A new pathway activated by the unfolded protein response: Involvement of the TGF-beta/Smad pathway in the dedifferentiating effect of ER stress in thyroid cells

Conditions perturbing the homeostasis of the endoplasmic reticulum (ER) cause accumulation of unfolded proteins and trigger ER stress. In PC Cl3 thyroid cells, thapsigargin and tunicamycin interfered with the folding of thyroglobulin causing accumulation of this very large secretory glycoprotein in the ER. Consequently, BiP and XBP-1 mRNAs were induced and spliced, respectively. In the absence of apoptosis, differentiation of PC Cl3 cells was inhibited. mRNA and protein levels of thyroid specific genes, thyroglobulin, thyroperoxidase and sodium/iodide symporter, and of thyroid transcription factors, TTF-1, TTF-2 and Pax-8, were dramatically downregulated. These effects were, at least in part, transcriptional. Moreover, they were selective and temporally distinct from the general and transient PERK-dependent translational inhibition. Thyroid dedifferentiation was accompanied by changes in the organization of polarized epithelial monolayer. E-cadherin mRNA downregulation, vimentin, α-smooth muscle actin, α(1)(I) collagen, and SNAI1/SIP1 mRNAs up-regulation, actin stress fibers formation, and loss of transepithelial resistance were found, confirming an epithelial-mesenchymal transition (EMT). The thyroid-specific and epithelial dedifferentiation by thapsigargin/tunicamycin were completely prevented by the Src-family kinases inhibitor PP2 and by stable expression of a dominant negative Src. Together, these data indicate that ER stress induces dedifferentiation and an EMT-like phenotype in thyroid cells via a Src-mediated signaling pathway.