Sambrano G R, Terpstra V, Steinberg D
Department of Medicine, University of California San Diego, La Jolla 92093-0682, USA.
Arterioscler Thromb Vasc Biol. 1997 Dec;17(12):3442-8. doi: 10.1161/01.atv.17.12.3442.
The binding and phagocytosis of oxidatively damaged red blood cells (OxRBCs) by mouse peritoneal macrophages can be inhibited by oxidatively modified LDL (OxLDL), implying some commonality at their receptor-binding domains. Studies from many different laboratories support the view that OxRBC binding is due to the disruption of plasma membrane phospholipid asymmetry and the subsequent exposure of phosphatidylserine (PS) on the outer membrane leaflet. Presumably, oxidation of LDL creates a surface structure on it in some way homologous to the PS-rich domain on OxRBCs. Apoptotic cells in some instances are also recognized because of PS exposure on the outer leaflet of the membrane, and apoptotic cells are a common feature of atherosclerotic lesions. In the present studies, the mechanisms of binding and internalization of cells recognized by virtue of their membrane PS were studied using OxRBCs or vanadate-treated erythrocytes (VaRBCs) as models. Disruption of phospholipid asymmetry with vanadate produced cells that were bound by macrophages in the same divalent cation-dependent manner as OxRBCs. However, whereas OxRBCs were rapidly phagocytosed, VaRBCs were not. Stimulation of mouse macrophages with phorbol myristate acetate resulted in a concentration-dependent induction of phagocytosis of bound VaRBCs, an effect that could be prevented by the protein kinase C inhibitor staurosporine. Because phagocytosis of OxRBCs occurred unassisted, we speculated that there must be additional membrane changes induced by oxidation (over and above the disruption of phospholipid asymmetry) that contribute to phagocytosis of OxRBCs, possibly resulting in the ligation of a distinct receptor that does not necessarily contribute to adherence. This proposal is supported by the finding that ligation of macrophage Fc gamma receptors by the anti-Fc gamma RII/RIII antibody 2.4G2 triggers the phagocytosis of bound VaRBCs. Phagocytosis is also triggered by subthreshold opsonization of VaRBC, i.e., by antibody concentrations that do not by themselves cause binding and phagocytosis of native RBCs. Finally, treatment with low concentrations of glutaraldehyde, which causes membrane protein cross-linking, promotes the phagocytosis of VaRBCs, but, at the low concentration used, has little or no effect on binding and phagocytosis of native RBCs. We suggest that the internalization of damaged cells, bound because of PS exposure, requires the cooperation of a PS-binding receptor with at least one additional receptor to trigger an intracellular signaling pathway to initiate phagocytosis.
氧化修饰的低密度脂蛋白（OxLDL）可抑制小鼠腹腔巨噬细胞对氧化损伤红细胞（OxRBCs）的结合与吞噬作用，这表明它们在受体结合域存在某些共性。许多不同实验室的研究支持这样一种观点，即OxRBC的结合是由于质膜磷脂不对称性的破坏以及随后磷脂酰丝氨酸（PS）在外膜小叶上的暴露。据推测，LDL的氧化以某种方式在其表面形成了一种与OxRBCs上富含PS的结构域同源的结构。在某些情况下，凋亡细胞也因膜外小叶上PS的暴露而被识别，且凋亡细胞是动脉粥样硬化病变的一个常见特征。在本研究中，以OxRBCs或钒酸盐处理的红细胞（VaRBCs）为模型，研究了因细胞膜PS而被识别的细胞的结合和内化机制。钒酸盐破坏磷脂不对称性产生的细胞，其与巨噬细胞的结合方式与OxRBCs相同，都依赖二价阳离子。然而，OxRBCs能被迅速吞噬，而VaRBCs则不能。用佛波酯乙酸肉豆蔻酯刺激小鼠巨噬细胞会导致结合的VaRBCs的吞噬作用呈浓度依赖性诱导，蛋白激酶C抑制剂星形孢菌素可阻止这种效应。由于OxRBCs的吞噬作用无需辅助即可发生，我们推测氧化作用一定还诱导了其他膜变化（除了磷脂不对称性的破坏之外），这些变化有助于OxRBCs的吞噬作用，可能导致一种独特受体的连接，而该受体不一定参与黏附。抗FcγRII/RIII抗体2.4G2对巨噬细胞Fcγ受体的连接会触发结合的VaRBCs的吞噬作用，这一发现支持了这一推测。VaRBC的亚阈值调理作用也能触发吞噬作用，即抗体浓度本身不会引起天然RBC的结合和吞噬，但却能引发吞噬作用。最后，用低浓度戊二醛处理可促进VaRBCs的吞噬作用，戊二醛会导致膜蛋白交联，但在所用的低浓度下，对天然RBC的结合和吞噬作用几乎没有影响。我们认为，因PS暴露而结合的受损细胞的内化需要PS结合受体与至少一种其他受体协同作用，以触发细胞内信号通路来启动吞噬作用。
