Mnk2 and Mnk1 Are Essential for Constitutive and Inducible Phosphorylation of Eukaryotic Initiation Factor 4E but Not for Cell Growth or Development

Abstract

Mnk1 and Mnk2 are protein kinases that are directly phosphorylated and activated by extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases and implicated in the regulation of protein synthesis through their phosphorylation of eukaryotic translation initiation factor 4E (eIF4E) at Ser209. To investigate their physiological functions, we generated mice lacking the Mnk1 or Mnk2 gene or both; the resulting KO mice were viable, fertile, and developed normally. In embryonic fibroblasts prepared from Mnk1-Mnk2 DKO mice, eIF4E was not detectably phosphorylated at Ser209, even when the ERK and/or p38 MAP kinases were activated. Analysis of embryonic fibroblasts from single KO mice revealed that Mnk1 is responsible for the inducible phosphorylation of eIF4E in response to MAP kinase activation, whereas Mnk2 mainly contributes to eIF4E's basal, constitutive phosphorylation. Lipopolysaccharide (LPS)-or insulininduced upregulation of eIF4E phosphorylation in the spleen, liver, or skeletal muscle was abolished in Mnk1 ؊/؊ mice, whereas the basal eIF4E phosphorylation levels were decreased in Mnk2 ؊/؊ mice. In Mnk1-Mnk2 DKO mice, no phosphorylated eIF4E was detected in any tissue studied, even after LPS or insulin injection. However, neither general protein synthesis nor cap-dependent translation, as assayed by a bicistronic reporter assay system, was affected in Mnk-deficient embryonic fibroblasts, despite the absence of phosphorylated eIF4E. Thus, Mnk1 and Mnk2 are exclusive eIF4E kinases both in cultured fibroblasts and adult tissues, and they regulate inducible and constitutive eIF4E phosphorylation, respectively. These results strongly suggest that eIF4E phosphorylation at Ser209 is not essential for cell growth during development.Mitogen-activated protein kinases (MAPKs) are activated by various extracellular signals, such as growth factors, stresses, and cytokines, and play crucial roles in the determination of cell fate through proliferation, differentiation, survival, and apoptosis (1,5,24,38) . Three classes of MAPK families, the ERK, Jun N-terminal kinase/stress-activated protein kinase, and p38 MAPK, are differentially activated depending on the signaling context and in turn phosphorylate target proteins, which include transcription factors and protein kinases. These proteins can be common targets for subsets of MAPK proteins or specific targets for individual MAPKs. The direct downstream protein kinases, comprehensively called the MAPKactivated protein kinase (MAPKAPK) family, can be categorized into four subclasses, the Rsk, MK, Mnk, and Msk families. The Rsks (Rsk1, Rsk2, and Rsk3) are activated specifically by ERKs, whereas the MKs (MK2/MAPKAPK2, MK3/ MAPKAPK3/3pK, and MK5/PRAK) are activated mainly by p38 MAPK in vivo. In contrast, the Mnks (Mnk1 and Mnk2) and Msks (Msk1 and Msk2) are targeted in vivo by both the ERK and p38 MAPK pathways, resulting in the activation of Mnks and Msks by a broad spectrum of extracellular stimuli.Mnk1 (MAPK signal-integrating kinase 1/MAPK-int...

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Selective inhibition of MEK1/2 reveals a differential requirement for ERK1/2 signalling in the regulation of HIF-1 in response to hypoxia and IGF-1

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