Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, Montreal, Quebec H2T 1E2, Canada; Department of Medicine, McGill University, Montreal, Quebec H3G 1Y6, Canada.
RNA. 2012 Mar;18(3):547-56. doi: 10.1261/rna.029322.111. Epub 2012 Jan 23.
Ferritin stores and detoxifies an excess of intracellular iron, and thereby plays an important role in the metabolism of this metal. As unshielded iron promotes oxidative stress, ferritin is crucial in maintaining cellular redox balance and may also modulate cell growth, survival, and apoptosis. The expression of ferritin is controlled by transcriptional and post-transcriptional mechanisms. In light of the well-established transcriptional induction of ferritin by inflammatory signals, we examined how ferritin mRNA translation responds to stress conditions. We first used HT1080 fibrosarcoma cells engineered for coumermycin-inducible expression of PKR, a stress kinase that inhibits protein synthesis in virus-infected cells by phosphorylating eIF2α. In this setting, iron triggered partial ferritin mRNA translation despite a PKR-induced global shutdown in protein synthesis. Moreover, iron-mediated ferritin synthesis was evident in cells infected with an attenuated strain of poliovirus; when eIF4GI was cleaved, eIF2α was phosphorylated, and host protein synthesis was inhibited. Under global inhibition of protein synthesis or specific inhibition of ferritin mRNA translation in cells overexpressing PKR or IRP1, respectively, we demonstrate association of ferritin mRNA with heavy polysomes. Further experiments revealed that the 5' untranslated region (5' UTR) of ferritin mRNA contains a bona fide internal ribosomal entry site (IRES). Our data are consistent with the existence of an alternative, noncanonical mechanism for ferritin mRNA translation, which may primarily operate under stress conditions to protect cells from oxidative stress.
铁蛋白储存和解毒细胞内过量的铁,因此在这种金属的代谢中起着重要作用。由于无屏蔽的铁会促进氧化应激,铁蛋白对于维持细胞氧化还原平衡至关重要,它还可能调节细胞生长、存活和凋亡。铁蛋白的表达受转录和转录后机制的控制。鉴于炎症信号对铁蛋白的转录诱导已得到充分证实,我们研究了铁蛋白 mRNA 翻译如何响应应激条件。我们首先使用 HT1080 纤维肉瘤细胞,该细胞经过工程改造,可诱导表达 PKR,这是一种应激激酶,可通过磷酸化 eIF2α 来抑制病毒感染细胞中的蛋白质合成。在这种情况下,尽管 PKR 诱导了蛋白质合成的全局关闭,但铁仍会触发部分铁蛋白 mRNA 翻译。此外,在感染弱化的脊髓灰质炎病毒的细胞中,铁介导的铁蛋白合成是明显的;当 eIF4GI 被切割、eIF2α 被磷酸化且宿主蛋白合成被抑制时。在过表达 PKR 或 IRP1 的细胞中进行蛋白质合成的全局抑制或铁蛋白 mRNA 翻译的特异性抑制时,我们证明铁蛋白 mRNA 与重多核糖体结合。进一步的实验表明,铁蛋白 mRNA 的 5'非翻译区(5'UTR)含有一个真正的内部核糖体进入位点(IRES)。我们的数据与铁蛋白 mRNA 翻译存在替代的非规范机制的存在一致,该机制主要在应激条件下发挥作用,以保护细胞免受氧化应激。