Zhu Shaohua, Cao Jiani, Sun Hongyan, Liu Kun, Li Yaqiong, Zhao Tongbiao
School of Biological Sciences, University of Science and Technology of China, Hefei 230026, China.
State Key Laboratory of Stem Cell and Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China.
Sci Rep. 2016 Aug 3;6:31085. doi: 10.1038/srep31085.
Pluripotent stem cells (PSCs), including embryonic and induced pluripotent stem cells (iPSCs), show atypical cell cycle regulation characterized by a high proliferation rate and a shorter G1 phase compared with somatic cells. The mechanisms by which somatic cells remodel their cell cycle to achieve the high proliferation rate of PSCs during reprogramming are unclear. Here we identify that the Ink4 protein p18, which is expressed at high levels in somatic cells but at low levels in PSCs, is a roadblock to successful reprogramming. Mild inhibition of p18 expression enhances reprogramming efficiency, while ectopic expression of p18 completely blocks reprogramming. Mechanistic studies show that expression of wild-type p18, but not a p18(D68N) mutant which cannot inhibit Cdk4/6, down-regulates expression of Cdk4/6 target genes involved in DNA synthesis (TK, TS, DHFR, PCNA) and cell cycle regulation (CDK1 and CCNA2) and thus inhibits reprogramming. These results indicate that p18 blocks reprogramming by targeting Cdk4/6-mediated cell cycle regulation. Taken together, our results define a novel pathway that inhibits somatic cell reprogramming, and provide a new target to enhance reprogramming efficiency.
多能干细胞（PSC），包括胚胎干细胞和诱导多能干细胞（iPSC），表现出非典型的细胞周期调控，其特征在于与体细胞相比具有高增殖率和较短的G1期。在重编程过程中，体细胞重塑其细胞周期以实现PSC高增殖率的机制尚不清楚。在这里，我们发现Ink4蛋白p18在体细胞中高水平表达，但在PSC中低水平表达，是成功重编程的一个障碍。轻度抑制p18表达可提高重编程效率，而p18的异位表达则完全阻断重编程。机制研究表明，野生型p18的表达，而不是不能抑制Cdk4/6的p18（D68N）突变体，下调参与DNA合成（TK、TS、DHFR、PCNA）和细胞周期调控（CDK1和CCNA2）的Cdk4/6靶基因的表达，从而抑制重编程。这些结果表明，p18通过靶向Cdk4/6介导的细胞周期调控来阻断重编程。综上所述，我们的结果定义了一条抑制体细胞重编程的新途径，并为提高重编程效率提供了一个新靶点。
