Ingalls R R, Monks B G, Golenbock D T
The Maxwell Finland Laboratory for Infectious Diseases, Boston Medical Center, Boston, Massachusetts 02118, USA.
J Biol Chem. 1999 May 14;274(20):13993-8. doi: 10.1074/jbc.274.20.13993.
The activation of phagocytes by lipopolysaccharide (LPS) has been implicated in the pathogenesis of Gram-negative sepsis. Although the interaction between CD14 and LPS is a key event in the signaling cascade, the molecular mechanism by which cellular activation occurs remains obscure. We hypothesized that the main function of CD14 was to bind LPS and transfer it to a second receptor, which then initiates the subsequent signal for cellular activation. Thus, surface binding of LPS to the cell membrane would be the critical step that CD14 carries out. To test this hypothesis, we examined the activity of two other proteins known to bind LPS, lipopolysaccharide-binding protein and bactericidal/permeability-increasing protein. We found that when these normally soluble proteins were expressed in Chinese hamster ovary-K1 fibroblasts as glycosylphosphatidylinositol-anchored proteins, both could substitute for CD14 in initiating LPS signaling. Pharmacological studies with synthetic lipid A analogues demonstrated that these surface expressed LPS-binding proteins had characteristics that were qualitatively identical to membrane CD14. These data support the hypothesis that a receptor distinct from CD14 functions as the actual signal transducer and suggest that surface binding of LPS to the cell membrane is the crucial first step for initiating downstream signaling events.
脂多糖（LPS）激活吞噬细胞与革兰氏阴性菌败血症的发病机制有关。尽管CD14与LPS之间的相互作用是信号级联反应中的关键事件，但细胞激活发生的分子机制仍不清楚。我们推测CD14的主要功能是结合LPS并将其转移至第二个受体，该受体随后启动细胞激活的后续信号。因此，LPS与细胞膜的表面结合将是CD14执行的关键步骤。为验证这一假设，我们检测了另外两种已知可结合LPS的蛋白，即脂多糖结合蛋白和杀菌/通透性增加蛋白的活性。我们发现，当这些通常为可溶性的蛋白在中国仓鼠卵巢-K1成纤维细胞中作为糖基磷脂酰肌醇锚定蛋白表达时，二者均可替代CD14启动LPS信号传导。用合成类脂A类似物进行的药理学研究表明，这些表面表达的LPS结合蛋白具有与膜CD14在性质上相同的特征。这些数据支持了以下假设：一种不同于CD14的受体作为实际的信号转导器发挥作用，并表明LPS与细胞膜的表面结合是启动下游信号事件的关键第一步。
