Translational control of the sterol-regulatory transcription factor SREBP-1 mRNA in response to serum starvation or ER stress is mediated by an internal ribosome entry site
Translational control of the sterol-regulatory transcription factor SREBP-1 mRNA in response to serum starvation or ER stress is mediated by an internal ribosome entry site
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DOI：
10.1042/bj20091827
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发表时间：
2010-08-01
影响因子：
4.1
通讯作者：
Siculella, Luisa
中科院分区：
文献类型：
--
作者：
Damiano, Fabrizio;Alemanno, Simone;Siculella, Luisa
SREBP（甾醇调节元件结合蛋白）是一个转录因子家族，可调节与内源性胆固醇、脂肪酸、三酰甘油和磷脂合成有关的几种酶的表达。在本研究中，报告了 SREBP-1 在翻译水平上调节的证据。使用多种实验方法，我们已经证明 SREBP-1a mRNA 的 5'-UTR（非翻译区）包含 IRES（内部核糖体进入位点）。使用插入双顺反子报告载体的 SREBP-1 5'-UTR 进行的转染实验显示，通过帽独立机制，下游顺反子翻译显着增加。在同一载体中插入 SREBP-1c 5'-UTR 也刺激了下游顺反子的翻译，但观察到的效果至少部分归因于隐秘的启动子活性。尽管蛋白质合成总体减少，但血清饥饿等细胞应激条件导致 Hep G2 和 HeLa 细胞中 SREBP-1 前体和成熟形式的水平增加，而 SREBP-1 mRNA 水平不受血清饥饿影响。使用双顺反子构建体进行的转染实验表明，血清饥饿对帽子依赖性翻译的影响比IRES介导的翻译更大。毒胡萝卜素和衣霉素诱导的 UPR（未折叠蛋白反应）还通过 IRES 介导的帽独立翻译增加了 Hep G2 细胞中的 SREBP-I 表达。总体而言，这些发现表明SREBP-I a 5'-UTR中IRES的存在使得翻译能够在抑制帽依赖性翻译的条件下得以维持。
SREBPs (sterol-regulatory-element-binding proteins) are a family of transcription factors that modulate the expression of several enzymes implicated in endogenous cholesterol, fatty acid, triacylelycerol and phospholipid synthesis. In the present study, evidence for SREBP-1 regulation at the translational level is reported. Using, several experimental approaches, we have demonstrated that the 5'-UTR (untranslated region) of the SREBP-1 a mRNA contains an IRES (internal ribosome entry site). Transfection experiments with the SREBP-1 a 5'-UTR inserted in a dicistronic reporter vector showed a remarkable increase in the downstream cistron translation, through a cap-independent mechanism. Insertion of the SREBP-1c 5'-UTR in the same vector also stimulated the translation of the downstream cistron, but the observed effect can be ascribed, at least in part, to a cryptic promoter activity. Cellular stress conditions, such as serum starvation, caused an increase in the level of SREBP-1 precursor and mature form in both Hep G2 and HeLa cells, despite the overall reduction in protein synthesis, whereas mRNA levels for SREBP-1 were unaffected by serum starvation. Transfection experiments carried out with a dicistronic construct demonstrated that the cap-dependent translation was affected more than IRES-mediated translation by serum starvation. The thapsigargin-and tunicamycin-induced UPR (unfolded protein response) also increased SREBP-I expression in Hep G2 cells, through the cap-independent translation mediated by IRES. Overall, these findings indicate that the presence of IRES in the SREBP-I a 5'-UTR allows translation to be maintained under conditions that are inhibitory to cap-dependent translation.