Pajalunga Deborah, Franzolin Elisa, Stevanoni Martina, Zribi Sara, Passaro Nunzia, Gurtner Aymone, Donsante Samantha, Loffredo Daniela, Losanno Lidia, Bianchi Vera, Russo Antonella, Rampazzo Chiara, Crescenzi Marco
Department of Cell Biology and Neurosciences, Italian National Institute of Health, Rome, Italy.
Department of Biology, University of Padua, Padua, Italy.
Cell Death Differ. 2017 May;24(5):774-784. doi: 10.1038/cdd.2017.4. Epub 2017 Feb 10.
Terminally differentiated cells are defined by their inability to proliferate. When forced to re-enter the cell cycle, they generally cannot undergo long-term replication. Our previous work with myotubes has shown that these cells fail to proliferate because of their intrinsic inability to complete DNA replication. Moreover, we have reported pronounced modifications of deoxynucleotide metabolism during myogenesis. Here we investigate the causes of incomplete DNA duplication in cell cycle-reactivated myotubes (rMt). We find that rMt possess extremely low levels of thymidine triphosphate (dTTP), resulting in very slow replication fork rates. Exogenous administration of thymidine or forced expression of thymidine kinase increases deoxynucleotide availability, allowing extended and faster DNA replication. Inadequate dTTP levels are caused by selective, differentiation-dependent, cell cycle-resistant suppression of genes encoding critical synthetic enzymes, chief among which is thymidine kinase 1. We conclude that lack of dTTP is at least partially responsible for the inability of myotubes to proliferate and speculate that it constitutes an emergency barrier against unwarranted DNA replication in terminally differentiated cells.
终末分化细胞的定义是它们无法增殖。当被迫重新进入细胞周期时，它们通常无法进行长期复制。我们之前对肌管的研究表明，这些细胞由于其内在无法完成DNA复制而无法增殖。此外，我们报道了在肌生成过程中脱氧核苷酸代谢的显著变化。在这里，我们研究细胞周期重新激活的肌管（rMt）中DNA复制不完全的原因。我们发现rMt中三磷酸胸苷（dTTP）水平极低，导致复制叉速率非常缓慢。外源性给予胸苷或强制表达胸苷激酶可增加脱氧核苷酸的可用性，从而使DNA复制得以延长且更快。dTTP水平不足是由对编码关键合成酶的基因进行选择性、分化依赖性、细胞周期抗性抑制所致，其中主要的是胸苷激酶1。我们得出结论，dTTP的缺乏至少部分导致了肌管无法增殖，并推测这构成了终末分化细胞中防止不必要DNA复制的一道紧急屏障。
