The aim of our study was to investigate the impact of macroautophagy on exosome secretion. Exosomes are small membrane vesicles released in the extracellular space upon fusion of multivesicular endosomes with the plasma membrane. They were initially discovered as a way to remodel the reticulocyte plasma membrane before entering the blood circulation (Current Opinion in Hematology 2010, 17:177-183) and are now essentially studied as mediators of intercellular communication. Using iTRAQ proteomics, we compared the protein composition of purified exosomes secreted by cells impaired or not for macroautophagy by Atg5 depletion, during serum starvation conditions or complete medium culture. We show that the absence of serum modifies exosomal content, especially inducing secretion of two cytoplasmic protein complexes, namely proteasomal 19S regulatory particle (RP) and components of noncanonical translation preinitiation complex (PIC). This process is enhanced when autophagy is impaired by Atg5 depletion. Moreover, we show that the proteasome 20S core particle (CP) is released in the extracellular space. However, in striking contrast to what seen for its 19S RP regulator, release is independent of the exosomal vesicles, Atg5 expression and cell culture conditions. Exosome secretion can thus be considered as a cell process that participates in and reflects cell homeostasis, and care must be taken when studying potential extracellular function of exosomes due to the possible copurification of proteasome 20S CP.

中文翻译：

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血清饥饿时，外泌体内分泌蛋白酶体19S RP和翻译预启动复合体。

我们研究的目的是研究巨自噬对外泌体分泌的影响。外泌体是多囊泡内体与质膜融合后释放到细胞外空间的小膜囊泡。最初发现它们是在进入血液循环之前重塑网状细胞质膜的一种方法（Current Opinion in Hematology 2010，17：177-183），现在已被作为细胞间通讯的媒介进行了研究。使用iTRAQ蛋白质组学，我们比较了在血清饥饿条件下或完全培养基培养过程中，Atg5耗竭的受损或未受损的细胞分泌的纯化外泌体的蛋白质组成。我们表明，血清的缺乏会改变外泌体的含量，尤其是诱导两种胞浆蛋白复合物的分泌，即蛋白酶体19S调节颗粒（RP）和非规范翻译预起始复合物（PIC）的成分。当Atg5耗尽削弱自噬时，此过程会得到增强。此外，我们表明蛋白酶体20S核心粒子（CP）被释放到细胞外空间。但是，与19S RP调节剂形成鲜明对比的是，释放与外泌体囊泡，Atg5表达和细胞培养条件无关。因此，可以将外泌体分泌视为参与并反映细胞稳态的细胞过程，由于可能共纯化蛋白酶体20S CP，因此在研究外泌体的潜在胞外功能时必须小心。此外，我们表明蛋白酶体20S核心粒子（CP）被释放到细胞外空间。但是，与19S RP调节剂形成鲜明对比的是，释放与外泌体囊泡，Atg5表达和细胞培养条件无关。因此，可以将外泌体分泌视为参与并反映细胞稳态的细胞过程，由于可能共纯化蛋白酶体20S CP，因此在研究外泌体的潜在胞外功能时必须小心。此外，我们表明蛋白酶体20S核心粒子（CP）被释放到细胞外空间。但是，与19S RP调节剂形成鲜明对比的是，释放与外泌体囊泡，Atg5表达和细胞培养条件无关。因此，可以将外泌体分泌视为参与并反映细胞稳态的细胞过程，由于可能共纯化蛋白酶体20S CP，因此在研究外泌体的潜在胞外功能时必须小心。