Reversal of Fibrosis in Chronic Rejection in Mouse Airways through Combined Relaxin and Lysyl Oxidase Inhibitor Therapy

Abstract

The leading cause of death in lung transplant recipients is the development of chronic rejection, signified by obliterative bronchiolitis, which is histologically characterized by a fibrotic process that leads to slow narrowing of the airways. The fibrosis occurs when activated fibroblasts deposit excess extracellular matrix (ECM), which compromises gas exchange. Accumulation of ECM proteins in chronic rejected allograft in lung transplant may increase the stiffness of tissue and result in further unbalance of protein turnover and fibrous obliteration of the airway. We hypothesized that relaxin, a hormone that helps pelvic and cervical expansion and relaxation during pregnancy, could ameliorate fibrosis in chronic rejection. Relaxin might prohibit fibroblast-to-myofibroblast differentiation and fibroblast contraction, thus inhibiting fibrosis. These effects can be amplified by combining with lysyl oxidase inhibitor, β-aminoproprilnitrile (BAPN), which functions by inhibiting the cross-linking of fiber-forming stiff collagen, which decreases the ECM stiffness. We test in vitro by transplanting tracheas from major histocompatibility complex-mismatched mice to C57BL/6 mice treated with recombinant relaxin combined with BAPN after chronic rejection. The combined treatment of relaxin and BAPN attenuates airway fibrosis, decreases cross-linked collagen deposition, and promotes re-epithelialization. Relaxin inhibits fibroblast-to-myofibroblast differentiation and procollagen deposition in fibroblasts plated on matrix with intermediate stiffness, but not high stiffness. Relaxin also decreases fibroblast contractility, evidenced by decreased myosin light chain protein phosphorylation, through up-regulating prostaglandin E2. The beneficial effects of combined treatment might result from the regulation of cell contractility by relaxin and the decreased ECM stiffness caused by inhibition of collagen cross-linking by BAPN.