Cyclic nucleotide phosphodiesterase 5 and sildenafil: promises realized

Sildenafil Increases the p50 and Shifts the Oxygen-Hemoglobin Dissociation Curve to the Right

INTRODUCTION

Sildenafil (Viagra®) is a selective phosphodiesterase type 5 (PDE5) inhibitor that block the breakdown of cyclic guanyl monophosphate (cGMP) leading to relaxation of the smooth muscles of the corpus cavernous and an increase in blood flow resulting in penile erection. It is hypothesized that sildenafil will increase the release of oxygen from erythrocytes and shift the oxygen-hemoglobin curve to the right.

AIM

The aim of this study was to investigate the effect of varying doses of sildenafil on the p50 of the oxygen-hemoglobin dissociation curve in blood samples from eight (8) healthy adult male volunteers with normal hemoglobin HbAA.

METHOD

The hemox-analyzer was used to generate the p50 and the oxygen-hemoglobin dissociation curves.

MAIN OUTCOME MEASURES

The effect of different doses of sildenafil on the p50 values and shift of the oxygen-hemoglobin curve were the main outcome measures.

RESULT

Sildenafil caused a statistically significant increase in the p50 values and rightward shift of the oxygen-hemoglobin dissociation curve.

CONCLUSION

Sildenafil caused a dose-dependent increase in the release of oxygen from the erythrocytes as shown by the increased p50 values and rightward shift of the oxygen-hemoglobin dissociation curve. Ellis SS and Pepple DJ. Sildenafil increases the p50 and shifts the oxygen-hemoglobin dissociation curve to the right.

Modulation of soluble guanylate cyclase for the treatment of erectile dysfunction

Nitric oxide (NO) is the principal mediator of penile erection, and PDE-5 inhibitors are the first-line agents used to treat erectile dysfunction (ED). When NO formation or bioavailability is decreased by oxidative stress and PDE-5 inhibitors are no longer effective, a new class of agents called soluble guanylate cyclase (sGC) stimulators like BAY 41-8543 will induce erection. sGC stimulators bind to the normally reduced, NO-sensitive form of sGC to increase cGMP formation and promote erection. The sGC stimulators produce normal erectile responses when NO formation is inhibited and the nerves innervating the corpora cavernosa are damaged. However, with severe oxidative stress, the heme iron on sGC can be oxidized, rendering the enzyme unresponsive to NO or sGC stimulators. In this pathophysiological situation, another newly developed class of agents called sGC activators can increase the catalytic activity of the oxidized enzyme, increase cGMP formation, and promote erection. The use of newer agents that stimulate or activate sGC to promote erection and treat ED is discussed in this brief review article.

Phosphodiesterase type 5 (PDE5) in the adipocyte: a novel player in fat metabolism?

Phosphodiesterase type 5 (PDE5) is expressed in many tissues (e.g. heart, lung, pancreas, penis) and plays a specific role in hydrolyzing cyclic guanosine monophosphate (cGMP). In adipocytes, cGMP regulates crucial functions by activating cGMP-dependent protein kinase (PKG). Interestingly, PDE5 was recently identified in adipose tissue, although its role remains unclear. Its inhibition, however, was recently shown to affect adipose differentiation and aromatase function. This review summarizes evidence supporting a role for the PDE5-regulated cGMP/PKG system in adipose tissue and its effects on adipocyte function. A better elucidation of the role of PDE5 in the adipocyte could reveal new therapeutic strategies for fighting obesity and metabolic syndrome.

Target profiling of a small library of phosphodiesterase 5 (PDE5) inhibitors using chemical proteomics

Inhibitors of phosphodiesterase 5 (PDE5) are widely used for the treatment of erectile dysfunction and pulmonary hypertension. The commercially available inhibitors are effective, well-tolerated drugs, but differ in their phosphodiesterase specificity. To explore and manipulate the specificity of PDE5 inhibitors, a small library of four inhibitors was synthesized using the structure of known PDE5 inhibitors as a scaffold. Their inhibitory potency towards PDE5 and related family members was evaluated. Next, they were immobilized on a matrix to perform affinity pull-down assays in rat testis tissue, followed by mass spectrometric (MS) analysis. By using unique peptide spectral counts of identified proteins in the MS analysis, we were able to assess the relative binding of these inhibitors to a large set of proteins, allowing the determination of their selectivity profiles in vitro. For selected proteins of interest, the results were verified using quantitative isotopic dimethyl labeling and immunoblotting, and isothermal titration calorimetry (ITC). For the PDE5 inhibitors, our data reveal that even slight chemical modifications can bias their selectivity significantly towards other interacting proteins, opening up the potential of these compounds to be used as scaffolds for the development of inhibitors for new protein targets. In a broad sense, we demonstrate that the combination of chemical proteomics and unique peptide spectral counting allows for the confident and facile analysis of the differential interactome of bioactive small molecules.

Exposure to phosphodiesterase type 5 inhibitors stimulates aromatase expression in human adipocytes in vitro

INTRODUCTION

Prolonged tadalafil administration in men with erectile dysfunction is associated with increased testosterone (T): estradiol (E(2)) ratio mainly related to reduction of E(2) levels.

AIM

To investigate the presence of phosphodiesterase type 5 (PDE5) isoenzyme in primary human visceral adipocytes and whether different PDE5 inhibitors (PDE5i) could directly modulate aromatase (ARO) expression in differentiated human visceral adipocytes in culture.

MAIN OUTCOME MEASURES

PDE5 mRNA and protein expression in primary human visceral adipocytes as well as mRNA and protein expression of ARO, with functional activity after selective PDE5 blockade by tadalafil and sildenafil.

METHODS

Purified primary human visceral pre-adipocytes were differentiated ex vivo and were exposed to tadalafil or sildenafil (1 µM) for different intervals of time (6-12-24-96 hours). ARO mRNA content and expression were measured by Western Blot and quantitative reverse transcription-polymerase chain reaction (qRT-PCR), respectively. T and E(2) in supernatants were measured by ELISA also in the presence of letrozole.

RESULTS

Differentiated adipocytes were found to express detectable levels of PDE5 transcripts. Acute exposure (6 hours) to both PDE5i tadalafil and sildenafil increased ARO mRNA expression by 4.7- and 2.8-fold, respectively (P < 0.001). ARO mRNA and protein levels were increased by the treatment with PDE5i in a time- and dose-dependent manner. Such effect was mimicked by 8-bromo-cGMP but was lost after 24 and 96 hours; differently, the PDE3B specific inhibitor milrinone (1 µM), displayed no effect. Accordingly, long-term exposure (24 and 96 hours) to PDE5i caused a significant increase in E(2) concentrations in the supernatant (1.7 and 2 fold, respectively; P < 0.001), with a parallel reduction of T (15% and 30%, respectively; P < 0.001). Such effect was reversed by the co-incubation with the specific ARO-inhibitor letrozole.

CONCLUSIONS

Our results demonstrate that PDE5 is expressed in human visceral adipocytes and that acute exposure to PDE5i selectively stimulates ARO expression, which is related to a specific PDE5 blockade. We speculate that modulation of ARO activity by PDE5i could be one of the mechanisms responsible, at least in part, for the beneficial effects of PDE5i on endothelial and metabolic functions.

Characterization of the structures of phosphodiesterase 10 binding with adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate by hybrid quantum mechanical/molecular mechanical calculations

Quantum mechanical/molecular mechanical (QM/MM) geometry optimizations of the X-ray crystal structures of PDE10-AMP (PDB code 2OUN ) and PDE10-GMP (PDB code 2OUQ ) complexes have been performed to characterize the state of the AMP and GMP products, respectively. Results show that only one phosphate oxygen atom (O1) is protonated for both AMP and GMP product complexes. In addition, QM/MM calculations have resolved the orientation of the amide group of Gln726 in PDE10-GMP which was in conflict with the assignment of the guanine group of GMP in the X-ray crystal structure. Calculations reveal that the amide oxygen and nitrogen atom of Gln726 are rotated 180 degrees, resulting in two strong hydrogen bonds formed between the amide group of Gln726 and the guanine group of GMP. Binding free energy calculations for both QM/MM-optimized structures confirm the new conformational assignment of Gln726 in PDE10-GMP. The calculated binding free energy of the rotated structure is approximately 22 kcal/mol lower than the X-ray crystal assignment. The lower energy is mainly derived from the formation of two hydrogen bonds between the amide group of Gln726 and the guanine group of GMP. This implies that the orientation of the amide oxygen and nitrogen atoms in PDE10-AMP is different from PDE10-GMP. Finally, our results help to understand why PDE10 can hydrolyze both cAMP and cGMP.

Dynamic structures of phosphodiesterase-5 active site by combined molecular dynamics simulations and hybrid quantum mechanical/molecular mechanical calculations

Various quantum mechanical/molecular mechanical (QM/MM) geometry optimizations starting from an x-ray crystal structure and from the snapshot structures of constrained molecular dynamics (MD) simulations have been performed to characterize two dynamically stable active site structures of phosphodiesterase-5 (PDE5) in solution. The only difference between the two PDE5 structures exists in the catalytic, second bridging ligand (BL2) which is HO- or H2O. It has been shown that, whereas BL2 (i.e. HO-) in the PDE5(BL2 = HO-) structure can really bridge the two positively charged metal ions (Zn2+ and Mg2+), BL2 (i.e. H2O) in the PDE5(BL2 = H2O) structure can only coordinate Mg2+. It has been demonstrated that the results of the QM/MM geometry optimizations are remarkably affected by the solvent water molecules, the dynamics of the protein environment, and the electronic embedding charges of the MM region in the QM part of the QMM/MM calculation. The PDE5(BL2 = H2O) geometries optimized by using the QM/MM method in different ways show strong couplings between these important factors. It is interesting to note that the PDE5(BL2 = HO-) and PDE5(BL2 = H2O) geometries determined by the QM/MM calculations neglecting these three factors are all consistent with the corresponding geometries determined by the QM/MM calculations that account for all of these three factors. These results suggest the overall effects of these three important factors on the optimized geometries can roughly cancel out. However, the QM/MM calculations that only account for some of these factors could lead to considerably different geometries. These results might be useful also in guiding future QM/MM geometry optimizations on other enzymes.

Characterization of a catalytic ligand bridging metal ions in phosphodiesterases 4 and 5 by molecular dynamics simulations and hybrid quantum mechanical/molecular mechanical calculations

Cyclic nucleotide phosphodiesterases (PDEs) constitute a large superfamily of enzymes regulating concentrations of intracellular second messengers cAMP and cGMP through PDE-catalyzed hydrolysis. Although three-dimensional x-ray crystal structures of PDE4 and PDE5 have been reported, it is uncertain whether a critical, second bridging ligand (BL2) in the active site is H2O or HO- because hydrogen atoms cannot be determined by x-ray diffraction. The identity of BL2 is theoretically determined by performing molecular dynamics simulations and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations, for the first time, on the protein structures resolved by x-ray diffraction. The computational results confirm our previous suggestion, which was based on QM calculations on a simplified active site model, that BL2 in PDE4 should be HO-, rather than H2O, serving as the nucleophile to initialize the catalytic hydrolysis of cAMP. The molecular dynamics simulations and QM/MM calculations on PDE5 demonstrate for the first time that the BL2 in PDE5 should also be HO- rather than H2O as proposed in recently published reports on the x-ray crystal structures, which serves as the nucleophile to initialize the PDE5-catalyzed hydrolysis of cGMP. These fundamental structural insights provide a rational basis for future structure-based drug design targeting PDEs.

Efficacy and selectivity of phosphodiesterase-targeted drugs in inhibiting photoreceptor phosphodiesterase (PDE6) in retinal photoreceptors

PURPOSE

Phosphodiesterase (PDE) inhibitors are important therapeutic agents, but their effects on photoreceptor PDE (PDE6) and photoreceptor cells are poorly understood. The potency and selectivity of various classes of PDE inhibitors on purified rod and cone PDE6 and on intact rod outer segments (ROS) were characterized.

METHODS

The inhibition constant (K(i)) of isozyme-selective PDE inhibitors was determined for purified rod and cone PDE6. Perturbations of cGMP levels in isolated ROS suspensions by PDE inhibitors were quantitated by a cGMP enzyme-linked immunoassay.

RESULTS

Most PDE5-selective inhibitors were excellent PDE6 inhibitors. Vardenafil, a potent PDE5 inhibitor (K(i) = 0.2 nM), was the most potent PDE6 inhibitor tested (K(i) = 0.7 nM). Zaprinast was the only drug that inhibited PDE6 more potently than did PDE5. PDE1-selective inhibitors were equally effective in inhibiting PDE6. In intact ROS, PDE inhibitors elevated cGMP levels, but none fully inhibited PDE6. Their potency for elevating cGMP levels in ROS was much lower than their ability to inhibit the purified enzyme. Competition between PDE5/6-selective drugs and the inhibitory gamma-subunit for the active site of PDE6 is proposed to reduce the effectiveness of drugs at the enzyme-active site.

CONCLUSIONS

Several classes of PDE inhibitors inhibit PDE6 equally as well as the PDE family to which they are targeted. In intact ROS, high PDE6 concentrations, binding of the gamma-subunit to the active site, and calcium feedback mechanisms attenuate the effectiveness of PDE inhibitors to inhibit PDE6 and disrupt the cGMP signaling pathway during visual transduction.

Expression and functional activity of phosphodiesterase type 5 in human and rabbit vas deferens

The molecular mechanisms underlying the regulation of vas deferens (VD) motility and semen emission are still poorly understood. We now report evidence on VD expression of phosphodiesterase type 5 (PDE5), which regulates nitric oxide (NO)-induced relaxation and cGMP breakdown in smooth muscle cells. In human VD, the PDE5 abundance was relatively high (>3 x 10(6) molecules/microg total RNA), although 10-fold lower than in corpora cavernosa (CC). Also cGMP metabolising activity was higher in CC than in VD. However, both tissues share the same sensitivity to a broad panel of cGMP-related PDE inhibitors: sildenafil, tadalafil, dipyridamole, zaprinast, vinpocetine, EHNA and cilostamide. Based on the rank order of potency of these PDE inhibitors, we found that the cGMP metabolizing activity in human VD mostly corresponds to PDE5. PDE5 was immunolocalized in all the muscular layers of human and rabbit VD and was found to be negatively involved in regulating NO-induced relaxation. In addition, by using a rabbit model of hypogonadotropic hypogonadism, we found that PDE5 gene expression and activity are androgen-dependent in VD, as previously demonstrated in CC. In fact, the sensitivity to a NO-donor (NCX4040), its enhancement by PDE5 inhibitors and the PDE5-related cGMP breakdown were all affected by androgen manipulation. Our results provide a hypothesis explaining the beneficial effects of PDE inhibitors in patients with rapid ejaculation.

Androgens regulate phosphodiesterase type 5 expression and functional activity in corpora cavernosa

By real-time RT-PCR and Western blot analysis, we found that phosphodiesterase type 5 (PDE5) mRNA and protein abundance was several fold higher in human male than in female reproductive tracts. The highest mRNA level (>1 x 10(7) molecules/microg total RNA) was detected in human corpora cavernosa (CC), where PDE5 protein was immunolocalized in both muscular and endothelial compartment. The possible role of androgens in regulating PDE5 expression was studied using a previously established rabbit model of hypogonadotropic hypogonadism. In this model, hypogonadism reduced, and testosterone (T) supplementation restored, CC PDE5 gene and protein expression. In addition, T supplementation completely rescued and even enhanced cyclic GMP conversion to metabolites, without changing IC(50) for sildenafil (IC(50) = 2.16 +/- 0.62 nm). In control CC strips, sildenafil dose-dependently increased relaxation induced by electrical field stimulation, with EC(50) = 3.42 +/- 1.7 nm. Hypogonadism reduced, and T increased, sildenafil effect on electrical field stimulation, again without changing their relative EC(50) values. CC sensitivity to the NO-donor NCX4040 was greater in hypogonadal rabbit strips than in control or T-treated counterparts. Moreover, sildenafil enhanced NCX4040 effect in eugonadal rabbit strips but not in hypogonadal ones. This suggests that androgens up-regulate PDE5 in rabbit penis. We also measured PDE5 gene expression and metabolic activity in human CC from male-to-female transsexual individuals, chronically treated with estrogens and cyproterone acetate. Comparing the observed values vs. eugonadal controls, PDE5 mRNA, protein, and functional activity were significantly reduced. In conclusion, we demonstrated, for the first time, that androgens positively regulate PDE5, thus providing a possible explanation about the highest abundance of this enzyme in male genital tract.