Thuerauf Donna J, Morrison Lisa, Glembotski Christopher C
San Diego State University Heart Institute and Department of Biology, San Diego State University, San Diego, CA 92182, USA.
J Biol Chem. 2004 May 14;279(20):21078-84. doi: 10.1074/jbc.M400713200. Epub 2004 Feb 18.
The endoplasmic reticulum (ER) transmembrane proteins, ATF6alpha and ATF6beta, are cleaved in response to ER stress, which can be induced by tunicamycin. The resulting N-terminal fragments of both ATF6 isoforms, which have conserved basic leucine-zipper and DNA binding domains but divergent transcriptional activation domains, translocate to the nucleus where they bind to ER stress-response elements (ERSE) in ER stress-response genes (ERSRG), such as GRP78. Although it is known that ATF6alpha is a potent activator of ERSRGs, the transcriptional potency and functions of ATF6beta remain to be explored. Accordingly, N-terminal fragments of each ATF6 isoform (N-ATF6alpha and N-ATF6beta) were overexpressed in HeLa cells and the effects on GRP78 induction were assessed. When expressed at similar levels, N-ATF6alpha conferred approximately 200-fold greater GRP78 promoter activation than N-ATF6beta. Because ER stress activates nuclear translocation of both ATF6alpha and beta and because both bind to ERSEs, the effect of co-expressing them on GRP78 induction was assessed. Surprisingly, N-ATF6beta inhibited N-ATF6alpha-mediated GRP78 promoter activation in a dominant-negative manner. Moreover, N-ATF6beta inhibited TN-mediated GRP78 promoter activation, which requires endogenous ATF6alpha. ATF6 isoform-specific small inhibitory RNAs were used to show that, as expected, endogenous ATF6alpha was required for maximal ERSRG induction; however, endogenous ATF6beta moderated ERSRG induction. These results indicate that compared with ATF6alpha, ATF6beta is a very poor activator of ERSRG induction and it represses ATF6alpha-mediated ERSRG induction. Thus, ATF6beta may serve as a transcriptional repressor functioning in part to regulate the strength and duration of ATF6alpha-mediated ERSRG activation during the ER stress response.
内质网（ER）跨膜蛋白ATF6α和ATF6β会在内质网应激反应中被切割，衣霉素可诱导这种应激反应。两种ATF6亚型产生的N端片段，具有保守的碱性亮氨酸拉链和DNA结合结构域，但转录激活结构域不同，它们会转移至细胞核，在那里与内质网应激反应基因（ERSRG）如GRP78中的内质网应激反应元件（ERSE）结合。尽管已知ATF6α是ERSRGs的有效激活剂，但ATF6β的转录活性和功能仍有待探索。因此，在HeLa细胞中过表达了每种ATF6亚型的N端片段（N-ATF6α和N-ATF6β），并评估了其对GRP78诱导的影响。当以相似水平表达时，N-ATF6α赋予GRP78启动子的激活能力比N-ATF6β大200倍左右。由于内质网应激会激活ATF6α和β的核转位，且二者都与ERSEs结合，因此评估了共表达它们对GRP78诱导的影响。令人惊讶的是，N-ATF6β以显性负性方式抑制N-ATF6α介导的GRP78启动子激活。此外，N-ATF6β抑制了需要内源性ATF6α的衣霉素介导的GRP78启动子激活。使用ATF6亚型特异性小干扰RNA表明，正如预期的那样，最大程度的ERSRG诱导需要内源性ATF6α；然而，内源性ATF6β会调节ERSRG诱导。这些结果表明，与ATF6α相比，ATF6β是ERSRG诱导的非常弱的激活剂，并且它会抑制ATF6α介导的ERSRG诱导。因此，ATF6β可能作为一种转录抑制因子，部分作用是在内质网应激反应期间调节ATF6α介导的ERSRG激活的强度和持续时间。
