Tang H, Hornstein E, Stolovich M, Levy G, Livingstone M, Templeton D, Avruch J, Meyuhas O
Department of Biochemistry, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.
Mol Cell Biol. 2001 Dec;21(24):8671-83. doi: 10.1128/MCB.21.24.8671-8683.2001.
Vertebrate TOP mRNAs contain an oligopyrimidine tract at their 5' termini (5'TOP) and encode components of the translational machinery. Previously it has been shown that they are subject to selective translational repression upon growth arrest and that their translational behavior correlates with the activity of S6K1. We now show that the translation of TOP mRNAs is rapidly repressed by amino acid withdrawal and that this nutritional control depends strictly on the integrity of the 5'TOP motif. However, neither phosphorylation of ribosomal protein (rp) S6 nor activation of S6K1 per se is sufficient to relieve the translational repression of TOP mRNAs in amino acid-starved cells. Likewise, inhibition of S6K1 activity and rpS6 phosphorylation by overexpression of dominant-negative S6K1 mutants failed to suppress the translational activation of TOP mRNAs in amino acid-refed cells. Furthermore, TOP mRNAs were translationally regulated by amino acid sufficiency in embryonic stem cells lacking both alleles of the S6K1 gene. Inhibition of mTOR by rapamycin led to fast and complete repression of S6K1, as judged by rpS6 phosphorylation, but to only partial and delayed repression of translational activation of TOP mRNAs. In contrast, interference in the phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathway by chemical or genetic manipulations blocked rapidly and completely the translational activation of TOP mRNAs. It appears, therefore, that translational regulation of TOP mRNAs, at least by amino acids, (i) is fully dependent on PI3-kinase, (ii) is partially sensitive to rapamycin, and (iii) requires neither S6K1 activity nor rpS6 phosphorylation.
脊椎动物的TOP mRNA在其5'末端含有一个寡嘧啶序列（5'TOP），并编码翻译机制的组成部分。此前已经表明，它们在生长停滞时会受到选择性翻译抑制，并且其翻译行为与S6K1的活性相关。我们现在表明，氨基酸剥夺会迅速抑制TOP mRNA的翻译，并且这种营养控制严格依赖于5'TOP基序的完整性。然而，核糖体蛋白（rp）S6的磷酸化或S6K1本身的激活都不足以缓解氨基酸饥饿细胞中TOP mRNA的翻译抑制。同样，通过过表达显性负性S6K1突变体来抑制S6K1活性和rpS6磷酸化，也未能抑制氨基酸再喂养细胞中TOP mRNA的翻译激活。此外，在缺乏S6K1基因两个等位基因的胚胎干细胞中，TOP mRNA的翻译受氨基酸充足性的调节。用雷帕霉素抑制mTOR导致rpS6磷酸化判断的S6K1快速且完全抑制，但仅导致TOP mRNA翻译激活的部分和延迟抑制。相反，通过化学或基因操作干扰磷脂酰肌醇3激酶（PI3激酶）介导的途径，会迅速且完全阻断TOP mRNA的翻译激活。因此，看来TOP mRNA的翻译调节，至少由氨基酸调节时，（i）完全依赖于PI3激酶，（ii）对雷帕霉素部分敏感，并且（iii）既不需要S6K1活性也不需要rpS6磷酸化。
