Mitogen-activated protein kinases interacting kinases are autoinhibited by a reprogrammed activation segment

Structural insights into trypanosomatid Mnk kinase orthologues (kMnks) suggest altered mechanism in the kinase domain

Mitogen-activated protein kinase (MAPK) interacting protein kinases (Mnk1 and Mnk2) mediated phosphorylation of the eukaryotic initiation factor eIF4E is an important translation initiation control, in Mnk-mediated oncogenic activity and other disease conditions. Thus, Mnk kinases are an important target for therapy. Trypanosomatids are a class of kinetoplastids, some of which are protozoan parasites and cause diseases in humans. While protein translation initiation is well understood in eukaryotes and prokaryotes, there is a lack of sufficient structural information of this process in trypanosomatids. Here, we report that trypanosomatids have one orthologue of Mnk kinase with low overall sequence homology but high homology in the kinase domain and an additional C-terminal domain containing putative calmodulin binding site(s). We show that while many of the domains and motifs are conserved, homology modeling/structure prediction, docking analysis and molecular dynamics simulation studies suggest that trypanosomatid kMnk kinases, kinase domains are present in DFG-in conformation as opposed to the auto-inhibited DFD-out conformation of un-phosphorylated human Mnk1. Furthermore, we observed that several regulatory features are different in trypanosomatid kMnk kinases. Our study indicates that mechanism and regulation in the kinase domain of trypanosomatid kMnks are likely to be altered, and that they can be important drug targets.

Discovery of D25, a Potent and Selective MNK Inhibitor for Sepsis-Associated Acute Spleen Injury

Mitogen-activated protein kinase-interacting protein kinases (MNKs) and phosphorylate eukaryotic initiation factor 4E (p-eIF4E) play a critical role in regulating mRNA translation and protein synthesis associated with the development of cancer, metabolism, and inflammation. This study undertakes the modification of a 4-(3-(piperidin-4-yl)-1H-pyrazol-5-yl)pyridine structure, leading to the discovery of 4-(3-(piperidin-4-yl)-1H-pyrazol-5-yl)-1H-pyrrolo[2,3-b]pyridine (D25) as a potent and selective MNK inhibitor. D25 demonstrated inhibitory activity, with IC50 values of 120.6 nM for MNK1 and 134.7 nM for MNK2, showing exceptional selectivity. D25 inhibited the expression of pro-inflammation cytokines in RAW264.7 cells, such as inducible NO synthase, cyclooxygenase-2, and interleukin-6 (IL-6). In the lipopolysaccharide-induced sepsis mouse model, D25 significantly reduced p-eIF4E in spleen tissue and decreased the expression of tumor necrosis factor α, interleukin-1β, and IL-6, and it also reduced the production of reactive oxygen species, resulting in improved organ injury caused by inflammation. This suggests that D25 may provide a potential treatment for sepsis and sepsis-associated acute spleen injury.

Medicinal chemistry approaches to target the MNK-eIF4E axis in cancer

Aberrant translation of proteins that promote cell proliferation is an essential factor that defines oncogenic processes and cancer. The process for ribosomal translation of proteins from mRNA requires an essential initiation step which is controlled by the protein eIF4E, which binds the RNA 5'-cap and forms the eIF4F complex that subsequently translates protein. Typically, eIF4E is activated by phosphorylation on Ser209 by MNK1 and MNK2 kinases. Substantial work has shown that eIF4E and MNK1/2 are dysregulated in many cancers and this axis has therefore become an active area of interest for developing new cancer therapeutics. This review summarizes and discusses recent work to develop small molecules that target different steps in the MNK-eIF4E axis as potential cancer therapeutics. The aim of this review is to cover the breadth of different molecular approaches being taken and the medicinal chemistry basis for their optimization and testing as new cancer therapeutics.

Discovery of novel natural products as dual MNK/PIM inhibitors for acute myeloid leukemia treatment: Pharmacophore modeling, molecular docking, and molecular dynamics studies

MNK-2 and PIM-2 kinases play an indispensable role in cell proliferation signaling pathways linked to tyrosine kinase inhibitors resistance. In this study, pharmacophore modeling studies have been conducted on the co-crystalized ligands of MNK-2 and PIM-2 enzyme crystal structures to determine the essential features required for the identification of potential dual inhibitors. The obtained pharmacophore features were then screened against a library of 270,540 natural products from the ZINC database. The matched natural molecules were docked into the binding sites of MNK-2 and PIM-2 enzymes. The compounds with high docking scores with the two enzymes were further subjected to MM-GBSA calculations and ADME prediction. This led to the identification of compound 1 (ZINC000085569211), compound 2 (ZINC000085569178), and compound 3 (ZINC000085569190), with better docking scores compared to the reference co-crystallized ligands of MNK-2 and PIM-2. Moreover, compounds 1‒3 displayed better MM-GBSA binding free energies compared to the reference ligands. Finally, molecular dynamics (MD) study was used to assess the interaction stability of the compounds with MNK-2. To this end, compounds 1 and 3 bound strongly to the target during the whole period of MD simulation. The findings of the current study may further help the researchers in the discovery of novel molecules against MNK-2 and PIM-2.

Design, Synthesis and Evaluation of Novel Phorbazole C Derivatives as MNK Inhibitors through Virtual High-Throughput Screening

MNKs (mitogen-activated protein kinase-interacting protein kinases) phosphorylate eIF4E at Ser209 to control the translation of certain mRNAs and regulate the process of cell proliferation, cell migration and invasion, etc. Development of MNK inhibitors would be an effective treatment for related diseases. We used the MarineChem3D database to identify hit compounds targeting the protein MNK1 and MNK2 through high-throughput screening. Compounds from the phorbazole family showed good interactions with MNK1, and phorbazole C was selected as our hit compound. By analyzing the binding mode, we designed and synthesized 29 derivatives and evaluated their activity against MNKs, of which, six compounds showed good inhibition to MNKs. We also confirmed three interactions between this kind of compound and MNK1, which are vital for the activity. In conclusion, we report series of novel MNK inhibitors inspired from marine natural products and their relative structure–activity relationship. This will provide important information for further developing MNK inhibitors based on this kind of structure.

Synthesis, structural characterization, DFT calculations, molecular docking, and molecular dynamics simulations of a novel ferrocene derivative to unravel its potential antitumor activity

In this article, we describe a set of subsequent five-steps chemical reactions to synthesize a ferrocene derivative named 1-(5-(diphenylphosphaneyl)cyclopenta-1,3-dien-1-yl)ethyl)imino)-1,3-dihydroisobenzofuran-5-yl)methanol (compound 10). Structural characterization of 10 and its intermediate products was also performed and reported to attest to their formation. A molecular docking study was performed to propose the novel synthesized ferrocene derivative (10) as a potential antitumor candidate targeting the mitogen-activated protein (MAP) kinases interacting kinase (Mnk) 1. The computed docking score of (10) at -9.50 kcal/mol compared to the native anticancer staurosporine at -8.72 kcal/mol postulated a promising anticancer activity. Also, molecular dynamics (MD) simulations were carried out for 500 ns followed by MM-GBSA-binding free energy calculations for both the docked complexes of ferrocene and staurosporine to give more deep insights into their dynamic behavior in physiological conditions. Furthermore, DFT calculations were performed to unravel some of the physiochemical characteristics of the ferrocene derivative (10). The quantum mechanics calculations shed the light on some of the structural and electrochemical configurations of (10) which would open the horizon for further investigation. HighlightsThe synthesis of a ferrocene derivative named 1-(5-(diphenylphosphaneyl)cyclopenta-1,3-dien-1-yl)ethyl)imino)-1,3-dihydroisobenzofuran-5-yl)methanol (compound 10) was described.Structural characterizations of ferrocene derivative (10) and its intermediate products were also performed.DFT calculations, molecular docking, molecular dynamics, and MM-GBSA calculations were carried out.Computational studies revealed the antitumor potential of ferrocene derivative (10) through targeting and inhibiting mitogen-activated protein (MAP) kinases interacting kinase (Mnk) 1.Communicated by Ramaswamy H. Sarma.

Overcoming Paradoxical Kinase Priming by a Novel MNK1 Inhibitor

Targeting the kinases MNK1 and MNK2 has emerged as a valuable strategy in oncology. However, most of the advanced inhibitors are acting in an adenosine triphosphate (ATP)-competitive mode, precluding the evaluation of different binding modes in preclinical settings. Using rational design, we identified and validated the 4,6-diaryl-pyrazolo[3,4-b]pyridin-3-amine scaffold as the core for MNK inhibitors. Signaling pathway analysis confirmed a direct effect of the hit compound EB1 on MNKs, and in line with the reported function of these kinases, EB1 only affects the growth of tumor but not normal cells. Molecular modeling revealed the binding of EB1 to the inactive conformation of MNK1 and the interaction with the specific DFD motif. This novel mode of action appears to be superior to the ATP-competitive inhibitors, which render the protein in a pseudo-active state. Overcoming this paradoxical activation of MNKs by EB1 represents therefore a promising starting point for the development of a novel generation of MNK inhibitors.

Optimization of 4,6-Disubstituted Pyrido[3,2-d]pyrimidines as Dual MNK/PIM Inhibitors to Inhibit Leukemia Cell Growth

Mitogen-activated protein kinase-interacting kinases (MNKs) and provirus integration in maloney murine leukemia virus kinases (PIMs) are downstream enzymes of cell proliferation signaling pathways associated with the resistance of tyrosine kinase inhibitors. MNKs and PIMs have complementary effects to regulate cap-dependent translation of oncoproteins. Dual inhibitors of MNKs and PIMs have not been developed. We developed a novel 4,6-disubstituted pyrido[3,2-d]pyrimidine compound 21o with selective inhibition of MNKs and PIMs. The IC50's of 21o to inhibit MNK1 and MNK2 are 1 and 7 nM and those to inhibit PIM1, PIM2, and PIM3 are 43, 232, and 774 nM, respectively. 21o inhibits the growth of myeloid leukemia K562 and MOLM-13 cells with GI50's of 2.1 and 1.2 μM, respectively. 21o decreases the levels of p-eIF4E and p-4EBP1, the downstream products of MNKs and PIMs, as well as cap-dependent proteins c-myc, cyclin D1, and Mcl-1. 21o inhibits the growth of MOLM-13 cell xenografts without causing evident toxicity. 21o represents an innovative dual MNK/PIM inhibitor with a good pharmacokinetic profile.

Update on the Development of MNK Inhibitors as Therapeutic Agents

Mitogen-activated protein kinase-interacting kinases 1 and 2 (MNK1/2) represent a central class of enzymes that are activated by extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases. MNK1 and MNK2 coordinate cellular signaling, control production of inflammatory chemokines, and regulate cell proliferation and survival. MNK1/2 are referred to as serine/threonine kinases as they phosphorylate serine or threonine residues on their substrates. Upon activation, MNK1/2 phosphorylate eukaryotic translation initiation factor 4E (eIF4E) at Ser209, which in turn initiates ribosome assembly and protein translation. Deleterious overexpression of MNK1/2 and/or eIF4E have been reported in several diseases including cancers, neurological disorders, autism, and inflammation. Recently, there have been intense efforts toward the development of potent and selective inhibitors of MNK1/2 in both academia and industry. Herein, we review the current understanding of the structural and biological aspects of MNK1/2 and provide an update of pharmacological inhibitors of MNK1/2 including candidates in clinical trials.

Deep computational analysis details dysregulation of eukaryotic translation initiation complex eIF4F in human cancers

eIF4F plays diverse roles in human cancers, which complicate the development of an overarching understanding of its functional and regulatory impacts across tumor types. Typically, eIF4F drives initiation from the mRNA 5' end (cap) and is composed of eIF4G1, eIF4A1, and cap-binding eIF4E. Cap-independent initiation is possible without eIF4E, from internal ribosomal entry sites (IRESs). By analyzing large public datasets, we found that cancers selectively overexpress EIF4G1 more than EIF4E. That expression imbalance supports EIF4G1 as a prognostic indicator in patients with cancer. It also attenuates "housekeeping" pathways that are usually regulated in a tissue-specific manner via cap-dependent initiation in healthy tissues and reinforce regulation of cancer-preferred pathways in cap-independent contexts. Cap-independent initiation is mechanistically attributable to eIF4G1 hyperphosphorylation that promotes binding to eIF4A1 and reduced eIF4E availability. Collectively, these findings reveal a novel model of dysregulated eIF4F function and highlight the clinical relevance of cap-(in)dependent initiation in cancer.

Effect of Neuroprotective Magnesium Sulfate Treatment on Brain Transcription Response to Hypoxia Ischemia in Neonate Mice

MgSO₄ is widely used in the prevention of preterm neurological disabilities but its modes of action remain poorly established. We used a co-hybridization approach using the transcriptome in 5-day old mice treated with a single dose of MgSO₄ (600 mg/kg), and/or exposed to hypoxia-ischemia (HI). The transcription of hundreds of genes was altered in all the groups. MgSO₄ mainly produced repressions culminating 6 h after injection. Bio-statistical analysis revealed the repression of synaptogenesis and axonal development. The putative targets of MgSO₄ were Mnk1 and Frm1. A behavioral study of adults did not detect lasting effects of neonatal MgSO₄ and precluded NMDA-receptor-mediated side effects. The effects of MgSO₄ plus HI exceeded the sum of the effects of separate treatments. MgSO₄ prior to HI reduced inflammation and the innate immune response probably as a result of cytokine inhibition (Ccl2, Ifng, interleukins). Conversely, MgSO₄ had little effect on HI-induced transcription by RNA-polymerase II. De novo MgSO₄-HI affected mitochondrial function through the repression of genes of oxidative phosphorylation and many NAD-dehydrogenases. It also likely reduced protein translation by the repression of many ribosomal proteins, essentially located in synapses. All these effects appeared under the putative regulatory MgSO₄ induction of the mTORC2 Rictor coding gene. Lasting effects through Sirt1 and Frm1 could account for this epigenetic footprint.

Progress in developing MNK inhibitors

The MNKs (mitogen-activated protein kinase-interacting protein kinases) phosphorylate eIF4E (eukaryotic initiation factor 4 E) at serine 209; eIF4E plays an important role in the translation of cytoplasmic mRNAs, all of which possess a 5' 'cap' structure to which eIF4E binds. Elevated levels of eIF4E, p-eIF4E and/or the MNK protein kinases have been found in many types of cancer, including solid tumors and leukemia. MNKs also play a role in metabolic disease. Regulation of the activities of MNKs (MNK1 and MNK2), control the phosphorylation of eIF4E, which in turn has a close relationship with the processes of tumor development, cell migration and invasion, and energy metabolism. MNK knock-out mice display no adverse effects on normal cells or phenotypes suggesting that MNK may be a potentially safe targets for the treatment of various cancers. Several MNK inhibitors or 'degraders' have been identified. Initially, some of the inhibitors were developed from natural products or based on other protein kinase inhibitors which inhibit multiple kinases. Subsequently, more potent and selective inhibitors for MNK1/2 have been designed and synthesized. Currently, three inhibitors (BAY1143269, eFT508 and ETC-206) are in various stages of clinical trials for the treatment of solid cancers or leukemia, either alone or combined with inhibitors of other protein kinase. In this review, we summarize the diverse MNK inhibitors that have been reported in patents and other literature, including those with activities in vitro and/or in vivo.

Mnk inhibitors: a patent review

The alteration of mRNA translation has a crucial role in defining the changes in cellular proteome. The phosphorylation of eukaryotic initiation factor 4E by mitogen-activated protein kinase-interacting kinases (Mnks) leads to the release and translation of mRNAs of specific oncogenic proteins. In recent years, the efforts made by the pharmaceutical industry to develop novel chemical skeletons to create potent and selective Mnk inhibitors have been fruitful. The pyridone-aminal scaffold has been utilized to generate several series of Mnk inhibitors presented in multiple patent applications and research articles. Tomivosertib (eFT508) is one of the molecules with such scaffold. It is one of the first two Mnk inhibitors that entered clinical trials, and has displayed momentous activity against several solid and hematological cancers. The present compilation provides a succinct review of the current state of development of pyridone-aminal-derived Mnk inhibitors through the analysis of relevant patent applications filed in the last 5 years.

Discovery of indazole-pyridinone derivatives as a novel class of potent and selective MNK1/2 kinase inhibitors that protecting against endotoxin-induced septic shock

The mitogen-activated protein kinase (MAPK)-interacting kinases 1 and 2 (MNKs 1/2) and their downstream target eIF4E, play a role in oncogenic transformation, progression and metastasis. These results provided rationale for development of first MNKs inhibitors, currently in clinical trials for cancer treatment. Inhibitors of the MNKs/eIF4E pathway are also proposed as treatment strategy for inflammatory conditions. Here we present results of optimization of indazole-pyridinone derived MNK1/2 inhibitors among which compounds 24 and 26, selective and metabolically stable derivatives. Both compounds decreased levels of eIF4E Ser206 phosphorylation (pSer209-eIF4E) in MOLM16 cell line. When administered in mice compounds 24 and 26 significantly improved survival rates of animals in the endotoxin lethal dose challenge model, with concomitant reduction of proinflammatory cytokine levels - TNFα and IL-6 in serum. Identified MNK1/2 inhibitors represent a novel class of immunomodulatory compounds with a potential for the treatment of inflammatory diseases including sepsis.

Synthesis of Novel Biocompatible Thienopyrimidine Chromophores with Aggregation-Induced Emission Sensitive to Molecular Aggregation

Biocompatible luminogens with aggregation-induced emission (AIE) have several applications in the biology field, such as in detecting biomacromolecules bioprobes and in bio-imaging. Due to their bioactivities and light-emitting properties, many heterocyclic compounds are good candidates for such applications. However, heterocyclic π-conjugated systems with AIE behavior remain rare as strong intermolecular π-π interactions usually quench their emission. In this work, new thienopyrimidine heterocyclic compounds were synthesized and their structures were verified by elemental analysis and Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and 13C NMR spectra. The photophysical properties of some compounds were investigated in the solution and solid states. Density functional theory calculations were also performed to confirm the observed photophysical properties of the compounds. The studied dyes displayed AIE properties with spectral shapes related to the aggregate structure and a quantum yield up to 10.8%. The emission efficiency of the powder is attributed to the incorporation of multiply rotatable and twisted aryl groups to the fused heterocyclic moieties. The dyes also showed high thermal stability and potent antimicrobial activities against numerous bacterial and fungal strains. Additionally, the cytotoxicity of the new compounds was evaluated against the Caco-2 cell line, and molecular docking was used to investigate the binding conformation of the most effective compound with the MNK2 enzyme. Therefore, the presented structures may potentially be used for bioapplications.

Mechanism of auto-inhibition and activation of Mec1ATR checkpoint kinase

In response to DNA damage or replication fork stalling, the basal activity of Mec1^ATR is stimulated in a cell-cycle-dependent manner, leading to cell-cycle arrest and the promotion of DNA repair. Mec1^ATR dysfunction leads to cell death in yeast and causes chromosome instability and embryonic lethality in mammals. Thus, ATR is a major target for cancer therapies in homologous recombination-deficient cancers. Here we identify a single mutation in Mec1, conserved in ATR, that results in constitutive activity. Using cryo-electron microscopy, we determine the structures of this constitutively active form (Mec1(F2244L)-Ddc2) at 2.8 Å and the wild type at 3.8 Å, both in complex with Mg²⁺-AMP-PNP. These structures yield a near-complete atomic model for Mec1-Ddc2 and uncover the molecular basis for low basal activity and the conformational changes required for activation. Combined with biochemical and genetic data, we discover key regulatory regions and propose a Mec1 activation mechanism.

The MNK1/2-eIF4E Axis as a Potential Therapeutic Target in Melanoma

: Melanoma is a type of skin cancer that originates in the pigment-producing cells of the body known as melanocytes. Most genetic aberrations in melanoma result in hyperactivation of the mitogen activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways. We and others have shown that a specific protein synthesis pathway known as the MNK1/2-eIF4E axis is often dysregulated in cancer. The MNK1/2-eIF4E axis is a point of convergence for these signaling pathways that are commonly constitutively activated in melanoma. In this review we consider the functional implications of aberrant mRNA translation in melanoma and other malignancies. Moreover, we discuss the consequences of inhibiting the MNK1/2-eIF4E axis on the tumor and tumor-associated cells, and we provide important avenues for the utilization of this treatment modality in combination with other targeted and immune-based therapies. The past decade has seen the increased development of selective inhibitors to block the action of the MNK1/2-eIF4E pathway, which are predicted to be an effective therapy regardless of the melanoma subtype (e.g., cutaneous, acral, and mucosal).

Deeping in the Role of the MAP-Kinases Interacting Kinases (MNKs) in Cancer

The mitogen-activated protein kinase (MAPK)-interacting kinases (MNKs) are involved in oncogenic transformation and can promote metastasis and tumor progression. In human cells, there are four MNKs isoforms (MNK1a/b and MNK2a/b), derived from two genes by alternative splicing. These kinases play an important role controlling the expression of specific proteins involved in cell cycle, cell survival and cell motility via eukaryotic initiation factor 4E (eIF4E) regulation, but also through other substrates such as heterogeneous nuclear ribonucleoprotein A1, polypyrimidine tract-binding protein-associated splicing factor and Sprouty 2. In this review, we provide an overview of the role of MNK in human cancers, describing the studies conducted to date to elucidate the mechanism involved in the action of MNKs, as well as the development of MNK inhibitors in different hematological cancers and solid tumors.

FLI1 promotes protein translation via the transcriptional regulation of MKNK1 expression

The disruption of protein translation machinery is a common feature of cancer initiation and progression, and drugs that target protein translation offer new avenues for therapy. The translation initiation factor, eukaryotic initiation factor 4E (eIF4E), is induced in a number of cancer cell lines and is one such candidate for therapeutic intervention. Friend leukemia integration 1 (FLI1) is a potent oncogenic transcription factor that promotes various types of cancer by promoting several hallmarks of cancer progression. FLI1 has recently been implicated in protein translation through yet unknown mechanisms. This study identified a positive association between FLI1 expression and mitogen-activated protein kinase (MAPK)-interacting serine/threonine kinase1 (MKNK1), the immediate upstream regulator of the eIF4E initiation factor. The short hairpin RNA (shRNA)-mediated silencing or overexpression of FLI1 in leukemic cell lines downregulated or upregulated MKNK1 expression, respectively. Promoter analysis identified a potent FLI1 binding site in the regulatory region of the MKNK1 promoter. In transient transfection experiments, FLI1 increased MKNK1 promoter activity, which was blocked by mutating the FLI1 binding site. FLI1 specifically affected the expression of MKNK1, but not that of MKNK2. The siRNA-mediated downregulation of MKNK1 downregulated the expression of survivin (BIRC5) and significantly suppressed cell proliferation in culture. FLI1 inhibitory compounds were shown to downregulate this oncogene through the suppression of MAPK/extracellular-regulated kinase (ERK) signaling and the subsequent activation of miR-145, leading to a lower MKNK1 expression and the suppression of leukemic growth. These results uncover a critical role for FLI1 in the control of protein translation and the importance of targeting its function and downstream mediators, such as MKNK1, for cancer therapy.

Discovery of novel Mnk inhibitors using mutation-based induced-fit virtual high-throughput screening

Mnk kinases (Mnk1 and 2) are downstream effectors of Map kinase pathways and regulate phosphorylation of eukaryotic initiation factor 4E. Engagement of the Mnk pathway is critical in acute myeloid leukemia (AML) leukemogenesis and Mnk inhibitors have potent antileukemic properties in vitro and in vivo, suggesting that targeting Mnk kinases may provide a novel approach for treating AML. Here, we report the development and application of a mutation-based induced-fit in silico screen to identify novel Mnk inhibitors. The Mnk1 structure was modeled by temporarily mutating an amino acid that obstructs the ATP-binding site in the Mnk1 crystal structure while carrying out docking simulations of known inhibitors. The hit compounds display activity in Mnk biochemical and cellular assays, including acute myeloid leukemia progenitors. This approach will enable further rational structure-based drug design of new Mnk inhibitors and potentially novel ways of therapeutically targeting this kinase.

Disordered Protein Kinase Regions in Regulation of Kinase Domain Cores

Since publication of the crystal structure of protein kinase (PK)A three decades ago, a structural portrait of the conserved kinase core has been drawn. The next challenge is to elucidate structures of full-length kinases and to address the intrinsically disordered regions (IDRs) that typically flank the core as well as the small linear motifs (SLiMs) that are embedded within the IDRs. It is increasingly apparent that unstructured regions integrate the kinase catalytic chassis into multienzyme-based regulatory networks. The extracellular signal-regulated kinase-ribosomal S6 PK-phosphoinositide-dependent kinase (ERK-RSK-PDK) complex is an excellent example to demonstrate how IDRs and SLiMs govern communication between four different kinase catalytic cores to mediate activation and how in molecular terms these promote the formation of kinase heterodimers in a context dependent fashion.

Influence of pharmacological and epigenetic factors to suppress neurotrophic factors and enhance neural plasticity in stress and mood disorders

Stress-induced major depression and mood disorders are characterized by behavioural abnormalities and psychiatric illness, leading to disability and immature mortality worldwide. Neurobiological mechanisms of stress and mood disorders are discussed considering recent findings, and challenges to enhance pharmacological effects of antidepressant, and mood stabilizers. Pharmacological enhancement of ketamine and scopolamine regulates depression at the molecular level, increasing synaptic plasticity in prefrontal regions. Blood-derived neurotrophic factors facilitate mood-deficit symptoms. Epigenetic factors maintain stress-resilience in hippocampal region. Regulation of neurotrophic factors blockades stress, and enhances neuronal survival though it paralyzes limbic regions. Molecular agents and neurotrophic factors also control behavioral and synaptic plasticity in addiction and stress disorders. Future research on neuronal dynamics and cellular actions can be directed to obtain the etiology of synaptic dysregulation in mood disorder and stress. For the first time, the current review contributes to the literature of synaptic plasticity representing the role of epigenetic mechanisms and glucocorticoid receptors to predict depression and anxiety in clinical conditions.

Optimization of Selective Mitogen-Activated Protein Kinase Interacting Kinases 1 and 2 Inhibitors for the Treatment of Blast Crisis Leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disease caused by bcr-abl1, a constitutively active tyrosine kinase fusion gene responsible for an abnormal proliferation of leukemic stem cells (LSCs). Inhibition of BCR-ABL1 kinase activity offers long-term relief to CML patients. However, for a proportion of them, BCR-ABL1 inhibition will become ineffective at treating the disease, and CML will progress to blast crisis (BC) CML with poor prognosis. BC-CML is often associated with excessive phosphorylated eukaryotic translation initiation factor 4E (eIF4E), which renders LSCs capable of proliferating via self-renewal, oblivious to BCR-ABL1 inhibition. In vivo, eIF4E is exclusively phosphorylated on Ser209 by MNK1/2. Consequently, a selective inhibitor of MNK1/2 should reduce the level of phosphorylated eIF4E and re-sensitize LSCs to BCR-ABL1 inhibition, thus hindering the proliferation of BC LSCs. We report herein the structure-activity relationships and pharmacokinetic properties of a selective MNK1/2 inhibitor clinical candidate, ETC-206, which in combination with dasatinib prevents BC-CML LSC self-renewal in vitro and enhances dasatinib antitumor activity in vivo.

A novel inhibitor stabilizes the inactive conformation of MAPK-interacting kinase 1

Mitogen-activated protein kinase (MAPK)-interacting kinases 1 (Mnk1) and 2 (Mnk2) modulate translation initiation through the phosphorylation of eukaryotic translation initiation factor 4E, which promotes tumorigenesis. However, Mnk1 and Mnk2 are dispensable in normal cells, suggesting that the inhibition of Mnk1 and Mnk2 could be effective in cancer therapy. To provide a structural basis for Mnk1 inhibition, a novel Mnk1 inhibitor was discovered and the crystal structure of Mnk1 in complex with this inhibitor was determined. The crystal structure revealed that the inhibitor binds to the autoinhibited state of Mnk1, stabilizing the Mnk-specific DFD motif in the DFD-out conformation, thus preventing Mnk1 from switching to the active conformation and thereby inhibiting the kinase activity. These results provide a valuable platform for the structure-guided design of Mnk1 inhibitors.

Small Molecules Targeting the Inactive Form of the Mnk1/2 Kinases

Overexpression of the eukaryotic initiation factor 4E (eIF4E) is linked to a variety of cancers. Both mitogen-activated protein kinases-interacting kinases 1 and 2 (Mnk1/2) activate the oncogene eIF4E through posttranslational modification (phosphorylating it at the conserved Ser209). Inhibition of Mnk prevents eIF4E phosphorylation, making the Mnk-eIF4E axis a potential therapeutic target for oncology. Recently, the design and synthesis of a series of novel potent compounds inhibiting the Mnk1/2 kinases were carried out in-house. Here, we describe computational models of the interactions between Mnk1/2 kinases and these inhibitors. Molecular modeling combined with free energy calculations show that these compounds bind to the inactive forms of the kinases. All compounds adopt similar conformations in the catalytic sites of both kinases, stabilized by hydrogen bonds with the hinge regions and with the catalytic Lys78 (Mnk1) and Lys113 (Mnk2). These hydrogen bond interactions clearly play a critical role in determining the conformational stability and potency of the compounds. We also find that van der Waals interactions with an allosteric pocket are key to their binding and potency. Two distinct hydration sites that appear to further stabilize the ligand binding/interactions were observed. Critically, the inclusion of explicit water molecules in the calculations results in improving the agreement between calculated and experimental binding free energies.

MNK Controls mTORC1:Substrate Association through Regulation of TELO2 Binding with mTORC1

The mechanistic target of rapamycin (mTOR) integrates numerous stimuli and coordinates the adaptive response of many cellular processes. To accomplish this, mTOR associates with distinct co-factors that determine its signaling output. While many of these co-factors are known, in many cases their function and regulation remain opaque. The MAPK-interacting kinase (MNK) contributes to rapamycin resistance in cancer cells. Here, we demonstrate that MNK sustains mTORC1 activity following rapamycin treatment and contributes to mTORC1 signaling following T cell activation and growth stimuli in cancer cells. We determine that MNK engages with mTORC1, promotes mTORC1 association with the phosphatidyl inositol 3' kinase-related kinase (PIKK) stabilizer, TELO2, and facilitates mTORC1:substrate binding. Moreover, our data suggest that DEPTOR, the endogenous inhibitor of mTOR, opposes mTORC1:substrate association by preventing TELO2:mTORC1 binding. Thus, MNK orchestrates counterbalancing forces that regulate mTORC1 enzymatic activity.

Unveiling new chemical scaffolds as Mnk inhibitors

The discovery of small molecules that selectively inhibit Mnks is considered of paramount importance towards deciphering the exact role of these proteins in carcinogenesis and to further validate them as anti-cancer drug targets. However, the dearth of structural information of Mnks is a major hurdle. This study unveils the 7H-pyrrolo[2,3-d]pyrimidine derivatives as potent inhibitors of Mnks. ATP and substrate competition assays showed that this scaffold interacts with the ATP binding site, but not with the substrate site. Screened against a panel of cancer cells, Mnk inhibitors were most potent against MV4-11 acute myeloid leukemia cells. The induction of apoptosis was shown to be mediated by downregulation of Mcl-1.

Identifying three-dimensional structures of autophosphorylation complexes in crystals of protein kinases

Protein kinase autophosphorylation is a common regulatory mechanism in cell signaling pathways. Crystal structures of several homomeric protein kinase complexes have a serine, threonine, or tyrosine autophosphorylation site of one kinase monomer located in the active site of another monomer, a structural complex that we call an "autophosphorylation complex." We developed and applied a structural bioinformatics method to identify all such autophosphorylation complexes in x-ray crystallographic structures in the Protein Data Bank (PDB). We identified 15 autophosphorylation complexes in the PDB, of which five complexes had not previously been described in the publications describing the crystal structures. These five complexes consist of tyrosine residues in the N-terminal juxtamembrane regions of colony-stimulating factor 1 receptor (CSF1R, Tyr(561)) and ephrin receptor A2 (EPHA2, Tyr(594)), tyrosine residues in the activation loops of the SRC kinase family member LCK (Tyr(394)) and insulin-like growth factor 1 receptor (IGF1R, Tyr(1166)), and a serine in a nuclear localization signal region of CDC-like kinase 2 (CLK2, Ser(142)). Mutations in the complex interface may alter autophosphorylation activity and contribute to disease; therefore, we mutated residues in the autophosphorylation complex interface of LCK and found that two mutations impaired autophosphorylation (T445V and N446A) and mutation of Pro(447) to Ala, Gly, or Leu increased autophosphorylation. The identified autophosphorylation sites are conserved in many kinases, suggesting that, by homology, these complexes may provide insight into autophosphorylation complex interfaces of kinases that are relevant drug targets.

Identification of a Highly Conserved Allosteric Binding Site on Mnk1 and Mnk2

Elevated levels of phosphorylated eukaryotic initiation factor 4E (eIF4E) have been implicated in many tumor types, and mitogen activated protein kinase-interacting kinases (Mnks) are the only known kinases that phosphorylate eIF4E at Ser209. The phosphorylation of eIF4E is essential for oncogenic transformation but is of no significance to normal growth and development. Pharmacological inhibition of Mnks therefore provides a nontoxic and effective strategy for cancer therapy. However, a lack of specific Mnk inhibitors has confounded pharmacological target validation and clinical development. Herein, we report the identification of a novel series of Mnk inhibitors and their binding modes. A systematic workflow has been established to distinguish between type III and type I/II inhibitors. A selection of 66 compounds was tested for Mnk1 and Mnk2 inhibition, and 9 out of 20 active compounds showed type III interaction with an allosteric site of the proteins. Most of the type III inhibitors exhibited dual Mnk1 and Mnk2 activities and demonstrated potent antiproliferative properties against the MV4-11 acute myeloid leukemia cell line. Interestingly, ATP-/substrate-competitive inhibitors were found to be highly selective for Mnk2, with little or no activity for Mnk1. Our study suggests that Mnk1 and Mnk2 share a common structure of the allosteric inhibitory binding site but possess different structural features of the ATP catalytic domain. The findings will assist in the future design and development of Mnk targeted anticancer therapeutics.

Discovery of 4-(dihydropyridinon-3-yl)amino-5-methylthieno[2,3-d]pyrimidine derivatives as potent Mnk inhibitors: synthesis, structure-activity relationship analysis and biological evaluation

Phosphorylation of the eukaryotic initiation factor 4E (eIF4E) by mitogen-activated protein kinase (MAPK)-interacting kinases (Mnks) is essential for oncogenesis but unnecessary for normal development. Thus, pharmacological inhibition of Mnks may offer an effective and non-toxic anti-cancer therapeutic strategy. Herein, we report the discovery of 4-(dihydropyridinon-3-yl)amino-5-methylthieno[2,3-d]pyrimidine derivatives as potent Mnk inhibitors. Docking study of 7a in Mnk2 suggests that the compound is stabilised in the ATP binding site through multiple hydrogen bonds and hydrophobic interaction. Cellular mechanistic studies on MV-4-11 cells with leads 7a, 8e and 8f reveal that they are able to down-regulate the phosphorylated eIF4E, Mcl-1 and cyclin D1, and induce apoptosis.

Dynamical insights of Mnk2 kinase activation by phosphorylation to facilitate inhibitor discovery

Aim: Mitogen-activated protein kinase-interacting kinases (Mnks) are emerging anticancer targets. Mnks feature unique structural features, enhancing their importance for selective inhibitor discovery. Nonetheless, the lack of structural details obstruct the development of selective Mnk inhibitors. Results: We disclose the first complete structure model of the activated state of Mnk2. Using all-atom accelerated molecular dynamics, we also demonstrate that its activation by phosphorylation grants access to distinct activation loop conformations, steering the inactive-to-active conformational transformation. Then we propose the binding mode of CGP57380 to active Mnk2, and evaluate key interactions that could be critical for future Mnk-targeted inhibitors. Conclusion: Critical insights of the Mnk2 activation process are gained, while providing a platform for designing Mnk-targeted anticancer agents.

Probing the binding mechanism of Mnk inhibitors by docking and molecular dynamics simulations

Mitogen-activated protein kinases-interacting kinase 1 and 2 (Mnk1/2) activate the oncogene eukaryotic initiation factor 4E (eIF4E) by phosphorylation. High level of phosphorylated eIF4E is associated with various types of cancers. Inhibition of Mnk prevents eIF4E phosphorylation, making them potential therapeutic targets for cancer. Recently, we have designed and synthesized a series of novel imidazopyridine and imidazopyrazine derivatives that inhibit Mnk1/2 kinases with a potency in the nanomolar to micromolar range. In the current work we model the inhibition of Mnk kinase activity by these inhibitors using various computational approaches. Combining homology modeling, docking, molecular dynamics simulations, and free energy calculations, we find that all compounds bind similarly to the active sites of both kinases with their imidazopyridine and imidazopyrazine cores anchored to the hinge regions of the kinases through hydrogen bonds. In addition, hydrogen bond interactions between the inhibitors and the catalytic Lys78 (Mnk1), Lys113 (Mnk2) and Ser131 (Mnk1), Ser166 (Mnk2) appear to be important for the potency and stability of the bound conformations of the inhibitors. The computed binding free energies (ΔGPred) of these inhibitors are in accord with experimental bioactivity data (pIC50) with correlation coefficients (r(2)) of 0.70 and 0.68 for Mnk1 and Mnk2 respectively. van der Waals energies and entropic effects appear to dominate the binding free energy (ΔGPred) for each Mnk-inhibitor complex studied. The models suggest that the activities of these small molecule inhibitors arise from interactions with multiple residues in the active sites, particularly with the hydrophobic residues.

Induction of viral, 7-methyl-guanosine cap-independent translation and oncolysis by mitogen-activated protein kinase-interacting kinase-mediated effects on the serine/arginine-rich protein kinase

Protein synthesis, the most energy-consuming process in cells, responds to changing physiologic priorities, e.g., upon mitogen- or stress-induced adaptations signaled through the mitogen-activated protein kinases (MAPKs). The prevailing status of protein synthesis machinery is a viral pathogenesis factor, particularly for plus-strand RNA viruses, where immediate translation of incoming viral RNAs shapes host-virus interactions. In this study, we unraveled signaling pathways centered on the ERK1/2 and p38α MAPK-interacting kinases MNK1/2 and their role in controlling 7-methyl-guanosine (m(7)G) "cap"-independent translation at enterovirus type 1 internal ribosomal entry sites (IRESs). Activation of Raf-MEK-ERK1/2 signals induced viral IRES-mediated translation in a manner dependent on MNK1/2. This effect was not due to MNK's known functions as eukaryotic initiation factor (eIF) 4G binding partner or eIF4E(S209) kinase. Rather, MNK catalytic activity enabled viral IRES-mediated translation/host cell cytotoxicity through negative regulation of the Ser/Arg (SR)-rich protein kinase (SRPK). Our investigations suggest that SRPK activity is a major determinant of type 1 IRES competency, host cell cytotoxicity, and viral proliferation in infected cells.

IMPORTANCE

We are targeting unfettered enterovirus IRES activity in cancer with PVSRIPO, the type 1 live-attenuated poliovirus (PV) (Sabin) vaccine containing a human rhinovirus type 2 (HRV2) IRES. A phase I clinical trial of PVSRIPO with intratumoral inoculation in patients with recurrent glioblastoma (GBM) is showing early promise. Viral translation proficiency in infected GBM cells is a core requirement for the antineoplastic efficacy of PVSRIPO. Therefore, it is critically important to understand the mechanisms controlling viral cap-independent translation in infected host cells.

Type III effector NleH2 from Escherichia coli O157:H7 str. Sakai features an atypical protein kinase domain

The crystal structure of a C-terminal domain of enterohemorrhagic Escherichia coli type III effector NleH2 has been determined to 2.6 Å resolution. The structure resembles those of protein kinases featuring the catalytic, activation, and glycine-rich loop motifs and ATP-binding site. The position of helix αC and the lack of a conserved arginine within an equivalent HRD motif suggested that the NleH2 kinase domain's active conformation might not require phosphorylation. The activation segment markedly contributed to the dimerization interface of NleH2, which can also accommodate the NleH1-NleH2 heterodimer. The C-terminal PDZ-binding motif of NleH2 provided bases for interaction with host proteins.

MAP kinase-interacting kinases--emerging targets against cancer

Mitogen-activated protein kinase (MAPK)-interacting kinases (Mnks) regulate the initiation of translation through phosphorylation of eukaryotic initiation factor 4E (eIF4E). Mnk-mediated eIF4E activation promotes cancer development and progression. While the phosphorylation of eIF4E is necessary for oncogenic transformation, the kinase activity of Mnks seems dispensable for normal development. For this reason, pharmacological inhibition of Mnks could represent an ideal mechanism-based and nontoxic therapeutic strategy for cancer treatment. In this review, we discuss the current understanding of Mnk biological roles, structures, and functions, as well as clinical implications. Importantly, we propose different strategies for identification of highly selective small molecule inhibitors of Mnks, including exploring a structural feature of their kinase domain, DFD motif, which is unique within the human kinome. We also argue that a combined targeting of Mnks and other pathways should be considered given the complexity of cancer.

Insights into the Importance of DFD-Motif and Insertion I1 in Stabilizing the DFD-Out Conformation of Mnk2 Kinase

Human mitogen-activated protein kinases (MAPK)-interacting kinases 1 and 2 (Mnk1/2) are promising anticancer targets. Mnks possess special insertions and a DFD-motif that are distinct from other kinases. Crystallographic studies of Mnk1/2 have revealed that the DFD-motif adopts the DFG/D-out conformation in which residue F227 flips into the ATP binding pocket. This is rarely observed in other kinases. Although the DFG-out conformation has attracted great interest for designing selective inhibitors, structural requirements for binding and the mechanism governing the DFG-out conformation remain unclear. This work presents for the first time the applicability of 3D models of Mnk2 protein in studying conformational changes by utilizing homology modeling and molecular dynamics simulations. The study reveals that the interactions between residue K234 of insertion I1 and D226 of the DFD motif play a key role in inducing and stabilizing the DFD-out conformation. The structural features will aid in the rational design of Mnk2 inhibitors.

Targeting the unactivated conformations of protein kinases for small molecule drug discovery

BACKGROUND

The number of drugs in active clinical development or on the market that target the unactivated conformational states of protein kinases is growing and represents a significant portion of kinase research at biopharmaceutical companies. These non-classical kinase inhibitors have a mode of action which may overcome some of the liabilities of classical ATP-site inhibitors that substantially overlap the space that ATP occupies in the activated kinase.

OBJECTIVE

This review will discuss state-of-the-art methods of inhibiting protein kinases by targeting the unactivated conformations of the enzyme with small molecules directed to the ATP binding region.

METHODS

Biochemical and structural biology publications and public domain crystal structures were evaluated to identify key concepts in drug discovery for unactivated protein kinase inhibitors that target the ATP binding region.

CONCLUSION

The potential for enhanced selectivity, potency and duration of pharmacological action may allow non-classical kinase therapeutics to be used for chronic dosing in non-life-threatening indications. Moreover, by targeting additional conformational space on the kinase protein it is possible that new chemical matter will be discovered such that current intellectual property limitations on traditional ATP-site chemical scaffolds may be circumvented.

LY2801653 is an orally bioavailable multi-kinase inhibitor with potent activity against MET, MST1R, and other oncoproteins, and displays anti-tumor activities in mouse xenograft models

The HGF/MET signaling pathway regulates a wide variety of normal cellular functions that can be subverted to support neoplasia, including cell proliferation, survival, apoptosis, scattering and motility, invasion, and angiogenesis. MET over-expression (with or without gene amplification), aberrant autocrine or paracrine ligand production, and missense MET mutations are mechanisms that lead to activation of the MET pathway in tumors and are associated with poor prognostic outcome. We report here preclinical development of a potent, orally bioavailable, small-molecule inhibitor LY2801653 targeting MET kinase. LY2801653 is a type-II ATP competitive, slow-off inhibitor of MET tyrosine kinase with a dissociation constant (Ki) of 2 nM, a pharmacodynamic residence time (Koff) of 0.00132 min(-1) and t1/2 of 525 min. LY2801653 demonstrated in vitro effects on MET pathway-dependent cell scattering and cell proliferation; in vivo anti-tumor effects in MET amplified (MKN45), MET autocrine (U-87MG, and KP4) and MET over-expressed (H441) xenograft models; and in vivo vessel normalization effects. LY2801653 also maintained potency against 13 MET variants, each bearing a single-point mutation. In subsequent nonclinical characterization, LY2801653 was found to have potent activity against several other receptor tyrosine oncokinases including MST1R, FLT3, AXL, MERTK, TEK, ROS1, DDR1/2 and against the serine/threonine kinases MKNK1/2. The potential value of MET and other inhibited targets within a number of malignancies (such as colon, bile ducts, and lung) is discussed. LY2801653 is currently in phase 1 clinical testing in patients with advanced cancer (trial I3O-MC-JSBA, NCT01285037).

Targeting Mnks for cancer therapy

Deregulation of protein synthesis is a common event in human cancer and a key player in translational control is eIF4E. Elevated expression levels of eIF4E promote cancer development and progression. Recent findings suggest that eIF4E activity is a key determinant of the PI3K/Akt/mTOR and Ras/Raf/MEK/ERK mediated tumorigenic activity and targeting eIF4E should have a major impact on these pathways in human cancer. The function of eIF4E is modulated through phosphorylation of a conserved serine (Ser209) by Mnk1 and Mnk2 downstream of ERK. While the phosphorylation event is necessary for oncogenic transformation, it seems to be dispensable for normal development. Hence, pharmacologic Mnk inhibitors may provide non-toxic and effective anti-cancer strategy. Strong circumstantial evidence indicates that Mnk inhibition presents attractive therapeutic potential, but the lack of selective Mnk inhibitors has so far confounded pharmacological target validation and clinical development.

Exploring aigialomycin d and its analogues as protein kinase inhibitors for cancer targets

The natural product aigialomycin D (1) is a member of the resorcylic acid lactone (RAL) family possessing protein kinase inhibitory activities. This paper describes the synthesis of aigialomycin D and a series of its analogues and their activity for the inhibition of protein kinases related to cancer pathways. A preliminary study of these compounds in the inhibition of CDK2/cyclin A kinase has found that aigialomycin D and analogues 11 and 23 are moderate CDK2/cyclin A inhibitors with IC50 values of ca. 20 μM. Kinase profiling of aigialomycin D against a panel of kinases has led to the identification of MNK2 as a promising target (IC50 = 0.45 μM), and preliminary structure-activity relationship studies have been carried out to identify the essential functional groups for activity.

Regulation of eukaryotic initiation factor 4E (eIF4E) phosphorylation by mitogen-activated protein kinase occurs through modulation of Mnk1-eIF4G interaction

The m(7)G cap binding protein eukaryotic initiation factor 4E (eIF4E) is a rate-limiting determinant of protein synthesis. Elevated eIF4E levels, commonly associated with neoplasia, promote oncogenesis, and phosphorylation of eIF4E at Ser209 is critical for its tumorigenic potential. eIF4E phosphorylation is catalyzed by mitogen-activated protein kinase (MAPK)-interacting serine/threonine kinase (Mnk), a substrate of Erk1/2 and p38 MAPKs. Interaction with the scaffolding protein eIF4G, which also binds eIF4E, brings Mnk and its substrate into physical proximity. Thus, Mnk-eIF4G interaction is important for eIF4E phosphorylation. Through coimmunoprecipitation assays, we showed that MAPK-mediated phosphorylation of the Mnk1 active site controls eIF4G binding. Utilizing a naturally occurring splice variant, we demonstrated that the C-terminal domain of Mnk1 restricts its interaction with eIF4G, preventing eIF4E phosphorylation in the absence of MAPK signaling. Furthermore, using a small-molecule Mnk1 inhibitor and kinase-dead mutant, we established that Mnk1 autoregulates its interaction with eIF4G, releasing itself from the scaffold after phosphorylation of its substrate. Our findings indicate tight control of eIF4E phosphorylation through modulation of Mnk1-eIF4G interaction.

Proteus in the world of proteins: conformational changes in protein kinases

The 512 protein kinases encoded by the human genome are a prime example of nature's ability to create diversity by introducing variations to a highly conserved theme. The activity of each kinase domain is controlled by layers of regulatory mechanisms involving different combinations of post-translational modifications, intramolecular contacts, and intermolecular interactions. Ultimately, they all achieve their effect by favoring particular conformations that promote or prevent the kinase domain from catalyzing protein phosphorylation. The central role of kinases in various diseases has encouraged extensive investigations of their biological function and three-dimensional structures, yielding a more detailed understanding of the mechanisms that regulate protein kinase activity by conformational changes. In the present review, we discuss these regulatory mechanisms and show how conformational changes can be exploited for the design of specific inhibitors that lock protein kinases in inactive conformations. In addition, we highlight recent developments to monitor ligand-induced structural changes in protein kinases and for screening and identifying inhibitors that stabilize enzymatically incompetent kinase conformations.

The C-terminal domain of Mnk1a plays a dual role in tightly regulating its activity

The human family of MAPK (mitogen-activated protein kinase) signal-integrating kinases (Mnks) comprises four related proteins derived from two genes by alternative splicing. The MNK1 gene gives rise to two proteins, Mnk1a and Mnk1b, which possess distinct C-termini and properties. Despite lacking the C-terminal MAPK-binding site, Mnk1b shows higher basal activity than Mnk1a. In contrast, the activity of Mnk1a is tightly regulated by signalling through ERK (extracellular-signal-regulated kinase) and p38 MAPK. We show that the short C-terminus of Mnk1b confers on it a 'default' behaviour of substantial, but unregulated, activity. In contrast, the longer C-terminus of Mnk1a represses the basal activity and T (activation)-loop phosphorylation of this isoenzyme while allowing both properties to be stimulated by upstream MAPK signalling. Two features of the C-terminus of Mnk1a appear to account for this behaviour: the known MAPK-binding site and a region (predicted to be alpha-helical) which occludes access to the catalytic domain and the T-loop. The activation of Mnk1a results in a marked conformational change leading to a more 'open' structure. We also identified a conserved phenylalanine residue in an Mnk-specific insert as playing a key role in governing the ease with which Mnk1a can be phosphorylated. These studies help to identify the features that give rise to the diverse properties of human Mnk isoforms.

Type I interferon (IFN)-dependent activation of Mnk1 and its role in the generation of growth inhibitory responses

We provide evidence for the existence of an IFN-regulated cellular pathway involving the mitogen-activated protein kinase (MAPK)-integrating kinase (Mnk) 1. Our data demonstrate that type I (alpha, beta) IFNs induce phosphorylation/activation of Mnk1, which, in turn, regulates phosphorylation of the eukaryotic initiation factor 4E (eIF4E) on Ser-209. Such Mnk activation depends on upstream engagement of Jak1, and requires downstream activation of the Mek/Erk MAPK pathway. In studies using double Mnk1-/-Mnk2-/- knockout mouse embryonic fibroblasts (MEFs), we found that engagement of Mnk kinases is essential for mRNA translation of the Isg15 and Isg54 genes, suggesting an important role for this pathway in mRNA translation of IFN-stimulated genes (ISGs). Importantly, our data demonstrate that pharmacological inhibition of Mnk kinases or siRNA-mediated knockdown of Mnk1 and Mnk2 results in partial reversal of the suppressive effects of IFNalpha on normal and leukemic hematopoietic progenitors, establishing a key role for this pathway in the generation of the growth inhibitory effects of type I IFNs. Together, our findings establish that the Mnk/eIF4E kinase pathway is activated in an IFN-inducible manner and plays important roles in mRNA translation for ISGs and generation of IFN-inducible anti-proliferative responses.