Blocking of the Ubiquitin-Proteasome System Prevents Inflammation-Induced Bone Loss by Accelerating M-CSF Receptor c-Fms Degradation in Osteoclast Differentiation.

Kyunghee Lee,Mi Kim,Han-Sung Kim,Daewon Jeong,Heejin Ahn,Hong-In Shin

doi:10.3390/ijms18102054

Abstract

Anti-osteoporotic activity of a blocker of the ubiquitin-proteasome system, bortezomib, has known to be achieved by directly opposed action in increased bone formation by osteoblasts and in decreased bone destruction by osteoclasts. However, the mechanisms underlying the proteasome blocker inhibition of osteoclast differentiation and function are not fully understood. Here, we observed that proteasome inhibitors, such as MG132 and bortezomib, in osteoclasts accelerated the degradation of c-Fms, a cognate receptor of macrophage colony-stimulating factor (M-CSF), and did not affect the amount of receptor activator of nuclear factor kappa-B (RANK), a receptor of receptor activator of nuclear factor kappa-B ligand (RANKL). c-Fms degradation induced by proteasome inhibitors was controlled by the activation of p38/tumor necrosis factor-alpha converting enzyme (TACE)-mediated regulated intramembrane proteolysis (RIPping). This was validated through the restoration of c-Fms using specific inhibitors of p38 and TACE, and a stimulation of p38-dependent TACE. In addition, c-Fms degradation by proteasome inhibition completely blocked M-CSF-mediated intrinsic signalling and led to the suppression of osteoclast differentiation and bone resorption. In a mouse model with intraperitoneal administration of lipopolysaccharide (LPS) that stimulates osteoclast formation and leads to bone loss, proteasome blockers prevented LPS-induced inflammatory bone resorption due to a decrease in the number of c-Fms-positive osteoclasts. Our study showed that accelerating c-Fms proteolysis by proteasome inhibitors may be a therapeutic option for inflammation-induced bone loss.

Highlights

Macrophage colony-stimulating factor (M-CSF) is a critical cytokine that regulates the survival, proliferation, and the differentiation of monocytes and macrophages [1]
To study the effect of proteasome inhibition on osteoclast differentiation, osteoclast progenitors were differentiated into osteoclasts by culture with M-CSF and receptor activator of nuclear factor kappa-B ligand (RANKL) in the presence or absence of proteasome inhibitors MG132 and bortezomib
Intraperitoneal injection of LPS as an inducer of inflammation in mice resulted in an osteoporotic phenotype characterized by a decreased bone mineral density (BMD), bone volume (BV/TV), connectivity density, and trabecular number, as well immunohistochemical studies showed significantly lower levels of c-Fms-positive osteoclasts on the trabecular bone surface of mice treated with MG132 when compared with those of control-treated mice (Figure 5B, lower panel). c-Fms immunoreactivity was increased in LPS-treated mice, but was significantly decreased in mice treated with both MG132 and LPS

Summary

Introduction

Macrophage colony-stimulating factor (M-CSF) is a critical cytokine that regulates the survival, proliferation, and the differentiation of monocytes and macrophages [1]. C-Fms degradation is known to result from lysosomal degradation or a regulated intramembrane proteolysis (RIPping) process [5,6,7,8]. Upon the binding of M-CSF, the c-Fms receptor undergoes internalization and degradation in the lysosome, attenuating or preventing signalling for survival and proliferation [1,9]. C-Fms undergoes a RIPping process through the activation of two consecutive cleavage events, namely ectodomain shedding by tumour necrosis factor-alpha converting enzyme (TACE) and intramembrane cleavage by γ-secretase [6,10]. M-CSF/c-Fms regulates the development of mammary tumors to malignancy by promoting the infiltration and function of tumour-associated macrophages [16]. In a collagen-induced arthritis model, the inhibition of c-M-CSF/c-Fms signalling by two inhibitors, imatinib and GW2580, prevented autoimmune arthritis, macrophage infiltration into synovial joints, osteoclast differentiation, and bone resorption [17]. A blockade of the c-Fms signalling pathway by using a monoclonal antibody raised against c-Fms was shown to protect against early-stage atherosclerosis [18]

Methods

Results

Conclusion