Because of lipopolysaccharide (LPS)-mediated effects on osteoclast differentiation and bone loss, periprosthetic joint infection (PJI) caused by Gram-negative bacteria increases the risk of aseptic loosening after reimplantation. Synovial fluid interleukin-16 (IL-16) expression was higher in patients with PJI than in patients without joint infection. Thus, we explored the effects of IL-16 on bone. We investigated whether IL-16 modulates osteoclast or osteoblast differentiation in vitro. An LPS-induced bone loss mice model was used to explore the possible advantages of IL-16 inhibition for the prevention of bone loss. IL-16 directly activated p38 and c-Jun N-terminal kinase (JNK)/mitogen-activated protein kinase (MAPK) signaling and increased osteoclast activation markers, including tartrate-resistant acid phosphatase (TRAP), cathepsin K, and nuclear factor of activated T cells 1 (NFATc1). IL-16 directly caused monocytes to differentiate into TRAP-positive osteoclast-like cells through NFATc1 activation dependent on JNK/MAPK signaling. Moreover, IL-16 did not alter alkaline phosphatase activity or calcium deposition during osteoblastic differentiation. Finally, IL-16 inhibition prevented LPS-induced trabecular bone loss and osteoclast activation in vivo. IL-16 directly increased osteoclast activation through the JNK/NFATc1 pathway. IL-16 inhibition could represent a new strategy for treating infection-associated bone loss.Int. J. Mol. Sci. 2020, 21, 2904 2 of 15 of differentiation 4 (CD4 + ) lymphocytes, monocytes, and eosinophils [11,12]. IL-16 is synthesized as a precursor protein (pro-IL-16) of approximately 68 kDa. Pro-IL-16 is cleaved by activated caspase-3 and then IL-16 generation [13]. CD4 serves as a signaling transducing receptor for IL-16 signaling. Increased intracytoplasmic calcium and inositol trisphosphate can be initiated when IL-16 interacts with CD4 receptors, and this subsequently activates stress-activated protein kinases (SAPKs) p46 and p54. Stimulation with IL-16 also activates members of the mitogen-activated protein kinase (MAPK) family, namely c-Jun N-terminal kinase (JNK) and p38 but does not activate extracellular signal-regulated kinase (ERK)-1 or ERK-2 [14]. IL-16 can activate monocytes and stimulate the secretion of inflammatory cytokines, chiefly tumor necrosis factor (TNF)-α, IL-1β, 15,16]. Currently, these cytokines are believed to promote the development of septic arthritis [17][18][19].PJI can result from Gram-positive (GP) or Gram-negative (GN) bacterial infections. Our previous retrospective study revealed that GN infections were associated with an increased risk of aseptic loosening as compared with GP infections [20]. The cell walls of GP bacteria contain lipoteichoic acid (LTA) components, whereas those of GN bacteria contain components of lipopolysaccharide (LPS). In studies of mice receiving intrafemoral injections of LPS or LTA, LPS reduced both the number of trabeculae and bone mineral density, whereas LTA did not have this effect [20]. In addition, disruption of the bal...

Lipoteichoic acid (LTA) is a cell wall component of Gram-positive bacteria. Limited data suggest that LTA is beneficial for bone regeneration in vitro. Thus, we used a mouse model of femoral defects to explore the effects of LTA on bone healing in vivo. Micro-computed tomography analysis and double-fluorochrome labeling were utilized to examine whether LTA can accelerate dynamic bone formation in vivo. The effects of LTA on osteoblastogenesis and osteoclastogenesis were also studied in vitro. LTA treatment induced prompt bone bridge formation, rapid endochondral ossification, and accelerated healing of fractures in mice with femoral bone defects. In vitro, LTA directly enhanced indicators of osteogenic factor-induced MC3T3-E1 cell differentiation, including alkaline phosphatase activity, calcium deposition and osteopontin expression. LTA also inhibited osteoclast activation induced by receptor activator of nuclear factor-kappa B ligand. We identified six molecules that may be associated with LTA-accelerated bone healing: monocyte chemoattractant protein 1, chemokine (C-X-C motif) ligand 1, cystatin C, growth/differentiation factor 15, endostatin and neutrophil gelatinase-associated lipocalin. Finally, double-fluorochrome, dynamic-labeling data indicated that LTA significantly enhanced bone-formation rates in vivo. In conclusion, our findings suggest that LTA has promising bone-regeneration properties.

Blood supply interruption induces hypoxia and reduces serum provision to cause ischemia-induced osteonecrosis, including avascular osteonecrosis of the femoral head (ONFH). Oxygen deficiency (hypoxia) is known to induce different expression patterns in osteoblasts and osteoclasts, which have been extensively studied. However, the effects of serum insufficiency in nutrients, growth factors, and hormones on osteoblast and osteoclast activity in the damaged area and nearby regions remain poorly understood. In this study, the expression of osteoblast and osteoclast marker proteins was elucidated through in vitro and ex vivo studies. The results indicate that serum insufficiency accelerates the formation of monocyte-derived osteoclasts. The combined effect of serum insufficiency and hypoxia (mimicking ischemia) suppressed the activity of alkaline phosphatase and calcification in osteoblasts after the stimulation of osteogenic growth factors. Serum insufficiency increased the activity of tartrate-resistant acid phosphatase, expression of phosphorylated extracellular signal-regulated kinases, and production of reactive oxygen species in monocyte-derived osteoclasts in the absence of receptor activator of nuclear factor kappa-Β ligand stimulation. The findings indicate that changes in the expression of osteoblast and osteoclast markers in necrotic bone extracts were similar to those observed during an in vitro study. These results also suggest that serum insufficiency may be involved in the regulation of osteoclast formation in patients with ONFH.

Osteoarthritis (OA) is a degenerative joint disease that may cause joint inflammation, stiffness, and pain. Current therapy for OA involves symptomatic treatment, mainly pain management. Therefore, it does not repair degenerated cartilage or attenuate joint inflammation. Because articular cartilage cannot heal or regenerate these tissues, tissue regeneration remains one of the most important objectives of new and potential OA therapeutics. The main features of adult mesenchymal stem cells (MSCs) are simple acquisition from adult tissues, rapid proliferation in vitro, immunomodulation in vivo, and lasting existence in the host, which are beneficial for OA treatment. In the past 15 years, adult MSCs, including bone marrow-, adipose-, and synovial membrane-derived MSCs, and their secretome have been successfully used in different animal (preclinical) models or in genetic manipulation for regenerating degenerated cartilage, reducing inflammation, or relieving pain. Furthermore, the implantation of adult MSCs showed pain reduction, anti-inflammation, and cartilage protection or healing in early-phase clinical trials. Adult MSCs are the most extensively explored as potential regenerative medicine for OA because of their efficacy in chondrocyte differentiation and their immunomodulatory properties. In this review paper, we highlighted current knowledge and future perspectives regarding preclinical tests, clinical application, and MSC-based/related products for curing OA.