Göpfert Ulrich, Kullmann Michael, Hengst Ludger
Max-Planck-Institut für Biochemie, Am Klopferspitz 18a, D-82152 Martinsried, Germany.
Hum Mol Genet. 2003 Jul 15;12(14):1767-79. doi: 10.1093/hmg/ddg177.
p27(Kip1) regulates cell proliferation by binding to and modulating the activity of cyclin-dependent kinases. The CDK inhibitor is haploinsufficient for tumor suppression and reduced p27 activity is fundamental for the development of many human malignancies. Consistently, reduced p27 protein provides independent prognostic information in various tumors including breast, prostate, colon and gastric carcinomas. In normal cells, p27 protein increases in growth arrest but also oscillates during cell cycle progression. Expression of p27 is regulated through mechanisms including transcription, translation and ubiquitin-mediated degradation. Each of these pathways may contribute to deregulation of p27 in hyperproliferative diseases. p27 translation increases in proliferating cells during G(1) phase and declines as cells enter S phase. To investigate the mechanisms of p27 translational control, we analyzed fragments of the p27 transcript for their contribution to cell cycle regulated translation. We found that an element in the p27 5'-UTR can render reporter translation cell cycle sensitive with maximal translation in G1-arrested cells. This novel element of 114 nt contains a G/C-rich hairpin domain that is predicted to form multiple stable stemloops and also overlaps with a small upstream ORF (uORF). Both structures contribute to cell cycle-regulated translation. The uORF can be translated in vitro and its sequence and position are highly conserved in mice and chickens. Interestingly, the precise sequence or the length of the uORF-encoded peptide are not important for p27 translation, consistent with the idea that ribosomal recruitment to its initiation codon rather than the translation product itself contributes to the regulation.
p27（Kip1）通过与细胞周期蛋白依赖性激酶结合并调节其活性来调控细胞增殖。这种细胞周期蛋白依赖性激酶抑制剂在肿瘤抑制方面存在单倍剂量不足的情况，而p27活性降低是许多人类恶性肿瘤发生发展的根本原因。同样，p27蛋白水平降低在包括乳腺癌、前列腺癌、结肠癌和胃癌在内的多种肿瘤中提供了独立的预后信息。在正常细胞中，p27蛋白在生长停滞时增加，但在细胞周期进程中也会振荡。p27的表达通过包括转录、翻译和泛素介导的降解等机制进行调控。这些途径中的每一条都可能导致p27在过度增殖性疾病中的失调。p27的翻译在G1期的增殖细胞中增加，而随着细胞进入S期则下降。为了研究p27翻译控制的机制，我们分析了p27转录本的片段对细胞周期调控翻译的贡献。我们发现p27 5'-UTR中的一个元件可使报告基因的翻译对细胞周期敏感，在G1期停滞的细胞中翻译量最大。这个114 nt的新元件包含一个富含G/C的发夹结构域，预计会形成多个稳定的茎环结构，并且还与一个小的上游开放阅读框（uORF）重叠。这两种结构都有助于细胞周期调控的翻译。uORF可以在体外翻译，其序列和位置在小鼠和鸡中高度保守。有趣的是，uORF编码肽的精确序列或长度对p27的翻译并不重要，这与核糖体募集到其起始密码子而非翻译产物本身参与调控的观点一致。
