Idica Adam, Sevrioukov Evgueni A, Zisoulis Dimitrios G, Hamdorf Matthias, Daugaard Iben, Kadandale Pavan, Pedersen Irene M
From the Department of Molecular Biology and Biochemistry, Francisco J. Ayala School of Biological Sciences, University of California, Irvine, California 92697.
From the Department of Molecular Biology and Biochemistry, Francisco J. Ayala School of Biological Sciences, University of California, Irvine, California 92697
J Biol Chem. 2017 Dec 15;292(50):20494-20508. doi: 10.1074/jbc.M117.807677. Epub 2017 Oct 3.
Repetitive elements, including LINE-1 (L1), comprise approximately half of the human genome. These elements can potentially destabilize the genome by initiating their own replication and reintegration into new sites (retrotransposition). In somatic cells, transcription of L1 elements is repressed by distinct molecular mechanisms, including DNA methylation and histone modifications, to repress transcription. Under conditions of hypomethylation ( in tumor cells), a window of opportunity for L1 derepression arises, and additional restriction mechanisms become crucial. We recently demonstrated that the microRNA miR-128 represses L1 activity by directly binding to L1 ORF2 RNA. In this study, we tested whether miR-128 can also control L1 activity by repressing cellular proteins important for L1 retrotransposition. We found that miR-128 targets the 3' UTR of nuclear import factor transportin 1 (TNPO1) mRNA. Manipulation of miR-128 and TNPO1 levels demonstrated that induction or depletion of TNPO1 affects L1 retrotransposition and nuclear import of an L1-ribonucleoprotein complex (using L1-encoded ORF1p as a proxy for L1-ribonucleoprotein complexes). Moreover, TNPO1 overexpression partially reversed the repressive effect of miR-128 on L1 retrotransposition. Our study represents the first description of a protein factor involved in nuclear import of the L1 element and demonstrates that miR-128 controls L1 activity in somatic cells through two independent mechanisms: direct binding to L1 RNA and regulation of a cellular factor necessary for L1 nuclear import and retrotransposition.
包括LINE-1（L1）在内的重复元件约占人类基因组的一半。这些元件可能通过启动自身复制并重新整合到新位点（逆转座）来破坏基因组的稳定性。在体细胞中，L1元件的转录受到包括DNA甲基化和组蛋白修饰在内的不同分子机制的抑制，从而抑制转录。在低甲基化条件下（如肿瘤细胞中），L1去抑制的机会窗口出现，其他限制机制变得至关重要。我们最近证明，微小RNA miR-128通过直接结合L1 ORF2 RNA来抑制L1活性。在本研究中，我们测试了miR-128是否也能通过抑制对L1逆转座重要的细胞蛋白来控制L1活性。我们发现miR-128靶向核输入因子运输蛋白1（TNPO1）mRNA的3'UTR。对miR-128和TNPO1水平的操作表明，TNPO1的诱导或缺失会影响L1逆转座以及L1核糖核蛋白复合物的核输入（使用L1编码的ORF1p作为L1核糖核蛋白复合物的替代物）。此外，TNPO1的过表达部分逆转了miR-128对L1逆转座的抑制作用。我们的研究首次描述了参与L1元件核输入的一种蛋白质因子，并证明miR-128通过两种独立机制控制体细胞中的L1活性：直接结合L1 RNA以及调节L1核输入和逆转座所需的一种细胞因子。
