Aberrant expression of extracellular signal-regulated kinase 5 in human prostate cancer

Extracellular signal-regulated protein kinase 5 modulates the spindle assembly to coordinate the oocyte meiotic maturation

Extracellular signal-regulated protein kinase 5 (Erk5), a member of the mitogen-activated protein kinase (MAPK) family, is ubiquitously expressed in all eukaryotic cells and is implicated in the various mitotic processes such as cell survival, proliferation, migration, and differentiation. However, the potential functional roles of Erk5 in oocyte meiosis have not been fully determined. In this study, we document that ERK5 participates in the meiotic maturation of mouse oocytes by regulating the spindle assembly to ensure the meiotic progression. We unexpectedly found that phosphorylated ERK5 was localized in the spindle pole region at metaphase I and II stages by immunostaining analysis. Inhibition of ERK5 activity using its specific inhibitor XMD8-92 dramatically reduced the incidence of first polar body extrusion. In addition, inhibition of ERK5 evoked the spindle assembly checkpoint to arrest oocytes at metaphase I stage by impairing the spindle assembly, chromosome alignment and kinetochore-microtubule attachment. Mechanically, over-strengthened microtubule stability was shown to disrupt the microtubule dynamics and thus compromise the spindle assembly in ERK5-inhibited oocytes. Conversely, overexpression of ERK5 caused decreased level of acetylated α-tubulin and spindle defects. Collectively, we conclude that ERK5 plays an important role in the oocyte meiotic maturation by regulating microtubule dynamics and spindle assembly.

Bone and Extracellular Signal-Related Kinase 5 (ERK5)

Bones are vital for anchoring muscles, tendons, and ligaments, serving as a fundamental element of the human skeletal structure. However, our understanding of bone development mechanisms and the maintenance of bone homeostasis is still limited. Extracellular signal-related kinase 5 (ERK5), a recently identified member of the mitogen-activated protein kinase (MAPK) family, plays a critical role in the pathogenesis and progression of various diseases, especially neoplasms. Recent studies have highlighted ERK5's significant role in both bone development and bone-associated pathologies. This review offers a detailed examination of the latest research on ERK5 in different tissues and diseases, with a particular focus on its implications for bone health. It also examines therapeutic strategies and future research avenues targeting ERK5.

MAP kinase ERK5 modulates cancer cell sensitivity to extrinsic apoptosis induced by death-receptor agonists

Death receptor ligand TRAIL is a promising cancer therapy due to its ability to selectively trigger extrinsic apoptosis in cancer cells. However, TRAIL-based therapies in humans have shown limitations, mainly due inherent or acquired resistance of tumor cells. To address this issue, current efforts are focussed on dissecting the intracellular signaling pathways involved in resistance to TRAIL, to identify strategies that sensitize cancer cells to TRAIL-induced cytotoxicity. In this work, we describe the oncogenic MEK5-ERK5 pathway as a critical regulator of cancer cell resistance to the apoptosis induced by death receptor ligands. Using 2D and 3D cell cultures and transcriptomic analyses, we show that ERK5 controls the proteostasis of TP53INP2, a protein necessary for full activation of caspase-8 in response to TNFα, FasL or TRAIL. Mechanistically, ERK5 phosphorylates and induces ubiquitylation and proteasomal degradation of TP53INP2, resulting in cancer cell resistance to TRAIL. Concordantly, ERK5 inhibition or genetic deletion, by stabilizing TP53INP2, sensitizes cancer cells to the apoptosis induced by recombinant TRAIL and TRAIL/FasL expressed by Natural Killer cells. The MEK5-ERK5 pathway regulates cancer cell proliferation and survival, and ERK5 inhibitors have shown anticancer activity in preclinical models of solid tumors. Using endometrial cancer patient-derived xenograft organoids, we propose ERK5 inhibition as an effective strategy to sensitize cancer cells to TRAIL-based therapies.

The significance of ERK5 catalytic-independent functions in disease pathways

Extracellular signal-regulated kinase 5 (ERK5), also known as BMK1 or MAPK7, represents a recent addition to the classical mitogen-activated protein kinase (MAPK) family. This family includes well-known members such as ERK1/2, c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK), as well as atypical MAPKs such as ERK3, ERK4, ERK7 (ERK8), and Nemo-like kinase (NLK). Comprehensive reviews available elsewhere provide detailed insights into ERK5, which interested readers can refer to for in-depth knowledge (Nithianandarajah-Jones et al., 2012; Monti et al., Cancers (Basel), 2022, 14). The primary aim of this review is to emphasize the essential characteristics of ERK5 and shed light on the intricate nature of its activation, with particular attention to the catalytic-independent functions in disease pathways.

Parallel Optimization of Potency and Pharmacokinetics Leading to the Discovery of a Pyrrole Carboxamide ERK5 Kinase Domain Inhibitor

The nonclassical extracellular signal-related kinase 5 (ERK5) mitogen-activated protein kinase pathway has been implicated in increased cellular proliferation, migration, survival, and angiogenesis; hence, ERK5 inhibition may be an attractive approach for cancer treatment. However, the development of selective ERK5 inhibitors has been challenging. Previously, we described the development of a pyrrole carboxamide high-throughput screening hit into a selective, submicromolar inhibitor of ERK5 kinase activity. Improvement in the ERK5 potency was necessary for the identification of a tool ERK5 inhibitor for target validation studies. Herein, we describe the optimization of this series to identify nanomolar pyrrole carboxamide inhibitors of ERK5 incorporating a basic center, which suffered from poor oral bioavailability. Parallel optimization of potency and in vitro pharmacokinetic parameters led to the identification of a nonbasic pyrazole analogue with an optimal balance of ERK5 inhibition and oral exposure.

Clinical Significance and Regulation of ERK5 Expression and Function in Cancer

Extracellular signal-regulated kinase 5 (ERK5) is a unique kinase among MAPKs family members, given its large structure characterized by the presence of a unique C-terminal domain. Despite increasing data demonstrating the relevance of the ERK5 pathway in the growth, survival, and differentiation of normal cells, ERK5 has recently attracted the attention of several research groups given its relevance in inflammatory disorders and cancer. Accumulating evidence reported its role in tumor initiation and progression. In this review, we explore the gene expression profile of ERK5 among cancers correlated with its clinical impact, as well as the prognostic value of ERK5 and pERK5 expression levels in tumors. We also summarize the importance of ERK5 in the maintenance of a cancer stem-like phenotype and explore the major known contributions of ERK5 in the tumor-associated microenvironment. Moreover, although several questions are still open concerning ERK5 molecular regulation, different ERK5 isoforms derived from the alternative splicing process are also described, highlighting the potential clinical relevance of targeting ERK5 pathways.

ERK5 modulates IL-6 secretion and contributes to tumor-induced immune suppression

Tumors exhibit a variety of strategies to dampen antitumor immune responses. With an aim to identify factors that are secreted from tumor cells, we performed an unbiased mass spectrometry-based secretome analysis in lung cancer cells. Interleukin-6 (IL-6) has been identified as a prominent factor secreted by tumor cells and cancer-associated fibroblasts isolated from cancer patients. Incubation of dendritic cell (DC) cultures with tumor cell supernatants inhibited the production of IL-12p70 in DCs but not the surface expression of other activation markers which is reversed by treatment with IL-6 antibody. Defects in IL-12p70 production in the DCs inhibited the differentiation of Th1 but not Th2 and Th17 cells from naïve CD4⁺ T cells. We also demonstrate that the classical mitogen-activated protein kinase, ERK5/MAPK7, is required for IL-6 production in tumor cells. Inhibition of ERK5 activity or depletion of ERK5 prevented IL-6 production in tumor cells, which could be exploited for enhancing antitumor immune responses.

Extracellular Signal-Regulated Kinase 5 Regulates the Malignant Phenotype of Cholangiocarcinoma Cells

BACKGROUND AND AIMS

Cholangiocarcinoma (CCA) is characterized by high resistance to chemotherapy and poor prognosis. Several oncogenic pathways converge on activation of extracellular signal-regulated kinase 5 (ERK5), whose role in CCA has not been explored. The aim of this study was to investigate the role of ERK5 in the biology of CCA.

APPROACH AND RESULTS

ERK5 expression was detected in two established (HuCCT-1 and CCLP-1) and two primary human intrahepatic CCA cell lines (iCCA58 and iCCA60). ERK5 phosphorylation was increased in CCA cells exposed to soluble mediators. In both HuCCT-1 and CCLP-1 cells, ERK5 was localized in the nucleus, and exposure to fetal bovine serum (FBS) further increased the amount of nuclear ERK5. In human CCA specimens, ERK5 mRNA expression was increased in tumor cells and positively correlated with portal invasion. ERK5 protein levels were significantly associated with tumor grade. Growth, migration, and invasion of CCA cells were decreased when ERK5 was silenced using specific short hairpin RNA (shRNA). The inhibitory effects on CCA cell proliferation, migration and invasion were recapitulated by treatment with small molecule inhibitors targeting ERK5. In addition, expression of the angiogenic factors VEGF and angiopoietin 1 was reduced after ERK5 silencing. Conditioned medium from ERK5-silenced cells had a lower ability to induce tube formation by human umbilical vein endothelial cells and to induce migration of myofibroblasts and monocytes/macrophages. In mice, subcutaneous injection of CCLP-1 cells silenced for ERK5 resulted in less frequent tumor development and smaller size of xenografts compared with cells transfected with nontargeting shRNA.

CONCLUSIONS

ERK5 is a key mediator of growth and migration of CCA cells and supports a protumorigenic crosstalk between the tumor and the microenvironment.

Small molecule ERK5 kinase inhibitors paradoxically activate ERK5 signalling: be careful what you wish for…

ERK5 is a protein kinase that also contains a nuclear localisation signal and a transcriptional transactivation domain. Inhibition of ERK5 has therapeutic potential in cancer and inflammation and this has prompted the development of ERK5 kinase inhibitors (ERK5i). However, few ERK5i programmes have taken account of the ERK5 transactivation domain. We have recently shown that the binding of small molecule ERK5i to the ERK5 kinase domain stimulates nuclear localisation and paradoxical activation of its transactivation domain. Other kinase inhibitors paradoxically activate their intended kinase target, in some cases leading to severe physiological consequences highlighting the importance of mitigating these effects. Here, we review the assays used to monitor ERK5 activities (kinase and transcriptional) in cells, the challenges faced in development of small molecule inhibitors to the ERK5 pathway, and classify the molecular mechanisms of paradoxical activation of protein kinases by kinase inhibitors.

Non-'classical' MEKs: A review of MEK3-7 inhibitors

The MAPK pathways are an enduring area of interest due to their essential roles in cell processes. Increased expression and activity can lead to a multitude of diseases, sparking research efforts in developing inhibitors against these kinases. Though great strides have been made in developing MEK1/2 inhibitors, there is a notable lack of chemical probes for MEK3-7, given their central role in stimuli response, cell growth, and development. This review summarizes the progress that has been made on developing small molecule probes for MEK3-7, the specific disease states in which they have been studied, and their potential to become novel therapeutics.

Activator of G protein signaling 3 modulates prostate tumor development and progression

Prostate cancer (PCa) is a leading cause of cancer death among men, with greater prevalence of the disease among the African American population in the USA. Activator of G-protein signaling 3 (AGS3/G-protein signaling modulator 1) was shown to be overexpressed in prostate adenocarcinoma relative to the prostate gland. In this study, we investigated the correlation between AGS3 overexpression and PCa malignancy. Immunoblotting analysis and real-time quantitative-PCR showed increase in AGS3 expression in the metastatic cell lines LNCaP (~3-fold), MDA PCa 2b (~2-fold), DU 145 (~2-fold) and TRAMP-C1 (~20-fold) but not in PC3 (~1-fold), relative to control RWPE-1. Overexpression of AGS3 in PC3, LNCaP and MDA PCa 2b enhanced tumor growth. AGS3 contains seven tetratricopeptide repeats (TPR) and four G-protein regulatory (GPR) motifs. Overexpression of the TPR or the GPR motifs in PC3 cells had no effect in tumor growth. Depletion of AGS3 in the TRAMP-C1 cells (TRAMP-C1-AGS3-/-) decreased cell proliferation and delayed wound healing and tumor growth in both C57BL/6 (~3-fold) and nude mice xenografts, relative to control TRAMP-C1 cells. TRAMP-C1-AGS3-/- tumors also exhibited a marked increase (~5-fold) in both extracellular signal-regulated kinase (ERK) 1/2 and P38 mitogen-activated protein kinase (MAPK) activation, which correlated with a significant increase (~3-fold) in androgen receptor (AR) expression, relative to TRAMP-C1 xenografts. Interestingly, overexpression of AGS3 in TRAMP-C1-AGS3-/- cells inhibited ERK activation and AR overexpression as compared with control TRAMP-C1 cells. Taken together, the data indicate that the effect of AGS3 in prostate cancer development and progression is probably mediated via a MAPK/AR-dependent pathway.

SUMOylation Is Required for ERK5 Nuclear Translocation and ERK5-Mediated Cancer Cell Proliferation

The MAP kinase ERK5 contains an N-terminal kinase domain and a unique C-terminal tail including a nuclear localization signal and a transcriptional activation domain. ERK5 is activated in response to growth factors and stresses and regulates transcription at the nucleus by either phosphorylation or interaction with transcription factors. MEK5-ERK5 pathway plays an important role regulating cancer cell proliferation and survival. Therefore, it is important to define the precise molecular mechanisms implicated in ERK5 nucleo-cytoplasmic shuttling. We previously described that the molecular chaperone Hsp90 stabilizes and anchors ERK5 at the cytosol and that ERK5 nuclear shuttling requires Hsp90 dissociation. Here, we show that MEK5 or overexpression of Cdc37—mechanisms that increase nuclear ERK5—induced ERK5 Small Ubiquitin-related Modifier (SUMO)-2 modification at residues Lys6/Lys22 in cancer cells. Furthermore, mutation of these SUMO sites abolished the ability of ERK5 to translocate to the nucleus and to promote prostatic cancer PC-3 cell proliferation. We also show that overexpression of the SUMO protease SENP2 completely abolished endogenous ERK5 nuclear localization in response to epidermal growth factor (EGF) stimulation. These results allow us to propose a more precise mechanism: in response to MEK5 activation, ERK5 SUMOylation favors the dissociation of Hsp90 from the complex, allowing ERK5 nuclear shuttling and activation of the transcription.

Reversal of ABCB1-related multidrug resistance by ERK5-IN-1

Background

Inhibition of ABC transporters is considered the most effective way to circumvent multidrug resistance (MDR). In the present study, we evaluated the MDR modulatory potential of ERK5-IN-1, a potent extracelluar signal regulated kinase 5 (ERK5) inhibitor.

Methods

The cytotoxicity and MDR reversal effect of ERK5-IN-1 were assessed by MTT assay. The KBv200-inoculated nude mice xenograft model was used for the in vivo study. Doxorubicin efflux and accumulation were measured by flow cytometry. The modulation of ABCB1 activity was measured by colorimetric ATPase assay and [¹²⁵I]-iodoarylazidoprazosin (IAAP) photolabeling assay. Effect of ERK5-IN-1 on expression of ABCB1 and its downstream markers was measured by PCR and/or Western blot. Cell surface expression and subcellular localization of ABCB1 were tested by flow cytometry and immunofluorescence.

Results

Our results showed that ERK5-IN-1 significantly increased the sensitivity of vincristine, paclitaxel and doxorubicin in KBv200, MCF7/adr and HEK293/ABCB1 cells, respectively. This effect was not found in respective drug sensitive parental cell lines. Moreover, in vivo combination studies showed that ERK5-IN-1 effectively enhanced the antitumor activity of paclitaxel in KBv200 xenografts without causing addition toxicity. Mechanistically, ERK5-IN-1 increased intracellular accumulation of doxorubicin dose dependently by directly inhibiting the efflux function of ABCB1. ERK5-IN-1 stimulated the ABCB1 ATPase activity and inhibited the incorporation of [¹²⁵I]-iodoarylazidoprazosin (IAAP) into ABCB1 in a concentration-dependent manner. In addition, ERK5-IN-1 treatment neither altered the expression level of ABCB1 nor blocked the phosphorylation of downstream Akt or Erk1/2. No significant reversal effect was observed on ABCG2-, ABCC1-, MRP7- and LRP-mediated drug resistance.

Conclusions

Collectively, these results indicated that ERK5-IN-1 efficiently reversed ABCB1-mediated MDR by competitively inhibiting the ABCB1 drug efflux function. The use of ERK5-IN-1 to restore sensitivity to chemotherapy or to prevent resistance could be a potential treatment strategy for cancer patients.

Targeted Avenues for Cancer Treatment: The MEK5-ERK5 Signaling Pathway

Twenty years have passed since extracellular signal-regulated kinase 5 (ERK5) and its upstream activator, mitogen-activated protein kinase 5 (MEK5), first emerged onto the cancer research scene. Although we have come a long way in defining the liaison between dysregulated MEK5-ERK5 signaling and the pathogenesis of epithelial and nonepithelial malignancies, selective targeting of this unique pathway remains elusive. Here, we provide an updated review of the existing evidence for a correlation between aberrant MEK5-ERK5 (phospho)proteomic/transcriptomic profiles, aggressive cancer states, and poor patient outcomes. We then focus on emerging insights from preclinical models regarding the relevance of upregulated ERK5 activity in promoting tumor growth, metastasis, therapy resistance, undifferentiated traits, and immunosuppression, highlighting the opportunities, prospects, and challenges of selectively blocking this cascade for antineoplastic treatment and chemosensitization.

Beyond Kinase Activity: ERK5 Nucleo-Cytoplasmic Shuttling as a Novel Target for Anticancer Therapy

The importance of mitogen-activated protein kinases (MAPK) in human pathology is underlined by the relevance of abnormalities of MAPK-related signaling pathways to a number of different diseases, including inflammatory disorders and cancer. One of the key events in MAPK signaling, especially with respect to pro-proliferative effects that are crucial for the onset and progression of cancer, is MAPK nuclear translocation and its role in the regulation of gene expression. The extracellular signal-regulated kinase 5 (ERK5) is the most recently discovered classical MAPK and it is emerging as a possible target for cancer treatment. The bigger size of ERK5 when compared to other MAPK enables multiple levels of regulation of its expression and activity. In particular, the phosphorylation of kinase domain and C-terminus, as well as post-translational modifications and chaperone binding, are involved in ERK5 regulation. Likewise, different mechanisms control ERK5 nucleo-cytoplasmic shuttling, underscoring the key role of ERK5 in the nuclear compartment. In this review, we will focus on the mechanisms involved in ERK5 trafficking between cytoplasm and nucleus, and discuss how these processes might be exploited to design new strategies for cancer treatment.

Targeting MEK5 impairs nonhomologous end-joining repair and sensitizes prostate cancer to DNA damaging agents

Radiotherapy is commonly used to treat a variety of solid human tumors, including localized prostate cancer. However, treatment failure often ensues due to tumor intrinsic or acquired radioresistance. Here we find that the MEK5/ERK5 signaling pathway is associated with resistance to genotoxic stress in aggressive prostate cancer cells. MEK5 knockdown by RNA interference sensitizes prostate cancer cells to ionizing radiation (IR) and etoposide treatment, as assessed by clonogenic survival and short-term proliferation assays. Mechanistically, MEK5 downregulation impairs phosphorylation of the catalytic subunit of DNA-PK at serine 2056 in response to IR or etoposide treatment. Although MEK5 knockdown does not influence the initial appearance of radiation- and etoposide-induced γH2AX and 53BP1 foci, it markedly delays their resolution, indicating a DNA repair defect. A cell-based assay shows that non-homologous end joining (NHEJ) is compromised in cells with ablated MEK5 protein expression. Finally, MEK5 silencing combined with focal irradiation causes strong inhibition of tumor growth in mouse xenografts, compared with MEK5 depletion or radiation alone. These findings reveal a convergence between MEK5 signaling and DNA repair by NHEJ in conferring resistance to genotoxic stress in advanced prostate cancer and suggest targeting MEK5 as an effective therapeutic intervention in the management of this disease.

Identification of a novel orally bioavailable ERK5 inhibitor with selectivity over p38α and BRD4

Extracellular regulated kinase 5 (ERK5) signalling has been implicated in driving a number of cellular phenotypes including endothelial cell angiogenesis and tumour cell motility. Novel ERK5 inhibitors were identified using high throughput screening, with a series of pyrrole-2-carboxamides substituted at the 4-position with an aroyl group being found to exhibit IC50 values in the micromolar range, but having no selectivity against p38α MAP kinase. Truncation of the N-substituent marginally enhanced potency (∼3-fold) against ERK5, but importantly attenuated inhibition of p38α. Systematic variation of the substituents on the aroyl group led to the selective inhibitor 4-(2-bromo-6-fluorobenzoyl)-N-(pyridin-3-yl)-1H-pyrrole-2-carboxamide (IC50 0.82 μM for ERK5; IC50 > 120 μM for p38α). The crystal structure (PDB 5O7I) of this compound in complex with ERK5 has been solved. This compound was orally bioavailable and inhibited bFGF-driven Matrigel plug angiogenesis and tumour xenograft growth. The selective ERK5 inhibitor described herein provides a lead for further development into a tool compound for more extensive studies seeking to examine the role of ERK5 signalling in cancer and other diseases.

Pharmacological inhibition of the MEK5/ERK5 and PI3K/Akt signaling pathways synergistically reduces viability in triple-negative breast cancer

Triple-negative breast cancers (TNBCs) represent 15% to 20% of all breast cancers and are often associated with poor prognosis. The lack of targeted therapies for TNBCs contributes to higher mortality rates. Aberrations in the phosphoinositide-3-kinase (PI3K) and mitogen-activated protein kinase pathways have been linked to increased breast cancer proliferation and survival. It has been proposed that these survival characteristics are enhanced through compensatory signaling and crosstalk mechanisms. While the crosstalk between PI3K and extracellular signal-regulated kinase 1/2 (ERK1/2) pathways has been characterized in several systems, new evidence suggests that MEK5/ERK5 signaling is a key component in the proliferation and survival of several aggressive cancers. In this study, we examined the effects of dual inhibition of PI3K/protein kinase B (Akt) and MEK5/ERK5 in the MDA-MB-231, BT-549, and MDA-MB-468 TNBC cell lines. We used the Akt inhibitor ipatasertib, ERK5 inhibitors XMD8-92 and AX15836, and the novel MEK5 inhibitor SC-1-181 to investigate the effects of dual inhibition. Our results indicated that dual inhibition of PI3K/Akt and MEK5/ERK5 signaling was more effective at reducing the proliferation and survival of TNBCs than single inhibition of either pathway alone. In particular, a loss of Bad phosphorylation at two distinct sites was observed with dual inhibition. Furthermore, the inhibition of both pathways led to p21 restoration, decreased cell proliferation, and induced apoptosis. In addition, the dual inhibition strategy was determined to be synergistic in MDA-MB-231 and BT-549 cells and was relatively nontoxic in the nonneoplastic MCF-10 cell line. In summary, the results from this study provide a unique prospective into the utility of a novel dual inhibition strategy for targeting TNBCs.

Impact of ERK5 on the Hallmarks of Cancer

Extracellular signal-regulated kinase 5 (ERK5) belongs to the mitogen-activated protein kinase (MAPK) family that consists of highly conserved enzymes expressed in all eukaryotic cells and elicits several biological responses, including cell survival, proliferation, migration, and differentiation. In recent years, accumulating lines of evidence point to a relevant role of ERK5 in the onset and progression of several types of cancer. In particular, it has been reported that ERK5 is a key signaling molecule involved in almost all the biological features of cancer cells so that its targeting is emerging as a promising strategy to suppress tumor growth and spreading. Based on that, in this review, we pinpoint the hallmark-specific role of ERK5 in cancer in order to identify biological features that will potentially benefit from ERK5 targeting.

Nuclear ERK: Mechanism of Translocation, Substrates, and Role in Cancer

The extracellular signal-regulated kinases 1/2 (ERK) are central signaling components that regulate stimulated cellular processes such as proliferation and differentiation. When dysregulated, these kinases participate in the induction and maintenance of various pathologies, primarily cancer. While ERK is localized in the cytoplasm of resting cells, many of its substrates are nuclear, and indeed, extracellular stimulation induces a rapid and robust nuclear translocation of ERK. Similarly to other signaling components that shuttle to the nucleus upon stimulation, ERK does not use the canonical importinα/β mechanism of nuclear translocation. Rather, it has its own unique nuclear translocation signal (NTS) that interacts with importin7 to allow stimulated shuttling via the nuclear pores. Prevention of the nuclear translocation inhibits proliferation of B-Raf- and N/K-Ras-transformed cancers. This effect is distinct from the one achieved by catalytic Raf and MEK inhibitors used clinically, as cells treated with the translocation inhibitors develop resistance much more slowly. In this review, we describe the mechanism of ERK translocation, present all its nuclear substrates, discuss its role in cancer and compare its translocation to the translocation of other signaling components. We also present proof of principle data for the use of nuclear ERK translocation as an anti-cancer target. It is likely that the prevention of nuclear ERK translocation will eventually serve as a way to combat Ras and Raf transformed cancers with less side-effects than the currently used drugs.

The Pharmacological Inhibition of ERK5 Enhances Apoptosis in Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is a fatal hematological malignancy which is resistant to a variety of chemotherapy drugs. Extracellular signal-regulated kinase 5 (ERK5) plays a novel role in chemoresistance in some cancer cells and this pathway is a central mediator of cell survival and apoptotic regulation. The aim of this study was to investigate the effect of ERK5 inhibitor, XMD8-92, on proliferation and apoptosis in AML cell lines. Findings showed that XMD8-92 inhibited the activation of ERK5 by G-CSF and decreased the expression of c-Myc and Cyclin D1. The treatment of XMD8-92 reduced the phosphorylation of ERK5 leading to a distinct inhibition of cell proliferation and increased apoptosis in Kasumi-1 and HL-60 cells. Taken together, our study suggests that the inhibition of ERK5 by XMD8-92 can trigger apoptosis and inhibit proliferation in AMLs. Therefore, the inhibition of ERK5 may be an effective adjuvant in AML chemotherapy.

Targeting the Extracellular Signal-Regulated Kinase 5 Pathway to Suppress Human Chronic Myeloid Leukemia Stem Cells

Tyrosine kinase inhibitors (TKi) are effective against chronic myeloid leukemia (CML), but their inefficacy on leukemia stem cells (LSCs) may lead to relapse. To identify new druggable targets alternative to BCR/ABL, we investigated the role of the MEK5/ERK5 pathway in LSC maintenance in low oxygen, a feature of bone marrow stem cell niches. We found that MEK5/ERK5 pathway inhibition reduced the growth of CML patient-derived cells and cell lines in vitro and the number of leukemic cells in vivo. Treatment in vitro of primary CML cells with MEK5/ERK5 inhibitors, but not TKi, strikingly reduced culture repopulation ability (CRA), serial colony formation ability, long-term culture-initiating cells (LTC-ICs), and CD26-expressing cells. Importantly, MEK5/ERK5 inhibition was effective on CML cells regardless of the presence or absence of imatinib, and did not reduce CRA or LTC-ICs of normal CD34+ cells. Thus, targeting MEK/ERK5 may represent an innovative therapeutic approach to suppress CML progenitor/stem cells.

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ERK5 is constitutively active in chronic myeloid leukemia (CML) cells

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ERK5 pathway inhibition reduces the growth of CML cells in vitro and in vivo

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ERK5 pathway inhibition strikingly reduces CML progenitor/stem cell maintenance

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The combination of ERK5i with imatinib reduces the expression of stem cell proteins

Establishment of a nine-gene prognostic model for predicting overall survival of patients with endometrial carcinoma

Endometrial carcinoma (EC) is the most common malignant tumor of the female genital tract in developed countries. The prognosis of early stage EC is favorable, but a subset faces high risk of cancer progression or recurrence. EC has a poor prognosis upon progression to advanced or metastatic stages. Therefore, our goal is to build a robust prognostic model for predicting overall survival (OS) in EC patients. In this study, 1571 genes were identified as being associated with OS based on genomewide expression profiles using a training dataset. Kyoto Encyclopedia of Genes and Genomes enrichment analysis revealed that these genes were involved in various cancer-related signaling pathways. Nine signature genes were further selected using stepwise selection, and their potential role in the development of EC was demonstrated by performing differential expression analysis between EC and normal uterine tissues. A prognostic model that aggregated these nine signature genes was ultimately established and effectively divided EC patients into two risk groups. OS for patients in the high-risk group was significantly poorer compared with that of the low-risk group. This nine-gene model was subsequently validated and evaluated using the TCGA dataset and shown to have a high discriminating power to distinguish EC patients with an elevated risk of mortality based on the FIGO staging system and other prognostic factors. This study provides a novel prognostic model for the identification of EC patients with elevated risk of mortality and will help to improve our understanding of the underlying mechanisms involved in prognostic EC factors.

ERK5 is activated by oncogenic BRAF and promotes melanoma growth

Malignant melanoma is among the most aggressive cancers and its incidence is increasing worldwide. Targeted therapies and immunotherapy have improved the survival of patients with metastatic melanoma in the last few years; however, available treatments are still unsatisfactory. While the role of the BRAF-MEK1/2-ERK1/2 pathway in melanoma is well established, the involvement of mitogen-activated protein kinases MEK5-ERK5 remains poorly explored. Here we investigated the function of ERK5 signaling in melanoma. We show that ERK5 is consistently expressed in human melanoma tissues and is active in melanoma cells. Genetic silencing and pharmacological inhibition of ERK5 pathway drastically reduce the growth of melanoma cells and xenografts harboring wild-type (wt) or mutated BRAF (V600E). We also found that oncogenic BRAF positively regulates expression, phosphorylation, and nuclear localization of ERK5. Importantly, ERK5 kinase and transcriptional transactivator activities are enhanced by BRAF. Nevertheless, combined pharmacological inhibition of BRAFV600E and MEK5 is required to decrease nuclear ERK5, that is critical for the regulation of cell proliferation. Accordingly, combination of MEK5 or ERK5 inhibitors with BRAFV600E inhibitor vemurafenib is more effective than single treatments in reducing colony formation and growth of BRAFV600E melanoma cells and xenografts. Overall, these data support a key role of the ERK5 pathway for melanoma growth in vitro and in vivo and suggest that targeting ERK5, alone or in combination with BRAF-MEK1/2 inhibitors, might represent a novel approach for melanoma treatment.

PD2/PAF1 at the Crossroads of the Cancer Network

Pancreatic differentiation 2 (PD2)/RNA polymerase II-associated factor 1 (PAF1) is the core subunit of the human PAF1 complex (PAF1C) that regulates the promoter-proximal pausing of RNA polymerase II as well as transcription elongation and mRNA processing and coordinates events in mRNA stability and quality control. As an integral part of its transcription-regulatory function, PD2/PAF1 plays a role in posttranslational histone covalent modifications as well as regulates expression of critical genes of the cell-cycle machinery. PD2/PAF1 alone, and as a part of PAF1C, provides distinct roles in the maintenance of self-renewal of embryonic stem cells and cancer stem cells, and in lineage differentiation. Thus, PD2/PAF1 malfunction or its altered abundance is likely to affect normal cellular functions, leading to disease states. Indeed, PD2/PAF1 is found to be upregulated in poorly differentiated pancreatic cancer cells and has the capacity for neoplastic transformation when ectopically expressed in mouse fibroblast cells. Likewise, PD2/PAF1 is upregulated in pancreatic and ovarian cancer stem cells. Here, we concisely describe multifaceted roles of PD2/PAF1 associated with oncogenic transformation and implicate PD2/PAF1 as an attractive target for therapeutic development to combat malignancy. Cancer Res; 78(2); 313-9. ©2018 AACR.

MEK5/ERK5 signaling inhibition increases colon cancer cell sensitivity to 5-fluorouracil through a p53-dependent mechanism

The MEK5/ERK5 signaling pathway is emerging as an important contributor to colon cancer onset, progression and metastasis; however, its relevance to chemotherapy resistance remains unknown. Here, we evaluated the impact of the MEK5/ERK5 cascade in colon cancer cell sensitivity to 5-fluorouracil (5-FU). Increased ERK5 expression was correlated with poor overall survival in colon cancer patients. In colon cancer cells, 5-FU exposure impaired endogenous KRAS/MEK5/ERK5 expression and/or activation. In turn, MEK5 constitutive activation reduced 5-FU-induced cytotoxicity. Using genetic and pharmacological approaches, we showed that ERK5 inhibition increased caspase-3/7 activity and apoptosis following 5-FU exposure. Mechanistically, this was further associated with increased p53 transcriptional activation of p21 and PUMA. In addition, ERK5 inhibition increased the response of HCT116 p53^+/+ cells to 5-FU, but failed to sensitize HCT116 p53^−/− cells to the cytotoxic effects of this chemotherapeutic agent, suggesting a p53-dependent axis mediating 5-FU sensitization. Finally, ERK5 inhibition using XMD8-92 was shown to increase the antitumor effects of 5-FU in a murine subcutaneous xenograft model, enhancing apoptosis while markedly reducing tumor growth. Collectively, our results suggest that ERK5-targeted in hibition provides a promising therapeutic approach to overcome resistance to 5-FU-based chemotherapy and improve colon cancer treatment.

Analysis of Nkx3.1:Cre-driven Erk5 deletion reveals a profound spinal deformity which is linked to increased osteoclast activity

Extracellular signal-regulated protein kinase 5 (ERK5) has been implicated during development and carcinogenesis. Nkx3.1-mediated Cre expression is a useful strategy to genetically manipulate the mouse prostate. While grossly normal at birth, we observed an unexpected phenotype of spinal protrusion in Nkx3.1:Cre;Erk5 fl/fl (Erk5 fl/fl) mice by ~6-8 weeks of age. X-ray, histological and micro CT (µCT) analyses showed that 100% of male and female Erk5 fl/fl mice had a severely deformed curved thoracic spine, with an associated loss of trabecular bone volume. Although sex-specific differences were observed, histomorphometry measurements revealed that both bone resorption and bone formation parameters were increased in male Erk5 fl/fl mice compared to wild type (WT) littermates. Osteopenia occurs where the rate of bone resorption exceeds that of bone formation, so we investigated the role of the osteoclast compartment. We found that treatment of RANKL-stimulated primary bone marrow-derived macrophage (BMDM) cultures with small molecule ERK5 pathway inhibitors increased osteoclast numbers. Furthermore, osteoclast numbers and expression of osteoclast marker genes were increased in parallel with reduced Erk5 expression in cultures generated from Erk5 fl/fl mice compared to WT mice. Collectively, these results reveal a novel role for Erk5 during bone maturation and homeostasis in vivo.

Increased T-cell Infiltration Elicited by Erk5 Deletion in a Pten-Deficient Mouse Model of Prostate Carcinogenesis

Prostate cancer does not appear to respond to immune checkpoint therapies where T-cell infiltration may be a key limiting factor. Here, we report evidence that ablating the growth regulatory kinase Erk5 can increase T-cell infiltration in an established Pten-deficient mouse model of human prostate cancer. Mice that were doubly mutant in prostate tissue for Pten and Erk5 (prostate DKO) exhibited a markedly increased median survival with reduced tumor size and proliferation compared with control Pten-mutant mice, the latter of which exhibited increased Erk5 mRNA expression. A comparative transcriptomic analysis revealed upregulation in prostate DKO mice of the chemokines Ccl5 and Cxcl10, two potent chemoattractants for T lymphocytes. Consistent with this effect, we observed a relative increase in a predominantly CD4+ T-cell infiltrate in the prostate epithelial and stroma of tumors from DKO mice. Collectively, our results offer a preclinical proof of concept for ERK5 as a target to enhance T-cell infiltrates in prostate cancer, with possible implications for leveraging immune therapy in this disease. Cancer Res; 77(12); 3158-68. ©2017 AACR.

Oncogenic signaling of MEK5-ERK5

Mitogen-activated protein kinases (MAPKs) regulate diverse cellular processes including proliferation, cell survival, differentiation, and apoptosis. While conventional MAPK constituents have well-defined roles in oncogenesis, the MEK5 pathway has only recently emerged in cancer research. In this review, we consider the MEK5 signaling cascade, focusing specifically on its involvement in drug resistance and regulation of aggressive cancer phenotypes. Moreover, we explore the role of MEK5/ERK5 in tumorigenesis and metastatic progression, discussing the discrepancies in preclinical studies and assessing its viability as a therapeutic target for anti-cancer agents.

Implication of VHL, ERK5, and HIF-1alpha in clear cell renal cell carcinoma: Molecular basis

OBJECTIVES

To determine the expression status of several proteins related to VHL gene function and its relationship with common clinicopathological parameters.

MATERIAL AND METHODS

Observational, analytical, cross-sectional study with 50 patients diagnosed with clear cell renal cell carcinoma. The study analyzed VHL mutations and hypermethylation as well as protein expression of VHL, CA-IX, HIF-1alpha, VEGF, ERK1/2, and ERK5, relating them to clinical variables. A bivariate and multivariate descriptive logistical regression analysis was performed, using the presence of metastasis at diagnosis as dependent variable.

RESULTS

The study identified 13 (26%) VHL mutations related to nuclear grade (P = 0.036). VHL hypermethylation was found in 20% of cases. VHL expression was associated with the presence of mutations (P = 0.013), and the absence of expression was associated with nuclear grade and the presence of metastasis (P<0.05). HIF-1alpha was negative in only 5 cases. Vascular endothelial growth factor (VEGF) was positive in 31 of 47 cases and was associated with Fuhrman nuclear grade, presence of metastasis, and stage (P<0.05). ERK5 expression was increased in 58% of cases and associated with the presence of metastasis and more advanced stages (P<0.05). In the logistic regression analysis, the only variable remaining in the model was VEGF expression (P = 0.014).

CONCLUSIONS

VEGF has prognostic value in clear cell renal cell carcinoma, and ERK5 may be a new prognostic marker in this type of tumor owing to its relationship with metastasis and more advanced stages.

Nuclear ERK5 inhibits progression of leukemic monocytes to macrophages by regulating the transcription factor PU.1 and heat shock protein HSP70

Differentiation therapy can supplement the therapy of APL, but other subtypes of AML are treated principally with cytotoxic agents, with few lasting remissions. While the induction of monocyte followed by macrophage differentiation by vitamin D derivatives (VDDs) is dramatic in cultured AML cells of all subtypes, attempts to translate this to the clinic have not been effective. Thus, better understanding of the mechanisms underlying VDD-induced differentiation may improve this approach. The key events in this form of differentiation include increased expression of CD11b, and the transcription factor PU.1 is known to be a part of this process. We show here that in the transition of monocytes to macrophages induced by a VDD, ERK5, a member of the MAPK family of signaling molecules, prevents PU.1 expression. However, upon ERK5 inhibition PU.1 protein is stabilized by HSP70.Thus, ERK5 may be a target for manipulation of the immunoregulatory actions of macrophages in cancer.

ERK5 and Cell Proliferation: Nuclear Localization Is What Matters

ERK5, the last MAP kinase family member discovered, is activated by the upstream kinase MEK5 in response to growth factors and stress stimulation. MEK5-ERK5 pathway has been associated to different cellular processes, playing a crucial role in cell proliferation in normal and cancer cells by mechanisms that are both dependent and independent of its kinase activity. Thus, nuclear ERK5 activates transcription factors by either direct phosphorylation or acting as co-activator thanks to a unique transcriptional activation TAD domain located at its C-terminal tail. Consequently, ERK5 has been proposed as an interesting target to tackle different cancers, and either inhibitors of ERK5 activity or silencing the protein have shown antiproliferative activity in cancer cells and to block tumor growth in animal models. Here, we review the different mechanisms involved in ERK5 nuclear translocation and their consequences. Inactive ERK5 resides in the cytosol, forming a complex with Hsp90-Cdc37 superchaperone. In a canonical mechanism, MEK5-dependent activation results in ERK5 C-terminal autophosphorylation, Hsp90 dissociation, and nuclear translocation. This mechanism integrates signals such as growth factors and stresses that activate the MEK5-ERK5 pathway. Importantly, two other mechanisms, MEK5-independent, have been recently described. These mechanisms allow nuclear shuttling of kinase-inactive forms of ERK5. Although lacking kinase activity, these forms activate transcription by interacting with transcription factors through the TAD domain. Both mechanisms also require Hsp90 dissociation previous to nuclear translocation. One mechanism involves phosphorylation of the C-terminal tail of ERK5 by kinases that are activated during mitosis, such as Cyclin-dependent kinase-1. The second mechanism involves overexpression of chaperone Cdc37, an oncogene that is overexpressed in cancers such as prostate adenocarcinoma, where it collaborates with ERK5 to promote cell proliferation. Although some ERK5 kinase inhibitors have shown antiproliferative activity it is likely that those tumors expressing kinase-inactive nuclear ERK5 will not respond to these inhibitors.

High-Throughput Screening and Hit Validation of Extracellular-Related Kinase 5 (ERK5) Inhibitors

The extracellular-related kinase 5 (ERK5) is a promising target for cancer therapy. A high-throughput screen was developed for ERK5, based on the IMAP FP progressive binding system, and used to identify hits from a library of 57 617 compounds. Four distinct chemical series were evident within the screening hits. Resynthesis and reassay of the hits demonstrated that one series did not return active compounds, whereas three series returned active hits. Structure-activity studies demonstrated that the 4-benzoylpyrrole-2-carboxamide pharmacophore had excellent potential for further development. The minimum kinase binding pharmacophore was identified, and key examples demonstrated good selectivity for ERK5 over p38α kinase.

The MEK5/ERK5 signalling pathway in cancer: a promising novel therapeutic target

Conventional mitogen-activated protein kinase (MAPK) family members are among the most sought-after oncogenic effectors for the development of novel human cancer treatment strategies. MEK5/ERK5 has been the less-studied MAPK subfamily, despite its increasingly demonstrated relevance in the growth, survival, and differentiation of normal cells. MEK5/ERK5 signalling has already been proposed to have pivotal roles in several cancer hallmarks, and to mediate the effects of a range of oncogenes. Accumulating evidence indicates the contribution of MEK5/ERK5 signalling to therapy resistance and the benefits of using MEK5/ERK5 inhibitory strategies in the treatment of human cancer. Here, we explore the major known contributions of MEK5/ERK5 signalling to the onset and progression of several types of cancer, and highlight the potential clinical relevance of targeting MEK5/ERK5 pathways.

MEK5 overexpression is associated with the occurrence and development of colorectal cancer

BACKGROUND

Mitogen/extracellular signal-regulated kinase kinase-5 (MEK5) has been confirmed to play a pivotal role in tumor carcinogenesis and progression. However, few studies have investigated the role of MEK5 in colorectal cancer (CRC).

METHODS

MEK5 expression was determined by immunohistochemistry (IHC) in tissue microarrays (TMAs) containing 2 groups of tissues, and western blotting was used to confirm MEK5 expression in 8 cases of primary CRC tissues and paired normal mucosa. RNA interference was used to verify the biological function of MEK5 gene in the development of CRC.

RESULTS

IHC revealed the expression of MEK5 was higher in tumor tissues (38.1 %), compared with adjacent normal tissue (8.3 %). Western blot showed that, MEK5 expression was upregulated in CRC tumor tissues compared with normal tissue. Analysis of clinical pathology parameters indicated MEK5 overexpression was significantly correlated with the depth of invasion, lymph node metastasis, distant metastasis and histological grade. Survival analysis revealed that MEK5 overexpression negatively correlated with cancer-free survival (hazard ratio 1.64, P = 0.017). RNA interference-mediated knockdown of MEK5 in SW480 colon cancer cells decreased their proliferation, division, migration and invasiveness in vitro and slowed down tumors growth in mice engrafted with the cells.

CONCLUSION

MEK5 plays an important role in CRC progression and may be a potential molecular target for the treatment of CRC.

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.

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.

ERK5 negatively regulates tobacco smoke-induced pulmonary epithelial-mesenchymal transition

As the primary cause of lung cancer, tobacco smoke (TS) promotes the initiation and progression of lung tumorigenesis. Epithelial-mesenchymal transition (EMT) is a crucial process involved in cell malignant transformation. The role of ERK5, the lesser studied member of MAPKs family, in regulating TS-triggered pulmonary EMT has not been investigated. Normal human bronchial epithelial cells and BALB/c mice were used as in vitro and in vivo TS exposure models. Exposure of normal human bronchial epithelial cells to TS for 7 days induced morphological change, enhanced migratory and invasive capacities, reduced epithelial marker expression and increased mesenchymal marker expression. Importantly, we demonstrated for the first time that ERK5 negatively regulated TS-mediated lung epithelial EMT, as evidenced by the findings that TS suppressed ERK5 activation, and that TS-triggered EMT was mimicked with ERK5 inhibition and reversed by ERK5 overexpression. The negative regulation of ERK5 on pulmonary EMT was further confirmed in mice exposed to TS for 12 weeks. Taken together, our data suggest that ERK5 negatively regulates TS-mediated pulmonary EMT. These findings provide new insight into the molecular mechanisms of TS-associated lung tumorigenesis and may open up new avenues in the search for potential target of lung cancer intervention.

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.

Aberrant MEK5/ERK5 signalling contributes to human colon cancer progression via NF-κB activation

This study was designed to evaluate MEK5 and ERK5 expression in colon cancer progression and to ascertain the relevance of MEK5/ERK5 signalling in colon cancer. Expression of MEK5 and ERK5 was evaluated in 323 human colon cancer samples. To evaluate the role of MEK5/ERK5 signalling in colon cancer, we developed a stable cell line model with differential MEK5/ERK5 activation. Impact of differential MEK5/ERK5 signalling was evaluated on cell cycle progression by flow cytometry and cell migration was evaluated by wound healing and transwell migration assays. Finally, we used an orthotopic xenograft mouse model of colon cancer to assess tumour growth and progression. Our results demonstrated that MEK5 and ERK5 are overexpressed in human adenomas (P<0.01) and adenocarcinomas (P<0.05), where increased ERK5 expression correlated with the acquisition of more invasive and metastatic potential (P<0.05). Interestingly, we observed a significant correlation between ERK5 expression and NF-κB activation in human adenocarcinomas (P<0.001). We also showed that ERK5 overactivation significantly accelerated cell cycle progression (P<0.05) and increased cell migration (P<0.01). Furthermore, cells with overactivated ERK5 displayed increased NF-κB nuclear translocation and transcriptional activity (P<0.05), together with increased expression of the mesenchymal marker vimentin (P<0.05). We further demonstrated that increased NF-κB activation was associated with increased IκB phosphorylation and degradation (P<0.05). Finally, in the mouse model, lymph node metastasis was exclusively seen in orthotopically implanted tumours with overactivated MEK5/ERK5, and not in tumours with inhibited MEK5/ERK5. Our results suggested that MEK5/ERK5/NF-κB signalling pathway is important for tumour onset, progression and metastasis, possibly representing a novel relevant therapeutic target in colon cancer treatment.

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.

Overexpression of MEKK2 is associated with colorectal carcinogenesis

Mitogen-activated protein kinase kinase kinase 2 (MEKK2) is an important upstream mediator of the extracellular signal-regulated kinase 5 signaling cascade that is essential for a number of cellular functions, including mitogenesis, differentiation and oncogenic transformation. Using western blotting to examine MEKK2 expression in 16 cases of primary colorectal cancer (CRC) lesions with paired normal mucosa, it was identified that MEKK2 is highly expressed in CRC lesions compared with that of the normal mucosa. Immunohistochemistry of 24 normal mucosa, 24 adenoma and 96 adenocarcinoma colorectal specimens indicated that the expression of MEKK2 was significantly increased in the adenoma and carcinoma specimens compared with that of the normal mucosa cases (P<0.0001 for both). However, no significant differences were detected in MEKK2 expression between the carcinoma and adenoma specimens (P=0.85). Similarly, no correlations were identified between MEKK2 expression and clinicopathological features, including gender, age, body mass index, histological differentiation, depth of invasion, lymph node metastasis, UICC stage and K-ras mutations (P>0.05). The present study demonstrated that MEKK2 functions as a promotive factor in CRC.

X-ray crystal structure of ERK5 (MAPK7) in complex with a specific inhibitor

The protein kinase ERK5 (MAPK7) is an emerging drug target for a variety of indications, in particular for cancer where it plays a key role mediating cell proliferation, survival, epithelial–mesenchymal transition, and angiogenesis. To date, no three-dimensional structure has been published that would allow rational design of inhibitors. To address this, we determined the X-ray crystal structure of the human ERK5 kinase domain in complex with a highly specific benzo[e]pyrimido[5,4-b]diazepine-6(11H)-one inhibitor. The structure reveals that specific residue differences in the ATP-binding site, compared to the related ERKs p38s and JNKs, allow for the development of ERK5-specific inhibitors. The selectivity of previously observed ERK5 inhibitors can also be rationalized using this structure, which provides a template for future development of inhibitors with potential for treatment of disease.

Mir143 expression inversely correlates with nuclear ERK5 immunoreactivity in clinical prostate cancer

Background:

Aberrant mitogen/extracellular signal-regulated kinase 5 (MEK5)–extracellular signal-regulated protein kinase 5 (ERK5)-mediated signalling has been implicated in a number of tumour types including prostate cancer (CaP). The mechanism for ERK5 activation in CaP remains to be fully elucidated. Studies have recently implicated the role of microRNA (miRNA) mir143 expression in the regulation of ERK5 expression.

Methods:

We utilised a tissue microarray (TMA) of 530 CaP cores from 168 individual patients and stained for both mir143 and ERK5. These TMAs were scored by a combination of observer and automated methods.

Results:

We observed a strong inverse relation between ERK5 and mir143, which manifested itself most strongly in the subgroup of 417 cores with non-zero mir143 and ERK5 immunoreactivity, or with only one of mir143 or ERK5 being zero (cc=0.2558 and P<0.0001). Mir143 neither correlate with Gleason scores or prostate-specific antigen levels, nor was it a predictor of disease-specific survival on univariate analysis.

Conclusion:

Although the mechanism for ERK5 activation in CaP remains to be fully elucidated, we have further validated the potential role of mir143 in regulating ERK5 levels in the clinical context. In addition, we demonstrate that the automated counting method for nuclear ERK5 is a clinically useful alterative to observer counting method in patient stratification in the context of ERK5 targeting therapy.

Extracellular signal-regulated kinase 5: a potential therapeutic target for malignant mesotheliomas

PURPOSE

Malignant mesothelioma is a devastating disease with a need for new treatment strategies. In the present study, we showed the importance of extracellular signal-regulated kinase 5 (ERK5) in malignant mesothelioma tumor growth and treatment.

EXPERIMENTAL DESIGN

ERK5 as a target for malignant mesothelioma therapy was verified using mesothelial and mesothelioma cell lines as well as by xenograft severe combined immunodeficient (SCID) mouse models.

RESULTS

We first showed that crocidolite asbestos activated ERK5 in LP9 cells and mesothelioma cell lines exhibit constitutive activation of ERK5. Addition of doxorubicin resulted in further activation of ERK5 in malignant mesothelioma cells. ERK5 silencing increased doxorubicin-induced cell death and doxorubicin retention in malignant mesothelioma cells. In addition, shERK5 malignant mesothelioma lines exhibited both attenuated colony formation on soft agar and invasion of malignant mesothelioma cells in vitro that could be related to modulation of gene expression linked to cell proliferation, apoptosis, migration/invasion, and drug resistance as shown by microarray analysis. Most importantly, injection of shERK5 malignant mesothelioma cell lines into SCID mice showed significant reduction in tumor growth using both subcutaneous and intraperitoneal models. Assessment of selected human cytokine profiles in peritoneal lavage fluid from intraperitoneal shERK5 and control tumor-bearing mice showed that ERK5 was critical in regulation of various proinflammatory (RANTES/CCL5, MCP-1) and angiogenesis-related (interleukin-8, VEGF) cytokines. Finally, use of doxorubicin and cisplatin in combination with ERK5 inhibition showed further reduction in tumor weight and volume in the intraperitoneal model of tumor growth.

CONCLUSION

ERK5 inhibition in combination with chemotherapeutic drugs is a beneficial strategy for combination therapy in patients with malignant mesothelioma.

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.

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.