The nucleus, a site for signal termination by sequestration and inactivation of p42/p44 MAP kinases

Improving Split Reporters of Protein-Protein Interactions through Orthology-Based Protein Engineering

Protein-protein interactions (PPIs) can be detected through selective complementation of split fluorescent reporters made of two complementary fragments that reassemble into a functional fluorescent reporter when in close proximity. We previously introduced splitFAST, a chemogenetic PPI reporter with rapid and reversible complementation. Here, we present the engineering of splitFAST2, an improved reporter displaying higher brightness, lower self-complementation, and higher dynamic range for optimal monitoring of PPI using an original protein engineering strategy that exploits proteins with orthology relationships. Our study allowed the identification of a system with improved properties and enabled a better understanding of the molecular features controlling the complementation properties. Because of the rapidity and reversibility of its complementation, its low self-complementation, high dynamic range, and improved brightness, splitFAST2 is well suited to study PPI with high spatial and temporal resolution, opening great prospects to decipher the role of PPI in various biological contexts.

N-terminal alanine-rich (NTAR) sequences drive precise start codon selection resulting in elevated translation of multiple proteins including ERK1/2

We report the discovery of N-terminal alanine-rich sequences, which we term NTARs, that act in concert with their native 5'-untranslated regions to promote selection of the proper start codon. NTARs also facilitate efficient translation initiation while limiting the production of non-functional polypeptides through leaky scanning. We first identified NTARs in the ERK1/2 kinases, which are among the most important signaling molecules in mammals. Analysis of the human proteome reveals that hundreds of proteins possess NTARs, with housekeeping proteins showing a particularly high prevalence. Our data indicate that several of these NTARs act in a manner similar to those found in the ERKs and suggest a mechanism involving some or all of the following features: alanine richness, codon rarity, a repeated amino acid stretch and a nearby second AUG. These features may help slow down the leading ribosome, causing trailing pre-initiation complexes (PICs) to pause near the native AUG, thereby facilitating accurate translation initiation. Amplification of erk genes is frequently observed in cancer, and we show that NTAR-dependent ERK protein levels are a rate-limiting step for signal output. Thus, NTAR-mediated control of translation may reflect a cellular need to precisely control translation of key transcripts such as potential oncogenes. By preventing translation in alternative reading frames, NTAR sequences may be useful in synthetic biology applications, e.g. translation from RNA vaccines.

Non-catalytic allostery in α-TAT1 by a phospho-switch drives dynamic microtubule acetylation

Spatiotemporally dynamic microtubule acetylation underlies diverse physiological and pathological events. Despite its ubiquity, the molecular mechanisms that regulate the sole microtubule acetylating agent, α-tubulin-N-acetyltransferase-1 (α-TAT1), remain obscure. Here, we report that dynamic intracellular localization of α-TAT1 along with its catalytic activity determines efficiency of microtubule acetylation. Specifically, we newly identified a conserved signal motif in the intrinsically disordered C-terminus of α-TAT1, consisting of three competing regulatory elements-nuclear export, nuclear import, and cytosolic retention. Their balance is tuned via phosphorylation by CDK1, PKA, and CK2, and dephosphorylation by PP2A. While the unphosphorylated form binds to importins and resides both in cytosol and nucleus, the phosphorylated form binds to specific 14-3-3 adapters and accumulates in the cytosol for maximal substrate access. Unlike other molecules with a similar phospho-regulated signal motif, α-TAT1 uniquely uses the nucleus as a hideout. This allosteric spatial regulation of α-TAT1 function may help uncover a spatiotemporal code of microtubule acetylation in normal and aberrant cell behavior.

A split fluorescent reporter with rapid and reversible complementation

Interactions between proteins play an essential role in metabolic and signaling pathways, cellular processes and organismal systems. We report the development of splitFAST, a fluorescence complementation system for the visualization of transient protein-protein interactions in living cells. Engineered from the fluorogenic reporter FAST (Fluorescence-Activating and absorption-Shifting Tag), which specifically and reversibly binds fluorogenic hydroxybenzylidene rhodanine (HBR) analogs, splitFAST displays rapid and reversible complementation, allowing the real-time visualization of both the formation and the dissociation of a protein assembly.

Extracellular-Signal Regulated Kinase: A Central Molecule Driving Epithelial-Mesenchymal Transition in Cancer

Epithelial-mesenchymal transition (EMT) is a reversible cellular process, characterized by changes in gene expression and activation of proteins, favoring the trans-differentiation of the epithelial phenotype to a mesenchymal phenotype. This process increases cell migration and invasion of tumor cells, progression of the cell cycle, and resistance to apoptosis and chemotherapy, all of which support tumor progression. One of the signaling pathways involved in tumor progression is the MAPK pathway. Within this family, the ERK subfamily of proteins is known for its contributions to EMT. The ERK subfamily is divided into typical (ERK 1/2/5), and atypical (ERK 3/4/7/8) members. These kinases are overexpressed and hyperactive in various types of cancer. They regulate diverse cellular processes such as proliferation, migration, metastasis, resistance to chemotherapy, and EMT. In this context, in vitro and in vivo assays, as well as studies in human patients, have shown that ERK favors the expression, function, and subcellular relocalization of various proteins that regulate EMT, thus promoting tumor progression. In this review, we discuss the mechanistic roles of the ERK subfamily members in EMT and tumor progression in diverse biological systems.

ERK/MAPK signaling and autism spectrum disorders

The MAPK pathway is a prominent intracellular signaling pathway regulating various intracellular functions. Components of this pathway are mutated in a related collection of congenital syndromes collectively referred to as neuro-cardio-facio-cutaneous syndromes (NCFC) or Rasopathies. Recently, it has been appreciated that these disorders are associated with autism spectrum disorders (ASD). In addition, idiopathic ASD has also implicated the MAPK signaling cascade as a common pathway that is affected by many of the genetic variants that have been found to be linked to ASDs. This chapter describes the components of the MAPK pathway and how it is regulated. Furthermore, this chapter will highlight the various functions of the MAPK pathway during both embryonic development of the central nervous system (CNS) and its roles in neuronal physiology and ultimately, behavior. Finally, we will summarize the perturbations to MAPK signaling in various models of autism spectrum disorders and Rasopathies to highlight how dysregulation of this pivotal pathway may contribute to the pathogenesis of autism.

Extracellular-Regulated Kinases: Signaling From Ras to ERK Substrates to Control Biological Outcomes

The extracellular-regulated kinases ERK1 and ERK2 are evolutionarily conserved, ubiquitous serine-threonine kinases that are involved in regulating cellular signaling in both normal and pathological conditions. Their expression is critical for development and their hyperactivation is a major factor in cancer development and progression. Since their discovery as one of the major signaling mediators activated by mitogens and Ras mutation, we have learned much about their regulation, including their activation, binding partners and substrates. In this review I will discuss some of what has been discovered about the members of the Ras to ERK pathway, including regulation of their activation by growth factors and cell adhesion pathways. Looking downstream of ERK activation I will also highlight some of the many ERK substrates that have been discovered, including those involved in feedback regulation, cell migration and cell cycle progression through the control of transcription, pre-mRNA splicing and protein synthesis.

ERK1/2/MAPK pathway-dependent regulation of the telomeric factor TRF2

Telomere stability is a hallmark of immortalized cells, including cancer cells. While the telomere length is maintained in most cases by the telomerase, the activity of a protein complex called Shelterin is required to protect telomeres against unsuitable activation of the DNA damage response pathway. Within this complex, telomeric repeat binding factor 2 (TRF2) plays an essential role by blocking the ataxia telangiectasia-mutated protein (ATM) signaling pathway at telomeres and preventing chromosome end fusion. We showed that TRF2 was phosphorylated in vitro and in vivo on serine 323 by extracellular signal-regulated kinase (ERK1/2) in both normal and cancer cells. Moreover, TRF2 and activated ERK1/2 unexpectedly interacted in the cytoplasm of tumor cells and human tumor tissues. The expression of non-phosphorylatable forms of TRF2 in melanoma cells induced the DNA damage response, leading to growth arrest and tumor reversion. These findings revealed that the telomere stability is under direct control of one of the major pro-oncogenic signaling pathways (RAS/RAF/MEK/ERK) via TRF2 phosphorylation.

Implications of a peroxisome proliferator-activated receptor alpha (PPARα) ligand clofibrate in breast cancer

Inflammatory and invasive breast cancers are aggressive and require better understanding for the development of new treatments and more accurate prognosis. Here, we detected high expression of PPARα in human primary inflammatory (SUM149PT) and highly invasive (SUM1315MO2) breast cancer cells, and tissue sections of human breast cancer. PPARα ligands are clinically used to treat dyslipidemia. Among lipid lowering drugs clofibrate, fenofibrate and WY14643, clofibrate showed high chemo-sensitivity towards breast cancer cells. Clofibrate treatment significantly induced PPARα DNA binding activity, and remarkably reduced cyclooxygenase-2/PGE2 and 5-lipoxygenase/LTB4 inflammatory pathways. Clofibrate treatment reduced the proliferation of breast cancer cells probably by inhibiting NF-κB and ERK1/2 activation, reducing cyclinD1, cyclinA, cyclinE, and inducing pro-apoptotic P21 levels. Surprisingly, the expression of lipogenic pathway genes including SREBP-1c (sterol regulatory element-binding protein-1c), HMG-CoA synthase, SPTLC1 (serine palmitoyltransferase long-chain), and Acyl-CoA oxidase (ACO) decreased with a concurrent increase in fatty acid oxidation genes such as CPT-1a (carnitine palmitoyltransferase 1a) and SREBP-2 (Sterol regulatory element-binding protein-2). Clofibrate treatment induced secretion of free fatty acids and effectively decreased the level of phosphorylated active form of fatty acid synthase (FASN), an enzyme catalyzing de novo synthesis of fatty acids. High level of coactivators steroid receptor coactivator-1 (SRC-1) and histone acetylase CBP-300 (CREB binding protein-300) were observed in the nuclear complexes of clofibrate treated breast cancer cells. These findings implicate that stimulating PPARα by safe, well-tolerated, and clinically approved clofibrate may provide a safer and more effective strategy to target the signaling, lipogenic, and inflammatory pathways in aggressive forms of breast cancer.

Heterochromatin protein (HP)1γ is not only in the nucleus but also in the cytoplasm interacting with actin in both cell compartments

Confocal and electron microscopy images, and WB analysis of cellular fractions revealed that HP1γ is in the nucleus but also in the cytoplasm of C2C12 myoblasts, myotubes, skeletal and cardiac muscles, N2a, HeLa and HEK293T cells. Signal specificity was tested with different antibodies and by HP1γ knockdown. Leptomycin B treatment of myoblasts increased nuclear HP1γ, suggesting that its nuclear export is Crm-1-dependent. HP1γ exhibited a filamentous pattern of staining partially co-localizing with actin in the cytoplasm of myotubes and myofibrils. Immunoelectron microscopic analysis showed high-density immunogold particles that correspond to HP1γ localized to the Z-disk and A-band of the sarcomere of skeletal muscle. HP1γ partially co-localized with actin in C2C12 myotubes and murine myofibrils. Importantly, actin co-immunoprecipitated with HP1γ in the nuclear and cytosolic fractions of myoblasts. Actin co-immunoprecipitated with HP1γ in myoblasts incubated in the absence or presence of the actin depolymerizing agent cytochalasin D, suggesting that HP1γ may interact with G-and F-actin. In the cytoplasm, HP1γ was associated to the perinuclear actin cap that controls nuclear shape and position. In the nucleus, re-ChIP assays showed that HP1γ-actin associates to the promoter and transcribed regions of the house keeping gene GAPDH, suggesting that HP1γ may function as a scaffold protein for the recruitment of actin to control gene expression. When HP1γ was knocked-down, myoblasts were unable to differentiate or originated thin myotubes. In summary, HP1γ is present in the nucleus and the cytoplasm interacting with actin, a protein complex that may exert different functions depending on its subcellular localization.

Biphasic activation of extracellular signal-regulated kinase (ERK) 1/2 in epidermal growth factor (EGF)-stimulated SW480 colorectal cancer cells

Cancer cells have different characteristics due to the genetic differences where these unique features may strongly influence the effectiveness of therapeutic interventions. Here, we show that the spontaneous reactivation of extracellular signalregulated kinase (ERK), distinct from conventional ERK activation, represents a potent mechanism for cancer cell survival. We studied ERK1/2 activation in vitro in SW480 colorectal cancer cells. Although ERK signaling tends to be transiently activated, we observed the delayed reactivation of ERK1/2 in epidermal growth factor (EGF)-stimulated SW480 cells. This effect was observed even after EGF withdrawal. While phosphorylated ERK1/2 translocated into the nucleus following its primary activation, it remained in the cytoplasm during late-phase activation. The inhibition of primary ERK1/2 activation or protein trafficking, blocked reactivation and concurrently increased caspase 3 activity. Our results suggest that the biphasic activation of ERK1/2 plays a role in cancer cell survival; thus, regulation of ERK1/2 activation may improve the efficacy of cancer therapies that target ERK signaling. [BMB Reports 2016; 49(4): 220-225]

Dual-specificity phosphatase 5 controls the localized inhibition, propagation, and transforming potential of ERK signaling

Deregulated extracellular signal-regulated kinase (ERK) signaling drives cancer growth. Normally, ERK activity is self-limiting by the rapid inactivation of upstream kinases and delayed induction of dual-specificity MAP kinase phosphatases (MKPs/DUSPs). However, interactions between these feedback mechanisms are unclear. Here we show that, although the MKP DUSP5 both inactivates and anchors ERK in the nucleus, it paradoxically increases and prolongs cytoplasmic ERK activity. The latter effect is caused, at least in part, by the relief of ERK-mediated RAF inhibition. The importance of this spatiotemporal interaction between these distinct feedback mechanisms is illustrated by the fact that expression of oncogenic BRAF^V600E, a feedback-insensitive mutant RAF kinase, reprograms DUSP5 into a cell-wide ERK inhibitor that facilitates cell proliferation and transformation. In contrast, DUSP5 deletion causes BRAF^V600E-induced ERK hyperactivation and cellular senescence. Thus, feedback interactions within the ERK pathway can regulate cell proliferation and transformation, and suggest oncogene-specific roles for DUSP5 in controlling ERK signaling and cell fate.

Using 3D Culture of Primary Mammary Epithelial Cells to Define Molecular Entities Required for Acinus Formation: Analyzing MAP Kinase Phosphatases

Three-dimensional (3D) cell cultures on reconstituted basement membrane (rBM) enable the study of complex interactions between extracellular matrix (ECM) components and epithelial cells, which are crucial for the establishment of cell polarity and functional development of epithelia. 3D cultures of mammary epithelial cells (MECs) on Matrigel (a laminin-rich ECM derived from the Engelbreth-Holm-Swarm (EHS) murine tumor) promote interactions of MECs with the matrix via integrins, leading to formation of spherical monolayers of polarized cells surrounding a hollow lumen (acini). Acini closely resemble mammary alveoli found in the mammary gland. Thus, it is possible to study ECM-cell interactions and signalling pathways that regulate formation and maintenance of tissue-specific shape and functional differentiation of MECs in 3D under in vitro conditions. Here we present experimental protocols used to investigate the role of mitogen-activated protein kinase phosphatases (MKPs) during development of the alveoli-like structures by primary mouse mammary epithelial cells (PMMEC) cultured on Matrigel. We present detailed protocols for PMMEC isolation, and establishment of 3D cultures using an "on top" method, use of specific kinase and phosphatases inhibitors (PD98059 and pervanadate, respectively) administered at different stages of acinus development, and give examples of analyses carried out post-culture (Western blot, immunofluorescence staining, and confocal imaging).

ERK1 and ERK2 Map Kinases: Specific Roles or Functional Redundancy?

The MAP kinase signaling cascade Ras/Raf/MEK/ERK has been involved in a large variety of cellular and physiological processes that are crucial for life. Many pathological situations have been associated to this pathway. More than one isoform has been described at each level of the cascade. In this review we devoted our attention to ERK1 and ERK2, which are the effector kinases of the pathway. Whether ERK1 and ERK2 specify functional differences or are in contrast functionally redundant, constitutes an ongoing debate despite the huge amount of studies performed to date. In this review we compiled data on ERK1 vs. ERK2 gene structures, protein sequences, expression levels, structural and molecular mechanisms of activation and substrate recognition. We have also attempted to perform a rigorous analysis of studies regarding the individual roles of ERK1 and ERK2 by the means of morpholinos, siRNA, and shRNA silencing as well as gene disruption or gene replacement in mice. Finally, we comment on a recent study of gene and protein evolution of ERK isoforms as a distinct approach to address the same question. Our review permits the evaluation of the relevance of published studies in the field especially when measurements of global ERK activation are taken into account. Our analysis favors the hypothesis of ERK1 and ERK2 exhibiting functional redundancy and points to the concept of the global ERK quantity, and not isoform specificity, as being the essential determinant to achieve ERK function.

Novel Reporter for Faithful Monitoring of ERK2 Dynamics in Living Cells and Model Organisms

Uncoupling of ERK1/2 phosphorylation from subcellular localization is essential towards the understanding of molecular mechanisms that control ERK1/2-mediated cell-fate decision. ERK1/2 non-catalytic functions and discoveries of new specific anchors responsible of the subcellular compartmentalization of ERK1/2 signaling pathway have been proposed as regulation mechanisms for which dynamic monitoring of ERK1/2 localization is necessary. However, studying the spatiotemporal features of ERK2, for instance, in different cellular processes in living cells and tissues requires a tool that can faithfully report on its subcellular distribution. We developed a novel molecular tool, ERK2-LOC, based on the T2A-mediated coexpression of strictly equimolar levels of eGFP-ERK2 and MEK1, to faithfully visualize ERK2 localization patterns. MEK1 and eGFP-ERK2 were expressed reliably and functionally both in vitro and in single living cells. We then assessed the subcellular distribution and mobility of ERK2-LOC using fluorescence microscopy in non-stimulated conditions and after activation/inhibition of the MAPK/ERK1/2 signaling pathway. Finally, we used our coexpression system in Xenopus laevis embryos during the early stages of development. This is the first report on MEK1/ERK2 T2A-mediated coexpression in living embryos, and we show that there is a strong correlation between the spatiotemporal subcellular distribution of ERK2-LOC and the phosphorylation patterns of ERK1/2. Our approach can be used to study the spatiotemporal localization of ERK2 and its dynamics in a variety of processes in living cells and embryonic tissues.

Proper Level of Cytosolic Disabled-1, Which Is Regulated by Dual Nuclear Translocation Pathways, Is Important for Cortical Neuronal Migration

Disabled-1 (Dab1) is an essential intracellular protein in the Reelin pathway. It has a nuclear localization signal (NLS; hereafter referred to as "NLS1") and 2 nuclear export signals, and shuttles between the nucleus and the cytoplasm. In this study, we found that Dab1 has an additional unidentified NLS, and that the Dab1 NLS1 mutant could translocate to the nucleus in an unconventional ATP/temperature-dependent and cytoplasmic factor/RanGTP gradient-independent manner. Additional mutations in the NLS1 mutant revealed that K(67) and K(69) are important for the nuclear transport. Furthermore, an excess of the intracellular domain of the Reelin receptors inhibited the nuclear translocation of Dab1. An in utero electroporation study showed that a large amount of Dab1 in the cytoplasm in migrating neurons inhibited the migration, and that forced transport of Dab1 into the nucleus attenuated this inhibitory effect. In addition, rescue experiments using yotari, an autosomal recessive mutant of dab1, revealed that cells expressing Dab1 NLS1 mutant tend to distribute at more superficial positions than those expressing wild-type Dab1. Taken together, these findings suggest that Dab1 has at least 2 NLSs, and that the regulation of the subcellular localization of Dab1 is important for the proper migration of excitatory neurons.

Cellular reprogramming through mitogen-activated protein kinases

Mitogen-activated protein kinase (MAPK) cascades are conserved eukaryote signaling modules where MAPKs, as the final kinases in the cascade, phosphorylate protein substrates to regulate cellular processes. While some progress in the identification of MAPK substrates has been made in plants, the knowledge on the spectrum of substrates and their mechanistic action is still fragmentary. In this focused review, we discuss the biological implications of the data in our original paper (Sustained mitogen-activated protein kinase activation reprograms defense metabolism and phosphoprotein profile in Arabidopsis thaliana; Frontiers in Plant Science 5: 554) in the context of related research. In our work, we mimicked in vivo activation of two stress-activated MAPKs, MPK3 and MPK6, through transgenic manipulation of Arabidopsis thaliana and used phosphoproteomics analysis to identify potential novel MAPK substrates. Here, we plotted the identified putative MAPK substrates (and downstream phosphoproteins) as a global protein clustering network. Based on a highly stringent selection confidence level, the core networks highlighted a MAPK-induced cellular reprogramming at multiple levels of gene and protein expression-including transcriptional, post-transcriptional, translational, post-translational (such as protein modification, folding, and degradation) steps, and also protein re-compartmentalization. Additionally, the increase in putative substrates/phosphoproteins of energy metabolism and various secondary metabolite biosynthesis pathways coincides with the observed accumulation of defense antimicrobial substances as detected by metabolome analysis. Furthermore, detection of protein networks in phospholipid or redox elements suggests activation of downstream signaling events. Taken in context with other studies, MAPKs are key regulators that reprogram cellular events to orchestrate defense signaling in eukaryotes.

A single mechanism can explain network-wide insulin resistance in adipocytes from obese patients with type 2 diabetes

The response to insulin is impaired in type 2 diabetes. Much information is available about insulin signaling, but understanding of the cellular mechanisms causing impaired signaling and insulin resistance is hampered by fragmented data, mainly obtained from different cell lines and animals. We have collected quantitative and systems-wide dynamic data on insulin signaling in primary adipocytes and compared cells isolated from healthy and diabetic individuals. Mathematical modeling and experimental verification identified mechanisms of insulin control of the MAPKs ERK1/2. We found that in human adipocytes, insulin stimulates phosphorylation of the ribosomal protein S6 and hence protein synthesis about equally via ERK1/2 and mTORC1. Using mathematical modeling, we examined the signaling network as a whole and show that a single mechanism can explain the insulin resistance of type 2 diabetes throughout the network, involving signaling both through IRS1, PKB, and mTOR and via ERK1/2 to the nuclear transcription factor Elk1. The most important part of the insulin resistance mechanism is an attenuated feedback from the protein kinase mTORC1 to IRS1, which spreads signal attenuation to all parts of the insulin signaling network. Experimental inhibition of mTORC1 using rapamycin in adipocytes from non-diabetic individuals induced and thus confirmed the predicted network-wide insulin resistance.

Salt-induced subcellular kinase relocation and seedling susceptibility caused by overexpression of Medicago SIMKK in Arabidopsis

Dual-specificity mitogen-activated protein kinases kinases (MAPKKs) are the immediate upstream activators of MAPKs. They simultaneously phosphorylate the TXY motif within the activation loop of MAPKs, allowing them to interact with and regulate multiple substrates. Often, the activation of MAPKs triggers their nuclear translocation. However, the spatiotemporal dynamics and the physiological consequences of the activation of MAPKs, particularly in plants, are still poorly understood. Here, we studied the activation and localization of the Medicago sativa stress-induced MAPKK (SIMKK)-SIMK module after salt stress. In the inactive state, SIMKK and SIMK co-localized in the cytoplasm and in the nucleus. Upon salt stress, however, a substantial part of the nuclear pool of both SIMKK and SIMK relocated to cytoplasmic compartments. The course of nucleocytoplasmic shuttling of SIMK correlated temporally with the dual phosphorylation of the pTEpY motif. SIMKK function was further studied in Arabidopsis plants overexpressing SIMKK-yellow fluorescent protein (YFP) fusions. SIMKK-YFP plants showed enhanced activation of Arabidopsis MPK3 and MPK6 kinases upon salt treatment and exhibited high sensitivity against salt stress at the seedling stage, although they were salt insensitive during seed germination. Proteomic analysis of SIMKK-YFP overexpressors indicated the differential regulation of proteins directly or indirectly involved in salt stress responses. These proteins included catalase, peroxiredoxin, glutathione S-transferase, nucleoside diphosphate kinase 1, endoplasmic reticulum luminal-binding protein 2, and finally plasma membrane aquaporins. In conclusion, Arabidopsis seedlings overexpressing SIMKK-YFP exhibited higher salt sensitivity consistent with their proteome composition and with the presumptive MPK3/MPK6 hijacking of the salt response pathway.

Tay bridge is a negative regulator of EGFR signalling and interacts with Erk and Mkp3 in the Drosophila melanogaster wing

The regulation of Extracellular regulated kinase (Erk) activity is a key aspect of signalling by pathways activated by extracellular ligands acting through tyrosine kinase transmembrane receptors. In this process, participate proteins with kinase activity that phosphorylate and activate Erk, as well as different phosphatases that inactivate Erk by de-phosphorylation. The state of Erk phosphorylation affects not only its activity, but also its subcellular localization, defining the repertoire of Erk target proteins, and consequently, the cellular response to Erk. In this work, we characterise Tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge is a large nuclear protein with a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus.

Extracellular regulated kinases (Erk) mediate signalling by pathways activated by tyrosine kinase transmembrane receptors. The level of activated Erk depends on a highly regulated balance between cytoplasmic kinases and nuclear/cytoplasmic phosphatases, which determine the state of Erk phosphorylation. This affects Erk activity and its subcellular localization, defining the repertoire of Erk targets, and consequently, the cellular response to Erk. In this work, we use a genetic approach to characterise the gene tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge has a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus. These results could provide important insights into the clinical consequences of AUTS2 mutations in humans, which are related to behavioural perturbations including autism, mental retardation, Attention Deficit Hyperactivity Disorder and alcohol drinking behaviour.

Early phase of plasticity-related gene regulation and SRF dependent transcription in the hippocampus

Hippocampal organotypic cultures are a highly reliable in vitro model for studying neuroplasticity: in this paper, we analyze the early phase of the transcriptional response induced by a 20 µM gabazine treatment (GabT), a GABA-Ar antagonist, by using Affymetrix oligonucleotide microarray, RT-PCR based time-course and chromatin-immuno-precipitation. The transcriptome profiling revealed that the pool of genes up-regulated by GabT, besides being strongly related to the regulation of growth and synaptic transmission, is also endowed with neuro-protective and pro-survival properties. By using RT-PCR, we quantified a time-course of the transient expression for 33 of the highest up-regulated genes, with an average sampling rate of 10 minutes and covering the time interval [10∶90] minutes. The cluster analysis of the time-course disclosed the existence of three different dynamical patterns, one of which proved, in a statistical analysis based on results from previous works, to be significantly related with SRF-dependent regulation (p-value<0.05). The chromatin immunoprecipitation (chip) assay confirmed the rich presence of working CArG boxes in the genes belonging to the latter dynamical pattern and therefore validated the statistical analysis. Furthermore, an in silico analysis of the promoters revealed the presence of additional conserved CArG boxes upstream of the genes Nr4a1 and Rgs2. The chip assay confirmed a significant SRF signal in the Nr4a1 CArG box but not in the Rgs2 CArG box.

Dual-specificity MAP kinase phosphatases (MKPs): shaping the outcome of MAP kinase signalling

Dual-specificity MAP kinase phosphatases (MKPs) provide a complex negative regulatory network that acts to shape the duration, magnitude and spatiotemporal profile of MAP kinase activities in response to both physiological and pathological stimuli. Individual MKPs may exhibit either exquisite specificity towards a single mitogen-activated protein kinase (MAPK) isoform or be able to regulate multiple MAPK pathways in a single cell or tissue. They can act as negative feedback regulators of MAPK activity, but can also provide mechanisms of crosstalk between distinct MAPK pathways and between MAPK signalling and other intracellular signalling modules. In this review, we explore the current state of knowledge with respect to the regulation of MKP expression levels and activities, the mechanisms by which individual MKPs recognize and interact with different MAPK isoforms and their role in the spatiotemporal regulation of MAPK signalling.

Mechanisms of GnRH-induced extracellular signal-regulated kinase nuclear localization

Gonadotropin-releasing hormone receptors (GnRHR) mediate activation and nuclear translocation of the extracellular signal regulated kinases 1 and 2 (ERK) by phosphorylation on the TEY motif. This is necessary for GnRH to initiate transcriptional programmes controlling fertility, but mechanisms that govern ERK targeting are unclear. Using automated microscopy to explore ERK regulation in single cells, we find that GnRHR activation induces marked redistribution of ERK to the nucleus and that this effect can be uncoupled from the level of TEY phosphorylation of ERK. Thus, 5 min stimulation with 100 nM GnRH increased phospho-ERK levels (from 89 ± 34 to 555 ± 45 arbitrary fluorescence units) and increased the nuclear:cytoplasmic (N:C) ERK ratio (from 1.36 ± 0.06 to 2.16 ± 0.05) in the whole cell population, but it also significantly increased N:C ERK in cells binned according to phospho-ERK levels. This phosphorylation unattributable component of the ERK translocation response occurs at a broad range of GnRHR expression levels, in the presence of tyrosine phosphatase and protein synthesis inhibitors, and in ERK mutants unable to undergo catalytic activation. It also occurred in mutants incapable of binding the DEF (docking site for ERK, F/Y-X-F/Y-P) domains found in many ERK binding partners. It was however, reduced by MEK or PKC inhibition and by mutations preventing TEY phosphorylation or that abrogate ERK binding to D (docking) domain partners. We therefore show that TEY phosphorylation of ERK is necessary, but not sufficient for the full nuclear localization response. We further show that this "phosphorylation unattributable" component of GnRH-mediated ERK nuclear translocation requires both PKC activity and association with partner proteins via the D-domain.

Mathematical modeling reveals the functional implications of the different nuclear shuttling rates of Erk1 and Erk2

The mitogen-activated protein kinase (MAPK) family of proteins is involved in regulating cellular fates such as proliferation, differentiation and apoptosis. In particular, the dynamics of the Erk/Mek system, which has become the canonical example for MAPK signaling systems, have attracted considerable attention. Erk is encoded by two genes, Erk1 and Erk2, that until recently had been considered equivalent as they differ only subtly at the sequence level. However, these proteins exhibit radically different trafficking between cytoplasm and nucleus and this fact may have functional implications. Here we use spatially resolved data on Erk1/2 to develop and analyze spatio-temporal models of these cascades, and we discuss how sensitivity analysis can be used to discriminate between mechanisms. Our models elucidate some of the factors governing the interplay between signaling processes and the Erk1/2 localization in different cellular compartments, including competition between Erk1 and Erk2. Our approach is applicable to a wide range of signaling systems, such as activation cascades, where translocation of molecules occurs. Our study provides a first model of Erk1 and Erk2 activation and their nuclear shuttling dynamics, revealing a role in the regulation of the efficiency of nuclear signaling.

Regulation of MEK/ERK pathway output by subcellular localization of B-Raf

The strength and duration of intracellular signalling pathway activation is a key determinant of the biological outcome of cells in response to extracellular cues. This has been particularly elucidated for the Ras/Raf/MEK [mitogen-activated growth factor/ERK (extracellular-signal-regulated kinase) kinase]/ERK signalling pathway with a number of studies in fibroblasts showing that sustained ERK signalling is a requirement for S-phase entry, whereas transient ERK signalling does not have this capability. A major unanswered question, however, is how a cell can sustain ERK activation, particularly when ERK-specific phosphatases are transcriptionally up-regulated by the pathway itself. A major point of ERK regulation is at the level of Raf, and, to sustain ERK activation in the presence of ERK phosphatases, sustained Raf activation is a requirement. Three Raf proteins exist in mammals, and the activity of all three is induced following growth factor stimulation of cells, but only B-Raf activity is maintained at later time points. This observation points to B-Raf as a regulator of sustained ERK activation. In the present review, we consider evidence for a link between B-Raf and sustained ERK activation, focusing on a potential role for the subcellular localization of B-Raf in this key physiological event.

ERK1/2 MAP kinases: structure, function, and regulation

ERK1 and ERK2 are related protein-serine/threonine kinases that participate in the Ras-Raf-MEK-ERK signal transduction cascade. This cascade participates in the regulation of a large variety of processes including cell adhesion, cell cycle progression, cell migration, cell survival, differentiation, metabolism, proliferation, and transcription. MEK1/2 catalyze the phosphorylation of human ERK1/2 at Tyr204/187 and then Thr202/185. The phosphorylation of both tyrosine and threonine is required for enzyme activation. Whereas the Raf kinase and MEK families have narrow substrate specificity, ERK1/2 catalyze the phosphorylation of hundreds of cytoplasmic and nuclear substrates including regulatory molecules and transcription factors. ERK1/2 are proline-directed kinases that preferentially catalyze the phosphorylation of substrates containing a Pro-Xxx-Ser/Thr-Pro sequence. Besides this primary structure requirement, many ERK1/2 substrates possess a D-docking site, an F-docking site, or both. A variety of scaffold proteins including KSR1/2, IQGAP1, MP1, β-Arrestin1/2 participate in the regulation of the ERK1/2 MAP kinase cascade. The regulatory dephosphorylation of ERK1/2 is mediated by protein-tyrosine specific phosphatases, protein-serine/threonine phosphatases, and dual specificity phosphatases. The combination of kinases and phosphatases make the overall process reversible. The ERK1/2 catalyzed phosphorylation of nuclear transcription factors including those of Ets, Elk, and c-Fos represents an important function and requires the translocation of ERK1/2 into the nucleus by active and passive processes involving the nuclear pore. These transcription factors participate in the immediate early gene response. The activity of the Ras-Raf-MEK-ERK cascade is increased in about one-third of all human cancers, and inhibition of components of this cascade by targeted inhibitors represents an important anti-tumor strategy. Thus far, however, only inhibition of mutant B-Raf (Val600Glu) has been found to be therapeutically efficacious.

Oncogenic KRAS and BRAF activation of the MEK/ERK signaling pathway promotes expression of dual-specificity phosphatase 4 (DUSP4/MKP2) resulting in nuclear ERK1/2 inhibition

Gain-of-function mutations in KRAS and BRAF genes are found in up to 50% of colorectal cancers. These mutations result in the activation of the BRAF/MEK signaling pathway culminating in the stimulation of ERK1/2 mitogen-activated protein kinases. Upon activation, ERK1/2 translocate from the cytoplasm to the nucleus. This process has been shown to be required for the induction of many cellular responses, although the molecular mechanisms regulating ERK nuclear function, especially under oncogenic stimulation, remain to be explored. Herein, we examined the spatiotemporal regulation of ERK1/2 activity upon oncogenic activation of KRAS(G12V) and BRAF(V600E) in normal intestinal epithelial crypt cells (IECs). Results demonstrate that expression of these oncogenes markedly stimulated ERK1/2 activities and morphologically transformed IECs. Importantly however, ERK phosphorylation was not observed in the nucleus, but restricted to the cytoplasm of KRAS(G12V)- and BRAF(V600E)-transformed IECs. The absence of nuclear ERK phosphorylation was due to a vanadate-sensitive phosphatase activity. Nuclear ERK dephosphorylation was found to be tightly correlated with the rapid expression of DUSP4 phosphatase induced in an MEK-dependent manner. In addition, MEK-dependent phosphorylation of T361, T363, S390 and S395 residues highly stabilized DUSP4 protein. Finally, in human colorectal cancer cells, ERK1/2 activities were also confined to the cytoplasm and treatment with pervanadate reactivated ERK1/2 in the nucleus. Accordingly, DUSP4 mRNAs were found to be highly expressed, in an MEK-dependent manner, in all colorectal cancer cells analyzed. These findings indicate that DUSP4 functions as part of a negative feedback mechanism in the control of the duration and magnitude of nuclear ERK activation during intestinal tumorigenesis.

Stimulus-induced uncoupling of extracellular signal-regulated kinase phosphorylation from nuclear localization is dependent on docking domain interactions

Many stimuli activate the extracellular signal-regulated kinase (ERK) by phosphorylation on the TEY motif. Activated ERK characteristically accumulates in the nucleus, but the underlying mechanisms involved are unclear. Using automated microscopy to explore ERK regulation in single intact cells, we find that, when protein kinase C or epidermal growth factor receptors are activated, a substantial fraction of the ERK nuclear localization response is uncoupled from TEY phosphorylation. This phosphorylation-unattributable nuclear localization response occurs in the presence of inhibitors of tyrosine phosphatases and protein synthesis. It was also evident with a catalytically inactive ERK2-GFP mutant, and with a mutant incapable of binding the DEF (docking site for ERK, F/Y-X-F/Y-P) domains found in many ERK binding partners. It was, however, reduced by MEK inhibition and by mutations preventing either TEY phosphorylation or D (docking)-domain-dependent ERK binding (D319N). Thus, we show that MEK-catalysed ERK phosphorylation is necessary but not sufficient for the full nuclear localization response: there is an additional phosphorylation-unattributable component of the response that does not reflect induced expression of nuclear anchors and is independent of ERK catalytic activity or DEF-domain binding. It is, however, dependent upon D-domain binding, highlighting distinct roles of ERK motifs during nuclear targeting.

Light-activated kinases enable temporal dissection of signaling networks in living cells

We report a general strategy for creating protein kinases in mammalian cells that are poised for very rapid activation by light. By photoactivating a caged version of MEK1, we demonstrate the specific, rapid, and receptor independent activation of an artificial subnetwork within the Raf/MEK/ERK pathway. Time-lapse microscopy allowed us to precisely characterize the kinetics of elementary steps in the signaling cascade and provided insight into adaptive feedback and rate-determining processes in the pathway.

Insulin-like growth factor-I activates extracellularly regulated kinase to regulate the p450 side-chain cleavage insulin-like response element in granulosa cells

IGF regulates steroidogenesis in granulosa cells through expression of the cytochrome P450 side-chain cleavage enzyme (P450scc) (CYP11A1), the rate-limiting enzyme in this biosynthetic process. We showed previously that the polypyrimidine tract-binding protein-associated splicing factor (PSF) acts as a repressor, whereas Sp1 is an activator, of P450 gene expression. The aim of the present study was to investigate IGF-stimulated ERK signaling regulating P450scc gene expression in the immortalized porcine granulosa cell line JC-410. We used a reporter gene under control of the IGF response element from the P450scc promoter. Inhibition of ERK phosphorylation with U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] blocked IGF-I induction of IGF response element reporter gene activity. Western blotting revealed that IGF-I treatment resulted in phosphorylation of ERK that was specifically inhibited by U0126. ERK activation led to phosphorylation of T739 (an ERK site) on Sp1 that was diminished by U0126 or overexpression of PSF. Coimmunoprecipitation and Western blotting of nuclear extracts showed that phosphorylated ERK (pERK) bound PSF under basal conditions. IGF-I caused dissociation of pERK from PSF. Finally, chromatin immunoprecipitation analysis showed that PSF and Sp1 constitutively occupy the P450scc promoter independent of IGF-I treatment. These events provide a potential molecular mechanism for release of PSF repression of P450scc expression by dissociation of pERK and subsequent pERK-mediated phosphorylation of Sp1 to drive transcriptional induction of the P450scc gene in the absence of altered binding of PSF or Sp1 to the promoter. Understanding IGF-I regulation of these critical ovarian signaling pathways is the first step to delineating ovarian hyperstimulation syndromes such as polycystic ovarian syndrome.

A quantitative image cytometry technique for time series or population analyses of signaling networks

Background

Modeling of cellular functions on the basis of experimental observation is increasingly common in the field of cellular signaling. However, such modeling requires a large amount of quantitative data of signaling events with high spatio-temporal resolution. A novel technique which allows us to obtain such data is needed for systems biology of cellular signaling.

Methodology/Principal Findings

We developed a fully automatable assay technique, termed quantitative image cytometry (QIC), which integrates a quantitative immunostaining technique and a high precision image-processing algorithm for cell identification. With the aid of an automated sample preparation system, this device can quantify protein expression, phosphorylation and localization with subcellular resolution at one-minute intervals. The signaling activities quantified by the assay system showed good correlation with, as well as comparable reproducibility to, western blot analysis. Taking advantage of the high spatio-temporal resolution, we investigated the signaling dynamics of the ERK pathway in PC12 cells.

Conclusions/Significance

The QIC technique appears as a highly quantitative and versatile technique, which can be a convenient replacement for the most conventional techniques including western blot, flow cytometry and live cell imaging. Thus, the QIC technique can be a powerful tool for investigating the systems biology of cellular signaling.

The subcellular localization of MEK and ERK--a novel nuclear translocation signal (NTS) paves a way to the nucleus

The ERK cascade is a central signaling pathway that regulates a large number of intracellular processes including proliferation, differentiation, development and also survival or apoptosis. The induction of so many distinct and even opposing cellular processes raises the question as to how the signaling specificity of the cascade is regulated. In the past few years, subcellular localization of components of the ERK cascade was shown to play an important role in specificity determination. Here we describe the dynamic subcellular localization of Raf kinases, MEKs, and particularly ERKs, which translocate into the nucleus during many cellular processes to induce transcription. We also describe in details the recent identification of a novel nuclear translocation mechanism for ERKs, which is based on a nuclear translocation sequence (NTS) within their kinase insert domain (KID). Phosphorylation of this domain, mainly upon stimulation, allows ERKs to interact with the nuclear importing protein - importin7, which mediates the penetration of the interacting ERKs into the nucleus via nuclear pores. Interestingly, the NTS is not specific to ERKs, and seems to be a general signal for regulating nuclear accumulation of various proteins, including MEKs, upon their stimulation. Better understanding of this mechanism may clarify the role of the massive nuclear translocation of many regulatory proteins shortly after their stimulation.

Cell fate decisions are specified by the dynamic ERK interactome

Extracellular signal-regulated kinase (ERK) controls fundamental cellular functions, including cell fate decisions. In PC12, cells shifting ERK activation from transient to sustained induces neuronal differentiation. As ERK associates with both regulators and effectors, we hypothesized that the mechanisms underlying the switch could be revealed by assessing the dynamic changes in ERK-interacting proteins that specifically occur under differentiation conditions. Using quantitative proteomics, we identified 284 ERK-interacting proteins. Upon induction of differentiation, 60 proteins changed their binding to ERK, including many proteins that were not known to participate in differentiation. We functionally characterized a subset, showing that they regulate the pathway at several levels and by different mechanisms, including signal duration, ERK localization, feedback, crosstalk with the Akt pathway and differential interaction and phosphorylation of transcription factors. Integrating these data with a mathematical model confirmed that ERK dynamics and differentiation are regulated by distributed control mechanisms rather than by a single master switch.

ERK nuclear translocation is dimerization-independent but controlled by the rate of phosphorylation

Upon activation, ERKs translocate from the cytoplasm to the nucleus. This process is required for the induction of many cellular responses, yet the molecular mechanisms that regulate ERK nuclear translocation are not fully understood. We have used a mouse embryo fibroblast ERK1-knock-out cell line expressing green fluorescent protein (GFP)-tagged ERK1 to probe the spatio-temporal regulation of ERK1. Real time fluorescence microscopy and fluorescence correlation spectroscopy revealed that ERK1 nuclear accumulation increased upon serum stimulation, but the mobility of the protein in the nucleus and cytoplasm remained unchanged. Dimerization of ERK has been proposed as a requirement for nuclear translocation. However, ERK1-Delta4, the mutant shown consistently to be dimerization-deficient in vitro, accumulated in the nucleus to the same level as wild type (WT), indicating that dimerization of ERK1 is not required for nuclear entry and retention. Consistent with this finding, energy migration Förster resonance energy transfer and fluorescence correlation spectroscopy measurements in living cells did not detect dimerization of GFP-ERK1-WT upon activation. In contrast, the kinetics of nuclear accumulation and phosphorylation of GFP-ERK1-Delta4 were slower than that of GFP-ERK1-WT. These results indicate that the differential shuttling behavior of the mutant is a consequence of delayed phosphorylation of ERK by MEK rather than dimerization. Our data demonstrate for the first time that a delay in cytoplasmic activation of ERK is directly translated into a delay in nuclear translocation.

GLP-1 mediates antiapoptotic effect by phosphorylating Bad through a beta-arrestin 1-mediated ERK1/2 activation in pancreatic beta-cells

Strategies based on activating GLP-1 receptor (GLP-1R) are intensively developed for the treatment of type 2 diabetes. The exhaustive knowledge of the signaling pathways linked to activated GLP-1R within the beta-cells is of major importance. In beta-cells, GLP-1 activates the ERK1/2 cascade by diverse pathways dependent on either Galpha(s)/cAMP/cAMP-dependent protein kinase (PKA) or beta-arrestin 1, a scaffold protein. Using pharmacological inhibitors, beta-arrestin 1 small interfering RNA, and islets isolated from beta-arrestin 1 knock-out mice, we demonstrate that GLP-1 stimulates ERK1/2 by two temporally distinct pathways. The PKA-dependent pathway mediates rapid and transient ERK1/2 phosphorylation that leads to nuclear translocation of the activated kinases. In contrast, the beta-arrestin 1-dependent pathway produces a late ERK1/2 activity that is restricted to the beta-cell cytoplasm. We further observe that GLP-1 phosphorylates the cytoplasmic proapoptotic protein Bad at Ser-112 but not at Ser-155. We find that the beta-arrestin 1-dependent ERK1/2 activation engaged by GLP-1 mediates the Ser-112 phosphorylation of Bad, through p90RSK activation, allowing the association of Bad with the scaffold protein 14-3-3, leading to its inactivation. beta-Arrestin 1 is further found to mediate the antiapoptotic effect of GLP-1 in beta-cells through the ERK1/2-p90RSK-phosphorylation of Bad. This new regulatory mechanism engaged by activated GLP-1R involving a beta-arrestin 1-dependent spatiotemporal regulation of the ERK1/2-p90RSK activity is now suspected to participate in the protection of beta-cells against apoptosis. Such signaling mechanism may serve as a prototype to generate new therapeutic GLP-1R ligands.

VEGF autoregulates its proliferative and migratory ERK1/2 and p38 cascades by enhancing the expression of DUSP1 and DUSP5 phosphatases in endothelial cells

Vascular endothelial growth factor (VEGF) is a key angiogenic factor that regulates proliferation and migration of endothelial cells via phosphorylation of extracellular signal-regulated kinase-1/2 (ERK1/2) and p38, respectively. Here, we demonstrate that VEGF strongly induces the transcription of two dual-specificity phosphatase (DUSP) genes DUSP1 and DUSP5 in endothelial cells. Using fluorescence microscopy, fluorescence lifetime imaging (FLIM), and fluorescence cross-correlation spectroscopy (FCCS), we found that DUSP1/mitogen-activated protein kinases phosphatase-1 (MKP-1) was localized in both the nucleus and cytoplasm of endothelial cells, where it existed in complex with p38 (effective dissociation constant, K(D)(eff), values of 294 and 197 nM, respectively), whereas DUSP5 was localized in the nucleus of endothelial cells in complex with ERK1/2 (K(D)(eff) 345 nM). VEGF administration affected differentially the K(D)(eff) values of the DUSP1/p38 and DUSP5/ERK1/2 complexes. Gain-of-function and lack-of-function approaches revealed that DUSP1/MKP-1 dephosphorylates primarily VEGF-phosphorylated p38, thereby inhibiting endothelial cell migration, whereas DUSP5 dephosphorylates VEGF-phosphorylated ERK1/2 inhibiting proliferation of endothelial cells. Moreover, DUSP5 exhibited considerable nuclear anchoring activity on ERK1/2 in the nucleus, thereby diminishing ERK1/2 export to the cytoplasm decreasing its further availability for activation.

Anthrax lethal toxin impairs IL-8 expression in epithelial cells through inhibition of histone H3 modification

Lethal toxin (LT) is a critical virulence factor of Bacillus anthracis, the etiological agent of anthrax, whose pulmonary form is fatal in the absence of treatment. Inflammatory response is a key process of host defense against invading pathogens. We report here that intranasal instillation of a B. anthracis strain bearing inactive LT stimulates cytokine production and polymorphonuclear (PMN) neutrophils recruitment in lungs. These responses are repressed by a prior instillation of an LT preparation. In contrast, instillation of a B. anthracis strain expressing active LT represses lung inflammation. The inhibitory effects of LT on cytokine production are also observed in vitro using mouse and human pulmonary epithelial cells. These effects are associated with an alteration of ERK and p38-MAPK phosphorylation, but not JNK phosphorylation. We demonstrate that although NF-κB is essential for IL-8 expression, LT downregulates this expression without interfering with NF-κB activation in epithelial cells. Histone modifications are known to induce chromatin remodelling, thereby enhancing NF-κB binding on promoters of a subset of genes involved in immune response. We show that LT selectively prevents histone H3 phosphorylation at Ser 10 and recruitment of the p65 subunit of NF-κB at the IL-8 and KC promoters. Our results suggest that B. anthracis represses the immune response, in part by altering chromatin accessibility of IL-8 promoter to NF-κB in epithelial cells. This epigenetic reprogramming, in addition to previously reported effects of LT, may represent an efficient strategy used by B. anthracis for invading the host.

Bacillus anthracis, the etiological agent of anthrax, can infect mammals either accidentally or as a potential consequence of a terrorism threat. Pulmonary infection is a life-threatening form of the disease, causing a near 100% mortality rate in the absence of appropriate therapy. Thus, it is important to understand the mechanisms of host defense against B. anthracis. We examined the effects of various B. anthracis strains on lung inflammation in a mouse model of pulmonary anthrax and on human lung epithelial cells, the first barrier of lung against invading pathogens. We showed that a B. anthracis strain expressing lethal toxin inhibits inflammation. In contrast, a strain in which this toxin has been inactivated induces lung inflammation. We next examined the mechanisms involved in the inhibitory effect of lethal toxin. We showed that B. anthracis injects lethal toxin into epithelial cells, blocks the molecules associated on the chromosome, and thus represses production of mediators involved in inflammation. As the latter is a key process in host defense, its alteration by lethal toxin predisposes the host to infection by B. anthracis. This effect on the chromosomal machinery may represent an efficient strategy used by B. anthracis for invading the host.

Gonadotropin-releasing hormone and protein kinase C signaling to ERK: spatiotemporal regulation of ERK by docking domains and dual-specificity phosphatases

Activated ERK translocates to the nucleus to regulate transcription. Spatiotemporal aspects of this response dictate biological consequences and are influenced by dual-specificity phosphatases (DUSPs) that can scaffold and dephosphorylate ERK. In HeLa cells, GnRH causes transient and protein kinase C (PKC)-dependent ERK activation, but termination mechanisms are unknown. We now explore DUSP roles using short inhibitory RNA to knock down endogenous ERK, adenoviruses to express GnRH receptors and add-back ERK2-GFP, and automated microscopy to monitor ERK location and activation. GnRH caused rapid and transient increases in dual phosphorylated ERK2 (ppERK2) and nuclear to cytoplasmic ERK2-green fluorescent protein (GFP) ratio, whereas responses to a PKC-activating phorbol ester were more sustained. In cells expressing D319N ERK2-GFP (D319N mutation impairs docking-domain-dependent binding to DUSPs), GnRH caused more sustained increases in ppERK2 and nuclear to cytoplasmic ERK2-GFP ratio and also had more pronounced effects on Egr-1 luciferase (a transcriptional reporter for ERK activation). Cycloheximide caused more sustained effects of GnRH and phorbol ester on ppERK, suggesting termination by nuclear-inducible DUSPs. GnRH also increased expression of nuclear-inducible DUSP1 and -4, but their knockdown did not alter GnRH-mediated ERK signaling. Screening a short inhibitory RNA library targeting 16 DUSPs (nuclear-inducible DUSPs, cytoplasmic ERK MAPK phosphatases, c-Jun N-terminal kinase/p38 MAPK phosphatases, and atypical DUSPs) revealed GnRH effects to be influenced by DUSPs 5, 9, 10, 16, and 3 (i.e. by each DUSP class). Thus, GnRH-mediated ERK responses (like PKC-mediated ERK responses) are dependent on protein neosynthesis and docking-domain-dependent binding, but for GnRH activation (unlike PKC activation), this does not reflect dependence on nuclear-inducible DUSPs. Termination of these GnRH effects is apparently dependent upon a preexisting rapid turnover protein.

The N-terminal domain of ERK1 accounts for the functional differences with ERK2

The Extracellular Regulated Kinase 1 and 2 transduce a variety of extracellular stimuli regulating processes as diverse as proliferation, differentiation and synaptic plasticity. Once activated in the cytoplasm, ERK1 and ERK2 translocate into the nucleus and interact with nuclear substrates to induce specific programs of gene expression. ERK1/2 share 85% of aminoacid identity and all known functional domains and thence they have been considered functionally equivalent until recent studies found that the ablation of either ERK1 or ERK2 causes dramatically different phenotypes. To search a molecular justification of this dichotomy we investigated whether the different functions of ERK1 and 2 might depend on the properties of their cytoplasmic-nuclear trafficking. Since in the nucleus ERK1/2 is predominantly inactivated, the maintenance of a constant level of nuclear activity requires continuous shuttling of activated protein from the cytoplasm. For this reason, different nuclear-cytoplasmic trafficking of ERK1 and 2 would cause a differential signalling capability. We have characterised the trafficking of fluorescently tagged ERK1 and ERK2 by means of time-lapse imaging in living cells. Surprisingly, we found that ERK1 shuttles between the nucleus and cytoplasm at a much slower rate than ERK2. This difference is caused by a domain of ERK1 located at its N-terminus since the progressive deletion of these residues converted the shuttling features of ERK1 into those of ERK2. Conversely, the fusion of this ERK1 sequence at the N-terminus of ERK2 slowed down its shuttling to a similar value found for ERK1. Finally, computational, biochemical and cellular studies indicated that the reduced nuclear shuttling of ERK1 causes a strong reduction of its nuclear phosphorylation compared to ERK2, leading to a reduced capability of ERK1 to carry proliferative signals to the nucleus. This mechanism significantly contributes to the differential ability of ERK1 and 2 to generate an overall signalling output.

The expression of estrogen receptor isoforms alpha, beta and insulin-like growth factor-I in uterine leiomyoma

BACKGROUND

A prospective study focusing on leiomyoma was conducted to investigate the expression of estrogen receptor-alpha (ER alpha), estrogen receptor-beta (ER beta) and insulin-like growth factor-I (IGF-I) to elucidate their roles in the pathogenesis of leiomyoma.

METHODS

Twenty-one leiomyomal and six paired myometrial tissues were included in the study. The levels of ER alpha, ER beta and IGF-I mRNAs in leiomyoma and adjacent myometrium were analyzed by quantitative reverse transcription-polymerase chain reaction.

RESULTS

All tissues showed a higher ER alpha expression level. The correlation between ER alpha mRNA and IGF-I mRNA levels was significantly positive (r = 0.527, p = 0.014). The correlation between ER beta mRNA and IGF-I mRNA levels was also positive, but not statistically significant. In six paired leiomyoma and myometrium samples, the levels of ER alpha and ER beta mRNA exhibited no difference, and IGF-I mRNA levels were elevated in leiomyoma compared with paired myometrium (p < 0.05).

CONCLUSIONS

The expression of IGF-I mRNA increases in leiomyoma and ER alpha mRNA is positively correlated with IGF-I mRNA, which implies that estrogen upregulates the gene encoding IGF-I through ER alpha in leiomyoma; ER beta may also be involved in the IGF-I signaling pathway in leiomyoma.

Systems analysis of MAPK signal transduction

For more than a decade, the MAPK (mitogen-activated protein kinase) cascade has been studied using mathematical modelling and quantitative experimentation [1]. The MAPK cascade relays the presence of extracellular stimuli such as growth hormones to the nucleus and controls the expression of hundreds of genes. MAPKs control major cell fate decisions such as proliferation, differentiation and apoptosis, mainly by inducing alterations in gene expression. In this chapter, we discuss how systems biology analysis provides insights into the functioning of this cascade. We show how this pathway assists the cell in responding properly to extracellular cues by filtering out sub-threshold stimuli, while efficiently transmitting physiologically relevant inputs. Several different receptors signal through the MAPK pathway even though they elicit opposite biological responses, thus raising the question of how specificity is achieved in MAPK signalling. Experimental studies revealed that specific biological responses are encoded by quantitative aspects of the MAPK signal such as amplitude or duration. We discuss mechanisms that enable the pathway to generate quantitatively different signals, and also explain how different signals are interpreted by the downstream gene expression machinery.

Spatiotemporal regulation of ERK2 by dual specificity phosphatases

Although many stimuli activate extracellular signal-regulated kinases 1 and 2 (ERK1/2), the kinetics and compartmentalization of ERK1/2 signals are stimulus-dependent and dictate physiological consequences. ERKs can be inactivated by dual specificity phosphatases (DUSPs), notably the MAPK phosphatases (MKPs) and atypical DUSPs, that can both dephosphorylate and scaffold ERK1/2. Using a cell imaging model (based on knockdown of endogenous ERKs and add-back of wild-type or mutated ERK2-GFP reporters), we explored possible effects of DUSPs on responses to transient or sustained ERK2 activators (epidermal growth factor and phorbol 12,13-dibutyrate, respectively). For both stimuli, a D319N mutation (which impairs DUSP binding) increased ERK2 activity and reduced nuclear accumulation. These stimuli also increased mRNA levels for eight DUSPs. In a short inhibitory RNA screen, 12 of 16 DUSPs influenced ERK2 responses. These effects were evident among nuclear inducible MKP, cytoplasmic ERK MKP, JNK/p38 MKP, and atypical DUSP subtypes and, with the exception of the nuclear inducible MKPs, were paralleled by corresponding changes in Egr-1 luciferase activation. Simultaneous removal of all JNK/p38 MKPs or nuclear inducible MKPs revealed them as positive and negative regulators of ERK2 signaling, respectively. The effects of JNK/p38 MKP short inhibitory RNAs were not dependent on protein neosynthesis but were reversed in the presence of JNK and p38 kinase inhibitors, indicating DUSP-mediated cross-talk between MAPK pathways. Overall, our data reveal that a large number of DUSPs influence ERK2 signaling. Together with the known tissue-specific expression of DUSPs and the importance of ERK1/2 in cell regulation, our data support the potential value of DUSPs as targets for drug therapy.

Epidermal growth factor receptor and protein kinase C signaling to ERK2: spatiotemporal regulation of ERK2 by dual specificity phosphatases

Spatiotemporal aspects of ERK activation are stimulus-specific and dictate cellular consequences. They are dependent upon dual specificity phosphatases (DUSPs) that bind ERK via docking domains and can both inactivate and anchor ERK in cellular compartments. Using high throughput fluorescence microscopy in combination with a system where endogenous ERKs are removed and replaced with wild-type or mutated ERK2-green fluorescent protein (GFP), we show that ERK2 activation responses to epidermal growth factor (EGF) and protein kinase C (PKC) are transient and sustained, respectively. PKC-mediated ERK2 activation is associated with prolonged nuclear localization in the dephosphorylated form, whereas EGF-stimulated ERK2 activation mediates only transient nuclear accumulation. By using short inhibitory RNAs to nuclear inducible DUSP1, -2, or -4 (alone or in combination), we demonstrate that all three of these enzymes contribute to the dephosphorylation of PKC (but not EGF)-activated ERK2 in the nucleus but that they have opposing effects on localization. DUSP2 and -4 inactivate and anchor ERK2, whereas DUSP1 dephosphorylates ERK in the nucleus but allows its traffic back to the cytoplasm. Overexpression of DUSP1, -2, or -4 prevented ERK2 activation, but only DUSP2 and -4 caused ERK2-GFP nuclear accumulation or could be immunoprecipitated with ERK2. Furthermore, protein synthesis inhibition or replacement of wild-type ERK2-GFP with docking domain mutants selectively increased PKC effects on ERK activity and altered ERK2-GFP localization. These mutations also impaired the ability of ERK2-GFP to bind DUSP2 and -4. Together, our data reveal a novel, stimulus-specific, and phosphatase-specific mechanism of ERK2 regulation in the nucleus by DUSP1, -2, and -4.

Endosulfan decreases cell growth and apoptosis in human HaCaT keratinocytes: partial ROS-dependent ERK1/2 mechanism

Endosulfan is an organochlorine insecticide described as a potential carcinogen in humans. This insecticide was recently reported to alter the mitogen-activated protein (MAP) kinase signaling pathways and is suspected to affect cell growth and differentiation in human keratinocytes. This study was designed to assess the mitogenic, apoptogenic, and genotoxic effects of endosulfan on the HaCaT cell line. We first found that 25 microM endosulfan led to persistent extracellular signal-regulated kinase (ERK)1/2 phosphorylation with an accumulation of the phosphorylated form in the nucleus, probably caused by MAP kinase phosphatase (MKP) inhibition. As previously described under sustained ERK1/2 activation, cell growth was decreased: delayed confluency and 35% decrease of BrdU incorporation was demonstrated in endosulfan-treated keratinocytes. In addition, endosulfan has been shown to generate transient reactive oxygen species (ROS), and blocking this oxidative stress by N-acetyl cysteine (NAC) strongly prevented both persistent nuclear ERK1/2 phosphorylation and cell growth decrease. Additional experiments demonstrated that unchanged endosulfan rather than its metabolites has mutagenic effects (Ames positive without S9) and increased DNA strand breaks (Comet assay) in HaCaT cells, via a ROS-dependent mechanism. Therefore, to assess the putative pro-apoptotic response of damaged cells, caspases 3/7 activity and poly(ADP-ribose)-polymerase (PARP) cleavage were measured. The results clearly indicated that endosulfan inhibited both spontaneous and staurosporine-induced apoptosis. Taken together, these findings strongly support that endosulfan induces ROS generation leading to sustained ERK1/2 phosphorylation and decrease in cell growth. Moreover, endosulfan was found to inhibit apoptosis and this could contribute to mutant cell survival and therefore have possible carcinogenic effects.