Claudins in cancer: bench to bedside

Claudins: from gatekeepers of epithelial integrity to potential targets in hepato-pancreato-biliary cancers

Claudins, a family of tetraspan transmembrane proteins, are critical to the integrity of tight junctions in epithelia and endothelia, influencing cellular processes such as development, differentiation, and apoptosis. Abnormal claudin expression is associated with various malignancies, particularly affecting tissue architecture and potentially facilitating tumor invasion and metastasis. In this comprehensive review, we explore the multifaceted functions of claudins: their expression, specific roles in cancer with a focus on hepato-pancreato-biliary malignancies and highlight their potential as therapeutic targets. We discuss current claudin-targeted therapies, including monoclonal antibodies, antibody-drug conjugates, bispecific T-cell engager and chimeric antigen receptor T-cell therapies. These approaches show promise in pre-clinical and clinical studies, particularly in hepato-pancreato-biliary cancers with large unmet needs. Despite these early signs of efficacy, challenges remain in effectively targeting these proteins due to their structural resemblance and overlapping functions.

Expression and Targeted Application of Claudins Family in Hepatobiliary and Pancreatic Diseases

Hepatobiliary and pancreatic diseases are becoming increasingly common worldwide and associated cancers are prone to recurrence and metastasis. For a more accurate treatment, new therapeutic strategies are urgently needed. The claudins (CLDN) family comprises a class of membrane proteins that are the main components of tight junctions, and are essential for forming intercellular barriers and maintaining cellular polarity. In mammals, the claudin family contains at least 27 transmembrane proteins and plays a major role in mediating cell adhesion and paracellular permeability. Multiple claudin proteins are altered in various cancers, including gastric cancer (GC), esophageal cancer (EC), hepatocellular carcinoma (HCC), pancreatic cancer (PC), colorectal cancer (CRC) and breast cancer (BC). An increasing number of studies have shown that claudins are closely associated with the occurrence and development of hepatobiliary and pancreatic diseases. Interestingly, claudin proteins exhibit different effects on cancer progression in different tumor tissues, including tumor suppression and promotion. In addition, various claudin proteins are currently being studied as potential diagnostic and therapeutic targets, including claudin-3, claudin-4, claudin-18.2, etc. In this article, the functional phenotype, molecular mechanism, and targeted application of the claudin family in hepatobiliary and pancreatic diseases are reviewed, with an emphasis on claudin-1, claudin-4, claudin-7 and claudin-18.2, and the current situation and future prospects are proposed.

CLDN6 inhibits breast cancer growth and metastasis through SREBP1-mediated RAS palmitoylation

BACKGROUND

Breast cancer (BC) ranks as the third most fatal malignant tumor worldwide, with a strong reliance on fatty acid metabolism. CLDN6, a candidate BC suppressor gene, was previously identified as a regulator of fatty acid biosynthesis; however, the underlying mechanism remains elusive. In this research, we aim to clarify the specific mechanism through which CLDN6 modulates fatty acid anabolism and its impact on BC growth and metastasis.

METHODS

Cell function assays, tumor xenograft mouse models, and lung metastasis mouse models were conducted to evaluate BC growth and metastasis. Human palmitic acid assay, triglyceride assay, Nile red staining, and oil red O staining were employed to investigate fatty acid anabolism. Reverse transcription polymerase chain reaction (RT-PCR), western blot, immunohistochemistry (IHC) assay, nuclear fractionation, immunofluorescence (IF), immunoprecipitation and acyl-biotin exchange (IP-ABE), chromatin immunoprecipitation (ChIP), dual luciferase reporter assay, and co-immunoprecipitation (Co-IP) were applied to elucidate the underlying molecular mechanism. Moreover, tissue microarrays of BC were analyzed to explore the clinical implications.

RESULTS

We identified that CLDN6 inhibited BC growth and metastasis by impeding RAS palmitoylation both in vitro and in vivo. We proposed a unique theory suggesting that CLDN6 suppressed RAS palmitoylation through SREBP1-modulated de novo palmitic acid synthesis. Mechanistically, CLDN6 interacted with MAGI2 to prevent KLF5 from entering the nucleus, thereby restraining SREBF1 transcription. The downregulation of SREBP1 reduced de novo palmitic acid synthesis, hindering RAS palmitoylation and subsequent endosomal sorting complex required for transport (ESCRT)-mediated plasma membrane localization required for RAS oncogenic activation. Besides, targeting inhibition of RAS palmitoylation synergized with CLDN6 to repress BC progression.

CONCLUSIONS

Our findings provide compelling evidence that CLDN6 suppresses the palmitic acid-induced RAS palmitoylation through the MAGI2/KLF5/SREBP1 axis, thereby impeding BC malignant progression. These results propose a new insight that monitoring CLDN6 expression alongside targeting inhibition of palmitic acid-mediated palmitoylation could be a viable strategy for treating oncogenic RAS-driven BC.

Claudin-12: guardian of the tissue barrier or friend of tumor cells

Tight junctions (TJs) are an important component of cellular connectivity. Claudin family proteins, as a constituent of TJs, determine their barrier properties, cell polarity and paracellular permeability. Claudin-12 is an atypical member of the claudin family, as it belongs to the group of non-classical claudins that lack a PDZ-binding domain. It has been shown that claudin-12 is involved in paracellular Ca2+ transients and it is present in normal and hyperplastic tissues in addition to neoplastic tissues. Dysregulation of claudin-12 expression has been reported in various cancers, suggesting that this protein may play an important role in cancer cell migration, invasion, and metastasis. Some studies have shown that claudin-12 gene functions as a tumor suppressor, but others have reported that overexpression of claudin-12 significantly increases the metastatic properties of various tumor cells. Investigating this dual role of claudin-12 is of utmost importance and should therefore be studied in detail. The aim of this review is to provide an overview of the information available to date on claudin-12, including its structure, expression in various tissues and substances that may affect it, with a final focus on its role in cancer.

Comprehensive landscape of junctional genes and their association with overall survival of patients with lung adenocarcinoma

Objectives

Junctional proteins are involved in tumorigenesis. Therefore, this study aimed to investigate the association between junctional genes and the prognosis of patients with lung adenocarcinoma (LUAD).

Methods

Transcriptome, mutation, and clinical data were retrieved from The Cancer Genome Atlas (TCGA). "Limma" was used to screen differentially expressed genes. Moreover, Kaplan-Meier survival analysis was used to identify junctional genes associated with LUAD prognosis. The junctional gene-related risk score (JGRS) was generated based on multivariate Cox regression analysis. An overall survival (OS) prediction model combining the JGRS and clinicopathological properties was proposed using a nomogram and further validated in the Gene Expression Omnibus (GEO) LUAD cohort.

Results

To our knowledge, this study is the first to demonstrate the correlation between the mRNA levels of 14 junctional genes (CDH15, CDH17, CDH24, CLDN6, CLDN12, CLDN18, CTNND2, DSG2, ITGA2, ITGA8, ITGA11, ITGAL, ITGB4, and PKP3) and clinical outcomes of patients with LUAD. The JGRS was generated based on these 14 genes, and a higher JGRS was associated with older age, higher stage levels, and lower immune scores. Thus, a prognostic prediction nomogram was proposed based on the JGRS. Internal and external validation showed the good performance of the prediction model. Mechanistically, JGRS was associated with cell proliferation and immune regulatory pathways. Mutational analysis revealed that more somatic mutations occurred in the high-JGRS group than in the low-JGRS group.

Conclusion

The association between junctional genes and OS in patients with LUAD demonstrated by our "TCGA filtrating and GEO validating" model revealed a new function of junctional genes.

Surface Proteome of Extracellular Vesicles and Correlation Analysis Reveal Breast Cancer Biomarkers

Breast cancer (BC) is the second most frequently diagnosed cancer and accounts for approximately 25% of new cancer cases in Canadian women. Using biomarkers as a less-invasive BC diagnostic method is currently under investigation but is not ready for practical application in clinical settings. During the last decade, extracellular vesicles (EVs) have emerged as a promising source of biomarkers because they contain cancer-derived proteins, RNAs, and metabolites. In this study, EV proteins from small EVs (sEVs) and medium EVs (mEVs) were isolated from BC MDA-MB-231 and MCF7 and non-cancerous breast epithelial MCF10A cell lines and then analyzed by two approaches: global proteomic analysis and enrichment of EV surface proteins by Sulfo-NHS-SS-Biotin labeling. From the first approach, proteomic profiling identified 2459 proteins, which were subjected to comparative analysis and correlation network analysis. Twelve potential biomarker proteins were identified based on cell line-specific expression and filtered by their predicted co-localization with known EV marker proteins, CD63, CD9, and CD81. This approach resulted in the identification of 11 proteins, four of which were further investigated by Western blot analysis. The presence of transmembrane serine protease matriptase (ST14), claudin-3 (CLDN3), and integrin alpha-7 (ITGA7) in each cell line was validated by Western blot, revealing that ST14 and CLDN3 may be further explored as potential EV biomarkers for BC. The surface labeling approach enriched proteins that were not identified using the first approach. Ten potential BC biomarkers (Glutathione S-transferase P1 (GSTP1), Elongation factor 2 (EEF2), DEAD/H box RNA helicase (DDX10), progesterone receptor (PGR), Ras-related C3 botulinum toxin substrate 2 (RAC2), Disintegrin and metalloproteinase domain-containing protein 10 (ADAM10), Aconitase 2 (ACO2), UTP20 small subunit processome component (UTP20), NEDD4 binding protein 2 (N4BP2), Programmed cell death 6 (PDCD6)) were selected from surface proteins commonly identified from MDA-MB-231 and MCF7, but not identified in MCF10A EVs. In total, 846 surface proteins were identified from the second approach, of which 11 were already known as BC markers. This study supports the proposition that Evs are a rich source of known and novel biomarkers that may be used for non-invasive detection of BC. Furthermore, the presented datasets could be further explored for the identification of potential biomarkers in BC.

TMEM25 is a Par3-binding protein that attenuates claudin assembly during tight junction development

The tight junction (TJ) in epithelial cells is formed by integral membrane proteins and cytoplasmic scaffolding proteins. The former contains the claudin family proteins with four transmembrane segments, while the latter includes Par3, a PDZ domain-containing adaptor that organizes TJ formation. Here we show the single membrane-spanning protein TMEM25 localizes to TJs in epithelial cells and binds to Par3 via a PDZ-mediated interaction with its C-terminal cytoplasmic tail. TJ development during epithelial cell polarization is accelerated by depletion of TMEM25, and delayed by overexpression of TMEM25 but not by that of a C-terminally deleted protein, indicating a regulatory role of TMEM25. TMEM25 associates via its N-terminal extracellular domain with claudin-1 and claudin-2 to suppress their cis- and trans-oligomerizations, both of which participate in TJ strand formation. Furthermore, Par3 attenuates TMEM25-claudin association via binding to TMEM25, implying its ability to affect claudin oligomerization. Thus, the TJ protein TMEM25 appears to negatively regulate claudin assembly in TJ formation, which regulation is modulated by its interaction with Par3.

"Hurdles race for CAR T-cell therapy in digestive tract cancer"

Digestive tract cancers (DTC) belong to the most investigated family of tumors. The incidence, prevalence, and mortality rate of DTC remain high, especially for patients with pancreatic cancer. Even though immunotherapy such as immune checkpoint inhibitors (ICI) have revolutionized the treatment of solid cancer types, ICI are still restricted to a very small group of patients and seem to be more efficacious in combination with chemotherapy. Cellular immunotherapy such as CAR T-cell therapy has entered clinical routine in hematological malignancies with outstanding results. There is growing interest on translating this kind of immunotherapy and success into patients with solid malignancies, such as DTC. This review attempts to describe the major advances in preclinical and clinical research with CAR T cells in DTC, considering the most relevant hurdles in each subtype of DTC.

Claudin expression in pulmonary adenoid cystic carcinoma and mucoepidermoid carcinoma

Background: Although the expression of tight junction protein claudins (CLDNs) is well known in common histological subtypes of lung cancer, it has not been investigated in rare lung cancers. The aim of our study was to examine the expression of different CLDNs in pulmonary salivary gland tumors. Methods: 35 rare lung cancers including pathologically confirmed 12 adenoid cystic carcinomas (ACCs) and 23 mucoepidermoid carcinomas (MECs) were collected retrospectively. Immunohistochemical (IHC) staining was performed on formalin fixed paraffin embedded (FFPE) tumor tissues, and CLDN1, -2, -3, -4, -5, -7, and -18 protein expressions were analyzed. The levels of immunopositivity were determined with H-score. Certain pathological characteristics of ACC and MEC samples (tumor grade, presence of necrosis, presence of blood vessel infiltration, and degree of lymphoid infiltration) were also analyzed. Results: CLDN overexpression was observed in both tumor types, especially in CLDN2, -7, and -18 IHC. Markedly different patterns of CLDN expression were found for ACC and MEC tumors, especially for CLDN1, -2, -4, and -7, although none of these trends remained significant after correction for multiple testing. Positive correlations between expressions of CLDN2 and -5, CLDN3 and -4, and CLDN5 and -18 were also demonstrated. Tumors of never-smokers presented lower levels of CLDN18 than tumors of current smokers (p-value: 0.003). Conclusion: This is the first study to comprehensively describe the expression of different CLDNs in lung ACC and MEC. Overexpression of certain CLDNs may pave the way for targeted anti-claudin therapy in these rare histological subtypes of lung cancer.

Emerging roles of transmembrane-type tight junction proteins in cancers

Tight junctions (TJs) are the most apical components of the cell-cell adhesion machinery in epithelial and endothelial cells and they play essential roles in homeostasis. Recent studies have revealed that aberrant expression of tight junction proteins (TJPs) is frequently observed in various type of cancers. Here we review cancer-associated aberrant expression of TJPs with focus on transmembrane-type TJPs including claudins, junctional adhesion molecule-A (JAM-A), and occludin. Some transmembrane-type TJPs are upregulated at the early neoplastic stage and their expression persists during dedifferentiation. Aberrant expression of TJPs contributes to proliferation, invasion, and dysregulated signaling of cancer cells. In addition to an increase in their expression level, their localization is altered from a TJ-restricted pattern to distribution throughout the whole cell membrane, making them suitable as therapeutic targets. Extracellular domains of transmembrane-type TJPs can be approached by target drugs not only from the lumen side (apical side) but also from the extracellular matrix side (basal side), including blood vessels. Aberrantly expressed TJPs are potential useful diagnostic markers as well as therapeutic targets for cancers.

Claudin-4: A New Molecular Target for Epithelial Cancer Therapy

Claudin-4 (CLDN4) is a key component of tight junctions (TJs) in epithelial cells. CLDN4 is overexpressed in many epithelial malignancies and correlates with cancer progression. Changes in CLDN4 expression have been associated with epigenetic factors (such as hypomethylation of promoter DNA), inflammation associated with infection and cytokines, and growth factor signaling. CLDN4 helps to maintain the tumor microenvironment by forming TJs and acts as a barrier to the entry of anticancer drugs into tumors. Decreased expression of CLDN4 is a potential marker of epithelial-mesenchymal transition (EMT), and decreased epithelial differentiation due to reduced CLDN4 activity is involved in EMT induction. Non-TJ CLDN4 also activates integrin beta 1 and YAP to promote proliferation, EMT, and stemness. These roles in cancer have led to investigations of molecular therapies targeting CLDN4 using anti-CLDN4 extracellular domain antibodies, gene knockdown, clostridium perfringens enterotoxin (CPE), and C-terminus domain of CPE (C-CPE), which have demonstrated the experimental efficacy of this approach. CLDN4 is strongly involved in promoting malignant phenotypes in many epithelial cancers and is regarded as a promising molecular therapeutic target.

Overexpression of CLDN16 in ovarian cancer is modulated by PI3K and PKC pathways

Epithelial ovarian cancer (EOC) is the gynecological malignant tumor of poorest prognosis and higher mortality rate. Chemotherapy is the base of high-grade serous ovarian cancer (HGSOC) treatment; however, it favors the emergence of chemoresistance and metastasis. Thus, there is an urge to search for new therapeutic targets, such as proteins related to cellular proliferation and invasion. Herein, we investigated the expression profile of claudin-16 (CLDN16 protein and CLDN16 transcript) and its possible functions in EOC. In silico analysis of CLDN16 expression profile was performed using data extracted from GENT2 and GEPIA2 platforms. A retrospective study was carried out with 55 patients to evaluate the expression of CLDN16. The samples were evaluated by immunohistochemistry, immunofluorescence, qRT-PCR, molecular docking, sequencing, and immunoblotting assays. Statistical analyzes were performed using Kaplan-Meier curves, one-way ANOVA, Turkey posttest. Data were analyzed using GraphPad Prism 8.0. In silico experiments showed that CLDN16 is overexpressed in EOC. 80.0% of all EOC types overexpressed CLDN16, of which in 87% of the cases the protein is restricted to cellular cytoplasm. CLDN16 expression was not related to tumor stage, tumor cells differentiation status, tumor responsiveness to cisplatin, or patients' survival rate. When compared to data obtained from in silico analysis regarding EOC stage and degree of differentiation, differences were found in the former but not in the later, neither in survival curves. CLDN16 expression in HGSOC OVCAR-3 cells increased by 1.95-fold (p < 0.001), 2.32-fold (p < 0.001), and 6.57-fold (p < 0.001) via PKC, PI3K, and estrogen pathways, respectively. Altogether, our results suggest that despite the low number of samples included in our in vitro studies, adding to the expression profile findings, we provided a comprehensive study of CLDN16 expression in EOC. Therefore, we hypothesize that CLDN16 is a potential target in the diagnosis and treatment of the disease.

Tight junction gene expression in salivary gland tumors

Salivary gland neoplasms comprise a heterogeneous group of lesions with multiple histological subtypes, each with distinct growth patterns, resulting in a spectrum of tumor-specific prognoses; pleomorphic adenoma (PA) and mucoepidermoid carcinoma (MEC) are the most common representatives of these neoplasms. Many studies have associated specific profiles of membrane and adhesion molecules in salivary gland tissues; these profiles appear to be relevant in tumor biology as well as be interpreted as fingerprints for tumor classification, diagnostic prognostic and therapeutic targets. One of these membrane molecule complexes are the tight junctions, composed by various proteins, in which claudins are protagonists. The aim of this study was to evaluate the expressions of genes that encode tight junction proteins (CLDN-1, -3, -4, -5, -7, and -11, occludin [OCLN], zonula occludens [TJP1, TJP2, and TJP3] and junctional adhesion molecule A [F11R]) in MEC and PA using real time RT-PCR. We observed high expression of CLDN-1 and -7 and low expression of CLDN-3, -11 and TJP2 in MEC compared to PA. PA samples demonstrated high OCLN expression when compared to MEC. CRTC1::MAML2 fusion was detected in 12 of 20 (60.0%) MEC samples and was associated with CLDN7 expression, while the absence of fusion was associated with high histological grade. Increased CLDN5 expression was associated with submandibular gland tumors. This study demonstrated differential expressions of genes encoding tight junction constituent proteins and their associations with tumor characteristics, suggesting their potential future role as diagnostic and prognostic markers.

Gene Expression Analysis of Biphasic Pleural Mesothelioma: New Potential Diagnostic and Prognostic Markers

Biphasic is the second most common histotype of pleural mesothelioma (PM). It shares epithelioid and sarcomatoid features and is challenging to diagnose. The aim of this study was to identify biphasic PM markers to improve subtyping and prognosis definition. The expression levels of 117 cancer genes, evaluated using the nanoString system, were compared between the three major histotypes (epithelioid, sarcomatoid, and biphasic), and expression differences within biphasic PM were evaluated in relation to the percentage of epithelioid components. Biphasic PM overexpressed CTNNA1 and TIMP3 in comparison to sarcomatoid, and COL16A1 and SDC1 in comparison to epithelioid PM. CFB, MSLN, CLDN15, SERPINE1, and PAK4 were deregulated among all histotypes, leading to the hypothesis of a gradual expression from epithelioid to sarcomatoid PM. According to gene expression, biphasic PM samples were divided in two clusters with a significant difference in the epithelioid component. ADCY4, COL1A1, and COL4A2 were overexpressed in the biphasic group with a low percentage of epithelioid component. Survival analysis using TCGA data showed that high COL1A1 and COL4A2 expression levels correlate with poor survival in PM patients. Herein, we identified markers with the potential to improve diagnosis and prognostic stratification of biphasic PM, which is still an orphan tumor.

Aberrant expression of claudin-6 contributes to malignant potentials and drug resistance of cervical adenocarcinoma

Recent studies have revealed that aberrant expression of tight junction (TJ) proteins is a hallmark of various solid tumors and it is recognized as a useful therapeutic target. Claudin‐6 (CLDN6), a member of the family of TJ transmembrane proteins, is an ideal therapeutic target because it is not expressed in human adult normal tissues. In this study, we found that CLDN6 is highly expressed in uterine cervical adenocarcinoma (ADC) and that high CLDN6 expression was correlated with lymph node metastasis and lymphovascular infiltration and was an independent prognostic factor. Shotgun proteome analysis revealed that cell‐cell adhesion‐related proteins and drug metabolism‐associated proteins (aldo‐keto reductase [AKR] family proteins) were significantly increased in CLDN6‐overexpressing cells. Furthermore, overexpression of CLDN6 enhanced cell‐cell adhesion properties and attenuated sensitivity to anticancer drugs including doxorubicin, daunorubicin, and cisplatin. Taken together, the results indicate that aberrant expression of CLDN6 enhances malignant potentials and drug resistance of cervical ADC, possibly due to increased cell‐cell adhesion properties and drug metabolism. Our findings provide an insight into a new therapeutic strategy, a CLDN6‐targeting therapy, against cervical ADC.

Intestinal Claudin-7 deficiency impacts the intestinal microbiota in mice with colitis

Background

Intestinal epithelial cells form a physical barrier that protects the intestine against the intestinal microbiota through tight junctions (TJs) and adhesive junctions, while barrier disruption may lead to inflammatory bowel disease (IBD). Claudin-7 (Cldn7) has been implicated in this protection as an important member of TJs. Here, we experimentally study the effect of Cldn7 deletion on intestinal microbiota in colitis.

Methods

Colitis model was established based on inducible intestinal conditional Cldn7 gene knockout mice (Cldn7fl/fl; villin-CreERT2), by feeding with dextran sodium sulfate (DSS). AB-PAS staining and immunohistochemical staining of Muc2 mucin were used to detect the effect of Cldn7 deficiency on the mucus layer of mice with colitis, and fluorescence in situ hybridization was used to detect how Cldn7 promotes spatial separation of the gut microbiota from the host. The microbiota population was characterized by high-throughput 16S rRNA gene sequencing of DNA extracted from fecal samples.

Results

Compared with the controls, Cldn7 knockout increased susceptibility to colitis, including greater degree of weight loss, colon shortening, and a significantly higher disease activity index score. DSS-treated Cldn7 knockout mice promoted the migration of bacteria to the intestinal epithelium to some extent by damaging the intestinal mucus layer. Sequencing of 16S rRNA showed that DSS-treated Cldn7 knockout mice reduced the gut microbiota diversity and had greater relative abundance of Escherichia coli. LEfSe analysis indicated that Escherichia coli may be the key bacteria in Cldn7 knockout mice during DSS-induced colitis. Furthermore, the Tax4Fun analysis predicted that DSS-treated Cldn7 knockout mice enriched for microbiota impacting infectious diseases, immune system and metabolic functions.

Conclusions

Our data suggests an association between intestinal Cldn7 knockout and microbiota dysbiosis during inflammatory events.

Transcriptional and Epigenetic Bioinformatic Analysis of Claudin-9 Regulation in Gastric Cancer

Gastric cancer is a heterogeneous disease that represents 5% to 10% of all new cancer cases worldwide. Advances in histological diagnosis and the discovery of new genes have admitted new genomic classifications. Nevertheless, the bioinformatic analysis of gastric cancer databases has favored the detection of specific differentially expressed genes with biological significance. Claudins, a family of proteins involved in tight junction physiology, have emerged as the key regulators of cellular processes, such as growth, proliferation, and migration, associated with cancer progression. The expression of Claudin-9 in the gastric cancer tissue has been linked to poor prognosis, however, its transcriptional and epigenetic regulations demand a more comprehensive analysis. Using the neural network promoter prediction, TransFact, Uniprot-KB, Expasy-SOPMA, protein data bank, proteomics DB, Interpro, BioGRID, String, and the FASTA protein sequence databases and software, we found the following: (1) the promoter sequence has an unconventional structure, including different transcriptional regulation elements distributed throughout it, (2) GATA 4, GATA 6, and KLF5 are the key regulators of Claudin-9 expression, (3) Oct1, NF-κB, AP-1, c-Ets-1, and HNF-3β have the higher binding affinity to the CLDN9 promoter, (4) Claudin-9 interacts with cell differentiation and development proteins, (5) CLDN9 is highly methylated, and (6) Claudin-9 expression is associated with poor survival. In conclusion, Claudin-9 is a protein that should be considered a diagnostic marker as its gene promoter region binds to the transcription factors associated with the deregulation of cell control, enhanced cell proliferation, and metastasis.

The Role of CAR-T Cells in Peritoneal Carcinomatosis from Gastric Cancer: Rationale, Experimental Work, and Clinical Applications

Cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) have shown poor effectiveness in treating peritoneal carcinomatosis (PC) of gastric origin with a high tumor burden (high peritoneal cancer index), though there are scarce therapy alternatives that are able to improve survival. In experimental studies, chimeric antigen receptor-T (CAR-T) cell therapy has shown encouraging results in gastric cancer and is currently being evaluated in several clinical trials. Regarding PC, CAR-T cell therapy has also proven useful in experimental studies, especially when administered intraperitoneally, as this route improves cell distribution and lifespan. Although these results need to be supported by ongoing clinical trials, CAR-T cells are a promising new therapeutic approach to peritoneal metastases from gastric cancer. In this review, we summarize the current evidence of the use of CAR-T cells in gastric cancer and PC of gastric origin.

Identification of Claudin-6 as a Molecular Biomarker in Pan-Cancer Through Multiple Omics Integrative Analysis

Claudin-6 (CLDN6) is one of the 27 family members of claudins and majorly involved in the tight junction and cell-to-cell adhesion of epithelial cell sheets, playing a significant role in cancer initiation and progression. To provide a more systematic and comprehensive dimension of identifying the diverse significance of CLDN6 in a variety of malignant tumors, we explored CLDN6 through multiple omics data integrative analysis, including gene expression level in pan-cancer and comparison of CLDN6 expression in different molecular subtypes and immune subtypes of pan-cancer, targeted protein, biological functions, molecular signatures, diagnostic value, and prognostic value in pan-cancer. Furthermore, we focused on uterine corpus endometrial carcinoma (UCEC) and further investigated CLDN6 from the perspective of the correlations with clinical characteristics, prognosis in different clinical subgroups, co-expression genes, and differentially expressed genes (DEGs), basing on discussing the validation of its established monoclonal antibody by immunohistochemical staining and semi-quantification reported in the previous study. As a result, CLDN6 expression differs significantly not only in most cancers but also in different molecular and immune subtypes of cancers. Besides, high accuracy in predicting cancers and notable correlations with prognosis of certain cancers suggest that CLDN6 might be a potential diagnostic and prognostic biomarker of cancers. Additionally, CLDN6 is identified to be significantly correlated with age, stage, weight, histological type, histologic grade, and menopause status in UCEC. Moreover, CLDN6 high expression can lead to a worse overall survival (OS), disease-specific survival (DSS), and progression-free interval (PFI) in UCEC, especially in different clinical subgroups of UCEC. Taken together, CLDN6 may be a remarkable molecular biomarker for diagnosis and prognosis in pan-cancer and an independent prognostic risk factor of UCEC, presenting to be a promising molecular target for cancer therapy.

Claudin 6: Therapeutic prospects for tumours, and mechanisms of expression and regulation (Review)

Tight junctions (TJs) are an important component of cell connectivity; they maintain cell polarity, permeability and adhesion, and participate in the regulation of cell proliferation and differentiation. The claudin (CLDN) family is integral to TJs, and CLDN6 is an important member of this family. Abnormal expression of CLDN6 can destroy the integrity of TJs through various mechanisms and can serve multiple roles in the occurrence and development of tumours. CLDN6 is widely expressed in various tumours but rarely expressed in healthy adult tissues. The aim of this review is to critically examine the recent literature on CLDN6, including its structure, expression in different tumours, regulatory mechanisms and therapeutic prospects. Although some conclusions are controversial, in certain tumours, such as liver, ovarian, endometrial and oesophageal cancer, and atypical teratoid/rhabdoid tumours, research consistently shows that CLDN6 is expressed in tumour tissues but is not expressed or is expressed at low levels in surrounding tissues. In these tumours, CLDN6 has potential as a carcinoembryonic antigen and a therapeutic target.

Claudins as biomarkers of differential diagnosis and prognosis of tumors

Claudins are a superfamily of transmembrane proteins, the optimal expression and localization of which are important for the normal physiological function of the epithelium and any imbalance may have pathological consequences. Not only insufficient but also excessive production of claudins in cancer cells, as well as their aberrant localization, equally manifest the formation of a malignant phenotype. Many works are distinguished by contradictory data, which demonstrate the action of the same claudins both in the role of tumor-growth suppressors and promoters in the same cancers. The most important possible causes of significant discrepancies in the results of the works are a considerable variability of sampling and the absence of a consistent approach both to the assessment of the immune reactivity of claudins and to the differential analysis of their subcellular localization. Combined, these drawbacks hinder the histological assessment of the link between claudins and tumor progression. In particular, ambiguous expression of claudins in breast cancer subtypes, revealed by various authors in immunohistochemical analysis, not only fails to facilitate the identification of the claudin-low molecular subtype but rather complicates these efforts. Research into the role of claudins in carcinogenesis has undoubtedly confirmed the potential value of this class of proteins as significant biomarkers in some cancer types; however, the immunohistochemical approach to the assessment of claudins still has limitations, needs standardization, and, to date, has not reached a diagnostic or a prognostic value.

Aberrant Claudin-6-Adhesion Signaling Promotes Endometrial Cancer Progression via Estrogen Receptor α

Cell adhesion proteins not only maintain tissue integrity, but also possess signaling abilities to organize diverse cellular events in a variety of physiologic and pathologic processes; however, the underlying mechanism remains obscure. Among cell adhesion molecules, the claudin (CLDN) family is often aberrantly expressed in various cancers, but the biological relevance and molecular basis for this observation have not yet been established. Here, we show that high CLDN6 expression accelerates cellular proliferation and migration in two distinct human endometrial cancer cell lines in vitro. Using a xenograft model, we also revealed that aberrant CLDN6 expression promotes tumor growth and invasion in endometrial cancer tissues. The second extracellular domain and Y196/200 of CLDN6 were required to recruit and activate Src-family kinases (SFK) and to stimulate malignant phenotypes. Knockout and overexpression of ESR1 in endometrial carcinoma cells showed that the CLDN6-adhesion signal links to estrogen receptor α (ERα) to advance tumor progression. In particular, aberrant CLDN6-ERα signaling contributed to collective cell behaviors in the leading front of endometrial cancer cells. Importantly, we demonstrate that CLDN6/SFK/PI3K-dependent AKT and SGK (serum- and glucocorticoid-regulated kinase) signaling in endometrial cancer cells targets Ser518 in the human ERα to activate ERα transcriptional activity in a ligand-independent manner, thereby promoting tumor progression. Furthermore, CLDN6, at least in part, also regulated gene expression in an ERα-independent manner. IMPLICATIONS: The identification of this machinery highlights regulation of the transcription factors by cell adhesion to advance tumor progression.

CLDN15 is a novel diagnostic marker for malignant pleural mesothelioma

Malignant mesothelioma is a cancer with a poor survival rate. It is difficult to diagnose mesotheliomas because they show a variety of histological patterns similar to those of various other cancers. However, since currently used positive markers for mesotheliomas may show false positives or false negatives, a novel mesothelial positive marker is required. In the present study, we screened 25 claudins and found that claudin-15 is expressed in the mesothelial cells. We made new rat anti-human claudin-15 (CLDN15) monoclonal antibodies that selectively recognize CLDN15, and investigated whether CLDN15 is a good positive marker for malignant pleural mesotheliomas (MPMs) using MPM tissue samples by immunohistochemistry and semi-quantification of the expression level using an immunoreactive score (IRS) method. Of 42 MPM samples, 83% were positive for CLDN15. The positive ratio was equal to or greater than other positive markers for MPMs including calretinin (81%), WT-1 (50%), and D2-40 (81%). In 50 lung adenocarcinoma sections, four cases were positive for CLDN15 and the specificity (92%) was comparable with other markers (90–100%). Notably, CLDN15 was rarely detected in 24 non-mesothelial tumors in the tissue microarray (12/327 cases). In conclusion, CLDN15 can be used in the clinical setting as a positive marker for MPM diagnosis.

The claudin-transcription factor signaling pathway

Claudins (CLDNs) represent major transmembrane proteins of tight junctions and contribute to the barrier function. They also serve as anchors for several signaling proteins, but the underlying molecular basis has yet to be established. The present review covers the recent progress in our understanding of the CLDN signaling pathway in health and disease. We discuss the functional relevance of phosphotyrosine motifs in the C-terminal cytoplasmic domain of CLDNs and define mutual regulation between CLDNs and Src-family kinases (SFKs). In addition, we focus on the crosstalk between CLDN and transcription factor signaling. We also describe how aberrant CLDN–transcription factor signaling promotes or inhibits cancer progression. We propose that a link between various cell adhesion molecules and transcription factors coordinates a range of physiological and pathological events via activation or suppression of target genes.

Tight Junction Protein Claudin-12 Is Involved in Cell Migration during Metastasis

Claudins are important components of the tight junctions determining barrier properties, cell polarity, and paracellular permeability. Although many functions of claudins in cancer cells have not been elucidated, recent studies have shown that claudins play an important role in cell migration and metastasis. Loss of epithelial/endothelial integrity, disruption of tight junctions, and increased paracellular leakage are often observed during metastasis. The aim of our study was to investigate the involvement of claudin-12 in the process of cell migration as well as to evaluate the possibility of using this protein as a specific target for the regulation of tumorigenesis. We have performed immunocytochemistry assays to detect the expression of claudin-12 in different epithelial/endothelial human cell lines, and selected three (A549, LS180, and HeLa) for further experiments. Using transwell chamber migration assays, we found that anti-claudin-12 antibodies inhibited both the migration and proliferation of claudin-12 expressing cells (A549 and LS180), inducing apoptosis, as well as the migration capacity of Jurkat cells through the monolayers formed from A549 or LS180 cells. In addition, co-cultures of Jurkat cells on monolayers from A549 or LS180 cells, in the presence of synthetic claudin-12 peptides representing the extracellular domains of the claudin-12 protein, also reduced the number of migrated Jurkat cells. Two of the tested peptides (p5 and p6) almost completely blocked the migration of Jurkat cells. All migrated Jurkat cells expressed LFA-1 and CD62L, but not CD44. Thus, claudin-12 is a suitable biomarker for tumor progression and metastasis and an attractive target for antitumor therapy. Anti-claudin-12 antibodies and competitive inhibitory peptides could be useful in the therapeutic approach applied to cancer metastasis in tissues expressing claudin-12.

Reduced Claudin-12 Expression Predicts Poor Prognosis in Cervical Cancer

Background: Within the claudin (CLDN) family, CLDN12 mRNA expression is altered in various types of cancer, but its clinicopathological relevance has yet to be established due to the absence of specific antibodies (Abs) with broad applications. Methods: We generated a monoclonal Ab (mAb) against human/mouse CLDN12 and verified its specificity. By performing immunohistochemical staining and semiquantification, we evaluated the relationship between CLDN12 expression and clinicopathological parameters in tissues from 138 cases of cervical cancer. Results: Western blot and immunohistochemical analyses revealed that the established mAb selectively recognized the CLDN12 protein. Twenty six of the 138 cases (18.8%) showed low CLDN12 expression, and the disease-specific survival (DSS) and recurrence-free survival rates were significantly decreased compared with those in the high CLDN12 expression group. We also demonstrated, via univariable and multivariable analyses, that the low CLDN12 expression represents a significant prognostic factor for the DSS of cervical cancer patients (HR 3.412, p = 0.002 and HR 2.615, p = 0.029, respectively). Conclusions: It can be concluded that a reduced CLDN12 expression predicts a poor outcome for cervical cancer. The novel anti-CLDN12 mAb could be a valuable tool to evaluate the biological relevance of the CLDN12 expression in diverse cancer types and other diseases.

Dihydrotanshinone, a Natural Diterpenoid, Preserves Blood-Retinal Barrier Integrity via P2X7 Receptor

Activation of P2X7 signaling, due to high glucose levels, leads to blood retinal barrier (BRB) breakdown, which is a hallmark of diabetic retinopathy (DR). Furthermore, several studies report that high glucose (HG) conditions and the related activation of the P2X7 receptor (P2X7R) lead to the over-expression of pro-inflammatory markers. In order to identify novel P2X7R antagonists, we carried out virtual screening on a focused compound dataset, including indole derivatives and natural compounds such as caffeic acid phenethyl ester derivatives, flavonoids, and diterpenoids. Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) rescoring and structural fingerprint clustering of docking poses from virtual screening highlighted that the diterpenoid dihydrotanshinone (DHTS) clustered with the well-known P2X7R antagonist JNJ47965567. A human-based in vitro BRB model made of retinal pericytes, astrocytes, and endothelial cells was used to assess the potential protective effect of DHTS against HG and 2′(3′)-O-(4-Benzoylbenzoyl)adenosine-5′-triphosphate (BzATP), a P2X7R agonist, insult. We found that HG/BzATP exposure generated BRB breakdown by enhancing barrier permeability (trans-endothelial electrical resistance (TEER)) and reducing the levels of ZO-1 and VE-cadherin junction proteins as well as of the Cx-43 mRNA expression levels. Furthermore, HG levels and P2X7R agonist treatment led to increased expression of pro-inflammatory mediators (TLR-4, IL-1β, IL-6, TNF-α, and IL-8) and other molecular markers (P2X7R, VEGF-A, and ICAM-1), along with enhanced production of reactive oxygen species. Treatment with DHTS preserved the BRB integrity from HG/BzATP damage. The protective effects of DHTS were also compared to the validated P2X7R antagonist, JNJ47965567. In conclusion, we provided new findings pointing out the therapeutic potential of DHTS, which is an inhibitor of P2X7R, in terms of preventing and/or counteracting the BRB dysfunctions elicited by HG conditions.

Aberrant expression of junctional adhesion molecule-A contributes to the malignancy of cervical adenocarcinoma by interaction with poliovirus receptor/CD155

Recent studies have shown that aberrant expression of tight junction proteins (TJP) contributes to malignant potential of various cancers. In the present study, we investigated the expression of junctional adhesion molecule-A (JAM-A), one of the transmembrane TJP, in uterine cervical adenocarcinoma and the significance of its expression for malignancy. Immunohistochemistry on human surgical specimens showed that JAM-A was aberrantly expressed in neoplastic regions including adenocarcinoma in situ (AIS). Knockout of JAM-A significantly suppressed cell proliferation and colony-forming and migration abilities. We also showed that an antibody specific to an extracellular region of JAM-A reduced cell proliferation ability and that loss of JAM-A increased drug sensitivity of cervical adenocarcinoma cells. Based on a comprehensive proteome analysis, we found that poliovirus receptor (PVR/CD155) was regulated by JAM-A and formed a physical interaction with JAM-A. In human surgical specimens, PVR/CD155 expression was significantly correlated with some clinicopathological features and prognosis of cervical adenocarcinoma. Interestingly, most of the PVR/CD155-positive cases expressed a high level of JAM-A, and patients with the expression pattern of PVR/CD155 positive/JAM-A high had significantly shorter periods of relapse-free survival (P = .00964) and overall survival (P = .0204) than those for the other patients. Our observations suggest that aberrant expression of JAM-A promotes malignancy of uterine cervical adenocarcinoma by regulation of PVR/CD155, and JAM-A is therefore a potential therapeutic target for this malignancy.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Prognostic Significance of Aberrant Claudin-6 Expression in Endometrial Cancer

BACKGROUND

Among the claudin (CLDN) family, CLDN6 exhibits aberrant expression in various cancers, but its biological relevance has not yet been established. We generated a monoclonal antibody (mAb) against human CLDN6 and verified its specificity. By immunohistochemical staining and semi-quantification, we evaluated the relationship between CLDN6 expression and clinicopathological parameters in tissues from 173 cases of endometrial cancer.

RESULTS

The established mAb selectively recognized CLDN6 protein. Ten of the 173 cases (5.8%) showed high CLDN6 expression (score 3+), whereas 19 (11.0%), 18 (10.4%) and 126 (72.4%) cases revealed low CLDN6 expression (score 2+, 1+ and 0, respectively). In addition, intratumor heterogeneity of CLDN6 expression was observed even in the cases with high CLDN6 expression. The 5-year survival rates in the high and low CLDN6 groups was approximately 30% and 90%, respectively. Among the clinicopathological factors, the high CLDN6 expression was significantly associated with surgical stage III/IV, histological grade 3, lymphovascular space involvement, lymph node metastasis and distant metastasis. Furthermore, the high CLDN6 expression was an independent prognostic marker for overall survival of endometrial cancer patients (hazard ratio 3.50, p = 0.014).

CONCLUSIONS

It can be concluded that aberrant CLDN6 expression is useful to predict poor outcome for endometrial cancer and might be a promising therapeutic target.

Epigenetic effects of low-level sodium arsenite exposure on human liver HepaRG cells

Chronic exposure to inorganic arsenic is associated with a variety of adverse health effects, including lung, bladder, kidney, and liver cancer. Several mechanisms have been proposed for arsenic-induced tumorigenesis; however, insufficient knowledge and many unanswered questions remain to explain the integrated molecular pathogenesis of arsenic carcinogenicity. In the present study, using non-tumorigenic human liver HepaRG cells, we investigated epigenetic alterations upon prolonged exposure to a noncytotoxic concentration of sodium arsenite (NaAsO₂). We demonstrate that continuous exposure of HepaRG cells to 1 µM sodium arsenite (NaAsO₂) for 14 days resulted in substantial cytosine DNA demethylation and hypermethylation across the genome, among which the claudin 14 (CLDN14) gene was hypermethylated and the most down-regulated gene. Another important finding was a profound loss of histone H3 lysine 36 (H3K36) trimethylation, which was accompanied by increased damage to genomic DNA and an elevated de novo mutation frequency. These results demonstrate that continuous exposure of HepaRG cells to a noncytotoxic concentration of NaAsO₂ results in substantial epigenetic abnormalities accompanied by several carcinogenesis-related events, including induction of epithelial-to-mesenchymal transition, damage to DNA, inhibition of DNA repair genes, and induction of de novo mutations. Importantly, this study highlights the intimate mechanistic link and interplay between two fundamental cancer-associated events, epigenetic and genetic alterations, in arsenic-associated carcinogenesis.

Claudin expression profile in flat wart and cutaneous squamous cell carcinoma in epidermodysplasia verruciformis

Epidermodysplasia verruciformis (EV) is a genodermatosis related to human beta-papillomavirus (beta-HPV), with a high risk of cutaneous squamous cell carcinoma (cSCC). Claudins are transmembrane proteins expressed in epithelia and may be altered during carcinogenesis. For a better understanding of the role of beta-HPV in cutaneous carcinogenesis, this claudin expression study was conducted on lesions of patients with and without EV. In this study, claudins-1, -2, -3, -4, -5, -7 and -11 expressions were analyzed by applying the immunohistochemistry technique, in samples of 108 normal skin, 39 flat warts and 174 cSCC. The cSCC samples were organized in tissue microarrays. We found that claudin-1 and claudin-3 focal expressions were associated with cSCC (p < 0.001), and claudin-2 focal or negative expression with flat wart (p < 0.001), in EV and NEV (non-EV) groups. For claudin-5, EV group showed a lower chance of focal and negative expression (p < 0.001), and its negative expression was associated with flat wart (p < 0.001) and lower mean age (p < 0.001). Claudins-4, -7 and -11 showed a diffuse expression in almost all studied samples. Our findings suggest that claudin-5 increased expression observed on normal skin, flat wart and cSCC showed association with EV. Claudin-1 and -3 down expression were also observed, but they could not be related to beta-HPV infection.

[Claudin-3 expression in gastric cancer]

Claudins are a family of transmembrane proteins which are essential for the formation and maintenance of epithelial tight junctions. Altered expression of claudins may lead to structural and functional damage of tight junctions, which plays an important role in tumorigenesis and cancer progression. The expression of claudin-3 in gastric cancer is not yet well understood.

AIM

To evaluate the expression of claudin-3 in gasric cancer and in adjacent normal mucosa and its association with clinical and pathological parameters.

SUBJECT AND METHODS

Tissue specimens from a total of 69 patients with gastric cancer were obtained. Immunohistochemical reactions were performed using mouse polyclonal antibodies to claudin-3.

RESULTS

The expression of claudin-3 in gastric cancer was significantly higher than in adjacent normal mucosa (p<0,05). The absence of claudin-3 was significantly associated with poor differentiation (p<0,05). An abnormal nuclear expression of claudin-3 was observed in 69.6% cases. A significant association was found between nuclear expression and the absence of membranous claudin-3 expression (p<0,05).

The Expression and Prognostic Significance of Claudin-8 and Androgen Receptor in Breast Cancer

Purpose

Claudin-8 (CLDN8) has been identified as an androgen-regulated gene in prostate cancer. However, the role of CLDN8 has not been fully explored in breast cancer. We aimed to explore the expression of CLDN8 and androgen receptor (AR), determine the correlation between CLDN8 and AR, assess the prognostic value of CLDN8 and AR co-expression, and investigate the possible CLDN8 expression molecular mechanism in breast cancer.

Materials and Methods

Twenty-eight pairs of fresh tumor tissues and adjacent noncancerous tissues were evaluated by Western blot for CLDN8. Then, 142 breast cancer samples were determined by immunohistochemistry for CLDN8 and AR. The association of clinicopathological features with CLDN8, AR and CLDN8, and AR co-expression was examined. The Cancer Genome Atlas (TCGA) was used to demonstrate the expression of CLDN8 and correlation between CLDN8 and AR. Kaplan–Meier survival analysis was performed to assess the prognostic impact of CLDN8 and AR co-expression. The mechanisms related to CLDN8 expression in breast cancer were explored by Gene Set Enrichment Analysis (GSEA).

Results

CLDN8 was downregulated in breast cancer tissues and positively correlated with none lymph node metastasis (P=0.016), low histological grade (P=0.006), positive ER (P=0.014), positive PR (P=0.003), low Ki-67 index (P=0.017) and molecular subtypes (P=0.012). CLDN8 level was significantly associated with AR level (r=0.348; P<0.001). CLDN8 and AR co-expression was positively correlated with none lymph node metastasis (P=0.007), low histological grade (P=0.017), positive ER (P=0.019), positive PR (P=0.015) and low Ki-67 index group (P=0.038). CLDN8 and AR co-expression had a better clinical prognosis.

Conclusion

The expression of CLDN8 is directly related to the expression of AR. CLDN8 and AR co-expression might be a potential prognostic evaluation factor for breast cancer patients.

Claudin 10 acts as a novel biomarker for the prognosis of patients with ovarian cancer

Ovarian cancer (OC) is one of the most fatal gynecological malignancies in the world and confers a poor 5-year survival rate. The present study was designed to discover novel prognostic markers for patients with OC in order to estimate disease metastasis or recurrence. Based on the large cohorts of transcriptome data from multicenter sources, a comprehensive analysis was performed to explore potential prognostic markers. A total of 269 differentially expressed genes were identified, of which 32 were upregulated and 237 downregulated in OC tissues compared with the corresponding expression in normal tissues. Kaplan-Meier analysis, log-rank test and nomogram analysis were employed to demonstrate that low expression levels of claudin 10 (CLDN10) were associated with a less favorable disease prognosis. The most promising prognostic marker for OC was subsequently selected. Additionally, the prognostic nomogram was constructed in order to assess the 5-year survival rate using CLDN10 expression as a prognostic marker for OC. Furthermore, gene set enrichment analysis and analysis of the tumor-associated competing endogenous RNA network were performed to elucidate the potential biological processes associated with CLDN10 expression. The current results indicated that CLDN10 may influence OC progression via transforming growth factor-β (TGF-β)- or WNT/β-catenin-induced epithelial-to-mesenchymal transition (EMT). The associations among CLDN10, microRNA-486-5p, TGF-β, WNT/β-catenin and EMT should be further investigated in future studies.

All Benign and Malignant Apocrine Breast Lesions Over-Express Claudin 1 and 3 and Are Negative for Claudin 4

Invasive apocrine carcinoma of the breast is an uncommon triple negative tumour that lacks a specific therapeutic target. Apocrine metaplasia of the breast shares common morphological features with apocrine carcinoma, and was previously found to consistently over-express claudin 1 and to lack claudin 4. This study was aimed at finding whether apocrine carcinoma, and other related apocrine breast lesions, have similar claudin profile. The immunohistochemical expression of claudin 1, 3 and 4 was studied in 11 cases of in situ and invasive apocrine breast carcinoma, 7 benign apocrine lesions and 45 consecutive morphologically non-apocrine triple negative breast carcinomas. All cases were also immunostained for Gross Cystic Disease Fluid Protein-15 (GCDFP-15), a marker for apocrine differentiation. Apocrine breast lesions maintained their expression pattern from benign through DCIS to invasive carcinoma; all showing strong expression of claudin 1 and 3 and absence of claudin 4. The same pattern of expression was seen in 2 out of the 45 morphologically non-apocrine tumours, but both showed strong positive staining for GCDFP-15. It is concluded that all benign and malignant apocrine lesions of the breast have a consistent pattern of claudin 1, 3 and 4 expression, suggesting the presence of a specific pathway for the development of invasive apocrine carcinoma. The over-expression of claudin 1 and 3 may have therapeutic implications as targets for managing apocrine cancers.

Claudin-9 enhances the metastatic potential of hepatocytes via Tyk2/Stat3 signaling

BACKGROUND/AIMS

We have previously identified a tight junction protein claudin-9 (CLDN9) as an upregulated gene in hepatocellular carcinoma (HCC) through an immunohistochemistry analysis. Here, we explore its function and clinical relevance in human HCC.

MATERIALS AND METHODS

Stable transfection of the hepatocyte line HL7702 with CLDN9 was confirmed by the real-time polymerase chain reaction (PCR), western blotting, and immunofluorescence. The impact of CLDN9 on the cell invasion and migration was assessed in vitro by a transwell assay and wound-healing experiment. Western blotting was used to determine the activation state of the Tyk2 (tyrosine kinase 2)/Stat3 (signal transducer and activator of transcription 3) pathway. Moreover, we used a Tyk2-RNAi assay to silence the expression of Tyk2 in CLDN9 expressing hepatocytes; subsequently, the impact of the Tyk2/Stat3 signaling pathway on the cell invasion and migration in vitro was assessed by a transwell assay and a wound-healing experiment. Furthermore, an immunohistochemistry method was utilized to explore the expression levels of CLDN9 and p-Stat3 in the HCC tissues and histologically non-neoplastic hepatic tissues.

RESULTS

We confirmed that the expression of CLDN9 significantly enhanced the metastatic ability of hepatocytes in vitro, and the activation of the Stat3 pathway by Tyk2 was an important mechanism by which CLDN9 promoted hepatocyte aggressiveness in HCC.

CONCLUSION

As an HCC proto-oncogene, CLDN9 affected the Stat3 signaling pathway via Tyk2 and ultimately enhanced the metastatic ability of hepatocytes.

Cell junction proteins: Crossing the glomerular filtration barrier in diabetic nephropathy

Diabetic nephropathy as a deleterious complication of diabetes mellitus and an important cause of end-stage renal failure is characterized by changes in the molecular and cellular levels. Cell-cell communication via the gap and tight junctions are involved in the pathogenesis of diseases such as diabetes and kidney failure. Studying cell junctions including gap junctions, tight junctions, and anchoring junctions within the nephron can be used as an early sign of diabetic nephropathy. Furthermore, cell junctions may be an upcoming target by pharmacological methods to improve treatments of diabetic nephropathy and pave the way to introduce promising therapeutic strategies based on cell-cell communications effects and its translation into clinical studies for the treatment of diabetic nephropathy.

Dichotomous roles of claudins as tumor promoters or suppressors: lessons from knockout mice

Claudins are a family of integral tight junction proteins that regulate paracellular permeability in polarized epithelia. Overexpression or reduction of claudins can both promote and limit cancer progression, revealing complex dichotomous roles for claudins depending on cellular context. In contrast, recent studies demonstrating tumor formation in claudin knockout mouse models indicate a role for several claudin family members in suppressing tumor initiation. For example, intestine-specific claudin-7 knockout mice spontaneously develop atypical hyperplasia and intestinal adenomas, while claudin-18 knockout mice develop carcinomas in the lung and stomach. Claudin-4, -11, and -15 knockout mice show increased cell proliferation and/or hyperplasia in urothelium, Sertoli cells, and small intestinal crypts, respectively, possibly a precursor to cancer development. Pathways implicated in both cell proliferation and tumorigenesis include Yap/Taz and insulin-like growth factor-1 receptor (IGF-1R)/Akt pathways, among others. Consistent with the tumor suppressive role of claudins shown in mice, in humans, claudin-low breast cancer has been described as a distinct entity with a poor prognosis, and claudin-18-Rho GTPase activating protein 26 (CLDN18-ARHGAP26) fusion protein as a driver gene aberration in diffuse-type gastric cancer due to effects on RhoA. Paradoxically, claudins have also garnered interest as targets for therapy, as they are sometimes aberrantly expressed in cancer cells, which may or may not promote cancer progression. For example, a chimeric monoclonal antibody which targets cells expressing claudin-18.2 through antibody-dependent cell-mediated cytotoxicity has shown promise in multiple phase II studies. In this review, we focus on new findings supporting a tumor suppressive role for claudins during cancer initiation.

The Cytoplasmic Expression Of CLDN12 Predicts An Unfavorable Prognosis And Promotes Proliferation And Migration Of Osteosarcoma

Background

To date, the impact and potential molecular mechanisms of CLDN12 and its association with malignancy in osteosarcoma have not been determined.

Materials and methods

In the present study, the expression profiles of CLDN12 in osteosarcoma cell lines and tissues were explored by immunohistochemistry. A fetal osteoblast cell line was transfected with a eukaryotic expression plasmid, and endogenous CLDN12 in osteosarcoma cells were silenced through an RNA interference (RNAi) method. These transfections were verified, and the activation state of Thr308 site in protein kinase B (Akt) was explored by Western blotting. Moreover, the malignant phenotype of osteosarcoma cells was evaluated by cell counting kit-8 (CCK-8), colony formation, Transwell, and wound-healing assays. Furthermore, osteoblast cells were treated with the phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002 to determine the impact of the PI3K/Akt signaling pathway on cell migration ability.

Results

The results revealed that CLDN12 was overexpressed and localized in the cytoplasm of osteosarcoma cells, and its overexpression was associated with an unfavorable prognosis, irrespective of tumor node metastasis stage. In addition, the knockdown of CLDN12 in cultured osteosarcoma cells markedly attenuated cell proliferation and migration, as indicated by the Cell Counting Kit-8 assay, colony formation assay, scratch wound healing assay and Transwell migration assay. The results also demonstrated that the overexpression of CLDN12 increased the activation of Thr308 site in Akt in fetal osteoblast cells, and the PI3K inhibitor LY294002 partially decreased CLDN12-promoted proliferation and metastasis.

Conclusion

In conclusion, the results of the present study indicated that CLDN12 promoted cell proliferation and migration through the PI3K/Akt signaling pathway in osteosarcoma cells, suggesting that CLDN12 may be a potential agent in the treatment of patients with osteosarcoma.

Impact of chemotherapy on the expression of claudins and cadherins in invasive breast cancer

The importance of the expression profile of claudins in the molecular classification of breast cancer (BC) is currently under investigation. Claudins, together with cadherins, serve an important role in the epithelial-mesenchymal transition and influence the chemosensitivity of cancer cells. Adjuvant chemotherapy is administered following surgical resection in selected cases of BC. Previous neoadjuvant chemotherapy may change the molecular profile of a tumour and subsequently also its chemosensitivity. In the current study, the expression of claudin-1, −3 and −4, E- and N-cadherin and the standard BC biomarkers [oestrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) and marker of proliferation Ki-67 (Ki-67)] in formalin-fixed, paraffin-embedded sections from 62 patients with invasive BC was analysed using immunohistochemistry prior to and following neoadjuvant chemotherapy. The results revealed increased expression of claudin-1 (P=0.03) and decreased expression of claudin-3 (P=0.005), PR (P<0.001) and Ki-67 (P=0.01) following the neoadjuvant therapy. No significant changes in the expression of ER, claudin-4 or E- and N-cadherin were observed following therapy. Furthermore, an association between the expression of claudin-1 and the standard BC markers (P<0.05) was identified. A high expression of claudin-1 was more frequently observed in the triple-negative BC cohort than in the cohort with positive ER, PR and/or HER2 before (P=0.04) and after chemotherapy (P=0.02). The expression of N-cadherin was associated with the expression of ER, PR, HER2 and tumour grade (P<0.05). A positive association between the expression of claudin-3 and E-cadherin (P=0.005) was observed. No association was found between the expression of E- and N-cadherin. In conclusion, significant changes in the expression of claudin-1 and −3 but not in the expression of claudin-4, E- and N-cadherin were observed in samples taken from patients with BC following chemotherapy. These findings indicate that claudins-1 and −3 serve a role in the response of BC to chemotherapy.

Potential for Tight Junction Protein-Directed Drug Development Using Claudin Binders and Angubindin-1

The tight junction (TJ) is an intercellular sealing component found in epithelial and endothelial tissues that regulates the passage of solutes across the paracellular space. Research examining the biology of TJs has revealed that they are complex biochemical structures constructed from a range of proteins including claudins, occludin, tricellulin, angulins and junctional adhesion molecules. The transient disruption of the barrier function of TJs to open the paracellular space is one means of enhancing mucosal and transdermal drug absorption and to deliver drugs across the blood–brain barrier. However, the disruption of TJs can also open the paracellular space to harmful xenobiotics and pathogens. To address this issue, the strategies targeting TJ proteins have been developed to loosen TJs in a size- or tissue-dependent manner rather than to disrupt them. As several TJ proteins are overexpressed in malignant tumors and in the inflamed intestinal tract, and are present in cells and epithelia conjoined with the mucosa-associated lymphoid immune tissue, these TJ-protein-targeted strategies may also provide platforms for the development of novel therapies and vaccines. Here, this paper reviews two TJ-protein-targeted technologies, claudin binders and an angulin binder, and their applications in drug development.

Estrogen receptor β inhibits breast cancer cells migration and invasion through CLDN6-mediated autophagy

Background

Estrogen receptor β (ERβ) has been reported to play an anti-cancer role in breast cancer, but the regulatory mechanism by which ERβ exerts this effect is not clear. Claudin-6 (CLDN6), a tight junction protein, acts as a tumor suppressor gene in breast cancer. Our previous studies have found that 17β-estradiol (E2) induces CLDN6 expression and inhibits MCF-7 cell migration and invasion, but the underlying molecular mechanisms are still unclear. In this study, we aimed to investigate the role of ERβ in this process and the regulatory mechanisms involved.

Methods

Polymerase chain reaction (PCR) and western blot were used to characterize the effect of E2 on the expression of CLDN6 in breast cancer cells. Chromatin immunoprecipitation (ChIP) assays were carried out to confirm the interaction between ERβ and CLDN6. Dual luciferase reporter assays were used to detect the regulatory role of ERβ on the promoter activity of CLDN6. Wound healing and Transwell assays were used to examine the migration and invasion of breast cancer cells. Western blot, immunofluorescence and transmission electron microscopy (TEM) were performed to detect autophagy. Xenograft mouse models were used to explore the regulatory effect of the CLDN6-beclin1 axis on breast cancer metastasis. Immunohistochemistry (IHC) was used to detect ERβ/CLDN6/beclin1 expression in breast cancer patient samples.

Results

Here, E2 upregulated the expression of CLDN6, which was mediated by ERβ. ERβ regulated CLDN6 expression at the transcriptional level. ERβ inhibited the migration and invasion of breast cancer cells through CLDN6. Interestingly, this effect was associated with CLDN6-induced autophagy. CLDN6 positively regulated the expression of beclin1, which is a key regulator of autophagy. Beclin1 knockdown reversed CLDN6-induced autophagy and the inhibitory effect of CLDN6 on breast cancer metastasis. Moreover, ERβ and CLDN6 were positively correlated, and the expression of CLDN6 was positively correlated with beclin1 in breast cancer tissues.

Conclusion

Overall, this is the first study to demonstrate that the inhibitory effect of ERβ on the migration and invasion of breast cancer cells was mediated by CLDN6, which induced the beclin1-dependent autophagic cascade.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1359-9) contains supplementary material, which is available to authorized users.

Targeting CLDN18.2 by CD3 Bispecific and ADC Modalities for the Treatments of Gastric and Pancreatic Cancer

Human CLDN18.2 is highly expressed in a significant proportion of gastric and pancreatic adenocarcinomas, while normal tissue expression is limited to the epithelium of the stomach. The restricted expression makes it a potential drug target for the treatment of gastric and pancreatic adenocarcinoma, as evidenced by efforts to target CLDN18.2 via naked antibody and CAR-T modalities. Herein we describe CLDN18.2-targeting via a CD3-bispecific and an antibody drug conjugate and the characterization of these potential therapeutic molecules in efficacy and preliminary toxicity studies. Anti-hCLDN18.2 ADC, CD3-bispecific and diabody, targeting a protein sequence conserved in rat, mouse and monkey, exhibited in vitro cytotoxicity in BxPC3/hCLDN18.2 (IC₅₀ = 1.52, 2.03, and 0.86 nM) and KATO-III/hCLDN18.2 (IC₅₀ = 1.60, 0.71, and 0.07 nM) respectively and inhibited tumor growth of pancreatic and gastric patient-derived xenograft tumors. In a rat exploratory toxicity study, the ADC was tolerated up to 10 mg/kg. In a preliminary assessment of tolerability, the anti-CLDN18.2 diabody (0.34 mg/kg) did not produce obvious signs of toxicity in the stomach of NSG mice 4 weeks after dosing. Taken together, our data indicate that targeting CLDN18.2 with an ADC or bispecific modality could be a valid therapeutic approach for the treatment of gastric and pancreatic cancer.

Identification of claudin‑1, ‑3, ‑7 and ‑8 as prognostic markers in human laryngeal carcinoma

Various genomic and epigenetic modifications that occur during the development of cancer act as potential biomarkers for early diagnosis and treatment. Previous studies have demonstrated abnormal expression of the claudin (CLDN) tight junction (TJ) proteins in numerous types of human cancer. Reverse transcription-quantitative polymerase chain reaction and western blotting were employed to investigate variations in the expression of the CLDN TJ proteins in laryngeal non-neoplastic tissues and laryngeal squamous carcinoma tissues. It was revealed that CLDN2, CLDN4, CLDN5, CLDN6, CLDN9, CLDN11 and CLDN12 were undetectable in laryngeal squamous carcinoma tissues and laryngeal non-neoplastic tissues. Additionally, CLDN10 was expressed in laryngeal squamous carcinoma tissues and laryngeal non-neoplastic tissues; however, no significant difference was reported. Conversely, the expression levels of CLDN1 and CLDN7 mRNA and protein were downregulated in laryngeal squamous carcinoma tissues compared with in adjacent non-neoplastic tissues, whereas those of CLDN3 and CLDN8 were upregulated. A total of 80 samples of laryngeal squamous carcinoma and non-neoplastic tissues were analyzed for the expression of CLDN1, −3, −7 and −8 via streptavidin-peroxidase immunohistochemical staining. It was revealed that the expression levels of CLDN1 and CLDN7 were downregulated in laryngeal squamous carcinoma tissues compared with in non-neoplastic mucosal tissues, whereas those of CLDN3 and CLDN8 were upregulated. Furthermore, the associations between CLDN expression and the clinicopathological factors of patients were analyzed. The expression levels of CLDN3 and CLDN7 were reported to be associated with distant metastasis and serve as potential predictors of poor prognosis. In conclusion, the findings of the present study demonstrated that the expression levels of CLDN1, −3, −7 and −8 varied between laryngeal squamous carcinoma tissues and non-neoplastic tissues. The expression levels of these CLDNs may be useful molecular markers for the diagnosis of laryngeal carcinoma, and determining the metastasis and prognosis of this disease.

Sulforaphane metabolites inhibit migration and invasion via microtubule-mediated Claudins dysfunction or inhibition of autolysosome formation in human non-small cell lung cancer cells

Both sulforaphane-cysteine (SFN-Cys) and sulforaphane-N-acetyl-l-cysteine (SFN-NAC) inhibited cancer migration and invasion, but the underlying mechanisms were not clear. Here we uncovered via tissue microarray assay that high expression of invasion-associated Claudin-5 was correlated to malignant grades in human non-small cell lung cancer (NSCLC) cells. Further, SFN-Cys (10 µM) induced the accumulated phosphorylation of ERK1/2, leading to downregulation of Claudin-5 and upregulation of Claudin-7, and the decrease of Claudin-1 in SK-1 cells and increase of Claudin-1 in A549 cells; knockdown of Claudin-5 significantly reduced invasion, whereas knockdown of Claudin-7 increased invasion; knockdown of Claudin-1 reduced invasion in SK-1 cells, whereas it increased invasion in A549 cells, indicating that SFN-Cys regulated Claudins and inhibited invasion depending on Claudin isotypes and cell types. Furthermore, immunofluorescence staining showed that SFN-Cys triggered microtubule disruption and knockdown of α-tubulin downregulated Claudin-1, 5, and 7, and inhibited migration and invasion, indicating that microtubule disruption contributed to invasive inhibition. Co-immunoprecipitation and confocal microscopy observation showed that SFN-Cys lowered the interaction between α-tubulin and Claudin-1 or 5, or 7. Meanwhile, Western blotting and immunofluorescence staining showed that SFN-NAC (15 µM) downregulated α-tubulin resulting in microtubule disruption; knockdown of α-tubulin increased SFN-NAC-induced LC3 II accumulation in SK-1 cells. Combined with the inhibitor of autolysosome formation, Bafilomycin A1 (100 nM), SFN-NAC inhibited invasion via accumulating LC3 II and blocking formation of autolysosome. Further, SFN-NAC upregulated microtubule-stabilizing protein Tau; knockdown of Tau reduced LC3 II/LC3 I inhibiting migration and invasion. These results indicated that SFN-Cys inhibited invasion via microtubule-mediated Claudins dysfunction, but SFN-NAC inhibited invasion via microtubule-mediated inhibition of autolysosome formation in human NSCLC cells.

Tight junction proteins in gastrointestinal and liver disease

Over the past two decades a growing body of evidence has demonstrated an important role of tight junction (TJ) proteins in the physiology and disease biology of GI and liver disease. On one side, TJ proteins exert their functional role as integral proteins of TJs in forming barriers in the gut and the liver. Furthermore, TJ proteins can also be expressed outside TJs where they play important functional roles in signalling, trafficking and regulation of gene expression. A hallmark of TJ proteins in disease biology is their functional role in epithelial-to-mesenchymal transition. A causative role of TJ proteins has been established in the pathogenesis of colorectal cancer and gastric cancer. Among the best characterised roles of TJ proteins in liver disease biology is their function as cell entry receptors for HCV-one of the most common causes of hepatocellular carcinoma. At the same time TJ proteins are emerging as targets for novel therapeutic approaches for GI and liver disease. Here we review our current knowledge of the role of TJ proteins in the pathogenesis of GI and liver disease biology and discuss their potential as therapeutic targets.

Claudin gene expression profiles and clinical value in colorectal tumors classified according to their molecular subtype

Purpose

Colorectal cancer (CRC) is a heterogeneous disease that can be classified into distinct molecular subtypes. The aims of this study were 1) to compare claudin (CLDN) gene expression in CRC samples and normal colon mucosa, and then in the different CRC molecular subtypes, and 2) to assess their prognostic value.

Patients and methods

CLDN expression in CRC samples was analyzed using gene expression data for a cohort of 143 primary CRC samples, and compared in the same CRC samples classified into different molecular subtypes (C1 to C6 according to the Marisa's classification, and CMS1 to CMS4 of the consensus classification). Comparison of CLDN expression in normal and tumor colon samples was also made on a smaller number of samples. Then, the relationship between CLDN expression profiles and overall survival (OS) and progression-free survival was examined.

Results

Compared with normal mucosa, CLDN1 and CLDN2 were upregulated, whereas CLDN5, 7, 8, and 23 were downregulated in CRC samples. Variations in CLDN expression profiles were observed mainly in the CMS2/C1 and CMS4/C4 subtypes. Overall, expression of CLDN2 or CLDN4 alone had a strong prognostic value that increased when they were associated. In the CMS4/C4 subtypes, lower expressions of CLDN11, CLDN12, and CLDN23 were associated with longer OS. Conversely, in the CMS2 and C1 subtypes, low CLDN23 expression was associated with shorter OS and progression-free survival, suggesting a dual role for CLDN23 as a tumor suppressor/promoter in CRC. CLDN6 and CLDN11 had a prognostic value in the CMS2 and C4 subtypes, respectively.

Conclusion

This analysis of CLDN gene expression profiles and prognostic value in CRC samples classified according to their molecular subtype shows that CRC heterogeneity must be taken into account when assessing CLDN potential value as prognostic markers or therapeutic targets.