Tokyo University of Pharmacy and Life Sciences, Japan.
FEBS J. 2010 May;277(9):2051-66. doi: 10.1111/j.1742-4658.2010.07620.x. Epub 2010 Mar 19.
Lactoferrin (LF) has been implicated in innate immunity. Here we reveal the signal transduction pathway responsible for human LF (hLF)-triggered nuclear factor-kappaB (NF-kappaB) activation. Endotoxin-depleted hLF induces NF-kappaB activation at physiologically relevant concentrations in the human monocytic leukemia cell line, THP-1, and in mouse embryonic fibroblasts (MEFs). In MEFs, in which both tumor necrosis factor receptor-associated factor 2 (TRAF2) and TRAF5 are deficient, hLF causes NF-kappaB activation at a level comparable to that seen in wild-type MEFs, whereas TRAF6-deficient MEFs show significantly impaired NF-kappaB activation in response to hLF. TRAF6 is known to be indispensable in leading to NF-kappaB activation in myeloid differentiating factor 88 (MyD88)-dependent signaling pathways, while the role of TRAF6 in the MyD88-independent signaling pathway has not been clarified extensively. When we examined the hLF-dependent NF-kappaB activation in MyD88-deficient MEFs, delayed, but remarkable, NF-kappaB activation occurred as a result of the treatment of cells with hLF, indicating that both MyD88-dependent and MyD88-independent pathways are involved. Indeed, hLF fails to activate NF-kappaB in MEFs lacking Toll-like receptor 4 (TLR4), a unique TLR group member that triggers both MyD88-depependent and MyD88-independent signalings. Importantly, the carbohydrate chains from hLF are shown to be responsible for TLR4 activation. Furthermore, we show that lipopolysaccharide-induced cytokine and chemokine production is attenuated by intact hLF but not by the carbohydrate chains from hLF. Thus, we present a novel model concerning the biological function of hLF: hLF induces moderate activation of TLR4-mediated innate immunity through its carbohydrate chains; however, hLF suppresses endotoxemia by interfering with lipopolysaccharide-dependent TLR4 activation, probably through its polypeptide moiety.
乳铁蛋白 (LF) 与先天免疫有关。在这里，我们揭示了负责人乳铁蛋白 (hLF) 触发核因子-κB (NF-κB) 激活的信号转导途径。在生理相关浓度下，去除内毒素的 hLF 可诱导人单核白血病细胞系 THP-1 和小鼠胚胎成纤维细胞 (MEF) 中 NF-κB 的激活。在 TRAF2 和 TRAF5 均缺失的 MEF 中，hLF 引起 NF-κB 的激活水平与野生型 MEF 相当，而 TRAF6 缺失的 MEF 对 hLF 的 NF-κB 激活明显受损。TRAF6 在髓样分化因子 88 (MyD88) 依赖性信号通路中对 NF-κB 的激活是不可或缺的，而 TRAF6 在 MyD88 非依赖性信号通路中的作用尚未得到广泛阐明。当我们在 MyD88 缺失的 MEF 中检测 hLF 依赖性 NF-κB 激活时，尽管延迟，但 hLF 处理细胞后仍会发生显著的 NF-κB 激活，表明 MyD88 依赖性和 MyD88 非依赖性途径均参与其中。事实上，hLF 不能在缺乏 Toll 样受体 4 (TLR4) 的 MEF 中激活 NF-κB，TLR4 是触发 MyD88 依赖性和 MyD88 非依赖性信号的独特 TLR 家族成员。重要的是，hLF 的糖链负责 TLR4 的激活。此外，我们还发现完整的 hLF 可减弱脂多糖诱导的细胞因子和趋化因子的产生，但 hLF 的糖链则不能。因此，我们提出了一个关于 hLF 生物学功能的新模型：hLF 通过其糖链诱导 TLR4 介导的先天免疫的适度激活；然而，hLF 通过干扰脂多糖依赖性 TLR4 激活来抑制内毒素血症，可能是通过其多肽部分。
