KFC, a Ste20-like kinase with mitogenic potential and capability to activate the SAPK/JNK pathway

Mass spectrometric analysis of host cell proteins interacting with dengue virus nonstructural protein 1 in dengue virus-infected HepG2 cells

Dengue virus (DENV) infection is a leading cause of the mosquito-borne infectious diseases that affect humans worldwide. Virus-host interactions appear to play significant roles in DENV replication and the pathogenesis of DENV infection. Nonstructural protein 1 (NS1) of DENV is likely involved in these processes; however, its associations with host cell proteins in DENV infection remain unclear. In this study, we used a combination of techniques (immunoprecipitation, in-solution trypsin digestion, and LC-MS/MS) to identify the host cell proteins that interact with cell-associated NS1 in an in vitro model of DENV infection in the human hepatocyte HepG2 cell line. Thirty-six novel host cell proteins were identified as potential DENV NS1-interacting partners. A large number of these proteins had characteristic binding or catalytic activities, and were involved in cellular metabolism. Coimmunoprecipitation and colocalization assays confirmed the interactions of DENV NS1 and human NIMA-related kinase 2 (NEK2), thousand and one amino acid protein kinase 1 (TAO1), and component of oligomeric Golgi complex 1 (COG1) proteins in virus-infected cells. This study reports a novel set of DENV NS1-interacting host cell proteins in the HepG2 cell line and proposes possible roles for human NEK2, TAO1, and COG1 in DENV infection.

Post-Translational Modifications of Kaposi's Sarcoma-Associated Herpesvirus Regulatory Proteins - SUMO and KSHV

KSHV latency can be envisioned as an outcome that is balanced between factors that promote viral gene expression and lytic replication against those that facilitate gene silencing and establish or maintain latency. A large body of work has focused on the activities of the key viral regulatory proteins involved in KSHV latent or lytic states. Moreover, recent studies have also begun to document the importance of epigenetic landscape evolution of the KSHV viral genome during latency and reactivation. However, one area of KSHV molecular virology that remains largely unanswered is the precise role of post-translational modifications on the activities of viral factors that function during latency and reactivation. In this review, we will summarize the post-translational modifications associated with three viral factors whose activities contribute to the viral state. The viral proteins discussed are the two major KSHV encoded transcription factors, K-Rta (KSHV replication and transcriptional activator) and K-bZIP (KSHV basic leucine zipper) and the viral latency-associated nuclear antigen (LANA). A special emphasis will be placed on the role of the sumoylation pathway in the modulation of the KSHV lifecycle. Newly uncovered small ubiquitin-like modifier (SUMO)-associated properties of LANA and K-Rta will also be presented, namely LANA histone targeting SUMO E3 ligase activity and K-Rta SUMO-targeted ubiquitin ligase function.

ERK/MAPK is essential for endogenous neuroprotection in SCN2.2 cells

Background

Glutamate (Glu) is essential to central nervous system function; however excessive Glu release leads to neurodegenerative disease. Strategies to protect neurons are underdeveloped, in part due to a limited understanding of natural neuroprotective mechanisms, such as those present in the suprachiasmatic nucleus (SCN). This study tests the hypothesis that activation of ERK/MAPK provides essential protection to the SCN after exposure to excessive Glu using the SCN2.2 cells as a model.

Methodology

Immortalized SCN2.2 cells (derived from SCN) and GT1-7 cells (neurons from the neighboring hypothalamus) were treated with 10 mM Glu in the presence or absence of the ERK/MAPK inhibitor PD98059. Cell death was assessed by Live/Dead assay, MTS assay and TUNEL. Caspase 3 activity was also measured. Activation of MAPK family members was determined by immunoblot. Bcl2, neuritin and Bid mRNA (by quantitative-PCR) and protein levels (by immunoblot) were also measured.

Principal Findings

As expected Glu treatment increased caspase 3 activity and cell death in the GT1-7 cells, but Glu alone did not induce cell death or affect caspase 3 activity in the SCN2.2 cells. However, pretreatment with PD98059 increased caspase 3 activity and resulted in cell death after Glu treatment in SCN2.2 cells. This effect was dependent on NMDA receptor activation. Glu treatment in the SCN2.2 cells resulted in sustained activation of the anti-apoptotic pERK/MAPK, without affecting the pro-apoptotic p-p38/MAPK. In contrast, Glu exposure in GT1-7 cells caused an increase in p-p38/MAPK and a decrease in pERK/MAPK. Bcl2-protein increased in SCN2.2 cells following Glu treatment, but not in GT1-7 cells; bid mRNA and cleaved-Bid protein increased in GT1-7, but not SCN2.2 cells.

Conclusions

Facilitation of ERK activation and inhibition of caspase activation promotes resistance to Glu excitotoxicity in SCN2.2 cells.

Significance

Further research will explore ERK/MAPK as a key molecule in the prevention of neurodegenerative processes.

Molecular cloning and characterisation of SmSLK, a novel Ste20-like kinase in Schistosoma mansoni

Serine/threonine kinases of the Ste20 group play important roles in various cellular functions such as growth, apoptosis and morphogenesis. This family includes p21-Activated Kinases (PAKs) and Germinal Center Kinases (GCKs) families which contain their kinase domain in the C-terminal and N-terminal position, respectively. Here, we report the characterisation of a novel Ste20-like kinase (SLK) in the helminth parasite Schistosoma mansoni (SmSLK). SmSLK belongs to the GCK subfamily and contains a conserved N-terminal Ste20-like catalytic domain and C-terminal coiled-coil structures homologous to mammalian Lymphocyte Oriented Kinase (LOK) and SLK kinases and described as regulatory domains in these proteins. Gene assembly was performed using S. mansoni sequences available from genomic databases and indicated that SmSLK is composed of 18 exons and present in one copy in the S. mansoni genome. RT-PCR experiments demonstrated an alternative splicing of SmSLK in the exon 9 encoding the hinge region between kinase and coiled-coil domains of SmSLK and showed the expression of both transcript isoforms (SmSLK and SmSLK-S in which exon 9 is deleted) in all the S. mansoni parasite stages. Most of the Ste20-related proteins are active kinases known to regulate mitogen-activated protein kinase (MAPK) cascades. We demonstrated the kinase activity of SmSLK and SmSLK-S and their capacity to activate the MAPK/Jun N-terminal kinase (JNK) pathway in human embryonic kidney (HEK) cells as well as in Xenopus oocytes. Immunofluorescence studies indicated that SmSLK proteins were abundant in the tegument of adult schistosomes. Therefore, these results indicate that SmSLK is a new member of the GCK protein family that could participate in the regulation of MAPK cascade activation during host-parasite interactions.

Crystal structure of the MAP3K TAO2 kinase domain bound by an inhibitor staurosporine

Mitogen-activated protein kinase (MAPK) signal transduction pathways are ubiquitous in eukaryotic cells, which transfer signals from the cell surface to the nucleus, controlling multiple cellular programs. MAPKs are activated by MAPK kinases [MAP2Ks or MAP/extracellular signal-regulated kinase (ERK) kinases (MEK)], which in turn are activated by MAPK kinase kinases (MAP3Ks). TAO2 is a MAP3K level kinase that activates the MAP2Ks MEK3 and MEK6 to activate p38 MAPKs. Because p38 MAPKs are key regulators of expression of inflammatory cytokines, they appear to be involved in human diseases such as asthma and autoimmunity. As an upstream activator of p38s, TAO2 represents a potential drug target. Here we report the crystal structure of active TAO2 kinase domain in complex with staurosporine, a broad-range protein kinase inhibitor that inhibits TAO2 with an IC50 of 3 mM. The structure reveals that staurosporine occupies the position where the adenosine of ATP binds in TAO2, and the binding of the inhibitor mimics many features of ATP binding. Both polar and nonpolar interactions contribute to the enzyme-inhibitor recognition. Staurosporine induces conformational changes in TAO2 residues that surround the inhibitor molecule, but causes very limited global changes in the kinase. The structure provides atomic details for TAO2-staurosporine interactions, and explains the relatively low potency of staurosporine against TAO2. The structure presented here should aid in the design of inhibitors specific to TAO2 and related kinases.

JNK inhibitory kinase is up-regulated in retinal ganglion cells after axotomy and enhances BimEL expression level in neuronal cells

Optic nerve transection results in retinal ganglion cell (RGC) death in adult mammals, after the alteration of gene expression of RGCs. To elucidate the molecular mechanism by which axotomy induces RGC death, we isolated the molecules up-regulated after optic nerve transection. One of these, axotomy-related [corrected] gene (ARG)357, an 898-amino-acid [corrected] protein containing a complete serine-threonine kinase domain, was isolated from a subtraction library of the rat retina. The sequence showed that this gene was a rat homolog of human c-Jun N-terminal kinase (JNK) inhibitory kinase and so belonged to the germinal center kinase-VIII subfamily of Sterile20s protein kinase. We designated ARG357 as rat JNK inhibitory kinase (JIK). Rat JIK was expressed ubiquitously in various tissues and was highly expressed in the retina, with selective expression in RGCs. After axotomy, BimEL and Hrk, which are BH3-only proteins, and rat JIK were up-regulated in RGCs. Overexpression of rat JIK in neuronal cells up-regulated the expression of BimEL, but not that of Hrk. These results indicate that JIK may contribute to axotomy-induced RGC death by up-regulating the expression of BH3-only protein.

Crystal structure of the TAO2 kinase domain: activation and specificity of a Ste20p MAP3K

TAO2 is a mitogen-activated protein kinase kinase kinase (MAP3K) that doubly phosphorylates and activates the MAP kinase kinases (MAP2Ks) MEK3 and MEK6. The structure of the kinase domain of TAO2 (1-320) has been solved in its phosphorylated active conformation. The structure, together with structure-based mutagenic analysis, reveals that positively charged residues in the substrate binding groove mediate the first step in the dual phosphorylation of MEK6, on the threonine residue in the motif DS*VAKT*I (*denotes phosphorylation site) of MEK6. TAO2 is a Ste20p homolog, and the structure of active TAO2, in comparison with that of low-activity p21-activated protein kinase (PAK1), a Ste20p-related MAP4K, reveals how this group of kinases is activated by phosphorylation. Finally, active TAO2 displays unusual interactions with ATP, involving, in part, a subgroup-specific C-terminal extension of TAO2. The observed interactions may be useful in making specific inhibitors of TAO kinases.

Comparative studies of a new subfamily of human Ste20-like kinases: homodimerization, subcellular localization, and selective activation of MKK3 and p38

The Sterile-20 or Ste20 family of serine/threonine kinases is a group of signaling molecules whose physiological roles within mammalian cells are just starting to be elucidated. Here, in this report we present the characterization of three human Ste20-like kinases with greater than 90% similarity within their catalytic domains that define a novel subfamily of Ste20s. Members of this kinase family include rat thousand and one (TAO1) and chicken KFC (kinase from chicken). For the lack of a consensus nomenclature in the literature, in this report, we shall call this family hKFC (for their homology to chicken KFC) and the three members hKFC-A, hKFC-B, and hKFC-C, respectively. These kinases have many similarities including an aminoterminal kinase domain, a serine-rich region, and a coiled-coil configuration within the C-terminus. All three kinases are able to activate the p38 MAP kinase pathway through the specific activation of the upstream MKK3 kinase. We also offer evidence, both theoretical and biochemical, showing that these kinases can undergo self-association. Despite these similarities, these kinases differ in tissue distribution, apparent subcellular localization, and feature structural differences largely within the carboxyl-terminal sequence.

MAPK pathways in radiation responses

Within the last 15 years, multiple new signal transduction pathways within cells have been discovered. Many of these pathways belong to what is now termed 'the mitogen-activated protein kinase (MAPK) superfamily.' These pathways have been linked to the growth factor-mediated regulation of diverse cellular events such as proliferation, senescence, differentiation and apoptosis. Based on currently available data, exposure of cells to ionizing radiation and a variety of other toxic stresses induces simultaneous compensatory activation of multiple MAPK pathways. These signals play critical roles in controlling cell survival and repopulation effects following irradiation, in a cell-type-dependent manner. Some of the signaling pathways activated following radiation exposure are those normally activated by mitogens, such as the 'classical' MAPK (also known as the ERK) pathway. Other MAPK pathways activated by radiation include those downstream of death receptors and procaspases, and DNA-damage signals, including the JNK and P38 MAPK pathways. The expression and release of autocrine growth factor ligands, such as (transforming growth factor alpha) and TNF-alpha, following irradiation can also enhance the responses of MAPK pathways in cells and, consequently, of bystander cells. Thus, the ability of radiation to activate MAPK signaling pathways may depend on the expression of multiple growth factor receptors, autocrine factors and Ras mutation. Enhanced basal signaling by proto-oncogenes such as K-/H-/N-RAS may provide a radioprotective and growth-promoting signal. In many cell types, this may be via the PI3K pathway; in others, this may occur through nuclear factor-kappa B or multiple MAPK pathways. This review will describe the enzymes within the known MAPK signaling pathways and discuss their activation and roles in cellular radiation responses.

Stress and radiation-induced activation of multiple intracellular signaling pathways

Exposure of cells to a variety of stresses induces compensatory activations of multiple intracellular signaling pathways. These activations can play critical roles in controlling cell survival and repopulation effects in a stress-specific and cell type-dependent manner. Some stress-induced signaling pathways are those normally activated by mitogens such as the EGFR/RAS/PI3K-MAPK pathway. Other pathways activated by stresses such as ionizing radiation include those downstream of death receptors, including pro-caspases and the transcription factor NFKB. This review will attempt to describe some of the complex network of signals induced by ionizing radiation and other cellular stresses in animal cells, with particular attention to signaling by growth factor and death receptors. This includes radiation-induced signaling via the EGFR and IGFI-R to the PI3K, MAPK, JNK, and p38 pathways as well as FAS-R and TNF-R signaling to pro-caspases and NFKB. The roles of autocrine ligands in the responses of cells and bystander cells to radiation and cellular stresses will also be discussed. Based on the data currently available, it appears that radiation can simultaneously activate multiple signaling pathways in cells. Reactive oxygen and nitrogen species may play an important role in this process by inhibiting protein tyrosine phosphatase activity. The ability of radiation to activate signaling pathways may depend on the expression of growth factor receptors, autocrine factors, RAS mutation, and PTEN expression. In other words, just because pathway X is activated by radiation in one cell type does not mean that pathway X will be activated in a different cell type. Radiation-induced signaling through growth factor receptors such as the EGFR may provide radioprotective signals through multiple downstream pathways. In some cell types, enhanced basal signaling by proto-oncogenes such as RAS may provide a radioprotective signal. In many cell types, this may be through PI3K, in others potentially by NFKB or MAPK. Receptor signaling is often dependent on autocrine factors, and synthesis of autocrine factors will have an impact on the amount of radiation-induced pathway activity. For example, cells expressing TGFalpha and HB-EGF will generate protection primarily through EGFR. Heregulin and neuregulins will generate protective signals through ERBB4/ERBB3. The impact on radiation-induced signaling of other autocrine and paracrine ligands such as TGFbeta and interleukin 6 is likely to be as complicated as described above for the ERBB receptors.

MST4, a new Ste20-related kinase that mediates cell growth and transformation via modulating ERK pathway

In this study, we report the cloning and characterization of a novel human Ste20-related kinase that we designated MST4. The 416 amino acid full-length MST4 contains an amino-terminal kinase domain, which is highly homologous to MST3 and SOK, and a unique carboxy-terminal domain. Northern blot analysis indicated that MST4 is highly expressed in placenta, thymus, and peripheral blood leukocytes. Wild-type but not kinase-dead MST4 can phosphorylate myelin basic protein in an in vitro kinase assay. MST4 specifically activates ERK but not JNK or p38 MAPK in transient transfected cells or in stable cell lines. Overexpression of dominant negative MEK1 or treatment with PD98059 abolishes MST4-induced ERK activity, whereas dominant-negative Ras or c-Raf-1 mutants failed to do so, indicating MST4 activates MEK1/ERK via a Ras/Raf-1 independent pathway. HeLa and Phoenix cell lines overexpressing wild-type, but not kinase-dead, MST4 exhibit increased growth rate and form aggressive soft-agar colonies. These phenotypes can be inhibited by PD98059. These results provide the first evidence that MST4 is biologically active in the activation of MEK/ERK pathway and in mediating cell growth and transformation.

Cloning and characterization of MST4, a novel Ste20-like kinase

MST4, a novel member of the germinal center kinase subfamily of human Ste20-like kinases, was cloned and characterized. Composed of a C-terminal regulatory domain and an N-terminal kinase domain, MST4 is most closely related to mammalian Ste20 kinase family member MST3. Both the kinase and C-terminal regulatory domains of MST4 are required for full activation of the kinase. Northern blot analysis indicates that MST4 is ubiquitously distributed, and the MST4 gene is localized to chromosome Xq26, a disease-rich region, by fluorescence in situ hybridization. Although some members of the MST4 family function as upstream regulators of mitogen-activated protein kinase cascades, expression of MST4 in 293 cells was not sufficient to activate or potentiate extracellular signal-regulated kinase, c-Jun N-terminal kinase, or p38 kinase. An alternatively spliced isoform of MST4 (MST4a) was isolated by yeast two-hybrid interaction with the catalytic domain of Raf from a human fetal brain cDNA library and also found in a variety of human fetal and adult tissues. MST4a lacks an exon encoding kinase subdomains IX-XI that stabilizes substrate binding. The existence of both MST4 isoforms suggests that the MST4 kinase activity is highly regulated, and MST4a may function as a dominant-negative regulator of the MST4 kinase.

Regulation of stress-responsive mitogen-activated protein (MAP) kinase pathways by TAO2

Previous studies demonstrated that in vitro the protein kinase TAO2 activates MAP/ERK kinases (MEKs) 3, 4, and 6 toward their substrates p38 MAP kinase and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK). In this study, we examined the ability of TAO2 to activate stress-sensitive MAP kinase pathways in cells and the relationship between activation of TAO2 and potential downstream pathways. Over-expression of TAO2 activated endogenous JNK/SAPK and p38 but not ERK1/2. Cotransfection experiments suggested that TAO2 selectively activates MEK3 and MEK6 but not MEKs 1, 4, or 7. Coimmunoprecipitation demonstrated that endogenous TAO2 specifically associates with MEK3 and MEK6 providing one mechanism for preferential recognition of MEKs upstream of p38. Sorbitol, and to a lesser extent, sodium chloride, Taxol, and nocodazole increased TAO2 activity toward itself and kinase-dead MEKs 3 and 6. Activation of endogenous TAO2 during differentiation of C2C12 myoblasts paralleled activation of p38 but not JNK/SAPK, consistent with the idea that TAO2 is a physiological regulator of p38 under certain circumstances.