|Lung cancer is a leading cause of cancer death for both men and women worldwide. The 5-year survival rate for lung cancer is only 15%, and it accounts for approximately 1.4 million deaths every year. Recently, it has been reported that changes in expression level of claudin proteins, a family of tight junction integral membrane proteins, has been associated with several types of cancer tissues. Claudin proteins are vital components of the tight junction complex, playing a crucial role in maintaining cell polarity, adhesion, and paracellular permeability. Studies have illustrated that claudin-7 expression is lost in head, neck, invasive breast cancer, and certain types of lung cancer. The objective of this study was to investigate apoptosis in a human lung adenocarcinoma cancer cell line transfected with claudin-7. We transfected claudin-7 cDNA tagged with myc into human NCI-H522 lung cancer cells, which have no detectable expression of claudin-7 protein. H522 cells transfected with vector alone were used as a control. Flow cytometry analysis demonstrated that cells transfected with claudin-7 had a significantly higher percentage of cell apoptosis when compared to that of vector transfected cell population. The cell viability by MTT, (3-(4,5-Dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide, a tetrazole), assay was significantly decreased and cell apoptosis by Annexin V assay was significantly increased in claudin-7 transfected cells compared to that of vector transfected cells after cisplatin treatment. Cisplatin is an anti-cancer drug clinically used to treat tumors in several tissues including lung tumors. Furthermore, we found that claudin-7 transfected cells exhibited upregulation of caspase-3, -8, and -9. Also, the phosphorylation of claudin-7 in the H522 cells was examined, and analyses revealed that claudin-7 was phosphorylated at serine 204 by protein kinase C. Comparison of phosphorylated and non- phosphorylated claudin-7 in the H522 cells showed decreased cell viability by the MTT assay suggesting that phosphorylation increases chemosensitivity to cisplatin treatment. We demonstrated that claudin-7 expression in H522 lung cancer cells increases chemosensitivity to cisplatin through increased activation of caspase pathways.