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Inflammation-induced bone destruction is a major treatment target in many inflammatory skeletal diseases. The aim of this study was to investigate if the cysteine proteinase inhibitors cystatin C, fungal cysteine proteinase inhibitor (E-64), and N-benzyloxycarbonyl-arginylleucyl-valyl-glycyl-diazomethane acetate (Z-RLVG-CHN2) can inhibit LPS-induced osteoclast formation. Mouse bone marrow macrophages (BMMs) were isolated and primed with receptor activator of NF-kappa B ligand (RANKL) for 24 h, followed by stimulation with LPS, with and without inhibitors. Adult mice were injected locally with LPS and then treated with E-64 and osteoclast formation assessed by the number of cathepsin K+ multinucleated cells. Cystatin C inhibited LPS-induced osteoclast formation time and concentration dependently (IC50 = 0.3 mu M). The effect was associated with decreased mRNA and protein expression of tartrate-resistant acid phosphatase (TRAP) and cathepsin K and of the osteoclastogenic transcription factors c-Fos and NFATc1. LPS-induced osteoclast formation on bone slices was also inhibited by cystatin C, resulting in decreased pit formation and release of bone matrix proteins. Similar data were obtained with E-64 and Z-RLVG-CHN2. Cystatin C was internalized in BMMs stimulated by LPS but not in unstimulated BMMs. Osteoclast formation induced by LPS was dependent on TNF-alpha, and the 3 inhibitors abolished LPS-induced TNF superfamily 2 (gene encoding TNF-alpha; Tnfsf2) mRNA expression without affecting Il1b, Il6, or oncostatin M (Osm) expression. Formation of osteoclasts in the skull bones after local LPS stimulation was inhibited by E-64. It is concluded that cysteine proteinase inhibitors effectively inhibit LPS-induced osteoclast formation in vivo and in vitro by inhibition of TNF-alpha expression. The targeting of cysteine proteinases might represent a novel treatment modality for prevention of inflammatory bone loss.

The cysteine proteinase inhibitor cystatin C inhibited RANKL-stimulated osteoclast formation in mouse bone marrow macrophage cultures, an effect associated with decreased mRNA expression of Acp5, Calcr, Ctsk, Mmp9, Itgb3, and Atp6i, without effect on proliferation or apoptosis. The effects were concentration dependent with half-maximal inhibition at 0.3 μM. Cystatin C also inhibited osteoclast formation when RANKL-stimulated osteoclasts were cultured on bone, leading to decreased formation of resorption pits. RANKL-stimulated cells retained characteristics of phagocytotic macrophages when cotreated with cystatin C. Three other cysteine proteinase inhibitors, cystatin D, Z-RLVG-CHN2 (IC50 0.1 μM), and E-64 (IC50 3 μM), also inhibited osteoclast formation in RANKL-stimulated macrophages. In addition, cystatin C, Z-RLVG-CHN2, and E-64 inhibited osteoclastic differentiation of RANKL-stimulated CD14(+) human monocytes. The effect by cystatin C on differentiation of bone marrow macrophages was exerted at an early stage after RANKL stimulation and was associated with early (4 h) inhibition of c-Fos expression and decreased protein and nuclear translocation of c-Fos. Subsequently, p52, p65, IκBα, and Nfatc1 mRNA were decreased. Cystatin C was internalized in osteoclast progenitors, a process requiring RANKL stimulation. These data show that cystatin C inhibits osteoclast differentiation and formation by interfering intracellularly with signaling pathways downstream RANK.