The Pervasive Effects of ER Stress on a Typical Endocrine Cell: Dedifferentiation, Mesenchymal Shift and Antioxidant Response in the Thyrocyte.

The endoplasmic reticulum stress and the unfolded protein response are triggered following an imbalance between protein load and protein folding. Until recently, two possible outcomes of the unfolded protein response have been considered: life or death. We sought to substantiate a third alternative, dedifferentiation, mesenchymal shift, and activation of the antioxidant response by using typical endocrine cells, i.e. thyroid cells. The thyroid is a unique system both of endoplasmic reticulum stress (a single protein, thyroglobulin represents the majority of proteins synthesized in the endoplasmic reticulum by the thyrocyte) and of polarized epithelium (the single layer of thyrocytes delimiting the follicle). Following endoplasmic reticulum stress, in thyroid cells the folding of thyroglobulin was disrupted. The mRNAs of unfolded protein response were induced or spliced (X-box binding protein-1). Differentiation was inhibited: mRNA levels of thyroid specific genes, and of thyroid transcription factors were dramatically downregulated, at least in part, transcriptionally. The dedifferentiating response was accompanied by an upregulation of mRNAs of antioxidant genes. Moreover, cadherin-1, and the thyroid (and kidney)-specific cadherin-16 mRNAs were downregulated, vimentin, and SNAI1 mRNAs were upregulated. In addition, loss of cortical actin and stress fibers formation were observed. Together, these data indicate that ER stress in thyroid cells induces dedifferentiation, loss of epithelial organization, shift towards a mesenchymal phenotype, and activation of the antioxidant response, highlighting, at the same time, a new and wide strategy to achieve survival following ER stress, and, as a sort of the other side of the coin, a possible new molecular mechanism of decline/loss of function leading to a deficit of thyroid hormones formation.

ER stress is associated with dedifferentiation and an epithelial-to-mesenchymal transition-like phenotype in PC Cl3 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, mRNAs encoding BiP and XBP-1 were induced and spliced, respectively. In the absence of apoptosis, differentiation of PC Cl3 cells was inhibited. mRNA and protein levels of the thyroid-specific genes encoding thyroglobulin, thyroperoxidase and the sodium/iodide symporter and of the genes encoding the 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 the polarized epithelial monolayer. Downregulation of the mRNA encoding E-cadherin, and upregulation of the mRNAs encoding vimentin, alpha-smooth muscle actin, alpha(1)(I) collagen and SNAI1/SIP1, together with formation of actin stress fibers and loss of trans-epithelial resistance were found, confirming an epithelial-mesenchymal transition (EMT). The thyroid-specific and epithelial dedifferentiation by thapsigargin or tunicamycin were completely prevented by the PP2 inhibitor of Src-family kinases 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 through a Src-mediated signaling pathway.