RNA splicing in regulation of nitric oxide receptor soluble guanylyl cyclase

Soluble Guanylate Cyclase β1 Subunit Represses Human Glioblastoma Growth

Malignant glioma is the most common and deadly brain tumor. A marked reduction in the levels of sGC (soluble guanylyl cyclase) transcript in the human glioma specimens has been revealed in our previous studies. In the present study, restoring the expression of sGCβ1 alone repressed the aggressive course of glioma. The antitumor effect of sGCβ1 was not associated with enzymatic activity of sGC since overexpression of sGCβ1 alone did not influence the level of cyclic GMP. Additionally, sGCβ1-induced inhibition of the growth of glioma cells was not influenced by treatment with sGC stimulators or inhibitors. The present study is the first to reveal that sGCβ1 migrated into the nucleus and interacted with the promoter of the TP53 gene. Transcriptional responses induced by sGCβ1 caused the G0 cell cycle arrest of glioblastoma cells and inhibition of tumor aggressiveness. sGCβ1 overexpression impacted signaling in glioblastoma multiforme, including the promotion of nuclear accumulation of p53, a marked reduction in CDK6, and a significant decrease in integrin α6. These anticancer targets of sGCβ1 may represent clinically important regulatory pathways that contribute to the development of a therapeutic strategy for cancer treatment.

Co-expression of soluble guanylyl cyclase subunits and PDE5A shRNA to elevate cellular cGMP level: A potential gene therapy for myocardial cell death

BACKGROUND: Genetic manipulation on the NO-sGC-cGMP pathway has been rarely achieved, partially due to complexity of the soluble guanylyl cyclase (sGC) enzyme. OBJECTIVE: We aim to develop gene therapy directly targeting the pathway to circumvent cytotoxicity and tolerance after prolonged use of NO-donors and the insufficiency of PDE inhibitors. METHODS: In this study, we constructed lentivirus vectors expressing GUCY1A3 and GUCY1B3 genes, which encoded the α1 and β1 subunits of soluble guanylyl cyclase (sGC), respectively, to enhance cGMP synthesis. We also constructed lentiviral vector harboring PDE5A shRNA to alleviate phosphodiesterase activity and cGMP degradation. RESULTS: Transductions of human HEK293 cells with the constructs were successful, as indicated by the fluorescent signal and altered gene expression produced by each vector. Overexpression of GUCY1A3 and GUCY1B3 resulted in increased sGC enzyme activity and elevated cGMP level in the cells. Expression of PDE5A shRNA resulted in decreased PDE5A expression and elevated cGMP level. Co-transduction of the three lentiviral vectors resulted in a more significant elevation of cGMP in HEK293 cells without obvious cytotoxicity. CONCLUSION: To the best of our knowledge, this is the first study to show that co-expression of exogenous subunits of the soluble guanylyl cyclase could form functional enzyme and increase cellular cGMP level in mammalian cells. Simultaneous expression of PDE5A shRNA could alleviate feedback up-regulation on PDE5A caused by cGMP elevation. Further studies are required to evaluate the effects of these constructs in vivo.

Maturation, inactivation, and recovery mechanisms of soluble guanylyl cyclase

Soluble guanylate cyclase (sGC) is a heme-containing heterodimeric enzyme that generates many molecules of cGMP in response to its ligand nitric oxide (NO); sGC thereby acts as an amplifier in NO-driven biological signaling cascades. Because sGC helps regulate the cardiovascular, neuronal, and gastrointestinal systems through its cGMP production, boosting sGC activity and preventing or reversing sGC inactivation are important therapeutic and pharmacologic goals. Work over the last two decades is uncovering the processes by which sGC matures to become functional, how sGC is inactivated, and how sGC is rescued from damage. A diverse group of small molecules and proteins have been implicated in these processes, including NO itself, reactive oxygen species, cellular heme, cell chaperone Hsp90, and various redox enzymes as well as pharmacologic sGC agonists. This review highlights their participation and provides an update on the processes that enable sGC maturation, drive its inactivation, or assist in its recovery in various settings within the cell, in hopes of reaching a better understanding of how sGC function is regulated in health and disease.

Differential expression of alternative transcripts of soluble guanylyl cyclase, GYCY1a3 and GUCY1b3 genes, in the malignant and benign breast tumors

Extensive alterations in splicing is one of the molecular indicator for human cancers. Soluble guanylyl cyclase (sGC), an obligatory heterodimer, is composed of α1 and β1 subunits. Each subunit is encoded by a separate gene, GUCY1a3 and GUCY1b3, correspondingly. sGC activity has been regulated by an alternative splicing and it has an important effect on the breast cancer. sGC alternative splicing has been evaluated in the 55 malignant, 25 benign and 30 normal breast tissues using qRT-PCR and RT-PCR. The differences between groups were analyzed by Mann-Whitney U. The expression of six different splice forms have been detected, three for α1 and three for β1 sGC. Expressions of Tr1, Tr2 β1 sGC and Tr7, Tr6 α1 sGC mRNA in the malignant breast tumors were significantly lower than those of benign and normal breast tissues. However, the expression of Tr3 α1 sGC mRNA was significantly higher than that of benign and normal tissues. Present data have provided some evidences for an alteration in the expression of α1 and β1 sGC alternative splicing forms which may contribute to the loss of sGC functions in the breast cancer. The observed information might be discussed by the cGMP status.

Nitric oxide induces epidermal stem cell de-adhesion by targeting integrin β1 and Talin via the cGMP signalling pathway

OBJECTIVE

Nitric oxide (NO) has emerged as a critical molecule in wound healing, but the mechanism underlying its activity is not well defined. Here, we explored the effect of NO on the de-adhesion of epidermal stem cells (ESCs) and the mechanism involved in this process.

METHODS

The effects of NO on isolated human and mouse ESCs cultured in the presence of different concentrations of the NO donor S-nitroso-N-acetyl penicillamine (SNAP) were evaluated in cell de-adhesion assays mediated by integrin β and collagen IV. Subsequently, changes in the expression of integrin β1 and the phosphorylation of Talin in response to different doses of SNAP were detected by Western blot analysis and real-time PCR in vitro. Furthermore, the roles of various soluble guanylyl cyclase (sGC)- and protein kinase G (PKG)-specific inhibitors and agonists in the effects of NO on ESC de-adhesion, integrin β1 expression and Talin phosphorylation were analysed. Moreover, the effects of NO on integrin β1 expression and sGC/cGMP/PKG signalling-mediated wound healing were detected in vivo using 5-bromo-2-deoxyuridine (BrdU) label-retaining cells (LRCs) in a scald model and an excision wound healing model, respectively.

RESULTS

SNAP promoted primary human and mouse ESC de-adhesion in a concentration-dependent manner in the integrin β1-and collagen IV-mediated adhesion assay, and this effect was suppressed by the sGC and PKG inhibitors. Additionally, integrin β1 expression and Talin phosphorylation at serine 425 (S425) were negatively correlated with SNAP levels, and this effect was blocked by the sGC and PKG inhibitors. Moreover, the roles of NO in integrin β1 expression and cGMP signalling pathway-mediated wound healing were confirmed in vivo.

CONCLUSION

Our data indicate that the stimulatory effects of NO on ESC de-adhesion related to integrin β1 expression and Talin phosphorylation were mediated by the cGMP signalling pathway, which is likely involved in wound healing.

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 Plays a Key Role in Ovariectomy-Induced Apoptosis in Anterior Pituitary: Interplay between Nitric Oxide Pathway and Estrogen

Changes in the estrogenic status produce deep changes in pituitary physiology, mainly because estrogens (E2) are one of the main regulators of pituitary cell population. Also, E2 negatively regulate pituitary neuronal nitric oxide synthase (nNOS) activity and expression and may thereby modulate the production of nitric oxide (NO), an important regulator of cell death and survival. Little is known about how ovary ablation affects anterior pituitary cell remodelling and molecular mechanisms that regulate this process have not yet been elucidated. In this work we used freshly dispersed anterior pituitaries as well as cell cultures from ovariectomized female rats in order to study whether E2 deficiency induces apoptosis in the anterior pituitary cells, the role of NO in this process and effects of E2 on the NO pathway. Our results showed that cell activity gradually decreases after ovariectomy (OVX) as a consequence of cell death, which is completely prevented by a pan-caspase inhibitor. Furthermore, there is an increase of fragmented nuclei and DNA cleavage thereby presenting the first direct evidence of the existence of apoptosis in the anterior pituitary gland after OVX. NO production and soluble guanylyl cyclase (sGC) expression in anterior pituitary cells increased concomitantly to the apoptosis. Inhibition of both, NO synthase (NOS) and sGC activities prevented the drop of cell viability after OVX, showing for the first time that increased NO levels and sGC activity observed post-OVX play a key role in the induction of apoptosis. Conversely, E2 and prolactin treatments decreased nNOS expression and activity in pituitary cells from OVX rats in a time- and E2 receptor-dependent manner, thus suggesting interplay between NO and E2 pathways in anterior pituitary.

Role of sGC-dependent NO signalling and myocardial infarction risk

The NO/cGMP pathway plays an important role in many physiological functions and pathophysiological conditions. In the last few years, several genetic and functional studies pointed to an underestimated role of this pathway in the development of atherosclerosis. Indeed, several genetic variants of key enzymes modulating the generation of NO and cGMP have been strongly associated with coronary artery disease and myocardial infarction risk. In this review, we aim to place the genomic findings on components of the NO/cGMP pathway, namely endothelial nitric oxide synthase, soluble guanylyl cyclase and phosphodiesterase 5A, in context of preventive and therapeutic strategies for treating atherosclerosis and its sequelae.

The fibrate gemfibrozil is a NO- and haem-independent activator of soluble guanylyl cyclase: in vitro studies

BACKGROUND AND PURPOSE

Fibrates are a class of drugs widely used to treat dyslipidaemias. They regulate lipid metabolism and act as PPARα agonists. Clinical trials demonstrate that besides changes in lipid profiles, fibrates decrease the incidence of cardiovascular events, with gemfibrozil exhibiting the most pronounced benefit. This study aims to characterize the effect of gemfibrozil on the activity and function of soluble guanylyl cyclase (sGC), the key mediator of NO signalling.

EXPERIMENTAL APPROACH

High-throughput screening of a drug library identified gemfibrozil as a direct sGC activator. Activation of sGC is unique to gemfibrozil and is not shared by other fibrates.

KEY RESULTS

Gemfibrozil activated purified sGC, induced endothelium-independent relaxation of aortic rings and inhibited platelet aggregation. Gemfibrozil-dependent activation was absent when the sGC haem domain was deleted, but was significantly enhanced when sGC haem was lacking or oxidized. Oxidation of sGC haem enhanced the vasoactive and anti-platelet effects of gemfibrozil. Gemfibrozil competed with the haem-independent sGC activators ataciguat and cinaciguat. Computational modelling predicted that gemfibrozil occupies the space of the haem group and interacts with residues crucial for haem stabilization. This is consistent with structure-activity data which revealed an absolute requirement for gemfibrozil's carboxyl group.

CONCLUSIONS AND IMPLICATIONS

These data suggest that in addition to altered lipid and lipoprotein state, the cardiovascular preventive benefits of gemfibrozil may derive from direct activation and protection of sGC function. A sGC-directed action may explain the more pronounced cardiovascular benefit of gemfibrozil observed over other fibrates and some of the described side effects of gemfibrozil.

Alternative splicing impairs soluble guanylyl cyclase function in aortic aneurysm

Nitric oxide (NO) receptor soluble guanylyl cyclase (sGC) is a key regulator of several important vascular functions and is important for maintaining cardiovascular homeostasis and vascular plasticity. Diminished sGC expression and function contributes to pathogenesis of several cardiovascular diseases. However, the processes that control sGC expression in vascular tissue remain poorly understood. Previous work in animal and cell models revealed the complexity of alternative splicing of sGC genes and demonstrated its importance in modulation of sGC function. The aim of this study was to examine the role of alternative splicing of α1 and β1 sGC in healthy and diseased human vascular tissue. Our study found a variety of α1 and β1 sGC splice forms expressed in human aorta. Their composition and abundance were different between samples of aortic tissue removed during surgical repair of aortic aneurysm and samples of aortas without aneurysm. Aortas with aneurysm demonstrated decreased sGC activity, which correlated with increased expression of dysfunctional sGC splice variants. In addition, the expression of 55-kDa oxidation-resistant α1 isoform B sGC (α1-IsoB) was significantly lower in aortic samples with aneurysm. The α1-IsoB splice variant was demonstrated to support sGC activity in aortic lysates. Together, our results suggest that alternative splicing contributes to diminished sGC function in vascular dysfunction. Precise understanding of sGC splicing regulation could help to design new therapeutic interventions and to personalize sGC-targeting therapies in treatments of vascular disease.

Nitric oxide and heat shock protein 90 activate soluble guanylate cyclase by driving rapid change in its subunit interactions and heme content

The chaperone heat shock protein 90 (hsp90) associates with signaling proteins in cells including soluble guanylate cyclase (sGC). hsp90 associates with the heme-free (apo) sGC-β1 subunit and helps to drive heme insertion during maturation of sGC to its NO-responsive active form. Here, we found that NO caused apo-sGC-β1 to rapidly and transiently dissociate from hsp90 and associate with sGC-α1 in cells. This NO response (i) required that hsp90 be active and that cellular heme be available and be capable of inserting into apo-sGC-β1; (ii) was associated with an increase in sGC-β1 heme content; (iii) could be mimicked by the heme-independent sGC activator BAY 60-2770; and (iv) was followed by desensitization of sGC toward NO, sGC-α1 disassociation, and reassociation with hsp90. Thus, NO promoted a rapid, transient, and hsp90-dependent heme insertion into the apo-sGC-β1 subpopulation in cells, which enabled it to combine with the sGC-α1 subunit to form the mature enzyme. The driving mechanism likely involves conformational changes near the heme site in sGC-β1 that can be mimicked by the pharmacologic sGC activator. Such dynamic interplay between hsp90, apo-sGC-β1, and sGC-α1 in response to NO is unprecedented and represent new steps by which cells can modulate the heme content and activity of sGC for signaling cascades.

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

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

Hydrogen peroxide alters splicing of soluble guanylyl cyclase and selectively modulates expression of splicing regulators in human cancer cells

Background

Soluble guanylyl cyclase (sGC) plays a central role in nitric oxide (NO)-mediated signal transduction in the cardiovascular, nervous and gastrointestinal systems. Alternative RNA splicing has emerged as a potential mechanism to modulate sGC expression and activity. C-α1 sGC is an alternative splice form that is resistant to oxidation-induced protein degradation and demonstrates preferential subcellular distribution to the oxidized environment of endoplasmic reticulum (ER).

Methodology/Principal Findings

Here we report that splicing of C-α1 sGC can be modulated by H₂O₂ treatment in BE2 neuroblastoma and MDA-MD-468 adenocarcinoma human cells. In addition, we show that the H₂O₂ treatment of MDA-MD-468 cells selectively decreases protein levels of PTBP1 and hnRNP A2/B1 splice factors identified as potential α1 gene splicing regulators by in silico analysis. We further demonstrate that down-regulation of PTBP1 by H₂O₂ occurs at the protein level with variable regulation observed in different breast cancer cells.

Conclusions/Significance

Our data demonstrate that H₂O₂ regulates RNA splicing to induce expression of the oxidation-resistant C-α1 sGC subunit. We also report that H₂O₂ treatment selectively alters the expression of key splicing regulators. This process might play an important role in regulation of cellular adaptation to conditions of oxidative stress.

The amino-terminus of nitric oxide sensitive guanylyl cyclase α₁ does not affect dimerization but influences subcellular localization

Background

Nitric oxide sensitive guanylyl cyclase (NOsGC) is a heterodimeric enzyme formed by an α- and a β₁-subunit. A splice variant (C-α₁) of the α₁-subunit, lacking at least the first 236 amino acids has been described by Sharina et al. 2008 and has been shown to be expressed in differentiating human embryonic cells. Wagner et al. 2005 have shown that the amino acids 61–128 of the α₁-subunit are mandatory for quantitative heterodimerization implying that the C-α₁-splice variant should lose its capacity to dimerize quantitatively.

Methodology/Principal Findings

In the current study we demonstrate preserved quantitative dimerization of the C-α₁-splice by co-purification with the β₁-subunit. In addition we used fluorescence resonance energy transfer (FRET) based on fluorescence lifetime imaging (FLIM) using fusion proteins of the β₁-subunit and the α₁-subunit or the C-α₁ variant with ECFP or EYFP. Analysis of the respective combinations in HEK-293 cells showed that the fluorescence lifetime was significantly shorter (≈0.3 ns) for α₁/β₁ and C-α₁/β₁ than the negative control. In addition we show that lack of the amino-terminus in the α₁ splice variant directs it to a more oxidized subcellular compartment.

Conclusions/Significance

We conclude that the amino-terminus of the α₁-subunit is dispensable for dimerization in-vivo and ex-vivo, but influences the subcellular trafficking.