Protein kinase A activity is required for depolarization-induced proline-rich tyrosine kinase 2 and mitogen-activated protein kinase activation in PC12 cells

Sequential gene regulatory events leading to glucocorticoid-evoked apoptosis of CEM human leukemic cells:interactions of MAPK, MYC and glucocorticoid pathways

Gene expression responses to glucocorticoid (GC) in the hours preceding onset of apoptosis were compared in three clones of human acute lymphoblastic leukemia CEM cells. Between 2 and 20h, all three clones showed increasing numbers of responding genes. Each clone had many unique responses, but the two responsive clones showed a group of responding genes in common, different from the resistant clone. MYC levels and the balance of activities between the three major groups of MAPKs are known important regulators of glucocorticoid-driven apoptosis in several lymphoid cell systems. Common to the two sensitive clones were changed transcript levels from genes that decrease amounts or activity of anti-apoptotic ERK/MAPK1 and JNK2/MAPK9, or of genes that increase activity of pro-apoptotic p38/MAPK14. Down-regulation of MYC and several MYC-regulated genes relevant to MAPKs also occurred in both sensitive clones. Transcriptomine comparisons revealed probable NOTCH-GC crosstalk in these cells.

How to awaken your nanomachines: Site-specific activation of focal adhesion kinases through ligand interactions

The focal adhesion kinase (FAK) and the related protein-tyrosine kinase 2-beta (Pyk2) are highly versatile multidomain scaffolds central to cell adhesion, migration, and survival. Due to their key role in cancer metastasis, understanding and inhibiting their functions are important for the development of targeted therapy. Because FAK and Pyk2 are involved in many different cellular functions, designing drugs with partial and function-specific inhibitory effects would be desirable. Here, we summarise recent progress in understanding the structural mechanism of how the tug-of-war between intramolecular and intermolecular interactions allows these protein 'nanomachines' to become activated in a site-specific manner.

Extracellular signal-regulated kinase and GEF-H1 mediate depolarization-induced Rho activation and paracellular permeability increase

Plasma membrane depolarization activates the Rho/Rho kinase (ROK) pathway and thereby enhances myosin light chain (MLC) phosphorylation, which in turn is thought to be a key regulator of paracellular permeability. However, the upstream mechanisms that couple depolarization to Rho activation and permeability changes are unknown. Here we show that three different depolarizing stimuli (high extracellular K(+) concentration, the lipophilic cation tetraphenylphosphonium, or l-alanine, which is taken up by electrogenic Na(+) cotransport) all provoke robust phosphorylation of ERK in LLC-PK1 and Madin-Darby canine kidney (MDCK) cells. Importantly, inhibition of ERK prevented the depolarization-induced activation of Rho. Searching for the underlying mechanism, we have identified the GTP/GDP exchange factor GEF-H1 as the ERK-regulated critical exchange factor responsible for the depolarization-induced Rho activation. This conclusion is based on our findings that 1) depolarization activated GEF-H1 but not p115RhoGEF, 2) short interfering RNA-mediated GEF-H1 silencing eliminated the activation of the Rho pathway, and 3) ERK inhibition prevented the activation of GEF-H1. Moreover, we found that the Na(+)-K(+) pump inhibitor ouabain also caused ERK, GEF-H1, and Rho activation, partially due to its depolarizing effect. Regarding the functional consequences of this newly identified pathway, we found that depolarization increased paracellular permeability in LLC-PK1 and MDCK cells and that this effect was mitigated by inhibiting myosin using blebbistatin or a dominant negative (phosphorylation incompetent) MLC. Taken together, we propose that the ERK/GEF-H1/Rho/ROK/pMLC pathway could be a central mechanism whereby electrogenic transmembrane transport processes control myosin phosphorylation and regulate paracellular transport in the tubular epithelium.

MAP-kinase-activated protein kinase 2 expression and activity is induced after neuronal depolarization

Mitogen-activated protein kinase-activated protein kinase (MK)2 is one of several downstream targets of p38 mitogen-activated protein kinase and has a well documented role in inflammation. Here, we describe a possible new function of MK2. We show that triggering depolarization by potassium chloride or increasing the cellular cAMP by forskolin treatment led to elevated levels of expression and activity of mouse MK2. In both treatments, the kinase inhibitor H89 completely prevented the up-regulation of MK2 at the transcript level. By the use of different cell lines we demonstrated that the induction of MK2 expression is characteristic of neuronal cells and is absent in fibroblasts, macrophages and kidney cells. In vivo, induction of a status epilepticus by systemic administration of the chemoconvulsant kainic acid resulted in markedly reduced neurodegeneration in the pyramidal layer of the hippocampus, dentate gyrus and hilus of MK2-deficient mice compared with wild-type mice. Together, our data suggest a possible role of MK2 in the cellular response after neuronal depolarization, in particular in excitotoxicity.

Effects of monocarboxylic acid-derived Cl−channel blockers on depolarization-activated potassium currents in rat ventricular myocytes

The effects of monocarboxylic acid-derived Cl(-) channel blockers on cardiac depolarization-activated K(+) currents were investigated. Membrane currents in rat ventricular myocytes were recorded using the whole-cell configuration of the patch-clamp technique. 5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and niflumic acid (NFA) induced an outward current at 0 mV. Both NPPB and NFA failed to induce any current when used intracellularly or after K(+) in the bath and pipette solutions was replaced by equimolar Cs(+). Voltage pulse protocols revealed that NPPB and NFA enhanced the steady-state K(+) current but inhibited the transient outward K(+) current. Genistein, a tyrosine kinase (PTK) inhibitor, inhibited NPPB- and NFA-induced outward current. Another PTK inhibitor, lavendustin A, produced a comparable effect. In contrast, the inactive analogue of genistein, daidzein, was ineffective. Orthovanadate, a tyrosine phosphatase inhibitor, markedly slowed the deactivation of the outward current induced by NPPB and NFA. The protein kinase A (PKA) inhibitor H-89 inhibited NPPB-induced outward current at 0 mV. In contrast, the protein kinase C (PKC) inhibitor H-7 was without significant effect on the action of NPPB. Pretreatment of the myocytes with genistein or H-89 prevented the enhancing effect of NPPB. Increasing intracellular Cl(-) from 22 to 132 mm slightly reduced NPPB-induced outward current at 0 mV. These results demonstrate that the monocarboxylic acid-derived Cl(-) channel blockers NPPB and NFA enhance cardiac steady-state K(+) current, and suggest that the enhancing effect of the Cl(-) channel blockers is mediated by stimulation of PKA and PTK signalling pathways.

Survival response-linked Pyk2 activation during potassium depletion-induced apoptosis of cerebellar granule neurons

Numerous extracellular stimuli trigger trans-autophosphorylation at Tyr402 of Pyk2, inducing its activation. Pyk2 is a key mediator of several signaling pathways and has been implicated in apoptosis induced by specific stress signals. We investigated whether Pyk2 participates in cerebellar granule neuron (CGN) apoptosis induced by the suppression of membrane depolarization. We demonstrate that shifting CGN cultures from 25 mM to 5 mM KCl-containing medium induces an early, transient 70% increase in phosphorylated Tyr402 and Tyr580 Pyk2 levels that is triggered by Ca(2+) released from intracellular stores and mediated by calmodulin (CaM). Overexpression of Pyk2 increases CGN survival after 24 h by 70% compared to the control, thus suggesting that Pyk2 is involved in an anti-apoptotic response to K+ lowering. Furthermore, we show that CGN grown in K25 medium exhibit detectable CaM-dependent Pyk2 activity. When silencing Pyk2 activity by expressing a dominant-negative form, only 40% of the transfected neurons were alive 24 h after transfection when compared to the control. Overall, the present findings demonstrate for the first time that Pyk2 is a critical mediator of CGN survival.

Corticotropin-releasing hormone induces vascular endothelial growth factor release from human mast cells via the cAMP/protein kinase A/p38 mitogen-activated protein kinase pathway

Mast cells are involved in allergic reactions but also in innate immunity and inflammation. Corticotropin-releasing hormone (CRH), the key regulator of the hypothalamic-pituitary-adrenal axis, also has proinflammatory effects, apparently through mast cells. We showed recently that CRH selectively stimulates human leukemic mast cells and human umbilical cord blood-derived mast cells to release newly synthesized vascular endothelial growth factor (VEGF) without release of either preformed mediators or cytokines. This effect was mediated through the activation of CRH receptor-1 and adenylate cyclase with increased intracellular cAMP. However, the precise mechanism by which CRH induces VEGF secretion has not yet been defined. Here, we show that CRH-induced VEGF release was dose-dependently inhibited by the specific protein kinase A inhibitor N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H89) or the p38 mitogen-activated protein kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580) but not by the specific inhibitor 2'-amino-3'-methoxyflavone (PD98059) of mitogen-activated protein kinase kinase, the upstream kinase of the extracellular signal-regulated protein kinase (ERK) or the c-Jun N-terminal kinase (JNK) inhibitor 1,9-pyrazoloanthrone anthra-(1,9-cd)pyrazol-6(2H)-one (SP600125). Furthermore, CRH significantly increased protein kinase A activity, which could be mimicked by the cell-permeable cAMP analog 8-bromo-cAMP, and was blocked by H89 or the adenylate cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purine-6-amine (SQ22536). CRH also induced rapid phosphorylation of p38 MAPK, which was mimicked by 8-bromo-cAMP and was inhibited by H89 or SB203580. CRH did not stimulate ERK or JNK phosphorylation and did not increase intracellular calcium levels. These results indicate that CRH induces VEGF release in human mast cells via selective activation of the cAMP/protein kinase A/p38 MAPK signaling pathway, thereby providing further insight into the molecular mechanism of how CRH affects the release of a key proinflammatory mediator.

Endothelin-1- and depolarization-induced differential regulation of cAMP response element-binding protein in proliferating and developed vascular smooth muscle

Regulation of the transcription factor cAMP response element-binding protein (CREB) is important for gene expression in vascular smooth muscle (VSM). This study has examined the intracellular regulation of CREB by endothelin-1 (ET-1) and depolarization in native VSM, comparing proliferative and fully differentiated phenotypes. Portal veins from neonatal (proliferative) and 6-week-old (differentiated) rats were stimulated with ET-1 or K(+). In both phenotypes, CREB activation was increased by ET-1 although the time course was prolonged in neonatal VSM. This paralleled a prolonged ET-1-induced [Ca(2+)](i) increase. ET-1-induced CREB activation was dependent on extracellular signal-regulated kinase 1/2 (ERK1/2) activation and inhibited by BAPTA but not by a calmodulin-dependent protein kinase (CamK) inhibitor. In contrast, CREB activation induced by depolarization in both neonatal and developed VSM was significantly reduced by CaMK inhibition and by ERK1/2 inhibition. Therefore, CREB activation is regulated differentially in VSM depending upon stimulus; however, this is not altered in different growth states.

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Urocortin, but not urocortin II, protects cultured hippocampal neurons from oxidative and excitotoxic cell death via corticotropin-releasing hormone receptor type I

Urocortin and urocortin II are members of the corticotropin-releasing hormone (CRH) family of neuropeptides that function to regulate stress responses. Two high-affinity G-protein-coupled receptors have been identified that bind CRH and/or urocortin I and II, designated CRHR1 and CRHR2, both of which are present in hippocampal regions of mammalian brain. The hippocampus plays an important role in regulating stress responses and is a brain region in which neurons are vulnerable during disease and stress conditions, including cerebral ischemia, Alzheimer's disease, and anxiety disorders. Here we report that urocortin exerts a potent protective action in cultured rat hippocampal neurons with concentrations in the range of 0.5-5.0 pm, increasing the resistance of the cells to oxidative (amyloid beta-peptide, 4-hydroxynonenal, ferrous sulfate) and excitotoxic (glutamate) insults. We observed that urocortin is 10-fold more potent than CRH in protecting hippocampal neurons from insult, whereas urocortin II is ineffective. RT-PCR and sequencing analyses revealed the presence of both CRHR1 and CRHR2 in the hippocampal cultures, with CRHR1 being expressed at much higher levels than CRHR2. Using subtype-selective CRH receptor antagonists, we provide evidence that the neuroprotective effect of exogenously added urocortin is mediated by CRHR1. Furthermore, we provide evidence that the signaling pathway that mediates the neuroprotective effect of urocortin involves cAMP-dependent protein kinase, protein kinase C, and mitogen-activated protein kinase. This is the first demonstration of a biological activity of urocortin in hippocampal neurons, suggesting a role for the peptide in adaptive responses of hippocampal neurons to potentially lethal oxidative and excitotoxic insults.

Electroconvulsive Shock Increases the Phosphorylation of Pyk2 in the Rat Hippocampus

Recently we reported the activation MAPKs, MEK, and Rafs by electroconvulsive shock (ECS) in the rat hippocampus. However, the upstream pathways for the activation of Raf-MEK-MAPK cascade after ECS have not been studied yet. Since the proline-rich tyrosine kinase 2 (Pyk2) and Src were reported to be involved in the activation of the MAPKs in neuronal cells, we examined tyrosine phosphorylation and activation of Pyk2 in the rat hippocampus after ECS. ECS transiently increased the phosphorylation of Pyk2 at multiple tyrosine residues (Tyr-402, Tyr-580, and Tyr-881). The phosphorylations reached the peak at 1 min and returned to basal level by 10 min after ECS. At 1 min after ECS, the binding of Pyk2 to Src and Grb2, and of Grb2 to Ras increased. These results suggested that ECS activates Pyk2, which then transmits the signal to MAPK cascade via Src, Grb2, and Ras in the rat hippocampus.