Fibrillar collagen I matrix remodelling in idiopathic pulmonary fibrosis: Are lysyl oxidases responsible?

Rationale: The development of fibrosis in Idiopathic Pulmonary Fibrosis (IPF) is a key feature and challenge in the treatment of the disease. The mechanisms of collagen I (COL1) reorganisation in the development of fibrosis, which may alter the stiffness of the tissue, are not well understood. Fibrillar COL1 has second harmonic generation (SHG) properties. SHG signals are coherent, propagating both in the forward (F) (primarily organized/mature collagen) and backward (B) (primarily disorganized/immature collagen) directions relative to the incident light. Collagen fibre maturity is governed by the degree of cross-linking and catalysed by enzymes from the Lysyl Oxidase family (LOX, LOXL1-4), which are implicated IPF pathology. The aim of this study was to investigate collagen I structural remodelling and the pathological role of Lysyl Oxidases in the development of fibrosis in IPF. Methods: Formalin fixed and paraffin embedded parenchymal tissues from non-diseased donors (n=8) and IPF explanted lungs (n=8) were analysed for SHG. F/B SHG signal ratio was used to determine the proportion of organized to disorganized collagen. Immunohistochemistry detected LOX and LOXL1 levels in tissues with single-blinded analysis to quantify colorimetric staining by computer-assisted image-analysis. Results: Increased F/B ratio was observed in IPF compared to non-diseased tissues (p=0.034) indicating increased COL1 fibre maturity in IPF. There was decreased LOX (p=0.009) and increased LOXL1 (p=0.031) expression in IPF. In lung tissues LOX negatively correlated with F/B ratio (p=0.0491, r2=0.249, n=16), while LOXL1 positively correlated with F/B ratio (p=0.003, r2=0.4898, n=16). Conclusion: Increased levels of COL1 fibre maturity in IPF could contribute to the increased stiffness of the tissue. LOXL1, but not LOX, may regulate the increased collagen fibre maturity in IPF tissues. These findings indicate there may be a specific role for LOX family members in the development of fibrosis in IPF