Buechler Robbie D, Peffley Dennis M
Department of Neurology, Duke University Medical Center, Erwin Road, Durham, North Carolina, USA.
Mol Carcinog. 2004 Sep;41(1):39-53. doi: 10.1002/mc.20039.
The rate-limiting enzyme for mevalonate synthesis in mammalian cells is 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Products of mevalonate synthesis are required for cell cycle progression as well as cell growth and survival. In tumor cells, HMG-CoA reductase is generally elevated because of attenuated sterol-mediated regulation of transcription. However, tumor cell HMG-CoA reductase remains sensitive to post-transcriptional regulation by mevalonate-derived isoprenoid intermediates of cholesterol synthesis. Isoprenoids suppress HMG-CoA reductase synthesis through a mechanism that reduces initiation of translation on HMG-CoA reductase mRNA. Because HMG-CoA reductase mRNA transcripts have 5'-untranslated regions (UTR) that are GC rich and contain stable secondary structure, we tested the hypothesis that overexpression of eIF4E would attenuate isoprenoid-mediated regulation of HMG-CoA reductase. eIF4E is elevated in many tumor cells and behaves as a proto-oncogene by aberrantly translating mRNAs whose translation is normally suppressed by 5-UTRs that are GC rich. A CHO cell line expressing high levels of eIF4E (rb4E) was developed by infecting cells with retroviruses containing a full-length mouse cDNA for eIF4E. Levels of reductase synthesis were elevated fivefold in rb4E cells compared to noninfected CHO cells; HMG-CoA reductase mRNA levels were not increased in rb4E cells compared to normal CHO cells. Total cellular protein synthesis was only increased by approximately 15% in rb4E cells compared to CHO cells. The mTOR inhibitor rapamycin lowered HMG-CoA reductase synthesis by 50 and 60% in rb4E and CHO cells, respectively; no equivalent effect was observed for HMG-CoA reductase mRNA levels with rapamycin treatment. These results indicate that HMG-CoA reductase mRNA is in a class of mRNAs with highly structured 5'-UTRs whose m(7)GpppX cap-dependent translation is closely linked to the rapamycin-sensitive mitogen activated pathway for protein synthesis.
哺乳动物细胞中，甲羟戊酸合成的限速酶是3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶。甲羟戊酸合成的产物是细胞周期进程、细胞生长和存活所必需的。在肿瘤细胞中，由于固醇介导的转录调控减弱，HMG-CoA还原酶通常会升高。然而，肿瘤细胞中的HMG-CoA还原酶对胆固醇合成中甲羟戊酸衍生的类异戊二烯中间体的转录后调控仍敏感。类异戊二烯通过减少HMG-CoA还原酶mRNA翻译起始的机制来抑制HMG-CoA还原酶的合成。由于HMG-CoA还原酶mRNA转录本具有富含GC且包含稳定二级结构的5'-非翻译区（UTR），我们检验了真核翻译起始因子4E（eIF4E）过表达会减弱类异戊二烯介导的HMG-CoA还原酶调控这一假说。eIF4E在许多肿瘤细胞中升高，并通过异常翻译通常被富含GC的5'-UTR抑制翻译的mRNA而表现为原癌基因。通过用含有全长小鼠eIF4E cDNA的逆转录病毒感染细胞，构建了表达高水平eIF4E的CHO细胞系（rb4E）。与未感染的CHO细胞相比，rb4E细胞中还原酶的合成水平提高了五倍；与正常CHO细胞相比，rb4E细胞中HMG-CoA还原酶mRNA水平未升高。与CHO细胞相比，rb4E细胞中总细胞蛋白合成仅增加了约15%。mTOR抑制剂雷帕霉素分别使rb4E细胞和CHO细胞中的HMG-CoA还原酶合成降低了50%和60%；雷帕霉素处理对HMG-CoA还原酶mRNA水平未观察到同等效果。这些结果表明，HMG-CoA还原酶mRNA属于一类5'-UTR具有高度结构化的mRNA，其m(7)GpppX帽依赖性翻译与雷帕霉素敏感的有丝分裂原激活蛋白合成途径密切相关。
