Chang Tsung-Cheng, Zeitels Lauren R, Hwang Hun-Way, Chivukula Raghu R, Wentzel Erik A, Dews Michael, Jung Jason, Gao Ping, Dang Chi V, Beer Michael A, Thomas-Tikhonenko Andrei, Mendell Joshua T
McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Proc Natl Acad Sci U S A. 2009 Mar 3;106(9):3384-9. doi: 10.1073/pnas.0808300106. Epub 2009 Feb 11.
Direct control of microRNA (miRNA) expression by oncogenic and tumor suppressor networks results in frequent dysregulation of miRNAs in cancer cells and contributes to tumorigenesis. We previously demonstrated that activation of the c-Myc oncogenic transcription factor (Myc) broadly influences miRNA expression and in particular leads to widespread miRNA down-regulation. miRNA transcripts repressed by Myc include several with potent tumor suppressor activity such as miR-15a/16-1, miR-34a, and let-7 family members. In this study, we have investigated mechanisms downstream of Myc that contribute to miRNA repression. Consistent with transcriptional down-regulation, Myc activity results in the decreased abundance of multiple miRNA primary transcripts. Surprisingly, however, primary transcripts encoding several let-7 miRNAs are not reduced in the high Myc state, suggesting a posttranscriptional mechanism of repression. The Lin-28 and Lin-28B RNA binding proteins were recently demonstrated to negatively regulate let-7 biogenesis. We now show that Myc induces Lin-28B expression in multiple human and mouse tumor models. Chromatin immunoprecipitation and reporter assays reveal direct association of Myc with the Lin-28B promoter resulting in transcriptional transactivation. Moreover, we document that activation of Lin-28B is necessary and sufficient for Myc-mediated let-7 repression. Accordingly, Lin-28B loss-of-function significantly impairs Myc-dependent cellular proliferation. These findings highlight an important role for Lin-28B in Myc-driven cellular phenotypes and uncover an orchestration of transcriptional and posttranscriptional mechanisms in Myc-mediated reprogramming of miRNA expression.
致癌和抑癌网络对微小RNA（miRNA）表达的直接控制导致癌细胞中miRNA频繁失调，并促进肿瘤发生。我们先前证明，致癌转录因子c-Myc（Myc）的激活广泛影响miRNA表达，尤其导致广泛的miRNA下调。被Myc抑制的miRNA转录本包括几种具有强大肿瘤抑制活性的转录本，如miR-15a/16-1、miR-34a和let-7家族成员。在本研究中，我们研究了Myc下游导致miRNA抑制的机制。与转录下调一致，Myc活性导致多种miRNA初级转录本丰度降低。然而，令人惊讶的是，在Myc高表达状态下，编码几种let-7 miRNA的初级转录本并未减少，这表明存在一种转录后抑制机制。最近证明，RNA结合蛋白Lin-28和Lin-28B对let-7生物合成具有负调控作用。我们现在表明，在多种人类和小鼠肿瘤模型中，Myc诱导Lin-28B表达。染色质免疫沉淀和报告基因检测揭示了Myc与Lin-28B启动子的直接关联，导致转录激活。此外，我们证明Lin-28B的激活对于Myc介导的let-7抑制是必要且充分的。因此，Lin-28B功能丧失显著损害Myc依赖的细胞增殖。这些发现突出了Lin-28B在Myc驱动的细胞表型中的重要作用，并揭示了Myc介导的miRNA表达重编程中，转录和转录后机制的协同作用。
