cAMP signaling in subcellular compartments

PDE2A2 regulates mitochondria morphology and apoptotic cell death via local modulation of cAMP/PKA signalling

cAMP/PKA signalling is compartmentalised with tight spatial and temporal control of signal propagation underpinning specificity of response. The cAMP-degrading enzymes, phosphodiesterases (PDEs), localise to specific subcellular domains within which they control local cAMP levels and are key regulators of signal compartmentalisation. Several components of the cAMP/PKA cascade are located to different mitochondrial sub-compartments, suggesting the presence of multiple cAMP/PKA signalling domains within the organelle. The function and regulation of these domains remain largely unknown. Here, we describe a novel cAMP/PKA signalling domain localised at mitochondrial membranes and regulated by PDE2A2. Using pharmacological and genetic approaches combined with real-time FRET imaging and high resolution microscopy, we demonstrate that in rat cardiac myocytes and other cell types mitochondrial PDE2A2 regulates local cAMP levels and PKA-dependent phosphorylation of Drp1. We further demonstrate that inhibition of PDE2A, by enhancing the hormone-dependent cAMP response locally, affects mitochondria dynamics and protects from apoptotic cell death.

Dynamic monitoring of Gi/o-protein-mediated decreases of intracellular cAMP by FRET-based Epac sensors

Analysis of G-protein-coupled receptor (GPCR) signaling, in particular of the second messenger cAMP that is tightly controlled by G_s- and G_i/o-proteins, is a central issue in biomedical research. The classical biochemical method to monitor increases in intracellular cAMP concentrations consists of a radioactive multicellular assay, which is well established, highly sensitive, and reproducible, but precludes continuous spatial and temporal assessment of cAMP levels in single living cells. For this purpose, Förster resonance energy transfer (FRET)-based Epac cAMP sensors are well suitable. So far, the latter sensors have been employed to monitor G_s-induced cAMP increases and it has remained elusive whether Epac sensors can reliably detect decreased intracellular cAMP levels as well. In this study, we systematically optimize experimental strategies employing FRET-based cAMP sensors to monitor G_i/o-mediated cAMP reductions. FRET experiments with adrenergic α_2A or μ opioid receptors and a set of different Epac sensors allowed for time-resolved, valid, and reliable detection of cAMP level decreases upon G_i/o-coupled receptor activation in single living cells, and this effect can be reversed by selective receptor antagonists. Moreover, pre-treatment with forskolin or 3-isobutyl-1-methylxanthine (IBMX) to artificially increase basal cAMP levels was not required to monitor G_i/o-coupled receptor activation. Thus, using FRET-based cAMP sensors is of major advantage when compared to classical biochemical and multi-cellular assays.

Multifunctional Mitochondrial Epac1 Controls Myocardial Cell Death

Rationale:

Although the second messenger cyclic AMP (cAMP) is physiologically beneficial in the heart, it largely contributes to cardiac disease progression when dysregulated. Current evidence suggests that cAMP is produced within mitochondria. However, mitochondrial cAMP signaling and its involvement in cardiac pathophysiology are far from being understood.

Objective:

To investigate the role of MitEpac1 (mitochondrial exchange protein directly activated by cAMP 1) in ischemia/reperfusion injury.

Methods and Results:

We show that Epac1 (exchange protein directly activated by cAMP 1) genetic ablation ( Epac1 −/− ) protects against experimental myocardial ischemia/reperfusion injury with reduced infarct size and cardiomyocyte apoptosis. As observed in vivo, Epac1 inhibition prevents hypoxia/reoxygenation–induced adult cardiomyocyte apoptosis. Interestingly, a deleted form of Epac1 in its mitochondrial-targeting sequence protects against hypoxia/reoxygenation–induced cell death. Mechanistically, Epac1 favors Ca 2+ exchange between the endoplasmic reticulum and the mitochondrion, by increasing interaction with a macromolecular complex composed of the VDAC1 (voltage-dependent anion channel 1), the GRP75 (chaperone glucose-regulated protein 75), and the IP3R1 (inositol-1,4,5-triphosphate receptor 1), leading to mitochondrial Ca 2+ overload and opening of the mitochondrial permeability transition pore. In addition, our findings demonstrate that MitEpac1 inhibits isocitrate dehydrogenase 2 via the mitochondrial recruitment of CaMKII (Ca 2+ /calmodulin-dependent protein kinase II), which decreases nicotinamide adenine dinucleotide phosphate hydrogen synthesis, thereby, reducing the antioxidant capabilities of the cardiomyocyte.

Conclusions:

Our results reveal the existence, within mitochondria, of different cAMP–Epac1 microdomains that control myocardial cell death. In addition, our findings suggest Epac1 as a promising target for the treatment of ischemia-induced myocardial damage.

Cyclic nucleotide imaging and cardiovascular disease

The universal second messengers cyclic nucleotides 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) play central roles in cardiovascular function and disease. They act in discrete, functionally relevant subcellular microdomains which regulate, for example, calcium cycling and excitation-contraction coupling. Such localized cAMP and cGMP signals have been difficult to measure using conventional biochemical techniques. Recent years have witnessed the advent of live cell imaging techniques which allow visualization of these functionally relevant second messengers with unprecedented spatial and temporal resolution at cellular, subcellular and tissue levels. In this review, we discuss these new imaging techniques and give examples how they are used to visualize cAMP and cGMP in physiological and pathological settings to better understand cardiovascular function and disease. Two primary techniques include the use of Förster resonance energy transfer (FRET) based cyclic nucleotide biosensors and nanoscale scanning ion conductance microscopy (SICM). These methods can provide deep mechanistic insights into compartmentalized cAMP and cGMP signaling.

Bicarbonate-sensing soluble adenylyl cyclase is present in the cell cytoplasm and nucleus of multiple shark tissues

The enzyme soluble adenylyl cyclase (sAC) is directly stimulated by bicarbonate (HCO₃⁻) to produce the signaling molecule cyclic adenosine monophosphate (cAMP). Because sAC and sAC‐related enzymes are found throughout phyla from cyanobacteria to mammals and they regulate cell physiology in response to internal and external changes in pH, CO₂, and HCO₃⁻, sAC is deemed an evolutionarily conserved acid‐base sensor. Previously, sAC has been reported in dogfish shark and round ray gill cells, where they sense and counteract blood alkalosis by regulating the activity of V‐type H⁺‐ ATPase. Here, we report the presence of sAC protein in gill, rectal gland, cornea, intestine, white muscle, and heart of leopard shark Triakis semifasciata. Co‐expression of sAC with transmembrane adenylyl cyclases supports the presence of cAMP signaling microdomains. Furthermore, immunohistochemistry on tissue sections, and western blots and cAMP‐activity assays on nucleus‐enriched fractions demonstrate the presence of sAC protein in and around nuclei. These results suggest that sAC modulates multiple physiological processes in shark cells, including nuclear functions.

G Protein Alpha S Subunit Promotes Cell Proliferation of Renal Cell Carcinoma with Involvement of Protein Kinase A Signaling

Heterotrimeric G proteins, which are composed of Gα and Gβγ subunits, transduce signals sensed by the coupled surface receptors. Aberrant expressions of G proteins have been observed in many cancer types. This study aimed to determine the expression level of the stimulatory G protein alpha S subunit (Gαs, the main transcript encoded by the GNAS locus) and its biological function in renal cell carcinoma (RCC). Western blotting and quantitative reverse transcription-PCR results show that Gαs expression dramatically increased in RCC cell lines (ACHN, GRC-1, and 786-O) compared to normal renal epithelial cells HK-2. Knockdown of Gαs by small interfering RNA (siRNA) caused a significant inhibition on proliferation of ACHN cells as indicated by MTT assay and colony formation assay. Overexpression of Gαs in HK-2 cells promoted cell proliferation and led to a higher level of intracellular cyclic adenosine monophosphate (cAMP) in response to parathyroid hormone (PTH) compared to the cells transfected with empty vector. Notably, the growth of HK-2 cells overexpressing Gαs was efficiently inhibited in the presence of protein kinase A (PKA) inhibitor H89. Furthermore, in a xenograft model by subcutaneous injection of ACHN cells, tumor growth was also suppressed by H89. Taken together, these results suggest that Gαs plays a tumor-promoting role in RCC and possibly acts through a PKA-dependent pathway. Our findings may provide new clues for target therapy for RCC in the future.

Systematic investigation of protein kinase A substrate proteins using on-chip protein kinase kinetic profiling

An on-chip protein kinase assay for profiling kinase kinetic parameters by introducing the substrate affinity (K_m) and the phosphorylation rate (V_p) under physiological conditions.

The Natural cAMP Elevating Compound Forskolin in Cancer Therapy: Is It Time?

Cancer is a major public health problem and the second leading cause of mortality around the world. Although continuous advances in the science of oncology and cancer research are now leading to improved outcomes for many cancer patients, novel cancer treatment options are strongly demanded. Naturally occurring compounds from a variety of vegetables, fruits, and medicinal plants have been shown to exhibit various anticancer properties in a number of in vitro and in vivo studies and represent an attractive research area for the development of new therapeutic strategies to fight cancer. Forskolin is a diterpene produced by the roots of the Indian plant Coleus forskohlii. The natural compound forskolin has been used for centuries in traditional medicine and its safety has also been documented in conventional modern medicine. Forskolin directly activates the adenylate cyclase enzyme, that generates cAMP from ATP, thus, raising intracellular cAMP levels. Notably, cAMP signaling, through the PKA-dependent and/or -independent pathways, is very relevant to cancer and its targeting has shown a number of antitumor effects, including the induction of mesenchymal-to-epithelial transition, inhibition of cell growth and migration and enhancement of sensitivity to conventional antitumor drugs in cancer cells. Here, we describe some features of cAMP signaling that are relevant to cancer biology and address the state of the art concerning the natural cAMP elevating compound forskolin and its perspectives as an effective anticancer agent. J. Cell. Physiol. 232: 922-927, 2017. © 2016 Wiley Periodicals, Inc.

Discovery of LRE1 as a specific and allosteric inhibitor of soluble adenylyl cyclase

The prototypical second messenger cAMP regulates a wide variety of physiological processes. It can simultaneously mediate diverse functions by acting locally in independently regulated microdomains. In mammalian cells, two types of adenylyl cyclase generate cAMP: G-protein-regulated transmembrane adenylyl cyclases and bicarbonate-, calcium- and ATP-regulated soluble adenylyl cyclase (sAC). Because each type of cyclase regulates distinct microdomains, methods to distinguish between them are needed to understand cAMP signaling. We developed a mass-spectrometry-based adenylyl cyclase assay, which we used to identify a new sAC-specific inhibitor, LRE1. LRE1 bound to the bicarbonate activator binding site and inhibited sAC via a unique allosteric mechanism. LRE1 prevented sAC-dependent processes in cellular and physiological systems, and it will facilitate exploration of the therapeutic potential of sAC inhibition.

Multiple isoforms for the catalytic subunit of PKA in the basal fungal lineage Mucor circinelloides

Protein kinase A (PKA) activity is involved in dimorphism of the basal fungal lineage Mucor. From the recently sequenced genome of Mucor circinelloides we could predict ten catalytic subunits of PKA. From sequence alignment and structural prediction we conclude that the catalytic core of the isoforms is conserved, and the difference between them resides in their amino termini. This high number of isoforms is maintained in the subdivision Mucoromycotina. Each paralogue, when compared to the ones form other fungi is more homologous to one of its orthologs than to its paralogs. All of these fungal isoforms cannot be included in the class I or II in which fungal protein kinases have been classified. mRNA levels for each isoform were measured during aerobic and anaerobic growth. The expression of each isoform is differential and associated to a particular growth stage. We reanalyzed the sequence of PKAC (GI 20218944), the only cloned sequence available until now for a catalytic subunit of M. circinelloides. PKAC cannot be classified as a PKA because of its difference in the conserved C-tail; it shares with PKB a conserved C2 domain in the N-terminus. No catalytic activity could be measured for this protein nor predicted bioinformatically. It can thus be classified as a pseudokinase. Its importance can not be underestimated since it is expressed at the mRNA level in different stages of growth, and its deletion is lethal.

The ATPase Inhibitory Factor 1 (IF1): A master regulator of energy metabolism and of cell survival

In this contribution we summarize most of the findings reported for the molecular and cellular biology of the physiological inhibitor of the mitochondrial H(+)-ATP synthase, the engine of oxidative phosphorylation (OXPHOS) and gate of cell death. We first describe the structure and major mechanisms and molecules that regulate the activity of the ATP synthase placing the ATPase Inhibitory Factor 1 (IF1) as a major determinant in the regulation of the activity of the ATP synthase and hence of OXPHOS. Next, we summarize the post-transcriptional mechanisms that regulate the expression of IF1 and emphasize, in addition to the regulation afforded by the protonation state of histidine residues, that the activity of IF1 as an inhibitor of the ATP synthase is also regulated by phosphorylation of a serine residue. Phosphorylation of S39 in IF1 by the action of a mitochondrial cAMP-dependent protein kinase A hampers its interaction with the ATP synthase, i.e., only dephosphorylated IF1 interacts with the enzyme. Upon IF1 interaction with the ATP synthase both the synthetic and hydrolytic activities of the engine of OXPHOS are inhibited. These findings are further placed into the physiological context to stress the emerging roles played by IF1 in metabolic reprogramming in cancer, in hypoxia and in cellular differentiation. We review also the implication of IF1 in other cellular situations that involve the malfunctioning of mitochondria. Special emphasis is given to the role of IF1 as driver of the generation of a reactive oxygen species signal that, emanating from mitochondria, is able to reprogram the nucleus of the cell to confer by various signaling pathways a cell-death resistant phenotype against oxidative stress. Overall, our intention is to highlight the urgent need of further investigations in the molecular and cellular biology of IF1 and of its target, the ATP synthase, to unveil new therapeutic strategies in human pathology. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Myocardial Response to Milrinone in Single Right Ventricle Heart Disease

OBJECTIVES

Empiric treatment with milrinone, a phosphodiesterase (PDE) 3 inhibitor, has become increasingly common in patients with single ventricle heart disease of right ventricular (RV) morphology (SRV); our objective was to characterize the myocardial response to PDE3 inhibition (PDE3i) in the pediatric population with SRV.

STUDY DESIGN

Cyclic adenosine monophosphate levels, PDE activity, and phosphorylated phospholamban (PLN) were determined in explanted human ventricular myocardium from nonfailing pediatric donors (n = 10) and pediatric patients transplanted secondary to SRV. Subjects with SRV were further classified by PDE3i treatment (n = 13 with PDE3i and n = 12 without PDE3i).

RESULTS

In comparison with nonfailing RV myocardium (n = 8), cyclic adenosine monophosphate levels are lower in patients with SRV treated with PDE3i (n = 12, P = .021). Chronic PDE3i does not alter total PDE or PDE3 activity in SRV myocardium. Compared with nonfailing RV myocardium, SRV myocardium (both with and without PDE3i) demonstrates equivalent phosphorylated PLN at the protein kinase A phosphorylation site.

CONCLUSIONS

As evidenced by preserved phosphorylated PLN, the molecular adaptation associated with SRV differs significantly from that demonstrated in pediatric heart failure because of dilated cardiomyopathy. These alterations support a pathophysiologically distinct mechanism of heart failure in pediatric patients with SRV, which has direct implications regarding the presumed response to PDE3i treatment in this population.

The Popeye Domain Containing Genes and their Function in Striated Muscle

The Popeye domain containing (POPDC) genes encode a novel class of cAMP effector proteins, which are abundantly expressed in heart and skeletal muscle. Here, we will review their role in striated muscle as deduced from work in cell and animal models and the recent analysis of patients carrying a missense mutation in POPDC1. Evidence suggests that POPDC proteins control membrane trafficking of interacting proteins. Furthermore, we will discuss the current catalogue of established protein-protein interactions. In recent years, the number of POPDC-interacting proteins has been rising and currently includes ion channels (TREK-1), sarcolemma-associated proteins serving functions in mechanical stability (dystrophin), compartmentalization (caveolin 3), scaffolding (ZO-1), trafficking (NDRG4, VAMP2/3) and repair (dysferlin) or acting as a guanine nucleotide exchange factor for Rho-family GTPases (GEFT). Recent evidence suggests that POPDC proteins might also control the cellular level of the nuclear proto-oncoprotein c-Myc. These data suggest that this family of cAMP-binding proteins probably serves multiple roles in striated muscle.

Intracellular tortuosity underlies slow cAMP diffusion in adult ventricular myocytes

Aims

3′,5′-Cyclic adenosine monophosphate (cAMP) signals in the heart are often confined to concentration microdomains shaped by cAMP diffusion and enzymatic degradation. While the importance of phosphodiesterases (degradative enzymes) in sculpting cAMP microdomains is well established in cardiomyocytes, less is known about cAMP diffusivity (D_cAMP) and factors affecting it. Many earlier studies have reported fast diffusivity, which argues against sharply defined microdomains.

Methods and results

[cAMP] dynamics in the cytoplasm of adult rat ventricular myocytes were imaged using a fourth generation genetically encoded FRET-based sensor. The [cAMP]-response to the addition and removal of isoproterenol (β-adrenoceptor agonist) quantified the rates of cAMP synthesis and degradation. To obtain a read out of D_cAMP, a stable [cAMP] gradient was generated using a microfluidic device which delivered agonist to one half of the myocyte only. After accounting for phosphodiesterase activity, D_cAMP was calculated to be 32 µm²/s; an order of magnitude lower than in water. Diffusivity was independent of the amount of cAMP produced. Saturating cAMP-binding sites with the analogue 6-Bnz-cAMP did not accelerate D_cAMP, arguing against a role of buffering in restricting cAMP mobility. cAMP diffused at a comparable rate to chemically unrelated but similar sized molecules, arguing for a common physical cause of restricted diffusivity. Lower mitochondrial density and order in neonatal cardiac myocytes allowed for faster diffusion, demonstrating the importance of mitochondria as physical barriers to cAMP mobility.

Conclusion

In adult cardiac myocytes, tortuosity due to physical barriers, notably mitochondria, restricts cAMP diffusion to levels that are more compatible with microdomain signalling.

Signalling at membrane contact sites: two membranes come together to handle second messengers

It is now clear that many intracellular signals result from multiple membrane-bound compartments acting in concert. Membrane contact sites, regions of close apposition between organelles, have emerged as major points of convergence during signalling, as these are places where material is exchanged. The material exchanged can be either water-insoluble molecules such as membrane lipids that are passed directly between organelles, or ions such as Ca(2+). Here we highlight new insights into the role of contacts in signalling by second messengers, including lipid traffic that underpins re-generation of IP3, the regulation of NAADP and store-operated Ca(2+) signals, and possible involvement in cyclic AMP signalling.

Isoproterenol increases histone deacetylase 6 expression and cell migration by inhibiting ERK signaling via PKA and Epac pathways in human lung cancer cells

Stress conditions are correlated with tumor growth, progression and metastasis. We hypothesized that stress signals might affect tumor progression via epigenetic control of gene expression and investigated the effects of stress signals on the expression levels of histone deacetylases (HDACs) and the underlying mechanisms of these effects in lung cancer cells. Treatment with isoproterenol (ISO), an analog of the stress signal epinephrine, increased the expression of HDAC6 protein and mRNA in H1299 lung cancer cells. ISO caused the deacetylation of α-tubulin and stimulated cell migration in an HDAC6-dependent manner. HDAC6 expression was increased by treatment with selective activators of cAMP-dependent protein kinase (PKA) or exchange protein activated by cAMP (Epac). ISO activated Rap1 via Epac, and constitutively active Rap1A increased the HDAC6 level; however, the knockdown of Rap1A decreased the 8-(4-cholorophenylthio)-2′-O-methyl-cAMP-induced increase in HDAC6 expression. Both PKA and Rap1A decreased c-Raf activation to inhibit extracellular signal-regulated kinase (ERK) signaling. Inhibition of ERK caused an increase in HDAC6 expression, and constitutively active MEK1 decreased the ISO-induced HDAC6 expression. We concluded that ISO increases HDAC6 expression via a PKA/Epac/ERK-dependent pathway that stimulates the migration of lung cancer cells. This study suggests that stress signals can stimulate the migration of cancer cells by inducing HDAC6 expression in lung cancer cells.

Simultaneous assessment of cAMP signaling events in different cellular compartments using FRET-based reporters

Several aspects of the cAMP signaling cascade, including the levels of the messenger itself and the activity of its main effector protein kinase A (PKA), can be measured in living cells, thanks to genetically encoded probes based on fluorescence resonance energy transfer (FRET). While these biosensors enable the assessment of cAMP or PKA activity with great spatial and temporal resolution, concomitant events triggered by the same stimuli at the same or other cellular compartments are not easily assessed. In this chapter we present a simple approach that allows the simultaneous measurement of cAMP and its actions in subcellular compartments of neighboring cells. As proof of principle, we compare cAMP signals and PKA activity in the cytosol of neighboring HEK cells. We propose that this flexible and powerful method can significantly improve the direct comparison of cAMP signals and their action in specific cellular domains.

Functional Characterization of PRKAR1A Mutations Reveals a Unique Molecular Mechanism Causing Acrodysostosis but Multiple Mechanisms Causing Carney Complex

The main target of cAMP is PKA, the main regulatory subunit of which (PRKAR1A) presents mutations in two genetic disorders: acrodysostosis and Carney complex. In addition to the initial recurrent mutation (R368X) of the PRKAR1A gene, several missense and nonsense mutations have been observed recently in acrodysostosis with hormonal resistance. These mutations are located in one of the two cAMP-binding domains of the protein, and their functional characterization is presented here. Expression of each of the PRKAR1A mutants results in a reduction of forskolin-induced PKA activation (measured by a reporter assay) and an impaired ability of cAMP to dissociate PRKAR1A from the catalytic PKA subunits by BRET assay. Modeling studies and sensitivity to cAMP analogs specific for domain A (8-piperidinoadenosine 3',5'-cyclic monophosphate) or domain B (8-(6-aminohexyl)aminoadenosine-3',5'-cyclic monophosphate) indicate that the mutations impair cAMP binding locally in the domain containing the mutation. Interestingly, two of these mutations affect amino acids for which alternative amino acid substitutions have been reported to cause the Carney complex phenotype. To decipher the molecular mechanism through which homologous substitutions can produce such strikingly different clinical phenotypes, we studied these mutations using the same approaches. Interestingly, the Carney mutants also demonstrated resistance to cAMP, but they expressed additional functional defects, including accelerated PRKAR1A protein degradation. These data demonstrate that a cAMP binding defect is the common molecular mechanism for resistance of PKA activation in acrodysosotosis and that several distinct mechanisms lead to constitutive PKA activation in Carney complex.

Genetically-encoded tools for cAMP probing and modulation in living systems

Intracellular 3′-5′-cyclic adenosine monophosphate (cAMP) is one of the principal second messengers downstream of a manifold of signal transduction pathways, including the ones triggered by G protein-coupled receptors. Not surprisingly, biochemical assays for cAMP have been instrumental for basic research and drug discovery for decades, providing insights into cellular physiology and guiding pharmaceutical industry. However, despite impressive track record, the majority of conventional biochemical tools for cAMP probing share the same fundamental shortcoming—all the measurements require sample disruption for cAMP liberation. This common bottleneck, together with inherently low spatial resolution of measurements (as cAMP is typically analyzed in lysates of thousands of cells), underpin the ensuing limitations of the conventional cAMP assays: (1) genuine kinetic measurements of cAMP levels over time in a single given sample are unfeasible; (2) inability to obtain precise information on cAMP spatial distribution and transfer at subcellular levels, let alone the attempts to pinpoint dynamic interactions of cAMP and its effectors. At the same time, tremendous progress in synthetic biology over the recent years culminated in drastic refinement of our toolbox, allowing us not only to bypass the limitations of conventional assays, but to put intracellular cAMP life-span under tight control—something, that seemed scarcely attainable before. In this review article we discuss the main classes of modern genetically-encoded tools tailored for cAMP probing and modulation in living systems. We examine the capabilities and weaknesses of these different tools in the context of their operational characteristics and applicability to various experimental set-ups involving living cells, providing the guidance for rational selection of the best tools for particular needs.

The spatio-temporal dynamics of PKA activity profile during mitosis and its correlation to chromosome segregation

The cyclic adenosine monophosphate dependent kinase protein (PKA) controls a variety of cellular processes including cell cycle regulation. Here, we took advantages of genetically encoded FRET-based biosensors, using an AKAR-derived biosensor to characterize PKA activity during mitosis in living HeLa cells using a single-cell approach. We measured PKA activity changes during mitosis. HeLa cells exhibit a substantial increase during mitosis, which ends with telophase. An AKAREV T>A inactive form of the biosensor and H89 inhibitor were used to ascertain for the specificity of the PKA activity measured. On a spatial point of view, high levels of activity near to chromosomal plate during metaphase and anaphase were detected. By using the PKA inhibitor H89, we assessed the role of PKA in the maintenance of a proper division phenotype. While this treatment in our hands did not impaired cell cycle progression in a drastic manner, inhibition of PKA leads to a dramatic increase in chromososme misalignement on the spindle during metaphase that could result in aneuploidies. Our study emphasizes the insights that can be gained with genetically encoded FRET-based biosensors, which enable to overcome the shortcomings of classical methologies and unveil in vivo PKA spatiotemporal profiles in HeLa cells.

Prostaglandin E2 Inhibits NLRP3 Inflammasome Activation through EP4 Receptor and Intracellular Cyclic AMP in Human Macrophages

PGE2 is a potent lipid mediator involved in maintaining homeostasis but also promotion of acute inflammation or immune suppression in chronic inflammation and cancer. Nucleotide-binding domain, leucine-rich repeat-containing protein (NLR)P3 inflammasome plays an important role in host defense. Uncontrolled activation of the NLRP3 inflammasome, owing to mutations in the NLRP3 gene, causes cryopyrin-associated periodic syndromes. In this study, we showed that NLRP3 inflammasome activation is inhibited by PGE2 in human primary monocyte-derived macrophages. This effect was mediated through PGE2 receptor subtype 4 (EP4) and an increase in intracellular cAMP, independently of protein kinase A or exchange protein directly activated by cAMP. A specific agonist of EP4 mimicked, whereas its antagonist or EP4 knockdown reversed, PGE2-mediated NLRP3 inhibition. PGE2 caused an increase in intracellular cAMP. Blockade of adenylate cyclase by its inhibitor reversed PGE2-mediated NLRP3 inhibition. Increase of intracellular cAMP by an activator of adenylate cyclase or an analog of cAMP, or a blockade of cAMP degradation by phosphodiesterase inhibitor decreased NLRP3 activation. Protein kinase A or exchange protein directly activated by cAMP agonists did not mimic, and their antagonists did not reverse, PGE2-mediated NLRP3 inhibition. Additionally, constitutive IL-1β secretion from LPS-primed PBMCs of cryopyrin-associated periodic fever syndromes patients was substantially reduced by high doses of PGE2. Moreover, blocking cytosolic phospholipase A2α by its inhibitor or small interfering RNA or inhibiting cyclooxygenase 2, resulting in inhibition of endogenous PGE2 production, caused an increase in NLRP3 inflammasome activation. Our results suggest that PGE2 might play a role in maintaining homeostasis during the resolution phase of inflammation and might serve as an autocrine and paracrine regulator.

Generation and functions of second messengers microdomains

A compelling example of the mechanisms by which the cells can organize and decipher complex and different functional activities is the convergence of a multitude of stimuli into signalling cascades, involving only few intracellular second messengers. The possibility of restricting these signalling events in distinct microdomains allows a fine and selective tuning of very different tasks. In this review, we will discuss the mechanisms that control the formation and the spatial distribution of Ca(2+) and cAMP microdomains, providing some examples of their functional consequences.

Cyclic AMP signaling reduces sirtuin 6 expression in non-small cell lung cancer cells by promoting ubiquitin-proteasomal degradation via inhibition of the Raf-MEK-ERK (Raf/mitogen-activated extracellular signal-regulated kinase/extracellular signal-regulated kinase) pathway

The cAMP signaling system regulates various cellular functions, including metabolism, gene expression, and death. Sirtuin 6 (SIRT6) removes acetyl groups from histones and regulates genomic stability and cell viability. We hypothesized that cAMP modulates SIRT6 activity to regulate apoptosis. Therefore, we examined the effects of cAMP signaling on SIRT6 expression and radiation-induced apoptosis in lung cancer cells. cAMP signaling in H1299 and A549 human non-small cell lung cancer cells was activated via the expression of constitutively active Gαs plus treatment with prostaglandin E2 (PGE2), isoproterenol, or forskolin. The expression of sirtuins and signaling molecules were analyzed by Western blotting. Activation of cAMP signaling reduced SIRT6 protein expression in lung cancer cells. cAMP signaling increased the ubiquitination of SIRT6 protein and promoted its degradation. Treatment with MG132 and inhibiting PKA with H89 or with a dominant-negative PKA abolished the cAMP-mediated reduction in SIRT6 levels. Treatment with PGE2 inhibited c-Raf activation by increasing inhibitory phosphorylation at Ser-259 in a PKA-dependent manner, thereby inhibiting downstream MEK-ERK signaling. Inhibiting ERK with inhibitors or with dominant-negative ERKs reduced SIRT6 expression, whereas activation of ERK by constitutively active MEK abolished the SIRT6-depleting effects of PGE2. cAMP signaling also augmented radiation-induced apoptosis in lung cancer cells. This effect was abolished by exogenous expression of SIRT6. It is concluded that cAMP signaling reduces SIRT6 expression by promoting its ubiquitin-proteasome-dependent degradation, a process mediated by the PKA-dependent inhibition of the Raf-MEK-ERK pathway. Reduced SIRT6 expression mediates the augmentation of radiation-induced apoptosis by cAMP signaling in lung cancer cells.

Activation of endogenous anti-inflammatory mediator cyclic AMP attenuates acute pyelonephritis in mice induced by uropathogenic Escherichia coli

The pathogenesis of pyelonephritis caused by uropathogenic Escherichia coli (UPEC) is not well understood. Here, we show that besides UPEC virulence, the severity of the host innate immune response and invasion of renal epithelial cells are important pathogenic factors. Activation of endogenous anti-inflammatory mediator cAMP significantly attenuated acute pyelonephritis in mice induced by UPEC. Administration of forskolin (a potent elevator of intracellular cAMP) reduced kidney infection (ie, bacterial load, tissue destruction); this was associated with attenuated local inflammation, as evidenced by the reduction of renal production of proinflammatory mediators, renal infiltration of inflammatory cells, and renal myeloperoxidase activity. In primary cell culture systems, forskolin not only down-regulated UPEC-stimulated production of proinflammatory mediators by renal tubular epithelial cells and inflammatory cells (eg, monocyte/macrophages) but also reduced bacterial internalization by renal tubular epithelial cells. Our findings clearly indicate that activation of endogenous anti-inflammatory mediator cAMP is beneficial for controlling UPEC-mediated acute pyelonephritis in mice. The beneficial effect can be explained at least in part by limiting excessive inflammatory responses through acting on both renal tubular epithelial cells and inflammatory cells and by inhibiting bacteria invasion of renal tubular epithelial cells.

cAMP signaling microdomains and their observation by optical methods

The second messenger cyclic AMP (cAMP) is a major intracellular mediator of many hormones and neurotransmitters and regulates a myriad of cell functions, including synaptic plasticity in neurons. Whereas cAMP can freely diffuse in the cytosol, a growing body of evidence suggests the formation of cAMP gradients and microdomains near the sites of cAMP production, where cAMP signals remain apparently confined. The mechanisms responsible for the formation of such microdomains are subject of intensive investigation. The development of optical methods based on fluorescence resonance energy transfer (FRET), which allow a direct observation of cAMP signaling with high temporal and spatial resolution, is playing a fundamental role in elucidating the nature of such microdomains. Here, we will review the optical methods used for monitoring cAMP and protein kinase A (PKA) signaling in living cells, providing some examples of their application in neurons, and will discuss the major hypotheses on the formation of cAMP/PKA microdomains.

Evidence for a causal link between adaptor protein PDZK1 downregulation and Na⁺/H⁺ exchanger NHE3 dysfunction in human and murine colitis

A dysfunction of the Na(+)/H(+) exchanger isoform 3 (NHE3) significantly contributes to the reduced salt absorptive capacity of the inflamed intestine. We previously reported a strong decrease in the NHERF family member PDZK1 (NHERF3), which binds to NHE3 and regulates its function in a mouse model of colitis. The present study investigates whether a causal relationship exists between the decreased PDZK1 expression and the NHE3 dysfunction in human and murine intestinal inflammation. Biopsies from the colon of patients with ulcerative colitis, murine inflamed ileal and colonic mucosa, NHE3-transfected Caco-2BBe colonic cells with short hairpin RNA (shRNA) knockdown of PDZK1, and Pdzk1-gene-deleted mice were studied. PDZK1 mRNA and protein expression was strongly decreased in inflamed human and murine intestinal tissue as compared to inactive disease or control tissue, whereas that of NHE3 or NHERF1 was not. Inflamed human and murine intestinal tissues displayed correct brush border localization of NHE3 but reduced acid-activated NHE3 transport activity. A similar NHE3 transport defect was observed when PDZK1 protein content was decreased by shRNA knockdown in Caco-2BBe cells or when enterocyte PDZK1 protein content was decreased to similar levels as found in inflamed mucosa by heterozygote breeding of Pdzk1-gene-deleted and WT mice. We conclude that a decrease in PDZK1 expression, whether induced by inflammation, shRNA-mediated knockdown, or heterozygous breeding, is associated with a decreased NHE3 transport rate in human and murine enterocytes. We therefore hypothesize that inflammation-induced loss of PDZK1 expression may contribute to the NHE3 dysfunction observed in the inflamed intestine.

Targeting protein kinase A in cancer therapy: an update

Protein Kinase A (PKA) is a well known member of the serine-threonin protein kinase superfamily. PKA, also known as cAMP-dependent protein kinase, is a multi-unit protein kinase that mediates signal transduction of G-protein coupled receptors through its activation upon cAMP binding. The widespread expression of PKA subunit genes, and the myriad of mechanisms by which cAMP is regulated within a cell suggest that PKA signaling is one of extreme importance to cellular function. It is involved in the control of a wide variety of cellular processes from metabolism to ion channel activation, cell growth and differentiation, gene expression and apoptosis. Importantly, since it has been implicated in the initiation and progression of many tumors, PKA has been proposed as a novel biomarker for cancer detection, and as a potential molecular target for cancer therapy. Here, we highlight some features of cAMP/PKA signaling that are relevant to cancer biology and present an update on targeting PKA in cancer therapy.

Regulation of mitochondrial poly(ADP-Ribose) polymerase activation by the β-adrenoceptor/cAMP/protein kinase A axis during oxidative stress

We investigated the regulation of mitochondrial poly(ADP-ribose) polymerase 1 (PARP1) by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) system during oxidative stress in U937 monocytes. Oxidative stress induced an early (10 minutes) mitochondrial DNA damage, and concomitant activation of PARP1 in the mitochondria. These early events were followed by a progressive mitochondrial oxidant production and nuclear PARP1 activation (by 6 hours). These processes led to a functional impairment of mitochondria, culminating in cell death of mixed (necrotic/apoptotic) type. β-Adrenoceptor blockade with propranolol or inhibition of its downstream cAMP/PKA signaling attenuated, while β-adrenoceptor agonists and cAMP/PKA activators enhanced, the oxidant-mediated PARP1 activation. In the presence of cAMP, recombinant PKA directly phosphorylated recombinant PARP1 on serines 465 (in the automodification domain) and 782 and 785 (both in the catalytic domain). Inhibition of the β-adrenergic receptor/cAMP/PKA axis protected against the oxidant-mediated cell injury. Propranolol also suppressed PARP1 activation in peripheral blood leukocytes during bacterial lipopolysaccharide (LPS)-induced systemic inflammation in mice. We conclude that the activation of mitochondrial PARP1 is an early, active participant in oxidant-induced cell death, which is under the control of β-adrenoceptor/cAMP/PKA axis through the regulation of PARP1 activity by PARP1 phosphorylation.

Soluble adenylyl cyclase in health and disease

The second messenger cAMP is integral for many physiological processes. Soluble adenylyl cyclase (sAC) was recently identified as a widely expressed intracellular source of cAMP in mammalian cells. sAC is evolutionary, structurally, and biochemically distinct from the G-protein-responsive transmembranous adenylyl cyclases (tmAC). The structure of the catalytic unit of sAC is similar to tmAC, but sAC does not contain transmembranous domains, allowing localizations independent of the membranous compartment. sAC activity is stimulated by HCO(3)(-), Ca²⁺ and is sensitive to physiologically relevant ATP fluctuations. sAC functions as a physiological sensor for carbon dioxide and bicarbonate, and therefore indirectly for pH. Here we review the physiological role of sAC in different human tissues with a major focus on the lung. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease, guest edited by J. Buck and L.R. Levin.

Immunocytochemistry: its applications and drawbacks for the study of gut neuroendocrinology

The ATPase Inhibitory Factor 1 (IF1) is the physiological inhibitor of the mitochondrial ATP synthase. Herein, we summarize the regulation of the expression and activity of IF1 as a main driver of the activity of oxidative phosphorylation (OXPHOS) in mammalian tissues. We emphasize that the expression of IF1, which is a mitochondrial protein with very short half-life, is tissue-specifically expressed and primarily controlled at posttranscriptional levels. Inhibition of the activity of IF1 as inhibitor of the ATP synthase under normal physiological conditions is exerted by phosphorylation of S39 by a cAMP-dependent PKA-like activity of mitochondria in response to different physiological cues. Conditional tissue-specific transgenic mice overexpressing IF1 in colon, or a mutant active version of IF1 (IF1-H49K) in liver or in neurons, revealed the inhibition of the ATP synthase and the reprograming of energy metabolism to an enhanced glycolysis. In the IF1-H49K models, the assembly/activity of complex IV and the superassembly of complex V are also affected. Moreover, the IF1-mediated inhibition of the ATP synthase generates a reactive oxygen species (mtROS) signal that switches on the expression of nuclear genes that facilitate adaptation to a restrained OXPHOS. In contrast to normal mice, metabolically preconditioned animals are partially protected from the action of cytotoxic agents by upgrading the activation of stress kinases and transcription factors involved in resolving metabolic adaptation, the antioxidant response, cell survival, and the immune response of the tissue microenvironment. Altogether, we stress a fundamental physiological function for the ATP synthase and its inhibitor in mitohormesis.