Oncogenic Raf-1 Disrupts Epithelial Tight Junctions via Downregulation of Occludin

Occludin is an integral membrane protein of the epithelial cell tight junction (TJ). Its potential role in coordinating structural and functional events of TJ formation has been suggested recently. Using a rat salivary gland epithelial cell line (Pa-4) as a model system, we have demonstrated that occludin not only is a critical component of functional TJs but also controls the phenotypic changes associated with epithelium oncogenesis. Transfection of an oncogenic Raf-1 into Pa-4 cells resulted in a complete loss of TJ function and the acquisition of a stratified phenotype that lacked cell-cell contact growth control. The expression of occludin and claudin-1 was downregulated, and the distribution patterns of ZO-1 and E-cadherin were altered. Introduction of the human occludin gene into Raf-1-activated Pa-4 cells resulted in reacquisition of a monolayer phenotype and the formation of functionally intact TJs. In addition, the presence of exogenous occludin protein led to a recovery in claudin-1 protein level, relocation of the zonula occludens 1 protein (ZO-1) to the TJ, and redistribution of E-cadherin to the lateral membrane. Furthermore, the expression of occludin inhibited anchorage-independent growth of Raf-1-activated Pa-4 cells in soft agarose. Thus, occludin may act as a pivotal signaling molecule in oncogenic Raf- 1-induced disruption of TJs, and regulates phenotypic changes associated with epithelial cell transformation.