Association between synthesis and release of cGMP and nitric oxide biosynthesis by hepatocytes

Multidrug resistance proteins preferentially regulate natriuretic peptide-driven cGMP signalling in the heart and vasculature

BACKGROUND AND PURPOSE

cGMP underpins the bioactivity of NO and natriuretic peptides and is key to cardiovascular homeostasis. cGMP-driven responses are terminated primarily by PDEs, but cellular efflux via multidrug resistance proteins (MRPs) might contribute. Herein, the effect of pharmacological blockade of MRPs on cGMP signalling in the heart and vasculature was investigated in vitro and in vivo.

EXPERIMENTAL APPROACH

Proliferation of human coronary artery smooth muscle cells (hCASMCs), vasorelaxation of murine aorta and reductions in mean arterial BP (MABP) in response to NO donors or natriuretic peptides were determined in the absence and presence of the MRP inhibitor MK571. The ability of MRP inhibition to reverse morphological and contractile deficits in a murine model of pressure overload-induced heart failure was also explored.

KEY RESULTS

MK571 attenuated hCASMC growth and enhanced the anti-proliferative effects of NO and atrial natriuretic peptide (ANP). MRP blockade caused concentration-dependent relaxations of murine aorta and augmented responses to ANP (and to a lesser extent NO). MK571 did not decrease MABP per se but enhanced the hypotensive actions of ANP and improved structural and functional indices of disease severity in experimental heart failure. These beneficial actions of MRP inhibition were associated with a greater intracellular:extracellular cGMP ratio in vitro and in vivo.

CONCLUSIONS AND IMPLICATIONS

MRP blockade promotes the cardiovascular functions of natriuretic peptides in vitro and in vivo, with more modest effects on NO. MRP inhibition may have therapeutic utility in cardiovascular diseases triggered by dysfunctional cGMP signalling, particularly those associated with altered natriuretic peptide bioactivity.

LINKED ARTICLES

This article is part of a themed issue on cGMP Signalling in Cell Growth and Survival. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.11/issuetoc.

Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Respiratory Syncytial Virus Infection Induces Expression of Inducible Nitric Oxide Synthase, CD3, and CD8 in Naturally Occurring Pneumonia in Lambs

Respiratory syncytial virus (RSV) is an RNA virus that belongs to the Pneumovirus genus of the Paramyxoviridae family. The aim of this study was to evaluate the expressions of inducible nitric oxide synthetase (iNOS), CD3 (pan T cells), and CD8 (cytotoxic T cells) in lamb lungs naturally infected with RSV using immunohistochemistry (IHC). For this purpose, 100 pneumonic and 10 control lung tissue samples were taken from lambs slaughtered in the slaughterhouse after macroscopic examination. The streptavidin– peroxidase method (ABC) was used for IHC staining, and it revealed RSV positivity in 18 of 100 examined lungs with pneumonia (18%). These positive cases were then immunostained for iNOS, CD3, and CD8, and compared to controls. In all these cases, an increase in iNOS expression (100%) was detected, the higher number of CD3+ T lymphocytes was detected in 14 (78%) cases while CD8+ T lymphocytes were detected in five (28%) cases, only. Given the increase of iNOS immunoexpression in all RSV-positive cases and increase in the number of CD3+ T lymphocytes in most cases, it was concluded that iNOS and CD3+ T lymphocytes play an important role in the immune response in lamb pneumonia with naturally occurring RSV infection. With this study, the role of the mentioned markers was evaluated for the first time in lambs naturally infected with RSV.

Stimulation of soluble guanylate cyclase exerts antiinflammatory actions in the liver through a VASP/NF-κB/NLRP3 inflammasome circuit

Fatty liver, which is an initial step in the development of more severe complications such as liver cirrhosis, is prevalent worldwide in our society. This study demonstrates that stimulation of soluble guanylate cyclase (sGC), an enzyme producing the second messenger cGMP, protects against the most common features of fatty liver, namely inflammation and fibrosis, in animal models of the disease. Our study also provides an explanation for this protection and describes how sGC stimulation blocks the inflammasome (a protein complex responsible for the production of the potent proinflammatory cytokine interleukin-1β) in liver macrophages. The results of this study support the investigation of sGC stimulators, which are already approved for treatment in other conditions, in patients with fatty liver disease.

Soluble guanylate cyclase (sGC) catalyzes the conversion of guanosine triphosphate into cyclic guanosine-3′,5′-monophosphate, a key second messenger in cell signaling and tissue homeostasis. It was recently demonstrated that sGC stimulation is associated with a marked antiinflammatory effect in the liver of mice with experimental nonalcoholic steatohepatitis (NASH). Here, we investigated the mechanisms underlying the antiinflammatory effect of the sGC stimulator praliciguat (PRL) in the liver. Therapeutic administration of PRL exerted antiinflammatory and antifibrotic actions in mice with choline-deficient l-amino acid-defined high-fat diet-induced NASH. The PRL antiinflammatory effect was associated with lower F4/80- and CX3CR1-positive macrophage infiltration into the liver in parallel with lower Ly6C^High- and higher Ly6C^Low-expressing monocytes in peripheral circulation. The PRL antiinflammatory effect was also associated with suppression of hepatic levels of interleukin (IL)-1β, NLPR3 (NACHT, LRR, and PYD domain-containing protein 3), ASC (apoptosis-associated speck-like protein containing a caspase-recruitment domain), and active cleaved-caspase-1, which are components of the NLRP3 inflammasome. In Kupffer cells challenged with the classical inflammasome model of lipopolysaccharide plus adenosine triphosphate, PRL inhibited the priming (expression of Il1b and Nlrp3) and blocked the release of mature IL-1β. Mechanistically, PRL induced the protein kinase G (PKG)-mediated phosphorylation of the VASP (vasodilator-stimulated phosphoprotein) Ser239 residue which, in turn, reduced nuclear factor-κB (NF-κB) activity and Il1b and Nlrp3 gene transcription. PRL also reduced active cleaved-caspase-1 levels independent of pannexin-1 activity. These data indicate that sGC stimulation with PRL exerts antiinflammatory actions in the liver through mechanisms related to a PKG/VASP/NF-κB/NLRP3 inflammasome circuit.

Circulating miRNAs Associated with Dysregulated Vascular and Trophoblast Function as Target-Based Diagnostic Biomarkers for Preeclampsia

Preeclampsia (PE) is a pregnancy-specific disorder associated with hypertension and proteinuria. Since there is no proven method to treat PE, early prediction and accurate diagnosis are essential for appropriate management of the disease. Thus, reliable biomarkers for diagnosing PE need to be identified and evaluated. We analyzed serum-soluble factors and miRNAs in 92 patients with PE and an equal number of healthy controls to identify new useful biomarkers for PE. Serum miR-31-5p, miR-155-5p, and miR-214-3p levels were significantly elevated in these patients and highly correlated with clinical symptoms of hypertension and proteinuria, whereas the miR-1290-3p level was decreased. The increased miRNAs were upregulated in an NF-κB-dependent manner and suppressed endothelial nitric oxide synthase (eNOS) and placental growth factor (PlGF) expression. The level of each miRNA had greater than 90% diagnostic accuracy, which was further increased by analyzing its ratio relative to that of miR-1290-3p. Taken together, the ratios of miR-31-5p, miR-155-5p, or miR-214-3p to miR-1290-3p may serve as reliable diagnostic or prognostic tools for PE, thereby providing evidence that these miRNAs are promising mechanism-based targets for designing therapeutic and preventive strategies for the clinical management of PE.

MRP4/ABCC4 As a New Therapeutic Target: Meta-Analysis to Determine cAMP Binding Sites as a Tool for Drug Design

MRP4 transports multiple endogenous and exogenous substances and is critical not only for detoxification but also in the homeostasis of several signaling molecules. Its dysregulation has been reported in numerous pathological disorders, thus MRP4 appears as an attractive therapeutic target. However, the efficacy of MRP4 inhibitors is still controversial. The design of specific pharmacological agents with the ability to selectively modulate the activity of this transporter or modify its affinity to certain substrates represents a challenge in current medicine and chemical biology. The first step in the long process of drug rational design is to identify the therapeutic target and characterize the mechanism by which it affects the given pathology. In order to develop a pharmacological agent with high specific activity, the second step is to systematically study the structure of the target and identify all the possible binding sites. Using available homology models and mutagenesis assays, in this review we recapitulate the up-to-date knowledge about MRP structure and aligned amino acid sequences to identify the candidate MRP4 residues where cyclic nucleotides bind. We have also listed the most relevant MRP inhibitors studied to date, considering drug safety and specificity for MRP4 in particular. This meta-analysis platform may serve as a basis for the future development of inhibitors of MRP4 cAMP specific transport.

Interferon β (IFN-β) Production during the Double-stranded RNA (dsRNA) Response in Hepatocytes Involves Coordinated and Feedforward Signaling through Toll-like Receptor 3 (TLR3), RNA-dependent Protein Kinase (PKR), Inducible Nitric Oxide Synthase (iNOS), and Src Protein

The sensing of double-stranded RNA (dsRNA) in the liver is important for antiviral defenses but can also contribute to sterile inflammation during liver injury. Hepatocytes are often the target of viral infection and are easily injured by inflammatory insults. Here we sought to establish the pathways involved in the production of type I interferons (IFN-I) in response to extracellular poly(I:C), a dsRNA mimetic, in hepatocytes. This was of interest because hepatocytes are long-lived and, unlike most immune cells that readily die after activation with dsRNA, are not viewed as cells with robust antimicrobial capacity. We found that poly(I:C) leads to rapid up-regulation of inducible nitric oxide synthase (iNOS), double-stranded RNA-dependent protein kinase (PKR), and Src. The production of IFN-β was dependent on iNOS, PKR, and Src and partially dependent on TLR3/Trif. iNOS and Src up-regulation was partially dependent on TLR3/Trif but entirely dependent on PKR. The phosphorylation of TLR3 on tyrosine 759 was shown to increase in parallel to IFN-β production in an iNOS- and Src-dependent manner, and Src was found to directly interact with TLR3 in the endosomal compartment of poly(I:C)-treated cells. Furthermore, we identified a robust NO/cGMP/PKG-dependent feedforward pathway for the amplification of iNOS expression. These data identify iNOS/NO as an integral component of IFN-β production in response to dsRNA in hepatocytes in a pathway that involves the coordinated activities of TLR3/Trif and PKR.

Cyclic nucleotide compartmentalization: contributions of phosphodiesterases and ATP-binding cassette transporters

Cyclic nucleotides [e.g., cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP)] are ubiquitous second messengers that affect multiple cell functions from maturation of the egg to cell division, growth, differentiation, and death. The concentration of cAMP can be regulated by processes within membrane domains (local regulation) as well as throughout a cell (global regulation). The phosphodiesterases (PDEs) that degrade cAMP have well-known roles in both these processes. It has recently been discovered that ATP-binding cassette (ABC) transporters contribute to both local and global regulation of cAMP. This regulation may require the formation of macromolecular complexes. Some of these transporters are ubiquitously expressed, whereas others are more tissue restricted. Because some PDE inhibitors are also ABC transporter inhibitors, it is conceivable that the therapeutic benefits of their use result from the combined inhibition of both PDEs and ABC transporters. Deciphering the individual contributions of PDEs and ABC transporters to such drug effects may lead to improved therapeutic benefits.

A role for cGMP in inducible nitric-oxide synthase (iNOS)-induced tumor necrosis factor (TNF) α-converting enzyme (TACE/ADAM17) activation, translocation, and TNF receptor 1 (TNFR1) shedding in hepatocytes

We and others have previously shown that the inducible nitric-oxide synthase (iNOS) and nitric oxide (NO) are hepatoprotective in a number of circumstances, including endotoxemia. In vitro, hepatocytes are protected from tumor necrosis factor (TNF) α-induced apoptosis via cGMP-dependent and cGMP-independent mechanisms. We have shown that the cGMP-dependent protective mechanisms involve the inhibition of death-inducing signaling complex formation. We show here that LPS-induced iNOS expression leads to rapid TNF receptor shedding from the surface of hepatocytes via NO/cGMP/protein kinase G-dependent activation and surface translocation of TNFα-converting enzyme (TACE/ADAM17). The activation of TACE is associated with the up-regulation of iRhom2 as well as the interaction and phosphorylation of TACE and iRhom2, which are also NO/cGMP/protein kinase G-dependent. These findings suggest that one mechanism of iNOS/NO-mediated protection of hepatocytes involves the rapid shedding of TNF receptor 1 to limit TNFα signaling.

Role of ischemic preconditioning in liver surgery and hepatic transplantation

INTRODUCTION

The purpose of this review is to summarize intraoperative surgical strategies available to decrease ischemia-reperfusion injury associated with liver resection and liver transplantation.

MATERIAL AND METHOD

We conducted a critical review of the literature evaluating the potential applications of hepatic ischemic preconditioning (IPC) for hepatic resection surgery and liver transplantation. In addition, we provide a basic bench-to-bedside summary of the liver physiology and cell signaling mechanisms that account for the protective effects seen with hepatic IPC.

Direct effects of nitric oxide on histamine release from rat enterochromaffin-like cells

The direct effects of nitric oxide (NO) on enterochromaffin-like (ECL) cells have not yet been demonstrated. In this study we investigated the direct effects of NO donors on rat ECL cells. The NO donor, NOR3 (10 and 100 microM), decreased gastrin-induced histamine release. 100 microM NOR3 increased cGMP levels and reduced gastrin-induced calcium influx. ODQ, an inhibitor of guanylate cyclase, completely blocked NOR3-induced inhibition of histamine release. These results suggest that NO inhibits gastric acid secretion via suppression of gastrin-induced histamine release through a pathway in which NO activates guanylate cyclase, in addition to increasing cGMP levels and reducing gastrin-induced calcium influx. The use of NO as a new type of gastric acid inhibitor that decreases histamine levels in the stomach would be beneficial as increased histamine levels resulting from use of a histamine H2 receptor antagonist or proton pump inhibitor have various effects on tumors and immunological functions.

Ischaemic preconditioning in transplantation and major resection of the liver

BACKGROUND

Ischaemia-reperfusion injury (IRI) contributes significantly to the morbidity and mortality of transplantation and major resection of the liver. Its severity is reduced by ischaemic preconditioning (IP), the precise mechanisms of which are not completely understood. This review discusses the pathophysiology and role of IP in this clinical setting.

METHODS

A Medline search was performed using the keywords 'ischaemic preconditioning', 'ischaemia-reperfusion injury', 'transplantation' and 'hepatic resection'. Additional articles were obtained from references within the papers identified by the Medline search.

RESULTS AND CONCLUSION

The mechanisms underlying hepatic IRI are complex, but IP reduces the severity of such injury in several animal models and in recent human trials. Increased understanding of the cellular processes involved in IP is of importance in the development of treatment strategies aimed at improving outcome after liver transplantation and major hepatic resection.

Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5)

Like other members of the multidrug resistance protein (MRP)/ABCC subfamily of ATP-binding cassette transporters, MRP4 (ABCC4) and MRP5 (ABCC5) are organic anion transporters. They have, however, the outstanding ability to transport nucleotides and nucleotide analogs. In vitro experiments using drug-selected or -transfected cells indicated that these transport proteins, when overexpressed, can lower the intracellular concentration of nucleoside/nucleotide analogs, such as the antiviral compounds PMEA (9-(2-phosphonylmethoxyethyl)adenine) or ganciclovir, and of anticancer nucleobase analogs, such as 6-mercaptopurine, after their conversion into the respective nucleotides. This may lead to an impaired ability of these compounds to inhibit virus replication or cell proliferation. It remains to be tested whether antiviral or anticancer chemotherapy based on nucleobase, nucleoside, or nucleotide precursors can be modulated by inhibition of MRP4 and MRP5. MRP4 also seems to be able to mediate the transport of conjugated steroids, prostaglandins, and glutathione. Furthermore, cyclic nucleotides (cyclic adenosine monophosphate and cyclic guanine monophosphate) are exported from cells by MRP4 and MRP5. This may modulate the intracellular concentration of these important mediators, besides the action of phosphodiesterases, as well as provide extracellular nucleotides for a possible paracrine action. In this line, tissue distribution and subcellular localization of MRP4 and MRP5 specifically in smooth muscle cells (MRP5), platelet-dense granules (MRP4), and nervous cells (MRP4 and MRP5), besides the capillary endothelium, point not only to a possible function of these transporters as exporters in cellular defense, but also to a physiological function in signaling processes.

Cyclic GMP transporters

The biokinetics of guanosine 3',5'-cyclic monophosphate (cGMP) is characterized by three distinct processes: synthesis by guanylate cyclases (GCs), conversion of cGMP to GMP by cyclic nucleotide phosphodiesterases (PDEs) and the excretion of unchanged cGMP by transport proteins in the cell membrane. Efflux is observed in virtually all cell types including cells which originate from brain. Studies of intact cells, in which metabolic inhibitors and probenecid reduced extrusion of cGMP and wherein cGMP was extruded against concentration gradients, indicated the existence of ATP requiring organic anion transport system(s). Functional studies of inside-out vesicles have revealed cGMP transport systems wherein translocation is coupled to hydrolysis of ATP. The extrusion of cGMP is inhibited by a number of unrelated compounds and this indicates that cGMP is substrate for multispecific transporters. Recent transfection studies suggest that members of the MRP (multidrug resistance protein) family; MRP4, MRP5 and MRP8 translocate cGMP across the cell membrane. Many of the MRPs have been detected in brain. In addition tertiary active transport by the organic anion transporter family has also been identified. At least one member (OAT1) shows relative high affinity for cGMP and is also expressed in brain. The biological significance of cGMP transporters has to be clarified. Their role in cGMP biokinetics, being responsible for one of the cellular elimination pathways, is well established. However, there is growing evidence that extracellular cGMP has effects on cell physiology and pathophysiology by an auto- or paracrine mechanism.

Expression and localization of the multidrug resistance protein 5 (MRP5/ABCC5), a cellular export pump for cyclic nucleotides, in human heart

The multidrug resistance protein 5 (MRP5/ABCC5) has been recently identified as cellular export pump for cyclic nucleotides with 3',5'-cyclic GMP (cGMP) as a high-affinity substrate. In view of the important role of cGMP for cardiovascular function, expression of this transport protein in human heart is of relevance. We analyzed the expression and localization of MRP5 in human heart [21 auricular (AS) and 15 left ventricular samples (LV) including 5 samples of dilated and ischemic cardiomyopathy]. Quantitative real-time polymerase chain reaction normalized to beta-actin revealed expression of the MRP5 gene in all samples (LV, 38.5 +/- 12.9; AS, 12.7 +/- 5.6; P < 0.001). An MRP5-specific polyclonal antibody detected a glycoprotein of approximately 190 kd in crude cell membrane fractions from these samples. Immunohistochemistry with the affinity-purified antibody revealed localization of MRP5 in cardiomyocytes as well as in cardiovascular endothelial and smooth muscle cells. Furthermore, we could detect MRP5 and ATP-dependent transport of [(3)H]cGMP in sarcolemma vesicles of human heart. Quantitative analysis of the immunoblots indicated an interindividual variability with a higher expression of MRP5 in the ischemic (104 +/- 38% of recombinant MRP5 standard) compared to normal ventricular samples (53 +/- 36%, P < 0.05). In addition, we screened genomic DNA from our samples for 20 single-nucleotide polymorphisms in the MRP5 gene. These results indicate that MRP5 is localized in cardiac and cardiovascular myocytes as well as endothelial cells with increased expression in ischemic cardiomyopathy. Therefore, MRP5-mediated cellular export may represent a novel, disease-dependent pathway for cGMP removal from cardiac cells.

MRP8, ATP-binding cassette C11 (ABCC11), is a cyclic nucleotide efflux pump and a resistance factor for fluoropyrimidines 2',3'-dideoxycytidine and 9'-(2'-phosphonylmethoxyethyl)adenine

MRP8 (ABCC11) is a recently identified cDNA that has been assigned to the multidrug resistance-associated protein (MRP) family of ATP-binding cassette transporters, but its functional characteristics have not been determined. Here we examine the functional properties of the protein using transfected LLC-PK1 cells. It is shown that ectopic expression of MRP8 reduces basal intracellular levels of cAMP and cGMP and enhances cellular extrusion of cyclic nucleotides in the presence or absence of stimulation with forskolin or SIN-1A. Analysis of the sensitivity of MRP8-overexpressing cells revealed that they are resistant to a range of clinically relevant nucleotide analogs, including the anticancer fluoropyrimidines 5'-fluorouracil (approximately 3-fold), 5'-fluoro-2'-deoxyuridine (approximately 5-fold), and 5'-fluoro-5'-deoxyuridine (approximately 3-fold), the anti-human immunodeficiency virus agent 2',3'-dideoxycytidine (approximately 6-fold) and the anti-hepatitis B agent 9'-(2'-phosphonylmethoxynyl)adenine (PMEA) (approximately 5-fold). By contrast, increased resistance was not observed for several natural product chemotherapeutic agents. In accord with the notion that MRP8 functions as a drug efflux pump for nucleotide analogs, MRP8-transfected cells exhibited reduced accumulation and increased efflux of radiolabeled PMEA. In addition, it is shown by the use of in vitro transport assays that MRP8 is able to confer resistance to fluoropyrimidines by mediating the MgATP-dependent transport of 5'-fluoro-2'-deoxyuridine monophosphate, the cytotoxic intracellular metabolite of this class of agents, but not of 5'-fluorouracil or 5'-fluoro-2'-deoxyuridine. We conclude that MRP8 is an amphipathic anion transporter that is able to efflux cAMP and cGMP and to function as a resistance factor for commonly employed purine and pyrimidine nucleotide analogs.

Nitric oxide (NO) pretreatment increases cytokine-induced NO production in cultured rat hepatocytes by suppressing GTP cyclohydrolase I feedback inhibitory protein level and promoting inducible NO synthase dimerization

Nitric oxide (NO) regulates the biological activity of many enzymes and other functional proteins as well as gene expression. In this study, we tested whether pretreatment with NO regulates NO production in response to cytokines in cultured rat hepatocytes. Hepatocytes were recovered in fresh medium for 24 h following pretreatment with the NO donor S-nitroso-N-acetyl-d,l-penicillamine (SNAP) and stimulated to express the inducible NO synthase (iNOS) with interleukin-1beta and interferon-gamma or transfected with the human iNOS gene. NO pretreatment resulted in a significant increase in NO production without changing iNOS expression for both conditions. This effect, which did not occur in macrophages and smooth muscle cells, was inhibited when NO was scavenged using red blood cells. Pretreatment with oxidized SNAP, 8-Br-cGMP, NO(2)(-), or NO(3)(-) did not increase the cytokine-induced NO production. SNAP pretreatment increased cytosolic iNOS activity measured only in the absence of exogenous tetrahydrobiopterin (BH(4)). SNAP pretreatment suppressed the level of GTP cyclohydrolase I (GTPCHI) feedback regulatory protein (GFRP) and increased GTPCHI activity without changing GTPCHI protein level. SNAP pretreatment also increased total cellular levels of biopterin and active iNOS dimer. These results suggest that SNAP pretreatment increased NO production from iNOS by elevating cellular BH(4) levels and promoting iNOS subunit dimerization through the suppression of GFRP levels and subsequent activation of GTPCHI.

Inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) expression in fulminant hepatic failure

BACKGROUND/AIMS

Inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) have important functions in inflammation and vasoregulation but their role in fulminant hepatic failure (FHF) is not well understood.

METHODS

Intrahepatic in situ staining and semi-quantification of iNOS and eNOS by immunohistochemistry in 25 patients with FHF, in 40 patients with chronic liver diseases (CLD) and in ten normal controls (NC).

RESULTS

Expression patterns of iNOS and eNOS differed. While in NC only faint iNOS expression was found in some Kupffer cells/macrophages and hepatocytes, eNOS was expressed constitutively in sinusoidal and vascular endothelial cells. In CLD, iNOS expression was induced in Kupffer cells/macrophages and hepatocytes, representing the main iNOS expressing cell types. Additionally, bile ducts, vascular endothelial cells and lymphocytes also expressed iNOS (P = 0.001). In contrast, no differences were found between eNOS expression in CLD and NC (P = 0.64). The same cell types expressed eNOS and iNOS in FHF but numbers of both were significantly enhanced, exceeding the levels seen in CLD (P < 0.001, P = 0.017).

CONCLUSIONS

Our data demonstrate that iNOS and eNOS are differently regulated in physiologic conditions and in liver disease. While eNOS seems to be involved in the physiological regulation of hepatic perfusion, strong upregulation of iNOS might contribute to inflammatory processes in FHF.

Atrial natriuretic peptide attenuates Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes

BACKGROUND & AIMS

Oscillations in cytosolic free Ca2+ concentration are a fundamental mechanism of intracellular signaling in hepatocytes. The aim of this study was to examine the effects of atrial natriuretic peptide (ANP) on cytosolic Ca2+ oscillations in rat hepatocytes.

METHODS

Cyclic guanosine monophosphate (cGMP) was measured by enzyme immunoassay. Cytosolic Ca2+ oscillations were recorded from single aequorin-injected hepatocytes. Ca2+ efflux from hepatocyte populations was measured by using extracellular fura-2. Ca2+ influx was estimated by Mn2+ quench of fluorescence of fura-2 dextran injected into single hepatocytes.

RESULTS

ANP attenuated cytosolic Ca2+ oscillations through a decrease in their frequency. In addition, ANP dramatically stimulated plasma membrane Ca2+ efflux and modestly inhibited basal Ca2+ influx. All of the observed effects of ANP were mimicked by the cGMP analogue 8-bromo-cGMP (8-Br-cGMP), and were prevented by inhibition of protein kinase G. In contrast, activation of cytosolic guanylyl cyclase by sodium nitroprusside had no effect on Ca2+ efflux, Ca2+ influx, or Ca2+ oscillations.

CONCLUSIONS

ANP decreases the frequency of Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes. Attenuation of oscillatory Ca2+ signaling in hepatocytes may represent a key role for ANP in vivo.

Role of MRP4 and MRP5 in biology and chemotherapy

Nucleotide efflux (especially cyclic nucleotides) from a variety of mammalian tissues, bacteria, and lower eukaryotes has been studied for several decades. However, the molecular identity of these nucleotide efflux transporters remained elusive, despite extensive knowledge of their kinetic properties and inhibitor profiles. Identification of the subfamily of adenosine triphosphate (ATP) binding cassette transporters, multidrug resistance protein (MRP) subfamily, permitted rapid advances because some recently identified MRP family members transport modified nucleotide analogs (ie, chemotherapeutic agents). We first identified, MRP4, based on its ability to efflux antiretroviral compounds, such as azidothymidine monophosphate (AZT-MP) and 9-(2-phosphonyl methoxyethyl) adenine (PMEA), in drug-resistant and also in transfected cell lines. MRP5, a close structural homologue of MRP4 also transported PMEA. MRP4 and MRP5 confer resistance to cytotoxic thiopurine nucleotides, and we demonstrate MRP4 expression varies among acute lymphoblastic leukemias, suggesting this as a factor in response to chemotherapy with these agents. The ability of MRP4 and MRP5 to transport 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) suggests they may play a biological role in cellular signaling by these nucleotides. Finally, we propose that MRP4 may also play a role in hepatic bile acid homeostasis because loss of the main bile acid efflux transporter, sister of P-glycoprotein (SPGP) aka bile-salt export pump (BSEP), leads to a strong compensatory upregulation in MRP4 expression. Cumulatively, these studies reveal that the ATP-binding cassette (ABC) transporters MRP4 and MRP5 have a unique role in biology and in chemotherapeutic response.

Interleukin-1beta stimulates cyclic GMP efflux in brain astrocytes

In rat brain astroglia-enriched cultures long-term treatment with interleukin-1beta induces NO release and stimulation of soluble guanylyl cyclase. The cGMP formed is recovered in the extracellular medium but not in the cell monolayer. The interleukin-1beta effect is mediated by type I receptor and potentiated by interferon-gamma. In cells treated with bacterial endotoxin a larger NO-dependent cGMP accumulation occurs only intracellularly, however a significant cGMP egression is observed when cells are co-treated with interleukin-1beta. The non-selective anion transport inhibitors probenecid and verapamil block cGMP efflux, indicating that interleukin-1beta stimulates a cGMP transporter.

Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4. Resistance to 6-mercaptopurine and 6-thioguanine

Human multidrug resistance protein 4 (MRP4) has recently been determined to confer resistance to the antiviral purine analog 9-(2-phosphonylmethoxyethyl)adenine and methotrexate. However, neither its substrate selectivity nor physiological functions have been determined. Here we report the results of investigations of the in vitro transport properties of MRP4 using membrane vesicles prepared from insect cells infected with MRP4 baculovirus. It is shown that expression of MRP4 is specifically associated with the MgATP-dependent transport of cGMP, cAMP, and estradiol 17-beta-D-glucuronide (E(2)17 beta G). cGMP, cAMP, and E(2)17 beta G are transported with K(m) and V(max) values of 9.7 +/- 2.3 microm and 2.0 +/- 0.3 pmol/mg/min, 44.5 +/- 5.8 microm and 4.1 +/- 0.4 pmol/mg/min, and 30.3 +/- 6.2 microm and 102 +/- 16 pmol/mg/min, respectively. Consistent with its ability to transport cyclic nucleotides, it is demonstrated that the MRP4 drug resistance profile extends to 6-mercaptopurine and 6-thioguanine, two anticancer purine analogs that are converted in the cell to nucleotide analogs. On the basis of its capacity to transport cyclic nucleotides and E(2)17 beta G, it is concluded that MRP4 may influence diverse cellular processes regulated by cAMP and cGMP and that its substrate range is distinct from that of any other characterized MRP family member.

Pancreastatin, a chromogranin A-derived peptide, inhibits DNA and protein synthesis by producing nitric oxide in HTC rat hepatoma cells

BACKGROUND/AIMS

Pancreastatin, a chromogranin A-derived peptide, has a counter-regulatory effect on insulin action. We have previously characterized pancreastatin receptor and signalling in rat liver and HTC hepatoma cells. A G alpha(q/11)-PLC-beta pathway leads to an increase in [Ca2+]i, PKC and mitogen activated protein kinase (MAPK) activation. These data suggested that pancreastatin might have a role in growth and proliferation, similar to other calcium-mobilizing hormones.

METHODS

DNA and protein synthesis were measured as [3H]-thymidine and [3H]-leucine incorporation. Nitric oxide (NO) was determined by the Griess method and cGMP production was quantified by enzyme-linked immunoassay.

RESULTS

Contrary to the expected results, we have found that pancreastatin inhibits protein and DNA synthesis in HTC hepatoma cells. On the other hand, when the activity of NO synthase was inhibited by N-monomethyl-L-arginine (NMLA), the inhibitory effect of pancreastatin on DNA and protein synthesis was not only reverted, but a dose-dependent stimulatory effect was observed, probably due to MAPK activation, since it was prevented by PD98059. These data strongly suggested the role of NO in the inhibitory effect of pancreastatin on protein and DNA synthesis, which is overcoming the effect on MAPK activation. Moreover, pancreastatin dose-dependently increased NO production in parallel to cyclic guanosine monophosphate (cGMP). Both effects were prevented by NMLA. Finally, an indirect effect of pancreastatin through the induction of apoptosis was ruled out.

CONCLUSIONS

Therefore, the NO and the cGMP produced by the NO-activated guanylate cyclase may mediate the dose-dependent inhibitory effect of pancreastatin on growth and proliferation in HTC hepatoma cells.

Animal models of hepadnavirus-associated hepatocellular carcinoma

Animal models of hepatitis B virus infection have been valuable for determining the mechanisms of hepadnavirus replication, for studies of pathogenesis, and for investigations of viral hepatocarcinogenesis. The woodchuck model also seems to be useful in the discovery and development of antiviral drugs to treat HBV infection and for testing new forms of immunotherapy. In particular, the woodchuck seems to be ideal for studying the effect of antiviral treatment and immunotherapy on the outcome of hepadnavirus infection and on survival. The median life expectancy of experimentally infected, chronic WHV carriers is approximately 29 months, and almost all develop HCC. New types of prophylaxis or therapy can be evaluated under controlled experimental conditions, in a relevant animal model, and within a reasonable time frame.

ATPase activity and transport by a cGMP transporter in human erythrocyte ghosts and proteoliposome-reconstituted membrane extracts

We previously described the [(3)H]cGMP-binding characteristics of a CHAPS-solubilized protein that we proposed to be a cGMP transporter. We now report the ATPase activity of the membrane-bound, solubilized and reconstituted form of a cGMP transporter. The membrane-bound protein of unsealed ghosts had a linear ATPase activity over a 120 min incubation period with optimal activity of about 400 pmol/mg/min. The apparent K(m) and V(max) for ATP were about 0.5 mM and 300 pmol/mg/min, respectively. When solubilized with CHAPS the specific activity of the protein was reduced to about 70 pmol/mg/min. Reconstitution of the CHAPS preparation into phospholipid bilayer using rapid detergent removal by Extracti-gel column resulted in proteoliposomes which had ATPase activity similar to that found in the erythrocyte membranes. The proteoliposomes displayed a linear ATP-dependent uptake of [(3)H]cGMP with an apparent K(m) value of 1. 0 microM. This low K(m)-uptake of [(3)H]cGMP in proteoliposomes was not affected by 10 microM of AMP, cAMP and GMP, but was completely abolished in the presence of the non-hydrolyzable ATP analogue, ATP-gamma-S. Some ATPase activation was also observed in the presence of 2 microM cAMP, but it is unclear whether this activity was coupled to the cGMP transporter. Our results show that the membrane protein responsible for cGMP transport has an ATPase activity and transports the cyclic nucleotide in the presence of ATP.

The multidrug resistance protein 5 functions as an ATP-dependent export pump for cyclic nucleotides

Cellular export of cyclic nucleotides has been observed in various tissues and may represent an elimination pathway for these signaling molecules, in addition to degradation by phosphodiesterases. In the present study we provide evidence that this export is mediated by the multidrug resistance protein isoform MRP5 (gene symbol ABCC5). The transport function of MRP5 was studied in V79 hamster lung fibroblasts transfected with a human MRP5 cDNA. An MRP5-specific antibody detected an overexpression of the glycoprotein of 185 +/- 15 kDa in membranes from MRP5-transfected cells and a low basal expression of hamster Mrp5 in control membranes. ATP-dependent transport of 3',5'-cyclic GMP at a substrate concentration of 1 micrometer was 4-fold higher in membrane vesicles from MRP5-transfected cells than in control membranes. This transport was saturable with a K(m) value of 2.1 micrometer. MRP5-mediated transport was also detected for 3',5'-cyclic AMP at a lower affinity, with a K(m) value of 379 micrometer. A potent inhibition of MRP5-mediated transport was observed by several compounds, known as phosphodiesterase modulators, including trequinsin, with a K(i) of 240 nm, and sildenafil, with a K(i) value of 267 nm. Thus, cyclic nucleotides are physiological substrates for MRP5; moreover, MRP5 may represent a novel pharmacological target for the enhancement of tissue levels of cGMP.

Inactivation of atrial natriuretic factor-stimulated cyclic guanosine 3',5'-monophosphate (cGMP) in UMR-106 osteoblast-like cells

Previous studies have suggested a role of cyclic guanosine 3', 5'-monophosphate (cGMP) in the differentiation and proliferation of osteoblasts. We studied the effect of ANF (atrial natriuretic factor) on intracellular cGMP accumulation, cGMP efflux, and cGMP-phosphodiesterase (PDE) activity in UMR-106 osteoblast-like cells. ANF rapidly increased both intracellular cGMP and cGMP efflux. ANF-stimulated intracellular cGMP peaked at 2 min in the absence and at 10 min in the presence of 0.25 mM 3-isobutyl-1-methylxanthine. Probenecid, an antagonist of anion transport, blocked the efflux of cGMP (IC(50) = 0.1 mM), ruling out simple diffusion as a mechanism of the efflux. cGMP-PDE activity was increased threefold in crude homogenates from ANF-treated cells (IC(50) = 23 nM). ANF-evoked stimulation of cGMP-PDE activity was reached simultaneously with the peak in intracellular cGMP. Separation of the PDEs by Q-Sepharose chromatography revealed three cGMP-hydrolyzing peaks. The first peak was sensitive to the PDE5 (cGMP-specific PDE) isoenzyme-selective inhibitor zaprinast (IC(50) = 0.45 microM). The second peak was stimulated fourfold by the addition of calcium/calmodulin, indicating the presence of PDE1. The third peak was sensitive to the PDE2 (cGMP-stimulated PDE) isoenzyme-selective inhibitor 9-[2-hydroxy-3-nonyl]adenine (EHNA) (IC(50) = 3 microM), and was activated by over 300% in the presence of 4 microM cGMP. Our results show that ANF-stimulated cGMP is released from UMR-106 cells by a probenecid-sensitive mechanism. ANF also stimulates cGMP hydrolysis by activating cGMP-PDE activity. Three distinct cGMP-hydrolyzing PDEs, namely PDE5, PDE1, and PDE2, are present in the studied cells.

Leukotriene C(4) (LTC(4)) does not share a cellular efflux mechanism with cGMP: characterisation of cGMP transport by uptake to inside-out vesicles from human erythrocytes

The transport of cGMP out of cells is energy requiring and has characteristics compatible with an ATP-energised anion pump. In the present study a model with inside-out vesicles from human erythrocytes was employed for further characterisation of the cGMP transporter. The uptake of leukotriene C(4) (LTC(4)), a substrate for multidrug resistance protein (MRP), was concentration-dependently inhibited by the leukotriene antagonist MK571 (IC(50)=110+/-20 nM), but cGMP was unable to inhibit LTC(4) uptake. Oxidised glutathione (GSSG) and glutathione S-conjugates caused a concentration-dependent inhibition of [(3)H]cGMP uptake with IC(50) of 2200+/-700 microM for GSSG, 410+/-210 microM for S-(p-nitrobenzyl)glutathione and 37+/-16 microM for S-decylglutathione, respectively. Antioxidants such as reduced glutathione and dithiothreitol did not influence transport for concentrations up to 100 microM, but both inhibited cGMP uptake with approx. 25% at 1 mM. The cGMP pump was sensitive to temperature without activity below 20 degrees C. The transport of cGMP was dependent on pH with maximal activity between pH 8.0 and 8.5. Calcium caused a concentration-dependent inhibition with IC(50) of 43+/-12 microM. Magnesium gave a marked activation in the range between 1 and 20 mM with maximum effect at 10 mM. The other divalent cations, Mn(2+) and Co(2+), were unable to substitute Mg(2+), but caused some activation at 1 mM. EDTA and EGTA stimulated cGMP transport concentration-dependently with 50% and 100% above control at 100 microM, respectively. The present study shows that the cGMP pump has properties compatible with an organic anion transport ATPase, without affinity for the MRP substrate LTC(4). However, the blockade of the cGMP transporter by glutathione S-conjugates suggests it is one of several GS-X pumps.

Induction of myocardial nitric oxide synthase by Coxsackie B3 virus in mice

BACKGROUND

Inducible nitric oxide synthase (iNOS) expression is regulated by cytokines. This study investigated whether Coxsackie group B virus (CVB) myocarditis resulted in an environment suitable for induction of NOS in the murine heart.

MATERIALS AND METHODS

Myocardium was removed from mice infected with CVB3 and from controls. Histology, reverse transcriptase polymerase reaction (RT-PCR) for murine iNOS, NOS enzyme activity and immunohistochemistry were assessed.

RESULTS

Histology revealed severe myocarditis 7 days after infection with CVB3 but not in controls. RT-PCR using primers for murine iNOS detected iNOS mRNA in infected mice but not in controls. Calcium-independent NOS activity increased by day 5 after infection with a peak at day 7. Calcium-dependent NOS activity was present throughout, with a trend to lower levels during peak calcium-independent activity. Immunohistochemistry revealed iNOS to be localized to inflammatory cells rather than to myocytes.

CONCLUSION

This study demonstrates the development of calcium-independent NOS activity and de novo gene transcription for iNOS in the murine myocardium in response to CVB3 infection. The nitric oxide produced at such high output may act at times as part of the immune defence as an antiviral agent and may be toxic to host tissue.

The parafascicular thalamic nucleus but not the prefrontal cortex facilitates the nitric oxide/cyclic GMP pathway in rat striatum

We investigated whether the parafascicular thalamic nucleus and the prefrontal cortex, the two major excitatory inputs to the striatum, modulate the nitric oxide/cyclic GMP pathway in rat striatum. Electrical stimulation (10 pulses of 0.5 ms, 10 V applied at 10 Hz, 140 microA) delivered bilaterally to the parafascicular thalamic nucleus for a total of 4, 10 and 20 min, time-dependently facilitated cyclic GMP output in the dorsal striatum of freely moving rats, assessed by trans-striatal microdialysis. Electrical stimulation to the prefrontal cortex for a total duration of 20 min did not affect striatal cyclic GMP levels. The facilitatory effect observed after electrical stimulation of the parafascicular thalamic nucleus was blocked by co-perfusion with tetrodotoxin, suggesting that the effect is mediated by neuronal process(es). The non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (30 microM infused into the dorsal striatum), and the competitive one, 3-[(R)-carboxypiperazin-4-yl]-propyl-phosphonic acid (50 microM infused), but not local perfusion of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM perfused locally), abolished the cyclic GMP response in the striatum. The nitric oxide synthase inhibitor, 7-nitroindazole, applied locally (1 mM), blocked the electrically evoked increase in striatal extracellular cyclic GMP. This increase was also prevented by local application (100 and 300 microM) of 1H-(1,2,4)-oxadiazolo-(4,3a)-quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The results provide direct functional evidence of selective thalamic facilitation of the nitric oxide/cyclic GMP pathway in the dorsal striatum, through activation of N-methyl-D-aspartate receptors.

Cyclic nucleotides and inducible nitric oxide synthesis in pulmonary artery smooth muscle

BACKGROUND

Nitric oxide (NO), cGMP, and cAMP affect the synthesis, metabolism, and cellular effects each other. We wanted to study how cGMP and cAMP interact to affect the induced synthesis of NO in response to interleukin-1 beta (IL-1 beta) in rat pulmonary artery smooth muscle cells. To further dissect the relative contributions of each cyclic nucleotide, and to detect any possible "crossover" effect of one cyclic nucleotide activating the other protein kinase, we tested how pharmacological inhibition of cGMP-dependent and cAMP-dependent protein kinases (PKG and PKA, respectively) affected responses.

MATERIALS AND METHODS

We tested the effects of IL-1 beta, dibutyryl (db)-cAMP (1-100 micro) and 8-bromo (Br)-cGMP (1 microM-1 mM) on NO synthesis in cultured rat pulmonary artery smooth muscle cells. Positive effects were then tested in the presence of KT5720 (10(-9)-10(-5) M), the pharmacological inhibitor of PKA, and KT5823 (10(-9)-10(-5) M), the pharmacological inhibitor of PKG. NO production was measured using the Greiss reaction, and mRNA abundance of the inducible NO synthase (iNOS), using semiquantitative RT-PCR.

RESULTS

IL-1 beta caused nitrite levels to increase nearly 10-fold over basal levels at 24 h (P < 0.05). Nitrite levels increased with the addition of either db-cAMP (100 microM, an 8-fold increase) or 8-Br-cGMP (100 microM, a 3-fold increase) to IL-1 beta (P < 0.05). PKA inhibition with KT5720 (10(-5) M) completely inhibited NO synthesis in response to the combination of IL-1 beta and cAMP, while KT5823 had less effect at all doses tested. NO synthesis in response to IL-1 beta plus cGMP also decreased to PKA inhibition, but not PKG inhibition, indicating that cGMP responses are a crossover effect. Both cAMP and cGMP in combination with IL-1 beta increased iNOS mRNA abundance above basal levels on reverse transcription polymerase chain reaction. KT5720, but not KT5823, decreased iNOS mRNA to basal levels.

CONCLUSION

Both cAMP and cGMP augment cytokine induction of NO synthesis through activation of PKA: cAMP does so directly; cGMP, through a crossover stimulation of PKA.

Nitric oxide and guanosine 3',5'-cyclic monophosphate stimulate bile secretion in isolated rat hepatocyte couplets, but not in isolated bile duct units

Nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cGMP) have recently been shown to stimulate bile acid-independent bile flow in the isolated perfused rat liver (IPRL). However, the cellular origin and mechanisms of this choleresis have not yet been determined. To address these questions, we examined the effects of NO and cGMP on bile secretion in isolated rat hepatocyte couplets (IRHC) and in isolated bile duct units (IBDU), both of which are isolated cell systems in which cell polarity is maintained and secretion can be measured directly. Changes in the area of the canalicular and ductular lumens were determined in IRHC and IBDU, respectively, as indicators of the rate of fluid secretion using video microscopy. In addition, Cl-/HCO3- exchanger activity in IBDU was evaluated by measuring changes in intracellular pH (pHi) after Cl- removal/readmission by microfluorometric methods. In the presence of HCO3-, both the NO donor, S-nitroso-acetyl-penicillamine (SNAP), and the cell-permeant cGMP analogue, dibutyryl cGMP (DBcGMP), stimulated canalicular bile secretion (P <.05), as did the cell-permeant cAMP analogue, dibutyryl cAMP (DBcAMP) (P <.05). Removal of HCO3- from the buffer completely abolished the choleretic effects of DBcGMP, but had no effect on NO-induced choleresis. In contrast, secretion in IBDU was not stimulated following incubations with SNAP or DBcGMP over 30 minutes, whereas DBcAMP and secretin, a cholangiocyte secretagogue and cAMP agonist, both had a marked effect on ductular secretion over this same time interval (P <.05). SNAP also had no effect on Cl-/HCO3- exchanger activity in IBDU, and inhibition of endogenous NO synthesis by NG-monomethyl-L-arginine (L-NMMA) did not alter secretin-induced stimulation of ductular bile secretion and Cl-/HCO3- exchanger activity. In summary, NO and cGMP stimulate bile secretion exclusively at the the level of hepatocytes, whereas cAMP mediates choleresis at both hepatocyte and bile duct levels. These findings may have important implications for the regulation of ductular bile secretion by hormones and neuropeptides, as well as under pathological conditions with increased hepatic NO synthesis.

Escherichia coli lipopolysaccharide downregulates soluble guanylate cyclase in pulmonary artery smooth muscle

The soluble isoform of guanylate cyclase (sGC) is activated by nitric oxide (NO) to form guanosine 3':5'-cyclic monophosphate (cGMP). Cyclic GMP levels cause smooth muscle relaxation and regulate vascular tone to various vascular beds, including the lung. Under conditions of cytokine excess the inducible synthesis of NO may result in cGMP overproduction, generalized vasodilation, and septic shock. In the pulmonary bed the opposite response may occur, pulmonary hypertension. We hypothesized that sGC activity becomes downregulated in the face of Escherichia coli lipopolysaccharide (LPS). We tested the effects of LPS on alpha1-subunit sGC mRNA abundance, Western analysis, and enzyme activity in cultured rat pulmonary artery smooth muscle cells. LPS increased extracellular cGMP production by pulmonary artery smooth muscle cells, with increased levels being first detectable at 3-6 h (10 microg/ml LPS) and exceeding 140 pmol/ml by 24 h (P < 0.05). The response was inhibited by 0.05 mM l-NG-monomethyl-l-arginine (l-NMA) and, in turn, restored by 1 mM l-arginine, indicating a NO synthase-dependent response. Pretreating cells with LPS for >/= 3 h inhibited subsequent cGMP synthesis in response to 10(-4) M SNAP for 60 min. Coincubating cells with 0.05 mM l-NMA also reversed this effect. Soluble GC enzyme activity in cells exposed to basal medium alone measured 0.74 pmol cGMP/ml per minute; activity in cells exposed to 10 microg/ml LPS for 24 h decreased to 0.04 pmol cGMP/ml per minute (P < 0.05). LPS pretreatment decreased sGC mRNA abundance and protein mass, but did not totally eliminate them. It is concluded that LPS affects cGMP synthesis at the level of enzyme activity, enzyme mass, and mRNA abundance. Over the short term (<24 h) LPS causes the synthesis of large amounts of cGMP. As the duration of exposure progresses (>/=3 h), mechanisms come into play that decrease cGMP production significantly and include decreases in mRNA abundance, enzyme mass, and enzyme activity.

Nitric Oxide Prevents IL-1β and IFN-γ-Inducing Factor (IL-18) Release from Macrophages by Inhibiting Caspase-1 (IL-1β-Converting Enzyme)

Procytokine processing by caspase-1 is required for the maturation and release of IL-1β and IFN-γ-inducing factor (IGIF) (or IL-18) from activated macrophages (Mφ). Nitric oxide (NO) has emerged as a potent inhibitor of cysteine proteases. Here, we tested the hypothesis that NO regulates cytokine release by inhibiting IL-1β-converting enzyme (ICE) or caspase-1 activity. Activated RAW264.7 cells released four to five times more IL-1β, but not TNF-α, in the presence of the NO synthase inhibitor NG-monomethyl-l-arginine. Stimulated peritoneal Mφ from wild-type mice (inducible NO synthase (iNOS)+/+) also released more IL-1β if exposed to NG-monomethyl-l-arginine, whereas Mφ from iNOS knockout mice (iNOS−/−) did not. Inhibition of NO synthesis in stimulated RAW264.7 cells also resulted in a threefold increase in intracellular caspase-1 activity. The NO donor S-nitroso-N-acetyl-dl-penicillamine inhibited caspase-1 activity in cells as well as the activity of purified recombinant caspase-1 and also prevented the cleavage of pro-IL-1β and pro-IGIF by recombinant caspase-1. The inhibition of caspase-1 by NO was reversible by the addition of DTT, which is consistent with S-nitrosylation as the mechanism of caspase-1 inhibition. An in vivo role for the regulation of caspase-1 by NO was established in iNOS knockout animals, which exhibited significantly higher plasma levels of IL-1β and IFN-γ than their wild-type counterparts at 10 h following LPS injection. Taken together, these data indicate that NO suppresses IL-1β and IGIF processing by inhibiting caspase-1 activity, providing evidence for a unique role for induced NO in regulating IL-1β and IGIF release.

Regulation of stellate cell proliferation by lipopolysaccharide: Role of endogenous nitric oxide

We studied the effect of lipopolysaccharide (LPS) on the proliferation of culture-stimulated rat stellate cells. DNA synthesis as determined by [³ H]-thymidine incorporation was significantly suppressed by up to 52% compared with the control culture in the presence of LPS (> 5 ng/mL). Such an inhibitory effect of LPS was dramatically augmented in the presence of interferon-γ (IFNγ). Lipopolysaccharide alone or in combination with IFNγ activated transcription factors AP-1 and NF-κB, and elicited nitric oxide (NO) production by stellate cells by inducing NO synthase. Inhibition of NO production by the addition of L-arginine antagonists to the culture, partially cancelled such an inhibitory effect of LPS and/or IFNγ on DNA synthesis without affecting the activation of AP-1 and NF-κB and the NO synthase level. The cellular level of cyclic guanosine monophosphate (cGMP) increased in response to LPS and IFNγ, and dibutyryl cGMP or 8-bromo-cGMP inhibited the incorporation of [³ H]-thymidine in a dose-dependent manner. These results indicate that LPS is potent in modulating stellate cell proliferation by some NO- and cGMP-dependent mechanism.

Adenovirus-mediated inducible nitric oxide synthase gene transfer inhibits hepatocyte apoptosis

BACKGROUND

Apoptosis limits hepatocyte viability in bioartificial livers in vitro and may contribute to liver dysfunction in vivo. Nitric oxide (NO) inhibits hepatocyte apoptosis; however, methods to deliver NO in a sustained manner to hepatocytes are limited. Here, we tested the feasibility of inducible NO synthase (iNOS) gene transfer as an approach to deliver an intracellular source of NO to inhibit spontaneous and tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis in cultured hepatocytes.

METHODS

An adenoviral vector carrying the human iNOS gene (AdiNOS) was used to overexpress iNOS in cultured rat hepatocytes. Spontaneous apoptosis was induced by prolonged culture (4 days), and stimulated apoptosis was induced by exposure to TNF-alpha + actinomycin D (TNF-alpha ActD). Nitrite (NO2-), cell viability, and cellular caspase-3-like protease activity were measured.

RESULTS

AdiNOS gene transfer resulted in sustained NO production and protected hepatocytes from spontaneous and TNF-alpha + ActD-induced apoptosis. Apoptosis was associated with increases in caspase-3-like protease activity, which was suppressed by iNOS gene transfer in an NO-dependent manner. Dithiothreitol partially reversed the NO-induced suppression of caspase-3-like activity, which is consistent with S-nitrosylation of caspase-3.

CONCLUSIONS

Adenovirus-mediated iNOS gene transfer effectively blocks spontaneous and TNF-alpha + ActD-induced cell killing in hepatocytes. iNOS gene transfer could be used to suppress apoptotic hepatocyte death in vitro and possibly in vivo.

Transcriptional activation of the haem oxygenase-1 gene by cGMP via a cAMP response element/activator protein-1 element in primary cultures of rat hepatocytes

The expression of the rate-limiting enzyme of haem degradation, haem oxygenase-1 (HO-1), can be induced by various stimuli, including lipopolysaccharide, tumour necrosis factor alpha and NO. The NO signal can be transmitted by cGMP, therefore this study was aimed at testing the activation of the HO-1 gene by cGMP. In primary cultures of rat hepatocytes, both HO-1 mRNA and protein were induced by the NO donor sodium nitroprusside and 8-bromo-cGMP. The HO-1 mRNA induction by cGMP was prevented by the specific protein kinase G inhibitor KT5823. The cGMP-dependent HO-1 mRNA induction was dose-dependent and transcriptionally regulated, as determined by studies with actinomycin D and a nuclear run-on assay. Cycloheximide lowered the cGMP-dependent induction of HO-1 mRNA to about one half. Luciferase reporter constructs driven by about 800 bp of the 5'-flanking region of the rat HO-1 gene were transiently transfected into primary rat hepatocytes; 8-bromo-cGMP caused a 6-fold induction, which was obliterated by deletion and mutation of the cAMP response element/activator protein-1 (CRE/AP-1) (-665/-654) site. Thus HO-1 induction by cGMP appears to be stimulated by the protein kinase G pathway and may be mediated mainly via a CRE/AP-1 element in the rat HO-1 promoter.

Galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide/cyclic GMP pathway in vivo in the rat ventral hippocampus

We investigated whether the neuropeptide galanin affects the nitric oxide synthase/cyclic GMP pathway in rat hippocampus by measuring in vivo the extracellular cyclic GMP levels during microdialysis. Galanin (2.5 and 3.5 nmol; i.c.v.) dose-dependently raised the extracellular levels of cyclic GMP in the ventral but not the dorsal hippocampus. The effect of 3.5 nmol galanin was blocked by local application of tetrodotoxin and inhibited by the high-affinity galanin antagonist M40 (galanin-[1-12]-Pro3-[Ala-Leu]2-Ala amide). The non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine maleate (30 microM infused into the ventral hippocampus or 0.2 mg/kg, i.p.) and the competitive one, 3-([R]-carboxypiperazin-4-yl)-propyl-phosphonic acid (50 microM infused), but not local perfusion of the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM) abolished the galanin-evoked cyclic GMP response in the hippocampus. Inhibitors of nitric oxide synthase, L-Arg(NO2)-OMe.HCl and 7-nitroindazole monosodium salt, applied locally, blocked the galanin-induced increase in hippocampal extracellular cyclic GMP. This increase was also prevented by local application of 1H-(1,2,4)oxadiazolo(4,3a) quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The galanin receptors mediating the rise in cyclic GMP reside outside the hippocampus, as galanin (0.35-3 nmol) locally applied had no effect. The results provide in vivo evidence that galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide synthase/cyclic GMP pathway in the ventral hippocampus, which may be of importance in memory processes.

Stimulation of cyclic guanosine monophosphate production by natriuretic peptide in human biliary cells

BACKGROUND & AIMS

Guanosine 3',5'-cyclic monophosphate (cGMP), whose production is stimulated by the interaction of nitric oxide, natriuretic peptides, and guanylin with their respective guanylate cyclases, activates secretion through ion channels in several epithelia. Cl- channels have been identified in the apical membrane of biliary epithelial cells. The aim of this study was to investigate the production of cGMP and its effects on Cl- permeability in biliary epithelial cells.

METHODS

Halide efflux measurement, whole-cell patch clamp recording, radioimmunoassay, and reverse-transcription polymerase chain reaction using two human biliary cell lines (H69 and Mz-ChA-1) were performed.

RESULTS

In cells equilibrated with 125I, bromo-cGMP stimulated halide efflux by 22%. In whole-cell patch clamp recordings, the addition of cGMP intracellularly, or of atrial natriuretic peptide extracellularly, stimulated inward currents at negative membrane potentials, consistent with Cl- efflux through open channels. In H69 cells, atrial and C-type natriuretic peptides stimulated production of cGMP. Mz-ChA-1 responded only to atrial natriuretic peptide. Both cell lines expressed messenger RNA for the guanylate cyclase type A receptor and the guanylate cyclase free-clearance receptor.

CONCLUSIONS

These data suggest that natriuretic peptide stimulates cGMP production in human biliary epithelial cells, which in turn may regulate ductular bile formation through the opening of Cl- channels.

Nitric oxide donor NOR 3 inhibits ketogenesis from oleate in isolated rat hepatocytes by a cyclic GMP-independent mechanism

Studies were conducted to clarify the effects of nitric oxide donors NOR 3 ((+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide, FK409), SIN-1 (3-morpholinosydnonimine) and SNAP (S-nitroso-N-acetylpenicillamine) on the accumulation of cGMP and cAMP and Ca2+ mobilization as well as ketogenesis from oleate in isolated rat hepatocytes. NOR 3 caused inhibition of ketogenesis from oleate along with stimulation of cGMP accumulation in rat hepatocytes, whereas SIN-1 and SNAP exerted no effect on ketogenesis despite their marked stimulation of cGMP accumulation. Although the nitric oxide trapping agent, carboxy-PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide), antagonized the stimulation by NOR 3 of cGMP accumulation, it failed to modulate the anti-ketogenic action of NOR 3. Furthermore, neither 8-bromoguanosine-3',5'-cyclic monophosphate nor N2,2'-O-dibutyrylguanosine-3',5'-cyclic monophosphate mimicked the anti-ketogenic action of NOR 3. It is concluded in the present study that NOR 3-induced inhibition of ketogenesis in rat hepatocytes is not mediated by cGMP. The present study revealed that the remaining structure of NOR 3 from which nitric oxide had been spontaneously released had no anti-ketogenic action. We first and clearly demonstrated that nitrite production was dramatically enhanced when NOR 3 was incubated in the presence of rat hepatocytes. The mechanism whereby NOR 3 inhibits ketogenesis in rat hepatocytes will be discussed.

Nitric oxide synthase inhibitor, aminoguanidine, reduces inflammation and demyelination produced by Theiler's virus infection

This study evaluated effects of the inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AG), on the neuropathology and clinical disease produced by Theiler's murine encephalomyelitis virus (TMEV) DA strain infection. Treatment with AG was started on day 7, 14, 28 or 66 post-inoculation and continued for a minimum of 21 days. Inflammation, demyelination and axonal necrosis were scored in a blinded fashion on spinal cord sections from each mouse. Reduction in inflammation, demyelination and axonal necrosis was observed in mice treated with AG. Apoptosis within the spinal cord parenchyma and perivascular cuffs was significantly decreased. AG treatment resulted in delayed onset of clinical disease.

Nitric oxide inhibits apoptosis by preventing increases in caspase-3-like activity via two distinct mechanisms

Nitric oxide (NO) has emerged as an important endogenous inhibitor of apoptosis, and here we report that NO prevents hepatocyte apoptosis initiated by the removal of growth factors or exposure to TNFalpha or anti-Fas antibody. We postulated that the mechanism of the inhibition of apoptosis by NO would include an effect on caspase-3-like protease activity. Caspase-3-like activity increased coincident with apoptosis due to all three stimuli, and treatment with the caspase-3-like protease inhibitor N-acetyl-Asp-Glu-Val-Asp-aldehyde inhibited both proteolytic activity and apoptosis. Endogenous or exogenous sources of NO prevented the increase in caspase-3-like activity in hepatocytes. Exposure of purified recombinant caspase-3 to an NO or NO+ donor inhibited proteolytic activity. Dithiothreitol (DTT), but not glutathione, reversed the inhibition of recombinant caspase-3 by NO. When lysates from cells stimulated to express inducible NO synthase or cells exposed to NO donors were incubated in DTT, caspase-3-like activity increased to about 55% of cells not exposed to a source of NO. Similarly, administration of an NO donor to rats treated with TNFalpha and D-galactosamine also prevented the increase in caspase-3-like activity as measured in liver homogenates. The effect of the NO donor was reversed by about 50% if the homogenate was incubated with DTT. TNFalpha-induced apoptosis and caspase-3-like activity were also reduced in cultured hepatocytes exposed to 8-bromo-cGMP, and both effects were inhibited by the cGMP-dependent kinase inhibitor KT5823. The suppression in caspase-3-like activity in hepatocytes exposed to an NO donor was partially blocked by an inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3, -a]quinoxalin-1-one, (ODQ), while the incubation of these lysates in DTT almost completely restored caspase-3-like activity to the level of TNFalpha-treated controls. These data indicate that NO prevents apoptosis in hepatocytes by either directly or indirectly inhibiting caspase-3-like activation via a cGMP-dependent mechanism and by direct inhibition of caspase-3-like activity through protein S-nitrosylation.

Inhibition of cytokine-induced inducible nitric oxide synthase expression by glucagon and cAMP in cultured hepatocytes

Addition of lipopolysaccharide plus interferon gamma, tumour necrosis factor alpha and interleukin 1 beta to cultured hepatocytes resulted in the induction of inducible nitric oxide synthase (iNOS) activity as measured by NO3(-)+NO2- formation, the conversion of L-[U-14C]arginine into citrulline and Western blotting of the iNOS protein. The inclusion of 1 microM glucagon during the induction period significantly decreased the effect of the cytokines on iNOS activity, the major effect being at the level of the total amount of protein, rather than alterations in substrate supply or covalent modification of the existing protein. In contrast, 1 microM insulin was without effect. The effect of glucagon was mediated via cAMP and could be mimicked by the presence of either dibutyryl cAMP or forskolin to activate adenylate cyclase directly. It was rapid in onset and long-lived, a 30 min pretreatment period protecting the cells from the induction of NO synthesis over the next 21 h in the presence of cytokines. Addition of glucagon at any time point up to 9 h after treatment of the cells with lipopolysaccharide plus the cytokines resulted in a significant inhibition of iNOS activity, glucagon being most potent when added during the first 3 h.

Tyrosine kinase inhibition ameliorates the hyperdynamic state and decreases nitric oxide production in cirrhotic rats with portal hypertension and ascites

Tumor necrosis factor-alpha (TNF) causes vasodilatation and a hyperdynamic state by activating nitric oxide (NO) synthesis. Tyrphostins, specific inhibitors of protein tyrosine kinase (PTK), block the signaling events induced by TNF and NO production. A hyperdynamic circulatory syndrome (HCS) is often observed in portal hypertension (PHT). TNF and NO seem to mediate these hemodynamic changes. The aim of this work was to study the effect of PTK inhibition on the systemic and portal hemodynamics, TNF and NO production, in cirrhotic rats with portal hypertension. Rats with liver cirrhosis induced by chronic inhalation of carbon tetrachloride were used. Animals were treated daily with tyrphostin AG 126 (alpha-cyano-(3-hydroxy-4-nitro) cinnamonitrile) or placebo for 5 d. Mean arterial pressure (MAP), heart rate (HR), and portal pressure (PP) were measured by indwelling catheters. Cardiac output (CI) and stroke volume (SV) were estimated by thermodilution, systemic vascular resistance (SVR) was calculated (MAP/CI), and portal systemic shunting (PSS) was quantitated using radioactive microspheres. Serum and mesenteric lymph node (MLN) TNF levels were measured using an immunoassay kit, and serum NOx was determined photometrically by its oxidation products. The AG 126-treated group showed a statistically significant increase in MAP and SVR, and decreases in CI, SV, MLN TNF, and serum NO oxidation products nitrite and nitrate (NOx) in comparison with the placebo-treated rats. No significant differences were noticed in HR, PP, PSS, or serum TNF. Significant correlations were observed between MAP and NOx, MAP and MLN TNF, PSS and NOx, and serum TNF and serum NOx. The HCS observed in PHT seems to be mediated, at least in part, by TNF and NO by the activation of PTKs and their signaling pathways. PTK activity inhibition ameliorates the hyperdynamic abnormalities that characterize animals with cirrhosis and PHT.

Involvement of nitric oxide synthesis in hepatic perturbations induced in rats by a necrogenic dose of thioacetamide

1. The biological actions of nitric oxide (NO), a highly diffusible and short-lived radical, range from signal transduction to cytotoxicity. The present study investigated whether NO is released in the course of liver necrosis and regeneration induced by a single necrogenic dose of thioacetamide (6.6 mmol kg-1 body wt) to rats. Samples of liver were obtained at 0, 3, 12, 24, 48, 72 and 96 h after thioacetamide administration. 2. Inducible nitric oxide synthase (iNOS) activity was determined in purified liver homogenates and a sharp 6 fold increase (P < 0.001) in iNOS activity was recorded at 48 h of intoxication, followed by a slight but progressive increase at 72 and 96 h. Changes in the expression of iNOS, as detected by its mRNA levels, were parallel to the NOS enzyme activity. Hepatocyte NO synthesis showed a progressive increase at 24, 48 and 72 h, to 8 (P < 0.001), 13 (P < 0.001) and 13 (P < 0.001) times the initial values, respectively. 3. In isolated Kupffer cells, where initial NO release was ten fold higher than in hepatocytes, a progressive increase was detected from 48 h which reached two fold of initial at 72 h of intoxication (192%; P < 0.001). Hepatic cyclic GMP concentration did not change significantly. However, mitochondrial aconitase activity decreased markedly at 12 and 24 h of intoxication showing a sharp increase towards normal values at 48 h which was maintained at 72 and 96 h. 4. As protein kinase C (PKC) is one of the likely candidates to mediate iNOS expression, translocation (activation) of PKC was assayed in hepatocytes, and a significant two fold increase (P < 0.001) between 48 and 96 h after thioacetamide intoxication was observed. When peritoneal macrophages from control rats were incubated with serum from thioacetamide-treated rats, a sharp increase in NO release was detected with serum obtained at 48 h, reaching at 96 h a value four fold (P < 0.001) that of the control. 5. These results suggest that iNOS activity and NO release play a role in the pathophysiological mechanisms that trigger post-necrotic hepatocellular regeneration following thioacetamide administration.

Induction of nitric oxide production by polyosides from the cell walls of Streptococcus mutans OMZ 175, a gram-positive bacterium, in the rat aorta

The cardiovascular dysfunctions associated with septic shock induced by gram-negative or gram-positive bacteria (gram-positive or gram-negative septic shock) are comparable. In gram-negative septic shock, lipopolysaccharide (LPS) induces nitric oxide (NO) synthase, which contributes to the vascular hypotension and hyporeactivity to vasoconstrictors. The role of NO in gram-positive septic shock and the nature of the bacterial wall components responsible for the vascular effects of gram-positive bacteria are not well known. This study investigated the vascular effects of cell wall serotype polyosides, rhamnose glucose polymers (RGPs), from Streptococcus mutans, in comparison with lipoteichoic acid (LTA) from Staphylococcus aureus, on the induction of NO synthase activity in the rat aorta. We show that 10 microg of both RGPs and LTA per ml induced hyporeactivity to noradrenaline, L-arginine-induced relaxation, increases of 2.2- and 7.8-fold, respectively, of cyclic GMP production, and increases of 7- and 12-fold in nitrite release. All of these effects appeared after several hours of incubation and were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase. Electron paramagnetic resonance spin trapping experiments demonstrated directly that RGPs and LTA induced NO overproduction (four- to eightfold, respectively) in rat aortic rings; this production was inhibited by L-NAME and prevented by dexamethasone. These results demonstrate directly the induction of NO production in vascular tissue by LTA and show that another, chemically different component of gram-positive bacteria can also have these properties. This result suggests that different components of the gram-positive bacterial wall could be implicated in the genesis of cardiovascular dysfunctions observed in gram-positive septic shock.

Effects of cytokine antagonists on the hepatic acute-phase response

Endotoxin-induced hepatic acute-phase protein synthesis has been thought to be primarily regulated through cytokines such as interleukin-1 (IL-1) and interleukin-6 (IL-6). Previously, it was found that a 23-kDa murine acute-phase protein, the lipopolysaccharide (LPS)-induced protein (LIP), was synthesized following treatment of hepatocytes in vitro with LPS. Since this protein was also induced by IL-1 and IL-6, the present studies were undertaken to determine if the effect of endotoxin was mediated through these cytokines. Primary cultures of murine hepatocytes were treated with LPS, IL-1, IL-6, or an LPS-stimulated macrophage supernatant in the presence or absence of the IL-1 receptor antagonist (IL-1 RA) and/or an anti-IL-6 antibody. The cells were then radiolabeled with [35S]methionine. LIP was detected by electrophoresis and autoradiography of the secreted proteins. In vitro, IL-1 RA completely inhibited the stimulation of LIP synthesis elicited by IL-1 and the macrophage supernatant, but did not affect LPS-stimulated synthesis of this protein. The anti-IL-6 antibody inhibited IL-6-triggered synthesis of LIP, but had no effect on LPS-stimulated synthesis. Hepatocytes isolated from mice treated in vivo with both IL-1 RA and LPS synthesized LIP to the same degree as hepatocytes isolated from mice treated with LPS alone. LPS-stimulated synthesis of LIP in vitro does not require IL-1 or IL-6 as an obligatory intermediate. These results are consistent with the hypothesis that endotoxin can directly stimulate hepatocyte acute-phase protein synthesis in the absence of cytokines.

A tutorial on the diffusibility and reactivity of free nitric oxide

In this review, I consider the quantitative consequences of the diffusion of free NO in determining its biological actions. Several studies have measured the extent to which NO diffuses away from an NO-producing cell, and the distance of its diffusion is quite large, on the order of 100-200 microm. This wide diffusibility is consistent also with the high value for its diffusion constant, 3300 microm2/s. Mathematical simulations based on this wide diffusibility suggest that, within spatial limits of approximately 0.3-0.4 mm, the actions of free NO are dictated by the total number of NO-producing cells within this location as opposed to where the NO-producing cells are located within this space. These results suggest that the actions of NO are surprisingly long range and the diffusion of NO is an important determinant of its biological actions. Thus, the effects of NO on individual target cells may be determined more by each cell's preprogrammed characteristic response to NO than by proximity to an NO source. In addition, scavenging of NO by hemoglobin in blood vessels should represent a significant sink for its scavenging, posing difficulty for the postulate that only free NO functions as EDRF.