Activation of a cGMP-stimulated cAMP phosphodiesterase by protein kinase C in a liver Golgi-endosomal fraction

Novel mechanism of cyclic nucleotide crosstalk mediated by PKG-dependent proteasomal degradation of the Hsp90 client protein phosphodiesterase 3A

Endothelial cAMP-specific phosphodiesterase PDE3A is one of the major negative regulators of the endothelial barrier function in acute lung injury models. However, the mechanisms underlying its regulation still need to be fully resolved. We show here that the PDE3A is a newly described client of the molecular chaperone heat shock protein 90 (hsp90). In endothelial cells (ECs), hsp90 inhibition by geldanamycin (GA) led to a disruption of the hsp90/PDE3A complex, followed by a significant decrease in PDE3A protein levels. The decrease in PDE3A protein levels was ubiquitin-proteasome-dependent and required the activity of the E3 ubiquitin ligase C terminus of Hsc70-interacting protein. GA treatment also enhanced the association of PDE3A with hsp70, which partially prevented PDE3A degradation. GA-induced decreases in PDE3A protein levels correlated with decreased PDE3 activity and increased cAMP levels in EC. We also demonstrated that protein kinase G-dependent phosphorylation of PDE3A at Ser654 can signal the dissociation of PDE3A from hsp90 and PDE3A degradation. This was confirmed by endogenous PDE3A phosphorylation and degradation in 8-Br-cGMP- or 8-CPT-cGMP- and Bay 41-8543-stimulated EC and comparisons of WT- and phospho-mimic S654D mutant PDE3A protein stability in transiently transfected HEK293 cells. In conclusion, we have identified a new mechanism of PDE3A regulation mediated by the ubiquitin-proteasome system. Further, the degradation of PDE3A is controlled by the phosphorylation of S654 and the interaction with hsp90. We speculate that targeting the PDE3A/hsp90 complex could be a therapeutic approach for acute lung injury.

Cyclic GMP and PKG Signaling in Heart Failure

Cyclic guanosine monophosphate (cGMP), produced by guanylate cyclase (GC), activates protein kinase G (PKG) and regulates cardiac remodeling. cGMP/PKG signal is activated by two intrinsic pathways: nitric oxide (NO)-soluble GC and natriuretic peptide (NP)-particulate GC (pGC) pathways. Activation of these pathways has emerged as a potent therapeutic strategy to treat patients with heart failure, given cGMP-PKG signaling is impaired in heart failure with reduced ejection fraction (HFrEF) and preserved ejection fraction (HFpEF). Large scale clinical trials in patients with HFrEF have shown positive results with agents that activate cGMP-PKG pathways. In patients with HFpEF, however, benefits were observed only in a subgroup of patients. Further investigation for cGMP-PKG pathway is needed to develop better targeting strategies for HFpEF. This review outlines cGMP-PKG pathway and its modulation in heart failure.

Nitration of protein kinase G-Iα modulates cyclic nucleotide crosstalk via phosphodiesterase 3A: Implications for acute lung injury

Phosphodiesterase 3A (PDE3A) selectively cleaves the phosphodiester bond of cAMP and is inhibited by cGMP, making it an important regulator of cAMP-cGMP signaling crosstalk in the pulmonary vasculature. In addition, the nitric oxide-cGMP axis is known to play an important role in maintaining endothelial barrier function. However, the potential role of protein kinase G-Iα (PKG-Iα) in this protective process is unresolved and was the focus of our study. We describe here a novel mechanism regulating PDE3A activity, which involves a PKG-Iα-dependent inhibitory phosphorylation of PDE3A at serine 654. We also show that this phosphorylation is critical for maintaining intracellular cAMP levels in the pulmonary endothelium and endothelial barrier integrity. In an animal model of acute lung injury (ALI) induced by challenging mice with lipopolysaccharide (LPS), an increase in PDE3 activity and a decrease in cAMP levels in lung tissue was associated with reduced PKG activity upon PKG-Iα nitration at tyrosine 247. The peroxynitrite scavenger manganese (III) tetrakis(1-methyl-4-pyridyl)porphyrin prevented this increase in PDE3 activity in LPS-exposed lungs. In addition, site-directed mutagenesis of PDE3A to replace serine 654 with alanine yielded a mutant protein that was insensitive to PKG-dependent regulation. Taken together, our data demonstrate a novel functional link between nitrosative stress induced by LPS during ALI and the downregulation of barrier-protective intracellular cAMP levels. Our data also provide new evidence that PKG-Iα is critical for endothelial barrier maintenance and that preservation of its catalytic activity may be efficacious in ALI therapy.

PDE2 Inhibits PKA-Mediated Phosphorylation of TFAM to Promote Mitochondrial Ca2+-Induced Colorectal Cancer Growth

Growing evidence indicates that the dysregulation of mitochondrial calcium (Ca²⁺) plays a critical role in the growth of tumor cells, including colorectal cancer (CRC). However, the underling mechanism is not fully elucidated. In this study, the regulatory effects of mitochondrial Ca²⁺ on phosphodiesterase 2 (PDE2)/cAMP/PKA axis and the phosphorylation of mitochondrial transcription factor A (TFAM) as well as the growth of CRC cells were systematically investigated both in vitro and in vivo. Our findings demonstrated that MCU-induced mitochondrial Ca²⁺ uptake activated mitochondrial PDE2 in CRC cells. Moreover, overexpression MCU in CRC led to a 1.9-fold increase in Ca²⁺ uptake compared to control cells. However, knockdown of MCU resulted in 1.5-fould decrease in Ca²⁺ uptake in mitochondria compared to the controls. Activation of mitochondrial PDE2 significantly inhibited the activity of mitochondrial protein kinase A (PKA), which subsequently leads to decreased phosphorylation of TFAM. Our data further revealed that PKA regulates the phosphorylation of TFAM and promotes the degradation of phosphorylated TFAM. Thus, TFAM protein levels accumulated in mitochondria when the activity of PKA was inhibited. Overall, this study showed that the overexpression of MCU enhanced CRC growth through promoting the accumulation of TFAM proteins in mitochondria. Conversely, knockdown of MCU in CRC cells resulted in decreased CRC growth. Collectively, these data suggest that the mitochondrial Ca²⁺-activated PDE2/cAMP/PKA axis plays a key role in regulating TFAM stability and the growth of CRC cells.

Therapeutic Implications for PDE2 and cGMP/cAMP Mediated Crosstalk in Cardiovascular Diseases

Phosphodiesterases (PDEs) are the principal superfamily of enzymes responsible for degrading the secondary messengers 3',5'-cyclic nucleotides cAMP and cGMP. Their refined subcellular localization and substrate specificity contribute to finely regulate cAMP/cGMP gradients in various cellular microdomains. Redistribution of multiple signal compartmentalization components is often perceived under pathological conditions. Thereby PDEs have long been pursued as therapeutic targets in diverse disease conditions including neurological, metabolic, cancer and autoimmune disorders in addition to numerous cardiovascular diseases (CVDs). PDE2 is a unique member of the broad family of PDEs. In addition to its capability to hydrolyze both cAMP and cGMP, PDE2 is the sole isoform that may be allosterically activated by cGMP increasing its cAMP hydrolyzing activity. Within the cardiovascular system, PDE2 serves as an integral regulator for the crosstalk between cAMP/cGMP pathways and thereby may couple chronically adverse augmented cAMP signaling with cardioprotective cGMP signaling. This review provides a comprehensive overview of PDE2 regulatory functions in multiple cellular components within the cardiovascular system and also within various subcellular microdomains. Implications for PDE2- mediated crosstalk mechanisms in diverse cardiovascular pathologies are discussed highlighting the prospective use of PDE2 as a potential therapeutic target in cardiovascular disorders.

Cyclic nucleotide phosphodiesterases: New targets in the metabolic syndrome?

Metabolic diseases have a tremendous impact on human morbidity and mortality. Numerous targets regulating adenosine monophosphate kinase (AMPK) have been identified for treating the metabolic syndrome (MetS), and many compounds are being used or developed to increase AMPK activity. In parallel, the cyclic nucleotide phosphodiesterase families (PDEs) have emerged as new therapeutic targets in cardiovascular diseases, as well as in non-resolved pathologies. Since some PDE subfamilies inactivate cAMP into 5'-AMP, while the beneficial effects in MetS are related to 5'-AMP-dependent activation of AMPK, an analysis of the various controversial relationships between PDEs and AMPK in MetS appears interesting. The present review will describe the various PDE families, AMPK and molecular mechanisms in the MetS and discuss the PDEs/PDE modulators related to the tissues involved, thus supporting the discovery of original molecules and the design of new therapeutic approaches in MetS.

Endogenous Gαq-Coupled Neuromodulator Receptors Activate Protein Kinase A

Protein kinase A (PKA) integrates inputs from G-protein-coupled neuromodulator receptors to modulate synaptic and cellular function. Gαs signaling stimulates PKA activity, whereas Gαi inhibits PKA activity. Gαq, on the other hand, signals through phospholipase C, and it remains unclear whether Gαq-coupled receptors signal to PKA in their native context. Here, using two independent optical reporters of PKA activity in acute mouse hippocampus slices, we show that endogenous Gαq-coupled muscarinic acetylcholine receptors activate PKA. Mechanistically, this effect is mediated by parallel signaling via either calcium or protein kinase C. Furthermore, multiple Gαq-coupled receptors modulate phosphorylation by PKA, a classical Gαs/Gαi effector. Thus, these results highlight PKA as a biochemical integrator of three major types of GPCRs and necessitate reconsideration of classic models used to predict neuronal signaling in response to the large family of Gαq-coupled receptors.

PDE2 at the crossway between cAMP and cGMP signalling in the heart

The cyclic nucleotides cAMP and cGMP are central second messengers in cardiac cells and critical regulators of cardiac physiology as well as pathophysiology. Consequently, subcellular compartmentalization allows for spatiotemporal control of cAMP/cGMP metabolism and subsequent regulation of their respective effector kinases PKA or PKG is most important for cardiac function in health and disease. While acute cAMP-mediated signalling is a mandatory prerequisite for the physiological fight-or-flight response, sustained activation of this pathway may lead to the progression of heart failure. In contrast, acute as well as sustained cGMP-mediated signalling can foster beneficial features, e.g. anti-hypertrophic and vasodilatory effects. These two signalling pathways seem to be intuitively counteracting and there is increasing evidence for a functionally relevant crosstalk between cAMP and cGMP signalling pathways on the level of cyclic nucleotide hydrolysing phosphodiesterases (PDEs). Among this diverse group of enzymes, PDE2 may fulfill a unique integrator role. Equipped with dual substrate specificity for cAMP as well as for cGMP, it is the only cAMP hydrolysing PDE, which is allosterically activated by cGMP. Recent studies have revealed strongly remodelled cAMP/cGMP microdomains and subcellular concentration profiles in different cardiac pathologies, leading to a putatively enhanced involvement of PDE2 in cAMP/cGMP breakdown and crosstalk compared to the other cardiac PDEs. This review sums up the current knowledge about molecular properties and regulation of PDE2 and explains the complex signalling network encompassing PDE2 in order to better understand the functional role of PDE2 in distinct cell types in cardiac health and disease. Moreover, this review gives an outlook in which way PDE2 may serve as a therapeutic target to treat cardiac disease.

Nanodomain Regulation of Cardiac Cyclic Nucleotide Signaling by Phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) form an 11-member superfamily comprising 100 different isoforms that regulate the second messengers cyclic adenosine or guanosine 3',5'-monophosphate (cAMP or cGMP). These PDE isoforms differ with respect to substrate selectivity and their localized control of cAMP and cGMP within nanodomains that target specific cellular pools and synthesis pathways for the cyclic nucleotides. Seven PDE family members are physiologically relevant to regulating cardiac function, disease remodeling of the heart, or both: PDE1 and PDE2, both dual-substrate (cAMP and cGMP) esterases; PDE3, PDE4, and PDE8, which principally hydrolyze cAMP; and PDE5A and PDE9A, which target cGMP. New insights regarding the different roles of PDEs in health and disease and their local signaling control are broadening the potential therapeutic utility for PDE-selective inhibitors. In this review, we discuss these PDEs, focusing on the different mechanisms by which they control cardiac function in health and disease by regulating intracellular nanodomains.

Delphinidin Inhibits Tumor Growth by Acting on VEGF Signalling in Endothelial Cells

The vasculoprotective properties of delphinidin are driven mainly by its action on endothelial cells. Moreover, delphinidin displays anti-angiogenic properties in both in vitro and in vivo angiogenesis models and thereby might prevent the development of tumors associated with excessive vascularization. This study was aimed to test the effect of delphinidin on melanoma-induced tumor growth with emphasis on its molecular mechanism on endothelial cells. Delphinidin treatment significantly decreased in vivo tumor growth induced by B16-F10 melanoma cell xenograft in mice. In vitro, delphinidin was not able to inhibit VEGFR2-mediated B16-F10 melanoma cell proliferation but it specifically reduced basal and VEGFR2-mediated endothelial cell proliferation. The anti-proliferative effect of delphinidin was reversed either by the MEK1/2 MAP kinase inhibitor, U-0126, or the PI3K inhibitor, LY-294002. VEGF-induced proliferation was reduced either by U-0126 or LY-294002. Under these conditions, delphinidin failed to decrease further endothelial cell proliferation. Delphinidin prevented VEGF-induced phosphorylation of ERK1/2 and p38 MAPK and decreased the expression of the transcription factors, CREB and ATF1. Finally, delphinidin was more potent in inhibiting in vitro cyclic nucleotide phosphodiesterases (PDEs), PDE1 and PDE2, compared to PDE3-PDE5. Altogether delphinidin reduced tumor growth of melanoma cell in vivo by acting specifically on endothelial cell proliferation. The mechanism implies an association between inhibition of VEGF-induced proliferation via VEGFR2 signalling, MAPK, PI3K and at transcription level on CREB/ATF1 factors, and the inhibition of PDE2. In conjunction with our previous studies, we demonstrate that delphinidin is a promising compound to prevent pathologies associated with generation of vascular network in tumorigenesis.

Tumour growth inhibition and anti-angiogenic effects using curcumin correspond to combined PDE2 and PDE4 inhibition

Vascular endothelial growth factor (VEGF) plays a major role in angiogenesis by stimulating endothelial cells. Increase in cyclic AMP (cAMP) level inhibits VEGF-induced endothelial cell proliferation and migration. Cyclic nucleotide phosphodiesterases (PDEs), which specifically hydrolyse cyclic nucleotides, are critical in the regulation of this signal transduction. We have previously reported that PDE2 and PDE4 up-regulations in human umbilical vein endothelial cells (HUVECs) are implicated in VEGF-induced angiogenesis and that inhibition of PDE2 and PDE4 activities prevents the development of the in vitro angiogenesis by increasing cAMP level, as well as the in vivo chicken embryo angiogenesis. We have also shown that polyphenols are able to inhibit PDEs. The curcumin having anti-cancer properties, the present study investigated whether PDE2 and PDE4 inhibitors and curcumin could have similar in vivo anti-tumour properties and whether the anti-angiogenic effects of curcumin are mediated by PDEs. Both PDE2/PDE4 inhibitor association and curcumin significantly inhibited in vivo tumour growth in C57BL/6N mice. In vitro, curcumin inhibited basal and VEGF-stimulated HUVEC proliferation and migration and delayed cell cycle progression at G0/G1, similarly to the combination of selective PDE2 and PDE4 inhibitors. cAMP levels in HUVECs were significantly increased by curcumin, similarly to rolipram (PDE4 inhibitor) and BAY-60-550 (PDE2 inhibitor) association, indicating cAMP-PDE inhibitions. Moreover, curcumin was able to inhibit VEGF-induced cAMP-PDE activity without acting on cGMP-PDE activity and to modulate PDE2 and PDE4 expressions in HUVECs. The present results suggest that curcumin exerts its in vitro anti-angiogenic and in vivo anti-tumour properties through combined PDE2 and PDE4 inhibition.

Cyclic nucleotide phosphodiesterase (PDE) isozymes as targets of the intracellular signalling network: benefits of PDE inhibitors in various diseases and perspectives for future therapeutic developments

Cyclic nucleotide phosphodiesterases (PDEs) that specifically inactivate the intracellular messengers cAMP and cGMP in a compartmentalized manner represent an important enzyme class constituted by 11 gene-related families of isozymes (PDE1 to PDE11). Downstream receptors, PDEs play a major role in controlling the signalosome at various levels of phosphorylations and protein/protein interactions. Due to the multiplicity of isozymes, their various intracellular regulations and their different cellular and subcellular distributions, PDEs represent interesting targets in intracellular pathways. Therefore, the investigation of PDE isozyme alterations related to various pathologies and the design of specific PDE inhibitors might lead to the development of new specific therapeutic strategies in numerous pathologies. This manuscript (i) overviews the different PDEs including their endogenous regulations and their specific inhibitors; (ii) analyses the intracellular implications of PDEs in regulating signalling cascades in pathogenesis, exemplified by two diseases affecting cell cycle and proliferation; and (iii) discusses perspectives for future therapeutic developments.

Concerted regulation of cGMP and cAMP phosphodiesterases in early cardiac hypertrophy induced by angiotensin II

Left ventricular hypertrophy leads to heart failure and represents a high risk leading to premature death. Cyclic nucleotides (cAMP and cGMP) play a major role in heart contractility and cyclic nucleotide phosphodiesterases (PDEs) are involved in different stages of advanced cardiac diseases. We have investigated their contributions in the very initial stages of left ventricular hypertrophy development. Wistar male rats were treated over two weeks by chronic infusion of angiotensin II using osmotic mini-pumps. Left cardiac ventricles were used as total homogenates for analysis. PDE1 to PDE5 specific activities and protein and mRNA expressions were explored.

Rats developed arterial hypertension associated with a slight cardiac hypertrophy (+24%). cAMP-PDE4 activity was specifically increased while cGMP-PDE activities were broadly increased (+130% for PDE1; +76% for PDE2; +113% for PDE5) and associated with increased expressions for PDE1A, PDE1C and PDE5A. The cGMP-PDE1 activation by Ca²⁺/CaM was reduced. BNP expression was increased by 3.5-fold, while NOX2 expression was reduced by 66% and AMP kinase activation was increased by 64%. In early cardiac hypertrophy induced by angiotensin II, all specific PDE activities in left cardiac ventricles were increased, favoring an increase in cGMP hydrolysis by PDE1, PDE2 and PDE5. Increased cAMP hydrolysis was related to PDE4. We observed the establishment of two cardioprotective mechanisms and we suggest that these mechanisms could lead to increase intracellular cGMP: i) increased expression of BNP could increase “particulate” cGMP pool; ii) increased activation of AMPK, subsequent to increase in PDE4 activity and 5′AMP generation, could elevate “soluble” cGMP pool by enhancing NO bioavailability through NOX2 down-regulation. More studies are needed to support these assumptions. Nevertheless, our results suggest a potential link between PDE4 and AMPK/NOX2 and they point out that cGMP-PDEs, especially PDE1 and PDE2, may be interesting therapeutic targets in preventing cardiac hypertrophy.

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

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

Eicosapentaenoic acid reduces ABCA1 serine phosphorylation and impairs ABCA1-dependent cholesterol efflux through cyclic AMP/protein kinase A signaling pathway in THP-1 macrophage-derived foam cells

ABCA1 is a key mediator of cholesterol efflux to apoA-I in cholesterol loaded macrophages, a first step of RCT in vivo. Unsaturated fatty acids can inhibit cholesterol efflux from macrophages by increasing degradation of ABCA1. However, the detailed mechanisms of ABCA1 regulation by unsaturated fatty acids are not fully understood. In the present study, we investigated the effects of EPA on ABCA1 expression and ABCA1-dependent cholesterol efflux and examined the role of cAMP/PKA pathway on the regulation of ABCA1 by EPA in THP-1 macrophage-derived foam cells. Results showed that EPA significantly destabilized ABCA1 protein and reduced ABCA1-dependent cholesterol efflux but had no effect on ABCA1 mRNA expression. We also revealed that EPA markedly reduced cAMP level and PKA activity and ABCA1 serine phosphorylation. PKA-specific activation by PKA agonist markedly compensated the down-regulation of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux by EPA, while, siRNA of PKA leaded to reduce of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux more significantly than EPA. However, EPA-Induced enhancement of degradation rate of ABCA1 protein did not change by treatment with PKA agonist or PKA-siRNA. These results provide evidence that EPA may have dual negative effects on ABCA1 activity by decreasing ABCA1 protein level and by reducing PKA-mediated ABCA1 serine phosphorylation in THP-1 macrophage-derived foam cells.

Control of cyclic AMP concentration in bovine endometrial stromal cells by arachidonic acid

Second messenger signalling through cyclic AMP (cAMP) plays an important role in the response of the endometrium to prostaglandin (PG) E(2) during early pregnancy. Arachidonic acid, which is a by-product of the luteolytic cascade in ruminants, is a potential paracrine signal from the epithelium to the stroma. We investigated the effects of arachidonic acid on the response of the stroma to PGE(2). cAMP was measured in bovine endometrial stromal cells treated with agents known to activate or inhibit adenylyl cyclase, protein kinase C (PKC) or phosphodiesterase (PDE). PGE(2) increased the intracellular cAMP concentration within 10 min, and this effect was attenuated by arachidonic acid and the PKC activator, 4beta-phorbol myristate acetate (PMA). The inhibitory effect of arachidonic acid on PGE(2)-induced cAMP accumulation was prevented by the PKC inhibitor, RO318425, and was absent in cells in which PKC had been downregulated by exposure to PMA for 24 h. The effect of arachidonic acid was also prevented by the PDE inhibitor, 3-isobutyl-1-methylxanthine. Arachidonic acid was shown by immunoblotting to prevent induction of cyclooxygenase-2 by PGE(2), forskolin or dibutyryl cAMP. The results indicate that arachidonic acid activates PDE through a mechanism involving PKC, counteracting a rise in intracellular cAMP in response to PGE(2). The data suggest that arachidonic acid antagonizes PGE(2) signalling through cAMP in the bovine endometrium, possibly acting to ensure a rapid return to oestrus in the case of failure of the maternal recognition of pregnancy.

Cytosolic and localized inhibition of phosphodiesterase by atrazine in swine tissue homogenates

Atrazine (ATR) significantly inhibited phosphodiesterase (PDE) in crude homogenates of swine heart, brain, and lung, but not liver or kidney tissues. Except for heart, PDE activities in the cytosolic fraction of the tissue homogenates were not affected by ATR. The inhibition of the PDE activity in the cytosol from heart homogenate was not significantly different between ATR and a non-specific PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Dixon plots of the crude tissue homogenates showed that heart and brain were inhibited via two different mechanisms (competitive or mixed inhibition, and noncompetitive inhibition, respectively), suggesting that ATR may be a semi-specific PDE inhibitor. Furthermore, in crude tissue homogenates, ATR did not inhibit PDE as effectively as IBMX suggesting that there are ATR-susceptible and ATR-nonsusceptible forms of PDE. Association constants for ATR were 55 microM for heart and 310 microM for brain. The stability of the activity of PDE was affected by freezing, requiring the use of only freshly prepared tissue homogenates.

Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents

Cyclic nucleotide phosphodiesterases (PDEs), which are ubiquitously distributed in mammalian tissues, play a major role in cell signaling by hydrolyzing cAMP and cGMP. Due to their diversity, which allows specific distribution at cellular and subcellular levels, PDEs can selectively regulate various cellular functions. Their critical role in intracellular signaling has recently designated them as new therapeutic targets for inflammation. The PDE superfamily represents 11 gene families (PDE1 to PDE11). Each family encompasses 1 to 4 distinct genes, to give more than 20 genes in mammals encoding the more than 50 different PDE proteins probably produced in mammalian cells. Although PDE1 to PDE6 were the first well-characterized isoforms because of their predominance in various tissues and cells, their specific contribution to tissue function and their regulation in pathophysiology remain open research fields. This concerns particularly the newly discovered families, PDE7 to PDE11, for which roles are not yet established. In many pathologies, such as inflammation, neurodegeneration, and cancer, alterations in intracellular signaling related to PDE deregulation may explain the difficulties observed in the prevention and treatment of these pathologies. By inhibiting specifically the up-regulated PDE isozyme(s) with newly synthesized potent and isozyme-selective PDE inhibitors, it may be potentially possible to restore normal intracellular signaling selectively, providing therapy with reduced adverse effects.

Negative feedback exerted by cAMP-dependent protein kinase and cAMP phosphodiesterase on subsarcolemmal cAMP signals in intact cardiac myocytes: an in vivo study using adenovirus-mediated expression of CNG channels

Intracardiac cAMP levels are modulated by hormones and neuromediators with specific effects on contractility and metabolism. To understand how the same second messenger conveys different information, mutants of the rat olfactory cyclic nucleotide-gated (CNG) channel alpha-subunit CNGA2, encoded into adenoviruses, were used to monitor cAMP in adult rat ventricular myocytes. CNGA2 was not found in native myocytes but was strongly expressed in infected cells. In whole cell patch-clamp experiments, the forskolin analogue L-858051 (L-85) elicited a non-selective, Mg2+ -sensitive current observed only in infected cells, which was thus identified as the CNG current (ICNG). The beta-adrenergic agonist isoprenaline (ISO) also activated ICNG, although the maximal efficiency was approximately 5 times lower than with L-85. However, ISO and L-85 exerted a similar maximal increase of the L-type Ca2+ current. The use of a CNGA2 mutant with a higher sensitivity for cAMP indicated that this difference is caused by the activation of a localized fraction of CNG channels by ISO. cAMP-dependent protein kinase (PKA) blockade with H89 or PKI, or phosphodiesterase (PDE) inhibition with IBMX, dramatically potentiated ISO- and L-85-stimulated ICNG. A similar potentiation of beta-adrenergic stimulation occurred when PDE4 was blocked, whereas PDE3 inhibition had a smaller effect (by 2-fold). ISO and L-85 increased total PDE3 and PDE4 activities in cardiomyocytes, although this effect was insensitive to H89. However, in the presence of IBMX, H89 had no effect on ISO stimulation of ICNG. This study demonstrates that subsarcolemmal cAMP levels are dynamically regulated by a negative feedback involving PKA stimulation of subsarcolemmal cAMP-PDE.

Sustained entry of Ca2+ is required to activate Ca2+-calmodulin-dependent phosphodiesterase 1A

Regulation of adenylyl cyclases (ACs) by Ca2+ requires capacitative Ca2+ entry (CCE) (Cooper, D. M. F. (2003) Biochem. J. 375, 517-529), but whether Ca2+-sensitive phosphodiesterases (PDEs) are similarly discriminating has never been addressed. In the present study, a variety of conditions were devised to manipulate [Ca2+]i so that we could ask whether PDE1 selectively responds to different modes of elevating [Ca2+]i, viz. Ca2+ released from intracellular stores and various modes of Ca2+ entry. In 1321N1 human astrocytoma cells, the endogenous PDE1 (identified as PDE1A by reverse transcriptase-PCR) was largely insensitive to Ca2+ released from carbachol-sensitive stores but was robustly stimulated by a similar rise in [Ca2+]i due to carbachol-induced Ca2+ influx. Gd3+, which effectively blocked thapsigargin-induced CCE and its effect on PDE1A, also inhibited the activation of PDE1A by carbachol-induced Ca2+ entry. However, non-selective ionomycin-mediated Ca2+ entry also activated PDE1A, so that, unlike Ca2+-sensitive ACs, PDE1A cannot discriminate between the different sources of Ca2+ entry. Fractionation of the cells revealed that the Ca2+-calmodulin-stimulated PDE activity was not present at the plasma membrane but was associated with the cytosol and the organellar compartments of the cell. Therefore, the apparent disparity between PDE1A and ACs is likely to be the consequence of their differential subcellular localization. Nevertheless, in a physiological context, where artificial modes of elevating [Ca2+]i are not available, as with ACs, a dependence on CCE would be evident, and it would be the duration of this influx of Ca2+ that would determine how long PDE1A was activated.

Subcellular localization of cyclic nucleotide phosphodiesterase type 10A variants, and alteration of the localization by cAMP-dependent protein kinase-dependent phosphorylation

Our previous studies have suggested that two phosphodiesterase type 10A (PDE10A) variants, PDE10A1 and PDE10A2 transcripts, are mainly expressed in humans and that PDE10A2 and PDE10A3 transcripts are major variants in rats. In the present study, immunoblot analysis demonstrated that PDE10A proteins, especially PDE10A2, are more abundant in membrane fractions than in cytosolic fractions of rat striatum. Recombinant PDE10A1 and PDE10A3 were produced only in cytosolic fractions of transfected PC12h cells. By contrast, recombinant PDE10A2 was present mainly in membrane fractions. This finding agreed well with the result of subcellular fractionation of PDE10A in rat striatum. Immunocytochemical analysis showed that PDE10A2 was localized in the Golgi apparatus of transfected PC12h cells. PDE10A2 was phosphorylated by cAMP-dependent protein kinase (PKA) at Thr16. Interestingly, recombinant protein of wild-type PDE10A2, but not PDE10A2 mutant with an Ala replacement at Thr16, was distributed to cytosolic fractions by co-transfection with a plasmid encoding the catalytic subunit of PKA. A PDE10A2 mutant with Glu substitution at Thr16, which can be a mimic of phosphorylation, was localized in the cytosolic fractions of transfected PC12h cells. These observations implied that phosphorylation of PDE10A2 at Thr16 by PKA caused alteration of subcellular localization of PDE10A2 from the Golgi apparatus to cytosol. It is hypothesized that cAMP signaling in the Golgi area and the cytosol in neurons is controlled through alteration of subcellular localization of PDE10A brought by activation of PKA in response to intracellular elevations of cAMP.

Insulin inhibits secretin-induced ductal secretion by activation of PKC alpha and inhibition of PKA activity

Insulin stimulates canalicular bile flow by interaction with hepatocytes. Insulin regulates the function of a number of epithelia through activation and membrane translocation of Ca(2+)-dependent PKC isoforms. No information exists regarding insulin regulation of ductal bile secretion. The aim of the study was to determine the role and mechanisms of action of insulin in the regulation of cholangiocyte secretion in BDL rats. We determined the subcellular localization of insulin receptor in cholangiocytes. We measured the effect of insulin on (1) secretin-stimulated cAMP levels in cholangiocytes and duct expansion in intrahepatic bile duct units (IBDUs) in the absence or presence of BAPTA/AM, H7 or rottlerin and (2) bile flow. We evaluated (1) if insulin effects are associated with activation of PKC alpha and (2) if activation of PKC causes inhibition of secretin-stimulated cAMP levels and PKA activity. We found insulin receptors only in the apical domain of cholangiocytes. Insulin inhibited secretin-induced choleresis and secretin-stimulated cholangiocyte cAMP levels. Insulin inhibited secretin-induced secretion in IBDUs when applied at the basolateral membrane or microinjected into IBDU lumen. Insulin inhibitory effects on cholangiocyte secretion were blocked by BAPTA/AM and H7. Insulin induced activation of PKC alpha, which decreased secretin-stimulated cAMP and PKA activity. In conclusion, insulin inhibited secretin-induced ductal secretion of BDL rats through activation of PKC and inhibition of secretin-stimulated cAMP and PKA activity. In conclusion, insulin counter-regulates cholangiocyte secretory processes in the BDL model, which is characterized by cholangiocyte proliferation.

Interaction of chelerythrine with inositol phosphate metabolism

Chelerythrine, a potent inhibitor of protein kinase C (PKC), was evaluated for its effect on inositol phosphate (IP) metabolism in newborn rat cardiomyocytes in culture. In a first step, we evaluated the effect of chelerythrine on IP accumulation in basal conditions. For a 10(-4) M dose, 5-phosphatase activity (which dephosphorylates IP3 into IP2) was completely blocked and we observed a large increase in IP accumulation limited to IP2 without any increase in IP3, strongly suggesting that chelerythrine at this dose modifies IP metabolism. At a lower dose (10(-5) M) of chelerythrine, which did not modify IP accumulation and 5-phosphatase activity in basal conditions, the response to angiotensin II stimulation was completely abolished by the addition of chelerythrine. We conclude thus that chelerythrine, even at 10(-5) M, interacts markedly with IP metabolism, and caution should be exerted when interpreting the results obtained with this drug, which is still currently used at this dose.

Activation of phosphodiesterase 5 and inhibition of guanylate cyclase by cGMP-dependent protein kinase in smooth muscle

The regulation of cGMP-specific phosphodiesterase (PDE) 5 and soluble guanylate cyclase (GC) by cGMP- and cAMP-dependent protein kinases (PKG and PKA respectively) was examined in gastric smooth muscle. The NO donor, sodium nitroprusside (SNP), stimulated PDE5 phosphorylation and activity, which was blocked by the selective PKG inhibitor, KT5823, resulting in an elevation of cGMP levels. Activation of PKA either directly by Sp-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole 3',5'-cyclic monophosphothioate, or via isoproterenol- and forskolin-dependent increase in cAMP, also caused an increase in PDE5 phosphorylation and activity, but only in the presence of cGMP; consistent with the dependence of PDE5 phosphorylation and activity on cGMP binding to allosteric sites in the regulatory domain of PDE5. The selective PKA inhibitors, myristoylated protein kinase inhibitor and H-89, blocked the increase in PDE5 phosphorylation and activity induced by PKA. SNP also stimulated soluble GC phosphorylation and activity. KT5823 abolished phosphorylation and augmented soluble GC activity, implying feedback inhibition of soluble GC by PKG-dependent phosphorylation. Phosphorylation by PKG was direct and could be induced in vitro. Activation of PKA had no effect on soluble GC. Thus cGMP levels are regulated by PKG- and PKA-dependent activation of PDE5 and PKG-specific inhibition of soluble GC.

Characterization of an in vivo hormonally regulated phosphodiesterase 3 (PDE3) associated with a liver Golgi-endosomal fraction

The biochemical properties of an in vivo hormonally regulated low Km cAMP phosphodiesterase (PDE) activity associated with a liver Golgi-endosomal (GE) fraction have been characterized. DEAE-Sephacel chromatography of a GE fraction solubilized by a lysosomal extract resulted in the sequential elution of three peaks of activity (numbered I, II, and III), while ion-exchange HPLC resolved five peaks of activity (numbered 1, 2, 3, 4, and 5). Based on the sensitivity of the eluted activity to cGMP and selected phosphodiesterase inhibitors, two phosphodiesterase isoforms were resolved: a cGMP-stimulated and EHNA-inhibited PDE2, eluted in DEAE-Sephacel peak I and HPLC peak 2 and a cGMP-, a cilostamide-, and ICI 118233-inhibited PDE3, eluted in DEAE-Sephacel peak III and HPLC peaks 3, 4, and 5. GE fractions isolated after acute treatments with insulin, tetraiodoglucagon, and growth hormone displayed an increase in phosphodiesterase activity relative to saline-injected controls, as did GE fractions from genetically obese and hyperinsulinemic rats relative to lean littermates. In all experimental rats, an increase in PDE3 activity associated with DEAE-Sephacel peak III and HPLC peaks 4 and 5 was observed relative to control animals. Furthermore, in genetically obese Zucker rats, an increase in the sensitivity of PDE activity to cilostamide and in the amount of PDE activity immunoprecipitated by an antibody to adipose tissue PDE3 was observed relative to lean littermates. These results extend earlier studies on isolated hepatocytes and show that liver PDE3 is the main if not sole PDE isoform activated by insulin, glucagon, and growth hormone in vivo.

Mechanism of action of the calcium-sensing receptor in human antral gastrin cells

BACKGROUND AND AIMS

Human G cells express the calcium-sensing receptor and respond to extracellular calcium by releasing gastrin. However, the receptor on G cells is insensitive to serum calcium levels. We investigated whether this is a result of differential regulation of signaling pathways compared with parathyroid or calcitonin cells.

METHODS

Gastrin release from primary cultures of human antral epithelial cells enriched for G cells (35%) was measured by radioimmunoassay. G cells were stimulated by increasing extracellular calcium concentration for 1 hour in the presence or absence of antagonists of specific intracellular signaling pathways. Intracellular calcium levels were monitored to evaluate the effect of the antagonists on calcium influx.

RESULTS

Inhibition of phospholipase C decreased calcium-stimulated gastrin release, but blockers of adenylate cyclase, phospholipase A(2), or mitogen-activated protein kinase had no effect. Inhibition of protein kinase C, nonselective cation channels, and phosphodiesterase increased basal and calcium-stimulated gastrin release while decreasing calcium influx. These data were consistent with basally active phosphodiesterase.

CONCLUSIONS

The calcium-sensing receptor on the G cell activates phospholipase C and opens nonselective cation channels, resulting in an influx of extracellular calcium. Protein kinase C isozymes expressed by the G cells play multiple roles regulating both gastrin secretion and phosphodiesterase activity.