Molecular Cancer Biology Program and Haartman Institute, University of Helsinki, Helsinki, Finland.
Oncogene. 2011 Feb 17;30(7):790-805. doi: 10.1038/onc.2010.469. Epub 2010 Oct 18.
The ubiquitin-proteasome pathway is essential for most cellular processes, including protein quality control, cell cycle, transcription, signaling, protein transport, DNA repair and stress responses. Hampered proteasome activity leads to the accumulation of polyubiquitylated proteins, endoplastic reticulum (ER) stress and even cell death. The ability of chemical proteasome inhibitors (PIs) to induce apoptosis is utilized in cancer therapy. During PI treatment, misfolded proteins accrue to cytoplasmic aggresomes. The formation of aggresome-like structures in the nucleus has remained obscure. We identify here a nucleolus-associated RNA-protein aggregate (NoA) formed by the inhibition of proteasome activity in mammalian cells. The aggregate forms within the nucleolus and is dependent on nucleolar integrity, yet is a separate structure, lacking nucleolar marker proteins, ribosomal RNA (rRNA) and rRNA synthesis activity. The NoAs contain polyadenylated RNA, conjugated ubiquitin and numerous nucleoplasmic proteasome target proteins. Several of these are key factors in oncogenesis, including transcription factors p53 and retinoblastoma protein (Rb), several cell cycle-regulating cyclins and cyclin-dependent kinases (CDKs), and stress response kinases ataxia-telangiectasia mutated (ATM) and Chk1. The aggregate formation depends on ubiquitin availability, as shown by modulating the levels of ubiquitin and deubiquitinases. Furthermore, inhibition of chromosome region maintenance 1 protein homolog (CRM1) export pathway aggravates the formation of NoAs. Taken together, we identify here a novel nuclear stress body, which forms upon proteasome inactivity within the nucleolus and is detectable in mammalian cell lines and in human tissue. These findings show that the nucleolus controls protein and RNA surveillance and export by the ubiquitin pathway in a previously unidentified manner, and provide mechanistic insight into the cellular effects of PIs.
泛素-蛋白酶体途径对于大多数细胞过程至关重要，包括蛋白质质量控制、细胞周期、转录、信号转导、蛋白质运输、DNA 修复和应激反应。蛋白酶体活性受阻会导致多泛素化蛋白的积累、内质网（ER）应激甚至细胞死亡。化学蛋白酶体抑制剂（PIs）诱导细胞凋亡的能力被用于癌症治疗。在 PI 治疗期间，错误折叠的蛋白质在细胞质中积累成聚集物。核内聚集物样结构的形成仍然不清楚。我们在这里鉴定了一种由哺乳动物细胞中蛋白酶体活性抑制引起的核仁相关 RNA-蛋白聚集体（NoA）。聚集体在核仁内形成，依赖于核仁的完整性，但它是一个独立的结构，缺乏核仁标记蛋白、核糖体 RNA（rRNA）和 rRNA 合成活性。NoAs 含有多聚腺苷酸化 RNA、缀合的泛素和许多核质蛋白酶体靶蛋白。其中一些是致癌的关键因素，包括转录因子 p53 和视网膜母细胞瘤蛋白（Rb）、几个细胞周期调节周期蛋白和细胞周期依赖性激酶（CDKs）以及应激反应激酶共济失调毛细血管扩张突变（ATM）和 Chk1。聚集体的形成依赖于泛素的可用性，这可以通过调节泛素和去泛素酶的水平来证明。此外，抑制染色体区域维持 1 蛋白同源物（CRM1）输出途径会加剧 NoAs 的形成。总之，我们在这里鉴定了一种新的核应激体，它在核仁内蛋白酶体失活时形成，可在哺乳动物细胞系和人类组织中检测到。这些发现表明，核仁以以前未被识别的方式通过泛素途径控制蛋白质和 RNA 的监视和输出，并为 PIs 的细胞效应提供了机制上的见解。
