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

Endothelial cell injury caused by Candida albicans is dependent on iron

Although it is known that Candida albicans causes endothelial cell injury, in vitro and in vivo, the mechanism by which this process occurs remains unknown. Iron is critical for the induction of injury in many types of host cells. Therefore, we investigated the role of iron in Candida-induced endothelial cell injury. We found that pretreatment of endothelial cells with the iron chelators phenanthroline and deferoxamine protected them from candidal injury, even though the organisms germinated and grew normally. Loading endothelial cells with iron reversed the cytoprotective effects of iron chelation. Moreover, chelation of endothelial cell iron significantly reduced phagocytosis of C. albicans by these cells, while candidal adherence to chelator-treated endothelial cells was slightly enhanced. Since endothelial cell phagocytosis of C. albicans is required for endothelial cell injury to occur, inhibition of phagocytosis is likely the principal mechanism of the cytoprotective effects of iron chelation. The production of toxic reactive oxygen intermediates by host cells is known to be inhibited by iron chelation. Therefore, we investigated whether treating endothelial cells with antioxidants could mimic the cytoprotective effects of iron chelation. Neither extracellular nor membrane-permeative antioxidants reduced candidal injury of endothelial cells. Furthermore, depleting endothelial cells of the endogenous antioxidant glutathione did not render them more susceptible to damage by C. albicans. These results suggest that candidal injury of endothelial cells is independent of the production of reactive oxygen intermediates and that the cytoprotective effects of iron chelation are not due to inhibition of the synthesis of these toxic intermediates.

Nitric oxide production contributes to the angiogenic properties of vascular endothelial growth factor in human endothelial cells

Vascular endothelial growth factor (VEGF) is a regulator of vasculogenesis and angiogenesis. To investigate the role of nitric oxide (NO) in VEGF-induced proliferation and in vitro angiogenesis, human umbilical vein endothelial cells (HUVEC) were used. VEGF stimulated the growth of HUVEC in an NO-dependent manner. In addition, VEGF promoted the NO-dependent formation of network-like structures in HUVEC cultured in three dimensional (3D) collagen gels. Exposure of cells to VEGF led to a concentration-dependent increase in cGMP levels, an indicator of NO production, that was inhibited by nitro-L-arginine methyl ester. VEGF-stimulated NO production required activation of tyrosine kinases and increases in intracellular calcium, since tyrosine kinase inhibitors and calcium chelators attenuated VEGF-induced NO release. Moreover, two chemically distinct phosphoinositide 3 kinase (PI-3K) inhibitors attenuated NO release after VEGF stimulation. In addition, HUVEC incubated with VEGF for 24 h showed an increase in the amount of endothelial NO synthase (eNOS) protein and the release of NO. In summary, both short- and long-term exposure of human EC to VEGF stimulates the release of biologically active NO. While long-term exposure increases eNOS protein levels, short-term stimulation with VEGF promotes NO release through mechanisms involving tyrosine and PI-3K kinases, suggesting that NO mediates aspects of VEGF signaling required for EC proliferation and organization in vitro.

Tetrahydrobiopterin modulates cyclooxygenase-2 expression in human mesangial cells

Tetrahydrobiopterin (BH4) biosynthetic pathways are stimulated under inflammatory conditions. The newly synthesized BH4 serves as a cofactor for optimal activity of inducible nitric oxide synthase (NOS2). In human mesangial cells (HMC), BH4 is also a limiting factor for NOS2 expression. In this study we show that BH4 availability can also play a modulatory role in the expression of cyclooxygenase 2 (COX-2) in HMC. Supplementing HMC with the BH4 donor sepiapterin potentiated IL-1beta/TNF-alpha-induced COX-2 expression by approximately 2-fold. This effect was abolished by methotrexate. In contrast, the NOS inhibitor L-NAME and the soluble guanylate cyclase inhibitor ODQ did not block sepiapterin amplification of COX-2 expression. Moreover, sepiapterin was found to modulate the tyrosine phosphorylation of several cellular substrates, an early event which occurred well before the induction of NOS2 could be evidenced. These findings suggest a role for BH4 in the modulation of mesangial cell responses to pro-inflammatory stimuli.

Inhalation of the nitric oxide synthase cofactor tetrahydrobiopterin in healthy volunteers

Pulmonary endothelial dysfunction is the hallmark of acute lung injury. Impaired pulmonary endothelial nitric oxide (NO) production in this event has been described. Tetrahydrobiopterin (BH4) is an essential cofactor for NO synthase and modulator of its activity. At high local concentrations, BH4 provokes local vasodilation in vivo in healthy individuals. At lower concentrations, BH4 selectively and locally restores disturbed NO-dependent vasodilation in patients with endothelial dysfunction. In this preliminary study, we therefore investigated the feasibility of BH4 inhalation in five healthy human volunteers. Inhalation of buffered, aqueous BH4-dihydrochloride solution was well tolerated; despite the buffer, BH4 stability was completely preserved. Resorption of inhaled BH4 was demonstrated by significantly increased BH4 levels in plasma and urine. Inhaled BH4 did not alter pulmonary function and had no effect on systemic hemodynamic values. Our data demonstrate that inhalation is a novel method for local BH4 administration, offering a basic therapeutic tool for investigation of restoration of impaired NO-dependent vasodilation due to pulmonary endothelial dysfunction.

Endothelial cell monolayers as a tool for studying microvascular pathophysiology

Endothelial cells contribute to a variety of biological responses that facilitate organ function. This critical role of the endothelial cell has resulted in the development of different in vitro models that utilize monolayers of cultured cells to simulate conditions that exist in the intact animal. This review focuses on endothelial cell monolayers as a model system for research on certain pathophysiological conditions affecting the gastrointestinal tract. The advantages and limitations of endothelial cell monolayers are addressed, along with evolving technologies and strategies that hold promise for extending the utility of this in vitro model for studies of gastrointestinal function and disease.

Cationic amino acid transporter-2 mRNA induction by tumor necrosis factor-alpha in vascular endothelial cells

Nitric oxide (NO) synthesis may be coupled to the activity of the cellular L-arginine transporter, namely the cationic amino acid transporter. The present study examined tumor necrosis factor (TNF)-alpha-induced alterations in the gene expression of the cationic amino acid transporter (CAT) and NO production in human umbilical vein endothelial cells. In quiescent endothelial cells, CAT-1 mRNA expression, determined by reverse transcription-polymerase chain reaction, was dominant to that of CAT-2. TNF-alpha (10 ng/ml for 1-24 h) induced a time-dependent increase in CAT-2 but not CAT-1 expression. Moreover, TNF-alpha (1-30 ng/ml) treatment for 6 h induced a concentration-dependent increase in CAT-2 mRNA expression. The upregulation of CAT-2 expression by TNF-alpha was associated with enhanced nitrite accumulation in the culture medium (70% increase compared with vehicle-treated cells at 24 h). Thus, induction of the cationic amino acid transporter may constitute one mechanism for the TNF-alpha-induced NO production in human umbilical vein endothelial cells.

Role of nitric oxide in poly(I-C)-induced endothelial cell expression of leukocyte adhesion molecules

Polyinosinic-polycytidylic acid [poly(I-C)] is a synthetic double-stranded RNA (dsRNA) that simulates a viral-infected state in cells. It has been shown that viral infection, as well as poly(I-C), stimulates leukocyte adhesion to endothelial cell (EC) monolayers and that this is mediated through the surface expression of the adhesion molecules E-selectin, vascular cell adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1. We have tested the involvement of nitric oxide (NO) in poly(I-C)-induced monocytic cell adhesion to human vascular EC. Using primary cultured EC for these studies, we confirmed the results from previous reports that these cells have higher basal levels of NO production than passaged cells. Poly(I-C)-induced monocytic cell adhesion to primary EC was concentration-dependently inhibited by 40-74% by the nitric oxide synthase (NOS) inhibitor NG-methyl-L-arginine (L-NMA), as well as three other NOS inhibitors, without significantly affecting interleukin-1 beta-induced adhesion. L-NMA inhibited poly(I-C)-induced surface expression of E-selectin and VCAM-1 by 25 and 45%, respectively, and mRNA levels of E-selectin and VCAM-1 by 62 and 74%, respectively. Primary EC transiently transfected with a plasmid containing an E-selectin promoter-driven luciferase reporter gene showed that L-NMA treatment reduced poly(I-C)-induced E-selectin promoter activity to basal levels. Electrophoretic mobility shift analysis indicated that poly(I-C)-induced nuclear factor-kappa B (NF-kappa B) binding to a radiolabeled oligonucleotide corresponding to the consensus NF-kappa B binding domain of the E-selectin promoter was decreased by L-NMA pretreatment. Hence, NO appears to augment E-selectin gene expression in response to poly(I-C) at the transcriptional level in vascular EC. Collectively, these data support the hypothesis that NO augments poly(I-C)-induced EC activation. These data suggest a novel role for NO as a response mediator in dsRNA-induced leukocyte adhesion to EC.

Expression and function of recombinant endothelial NO synthase in coronary artery smooth muscle cells

Smooth muscle cells (SMCs) play a key role in the pathogenesis of vascular diseases. The objectives of this study were to determine whether transfer of recombinant endothelial nitric oxide synthase (eNOS) gene to porcine coronary artery smooth muscle cell (CSMCs) would result in expression of a functional enzyme and to assess the effect of expression of eNOS on cell proliferation. CSMCs were transduced in vitro with adenoviral vectors encoding cDNA for eNOS (AdeNOS) and beta-galactosidase (Ad beta Gal). In contrast to Ad beta Gal- or sham-transduced cells, CSMCs transduced with AdeNOS stained positive with the NADPH-diaphorase stain, acquired calcium-dependent NOS activity (measured by the conversion of [3H]L-arginine to [3H]L-citrulline), had increasing cyclic 3',5' cGMP levels with increasing concentrations of the vector, and produced increased amounts of nitrite. cGMP production by AdeNOS-transduced cells was augmented by increasing intracellular levels of the eNOS cofactor tetrahydrobiopterin. CSMCs transduced with AdeNOS showed diminished serum-stimulated DNA synthesis as measured by thymidine uptake. Cell proliferation was diminished in AdeNOS-transduced CSMCs as assessed by cell counts 3 and 6 days after serum stimulation of quiescent CSMCs. The present study demonstrates that adenovirus-mediated gene transfer of eNOS to CSMCs results in the expression of a functional enzyme whose activity can be augmented by increasing intracellular levels of tetrahydrobiopterin. Expression of recombinant eNOS in CSMCs results in inhibition of serum-stimulated DNA synthesis and cell proliferation. These findings imply that eNOS gene transfer to SMCs may be a unique mode of increasing local NO production in the arterial wall.

Gene expression of natriuretic peptide receptors in rats with DOCA-salt hypertension

In our previous studies, we found that the atrial natriuretic peptide (ANP) binding and guanylyl cyclase activity of A-type natriuretic peptide receptors (NPR-A) were upregulated in renal papillae but downregulated in vascular tissues and glomeruli of rats with deoxycorticosterone acetate (DOCA)-salt hypertension [E. Nuglozeh, G. Gauquelin, R. Garcia, J. Tremblay, and E. L. Schiffrin. Am. J. Physiol. 259 (Renal Fluid Electrolyte Physiol. 28): F130-F137, 1990]. To further understand the molecular significance of these regulations, we measured the relative abundance of the transcripts of NPR-A and NPR-B by Northern blot in the aorta, mesenteric arteries, adrenal cortex, renal papillae, and lungs in DOCA-salt hypertensive and control rats. In renal papillae we also examined the translation and transcription of NPR-A by ribosome loading and run-on assay. Compared with controls, the steady-state levels of mRNA for NPR-A were increased in the aorta and mesenteric arteries but were decreased in the adrenal cortex and renal papillae in DOCA-salt-treated rats. NPR-B mRNA was decreased in the aorta, mesenteric arteries, and adrenal cortex in hypertensive rats. In lungs the mRNA for both receptors was unchanged. Translation of NPR-A mRNA, as assessed by ribosome loading, was reduced in renal papillae. Transcriptional activity of its gene was not detectable in these tissues. Guanosine 3',5'-cyclic monophosphate levels generated by NPR-A in renal papillae and by NPR-A and NPR-B in the adrenal cortex, aorta, and mesenteric arteries of DOCA-salt-treated rats remained increased in hypertension. The higher NPR-A activity in the presence of a lower level of its mRNA in renal papillae and the higher NPR-B activity in the presence of a lower level of its mRNA in the vasculature, adrenal cortex, and lungs can alternatively be explained by receptor stabilization or increased receptor recycling.

GTP cyclohydrolase I mRNA induction and tetrahydrobiopterin synthesis in human endothelial cells

The key role of tetrahydrobiopterin (BH4) in the synthesis of nitric oxide by human umbilical vein endothelial cells (HUVEC) has been demonstrated. We characterized the induction of BH4 synthesis in a cell line (ECV) derived from HUVEC and primary HUVEC. A significant induction of guanosine triphosphate cyclohydrolase I (GTPCH) mRNA was observed in response to TNF, IL-1beta, and IFNgamma in ECV and HUVEC. The induction of GTPCH mRNA was abolished by actinomycin D. The cytokines led to an increased accumulation of BH4 in ECV. This effect was prevented by 2,4-diamino-6-hydroxypyrimidine, a selective inhibitor of GTPCH, as well as by actinomycin D and by cycloheximide. Results provide evidence for an increase in GTPCH activity and in BH4 levels in response to immunostimulants in human endothelial cells.

Effect of ciprofloxacin on the accumulation of interleukin-6, interleukin-8, and nitrite from a human endothelial cell model of sepsis

OBJECTIVE

To determine the effect of the quinolone antibiotic ciprofloxacin, on interleukin-6, interleukin-8, and nitrite production by human endothelial cells.

DESIGN

Controlled cell culture experiments examining the immunomodulatory effects of an antibiotic.

SETTING

University research laboratory attached to a large teaching hospital.

SUBJECTS

A human endothelial cell line.

MEASUREMENTS AND MAIN RESULTS

Cells were incubated with tumor necrosis factor-alpha and interleukin-1beta in the presence of a range of ciprofloxacin concentrations. Interleukin-6, interleukin-8, and nitrite concentrations were measured in culture supernatants after 24 hrs using enzyme immunoassay. Ciprofloxacin decreased interleukin-6 accumulation (p = .001). Interleukin-8 was decreased at lower ciprofloxacin concentrations (p = .017) but was increased at 100 microg/mL (p = .0039). Ciprofloxacin had no effect on nitrite accumulation (p = .38).

CONCLUSIONS

Ciprofloxacin differentially modulates interleukin-6 and interleukin-8 expression. The response to infection is coordinated by a cascade of cytokines and other mediators. The widespread use of ciprofloxacin in patients with severe infections is likely to result in alterations in local concentrations of cytokines. Selective control of cytokine concentrations by antibacterial agents will clearly have important therapeutic implications and may be a future research consideration in antibiotic drug design.

Glomerular and vascular tissues do not down-regulate nitric oxide synthesis during protracted endotoxemia

Down-regulation of cytokines is implicated as an important component of the phenomenon of tolerance to bacterial products in humans and animals. Since many effects of endotoxin and cytokines are mediated by nitric oxide, this study was designed to evaluate in vivo the L-arginine:NO pathway in endotoxin tolerance. Sublethal injections of E. coli lipopolysaccharide (LPS, 1 mg/kg body wt, i.p.) were given to rats daily for five days. Blood levels of NO2-/NO3-, stable metabolites of nitric oxide (NO), significantly increased on day 1 (baseline, 89.64 +/- 40, day 1, 260.32 +/- 36 nmol/ml; P < 0.05) but returned to baseline levels on day 5 (77.60 +/- 5 nmol/ml). However, urinary NO2-/NO3- remained significantly elevated several-fold throughout the study period (baseline, 121.25 +/- 11.4, day 1, 899.35 +/- 15.8, day 5, 250.23 +/- 21.4 nmol/hr/100 g body wt). Glomeruli and aortae obtained from these rats showed increased NO production that was maintained at similar levels even at day 5 (glomeruli: control, 0.01 +/- 0.0, day 1, 22.4 +/- 0.3, and day 5, 22.0 +/- 2.5, P < 0.05 vs. control; aortae; control, 0.01 +/- 0.0, day 1, 4.3 +/- 2.2, and day 5, 5.4 +/- 1.0 nmol/hr/mg protein, P < 0.05 vs. control, respectively); this further increased significantly in response to in vitro LPS challenge. However, peritoneal macrophages, liver and spleen showed an initial increase in NO production that decreased significantly by the fifth day of LPS and could not be further stimulated by in vitro LPS challenge. Thus, in vivo NO synthesis is down-regulated during protracted LPS. Our results show that the process is relatively specific to the liver, spleen and macrophages, and is qualitatively and quantitatively different in vascular tissues such as aortae and glomeruli.

Characterization of a constitutive type III nitric oxide synthase in human U937 monocytic cells: stimulation by soluble CD23

The soluble cleavage fragment of the low-affinity immunoglobulin E (IgE) receptor/CD23 (sCD23 25000 MW) and antibodies directed against their receptors on monocytes, CD11b and CD11c, stimulate the production of nitric oxide (NO) by these cells and we have suggested that the enzyme involved could be related to the endothelial constitutive type III nitric oxide synthase (ecNOS). In the present work, we have analysed the characteristic properties of this NOS isoform in the model of the human promonocytic cells U937 By reverse-transcription polymerase chain reaction (RT-PCR), the presence of an mRNA coding for type III NOS was found in U937 cells and the corresponding protein was detected by immunofluorescence in permeabilized cells with a specific anti-ecNOS monoclonal antibody (mAb). Membrane extracts displayed a NOS activity dependent on the presence of calcium and calmodulin in the reaction medium and that was abrogated in the presence of EGTA. Recombinant soluble CD23 (25000 MW) was found to trigger an NO-dependent cGMP accumulation in these cells, which was abrogated by calcium chelators and inhibitors of the calcium/calmodulin complex. Moreover, sCD23 elicited a transient augmentation of intracytoplasmic free calcium concentration [Ca2+]i that was dependent on the presence of calcium in the external buffer and was prevented in the presence of EGTA, indicating that it was due to a calcium influx. In conclusion, human promonocytic cells such as U937 exhibit a functional type III NOS that can be stimulated by calcium-raising agents, such as sCD23.

Role of vascular nitric oxide in physiological and pathological conditions

This review describes the ability of certain diseases, such as essential hypertension, atherosclerosis, angina, and vasospasm, to reduce vascular nitric oxide (NO) formation or to increase its metabolism. In contrast, others, such as hypotension, sepsis, stroke, myocardial depression, and inflammatory responses, increase NO synthesis. The mechanism implicated in the changes in the formation and metabolism of NO are described. To prevent or treat these pathological processes, in which a deficiency in vascular NO formation plays a causative role, NO may be provided through methods such as direct NO administration or indirect NO supply through either NO donors or L-arginine, which facilitates NO formation.

Tetrahydrobiopterin, nitric oxide and regulation of cerebral arterial tone

Tetrahydrobiopterin is an essential cofactor required for activity of nitric oxide synthases. Existing evidence suggests that, during activation of constitutive and inducible isoforms of nitric oxide synthase, tetrahydrobiopterin is needed for allosteric and redox activation of enzymatic activity. However, precise mechanisms underlying the role of tetrahydrobiopterin in regulation of nitric oxide formation is not fully understood. In cerebral and peripheral arteries, increased availability of tetrahydrobiopterin can augment production of nitric oxide. In contrast, in arteries depleted of tetrahydrobiopterin, production of nitric oxide is impaired. Proinflammatory cytokines enhance mRNA expression of the rate-limiting enzyme of tetrahydrobiopterin biosynthesis, GTP cyclohydrolase I and stimulate production of tetrahydrobiopterin. The ability of vascular tissues to synthesize tetrahydrobiopterin plays an important role in regulation of nitric oxide synthase under physiological conditions as well as during inflammation and sepsis. More recent studies concerning expression and function of recombinant nitric oxide synthases suggest that availability of tetrahydrobiopterin is important for production of nitric oxide in genetically engineered blood vessels. In this review, mechanisms regulating availability of intracellular tetrahydrobiopterin and its role in control of vascular tone under physiological and pathological conditions will be discussed.

Quantitative measurement for endothelial constitutive nitric oxide synthase in cultured human endothelial cells

Constitutively expressed endothelial nitric oxide synthase (ecNOS) produces nitric oxide (NO) from L-arginine and is important for the maintenance of cardiovascular homeostasis. We report the development of a capture ELISA which is specific for ecNOS. The assay detection limit is 0.5 ng/ml ecNOS protein, allowing the measurement of ecNOS from as few as 6000 human endothelial cells cultured in 96-well microtiter plates. This ELISA has been used to measure a downregulation of ecNOS with 24-h TNFalpha treatment, consistent with results obtained by Western blot analysis. Quantitation of ecNOS in human endothelial cells showed a higher expression of ecNOS in human aortic endothelial cells (18.3+/-1.35 ng ecNOS per 10(6) cells, n = 3 donors) than in human umbilical vein endothelial cells (10.4+/-0.48 ng ecNOS per 10(6) cells, n = 3 donors). These studies demonstrate that this convenient, quantitative assay is currently the most sensitive method for investigating ecNOS protein regulation.

Tetrahydrobiopterin synthesis precedes nitric oxide-dependent inhibition of insulin secretion in INS-1 rat pancreatic beta-cells

Interleukin 1 (IL-1) induces pancreatic beta-cell dysfunction mainly due to overproduction of nitric oxide (NO). Since tetrahydrobiopterin (BH4) is a obligatory cofactor of NO synthases, we examined the temporal relationship of BH4 synthesis, NO production and insulin secretion in a pancreatic beta-cell line (INS-1) which was exposed to IL-1. IL-1 affected BH4 synthesis in a time- and concentration-dependent manner. At a concentration of 10 ng/ml IL-1 caused an increase in intracellular BH4 with peak levels being observed at 6 hours followed by a steady decline in the cellular BH4 content. The increase in BH4 synthesis was followed by enhanced NO production and, consecutively, inhibition of insulin secretion. The concentration-dependent regulation of BH4 synthesis, NO production and suppression of insulin secretion indicate a functional link between these parameters in pancreatic beta-cells.

Differential induction of constitutive and inducible nitric oxide synthases by distinct inflammatory stimuli in bovine aortic endothelial cells

Exposure to various combinations of cytokines and lipopolysaccharide (LPS) has been reported to increase NO production in vascular endothelial cells. The molecular entity of the newly expressed nitric oxide synthase (NOS) in endothelial cells, however, has not yet been examined in detail. In this report, we carried out biochemical characterizations and molecular identification of NOS isoform(s) expressed in cytokine/LPS-treated bovine aortic endothelial cells (BAEC). The increased NOS activity in tumor necrosis factor-alpha (TNF-alpha)/LPS-treated BAEC was localized mainly in the cytosolic fraction and Ca2+-independent, whereas that in interferon-alpha,beta(IFN-alpha,beta)/LPS-treated BAEC was preferentially in the membrane fraction and Ca2+-dependent, suggesting that TNF-alpha/LPS increased an inducible NOS (iNOS)-like activity, and IFN-alpha,beta/LPS increased an endothelial constitutive NOS (ecNOS)-like activity. Correspondingly, the different responses to the cytokine/LPS pretreatment were demonstrated in semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) using primers specific for iNOS or ecNOS, that is, TNF-alpha/LPS elicited the expression of iNOS mRNA whereas IFN-alpha,beta/LPS increased that of ecNOS mRNA. A nuclear run-on transcription assay and an inhibition experiment by actinomycin D indicated that the apparent increase of ecNOS in the IFN-alpha,beta/LPS-treated BAEC was at least in part ascribed to the transcriptional activation. The nucleotide sequences of the amplified PCR products in TNF-alpha/LPS- and IFN-alpha,beta/LPS-treated BAEC were 93% and 99% identical to the corresponding regions of human hepatocyte iNOS and bovine ecNOS, respectively. These findings indicated that, in cytokine/LPS-treated BAEC, two NOS isoforms whose molecular natures were closely homologous to the conventional isoforms of iNOS and ecNOS were differently induced in response to distinct inflammatory stimuli.

Cell dysfunction in atherosclerosis and the ischemic manifestations of coronary artery disease

Many of the cellular mechanisms and dysfunctions that underlie atherosclerotic plaque formation have been identified, including adverse interactions between atherogenic lipids and the arterial endothelium, loss of endothelium-dependent dilation, accumulation of inflammatory cells and mediators of inflammation in the intima of the arteries, and a decline in anticoagulant defenses. Several studies have shown that these mechanisms, which appear to be active throughout the pathogenesis and progression of atherosclerosis, are reversible within days, weeks, or months with effective lipid-lowering therapy. In addition, the findings of large-scale trials of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors suggest that the rapid improvement observed in trial participants is attributable to a reversal of endothelial and vascular wall dysfunctions rather than to a reduction in plaque size. The accumulated evidence indicates that improved endothelial function can benefit patients who have angina pectoris and/or are at risk for myocardial infarction. Current understanding of the cellular mechanisms of atherogenesis also suggests avenues of future research to refine treatment approaches and further improve outcomes for patients with coronary artery disease.

Sphingosine mediates the immediate negative inotropic effects of tumor necrosis factor-alpha in the adult mammalian cardiac myocyte

To determine whether activation of the neutral sphingomyelinase pathway was responsible for the immediate (<30 min) negative inotropic effects of tumor necrosis factor-alpha (TNF-alpha), we examined sphingosine levels in diluent and TNF-alpha-stimulated cardiac myocytes. TNF-alpha stimulation of adult feline cardiac myocytes provoked a rapid (<15 min) increase in the hydrolysis of [14C]sphingomyelin in cell-free extracts, as well as an increase in ceramide mass, consistent with cytokine-induced activation of the neutral sphingomyelinase pathway. High performance liquid chromatographic analysis of lipid extracts from TNF-alpha-stimulated cardiac myocytes showed that TNF-alpha stimulation produced a rapid (<30 min) increase in free sphingosine levels. Moreover, exogenous D-sphingosine mimicked the effects of TNF-alpha on intracellular calcium homeostasis, as well as the negative inotropic effects of TNF-alpha in isolated contracting myocytes; time course studies showed that exogenous D-sphingosine produced abnormalities in cell shortening that were maximal at 5 min. Finally, blocking sphingosine production using an inhibitor of ceramidase, n-oleoylethanolamine, completely abrogated the negative inotropic effects of TNF-alpha in isolated contracting cardiac myocytes. Additional studies employing biologically active ceramide analogs and sphingosine 1-phosphate suggested that neither the immediate precursor of sphingosine nor the immediate metabolite of sphingosine, respectively, were likely to be responsible for the immediate negative inotropic effects of TNF-alpha. Thus, these studies suggest that sphingosine mediates the immediate negative inotropic effects of TNF-alpha in isolated cardiac myocytes.

In situ characterization of inflammatory responses in the rectal mucosae of patients with shigellosis

Shigella species cause bacillary dysentery in humans by invading epithelial cells of the colonic mucosa leading to colonic epithelial cell destruction and inflammation. For further analysis of local gut inflammation, morphological changes and the potential involvement of mediators in regulatory mechanisms of cell activation and cell proliferation were studied immunohistochemically in rectal mucosal biopsies taken from patients during the acute phase of shigellosis and at convalescence. Rectal biopsies from 25 Shigella dysenteriae-1 and 10 Shigella flexneri-infected patients and from 40 controls were studied. The frequencies of proliferative cells (Ki67-positive cells), p53-immunostaining cells, and cells coexpressing Ki67 with CD3 or with p53 were analyzed. Immunostaining for the inducible nitric oxide synthase (iNOS) and the endothelial NOS was assessed. In addition, the frequencies of apoptotic cells and CD68+ cells that engulf apoptotic cells were assessed. By morphological grading, 20% of the patients had advanced inflammation (grade 3) in the acute phase; mild inflammation (grade 1) was seen in 37% of the patients at convalescence as well as in 10% of the controls. The findings in the present study suggest that in the acute phase of shigellosis inflammation is characterized by increased cell turnover in the lamina propria (LP) and the epithelium, increased iNOS expression in the surface epithelium, and apoptosis, which seems to be associated with LP macrophages. The findings also suggest that neither p53 nor iNOS are important factors for the induction of apoptosis in shigellosis. Expression of p53 may be related to early cell activation in crypt epithelium. Moreover, there is an indication of an active, low-level inflammatory process at convalescence. The results thus indicate that Shigella-induced inflammation is associated with a complex series of cellular reactions in the rectal gut mucosa which persist long after clinical symptoms have resolved.

Immunohistochemical localization of nitric oxide synthase in normal human skin: expression of endothelial-type and inducible-type nitric oxide synthase in keratinocytes

Nitric oxide (NO) is a critical mediator of various biological functions. NO is generated from L-arginine by nitric oxide synthase (NOS), which has three isoforms; endothelial-type NOS (eNOS) and brain-type NOS (bNOS) are constitutive enzymes, and inducible-type NOS (iNOS) is expressed after stimulation. We investigated the expression of NOS in normal human skin by an immunohistochemical technique and western blotting analysis. In human skin, epidermal keratinocytes and the outer root sheath were labeled with not only eNOS antibody but also with iNOS antibody. Both eNOS and iNOS protein in epidermal keratinocytes were confirmed by western blotting. eNOS immunoreactivity was observed in endothelial cells, fibroblasts, the arrector pili muscle, apocrine secretory gland, eccrine coiled duct, and eccrine secretory gland. bNOS immunoreactivity was observed in mast cells. No staining with anti-bNOS antibody was observed in any other cell type. Our present findings suggest that epidermal keratinocytes in normal human skin contain both eNOS and iNOS.

Tetrahydrobiopterin restores endothelial function in hypercholesterolemia

In hypercholesterolemia, impaired nitric oxide activity has been associated with increased nitric oxide degradation by oxygen radicals. Deficiency of tetrahydrobiopterin, an essential cofactor of nitric oxide synthase, causes both impaired nitric oxide activity and increased oxygen radical formation. In this study we tested whether tetrahydrobiopterin deficiency contributes to the decreased nitric oxide activity observed in hypercholesterolemic patients. Therefore, L-mono-methyl-arginine to inhibit basal nitric oxide activity, serotonin to stimulate nitric oxide activity, and nitroprusside as endothelium-independent vasodilator were infused in the brachial artery of 13 patients with familial hypercholesterolemia and 13 matched controls. The infusions were repeated during coinfusion of L-arginine (200 microg/kg/min), tetrahydrobiopterin (500 microg/min), or the combination of both compounds. Forearm vasomotion was assessed using forearm venous occlusion plethysmography and expressed as ratio of blood flow between measurement and control arm (M/C ratio). Tetrahydrobiopterin infusion alone did not alter M/C ratio. Both the attenuated L-mono-methyl-arginine-induced vasoconstriction as well as the impaired serotonin-induced vasodilation were restored in patients during tetrahydrobiopterin infusion. Tetrahydrobiopterin had no effect in controls. In conclusion, this study demonstrates restoration of endothelial dysfunction by tetrahydrobiopterin suppletion in hypercholesterolemic patients.

Tetrahydrobiopterin, a cofactor for nitric oxide synthase, produces endothelium-dependent dilation of mouse pial arterioles

BACKGROUND AND PURPOSE

Tetrahydrobiopterin (THBP) is an essential cofactor for nitric oxide synthase (NOS), which is responsible for the synthesis of the endothelium-derived relaxing factor (EDRF) responsible for mediating the vasorelaxation produced by acetylcholine (ACh). Previous publications suggest that EDRFACh is continuously synthesized and released from the endothelium of mouse pial arterioles. If so, then one may predict that addition of THBP will increase the local production of EDRFACh and produce an endothelium-dependent relaxation that can be blocked by application of a known inhibitor of NOS. This study tests the prediction.

METHODS

The pial vessels were observed at a continuously suffused craniotomy site by means of intravital television microscopy. The effects of topically applied THBP on diameter were monitored before and after endothelial damage and before and after local treatment with the NOS inhibitor N-guanidino-L-monomethyl arginine (LNMMA). The endothelial damage was produced by a helium-neon laser in the presence of Evans blue dye.

RESULTS

A dose-dependent relaxation was produced by 10(-3) and 10(-2) mol/L THBP. The response was virtually eliminated by endothelial injury. LNMMA 10(-6) mol/L also greatly inhibited dilation.

CONCLUSIONS

The data are consistent with all reports that THBP is a cofactor for constitutive endothelial NOS. The data are consonant with previous results suggesting that EDRFACh is continually synthesized and released. It appears that THBP increases this synthesis and consequently the local level of released EDRFACh. The continuous spontaneous synthesis/release of EDRFACh modulates basal tone and, according to other studies, helps maintain a platelet-free endothelial surface.

Inducible and endothelial nitric oxide synthase expression during development of transplant arteriosclerosis in rat aortic grafts

In the vascular system, distinct isoforms of nitric oxide synthase (NOS) generate nitric oxide (NO), which acts as a biological messenger. Its role in the development of transplant arteriosclerosis (TA) is still unclear. To investigate whether NO is involved in TA, we studied the expression of NOS isoforms, inducible NOS (iNOS) and endothelial NOS (eNOS), by immunohistochemistry and in situ hybridization during the first two post-transplantation months and their relation with cold ischemia (1 to 24 hours) and reperfusion injury using an aortic transplantation model in the rat. We found an increased iNOS expression in the intima and adventitia and a decreased expression in the media, whereas eNOS expression was not significantly altered during the development of TA. Co-localization studies suggested that iNOS-positive cells were vascular smooth muscle cells, monocyte-derived macrophages, and endothelial cells. Prolonged ischemic storage time resulted in an increase in eNOS expression in the neointima. In situ hybridization showed iNOS mRNA expression by vascular cells in the neointima and media. NO produced by iNOS and eNOS may be involved, at least in part, in the pathogenesis of TA in aortic grafts. Additional studies are needed to confirm the modulatory mechanism of NO during the development of TA.

The role of the NO pathway in the control of cardiac function

Nitric oxide (NO) acts as an autocrine- and paracrine-acting signaling autacoid that, among other functions, has been shown to regulate cardiac contractile responsiveness to beta-adrenergic and muscarinic cholinergic agonists. Nitric oxide (NO) is formed by the oxidation of one of two equivalent guanidino nitrogens in L-arginine by O2 to form NO and L-citrulline. This reaction is catalyzed by a family of enzymes termed NO synthases. Three distinct isoforms of NOS have been identified, each the product of a separate gene. Cellular constituents of cardiac muscle, including ventricular myocytes as well as microvascular endothelial cells, have been shown to express the "endothelial constitutive" isoform of NO synthase (ecNOS or NOS3) in vivo, and both cell types also express the NO synthase isoform induced by specific inflammatory cytokines (iNOS or NOS2) in vivo and in vitro. While NO-dependent intracellular signalling in cardiac myocytes clearly involves the activation of guanylate cyclase and downstream signalling by cGMP, there is accumulating evidence that non-cGMP-dependent regulatory signalling events are also initiated by NO. In addition, decreased contractile responsiveness of cardiac myocytes to beta-adrenergic agonists, following induction of NOS2 by inflammatory cytokines, requires the presence of insulin and the co-induction of enzymes responsible for production of tetrahydrobiopterin, a NOS co-factor. Inappropriate or excessive production of NO by cardiac myocytes and by microvascular endothelial cells likely contributes to the cardiac contractile dysfunction characteristic of the systemic inflammatory response syndrome and cardiac allograft rejection.

Effects of activating and deactivating cytokines on the functionally linked tetrahydrobiopterin. No pathways in vascular smooth muscle cells

The functional relationship of nitric oxide (NO) production and synthesis of tetrahydrobiopterin (BH4), the requisite cofactor for NO synthase, was investigated in rat aortic smooth muscles cells (SMC). Inflammatory cytokines induced BH4 and NO synthesis in different ratios, IL-1 beta induced mainly NO synthesis with concomitant but limiting amounts of BH4 for maximal NO production. TNF alpha did not induce NO synthesis but induced BH4 synthesis. IFN gamma was ineffective on both the induction of NO and BH4 synthesis. TGF beta downregulated NO production but did not affect BH4 biosynthesis. IL-4 and IL-10 had no effect on both BH4 and NO synthesis. Activating cytokines strongly synergized in induction of NO production, whereas endogenous BH4 production became insufficient for maximal NO synthesis. Exogenous cofactor in the form of sepiapterin or authentic BH4, but not the natural isomer 7-BH4, enhanced NO production twofold. Inhibition of BH4 synthesis with dicumarol abolished NO production that could be restored in the presence of BH4.

Stress activated cytokines and the heart

The ability of myocardium to successfully compensate for, and adapt to, stress ultimately determines whether the heart will decompensate and fail, or whether it will instead maintain preserved function. Despite the importance of the myocardial response to environmental stress, very little is known with respect to the biochemical mechanisms that are responsible for mediating and integrating the stress response in the heart. In the present review we will summarize recent experimental material which suggests that cytokines that are expressed within the myocardium in response to a environment injury, namely tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1) and interleukin-6 (IL-6), may play an important role in initiating and integrating homeostatic responses within the heart. However, these 'stress-activated' cytokines all have the potential to produce cardiac decompensation when expressed at sufficiently high concentrations. Accordingly, the theme that will emerge from this discussion is that the short-term expression of stress-activated cytokines within the heart may provide the heart with an adaptive response to stress, whereas long-term expression of these molecules may be frankly maladaptive by producing cardiac decompensation.

Phosphorylation of endothelial nitric oxide synthase in response to fluid shear stress

Endothelial cells release nitric oxide (NO) more potently in response to increased shear stress than to agonists which elevate intracellular free calcium concentration ([Ca2+]i). To determine mechanistic differences in the regulation of endothelial constitutive NO synthase (ecNOS), we measured NO production by bovine aortic endothelial cells exposed to shear stress in a laminar flow chamber or treated with Ca2+ ionophores in static culture. The kinetics of cumulative NO production varied strikingly: shear stress (25 dyne/cm2) stimulated a biphasic increase over control that was 13-fold at 60 minutes, whereas raising [Ca2+]i caused a monophasic 6-fold increase. We hypothesized that activation of a protein kinase cascade mediates the early phase of flow-dependent NO production. Immunoprecipitation of ecNOS showed a 210% increase in phosphorylation 1 minute after flow initiation, whereas there was no significant increase after Ca2+ ionophore treatment. Although ecNOS was not tyrosine-phosphorylated, the early phase of flow-dependent NO production was blocked by genistein, an inhibitor of tyrosine kinases. To determine the Ca2+ requirement for flow-dependent NO production, we measured [Ca2+]i with a novel flow-step protocol. [Ca2+]i increased with the onset of shear stress, but not after a step increase. However, the step increase in shear stress was associated with a potent biphasic increase in NO production rate and ecNOS phosphorylation. These studies demonstrate that shear stress can increase NO production in the absence of increased [Ca2+]i, and they suggest that phosphorylation of ecNOS may importantly modulate its activity during the imposition of increased shear stress.

A balance between nitric oxide and oxidants regulates mast cell-dependent neutrophil-endothelial cell interactions

Nitric oxide (NO) synthesis inhibition causes neutrophil adhesion to endothelium via a mast cell- and oxidant-dependent mechanism. The objective of this study was to delineate the cascade of events in the mast cell- and oxidant-induced neutrophil-endothelium interactions after NO synthesis inhibition. Mast cells were isolated and purified from the rat peritoneal cavity and coadministered with neutrophils to wells of endothelium. This system was treated with an NO synthesis inhibitor (NG-nitro-L-arginine methyl ester; L-NAME) for 60 minutes. L-NAME did not induce neutrophil-endothelium interactions in the absence of mast cells, but the addition of mast cells in a ratio as low as 1:50 mast cells to neutrophils was sufficient to induce a large increase in neutrophil adhesion to endothelium within 20 to 25 minutes. L-arginine, NO donors, and 8-bromo-cGMP reversed the L-NAME effect, whereas NG-nitro-D-arginine methyl ester alone had no proadhesive effect. The adhesion was inhibited by an anti-CD18 or an anti-intracellular adhesion molecule-1 antibody and a platelet-activating factor-receptor antagonist. Inhibition of NO in isolated endothelial monolayers induced oxidant release (reduction of cytochrome C) into extracellular fluid. The endothelium-derived superoxide contributed to the mast cell-induced adhesion, inasmuch as the extracellular antioxidant superoxide dismutase reduced the neutrophil adhesion response as did disruption of endothelial function. There was some direct activation of mast cells with L-NAME (independent of endothelium) inasmuch as intracellular calcium and oxidative stress increased within mast cells after L-NAME treatment, and this translated into increased neutrophil adhesion to nonendothelial substrata. These data demonstrate that depletion of NO increases oxidative stress within mast cells and endothelium and together these events promote neutrophil adhesion within the vasculature.

Lipopolysaccharide and interleukin 1 augment the effects of hypoxia and inflammation in human pulmonary arterial tissue

The combined effects of hypoxia and interleukin 1, lipopolysaccharide, or tumor necrosis factor alpha on the expression of genes encoding endothelial constitutive and inducible nitric oxide synthases, endothelin 1, interleukin 6, and interleukin 8 were investigated in human primary pulmonary endothelial cells and whole pulmonary artery organoid cultures. Hypoxia decreased the expression of constitutive endothelial nitric oxide synthase (NOS-3) mRNA and NOS-3 protein as compared with normoxic conditions. The inhibition of expression of NOS-3 corresponded with a reduced production of NO. A combination of hypoxia with bacterial lipopolysaccharide, interleukin 1 beta, or tumor necrosis factor alpha augmented both effects. In contrast, the combination of hypoxia and the inflammatory mediators superinduced the expression of endothelin 1, interleukin 6, and interleukin 8. Here, we have shown that inflammatory mediators aggravate the effect of hypoxia on the down-regulation of NOS-3 and increase the expression of proinflammatory cytokines in human pulmonary endothelial cells and whole pulmonary artery organoid cultures.

Accelerated atherosclerosis in mice lacking tumor necrosis factor receptor p55

TNF-alpha (TNF) is produced primarily from macrophages and promotes numerous inflammatory reactions associated with atherosclerosis including the induction of vascular adhesion molecules and the recruitment and proliferation of monocyte/macrophages. There are two receptors known to elicit TNF responses, termed p55 and p75. Since p55 is thought to play the primary role in inflammatory processes, we postulated that the absence of p55 in mice would protect against atherosclerosis. In contrast, C57BL/6 mice lacking p55 had aortic sinus lesion sizes 2.3-fold larger than C57BL/6 wild type mice when fed an atherogenic diet (37,123 +/- 3485 microm2 versus 16, 688 +/- 2887 microm2, respectively, p < 0.0004). Plasma lipid levels were not different between strains. A 3-fold increase in the uptake and degradation of acetylated low density lipoprotein for p55-null as compared with wild type mice was demonstrated in cultured peritoneal macrophages. Immunohistochemical staining for scavenger receptor protein in the aortic sinus was more intense in lesions from the p55-null mice as compared with wild type controls. Our results support the concept that increased scavenger receptor activity contributes to excessive fatty streak formation. We conclude that TNF p55 receptors protect against atherosclerotic lesion development in the mouse.

Effect of physiological concentrations of estradiol on PGI2 and NO in endothelial cells

OBJECTIVES

To elucidate the mechanisms by which estrogens protect against occlusive vascular disorders, we studied the effect of 17 beta-estradiol on the production of prostacyclin (PGI2) and nitric oxide (NO) in primary cultures of human umbilical vein endothelial cells (HUVECs).

METHODS

To study the effect of 17 beta-estradiol on PGI2 production, HUVECs were incubated in the absence and presence of 17 beta-estradiol (0.01-10 nmol/l) encapsulated within beta-cyclodextrin for 12 h in serum-free medium. To study the effect of 17 beta-estradiol (100 nmol/l) on maximal calcium-dependent NO production, we used different approaches. First, HUVECs were incubated with 2 mumol/l calcium ionophore A23187 with or without 17 beta-estradiol (100 nmol/l) for 24 h in serum-free medium. Second, HUVECs were preincubated with or without 17 beta-estradiol (100 nmol/l) for 12 h in medium supplemented with 2% fetal calf serum, and thereafter incubated in serum-free medium with 2 mumol/l of A23187 and with 100 nmol/l of 17 beta-estradiol (cells which contained 17 beta-estradiol during the preincubation period as well as cells which did not) or without it (only cells which did not contain 17 beta-estradiol during the preincubation period) for 6 h or 24 h.

RESULTS

17 beta-Estradiol (0.1 nmol/l) increased the concentration of 6-keto-prostaglandin F1 alpha, a stable metabolite of PGI2 in the incubation medium, by 16%, and no further increase occurred with higher 17 beta-estradiol concentrations. The stimulation was prevented by tamoxifen. 17 beta-Estradiol did not affect NO production in any of our experiments measured as accumulation of nitrate and nitrite in the experimental medium.

CONCLUSIONS

The stimulatory effect on PGI2 production of physiological concentrations of 17 beta-estradiol, shown now for the first time, may provide one explanation for the ability of 17 beta-estradiol to protect against occlusive vascular disorders.

Glucocorticoids regulate inducible nitric oxide synthase by inhibiting tetrahydrobiopterin synthesis and L-arginine transport

The cytokine-inducible isoform of nitric oxide synthase (iNOS or NOS2) plays an important role in the immune response to some pathogens. Within the heart, increased activity of NOS2 in cardiac microvascular endothelial cells (CMEC) also can diminish the contractile function of adjacent cardiac myocytes. Glucocorticoids, which are known to suppress cytokine induction of NOS2 in many cell types, caused only a moderate (approximately 20%) decline in NOS2 protein content and maximal activity measured in homogenates of cytokine-treated CMEC, but almost completely inhibited synthesis of nitrogen oxides (NOx) by intact cells. To determine whether glucocorticoids were inhibiting cellular NOx production by limiting the availability of NOS co-factors or substrate, the effect of dexamethasone on tetrahydrobiopterin (BH4) and L-arginine availability in cytokine-treated CMEC was examined. Dexamethasone prevented the coordinate induction of GTP cyclohydrolase I with NOS2 after exposure to interleukin-1beta and interferon-gamma and also the increase in intracellular BH4 content in cytokine-treated CMEC. Addition of BH4 overcame dexamethasone-mediated suppression of nitrite production. Dexamethasone also prevented a cytokine-mediated increase in L-arginine uptake into CMEC by suppressing the induction of the high affinity cationic amino acid transporters CAT-1 and CAT-2B and the low affinity CAT-2A transporter. In addition, dexamethasone also inhibited cytokine induction in CMEC of argininosuccinate synthase, the rate-limiting enzyme for the de novo synthesis of arginine from citrulline. Thus, glucocorticoids regulate NOx production following cytokine exposure in cardiac microvascular endothelial cells primarily by limiting BH4 and L-arginine availability.

Tetrahydrobiopterin biosynthesis in C6 glioma cells: induction of GTP cyclohydrolase I gene expression by lipopolysaccharide and cytokine treatment

The possibility that 5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis is stimulated in glial cells by treatment with lipopolysaccharide (LPS) and tumor necrosis factor (TNF-alpha) was examined in the astrocyte-derived C6 glioma cell line. Under basal culture conditions BH4 levels were found to be at the limit of detection. Concurrent treatment with 10 micrograms/ml LPS and 50 ng/ml TNF-alpha caused a time-dependent 13-fold increase in the levels of BH4. This treatment paradigm also induced nitric oxide synthase activity, as evidenced by increased levels of nitrite, an oxidized metabolite of NO, in the culture medium. LPS and TNF-alpha treatment led to a 25-fold increase in GTPCH enzyme activity, the first and rate-limiting enzyme in BH4 synthesis, and a corresponding 23-fold increase in GTPCH protein levels. Northern blot analysis showed that increased levels of GTPCH mRNA preceded changes in GTPCH protein, GTPCH enzyme activity and BH4 levels and reached a maximal of 44-fold that was sustained for at least 48 h. These results demonstrate that LPS and TNF-alpha stimulate de-novo BH4 biosynthesis and suggest that C6 cells offer a model system for studying the molecular events that control the induction of GTPCH gene expression and BH4 synthesis in glial cells.

Increased endothelial nitric oxide synthase activity in the hyperemic vessels of portal hypertensive rats

BACKGROUND/AIM

Portal hypertension is characterized by splanchnic hyperemia due to a reduction in mesenteric vascular resistance. Mediators of this hyperemia include nitric oxide. This is based on several reports indicating a marked splanchnic hyporesponsiveness in portal hypertension to vasoconstrictor stimuli both in vitro and in vivo, and a subsequent reversal using specific inhibitors of nitric oxide synthase. The objective of this study was to determine firstly whether the functional activity and/or expression of nitric oxide synthase is altered in portal hypertensive vasculature and secondly which isoenzyme form was responsible for the preferential response to nitric oxide blockade in these animals.

METHODS

We compared nitric oxide synthase functional activity in the hyperemic vasculature of sham and portal hypertensive rats (following partial portal vein ligation). Nitric oxide synthase activities were determined by measuring the conversion of L-arginine to citrulline using ion-exchange chromatography and the amount of immunodetectable nitric oxide synthase in sham and portal hypertensive vessels was determined by Western blot.

RESULTS

Ca(2+)-dependent nitric oxide synthase activity was significantly elevated (p < 0.05) in portal hypertensive particulate fractions from the superior mesenteric artery, thoracic aorta and portal vein. Vascular tissue cGMP levels and plasma nitrite levels were both significantly elevated in portal hypertension. Immunodetection with specific antisera raised against the inducible nitric oxide synthase demonstrated a lack of induction within the hyperemic vasculature. Immunodetection with antisera against endothelial nitric oxide synthase showed a significant increase in portal hypertensive portal vein only. These results demonstrate enhanced calcium-dependent nitric oxide synthase activity in portal hypertension hyperemic vessels concurrent with elevated tissue cGMP levels.

CONCLUSION

We conclude that enhanced endothelial nitric oxide synthesis may in part contribute to the hyperdynamic circulation of portal hypertension.

Monocyte-induced downregulation of nitric oxide synthase in cultured aortic endothelial cells

Since endothelium-dependent vasodilation is altered in atherosclerosis and enhanced monocyte/endothelial interactions are implicated in early atherosclerosis, we evaluated the effects of monocytes on the endothelial nitric oxide (NO) pathway by estimating release of biologically active NO from cultured endothelial cells and levels of constitutive NO synthase (ecNOS). NO release was estimated in a short-term bioassay using endothelial cell-induced cGMP accumulation in vascular smooth muscle (SM) cells. Exposure of SM cells to porcine aortic endothelial cells (PAECs) and human aortic endothelial cells (HAECs) produced large increases in SM cGMP content; this increase was prevented by NG-nitro-L-arginine methyl ester, the inhibitor of endothelial NOS. Confluent monolayers of PAECs and HAECs cocultured with monocytes also stimulated SM cGMP formation; however, NO release from these cultures was attenuated in a coculture time (2 to 48 hours)- and monocyte concentration (20 to 200 x 10(3) per well)-dependent manner. This effect of monocyte adhesion appeared to be selective for NO release since other biochemical pathways, such as atriopeptin-and isoproterenol-induced cyclic nucleotide accumulation within the endothelial cells, were not altered by monocytes. The effects of adherent monocytes on NO release were mimicked by monocyte-derived cytokines tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 alpha. Furthermore, the conditioned medium of monocytes contained significant quantities of these cytokines. Conditioned medium, as well as monocytes physically separated from the endothelial cells, attenuated NO release, suggesting that soluble factors may mediate the effects of monocytes. An IL-1 beta neutralizing antibody fully prevented the NO dysfunction in response to directly adherent monocytes. Superoxide dismutase, catalase, 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), and exogenous L-arginine failed to improve NO release, suggesting that oxidant stress-induced inactivation of NO or limited substrate availability were not primarily responsible for the inhibiting effects of monocytes. Western blot analysis revealed reduced quantities of ecNOS in monocyte/endothelium cocultures, as well as in HAECs treated with monocyte-conditioned medium or TNF-alpha. Thus, adhesion of monocytes to endothelial cells and monocyte-derived secretory products downregulate steady state levels of ecNOS, an event associated with attenuated release of biologically active NO. This mechanism may potentially contribute to diminished endothelium-dependent and NO-mediated vasodilation in early atherosclerosis.

Differential activation of mitogen-activated protein kinases by nitric oxide-related species

Many studies have identified nitric oxide (NO) and related chemical species (NOx) as having critical roles in neurotransmission, vasoregulation, and cellular signaling. Previous work in this laboratory has focused on elucidating the mechanism of NOx signaling in cells. We have demonstrated that NOx-induced activation of the guanine nucleotide-binding protein p21(ras) leads to nuclear translocation of the transcription factor NFkappaB. Here, we investigated whether intermediary signaling elements, namely the mitogen-activated protein (MAP) kinases, are involved in mediating NOx signaling. We found that NOx activates the extracellular signal-regulated kinase (ERK), p38, and c-Jun NH2-terminal kinase (JNK) subgroups of MAP kinases in human Jurkat T cells. JNK was found to be 100-fold more sensitive to NOx stimulation than p38 and ERK. In addition, the activation of JNK and p38 by NOx was more rapid than ERK activation. Depletion of intracellular glutathione augmented the NOx-induced increase in kinase activity. Furthermore, endogenous NO, generated from NO synthase, activated ERK, and NOx-induced MAP kinase activation was effectively blocked by the farnesyl transferase inhibitor alpha-hydroxyfarnesylphosphonic acid. These data support the hypothesis that critical signaling kinases, such as ERK, p38, and JNK, are activated by NO-related species and thus participate in NO signal transduction. These findings establish a role for multiple MAP kinase signaling pathways in the cellular response to NOx.

Purification and cloning of the GTP cyclohydrolase I feedback regulatory protein, GFRP

The activity of GTP cyclohydrolase I, the initial enzyme of the de novo pathway for biosynthesis of tetrahydrobiopterin, the cofactor required for aromatic amino acid hydroxylations and nitric oxide synthesis, is sensitive to end-product feedback inhibition by tetrahydrobiopterin. This inhibition by tetrahydrobiopterin is mediated by the GTP cyclohydrolase I feedback regulatory protein GFRP, previously named p35 (Harada, T., Kagamiyama, H., and Hatakeyama, K. (1993) Science 260, 1507-1510), and -phenylalanine specifically reverses the tetrahydrobiopterin-dependent inhibition. As a first step in the investigation of the physiological role of this unique mechanism of regulation, a convenient procedure has been developed to co-purify to homogeneity both GTP cyclohydrolase I and GFRP from rat liver. GTP cyclohydrolase I and GFRP exist in a complex which can be bound to a GTP-affinity column from which GTP cyclohydrolase I and GFRP are separately and selectively eluted. GFRP is dissociated from the GTP agarose-bound complex with 0.2 NaCl, a concentration of salt which also effectively blocks the tetrahydrobiopterin-dependent inhibitory activity of GFRP. GTP cyclohydrolase I is then eluted from the GTP-agarose column with GTP. Both GFRP and GTP cyclohydrolase I were then purified separately to near homogeneity by sequential high performance anion exchange and gel filtration chromatography. GFRP was found to have a native molecular mass of 20 kDa and consist of a homodimer of 9.5-kDa subunits. Based on peptide sequences obtained from purified GFRP, oligonucleotides were synthesized and used to clone a cDNA from a rat liver cDNA library by polymerase chain reaction-based methods. The cDNA contained an open reading frame that encoded a novel protein of 84 amino acids (calculated molecular mass 9665 daltons). This protein when expressed in Escherichia coli as a thioredoxin fusion protein had tetrahydrobiopterin-dependent GTP cyclohydrolase I inhibitory activity. Northern blot analysis indicated the presence of an 0.8-kilobase GFRP mRNA in most rat tissues, the amounts generally correlating with levels of GTP cyclohydrolase I and tetrahydrobiopterin. Thus, mRNA levels were relatively high in liver and kidney and somewhat lower in testis, heart, brain, and lung. These results suggest that GFRP is widely expressed and may play a role in regulating not only phenylalanine metabolism in the liver, but also the production of biogenic amine neurotransmitters as well as nitric oxide synthesis.

Effect of tetrahydrobiopterin on endothelial function in canine middle cerebral arteries

Tetrahydrobiopterin is an essential cofactor required for activation of NO synthase. However, in intact arteries, the exact role of tetrahydrobiopterin in the regulation of NO synthase activity is not fully understood. The present study was designed to determine the effect of increasing intracellular tetrahydrobiopterin levels on endothelial function in isolated canine middle cerebral arteries. The arterial segments were incubated in MEM for 24 hours at 37 degrees C in the presence or absence of a tetrahydrobiopterin precursor, sepiapterin (10(-4) mol/L), and/or superoxide dismutase (150 U/mL). The rings were suspended for isometric tension recording. Tetrahydrobiopterin levels were assayed by high-performance liquid chromatography. Production of cGMP was measured by radioimmunoassay. Incubation with sepiapterin markedly increased intracellular tetrahydrobiopterin levels. In sepiapterin-treated arteries, endothelium-dependent relaxations to calcium ionophore A23187 and intracellular cGMP levels were significantly reduced. Superoxide dismutase alone did not affect either relaxation to A23187 or production of cGMP. However, when arteries were incubated with superoxide dismutase plus sepiapterin, endothelium-dependent relaxations to A23187, as well as cGMP production, were significantly augmented. The augmentation of cGMP was observed in rings with (but not without) endothelium. Incubation of arteries in calcium-free medium almost abolished the synergistic effect of tetrahydrobiopterin and superoxide dismutase on cGMP production. These results demonstrate that increased availability of tetrahydrobiopterin may activate endothelial NO synthase. This effect appears to be critically dependent on the presence of superoxide dismutase.

Role of tetrahydrobiopterin availability in the regulation of nitric-oxide synthase expression in human mesangial cells

Human mesangial cells express an inducible form of nitric-oxide synthase (iNOS) after treatment with cytokines. Tetrahydrobiopterin (BH4), an essential cofactor for NOS, is required for cytokine-induced NO generation. We report here that BH4 is necessary not only for the activity but also for the expression of iNOS in human mesangial cells. Inhibition of de novo BH4 synthesis with 2,4-diamino-6-hydroxypyrimidine (DAHP) significantly attenuated iNOS activity as well as mRNA and protein expression in response to interleukin 1beta plus tumor necrosis factor alpha (IL-1beta/TNF-alpha). In contrast, sepiapterin, which provides BH4 through the pterin salvage pathway, strongly potentiated IL-1beta/TNF-alpha-induced iNOS expression and abrogated the inhibitory effect of DAHP. Inhibition of the pterin salvage pathway with methotrexate abolished sepiapterin potentiation of iNOS induction but did not alter the effect of IL-1beta/TNF-alpha. Determination of intracellular pteridines confirmed that sepiapterin markedly raised BH4 content, an effect that was blocked by methotrexate. These results suggest that BH4 availability plays an important role in the regulation of iNOS expression. The effect of BH4 appears to be mediated, at least in part, by an increase in mRNA stability, as indicated by the observation that DAHP shortened, whereas sepiapterin prolonged the half-life of IL-1beta/TNF-alpha-induced iNOS mRNA. Taken together, our results suggest that the biosynthesis of BH4 contributes to cytokine induction of iNOS expression in human mesangial cells through the stabilization of iNOS mRNA.

Differential regulation of L-arginine transport and nitric oxide production by vascular smooth muscle and endothelium

Since NO production is dependent on the availability of L-arginie, we examined whether L-arginine transport and NO synthesis are coregulated by vascular smooth muscle cells and endothelial cells cultured from the same vessel wall source. L-Arginine transport by both bovine aortic smooth muscle cells (BASMCs) and endothelial cells (BAECs) was primarily Na+ independent (approximately 70%) and was mediated by both a high- and low-affinity transport system. Treatment of BASMCs with tumor necrosis factor-alpha (TNF-alpha) or interleukin-1 beta (IL-1 beta) resulted in a significant increase in L-arginine transport (approximately 20%) and in the induction of NO release. Exposure of BASMCs to interferon gamma (IFN-gamma) or lipopolysaccharide (LPS) also stimulated NO release but did not affect L-arginine transport. In contrast, incubation of BAECs with TNF-alpha or LPS strikingly enhanced L-arginine uptake (2.5-fold), whereas IL-1 beta and IFN-gamma had no effect. Treatment of BAECs with any of the inflammatory mediators did not stimulate NO production. These results demonstrate that L-arginine uptake and NO synthesis by these cells are differentially regulated. In BASMCs, the coinduction of L-arginine transport and NO formation may function to provide increased levels of substrate to the cell during activation of the NO synthase enzyme. In contrast, the selective stimulation of L-arginine uptake in BAECs indicates that L-arginine transport is dissociated from NO generation in these cells.

Inhibition of nitric oxide synthase attenuates blood-brain barrier disruption during experimental meningitis

Increased permeability of the blood-brain (B-B) barrier is observed during meningitis. Preventing B-B barrier alterations is important because adverse neurological outcomes are correlated with breeches in barrier integrity. It was hypothesized that pathological production of nitric oxide (NO) contributes to B-B barrier disruption during meningitis in the rat. Experimental meningitis was induced by intracisternal (i.c.) administration of lipopolysaccharides (LPS) or vehicle. Groups of rats were concomitantly infused intravenously (i.v.) with saline or the NO synthase inhibitor, aminoguanidine (AG). Eight h after i.c. dosing, B-B barrier alterations were quantitated pharmacokinetically using [14C]sucrose. Serum and regional brain tissues were obtained 0-30 min after tracer dosing and sucrose influx transfer coefficients (Kin(app)) were calculated from the brain tissue data. Compared to the control groups (i.c. vehicle/i.v. saline), the Kin(app) of the i.c. LPS/i.v. saline group increased 1.6-2.1-fold in various brain regions, thus confirming previous observations of increased [14C]sucrose barrier penetration during meningeal inflammation. Remarkably, i.v. administration of AG to i.c. LPS-treated rats significantly inhibited meningeal NO synthesis and decreased Kin (app) permeability alterations in the B-B barrier, compared to i.c. LPS/i.v. saline-treated rats. Regional brain Kin (app) estimates in the i.c. LPS/i.v. AG group were similar to control groups (i.c. vehicle/i.v. AG and i.c. vehicle/i.v. saline). In conclusion, these data suggest the general concept that excessive NO production during neuroinflammatory diseases contributes to disruption of the blood-brain barrier.

Molecular cloning of endothelial, inducible nitric oxide synthase gene from rat aortic endothelial cell

We have isolated and sequenced clones of an inducible nitric oxide synthase (iNOS) from cDNA library of interleukin-1 beta-treated rat aortic endothelial cells (EC) completely free from other cell types. The cloned cDNA contains an ORF consisting of 3441 bp, which encodes 1147 amino acid residues. The deduced amino acid sequence contains putative binding sites for NADPH, FMN, FAD, calmodulin and heme. By comparison with amino acid sequences of other isoforms, rat EC iNOS is very similar (92% similarity) to mouse macrophage iNOS. There are four AUUUA motifs, potentially responsible for the instability of the mRNA, in 3'non-coding region of rat EC iNOS cDNA. Transient transfection of cultured rat vascular smooth-muscle cells with a full-length rat EC iNOS cDNA/SR alpha 296 construct by electroporation resulted in massive NO production in proportion to the doses of cDNA used. Northern blot analysis using rat EC iNOS cDNA as a probe revealed that cycloheximide treatment led to a marked accumulation of iNOS mRNA in the presence and absence of interleukin-1 beta. No appreciable decay in the cycloheximide-induced iNOS mRNA accumulation was observed, suggesting that blockade of de novo protein synthesis stabilizes mRNA. These results demonstrate that rat EC iNOS is identical (or very similar) to macrophage iNOS, and suggest that the EC iNOS gene is also regulated at the post-transcriptional level.