Pharmacological inhibition of BMK1 suppresses tumor growth through promyelocytic leukemia protein

Tumor cells with KRAS or BRAF mutations or ERK5/MAPK7 amplification are not addicted to ERK5 activity for cell proliferation

ERK5, encoded by MAPK7, has been proposed to play a role in cell proliferation, thus attracting interest as a cancer therapeutic target. While oncogenic RAS or BRAF cause sustained activation of the MEK1/2-ERK1/2 pathway, ERK5 is directly activated by MEK5. It has been proposed that RAS and RAF proteins can also promote ERK5 activation. Here we investigated the interplay between RAS-RAF-MEK-ERK and ERK5 signaling and studied the role of ERK5 in tumor cell proliferation in 2 disease-relevant cell models. We demonstrate that although an inducible form of CRAF (CRAF:ER*) can activate ERK5 in fibroblasts, the response is delayed and reflects feed-forward signaling. Additionally, oncogenic KRAS and BRAF do not activate ERK5 in epithelial cells. Although KRAS and BRAF do not couple directly to MEK5-ERK5, ERK5 signaling might still be permissive for proliferation. However, neither the selective MEK5 inhibitor BIX02189 or ERK5 siRNA inhibited proliferation of colorectal cancer cells harbouring KRAS(G12C/G13D) or BRAF(V600E). Furthermore, there was no additive or synergistic effect observed when BIX02189 was combined with the MEK1/2 inhibitor Selumetinib (AZD6244), suggesting that ERK5 was neither required for proliferation nor a driver of innate resistance to MEK1/2 inhibitors. Finally, even cancer cells with MAPK7 amplification were resistant to BIX02189 and ERK5 siRNA, showing that ERK5 amplification does not confer addiction to ERK5 for cell proliferation. Thus ERK5 signaling is unlikely to play a role in tumor cell proliferation downstream of KRAS or BRAF or in tumor cells with ERK5 amplification. These results have important implications for the role of ERK5 as an anti-cancer drug target.

Phosphorylation of BMK1 induces prostatic carcinoma cell proliferation by promoting entry into the S phase of the cell cycle

Big mitogen-activated protein kinase 1 (BMK1) is activated by mitogens and oncogenic signals, and is strongly implicated in tumorigenesis. In the present study, it was demonstrated that the activation of BMK1, but not extracellular signal-regulated kinase (ERK)1/2, can induce proliferation in prostate cancer cells. It was found that the proliferation of epidermal growth factor (EGF)-treated cells was accelerated by 40% compared with non-treated cells using a CCK8 assay. In addition, cell cycle analysis using flow cytometry showed that the proportion of cells in the S phase increased significantly in the BMK1-activated PC-3 cells, suggesting that the activation of BMK1 promotes entry into the S phase of the cell cycle in prostate cancer cells. Furthermore, western blot analysis indicated that EGF-mediated activation of BMK1, but not ERK1/2, participates in the proliferation and cell cycle regulation in prostate cancer cells. Furthermore, the effects of cell cycle regulation by the activation of BMK1 were associated with the increased expression levels of cyclin A and cyclin E, whereas the expression of cyclin B and cyclin D1 was unchanged in this process. Therefore, the present study demonstrated that the activation of BMK1 can induce proliferation by promoting entry into the S phase through the upregulation of cyclin A and cyclin E expression levels in prostate cancer cells.

The function, regulation and therapeutic implications of the tumor suppressor protein, PML

The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.

Alterations of Nuclear Architecture and Epigenetic Signatures during African Swine Fever Virus Infection

Viral interactions with host nucleus have been thoroughly studied, clarifying molecular mechanisms and providing new antiviral targets. Considering that African swine fever virus (ASFV) intranuclear phase of infection is poorly understood, viral interplay with subnuclear domains and chromatin architecture were addressed. Nuclear speckles, Cajal bodies, and promyelocytic leukaemia nuclear bodies (PML-NBs) were evaluated by immunofluorescence microscopy and Western blot. Further, efficient PML protein knockdown by shRNA lentiviral transduction was used to determine PML-NBs relevance during infection. Nuclear distribution of different histone H3 methylation marks at lysine’s 9, 27 and 36, heterochromatin protein 1 isoforms (HP1α, HPβ and HPγ) and several histone deacetylases (HDACs) were also evaluated to assess chromatin status of the host. Our results reveal morphological disruption of all studied subnuclear domains and severe reduction of viral progeny in PML-knockdown cells. ASFV promotes H3K9me3 and HP1β foci formation from early infection, followed by HP1α and HDAC2 nuclear enrichment, suggesting heterochromatinization of host genome. Finally, closeness between DNA damage response factors, disrupted PML-NBs, and virus-induced heterochromatic regions were identified. In sum, our results demonstrate that ASFV orchestrates spatio-temporal nuclear rearrangements, changing subnuclear domains, relocating Ataxia Telangiectasia Mutated Rad-3 related (ATR)-related factors and promoting heterochromatinization, probably controlling transcription, repressing host gene expression, and favouring viral replication.

A novel IL-17 signaling pathway controlling keratinocyte proliferation and tumorigenesis via the TRAF4-ERK5 axis

Wu et al. report a novel IL-17–mediated cascade via the IL-17R–TRAF4–ERK5 axis that directly stimulates keratinocyte proliferation and skin tumor formation in mice.

Although IL-17 is emerging as an important cytokine in cancer promotion and progression, the underlining molecular mechanism remains unclear. Previous studies suggest that IL-17 (IL-17A) sustains a chronic inflammatory microenvironment that favors tumor formation. Here we report a novel IL-17–mediated cascade via the IL-17R–Act1–TRAF4–MEKK3–ERK5 positive circuit that directly stimulates keratinocyte proliferation and tumor formation. Although this axis dictates the expression of target genes Steap4 (a metalloreductase for cell metabolism and proliferation) and p63 (a transcription factor for epidermal stem cell proliferation), Steap4 is required for the IL-17–induced sustained expansion of p63⁺ basal cells in the epidermis. P63 (a positive transcription factor for the Traf4 promoter) induces TRAF4 expression in keratinocytes. Thus, IL-17–induced Steap4-p63 expression forms a positive feedback loop through p63-mediated TRAF4 expression, driving IL-17–dependent sustained activation of the TRAF4–ERK5 axis for keratinocyte proliferation and tumor formation.

The mitogen-activated protein kinase ERK5 regulates the development and growth of hepatocellular carcinoma

OBJECTIVE

The extracellular signal-regulated kinase 5 (ERK5 or BMK1) is involved in tumour development. The ERK5 gene may be amplified in hepatocellular carcinoma (HCC), but its biological role has not been clarified. In this study, we explored the role of ERK5 expression and activity in HCC in vitro and in vivo.

DESIGN

ERK5 expression was evaluated in human liver tissue. Cultured HepG2 and Huh-7 were studied after ERK5 knockdown by siRNA or in the presence of the specific pharmacological inhibitor, XMD8-92. The role of ERK5 in vivo was assessed using mouse Huh-7 xenografts.

RESULTS

In tissue specimens from patients with HCC, a higher percentage of cells with nuclear ERK5 expression was found both in HCC and in the surrounding cirrhotic tissue compared with normal liver tissue. Inhibition of ERK5 decreased HCC cell proliferation and increased the proportion of cells in G0/G1 phase. These effects were associated with increased expression of p27 and p15 and decreased CCND1. Treatment with XMD8-92 or ERK5 silencing prevented cell migration induced by epidermal growth factor or hypoxia and caused cytoskeletal remodelling. In mouse xenografts, the rate of tumour appearance and the size of tumours were significantly lower when Huh-7 was silenced for ERK5. Moreover, systemic treatment with XMD8-92 of mice with established HCC xenografts markedly reduced tumour growth and decreased the expression of the proto-oncogene c-Rel.

CONCLUSIONS

ERK5 regulates the biology of HCC cells and modulates tumour development and growth in vivo. This pathway should be investigated as a possible therapeutic target in HCC.

ERK5 positively regulates cigarette smoke-induced urocystic epithelial-mesenchymal transition in SV‑40 immortalized human urothelial cells

Bladder cancer is universally acknowledged as a significant public health issue. Abundant evidence shows that cigarette smoke (CS) is the primary risk factor for bladder cancer. However, the mechanism of CS-induced bladder cancer has not been fully elucidated. CS-induced epithelial-mesenchymal transition (EMT) is critically involved in cell malignant transformation. The role of ERK5, the lesser studied member of the MAPK family, in regulating CS-triggered EMT has not yet been investigated. The objective of the present study was to investigate the regulatory role of ERK5 in CS-induced urocystic EMT. SV-40 immortalized normal human urothelial cells (SV-HUC-1) were used as in vitro CS exposure models. EMT phenotypic alterations were assessed by changes in cell morphology, invasive capacity, as well as expression of epithelial and mesenchymal markers. Protein and mRNA expression levels were analyzed by western blotting and quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). ERK5 inhibition studies were performed with a specific inhibitor. Exposure of SV-HUC-1 cells to CS induced morphological change, enhanced invasive capacity, reduced epithelial marker expression and increased mesenchymal marker expression. Importantly, we demonstrated for the first time that ERK5 positively regulated CS-mediated EMT in urothelial cells, as evidenced by the findings that CS promoted ERK5 activation, and that the CS-triggered alteration in the EMT phenotype was reversed by ERK5 inhibition.

Therapeutic potential of ERK5 targeting in triple negative breast cancer

Triple negative breast cancers (TNBCs) account for 15% of all breast cancers, and represent one of the most aggressive forms of the disease, exhibiting short relapse-free survival. In contrast to other breast cancer subtypes, the absence of knowledge about the etiopathogenic alterations that cause TNBCs force the use of chemotherapeutics to treat these tumors. Because of this, efforts have been devoted with the aim of incorporating novel therapies into the clinical setting. Kinases play important roles in the pathophysiology of several tumors, including TNBC. Since expression of the MAP kinase ERK5 has been linked to patient outcome in breast cancer, we analyzed the potential value of its targeting in TNBC. ERK5 was frequently overexpressed and active in samples from patients with TNBC, as well as in explants from mice carrying genetically-defined TNBC tumors. Moreover, expression of ERK5 was linked to a worse prognosis in TNBC patients. Knockdown experiments demonstrated that ERK5 supported proliferation of TNBC cells. Pharmacological inhibition of ERK5 with TG02, a clinical stage inhibitor which targets ERK5 and other kinases, inhibited cell proliferation by blocking passage of cells through G₁ and G₂, and also triggered apoptosis in certain TNBC cell lines. TG02 had significant antitumor activity in a TNBC xenograft model in vivo, and also augmented the activity of chemotherapeutic agents commonly used to treat TNBC. Together, these data indicate that ERK5 targeting may represent a valid strategy against TNBC, and support the development of trials aimed at evaluating the clinical effectiveness of drugs that block this kinase.

Extracellular signal-regulated kinase 5 promotes acute cellular and systemic inflammation

Inflammatory critical illness is a syndrome that is characterized by acute inflammation and organ injury, and it is triggered by infections and noninfectious tissue injury, both of which activate innate immune receptors and pathways. Although reports suggest an anti-inflammatory role for the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase 5 (ERK5), we previously found that ERK5 mediates proinflammatory responses in primary human cells in response to stimulation of Toll-like receptor 2 (TLR2). We inhibited the kinase activities and reduced the abundances of ERK5 and MEK5, a MAPK kinase directly upstream of ERK5, in primary human vascular endothelial cells and monocytes, and found that ERK5 promoted inflammation induced by a broad range of microbial TLR agonists and by the proinflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Furthermore, we found that inhibitors of MEK5 or ERK5 reduced the plasma concentrations of proinflammatory cytokines in mice challenged with TLR ligands or heat-killed Staphylococcus aureus, as well as in mice that underwent sterile lung ischemia-reperfusion injury. Finally, we found that inhibition of ERK5 protected endotoxemic mice from death. Together, our studies support a proinflammatory role for ERK5 in primary human endothelial cells and monocytes, and suggest that ERK5 is a potential therapeutic target in diverse disorders that cause inflammatory critical illness.

Dual role of ERK5 in the regulation of T cell receptor expression at the T cell surface

Regulation of the levels of the TCR/CD3 complex at the cell surface is critical to proper T cell development and mature T cell activation. We provide evidence that the MAPK ERK5 regulates the surface expression of the TCR/CD3 complex by controlling the degradation of the CD3ζ chain and the recovery of the complex after anti-CD3ε stimulation. ERK5 knockdown led to TCR/CD3 up-regulation at the cell surface and increased amounts of the CD3ζ chain. Inhibition of the MEK5-dependent phosphorylation status of the kinase domain of ERK5 in human T CD4(+) cells reduced CD3ζ ubiquitination and degradation, limiting TCR/CD3 down-regulation in anti-CD3-stimulated cells. Moreover, TCR/CD3 recovery at the cell surface, after anti-CD3ε treatment, is impaired by ERK5 knockdown or pharmacological inhibition of autophosphorylation in the ERK5 C-terminal region. ERK5 loss in thymocytes augmented cellular CD3ζ and increased cell surface levels of TCR/CD3 on CD4(+)CD8(+) thymocytes. This correlated with enhanced generation of CD4(+)CD8(-)CD25(+) thymocytes. Our findings define ERK5 as a novel kinase that modulates the levels of TCR/CD3 at the cell surface by promoting CD3ζ degradation and TCR/CD3 recovery after TCR stimulation.

The role of ERK5 in endothelial cell function

Extracellular-signal-regulated kinase 5 (ERK5), also termed big MAPK1 (BMK1), is the most recently discovered member of the mitogen-activated protein kinase (MAPK) family. It is expressed in a variety of tissues and is activated by a range of growth factors, cytokines and cellular stresses. Targeted deletion of Erk5 in mice has revealed that the ERK5 signalling cascade is critical for normal cardiovascular development and vascular integrity. In vitro studies have revealed that, in endothelial cells, ERK5 is required for preventing apoptosis, mediating shear-stress signalling and regulating tumour angiogenesis. The present review focuses on our current understanding of the role of ERK5 in regulating endothelial cell function.

Identification of ponatinib and other known kinase inhibitors with potent MEKK2 inhibitory activity

The kinase MEKK2 (MAP3K2) may play an important role in tumor growth and metastasis for several cancer types. Thus, targeting MEKK2 may represent a novel strategy for developing more effective therapies for cancer. In order to identify small molecules with MEKK2 inhibitory activity, we screened a collection of known kinase inhibitors using a high throughput MEKK2 intrinsic ATPase enzyme assay and confirmed activity of the most potent hits with this primary assay. We also confirmed activities of these known kinase inhibitors with an MEKK2 transphosphorylation slot blot assay using MKK6 as a substrate. We observed a good correlation in potencies between the two orthogonal MEKK2 kinase activity assay formats for this set of inhibitors. We report that ponatinib, AT9283, AZD7762, JNJ-7706621, PP121 and hesperadin had potent MEKK2 enzyme inhibitory activities ranging from 4.7 to 60 nM IC50. Ponatinib is an FDA-approved drug that potently inhibited MEKK2 enzyme activity with IC50 values of 10-16 nM. AT9283 is currently in clinical trials and produced MEKK2 IC50 values of 4.7-18 nM. This set of known kinase inhibitors represents some of the most potent in vitro MEKK2 inhibitors reported to date and may be useful as research tools. Although these compounds are not selective for MEKK2, the structures of these compounds give insight into pharmacophores that potently inhibit MEKK2 and could be used as initial leads to design highly selective inhibitors of MEKK2.

Identification and validation of dysregulated MAPK7 (ERK5) as a novel oncogenic target in squamous cell lung and esophageal carcinoma

Background

MAPK7/ERK5 (extracellular-signal-regulated kinase 5) functions within a canonical three-tiered MAPK (mitogen activated protein kinase) signaling cascade comprising MEK (MAPK/ERK kinase) 5, MEKK(MEK kinase) 2/3 and ERK5 itself. Despite being the least well studied of the MAPK-modules, evidence supports a role for MAPK7-signaling in the pathology of several cancer types.

Methods and results

Fluorescence in situ hybridization (FISH) analysis identified MAPK7 gene amplification in 4 % (3/74) of non-small cell lung cancers (NSCLC) (enriched to 6 % (3/49) in squamous cell carcinoma) and 2 % (2/95) of squamous esophageal cancers (sqEC). Immunohistochemical (IHC) analysis revealed a good correlation between MAPK7 gene amplification and protein expression. MAPK7 was validated as a proliferative oncogenic driver by performing in vitro siRNA knockdown of MAPK7 in tumor cell lines. Finally, a novel MEK5/MAPK7 co-transfected HEK293 cell line was developed and used for routine cell-based pharmacodynamic screening. Phosphorylation antibody microarray analysis also identified novel downstream pharmacodynamic (PD) biomarkers of MAPK7 kinase inhibition in tumor cells (pMEF2A and pMEF2D).

Conclusions

Together, these data highlight a broader role for dysregulated MAPK7 in driving tumorigenesis within niche populations of highly prevalent tumor types, and describe current efforts in establishing a robust drug discovery screening cascade.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1455-y) contains supplementary material, which is available to authorized users.

The Potential of Vitamin D-Regulated Intracellular Signaling Pathways as Targets for Myeloid Leukemia Therapy

The current standard regimens for the treatment of acute myeloid leukemia (AML) are curative in less than half of patients; therefore, there is a great need for innovative new approaches to this problem. One approach is to target new treatments to the pathways that are instrumental to cell growth and survival with drugs that are less harmful to normal cells than to neoplastic cells. In this review, we focus on the MAPK family of signaling pathways and those that are known to, or potentially can, interact with MAPKs, such as PI3K/AKT/FOXO and JAK/STAT. We exemplify the recent studies in this field with specific relevance to vitamin D and its derivatives, since they have featured prominently in recent scientific literature as having anti-cancer properties. Since microRNAs also are known to be regulated by activated vitamin D, this is also briefly discussed here, as are the implications of the emerging acquisition of transcriptosome data and potentiation of the biological effects of vitamin D by other compounds. While there are ongoing clinical trials of various compounds that affect signaling pathways, more studies are needed to establish the clinical utility of vitamin D in the treatment of cancer.

Palytoxin induces dissociation of HSP 27 oligomers through a p38 protein kinase pathway

Palytoxin (PlTX) induces a stress response in MCF-7 cells that involves the phosphorylation of HSP 27 at serines 15, 78, and 82 by an as yet undetermined mechanism. We have studied the involvement of major groups of the mitogen-activated protein kinase (MAPK) family in this molecular response and focused our analyses on the ERK1/2, JNK, p38 protein kinase (p38K), and ERK5 pathways. The results show that PlTX induces the activation of JNK and p38 kinase but not ERK1/2 and 5 in MCF-7 cells. Through the use of protein kinase inhibitors, we established that blocking p38K, but not JNK, prevents the phosphorylation of HSP 27 induced by PlTX and that MAPKAPK2 participates in the response induced by the toxin under our experimental conditions. The cell death response induced by PlTX was inhibited by preventing JNK phosphorylation but not by blocking p38K/MAPKAPK2 and HSP 27 phosphorylation. Sucrose density gradient centrifugation revealed that MCF-7 cell extracts contain a heterodisperse population of HSP 27, including oligomers and smaller forms. Treating MCF-7 cells with PlTX caused the dissociation of HSP 27 oligomers, and using inhibitors of the JNK and p38K pathways showed that the dissociation of HSP 27 oligomers induced by PlTX involves a p38K-dependent process. We conclude that the changes induced by PlTX in the HSP 27 stress response protein system proceed through a molecular mechanism involving the activation of the p38 kinase pathway and its substrate, MAPKAK2, leading to dissociation of HSP 27 oligomers and the stabilization of a cellular pool of monomers phosphorylated at serines 15, 78 and 82, which could play a protective role against the death response induced by PlTX.

Phosphorylation of ERK5 on Thr732 is associated with ERK5 nuclear localization and ERK5-dependent transcription

Extracellular signal-regulated kinases (ERKs) play critical roles in numerous cellular processes, including proliferation and differentiation. ERK5 contains a kinase domain at the N-terminal, and the unique extended C-terminal includes multiple autophosphorylation sites that enhance ERK5-dependent transcription. However, the impact of phosphorylation at the various sites remain unclear. In this study, we examined the role of phosphorylation at the ERK5 C-terminal. We found that a constitutively active MEK5 mutant phosphorylated ERK5 at the TEY motif, resulting in the sequential autophosphorylation of multiple C-terminal residues, including Thr732 and Ser769/773/775. However, when ERK1/2 was selectively activated by an oncogenic RAS mutant, ERK5 phosphorylation at Thr732 was induced without affecting the phosphorylation status at TEY or Ser769/773/775. The Thr732 phosphorylation was U0126-sensitive and was observed in a kinase-dead mutant of ERK5 as well, suggesting that ERK1/2 can phosphorylate ERK5 at Thr732. This phosphorylation was also promoted by epidermal growth factor and nerve growth factor in HEK293 and PC12 cells, respectively. The ERK5–T732A mutant was localized in the cytosol under basal conditions. In contrast, ERK5 phosphorylated at Thr732 via the RAS-ERK1/2 pathway and ERK5–T732E, which mimics the phosphorylated form, were localized in both the nucleus and cytosol. Finally, ER–32A and U0126 blocked ERK5-dependent MEF2C transcriptional activity. Based on these findings, we propose a novel cross-talk mechanism in which ERK1/2, following activation by growth factor stimulation, phosphorylates ERK5 at Thr732. This phosphorylation event is responsible for ERK5 nuclear localization and ERK5-dependent transcription.

ERK5 is a critical mediator of inflammation-driven cancer

Chronic inflammation is a hallmark of many cancers, yet the pathogenic mechanisms that distinguish cancer-associated inflammation from benign persistent inflammation are still mainly unclear. Here, we report that the protein kinase ERK5 controls the expression of a specific subset of inflammatory mediators in the mouse epidermis, which triggers the recruitment of inflammatory cells needed to support skin carcinogenesis. Accordingly, inactivation of ERK5 in keratinocytes prevents inflammation-driven tumorigenesis in this model. In addition, we found that anti-ERK5 therapy cooperates synergistically with existing antimitotic regimens, enabling efficacy of subtherapeutic doses. Collectively, our findings identified ERK5 as a mediator of cancer-associated inflammation in the setting of epidermal carcinogenesis. Considering that ERK5 is expressed in almost all tumor types, our findings suggest that targeting tumor-associated inflammation via anti-ERK5 therapy may have broad implications for the treatment of human tumors.

Enzyme inhibitor discovery by activity-based protein profiling

Eukaryotic and prokaryotic organisms possess huge numbers of uncharacterized enzymes. Selective inhibitors offer powerful probes for assigning functions to enzymes in native biological systems. Here, we discuss how the chemical proteomic platform activity-based protein profiling (ABPP) can be implemented to discover selective and in vivo-active inhibitors for enzymes. We further describe how these inhibitors have been used to delineate the biochemical and cellular functions of enzymes, leading to the discovery of metabolic and signaling pathways that make important contributions to human physiology and disease. These studies demonstrate the value of selective chemical probes as drivers of biological inquiry.

A Phospho-SIM in the Antiviral Protein PML is Required for Its Recruitment to HSV-1 Genomes

Herpes simplex virus type 1 (HSV-1) is a significant human pathogen that infects a large portion of the human population. Cells deploy a variety of defenses to limit the extent to which the virus can replicate. One such factor is the promyelocytic leukemia (PML) protein, the nucleating and organizing factor of nuclear domain 10 (ND10). PML responds to a number of stimuli and is implicated in intrinsic and innate cellular antiviral defenses against HSV-1. While the role of PML in a number of cellular pathways is controlled by post-translational modifications, the effects of phosphorylation on its antiviral activity toward HSV-1 have been largely unexplored. Consequently, we mapped phosphorylation sites on PML, mutated these and other known phosphorylation sites on PML isoform I (PML-I), and examined their effects on a number of PML's activities. Our results show that phosphorylation at most sites on PML-I is dispensable for the formation of ND10s and colocalization between PML-I and the HSV-1 regulatory protein, ICP0, which antagonizes PML-I function. However, inhibiting phosphorylation at sites near the SUMO-interaction motif (SIM) of PML-I impairs its ability to respond to HSV-1 infection. Overall, our data suggest that PML phosphorylation regulates its antiviral activity against HSV-1.

The kinase ALK stimulates the kinase ERK5 to promote the expression of the oncogene MYCN in neuroblastoma

Anaplastic lymphoma kinase (ALK) is an important molecular target in neuroblastoma. Although tyrosine kinase inhibitors abrogating ALK activity are currently in clinical use for the treatment of ALK-positive (ALK(+)) disease, monotherapy with ALK tyrosine kinase inhibitors may not be an adequate solution for ALK(+) neuroblastoma patients. Increased expression of the gene encoding the transcription factor MYCN is common in neuroblastomas and correlates with poor prognosis. We found that the kinase ERK5 [also known as big mitogen-activated protein kinase (MAPK) 1 (BMK1)] is activated by ALK through a pathway mediated by phosphoinositide 3-kinase (PI3K), AKT, MAPK kinase kinase 3 (MEKK3), and MAPK kinase 5 (MEK5). ALK-induced transcription of MYCN and stimulation of cell proliferation required ERK5. Pharmacological or RNA interference-mediated inhibition of ERK5 suppressed the proliferation of neuroblastoma cells in culture and enhanced the antitumor efficacy of the ALK inhibitor crizotinib in both cells and xenograft models. Together, our results indicate that ERK5 mediates ALK-induced transcription of MYCN and proliferation of neuroblastoma, suggesting that targeting both ERK5 and ALK may be beneficial in neuroblastoma patients.

Designed Macrocyclic Kinase Inhibitors

Cancer continues to present as an increasing and serious global unmet medical need in today's aging population.1 Macrocyclic kinase inhibitors have reached advanced clinical testing and are making an impact in oncologic conditions including myelofibrosis, lymphomas and leukemias. Rheumatoid arthritis (RA) is also beginning to be impacted with the first macrocycle having entered Phase I clinical evaluation in healthy volunteers. Increasing reports of innovative macrocycles in preclinical research are appearing in the literature. Desirable, selective, multi-kinase inhibitory profiles against specific kinases known to be abrogated in cancer, RA, and other diseases have been achieved in a first generation series of clinical stage compact small molecule macrocyclic kinase inhibitors. Herein we discuss their design, synthesis, structure activity relationships and assessment of the latest clinical data in a range of oncologic conditions. Macrocyclic kinase inhibitors have the potential to offer new hope to patients and their families.

ERK 5/MAPK pathway has a major role in 1α,25-(OH)2 vitamin D3-induced terminal differentiation of myeloid leukemia cells

Vitamin D derivatives, including its physiological form 1α,25(OH)2 vitamin D3 (1,25D), have anti-tumor actions demonstrated in cell culture and confirmatory epidemiological associations are frequently reported. However, their promise for use in the cancer clinic is still incompletely fulfilled, suggesting that a better understanding of the molecular events initiated by these compounds is needed for therapeutic advances. While ERK1/2 has been intensely investigated and is known to transmit signals for cell survival, growth, and differentiation, the role of other MAPK pathways has been studied sporadically. Therefore, we utilized acute myeloid leukemia (AML) cells in culture (HL60 and U937), to determine if ERK5 has a role in 1,25D-induced terminal differentiation which is distinct from the previously shown involvement of ERK1/2. We previously found that inhibition of kinase activity of ERK5 by specific pharmacological inhibitors BIX02189 or XMD8-92 results in higher expression of general myeloid marker CD11b, but a lower expression of the monocytic marker CD14. In contrast, the inhibition of the ERK1/2 pathway by PD98059 or U0126 reduced the expression of all differentiation markers studied. We report here for the first time that the differentiation changes induced by ERK5 inhibitors are accompanied by the inhibition of cell proliferation, and this occurs in the both G1 and G2 phases of the cell cycle. Of note, inhibition of ERK5 auto-phosphorylation by XMD8-92 results in a particularly robust cell cycle arrest in G2 phase in AML cells. This study provides a link between the 1,25D-elevated ERK5 pathway and changes in the cell cycle phase transitions in AML cells. Thus, combinations of vitamin D derivatives and ERK5 inhibitors may be more successful in cancer clinics than 1,25D or analogs alone. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

XMD8-92 inhibits pancreatic tumor xenograft growth via a DCLK1-dependent mechanism

XMD8-92 is a kinase inhibitor with anti-cancer activity against lung and cervical cancers, but its effect on pancreatic ductal adenocarcinoma (PDAC) remains unknown. Doublecortin-like kinase1 (DCLK1) is upregulated in various cancers including PDAC. In this study, we showed that XMD8-92 inhibits AsPC-1 cancer cell proliferation and tumor xenograft growth. XMD8-92 treated tumors demonstrated significant downregulation of DCLK1 and several of its downstream targets (including c-MYC, KRAS, NOTCH1, ZEB1, ZEB2, SNAIL, SLUG, OCT4, SOX2, NANOG, KLF4, LIN28, VEGFR1, and VEGFR2) via upregulation of tumor suppressor miRNAs let-7a, miR-144, miR-200a-c, and miR-143/145; it did not however affect BMK1 downstream genes p21 and p53. These data taken together suggest that XMD8-92 treatment results in inhibition of DCLK1 and downstream oncogenic pathways (EMT, pluripotency, angiogenesis and anti-apoptotic), and is a promising chemotherapeutic agent against PDAC.

ERK5 pathway regulates transcription factors important for monocytic differentiation of human myeloid leukemia cells

Mitogen-activated protein kinases (MAPKs) are important transducers of external signals for cell growth, survival, and other cellular responses including cell differentiation. Several MAPK cascades are known with the MEK1/2-ERK1/2, JNK, and p38MAPKs receiving most attention, but the role of MEK5-ERK5 in intracellular signaling deserves more scrutiny, as this pathway transmits signals that can complement ERK/2 signaling. We hypothesized that the ERK5 pathway plays a role in the control of monocytic differentiation, which is disturbed in myeloid leukemia. We therefore examined the cellular phenotype and key molecular events which occur when human myeloid leukemia cells, acute (AML) or chronic (CML), are forced to differentiate by vitamin D derivatives (VDDs). This study was performed using established cell lines HL60 and U937, and primary cultures of blasts from 10 patients with ML. We found that ERK5 and its direct downstream target transcription factor MEF2C are upregulated by 1,25D in parallel with monocytic differentiation. Further, inhibition of ERK5 activity by specific pharmacological agents BIX02189 and XMD8-92 alters the phenotype of these cells by reducing the abundance of the VDD-induced surface monocytic marker CD14, and concomitantly increasing surface expression of the general myeloid marker CD11b. Similar results were obtained when the expression of ERK5 was reduced by siRNA or short hairpin (sh) RNA. ERK5 inhibition resulted in an expected decrease in MEF2C activation. We also found that in AML cells the transcription factor C/EBPβ is positively regulated, while C/EBPα is negatively regulated by ERK5. These findings provide new understanding of dysregulated differentiation in human myeloid leukemia.

A positive feedback loop involving Erk5 and Akt turns on mesangial cell proliferation in response to PDGF

Platelet-derived growth factor BB and its receptor (PDGFRβ) play a pivotal role in the development of renal glomerular mesangial cells. Their roles in increased mesangial cell proliferation during mesangioproliferative glomerulonephritis have long been noted, but the operating logic of signaling mechanisms regulating these changes remains poorly understood. We examined the role of a recently identified MAPK, Erk5, in this process. PDGF increased the activating phosphorylation of Erk5 and tyrosine phosphorylation of proteins in a time-dependent manner. A pharmacologic inhibitor of Erk5, XMD8-92, abrogated PDGF-induced DNA synthesis and mesangial cell proliferation. Similarly, expression of dominant negative Erk5 or siRNAs against Erk5 blocked PDGF-stimulated DNA synthesis and proliferation. Inhibition of Erk5 attenuated expression of cyclin D1 mRNA and protein, resulting in suppression of CDK4-mediated phosphorylation of the tumor suppressor protein pRb. Expression of cyclin D1 or CDK4 prevented the dominant negative Erk5- or siErk5-mediated inhibition of DNA synthesis and mesangial cell proliferation induced by PDGF. We have previously shown that phosphatidylinositol 3-kinase (PI3-kinase) contributes to PDGF-induced proliferation of mesangial cells. Inhibition of PI3-kinase blocked PDGF-induced phosphorylation of Erk5. Since PI3-kinase acts through Akt, we determined the role of Erk5 on Akt phosphorylation. XMD8-92, dominant negative Erk5, and siErk5 inhibited phosphorylation of Akt by PDGF. Interestingly, we found inhibition of PDGF-induced Erk5 phosphorylation by a pharmacological inhibitor of Akt kinase and kinase dead Akt in mesangial cells. Thus our data unfold the presence of a positive feedback microcircuit between Erk5 and Akt downstream of PI3-kinase nodal point for PDGF-induced mesangial cell proliferation.

Interferon-γ induces leucine-rich repeat kinase LRRK2 via extracellular signal-regulated kinase ERK5 in macrophages

The gene encoding leucine-rich repeat kinase 2 (LRRK2) comprises a major risk factor for Parkinson's disease. Recently, it has emerged that LRRK2 plays important roles in the immune system. LRRK2 is induced by interferon-γ (IFN-γ) in monocytes, but the signaling pathway is not known. Here, we show that IFN-γ-mediated induction of LRRK2 was suppressed by pharmacological inhibition and RNA interference of the extracellular signal-regulated kinase 5 (ERK5). This was confirmed by LRRK2 immunostaining, which also revealed that the morphological responses to IFN-γ were suppressed by ERK5 inhibitor treatment. Both human acute monocytic leukemia THP-1 cells and human peripheral blood monocytes stimulated the ERK5-LRRK2 pathway after differentiation into macrophages. Thus, LRRK2 is induced via a novel, ERK5-dependent IFN-γ signal transduction pathway, pointing to new functions of ERK5 and LRRK2 in human macrophages. Leucine-rich repeat kinase 2 (LRRK2) is a major risk factor for the development of Parkinson's disease (PD). However, the role of LRRK2 in the affected neurons remains enigmatic. Recently, LRRK2 has been reported to be strongly expressed in the immune system. Here, we demonstrate that LRRK2 is induced by Interferon gamma via extracellular signal-regulated kinase 5 (ERK5) in macrophages, thus providing new insights in LRRK2 and ERK5 biology.

Novel roles for ERK5 and cofilin as critical mediators linking ERα-driven transcription, actin reorganization, and invasiveness in breast cancer

Cancer cell motility and invasiveness are fundamental characteristics of the malignant phenotype and are regulated through diverse signaling networks involving kinases and transcription factors. This study establishes an estrogen receptor (ERα)/MAPK (ERK5)/cofilin (CFL1) network that specifies the degree of breast cancer cell aggressiveness through coupling of actin reorganization and hormone receptor-mediated transcription. Using dominant negative and constitutively active forms, as well as small-molecule inhibitors of extracellular signal-regulated kinase (ERK)5 and MAP-ERK kinase (MEK)5, it was revealed that hormone activation of ERα determined the subcellular localization of ERK5, which functions as a coregulator of ERα-dependent gene transcription. Notably, ERK5 acted in concert with the actin remodeling protein, CFL1, and upon hormone exposure, both localized to active nuclear transcriptional hubs as verified by immunofluorescence and proximity ligation assays. Both ERK5 and CFL1 facilitated PAF1 recruitment to the RNA Pol II complex and both were required for regulation of gene transcription. In contrast, in cells lacking ERα, ERK5 and CFL1 localized to cytoplasmic membrane regions of high actin remodeling, promoting cell motility and invasion, thereby revealing a mechanism likely contributing to the generally poorer prognosis of patients with ERα-negative breast cancer. Thus, this study uncovers the dynamic interplay of nuclear receptor-mediated transcription and actin reorganization in phenotypes of breast cancer aggressiveness.

IMPLICATIONS

Identification of the ER/ERK5/CFL1 axis suggests new prognostic biomarkers and novel therapeutic avenues to moderate cancer aggressiveness.

ERK5/BMK1 is a novel target of the tumor suppressor VHL: implication in clear cell renal carcinoma

Extracellular signal-regulated kinase 5 (ERK5), also known as big mitogen-activated protein kinase (MAPK) 1, is implicated in a wide range of biologic processes, which include proliferation or vascularization. Here, we show that ERK5 is degraded through the ubiquitin-proteasome system, in a process mediated by the tumor suppressor von Hippel-Lindau (VHL) gene, through a prolyl hydroxylation-dependent mechanism. Our conclusions derive from transient transfection assays in Cos7 cells, as well as the study of endogenous ERK5 in different experimental systems such as MCF7, HMEC, or Caki-2 cell lines. In fact, the specific knockdown of ERK5 in pVHL-negative cell lines promotes a decrease in proliferation and migration, supporting the role of this MAPK in cellular transformation. Furthermore, in a short series of fresh samples from human clear cell renal cell carcinoma, high levels of ERK5 correlate with more aggressive and metastatic stages of the disease. Therefore, our results provide new biochemical data suggesting that ERK5 is a novel target of the tumor suppressor VHL, opening a new field of research on the role of ERK5 in renal carcinomas.

Phosphoproteomic evaluation of pharmacological inhibition of leucine-rich repeat kinase 2 reveals significant off-target effects of LRRK-2-IN-1

Genetic mutations in leucine-rich repeat kinase 2 (LRRK2) have been linked to autosomal dominant Parkinson's disease. The most prevalent mutation, G2019S, results in enhanced LRRK2 kinase activity that potentially contributes to the etiology of Parkinson's disease. Consequently, disease progression is potentially mediated by poorly characterized phosphorylation-dependent LRRK2 substrate pathways. To address this gap in knowledge, we transduced SH-SY5Y neuroblastoma cells with LRRK2 G2019S via adenovirus, then determined quantitative changes in the phosphoproteome upon LRRK2 kinase inhibition (LRRK2-IN-1 treatment) using stable isotope labeling of amino acids in culture combined with phosphopeptide enrichment and LC-MS/MS analysis. We identified 776 phosphorylation sites that were increased or decreased at least 50% in response to LRRK2-IN-1 treatment, including sites on proteins previously known to associate with LRRK2. Bioinformatic analysis of those phosphoproteins suggested a potential role for LRRK2 kinase activity in regulating pro-inflammatory responses and neurite morphology, among other pathways. In follow-up experiments, LRRK2-IN-1 inhibited lipopolysaccharide-induced tumor necrosis factor alpha (TNFα) and C-X-C motif chemokine 10 (CXCL10) levels in astrocytes and also enhanced multiple neurite characteristics in primary neuronal cultures. However, LRRK2-IN-1 had almost identical effects in primary glial and neuronal cultures from LRRK2 knockout mice. These data suggest LRRK2-IN-1 may inhibit pathways of perceived LRRK2 pathophysiological function independently of LRRK2 highlighting the need to use multiple pharmacological tools and genetic approaches in studies determining LRRK2 function.

Alteration of the ERK5 pathway by hydroxysafflor yellow A blocks expression of MEF2C in activated hepatic stellate cells in vitro: Potential treatment for hepatic fibrogenesis

Abstract Context: Hepatic fibrosis ultimately leads to cirrhosis if not treated effectively. Hepatic stellate cells (HSC) are a main mediator of hepatic fibrosis through the accumulation of extracellular matrix proteins. Suppression activation of passaged HSC has been proposed as therapeutic strategies for the treatment and prevention of hepatic fibrosis. Objective: To evaluate the effect of hydroxysafflor yellow A (HSYA), an active chemical compound derived from the flowers of Carthamus tinctorius L. (Compositae), on HSC inhibition, and to begin elucidating underlying mechanisms. Materials and methods: Primary HSCs were isolated from rats by in situ pronase/collagenase perfusion. Culture-activated HSCs were treated with or without HSYA at 30 μM in the presence or absence of PD98059 for 48 h, and then cell proliferation was measured by MTS assays. Messenger RNA (mRNA) expression was quantified by polymerase chain reaction, and protein was quantified by Western blots or enzyme-linked immunosorbent assays. Results: HSYA significantly inhibits culture-activated HSC proliferation in a dose-dependent and time-dependent manner with an IC50 value of 112.79 μM. HSYA (30 μM) induce the suppression of HSC activation, as indicated by decreases in contents of type I alpha collagen in HSC-cultured media and expression of α-smooth muscle actin protein in culture-activated HSC by 55 and 71%, respectively. HSYA (30 μM) also caused significant decreases in mRNA expression of type III alpha collagen in HSC by 28%. HSYA (30 μM) suppresses myocyte enhancer factor 2 C (MEF2C) expression both at its mRNA and protein levels by 60 and 61%, respectively. Further study demonstrated that HSYA (30 μM) caused significant decreases in p-ERK5 by 49%. Blocking extracellular signal-regulated protein kinase 5 (ERK5) activity by XMD 8--92, an ERK5 inhibitor, markedly abrogated the inhibitive effects of HSYA on HSC activation, and blocked the HSYA-mediated MEF2C down-regulation. Conclusions: HSYA suppress HSC activation by ERK5-mediated MEF2C down-regulation and makes it a potential candidate for prevention and treatment of hepatic fibrogenesis.

Structural determinants for ERK5 (MAPK7) and leucine rich repeat kinase 2 activities of benzo[e]pyrimido-[5,4-b]diazepine-6(11H)-ones

The benzo[e]pyrimido-[5,4-b]diazepine-6(11H)-one core was discovered as a novel ERK5 (also known as MAPK7 and BMK1) inhibitor scaffold, previously. Further structure-activity relationship studies of this scaffold led to the discovery of ERK5-IN-1 (26) as the most selective and potent ERK5 inhibitor reported to date. 26 potently inhibits ERK5 biochemically with an IC₅₀ of 0.162 ± 0.006 μM and in cells with a cellular EC₅₀ for inhibiting epidermal growth factor induced ERK5 autophosphorylation of 0.09 ± 0.03 μM. Furthermore, 26 displays excellent selectivity over other kinases with a KINOMEscan selectivity score (S₁₀) of 0.007, and exhibits exceptional bioavailability (F%) of 90% in mice. 26 will serve as a valuable tool compound to investigate the ERK5 signaling pathway and as a starting point for developing an ERK5 directed therapeutic agent.

MEK5/ERK5 signaling suppresses estrogen receptor expression and promotes hormone-independent tumorigenesis

Endocrine resistance and metastatic progression are primary causes of treatment failure in breast cancer. While mitogen activated protein kinases (MAPKs) are known to promote ligand-independent cell growth, the role of the MEK5-ERK5 pathway in the progression of clinical breast carcinoma remains poorly understood. Here, we demonstrated increased ERK5 activation in 30 of 39 (76.9%) clinical tumor samples, as well as across breast cancer cell systems. Overexpression of MEK5 in MCF-7 cells promoted both hormone-dependent and hormone-independent tumorigenesis in vitro and in vivo and conferred endocrine therapy resistance to previously sensitive breast cancer cells. Expression of MEK5 suppressed estrogen receptor (ER)α, but not ER-β protein levels, and abrogated downstream estrogen response element (ERE) transcriptional activity and ER-mediated gene transcription. Global gene expression changes associated with upregulation of MEK5 included increased activation of ER-α independent growth signaling pathways and promotion of epithelial-to-mesenchymal transition (EMT) markers. Taken together, our findings show that the MEK5-ERK5 pathway mediates progression to an ER(-), mesenchymal and endocrine therapy resistant phenotype. Given the need for new clinical therapeutic targets, our results demonstrate the therapeutic potential of targeting the MEK5-ERK5 pathway in breast cancer.

Profiling protein kinases and other ATP binding proteins in Arabidopsis using Acyl-ATP probes

Many protein activities are driven by ATP binding and hydrolysis. Here, we explore the ATP binding proteome of the model plant Arabidopsis thaliana using acyl-ATP (AcATP)(1) probes. These probes target ATP binding sites and covalently label lysine residues in the ATP binding pocket. Gel-based profiling using biotinylated AcATP showed that labeling is dependent on pH and divalent ions and can be competed by nucleotides. The vast majority of these AcATP-labeled proteins are known ATP binding proteins. Our search for labeled peptides upon in-gel digest led to the discovery that the biotin moiety of the labeled peptides is oxidized. The in-gel analysis displayed kinase domains of two receptor-like kinases (RLKs) at a lower than expected molecular weight, indicating that these RLKs lost the extracellular domain, possibly as a result of receptor shedding. Analysis of modified peptides using a gel-free platform identified 242 different labeling sites for AcATP in the Arabidopsis proteome. Examination of each individual labeling site revealed a preference of labeling in ATP binding pockets for a broad diversity of ATP binding proteins. Of these, 24 labeled peptides were from a diverse range of protein kinases, including RLKs, mitogen-activated protein kinases, and calcium-dependent kinases. A significant portion of the labeling sites could not be assigned to known nucleotide binding sites. However, the fact that labeling could be competed with ATP indicates that these labeling sites might represent previously uncharacterized nucleotide binding sites. A plot of spectral counts against expression levels illustrates the high specificity of AcATP probes for protein kinases and known ATP binding proteins. This work introduces profiling of ATP binding activities of a large diversity of proteins in plant proteomes. The data have been deposited in ProteomeXchange with the identifier PXD000188.

New Strategies to Direct Therapeutic Targeting of PML to Treat Cancers

The tumor suppressor function of the promyelocytic leukemia (PML) protein was first identified as a result of its dysregulation in acute promyelocytic leukemia, however, its importance is now emerging far beyond hematological neoplasms, to an extensive range of malignancies, including solid tumors. In response to stress signals, PML coordinates the regulation of numerous proteins, which activate fundamental cellular processes that suppress tumorigenesis. Importantly, PML itself is the subject of specific post-translational modifications, including ubiquitination, phosphorylation, acetylation, and SUMOylation, which in turn control PML activity and stability and ultimately dictate cellular fate. Improved understanding of the regulation of this key tumor suppressor is uncovering potential opportunities for therapeutic intervention. Targeting the key negative regulators of PML in cancer cells such as casein kinase 2, big MAP kinase 1, and E6-associated protein, with specific inhibitors that are becoming available, provides unique and exciting avenues for restoring tumor suppression through the induction of apoptosis and senescence. These approaches could be combined with DNA damaging drugs and cytokines that are known to activate PML. Depending on the cellular context, reactivation or enhancement of tumor suppressive PML functions, or targeted elimination of aberrantly functioning PML, may provide clinical benefit.

Canonical and kinase activity-independent mechanisms for extracellular signal-regulated kinase 5 (ERK5) nuclear translocation require dissociation of Hsp90 from the ERK5-Cdc37 complex

The mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase 5 (ERK5) plays a crucial role in cell proliferation, regulating gene transcription. ERK5 has a unique C-terminal tail which contains a transcriptional activation domain, and activates transcription by phosphorylating transcription factors and acting itself as a transcriptional coactivator. However, the molecular mechanisms that regulate its nucleocytoplasmatic traffic are unknown. We have used tandem affinity purification to identify proteins that interact with ERK5. We show that ERK5 interacts with the Hsp90-Cdc37 chaperone in resting cells, and that inhibition of Hsp90 or Cdc37 results in ERK5 ubiquitylation and proteasomal degradation. Interestingly, activation of cellular ERK5 induces Hsp90 dissociation from the ERK5-Cdc37 complex, leading to ERK5 nuclear translocation and activation of transcription, by a mechanism which requires the autophosphorylation at its C-terminal tail. Consequently, active ERK5 is no longer sensitive to Hsp90 or Cdc37 inhibitors. Cdc37 overexpression also induces Hsp90 dissociation and the nuclear translocation of a kinase-inactive form of ERK5 which retains transcriptional activity. This is the first example showing that ERK5 transcriptional activity does not require kinase activity. Since Cdc37 cooperates with ERK5 to promote cell proliferation, Cdc37 overexpression (as happens in some cancers) might represent a new, noncanonical mechanism by which ERK5 regulates tumor proliferation.

An ERK-p38 subnetwork coordinates host cell apoptosis and necrosis during coxsackievirus B3 infection

The host response to a virus is determined by intracellular signaling pathways that are modified during infection. These pathways converge as networks and produce interdependent phenotypes, making it difficult to link virus-induced signals and responses at a systems level. Coxsackievirus B3 (CVB3) infection induces death of cardiomyocytes, causing tissue damage and virus dissemination, through incompletely characterized host cell signaling networks. We built a statistical model that quantitatively predicts cardiomyocyte responses from time-dependent measurements of phosphorylation events modified by CVB3. Model analysis revealed that CVB3-stimulated cytotoxicity involves tight coupling between the host ERK and p38 MAPK pathways, which are generally thought to control distinct cellular responses. The kinase ERK5 requires p38 kinase activity and inhibits apoptosis caused by CVB3 infection. By contrast, p38 indirectly promotes apoptosis via ERK1/2 inhibition but directly causes CVB3-induced necrosis. Thus, the cellular events governing pathogenesis are revealed when virus-host programs are monitored systematically and deconvolved mathematically.

Post-translational modifications of PML: consequences and implications

The tumor suppressor promyelocytic leukemia protein (PML) predominantly resides in a structurally distinct sub-nuclear domain called PML nuclear bodies. Emerging evidences indicated that PML actively participates in many aspects of cellular processes, but the molecular mechanisms underlying PML regulation in response to stress and environmental cues are not complete. Post-translational modifications, such as SUMOylation, phosphorylation, acetylation, and ubiquitination of PML add a complex layer of regulation to the physiological function of PML. In this review, we discuss the fast-moving horizon of post-translational modifications targeting PML.

Regulation of the tumor suppressor PML by sequential post-translational modifications

Post-translational modifications (PTMs) regulate multiple biological functions of the promyelocytic leukemia (PML) protein and also the fission, disassembly, and rebuilding of PML nuclear bodies (PML-NBs) during the cell cycle. Pathway-specific PML modification patterns ensure proper signal output from PML-NBs that suit the specific functional requirements. Here we comprehensively review the signaling pathways and enzymes that modify PML and also the oncogenic PML-RARα fusion protein. Many PTMs occur in a hierarchical and timely organized fashion. Phosphorylation or acetylation constitutes typical starting points for many PML modifying events, while degradative ubiquitination is an irreversible end point of the modification cascade. As this hierarchical organization of PTMs frequently turns phosphorylation events as primordial events, kinases or phosphatases regulating PML phosphorylation may be interesting drug targets to manipulate the downstream modifications and thus the stability and function of PML or PML-RARα.

The extracellular-regulated protein kinase 5 (ERK5) promotes cell proliferation through the down-regulation of inhibitors of cyclin dependent protein kinases (CDKs)

Activation of the extracellular-regulated protein kinase 5 (ERK5) has been associated with mitogenic signal transduction. However, conflicting findings have challenged the idea that ERK5 is a critical regulator of cell proliferation. We have addressed this issue by testing the effect of the conditional loss of ERK5 in primary fibroblasts. We have discovered that ERK5 suppressed the expression of the cyclin dependent protein kinase (CDKs) inhibitors, p21 and p27, by decreasing mRNA and protein stability, respectively. As a result, low level CDK2 activity detected in ERK5-deficient cells correlated with a defect in G1 to S phase transition of the cell cycle. Similarly, we found that the malignant MDA-MB-231 human breast cancer cell line was dependent on ERK5 to proliferate. We propose that ERK5 blocks p21 expression in MDA-MB-231 cells via a mechanism that implicates c-Myc-dependent transcriptional regulation of the miR-17-92 cluster. Together with evidence that cancer patients with poor prognosis display a high level of expression of components of the ERK5 signaling pathway, these findings support the hypothesis that ERK5 can be a potential target for cancer therapy.

Development and validation of a high-throughput intrinsic ATPase activity assay for the discovery of MEKK2 inhibitors

The kinase MEKK2 (MAP3K2) has recently been implicated in tumor growth and metastasis. Thus, selective inhibition of MEKK2 may be a novel strategy for cancer therapy. To identify inhibitors of MEKK2 kinase activity, we have developed a novel activity assay for MEKK2 based on the discovery that recombinant purified MEKK2 has intrinsic ATPase activity. This MEKK2 ATPase assay was validated for enzyme identity and enzymatic purity by multiple methods including mass spectrometry analysis, testing different sources of MEKK2 and comparing ATPase assay IC50 data for multiple inhibitors to literature values and to IC50 data generated using MEKK2 binding and transphosphorylation assays. Taken together, these data indicated that genuine MEKK2 activity was being measured in this assay and no other ATPases contributed to the signal. A miniaturized version of the assay was validated for high-throughput screening, and compound libraries were screened. The screening hits generated comparable potencies in the MEKK2 intrinsic ATPase, binding, and transphosphorylation assays. We identified a novel MEKK2 inhibitor and confirmed that crizotinib and bosutinib are potent in vitro inhibitors of MEKK2 activity with IC50 values of <100 nM. Thus, this assay has utility for the discovery of small-molecule inhibitors of MEKK2 activity.

Defining MAP3 kinases required for MDA-MB-231 cell tumor growth and metastasis

Analysis of patient tumors suggests that multiple MAP3 kinases (MAP3Ks) are critical for growth and metastasis of cancer cells. MAP3Ks selectively control the activation of extracellular signal-regulated kinase 1/2 (ERK1/2), Jun N-terminal kinase (JNK), p38 and ERK5 in response to receptor tyrosine kinases and GTPases. We used MDA-MB-231 cells because of their ability to metastasize from the breast fat pad to distant lymph nodes for an orthotopic xenograft model to screen the function of seven MAP3Ks in controlling tumor growth and metastasis. Stable short hairpin RNA (shRNA) knockdown was used to inhibit the expression of each of the seven MAP3Ks, which were selected for their differential regulation of the MAPK network. The screen identified two MAP3Ks, MEKK2 and MLK3, whose shRNA knockdown caused significant inhibition of both tumor growth and metastasis. Neither MEKK2 nor MLK3 have been previously shown to regulate tumor growth and metastasis in vivo. These results demonstrated that MAP3Ks, which differentially activate JNK, p38 and ERK5, are necessary for xenograft tumor growth and metastasis of MDA-MB-231 tumors. The requirement for MAP3Ks signaling through multiple MAPK pathways explains why several members of the MAPK network are activated in cancer. MEKK2 was required for epidermal growth factor receptor and Her2/Neu activation of ERK5, with ERK5 being required for metastasis. Loss of MLK3 expression increased mitotic infidelity and apoptosis in vitro. Knockdown of MEKK2 and MLK3 resulted in increased apoptosis in orthotopic xenografts relative to control tumors in mice, inhibiting both tumor growth and metastasis; MEKK2 and MLK3 represent untargeted kinases in tumor biology for potential therapeutic development.

BMK1 is involved in the regulation of p53 through disrupting the PML-MDM2 interaction

Promyelocytic leukemia protein (PML) modulates the p53 tumor suppressor through its interaction with p53 and MDM2. We found that activated BMK1 preferentially associates with PML isoform IV and disrupts PML-MDM2 interaction. Doxorubicin, a common chemotherapeutic agent, is known to promote PML-mediated p53 activation in part by promoting PML-dependent MDM2 nucleolar sequestration. We discovered that BMK1 deactivation coupled with doxorubicin synergistically enhanced MDM2 nucleolar sequestration and, consequently, promoted PML-mediated p53 up-regulation leading to tumor cell apoptosis in vitro and tumor regression in vivo. Collectively, these results not only suggest that BMK1 activity plays a role in suppressing p53 by blocking the interaction between PML and MDM2 but also implicate that pharmacological BMK1 inhibitor should significantly enhance the anti-cancer capacity of doxorubicin-based chemotherapy.