Young Sara K, Shao Yu, Bidwell Joseph P, Wek Ronald C
From the Department of Biochemistry and Molecular Biology.
Department of Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis, Indiana 46202-5126.
J Biol Chem. 2016 Jun 24;291(26):13780-8. doi: 10.1074/jbc.M116.729830. Epub 2016 Apr 29.
The unfolded protein response (UPR) maintains protein homeostasis by governing the processing capacity of the endoplasmic reticulum (ER) to manage ER client loads; however, key regulators within the UPR remain to be identified. Activation of the UPR sensor PERK (EIFAK3/PEK) results in the phosphorylation of the α subunit of eIF2 (eIF2α-P), which represses translation initiation and reduces influx of newly synthesized proteins into the overloaded ER. As part of this adaptive response, eIF2α-P also induces a feedback mechanism through enhanced transcriptional and translational expression of Gadd34 (Ppp1r15A),which targets type 1 protein phosphatase for dephosphorylation of eIF2α-P to restore protein synthesis. Here we describe a novel mechanism by which Gadd34 expression is regulated through the activity of the zinc finger transcription factor NMP4 (ZNF384, CIZ). NMP4 functions to suppress bone anabolism, and we suggest that this occurs due to decreased protein synthesis of factors involved in bone formation through NMP4-mediated dampening of Gadd34 and c-Myc expression. Loss of Nmp4 resulted in an increase in c-Myc and Gadd34 expression that facilitated enhanced ribosome biogenesis and global protein synthesis. Importantly, protein synthesis was sustained during pharmacological induction of the UPR through a mechanism suggested to involve GADD34-mediated dephosphorylation of eIF2α-P. Sustained protein synthesis sensitized cells to pharmacological induction of the UPR, and the observed decrease in cell viability was restored upon inhibition of GADD34 activity. We conclude that NMP4 is a key regulator of ribosome biogenesis and the UPR, which together play a central role in determining cell viability during endoplasmic reticulum stress.
未折叠蛋白反应（UPR）通过控制内质网（ER）的加工能力来维持蛋白质稳态，以应对ER客户蛋白负载；然而，UPR中的关键调节因子仍有待确定。UPR传感器PERK（EIFAK3/PEK）的激活导致eIF2α亚基磷酸化（eIF2α-P），从而抑制翻译起始并减少新合成蛋白质流入过载的ER。作为这种适应性反应的一部分，eIF2α-P还通过增强Gadd34（Ppp1r15A）的转录和翻译表达诱导一种反馈机制，Gadd34靶向1型蛋白磷酸酶使eIF2α-P去磷酸化以恢复蛋白质合成。在此，我们描述了一种新机制，即通过锌指转录因子NMP4（ZNF384，CIZ）的活性来调节Gadd34的表达。NMP4的功能是抑制骨合成代谢，我们认为这是由于通过NMP4介导的Gadd34和c-Myc表达的抑制，参与骨形成的因子的蛋白质合成减少所致。Nmp4的缺失导致c-Myc和Gadd34表达增加，促进了核糖体生物合成和整体蛋白质合成。重要的是，在UPR的药理学诱导过程中，蛋白质合成通过一种涉及GADD34介导的eIF2α-P去磷酸化的机制得以维持。持续的蛋白质合成使细胞对UPR的药理学诱导敏感，并且在抑制GADD34活性后，观察到的细胞活力下降得以恢复。我们得出结论，NMP4是核糖体生物合成和UPR的关键调节因子，它们共同在内质网应激期间决定细胞活力方面发挥核心作用。
