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Oncogenic Raf-1 disrupts epithelial tight junctions via downregulation of occludin
1Department of Pharmaceutical Research and Development, Genentech Inc., South San Francisco, California 94080, USA.
Abstract:
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.
Insights
Occludin, a tight junction protein, regulates epithelial cell growth and prevents oncogenesis. Restoring occludin in cancer cells reverses tumor-like changes and restores cell adhesion.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Occludin is an integral membrane protein crucial for epithelial cell tight junctions (TJs).
- Its role in TJ formation and epithelial cell phenotype regulation is under investigation.
- Oncogenic transformations often involve disruptions in cell-cell adhesion and TJ integrity.
Purpose of the Study:
- To investigate the role of occludin in regulating epithelial cell phenotype during oncogenesis.
- To determine if occludin can reverse oncogene-induced changes in tight junctions and cell behavior.
- To explore occludin's function as a potential tumor suppressor in epithelial cells.
Main Methods:
- Utilized a rat salivary gland epithelial cell line (Pa-4) as a model system.
- Introduced oncogenic Raf-1 to induce transformation and analyzed TJ protein expression and localization.
- Transfected Raf-1-activated cells with the human occludin gene to assess functional recovery.
Main Results:
- Oncogenic Raf-1 expression disrupted TJ function, downregulated occludin and claudin-1, and altered ZO-1 and E-cadherin distribution.
- Introduction of occludin restored monolayer phenotype, TJ integrity, and cell-cell adhesion.
- Exogenous occludin normalized claudin-1 levels, ZO-1 localization, E-cadherin distribution, and inhibited anchorage-independent growth.
Conclusions:
- Occludin is a critical regulator of TJ structure and function, controlling phenotypic changes during oncogenesis.
- Occludin acts as a pivotal signaling molecule in Raf-1-induced TJ disruption and epithelial cell transformation.
- Occludin exhibits tumor-suppressive properties by inhibiting oncogenic transformation and restoring epithelial characteristics.