Genomic organization of alpha1 and beta1 subunits of the mammalian soluble guanylyl cyclase genes

Soluble guanylyl cyclase beta1 subunit targets epithelial-to-mesenchymal transition and downregulates Akt pathway in human endometrial and cervical cancer cells

Endometrial and cervical cancer are among the most frequently diagnosed malignancies globally. Nitric oxide receptor-soluble guanylyl cyclase (sGC) is a heterodimeric enzyme composed of two subunits, α1 and β1. Previously we showed that sGCα1 subunit promotes cell survival, proliferation, and migration, but the role of sGCβ1 subunit has not been addressed. The aim of the present work was to study the impact of sGCβ1 restoration in proliferation, survival, migration, and cell signaling in endometrial and cervical cancer cells. We found that sGCβ1 transcript levels are reduced in endometrial and cervical tumors vs normal tissues. We confirmed nuclear enrichment of sGCβ1, unlike sGCα1. Overexpression of sGCβ1 reduced cell viability and augmented apoptotic index. Cell migration and invasion were also negatively affected. All these sGCβ1-driven effects were independent of sGC enzymatic activity. sGCβ1 reduced the expression of epithelial-to-mesenchymal transition factors such as N-cadherin and β-catenin and increased the expression of E-cadherin. sGCβ1 impacted signaling in endometrial and cervical cancer cells through significant downregulation of Akt pathway affecting some of its main targets such as GSK-3β and c-Raf. Our results show for the first time that sGCβ1 exerts several antiproliferative actions in ECC-1 and HeLa cell lines by targeting key regulatory pathways.

Soluble Guanylyl Cyclase Alpha1 Subunit: A New Marker for Estrogenicity of Endocrine Disruptor Compounds

Endocrine-disrupting chemicals (EDCs) include widespread naturally occurring and synthetic substances in the environment that adversely affect humans and wildlife. Because of the increasing numbers of EDCs, screening methods and ideal biomarkers to determine EDC potencies at relevant environmental concentrations need to be drastically improved. Soluble guanylyl cyclase α1 subunit (sGCα1) is an abundant cytosolic protein ubiquitously expressed in most tissues. We previously showed that sGCα1 is specifically and highly up-regulated by estrogen (E2) in vivo and in vitro, even though it lacks estrogen-responsive elements. The aim of the present study was to evaluate sGCα1 protein expression as a potential marker for xenoestrogenic EDC exposure in the E2-responsive lactosomatotroph-derived pituitary cell line GH3. Cells were incubated with a wide variety of EDCs such as heavy metals and a metalloid, synthetic E2 derivatives, plastic byproducts, and pesticides at a range of doses including those with proven xenoestrogenic activity. We demonstrated that E2 increased sGCα1 expression in GH3 cells as well as in other E2-responsive tumor cell lines. Moreover, this effect was fully dependent on estrogen receptor (ER) activation. Importantly, sGCα1 protein levels were strongly up-regulated by all the EDCs tested, even by those exhibiting low or null ER binding capacity. We provide evidence that the in vitro sGCα1 protein assay may be a very sensitive and powerful tool to identify compounds with estrogenic activity, which could improve current mammalian-based screening methods. Environ Toxicol Chem 2019;38:2719-2728. © 2019 SETAC.

Transforming growth factor-β downregulates sGC subunit expression in pulmonary artery smooth muscle cells via MEK and ERK signaling

TGFβ activation during newborn lung injury decreases the expression of pulmonary artery smooth muscle cell (PASMC)-soluble guanylate cyclase (sGC), a critical mediator of nitric oxide signaling. Using a rat PASMC line (CS54 cells), we determined how TGFβ downregulates sGC expression. We found that TGFβ decreases sGC expression through stimulating its type I receptor; TGFβ type I receptor (TGFβR1) inhibitors prevented TGFβ-1-mediated decrease in sGCα1 subunit mRNA levels in the cells. However, TGFβR1-Smad mechanisms do not regulate sGC; effective knockdown of Smad2 and Smad3 expression and function did not protect sGCα1 mRNA levels during TGFβ-1 exposure. A targeted small-molecule kinase inhibitor screen suggested that MEK signaling regulates sGC expression in TGFβ-stimulated PASMC. TGFβ activates PASMC MEK/ERK signaling; CS54 cell treatment with TGFβ-1 increased MEK and ERK phosphorylation in a biphasic, time- and dose-dependent manner. Moreover, MEK/ERK activity appears to be required for TGFβ-mediated sGC expression inhibition in PASMC; MEK and ERK inhibitors protected sGCα1 mRNA expression in TGFβ-1-treated CS54 cells. Nuclear ERK activity is sufficient for sGC regulation; heterologous expression of a nucleus-retained, constitutively active ERK2-MEK1 fusion protein decreased CS54 cell sGCα1 mRNA levels. The in vivo relevance of this TGFβ-MEK/ERK-sGC downregulation pathway is suggested by the detection of ERK activation and sGCα1 protein expression downregulation in TGFβ-associated mouse pup hyperoxic lung injury, and the determination that ERK decreases sGCα1 protein expression in TGFβ-1-treated primary PASMC obtained from mouse pups. These studies identify MEK/ERK signaling as an important pathway by which TGFβ regulates sGC expression in PASMC.

Cytochrome b5 Reductase 3 Modulates Soluble Guanylate Cyclase Redox State and cGMP Signaling

RATIONALE

Soluble guanylate cyclase (sGC) heme iron, in its oxidized state (Fe³⁺), is desensitized to NO and limits cGMP production needed for downstream activation of protein kinase G-dependent signaling and blood vessel dilation.

OBJECTIVE

Although reactive oxygen species are known to oxidize the sGC heme iron, the basic mechanism(s) governing sGC heme iron recycling to its NO-sensitive, reduced state remain poorly understood.

METHODS AND RESULTS

Oxidant challenge studies show that vascular smooth muscle cells have an intrinsic ability to reduce oxidized sGC heme iron and form protein-protein complexes between cytochrome b5 reductase 3, also known as methemoglobin reductase, and oxidized sGC. Genetic knockdown and pharmacological inhibition in vascular smooth muscle cells reveal that cytochrome b5 reductase 3 expression and activity is critical for NO-stimulated cGMP production and vasodilation. Mechanistically, we show that cytochrome b5 reductase 3 directly reduces oxidized sGC required for NO sensitization as assessed by biochemical, cellular, and ex vivo assays.

CONCLUSIONS

Together, these findings identify new insights into NO-sGC-cGMP signaling and reveal cytochrome b5 reductase 3 as the first identified physiological sGC heme iron reductase in vascular smooth muscle cells, serving as a critical regulator of cGMP production and protein kinase G-dependent signaling.

The Role of Reactive Oxygen and Nitrogen Species in the Expression and Splicing of Nitric Oxide Receptor

SIGNIFICANCE

Nitric oxide (NO)-dependent signaling is critical to many cellular functions and physiological processes. Soluble guanylyl cyclase (sGC) acts as an NO receptor and mediates the majority of NO functions. The signaling between NO and sGC is strongly altered by reactive oxygen and nitrogen species. Recent Advances: Besides NO scavenging, sGC is affected by oxidation/loss of sGC heme, oxidation, or nitrosation of cysteine residues and phosphorylation. Apo-sGC or sGC containing oxidized heme is targeted for degradation. sGC transcription and the stability of sGC mRNA are also affected by oxidative stress.

CRITICAL ISSUES

Studies cited in this review suggest the existence of compensatory processes that adapt cellular processes to diminished sGC function under conditions of short-term or moderate oxidative stress. Alternative splicing of sGC transcripts is discussed as a mechanism with the potential to both enhance and reduce sGC function. The expression of α1 isoform B, a functional and stable splice variant of human α1 sGC subunit, is proposed as one of such compensatory mechanisms. The expression of dysfunctional splice isoforms is discussed as a contributor to decreased sGC function in vascular disease.

FUTURE DIRECTIONS

Targeting the process of sGC splicing may be an important approach to maintain the composition of sGC transcripts that are expressed in healthy tissues under normal conditions. Emerging new strategies that allow for targeted manipulations of RNA splicing offer opportunities to use this approach as a preventive measure and to control the composition of sGC splice isoforms. Rational management of expressed sGC splice forms may be a valuable complementary treatment strategy for existing sGC-directed therapies. Antioxid. Redox Signal. 26, 122-136.

Nitric oxide-sensitive guanylyl cyclase is differentially regulated by nuclear and non-nuclear estrogen pathways in anterior pituitary gland

17β-estradiol (E2) regulates hormonal release as well as proliferation and cell death in the pituitary. The main nitric oxide receptor, nitric oxide sensitive- or soluble guanylyl cyclase (sGC), is a heterodimer composed of two subunits, α and β, that catalyses cGMP formation. α1β1 is the most abundant and widely expressed heterodimer, showing the greater activity. Previously we have shown that E2 decreased sGC activity but exerts opposite effects on sGC subunits increasing α1 and decreasing β1 mRNA and protein levels. In the present work we investigate the mechanisms by which E2 differentially regulates sGC subunits' expression on rat anterior pituitary gland. Experiments were performed on primary cultures of anterior pituitary cells from adult female Wistar rats at random stages of estrous cycle. After 6 h of E2 treatment, α1 mRNA and protein expression is increased while β1 levels are down-regulated. E2 effects on sGC expression are partially dependent on de novo transcription while de novo translation is fully required. E2 treatment decreased HuR mRNA stabilization factor and increased AUF1 p37 mRNA destabilization factor. E2-elicited β1 mRNA decrease correlates with a mRNA destabilization environment in the anterior pituitary gland. On the other hand, after 6 h of treatment, E2-BSA (1 nM) and E2-dendrimer conjugate (EDC, 1 nM) were unable to modify α1 or β1 mRNA levels, showing that nuclear receptor is involved in E2 actions. However, at earlier times (3 h), 1 nM EDC causes a transient decrease of α1 in a PI3k-dependent fashion. Our results show for the first time that E2 is able to exert opposite actions in the anterior pituitary gland, depending on the activation of classical or non-classical pathways. Thus, E2 can also modify sGC expression through membrane-initiated signals bringing to light a new point of regulation in NO/sGC pathway.

Nitric oxide-cyclic GMP signaling in stem cell differentiation

The nitric oxide-cyclic GMP (NO-cGMP) pathway mediates important physiological functions associated with various integrative body systems including the cardiovascular and nervous systems. Furthermore, NO regulates cell growth, survival, apoptosis, proliferation, and differentiation at the cellular level. To understand the significance of the NO-cGMP pathway in development and differentiation, studies have been conducted both in developing embryos and in stem cells. Manipulation of the NO-cGMP pathway, by employing activators and inhibitors as pharmacological probes, and genetic manipulation of NO signaling components have implicated the involvement of this pathway in the regulation of stem cell differentiation. This review focuses on some of the work pertaining to the role of NO-cGMP in the differentiation of stem cells into cells of various lineages, particularly into myocardial cells, and in stem cell-based therapy.

RNA splicing in regulation of nitric oxide receptor soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a key protein in the nitric oxide (NO)/-cGMP signaling pathway. sGC activity is involved in a number of important physiological processes including smooth muscle relaxation, neurotransmission and platelet aggregation and adhesion. Regulation of sGC expression and activity emerges as a crucial factor in control of sGC function in normal and pathological conditions. Recently accumulated evidence strongly indicates that the regulation of sGC expression is a complex process modulated on several levels including transcription, post-transcriptional regulation, translation and protein stability. Presently our understanding of mechanisms governing regulation of sGC expression remains very limited and awaits systematic investigation. Among other ways, the expression of sGC subunits is modulated at the levels of mRNA abundance and transcript diversity. In this review we summarize available information on different mechanisms (including transcriptional activation, mRNA stability and alternative splicing) involved in the modulation of mRNA levels of sGC subunits in response to various environmental clues. We also summarize and cross-reference the information on human sGC splice forms available in the literature and in genomic databases. This review highlights the fact that the study of the biological role and regulation of sGC splicing will bring new insights to our understanding of NO/cGMP biology.

Muscarinic agonists acting through M2 acetylcholine receptors stimulate the migration of an NO-sensitive guanylyl cyclase to the plasma membrane of bovine tracheal smooth muscle

Muscarinic agonists acting on bovine tracheal smooth muscle (BTSM) induce two separate cGMP signals, one at 20 sec associated with NO-sensitive-soluble-guanylyl-cyclase (NO-sGC) and another at 60 sec, linked to natriuretic-peptide-GC. The 20-sec-cGMP novel cascade starts with mAChRs, via unknown components, activates an NO-sGC. To unravel this cascade, in crude membranes isolated from intact BTSM strips exposed to muscarinic agonists, we detected GC activities increments at 20 sec and 60 sec. The 20-sec-GC is a NO-sensitive-GC, identified as alpha(1)beta(1)-heterodimer. In reconstitution experiments with purified plasma membranes and cytosol, muscarinic agonists induced an NO-sGC migration in a dose-dependent manner, being inhibited by muscarinic antagonists displaying an M(2)AChR profile and blocked by PTX, suggesting the involvement of G(o)/G(i) proteins. The NO-sGC related to migration was isolated and identified as an alpha(1)beta(1)-heterodimer. This work shows that muscarinic agonists in BTSM induce a massive and selective alpha(1)beta(1)-NO-sGC migration from cytoplasm to plasma membranes being responsible for the 20-sec-cGMP signal.

KMUP-1 attenuates serum deprivation-induced neurotoxicity in SH-SY5Y cells: roles of PKG, PI3K/Akt and Bcl-2/Bax pathways

Aging populations with neurodegenerative disorders will gradually become a greater problem for society. Serum deprivation-induced cell death is recognized as one of the standard models for the study of neurotoxicity. Increasing evidence indicates that cGMP/PKG pathway may play a rescue role in serum deprivation-induced toxicity. The aim of this study was to investigate protective effects of KMUP-1, an enhancer of cGMP/PKG signaling on serum deprivation-induced neurotoxicity in SH-SY5Y neuroblastoma cells. Under normal serum condition, KMUP-1 enhanced protein expression of nNOS, PKG and sGCalpha1, increased intracellular cyclic GMP level, and attenuated PDE5 expression. KMUP-1 also increased expression of BDNF and Bcl-2, but it did not affect Bax expression. The phosphorylation of Akt and CREB induced by KMUP-1 was inhibited by tyrosine kinase (TrK) inhibitor K252a and phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, respectively. Under serum deprivation condition, flow cytometric analysis using Annexin V showed KMUP-1 increased cell viability, but lacked protective effects in the presence of nitric oxide synthase inhibitor l-NAME, PKG inhibitor Rp-8-pCPT-cGMPS or LY294002. KMUP-1 not only enhanced expression of nNOS, sGCalpha1, PKG, p-CREB, p-Akt and Bcl-2, but also attenuated Bax expression in serum deprivation-treated cultures. In conclusion, cGMP/PKG, PI3K/Akt/CREB and Bcl-2/Bax signals play critical roles in the neuroprotective effects of KMUP-1 on serum deprivation-induced toxicity.

Nitric oxide sensitive-guanylyl cyclase subunit expression changes during estrous cycle in anterior pituitary glands

17beta-estradiol (E2) exerts inhibitory actions on the nitric oxide pathway in rat adult pituitary glands. Previously, we reported that in vivo E2 acute treatment had opposite effects on soluble guanylyl cyclase (sGC) subunits, increasing alpha1- and decreasing beta1-subunit protein and mRNA expression and decreasing sGC activity in immature rats. Here we studied the E2 effect on sGC protein and mRNA expression in anterior pituitary gland from adult female rats to address whether the maturation of the hypothalamus-pituitary axis influences its effects and to corroborate whether these effects occur in physiological conditions such as during estrous cycle. E2 administration causes the same effect on sGC as seen in immature rats, and these effects are estrogen receptor dependent. These results suggest that E2 is the main effector of these changes. Since the sGC alpha-subunit increases while the sGC activity decreases, we studied if other less active isoforms of the sGC alpha-subunit are expressed. Here we show for the first time that sGCalpha2 and sGCalpha2 inhibitory (alpha2i) isoforms are expressed in this gland, but only sGCalpha2i mRNA increased after E2 acute treatment. Finally, to test whether E2 effects take place under a physiological condition, sGC subunit expression was monitored over estrous cycle. sGCalpha1, -beta1, and -alpha2i fluctuate along estrous cycle, and these changes are directly related with E2 level fluctuations rather than to NO level variations. These findings show that E2 physiologically regulates sGC expression and highlight a novel mechanism by which E2 downregulates sGC activity in rat anterior pituitary gland.

A short history of cGMP, guanylyl cyclases, and cGMP-dependent protein kinases

Here, we review the early studies on cGMP, guanylyl cyclases, and cGMP-dependent protein kinases to facilitate understanding of development of this exciting but complex field of research encompassing pharmacology, biochemistry, physiology, and molecular biology of these important regulatory molecules.

Characterization of the human alpha1 beta1 soluble guanylyl cyclase promoter: key role for NF-kappaB(p50) and CCAAT-binding factors in regulating expression of the nitric oxide receptor

Soluble guanylyl cyclase (sGC) is the principal receptor for NO and plays a ubiquitous role in regulating cellular function. This is exemplified in the cardiovascular system where sGC governs smooth muscle tone and growth, vascular permeability, leukocyte flux, and platelet aggregation. As a consequence, aberrant NO-sGC signaling has been linked to diseases including hypertension, atherosclerosis, and stroke. Despite these key (patho)physiological roles, little is known about the expressional regulation of sGC. To address this deficit, we have characterized the promoter activity of human α₁ and β₁ sGC genes in a cell type relevant to cardiovascular (patho)physiology, primary human aortic smooth muscle cells. Luciferase reporter constructs revealed that the 0.3- and 0.5-kb regions upstream of the transcription start sites were optimal for α₁ and β₁ sGC promoter activity, respectively. Deletion of consensus sites for c-Myb, GAGA, NFAT, NF-κB(p50), and CCAAT-binding factor(s) (CCAAT-BF) revealed that these are the principal transcription factors regulating basal sGC expression. In addition, under pro-inflammatory conditions, the effects of the strongest α₁ and β₁ sGC repressors were enhanced, and enzyme expression and activity were reduced; in particular, NF-κB(p50) is pivotal in regulating enzyme expression under such conditions. NO itself also elicited a cGMP-independent negative feedback effect on sGC promoter activity that is mediated, in part, via CCAAT-BF activity. In sum, these data provide a systematic characterization of the promoter activity of human sGC α₁ and β₁ subunits and identify key transcription factors that govern subunit expression under basal and pro-inflammatory (i.e. atherogenic) conditions and in the presence of ligand NO.

NO-cGMP signaling and regenerative medicine involving stem cells

Nitric oxide (NO) is a short lived diatomic free radical species synthesized by nitric oxide synthases (NOS). The physiological roles of NO depend on its local concentrations as well as availability and the nature of downstream target molecules. At low nanomolar concentrations, activation of soluble guanylyl cyclase (sGC) is the major event initiated by NO. The resulting elevation in the intracellular cyclic GMP (cGMP) levels serves as signals for regulating diverse cellular and physiological processes. The participation of NO and cGMP in diverse physiological processes is made possible through cell type specific spatio-temporal regulation of NO and cGMP synthesis and signal diversity downstream of cGMP achieved through specific target selection. Thus cyclic GMP directly regulates the activities of its downstream effectors such as Protein Kinase G (PKG), Cyclic Nucleotide Gated channels (CNG) and Cyclic nucleotide phosphodiesterases, which in turn regulate the activities of a number of proteins that are involved in regulating diverse cellular and physiological processes. Localization and activity of the NO-cGMP signaling pathway components are regulated by G-protein coupled receptors, receptor and non receptor tyrosine kinases, phosphatases and other signaling molecules. NO also serves as a powerful paracrine factor. At micromolar concentrations, NO reacts with superoxide anion to form reactive peroxinitrite, thereby leading to the oxidation of important cellular proteins. Extensive research efforts over the past two decades have shown that NO is an important modulator of axon outgrowth and guidance, synaptic plasticity, neural precursor proliferation as well as neuronal survival. Excessive NO production as that evoked by inflammatory signals has been identified as one of the major causative reasons for the pathogenesis of a number of neurodegenerative diseases such as ALS, Alzheimers and Parkinson diseases. Regenerative therapies involving transplantation of embryonic stem cells (ES cells) and ES cell derived lineage committed neural precursor cells have recently shown promising results in animal models of Parkinson disease (PD). Recent studies from our laboratory have shown that a functional NO-cGMP signaling system is operative early during the differentiation of embryonic stem cells. The cell type specific, spatio-temporally regulated NO-cGMP signaling pathways are well suited for inductive signals to use them for important cell fate decision making and lineage commitment processes. We believe that manipulating the NO-cGMP signaling system will be an important tool for large scale generation of lineage committed precursor cells to be used for regenerative therapies.

17 beta-estradiol modifies nitric oxide-sensitive guanylyl cyclase expression and down-regulates its activity in rat anterior pituitary gland

Previous studies showed that 17 beta-estradiol (17 beta-E2) regulates the nitric oxide (NO)/soluble guanylyl cyclase (sGC)/cGMP pathway in many tissues. Evidence from our laboratory indicates that 17 beta-E2 disrupts the inhibitory effect of NO on prolactin release, decreasing sGC activity and affecting the cGMP pathway in anterior pituitary gland of adult ovariectomized and estrogenized rats. To ascertain the mechanisms by which 17 beta-E2 affects sGC activity, we investigated the in vivo and in vitro effects of 17 beta-E2 on sGC protein and mRNA expression in anterior pituitary gland from immature female rats. In the present work, we showed that 17 beta-E2 acute treatment exerted opposite effects on the two sGC subunits, increasing alpha1 and decreasing beta1 subunit protein and mRNA expression. This action on sGC protein expression was maximal 6-9 h after 17 beta-E2 administration. 17beta-E2 also caused the same effect on mRNA expression at earlier times. Concomitantly, 17 beta-E2 dramatically decreased sGC activity 6 and 9 h after injection. These effects were specific of 17 beta-E2, because they were not observed with the administration of other steroids such as progesterone and 17 alpha-estradiol. This inhibitory action of 17beta-E2 on sGC also required the activation of estrogen receptor (ER), because treatment with the pure ER antagonist ICI 182,780 completely blocked 17 beta-E2 action. 17 beta-E2 acute treatment caused the same effects on pituitary cells in culture. These results suggest that 17 beta-E2 exerts an acute inhibitory effect on sGC in anterior pituitary gland by down-regulating sGC beta 1 subunit and sGC activity in a specific, ER-dependent manner.

Alterations of the expression and activity of midbrain nitric oxide synthase and soluble guanylyl cyclase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice

The study was aimed at investigating the expression and the activity of neuronal nitric oxide synthase, and of soluble guanylyl cyclase and phosphodiesterase activities that regulate guanosine 3',5'-cyclic monophosphate level in the midbrain, in a mouse model of PD using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections. Adult male mice of the C57/BL strain were given three i.p. injections of physiological saline or three i.p. injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine solution in physiological saline at 2 h intervals (summary 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine dose: 40 mg/kg), and were killed 3, 7, or 14 days later. mRNA, protein level, and/or activities of neuronal nitric oxide synthase, soluble guanylyl cyclase, phosphodiesterase and guanosine 3',5'-cyclic monophosphate were determined. Immunohistochemistry showed about 75% decrease in the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine showed increased midbrain guanylyl cyclase and total nitric oxide synthase activities at 3, 7, and 14 days post-treatment. The specific neuronal nitric oxide synthase inhibitor 7-nitroindazole (10 microM) and the specific inducible nitric oxide synthase inhibitor 1400W (10 microM) inhibited the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced excess in nitric oxide synthase activity by 63-70 and 13-25%, respectively. The increases in total midbrain nitric oxide synthase activity were accompanied by elevated guanosine 3',5'-cyclic monophosphate, enhanced expression of neuronal nitric oxide synthase and of the beta1 subunit of guanylyl cyclase at both mRNA and protein levels that persisted up to the end of the observation period, and by enhanced neuronal nitric oxide synthase and guanylyl cyclase beta1 immunoreactivities in substantia nigra pars compacta 7 and 14 days after the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The increases in guanylyl cyclase activity were found to occur exclusively due to increased maximal enzyme activity. No 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced change in phosphodiesterase activity has been detected in any brain region studied. 7-Nitroindazole prevented a significant increase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced midbrain guanosine 3',5'-cyclic monophosphate level and neurodegeneration of dopaminergic neurons. These results raise the possibility that the nitric oxide/guanylyl cyclase/guanosine 3',5'-cyclic monophosphate signaling pathway may play a role in maintaining dopaminergic neurons function in substantia nigra pars compacta.

Cyclic Guanosine 5′-Monophosphate-Dependent Protein Kinase II Is Induced by Luteinizing Hormone and Progesterone Receptor-Dependent Mechanisms in Granulosa Cells and Cumulus Oocyte Complexes of Ovulating Follicles

Cyclic GMP (cGMP)-dependent protein kinase II (Prkg2, cGK II) was identified as a potential target of the progesterone receptor (Nr3c3) in the mouse ovary based on microarray analyses. To document this further, the expression patterns of cGK II and other components of the cGMP signaling pathway were analyzed during follicular development and ovulation using the pregnant mare serum gonadotropin (PMSG)-human chorionic gonadotropin (hCG)-primed immature mice. Levels of cGK II mRNA were low in ovaries of immature mice, increased 4-fold in response to pregnant mare serum gonadotropin and 5-fold more within 12 h after hCG, the time of ovulation. In situ hybridization localized cGK II mRNA to granulosa cells and cumulus oocyte complexes of periovulatory follicles. In progesterone receptor (PR) null mice, cGK II mRNA was reduced significantly at 12 h after hCG in contrast to heterozygous littermates. In primary granulosa cell cultures, cGK II mRNA was induced by phorbol 12-myristate 13-acetate enhanced by adenoviral expression of PR-A and blocked by RU486 and trilostane. PR-A in the absence of phorbol 12-myristate 13-acetate was insufficient to induce cGK II. Expression of cGK I (Prkg1) was restricted to the residual tissue and not regulated by hormones. Guanylate cyclase-A (Npr1; GC-A) mRNA expression increased 6-fold by 4 h after hCG treatment in contrast to pregnant mare serum gonadotropin alone and was localized to granulosa cells of preovulatory follicles. Collectively, these data show for the first time that cGK II (not cGK I) and GC-A are selectively induced in granulosa cells of preovulatory follicles by LH- and PR-dependent mechanisms, thereby providing a pathway for cGMP function during ovulation.

Soluble guanylyl cyclase: more secrets revealed

Guanylyl cyclases (GCs) are enzymes that convert guanosine-5'-triphosphate (GTP) to cyclic guanosine-3',5'-monophosphate (cGMP). The second messenger cGMP participates in signaling by (1) stimulating the activity of kinases that belong to the protein kinase G family, (2) altering the conductance of cGMP-gated ion channels and (3) changing the activity of cGMP-regulated phosphodiesterases. In contrast to adenylyl cyclases which exist as membrane-bound molecules, guanylyl cyclases (GC) occur in both membrane-bound and cytosolic forms. The particulate GC (pGC) isoforms serve as receptors for natriuretic peptides, while soluble GC (sGC) is the "receptor" for nitric oxide (NO). In addition to the difference in ligands and subcellular organization, the two forms of GC also differ in that pGC exists in homodimeric form, while typically sGC occurs as a heterodimer. Herein, we will review the literature on sGC subunit structure and discuss the regulation of the enzyme at the transcriptional and post-translational level.

Differential Regulation of Soluble Guanylyl Cyclase Expression and Signaling by Collagens: Involvement of Integrin-Linked Kinase

Glomerular diseases are characterized by an abnormal synthesis of extracellular matrix proteins, such as collagen type I. Evidence that growth on collagen type I downregulates soluble guanylyl cyclase (sGC) expression and the responsiveness of human mesangial cells to nitric oxide (NO) by activating specific integrin signals involving integrin-linked kinase (ILK) is presented. Human mesangial cells were grown on collagen type I or IV for 24 to 72 h. Compared with collagen IV, growth on collagen I reduced the protein expression and NO-stimulated enzyme activity of sGC. This downregulation was effected at the level of transcription, because steady-state sGC mRNA expression was reduced on collagen I, but inhibition of transcription with Actinomycin D revealed no differences in transcript stability between the two culture conditions. Collagen I also reduced the capacity of cells to relax in response to NO after H2O2 challenge and inhibited NO-induced phosphorylation of vasodilator-activated phosphoprotein, a target of cyclic guanine monophosphate-dependent protein kinase. Examination of the surface expression of integrins, the receptors for extracellular matrix components, revealed that alpha1 and alpha2 integrin subunits were more abundant on cells that were grown on collagen IV and that surface expression of beta1 integrin did not vary with collagen type. However, growth on collagen I induced beta1 integrin to adopt its active conformation, and this activation of beta1 integrin was accompanied by increased activity of its downstream effector ILK. Dominant-negative suppression of ILK signaling relieved the suppression of sGC expression and NO-induced vasodilator-activated phosphoprotein phosphorylation induced by collagen I.

Effects of the JNK inhibitor anthra[1,9-cd]pyrazol-6(2H)-one (SP-600125) on soluble guanylyl cyclase alpha1 gene regulation and cGMP synthesis

The decreased expression of the nitric oxide (NO) receptor, soluble guanylyl cyclase (sGC), occurs in response to multiple stimuli in vivo and in cell culture and correlates with various disease states such as hypertension, inflammation, and neurodegenerative disorders. The ability to understand and modulate sGC expression and cGMP levels in any of these conditions could be a valuable therapeutic tool. We demonstrate herein that the c-Jun NH2-terminal kinase JNK II inhibitor anthra[1,9-cd]pyrazol-6(2H)-one (SP-600125) completely blocked the decreased expression of sGCalpha1-subunit mRNA by nerve growth factor (NGF) in PC12 cells. Inhibitors of the ERK and p38 MAPK pathways, PD-98059 and SB-203580, had no effect. SP-600125 also inhibited the NGF-mediated decrease in the expression of sGCalpha1 protein as well as sGC activity in PC12 cells. Other experiments revealed that decreased sGCalpha1 mRNA expression through a cAMP-mediated pathway, using forskolin, was not blocked by SP-600125. We also demonstrate that TNF-alpha/IL-1beta stimulation of rat fetal lung (RFL-6) fibroblast cells resulted in sGCalpha1 mRNA inhibition, which was blocked by SP-600125. Expression of a constitutively active JNKK2-JNK1 fusion protein in RFL-6 cells caused endogenous sGCalpha1 mRNA levels to decrease, while a constitutively active ERK2 protein had no effect. Collectively, these data demonstrate that SP-600125 may influence the intracellular levels of the sGCalpha1-subunit in certain cell types and may implicate a role for c-Jun kinase in the regulation of sGCalpha1 expression.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

Molecular dissection of mouse soluble guanylyl cyclase alpha1 promoter

Soluble guanylyl cyclase (sGC) is the only known receptor for nitric oxide (NO) and is downregulated in aging and hypertension. Little is known about sGC gene transcriptional regulation. In order to characterize the sGC transcriptional system, we cloned and sequenced the 5(') flanking region of mouse sGC alpha(1) gene (AY116663). Structurally, it is a non-canonical TATA-less promoter that we mapped to chromosome 3 with many putative regulation sites for Sp-1, NF-kappaB, and AP-1 transcription factors amongst others, and two (TG:CA)(n) dinucleotide microsatellites near the transcriptional start point. The cloned upstream sequence produced a 5-fold increase in luciferase activity in Cos7, HeLa, NIH3T3, and 293 cells as well as in mouse VSMC-like kidney mesangial cells. In the latter cell type, we showed that sGC alpha(1) promoter activity was dependent on the presence of its 5(') unstranslated region (5(')UTR).

CCAAT-binding factor regulates expression of the beta1 subunit of soluble guanylyl cyclase gene in the BE2 human neuroblastoma cell line

Soluble guanylyl cyclase (sGC) is a cytosolic enzyme producing the intracellular messenger cyclic guanosine monophosphate (cGMP) on activation with nitric oxide (NO). sGC is an obligatory heterodimer composed of alpha and beta subunits. We investigated human beta1 sGC transcriptional regulation in BE2 human neuroblastoma cells. The 5' upstream region of the beta1 sGC gene was isolated and analyzed for promoter activity by using luciferase reporter constructs. The transcriptional start site of the beta1 sGC gene in BE2 cells was identified. The functional significance of consensus transcriptional factor binding sites proximal to the transcriptional start site was investigated by site deletions in the 800-bp promoter fragment. The elimination of CCAAT-binding factor (CBF) and growth factor independence 1 (GFI1) binding cores significantly diminished whereas deletion of the NF1 core elevated the transcription. Electrophoretic mobility-shift assay (EMSA) and Western analysis of proteins bound to biotinated EMSA probes confirmed the interaction of GFI1, CBF, and NF1 factors with the beta1 sGC promoter. Treatment of BE2 cells with genistein, known to inhibit the CBF binding to DNA, significantly reduced protein levels of beta1 sGC by inhibiting transcription. In summary, our study represents an analysis of the human beta1 sGC promoter regulation in human neuroblastoma BE2 cells and identifies CBF as a critically important factor in beta1 sGC expression.

Effect of hypercholesterolemia on expression and function of vascular soluble guanylyl cyclase

BACKGROUND

Vasorelaxation to endothelial NO is mediated by activation of soluble guanylyl cyclase (sGC) and impaired by hypercholesterolemia in animals and humans. We investigated whether hypercholesterolemia impacts expression and function of sGC.

METHODS AND RESULTS

White New Zealand rabbits (n=10 per group) received a standard diet for 16 weeks (SD16) (n=20) or 32 weeks (SD32) and a cholesterol diet (7.5 g/kg) for 16 weeks (CD16) (n=20) or 32 weeks (CD32), respectively. Another group received cholesterol diet for 16 weeks followed by standard diet for 16 weeks (CD/SD). Aortic expression of the alpha1-subunit of sGC (sGC-alpha1) and beta1-subunit of sGC (sGC-beta1) was assessed by Western blot. Function was measured by aortic relaxation to S-Nitroso-N-acetyl-D, L-penicillamine (SNAP) and sGC activity in aortic cytosols. Hypercholesterolemia induced an upregulation of sGC-beta1 in CD16 (3.5+/-0.4-fold, P<0.001 versus SD16) and CD32 (4.0+/-0.4-fold, P<0.001 versus SD32). A similar increase was found for sGC-alpha1. In striking contrast, basal and NO-stimulated sGC activities in aortic cytosols of CD16 were only slightly enhanced (1.4-fold, P<0.05). Furthermore, the vasodilator potency of SNAP (EC50 in -logM) was 10-fold lower in CD16 (6.76a+/-0.09) than in SD16 (7.66+/-0.14, P<0.01). The increase of sGC expression was completely reversible, as indicated by comparable sGC-beta1 amounts in SD32 and CD/SD (1.2+/-0.1-fold, P>0.05). Immunohistochemical analysis suggests that a great portion of the overexpressed sGC is located in intimal lesions. Additional experiments showed that increased vascular superoxide production induced by 6-anilino-5,8-quinolinedione (LY85385) reduces sGC-activity but increases sGC-expression.

CONCLUSIONS

These results suggest that hypercholesterolemia induces a reversible overexpression of a dysfunctional vascular sGC, which may contribute to the pathogenesis of atherosclerosis.

Promoter activity of the 5'-flanking regions of medaka fish soluble guanylate cyclase alpha1 and beta1 subunit genes

We examined the spatial expression pattern of medaka fish (Oryzias latipes) soluble guanylate cyclase alpha(1) and beta(1) subunit genes, OlGCS-alpha(1) and OlGCS-beta(1), and characterized the 5'-flanking region required for expression of both genes by introducing various promoter-luciferase fusion-gene constructs into COS-1 cells and medaka fish embryos. The OlGCS-alpha(1) and OlGCS-beta(1) gene transcripts were detected in whole brain and kidney in 7-day and 9-day embryos. Primer-extension analysis demonstrated that there were no differences among various adult organs (brain, eye, kidney, ovary and testis) in the transcription start site of the OlGCS-alpha(1) and OlGCS-beta(1) genes. Neither gene contained the functional TATA box within its 5'-flanking region, and the basal promoter activity was found between nucleotides +33 and +42 in the OlGCS-alpha(1) gene and between nucleotides +146 and +155 in the OlGCS-beta(1) gene. In the assay of medaka fish embryos, the 5'-flanking region of the OlGCS-beta(1) gene exhibited lower promoter activity than that of the OlGCS-alpha(1) gene. In the experiments on dual-luciferase fusion-gene constructs, the 5'-flanking region of the OlGCS-alpha(1) gene connected to the 5'-flanking region of the OlGCS-beta(1) gene was introduced into medaka fish embryos, and the 5'-flanking regions of both subunit genes were shown to mutually influence each other's promoter activity.

Regional and age-dependent expression of the nitric oxide receptor, soluble guanylyl cyclase, in the human brain

Nitric oxide (NO), synthesized by neuronal NO synthase (NOS-I), plays essential physiological roles in the brain. The major molecular target for NO is soluble guanylyl cyclase (sGC), a heterodimeric hemoprotein composed of a larger alpha and a smaller beta subunit. Both subunits of sGC are needed to generate the second messenger cyclic GMP (cGMP). Here we show using subunit-specific antibodies and Western blot analysis that sGCalpha1 and sGCbeta1 protein subunits are present in all examined human brain regions. The relative distribution of the two subunits was similar and also correlated well with the known distribution of NOS-I. The highest expression levels of sGC were found in cortex, basal ganglia and the limbic system. These regions display the most prominent biochemical and histological changes during ageing. In cortex, a negative correlation between the amounts of sGC and age was found, while sex and post-mortem delay time did not affect sGC levels significantly. Our data suggest that sGCalpha1 and sGCbeta1 subunits are widely distributed in human brain, consistent with a major role in NO signaling. Moreover, the NO/cGMP pathway appears to be affected by ageing in the human brain.

Estradiol rapidly inhibits soluble guanylyl cyclase expression in rat uterus

Previous reports that investigated the regulation of the NO/soluble guanylyl cyclase (sGC)/cGMP pathway by estrogenic compounds have focused primarily on the levels of NO, NO-producing enzymes, and cGMP in various tissues. In this study, we demonstrate that 17beta-estradiol (E2) regulates the alpha(1) and beta(1) subunits of the NO receptor, sGC, at the mRNA and protein levels in rat uterus. Using real-time quantitative PCR, we found that within 1 h of in vivo E2 administration to rats, sGC mRNA levels begin to diminish. After 3 h, there is a maximal diminution of sGC mRNA expression (sGC alpha(1) 10% and sGC beta(1) 33% of untreated). This effect was blocked by the estrogen receptor antagonist, ICI 182,780, indicating that estrogen receptor is required. The effect of E2 also was observed in vitro with incubations of uterine tissue, indicating that the response does not depend on the secondary release of other hormones or factors from other tissues. Puromycin did not block the effect, suggesting the effects occur because of preexisting factors in uterine tissues and do not require new protein synthesis. Using immunoblot analysis, we found that sGC protein levels also were reduced by E2 over a similar time course as the sGC mRNA. We conclude that sGC plays a vital role in the NO/sGC/cGMP regulatory pathway during conditions of elevated estrogen levels in the rat uterus as a result of the reduction of sGC expression.

Molecular aspects of soluble guanylyl cyclase regulation

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme (comprised of alpha and beta subunits) that generates the intracellular second messenger cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP is subsequently important for the regulation of protein kinases, ion channels, and phosphodiesterases. Since recent evidence has demonstrated that heterodimerization of the alpha/beta subunits is essential for basal and stimulated enzymatic activity, the existence of several types of isoforms for each of the two subunits, along with their varying degrees of expression in different tissues, implies that multiple regulatory mechanisms exist for sGC. Yet, progress in studying and clarifying the regulatory processes that can alter sGC expression and activity has only slowly started being elucidated. In the following paper, we elaborate on sGC structure, function, and distribution along with recently described signaling pathways that modulate sGC gene expression.