The dual specificity phosphatases M3/6 and MKP-3 are highly selective for inactivation of distinct mitogen-activated protein kinases

Change of gene expression in intestinal epithelial cells under ischemia and anoxia in rats

AIM: To investigate the change of gene expression in intestinal epithelial cells (IECs) under ischemia and anoxia in rats and to search for the correlated genes with IECs injury.

METHODS: IECs injury model was induced by ischemia and anoxia, and the cells were divided into four groups: control group, ischemia group, anoxia group and ischemia and anoxia (I/A) group. Fluorescence reverse transcription was used to label mRNA. The cDNA microarray was used to detect the difference between gene expression of control group and experiment groups.

RESULTS: There were 207 genes differently expressed between control and ischemia group, of which 132 were down-regulated while 75 were up-regulated. One hundred and sixty-eight genes were differently expressed between control and anoxia group, of which 84 were down-regulated while 84 were up-regulated. Between control and I/A group, 321 genes were differently expressed, of which 97 were down-regulated while 224 were up-regulated.

CONCLUSION: cDNA microarray can be used to screen diversified gene expression related to injury under ischemia and anoxia, which brings some new clues for studying the mechanism of IECs injury.

Activation and signaling of the p38 MAP kinase pathway

The family members of the mitogen-activated protein (MAP) kinases mediate a wide variety of cellular behaviors in response to extracellular stimuli. One of the four main sub-groups, the p38 group of MAP kinases, serve as a nexus for signal transduction and play a vital role in numerous biological processes. In this review, we highlight the known characteristics and components of the p38 pathway along with the mechanism and consequences of p38 activation. We focus on the role of p38 as a signal transduction mediator and examine the evidence linking p38 to inflammation, cell cycle, cell death, development, cell differentiation, senescence and tumorigenesis in specific cell types. Upstream and downstream components of p38 are described and questions remaining to be answered are posed. Finally, we propose several directions for future research on p38.

Dynamic interaction between the dual specificity phosphatase MKP7 and the JNK3 scaffold protein beta-arrestin 2

JNK scaffold proteins bind JNK and upstream kinases to activate subsets of JNK and localize activated JNK to specific subcellular sites. We previously demonstrated that the dual specificity phosphatases (DSPs) MKP7 and M3/6 bind the scaffold JNK-interacting protein-1 (JIP-1) and inactivate the bound subset of JNK (1). The G protein-coupled receptor (GPCR) adaptor beta-arrestin 2 is also a JNK3 scaffold. It binds the upstream kinases ASK1 and MKK4 and couples stimulation of the angiotensin II receptor AT1aR to activation of a cytoplasmic pool of JNK3. Here we report that MKP7 also binds beta-arrestin 2 via amino acids 394-443 of MKP7, the same region that interacts with JIP-1. This region of MKP7 interacts with beta-arrestin 2 at a central region near the JNK binding domain. MKP7 dephosphorylates JNK3 bound to beta-arrestin 2, either following activation by ASK1 overexpression or following AT1aR stimulation. Initial AT1aR stimulation causes a rapid (within 5 min) dissociation of MKP7 from beta-arrestin 2. MKP7 then reassociates with beta-arrestin 2 on endocytic vesicles 30-60 min after initial receptor stimulation. This dynamic interaction between phosphatase and scaffold permits signal transduction through a module that binds both positive and negative regulators.

GeneChip analyses of global transcriptional responses of murine macrophages to the lethal toxin of Bacillus anthracis

We performed GeneChip analyses on RNA from Bacillus anthracis lethal toxin (LeTx)-treated RAW 264.7 murine macrophages to investigate global effects of anthrax toxin on host cell gene expression. Stringent analysis of data revealed that the expression of several mitogen-activated protein kinase kinase-regulatory genes was affected within 1.5 h post-exposure to LeTx. By 3.0 h, the expression of 103 genes was altered, including those involved in intracellular signaling, energy production, and protein metabolism.

Extracellular signal-regulated kinases phosphorylate mitogen-activated protein kinase phosphatase 3/DUSP6 at serines 159 and 197, two sites critical for its proteasomal degradation

Mitogen-activated protein (MAP) kinase phosphatases (MKPs) are dual-specificity phosphatases that dephosphorylate phosphothreonine and phosphotyrosine residues within MAP kinases. Here, we describe a novel posttranslational mechanism for regulating MKP-3/Pyst1/DUSP6, a member of the MKP family that is highly specific for extracellular signal-regulated kinase 1 and 2 (ERK1/2) inactivation. Using a fibroblast model in which the expression of either MKP-3 or a more stable MKP-3-green fluorescent protein (GFP) chimera was induced by tetracycline, we found that serum induces the phosphorylation of MKP-3 and its subsequent degradation by the proteasome in a MEK1 and MEK2 (MEK1/2)-ERK1/2-dependent manner. In vitro phosphorylation assays using glutathione S-transferase (GST)-MKP-3 fusion proteins indicated that ERK2 could phosphorylate MKP-3 on serines 159 and 197. Tetracycline-inducible cell clones expressing either single or double serine mutants of MKP-3 or MKP-3-GFP confirmed that these two sites are targeted by the MEK1/2-ERK1/2 module in vivo. Double serine mutants of MKP-3 or MKP-3-GFP were more efficiently protected from degradation than single mutants or wild-type MKP-3, indicating that phosphorylation of either serine by ERK1/2 enhances proteasomal degradation of MKP-3. Hence, double mutation caused a threefold increase in the half-life of MKP-3. Finally, we show that the phosphorylation of MKP-3 has no effect on its catalytic activity. Thus, ERK1/2 exert a positive feedback loop on their own activity by promoting the degradation of MKP-3, one of their major inactivators in the cytosol, a situation opposite to that described for the nuclear phosphatase MKP-1.

Feedback interactions between MKP3 and ERK MAP kinase control scleraxis expression and the specification of rib progenitors in the developing chick somite

Cells in the early vertebrate somite receive cues from surrounding tissues,which are important for their specification. A number of signalling pathways involved in somite patterning have been described extensively. By contrast,the interactions between cells from different regions within the somite are less well characterised. Here, we demonstrate that myotomally derived FGFs act through the MAPK signal transduction cascade and in particular, ERK1/2 to activate scleraxis expression in a population of mesenchymal progenitor cells in the dorsal sclerotome. We show that the levels of active,phosphorylated ERK protein in the developing somite are crucial for the expression of scleraxis and Mkp3. MKP3 is a dual specificity phosphatase and a specific antagonist of ERK MAP kinases and we demonstrate that in somites Mkp3 transcription depends on the presence of active ERK. Therefore, MKP3 and ERK MAP kinase constitute a negative feedback loop activated by FGF in sclerotomal progenitor cells. We propose that tight control of ERK signalling strength by MKP3 is important for the appropriate regulation of downstream cellular responses including the activation of scleraxis. We show that increased or decreased levels of phosphorylated ERK result in the loss of scleraxis transcripts and the loss of distal rib development, highlighting the importance of the MKP3-ERK-MAP kinase mediated feedback loop for cell specification and differentiation.

Specific inactivation and nuclear anchoring of extracellular signal-regulated kinase 2 by the inducible dual-specificity protein phosphatase DUSP5

The mechanisms which determine the nuclear accumulation and inactivation of the extracellular signal-regulated kinase 1 (ERK1) or ERK2 mitogen-activated protein (MAP) kinases are poorly understood. Here we demonstrate that DUSP5, an inducible nuclear phosphatase, interacts specifically with ERK2 via a kinase interaction motif (KIM) within its amino-terminal noncatalytic domain. This binding determines the substrate specificity of DUSP5 in vivo, as it inactivates ERK2 but not Jun N-terminal protein kinase or p38 MAP kinase. Using green fluorescent protein fusions, we identify within this same domain of DUSP5 a functional nuclear localization signal (NLS) which functions independently of the KIM. Moreover, we demonstrate that the expression of DUSP5 causes both nuclear translocation and sequestration of inactive ERK2. Nuclear anchoring is ERK2 specific and requires both interactions between the DUSP5 KIM and the common docking site of ERK2 and a functional NLS within DUSP5. Finally, the expression of a catalytically inactive mutant of DUSP5 also tethers ERK2 within the nucleus. Furthermore, this nuclear ERK2 is phosphorylated by MAP kinase kinase in response to growth factors and also activates transcription factor Elk-1. We conclude that DUSP5 is an inducible nuclear ERK-specific MAP kinase phosphatase that functions as both an inactivator of and a nuclear anchor for ERK2 in mammalian cells. In addition, our data indicate that the cytoplasm may not be an exclusive site of MAP kinase activation.

The p38 MAP kinase pathway and its biological function

p38 is a mitogen-activated protein (MAP) kinase with structural and functional characteristics that distinguish it from JNK and ERK MAP kinases. p38 activity is upregulated when cells are exposed to a variety of stimuli including bacterial pathogens, proinflammatory cytokines, certain growth factors, and other forms of environmental stress. By regulating downstream substrates that include protein kinases and transcription factors, p38 participates in transmission, amplification, and diversification of the extracellular signal, initiating several different cellular responses. Studies have revealed that activation of p38 pathway is related to many pathological changes that occur in the course of inflammatory/immunologic and cardiovascular diseases.

Mkp3 is a negative feedback modulator of Fgf8 signaling in the mammalian isthmic organizer

The pivotal mechanisms that govern the correct patterning and regionalization of the distinct areas of the mammalian CNS are driven by key molecules that emanate from the so-called secondary organizers at neural plate and tube stages. FGF8 is the candidate morphogenetic molecule to pattern the mesencephalon and rhombencephalon in the isthmic organizer (IsO). Recognizable relevance has been given to the intracellular pathways by which Fgf8 is regulated and modulated. In chick limb bud development, a dual mitogen-activated protein kinase phosphatase-3 (Mkp3) plays a role as a negative feedback modulator of Fgf8 signaling. We have investigated the role of Mkp3 and its functional relationship with the Fgf8 signaling pathway in the mouse IsO using gene transfer microelectroporation assays and protein-soaked bead experiments. Here, we demonstrate that MKP3 has a negative feedback action on the MAPK/ERK-mediated FGF8 pathway in the mouse neuroepithelium.

Matrix metalloproteinase secretion by gastric epithelial cells is regulated by E prostaglandins and MAPKs

Because matrix metalloproteinases (MMPs) play roles in inflammatory tissue injury, we asked whether MMP secretion by gastric epithelial cells may contribute to gastric injury in response to signals involved in Helicobacter pylori-induced inflammation and/or cyclooxygenase inhibition. Tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and epidermal growth factor (EGF) stimulated gastric cell MMP-1 secretion, indicating that MMP-1 secretion occurs in inflammatory as well as non-inflammatory situations. MMP-1 secretion required activation of the MAPK Erk and subsequent protein synthesis but was down-regulated by the alternate MAPK, p38. In contrast, secretion of MMP-13 was stimulated by TNF-alpha/IL-1beta but not EGF and was Erk-independent and mediated by p38. MMP-13 secretion was more rapid (peak, 6 h) than MMP-1 (peak > or =30 h) and only partly depended on protein synthesis, suggesting initial release of a pre-existing MMP-13 pool. Therefore, MMP-1 and MMP-13 secretion are differentially regulated by MAPKs. MMP-1 secretion was regulated by E prostaglandins (PGEs) in an Erk-dependent manner. PGEs enhanced Erk activation and MMP-1 secretion in response to EGF but inhibited Erk and MMP-1 when TNF-alpha and IL-1beta were the stimuli, indicating that the effects of PGEs on gastric cell responses are context-dependent. These data show that secretion of MMPs is differentially regulated by MAPKs and suggest mechanisms through which H. pylori infection and/or cyclooxygenase inhibition may induce epithelial cell signaling to contribute to gastric ulcerogenesis.

Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Intracellular expression profiling by laser capture microdissection: three novel components of the neuromuscular junction

The neuromuscular junction (NMJ) is a regionally specialized area of myofibers defined, in part, by specific gene expression from underlying myonuclei. We sought to obtain a more complete picture of the mRNA transcripts and proteins playing a role in NMJ formation and maintenance using laser capture microdissection (LCM) and to define expression profiles of the nuclear domain at the NMJ. NMJs (800) were isolated from normal mouse tibialis anterior muscle by LCM, with an equal amount of adjacent non-NMJ regions isolated. Many known components of the NMJ were found significantly differentially expressed. Three differentially expressed potential novel components of the NMJ were chosen for further study, and each was validated by immunostaining with and without blocking peptides (3/3), quantitative RT-PCR (3/3), and in situ hybridization (1/3). The three genes validated were dual-specificity phosphatase-6 (DUSP6), ribosomal receptor-binding protein-1 (RRBP1), and vacuolar protein sorting-26 (VPS26). Query of each of these novel components in a 27-time point in vivo muscle regeneration series showed expression commensurate with previously known NMJ markers (nestin, alpha-ACh receptor). Understanding and discovering elements responsible for the integrity and function of NMJs is relevant to understanding neuromuscular diseases such as spinal muscular atrophy. Our LCM-based mRNA expression profiling provided us with new means of identification of specific genes potentially responsible for NMJ stability and function and new candidates for involvement in disease pathogenesis.

An oncogenic KRAS2 expression signature identified by cross-species gene-expression analysis

Using advanced gene targeting methods, generating mouse models of cancer that accurately reproduce the genetic alterations present in human tumors is now relatively straightforward. The challenge is to determine to what extent such models faithfully mimic human disease with respect to the underlying molecular mechanisms that accompany tumor progression. Here we describe a method for comparing mouse models of cancer with human tumors using gene-expression profiling. We applied this method to the analysis of a model of Kras2-mediated lung cancer and found a good relationship to human lung adenocarcinoma, thereby validating the model. Furthermore, we found that whereas a gene-expression signature of KRAS2 activation was not identifiable when analyzing human tumors with known KRAS2 mutation status alone, integrating mouse and human data uncovered a gene-expression signature of KRAS2 mutation in human lung cancer. We confirmed the importance of this signature by gene-expression analysis of short hairpin RNA-mediated inhibition of oncogenic Kras2. These experiments identified both a pattern of gene expression indicative of KRAS2 mutation and potential effectors of oncogenic KRAS2 activity in human cancer. This approach provides a strategy for using genomic analysis of animal models to probe human disease.

Structure and regulation of MAPK phosphatases

MAP kinases (MAPKs), which control mitogenic signal transduction in all eukaryotic organisms, are inactivated by dual specificity MAPK phosphatases (DS-MKPs). Recent studies reveal that substrate specificity and enzymatic activity of MKPs are tightly controlled not only by the conserved C-terminal phosphatase domain but also by an N-terminal (NT) kinase-binding domain. Notably, MKPs that consist of a kinase-binding domain and a phosphatase domain exhibit little phosphatase activity in the absence of their physiological substrates. MKP binding to a specific MAPK results in enzymatic activation of the phosphatase in a substrate-induced activation mechanism. This direct coupling of inactivation of an MAPK to activation of an MKP provides a tightly controlled regulation that enables these two key enzymes to keep each other in check, thus guaranteeing the fidelity of signal transduction. This review discusses the recent understanding of structure and regulation of the large family of dual specificity MKPs, which can be divided into four subgroups according to their functional domains and mechanism of substrate recognition and enzymatic regulation. Moreover, detailed comparison of the structural basis between this unique substrate-induced activation mechanism and the common auto-inhibition mechanism is provided.

Reversible oxidation of ERK-directed protein phosphatases drives oxidative toxicity in neurons

Oxidative stress links diverse neuropathological conditions that include stroke, Parkinson's disease, and Alzheimer's disease and has been modeled in vitro with various paradigms that lead to neuronal cell death following the increased accumulation of reactive oxygen species. For example, immortalized neurons and immature primary cortical neurons undergo cell death in response to depletion of the antioxidant glutathione, which can be elicited by administration of glutamate at high concentrations. We have demonstrated previously that this glutamate-induced oxidative toxicity requires activation of the mitogen-activated protein kinase member ERK1/2, but the mechanisms by which this activation takes place in oxidatively stressed neurons are still not fully known. In this study, we demonstrate that during oxidative stress, ERK-directed phosphatases of both the serine/threonine- and tyrosine-directed classes are selectively and reversibly inhibited via a mechanism that is dependent upon the oxidation of cysteine thiols. Furthermore, the impact of ERK-directed phosphatases on ERK1/2 activation and oxidative toxicity in neurons was tested in a neuronal cell line and in primary cortical cultures. Overexpression of the highly ERK-specific phosphatase MKP3 and its catalytic mutant, MKP3 C293S, were neuroprotective in transiently transfected HT22 cells and primary neurons. The neuroprotective effect of the MKP3 C293S mutant, which enhances ERK1/2 phosphorylation but blocks its nuclear translocation, demonstrates the necessity for active ERK1/2 nuclear localization for oxidative toxicity in neurons. Together, these data implicate the inhibition of endogenous ERK-directed phosphatases as a mechanism that leads to aberrant ERK1/2 activation and nuclear accumulation during oxidative toxicity in neurons.

A role for ERKII in synaptic pattern selectivity on the time-scale of minutes

Stimulus reinforcement strengthens learning. Intervals between reinforcement affect both the kind of learning that occurs and the amount of learning. Stimuli spaced by a few minutes result in more effective learning than when massed together. There are several synaptic correlates of repeated stimuli, such as different kinds of plasticity and the amplitude of synaptic change. Here we study the role of signalling pathways in the synapse on this selectivity for spaced stimuli. Using the in vitro hippocampal slice technique we monitored long-term potentiation (LTP) amplitude in CA1 for repeated 100-Hz, 1-s tetani. We observe the highest LTP levels when the inter-tetanus interval is 5-10 min. We tested biochemical activity in the slice following the same stimuli, and found that extracellular signal-regulated kinase type II (ERKII) but not CaMKII exhibits a peak at about 10 min. When calcium influx into the slice is buffered using AM-ester calcium dyes, amplitude of the physiological and biochemical response is reduced, but the timing is not shifted. We have previously used computer simulations of synaptic signalling to predict such temporal tuning from signalling pathways. In the current study we consider feedback and feedforward models that exhibit temporal tuning consistent with our experiments. We find that a model incorporating post-stimulus build-up of PKM zeta acting upstream of mitogen-activated protein kinase is sufficient to explain the observed temporal tuning. On the basis of these combined experimental and modelling results we propose that the dynamics of PKM activation and ERKII signalling may provide a mechanism for functionally important forms of synaptic pattern selectivity.

Both nuclear-cytoplasmic shuttling of the dual specificity phosphatase MKP-3 and its ability to anchor MAP kinase in the cytoplasm are mediated by a conserved nuclear export signal

MAP kinase phosphatase (MKP)-3 is a cytoplasmic dual specificity protein phosphatase that specifically binds to and inactivates the ERK1/2 MAP kinases in mammalian cells. However, the molecular basis of the cytoplasmic localization of MKP-3 or its physiological significance is unknown. We have used MKP-3-green fluorescent protein fusions in conjunction with leptomycin B to show that the cytoplasmic localization of MKP-3 is mediated by a chromosome region maintenance-1 (CRM1)-dependent nuclear export pathway. Furthermore, the nuclear translocation of MKP-3 seen in the presence of leptomycin B is mediated by an active process, indicating that MKP-3 shuttles between the nucleus and cytoplasm. The amino-terminal noncatalytic domain of MKP-3 is both necessary and sufficient for nuclear export of the phosphatase and contains a single functional leucine-rich nuclear export signal (NES). Even though this domain of the protein also mediates the binding of MKP-3 to MAP kinase, we show that mutations of the kinase interaction motif which abrogate ERK2 binding do not affect MKP-3 localization. Conversely, mutation of the NES does not affect either the binding or phosphatase activity of MKP-3 toward ERK2, indicating that the kinase interaction motif and NES function independently. Finally, we demonstrate that the ability of MKP-3 to cause the cytoplasmic retention of ERK2 requires both a functional kinase interaction motif and NES. We conclude that in addition to its established function in the regulated dephosphorylation and inactivation of MAP kinase, MKP-3 may also play a role in determining the subcellular localization of its substrate. Our results reinforce the idea that regulatory proteins such as MKP-3 may play a key role in the spatio-temporal regulation of MAP kinase activity.

Regulation of c-Jun N-terminal Kinase and p38 Kinase Pathways in Endothelial Cells

The rapid and transient induction of E-selectin gene expression by inflammatory tumor necrosis factor (TNF)-alpha in endothelial cells is mediated by signaling pathways which involve c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) kinase pathways. To explore this regulation, we first observed that in the continuous presence of cytokine TNF, activation of JNK-1 in both nuclear and cytoplasmic compartments peaked at 15-30 min, with activity returning to uninduced levels by 60 min. Phosphorylation of both the p38 kinase and its molecular target, the nuclear transcription factor, activating transcription factor-2, were transient after TNF-alpha or interleukin (IL)-1beta induction. However, cycloheximide treatment prolonged the TNF-alpha-induced JNK-1 kinase activity beyond 60 min, suggesting that protein synthesis is required to limit this signaling cascade. We investigated the possible role of the dual-specificity phosphatases MAPK phosphatase (MKP)-1 and MKP-2 in limiting cytokine-induced MAPK signaling. Maximum induction of MKP-1 mRNA and nuclear protein levels by TNF-alpha or IL-1beta were noted at 60 min and their expression correlated with the termination of JNK kinase activity, whereas nuclear levels of MKP-2 were not significantly affected by treatment with TNF-alpha or IL-1beta. Transient overexpression of MKP-1 demonstrated significant specific inhibition of E-selectin promoter activity consistent with a regulatory role for dual-specificity phosphatases. Inhibition of MKP-1 expression through the use of small interfering RNAs prolonged the cytokine-induced p38 and JNK kinase phosphorylation. Our results suggest that endogenous inhibitors of the MAPK cascade, such as the dual-specificity phosphatases like MKP-1 may be important for the postinduction repression of MAPK activity and E-selectin transcription in endothelial cells. Thus, these inhibitors may play an important role in limiting the inflammatory effects of TNF-alpha and IL-1beta.

Mitogen-activated Protein Kinase Phosphatase-3 Is a Tumor Promoter Target in Initiated Cells That Express Oncogenic Ras

We have capitalized on the unique properties of the skin tumor promoter palytoxin, which does not activate protein kinase C, to investigate alternative mechanisms by which major signaling molecules can be modulated during carcinogenesis. We report here that palytoxin activates extracellular signal-regulated kinase (ERK) through a novel mechanism that involves inactivation of an ERK phosphatase in keratinocytes derived from initiated mouse skin (308 cells). Use of U0126 revealed that palytoxin requires the ERK kinase MEK to stimulate ERK activity, although palytoxin did not activate MEK. We found that 308 keratinocytes highly express mitogen-activated protein kinase phosphatase-3 (MKP-3), which selectively inactivates ERK. Palytoxin induced the loss of MKP-3 in a manner that corresponded to increased ERK phosphorylation. Complementary studies showed that sustained expression of exogenous MKP-3 inhibited palytoxin-stimulated ERK activation. As is characteristic of initiated keratinocytes, 308 cells express activated H-Ras. To investigate whether expression of oncogenic Ras is key to palytoxin-stimulated ERK activation, we determined how palytoxin affected ERK and MKP-3 in MCF10A human breast epithelial cells and in H-ras MCF10A cells, which stably express activated H-Ras. Palytoxin did not affect ERK activity in MCF10A cells, which had no detectable MKP-3. Like 308 cells, H-ras MCF10A cells highly express MKP-3. Strikingly, palytoxin stimulated ERK activity and induced a corresponding loss of MKP-3 in H-ras MCF10A cells. These studies indicate that in initiated cells palytoxin unleashes ERK activity by down-regulating MKP-3, an ERK inhibitor, and further suggest that MKP-3 may be a vulnerable target in cells that express oncogenic Ras.

MAP kinase phosphatase 3 (MKP3) interacts with and is phosphorylated by protein kinase CK2alpha

Mitogen-activated protein (MAP) kinases play a central role in controlling a wide range of cellular functions following their activation by a variety of extracellular stimuli. MAP kinase phosphatases (MKPs) represent a subfamily of dual specificity phosphatases, which negatively regulate MAP kinases. Although ERK2 activity is regulated by its phosphorylation state, MKP3 is regulated by physical interaction with ERK2, independent of its enzymatic activity (Camps, M., Nichols, A., Gillieron, C., Antonsson, B., Muda, M., Chabert, C., Boschert, U., and Arkinstall, S., (1998) Science 280, 1262-1265; Farooq, A., Chaturvedi, G., Mujtaba, S., Plotnikova, O., Zeng, L., Dhalluin, C., Ashton, R., and Zhou, M. M. (2001), Mol. Cell 7, 387-399; Zhou, B., and Zhang, Z. Y. (1999) J. Biol. Chem. 274, 35526-35534). The interaction of ERK2 and MKP3 allows the reciprocal cross-regulation of their catalytic activity. Indeed, MKP3 acts as a negative regulator on ERK2-MAP kinase signal transduction activity, representing thus a negative feedback for this MAPK pathway. To identify novel proteins able to complex MKP3, we used the yeast two-hybrid system. Here we report that MKP3 and protein kinase CK2 form a protein complex, which can include ERK2. The phosphatase activity of MKP3 is then slightly increased in vitro, whereas in transfected cells, ERK2 dephosphorylation is reduced. In addition, we demonstrated that CK2 selectively phosphorylates MKP3, suggesting cross-regulation between CK2alpha and MKP3, as well as a modulation of ERK2-MAPK signaling by CK2alpha via MKP3.

Toxoplasma gondii interferes with lipopolysaccharide-induced mitogen-activated protein kinase activation by mechanisms distinct from endotoxin tolerance

We show in this study that Toxoplasma gondii infection induces rapid activation of p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase 1/2, and stress-activated protein kinase/c-Jun N-terminal kinase MAPK, followed promptly by their deactivation in mouse macrophages. Nevertheless, when infected cells were subsequently subjected to LPS triggering, MAPK activation was severely defective, in particular in the case of p38 MAPK, which is required for LPS-triggered TNF-alpha and IL-12 production. Similar effects occurred during endotoxin tolerance, but the phenomena were distinct. LPS pretriggering failed to activate the major p38 MAPK kinase, MAPK kinase 3/6. Toxoplasma infection, in contrast, resulted in sustained activation of this kinase. Furthermore, endotoxin pre-exposure blocked IkappaBalpha degradation upon subsequent LPS triggering, but this was not the case for Toxoplasma preinfection. Endotoxin-mediated down-regulation of the LPS receptor, Toll-like receptor 4, has been suggested as one possible mechanism contributing to tolerance, and we found in this study that LPS down-modulated Toll-like receptor 4 expression. In contrast, Toxoplasma infection induced up-regulation of this pattern recognition receptor. Our results show that T. gondii blocks LPS-triggered cytokine production in part through MAPK inactivation, and that this occurs through pathways distinct from endotoxin-induced tolerance.

Inducible expression of a MAP kinase phosphatase-3-GFP chimera specifically blunts fibroblast growth and ras-dependent tumor formation in nude mice

The p42/p44 mitogen activated protein kinase (MAPK) pathway participates in a wide range of cellular programs including proliferation, migration, differentiation, and survival. Specific pharmacological inhibitors, like PD98059 and U0126, are often used to inhibit p42/p44 MAPK signaling. However, these inhibitors are not appropriate to study the function of these kinases in whole organisms. We thus developed an inducible system designed to inhibit p42/p44 MAPK activity through the expression of a phosphatase specific for these two kinases, the MAPK phosphatase 3 (MKP-3). A fibroblast cell line was established in which MKP-3 expression is controlled by tetracycline. Tetracycline-induced MKP-3 resulted in partial de-phosphorylation of p42/p44 MAPKs in serum-stimulated cells. However, we could improve MKP-3 stability and thereby the rate of MAPK de-phosphorylation, when the C-terminal end of MKP-3 was fused to the green fluorescent protein (GFP). Importantly, the fusion of GFP to MKP-3 did not alter the specificity of the phosphatase towards its MAPK substrates. We further show that conditional expression of MKP-3-GFP in this fibroblast cell line results in the inhibition of: (a) the phosphorylation of the p42/p44 MAPK substrates Elk1 and HIF-1alpha, (b) vascular endothelial growth factor (VEGF), cyclin D1, and c-fos gene transcription in response to MAPK pathway activation, and (c) cell proliferation. Finally, the MKP-3-GFP inducible cell line was transformed by Ha-ras and injected into nude mice. Treatment of mice with the tetracycline analog doxycycline resulted in a large delay in tumor emergence and growth as compared to the untreated control group, indicating that MKP-3-GFP activity is maintained in vivo. Altogether, these results show that inducible expression of MKP-3-GFP constitutes a valuable tool to study the role of p42/p44 MAPKs in various cellular responses in both cultured cell and animal models, a tool that may also be used to block unwanted cell growth in pathological conditions.

Nerve growth factor-dependent survival of CESS B cell line is mediated by increased expression and decreased degradation of MAPK phosphatase 1

The sIgG(+) lymphoblastoid B cell line CESS spontaneously produces a high amount of nerve growth factor (NGF) and expresses both high affinity (p140(Trk-A)) and low affinity (p75(NTR)) NGF receptors. Autocrine production of NGF maintains the survival of CESS cells through the continuous deactivation of p38 MAPK, an enzyme able to induce Bcl-2 phosphorylation and subsequent cytochrome c release and caspase activation. In this paper, we show that NGF induces transcriptional activation and synthesis of MAPK phosphatase 1 (MKP-1), a dual specificity phosphatase that dephosphorylates p38 MAPK, thus preventing Bcl-2 phosphorylation. Furthermore, NGF increases MKP-1 protein stability by preventing its degradation through the proteasome pathway. Following NGF stimulation, MKP-1 protein mainly localizes on mitochondria, suggesting an interaction with p38 MAPK in this compartment. Incubation of CESS cells with MKP-1-specific antisense oligonucleotides induces cell death, which was not prevented by exogenous NGF. By contrast, overexpression of native MKP-1, but not of its catalytically impaired form, inhibits apoptosis induced by NGF neutralization in CESS cells. Thus, the molecular mechanisms underlying the survival function of NGF in CESS B cell line predominantly consist in maintaining elevated levels of MKP-1 protein, which controls p38 MAPK activation.

Inflammatory cytokine expression is independent of the c-Jun N-terminal kinase/AP-1 signaling cascade in human neutrophils

In the last decade, the ability of neutrophils to generate proinflammatory cytokines has become firmly established. Because neutrophils typically infiltrate inflammatory sites in large numbers, they could significantly contribute to the cytokine environment and even represent a substantial source of cytokines in chronic inflammatory disorders in which they predominate over other cell types. To date, however, most studies have focused on identifying which mediators are produced by neutrophils, as opposed to elucidating the molecular bases underlying this process. We previously showed that most stimuli of cytokine production in neutrophils also activate NF-kappaB in these cells. In this report, we turned our attention to another transcription factor that plays a central role in inflammation, AP-1. Among Jun/Fos proteins, only JunD and c-Fos are abundantly expressed in neutrophils, and they are mainly cytoplasmic. Both the cellular levels and distribution of the Jun/Fos proteins remain unaffected by various neutrophil stimuli, including those that are known to increase the corresponding mRNA transcripts. Similarly, c-Jun N-terminal kinase (JNK) 1 is overwhelmingly cytoplasmic in neutrophils and does not translocate to the nucleus upon cell activation. Although JNK is not activatable under most circumstances, specific conditions do allow its phosphorylation in response to TNF. However, no experimental condition (even those leading to JNK activation) resulted in the induction of genuine AP-1 complexes in neutrophils. Accordingly, the potent JNK inhibitor, SP 600125, failed to inhibit inflammatory cytokine gene expression in neutrophils. Collectively, our findings strongly suggest that the JNK/AP-1 signaling pathway has little or no impact on the generation of inflammatory mediators in neutrophils.

Induction of apoptosis in human leukemia cells by the tyrosine kinase inhibitor adaphostin proceeds through a RAF-1/MEK/ERK- and AKT-dependent process

Effects of the tyrphostin tyrosine kinase inhibitor adaphostin (NSC 680410) have been examined in human leukemia cells (Jurkat, U937) in relation to mitochondrial events, apoptosis, and perturbations in signaling and cell cycle regulatory events. Exposure of cells to adaphostin concentrations > or =0.75 microM for intervals > or =6 h resulted in a pronounced release of cytochrome c and AIF, activation of caspase-9, -8, and -3, and apoptosis. These events were accompanied by the caspase-independent downregulation of Raf-1, inactivation of MEK1/2, ERK, Akt, p70S6K, dephosphorylation of GSK-3, and activation of c-Jun-N-terminal kinase (JNK) and p38 MAPK. Adaphostin also induced cleavage and dephosphorylation of pRb on CDK2- and CDK4-specific sites, as well as the caspase-dependent downregulation of cyclin D1. Inducible expression of a constitutively active MEK1 construct markedly diminished adaphostin-induced cytochrome c and AIF release, JNK activation, and apoptosis in Jurkat cells. Ectopic expression of Raf-1 or constitutively activated (myristolated) Akt also significantly attenuated adaphostin-induced apoptosis, but protection was less than that conferred by enforced activation of MEK. Lastly, antioxidants (e.g., L-N-acetylcysteine; L-NAC) opposed adaphostin-mediated mitochondrial dysfunction, Raf-1/MEK/ERK downregulation, JNK activation, and apoptosis. However, in contrast to L-NAC, enforced activation of MEK failed to block adaphostin-mediated ROS generation. Together, these findings demonstrate that the tyrphostin adaphostin induces multiple perturbations in signal transduction pathways in human leukemia cells, particularly inactivation of the cytoprotective Raf-1/MEK/ERK and Akt cascades, that culminate in mitochondrial injury, caspase activation, and apoptosis. They also suggest that adaphostin-related oxidative stress acts upstream of perturbations in these signaling pathways to trigger the cell death process.

c-Jun N-terminal kinases (JNK) antagonize cardiac growth through cross-talk with calcineurin-NFAT signaling

The c-Jun N-terminal kinase (JNK) branch of the mitogen-activated protein kinase (MAPK) signaling pathway regulates cellular differentiation, stress responsiveness and apoptosis in multicellular eukaryotic organisms. Here we investigated the functional importance of JNK signaling in regulating differentiated cellular growth in the post-mitotic myocardium. JNK1/2 gene-targeted mice and transgenic mice expressing dominant negative JNK1/2 were determined to have enhanced myocardial growth following stress stimulation or with normal aging. A mechanism underlying this effect was suggested by the observation that JNK directly regulated nuclear factor of activated T-cell (NFAT) activation in culture and in transgenic mice containing an NFAT-dependent luciferase reporter. Moreover, calcineurin Abeta gene targeting abrogated the pro-growth effects associated with JNK inhibition in the heart, while expression of an MKK7-JNK1 fusion protein in the heart partially reduced calcineurin-mediated cardiac hypertrophy. Collectively, these results indicate that JNK signaling antagonizes the differentiated growth response of the myocardium through direct cross-talk with the calcineurin-NFAT pathway. These results also suggest that myocardial JNK activation is primarily dedicated to modulating calcineurin-NFAT signaling in the regulation of differentiated heart growth.

The antidiabetic agent sodium tungstate activates glycogen synthesis through an insulin receptor-independent pathway

Sodium tungstate is a powerful antidiabetic agent when administered orally. In primary cultured hepatocytes, tungstate showed insulin-like actions, which led to an increase in glycogen synthesis and accumulation. However, this compound did not significantly alter the insulin receptor activation state or dephosphorylation rate in cultured cells (CHO-R) or in primary hepatocytes, in either short or long term treatments. In contrast, at low concentrations, tungstate induced a transient strong activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) after 5-10 min of treatment, in a similar way to insulin. Moreover, this compound did not significantly delay or inhibit the dephosphorylation of ERK1/2. ERK1/2 activation triggered a cascade of downstream events, which included the phosphorylation of p90rsk and glycogen synthase-kinase 3beta. Experiments with a specific inhibitor of ERK1/2 activation and kinase assays indicate that these proteins were directly involved in the stimulation of glycogen synthase and glycogen synthesis induced by tungstate without a direct involvement of protein kinase B (PKB/Akt). These results show a direct involvement of ERK1/2 in the mechanism of action of tungstate at the hepatic level.

MAPK phosphatase DUSP16/MKP-7, a candidate tumor suppressor for chromosome region 12p12–13, reduces BCR-ABL-induced transformation

Recurrent chromosome 12p deletions are associated with distinct tumor types and suggest the presence of a tumor suppressor gene (TSG). Previously, we mapped an EST with similarity to a protein tyrosine phosphatase to the minimally deleted region for all these neoplasms. The corresponding gene, DUSP16/MKP-7, was recently shown to code for a mitogen-activated protein kinase phosphatase, suggestive for a function as tumor suppressor. Overexpression of DUSP16 in BCR-ABL-transformed Rat-1 fibroblasts reduces their transforming capacity in vitro and in vivo via downregulation of BCR-ABL-induced JNK activation. A role for DUSP16 as a regulator of JNK signaling was further demonstrated via overexpression in Ba/F3 cells, which increased their antiapoptosis. However, no inactivating mutations could be detected in leukemia patients hemizygous for DUSP16, and the effect of hemizygosity on DUSP16 expression level could not be assessed due to the variability of DUSP16 transcript levels observed in leukaemia cell lines and in patients. Taken together, the functional data point to a context-dependent role for DUSP16 on cell transformation and apoptosis, reflecting the dual role of JNK, and therefore suggest that DUSP16 might be haploinsufficient for tumor suppression.

Alterations in subcellular localization of p38 MAPK potentiates endothelin-stimulated COX-2 expression in glomerular mesangial cells

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide with mitogenic actions linked to activation of tyrosine kinase signaling pathways. ET-1 induces cyclooxygenase-2 (COX-2), an enzyme that converts arachidonic acid to pro-inflammatory eicosanoids. Activation of each of the three major mitogen-activated protein kinase (MAPK) pathways, ERK1/2, JNK/SAPK, and p38 MAPK (p38), have been shown to enhance the expression of COX-2. Negative regulation of MAPK may occur via a family of dual specificity phosphatases referred to as mitogen-activated protein kinase phosphatases (MKP). The goal of this work was to test the hypothesis that wild type MKP-1 regulates the expression of ET-1-induced COX-2 expression by inhibiting the activation of p38 in cultured glomerular mesangial cells (GMC). An adenovirus expressing both wild type and a catalytically inactive mutant of MKP-1 (MKP-1/CS) were constructed to study ET-1-regulated MAPK signaling and COX-2 expression in cultured GMC. ET-1 stimulated the phosphorylation of ERK and p38 alpha MAPK and induced the expression of COX-2. Expression of COX-2 was partially blocked by U0126, a MEK inhibitor, and SB 203580, a p38 MAPK inhibitor. Adenoviral expression of MKP-1/CS augmented basal and ET-1-induced phosphorylation of p38 alpha MAPK with less pronounced effects on ERK1/2 phosphorylation. Ectopic expression of wild type MKP-1 blocked the phosphorylation of p38 alpha MAPK by ET-1 but increased the phosphorylation of p38 gamma MAPK. Co-precipitation studies demonstrated association of MKP-1 with p38 alpha MAPK and ERK1/2. Immunofluorescent image analysis demonstrated trapping of phospho-p38 MAPK in the cytoplasm by MKP-1/CS/green fluorescent protein. ET-1-stimulated expression of COX-2 was increased in MKP-1/CS versus LacZ or green fluorescent protein-infected control cells. These results indicate that MKP-1 demonstrates a relative selectivity for p38 alpha MAPK versus p38 gamma MAPK in GMC and is likely to indirectly regulate the expression of COX-2.

Gene profiling of a myeloma cell line reveals similarities and unique signatures among IL-6 response, N-ras-activating mutations, and coculture with bone marrow stromal cells

ANBL-6, a myeloma cell line, proliferates in response to interleukin 6 (IL-6) stimulation, coculture with bone marrow stromal cells, and when harboring a constitutively active mutant N-ras gene. Eighteen samples, including 4 IL-6-treated, 3 mutant N-ras-transfected, 3 normal stroma-stimulated, 2 multiple myeloma (MM) stroma-stimulated, and 6 untreated controls were profiled using microarrays interrogating 12 626 genes. Global hierarchical clustering analysis distinguished at least 6 unique expression signatures. Notably, the different stimuli altered distinct functional gene programs. Class comparison analysis (P =.001) revealed 138 genes (54% involved in cell cycle) that distinguished IL-6-stimulated versus nontreated samples. Eighty-seven genes distinguished stroma-stimulated versus IL-6-treated samples (22% encoded for extracellular matrix [ECM] proteins). A total of 130 genes distinguished N-ras transfectants versus IL-6-treated samples (26% involved in metabolism). A total of 157 genes, 20% of these involved in signaling, distinguished N-ras from stroma-interacting samples. All 3 stimuli shared 347 genes, mostly of metabolic function. Genes that distinguished MM1 from MM4 clinical groups were induced at least by one treatment. Notably, only 3 genes (ETV5, DUSP6, and KIAA0735) are uniquely induced in mutant ras-containing cells. We have demonstrated gene expression patterns in myeloma cells that distinguish an intrinsic genetic transformation event and patterns derived from both soluble factors and cell contacts in the bone marrow microenvironment.

Constitutive induction of p-Erk1/2 accompanied by reduced activities of protein phosphatases 1 and 2A and MKP3 due to reactive oxygen species during cellular senescence

The mechanism of senescence-associated cytoplasmic induction of p-Erk1/2 (SA-p-Erk1/2) proteins in human diploid fibroblasts was investigated. p-Erk1/2 proteins were efficiently dephosphorylated in vitro by protein phosphatases 1 and 2A (PP1/2A) and MAPK phosphatase 3 (MKP3). Specific activity of PP1/2A and MKP3 activity significantly decreased during cellular senescence, whereas their protein expression levels did not. To investigate possible mechanism of phosphatase inactivation, we measured reactive oxygen species (ROS) generation by fluorescence-activated cell sorting analysis and found it was much higher in mid-old cells than the young cells. Treating the young cells once with 1 mm H2O2 remarkably induced p-Erk1/2 expression; however, it was transient unless repeatedly treated until 72 h. Multiple treatment of the cells with 0.2 mm H2O2 significantly duplicated inactivation of PP1/2A; however, thiol-specific reagents could reverse the PP1/2A activities, suggesting the oxidation of cysteine molecule in PP1/2A by the increased ROS. When the cells were pretreated with 10 mm N-acetyl-l-cysteine for 1 h, Erk1/2 activation was completely blocked. To elucidate which cysteine residue and/or metal ion in PP1/2A was modified by H2O2, electrospray ionization-tandem mass spectrometry analyses were performed with purified PP1C-alpha and found Cys62-SO3H and Cys105-SO3H, implicating the tertiary structure perturbation. H2O2 inhibited purified PP1C-alpha activity by both oxidation of Cys residues and metal ion(s), evidenced by dithiothreitol and ascorbate-restoration assay. In summary, SA-p-Erk1/2 was most likely due to the oxidation of PP1/2A, which resulted from the continuous exposure of the cells to vast amounts of ROS generated during cellular senescence by oxidation of Cys62 and Cys105 in PP1C-alpha and metal ion(s).

Negative feedback regulation of FGF signaling levels by Pyst1/MKP3 in chick embryos

BACKGROUND

The importance of endogenous antagonists in intracellular signal transduction pathways is becoming increasingly recognized. There is evidence in cultured mammalian cells that Pyst1/MKP3, a dual specificity protein phosphatase, specifically binds to and inactivates ERK1/2 mitogen-activated protein kinases (MAPKs). High-level Pyst1/Mkp3 expression has recently been found at many sites of known FGF signaling in mouse embryos, but the significance of this association and its function are not known.

RESULTS

We have cloned chicken Pyst1/Mkp3 and show that high-level expression in neural plate correlates with active MAPK. We show that FGF signaling regulates Pyst1 expression in developing neural plate and limb bud by ablating and/or transplanting tissue sources of FGFs and by applying FGF protein or a specific FGFR inhibitor (SU5402). We further show by applying a specific MAP kinase kinase inhibitor (PD184352) that Pyst1 expression is regulated via the MAPK cascade. Overexpression of Pyst1 in chick embryos reduces levels of activated MAPK in neural plate and alters its morphology and retards limb bud outgrowth.

CONCLUSIONS

Pyst1 is an inducible antagonist of FGF signaling in embryos and acts in a negative feedback loop to regulate the activity of MAPK. Our results demonstrate both the importance of MAPK signaling in neural induction and limb bud outgrowth and the critical role played by dual specificity MAP kinase phosphatases in regulating developmental outcomes in vertebrates.

MKP3 mediates the cellular response to FGF8 signalling in the vertebrate limb

The mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and phosphatidylinositol-3-OH kinase (PI3K)/Akt pathways are involved in the regulatory mechanisms of several cellular processes including proliferation, differentiation and apoptosis. Here we show that during chick, mouse and zebrafish limb/fin development, a known MAPK/ERK regulator, Mkp3, is induced in the mesenchyme by fibroblast growth factor 8 (FGF8) signalling, through the PI3K/Akt pathway. This correlates with a high level of phosphorylated ERK in the apical ectodermal ridge (AER), where Mkp3 expression is excluded. Conversely, phosphorylated Akt is detected only in the mesenchyme. Constitutively active Mek1, as well as the downregulation of Mkp3 by small interfering RNA (siRNA), induced apoptosis in the mesenchyme. This suggests that MKP3 has a key role in mediating the proliferative, anti-apoptotic signalling of AER-derived FGF8.

Mitogen-activated protein kinases: new signaling pathways functioning in cellular responses to environmental stress

The mitogen-activated protein kinase (MAPK) superfamily consists of three main protein kinase families: the extracellular signal-regulated protein kinases (ERKs), the c-Jun N-terminal kinases (JNKs) and the p38 family of kinases. Each is proving to have major roles in the regulation of intracellular metabolism and gene expression and integral actions in many areas including growth and development, disease, apoptosis and cellular responses to external stresses. To date, this cellular signal transduction network has received relatively little attention from comparative biochemists, despite the high probability that MAPKs have critical roles in the adaptive responses to thermal, osmotic and oxygen stresses. The present article reviews the current understanding of the roles and regulation of ERKs, JNKs and p38, summarizes what is known to date about MAPK roles in animal models of anoxia tolerance, freeze tolerance and osmoregulation, and highlights the potential that studies of MAPK pathways have for advancing our understanding of the mechanisms of biochemical adaptation.

The JNK-interacting protein-1 scaffold protein targets MAPK phosphatase-7 to dephosphorylate JNK

The c-Jun N-terminal kinase (JNK) group of mitogen-activated protein kinases (MAPKs) are activated by pleiotropic signals including environmental stresses, growth factors, and hormones. A subset of JNK can bind to distinct scaffold proteins that also bind upstream kinases of the JNK pathway, allowing sequential kinase activation within a signaling module. The JNK-interacting protein-1 (JIP-1) scaffold protein specifically binds JNK, MAP kinase kinase 7, and members of the MLK family and is essential for stress-mediated JNK activation in neurones. Here we report that JIP-1 also binds the dual-specificity phosphatases MKP7 and M3/6 via a region independent of its JNK binding domain. The C-terminal region of MKP7, homologous to that of M3/6 but not other DSPs, is required for interaction with JIP-1. When MKP7 is bound to JIP-1 it reduces JNK activation leading to reduced phosphorylation of the JNK target c-Jun. These results indicate that the JIP-1 scaffold protein modulates JNK signaling via association with both protein kinases and protein phosphatases that target JNK.

Solution structure of the MAPK phosphatase PAC-1 catalytic domain. Insights into substrate-induced enzymatic activation of MKP

Inactivation of mitogen-activated protein kinases (MAPKs) by MAPK phosphatases (MKPs) is accomplished via substrate-induced activation of the latter enzymes; however, the structural basis for the underlying mechanism remains elusive. Here, we report the three-dimensional solution structure of the C-terminal phosphatase domain of the prototypical MKP PAC-1, determined when bound to phosphate. Structural and biochemical analyses reveal unique active site geometry of the enzyme important for binding to phosphorylated threonine and tyrosine of MAPK ERK2. Our study further demonstrates that the dynamic interaction between the N-terminal kinase binding domain and the C-terminal phosphatase domain of an MKP is directly coupled to MAPK-induced conformational change of the phosphatase active site, which is essential for eliciting its full enzymatic activity.

Molecular cloning and characterization of a novel human protein phosphatase, LMW-DSP3

Reversible phosphorylation is recognized to be a major mechanism for the control of intracellular events in eukaryotic cells. From a human fetal brain cDNA library, we isolated a cDNA clone encoding a novel dual specificity protein phosphatase, which showed 88% identity with previously reported mouse LMW-DSP3 at the amino acid level. The deduced protein had a single dual-specificity phosphatase catalytic domain, and lacked a cdc25 homology domain. LMW-DSP3 was expressed in the heart, lung, liver, and pancreas, and the expression level in the pancreas was highest. The LMW-DSP3 gene was located in human chromosome 2q32, and consisted of five exons spanning 21kb of human genomic DNA. LMW-DSP3 fused to GST showed phosphatase activity towards p-nitrophenyl phosphate which was optimal at pH 7.0 and 40 degrees C, and the activity was enhanced by Ca(2+) and Mn(2+). The phosphatase activity of LMW-DSP3 was inhibited by orthovanate. LMW-DSP3 showed phosphatase activity toward oligopeptides containing pSer/Thr and pTyr, indicating that LMW-DSP3 is a protein phosphatase with dual substrate specificity.

NMDA-mediated activation of the tyrosine phosphatase STEP regulates the duration of ERK signaling

The intracellular mechanism(s) by which a cell determines the duration of extracellular signal-regulated kinase (ERK) activation is not well understood. We have investigated the role of STEP, a striatal-enriched tyrosine phosphatase, in the regulation of ERK activity in rat neurons. Glutamate-mediated activation of NMDA receptors leads to the rapid but transient phosphorylation of ERK in cultured neurons. Here we show that activation of NMDA receptors led to activation of STEP, which limited the duration of ERK activity as well as its translocation to the nucleus and its subsequent downstream nuclear signaling. In neurons, STEP is phosphorylated and inactive under basal conditions. NMDA-mediated influx of Ca(2+), but not increased intracellular Ca(2+) from other sources, leads to activation of the Ca(2+)-dependent phosphatase calcineurin and the dephosphorylation and activation of STEP. We have identified an important mechanism involved in the regulation of ERK activity in neurons that highlights the complex interplay between serine/threonine and tyrosine kinases and phosphatases.

Protein kinases and their involvement in the cellular responses to genotoxic stress

Cells are constantly subjected to genotoxic stress, and much has been learned regarding their response to this type of stress during the past year. In general, the cellular genotoxic response can be thought to occur in three stages: (1) damage sensing; (2) activation of signal transduction pathways; (3) biological consequences and attenuation of the response. The biological consequences, in particular, include cell cycle arrest and cell death. Although our understanding of the molecular mechanisms underlying cellular genotoxic stress responses remains incomplete, many cellular components have been identified over the years, including a group of protein kinases that appears to play a major role. Various DNA-damaging agents can activate these protein kinases, triggering a protein phosphorylation cascade that leads to the activation of transcription factors, and altering gene expression. In this review, the involvement of protein kinases, particularly the mitogen-activated protein kinases (MAPKs), at different stages of the genotoxic response is discussed.

Cytosolic retention of phosphorylated extracellular signal-regulated kinase and a Rho-associated kinase-mediated signal impair expression of p21(Cip1/Waf1) in phorbol 12-myristate-13- acetate-induced apoptotic cells

In response to treatment with phorbol-12-myristate-13-acetate (PMA), the half-population of erythromyeloblast D2 cells, a cytokine-independent variant of TF-1 cells, displayed adhesion and differentiated into a monocyte/macrophage-like morphology, while the other half-population remained in suspension and underwent apoptosis. Expression of the cell cycle inhibitor p21(Cip1/Waf1) was induced after PMA treatment in the adherent cells but not in the proapoptotic cells. We investigated the mechanism responsible for the impairment of p21(Cip1/Waf1) induction in PMA-induced proapoptotic cells. We demonstrated that in PMA-induced adherent cells, upregulation of p21(Cip1/Waf1) requires the activation and nuclear translocation of phosphorylated extracellular signal-regulated kinase (phospho-ERK). Although ERK was phosphorylated to comparable levels in PMA-induced proapoptotic and adherent cells, nuclear distribution of phospho-ERK was seen only in the adherent, not in the proapoptotic cells. We also found that only PMA-induced proapoptotic cells contained the phosphorylated form of myosin light chain, which is dependent on Rho-associated kinase (ROCK) activation, and that expression of a dominant-active form of ROCK suppressed activation of the p21(Cip1/Waf1) promoter during PMA induction. Finally, we demonstrated that inhibition of ROCK restores nuclear distribution of phospho-ERK and activation of p21(Cip1/Waf1) expression. Based on these findings, we propose that a ROCK-mediated signal is involved in interfering with the process of ERK-mediated p21(Cip1/Waf1) induction in PMA-induced proapoptotic TF-1 and D2 cells.

A role for protein phosphatase-2A in p38 mitogen-activated protein kinase-mediated regulation of the c-Jun NH(2)-terminal kinase pathway in human neutrophils

Human neutrophil accumulation in inflammatory foci is essential for the effective control of microbial infections. Although exposure of neutrophils to cytokines such as tumor necrosis factor-alpha (TNFalpha), generated at sites of inflammation, leads to activation of MAPK pathways, mechanisms responsible for the fine regulation of specific MAPK modules remain unknown. We have previously demonstrated activation of a TNFalpha-mediated JNK pathway module, leading to apoptosis in adherent human neutrophils (Avdi, N. J., Nick, J. A., Whitlock, B. B., Billstrom, M. A., Henson, P. M., Johnson, G. L., and Worthen, G. S. (2001) J. Biol. Chem. 276, 2189-2199). Herein, evidence is presented linking regulation of the JNK pathway to p38 MAPK and the Ser/Thr protein phosphatase-2A (PP2A). Inhibition of p38 MAPK by SB 203580 and M 39 resulted in significant augmentation of TNFalpha-induced JNK and MKK4 (but not MKK7 or MEKK1) activation, whereas prior exposure to a p38-activating agent (platelet-activating factor) diminished the TNFalpha-induced JNK response. TNFalpha-induced apoptosis was also greatly enhanced upon p38 inhibition. Studies with a reconstituted cell-free system indicated the absence of a direct inhibitory effect of p38 MAPK on the JNK module. Neutrophil exposure to the Ser/Thr phosphatase inhibitors okadaic acid and calyculin A induced JNK activation. Increased phosphatase activity following TNFalpha stimulation was shown to be PP2A-associated and p38-dependent. Furthermore, PP2A-induced dephosphorylation of MKK4 resulted in its inactivation. Thus, in neutrophils, p38 MAPK, through a PP2A-mediated mechanism, regulates the JNK pathway, thus determining the extent and nature of subsequent responses such as apoptosis.

Phenylethyl isothiocyanate induces apoptotic signaling via suppressing phosphatase activity against c-Jun N-terminal kinase

Dietary isothiocyanates induce apoptosis in various cancer cell lines through a c-Jun N-terminal kinase (JNK)-dependent mechanism. We found that phenylethyl isothiocyanate (PEITC) was capable of inducing JNK activation and apoptosis in prostate cancer cell lines with distinct p53 statuses. PEITC induced JNK-mediated apoptotic signaling via a different pathway than that used by DNA-damaging agents, because genotoxicresistant LNCaP prostate cancer cells were equally sensitive to PEITC as parental LNCaP cells. PEITC did not induce significant MKK4 or MKK7 activation and did not activate JNK directly, suggesting that JNK and JNK upstream kinases are not primary targets of PEITC. The JNK dephosphorylation and inactivation rates were decreased in cells exposed to PEITC. Expression levels of M3/6, a JNK-specific phosphatase, were down-regulated by PEITC via a proteasome-dependent mechanism. Taken together, our data suggest that PEITC activates JNK through suppression of JNK dephosphorylation and that PEITC may be an alternative therapeutic agent for cancers that are resistant to genotoxic agents. This study also reveals that JNK phosphatases are potential targets for the development of novel cancer therapeutic agents.

The dual specificity JKAP specifically activates the c-Jun N-terminal kinase pathway

The involvement of dual specificity phosphatases (DSPs) in the mitogen-activated protein kinase (MAPK) signaling has been mostly limited to the inactivation of MAPKs by the direct dephosphorylation of the TXY motif within their activation loop. We report the cloning and characterization of a murine DSP, called JNK pathway-associated phosphatase (JKAP), which lacks the regulatory region present in most other MAP kinase phosphatases (MKPs) and is preferentially expressed in murine Lin(-)Sca-1(+) stem cells. Overexpression of JKAP in human embryonic kidney 293T cells specifically activated c-Jun N-terminal kinase (JNK) but not p38 and extracellular signal-regulated kinase 2. Overexpression of a mutant JKAP, JKAP-C88S, blocked tumor necrosis factor-alpha-induced JNK activation. Targeted gene disruption in murine embryonic stem cells abolished JNK activation by tumor necrosis factor-alpha and transforming growth factor-beta, but not by ultraviolet-C irradiation, indicating that JKAP is necessary for optimal JNK activation. JKAP associated with JNK and MKK7, but not SEK1, in vivo. However, JKAP did not interact with JNK in vitro, suggesting that JKAP exerts its effect on JNK in an indirect manner. Taken together, these studies identify a positive regulator for the JNK pathway and suggest a novel role for DSP in mitogen-activated protein kinase regulation.

Ras-MAP kinase signaling pathways and control of cell proliferation: relevance to cancer therapy

The mitogen-activated protein (MAP) kinase pathways represent several families of signal transduction cascades that mediate information provided by extracellular stimuli. MAP kinase pathways regulate a wide range of physiological responses, including cell proliferation, apoptosis, cell differentiation, and tissue development. Constitutive activation of MAP kinase proteins in experimental models has been shown to cause cell transformation and is implicated in tumorigenesis. Of clinical importance, MAP kinase pathways are regulated by Ras G-proteins, which are found to be mutated and constitutively active in approximately 30% of all human cancers. Thus, a major goal in the treatment of cancer is the development of specific compounds that target Ras and critical downstream signaling proteins responsible for uncontrolled cell growth. A variety of biochemical, molecular, and structural approaches have been used to develop drug compounds that target signaling proteins important for MAP kinase pathway activation. These compounds have been useful tools for identifying the mechanisms of MAP kinase pathway signaling and hold promise for clinical use. This review will present an overview of the major proteins involved in Ras and MAP kinase signaling pathways and their function in regulating cell cycle events and proliferation. In addition, some of the relevant compounds that have been developed to inhibit the activities of these proteins and MAP kinase signaling are discussed.

The specificity of extracellular signal-regulated kinase 2 dephosphorylation by protein phosphatases

The extracellular signal-regulated protein kinase 2 (ERK2) is the founding member of a family of mitogen-activated protein kinases (MAPKs) that are central components of signal transduction pathways for cell proliferation, stress responses, and differentiation. The MAPKs are unique among the Ser/Thr protein kinases in that they require both Thr and Tyr phosphorylation for full activation. The dual phosphorylation of Thr-183 and Tyr-185 in ERK2 is catalyzed by MAPK/ERK kinase 1 (MEK1). However, the identity and relative activity of protein phosphatases that inactivate ERK2 are less well established. In this study, we performed a kinetic analysis of ERK2 dephosphorylation by protein phosphatases using a continuous spectrophotometric enzyme-coupled assay that measures the inorganic phosphate produced in the reaction. Eleven different protein phosphatases, many previously suggested to be involved in ERK2 regulation, were compared, including tyrosine-specific phosphatases (PTP1B, CD45, and HePTP), dual specificity MAPK phosphatases (VHR, MKP3, and MKP5), and Ser/Thr protein phosphatases (PP1, PP2A, PP2B, PP2C alpha, and lambda PP). The results provide biochemical evidence that protein phosphatases display exquisite specificity in their substrate recognition and implicate HePTP, MKP3, and PP2A as ERK2 phosphatases. The fact that ERK2 inactivation could be carried out by multiple specific phosphatases shows that signals can be integrated into the pathway at the phosphatase level to determine the cellular response to external stimuli. Important insights into the roles of various protein phosphatases in ERK2 kinase signaling are obtained, and further analysis of the mechanism by which different protein phosphatases recognize and inactivate MAPKs will increase our understanding of how this kinase family is regulated.

Prolonged nuclear retention of activated extracellular signal-regulated kinase 1/2 is required for hepatocyte growth factor-induced cell motility

We examined the signaling pathway by which hepatocyte growth factor (HGF) induces cell motility, with special focus on the role of extracellular signal-regulated kinase (ERK) in the nucleus. We used Madin-Darby canine kidney cells overexpressing ERK2 because of their prominent motility response to HGF. HGF stimulation of the cells induces not only a rapid, marked, and sustained activation and rapid nuclear accumulation of ERK1/2, but also a prolonged nuclear retention of the activated ERK1/2. Interruption of the ERK1/2 activation by PD98059 treatment of the cells 30 min after HGF stimulation abolishes the HGF-induced cell motility. Enforced cytoplasmic retention of the activated ERK1/2 by the expression of an inactive form of MKP-3 cytoplasmic phosphatase inhibits the cell motility response. Although epidermal growth factor stimulation of the cells induces the activation and nuclear accumulation of ERK1/2, it does not induce the prolonged nuclear retention of the activated ERK1/2, and fails to induce cell motility. In the nucleus, activated ERK1/2 continuously phosphorylate Elk-1, leading to the prolonged expression of c-fos, which results in the expression of several genes such as matrix metalloproteinase (mmp)-9; MMP-9 activity is required for the induction of the cell motility response. Our results indicate that the sustained activity of ERK1/2 in the nucleus is required for the induction of HGF-induced cell motility.

Suppression of integrin activation by activated Ras or Raf does not correlate with bulk activation of ERK MAP kinase

The rapid modulation of ligand-binding affinity ("activation") is a central property of the integrin family of cell adhesion receptors. The Ras family of small GTP-binding proteins and their downstream effectors are key players in regulating integrin activation. H-Ras can suppress integrin activation in fibroblasts via its downstream effector kinase, Raf-1. In contrast, to H-Ras, a closely related small GTP-binding protein R-Ras has the opposite activity, and promotes integrin activation. To gain insight into the regulation of integrin activation by Ras GTPases, we created a series of H-Ras/R-Ras chimeras. We found that a 35-amino acid stretch of H-Ras was required for full suppressive activity. Furthermore, the suppressive chimeras were weak activators of the ERK1/2 MAP kinase pathway, suggesting that the suppression of integrin activation may be independent of the activation of the bulk of ERK MAP kinase. Additional data demonstrating that the ability of H-Ras or Raf-1 to suppress integrin activation was unaffected by inhibition of bulk ERK1/2 MAP kinase activation supported this hypothesis. Thus, the suppression of integrin activation is a Raf kinase induced regulatory event that can be mediated independently of bulk activation of the ERK MAP-kinase pathway.

Scaffold role of a mitogen-activated protein kinase phosphatase, SKRP1, for the JNK signaling pathway

Stress-activated protein kinase (SAPK) pathway-regulating phosphatase 1 (SKRP1) has been identified as a member of the mitogen-activated protein kinase (MAPK) phosphatase (MKP) family that interacts physically with the MAPK kinase (MAPKK) MKK7, a c-Jun N-terminal kinase (JNK) activator, and inactivates the MAPK JNK pathway. Although these findings indicated that SKRP1 contributes to the precise regulation of JNK signaling, it remains to be elucidated how SKRP1 is integrated into this pathway. We report that SKRP1 also plays a scaffold role for the JNK signaling, judged by the following observations. SKRP1 selectively formed the stable complexes with MKK7 but not with MKK4 and biphasically regulated the MKK7 activity and MKK7-induced gene transcription in vivo. Co-precipitation analysis between SKRP1 and MKK7-activating MAPKK kinases (MAPKKKs) revealed that SKRP1 also interacted with the MAPKKK, apoptosis signal-regulating kinase 1 (ASK1), but not with MAP kinase kinase kinase 1 (MEKK1). Consistent with these findings, SKRP1 expression increased the ASK1-MKK7 complexes in a dose-dependent manner and specifically enhanced the activation of MKK7 by ASK1. Thus, our findings are, to our knowledge, the first evidence to show that an MKP also functions as a scaffold protein for the particular MAPK signaling.

A novel dual specificity phosphatase SKRP1 interacts with the MAPK kinase MKK7 and inactivates the JNK MAPK pathway. Implication for the precise regulation of the particular MAPK pathway

Mitogen-activated protein kinases (MAPKs) are activated in response to various extracellular stimuli, and their activities are regulated by upstream activating kinases and protein phosphatases such as MAPK phosphatases (MKPs). We report the identification and characterization of a novel MKP termed SKRP1 (SAPK pathway-regulating phosphatase 1). It contains an extended active site sequence motif conserved in all MKPs but lacks a Cdc25 homology domain. Immunoblotting analysis revealed that SKRP1 is constitutively expressed, and its transcripts of 4.0 and 1.0 kb were detected in almost tissues examined. SKRP1 was highly specific for c-Jun N-terminal kinase (JNK) in vitro and effectively suppressed the JNK activation in response to tumor necrosis factor alpha or thapsigargin. Endogenous SKRP1 was present predominantly in the cytoplasm and co-localized with JNK. However, SKRP1 does not bind directly to its target JNK, but co-precipitation of SKRP1 with the MAPK kinase MKK7, a JNK activator, was found in vitro and in vivo. Furthermore, we found that SKRP1 did not interfere with the co-precipitation of MKK7 with JNK. Together, our findings indicate that SKRP1 interacts with its physiological substrate JNK through MKK7, thereby leading to the precise regulation of JNK activity in vivo.

c-Raf/MEK/ERK pathway controls protein kinase C-mediated p70S6K activation in adult cardiac muscle cells

p70S6 kinase (S6K1) plays a pivotal role in hypertrophic cardiac growth via ribosomal biogenesis. In pressure-overloaded myocardium, we show S6K1 activation accompanied by activation of protein kinase C (PKC), c-Raf, and mitogen-activated protein kinases (MAPKs). To explore the importance of the c-Raf/MAPK kinase (MEK)/MAPK pathway, we stimulated adult feline cardiomyocytes with 12-O-tetradecanoylphorbol-13-acetate (TPA), insulin, or forskolin to activate PKC, phosphatidylinositol-3-OH kinase, or protein kinase A (PKA), respectively. These treatments resulted in S6K1 activation with Thr-389 phosphorylation as well as mammalian target of rapamycin (mTOR) and S6 protein phosphorylation. Thr-421/Ser-424 phosphorylation of S6K1 was observed predominantly in TPA-treated cells. Dominant negative c-Raf expression or a MEK1/2 inhibitor (U0126) treatment showed a profound blocking effect only on the TPA-stimulated phosphorylation of S6K1 and mTOR. Whereas p38 MAPK inhibitors exhibited only partial effect, MAPK-phosphatase-3 expression significantly blocked the TPA-stimulated S6K1 and mTOR phosphorylation. Inhibition of mTOR with rapamycin blocked the Thr-389 but not the Thr-421/Ser-424 phosphorylation of S6K1. Therefore, during PKC activation, the c-Raf/MEK/extracellular signal-regulated kinase-1/2 (ERK1/2) pathway mediates both the Thr-421/Ser-424 and the Thr-389 phosphorylation in an mTOR-independent and -dependent manner, respectively. Together, our in vivo and in vitro studies indicate that the PKC/c-Raf/MEK/ERK pathway plays a major role in the S6K1 activation in hypertrophic cardiac growth.