Protein kinase A regulation of cAMP phosphodiesterase expression in rat skeletal myoblasts

Exposure to a specific time-varying electromagnetic field inhibits cell proliferation via cAMP and ERK signaling in cancer cells

Exposure to specific electromagnetic field (EMF) patterns can affect a variety of biological systems. We have shown that exposure to Thomas-EMF, a low-intensity, frequency-modulated (25-6 Hz) EMF pattern, inhibited growth and altered cell signaling in malignant cells. Exposure to Thomas-EMF for 1 h/day inhibited the growth of malignant cells including B16-BL6 mouse melanoma cells, MDA-MB-231, MDA-MB-468, BT-20, and MCF-7 human breast cancer and HeLa cervical cancer cells but did not affect non-malignant cells. The Thomas-EMF-dependent changes in cell proliferation were mediated by adenosine 3',5'-cyclic monophosphate (cAMP) and extracellular-signal-regulated kinase (ERK) signaling pathways. Exposure of malignant cells to Thomas-EMF transiently changed the level of cellular cAMP and promoted ERK phosphorylation. Pharmacologic inhibitors (SQ22536) and activators (forskolin) of cAMP production both blocked the ability of Thomas-EMF to inhibit cell proliferation, and an inhibitor of the MAP kinase pathway (PD98059) was able to partially block Thomas-EMF-dependent inhibition of cell proliferation. Genetic modulation of protein kinase A (PKA) in B16-BL6 cells also altered the effect of Thomas-EMF on cell proliferation. Cells transfected with the constitutively active form of PKA (PKA-CA), which interfered with ERK phosphorylation, also interfered with the Thomas-EMF effect on cell proliferation. The non-malignant cells did not show any EMF-dependent changes in cAMP levels, ERK phosphorylation, or cell growth. These data indicate that exposure to the specific Thomas-EMF pattern can inhibit the growth of malignant cells in a manner dependent on contributions from the cAMP and MAP kinase pathways. Bioelectromagnetics. 39;217-230, 2018. © 2017 Wiley Periodicals, Inc.

A novel thermoregulatory role for PDE10A in mouse and human adipocytes

Phosphodiesterase type 10A (PDE10A) is highly enriched in striatum and is under evaluation as a drug target for several psychiatric/neurodegenerative diseases. Preclinical studies implicate PDE10A in the regulation of energy homeostasis, but the mechanisms remain unclear. By utilizing small-animal PET/MRI and the novel radioligand [(18)F]-AQ28A, we found marked levels of PDE10A in interscapular brown adipose tissue (BAT) of mice. Pharmacological inactivation of PDE10A with the highly selective inhibitor MP-10 recruited BAT and potentiated thermogenesis in vivo In diet-induced obese mice, chronic administration of MP-10 caused weight loss associated with increased energy expenditure, browning of white adipose tissue, and improved insulin sensitivity. Analysis of human PET data further revealed marked levels of PDE10A in the supraclavicular region where brown/beige adipocytes are clustered in adults. Finally, the inhibition of PDE10A with MP-10 stimulated thermogenic gene expression in human brown adipocytes and induced browning of human white adipocytes. Collectively, our findings highlight a novel thermoregulatory role for PDE10A in mouse and human adipocytes and promote PDE10A inhibitors as promising candidates for the treatment of obesity and diabetes.

In mpkCCD cells, long-term regulation of aquaporin-2 by vasopressin occurs independent of protein kinase A and CREB but may involve Epac

Urine concentration involves the hormone vasopressin (AVP), which stimulates cAMP production in renal principal cells, resulting in translocation and transcription of aquaporin-2 (AQP2) water channels, greatly increasing the water permeability, leading to a concentrated urine. As cAMP levels decrease shortly after AVP addition, whereas AQP2 levels still increase and are maintained for days, we investigated in the present study the mechanism responsible for the AQP2 increase after long-term 1-desamino-8-d-arginine vasopressin (dDAVP) application using mouse collecting duct (mpkCCD) cells. While 30 min of dDAVP incubation strongly increased cAMP, cAMP was lower with 1 day and was even further reduced with 4 days of dDAVP, although still significantly higher than in control cells. One day of dDAVP incubation increased AQP2 promoter-dependent transcription, which was blocked by the protein kinase A (PKA) inhibitor H89. Moreover, phosphorylation of the cAMP-responsive element binding protein (CREB) and CRE-dependent transcription was observed after short-term dDAVP stimulation. With 4 days of dDAVP, AQP2 transcription remained elevated, but this was not blocked by H89, and CRE-dependent transcription and CREB phosphorylation were not increased. Exchange factor directly activated by cAMP (Epac) 1 and 2 were found to be endogenously expressed in mpkCCD cells. Application of dDAVP increased the expression of Epac1, while Epac2 was reduced. Incubation with a specific Epac activator after dDAVP pretreatment increased both AQP2 abundance and transcription compared with cells left unstimulated the last day. In conclusion, the PKA-CRE pathway is involved in the initial rise in AQP2 levels after dDAVP stimulation but not in the long-term effect of dDAVP. Instead, long-term regulation of AQP2 may involve the activation of Epac.

Localized cyclic AMP-dependent protein kinase activity is required for myogenic cell fusion

Multinucleated myotubes are formed by fusion of mononucleated myogenic progenitor cells (myoblasts) during terminal skeletal muscle differentiation. In addition, myoblasts fuse with myotubes, but terminally differentiated myotubes have not been shown to fuse with each other. We show here that an adenylate cyclase activator, forskolin, and other reagents that elevate intracellular cyclic AMP (cAMP) levels induced cell fusion between small bipolar myotubes in vitro. Then an extra-large myotube, designated a "myosheet," was produced by both primary and established mouse myogenic cells. Myotube-to-myotube fusion always occurred between the leading edge of lamellipodia at the polar end of one myotube and the lateral plasma membrane of the other. Forskolin enhanced the formation of lamellipodia where cAMP-dependent protein kinase (PKA) was accumulated. Blocking enzymatic activity or anchoring of PKA suppressed forskolin-enhanced lamellipodium formation and prevented fusion of multinucleated myotubes. Localized PKA activity was also required for fusion of mononucleated myoblasts. The present results suggest that localized PKA plays a pivotal role in the early steps of myogenic cell fusion, such as cell-to-cell contact/recognition through lamellipodium formation. Furthermore, the localized cAMP-PKA pathway might be involved in the specification of the fusion-competent areas of the plasma membrane in lamellipodia of myogenic cells.

cAMP regulates axon outgrowth and guidance during optic nerve regeneration in goldfish

Increased cAMP improves neuronal survival and axon regeneration in mammals. Here, we assess cAMP levels and identify activated pathways in a spontaneously regenerating central nervous system. Following optic nerve crush in goldfish, almost all retinal ganglion cells (RGC) survive and regenerate retinotectal topography. Goldfish received injections of a cAMP analogue (CPT-cAMP), a protein kinase A (PKA) inhibitor (KT5720), both compounds combined, or PBS (control). RGC survival in experimental groups was unaffected at any stage. The rate of axon regeneration was accelerated by the activator and decelerated both by the inhibitor and by combined injections, suggesting a PKA-dependent pathway. In addition, errors in regenerate retinotectal topography were observed when agents were applied in vivo and RGC response to the guidance cue ephrin-A5 in vitro was altered by the inhibitor. Our results highlight that therapeutic manipulation of cAMP levels to enhance axonal regeneration in mammals must ensure that topography, and consequently function, is not disrupted.

Intracellular targeting of phosphodiesterase-4 underpins compartmentalized cAMP signaling

The phosphodiesterase-4 (PDE4) enzyme belongs to a family of cAMP-dependent phosphodiesterases that provide the major means of hydrolyzing and, thereby, inactivating the key intracellular second messenger, cAMP. As such, PDE4s are central to the regulation of many diverse signaling processes that allow cells to respond to external stimuli. Four genes (4A, 4B, 4C, and 4D) encode around 20 distinct isoform members of the PDE4 family. Each isoform is characterized by a unique N-terminal region. PDE4s are multidomain metallohydrolases with each domain serving particular roles allowing them to be targeted to varying regions and organelles of intracellular space and regulated in distinct fashions by phosphorylation and protein-protein interaction. Although identical in catalytic function, each isoform locates to distinct regions within the cell so as to create and manage spatially distinct pools of cAMP. The multiplicity of partners associating with members of the four gene PDE4 family places these enzymes in key regulatory positions, permitting them to channel complex biological signals via fundamental signaling cohorts such as G-protein-coupled receptors (GPCRs), arrestins, A-kinase-anchoring proteins (AKAPs), and tyrosyl family kinases. The cAMP cascade has long been linked to cellular growth and embryogenesis and with this comes the implication that PDE4 may play considerable roles in the regulation of progeny development in maturing cells and tissues.

PGE(2) stimulates vascular smooth muscle cell proliferation via the EP2 receptor

Prostaglandins released by injured vascular tissue can modulate smooth muscle cell (SMC) proliferation. The mechanism of action of PGE(2) was investigated with porcine coronary artery SMCs obtained by explant culture. DNA and RNA syntheses exhibited a concentration-dependent increase following treatment of quiescent SMCs with PGE(2), while PGI(2) had no effect. PGE(2) also elevated PCNA expression, bromodeoxyuridine incorporation, and cell number, indicative of a hyperplastic growth response. Furthermore, induction of c-fos expression required activation of both phosphatidylinositol 3-kinase and mitogen-activated protein kinase. Contrary to these data, treatment of proliferating cells with PGE(2) caused a reduction in DNA synthesis. A role for PKA in either growth stimulation or inhibition was excluded. Interestingly, only quiescent SMCs expressed EP2 receptors, and the selective EP2 receptor agonist butaprost confirmed that this receptor was essential for growth stimulation and possibly inhibition. These data suggest that the growth state-dependent actions of PGE(2) on SMC proliferation may be mediated via the EP2 receptor.

Long PDE4 cAMP specific phosphodiesterases are activated by protein kinase A-mediated phosphorylation of a single serine residue in Upstream Conserved Region 1 (UCR1)

1. Challenge of COS1 cells with the adenylyl cyclase activator forskolin led to the activation of recombinant PDE4A8, PDE4B1, PDE4C2 and PDE4D5 cAMP-specific phosphodiesterase long isoforms. 2. Forskolin challenge did not activate mutant long PDE4 isoforms where the serine target residue (STR) within the protein kinase A (PKA) consensus phosphorylation site in Upstream Conserved Region 1 (UCR1) was mutated to alanine. 3. The PKA inhibitor, H89, ablated forskolin activation of wild-type long PDE4 isoforms. 4. Activated PKA caused the in vitro phosphorylation of recombinant wild-type long PDE4 isoforms, but not those where the STR was mutated to alanine. 5. An antiserum specific for the phosphorylated form of the STR detected a single immunoreactive band for recombinant long PDE4 isoforms expressed in COS1 cells challenged with forskolin. This was not evident in forskolin-challenged cells treated with H89. Neither was it evident in forskolin-challenged cells expressing long isoforms where the STR had been mutated to alanine. 6. In transfected COS cells challenged with forskolin, only the phosphorylated PDE4D3 long form showed a decrease in mobility in Western blotting analysis. This decreased mobility of PDE4D3 was ablated upon mutation of either of the two serine targets for PKA phosphorylation in this isoform, namely Ser54 in UCR1 and Ser13 in the isoform-specific N-terminal region. 7. Activation by forskolin challenge did not markedly alter the sensitivity of PDE4A8, PDE4B1, PDE4C2 and PDE4D5 to inhibition by rolipram. 8. Long PDE4 isoforms from all four sub-families can be phosphorylated by protein kinase A (PKA). This leads to an increase in their activity and may thus contribute to cellular desensitization processes in cells where these isoforms are selectively expressed.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Involvement of type 4 cAMP-phosphodiesterase in the myogenic differentiation of L6 cells

Myogenic cell differentiation is induced by Arg(8)-vasopressin, whereas high cAMP levels and protein kinase A (PKA) activity inhibit myogenesis. We investigated the role of type 4 phosphodiesterase (PDE4) during L6-C5 myoblast differentiation. Selective PDE4 inhibition resulted in suppression of differentiation induced by vasopressin. PDE4 inhibition prevented vasopressin-induced nuclear translocation of the muscle-specific transcription factor myogenin without affecting its overall expression level. The effects of PDE4 inhibition could be attributed to an increase of cAMP levels and PKA activity. RNase protection, reverse transcriptase PCR, immunoprecipitation, Western blot, and enzyme activity assays demonstrated that the PDE4D3 isoform is the major PDE4 expressed in L6-C5 myoblasts and myotubes, accounting for 75% of total cAMP-hydrolyzing activity. Vasopressin cell stimulation caused a biphasic increase of PDE4 activity, which peaked at 2 and 15 min and remained elevated for 48 h. In the continuous presence of vasopressin, cAMP levels and PKA activity were lowered. PDE4D3 overexpression increased spontaneous and vasopressin-dependent differentiation of L6-C5 cells. These results show that PDE4D3 plays a key role in the control of cAMP levels and differentiation of L6-C5 cells. Through the modulation of PDE4 activity, vasopressin inhibits the cAMP signal transduction pathway, which regulates myogenesis possibly by controlling the subcellular localization of myogenin.

The molecular biology of cyclic nucleotide phosphodiesterases

Recent progress in the field of cyclic nucleotides has shown that a large array of closely related proteins is involved in each step of the signal transduction cascade. Nine families of adenylyl cyclases catalyze the synthesis of the second messenger cAMP, and protein kinases A, the intracellular effectors of cAMP, are composed of four regulatory and three catalytic subunits. A comparable heterogeneity has been discovered for the enzymes involved in the inactivation of cyclic nucleotide signaling. In mammals, 19 different genes encode the cyclic nucleotide phosphodiesterases (PDEs), the enzymes that hydrolyze and inactivate cAMP and cGMP. This is only an initial level of complexity, because each PDE gene contains several distinct transcriptional units that give rise to proteins with subtle structural differences, bringing the number of the PDE proteins close to 50. The molecular biology of PDEs in Drosophila and Dictyostelium has shed some light on the role of PDE diversity in signaling and development. However, much needs to be done to understand the exact function of these enzymes, particularly during mammalian development and cell differentiation. With the identification and mapping of regulatory and targeting domains of the PDEs, modularity of the PDE structure is becoming an established tenet in the PDE field. The use of different transcriptional units and exon splicing of a single PDE gene generates proteins with different regulatory domains joined to a common catalytic domain, therefore expanding the array of isoforms with subtle differences in properties and sensitivities to different signals. The physiological context in which these different isoforms function is still largely unknown and undoubtedly will be a major area of expansion in the years to come.

Effects of dexamethosone and glucagon after long-term exposure on cyclic AMP phosphodiesterase 4 in cultured rat hepatocytes

67% of total cAMP phosphodiesterase activity (PDE) in cultured rat hepatocytes could be detected in the cytosol, 15% in plasma membrane, 15% in 'dense vesicle,' and 3% in endoplasmatic reticulum fractions. Up to 84% of the PDE activity of the cytosol is represented by the rolipram-sensitive PDE 4. ICI 118233-inhibited PDE 3 was found predominantly in membranes. We were able to show that dexamethasone acts on the PDE 4 in cytosolic and plasma membrane fractions whereas glucagon effected the PDE 4 of the cytosol and the PDE 3 in 'dense vesicle' membranes. Primary culture of hepatocytes was used to study long-term effects of dexamethasone and glucagon on PDE 4 activity. Addition of dexamethasone (0.1 microM) at the beginning of cultivation leads to a decrease of total PDE 4 activity whereas after 24 h precultivation no dexamethasone effect could be observed. Glucagon effects on PDE 4 were investigated in 20 h precultured hepatocytes. Maximal stimulation was achieved after 2 h of exposure. PDE 4 subtypes A, B , D and, to a lesser degree, subtype C could be detected by RT-PCR analysis. The results of semiquantitative RT-PCR show that the presence of dexamethasone during the first 24 h of cultivation reduced selectively the transcription of PDE 4D, whereas glucagon was without any effect. Also the translation of PDE 4D was reduced as shown in the Western blot. We would like to discuss the way that dexamethasone influences PDE 4D expression-most likely in combination with other factors such as cytokines--during the time of cell plating, whereas glucagon actions are part of metabolic regulations via phosphorylation reactions.

Selective up-regulation of phosphodiesterase-4 cyclic adenosine 3',5'-monophosphate (cAMP)-specific phosphodiesterase variants by elevated cAMP content in human myometrial cells in culture

In human myometrium, the modulation of intracellular cAMP content resulting from agonist-mediated stimulation of the receptor-adenylyl cyclase complex is largely influenced by the rate of cAMP hydrolysis by phosphodiesterase (PDE) isoenzymes. We have previously shown that the PDE4 family contributes to the predominant cAMP-hydrolyzing activity in human myometrium and that elevation of the PDE4B2 messenger RNA steady state level occurs in pregnant myometrial tissue. In the present study, we used a model of human myometrial cells in culture to determine whether an elevated cAMP concentration could influence PDE expression. As in myometrial tissue, high levels of PDE4 activity were detected in these smooth muscle cells. Long term treatment with 8-bromo-cAMP or forskolin resulted in a selective induction of PDE4B and of PDE4D short form messenger RNA variants. Concurrently, an increased immunoreactive signal for the PDE4B- and PDE4D-related isoenzymes was detected. This induction was consistent with an observed significant up-regulation of PDE4 activity. Accordingly, our results demonstrate that in human cultured myometrial cells, cAMP-elevating agents manipulate PDE4 activity through selective induction of synthesis of PDE4B and PDE4D short forms. Such a mechanism might have physiological importance during pregnancy by dampening hormonal stimulation and could thereby be involved in tolerance to the tocolytic effect of beta-adrenoceptor agonists.

Induction of phosphodiesterases 3B, 4A4, 4D1, 4D2, and 4D3 in Jurkat T-cells and in human peripheral blood T-lymphocytes by 8-bromo-cAMP and Gs-coupled receptor agonists. Potential role in beta2-adrenoreceptor desensitization

In this study, a potential mechanism of beta2-adrenoreceptor desensitization has been explored that is based upon the enhanced degradation of cAMP by phosphodiesterase (PDE). Pretreatment of Jurkat T-cells with 8-bromo cAMP (8-Br-cAMP) or prostaglandin E2 increased PDE3 and PDE4 activity in an actinomycin D- and cycloheximide-sensitive manner. This effect was associated with increased expression of HSPDE3B, HSPDE4A4, HSPDE4D1, HSPDE4D2, and HSPDE4D3 mRNA transcripts. Western analysis reproducibly labeled a band of immunoreactivity in vehicle-treated cells that corresponded to HSPDE4A4 (125 kDa). Although the intensity of this band was unchanged in cells treated with 8-Br-cAMP, additional 68-72-kDa proteins (HSPDE4D2, HSPDE4D1) were labeled that were not detected after vehicle. Similar results were obtained with T-lymphocytes exposed to 8-Br-cAMP and fenoterol. However, in those experiments HSPDE4A4 and HSPDE4D1 appeared to be equally expressed in vehicle- and treated cells, whereas HSPDE4D2 (72 kDa) was detected only after 8-Br-cAMP. The up-regulation of PDE activity in Jurkat T-cells abolished the ability of isoproterenol to elevate cAMP, which was partially reversed by the non-selective PDE inhibitor, 3-isobutyl-1-methylxanthine, and by the PDE3 and PDE4 inhibitors, Org 9935 and rolipram, respectively. Collectively, these data suggest that chronic treatment of T-cells with cAMP-elevating agents compromises beta2-adrenoreceptor-mediated cAMP accumulation by increasing the expression of HSPDE3B and HSPDE4D gene products.

Expression and regulation of mRNA for distinct isoforms of cAMP-specific PDE-4 in mitogen-stimulated and leukemic human lymphocytes

We reported previously that the gene for PDE-1B1 is induced in isolated human peripheral blood lymphocytes (HPBL) following mitogenic stimulation (Jiang, X., Li, J., Paskind, M., and Epstein, P.M. [1996] Proc. Natl. Acad. Sci. USA 93, 11,236-11,241). Using reverse transcription-polymerase chain reaction (RT-PCR), we investigated possible changes in the expression of the four genes for cAMP-specific phosphodiesterase (PDE-4A-D) in HPBL under the same conditions. Isolated, quiescent HPBL express mRNA for PDE-4B as the principal transcript. Following mitogenic stimulation with phytohemagglutinin (PHA), mRNA for PDE-4A and PDE-4D are clearly induced. HPBL appear not to express PDE-4C under resting or stimulated conditions. The PHA induced increase in PDE-1B1, PDE-4A, and PDE-4D mRNA is mimicked by incubation of HPBL with dibutyryl cAMP (dBcAMP) and 1-methyl-3-isobutylxanthine (IBMX). The B-lymphoblastoid cell line, RPMI 8392, and the T-leukemic cell line, Molt 4, express PDE-4A mRNA as the most abundant transcript, but incubation with dBcAMP and IBMX induces an increase in the expression of mRNA for PDE-4B in both of these cell lines, and in PDE-4D3 in the RPMI 8392 cell line. These studies demonstrate that expression of mRNA for PDE-1B1 and some of the subtypes of PDE-4 are induced in HPBL following mitogenic stimulation, possibly secondarily to elevation of cAMP induced by the mitogen. As already indicated for PDE-1B1, some of these subtypes of PDE-4 might also provide additional therapeutic targets for treatment of immunoproliferative disorders and immune dysfunction.

Alternative splicing of the high affinity cAMP-specific phosphodiesterase (PDE7A) mRNA in human skeletal muscle and heart

To further our understanding of the structure and function of phosphodiesterases of the newly identified family of high affinity cAMP-specific phosphodiesterases (PDE7), we identified and characterized the isozyme expressed in human skeletal muscle and the protein product of the previously isolated isozyme HCP1 (designated HSPDE7A1). We report the isolation of a cDNA encoding the full-length skeletal muscle isoform of human PDE7A (HSPDE7A2). The DNA sequence of this skeletal muscle cDNA indicates that PDE7A2 is a novel 5' splice variant of PDE7A encoding an isoform with a novel, hydrophobic N terminus. The 456-amino acid PDE7A2 protein is detected on Western blots as a band with an apparent mobility of 50 kDa. PDE7A2 is a high affinity cAMP-specific PDE (Km = 0.1 microM), which is localized to particulate cellular fractions. The PDE7A1 (HCP1) isozyme is detected on Western blots as a band with an apparent mobility of 57 kDa, demonstrating that the previously isolated HCP1 cDNA encodes the full-length PDE7A1 protein. The even distribution of PDE7A mRNA among fetal tissues and the relative abundance of its two mRNAs strongly suggest that the expression of PDE7A is regulated throughout development.

Induction of cyclic AMP and cyclic GMP 3':5'-cyclic nucleotide phosphodiesterase activities in neuroblastoma lines under differentiating conditions

It is now widely accepted that cyclic nucleotide phosphodiesterases (PDEs) play fundamental roles in signal transduction pathways; they show a remarkable molecular complexity, different tissue distribution and complex regulatory mechanisms. Here we report PDE isoforms expression in two dibutyryl cyclic AMP differentiated murine cell lines: the hybrid neuroblastoma-glioma 108CC15 and the parental neuroblastoma N18TG2. They differ in the ability to establish functional synapses, a feature present only in the former. Ionic exchange chromatography elution profiles of N18TG2 and 108CC15 undifferentiated cell extracts show two main peaks of activity. The first one hydrolyzes cyclic GMP and is specifically inhibited by Zaprinast, thus representing a member of the PDE5 family. The second peak hydrolyzes cyclic AMP and is significantly inhibited by rolipram, as all the PDE4 family members. The induction of differentiation by dibutyryl cyclic AMP in both clonal lines results in an increase of PDE activities only after 3 hr of treatment, suggesting that protein neosynthesis is involved. Interestingly in both clones, besides the increase in cyclic AMP hydrolyzing specific activity (3.1-fold in 108CC15 and 2.5-fold in N18TG2), we also observed an increase in cyclic GMP hydrolyzing activity (1.7-fold in 108CC15 and 4.3-fold in N18TG2). While the induction of PDE4, previously reported also in other cellular systems, could be considered as a feedback response to the higher cyclic AMP levels, this is not true for the isoform that hydrolyzes cyclic GMP. These data suggest that the induction of PDE isoforms in neuroblastoma cells could be related to the activation of neuronal differentiative pathway.

Recombinant expression of a type IV, cAMP-specific phosphodiesterase: characterization and structure-function studies of deletion mutants

A potential role for cAMP in regulating the differentiation of myoblasts has led us to examine the components of the cAMP signaling system, including the type IV, cAMP-specific phosphodiesterases. The full coding sequence of the phosphodiesterase PDE4D1 was inserted in the bacterial expression vector pGEX-KG. N- and C-terminal truncations were also placed in the same vector, allowing the expression and purification of glutathione S-transferase (GST)-PDE fusion proteins using glutathione-Sepharose. The purified PDE was active [V(max) = 318 +/- 18 nmol min(-1)(mg of protein)(-1)] and inhibited by RO 20-1724, rolipram, and MIX (IC50 values of 2, 0.4, and 40 microM, respectively). The requirement of PDE4D1 for a divalent cation was also examined. It was able to use Mg2+, Co2+, and Mn2+, but not Zn2+, suggesting that it is not a zinc hydrolase as has been proposed for other PDE types. Deletion of both C- and N-terminal regions affected the apparent native size of the enzyme. The C-terminal region was involved in dimer formation, whereas an N-terminal region was responsible for larger aggregates. Removal of the last 35 amino acids of an N-terminal 80-residue highly conserved region (UCR2) resulted in a 6-fold increase in PDE activity, providing evidence that this part of the molecule acts as an intramolecular inhibitor. The availability of a highly purified, enzymatically active protein in substantial quantities has allowed us to directly examine PDE4D1 for the first time.

Identification and tissue-specific expression of PDE7 phosphodiesterase splice variants

Type 7 cyclic nucleotide phosphodiesterases (PDE7s) are a newly described family of enzymes having high affinity and specificity for cAMP. However, little is known about their structure, function, or regulation. We have isolated a mouse skeletal muscle cDNA representing a new alternative splice variant (PDE7A2) of the PDE7 gene. The ORF encodes a 456-amino acid protein having a predicted molecular weight of 52.4 kDa. The 5' end of the mouse PDE7A2 is divergent from the 5' end of the human PDE7A1 sequence and is more hydrophobic. A comparison of the 5' ends of the two cDNA clones with human genomic sequence indicates that they represent alternate splice products rather than species variation. RNase protection analysis of several mouse tissues indicates that PDE7 is expressed widely with highest levels in skeletal muscle. HPLC fractionation and Western blot analysis of two human lymphocyte T-cell lines shows that an unknown PDE activity described by Ichimura and Kase [Ichimura, M. & Kase, H. (1993) Biochem. Biophys. Res. Commun. 193, 985-990] is most likely to be PDE7A1. A single immunoreactive band of approximately 55 kDa, which comigrates with PDE7A1, is seen in fractions of the HPLC profile containing this activity suggesting that the original human PDE7A1 clone contains a full-length ORF, and is not truncated at the 5' end as was originally postulated. In a human lymphocyte B-cell line and also in mouse skeletal muscle, a large amount of PDE7 mRNA but little PDE7 protein or activity is expressed suggesting that the translation or stability of PDE7 protein may be highly regulated in these tissues.

Phosphorylation and activation of a cAMP-specific phosphodiesterase by the cAMP-dependent protein kinase. Involvement of serine 54 in the enzyme activation

A cAMP-specific phosphodiesterase (PDE4D3) is activated in rat thyroid cells by TSH through a cAMP-dependent phosphorylation (Sette, C., Iona, S., and Conti, M.(1994) J. Biol. Chem. 269, 9245-9252). This short term activation may be involved in the termination of the hormonal stimulation and/or in the induction of desensitization. Here, we have further characterized the protein kinase A (PKA)-dependent phosphorylation of this PDE4D3 variant and identified the phosphorylation site involved in the PDE activation. The PKA-dependent incorporation of phosphate in the partially purified, recombinant rat PDE4D3 followed a time course similar to that of activation. Half-maximal activation of the enzyme was obtained with 0.6 microM ATP and 30 nM of the catalytic subunit of PKA. Phosphorylation altered the Vmax of the PDE without affecting the Km for cAMP. Phosphorylation also modified the Mg2+ requirements and the pattern of inhibition by rolipram. Cyanogen bromide cleavage of the 32P-labeled rat PDE4D3 yielded two or three major phosphopeptide bands, providing a first indication that the enzyme may be phosphorylated at multiple sites in a cell-free system. Site-directed mutagenesis was performed on the serine residues present at the amino terminus of this PDE in the context of preferred motifs for PKA phosphorylation. The PKA-dependent incorporation of 32P was reduced to the largest extent in mutants with both Ser13 --> Ala and Ser54 --> Ala substitutions, confirming the presence of more than one phosphorylation site in rat PDE4D3. While substitution of serine 13 with alanine did not affect the activation by PKA, substitution of Ser54 completely suppressed the kinase activation. Similar conclusions were reached with wild type and mutated PDE4D3 proteins expressed in MA-10 cells, where the endogenous PKA was activated by dibutyryl cAMP. Again, the PDE with the Ser54 --> Ala substitution could not be activated by the endogenous PKA in the intact cell. These findings support the hypothesis that the PDE4D3 variant contains a regulatory domain target for phosphorylation at the amino terminus of the protein and that Ser54 in this domain plays a crucial role in activation.

Identification of phosphodiesterase IV activity and its cyclic adenosine monophosphate-dependent up-regulation in a human keratinocyte cell line (HaCaT)

Cellular activity of cyclic adenosine monophosphate (cAMP)-degrading phosphodiesterases (PDEs) is of crucial importance for the regulation of cAMP levels. However, PDE isoenzymes in human keratinocytes have not been characterized previously. In the present study, the PDE isoenzyme activity profile of the human keratinocyte cell line HaCaT was investigated by PDE activity measurements. In addition, the cAMP-mediated regulation of PDE activities was examined. The isoenzymes PDE IV and PDE V activities were identified in HaCaT cell homogenates by activity measurements and were found to be preferentially located in the soluble fraction. Long-term exposure of HaCaT cells to cAMP-elevating agents (e.g., rolipram, salbutamol, forskolin) triggered a maximum threefold up-regulation of PDE IV activity, whereas PDE V activity was not affected. The PDE IV inhibitor rolipram synergistically amplified PDE IV up-regulation by beta 2-receptor agonists. Experiments applying protein kinase A activators and inhibitors as well as actinomycin D and cycloheximide indicated that de novo mRNA and protein synthesis were at least partly involved in PDE IV up-regulation. Functionally, the enhanced PDE IV activity was reflected by an impaired cAMP response to salbutamol. This hyporesponsiveness toward the beta 2-adrenoceptor agonists was partly reversed by rolipram. This study describes a cAMP-dependent long-term up-regulation of PDE IV in HaCaT cells, which is at least partly reflected by a simultaneous reduced cAMP response to a beta-agonist.

Expression and regulation of human and rat phosphodiesterase type IV isogenes

Type IV phosphodiesterases (PDE IV) specifically hydrolyze cAMP and are inhibited by rolipram. RT-PCR was applied to analyze the expression patterns of mRNAs for four cloned human and rat phosphodiesterase type IV isogenes (PDE IV-A, -B, -C and -D). Although these patterns were mostly coincident for the human and rat PDE IV genes, some differences were found between the two species. PDE IV-A expression was detectable in human blood but not in rat blood, suggesting a species-specific difference in the expression of this PDE IV isogene. PDE IV-C was neither detected in human or rat blood nor in different cell populations of the human immune system. It is further demonstrated that the PDE IV isogene expression is differentially regulated by cAMP in different cell types.