Calmodulin is a subunit of nitric oxide synthase from macrophages

Inducible nitric oxide synthase gene expression in brain following cerebral ischemia

Cerebral ischemia is followed by a local inflammatory response that is thought to participate in the extension of the tissue damage occurring in the postischemic period. However, the mechanisms whereby the inflammation contributes to the progression of the damage have not been fully elucidated. In models of inflammation, expression of the inducible isoform of nitric oxide synthase (iNOS) is responsible for cytotoxicity through the production of large amounts of nitric oxide (NO). In this study, therefore, we sought to establish whether iNOS is expressed in the ischemic brain. Rats were killed 6 h to 7 days after occlusion of the middle cerebral artery. iNOS expression in the ischemic area was determined by reverse-transcription polymerase chain reaction. Porphobilinogen deaminase mRNA was detected in the same sample and used for normalization. In the ischemic brain, there was expression of iNOS mRNA that began at 12 h, peaked at 48 h, and returned to baseline at 7 days (n = 3/time point). iNOS mRNA expression paralleled the time course of induction of iNOS catalytic activity, determined by the citrulline assay (17.4 +/- 4.4 pmol citrulline/micrograms protein/min at 48 h; mean +/- SD; n = 5 per time point). iNOS immunoreactivity was seen in neutrophils at 48-96 h after ischemia. The data provide molecular, biochemical, and immunocytochemical evidence of iNOS induction following focal cerebral ischemia. These findings, in concert with our recent demonstration that inhibition of iNOS reduces infarct volume in the same stroke model, indicate that NO production may play an important pathogenic role in the progression of the tissue damage that follows cerebral ischemia.

Nitric oxide synthesis in retinal photoreceptor cells

Nitric oxide (NO) is known to be synthesized in several tissues and to increase the formation of cyclic GMP through the activation of soluble guanylate cyclases. Since cyclic GMP plays an important role in visual transduction, we investigated the presence of nitric oxide synthesizing activity in retinal rod outer segments. Bovine rod outer segments were isolated intact and separated into membrane and cytosolic fractions. Nitric oxide synthase activity was assayed by measuring the conversion of L-arginine to L-citrulline. Both membrane and cytosolic fractions were active in the presence of calcium and calmodulin. The activity in both fractions was stimulated by the nitric oxide synthase cofactors FAD, FMN, and tetrahydrobiopterin and inhibited by the L-arginine analog, L-monomethyl arginine. The Km for L-arginine was similar, about 5 microM for the enzyme in both fractions. However, the two fractions differed in their calcium/calmodulin dependence: the membrane fraction exhibited basal activity even in the absence of added calcium and calmodulin while the cytosolic fraction was inactive. But the activity increased in both fractions when supplemented with calcium/calmodulin: in membranes from about 40 to 110 fmol/min/mg of protein and in the cytosol from near zero to about 350 fmol/min/mg of protein in assays carried out at 0.3 microM L-arginine. The two enzymes also responded differently to detergent: the activity of the membrane enzyme was doubled by Triton X-100 while that of the cytosolic enzyme was unaffected. These results show that NO is produced by cytosolic and membrane-associated enzymes with distinguishable properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide. Novel biology with clinical relevance

OBJECTIVE

The author provides the reader with a view of the regulation and function of nitric oxide (NO), based on the three distinct enzyme isoforms that synthesize NO.

SUMMARY BACKGROUND DATA

Nitric oxide is a short-lived molecule exhibiting functions as diverse as neurotransmission and microbial killing. Recent advances in the characterization of the enzymes responsible for NO synthesis and in the understanding of how NO interacts with targets have led to new insights into the many facets of this diverse molecule.

METHODS

Nitric oxide is produced by one of three enzyme isoforms of NO synthesis. These enzymes vary considerably in their distribution, regulation, and function. Accordingly, the NO synthesis or lack of NO production will have consequences unique to that isoform. Therefore, this review summarizes the regulation and function of NO generated by each of the three isoforms.

RESULTS

Nitric oxide exhibits many unique characteristics that allow this molecule to perform so many functions. The amount, duration, and location of the NO synthesis will depend on the isoform of NO synthase expressed. For each isoform, there probably are disease processes in which deficiency states exist. For induced NO synthesis, states of overexpression exist.

CONCLUSIONS

Understanding the regulation and function of the enzymes that produce NO and the unique characteristics of each enzyme isoform is likely to lead to therapeutic approaches to prevent or treat a number of diseases.

Enzymatic characterisation of recombinant murine inducible nitric oxide synthase

A complementary DNA (cDNA) encoding murine inducible nitric oxide synthase was cloned from activated J774 macrophages. Expression of this cDNA in a baculovirus-insect cell system allowed comparison of the recombinant enzyme with the native homologue. Western blot analysis of activated J774 and baculovirus-infected insect cell cytosols demonstrated reactivity against a protein of 135 kDa. Kinetic studies on the recombinant and native enzymes revealed an absolute requirement for L-arginine and NADPH in order to achieve full activity. In addition, both enzymes were found to have similar maximum velocities and Km values for these two substrates. The nitric oxide synthase antagonists N-guanidino monomethyl L-arginine and N-iminoethyl L-ornithine inhibited both enzymes at a similar rate. Furthermore, comparable concentrations of inhibitor were required to achieve half maximal enzyme inhibition. These results indicate that recombinant inducible NO synthase appears to be pharmacologically indistinguishable from the native enzyme.

Inhibitory effect of cyclosporin A and FK506 on nitric oxide production by cultured macrophages. Evidence of a direct effect on nitric oxide synthase activity

Casein-elicited peritoneal macrophages from mice were cultured either alone or with interferon-gamma (IFN-gamma) and bacterial lipopolysaccharide (LPS), and the effect of cyclosporin A (CsA) and FK506 on NO2- production (due technical difficulties NO2- was taken as the index for NO) was analysed. We observed an inhibitory effect of CsA and FK506 on NO2- production. The IC50 for NO2- production by casein-elicited macrophages was 0.1 microgram/ml for CsA and 0.3 microgram/ml FK506. The effect of both drugs was dose-dependent and was more clear in non-stimulated macrophages. The presence of IFN-gamma and LPS in the culture increased NO2- production by casein-elicited macrophages and partially eliminated the inhibition exerted by CsA and FK506. Both drugs acted directly on the nitric oxide synthase (NOS), since CsA and FK506 reduced by 35% and by 17%, respectively, NOS activity in the crude cytosolic fraction. However, CsA and FK506 did not alter 14CO2 production from [1-14C]glucose, suggesting that the pentose monophosphate pathway activity was not modified. These data add new insight into the interpretation of the immunosuppressive properties of both drugs.

Immunohistochemical localization of endothelial nitric oxide synthase in human villous and extravillous trophoblast populations and expression during syncytiotrophoblast formation in vitro

We have examined the distribution of the endothelial isoform of nitric oxide synthase (eNOS) in villous and extravillous trophoblast populations by immunohistochemistry and have further studied expression of eNOS during differentiation of cytotrophoblast into syncytiotrophoblast in culture. In first trimester villous tissue, NADPH diaphorase activity and eNOS immunostaining were present in syncytiotrophoblast but not the progenitor cytotrophoblast layer. Extravillous trophoblast in the basal plate of the placenta was identified by anticytokeratin immunostaining and displayed NADPH diaphorase activity, but not eNOS immunostaining. Both amnion epithelial cells and chorion cytotrophoblast had NADPH diaphorase activity but no eNOS immunostaining, whereas eNOS immunostaining was seen in the fibroblast layer of amnion. Purified villous cytotrophoblast cells from term placentae aggregated and fused to form a syncytium with increasing time in culture as assessed by antidesmosomal protein and antinuclear antibody immunostaining. Following 24 h in culture, the majority of cells were still mononucleate cytotrophoblast which did not display eNOS immunostaining, whereas a few syncytial aggregates had formed which were both eNOS positive and hPL positive. By 3 to 5 days in culture, the majority of cells were present as syncytiotrophoblast. However, eNOS and hPL immunostaining was more diffuse and not all syncytial aggregates were positive. Of the trophoblast populations, only syncytiotrophoblast appears to express eNOS. Differentiation of cytotrophoblast into syncytiotrophoblast is associated with eNOS expression.

Inhibition of constitutive endothelial NO-synthase activity by tannin and quercetin

The effect of natural polyphenols on three isoforms of NO-synthase was investigated. Among the compounds tested, tannin was the most potent, inhibiting endothelial constitutive NO synthase (eNOS) with an IC50 of 2.2 microM. Other NOS isoforms (i.e. neuronal constitutive NOS and smooth muscle inducible NOS) were also inhibited but at much higher concentrations (selectivity ratio of approx. 20-30). Quercetin was also an effective but less potent inhibitor of eNOS (IC50 = 220 microM). The kinetics of tannin inhibition were investigated to gather information on the mechanism of action. Tannin did not interfere with the interaction of the enzyme with the co-substrates L-arginine and NADPH nor with the cofactor tetrahydrobiopterin. The inhibition level was also independent of free Ca2+ concentration as well as of the presence of high exogenous calmodulin concentrations.

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Nitric oxide: an ancestral immunocyte effector molecule

The presence and the role of nitric oxide synthase (NOS) were investigated in the molluscan hemocytes by immunocytochemical, biochemical and functional approaches. Using an anti-NOS polyclonal antibody, immunoreactivity was observed in the hemocytes, and this reactivity increased after stimulation of the animals with Escherichia coli, indicating that this enzyme is inducible. The NOS inducibility was also histochemically demonstrated by detection of NADPH-diaphorase activity. Biochemical studies show that the enzyme is 70% cytoplasmatic and 30% membrane bound and that the inducible form is mainly cytoplasmatic. The nitrite + nitrate and citrulline formation, the inhibition by N omega-nitro-L-arginine, the Km value for arginine, the calcium and co-enzyme dependence show that the molluscan NOS shares the same properties as the NOS isoenzymes so far studied. However, it cannot be identified with any of these enzymes. It appears to be in some way similar to an inducible form of human hepatocyte NOS. Also cytokines are able to induce NOS. In vitro studies have shown that hemocytes produce nitric oxide (NO), a bactericide substance, and that there is a relationship between the NO system and phagocytosis. The presence of NO in the invertebrate hemocyte demonstrates that critical molecules have been conserved over the course of evolution.

Detection of endothelial cell-derived nitric oxide: current trends and future directions

The vascular endothelium is a significant site of NO release that inhibits cellular adhesion and maintains a non-thrombogenic surface. Use of newly described technology suggests for the first time that the maximal release of NO induced by cNOS and iNOS activation may be quite similar, implying that it is the duration of NO release and not the concentration of NO produced from stimulated endothelial cells that accounts for the different biological activities of the enzymes. The respective roles of cNOS and iNOS must be carefully evaluated since both enzymes may have potent biological effects at local sites of production.

Role of nitric oxide in cell-mediated tumor cytotoxicity

Strong and increasing evidence shows that nitric oxide (NO) contributes to immune function, and in particular to 'non-specific host defense'. The aim of the present review was to focus the current understanding of the role of NO as a biochemical effector of L-arginine-dependent cell-mediated immune responses to neoplastic cells in vitro and in vivo. The cytokine-inducible nitric oxide synthase (NOS) seems to mainly be implicated in the cytotoxic activity of almost all the effector cells involved in tumor cell killing. The cytotoxic actions of NO against tumor cells appear to be related mainly to inhibition of several heme-containing enzymes of the mitochondrial electron transport complex and the citric acid cycle.

Structure and function of nitric oxide synthases

Nitric oxide (NO), which accounts for the biological activity of endothelium-derived relaxing factor, is now thought to play a variety of roles in the nervous system and in immunologic reactions. NO is synthesized from L-arginine by nitric oxide synthase (NOS). There are three isoforms of NOS; type I (neuronal), type II (inducible), and type III (endothelial). The fundamental structure of the three isoforms, which contain calmodulin-, FMN-, FAD-, and NADPH-binding domains, is the same. Calmodulin is already bound to inducible NOS without requiring Ca2+, while the others are Ca2+/calmodulin-dependent. Endothelial NOS is bound to membranes by N-myristoylation, while the other isoforms are soluble. The human endothelial NOS gene has been cloned. It has several highly repetitive regions which could provide potential sites for DNA polymorphism. It might be of interest to examine the relationship between such polymorphism and cardiovascular disorders.

Macrophage synthesis of nitric oxide in the mouse mixed leucocyte reaction

The production of nitric oxide (.N = O) in the splenocyte mixed leucocyte reaction (MLR) results in inhibition of allospecific lymphocyte effector function. In order to more clearly define the circumstances which promote .N = O synthesis in the MLR, responder accessory cell depleted spleen cells (ACDSC) were co-cultured with allogeneic macrophage cell lines or peritoneal macrophages. .N = O synthesis and C57BL/6 (H-2b) ACDSC proliferation were concurrently monitored in cultures comparing RAW 264.7 (H-2d, a high .N = O producer), P388D1 (H-2d, a low .N = O and BALB/c (H-2d) peritoneal macrophages as allogeneic antigen presenting cells (APC). A concentration-dependent increase in lymphocyte proliferation was observed in the presence of 1 x 10(4) to 1 x 10(5) P388D1. In contrast, addition of NG-monomethyl-L-arginine (NMA), a competitive inhibitor of .N = O synthase, was necessary in order to observe lymphocyte proliferation in the presence of increasing numbers of RAW 264.7 and BALB/c peritoneal macrophages. The addition of both anti-IL-2 and anti-IFN gamma (interferon-gamma) monoclonal antibodies inhibited .N = O synthesis in alloantigen-stimulated cultures. The IFN gamma induced expression of class II antigen, as well as the constitutive expression of class I antigen, on RAW 264.7 was similar in the presence or absence of NMA, indicating that induction of .N = O synthesis by IFN gamma does not inhibit H-2 antigen expression. Thus, cytokines produced as a result of alloimmune interaction initiate macrophage .N = O synthesis. However, allogeneic APC function, as assessed by H-2 antigen expression and subsequent stimulatory capacity of MLR, is not affected by initiation of the .N = O pathway.

Pteridine biosynthesis and nitric oxide synthase in Physarum polycephalum

Physarum polycephalum, an acellular slime mould, serves as a model system to study cell-cycle-dependent events since nuclear division is naturally synchronous. This organism was shown to release isoxanthopterin which is structurally related to tetrahydrobiopterin, a cofactor of aromatic amino acid hydroxylases and of nitric oxide synthases (NOSs) (EC 1.14.13.39). Here, we studied Physarum pteridine biosynthesis in more detail and found that high amounts of tetrahydrobiopterin are produced and NOS activity is expressed. Physarum pteridine biosynthesis is peculiar in as much as 7,8-dihydroneopterin aldolase (EC 4.1.2.25), an enzyme of folic acid biosynthesis usually not found in organisms producing tetrahydrobiopterin, is detected in parallel. NOS purified from Physarum depends on NADPH, tetrahydrobiopterin and flavins. Enzyme activity is independent of exogenous Ca2+ and is inhibited by arginine analogues. The purified enzyme (with a molecular mass of 130 kDa) contains tightly bound tetrahydrobiopterin and flavins. During the synchronous cell cycle of Physarum, pteridine biosynthesis increases during S-phase whereas NOS activity peaks during mitosis, drops at telophase and peaks again during early S-phase. Our results characterize Physarum pteridine biosynthesis and NOS and suggest a possible link between NOS activity and mitosis.

Duration of expression of inducible nitric oxide synthase in glial cells

Lipopolysaccharide (LPS) or a combination of interleukin (IL)-1 beta and interferon (IFN)-gamma cause transcriptional induction of a calcium-independent nitric oxide synthase (NOS) in astrocytes and C6 glioma cells. LPS induction of NOS in C6 cells was evidenced by a small amount of nitrite accumulation as compared with cells exposed to IL-1 beta/IFN-gamma, but the maximal NOS activity achieved (as revealed by cGMP formation) was the same. The NOS activity induced by LPS in C6 cells was maximal at 4 to 8 hr and then rapidly decreased, while NOS activity induced by IL-1 beta/IFN-gamma slowly decreased after 4 hr. In addition, the effects of re-presenting IL-1 beta/IFN-gamma to both astrocytes and C6 cells after maximal induction of activity of the inducible form of NOS were studied. The re-addition of cytokines prolonged both NOS mRNA expression and also enzyme activity, suggesting effects at either the transcriptional (further induction) or translational level (mRNA stability). These results imply that the time course of NO production by induced astrocytes depends both upon the nature of the inducing stimulus and the frequency of the cells' exposure to it.

Constitutive and inducible nitric oxide synthase gene expression, regulation, and activity in human lung epithelial cells

Histochemical activity and immunoreactivity of nitric oxide synthase (NOS, EC 1.14.13.39) have been recently demonstrated in human lung epithelium. However, the molecular nature of NOS and the regulation and function of the enzyme(s) in the airway is not known. A549 cells (human alveolar type II epithelium-like), BEAS 2B cells (transformed human bronchial epithelial cells), and primary cultures of human bronchial epithelial cells all exhibited constitutive NOS activity that was calcium dependent and inhibitable by the NOS inhibitor NG-monomethyl-L-arginine. Nitric oxide production by epithelial cells was enhanced by culture in the presence of interferon gamma, interleukin 1 beta, tumor necrosis factor alpha, and lipopolysaccharide; the NOS activity expressed under these conditions showed less dependence on calcium, reminiscent of other inducible forms of NOS. Two distinct NOS mRNA species, homologous to previously identified constitutive brain (type I) and inducible hepatic (type II) NOS, were demonstrated by reverse transcription-polymerase chain reaction in all cell lines. Northern analysis confirmed the expression of inducible NOS mRNA. Cell culture with epidermal growth factor, a principal regulator of epithelial cell function, decreased inducible NOS activity by posttranscriptional action but did not affect constitutive NOS activity. The coexistence of constitutive and inducible NOS in human alveolar and bronchial epithelial cells is consistent with a complex mechanism evolved by epithelial cells to protect the host from microbial assault at the air/surface interface while shielding the host from the induction of airway hyperreactivity.

Role of interferon regulatory factor 1 in induction of nitric oxide synthase

Interferon gamma (IFN-gamma) interacts synergistically with bacterial lipopolysaccharide (LPS) to induce transcription of iNOS, the isoform of nitric oxide synthase whose activity is independent of elevated Ca2+ and exogenous calmodulin. To define a cis-acting element mediating IFN-gamma-dependent synergy, we made deletions in iNOS promoter constructs fused to reporter genes, transfected RAW 264.7 macrophages, and treated the cells with IFN-gamma and/or LPS. This analysis implicated the region from positions -951 to -911, a cluster of four enhancer elements known to bind IFN-gamma-responsive transcription factors, including an interferon regulatory factor binding site (IRF-E) at nucleotides -913 to -923. Site-specific substitution of two conserved nucleotides within IRF-E in the context of the full-length iNOS promoter ablated IFN-gamma's contribution to synergistic enhancement of transcription. Electromobility shift assays performed with a probe containing IRF-E revealed the existence of a complex in nuclei of RAW 264.7 macrophages that was present only after treatment with IFN-gamma, which reacted specifically with anti-IRF-1 immunoglobulin G and which included a species migrating at 40-45 kD, consistent with the apparent molecular weight of murine IRF-1. Thus, the synergistic contribution of IFN-gamma to transcription of iNOS in RAW 264.7 macrophages requires that IRF-1 bind to IRF-E in the iNOS promoter. In conjunction with the work of Kamijo et al. (Kamijo, R., H. Harada, T. Matsuyama, M. Bosland, J. Gerecitano, D. Shapiro, J. Le, K. S. Im, T. Kimura, S. Green et al. 1994. Science [Wash. DC]. 263:1612), these findings identify iNOS as the first gene that requires IRF-1 for IFN-gamma-dependent transcriptional regulation.

Induction of myocardial nitric oxide synthase by cardiac allograft rejection

Cardiac transplantation, effective therapy for end-stage heart failure, is frequently complicated by allograft rejection, the mechanisms of which remain incompletely understood. Nitric oxide (NO), a vasodilator which is cytotoxic and negatively inotropic, can be produced in large amounts by an inducible NO synthase (iNOS) in response to cytokines. To investigate whether iNOS is induced during cardiac allograft rejection, hearts from Lewis or Wistar-Furth rats were transplanted into Lewis recipients. At day 5, allogeneic grafts manifested reduced contractility and histologic evidence of rejection (inflammatory infiltrate, edema, necrosis of myocytes). The mRNA for iNOS and iNOS protein were detected in ventricular homogenates and in isolated cardiac myocytes from rejecting allogeneic grafts but not in tissue and myocytes from syngeneic control grafts. Immunocytochemistry showed increased iNOS staining in infiltrating macrophages and in microvascular endothelial cells and cardiac muscle fibers and also in isolated purified cardiac myocytes from the rejecting allografts. Using a myocardial cytosolic iNOS preparation, nitrite formation from L-arginine and [3H] citrulline formation from [3H]L-arginine were increased significantly in the rejecting allogeneic grafts (P < 0.01). Myocardial cyclic GMP was also increased significantly (P < 0.05). The data indicate myocardial iNOS mRNA, protein and enzyme activity are induced in infiltrating macrophages and cardiac myocytes of the rejecting allogeneic grafts. Synthesis of NO by iNOS may contribute to myocyte necrosis and ventricular failure during cardiac allograft rejection.

Nitric oxide in the kidney: synthesis, localization, and function

The characterization and cloning of constitutive and inducible nitric oxide (NO)-synthesizing enzymes and the development of specific inhibitors of the L-arginine NO pathway have provided powerful tools to define the role of NO in renal physiology and pathophysiology. There is increasing evidence that endothelium-derived NO is tonically synthesized within the kidney and that NO plays a crucial role in the regulation of renal hemodynamics and excretory function. Bradykinin and acetylcholine induce renal vasodilation by increasing NO synthesis, which in turn leads to enhancement of diuresis and natriuresis. The blockade of basal NO synthesis has been shown to result in decreases of renal blood flow and sodium excretion. These effects are partly mediated by an interaction between NO and the renin angiotensin system. Intrarenal inhibition of NO synthesis leads to reduction of sodium excretory responses to changes in renal arterial pressure without an effect on renal autoregulation, suggesting that NO exerts a permissive or a mediatory role in pressure natriuresis. Nitric oxide released from the macula densa may modulate tubuloglomerular feedback response by affecting afferent arteriolar constriction. Nitric oxide produced in the proximal tubule possibly mediates the effects of angiotensin on tubular reabsorption. In the collecting duct, an NO-dependent inhibition of solute transport is suggested. The L-arginine NO pathway is also active in the glomerulus. Under pathologic conditions such as glomerulonephritis, NO generation is markedly enhanced due to the induction of NO synthase, which is mainly derived from infiltrating macrophages. An implication of NO in the mechanism of proteinuria, thrombosis mesangial proliferation, and leukocyte infiltration is considered. In summary, the data presented on NO and renal function have an obvious clinical implication. A role for NO in glomerular pathology has been established. Nitric oxide is the only vasodilator that closely corresponds to the characteristics of essential hypertension. Using chronic NO blockade, models of systemic hypertension will provide new insights into mechanisms of the development of high blood pressure.

Role of NO in vascular smooth muscle and cardiac muscle function

NO is a key transducer of a vasodilator message from the endothelium to vascular smooth muscle. Recently, its actions as a negative inotrope in cardiac muscle have been discovered. In the vasculature, it is synthesized under physiological conditions following activation of a low-output, Ca(2+)-dependent NO synthase (NOS) in endothelial cells. Immune activation triggers the expression of a high-output, Ca(2+)-independent NOS in the vasculature and myocardium, causing the overproduction of NO and significant cardiovascular dysfunction. In this article, Richard Schultz and Chris Triggle briefly review recent findings concerning the role of NO, and other endothelium-derived factors, in vascular smooth muscle function and consider the consequences of its production in the heart.

Induction of nitric oxide synthase gene by interleukin-1 beta in cultured rat cardiocytes

BACKGROUND

Impaired myocardial contractility in septic shock is protracting, which may be caused by cytokine-induced nitric oxide (NO) synthesis in the heart. However, the cellular mechanism by which cytokines induce nitric oxide synthase (NOS) in cardiocytes remains obscure.

METHODS AND RESULTS

We studied the effect of human recombinant interleukin-1 beta (IL-1 beta) on synthesis of NO2-/NO3- (NOx) and the expression of NOS mRNA and protein in cultured neonatal rat cardiocytes. IL-1 beta dose-dependently (0.1 to 10 ng/mL) stimulated NOx production as a function of time (6 to 48 hours). Northern blot analysis using complementary DNAs for rat brain-type constitutive (c) NOS and mouse macrophage-type inducible (i) NOS as probes showed that IL-1 beta induced expression of mRNA for iNOS but not for cNOS, starting after 6 hours and reaching a maximum after 48 hours in cardiocytes. IL-1 beta similarly induced iNOS mRNA expression in cultured adult rat cardiocytes in a time-dependent manner. Western blot analysis using specific antibody against the N-terminal fragment of mouse iNOS revealed the expression of 130-kD iNOS-like protein in IL-1 beta-treated cardiocytes. Northern blotting and immunocytochemical study revealed that IL-1 beta-induced iNOS mRNA and iNOS-like immunoreactivity were exclusively localized to cardiac myocytes but also to nonmyocytes, to a lesser extent. NG-mono-methyl-L-arginine, an NOS inhibitor, completely blocked the IL-1 beta-induced NOx production, whose effect was reversed by L-arginine but not by D-arginine. Dexamethasone inhibited the IL-1 beta-induced NOx production as well as iNOS mRNA expression. Cycloheximide and actinomycin D completely inhibited the IL-1 beta-induced NOx production and iNOS mRNA expression. Neither a calmodulin inhibitor (W-7), a protein kinase C inhibitor (calphostin C), nor a Ca2+ channel antagonist (nicardipine) showed any effect on the IL-1 beta-induced NOx production.

CONCLUSIONS

These data demonstrate that IL-1 beta induces macrophage-type iNOS mRNA expression mainly by cardiac myocytes but also by nonmyocytes to a lesser extent, and subsequent de novo protein synthesis of iNOS leads to excessive local production of NO by cardiocytes.

Roles for protein kinases in the induction of nitric oxide synthase in astrocytes

Lipopolysaccharide (LPS) or a combination of interferon (IFN)-gamma and interleukin (IL)-1 beta can induce a calcium-independent nitric oxide synthase (iNOS) in astrocyte cultures (Simmons and Murphy: J Neurochem 59:897, 1992; Eur J Neurosci 5:825, 1993; Galea et al: Proc Natl Acad Sci USA 89:10945, 1992). This induction can be measured by assaying cyclic GMP levels in the cultures, which correlates with, but is more sensitive than, measurement of nitrite accumulation. To study potential second-messenger systems involved in the induction of iNOS, phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, and various protein kinase inhibitors were employed. PMA induced a time-, dose-, and L-arginine-dependent increase in cyclic GMP, which could be inhibited by dexamethasone or actinomycin D. This induction could be dramatically increased by concurrent treatment with IFN-gamma. The presence of iNOS mRNA could be demonstrated by hybridization with a specific cDNA probe. H7 (a non-specific serine/threonine kinase inhibitor) but not H89 (a more specific PKA inhibitor) prevented induction by all agents. However, downregulation of PKC or pretreatment with the PKC inhibitor calphostin C did not prevent the induction by LPS or cytokines, suggesting that PKC is not necessary for iNOS induction by these mediators. Additionally, genistein (a nonspecific tyrosine kinase inhibitor) could prevent induction by all agents, but the more specific inhibitor, tyrphostin, attenuated only NOS induction by LPS. These results suggest that activation of PKC can lead to, but is not necessary for, the induction of NOS in astrocytes and that there is a potential role for tyrosine kinases in NOS induction by LPS.(ABSTRACT TRUNCATED AT 250 WORDS)

New insights into the regulation of inducible nitric oxide synthesis

Recent studies have identified the induction of nitric oxide (NO) synthesis in many cell types as part of the host response to sepsis and inflammation. Induced NO can have a variety of effects which may be detrimental or beneficial during sepsis or inflammation, depending on amount, duration, and anatomic site of synthesis. As arginine is the only physiological nitrogen donor for NO synthesis, metabolism of this amino acid may play an important role in regulation of NO synthesis during sepsis. This review will discuss the roles NO plays in sepsis and the potential impact of arginine metabolism on NO synthesis.

Nitric oxide synthase isozymes. Characterization, purification, molecular cloning, and functions

Three isozymes of nitric oxide (NO) synthase (EC 1.14.13.39) have been identified and the cDNAs for these enzymes isolated. In humans, isozymes I (in neuronal and epithelial cells), II (in cytokine-induced cells), and III (in endothelial cells) are encoded for by three different genes located on chromosomes 12, 17, and 7, respectively. The deduced amino acid sequences of the human isozymes show less than 59% identity. Across species, amino acid sequences for each isoform are well conserved (> 90% for isoforms I and III, > 80% for isoform II). All isoforms use L-arginine and molecular oxygen as substrates and require the cofactors NADPH, 6(R)-5,6,7,8-tetrahydrobiopterin, flavin adenine dinucleotide, and flavin mononucleotide. They all bind calmodulin and contain heme. Isoform I is constitutively present in central and peripheral neuronal cells and certain epithelial cells. Its activity is regulated by Ca2+ and calmodulin. Its functions include long-term regulation of synaptic transmission in the central nervous system, central regulation of blood pressure, smooth muscle relaxation, and vasodilation via peripheral nitrergic nerves. It has also been implicated in neuronal death in cerebrovascular stroke. Expression of isoform II of NO synthase can be induced with lipopolysaccharide and cytokines in a multitude of different cells. Based on sequencing data there is no evidence for more than one inducible isozyme at this time. NO synthase II is not regulated by Ca2+; it produces large amounts of NO that has cytostatic effects on parasitic target cells by inhibiting iron-containing enzymes and causing DNA fragmentation. Induced NO synthase II is involved in the pathophysiology of autoimmune diseases and septic shock. Isoform III of NO synthase has been found mostly in endothelial cells. It is constitutively expressed, but expression can be enhanced, eg, by shear stress. Its activity is regulated by Ca2+ and calmodulin. NO from endothelial cells keeps blood vessels dilated, prevents the adhesion of platelets and white cells, and probably inhibits vascular smooth muscle proliferation.

Dysmorphogenic effects of nitric oxide (NO) and NO-synthase inhibition: studies with intra-amniotic injections of sodium nitroprusside and NG-monomethyl-L-arginine

Sodium nitroprusside (SNP), a chemical that is readily converted to nitric oxide (NO) in biological systems, was microinjected into the amniotic fluids of cultured whole rat conceptuses on day 10.5 of gestation and dysmorphogenic/embryotoxic effects were evaluated after a 24 hr incubation period. Injections of 217 ng/embryo (approximately 800 microM) resulted in whitened zones of dead cells in a discretely circumscribed region within the mesencephalon closely associated with the neural tube. These zones were observed with a high incidence after SNP microinjections and were referred to as "white caps" because of their microscopic appearance. At higher concentrations, the whitened zone extended into the rhombencephalon and occasionally appeared to extend the full length of the dorsal midline. The whitened zones of tissue separated readily from the apparently normal underlying tissues upon removal or disturbance of the amniotic membrane. Coinjection of ferrous hemoglobin with SNP selectively prevented the appearance of "white caps" but not other embryotoxic manifestations. Microinjections of the breakdown products of light-exposed SNP elicited generalized embryotoxicity but "white caps" were not observed. In separate experiments, we found that embryonic enzymes catalyzed significant conversion of arginine to citrulline, indicating expression of NO-synthase during organogenesis. NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO-synthase, was microinjected (50-150 ng/embryo; approximately 200-600 microM) on day 10.5 of gestation and produced malformations that differed markedly from those elicited by SNP. Failure of anterior and posterior neural tube closure and profound underdevelopment of the hyoid arch and optic cup were observed at concentrations that produced no apparent growth deficit. These studies with SNP and L-NMMA indicated that both an excess and a deficiency of NO can be embryotoxic/dysmorphogenic and suggest important roles for optimal levels of NO and NO synthases in normal embryonic development.

Excitotoxicity, free radicals, and cell membrane changes

Neuronal injury resulting from glutamate receptor-mediated excitotoxicity has been implicated in a wide spectrum of neurological disease states, including ischemia, central nervous system trauma, and some types of neurodegenerative diseases. Excitotoxicity may interact with other pathophysiological processes to enhance neuronal injury; for example, excess glutamate release due to free radicals generated during the immune response to infection might initiate secondary excitotoxicity, and intracellular pathways that contribute to neuronal destruction may be common to both excitotoxic and nonexcitotoxic injury processes. Defining the contribution of excitotoxicity to neuronal damage in acute zoster infection and post-herpetic neuralgia may provide one means of reducing morbidity from this often debilitating disease.

Regulation of nitric oxide synthesis by proinflammatory cytokines in human umbilical vein endothelial cells. Elevations in tetrahydrobiopterin levels enhance endothelial nitric oxide synthase specific activity

We have examined cytokine regulation of nitric oxide synthase (NOS) in human umbilical vein endothelial cells (HUVEC). 24-h treatment with IFN-gamma (200 U/ml) plus TNF (200 U/ml) or IL-1 beta (5 U/ml) increased NOS activity in HUVEC lysates, measured as conversion of [14C]L-arginine to [14C]L-citrulline. Essentially, all NOS activity in these cells was calcium dependent and membrane associated. Histamine-induced nitric oxide release, measured by chemiluminescence, was greater in cytokine-treated cells than in control cells. Paradoxically, steady-state mRNA levels of endothelial NOS fell by 94 +/- 2.0% after cytokine treatment. Supplementation of HUVEC lysates with exogenous tetrahydrobiopterin (3 microM) greatly increased total NOS activity, and under these assay conditions, cytokine treatment decreased maximal NOS activity. IFN-gamma plus TNF or IL-1 beta increased endogenous tetrahydrobiopterin levels and GTP cyclohydrolase I activity, the rate-limiting enzyme of tetrahydrobiopterin synthesis. Intracellular tetrahydrobiopterin levels were higher in freshly isolated HUVEC than in cultured cells, but were still limiting. We conclude that inflammatory cytokines increase NOS activity in cultured human endothelial cells by increasing tetrahydrobiopterin levels in the face of falling total enzyme; similar regulation appears possible in vivo.

Involvement of nitric oxide synthase in antiproliferative activity of macrophages: induction of the enzyme requires two different kinds of signal acting synergistically

Activated rodent macrophages inhibit micro-organism and tumour cell growth through a high output of nitric oxide; generated by an isoform of nitric oxide synthase which is induced, for example, in murine macrophages, by concomitant stimulation with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS). We show here that LPS could be replaced as a co-stimulant by the mycobacterial derivative muramyl dipeptide (MDP) in macrophages, and by interleukin-1 (IL-1) in EMT-6 adenocarcinoma cells. Moreover, our results indicate that nitric oxide synthase RNA synthesis required either simultaneous or sequential exposure to IFN-gamma and MDP/IL-1; whereas exposure to MDP/IL-1 followed by exposure to IFN-gamma was ineffective. Thus, two kinds of signal could be distinguished: IFN-gamma on the one hand, acting first in an irreversible way, and LPS, MDP, IL-1 on the other hand, which seemed to be permanently required for continuous transcription of the nitric oxide synthase gene.

Location of an inducible nitric oxide synthase mRNA in the normal kidney

An inducible nitric oxide synthase (iNOS) mRNA was found primarily in the outer medulla of normal rat kidney. Identification of the mRNA was based upon the specificity of the oligonucleotide primers used for PCR amplification, PCR-Southern blot analysis and the nucleic acid sequence of the cloned PCR product. In addition to the outer medulla, glomeruli prepared from normal rat kidney contained significant amounts of an iNOS mRNA. These results suggest that there may be tonic influences in the outer medulla of the normal rat kidney resulting in the "steady-state" presence of an iNOS mRNA. Cortical tubules and the inner medulla were found to contain detectable but lesser amounts of the iNOS mRNA. The outer medulla was microdissected into proximal straight tubule (PST), medullary thick ascending limb (MTAL), medullary collecting duct (MCD) and vasa recta bundle (VRB). The iNOS mRNA was found primarily in the MTAL with minor amounts in the MCD and VRB of normal rat kidney. Animals were injected with lipopolysaccharide (LPS) and sacrificed 24 hours later. Treatment with LPS caused at least a 20-fold increase in the amount of iNOS mRNA in the liver or in macrophages isolated from the peritoneum. Endotoxin treatment led to over a 10-fold increase in iNOS mRNA content in glomeruli and the inner medulla. The iNOS mRNA level of the outer medulla was increased two- to threefold due to LPS treatment.

Effect of anti-fungal imidazoles on mRNA levels and enzyme activity of inducible nitric oxide synthase

1. Experiments were performed to examine the effects of anti-fungal imidazole compounds (clotrimazole, econazole and miconazole) on the induction of nitric oxide (NO) synthase and subsequent production of NO in the cultured murine monocyte/macrophage cell line J774 using a specific cDNA probe for inducible NO synthase mRNA and by monitoring nitrite production. 2. Stimulation of J774 cells with lipopolysaccharide (LPS, 10 micrograms ml-1) resulted in the induction of NO synthase activity as determined by nitrite accumulation in the culture medium (48 +/- 3 nmol per 10(6) cells over 24 h). Production of nitrite was inhibited by co-incubation of cells with LPS (10 micrograms ml-1) and either dexamethasone (10 microM) or NG-monomethyl-L-arginine (L-NMMA; 0.1 mM), however, only L-NMMA was an effective inhibitor of nitrite production when added after induction of NO synthase had occurred. 3. Co-incubation of J774 cells with LPS (10 micrograms ml-1) and either clotrimazole, econazole or miconazole (1-10 microM) resulted in a concentration-dependent inhibition of nitrite production over the subsequent 24 h without any evidence for a cytotoxic effect. However, addition of these imidazoles after induction of NO synthase did not inhibit nitrite production. 4. Messenger RNA for inducible NO synthase was not detected in unstimulated J774 cells. Treatment with LPS (10 micrograms ml-1) for 4 h resulted in significant expression of mRNA for inducible NO synthase which was not altered in the presence of econazole (10 microM) but was reduced significantly by dexamethasone (10 microM). 5. These results demonstrate that anti-fungal imidazoles inhibit the production of nitric oxide by cultured J774 cells by a mechanism which appears to differ from that of dexamethasone and substrate type inhibitors of NO synthase. Furthermore, the presence of mRNA for NO synthase does not indicate the presence of functionally active NO synthase.

Nitric oxide synthases in mammals

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