The phosphatidylinositol 3-kinase-mediated production of interferon-β is critical for the lipopolysaccharide inhibition of osteoclastogenesis

Staphylococci planktonic and biofilm environments differentially affect osteoclast formation

INTRODUCTION

The pathophysiology of chronic implant-related bone infections is characterized by an increase in osteoclast numbers and enhanced bone resorption. Biofilms are a major reason for chronicity of such infections as the biofilm matrix protects bacteria against antibiotics and impairs the function of immune cells. Macrophages are osteoclast precursor cells and therefore linked to inflammation and bone destruction.

OBJECTIVE AND METHOD

Investigations on the impact of biofilms on the ability of macrophages to form osteoclasts are yet missing and we, therefore, analyzed the effect of Staphylococcus aureus (SA) and Staphylococcus epidermidis (SE) planktonic and biofilm environments on osteoclastogenesis using RAW 264.7 cells and conditioned media (CM).

RESULTS

Priming with the osteoclastogenic cytokine RANKL before CM addition enabled the cells to differentiate into osteoclasts. This effect was highest in SE planktonic or SA biofilm CM. Simultaneous stimulation with CM and RANKL, however, suppressed osteoclast formation and resulted in formation of inflammation-associated multinucleated giant cells (MGCs) which was most pronounced in SE planktonic CM.

CONCLUSION

Our data indicate that the biofilm environment and its high lactate levels are not actively promoting osteoclastogenesis. Hence, the inflammatory immune response against planktonic bacterial factors through Toll-like receptors seems to be the central cause for the pathological osteoclast formation. Therefore, immune stimulation or approaches that aim at biofilm disruption need to consider that this might result in enhanced inflammation-mediated bone destruction.

The Role of Toll-Like Receptors in Osteoclastogenesis

Bone homeostasis is maintained by a balance in the levels of osteoclast and osteoblast activity. Osteoclasts are bone-resorbing cells and have been shown to act as key players in various osteolytic diseases. Osteoclasts differentiate from monocyte/macrophage lineage cells in the presence of receptor activator of nuclear factor-κB ligand and macrophage colony-stimulating factor. Osteoblasts support osteoclastogenesis by producing several osteoclast differentiation factors. Toll-like receptors (TLRs) are members of the pattern recognition receptor family that are involved in recognizing pathogen-associated molecular patterns and damage-associated molecular patterns in response to pathogen infection. TLRs regulate osteoclastogenesis and bone resorption through either the myeloid differentiation primary response 88 or the Toll/interleukin-1 receptor domain-containing adapter-inducing interferon-β signaling pathways. Since osteoclasts play a central role in the progression of osteolytic diseases, extensive research focusing on TLR downstream signaling in these cells should be conducted to advance the development of effective TLR modulators. In this review, we summarize the currently available information on the role of TLRs in osteoclast differentiation and osteolytic diseases.

Endogenous Collagenases Regulate Osteoclast Fusion

The precise regulation of osteoclast differentiation and function is crucial for the maintenance of healthy bone. Despite several reports of collagenase expression in bone tissues, the precise isoform expression as well as the role in osteoclasts are still unclear. In the present report, the expression of matrix metalloprotease (MMP)8 and MMP13 was confirmed in mouse bone marrow macrophage osteoclast precursors. The mRNA and protein expressions of both collagenases were significantly reduced by receptor activator of nuclear factor κB ligand (RANKL) stimulation. Notably, either inhibition of MMP expression by siRNA or treatment of cells with collagenase inhibitor Ro 32-3555 significantly augmented osteoclast fusion and resorption activity without affecting the osteoclast number. The inhibition of collagenase by Ro 32-3555 increased the expression of osteoclast fusion genes, Atp6v0d2 and Dcstamp, without affecting nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) protein expression. The enhanced osteoclast fusion by collagenase inhibition appears to be mediated through an extracellular signal regulated kinase (ERK)-dependent pathway. Collectively, these data provide novel information on the regulation of osteoclast fusion process.

Cyclic Dinucleotides Inhibit Osteoclast Differentiation Through STING-Mediated Interferon-β Signaling

Cyclic dinucleotides (CDNs), such as cyclic diadenylate monophosphate and cyclic diguanylate monophosphate, are commensal bacteria-derived second messengers in the gut that modulate bacterial survival, colonization, and biofilm formation. Recently, CDNs have been discovered to have an immunomodulatory activity by inducing the expression of type I interferon (IFN) through STING signaling pathway in macrophages. Because CDNs are possibly absorbed and delivered into the bone marrow, where bone-resorbing osteoclasts are derived from monocyte/macrophage lineages, CDNs could affect bone metabolism by regulating osteoclast differentiation. In this study, we investigated the effect of CDNs on the differentiation and function of osteoclasts and osteoblasts. When bone marrow-derived macrophages (BMMs) were differentiated into osteoclasts with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) in the presence of CDNs, the differentiation was inhibited by CDNs in a dose-dependent manner. In contrast, CDNs did not influence the differentiation of committed osteoclasts or osteoblast precursors. STING signaling pathway appeared to be critical for CDNs-mediated inhibition of osteoclast differentiation since CDNs induced the phosphorylation of TBK1 and IRF3, a representative feature of STING activation, and osteoclast differentiation was restored in STING knockdown BMMs with siRNA. Moreover, CDNs increased the mRNA expression of STING-meditated IFN-β, which is a negative regulator of osteoclastogenesis. In addition, CDNs also induced the phosphorylation of STAT1, which mediates IFN-α/β receptor (IFNAR) signal transduction. The inhibitory effects of CDNs on osteoclast differentiation were not observed in the presence of antibody blocking IFNAR or in macrophages derived from IFNAR1^-/- mice. Experiments using a mouse calvarial implantation model showed that RANKL-induced bone resorption was inhibited by CDNs. Taken together, these results suggest that CDNs inhibit osteoclast differentiation and bone resorption through induction of IFN-β via the STING signaling pathway. © 2019 American Society for Bone and Mineral Research.

Evidence that contamination by lipopolysaccharide confounds in vitro studies of adiponectin activity in bone

Adiponectin, a hormone produced and secreted from adipose tissue, circulates at levels that are inversely related to visceral fat mass and bone mineral density. Adiponectin receptors are expressed in bone cells, and several studies have shown that adiponectin affects bone phenotype and might play a role in the cross talk between fat and bone tissues. In the current study, we determined global changes in gene expression induced by adiponectin in mouse bone marrow cells, in order to identify the molecular mechanisms that mediate adiponectin's effect to inhibit osteoclast differentiation in these cultures. The gene signature that was produced by microarray analysis was very similar to a signature produced by activation of type I interferons (IFN), and we therefore tested the hypothesis that the adiponectin preparation, although marketed as "lipopolysaccharide (LPS) free", was contaminated with LPS that induced an IFN response in the bone marrow cells. Heat inactivation of the adiponectin preparation and the use of small interfering RNA to knockdown the AdipoR1 receptor had not diminished the activity of the adiponectin preparation to induce the IFN target genes Ccl5 and Irf7. Thus, the changes in gene expression determined in the bone marrow cultures are likely to be the result of a combination of adiponectin and LPS effects. Our study suggests that the purity of commercially available proteins needs to be verified and that experimental results of adiponectin activity in vitro should be interpreted cautiously.