title: MEK signaling is required for phosphorylation of eIF2alpha following amino acid limitation of HepG2 human hepatoma cells.
abstract: The mammalian amino acid response (AAR) pathway is up-regulated by protein or amino acid depletion. This pathway involves detection of uncharged tRNA by the GCN2 kinase, phosphorylation of the translation initiation factor eIF2alpha (eukaryotic initiation factor 2alpha), 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 eIF2alpha 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 eIF2alpha phosphorylation. Inhibition of protein phosphatase 1 action on phospho-eIF2alpha by knockdown of GADD34 did not block the sensitivity to PD98059, suggesting that MEK functions to enhance GCN2-dependent eIF2alpha 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.
authors.affiliation: Department of Biochemistry and Molecular Biology, Genetics Institute, Shands Cancer Center and Center for Nutritional Sciences, University of Florida College of Medicine, Gainesville, Florida 32610, USA.
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Thiaville
authors.forename: Yuan-Xiang
authors.initials: YX
authors.lastname: Pan
authors.forename: Altin
authors.initials: A
authors.lastname: Gjymishka
authors.forename: Can
authors.initials: C
authors.lastname: Zhong
authors.forename: Randal J
authors.initials: RJ
authors.lastname: Kaufman
authors.forename: Michael S
authors.initials: MS
authors.lastname: Kilberg
mesh_terms: D000818:Animals; D000943:Antigens, Differentiation / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D005419:Flavonoids / Q000494:pharmacology; D006801:Humans; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D008954:Models, Biological; D010766:Phosphorylation; D014176:Protein Biosynthesis; D054645:Protein Phosphatase 1; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism* / Q000502:physiology*; D034741:RNA, Small Interfering / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D000943:Antigens, Differentiation; D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D015852:Eukaryotic Initiation Factor-2; D005419:Flavonoids; D034741:RNA, Small Interfering; C529271:EIF2AK4 protein, human; C070634:Eif2ak4 protein, mouse; D017346:Protein Serine-Threonine Kinases; D020929:Mitogen-Activated Protein Kinase Kinases; C087519:PPP1R15A protein, human; D054645:Protein Phosphatase 1; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one