Smrz Daniel, Dráberová L'ubica, Dráber Petr
Department of Signal Transduction, Institute of Molecular Genetics, Academy of Sciences of the Czech Republic, CZ 142 20 Prague 4, Czech Republic.
J Biol Chem. 2007 Apr 6;282(14):10487-97. doi: 10.1074/jbc.M611090200. Epub 2007 Feb 5.
The exposure of phosphatidylserine (PS) on the cell surface is a general marker of apoptotic cells. Non-apoptotic PS externalization is induced by several activation stimuli, including engagement of immunoreceptors. Immune cells can also be activated by aggregation of glycosylphosphatidylinositol-anchored proteins (GPI-APs). However, it is unknown whether cell triggering through these proteins, lacking transmembrane and cytoplasmic domains, also leads to PS externalization. Here we show that engagement of GPI-APs in rodent mast cells induces a rapid and reversible externalization of PS by a non-apoptotic mechanism. PS externalization triggered by GPI-AP-specific monoclonal antibodies was dependent on the activity of H(+)-ATP synthase and several other enzymes involved in mast cell signaling but was independent of cell degranulation, free cytoplasmic calcium up-regulation, and a decrease in lipid packing as determined by merocyanine 540 binding. Surprisingly, disruption of actin cytoskeleton by latrunculin B or plasma membrane integrity by methyl-beta-cyclodextrin had opposite effects on PS externalization triggered through GPI-AP or the high affinity IgE receptor. We further show that PS externalization mediated by GPI-APs was also observed in some other cells, and its extent varied with antibodies used. Interestingly, effects of different antibodies on PS externalization were additive, indicating that independent stimuli converge onto a signaling pathways leading to PS externalization. Our findings identify the cell surface PS exposure induced through GPI-AP as a distinct mechanism of cell signaling. Such a mechanism could contribute to "inside-out" signaling in response to pathogens and other external activators and/or to initiation of other functions associated with PS externalization.
细胞表面磷脂酰丝氨酸（PS）的暴露是凋亡细胞的一个普遍标志。非凋亡性PS外化可由多种激活刺激诱导，包括免疫受体的结合。免疫细胞也可通过糖基磷脂酰肌醇锚定蛋白（GPI-APs）的聚集而被激活。然而，尚不清楚通过这些缺乏跨膜和细胞质结构域的蛋白质触发细胞是否也会导致PS外化。在这里，我们表明啮齿动物肥大细胞中GPI-APs的结合通过非凋亡机制诱导PS快速且可逆的外化。GPI-AP特异性单克隆抗体触发的PS外化依赖于H(+)-ATP合酶和肥大细胞信号传导中涉及的其他几种酶的活性，但与细胞脱颗粒、游离细胞质钙上调以及通过部花青540结合测定的脂质堆积减少无关。令人惊讶的是，用拉特罗毒素B破坏肌动蛋白细胞骨架或用甲基-β-环糊精破坏质膜完整性对通过GPI-AP或高亲和力IgE受体触发的PS外化有相反的影响。我们进一步表明，在其他一些细胞中也观察到了由GPI-APs介导的PS外化，其程度因所用抗体而异。有趣的是，不同抗体对PS外化的影响是相加的，表明独立的刺激汇聚到导致PS外化的信号通路。我们的发现确定了通过GPI-AP诱导的细胞表面PS暴露是一种独特的细胞信号传导机制。这种机制可能有助于响应病原体和其他外部激活剂的“由内向外”信号传导和/或启动与PS外化相关的其他功能。
