Oxidative damage and TGF-β differentially induce lung epithelial cell sonic hedgehog and tenascin-C expression: implications for the regulation of lung remodelling in idiopathic interstitial lung disease

TLDR

Idiopathic interstitial lung diseases are marked by inflammation, Type‑II alveolar epithelial hyperplasia, and progressive fibrotic remodeling, yet the initiating and sustaining signals remain undefined. The study aimed to determine whether hedgehog signaling is functional and whether tenascin‑C is expressed in activated Type‑II alveolar epithelial cells across UIP, NSIP, and COP. Using real‑time RT‑PCR, immunohistochemistry, and in‑vitro assays, the authors examined how oxidative damage and TGF‑β influence SHH and tenascin‑C production in a Type‑II AEC cell line. SHH signaling and tenascin‑C were present in all three iILDs, with SHH most active at fibroblastic foci in UIP; Type‑II AECs constitutively secreted SHH but not tenascin‑C, oxidative injury stimulated SHH release while TGF‑β inhibited it, and both stimuli upregulated tenascin‑C mRNA but only TGF‑β induced a distinct protein isoform.

Abstract

Idiopathic interstitial lung diseases (iILDs) are characterized by inflammation, hyperplasia of Type-II alveolar epithelial cells (AECs) and lung remodelling often with progressive fibrosis. It remains unclear which signals initiate iILD and/or maintain the disease processes. Using real-time RT-PCR and immunohistochemistry on archival biopsies of three patterns of iILD (usual interstitial pneumonitis/UIP, non-specific interstitial pneumonitis/NSIP and cryptogenic organizing pneumonia/COP) we investigated whether hedgehog signalling (previously associated with lung damage and repair) was functional and whether the damage associated extracellular matrix protein tenascin-C was present in activated Type-II AECs in all three iILDs. Using tissue culture, protein and mRNA detection we also determined how two signals (oxidative damage and TGF-β) associated with iILD pathogenesis affected Sonic hedgehog (SHH) and tenascin-C production by a Type-II AEC cell line. We report that SHH pathway and tenascin-C mRNA and proteins were found in UIP, NSIP and COP. SHH signalling was most active at sites of immature organizing fibrous tissue (fibroblastic foci) in UIP. In vitro Type-II AECs constitutively secrete SHH but not tenascin-C. Oxidative injury stimulated SHH release whereas TGF-β inhibited it. TGF-β and oxidative damage both upregulated tenascin-C mRNA but only TGF-β induced synthesis and release of a distinct protein isoform. SHH signalling is active in Type-II AECs from three types of ILD and all three express tenascin-C.