Shi W X, Chammas R, Varki N M, Powell L, Varki A
Glycobiology Program, UCSD Cancer Center, the Division of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, California 92093, USA.
J Biol Chem. 1996 Dec 6;271(49):31526-32. doi: 10.1074/jbc.271.49.31526.
O-Acetylation of the 9-hydroxyl group of sialic acids has been suggested to modify various recognition phenomena involving these molecules, but direct proof has been lacking in most situations. In the accompanying paper (Shi, W.-X., Chammas, R., and Varki, A. (1996) J. Biol. Chem. 261, 31517-31525), we report that the extent of 9-O-acetylation of cell surface sialic acids on murine erythroleukemia (MEL) cells can be modified by various manipulations, including differentiation, nocodazole treatment, and 9-O-acetyl esterase treatment. We have used this system to explore the putative roles of 9-O-acetylation in modulating alternative pathway complement activation, I-type lectin binding, and tissue homing. MEL cells are shown to be sensitive to lysis in vitro by the alternative pathway of human complement. Induced differentiation of the MEL cells causes resistance to lysis, and this correlates directly with extent of decrease in 9-O-acetylation. A similar resistance to alternative pathway lysis can be obtained by selective enzymatic removal of 9-O-acetyl groups from sialic acids. Conversely, the increase in cell surface 9-O-acetylation caused by nocodazole treatment correlates with increased sensitivity to alternative pathway lysis. Thus, a 9-O-acetyl group added to the side chain of cell surface sialic acids may abrogate its normal function in restricting alternative pathway activation. Indeed, the binding of human complement factor H, a negative regulator of the alternative pathway, is shown to be blocked by O-acetylation of the sialic acids on MEL cells. MEL cells are also shown to have cell surface ligands for the I-type lectins sialoadhesin and CD22. Sialoadhesin (but not CD22) binding is selectively enhanced by differentiation-induced loss of cell surface 9-O-acetylation and by direct enzymatic removal of the ester groups. Thus, some sialoadhesin ligands are masked by 9-O-acetylation, presumably because the side chain is required for recognition. Since sialoadhesin is expressed on some macrophages in vivo, we reasoned that tissue homing of MEL cells might be affected by O-acetylation. Indeed, enzymatic removal of cell surface 9-O-acetyl groups alters the tissue distribution of intravenously injected cells. In particular, de-O-acetylation caused significant increase in homing to the liver and spleen. These data demonstrate that cell surface 9-O-acetylation can affect a variety of biological recognition phenomena and provide a system for further exploration of the specific molecular mechanisms involved.
有人提出，唾液酸9-羟基的O-乙酰化作用可改变涉及这些分子的各种识别现象，但在大多数情况下一直缺乏直接证据。在随附的论文中（Shi, W.-X., Chammas, R., and Varki, A. (1996) J. Biol. Chem. 261, 31517 - 31525），我们报告了小鼠红白血病（MEL）细胞表面唾液酸的9-O-乙酰化程度可通过多种操作进行改变，包括分化、诺考达唑处理和9-O-乙酰酯酶处理。我们利用这个系统来探究9-O-乙酰化在调节替代途径补体激活、I型凝集素结合和组织归巢中的假定作用。结果显示，MEL细胞在体外对人补体的替代途径裂解敏感。MEL细胞的诱导分化使其产生抗裂解能力，这与9-O-乙酰化程度的降低直接相关。通过从唾液酸中选择性地酶促去除9-O-乙酰基团，也可获得类似的对替代途径裂解的抗性。相反，诺考达唑处理导致的细胞表面9-O-乙酰化增加与对替代途径裂解的敏感性增加相关。因此，添加到细胞表面唾液酸侧链上的9-O-乙酰基团可能会消除其在限制替代途径激活中的正常功能。实际上，已表明替代途径的负调节因子人补体因子H与MEL细胞上唾液酸的O-乙酰化结合被阻断。还显示MEL细胞具有I型凝集素唾液酸黏附素和CD22的细胞表面配体。分化诱导的细胞表面9-O-乙酰化丧失以及酯基团的直接酶促去除可选择性增强唾液酸黏附素（而非CD22）的结合。因此，一些唾液酸黏附素配体被9-O-乙酰化所掩盖，推测是因为识别需要侧链。由于唾液酸黏附素在体内某些巨噬细胞上表达，我们推断MEL细胞的组织归巢可能受O-乙酰化影响。实际上，细胞表面9-O-乙酰基团的酶促去除改变了静脉注射细胞的组织分布。特别是，去O-乙酰化导致归巢至肝脏和脾脏的显著增加。这些数据表明，细胞表面9-O-乙酰化可影响多种生物识别现象，并为进一步探索所涉及的具体分子机制提供了一个系统。
