Inflammation of human bronchial epithelia (HBE) activates the endoplasmic reticulum (ER) stress transducer inositolrequiring enzyme 1 (IRE1)α, resulting in IRE1α-mediated cytokine production. Previous studies demonstrated ubiquitous expression of IRE1α and gut-restricted expression of IRE1β.We found that IRE1β is also expressed in HBE, is absent in human alveolar cells, and is upregulated in cystic fibrosis and asthmatic HBE. Studies with Ire1β−/− mice and Calu-3 airway epithelia exhibiting IRE1β knockdown or overexpression revealed that IRE1β is expressed in airway mucous cells, is functionally required for airway mucin production, and this function is specific for IRE1β vs. IRE1α. IRE1β-dependent mucin production is mediated, at least in part, by activation of the transcription factor X-box binding protein-1 (XBP-1) and the resulting XBP-1-dependent transcription of anterior gradient homolog 2, a gene implicated in airway and intestinal epithelial mucin production. These novel findings suggest that IRE1β is a potential mucous cell-specific therapeutic target for airway diseases characterized by mucin overproduction.

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Rationale: The goal was to connect elements of idiopathic pulmonary fibrosis (IPF) pathogenesis, including chronic endoplasmic reticulum stress in respiratory epithelia associated with injury/inflammation and remodeling, distal airway mucus obstruction and honeycomb cyst formation with accumulation of MUC5B (mucin 5B), and associations between IPF risk and polymorphisms in the MUC5B promoter. Objectives: To test whether the endoplasmic reticulum (ER) stress sensor protein ERN2 (ER-to-nucleus signaling 2) and its downstream effector, the spliced form of XBP1S (X-box-binding protein 1), regulate MUC5B expression and differentially activate the MUC5B promoter variant in respiratory epithelia. Methods: Primary human airway epithelial (HAE) cells, transgenic mouse models, human IPF lung tissues, and cell lines expressing XBP1S and MUC5B promoters were used to explore relationships between the ERN2/XBP1S pathway and MUC5B. An inhibitor of the pathway, KIRA6, and XBP1 CRISPR-Cas9 were used in HAE cells to explore therapeutic potential. Measurements and Main Results: ERN2 regulated MUC5B and MUC5AC mRNAs. Downstream XBP1S selectively promoted MUC5B expression in vitro and in distal murine airway epithelia in vivo. XBP1S bound to the proximal region of the MUC5B promoter and differentially upregulated MUC5B expression in the context of the MUC5B promoter rs35705950 variant. High levels of ERN2 and XBP1S were associated with excessive MUC5B mRNAs in distal airways of human IPF lungs. Cytokine-induced MUC5B expression in HAE cells was inhibited by KIRA6 and XBP1 CRISPR-Cas9. Conclusions: A positive feedback bistable ERN2-XBP1S pathway regulates MUC5B-dominated mucus obstruction in IPF, providing an unfolded protein response-dependent mechanism linking the MUC5B promoter rs35705950 polymorphism with IPF pathogenesis. Inhibiting ERN2dependent pathways/elements may provide a therapeutic option for IPF.