Pickering M T, Stadler B M, Kowalik T F
Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, MA 1655, USA.
Oncogene. 2009 Jan 8;28(1):140-5. doi: 10.1038/onc.2008.372. Epub 2008 Oct 6.
The stringent regulation of cell cycle progression helps to maintain genetic stability in cells. MicroRNAs (miRNAs) are critical regulators of gene expression in diverse cellular pathways, including developmental patterning, hematopoietic differentiation and antiviral defense. Here, we show that two c-Myc-regulated miRNAs, miR-17 and miR-20a, govern the transition through G1 in normal diploid human cells. Inhibition of these miRNAs leads to a G1 checkpoint due to an accumulation of DNA double-strand breaks, resulting from premature temporal accumulation of the E2F1 transcription factor. Surprisingly, gross changes in E2F1 levels were not required to initiate the DNA damage response and checkpoint, as these responses could occur with a less than twofold change in E2F1 protein levels. Instead, our findings indicate that the precise timing of E2F1 expression dictates S-phase entry and that accurate timing of E2F1 accumulation requires converging signals from the Rb/E2F pathway and the c-Myc-regulated miR-17 and miR-20a miRNAs to circumvent a G1 checkpoint arising from the untimely accumulation of E2F1. These data provide a mechanistic view of miRNA-based regulation of E2F1 in the context of the emerging model that miRNAs coordinate the timing of cell cycle progression.
细胞周期进程的严格调控有助于维持细胞内的遗传稳定性。微小RNA（miRNA）是多种细胞途径中基因表达的关键调节因子，这些途径包括发育模式形成、造血分化和抗病毒防御。在此，我们表明，两种由c-Myc调控的miRNA，即miR-17和miR-20a，在正常二倍体人类细胞中控制着G1期的转换。抑制这些miRNA会由于DNA双链断裂的积累而导致G1期检查点的出现，这种积累是由E2F1转录因子的过早暂时积累引起的。令人惊讶的是，引发DNA损伤反应和检查点并不需要E2F1水平的大幅变化，因为这些反应可以在E2F1蛋白水平变化不到两倍的情况下发生。相反，我们的研究结果表明，E2F1表达的精确时间决定了S期的进入，并且E2F1积累的准确时间需要来自Rb/E2F途径以及由c-Myc调控的miR-17和miR-20a miRNA的汇聚信号，以规避因E2F1的过早积累而产生的G1期检查点。这些数据在miRNA协调细胞周期进程时间的新模型背景下，提供了基于miRNA对E2F1调控的机制观点。
