γ-Glutamyltranspeptidase is an endogenous activator of Toll-like receptor 4-mediated osteoclastogenesis.

Sawako Moriwaki,Mutsumi Miyauchi,Keiko Suzuki,Keigo Shibayama,Shumpei Niida,Takashi Takata,Takeshi Into

doi:10.1038/srep35930

Abstract

Chronic inflammation-associated bone destruction, which is observed in rheumatoid arthritis (RA) and periodontitis, is mediated by excessive osteoclastogenesis. We showed previously that γ-glutamyltranspeptidase (GGT), an enzyme involved in glutathione metabolism, acts as an endogenous activator of such pathological osteoclastogenesis, independent of its enzymatic activity. GGT accumulation is clinically observed in the joints of RA patients, and, in animals, the administration of recombinant GGT to the gingival sulcus as an in vivo periodontitis model induces an increase in the number of osteoclasts. However, the underlying mechanisms of this process remain unclear. Here, we report that Toll-like receptor 4 (TLR4) recognizes GGT to activate inflammation-associated osteoclastogenesis. Unlike lipopolysaccharide, GGT is sensitive to proteinase K treatment and insensitive to polymyxin B treatment. TLR4 deficiency abrogates GGT-induced osteoclastogenesis and activation of NF-κB and MAPK signaling in precursor cells. Additionally, GGT does not induce osteoclastogenesis in cells lacking the signaling adaptor MyD88. The administration of GGT to the gingival sulcus induces increased osteoclastogenesis in wild-type mice, but does not induce it in TLR4-deficient mice. Our findings elucidate a novel mechanism of inflammation-associated osteoclastogenesis, which involves TLR4 recognition of GGT and subsequent activation of MyD88-dependent signaling.

Highlights

Inflammation represents a protective immunovascular response of tissues to harmful stimuli; when not strictly controlled, it may lead to chronic persistent inflammation[1]
We show that a number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclast-like cells (OCLs) were generated by recombinant human GGT1 protein (rGGT) stimulation (Fig. 1)
Our results suggest that extracellular GGT represents a bone metabolism-associated damage-associated molecular patterns (DAMPs)

Summary

Introduction

Inflammation represents a protective immunovascular response of tissues to harmful stimuli; when not strictly controlled, it may lead to chronic persistent inflammation[1]. GGT is produced as an ectoenzyme, localized at the surface of various types of cells, but it can be found in body fluids, including serum[20] Even though it has not been determined how GGT is released extracellularly, previous studies have demonstrated that the production of both types of GGT can be elevated in response to oxidative stress[21,22]. In the collagen-induced arthritis mouse model, the administration of monoclonal antibody against GGT to the arthritic joints attenuates bone destruction through suppression of excessive osteoclast formation that is associated with inflammatory reactions[23]. These observations indicate that extracellularly released GGT possesses osteoclastogenic activity, but the underlying mechanisms have not been elucidated. Our findings propose an important mechanism of the recognition of extracellular GGT by TLR4, which may be involved in the development of inflammatory diseases accompanied with excessive osteoclastogenesis and bone destruction

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