| Co-Investigator(Kenkyū-buntansha) |
KOBAYASHI Yasuhiro Institute of Oral Science, Calcified Tissue Research, Associate Professor, 総合歯科医学研究所, 助教授 (20264252)
HIRAOKA Yukihiko Institute of Oral Science, Calcified Tissue Research, Professor, 総合歯科医学研究所, 教授 (20097512)
TAKAHASHI Naoyuki Institute of Oral Science, Calcified Tissue Research, Professor, 総合歯科医学研究所, 教授 (90119222)
MIZOGUCHI Toshihide Institute of Oral Science, Calcified Tissue Research, Research Assistant, 総合歯科医学研究所, 助手 (90329475)
OKUMURA Higeki School of Dentistry, Biochemistry, Research Assistant, 歯学部, 助手 (80350825)
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| Research Abstract |
Lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, is proposed to be a potent stimulator of bone resorption in inflammatory diseases caused by bacreria. Bacterial lipoprotein/lipopeptides are also pathogen-specific molecular patterns. Recently, toll-like receptor 4 (TLR4) was identified as the signaling receptor for LPS. In addition, TLR6 associate with TLR2, and the complex of TLR6 and TLR2 recognizes diacylated mycoplasmal lipopeptides. The signaling cascade of TLR is believed to be similar to that of IL-1 receptors (IL- 1R), because both TLR and IL-1R use myeloid differentiation factor 88 (MyD88) as a common cytoplasmic signaling molecule. However, accumulating evidence also demonstrates the existence of MyD88-independent pathways, which may explain unique biological responses of individual TLR and IL-1R. Using MyD88-deficient (-/-) mice, we explored the involvement of MyD88-mediated signals in osteoclast formation. LPS, synthetic lipopept
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ide (FSL-1), IL-1a and 1.25(OH)_2D_3 all stimulated osteoclast formation in co-cultures of primary osteoblasts and bone marrow cells obtained from wild-type mice. Osteoprotegerin, a decoy receptor of RANKL, completely inhibited the osteoclast formation in the co-culture. In contrasts, LPS, lipopeptide and IL-1a failed to induce the osteoclast formation in the co-culture of Myd88 (-/-) mice-derived osteoblasts and bone marrow cells, though 1.25(OH)_2D_3 stimulated osteoclast formation even in the MyD88 (-/-) co-culture. RT-PCR analysis showed that primary osteoblasts obtained from both wild type and MyD88 (-/-) mice similarly expressed TLR2, TLR4, TLR6 and IL-1R mRNAs. LPS, lipopeptide and IL-1a stimulated expression of RANKL mRNA within 24 hr in primary osteoblasts obtained from wild-type mice but not in those from MyD88 (-/-) mice. Similarly, LPS and IL-1a stimulated phosphorylation of ERK in wild-type osteoblasts but not MyD88 (-/-) osteoblasts. Hemopoietic cells obtained from MyD88 (-/-) mice and those from wild-type mice similarly differentiated into osteoclasts within 3 days in response to RANKL and M-CSF. These results suggest that the MyD88-mediated signaling pathway is essentially involved in osteoclast formation induced by LPS, lipopeptide and IL-1a through the RANKL expression by osteoblasts. Less
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