MEK-1 activates C-Raf through a Ras-independent mechanism
MEK-1 activates C-Raf through a Ras-independent mechanism
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DOI：
10.1016/j.bbamcr.2013.01.015
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发表时间：
2013-05-01
影响因子：
5.1
通讯作者：
Tzivion, Guri
中科院分区：
文献类型：
--
作者：
Leicht, Deborah T.;Balan, Vitaly;Tzivion, Guri
C-Raf 是 Ras-Raf-MEK-ERK 丝裂原激活蛋白激酶 (MAPK) 信号通路的成员，该通路在多种生理过程中发挥关键作用，并在许多人类癌症中上调。 C-Raf 激活涉及与 Ras 的结合、磷酸化的增加以及与辅助因子的相互作用。在这里，我们描述了 C-Raf 通过其下游靶标 MEK 激活的不依赖于 Ras 的体内途径。通过 P-32 代谢标记和 2D 磷酸肽图谱实验，我们表明 MEK 可使 C-Raf 磷酸化增加多达 10 倍。这种增加与 C-Raf 激酶激活相关，与生长因子刺激所见的活性相匹配。因此，野生型 C-Raf 和 MEK 的共表达足以实现 ERK 的完全和组成型激活。值得注意的是，MEK 激活 C-Raf 的能力完全独立于 Ras，因为对生长因子无反应的 Ras 结合受损的突变体或 Ras 被 MEK 完全激活。 MEK 激活 C-Raf 的能力仅部分依赖于 MEK 激酶活性，但需要 MEK 与 C-Raf 结合，表明这种结合导致构象变化，从而增加 C-Raf 对磷酸化和激活的敏感性，或导致磷酸化活性形式的稳定。这些发现提出了一种新的 Ras 独立机制，用于激活 C-Raf 和 MAPK 通路，而不需要该通路发生突变。这种机制在病理状况或过度表达 C-Raf 和 MEK 的癌症中或在 C-Raf-MEK 相互作用因 RKIP 和 MST2 下调而增强的情况下可能具有重要意义。 (C) 2013 Elsevier B.V. 保留所有权利。
C-Raf is a member of the Ras-Raf-MEK-ERK mitogen-activated protein kinase (MAPK) signaling pathway that plays key roles in diverse physiological processes and is upregulated in many human cancers. C-Raf activation involves binding to Ras, increased phosphorylation and interactions with co-factors. Here, we describe a Ras-independent in vivo pathway for C-Raf activation by its downstream target MEK. Using P-32-metabolic labeling and 2D-phosphopeptide mapping experiments, we show that MEK increases C-Raf phosphorylation by up-to 10-fold. This increase was associated with C-Raf kinase activation, matching the activity seen with growth factor stimulation. Consequently, coexpression of wildtype C-Raf and MEK was sufficient for full and constitutive activation of ERK. Notably, the ability of MEK to activate C-Raf was completely Ras independent, since mutants impaired in Ras binding that are irresponsive to growth factors or Ras were fully activated by MEK. The ability of MEK to activate C-Raf was only partially dependent on MEK kinase activity but required MEK binding to C-Raf, suggesting that the binding results in a conformational change that increases C-Raf susceptibility to phosphorylation and activation or in the stabilization of the phosphorylated-active form. These findings propose a novel Ras-independent mechanism for activating the C-Raf and the MAPK pathway without the need for mutations in the pathway. This mechanism could be of significance in pathological conditions or cancers overexpressing C-Raf and MEK or in conditions where C-Raf-MEK interaction is enhanced due to the down-regulation of RKIP and MST2. (C) 2013 Elsevier B.V. All rights reserved.