Takeshi Ueda

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 eIF4Es basal, constitutive phosphorylation. Lipopolysaccharide (LPS)- or insulin-induced 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.

Eukaryotic Translation Initiation Factor 4F Architectural Alterations Accompany Translation Initiation Factor Redistribution in Poxvirus-Infected Cells

ABSTRACT Despite their self-sufficient ability to generate capped mRNAs from cytosolic DNA genomes, poxviruses must commandeer the critical eukaryotic translation initiation factor 4F (eIF4F) to recruit ribosomes. While eIF4F integrates signals to control translation, precisely how poxviruses manipulate the multisubunit eIF4F, composed of the cap-binding eIF4E and the RNA helicase eIF4A assembled onto an eIF4G platform, remains obscure. Here, we establish that the poxvirus infection of normal, primary human cells destroys the translational repressor eIF4E binding protein (4E-BP) and promotes eIF4E assembly into an active eIF4F complex bound to the cellular polyadenylate-binding protein (PABP). Stimulation of the eIF4G-associated kinase Mnk1 promotes eIF4E phosphorylation and enhances viral replication and protein synthesis. Remarkably, these eIF4F architectural alterations are accompanied by the concentration of eIF4E and eIF4G within cytosolic viral replication compartments surrounded by PABP. This demonstrates that poxvirus infection redistributes, assembles, and modifies core and associated components of eIF4F and concentrates them within discrete subcellular compartments. Furthermore, it suggests that the subcellular distribution of eIF4F components may potentiate the complex assembly.

Regulation of Arsenic Trioxide-induced Cellular Responses by Mnk1 and Mnk2

Arsenic trioxide (As2O3) is a potent inducer of apoptosis of malignant cells in vitro and in vivo, but the precise mechanisms by which it mediates such effects are not well defined. We provide evidence that As2O3 induces phosphorylation/activation of the MAPK signal-integrating kinases (Mnks) 1 and 2 in leukemia cell lines. Such activation is defective in cells with targeted disruption of the p38α MAPK gene, indicating that it requires upstream engagement of the p38 MAPK pathway. Studies using Mnk1–/– or Mnk2–/–, or double Mnk1–/–Mnk2–/– knock-out cells, establish that activation of Mnk1 and Mnk2 by arsenic trioxide regulates downstream phosphorylation of the eukaryotic initiation factor 4E at Ser-209. Importantly, arsenic-induced apoptosis is enhanced in cells with targeted disruption of the Mnk1 and/or Mnk2 genes, suggesting that these kinases are activated in a negative-feedback regulatory manner, to control generation of arsenic trioxide responses. Consistent with this, pharmacological inhibition of Mnk activity enhances the suppressive effects of arsenic trioxide on primary leukemic progenitors from patients with acute leukemias. Taken together, these findings indicate an important role for Mnk kinases, acting as negative regulators for signals that control generation of arsenic trioxide-dependent apoptosis and antileukemic responses.

Loss of MNK function sensitizes fibroblasts to serum-withdrawal induced apoptosis

Map kinase‐interacting protein kinases 1 and 2 (MNK1, MNK2) function downstream of p38 and ERK MAP kinases, but there are large gaps in our knowledge of how MNKs are regulated and function. Mice deleted of both genes are apparently normal, suggesting that MNKs function in adaptive pathways during stress. Here, we show that mouse embryo fibroblasts (MEFs) obtained from mnk1 (–/–)/mnk2 (–/–) as well as mnk1 (–/–) and mnk2 (–/–) mice are sensitized to caspase‐3 activation upon withdrawal of serum in comparison to wild‐type cells. Caspase‐3 cleavage occurs with all cells in the panel, but most rapidly and robustly in cells derived from mice lacking both MNK genes. Treatment of wild‐type MEFs in the panel with a compound (CGP57380) that inhibits MNK1 and MNK2 sensitizes wild‐type cells for serum‐withdrawal induced apoptosis, suggesting that sensitization is due to loss of MNK function and not to a secondary event. Reintroduction of wild‐type MNK1 in the double knockout MEFs results in decreased sensitivity to serum withdrawal that is not observed for wild‐type MNK2, or the kinase dead variant. Our work identifies MNKs as kinases involved in anti‐apoptotic signaling in response to serum withdrawal.

Critical role of the p400/mDomino chromatin‐remodeling ATPase in embryonic hematopoiesis

The SWI2/SNF2 family ATPase, p400/mDomino, is a core subunit of a large chromatin‐remodeling complex, and is currently suggested to play a unique function in histone variant exchange, a process by which chromatin structure is altered. Here, we investigated the role of p400/mDomino in mammalian development by generating mutant mice with a targeted deletion of the N‐terminal domain of p400/mDomino (referred to as mDomΔN/ΔN). The mDomΔN/ΔN mice died on embryonic day 11.5 (E11.5), and displayed an anemic appearance and slight deformity of the neural tube. DNA microarray and quantitative RT‐PCR analyses revealed that all of the embryonic globin genes and a globin chaperone gene were poorly expressed in the mDomΔN/ΔN embryo and yolk sac on E8.5, indicating that primitive erythropoiesis was impaired. A hematopoietic colony assay indicated that the hematopoietic activity of the yolk sac was significantly blocked in the mutant mice. We also found that the expression of a limited set of Hox genes, including Hoxa7, Hoxa9 and Hoxb9, was drastically enhanced in the mDomΔN/ΔN yolk sacs. These results suggest that p400/mDomino plays a critical role in embryonic hematopoiesis by regulating the expression of developmentally essential genes such as those in the Hox gene cluster.

Stenosis of pulmonary veins: Report of a patient corrected surgically.

Abstract A 15-year-old boy is reported who underwent surgical correction for stenosis of the right upper and left pulmonary veins at their junction with the left atrium and associated atrial septal defect, ventricular septal defect, and severe pulmonary hypertension. The literature dealing with stenosis of the pulmonary veins is reviewed and the etiology of the lesion and feasibility of preoperative diagnosis are discussed.

A SWI2/SNF2‐type ATPase/helicase protein, mDomino, interacts with myeloid zinc finger protein 2A (MZF‐2A) to regulate its transcriptional activity

Background: The myeloid zinc finger protein 2A (MZF‐2A) is a Krüppel‐type C2H2 zinc finger transcription factor expressed in myeloid cells and involved in the growth, differentiation and tumorigenesis of myeloid progenitors. Previously we identified a 180 amino acid domain in MZF‐2A which is responsible for the transcriptional activation of MZF‐2A. To understand the mechanism of the MZF‐2A‐dependent transcriptional activation, we screened for molecules that interact with the transactivation domain (TAD) of MZF‐2A.