Claudin-7 increases chemosensitivity to cisplatin through the upregulation of caspase pathway in human NCI-H522 lung cancer cells

Claudins are a family of tight junction (TJ) integral membrane proteins that play a crucial role in maintaining cell polarity, adhesion, and paracellular permeability. Changes in expression levels of claudin proteins have been associated with human lung cancer. Previously, we have reported that claudin-7 expression is significantly downregulated in human lung carcinomas. To investigate the role of claudin-7 in lung cancer cells after anti-cancer drug treatments, we transfected claudin-7 cDNA into human NCI-H522 lung cancer cells, which have no detectable expression of claudin-7 protein. 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 assayed by MTT and Annexin V was significantly decreased and cell apoptosis was dramatically 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. Most importantly, after cisplatin treatment, the expression levels of cleaved caspase-3, -8, and poly adenosine 5-diphosphate ribose polymerase (PARP) were much higher in claudin-7 transfected cells than in control cells. Furthermore, using the site-directed mutagenesis approach, we identified that claudin-7 was phosphorylated at serine 204 by protein kinase C. Non-phosphorylated claudin-7 mutant showed increased cell viability, suggesting that phosphorylation increases chemosensitivity to cisplatin treatment. We concluded that claudin-7 expression in H522 lung cancer cells increases chemosensitivity to cisplatin through the increased activation of caspase pathway.