TLR2-dependent modulation of osteoclastogenesis by Porphyromonas gingivalis through differential induction of NFATc1 and NF-kappaB.
ABSTRACT Osteolytic diseases, including rheumatoid arthritis, osteomyelitis, and periodontitis, are usually associated with bacterial infections. However, the precise mechanisms by which bacteria induce bone loss still remain unclear. Evidence exists that Toll-like receptor (TLR) signaling regulates both inflammation and bone metabolism and that the receptor activator of NF-κB ligand (RANKL) and its receptor RANK are the key regulators for bone remodeling and for the activation of osteoclasts. Here, we investigate the direct effects of the periodontal pathogen Porphyromonas gingivalis on osteoclast differentiation and show that P. gingivalis differentially modulates RANKL-induced osteoclast formation contingent on the state of differentiation of osteoclast precursors. In addition, although an optimal induction of cytokines by P. gingivalis is dependent on TLR2 and TLR4, as well as myeloid differentiation factor 88 and Toll/IL-1R domain-containing adaptor-inducing IFN-β, P. gingivalis utilizes TLR2/ myeloid differentiation factor 88 in modulating osteoclast differentiation. P. gingivalis modulates RANKL-induced osteoclast formation by differential induction of NFATc1 and c-Fos. More importantly, RANKL-mediated lineage commitment also has an impact on P. gingivalis-induced cytokine production. RANKL inhibits P. gingivalis-induced cytokine production by down-regulation of TLR/NF-κB and up-regulation of NFATc1. Our findings reveal novel aspects of the interactions between TLR and RANK signaling and provide a new model for understanding the mechanism underlying the pathogenesis of bacteria-mediated bone loss.
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ABSTRACT: The innate host response to lipopolysaccharide (LPS) obtained from Porphyromonas gingivalis is unusual in that different studies have reported that it can be an agonist for Toll-like receptor 2 (TLR2) as well as an antagonist or agonist for TLR4. In this report it is shown that P. gingivalis LPS is highly heterogeneous, containing more lipid A species than previously described. In addition, purification of LPS can preferentially fractionate these lipid A species. It is shown that an LPS preparation enriched for lipid A species at m/z 1,435 and 1,450 activates human and mouse TLR2, TLR2 plus TLR1, and TLR4 in transiently transfected HEK 293 cells coexpressing membrane-associated CD14. The HEK cell experiments further demonstrated that cofactor MD-2 was required for functional engagement of TLR4 but not of TLR2 nor TLR2 plus TLR1. In addition, serum-soluble CD14 effectively transferred P. gingivalis LPS to TLR2 plus TLR1, but poorly to TLR4. Importantly, bone marrow cells obtained from TLR2(-/-) and TLR4(-/-) mice also responded to P. gingivalis LPS in a manor consistent with the HEK results, demonstrating that P. gingivalis LPS can utilize both TLR2 and TLR4. No response was observed from bone marrow cells obtained from TLR2 and TLR4 double-knockout mice, demonstrating that P. gingivalis LPS activation occurred exclusively through either TLR2 or TLR4. Although the biological significance of the different lipid A species found in P. gingivalis LPS preparations is not currently understood, it is proposed that the presence of multiple lipid A species contributes to cell activation through both TLR2 and TLR4.Infection and Immunity 10/2004; 72(9):5041-51. · 4.07 Impact Factor
Article: Toll-like receptor signalling.[show abstract] [hide abstract]
ABSTRACT: One of the mechanisms by which the innate immune system senses the invasion of pathogenic microorganisms is through the Toll-like receptors (TLRs), which recognize specific molecular patterns that are present in microbial components. Stimulation of different TLRs induces distinct patterns of gene expression, which not only leads to the activation of innate immunity but also instructs the development of antigen-specific acquired immunity. Here, we review the rapid progress that has recently improved our understanding of the molecular mechanisms that mediate TLR signalling.Nature reviews. Immunology 08/2004; 4(7):499-511. · 33.13 Impact Factor
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ABSTRACT: Osteoclasts are multinucleated cells of monocyte/macrophage origin that degrade bone matrix. The differentiation of osteoclasts is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor (NF)-kappaB ligand (RANKL), as well as macrophage colony-stimulating factor (M-CSF). Congenital lack of osteoclasts causes osteopetrosis, investigation of which has provided insights into the essential molecules for osteoclastogenesis, including TNF receptor-associated factor (TRAF) 6, NF-kappaB and c-Fos. In addition, genome-wide screening techniques have shed light on an additional set of gene products such as nuclear factor of activated T cells (NFAT) c1. Here we summarize the efforts to understand the sequential molecular events induced by RANKL during osteoclast differentiation. RANKL binds to its receptor RANK, which recruits adaptor molecules such as TRAF6. TRAF6 activates NF-kappaB, which is important for the initial induction of NFATc1. NFATc1 is activated by calcium signaling and binds to its own promoter, thus switching on an autoregulatory loop. An activator protein (AP)-1 complex containing c-Fos is required for the autoamplification of NFATc1, enabling the robust induction of NFATc1. Finally, NFATc1 cooperates with other transcriptional partners to activate osteoclast-specific genes. NFATc1 autoregulation is controlled by an epigenetic mechanism, which has profound implications for an understanding of the general mechanism of irreversible cell fate determination. From the clinical point of view, RANKL signaling pathway has promise as a strategy for suppressing the excessive osteoclast formation characteristic of a variety of bone diseases.Bone 03/2007; 40(2):251-64. · 3.82 Impact Factor