Actin and ERK1/2-CEBPß signaling mediates phagocytosis-induced innate immune response of osteoprogenitor cells.

Wear particles at the host bone-implant interface are a major challenge for successful bone implant arthoplasties. Current understanding of aseptic loosening consists of macrophage-mediated inflammatory responses and increasing osteoclastogenesis, which lead to an imbalance between bone formation and resorption. Despite its significant role in bone regeneration and implant osteointegration, the osteoprogenitor response to wear particles has been examined recent years. More specifically, the intracellular mechanism of osteoprogenitor mediated inflammation has not been fully elucidated. In this study, we examined the role of osteoprogenitors and the cellular mechanism by which metal wear particles elicit an inflammatory cascade. Through both in vivo and in vitro experiments, we have demonstrated that osteoprogenitor cells are capable of initiating inflammatory responses by phagocytosing wear particles, which lead to subsequent accumulation of macrophages and osteoclastogenesis, and the ERK_CEBP/ß intracellular signaling is a key inflammatory pathway that links phagocytosis of wear particles to inflammatory gene expression in osteoprogenitors. AZD6244 treatment, a potent inhibitor of the ERK pathway, attenuated particle mediated inflammatory osteolysis both in vivo and in vitro. This study advances our understanding of the mechanisms of osteoprogenitor-mediated inflammation, and provides further evidence that the ERK_CEBP/ß pathway may be a suitable therapeutic target in the treatment of inflammatory osteolysis.

Nuclear presence of nuclear factor of activated T cells (NFAT) c3 and c4 is required for Toll-like receptor-activated innate inflammatory response of monocytes/macrophages.

Nuclear factor of activated T cells (NFATs) are crucial transcription factors that tightly control proinflammatory cytokine expression for adaptive immunity in T and B lymphocytes. However, little is known about the role of NFATs for innate immunity in macrophages. In this study, we report that NFAT is required for Toll-like receptor (TLR)-initiated innate immune responses in bone marrow-derived macrophages (BMMs). All TLR ligand stimulation including LPS, a TLR4 ligand, and Pam(3)CSK(4), a TLR1/2 ligand, induced expression of TNF which was inhibited by VIVIT, an NFAT-specific inhibitor peptide. BMMs from NFATc4 knock-out mouse expressed less TNF than wild type. Despite apparent association between NFAT and TNF, LPS did not directly activate NFAT based on NFAT-luciferase reporter assay, whereas NF-κB was inducibly activated by LPS. Instead, macrophage exhibited constitutive NFAT activity which was not increased by LPS and was decreased by VIVIT. Immunocytochemical examination of NFATc1-4 of BMMs exhibited nuclear localization of NFATc3/c4 regardless of LPS stimulation. LPS stimulation did not cause nuclear translocation of NFATc1/c2. Treatment with VIVIT resulted in nuclear export of NFATc3/c4 and inhibited TLR-activated TNF expression, suggesting that nuclear residence of NFATc is required for TLR-related innate immune response. Chromatin immunoprecipitation (ChIP) assay using anti-RNA polymerase II (PolII) antibody suggested that VIVIT decreased PolII binding to TNF gene locus, consistent with VIVIT inhibition of LPS-induced TNF mRNA expression. This study identifies a novel paradigm of innate immune regulation rendered by NFAT which is a well known family of adaptive immune regulatory proteins.