Department of Genetics & Tumor Cell Biology, St. Jude Children's Research Hospital, Memphis, Tennessee, United States of America.
PLoS One. 2010 Sep 2;5(9):e12521. doi: 10.1371/journal.pone.0012521.
Inadequate extracellular conditions can adversely affect the environment of the ER and impinge on the maturation of nascent proteins. The resultant accumulation of unfolded proteins activates a signal transduction pathway, known as the unfolded protein response, which serves primarily to protect the cell during stress and helps restore homeostasis to the ER.
Microarray analysis of the unfolded protein response in a human medulloblastoma cell line treated with thapsigargin revealed that, in addition to known targets, a large number of proangiogenic factors were up-regulated. Real-Time PCR analyses confirmed that four of these factors, VEGFA, FGF2, angiogenin and IL8, were transcriptionally up-regulated in multiple cell lines by various ER stress inducers. Our studies on VEGFA regulation revealed that XBP-1(S), a UPR-inducible transcription factor, bound to two regions on the VEGFA promoter, and analysis of XBP-1 null mouse embryonic fibroblasts revealed that it contributes to VEGFA expression in response to ER stress. ATF4, another UPR-inducible transcription factor, also binds to the VEGFA gene, although its contribution to VEGFA transcription appeared to be fairly modest. We also found that VEGFA mRNA stability is increased in response to UPR activation, via activation of AMP kinase, demonstrating that increased mRNA levels occur at two regulatory points. In keeping with the mRNA levels, we found that VEGFA protein is secreted at levels as high as or higher than that achieved in response to hypoxia.
Our results indicate that the UPR plays a significant role in inducing positive regulators of angiogenesis. It also regulates VEGFA expression at transcriptional, post-transcriptional and post-translational levels and is likely to have widespread implications for promoting angiogenesis in response to normal physiological cues as well as in pathological conditions like cancer.
细胞外环境不足会对内质网的环境产生不利影响，并影响新生蛋白质的成熟。未折叠蛋白质的积累会激活一种信号转导途径，称为未折叠蛋白反应，主要作用是在应激过程中保护细胞，并帮助内质网恢复内稳态。
用 thapsigargin 处理人髓母细胞瘤细胞系的未折叠蛋白反应的微阵列分析显示，除了已知的靶点外，大量促血管生成因子上调。实时 PCR 分析证实，在多种细胞系中，这四种因子（VEGFA、FGF2、血管生成素和 IL8）被各种内质网应激诱导物转录上调。我们对 VEGFA 调节的研究表明，UPR 诱导的转录因子 XBP-1(S)结合到 VEGFA 启动子的两个区域，对 XBP-1 缺失的小鼠胚胎成纤维细胞的分析表明，它有助于 VEGFA 在 ER 应激时的表达。另一种 UPR 诱导的转录因子 ATF4 也结合到 VEGFA 基因上，尽管其对 VEGFA 转录的贡献似乎相当有限。我们还发现，VEGFA mRNA 稳定性在 UPR 激活时通过激活 AMP 激酶而增加，表明在两个调节点增加了 mRNA 水平。与 mRNA 水平一致，我们发现 VEGFA 蛋白的分泌水平与低氧反应时一样高，甚至更高。
我们的结果表明，UPR 在诱导血管生成的正调节剂方面起着重要作用。它还在转录、转录后和翻译后水平调节 VEGFA 表达，并且可能对响应正常生理信号以及癌症等病理条件下促进血管生成具有广泛的意义。
