Division of Medical Protein Chemistry, Department of Translational Medicine, Lund University, 21428 Malmö, Sweden.
Department of Molecular Biology and Genetics, Aarhus University, 8000 Aarhus, Denmark.
J Immunol. 2020 May 15;204(10):2779-2790. doi: 10.4049/jimmunol.1900372. Epub 2020 Apr 6.
We identified apolipoprotein E (ApoE) as one of the proteins that are found in complex with complement component C4d in pooled synovial fluid of rheumatoid arthritis (RA) patients. Immobilized human ApoE activated both the classical and the alternative complement pathways. In contrast, ApoE in solution demonstrated an isoform-dependent inhibition of hemolysis and complement deposition at the level of sC5b-9. Using electron microscopy imaging, we confirmed that ApoE interacts differently with C1q depending on its context; surface-bound ApoE predominantly bound C1q globular heads, whereas ApoE in a solution favored the hinge/stalk region of C1q. As a model for the lipidated state of ApoE in lipoprotein particles, we incorporated ApoE into phosphatidylcholine/phosphatidylethanolamine liposomes and found that the presence of ApoE on liposomes increased deposition of C1q and C4b from serum when analyzed using flow cytometry. In addition, posttranslational modifications associated with RA, such as citrullination and oxidation, reduced C4b deposition, whereas carbamylation enhanced C4b deposition on immobilized ApoE. Posttranslational modification of ApoE did not alter C1q interaction but affected binding of complement inhibitors factor H and C4b-binding protein. This suggests that changed ability of C4b to deposit on modified ApoE may play an important role. Our data show that posttranslational modifications of ApoE alter its interactions with complement. Moreover, ApoE may play different roles in the body depending on its solubility, and in diseased states such as RA, deposited ApoE may induce local complement activation rather than exert its typical role of inhibition.
我们鉴定出载脂蛋白 E（ApoE）是类风湿关节炎（RA）患者关节滑液中与补体成分 C4d 形成复合物的蛋白之一。固定化的人 ApoE 可激活经典和替代补体途径。相比之下，溶液中的 ApoE 表现出依赖于同工型的抑制作用，即在 sC5b-9 水平抑制溶血和补体沉积。通过电子显微镜成像，我们证实 ApoE 与 C1q 的相互作用因环境而异；表面结合的 ApoE 主要结合 C1q 球状头部，而溶液中的 ApoE 则有利于 C1q 的铰链/柄部区域。作为 ApoE 在脂蛋白颗粒中酯化状态的模型，我们将 ApoE 掺入磷脂酰胆碱/磷脂酰乙醇胺脂质体中，发现使用流式细胞术分析时，脂质体上存在 ApoE 会增加来自血清的 C1q 和 C4b 的沉积。此外，与 RA 相关的翻译后修饰，如瓜氨酸化和氧化，会减少 C4b 的沉积，而氨甲酰化则会增强固定化 ApoE 上 C4b 的沉积。ApoE 的翻译后修饰不会改变 C1q 的相互作用，但会影响补体抑制剂因子 H 和 C4b 结合蛋白的结合。这表明 C4b 在修饰的 ApoE 上沉积能力的改变可能发挥重要作用。我们的数据表明，ApoE 的翻译后修饰改变了其与补体的相互作用。此外，ApoE 可能根据其可溶性在体内发挥不同的作用，在 RA 等疾病状态下，沉积的 ApoE 可能会诱导局部补体激活，而不是发挥其典型的抑制作用。
