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The apical membrane protein podocalyxin acts as an anti-adhesin in epithelial ovarian carcinoma Cipollone, Jane Alexandra

Abstract

Podocalyxin is a heavily sialylated and sulfated integral membrane protein with anti-adhesive properties that contributes to apical domain formation in normal, polarized epithelial cells. Immunohistochemical analysis of an outcome-linked ovarian tumour microarray (TMA) revealed that podocalyxin was expressed in the majority of lesions but showed no significant correlation with poor outcome. Podocalyxin expression was most prevalent in serous tumours where it was often apically localized in low-grade well-differentiated tumours. In the normal ovary, podocalyxin expression was moderate and localized to the free peritoneal surface of normal ovarian surface epithelia (OSE), serving to distinguish the OSE from its underlying stroma. Interestingly, podocalyxin was apically localized in dysplastic, columnar OSE suggesting that it may serve as a marker of polarity early in the carcinogenic process. After characterizing podocalyxin expression in a variety of ovarian carcinoma cell lines, OVCAR-3, a low podocalyxin expressing cell line, was forced to overexpress the mouse podocalyxin cDNA. Forced overexpression of podocalyxin did not disrupt cellcell junctions in polarized monolayers. However, it did disrupt cohesive cell-cell aggregation in suspension. In addition, podocalyxin overexpression in depolarized monolayers disrupted celhsubtratum adhesion. Podocalyxin overexpression caused a marked decrease in cell adhesion to fibronectin, mesothelial cells and to anti-β1 integrin antibody-coated wells, suggesting that high levels of podocalyxin may interfere with integrin engagement. To investigate the consequences of podocalyxin overexpression on β1 integrin localization in polarized monolayers and 3D cell clusters, dual immunofluorescence staining was performed for podocalyxin/β1 integrin. Podocalxyin overexpressing monolayers targetted podocalyxin to the apical membrane, resulting in a subtle depletion of β1 integrin from this site. In non-polar 3D cultures, podocalyxin overexpressing cell clusters mislocalized podocalxyin to the cell-matrix interface causing β1 integrin depletion from this membrane domain. Therefore, as a cell-ECM disrupting anti-adhesin, mislocalized podocalyxin may promote the dissemination of primary tumour nodules into the peritoneal cavity by interfering with integrin engagement at the tumounECM interface.

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