In the early steps of metastasis, carcinoma cells undergo through a dedifferentiation process called ‘epithelial-mesenchymal transition’ (EMT). Among all phenotypic changes associated with this transition, the reduction or loss of keratin expression is often considered as a histological and biochemical feature for tumor cells that are going through an EMT. Keratins are epithelial-specific intermediate filament proteins which are expressed in a tissue-specific manner. Keratins 8 and 18 (K8/18) are typically co-expressed as the primary keratin pair in simple epithelial cells and their expression is maintained during malignant transformation until the tumor become invasive. As part of the cytoskeleton, keratins are important for the integrity of epithelial cells and tissues. Moreover, a number of keratins are involved in intracellular signaling pathways which regulates cell growth and death. In recent years, studies provided evidence that keratins should not be considered only as markers but also as regulators of cancer cell signaling.In the present study, we investigated whether K8/18 expression play an active role in EMT. We used RNA interference approach targeted against K8/18 to mimic keratin loss during EMT in two different epithelial cancer cell lines. Using in vitro wound healing and matrigel invasion assays, we observed that K8/18 stable knockdown increases collective migration and invasiveness of epithelial cancer cells without modulating EMT markers. Interestingly, we identified the tight junction protein claudin1 as a regulator of these processes. Indeed, claudin1 is highly increased and mislocalized in K8/18-depleted cells and it promotes membrane targeting and activation of Akt1 and Akt3. Claudin1 also induces NFkB nuclear translocation in a PI3K dependent manner. Therefore, NFkB transcriptional activity is increased in K8/18-knockdown cells as shown by luciferase reporter assay, leading to MMP2 and MMP9 expression. This led to our proposal of a model in which K8/18 loss promotes collective migration of epithelial cancer cell in a different way than an EMT, by increasing directional migration and intercellular cohesion. At the leading edge of the migrating cell sheet, K8/18 loss improves PI3K/Akt activation involved in the direction of movement and local matrix degradation through NFkB activity. In the following rows, K8/18 knockdown increases claudin1 at the cell membrane which strengthens cell cohesion.To our knowledge, these results represent the first indication that K8/18 can influence the phenotype of epithelial cancer cells at a transcriptional level and strongly indicate that in cancer, keratins should not be considered only as markers of differentiation but also as key regulators in cancer progression.
Citation Format: Anne-Marie Fortier, Monique Cadrin, Eric Asselin. Claudin1 induces cell motility and invasion through PI3K/Akt/NFkB signaling in keratin-depleted carcinoma cell lines. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5160. doi:10.1158/1538-7445.AM2013-5160