TGFβ Induces Epithelial-Mesenchymal Transition of Thyroid Cancer Cells by Both the BRAF/MEK/ERK and Src/FAK Pathways

The epithelial-mesenchymal transition (EMT) is a crucial process in tumour progression, by which epithelial cells acquire a mesenchymal phenotype, increasing its motility and the ability to invade distant sites. Here, we describe the molecular mechanisms by which (V600E)BRAF, TGF beta and the Src/FAK complex cooperatively regulate EMT induction and cell motility of anaplastic thyroid cancer cells. Analysis of EMT marker levels reveals a positive correlation between TGF beta and Snail expression, with a concomitant downregulation of E-cadherin, accompanied by an increase of cell migration and invasion. Furthermore, we show that (V600E)BRAF depletion by siRNA or inhibition of its activity by treatment with its inhibitor PLX4720 reverses the TGF beta-mediated effects on Snail, E-cadherin, migration and invasion. Moreover, (V600E)BRAF induces TGF beta secretion through a MEK/ERK-dependent mechanism. In addition, TGF beta activates the Src/FAK complex, which in turn regulates the expression of Snail and E-cadherin as well as cell migration. The inhibition of Src with the inhibitor SU6656 or abrogation of FAK expression with a specific siRNA reverses the TGF beta-induced effects. Interestingly, we demonstrate that activation of the Src/FAK complex by TGF beta is independent of (V600E)BRAF signalling, since inhibition of this oncogene does not affect its phosphorylation. Our data strongly suggest that TGF beta induces EMT and aggressiveness of thyroid cancer cells by parallel mechanisms involving both the (V600E)BRAF/MEK/ERK and Src/FAK pathways independently. Thus, we describe novel functions for Src/FAK in mediating the EMT program and aggressiveness regulated by TGF beta, establishing the inhibition of these proteins as a possible effective approach in preventing tumour progression of (V600E)BRAF-expressing thyroid tumours. (C) 2015 Wiley Periodicals, Inc.