Matrix metalloproteinase-7 and claudin-7 as novel identified therapeutic targets for restoration of intestinal epithelial barrier in inflammatory bowel diseases

Intestinal tight junction disruption and mucosal immune dysregulation contribute to pathogenesis and progression of inflammatory bowel diseases (IBD). A proteolytic enzyme matrix metalloproteinase 7 (MMP-7), which is highly expressed in intestinal tissue, is implicated to IBD and other immune overactivation-associated diseases. In the issue of the Frontiers in Immunology, Ying Xiao and colleagues demonstrate that MMP-7-mediated claudin-7 degradation promotes IBD pathogenesis and disease progression. Therefore, inhibition of MMP-7 enzymatic activity can be a therapeutic strategy for the treatment of IBD.

Hypoxia-inducible microRNA-155 negatively regulates epithelial barrier in eosinophilic esophagitis by suppressing tight junction claudin-7

MicroRNA (miRNA)-mediated mRNA regulation directs many homeostatic and pathological processes, but how miRNAs coordinate aberrant esophageal inflammation during eosinophilic esophagitis (EoE) is poorly understood. Here, we report a deregulatory axis where microRNA-155 (miR-155) regulates epithelial barrier dysfunction by selectively constraining tight junction CLDN7 (claudin-7). MiR-155 is elevated in the esophageal epithelium of biopsies from patients with active EoE and in cell culture models. MiR-155 localization using in situ hybridization (ISH) in patient biopsies and intra-epithelial compartmentalization of miR-155 show expression predominantly within the basal epithelia. Epithelial miR-155 activity was evident through diminished target gene expression in 3D organotypic cultures, particularly in relatively undifferentiated basal cell states. Mechanistically, generation of a novel cell line with enhanced epithelial miR-155 stable overexpression induced a functionally deficient epithelial barrier in 3D air-liquid interface epithelial cultures measured by transepithelial electrical resistance (TEER). Histological assessment of 3D esophageal organoid cultures overexpressing miR-155 showed notable dilated intra-epithelial spaces. Unbiased RNA-sequencing analysis and immunofluorescence determined a defect in epithelial barrier tight junctions and revealed a selective reduction in the expression of critical esophageal tight junction molecule, claudin-7. Together, our data reveal a previously unappreciated role for miR-155 in mediating epithelial barrier dysfunction in esophageal inflammation.

Three-Dimensional Culture of Murine Colonic Crypts to Study Intestinal Stem Cell Function Ex Vivo

The intestinal epithelium regenerates every 5-7 days, and is controlled by the intestinal epithelial stem cell (IESC) population located at the bottom of the crypt region. IESCs include active stem cells, which self-renew and differentiate into various epithelial cell types, and quiescent stem cells, which serve as the reserve stem cells in the case of injury. Regeneration of the intestinal epithelium is controlled by the self-renewing and differentiating capabilities of these active IESCs. In addition, the balance of the crypt stem cell population and maintenance of the stem cell niche are essential for intestinal regeneration. Organoid culture is an important and attractive approach to studying proteins, signaling molecules, and environmental cues that regulate stem cell survival and functions. This model is less expensive, less time-consuming, and more manipulatable than animal models. Organoids also mimic the tissue microenvironment, providing in vivo relevance. The present protocol describes the isolation of colonic crypts, embedding these isolated crypt cells into a three-dimensional gel matrix system and culturing crypt cells to form colonic organoids capable of self-organization, proliferation, self-renewal, and differentiation. This model allows one to manipulate the environment-knocking out specific proteins such as claudin-7, activating/deactivating signaling pathways, etc.-to study how these effects influence the functioning of colonic stem cells. Specifically, the role of tight junction protein claudin-7 in colonic stem cell function was examined. Claudin-7 is vital for maintaining intestinal homeostasis and barrier function and integrity. Knockout of claudin-7 in mice induces an inflammatory bowel disease-like phenotype exhibiting intestinal inflammation, epithelial hyperplasia, weight loss, mucosal ulcerations, epithelial cell sloughing, and adenomas. Previously, it was reported that claudin-7 is required for intestinal epithelial stem cell functions in the small intestine. In this protocol, a colonic organoid culture system is established to study the role of claudin-7 in the large intestine.

A Claudin-Based Molecular Signature Identifies High-Risk, Chemoresistant Colorectal Cancer Patients

Identifying molecular characteristics that are associated with aggressive cancer phenotypes through gene expression profiling can help predict treatment responses and clinical outcomes. Claudins are deregulated in colorectal cancer (CRC). In CRC, increased claudin-1 expression results in epithelial-to-mesenchymal transition and metastasis, while claudin-7 functions as a tumor suppressor. In this study, we have developed a molecular signature based on claudin-1 and claudin-7 associated with poor patient survival and chemoresistance. This signature was validated using an integrated approach including publicly available datasets and CRC samples from patients who either responded or did not respond to standard-of-care treatment, CRC cell lines, and patient-derived rectal and colon tumoroids. Transcriptomic analysis from a patient dataset initially yielded 23 genes that were differentially expressed along with higher claudin-1 and decreased claudin-7. From this analysis, we selected a claudins-associated molecular signature including PIK3CA, SLC6A6, TMEM43, and ASAP-1 based on their importance in CRC. The upregulation of these genes and their protein products was validated using multiple CRC patient datasets, in vitro chemoresistant cell lines, and patient-derived tumoroid models. Additionally, blocking these genes improved 5-FU sensitivity in chemoresistant CRC cells. Our findings propose a new claudin-based molecular signature that associates with poor prognosis as well as characteristics of treatment-resistant CRC including chemoresistance, metastasis, and relapse.

Nobiletin Attenuates DSS-Induced Intestinal Barrier Damage through the HNF4α-Claudin-7 Signaling Pathway

The intestinal epithelium barrier functions to protect human bodies from damages such as harmful microorganisms, antigens, and toxins. In this study, we evaluated the protective effect and molecular mechanism of a dominant polymethoxyflavone nobiletin (NOB) from tangerine peels on intestinal epithelial integrity. The results from transepithelial electrical resistance (TEER) suggested that NOB pretreatment counteracts epithelial injury induced by inflammatory cytokines (TEER value in 48 h: vehicle, 135.6 ± 3.9 Ω/cm²; TNF-α + IL-1β, 90.7 ± 0.5 Ω/cm²; 10 μM NOB + TNF-α + IL-1β, 126.1 ± 0.8 Ω/cm²; 100 μM NOB + TNF-α + IL-1β, 125.3 ± 0.5 Ω/cm². P < 0.001). Clinical and pathological test results suggested that administration of NOB effectively alleviates intestinal barrier injury induced by dextran sulfate sodium (DSS) as evidenced by the length of colon villi on day 7 (control, 253.7 ± 4.8 μm, DSS 131.6 ± 4.6 μm, NOB + DSS, 234.5 ± 5.1 μm. P < 0.001). Interestingly, when screening tight junction molecules for intestinal barrier integrity, we observed that independent treatment with NOB sharply increased claudin-7 levels (ratio of claudin-7 over GAPDH: control, 1.0 ± 0.06; DSS, 0.02 ± 0.001; NOB + DSS, 0.3 ± 0.07. P < 0.001), which was previously suppressed upon DSS stimulation. Furthermore, hepatocyte nuclear factor 4α (HNF-4α) transcriptional regulation of claudin-7 contributed to intestinal barrier homeostasis. Therefore, our study suggests potential intestinal protective strategies based on polymethoxyflavones of aged tangerine peels.

Upregulation of Claudin-7 Expression by Angiotensin II in Colonic Epithelial Cells of Mice Fed with NaCl-Depleted Diets

Dietary NaCl depletion increases Na⁺ and Cl⁻ absorption in the colon, but the mechanisms are not fully understood. So far, we reported that the expression of claudin-7 (CLDN7), a tight junction (TJ) protein, was upregulated in the mice fed with NaCl-depleted diets, but the regulatory mechanism has not been clarified. Here, we found that angiotensin II (ANGII) increases the mRNA level of CLDN7, which was inhibited by losartan, a type 1 ANGII (AT1) receptor antagonist. Immunofluorescence measurement showed that CLDN7 is colocalized with zonula occludens-1 at the TJ in untreated and ANGII-treated cells. ANGII decreased transepithelial electrical resistance (TER) and increased permeability to C1⁻ without affecting permeability to lucifer yellow, a paracellular flux marker. In contrast, TER was increased by CLDN7 knockdown in the absence and presence of ANGII. ANGII increased the nuclear distribution of phosphorylated p65 subunit of NF-κB, which was inhibited by losartan. The ANGII-induced elevation of CLDN7 expression was blocked by BAY 11-7082 (BAY), an NF-κB inhibitor. Luciferase reporter assay showed that ANGII increases promoter activity of CLDN7, which was inhibited by the treatment with losartan or BAY, and introduction of mutations in κB-binding motifs in the promoter. The binding of p65 on the promoter region of CLDN7 was increased by ANGII, which was inhibited by losartan and BAY in chromatin immunoprecipitation assay. Our data suggest that ANGII acts on AT1 receptor and increases paracellular permeability to Cl⁻ mediated by the elevation of CLDN7 expression in the colon.

Catechin and Procyanidin B2 Modulate the Expression of Tight Junction Proteins but Do Not Protect from Inflammation-Induced Changes in Permeability in Human Intestinal Cell Monolayers

The possibility of counteracting inflammation-related barrier defects with dietary compounds such as (poly)phenols has raised much interest, but information is still scarce. We have investigated here if (+)-catechin (CAT) and procyanidin B₂ (PB₂), two main dietary polyphenols, protect the barrier function of intestinal cells undergoing inflammatory stress. The cell model adopted consisted of co-cultured Caco-2 and HT29-MTX cells, while inflammatory conditions were mimicked through the incubation of epithelial cells with the conditioned medium of activated macrophages (MCM). The epithelial barrier function was monitored through trans-epithelial electrical resistance (TEER), and ROS production was assessed with dichlorofluorescein, while the expression of tight-junctional proteins and signal transduction pathways were evaluated with Western blot. The results indicated that MCM produced significant oxidative stress, the activation of NF-κB and MAPK pathways, a decrease in occludin and ZO-1 expression, and an increase in claudin-7 (CL-7) expression, while TEER was markedly lowered. Neither CAT nor PB₂ prevented oxidative stress, transduction pathways activation, ZO-1 suppression, or TEER decrease. However, PB₂ prevented the decrease in occludin expression and both polyphenols produced a huge increase in CL-7 abundance. It is concluded that, under the conditions adopted, CAT and PB₂ do not prevent inflammation-dependent impairment of the epithelial barrier function of intestinal cell monolayers. However, the two compounds modify the expression of tight-junctional proteins and, in particular, markedly increase the expression of CL-7. These insights add to a better understanding of the potential biological activity of these major dietary flavan-3-ols at intestinal level.

Trophoblast cell surface antigen 2 (Trop-2) phosphorylation by protein kinase C α/δ (PKCα/δ) enhances cell motility

Dysfunction of tight junctions is a critical step during the initial stage of tumor progression. Trophoblast cell surface antigen 2 (Trop-2) belongs to the family of tumor-associated calcium signal transducer (TACSTD) and is required for the stability of claudin-7 and claudin-1, which are often dysregulated or lost in carcinogenesis. Here, we investigated the effects of Trop-2 phosphorylation on cell motility. Analyses using HCT116 cells expressing WT Trop-2 (HCT116/WT) or Trop-2 alanine-substituted at Ser-303 (HCT116/S303A) or Ser-322 (HCT116/S322A) revealed that Trop-2 is phosphorylated at Ser-322. Furthermore, coimmunoprecipitation and Transwell assays indicated that Trop-2 S322A interacted with claudin-7 the strongest, and a phosphomimetic variant, Trop-2 S322E, the weakest and that HCT116/S322E cells have the highest motility and HCT116/S322A cells the lowest. All cell lines had similar levels of claudin-7 mRNA, but levels of claudin-7 protein were markedly decreased in the HCT116/S322E cells, suggesting posttranscriptional control of claudin-7. Moreover, claudin-7 was clearly localized to cell-cell borders in HCT116/S322A cells but was diffusely distributed on the membrane and partially localized in the cytoplasm of HCT116/S322E and HCT116/WT cells. These observations suggested that Trop-2 phosphorylation plays a role in the decrease or mislocalization of claudin-7. Using protein kinase C (PKC) inhibitors and PKC-specific siRNAs, we found that PKCα and PKCδ are responsible for Trop-2 phosphorylation. Of note, chemical PKC inhibition and PKCα- and PKCδ-specific siRNAs reduced motility. In summary, our findings provide evidence that Trop-2 is phosphorylated at Ser-322 by PKCα/δ and that this phosphorylation enhances cell motility and decreases claudin-7 localization to cellular borders.

Claudin-7 modulates cell-matrix adhesion that controls cell migration, invasion and attachment of human HCC827 lung cancer cells

Claudins are a family of tight junction proteins, and serve important roles in epithelial barrier, selective ion transports and cancer metastasis. Although the exact role of claudin-7 in human lung cancer has not been completely elucidated, recent clinical studies have demonstrated that claudin-7 is associated with the survival of patients with lung cancer. Our previous studies have demonstrated that claudin-7 forms a protein complex with integrin β1 in human lung cancer cells. The knockdown (KD) of claudin-7 by short hairpin RNA (shRNA) reduced integrin β1 expression and increased the cell proliferative rate, whereas claudin-7 re-expression in the KD cells decreased the cell proliferation. It is unknown as to whether claudin-7 and integrin β1 regulate cell proliferation and invasion synergistically or independently. In the present study, it was observed that ectopic expression of integrin β1 in claudin-7 KD lung cancer cells did not reduce the cell proliferation. However, integrin β1-transfected cells migrated more effectively in wound healing and cell invasion assays and were more adhesive in a cell attachment assay when compared with those of claudin-7 KD cells. This indicates that claudin-7 controls cell proliferation, while cell attachment and motility were regulated partially through integrin β1. Additionally, claudin-7 overexpression in claudin-7 KD cells resulted in an improved ability to attach to the surface of cell culture plates and a higher expression of focal adhesion proteins when compared with claudin-7 non-KD control cells, which supports the role of claudin-7 in cell adhesion and motility. Taken together, these data suggest that claudin-7 regulates cell motility through integrin β1, providing additional insight into the roles of claudins in carcinogenesis and cancer cell metastasis.

Impaired Localization of Claudin-11 in Endometriotic Epithelial Cells Compared to Endometrial Cells

Claudins are the major components of tight junctions and are often deregulated in human cancer, permitting escape of cancer cells along with the acquisition of invasive properties. Similarly, endometrial cells also show invasive capabilities; however, the role of tight junctions in endometriosis has only rarely been examined. In this study, we analyzed the protein expression and localization of claudin-7 and claudin-11 in human eutopic and ectopic endometrium and endometrial cell lines. We identified claudin-7 primarily at the basolateral junctions of the glandular epithelial cells in eutopic endometrium as well as in the ectopic lesions in nearly all glands and cysts. Quantification of claudin-7 localization by HSCORE showed a slight increase in peritoneal and deep infiltrating endometriosis (DIE) compared to eutopic endometrium. In contrast, claudin-11 was localized mainly in the apicolateral junctions in nearly all glandular epithelial cells of the eutopic endometrium. Interestingly, we observed a deregulation of claudin-11 localization to a basal or basolateral localization in ovarian (P < .001), peritoneal (P < .01), and DIE (P < .05) and a moderately decreased abundance in ovarian endometriosis. In endometrial cell lines, claudin-7 was only present in epithelial Ishikawa cells, and silencing by small-interfering RNA increased cell invasiveness. In contrast, claudin-11 could be demonstrated in Ishikawa and endometriotic 12Z and 49Z cells. Silencing of claudin-11 decreased invasiveness of 12Z slightly but significantly in 49Z. We suggest that although claudin-7 and claudin-11 can be found in nearly all eutopic and ectopic epithelial cells, the impaired localization of claudin-11 in ectopic endometrium might contribute to the pathogenesis of endometriosis.

Emerging clinical significance of claudin-7 in colorectal cancer: a review

Tight junctions (TJs) play an important role in maintaining cell polarity and regulating cell permeability. In recent years, many studies have shown that TJ proteins, especially claudin-7, are closely related to inflammation and the development of various malignant tumors. Claudin-7 plays a significant role in maintaining the physiological functions and pathological conditions of the TJ barrier. The dysregulation of claudin-7 plays a tumor suppressor role or conversely has carcinogenic effects in different target tissues or cells, but the exact underlying mechanism is still unclear. In this review, we will summarize the expression pattern of claudin-7 in tumors, focusing on the expression and regulation of claudin-7 in colorectal cancer and discussing the correlation between claudin-7 and invasion, metastasis and epithelial–mesenchymal transition (EMT) in colorectal cancer. The construction of Cldn7^−/− mice and conventional claudin-7 knockout mouse models has helped determine the mechanisms by which claudin-7 promotes tumorigenesis. Elucidation of the expression and subcellular localization of claudin-7 under pathological conditions will help develop claudin-7 as a useful biomarker for detecting and diagnosing cancer, and thus may help combat the occurrence, development, and invasion of cancers.

TGF-β1 alters esophageal epithelial barrier function by attenuation of claudin-7 in eosinophilic esophagitis

Barrier dysfunction has been implicated in the pathophysiology of eosinophilic esophagitis (EoE). Transforming growth factor-β1 (TGF-β1), a potent pleiotropic molecule, is increased in EoE; however, no study has evaluated its influence on esophageal epithelial barrier. We hypothesized that TGF-β1 regulates barrier dysfunction in EoE. We aimed to determine the role of TGF-β1 in the epithelial barrier in models of EoE. To examine the impact of TGF-β1 on esophageal barrier, immortalized human esophageal epithelial (EPC2-hTERT) cells were exposed to TGF-β1 during the three-dimensional air-liquid interface (3D-ALI) model in vitro. TGF-β1 exposure diminished EPC2-hTERT barrier function as measured by transepithelial electrical resistance (TEER) and 3 kDa Fluorescein isothiocyanate dextran paracellular flux (FITC Flux), and hematoxylin and eosin (H&E) assessment revealed prominent cellular separation. In analysis of epithelial barrier molecules, TGF-β1 led to the specific reduction in expression of the tight-junction molecule, claudin-7 (CLDN7), and this was prevented by TGF-β-receptor I inhibitor. Short hairpin ribonucleic acid (shRNA)-mediated CLDN7 knockdown diminished epithelial barrier function, whereas CLDN7 overexpression resulted in protection from TGF-β1-mediated barrier dysfunction. In pediatric EoE biopsies CLDN7 expression was decreased and altered localization was observed with immunofluorescence analysis, and the TGF-β1 downstream transcription factor, phosphorylated SMAD2/3 (pSMAD2/3), was increased. Our data suggest that TGF-β1 participates in esophageal epithelial barrier dysfunction through CLDN7 dysregulation.

Nuclear expression of claudin-3 in human colorectal adenocarcinoma cell lines and tissues

Claudins are members of a large family of transmembrane proteins, which are essential for the formation of tight junctions and have a significant effect on the biological behavior of tumor progression. Previous studies have demonstrated that several claudins show aberrant expression patterns in numerous types of cancer. The present study investigated the expression and localization of claudin-3 and claudin-7 in human colorectal adenocarcinoma cell lines and tissues. The protein expression levels of claudin-3 and claudin-7 were determined using immunocytochemical and immunohistochemical staining. Claudin-3, but not claudin-7, exhibited nuclear localization in the human colorectal adenocarcinoma Caco-2 and SW620 cell lines. Surgically resected colorectal adenocarcinoma tissue specimens were obtained, and the associations between the expression of claudin-3 or claudin-7 and various clinicopathological parameters were analyzed. The membranous expression rates of claudin-3 and claudin-7 were 58.0 and 50.0%, while their nuclear expression rates were 22.0 and 2.0%, respectively. The membranous expression of claudin-3 and claudin-7 was not associated with any clinicopathological factors, whereas the nuclear expression of claudin-3 was associated with histological type and was significantly increased in colorectal mucinous adenocarcinomas compared with that in well- to moderately-differentiated colorectal adenocarcinomas (P<0.01). However, no associations were observed between the nuclear expression of claudin-7 and any clinicopathological parameter. In conclusion, the nuclear expression of claudin-3 in colorectal mucinous adenocarcinoma may be involved in the biological transformation of tumors. The results from the present study indicated that claudin-3 is an important protein associated with histological type and has potential as a prognostic marker. Although the mechanisms underlying the nuclear localization of claudin-3 in tumorigenesis have not yet been elucidated in detail, the present results indicated the potential of claudin-3 as a histopathological biomarker for colorectal adenocarcinomas.

Clinical significance of serum claudin-1 and claudin-7 levels in patients with colorectal cancer

The present study aimed to investigate the serum levels and clinical relevance of claudin (CLDN) 1 and CLDN7 in patients with colorectal cancer (CRC). A total of 140 patients with a pathologically confirmed diagnosis of CRC were enrolled in this study. The serum levels of CLDN1 and CLDN7 were determined using the solid-phase sandwich ELISA method. A total of 40 healthy age- and gender-matched controls were included in the analysis. The median age of the patients was 60 years (range, 24-84 years). The localization of the tumor in the majority of the patients was the colon (n=81, 58%). Of the 55 metastatic patients who received palliative chemotheraphy, 31% were chemotherapy-responsive. The baseline median serum CLDN1 and CLDN7 levels were significantly lower in non-metastatic and metastatic patients compared with those in healthy controls (CLND1, P=0.008 and 0.002; and CLND7, P=0.002 and 0.002, respectively). Moreover, known clinical variables, including poor performance status and high carcinoembryonic antigen (CEA) levels were found to be associated with lower serum CLDN1 concentrations for all patients (P=0.03 and P=0.03, respectively). High T stage and high CEA levels were found to be correlated with lower serum CLDN7 concentrations for all patients (P=0.04 and 0.03, respectively). A correlation was identified between CLDN1 and CLDN7 levels in non-metastatic and metastatic CRC patients (both P-values <0.001). Our study results did not reveal any statistical significance for serum CLDN1 or CLND7 concentrations regarding progression-free and overall survival rate. Therefore, reduced serum levels of CLDN1 and CLND7 may be useful markers in the differential diagnosis of CRC.

Expression of claudin-5, -7, -8 and -9 in cervical carcinoma tissues and adjacent non-neoplastic tissues

Recent data indicate that the tight junction proteins are abnormally regulated in several human cancers and the expression of these proteins is involved in the etiology and progression of cancer. To explore the expression distinction of the tight junction proteins claudin-5, -7, -8 and -9 in the adjacent non-neoplastic tissues and cervical carcinoma tissues, 72 cervical carcinoma tissues and the samples of non-neoplastic tissues adjacent to the tumors were examined for expression of claudin-5, -7, -8 and -9 by streptavidin-perosidase immunohistochemical staining method. The positive expression rates of claudin-5 in cervical carcinoma tissues and adjacent non-neoplastic tissues were 31.9% (23/72) and 51.4% (37/72) respectively (P < 0.05). The positive expression rates of claudin-7 in cervical carcinoma tissues and adjacent non-neoplastic tissues were 47.2% and 50.0% respectively (P = 1.000). The positive expression rates of claudin-8 in cervical carcinoma tissues and adjacent non-neoplastic tissues were 54.2 % and 27.8% respectively (P < 0.01). The positive expression rates of claudin-9 in cervical carcinoma tissues and adjacent non-neoplastic tissues were 38.9% and 56.9% respectively (P < 0.05). Thus in our study, the expression of claudin-5 and claudin-9 was down-regulated while the expression of claudin-8 was up-regulated in cervical carcinoma tissues compared with adjacent non-neoplastic tissues. The expression of claudin-7 has no obviously difference between cervical carcinoma tissues and adjacent non-neoplastic tissues. In addition, correlations between claudin-5, -8 and -9 expression with lymphatic metastasis were observed. Our study reveals that the expression of claudin-5, -8 and -9 altered between in cervical carcinoma tissues and adjacent non-neoplastic tissues.

Downregulation of claudin-7 potentiates cellular proliferation and invasion in endometrial cancer

Claudin-7, a tight junction protein, has been demonstrated to be abnormally regulated in several types of human cancer. The present study aimed to investigate the expression and function of claudin-7 in endometrial cancer. In total, 31 pairs of endometrial cancer samples and their adjacent normal tissues were used to detect the expression of claudin-7 by immunohistochemical staining. Compared with the corresponding normal tissues, 45.2% of the endometrial cancer tissues exhibited weak or absent claudin-7 protein expression. Low levels of claudin-7 were correlated with a late tumor stage (P=0.023) and low histological grade (P=0.018). Claudin-7 was either overexpressed in AN3CA endometrial cancer cells, via plasmid cDNA transfection, or silenced by RNA interference in Ishikawa cells. Following either type of experimental manipulation, cellular proliferation and invasion were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, wound healing and transwell assays, respectively. The silencing of claudin-7 significantly increased cellular proliferation (P=0.032) and invasion (P=0.020) rates. Consistent with these results, the increased expression of claudin-7 decreased the proliferation (P=0.021) and invasiveness (P=0.012) of the AN3CA cells. A low expression of claudin-7 in the endometrial cancer cells was indicative of a late tumor stage and low histological grade. Additionaly, restoration of claudin-7 inhibited the proliferation and invasion of endometrial cancer cells, thus providing a potential therapeutic strategy.

Claudin-7 inhibits human lung cancer cell migration and invasion through ERK/MAPK signaling pathway

Tight junctions are the most apical component of the junctional complex critical for epithelial cell barrier and polarity functions. Although its disruption is well documented during cancer progression such as epithelial-mesenchymal transition, molecular mechanisms by which tight junction integral membrane protein claudins affect this process remain largely unknown. In this report, we found that claudin-7 was normally expressed in bronchial epithelial cells of human lungs but was either downregulated or disrupted in its distribution pattern in lung cancer. To investigate the function of claudin-7 in lung cancer cells, we transfected claudin-7 cDNA into NCI-H1299, a human lung carcinoma cell line that has no detectable claudin-7 expression. We found that claudin-7 expressing cells showed a reduced response to hepatocyte growth factor (HGF) treatment, were less motile, and formed fewer foot processes than the control cells did. In addition, cells transfected with claudin-7 dramatically decreased their invasive ability after HGF treatment. These effects were mediated through the MAPK signaling pathway since the phosphorylation level of ERK1/2 was significantly lower in claudin-7 transfected cells than in control cells. PD98059, a selective inhibitor of ERK/MAPK pathway, was able to block the motile effect. Claudin-7 formed stable complexes with claudin-1 and -3 and was able to recruit them to the cell-cell junction area in claudin-7 transfected cells. When control and claudin-7 transfected cells were inoculated into nude mice, claudin-7 expressing cells produced smaller tumors than the control cells. Taken together, our study demonstrates that claudin-7 inhibits cell migration and invasion through ERK/MAPK signaling pathway in response to growth factor stimulation in human lung cancer cells.

Detection of tight junction barrier function in vivo by biotin

Tight junctions (TJs) are the most apical component of the junctional complexes in mammalian epithelial cells and form selective paracellular barriers restricting the passage of solutes and ions across the epithelial sheets. Claudins, a TJ integral membrane protein family, play a critical role in regulating paracellular barrier permeability. In the in vitro cell culture system, transepithelial electrical resistance (TER) measurement and the flux of radioisotope or fluorescent labeled molecules with different sizes have been widely used to determine the TJ barrier function. In the in vivo system, the tracer molecule Sulfo-NHS-Biotin was initially used in Xenopus embryo system and subsequently was successfully applied to a number of animal tissues in situ and in different organisms under the experimental conditions to examine the functional integrity of TJs by several laboratories. In this chapter, we will describe the detailed procedures of applying biotin as a paracellular tracer molecule to different in vivo systems to assay TJ barrier function.

Renal salt wasting and chronic dehydration in claudin-7-deficient mice

Claudin-7, a member of the claudin family, is highly expressed in distal nephrons of kidneys and has been reported to be involved in the regulation of paracellular Cl(-) permeability in cell cultures. To investigate the role of claudin-7 in vivo, we generated claudin-7 knockout mice (Cln7(-/-)) by the gene-targeting deletion method. Here we report that Cln7(-/-) mice were born viable, but died within 12 days after birth. Cln7(-/-) mice showed severe salt wasting, chronic dehydration, and growth retardation. We found that urine Na(+), Cl(-), and K(+) were significantly increased in Cln7(-/-) mice compared with that of Cln7(+/+) mice. Blood urea nitrogen and hematocrit were also significantly higher in Cln7(-/-) mice. The wrinkled skin was evident when Cln7(-/-) mice were approximately 1 wk old, indicating that they suffered from chronic fluid loss. Transepidermal water loss measurements showed no difference between Cln7(+/+) and Cln7(-/-) skin, suggesting that there was no transepidermal water barrier defect in Cln7(-/-) mice. Claudin-7 deletion resulted in the dramatic increase of aldosterone synthase mRNA level as early as 2 days after birth. The significant increases of epithelial Na(+) channel alpha, Na(+)-Cl(-) cotransporter, and aquaporin 2 mRNA levels revealed a compensatory response to the loss of electrolytes and fluid in Cln7(-/-) mice. Na(+)-K(+)-ATPase alpha(1) expression level was also greatly increased in distal convoluted tubules and collecting ducts where claudin-7 is normally expressed. Our study demonstrates that claudin-7 is essential for NaCl homeostasis in distal nephrons, and the paracellular ion transport pathway plays indispensable roles in keeping ionic balance in kidneys.

Expression of claudin-7 in benign kidney and kidney tumors

Claudins, a family of tight junction-related transmembrane proteins, have been implicated in the pathogenesis of various human neoplasms. Expression of claudin-7 was increased in chromophobe renal cell carcinoma in a recent oligonucleotide microarray study. We studied the expression of claudin-7 in benign and neoplastic kidneys by immunohistochemical staining. Distal nephron (distal convoluted tubule and thick ascending limb of Henle's loop) epithelium showed strong membranous staining in 100% (174/174) of the cases. Chromophobe renal cell carcinoma was positive for claudin-7 expression in 100% (36/36) of cases, while papillary renal cell carcinoma, oncocytoma and clear cell renal cell carcinoma were positive in 90% (71/80), 45% (21/47) and 7% (7/98) of the cases, respectively. Differential expression of Claudin-7 in different types of renal cell neoplasms can be useful in their differential diagnosis, particularly when used in a panel of markers. In addition, results from this study support previous reports of distal nephron origin for chromophobe renal cell carcinoma and oncocytoma. The data also suggest that, as far as claudin-7 expression is concerned, papillary renal cell carcinoma may be more closely related to the distal nephron, rather than the proximal nephron.

Claudin-7 is highly expressed in chromophobe renal cell carcinoma and renal oncocytoma

Claudin-7 has recently been suggested to be a distal nephron marker. We tested the possibility that expression of claudin-7 could be used as a marker of renal tumors originating from the distal nephron. We examined the immunohistochemical expression of claudin-7 and parvalbumin in 239 renal tumors, including 179 clear cell renal cell carcinoma (RCC)s, 29 papillary RCCs, 20 chromophobe RCCs, and 11 renal oncocytomas. In addition, the methylation specific-PCR (MSP) of claudin-7 was performed. Claudin-7 and parvalbumin immunostains were positive in 3.4%, 7.8% of clear cell RCCs, 34.5%, 31.0% of papillary RCCs, 95.0%, 80.0% of chromophobe RCCs, and 72.7%, 81.8% of renal oncocytomas, respectively. The sensitivity and specificity of claudin-7 in diagnosing chromophobe RCC among subtypes of RCC were 95.0% and 92.3%. Those of parvalbumin were 80.0% and 88.9%. The expression pattern of claudin-7 was mostly diffuse in chromophobe RCC and was either focal or diffuse in oncocytoma. All of the cases examined in the MSP revealed the presence of unmethylated promoter of claudin-7 without regard to claudin-7 immunoreactivity. Hypermethylation of the promoter might not be the underlying mechanism for loss of its expression in RCC. Claudin-7 can be used as a useful diagnostic marker in diagnosing chromophobe RCC and oncocytoma.