Pharmacologic inhibition of IRE1α-dependent decay protects alveolar epithelial identity and prevents pulmonary fibrosis in mice.

Stress-induced epithelial plasticity is central to lung regeneration, fibrosis, and malignancy, but how cellular stress leads to differentiation is incompletely understood. Here, we found a central role for IRE1α, a conserved mediator of the unfolded protein response (UPR), in stimulating the plasticity of alveolar type 2 (AT2) cells. In single-cell RNA-seq, IRE1α activity was associated with loss of AT2 identity and progression toward a damage-associated transitional state unique to fibrosis. AT2 plasticity required destructive regulated IRE1α-dependent decay (RIDD), which we demonstrated by deploying PAIR2, a kinase modulator that inhibits RIDD while preserving IRE1α's adaptive XBP1 mRNA splicing activity. In vivo, selective inhibition of RIDD with PAIR2 reduced AT2 differentiation into profibrotic transitional cells and protected mice from bleomycin-induced pulmonary fibrosis. Mechanistically, we identified the Fgfr2 mRNA as a direct and regulated substrate for IRE1α's RNase in primary AT2 cells and in a biochemically reconstituted cell-free system. Loss of Fgf signaling caused AT2 differentiation, while gain of signaling protected cells from IRE1α-induced differentiation. We propose that IRE1α downregulates Fgf signaling through RIDD, provoking loss of AT2 identity and differentiation towards a profibrotic phenotype. Thus, IRE1α's RIDD activity emerges as a novel target for treatment of pulmonary fibrosis and potentially other diseases driven by aberrant epithelial cell plasticity.