Bianchini Andrea, Loiarro Maria, Bielli Pamela, Busà Roberta, Paronetto Maria Paola, Loreni Fabrizio, Geremia Raffaele, Sette Claudio
Department of Public Health and Cell Biology, University of Rome Tor Vergata, Rome, Italy.
Carcinogenesis. 2008 Dec;29(12):2279-88. doi: 10.1093/carcin/bgn221. Epub 2008 Sep 22.
Deregulation of the phosphatidyl inositol trisphosphate kinase/AKT/mammalian target of rapamycin (mTOR) and RAS/mitogen-activated protein kinase (MAPK)/MNK pathways frequently occurs in human prostate carcinomas (PCas) and leads to aberrant modulation of messenger RNA (mRNA) translation. We have investigated the relative contribution of these pathways to translational regulation and proliferation of PCa cells. MNK-dependent phosphorylation of eIF4E is elevated in DU145 cells, which have low basal levels of AKT/mTOR activity due to the expression of the tumor suppressor PTEN. In contrast, eIF4E phosphorylation is low in PC3 and LNCaP cells with mutated PTEN and constitutively active AKT/mTOR pathway, but it can be strongly induced through inhibition of mTOR activity by rapamycin or serum depletion. Remarkably, we found that inhibition of MNKs strongly reduced the polysomal recruitment of terminal oligopyrimidine messenger RNAs (TOP mRNAs), which are known targets of mTOR-dependent translational control. Pull-down assays of the eIF4F complex indicated that translation initiation was differently affected by inhibition of MNKs and mTOR. In addition, concomitant treatment with MNK inhibitor and rapamycin exerted additive effects on polysomal recruitment of TOP mRNAs and protein synthesis. The MNK inhibitor was more effective than rapamycin in blocking proliferation of PTEN-expressing cells, whereas combination of the two inhibitors suppressed cell cycle progression in both cell lines. Microarray analysis showed that MNK affected translation of mRNAs involved in cell cycle progression. Thus, our results indicate that a balance between the activity of the AKT/mTOR and the MAPK/MNK pathway in PCa cells maintains a defined translational level of specific mRNAs required for ribosome biogenesis, cell proliferation and stress response and might confer to these cells the ability to overcome negative insults.
磷脂酰肌醇三磷酸激酶/蛋白激酶B/雷帕霉素哺乳动物靶蛋白（mTOR）和RAS/丝裂原活化蛋白激酶（MAPK）/MNK通路的失调在人类前列腺癌（PCa）中频繁发生，并导致信使核糖核酸（mRNA）翻译的异常调节。我们研究了这些通路对PCa细胞翻译调控和增殖的相对贡献。在DU145细胞中，由于肿瘤抑制因子PTEN的表达，AKT/mTOR活性的基础水平较低，而eIF4E的MNK依赖性磷酸化水平升高。相反，在PTEN突变且AKT/mTOR通路组成性激活的PC3和LNCaP细胞中，eIF4E磷酸化水平较低，但通过雷帕霉素抑制mTOR活性或血清饥饿可强烈诱导其磷酸化。值得注意的是，我们发现抑制MNKs可显著降低末端寡嘧啶信使核糖核酸（TOP mRNA）的多核糖体募集，而TOP mRNA是mTOR依赖性翻译控制的已知靶点。eIF4F复合物的下拉分析表明，MNKs和mTOR的抑制对翻译起始的影响不同。此外，MNK抑制剂和雷帕霉素联合处理对TOP mRNA的多核糖体募集和蛋白质合成具有累加效应。MNK抑制剂在阻断PTEN表达细胞的增殖方面比雷帕霉素更有效，而两种抑制剂联合使用可抑制两种细胞系的细胞周期进程。微阵列分析表明，MNK影响参与细胞周期进程的mRNA的翻译。因此，我们的结果表明，PCa细胞中AKT/mTOR和MAPK/MNK通路活性之间的平衡维持了核糖体生物合成、细胞增殖和应激反应所需的特定mRNA的特定翻译水平，并可能赋予这些细胞克服负面损伤的能力。
