cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Progesterone directly and rapidly inhibits GnRH neuronal activity via progesterone receptor membrane component 1

GnRH neurons are essential for reproduction, being an integral component of the hypothalamic-pituitary-gonadal axis. Progesterone (P4), a steroid hormone, modulates reproductive behavior and is associated with rapid changes in GnRH secretion. However, a direct action of P4 on GnRH neurons has not been previously described. Receptors in the progestin/adipoQ receptor family (PAQR), as well as progesterone receptor membrane component 1 (PgRMC1) and its partner serpin peptidase inhibitor, clade E (nexin, plasminogen activator inhibitor type 1) mRNA binding protein 1 (SERBP1), have been shown to mediate rapid progestin actions in various tissues, including the brain. This study shows that PgRMC1 and SERBP1, but not PAQR, are expressed in prenatal GnRH neurons. Expression of PgRMC1 and SERBP1 was verified in adult mouse GnRH neurons. To investigate the effect of P4 on GnRH neuronal activity, calcium imaging was used on primary GnRH neurons maintained in explants. Application of P4 significantly decreased the activity of GnRH neurons, independent of secretion of gamma-aminobutyric acidergic and glutamatergic input, suggesting a direct action of P4 on GnRH neurons. Inhibition was not blocked by RU486, an antagonist of the classic nuclear P4 receptor. Inhibition was also maintained after uncoupling of the inhibitory regulative G protein (G(i/o)), the signal transduction pathway used by PAQR. However, AG-205, a PgRMC1 ligand and inhibitor, blocked the rapid P4-mediated inhibition, and inhibition of protein kinase G, thought to be activated downstream of PgRMC1, also blocked the inhibitory activity of P4. These data show for the first time that P4 can act directly on GnRH neurons through PgRMC1 to inhibit neuronal activity.

Extracellular ATP signaling via P2X(4) receptor and cAMP/PKA signaling mediate ATP oscillations essential for prechondrogenic condensation

Prechondrogenic condensation is the most critical process in skeletal patterning. A previous study demonstrated that ATP oscillations driven by Ca(2+) oscillations play a critical role in prechondrogenic condensation by inducing oscillatory secretion. However, it remains unknown what mechanisms initiate the Ca(2+)-driven ATP oscillations, mediate the link between Ca(2+) and ATP oscillations, and then result in oscillatory secretion in chondrogenesis. This study has shown that extracellular ATP signaling was required for both ATP oscillations and prechondrogenic condensation. Among P2 receptors, the P2X(4) receptor revealed the strongest expression level and mediated ATP oscillations in chondrogenesis. Moreover, blockage of P2X(4) activity abrogated not only chondrogenic differentiation but also prechondrogenic condensation. In addition, both ATP oscillations and secretion activity depended on cAMP/PKA signaling but not on K(ATP) channel activity and PKC or PKG signaling. This study proposes that Ca(2+)-driven ATP oscillations essential for prechondrogenic condensation is initiated by extracellular ATP signaling via P2X(4) receptor and is mediated by cAMP/PKA signaling and that cAMP/PKA signaling induces oscillatory secretion to underlie prechondrogenic condensation, in cooperation with Ca(2+) and ATP oscillations.

A VASP-Rac-soluble guanylyl cyclase pathway controls cGMP production in adipocytes

The ubiquitous second messenger cyclic guanosine monophosphate (cGMP) plays an important role in metabolism and promotes brown adipocyte differentiation. We showed that ablation of the gene encoding vasodilator-stimulated phosphoprotein (VASP), a major downstream component of the cGMP signaling cascade, increased cellular cGMP content in brown and white adipocytes and mouse embryonic fibroblasts. VASP-deficient cells showed increased activation of Rac1, which in turn increased the abundance of the cGMP-producing enzyme soluble guanylyl cyclase (sGC), the main receptor for nitric oxide. Consequently, loss of VASP caused increased cGMP concentrations and enhanced brown adipocyte differentiation. Consistent with the in vitro data, we found increased energy expenditure in VASP-deficient mice and exposure to cold triggered enhanced lipolysis and cellular respiration in VASP-deficient brown fat cells. In addition, VASP-deficient mice exhibited increased development of brown-like adipocytes in white fat. Our data revealed that a VASP to Rac to sGC negative feedback loop limited cGMP production, thereby regulating adipogenesis and energy homeostasis.

Redox Regulation of Protein Function via Cysteine S-Nitrosylation and Its Relevance to Neurodegenerative Diseases

Debilitating neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), can be attributed to neuronal cell damage in specific brain regions. An important hallmark of these diseases is increased oxidative and nitrosative stress that occurs via overproduction of highly reactive free radicals known as reactive oxygen species (ROS) and reactive nitrogen species (RNS). These molecules are normally removed by cellular antioxidant systems. Under physiological conditions, ROS/RNS are present at low levels, mediating several neurotrophic and neuroprotective signaling pathways. In contrast, under pathological conditions, there is a pronounced increase in ROS/RNS generation, impairing normal neurological function. Nitric oxide (NO) is one such molecule that functions as a signaling agent under physiological conditions but causes nitrosative stress under pathological conditions due to its enhanced production. As first reported by our group and colleagues, the toxic effects of NO can be in part attributed to thiol S-nitrosylation, a posttranslational modification of cysteine residues on specific proteins. Here, we review several reports appearing over the past decade showing that S-nitrosylation of an increasing number of proteins compromises important cellular functions, including mitochondrial dynamics, endoplasmic reticulum (ER) protein folding, and signal transduction, thereby promoting synaptic damage, cell death, and neurodegeneration.

Cell membrane disruption stimulates NO/PKG signaling and potentiates cell membrane repair in neighboring cells

Resealing of a disrupted plasma membrane at the micron-diameter range requires Ca²⁺-regulated exocytosis. Repeated membrane disruptions reseal more quickly than the initial wound, and this potentiation of membrane resealing persists for at least 24 hours after the initial wound. Long-term potentiation of membrane resealing requires CREB-dependent gene expression, which is activated by the PKC- and p38 MAPK-dependent pathway in a wounded cell. The present study demonstrates that membrane resealing is potentiated in both wounded and neighboring cells in MDCK cells. Wounding of cells expressing CREB133, a mutant variant of CREB, does not show the potentiated response of cell membrane resealing in either wounded or neighboring cells. Furthermore, wounding of cells induces CREB phosphorylation, not only in wounded cells, but also in neighboring cells. Inhibition of the nitric oxide/PKG signaling pathway suppresses CREB phosphorylation in neighboring cells, but not in wounded cells. The potentiation of membrane resealing in neighboring cells is suppressed if the nitric oxide/PKG pathway is inhibited during the initial wound. Together, these results suggest that the nitric oxide/PKG pathway stimulates CREB phosphorylation in neighboring cells so that subsequent cell membrane disruptions of the neighboring cells reseal more quickly.

Pathological cardiac hypertrophy alters intracellular targeting of phosphodiesterase type 5 from nitric oxide synthase-3 to natriuretic peptide signaling

BACKGROUND

In the normal heart, phosphodiesterase type 5 (PDE5) hydrolyzes cGMP coupled to nitric oxide- (specifically from nitric oxide synthase 3) but not natriuretic peptide (NP)-stimulated guanylyl cyclase. PDE5 is upregulated in hypertrophied and failing hearts and is thought to contribute to their pathophysiology. Because nitric oxide signaling declines whereas NP-derived cGMP rises in such diseases, we hypothesized that PDE5 substrate selectivity is retargeted to blunt NP-derived signaling.

METHODS AND RESULTS

Mice with cardiac myocyte inducible PDE5 overexpression (P5(+)) were crossed to those lacking nitric oxide synthase 3 (N3(-)), and each model, the double cross, and controls were subjected to transaortic constriction. P5(+) mice developed worse dysfunction and hypertrophy and enhanced NP stimulation, whereas N3(-) mice were protected. However, P5(+)/N3(-) mice behaved similarly to P5(+) mice despite the lack of nitric oxide synthase 3-coupled cGMP generation, with protein kinase G activity suppressed in both models. PDE5 inhibition did not alter atrial natriuretic peptide-stimulated cGMP in the resting heart but augmented it in the transaortic constriction heart. This functional retargeting was associated with PDE5 translocation from sarcomeres to a dispersed distribution. P5(+) hearts exhibited higher oxidative stress, whereas P5(+)/N3(-) hearts had low levels (likely owing to the absence of nitric oxide synthase 3 uncoupling). This highlights the importance of myocyte protein kinase G activity as a protection for pathological remodeling.

CONCLUSIONS

These data provide the first evidence for functional retargeting of PDE5 from one compartment to another, revealing a role for natriuretic peptide-derived cGMP hydrolysis by this esterase in diseased heart myocardium. Retargeting likely affects the pathophysiological consequence and the therapeutic impact of PDE5 modulation in heart disease.

H₂S signalling through protein sulfhydration and beyond

Hydrogen sulfide (H(2)S) has recently emerged as a mammalian gaseous messenger molecule, akin to nitric oxide and carbon monoxide. H(2)S is predominantly formed from Cys or its derivatives by the enzymes cystathionine β-synthase and cystathionine γ-lyase. One of the mechanisms by which H(2)S signals is by sulfhydration of reactive Cys residues in target proteins. Although analogous to protein nitrosylation, sulfhydration is substantially more prevalent and usually increases the catalytic activity of targeted proteins. Physiological actions of sulfhydration include the regulation of inflammation and endoplasmic reticulum stress signalling as well as of vascular tension.

Intracellular nitric oxide mediates neuroproliferative effect of neuropeptide y on postnatal hippocampal precursor cells

Neuropeptide Y (NPY) is widely expressed in the central and peripheral nervous systems and is proliferative for a range of cells types in vitro. NPY plays a key role in regulating adult hippocampal neurogenesis in vivo under both basal and pathological conditions, although the underlying mechanisms are largely unknown. We have investigated the role of nitric oxide (NO) on the neurogenic effects of NPY. Using postnatal rat hippocampal cultures, we show that the proliferative effect of NPY on nestin(+) precursor cells is NO-dependent. As well as the involvement of neuronal nitric-oxide synthase, the proliferative effect is mediated via an NO/cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (PKG) and extracellular signal-regulated kinase (ERK) 1/2 signaling pathway. We show that NPY-mediated intracellular NO signaling results in an increase in neuroproliferation. By contrast, extracellular NO had an opposite, inhibitory effect on proliferation. The importance of the NO-cGMP-PKG signaling pathway in ERK1/2 activation was confirmed using Western blotting. This work unites two significant modulators of hippocampal neurogenesis within a common signaling framework and provides a mechanism for the independent extra- and intracellular regulation of postnatal neural precursors by NO.

The pharmacological treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening and progressive disease of various origins characterized by pulmonary vascular remodeling that leads to increased pulmonary vascular resistance and pulmonary arterial pressure, most often resulting in right-sided heart failure. The most common symptom at presentation is breathlessness, with impaired exercise capacity as a hallmark of the disease. Advances in understanding the pathobiology over the last 2 decades have led to therapies (endothelin receptor antagonists, phosphodiesterase type 5 inhibitors, and prostacyclins or analogs) initially directed at reversing the pulmonary vasoconstriction and more recently directed toward reversing endothelial cell dysfunction and smooth muscle cell proliferation. Despite these advances, disease progression is common even with use of combination regimens targeting multiple mechanistic pathways. Overall 5-year survival for PAH has increased significantly from approximately 30% in the 1980s to approximately 60% at present, yet remains abysmal. This review summarizes the mechanisms of action, clinical data, and regulatory histories of approved PAH therapies and describes the latest agents in late-stage clinical development.

Nitric oxide increases susceptibility of Toll-like receptor-activated macrophages to spreading Listeria monocytogenes

Toll-like receptor (TLR) stimulation activates macrophages to resist intracellular pathogens. Yet, the intracellular bacterium Listeria monocytogenes (Lm) causes lethal infections in spite of innate immune cell activation. Lm uses direct cell-cell spread to disseminate within its host. Here, we have shown that TLR-activated macrophages killed cell-free Lm but failed to prevent infection by spreading Lm. Instead, TLR signals increased the efficiency of Lm spread from "donor" to "recipient" macrophages. This enhancement required nitric oxide (NO) production by nitric oxide synthase-2 (NOS2). NO increased Lm escape from secondary vacuoles in recipient cells and delayed maturation of phagosomes containing membrane-like particles that mimic Lm-containing pseudopods. NO also promoted Lm spread during systemic in vivo infection, as shown by the fact that inhibition of NOS2 with 1400W reduced spread-dependent Lm burdens in mouse livers. These findings reveal a mechanism by which pathogens capable of cell-cell spread can avoid the consequences of innate immune cell activation by TLR stimuli.

Hydrogen sulfide and nitric oxide are mutually dependent in the regulation of angiogenesis and endothelium-dependent vasorelaxation

Hydrogen sulfide (H(2)S) is a unique gasotransmitter, with regulatory roles in the cardiovascular, nervous, and immune systems. Some of the vascular actions of H(2)S (stimulation of angiogenesis, relaxation of vascular smooth muscle) resemble those of nitric oxide (NO). Although it was generally assumed that H(2)S and NO exert their effects via separate pathways, the results of the current study show that H(2)S and NO are mutually required to elicit angiogenesis and vasodilatation. Exposure of endothelial cells to H(2)S increases intracellular cyclic guanosine 5'-monophosphate (cGMP) in a NO-dependent manner, and activated protein kinase G (PKG) and its downstream effector, the vasodilator-stimulated phosphoprotein (VASP). Inhibition of endothelial isoform of NO synthase (eNOS) or PKG-I abolishes the H(2)S-stimulated angiogenic response, and attenuated H(2)S-stimulated vasorelaxation, demonstrating the requirement of NO in vascular H(2)S signaling. Conversely, silencing of the H(2)S-producing enzyme cystathionine-γ-lyase abolishes NO-stimulated cGMP accumulation and angiogenesis and attenuates the acetylcholine-induced vasorelaxation, indicating a partial requirement of H(2)S in the vascular activity of NO. The actions of H(2)S and NO converge at cGMP; though H(2)S does not directly activate soluble guanylyl cyclase, it maintains a tonic inhibitory effect on PDE5, thereby delaying the degradation of cGMP. H(2)S also activates PI3K/Akt, and increases eNOS phosphorylation at its activating site S1177. The cooperative action of the two gasotransmitters on increasing and maintaining intracellular cGMP is essential for PKG activation and angiogenesis and vasorelaxation. H(2)S-induced wound healing and microvessel growth in matrigel plugs is suppressed by pharmacological inhibition or genetic ablation of eNOS. Thus, NO and H(2)S are mutually required for the physiological control of vascular function.

Familial diarrhea syndrome caused by an activating GUCY2C mutation

BACKGROUND

Familial diarrhea disorders are, in most cases, severe and caused by recessive mutations. We describe the cause of a novel dominant disease in 32 members of a Norwegian family. The affected members have chronic diarrhea that is of early onset, is relatively mild, and is associated with increased susceptibility to inflammatory bowel disease, small-bowel obstruction, and esophagitis.

METHODS

We used linkage analysis, based on arrays with single-nucleotide polymorphisms, to identify a candidate region on chromosome 12 and then sequenced GUCY2C, encoding guanylate cyclase C (GC-C), an intestinal receptor for bacterial heat-stable enterotoxins. We performed exome sequencing of the entire candidate region from three affected family members, to exclude the possibility that mutations in genes other than GUCY2C could cause or contribute to susceptibility to the disease. We carried out functional studies of mutant GC-C using HEK293T cells.

RESULTS

We identified a heterozygous missense mutation (c.2519G→T) in GUCY2C in all affected family members and observed no other rare variants in the exons of genes in the candidate region. Exposure of the mutant receptor to its ligands resulted in markedly increased production of cyclic guanosine monophosphate (cGMP). This may cause hyperactivation of the cystic fibrosis transmembrane regulator (CFTR), leading to increased chloride and water secretion from the enterocytes, and may thus explain the chronic diarrhea in the affected family members.

CONCLUSIONS

Increased GC-C signaling disturbs normal bowel function and appears to have a proinflammatory effect, either through increased chloride secretion or additional effects of elevated cellular cGMP. Further investigation of the relevance of genetic variants affecting the GC-C-CFTR pathway to conditions such as Crohn's disease is warranted. (Funded by Helse Vest [Western Norway Regional Health Authority] and the Department of Science and Technology, Government of India.).

Hydrogen peroxide enhances vasodilatation by increasing dimerization of cGMP-dependent protein kinase type Iα

BACKGROUND

cGMP-dependent protein kinase type I (PKG I) plays a key role in vasodilatation caused by cGMP-elevating agents. It is a homodimer in mammalian cells, existing as 2 isoforms, Iα and Iβ. The aim of the present study was both to determine whether PKG I dimerization and activity are modulated by hydrogen peroxide (H(2)O(2)) and its influence on vasodilatation.

METHODS AND RESULTS

The dimers and monomers of total PKG I and PKG Iβ were analyzed by Western blotting. PKG I activity was assayed by measuring the incorporation of (32)P into BPDEtide. Changes in vessels tension were determined by organ chamber technique. In isolated porcine coronary arteries, H(2)O(2) increased the dimers of total PKG I in a concentration-dependent manner, but had no effect on dimerization of PKG Iβ. The dimerization of PKG I caused by H(2)O(2) was prevented by catalase but not by deferoxamine and tiron. H(2)O(2) promoted the translocation of PKG I from cytoplasm to membrane. H(2)O(2) enhanced the activity of PKG I and relaxations of porcine coronary arteries to the nitric oxide donor and 8-Br-cGMP. Inhibition of catalase under in vivo conditions significantly decreased rat mean arterial pressure, which was associated with increased dimerization of PKG I.

CONCLUSIONS

The present study suggests that H(2)O(2) may enhance the activity of PKG Iα-and PKG I-dependent vasodilatation via increased dimerization of the enzyme.

Insight into the rescue of oxidized soluble guanylate cyclase by the activator cinaciguat

Nitric oxide (NO) signaling mediates many important physiological processes through the receptor soluble guanylate cyclase (sGC). Under disease conditions sGC heme can be oxidized resulting in NO insensitivity. Here, we show that the therapeutic compound cinaciguat (Cin) rescues dysfunctional sGC by direct displacement of the oxidized heme.

Transient rise of serum testosterone level after single sildenafil treatment of adult male rats

INTRODUCTION

Phosphodiesterase type 5 (PDE5) inhibitors have been established in therapy for a variety of physiological disorders including erectile dysfunction. Despite its popularity and wide usage in erectile dysfunction treatment, the short-term effect of PDE5 inhibition on Leydig cell functionality and testosterone dynamics is missing.

AIM

This study was designed to assess the acute in vivo effects of sildenafil citrate (Viagra) treatment on testosterone production.

METHODS

Male adult rats were given sildenafil (1.25 mg/kg BW) per os, and testosterone production were analyzed 30, 60, 120, and 180 minutes after treatment. Additionally, in vitro effect of sildenafil extract on Leydig cell steroidogenesis was estimated.

MAIN OUTCOME MEASURES

The formation of testicular interstitial fluid (TIF), and testosterone, cyclic guanosine monophosphate (cGMP), cyclic adenosine monophosphate (cAMP) content was followed. Occurrence and phosphorylation of mature steroidogenic acute regulatory protein (StAR) and interaction with protein kinase G 1 (PRKG1) were assessed by immunoprecipitation and Western blot.

RESULTS

Serum testosterone was increased 60 and 120 minutes after sildenafil treatment. In 60 minutes, TIF volume was doubled and stayed increased till the end of the experimental period. cGMP and testosterone content in TIF were increased 30 minutes after treatment, and cAMP decreased in 60 minutes. Further, sildenafil-induced stimulation of testosterone production was abolished by ex vivo addition of PRKG1 inhibitor but not by protein kinase A inhibitor. Sildenafil treatment increased the level of phosphorylated and total StAR protein. Moreover, co-immunoprecipitation of StAR and PRKG1 was increased following sildenafil treatment suggesting the active role of this kinase in initiation of testosterone synthesis. Additionally, sildenafil extract applied in vitro on primary Leydig cell culture increased cGMP accumulation and testosterone production in time- and dose-dependent manner without effect on cAMP level.

CONCLUSION

Acute sildenafil treatment enlarged TIF volume but also stimulated testosterone production which may be significant considering the positive testosterone effect in regulation of sexual activity.

Neuronal nitric oxide synthase inhibition attenuates the development of L-DOPA-induced dyskinesia in hemi-Parkinsonian rats

Long-term treatment with the dopamine precursor levodopa (l-DOPA) frequently induces dyskinesia in Parkinson's disease patients, which is a major complication of this therapy. Previous studies using animal models show that repeated administration of l-DOPA results in alterations of some signaling molecules, including ΔFosB, phospho-DARPP32 and phosoho-GluA1 (also referred to as GluR1 or GluR-A) AMPA receptor subunits. Moreover, an in vivo microdialysis study showed that l-DOPA increases nitric oxide (NO) production in the striatum. However, it is not known whether NO is involved in the development of dyskinesia. The present study examined the effects of NOS inhibitors on the development of l-DOPA-induced dyskinesia in the rats. Dyskinesia symptoms were triggered by daily administration of l-DOPA for 3-4weeks in unilateral 6-hydroxydopamine lesioned rats. Repeated treatments, 30min prior l-DOPA administration, of the nonselective NOS inhibitor, N(G)-nitro-l-arginine methyl ester, and the nNOS inhibitor 7-nitroindazole, but not the inducible NOS inhibitor aminoguanidine, attenuated the development of l-DOPA-induced dyskinesia. In agreement with the behavioral analysis, 7-nitroindazole reduced the l-DOPA-induced increases in ΔFosB, phospho-DARPP32 and phospho-GluA1 AMPA receptor subunit levels in the striatum of 6-hydroxydopamine-lesioned rats. Furthermore, aminoguanidine did not affect ΔFosB or phospho-GluA1 AMPA receptor subunit levels. These findings suggest that nNOS-derived NO is involved in the development of l-DOPA-induced dyskinesia through a post-synaptic mechanism.

The problem of nitrogen disposal in the obese

Amino-N is preserved because of the scarcity and nutritional importance of protein. Excretion requires its conversion to ammonia, later incorporated into urea. Under conditions of excess dietary energy, the body cannot easily dispose of the excess amino-N against the evolutively adapted schemes that prevent its wastage; thus ammonia and glutamine formation (and urea excretion) are decreased. High lipid (and energy) availability limits the utilisation of glucose, and high glucose spares the production of ammonium from amino acids, limiting the synthesis of glutamine and its utilisation by the intestine and kidney. The amino acid composition of the diet affects the production of ammonium depending on its composition and the individual amino acid catabolic pathways. Surplus amino acids enhance protein synthesis and growth, and the synthesis of non-protein-N-containing compounds. But these outlets are not enough; consequently, less-conventional mechanisms are activated, such as increased synthesis of NO∙ followed by higher nitrite (and nitrate) excretion and changes in the microbiota. There is also a significant production of N(2) gas, through unknown mechanisms. Health consequences of amino-N surplus are difficult to fathom because of the sparse data available, but it can be speculated that the effects may be negative, largely because the fundamental N homeostasis is stretched out of normalcy, forcing the N removal through pathways unprepared for that task. The unreliable results of hyperproteic diets, and part of the dysregulation found in the metabolic syndrome may be an unwanted consequence of this N disposal conflict.

The nitric oxide pathway participates in lordosis behavior induced by central administration of leptin

Intracerebroventricular (icv) administration of leptin facilitates lordosis behavior in ad libitum-fed, estrogen-primed rats. The cellular mechanism involved in this response is unknown. The present study tested the hypothesis that the nitric oxide-guanylyl cyclase, cGMP-dependent protein kinase (PKG) pathway is involved in the facilitation of lordosis behavior induced by the central administration of leptin. We tested the importance of the nitric oxide/cGMP pathway for lordosis stimulation by either icv infusion of a nitric oxide synthase inhibitor (L-NAME) or a nitric oxide-dependent, soluble guanylyl cyclase inhibitor (ODQ) 30 min before leptin administration (1 μg). This dose of leptin reliably induced lordosis behavior in ovariectomized estradiol benzoate treated rats. The lordosis induced by leptin at 1 and 2h after infusion was significantly reduced by the previous injection of either L-NAME or by ODQ. Intracerebroventricular infusion of the PKG inhibitor (KT5823) 30 min before leptin infusion, also significantly inhibited the lordosis behavior induced by leptin at 1 and 2h after hormone administration. These data support the hypothesis that the nitric oxide/cGMP/PKG pathway is involved in the facilitation of lordosis by leptin in estrogen-primed female rats.

Polymer-Based Nitric Oxide Therapies: Recent Insights for Biomedical Applications

Since the discovery of nitric oxide (NO) in the 1980s, this cellular messenger has been shown to participate in diverse biological processes such as cardiovascular homeostasis, immune response, wound healing, bone metabolism, and neurotransmission. Its beneficial effects have prompted increased research in the past two decades, with a focus on the development of materials that can locally release NO. However, significant limitations arise when applying these materials to biomedical applications. This Feature Article focuses on the development of NO-releasing and NO-generating polymeric materials (2006-2011) with emphasis on recent in vivo applications. Results are compared and discussed in terms of NO dose, release kinetics, and biological effects, in order to provide a foundation to design and evaluate new NO therapies.

Platelet hyperaggregability in high-fat fed rats: a role for intraplatelet reactive-oxygen species production

Background

Adiposity greatly increases the risk of atherothrombotic events, a pathological condition where a chronic state of oxidative stress is reported to play a major role. This study aimed to investigate the involvement of (NO)-soluble guanylyl cyclase (sGC) signaling pathway in the platelet dysfunction from high fat-fed (HFF) rats.

Methods

Male Wistar rats were fed for 10 weeks with standard chow (SCD) or high-fat diet (HFD). ADP (10 μM)- and thrombin (100 mU/ml)-induced washed platelet aggregation were evaluated. Measurement of intracellular levels of ROS levels was carried out using flow cytometry. Cyclic GMP levels were evaluated using ELISA kits.

Results

High-fat fed rats exhibited significant increases in body weight, epididymal fat, fasting glucose levels and glucose intolerance compared with SCD group. Platelet aggregation induced by ADP (n = 8) and thrombin from HFD rats (n = 8) were significantly greater (P < 0.05) compared with SCD group. Platelet activation with ADP increased by 54% the intraplatelet ROS production in HFD group, as measured by flow cytometry (n = 6). N-acetylcysteine (NAC; 1 mM) and PEG-catalase (1000 U/ml) fully prevented the increased ROS production and platelet hyperaggregability in HFD group. The NO donors sodium nitroprusside (SNP; 10 μM) and SNAP (10 μM), as well as the NO-independent soluble guanylyl cyclase stimulator BAY 41-2272 (10 μM) inhibited the platelet aggregation in HFD group with lower efficacy (P < 0.05) compared with SCD group. The cGMP levels in response to these agents were also markedly lower in HFD group (P < 0.05). The prostacyclin analogue iloprost (1 μM) reduced platelet aggregation in HFD and SCD rats in a similar fashion (n = 4).

Conclusions

Metabolic abnormalities as consequence of HFD cause platelet hyperaggregability involving enhanced intraplatelet ROS production and decreased NO bioavailability that appear to be accompanied by potential defects in the prosthetic haem group of soluble guanylyl cyclase.

β-Carotene and lycopene affect endothelial response to TNF-α reducing nitro-oxidative stress and interaction with monocytes

SCOPE

Cardiovascular disease (CVD) is associated with vascular oxidative imbalance and inflammation. Increased reactive oxygen species (ROS) generation is associated with a functional inactivation of nitric oxide (NO) due to the reaction with O₂⁻, leading to peroxynitrite (ONOO⁻) formation and subsequent reduction in the beneficial effect of vascular NO bioavailability. Carotenoids'-rich diets have been associated with decreased risk of CVD, but the underlying mechanism is still unknown.

METHODS AND RESULTS

In human umbilical vein endothelial cells (HUVECs), both β-carotene (BC) or lycopene (Lyc) significantly affected tumor necrosis factor-α (TNF-α)-induced inflammation, being associated with a significant decrease in the generation of ROS (spectrofluorometry) and nitrotyrosine (an index of ONOO⁻ formation, cytofluorimetry), an increased NO/cGMP (cyclic guanosine monophosphate) levels (EIA), and a down-regulation of NF-κB-dependent adhesion molecule expression (Western blot and EMSA) and monocyte-HUVEC interaction (adhesion assay). Our results indicate that BC or Lyc treatment reduce the inflammatory response in TNF-α-treated HUVECs. This is due to the redox balance protection and to the maintenance of NO bioavailability.

CONCLUSION

Our observations provide background for a novel mechanism for carotenoids' anti-inflammatory activity in the vasculature and may contribute to a better understanding of the protective effects of carotenoid-rich diets against CVD risk.

Pro-survival effects of 17β-estradiol on osteocytes are mediated by nitric oxide/cGMP via differential actions of cGMP-dependent protein kinases I and II

Estrogens promote bone health in part by increasing osteocyte survival, an effect that requires activation of the protein kinases Akt and ERK1/2, but the molecular mechanisms involved are only partly understood. Because estrogens increase nitric oxide (NO) synthesis and NO can have anti-apoptotic effects, we examined the role of NO/cGMP signaling in estrogen regulation of osteocyte survival. Etoposide-induced death of MLO-Y4 osteocyte-like cells, assessed by trypan blue staining, caspase-3 cleavage, and TUNEL assays, was completely prevented when cells were pre-treated with 17β-estradiol. This protective effect was mimicked when cells were pre-treated with a membrane-permeable cGMP analog and blocked by pharmacological inhibitors of NO synthase, soluble guanylate cyclase, or cGMP-dependent protein kinases (PKGs), supporting a requirement for NO/cGMP/PKG signaling downstream of 17β-estradiol. siRNA-mediated knockdown and viral reconstitution of individual PKG isoforms demonstrated that the anti-apoptotic effects of estradiol and cGMP were mediated by PKG Iα and PKG II. Akt and ERK1/2 activation by 17β-estradiol required PKG II, and cGMP mimicked the effects of estradiol on Akt and ERK, including induction of ERK nuclear translocation. cGMP induced BAD phosphorylation on several sites, and experiments with phosphorylation-deficient BAD mutants demonstrated that the anti-apoptotic effects of cGMP and 17β-estradiol required BAD phosphorylation on Ser(136) and Ser(155); these sites were targeted by Akt and PKG I, respectively, and regulate BAD interaction with Bcl-2. In conclusion, 17β-estradiol protects osteocytes against apoptosis by activating the NO/cGMP/PKG cascade; PKG II is required for estradiol-induced activation of ERK and Akt, and PKG Iα contributes to pro-survival signaling by directly phosphorylating BAD.

Role of PDE3A in regulation of cell cycle progression in mouse vascular smooth muscle cells and oocytes: implications in cardiovascular diseases and infertility

Phosphodiesterase-3 (PDE3) is a major cAMP-hydrolyzing PDE in vascular smooth muscle cells (VSMCs) and oocytes. The exact role and contribution of the two PDE3 isoforms, PDE3A and PDE3B, in VSMC growth regulation and oocyte maturation was examined using PDE3A (3A) and PDE3B (3B) knockout (KO) mouse models. PDE3A-deficient VSMCs exhibit marked reduction in mitogen-induced cell growth due to cell cycle arrest at G₀-G₁ phase, which resulted from dysregulation of cAMP/protein kinase A (PKA)-activated and mitogen-activated protein kinase (MAPK)-signaling pathways, as well as from alterations in key cell cycle regulatory proteins. Similarly, PDE3A-deficient oocytes exhibit cell cycle arrest at G₂/M phase because increased cAMP/PKA signaling in KO oocytes most likely inhibits Cdc25B-catalyzed dephosphorylation/activation of Cdc2 (maturation promoting factor (MPF)), a key regulator of G₂/M transition.

Differential activation of cAMP- and cGMP-dependent protein kinases by cyclic purine and pyrimidine nucleotides

The cyclic purine nucleotides cAMP and cGMP are well-characterized second messengers and activators of PKA and PKG, respectively. In contrast, the functions of the cyclic pyrimidine nucleotides cCMP and cUMP are poorly understood. cCMP induces relaxation of smooth muscle via PKGI, and phosphodiesterases differentially hydrolyze cNMPs. Here, we report that cNMPs differentially activate PKA isoforms and PKGIα. The combination of cCMP with cAMP reduced the EC(50) of cAMP for PKA. PKGIα exhibited higher specificity for the cognate cNMP than PKA. Our data support a role of cCMP and cUMP as second messengers.

Target highlights in CASP9: Experimental target structures for the critical assessment of techniques for protein structure prediction

One goal of the CASP community wide experiment on the critical assessment of techniques for protein structure prediction is to identify the current state of the art in protein structure prediction and modeling. A fundamental principle of CASP is blind prediction on a set of relevant protein targets, that is, the participating computational methods are tested on a common set of experimental target proteins, for which the experimental structures are not known at the time of modeling. Therefore, the CASP experiment would not have been possible without broad support of the experimental protein structural biology community. In this article, several experimental groups discuss the structures of the proteins which they provided as prediction targets for CASP9, highlighting structural and functional peculiarities of these structures: the long tail fiber protein gp37 from bacteriophage T4, the cyclic GMP-dependent protein kinase Iβ dimerization/docking domain, the ectodomain of the JTB (jumping translocation breakpoint) transmembrane receptor, Autotaxin in complex with an inhibitor, the DNA-binding J-binding protein 1 domain essential for biosynthesis and maintenance of DNA base-J (β-D-glucosyl-hydroxymethyluracil) in Trypanosoma and Leishmania, an so far uncharacterized 73 residue domain from Ruminococcus gnavus with a fold typical for PDZ-like domains, a domain from the phycobilisome core-membrane linker phycobiliprotein ApcE from Synechocystis, the heat shock protein 90 activators PFC0360w and PFC0270w from Plasmodium falciparum, and 2-oxo-3-deoxygalactonate kinase from Klebsiella pneumoniae.

Hypoxic regulation of pulmonary vascular smooth muscle cyclic guanosine monophosphate-dependent kinase by the ubiquitin conjugating system

We previously reported that hypoxia attenuates nitric oxide-cyclic guanosine monophosphate (NO-cGMP)-mediated fetal pulmonary vessel relaxation by inhibiting cGMP-dependent protein kinase 1 (PKG1) activity, but not all the mechanisms by which acute hypoxia inhibits PKG1 activity have been delineated. Here we demonstrate for the first time, to the best of our knowledge, that acute hypoxia induces an accumulation of ubiquitinated PKG1 in ovine fetal and newborn pulmonary artery smooth muscle cells. Such a modification was not evident in ovine fetal systemic (cerebral) artery smooth muscle cells. The accumulation of polyubiquitinated PKG1 observed after 4 hours of hypoxia was affected neither by the activation of PKG1 kinase activity with the cell-permeable cGMP analogue 8-bromo-cGMP, nor by its inhibition with DT-3 in fetal pulmonary artery smooth muscle cells. Ubiquitinated PKG1α was unable to bind the cGMP analogue 8-(2-aminoethyl)thioguanosine-3',5' (AET)-cGMP, a ligand for the unmodified protein. Inhibition of the proteasomal complex with MG132 led to the accumulation of polyubiquitinated PKG1 in normoxia, indicating the involvement of the ubiquitin-26S proteasomal system in degradation and clearance of this protein under normoxic conditions. The ubiquitinated PKG1 under hypoxic conditions, however, was not predominantly targeted for proteasomal degradation. Importantly, reoxygenation reversed the acute hypoxia-induced accumulation of ubiquitinated PKG1. Our results suggest that the PKG1 ubiquitination induced by acute hypoxia plays a unique role in the regulation of the pulmonary vascular smooth muscle cell vasoreactivity and relaxation mediated by the NO-cGMP-PKG1 pathway.

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.

Sub-cellular targeting of constitutive NOS in health and disease

Constitutive nitric oxide synthases (NOSs) are ubiquitous enzymes that play a pivotal role in the regulation of myocardial function in health and disease. The discovery of both a neuronal NOS (nNOS) and an endothelial NOS (eNOS) isoform in the myocardium and the availability of genetically modified mice with selective eNOS or nNOS gene deletion have been of crucial importance for understanding the role of constitutive nitric oxide (NO) production in the myocardium. eNOS and nNOS are homologous in structure and utilize the same co-factors and substrates; however, they differ in their subcellular localization, regulation, and downstream signaling, all of which may account for their distinct effects on excitation-contraction coupling. In particular, eNOS-derived NO has been reported to increase left ventricular (LV) compliance, attenuate beta-adrenergic inotropy and enhance parasympathetic/muscarinic responses, and mediate the negative inotropic response to β3 adrenoreceptor stimulation via cGMP-dependent signaling. Conversely, nNOS-derived NO regulates basal myocardial inotropy and relaxation by inhibiting the sarcolemmal Ca(2+) current (I(Ca)) and promoting protein kinase A-dependent phospholamban (PLN) phosphorylation, independent of cGMP. By inhibiting the activity of myocardial oxidase systems, nNOS regulates the redox state of the myocardium and contributes to maintain eNOS "coupled" activity. After myocardial infarction, up-regulation of myocardial nNOS attenuates adverse remodeling and prevents arrhythmias whereas uncoupled eNOS activity in murine models of left ventricular pressure overload accelerates the progress towards heart failure. Here we review the evidence in support of the idea that NOS subcellular localization, mode of activation, and downstream signaling account for the diverse and highly specialized actions of NO in the heart. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

PDE5 inhibitors: targeting erectile dysfunction in diabetics

Erectile dysfunction (ED) is strongly linked to cardiovascular disease (CVD), especially in diabetics. ED is associated with deleterious changes in the overall vasculature and is recognized as an indicator of higher risk for adverse cardiovascular events. Endothelial dysfunction, vascular smooth muscle changes and increased fibrosis are indicated as major players in both ED and CVD. ED in diabetics is more refractory to acute treatment with phosphodiesterase-5 (PDE5) inhibitors (Viagra, Cialis, Levitra, Zydena) than in non-diabetics, but recent studies indicate that chronic administration of these drugs improves endothelial function, preserves vascular smooth muscle and decreases fibrotic changes. Use of PDE5 inhibitors in pre-diabetic and diabetic men may protect cardiovascular health, including vascular function in penile tissues.

Gender bias in gastroparesis: is nitric oxide the answer?

Accumulating evidence suggests that gender-related differences are prominent in gastric motility functions in both health and disease. Women are more susceptible to gastroparesis than men. Though the mechanism(s) involved are not fully understood, impairment of the nitrergic system is one of the main factors responsible for the disease. Uncoupling of neuronal nitric oxide synthase (nNOS) causes a decreased synthesis of NO leading to a reduction in smooth muscle relaxation. Tetrahydrobiopterin (BH(4)) (an essential cofactor for nNOS) is a key regulator of nNOS activity for stomach dysfunction and gastroparesis. In addition, BH(4) has been shown to be a potent antioxidant and anti-inflammatory agent. Well established by results from our laboratory, a diminished intracellular (BH(4):total biopterin) ratio in diabetic female rats significantly impairs nNOS activity and function. Recent research has been focused on BH(4) biosynthesis and gastroparesis because reduced BH(4) cofactor levels can alter the production of NO by nNOS. Researchers are now paying more attention to the possibility of using BH(4) as a therapeutic strategy in gastroparesis. The purpose of this review is to provide an overview of the regulation and function of nNOS by sex hormones and BH(4) and its potential role in the treatment of gastroparesis.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Interaction between nitric oxide signaling and gap junctions: effects on vascular function

Nitric oxide signaling, through eNOS (or possibly nNOS), and gap junction communication are essential for normal vascular function. While each component controls specific aspects of vascular function, there is substantial evidence for cross-talk between nitric oxide signaling and the gap junction proteins (connexins), and more recently, protein-protein association between eNOS and connexins. This review will examine the evidence for interaction between these pathways in normal and diseased arteries, highlight the questions that remain about the mechanisms of their interaction, and explore the possible interaction between nitric oxide signaling and the newly discovered pannexin channels. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Neuronal nitric oxide contributes to neuroplasticity-associated protein expression through cGMP, protein kinase G, and extracellular signal-regulated kinase

Nitric oxide (NO) synthesized by neuronal NO synthase (nNOS) has long been implicated in brain plasticity. However, it is unclear how this short-lived mediator contributes to the long-term molecular changes underlying neuroplasticity, which typically require activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) signaling pathway and gene expression. To address this issue, we used a neuroplasticity model based on treatment of neuronal cultures with bicuculline and a model of experience-dependent plasticity in the barrel cortex. In neuronal cultures, NOS inhibition attenuated the bicuculline-induced activation of ERK and the expression of c-Fos, Egr-1, Arc, and brain-derived neurotrophic factor (BDNF), proteins essential for neuroplasticity. Furthermore, inhibition of the NO target soluble guanylyl cyclase or of the cGMP effector kinase protein kinase G (PKG) reduced both ERK activation and plasticity-related protein expression. NOS inhibition did not affect phosphorylation of cAMP response element-binding protein (CREB), a well-established ERK nuclear target, but it attenuated the nuclear accumulation of the CREB coactivator TORC1 and suppressed the activation of Elk-1, another transcription factor target of ERK. Consistent with these in vitro observations, induction of c-Fos, Egr-1, and BDNF was attenuated in the D1 cortical barrel of nNOS(-/-) mice subjected to single whisker experience. These results establish nNOS-derived NO as a key factor in the expression of proteins involved in neuroplasticity, an effect mediated through cGMP, PKG, and ERK signaling. These actions of NO do not depend on CREB phosphorylation but may involve TORC1 and Elk-1. Our data unveil a previously unrecognized link between neuronal NO and the molecular machinery responsible for the sustained synaptic changes underlying neuroplasticity.

Nitric oxide signaling: classical, less classical, and nonclassical mechanisms

Although nitric oxide (NO) was identified more than 150 years ago and its effects were clinically tested in the form of nitroglycerine, it was not until the decades of 1970-1990 that it was described as a gaseous signal transducer. Since then, a canonical pathway linked to cyclic GMP (cGMP) as its quintessential effector has been established, but other modes of action have emerged and are now part of the common body of knowledge within the field. Classical (or canonical) signaling involves the selective activation of soluble guanylate cyclase, the generation of cGMP, and the activation of specific kinases (cGMP-dependent protein kinases) by this cyclic nucleotide. Nonclassical signaling alludes to the formation of NO-induced posttranslational modifications (PTMs), especially S-nitrosylation, S-glutathionylation, and tyrosine nitration. These PTMs are governed by specific biochemical mechanisms as well as by enzymatic systems. In addition, a less classical but equally important pathway is related to the interaction between NO and mitochondrial cytochrome c oxidase, which might have important implications for cell respiration and intermediary metabolism. Cross talk trespassing these necessarily artificial conceptual boundaries is progressively being identified and hence an integrated systems biology approach to the comprehension of NO function will probably emerge in the near future.

Phosphodiesterase type 5 inhibitors in the management of non-neurogenic male lower urinary tract symptoms: critical analysis of current evidence

CONTEXT

A large body of epidemiologic data suggests a causal relationship between lower urinary tract symptoms (LUTS) and erectile dysfunction (ED). Recently reported studies on phosphodiesterase type 5 inhibitors (PDE5-Is) and LUTS have further contributed to the understanding of mechanisms involved in this relationship and of potential treatment options.

OBJECTIVE

A nonsystematic descriptive review was performed to summarize the literature concerning the role of PDE5-Is in men with LUTS, particularly looking at data derived from clinical trials in relation to the different PDE5-Is or their association with α-blockers.

EVIDENCE ACQUISITION

A comprehensive electronic search was conducted in October 2010 using the Medline database to identify all publications relating to ED and BPH and treatment with sildenafil, vardenafil, tadalafil, udenafil, UK-369003, and combination therapy with alfuzosin and tamsulosin.

EVIDENCE SYNTHESIS

In studies in which either ED or LUTS was the entry criterion, sildenafil appears to improve both erectile function and LUTS in subjects with ED. Placebo-controlled trials of tadalafil and vardenafil showed improvement of LUTS secondary to benign prostatic hyperplasia (BPH), but none of the studies showed a significant effect on urodynamic measures. Exploratory studies with UK-369003 showed improvements in LUTS and ED. Sildenafil or tadalafil associated with alfuzosin or tamsulosin showed greater benefits for the combination therapy for both LUTS and ED. The coadministration of udenafil and an α-blocker in patients with BPH and ED also appeared to improve both LUTS and ED severity.

CONCLUSIONS

Consistent evidence of improvements in LUTS has been shown with PDE5-Is, either alone or in combination with α-blockers. However, effects on urodynamics or objective measures of urinary flow are lacking. Further areas of research include investigation of mechanism of PDE5-Is, urodynamic studies, identification of new efficacy end points, head-to-head comparison with standard of care, potential benefit of add-on treatment, and long-term outcomes.

Sinusoidal electromagnetic field stimulates rat osteoblast differentiation and maturation via activation of NO-cGMP-PKG pathway

Nitric oxide (NO) is an important intracellular and intercellular messenger, critically affecting bone metabolism. The purpose of this research is to investigate whether the effect of sinusoidal electromagnetic field (SEMF) on the differentiation and maturation of osteoblasts is mediated by the NO-cGMP-PKG signal pathway. We examined the impact of SEMF on nitric oxide synthase (NOS) activity, and found that L-NAME, nitric oxide synthase's inhibitor, prevents SEMF-mediated increase in NOS activity and NO levels. We showed that an inhibitor of soluble guanylyl cyclase (ODQ) blocks the increase in cGMP levels triggered by exposure to SEMF. The inhibitor PDE5, which hydrolyzes 3',5'-cyclic-GMP to 5'-GMP, prevents the SEMF's stimulation of PKG activity. We also blocked the NO-cGMP-PKG pathway to determine whether the maturation and mineralization of osteoblasts, stimulated by SEMF, would be inhibited. This was evaluated by measuring alkaline phosphatase (ALP) activity, osterix gene expression and mineralized bone modulus. After treatment with SEMF, the NOS activity increases in comparison with the control group (P<0.01), reaching the highest level after 0.5h. Osterix gene expression, ALP activity and mineralized bone nodules in the SEMF experimental group also increase significantly. However, these effects are partially blocked in the L-NAME treated cultures. Surprisingly, all the osteogenic markers in the SEMF+L-NAME group were slightly higher than in the control culture, but lower than in the cells exposed to SEMF only. We conclude that the NO-cGMP-PKG signal pathway is activated by SEMF treatment, the stimulatory effect of SEMF on the differentiation and mineralization of osteoblasts is attenuated when the pathway is blocked.

Nitric oxide regulates pulmonary vascular smooth muscle cell expression of the inducible cAMP early repressor gene

Nitric oxide (NO) regulates vascular smooth muscle cell (VSMC) structure and function, in part by activating soluble guanylate cyclase (sGC) to synthesize cGMP. The objective of this study was to further characterize the signaling mechanisms by which NO regulates VSMC gene expression using transcription profiling. DNA microarrays were hybridized with RNA extracted from rat pulmonary artery smooth muscle cells (RPaSMC) exposed to the NO donor compound, S-nitroso-glutathione (GSNO). Many of the genes, whose expression was induced by GSNO, contain a cAMP-response element (CRE), of which one encoded the inducible cAMP early repressor (ICER). sGC and cAMP-dependent protein kinase, but not cGMP-dependent protein kinase, were required for NO-mediated phosphorylation of CRE-binding protein (CREB) and induction of ICER gene expression. Expression of a dominant-negative CREB in RPaSMC prevented the NO-mediated induction of CRE-dependent gene transcription and ICER gene expression. Pre-treatment of RPaSMC with the intracellular calcium (Ca(2+)) chelator, BAPTA-AM, blocked the induction of ICER gene expression by GSNO. The store-operated Ca(2+) channel inhibitors, 2-ABP, and SKF-96365, reduced the GSNO-mediated increase in ICER mRNA levels, while 2-ABP did not inhibit GSNO-induced CREB phosphorylation. Our results suggest that induction of ICER gene expression by NO requires both CREB phosphorylation and Ca(2+) signaling. Transcription profiling of RPaSMC exposed to GSNO revealed important roles for sGC, PKA, CREB, and Ca(2+) in the regulation of gene expression by NO. The induction of ICER in GSNO-treated RPaSMC highlights a novel cross-talk mechanism between cGMP and cAMP signaling pathways.

Conformational conversion of PDE5 by incubation with sildenafil or metal ion is accompanied by stimulation of allosteric cGMP binding

Phosphodiesterase-5 (PDE5) is a dimer containing a cGMP-specific catalytic domain and an allosteric cGMP-binding subdomain (GAF A) on each subunit. PDE5 exhibits three conformational forms that can be separated by Native PAGE and are denoted as Bands 1, 2, and 3 in decreasing order of mobility. A preparation comprised mainly of Band 2 PDE5 was partially converted to Band 3 PDE5 by 1h incubation with cGMP or the PDE5-specific inhibitors sildenafil, vardenafil, or tadalafil, but not with cAMP, milrinone (PDE3-specific), or rolipram (PDE4-specific). Band 2 PDE5 was converted almost entirely to Band 3 PDE5 by overnight incubation with sildenafil at 30°C. This time-dependent conversion was accompanied by a 7-fold increase in allosteric cGMP-binding activity, suggesting that Band 3 PDE5 is a much more active form than Band 2 PDE5 for allosteric cGMP binding. Conversion of Band 2 PDE5 to Band 3 PDE5 occurred faster by pre-incubation with cGMP, which binds to both the allosteric and catalytic sites of PDE5, than with catalytic site-specific sildenafil. Overnight incubation of a Band 2/Band 3 PDE5 mixture with EDTA caused time-dependent conversion to Band 1 PDE5 (apoenzyme), and this conversion was accompanied by a 50% loss in cGMP-binding activity. After incubation with EDTA, addition of Mn(++) or Mg(++) caused reversion of Band 1 to a Band 2/Band 3 PDE5 mixture in which Band 3 PDE5 predominated. This reversion was accompanied by a 3-fold increase in allosteric cGMP-binding activity. The combination of results implied that physiological conversion of Band 2 to Band 3 PDE5 by cGMP and/or divalent metal ion occupancy of the catalytic domain would increase allosteric cGMP binding to the enzyme. This conversion would produce a greater negative feedback effect on cGMP action by increasing sequestration of cGMP at the allosteric cGMP-binding site of PDE5 and by increasing cGMP degradation at the catalytic site of the enzyme. This conversion would also increase PDE5 inhibitor binding to the enzyme.

CEACAM1 creates a pro-angiogenic tumor microenvironment that supports tumor vessel maturation

We have studied the effects of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) on tumor angiogenesis in murine ductal mammary adenocarcinomas. We crossed transgenic mice with whey acidic protein promoter-driven large T-antigen expression (WAP-T mice) with oncogene-induced mammary carcinogenesis with CEACAM1null mice, and with Tie2-Ceacam1 transgenics, in which the Tie2 promoter drives endothelial overexpression of CEACAM1 (WAP-T × CEACAM1(endo+) mice), and analyzed tumor vascularization, angiogenesis and vessel maturation in these mice. Using flat-panel volume computed tomography (fpVCT) and histology, we found that WAP-T × CEACAM1(endo+) mice exhibited enhanced tumoral vascularization owing to CEACAM1(+) vessels in the tumor periphery, and increased intratumoral angiogenesis compared with controls. In contrast, vascularization of CEACAM1null/WAP-T-derived tumors was poor, and tumor vessels were dilated, leaky and showed poor pericyte coverage. Consequently, the tumoral vasculature could not be visualized in CEACAM1null/WAP-T mice by fpVCT, and we observed poor organization of the perivascular extracellular matrix (ECM), accompanied by the accumulation of collagen IV-degrading matrix metalloproteinase 9(+) (MMP9(+)) leukocytes and stromal cells. Vascular instability and alterations in ECM structure were accompanied by a significant increase in pulmonary metastases in CEACAM1null/WAP-T mice, whereas only occasional metastases were observed in CEACAM1(+) hosts. In CEACAM1(+) hosts, intratumoral vessels did not express CEACAM1, but they were intact, extensively covered with pericytes and framed by a well-organized perivascular ECM. MMP9(+) accessory cells were largely absent. Orthotopic transplantation of primary WAP-T- and CEACAM1null/WAP-T tumors into all three mouse lines confirmed that a CEACAM1(+) host environment is a prerequisite for productive angiogenic remodeling of the tumor microenvironment. Hence, CEACAM1 expression in the tumor periphery determines the vascular phenotype in a tumor, whereas systemic absence of CEACAM1 interferes with the formation of an organized tumor matrix and intratumoral vessel maturation.