ER Stress is Involved in Epithelial-To-Mesenchymal Transition of Alveolar Epithelial Cells Exposed to a Hypoxic Microenvironment.

Eva Delbrel,Yurdagül Uzunhan,Dominique Marchant,Emilie Boncoeur,Abdoulaye Soumare,Carole Planès,Thomas Gille

doi:10.3390/ijms20061299

Abstract

Background: Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal interstitial lung disease of unknown origin. Alveolar epithelial cells (AECs) play an important role in the fibrotic process as they undergo sustained endoplasmic reticulum (ER) stress, and may acquire a mesenchymal phenotype through epithelial-to-mesenchymal transition (EMT), two phenomena that could be induced by localized alveolar hypoxia. Here we investigated the potential links between hypoxia, ER stress and EMT in AECs. Methods: ER stress and EMT markers were assessed by immunohistochemistry, western blot and qPCR analysis, both in vivo in rat lungs exposed to normoxia or hypoxia (equivalent to 8% O2) for 48 h, and in vitro in primary rat AECs exposed to normoxia or hypoxia (1.5% O2) for 2–6 days. Results: Hypoxia induced expression of mesenchymal markers, pro-EMT transcription factors, and the activation of ER stress markers both in vivo in rat lungs, and in vitro in AECs. In vitro, pharmacological inhibition of ER stress by 4-PBA limited hypoxia-induced EMT. Calcium chelation or hypoxia-inducible factor (HIF) inhibition also prevented EMT induction under hypoxic condition. Conclusions: Hypoxia and intracellular calcium are both involved in EMT induction of AECs, mainly through the activation of ER stress and HIF signaling pathways.

Highlights

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal interstitial lung disease of unknown origin
Our results provide evidence that endoplasmic reticulum (ER) stress and hypoxia-inducible factor (HIF)-1 signaling pathways are both involved in the induction of epithelial-to-mesenchymal transition (EMT) of Alveolar epithelial cells (AECs) exposed to hypoxia, and suggest that any disturbance of intracellular calcium homeostasis plays a crucial role in this process
As ER stress inhibition or calcium chelation prevented the effect of hypoxia on zonula occludens-1 (ZO-1) and transcription factor-1 (TTF1) expression, we investigated the impact of unfolded protein response (UPR) pathways inhibition or calcium chelation on hypoxia-inducible factor 1α (HIF-1α) stabilization and HIF capacity to transactivate the hypoxia responsive elements (HRE) upstream the luciferase gene in response to hypoxia (Figure 5B,C)

Summary

Introduction

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal interstitial lung disease of unknown origin. In IPF lungs, some alveolar epithelial cells (AECs) become hyperplastic with abnormal activation and production of profibrotic factors such as TGF-β1, while other AECs undergo massive apoptosis. AECs may even acquire a mesenchymal phenotype through a process of epithelial-to-mesenchymal transition (EMT) [2,3,4]. Alveolar epithelial cells (AECs) play an important role in the fibrotic process as they undergo sustained endoplasmic reticulum (ER) stress, and may acquire a mesenchymal phenotype through epithelial-to-mesenchymal transition (EMT), two phenomena that could be induced by localized alveolar hypoxia. Calcium chelation or hypoxia-inducible factor (HIF) inhibition prevented EMT induction under hypoxic condition. Conclusions: Hypoxia and intracellular calcium are both involved in EMT induction of AECs, mainly through the activation of ER stress and HIF signaling pathways

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