Localized effects of cAMP mediated by distinct routes of protein kinase A

Cyclic nucleotide phosphodiesterases as targets for treatment of haematological malignancies

The cAMP signalling pathway has emerged as a key regulator of haematopoietic cell proliferation, differentiation and apoptosis. In parallel, general understanding of the biology of cyclic nucleotide PDEs (phosphodiesterases) has advanced considerably, revealing the remarkable complexity of this enzyme system that regulates the amplitude, kinetics and location of intracellular cAMP-mediated signalling. The development of therapeutic inhibitors of specific PDE gene families has resulted in a growing appreciation of the potential therapeutic application of PDE inhibitors to the treatment of immune-mediated illnesses and haematopoietic malignancies. This review summarizes the expression and function of PDEs in normal haematopoietic cells and the evidence that family-specific inhibitors will be therapeutically useful in myeloid and lymphoid malignancies.

Phosphoinositide 3-kinasegamma (PI3Kgamma) controls L-type calcium current (ICa,L) through its positive modulation of type-3 phosphodiesterase (PDE3)

The modulation of L-type calcium current (ICa,L) is mainly due to mediators acting through activation of G protein-coupled receptors (GPCR) and different protein kinases; among them, phosphoinositide 3-kinasegamma (PI3Kgamma) has been recently discovered to play an important role in the regulation of cardiac contractility and beta-adrenergic signal transduction. Recent reports have demonstrated that, in the heart, different subtypes of beta-adrenergic receptors are coupled to both Gi and/or Gs proteins. While beta1-adrenergic receptors (beta1-AR) couple only to Gs and evoke a strong ICa,L, beta2-adrenergic receptors (beta2-AR) can activate both Gs and Gi proteins and trigger only a limited ICa,L. Here we demonstrate that (i) PI3Kgamma-/- ventricular myocytes are characterized by an higher basal ICa,L density, even if the responsiveness of adenylyl cyclase to Forskolin is comparable to that observed in PI3Kgamma+/+ cardiomyocytes; (ii) both in basal conditions and after beta-AR stimulation, the activity of phosphodiesterase (PDE) type 3 depends on PI3Kgamma; (iii) in PI3Kgamma-/- cardiac myocytes, specific stimulation of beta2-AR is followed by a increase in ICa,L stronger than in wild-type controls. Taken together, our results suggest that the higher values of ICa,L observed both in basal conditions and after beta-AR stimulation in PI3Kgamma-/- ventricular myocytes are mainly due to a positive modulation of PDE3 activity exerted by PI3Kgamma. As observed in PI3Kgamma-/- neonatal cardiomyocytes, cells lacking PI3Kgamma are more sensitive to stimulation of beta2-adrenergic receptors.

Keynote review: phosphodiesterase-4 as a therapeutic target

Cyclic AMP (cAMP) is a key second messenger in all cells. It is compartmentalized within cells and its levels are controlled, as a result of spatially discrete signaling cassettes controlling its generation, detection and degradation. Underpinning compartmentalized cAMP signaling are approximately 20 members of the phosphodiesterase-4 (PDE4) family. The selective inhibition of this family generates profound, functional effects and PDE4 inhibitors are currently under development to provide potential, novel therapeutics for the treatment of inflammatory diseases, such as asthma, chronic obstructive pulmonary disease and psoriasis, as well as treating depression and serving as cognitive enhancers. Here, we delineate the range of PDE4 isoforms, their role in signaling, their structural biology and related preclinical and clinical pharmacology.

Novel, isotype-specific sensors for protein kinase A subunit interaction based on bioluminescence resonance energy transfer (BRET)

Homogeneous protein-protein interaction assays without the need of a separation step are an essential tool to unravel signal transduction events in live cells. We have established an isoform specific protein kinase A (PKA) subunit interaction assay based on bioluminescence resonance energy transfer (BRET). Tagging human Ralpha(I)-, Ralpha(II)-, as well as Calpha-subunits of PKA with Renilla luciferase (Rluc) as the bioluminescent donor or with green fluorescent protein (GFP2) as the energy acceptor, respectively, allows to directly probe PKA subunit interaction in living cells as well as in total cell extracts in order to study side by side PKA type I versus type II holoenzyme dynamics. Several novel, genetically encoded cAMP sensors and-for the first time PKA type I sensors-were generated. When C- and R-subunits are assembled to the respective holoenzyme complexes inside the cell, BRET occurs with a signal up to three times above the background. An increase of endogenous cAMP levels as well as treatment with the cAMP analog 8-Br-cAMP is reflected by a dose-dependent BRET signal reduction in cells expressing wild type proteins. In contrast to type II, the dissociation of the PKA type I holoenzyme complex was never complete in cells with maximally elevated cAMP levels. Both sensors dissociated completely upon treatment with 8-Br-cAMP after cell lysis, consistent with in vitro activation assays using holoenzymes assembled from purified PKA subunits. Interestingly, incubation of cells with the PKA antagonist Rp-8-Br-cAMPS leads to a significant BRET signal increase in cells expressing PKA type I or type II isoforms, indicating a stabilization of the holoenzyme complexes in vivo. Mutant RI subunits with reduced (hRIalpha-R210K) or abolished (hRIalpha-G200E/G324E) cAMP binding capability were studied to quantify maximal signal to noise ratios for the RI-BRET sensor. Utilizing BRET we demonstrate that PKA type II holoenzyme was rendered insensitive to beta-adrenergic receptor stimulation with isoproterenol when anchoring to the plasma membrane of COS-7 cells was disrupted by either using Ht31 peptide or by depletion of membrane cholesterol.

PKCtheta and PKA are antagonistic partners in the NF-AT transactivation pathway of primary mouse CD3+ T lymphocytes

We here investigate the crosstalk of PKC and PKA signaling during primary CD3(+) T-lymphocyte activation using pharmacologic inhibitors and activators in combination with our established panel of PKC isotype-deficient mouse T cells in vitro. PKCtheta and PKA inversely affect the CD3/CD28-induced IL-2 expression, whereas other PKC isotypes are dispensable in this signaling pathway. Gene ablation of PKCtheta selectively results in a profound reduction of IL-2 production; however, complete abrogation of IL-2 production in these PKCtheta(-/-) T cells was achieved only by simultaneous coactivation of the cAMP/PKA pathway in CD3(+) T cells. Conversely, the reduced IL-2 production in PKC inhibitor-treated T cells can be rescued by inhibition of the cAMP/PKA pathway in wild-type but not in PKCtheta(-/-) T cells. Mechanistically, the cAMP/PKA and PKCtheta pathways converge at the level of NF-AT, as shown by DNA binding analysis. The combined increase in PKA and decrease in PKCtheta activity leads to an enhanced inhibition of nuclear NF-AT translocation. This PKCtheta/PKA crosstalk significantly affects neither the NF-kappaB, the AP-1, nor the CREB pathways. Taken together, this opposite effect between the positive PKCtheta and the negative cAMP/PKA signaling pathways appears rate limiting for NF-AT transactivation and IL-2 secretion responses of CD3(+) T lymphocytes.

Spatial organisation of AKAP18 and PDE4 isoforms in renal collecting duct principal cells

A plethora of stimuli including hormones and neurotransmitters mediate a rise of the cellular level of cAMP and thereby activation of protein kinase A (PKA). PKA phosphorylates and thereby modulates the activity of a wide range of cellular targets. It is now appreciated that different stimuli induce the activation of PKA at specific sites where the kinase phosphorylates particular substrates in close proximity. The tethering of PKA to cellular compartments is facilitated by A kinase-anchoring proteins (AKAPs). The incorporation of phosphodiesterases (PDEs) into AKAP-based signalling complexes provides gradients of cAMP that regulate PKA activity locally. An example for a process depending on compartmentalised cAMP/PKA signalling is the arginine-vasopressin (AVP)-mediated water reabsorption in renal collecting duct principal cells. Upon activation through AVP, PKA phosphorylates the water channel aquaporin-2 (AQP-2) located on intracellular vesicles. The phosphorylation triggers the redistribution of AQP2 to the plasma membrane. AKAP-anchored PKA has been shown to be involved in AQP2 shuttling. Here, AKAP18 isoforms and members of the PDE4 family of PDEs are shown to be differentially localised in renal principal cells.

Phosphodiesterases and compartmentalized cAMP signalling in the heart

A key feature of the cAMP/cAMP-dependent protein kinase (PKA) transduction system is the compartmentalisation of its signalling enzymes and effectors. Given the large diversity of PKA targets within cardiac cells a precisely regulated and confined activity of such signalling pathway is essential for specificity of response. This appears to be achieved through the generation of local pools of high cAMP and activation of PKA at discrete subcellular locations. Phosphodiesterases (PDEs) are the only route for degrading cAMP and are thus poised to regulate intracellular cAMP gradients. Their spatial confinement to discrete compartments and functional coupling to individual receptors provides an efficient way to control local [cAMP](i) in a stimulus-specific manner. A better understanding of the distinctive role that individual PDEs play in shaping the cAMP signal in heart cells may lead to the development of new strategies for selective pharmacologic manipulation of cAMP signalling in defined functional domains.

Histamine H2 receptor overexpression induces U937 cell differentiation despite triggered mechanisms to attenuate cAMP signalling

Knowing that cell-surface receptors that recognize and respond to extracellular stimuli are key components for the regular communication between individual cells required for the survival of any living organism, the aim of the present work was to investigate the effect of H2R overexpression on the U937 signal transduction pathway and its consequences on cell proliferation and differentiation. The overexpression of H2R led to an increase in cAMP basal levels, a leftward shift of agonist concentration-response curves, and similar maximal response to agonist treatment, suggesting that overexpressed H2Rs act as functional spare receptors. In this system cells triggered several mechanisms tending to restore cAMP basal levels to those of the naïve cells. H2R overexpression induced PDE activity stimulation and GRK2 overexpression. In spite of the onset of these regulatory mechanisms, H2 agonist and rolipram treatments induced the terminal differentiation of the H2R overexpressed clone, conversely to the naïve cells. Present findings show that stably H2R overexpression alters cAMP signalling as the result of not only the amounts of second messenger generated but also the activation or upregulation of various components of signalling cascade, leading to an adapted biologically unique system. This adaptation may represent an advantage or a disadvantage, depending on the biological system, but in any case, the existence of compensatory mechanisms should be considered when a clinical treatment is designed.

Hypoxia-induced remodelling of PDE4 isoform expression and cAMP handling in human pulmonary artery smooth muscle cells

Human pulmonary artery smooth muscle cells (hPASM cells) express PDE4A10, PDE4A11, PDE4B2, PDE4C and PDE4D5 isoforms. Hypoxia causes a transient up-regulation of PDE4B2 that reaches a maximum after 7 days and sustained up-regulation of PDE4A10/11 and PDE4D5 over 14 days in hypoxia. Seven days in hypoxia increases both intracellular cAMP levels, protein kinase A (PKA) activity and activated, phosphorylated extracellular signal regulated kinase (pERK) but does not alter either PKA isoform expression or total cAMP phosphodiesterase-4 (PDE4) activity or cAMP phosphodiesterase-3 (PDE3) activity. Both the cyclooxygenase inhibitor, indomethacin and the ERK inhibitors, UO126 and PD980589 reverse the hypoxia-induced increase in intracellular cAMP levels back to those seen in normoxic hPASM cells. Challenge of normoxic hPASM cells with prostaglandin E(2) (PGE(2)) elevates cAMP to levels comparable to those seen in hypoxic cells but fails to increase intracellular cAMP levels in hypoxic hPASM cells. The adenylyl cyclase activator, forskolin increases cAMP levels in both normoxic and hypoxic hPASM cells to comparable elevated levels. Challenge of hypoxic hPASM cells with indomethacin attenuates total PDE4 activity whilst challenge with UO126 increases total PDE4 activity. We propose that the hypoxia-induced activation of ERK initiates a phospholipase A(2)/COX-driven autocrine effect whereupon PGE(2) is generated, causing the activation of adenylyl cyclase and increase in intracellular cAMP. Despite the hypoxia-induced increases in the expression of PDE4A10/11, PDE4B2 and PDE4D5 and activation of certain of these long PDE4 isoforms through PKA phosphorylation, we suggest that the failure to see any overall increase in PDE4 activity is due to ERK-mediated phosphorylation and inhibition of particular PDE4 long isoforms. Such hypoxia-induced increase in expression of PDE4 isoforms known to interact with certain signalling scaffold proteins may result in alterations in compartmentalised cAMP signalling. The hypoxia-induced increase in cAMP may represent a compensatory protective mechanism against hypoxia-induced mitogens such as endothelin-1 and serotonin.

A FYVE-containing unusual cyclic nucleotide phosphodiesterase from Trypanosoma cruzi

Cyclic-nucleotide-specific phosphodiesterases (PDEs) are key players in the intracellular signaling pathways of the important human pathogen Trypanosoma cruzi. We report herein the identification of an unusual PDE from this protozoal organism. This enzyme, TcrPDEC, is a member of the class I PDEs, as determined from the presence of a characteristic signature sequence and from the conservation of a number of functionally important amino acid residues within its catalytic domain. Class I PDEs include a large number of PDEs from eukaryotes, among them all 11 human PDE families. Unusually for an enzyme of this class, TcrPDEC contains a FYVE-type domain in its N-terminal region, followed by two closely spaced coiled-coil domains. Its catalytic domain is located in the middle of the polypeptide chain, whereas all other class I enzymes contain their catalytic domains in their C-terminal parts. TcrPDEC can complement a PDE-deficient yeast strain. Unexpectedly for a kinetoplastid PDE, TcrPDEC is a dual-specificity PDE that accepts both cAMP and cGMP as its substrates.

Termination of cAMP signals by Ca2+ and G(alpha)i via extracellular Ca2+ sensors: a link to intracellular Ca2+ oscillations

Termination of cyclic adenosine monophosphate (cAMP) signaling via the extracellular Ca²⁺-sensing receptor (CaR) was visualized in single CaR-expressing human embryonic kidney (HEK) 293 cells using ratiometric fluorescence resonance energy transfer–dependent cAMP sensors based on protein kinase A and Epac. Stimulation of CaR rapidly reversed or prevented agonist-stimulated elevation of cAMP through a dual mechanism involving pertussis toxin–sensitive Gα_i and the CaR-stimulated increase in intracellular [Ca²⁺]. In parallel measurements with fura-2, CaR activation elicited robust Ca²⁺ oscillations that increased in frequency in the presence of cAMP, eventually fusing into a sustained plateau. Considering the Ca²⁺ sensitivity of cAMP accumulation in these cells, lack of oscillations in [cAMP] during the initial phases of CaR stimulation was puzzling. Additional experiments showed that low-frequency, long-duration Ca²⁺ oscillations generated a dynamic staircase pattern in [cAMP], whereas higher frequency spiking had no effect. Our data suggest that the cAMP machinery in HEK cells acts as a low-pass filter disregarding the relatively rapid Ca²⁺ spiking stimulated by Ca²⁺-mobilizing agonists under physiological conditions.

Solution scattering reveals large differences in the global structures of type II protein kinase A isoforms

Isoform diversity within the protein kinase A (PKA) family is achieved by catalytic (C) subunits binding to different isoforms of regulatory subunit homodimers (R2). In a previous small-angle X-ray scattering study, we showed that the type Ialpha R2 homodimer has a distinctive Y-shaped structure, while the IIalpha and IIbeta homodimers are highly flexible and extended in solution. Here we present the results of X-ray scattering experiments on different isoforms of the PKA holoenzyme (R2C2) and show that the type IIbeta R2 homodimer undergoes a dramatic compaction upon binding C subunits that involves a 10A reduction in radius of gyration (from 56 to 46 A) and a 35 A shortening of the maximum linear dimension (from 180-145 A). In contrast, the type IIalpha R2 homodimer shows very little change in these structural parameters and remains extended upon C-subunit binding. This large difference is surprising given the highly conserved sequence and domain organization for the different R isoforms. A mutant RIIbeta holoenzyme and an RIIalpha/RIIbeta chimera were used to explore the role of the sequence linking different functional domains within RIIbeta in the observed C subunit-induced compaction. Structural modeling was used to aid in interpreting the scattering results in terms of the role of inter-domain and inter-subunit contacts in determining the global conformations of the different isoforms. The results provide an important structural foundation for understanding isoform-specific PKA localization and signaling.

Calcium-induced acrosomal exocytosis requires cAMP acting through a protein kinase A-independent, Epac-mediated pathway

Epac, a guanine nucleotide exchange factor for the small GTPase Rap, binds to and is activated by the second messenger cAMP. In sperm, there are a number of signaling pathways required to achieve egg-fertilizing ability that depend upon an intracellular rise of cAMP. Most of these processes were thought to be mediated by cAMP-dependent protein kinases. Here we report a new dependence for the cAMP-induced acrosome reaction involving Epac. The acrosome reaction is a specialized type of regulated exocytosis leading to a massive fusion between the outer acrosomal and the plasma membranes of sperm cells. Ca2+ is the archetypical trigger of regulated exocytosis, and we show here that its effects on acrosomal release are fully mediated by cAMP. Ca2+ failed to trigger acrosomal exocytosis when intracellular cAMP was depleted by an exogenously added phosphodiesterase or when Epac was sequestered by specific blocking antibodies. The nondiscriminating dibutyryl-cAMP and the Epac-selective 8-(p-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate analogues triggered the acrosome reaction in the effective absence of extracellular Ca2+. This indicates that cAMP, via Epac activation, has the ability to drive the whole cascade of events necessary to bring exocytosis to completion, including tethering and docking of the acrosome to the plasma membrane, priming of the fusion machinery, mobilization of intravesicular Ca2+, and ultimately, bilayer mixing and fusion. cAMP-elicited exocytosis was sensitive to anti-alpha-SNAP, anti-NSF, and anti-Rab3A antibodies, to intra-acrosomal Ca2+ chelators, and to botulinum toxins but was resistant to cAMP-dependent protein kinase blockers. These experiments thus identify Epac in human sperm and evince its indispensable role downstream of Ca2+ in exocytosis.

Inhibitors of signal transduction protein kinases as targets for cancer therapy

Cancer development requires that tumour cells attain several capabilities, including increased replicative potentials, anchorage and growth-factor independency, evasion of apoptosis, angiogenesis and metastasis. Many of these processes involve the actions of protein kinases, which have emerged as key regulators of all aspects of neoplasia. Perturbed protein kinase activity is repeatedly found to be associated with human malignancies, making these proteins attractive targets for anti-cancer therapy. The last decade has witnessed an exponential increase in the development of specific small protein kinase inhibitors. Many of them are in clinical trials in patients with different types of cancer and some are successfully used in clinic. This review describes different approaches that are currently applied to develop such specific protein kinase inhibitors and provides an overview of protein kinase inhibitors that are currently in clinical trials or are administered in the clinic. Focus is directed on inhibitors against receptor tyrosine kinases and protein kinases participating in the signalling cascades.

PRKAR1A mutations in primary pigmented nodular adrenocortical disease

Primary Pigmented Nodular Adrenocortical Disease (PPNAD) is a rare primary bilateral adrenal defect causing corticotropin-independent Cushing's syndrome. It occurs mainly in children and young adults. Macroscopic appearance of the adrenals is characteristic with small pigmented micronodules observed in the cortex. PPNAD is most often diagnosed in patients with Carney complex (CNC), but it can also be observed in patients without other manifestations or familial history (isolated PPNAD). The CNC is an autosomal dominant multiple neoplasia syndrome characterized by the association of myxoma, spotty skin pigmentation and endocrine overactivity. One of the putative CNC genes has been identified as the gene of the regulatory R1A subunit of protein kinase A (PRKAR1A), located at 17q22-24. Germline heterozygous inactivating mutations of PRKAR1A have been reported in about 45% of patients with CNC, and up to 80% of CNC patients with Cushing's syndrome due to PPNAD. Interestingly, such inactivating germline PRKAR1A mutations have also been found in patients with isolated PPNAD. The hot spot PRKAR1A mutation termed c.709[-7-2]del6 predisposes mostly to isolated PPNAD, and is the first clear genotype/phenotype correlation described for this gene. Somatic inactivating mutations of PRKAR1A have been observed in macronodules of PPNAD and in sporadic cortisol secreting adrenal adenomas. Isolated PPNAD is a genetic heterogenous disease, and recently inactivating mutations of the gene of the phosphodiesterase 11A4 (PDE11A4) located at 2q31-2q35 have been identified in patients without PRKAR1A mutations. Interestingly, both PRKAR1A and PDE11A gene products control the cAMP signaling pathway, which can be altered at various levels in endocrine tumors.

Mitochondrial AKAP121 links cAMP and src signaling to oxidative metabolism

AKAP121 focuses distinct signaling events from membrane to mitochondria by binding and targeting cAMP-dependent protein kinase (PKA), protein tyrosine phosphatase (PTPD1), and mRNA. We find that AKAP121 also targets src tyrosine kinase to mitochondria via PTPD1. AKAP121 increased src-dependent phosphorylation of mitochondrial substrates and enhanced the activity of cytochrome c oxidase, a component of the mitochondrial respiratory chain. Mitochondrial membrane potential and ATP oxidative synthesis were enhanced by AKAP121 in an src- and PKA-dependent manner. Finally, siRNA-mediated silencing of endogenous AKAP121 drastically impaired synthesis and accumulation of mitochondrial ATP. These findings indicate that AKAP121, through its role in enhancing cAMP and tyrosine kinase signaling to distal organelles, is an important regulator in mitochondrial metabolism.

Signal transduction gRABs attention

Rab proteins are small GTPases involved in the regulation of vesicular membrane traffic. Research done in the past years has demonstrated that some of these proteins are under the control of signal transduction pathways. Still, several recent papers point out to a new unexpected role for this family of Ras-related proteins, as potential regulators of intracellular signaling pathways. In particular, several evidence indicate that members of the Rab family of small GTPases, through their effectors, are key molecules participating to the regulation of numerous signal transduction pathways profoundly influencing cell proliferation, cell nutrition, innate immune response, fragmentation of compartments during mitosis and apoptosis. Even more surprisingly, direct involvement of Rab proteins in signaling to the nucleus has been demonstrated. This review will focus on aspects of Rab proteins function connected to signal transduction and, in particular, connections between membrane traffic and other cell pathways will be examined.

PKA-mediated phosphorylation regulates the function of activation-induced deaminase (AID) in B cells

During humoral immune responses, two distinct genetic modification events diversify the Ig genes in germinal center (GC) B cells: somatic hypermutation and class switch recombination (CSR). Both processes require the activity of activation-induced cytidine deaminase (AID), an enzyme expressed specifically in GC B cells. However, the mechanisms that regulate AID activity are largely unknown. Here we report that protein kinase A (PKA) phosphorylates AID and regulates its activity in GC B cells. AID physically interacts with the PKA holoenzyme in the cytoplasm and is phosphorylated by the PKA catalytic subunit at specific residues. AID phosphorylation is required for CSR, because substitution of the two phosphorylation targets impairs its ability to rescue CSR in AID-deficient B cells. Pharmacologic inhibition of PKA prevents isotype class switching in a murine B-cell lymphoma cell line; conversely, B cells from mice where PKA activity is made constitutive by conditional deletion of the PKA regulatory subunit gene display enhanced CSR. These findings implicate PKA in the regulation of AID function and suggest that the control of T cell-dependent immune responses may be modulated, via AID, by signals that activate PKA.

Rearrangements in a hydrophobic core region mediate cAMP action in the regulatory subunit of PKA

cAMP-dependent protein kinase (PKA) forms an inactive heterotetramer of two regulatory (R; with two cAMP-binding domains A and B each) and two catalytic (C) subunits. Upon the binding of four cAMP molecules to the R dimer, the monomeric C subunits dissociate. Based on sequence analysis of cyclic nucleotide-binding domains in prokaryotes and eukaryotes and on crystal structures of cAMP-bound R subunit and cyclic nucleotide-free Epac (exchange protein directly activated by cAMP), four amino acids were identified (Leu203, Tyr229, Arg239 and Arg241) and probed for cAMP binding to the R subunits and for R/C interaction. Arg239 and Arg241 (mutated to Ala and Glu) displayed no differences in the parameters investigated. In contrast, Leu203 (mutated to Ala and Trp) and Tyr229 (mutated to Ala and Thr) exhibited up to 30-fold reduced binding affinity for the C subunit and up to 120-fold reduced binding affinity for cAMP. Tyr229Asp showed the most severe effects, with 350-fold reduced affinity for cAMP and no detectable binding to the C subunit. Based on these results and structural data in the cAMP-binding domain, a switch mechanism via a hydrophobic core region is postulated that is comparable to an activation model proposed for Epac.

Compartmentalized phosphodiesterase-2 activity blunts beta-adrenergic cardiac inotropy via an NO/cGMP-dependent pathway

beta-Adrenergic signaling via cAMP generation and PKA activation mediates the positive inotropic effect of catecholamines on heart cells. Given the large diversity of protein kinase A targets within cardiac cells, a precisely regulated and confined activity of such signaling pathway is essential for specificity of response. Phosphodiesterases (PDEs) are the only route for degrading cAMP and are thus poised to regulate intracellular cAMP gradients. Their spatial confinement to discrete compartments and functional coupling to individual receptors provides an efficient way to control local [cAMP]i in a stimulus-specific manner. By performing real-time imaging of cyclic nucleotides in living ventriculocytes we identify a prominent role of PDE2 in selectively shaping the cAMP response to catecholamines via a pathway involving beta3-adrenergic receptors, NO generation and cGMP production. In cardiac myocytes, PDE2, being tightly coupled to the pool of adenylyl cyclases activated by beta-adrenergic receptor stimulation, coordinates cGMP and cAMP signaling in a novel feedback control loop of the beta-adrenergic pathway. In this, activation of beta3-adrenergic receptors counteracts cAMP generation obtained via stimulation of beta1/beta2-adrenoceptors. Our study illustrates the key role of compartmentalized PDE2 in the control of catecholamine-generated cAMP and furthers our understanding of localized cAMP signaling.

Overexpression of RhoA-GTP induces activation of the Epidermal Growth Factor Receptor, dephosphorylation of focal adhesion kinase and increased motility in breast cancer cells

Rho GTPases are overexpressed in human tumors and are involved in a variety of cellular processes such as organization of the actin cytoskeleton, cell-cell contact and malignant transformation. EGFR activation plays a key role in the acquisition of motile properties in carcinoma cells, and it has been proposed that downregulation of FAK activity is one of its most relevant consequences. In the present study, using mammary MCF-7 cells, we demonstrated that overexpression of the active form of the small GTPase RhoA induced the activation of EGFR by a phenomenon that depends on the activity of a metalloproteinase (MMP), which presumably cleaves a membrane-bound EGFR ligand. The EGFR tyrosine phosphorylation correlates with ERK1,2 activation and the stimulation of urokinase production. An aggressive mammary cell line (MDA-MB-231) that overexpresses both RhoA and EGFR in their active forms also displayed an MMP-dependent activation mechanism of EGFR. RhoA-GTP-transfected cells showed a cortical array of F-actin, rounded morphology, reduced spreading potential and a dephosphorylation of FAK that was released by integrin-dependent fibronectin adhesion and a specific EGFR tyrosine kinase inhibitor. Our results suggest that the MMP-dependent EGFR activation observed in V14 RhoA cells represents the starting point of a signaling route that promotes cell motility by activation of ERK1,2 and further enhancement of proteases production.

Effects of forskolin on Kupffer cell production of interleukin-10 and tumor necrosis factor alpha differ from those of endogenous adenylyl cyclase activators: possible role for adenylyl cyclase 9

Proinflammatory cytokines like tumor necrosis factor alpha (TNF-alpha) that are released from Kupffer cells may trigger liver inflammation and damage. Hence, endogenous mechanisms for limiting TNF-alpha expression are crucial for avoiding the development of sepsis. Such mechanisms include the anti-inflammatory actions of interleukin-10 (IL-10) as well as signaling induced by the intracellular second messenger cyclic AMP (cAMP). Kupffer cells express several receptors that activate cAMP synthesis, including E-prostanoid receptors and beta-adrenergic receptors. The expression and role of specific adenylyl cyclases in the inhibition of Kupffer cell activation have so far not been subject to study. Pretreatment of rat Kupffer cell cultures with cAMP analogues [8-(4-chlorophenyl)-thio-cAMP], adenylyl cyclase activator (forskolin), or ligands for G-coupled receptors (isoproterenol or prostaglandin E2) 30 min before the addition of lipopolysaccharide (LPS) (1 microg/ml) caused attenuated TNF-alpha levels in culture medium (forskolin/isoproterenol, P < or = 0.05; prostaglandin E2, P < or = 0.01). Forskolin also reduced IL-10 mRNA and protein (P < or = 0.05), which was not observed with the other cAMP-inducing agents. Furthermore, we found that rat Kupffer cells express high levels of the forskolin-insensitive adenylyl cyclase 9 compared to whole liver and that this expression is down-regulated by LPS (P < or = 0.05). We conclude that regulation of TNF-alpha and IL-10 in Kupffer cells depends on the mechanism by which cAMP is elevated. Forskolin and prostaglandin E2 differ in their effects, which suggests a possible role of forskolin-insensitive adenylyl cyclases like adenylyl cyclase 9.

Imaging the cAMP-dependent signal transduction pathway

In recent years, the development of new technologies based on the green fluorescent protein and FRET (fluorescence resonance energy transfer) has introduced a new perspective in the study of cAMP signalling. Real-time imaging of fluorescent biosensors is making it possible to visualize cAMP dynamics directly as they happen in intact, living cells, providing important and original insights for our understanding of the spatiotemporal organization of the cAMP/PKA (protein kinase A) signalling pathway.

Beta-arrestin-recruited phosphodiesterase-4 desensitizes the AKAP79/PKA-mediated switching of beta2-adrenoceptor signalling to activation of ERK

Using combined dominant-negative and siRNA (small interfering RNA)-mediated knockdown strategies, the functional importance of specific PDE4 (phosphodiesterase-4) isoforms in modifying signalling through the beta2-AR (beta2-adrenoceptor) has been uncovered. The PDE4D5 isoform preferentially interacts with the signalling scaffold protein beta-arrestin and is thereby recruited to the beta2-AR upon agonist challenge. Delivery of an active PDE to the site of cAMP synthesis at the plasma membrane specifically attenuates the activity of a pool of PKA (protein kinase A) that is tethered to the beta2-AR via AKAP79 (A-kinase anchoring protein 79). The specific functional role of this anchored PKA is to phosphorylate the beta2-AR and allow it to switch its coupling with G(i) and thereby activation of ERK (extracellular-signal-regulated kinase). Our studies uncover a novel facet of the regulation of beta2-AR signalling by showing that beta-arrestin-recruited PDE4 provides the means of desensitizing the agonist-dependent coupling of beta2-AR with G(i) and its consequential activation of ERK.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

Compartmentalized cAMP signalling regulates vasopressin-mediated water reabsorption by controlling aquaporin-2

The cAMP/PKA (protein kinase A) signalling pathway is activated by a plethora of stimuli. To facilitate the specificity of a cellular response, signal transduction complexes are formed and segregated to discrete sites (compartmentalization). cAMP/PKA signalling compartments are maintained by AKAPs (A-kinase anchoring proteins) which bind PKA and other signalling proteins, and by PDEs (phosphodiesterases). The latter hydrolyse cAMP and thus limit its diffusion and terminate PKA activity. An example of a cAMP-dependent process requiring compartmentalization of cAMP/PKA signals is arginine-vasopressin-regulated water reabsorption in renal principal cells. A detailed understanding of the protein interactions within a signal transduction complex offers the possibility to design agents influencing PKA binding to a specific AKAP, the targeting of an AKAP or the interactions of AKAPs with other signalling molecules. The ability to specifically modulate selected branches of a signal transduction pathway would greatly advance basic research, and may lead to new drugs suitable for the treatment of diseases caused by dysregulation of anchored PKA signalling (e.g. renal and cardiovascular diseases).

Modulation of monocarboxylic acid transporter-1 kinetic function by the cAMP signaling pathway in rat brain endothelial cells

MCT1 (monocarboxylic acid transporter 1) facilitates bidirectional monocarboxylic acid transport across membranes. MCT1 function and regulation have not been characterized previously in cerebral endothelial cells but may be important during normal cerebral energy metabolism and during brain diseases such as stroke. Here, by using the cytoplasmic pH indicator 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein-acetoxymethyl ester, the initial rates of monocarboxylate-dependent cytoplasmic acidification were measured as an indication of MCT1 kinetic function in vitro using the rat brain endothelial cell (RBE4) model of blood-brain transport. The initial rate of L-lactate-dependent acidification was significantly inhibited by 5-10-min incubations with agonists of intracellular cAMP-dependent cell signaling pathways as follows: dibutyryl cAMP, forskolin, and isoproterenol. Isoproterenol reduced V(max) but did not affect K(m) values. The effects of forskolin were completely reversed by the protein kinase A inhibitor H89, whereas H89 alone increased transport rates. Cytoplasmic cAMP levels, measured by radioimmunoassay, were increased by forskolin or isoproterenol, and the effect of isoproterenol was inhibited by propranolol. MCT1-independent intracellular pH control mechanisms did not contribute to the forskolin or H89 effects on MCT1 kinetic function as determined with amiloride, monocarboxylate-independent acid loading, or the transport inhibitor alpha-cyano-4-hydroxycinnamate. The data demonstrate the direct modulation of MCT1 kinetic function in cerebral endothelial cells by agents known to affect the beta-adrenergic receptor/adenylyl cyclase/cAMP/protein kinase A intracellular signaling pathway.

Nerve growth factor regulates substance P in adult sensory neurons through both TrkA and p75 receptors

Expression of the nociceptive peptide, substance P (SP) is regulated by the neurotrophin, nerve growth factor (NGF), and exogenous exposure to high levels of NGF increases its cellular content and release. NGF utilizes two receptors, the NGF-specific tyrosine kinase receptor, TrkA, and also the non-specific neurotrophin receptor, p75(NTR) (p75). The purpose of this study is to determine the relative involvement of these receptors in nociception. To investigate the role of TrkA in SP signaling, sensory neurons from adult rats were grown in vitro and exposed to a TrkA-blocking antibody. Pretreatment with the antibody inhibited NGF-induced SP elevation. Furthermore, when neurons were exposed to K252a, a relatively specific TrkA kinase inhibitor, the NGF effect on SP was also inhibited. K252a did not prevent SP up-regulation in cells exposed to forskolin or glial cell line-derived neurotrophic factor (GDNF), two agents which increase SP expression independently of TrkA. When p75 was blocked by antiserum, SP up-regulation by NGF was also inhibited. The antiserum neither impacted neuronal survival or basal levels of SP expression, nor did it inhibit SP up-regulation induced by forskolin. Two other neurotrophins, which are also ligands for p75, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) did not block NGF-induced SP up-regulation, raising the possibility that activated p75 is able to cooperate in SP regulation regardless of which neurotrophin ligand occupies it. Our data suggest that NGF up-regulation of SP expression requires the involvement of both TrkA and p75, although the specific contribution of each receptor to SP signaling remains to be determined.

Glucose represses connexin36 in insulin-secreting cells

The gap-junction protein connexin36 (Cx36) contributes to control the functions of insulin-producing cells. In this study, we investigated whether the expression of Cx36 is regulated by glucose in insulin-producing cells. Glucose caused a significant reduction of Cx36 in insulin-secreting cell lines and freshly isolated pancreatic rat islets. This decrease appeared at the mRNA and the protein levels in a dose- and time-dependent manner. 2-Deoxyglucose partially reproduced the effect of glucose, whereas glucosamine, 3-O-methyl-D-glucose and leucine were ineffective. Moreover, KCl-induced depolarization of beta-cells had no effect on Cx36 expression, indicating that glucose metabolism and ATP production are not mandatory for glucose-induced Cx36 downregulation. Forskolin mimicked the repression of Cx36 by glucose. Glucose or forskolin effects on Cx36 expression were not suppressed by the L-type Ca(2+)-channel blocker nifedipine but were fully blunted by the cAMP-dependent protein kinase (PKA) inhibitor H89. A 4 kb fragment of the human Cx36 promoter was identified and sequenced. Reporter-gene activity driven by various Cx36 promoter fragments indicated that Cx36 repression requires the presence of a highly conserved cAMP responsive element (CRE). Electrophoretic-mobility-shift assays revealed that, in the presence of a high glucose concentration, the binding activity of the repressor CRE-modulator 1 (CREM-1) is enhanced. Taken together, these data provide evidence that glucose represses the expression of Cx36 through the cAMP-PKA pathway, which activates a member of the CRE binding protein family.

The AID antibody diversification enzyme is regulated by protein kinase A phosphorylation

Antibodies, which are produced by B-lineage cells, consist of immunoglobulin heavy (IgH) and light (IgL) chains that have amino-terminal variable regions and carboxy-terminal constant regions. In response to antigens, B cells undergo two types of genomic alterations to increase antibody diversity. Affinity for antigen can be increased by introduction of point mutations into IgH and IgL variable regions by somatic hypermutation. In addition, antibody effector functions can be altered by changing the expressed IgH constant region exons through IgH class switch recombination (CSR). Somatic hypermutation and CSR both require the B-cell-specific activation-induced cytidine deaminase protein (AID), which initiates these reactions through its single-stranded (ss)DNA-specific cytidine deaminase activity. In biochemical assays, replication protein A (RPA), a ssDNA-binding protein, associates with phosphorylated AID from activated B cells and enhances AID activity on transcribed double-stranded (ds)DNA containing somatic hypermutation or CSR target sequences. This AID-RPA association, which requires phosphorylation, may provide a mechanism for allowing AID to access dsDNA targets in activated B cells. Here we show that AID from B cells is phosphorylated on a consensus protein kinase A (PKA) site and that PKA is the physiological AID kinase. Thus, AID from non-lymphoid cells can be functionally phosphorylated by recombinant PKA to allow interaction with RPA and promote deamination of transcribed dsDNA substrates. Moreover, mutation of the major PKA phosphorylation site of AID preserves ssDNA deamination activity, but markedly reduces RPA-dependent dsDNA deamination activity and severely impairs the ability of AID to effect CSR in vivo. We conclude that PKA has a critical role in post-translational regulation of AID activity in B cells.

Inhibition of pro-inflammatory cytokine receptor signalling by cAMP in vascular endothelial cells

The anti-inflammatory effects of the prototypical second messenger cAMP have been extensively documented in multiple cell types. However, in many instances, the molecular mechanisms by which cAMP elevation disrupts specific pro-inflammatory signalling cascades are unknown. In this review, we will describe the importance of the JAK–STAT (where JAK stands for Janus kinase and STAT for signal transducer and activator of transcription) signalling pathway in vascular endothelial cell function, outline key inhibitory processes that serve to reduce cytokine-stimulated tyrosine phosphorylation and activation of STAT proteins, and discuss possible mechanisms by which intracellular cAMP sensors could interact with these inhibitory processes to diminish cytokine receptor-mediated pro-inflammatory signalling.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Glucose-dependent activation of protein kinase A activity in Saccharomyces cerevisiae and phosphorylation of its TPK1 catalytic subunit

Protein kinase A (PKA), in yeast, plays a major role in controlling metabolism and gene expression in connection with the available nutrient conditions. We here measure, for the first time, a transient change in the in vivo PKA activity, along a cAMP peak produced by 100 mM glucose addition to glycerol-growing cells as well as a change in the phosphorylation state of its catalytic subunit (Tpk1p) following PKA activation. PKA activity was measured in situ in permeabilized cells, preserving its intracellular localization. Comparison of total PKA activity, measured in situ in permeabilized cells with data obtained from in vitro assays in crude extracts, underscores the inhibitory potency of the regulatory subunit within the cell. Tpk1p phosphorylation was detected through non-denaturing gel electrophoresis. Phosphorylation of Tpk1p increases its specificity constant toward kemptide substrate. The use of mutants of the cAMP pathway showed that phosphorylation depends on the activation of PKA via the G-protein coupled receptor pathway triggered by glucose. The phosphorylation state of Tpk1p was followed during the diauxic shift. Tpk1p phosphorylation is dynamic and reversible: its up-regulation correlates with a fully fermentative metabolism, while its down-regulation with stationary phase or respiratory metabolism. Reversible phosphorylation can thus be considered a new control mechanism possibly pointing to a fine-tuning of PKA activity in response to environmental conditions.

Characterization of a Novel Chemokine-Containing Storage Granule in Endothelial Cells: Evidence for Preferential Exocytosis Mediated by Protein Kinase A and Diacylglycerol

We have recently shown that several proinflammatory chemokines can be stored in secretory granules of endothelial cells (ECs). Subsequent regulated exocytosis of such chemokines may then enable rapid recruitment of leukocytes to inflammatory sites. Although IL-8/CXCL8 and eotaxin-3/CCL26 are sorted to the rod-shaped Weibel-Palade body (WPB), we found that GROalpha/CXCL1 and MCP-1/CCL2 reside in small granules that, similarly to the WPB, respond to secretagogue stimuli. In the present study, we report that GROalpha and MCP-1 colocalized in 50- to 100-nm granules, which occur throughout the cytoplasm and at the cell cortex. Immunofluorescence confocal microscopy revealed no colocalization with multimerin or tissue plasminogen activator, i.e., proteins that are released from small granules of ECs by regulated exocytosis. Moreover, the GROalpha/MCP-1-containing granules were Rab27-negative, contrasting the Rab27-positive, WPB. The secretagogues PMA, histamine, and forskolin triggered distinct dose and time-dependent responses of GROalpha release. Furthermore, GROalpha release was more sensitive than IL-8 release to inhibitors and activators of PKA and PKC but not to an activator of Epac, a cAMP-regulated GTPase exchange factor, indicating that GROalpha release is regulated by molecular adaptors different from those regulating exocytosis of the WPB. On the basis of these findings, we designated the GROalpha/MCP-1-containing compartment the type 2 granule of regulated secretion in ECs, considering the WPB the type 1 compartment. In conclusion, we propose that the GROalpha/MCP-1-containing type 2 granule shows preferential responsiveness to important mediators of EC activation, pointing to the existence of selective agonists that would allow differential release of selected chemokines.

Mutations in rugose promote cell type-specific apoptosis in the Drosophila eye

RUGOSE (RG): encodes an A kinase anchor protein and was isolated as a genetic interactor of the Notch and epidermal growth factor receptor (EGFR) pathways during eye development in Drosophila. rg mutants display a small, rough eye phenotype primarily caused by the loss of cone cells. Here we show that the basis of this phenotype is cell type-specific apoptosis rather than transformation and hence can be rescued by reduction of proapoptotic signals. Moreover, a nearly complete rescue is observed by an increased Notch signal suggesting an antiapoptotic function of Notch in this developmental context. Cone cell loss in rg mutants is accompanied by enhanced Jun N-terminal kinase activity and, concomitantly, by a reduction of EGFR signalling activity. Together, these findings support the idea that rg plays an important role in the integration of different signals required for the exact regulation of cone cell development and survival.

Differential coronary microvascular exchange responses to adenosine: roles of receptor and microvessel subtypes

OBJECTIVE

To assess the role of adenosine receptors in the regulation of coronary microvascular permeability to porcine serum albumin (P(s)(PSA)).

METHODS

Solute flux was measured in single perfused arterioles and venules isolated from pig hearts using fluorescent dye-labeled probes by microspectro-fluorometry. Messenger RNA, protein, and cellular distribution of adenosine receptors in arterioles and venules were analyzed by RT-PCR, immunoblot, and immunofluorescence.

RESULTS

Control venule P(s)(PSA) (10.7 +/- 4.8 x 10(- 7) cm x s(- 1)) was greater than that of arterioles (6.4+/- 2.8 x 10(-7) cm . s(-1); p < .05). Arteriolar P(s)(PSA) decreased (p < .05) with adenosine suffusion over the range from 10(- 8) to 10(-5) M, while venular P(s)(PSA) did not change. The nonselective A(1) and A(2) receptor antagonist, 8-(p-sulfophenyl) theophylline, blocked the adenosine-induced decrease in arteriolar P(s)(PSA). Messenger RNA for adenosine A(1), A(2A), A(2B), and A(3) receptors was expressed in arterioles and venules. Protein for A(1), A(2A), and A(2B), but not A(3), was detected in both microvessel types and was further demonstrated on vascular endothelial cells.

CONCLUSION

Arteriolar P(s)(PSA) decreases with adenosine suffusion but not venular P(s)(PSA). Adenosine A(1), A(2A), and A(2B) receptors are expressed in both arterioles and venules. Selective receptor-linked cellular signaling mechanisms underlying the regulation of permeability remain to be determined.

Coordinate regulation of forskolin-induced cellular proliferation in macrophages by protein kinase A/cAMP-response element-binding protein (CREB) and Epac1-Rap1 signaling: effects of silencing CREB gene expression on Akt activation

In this study, we have examined the role of two cAMP downstream effectors protein kinase A (PKA) and Epac, in forskolin-induced macrophage proliferation. Treatment of macrophages with forskolin enhanced [(3)H]thymidine uptake and increased cell number, and both were profoundly reduced by prior treatment of cells with H-89, a specific PKA inhibitor. Incubation of macrophages with forskolin triggered the activation of Akt, predominantly by phosphorylation of Ser-473, as measured by Western blotting and assay of its kinase activity. Akt activation was significantly inhibited by LY294002 and wortmannin, specific inhibitors of phosphatidylinositol 3-kinase, but not by H-89. Incubation of macrophages with forskolin also increased Epac1 and Rap1.GTP. Immunoprecipitation of Epac1 in forskolin-stimulated cells co-immunoprecipitated Rap1, p-Akt(Thr-308), and p-Akt(Ser-473). Silencing of CREB gene expression by RNA interference prior to forskolin treatment not only decreased CREB protein and its phosphorylation at Ser-133, but also phosphorylation of Akt at Ser-473, and Thr-308. Concomitantly, this treatment inhibited [(3)H]thymidine uptake and reduced forskolin-induced proliferation of macrophages. Forskolin treatment also inhibited activation of the apoptotic mechanism while promoting up-regulation of the anti-apoptotic pathway. We conclude that forskolin mediates cellular proliferation via cAMP-dependent activation of both PKA and Epac.

Phosphorylation of the A-kinase-anchoring Protein Yotiao Contributes to Protein Kinase A Regulation of a Heart Potassium Channel

Regulation of the heart by the sympathetic nervous system, fundamental to the physiological response to stress and exercise, requires coordinated phosphorylation of multiple downstream molecular targets, including the I(Ks) (slowly activating potassium current) channel. Sympathetic nervous system stimulation increases intracellular cAMP for which targeted regulation is directed in large part by distinct scaffold or anchoring proteins. Yotiao is an A-kinase-anchoring protein (AKAP) that recruits the cyclic AMP-dependent protein kinase (protein kinase A (PKA)) and protein phosphatase 1 to the carboxyl terminus of the I(Ks) channel to form a molecular complex and control its phosphorylation state, crucial to the cardiac cellular response to sympathetic nervous system stimulation. Here we report that Yotiao itself is a substrate for PKA phosphorylation, and we identify a Yotiao amino-terminal (N-T) residue (Ser-43) that is PKA-phosphorylated in response to beta-adrenergic receptor stimulation. The replacement of Ser-43 by Ala ablates the PKA phosphorylation of N-T Yotiao and markedly diminishes the functional response of the wild type and pseudo-phosphorylated I(Ks) channel to cAMP but neither prevents the PKA phosphorylation of KCNQ1 nor its binding to Yotiao. These results suggest, for the first time, a critical role for the PKA phosphorylation of an AKAP in the functional regulation of an ion channel protein and postphosphorylation allosteric modulation of the I(Ks) channel by Yotiao.

Arrestin times for compartmentalised cAMP signalling and phosphodiesterase-4 enzymes

Various methods reveal that cyclic AMP (cAMP) signalling in cells is compartmentalised. These methods use FRET probes based upon either protein kinase A (PKA) or EPAC, cAMP-gated ion channels, or the selective activation of AKAP-anchored PKA isoforms. The basis of compartmentalisation involves point sources of cAMP generation within sub-domains of the plasma membrane coupled to degradation by spatially segregated, anchored forms of cAMP phosphodiesterases. cAMP-specific phosphodiesterase-4 (PDE4) isoforms play a central role in determining compartmentalisation, as exemplified in cardiac myocytes and T cells. The PKA phosphorylation status of the beta2-adrenoreceptor, and hence its ability to switch its signalling from G(s) to G(i) and thus to activate ERK, is regulated dynamically by the agonist-stimulated recruitment of PDE4 to the receptor in complex with beta-arrestin. The co-receptor CD28 enhances signalling through the T-cell receptor by recruiting a PDE4/beta-arrestin complex, which then attenuates PKA phosphorylation of Csk.

cAMP-PKA signaling to the mitochondria: protein scaffolds, mRNA and phosphatases

Energy metabolism and, specifically, the coupling of mitochondria to growth and survival is controlled by the cAMP-PKA pathway in yeast. In higher eukaryotes, cAMP signaling originating at the plasma membrane is distributed to different subcellular districts by cAMP waves received by PKA bound to PKA anchor proteins (AKAPs) tethered to these compartments. This review focuses on the subgroup of AKAPs that anchor PKA to the mitochondrial outer membrane (mtAKAPs). Only PKA anchored to mtAKAPs can efficiently transmit cAMP signals to mitochondria. mtAKAP complexes are remarkably heterogeneous. In addition to PKA regulatory subunits, they may include mRNAs, tyrosine phosphatase(s) and tyrosine kinase(s). Selective regulation of these components by cAMP-PKA integrates various signal transduction pathways and can determine which subcellular compartment receives the signal. Unveiling the interactions among the components of these large complexes will shed light on how cAMP and PKA regulate vital mitochondrial processes.

RNA silencing identifies PDE4D5 as the functionally relevant cAMP phosphodiesterase interacting with beta arrestin to control the protein kinase A/AKAP79-mediated switching of the beta2-adrenergic receptor to activation of ERK in HEK293B2 cells

PDE4B and PDE4D provide >90% of PDE4 cAMP phosphodiesterase activity in human embryonic kidney (HEK293B2) cells. Their selective small interference RNA (siRNA)-mediated knockdown potentiates isoprenaline-stimulated protein kinase A (PKA) activation. Whereas endogenous PDE4D co-immunoprecipitates with beta arrestin, endogenous PDE4B does not, even upon PDE4D knockdown. Ectopic overexpression of PDE4B2 confers co-immunoprecipitation with beta arrestin. Knockdown of PDE4D, but not PDE4B, amplifies isoprenaline-stimulated phosphorylation of the beta2-adrenergic receptor (beta2-AR) by PKA and activation of extracellular signal-regulated kinase (ERK) through G(i). Isoform-selective knockdown identifies PDE4D5 as the functionally important species regulating isoprenaline stimulation of both these processes. Ht31-mediated disruption of the tethering of PKA to AKAP scaffold proteins attenuates isoprenaline activation of ERK, even upon PDE4D knockdown. Selective siRNA-mediated knockdown identifies AKAP79, which is constitutively associated with the beta2-AR, rather than isoprenaline-recruited gravin, as being the functionally relevant AKAP in this process. Isoprenaline-stimulated membrane recruitment of PDE4D is ablated upon beta arrestin knockdown. A mutation that compromises interactions with beta arrestin prevents catalytically inactive PDE4D5 from performing a dominant negative role in potentiating isoprenaline-stimulated ERK activation. Beta arrestin-recruited PDE4D5 desensitizes isoprenaline-stimulated PKA phosphorylation of the beta2-AR and the consequential switching of its signaling to ERK. The ability to observe a cellular phenotype upon PDE4D5 knockdown demonstrates that other PDE4 isoforms, expressed at endogenous levels, are unable to afford rescue in HEK293B2 cells.

Chronic Delta9-tetrahydrocannabinol treatment produces antinociceptive tolerance in mice without altering protein kinase A activity in mouse brain and spinal cord

The present study investigated the effect of different levels of Delta-9-tetrahydrocannabinol (Delta(9)-THC) antinociceptive tolerance on Protein Kinase A (PKA) activity in mouse brain and spinal cord. To strengthen this investigation, a positive control was developed to demonstrate the assay utilized in this study was sensitive enough to detect an increase in PKA activity in the anatomical regions utilized in this study. The membrane-permeant and phosphodiesterase-resistant cAMP analog 8-Bromoadenosine-3',5'-cyclic monophosphorothioate, Sp-isomer (Sp-8-Br-cAMPS) was utilized for the development of this positive control and this compound produced an increase in PKA activity in several mouse brain regions (i.c.v.) and lumbar spinal cord (i.t.) following its administration. Models were then developed in which mice expressed either a 13-fold or 49-fold level of Delta(9)-THC antinociceptive tolerance following chronic treatment with 10mg/kg Delta(9)-THC or 80mg/kg Delta(9)-THC for 6.5 days. Basal and total cytosolic and particulate PKA activities were measured directly in homogenates from the striatum, hippocampus, cerebellum, cortex and lumbar spinal cord. Results from this study indicate that chronic exposure to Delta(9)-THC does not produce an increase in PKA activity in these mouse brain regions or spinal cord. Future work is needed to determine the role of PKA in cannabinoid tolerance in mice.

The long and short of vascular smooth muscle phosphodiesterase-4 as a putative therapeutic target

In this issue, Tilley and Maurice (p. 596) show that differentiation of vascular smooth muscle cells to a proliferative phenotype is associated with a profound up-regulation of specific phosphodiesterase-4 (PDE4) isoforms because of increased histone acetylation. The increased PDE4 activity is seen as preventing cAMP from inhibiting the enhanced proliferation, migration, and production of extracellular matrix seen in activated VSMC. This Perspective examines the proposal that selective inhibition of PDE4D1/2 could find use in adjunctive pharmacotherapy after percutaneous coronary interventions and, in addition, discusses the recent genetic evidence that PDE4D7 may provide a therapeutic target in stroke.

Phosphorylation of claudin-3 at threonine 192 by cAMP-dependent protein kinase regulates tight junction barrier function in ovarian cancer cells

Claudins are integral membrane proteins essential in the formation and function of tight junctions (TJs). Disruption of TJs, which have essential roles in cell permeability and polarity, is thought to contribute to epithelial tumorigenesis. Claudin-3 and -4 are frequently overexpressed in ovarian cancer, but the molecular pathways involved in the regulation of these proteins are unclear. Interestingly, several studies have demonstrated a role for phosphorylation in the regulation of TJ complexes, although evidence for claudin phosphorylation is scarce. Here, we showed that claudin-3 and -4 can be phosphorylated in ovarian cancer cells. In vitro phosphorylation assays using glutathione S-transferase fusion constructs demonstrated that the C terminus of claudin-3 is an excellent substrate for cAMP-dependent protein kinase (PKA). Using site-directed mutagenesis, we identified a PKA phosphorylation site at amino acid 192 in the C terminus of claudin-3. Overexpression of the protein containing a T192D mutation, mimicking the phosphorylated state, resulted in a decrease in TJ strength in ovarian cancer cell line OVCA433. Our results suggest that claudin-3 phosphorylation by PKA, a kinase frequently activated in ovarian cancer, may provide a mechanism for the disruption of TJs in this cancer. In addition, our findings may have general implications for the regulation of TJs in normal epithelial cells.

Compartmentalisation of phosphodiesterases and protein kinase A: opposites attract

Understanding the molecular organisation of intracellular signalling pathways is a topic of considerable research interest. Since many signalling enzymes are widely distributed and have several substrates, a critical component in signal transduction is the control of specificity. This is achieved, in part by the assembly of multiprotein complexes where clusters of signalling enzymes create focal points to disseminate the intracellular action of many hormones. This is particularly true for the cAMP dependent protein kinase (PKA) that is localised throughout the cell via its association with A-kinase anchoring proteins (AKAPs). Recent data suggest that some AKAPs also interact with phosphodiesterases (PDEs). Compartmentalisation of PDEs not only provides an elegant means to control PKA activation by monitoring the local cAMP flux, but also serves to concentrate and segregate the action of these important regulatory enzymes.

Alterations in brain Protein Kinase A activity and reversal of morphine tolerance by two fragments of native Protein Kinase A inhibitor peptide (PKI)

Two peptide fragments of native Protein Kinase A inhibitor (PKI), PKI-(6-22)-amide and PKI-(Myr-14-22)-amide, significantly reversed low-level morphine antinociceptive tolerance in mice. The inhibition of Protein Kinase A (PKA) activity by both peptide fragments was then measured in specific brain regions (thalamus, periaqueductal gray (PAG), and medulla) and in lumbar spinal cord (LSC), which in previous studies have been shown to play a role in morphine-induced analgesia. In drug naive animals, cytosolic PKA activity was greater than particulate PKA activity in each region, while cytosolic and particulate PKA activities were greater in thalamus and PAG compared to medulla and LSC. The addition of both peptides to homogenates from each region completely abolished cytosolic and particulate PKA activities in vitro. Following injection into the lateral ventricle of the brain of drug naive mice and morphine-tolerant mice, both peptides inhibited PKA activity in the cytosolic, but not the particulate fraction of LSC. In addition, cytosolic and particulate PKA activities were inhibited by both peptides in thalamus. These results demonstrate that the inhibition of PKA reverses morphine tolerance. Moreover, the inhibition of PKA activity in specific brain regions and LSC from morphine-tolerant mice by PKI analogs administered i.c.v. is evidence that PKA plays a role in morphine tolerance.

Regulation of neuroprotective activity of myocyte-enhancer factor 2 by cAMP-protein kinase A signaling pathway in neuronal survival

The transcription factor myocyte-enhancer factor 2 (MEF2) has been shown to be required for the survival of different types of neurons. However, the death- or survival-inducing second messenger pathways that regulate MEF2 activity remain to be fully elucidated. Membrane depolarization by KCl induces neuronal survival that is dependent upon MEF2-mediated gene transactivation. Here we report that membrane depolarizationinduced activation of MEF2 requires the cAMP-protein kinase A (PKA) pathway. Inhibition of the activity of cAMP-PKA pathway attenuates membrane depolarization-induced activation of MEF2 activity and neuronal survival, whereas enhancing the activity of this pathway prevents KCl withdrawal-induced inhibition of MEF2 and neuronal apoptosis. Moreover, PKA directly phosphorylates MEF2 at Thr-20 in vitro to increase MEF2 DNA binding activity. A mutation of Thr-20 to Ala renders MEF2 resistant to PKA phosphorylation in vitro and reduces its DNA binding activity. Transfection of this T20A mutant blocks survival and induces apoptosis in cultured cortical and cerebellar granule neurons. This study identifies the transcription factor MEF2 as a target of cAMP-PKA pathway and demonstrates that PKA phosphorylation of MEF2 is a key step in modulating its DNA binding activity and ability to promote neuronal survival.

PDE4 inhibitors as new anti-inflammatory drugs: effects on cell trafficking and cell adhesion molecules expression

Phosphodiesterase 4 (PDE4) is a major cyclic AMP-hydrolyzing enzyme in inflammatory and immunomodulatory cells. The wide range of inflammatory mechanisms under control by PDE4 points to this isoenzyme as an attractive target for new anti-inflammatory drugs. Selective inhibitors of PDE4 have demonstrated a broad spectrum of anti-inflammatory activities including the inhibition of cellular trafficking and microvascular leakage, cytokine and chemokine release from inflammatory cells, reactive oxygen species production, and cell adhesion molecule expression in a variety of in vitro and in vivo experimental models. The initially detected side effects, mainly nausea and emesis, appear at least partially overcome by the 'second generation' PDE4 inhibitors, some of which like roflumilast and cilomilast are in the later stages of clinical development for treatment of chronic obstructive pulmonary disease. These new drugs may also offer opportunities for treatment of other inflammatory diseases.