Direct activation of the stress-activated protein kinase (SAPK) and extracellular signal-regulated protein kinase (ERK) pathways by an inducible mitogen-activated protein Kinase/ERK kinase kinase 3 (MEKK) derivative

MEKK3 in hybrid snakehead (Channa maculate ♀ ×Channa argus ♂): Molecular characterization and immune response to infection with Nocardia seriolae and Aeromonas schubertii

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 3 (MEKK3) is a serine/threonine protein kinase that acts as a key regulator and is widely involved in various innate and acquired immune signaling pathways. In this study, we first cloned the complete open reading frame (ORF) of the MEKK3 gene (named CcMEKK3) in a hybrid snakehead (Channa maculate ♀ × Channa argus ♂). The full-length ORF of CcMEKK3 is 1851 bp, and encodes a putative protein of 616 amino acids containing a serine/threonine kinase catalytic (S-TKc) domain and a Phox and Bem1p (PB1) domain. A sequence alignment and phylogenetic tree analysis showed that CcMEKK3 is highly conserved relative to the MEKK3 proteins of other teleost species. CcMEKK3 was constitutively expressed in all the healthy hybrid snakehead tissues tested, with greatest expression in the immune tissues, such as the head kidney and spleen. The expression of CcMEKK3 was usually upregulated in the head kidney, spleen, and liver at different time points after infection with Nocardia seriolae or Aeromonas schubertii. Similarly, the dynamic expression levels of CcMEKK3 in head kidney leukocytes after stimulation revealed that CcMEKK3 was induced by LTA, LPS, and poly(I:C). In the subcellular localization analysis, CcMEKK3 was evenly distributed in the cytoplasm of HEK293T cells, and its overexpression significantly promoted the activities of NF-κB and AP-1. These results suggest that CcMEKK3 is involved in the immune defense against these two pathogens, and plays a crucial role in activating the NF-κB and MAPK signaling pathways.

Concurrent targeting of MAP3K3 and BRD4 by miR-3140-3p overcomes acquired resistance to BET inhibitors in neuroblastoma cells

Neuroblastoma (NB) harboring MYCN amplification is a refractory disease with a poor prognosis. As BRD4, an epigenetic reader belonging to the bromodomain and extra terminal domain (BET) family, drives transcription of MYCN in NB cells, BET inhibitors (BETis) are considered useful for NB therapy. However, clinical trials of BETis suggested that early acquired resistance to BETis limits their therapeutic benefit. MicroRNAs are small non-coding RNAs that mediate post-transcriptional silencing of target genes. We previously identified miR-3140-3p as a potent candidate for nucleic acid therapeutics for cancer, which directly targets BRD4. We demonstrated that miR-3140-3p suppresses tumor cell growth in MYCN-amplified NB by downregulating MYCN and MYC through BRD4 suppression. We established BETi-acquired resistant NB cells to evaluate the mechanism of resistance to BETi in NB cells. We revealed that activated ERK1/2 stabilizes MYCN protein by preventing ubiquitin-mediated proteolysis via phosphorylation of MYCN at Ser62 in BETi-acquired resistant NB cells, thereby attenuating the effects of BETi in these cells. miR-3140-3p efficiently downregulated MYCN expression by directly targeting the MAP3K3-ERK1/2 pathway in addition to BRD4 suppression, inhibiting tumor cell growth in BETi-acquired resistant NB cells. This study suggests that miR-3140-3p has the potential to overcome resistance to BETi in NB.

Late-onset multiple venous malformations confined to the upper limb: link to somatic MAP3K3 mutations

Venous (cavernous) malformations are commonly seen in the upper limb. Almost all venous malformations are congenital. They may be sporadic, familial, or syndromic. Late-onset, multiple venous malformations confined to the upper limb are rare. Lesions present after puberty. All previously reported cases were located subcutaneously and were small in size. The condition is non-hereditary and non-syndromic. We present a unique series of eight patients with this rare condition. Unique features included the presence of large malformations (up to 20 cm in diameter) and the presence of subfascial lesions causing nerve compression. Surgical excision was curative. Mutational analysis in one patient identified a novel somatic MAP3K3 gene mutation (c.1723T > C, p.Tyr 575 His) in the affected veins. The encoded MAP3K3 protein is known to accelerate the RAS pathway of cellular proliferation.Level of evidence: IV.

Platelet MEKK3 regulates arterial thrombosis and myocardial infarct expansion in mice

MAPKs play important roles in platelet activation. However, the molecular mechanisms by which MAPKs are regulated in platelets remain largely unknown. Real-time polymerase chain reaction and western blot data showed that MEKK3, a key MAP3K family member, was expressed in human and mouse platelets. Then, megakaryocyte/platelet-specific MEKK3-deletion (MEKK3^-/- ) mice were developed to elucidate the platelet-related function(s) of MEKK3. We found that agonist-induced aggregation and degranulation were reduced in MEKK3^-/- platelets in vitro. MEKK3 deficiency significantly impaired integrin αIIbβ3-mediated inside-out signaling but did not affect the outside-in signaling. At the molecular level, MEKK3 deficiency led to severely impaired activation of extracellular signal-regulated kinases 1/2 (ERK1/2) and c-Jun NH₂-terminal kinase 2 but not p38 or ERK5. In vivo, MEKK3^-/- mice showed delayed thrombus formation following FeCl₃-induced carotid artery injury. Interestingly, the tail bleeding time was normal in MEKK3^-/- mice. Moreover, MEKK3^-/- mice had fewer microthrombi, reduced myocardial infarction (MI) size, and improved post-MI heart function in a mouse model of MI. These results suggest that MEKK3 plays important roles in platelet MAPK activation and may be used as a new effective target for antithrombosis and prevention of MI expansion.

Association of single-nucleotide polymorphisms in NAT9 and MAP3K3 genes with litter size traits in Berkshire pigs

Litter size is an economically important trait in the pig industry. We aimed to identify genetic markers associated with litter size, which can be used in breeding programs for improving reproductive traits. Single-nucleotide polymorphisms (SNPs) of Berkshire pigs in the N-acetyltransferase 9 (NAT9) and Mitogen-activated protein kinase kinase kinase 3 (MAP3K3) genes were from RNA sequencing results, and already exist in the databank (NCBI), and were confirmed by polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP). A total of 272 Berkshire sows were used to examine the genotype, and their association with litter size traits was analyzed. The NAT9 SNP was located in chromosome 12 exon 640 mRNA (A > G) and the MAP3K3 SNP was located in chromosome 12 intron 11 (80, C > T). Association analysis indicated that the GG genotype of NAT9 and the CT genotype of MAP3K3 had the highest values for litter size traits. The GG genotype expressed higher levels of NAT9 mRNA in the endometrium than the other genotypes did, and a positive correlation was found between litter size traits and NAT9, but not MAP3K3 expression level. These results indicate that the NAT9 and MAP3K3 can be used as candidate genes applicable in breeding program for the improvement of litter size traits in Berkshire pigs.

De Novo Missense Variants in TRAF7 Cause Developmental Delay, Congenital Anomalies, and Dysmorphic Features

TRAF7 is a multi-functional protein involved in diverse signaling pathways and cellular processes. The phenotypic consequence of germline TRAF7 variants remains unclear. Here we report missense variants in TRAF7 in seven unrelated individuals referred for clinical exome sequencing. The seven individuals share substantial phenotypic overlap, with developmental delay, congenital heart defects, limb and digital anomalies, and dysmorphic features emerging as key unifying features. The identified variants are de novo in six individuals and comprise four distinct missense changes, including a c.1964G>A (p.Arg655Gln) variant that is recurrent in four individuals. These variants affect evolutionarily conserved amino acids and are located in key functional domains. Gene-specific mutation rate analysis showed that the occurrence of the de novo variants in TRAF7 (p = 2.6 × 10^-3) and the recurrent de novo c.1964G>A (p.Arg655Gln) variant (p = 1.9 × 10^-8) in our exome cohort was unlikely to have occurred by chance. In vitro analyses of the observed TRAF7 mutations showed reduced ERK1/2 phosphorylation. Our findings suggest that missense mutations in TRAF7 are associated with a multisystem disorder and provide evidence of a role for TRAF7 in human development.

Fisetin inhibits the growth and migration in the A549 human lung cancer cell line via the ERK1/2 pathway

Lung cancer is the most prevalent malignant tumor type in the developed world and the discovery of novel anti-tumor drugs is a research hotspot. Fisetin, a naturally occurring flavonoid, has been reported to have anti-cancer effects in multiple tumor types. The present study found that fisetin inhibited the growth and migration of non-small cell lung cancer in vitro. MTT, wound-healing, cell-matrix adhesion and Transwell assays were performed and demonstrated that fisetin suppressed proliferation, migration, adhesion and invasion, respectively. Flow cytometric analysis indicated that fisetin induced apoptosis in the A549 cell line by decreasing the expression of c-myc, cyclin-D1, cyclooxygenase-2, B cell lymphoma-2, CXC chemokine receptor type 4, cluster of differentiation 44 and metalloproteinase-2/9, increasing the expression of cyclin dependent kinase inhibitor (CDKN) 1A/B, CDKN2D and E-cadherin and increasing the activity of caspase-3/9 via targeting the extracellular signal-regulated kinase signaling pathway. The results provided comprehensive evidence for the anti-tumor effects of fisetin in non-small cell lung cancer in vitro, which may provide a novel approach for clinical treatment.

A mutational comparison of adult and adolescent and young adult (AYA) colon cancer

BACKGROUND

It is possible that the relative lack of progress in treatment outcomes among adolescent and young adult (AYA) patients with cancer is caused by a difference in disease biology compared with the corresponding diseases in younger and older individuals. There is evidence that colon cancer is more aggressive and has a poorer prognosis in AYA patients than in older adult patients.

METHODS

To further understand the molecular basis for this difference, whole-exome sequencing was conducted on a cohort of 30 adult, 30 AYA, and 2 pediatric colon cancers.

RESULTS

A statistically significant difference in mutational frequency was observed between AYA and adult samples in 43 genes, including ROBO1, MYC binding protein 2 (MYCBP2), breast cancer 2 (early onset) (BRCA2), MAP3K3, MCPH1, RASGRP3, PTCH1, RAD9B, CTNND1, ATM, NF1; KIT, PTEN, and FBXW7. Many of these mutations were nonsynonymous, missense, stop-gain, or frameshift mutations that were damaging. Next, RNA sequencing was performed on a subset of the samples to confirm the mutations identified by exome sequencing. This confirmation study verified the presence of a significantly greater frequency of damaging mutations in AYA compared with adult colon cancers for 5 of the 43 genes (MYCBP2, BRCA2, PHLPP1, TOPORS, and ATR).

CONCLUSIONS

The current results provide the rationale for a more comprehensive study with a larger sample set and experimental validation of the functional impact of the identified variants along with their contribution to the biologic and clinical characteristics of AYA colon cancer. Cancer 2018;124:1070-82. © 2017 American Cancer Society.

Genome-Wide Association and Replication Study of Hepatotoxicity Induced by Antiretrovirals Alone or with Concomitant Anti-Tuberculosis Drugs

Drug-induced hepatotoxicity (DIH) is a common adverse event that is associated with both antiretroviral (ARV) and anti-tuberculosis drugs (ATD). Moreover, the genetic variations predisposing ARV- and ARV-ATD-induced liver toxicity in African populations are not well investigated, despite the two diseases being the major global health problems in sub-Saharan Africa. We performed a genome-wide association study (GWAS) and replication study to identify the genetic variants linked to the risk of developing DIH due to ARV drugs alone, and ARV-ATD co-treatment in Ethiopian HIV-positive patients. Treatment-naïve newly diagnosed HIV patients (n = 719) with or without tuberculosis (TB) co-infection were enrolled prospectively and received efavirenz-based ARV therapy with or without rifampicin-based short course ATD, respectively. Whole-genome genotyping was performed by using the Illumina Omni Express Exome Bead Chip genotyping array with 951,117 single nucleotide polymorphisms (SNPs) on a total of 41 cases of DIH, and 452 people without DIH (treatment tolerants). The replication study was carried out for 100 SNPs with the lowest p-values (top SNPs) by using an independent cohort consisting of 18 DIH cases and 208 treatment tolerants. We identified a missense SNP rs199650082 (2756G→A, R919Q, p = 1.4 × 10^-6, odds ratio [OR] = 18.2, 95% confidence interval [CI] = 7.1-46.9) in an endoplasmic reticulum to the nucleus signaling-1 (ERN1) gene on chromosome 17 to be associated with DIH in the ARV-only cohort. In the ARV-ATD co-treatment groups, rs4842407, a long intergenic noncoding RNAs (lincRNAs) transcript variant on chromosome 12, was associated with DIH (p = 5.3 × 10^-7, OR = 5.4, 95% CI = 2.8-10.3). These genetic variants that are putatively associated with DIH due to ARV drugs alone and ARV-ATD co-treatment establish a foundation for future personalized medicine in people with HIV and TB and call for larger studies in independent populations.

RNAi screening identifies Trypanosoma brucei stress response protein kinases required for survival in the mouse

Protein kinases (PKs) are a class of druggable targets in Trypanosoma brucei, the causative agent of Human African Trypanosomiasis (sleeping sickness), yet little is known about which PKs are essential for survival in mammals. A recent kinome-wide RNAi screen with 176 individual bloodstream form Trypanosoma brucei lines identified PKs required for proliferation in culture. In order to assess which PKs are also potential virulence factors essential in vivo, lines were pooled, inoculated into mice, and screened for loss of fitness after 48 h RNAi. The presence of trypanosomes in the bloodstream was assessed using RNAi target sequencing (RITseq) and compared to growth in culture. We identified 49 PKs with a significant loss of fitness in vivo in two independent experiments, and a strong correlation between in vitro and in vivo loss of fitness for the majority. Nine PKs had a more pronounced growth defect in vivo, than in vitro. Amongst these PKs were several with putative functions related to stress responses mediated through the PI3K/TOR or MAPK signaling cascades, which act to protect the parasite from complement-mediated and osmotic lysis. Identification of these virulence-associated PKs provides new insights into T. brucei-host interaction and reveals novel potential protein kinase drug targets.

The role of long noncoding RNA-LET in cell proliferation and invasion of nasopharyngeal carcinoma and its mechanism

LncRNA-LET, a recently identified long noncoding RNA, has been shown to act as a tumor suppressor; however, its biological function and mechanism have not been fully investigated. Our research found that there was less expression of LET in nasopharyngeal carcinoma (NPC) tissues than normal tissues and that LET might inhibit proliferation, adhesion and invasion of NPC in vitro by enhancing its expression. By contrast, decreased LET expression could promote the proliferation, adhesion and invasion of NPC. In addition, the expression profiles of related genes and MAPK/ERK pathway were also regulated effectively via overexpression or silencing of LET. This result provides comprehensive evidence of LET’s antitumor effect on NPC in vitro, which might provide a new approach for clinical treatment.

Somatic Variations in Cervical Cancers in Indian Patients

There are very few reports that describe the mutational landscape of cervical cancer, one of the leading cancers in Indian women. The aim of the present study was to investigate the somatic mutations that occur in cervical cancer. Whole exome sequencing of 10 treatment naïve tumour biopsies with matched blood samples, from a cohort of Indian patients with locally advanced disease, was performed. The data revealed missense mutations across 1282 genes, out of 1831 genes harbouring somatic mutations. These missense mutations (nonsynonymous + stop-gained) when compared with pre-existing mutations in the COSMIC database showed that 272 mutations in 250 genes were already reported although from cancers other than cervical cancer. More than 1000 novel somatic variations were obtained in matched tumour samples. Pathways / genes that are frequently mutated in various other cancers were found to be mutated in cervical cancers. A significant enrichment of somatic mutations in the MAPK pathway was observed, some of which could be potentially targetable. This is the first report of whole exome sequencing of well annotated cervical cancer samples from Indian women and helps identify trends in mutation profiles that are found in an Indian cohort of cervical cancer.

Molecular mechanisms of OLIG2 transcription factor in brain cancer

Oligodendrocyte lineage transcription factor 2 (OLIG2) plays a pivotal role in glioma development. Here we conducted a comprehensive study of the critical gene regulatory networks involving OLIG2. These include the networks responsible for OLIG2 expression, its translocation to nucleus, cell cycle, epigenetic regulation, and Rho-pathway interactions. We described positive feedback loops including OLIG2: loops of epigenetic regulation and loops involving receptor tyrosine kinases. These loops may be responsible for the prolonged oncogenic activity of OLIG2. The proposed schemes for epigenetic regulation of the gene networks involving OLIG2 are confirmed by patient survival (Kaplan-Meier) curves based on the cancer genome atlas (TCGA) datasets. Finally, we elucidate the Coherent-Gene Modules (CGMs) networks-framework of OLIG2 involvement in cancer. We showed that genes interacting with OLIG2 formed eight CGMs having a set of intermodular connections. We showed also that among the genes involved in these modules the most connected hub is EGFR, then, on lower level, HSP90 and CALM1, followed by three lower levels including epigenetic genes KDM1A and NCOR1. The genes on the six upper levels of the hierarchy are involved in interconnections of all eight CGMs and organize functionally defined gene-signaling subnetworks having specific functions. For example, CGM1 is involved in epigenetic control. CGM2 is significantly related to cell proliferation and differentiation. CGM3 includes a number of interconnected helix-loop-helix transcription factors (bHLH) including OLIG2. Many of these TFs are partially controlled by OLIG2. The CGM4 is involved in PDGF-related: angiogenesis, tumor cell proliferation and differentiation. These analyses provide testable hypotheses and approaches to inhibit OLIG2 pathway and relevant feed-forward and feedback loops to be interrogated. This broad approach can be applied to other TFs.

The cerebral cavernous malformation proteins CCM2L and CCM2 prevent the activation of the MAP kinase MEKK3

Three genes, CCM1, CCM2, and CCM3, interact genetically and biochemically and are mutated in cerebral cavernous malformations (CCM). A recently described member of this CCM family of proteins, CCM2-like (CCM2L), has high homology to CCM2. Here we show that its relative expression in different tissues differs from that of CCM2 and, unlike CCM2, the expression of CCM2L in endothelial cells is regulated by density, flow, and statins. In vitro, both CCM2L and CCM2 bind MEKK3 in a complex with CCM1. Both CCM2L and CCM2 interfere with MEKK3 activation and its ability to phosphorylate MEK5, a downstream target. The in vivo relevance of this regulation was investigated in zebrafish. A knockdown of ccm2l and ccm2 in zebrafish leads to a more severe "big heart" and circulation defects compared with loss of function of ccm2 alone, and also leads to substantial body axis abnormalities. Silencing of mekk3 rescues the big heart and body axis phenotype, suggesting cross-talk between the CCM proteins and MEKK3 in vivo. In endothelial cells, CCM2 deletion leads to activation of ERK5 and a transcriptional program that are downstream of MEKK3. These findings suggest that CCM2L and CCM2 cooperate to regulate the activity of MEKK3.

c-KIT signaling is targeted by pathogenic Yersinia to suppress the host immune response

Background

The pathogenic Yersinia species exhibit a primarily extracellular lifestyle through manipulation of host signaling pathways that regulate pro-inflammatory gene expression and cytokine release. To identify host genes that are targeted by Yersinia during the infection process, we performed an RNA interference (RNAi) screen based on recovery of host NF-κB-mediated gene activation in response to TNF-α stimulation upon Y. enterocolitica infection.

Results

We screened shRNAs against 782 genes in the human kinome and 26 heat shock genes, and identified 19 genes that exhibited ≥40% relative increase in NF-κB reporter gene activity. The identified genes function in multiple cellular processes including MAP and ERK signaling pathways, ion channel activity, and regulation of cell growth. Pre-treatment with small molecule inhibitors specific for the screen hits c-KIT and CKII recovered NF-κB gene activation and/or pro-inflammatory TNF-α cytokine release in multiple cell types, in response to either Y. enterocolitica or Y. pestis infection.

Conclusions

We demonstrate that pathogenic Yersinia exploits c-KIT signaling in a T3SS-dependent manner to downregulate expression of transcription factors EGR1 and RelA/p65, and pro-inflammatory cytokines. This study is the first major functional genomics RNAi screen to elucidate virulence mechanisms of a pathogen that is primarily dependent on extracellular-directed immunomodulation of host signaling pathways for suppression of host immunity.

Mammalian MAPK signal transduction pathways activated by stress and inflammation: a 10-year update

The mammalian stress-activated families of mitogen-activated protein kinases (MAPKs) were first elucidated in 1994, and by 2001, substantial progress had been made in identifying the architecture of the pathways upstream of these kinases as well as in cataloguing candidate substrates. This information remains largely sound. Nevertheless, an informed understanding of the physiological and pathophysiological roles of these kinases remained to be accomplished. In the past decade, there has been an explosion of new work using RNAi in cells, as well as transgenic, knockout and conditional knockout technology in mice that has provided valuable insight into the functions of stress-activated MAPK pathways. These findings have important implications in our understanding of organ development, innate and acquired immunity, and diseases such as atherosclerosis, tumorigenesis, and type 2 diabetes. These new developments bring us within striking distance of the development and validation of novel treatment strategies. Herein we first summarize the molecular components of the mammalian stress-regulated MAPK pathways and their regulation as described thus far. We then review some of the in vivo functions of these pathways.

Conditional deletion of Ccm2 causes hemorrhage in the adult brain: a mouse model of human cerebral cavernous malformations

Cerebral cavernous malformations (CCM) are irregularly shaped and enlarged capillaries in the brain that are prone to hemorrhage, resulting in headaches, seizures, strokes and even death in patients. The disease affects up to 0.5% of the population and the inherited form has been linked to mutations in one of three genetic loci, CCM1, CCM2 and CCM3. To understand the pathophysiology underlying the vascular lesions in CCM, it is critical to develop a reproducible mouse genetic model of this disease. Here, we report that limited conditional ablation of Ccm2 in young adult mice induces observable neurological dysfunction and reproducibly results in brain hemorrhages whose appearance is highly reminiscent of the lesions observed in human CCM patients. We first demonstrate that conventional or endothelial-specific deletion of Ccm2 leads to fatal cardiovascular defects during embryogenesis, including insufficient vascular lumen formation as well as defective arteriogenesis and heart malformation. These findings confirm and extend prior studies. We then demonstrate that the inducible deletion of Ccm2 in adult mice recapitulates the CCM-like brain lesions in humans; the lesions display disrupted vascular lumens, enlarged capillary cavities, loss of proper neuro-vascular associations and an inflammatory reaction. The CCM lesions also exhibit damaged neuronal architecture, the likely cause of neurologic defects, such as ataxia and seizure. These mice represent the first CCM2 animal model for CCM and should provide the means to elucidate disease mechanisms and evaluate therapeutic strategies for human CCM.

MEKK3 regulates IFN-gamma production in T cells through the Rac1/2-dependent MAPK cascades

MEKK3 is a conserved Ser/Thr protein kinase belonging to the MAPK kinase kinase (MAP3K) family. MEKK3 is constitutively expressed in T cells, but its function in T cell immunity has not been fully elucidated. Using Mekk3 T cell conditional knockout (T-cKO) mice, we show that MEKK3 is required for T cell immunity in vivo. Mekk3 T-cKO mice had reduced T cell response to bacterial infection and were defective in clearing bacterial infections. The Ag-induced cytokine production, especially IFN-γ production, was impaired in Mekk3-deficient CD4 T cells. The TCR-induced ERK1/2, JNK, and p38 MAPKs activation was also defective in Mekk3-deficient CD4 T cells. In vitro, MEKK3 is not required for Th1 and Th2 cell differentiation. Notably, under a nonpolarizing condition (Th0), Mekk3 deficiency led to a significant reduction of IFN-γ production in CD4 T cells. Furthermore, the IL-12/IL-18-driven IFN-γ production and MAPK activation in Mekk3-deficient T cells was not affected suggesting that MEKK3 may selectively mediate the TCR-induced MAPK signals for IFN-γ production. Finally, we found that MEKK3 activation by TCR stimulation requires Rac1/2. Taken together, our study reveals a specific role of MEKK3 in mediating the TCR signals for IFN-γ production.

New insights into the biogenesis of nuclear RNA polymerases?

More than 30 years of research on nuclear RNA polymerases (RNAP I, II, and III) has uncovered numerous factors that regulate the activity of these enzymes during the transcription reaction. However, very little is known about the machinery that regulates the fate of RNAPs before or after transcription. In particular, the mechanisms of biogenesis of the 3 nuclear RNAPs, which comprise both common and specific subunits, remains mostly uncharacterized and the proteins involved are yet to be discovered. Using protein affinity purification coupled to mass spectrometry (AP-MS), we recently unraveled a high-density interaction network formed by nuclear RNAP subunits from the soluble fraction of human cell extracts. Validation of the dataset using a machine learning approach trained to minimize the rate of false positives and false negatives yielded a high-confidence dataset and uncovered novel interactors that regulate the RNAP II transcription machinery, including a set of proteins we named the RNAP II-associated proteins (RPAPs). One of the RPAPs, RPAP3, is part of an 11-subunit complex we termed the RPAP3/R2TP/prefoldin-like complex. Here, we review the literature on the subunits of this complex, which points to a role in nuclear RNAP biogenesis.

Isolation of diapause-regulated genes from the flesh fly, Sarcophaga crassipalpis by suppressive subtractive hybridization

Subtractive suppressive hybridization (SSH) was used to characterize the diapause transcriptome of the flesh fly Sarcophaga crassipalpis. Through these efforts, we isolated 97 unique clones which were used as probes in northern hybridization to assess their expression during diapause. Of these, 17 were confirmed to be diapause upregulated and 1 was diapause downregulated, while 12 were shown to be unaffected by diapause in this species. The diapause upregulated genes fall into several broad categories including heat shock proteins, heavy metal responsive genes, neuropeptides, structural genes, regulatory elements, and several genes of unknown function. In combination with other large-scale analyses of gene expression during diapause, this study assists in the characterization of the S. crassipalpis diapause transcriptome, and begins to identify common elements involved in diapause across diverse taxa.

Vasculogenesis driven by bone marrow-derived cells is essential for growth of Ewing's sarcomas

The role of vasculogenesis as opposed to angiogenesis in tumor formation has been little explored genetically. Endothelial cells that lack the MEK kinase MEKK3 cannot form vessels. In this study, we employed mice with hematopoietic deletions of the Mekk3 gene to evaluate the importance of vasculogenesis in the formation of Ewing's sarcoma tumors. Bone marrow cells (BM) from LacZ(+) Mekk3-deficient conditional knockout mice (Mekk3(Deltaflox/-) mice) were transplanted into irradiated nude mice before injection of Ewing's sarcoma cells. Because the grafted Mekk3(Deltaflox/-) BM cells cannot contribute to vessel development in the same way as the host Mekk3(+/+) endothelial cells, angiogenesis is normal in the model whereas vasculogenesis is impaired. Four weeks after BM transplant, Ewing's sarcoma TC71 or A4573 cells were injected, and tumor growth and vessel density were compared. Strikingly, chimeric mice transplanted with Mekk3(Deltaflox/-) BM exhibited a reduction in tumor growth and vessel density compared with mice transplanted with Mekk3(Deltaflox/+) BM cells. Mekk3(Deltaflox/-) cells that were LacZ positive were visualized within the tumor; however, few of the LacZ(+) cells colocalized with either CD31(+) endothelial cells or desmin(+) pericytes. Quantification of double-positive LacZ(+) and CD31(+) endothelial cells or LacZ(+) and desmin(+) pericytes confirmed that chimeric mice transplanted with Mekk3(Deltaflox/-) BM were impaired for tumor vessel formation. In contrast, siRNA-mediated knockdown of Mekk3 in TC71 Ewing's sarcoma cells had no effect on tumor growth or vessel density. Our findings indicate that vasculogenesis is critical in the expansion of the tumor vascular network.

PB1 domain interaction of p62/sequestosome 1 and MEKK3 regulates NF-kappaB activation

p62/Sequestosome 1 is a scaffold protein involved in the regulation of autophagy, trafficking of proteins to the proteasome, and activation of NF-kappaB. p62 encodes an N-terminal PB1 domain in addition to the ZZ domain, TRAF6-binding domain, LC3 interaction region, and ubiquitin-associated domain, each critical for the physiological function of p62. PB1 domains have a beta-grasp topology where the front end of one PB1 domain binds the back end of a second PB1 domain. The p62 PB1 domain homodimerizes as well as heterodimerizes with other PB1 domains. The front end of the PB1 domain in p62 binds the PB1 domain of atypical protein kinases C, the MAPK kinase, MEK5, and the NBR1 protein. Other than its role in homodimerization, the rear end acidic cluster region of the p62 PB1 domain had no previous defined binding partners. Herein, we demonstrate that the rear end acidic cluster region of the p62 PB1 domain binds the front end basic region of the MAPK kinase kinase, MEKK3. p62 and MEKK3 co-localize in speckles or aggregates that are centers for organizing TRAF6-regulated NF-kappaB signaling and the assembly of polyubiquinated proteins sorting to sequestosomes and proteasomes. The p62-MEKK3 complex binds TRAF6, which regulates the ubiquitination of the IKK complex and NF-kappaB activation. p62 is required for the association of MEKK3 with TRAF6 and short hairpin RNA knockdown of p62 inhibits IL-1 and MEKK3 activation of NF-kappaB. The rear end acidic cluster of the p62 PB1 domain is used to organize cytosolic aggregates or speckles-associated TRAF6-p62-MEKK3 complex for control of NF-kappaB activation.

MEKK3 is essential for lymphopenia-induced T cell proliferation and survival

T cell homeostasis is crucial for maintaining an efficient and balanced T cell immunity. The interaction between TCR and self peptide (sp) MHC ligands is known to be the key driving force in this process, and it is believed to be functionally and mechanistically different from that initiated by the antigenic TCR stimulation. Yet, very little is known about the downstream signaling events triggered by this TCR-spMHC interaction and how they differ from those triggered by antigenic TCR stimulation. In this study, we show that T cell conditional ablation of MEKK3, a Ser/Thr kinase in the MAPK cascade, causes a significant reduction in peripheral T cell numbers in the conditional knockout mice, but does not perturb thymic T cell development and maturation. Using an adoptive mixed transfer method, we show that MEKK3-deficient T cells are severely impaired in lymphopenia-induced cell proliferation and survival. Interestingly, the Ag-induced T cell proliferation proceeds normally in the absence of MEKK3. Finally, we found that the activity of ERK1/2, but not p38 MAPK, was attenuated during the lymphopenia-driven response in MEKK3-deficient T cells. Together, these data suggest that MEKK3 may play a crucial selective role for spMHC-mediated T cell homeostasis.

Regulation of NF-kappaB-dependent T cell activation and development by MEKK3

The serine/threonine kinase MEKK3, also known as mitogen-activated protein kinase kinase kinase 3, is a critical activator of the transcription factor NF-kappaB in innate immunity. However, the physiological function of MEKK3 in adaptive immunity is unclear. Here we report that following TCR signaling, MEKK3 positively regulated the kinase, IkappaB kinase, leading to NF-kappaB activation. T cells lacking MEKK3 were defective in TCR-induced and cytokine-induced responses. Furthermore, T cell-specific deletion of MEKK3 resulted in reduced numbers of thymocytes and peripheral T cells. Thus, our results provide genetic evidence that MEKK3 plays a crucial role in adaptive immunity.

MAPK signalling in cardiovascular health and disease: molecular mechanisms and therapeutic targets

Intracellular MAPK (mitogen-activated protein kinase) signalling cascades probably play an important role in the pathogenesis of cardiac and vascular disease. A substantial amount of basic science research has defined many of the details of MAPK pathway organization and activation, but the role of individual signalling proteins in the pathogenesis of various cardiovascular diseases is still being elucidated. In the present review, the role of the MAPKs ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase) and p38 MAPK in cardiac hypertrophy, cardiac remodelling after myocardial infarction, atherosclerosis and vascular restenosis will be examined, with attention paid to genetically modified murine model systems and to the use of pharmacological inhibitors of protein kinases. Despite the complexities of this field of research, attractive targets for pharmacological therapy are emerging.

Grifolin, a potential antitumor natural product from the mushroom Albatrellus confluens, induces cell-cycle arrest in G1 phase via the ERK1/2 pathway

Grifolin, a natural product isolated from the mushroom Albatrellus confluens, has been shown to inhibit the growth of some cancer cell lines and induce significant apoptosis. However, the molecular targets and the signaling mechanism underlying the anticancer effect of this compound are not completely understood. Here, we undertook a gene expression profiling study to identify novel targets of grifolin. We found that the effect of grifolin on the human nasopharyngeal carcinoma cell line CNE1 occurs primarily via the ERK1/2 pathway. At high doses, both the ERK1/2 and the ERK5 pathways may be involved in the inhibition. Because inhibition of the ERK1/2 or the ERK5 pathway has been associated with cell-cycle arrest and growth inhibition, we evaluated the cell cycle distribution after grifolin treatment. We found that grifolin significantly caused cell-cycle arrest in G1 phase. To investigate the underlying mechanisms, G1-related proteins were assayed by Western blotting. Following grifolin treatment, a concomitant inhibition of cyclin D1, cyclin E, CDK4 expression, and subsequent reduction in pRB phosphorylation occurred. Meanwhile, grifolin treatment also resulted in a significant upregulation of CKI (p19INK4D). These results suggest that the inhibition of the ERK1/2 or the ERK5 pathway is responsible for at least part of the induction of cell-cycle arrest in G1 phase by grifolin. These results are significant in that they provide a mechanistic framework for further exploring the use of grifolin as a novel antitumor agent.

Homeostatic interactions between MEKK3 and TAK1 involved in NF-kappaB signaling

Several members of the mitogen-activated protein kinase kinase kinase (MAP3K) family including MEKK3 and TGFbeta-activating kinase (TAK1) play nonredundant roles in activation of the NF-kappaB transcription factor. However, the mechanism by which MEKK3 mediates NF-kappaB signaling is not fully understood. In this report we investigate the association of murine MEKK3 with other proteins and their roles in NF-kappaB activation. Using tandem affinity purification TAK1 was identified as an endogenous protein that interacts with MEKK3. MEKK3-TAK1 interactions were confirmed by fluorescence resonance energy transfer and coimmunoprecipitation. MEKK3-TAK1 complexes contain non-phosphorylated forms of both molecules. Expression of non-phosphorylated TAK1 interferes with MEKK3 phosphorylation and NF-kappaB reporter activity induced by transient MEKK3 expression or TNFalpha stimulation. Addition of TAB1 facilitates TAK1 autophosphorylation and reverses the inhibitory effects of TAK1 on MEKK3 phosphorylation and NF-kappaB signal transduction in human 293 cells and TAK1 deficient mouse embryonic fibroblasts. The data provide insights into the homeostatic interactions that maintain basal NF-kappaB levels by holding the enzymes MEKK3 and TAK1 in their inactive state.

MEKK3 is required for endothelium function but is not essential for tumor growth and angiogenesis

Mitogen-activated protein kinase kinase kinase 3 (MEKK3) plays an essential role in embryonic angiogenesis, but its role in tumor growth and angiogenesis is unknown. In this study, we further investigated the role of MEKK3 in embryonic angiogenesis, tumor angiogenesis, and angiogenic factor production. We found that endothelial cells from Mekk3-deficient embryos showed defects in cell proliferation, apoptosis, and interactions with myocardium in the heart. We also found that MEKK3 is required for angiopoietin-1 (Ang1)-induced p38 and ERK5 activation. To study the role of MEKK3 in tumor growth and angiogenesis, we established both wild-type and Mekk3-deficient tumor-like embryonic stem cell lines and transplanted them subcutaneously into nude mice to assess their ability to grow and induce tumor angiogenesis. Mekk3-deficient tumors developed and grew similarly as control Mekk3 wild-type tumors and were also capable of inducing tumor angiogenesis. In addition, we found no differences in the production of VEGF in Mekk3-deficient tumors or embryos. Taken together, our results suggest that MEKK3 plays a critical role in Ang1/Tie2 signaling to control endothelial cell proliferation and survival and is required for endothelial cells to interact with the myocardium during early embryonic development. However, MEKK3 is not essential for tumor growth and angiogenesis.

Hepatitis B virus x protein induces perinuclear mitochondrial clustering in microtubule- and Dynein-dependent manners

The hepatitis B virus (HBV) X protein (HBx) is thought to play a key role in HBV replication and the development of liver cancer. It became apparent that HBx induces mitochondrial clustering at the nuclear periphery, but the molecular basis for mitochondrial clustering is not understood. Since mitochondria move along the cytoskeleton as a cargo of motor proteins, we hypothesized that mitochondrial clustering induced by HBx occurs by an altered intracellular motility. Here, we demonstrated that the treatment of HBx-expressing cells with a microtubule-disrupting drug (nocodazole) abrogated mitochondrial clustering, while the removal of nocodazole restored clustering within 30 to 60 min, indicating that mitochondrial transport is occurring in a microtubule-dependent manner. The addition of a cytochalasin D-disrupting actin filament, however, did not measurably affect mitochondrial clustering. Mitochondrial clustering was further studied by observations of HBV-related hepatoma cells and HBV-replicating cells. Importantly, the abrogation of the dynein activity in HBx-expressing cells by microinjection of a neutralizing anti-dynein intermediate-chain antibody, dynamitin overexpression, or the addition of a dynein ATPase inhibitor significantly suppressed the mitochondrial clustering. In addition, HBx induced the activation of the p38 mitogen-activated protein kinase (MAPK) and inhibition of the p38 kinase activity by SB203580-attenuated HBx-induced mitochondrial clustering. Taken together, HBx activation of the p38 MAPK contributed to the increase in the microtubule-dependent dynein activity. The data suggest that HBx plays a novel regulatory role in subcellular transport systems, perhaps facilitating the process of maturation and/or assembly of progeny particles during HBV replication. Furthermore, mitochondrion aggregation induced by HBx may represent a cellular process that underlies disease progression during chronic viral infection.

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.

Multiple Toll-like receptor ligands induce an IL-6 transcriptional response in skeletal myocytes

Toll-like receptors (TLRs) comprise a critical sentinel that monitors body compartments for the presence of pathogens. Skeletal muscle expresses TLRs and responds to pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), by mounting an innate immune response. In the present study, we used C2C12myocytes as a model system for skeletal muscle during infection. C2C12cells responded to LPS in a time frame and with a pattern of gene expression that faithfully mimicked the response of skeletal muscle to LPS in vivo. LPS from a variety of Escherichia coli serotypes stimulated IL-6 synthesis. C2C12cells expressed TLR1–7, but not TLR8 or TLR9, mRNA by RT-PCR. A synthetic tripalmitoylated cysteine-, serine-, and lysine-containing peptide (Pam) and LPS from Porphyromonas gingivalis, two TLR2 ligands, also stimulated IL-6 expression. LPS and Pam stimulated luciferase activity driven from NF-κB and IL-6 promoter-containing plasmids, and this response was blunted when the NF-κB binding site was mutated. LPS- and Pam-stimulated IL-6 expression was inhibited by the proteasome inhibitor MG-132 and the IκB kinase-2 (IKK2) inhibitor 2-[(aminocarbonyl)amino]-5-(4-fluorophenyl)-3-thiophenecarboxamide (TPCA-1). Pam-stimulated NF-κB and IL-6 promoter activities were disrupted by a dominant-negative form of TLR2, but not TLR4. Local injection of LPS or Pam into the gastrocnemius muscle stimulated IL-6 mRNA expression in the injected, but not the contralateral, muscle. The LPS- but not Pam-stimulated expression of IL-6 mRNA was blunted in skeletal muscle of mice carrying an inactivating mutation in TLR4. The data suggest that skeletal muscle and muscle cells recognize pathogen-associated molecules with specific TLRs to initiate an IL-6 transcriptional response.

Phosphorylation of Serine 526 Is Required for MEKK3 Activity, and Association with 14-3-3 Blocks Dephosphorylation

MAPK/ERK kinase kinase 3 (MEKK3) is a mitogen-activated protein kinase kinase kinase (MAP3K) that functions upstream of the MAP kinases and IkappaB kinase. Phosphorylation is believed to be a critical component for MEKK3-dependent signal transduction, but little is known about the phosphorylation sites of this MAP3K. To address this question, point mutations were introduced in the activation loop (T-loop), substituting alanine for serine or threonine, and the mutants were transfected into HEK293 Epstein-Barr virus nuclear antigen cells. MEKK3-dependent activation of an NF-kappaB reporter gene as well as ERK, JNK, and p38 MAP kinases correlated with a requirement for serine at position 526. Constitutively active mutants of MEKK3, consisting of S526D and S526E, were capable of activating a NF-kappaB luciferase reporter gene as well as ERK and MEK, suggesting that a negative charge at Ser526 was necessary for MEKK3 activity and implicating Ser526 as a phosphorylation site. An antibody was developed that specifically recognized phospho-Ser526 of MEKK3 but did not recognize the S526A point mutant. The catalytically inactive (K391M) mutant of MEKK3 was not phosphorylated at Ser526, indicating that phosphorylation of Ser526 occurs via autophosphorylation. Endogenous MEKK3 was phosphorylated on Ser526 in response to osmotic stress. In addition, phosphorylation of Ser526 was required for MKK6 phosphorylation in vitro, whereas dephosphorylation of Ser526 was mediated by protein phosphatase 2A and sensitive to okadaic acid and sodium fluoride. Finally, the association between MEKK3 and 14-3-3 was dependent on Ser526 and prevented dephosphorylation of Ser526. In summary, Ser526 of MEKK3 is an autophosphorylation site within the T-loop that is regulated by PP2A and 14-3-3 proteins.

Identification of MEKK2/3 serine phosphorylation site targeted by the Toll-like receptor and stress pathways

Members of the mitogen-activated protein kinase kinase kinase (MAP3K) family are crucial for the Toll-like receptor (TLR) signaling and cellular stress responses. However, the molecular mechanisms underlying the TLR- and cellular stress-mediated MAP3K activation remain largely unknown. In this study, we identified a key regulatory phosphorylation site, serine 519 and serine 526, in MAP3K MEKK2 and MEKK3, respectively. Mutation of this serine to an alanine severely impaired MEKK2/3 activation. We generated an anti-p-MEKK2/3 antibody and used this antibody to demonstrate that lipopolysaccharide induced MEKK2 and MEKK3 phosphorylation on their regulatory serine. We found that the serine phosphorylation was crucial for TLR-induced interleukin 6 production and this process is regulated by TRAF6, a key adaptor molecule for the TLR pathway. We further demonstrated that many, but not all, MAPK agonists induced the regulatory serine phosphorylation, suggesting an involvement of different MAP3Ks in activation of the MAPK cascades leading to different cellular responses. In conclusion, this study reveals a novel molecular mechanism for MEKK2/3 activation by the TLR and cellular stress pathways.

Saikosaponin-d inhibits T cell activation through the modulation of PKCtheta, JNK, and NF-kappaB transcription factor

The effects of saikosaponin-d, a triterpene saponin derived from Bupleurum falcatum L. (Umbelliferae), on the signaling pathways of T cell activation were examined. The results showed that saikosaponin-d potently suppressed both early (CD69) and late (CD71) expressions of mouse T cells stimulated with Con A or PMA. It interfered with PKCtheta translocation from cytosol to membrane fraction and inhibited the phosphorylations of IkappaBalpha and JNK, but not ERK, in PMA-activated mouse T cells. Additionally, it inhibited PMA and ionomycin-stimulated IL-2 production in mouse T cells. In summary, these results indicate that the mechanism by which saikosaponin-d inhibits T cell activation would involve the suppression of CD69 and CD71 expressions and IL-2 production, and the modulation of PKC pathway through PKCtheta, JNK, and NF-kappaB transcription factor. This may herald a novel approach for further studies of saikosaponin-d as a candidate for use in the treatment of inflammatory and autoimmune diseases.

hKSR-2 inhibits MEKK3-activated MAP kinase and NF-kappaB pathways in inflammation

Kinase suppressor of ras (KSR) and MEKK3 (MAP kinase kinase kinase) are integral members of the MAP kinase pathway. We have recently identified a new isoform of the KSR family named human kinase suppressor of ras-2 (hKSR-2), and demonstrated that hKSR-2 negatively regulates Cot, a MAP3K family member which is important in inflammation and oncogenesis [P.L. Channavajhala, L. Wu, J.W. Cuozzo, J.P. Hall, W. Liu, L.L. Lin, Y. Zhang, J. Biol. Chem. 278 (2003) 47089-47097]. In this report, we provide evidence that hKSR-2 also regulates the activity of MEKK3 (another MAP3K family member) in HEK-293T cells. We demonstrate that hKSR-2 is a negative regulator of MEKK3-mediated activation of MAP kinase (specifically ERK and JNK) and NF-kappaB pathways, and concurrently inhibits MEKK3-mediated interleukin-8 production. We find that while hKSR-2 blocks MEKK3 activation, it has little to no effect on other members of the MAP3K family, including MEKK4, TAK1, and Ras-Raf, suggesting that its effects are selective.

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.

Restoration of NF-kappaB activation by tumor necrosis factor alpha receptor complex-targeted MEKK3 in receptor-interacting protein-deficient cells

Receptor-interacting protein (RIP) plays a critical role in tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB activation. However, the mechanism by which RIP mediates TNF-alpha-induced signal transduction is not fully understood. In this study, we reconstituted RIP-deficient Jurkat T cells with a fusion protein composed of full-length MEKK3 and the death domain of RIP (MEKK3-DD). In these cells, MEKK3-DD substitutes for RIP and directly associates with TRADD in TNF receptor complexes following TNF-alpha stimulation. We found that TNF-alpha-induced NF-kappaB activation was fully restored by MEKK3-DD in these cells. In contrast, expression of a fusion protein composed of NEMO, a component of the IkappaB kinase complex, and the death domain of RIP (NEMO-DD) cannot restore TNF-alpha-induced NF-kappaB activation in RIP-deficient cells. These results indicate that the role of RIP is to specifically recruit MEKK3 to the TNF-alpha receptor complex, whereas the forced recruitment of NEMO to the TNF-alpha receptor complex is insufficient for TNF-alpha-induced NF-kappaB activation. Although MEKK2 has a high degree of homology with MEKK3, MEKK2-DD, unlike MEKK3-DD, also fails to restore TNF-alpha-induced NF-kappaB activation in RIP-deficient cells, indicating that RIP-dependent recruitment of MEKK3 plays a specific role in TNF-alpha signaling.

NF-kappaB and AP-1 connection: mechanism of NF-kappaB-dependent regulation of AP-1 activity

Nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1) transcription factors regulate many important biological and pathological processes. Activation of NF-kappaB is regulated by the inducible phosphorylation of NF-kappaB inhibitor IkappaB by IkappaB kinase. In contrast, Fos, a key component of AP-1, is primarily transcriptionally regulated by serum responsive factors (SRFs) and ternary complex factors (TCFs). Despite these different regulatory mechanisms, there is an intriguing possibility that NF-kappaB and AP-1 may modulate each other, thus expanding the scope of these two rapidly inducible transcription factors. To determine whether NF-kappaB activity is involved in the regulation of fos expression in response to various stimuli, we analyzed activity of AP-1 and expression of fos, fosB, fra-1, fra-2, jun, junB, and junD, as well as AP-1 downstream target gene VEGF, using MDAPanc-28 and MDAPanc-28/IkappaBalphaM pancreatic tumor cells and wild-type, IKK1-/-, and IKK2-/- murine embryonic fibroblast cells. Our results show that elk-1, a member of TCFs, is one of the NF-kappaB downstream target genes. Inhibition of NF-kappaB activity greatly decreased expression of elk-1. Consequently, the reduced level of activated Elk-1 protein by extracellular signal-regulated kinase impeded constitutive, serum-, and superoxide-inducible c-fos expression. Thus, our study revealed a distinct and essential role of NF-kappaB in participating in the regulation of elk-1, c-fos, and VEGF expression.

Activation of extracellular signal-regulated kinase ERK after hypo-osmotic stress in renal epithelial A6 cells

Activation of mitogen-activated protein (MAP) kinases has been reported to occur after a hypo-osmotic cell swelling in various types of cells. In renal epithelial A6 cells, the hypo-osmotic shock induced a rapid increase in the phosphorylation of an extracellular signal-regulated kinase (ERK)-like protein that was maximal 10 min after osmotic stress. Activation of ERK was significantly increased when hypo-osmotic stress was performed in the absence of extracellular Ca2+, a condition that inhibits regulatory volume decrease (RVD). Exposure of cells to PD98059, an inhibitor of the MAP kinase kinase MEK, at a concentration that fully cancelled ERK activation, did not inhibit RVD. On the contrary, RVD was abolished when osmotic shock was induced in the presence of SB203580, an inhibitor of stress-activated protein kinases (SAPKs). These results suggest that different MAP kinases are activated after hypo-osmotic stress in A6 cells. SAPKs would be involved in the control of RVD, while ERK would lead to later events, such as gene expression or energy metabolism.

TRAF7 Potentiates MEKK3-induced AP1 and CHOP Activation and Induces Apoptosis

The tumor necrosis factor receptor-associated factor (TRAF) protein family members are critically involved in activation of NF-kappaB, JNK, and p38 activation triggered by tumor necrosis factor (TNF) receptor family members and toll/interleukin-1 receptor (TIR)-containing receptors. TRAF proteins (except for TRAF1) contain an N-terminal RING finger domain that is essential for their functions. In this report, we identified a protein designated as TRAF7, which contains a RING finger domain and a zinc finger domain that are mostly conserved with those of TRAFs. TRAF7 also contains seven WD40 repeats at its C terminus. TRAF7 specifically interacted with MEKK3 and potentiated MEKK3-mediated AP1 and CHOP activation. Depletion of TRAF7 by antisense RNA inhibited MEKK3-mediated AP1 and CHOP activation. Moreover, overexpression of TRAF7 induced caspase-dependent apoptosis. Domain mapping experiments indicated that TRAF7 potentiated MEKK3-mediated AP1 and CHOP activation and induced apoptosis through distinct domains. Our studies identified a novel TRAF family member that is involved in MEKK3 signaling and apoptosis.

The death domain kinase RIP1 is essential for tumor necrosis factor alpha signaling to p38 mitogen-activated protein kinase

The cytokine tumor necrosis factor alpha (TNF-alpha) stimulates the NF-kappaB, SAPK/JNK, and p38 mitogen-activated protein (MAP) kinase pathways by recruiting RIP1 and TRAF2 proteins to the tumor necrosis factor receptor 1 (TNFR1). Genetic studies have revealed that RIP1 links the TNFR1 to the IkappaB kinase (IKK) complex, whereas TRAF2 couples the TNFR1 to the SAPK/JNK cascade. In transfection studies, RIP1 and TRAF2 stimulate p38 MAP kinase activation, and dominant-negative forms of RIP1 and TRAF2 inhibit TNF-alpha-induced p38 MAP kinase activation. We found TNF-alpha-induced p38 MAP kinase activation and interleukin-6 (IL-6) production impaired in rip1(-/-) murine embryonic fibroblasts (MEF) but unaffected in traf2(-/-) MEF. Yet, both rip1(-/-) and traf2(-/-) MEF exhibit a normal p38 MAP kinase response to inducers of osmotic shock or IL-1alpha. Thus, RIP1 is a specific mediator of the p38 MAP kinase response to TNF-alpha. These studies suggest that TNF-alpha-induced activation of p38 MAP kinase and SAPK/JNK pathways bifurcate at the level of RIP1 and TRAF2. Moreover, endogenous RIP1 associates with the MAP kinase kinase kinase (MAP3K) MEKK3 in TNF-alpha-treated cells, and decreased TNF-alpha-induced p38 MAP kinase activation is observed in Mekk3(-/-) cells. Taken together, these studies suggest a mechanism whereby RIP1 may mediate the p38 MAP kinase response to TNF-alpha, by recruiting the MAP3K MEKK3.

Differential regulation of interleukin 1 receptor and Toll-like receptor signaling by MEKK3

Interleukin 1 receptor (IL-1R) and Toll-like receptors (TLRs) induce inflammatory genes through the complex of MyD88, IL-1R-associated protein kinase (IRAK) and tumor necrosis factor receptor-associated factor 6 (TRAF6), which is believed to function 'upstream' of the cascades of IkappaB kinase (IKK) and nuclear factor-kappaB (NF-kappaB); extracellular signal-regulated protein kinase (ERK); c-Jun N-terminal kinase (JNK); and p38 mitogen-activated protein kinase (MAPK). Here we show that MAPK-ERK kinase kinase (MEKK3) is an essential signal transducer of the MyD88-IRAK-TRAF6 complex in IL-1R-TLR4 signaling. MEKK3 forms a complex with TRAF6 in response to IL-1 and lipopolysaccharide (LPS) but not CpG, and is required for IL-1R- and TLR4-induced IL-6 production. Furthermore, MEKK3 is crucial for IL-1- and LPS-induced activation of NF-kappaB and JNK-p38 but not ERK, indicating that MAPKs are differentially activated during IL-1R-TLR4 signaling. These data demonstrate that MEKK3 is crucial for IL-1R and TLR4 signaling through the IKK-NF-kappaB and JNK-p38 MAPK pathways.*Note: In the version of this article originally published online, the third author's name was incorrect. The correct author name should be Yong Lin. This error has been corrected for the HTML and print versions of this article.

PAK1 phosphorylation of MEK1 regulates fibronectin-stimulated MAPK activation

Activation of the Ras-MAPK signal transduction pathway is necessary for biological responses both to growth factors and ECM. Here, we provide evidence that phosphorylation of S298 of MAPK kinase 1 (MEK1) by p21-activated kinase (PAK) is a site of convergence for integrin and growth factor signaling. We find that adhesion to fibronectin induces PAK1-dependent phosphorylation of MEK1 on S298 and that this phosphorylation is necessary for efficient activation of MEK1 and subsequent MAPK activation. The rapid and efficient activation of MEK and phosphorylation on S298 induced by cell adhesion to fibronectin is influenced by FAK and Src signaling and is paralleled by localization of phospho-S298 MEK1 and phospho-MAPK staining in peripheral membrane-proximal adhesion structures. We propose that FAK/Src-dependent, PAK1-mediated phosphorylation of MEK1 on S298 is central to the organization and localization of active Raf-MEK1-MAPK signaling complexes, and that formation of such complexes contributes to the adhesion dependence of growth factor signaling to MAPK.

To die or not to die for neurons in ischemia, traumatic brain injury and epilepsy: a review on the stress-activated signaling pathways and apoptotic pathways

After a severe episode of ischemia, traumatic brain injury (TBI) or epilepsy, it is typical to find necrotic cell death within the injury core. In addition, a substantial number of neurons in regions surrounding the injury core have been observed to die via the programmed cell death (PCD) pathways due to secondary effects derived from the various types of insults. Apart from the cell loss in the injury core, cell death in regions surrounding the injury core may also contribute to significant losses in neurological functions. In fact, it is the injured neurons in these regions around the injury core that treatments are targeting to preserve. In this review, we present our cumulated understanding of stress-activated signaling pathways and apoptotic pathways in the research areas of ischemic injury, TBI and epilepsy and that gathered from concerted research efforts in oncology and other diseases. However, it is obvious that our understanding of these pathways in the context of acute brain injury is at its infancy stage and merits further investigation. Hopefully, this added research effort will provide a more detailed knowledge from which better therapeutic strategies can be developed to treat these acute brain injuries.

Phosphorylation of the stress-activated protein kinase, MEKK3, at serine 166

Much effort has focused on the identification of MAPK cascades that are activated by the MEKK family of protein kinases. However, direct phosphorylation and regulation of the MEKK proteins has not been shown. To address this question, we have expressed recombinant (His)6FLAG.MEKK3 in Sf9 insect cells and tethered the purified protein to Ni-Sepharose so that we could precipitate interacting proteins and then identify such proteins by liquid chromatography and mass spectrometry (LC-MS). We identified 14-3-3 proteins as interacting with MEKK3, which suggested that (His)6FLAG.MEKK3 was phosphorylated on serine since 14-3-3 proteins are known to associate with phosphorylated proteins. We identified two phosphorylated amino acids at Ser166 and Ser337 of tryptic peptides derived from (His)6FLAG.MEKK3 by using LC-MS. Antibodies were developed that recognize the specific phosphorylated amino acid and with these antibodies, we demonstrate that various stimuli (tumor necrosis factor, arsenite, forskolin, and serum) promote phosphorylation of Ser166 and Ser337. However, neither of these phosphorylated amino acids is required for association with 14-3-3 protein or regulation of MEKK3-dependent ERK and JNK activity. Nonetheless, these results suggest that MEKK3 is a convergence point of multiple upstream signaling pathways.

BAFF-induced NEMO-independent processing of NF-kappa B2 in maturing B cells

NF-kappa B is usually activated by signal-induced, ubiquitin-mediated degradation of its inhibitor, I kappa B. This process is initiated by phosphorylation of I kappa B by the I kappa B kinase (IKK) complex, predominantly by the IKK beta catalytic subunit, and requires the regulatory subunit IKK gamma (NEMO). Another activation pathway, with no known physiological inducers, involves ubiquitin-mediated processing of the NF-kappa B2 inhibitory protein p100 and is dependent on phosphorylation of p100 by IKK alpha. We show here that B cell-activating factor (BAFF) activates this second pathway and that this requires the BAFF receptor (BAFF-R), the NF-kappa B-inducing kinase (NIK) and protein synthesis, but not NEMO. This NEMO-independent cascade is physiologically relevant for the survival and, hence, progression of maturing splenic B cells.

Interaction of Rac exchange factors Tiam1 and Ras-GRF1 with a scaffold for the p38 mitogen-activated protein kinase cascade

Tiam1 and Ras-GRF1 are guanine nucleotide exchange factors (GEFs) that activate the Rac GTPase. The two GEFs have similar N-terminal regions containing pleckstrin homology domains followed by coiled-coils and additional sequences that function together to allow regulated GEF activity. Here we show that this N-terminal region of both proteins binds to the scaffold protein IB2/JIP2. IB2/JIP2 is a scaffold for the p38 mitogen-activated protein (MAP) kinase cascade because it binds to the Rac target MLK3, the MAP kinase kinase MKK3, and the p38 MAP kinase. Expression of IB2/JIP2 in cells potentiates the ability of Tiam1 or Ras-GRF1 to activate the p38 MAP kinase cascade but not the Jnk MAP kinase cascade. In addition, Tiam1 or Ras-GRF1 binding to IB2/JIP2 increases the association of the components of the p38 MAP kinase signaling cassette with IB2/JIP2 in cells and activates scaffold-associated p38. These findings imply that Tiam1 and Ras-GRF1 can contribute to Rac signaling specificity by their ability to form a complex with a scaffold that binds components of one of the many known Rac effector pathways.

Constitutive nuclear factor kappaB activity is required for survival of activated B cell-like diffuse large B cell lymphoma cells

Gene expression profiling has revealed that diffuse large B cell lymphoma (DLBCL) consists of at least two distinct diseases. Patients with one DLBCL subtype, termed activated B cell-like (ABC) DLBCL, have a distinctly inferior prognosis. An untapped potential of gene expression profiling is its ability to identify pathogenic signaling pathways in cancer that are amenable to therapeutic attack. The gene expression profiles of ABC DLBCLs were notable for the high expression of target genes of the nuclear factor (NF)-kappaB transcription factors, raising the possibility that constitutive activity of the NF-kappaB pathway may contribute to the poor prognosis of these patients. Two cell line models of ABC DLBCL had high nuclear NF-kappaB DNA binding activity, constitutive IkappaB kinase (IKK) activity, and rapid IkappaB(alpha) degradation that was not seen in cell lines representing the other DLBCL subtype, germinal center B-like (GCB) DLBCL. Retroviral transduction of a super-repressor form of IkappaBalpha or dominant negative forms of IKKbeta was toxic to ABC DLBCL cells but not GCB DLBCL cells. DNA content analysis showed that NF-kappaB inhibition caused both cell death and G1-phase growth arrest. These findings establish the NF-kappaB pathway as a new molecular target for drug development in the most clinically intractable subtype of DLBCL and demonstrate that the two DLBCL subtypes defined by gene expression profiling utilize distinct pathogenetic mechanisms.