ER stress is involved in alveolar epithelial cell dysfunction in hypoxia

Introduction: Markers of hypoxia (HIF-1α) and Endoplasmic Reticulum (ER) stress are present in alveolar epithelial cells (AEC) of patients with Idiopathic Pulmonary Fibrosis (IPF). However, the origin and specific contribution of ER stress remain unclear in this disease. Aims: To investigate the link between hypoxic and ER stress pathways in the context of alveolar cell dysfunction. Methods: Impact of hypoxia on cell dysfunction was evaluated in a rat hypoxic model. ER stress modulators were used to understand the relationship between ER stress, apoptosis, and epithelial-mesenchymal transition (EMT) induction in primary rat AEC exposed to hypoxia. The role of HIF-1α on the unfolded protein response pathways and Chop promoter activation was documented through the overexpression of HIF-1α in AEC. Then, crosstalk between the hypoxic and ER stress pathways was analysed by co-immunoprecipitation on hypoxic-AEC, or by proximity ligation assay in pathologic lungs. Results:In vivo, 24h-hypoxia exposure induced ER stress and apoptosis markers specifically in AEC. Loss of epithelial phenotype was observed at 48h. In vitro, ER stress inhibitors limited apoptosis and EMT induction by hypoxia whereas calcium chelation limits only hypoxia-induced EMT. Overexpression of HIF-1α was able to induce ATF4, ATF6α-XBP1s transactivation activity and to enhance Chop promoter activity. Finally, the co-expression of CHOP and HIF-1α was observed in mice with bleomycin-induced pulmonary fibrosis and in IPF lungs. Conclusions: Our results suggest that hypoxic microenvironment and expression of HIF-1α could trigger ER stress in AEC, which may promote the development of the fibrosing process.