Rosenwald I B, Kaspar R, Rousseau D, Gehrke L, Leboulch P, Chen J J, Schmidt E V, Sonenberg N, London I M
Harvard-Massachusetts Institutes of Technology Division of Health Sciences and Technology, Cambridge 02139, USA.
J Biol Chem. 1995 Sep 8;270(36):21176-80. doi: 10.1074/jbc.270.36.21176.
Regulation of the cell cycle is orchestrated by cyclins and cyclin-dependent kinases. We have demonstrated previously that overexpression of eukaryotic translation initiation factor 4E (eIF-4E) in NIH 3T3 cells growing in 10% fetal calf serum leads to highly elevated levels of cyclin D1 protein without significant increase in cyclin D1 mRNA levels, suggesting that a post-transcriptional mechanism is involved. (Rosenwald, I. B., Lazaris-Karatzas, A., Sonenberg, N., and Schmidt, E. V. (1993) Mol. Cell. Biol. 13, 7358-7363). In the present research, we did not find any significant effect of eIF-4E on polysomal distribution of cyclin D1 mRNA. However, the total amount of cyclin D1 mRNA associated with polysomes was significantly increased by eIF-4E overexpression. Further, we determined that the levels of both cyclin D1 protein and mRNA are increased in serum-deprived cells overexpressing eIF-4E. Nuclear run-on experiments demonstrated that the rate of the cyclin D1 transcription is not down-regulated in serum-deprived cells overexpressing eIF-4E. Thus, elevated levels of eIF-4E may lead to increased transcription of the cyclin D1 gene, and this effect becomes visible when serum deprivation down-regulates the rate of cyclin D1 mRNA synthesis in control cells. However, artificial overexpression of cyclin D1 mRNA in serum-deprived cells in the absence of eIF-4E overexpression did not cause the elevation of cyclin D1 protein, and this overexpressed cyclin D1 mRNA accumulated in the nucleus, suggesting that one post-transcriptional role of eIF-4E is to transport cyclin D1 mRNA from the nucleus to cytoplasmic polysomes.
细胞周期的调控由细胞周期蛋白和细胞周期蛋白依赖性激酶精心安排。我们之前已经证明，在含10%胎牛血清的条件下生长的NIH 3T3细胞中，真核翻译起始因子4E（eIF - 4E）的过表达导致细胞周期蛋白D1蛋白水平大幅升高，而细胞周期蛋白D1 mRNA水平没有显著增加，这表明涉及一种转录后机制。（罗森瓦尔德，I. B.，拉扎里斯 - 卡拉察斯，A.，索嫩伯格，N.，和施密特，E. V.（1993年）《分子与细胞生物学》13卷，7358 - 7363页）。在本研究中，我们未发现eIF - 4E对细胞周期蛋白D1 mRNA的多核糖体分布有任何显著影响。然而，eIF - 4E过表达显著增加了与多核糖体相关的细胞周期蛋白D1 mRNA的总量。此外，我们确定在过表达eIF - 4E的血清饥饿细胞中，细胞周期蛋白D1蛋白和mRNA的水平均升高。细胞核连续转录实验表明，在过表达eIF - 4E的血清饥饿细胞中，细胞周期蛋白D1的转录速率并未下调。因此，eIF - 4E水平升高可能导致细胞周期蛋白D1基因转录增加，当血清饥饿下调对照细胞中细胞周期蛋白D1 mRNA合成速率时，这种效应变得明显。然而，在没有eIF - 4E过表达的情况下，在血清饥饿细胞中人工过表达细胞周期蛋白D1 mRNA并未导致细胞周期蛋白D1蛋白升高，并且这种过表达的细胞周期蛋白D1 mRNA在细胞核中积累，这表明eIF - 4E的一个转录后作用是将细胞周期蛋白D1 mRNA从细胞核转运到细胞质多核糖体。
