Expression and expressional control of nitric oxide synthases in various cell types

A critical role for inducible nitric oxide synthase in host survival following coxsackievirus B4 infection

Coxsackieviral infections have been linked etiologically to multiple diseases. The serotype CB4 is associated with acute pancreatitis and autoimmune type 1 diabetes. To delineate the mechanisms of host survival after an acute infection with CB4 (strain E2), we have investigated the role of nitric oxide (NO), generated by the inducible form of nitric oxide synthase (NOS2), in viral clearance and pancreatic beta-cell maintenance. Mice deficient in NOS2 (NOS2-/- mice) and their wild-type (wt) counterparts were injected with CB4, after which both groups developed severe pancreatitis, hepatitis, and hypoglycemia within 3 days. Within 4 to 7 days postinfection (p.i.), most of the NOS2-/- mice died and at a strikingly higher mortality rate than wt mice. Histological examination of pancreata from both infected NOS2-/- and infected wt mice revealed early and complete destruction of the pancreatic acinar tissue, but intact, insulin-stained islets. When examined up to 8 weeks p.i., neither surviving NOS2-/-mice nor surviving wt mice developed hyperglycemia. However, the clearance of infectious CB4 was different between the mice. The spleens of NOS2-/- survivors were cleared of infectious virus with kinetics similar to that of wt mice, but the livers, pancreata, kidneys, and hearts of the NOS2-/- groups cleared virus more slowly than those of the wt group. This delayed clearance was particularly prominent in the livers of infected NOS2-/- mice, which also showed prolonged histopathological features of viral hepatitis. Taken together, this outcome suggests that NOS2 (and NO) is not required for the prevention of pancreatic beta-cell depletion after CB4 infection. Instead the critical actions of NOS2 apparently occur early in the host immune response, allowing mice to survive and clear virus. Moreover, the data support the existence of an organ-specific dependency on NO for a rapid clearance of CB4.

GnRH agonist-suppressed expression of nitric oxide synthases and generation of peroxynitrite in adenomyosis

Because overproduction of nitric oxide (NO) and peroxynitrite is known to cause tissue injury, the expression of NO synthases (NOS) and generation of peroxynitrite were investigated in adenomyosis. Immunoreactivities to endothelial and inducible NOS demonstrated phase-dependent changes in normal endometrium, and in eutopic endometrium of adenomyosis. However, NOS were expressed throughout the menstrual cycle in ectopic endometrium from the majority of patients with adenomyosis. Nitrotyrosine, a footprint of peroxynitrite, was detected concomitantly with NOS protein. This suggested that high doses of NO and superoxide are produced in the ectopic endometrium, presumably by stimulation with bioactive molecules such as cytokines and growth factors. The expression of NOS and generation of peroxynitrite were markedly reduced by administration of gonadotrophin-releasing hormone agonists (GnRHa). The suppression of serum concentrations of nitrite/nitrate, stable metabolites of NO, by long-term administration of GnRHa was also demonstrated. The suppression of synthesis of NO and/or peroxynitrite may be part of both the therapeutic and adverse effects of GnRHa therapy.

Induction of MRP1 and gamma-glutamylcysteine synthetase gene expression by interleukin 1beta is mediated by nitric oxide-related signalings in human colorectal cancer cells

Treatment of human colorectal cancer cells HT29 with interleukin 1beta (IL-1beta) induces expression of the multidrug resistance protein (MRP1) gene encoding the ATP-dependent glutathione S-conjugate export (GS-X) pump and the gamma-glutamylcysteine synthetase (gamma-GCSh) gene encoding heavy (catalytic) subunit of gamma-glutamylcysteine synthetase, the rate-limiting enzyme for the biosynthesis of glutathione (GSH). The induction can be suppressed by N(G)-methyl-L-arginine, a specific inhibitor of nitric oxide synthase (NOS). These results suggest that IL-1beta-mediated MRP1 and gamma-GCSh induction involve nitric oxide (NO) -related signaling. Further supports to the involvement of NO in the induction of MRP1 and gamma-GCSh expression are made by the following observations. (i) Expression of MRP1 and gamma-GCSh genes were induced by treating the cells with NO donors, i.e., S-nitro-N-acetyl-D,L-penicillamide (SNAP) and S-nitroso-L-glutathione, in a concentration-dependent manner. (ii) Ectopic expression of inducible NOS (iNOS) activity by transfecting expressible recombinant iNOS cDNA encoding functional iNOS but not the nonfunctional version resulted in elevated expression of MRP1 and gamma-GCSh. We also demonstrated that HT-29 cells treated with either 1L-1beta or SNAP induced ceramide production, and addition of C2 or C6 ceramides into cultured HT-29 cells resulted in induction of gamma-GCSh but not MRP1 expression. Collectively, our results demonstrate that induction of MRP1 and gamma-GCSh by IL-1beta is regulated, at least in part, by an NO-related signaling, and induction of gamma-GCSh is by NO-related ceramide signaling.

Expression of neuronal nitric oxide synthase/NADPH-diaphorase during olfactory deafferentation and regeneration

Neuronal nitric oxide synthase (nNOS) expression can be regulated under natural or experimental conditions. This work aims at elucidating whether the expression of nNOS or its related NADPH-diaphorase (ND) activity are modified by manipulation of the normal inputs to neurons. We used the olfactory bulbs from two mouse strains, BALB and CD1, because they show divergences in their synapse patterns, and these differences affect periglomerular cells, interneurons expressing tyrosine hydroxylase or nNOS/ND. The olfactory inputs to these neurons can be disrupted by inhalation of methyl bromide. The effect of this gas on olfactory axons, as well as the synaptic features in both mouse strains, were studied using electron microscopy. The changes in expression were analysed qualitatively and quantitatively at different times after lesion to nine topographical regions of the olfactory bulb. Methyl bromide inhalation induced a degeneration of olfactory axons in both strains, but had different effects on the expression of nNOS/ND and tyrosine hydroxylase. In BALB mice, where periglomerular cells do not receive direct inputs from olfactory axons, no changes were detected in tyrosine hydroxylase or in ND expression. In CD1 periglomerular cells, where olfactory axons establish direct synapses, a significant down-regulation of both markers was observed. These changes were observed differentially across the olfactory bulb, being more pronounced in rostral regions and more acute for ND than for tyrosine hydroxylase. Our results indicate that the synaptic inputs influence the expression of ND activity related to nNOS and that the activation of the enzyme is more severely affected than its protein expression.

Immune-inflammatory mechanisms in IFNgamma-mediated anti-tumor activity

IFNgamma is a functionally pleiotropic cytokine which shows considerable potency in promoting anti-tumor functions in vivo. Despite limited efficacy when delivered systemically either to experimental animals or patients, IFNgamma appears to play an important and perhaps critical role in directing the development of immune-mediated tumor destruction when expressed within the tumor bed. This has been demonstrated both by use of tumor cells transduced to express IFNgamma and by the use of IL-12 which is able, at least is murine models, to promote an IFNgamma-dependent, T cell mediated anti-tumor response. Recent studies indicate that the therapeutic efficacy of IFNgamma in tumor models depends critically upon the ability of the tumor cells themselves to respond to IFNgamma. Though IFNgamma is able to induce anti-viral activity and has direct anti-proliferative effects on some tumor cell lines, immunomodulatory function also appears to be an important component of its anti-tumor action. This is mediated through the action of several different classes of IFNgamma-inducible gene expression which control antigen processing and presentation, leukocyte trafficking, and indirect tumor cytotoxicity.

Functional expression of NOS 1 in vascular smooth muscle

Substances that increase intracellular calcium concentration ([Ca(2+)](i)), such as serotonin, are known to induce vascular smooth muscle (VSM) contraction. However, increases in [Ca(2+)](i) also activate Ca(2+)/calmodulin-dependent nitric oxide synthases (NOS), which leads to increases in cGMP and activation of cGMP-dependent protein kinase (PKG). One recently identified substrate protein of PKG is the small heat shock protein, HSP20. The purpose of this study was to determine if serotonin activates a Ca(2+)-dependent NOS in VSM. Strips of bovine carotid arterial smooth muscle denuded of endothelium were stimulated with serotonin in the presence and absence of the nonspecific NOS inhibitor N-monomethyl-L-arginine (L-NMMA). Activation of NOS was determined by increases in cGMP and in the phosphorylation of HSP20. Immunohistochemical and Western blotting techniques were performed to identify specific NOS isoforms in bovine carotid arterial smooth muscle preparations. Serotonin stimulation led to significant increases in cGMP and in the phosphorylation of HSP20, which were inhibited by pretreatment with L-NMMA. Antibodies against NOS 1 stained the media of bovine carotid and human renal arteries, whereas antibodies against NOS 3 stained only the endothelium. Additionally, the conversion of radiolabeled L-arginine to L-citrulline NOS activity demonstrated a consistent amount of activity present in the endothelium-denuded smooth muscle preparations that was reduced by 99% with an NOS 1 specific inhibitor. Finally, an NOS 1 specific inhibitor, 7-nitroindazole, augmented contractions induced by high extracellular KCl. This study demonstrates that NOS 1 is present in VSM and may effect physiological contractile responses.

Novel experimental strategies to prevent the development of type 1 diabetes mellitus

Type 1 diabetes is an autoimmune disease leading to extensive destruction of the pancreatic beta-cells. Our research focusses on the role of beta-cells during the course of the disease, aiming at finding novel strategies to enhance beta-cell resistance against the cytotoxic damage inflicted by the immune system. Special attention has been paid to the possibility that cytokines released by the immune cells infiltrating the pancreatic islets can directly suppress and kill beta-cells. Certain cytokines (interleukin-1beta, tumor necrosis factor-alpha and interferon-gamma) either alone or in combination, are able to activate signal transduction pathways in beta-cells leading to transcription factor activation and de novo gene expression. In this context, it has been found that induction of inducible nitric oxide synthase mediates an elevated production of nitric oxide, which impairs mitochondrial function and causes DNA damage eventually leading to apoptosis and necrosis. However, other induced proteins SUCH AS heat shock protein 70 and superoxide dismutase may reflect a defense reaction elicited in the beta-cells by the cytokines. Our strategy is to further seek for proteins involved in both destruction and protection of beta-cells. Based on this knowledge, we plan to apply gene therapeutic approaches to increase expression of protective genes in beta-cells. If this is feasible we will then evaluate the function and survival of such modified beta-cells in animal models of type 1 diabetes such as the NOD mouse. The long-term goal for this research line is to find novel approaches to influence beta-cell resistance in humans at risk of developing type 1 diabetes.

Formation of nitric oxide-derived oxidants by myeloperoxidase in monocytes: pathways for monocyte-mediated protein nitration and lipid peroxidation In vivo

Protein nitration and lipid peroxidation are implicated in the pathogenesis of atherosclerosis; however, neither the cellular mediators nor the reaction pathways for these events in vivo are established. In the present study, we examined the chemical pathways available to monocytes for generating reactive nitrogen species and explored their potential contribution to the protein nitration and lipid peroxidation of biological targets. Isolated human monocytes activated in media containing physiologically relevant levels of nitrite (NO(2)(-)), a major end product of nitric oxide ((*)NO) metabolism, nitrate apolipoprotein B-100 tyrosine residues and initiate LDL lipid peroxidation. LDL nitration (assessed by gas chromatography-mass spectrometry quantification of nitrotyrosine) and lipid peroxidation (assessed by high-performance liquid chromatography with online tandem mass spectrometric quantification of distinct products) required cell activation and NO(2)(-); occurred in the presence of metal chelators, superoxide dismutase (SOD), and scavengers of hypohalous acids; and was blocked by myeloperoxidase (MPO) inhibitors and catalase. Monocytes activated in the presence of the exogenous (*)NO generator PAPA NONOate (Z-[N-(3-aminopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2- diolate) promoted LDL protein nitration and lipid peroxidation by a combination of pathways. At low rates of (*)NO flux, both protein nitration and lipid peroxidation were inhibited by catalase and peroxidase inhibitors but not SOD, suggesting a role for MPO. As rates of (*)NO flux increased, both nitrotyrosine formation and 9-hydroxy-10,12-octadecadienoate/9-hydroperoxy-10,12-octadecadieno ic acid production by monocytes became insensitive to the presence of catalase or peroxidase inhibitors, but they were increasingly inhibited by SOD and methionine, suggesting a role for peroxynitrite. Collectively, these results demonstrate that monocytes use distinct mechanisms for generating (*)NO-derived oxidants, and they identify MPO as a source of nitrating intermediates in monocytes.

Potential functional significance of brain-type and muscle-type nitric oxide synthase I expressed in adventitia and media of rat aorta

Skeletal muscle and myocardium express microNOS I, an elongated splice variant of neuronal-type nitric oxide (NO) synthase (NOS I), and NOS III, endothelial-type NO synthase, respectively. This study was designed to elucidate whether vascular smooth muscle also contains a constitutively expressed NO synthase isoform. In the rat, microNOS I contains an insert of 102 nucleotides after nucleotide 2865 of the cDNA, yielding a protein of 164 kd. Reverse transcription-polymerase chain reaction with primers flanking this insert and with insert-specific primers indicated that endothelium-denuded rat aorta expresses both brain-type NOS I and microNOS I. RNase protection analyses with an antisense RNA probe overlapping the microNOS I insert detected significant amounts of NOS I mRNA and lesser amounts of microNOS I mRNA in endothelium-denuded aorta. Western blots using a specific polyclonal antibody recognizing NOS I and microNOS I showed a major band of the 160-kd NOS I and a lesser band of a slightly larger protein in endothelium-denuded aorta. Immunohistochemistry demonstrated low levels of NOS I/microNOS I immunoreactivity in the medial layer of rat aorta, whereas the endothelium expressed only NOS III immunoreactivity. When the adventitia also was removed, NOS I and microNOS I mRNA decreased markedly but remained detectable in the medial layer. In functional experiments with endothelium-denuded rat aortic rings (that contained no NOS III), contractions induced by KCl were markedly increased in the presence of the NOS inhibitor N(G)-nitro-L-arginine. These data demonstrate that 2 subforms of NOS I are expressed in nonendothelial components of rat aorta: NOS I and lesser amounts of microNOS I. Under certain conditions, this NOS I/microNOS I expression could serve as a backup system to the functionally predominant NOS III.

Nitric Oxide and the Th2 Response Combine to Prevent Severe Hepatic Damage During Schistosoma mansoni Infection

During infection with Schistosoma mansoni, NO production increases following the deposition of parasite eggs in the liver. In wild-type C57BL/6 mice, NO levels peak during the sixth week of infection and are subsequently down-regulated. Inducible NO synthase (iNOS) mRNA was found in diseased liver tissue along with TNF-α and IFN-γ, which are known promoters of iNOS expression. Mice treated with aminoguanidine, a selective inhibitor of iNOS, exhibited cachexia and exacerbated liver pathology, suggesting that NO limits hepatocyte damage when the liver is first exposed to eggs. Hepatic iNOS is up-regulated in SCID mice, indicating that NO production is part of an innate response. Studies with infected highly susceptible IL-4−/− mice revealed that prolonged NO production is in itself deleterious and that a major function of the Th2 response, which is severely compromised in the absence of IL-4, is to regulate NO production. In these animals, plasma NO levels are high compared with those in infected wild-type mice and remain elevated until death. Nevertheless, the underlying importance of NO is illustrated by the finding that aminoguanidine treatment leads to more severe liver disease and reduced time to death in infected IL-4−/− mice.

Leukemia cell differentiation: cellular and molecular interactions of retinoids and vitamin D

1. The conventional approach to treatment of acute myeloid leukemia has been the use of chemotherapy, which although being cytotoxic to malignant clones, is also cytodestructive to normal cells. In addition, some leukemia cells develop resistance to chemotherapy and are therefore difficult to eradicate. 2. Differentiation therapy, whereby immature cells are induced to attain a mature phenotype by differentiation agents, has provided an alternative strategy in the treatment of hyperproliferative disorders. This has been highlighted by the use of all-trans retinoic acid (ATRA) in the treatment of acute promyelocytic leukemia (APL). 3. Another differentiation agent, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), directs monocytic maturation of normal and leukemic cells. Cellular studies have revealed that combinations of vitamin D derivatives and retinoids such as ATRA and 9-cis retinoic acid (9-cis RA) exhibit cooperative effects on differentiation in established leukemia cell lines such as HL-60, U937, and NB4. Furthermore, vitamin D compounds, although not able to induce apoptosis when used alone, potentiate apoptosis induced by 9-cis RA in HL-60 cells and differentially regulate the expression of the apoptosis-related gene products bcl-2 and bax. The molecular mechanisms involved in regulating differentiation and apoptosis by these agents are mediated through the interactions of the nuclear receptors for vitamin D (VDR), ATRA (RAR), and 9-cis RA (RXR), which are able to form homo- or heterodimeric complexes and transcriptionally activate or repress target gene expression. 4. There is evidence to suggest that nitric oxide may also play a role in leukemic cell differentiation and that 1,25(OH)2D3 may influence endogenous nitric oxide production either by directly increasing tumor necrosis factor-alpha (TNF-alpha) or through a secondary mediator such as the C-type lectin CD23.

Effects of hyperoxia on nitric oxide synthase expression, nitric oxide activity, and lung injury in rat pups

Although hyperoxic exposure is an important contributor to the development of bronchopulmonary dysplasia and nitric oxide (NO) has been implicated in the pulmonary response to oxygen, the role of NO in mediating chronic neonatal lung injury is unclear. Therefore, rat pups were exposed to normoxia or hyperoxia (>95% O2) from d 21 to 29. After the rats were killed, their lungs were removed for analysis of nitric oxide synthase (NOS) expression, NO activity as measured by 3',5'-cyclic guanosine monophosphate (cGMP) assay, and lung pathology. Hyperoxia caused 5-fold and 2-fold increases in inducible (i) NOS and endothelial (e) NOS levels, respectively. NO activity was assessed by measuring cGMP levels after normoxic or hyperoxic exposure in the presence and absence of NOS blockade with either aminoguanidine (AG) or Nomega-nitro-L-arginine (L-NNA). cGMP levels were elevated in hyperoxic versus normoxic rats (287+/-15 versus 106+/-9 pmol/mg protein, respectively, p < 0.001), and this increase in cGMP was attenuated after NOS blockade with either AG or L-NNA. Hyperoxic exposure significantly increased lung/body weight ratios and induced histologic changes of interstitial and alveolar edema; however, these hyperoxia-induced histologic changes were not altered by NOS blockade with AG or L-NNA. We conclude that hyperoxic exposure of rat pups up-regulated both iNOS and eNOS and increased NO activity as measured by cGMP levels derived from both iNOS and eNOS. Blockade of NOS reduced cGMP levels in the hyperoxic rat pups; however, it did not seem to reverse the pathologic consequences of hyperoxic exposure.

Expressional down-regulation of neuronal-type nitric oxide synthase I by glucocorticoids in N1E-115 neuroblastoma cells

Neuronal-type nitric oxide synthase (NOS I) is involved in ischemia-induced brain damage, and glucocorticoids have been reported to protect from brain damage. This prompted us to investigate if the activity or expression of NOS I was influenced by glucocorticoids. We used the murine neuroblastoma cell line N1E-115 as our experimental model. Short-term incubation (30 min) of the N1E-115 cells with dexamethasone (10 nM to 1 microM) or hydrocortisone (100 nM to 10 microM) did not change the enzymatic activity of NOS I. However, the glucocorticoids inhibited NOS I mRNA expression in a concentration-dependent fashion (down to 53.3 +/- 2. 5% of control). In time-course experiments with 100 nM dexamethasone, maximum down-regulation of NOS I mRNA was seen after 24 hr (55.6 +/- 6.3% of control). Similar effects were seen with 10 microM hydrocortisone. The effect of 100 nM dexamethasone was completely reversed by 1 microM of the glucocorticoid receptor antagonist mifepristone. In experiments with actinomycin D (10 microg/ml), the half-life of the NOS I mRNA was determined to be approximately 12 hr and remained unchanged after glucocorticoid incubation. Nuclear run-on analyses indicated that the decrease in NOS I mRNA was the result of a glucocorticoid-induced inhibition of NOS I gene transcription. In Western blots, the 160-kDa NOS I protein band was down-regulated to 68.5 +/- 8.4% of control after an incubation of the N1E-115 cells with 100 nM dexamethasone for 26 hr. Similarly, NO production was down-regulated to 57.8 +/- 8.7% of control. These data demonstrate that glucocorticoids reduce the expression of NOS I without changing its activity.

Cross-tolerance between endogenous nitric oxide and exogenous nitric oxide donors

It is still unclear whether cross-tolerance develops between endogenously produced nitric oxide and exogenous nitric oxide donors. Thus, cGMP accumulation was determined in cultured aortic smooth muscle cells exposed to a nitric oxide source. Exposure of human, rat, rabbit, porcine or bovine smooth muscle cells to sodium nitroprusside led to a time- and concentration-dependent development of tolerance. In rat aortic smooth muscle cells, cross-tolerance developed between the sodium nitroprusside and S-nitroso-N-acetylpenicillamine, but not between sodium nitroprusside and atriopeptin. In addition, when rat aortic smooth muscle cells were treated with endotoxin or interleukin-1beta, they displayed lower sodium nitroprusside-induced cGMP accumulation as compared to control cells. When rat aortic smooth muscle cells were exposed to sodium nitroprusside for 12 h they displayed a decreased ability to accumulate cGMP in response to endothelium-derived nitric oxide released from bovine aortic endothelial cells. In addition, co-cultures of rat aortic smooth muscle cells with bovine aortic endothelial cells showed an L-nitroarginine methylester-sensitive decrease in sodium nitroprusside-induced cGMP accumulation compared to single rat aortic smooth muscle cell cultures. We conclude that cross-tolerance between endothelium-derived nitric oxide and exogenously applied nitric oxide donors occurs in vitro.

Functional coupling of nitric oxide synthase and soluble guanylyl cyclase in controlling catecholamine secretion from bovine chromaffin cells

This study was designed to evaluate whether the enzymes of the nitric oxide/cyclic-GMP pathway, nitric oxide synthase and soluble guanylyl cyclase, are functionally coupled in controlling catecholamine secretion in primary cultures of bovine chromaffin cells. In immunocytochemical studies, 80-85% of the tyrosine hydroxylase-positive chromaffin cells also possessed phenylethanolamine-N-methyltransferase, f1p4cating their capability to synthesize epinephrine. Immunoreactivity for neuronal-type nitric oxide synthase was found in over 90% of all chromaffin cells. Reverse transcription-polymerase chain reaction also demonstrated neuronal-type nitric oxide synthase messenger RNA. Immunoreactivity for soluble guanylyl cyclase was detectable in over 95% of chromaffin cells. Double-labeling immunofluorescence studies co-localized neuronal-type nitric oxide synthase and soluble guanylyl cyclase with tyrosine hydroxylase and phenylethanolamine-N-methyltransferase in the majority of chromaffin cells. Chromaffin cells possessed basal nitric oxide synthase activity which could be stimulated by acetylcholine and inhibited by NG-nitro-L-arginine methyl ester. Activation of soluble guanylyl cyclase by endogenously synthesized nitric oxide or the nitric oxide donor compound sodium nitroprusside was blocked by the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Catecholamine release and the increase in cytosolic Ca2+ concentration evoked by acetylcholine were enhanced by inhibitors of the endogenous nitric oxide/cyclic-GMP pathway such as NG-nitro-L-arginine methyl ester, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and the protein kinase G inhibitor Rp-8-pCPT-cGMPS. These data indicate that chromaffin cells possess an autocrine nitric oxide/cyclic-GMP pathway tonically controlling the inhibition of catecholamine release.

Immunohistochemical localization of nitric oxide synthase and the oxidant peroxynitrite in lung transplant recipients with obliterative bronchiolitis

BACKGROUND

Obliterative bronchiolitis (OB) is a disease affecting a large percentage of lung and heart-lung transplant recipients. Histologically, the disease is characterized by inflammation, cellular proliferation, and obliteration of terminal airways.

METHODS

We investigated the production of inducible and constitutive nitric oxide synthases and peroxynitrite by immunohistochemistry in the lungs of control subjects (n=14) compared with those of transplant recipients with OB (n=8).

RESULTS

Strong immunoreactivity for inducible nitric oxide synthase and nitrotyrosine, a marker of protein nitration by peroxynitrite, was seen in inflammatory cells, airway epithelium, and vascular endothelium of patients with OB, compared with little immunoreactivity in control lungs. Immunoreactivity for constitutive nitric oxide synthase was abundant in the airway epithelium and vascular endothelium of control lungs, however, it was decreased in airway epithelial cells and arterial endothelial cells of patients with OB.

CONCLUSIONS

We conclude that increased formation of the potent oxidant peroxynitrite and decreased production of endothelial nitric oxide may contribute to the functional and morphological abnormalities of OB.

Nitric oxide increases tumor necrosis factor production in differentiated U937 cells by decreasing cyclic AMP

Nitric oxide (NO) increases tumor necrosis factor (TNF) synthesis in human peripheral blood mononuclear cells by a cGMP-independent mechanism. NO has been shown to inhibit adenylate cyclase in cell membranes. Since cAMP down-regulates TNF transcription, we examined the possibility that NO enhances TNF synthesis by decreasing cAMP. U937 cells were induced to differentiate using phorbol myristate acetate (100 nM for 48 h) and then were incubated for 24 h with sodium nitroprusside (SNP) or S-nitroso-N-acetylpenicillamine (SNAP). These NO donors increased TNF production (7.0- and 15.6-fold, respectively, at 500 microM) in a dose-dependent manner (p = 0.002). However, SNP and SNAP did not elevate cGMP levels in U937 cell cultures, and the cGMP analog, 8-bromo-cGMP, had no effect on TNF production. In contrast, SNP (p = 0.001) and SNAP (p = 0.009) decreased intracellular cAMP levels by up to 51.5% over 24 h and, in the presence of a phosphodiesterase inhibitor, blunted isoproterenol-stimulated increases in cAMP by 21.8% (p = 0.004) and 27.6% (p = 0.008), respectively. H89, an inhibitor of cAMP-dependent protein kinase, dose dependently increased TNF production in phorbol myristate acetate-differentiated U937 cells in the absence (6.5-fold at 30 microM; p = 0.035), but not in the presence (p = 0.77) of SNAP. Conversely, the cAMP analog dibutyryl cAMP (Bt2cAMP) blocked SNAP-induced TNF production (p = 0.001). SNP and SNAP (500 microM) increased relative TNF mRNA levels by 57.5% (p = 0.045) and 66.2% (p = 0.001), respectively. This effect was prevented by Bt2cAMP. These results indicate that NO up-regulates TNF production by decreasing intracellular cAMP.

Nitric oxide regulates cell proliferation during Drosophila development

Cell division and subsequent programmed cell death in imaginal discs of Drosophila larvae determine the final size of organs and structures of the adult fly. We show here that nitric oxide (NO) is involved in controlling the size of body structures during Drosophila development. We have found that NO synthase (NOS) is expressed at high levels in developing imaginal discs. Inhibition of NOS in larvae causes hypertrophy of organs and their segments in adult flies, whereas ectopic expression of NOS in larvae has the opposite effect. Blocking apoptosis in eye imaginal discs unmasks surplus cell proliferation and results in an increase in the number of ommatidia and component cells of individual ommatidia. These results argue that NO acts as an antiproliferative agent during Drosophila development, controlling the balance between cell proliferation and cell differentiation.