Browsing by Subject "Eukaryotic Initiation Factor-3"

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    EIF3i Promotes Colon Oncogenesis by Regulating COX-2 Protein Synthesis and β-Catenin Activation
    (Nature Publishing Group, 2014-08-07) Qi, Jing; Dong, Zizheng; Liu, Jianguo; Zhang, Jian-Ting; Department of Pharmacology and Toxicology, IU School of Medicine
    Translational control of gene expression has recently been recognized as an important mechanism controlling cell proliferation and oncogenesis and it mainly occurs in the initiation step of protein synthesis that involves multiple eukaryotic initiation factors (eIFs). Many eIFs have been found to have aberrant expression in human tumors and the aberrant expression may contribute to oncogenesis. However, how these previously considered house-keeping proteins are potentially oncogenic remains elusive. In this study, we investigated the expression of eIF3i in human colon cancers, tested its contribution to colon oncogenesis, and determined the mechanism of eIF3i action in colon oncogenesis. We found that eIF3i expression was up-regulated in both human colon adenocarcinoma and adenoma polyps as well as in model inducible colon tumorigenic cell lines. Over-expression of ectopic eIF3i in intestinal epithelial cells causes oncogenesis by directly up-regulating synthesis of COX-2 protein and activates the β-catenin/TCF4 signaling pathway that mediates the oncogenic function of eIF3i. Together, we conclude that eIF3i is a proto-oncogene that drives colon oncogenesis by translationally up-regulating COX-2 and activating β-catenin signaling pathway. These findings imply that protooncogenic eIFs likely exert their tumorigenic function by regulating/altering the synthesis level of down-stream tumor suppressor or oncogenes.
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    The roles of stress-activated Sty1 and Gcn2 kinases and proto-oncoprotein homologue Int6/eIF3e in responses to endogenous oxidative stress during histidine starvation
    (Elsevier, 2010-11-26) Nemoto, Naoki; Udagawa, Tsuyoshi; Ohira, Takahiro; Jiang, Li; Hirota, Kouji; Wilkinson, Caroline R. M.; Bähler, Jürg; Jones, Nic; Ohta, Kunihiro; Wek, Ronald C.; Asano, Katsura; Department of Biochemistry & Molecular Biology, IU School of Medicine
    In fission yeast, Sty1 and Gcn2 are important protein kinases regulating gene expression in response to amino acid starvation. The translation factor subunit eIF3e/Int6 promotes the Sty1-dependent response by increasing the abundance of Atf1, a transcription factor targeted by Sty1. While Gcn2 promotes expression of amino acid biosynthesis enzymes, the mechanism and function for Sty1 activation and Int6/eIF3e involvement during this nutrient stress is not understood. Here we show that mutants lacking sty1+ or gcn2+ display reduced viabilities during histidine depletion stress in a manner suppressible by the antioxidant, N-acetyl cysteine, suggesting that these protein kinases function to alleviate endogenous oxidative damage generated during nutrient starvation. Int6/eIF3e also promotes cell viability by a mechanism involving stimulation of the Sty1 response to oxidative damage. In further support of these observations, microarray data suggests that, during histidine starvation, int6Δ increases the duration of Sty1-activated gene expression linked to oxidative stress due to the initial attenuation of Sty1-dependent transcription. Moreover, loss of gcn2 induces the expression of a new set of genes not activated in wild-type cells starved for histidine. These genes encode heatshock proteins, redox enzymes and proteins involved in mitochondrial maintenance, in agreement with the idea that oxidative stress is imposed onto gcn2Δ cells. Furthermore, the early Sty1 activation promotes a rapid Gcn2 activation on histidine starvation. These results suggest that Gcn2, Sty1, and Int6/eIF3e are functionally integrated and cooperate to respond to oxidative stress that is generated during histidine starvation.