Stimulation of specific types of Gs-stimulated adenylyl cyclases by phorbol ester treatment

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

Allosteric Inhibition of Adenylyl Cyclase Type 5 by G-Protein: A Molecular Dynamics Study

Adenylyl cyclases (ACs) have a crucial role in many signal transduction pathways, in particular in the intricate control of cyclic AMP (cAMP) generation from adenosine triphosphate (ATP). Using homology models developed from existing structural data and docking experiments, we have carried out all-atom, microsecond-scale molecular dynamics simulations on the AC5 isoform of adenylyl cyclase bound to the inhibitory G-protein subunit Gαi in the presence and in the absence of ATP. The results show that Gαi has significant effects on the structure and flexibility of adenylyl cyclase, as observed earlier for the binding of ATP and Gsα. New data on Gαi bound to the C1 domain of AC5 help explain how Gαi inhibits enzyme activity and obtain insight on its regulation. Simulations also suggest a crucial role of ATP in the regulation of the stimulation and inhibition of AC5.

Activation of the Ca2+ sensing receptor and the PKC/WNK4 downstream signaling cascade induces incorporation of ZO-2 to tight junctions and its separation from 14-3-3

Zonula occludens-2 (ZO-2) is a tight junction (TJ) cytoplasmic protein, whose localization varies according to cell density and Ca²⁺ in the media. In cells cultured in low calcium (LC), ZO-2 displays a diffuse cytoplasmic distribution, but activation of the Ca²⁺ sensing receptor (CaSR) with Gd³⁺ triggers the appearance of ZO-2 at the cell borders. CaSR downstream signaling involves activation of protein kinase C, which phosphorylates and activates with no lysine kinase-4 that phosphorylates ZO-2 inducing its concentration at TJs. In LC, ZO-2 is protected from degradation by association to 14-3-3 proteins. When monolayers are transferred to normal calcium, the complexes ZO-2/14-3-3ζ and ZO-2/14-3-3σ move to the cell borders and dissociate. The 14-3-3 proteins are then degraded in proteosomes, whereas ZO-2 integrates to TJs. From the plasma membrane residual ZO-2 is endocyted and degradaded in lysosomes. The unique region 2 of ZO-2, and S261 located within a nuclear localization signal, are critical for the interaction with 14-3-3 ζ and σ and for the efficient nuclear importation of ZO-2. These results explain the molecular mechanism through which extracellular Ca²⁺ triggers the appearance of ZO-2 at TJs in epithelial cells and reveal the novel interaction between ZO-2 and 14-3-3 proteins, which is critical for ZO-2 protection and intracellular traffic.

A molecular dynamics study of adenylyl cyclase: The impact of ATP and G-protein binding

Adenylyl cyclases (ACs) catalyze the biosynthesis of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP) and play an important role in many signal transduction pathways. The enzymatic activity of ACs is carefully controlled by a variety of molecules, including G-protein subunits that can both stimulate and inhibit cAMP production. Using homology models developed from existing structural data, we have carried out all-atom, microsecond-scale molecular dynamics simulations on the AC5 isoform of adenylyl cyclase and on its complexes with ATP and with the stimulatory G-protein subunit Gsα. The results show that both ATP and Gsα binding have significant effects on the structure and flexibility of adenylyl cyclase. New data on ATP bound to AC5 in the absence of Gsα notably help to explain how Gsα binding enhances enzyme activity and could aid product release. Simulations also suggest a possible coupling between ATP binding and interactions with the inhibitory G-protein subunit Gαi.

cAMP Signaling Compartmentation: Adenylyl Cyclases as Anchors of Dynamic Signaling Complexes

It is widely accepted that cAMP signaling is compartmentalized within cells. However, our knowledge of how receptors, cAMP signaling enzymes, effectors, and other key proteins form specific signaling complexes to regulate specific cell responses is limited. The multicomponent nature of these systems and the spatiotemporal dynamics involved as proteins interact and move within a cell make cAMP responses highly complex. Adenylyl cyclases, the enzymatic source of cAMP production, are key starting points for understanding cAMP compartments and defining the functional signaling complexes. Three basic elements are required to form a signaling compartment. First, a localized signal is generated by a G protein-coupled receptor paired to one or more of the nine different transmembrane adenylyl cyclase isoforms that generate the cAMP signal in the cytosol. The diffusion of cAMP is subsequently limited by several factors, including expression of any number of phosphodiesterases (of which there are 24 genes plus spice variants). Finally, signal response elements are differentially localized to respond to cAMP produced within each locale. A-kinase-anchoring proteins, of which there are 43 different isoforms, facilitate this by targeting protein kinase A to specific substrates. Thousands of potential combinations of these three elements are possible in any given cell type, making the characterization of cAMP signaling compartments daunting. This review will focus on what is known about how cells organize cAMP signaling components as well as identify the unknowns. We make an argument for adenylyl cyclases being central to the formation and maintenance of these signaling complexes.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

PKCβII acts downstream of chemoattractant receptors and mTORC2 to regulate cAMP production and myosin II activity in neutrophils

mTORC2 has been shown to be involved in cytoskeletal regulation, but the mechanisms by which this takes place are poorly understood. This study shows that PKCβII is specifically required for mTORC2-dependent activation of adenylyl cyclase 9 and back retraction during neutrophil chemotaxis to chemoattractants.

Chemotaxis is a process by which cells polarize and move up a chemical gradient through the spatiotemporal regulation of actin assembly and actomyosin contractility, which ultimately control front protrusions and back retractions. We previously demonstrated that in neutrophils, mammalian target of rapamycin complex 2 (mTORC2) is required for chemoattractant-mediated activation of adenylyl cyclase 9 (AC9), which converts ATP into cAMP and regulates back contraction through MyoII phosphorylation. Here we study the mechanism by which mTORC2 regulates neutrophil chemotaxis and AC9 activity. We show that inhibition of protein kinase CβII (PKCβII) by CPG53353 or short hairpin RNA knockdown severely inhibits chemoattractant-induced cAMP synthesis and chemotaxis in neutrophils. Remarkably, PKCβII-inhibited cells exhibit specific and severe tail retraction defects. In response to chemoattractant stimulation, phosphorylated PKCβII, but not PKCα, is transiently translocated to the plasma membrane, where it phosphorylates and activates AC9. mTORC2-mediated PKCβII phosphorylation on its turn motif, but not its hydrophobic motif, is required for membrane translocation of PKCβII. Inhibition of mTORC2 activity by Rictor knockdown not only dramatically decreases PKCβII activity, but it also strongly inhibits membrane translocation of PKCβII. Together our findings show that PKCβII is specifically required for mTORC2-dependent AC9 activation and back retraction during neutrophil chemotaxis.

Non-raft adenylyl cyclase 2 defines a cAMP signaling compartment that selectively regulates IL-6 expression in airway smooth muscle cells: differential regulation of gene expression by AC isoforms

Adenylyl cyclase (AC) isoforms differ in their tissue distribution, cellular localization, regulation, and protein interactions. Most cell types express multiple AC isoforms. We hypothesized that cAMP produced by different AC isoforms regulates unique cellular responses in human bronchial smooth muscle cells (BSMC). Overexpression of AC2, AC3, or AC6 had distinct effects on forskolin (Fsk)-induced expression of a number of known cAMP-responsive genes. These data show that different AC isoforms can differentially regulate gene expression. Most notable, overexpression and activation of AC2 enhanced interleukin 6 (IL-6) expression, but overexpression of AC3 or AC6 had no effect. IL-6 production by BSMC was induced by Fsk and select G protein-coupled receptor (GPCR) agonists, though IL-6 levels did not directly correlate with global cAMP levels. Treatment with PKA selective 6-Bnz-cAMP or Epac selective 8-CPT-2Me-cAMP cAMP analogs revealed a predominant role for PKA in cAMP-mediated induction of IL-6. IL-6 promoter mutations demonstrated that AP-1 and CRE transcription sites were required for Fsk to stimulate IL-6 expression. Our present study defines an AC2 cAMP signaling compartment that specifically regulates IL-6 expression in BSMC via Epac and PKA and demonstrates that other AC isoforms are excluded from this pool.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Muscarinic receptors stimulate AC2 by novel phosphorylation sites, whereas Gβγ subunits exert opposing effects depending on the G-protein source

Direct phosphorylation of AC2 (adenylyl cyclase 2) by PKC (protein kinase C) affords an opportunity for AC2 to integrate signals from non-canonical pathways to produce the second messenger, cyclic AMP. The present study shows that stimulation of AC2 by pharmacological activation of PKC or muscarinic receptor activation is primarily the result of phosphorylation of Ser⁴⁹⁰ and Ser⁵⁴³, as opposed to the previously proposed Thr¹⁰⁵⁷. A double phosphorylation-deficient mutant (S490/543A) of AC2 was insensitive to PMA (phorbol myristic acid) and CCh (carbachol) stimulation, whereas a double phosphomimetic mutant (S490/543D) mimicked the activity of PKC-activated AC2. Putative Gβγ-interacting sites are in the immediate environment of these PKC phosphorylation sites (Ser⁴⁹⁰ and Ser⁵⁴³) that are located within the C1b domain of AC2, suggesting a significant regulatory importance of this domain. Consequently, we examined the effect of both G_q-coupled muscarinic and G_i-coupled somatostatin receptors. Employing pharmacological and FRET (fluorescence resonance energy transfer)-based real-time single cell imaging approaches, we found that Gβγ released from the G_q-coupled muscarinic receptor or G_i-coupled somatostatin receptors exert inhibitory or stimulatory effects respectively. These results underline the sophisticated regulatory capacities of AC2, in not only being subject to regulation by PKC, but also and in an opposite manner to Gβγ subunits, depending on their source.

Choreographing the adenylyl cyclase signalosome: sorting out the partners and the steps

Adenylyl cyclases are a ubiquitous family of enzymes and are critical regulators of metabolic and cardiovascular function. Multiple isoforms of the enzyme are expressed in a range of tissues. However, for many processes, the adenylyl cyclase isoforms have been thought of as essentially interchangeable, with their impact more dependent on their common actions to increase intracellular cyclic adenosine monophosphate content regardless of the isoform involved. It has long been appreciated that each subfamily of isoforms demonstrate a specific pattern of "upstream" regulation, i.e., specific patterns of ion dependence (e.g., calcium-dependence) and specific patterns of regulation by kinases (protein kinase A (PKA), protein kinase C (PKC), raf). However, more recent studies have suggested that adenylyl cyclase isoform-selective patterns of signaling are a wide-spread phenomenon. The determinants of these selective signaling patterns relate to a number of factors, including: (1) selective coupling of specific adenylyl cyclase isoforms with specific G protein-coupled receptors, (2) localization of specific adenylyl cyclase isoforms in defined structural domains (AKAP complexes, caveolin/lipid rafts), and (3) selective coupling of adenylyl cyclase isoforms with specific downstream signaling cascades important in regulation of cell growth and contractility. The importance of isoform-specific regulation has now been demonstrated both in mouse models as well as in humans. Adenylyl cyclase has not been viewed as a useful target for therapeutic regulation, given the ubiquitous expression of the enzyme and the perceived high risk of off-target effects. Understanding which isoforms of adenylyl cyclase mediate distinct cellular effects would bring new significance to the development of isoform-specific ligands to regulate discrete cellular actions.

Temporal expression of calcium/calmodulin-dependent adenylyl cyclase isoforms in rat articular chondrocytes: RT-PCR and immunohistochemical localization

A multitude of signalling cascades are implicated in the homeostasis of articular chondrocytes. However, the identity of these signalling pathways is not fully established. The 3, 5'-cyclic AMP-mediated signalling system is considered to be a prototype. Adenylyl cyclase (AC) is an effector enzyme responsible for the synthesis of cAMP. There are 10 mammalian AC isoforms and some of these are differentially regulated by calcium/calmodulin (Ca²(+) /CaM). Ca²(+) is known to play an important role in the development and maintenance of skeletal tissues. Ca²(+) /CaM-dependent AC isoforms and their temporal expression in articular chondrocytes in rats were identified using RT-PCR and immunohistochemistry techniques. All Ca²(+) /CaM-dependent AC isoforms were expressed in chondrocytes from all age groups examined. Each isoform was differentially expressed in developing and adult articular chondrocytes. Generally, expression of AC isoforms was observed to increase with age, but the increase was not uniform for all Ca²(+) /CaM-dependent AC isoforms. Expression of Ca²(+) /CaM-dependent AC isoforms along with other signalling molecules known to be present in articular chondrocytes indicate complicated and multifactorial signalling cascades involved in the development and homeostasis of articular cartilage. The significance of these findings in terms of articular chondrocyte physiology is discussed.

Direct demonstration of discrete Ca2+ microdomains associated with different isoforms of adenylyl cyclase

Ca(2+)-sensitive adenylyl cyclases (ACs) orchestrate dynamic interplay between Ca(2+) and cAMP that is a crucial feature of cellular homeostasis. Significantly, these ACs are highly selective for capacitative Ca(2+) entry (CCE) over other modes of Ca(2+) increase. To directly address the possibility that these ACs reside in discrete Ca(2+) microdomains, we tethered a Ca(2+) sensor, GCaMP2, to the N-terminus of Ca(2+)-stimulated AC8. GCaMP2-AC8 measurements were compared with global, plasma membrane (PM)-targeted or Ca(2+)-insensitive AC2-targeted GCaMP2. In intact cells, GCaMP2-AC8 responded rapidly to CCE, but was largely unresponsive to other types of Ca(2+) rise. The global GCaMP2, PM-targeted GCaMP2 and GCaMP2-AC2 sensors reported large Ca(2+) fluxes during Ca(2+) mobilization and non-specific Ca(2+) entry, but were less responsive to CCE than GCaMP2-AC8. Our data reveal that different AC isoforms localize to distinct Ca(2+)-microdomains within the plasma membrane. AC2, which is regulated via protein kinase C, resides in a microdomain that is exposed to a range of widespread Ca(2+) signals seen throughout the cytosol. By contrast, a unique Ca(2+) microdomain surrounds AC8 that promotes selectivity for Ca(2+) signals arising from CCE, and optimizes CCE-mediated cAMP synthesis. This direct demonstration of discrete compartmentalized Ca(2+) signals associated with specific signalling proteins provides a remarkable insight into the functional organization of signalling microdomains.

Light modulates the melanophore response to alpha-MSH in Xenopus laevis: an analysis of the signal transduction crosstalk mechanisms involved

Melanin granule (melanosome) dispersion within Xenopus laevis melanophores is evoked either by light or alpha-MSH. We have previously demonstrated that the initial biochemical steps of light and alpha-MSH signaling are distinct, since the increase in cAMP observed in response to alpha-MSH was not seen after light exposure. cAMP concentrations in response to alpha-MSH were significantly lower in cells pre-exposed to light as compared to the levels in dark-adapted melanophores. Here we demonstrate the presence of an adenylyl cyclase (AC) in the Xenopus melanophore, similar to the mammalian type IX which is inhibited by Ca(2+)-calmodulin-activated phosphatase. This finding supports the hypothesis that the cyclase could be negatively modulated by a light-promoted Ca(2+) increase. In fact, the activity of calcineurin PP2B phosphatase was increased by light, which could result in AC IX inhibition, thus decreasing the response to alpha-MSH. St-Ht31, a disrupting agent of protein kinase A (PKA)-anchoring kinase A protein (AKAP) complex totally blocked the melanosome dispersing response to alpha-MSH, but did not impair the photo-response in Xenopus melanophores. Sequence comparison of a melanophore AKAP partial clone with GenBank sequences showed that the anchoring protein was a gravin-like adaptor previously sequenced from Xenopus non-pigmentary tissues. Co-immunoprecipitation of Xenopus AKAP and the catalytic subunit of PKA demonstrated that PKA is associated with AKAP and it is released in the presence of alpha-MSH. We conclude that in X. laevis melanophores, AKAP12 (gravin-like) contains a site for binding the inactive PKA thus compartmentalizing PKA signaling and also possesses binding sites for PKC. Light diminishes alpha-MSH-induced increase of cAMP by increasing calcineurin (PP2B) activity, which in turn inhibits adenylyl cyclase type IX, and/or by activating PKC, which phosphorylates the gravin-like molecule, thus destabilizing its binding to the cell membrane.

Protein kinases A and C regulate receptor-mediated increases in cAMP in rabbit erythrocytes

Activation of the beta-adrenergic receptor (beta-AR) or the prostacyclin receptor (IPR) results in increases in cAMP and ATP release from erythrocytes. cAMP levels depend on a balance between synthesis via adenylyl cyclase and hydrolysis by phosphodiesterases (PDEs). Previously, we reported that cAMP increases associated with activation of the beta-AR and IPR in rabbit and human erythrocytes are tightly regulated by distinct PDEs. Importantly, inhibitors of these PDEs potentiated both increases in cAMP and ATP release. It has been shown that increases in protein kinase (PK) activity can activate PDE3 and PDE4. Both PKA and PKC are present in the erythrocyte and can phosphorylate and activate these PDEs. Here we investigate the hypothesis that PKA regulates PDE activity associated with the beta-AR and both PKA and PKC regulate the PDE activity associated with the IPR in rabbit erythrocytes. Pretreatment of erythrocytes with the PKA inhibitor, H89 (10 microM), in the presence of the PDE4 inhibitor, rolipram (10 microM), augmented isoproterenol (1 microM)-induced cAMP increases. In contrast, in the presence of the PDE3 inhibitor, cilostazol (10 microM), pretreatment of erythrocytes with either H89 (1 microM) or two chemically dissimilar inhibitors of PKC, calphostin C (1 microM) or GFX109203X (1 microM), potentiated iloprost (1 microM)-induced cAMP increases. Furthermore, pretreatment of erythrocytes with both H89 and GFX109203X in the presence of cilostazol augmented the iloprost-induced increases in cAMP to a greater extent than either PK inhibitor individually. These results support the hypothesis that PDEs associated with receptor-mediated increases in cAMP in rabbit erythrocytes are regulated by kinases specific to the receptor's signaling pathway.

Molecular mechanisms underlying a cellular analog of operant reward learning

Operant conditioning is a ubiquitous but mechanistically poorly understood form of associative learning in which an animal learns the consequences of its behavior. Using a single-cell analog of operant conditioning in neuron B51 of Aplysia, we examined second-messenger pathways engaged by activity and reward and how they may provide a biochemical association underlying operant learning. Conditioning was blocked by Rp-cAMP, a peptide inhibitor of PKA, a PKC inhibitor, and by expressing a dominant-negative isoform of Ca2+-dependent PKC (apl-I). Thus, both PKA and PKC were necessary for operant conditioning. Injection of cAMP into B51 mimicked the effects of operant conditioning. Activation of PKC also mimicked conditioning but was dependent on both cAMP and PKA, suggesting that PKC acted at some point upstream of PKA activation. Our results demonstrate how these molecules can interact to mediate operant conditioning in an individual neuron important for the expression of the conditioned behavior.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

The MAPK pathway is required for depolarization-induced "promiscuous" immediate-early gene expression but not for depolarization-restricted immediate-early gene expression in neurons

Depolarization, growth factors, neurotrophins, and other stimuli induce expression of immediate early genes (IEGs) in neurons. We identified a subset of IEGs, IPD-IEGs, which are induced preferentially by depolarization, but not by neurotrophins or growth factors, in PC12 cells. The "promiscuous" IEGs Egr1 and c-fos, induced by growth factors and neurotrophins, in addition to depolarization, require activation of the MAP kinase signaling pathway for induction in response to KCl depolarization in PC12 cells; MEK1/2 inhibitors block KCl-induced Egr1 and c-fos expression. In contrast, MEK1/2 inhibition has no effect on KCl-induced expression of the known IPD-IEGs in PC12 cells. Additional "candidate" IDP-IEGs were identified by a microarray comparison of genes induced by KCl in the presence vs. the absence of an MEK1/2 inhibitor in PC12 cells. Northern blot analyses demonstrated that representative newly identified candidate IPD-IEGs, as with the known IPD-IEGs, are also induced by a MAP kinase- independent pathway in response to depolarization, both in PC12 cells and in rat primary cortical neurons. Nerve growth factor and epidermal growth factor are unable to induce the expression of the Crem/Icer, Nur77, Nor1, Rgs2, Dusp1 (Mkp1), and Dscr1 genes in PC12 cells, validating their identification as IPD-IEGs. Inhibiting calcium/calmodulin-dependent kinase II (CaMKII), calcineurin, or protein kinase A (PKA) activity prevents KCl-induced IPD-IEG mRNA accumulation, suggesting that the IPD-IEG genes are induced by depolarization in neurons via a combination of calcineurin/PKA- and CaMKII-dependent pathways.

"Soluble" adenylyl cyclase-generated cyclic adenosine monophosphate promotes fast migration in PC12 cells

In a model for neuronal movement, PC12 cells undergo fast migration in response to nerve growth factor (NGF) and phorbol ester (PMA). We previously showed that NGF increases intracellular cAMP via activation of soluble adenylyl cyclase (sAC). In this report, we demonstrate that sAC activation is an essential component of NGF- + PMA-induced fast migration in PC12 cells. Interestingly, PMA also raises intracellular cAMP but does so by stimulating transmembrane adenylyl cyclases (tmAC); however, this tmAC-generated cAMP does not contribute to fast migration. Therefore, cells must possess independent pools of cAMP capable of modulating distinct functions.

Plasticity of adenylyl cyclase-related signaling sequelae after long-term morphine treatment

Adaptations to long-term morphine treatment resulting in tolerance are protective by counteracting the consequences of sustained opioid receptor activation. Consequently, the manifestation of specific adenylyl cyclase (AC)-related neurochemical sequelae of long-term morphine treatment should depend on the consequences of short-term mu-opioid receptor (MOR) activation. We tested this by comparing complementary chemical sequelae of long-term morphine treatment among cells in which short-term MOR activation inhibited instead of stimulated AC activity. Short-term activation of MOR in Chinese hamster ovary (CHO) cells stably transfected with MOR (MOR-CHO) inhibits AC activity. Long-term morphine treatment of these cells increased AC and Gbeta phosphorylation, membrane protein kinase Cgamma (PKCgamma) translocation, and MOR G(s) association. All converge, shifting the consequences of short-term MOR activation from Galpha(i)/Galpha(o) inhibitory to AC stimulatory signaling. In contrast, overexpression of the Gbetagamma-stimulated AC isoform AC2 (which converted MOR-coupled inhibition to stimulation of AC) eliminated or reversed these adaptations to long-term morphine treatment; it negated the increase in Gbeta phosphorylation and PKCgamma translocation while reversing the increase in AC phosphorylation and MOR G(s) association. These adaptations greatly attenuated MOR-coupled stimulation of AC activity. Altered overexpression of AC protein per se was not a confounding factor because MOR-CHO overexpressing AC1, which is inhibited by short-term MOR activation, manifested adaptations to long-term morphine treatment qualitatively identical with those of MOR-CHO. These results reveal that adaptations elicited by long-term morphine treatment depend on the effects of short-term MOR activation. This dynamic and pliable nature of tolerance mechanisms could represent a new paradigm for pharmacotherapeutics.

Modulation of adenylyl cyclase activity in young and adult rat brain cortex. Identification of suramin as a direct inhibitor of adenylyl cyclase

Adenylyl cyclase (AC) in brain cortex from young (12-day-old) rats exhibits markedly higher activity than in adult (90-day-old) animals. In order to find some possibly different regulatory features of AC in these two age groups, here we modulated AC activity by dithiothreitol (DTT), Fe(2+), ascorbic acid and suramin. We did not detect any substantial difference between the effects of all these tested agents on AC activity in cerebrocortical membranes from young and adult rats, and the enzyme activity was always about two-fold higher in the former preparations. Nevertheless, several interesting findings have come out of these investigations. Whereas forskolin- and Mn(2+)-stimulated AC activity was significantly enhanced by the addition of DTT, increased concentrations of Fe(2+) ions or ascorbic acid substantially suppressed the enzyme activity. Lipid peroxidation induced by suitable combinations of DTT/Fe(2+) or by ascorbic acid did not influence AC activity. We have also observed that PKC- or protein tyrosine kinase-mediated phosphorylation apparently does not play any significant role in different activity of AC determined in cerebrocortical preparations from young and adult rats. Our experiments analysing the presumed modulatory role of suramin revealed that this pharmacologically important drug may act as a direct inhibitor of AC. The enzyme activity was diminished to the same extent by suramin in membranes from both tested age groups. Our present data show that AC is regulated similarly in brain cortex from both young and adult rats, but its overall activity is much lower in adulthood.

Regulatory properties of adenylate cyclases type 5 and 6: A progress report

Adenylate cyclases (AC) type 5 and 6 comprise the calcium-inhibited family of adenylate cyclase isoforms. Here we review recent discoveries in the regulation of AC5 and AC6 with a focus on posttranslational modifications including glycosylation, nitrosylation, and phosphorylation by the cyclic AMP-dependent protein kinase (PKA), protein kinase C (PKC), and Raf1. We also describe novel signaling interactions such as Galpha(q)-mediated potentiation of AC6 activation. Novel regulators of AC5 and AC6, including small molecules and proteins that physically interact with AC5 and AC6 such as snapin, regulator of G protein signaling 2 (RGS2), protein associated with myc (PAM), and caveolin peptides are discussed. We also describe several recent studies that demonstrate the usefulness of transgenic or adenoviral overexpression of AC5 and AC6 in models for disease states such as cardiovascular hypertrophy. The discovery of novel regulatory mechanisms for AC5 and AC6 and their potential role in crucial physiological processes provide new avenues for research into therapeutic interventions targeting the cyclic AMP pathway.

D2 dopamine receptor-induced sensitization of adenylyl cyclase type 1 is G alpha(s) independent

Acute activation of D2 dopamine receptors inhibits adenylyl cyclase (EC 4.6.1.1), whereas persistent activation of these inhibitory receptors results in a compensatory increase in cyclic AMP accumulation. This sensitization of adenylyl cyclase is thought to involve enhanced Galpha(s)-adenylyl cyclase interactions; however, the absolute requirement of Galpha(s) has not been determined. The present study used a Galpha(s)-deficient cell line to examine directly the role of Galpha(s) in D2 dopamine receptor-induced sensitization of recombinant adenylyl cyclase type 1 (AC1) and 5 (AC5). In acute experiments, quinpirole activation of the D2 dopamine receptor inhibited AC1 and AC5 activity, indicating that the acute regulatory properties of AC1 and AC5 were retained in the absence of Galpha(s). Subsequent experiments revealed that short-term (2 h) activation of the D2 dopamine receptor resulted in significantly enhanced forskolin-stimulated AC1 activity in the absence of Galpha(s), whereas sensitization of forskolin-stimulated AC5 activity appeared to require Galpha(s). The Galpha(s)-independent sensitization of AC1 was explored further using AC1-selective activation protocols (A23187 and CCE) following short- and long-term agonist treatment. These studies revealed that persistent activation of D2 dopamine receptors sensitized AC1 activity to Ca2+ stimulation in cells devoid of endogenous Galpha(s) and demonstrate directly that sensitization of AC1 is Galpha(s)-independent.

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.

Involvement of histamine H1 and H2 receptors in the regulation of STAT-1 phosphorylation: inverse agonism exhibited by the receptor antagonists

Signal transducer and activator of transcription-1 (STAT1) is a latent signal transducer protein which, on phosphorylation, is translocated from the cytoplasm to the nucleus and is subsequently activated. This study was designed to determine the involvement of histamine receptors in histamine-mediated effect on STAT1 phosphorylation. It is known that the actions of histamine mediated through H1 and H2 receptors are dependent on their respective downstream pathways, Ca(2+)-PKC and cAMP-PKA. In this study, we investigated the significance of PKA in STAT1 phosphorylation. C57BL/6 mouse splenocytes were isolated and treated with histamine (10(-7)-10(-4) M) and then activated with PMA (phorbol 12 myristate 13-acetate) plus ionomycin. The phosphorylated STAT1 levels were analyzed by immunoblotting. Histamine receptor agonists amthamine and betahistine, histamine receptor antagonists pyrilamine maleate, tripelennamine, ranitidine, cimetidine and thioperamide, cAMP agonists N(6), 2'-0-dibutyryladenosine-3',5'-cyclic monophosphate sodium salt (db-cAMP) and forskolin, protein kinase A inhibitors N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinoline-sulfonamide (H89) and Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (RpcAMPs) and tyrosine kinase inhibitor tyrphostin were used to identify the upstream signal transduction pathways. We observed that histamine augmented the phosphorylation of STAT1 through both H1 and H2 receptors. Furthermore, H1 and H2 receptor antagonists displayed inverse agonism. Ca(2+)-PKC-induced phosphorylation of STAT1 was completely inhibited by H89 and significantly inhibited by RpcAMPs. DbcAMP and forskolin augmented the Ca(2+)-PKC-induced STAT1 phosphorylation thus suggesting a convergent crosstalk between the two histamine receptor signaling pathways, PKA and PKC.

Interferon-gamma down-regulates adenosine 2b receptor-mediated signaling and short circuit current in the intestinal epithelia by inhibiting the expression of adenylate cyclase

Adenosine is an endogenous signaling molecule that is highly up-regulated in inflammatory states. Adenosine acts through the A2b receptor, a G protein-coupled receptor that couples positively to Galpha(s) and activates adenylate cyclase. This leads to cAMP-mediated electrogenic chloride secretion in intestinal epithelia. To better understand the regulation of the A2b receptor in intestinal epithelia, we studied the effects of interferon-gamma (IFN-gamma), a potent immunomodulatory cytokine, in the T84 cell line. Pretreatment of cells with 500 units/ml IFN-gamma for 12 h inhibited an adenosine-induced short circuit current (Isc) without affecting the transepithelial resistance. Under these conditions, IFN-gamma did not inhibit the protein expression or membrane recruitment of the A2b receptor, shown to be essential for its function. Interestingly, IFN-gamma inhibited cAMP levels as well as its downstream signaling pathway as shown by the inhibition of adenosine-induced phosphorylation of cAMP response element-binding protein and protein kinase A activity. Similar studies with forskolin, a direct activator of adenylate cyclase, also demonstrated inhibition of cAMP and its downstream response by IFN-gamma. However, IFN-gamma did not affect secretory responses to the calcium-dependent secretagogue carbachol or cAMP analog 8-bromo-cAMP, indicating that normal secretory responses to adequate second messengers in IFN-gamma-treated cells are achievable. Moreover, IFN-gamma inhibited the expression of adenylate cyclase isoforms 5 and 7. In conclusion, we demonstrate that IFN-gamma down-regulates adenosine-mediated signaling possibly through the direct inhibition of adenylate cyclase expression. We propose that IFN-gamma may acutely affect global cAMP-mediated responses in the intestinal epithelia, thereby decreasing secretory responses, which may consequently aggravate inflammatory processes.

Protein kinase C and epidermal growth factor stimulation of Raf1 potentiates adenylyl cyclase type 6 activation in intact cells

Adenylyl cyclase type 6 (AC6) activity is inhibited by protein kinase C (PKC) in vitro; however, in intact cells, PKC activation does not inhibit the activity of transiently expressed AC6. To investigate the effects of PKC activation on AC6 activity in intact cells, we constructed human embryonic kidney (HEK) 293 cells that stably express wild-type AC6 (AC6-WT) or an AC6 mutant lacking a PKC and cyclic AMP-dependent protein kinase (PKA) phosphorylation site, Ser674 (AC6-S674A). In contrast to in vitro observations, we observed a PKC-mediated enhancement of forskolin- and isoproterenol-stimulated cyclic AMP accumulation in HEK-AC6 cells. Phorbol 12-myristate 13-acetate also potentiated cyclic AMP accumulation in cells expressing endogenous AC6, including Chinese hamster ovary cells and differentiated Cath.a differentiated cells. In HEK-AC6-S674A cells, the potentiation of AC6 stimulation was significantly greater than in cells expressing AC6-WT. The positive effect of PKC activation on AC6 activity seemed to involve Raf1 kinase because the Raf1 inhibitor 3-(3,5-dibromo-4-hydroxybenzylidene-5-iodo-1,3-dihydro-indol-2-one (GW5074) inhibited the PKC potentiation of AC6 activity. Furthermore, the forskolin-stimulated activity of a recombinant AC6 in which the putative Raf1 regulatory sites have been eliminated was not potentiated by activation of PKC. The ability of Raf1 to regulate AC6 may involve a direct interaction because AC6 and a constitutively active Raf1 construct were coimmunoprecipitated. In addition, we report that epidermal growth factor receptor activation also enhances AC6 signaling in a Raf1-dependent manner. These data suggest that Raf1 potentiates drug-stimulated cyclic AMP accumulation in cells expressing AC6 after activation of multiple signaling pathways.

Isoform-specific regulation of adenylyl cyclase: a potential target in future pharmacotherapy

Adenylyl cyclase (AC) is a target enzyme of multiple G-protein-coupled receptors (GPCRs). In the past decade, the cloning, structure and biochemical properties of nine AC isoforms were reported, and each isoform of AC shows distinct patterns of tissue distribution and biochemical/pharmacological properties. In addition to the conventional regulators of this enzyme, such as calmodulin (CaM) or PKC, novel regulators, for example, caveolin, have been identified. Most importantly, these regulators work on AC in an isoform dependent manner. Recent studies have demonstrated that certain classic AC inhibitors, i.e., P-site inhibitors, show an isoform-dependent inhibition of AC. The side chain modifications of forskolin, a diterpene extract from Coleus forskolii, markedly enhance its isoform selectivity. When taken together, these findings suggest that it is feasible to develop new pharmacotherapeutic agents that target AC isoforms to regulate various neurohormonal signals in a highly tissue-/organ-specific manner.

Expression of adenylyl cyclase isoforms in neutrophils

In the present study, we have identified the expression of adenylyl cyclase (AC) isoforms in rat neutrophils according to the mRNA analysis and the distinct mode of regulation of isoform activity. Agarose gel electrophoresis of reverse transcription-polymerase chain reaction (RT-PCR)-amplified products resulted in a single band of the expected size for each product with nucleotide sequences corresponding to AC1 to AC9. AC1 was abundant, while AC2, 6 and 9 were of moderate expression among the AC isoforms in neutrophils based on the quantitative real-time RT-PCR analysis. Exposure of neutrophils to Ca(2+) ionophore A23187, isoproterenol and forskolin stimulated cellular cyclic AMP accumulation. EDTA and the calmodulin (CaM) antagonist, trifluoperazine, prevented the A23187-induced response. Pretreatment with pertussis toxin (PTX) inhibited the alpha(2)-adrenergic agonist, UK14304-induced cellular cyclic AMP elevation. In addition, UK14304 augmented the cyclic AMP elevation when cells were stimulated by isoproterenol. Phorbol 12-myristate 13-acetate (PMA) attenuated the augmentation response of UK14304 and isoproterenol. Treatment of the membrane preparations from rat neutrophils with Ca(2+)/CaM, forskolin, isoproterenol, GTPgammaS or Gbetagamma all increased cyclic AMP production. The addition of protein kinase C (PKC) catalytic fragment and Gbetagamma augmented the Ca(2+)/CaM- and isoproterenol-stimulated AC activity, respectively. However, forskolin and the activated protein kinase A (PKA) attenuated the GTPgammaS- and isoproterenol-stimulated AC activity, respectively. KT5720, a PKA inhibitor, reversed the inhibition by PKA. Taken together, these data suggest the presence of four groups of AC isoforms in rat neutrophils.

Interaction between alpha2-autoreceptors and receptors mediating the effects of angiotensin II and bradykinin in the heart of newborn rats

The interaction between alpha2-autoreceptors and receptors for angiotensin II and bradykinin was studied in the heart of newborn rats. The tissues were labelled with [3H]noradrenaline and then superfused with cocaine-containing medium and stimulated electrically. Angiotensin II (10-300 nM) and bradykinin (3-100 nM) enhanced the evoked overflow of tritium, the maximum increase reaching 63.2% and 87.1%, respectively. Blockade of alpha2-adrenoceptors by 100 nM yohimbine reduced, and that by 1 microM abolished, the effect of both angiotensin II and bradykinin. On the contrary, chelerythrine and staurosporine--blockers of protein kinase C--as well as forskolin, an activator of adenylyl cyclase and a blocker of phosphodiesterase, markedly enhanced the facilitatory effect of angiotensin II and bradykinin. We conclude that: (1) alpha2-autoreceptors are present in the heart of newborn rats which interact with prejunctional receptors for angiotensin II and bradykinin also present in the rat heart at that age; (2) the facilitatory influence of chelerythrine and staurosporine on the one hand and that of forskolin on the other hand suggests a link between protein kinase C and cyclicAMP pathways.

Astrocyte mGlu(2/3)-mediated cAMP potentiation is calcium sensitive: studies in murine neuronal and astrocyte cultures

Signal transduction mechanisms of group II metabotropic glutamate receptors (mGlu(2/3)) remains a matter of some controversy, therefore we sought to gain new insights into its regulation by studying cAMP production in cultured neurons and astrocytes, and by examining inter-relationships of mGlu(2/3)-induced signalling with cellular calcium and various signalling cascades. mGlu(2/3) agonists 2R,4R-4-aminopyrrolidine-2,4-dicarboxylic acid (2R,4R-APDC) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) inhibited 10 microM forskolin-stimulated production of cAMP in murine cortical neurons, striatal neurons and forebrain astrocytes in the absence of extracellular Ca(2+). These agonists potentiated cAMP production in the presence of 1.8 mM Ca(2+) in astrocytes only. This potentiation was dependent on the extracellular Ca(2+) concentration (0.001-10 mM) and inhibited by the mGlu(2/3) antagonist LY341495 (1 microM), adenosine deaminase (1 U/ml) and the adenosine A(2A) receptor antagonist ZM241385 (1 microM). Pre-incubation with the phospholipase C (PLC) inhibitor U73122 (10 microM), L-type Ca(2+)-channel blockers nifedipine (1 microM) and nimodipine (1 microM), the calmodulin kinase II (CaMKII) inhibitor KN-62 (10 microM) or pertussis toxin (100 ng/ml) inhibited this potentiation. In the absence of 1.8 mM Ca(2+), thapsigargin (1 microM) facilitated the potentiation of cAMP production. Measurement of the Ca(2+)-binding dye Fluo-3/AM showed that, compared to Ca(2+)-free conditions, thapsigargin and 1.8 mM Ca(2+) elevated [Ca(2+)](i) in astrocytes; the latter effect being prevented by L-type Ca(2+)-channel blockers. Potentiation of cAMP production was also demonstrated when astrocytes were stimulated with the beta-adrenoceptor agonist isoprenaline (10 microM) in the presence of 1.8 mM Ca(2+), but not with the adenosine agonist NECA (10 microM) or the group I mGlu receptor agonist DHPG (100 microM). BaCl(2) (1.8 mM) in place of Ca(2+) did not facilitate forskolin-stimulated mGlu(2/3)-potentiation of cAMP. In short, this study in astrocytes demonstrates that under physiological Ca(2+) and adenylate cyclase stimulation an elevation of cAMP production is achieved that is mediated by PLC/IP(3)- and CaMKII-dependent pathways and results in the release of endogenous adenosine which acts at G(s) protein-coupled A(2A) receptors. These findings provide new insights into mGlu(2/3) signalling in astrocytes versus neurons, and which could determine the functional phenotypy of astrocytes under physiological and pathological conditions.

Human airway smooth muscle expresses 7 isoforms of adenylyl cyclase: a dominant role for isoform V

Adenylyl cyclases are a nine-member family of differentially regulated enzymes responsible for the synthesis of cAMP. cAMP is an important second messenger that contributes to the regulation of airway smooth muscle tone. However, little is known regarding the expression and regulation of adenylyl cyclase isoforms in airway smooth muscle cells. Nondegenerate specific primers were designed for all nine known isoforms of human adenylyl cyclase. RT-PCR experiments were performed using total RNA extracted from whole human brain (positive control), whole rat brain (negative control), whole human trachea, human airway smooth muscle, and primary cultures of human airway smooth muscle cells. Seven of the nine known isoforms of adenylyl cyclase (isoforms I, III-VII, and IX) were expressed at the mRNA level in both human airway smooth muscle and primary cultures of human airway smooth muscle cells. Immunoblot and adenylyl cyclase functional assay indicated that isoform V is likely among the functionally predominant isoforms of adenylyl cyclase in human airway smooth muscle. These results suggest that multiple isoforms of adenylyl cyclase enzymes are coexpressed in human airway smooth muscle cells and that isoform V is among the functionally important isoforms.

Adenylyl cyclase isoforms and signal integration in models of vascular smooth muscle cells

Adenylyl cyclases present a potential focal point for signal integration in vascular smooth muscle cells (VSMC) influencing contractile state and cellular responses to vessel wall injury. In the present study, we examined the influence of the vasoactive peptide arginine vasopressin (AVP) on cAMP regulation in primary cultures of rat aortic VSMC and in the A7r5 arterial smooth muscle cell line. In cultured VSMC and A7r5 cells, AVP had no effect on basal cAMP but differentially affected beta-adrenergic receptor-induced activation of adenylyl cyclase. AVP synergistically increased (twofold) isoproterenol-stimulated cAMP production in VSMC but inhibited the effect of isoproterenol (50%) in the A7r5 cell line. The effects of AVP in both preparations were blocked when cells were pretreated with a selective V(1) vasopressin receptor antagonist. Moreover, the actions of AVP in both models were dependent on release of intracellular Ca(2+) and were mimicked by elevation of Ca(2+) with the ionophore A23187, suggesting that the responses to AVP involve Ca(2+)-mediated regulation of adenylyl cyclase stimulation. Adenylyl cyclase types I, III, and VIII are stimulated by Ca(2+)/calmodulin, whereas types V and VI are directly inhibited by Ca(2+). RNA blot analysis for effector isotypes indicated that both VSMC and A7r5 cells expressed types III, V, and VI. VSMC also expressed mRNA for type IV and VIII effectors, which could account for the cell-specific responses to peptide hormone and Ca(2+).

Adenylyl cyclase 3 mediates prostaglandin E(2)-induced growth inhibition in arterial smooth muscle cells

Arterial smooth muscle cell (SMC) proliferation contributes to a number of vascular pathologies. Prostaglandin E(2) (PGE(2)), produced by the endothelium and by SMCs themselves, acts as a potent SMC growth inhibitor. The growth-inhibitory effects of PGE(2) are mediated through activation of G-protein-coupled membrane receptors, activation of adenylyl cyclases (ACs), formation of cAMP, and subsequent inhibition of mitogenic signal transduction pathways in SMCs. Of the 10 different mammalian AC isoforms known today, seven isoforms (AC2-7 and AC9) are expressed in SMCs from various species. We show that, despite the presence of several different AC isoforms, the principal AC isoform activated by PGE(2) in human arterial SMCs is a calmodulin kinase II-inhibited AC with characteristics similar to those of AC3. AC3 is expressed in isolated human arterial SMCs and in intact aorta. We further show that arterial SMCs isolated from AC3-deficient mice are resistant to PGE(2)-induced growth inhibition. In summary, AC3 is the principal AC isoform activated by PGE(2) in arterial SMCs, and AC3 mediates the growth-inhibitory effects of PGE(2). Because AC3 activity is inhibited by intracellular calcium through calmodulin kinase II, AC3 may serve as an important integrator of growth-inhibitory signals that stimulate cAMP formation and growth factors that increase intracellular calcium.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Characterization of adenylyl cyclase isoforms in rat peripheral pulmonary arteries

Activation of adenylyl cyclase (AC), of which there are 10 diversely regulated isoforms, is important in regulating pulmonary vascular tone and remodeling. Immunohistochemistry in rat lungs demonstrated that AC2, AC3, and AC5/6 predominated in vascular and bronchial smooth muscle. Isoforms 1, 4, 7, and 8 localized to the bronchial epithelium. Exposure of animals to hypoxia did not change the pattern of isoform expression. RT-PCR confirmed mRNA expression of AC2, AC3, AC5, and AC6 and demonstrated AC7 and AC8 transcripts in smooth muscle. Western blotting confirmed the presence of AC2, AC3, and AC5/6 proteins. Functional studies provided evidence of cAMP regulation by Ca(2+) and protein kinase C-activated but not G(i)-inhibited pathways, supporting a role for AC2 and a Ca(2+)-stimulated isoform, AC8. However, NKH-477, an AC5-selective activator, was more potent than forskolin in elevating cAMP and inhibiting serum-stimulated [(3)H]thymidine incorporation, supporting the presence of AC5. These studies demonstrate differential expression of AC isoforms in rat lungs and provide evidence that AC2, AC5, and AC8 are functionally important in cAMP regulation and growth pathways in pulmonary artery myocytes.