Transcriptional Regulation of VEGF-A by the Unfolded Protein Response Pathway

BackgroundAngiogenesis is crucial to many physiological and pathological processes including development and cancer cell survival. Vascular endothelial growth factor-A (VEGFA) is the predominant mediator of angiogenesis in the VEGF family. During development, adverse environmental conditions like nutrient deprivation, hypoxia and increased protein secretion occur. IRE1α, PERK, and ATF6α, master regulators of the unfolded protein response (UPR), are activated under these conditions and are proposed to have a role in mediating angiogenesis.
Principal FindingsHere we show that IRE1α, PERK, and ATF6α powerfully regulate VEGFA mRNA expression under various stress conditions. In Ire1α−/− and Perk−/− mouse embryonic fibroblasts and ATF6α-knockdown HepG2 cells, induction of VEGFA mRNA by endoplasmic reticulum stress is attenuated as compared to control cells. Embryonic lethality of Ire1α−/− mice is due to the lack of VEGFA induction in labyrinthine trophoblast cells of the developing placenta. Rescue of IRE1α and PERK in Ire1α−/− and Perk−/− cells respectively, prevents VEGFA mRNA attenuation. We further report that the induction of VEGFA by IRE1α, PERK and ATF6 involves activation of transcription factors, spliced-XBP-1, ATF4 and cleaved ATF6 respectively.
Conclusions/SignificanceOur results reveal that the IRE1α-XBP-1, PERK-ATF4, and ATF6α pathways constitute novel upstream regulatory pathways of angiogenesis by modulating VEGF transcription. Activation of these pathways helps the rapidly growing cells to obtain sufficient nutrients and growth factors for their survival under the prevailing hostile environmental conditions. These results establish an important role of the UPR in angiogenesis.

研究背景
血管生成对包括胚胎发育与癌细胞存活在内的诸多生理及病理过程均至关重要。血管内皮生长因子-A（Vascular endothelial growth factor-A, VEGFA）是VEGF家族中介导血管生成的核心因子。在生物体发育过程中，常会出现营养匮乏、缺氧及蛋白分泌过载等不良环境应激。内质网未折叠蛋白反应（unfolded protein response, UPR）的核心调控因子IRE1α、PERK与ATF6α会在这类应激条件下被激活，且被证实参与介导血管生成过程。

主要研究结果
本研究证实，IRE1α、PERK及ATF6α可在多种应激条件下强效调控VEGFA mRNA的表达水平。相较于对照细胞，在Ire1α−/−与Perk−/−小鼠胚胎成纤维细胞，以及ATF6α敲低的HepG2细胞中，内质网应激诱导的VEGFA mRNA上调效应被显著削弱。Ire1α−/−小鼠的胚胎致死表型，源于发育胎盘的迷路层滋养层细胞无法正常诱导VEGFA的表达。分别在Ire1α−/−与Perk−/−细胞中恢复IRE1α与PERK的表达，可逆转VEGFA mRNA的表达下调缺陷。本研究进一步揭示，IRE1α、PERK与ATF6α对VEGFA的诱导作用，分别通过激活转录因子剪接型XBP-1（spliced-XBP-1）、ATF4及裂解型ATF6实现。

结论与意义
本研究结果表明，IRE1α-XBP-1、PERK-ATF4及ATF6α通路通过调控VEGF的转录过程，构成了血管生成的新型上游调控通路。在恶劣的环境应激条件下，这些通路的激活可帮助快速增殖的细胞获取充足的营养与生长因子以维持存活。本研究证实了未折叠蛋白反应在血管生成过程中的重要生物学功能。