Eley Helen L, Tisdale Michael J
Nutritional Biomedicine, School of Life and Health Sciences, Aston University, Birmingham B4 7ET, United Kingdom.
J Biol Chem. 2007 Mar 9;282(10):7087-97. doi: 10.1074/jbc.M610378200. Epub 2007 Jan 9.
Both proteolysis-inducing factor (PIF) and angiotensin II have been shown to produce a depression in protein synthesis in murine myotubes concomitant with an increased phosphorylation of eukaryotic initiation factor 2 (eIF2alpha). Both PIF and angiotensin II were shown to induce autophosphorylation of the RNA-dependent protein kinase (PKR), and an inhibitor of this enzyme completely attenuated the depression in protein synthesis and prevented the induction of eIF2alpha phosphorylation. The PKR inhibitor also completely attenuated the increase in protein degradation induced by PIF and angiotensin II and prevented the increase in proteasome expression and activity. To confirm these results myotubes were transfected with plasmids that express either wild-type PKR, or a catalytically inactive PKR variant, PKRDelta6. Myotubes expressing PKRDelta6 showed no increase in eIF2alpha phosphorylation in response to PIF or angiotensin II, no depression in protein synthesis, and no increase in protein degradation or increase in proteasome expression. Induction of the ubiquitin-proteasome pathway by PIF and angiotensin II has been linked to activation of the transcription factor nuclear factor-kappaB (NF-kappaB). Inhibition of PKR prevented nuclear migration of NF-kappaB in response to both PIF and angiotensin II, by preventing degradation of the inhibitor protein I-kappaB. Phosphorylation of PKR and eIF2alpha was also significantly increased in the gastrocnemius muscle of weight losing mice bearing the MAC16 tumor, suggesting that a similar process may be operative in cancer cachexia. These results provide a link between the depression of protein synthesis in skeletal muscle and the increase in protein degradation.
蛋白水解诱导因子（PIF）和血管紧张素II均已被证明可使小鼠肌管中的蛋白质合成受到抑制，同时真核起始因子2（eIF2α）的磷酸化增加。PIF和血管紧张素II均被证明可诱导RNA依赖性蛋白激酶（PKR）的自磷酸化，并且该酶的抑制剂可完全减轻蛋白质合成的抑制，并阻止eIF2α磷酸化的诱导。PKR抑制剂还可完全减轻PIF和血管紧张素II诱导的蛋白质降解增加，并阻止蛋白酶体表达和活性的增加。为了证实这些结果，用表达野生型PKR或催化无活性的PKR变体PKRDelta6的质粒转染肌管。表达PKRDelta6的肌管在对PIF或血管紧张素II的反应中未显示eIF2α磷酸化增加，未出现蛋白质合成抑制，也未出现蛋白质降解增加或蛋白酶体表达增加。PIF和血管紧张素II对泛素-蛋白酶体途径的诱导与转录因子核因子-κB（NF-κB）的激活有关。抑制PKR可通过阻止抑制剂蛋白I-κB的降解来阻止NF-κB响应PIF和血管紧张素II的核迁移。在携带MAC16肿瘤的体重减轻小鼠的腓肠肌中，PKR和eIF2α的磷酸化也显著增加，这表明类似的过程可能在癌症恶病质中起作用。这些结果在骨骼肌蛋白质合成抑制和蛋白质降解增加之间建立了联系。
