S100A8 enhances osteoclastic bone resorption in vitro through activation of Toll‐like receptor 4: Implications for bone destruction in murine antigen‐induced arthritis

Abstract

Objective. Rheumatoid arthritis, which is associated with elevated levels of S100A8 and S100A9, is characterized by severe bone erosions caused by enhanced osteoclast formation and activity. The aim of the present study was to investigate the role of S100A8 and S100A9 in osteoclastic bone destruction in murine antigen-induced arthritis (AIA).Methods. Bone destruction was analyzed in the arthritic knee joints of S100A9-deficient mice in which S100A8 protein expression was also lacking, and in wild-type (WT) controls. Osteoclast precursors from S100A9-deficient and WT mice were differentiated into osteoclasts in vitro. Additionally, precursors were stimulated with S100A8, S100A9, or S100A8/A9 during osteoclastogenesis. Receptor involvement was investigated using an anti-receptor for advanced glycation end products (anti-RAGE)-blocking antibody, soluble RAGE, or Toll-like receptor 4 (TLR-4)-deficient osteoclast precursors. The formation of osteoclasts and actin rings, the regulation of osteoclast markers, and bone resorption were analyzed.Results. Bone erosions and cathepsin K staining were significantly suppressed in S100A9-deficient mice after AIA induction. However, osteoclast precursors from S100A9-deficient mice developed normally into functional osteoclasts, which excludes a role for intrinsic S100A8/A9. In contrast to the results observed with S100A9 and S100A8/A9, the addition of S100A8 during osteoclastogenesis resulted in stimulation of osteoclast formation in conjunction with enhanced actin ring formation and increased bone resorption. Analysis of the putative receptor for S100A8 in osteoclastogenesis revealed that osteoclast differentiation and function could not be inhibited by blocking RAGE, whereas the increase in osteoclast numbers and enhanced bone resorption were completely abrogated using TLR-4-deficient osteoclast precursors.Conclusion. These results demonstrate that S100A8 stimulated osteoclast formation and activity and suggest that both S100A8 and TLR-4 are important factors in mediating osteoclastic bone destruction in experimental arthritis.Bone erosions are an important hallmark of rheumatoid arthritis (RA) and typically result from enhanced formation and activity of osteoclasts in affected joints. Osteoclasts originate from hematopoietic precursors of the monocyte/macrophage lineage that differentiate into multinucleated osteoclasts under the

Myeloid skewing in murine autoimmune arthritis occurs in hematopoietic stem and primitive progenitor cells

“…43 This is significant because exogenous stimulation of TLR4 using LPS drives myeloid differentiation of KSL cells. 21,44 Furthermore, S100a8, acting through TLR4, has been shown to promote osteoclastogenesis 33 and up-regulation of Fc␥RIII/IIb 45 ; we have shown that both phenomena occur in arthritic KSL cells (Figures 1E, 3C).…”

“…In addition, we have shown that arthritic KSL cells have increased osteoclastogenic activity and increased cell-surface Fc␥RIII/IIb, both previously described effects of S100a8 on less primitive hematopoietic cells. 33,45 Therefore, it is possible that endogenous S100a8 produced by arthritic HSCs promote the myeloid priming of these cells in an autocrine manner. This might occur through TLR4 but could also occur via RAGE, the other signaling receptor for S100a8.…”

“…33,43,45 In vivo or in vitro TLR4 stimulation leads to diminution of the cell-surface TLR4 protein signal by receptor internalization or altered conformation. 21,46,47 We found that while TLR4 was detectable on KSL cells from nonarthritic mice as previously reported, 21 the TLR4 signal on the surface of KRNxG7 arthritic KSL cells was reduced to close to background levels ( Figure 5B).…”

Expression of the Hutchinson-Gilford Progeria Mutation during Osteoblast Development Results in Loss of Osteocytes, Irregular Mineralization, and Poor Biomechanical Properties

“…S100 proteins belong to the superfamily of calcium binding proteins and are involved in damage associated molecular patterns (36). Among the S100 proteins, S100A8, S100A9, and S100A4 have been shown to be associated with osteoblast differentiation, and S100A4 has been shown to be a negative regulator for bone mineralization (37)(38)(39). We performed expression analysis and found a trend toward higher levels of S100A8 and S100A9 in HGPS mice compared with control animals, even though not significantly different (Fig.…”

Calprotectin in rheumatic diseases

“…59 Osteoclast differentiation is also affected by CLP primarily through a TLR4-mediated signal and S100A8-induced osteoclastic bone resorption in a murine model. 60 CLP levels are higher in the serum of RA patients compared with healthy subjects or patients with osteoarthritis (OA), spondyloarthritis (SpA), systemic lupus erythematosus (SLE), and pseudogout. 47,61,62 Still, no cut-off levels have been identified to help in the diagnosis of RA.…”