Cgmp In Pharmacology

[Lauro Pericles]

The second messenger cyclic GMP (cGMP) is known over 50 years, but its general recognition was overshadowed for a long time by the second messengers cyclic AMP and Ca2+. Interest in cGMP increased when it was discovered that NO activates soluble guanylyl cyclase and raises thereby intracellular cGMP concentrations. Soon thereafter, it was found that organic nitrates, compounds that were used to treat angina pectoris, released NO and enhanced cellular cGMP levels. Already in the early 1970, the first target of cGMP was identified, cGMP-dependent protein kinase (cGK), and 15 years later, the second target, the cGMP-gated ion channel in the retina, the first described cyclic nucleotide-gated ion channel (CNG channel). About this time, it was known that cGMP could regulate the cellular concentration of cAMP through several phosphodiesterases (PDEs). Between 1980 and 1985, it was shown that the atria released a peptide, now known as atrial natriuretic peptide (ANP) that caused diuresis and blood pressure lowering by activation of the particulate guanylyl cyclase, a membrane bound enzyme, which is not activated by NO. Since then, it became clear that the cGMP signaling machinery is an important regulatory system in human conditions such as fear processing, cerebellar motor control, pulmonary hypertension, cardiac failure, hypertension, and erectile dysfunction. The rapidly expanding cGMP field had its first successful gathering in 2003 that suggested that new compounds may be available that elevated cellular cGMP levels and could emerge as valuable new therapeutic options in some of the above mentioned diseases. Since then, the cGMP field developed significantly by the analysis of various mouse models, crystal structures, and new regulatory pathways modulated by oxygen radicals. In 2011, the fifth cGMP symposium was held in Halle (Saale), Germany (www.cyclicgmp.net). This report summarizes the new data on molecular mechanisms, (patho)physiological relevance, and therapeutic potentials of the cGMP signaling system that were presented at this meeting. We apologize to our colleagues whose contributions were not enclosed because of space limitations.

Nesiritide, a recombinant form of human BNP, is approved for use in the treatment of heart failure in the USA (Boerrigter et al. 2009). However, despite its favorable cardiorenal properties (e.g., natriuresis, diuresis, inhibition of renin and aldosterone, antihypertrophic, antifibrotic effects, etc.), BNP also serves as a potent vasodilator and may decrease renal perfusion and compromise renal function in heart failure patients. Thus, there is a need for novel NPs with favorable cardiorenal properties. John Burnett (Rochester, NY, USA) described a chimeric NP, CD-NP (cenderitide), comprising CNP-22 and the C terminus of Dendroaspis NP (DNP) (McKie et al. 2010) (Fig. 1). CD-NP activates GC-A and GC-B receptors, increases natriuresis, urinary flow, and glomerular filtration rate, yet is less hypotensive than BNP. In patients with acute heart failure, continuous infusion of CD-NP unloaded the heart without altering blood pressure. Therefore, designer NPs offer advantages over natural NPs and may represent a novel therapeutic strategy in the treatment of heart failure.

Hypertension is still a major cause for stroke, heart failure, myocardial infarction, and chronic kidney disease. Blood pressure setting includes hereditary features, but genetic factors have been difficult to identify at the population. Human genetic studies could indicate novel physiological mechanisms that underlie blood pressure. Christopher Newton-Cheh (Boston, MA, USA) used a candidate gene association study of common variants across the NPPA-NPPB locus to identify genetic variants that influence atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) level (Kato et al. 2011; Newton-Cheh et al. 2009a,b). The minor alleles of three noncoding SNPs were associated with higher amounts of ANP and BNP in 14,515 individuals of European ancestry. Two alleles correlating with higher ANP/BNP concentrations were associated with lower systolic and diastolic blood pressure as well as lower odds of hypertension in 29,717 individuals. Thus, the ANP-BNP/pGC/cGMP axis is important in the regulation of BP in humans (Ehret et al. 2011). Further ongoing genome wide association studies (GWAS) have identified additional novel gene loci. These findings provide new insights into the genetics and biology of blood pressure regulation and suggest potential novel therapeutic pathways for cardiovascular disease prevention.

Soluble guanylyl cyclases (sGC) are intracellularly located heterodimers consisting of an α-subunit (α1 or α2) and a heme-containing β-subunit (β1 or β2). The α1β1 and the heterodimer is the dominantly found sGC in most tissues, whereas α2β1 is the major sGC in brain, uterus, and placenta. NO activates sGC by binding to the heme-moiety and thereby inducing cGMP synthesis. The search of new drugs that enhance sGC activity lead to two different classes of compounds: sGC stimulators, which act NO independently and heme dependently, and sGC activators, which function NO and heme independently.

Marco Guazzi (Milano, Italy) further confirmed the importance of elevating myocardial cGMP in the treatment of diastolic heart failure demonstrating an ability of the PDE5 inhibitor, sildenafil, to show reduction in pulmonary vascular tone and RV hemodynamic burden. Significantly improved pulmonary hemodynamics, right ventricular contractility, and chamber dimensions were observed at 6 and 12 months of follow-up, which was paralleled by an improvement in quality of life and clinical status (Guazzi et al. 2011). Further large-scale morbidity and mortality studies are needed to approve whether PDE5 inhibition may effectively impact the appearance of heart failure due to diastolic origin.

Intravascular red cell hemolysis affects NO-redox homeostasis and causes endothelial dysfunction, platelet activation and vasculopathy. The released cell-free plasma hemoglobin reacts with NO in a 1:1 ratio (Donadee et al. 2011). This NO consumption alters vascular function in subjects with sickle cell anemia. Marc Gladwin (Pittsburg, PA, USA) investigated the therapeutic potential to bypass NO scavenging via direct pharmacological induction of sGC. He showed that sGC activators or sGC stimulators could reverse free-hemoglobin-mediated vasoconstriction. The results propose new mechanisms for endothelial injury and impaired vascular function associated with red blood cell storage lesions and hemolysis. Furthermore, theses studies might indicate a new therapeutic approach restoring cGMP levels during conditions, which excite NO scavenging.

Oleg Evgenov (Boston, MA, USA) suggested that stimulation of sGC might represent a new modality for treating pulmonary fibrosis and related conditions as patients with pulmonary fibrosis develop pulmonary hypertension (PH), in part due to impaired production of endogenous NO that activates sGC. Pharmacological stimulation of sGC with riociguat attenuates pulmonary fibrosis, PH, right ventricular hypertrophy, and mortality in bleomycin-exposed mice (Evgenov et al. 2011).

An extraordinary Evening Lecture has been presented by Paul Vanhoutte (Hongkong, China) on endothelial dysfunction and vascular disease. Endothelial cells regulate vascular tone by releasing various contracting and relaxing factors including NO, arachidonic acid metabolites (derived from cyclooxygenases, lipoxygenases, and cytochrome P450 monooxygenases), reactive oxygen species, and vasoactive peptides. Additionally, another pathway associated with the hyperpolarization of the underlying smooth muscle cells plays a predominant role in resistance arteries. Endothelial dysfunction is a multifaceted disorder, which has been associated with hypertension of diverse etiologies involving not only alterations of the NO/sGC/cGMP pathway but also reduced endothelium-dependent hyperpolarizations and enhanced production of contracting factors, particularly vasoconstrictor prostanoids. Vanhoutte (2009) highlighted these different endothelial pathways as potential drug targets for novel treatments in hypertension and the associated endothelial dysfunction and end-organ damage.

New insights were elucidated in the protein structures and regulatory pathways of the GC/cGMP system, which enhance the understanding of the (patho)physiological consequences of this signaling pathway. Mammalian soluble guanylyl cyclase (sGC) is a heterodimer consisting of an α- and β-subunit (see above). The C terminus of each subunit includes a catalytic domain, and the active site is arranged from both subunits. The subunits contain a PAS-like domain and a predicted helical region, (Derbyshire and Marletta 2009; Mergia et al. 2009). N-termini of α- and β-subunits are homologous to the H-NOX (heme-nitric oxide/oxygen) family of proteins. The N terminus of the β-subunit contains a ferrous heme cofactor acting as a NO receptor. Ferric heme-oxidized sGC has low activity. The NO complex of the re-reduced heme generates a desensitized, low-activity state of sGC. The molecular mechanism for this desensitization involves site specific S-nitrosation. sGC activation induce slight and complex conformational changes, which are still only partially understood. Some insights were revealed by analyzing the complexes of cinaciguat (BAY 58-2667), a sGC activator, bound to Nostoc H-NOX domain, a homolog of sGC (Martin et al. 2010). Heme forms a covalent inhibitory bond between the Fe and H105 that is part of the αF helix. Cinaciguat does not form this covalent bond yet mimics the rest of the heme features and activates the heme free sGC. Cinaciguat is in this respect similar to protoporphyrin IX that lacks Fe, but is also a good sGC activator. These insights are crucial for the understanding of the action of the new drugs. Hence, further detailed structural and mechanistic analysis will be an important field to enhance the knowledge of sGC physiology.

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