Tight Junction Proteins Claudin-3 and Claudin-4 Are Frequently Overexpressed in Ovarian Cancer but Not in Ovarian Cystadenomas

Purpose: Claudin proteins represent a large family of integral membrane proteins crucial for tight junction (TJ) formation and function. Claudins have been shown to be up-regulated in various cancers and have been suggested as possible biomarkers and targets for cancer therapy. Because claudin-3 and claudin-4 have been proposed to be expressed in epithelial ovarian cancer, we have performed a detailed analysis of CLDN3 and CLDN4 expression in a panel of ovarian tumors of various subtypes and cell lines. We also investigated whether high expression of claudin-3 and claudin-4 was associated with TJ function in ovarian cancer cells. Experimental Design: RNA was obtained from a panel of 39 microdissected epithelial ovarian tumors of various histological subtypes for real-time reverse transcription-PCR analysis. In addition, a total of 70 cases of ovarian carcinomas, ovarian cysts, and normal ovarian epithelium from a tissue array were analyzed by immunohistochemistry. Finally, a panel of cell lines was used for Western analysis of claudin expression and TJ permeability studies. Results: Although expressed at low levels in some normal human tissues, including the ovary, CLDN3 and CLDN4 are highly up-regulated in epithelial ovarian cancers of all subtypes. Immunohistochemical analyses using our ovarian tissue array confirmed the high level of expression of claudin-3 and claudin-4 in the majority of ovarian carcinomas, including many tumors exhibiting cytoplasmic staining. Ovarian cystadenoma did not frequently overexpress these proteins, suggesting that the expression of these proteins is associated with malignancy. In ovarian cancer cell lines, claudin-3 and claudin-4 expression was not associated with functional TJs as measured by transepithelial electrical resistance. Conclusions: These results show that CLDN3 and CLDN4 are frequently up-regulated in ovarian tumors and cell lines and may represent novel markers for this disease. Overexpression of these genes in ovarian cancer also suggests interesting scenarios for the involvement of TJ in tumorigenesis. A better knowledge of the mechanisms underlying ovarian tumorigenesis will likely result in the development of novel approaches for the diagnosis and therapy of this deadly disease.