Seiser C, Teixeira S, Kühn L C
Swiss Institute for Experimental Cancer Research, Epalinges.
J Biol Chem. 1993 Jun 25;268(18):13074-80.
Interleukin-2 (IL-2) controls the proliferation of the murine T cell line B6.1 and induces transferrin receptor (TfR) mRNA steady-state levels 50-fold when added to arrested, IL-2-deprived cells. In addition, TfR mRNA is post-transcriptionally regulated by intracellular iron. Low iron levels activate a cytoplasmic RNA-binding protein, called iron regulatory factor (IRF) or iron-responsive element-binding protein, which coordinately stabilizes TfR mRNA and inhibits ferritin mRNA translation. Since ferritin expression is known to be modulated by cytokines, we decided to investigate the mechanism by which IL-2 activates TfR gene expression in B6.1 cells. Induction by IL-2 of both nuclear and cytoplasmic TfR RNA was compared with run-on transcription rates in isolated nuclei. The results revealed a 3-fold increase in TfR gene transcription and a 6-fold rise in nuclear TfR RNA reaching its steady-state level within 2 h. The main accumulation of mature mRNA in the cytoplasm occurred after 6 h in parallel with the activation of IRF. However, stimulation of IRF binding activity by the iron chelator desferrioxamine, in the absence of IL-2, failed to induce TfR mRNA. Moreover, deprivation of growing B6.1 cells of IL-2 resulted in cell arrest and a rapid decay of TfR mRNA, which was not prevented by the activation of IRF with desferrioxamine. TfR mRNA stabilization appears, therefore, to depend on IL-2. We conclude that TfR mRNA expression is controlled by at least three steps at the onset of cell proliferation: (i) the growth factor-dependent activation of transcription; (ii) mRNA stabilization by IRF in the cytoplasm; and (iii) an additional IL-2-dependent activity which prevents TfR mRNA degradation. Our results indicate that expression of TfR, like ferritin, is controlled by both iron and cytokines.
白细胞介素-2（IL-2）可控制小鼠T细胞系B6.1的增殖，当添加到停滞的、缺乏IL-2的细胞中时，可诱导转铁蛋白受体（TfR）mRNA稳态水平升高50倍。此外，TfR mRNA受细胞内铁的转录后调控。低铁水平会激活一种细胞质RNA结合蛋白，称为铁调节因子（IRF）或铁反应元件结合蛋白，它可协同稳定TfR mRNA并抑制铁蛋白mRNA的翻译。由于已知铁蛋白表达受细胞因子调节，我们决定研究IL-2激活B6.1细胞中TfR基因表达的机制。将IL-2对核内和细胞质TfR RNA的诱导与分离细胞核中的连续转录率进行了比较。结果显示，TfR基因转录增加了3倍，核内TfR RNA在2小时内达到稳态水平时增加了6倍。成熟mRNA在细胞质中的主要积累在6小时后发生，与IRF的激活同时出现。然而，在没有IL-2的情况下，铁螯合剂去铁胺对IRF结合活性的刺激未能诱导TfR mRNA。此外，剥夺生长中的B6.1细胞的IL-2会导致细胞停滞和TfR mRNA的快速降解，而去铁胺激活IRF并不能阻止这种降解。因此，TfR mRNA的稳定似乎依赖于IL-2。我们得出结论，TfR mRNA的表达在细胞增殖开始时至少受三个步骤的控制：（i）生长因子依赖性的转录激活；（ii）IRF在细胞质中对mRNA的稳定作用；（iii）一种额外的IL-2依赖性活性，可防止TfR mRNA降解。我们的结果表明，TfR的表达与铁蛋白一样，受铁和细胞因子的共同控制。
