Role of MEKK2-MEK5 in the regulation of TNF-alpha gene expression and MEKK2-MKK7 in the activation of c-Jun N-terminal kinase in mast cells

The role of deacetylase SIRT1 in allergic diseases

The silent information regulator sirtuin 1 (SIRT1) protein is an NAD+-dependent class-III lysine deacetylase that serves as an important post-transcriptional modifier targeting lysine acetylation sites to mediate deacetylation modifications of histones and non-histone proteins. SIRT1 has been reported to be involved in several physiological or pathological processes such as aging, inflammation, immune responses, oxidative stress and allergic diseases. In this review, we summarized the regulatory roles of SIRT1 during allergic disorder progression. Furthermore, we highlight the therapeutic effects of targeting SIRT1 in allergic diseases.

Functions of MAP3Ks in antiviral immunity

Immune signal transduction is crucial to the body's defense against viral infection. Recognition of pathogen-associated molecular patterns by pattern recognition receptors (PRRs) activates the transcription of interferon regulators and nuclear factor-κB (NF-κB); this promotes the release of interferons and inflammatory factors. Efficient regulation of type I interferon and NF-κB signaling by members of the mitogen-activated protein (MAP) kinase kinase kinase (MAP3K) family plays an important role in antiviral immunity. Elucidating the specific roles of MAP3K activation during viral infection is essential to develop effective antiviral therapies. In this review, we outline the specific regulatory mechanisms of MAP3Ks in antiviral immunity and discuss the feasibility of targeting MAP3Ks for the treatment of virus-induced diseases.

CircRNA PDE3B regulates tumorigenicity via the miR-136-5p/MAP3K2 axis of esophageal squamous cell carcinoma

BACKGROUND

CircRNA has a covalently closed circular conformation and a stable structure. However, the exact role of circRNA in esophageal squamous cell carcinoma (ESCC) remains uncertain. The purpose of this study was to explore the role of hsa_circ_0000277 (circ_PDE3B) in ESCC.

METHODS

The expression levels of circ_PDE3B, miR-136-5p and mitogen-activated protein kinase kinase kinase 2 (MAP3K2) in ESCC tissues and cells were detected by quantitative real-time polymerase chain reaction (qRT-PCR) or western blot. The proliferation ability of EC9706 and KYSE30 cells was detected by clonal formation, 5-ethynyl-2'-deoxyuridine (EdU) and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assays. Flow cytometry was used to detect the apoptosis rate of cells. Transwell assay was used to detect the invasion ability of EC9706 and KYSE3 cells. The relationship between miR-136-5p and circ_PDE3B or MAP3K2 was verified by dual-luciferase reporter assay and RNA pull-down, and the effect of circ_PDE3B on tumor growth in vivo was explored through tumor transplantation experiment. Immunohistochemistry (IHC) assay was used to detect MAP3K2 and Ki67 expression in mice tumor tissues.

RESULTS

The results showed that circ_PDE3B was highly expressed in ESCC tissues and cells. Downregulated circ_PDE3B expression in ESCC cells significantly reduced cell proliferation, migration and invasion. Circ_PDE3B served as a sponge for miR-136-5p, and miR-136-5p inhibition reversed the roles of circ_PDE3B knockdown in ESCC cells. MAP3K2 was a direct target of miR-136-5p, and miR-136-5p targeted MAP3K2 to inhibit the malignant behaviors of ESCC cells. Furthermore, circ_PDE3B regulated MAP3K2 expression by sponging miR-136-5p. Importantly, circ_PDE3B knockdown inhibited tumor growth in vivo.

CONCLUSIONS

In conclusion, circ_PDE3B acted as oncogenic circRNA in ESCC and accelerated ESCC progression by adsorption of miR-136-5p and activation of MAP3K2, supporting circ_PDE3B as a potential therapeutic target for ESCC.

E3 ubiquitin ligase NEDD4L negatively regulates inflammation by promoting ubiquitination of MEKK2

Aberrant activation of inflammation signaling triggered by tumor necrosis factor α (TNF‐α), interleukin‐1 (IL‐1), and interleukin‐17 (IL‐17) is associated with immunopathology. Here, we identify neural precursor cells expressed developmentally down‐regulated gene 4‐like (NEDD4L), a HECT type E3 ligase, as a common negative regulator of signaling induced by TNF‐α, IL‐1, and IL‐17. NEDD4L modulates the degradation of mitogen‐activated protein kinase kinase kinase 2 (MEKK2) via constitutively and directly binding to MEKK2 and promotes its poly‐ubiquitination. In interleukin‐17 receptor (IL‐17R) signaling, Nedd4l knockdown or deficiency enhances IL‐17‐induced p38 and NF‐κB activation and the production of proinflammatory cytokines and chemokines in a MEKK2‐dependent manner. We further show that IL‐17‐induced MEKK2 Ser520 phosphorylation is required not only for downstream p38 and NF‐κB activation but also for NEDD4L‐mediated MEKK2 degradation and the subsequent shutdown of IL‐17R signaling. Importantly, Nedd4l‐deficient mice show increased susceptibility to IL‐17‐induced inflammation and aggravated symptoms of experimental autoimmune encephalomyelitis (EAE) in IL‐17R signaling‐dependent manner. These data suggest that NEDD4L acts as an inhibitor of IL‐17R signaling, which ameliorates the pathogenesis of IL‐17‐mediated autoimmune diseases.

Allicin regulates Treg/Th17 balance in mice with collagen-induced arthritis by increasing the expression of MEKK2 protein

To study the role of Allicin in regulating Treg/Th17 ratio in splenic lymphocyte by increasing the expression of MEKK2 protein in MAPK signaling pathway, and to explore the mechanism of immune response in mice with collagen-induced arthritis (CIA). Mouse CIA model was induced by chicken collagen type II, and experimental mice were randomly divided into NC group, Model group, and Allicin group. HE staining was used to compare the degree of joint pathological damage in mice of each group, and Masson staining to observe the proliferation of collagen tissue in each group. Flow cytometry detected Treg/Th17 ratio in splenic lymphocytes. Furthermore, RT-PCR and WB were used to detect the mRNA and protein expression of related transcription factors and inflammatory factors Foxp3, ROR-γt, and IL-17A, as well as MEK2 protein expression in splenic lymphocytes. The results showed that Allicin treatment could reduce the severity of arthritis and the proliferation of collagen fibers on the surface of cartilage and bone joints in CIA mice. Compared with NC group, Treg decreased and Th17 increased in spleen lymphocyte of Model group (p < .01); after Allicin treatment, Treg increased while Th17 decreased significantly (p < .01). Meanwhile, MEKK2 protein expression in spleen lymphocyte of Model group decreased compared to that in NC group (p < .01), and MEK2 protein expression increased significantly after Allicin treatment (p < .01). To sum up, the present study suggests that MEKK2 protein plays an important role in the pathogenesis of CIA model. In terms of mechanism, Allicin may play a therapeutic role in rheumatoid arthritis (RA) by increasing the expression of MEKK2 protein and affecting Treg/Th17 ratio.

Thymic Stromal Lymphopoietin Promotes MRGPRX2-Triggered Degranulation of Skin Mast Cells in a STAT5-Dependent Manner with Further Support from JNK

Thymic stromal lymphopoietin (TSLP) is released by epithelial cells following disturbed homeostasis to act as “alarmin” and driver of Th2-immunity. Aberrant TSLP expression is a hallmark of atopic diseases, including atopic dermatitis (AD). Mast cells (MCs) are overabundant in AD lesions and show signs of degranulation, but it remains unknown whether TSLP contributes to granule discharge. Degranulation of skin MCs proceeds via two major routes, i.e., FcεRI-dependent (allergic) and MRGPRX2-mediated (pseudo-allergic/neurogenic). Evidence is accumulating that MRGPRX2 may be crucial in the context of skin diseases, including eczema. The current study reveals TSLP as a novel priming factor of human skin MCs. Interestingly, TSLP selectively cooperates with MRGPRX2 to support granule discharge, while it does not impact spontaneous or FcεRI-driven exocytosis. TSLP-assisted histamine liberation triggered by compound 48/80 or Substance P, two canonical MRGPRX2 agonists, was accompanied by an increase in CD107a+ cells (a MC activation marker). The latter process was less potent, however, and detectable only at the later of two time points, suggesting TSLP may prolong opening of the granules. Mechanistically, TSLP elicited phosphorylation of STAT5 and JNK in skin MCs and the reinforced degranulation critically depended on STAT5 activity, while JNK had a contributory role. Results from pharmacological inhibition were confirmed by RNA-interference, whereby silencing of STAT5 completely abolished the priming effect of TSLP on MRGPRX2-mediated degranulation. Collectively, TSLP is the first factor to favor MRGPRX2- over FcεRI-triggered MC activation. The relevance of TSLP, MCs and MRGPRX2 to pruritis and atopic skin pathology indicates broad repercussions of the identified connection.

Biochanin A protects against PM2.5-induced acute pulmonary cell injury by interacting with the target protein MEK5

Epidemiological studies have shown that exposure to ambient fine particulate matter (PM_2.5) is associated with an increased risk for cardiopulmonary diseases. The MEK5/ERK5 and NF-κB signaling pathways are closely related to the regulation of acute pulmonary cell injury (APCI) and may play an important role in the underlying pathophysiological mechanisms. Related studies have shown that Biochanin A (BCA) effectively interferes with APCI, but the underlying mechanism through which this occurs is not fully understood. Previously, based on proteomic and bioinformatic research, we found the indispensable role of MEK5 in mediating remission effects of BCA against PM_2.5-induced lung toxicity. Therefore, using A549 adenocarcinoma human alveolar basal epithelial cells (A549 cells), we combined western blot and qRT-PCR to study the protective signaling pathways induced by BCA, indicating that MEK5/ERK5 and NF-κB are both involved in mediating APCI in response to PM_2.5, and MEK5/ERK5 positively activated NF-κB and its downstream cellular regulatory factors. BCA significantly suppressed PM_2.5-induced upregulation of MEK5/ERK5 expression and phosphorylation and activation of NF-κB. Furthermore, due to the specificity of the MEK5/ERK5 protein structure, the binding sites and binding patterns of BCA and MEK5 were analyzed using molecular docking correlation techniques, which showed that there are stable hydrogen bonds between BCA and the PB1 domain of MEK5 as well as its kinase domain. BCA forms a stable complex with MEK5, which has potential effects on MEKK2/3-MEK5-ERK5 ternary interactions, p62/αPKC-mediated NF-κB regulation, and inhibition of MEK5 target protein phosphorylation. Therefore, our study suggests that MEK5 is an important regulator of intracellular signaling of APCI in response to PM_2.5 exposure. BCA may exert anti-APCI activity by targeting MEK5 to inhibit activation of the MEK5/ERK5/NF-κB signaling pathway.

IFN-γ-inducible antiviral responses require ULK1-mediated activation of MLK3 and ERK5

It is well established that activation of the transcription factor signal transducer and activator of transcription 1 (STAT1) is required for the interferon-γ (IFN-γ)-mediated antiviral response. Here, we found that IFN-γ receptor stimulation also activated Unc-51-like kinase 1 (ULK1), an initiator of Beclin-1-mediated autophagy. Furthermore, the interaction between ULK1 and the mitogen-activated protein kinase kinase kinase MLK3 (mixed lineage kinase 3) was necessary for MLK3 phosphorylation and downstream activation of the kinase ERK5. This autophagy-independent activity of ULK1 promoted the transcription of key antiviral IFN-stimulated genes (ISGs) and was essential for IFN-γ-dependent antiviral effects. These findings define a previously unknown IFN-γ pathway that appears to be a key element of the antiviral response.

Upregulation of MEK5 by Stat3 promotes breast cancer cell invasion and metastasis

Mitogen extracellular-signal-regulated kinase kinase 5 (MEK5) plays an important role in promoting cell proliferation and tumorigenesis. The aberrant expression of MEK5 has been reported in various malignant diseases including cancers of breast, prostate, lung, colorectal and brain. However, the function and regulation of MEK5 signaling pathway are ambiguous and remain elusive with respect to its oncogenic roles in various cancers, especially in the regulation of the initiation and progression of cancer invasion and metastasis. Ectopic expression of MEK5 or knockdown of MEK5 by shRNA with in vitro cell based models demonstrated the role of MEK5 in regulation of epithelial mesenchymal transition (EMT) and breast cancer invasion and metastasis. Here, we show that MEK5 upregulated by Stat3 promotes breast cancer cell invasion through EMT. Further study demonstrated that Stat3 could bind to promoter region of MEK5 and enhanced MEK5 transcription and expression. In addition, the phosphorylation of MEK5 significantly increased in breast cancer cells corresponding to metastatic capability of breast cancer cells. The depletion of MEK5 by shRNA significantly decreased breast cancer invasion. Ectopic expression of MEK5 could confer non-invasive breast cancer cells to become invasion capable cells. Moreover, the phosphorylation of Erk5, a MEK5-regulated downstream kinase, was also upregulated consistent with the increased level of active MEK5. Our studies provide insights into a molecular mechanism by which MEK5 transcriptionally upregulated by Stat3 augments breast cancer cell EMT, which subsequently enhances cancer cell invasion and metastasis. This finding may suggest that Stat3 and MEK5/Erk5 pathways could be an effective therapeutic target for inhibition of breast cancer invasion and metastasis.

MicroRNA-186 suppresses cell proliferation and metastasis through targeting MAP3K2 in non-small cell lung cancer

MicroRNAs are a class of small endogenous non-coding RNAs that play crucial roles in the initiation and progression of human cancers. miR-186 was found decreased in various human malignancies and function as a tumor suppressor. However, the regulating mechanism of miR-186 in growth and metastasis of human non-small cell lung cancer (NSCLC) is still poorly understood. We investigated the role of miR-186 in the growth and metastasis of human NSCLC. In the present study, we found that miR-186 was significantly decreased in lung cancer tissues and cells. Furthermore, overexpression of miR-186 suppressed lung cancer cell proliferation, migration and invasion, and induced cell apoptosis. Moreover, we found that confirmed mitogen-activated protein kinase kinase kinase 2 (MAP3K2) protein was increased in lung cancer tissues and confirmed that MAP3K2 is a target gene of miR-186. In addition, knockdown of MAP3K2 by RNA interference inhibited lung cancer cell proliferation, migration and invasion, and promoted cell apoptosis in vitro. Furthermore, we observed tthat the overexpression of MAP3K2 partially reversed the inhibitory effect of miR-186 on the proliferation and metastasis of A549 and HCC827 cell lines. Taken together, our data indicated that miR-186 regulates lung cancer growth and metastasis through suppressing MAP3K2 expression, at least partly. Therefore, miR-186-MAP3K2 may represent a new and useful potential clinical treatment and diagnosis target for NSCLC.

miRNA-106a directly targeting RARB associates with the expression of Na(+)/I(-) symporter in thyroid cancer by regulating MAPK signaling pathway

BACKGROUND

Serum miRNAs profiles between papillary thyroid carcinoma (PTC) patients with non-(131)I and (131)I-avid lung metastases are differentially expressed. These miRNAs have to be further validated and the role of these miRNAs in the molecular function level of thyroid cancer cell lines has not been investigated.

METHODS

Expression levels of six identified miRNAs were assessed via quantitative real-time PCR (qRT-PCR) in the serum of eligible patients. Dual-luciferase reporter assay was used to determine the potential target of miR-106a. Cell viability and apoptosis were evaluated by MTT assay and flow cytometry analysis, respectively. The change of gene expression was detected by qRT-PCR and western blotting analysis. In vitro iodine uptake assay was conducted by a γ-counter.

RESULTS

Compared to PTC patients with (131)I-avid lung metastases, miR-106a was up-regulated in the serum of patients with non-(131)I-avid lung metastases. The results of dual-luciferase reporter assay demonstrated that miR-106a directly targeted retinoic acid receptor beta (RARB) 3'-UTR. miR-106a-RARB promoted viability of thyroid cancer cells by regulating MEKK2-ERK1/2 and MEKK2-ERK5 pathway. miR-106a-RARB inhibited apoptosis of thyroid cancer cells by regulating ASK1-p38 pathway. Moreover, miR-106a-RARB could regulate the expression of sodium iodide symporter, TSH receptor and alter the iodine uptake function of thyroid cancer cells.

CONCLUSIONS

miRNA-106a, directly targeting RARB, associates with the viability, apoptosis, differentiation and the iodine uptake function of thyroid cancer cell lines by regulating MAPK signaling pathway in vitro. These findings in the present study may provide new strategies for the diagnosis and treatment in radioiodine-refractory differentiated thyroid carcinoma.

Interaction with the Paxillin LD1 Motif Relieves MEKK2 Auto-inhibition

The cell signaling molecule MEK kinase 2 (MEKK2) is a key upstream regulator of MAPK activity that regulates numerous cellular functions, but the mechanisms that control MEKK2 activity are not well understood. Recently, we reported that MEKK2 both binds and promotes ubiquitylation of the scaffold protein paxillin, and thereby modulates the composition of adhesion complexes. In this study, we have extended our examination of this interaction and report that recombinant paxillin is sufficient to induce MEKK2 auto-phosphorylation. Furthermore, we utilize siRNA-mediated paxillin expression knockdown to reveal that MEKK2 activity is reduced in paxillin-deficient cells. Finally, we show that the paxillin leucine-rich motif 1 (LD1) is sufficient to bind to the MEKK2 amino terminal region and activate MEKK2. Taken together, our results show for the first time that paxillin association promotes MEKK2 activation and reveal the existence of a novel bi-directional regulatory relationship between MEKK2 and paxillin.

MEKK2 kinase association with 14-3-3 protein regulates activation of c-Jun N-terminal kinase

MEKK2 (MAP/ERK kinase kinase-2) is a serine/threonine kinase that belongs to the MEKK/STE11 family of MAP kinase kinase kinases (MAP(3)Ks). MEKK2 integrates stress and mitogenic signals to the activation of NF-κB, JNK1/2, p38, and ERK5 pathways. We have found that MEKK2 is regulated through a phosphorylation-dependent association with 14-3-3, a group of adapters that modulate dimerization and association between proteins. We found that MEKK2 was phosphorylated at Thr-283, which resulted in decreased activation loop phosphorylation at Ser-519 and consequently reduced activity. Mechanistically, we found that MEKK2 associated with inactive MEKK2 in the absence of 14-3-3 binding, which led to trans-autophosphorylation of Ser-519. Enforced binding with 14-3-3 reduced Ser-519 trans-autophosphorylation. Expression of T283A MEKK2 within a MEKK2(-/-) background enhanced stress-activated c-Jun N-terminal kinase activity while elevating IL-6 expression, but also reduced ERK activation with a corresponding reduced proliferation rate. These results indicate that Thr-283 phosphorylation is an important regulatory mechanism for MEKK2 activation.

Applying ligands profiling using multiple extended electron distribution based field templates and feature trees similarity searching in the discovery of new generation of urea-based antineoplastic kinase inhibitors

This study provides a comprehensive computational procedure for the discovery of novel urea-based antineoplastic kinase inhibitors while focusing on diversification of both chemotype and selectivity pattern. It presents a systematic structural analysis of the different binding motifs of urea-based kinase inhibitors and the corresponding configurations of the kinase enzymes. The computational model depends on simultaneous application of two protocols. The first protocol applies multiple consecutive validated virtual screening filters including SMARTS, support vector-machine model (ROC = 0.98), Bayesian model (ROC = 0.86) and structure-based pharmacophore filters based on urea-based kinase inhibitors complexes retrieved from literature. This is followed by hits profiling against different extended electron distribution (XED) based field templates representing different kinase targets. The second protocol enables cancericidal activity verification by using the algorithm of feature trees (Ftrees) similarity searching against NCI database. Being a proof-of-concept study, this combined procedure was experimentally validated by its utilization in developing a novel series of urea-based derivatives of strong anticancer activity. This new series is based on 3-benzylbenzo[d]thiazol-2(3H)-one scaffold which has interesting chemical feasibility and wide diversification capability. Antineoplastic activity of this series was assayed in vitro against NCI 60 tumor-cell lines showing very strong inhibition of GI₅₀ as low as 0.9 uM. Additionally, its mechanism was unleashed using KINEX™ protein kinase microarray-based small molecule inhibitor profiling platform and cell cycle analysis showing a peculiar selectivity pattern against Zap70, c-src, Mink1, csk and MeKK2 kinases. Interestingly, it showed activity on syk kinase confirming the recent studies finding of the high activity of diphenyl urea containing compounds against this kinase. Allover, the new series, which is based on a new kinase scaffold with interesting chemical diversification capabilities, showed that it exhibits its “emergent” properties by perturbing multiple unexplored kinase pathways.

Assessing developmental roles of MKK4 and MKK7 in vitro

In vivo gene knockout studies in mice have revealed essential roles of the mitogen-activated protein kinases (MAPKs) in embryogenesis, but due to early lethality of the knockout embryos, the underlying mechanisms and specific developmental programs regulated by the MAPK pathways have remained largely unknown. In vitro differentiation of mouse embryonic stem cells (ESCs) have opened new possibilities for understanding lineage segregation and gene function in the developmental stages that are not normally accessible in vivo. Building on this technology, in combination with gene knockout cells, we investigated the roles of MKK4 and MKK7, two upstream kinases of the MAPKs, in early lineage specification. Our results show that MKK4 and MKK7 differentially regulate the JNK and p38 MAPKs and make distinct contributions to differentiation programs. In vitro ESC differentiation is a valuable system to investigate the molecular and signaling mechanisms of early embryogenesis.

Beside the MEF2 axis: unconventional functions of HDAC4

The class IIa deacetylase HDAC4 is unequivocally known as a negative regulator of MEF2-dependent transcription. In the past years several works have allowed us to understand how different signals, mirroring specific environmental circumstances keep in check the repressive action of HDAC4 against MEF2s. At the same time, pieces of evidence have gradually accumulated about HDAC4 activities emancipated from MEF2s. The aim of this review is to discuss about these "unconventional functions" of HDAC4.

A direct HDAC4-MAP kinase crosstalk activates muscle atrophy program

Prolonged deficits in neural input activate pathological muscle remodeling, leading to atrophy. In denervated muscle, activation of the atrophy program requires HDAC4, a potent repressor of the master muscle transcription factor MEF2. However, the signaling mechanism that connects HDAC4, a protein deacetylase, to the atrophy machinery remains unknown. Here, we identify the AP1 transcription factor as a critical target of HDAC4 in neurogenic muscle atrophy. In denervated muscle, HDAC4 activates AP1-dependent transcription, whereas AP1 inactivation recapitulates HDAC4 deficiency and blunts the muscle atrophy program. We show that HDAC4 activates AP1 independently of its canonical transcriptional repressor activity. Surprisingly, HDAC4 stimulates AP1 activity by activating the MAP kinase cascade. We present evidence that HDAC4 binds and promotes the deacetylation and activation of a key MAP3 kinase, MEKK2. Our findings establish an HDAC4-MAPK-AP1 signaling axis essential for neurogenic muscle atrophy and uncover a direct crosstalk between acetylation- and phosphorylation-dependent signaling cascades.

MicroRNA-520b inhibits growth of hepatoma cells by targeting MEKK2 and cyclin D1

Growing evidence indicates that the deregulation of microRNAs (miRNAs) contributes to the tumorigenesis. We previously revealed that microRNA-520b (miR-520b) was involved in the complement attack and migration of breast cancer cells. In this report, we show that miR-520b is an important miRNA in the development of hepatocellular carcinoma (HCC). Our data showed that the expression levels of miR-520b were significantly reduced in clinical HCC tissues and hepatoma cell lines. We observed that the introduction of miR-520b dramatically suppressed the growth of hepatoma cells by colony formation assays, 5-ethynyl-2-deoxyuridine (EdU) incorporation assays and 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Moreover, ectopic expression of miR-520b was able to inhibit the growth of hepatoma cells in nude mice. Further studies revealed that the mitogen-activated protein kinase kinase kinase 2 (MEKK2) and cyclin D1 were two of direct target genes of miR-520b. Silencing of MEKK2 or cyclin D1 was able to inhibit the growth of hepatoma cells in vitro and in vivo, which is consistent with the effect of miR-520b overexpression on the growth of hepatoma cells. In addition, miR-520b significantly decreased the phosphorylation levels of c-Jun N-terminal kinase (p-JNK, a downstream effector of MEKK2) or retinoblastoma (p-Rb, a downstream effector of cyclin D1). In conclusion, miR-520b is able to inhibit the growth of hepatoma cells by targeting MEKK2 or cyclin D1 in vitro and in vivo. Our findings provide new insights into the role of miR-520b in the development of HCC, and implicate the potential application of miR-520b in cancer therapy.

IL-17A stimulates granulocyte colony-stimulating factor production via ERK1/2 but not p38 or JNK in human renal proximal tubular epithelial cells

We investigated the potential role of IL-17A in the induction of granulocyte colony-stimulating factor (G-CSF), a critical granulopoietic growth factor, in human renal proximal tubular epithelial cells. Human renal proximal tubular cells (HK-2, ATCC) were used to characterize the effects of IL-17A or IL-17F on G-CSF production, using ELISA, real-time RT-PCR, and immunoblotting. The cell surface expression of IL-17 receptors (IL-17Rs) was analyzed by flow cytometry. IL-17A stimulation of proximal tubular cells led to a dose- and time-dependent increase in secreted G-CSF. This effect was dependent on mRNA transcription and protein translation. Real-time RT-PCR demonstrated that G-CSF mRNA expression reached a maximum level at 6 h following IL-17A stimulation and that this increase was dose dependent. Both IL-17RA and IL-17RC were expressed on proximal tubular cells. IL-17A also enhanced TNF-α- or IL-1β-mediated G-CSF secretion from cells. Additionally, IL-17A induced MAPK (ERK1/2 but not p38 MAPK or JNK) activation, and pharmacological inhibitors of MEK1/2 (U0126) but not of p38 MAPK (SB203580) or JNK (SP600125), significantly blocked the IL-17A-mediated G-CSF release. We demonstrated the potential ability of IL-17A to induce G-CSF in renal proximal tubular cells. It is proposed that IL-17A may play an important role in neutrophil transmigration and activation via stimulation of G-CSF in tubular injury.

Hepatitis B virus X protein promotes hepatoma cell proliferation via upregulation of MEKK2

AIM

To investigate the mechanism underlying the increase of hepatoma cell proliferation by hepatitis B virus X protein (HBx).

METHODS

HepG2, H7402 and HepG2.2.15 cells, which constitutively replicated hepatitis B virus were used. The effects of HBx on hepatoma cell proliferation were examined using 5-ethynyl-2-deoxyuridine (EdU) incorporation assay and MTT assay. The expression level of MEKK2 was measured using RT-PCR, Western blot and luciferase reporter gene assay. The activity of activator protein 1 (AP-1) was detected using luciferase reporter gene assay. The phosphorylation levels of JNK and c-Jun were measured using Western blot. The expression levels of HBx and MEKK2 in 11 clinical hepatocellular carcinoma (HCC) tissues were measured using real time PCR and Western blot. In addition, the expression of MEKK2 in 95 clinical HCC tissues was examined using immunohistochemistry.

RESULTS

HBx significantly enhanced HepG2-X cell proliferation. In HepG2-X, H7402-X and HepG2.2.15 cells, the expression level of MEKK2 was remarkably increased. In HepG2.2.15 cells, HBx was found to activate JNK and AP-1, which were the downstream effectors of MEKK2 in HepG2-X and HepG2.2.15 cells. In 11 clinical HCC tissues, both HBx and MEKK2 expression levels were remarkably increased, as compared to those in the corresponding peritumor tissues. In 95 clinical HCC tissues, the rate of detection of MEKK2 was 85.3%.

CONCLUSION

HBx promotes hepatoma cell proliferation via upregulating MEKK2, which may be involved in hepatocarcinogenesis.

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.

IL-17A and IL-17F stimulate chemokines via MAPK pathways (ERK1/2 and p38 but not JNK) in mouse cultured mesangial cells: synergy with TNF-α and IL-1β

We investigated the role of IL-17 family members IL-17A and IL-17F in the induction of chemokines in mouse cultured mesangial cells (SV40 MES 13 cells). We evaluated the expression of the chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-2 (MIP-2) by ELISA and real-time RT-PCR (Q-PCR). Activation of MAPK was assessed by immunoblotting. IL-17RA and IL-17RC were inhibited by small interfering RNA (siRNA). We found that IL-17A or IL-17F stimulation of mesangial cells led to both a dose- and time-dependent increase in MCP-1 and MIP-2 release. This effect was dependent on mRNA transcription and protein translation. Both also enhanced TNF-α- and IL-1β-mediated MCP-1 and MIP-2 release in the cells. Additionally, we observed that IL-17A and IL-17F induced MAPK (p38 MAPK, ERK1/2, and JNK) activation and that pharmacological inhibitors of p38 MAPK (SB203580) and ERK1/2 (U0126), but not JNK (SP600125), blocked the IL-17A/IL-17F-mediated MCP-1 and MIP-2 release. Mesangial cells expressed IL-17RA and IL-17RC, and the IL-17A-mediated MCP-1 and MIP-2 release was significantly blocked by soluble IL-17RA. Furthermore, inhibition of either IL-17RA or IL-17RC expression via siRNA led to significant reduction of IL-17A/IL-17F-stimulated chemokine production. We conclude that IL-17A and IL-17F induce the production of chemokines MCP-1 and MIP-2 via MAPK pathways (p38 MAPK and ERK1/2), as well as mRNA transcription and protein translation and have synergistic effects with TNF-α and IL-1β in cultured mesangial cells.

NF-kappaB-dependent induction of cathelicidin-related antimicrobial peptide in murine mast cells by lipopolysaccharide

BACKGROUND

An important aspect of the innate immune response to pathogens is the production of anti-microbial peptides such as cathelicidin-related antimicrobial peptide (CRAMP), the murine homologue of human cathelicidin LL-37. In this study, mechanisms regulating LPS-induction of CRAMP gene expression in mast cells were investigated. NF-kappaB and MAPK pathways were the focus of investigation.

METHODS

Mouse bone marrow-derived mast cells were grown in culture and stimulated with LPS. MAPKs and NF-kappaB were monitored by immunoblot analysis. ERK, JNK and p38 MAPK were inhibited using siRNAs or a pharmacological inhibitor. Accumulation of the p65 component of NF-kappaB was inhibited by siRNA and NF-kappaB activation was inhibited by overexpression of I kappaB alpha. MEKK2 or MEKK3 were overexpressed by transfection. The effects of all of these treatments on CRAMP gene expression were monitored by RT-PCR.

RESULTS

Inhibition of ERK, JNK or p38 MAPK had little discernible effect on LPS-inducible CRAMP gene expression. Overexpression of MEKK2 or MEKK3 likewise had little impact. However, inhibition of the accumulation of p65 NF-kappaB prevented LPS-induced CRAMP mRNA. An important role for NF-kappaB in CRAMP gene expression was confirmed by overexpression of I kappaB alpha, which reduced both basal and induced levels of CRAMP mRNA.

CONCLUSIONS

NF-kappaB, but not MAPKs, plays an important role in LPS-mediated induction of CRAMP gene in mast cells. Defects which inhibit NF-kappaB activity may increase susceptibility to bacterial and viral pathogens which are sensitive to cathelicidins.

MAP3Ks as central regulators of cell fate during development

The cytoplasmic serine/threonine kinases transduce extracellular signals into regulatory events that impact cellular responses. The induction of one kinase triggers the activation of several downstream kinases, leading to the regulation of transcription factors to affect gene function. This arrangement allows for the kinase cascade to be amplified, and integrated according to the cellular context. An upstream mitogen or growth factor signal initiates a module of three kinases: a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK; e.g., Raf) that phosphorylates and activates a MAP kinase kinase (MAPKK; e.g., MEK) and finally activation of MAP kinase (MAPK; e.g., ERK). Thus, this MAP3K-MAP2K-MAPK module represents critical effectors that regulate extracellular stimuli into cellular responses, such as differentiation, proliferation, and apoptosis all of which function during development. There are 21 characterized MAP3Ks that activate known MAP2Ks, and they function in many aspects of developmental biology. This review summarizes known transduction routes linked to each MAP3K and highlights mouse models that provide clues to their physiological functions. This perspective reveals that some of these MAP3K effectors may have redundant functions, and also serve as unique nexus depending on the context of the signaling pathway.

Synoviocyte innate immune responses: I. Differential regulation of interferon responses and the JNK pathway by MAPK kinases

JNK is a key regulator of matrix metalloproteinase production in rheumatoid arthritis. It is regulated by two upstream kinases known as MKK4 and MKK7. Previous studies demonstrated that only MKK7 is required for cytokine-mediated JNK activation and matrix metalloproteinase expression in cultured fibroblast-like synoviocytes (FLS). However, the functions of MKK4 and MKK7 in synoviocyte innate immune responses have not been determined. TNF, peptidoglycan (PGN), and LPS stimulation led to higher and more prolonged MKK7 phosphorylation compared with MKK4 in FLS. However, this pattern was reversed in poly(I-C) stimulated cells. siRNA knockdown studies showed that TNF, PGN, and LPS-induced JNK and c-Jun phosphorylation are MKK7 dependent, while poly(I-C) responses require both MKK4 and MKK7. Poly(I-C)-induced expression of IP-10, RANTES, and IFN-beta mRNA was decreased in MKK4- or MKK7-deficient FLS. However, MKK4 and MKK7 deficiency did not affect phosphorylation of IkappaB kinase-related kinases in the TLR3 signaling pathway. MKK7, but not MKK4 deficiency, significantly decreased poly(I-C)-mediated IRF3 dimerization, DNA binding, and IFN-sensitive response element-mediated gene transcription. These results were mimicked by the JNK inhibitor SP600125, indicating that JNK can directly phosphorylate IRF3. In contrast, deficiency of either MKK4 or MKK7 decreased AP-1 transcriptional activity. Therefore, JNK is differentially regulated by MKK4 and MKK7 depending on the stimulus. MKK7 is the primary activator of JNK in TNF, LPS, and PGN responses. However, TLR3 requires both MKK4 and MKK7, with the former activating c-Jun and the latter activating both c-Jun and IRF3 through JNK-dependent mechanisms.

Vascular endothelial growth factor induces MEF2C and MEF2-dependent activity in endothelial cells

PURPOSE

Vascular endothelial growth factor is a key regulator of physiological and pathologic angiogenesis. Although much is known about the major upstream signaling pathways of VEGF in endothelial cells, less is known about key transcription factors involved in VEGF action. The transcription factor myocyte enhancer factor (MEF)-2C is thought to play an important role in vasculogenesis and angiogenesis during vascular development. The purpose of this study was to investigate the regulation of MEF2C expression and MEF2-dependent activity in endothelial cells by VEGF.

METHODS

Expression of MEF2C in human retinal endothelial cells and human umbilical vein endothelial cells was assayed by real-time PCR and Western blot. VEGF regulation of MEF2-dependent transcription was studied using an MEF2-luciferase reporter construct containing three copies of a high-affinity MEF2 binding site. The effect of MEF2 on endothelial cell migration was evaluated using a dominant-negative MEF2C mutant.

RESULTS

VEGF induced MEF2C expression in a dose- and time-dependent fashion. This induction was completely abrogated by the inhibition of protein kinase C and was partially blocked by the inhibition of PKC-beta and PKC-delta. In addition to regulating MEF2C expression, VEGF stimulated transcription from an MEF2-dependent promoter. VEGF stimulation of transcription was significantly reduced by the inhibition of calcineurin, CaMKII, p38 MAPK, and PKC, but not by the inhibition of ERK1/2 or BMK1/ERK5. Transfection of a dominant-negative mutant of MEF2C significantly inhibited VEGF-stimulated endothelial cell migration.

CONCLUSIONS

These results implicate VEGF as a key regulator of MEF2C and suggest that MEF2 may be an important mediator of VEGF in endothelial cells.

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.

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 Basic Protocol 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.

Negative regulation of MEKK1/2 signaling by serine-threonine kinase 38 (STK38)

Mitogen-activated protein kinases (MAPKs) are activated through the kinase cascades of MAPK, MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK). MAPKKKs phosphorylate and activate their downstream MAPKKs, which in turn phosphorylate and activate their downstream MAPKs. MAPKKK proteins relay upstream signals through the MAPK cascades to induce cellular responses. However, the molecular mechanisms by which given MAPKKKs are regulated remain largely unknown. Here, we found that serine-threonine protein kinase 38, STK38, physically interacts with the MAPKKKs MEKK1 and MEKK2 (MEKK1/2). The carboxy terminus, including the catalytic domain, but not the amino terminus of MEKK1/2 was necessary for the interaction with STK38. STK38 inhibited MEKK1/2 activation without preventing MEKK1/2 binding to its substrate, SEK1. Importantly, STK38 suppressed the autophosphorylation of MEKK2 without interfering with MEKK2 dimer formation, and converted MEKK2 from its phosphorylated to its nonphosphorylated form. The negative regulation of MEKK1/2 was not due to its phosphorylation by STK38. On the other hand, stk38 short hairpin RNA enhanced sorbitol-induced activation of MEKK2 and phosphorylation of the downstream MAPKKs, MKK3/6. Taken together, our results indicate that STK38 negatively regulates the activation of MEKK1/2 by direct interaction with the catalytic domain of MEKK1/2, suggesting a novel mechanism of MEKK1/2 regulation.

The Src family kinase Hck regulates mast cell activation by suppressing an inhibitory Src family kinase Lyn

IgE/antigen-dependent mast cell activation plays a central role in immediate hypersensitivity and other allergic reactions. The Src family tyrosine kinase (SFK) Lyn is activated by the cross-linking of high-affinity IgE receptors (FcepsilonRI). Activated Lyn phosphorylates the FcepsilonRI subunits, beta and gamma, leading to subsequent activation of various signaling pathways. Lyn also plays a negative regulatory function by activating negative regulatory molecules. Another SFK, Fyn, also contributes to mast cell degranulation by inducing Gab2-dependent microtubule formation. Here we show that a third SFK, Hck, plays a critical role in mast cell activation. Degranulation and cytokine production are reduced in FcepsilonRI-stimulated hck(-/-) mast cells. The reduced degranulation can be accounted for by defects in Gab2 phosphorylation and microtubule formation. Importantly, Lyn activity is elevated in hck(-/-) cells, leading to increased phosphorylation of several negative regulators. However, positive regulatory events, such as activation of Syk, Btk, JNK, p38, Akt, and NF-kappaB, are substantially reduced in hck(-/-) mast cells. Analysis of lyn(-/-)hck(-/-), lyn(-/-)FcepsilonRIbeta(-/-), and hck(-/-)FcepsilonRIbeta(-/-) cells shows that Hck exerts these functions via both Lyn-dependent and Lyn-independent mechanisms. Thus, this study has revealed a hierarchical regulation among SFK members to fine-tune mast cell activation.

Noncanonical function of MEKK2 and MEK5 PB1 domains for coordinated extracellular signal-regulated kinase 5 and c-Jun N-terminal kinase signaling

MEKK2 and MEK5 encode Phox/Bem1p (PB1) domains that heterodimerize with one another. MEKK2, MEK5, and extracellular signal-related kinase 5 (ERK5) form a ternary complex through interactions involving the MEKK2 and MEK5 PB1 domains and a 34-amino-acid C-terminal extension of the MEK5 PB1 domain. This C-terminal extension encodes an ERK5 docking site required for MEK5 activation of ERK5. The PB1 domains bind in a front-to-back arrangement, with a cluster of basic amino acids in the front of the MEKK2 PB1 domain binding to the back-end acidic clusters of the MEK5 PB1 domain. The C-terminal moiety, including the acidic cluster of the MEKK2 PB1 domain, is not required for MEK5 binding and binds MKK7. Quiescent MEKK2 preferentially binds MEK5, and MEKK2 activation results in ERK5 activation. Activated MEKK2 binds and activates MKK7, leading to JNK activation. The findings define how the MEKK2 and MEK5 PB1 domains are uniquely used for differential binding of two mitogen-activated protein kinase kinases, MEK5 and MKK7, for the coordinated control of ERK5 and c-Jun N-terminal kinase activation.

Gab2 antisense oligonucleotide blocks rat basophilic leukemic cell functions

Adapter molecule Grb2-associated binder-like protein 2 (Gab2) plays a critical role in FcepsilonRI-induced mast cell degranulation and activation. The present study aimed to investigate the pharmacological effects of an antisense oligonucleotide (ASO) targeted at Gab2 on the immune responses of rat basophilic leukemic (RBL)-2H3 cells. Gab2 ASOs were rationally designed and transfected into RBL-2H3 cells. Gab2 mRNA and protein knockdown was confirmed by real-time RT-PCR and immunoblotting, respectively. Effects of Gab2 ASO on FcepsilonRI-induced release of histamine and beta-hexosaminidase was measured by EIA and an enzymatic assay, respectively; signaling events by immunoblotting; and cytokine mRNA expression by RT-PCR. Effects of Gab2 ASO on cell adhesion and migration were performed on fibronectin-coated 96-well plate and transwells cell culture chambers, respectively. We have characterized a phosphorothioate-modified ASO targeted at Gab2 mRNA that was able to knockdown Gab2 mRNA and protein in RBL-2H3 cells. Gab2 ASO significantly blocked IgE-mediated mast cell release of beta-hexosaminidase and histamine; phosphorylation of Akt, p38 mitogen-activated protein kinase and PKCdelta; and up-regulation of cytokine mRNA levels (e.g. IL-4, -6, -9 and -13, and TNF-alpha). In addition, Gab2 ASO markedly prevented mast cell adhesion to fibronectin-coated plates and restrained random migration of RBL-2H3 cells in cell culture chambers. Our findings show that Gab2 knockdown in RBL-2H3 cells by ASO strategy can suppress many aspects of the mast cell functions and, therefore, a selective Gab2 ASO may have therapeutic potential for mast cell-dependent allergic disorders.

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

MEKK3 is essential for lipopolysaccharide-induced interleukin-6 and granulocyte-macrophage colony-stimulating factor production in macrophages

Mitogen-activated protein/ERK kinase kinase 3 (MEKK3) is a Ser/Thr protein kinase belonging to the MEKK/STE11 subgroup of the MAP3K family. Recently, we found that MEKK3 plays a critical role in interleukin-1 (IL-1) receptor and Toll-like receptor 4 signalling using established primary mouse embryonic fibroblast (MEF) cell lines. However, the function of MEKK3 in immune cells has not been studied because germ-line MEKK3 knockout mice are embryonically lethal between embryonic days 10 and 11. In this study, we used small interference RNA to the mouse Mekk3 gene to specifically knock down MEKK3 expression in the macrophage line Raw264.7. We found that the lipopolysaccharide-induced IL-6 and granulocyte-macrophage colony-stimulating factor (GM-CSF) production was dramatically decreased in MEKK3 knockdown cells whereas the tumour necrosis factor-alpha and IL-1beta production were not affected. We also observed that the ERK1/2, p38 and JNK MAPK induction in MEKK3 knockdown cells were moderately inhibited within the first 60 min of stimulation, while the ERK and p38 were more severely inhibited after 2-4 hr of stimulation. Degradation of IkappaBalpha was also partially blocked in MEKK3 knockdown cells. Notably, the impairment in IL-6 and GM-CSF production in the MEKK3 knockdown cells was restored by reintroducing a human Mekk3 cDNA that could not be targeted by mouse Mekk3-siRNAs. In conclusion, this study showed that MEKK3 is a crucial and specific regulator of the proinflammatory cytokines IL-6 and GM-CSF in macrophages and provided a novel method for investigating MEKK3 function in other immune cells.

Brain-derived Neurotrophic Factor Activates ERK5 in Cortical Neurons via a Rap1-MEKK2 Signaling Cascade

The extracellular signal-regulated kinase 5 (ERK5) is activated in neurons of the central nervous system by neurotrophins including brain-derived neurotrophic factor (BDNF). Although MEK5 is known to mediate BDNF stimulation of ERK5 in central nervous system neurons, other upstream signaling components have not been identified. Here, we report that BDNF induces a sustained activation of ERK5 in rat cortical neurons and activates Rap1, a small GTPase, as well as MEKK2, a MEK5 kinase. Our data indicate that activation of Rap1 or MEKK2 is sufficient to stimulate ERK5, whereas inhibition of either Rap1 or MEKK2 attenuates BDNF activation of ERK5. Furthermore, BDNF stimulation of MEKK2 is regulated by Rap1. Our evidence also indicates that Ras and MEKK3, a MEK5 kinase in non-neuronal cells, do not play a significant role in BDNF activation of ERK5. This study identifies Rap1 and MEKK2 as critical upstream signaling molecules mediating BDNF stimulation of ERK5 in central nervous system neurons.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

Cyclic AMP selectively uncouples mitogen-activated protein kinase cascades from activating signals

Cells integrate signals to select the appropriate response from an array of possible outcomes. Signal integration causes the reorganization of signaling pathways by undescribed events. To analyze the molecular changes in signaling pathways that elicit different responses, we focused on the interaction between cyclic AMP (cAMP) and growth factors. We show that the activation of extracellular signal-regulated kinase 5 (ERK5), but not ERK1/2, by growth factors is disrupted by cAMP through cAMP-dependent protein kinase (PKA). Activation of MEKK2, a mitogen-activated protein (MAP) kinase kinase kinase upstream of ERK5 that is required for growth factor activation of ERK5, is also disrupted by PKA. Transcription of c-Jun is induced by ERK5, and like ERK5, c-Jun induction is also blocked by cAMP. Transcription from the serum response element, like activation of ERK1/2, is not blocked by cAMP. Collectively, these results support a model in which cAMP shapes the growth factor-induced cellular response through PKA-dependent uncoupling of selected MAP kinase cascades from activating signals.

IL-3-mediated TNF production is necessary for mast cell development

Mouse mast cell development and survival are largely controlled by the cytokines IL-3 and stem cell factor (SCF). We have found that IL-3 stimulation of bone marrow cells induces the production of TNF via a PI3K- and MAPK kinase/ERK-dependent pathway. Specifically, Mac-1-positive cells were responsible for TNF production, which peaked on days 7-10 of culture and decreased rapidly thereafter. The importance of IL-3-induced TNF secretion was demonstrated by the failure of TNF-deficient bone marrow cells to survive for >3 wk when cultured in IL-3 and SCF, a defect that was reversed by the addition of soluble TNF. The development of human mast cells from bone marrow progenitors was similarly hampered by the addition of TNF-blocking Abs. Cell death was due to apoptosis, which occurred with changes in mitochondrial membrane potential and caspase activation. Apoptosis appeared to be due to loss of IL-3 signaling, because TNF-deficient cells were less responsive than their wild-type counterparts to IL-3-mediated survival. In vitro cultured mast cells from TNF-deficient mice also demonstrated reduced expression of the high affinity IgE receptor, which was restored to normal levels by the addition of soluble TNF. Finally, TNF-deficient mice demonstrated a 50% reduction in peritoneal mast cell numbers, indicating that TNF is an important mast cell survival factor both in vitro and in vivo.

The MAP kinase ERK5 binds to and phosphorylates p90 RSK

We showed previously that p90 RSK was activated in cells expressing an activated mutant of MEK5, the activator of the MAP kinase ERK5. Based on the following evidence, we suggest that ERK5 can directly activate RSK in cells. ERK5 binds to RSK in vitro and co-immunoprecipitates from cell extracts; activation of ERK5 weakens its binding to RSK, suggesting that RSK is released upon activation. Phosphorylation of RSK by ERK5 in vitro causes its activation, indicating that RSK is a substrate of ERK5. In cells activation of ERK5 but not p38 or the c-Jun N-terminal kinase is associated with RSK activation. The large C-terminal domain of ERK5 is not required for binding or activation of RSK by ERK5; however, the common docking or CD domain of ERK5 and the docking or D domain of RSK are important for their association.

Involvement of mitogen-activated protein kinase and NF-κB activation in Ca2+-induced IL-8 production in human mast cells

Interleukin-8 (IL-8) is a potent proinflammatory chemokine that plays an important role in inflammation by activating and recruiting neutrophils, lymphocytes, and eosinophils. To demonstrate the effect of intracellular Ca(2+) on IL-8 production and related signaling, we stimulated human mast cell line HMC-1 with either calcium ionophore A23187 or thapsigargin. Increase of intracellular Ca(2+) resulted in inducing IL-8 gene expression and protein secretion, and addition of EGTA or BAPTA/AM before Ca(2+) stimulation inhibited the induction of IL-8 production. Intracellular Ca(2+) triggered the activation of mitogen-activated protein kinase (MAPK) in HMC-1, especially p42 and p44 isoforms of extracellular signal-regulated kinase (ERK) and p38 MAPK, but not c-Jun N-terminal kinase (JNK). Pretreatment of MAPK inhibitors (PD98059 and SB203580) markedly blocked Ca(2+)-induced IL-8 production from cells, and anti-inflammatory drugs, such as dexamethasone and cyclosporin A, partially inhibited the activation of ERK1/2. We determined that increased Ca(2+) activates the nuclear translocation of the transcription factor NF-kappaB. NF-kappaB inhibitors blocked the ability of Ca(2+) to induce IL-8 production, and the activation of NF-kappaB was required for intracellular Ca(2+)-induced up-regulation of IL-8. These results suggest that increased intracellular Ca(2+) stimulated p38 and ERK1/2 MAPK signaling cascades result in NF-kappaB activation and IL-8 production in HMC-1 cells. This study is the first to identify the intracellular signaling pathways involved in the Ca(2+)-mediated up-regulation of IL-8 synthesis and release from HMC-1 cells.

Protein kinase C α, βI, and βII isozymes regulate cytokine production in mast cells through MEKK2/ERK5-dependent and -independent pathways

Regulation of MAPK pathways by PKC isoforms was examined in murine bone marrow-derived mast cells (BMMCs). The PKCalpha, betaI, and betaII isoforms showed the most robust activation after FcepsilonR1-mediated stimulation by anti-ovalbumin specific IgE and ovalbumin (IgE-ova). PKCalpha, betaI, and betaII were all involved in activation of JNK, MEKK2, and ERK5, with differential relative contributions of each isoform to specific MAPK pathway components. BMMCs from mice lacking MEKK2 showed reduced production (50-60%) of IL-6, IL-13, and TNF-alpha after stimulation, demonstrating MEKK2-dependent and -independent pathways for cytokine production. Cytokine production was stimulated by over-expression of PKC in cells from MEKK2-deficient and wild-type mice. Activation of ERK5 did not occur in BMMCs lacking MEKK2, indicating that MEKK2-independent cytokine production was also ERK5-independent. Since MAPK modules differentially regulate mast cell functions, including degranulation and cytokine production, it is suggested that specific functions could be targeted by inhibiting specific PKC isoforms.

Inhibition of spleen tyrosine kinase prevents mast cell activation and airway hyperresponsiveness

RATIONALE

Spleen tyrosine kinase (Syk) is important for Fc and B-cell receptor-mediated signaling.

OBJECTIVE

To determine the activity of a specific Syk inhibitor (R406) on mast cell activation in vitro and on the development of allergen-induced airway hyperresponsiveness (AHR) and inflammation in vivo.

METHODS

AHR and inflammation were induced after 10 d of allergen (ovalbumin [OVA]) exposure exclusively via the airways and in the absence of adjuvant. This approach was previously established to be IgE, FcepsilonRI, and mast cell dependent. Alternatively, mice were passively sensitized with OVA-specific IgE, followed by limited airway challenge. In vitro, the inhibitor was added to cultures of IgE-sensitized bone marrow-derived mast cells (BMMCs) before cross-linking with allergen.

RESULTS

The inhibitor prevented OVA-induced degranulation of passively IgE-sensitized murine BMMCs and inhibited the production of interleukin (IL)-13, tumor necrosis factor alpha, IL-2, and IL-6 in these sensitized BMMCs. When administered in vivo, R406 inhibited AHR, which developed in BALB/c mice exposed to aerosolized 1% OVA for 10 consecutive d (20 min/d), as well as pulmonary eosinophilia and goblet cell metaplasia. A similar inhibition of AHR was demonstrated in mice passively sensitized with OVA-specific IgE and exposed to limited airway challenge.

CONCLUSION

This study delineates a functional role for Syk in the development of mast cell- and IgE-mediated AHR and airway inflammation, and these results indicate that inhibition of Syk may be a target in the treatment of allergic asthma.

Btk plays a crucial role in the amplification of Fc epsilonRI-mediated mast cell activation by kit

Stem cell factor (SCF) acts in synergy with antigen to enhance the calcium signal, degranulation, activation of transcription factors, and cytokine production in human mast cells. However, the underlying mechanisms for this synergy remain unclear. Here we show, utilizing bone marrow-derived mast cells (BMMCs) from Btk and Lyn knock-out mice, that activation of Btk via Lyn plays a key role in promoting synergy. As in human mast cells, SCF enhanced degranulation and cytokine production in BMMCs. In Btk-/- BMMCs, in which there was a partial reduction in the capacity to degranulate in response to antigen, SCF was unable to enhance the residual antigen-mediated degranulation. Furthermore, as with antigen, the ability of SCF to promote cytokine production was abrogated in the Btk-/- BMMCs. The impairment of responses in Btk-/- cells correlated with an inability of SCF to augment phospholipase Cgamma1 activation and calcium mobilization, and to phosphorylate NFkappaB and NFAT for cytokine gene transcription in these cells. Similar studies with Lyn-/- and Btk-/-/Lyn-/- BMMCs indicated that Lyn was a regulator of Btk for these responses. These data demonstrate, for the first time, that Btk is a key regulator of a Kit-mediated amplification pathway that augments Fc epsilonRI-mediated mast cell activation.

Mip1, an MEKK2-interacting protein, controls MEKK2 dimerization and activation

Mitogen-activated protein kinase (MAPK) cascades are central components of the intracellular signaling networks used by eukaryotic cells to respond to a wide spectrum of extracellular stimuli. An MAPK is activated by an MAPK kinase, which in turn is activated by an MAPK kinase kinase (MAP3K). However, little is known about the molecular aspects of the regulation and activation of large numbers of MAP3Ks that are crucial in relaying upstream receptor-mediated signals through the MAPK cascades to induce various physiological responses. In this study, we identified a novel MEKK2-interacting protein, Mip1, that regulates MEKK2 dimerization and activation by forming a complex with inactive and nonphosphorylated MEKK2. In particular, Mip1 prevented MEKK2 activation by blocking MEKK2 dimer formation, which in turn blocked JNKK2, c-Jun N-terminal kinase 1 (JNK1), extracellular signal-regulated kinase 5, and AP-1 reporter gene activation by MEKK2. Furthermore, we found that the endogenous Mip1-MEKK2 complex was dissociated transiently following epidermal growth factor stimulation. In contrast, the knockdown of Mip1 expression by siRNA augmented the MEKK2-mediated JNK and AP-1 reporter activation. Together, our data suggest a novel model for MEKK2 regulation and activation.

Wiring diagrams of MAPK regulation by MEKK1, 2, and 3

Mitogen-activated protein kinase (MAPK) pathways are activated by a plethora of stimuli. The literature is filled with papers describing the activation of different MAPKs by almost any stimulus or insult imaginable to cells. In this review, we use signal transduction wiring diagrams to illustrate putative upstream regulators for the MAPK kinase kinases, MEKK1, 2, and 3. Targeted gene disruption of MEKK1, 2, or 3 defined phenotypes for each MEKK associated with loss of specific MAPK regulation. Genetic analysis of MEKK function clearly defines specific components of the wiring diagram that require MEKK1, 2, or 3 for physiological responses. We propose that signal transduction network wiring diagrams are valuable tools for hypothesis building and filtering physiologically relevant phenotypic responses from less connected protein relations in the regulation of MAPK pathways.

Dimerization through the Catalytic Domain Is Essential for MEKK2 Activation

Mitogen-activated protein kinase (MAPK) cascades are the central components of the intracellular signaling networks that eukaryotic cells use to respond to a wide spectrum of extracellular stimuli. MAPKs are activated through a module consisting of a MAPK, a MAPK kinase (MKK), and a MKK kinase (MAP3K). Because of its unique position in the MAPK module, a MAP3K is crucial in relaying the upstream receptor-mediated signals through the MAPK cascades to induce physiological responses. Yet, the underlying molecular mechanism of MAP3K regulation and activation remains largely unknown. In this study, we demonstrated that MAP3K MEKK2 activation requires dimerization. We mapped the MEKK2 dimerization motif in its catalytic domain and showed that the NH2-terminal region is not required for MEKK2 dimer formation. We also found that the inactive, non-phosphorylated MEKK2 formed significantly more dimers than the phosphorylated and, hence, active MEKK2. Moreover, prevention of MEKK2 dimer formation inhibited MEKK2-mediated JNK activation. Using a chemical-induced dimerization system, we further demonstrated that MEKK2 dimer formation in vivo augmented MEKK2-dependent JNK activation and JNK/AP-1 reporter gene transcription. Together, these results suggest a novel mechanism underlying MEKK2 regulation and activation.

WNK1: analysis of protein kinase structure, downstream targets, and potential roles in hypertension

The WNK kinases are a recently discovered family of serine-threonine kinases that have been shown to play an essential role in the regulation of electrolyte homeostasis. Intronic deletions in the WNK1 gene result in its overexpression and lead to pseudohypoaldosteronism type II, a disease with salt-sensitive hypertension and hyperkalemia. This review focuses on the recent evidence elucidating the structure of the kinase domain of WNK1 and functions of these kinases in normal and disease physiology. Their functions have implications for understanding the biochemical mechanism that could lead to the retention or insertion of proteins in the plasma membrane. The WNK kinases may be able to influence ion homeostasis through its effects on synaptotagmin function.

Stat3 upregulates MEK5 expression in human breast cancer cells

The constitutive activation of signal transducer and activator of transcription 3 (Stat3) is frequently detected in breast cancer cell lines but not in normal breast epithelial cells. Stat3 has been classified as an oncogene, because constitutively active Stat3 can mediate oncogenic transformation in cultured cells and tumor formation in nude mice. Since Stat3 appears to play an important role in breast cancer, it is of interest to investigate Stat3-regulated genes and elucidate Stat3-mediated oncogenesis. In this study, we investigated the Stat3-regulated genes in human breast epithelial cells. Upon overexpression of Stat3-C, a constitutively active Stat3 form, in nonmalignant telomerase immortalized breast (TERT) cells, the total mRNA was extracted and subjected to Affymetrix microarray analysis. Our results showed that mitogen-activated protein kinase kinase 5 (MEK5) was markedly induced (more than 22-fold increase, P<0.001) by Stat3-C expression. RT-PCR result also demonstrated that MEK5 mRNA was significantly induced by Stat3-C in TERT cells. The upregulation of MEK5 by Stat3-C was further confirmed by Western blot in MCF10A breast epithelial cells. Furthermore, in MDA-MB-435s breast carcinoma cells, which express high levels of activated Stat3 and MEK5, MEK5 protein was significantly reduced by using Stat3 short interfering RNA. The reduction of MEK5 was consistent with Stat3 knockdown in this breast carcinoma cell line. We also investigated MEK5 expression in different breast carcinoma cell lines and breast cancer tissues using tissue array analysis. Compared with nonmalignant breast epithelial cells or normal tissues without constitutively active Stat3 signaling, MEK5 protein levels are remarkably higher in breast carcinoma cell lines and cancer tissues with constitutively activated Stat3. Taken together, our findings suggest that constitutively active Stat3 upregulates MEK5 in the breast epithelial cells. MEK5 may be one of the Stat3-regulated genes and plays its essential roles in oncogenesis mediated by aberrantly activated Stat3 signaling in breast carcinomatosis and malignancies.

Counting on mitogen-activated protein kinases—ERKs 3, 4, 5, 6, 7 and 8

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.

Role of the BMK1/ERK5 signaling pathway: lessons from knockout mice

Mitogen-activated protein (MAP) kinase cascades play a central role in mediating extracellular stimuli-induced intracellular signaling during cell activation. The fourth and least studied mammalian MAP kinase pathway, big MAP kinase 1 (BMK1), also known as extracellular signal regulated kinase 5 (ERK5), is activated in response to growth factors and stress. Activation of this signaling pathway has been implicated not only in physiological functions such as cell survival, proliferation and differentiation but also in pathological processes such as carcinogenesis, cardiac hypertrophy and atherosclerosis. In recent years a series of gene-targeted mice lacking components within the BMK1 cascade have been generated, which have enabled us to investigate the role of the BMK1 pathway within different tissues. Analyses of these knockout mice have led to major discoveries in the role of BMK1 signaling in angiogenesis and in cardiac development. Moreover, studies using conditional BMK1 knockout mice, which circumvent the early embryonic lethality of BMK1 knockouts, have unveiled the importance of BMK1 in endothelial survival and maintenance of vascular integrity during adulthood. Here we summarize current understanding of the function of BMK1, as well as include new data generated from a series of tissue-specific BMK1 knockout mice in an attempt to dissect the role of the BMK1 pathway in various cell types in animals.