Signal transduction of RANK and Toll-like receptor in alveolar bone destruction

在细菌引起的炎症性疾病中，脂多糖是一种有效的骨吸收刺激因子。细菌脂蛋白/脂肽也是病原体特有的分子模式。Toll样受体4(TLR4)是脂多糖的信号受体。TLR6与TLR2的络合物识别二酰基脂多肽。由于TLR和IL-1受体都使用MyD88作为共同的信号分子，所以TLR的信号级联与IL-1受体相似。Toll-IL-1受体结构域适配子诱导的干扰素-γ(TRIF)介导的信号也被证明参与了内毒素诱导的MyD88非依赖途径。利用MyD88缺陷(MyD88；-/-&gt；)小鼠和TRIF缺陷(TRIF^&lt；-/-&gt；)小鼠，我们研究了MyD88和TRIF在破骨细胞分化和功能中的作用。脂多糖、二酰基脂肽(DL)和IL-1可促进来自TRIF^&lt；-/-&gt；小鼠成骨细胞和造血细胞共培养的破骨细胞生成，但不能刺激MyD88^&lt；-/-&gt；小鼠的破骨细胞生成。MyD88；-/-&gt；m…的骨髓造血细胞更多的ICE和TRIF^&lt；-/-&gt；小鼠同样在RANKL加M-CSF的作用下分化为破骨细胞。脂多糖、DL和IL-1可刺激成骨细胞RANKL mRNA的表达，但对MyD88；-/-&gt；成骨细胞RANKL的表达无明显影响，提示只有MyD88介导的信号在成骨细胞RANKL表达中起重要作用。这一发现特别有趣，因为依赖MyD88和依赖TRIF的通路对于内毒素诱导巨噬细胞产生细胞因子都是必不可少的。事实上，内毒素未能刺激TRIF^&lt；-/-&gt；骨髓巨噬细胞产生IL-6。但内毒素可刺激TRIF^&lt；-/-&gt；成骨细胞产生IL-6。脂多糖和IL-1可促进破骨细胞存活，但不能促进破骨细胞存活。DL不支持破骨细胞的存活，因为破骨细胞中缺乏TLR6。TRIF相关的适配器分子(TRAM)主要参与TRIF介导的信号通路。有趣的是，巨噬细胞同时表达TRIF和TRAM基因，而成骨细胞和破骨细胞仅表达TRIF基因。在内毒素诱导的成骨细胞中RANKL和IL-6的表达以及破骨细胞的存活不需要TRIF介导的信号，这可能与成骨细胞和破骨细胞中缺乏TRAM的表达有关。骨组织形态计量学显示，MyD88^&lt；-/-&gt；小鼠表现出低周转率的骨质疏松，骨吸收和骨形成减少。这些结果表明，MyD88介导的信号对IL-1和TLR配体诱导的破骨细胞的形成和功能是必不可少的，MyD88在生理学上参与了骨转换。较少

LPS is a potent stimulator of bone resorption in inflammatory diseases caused by bacteria. Bacterial lipoprotein/lipopeptides are also pathogen-specific molecular patterns. Toll-like receptor 4 (TLR4) is identified as the signaling receptor for LPS. The complex of TLR6 and TLR2 recognizes diacyl lipopeptide. The signaling cascade of TLR is similar to that of IL-1 receptors, because both TLR and IL-1receptors use MyD88 as a common signaling molecule. Toll-IL-1 receptor domain-containing adapter inducing interferon-γ (TRIF)-mediated signals are also shown to be involved in LPS-induced MyD88-independent pathway. Using MyD88-deficient (MyD88^<-/->) mice and TRIF-deficient (TRIF^<-/->) mice, we examined roles of MyD88 and TRIF in osteoclast differentiation and function. LPS, diacyl lipopeptide (DL) and IL-1 stimulated osteoclastogenesis in co-cultures of osteoblasts and hemopoietic cells obtained from TRIF^<-/-> mice but not MyD88^<-/-> mice. Bone marrow hemopoietic cells from MyD88^<-/-> m … More ice and TRIF^<-/-> mice similarly differentiated into osteoclasts in response to RANKL plus M-CSF. LPS, DL and IL-1 stimulated RANKL mRNA expression in TRIF^<-/-> osteoblasts but not MyD88^<-/-> osteoblasts, suggesting that only MyD88-mediated signal in osteoblasts was important for RANKL expression in response to those factors. This finding was particularly interesting, because both MyD88-dependent and TRIF-dependent pathways are essential for LPS-induced cytokine production in macrophages. Indeed, LPS failed to stimulate IL-6 production in TRIF^<-/-> bone marrow macrophages. But, LPS could stimulate IL-6 production in TRIF^<-/-> osteoblasts. LPS and IL-1 enhanced the survival of TRIF^<-/-> osteoclasts but not MyD88^<-/-> osteoclasts. DL did not support the survival of osteoclasts, because of the lack of TLR6 in osteoclasts. TRIF-related adaptor molecule (TRAM) was shown to be essentially involved in the TRIF-mediated signaling pathway. Interestingly, macrophages expressed both TRIF and TRAM mRNAs, while osteoblasts and osteoclasts expressed only TRIF mRNA. The fact that TRIF-mediated signals are not required for LPS-induced RANKL and IL-6 expression in osteoblasts and for osteoclast survival may be related to the lack of TRAM expression in osteoblasts and osteoclasts. Bone histomorphometry showed that MyD88^<-/-> mice exhibited low turnover osteoporosis with reduced bone resorption and formation. These results suggest that the MyD88-mediated signal is essential for the osteoclastogenesis and function induced by IL-1 and TLR ligands, and that MyD88 is physiologically involved in bone turnover. Less