Extracellular Matrix Cross-Linking Enhances Fibroblast Growth and Protects against Matrix Proteolysis in Lung Fibrosis. 
Idiopathic pulmonary fibrosis (IPF) is characterized by accumulation of extracellular matrix (ECM) proteins and fibroblast proliferation. ECM cross-linking enzymes have been implicated in fibrotic diseases, and we hypothesized that the ECM in IPF is abnormally cross-linked, which enhances fibroblast growth and resistance to normal ECM turnover. We used a combination of in vitro ECM preparations and in vivo assays to examine the expression of cross-linking enzymes and the effect of their inhibitors on fibroblast growth and ECM turnover. Lysyl oxidase-like 1 (LOXL1), LOXL2, LOXL3, and LOXL4 were expressed equally in control and IPF-derived fibroblasts. Transglutaminase 2 was  more strongly expressed in IPF fibroblasts. LOXL2-, transglutaminase 2-, and transglutaminase-generated cross-links were strongly expressed in IPF lung tissue. Fibroblasts grown on IPF ECM had higher LOXL3 protein expression and transglutaminase activity than those grown on control ECM. IPF-derived ECM also enhanced fibroblast adhesion and proliferation compared with control ECM. Inhibition of lysyl oxidase and transglutaminase activity during ECM formation affected ECM structure as visualized by electron microscopy, and it reduced the enhanced fibroblast adhesion and proliferation of IPF ECM to control levels. Inhibition of transglutaminase, but not of lysyl oxidase, activity enhanced the turnover of ECM in vitro. In bleomycin-treated mice, during the postinflammatory  fibrotic phase, inhibition of transglutaminases was associated with a reduction in whole-lung collagen. Our findings suggest that the ECM in IPF may enhance pathological cross-linking, which contributes to increased fibroblast growth and  resistance to normal ECM turnover to drive lung fibrosis.