BMK1 mediates growth factor-induced cell proliferation through direct cellular activation of serum and glucocorticoid-inducible kinase

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.

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.

Etiopathogenic role of ERK5 signaling in sarcoma: prognostic and therapeutic implications

Sarcomas constitute a heterogeneous group of rare and difficult-to-treat tumors that can affect people of all ages, representing one of the most common forms of cancer in childhood and adolescence. Little is known about the molecular entities involved in sarcomagenesis. Therefore, the identification of processes that lead to the development of the disease may uncover novel therapeutic opportunities. Here, we show that the MEK5/ERK5 signaling pathway plays a critical role in the pathogenesis of sarcomas. By developing a mouse model engineered to express a constitutively active form of MEK5, we demonstrate that the exclusive activation of the MEK5/ERK5 pathway can promote sarcomagenesis. Histopathological analyses identified these tumors as undifferentiated pleomorphic sarcomas. Bioinformatic studies revealed that sarcomas are the tumors in which ERK5 is most frequently amplified and overexpressed. Moreover, analysis of the impact of ERK5 protein expression on overall survival in patients diagnosed with different sarcoma types in our local hospital showed a 5-fold decrease in median survival in patients with elevated ERK5 expression compared with those with low expression. Pharmacological and genetic studies revealed that targeting the MEK5/ERK5 pathway drastically affects the proliferation of human sarcoma cells and tumor growth. Interestingly, sarcoma cells with knockout of ERK5 or MEK5 were unable to form tumors when engrafted into mice. Taken together, our results reveal a role of the MEK5/ERK5 pathway in sarcomagenesis and open a new scenario to be considered in the treatment of patients with sarcoma in which the ERK5 pathway is pathophysiologically involved.

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 signalling pathway is a novel target of sorafenib: Implication in EGF biology

Sorafenib is a multikinase inhibitor widely used in cancer therapy with an antitumour effect related to biological processes as proliferation, migration or invasion, among others. Initially designed as a Raf inhibitor, Sorafenib was later shown to also block key molecules in tumour progression such as VEGFR and PDGFR. In addition, sorafenib has been connected with key signalling pathways in cancer such as EGFR/EGF. However, no definitive clue about the molecular mechanism linking sorafenib and EGF signalling pathway has been established so far. Our data in HeLa, U2OS, A549 and HEK293T cells, based on in silico, chemical and genetic approaches demonstrate that the MEK5/ERK5 signalling pathway is a novel target of sorafenib. In addition, our data show how sorafenib is able to block MEK5-dependent phosphorylation of ERK5 in the Ser218/Tyr220, affecting the transcriptional activation associated with ERK5. Moreover, we demonstrate that some of the effects of this kinase inhibitor onto EGF biological responses, such as progression through cell cycle or migration, are mediated through the effect exerted onto ERK5 signalling pathway. Therefore, our observations describe a novel target of sorafenib, the ERK5 signalling pathway, and establish new mechanistic insights for the antitumour effect of this multikinase inhibitor.

Curcumin and Carnosic Acid Cooperate to Inhibit Proliferation and Alter Mitochondrial Function of Metastatic Prostate Cancer Cells

Anticancer activities of plant polyphenols have been demonstrated in various models of neoplasia. However, evidence obtained in numerous in vitro studies indicates that proliferation arrest and/or killing of cancer cells require quite high micromolar concentrations of polyphenols that are difficult to reach in vivo and can also be (geno)toxic to at least some types of normal cells. The ability of certain polyphenols to synergize with one another at low concentrations can be used as a promising strategy to effectively treat human malignancies. We have recently reported that curcumin and carnosic acid applied at non-cytotoxic concentrations synergistically cooperate to induce massive apoptosis in acute myeloid leukemia cells, but not in normal hematopoietic and non-hematopoietic cells, via sustained cytosolic calcium overload. Here, we show that the two polyphenols can also synergistically suppress the growth of DU145 and PC-3 metastatic prostate cancer cell cultures. However, instead of cell killing, the combined treatment induced a marked inhibition of cell proliferation associated with G₀/G₁ cell cycle arrest. This was preceded by transient elevation of cytosolic calcium levels and prolonged dissipation of the mitochondrial membrane potential, without generating oxidative stress, and was associated with defective oxidative phosphorylation encompassing mitochondrial dysfunction. The above effects were concomitant with a significant downregulation of mRNA and protein expression of the oncogenic kinase SGK1, the mitochondria-hosted mTOR component. In addition, a moderate decrease in SGK1 phosphorylation at Ser422 was observed in polyphenol-treated cells. The mTOR inhibitor rapamycin produced a similar reduction in SGK1 mRNA and protein levels as well as phosphorylation. Collectively, our findings suggest that the combination of curcumin and carnosic acid at potentially bioavailable concentrations may effectively target different types of cancer cells by distinct modes of action. This and similar combinations merit further exploration as an anticancer modality.

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.

Cutaneous and hepatic vascular lesions due to a recurrent somatic GJA4 mutation reveal a pathway for vascular malformation

The term “cavernous hemangioma” has been used to describe vascular anomalies with histology featuring dilated vascular spaces, vessel walls consisting mainly of fibrous stromal bands lined by a layer of flattened endothelial cells, and an irregular outer rim of interrupted smooth muscle cells. Hepatic hemangiomas (HHs) and cutaneous venous malformations (VMs) share this histologic pattern, and we examined lesions in both tissues to identify genetic drivers. Paired whole-exome sequencing (WES) of lesional tissue and normal liver in HH subjects revealed a recurrent GJA4 c.121G>T (p.Gly41Cys) somatic mutation in four of five unrelated individuals, and targeted sequencing in paired tissue from 9 additional HH individuals identified the same mutation in 8. In cutaneous lesions, paired targeted sequencing in 5 VMs and normal epidermis found the same GJA4 c.121G>T (p.Gly41Cys) somatic mutation in three. GJA4 encodes gap junction protein alpha 4, also called connexin 37 (Cx37), and the p.Gly41Cys mutation falls within the first transmembrane domain at a residue highly conserved among vertebrates. We interrogated the impact of the Cx37 mutant via lentiviral transduction of primary human endothelial cells. We found that the mutant induced changes in cell morphology and activated serum/glucocorticoid-regulated kinase 1 (SGK1), a serine/threonine kinase known to regulate cell proliferation and apoptosis, via non-canonical activation. Treatment with spironolactone, an inhibitor of angiogenesis, suppressed mutant SGK1 activation and reversed changes in cell morphology. These findings identify a recurrent somatic GJA4 c.121G>T mutation as a driver of hepatic and cutaneous VMs, revealing a new pathway for vascular anomalies, with spironolactone a potential pathogenesis-based therapy.

Serum and Glucocorticoid-Inducible Kinase 1 (SGK1) in NSCLC Therapy

Non-small cell lung cancer (NSCLC) remains the most prevalent and one of the deadliest cancers worldwide. Despite recent success, there is still an urgent need for new therapeutic strategies. It is also becoming increasingly evident that combinatorial approaches are more effective than single modality treatments. This review proposes that the serum and glucocorticoid-inducible kinase 1 (SGK1) may represent an attractive target for therapy of NSCLC. Although ubiquitously expressed, SGK1 deletion in mice causes only mild defects of ion physiology. The frequent overexpression of SGK1 in tumors is likely stress-induced and provides a therapeutic window to spare normal tissues. SGK1 appears to promote oncogenic signaling aimed at preserving the survival and fitness of cancer cells. Most importantly, recent investigations have revealed the ability of SGK1 to skew immune-cell differentiation toward pro-tumorigenic phenotypes. Future studies are needed to fully evaluate the potential of SGK1 as a therapeutic target in combinatorial treatments of NSCLC. However, based on what is currently known, SGK1 inactivation can result in anti-oncogenic effects both on tumor cells and on the immune microenvironment. A first generation of small molecules to inactivate SGK1 has already been already produced.

Behavioral effects of SGK1 knockout in VTA and dopamine neurons

Drugs of abuse cause significant neuroadaptations within the ventral tegmental area (VTA), with alterations in gene expression tied to changes in reward behavior. Serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription, catalytic activity, and phosphorylation are upregulated in the VTA by chronic cocaine or morphine treatment, positioning SGK1 as a critical mediator of reward behavior. Using transgenic mouse models, we investigated the effect of SGK1 knockout in the VTA and in dopamine (DA) neurons to evaluate the necessity of protein expression for natural and drug reward behaviors. SGK1 knockdown in the VTA did not impact reward behaviors. Given VTA cellular heterogeneity, we also investigated a DA neuron-specific SGK1 knockout (KO). DA SGK1 KO significantly decreased body weight of adult mice as well as increased general locomotor activity; however, reward behaviors were similarly unaltered. Given that SGK1 mutants virally overexpressed in the VTA are capable of altering drug-associated behavior, our current results suggest that changes in SGK1 protein signaling may be distinct from expression. This work yields novel information on the impact of SGK1 deletion, critical for understanding the role of SGK1 signaling in the central nervous system and evaluating SGK1 as a potential therapeutic target for treatment of substance use disorders.

The PI3K-AKT-mTOR Pathway and Prostate Cancer: At the Crossroads of AR, MAPK, and WNT Signaling

Oncogenic activation of the phosphatidylinositol-3-kinase (PI3K), protein kinase B (PKB/AKT), and mammalian target of rapamycin (mTOR) pathway is a frequent event in prostate cancer that facilitates tumor formation, disease progression and therapeutic resistance. Recent discoveries indicate that the complex crosstalk between the PI3K-AKT-mTOR pathway and multiple interacting cell signaling cascades can further promote prostate cancer progression and influence the sensitivity of prostate cancer cells to PI3K-AKT-mTOR-targeted therapies being explored in the clinic, as well as standard treatment approaches such as androgen-deprivation therapy (ADT). However, the full extent of the PI3K-AKT-mTOR signaling network during prostate tumorigenesis, invasive progression and disease recurrence remains to be determined. In this review, we outline the emerging diversity of the genetic alterations that lead to activated PI3K-AKT-mTOR signaling in prostate cancer, and discuss new mechanistic insights into the interplay between the PI3K-AKT-mTOR pathway and several key interacting oncogenic signaling cascades that can cooperate to facilitate prostate cancer growth and drug-resistance, specifically the androgen receptor (AR), mitogen-activated protein kinase (MAPK), and WNT signaling cascades. Ultimately, deepening our understanding of the broader PI3K-AKT-mTOR signaling network is crucial to aid patient stratification for PI3K-AKT-mTOR pathway-directed therapies, and to discover new therapeutic approaches for prostate cancer that improve patient outcome.

Serum- and glucocorticoid-inducible kinase 1 activity reduces dendritic spines in dorsal hippocampus

The hippocampus has a well-known role in mediating learning and memory, and its function can be directly regulated by both stress and glucocorticoid receptor activation. Hippocampal contributions to learning are thought to be dependent on changes in the plasticity of synapses within specific subregions, and these functional changes are accompanied by morphological changes in the number and shape of dendritic spines, the physical correlates of these glutamatergic synapses. Serum- and glucocorticoid-inducible kinase 1 (SGK1) regulates dendritic spine morphology in the prefrontal cortex, and modulation of SGK1 expression in mouse hippocampus regulates learning. However, the role of SGK1 in dendritic spine morphology within the CA1 and dentate gyrus regions of the hippocampus are unknown. Thus, herpes simplex viral vectors expressing GFP and various SGK1 constructs, including wild type SGK1, a catalytically inactive version of SGK1 (K127Q), and a phospho-defective version of SGK1 (S78A), were infused into the hippocampus of adult mice and confocal fluorescent microscopy was used to visualize dendritic spines. We show that increasing expression of SGK1 in the dentate gyrus increased the total number of spines, driven primarily by an increase in mushroom spines, while decreasing SGK1 activity (K127Q) in the CA1 region increased the total number of dendritic spines, driven by a significant increase in mushroom and stubby spines. The differential effects of SGK1 in these regions may be mediated by the interactions of SGK1 with multiple pathways required for spine formation and stability. As the formation of mature synapses is a crucial component of learning and memory, this indicates that SGK1 is a potential target in the pathway underlying stress-associated changes in cognition and memory.

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.

SGK regulates pH increase and cyclin B-Cdk1 activation to resume meiosis in starfish ovarian oocytes

Tight regulation of intracellular pH (pH_i) is essential for biological processes. Fully grown oocytes, having a large nucleus called the germinal vesicle, arrest at meiotic prophase I. Upon hormonal stimulus, oocytes resume meiosis to become fertilizable. At this time, the pH_i increases via Na⁺/H⁺ exchanger activity, although the regulation and function of this change remain obscure. Here, we show that in starfish oocytes, serum- and glucocorticoid-regulated kinase (SGK) is activated via PI3K/TORC2/PDK1 signaling after hormonal stimulus and that SGK is required for this pH_i increase and cyclin B-Cdk1 activation. When we clamped the pH_i at 6.7, corresponding to the pH_i of unstimulated ovarian oocytes, hormonal stimulation induced cyclin B-Cdk1 activation; thereafter, oocytes failed in actin-dependent chromosome transport and spindle assembly after germinal vesicle breakdown. Thus, this SGK-dependent pH_i increase is likely a prerequisite for these events in ovarian oocytes. We propose a model that SGK drives meiotic resumption via concomitant regulation of the pH_i and cell cycle machinery.

PLPP/CIN-mediated NEDD4-2 S448 dephosphorylation regulates neuronal excitability via GluA1 ubiquitination

Neuronal precursor cell expressed developmentally downregulated 4-2 (NEDD4-2) is an E3 ubiquitin ligase to regulate ion transport by controlling cellular trafficking/endocytosis and lysosomal degradation of ion channels and transporters. Thus, NEDD4-2 is relevant to neuronal excitability and epileptic encephalopathies in human patients. However, the regulatory molecules for NEDD4-2 dephosphorylation have been still elusive. Here, we demonstrate that pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN) specifically dephosphorylated NEDD4-2 serine (S) 448 site. PLPP/CIN deletion inhibited NEDD4-2 ubiquitination, and diminished the responsiveness of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor (AMPAR) by facilitating NEDD4-2-mediated ubiquitination of GluA1 subunit under physiological condition. PLPP/CIN overexpression reversed these effects. These PLPP/CIN-mediated processes were required for the increased seizure severity and its progression in response to kainic acid (KA). Therefore, we suggest the novel function of PLPP/CIN as a NEDD4-2 phosphatase, which may be a potential therapeutic target for NEDD4-2-associated diseases as well as various neurological and psychiatric disorders, including epilepsy.

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.

Molecular regulation of ERK5 in development of diabetic retinopathy

Diabetic retinopathy (DR) is a major complication of diabetes, and causes pathological changes in retina blood vessels, as the most common cause of vision loss. Extracellular-signal-regulated kinase 5 (ERK5) is the newest discovered member in the mitogen-activated protein kinases (MAPKs) family, and recent evidence demonstrates an essential role of ERK5 signaling in the angiogenesis. However, whether ERK5 signaling may regulate DR development is unknown. Here, we used a streptozocin (STZ)-induce mouse DR model to investigate this question. We detected significant increases in the phosphorylation of ERK5, a signature of ERK5 activation in the purified retinal endothelial cells in DR mice, compared to control mice. In vivo suppression of ERK5 phosphorylation through administration of a specific inhibitor of ERK5 activation, BIX02189, did not prevent the occurrence of STZ-induced diabetes in mice, but significantly alleviated the severity of DR, seemingly through attenuating the retina neovascularization. Thus, our study suggests a previously unappreciated role of ERK5 signaling in DR development.

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.

Protein kinases: mechanisms and downstream targets in inflammation-mediated obesity and insulin resistance

Obesity-induced low-grade inflammation (metaflammation) impairs insulin receptor signaling. This has been implicated in the development of insulin resistance. Insulin signaling in the target tissues is mediated by stress kinases such as p38 mitogen-activated protein kinase, c-Jun NH2-terminal kinase, inhibitor of NF-kB kinase complex β (IKKβ), AMP-activated protein kinase, protein kinase C, Rho-associated coiled-coil containing protein kinase, and RNA-activated protein kinase. Most of these kinases phosphorylate several key regulators in glucose homeostasis. The phosphorylation of serine residues in the insulin receptor and IRS-1 molecule results in diminished enzymatic activity in the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. This has been one of the key mechanisms observed in the tissues that are implicated in insulin resistance especially in type 2 diabetes mellitus (T2-DM). Identifying the specific protein kinases involved in obesity-induced chronic inflammation may help in developing the targeted drug therapies to minimize the insulin resistance. This review is focused on the protein kinases involved in the inflammatory cascade and molecular mechanisms and their downstream targets with special reference to obesity-induced T2-DM.

DNA damage induced apoptosis suppressor (DDIAS) is upregulated via ERK5/MEF2B signaling and promotes β-catenin-mediated invasion

DNA damage induced apoptosis suppressor (DDIAS) is an anti-apoptotic protein that promotes cancer cell survival. We previously reported that DDIAS is transcriptionally activated by nuclear factor of activated T cells 2 (NFATc1). However, the upstream regulation of DDIAS expression by growth factors has not been studied. Here, we demonstrate that DDIAS expression is induced by extracellular signal-regulated kinase 5 (ERK5) and myocyte enhancer factor 2B (MEF2B) in response to epidermal growth factor (EGF) and that it positively regulates β-catenin signaling in HeLa cells. The genetic or pharmacological inhibition of ERK5 suppressed DDIAS induction following EGF exposure and the overexpression of constitutively active MEK5 (CA-MEK5) enhanced DDIAS expression. In chromatin immunoprecipitation assays, MEF2B, a downstream target of ERK5, exhibited sequence-specific binding to a MEF2 binding site in the DDIAS promoter following treatment with EGF. The overexpression of MEF2B increased the EGF-mediated induction of DDIAS expression, whereas the knockdown of MEF2B impaired this effect. Furthermore, DDIAS promoted invasion by increasing β-catenin expression at the post-translational level in response to EGF, suggesting that DDIAS plays a crucial role in the metastasis of cancer cells by regulating β-catenin expression. It is unlikely that MEF2B and NFATc1 cooperatively regulate DDIAS transcription in response to EGF. Collectively, EGF activates the ERK5/MEF2 pathway, which in turn induces DDIAS expression to promote cancer cell invasion by activating β-catenin target genes.

Platelet-derived growth factor (PDGF)-induced activation of Erk5 MAP-kinase is dependent on Mekk2, Mek1/2, PKC and PI3-kinase, and affects BMP signaling

Platelet-derived growth factor-BB (PDGF-BB) binds to its tyrosine kinase receptors (PDGFRs) and stimulates mitogenicity and survival of cells of mesenchymal origin. Activation of PDGFRs initiates a number of downstream signaling pathways, including phosphatidyl 3'-inositol kinase (PI3-kinase), phospholipase Cγ and MAP kinase pathways. In this report, we show that Erk5 MAP kinase is activated in response to PDGF-BB in the smooth muscle cell line MOVAS in a manner dependent on Mekk2, Mek1/2, Mek5, PI3-kinase and protein kinase C (PKC). The co-operation of Mek1/2 and Mekk2 in the activation of Erk5, suggests a close co-regulation between the Erk1/2 and Erk5 MAP kinase pathways. Furthermore, we found that classical PKCs are important for Erk5 activation. In addition, we found that PKCζ interacts with Erk5 and may exert a negative feed-back effect. We observed no nuclear accumulation of Erk5 in response to PDGF-BB stimulation, however, we identified a mechanism by which cytoplasmic Erk5 influences gene expression; Erk5 was essential for PDGF-BB-mediated Smad1/5/8 signaling by stimulating release and/or activation of bone morphogenetic protein(s) (BMPs). Thus, PDGF-BB-induced Erk5 activation involves parallel stimulatory and inhibitory pathways and promotes Smad1/5/8 signaling.

Estrogen receptor β regulates endometriotic cell survival through serum and glucocorticoid-regulated kinase activation

OBJECTIVE

To determine the expression and biological roles of serum and glucocorticoid-regulated kinase (SGK1) in tissues and cells from patients with endometriosis and from healthy control subjects.

DESIGN

Case-control.

SETTING

University research setting.

PATIENT(S)

Premenopausal women.

INTERVENTION(S)

Endometriotic tissues were obtained from women with ovarian endometriosis, and normal endometrial tissues were obtained from women undergoing hysterectomy for benign conditions.

MAIN OUTCOME MEASURE(S)

Expression levels of SGK1, the role of SGK1 in endometriosis pathology, and regulation of SGK1 by estrogen receptor (ER) β.

RESULT(S)

Transcript and protein levels of SGK1 were significantly higher in endometriotic tissues and cells compared with normal endometrium. SGK1 mRNA and protein levels were stimulated by E2, by the ERβ-selective agonist diarylpropionitrile, and by prostaglandin E2. SGK1 was transcriptionally regulated by ERβ based on small interfering RNA knockdown and chromatin immunoprecipitation of ERβ followed by quantitative polymerase chain reaction. SGK1 knockdown led to increased cleavage of poly(ADP-ribose) polymerase, and SGK1 activation was correlated with the phosphorylation of FOXO3a, a proapoptotic factor.

CONCLUSION(S)

ERβ leads to SGK1 overexpression in endometriosis, which contributes to the survival of endometriotic lesions through inhibition of apoptosis.

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

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

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.

Combinatorial therapy with tamoxifen and trifluoperazine effectively inhibits malignant peripheral nerve sheath tumor growth by targeting complementary signaling cascades

Chemotherapeutic agents effective against malignant peripheral nerve sheath tumors (MPNSTs) are urgently needed. We recently found that tamoxifen potently impedes xenograft growth. In vitro, tamoxifen inhibits MPNST proliferation and survival in an estrogen receptor-independent manner; these effects are phenocopied by the calmodulin inhibitor trifluoperazine. The present study was performed to establish the mechanism of action of tamoxifen in vivo and optimize its therapeutic effectiveness. To determine if tamoxifen has estrogen receptor-dependent effects in vivo, we grafted MPNST cells in castrated and ovariectomized mice; xenograft growth was unaffected by reductions in sex hormones. To establish whether tamoxifen and trifluoperazine additively or synergistically impede MPNST growth, mice xenografted with neurofibromatosis type 1-associated or sporadic MPNST cells were treated with tamoxifen, trifluoperazine, or both drugs for 30 days. Both monotherapies inhibited graft growth by 50%, whereas combinatorial treatment maximally reduced graft mass by 90% and enhanced decreases in proliferation and survival. Kinomic analyses showed that tamoxifen and trifluoperazine have both shared and distinct targets in MPNSTs. In addition, trifluoperazine prevented tamoxifen-induced increases in serum/glucocorticoid regulated kinase 1, a protein linked to tamoxifen resistance. These findings suggest that combinatorial therapy with tamoxifen and trifluoperazine is effective against MPNSTs because these agents target complementary pathways that are essential for MPNST pathogenesis.

Increased expression and activation of serum- and glucocorticoid-inducible kinase-1 (SGK1) by cadmium in HK-2 renal proximal tubular epithelial cells

In HK-2 cells exposed to cadmium chloride (CdCl2), the level of serum- and glucocorticoid-inducible kinase-1 (SGK1) protein is increased, but the levels of SGK2 and SGK3 proteins are not. Phosphorylation of SGK1 protein is also observed. Treatment with actinomycin D abolished CdCl2-induced elevation of SGK1 mRNA level. Treatment with actinomycin D or cycloheximide suppressed SGK1 protein levels in cells exposed to CdCl2. Treatment with SGK1 inhibitor EMD638683 or knockdown of SGK1 with siRNA suppressed CdCl2-induced phosphorylation of N-Myc downstream-regulated kinase 1 (NDRG1). These results indicate that cadmium induces the transcriptional upregulation of SGK1 expression and regulates NDRG1 in HK-2 cells.

Morphine and cocaine increase serum- and glucocorticoid-inducible kinase 1 activity in the ventral tegmental area

Drugs of abuse modulate the function and activity of the mesolimbic dopamine circuit. To identify novel mediators of drug-induced neuroadaptations in the ventral tegmental area (VTA), we performed RNA sequencing analysis on VTA samples from mice administered repeated saline, morphine, or cocaine injections. One gene that was similarly up-regulated by both drugs was serum- and glucocorticoid-inducible kinase 1 (SGK1). SGK1 activity, as measured by phosphorylation of its substrate N-myc downstream regulated gene (NDRG), was also increased robustly by chronic drug treatment. Increased NDRG phosphorylation was evident 1 but not 24 h after the last drug injection. SGK1 phosphorylation itself was similarly modulated. To determine the role of increased SGK1 activity on drug-related behaviors, we over-expressed constitutively active (CA) SGK1 in the VTA. SGK1-CA expression reduced locomotor sensitization elicited by repeated cocaine, but surprisingly had the opposite effect and promoted locomotor sensitization to morphine, without affecting the initial locomotor responses to either drug. SGK1-CA expression did not significantly affect morphine or cocaine conditioned place preference, although there was a trend toward increased conditioned place preference with both drugs. Further characterizing the role of this kinase in drug-induced changes in VTA may lead to improved understanding of neuroadaptations critical to drug dependence and addiction. We find that repeated, but not acute, morphine or cocaine administration induces an increase in serum- and glucocorticoid-inducible kinase (SGK1) gene expression and activity in the ventral tegmental area (VTA). This increase in SGK1 activity may play a role in drug-dependent behaviors and suggests a novel signaling cascade for potential intervention in drug dependence and addiction.

Glucose activates TORC2-Gad8 protein via positive regulation of the cAMP/cAMP-dependent protein kinase A (PKA) pathway and negative regulation of the Pmk1 protein-mitogen-activated protein kinase pathway

The target of rapamycin (TOR) kinase belongs to the highly conserved eukaryotic family of phosphatidylinositol 3-kinase-related kinases. TOR proteins are found at the core of two evolutionary conserved complexes, known as TORC1 and TORC2. In fission yeast, TORC2 is dispensable for proliferation under optimal growth conditions but is required for starvation and stress responses. TORC2 has been implicated in a wide variety of functions; however, the signals that regulate TORC2 activity have so far remained obscure. TORC2 has one known direct substrate, the AGC kinase Gad8, which is related to AKT in human cells. Gad8 is phosphorylated by TORC2 at Ser-546 (equivalent to AKT Ser-473), leading to its activation. Here, we show that glucose is necessary and sufficient to induce Gad8 Ser-546 phosphorylation in vivo and Gad8 kinase activity in vitro. The glucose signal that activates TORC2-Gad8 is mediated via the cAMP/PKA pathway, a major glucose-sensing pathway. By contrast, Pmk1, similar to human extracellular signal-regulated kinases and a major stress-induced mitogen activated protein kinase (MAPK) in fission yeast, inhibits TORC2-dependent Gad8 phosphorylation and activation. Inhibition of TORC2-Gad8 also occurs in response to ionic or osmotic stress, in a manner dependent on the cAMP/PKA and Pmk1-MAPK signaling pathways. Our findings highlight the significance of glucose availability in regulation of TORC2-Gad8 and indicate a novel link between the cAMP/PKA, Pmk1/MAPK, and TORC2-Gad8 signaling.

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

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

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

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

High-glucose-based peritoneal dialysis solution induces the upregulation of VEGF expression in human peritoneal mesothelial cells: The role of pleiotrophin

The aim of the present study was to investigate the effect of a high-glucose-based peritoneal dialysis solution (HGPDS) on the expression of pleiotrophin (PTN) and vascular endothelial growth factor (VEGF) in human peritoneal mesothelial cells (HPMCs) and the mechanisms through which fluvastatin (Flu) protects the peritoneal membrane in continuous ambulatory peritoneal dialysis (CAPD). HPMCs were cultured with HGPDS, Flu (10-8‑10-6 mol/l) and PTN (10‑30 nmol/l). The expression of PTN and VEGF was examined at the mRNA and protein level. To define the role of PTN in the regulation of VEGF expression, HPMCs were cultured with HGPDS in the presence or absence of the blocking peptide of PTN. The signaling pathways involved in PTN synthesis induced by HGPDS were also characterized. The phenotypic characteristics of HPMCs were observed under a light microscope. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetry and the mRNA and protein expression of PTN, VEGF and ERK1/2 was assessed by RT‑PCR and the western blot analysis, respectively. Following incubation with HGPDS for 48 h, the morphology of the HPMCs changed from a typical cobblestone‑like appearance to a fibroblast‑like phenotype. The same alteration in the morphology of the HPMCs also occurred following incubation with 20 nmol/l PTN. Flu (10-6 mol/l), GSK650394 [a competitive inhibitor of serum/glucocorticoid-regulated kinase 1 (SGK1), 10-5 mol/l] and PD98059 (a competitive inhibitor of ERK1/2, 10-5 mol/l) improved the negative changes in cell morphology induced by HGPDS. The results of MTT assay revealed that the reduction in HPMC viability occurred in the groups treated with HGPDS and this reduction was partially restored by Flu, GSK650394 and PD98059. A significant improvement in cell viability, which had been decreased by HGPDS, was observed following treatment with Flu (10-6 mol/l), PD98059 (10-5 mol/l) or GSK650394 (10-5 mol/l) (P<0.05). Compared with the control, the mRNA and protein expression of PTN and VEGF significantly increased in the HPMCs treated with HGPDS (P<0.05). GSK650394 and PD98059 significantly decreased the high mRNA and protein expression levels of PTN and VEGF induced by HGPDS (P<0.05) and Flu had the same inhibitory effect as GSK650394 and PD98059 in a dose‑dependent manner (P<0.05). The mRNA and protein expression of VEGF increased following the incubation of HPMCs with 20 nmol/l PTN. By contrast, the mRNA and protein expression levels of VEGF in the HPMCs decreased in the presence of the blocking peptide of PTN. The results from the present study indicated that HGPDS increased the expression of PTN and VEGF in the HPMCs, and this increase was attenuated by Flu, GSK650394 and PD98059. The protein expression of phosphorylated ERK1/2 (p-ERK1/2) was decreased by GSK650394 in the HPMCs treated with HGPDS. Taken together, the protective effects of Flu in HPMCs may be partially achieved through the SGK1‑ERK1/2 signaling pathway.

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

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

ERK5: structure, regulation and function

Extracellular signal-regulated kinase 5 (ERK5), also termed big mitogen-activated protein kinase-1 (BMK1), is the most recently identified member of the mitogen-activated protein kinase (MAPK) family and consists of an amino-terminal kinase domain, with a relatively large carboxy-terminal of unique structure and function that makes it distinct from other MAPK members. It is ubiquitously expressed in numerous tissues and is activated by a variety of extracellular stimuli, such as cellular stresses and growth factors, to regulate processes such as cell proliferation and differentiation. Targeted deletion of Erk5 in mice has revealed that the ERK5 signalling cascade plays a critical role in cardiovascular development and vascular integrity. Recent data points to a potential role in pathological conditions such as cancer and tumour angiogenesis. This review focuses on the physiological and pathological role of ERK5, the regulation of this kinase and the recent development of small molecule inhibitors of the ERK5 signalling cascade.

BMK1 kinase suppresses epithelial-mesenchymal transition through the Akt/GSK3β signaling pathway

Epithelial-mesenchymal transition (EMT) plays a crucial role in the development of cancer metastasis. The mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase, c-jun-NH(2)-kinase, and p38 have been implicated in promoting EMT, but a role for the MAP kinase BMK1 has not been studied. Here, we report that BMK1 signaling suppresses EMT. BMK1 elevation augmented E-cadherin-mediated cell-cell adhesion, downregulated mesenchymal markers, and decreased cell motility. Conversely, BMK1 silencing attenuated E-cadherin-mediated cell-cell adhesion, upregulated mesenchymal markers, and stimulated cell motility. BMK1 depletion dramatically increased the accumulation of endogenous Snail in the nuclear compartment. Snail accumulation was mediated by Akt/GSK3β signaling, which was activated by a modulation in the expression of the mTOR inhibitor DEPTOR. In support of these observations, BMK1 depletion promoted metastasis in vivo. Together, our findings reveal a novel mechanism of EMT control via mTOR/Akt inhibition that suppresses cancer metastasis.

MEK5/ERK5 pathway: the first fifteen years

While conventional MAP kinase pathways are one of the most highly studied signal transduction molecules, less is known about the MEK5 signaling pathway. This pathway has been shown to play a role in normal cell growth cycles, survival and differentiation. The MEK5 pathway is also believed to mediate the effects of a number of oncogenes. MEK5 is the upstream activator of ERK5 in many epithelial cells. Activation of the MEK-MAPK pathway is a frequent event in malignant tumor formation and contributes to chemoresistance and anti-apoptotic signaling. This pathway may be involved in a number of more aggressive, metastatic varieties of cancer due to its role in cell survival, proliferation and EMT transitioning. Further study of this pathway may lead to new prognostic factors and new drug targets to combat more aggressive forms of cancer.

Mitogen-activated protein kinases in mammalian oxidative stress responses

All aerobic organisms are exposed to oxidative stress during their lifetime and are required to respond appropriately for maintenance of their survival and homeostasis. Sustained exposure to oxidative stress has devastating effects in organisms, and, not surprisingly, oxidative stress has been implicated in numerous human diseases. Therefore, an understanding of how mammals respond to oxidative stress is crucial both biologically and clinically. Intracellular signaling pathways, which are activated in response to excessive oxygen radicals, play essential roles in overcoming oxidative stress. The mitogen-activated protein kinase (MAPK) signaling pathways are involved in diverse physiological processes, and are critical for induction of oxidative stress responses. In this review, we will discuss the physiological roles of MAPKs in oxidative stress, the upstream signaling pathways leading to MAPK activation, their regulation, and the MAPK downstream substrates, with a focus on mammalian systems.

Targeting the BMK1 MAP kinase pathway in cancer therapy

The big mitogen activated protein kinase 1 (BMK1) pathway is the most recently discovered and least-studied mammalian mitogen-activated protein (MAP) kinase cascade, ubiquitously expressed in all types of cancer cells tested so far. Mitogens and oncogenic signals strongly activate this cellular MAP kinase pathway, thereby passing down proliferative, survival, chemoresistance, invasive, and angiogenic signals in tumor cells. Recently, several pharmacologic small molecule inhibitors of this pathway have been developed. Among them, the BMK1 inhibitor XMD8-92 blocks cellular BMK1 activation and significantly suppresses tumor growth in lung and cervical tumor models and is well tolerated in animals. On the other hand, MEK5 inhibitors, BIX02188, BIX02189, and compound 6, suppress cellular MEK5 activity, but no data exist to date on their effectiveness in animals.

The role of specific mitogen-activated protein kinase signaling cascades in the regulation of steroidogenesis

Mitogen-activated protein kinases (MAPKs) comprise a family of serine/threonine kinases that are activated by a large variety of extracellular stimuli and play integral roles in controlling many cellular processes, from the cell surface to the nucleus. The MAPK family includes four distinct MAPK cascades, that is, extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK, c-Jun N-terminal kinase or stress-activated protein kinase, and ERK5. These MAPKs are essentially operated through three-tiered consecutive phosphorylation events catalyzed by a MAPK kinase kinase, a MAPK kinase, and a MAPK. MAPKs lie in protein kinase cascades. The MAPK signaling pathways have been demonstrated to be associated with events regulating the expression of the steroidogenic acute regulatory protein (StAR) and steroidogenesis in steroidogenic tissues. However, it has become clear that the regulation of MAPK-dependent StAR expression and steroid synthesis is a complex process and is context dependent. This paper summarizes the current level of understanding concerning the roles of the MAPK signaling cascades in the regulation of StAR expression and steroidogenesis in different steroidogenic cell models.

The MAPK cascades: signaling components, nuclear roles and mechanisms of nuclear translocation

The MAPK cascades are central signaling pathways that regulate a wide variety of stimulated cellular processes, including proliferation, differentiation, apoptosis and stress response. Therefore, dysregulation, or improper functioning of these cascades, is involved in the induction and progression of diseases such as cancer, diabetes, autoimmune diseases, and developmental abnormalities. Many of these physiological, and pathological functions are mediated by MAPK-dependent transcription of various regulatory genes. In order to induce transcription and the consequent functions, the signals transmitted via the cascades need to enter the nucleus, where they may modulate the activity of transcription factors and chromatin remodeling enzymes. In this review, we briefly cover the composition of the MAPK cascades, as well as their physiological and pathological functions. We describe, in more detail, many of the important nuclear activities of the MAPK cascades, and we elaborate on the mechanisms of ERK1/2 translocation into the nucleus, including the identification of their nuclear translocation sequence (NTS) binding to the shuttling protein importin7. Overall, the nuclear translocation of signaling components may emerge as an important regulatory layer in the induction of cellular processes, and therefore, may serve as targets for therapeutic intervention in signaling-related diseases such as cancer and diabetes. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Second AKT: the rise of SGK in cancer signalling

The serum and glucocorticoid kinase (SGK) family of serine/threonine kinases consists of three isoforms, SGK-1, SGK-2 and SGK-3. This family of kinases is highly homologous to the AKT kinase family, sharing similar upstream activators and downstream targets. SGKs have been implicated in the regulation of cell growth, proliferation, survival and migration: cellular processes that are dysregulated in cancer. Furthermore, SGKs lie downstream of phosphoinositide-3-kinase (PI3Kinase) signalling and interact at various levels with RAS/RAF/ERK signalling, two pathways that are involved in promoting tumorigenesis. Recent evidence suggests that mutant PI3Kinase can induce tumorigenesis through an AKT-independent but SGK3-dependent mechanism, thus implicating SGKs as potential players in malignant transformation. Here, we will review the current state of knowledge on the regulation of the SGKs and their role in normal cell physiology and transformation with a particular focus on SGK3.

Pharmacological inhibition of BMK1 suppresses tumor growth through promyelocytic leukemia protein

BMK1 is activated by mitogens and oncogenic signals and, thus, is strongly implicated in tumorigenesis. We found that BMK1 interacted with promyelocytic leukemia protein (PML), and inhibited its tumor-suppressor function through phosphorylation. Furthermore, activated BMK1 notably inhibited PML-dependent activation of p21. To further investigate the BMK-mediated inhibition of the tumor suppressor activity of PML in tumor cells, we developed a small-molecule inhibitor of the kinase activity of BMK1, XMD8-92. Inhibition of BMK1 by XMD8-92 blocked tumor cell proliferation in vitro and significantly inhibited tumor growth in vivo by 95%, demonstrating the efficacy and tolerability of BMK1-targeted cancer treatment in animals.

Key signalling nodes in mammary gland development and cancer. Mitogen-activated protein kinase signalling in experimental models of breast cancer progression and in mammary gland development

Seven classes of mitogen-activated protein kinase (MAPK) intracellular signalling cascades exist, four of which are implicated in breast disease and function in mammary epithelial cells. These are the extracellular regulated kinase (ERK)1/2 pathway, the ERK5 pathway, the p38 pathway and the c-Jun N-terminal kinase (JNK) pathway. In some forms of human breast cancer and in many experimental models of breast cancer progression, signalling through the ERK1/2 pathway, in particular, has been implicated as being important. We review the influence of ERK1/2 activity on the organised three-dimensional association of mammary epithelial cells, and in models of breast cancer cell invasion. We assess the importance of epidermal growth factor receptor family signalling through ERK1/2 in models of breast cancer progression and the influence of ERK1/2 on its substrate, the oestrogen receptor, in this context. In parallel, we consider the importance of these MAPK-centred signalling cascades during the cycle of mammary gland development. Although less extensively studied, we highlight the instances of signalling through the p38, JNK and ERK5 pathways involved in breast cancer progression and mammary gland development.

SGK1 phosphorylation of IkappaB Kinase alpha and p300 Up-regulates NF-kappaB activity and increases N-Methyl-D-aspartate receptor NR2A and NR2B expression

Serum- and glucocorticoid-inducible kinase 1 (SGK1) is a downstream target of phosphatidylinositol 3-kinase signaling, and it regulates various cellular and physiological functions, but the SGK1 substrate proteins and genes regulated by SGK1 are less known. Here we have identified IkappaB kinase alpha (IKKalpha) as a novel substrate of SGK1 by using biochemical and bioinformatic approaches. SGK1 directly phosphorylates IKKalpha at Thr-23 and indirectly activates IKKalpha at Ser-180. Furthermore, SGK1 enhanced nuclear factor kappaB (NF-kappaB) activity and up-regulated N-methyl-d-aspartate receptor NR2A and NR2B expression through activation of IKKalpha at Thr-23 and Ser-180, and these two residues play an equally important role in mediating these effects of SGK1. Although SGK1 does not phosphorylate IKKbeta, IKKbeta activity is still required for IKK complex activation and for SGK1 phosphorylation and activation of NF-kappaB. In addition, SGK1 increased the acetylation of NF-kappaB through phosphorylation of p300 at Ser-1834, and this also leads to NF-kappaB activation and NR2A and NR2B expression. Moreover, an endogenous stimulus of SGK1, insulin, increased IKKalpha and NF-kappaB phosphorylation as well as NF-kappaB acetylation and NF-kappaB activity, but SGK1 small interfering RNA transfection blocked these effects of insulin. In examination of the functional significance of the SGK1-IKKalpha-NF-kappaB signaling pathway, we found that transfection of the IKKalpha double mutant (IKKalphaT23A/S180A) to rat hippocampus antagonized SGK-1-mediated spatial memory facilitation. Our results together demonstrated novel substrate proteins of SGK1 and novel SGK1 signaling pathways. Activation of these signaling pathways enhances NR2A and NR2B expression that is implicated in neuronal plasticity.

Advance in research of genes associated with peptic ulcer

At present, studies about peptic ulcer have developed from cellular level to molecular level. More and more genes associated with peptic ulcer have been discovered, including human leukocyte antigen (HLA) genes, epidermal growth factor (EGF) genes, apoptosis-associated genes, calcitonin genes and trefoil peptide genes. These genes play important roles in the development and healing of peptic ulcer, providing theoretical basis for clinical diagnosis and cure of peptic ulcer.

Homogeneous time-resolved fluorescence resonance energy transfer assay for measurement of Phox/Bem1p (PB1) domain heterodimerization

Twenty human proteins encode Phox/Bem1p (PB1) domains, which are involved in forming protein heterodimers. MEKK2, MEKK3, and MEK5 are 3 serine-threonine protein kinases that have PB1 domains. MEKK2, MEKK3, and MEK5 are the MAP3Ks and the MAP2K in the ERK5 mitogen-activated protein kinase (MAPK) signaling module. ERK5 is a critical MAPK for both development of the vasculature and vascular homeostasis in the adult, but no other MAPK has been shown to be critical in vascular maintenance in the adult animal. MEKK2 and MEKK3 are the only MAP3Ks shown to physically interact with and activate the MEK5-ERK5 signaling module. Interaction of MEKK2 or MEKK3 with MEK5 is mediated by heterodimerization of the MEKK2 (or MEKK3) PB1 and MEK5 PB1 domains. The authors have developed a homogeneous, time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor PB1-PB1 domain heterodimerization. The assay uses a europium-chelate conjugated GST-MEK5 PB1 domain chimera, biotinylated MEKK2 PB1 domain, and streptavidin-Cy5. Interaction of the MEKK2 and MEK5 PB1 domains gives a robust FRET signal (Z' factor = 0.93), which is completely abrogated by mutation of 2 acidic residues (64D65E-->AA) within the MEK5 PB1 domain that causes loss of stable PB1-PB1 domain interaction. This assay can be used to study the specificity of PB1-PB1 domain interactions and to screen for molecules that can regulate MEKK2/MEKK3-MEK5 interactions. Disruption of PB1 domain interactions represents a novel approach for selectively regulating the ERK5 signaling pathway independent of kinase active site-directed adenosine triphosphate competitive inhibitors.

Hsp90 regulates the phosphorylation and activity of serum- and glucocorticoid-regulated kinase-1

SGK-1 (serum- and glucocorticoid-regulated kinase-1), a member of the AGC protein kinase family, plays an important role in regulating ion channel expression and contributes to malignant epithelial cell proliferation and survival. SGK-1 activity is regulated on three levels: transcriptional induction following a variety of environmental and intracellular stresses, proteasomal degradation, and phosphorylation. Here we report that phosphoinositide 3-kinase (PI3K)-dependent phosphorylation of SGK-1 requires formation of a complex between SGK-1 and heat-shock protein 90 (Hsp90). Inactivation of Hsp90 by geldanamycin led to decreased SGK-1 phosphorylation independently of increased proteasomal protein degradation, and inhibition of PI3K activity by LY294002 appeared to eliminate SGK-1 phosphorylation at the same residues as those affected by geldanamycin treatment. Interestingly, geldanamycin-targeted phosphorylation sites were not limited to the known conserved PI3K-dependent sites Thr-256 and Ser-422 in SGK-1 but included additional unknown PI3K-dependent residues. Inhibition of Hsp90 also resulted in a complete loss of SGK-1 kinase activity, suggesting that Hsp90 activity is essential for regulating the PI3K/SGK-1 pathway.

The detection of MAPK signaling

Mitogen-activated protein kinase (MAPK) cascades are central pathways that participate in the intracellular transmission of extracellular signals. Each of the MAPK signaling cascades seems to consist of three to five tiers of protein kinases that sequentially activate each other by phosphorylation. Since the majority of MAPK cascade components are kinases, the methods used to detect their activation involve determining phosphorylation state and protein kinase activities. The primary method describes the use of immunoblotting with specific anti-phospho antibody to detect activation of MAPK components. Alternative methods described are immunoprecipitation of desired protein kinases followed by phosphorylation of specific substrates and the use of an in-gel kinase assay. These methods have proven useful in the study of the MAPK signaling cascades.