Abstract
Osteoprotegerin (tnfsf11b, OPG) is a soluble member of the TNF superfamily originally described as an important regulator of osteoclastogenesis almost 20 years ago. OPG is a heparin-binding secreted glycoprotein that exists as a 55–62 kDa monomer or a 110–120 kDa disulphide-linked homodimer. Acting as a soluble decoy receptor for RANKL, OPG actively regulates RANK signalling, and thereby osteoclastogenesis. OPG has subsequently been shown to also be a decoy receptor TNF related apoptosis inducing-ligand (tnfsf10, TRAIL, Apo2L). TRAIL is a type II transmembrane protein that is widely expressed in a variety of human tissues, including the spleen, lung, and prostate. Through binding to TRAIL, OPG can inhibit TRAIL-induced apoptosis of cancer cells. More recently, OPG has been demonstrated to be secreted by, and influence, vascular smooth muscle cells phenotype particularly related to vascular calcification and pulmonary vascular remodelling. In pulmonary artery smooth muscle cell (PASMC) suppression of BMP, induction of 5-HT and IL-1 signalling have been shown to stimulate the release of OPG in vitro, which causes cell migration and proliferation. Patients with idiopathic PAH (IPAH) demonstrate increased circulating and tissue levels of OPG, and circulating serum levels predict survival. In pre-clinical models, OPG levels correlate with disease severity. Since OPG is a naturally circulating protein, we are investigating the potential of novel biologic antibody therapies to rescue PAH phenotype in disease models. Further pre-clinical and mechanistic data are forthcoming, but we believe current published data identify OPG as an exciting and novel therapeutic target in PAH.
Highlights
Since its discovery nearly 20 years ago, there have been significant advances in our understanding of the role of osteoprotegerin in health and disease
Osteoclastogenesis requires binding of RANKL, a type 2 homotrimeric transmembrane protein expressed on mature osteoblasts, to its receptor, RANK, a type 1 homotrimeric transmembrane protein expressed on osteoclast precursor cells (Hofbauer & Schoppet, 2004; Boyce & Xing, 2007; Vitovski et al, 2007) (Fig. 1)
Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL, Apo2L) is a type II transmembrane protein that is widely expressed in a variety of human tissues, including the spleen, lung, and prostate
Summary
Since its discovery nearly 20 years ago, there have been significant advances in our understanding of the role of osteoprotegerin in health and disease. Osteoclastogenesis requires binding of RANKL, a type 2 homotrimeric transmembrane protein expressed on mature osteoblasts, to its receptor, RANK, a type 1 homotrimeric transmembrane protein expressed on osteoclast precursor cells (Hofbauer & Schoppet, 2004; Boyce & Xing, 2007; Vitovski et al, 2007) (Fig. 1). Work undertaken by Benslimane–Ahmim and colleagues has shown that OPG induces the migration and differentiation of endothelial colony-forming cells into cord-like structures, promotes fibroblast growth factor-2 (FGF2)-induced neo-angiogenesis in vivo, and increases endothelial colony-forming cell adhesion to fibronectin in vitro (Benslimane-Ahmim et al, 2013)
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