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剂。细菌脂蛋白/脂肽也是病原体特异性的分子模式。toll样受体4 （TLR4）被认为是LPS的信号受体。TLR6和TLR2复合物可识别二酰基脂肽。TLR的信号级联与IL-1受体的信号级联相似，因为TLR和IL-1受体都使用MyD88作为共同的信号分子。Toll-IL-1受体结构域适配器诱导的干扰素-γ (TRIF)介导的信号也被证明参与了lps诱导的myd88独立通路。使用MyD88缺失（MyD88^<-/->）小鼠和TRIF缺失（TRIF^<-/->）小鼠，我们研究了MyD88和TRIF在破骨细胞分化和功能中的作用。LPS、二酰基脂肽（DL）和IL-1可刺激TRIF^<-/->小鼠的成骨细胞和造血细胞的共培养，而MyD88^<-/->小鼠则无此作用。MyD88^<-/-> m . More ice和TRIF^<-/->小鼠骨髓造血细胞在RANKL + m - csf作用下相似地分化为破骨细胞。LPS、DL和IL-1可刺激TRIF^<-/->成骨细胞中RANKL mRNA的表达，而对MyD88^<-/->成骨细胞无影响，提示成骨细胞中只有MyD88介导的信号对RANKL的表达起重要作用。这一发现特别有趣，因为myd88依赖途径和trif依赖途径都是巨噬细胞中lps诱导的细胞因子产生所必需的。事实上，LPS不能刺激TRIF^<-/->骨髓巨噬细胞产生IL-6。而脂多糖可以刺激TRIF^<-/->成骨细胞产生IL-6。LPS和IL-1可提高TRIF^<-/->破骨细胞的存活率，但对MyD88^<-/->破骨细胞无作用。DL不支持破骨细胞存活，因为破骨细胞中缺乏TLR6。trf相关适配分子（TRAM）在trf介导的信号通路中起重要作用。有趣的是，巨噬细胞同时表达TRIF和TRAM mRNA，而成骨细胞和破骨细胞仅表达TRIF mRNA。成骨细胞中脂多糖诱导的RANKL和IL-6表达以及破骨细胞存活不需要trif介导的信号，这可能与成骨细胞和破骨细胞中缺乏TRAM表达有关。骨组织形态学测量显示MyD88^<-/->小鼠表现为低周转骨质疏松症，骨吸收和骨形成减少。这些结果表明，MyD88介导的信号对于IL-1和TLR配体诱导的破骨细胞发生和功能至关重要，MyD88在生理上参与骨转换。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