INSERM, U848, Villejuif, France.
Autophagy. 2013 Mar;9(3):415-7. doi: 10.4161/auto.22910. Epub 2012 Dec 7.
A chemical screen designed to identify novel inducers of autophagy led to the discovery that signal transducer and activator of transcription 3 (STAT3) inhibitors can potently stimulate the autophagic flux. Although STAT3 is best known as a pro-inflammatory and oncogenic transcription factor, mechanistic analyses revealed that autophagy is regulated by the cytoplasmic, not nuclear, pool of STAT3. Cytoplasmic STAT3 normally interacts with the eukaryotic translation initiation factor 2, subunit 1α, 35kDa (EIF2S1/eIF2α) kinase 2/protein kinase, RNA-activated (EIF2AK2/PKR), a sensor of double-stranded RNA. This interaction, which could be recapitulated using recombinant proteins in pull-down experiments, involves the catalytic domain of EIF2AK2 as well as the SH2 domain of STAT3, which can adopt a fold similar to that of EIF2S1. Thus, STAT3 may act as a competitive inhibitor of EIF2AK2. Indeed, pharmacological or genetic inhibition of STAT3 stimulates EIF2AK2-dependent EIF2S1 phosphorylation and autophagy. Conversely, the overexpression of wild-type STAT3 as well as of STAT3 mutants that cannot be phosphorylated by JAK2 or are excluded from the nucleus inhibits autophagy. However, STAT3 mutants that fail to interact with EIF2AK2 are unable to suppress autophagy. Both STAT3-targeting agents (i.e., Stattic, JSI-124 and WP1066) and EIF2AK2 activators (such as the double-strand RNA mimetic polyinosinic:polycytidylic acid) are capable of disrupting the inhibitory interaction between STAT3 and EIF2AK2 in cellula, yet only the latter does so in cell-free systems in vitro. A further screen designed to identify EIF2AK2-dependent autophagy inducers revealed that several fatty acids including palmitate trigger autophagy via a pathway that involves the disruption of the STAT3-EIF2AK2 complex as well as the phosphorylation of mitogen-activated protein kinase 8/c-Jun N-terminal kinase 1 (MAPK8/JNK1) and EIF2S1. These results reveal an unsuspected crosstalk between cellular metabolism (fatty acids), pro-inflammatory signaling (STAT3), innate immunity (EIF2AK2), and translational control (EIF2S1) that regulates autophagy.
一种旨在鉴定新的自噬诱导物的化学筛选方法导致发现信号转导和转录激活因子 3（STAT3）抑制剂可以强烈刺激自噬通量。尽管 STAT3 作为一种促炎和致癌转录因子而被广泛研究，但机制分析表明自噬是由细胞质而非核内的 STAT3 池调节的。细胞质 STAT3 通常与真核翻译起始因子 2、亚基 1α、35kDa（EIF2S1/eIF2α）激酶 2/蛋白激酶，RNA 激活（EIF2AK2/PKR）相互作用，后者是双链 RNA 的传感器。这种相互作用可以在 pull-down 实验中使用重组蛋白重现，涉及 EIF2AK2 的催化结构域以及 STAT3 的 SH2 结构域，后者可以采用与 EIF2S1 相似的折叠方式。因此，STAT3 可能作为 EIF2AK2 的竞争性抑制剂发挥作用。事实上，STAT3 的药理学或遗传学抑制可刺激 EIF2AK2 依赖性 EIF2S1 磷酸化和自噬。相反，野生型 STAT3 的过表达以及不能被 JAK2 磷酸化或不能进入细胞核的 STAT3 突变体抑制自噬。然而，不能与 EIF2AK2 相互作用的 STAT3 突变体不能抑制自噬。STAT3 靶向剂（即 Stattic、JSI-124 和 WP1066）和 EIF2AK2 激活剂（如双链 RNA 模拟物聚肌苷酸：聚胞苷酸）都能够在细胞内破坏 STAT3 和 EIF2AK2 之间的抑制性相互作用，但只有后者在体外无细胞系统中如此。进一步设计的鉴定 EIF2AK2 依赖性自噬诱导物的筛选显示，包括棕榈酸在内的几种脂肪酸通过涉及破坏 STAT3-EIF2AK2 复合物以及丝裂原激活蛋白激酶 8/c-Jun N-末端激酶 1（MAPK8/JNK1）和 EIF2S1 磷酸化的途径触发自噬。这些结果揭示了细胞代谢（脂肪酸）、促炎信号（STAT3）、先天免疫（EIF2AK2）和翻译控制（EIF2S1）之间一种意想不到的串扰，调节自噬。
