Gille H, Strahl T, Shaw P E
Max-Planck-Institut für Immunbiologie, Spemann Laboratories, Freiburg, Germany.
Curr Biol. 1995 Oct 1;5(10):1191-200. doi: 10.1016/s0960-9822(95)00235-1.
The mammalian response to stress results in the activation of stress-activated protein kinases (also known as cJun N-terminal kinases; SAPKs or JNKs), which are a sub-group of the mitogen-activated protein (MAP) kinase family. The SAPKs are involved in the upregulation of activity of the transcription factor AP-1 by post-translational modification of two of its components, cJun and ATF2. AP-1 activity can also be elevated by increased expression of the Fos protein, a further AP-1 component. Elk-1 (also called p62TCF), a transcription factor involved in the induction of the expression from the c-fos promoter through the promoter's serum response element, is known to be activated as a result of phosphorylation by the MAP kinases ERK1 and ERK2. However, induction of c-fos expression in response to noxious agents takes place in the absence of ERK activation. We therefore investigated whether SAPKs similarly upregulate c-fos expression by phosphorylating Elk-1.
Elk-1 is activated in response to stimuli other than mitogenic signals. Both p46SAPK and p54SAPK interact physically with, and phosphorylate, Elk-1. The capacity of Elk-1 to form a ternary complex with serum response factor in vitro is thereby elevated. In vivo, selective activation of SAPKs stimulates formation of the ternary complex containing Elk-1, serum response factor and the serum response element, and enhances Elk-1-dependent transcription. Expression of the SAPK upstream-activator kinase, MEKK1, induces SAPK activation and c-fos transcription in the absence of ERK activity. Phosphopeptide mapping of Elk-1 phosphorylated with p46SAPK or p54SAPK reveals Ser383, a residue critical for ternary complex formation and transcriptional activation, to be the major phosphorylation site.
Elk-1 responds to stress-induced, as well as mitogenic, signals by stimulating c-fos transcription through the serum response element. Phosphorylation of Elk-1 by SAPKs and the ensuing expression of Fos protein thus constitutes an additional mechanism by which cells can upregulate AP-1 activity in response to stress.
哺乳动物对应激的反应会导致应激激活蛋白激酶（也称为cJun氨基末端激酶；SAPKs或JNKs）的激活，它们是丝裂原激活蛋白（MAP）激酶家族的一个亚组。SAPKs通过对转录因子AP-1的两个组分cJun和ATF2进行翻译后修饰，参与上调其活性。Fos蛋白（AP-1的另一个组分）表达增加也可提高AP-1活性。已知转录因子Elk-1（也称为p62TCF）通过MAP激酶ERK1和ERK2的磷酸化而被激活，它通过启动子的血清反应元件参与诱导c-fos启动子的表达。然而，在没有ERK激活的情况下，对有害刺激的反应中也会发生c-fos表达的诱导。因此，我们研究了SAPKs是否同样通过磷酸化Elk-1来上调c-fos表达。
Elk-1在有丝分裂信号以外的刺激下被激活。p46SAPK和p54SAPK都与Elk-1发生物理相互作用并使其磷酸化。这从而提高了Elk-1在体外与血清反应因子形成三元复合物的能力。在体内，SAPKs的选择性激活刺激了包含Elk-1、血清反应因子和血清反应元件的三元复合物的形成，并增强了Elk-1依赖性转录。SAPK上游激活激酶MEKK1的表达在没有ERK活性的情况下诱导SAPK激活和c-fos转录。用p46SAPK或p54SAPK磷酸化的Elk-1的磷酸肽图谱显示，Ser383是三元复合物形成和转录激活的关键残基，是主要的磷酸化位点。
Elk-1通过血清反应元件刺激c-fos转录，对应激诱导信号以及有丝分裂信号作出反应。因此，SAPKs对Elk-1的磷酸化以及随后Fos蛋白的表达构成了细胞在应激反应中上调AP-1活性的另一种机制。
