Osteoprotegerin and RANKL differentially regulate angiogenesis and endothelial cell function

Abstract

Osteoprotegerin (OPG) a soluble tumor necrosis factor receptor family molecule protects endothelial cells from apoptosis in vitro and promotes neovascularization in vivo. In this study, we assessed the role of OPG and its ligands, receptor activator of nuclear factor-kappaB ligand (RANKL) and tumor necrosis factor-related apoptosis inducing ligand (TRAIL), in microvessel formation using the rat aortic ring model of angiogenesis. OPG was found to promote a twofold increase in angiogenic sprouting in the aortic ring model, and this effect was inhibited by pre-incubation with a fivefold molar excess of either RANKL or TRAIL. While TRAIL had no effect upon angiogenesis on its own, RANKL was found to potently inhibit basal and vascular endothelial growth factor-induced angiogenesis. OPG increased the rate of endothelial cell proliferation in sprouting microvessels; in contrast, RANKL inhibited proliferation. RANKL was found to induce endothelial apoptosis at days 6, 7, and 10 in the aortic ring model and after incubation with human umbilical vein endothelial cells (HUVECs). Signaling studies showed that OPG induced ERK1/2 and Akt phosphorylation in HUVECs while RANKL had no effect. Our results indicate that OPG is a positive regulator of microvessel formation, while RANKL is an angiogenic inhibitor due to effects on regulation of endothelial cell proliferation, apoptosis, and signaling.

Osteoclasts are important for bone angiogenesis

“…[32][33][34] Further, OPG can stimulate angiogenesis or endothelial cell survival in cell culture or aortic ring explants. 35,36 In agreement with these findings, when equivalent concentrations of OPG to those used to inhibit metatarsal angiogenesis were tested in the HUVEC/fibroblast coculture angiogenesis assay, which lacks osteoclasts, we observed a slight stimulation rather than any inhibition of angiogenesis in the HUVEC cultures (data not shown). It is unclear whether PTHrP can directly inhibit or stimulate endothelial cells.…”

Osteoprotegerin: Multiple partners for multiple functions

“…As mentioned above, OPG exerts an antiapoptotic effect towards TRAIL and decreases in consequence the number of apoptotic bodies that may serve as nucleation sites for passive mineralization (Fleishman et al 1990;Benslimane-Ahmim et al 2011). Several studies have shown also that recombinant OPG can promote the survival/proliferation of mature vascular endothelial cells (Malyankar et al 2000;Pritzker et al 2004;Cross et al 2006;Kobayashi-Sakamoto et al 2006;McGonigle et al 2009). Similarly, Lawrie et al showed OPG promotes the proliferation and the migration of pulmonary artery smooth muscle cell, and thus evidenced an important role of OPG in the pathogenesis of pulmonary arterial hypertension (Lawrie et al 2008).…”

Bone metastases in gastric cancer follow a RANKL-independent mechanism

“…Previous data reported an inhibitory role of RANKL in angiogenesis (14,15), which could explain its lower serum levels in patients compared to controls. To evaluate angiogenesis, we administered serum VEGF, which is a key angiogenic factor mediating neo-vascularization and could act as a surrogate marker of tumor angiogenesis (40).…”

“…In addition to the known role of RANKL in the skeletal and immune systems (13), recent studies also reported a role of RANKL in angiogenesis. Certain authors support an inhibitory role of RANKL (14,15), whereas others showed a stimulating action on angiogenesis (16). Contrary to RANKL, VEGF is known as a potent mitogen angiogenic factor and is one of the most important molecules involved in the vascularization of bone tissue (17), thus its serum levels are considered an index of angiogenesis.…”