Interaction of osteoprotegerin with fibulin-1 is essential for extracellular matrix deposition in fibrotic lung tissue

Idiopathic pulmonary fibrosis (IPF) is a progressive, fatal lung disease with an unclear cause and no cure. Osteoprotegerin (OPG), a well-known regulator of bone extracellular matrix (ECM), is increased in pulmonary fibrosis and this is associated with lower lung function and IPF progression. Fibulin-1, a key ECM protein, is increased in lung fibroblasts and lung tissues from patients with IPF and high levels are predictive of a poor prognosis for these patients. We found a positive correlation between levels of OPG and fibulin-1 in serum from patients with IPF, but it is unknown if these proteins interact. This study aimed at investigating interactions between OPG and fibulin-1 in lung fibrosis, hypothesizing that OPG may form a complex with fibulin-1 thereby possibly influencing lung fibrosis. We investigated intracellular interactions between OPG and fibulin-1 in human lung fibroblasts, showing neither OPG nor fibulin-1 expression were dependent on each other. However, OPG protein deposition was significantly decreased in lung tissues of fibulin-1 knockout mice (fibulin-1-/-)compared to controls suggesting extracellular interactions. Subsequently, we detected colocalization of both proteins in both IPF and control lung tissues using immunofluorescence, with both of them also colocalizing with latent TGF{beta} binding protein 1 (LTBP1). Proximity ligation analyses confirmed close proximity of OPG to fibulin-1 and fibulin-1 to LTBP1, particularly in the interstitial regions of both control and IPF lung tissue, but not OPG and LTBP1. In conclusion, we found that OPG, fibulin-1 and LTBP1 seem to form a complex, with fibulin-1 playing a bridging role between OPG and LTBP1, in the extracellular environment. This complex may play a regulatory role in deposition of extracellular matrix in lung tissue.