Regulation of arsenic trioxide-induced cellular responses by Mnk1 and Mnk2

Mechanistic update of Trisenox in blood cancer

Acute promyelocytic leukemia (APL)/blood cancer is M3 type of acute myeloid leukemia (AML) formed inside bone marrow through chromosomal translocation mutation usually between chromosome 15 & 17. It accounts around 10% cases of AML worldwide. Trisenox (TX/ATO) is used in chemotherapy for treatment of all age group of APL patients with highest efficacy and survival rate for longer period. High concentration of TX inhibits growth of APL cells by diverse mechanism however, it cures only PML-RARα fusion gene/oncogene containing APL patients. TX resistant APL patients (different oncogenic make up) have been reported from worldwide. This review summarizes updated mechanism of TX action via PML nuclear bodies formation, proteasomal degradation, autophagy, p53 activation, telomerase activity, heteromerization of pRb & E2F, and regulation of signaling mechanism in APL cells. We have also provided important information of combination therapy of TX with other molecules mechanism of action in acute leukemia cells. It provides updated information of TX action for researcher which may help finding new target for further research in APL pathophysiology or new TX resistant APL patients drug designing.

Global signalling network analysis of luminal T47D breast cancer cells in response to progesterone

BACKGROUND

Breast cancer cells enter into the cell cycle following progestin exposure by the activation of signalling cascades involving a plethora of enzymes, transcription factors and co-factors that transmit the external signal from the cell membrane to chromatin, ultimately leading to a change of the gene expression program. Although many of the events within the signalling network have been described in isolation, how they globally team up to generate the final cell response is unclear.

METHODS

In this study we used antibody microarrays and phosphoproteomics to reveal a dynamic global signalling map that reveals new key regulated proteins and phosphor-sites and links between previously known and novel pathways. T47D breast cancer cells were used, and phospho-sites and pathways highlighted were validated using specific antibodies and phenotypic assays. Bioinformatic analysis revealed an enrichment in novel signalling pathways, a coordinated response between cellular compartments and protein complexes.

RESULTS

Detailed analysis of the data revealed intriguing changes in protein complexes involved in nuclear structure, epithelial to mesenchyme transition (EMT), cell adhesion, as well as transcription factors previously not associated with breast cancer cell proliferation. Pathway analysis confirmed the key role of the MAPK signalling cascade following progesterone and additional hormone regulated phospho-sites were identified. Full network analysis shows the activation of new signalling pathways previously not associated with progesterone signalling in T47D breast cancer cells such as ERBB and TRK. As different post-translational modifications can mediate complex crosstalk mechanisms and massive PARylation is also rapidly induced by progestins, we provide details of important chromatin regulatory complexes containing both phosphorylated and PARylated proteins.

CONCLUSIONS

This study contributes an important resource for the scientific community, as it identifies novel players and connections meaningful for breast cancer cell biology and potentially relevant for cancer management.

Genome-wide translation patterns in gliomas: An integrative view

Gliomas are the most frequent tumors of the central nervous system (CNS) and include the highly malignant glioblastoma (GBM). Characteristically, gliomas have translational control deregulation related to overactivation of signaling pathways such as PI3K/AKT/mTORC1 and Ras/ERK1/2. Thus, mRNA translation appears to play a dominant role in glioma gene expression patterns. The, analysis of genome-wide translated transcripts, together known as the translatome, may reveal important information for understanding gene expression patterns in gliomas. This review provides a brief overview of translational control mechanisms altered in gliomas with a focus on the current knowledge related to the translatomes of glioma cells and murine glioma models. We present an integrative meta-analysis of selected glioma translatome data with the aim of identifying recurrent patterns of gene expression preferentially regulated at the level of translation and obtaining clues regarding the pathological significance of these alterations. Re-analysis of several translatome datasets was performed to compare the translatomes of glioma models with those of their non-tumor counterparts and to document glioma cell responses to radiotherapy and MNK modulation. The role of recurrently altered genes in the context of translational control and tumorigenesis are discussed.

The Differential Phosphorylation-Dependent Signaling and Glucose Immunometabolic Responses Induced during Infection by Salmonella Enteritidis and Salmonella Heidelberg in Chicken Macrophage-like cells

Salmonella is a burden to the poultry, health, and food safety industries, resulting in illnesses, food contamination, and recalls. Salmonella enterica subspecies enterica Enteritidis (S. Enteritidis) is one of the most prevalent serotypes isolated from poultry. Salmonella enterica subspecies enterica Heidelberg (S. Heidelberg), which is becoming as prevalent as S. Enteritidis, is one of the five most isolated serotypes. Although S. Enteritidis and S. Heidelberg are almost genetically identical, they both are capable of inducing different immune and metabolic responses in host cells to successfully establish an infection. Therefore, using the kinome peptide array, we demonstrated that S. Enteritidis and S. Heidelberg infections induced differential phosphorylation of peptides on Rho proteins, caspases, toll-like receptors, and other proteins involved in metabolic- and immune-related signaling of HD11 chicken macrophages. Metabolic flux assays measuring extracellular acidification rate (ECAR) and oxygen consumption rate (OCR) demonstrated that S. Enteritidis at 30 min postinfection (p.i.) increased glucose metabolism, while S. Heidelberg at 30 min p.i. decreased glucose metabolism. S. Enteritidis is more invasive than S. Heidelberg. These results show different immunometabolic responses of HD11 macrophages to S. Enteritidis and S. Heidelberg infections.

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.

Induction of MNK Kinase-dependent eIF4E Phosphorylation by Inhibitors Targeting BET Proteins Limits Efficacy of BET Inhibitors

BET inhibitors (BETi), which target transcription of key oncogenic genes, are currently being evaluated in early-phase clinical trials. However, because BETis show limited single-agent activity, there is increasing interest in identifying signaling pathways to enhance the efficacy of BETis. Here, we demonstrate increased MNK kinase-dependent eIF4E phosphorylation following treatment with BETis, indicating activation of a prosurvival feedback mechanism in response to BETis. BET PROTACs, which promote degradation of BET proteins, also induced eIF4E phosphorylation in cancer cells. Mechanistically, we show that the effect of BETis on MNK-eIF4E phosphorylation was mediated by p38 MAPKs. We also show that BETis suppressed RacGAP1 to induce Rac signaling-mediated eIF4E phosphorylation. Significantly, MNK inhibitors and MNK1/2 knockdown enhanced the efficacy of BETis in suppressing proliferation of cancer cells in vitro and in a syngeneic mouse model. Together, these results demonstrate a novel prosurvival feedback signaling induced by BETis, providing a mechanistic rationale for combination therapy with BET and MNK inhibitors for synergistic inhibition of cancer cells.

Differential Response of Glioma Stem Cells to Arsenic Trioxide Therapy Is Regulated by MNK1 and mRNA Translation

Mesenchymal (MES) and proneural (PN) are two distinct glioma stem cell (GSC) populations that drive therapeutic resistance in glioblastoma (GBM). We screened a panel of 650 small molecules against patient-derived GBM cells to discover compounds targeting specific GBM subtypes. Arsenic trioxide (ATO), an FDA-approved drug that crosses the blood-brain barrier, was identified as a potent PN-specific compound in the initial screen and follow-up validation studies. Furthermore, MES and PN GSCs exhibited differential sensitivity to ATO. As ATO has been shown to activate the MAPK-interacting kinase 1 (MNK1)-eukaryotic translation initiation factor 4E (eIF4E) pathway and subsequent mRNA translation in a negative regulatory feedback manner, the mechanistic role of ATO resistance in MES GBM was explored. In GBM cells, ATO-activated translation initiation cellular events via the MNK1-eIF4E signaling axis. Furthermore, resistance to ATO in intracranial PDX tumors correlated with high eIF4E phosphorylation. Polysomal fractionation and microarray analysis of GBM cells were performed to identify ATO's effect on mRNA translation and enrichment of anti-apoptotic mRNAs in the ATO-induced translatome was found. Additionally, it was determined that MNK inhibition sensitized MES GSCs to ATO in neurosphere and apoptosis assays. Finally, examination of the effect of ATO on patients from a phase I/II clinical trial of ATO revealed that PN GBM patients responded better to ATO than other subtypes as demonstrated by longer overall and progression-free survival.Implications: These findings raise the possibility of a unique therapeutic approach for GBM, involving MNK1 targeting to sensitize MES GSCs to drugs like arsenic trioxide. Mol Cancer Res; 16(1); 32-46. ©2017 AACR.

Quercetin and Cisplatin combined treatment altered cell cycle and mitogen activated protein kinase expressions in malignant mesotelioma cells

Background

Malignant mesothelioma is a locally aggressive and highly lethal neoplasm of pleural, peritoneal and pericardial mesothelial cells without successful therapy. Previously, we reported that Quercetin in combination with Cisplatin inhibits cell proliferation and activates caspase-9 and -3 enzymes in different malignant mesothelioma cell lines. Moreover, Quercetin + Cisplatin lead to accumulation of both SPC111 and SPC212 cell lines in S phase.

Methods

In present work, 84 genes involved in cell growth and proliferation have analysed by using RT²-PCR array system and protein profile of mitogen activated protein kinase (MAPK) family proteins investigated by western blots.

Results

Our results showed that Quercetin and Quercetin + Cisplatin modulated gene expression of cyclins, cyclin dependent kinases and cyclin dependent kinases inhibitors. In addition genes involved in JNK, p38 and MAPK/ERK pathways were up regulated. Moreover, while p38 and JNK phosphorylations were increased, ERK phosphorylations were decreased after using Quercetin + Cisplatin.

Conclusion

This research has clarified our previous results and detailed mechanism of anti-carcinogenic potential of Quercetin alone and incombination with Cisplatin on malignant mesothelioma cells.

Arsenic Triglutathione [As(GS)3] Transport by Multidrug Resistance Protein 1 (MRP1/ABCC1) Is Selectively Modified by Phosphorylation of Tyr920/Ser921 and Glycosylation of Asn19/Asn23

The ATP-binding cassette (ABC) transporter multidrug resistance protein 1 (MRP1/ABCC1) is responsible for the cellular export of a chemically diverse array of xenobiotics and endogenous compounds. Arsenic, a human carcinogen, is a high-affinity MRP1 substrate as arsenic triglutathione [As(GS)3]. In this study, marked differences in As(GS)3 transport kinetics were observed between MRP1-enriched membrane vesicles prepared from human embryonic kidney 293 (HEK) (Km 3.8 µM and Vmax 307 pmol/mg per minute) and HeLa (Km 0.32 µM and Vmax 42 pmol/mg per minute) cells. Mutant MRP1 lacking N-linked glycosylation [Asn19/23/1006Gln; sugar-free (SF)-MRP1] expressed in either HEK293 or HeLa cells had low Km and Vmax values for As(GS)3, similar to HeLa wild-type (WT) MRP1. When prepared in the presence of phosphatase inhibitors, both WT- and SF-MRP1-enriched membrane vesicles had a high Km value for As(GS)3 (3-6 µM), regardless of the cell line. Kinetic parameters of As(GS)3 for HEK-Asn19/23Gln-MRP1 were similar to those of HeLa/HEK-SF-MRP1 and HeLa-WT-MRP1, whereas those of single glycosylation mutants were like those of HEK-WT-MRP1. Mutation of 19 potential MRP1 phosphorylation sites revealed that HEK-Tyr920Phe/Ser921Ala-MRP1 transported As(GS)3 like HeLa-WT-MRP1, whereas individual HEK-Tyr920Phe- and -Ser921Ala-MRP1 mutants were similar to HEK-WT-MRP1. Together, these results suggest that Asn19/Asn23 glycosylation and Tyr920/Ser921 phosphorylation are responsible for altering the kinetics of MRP1-mediated As(GS)3 transport. The kinetics of As(GS)3 transport by HEK-Asn19/23Gln/Tyr920Glu/Ser921Glu were similar to HEK-WT-MRP1, indicating that the phosphorylation-mimicking substitutions abrogated the influence of Asn19/23Gln glycosylation. Overall, these data suggest that cross-talk between MRP1 glycosylation and phosphorylation occurs and that phosphorylation of Tyr920 and Ser921 can switch MRP1 to a lower-affinity, higher-capacity As(GS)3 transporter, allowing arsenic detoxification over a broad concentration range.

Translational control of PML contributes to TNFα-induced apoptosis of MCF7 breast cancer cells and decreased angiogenesis in HUVECs

The tumor suppressor protein promyelocytic leukemia (PML) is a key regulator of inflammatory responses and tumorigenesis and functions through the assembly of subnuclear structures known as PML nuclear bodies (NBs). The inflammation-related cytokine tumor necrosis factor-α (TNFα) is known to induce PML protein accumulation and PML NB formation that mediate TNFα-induced cell death in cancer cells and inhibition of migration and capillary tube formation in endothelial cells (ECs). In this study, we uncover a novel mechanism of PML gene regulation in which the p38 MAPK and its downstream kinase MAP kinase-activated protein kinase 1 (MNK1) mediate TNFα-induced PML protein accumulation and PML NB formation. The mechanism includes the presence of an internal ribosome entry site (IRES) found within the well-conserved 100 nucleotides upstream of the PML initiation codon. The activity of the PML IRES is induced by TNFα in a manner that involves MNK1 activation. It is proposed that the p38–MNK1–PML network regulates TNFα-induced apoptosis in breast cancer cells and TNFα-mediated inhibition of migration and capillary tube formation in ECs.

Direct binding of arsenic trioxide to AMPK and generation of inhibitory effects on acute myeloid leukemia precursors

Arsenic trioxide (As2O3) exhibits potent antineoplastic effects and is used extensively in clinical oncology for the treatment of a subset of patients with acute myeloid leukemia (AML). Although As2O3 is known to regulate activation of several signaling cascades, the key events, accounting for its antileukemic properties, remain to be defined. We provide evidence that arsenic can directly bind to cysteine 299 in AMPKα and inhibit its activity. This inhibition of AMPK by arsenic is required in part for its cytotoxic effects on primitive leukemic progenitors from patients with AML, while concomitant treatment with an AMPK activator antagonizes in vivo the arsenic-induced antileukemic effects in a xenograft AML mouse model. A consequence of AMPK inhibition is activation of the mTOR pathway as a negative regulatory feedback loop. However, when AMPK expression is lost, arsenic-dependent activation of the kinase RSK downstream of MAPK activity compensates the generation of regulatory feedback signals through phosphorylation of downstream mTOR targets. Thus, therapeutic regimens with As2O3 will need to include inhibitors of both the mTOR and RSK pathways in combination to prevent engagement of negative feedback loops and maximize antineoplastic responses.

Nanomedicine strategies for hematological malignancies: what is next?

The major obstacle in treating cancer depends on the low therapeutic index of most anticancer drugs. The lack of specificity, coupled with the large volumes of distribution, translates into a nonpreferential distribution of anticancer drugs to the tumor. Accordingly, the dose of the anticancer drug that is achievable within tumor is limited, resulting in suboptimal treatment and unwanted toxicity. Nanoparticles applied as drug-delivery systems are submicron-sized (3–200 nm) particles, that can enhance the selectivity of the active drug to cancer cells through a change of its pharmacokinetic profile, while avoiding toxicity in normal cells. This review will discuss the current uses of nanodrugs in hematology, with a focus on the most promising nanoparticles in development for the treatment of hematologic tumors.

Resveratrol enhances the suppressive effects of arsenic trioxide on primitive leukemic progenitors

Efforts to enhance the antileukemic properties of arsenic trioxide are clinically relevant and may lead to the development of new therapeutic approaches for the management of certain hematological malignancies. We provide evidence that concomitant treatment of acute myeloid leukemia (AML) cells or chronic myeloid leukemia (CML) cells with resveratrol potentiates arsenic trioxide-dependent induction of apoptosis. Importantly, clonogenic assays in methylcellulose demonstrate potent suppressive effects of the combination of these agents on primitive leukemic progenitors derived from patients with AML or CML. Taken together, these findings suggest that combinations of arsenic trioxide with resveratrol may provide an approach for targeting of early leukemic precursors and, possibly, leukemia initiating stem cells.

Nano-encapsulation of arsenic trioxide enhances efficacy against murine lymphoma model while minimizing its impact on ovarian reserve in vitro and in vivo

Advances in cancer therapy have increased the rate of survival of young cancer patients; however, female lymphoma patients frequently face a temporary or permanent loss of fertility when treated with traditional cytotoxic agents. The potential loss of fertility is an important concern that can influence treatment decisions for many premenopausal cancer patients. The negative effect of chemotherapeutic agents and treatment protocols to patients’ fertility–referred to as fertotoxicity–are thus an increasingly important cancer survivorship issue. We have developed a novel nanoscale formulation of arsenic trioxide, a potent drug for treatment of hematological malignancies, and demonstrate that it has significantly better activity in a murine lymphoma model than the free drug. In parallel, we have developed a novel in vitro assay of ovarian follicle function that predicts in vivo ovarian toxicity of therapeutic agents. Our results reveal that the nanotherapeutic agent is not only more active against lymphoma, but is fertoprotective, i.e., it is much less deleterious to ovarian function than the parent drug. Thus, our in vitro assay allows rapid evaluation of both established and experimental anticancer drugs on ovarian reserve and can inform the selection of efficacious and fertility-sparing treatment regimens for reproductive-age women diagnosed with cancer.

Inhibition of Mnk kinase activity by cercosporamide and suppressive effects on acute myeloid leukemia precursors

Mnk kinases regulate the phosphorylation and activation of the eukaryotic initiation factor 4E (eIF4E), a protein that plays key roles in the initiation of messenger RNA translation and whose activity is critical for various cellular functions. eIF4E is deregulated in acute myeloid leukemia (AML), and its aberrant activity contributes to leukemogenesis. We determined whether cercosporamide, an antifungal agent that was recently shown to act as a unique Mnk inhibitor, exhibits antileukemic properties. Treatment of AML cells with cercosporamide resulted in a dose-dependent suppression of eIF4E phosphorylation. Such suppression of Mnk kinase activity and eIF4E phosphorylation by cercosporamide resulted in dose-dependent suppressive effects on primitive leukemic progenitors (CFU-L) from AML patients and enhanced the antileukemic properties of cytarabine (Ara-C) or mammalian target of rapamycin (mTOR) complex 1 inhibition. Similarly, the combination of cercosporamide with cytarabine resulted in enhanced antileukemic responses in a xenograft mouse model in vivo. Altogether, this work demonstrates that the unique Mnk inhibitor cercosporamide suppresses phosphorylation of eIF4E and exhibits antileukemic effects, in support of future clinical-translational efforts involving combinations of Mnk inhibitors with cytarabine and/or mTOR inhibitors for the treatment of AML.

Regulation of the kinase RSK1 by arsenic trioxide and generation of antileukemic responses

Arsenic Trioxide (As₂O₃) is one of the most effective agents in the treatment of acute promyelocytic leukemia (APL), but has no significant efficacy in other forms of AML. The mechanisms of relative resistance of non-APL cells are not well understood, but emerging evidence suggests that activation of negative feedback regulatory loops and pathways contributes to such resistance. We provide evidence that a signaling cascade involving the kinase RSK1 is engaged in a negative feedback manner during arsenic-treatment of cells and exhibits regulatory effects on growth and survival of AML cells in response to treatment with As₂O₃. Our data demonstrate that pharmacological inhibition or molecular disruption of expression of RSK1 enhances As₂O₃-dependent apoptosis and/or growth inhibition of AML cells. Importantly, combination of a pharmacological inhibitor of RSK and As₂O₃ results in enhanced suppression of primary AML leukemic progenitors. Altogether, our findings suggest an important regulatory role for RSK1 in the generation of the effects of As₂O₃ in AML cells. They also raise the potential of RSK1 targeting in combination with As₂O₃ as a novel approach to promote antileukemic responses.

p38α mitogen-activated protein kinase depletion and repression of signal transduction to translation machinery by miR-124 and -128 in neurons

The p38α to p38δ mitogen-activated protein kinases (MAPKs) are central regulatory nodes coordinating acute stress and inflammatory responses. Their activation leads to rapid adjustment of protein synthesis, for instance translational induction of proinflammatory cytokines. The only known direct link of p38 to translation machinery is the MAPK signal-integrating kinase Mnk. Only p38α and p38β transcripts are ubiquitously expressed. These mRNAs encode highly conserved proteins that equally phosphorylate recombinant Mnk1 in vitro. We discovered that expression of the p38α protein, but not the p38β isoform, is suppressed in the brain. This is due to p38α depletion by two neuron-selective microRNAs (miRNAs), miR-124 and -128. Suppression of p38α protein was reversed by miR-124/-128 antisense oligonucleotides in primary explant neuronal cultures. Targeted p38α depletion reduced Mnk1 activation, which cannot be compensated by p38β. Our research shows that p38α alone controls acute stress and cytokine signaling from p38 MAPK to translation machinery. This regulatory axis is greatly diminished in neurons, which may insulate brain physiology and function from p38α-Mnk1-mediated signaling.

Autophagic degradation of the BCR-ABL oncoprotein and generation of antileukemic responses by arsenic trioxide

We provide evidence that arsenic trioxide (As(2)O(3)) targets the BCR-ABL oncoprotein via a novel mechanism involving p62/SQSTM1-mediated localization of the oncoprotein to the autolysosomes and subsequent degradation mediated by the protease cathepsin B. Our studies demonstrate that inhibitors of autophagy or cathepsin B activity and/or molecular targeting of p62/SQSTM1, Atg7, or cathepsin B result in partial reversal of the suppressive effects of AS(2)O(3) on BCR-ABL expressing leukemic progenitors, including primitive leukemic precursors from chronic myelogenous leukemia (CML) patients. Altogether, these findings indicate that autophagic degradation of BCR-ABL is critical for the induction of the antileukemic effects of As(2)O(3) and raise the potential for future therapeutic approaches to target BCR-ABL expressing cells by modulating elements of the autophagic machinery to promote BCR-ABL degradation.

A new era for an ancient drug: arsenic trioxide and Hedgehog signaling

Arsenic has been used for ages as a therapeutic agent. Currently, it is an FDA approved drug to treat acute promyelocytic leukemia where it leads to degradation of the PML-RAR fusion protein. It has been shown to have various other targets in cells such as JNK, NFκB, thioredoxin reductase, and MAPK pathways. Most of its effects in cells have been through arsenic's ability to bind to thiol groups in cysteine residues. Recent evidence has shown that arsenic can inhibit the Hedgehog pathway by inhibiting GLI proteins. The proposed mechanism of action is through direct binding. Potential binding sites include the critical cysteine residues in GLI zinc finger domains. The role of the Hedgehog pathway has been implicated in many cancers such as basal cell carcinoma, medulloblastoma, Ewing sarcoma, and rhabdoid tumors. Current Hedgehog pathway inhibitors have been fraught with resistance issues and so arsenic trioxide may provide an alternative therapy when combined with these other inhibitors or after acquired resistance.

Arsenic oxide targets stem cell marker CD133/prominin-1 in gallbladder carcinoma

CD133+ tumor cells are responsible for the initiation, propagation and recurrence of tumors, which raises the question of how to effectively target CD133+ tumor cells. Arsenic trioxide (As2O3) has considerable efficacy in treating solid tumors with induction of apoptosis. Here, we found that purified CD133+ gallbladder carcinoma cells are highly resistant to conventional chemotherapy. However, As2O3 effectively induces CD133+ gallbladder carcinoma cells apoptosis. Treatment with As2O3 reduces CD133 expression at transcriptional levels. Furthermore, the ectopic expression of CD133 attenuated the apoptotic effect of As2O3 on cells through activation of AKT signaling pathways. Collectively, As2O3 effectively targets CD133 in gallbladder carcinoma, providing a new mechanism of As2O3-induced cell apoptosis and a better understanding of drug resistance in gallbladder carcinoma.

Interferon regulatory factor-1 binds c-Cbl, enhances mitogen activated protein kinase signaling and promotes retinoic acid-induced differentiation of HL-60 human myelo-monoblastic leukemia cells

All-trans retinoic acid (RA) and interferons (IFNs) have efficacy in treating certain leukemias and lymphomas, respectively, motivating interest in their mechanism of action to improve therapy. Both RA and IFNs induce interferon regulatory factor-1 (IRF-1). We find that in HL-60 myeloblastic leukemia cells which undergo mitogen activated protien kinase (MAPK)-dependent myeloid differentiation in response to RA, IRF-1 propels differentiation. RA induces MAPK-dependent expression of IRF-1. IRF-1 binds c-Cbl, a MAPK related adaptor. Ectopic IRF-1 expression causes CD38 expression and activation of the Raf/MEK/ERK axis, and enhances RA-induced differentiation by augmenting CD38, CD11b, respiratory burst and G0 arrest. Ectopic IRF-1 expression also decreases the activity of aldehyde dehydrogenase 1, a stem cell marker, and enhances RA-induced ALDH1 down-regulation. Interestingly, expression of aryl hydrocarbon receptor (AhR), which is RA-induced and known to down-regulate Oct4 and drive RA-induced differentiation, also enhances IRF-1 expression. The data are consistent with a model whereby IRF-1 acts downstream of RA and AhR to enhance Raf/MEK/ERK activation and propel differentiation.

MAP kinase-interacting kinase 1 regulates SMAD2-dependent TGF-β signaling pathway in human glioblastoma

Glioblastoma multiforme (GBM) is the most common aggressive brain cancer with a median survival of approximately 1 year. In a search for novel molecular targets that could be therapeutically developed, our kinome-focused microarray analysis identified the MAP (mitogen-activated protein) kinase-interacting kinase 1 (MNK1) as an attractive theranostic candidate. MNK1 overexpression was confirmed in both primary GBMs and glioma cell lines. Inhibition of MNK1 activity in GBM cells by the small molecule CGP57380 suppressed eIF4E phosphorylation, proliferation, and colony formation whereas concomitant treatment with CGP57380 and the mTOR inhibitor rapamycin accentuated growth inhibition and cell-cycle arrest. siRNA-mediated knockdown of MNK1 expression reduced proliferation of cells incubated with rapamycin. Conversely, overexpression of full-length MNK1 reduced rapamycin-induced growth inhibition. Analysis of polysomal profiles revealed inhibition of translation in CGP57380 and rapamycin-treated cells. Microarray analysis of total and polysomal RNA from MNK1-depleted GBM cells identified mRNAs involved in regulation of TGF-β pathway. Translation of SMAD2 mRNA as well as TGF-β-induced cell motility and vimentin expression was regulated by MNK1 signaling. Tissue microarray analysis revealed a positive correlation between the immunohistochemical staining of MNK1 and SMAD2. Taken together, our findings offer insights into how MNK1 pathways control translation of cancer-related mRNAs including SMAD2, a key component of the TGF-β signaling pathway. Furthermore, they suggest MNK1-controlled translational pathways in targeted strategies to more effectively treat GBM.

Essential role for Mnk kinases in type II interferon (IFNgamma) signaling and its suppressive effects on normal hematopoiesis

IFNγ exhibits potent antitumor effects and plays important roles in the innate immunity against cancer. However, the mechanisms accounting for the antiproliferative effects of IFNγ still remain to be elucidated. We examined the role of Mnk1 (MAPK-interacting protein kinase 1) in IFNγ signaling. Our data demonstrate that IFNγ treatment of sensitive cells results in engagement of Mnk1, activation of its kinase domain, and downstream phosphorylation of the cap-binding protein eIF4E on Ser-209. Such engagement of Mnk1 plays an important role in IFNγ-induced IRF-1 (IFN regulatory factor 1) gene mRNA translation/protein expression and is essential for generation of antiproliferative responses. In studies aimed to determine the role of Mnk1 in the induction of the suppressive effects of IFNs on primitive hematopoietic progenitors, we found that siRNA-mediated Mnk1/2 knockdown results in partial reversal of the suppressive effects of IFNγ on human CD34+-derived myeloid (CFU-GM) and erythroid (BFU-E) progenitors. These findings establish a key role for the Mnk/eIF4E pathway in the regulatory effects of IFNγ on normal hematopoiesis and identify Mnk kinases as important elements in the control of IFNγ-inducible ISG mRNA translation.

Mnk mediates integrin α6β4-dependent eIF4E phosphorylation and translation of VEGF mRNA

It was previously shown that integrin α6β4 contributes to translation of cancer-related mRNAs such as VEGF via initiation factor eIF4E. In this study, we found that integrin α6β4 regulates the activity of eIF4E through the Ser/Thr kinase Mnk. Although a role for Mnk in various aspects of cancer progression has been established, a link between integrin and Mnk activity has not. Here we show that Mnk1 is a downstream effector of integrin α6β4 and mediates the α6β4 signaling, important for translational control. Integrin α6β4 signals through MEK and p38 MAPK to increase phosphorylation of Mnk1 and eIF4E. Inhibition of Mnk1 activity by CGP57380 or downregulation by shRNA blocks α6β4-dependent translation of VEGF mRNA. Our studies suggest that Mnk1 could be a therapeutic target in cancers where the integrin α6β4 level is high.

Negative regulatory effects of Mnk kinases in the generation of chemotherapy-induced antileukemic responses

Mnk kinases are downstream effectors of mitogen-activated protein kinase pathways and mediate phosphorylation of the eukaryotic initiation factor (eIF4E), a protein that plays a key role in the regulation of mRNA translation and is up-regulated in acute myeloid leukemia (AML). We determined the effects of chemotherapy (cytarabine) on the activation status of Mnk in AML cells and its role in the generation of antileukemic responses. A variety of experimental approaches were used, including immunoblotting, apoptosis assays, small interfering RNA (siRNA)-mediated knockdown of proteins, and clonogenic hematopoietic progenitor assays in methylcellulose. Cytarabine induced phosphorylation/activation of Mnk and Mnk-mediated phosphorylation of eIF4E on Ser209, as evidenced by studies involving pharmacological inhibition of Mnk or experiments using cells with targeted disruption of Mnk1 and Mnk2 genes. To assess the functional relevance of cytarabine-inducible engagement of Mnk/eIF4E pathway, the effects of pharmacological inhibition of Mnk on cytarabine-mediated suppression of primitive leukemic progenitors [leukemic colony forming unit (CFU-L)] were examined. Concomitant treatment of cells with a pharmacological inhibitor of Mnk or siRNA-mediated knockdown of Mnk1/2 strongly enhanced the suppressive effects of low cytarabine concentrations on CFU-L. It is noteworthy that the mammalian target of rapamycin (mTOR) inhibitor rapamycin also induced phosphorylation of eIF4E in a Mnk-dependent manner, whereas inhibition strongly enhanced its antileukemic effects. These data demonstrate that Mnk kinases are activated in a negative-feedback regulatory manner in response to chemotherapy and impair the generation of antileukemic responses. They also identify this pathway as a novel target for the design of new approaches to enhance the antileukemic effects of chemotherapy or mTOR inhibitors in AML.

Autophagy is a critical mechanism for the induction of the antileukemic effects of arsenic trioxide

Arsenic trioxide (As(2)O(3)) exhibits potent antitumor effects in vitro and in vivo, but the precise mechanisms by which it generates such responses are not well understood. We provide evidence that As(2)O(3) is a potent inducer of autophagy in leukemia cells. Such induction of autophagy by As(2)O(3) appears to require activation of the MEK/ERK pathway but not the AKT/mammalian target of rapamycin or JNK pathways. In efforts to understand the functional relevance of arsenic-induced autophagy, we found that pharmacological inhibitors of autophagy or molecular targeting of beclin 1 or Atg7 results in reversal of the suppressive effects of As(2)O(3) on leukemic cell lines and primary leukemic progenitors from acute myelogenous leukemia patients. Altogether, our data provide direct evidence that autophagic cell death is critical for the generation of the effects of As(2)O(3) on acute myelogenous leukemia cells and raise the potential of modulation of elements of the autophagic machinery as an approach to enhance the antitumor properties of As(2)O(3) and possibly other heavy metal derivatives.

Arsenic trioxide-dependent activation of thousand-and-one amino acid kinase 2 and transforming growth factor-beta-activated kinase 1

Arsenic trioxide (As(2)O(3)) has potent antileukemic properties in vitro and in vivo, but the mechanisms by which it generates its effects on target leukemic cells are not well understood. Understanding cellular mechanisms and pathways that are activated in leukemic cells to control the generation of As(2)O(3) responses should have important implications in the development of novel approaches using As(2)O(3) for the treatment of leukemias. In this study, we used immunoblotting and immune complex kinase assays to provide evidence that the kinases thousand-and-one amino acid kinase 2 (TAO2) and transforming growth factor-beta-activated kinase 1 (TAK1) are rapidly activated in response to treatment of acute leukemia cells with As(2)O(3). Such activation occurs after the generation of reactive oxygen species and regulates downstream engagement of the p38 mitogen-activated protein kinase. Our studies demonstrate that siRNA-mediated knockdown of TAO2 or TAK1 or pharmacological inhibition of TAK1 enhances the suppressive effects of As(2)O(3) on KT-1-derived leukemic progenitor colony formation and on primary leukemic progenitors from patients with acute myelogenous leukemia. These results indicate key negative-feedback regulatory roles for these kinases in the generation of the antileukemic effects of As(2)O(3). Thus, molecular or pharmacological targeting of these kinases may provide a novel approach to enhance the generation of arsenic-dependent antileukemic responses.

Biological responses to arsenic compounds

Arsenic is a metalloid that generates various biological effects on cells and tissues. Depending on the specific tissue exposed and the time and degree of exposure, diverse responses can be observed. In humans, prolonged and/or high dose exposure to arsenic can have a variety of outcomes, including the development of malignancies, severe gastrointestinal toxicities, diabetes, cardiac arrhythmias, and death. On the other hand, one arsenic derivative, arsenic trioxide (As(2)O(3)), has important antitumor properties. This agent is a potent inducer of antileukemic responses, and it is now approved by the Food and Drug Administration for the treatment of acute promyelocytic leukemia in humans. The promise and therapeutic potential of arsenic and its various derivatives have been exploited for hundreds of years. Remarkably, research focused on the potential use of arsenic compounds in the treatment of human diseases remains highly promising, and it is an area of active investigation. An emerging approach of interest and therapeutic potential involves efforts to target and block cellular pathways activated in a negative feedback manner during treatment of cells with As(2)O(3). Such an approach may ultimately provide the means to selectively enhance the suppressive effects of this agent on malignant cells and render normally resistant tumors sensitive to its antineoplastic properties.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.