Importin subunit beta-1 mediates ERK5 nuclear translocation, and its inhibition synergizes with ERK5 kinase inhibitors in reducing cancer cell proliferation

The mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase 5 (ERK5) is emerging as a promising target in cancer. Indeed, alterations of the MEK5/ERK5 pathway are present in many types of cancer, including melanoma. One of the key events in MAPK signalling is MAPK nuclear translocation and its subsequent regulation of gene expression. Likewise, the effects of ERK5 in supporting cancer cell proliferation have been linked to its nuclear localization. Despite many processes regulating ERK5 nuclear translocation having been determined, the nuclear transporters involved have not yet been identified. Here, we investigated the role of importin subunit alpha (α importin) and importin subunit beta-1 (importin β1) in ERK5 nuclear shuttling to identify additional targets for cancer treatment. Either importin β1 knockdown or the α/β1 importin inhibitor ivermectin reduced the nuclear amount of overexpressed and endogenous ERK5 in HEK293T and A375 melanoma cells, respectively. These results were confirmed in single-molecule microscopy in HeLa cells. Moreover, immunofluorescence analysis showed that ivermectin impairs epidermal growth factor (EGF)-induced ERK5 nuclear shuttling in HeLa cells. Both co-immunoprecipitation experiments and proximity ligation assay provided evidence that ERK5 and importin β1 interact and that this interaction is further induced by EGF administration and prevented by ivermectin treatment. The combination of ivermectin and the ERK5 inhibitor AX15836 synergistically reduced cell viability and colony formation ability in A375 and HeLa cells and was more effective than single treatments in preventing the growth of A375 and HeLa spheroids. The increased reduction of cell viability upon the same combination was also observed in patient-derived metastatic melanoma cells. The combination of ivermectin and ERK5 inhibitors other than AX15836 provided similar effects on cell viability. The identification of importin β1 as the nuclear transporter of ERK5 may be exploited for additional ERK5-inhibiting strategies for cancer therapy.

Design, Synthesis, and Biological Evaluation of Proteolysis-Targeting Chimeras as Highly Selective and Efficient Degraders of Extracellular Signal-Regulated Kinase 5

Extracellular signal-regulated kinase 5 (ERK5) is recognized as a key member of the mitogen-activated protein kinase family and is involved in tumor growth, migration, and angiogenesis. However, the results of ERK5 inhibition in multiple studies are controversial, and a highly specific ERK5-targeting agent is required to confirm physiological functions. Using proteolysis-targeting chimera technology, we designed the selective ERK5 degrader PPM-3 and examined its biological effect on cancer cells. Interestingly, the selective degradation of ERK5 with PPM-3 did not influence tumor cell growth directly. Based on proteomics analysis, the ERK5 deletion may be associated with tumor immunity. PPM-3 influences tumor development by affecting the differentiation of macrophages. Therefore, PPM-3 is an effective small-molecule tool for studying ERK5 and a promising immunotherapy drug candidate.

Pathophysiological Impact of the MEK5/ERK5 Pathway in Oxidative Stress

Oxidative stress regulates many physiological and pathological processes. Indeed, a low increase in the basal level of reactive oxygen species (ROS) is essential for various cellular functions, including signal transduction, gene expression, cell survival or death, as well as antioxidant capacity. However, if the amount of generated ROS overcomes the antioxidant capacity, excessive ROS results in cellular dysfunctions as a consequence of damage to cellular components, including DNA, lipids and proteins, and may eventually lead to cell death or carcinogenesis. Both in vitro and in vivo investigations have shown that activation of the mitogen-activated protein kinase kinase 5/extracellular signal-regulated kinase 5 (MEK5/ERK5) pathway is frequently involved in oxidative stress-elicited effects. In particular, accumulating evidence identified a prominent role of this pathway in the anti-oxidative response. In this respect, activation of krüppel-like factor 2/4 and nuclear factor erythroid 2-related factor 2 emerged among the most frequent events in ERK5-mediated response to oxidative stress. This review summarizes what is known about the role of the MEK5/ERK5 pathway in the response to oxidative stress in pathophysiological contexts within the cardiovascular, respiratory, lymphohematopoietic, urinary and central nervous systems. The possible beneficial or detrimental effects exerted by the MEK5/ERK5 pathway in the above systems are also discussed.

Modulation of ERK5 Activity as a Therapeutic Anti-Cancer Strategy

The extracellular signal-regulated kinase 5 (ERK5) signaling pathway is one of four conventional mitogen-activated protein (MAP) kinase pathways. Genetic perturbation of ERK5 has suggested that modulation of ERK5 activity may have therapeutic potential in cancer chemotherapy. This Miniperspective examines the evidence for ERK5 as a drug target in cancer, the structure of ERK5, and the evolution of structurally distinct chemotypes of ERK5 kinase domain inhibitors. The emerging complexities of ERK5 pharmacology are discussed, including the confounding phenomenon of paradoxical ERK5 activation by small-molecule ERK5 inhibitors. The impact of the recent development and biological evaluation of potent and selective bifunctional degraders of ERK5 and future opportunities in ERK modulation are also explored.

VEGF Stimulates Activation of ERK5 in the Absence of C-Terminal Phosphorylation Preventing Nuclear Localization and Facilitating AKT Activation in Endothelial Cells

Extracellular-signal-regulated kinase 5 (ERK5) is critical for normal cardiovascular development. Previous studies have defined a canonical pathway for ERK5 activation, showing that ligand stimulation leads to MEK5 activation resulting in dual phosphorylation of ERK5 on Thr218/Tyr220 residues within the activation loop. ERK5 then undergoes a conformational change, facilitating phosphorylation on residues in the C-terminal domain and translocation to the nucleus where it regulates MEF2 transcriptional activity. Our previous research into the importance of ERK5 in endothelial cells highlighted its role in VEGF-mediated tubular morphogenesis and cell survival, suggesting that ERK5 played a unique role in endothelial cells. Our current data show that in contrast to EGF-stimulated HeLa cells, VEGF-mediated ERK5 activation in human dermal microvascular endothelial cells (HDMECs) does not result in C-terminal phosphorylation of ERK5 and translocation to the nucleus, but instead to a more plasma membrane/cytoplasmic localisation. Furthermore, the use of small-molecule inhibitors to MEK5 and ERK5 shows that instead of regulating MEF2 activity, VEGF-mediated ERK5 is important for regulating AKT activity. Our data define a novel pathway for ERK5 activation in endothelial cells leading to cell survival.

ERK5 Signalling and Resistance to ERK1/2 Pathway Therapeutics: The Path Less Travelled?

The RAS-regulated RAF-MEK1/2-ERK1/2 signalling pathway is frequently de-regulated in human cancer. Melanoma in particular exhibits a high incidence of activating BRAFV600E/K and NRASQ61L/K mutations and such cells are addicted to the activity of these mutant oncoproteins. As a result three different BRAF inhibitors (BRAFi) have now been approved for BRAFV600E/K- mutant melanoma and have transformed the treatment of this disease. Despite this, clinical responses are typically transient as tumour cells develop resistance. These resistance mechanisms frequently involve reinstatement of ERK1/2 signalling and BRAFi are now deployed in combination with one of three approved MEK1/2 inhibitors (MEKi) to provide more durable, but still transient, clinical responses. Furthermore, inhibitors to ERK1/2 (ERK1/2i) have also been developed to counteract ERK1/2 signalling. However, recent studies have suggested that BRAFi/MEKi and ERK1/2i resistance can arise through activation of a parallel signalling pathway leading to activation of ERK5, an unusual protein kinase that contains both a kinase domain and a transcriptional transactivation domain. Here we review the evidence supporting ERK5 as a mediator of BRAFi/MEKi and ERK1/2i resistance. We also review the challenges in targeting ERK5 signalling with small molecules, including paradoxical activation of the transcriptional transactivation domain, and discuss new therapeutic modalities that could be employed to target ERK5.

BAY-885, a mitogen-activated protein kinase kinase 5 inhibitor, induces apoptosis by regulating the endoplasmic reticulum stress/Mcl-1/Bim pathway in breast cancer cells

The mitogen-activated protein kinase kinase 5 (MEK5)/extracellular signal-regulated kinase 5 (ERK5) axis has been reported to promote tumorigenesis in breast cancer (BC). Therefore, targeting the MEK5/ERK5 axis is a potential strategy against BC. BAY-885 is a novel inhibitor of ERK5; however, to date, its anti-tumor effects in BC have not been investigated. This study aimed to assess the anti-tumor effects of BAY-885 in BC and identify its underlying mechanisms of action. Unlike other ERK5 inhibitors, which frequently failed to mimic ERK5 genetic ablation phenotypes, the BAY-885 treatment effectively recapitulated ERK5 depletion effects in BC cells. Results revealed that BAY-885 affected the viability and induced apoptosis in BC cells. Moreover, the BAY-885-mediated downregulation of myeloid cell leukemia-1 (Mcl-1) and upregulation of Bim were dependent on ERK5 inhibition. Furthermore, BAY-885 triggered activation of endoplasmic reticulum (ER) stress, which further led to the upregulation of Bim and downregulation of Mcl-1. ER stress was induced in an ERK5 inhibition-dependent manner. These findings suggested that BAY-885 induced apoptosis in BC cells via ER stress/Mcl-1/Bim axis, suggesting that BAY-885 may serve as a therapeutic agent for BC.

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.

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.

The MEK5/ERK5 Pathway in Health and Disease

The MEK5/ERK5 mitogen-activated protein kinases (MAPK) cascade is a unique signaling module activated by both mitogens and stress stimuli, including cytokines, fluid shear stress, high osmolarity, and oxidative stress. Physiologically, it is mainly known as a mechanoreceptive pathway in the endothelium, where it transduces the various vasoprotective effects of laminar blood flow. However, it also maintains integrity in other tissues exposed to mechanical stress, including bone, cartilage, and muscle, where it exerts a key function as a survival and differentiation pathway. Beyond its diverse physiological roles, the MEK5/ERK5 pathway has also been implicated in various diseases, including cancer, where it has recently emerged as a major escape route, sustaining tumor cell survival and proliferation under drug stress. In addition, MEK5/ERK5 dysfunction may foster cardiovascular diseases such as atherosclerosis. Here, we highlight the importance of the MEK5/ERK5 pathway in health and disease, focusing on its role as a protective cascade in mechanical stress-exposed healthy tissues and its function as a therapy resistance pathway in cancers. We discuss the perspective of targeting this cascade for cancer treatment and weigh its chances and potential risks when considering its emerging role as a protective stress response pathway.

Inhibition of MEK5/ERK5 signaling overcomes acquired resistance to the third generation EGFR inhibitor, osimertinib, via enhancing Bim-dependent apoptosis

The promising therapeutic efficacy of the third generation EGFR inhibitor, osimertinib (AZD9291), for the treatment of patients with EGFR-mutant non-small cell lung cancer (NSCLC) has been demonstrated in the clinic both as first-line and second line therapy. However, inevitable acquired resistance limits its long-term benefit to patients and is thus a significant clinical challenge. The current study focuses on studying the potential role of targeting MEK5-ERK5 signaling in overcoming acquired resistance to osimertinib. Osimertinib and other third generation EGFR inhibitors exerted a rapid and sustained suppressive effect on ERK5 phosphorylation primarily in EGFR-mutant NSCLC cell lines and lost this activity in some osimertinib-resistant cell lines. Osimertinib combined with either ERK5 or MEK5 inhibitors synergistically decreased the survival of osimertinib-resistant cell lines with enhanced induction of apoptosis primarily via augmenting Bim expression. Moreover, the combination effectively inhibited the growth of osimertinib-resistant xenografts in vivo. Together, these findings suggest the potential role of MEK5-ERK5 signaling in modulating development of acquired resistance to osimertinib and value of targeting this signaling as a potential strategy in overcoming acquired resistance to osimertinib and possibly other third generation EGFR inhibitors.

A walk-through MAPK structure and functionality with the 30-year-old yeast MAPK Slt2

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved signaling proteins involved in the regulation of most eukaryotic cellular processes. They are downstream components of essential signal transduction pathways activated by the external stimuli, in which the signal is conveyed through phosphorylation cascades. The excellent genetic and biochemical tractability of simple eukaryotes such as Saccharomyces cerevisiae has significantly contributed to gain fundamental information into the physiology of these key proteins. The budding yeast MAPK Slt2 was identified 30 years ago and was later revealed as a fundamental element of the cell wall integrity (CWI) pathway, one of the five MAPK routes of S. cerevisiae. As occurs with other MAPKs, whereas Slt2 displays the core typical structural traits of eukaryotic protein kinases, it also features conserved domains among MAPKs that allow an exquisite spatio-temporal regulation of their activity and binding to activating kinases, downregulatory phosphatases, or nuclear transcription factors. Additionally, Slt2 bears a regulatory extra C-terminal tail unique among S. cerevisiae MAPKs. Here, we review the structural and functional basis for the signaling role of Slt2 in the context of the molecular architecture of this important family of protein kinases.

Paradoxical activation of the protein kinase-transcription factor ERK5 by ERK5 kinase inhibitors

The dual protein kinase-transcription factor, ERK5, is an emerging drug target in cancer and inflammation, and small-molecule ERK5 kinase inhibitors have been developed. However, selective ERK5 kinase inhibitors fail to recapitulate ERK5 genetic ablation phenotypes, suggesting kinase-independent functions for ERK5. Here we show that ERK5 kinase inhibitors cause paradoxical activation of ERK5 transcriptional activity mediated through its unique C-terminal transcriptional activation domain (TAD). Using the ERK5 kinase inhibitor, Compound 26 (ERK5-IN-1), as a paradigm, we have developed kinase-active, drug-resistant mutants of ERK5. With these mutants, we show that induction of ERK5 transcriptional activity requires direct binding of the inhibitor to the kinase domain. This in turn promotes conformational changes in the kinase domain that result in nuclear translocation of ERK5 and stimulation of gene transcription. This shows that both the ERK5 kinase and TAD must be considered when assessing the role of ERK5 and the effectiveness of anti-ERK5 therapeutics.

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.

Discovery of a Gatekeeper Residue in the C-Terminal Tail of the Extracellular Signal-Regulated Protein Kinase 5 (ERK5)

The extracellular signal-regulated protein kinase 5 (ERK5) is a non-redundant mitogen-activated protein kinase (MAPK) that exhibits a unique C-terminal extension which comprises distinct structural and functional properties. Here, we sought to elucidate the significance of phosphoacceptor sites in the C-terminal transactivation domain of ERK5. We have found that Thr732 acted as a functional gatekeeper residue controlling C-terminal-mediated nuclear translocation and transcriptional enhancement. Consistently, using a non-bias quantitative mass spectrometry approach, we demonstrated that phosphorylation at Thr732 conferred selectivity for binding interactions of ERK5 with proteins related to chromatin and RNA biology, whereas a number of metabolic regulators were associated with full-length wild type ERK5. Additionally, our proteomic analysis revealed that phosphorylation of the Ser730-Glu-Thr732-Pro motif could occur independently of dual phosphorylation at Thr218-Glu-Tyr220 in the activation loop. Collectively, our results firmly establish the significance of C-terminal phosphorylation in regulating ERK5 function. The post-translational modification of ERK5 on its C-terminal tail might be of particular relevance in cancer cells where ERK5 has be found to be hyperphosphoryated.

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.

Cyclin-dependent kinase 1 activity coordinates the chromatin associated state of Oct4 during cell cycle in embryonic stem cells

Cyclin-dependent kinase 1 (Cdk1) is indispensable for embryonic stem cell (ESC) maintenance and embryo development. Even though some reports have described a connection between Cdk1 and Oct4, there is no evidence that Cdk1 activity is directly linked to the ESC pluripotency transcription program. We recently reported that Aurkb/PP1-mediated Oct4 resetting is important to cell cycle maintenance and pluripotency in mouse ESCs (mESCs). In this study, we show that Cdk1 is an upstream regulator of the Oct4 phosphorylation state during cell cycle progression, and it coordinates the chromatin associated state of Oct4 for pluripotency-related gene expression within the cell cycle. Upon entry into mitosis, Aurkb in the chromosome passenger complex becomes fully activated and PP1 activity is inhibited downstream of Cdk1 activation, leading to sustaining Oct4(S229) phosphorylation and dissociation of Oct4 from chromatin during the mitotic phase. Cdk1 inhibition at the mitotic phase abnormally results in Oct4 dephosphorylation, chromosome decondensation and chromatin association of Oct4, even in replicated chromosome. Our study results suggest a molecular mechanism by which Cdk1 directly links the cell cycle to the pluripotency transcription program in mESCs.

TRAIL receptor activation overcomes resistance to trastuzumab in HER2 positive breast cancer cells

The appearance of resistance to the anti-HER2 targeted drug trastuzumab constitutes, nowadays, an important challenge in the oncology clinic. To fight such resistance, we searched for potential vulnerabilities in cells resistant to that drug. To that end, we used cell lines primary resistant to trastuzumab, as well as cells made secondarily resistant to the drug upon continuous exposure. Using genomic and proteomic approaches, a deregulation in cell death pathways was identified in trastuzumab-resistant cells. More precisely, an increased response to the death factor TRAIL, caused by an increase in the cellular receptors for this factor, was observed. In parallel, a decrease in inhibitory components of the pathway was detected. This combination produces a more efficient assembly of the functional complex in the trastuzumab-resistant cells that translates in the observed increased response to TRAIL. Analysis of HER2 positive patient samples confirmed deregulation of this pathway in trastuzumab-resistant patients. Taken together our data identify a vulnerability of trastuzumab-resistant cells that could be used to design new targeted therapies in that context.

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.

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

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.

Antitumoral effect of Ocoxin in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is becoming one of the most prevalent types of cancer worldwide. The most efficient types of treatment at present include surgical resection and liver transplantation, but these treatments may only be used in a small percentage of patients. In order to identify novel therapeutic strategies for this disease, the present study explored the potential antitumoral effect of Ocoxin® oral solution (OOS) in HCC. OOS inhibited the proliferation of HCC cell lines in a time- and dose-dependent manner, being more efficient when used in combination with sorafenib, a standard of care treatment for patients diagnosed with advanced-stage disease. Mechanistic studies indicated that the effect of OOS was due to the induction of cell cycle arrest rather than the stimulation of apoptotic cell death. The cell cycle was slowed down in all phases in the HCC cell lines treated with OOS. Finally, when tested in animal models of HCC, OOS reduced tumor progression through the induction of necrosis in xenograft tumor models. Considering the poor prognosis and high resistance to antitumor treatments of HCC, the antiproliferative action of OOS, particularly in combination with sorafenib, provides the opportunity to investigate the effect of combined therapy in a clinical setting.

CDK5 functions as a tumor promoter in human colorectal cancer via modulating the ERK5-AP-1 axis

Abnormal expression of cyclin-dependent kinase 5 (CDK5) has been found in several human cancers, whereas the role of CDK5 in the malignant development of colorectal cancer (CRC) has not been well characterized. Here we investigated the role of CDK5 in CRC and found that its expression was much higher in CRC tissues than that in normal tissues with a higher expression level of CDK5 closely correlating to advanced American Joint Committee on Cancer (AJCC) stage, poor differentiation, increased tumor size and poor prognosis of CRC. Biological function experiments showed that CDK5 regulated CRC cell proliferation and metastasis ability. Whole-genome microarray analysis, co-immunoprecipitation, in vitro kinase assay, western blotting, luciferase reporter assays and electrophoretic mobility shift assay (EMSA) showed that CDK5 could directly phosphorylate ERK5 at threonine (Thr) 732 and finally modulate the oncogenic ERK5–AP-1 axis. Further researches showed that CDK5–ERK5–AP-1 axis could promote progression of CRC carcinogenesis and had a significant correlation in human CRC samples. In summary, this study revealed the functional and mechanistic links between CDK5 and the oncogenic ERK5–AP-1 signaling pathway in the pathogenesis of CRC. These findings suggest that CDK5 has an important role in CRC development and may serve as a potential therapeutic target for CRC.

ERK5 kinase activity is dispensable for cellular immune response and proliferation

Unlike other members of the MAPK family, ERK5 contains a large C-terminal domain with transcriptional activation capability in addition to an N-terminal canonical kinase domain. Genetic deletion of ERK5 is embryonic lethal, and tissue-restricted deletions have profound effects on erythroid development, cardiac function, and neurogenesis. In addition, depletion of ERK5 is antiinflammatory and antitumorigenic. Small molecule inhibition of ERK5 has been shown to have promising activity in cell and animal models of inflammation and oncology. Here we report the synthesis and biological characterization of potent, selective ERK5 inhibitors. In contrast to both genetic depletion/deletion of ERK5 and inhibition with previously reported compounds, inhibition of the kinase with the most selective of the new inhibitors had no antiinflammatory or antiproliferative activity. The source of efficacy in previously reported ERK5 inhibitors is shown to be off-target activity on bromodomains, conserved protein modules involved in recognition of acetyl-lysine residues during transcriptional processes. It is likely that phenotypes reported from genetic deletion or depletion of ERK5 arise from removal of a noncatalytic function of ERK5. The newly reported inhibitors should be useful in determining which of the many reported phenotypes are due to kinase activity and delineate which can be pharmacologically targeted.

Erk5 contributes to maintaining the balance of cellular nucleotide levels and erythropoiesis

An adequate supply of nucleotides is essential for accurate DNA replication, and inappropriate deoxyribonucleotide triphosphate (dNTP) concentrations can lead to replication stress, a common source of DNA damage, genomic instability and tumourigenesis. Here, we provide evidence that Erk5 is necessary for correct nucleotide supply during erythroid development. Mice with Erk5 knockout in the haematopoietic lineage showed impaired erythroid development in bone marrow, accompanied by altered dNTP levels and increased DNA mutagenesis in erythroid progenitors as detected by exome sequencing. Moreover, Erk5-depleted leukemic Jurkat cells presented a marked sensitivity to thymidine-induced S phase stalling, as evidenced by increased H2AX phosphorylation and apoptosis. The increase in thymidine sensitivity correlated with a higher dTTP/dCTP ratio. These results indicate that Erk5 is necessary to maintain the balance of nucleotide levels, thus preventing dNTP misincorporation and DNA damage in proliferative erythroid progenitors and leukemic Jurkat T cells.

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.

Erk5 Is a Key Regulator of Naive-Primed Transition and Embryonic Stem Cell Identity

Embryonic stem cells (ESCs) can self-renew or differentiate into any cell type, a phenomenon known as pluripotency. Distinct pluripotent states, termed naive and primed pluripotency, have been described. However, the mechanisms that control naive-primed pluripotent transition are poorly understood. Here, we perform a targeted screen for kinase inhibitors, which modulate the naive-primed pluripotent transition. We find that XMD compounds, which selectively inhibit Erk5 kinase and BET bromodomain family proteins, drive ESCs toward primed pluripotency. Using compound selectivity engineering and CRISPR/Cas9 genome editing, we reveal distinct functions for Erk5 and Brd4 in pluripotency regulation. We show that Erk5 signaling maintains ESCs in the naive state and suppresses progression toward primed pluripotency and neuroectoderm differentiation. Additionally, we identify a specialized role for Erk5 in defining ESC lineage selection, whereby Erk5 inhibits a cardiomyocyte-specific differentiation program. Our data therefore reveal multiple critical functions for Erk5 in controlling ESC identity.

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.

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.

Endothelial protective genes induced by statin are mimicked by ERK5 activation as triggered by a drug combination of FTI-277 and GGTI-298

BACKGROUND

Statins are potent inhibitors of cholesterol biosynthesis and are clinically beneficial in preventing cardiovascular diseases, however, the therapeutic utility of these drugs is limited by myotoxicity. Here, we explored the mechanism of statin-mediated activation of ERK5 in the human endothelium with the goal of identifying compounds that confer endothelial protection but are nontoxic to muscle.

METHODS

An ERK5-one hybrid luciferase reporter transfected into COS-7 cells with pharmacological and molecular manipulations dissected the signaling pathway leading to statin activation of ERK5. qRT-PCR of HUVEC cells documented the transcriptional activation of endothelial-protective genes. Lastly, morphological and cellular ATP analysis, and induction of atrogin-1 in C2C12 myotubes were used to assess statin-induced myopathy.

RESULTS

Statin activation of ERK5 is dependent on the cellular reduction of GGPPs. Furthermore, we found that the combination of FTI-277 (inhibitor of farnesyl transferase) and GGTI-298 (inhibitor of geranylgeranyl transferase I) mimicked the statin-mediated activation of ERK5. FTI-277 and GGTI-298 together recapitulated the beneficial effects of statins by transcriptionally upregulating anti-inflammatory mediators such as eNOS, THBD, and KLF2. Finally, C2C12 skeletal myotubes treated with both FTI-277 and GGTI-298 evoked less morphological and cellular changes recognized as biomarkers of statin-associated myopathy.

CONCLUSIONS

Statin-induced endothelial protection and myopathy are mediated by distinct metabolic intermediates and co-inhibition of farnesyl transferase and geranylgeranyl transferase I confer endothelial protection without myopathy.

GENERAL SIGNIFICANCE

The combinatorial FTI-277 and GGTI-298 drug regimen provides a promising alternative avenue for endothelial protection without myopathy.

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.

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.

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.

Suppression of protein kinase C theta contributes to enhanced myogenesis in vitro via IRS1 and ERK1/2 phosphorylation

Background

Differentiation and fusion of skeletal muscle myoblasts into multi-nucleated myotubes is required for neonatal development and regeneration in adult skeletal muscle. Herein, we report novel findings that protein kinase C theta (PKCθ) regulates myoblast differentiation via phosphorylation of insulin receptor substrate-1 and ERK1/2.

Results

In this study, PKCθ knockdown (PKCθ^shRNA) myotubes had reduced inhibitory insulin receptor substrate-1 ser1095 phosphorylation, enhanced myoblast differentiation and cell fusion, and increased rates of protein synthesis as determined by [³H] phenylalanine incorporation. Phosphorylation of insulin receptor substrate-1 ser632/635 and extracellular signal-regulated kinase1/2 (ERK1/2) was increased in PKCθ^shRNA cells, with no change in ERK5 phosphorylation, highlighting a PKCθ-regulated myogenic pathway. Inhibition of PI3-kinase prevented cell differentiation and fusion in control cells, which was attenuated in PKCθ^shRNA cells. Thus, with reduced PKCθ, differentiation and fusion occur in the absence of PI3-kinase activity. Inhibition of the ERK kinase, MEK1/2, impaired differentiation and cell fusion in control cells. Differentiation was preserved in PKCθ^shRNA cells treated with a MEK1/2 inhibitor, although cell fusion was blunted, indicating PKCθ regulates differentiation via IRS1 and ERK1/2, and this occurs independently of MEK1/2 activation.

Conclusion

Cellular signaling regulating the myogenic program and protein synthesis are complex and intertwined. These studies suggest that PKCθ regulates myogenic and protein synthetic signaling via the modulation of IRS1and ERK1/2 phosphorylation. Myotubes lacking PKCθ had increased rates of protein synthesis and enhanced myotube development despite reduced activation of the canonical anabolic-signaling pathway. Further investigation of PKCθ regulated signaling may reveal important interactions regulating skeletal muscle health in an insulin resistant state.

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.

Active kinase profiling, genetic and pharmacological data define mTOR as an important common target in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is an aggressive form of breast cancer. Despite response to chemotherapy, relapses are frequent and resistance to available treatments is often seen in the metastatic setting. Therefore, identification of new therapeutic targets is required. With this aim, we have profiled the activation status of 44 receptor tyrosine kinases (RTKs) and their major signaling pathways in patient-derived TNBC tumors. Frequent co-activation of several RTKs as well as the extracellular signal-regulated protein kinases 1 and 2 (Erk1/2) and mammalian target of rapamycin (mTOR) routes was found. Pharmacologic targeting of the activated kinases indicated that agents that attack the mTOR route are more potent and efficient antitumoral treatments than agents targeting RTKs. mTOR signals through two multiprotein complexes, mTORC1 and mTORC2. We used a genetic approach to explore the contribution of each of the two mTOR branches to the regulation of cell number of TNBC cells. RNA interference experiments indicated that mTORC1 predominated over mTORC2 in the control of TNBC cell proliferation. Moreover, RNA interference of mTOR had a superior antiproliferative action than separately acting on mTORC1 or mTORC2. To analyze the relevance of mTOR targeting in vivo, we used mice with TNBC. Treatment of these mice with BEZ235, a drug that targets mTOR, slowed tumor growth. Mechanistically, BEZ235 delayed cell cycle progression without affecting cell viability. Our results show that TNBCs are particularly sensitive to inhibition of the mTOR pathway, and indicate that mTOR targeting may be a more efficient anti-TNBC therapy than exclusively acting on the mTORC1 branch of the pathway. This is relevant as most mTOR inhibitors used in the clinic act on mTORC1. Collectively with the fact that BEZ235 synergized with drugs commonly used in the treatment of TNBC, our data support the clinical development of agents that act on mTOR as a therapy for this disease.

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.

ERK5 and its role in tumour development

The MEK5 [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase 5]/ERK5 pathway is the least well studied MAPK signalling module. It has been proposed to play a role in the pathology of cancer. In the present paper, we review the role of the MEK5/ERK5 pathway using the 'hallmarks of cancer' as a framework and consider how this pathway is deregulated. As well as playing a key role in endothelial cell survival and tubular morphogenesis during tumour neovascularization, ERK5 is also emerging as a regulator of tumour cell invasion and migration. Several oncogenes can stimulate ERK5 activity, and protein levels are increased by a novel amplification at chromosome locus 17p11 and by down-regulation of the microRNAs miR-143 and miR-145. Together, these finding underscore the case for further investigation into understanding the role of ERK5 in cancer.

Protein kinase C (PKC)ζ-mediated Gαq stimulation of ERK5 protein pathway in cardiomyocytes and cardiac fibroblasts

Gq-coupled G protein-coupled receptors (GPCRs) mediate the actions of a variety of messengers that are key regulators of cardiovascular function. Enhanced Gα(q)-mediated signaling plays an important role in cardiac hypertrophy and in the transition to heart failure. We have recently described that Gα(q) acts as an adaptor protein that facilitates PKCζ-mediated activation of ERK5 in epithelial cells. Because the ERK5 cascade is known to be involved in cardiac hypertrophy, we have investigated the potential relevance of this pathway in cardiovascular Gq-dependent signaling using both cultured cardiac cell types and chronic administration of angiotensin II in mice. We find that PKCζ is required for the activation of the ERK5 pathway by Gq-coupled GPCR in neonatal and adult murine cardiomyocyte cultures and in cardiac fibroblasts. Stimulation of ERK5 by angiotensin II is blocked upon pharmacological inhibition or siRNA-mediated silencing of PKCζ in primary cultures of cardiac cells and in neonatal cardiomyocytes isolated from PKCζ-deficient mice. Moreover, upon chronic challenge with angiotensin II, these mice fail to promote the changes in the ERK5 pathway, in gene expression patterns, and in hypertrophic markers observed in wild-type animals. Taken together, our results show that PKCζ is essential for Gq-dependent ERK5 activation in cardiomyocytes and cardiac fibroblasts and indicate a key cardiac physiological role for the Gα(q)/PKCζ/ERK5 signaling axis.

Deficient spindle assembly checkpoint in multiple myeloma

Multiple myeloma (MM) is a hematological disease characterized by an abnormal accumulation of plasma cells in the bone marrow. These cells have frequent cytogenetic abnormalities including translocations of the immunoglobulin heavy chain gene and chromosomal gains and losses. In fact, a singular characteristic differentiating MM from other hematological malignancies is the presence of a high degree of aneuploidies. As chromosomal abnormalities can be generated by alterations in the spindle assembly checkpoint (SAC), the functionality of such checkpoint was tested in MM. When SAC components were analyzed in MM cell lines, the RNA levels of most of them were conserved. Nevertheless, the protein content of some key constituents was very low in several cell lines, as was the case of MAD2 or CDC20 in RPMI-8226 or RPMI-LR5 cells. The recovery of their cellular content did not substantially affect cell growth, but improved their ability to segregate chromosomes. Finally, SAC functionality was tested by challenging cells with agents disrupting microtubule dynamics. Most of the cell lines analyzed exhibited functional defects in this checkpoint. Based on the data obtained, alterations both in SAC components and their functionality have been detected in MM, pointing to this pathway as a potential target in MM treatment.