Molecular mechanisms of altered membrane microdomain sensitivity leading to initiation of the arachidonic acid metabolism

在这项研究中，我们研究了细胞膜微域的扰动导致各种哺乳动物分泌磷脂酶A2（sPLA2）酶启动花生四烯酸（AA）释放的分子机制。我们根据sPLA2s不同的膜微域敏感性将其细分为三类：(i) sPLA2-IIA型，作用于活化细胞的诱导膜；（ii） sPLA2-X型，作用于静止细胞的未修饰膜；（iii） spla2 - v型，它同时具有sPLA2-IIA和sPLA2-X型的性质。sPLA2-IIA和相关酶（IID和IIE）结合到筏/小泡中富集的硫酸肝素蛋白聚糖（HSPG） glypican上，并内化到核周膜烷室中，在那里它们表现出释放aa的功能（HSPG穿梭途径）。sPLA2-X由于无法结合HSPG而无法利用HSPG穿梭途径，但它对磷脂酰胆碱（PC）具有高亲和力，可以作用于富含PC的质膜外小叶（外质膜（EPM）途径）。sPLA2-V对HSPG和PC都具有高亲和力，因此能够利用这两种途径。尽管sPLA2-IIF微弱地诱导基于EPM途径的AA释放，但其功能在活化细胞中大大促进，该酶可能通过其独特的c端延伸与受干扰的质膜微域相互作用。sPLA2III由sPLA2中心结构域和独特的c端结构域组成，能够利用EPM途径。此外，高阳离子的n端和c端结构域允许sPLA2-III结合阴离子HSPG，从而进入HSPG穿梭路线。总的来说，我们目前的分析表明，各种sPLA2酶的细胞作用受到其酶学性质和亚细胞定位的关键影响，后者与细胞膜的动力学密切相关。

In this study, we have investigated the molecular mechanisms of the perturbation of cellular membrane microdomains leading to the initiation of arachidonic acid (AA) release by various mammalian secretory phospholipase A2(sPLA2) enzymes. We have subdivided sPLA2s into three classes in terms of their different membrane microdomain sensisivities: (i) the sPLA2-IIA type, which acts on elicited membranes in activated cells;(ii) the sPLA2-X type, which acts on unmodified membranes in quiescent cells;and (iii) the sPLA-V type, which displays both the sPLA2-IIA and sPLA2-X type properties. sPLA2-IIA and related enzymes (IID and IIE) bind to the heparan sulfate proteoglycan (HSPG) glypican that is enriched in the rafts/caveolae,and are internalized into the perinuclear membtane compartments where they exhibit the AA-releasing function(the HSPG-shuttling pathway). sPLA2-X is unable to utilize the HSPG-shuttling pathway because of its inability to bind HSPG,yet it has a high affinity for phosphatidylcholine(PC) and can act on the PC-rich outer leaflet of the plasma membrane (the external plasma membrane (EPM) pathway). sPLA2-V has high affinity for both HSPG and PC, thereby being able to utilize both pathways, Although sPLA2-IIF weakly elicits the EPM pathway-based AA release, its function is greatly faciliated in activated cells, where this enzyme may interacts with the perturbed plasma membrame microdomain through its unique C-terminal extension. sPLA2III,which is composed of the central sPLA2 domain flanked with unique C-terminal domains, is capable of utilizing the EPM pathway. In addition, the highly cationic N-and C-terminal domains allow sPLA2-III to bind anionic HSPG and thus to enter the HSPG-shuttiling route. Collectively, our present analyses have revealed that the cellular actions of various sPLA2 enzymes are crucially affected by their anzymatic properties and subcellular localization, the latter of which is tightly linked with the dymanics of cellular membranes.