MEK signaling is required for phosphorylation of eIF2α following amino acid limitation of HepG2 human hepatoma cells
MEK signaling is required for phosphorylation of eIF2α following amino acid limitation of HepG2 human hepatoma cells
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DOI：
10.1074/jbc.m708320200
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发表时间：
2008-04-18
影响因子：
4.8
通讯作者：
Kilberg, Michael S.
中科院分区：
文献类型：
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作者：
Thiaville, Michelle M.;Pan, Yuan-Xiang;Kilberg, Michael S.
哺乳动物的氨基酸反应(AAR)途径被蛋白质或氨基酸耗竭上调。这个途径包括通过GCN2激酶检测不带电荷的tRNA，翻译起始因子eIF2α(真核细胞起始因子2α)的磷酸化，以及通过随后的翻译控制，增强转录因子ATF4的从头合成。目前的研究表明，PD98059或U0126抑制人肝癌细胞中MEK的激活，阻断了由氨基酸限制引起的eIF2α和ATF4合成的磷酸化增加，表明AAR需要激活MEK-ERK通路。JNK或p38 MAPK通路的抑制剂无效。因此，抑制MEK的激活阻止了ATF4靶基因的转录诱导，但ATF4蛋白的过度表达挽救了这种诱导。此外，剥夺氨基酸后ERK磷酸化的增强需要GCN2激酶活性和eIF2α的磷酸化。通过敲除Gadd34抑制蛋白磷酸酶1对磷酸化eIF2α的作用并不能阻断对PD98059的敏感性，提示MEK的作用是增强GCN2依赖的eIF2α的磷酸化，而不是抑制去磷酸化。总之，这些结果证明了MEK-ERK MAPK信号通路和氨基酸应激激活通路之间的关键相互依赖关系。
The mammalian amino acid response (AAR) pathway is upregulated by protein or amino acid depletion. This pathway involves detection of uncharged tRNA by the GCN2 kinase, phosphorylation of the translation initiation factor eIF2 alpha (eukaryotic initiation factor 2 alpha), and, through subsequent translational control, enhanced de novo synthesis of the transcription factor ATF4. The present studies demonstrate that inhibition of MEK activation in HepG2 human hepatoma cells by PD98059 or U0126 blocked the increased phosphorylation of eIF2 alpha and ATF4 synthesis triggered by amino acid limitation, showing that the AAR requires activation of the MEK-ERK pathway. Inhibitors of the JNK or p38 MAPK pathways were ineffective. Consequently, inhibition of MEK activation blocked transcriptional induction of ATF4 target genes, but the induction was rescued by overexpression of ATF4 protein. Furthermore, the enhanced ERK phosphorylation following amino acid deprivation required GCN2 kinase activity and eIF2 alpha phosphorylation. Inhibition of protein phosphatase 1 action on phospho-eIF2 alpha by knockdown of GADD34 did not block the sensitivity to PD98059, suggesting that MEK functions to enhance GCN2-dependent eIF2 alpha phosphorylation rather than suppressing dephosphorylation. Collectively, these results document a critical interdependence between the MEK-ERK MAPK signaling pathway and the amino acid stress-activated pathway.