Effect of oral organic nitrates on expression and activity of vascular soluble guanylyl cyclase

Non-canonical chemical feedback self-limits nitric oxide-cyclic GMP signaling in health and disease

Nitric oxide (NO)-cyclic GMP (cGMP) signaling is a vasoprotective pathway therapeutically targeted, for example, in pulmonary hypertension. Its dysregulation in disease is incompletely understood. Here we show in pulmonary artery endothelial cells that feedback inhibition by NO of the NO receptor, the cGMP forming soluble guanylate cyclase (sGC), may contribute to this. Both endogenous NO from endothelial NO synthase and exogenous NO from NO donor compounds decreased sGC protein and activity. This effect was not mediated by cGMP as the NO-independent sGC stimulator, or direct activation of cGMP-dependent protein kinase did not mimic it. Thiol-sensitive mechanisms were also not involved as the thiol-reducing agent N-acetyl-L-cysteine did not prevent this feedback. Instead, both in-vitro and in-vivo and in health and acute respiratory lung disease, chronically elevated NO led to the inactivation and degradation of sGC while leaving the heme-free isoform, apo-sGC, intact or even increasing its levels. Thus, NO regulates sGC in a bimodal manner, acutely stimulating and chronically inhibiting, as part of self-limiting direct feedback that is cGMP independent. In high NO disease conditions, this is aggravated but can be functionally recovered in a mechanism-based manner by apo-sGC activators that re-establish cGMP formation.

More answers to the still unresolved question of nitrate tolerance

Organic nitrates are traditionally felt to be a safe adjuvant in the chronic therapy of patients with coronary artery disease. Despite their long use, progress in the understanding of the pharmacology and mechanism of action of these drugs has been achieved only in the last two decades, with the identification of the role of oxidative stress in the pathophysiology of nitrate tolerance, with, the discovery of the ancillary effects of nitrates, and with the demonstration that nitrate therapy has important chronic side effects that might modify patients' prognosis. These advances are however mostly confined to the molecular level or to studies in healthy volunteers, and the true impact of organic nitrates on clinical outcome remains unknown. Complicating this issue, evidence supports the existence of important differences among the different drugs belonging to the group, and there are reasons to believe that the nitrates should not be treated as a homogeneous class. As well, the understanding of the effects of alternative nitric oxide (NO) donors is currently being developed, and future studies will need to test whether the properties of these new medications may compensate and prevent the abnormalities imposed by chronic nitrate therapy. Intermittent therapy with nitroglycerin and isosorbide mononitrate is now established in clinical practice, but they should neither be considered a definitive solution to the problem of nitrate tolerance. Both these strategies are not deprived of complications, and should currently be seen as a compromise rather than a way fully to exploit the benefits of NO donor therapy.

The effects of bupivacaine injection and oral nitric oxide on extraocular muscle in the rabbit

AIM

Nitric oxide (NO) plays a key role in muscle regeneration, which is the primary response, observed during bupivacaine-induced extraocular muscle (EOM) hypertrophy. Our aims were to investigate the effects of bupivacaine injection into the rabbit EOM and the interaction with NO.

MATERIALS AND METHODS

Superior rectus (SR) muscles of 24 New Zealand albino rabbits were studied. Single muscle twitch tension (SMTT) and tetanic muscle tensions at 50, 75, and 100 Hz were recorded using a 15 V stimuli. The rabbits were equally allocated into three groups. Measurements were performed without any drug treatments in group 1. In groups 2 and 3, bupivacaine, 0.5 ml of a 0.50 % solution, was injected into the EOM, and after 21 days, measurements were performed. Oral isosorbide dinitrate (NO donor) at 20 mg/day was given each day prior to measurements in group 3.

RESULTS

SMTTs were 69.9 (66.7-77.6), 187.7 (114.9-252.1) and 204.2 (135.3-311.6) mg in groups 1, 2, and 3 respectively. SMTTs for both groups 2 and 3 were significantly higher than that for group 1 (p < 0.05). Compared with group 1, group 2 exhibited a 3.8-11.7 % increase in the tetanic tensions at 50, 75, and 100 Hz, but none of these differences were statistically significant. The increase was 47.5-137.5 % in group 3 relative to group 2, and the differences were statistically significant except at 100 Hz. The enlargement of the muscle fibers after bupivacaine injection was shown histopathologically.

CONCLUSION

Bupivacaine injection increased the EOM tension in rabbits to some extent. NO augmented the effect of bupivacaine.

Hypoxia induces downregulation of soluble guanylyl cyclase β1 by miR-34c-5p

Soluble guanylyl cyclase (sGC) is the principal receptor for nitric oxide (NO) and crucial for the control of various physiological functions. The β1 subunit of sGC is obligatory for the biological stability and activity of the sGC heterodimer. MicroRNAs (miRNAs) are important regulators of gene expression and exert great influences on diverse biological activities. The aim of the present study was to determine whether or not the expression of sGCβ1 is specifically regulated by miRNAs. We report that miR-34c-5p directly targets sGCβ1 under hypoxia. Bioinformatics analysis of the sGCβ1 3'-untranslated region (3'-UTR) revealed a putative binding site for miR-34b-5p and miR-34c-5p, but only miR-34c-5p inhibited luciferase activity through interaction with sGCβ1 3'-UTR in HEK293T cells. Site-directed mutagenesis of the putative miR-34c-5p binding site abolished the negative regulation of luciferase expression. Overexpression of miR-34c-5p repressed the expression of sGCβ1 in stable cell lines, which was reversed by miR-34c-5p-specific sponge. Inoculation of mouse lung tissues in vitro with lentivirus bearing miR-34c-5p significantly decreased both the expression of sGCβ1 and NO-stimulated sGC activity, which was also rescued by miR-34c-5p-specific sponge. Furthermore, we identified the putative Sp1-binding site in the promoter region of miR-34c-5p. Luciferase reporter constructs revealed that Sp1 directly binds to the wild-type promoter of miR-34c-5p, which was confirmed by chromatin immunoprecipitation. In summary, these findings reveal that miR-34c-5p directly regulates sGCβ1 expression, and they identify the key transcription factor Sp1 that governs miR-34c-5p expression during hypoxia.

Reactive oxygen species as cardiovascular mediators: lessons from endothelial-specific protein overexpression mouse models

The term reactive oxygen species (ROS) summarizes several small chemical compounds such as superoxide, peroxynitrite, hydrogen peroxide and nitric oxide. The stoichiometry of the chemical reactions underlying generation and metabolism is subject of tight enzymatic regulation resulting in well balanced steady-state concentrations throughout the healthy body. ROS are short-lived and usually active at the site of production only, e.g. in vascular endothelial cells. Although an increase of vascular ROS-production is considered an important pathogenic factor in cardiovascular diseases, there is evidence for physiological or even beneficial effects as well. We have generated several transgenic mice using the Tie-2 promotor which expresses an enzyme of interest specifically in vascular endothelial cells. Here, we review some results obtained with mice carrying a Tie-2-driven overexpression of catalase or endothelial nitric oxide synthase (eNOS). Tie-2-catalase mice have a strongly reduced steady-state concentration of vascular hydrogen peroxide and show profound hypotension that is not dependent on the bioavailability of endothelial nitric oxide but is completely reversible by treatment with the catalase inhibitor aminotriazole. A similar hypotension was observed in transgenic mice with an endothelial-specific overexpression of eNOS but this hypotension is entirely dependent on vascular eNOS activity. These observations suggest a tonic effect of hydrogen peroxide on vascular smooth muscle. Further studies suggested that hydrogen peroxide promotes the exercise-induced increase of vascular eNOS expression and inhibits the release of endothelial progenitor cells induced by exercise training. In summary, our data support the concept of a dual role of ROS in the vascular system.