Anthony Tracy G, McDaniel Brent J, Byerley Rachel L, McGrath Barbara C, Cavener Douglas R, McNurlan Margaret A, Wek Ronald C
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Evansville, IN 47712, USA.
J Biol Chem. 2004 Aug 27;279(35):36553-61. doi: 10.1074/jbc.M404559200. Epub 2004 Jun 22.
In eukaryotic cells, amino acid depletion reduces translation by a mechanism involving phosphorylation of eukaryotic initiation factor-2 (eIF2). Herein we describe that mice lacking the eIF2 kinase, general control nonderepressible 2 (GCN2) fail to alter the phosphorylation of this initiation factor in liver, and are moribund in response to dietary leucine restriction. Wild-type (GCN2(+/+)) and two strains of GCN2 null (GCN2(-/-)) mice were provided a nutritionally complete diet or a diet devoid of leucine or glycine for 1 h or 6 days. In wild-type mice, dietary leucine restriction resulted in loss of body weight and liver mass, yet mice remained healthy. In contrast, a significant proportion of GCN2(-/-) mice died within 6 days of the leucine-deficient diet. Protein synthesis in wild-type livers was decreased concomitant with increased phosphorylation of eIF2 and decreased phosphorylation of 4E-BP1 and S6K1, translation regulators controlled nutritionally by mammalian target of rapamycin. Whereas translation in the liver was decreased independent of GCN2 activity in mice fed a leucine-free diet for 1 h, protein synthesis in GCN2(-/-) mice at day 6 was enhanced to levels measured in mice fed the complete diet. Interestingly, in addition to a block in eIF2 phosphorylation, phosphorylation of 4E-BP1 and S6K1 was not decreased in GCN2(-/-) mice deprived of leucine for 6 days. This suggests that GCN2 activity can also contribute to nutritional regulation of the mammalian target of rapamycin pathway. As a result of the absence of these translation inhibitory signals, liver weights were preserved and instead, skeletal muscle mass was reduced in GCN2(-/-) mice fed a leucine-free diet. This study indicates that loss of GCN2 eIF2 kinase activity shifts the normal maintenance of protein mass away from skeletal muscle to provide substrate for continued hepatic translation.
在真核细胞中，氨基酸耗竭通过一种涉及真核起始因子2（eIF2）磷酸化的机制来降低翻译水平。在此我们描述，缺乏eIF2激酶即一般控制非抑制性2（GCN2）的小鼠，肝脏中该起始因子的磷酸化未发生改变，并且在饮食亮氨酸限制的情况下濒死。给野生型（GCN2(+/+)）和两株GCN2基因敲除（GCN2(-/-)）小鼠提供营养完全的饮食或不含亮氨酸或甘氨酸的饮食1小时或6天。在野生型小鼠中，饮食亮氨酸限制导致体重和肝脏质量下降，但小鼠仍保持健康。相比之下，相当一部分GCN2(-/-)小鼠在缺乏亮氨酸饮食的6天内死亡。野生型肝脏中的蛋白质合成减少，同时eIF2的磷酸化增加，4E-BP1和S6K1的磷酸化减少，4E-BP1和S6K1是受雷帕霉素哺乳动物靶标营养调控的翻译调节因子。虽然在喂食无亮氨酸饮食1小时的小鼠中，肝脏中的翻译减少与GCN2活性无关，但在第6天GCN2(-/-)小鼠中的蛋白质合成增强至喂食完全饮食小鼠中测得的水平。有趣的是，除了eIF2磷酸化受阻外，在缺乏亮氨酸6天的GCN2(-/-)小鼠中，4E-BP1和S6K1的磷酸化并未降低。这表明GCN2活性也可能有助于雷帕霉素哺乳动物靶标途径的营养调节。由于缺乏这些翻译抑制信号，喂食无亮氨酸饮食的GCN2(-/-)小鼠的肝脏重量得以保留，相反，骨骼肌质量减少。这项研究表明，GCN2 eIF2激酶活性的丧失将蛋白质质量的正常维持从骨骼肌转移，以为肝脏的持续翻译提供底物。
