is responsible for the epithelial permeability barrier at tricellular contacts and contributes to various human airway and intestinal inflammatory diseases. In this review, we indicate the mechanisms including angulin-1/LSR and multiple signaling in dysfunction of the epithelial permeability barrier induced by HMGB1 in 2.5D culture of human airway and intestinal epithelial cells.

Translocation of LSR from tricellular corners causes macropinocytosis at cell-cell interface as a trigger for breaking out of contact inhibition

Withdrawal from contact inhibition is necessary for epithelial cancer precursor cells to initiate cell growth and motility. Nevertheless, little is understood about the mechanism for the sudden initiation of cell growth under static conditions. We focused on cellular junctions as one region where breaking out of contact inhibition occurs. In well-differentiated endometrial cancer cells, Sawano, the ligand administration for tricellular tight junction protein LSR, which transiently decreased the robust junction property, caused an abrupt increase in cell motility and consequent excessive multilayered cell growth despite being under contact inhibition conditions. We observed that macropinocytosis essentially and temporarily occurred as an antecedent event for the above process at intercellular junctions without disruption of the junction apparatus but not at the apical plasma membrane. Collectively, we concluded that the formation of macropinocytosis, which is derived from tight junction-mediated signaling, was triggered for the initiation of cell growth in static precancerous epithelium.

Effects of HMGB1 on Tricellular Tight Junctions via TGF-β Signaling in Human Nasal Epithelial Cells

The airway epithelium of the human nasal mucosa acts as a physical barrier that protects against inhaled substances and pathogens via bicellular and tricellular tight junctions (bTJs and tTJs) including claudins, angulin-1/LSR and tricellulin. High mobility group box-1 (HMGB1) increased by TGF-β1 is involved in the induction of nasal inflammation and injury in patients with allergic rhinitis, chronic rhinosinusitis, and eosinophilic chronic rhinosinusitis. However, the detailed mechanisms by which this occurs remain unknown. In the present study, to investigate how HMGB1 affects the barrier of normal human nasal epithelial cells, 2D and 2.5D Matrigel culture of primary cultured human nasal epithelial cells were pretreated with TGF-β type I receptor kinase inhibitor EW-7197 before treatment with HMGB1. Knockdown of angulin-1/LSR downregulated the epithelial barrier. Treatment with EW-7197 decreased angulin-1/LSR and concentrated the expression at tTJs from bTJs and increased the epithelial barrier. Treatment with a binder to angulin-1/LSR angubindin-1 decreased angulin-1/LSR and the epithelial barrier. Treatment with HMGB1 decreased angulin-1/LSR and the epithelial barrier. In 2.5D Matrigel culture, treatment with HMGB1 induced permeability of FITC-dextran (FD-4) into the lumen. Pretreatment with EW-7197 prevented the effects of HMGB1. HMGB1 disrupted the angulin-1/LSR-dependent epithelial permeability barriers of HNECs via TGF-β signaling in HNECs.

Effects of histone deacetylase inhibitors Tricostatin A and Quisinostat on tight junction proteins of human lung adenocarcinoma A549 cells and normal lung epithelial cells

Histone deacetylase (HDAC) inhibitors have a potential therapeutic role for non-small cell lung cancer (NSCLC). However, more preclinical studies of HDAC inhibitors in NSCLC and normal lung epithelial cells are required to evaluate their antitumor activities and mechanisms. The bicellular tight junction molecule claudin-2 (CLDN-2) is highly expressed in lung adenocarcinoma tissues and increase the proliferation of adenocarcinoma cells. Downregulation of the tricellular tight junction molecule angulin-1/LSR induces malignancy via EGF-dependent CLDN-2 and TGF-β-dependent cellular metabolism in human lung adenocarcinoma cells. In the present study, to investigate the detailed mechanisms of the antitumor activities of HDAC inhibitors in lung adenocarcinoma, human lung adenocarcinoma A549 cells and normal lung epithelial cells were treated with the HDAC inibitors Trichostatin A (TSA) and Quisinostat (JNJ-2648158) with or without TGF-β. Both HDAC inhibitors increased anguin-1/LSR, decrease CLDN-2, promoted G1 arrest and prevented the migration of A549 cells. Furthermore, TSA but not Quisinostat with or without TGF-β induced cellular metabolism indicated as the mitochondrial respiration measured using the oxygen consumption rate. In normal human lung epithelial cells, treatment with TSA and Quisinostat increased expression of LSR and CLDN-2 and decreased that of CLDN-1 with or without TGF-β in 2D culture. Quisinostat but not TSA with TGF-β increased CLDN-7 expression in 2D culture. Both HDAC inhibitors prevented disruption of the epithelial barrier measured as the permeability of FD-4 induced by TGF-β in 2.5D culture. TSA and Quisinostat have potential for use in therapy for lung adenocarcinoma via changes in the expression of angulin-1/LSR and CLDN-2.

LSR Promotes Cell Proliferation and Invasion in Lung Cancer

The lipolysis-stimulated lipoprotein receptor (LSR) displays an important regulatory role in cancer. However, the association between LSR and lung cancer is still elusive. Here, the candidate oncogene LSR on Ch.9q was obtained and assessed by bioinformatics analysis of The Cancer Genome Atlas (TCGA) dataset of lung cancer. We conducted clinical pathology and survival analysis based on the lung cancer database. We assessed the biological effects of LSR in lung cancer cells on cell proliferation. Our data indicated that LSR was upregulated in lung cancer cells. Meanwhile, LSR was identified in this study to be a poor prognostic factor, and its high expression exhibited relations with grades, stages, and nodal metastasis status. Using in vitro analysis, our data revealed that LSR could promote lung cancer progression by regulating cell proliferation, migration, and invasion. In our study, our data demonstrated that LSR was a tumor promoter for lung cancer and was a potential biomarker and target for lung cancer prognosis and treatment.

The Role of Tricellular Junctions in the Transport of Macromolecules Across Endothelium

PURPOSE

Transport of water and solutes across vascular endothelium is important in normal physiology and critical in the development of various diseases, including atherosclerosis. However, there is debate about the routes for such transport. We recently showed that an albumin-sized tracer crossed endothelium at bicellular and tricellular junctions, a tracer having the size of high density lipoprotein crossed only through tricellular junctions, and a tracer with the size of low density lipoprotein was unable to cross by either route and instead traversed the cells themselves. Here we review previous work on the structure and function of tricellular junctions. We then describe a study in which we assessed the role of such junctions in the transport of an albumin-sized tracer.

METHODS

We examined normal endothelial monolayers, the effect of agonists that modify their permeability, and the influence of different patterns of shear stress.

RESULTS

Under normal conditions, approximately 85% of transendothelial transport occurred through tricellular junctions. This fraction was unchanged when permeability was reduced by sphingosine-1-phosphate or increased by thrombin, and also did not differ between endothelium exposed to multidirectional as opposed to uniaxial shear stress despite a > 50% difference in permeability.

CONCLUSION

These data show that tricellular junctions dominate normal transport of this tracer and largely determine influences of agonists and shear. The effects were attributable to changes in both the number and conductivity of the junctions. Further investigation of these structures will lead to increased understanding of endothelial barrier function and may suggest new therapeutic strategies in disease.

Tight Junction Modulating Bioprobes for Drug Delivery System to the Brain: A Review

The blood-brain barrier (BBB), which is composed of endothelial cells, pericytes, astrocytes, and neurons, separates the brain extracellular fluid from the circulating blood, and maintains the homeostasis of the central nervous system (CNS). The BBB endothelial cells have well-developed tight junctions (TJs) and express specific polarized transport systems to tightly control the paracellular movements of solutes, ions, and water. There are two types of TJs: bicellular TJs (bTJs), which is a structure at the contact of two cells, and tricellular TJs (tTJs), which is a structure at the contact of three cells. Claudin-5 and angulin-1 are important components of bTJs and tTJs in the brain, respectively. Here, we review TJ-modulating bioprobes that enable drug delivery to the brain across the BBB, focusing on claudin-5 and angulin-1.

Role of tight junctions in the epithelial-to-mesenchymal transition of cancer cells

The epithelial-mesenchymal transition (EMT) is an essential step in cancer progression. Epithelial cells possess several types of cell-cell junctions, and tight junctions are known to play important roles in maintaining the epithelial program. EMT is characterized by a loss of epithelial markers, including E-cadherin and tight junction proteins. Somewhat surprisingly, the evidence is accumulating that upregulated expression of tight junction proteins plays an important role in the EMT of cancer cells. Tight junctions have distinct tissue-specific and cancer-specific regulatory mechanisms, enabling them to play different roles in EMT. Tight junctions and related signaling pathways are attractive targets for cancer treatments; signal transduction inhibitors and monoclonal antibodies for tight junction proteins may be used to suppress EMT, invasion, and metastasis. Here we review the role of bicellular and tricellular tight junction proteins during EMT. Further investigation of regulatory mechanisms of tight junctions during EMT in cancer cells will inform the development of biomarkers for predicting prognosis as well as novel therapies.

Increase in Epithelial Permeability and Cell Metabolism by High Mobility Group Box 1, Inflammatory Cytokines and TPEN in Caco-2 Cells as a Novel Model of Inflammatory Bowel Disease

High mobility group box 1 protein (HMGB1) is involved in the pathogenesis of inflammatory bowel disease (IBD). Patients with IBD develop zinc deficiency. However, the detailed roles of HMGB1 and zinc deficiency in the intestinal epithelial barrier and cellular metabolism of IBD remain unknown. In the present study, Caco-2 cells in 2D culture and 2.5D Matrigel culture were pretreated with transforming growth factor-β (TGF-β) type 1 receptor kinase inhibitor EW-7197, epidermal growth factor receptor (EGFR) kinase inhibitor AG-1478 and a TNFα antibody before treatment with HMGB1 and inflammatory cytokines (TNFα and IFNγ). EW-7197, AG-1478 and the TNFα antibody prevented hyperpermeability induced by HMGB1 and inflammatory cytokines in 2.5D culture. HMGB1 affected cilia formation in 2.5D culture. EW-7197, AG-1478 and the TNFα antibody prevented the increase in cell metabolism induced by HMGB1 and inflammatory cytokines in 2D culture. Furthermore, ZnSO₄ prevented the hyperpermeability induced by zinc chelator TPEN in 2.5D culture. ZnSO₄ and TPEN induced cellular metabolism in 2D culture. The disruption of the epithelial barrier induced by HMGB1 and inflammatory cytokines contributed to TGF-β/EGF signaling in Caco-2 cells. The TNFα antibody and ZnSO₄ as well as EW-7197 and AG-1478 may have potential for use in therapy for IBD.

HMGB1 enhances epithelial permeability via p63/TGF-β signaling in lung and terminal bronchial epithelial cells

High mobility group box 1 (HMGB1) is involved in the induction of airway inflammation and injury in patients with chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). HMGB1 increased by transforming growth factor-β1 (TGF-β1), impairs airway epithelial barrier function in the lung. In the present study, to investigate how HMGB1 affects the barrier of normal human lung epithelial (HLE) cells, monolayer cells (2D culture) and bronchial-like spheroid cells (2.5 D Matrigel culture), which have lumen formation, were pretreated with TGF-β type I receptor kinase inhibitor EW-7197 before treatment with HMGB1. In 2D culture, treatment with HMGB1 decreased expression of angulin-1/LSR, TRIC and CLDN-1, -4, -7 and increased that of CLDN-2. Pretreatment with EW-7197 prevented the changes of all tight junction molecules induced by HMGB1. In 2.5D Matrigel culture, treatment with HMGB1 induced permeability of FITC-dextran (FD-4) into the lumen, whereas pretreatment with EW-7197 prevented the hyperpermeability of FD-4 into the lumen caused by HMGB1. In 2.5D Matrigel culture, knockdown of transcription factor p63 prevented the hyperpermeability induced by HMGB1 as well as pretreatment with EW-7197. In the 2D culture of HLE cells with HMGB1, knockdown of p63 increased the level of angulin-1/LSR and CLDN-4, while pretreatment with EW-7197 enhanced the increase of CLDN-4 induced by knockdown of p63. Immunohistochemical analysis of IPF, CLDN-2, HMGB1 and p63 revealed that their levels were higher in the regenerative epithelium of the terminal bronchial region than in normal epithelium. HMGB1 induces epithelial permeability of HLE cells via p63/TGF-β signaling in normal lung and IPF.

HMGB1-downregulated angulin-1/LSR induces epithelial barrier disruption via claudin-2 and cellular metabolism via AMPK in airway epithelial Calu-3 cells

A non-histone chromatin-associated protein, high mobility group box 1 (HMGB1), which impairs the airway epithelial barrier, is involved in the induction of airway inflammation in patients with allergy, asthma, chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF). Tricellular tight junctions (tTJs) form at the convergence of bicellular tight junctions (bTJs). Angulin-1/lipolysis-stimulated lipoprotein receptor (LSR) is a novel molecule present at tricellular contacts and contributes to the epithelial barrier and cellular metabolism. Adenosine monophosphate-activated protein kinase (AMPK) is a central metabolic regulator and has a reciprocal association with TJs. In the present study, to examine how HMGB1 contributes to airway epithelial barrier disruption and the cellular metabolism indicated as mitochondrial respiration, bronchial epithelial Calu-3 cells were transfected with siRNAs of angulin-1/LSR or treated with HMGB1 and the relationship between HMGB1 and angulin-1/LSR was investigated. Knockdown of angulin-1/LSR upregulated the expression of the tight junction molecule claudin-2, AMPK activity, and mitochondrial respiration, and downregulated the epithelial barrier. Treatment with HMGB1 downregulated angulin-1/LSR expression and the epithelial barrier, and upregulated claudin-2 expression, AMPK activity and mitochondrial respiration. Treatment with EW-7197, a transforming growth factor-β (TGF-β) type I receptor kinase inhibitor, prevented all the effects of HMGB1 in Calu-3 cells. HMGB1-downregulated angulin-1/LSR induced epithelial barrier disruption via claudin-2 and cellular metabolism via AMPK in airway epithelial Calu-3 cells. The effects of HMGB1 contribute to TGF-β signaling and EW-7197 shows potential for use in therapy for HMGB1-induced airway inflammation.

Post-translational modifications of tight junction transmembrane proteins and their direct effect on barrier function

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination or glycosylation are processes affecting the conformation, stability, localization and function of proteins. There is clear evidence that PTMs can act upon tight junction (TJ) proteins, thus modulating epithelial barrier function. Compared to transcriptional or translational regulation, PTMs are rapid and more dynamic processes so in the context of barrier maintenance they might be essential for coping with changing environmental or external impacts. The aim of this review is to extract literature deciphering PTMs in TJ proteins directly contributing to epithelial barrier changes in permeability to ions and macromolecules. It is not intended to cover the entire scope of PTMs in TJ proteins and should rather be understood as a digest of TJ protein modifications directly resulting in the tightening or opening of the epithelial barrier.