MEK signaling is required for phosphorylation of eIF2a 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 eIF2α, and enhanced de novo synthesis of the transcription factor ATF4. The present studies demonstrate that inhibition of MEK activation, by treatment of HepG2 human hepatoma cells with PD98059, blocked the increased phosphorylation of eIF2α and ATF4 synthesis triggered by amino acid limitation, showing that AAR signaling requires the MEK‐ERK pathway. Consequently, inhibition of MEK activation blocked transcriptional induction of ATF4 target genes, but the induction was rescued by over‐expression of ATF4 protein. Furthermore, the enhanced ERK phosphorylation following amino acid deprivation required GCN2 kinase activity, but not eIF2α phosphorylation. Inhibition of protein phosphatase 1 action on phospho‐eIF2α, by siRNA knockdown of GADD34, did not block the sensitivity to PD98059 suggesting that the MEK‐ERK pathway functions to enhance GCN2‐dependent eIF2α phosphorylation, rather than to suppress dephosphorylation. These results document a critical interdependence between the MEK‐ERK MAPK signaling pathway and the AAR pathway. Funded by grants from the NIH (DK‐52064, DK‐70647).