Characterization of the rolipram-sensitive, cyclic AMP-specific phosphodiesterases: identification and differential expression of immunologically distinct forms in the rat brain

Splice variants of the cyclic nucleotide phosphodiesterase PDE4D are differentially expressed and regulated in rat tissue

Cyclic nucleotide PDE4 (phosphodiesterase 4) inhibitors are being developed as potent anti-inflammatory drugs for use in chronic lung diseases, but the complexity of the PDE4 family has hampered this process. The four genes comprising the PDE4 family, PDE4A, PDE4B, PDE4C and PDE4D, are all expressed as multiple splice variants. The most widely used criterion to identify PDE4 variants expressed endogenously is their migration on SDS/PAGE. However, when a PDE4D3-selective antibody was used for immunoprecipitation, the pattern of expression obtained did not confirm the expression predicted by SDS/PAGE. This observation, together with the recent discovery of additional PDE4D transcripts, prompted us to re-evaluate the pattern of expression of these variants. The nine rat PDE4D splice variants, PDE4D1 to PDE4D9, were cloned, their electrophoretic properties compared, and their in vivo mRNA and protein levels determined. Using this approach, we found that the pattern of distribution of the PDE4D splicing variants is more complex than previously reported. Multiple variants co-migrate in single immunoreactive bands, and variant-selective antibodies were necessary to discriminate between splice variants. Tissues that were thought to express only PDE4D3, express three closely related proteins, with PDE4D8 and PDE4D9 as the predominantly expressed forms. In addition, activation of cAMP signalling produces phosphorylation and activation of variants other than PDE4D3, and expression of PDE4D mRNA does not always correlate with the pattern of protein expression. As PDE4 inhibitors have different affinities for distinct PDE4D splicing variants, our results indicate that a better definition of the pattern of PDE4 expression is required for target validation.

AKAP3 selectively binds PDE4A isoforms in bovine spermatozoa

Cyclic AMP plays an important role in regulating sperm motility and acrosome reaction through activation of cAMP-dependent protein kinase A (PKA). Phosphodiesterases (PDEs) modulate the levels of cyclic nucleotides by catalyzing their degradation. Although PDE inhibitors specific to PDE1 and PDE4 are known to alter sperm motility and capacitation in humans, little is known about the role or subcellular distribution of PDEs in spermatozoa. The localization of PKA is regulated by A-kinase anchoring proteins (AKAPs), which may also control the intracellular distribution of PDE. The present study was undertaken to investigate the role and localization of PDE4 during sperm capacitation. Addition of Rolipram or RS25344, PDE4-specific inhibitors significantly increased the progressive motility of bovine spermatozoa. Immunolocalization techniques detected both PDE4A and AKAP3 (formerly known as AKAP110) in the principal piece of bovine spermatozoa. The PDE4A5 isoform was detected primarily in the Triton X-100-soluble fraction of caudal epididymal spermatozoa. However, in ejaculated spermatozoa it was seen primarily in the SDS-soluble fraction, indicating a shift in PDE4A5 localization into insoluble organelles during sperm capacitation. AKAP3 was detected only in the SDS-soluble fraction of both caudal and ejaculated sperm. Immunoprecipitation experiments using COS cells cotransfected with AKAP3 and either Pde4a5 or Pde4d provide evidence that PDE4A5 but not PDE4D interacts with AKAP3. Pulldown assays using sperm cell lysates confirm this interaction in vitro. These data suggest that AKAP3 binds both PKA and PDE4A and functions as a scaffolding protein in spermatozoa to regulate local cAMP concentrations and modulate sperm functions.

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.

Phosphodiesterase 4D Forms a cAMP Diffusion Barrier at the Apical Membrane of the Airway Epithelium

We demonstrated previously that Calu-3 airway epithelial cells sense adenosine on their luminal surface through adenosine A2B receptors coupled to adenylyl cyclase. Occupancy of these receptors leads to activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel through protein kinase A (PKA) anchored at the apical membrane. Because luminal A2B receptor activation does not raise total cellular cAMP levels, we hypothesized that activation of phosphodiesterases (PDEs) confines cAMP generated by apical A2B receptors to a microdomain that includes the CFTR channel. Using reverse transcription-PCR, Western blotting, and activity measurements, PDE4D was identified as the major PDE species in airway epithelia. Consistent with these results, inhibitors of PDE4, but not PDE3, selectively abolished the lateral confinement of cAMP signaling in apical membrane patches during cell-attached recordings. Furthermore, stimulation of the CFTR in excised apical patches by rolipram and RS25344 indicated that PDE4 is localized in close proximity to the CFTR channel. Indeed, immunohistochemistry of human airway sections revealed that PDE4D is localized in the apical domain of the cell. PDE4 was activated after luminal adenosine exposure in a PKA-dependent manner. Because PDE4 activity is positively regulated by PKA, our results support a model whereby the PDE diffusion barrier is proportional to the degree of receptor stimulation. These findings underscore the concept that subcellular localization of individual PDE isozymes is an important mechanism for confining cAMP signaling to functional domains within cells.

Activation and induction of cyclic AMP phosphodiesterase (PDE4) in rat pulmonary microvascular endothelial cells

Regulation of the rolipram-sensitive cAMP-specific phosphodiesterase 4 (PDE4) gene family was studied in rat pulmonary microvascular endothelial cells (RPMVECs). Total PDE4 hydrolysis was increased within 10 min after addition of forskolin (10 microM), reached a maximum at 20-40 min, and then gradually declined in the cells. A similar activation of PDE4 activity was observed using a protein kinase A (PKA) activator, N(6)-monobutyryl cAMP. Both the forskolin and the N(6)-monobutyryl cAMP activated PDE4 activities were blocked by the PKA-specific inhibitor, H89. This forskolin-stimulated and PKA-mediated short-term activation of PDE4 activity was further confirmed by in vitro phosphorylation of 87kDa PDE4A6 and 83kDa PDE4B3 polypeptides using exogenous PKA Calpha. Increased immunoreactivity of phosphorylated PDE4A6 in situ was detected in Western blots by a PDE4A-phospho antibody specific to the putative PKA phosphorylation sites. Following long-term treatment of RPMVECs with rolipram and forskolin medium (RFM) for more than 60 days, PDE4 activity reached ten-fold higher values than control RPMVECS with twenty-fold increases detected in intracellular cAMP content. The RFM cells showed increased immunoreactivities of the constitutive 4A6 and 4B3 isoforms plus two novel splice variants at 101kDa (4B1) and 71kDa (4B2). Treatment with H89 did not inhibit the PDE4 elevation in RFM cells. In addition to the increased levels of PDE4 in RFM cells, immunofluorescence showed a translocation of PDE4A and 4B to a nuclear region, which was normally not observed in RPMVECs. The PDE4 activity in RFM cells decayed rapidly with an even faster decline of intracellular cAMP content when forskolin/rolipram were removed from the medium. These results suggest that both the activation (short-term) and induction (long-term) of PDE4A/4B isoforms in RPMVECs are closely modulated by the intracellular cAMP content via both post-translational and synthetic mechanisms.

Role of PDE4 in superoxide anion generation through p44/42MAPK regulation: a cAMP and a PKA-independent mechanism

We investigated the generation of reactive oxygen species (ROS) from bronchoalveolar lavage (BAL) cells of either control or LPS-exposed rats and the effects of PDE4 inhibitors on ROS production. The PDE4 inhibitors, rolipram and Ariflo (cilomilast, SB 207499) dose-dependently (0.1-10 microm) inhibited fMLP-induced superoxide anion (O(2)(*-)) production (IC(50)s: 0.03 and 0.55 microm, respectively) in BAL cells of Wistar rats collected 3 h after an LPS-aerosol (200 micrograms ml(-1), 1 h). These BAL contained 85-95% neutrophils (BAL cells enriched in neutrophils). In contrast, BAL cells collected at the end of the challenge contained only macrophages and in these conditions, rolipram and Ariflo (0.1-10 microm) could only inhibit 25 and 45% of fMLP-induced O(2)(*-) release, respectively. We also observed that the inhibition of p44/42(MAPK) by PD98059 (1-10 microm) increased O(2)(*-) release by BAL cells enriched in neutrophils, but not by macrophages, and prevented the inhibition of O(2)(*-) production induced by PDE4 inhibitors. Western blot analysis showed that PDE4 inhibitors strongly activated p44/42(MAPK) in BAL cells enriched in neutrophils but not in macrophages. And in these cells, PDE4 and p44/42(MAPK) were co-immunoprecipitated by a polyclonal anti-PDE4 antibody. The following cell permeable-cAMP analogues, dbcAMP (10 microm-1 mm), 8-CPT-cAMP (1 mm) and 8-pMeOPT-2'-O-Me-cAMP (0.5 mm), could not reduce fMLP-induced O(2)(*-) production and both PKA inhibitors, PKA inhibitor 14-22 amide myristoylated (50 nm-1 microm) and H-89 (100 nm-1 microm), did not affect the decrease of O(2)(*-) release induced by PDE4 inhibitors in BAL cells enriched in neutrophils. These data suggest that PDE4 inhibitors decreased fMLP-induced O(2)(*-) release in BAL cells enriched in neutrophils but not in macrophages, through p44/42(MAPK) activation by a cAMP- and a PKA-independent mechanism.

PDE4D plays a critical role in the control of airway smooth muscle contraction

The airways of mice deficient in the cAMP phosphodiesterase PDE4D gene are refractory to muscarinic cholinergic stimulation. This study was undertaken to determine whether altered smooth muscle contractility causes the PDE4D-/- phenotype. A major disruption in contractility was observed in isolated PDE4D-/- tracheas, with a 60% reduction in maximal tension and a fivefold decrease in sensitivity to muscarinic cholinergic agonists. Conversely, responses to KCl or arginine vasopressin were unaffected. PDE4D is the predominant PDE4 form in tracheal extracts and PDE4D mRNA is expressed in smooth muscle where muscarinic binding sites are most abundant. Cyclic AMP accumulation in response to acute G(s)alpha-coupled receptor stimulation was increased up to fourfold in the airway of PDE4D-/- mice when compared with wild-type. This increase in cAMP was associated with an increased sensitivity to PGE2-induced relaxation of the PDE4D-/-tracheas. Furthermore, a blockade of prostanoid accumulation in PDE4D-/- tracheas restored the response to muscarinic cholinergic stimulation in vitro and in vivo. These results demonstrate that PDE4D plays a key role in balancing relaxant and contracting cues in airway smooth muscle, suggesting that natural mutations in the PDE4D gene have profound effects on airway tone.

IGF-I-induced differentiation of L6 myogenic cells requires the activity of cAMP-phosphodiesterase

Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished myogenic differentiation induced by low-serum medium and IGF-I. L6-C5 cells cultured in low-serum medium displayed a PDE4 activity higher than cells cultured in serum-free medium, a condition not sufficient to induce differentiation. In the presence of serum, PDE4D3, the major isoform natively expressed in L6-C5 cells, translocated to a Triton-insoluble fraction, which increased the PDE specific activity of the fraction, and exhibited a Mr shift typical of phosphorylation of this isoform. Furthermore, serum promoted the localization of PDE4D3 to a vesicular subcellular compartment. In L6-C5 cells, IGF-I is a stronger inducer of myogenic differentiation in the presence than in absence of serum. Its ability to trigger differentiation in the absence of serum was restored by overexpressing wild-type PDE4D3, but not a phosphorylation-insensitive mutant. This finding was confirmed in single cells overexpressing a GFP-PDE4D3 fusion protein by assessing nuclear accumulation of myogenin in both L6-C5 and L6-E9. Overexpression of other PDE isoforms was less efficient, confirming that PDE4D3 is the physiologically relevant phosphodiesterase isoform in the control of myogenesis. These results show that downregulation of cAMP signaling through cAMP-phosphodiesterase stimulation is a prerequisite for induction of myogenesis.

Molecular cloning and subcellular distribution of the novel PDE4B4 cAMP-specific phosphodiesterase isoform

We have isolated cDNAs encoding PDE4B4, a new cAMP-specific phosphodiesterase (PDE4) isoform with novel properties. The amino acid sequence of PDE4B4 demonstrates that it is encoded by the PDE4B gene, but that it differs from the previously isolated PDE4B1, PDE4B2 and PDE4B3 isoforms by the presence of a novel N-terminal region of 17 amino acids. PDE4B4 contains both of the upstream conserved region 1 (UCR1) and UCR2 regulatory units that are characteristic of 'long' PDE4 isoforms. RNase protection demonstrated that PDE4B4 mRNA is expressed preferentially in liver, skeletal muscle and various regions of the brain, which differs from the pattern of tissue distribution of the other known PDE4B long forms, PDE4B1 and PDE4B3. Expression of PDE4B4 cDNA in COS7 cells produced a protein of 85 kDa under denaturing conditions. Subcellular fractionation of recombinant, COS7-cell expressed PDE4B4 showed that the protein was localized within the cytosol, which was confirmed by confocal microscopic analysis of living COS7 cells transfected with a green fluorescent protein-PDE4B4 chimaera. PDE4B4 exhibited a K(m) for cAMP of 5.4 microM and a V(max), relative to that of the long PDE4B1 isoform, of 2.1. PDE4B4 was inhibited by the prototypical PDE4 inhibitor rolipram [4-[3-(cyclopentoxyl)-4-methoxyphenyl]-2-pyrrolidinone] with an IC(50) of 83 nM. Treatment of COS7 cells with forskolin, to elevate cAMP levels, produced activation of PDE4B4, which was associated with the phosphorylation of PDE4B4 on Ser-56 within UCR1. The unique tissue distribution and intracellular targeting of PDE4B4 suggests that this isoform may have a distinct functional role in regulating cAMP levels in specific cell types.

Deletion of phosphodiesterase 4D in mice shortens alpha(2)-adrenoceptor-mediated anesthesia, a behavioral correlate of emesis

A combination of pharmacological and genetic approaches was used to determine the role of type 4 cAMP-specific cyclic nucleotide phosphodiesterase 4 (PDE4) in reversing alpha(2)-adrenoceptor-mediated anesthesia, a behavioral correlate of emesis in non-vomiting species. Among the family-specific PDE inhibitors, PDE4 inhibitors reduced the duration of xylazine/ketamine-induced anesthesia in mice, with no effect on pentobarbital-induced anesthesia. The rank order of the PDE4 inhibitors tested was 6-(4-pyridylmethyl)-8-(3-nitrophenyl)quinoline (PMNPQ) > (R)-rolipram > (S)-rolipram >> (R)-N-[4-[1-(3-cyclopentyloxy-4-methoxyphenyl)-2-(4-pyridyl)ethyl]phenyl]N'-ethylurea (CT-2450). The specific roles of PDE4B and PDE4D in this model were studied using mice deficient in either subtype. PDE4D-deficient mice, but not PDE4B-deficient mice, had a shorter sleeping time than their wild-type littermates under xylazine/ketamine-induced anesthesia, but not under that induced with pentobarbital. Concomitantly, rolipram-sensitive PDE activity in the brain stem was decreased only in PDE4D-deficient mice compared with their wild-type littermates. While PMNPQ significantly reduced the xylazine/ketamine-induced anesthesia period in wild-type mice and in PDE4B-null mice, it had no effect in PDE4D-deficient mice. These findings strongly support the hypothesis that inhibition of PDE4D is pivotal to the anesthesia-reversing effect of PMNPQ and is likely responsible for emesis induced by PDE4 inhibitors.

Differential distribution of PDE4D splice variant mRNAs in rat brain suggests association with specific pathways and presynaptical localization

cAMP plays an important role as a second-messenger molecule controlling multiple cellular processes. Its hydrolysis provides an important mechanism by which cAMP levels are regulated. This is performed by a large multigene family of cyclic nucleotide phosphodiesterases (PDEs). Members of the PDE4 enzyme family are selectively inhibited by rolipram. Five different mRNA splice forms for PDE4D have been isolated. Here, we analyzed the regional distribution of the mRNAs coding for the splice variants PDE4D1, PDE4D2, PDE4D3, PDE4D4, and PDE4D5 in the rat brain by in situ hybridization histochemistry using specific radiolabeled oligonucleotides. We found that all five splice variants showed a distinct distribution pattern and, in some cases, in association with specific brain pathways. The most relevant differences were in hippocampal formation, medial habenula, basal ganglia, and area postrema, at both the regional and cellular level. The dorsal and median raphe nuclei exclusively contained PDE4D2 mRNA transcripts, probably located on serotonergic cells. PDE4D1 mRNA was expressed in some white matter cells. PDE4D1 and PDE4D2 mRNA splice forms presented a similar distribution in the area postrema, whereas for PDE4D4 and PDE4D5 the cellular distribution presented a complementary pattern. The differential expression of PDE4D mRNA splice variants in the area postrema is consistent with their possible involvement in emesis control and suggests new molecular targets for a more selective drug design.

Reciprocal regulation of calcium dependent and calcium independent cyclic AMP hydrolysis by protein phosphorylation

The hydrolysis of cyclic nucleotide second messengers takes place through multiple cyclic nucleotide phosphodiesterases (PDEs). The significance of this diversification is not fully understood. Here we report the differential regulation of low K(m) Ca2+-activated (PDE1C) and Ca2+-independent, rolipram-sensitive (PDE4) PDEs by protein phosphorylation in the neuroendocrine cell line AtT20. Incubation of cells with 8-(4-chlorophenylthio)-cyclic AMP (CPT-cAMP) enhanced PDE4 and reduced PDE1C activity. These effects were blocked by H89 indicating mediation by cAMP-dependent protein kinase (PKA), furthermore in broken cell preparations PKA produced the same reciprocal changes of PDE activities. Calyculin A, an inhibitor of protein phosphatases 1 and 2 A, stimulated PDE4 and enhanced the inhibitory effect of CPT-cAMP on PDE1C. The reduction of PDE1C activity was characterized by a marked attenuation of the activation by Ca2+/calmodulin. Stimulation of PDE4 activity by CPT-cAMP or calyculin A was attributable to PDE4D3 and these effects could also be reproduced in human embryonic kidney cells expressing epitope-tagged PDE4D3. Together, these data show reciprocal regulation of PDE1C and PDE4D by PKA, which represents a novel scheme for plasticity in intracellular signalling.

Regulation of cyclic AMP in rat pulmonary microvascular endothelial cells by rolipram-sensitive cyclic AMP phosphodiesterase (PDE4)

We report here studies on the regulation of the metabolism of adenosine 3',5'-monophosphate (cAMP) in established and primary cultures of rat pulmonary microvascular endothelial cells (RPMVEC). Inhibition by rolipram, a selective inhibitor of cAMP phosphodiesterase (PDE) of the PDE4 gene family, was required to achieve maximal cAMP accumulation induced by direct or receptor-mediated adenylate cyclase activation when measured by [3H]-adenine prelabeling. Rolipram increased cAMP accumulation more effectively than did forskolin, isoproterenol, or adenosine derivatives alone, although extensive synergy was seen with combined agents. High-affinity PDE4 inhibitors, but not low-affinity or non-selective inhibitors, were effective inducers of cAMP accumulation in intact cells. The maximum effects (i.e. intrinsic activities) of these agents in the intact cell did not correlate with their in vitro PDE4 inhibitory affinities. RPMVEC were shown to express almost exclusively the PDE4 gene family isoforms A6 and B3. Guanosine 3',5'-monophosphate hydrolysis, observed in other types of endothelial cells was not found in early or late passage RPMVEC. Reverse transcription-polymerase chain reaction identification of mRNAse supported these conclusions with the exception that PDE2 and PDE4D mRNA isoform transcripts were present. These studies also support the conclusion that the mechanism of rolipram reversal of rat lung ischemia-reperfusion-induced permeability involves PDE4 inhibition in the microvascular endothelial cells of the lung.

Cyclic AMP specific phosphodiesterase activity and colon cancer cell motility

To investigate mechanisms for regulation of intracellular cAMP involved in cancer cell invasion, phosphodiesterase (PDE) activity in a colon cancer cell line, DLD-1, was studied. Activities of PDE 2, 4, and 5 were detected in DLD-1 cells by pharmacological approach. Specific and cell permeable inhibitors for those PDEs were used to determine which PDE is responsible for cAMP turnover involved in cancer cell motility. Treatment of DLD-1 cells with rolipram and Ro-20-1724 inhibitors for PDE 4, elevated intracellular cAMP contents three to five times of control. EHNA, an inhibitor for PDE 2, and zaprinast. an inhibitor for PDE 5, did not affect cAMP levels. To assess cellular motility, we utilized chemotaxis assay. EHNA and zaprinast did not suppress serum-induced chemotaxis. In contrast, rolipram and Ro-20-1724, suppressed chemotaxis in a dose dependent fashion. These suggest that PDE 4 plays a critical role in regulating intracellular cAMP levels of colon cancer cells and is involved in cancer invasion. PDE 4 can be a novel target of anti-invasion drug.

Localization of phosphodiesterase-4 isoforms in the medulla and nodose ganglion of the squirrel monkey

Pre-clinical and clinical studies are currently underway to evaluate the potential of phosphodiesterase-4 (PDE4) inhibitors for the treatment of chronic obstructive pulmonary disease and other inflammatory conditions of the airways. The most common side effect associated with this class of compounds is emesis. The squirrel monkey provides a model for evaluating the efficacy of PDE4 inhibitors and their emetic potential. The distribution of three PDE4 isoforms (A, C and D) has been investigated in the squirrel monkey medulla and nodose ganglion to determine which isoform(s) could be responsible for the emetic adverse effects. The distribution of PDE4 isoforms was delineated using immunohistochemistry with antibodies specific for PDE4A, PDE4C and PDE4D and by in situ hybridization with isoform-selective riboprobes. PDE4A was present in the medulla where expression was mostly restricted to glial cells and the vasculature. PDE4C was not detected in either the medulla or nodose ganglion. Finally, the PDE4D isoform was localized to neurons in the nodose ganglion and found through many structures of medulla including the area postrema, neurons of the nucleus tractus solitarius and locus coeruleus. These data are consistent with a role for PDE4D in the emetic response.

Role of phosphodiesterase type 3A in rat oocyte maturation

It is generally accepted that cyclic nucleotides are key signaling molecules in the control of oocyte meiotic resumption. Given the role of phosphodiesterases (PDEs) in cyclic nucleotide degradation, this study was undertaken to investigate the properties and regulation of PDEs expressed in rat oocytes. Cilostamide-sensitive PDE3 was the major activity detected in denuded oocytes, whereas no PDE3 activity could be detected in cumulus cells. Moreover, comparable levels of PDE3 activity were measured in cumulus-oocyte complexes (COCs) and in denuded oocytes. The oocyte PDE was recovered in the soluble fraction of the homogenate and immunoprecipitated with a specific PDE3A antibody. A significant and transient increase (P < 0.05) in PDE3 activity was measured in the oocytes after 30 min of culture (70 min after isolation) compared with immediately after collection (10 min after isolation). Conversely, no changes in activity were observed when denuded oocytes or cumulus cells were incubated for up to 130 min. Evaluation of oocyte maturation indicated that only 10% of oocytes had resumed meiosis at the peak of the PDE3 activity. A significant increase (P < 0.05) in PDE3 activity was measured in COCs when follicle-enclosed oocytes were cultured in the presence of hCG. Again, this increase preceded oocyte maturation. In conclusion, these data demonstrate that PDE3A is the major PDE form expressed in mammalian oocytes. PDE3A activity increases prior to resumption of meiosis in both spontaneous and gonadotropin-stimulated maturation. These findings strongly support the hypothesis that an increase in oocyte PDE3A activity is one of the intraoocyte mechanisms controlling resumption of meiosis in rat oocytes, at least in vitro.

Cyclic nucleotide phosphodiesterases

Cyclic nucleotide second messengers (cAMP and cGMP) play a central role in signal transduction and regulation of physiologic responses. Their intracellular levels are controlled by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Continuing advances in our understanding of the molecular pharmacology of these enzymes has led to the development of selective inhibitors as therapeutic agents for disease states ranging from cancer and heart failure to depression and sexual dysfunction. Several PDE types have been identified as therapeutic targets for immune/inflammatory diseases. This article briefly reviews the available in vitro, preclinical, and clinical data supporting the potential for selective PDE inhibitors as immunomodulatory agents.

Phosphodiesterase 4D and protein kinase a type II constitute a signaling unit in the centrosomal area

The mediation of cAMP effects by specific pools of protein kinase A (PKA) targeted to distinct subcellular domains raises the question of how inactivation of the cAMP signal is achieved locally and whether similar targeting of phosphodiesterases (PDEs) to sites of cAMP/PKA action could be observed. Here, we demonstrate that Sertoli cells of the testis contain an insoluble PDE4D3 isoform, which is shown by immunofluorescence to target to centrosomes. Staining of PDE4D and PKA shows co-localization of PDE4D with PKA-RIIalpha and RIIbeta in the centrosomal region. Co-precipitation of RII subunits and PDE4D3 from cytoskeletal extracts indicates a physical association of the two proteins. Distribution of PDE4D overlaps with that of the centrosomal PKA-anchoring protein, AKAP450, and AKAP450, PDE4D3, and PKA-RIIalpha co-immunoprecipitate. Finally, both PDE4D3 and PKA co-precipitate with a soluble fragment of AKAP450 encompassing amino acids 1710 to 2872 when co-expressed in 293T cells. Thus, a centrosomal complex that includes PDE4D and PKA constitutes a novel signaling unit that may provide accurate spatio-temporal modulation of cAMP signals.

Myomegalin is a novel protein of the golgi/centrosome that interacts with a cyclic nucleotide phosphodiesterase

Subcellular targeting of the components of the cAMP-dependent pathway is thought to be essential for intracellular signaling. Here we have identified a novel protein, named myomegalin, that interacts with the cyclic nucleotide phosphodiesterase PDE4D, thereby targeting it to particulate structures. Myomegalin is a large 2,324-amino acid protein mostly composed of alpha-helical and coiled-coil structures, with domains shared with microtubule-associated proteins, and a leucine zipper identical to that found in the Drosophila centrosomin. Transcripts of 7.5-8 kilobases were present in most tissues, whereas a short mRNA of 2.4 kilobases was detected only in rat testis. A third splicing variant was expressed predominantly in rat heart. Antibodies against the deduced sequence recognized particulate myomegalin proteins of 62 kDa in testis and 230-250 kDa in heart and skeletal muscle. Immunocytochemistry and transfection studies demonstrate colocalization of PDE4D and myomegalin in the Golgi/centrosomal area of cultured cells, and in sarcomeric structures of skeletal muscle. Myomegalin expressed in COS-7 cells coimmunoprecipitated with PDE4D3 and sequestered it to particulate structures. These findings indicate that myomegalin is a novel protein that functions as an anchor to localize components of the cAMP-dependent pathway to the Golgi/centrosomal region of the cell.

Increased cGMP phosphodiesterase activity mediates renal resistance to ANP in rats with bile duct ligation

BACKGROUND

Liver disease resulting from common bile duct ligation (CBDL) causes abnormal sodium metabolism that is manifested by resistance to the natriuretic action of atrial natriuretic peptide (ANP). This resistance is corrected both in vitro and in vivo by zaprinast, a selective inhibitor of a guanosine cyclic-3'-5'-monophosphate (cGMP)-specific phosphodiesterase (PDE5). Several other PDEs with affinity for cGMP are expressed in kidney and could also be involved in this response.

METHODS

We measured cGMP hydrolysis in inner medullary collecting duct (IMCD) cell homogenates from kidneys of sham-operated and CBDL rats and quantitated the amount of PDE5 protein by Western blotting and immunoprecipitation studies. We also characterized ANP responsiveness in vivo of kidneys of anesthetized sham and CBDL rats by measuring sodium excretion before and after volume expansion (VE).

RESULTS

Kinetic analysis of PDE5 activity in homogenates of IMCD cells isolated from kidneys of sham-operated rats indicated a Vmax of 85.3 +/- 1.7 versus 157 +/- 2.9 pmol/mg/min from CBDL rats (P < 0.01), without a difference in Km. Enzyme activity was inhibited competitively by 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazol[3,4d]-pyrimidin-4-(5H)-one (DMPPO), a potent and specific inhibitor of PDE5, with an apparent Ki of 4.5 +/- 0.7 and 4.9 +/- 0.7 nmol/L and an IC50 of 6.1 +/- 0.8 and 8.7 +/- 0.7 nmol/L in sham and CBDL rats, respectively (P = NS). DMPPO exhibited very poor inhibitory activity against the calcium-calmodulin-dependent PDE1 in IMCD homogenates from sham rats (Ki 1.3 +/- 0.1 micromol/L and IC50 1.9 +/- 0.2 micromol/L). Western analysis using an antiserum made against bovine lung PDE5 revealed a twofold increase in PDE5 protein in cytosolic extracts from IMCD of CBDL rat kidneys compared with sham-operated controls, and immunoprecipitation studies indicated that the increase in PDE5 protein accounted for the observed increase in cGMP hydrolysis. DMPPO (10 nmol/L) normalized the blunted ANP-dependent cGMP accumulation by IMCD cells from CBDL rats in vitro. Intrarenal infusion of DMPPO (0.5 nmol/min) in CBDL rats corrected both the impaired natriuretic response to VE and the blunted VE-related increase in urinary cGMP excretion from the infused, but not the contralateral kidney.

CONCLUSION

These results demonstrate that renal resistance to ANP in CBDL rats is accompanied by heightened activity of PDE5, which is due largely to an increase in PDE5 protein. Other PDEs could contribute only a minor part to the enhanced cGMP hydrolysis observed in kidneys of CBDL rats. This PDE5-dependent ANP resistance may represent an important contributor to the sodium retention of liver disease.

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.

Noradrenergic lesions differentially alter the expression of two subtypes of low Km cAMP-sensitive phosphodiesterase type 4 (PDE4A and PDE4B) in rat brain

This study examined the effects of selective, central noradrenergic dennervation with 6-hydroxydopamine (6-OHDA) on the expression of type 4 phosphodiesterases (PDE4). Twenty-one days following i.c.v. injection of 6-OHDA (200 microg) hypothalamus, neostriatum, and cerebellum were dissected. Infusion of 6-OHDA reduced norepinephrine (NE) content in all the brain areas examined (to 17%, 76% and 16% of sham-operated controls in hypothalamus, striatum, and cerebellum, respectively). 6-OHDA injections also reduced dopamine levels in hypothalamus (53%) and neostriatum (68%). Administration of desipramine (20 mg/kg, i.p.) 30 min prior to 6-OHDA injection protected neostriatal and cerebellar noradrenergic neurons NE levels (110-122% of the control levels). Desipramine partially attenuated the 6-OHDA-mediated decrease in NE content of hypothalamus, but had little or no effect on either striatal or hypothalamic dopamine (DA) levels. Western blot analysis using a PDE4A-selective antibody revealed three major bands (109 kDa PDE4A5, 102 kDa PDE4AX and 76 kDa PDE4A1) in hypothalamus and striatum. Infusion of 6-OHDA decreased the expression of PDE4A5 and PDE4AX but not of PDE4A1 in hypothalamus, as determined by quantitative Western blotting. Pretreatment of rats with desipramine attenuated the 6-OHDA-induced down-regulation of PDE4A5 and PDE4AX bands in hypothalamus. The PDE4B selective antibody K118 labels 5 major bands in all the brain regions studied. One hundred kDa PDE4B3, 86 kDa PDE4B2 and a 78 kDa PDE4B band was identified using recombinant proteins. Treatment of rats with 6-OHDA resulted in a 52% decrease in the PDE4B3 and 58% decrease in 78 kDa PDE4B variant in hypothalamus; administration of desipramine attenuated the 6-OHDA-induced down-regulation of both PDE4B variants. Neither 6-OHDA nor desipramine altered striatal PDE4A or PDE4B isozymes. In contrast, cerebellar PDE4B3 variant is up-regulated by 6-OHDA treatment and were partially normalized to control values by desipramine pretreatment. These data demonstrate that PDE4 subtypes are differentially regulated by presynaptic noradrenergic activity and may play an important role in the maintaining homeostasis of noradrenergic signal transduction in rat brain.

Absence of muscarinic cholinergic airway responses in mice deficient in the cyclic nucleotide phosphodiesterase PDE4D

Muscarinic cholinergic signaling plays an essential role in the control of the normal airway functions and in the development of pulmonary pathologies including asthma. In this paper we demonstrate that the airways of mice deficient in a cAMP-specific phosphodiesterase (PDE4D) are no longer responsive to cholinergic stimulation. Airway hyperreactivity that follows exposure to antigen was also abolished in PDE4D(-/-) mice, despite an apparently normal lung inflammatory infiltration. The loss of cholinergic responsiveness was specific to the airway, not observed in the heart, and was associated with a loss of signaling through muscarinic receptors with an inability to decrease cAMP accumulation. These findings demonstrate that the PDE4D gene plays an essential role in cAMP homeostasis and cholinergic stimulation of the airway, and in the development of hyperreactivity. In view of the therapeutic potentials of PDE4 inhibitors, our findings provide the rationale for novel strategies that target a single PDE isoenzyme.

Conformational difference between PDE4 apoenzyme and holoenzyme

The type 4 cAMP-specific phosphodiesterases (PDE4s) are Mg(2+)-dependent hydrolases that catalyze the hydrolysis of 3', 5'-cAMP to AMP. Previous studies indicate that PDE4 exists in two conformations that bind the inhibitor rolipram with affinities differing by more than 100-fold. Here we report that these two conformations are the consequence of PDE4 binding to its metal cofactor such as Mg(2+). Using a fluorescence resonance energy transfer (FRET)-based equilibrium binding assay, we identified that L-791,760, a fluorescent inhibitor, binds to the apoenzyme (free enzyme) and the holoenzyme (enzyme bound to Mg(2+)) with comparable affinities (K(d) approximately 30 nM). By measuring the displacement of the bound L-791,760, we have also identified that other inhibitors bind differentially with the apoenzyme and the holoenzyme depending upon their structure. CDP-840, SB-207499, and RP-73401 bind preferentially to the holoenzyme. The conformational-sensitive inhibitor (R)-rolipram binds to the holoenzyme and apoenzyme with affinities (K(d)) of 5 and 300 nM, respectively. In contrast to its high affinity (K(d) approximately 2 microM) and active holoenzyme complex, cAMP binds to the apoenzyme nonproductively with a reduced affinity (K(d) approximately 170 microM). These results demonstrate that cofactor binding to PDE4 is responsible for eliciting its high-affinity interaction with cAMP and the activation of catalysis.

Short term feedback regulation of cAMP in FRTL-5 thyroid cells. Role of PDE4D3 phosphodiesterase activation

Together with a transient accumulation of intracellular cAMP, thyrotropin (TSH) stimulation of the FRTL-5 thyroid cell induces phosphorylation and activation of a cAMP-specific phosphodiesterase (PDE4D3). Here we have investigated the impact of PDE4D3 activation on hormone responsiveness. Stimulation of FRTL-5 cells with TSH caused an increase in PDE activity within 3 min, with a maximal stimulation reached after 5 min. Preincubation with the protein kinase A (PKA) inhibitor H89 or (R(p))-cAMPS, but not with the inactive isomer H85, blocked this activation. Preincubation with PKA inhibitors also blocked the shift in mobility of the PDE4D3 protein. Under these conditions, H89, but not H85, potentiated the cAMP accumulation induced by TSH. Incubation of FRTL-5 cells with the PKA activator 8-(4-chlorophenylthio)adenosine-cAMP caused an increase in PDE activity and a decrease in the endogenous cAMP, confirming the presence of a PKA-PDE feedback loop. MA-10 Leydig tumor cells stably transfected with either a wild type PDE4D3 or a PDE4D3 with mutations in the PKA phosphorylation sites showed an increase in PDE activity when compared with control cells. Human choriogonadotropin or Bt(2)cAMP treatment induced a stimulation of PDE activity in cells transfected with wild type PDE4D3, whereas the activation was absent in mutant- and control-transfected cells. The increase in cAMP accumulation elicited by human choriogonadotropin was reduced in cells transfected with the wild type PDE4D3, but not in cells transfected with the mutant PDE. Rolipram, a specific inhibitor of PDE4, restored the cAMP accumulation in the PDE4D3-transfected cells. These data provide evidence that a rapid activation of PDE4D3 is one of the mechanisms determining the intensity of the cAMP signal.

Phosphodiesterase 4 (PDE4) inhibitors in asthma and chronic obstructive pulmonary disease (COPD)

Phosphodiesterases (PDE) are a family of enzymes responsible for the metabolism of the intracellular second messengers cyclic AMP and cyclic GMP. PDE4 is a cyclic AMP specific PDE that is the major if not sole cyclic AMP metabolizing enzymes found in inflammatory and immune cells, and contributes significantly to cyclic AMP metabolism in smooth muscles. Based on its cellular and tissue distribution and the demonstration that selective inhibitors of this isozyme reduce bronchoconstriction in animals and suppress the activation of inflammatory cells, PDE4 has become an important molecular target for the development of novel therapies for asthma and COPD. This chapter will review the evidence demonstrating the ability of PDE4 inhibitors to modify airway obstruction, airway inflammation and airway remodelling and hyperreactivity, will present some preliminary findings obtained with theses compounds in clinical trials and and will discuss experimental approaches designed to identify novel compounds that maintain the beneficial activity of the initial selective PDE4 inhibitors but with a reduced tendency of elicit the gastrointestinal side effects observed with this class of compounds.

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.

Characterization of cyclic nucleotide phosphodiesterase isoforms associated to isolated cardiac nuclei

The identity and location of nuclear cyclic nucleotide phosphodiesterases (PDE) has yet to be ascertained. Intact cardiac nuclei and subnuclear fractions from ovine hearts were isolated to determine cAMP-specific PDE activity which was 3-fold greater than that of cGMP PDE, the latter being insensitive to Ca-calmodulin and zaprinast. Specific hydrolytic activities of the cardiac nuclear envelopes (NE) were similar to those measured in the corresponding intact nuclei, thus suggesting that most PDE activity is associated with the nuclear membrane. Moreover, the main hydrolytic activities in cardiac nuclei were attributed to PDE4 (56%) and PDE3 (44%). The pharmacological sensitivity of each isoform in terms of IC(50), K(m) and K(i) values was typical of previously characterized cardiac PDE 3 and 4 isoforms. PDE2 (cGMP-stimulated PDE) represented a minor component (8-9%) of total hydrolytic activity. Solubilization of nuclear envelopes and HPLC separation also yielded rolipram-sensitive PDE activities. Upon 1% Triton X-100 extractions, the presence of PDE3 and PDE4 was revealed in a low speed, nucleopore complex-enriched, P1 pellet. In addition, Western blot analysis demonstrated the presence of PDE4B and PDE4D subtypes in the nuclei as well as enrichment in NE. However, in the same preparations, the presence of PDE4A could not be ascertained. Altogether, these results suggest an intrinsic and predominant association of these nuclear PDEs with the NE and much likely with nucleopore complexes.

Impaired growth and fertility of cAMP-specific phosphodiesterase PDE4D-deficient mice

In eukaryotic cells, the inactivation of the cyclic nucleotide signal depends on a complex array of cyclic nucleotide phosphodiesterases (PDEs). Although it has been established that multiple PDE isoenzymes with distinct catalytic properties and regulations coexist in the same cell, the physiological significance of this remarkable complexity is poorly understood. To examine the role of a PDE in cAMP signaling in vivo, we have inactivated the type 4 cAMP-specific PDE (PDE4D) gene, a mammalian homologue of the Drosophila dunce. This isoenzyme is involved in feedback regulation of cAMP levels. Mice deficient in PDE4D exhibit delayed growth as well as reduced viability and female fertility. The decrease in fertility of the null female is caused by impaired ovulation and diminished sensitivity of the granulosa cells to gonadotropins. These pleiotropic phenotypes demonstrate that PDE4D plays a critical role in cAMP signaling and that the activity of this isoenzyme is required for the regulation of growth and fertility.

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 noradrenergic lesions on the development of rolipram-sensitive, low-K(m), cyclic AMP specific phosphodiesterase in rat brain

Rolipram-sensitive, low-K(m)80% loss of norepinephrine in cerebral cortex) without affecting dopaminergic systems. The lesions resulted in temporary reduction of PDE4 activity in cerebral cortex, cerebellum and brainstem. Lesions in the adult rats, on the other hand, did not alter PDE4 activity. Decreased PDE4 activity by neonatal noradrenergic lesions was due to a decrease in the V(max) of cAMP hydrolysis by PDE4, and not a change in the K(m) values. Immunoblot analysis showed that decreased PDE4 activity in cerebellum was associated with reduced expression of PDE4A5, PDE4A1, and several PDE4B variants. No change in the expression of any PDE4 subtype in cerebral cortex was detected with the antibodies used in this study. Neither the permanent loss of noradrenergic innervation in cerebral cortex, nor the permanent noradrenergic hyperinnervation in brainstem was accompanied by any permanent change in PDE4 activity. Decreasing PDE4 activity early after neonatal noradrenergic lesions might be important in maintaining constant concentrations of cAMP, which is critical for the cellular proliferation and differentiation that is active during this period.

Novel strategies for pharmacotherapy of depression

Modulating monoamine activity as a therapeutic strategy continues to dominate antidepressant research, with a recent emphasis on agents with multiple targets, including combined serotonin/noradrenaline re-uptake inhibitors and numerous serotonin receptor ligands. An important new development has been the emergence of potential novel mechanisms of action, notably modulation of the activity of neuropeptides substance P and corticotrophin-releasing factor, and the intracellular messenger cyclic adenosine monophosphate. Efforts in this area have recently been rewarded by the demonstration of antidepressant efficacy of the substance P receptor antagonist MK-0869.

Phosphodiesterase 4 inhibitors as novel anti-inflammatory agents

Preclinical and clinical studies of phosphodiesterase 4 inhibitors have shown that these agents may find utility in a wide range of inflammatory disorders, including asthma, chronic obstructive pulmonary disease, atopic dermatitis, rheumatoid arthritis, multiple sclerosis and various neurological disorders. The future of this class of drugs will depend upon the ability to demonstrate a reasonable safety margin against emesis and other typical phosphodieserase (PDE4) side effects, as well as in identification of the inflammatory disorder(s) most relevant to PDE4 inhibition.

Activation of the cAMP-specific phosphodiesterase PDE4D3 by phosphorylation. Identification and function of an inhibitory domain

Splicing variants of type 4 phosphodiesterases (PDE4) are regulated by phosphorylation. In these proteins, a conserved region is located between the amino-terminal domain, which is the target for phosphorylation, and the catalytic domain. Previous studies have indicated that nested deletions encompassing this region cause an increase in catalytic activity, suggesting this domain exerts an inhibitory constraint on catalysis. Here, we have further investigated the presence and function of this domain. A time-dependent increase in hydrolytic activity was observed when PDE4D3 from FRTL-5 cells was incubated with the endoproteinase Lys-C. The activation was abolished by protease inhibitors and was absent when a phosphorylated enzyme was used. Western blot analysis with PDE4D-specific antibodies indicated the Lys-C treatment separates the catalytic domain of PDE4D3 from the inhibitory domain. Incubation with antibodies recognizing an epitope within this domain caused a 3- to 4-fold increase in activity of native or recombinant PDE4D3. Again, PDE activation by these antibodies had properties similar to, and not additive with, the activation by protein kinase A phosphorylation. An interaction between the inhibitory domain and both regulatory and catalytic domains of PDE4D3 was detected by the yeast two-hybrid system. Mutations of Ser54 to Ala in the regulatory domain decreased or abolished this interaction, whereas mutations of Ser54 to the negatively charged Asp strengthened it. These data strongly support the hypothesis that an inhibitory domain is present in PDE4D and that phosphorylation of the regulatory domain causes activation of the enzyme by modulating the interaction between inhibitory and catalytic domains.

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.

Type 4 cyclic adenosine monophosphate-specific phosphodiesterases are expressed in discrete subcellular compartments during rat spermiogenesis

The type 4 cAMP-specific phosphodiesterases (PDE4) are a family of closely related enzymes with similar catalytic domains and divergent amino- and carboxyl-terminus domains. Multiple PDE proteins with heterogeneous amino termini are derived from each gene. To understand the significance of this heterogeneity, the expression and localization of variants derived from PDE4A and PDE4D genes was investigated during spermatogenesis in the rat. RNase protection analysis with mRNA for testes at different ages of development showed that two transcripts (PDE4D1 and PDE4D2) are expressed at day 10 and 15 of age and become undetectable thereafter. An additional PDE4D transcript appears at day 30 and increased during testid maturation. This latter transcript codes for a long variant of the PDE4D gene and is expressed in germ cells as demonstrated by RNase protection with RNA from isolated pachytene spermatocytes and round spermatids. The presence of a corresponding PDE4D protein with a molecular mass of 98 kDa was established by immunoprecipitation and Western blot analysis with antibodies specific for PDE4D and by immunoaffinity chromatography purification of the 98 kDa variant from isolated germ cells. PDE4A transcripts were also expressed in pachytene spermatocytes and round spermatids. Two polypeptides encoded by these PDE4A transcripts were expressed in pachytene spermatocytes, reached a maximum in round spermatids, and declined thereafter. Immunofluorescence analysis demonstrated a localization of the PDE4D protein in the manchette and in a periacrosomal region of the developing spermatid, a localization confirmed by immunogold electron microscopy. Conversely, the PDE4A was mostly soluble in the cytoplasm of round spermatids. These data demonstrate that PDE4D and PDE4A variants are expressed at different stages and localized in distinct subcellular structures of developing spermatids. Different properties of the mRNAs derived from the two genes and localization signals are responsible for the temporal and spatial expression of the different PDE4 isoenzymes.

Chronic antidepressant administration increases the expression of cAMP-specific phosphodiesterase 4A and 4B isoforms

The influence of chronic antidepressant administration on expression of the three major phosphodiesterase (PDE) 4 subtypes found in brain (PDE4A, PDE4B, and PDE4D) was examined. The treatments tested included representatives of four major classes of antidepressants: selective reuptake inhibitors of serotonin (sertraline and fluoxetine) or norepinephrine (desipramine), a monoamine oxidase inhibitor (tranylcypromine), and electroconvulsive seizure. Expression of PDE4A and PDE4B, but not PDE4D, mRNA and immunoreactivity were significantly increased in rat frontal cortex by chronic administration of each of the four classes of antidepressants. We also found that antidepressant administration significantly increased the expression of PDE4B mRNA in the nucleus accumbens, a brain region thought to mediate pleasure and reward that could also contribute to the anhedonia often observed in depressed patients. In contrast, expression of PDE4A and PDE4B were not influenced by short-term treatment (1 or 7 d) and were not influenced by chronic administration of nonantidepressant psychotropic drugs (cocaine or haloperidol), demonstrating the time dependence and pharmacological specificity of these effects. Upregulation of PDE4A and PDE4B may represent a compensatory response to antidepressant treatment and activation of the cAMP system. The possibility that targeted inhibition of these PDE4 subtypes may produce an antidepressant effect is discussed.

Ontogeny of rolipram-sensitive, low-K(m), cyclic AMP-specific phosphodiesterase in rat brain

The postnatal development of rolipram-sensitive, low-K(m), cyclic AMP-specific phosphodiesterase (PDE4) was investigated in discrete regions of rat brain using a PDE4 activity assay and immunoblot analyses with K116, a PDE4 antibody. The Vmax for cyclic AMP hydrolysis by PDE4 was lower at birth when compared to adult levels in cerebral cortex, cerebellum, and neostriatum. K(m) values for cyclic AMP hydrolysis by PDE4, in contrast, did not change throughout the observed period in any brain region tested. The developmental patterns for PDE4 were significantly different among the examined brain regions. PDE4 activity in olfactory bulb and hippocampus also was found to be lower at birth in comparison to adult levels. Immunoblot analyses showed that developmental patterns of PDE4 were significantly different for the various subtypes, and also varied substantially across brain regions. The results suggest that PDE4 might be differentially regulated by different ontogenetic events.

Release of cAMP gating by the alpha6beta4 integrin stimulates lamellae formation and the chemotactic migration of invasive carcinoma cells

The alpha6beta4 integrin promotes carcinoma in-vasion by its activation of a phosphoinositide 3-OH (PI3-K) signaling pathway (Shaw, L.M., I. Rabinovitz, H.H.-F. Wang, A. Toker, and A.M. Mercurio. Cell. 91: 949-960). We demonstrate here using MDA-MB-435 breast carcinoma cells that alpha6beta4 stimulates chemotactic migration, a key component of invasion, but that it has no influence on haptotaxis. Stimulation of chemotaxis by alpha6beta4 expression was observed in response to either lysophosphatidic acid (LPA) or fibroblast conditioned medium. Moreover, the LPA-dependent formation of lamellae in these cells is dependent upon alpha6beta4 expression. Both lamellae formation and chemotactic migration are inhibited or "gated" by cAMP and our results reveal that a critical function of alpha6beta4 is to suppress the intracellular cAMP concentration by increasing the activity of a rolipram-sensitive, cAMP-specific phosphodiesterase (PDE). This PDE activity is essential for lamellae formation, chemotactic migration and invasion based on data obtained with PDE inhibitors. Although PI3-K and cAMP-specific PDE activities are both required to promote lamellae formation and chemotactic migration, our data indicate that they are components of distinct signaling pathways. The essence of our findings is that alpha6beta4 stimulates the chemotactic migration of carcinoma cells through its ability to influence key signaling events that underlie this critical component of carcinoma invasion.

Subcellular localization of rolipram-sensitive, cAMP-specific phosphodiesterases. Differential targeting and activation of the splicing variants derived from the PDE4D gene

Biochemical and immunofluorescence analyses revealed that phosphodiesterase variants encoded by the PDE4D gene are targeted to discrete subcellular structures. In quiescent FRTL-5 thyroid cells, the rolipram-sensitive phosphodiesterase (PDE) activity (cAMP-PDE) was recovered both in the soluble and particulate fractions of the homogenate. Although an immunoreactive 93-kDa PDE (PDE4D3) variant was recovered in both compartments, a 105-kDa variant with the properties of PDE4D4 was recovered mostly in the particulate fraction. The PDE4D3 form was readily solubilized with nonionic detergents. Conversely, the PDE4D4 form required buffers containing ionic detergents for extraction, suggesting that different mechanisms target these variants to insoluble structures. A 15-min stimulation with thyroid-stimulating hormone (TSH) led to an activation of the cAMP-PDE in both compartments and was correlated with a shift in electrophoretic mobility of the PDE4D3 polypeptide. Long term incubation with TSH caused an increase of the PDE activity in the soluble fraction and the appearance of a 68-kDa immunoreactive polypeptide with the properties of PDE4D2. Immunofluorescence analysis showed, in addition to diffuse staining, a signal localized on regions adjacent to the plasma membrane on cytoskeletal structures and in a perinuclear region of quiescent cells. Long term incubation with TSH caused an increase in the immunofluorescence signal in the soluble compartment. These data demonstrate that three PDE4D splicing variants are targeted to discrete subcellular compartments and that hormones cause the activation of these isoforms in a temporally and spatially dependent manner.