Institute of Medical Biochemistry and Molecular Biology, Universitätsmedizin Greifswald, Greifswald, Germany.
Front Immunol. 2019 Dec 18;10:2900. doi: 10.3389/fimmu.2019.02900. eCollection 2019.
Proteostasis is critical for cells to maintain the balance between protein synthesis, quality control, and degradation. This is particularly important for myeloid cells of the central nervous system as their immunological function relies on proper intracellular protein turnover by the ubiquitin-proteasome system. Accordingly, disruption of proteasome activity due to, e.g., loss-of-function mutations within genes encoding proteasome subunits, results in systemic autoinflammation. On the molecular level, pharmacological inhibition of proteasome results in endoplasmic reticulum (ER) stress-activated unfolded protein response (UPR) as well as an induction of type I interferons (IFN). Nevertheless, our understanding as to whether and to which extent UPR signaling regulates type I IFN response is limited. To address this issue, we have tested the effects of proteasome dysfunction upon treatment with proteasome inhibitors in primary murine microglia and microglia-like cell line BV-2. Our data show that proteasome impairment by bortezomib is a stimulus that activates all three intracellular ER-stress transducers activation transcription factor 6, protein kinase R-like endoplasmic reticulum kinase and inositol-requiring protein 1 alpha (IRE1α), causing a full activation of the UPR. We further demonstrate that impaired proteasome activity in microglia cells triggers an induction of IFNβ1 in an IRE1-dependent manner. An inhibition of the IRE1 endoribonuclease activity significantly attenuates TANK-binding kinase 1-mediated activation of type I IFN. Moreover, interfering with TANK-binding kinase 1 activity also compromised the expression of C/EBP homologous protein 10, thereby emphasizing a multilayered interplay between UPR and type IFN response pathway. Interestingly, the induced protein kinase R-like endoplasmic reticulum kinase-activation transcription factor 4-C/EBP homologous protein 10 and IRE1-X-box-binding protein 1 axes caused a significant upregulation of proinflammatory cytokine interleukin 6 expression that exacerbates STAT1/STAT3 signaling in cells with dysfunctional proteasomes. Altogether, these findings indicate that proteasome impairment disrupts ER homeostasis and triggers a complex interchange between ER-stress sensors and type I IFN signaling, thus inducing in myeloid cells a state of chronic inflammation.
蛋白质稳态对于细胞维持蛋白质合成、质量控制和降解之间的平衡至关重要。这对于中枢神经系统的髓样细胞尤为重要，因为它们的免疫功能依赖于泛素-蛋白酶体系统对细胞内蛋白质周转的适当调节。因此，由于编码蛋白酶体亚基的基因丧失功能突变导致蛋白酶体活性的破坏，会导致全身自身炎症。在分子水平上，蛋白酶体的药理学抑制导致内质网（ER）应激激活未折叠蛋白反应（UPR）以及 I 型干扰素（IFN）的诱导。然而，我们对于 UPR 信号是否以及在何种程度上调节 I 型 IFN 反应的理解是有限的。为了解决这个问题，我们已经测试了在原代小鼠小胶质细胞和小胶质细胞样细胞系 BV-2 中用蛋白酶体抑制剂处理时蛋白酶体功能障碍的影响。我们的数据表明，硼替佐米引起的蛋白酶体损伤是一种刺激物，可激活三种细胞内 ER 应激传感器激活转录因子 6、蛋白激酶 R 样内质网激酶和肌醇需求蛋白 1α（IRE1α），导致 UPR 的完全激活。我们进一步证明，小胶质细胞中蛋白酶体活性的受损以 IRE1 依赖性方式触发 IFNβ1 的诱导。IRE1 内切核酸酶活性的抑制显著减弱 TANK 结合激酶 1 介导的 I 型 IFN 的激活。此外，干扰 TANK 结合激酶 1 活性也会损害 C/EBP 同源蛋白 10 的表达，从而强调 UPR 和 I 型 IFN 反应途径之间的多层次相互作用。有趣的是，诱导的蛋白激酶 R 样内质网激酶-激活转录因子 4-C/EBP 同源蛋白 10 和 IRE1-X 盒结合蛋白 1 轴导致促炎细胞因子白细胞介素 6 表达的显著上调，从而加剧了具有功能障碍蛋白酶体的细胞中 STAT1/STAT3 信号。总之，这些发现表明，蛋白酶体损伤破坏了 ER 稳态，并触发了 ER 应激传感器和 I 型 IFN 信号之间的复杂相互作用，从而在髓样细胞中诱导慢性炎症状态。
