# ER Stress Is Associated with a “Mesenchymal Drift” in an Anaplastic Thyroid Carcinoma Cell Line

## TL;DR

High ER stress in anaplastic thyroid cancer cells leads to increased malignancy through a mesenchymal drift, where cells become more invasive and lose epithelial traits.

## Contribution

The study reveals that ER stress promotes a mesenchymal drift in anaplastic thyroid cancer cells, enhancing their malignant properties.

## Key findings

- ER stress in anaplastic thyroid cancer cells leads to increased single-cell motility and invasion.
- Adapted cells show high expression of mesenchymal markers like vimentin and fibronectin.
- ER stress selects for a more malignant cell population with suppressed UPR and stress kinase activation.

## Abstract

A particular type of cellular stress that hits the endoplasmic reticulum (ER) may cause death or survival, depending on several factors, one of which is the intensity of the stress. However, data obtained by our group and others on normal and differentiated cells show that survival may be achieved at the expense of differentiation and, in thyroid epithelial cells, thyroid-specific proteins disappear, while mesenchymal markers appear in a variable number and extent, causing a “mesenchymal shift”. We hypothesize that these findings may be even more significant in a cancer context, where ER stress is linked to an exacerbation of the malignant properties of cancer cells, a process known as cancer progression. Here, we show that anaplastic thyroid cancer cells subjected to ER stress display an exacerbation of the malignant phenotype, with increased single-cell motility, invasion, and high-level mesenchymal protein expression in the moving component that has lost cell–cell contacts, features that may be defined as “mesenchymal drift”.

Background/Objectives: The tumor microenvironment (TME) plays a crucial role in the progression of the malignant phenotype through several mechanisms, such as hypoxia and nutrient deprivation, among others. These insults activate several intracellular pathways, and among them are ER stress and the unfolded protein response (UPR). Our aim was to assess if a specific ER stress inducer causes an exacerbation of the malignant phenotype of anaplastic thyroid carcinoma (ATC) cells. Methods: We used an ATC cell line, FRO cells, that had not undergone a full Epithelial–Mesenchymal Transition (EMT) and an ER stress-adapted cell line derived from FRO cells, A400 cells. Western blot, immunofluorescence, scratch, and invasion assays were used to evaluate the response of the FRO and A400 cells to ER stress. Results: The FRO cells were subjected to high-level ER stress caused by 400 ng/mL of tunicamycin (Tn). This caused the death of a large fraction of cells, but eventually a population emerged that we called A400 cells. Following an over challenge with Tn, the adapted population showed suppression of the UPR, apoptosis, and stress kinase activation. Moreover, the adapted population showed an exacerbation of mesenchymal features with a more invasive phenotype. At the level of a single cell, the adapted cells, caught in the act of moving, showed high-level expressions of vimentin (VIM), fibronectin (FN), and N-cadherin. Conclusions: High-level ER stress acts as a selection factor favoring the emergence of a cell population showing “mesenchymal drift” with a more malignant phenotype.