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Human osteosarcoma (OS) represents one of the most common primary sarcomas often originating in the metaphyses of long bones. However, its underlying molecular pathogenesis is still only vaguely understood. Several tight junction proteins were shown to be associated with and involved in tumorigenesis. This study is aimed to evaluate the role of Claudin 8 (CLDN8) in human OS. Lentivirus-based short hairpin RNA targeting CLDN8 specifically depleted its endogenous expression in U2OS and SW1353 OS cells, with a reduction by 97.7% and 89.3%, respectively, in contrast to control. Depletion of CLDN8 led to a significant diminution in cell viability and proliferation. To test the mechanism by which CLDN8 modulates cell proliferation, the flow cytometry assay and apoptosis assay were performed and confirmed that G1-S transition was blocked and a strong proapoptotic effect was induced in U2OS cells by CLDN8 knockdown. These data demonstrate that CLDN8 plays an essential role in OS proliferation in vitro, which will provide a new opportunity for discovering and identifying novel effective treatment strategies.

Disruption or loss of tight junction structure and function is associated with tumor growth, invasion and metastasis in tumors of human epithelial origin. Since claudin is the most important backbone protein of tight junctions, the downregulation or loss of claudin expression is hypothesized to be important for tumor development and metastasis. In the current study, RNA interference (RNAi) was used to knock down the expression of claudin‑6 to investigate the effect of claudin‑6 downregulation on the malignant phenotype in the human breast epithelium cell line HBL‑100. The junctional function was investigated by measuring the transepithelial electrical resistance across the confluent epithelial cell layer. Manual cell counting and wound healing assays were performed to examine cell proliferation and migration. Changes in matrix metalloproteinase‑2 (MMP‑2) expression and activity were examined by reverse transcription polymerase chain reaction (RT‑PCR) and gelatin zymography. The expression of p38 mitogen‑activated protein kinases (MAPKs) and phosphorylated p38 MAPK were measured by western blot analysis. Claudin‑6 knockdown resulted in significantly lower transepithelial electrical resistance (P<0.001), higher growth rate (P<0.001) and migratory ability (P<0.001) accompanied with an increased MMP‑2 expression and activity (P<0.001). Furthermore, a decreased expression of phosphorylated p38 MAPK (P<0.001) was detected in HBL‑100 cells. These observations support the hypothesis that a decreased expression of claudin‑6 contributes to the malignant progression of human breast cancer.

Background: Claudins are known as tight junction proteins, and their expression pattern in gastric cancer is still controversial. The relationship between the expression patterns of tight junction proteins and tumor proliferation in early gastric cancer is still far from clear., Aims: To investigate the expression patterns of claudin-18 and Ki-67 in early gastric cancer at the invasive front and surrounding normal gastric mucosa and to investigate the biological function of claudin-18 in the proliferation and invasion of cancer cells., Methods: Seventy-five early gastric cancer lesions removed via endoscopic mucosal resection or endoscopic submucosal resection were evaluated. All gastric cancer lesions were diagnosed as differentiated adenocarcinoma using the Japanese Classification of Gastric Carcinoma. To assess epithelial proliferation, immunostaining with Ki-67 was performed, and the labeling index was calculated. To assess the expression of epithelial tight junction proteins, immunofluorescent staining of claudin-18 was performed. The immunoreactivity of claudin-18 was graded according to the number of stained cells. Correlation analysis was performed by Spearman's rank correlation coefficient. Transfection of claudin-18 small interfering RNA (siRNA) was accomplished in MKN74, a claudin-18-positive gastric cancer cell line, to investigate the effect of claudin-18 on proliferation and invasion of cancer cells., Results: Claudin-18 was significantly down-regulated in gastric cancer compared to surrounding gastric normal mucosa or intestinal metaplasia. The Ki-67 labeling index of gastric cancer at the invasive front was inversely correlated with the claudin-18 level, but that at the mucosal lesion was not correlated. Claudin-18 knockdown significantly promoted the proliferation of MKN74 compared with control siRNA-transfected cells. MKN74 invasion increased significantly with claudin-18 siRNA transfection compared with control siRNA transfection., Conclusions: Down-regulation of claudin-18 is associated with the proliferative potential at the invasive front of gastric cancer, suggesting that it has a pivotal role in gastric cancer progression.