BACKGROUND: The CLDN18 gene, encoding claudin 18.1 and claudin 18.2, is a key component of tight junction strands in epithelial cells that form a paracellular barrier that is critical in Stomach Adenocarcinoma (STAD).
METHODS: Our study included 1,095 patients with proven STAD, 415 from The Cancer Genome Atlas (TCGA) cohort and 680 from the Gene Expression Omnibus database. We applied various analyses, including gene set enrichment analysis, pathway analysis, and in vitro drug screening to evaluate survival, immune cells, and genes and gene sets associated with cancer progression, based on CLDN18 expression levels. Gradient boosting machine learning (70% for training, 15% for validation, and 15% for testing) was used to evaluate the impact of CLDN18 on survival and develop a survival prediction model.
RESULTS: High CLDN18 expression correlated with worse survival in lymphocyte-poor STAD, accompanied by decreased helper T cells, altered metabolic genes, low necrosis-related gene expression, and increased tumor proliferation. CLDN18 expression showed associations with gene sets associated with various stomach, breast, ovarian, and esophageal cancers, while pathway analysis linked CLDN18 to immunity. Incorporating CLDN18 expression improved survival prediction in a machine learning model. Notably, nutlin-3a and niraparib effectively inhibited high CLDN18-expressing gastric cancer cells in drug screening.
CONCLUSIONS: Our study provides a comprehensive understanding of the biological role of CLDN18-based bioinformatics and machine learning analysis in STAD, shedding light on its prognostic significance and potential therapeutic implications. To fully elucidate the molecular intricacies of CLDN18, further investigation is warranted, particularly through in vitro and in vivo studies.

Angiogenesis is essential for the growth and metastasis of several malignant tumors including colorectal cancer (CRC). The molecular mechanism underlying CRC angiogenesis has not been fully elucidated. Emerging evidence indicates that secreted microRNAs (miRNAs) may mediate the intercellular communication between tumor cells and neighboring endothelial cells to regulate tumor angiogenesis. In addition, exosomes have been shown to carry and deliver miRNAs to regulate angiogenesis. miRNA N-72 is a novel miRNA that plays a regulatory role in the EGF-induced migration of human amnion mesenchymal stem cells. However, the relation between miRNA N-72 and cancer remains unclear. We here found that CRC cells could secrete miRNA N-72. A high miRNA N-72 level was detected in the serum of CRC patients and the cultured CRC cells. Moreover, the CRC cell-secreted miRNA N-72 could promote the migration, tubulogenesis, and permeability of endothelial cells. In addition, the mouse xenograft model was used to verify the facilitating effects of miRNA N-72 on CRC growth, angiogenesis, and metastasis in vivo. Further mechanism analysis revealed that CRC cell-secreted miRNA N-72 could be delivered into endothelial cells via exosomes, which then inhibited cell junctions of endothelial cells by targeting CLDN18 and consequently promoted angiogenesis. Our findings reveal a novel mechanism of CRC angiogenesis and highlight the potential of secreted miRNA N-72 as a therapeutic target and a biomarker for CRC.

BACKGROUND:  Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related deaths worldwide, and is characterized by insidious onset, rapid progression, and poor prognosis. Immunotherapy is a first-line treatment for advanced HCC. The identification of immune-related prognostic markers may be an effective strategy to predict and improve clinical response rate of immunotherapy.
METHODS:  The DESeq2, edgeR, and limma R packages were used to compare the transcriptomes of HCC with different prognoses. Cancer-related databases such as UALCAN, TNMplot, GEPIA, muttarget and Human Protein Atlas (HPA), and the Kaplan-Meier Plotter platform were used to analyze the relationship between CLDN18 and the clinical characteristics, as well as prognosis of HCC. The co-expressed genes of CLDN18 were obtained from LinkedOmics platform, and GO functional enrichment and KEGG pathway analysis were performed. The CIBERSORT, TIMER, Timer 2.0 and TISIDB algorithms were used to analyze immune infiltration.
RESULTS:  CLDN18 was differentially expressed in HCC patients with different prognoses, and its expression level in PBMC was positively correlated with the stage of BCLC. In addition, CLDN18 was significantly overexpressed in HCC tumor tissues compared to adjacent non-tumor tissues, which was consistent with PBMC sequencing results and immunohistochemical data from human protein profiles. CLDN18 was also positively correlated with HCC staging and grading, and high expression levels of CLDN18 predicted shorter overall survival. Functional annotation of CLDN18 in HCC revealed enrichment of the cellular senescence and protein activation cascade, along with biological processes such as cell cycle, inflammatory response, and cellular ketone metabolism. In addition, CLDN18 was also associated with tumor infiltrating immune cells, suppressive immune cell markers, T lymphocyte depletion and activation of HCC, and low expression of CLDN18 was associated with higher CD8 + T cell infiltration and better survival rates.
CONCLUSIONS: CLDN18 is a potential prognostic marker and immunotherapeutic target for HCC.

BACKGROUND: CLDN encodes a member of the claudin family. Claudin is a tight junction protein that is mainly involved in cell migration. Claudin family is of interest as a potential therapeutic target. Claudin18.2 is one of its important isoforms and is mainly expressed in the stomach. Its expression and prognosis in lung adenocarcinoma remain unknown. The aim of this study was to investigate the correlation between CLDN18 and claudin18.2 expression and prognosis in lung adenocarcinoma.
METHODS: Two cohorts were introduced in this study: one from The Cancer Genome Atlas (TCGA) CLDN18 mRNA public data (TCGA-LUAD, N = 551); the other from 1079 cases of lung adenocarcinoma diagnosed at the Fourth Hospital of Hebei Medical University, China, with immunohistochemical (IHC) detection of claudin18.2 in tissue microarrays. the IHC-positive cases were again verified by fluorescence in situ hybridization (FISH).
RESULTS: The mRNA expression of CLDN18 was significantly lower in lung adenocarcinoma tissues than in normal lung tissues (P < 0.05). Among 1079 Chinese lung adenocarcinoma cases, the overall positive rate of IHC for Claudin18.2 was 7.78% (84/1079). Among those positive for IHC, the positive rate of FISH was 11.9% (10/84), which accounted for 0.9% of the total number of cases (10/1079). To explore the best scoring scheme for Claudin 18.2, we used a four-group (IHC4) and two-group (IHC2) scoring method for evaluation. We found that IHC4 better explained Claudin 18.2 expression and helped us to find specific differences in clinical factors for weak, moderate and strong Claudin 18.2 expression. This difference was not discernible in the IHC2 score. By survival analysis, we found that Claudin 18.2 (IHC4) was able to stratify the prognosis of lung adenocarcinoma patients, with strongly positive patients having a better prognosis than the other subgroups (p < 0.05). We also found that patients with EGFR wild type or PD-L1 < 1% accompanied by strong positive claudin18.2 had a significantly better prognosis than other subgroups (P < 0.05).
CONCLUSIONS: Claudin18.2 (IHC4) better reveals the clinical and prognostic characteristics of patients with lung adenocarcinoma. Patients with EGFR wild type and PD-L1 < 1% have a better prognosis and partially overlap with claudin18.2 expression, so claudin18.2 may also be an important biomarker for lung adenocarcinoma testing, which is particularly important for EGFR wild type and PD-L1 < 1%.

A classification system for endocervical adenocarcinoma (ECA) based on high-risk human papillomavirus (HPV) status has been established; however, the immunohistochemical markers distinguishing HPV-independent and HPV-associated ECAs have not been fully described. Here, we aimed to characterize ECA immunopathological features.
We evaluated the immunohistochemical profile of CLDN18, CDX2, PAX8, p16, p53, and CEA in 60 ECAs comprising 10 HPV-independent ECAs and 50 HPV-associated ECAs. Both the membranous and nuclear expression levels of CLDN18 were analyzed.
Membranous CLDN18 (CLDN18 [M]) was found to be expressed in the mucinous epithelium of all HPV-independent ECAs, including eight gastric-type ECAs (G-ECAs), one endometrioid ECA, and one clear cell ECA, but no nuclear CLDN18 (CLDN18 [N]) expression was detected in HPV-independent ECAs. Among HPV-associated ECAs, CLDN18 (M) expression levels in intestinal-type (I-ECAs) and usual-type ECAs (U-ECAs) were significantly different from those in invasive stratified mucin-producing (iSMILE) carcinomas (p = 0.036). Positive CLDN18 (M) staining was present in 55.6% (5/9) of intestinal-type and 39.4% (13/33) of usual-type ECAs and was not present in iSMILE ECAs. Silva pattern C cancers expressed higher levels of CLDN18 (M) than Silva pattern A and B cancers (p = 0.004), whereas the CLDN18 (N) expression levels in cancers showing Silva pattern A were significantly higher than those in cancers exhibiting Silva patterns B and C (p < 0.001).
Membranous CLDN18 is expressed in ECAs and is particularly frequently expressed in HPV-independent ECAs, and membranous CLDN18 expression has potential as a therapeutic target. Nuclear staining of CLDN18 is a new immunohistochemical marker for diagnosing Silva pattern A HPV-associated ECAs and is associated with a good prognosis. Further studies should investigate the therapeutic and prognostic significance of membranous and nuclear CLDN18 expression and develop a related test that can be implemented in the clinical evaluation of ECAs.

Despite recent improvements in diagnostic ability and treatment strategies, advanced gastric cancer (GC) has a high frequency of recurrence and metastasis, with poor prognosis. To improve the treatment results of GC, the search for new treatment targets from proteins related to epithelial-mesenchymal transition (EMT) and cell-cell adhesion is currently being conducted. EMT plays an important role in cancer metastasis and is initiated by the loss of cell-cell adhesion, such as tight junctions (TJs), adherens junctions, desmosomes, and gap junctions. Among these, claudins (CLDNs) are highly expressed in some cancers, including GC. Abnormal expression of CLDN1, CLDN2, CLDN3, CLDN4, CLDN6, CLDN7, CLDN10, CLDN11, CLDN14, CLDN17, CLDN18, and CLDN23 have been reported. Among these, CLDN18 is of particular interest. In The Cancer Genome Atlas, GC was classified into four new molecular subtypes, and CLDN18-ARHGAP fusion was observed in the genomically stable type. An anti-CLDN18.2 antibody drug was recently developed as a therapeutic drug for GC, and the results of clinical trials are highly predictable. Thus, CLDNs are highly expressed in GC as TJs and are expected targets for new antibody drugs. Herein, we review the literature on CLDNs, focusing on CLDN18 in GC.

Claudin 18 (CLDN18) is a transmembrane protein that localizes to apical regions to form tight junction complexes. Abnormal expression of CLDN18 has been reported in gastric cancer (GC). The expression, genetic alterations, and prognostic role of CLDN18 were analyzed using public data from The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO), and Human Protein Atlas (HPA) databases using multiple online tools. The biological network of CLDN18 was determined using GeneMANIA. Expression of CLDN18 was restricted to lung and stomach in normal tissues, was significantly downregulated in GC, but was ectopically overexpressed in some other cancer types. There was no correlation between mRNA expression of CLDN18 and the clinicopathology of GC, although expression was higher in the Epstein-Barr virus (EBV)-positive subgroup than other subgroups. Genetic alteration of CLDN18 was not a common event in GC; the main alteration was gene fusion with ARHGAP26. CLDN18 expression did not predict the overall survival (OS) of GC patients. This study summarizes the expression features of CLDN18 in GC and suggests it may serve as a biomarker and therapy target for GC.

OBJECTIVE: Gallbladder cancer (GBC) is an aggressive tumour that is usually diagnosed at advanced stages and is characterised by a poor prognosis. Using public data of normal human tissues, we found that mRNA and protein levels of mucin 5B (MUC5B) and carbonic anhydrase 9 (CA9) were highly increased in gallbladder tissues. In addition, previous evidence has shown that claudin 18 (CLDN18) protein expression is higher in GBC. The aim of this study was to perform an analysis of these cell surface proteins during the histological progression of GBC in order to identify their theranostic potential.
RESULTS: MUC5B expression, CA9 expression and CLDN18 expression were examined by immunohistochemistry in a series of 179 chronic cholecystitis (including 16 metaplastic tissues), 15 dysplasia and 217 GBC samples by the use of tissue microarray analysis. A composite staining score was calculated from staining intensity and percentage of positive cells. Immunohistochemical analysis showed high expression of MUC5B and CA9 among normal epithelium, metaplastic tissues, and dysplastic tissues. However, expression of both proteins was observed in roughly 50% of GBC samples. In contrast, CLDN18 was absent in normal epithelium, but its expression was higher in metaplastic cells. Among GBC cases, approximately half showed high CLDN18 expression. No associations were found between MUC5B, CA9 and CLDN18 expression and any clinicopathological features.
CONCLUSIONS: CLDN18 is a new metaplasia marker in gallbladder tissues, and is conserved in approximately half of GBC cases. MUC5B and CA9 are highly conserved during GBC histological progression. The three markers are potential theranostic markers, in particular CA9 and CLDN18, for which there are already targeted therapies available.

Patients with gastric cancer typically face gastrectomies even when few or no nodal metastases are reported. Current procedures poorly predict lymphatic metastases; thus, evaluation of target molecules expressed on cancer cell membranes is necessary for in vivo detection. However, marker development is limited by the intratumoral heterogeneity of gastric cancer cells. In this study, multiple gene expression arrays of 42 systemic normal tissue samples and 56 gastric cancer samples were used to investigate two adhesion molecules, cadherin 17 (CDH17) and claudin 18 (CLDN18), which are intestinal and gastric markers, respectively. Expression of CDH17 and CLDN18 was partially redundant, but overlapped in 50 of 56 cases (89.3%). Tissue microarrays constructed using primary lesions and nodal metastases of 106 advanced gastric cancers revealed CDH17 and CLDN18 expression in 98 positive cases of 106 (92%). Hierarchical clustering classified gastric cancers into three subgroups, CDH17(++)/CLDN18(+/-), CDH17(++)/CLDN18(++) or CDH17(+)/CLDN18(+), and CDH17(-)/CLDN18(++/+/-). Whole tissue sections displayed strong, homogeneous staining for CDH17 and CLDN18. Together, these results indicate that CDH17 and CLDN18 are useful target molecules; moreover, their coupling can aid in the comprehensive detection and localization of gastric cancer metastases in vivo to overcome challenges associated with intratumoral heterogeneity.