Nitric oxide and cyclic GMP as pro- and anti-apoptotic agents

Common and specific proteins and pathways in heart and cerebral ischemia

OBJECTIVE

To understand the similarities and differences between acute ischemic stroke and acute myocardial infarction (AMI) to help in the development of specific or common treatment strategies.

METHODS

Using an aptamer-based proteomic array, we measured and compared 1310 circulating proteins in the blood of 40 patients with AIS, 9 patients with AMI, and 31 healthy controls. Pathway enrichment analysis was performed using GSEA and g:profiler.

RESULTS

Ninety-four proteins were differentially expressed in AIS, and 284 were differentially expressed in AMI. Of these, 8 were specific to cerebral ischemia, and 197 were specific to myocardial infarction. Forty-two proteins were altered in both ischemia processes. Most altered pathways in AIS could be classified as immune response, cell cycle processing, molecular transport, or signaling. Pathways altered in AMI were mostly related to lipid metabolism and transport, highlighting cholesterol metabolic processes and estrogen signaling. In both types of ischemia, we found pathways related to metabolism, specifically purine metabolism, and signaling processes, such as TNF signaling or MAPK1/3.

CONCLUSIONS

The present study revealed proteins and pathways that were specifically altered in cerebral ischemia, in cardiac ischemia, or in both diseases, providing information on the similarities and differences of ischemic conditions. The role of common and specific proteins and pathways should be explored in detail to find possible therapeutic targets.

Active nNOS Is Required for Grp94-Induced Antioxidant Cytoprotection: A Lesson from Myogenic to Cancer Cells

The endoplasmic reticulum (ER) chaperone Grp94/gp96 appears to be involved in cytoprotection without being required for cell survival. This study compared the effects of Grp94 protein levels on Ca²⁺ homeostasis, antioxidant cytoprotection and protein–protein interactions between two widely studied cell lines, the myogenic C2C12 and the epithelial HeLa, and two breast cancer cell lines, MDA-MB-231 and HS578T. In myogenic cells, but not in HeLa, Grp94 overexpression exerted cytoprotection by reducing ER Ca²⁺ storage, due to an inhibitory effect on SERCA2. In C2C12 cells, but not in HeLa, Grp94 co-immunoprecipitated with non-client proteins, such as nNOS, SERCA2 and PMCA, which co-fractionated by sucrose gradient centrifugation in a distinct, medium density, ER vesicular compartment. Active nNOS was also required for Grp94-induced cytoprotection, since its inhibition by L-NNA disrupted the co-immunoprecipitation and co-fractionation of Grp94 with nNOS and SERCA2, and increased apoptosis. Comparably, only the breast cancer cell line MDA-MB-231, which showed Grp94 co-immunoprecipitation with nNOS, SERCA2 and PMCA, increased oxidant-induced apoptosis after nNOS inhibition or Grp94 silencing. These results identify the Grp94-driven multiprotein complex, including active nNOS as mechanistically involved in antioxidant cytoprotection by means of nNOS activity and improved Ca²⁺ homeostasis.

Soluble guanylate cyclase signalling mediates etoposide resistance in progressing small cell lung cancer

Small cell lung cancer (SCLC) has a 5-year survival rate of <7%. Rapid emergence of acquired resistance to standard platinum-etoposide chemotherapy is common and improved therapies are required for this recalcitrant tumour. We exploit six paired pre-treatment and post-chemotherapy circulating tumour cell patient-derived explant (CDX) models from donors with extensive stage SCLC to investigate changes at disease progression after chemotherapy. Soluble guanylate cyclase (sGC) is recurrently upregulated in post-chemotherapy progression CDX models, which correlates with acquired chemoresistance. Expression and activation of sGC is regulated by Notch and nitric oxide (NO) signalling with downstream activation of protein kinase G. Genetic targeting of sGC or pharmacological inhibition of NO synthase re-sensitizes a chemoresistant CDX progression model in vivo, revealing this pathway as a mediator of chemoresistance and potential vulnerability of relapsed SCLC.

Long-term inhibition of PDE5 ameliorates aging-induced changes in rat testis

NO-cGMP signaling pathway has been implicated in reduction of testicular steroidogenesis during aging. Here we analyzed the effect of PDE5 inhibition on old testicular phenotype formation. The old phenotype exhibited low testosterone and increased nitrite levels in circulation, increased cGMP accumulation in testicular interstitial fluid (TIF), progressive atrophy of testicular seminiferous tubules and enlargement of interstitial area followed by rise in blood vessel density and slight increase in the number of Leydig cells and macrophages. Leydig cells have reduced steroidogenic capacity, increased MAP kinases expression (MEK, ERK1/2, JNK) and antiapoptotic PRKG1 and AKT, suggesting increased proliferation/survival and accumulation of senescent Leydig cells in testis. In 12 month-old rats, a long-term treatment with sildenafil (PDE5 inhibitor) normalized testosterone/nitrite levels in circulation and cGMP accumulation in TIF; improved Leydig cell steroidogenic capacity; decreased MEK, ERK1/2 and PRKG1 expression; prevented an increase in the Leydig cells number and atrophy of seminiferous tubules leading to histological appearance of young rat testes. In 18 month-old rats, long-term PDE5 inhibition partially recovered testosterone and nitrite levels in serum; normalized PRKG1 expression without effect on MEK and ERK1/2; and slowed down Leydig cell and macrophage accumulation and regressive tubular changes. Culturing of primary Leydig cells from aged rats in presence of PDE5-inhibitor stimulated steroidogenic and MAPK gene expression. Taking together, results indicate that cGMP targeting alter both steroidogenesis and signaling pathways associated with cell proliferation/survival. The long-term PDE5 inhibition improves testicular steroidogenesis and slows-down regressive changes in testes during aging.

Phosphodiesterase 5 Inhibition Limits Doxorubicin-induced Heart Failure by Attenuating Protein Kinase G Iα Oxidation

Phosphodiesterase 5 (PDE5) inhibitors limit myocardial injury caused by stresses, including doxorubicin chemotherapy. cGMP binding to PKG Iα attenuates oxidant-induced disulfide formation. Because PDE5 inhibition elevates cGMP and protects from doxorubicin-induced injury, we reasoned that this may be because it limits PKG Iα disulfide formation. To investigate the role of PKG Iα disulfide dimerization in the development of apoptosis, doxorubicin-induced cardiomyopathy was compared in male wild type (WT) or disulfide-resistant C42S PKG Iα knock-in (KI) mice. Echocardiography showed that doxorubicin treatment caused loss of myocardial tissue and depressed left ventricular function in WT mice. Doxorubicin also reduced pro-survival signaling and increased apoptosis in WT hearts. In contrast, KI mice were markedly resistant to the dysfunction induced by doxorubicin in WTs. In follow-on experiments the influence of the PDE5 inhibitor tadalafil on the development of doxorubicin-induced cardiomyopathy in WT and KI mice was investigated. In WT mice, co-administration of tadalafil with doxorubicin reduced PKG Iα oxidation caused by doxorubicin and also protected against cardiac injury and loss of function. KI mice were again innately resistant to doxorubicin-induced cardiotoxicity, and therefore tadalafil afforded no additional protection. Doxorubicin decreased phosphorylation of RhoA (Ser-188), stimulating its GTPase activity to activate Rho-associated protein kinase (ROCK) in WTs. These pro-apoptotic events were absent in KI mice and were attenuated in WTs co-administered tadalafil. PKG Iα disulfide formation triggers cardiac injury, and this initiation of maladaptive signaling can be blocked by pharmacological therapies that elevate cGMP, which binds kinase to limit its oxidation.

Nitric oxide and reactive oxygen species: Clues to target oxidative damage repair defective breast cancers

The identification of various biomolecules in cancer progression and therapy has led to the exploration of the roles of two cardinal players, namely Nitric Oxide (NO) and Reactive Oxygen Species (ROS) in cancer. Both ROS and NO display bimodal fashions of functional activity in a concentration dependent manner, by inducing either pro- or anti- tumorigenic signals. Researchers have identified the potential capability of NO and ROS in therapies owing to their role in eliciting pro-apoptotic signals at higher concentrations and their ability to sensitize cancer cells to one another as well as to other therapeutics. We review the prospects of NO and ROS in cancer progression and therapy, and analyze the role of a combinatorial therapy wherein an NO donor (SNAP) is used to sensitize the oxidative damage repair defective, triple negative breast cancer cells (HCC 1937) to a potent ROS inducer. Preliminary findings support the potential to employ various combinatorial regimes for anti-cancer therapies with regard to exploiting the chemo-sensitization property of NO donors.

Redox regulation of cGMP-dependent protein kinase Iα in the cardiovascular system

Elevated levels of oxidants in biological systems have been historically referred to as “oxidative stress,” a choice of words that perhaps conveys an imbalanced view of reactive oxygen species in cells and tissues. The term stress suggests a harmful role, whereas a contemporary view is that oxidants are also crucial for the maintenance of homeostasis or adaptive signaling that can actually limit injury. This regulatory role for oxidants is achieved in part by them inducing oxidative post-translational modifications of proteins which may alter their function or interactions. Such mechanisms allow changes in cell oxidant levels to be coupled to regulated alterations in enzymatic function (i.e., signal transduction), which enables “redox signaling.” In this review we focus on the role of cGMP-dependent protein kinase (PKG) Ia disulfide dimerisation, an oxidative modification that is induced by oxidants that directly activates the enzyme, discussing how this impacts on the cardiovascular system. Additionally, how this oxidative activation of PKG may coordinate with or differ from classical activation of this kinase by cGMP is also considered.

Neurogenesis and inflammation after ischemic stroke: what is known and where we go from here

This review covers the pathogenesis of ischemic stroke and future directions regarding therapeutic options after injury. Ischemic stroke is a devastating disease process affecting millions of people worldwide every year. The mechanisms underlying the pathophysiology of stroke are not fully understood but there is increasing evidence demonstrating the contribution of inflammation to the drastic changes after cerebral ischemia. This inflammation not only immediately affects the infarcted tissue but also causes long-term damage in the ischemic penumbra. Furthermore, the interaction between inflammation and subsequent neurogenesis is not well understood but the close relationship between these two processes has garnered significant interest in the last decade or so. Current approved therapy for stroke involving pharmacological thrombolysis is limited in its efficacy and new treatment strategies need to be investigated. Research aimed at new therapies is largely about transplantation of neural stem cells and using endogenous progenitor cells to promote brain repair. By understanding the interaction between inflammation and neurogenesis, new potential therapies could be developed to further establish brain repair mechanisms.

Effect of phosphodiesterase-5 inhibition on apoptosis and beta amyloid load in aged mice

Age-related cognitive decline is accompanied by an increase of neuronal apoptosis and a dysregulation of neuroplasticity-related molecules such as brain-derived neurotrophic factor and neurotoxic factors including beta amyloid (Aβ) peptide. Because it has been previously demonstrated that phosphodiesterase-5 inhibitors (PDE5-Is) protect against hippocampal synaptic dysfunction and memory deficits in mouse models of Alzheimer's disease and physiological aging, we investigated the effect of a treatment with the PDE5-I, sildenafil, on cell death, pro- and antiapoptotic molecules, and Aβ production. We demonstrated that chronic intraperitoneal injection of sildenafil (3 mg/kg for 3 weeks) decreased terminal deoxyuridine triphosphate nick end labeling-positive cells in the CA1 hippocampal area of 26-30-month-old mice, downregulating the proapoptotic proteins, caspase-3 and B-cell lymphoma 2-associated X, and increasing antiapoptotic molecules such as B-cell lymphoma protein-2 and brain-derived neurotrophic factor. Also, sildenafil reverted the shifting of amyloid precursor protein processing toward Aβ42 production and the increase of the Aβ42:Aβ40 ratio in aged mice. Our data suggest that PDE5-I might be beneficial to treat age-related detrimental features in a physiological mouse model of aging.

The role of oxidative stress in the development of cisplatin resistance in epithelial ovarian cancer

OBJECTIVE

To investigate the role of oxidative stress in the development of cisplatin resistance in epithelial ovarian cancer (EOC).

METHODS

Two parent EOC cell lines (MDAH-2774 and SKOV-3) and their chemoresistant counterparts (cisplatin, 50 µmol/L) were used. Total RNA was extracted and subjected to real-time reverse transcriptase polymerase chain reaction to evaluate the expression of glutathione reductase (GSR) and inducible nitric oxide synthase (iNOS), as well as nitrate/nitrite levels. Analysis of variance was used for main effects and Tukey for post hoc analysis at P < .05 for statistical significance.

RESULTS

Both cisplatin resistant cell lines displayed a significant decrease in GSR messenger RNA (mRNA) levels and activity (P < .01). As compared to sensitive controls, nitrate/nitrite levels were significantly higher in SKOV-3 cisplatin resistant cells while iNOS mRNA levels were significantly higher in MDAH-2774 cisplatin resistant cells (P < .05).

CONCLUSION

Our data suggest that the development of cisplatin resistance tilts the balance toward a pro-oxidant state in EOC.

Cyclic GMP/protein kinase G type-Iα (PKG-Iα) signaling pathway promotes CREB phosphorylation and maintains higher c-IAP1, livin, survivin, and Mcl-1 expression and the inhibition of PKG-Iα kinase activity synergizes with cisplatin in non-small cell lung cancer cells

Previously, our laboratory showed that nitric oxide (NO)/cyclic GMP (cGMP)/protein kinase G type-Iα (PKG-Iα) signaling pathway plays an important role in preventing spontaneous apoptosis and promoting cell proliferation in both normal cells (bone marrow stromal cells and vascular smooth muscle cells) and certain cancer cells (ovarian cancer cells). In the present study, we investigated the novel role of the cGMP/PKG-Iα pathway in preventing spontaneous apoptosis, promoting colony formation and regulating phosphorylation of cAMP response element binding (CREB) protein and protein expression of inhibitor of apoptosis proteins (IAPs) and anti-apoptotic Bcl-2-related proteins in NCI-H460 and A549 non-small cell lung cancer (NSCLC) cells. 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), which blocks endogenous NO-induced activation of cGMP/PKG-Iα, induced apoptosis and decreased colony formation. ODQ also decreased CREB ser133 phosphorylation and protein expression of c-IAP1, livin, and survivin. DT-2 (inhibitor of PKG-Iα kinase activity) increased apoptosis by twofold and decreased CREB ser133 phosphorylation and c-IAP1, livin, and survivin expression. Gene knockdown of PKG-Iα expression using small-interfering RNA increased apoptosis and decreased CREB ser133 phosphorylation, and c-IAP1, livin, survivin, and Mcl-1 expression. Inhibition of PKG-Iα kinase activity with DT-2 dramatically enhanced pro-apoptotic effects of the chemotherapeutic agent cisplatin. Combined treatment of DT-2 and cisplatin increased apoptosis compared with cisplatin or DT-2 alone, showing a synergistic effect. The data suggest that the PKG-Iα kinase activity is necessary for maintaining higher levels of CREB phosphorylation at ser133 and protein expression of c-IAP1, livin, survivin, and Mcl-1, preventing spontaneous apoptosis and promoting colony formation in NSCLC cells, which may limit the effectiveness of chemotherapeutic agents like cisplatin.

(-)-Epigallocatechin-3-gallate protects against NO-induced ototoxicity through the regulation of caspase- 1, caspase-3, and NF-κB activation

Excessive nitric oxide (NO) production is toxic to the cochlea and induces hearing loss. However, the mechanism through which NO induces ototoxicity has not been completely understood. The aim of this study was to gain further insight into the mechanism mediating NO-induced toxicity in auditory HEI-OC1 cells and in ex vivo analysis. We also elucidated whether and how epigallocatechin-3-gallate (EGCG), the main component of green tea polyphenols, regulates NO-induced auditory cell damage. To investigate NO-mediated ototoxicity, S-nitroso-N-acetylpenicillamine (SNAP) was used as an NO donor. SNAP was cytotoxic, generating reactive oxygen species, releasing cytochrome c, and activating caspase-3 in auditory cells. NO-induced ototoxicity also mediated the nuclear factor (NF)-κB/caspase-1 pathway. Furthermore, SNAP destroyed the orderly arrangement of the 3 outer rows of hair cells in the basal, middle, and apical turns of the organ of Corti from the cochlea of Sprague-Dawley rats at postnatal day 2. However, EGCG counteracted this ototoxicity by suppressing the activation of caspase-3/NF-κB and preventing the destruction of hair cell arrays in the organ of Corti. These findings may lead to the development of a model for pharmacological mechanism of EGCG and potential therapies against ototoxicity.

Essential roles of the nitric oxide (no)/cGMP/protein kinase G type-Iα (PKG-Iα) signaling pathway and the atrial natriuretic peptide (ANP)/cGMP/PKG-Iα autocrine loop in promoting proliferation and cell survival of OP9 bone marrow stromal cells

Inappropriate signaling conditions within bone marrow stromal cells (BMSCs) can lead to loss of BMSC survival, contributing to the loss of a proper micro-environmental niche for hematopoietic stem cells (HSCs), ultimately causing bone marrow failure. In the present study, we investigated the novel role of endogenous atrial natriuretic peptide (ANP) and the nitric oxide (NO)/cGMP/protein kinase G type-Iα (PKG-Iα) signaling pathway in regulating BMSC survival and proliferation, using the OP9 BMSC cell line commonly used for facilitating the differentiation of HSCs. Using an ANP-receptor blocker, endogenously produced ANP was found to promote cell proliferation and prevent apoptosis. NO donor SNAP (S-nitroso-N-acetylpenicillamine) at low concentrations (10 and 50 µM), which would moderately stimulate PKG activity, protected these BMSCs against spontaneous apoptosis. YC-1, a soluble guanylyl cyclase (sGC) activator, decreased the levels of apoptosis, similar to the cytoprotective effects of low-level NO. ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one), which blocks endogenous NO-induced activation of sGC and thus lowers endogenous cGMP/PKG activity, significantly elevated apoptotic levels by 2.5- and three-fold. Pre-incubation with 8-Bromo-cGMP or ANP, which bypass the ODQ block, almost completely prevented the ODQ-induced apoptosis. A highly-specific PKG inhibitor, DT-3, at 20, and 30 µM, caused 1.5- and two-fold increases in apoptosis, respectively. ODQ and DT-3 also decreased BMSCs proliferation and colony formation. Small Interfering RNA gene knockdown of PKG-Iα increased apoptosis and decreased proliferation in BMSCs. The data suggest that basal NO/cGMP/PKG-Iα activity and autocrine ANP/cGMP/PKG-Iα are necessary for preserving OP9 cell survival and promoting cell proliferation and migration.

Protein kinase G type Ialpha activity in human ovarian cancer cells significantly contributes to enhanced Src activation and DNA synthesis/cell proliferation

Previously, we showed that basal activity of nitric oxide (NO)/cyclic GMP (cGMP)/protein kinase G (PKG) signaling pathway protects against spontaneous apoptosis and confers resistance to cisplatin-induced apoptosis in human ovarian cancer cells. The present study determines whether basal PKG kinase activity regulates Src family kinase (SFK) activity and proliferation in these cells. PKG-Ialpha was identified as predominant isoform in both OV2008 (cisplatin-sensitive, wild-type p53) and A2780cp (cisplatin-resistant, mutated p53) ovarian cancer cells. In both cell lines, ODQ (inhibitor of endogenous NO-induced cGMP biosynthesis), DT-2 (highly specific inhibitor of PKG-Ialpha kinase activity), and PKG-Ialpha knockdown (using small interfering RNA) caused concentration-dependent inhibition of DNA synthesis (assessed by bromodeoxyuridine incorporation), indicating an important role of basal cGMP/PKG-Ialpha kinase activity in promoting cell proliferation. DNA synthesis in OV2008 cells was dependent on SFK activity, determined using highly selective SFK inhibitor, 4-(4'-phenoxyanilino)-6,7-dimethoxyquinazoline (SKI-1). Studies using DT-2 and PKG-Ialpha small interfering RNA revealed that SFK activity was dependent on PKG-Ialpha kinase activity. Furthermore, SFK activity contributed to endogenous tyrosine phosphorylation of PKG-Ialpha in OV2008 and A2780cp cells. In vitro coincubation of recombinant human c-Src and PKG-Ialpha resulted in c-Src-mediated tyrosine phosphorylation of PKG-Ialpha and enhanced c-Src autophosphorylation/activation, suggesting that human c-Src directly tyrosine phosphorylates PKG-Ialpha and the c-Src/PKG-Ialpha interaction enhances Src kinase activity. Epidermal growth factor-induced stimulation of SFK activity in OV2008 cells increased PKG-Ialpha kinase activity (indicated by Ser(239) phosphorylation of the PKG substrate vasodilator-stimulated phosphoprotein), which was blocked by both SKI-1 and SU6656. The data suggest an important role of Src/PKG-Ialpha interaction in promoting DNA synthesis/cell proliferation in human ovarian cancer cells. The NO/cGMP/PKG-Ialpha signaling pathway may provide a novel therapeutic target for disrupting ovarian cancer cell proliferation.

Glial cells as sources and targets of natriuretic peptides

Natriuretic peptides and their receptors are widely expressed in mammalian CNS and increasing evidence implicates them in the regulation of neural development, synaptic transmission and processing of information, and neuroprotection. Although the peptides have been mainly localized in neuronal populations they are also produced in glial cells. Astroglia and microglia also express functional natriuretic peptide receptors that can regulate important physiological responses. In this article we review evidence on the localization of natriuretic peptides and their receptors in astroglial and microglial cells and summarize data supporting the participation of this signalling system in neuron-glia and glia-brain blood vessel communication relevant to CNS function.

Cyclic GMP protects human macrophages against peroxynitrite-induced apoptosis

Background

Nitric oxide (NO) can be both pro- and anti-apoptotic in various cell types, including macrophages. This apparent paradox may result from the actions of NO-related species generated in the microenvironment of the cell, for example the formation of peroxynitrite (ONOO^-). In this study we have examined the ability of NO and ONOO^- to evoke apoptosis in human monocyte-derived macrophages (MDMϕ), and investigated whether preconditioning by cyclic guanosine monophosphate (cGMP) is able to limit apoptosis in this cell type.

Methods

Characterisation of the NO-related species generated by (Z)-1- [2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA/NO) and 1,2,3,4-oxatriazolium, 5-amino-3-(3,4-dichlorophenyl)-, chloride (GEA-3162) was performed by electrochemistry using an isolated NO electrode and electron paramagnetic resonance (EPR) spectrometry. Mononuclear cells were isolated from peripheral blood of healthy volunteers and cultured to allow differentiation into MDMϕ. Resultant MDMϕ were treated for 24 h with DETA/NO (100 – 1000 μM) or GEA-3162 (10 – 300 μM) in the presence or absence of BAY 41–2272 (1 μM), isobutylmethylxanthine (IBMX; 1 μM), 1H- [1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ; 20 μM) or 8-bromo-cGMP (1 mM). Apoptosis in MDMϕ was assessed by flow cytometric analysis of annexin V binding in combination with propidium iodide staining.

Results

Electrochemistry and EPR revealed that DETA/NO liberated free NO radical, whilst GEA-3162 concomitantly released NO and O₂^-, and is therefore a ONOO^- generator. NO (DETA/NO) had no effect on cell viability, but ONOO^- (GEA-3162) caused a concentration-dependent induction of apoptosis in MDMϕ. Preconditioning of MDMϕ with NO in combination with the phosphodiesterase inhibitor, 3-Isobutyl-1-methylxanthine (IBMX), or the NO-independent stimulator of soluble guanylate cyclase, BAY 41–2272, significantly attenuated ONOO^--induced apoptosis in a cGMP-dependent manner.

Conclusion

These results demonstrate disparities between the ability of NO and ONOO^- to induce apoptosis in human MDMϕ. Furthermore, this study provides evidence for a novel cGMP-dependent pre-conditioning mechanism to limit ONOO^--induced apoptosis in human MDMϕ.

Cisplatin alters nitric oxide synthase levels in human ovarian cancer cells: involvement in p53 regulation and cisplatin resistance

The present study determines if (1) basal protein levels of nitric oxide (NO) synthases (eNOS, iNOS, and nNOS) are different in cisplatin-sensitive (OV2008) and counterpart cisplatin-resistant (C13^*) human ovarian cancer cells, (2) cisplatin alters NOS levels, (3) NO donor causes apoptosis and p53 upregulation, (4) NO donor sensitises C13^* cells to cisplatin via p53 upregulation (determined by p53 siRNA gene-knockdown), and (5) inhibition of endogenous NOS alters cisplatin-induced apoptosis. Basal iNOS levels were higher in OV2008 cells than in C13^* cells. Cisplatin upregulated iNOS, but dramatically reduced eNOS and nNOS, in OV2008 cells only. Failure of cisplatin to upregulate iNOS and downregulate eNOS/nNOS in cisplatin-resistant C13^* cells may be an aetiological factor in the development of resistance. The NO donor S-nitroso-N-acetylpenicillamine (SNAP) increased p53 protein levels and induced apoptosis in both cell types, and enhanced cisplatin-induced apoptosis in C13^* cells in a p53-dependent manner (i.e., enhancement blocked by p53 siRNA). Specific iNOS inhibitor 1400W partially blocked cisplatin-induced apoptosis in OV2008 cells. In cisplatin-resistant C13^* cells, blocking all NOSs with N^G-amino-L-arginine dramatically changed these cells from cisplatin-resistant to cisplatin-sensitive, greatly potentiating cisplatin-induced apoptosis. The data suggest important roles for the three NOSs in regulating chemoresistance to cisplatin in ovarian cancer cells.

Cyclic GMP and protein kinase-G in myocardial ischaemia-reperfusion: opportunities and obstacles for survival signaling

It is clear that multiple signalling pathways regulate the critical balance between cell death and survival in myocardial ischaemia-reperfusion. Recent attention has focused on the activation of survival or salvage kinases, particularly during reperfusion, as a common mechanism of many cardioprotective interventions. The phosphatidyl inositol 3'-hydroxy kinase/Akt complex (PI3K/Akt) and p42/p44 mitogen-activated protein kinase cascades have been widely promoted in this respect but the cyclic guanosine 3',5'-monophosphate/cGMP-dependent protein kinase (cGMP/PKG) signal transduction cassette has been less systematically investigated as a survival cascade. We propose that activation of the cGMP/PKG signalling pathway, following activation of soluble or particulate guanylate cyclases, may play a pivotal role in survival signalling in ischaemia-reperfusion, especially in the classical preconditioning, delayed preconditioning and postconditioning paradigms. The resurgence of interest in reperfusion injury, largely as a result of postconditioning-related research, has confirmed that the cGMP/PKG pathway is a pivotal salvage mechanism in reperfusion. Numerous studies suggest that the infarct-limiting effects of preconditioning and postconditioning, exogenously donated nitric oxide (NO), natriuretic peptides, phosphodiesterase inhibitors, and other diverse drugs and mediators such as HMG co-A reductase inhibitors (statins), Rho-kinase inhibitors and adrenomedullin, whether given before and during ischaemia, or specifically at the onset of reperfusion, may be mediated by activation or enhancement of the cGMP pathway, either directly or indirectly via endogenous NO generation downstream of PI3K/Akt. Putative mechanisms of protection include PKG regulation of Ca(2+) homeostasis through the modification of sarcoplasmic reticulum Ca(2+) uptake mechanisms, and PKG-induced opening of ATP-sensitive K(+) channels during ischaemia and/or reperfusion. At present, significant technical obstacles in defining the precise roles played by cGMP/PKG signalling include the heavy reliance on pharmacological PKG inhibitors of uncertain selectivity, difficulties in determining PKG activity in intact tissue, and the growing recognition that intracellular compartmentalisation of the cGMP pool may contribute markedly to the nucleotide's biological actions and biochemical determination. Overall, the body of experimental evidence suggests that cGMP/PKG survival signalling ameliorates irreversible injury associated with ischaemia-reperfusion and may be a tractable therapeutic target.

Nitric oxide synthase regulation and diversity: Implications in Parkinson’s disease

Nitric oxide (NO) is a janus faced chemical messenger, which, in the recent years, has been the focus of neurobiologists for its involvement in neurodegenerative disorders in particular, Parkinson's disease (PD). Nitric oxide synthase, the key enzyme involved in NO production exists in three known isoforms. The neuronal and inducible isoforms have been implicated in the pathogenesis of PD. These enzymes are subject to complex expressional and functional regulation involving mRNA diversity, phosphorylation and protein interaction. In the recent years, mRNA diversity and polymorphisms have been identified in the NOS isoforms. Some of these genetic variations have been associated with PD, indicating an etiological role for the NOS genes. This review mainly focuses on the NOS genes - their differential regulation and genetic heterogeneity, highlighting their significance in the pathobiology of PD.

Regulation of p53 and suppression of apoptosis by the soluble guanylyl cyclase/cGMP pathway in human ovarian cancer cells

Dysregulated apoptosis plays a critical role in the development of a number of aberrant cellular processes, including tumorigenesis and chemoresistance. However, the mechanisms that govern the normal apoptotic program are not completely understood. Soluble guanylyl cyclase (sGC) and cyclic guanosine monophosphate (cGMP) promote mammalian cell viability via an unknown mechanism and p53 status is a key determinant of cell fate in human ovarian cancer cells. Whether an interaction exists between these two determinants of cell fate is unknown. We hypothesized that basal sGC activity reduces p53 content and attenuates p53-dependent apoptosis in human ovarian cancer cells. Suppression of sGC activity with the specific inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) lowered cGMP content, and increased p53 protein content and induced apoptosis in three ovarian cancer cell lines, effects which were attenuated by the cGMP analog 8-Br-cGMP and by Atrial Natriuretic Factor, an activator of particulate guanylyl cyclase, which circumvent the inhibition of sGC. ODQ prolonged p53 half-life, induced phosphorylation of p53 on Ser15, and upregulated the p53-dependent gene products p21, murine double minute-2, and the proapoptotic, p53-responsive gene product Bax. ODQ activated caspase-3, and ODQ-induced apoptosis was inhibited by overexpression of X-linked inhibitor of apoptosis Protein. Pretreatment with the specific p53 inhibitor pifithrin or downregulation of p53 using a specific small inhibitory RNA significantly attenuated ODQ-induced apoptosis. Moreover, ODQ-induced upregulation of p21 and Bax and ODQ-induced apoptosis were significantly reduced in a p53 mutant cell line relative to the wild-type parental cell line. Thus, the current study establishes that basal sGC/cGMP activity regulates p53 protein stability, content, and function, possibly by altering p53 phosphorylation and stabilization, and promotes cell survival in part through regulation of caspase-3 and p53.

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.

Exaggerated production of nitric oxide (NO) and increases in inducible NO-synthase mRNA levels induced by the pro-inflammatory cytokine interleukin-1beta in vascular smooth muscle cells of elderly rats

Nitric oxide (NO) is produced at high levels by inducible nitric oxide synthase (iNOS) during inflammation and other pathological conditions, contributing to development of cardiovascular diseases. The present study determined if aging affects the ability of interleukin-1beta (IL-1beta), a pro-inflammatory cytokine, to induce increased NO production (assessed by Griess reaction) and iNOS mRNA levels (assessed by RT-PCR/agarose gel electrophoresis) in vascular smooth muscle cells (VSMCs) from young (3-month-old) and elderly (20-22-month-old) rats. The VSMCs cells were used only in early passages (passages 0 and 1) to avoid phenotypic modulation. To uncover subtle differences in basal iNOS mRNA levels in VSMCs of young and elderly rats, RT-PCR products were also analyzed by a new ultrasensitive technique using capillary electrophoresis with laser-induced fluorescence detector (CE-LIF). IL-1beta (5 ng/ml) significantly (P < 0.05) increased NO production 3.7-fold in elderly female VSMCs and 6.7-fold in elderly male VSMCs, but had no detectable effect in young female and male VSMCs. Basal iNOS mRNA levels (assessed by RT-PCR/CE-LIF) were dramatically higher in VSMCs of elderly male rats compared to young ones. In general, VSMCs of elderly rats showed much greater sensitively to iNOS-inducing actions of IL-1beta. These data give new insight into effects of aging on iNOS expression in VSMCs, showing dramatic increases in both basal and stimulated iNOS mRNA levels, which may contribute to the development of vascular diseases in the elderly.

Preincubation with atrial natriuretic peptide protects NG108-15 cells against the toxic/proapoptotic effects of the nitric oxide donor S-nitroso- N-acetylpenicillamine

The TUNEL method is used to quantify the proapoptotic effects of an NO donor, S-nitroso- N-acetylpenicillamine (SNAP), in NG108-15 cells. Unlike sodium nitroprusside used in previous studies, SNAP does not release cyanide along with NO, thus NO toxicity was determined without concurrent cyanide toxicity. The present study also determined if pretreatment with ANP could protect against NO-induced apoptosis in NG108-15 cells. Cell death at 24 h following SNAP treatment was associated with apoptotic DNA fragmentation. SNAP at 0.5, 0.75, 1.0, and 2.0 mM caused significant (P<0.05) increases in the percentage of TUNEL-labeled cells from a control of 0.90% to 6.19%, 6.36%, 7.25%, and 15.1%, respectively. Thus, SNAP caused concentration-dependent induction of apoptosis in NG108-15 cells. SNAP-induced apoptosis was confirmed by morphological changes and increased levels of polynucleosome-sized fragments of DNA assessed by capillary electrophoresis. Preincubation for 24 h with ANP at 0.01, 0.1, and 1.0 microM, before the SNAP, significantly (P<0.05) decreased the percentage of labeled cells from 7.25% to 5.10%, 4.36%, and 3.24% in the presence of SNAP (1 mM) and from 15.1% to 7.91%, 6.64%, and 5.60% in the presence of SNAP (2 mM), respectively, representing protection of 24.0%, 34.0%, and 57.0% against SNAP (1 mM) and 26.0%, 37.0%, and 50.9% against SNAP (2 mM). Thus, prior activation of a cGMP-mediated neuroprotective mechanism induced by ANP appears to counterbalance, at least partially, the proapoptotic effects of excess NO. This neuroprotective mechanism involving cGMP may be especially important in protecting against the development of neurodegenerative diseases in which excess NO is thought to contribute to neuronal apoptosis.