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Selective activation of mitogen-activated protein (MAP) kinase kinase 3 and p38alpha MAP kinase is essential for cyclic AMP-dependent UCP1 expression in adipocytes.

The sympathetic nervous system regulates the activity and expression of uncoupling protein 1 (UCP1) through the three beta-adrenergic receptor subtypes and their ability to raise intracellular cyclic AMP (cAMP) levels. Unexpectedly, we recently discovered that the cAMP-dependent regulation of multiple genes in brown adipocytes, including Ucp1, occurred through the p38 mitogen-activated protein kinases (MAPK) (W. Cao, K. W. Daniel, J. Robidoux, P. Puigserver, A. V. Medvedev, X. Bai, L. M. Floering, B. M. Spiegelman, and S. Collins, Mol. Cell. Biol. 24:3057-3067, 2004). However, no well-defined pathway linking cAMP accumulation or cAMP-dependent protein kinase (PKA) to p38 MAPK has been described. Therefore, in the present study using both in vivo and in vitro models, we have initiated a retrograde approach to define the required components, beginning with the p38 MAPK isoforms themselves and the MAP kinase kinase(s) that regulates them. Our strategy included ectopic expression of wild-type and mutant kinases as well as targeted inhibition of gene expression using small interfering RNA. The results indicate that the beta-adrenergic receptors and PKA lead to a highly selective activation of the p38alpha isoform of MAPK, which in turn promotes Ucp1 gene transcription. In addition, this specific activation of p38alpha relies solely on the presence of MAP kinase kinase 3, despite the expression in brown fat of MKK3, -4, and -6. Finally, of the three scaffold proteins of the JIP family expressed in brown adipocytes, only JIP2 co-immunoprecipitates p38alpha MAPK and MKK3. Therefore, in the brown adipocyte the recently described scaffold protein JIP2 assembles the required factors MKK3 and p38alpha MAPK linking PKA to the control of thermogenic gene expression.

Adaptor Proteins, Signal Transducing↗

Redox regulation of MAP kinase phosphatase 3.

MAP kinase phosphatase 3 (MKP3) is a protein tyrosine phosphatase (PTP) for which in vivo evidence suggests that regulation can occur by oxidation and/or reduction of the active site cysteine. Using kinetics and mass spectrometry, we have probed the biochemical details of oxidation of the active site cysteine in MKP3, with particular focus on the mechanism of protection from irreversible inactivation to the sulfinic or sulfonic acid species. Like other PTPs, MKP3 was found to be rapidly and reversibly inactivated by mild treatment with hydrogen peroxide. We demonstrate that unlike the case for some PTPs, the sulfenic acid of the active site cysteine in MKP3 is not stabilized in the active site but instead is rapidly trapped in a re-reducible form. Unlike the case for other PTPs, the sulfenic acid in MKP3 does not form a sulfenyl-amide species with its neighboring residue or a disulfide with a single proximate cysteine. Instead, multiple cysteines distributed in both the N-terminal substrate-binding domain (Cys147 in particular) and the C-terminal catalytic domain (Cys218) are capable of rapidly and efficiently trapping the sulfenic acid as a disulfide. Our results extend the diversity of mechanisms utilized by PTPs to prevent irreversible oxidation of their active sites and expand the role of the N-terminal substrate recognition domain in MKP3 to include redox regulation.

Amides↗

Interaction of the Flt-1 tyrosine kinase receptor with the p85 subunit of phosphatidylinositol 3-kinase. Mapping of a novel site involved in binding.

We have examined the interactions of the p85 regulatory subunit of phosphatidylinositol 3-kinase with the endothelium-specific Flt-1 receptor tyrosine kinase using the yeast two-hybrid system. We find that both the amino- and carboxyl-terminal SH2 domains of p85 bind to Flt-1. We have performed site-directed mutagenesis on the carboxyl-terminal tail of the Flt-1 receptor in order to identify the site(s) that is responsible for the p85 interactions. A single tyrosine to phenylalanine change at position 1213 inhibits the binding of both p85 SH2 domains. Phosphopeptide mapping of the wild type and mutant protein expressed in insect cells verifies that this amino acid is a target for autophosphorylation. The amino acids following this tyrosine are VNA and thus define a novel binding site for p85.

Amino Acid Sequence↗

Defective IL-12 production in mitogen-activated protein (MAP) kinase kinase 3 (Mkk3)-deficient mice.

The p38 mitogen-activated protein kinase (MAPK) pathway, like the c-Jun N-terminal kinase (JNK) MAPK pathway, is activated in response to cellular stress and inflammation and is involved in many fundamental biological processes. To study the role of the p38 MAPK pathway in vivo, we have used homologous recombination in mice to inactivate the Mkk3 gene, one of the two specific MAPK kinases (MAPKKs) that activate p38 MAPK. Mkk3(-/-) mice were viable and fertile; however, they were defective in interleukin-12 (IL-12) production by macrophages and dendritic cells. Interferon-gamma production following immunization with protein antigens and in vitro differentiation of naive T cells is greatly reduced, suggesting an impaired type I cytokine immune response. The effect of the p38 MAPK pathway on IL-12 expression is at least partly transcriptional, since inhibition of this pathway blocks IL-12 p40 promoter activity in macrophage cell lines and IL-12 p40 mRNA is reduced in MKK3-deficient mice. We conclude that the p38 MAP kinase, activated through MKK3, is required for the production of inflammatory cytokines by both antigen-presenting cells and CD4(+) T cells.

Animals↗

MAP kinase kinase kinases and innate immunity.

Toll-like receptors, which respond to invariant microbial molecules, and receptors for the proinflammatory cytokines tumour necrosis factor and interleukin-1 are crucial for initiation and regulation of innate immune responses. These receptors activate each of the major mitogen-activated protein (MAP) kinase subtypes, extracellular signal-regulated protein kinases, c-Jun amino-terminal kinases and p38 MAP kinases, which are crucial for cell survival and controlling the expression of immune mediators. Here we discuss recent studies characterizing the specific MAP kinase kinase kinases (MAP 3-kinases) that link MAP kinases to receptors involved in innate immunity and the mechanisms by which the activity of MAP 3-kinases is regulated by such receptors.

Animals↗

Involvement of MAP kinase in angiotensin II-induced phosphorylation and intracellular targeting of neuronal AT1 receptors.

MAP kinase stimulation is a key signaling event in the AT1 receptor (AT1R)-mediated chronic stimulation of tyrosine hydroxylase and norepinephrine transporter in brain neurons by angiotensin II (Ang II). In this study, we investigated the involvement of MAP kinase in AT1R phosphorylation to further our understanding of these persistent neuromodulatory actions of Ang II. Ang II caused a time-dependent phosphorylation of neuronal AT1R. This phosphorylation was associated with internalization and translocation of AT1R into the nucleus. MAP kinase also stimulated phosphorylation of neuronal AT1R. The conclusion that MAP kinase participates in neuronal AT1R phosphorylation and its targeting into the nucleus is supported further by the following. (1) MAP kinase-mediated phosphorylation of AT1R was blocked by the AT1R antagonist losartan; (2) AT1R co-immunoprecipitated with MAP kinase; (3) MAP kinase-kinase inhibitor PD98059 attenuated Ang II-induced phosphorylation of AT1R; and (4) PD98059 blocked Ang II-induced nuclear translocation of AT1Rs. In summary, these observations demonstrate that Ang II-induced phosphorylation of AT1R is mediated by its activation of MAP kinase. A possible role of AT1R translocation into the nucleus on persistent neuromodulatory actions of Ang II has been discussed.

1-Sarcosine-8-Isoleucine Angiotensin II↗

The PI 3-kinase-Rac-p38 MAP kinase pathway is involved in the formation of signet-ring cell carcinoma.

Signet-ring cell carcinoma is classified in poorly differentiated adenocarcinoma with an aggressive nature and a poor prognosis. We have shown that the activation of PI 3-kinase in highly differentiated adenocarcinomas induces loss of cell-cell contact and formation of vacuoles, giving phenotypes similar to those of signet-ring cell lines. SB203580, a potent p38 MAP kinase inhibitor, blocked this transition, and expression of an active form of MKK6 (MKK6DA), an activator of p38 MAP kinase, gave effects similar to those induced by expression of the active form of PI 3-kinase (BD110), although formation of large vacuoles was not induced. Activation of MKK3, another activator of p38 MAP kinase, was activated in native signet-ring carcinoma cell lines. Anchorage-independent growth of signet-ring cell lines was inhibited by LY294002 or SB203580. These results suggest that p38 MAP kinase is functioning downstream of PI 3-kinase in signaling of the malignant phenotype. Secretion of mucins was enhanced in BD110-expressing cells, but not in MKK6DA-expressing cells, suggesting that secretion of mucins is independent of the MKK6-p38 MAP kinase cascade. Thus, there may be at least two pathways, p38 MAP kinase-dependent and -independent, which are involved in regulation of cell-cell contact and the protein secretion system, respectively.

Adenocarcinoma↗

Rac-MEKK3-MKK3 scaffolding for p38 MAPK activation during hyperosmotic shock.

Sensing the osmolarity of the environment is a critical response for all organisms. Whereas bacteria will migrate away from high osmotic conditions, most eukaryotic cells are not motile and use adaptive metabolic responses for survival. The p38 MAPK pathway is a crucial mediator of survival during cellular stress. We have discovered a novel scaffold protein that binds to actin, the GTPase Rac, and the upstream kinases MEKK3 and MKK3 in the p38 MAPK phospho-relay module. RNA interference (RNAi) demonstrates that MEKK3 and the scaffold protein are required for p38 activation in response to sorbitol-induced hyperosmolarity. FRET identifies a cytoplasmic complex of the MEKK3 scaffold protein that is recruited to dynamic actin structures in response to sorbitol treatment. Through its ability to bind actin, relocalize to Rac-containing membrane ruffles and its obligate requirement for p38 activation in response to sorbitol, we have termed this protein osmosensing scaffold for MEKK3 (OSM). The Rac-OSM-MEKK3-MKK3 complex is the mammalian counterpart of the CDC42-STE50-STE11-Pbs2 complex in Saccharomyces cerevisiae that is required for the regulation of p38 activity.

Actin Cytoskeleton↗

Involvement of p38 mitogen-activated protein kinase in different stages of thymocyte development.

Positive selection of thymocytes during T-cell development is mediated by T-cell receptor (TCR)-activated signals. For different mitogen-activated protein kinases (MAPKs) activated by TCR complex, a selective involvement of extracellular signal-regulated kinase, but not p38 MAPK, in positive selection has been suggested. Using transgenic mice with dominant-negative mutation of both MAP kinase kinase 3 (MMK3) and MKK6, we obtained mice with different extents of inhibition of p38 MAPK activation. Partial inhibition of p38 MAPK impaired CD4(-)CD8(-) thymocyte development and T-cell proliferation, but not positive selection. Interference with thymocyte positive selection was observed in mice with effective suppression of p38 MAPK. Our results suggest that, in addition to early thymocyte development, p38 is involved in positive selection.

Animals↗

Insulin- and mitogen-activated protein kinase-mediated phosphorylation and activation of peroxisome proliferator-activated receptor gamma.

Peroxisome proliferator-activated receptor (PPAR) gamma plays an important role in adipocyte differentiation and the regulation of adipocyte gene expression. Insulin also serves to promote adipogenesis. We report that insulin and a PPARgamma ligand (thiazolidinedione (TZD)) stimulate in a synergistic manner the expression of an adipocyte-specific gene (aP2) in rat adipocytes and 3T3-L1 cells. Potential cross-talk between insulin signaling and PPARgamma was studied in Chinese hamster ovary cells expressing insulin receptors (CHO.T), PPARgamma, and reporter genes. Both TZD and insulin independently stimulated PPARgamma-mediated transactivation of aP2 promoter-luciferase reporter genes; both agents combined resulted in a synergistic effect. Co-transfection of CHO.T cells with dominant-negative mitogen-activated protein (MAP) kinase-kinase (MKK1) abrogated both insulin- and TZD-mediated activation of PPARgamma; transactivation was markedly increased in cells co-transfected with constitutively active MKK1. Both insulin and constitutively active MKK1 also stimulated 32P incorporation into PPARgamma in vivo. The conclusions are: 1) Insulin synergizes with a PPARgamma ligand and can activate the receptor in a ligand-independent fashion. 2) PPARgamma is phosphorylated in vivo by insulin stimulation or activation of the MAP kinase pathway. 3) MAP kinase is an important mediator of cross-talk between insulin signal transduction pathways and PPARgamma function.

3T3 Cells↗

Structural basis of docking interactions between ERK2 and MAP kinase phosphatase 3.

Mitogen-activated protein (MAP) kinases are central components of signal transduction pathways for cell proliferation, stress responses, and differentiation. Signaling efficiency and specificity are modulated in large part by docking interactions between individual MAP kinase and the kinase interaction motif (KIM), (R/K)(2-3)-X(1-6)-Phi(A)-X-Phi(B), in its cognate kinases, phosphatases, scaffolding proteins, and substrates. We have determined the crystal structure of extracellular signal-regulated protein kinase 2 bound to the KIM peptide from MAP kinase phosphatase 3, an extracellular signal-regulated protein kinase 2-specific phosphatase. The structure reveals that the KIM docking site, situated in a noncatalytic region opposite of the kinase catalytic pocket, is comprised of a highly acidic patch and a hydrophobic groove, which engage the basic and Phi(A)-X-Phi(B) residues, respectively, in the KIM sequence. The specific docking interactions observed in the structure consolidate all known biochemical data. In addition, structural comparison indicates that the KIM docking site is conserved in all MAP kinases. The results establish a structural model for understanding how MAP kinases interact with their regulators and substrates and provide new insights into how MAP kinase docking specificity can be achieved.

Amino Acid Motifs↗

Involvement of JAK2, but not PI 3-kinase/Akt and MAP kinase pathways, in anti-apoptotic signals of GM-CSF in human eosinophils.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) transmits anti-apoptotic signals in eosinophils and is involved in tissue eosinophilia at the site of allergic inflammation. We determined whether phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein kinase (MAP kinase) are involved in anti-apoptotic signals of GM-CSF in eosinophils. GM-CSF phosphorylated Akt, a downstream component of PI 3-kinase, and MAP kinases (ERK1 and ERK2) at 10 min after stimulation in eosinophils. GM-CSF prevented eosinophil apoptosis and sustained its survival during the 5-day culture. However, neither two PI-3 kinase inhibitors, wortmannin and LY294002, nor MEK inhibitor PD98059 inhibited GM-CSF-induced survival of eosinophils, although wortmannin and PD98059 inhibited GM-CSF-induced Akt phosphorylation and MAP kinase activation in eosinophils, respectively. In contrast, JAK2 inhibitor AG-490 inhibited both GM-CSF-induced JAK2 phosphorylation and cell survival in eosinophils. These results indicate that activation of JAK2, but not activation of PI 3-kinase/Akt and MAP kinase pathways, is critical for anti-apoptotic signals of GM-CSF in human eosinophils. Our findings suggest that manipulation of JAK2 activation would be useful for the treatment of allergic disorders.

Androstadienes↗

MEK kinases are regulated by EGF and selectively interact with Rac/Cdc42.

MEK kinases (MEKKs) 1, 2, 3 and 4 are members of sequential kinase pathways that regulate MAP kinases including c-Jun NH2-terminal kinases (JNKs) and extracellular regulated kinases (ERKs). Confocal immunofluorescence microscopy of COS cells demonstrated differential MEKK subcellular localization: MEKK1 was nuclear and in post-Golgi vesicular-like structures; MEKK2 and 4 were localized to distinct Golgi-associated vesicles that were dispersed by brefeldin A. MEKK1 and 2 were activated by EGF, and kinase-inactive mutants of each MEKK partially inhibited EGF-stimulated JNK activity. Kinase-inactive MEKK1, but not MEKK2, 3 or 4, strongly inhibited EGF-stimulated ERK activity. In contrast to MEKK2 and 3, MEKK1 and 4 specifically associated with Rac and Cdc42 and kinase-inactive mutants blocked Rac/Cdc42 stimulation of JNK activity. Inhibitory mutants of MEKK1-4 did not affect p21-activated kinase (PAK) activation of JNK, indicating that the PAK-regulated JNK pathway is independent of MEKKs. Thus, in different cellular locations, specific MEKKs are required for the regulation of MAPK family members, and MEKK1 and 4 are involved in the regulation of JNK activation by Rac/Cdc42 independent of PAK. Differential MEKK subcellular distribution and interaction with small GTP-binding proteins provides a mechanism to regulate MAP kinase responses in localized regions of the cell and to different upstream stimuli.

Animals↗

Serum amyloid A induces IL-8 secretion through a G protein-coupled receptor, FPRL1/LXA4R.

Host response to injury and infection is accompanied by a rapid rise in the blood of acute-phase proteins such as serum amyloid A (SAA). Although SAA has been used as a marker for inflammatory diseases, its role in the modulation of inflammation and immunity has not been defined. Human neutrophils respond to SAA with secretion of the proinflammatory cytokines interleukin 8 (IL-8) and, to a lesser extent, tumor necrosis factor alpha (TNF-alpha). The induction of IL-8 secretion by SAA involves both transcription and translation and correlates with activation of nuclear factor kappaB (NF-kappaB). The proximal signaling events induced by SAA include mobilization of intracellular Ca(2+) and activation of the mitogen-activated protein kinases ERK1/2 and p38, both required for the induced IL-8 secretion. Pertussis toxin effectively blocks SAA-induced IL-8 secretion indicating involvement of a Gi-coupled receptor. Overexpression of FPRL1/LXA4R in HeLa cells results in a significant increase of the expression of NF-kappaB and IL-8 luciferase reporters by SAA, and an antibody against the N-terminal domain of FPRL1/LXA4R inhibits IL-8 secretion. Lipoxin A4, which binds to FPRL1/LXA4R specifically, decreases SAA-induced IL-8 secretion significantly. Collectively, these results indicate that the cytokine-like property of SAA is manifested through activation of the Gi-coupled FPRL1/LXA4R, which has been known to mediate the anti-inflammatory effects of lipoxin A4. The ability of FPRL1/LXA4R to mediate 2 drastically different and opposite functions suggests that it plays a role in the modulation of inflammatory and immune responses.

Apolipoproteins↗

Knock-down of POSH expression is neuroprotective through down-regulating activation of the MLK3-MKK4-JNK pathway following cerebral ischaemia in the rat hippocampal CA1 subfield.

We investigated the expression and subcellular localization of the multidomain protein POSH (plenty of SH3s) by immunohistochemistry and western blot analysis, as well as its role in the selective activation of mixed-lineage kinases (MLKs) 3, MAP kinase kinase (MKK) 4, c-Jun N-terminal kinases (JNKs) and the c-Jun signalling cascade in the rat hippocampal CA1 region following cerebral ischaemia. Our results indicated that the cytosol immunoreactivity of POSH was strong in the CA1-CA3 pyramidal cell but weak in the DG granule cell of the rat hippocampus both in sham control and after reperfusion. Co-immunoprecipitation experiments showed that the interactions of MLK3, MKK4 and phospho-JNKs with POSH were persistently enhanced during the early (30 min) and the later reperfusion period (from 1 to 3 days) compared with sham controls. Consistently, MLK3-MKK4-JNK activation was rapidly increased with peaks both at 30 min and 3 days of reperfusion. Intracerebroventricular infusion of POSH antisense oligodeoxynucleotides (AS-ODNs) not only significantly reduced the protein level of POSH, markedly decreased its interactions with MLK3, MKK4 and phospho-JNKs, but also attenuated the activation of the JNK signalling pathway. In addition, infusion of POSH AS-ODNs significantly increased the neuronal density in the CA1 region at 5 days of reperfusion. Our results suggest that POSH might serve as a scaffold mediating JNK signalling activation in the hippocampal CA1 region following cerebral ischaemia, and POSH AS-ODNs exerts its protective effects on ischaemic injury through a mechanism of inhibition of the MLK3-MKK4-JNK signalling pathway, involving c-Jun and caspase 3 activation.

Adaptor Proteins, Signal Transducing↗

Delta MEKK3:ER* activation induces a p38 alpha/beta 2-dependent cell cycle arrest at the G2 checkpoint.

Whilst many studies have examined the role of the MAP Kinases in regulating the G1-->S transition, much less is known about the function of these pathways in regulating other cell cycle transitions. Stimulation of the conditional mutant Delta MEKK3:ER* in asynchronous hamster (CCl39) and rat (Rat-1) fibroblasts resulted in the strong activation of endogenous JNK and p38 but only a weak activation of ERK. Activation of Delta MEKK3:ER* inhibited cell proliferation through a combination of an initial G1 and G2 cell cycle arrest, followed by a delayed onset of apoptosis. When cells were synchronized in S phase with aphidicolin and then released, activation of Delta MEKK3:ER* resulted in the up-regulation of p21(CIP1) and a pronounced inhibition of cyclin A/CDK2 and cyclin B1/CDK1 kinase activity. Analysis of mitotic figures indicated that cells failed to enter mitosis, arresting late in G2. Delta MEKK3:ER*-mediated CDK inhibition and G2 arrest did not absolutely require p21(CIP1), since both events were observed in Rat-1 cells in which p21(CIP1) is transcriptionally silenced due to promoter methylation. Rather, CDK inhibition was associated with a down-regulation of cyclin A and B1 expression. Finally, application of the p38 inhibitor SB203580 partially restored cyclin B associated kinase activity and allowed cells to proceed through mitosis into the next G1 phase, suggesting that activation of the p38 alpha/beta 2 pathway can promote a G2 cell cycle arrest.

Animals↗

Opposing effects of ERK and JNK-p38 MAP kinases on apoptosis.

Apoptosis plays an important role during neuronal development, and defects in apoptosis may underlie various neurodegenerative disorders. To characterize molecular mechanisms that regulate neuronal apoptosis, the contributions to cell death of mitogen-activated protein (MAP) kinase family members, including ERK (extracellular signal-regulated kinase), JNK (c-JUN NH2-terminal protein kinase), and p38, were examined after withdrawal of nerve growth factor (NGF) from rat PC-12 pheochromocytoma cells. NGF withdrawal led to sustained activation of the JNK and p38 enzymes and inhibition of ERKs. The effects of dominant-interfering or constitutively activated forms of various components of the JNK-p38 and ERK signaling pathways demonstrated that activation of JNK and p38 and concurrent inhibition of ERK are critical for induction of apoptosis in these cells. Therefore, the dynamic balance between growth factor-activated ERK and stress-activated JNK-p38 pathways may be important in determining whether a cell survives or undergoes apoptosis.

Alkaloids↗

Inhibition of protein kinase Calpha prevents endothelial cell migration and vascular tube formation in vitro and myocardial neovascularization in vivo.

Although protein kinase C (PKC) activation is required for endothelial cell (EC) growth, migration, adhesion, and vessel formation, the role of individual PKC isoenzymes in these events is not defined. Because PKCalpha has been previously linked with enhanced EC migration and response to angiogenic growth factors, we characterized a specific phosphorothioate-modified 21-mer antisense PKCalpha (AS-PKCalpha). AS-PKCalpha (500 nmol/L) prevented the expression of PKCalpha protein by 90% in human ECs and did not reduce the expression of any other PKC isoenzyme. AS-PKCalpha reduced human EC migration by 64% compared with its control oligonucleotide in a "scratch" wounding assay, and AS-PKCalpha reduced human EC adhesion to the extracellular matrix protein vitronectin by 18%. Phosphorylation of mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2) induced by vascular endothelial growth factor was inhibited by 30% in human ECs transfected with AS-PKCalpha. Compared with control, AS-PKCalpha also reduced the number of EC tubes formed in a 3D type I collagen gel assay by 37.5%. Finally, using an osmotic minipump, we infused AS-PKCalpha into mice in which myocardial infarction was induced by coronary ligation and found that the oligonucleotide was primarily taken up by intramyocardial blood vessels. Compared with the results with control oligonucleotide, AS-PKCalpha oligonucleotide inhibited the number of anti-PKCalpha-stained blood vessels by 48% and reduced the total vessel number by 72% as well. In conclusion, the expression of PKCalpha is required for full EC migration, adhesion to vitronectin, vascular endothelial growth factor-induced extracellular signal-regulated kinase activation, and tube formation and is likely to be of importance in myocardial angiogenesis in vivo after ischemia.

Animals↗