Osteocytic cells exposed to titanium particles increase sclerostin expression and inhibit osteoblastic cell differentiation mostly via direct cell-to-cell contact

Titanium Particles Activate Osteocytic Connexin 43 to Induce Oxidative Stress and Osteoclastogenesis Through the JAK-STAT Pathway

Aims: Periprosthetic osteolysis (PPO), a leading cause of aseptic loosening in joint replacement, arose from complex interactions among osteoblasts, osteoclasts, and osteocytes. Given the pivotal role of connexin 43 (Cx43) in osteocyte communication and bone remodeling, investigating its function was essential for understanding the mechanisms of osteolysis. Our previous studies showed that titanium (Ti) particles increased Cx43 expression in osteocytes. However, the role of Cx43 in osteolysis remained unclear. This study investigated the role of Cx43-mediated regulation of osteocytes on osteoclastogenesis in wear debris-induced osteolysis. Results: Using Dmp1-cre conditional Cx43 knockout mice and the MLO-Y4 osteocyte cell line, we demonstrated that Cx43 deficiency reduced bone resorption and osteoclastogenesis, thereby improving bone remodeling in a Ti particle-induced osteolysis model. Sequencing analysis revealed that Cx43 gene expression changes might be linked to oxidative stress and the Janus Kinase (JAK)-STAT pathway. Elevated Cx43 expression in osteocytes stimulated by Ti particles increased STAT1 protein phosphorylation, induced oxidative stress, elevated the Receptor Activator of Nuclear Factor Kappa-Β Ligand (RANKL)/Osteoprotegerin (OPG) ratio, and promoted osteoclast activation and bone resorption. Conversely, Cx43 gene knockout decreased STAT1 protein phosphorylation and enhanced Nuclear Factor Erythroid 2-Related Factor 2 (NrF2) protein expression. Blocking the JAK-STAT signaling pathway activated by Cx43 increased NrF2 expression, reduced reactive oxygen species levels, and subsequently decreased the RANKL/OPG ratio. Innovation and Conclusions: This study identified a novel mechanism where Cx43 in osteocytes promoted osteoclastogenesis through JAK-STAT pathway activation and oxidative stress in wear debris-induced osteolysis. These findings highlighted the critical role of Cx43 in bone resorption and suggested targeting Cx43 or the JAK-STAT pathway as potential therapeutic strategies to mitigate osteolysis and improve implant longevity. Antioxid. Redox Signal. 00, 000-000.

Osteocytes Exposed to Titanium Particles Inhibit Osteoblastic Cell Differentiation via Connexin 43

Periprosthetic osteolysis (PPO) induced by wear particles is the most severe complication of total joint replacement; however, the mechanism behind PPO remains elusive. Previous studies have shown that osteocytes play important roles in wear-particle-induced osteolysis. In this study, we investigated the effects of connexin 43 (Cx43) on the regulation of osteocyte-to-osteoblast differentiation. We established an in vivo murine model of calvarial osteolysis induced by titanium (Ti) particles. The osteolysis characteristic and osteogenesis markers in the osteocyte-selective Cx43 (CKO)-deficient and wild-type (WT) mice were observed. The calvarial osteolysis induced by Ti particles was partially attenuated in CKO mice. The expression of β-catenin and osteogenesis markers increased significantly in CKO mice. In vitro, the osteocytic cell line MLO-Y4 was treated with Ti particles. The co-culturing of MLO-Y4 cells with MC3T3-E1 osteoblastic cells was used to observe the effects of Ti-treated osteocytes on osteoblast differentiation. When Cx43 of MLO-Y4 cells was silenced or overexpressed, β-catenin was detected. Additionally, co-immunoprecipitation detection of Cx43 and β-catenin binding in MLO-Y4 cells and MC3T3-E1 cells was performed. Finally, β-catenin expression in MC3T3-E1 cells and osteoblast differentiation were evaluated after 18α-glycyrrhetinic acid (18α-GA) was used to block the intercellular communication of Cx43 between MLO-Y4 and MC3T3-E1 cells. Ti particles increased Cx43 expression and decreased β-catenin expression in MLO-Y4 cells. The silencing of Cx43 increased the β-catenin expression, and the over-expression of Cx43 decreased the β-catenin expression. In the co-culture model, Ti treatment of MLO-Y4 cells inhibited the osteoblastic differentiation of MC3T3-E1 cells and Cx43 silencing in MLO-Y4 cells attenuated the inhibitory effects on osteoblastic differentiation. With Cx43 silencing in the MLO-Y4 cells, the MC3T3-E1 cells, co-cultured alongside MLO-Y4, displayed decreased Cx43 expression, increased β-catenin expression, activation of Runx2, and promotion of osteoblastic differentiation in vitro co-culture. Finally, Cx43 expression was found to be negatively correlated to the activity of the Wnt signaling pathway, mostly through the Cx43 binding of β-catenin from its translocation to the nucleus. The results of our study suggest that Ti particles increased Cx43 expression in osteocytes and that osteocytes may participate in the regulation of osteoblast function via the Cx43 during PPO.

SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis

The most common cause for prosthetic revision surgery is wear particle-induced periprosthetic osteolysis, which leads to aseptic loosening of the prosthesis. Both SOST gene and its synthetic protein, sclerostin, are hallmarks of osteocytes. According to our previous findings, blocking SOST induces bone formation and protects against bone loss and deformation caused by titanium (Ti) particles by activating the Wnt/β-catenin cascade. Although SOST has been shown to influence osteoblasts, its ability to control wear-particle-induced osteolysis via targeting osteoclasts remains unclear. Mice were subjected to development of a cranial osteolysis model. Micro CT, HE staining, and TRAP staining were performed to evaluate bone loss in the mouse model. Bone marrow-derived monocyte-macrophages (BMMs) made from the C57BL/6 mice were exposed to the medium of MLO-Y4 (co-cultured with Ti particles) to transform them into osteoclasts. Bioinformatics methods were used to predict and validate the interaction among SOST, Wnt/β-catenin, RANKL/OPG, TNF-α, and IL-6. Local bone density and bone volume improved after SOST inhibition, both the number of lysis pores and the rate of skull erosion decreased. Histological research showed that β-catenin and OPG expression were markedly increased after SOST inhibition, whereas TRAP and RANKL levels were markedly decreased. In-vitro, Ti particle treatment elevated the expression of sclerostin, suppressed the expression of β-catenin, and increased the RANKL/OPG ratio in the MLO-Y4 cell line. TNF-α and IL-6 also elevated after treatment with Ti particles. The expression levels of NFATc1, CTSK, and TRAP in osteoclasts were significantly increased, and the number of positive cells for TRAP staining was increased. Additionally, the volume of bone resorption increased at the same time. In contrast, when SOST expression was inhibited in the MLO-Y4 cell line, these effects produced by Ti particles were reversed. All the results strongly show that SOST inhibition triggered the osteocyte Wnt/β-catenin signaling cascade and prevented wear particle-induced osteoclastogenesis, which might reduce periprosthetic osteolysis. KEY MESSAGES: SOST is a molecular regulator in maintaining bone homeostasis. SOST plays in regulating bone homeostasis through the Wnt/β-catenin signaling pathway. SOST gene suppression stimulates osteocyte Wnt/β-catenin signaling to prevent bone resorption and attenuates particle-induced osteolysis.

Bimetallic Metal-Organic Framework for Mitigating Aseptic Osteolysis

The disability rate of joint diseases can be reduced by the use of artificial joints, but joint loosening at a late state limits the lifespan and surgical efficacy of the joints. Wear particles can be recognized by macrophages and induce cells to produce reactive oxygen species (ROS) and inflammatory factors, causing persistent inflammation and decreased osteogenic activity, which ultimately leads to loosening of joint prostheses. Here, the platinum (Pt) nanozymes with excellent ROS scavenging and anti-inflammatory capabilities were encapsulated in zinc imidazolium zeolite framework-8 (ZIF-8), and then the osteogenic active element lanthanum (La) was introduced through ion exchange to finally construct a bimetallic metal-organic framework (Pt@ZIF-8@La). In vitro and in vivo experiments demonstrated that this multifunctional nanoplatform possessed the functions of efficient scavenging of ROS, immune regulation, and promotion of osteogenic differentiation. Meanwhile, the mechanism is explored that Pt@ZIF-8@La can also promote osteogenic mineralization by upregulating the ratio of the osteoprotegerin (OPG)/receptor activator of the NF-κB ligand (RANKL), which can achieve a synergistic therapeutic effect of immunomodulation and osteogenesis, thereby realizing the purpose of relieving aseptic osteolysis.