Adenylyl cyclases as innovative therapeutic goals

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

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.

Host cell cAMP-Epac-Rap1b pathway inhibition by hawthorn extract as a potential target against Trypanosoma cruzi infection

Although the two drugs currently available for the treatment of Chagas disease, Benznidazole and Nifurtimox, have proven to be effective in the acute phase of the disease, the 60-90-day treatment leads to high toxicity and unwanted side effects, presenting, in addition, a low efficacy in the chronic phase of the disease. For this reason, new therapies that are more effective are needed. In this regard, we have recently shown that the inhibition of the Epac-Rap1b pathway suppressed the cAMP-mediated host cell invasion by Trypanosoma cruzi. Interestingly, it has been described that vitexin, a natural flavone that protects against ischemia-reperfusion damage, acts by inhibiting the expression of Epac and Rap1 proteins. Vitexin can be found in plants of the genus Crataegus spp., traditionally known as hawthorn, which are of great interest considering their highly documented use as cardio-protectors. Pre-treating cells with an extract of Crataegus oxyacantha produced levels of T. cruzi invasion comparable to the ones observed for the commercially available Epac1-specific inhibitor, ESI-09. In addition, extract-treated cells exhibited a decrease in the activation of Rap1b, suggesting that the effects of the extract would be mediated by the inhibition of the cAMP-Epac-Rap1 signaling pathway. Using HPLC-HRMS2, we could confirm the presence of vitexin, and other flavones that could act as inhibitors of Epac/Rap1b, in the extracts of C. oxyacantha. Most significantly, when cells were treated with the extract of C. oxyacantha in conjunction with Nifurtimox, an increased modulation of invasion was observed.

Isoforskolin, an adenylyl cyclase activator, attenuates cigarette smoke-induced COPD in rats

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is characterized by limited airflow due to pulmonary and alveolar abnormalities from exposure to cigarette smoke (CS). Current therapeutic drugs are limited and the development of novel treatments to prevent disease progression is challenging. Isoforskolin (ISOF) from the plant Coleus forskohlii is an effective activator of adenylyl cyclase (AC) isoforms. Previously we found ISOF could attenuate acute lung injury in animal models, while the effect of ISOF on COPD has not been elucidated.

PURPOSE

In this study, we aimed to evaluate the efficacy of ISOF on COPD and reveal its potential mechanisms.

METHODS

A rat model of COPD was established by long-term exposure to CS, then the rats were orally administered with ISOF (0.5, 1 and 2 mg/kg). The pulmonary function, lung morphology, inflammatory cells and cytokines in serum or bronchoalveolar lavage fluid (BALF) were evaluated. Transcriptomics, proteomics and network pharmacology analysis were utilized to identify potential mechanisms of ISOF. Droplet digital PCR was used to detect the mRNA expression of AC1-10 in donor lung tissues. AC activation was determined in recombinant human embryonic kidney 293 (HEK293) cells stably expressing human AC isoforms. In addition, ISOF caused trachea relaxation ex vivo were assessed in isolated trachea rings from guinea pigs.

RESULTS

ISOF significantly ameliorated pathological damage of lung tissue and improved pulmonary function in COPD rats. ISOF treatment decreased the number of inflammatory cells in peripheral blood, and also the levels of pro-inflammatory cytokines in serum and BALF. Consistent with omics-based analyses, ISOF markedly downregulated the mTOR level in lung tissue. Flow cytometry analysis revealed that ISOF treatment reduced the ratio of Th17/Treg cells in peripheral blood. Furthermore, the expression levels of AC1 and AC2 are relatively higher than other AC isoforms in normal lung tissues, and ISOF could potently activate AC1 and AC2 in vitro and significantly relax isolated guinea pig trachea.

CONCLUSION

Collectively, our studies suggest that ISOF exerts its anti-COPD effect by improving lung function, anti-inflammation and trachea relaxation, which may be related to AC activation, mTOR signaling and Th17/Treg balance.

Discovery of a potent and selective adenylyl cyclase type 8 agonist by docking-based virtual screening

Adenylyl cyclases (ACs), which are responsible for catalyzing the conversion of adenosine triphosphate (ATP) into the second messenger cyclic adenosine monophosphate (cAMP), play a critical role in cell signal transduction. In this study, a combined approach involving docking-based virtual screening, with the combination of homology modeling followed by an in-vitro, and cell-based biological assay have been performed for discovering a class of novel potent and selective isoform adenylyl cyclase type 8 (AC8) agonist. The computer-aided virtual screening was used to identify fourteen virtual cluster compounds as potential hits which were further subjected to rigorous bioassays. A novel hit compound VHC-7 (ethyl 3-(2,4-dichlorobenzyl)-2-oxoindoline-3-carboxylate) was identified as a highly potent selective AC8 agonist with EC₅₀ value of 0.1052 ± 0.038 µM. Remarkably, the molecule herein reported can be explored further to discover greater number of hit compounds with better pharmacokinetic properties as well as to serve as a promising novel hit agonist of AC8 for the treatment of various central nervous system disorders and its associated diseases.

The Role of Cyclic AMP Signaling in Cardiac Fibrosis

Myocardial stress and injury invariably promote remodeling of the cardiac tissue, which is associated with cardiomyocyte death and development of fibrosis. The fibrotic process is initially triggered by the differentiation of resident cardiac fibroblasts into myofibroblasts. These activated fibroblasts display increased proliferative capacity and secrete large amounts of extracellular matrix. Uncontrolled myofibroblast activation can thus promote heart stiffness, cardiac dysfunction, arrhythmias, and progression to heart failure. Despite the well-established role of myofibroblasts in mediating cardiac disease, our current knowledge on how signaling pathways promoting fibrosis are regulated and coordinated in this cell type is largely incomplete. In this respect, cyclic adenosine monophosphate (cAMP) signaling acts as a major modulator of fibrotic responses activated in fibroblasts of injured or stressed hearts. In particular, accumulating evidence now suggests that upstream cAMP modulators including G protein-coupled receptors, adenylyl cyclases (ACs), and phosphodiesterases (PDEs); downstream cAMP effectors such as protein kinase A (PKA) and the guanine nucleotide exchange factor Epac; and cAMP signaling organizers such as A-kinase anchoring proteins (AKAPs) modulate a variety of fundamental cellular processes involved in myocardial fibrosis including myofibroblast differentiation, proliferation, collagen secretion, and invasiveness. The current review will discuss recent advances highlighting the role of cAMP and AKAP-mediated signaling in regulating pathophysiological responses controlling cardiac fibrosis.

p53 regulates mitochondrial dynamics by inhibiting Drp1 translocation into mitochondria during cellular senescence

Cellular senescence acts as an important barrier to tumorigenesis by eliminating precancerous cells. Previous studies have shown an essential role of the tumor suppressor p53 in cellular senescence, but how p53 induces cellular senescence is not fully understood. We found that p53 promoted the formation of highly interconnected and elongated mitochondria prior to the onset of cellular senescence. The inhibition of mitochondrial elongation upon p53 expression suppressed cellular senescence, suggesting that mitochondrial elongation is required for the induction of p53-dependent senescence. p53-induced mitochondrial elongation resulted in mitochondrial dysfunction and subsequent increases in intracellular reactive oxygen species (ROS) levels, an important mediator of cellular senescence. Mechanistically, the inhibitory phosphorylation of Drp1 Ser637 increased upon p53 expression, suppressing the translocation of Drp1 into mitochondria. The transcriptional function of p53 was crucial for controlling the inhibitory phosphorylation of Drp1, whereas p21 was nonessential. Protein kinase A (PKA) activity was responsible for p53-mediated Drp1 Ser637 phosphorylation and mitochondrial dysfunction. Taken together, these results suggest that p53 regulates mitochondrial dynamics through the PKA-Drp1 pathway to induce cellular senescence.

A new family of sesterterpenoids isolated around the Pacific Rim

Covering: 2009 up to the end of 2017 There has been a recent eruption in the number of known marine sesterterpenoids which have been isolated from Pacific Rim marine organisms. These compounds have novel and unusual structures that exhibit incredibly potent and varied bioactivities. This review details the isolation, biological testing and prospects for this exciting new family with discussion of their potential biogenetic origins.

Natural products as modulators of the cyclic-AMP pathway: evaluation and synthesis of lead compounds

It is now well recognized that the normal cellular response in mammalian cells is critically regulated by the cyclic-AMP (cAMP) pathway through the appropriate balance of adenylyl cyclase (AC) and phosphodiesterase-4 (PDE4) activities. Dysfunctions in the cAMP pathway have major implications in various diseases like CNS disorders, inflammation and cardiac syndromes and, hence, the modulation of cAMP signalling through appropriate intervention of AC/PDE4 activities has emerged as a promising new drug discovery strategy of current interest. In this context, synthetic small molecules have had limited success so far and therefore parallel efforts on natural product leads have been actively pursued. The early promise of using the diterpene forskolin and its semi-synthetic analogs as AC activators has given way to new leads in the last decade from novel natural products like the marine sesterterpenoids alotaketals and ansellones and the 9,9'-diarylfluorenone cored selaginpulvilins, etc. and their synthesis has drawn much attention. This review captures these contemporary developments, particularly total synthesis campaigns and structure-guided analog design in the context of AC and PDE-4 modulating attributes and the scope for future possibilities.

α-Cedrene protects rodents from high-fat diet-induced adiposity via adenylyl cyclase 3

OBJECTIVE

The increasing global prevalence of obesity and its associated disorders points to an urgent need for the development of novel and effective strategies for the prevention of weight gain. Here, we investigated the potential of α-cedrene, a volatile sesquiterpene compound derived from cedarwood oil, in regulation of obesity and delineated the mechanisms involved.

METHODS

For the prevention of obesity, C57BL/6 N mice were fed a high-fat diet (HFD) and were orally administered either with vehicle or α-cedrene for 8 weeks. For the therapy of obesity, obese Sprague Dawley rats, induced by a HFD for 8 weeks, were orally treated either with vehicle or α-cedrene for 12 weeks. To determine whether the action of α-cedrene was Adcy3 dependent, Adcy3 heterozygous null mice (Adcy3^+/-) and wild-type controls were fed either HFD or α-cedrene supplemented HFD for 17 weeks.

RESULTS

Oral α-cedrene administration prevented or reversed HFD-induced obesity and abnormal metabolic aberrations in rodents, without affecting their food intake. Downregulation of Adcy3 expression by small interfering RNA abrogated the beneficial effects of α-cedrene on the oxygen consumption rate and intracellular lipid accumulation in 3T3-L1 adipocytes. Similarly, in Adcy3^+/- mice, the α-cedrene-driven suppression of body weight gain observed in wild-type mice was substantially (~50%) attenuated. Expression of thermogenic and lipid oxidation genes was increased in adipose tissues of α-cedrene-treated mice, with concomitant downregulation of adipogenic gene expression. These beneficial molecular changes elicited by α-cedrene were blunted in adipose tissues of Adcy3^+/- mice.

CONCLUSIONS

Our results highlight the potential of α-cedrene for antiobesity interventions and suggest that the antiobesity effect of α-cedrene is mediated by Adcy3 in adipose tissues.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

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.

Comparative electrostatic analysis of adenylyl cyclase for isoform dependent regulation properties

The enzyme adenylyl cyclase (AC) plays a pivotal role in a variety of signal transduction pathways inside the cell, where it catalyzes the cyclization of adenosine triphosphate (ATP) into the second-messenger cyclic adenosine monophosphate (cAMP). Among other roles, AC regulates processes involved in neural plasticity, innervation of smooth muscles of the heart and the endocrine system of the pancreas. The functional diversity of AC is manifested in its different isoforms, each having a specific regulation pattern. There is an increasing amount of data available concerning the regulatory properties of AC isoforms, however little is known about the interactions on a structural level. Here, we conducted a comparative electrostatic analysis of the catalytic domains of all nine transmembrane AC isoforms with the aim of detecting, verifying and predicting the binding sites of molecular regulators on AC. The results provide support for the positioning of the binding site of the inhibitory protein G_i α at a pseudo-symmetric position to the stimulatory G_s α binding site. They also provide a structural interpretation of the Gβγ interaction with ACs 2, 4, and 7 and suggest a new binding site for RGS2. Comparison of the small molecule binding sites on AC shows that overall they have high electrostatic similarity, but regions of electrostatic differences are identified. These could provide a basis for the development of novel compounds with isoform-specific modulatory effects on AC. Proteins 2016; 84:1844-1858. © 2016 Wiley Periodicals, Inc.

Differences in the control of basal L-type Ca(2+) current by the cyclic AMP signaling cascade in frog, rat, and human cardiac myocytes

β-adrenergic receptors (β-ARs) mediate the positive inotropic effects of catecholamines by cAMP-dependent phosphorylation of the L-type Ca(2+) channels (LTCCs), which provide Ca(2+) for the initiation and regulation of cell contraction. The overall effect of cAMP-modulating agents on cardiac calcium current (I Ca,L) and contraction depends on the basal activity of LTCCs which, in turn, depends on the basal activities of key enzymes involved in the cAMP signaling cascade. Our current work is a comparative study demonstrating the differences in the basal activities of β-ARs, adenylyl cyclase, phosphodiesterases, phosphatases, and LTCCs in the frog and rat ventricular and human atrial myocytes. The main conclusion is that the basal I Ca,L, and consequently the contractile function of the heart, is secured from unnecessary elevation of its activity and energy consumption at the several "checking-points" of cAMP-dependent signaling cascade and the loading of these "checking-points" may vary in different species and tissues.

The cAMP Pathway as Therapeutic Target in Autoimmune and Inflammatory Diseases

Nucleotide signaling molecules contribute to the regulation of cellular pathways. In the immune system, cyclic adenosine monophosphate (cAMP) is well established as a potent regulator of innate and adaptive immune cell functions. Therapeutic strategies to interrupt or enhance cAMP generation or effects have immunoregulatory potential in autoimmune and inflammatory disorders. Here, we provide an overview of the cyclic AMP axis and its role as a regulator of immune functions and discuss the clinical and translational relevance of interventions with these processes.

Adenylyl cyclases 5 and 6 underlie PIP3-dependent regulation

Many different neurotransmitters and hormones control intracellular signaling by regulating the production of the second messenger cAMP. The function of the broadly expressed adenylyl cyclases (ACs) 5 and 6 is regulated by either stimulatory or inhibitory G proteins. By analyzing a well-known rebound stimulation phenomenon after withdrawal of Gi protein in atrial myocytes, we discovered that AC5 and -6 are tightly regulated by the second messenger PIP3. By monitoring cAMP levels in real time by means of Förster resonance energy transfer (FRET)-based biosensors, we reproduced the rebound stimulation in a heterologous expression system specifically for AC5 or -6. Strikingly, this cAMP rebound stimulation was completely blocked by the PI3K inhibitor wortmannin, both in atrial myocytes and in transfected human embryonic kidney cells. Similar effects were observed by heterologous expression of the PIP3 phosphatase and tensin homolog (PTEN). However, general kinase inhibitors or inhibitors of Akt had no effect, suggesting a PIP3-dependent mechanism. These findings demonstrate the existence of a novel general pathway for regulation of AC5 and -6 activity via PIP3 that leads to pronounced alterations of cytosolic cAMP levels.

The role of adenylyl cyclase isoform 6 in β-adrenoceptor signalling in murine airways

BACKGROUND AND PURPOSE

Adenylyl cyclase (AC) is a key signalling enzyme for many GPCRs and catalyses the conversion of ATP to cAMP which, in turn, is a crucial determinant of many biological responses. β-Adrenoceptor agonists are prescribed as bronchodilators for asthma and chronic obstructive pulmonary disease, and it is commonly assumed that they elicit their actions via AC-dependent production of cAMP. However, empirical evidence in support of this is lacking and the exact mechanism by which these drugs acts remains elusive. This is partly due to the existence of at least 10 different isoforms of AC and the absence of any truly selective pharmacological inhibitors. Here, we have used genetically modified mice and model systems to establish the role of AC isoforms in the airway responses to β-adrenoceptor agonists.

EXPERIMENTAL APPROACH

Receptors mediating responses to β-adrenoceptor agonists in airway smooth muscle (ASM) and sensory nerve were identified in isolated tissue systems. Expression of mRNA for the AC isoforms in ASM and neurones was determined by qPCR. Functional responses were assessed in AC isoform KO mice and wild-type controls.

KEY RESULTS

Airway and vagal tissue expressed mRNA for various isoforms of AC. AC6 was the most prominent isoform. Responses to β-adrenoceptor agonists in tissues from AC6 KO mice were virtually abolished.

CONCLUSIONS AND IMPLICATIONS

AC6 played a critical role in relaxation of ASM to β1 -adrenoceptor agonists and in modulation of sensory nerves by β1-3 -adrenoceptor agonists. These results further unravel the signalling pathway of this extensively prescribed class of medicine.

Functional non-nucleoside adenylyl cyclase inhibitors

In this study, we describe the synthesis of novel functional non-nucleoside adenylyl cyclase inhibitors, which can be easily modified with thiol containing biomolecules such as tumour targeting structures. The linkage between inhibitor and biomolecule contains cleavable bonds to enable efficient intracellular delivery in the reductive milieu of the cytosol as well as in the acidic environment within endosomes and lysosomes. The suitability of this synthetic approach was shown by the successful bioconjugation of a poor cell-permeable inhibitor with a cell-penetrating peptide. Additionally, we have demonstrated the excellent inhibitory effect of the compounds presented here in a live-cell Förster resonance energy transfer-based assay in human embryonic kidney cells.

Cyclic AMP synthesis and hydrolysis in the normal and failing heart

Cyclic AMP regulates a multitude of cellular responses and orchestrates a network of intracellular events. In the heart, cAMP is the main second messenger of the β-adrenergic receptor (β-AR) pathway producing positive chronotropic, inotropic, and lusitropic effects during sympathetic stimulation. Whereas short-term stimulation of β-AR/cAMP is beneficial for the heart, chronic activation of this pathway triggers pathological cardiac remodeling, which may ultimately lead to heart failure (HF). Cyclic AMP is controlled by two families of enzymes with opposite actions: adenylyl cyclases, which control cAMP production and phosphodiesterases, which control its degradation. The large number of families and isoforms of these enzymes, their different localization within the cell, and their organization in macromolecular complexes leads to a high level of compartmentation, both in space and time, of cAMP signaling in cardiac myocytes. Here, we review the expression level, molecular characteristics, functional properties, and roles of the different adenylyl cyclase and phosphodiesterase families expressed in heart muscle and the changes that occur in cardiac hypertrophy and failure.

Regulating the ubiquitin/proteasome pathway via cAMP-signaling: neuroprotective potential

The cAMP-signaling pathway has been under intensive investigation for decades. It is a wonder that such a small simple molecule like cAMP can modulate a vast number of diverse processes in different types of cells. The ubiquitous involvement of cAMP-signaling in a variety of cellular events requires tight spatial and temporal control of its generation, propagation, compartmentalization, and elimination. Among the various steps of the cAMP-signaling pathway, G-protein-coupled receptors, adenylate cyclases, phosphodiesterases, the two major cAMP targets, i.e., protein kinase A and exchange protein activated by cAMP, as well as the A-kinase anchoring proteins, are potential targets for drug development. Herein we review the recent progress on the regulation and manipulation of different steps of the cAMP-signaling pathway. We end by focusing on the emerging role of cAMP-signaling in modulating protein degradation via the ubiquitin/proteasome pathway. New discoveries on the regulation of the ubiquitin/proteasome pathway by cAMP-signaling support the development of new therapeutic approaches to prevent proteotoxicity in chronic neurodegenerative disorders and other human disease conditions associated with impaired protein turnover by the ubiquitin/proteasome pathway and the accumulation of ubiquitin-protein aggregates.

High glucose-induced barrier impairment of human retinal pigment epithelium is ameliorated by treatment with Goji berry extracts through modulation of cAMP levels

Human retinal pigment epithelium cells were used to investigate the mechanisms underlying blood-retinal barrier disruption under conditions of chronic hyperglycemia. The treatment with 25 mM glucose caused a rapid drop in the transepithelial electrical resistance (TEER), which was reversed by the addition of either a methanolic extract from Goji (Lycium barbarum L.) berries or its main component, taurine. Intracellular cAMP levels increased concurrently with the high glucose-induced TEER decrease, and were correlated to an increased activity of the cytosolic isoform of the enzyme adenylyl cyclase. The treatment with plant extract or taurine restored control levels. Data are discussed in view of a possible prevention approach for diabetic retinopathy.

Adenylyl cyclase regulation in heart failure due to myocardial infarction in rats

Cardiac adenylyl cyclase (AC) activity was described to be differentially regulated in left and right ventricles (LVs and RVs) of rats with heart failure (HF) due to LV myocardial infarction (MI) (Sethi et al. Am J Physiol 272:H884-H893, 1997). AC activities in LVs and RVs were increased and decreased respectively in rats 8 and 16 weeks post MI under basal and stimulatory conditions including AC activation via β-adrenergic receptors (β-ARs), stimulatory G protein (Gs), and direct AC activation with forskolin (FS). The current study aimed to detect alterations in rat heart AC activities in a comparable model of HF 9 weeks post LV MI. Therefore, cardiac AC activities were measured under basal and β-AR-, Gs-, or FS-stimulated conditions as well as under inhibition with various MANT [2'(3')-O-(N-methylanthraniloyl)]-nucleotide AC inhibitors and the P-site AC inhibitors NKY80 [2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone] and vidarabine (9-β-D-arabinosyladenine, AraAde). Basal and stimulated AC activities along with AC inhibition experiments did not reveal evidence for changes in AC activity in LVs and RVs from MI group animals despite the presence of congestive HF. However, our study is indeterminate. Further studies are required to identify the factors responsible for previously described changes in cardiac AC activity in MI induced HF and to elucidate the role of altered AC regulation in the pathophysiology of HF. In order to detect small changes in AC regulation, larger group sizes than the ones used in our present study are required.

Dynamics of Gαi1 interaction with type 5 adenylate cyclase reveal the molecular basis for high sensitivity of Gi-mediated inhibition of cAMP production

Many physiological and pathophysiological processes are regulated by cAMP. Different therapies directly or indirectly influence the cellular concentration of this second messenger. A wide variety of receptors either activates or inhibits adenylate cyclases in order to induce proper physiological responses. A key event in this signalling system is the direct and dynamic interaction of Gαi1 subunits with adenylate cyclases. We established a FRET-based assay between G-protein subunits and AC5 (type 5 adenylate cyclase) and monitored receptor-stimulated interactions between Gαi1 and AC5 in single intact cells with high temporal resolution. We observed that FRET between Gαi1 and AC5 developed at much lower concentration of agonist compared with the overall Gi-protein activity resulting in a left-shift of the concentration-response curve by approximately one order of magnitude. Furthermore, Gi1-protein-mediated attenuation of AC5-dependent increases in cAMP occurred at comparable low concentrations of agonist. On analysing the dynamics we found the dissociation of the Gαi1 subunits and AC5 to occur significantly slower than the G-protein deactivation and to be insensitive to RGS4 (regulator of G-protein signalling type 4) expression. This led us to the conclusion that AC5, by binding active Gαi1, interferes with G-protein deactivation and reassembly and thereby might sensitize its own regulation.

Development of a high-throughput screening paradigm for the discovery of small-molecule modulators of adenylyl cyclase: identification of an adenylyl cyclase 2 inhibitor

Adenylyl cyclase (AC) isoforms are implicated in several physiologic processes and disease states, but advancements in the therapeutic targeting of AC isoforms have been limited by the lack of potent and isoform-selective small-molecule modulators. The discovery of AC isoform-selective small molecules is expected to facilitate the validation of AC isoforms as therapeutic targets and augment the study of AC isoform function in vivo. Identification of chemical probes for AC2 is particularly important because there are no published genetic deletion studies and few small-molecule modulators. The present report describes the development and implementation of an intact-cell, small-molecule screening approach and subsequent validation paradigm for the discovery of AC2 inhibitors. The NIH clinical collections I and II were screened for inhibitors of AC2 activity using PMA-stimulated cAMP accumulation as a functional readout. Active compounds were subsequently confirmed and validated as direct AC2 inhibitors using orthogonal and counterscreening assays. The screening effort identified SKF-83566 [8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrobromide] as a selective AC2 inhibitor with superior pharmacological properties for selective modulation of AC2 compared with currently available AC inhibitors. The utility of SKF-83566 as a small-molecule probe to study the function of endogenous ACs was demonstrated in C2C12 mouse skeletal muscle cells and human bronchial smooth muscle cells.

Isoform selectivity of adenylyl cyclase inhibitors: characterization of known and novel compounds

Nine membrane-bound adenylyl cyclase (AC) isoforms catalyze the production of the second messenger cyclic AMP (cAMP) in response to various stimuli. Reduction of AC activity has well documented benefits, including benefits for heart disease and pain. These roles have inspired development of isoform-selective AC inhibitors, a lack of which currently limits exploration of functions and/or treatment of dysfunctions involving AC/cAMP signaling. However, inhibitors described as AC5- or AC1-selective have not been screened against the full panel of AC isoforms. We have measured pharmacological inhibitor profiles for all transmembrane AC isoforms. We found that 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22,536), 2-amino-7-(furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), and adenine 9-β-d-arabinofuranoside (Ara-A), described as supposedly AC5-selective, do not discriminate between AC5 and AC6, whereas the putative AC1-selective inhibitor 5-[[2-(6-amino-9H-purin-9-yl)ethyl]amino]-1-pentanol (NB001) does not directly target AC1 to reduce cAMP levels. A structure-based virtual screen targeting the ATP binding site of AC was used to identify novel chemical structures that show some preference for AC1 or AC2. Mutation of the AC2 forskolin binding pocket does not interfere with inhibition by SQ22,536 or the novel AC2 inhibitor, suggesting binding to the catalytic site. Thus, we show that compounds lacking the adenine chemical signature and targeting the ATP binding site can potentially be used to develop AC isoform-specific inhibitors, and discuss the need to reinterpret literature using AC5/6-selective molecules SQ22,536, NKY80, and Ara-A.

Luciferase reporter gene assay on human, murine and rat histamine H4 receptor orthologs: correlations and discrepancies between distal and proximal readouts

The investigation of the (patho)physiological role of the histamine H4 receptor (H4R) and its validation as a possible drug target in translational animal models are compromised by distinct species-dependent discrepancies regarding potencies and receptor subtype selectivities of the pharmacological tools. Such differences were extremely pronounced in case of proximal readouts, e. g. [(32)P]GTPase or [(35)S]GTPγS binding assays. To improve the predictability of in vitro investigations, the aim of this study was to establish a reporter gene assay for human, murine and rat H4Rs, using bioluminescence as a more distal readout. For this purpose a cAMP responsive element (CRE) controlled luciferase reporter gene assay was established in HEK293T cells, stably expressing the human (h), the mouse (m) or the rat (r) H4R. The potencies and efficacies of 23 selected ligands (agonists, inverse agonists and antagonists) were determined and compared with the results obtained from proximal readouts. The potencies of the examined ligands at the human H4R were consistent with reported data from [(32)P]GTPase or [(35)S]GTPγS binding assays, despite a tendency toward increased intrinsic efficacies of partial agonists. The differences in potencies of individual agonists at the three H4R orthologs were generally less pronounced compared to more proximal readouts. In conclusion, the established reporter gene assay is highly sensitive and reliable. Regarding discrepancies compared to data from functional assays such as [(32)P]GTPase and [(35)S]GTPγS binding, the readout may reflect multifactorial causes downstream from G-protein activation, e.g. activation/amplification of or cross-talk between different signaling pathways.

Adenosine and prostaglandin e2 production by human inducible regulatory T cells in health and disease

Regulatory T cells (Treg) play a key role in maintaining the balance of immune responses in human health and in disease. Treg come in many flavors and can utilize a variety of mechanisms to modulate immune responses. In cancer, inducible (i) or adaptive Treg expand, accumulate in tissues and peripheral blood of patients, and represent a functionally prominent component of CD4+ T lymphocytes. Phenotypically and functionally, iTreg are distinct from natural (n) Treg. A subset of iTreg expressing ectonucleotidases CD39 and CD73 is able to hydrolyze ATP to 5′-AMP and adenosine (ADO) and thus mediate suppression of those immune cells which express ADO receptors. iTreg can also produce prostaglandin E₂ (PGE₂). The mechanisms responsible for iTreg-mediated suppression involve binding of ADO and PGE₂ produced by iTreg to their respective receptors expressed on T effector cells (Teff), leading to the up-regulation of adenylate cyclase and cAMP activities in Teff and to their functional inhibition. The potential for regulating these mechanisms by the use of pharmacologic inhibitors to relieve iTreg-mediated suppression in cancer suggests the development of therapeutic strategies targeting the ADO and PGE₂ pathways.

Short- and long-term regulation of intestinal Na+/H+ exchange activity associated with TLR2 receptor activation is independent of nuclear factor-κB signaling

Type 2 Toll-like receptors (TLR2s) are expressed in cell membranes and recognize a wide range of pathogen-associated molecular patterns derived from bacteria, such as lipoteichoic acid (LTA). The aim of this study was to evaluate the effect of TLR2 activation by LTA on the activity of type 1 Na(+)/H(+) exchanger (NHE) in T84 intestinal epithelial cells. Short-term (0.5 hour) and long-term (18 hours) TLR2 activation significantly inhibited NHE1 activity in a concentration-dependent manner (0.01-100 µg/ml; -7 ± 3 to -21 ± 3% and 3 ± 3 to -21 ± 3% of control values, respectively). S3226 [3-[2-(3-guanidino-2-methyl-3-oxopropenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride], an NHE3-selective inhibitor, did not affect the inhibitory effect on NHE activity. LTA-induced NHE inhibition did not occur in the presence ofethylisopropylamiloride (an NHE1 inhibitor). Long-term TLR2 activation decreased NHE1 affinity for Na(+) (Km= 64.98 ± 1.67 mM) compared with control (Km= 20.44 ± 0.54 mM) without changes in Vmax values. After TLR2 activation, we observed tyrosine-protein kinase (SRC) activation, phosphatidylinositol 3-kinase (PI3K) recruitment, and adenylyl cyclase (AC3) phosphorylation. The total amount of AC3 increased (23 ± 8% of control) after long-term treatment with LTA. Anti-AC3 small interfering RNA prevented LTA-induced NHE1 inhibition, similar to that observed with the AC3 inhibitor KH7 [(±)-2-(1H-benzimidazol-2-ylthio)propanoic acid 2-[(5-bromo-2-hydroxyphenyl)methylene]hydrazide]. A significant increase in cAMP levels (32 ± 3% and 14 ± 2% after short- and long-term stimulation, respectively) was detected, and inhibition of protein kinase A (PKA), phospholipase C (PLC), and downregulation of protein kinase C (PKC) prevented NHE1 inhibition. Inhibition of nuclear factor-κΒ (NF-κB) failed to revert NHE1 inhibition. We concluded that activation of TLR2 reduces NHE1 activity in epithelial cells through an alternative pathway that is unrelated to NF-κB, which involves SCR, PI3K, AC3, PKA, PLC, and PKC.

Similarly potent inhibition of adenylyl cyclase by P-site inhibitors in hearts from wild type and AC5 knockout mice

Adenylyl cyclase type 5 (AC5) was described as major cardiac AC isoform. The knockout of AC5 (AC5KO) exerted cardioprotective effects in heart failure. Our study explored the impact of AC5KO on mouse heart AC activities and evaluated putative AC5-selective inhibitors. In cardiac membranes from AC5KO mice, basal AC activity was decreased, while AC stimulation was intact. The putative AC5-selective P-site inhibitors SQ22,536 [9-(tetra-hydro-2-furanyl)-9H-purin-6-amine], vidarabine (9-β-D-arabinosyladenine) and NKY80 [2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone] inhibited recombinant AC5 more potently than AC2 and AC1, but selectivity was only modest (∼4-40-fold). These compounds inhibited cardiac AC from WT and AC5KO mice with similar potencies. In conclusion, AC regulation in AC5KO hearts was unimpaired, questioning the supposed dominant role of AC5 in the heart. Moreover, the AC inhibitors SQ22,536, NKY80 and vidarabine lack adequate selectivity for AC5 and, therefore, do not present suitable tools to study AC5-specific functions.

Total synthesis and structure-activity relationship study of the potent cAMP signaling agonist (-)-alotaketal A

A detailed account of the first total synthesis of alotaketal A, a tricyclic spiroketal sesterterpenoid that potently activates the cAMP signaling pathway, is provided. The synthesis employs both intra- and intermolecular reductive allylation of esters for assembling one of the fragments and their coupling. A Hg(OAc)2-mediated allylic mercuration is used to introduce the C22-hydroxyl group. The subtle influence of substituents over the course of the spiroketalization process is revealed. The synthesis confirms the relative and absolute stereochemistry of (-)-alotaketal A and allows verification of alotaketal A's effect over cAMP signaling using reporter-based FRET imaging assays with HEK 293T cells. Our studies also revealed alotaketal A's unique activity in selectively targeting nuclear PKA signaling in living cells.

Biological roles of cAMP: variations on a theme in the different kingdoms of life

Cyclic AMP (cAMP) plays a key regulatory role in most types of cells; however, the pathways controlled by cAMP may present important differences between organisms and between tissues within a specific organism. Changes in cAMP levels are caused by multiple triggers, most affecting adenylyl cyclases, the enzymes that synthesize cAMP. Adenylyl cyclases form a large and diverse family including soluble forms and others with one or more transmembrane domains. Regulatory mechanisms for the soluble adenylyl cyclases involve either interaction with diverse proteins, as happens in Escherichia coli or yeasts, or with calcium or bicarbonate ions, as occurs in mammalian cells. The transmembrane cyclases can be regulated by a variety of proteins, among which the α subunit and the βγ complex from G proteins coupled to membrane receptors are prominent. cAMP levels also are controlled by the activity of phosphodiesterases, enzymes that hydrolyze cAMP. Phosphodiesterases can be regulated by cAMP, cGMP or calcium-calmodulin or by phosphorylation by different protein kinases. Regulation through cAMP depends on its binding to diverse proteins, its proximal targets, this in turn causing changes in a variety of distal targets. Specifically, binding of cAMP to regulatory subunits of cAMP-dependent protein kinases (PKAs) affects the activity of substrates of PKA, binding to exchange proteins directly activated by cAMP (Epac) regulates small GTPases, binding to transcription factors such as the cAMP receptor protein (CRP) or the virulence factor regulator (Vfr) modifies the rate of transcription of certain genes, while cAMP binding to ion channels modulates their activity directly. Further studies on cAMP signalling will have important implications, not only for advancing fundamental knowledge but also for identifying targets for the development of new therapeutic agents.

Gα(h)/transglutaminase-2 activity is required for maximal activation of adenylylcyclase 8 in human and rat glioma cells

Gα(h) (or transglutaminase-2 (TG2)) is an atypical guanine nucleotide binding-protein that associates with G protein-coupled receptors. TG2 also exerts transglutaminase activity that catalyzes posttranslational protein cross-linking with the formation of ε-(γ-glutamyl) lysine or (γ-glutamyl) polyamine bonds. Here, the role of Gα(h)/TG2 in signal transduction in glial cells was examined in detail. In 1321N1 human astrocytoma cells that lack Gα(h)/TG2, overexpression of Gα(h)/TG2 caused an enhancement of cAMP accumulation stimulated with the β-adrenergic receptor agonist, isoproterenol, or the adenylylcyclase activator, forskolin. This cAMP-enhancement was reversed by the TG2 inhibitor, ERW1069. In rat C6 glioma cells that express endogenous Gα(h)/TG2, cAMP accumulation induced by isoproterenol or forskolin was significantly inhibited by overexpression of Gα(h)/TG2-C277V, a dominant-negative mutant that lacks transglutaminase activity, but was not inhibited by the Gα(h)/TG2-S171E mutant that cannot bind GTP/GDP. These results suggest Gα(h)/TG2 potentiates adenylylcyclase activity by its transglutaminase activity and not by its G-protein activity. Gα(h)/TG2 also increased the activities of the cAMP response element and interleukin-6 promoter, accompanied by an of cAMP in both glioma cells. Since adenylylcyclase 8 plays a major role in cAMP production, we focused on post-translational modification of adenylylcyclase 8 by Gα(h)/TG2. Adenylylcyclase 8 is expressed in both 1321N1 and C6 cells; however, Gα(h)/TG2 affected neither adenylylcyclase 8 expression levels, glycosylation, nor dimerization status. In contrast, pentylamine, a substrate of Gα(h)/TG2, was incorporated into adenylylcyclase 8 in a transglutaminase activity-dependent manner. Taking these results together, Gα(h)/TG2 promotes cAMP production accompanied by a modification of adenylylcyclase 8 in glioma cells.

Mechanisms of apoptosis by the tumor suppressor Par-4

Par-4 is a pro-apoptotic, tumor suppressor protein that induces apoptosis selectively in cancer cells. Endoplasmic reticulum-stress and higher levels of protein kinase A in tumor cells confer the coveted feature of cancer selective response to extracellular and intracellular Par-4, respectively. Recent studies have shown that systemic Par-4 confers resistance to tumor growth in mice, and that tumor-resistance is transferable by bone-marrow transplantation. Moreover, recombinant Par-4 inhibits the growth of tumors in mice. As systemic Par-4 induces apoptosis via cell surface GRP78, strategies that promote GRP78 trafficking to the cell surface are expected sensitize cancer cells to circulating levels of Par-4. This review illustrates the domains and mechanisms by which Par-4 orchestrates the apoptotic process in both cell culture models and in physiological settings.

The role of the adenosinergic pathway in immunosuppression mediated by human regulatory T cells (Treg)

Tumor-induced dysfunction of immune cells is a common problem in cancer. Tumors induce immune suppression by many different mechanisms, including accumulation of regulatory T cells (Treg). Adaptive Treg (Tr1) generated in the tumor microenvironment express CD39 and CD73 ectonucleotidases, produce adenosine and are COX2+PGE2+. Adenosine and PGE2 produced by Tr1 or tumor cells bind to their respective receptors on the surface of T effector cells (Teff) and cooperate in up-regulating cytosolic 3'5'-cAMP levels utilizing adenylyl cyclase isoform 7 (AC-7). In Teff, increased cAMP mediates suppression of anti-tumor functions. Treg, in contrast to Teff, seem to require high cAMP levels for mediating suppression. This differential requirement of Treg and Teff for cAMP offers an opportunity for pharmacologic interventions using selected inhibitors of the adenosine/PGE2 pathways. Blocking of adenosine/PGE2 production by Tr1 or blocking binding of these factors to their receptors on T cells or inhibition of cAMP synthesis in Teff all represent novel therapeutic strategies that used in combination with conventional therapies could restore anti-tumor functions of Teff . At the same time, these inhibitors could disarm Tr1 cells by depriving them of the factors promoting their generation and activity or by down-regulating 3'5'-cAMP levels. Thus, the pharmacologic control of Treg-Teff interactions offers a novel strategy for restoration of anti-tumor Teff functions and silencing of Treg. Used in conjunction with anti-cancer drugs or with immune therapies, this strategy has a potential to improve therapeutic effects by preventing or reversing tumor-induced immune suppression.

Total synthesis of the potent cAMP signaling agonist (-)-alotaketal A

We have developed a convergent synthetic route to the potent cAMP signaling agonist (-)-alotaketal A that employs two stages of SmI(2)-mediated reductive allylation reactions for assembling the polycycle and fragment coupling. Also notable are a Hg(OAc)(2)-mediated selective alkene oxidation and the subtlety of the formation of the unprecedented spiroketal ring system. The probes AKAR4 and ICUE3 were used to evaluate the cAMP singaling agonistic activity of (-)-alotaketal A and elucidate its structure-activity relationship.

Targeting neuronal adenylyl cyclase for the treatment of chronic pain

Pain research is currently undergoing dramatic changes. In the area of basic pain research, new discoveries have been made towards the understanding of pain transmission, modulation and plasticity. However, many of these basic discoveries have not yet led to the development of new drugs for the treatment of chronic pain. One major reason for this disconnection is the lack of translational research and drug discovery based directly on the novel pain mechanism. In this review, I focus on activity-dependent potentiation in pain-related cortical areas and recent translational research on adenylyl cyclase subtype 1 (AC1) as a novel target for treating chronic pain. In particular, I discuss the AC1 inhibitor, NB001, which produces powerful analgesic effects in animal models of chronic pain by inhibiting chronic pain-related cortical potentiation.

Signal transduction in Trypanosoma cruzi

Signal transduction plays a key role in regulating important functions in both multicellular and unicellular organisms and largely controls the manner in which cells respond to stimuli. Signal transduction pathways coordinate the functions in different type of cells in animals and control the growth and differentiation in unicellular organisms. Intracellular signal transduction pathways are largely activated by second messenger molecules. Trypanosoma cruzi has a complex life cycle involving four morphogenetic stages with various second messenger systems able to regulate its growth and differentiation. Signal transduction often alters the status of phosphorylation in target proteins and thus alters the activities of these proteins. In this review, two major signal transduction pathways, cyclic AMP-dependent pathway and mitogen-activated protein kinase pathway, are discussed. Protein phosphatases are also discussed due to their importance in dephosphorylating target proteins and terminating signal transduction. Understanding of the unique pathways in this pathogen may lead to the development of novel therapeutic agents.

Altered axonal targeting and short-term plasticity in the hippocampus of Disc1 mutant mice

Carefully designed animal models of genetic risk factors are likely to aid our understanding of the pathogenesis of schizophrenia. Here, we study a mouse strain with a truncating lesion in the endogenous Disc1 ortholog designed to model the effects of a schizophrenia-predisposing mutation and offer a detailed account of the consequences that this mutation has on the development and function of a hippocampal circuit. We uncover widespread and cumulative cytoarchitectural alterations in the dentate gyrus during neonatal and adult neurogenesis, which include errors in axonal targeting and are accompanied by changes in short-term plasticity at the mossy fiber/CA3 circuit. We also provide evidence that cAMP levels are elevated as a result of the Disc1 mutation, leading to altered axonal targeting and dendritic growth. The identified structural alterations are, for the most part, not consistent with the growth-promoting and premature maturation effects inferred from previous RNAi-based Disc1 knockdown. Our results provide support to the notion that modest disturbances of neuronal connectivity and accompanying deficits in short-term synaptic dynamics is a general feature of schizophrenia-predisposing mutations.

Characterization of a hyperactive Cyr1 mutant reveals new regulatory mechanisms for cellular cAMP levels in Candida albicans

The adenylyl cyclase Cyr1 plays a pivotal role in regulating virulence traits in the human fungal pathogen Candida albicans. Although a diverse range of signals are known to activate Cyr1, it remains unclear how low activity is maintained in the absence of stimuli. To uncover negative regulatory elements, we designed a genetic screen to identify mutations in Cyr1 that increase its catalytic activity. We found such a mutant carrying a single Glu1541 to Lys substitution in a conserved motif C-terminal to the catalytic domain. This E1541K mutation caused constitutive filamentous growth, hypersensitivity to stress, resistance to farnesol and overproduction of riboflavin. The mutant phenotype depends on Cap1 and Ras1, two known positive regulators of Cyr1, and the filamentous growth requires Hgc1, a key promoter of hyphal growth. Strikingly, expressing a truncated version of the mutant protein lacking the entire region N-terminal to the catalytic domain in cyr1Δ cells caused a fivefold increase in the cellular cAMP level. Such cells exhibited dramatic enlargement, cytokinetic defects, G1 arrest and impaired hyphal development. Thus, our studies have revealed novel regulatory elements in Cyr1 that normally repress Cyr1 activity to prevent the toxicity of unregulated high cAMP levels.

Targeting human inducible regulatory T cells (Tr1) in patients with cancer: blocking of adenosine-prostaglandin E₂ cooperation

INTRODUCTION

Emerging data suggest that human inducible regulatory T cells (Tr1) produce adenosine and prostaglandin E(2) and that these factors cooperate in mediating immune suppression.

AREAS COVERED

Human Tr1 present in human tumors or blood of cancer patients express ectonucleotidases, CD39 and/or CD73, hydrolyze ATP to adenosine and are COX-2 positive. Expression of CD39 and/or CD73 on human tumors favors expansion and suppressor functions of Tr1. Adenosine and PGE(2) signal via adenosine 2A receptor (A(2A)R) and prostaglandin E(2) receptor 2 (EP(2)R) expressed on effector T (Teff) cells, suppressing their anti-tumor functions by a common mechanism involving upregulation of cytosolic cAMP levels and protein kinase A (PKA) type I activation. The frequency and activity of circulating CD4(+)CD39(+) and CD4(+)COX-2(+) Treg subsets increase in advanced disease and also following oncologic therapies.

EXPERT OPINION

Pharmacologic blocking of adenosine-PGE(2) collaboration provides a clinically-feasible strategy for disarming of Treg. Used in conjunction with conventional anti-cancer drugs or immune interventions, pharmacologic inhibitors could improve outcome of oncologic therapies.

LRP6 mediates cAMP generation by G protein-coupled receptors through regulating the membrane targeting of Gα(s)

Ligand binding to certain heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) stimulates the rapid synthesis of cyclic adenosine monophosphate (cAMP) through the G protein α(s) subunit, which activates adenylyl cyclase (AC). We found that the transmembrane receptor low-density lipoprotein receptor-related protein 6 (LRP6), a co-receptor for Wnt proteins, bound to the Gα(s)βγ heterotrimer and that knockdown of LRP6 attenuated cAMP production by various GPCRs, including parathyroid hormone receptor 1 (PTH1R). Knockdown of LRP6 disrupted the localization of Gα(s) to the plasma membrane, which led to a decrease in the extent of coupling of Gα(s) to PTH1R and inhibited the production of cAMP and the activation of cAMP-dependent protein kinase (PKA) in response to PTH. PKA phosphorylated LRP6, which enhanced the binding of Gα(s) to LRP6, its localization to the plasma membrane, and the production of cAMP in response to PTH. Decreased PTH-dependent cAMP production was observed in single cells in which LRP6 was knocked down or mutated at the PKA site by monitoring the cAMP kinetics. Thus, we suggest that the binding of Gα(s) to LRP6 is required to establish a functional GPCR-Gα(s)-AC signaling pathway for the production of cAMP, providing an additional regulatory component to the current GPCR-cAMP paradigm.

Fluid transport and cystogenesis in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. The development and enlargement of cysts in ADPKD requires tubular cell proliferation, abnormalities in the extracellular matrix and transepithelial fluid secretion. Multiple studies have suggested that fluid secretion across ADPKD cyst-lining cells is driven by the transepithelial secretion of chloride, mediated by the apical CFTR channel and specific basolateral transporters. The whole secretory process is stimulated by increased levels of cAMP in the cells, probably reflecting modifications in the intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor. This review will focus on the pathophysiology of fluid secretion in ADPKD cysts, starting with classic, morphological and physiological studies that were followed by investigations of the molecular mechanisms involved and therapeutic trials targeting these pathways in cellular and animal models and ADPKD patients. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Lipoic acid stimulates cAMP production via G protein-coupled receptor-dependent and -independent mechanisms

Lipoic acid (LA) is a naturally occurring fatty acid that exhibits anti-oxidant and anti-inflammatory properties and is being pursued as a therapeutic for many diseases including multiple sclerosis, diabetic polyneuropathy and Alzheimer's disease. We previously reported on the novel finding that racemic LA (50:50 mixture of R-LA and S-LA) stimulates cAMP production, activates prostanoid EP2 and EP4 receptors and adenylyl cyclases (AC), and suppresses activation and cytotoxicity in NK cells. In this study, we present evidence that furthers our understanding of the mechanisms of action of LA. Using various LA derivatives, such as dihydrolipoic acid (DHLA), S,S-dimethyl lipoic acid (DMLA) and lipoamide (LPM), we discovered that only LA is capable of stimulating cAMP production in NK cells. Furthermore, there is no difference in cAMP production after stimulation with either R-LA, S-LA or racemic LA. Competition and synergistic studies indicate that LA may also activate AC independent of the EP2 and EP4 receptors. Pretreatment of PBMCs with KH7 (a specific peptide inhibitor of soluble AC) and the calcium inhibitor (Bapta) prior to LA treatment resulted in reduced cAMP levels, suggesting that soluble AC and calcium signaling mediate LA stimulation of cAMP production. In addition, pharmacological inhibitor studies demonstrate that LA also activates other G protein-coupled receptors, including histamine and adenosine but not the β-adrenergic receptors. These novel findings provide information to better understand the mechanisms of action of LA, which can help facilitate the use of LA as a therapeutic for various diseases.

Lipoic acid attenuates inflammation via cAMP and protein kinase A signaling

Background

Abnormal regulation of the inflammatory response is an important component of diseases such as diabetes, Alzheimer's disease and multiple sclerosis (MS). Lipoic acid (LA) has been shown to have antioxidant and anti-inflammatory properties and is being pursued as a therapy for these diseases. We first reported that LA stimulates cAMP production via activation of G-protein coupled receptors and adenylyl cyclases. LA also suppressed NK cell activation and cytotoxicity. In this study we present evidence supporting the hypothesis that the anti-inflammatory properties of LA are mediated by the cAMP/PKA signaling cascade. Additionally, we show that LA oral administration elevates cAMP levels in MS subjects.

Methodology/Principal Findings

We determined the effects of LA on IL-6, IL-17 and IL-10 secretion using ELISAs. Treatment with 50 µg/ml and 100 µg/ml LA significantly reduced IL-6 levels by 19 and 34%, respectively, in T cell enriched PBMCs. IL-17 levels were also reduced by 35 and 50%, respectively. Though not significant, LA appeared to have a biphasic effect on IL-10 production. Thymidine incorporation studies showed LA inhibited T cell proliferation by 90%. T-cell activation was reduced by 50% as measured by IL-2 secretion. Western blot analysis showed that LA treatment increased phosphorylation of Lck, a downstream effector of protein kinase A. Pretreatment with a peptide inhibitor of PKA, PKI, blocked LA inhibition of IL-2 and IFN gamma production, indicating that PKA mediates these responses. Oral administration of 1200 mg LA to MS subjects resulted in increased cAMP levels in PBMCs four hours after ingestion. Average cAMP levels in 20 subjects were 43% higher than baseline.

Conclusions/Significance

Oral administration of LA in vivo resulted in significant increases in cAMP concentration. The anti-inflammatory effects of LA are mediated in part by the cAMP/PKA signaling cascade. These novel findings enhance our understanding of the mechanisms of action of LA.

cAMP-dependent protein kinase from Plasmodium falciparum: an update

One of the most important public health problems in the world today is the emergence and dissemination of drug-resistant malaria parasites. Plasmodium falciparum is the causative agent of the most lethal form of human malaria. New anti-malarial strategies are urgently required, and their design and development require the identification of potential therapeutic targets. However, the molecular mechanisms controlling the life cycle of the malaria parasite are still poorly understood. The published genome sequence of P. falciparum and previous studies have revealed that several homologues of eukaryotic signalling proteins, such as protein kinases, are relatively conserved. Protein kinases are now widely recognized as important drug targets in protozoan parasites. Cyclic AMP-dependent protein kinase (PKA) is implicated in numerous processes in mammalian cells, and the regulatory mechanisms of the cAMP pathway have been characterized. P. falciparum cAMP-dependent protein kinase plays an important role in the parasite's life cycle and thus represents an attractive target for the development of anti-malarial drugs. In this review, we focus on the P. falciparum cAMP/PKA pathway to provide new insights and an improved understanding of this signalling cascade.