Contrasting responses of control and fibrotic lung fibroblasts to fibrotic stimuli: the role of osteoprotegerin in extracellular matrix remodeling

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal lung disease characterized by dysregulated wound healing, leading to excessive production of extracellular matrix (ECM) proteins, particularly collagen, by activated fibroblasts. Osteoprotegerin (OPG), a soluble protein associated with ECM remodeling in bone, has been found in higher levels in serum and lung tissue of IPF patients compared to controls. However, whether OPG is associated with ECM remodeling by fibroblasts in lung fibrosis remains poorly understood. In this study we assessed differences in OPG levels in ECM-derived hydrogels from non-disease controls and patients with IPF encapsulated with primary human lung fibroblasts from control and patients with IPF (7 days). OPG levels were assessed in a collected culture medium and stained lung tissue sections, and related to biomechanical properties. Fibrotic human lung fibroblasts secreted and deposited more OPG compared to controls, which was observed in both non-diseased and IPF ECM hydrogels. OPG levels correlated significantly with matrix stiffness and collagen fiber structural properties, but only in IPF hydrogels, independent of fibroblast source. Strikingly, we also observed opposite correlations for control and fibrotic fibroblasts in fibrotic hydrogels. Control fibroblasts within a fibrotic microenvironment, secreted less OPG when hydrogel stiffness increased and deposited more OPG when collagen fibers were more aligned. Interestingly, fibrotic fibroblasts secreted more OPG when collagen fiber length increased and when collagen fiber curvature increased. Overall, our findings suggest that control and fibrotic lung fibroblasts exhibited opposite behavior with respect to OPG and ECM remodeling under fibrotic conditions and this may have consequences for matrix stiffness