Regulation of cGMP-specific phosphodiesterase (PDE5) phosphorylation in smooth muscle cells

Phosphodiesterase type 5 as a target for the treatment of hypoxia-induced pulmonary hypertension

BACKGROUND

Phosphodiesterase type 5 (PDE5) is a novel therapeutic target for the treatment of pulmonary hypertension. This study examined the distribution of PDE5 in normal and hypoxic lung and the effect of chronic PDE5 inhibition with sildenafil, initiated before and during exposure to hypoxia, on pulmonary artery pressure (PAP) and structure.

METHODS AND RESULTS

Sprague-Dawley rats were exposed to hypoxia (10% O2) for up to 42 days. PAP, measured continuously by telemetry, increased gradually by 20 to 40 mm Hg, reaching a plateau between 10 and 14 days, and declined to normal levels on return to normoxia. PDE5 immunoreactivity was localized to smooth muscle cells in the medial layer of pulmonary arteries and veins in the normal lung and in distal muscularized arteries (<25 microm diameter) after hypoxia-induced pulmonary hypertension. Sildenafil (25 or 75 mg x kg(-1) x d(-1)) given before hypoxia produced marked dose-dependent inhibition in the rise of PAP (60% to 90% reduction; P<0.0001) and vascular muscularization (28.4+/-5.0% reduction; P<0.001). When begun after 14 days of hypoxia, sildenafil significantly reduced PAP (30% reduction; P<0.0001) and partially reversed pulmonary artery muscularization (39.9+/-4.9% reduction; P<0.001).

CONCLUSIONS

PDE5 is found throughout the muscularized pulmonary vascular tree, including in newly muscularized distal pulmonary arteries exposed to hypoxia. PDE5 inhibition attenuates the rise in PAP and vascular remodeling when given before chronic exposure to hypoxia and when administered as a treatment during ongoing hypoxia-induced pulmonary hypertension.

cGMP-dependent protein kinase protects cGMP from hydrolysis by phosphodiesterase-5

The physiological effects of cGMP are largely determined by the activities of intracellular receptors, including cGMP-dependent protein kinase (PKG) and cGMP-binding cyclic nucleotide phosphodiesterases (PDEs), and the distribution of cGMP among these receptors dictates activity of the signalling pathway. In the present study, the effects of PKG-Ialpha or PKG-Ibeta on the rate of cGMP hydrolysis by the isolated PDE5 catalytic domain were examined. PKG-Ialpha strongly inhibited cGMP hydrolysis with an IC(50) value of 217 nM, which is similar to the physiological concentration of PKG in pig coronary artery reported previously. By contrast, PKG-Ibeta, which has lower affinity for cGMP than does PKG-Ialpha, inhibited cGMP hydrolysis with an IC(50) of approx. 1 microM. Inhibition by PKG-Ialpha was more effective than that by PKG-Ibeta, consistent with their relative affinities for cGMP. Autophosphorylation of PKGs increased their cGMP-binding affinities and their inhibitory effects on PDE5 hydrolysis of cGMP. Autophosphorylation of PKG-Ibeta increased its inhibitory potency on PDE5 hydrolysis of cGMP by 10-fold compared with a 2-fold increase upon autophosphorylation of PKG-Ialpha. The results indicate that cGMP bound to allosteric cGMP-binding sites of PKG is protected from hydrolysis by PDE5 and that persistent protection of cGMP by either non-phosphorylated or autophosphorylated PKGs may be a positive-feedback control to sustain cGMP signalling.

[3H]sildenafil binding to phosphodiesterase-5 is specific, kinetically heterogeneous, and stimulated by cGMP

Sildenafil (Viagra) potentiates penile erection by acting as a nonhydrolyzable analog of cGMP and competing with this nucleotide for catalysis by phosphodiesterase-5 (PDE5), but the characteristics of direct binding of radiolabeled sildenafil to PDE5 have not been determined. [3H]Sildenafil binding to PDE5 was retained when filtered through nitrocellulose or glass-fiber membranes. Binding was inhibited by excess sildenafil, 2-(2-methylpyridin-4-yl)methyl-4-(3,4,5-trimethoxyphenyl)-8-(pyrimidin-2-yl)methoxy-1,2-dihydro-1-oxo-2,7-naphthyridine-3-carboxylic acid methyl ester hydrochloride (T-0156), 3-isobutyl-1-methylxanthine, EDTA, or cGMP, but not by cAMP or 5'-GMP. PDE5 was the only [3H]sildenafil binding protein detected in human lung extract. Using purified recombinant PDE5, [3H]sildenafil exchange dissociation yielded two components with t1/2 values of 1 and 14 min and corresponding calculated KD values of 12 and 0.83 nM, respectively. This implied the existence of two conformers of the PDE5 catalytic site. [3H]Sildenafil binding isotherm of PDE5 indicated KD was 8.3 to 13.3 nM, and low cGMP decreased the KD to 4.8 nM but only slightly increased Bmax to a maximum of 0.61 mol/mol-subunit. Results suggest that these effects occur via cGMP binding to the allosteric cGMP binding sites of PDE5. Results imply that by inhibiting PDE5 and thereby increasing cGMP, sildenafil accentuates its own binding affinity for PDE5, which further elevates cGMP. The data also indicate that after physiological elevation, cGMP may directly stimulate the catalytic site by binding to the allosteric cGMP-binding sites of PDE5, thus causing negative feedback on this pathway.

PDE5 inhibitor sildenafil citrate augments endothelium-dependent vasodilation in smokers

Smoking is associated with endothelial dysfunction. The purpose of this study was to determine the effect of sildenafil, an inhibitor of phosphodiesterase type 5 (PDE5), on endothelial function in smokers. We evaluated the forearm blood flow (FBF) responses to acetylcholine (ACh), an endothelium-dependent vasodilator, and to sodium nitroprusside (SNP), an endothelium-independent vasodilator, before and after oral sildenafil administration (100 mg) with a strain-gauge plethysmograph in 10 young healthy male smokers and 10 young healthy male nonsmokers. FBF response to ACh was lower in smokers than in nonsmokers. The vasodilatory effects of SNP were similar in both groups. Sildenafil increased the FBF response to ACh from 9.3+/-2.0 to 12.5+/-3.5 mL/min per 100 mL tissue in smokers and from 12.6+/-5.6 to 19.6+/-8.4 mL/min per 100 mL tissue in nonsmokers, and it increased the response to SNP from 13.3+/-3.9 to 15.1+/-4.3 mL/min per 100 mL tissue in smokers and from 14.8+/-5.2 to 18.4+/-6.0 mL/min/100 mL tissue in nonsmokers (P<0.05 for all). The ratio of maximal ACh-stimulated FBF expressed as a ratio of maximal SNP-stimulated FBF significantly increased after administration of sildenafil in both groups. Infusion of NG-monomethyl-L-arginine, a nitric oxide synthase inhibitor, abolished sildenafil-induced augmentation of the FBF response to ACh in both groups. The findings suggest that endothelial function is impaired in smokers compared with that in nonsmokers, that inhibition of PDE5 by sildenafil significantly increases nitric oxide-mediated vasodilation, and that the activities of PDE5 in smokers and nonsmokers may be similar.

The GAFa domains of rod cGMP-phosphodiesterase 6 determine the selectivity of the enzyme dimerization

Retinal rod cGMP phosphodiesterase (PDE6 family) is the effector enzyme in the vertebrate visual transduction cascade. Unlike other known PDEs that form catalytic homodimers, the rod PDE6 catalytic core is a heterodimer composed of alpha and beta subunits. A system for efficient expression of rod PDE6 is not available. Therefore, to elucidate the structural basis for specific dimerization of rod PDE6, we constructed a series of chimeric proteins between PDE6alphabeta and PDE5, which contain the N-terminal GAFa/GAFb domains, or portions thereof, of the rod enzyme. These chimeras were co-expressed in Sf9 cells in various combinations as His-, myc-, or FLAG-tagged proteins. Dimerization of chimeric PDEs was assessed using gel filtration and sucrose gradient centrifugation. The composition of formed dimeric enzymes was analyzed with Western blotting and immunoprecipitation. Consistent with the selectivity of PDE6 dimerization in vivo, efficient heterodimerization was observed between the GAF regions of PDE6alpha and PDE6beta with no significant homodimerization. In addition, PDE6alpha was able to form dimers with the cone PDE6alpha' subunit. Furthermore, our analysis indicated that the PDE6 GAFa domains contain major structural determinants for the affinity and selectivity of dimerization of PDE6 catalytic subunits. The key dimerization selectivity module of PDE6 has been localized to a small segment within the GAFa domains, PDE6alpha-59-74/PDE6beta-57-72. This study provides tools for the generation of the homodimeric alphaalpha and betabeta enzymes that will allow us to address the question of functional significance of the unique heterodimerization of rod PDE6.

Direct activation of PDE5 by cGMP: long-term effects within NO/cGMP signaling

In platelets, the nitric oxide (NO)-induced cGMP response is indicative of a highly regulated interplay of cGMP formation and cGMP degradation. Recently, we showed that within the NO-induced cGMP response in human platelets, activation and phosphorylation of phosphodiesterase type 5 (PDE5) occurred. Here, we identify cyclic GMP-dependent protein kinase I as the kinase responsible for the NO-induced PDE5 phosphorylation. However, we demonstrate that cGMP can directly activate PDE5 without phosphorylation in platelet cytosol, most likely via binding to the regulatory GAF domains. The reversal of activation was slow, and was not completed after 60 min. Phosphorylation enhanced the cGMP-induced activation, allowing it to occur at lower cGMP concentrations. Also, in intact platelets, a sustained NO-induced activation of PDE5 for as long as 60 min was detected. Finally, the long-term desensitization of the cGMP response induced by a low NO concentration reveals the physiological relevance of the PDE5 activation within NO/cGMP signaling. In sum, we suggest NO-induced activation and phosphorylation of PDE5 as the mechanism for a long-lasting negative feedback loop shaping the cGMP response in human platelets in order to adapt to the amount of NO available.

PDE5 is converted to an activated state upon cGMP binding to the GAF A domain

cGMP-specific, cGMP-binding phosphodiesterase (PDE5) regulates such physiological processes as smooth muscle relaxation and neuronal survival. PDE5 contains two N-terminal domains (GAF A and GAF B), but the functional roles of these domains have not been determined. Here we show that recombinant PDE5 is activated directly upon cGMP binding to the GAF A domain, and this effect does not require PDE5 phosphorylation. PDE5 exhibited time- and concentration-dependent reversible activation in response to cGMP, with the highest activation (9- to 11-fold) observed at low substrate concentrations (0.1 micro M cGMP). A monoclonal antibody directed against GAF A blocked cGMP binding, prevented PDE5 activation and decreased basal activity, revealing that PDE5 in its non-activated state has low intrinsic catalytic activity. Activated PDE5 showed higher sensitivity towards sildenafil than non-activated PDE5. The stimulatory effect of cGMP binding on the catalytic activity of PDE5 suggests that this mechanism of enzyme activation may be common among other GAF domain-containing proteins. The data also suggest that development of agonists and antagonists of PDE5 activity based on binding to this site might be possible.

Functional interplay between angiotensin II and nitric oxide: cyclic GMP as a key mediator

Angiotensin II (Ang II) and nitric oxide (NO) signaling pathways mutually regulate each other by multiple mechanisms. Ang II regulates the expression of NO synthase and NO production, whereas NO downregulates the Ang II type I (AT1) receptor. In addition, downstream effectors of Ang II and NO signaling pathways also interact with each other. A feedback mechanism between Ang II and NO is critical for normal vascular structure and function. Imbalance of Ang II and NO has been implicated in the pathophysiology of many vascular diseases. In this review, we focus on the diverse ways in which Ang II and NO interact and the importance of the balance between the signaling pathways activated by these mediators.

Signaling through NO and cGMP-dependent protein kinases

The gaseous molecule nitric oxide (NO) modulates a large variety of physiological functions including vascular tone, intestinal motility, platelet aggregation, proliferation, apoptosis, and neurotransmission. NO initiates diverse cellular signaling cascades which comprise nitrosylation of proteins, adenosine 5'-diphosphate (ADP)-ribosylation, or stimulation of soluble guanylyl cyclases which catalyze intracellular guanosine 3',5'-cyclic monophosphate (cGMP) synthesis. cGMP activates cGMP-dependent protein kinases (cGK) which mediate localized and global signaling. Furthermore, cGMP regulates the activity of phosphodiesterases (PDE) which modulate the duration and amplitude of cyclic nucleotide signaling. Two different types of cGK are expressed in mammals, cGKI and cGKII. Activation of the NO/cGMP/cGKI pathway induces relaxation of smooth muscle by lowering the cytosolic calcium level and/or by calcium desensitization of the contractile elements.

Immune response to parasitic attack: evolution of a pulsed character

Pulsed characters are temporary biochemical, cellular, or structural changes produced in response to environmental or biotic challenge. For example, response to parasitic attack develops as a pulse of defensive chemicals or cells that typically decay after the parasite has been controlled. Almost all theories for the genetic variability of characters assume measurements on static characters. This paper presents theoretical tools to examine optimal control variables for pulsed characters and the expected level of genetic variability in those control variables. The example of host immune response to parasitic attack is used to develop the theory.

Phosphorylation of isolated human phosphodiesterase-5 regulatory domain induces an apparent conformational change and increases cGMP binding affinity

Substrate binding to the phosphodiesterase-5 (PDE5) catalytic site increases cGMP binding to the regulatory domain (R domain). The latter promotes PDE5 phosphorylation by cyclic nucleotide-dependent protein kinases, which activates catalysis, enhances allosteric cGMP binding, and causes PDE5A1 to apparently elongate. A human PDE5A1 R domain fragment (Val(46)-Glu(539)) containing the phosphorylation site (Ser(102)) and allosteric cGMP-binding sites was studied. The rate, cGMP dependence, and stoichiometry of phosphorylation of the PDE5 R domain by the catalytic subunit of cAMP-dependent protein kinase are comparable with that of the holoenzyme. Migration in native polyacrylamide gels suggests that either cGMP binding or phosphorylation produces distinct conformers of the R domain. Phosphorylation of the R domain increases affinity for cGMP approximately 10-fold (K(D) values 97.8 +/- 17 and 10.0 +/- 0.5 nm for unphospho- and phospho-R domains, respectively). [(3)H]cGMP dissociates from the phospho-R domain with a single rate (t(12) = 339 +/- 30 min) compared with the biphasic pattern of the unphospho-R domain (t(12) = 39.0 +/- 4.8 and 265 +/- 28 min, for the fast and slow components, respectively). Thus, cGMP-directed regulation of PDE5 phosphorylation and the resulting increase in cGMP binding affinity occur largely within the R domain. Conformational change(s) elicited by phosphorylation of the R domain within the PDE5 holoenzyme may also cause or participate in stimulating catalysis.

Calcium-independent and cAMP-dependent modulation of soluble guanylyl cyclase activity by G protein-coupled receptors in pituitary cells

It is well established that G protein-coupled receptors stimulate nitric oxide-sensitive soluble guanylyl cyclase by increasing intracellular Ca(2+) and activating Ca(2+)-dependent nitric-oxide synthases. In pituitary cells receptors that stimulated adenylyl cyclase, growth hormone-releasing hormone, corticotropin-releasing factor, and thyrotropin-releasing hormone also stimulated calcium signaling and increased cGMP levels, whereas receptors that inhibited adenylyl cyclase, endothelin-A, and dopamine-2 also inhibited spontaneous calcium transients and decreased cGMP levels. However, receptor-controlled up- and down-regulation of cyclic nucleotide accumulation was not blocked by abolition of Ca(2+) signaling, suggesting that cAMP production affects cGMP accumulation. Agonist-induced cGMP accumulation was observed in cells incubated in the presence of various phosphodiesterase and soluble guanylyl cyclase inhibitors, confirming that G(s)-coupled receptors stimulated de novo cGMP production. Furthermore, cholera toxin (an activator of G(s)), forskolin (an activator of adenylyl cyclase), and 8-Br-cAMP (a permeable cAMP analog) mimicked the stimulatory action of G(s)-coupled receptors on cGMP production. Basal, agonist-, cholera toxin-, and forskolin-stimulated cGMP production, but not cAMP production, was significantly reduced in cells treated with H89, a protein kinase A inhibitor. These results indicate that coupling seven plasma membrane-domain receptors to an adenylyl cyclase signaling pathway provides an additional calcium-independent and cAMP-dependent mechanism for modulating soluble guanylyl cyclase activity in pituitary cells.

Direct interaction of the inhibitory gamma-subunit of Rod cGMP phosphodiesterase (PDE6) with the PDE6 GAFa domains

Retinal rod and cone cGMP phosphodiesterases (PDE6 family) function as the effector enzyme in the vertebrate visual transduction cascade. The activity of PDE6 catalytic subunits is controlled by the Pgamma-subunits. In addition to the inhibition of cGMP hydrolysis at the catalytic sites, Pgamma is known to stimulate a noncatalytic binding of cGMP to the regulatory GAFa-GAFb domains of PDE6. The latter role of Pgamma has been attributed to its polycationic region. To elucidate the structural basis for the regulation of cGMP binding to the GAF domains of PDE6, a photoexcitable peptide probe corresponding to the polycationic region of Pgamma, Pgamma-21-45, was specifically cross-linked to rod PDE6alphabeta. The site of Pgamma-21-45 cross-linking was localized to Met138Gly139 within the PDE6alpha GAFa domain using mass spectrometric analysis. Chimeras between PDE5 and cone PDE6alpha', containing GAFa and/or GAFb domains of PDE6alpha' have been generated to probe a potential role of the GAFb domains in binding to Pgamma. Analysis of the inhibition of the PDE5/PDE6alpha' chimeras by Pgamma supported the role of PDE6 GAFa but not GAFb domains in the interaction with Pgamma. Our results suggest that a direct binding of the polycationic region of Pgamma to the GAFa domains of PDE6 may lead to a stabilization of the noncatalytic cGMP-binding sites.