XBP1S Regulates MUC5B in a Promoter Variant-Dependent Pathway in Idiopathic Pulmonary Fibrosis Airway Epithelia.

Abstract

<b>Rationale:</b> 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 <i>MUC5B</i> promoter. <b>Objectives:</b> 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 <i>MUC5B</i> promoter variant in respiratory epithelia. <b>Methods:</b> Primary human airway epithelial (HAE) cells, transgenic mouse models, human IPF lung tissues, and cell lines expressing XBP1S and <i>MUC5B</i> promoters were used to explore relationships between the ERN2/XBP1S pathway and MUC5B. An inhibitor of the pathway, KIRA6, and <i>XBP1</i> CRISPR-Cas9 were used in HAE cells to explore therapeutic potential. <b>Measurements and Main Results:</b> ERN2 regulated <i>MUC5B</i> and <i>MUC5AC</i> mRNAs. Downstream XBP1S selectively promoted MUC5B expression <i>in vitro</i> and in distal murine airway epithelia <i>in vivo</i>. XBP1S bound to the proximal region of the <i>MUC5B</i> promoter and differentially upregulated MUC5B expression in the context of the <i>MUC5B</i> promoter rs35705950 variant. High levels of <i>ERN2</i> and <i>XBP1S</i> were associated with excessive <i>MUC5B</i> mRNAs in distal airways of human IPF lungs. Cytokine-induced <i>MUC5B</i> expression in HAE cells was inhibited by KIRA6 and <i>XBP1</i> CRISPR-Cas9. <b>Conclusions:</b> A positive feedback bistable ERN2-XBP1S pathway regulates MUC5B-dominated mucus obstruction in IPF, providing an unfolded protein response-dependent mechanism linking the <i>MUC5B</i> promoter rs35705950 polymorphism with IPF pathogenesis. Inhibiting ERN2-dependent pathways/elements may provide a therapeutic option for IPF.