Fan Xiaoxuan, Krahling Stephen, Smith Douglas, Williamson Patrick, Schlegel Robert A
Department of Biochemistry and Molecular Biology, Penn State University, University Park, Pennsylvania 16802, USA.
Mol Biol Cell. 2004 Jun;15(6):2863-72. doi: 10.1091/mbc.e03-09-0670. Epub 2004 Apr 2.
When cells undergo apoptosis, or programmed cell death, they expose phosphatidylserine (PS) on their surface. Macrophages that efficiently phagocytose apoptotic cells also express PS on their surface, although at a lower level. The PS exposed on both cells is required for phagocytosis, because uptake is inhibited by masking PS on either cell with annexin V, a PS-binding protein. The inhibition is not additive, suggesting that the exposed PS molecules on the two cells participate in a common process. We asked whether this dual requirement reflects bridging of the target cell and macrophage by bivalent, PS-binding annexins. Monoclonal antibodies (mAbs) against annexins I or II stained a variety of live phagocytes. Apoptotic Jurkat T lymphocytes and human peripheral T lymphocytes, but not apoptotic thymocytes, were stained by anti-annexin I but not II. Phagocytosis of apoptotic targets was inhibited by mAbs to annexins I or II, or by pretreatment of macrophages with the same mAbs. Pretreatment of apoptotic thymocytes had no effect, whereas pretreating Jurkat cells with anti-annexin I or removing annexin I with EGTA was inhibitory. Annexin bridging is vectorial, because annexin is bound to PS molecules on targets but not on macrophages, suggesting annexins serve as both ligand and receptor in promoting phagocytosis.
当细胞发生凋亡，即程序性细胞死亡时，它们会在表面暴露磷脂酰丝氨酸（PS）。能够有效吞噬凋亡细胞的巨噬细胞表面也表达PS，不过水平较低。吞噬作用需要两个细胞表面都暴露PS，因为用PS结合蛋白膜联蛋白V封闭任何一个细胞表面的PS都会抑制摄取。这种抑制作用没有累加效应，这表明两个细胞表面暴露的PS分子参与了一个共同过程。我们探究了这种双重需求是否反映了二价的、结合PS的膜联蛋白在靶细胞和巨噬细胞之间形成的桥连作用。针对膜联蛋白I或II的单克隆抗体（mAb）可对多种活吞噬细胞进行染色。抗膜联蛋白I而非抗膜联蛋白II可对凋亡的Jurkat T淋巴细胞和人外周血T淋巴细胞进行染色，但不能对凋亡的胸腺细胞进行染色。针对膜联蛋白I或II的mAb，或者用相同的mAb预处理巨噬细胞，均可抑制对凋亡靶标的吞噬作用。对凋亡胸腺细胞进行预处理没有效果，而用抗膜联蛋白I预处理Jurkat细胞或用乙二醇双乙醚四乙酸（EGTA）去除膜联蛋白I则具有抑制作用。膜联蛋白桥连具有方向性，因为膜联蛋白与靶细胞而非巨噬细胞表面的PS分子结合，这表明膜联蛋白在促进吞噬作用中既作为配体又作为受体发挥作用。
