L-arginine depletion by arginase reduces nitric oxide production in endotoxic shock: an electron paramagnetic resonance study

The Role of PPAR Alpha in the Modulation of Innate Immunity

Peroxisome proliferator-activated receptor α is a potent regulator of systemic and cellular metabolism and energy homeostasis, but it also suppresses various inflammatory reactions. In this review, we focus on its role in the regulation of innate immunity; in particular, we discuss the PPARα interplay with inflammatory transcription factor signaling, pattern-recognition receptor signaling, and the endocannabinoid system. We also present examples of the PPARα-specific immunomodulatory functions during parasitic, bacterial, and viral infections, as well as approach several issues associated with innate immunity processes, such as the production of reactive nitrogen and oxygen species, phagocytosis, and the effector functions of macrophages, innate lymphoid cells, and mast cells. The described phenomena encourage the application of endogenous and pharmacological PPARα agonists to alleviate the disorders of immunological background and the development of new solutions that engage PPARα activation or suppression.

Pharmacologic approaches to amino acid depletion for cancer therapy

Cancer cells undergo metabolic reprogramming to support increased demands in bioenergetics and biosynthesis and to maintain reactive oxygen species at optimum levels. As metabolic alterations are broadly observed across many cancer types, metabolic reprogramming is considered a hallmark of cancer. A metabolic alteration commonly seen in cancer cells is an increased demand for certain amino acids. Amino acids are involved in a wide range of cellular functions, including proliferation, redox balance, bioenergetic and biosynthesis support, and homeostatic functions. Thus, targeting amino acid dependency in cancer is an attractive strategy for a number of cancers. In particular, pharmacologically mediated amino acid depletion has been evaluated as a cancer treatment option for several cancers. Amino acids that have been investigated for the feasibility of drug-induced depletion in preclinical and clinical studies for cancer treatment include arginine, asparagine, cysteine, glutamine, lysine, and methionine. In this review, we will summarize the status of current research on pharmacologically mediated amino acid depletion as a strategy for cancer treatment and potential chemotherapeutic combinations that synergize with amino acid depletion to further inhibit tumor growth and progression.

Granulocytic myeloid-derived suppressor cells inversely correlate with plasma arginine and overall survival in critically ill patients

Critically ill patients display a state of immunosuppression that has been attributed in part to decreased plasma arginine concentrations. However, we and other authors have failed to demonstrate a clinical benefit of L-arginine supplementation. We hypothesize that, in these critically ill patients, these low plasma arginine levels may be secondary to the presence of granulocytic myeloid-derived suppressor cells (gMDSC), which express arginase known to convert arginine into nitric oxide (NO) and citrulline. Indeed, in a series of 28 non-surgical critically ill patients, we showed a dramatic increase in gMDSC compared to healthy subjects (P = 0·0002). A significant inverse correlation was observed between arginine levels and gMDSC (P = 0·01). As expected, gMDSC expressed arginase preferentially in these patients. Patients with high gMDSC levels on admission to the medical intensive care unit (MICU) presented an increased risk of death at day 7 after admission (P = 0·02). In contrast, neither plasma arginine levels, monocytic MDSC levels nor neutrophil levels were associated with overall survival at day 7. No relationship was found between body mass index (BMI) or simplified acute physiology score (SAPS) score, sequential organ failure assessment (SOFA) score or gMDSC levels, eliminating a possible bias concerning the direct prognostic role of these cells. As gMDSC exert their immunosuppressive activity via multiple mechanisms [production of prostaglandin E2 (PGE2 ), interleukin (IL)-10, arginase, etc.], it may be more relevant to target these cells, rather than simply supplementing with L-arginine to improve immunosuppression and its clinical consequences observed in critically ill patients.

Arginase-1 deficiency regulates arginine concentrations and NOS2-mediated NO production during endotoxemia

Rationale and objective

Arginase-1 is an important component of the intricate mechanism regulating arginine availability during immune responses and nitric oxide synthase (NOS) activity. In this study Arg1^fl/fl/Tie2-Cre^tg/− mice were developed to investigate the effect of arginase-1 related arginine depletion on NOS2- and NOS3-dependent NO production and jejunal microcirculation under resting and endotoxemic conditions, in mice lacking arginase-1 in endothelial and hematopoietic cells.

Methods and Results

Arginase-1-deficient mice as compared with control mice exhibited higher plasma arginine concentration concomitant with enhanced NO production in endothelial cells and jejunal tissue during endotoxemia. In parallel, impaired jejunal microcirculation was observed in endotoxemic conditions. Cultured bone-marrow-derived macrophages of arginase-1 deficient animals also presented a higher inflammatory response to endotoxin than control littermates. Since NOS2 competes with arginase for their common substrate arginine during endotoxemia, Nos2 deficient mice were also studied under endotoxemic conditions. As Nos2^−/− macrophages showed an impaired inflammatory response to endotoxin compared to wild-type macrophages, NOS2 is potentially involved. A strongly reduced NO production in Arg1^fl/fl/Tie2-Cre^tg/− mice following infusion of the NOS2 inhibitor 1400W further implicated NOS2 in the enhanced capacity to produce NO production Arg1^fl/fl/Tie2-Cre^tg/− mice.

Conclusions

Reduced arginase-1 activity in Arg1^fl/fl/Tie2-Cre^tg/− mice resulted in increased inflammatory response and NO production by NOS2, accompanied by a depressed microcirculatory flow during endotoxemia. Thus, arginase-1 deficiency facilitates a NOS2-mediated pro-inflammatory activity at the expense of NOS3-mediated endothelial relaxation.

Low plasma citrulline levels are associated with acute respiratory distress syndrome in patients with severe sepsis

INTRODUCTION

The role of nitric oxide synthase (NOS) in the pathophysiology of acute respiratory distress syndrome (ARDS) is not well understood. Inducible NOS is upregulated during physiologic stress; however, if NOS substrate is insufficient then NOS can uncouple and switch from NO generation to production of damaging peroxynitrites. We hypothesized that NOS substrate levels are low in patients with severe sepsis and that low levels of the NOS substrate citrulline would be associated with end organ damage including ARDS in severe sepsis.

METHODS

Plasma citrulline, arginine and ornithine levels and nitrate/nitrite were measured at baseline in 135 patients with severe sepsis. ARDS was diagnosed by consensus definitions.

RESULTS

Plasma citrulline levels were below normal in all patients (median 9.2 uM, IQR 5.2 - 14.4) and were significantly lower in ARDS compared to the no ARDS group (6.0 (3.3 - 10.4) vs. 10.1 (6.2 - 16.6), P = 0.002). The rate of ARDS was 50% in the lowest citrulline quartile compared to 15% in the highest citrulline quartile (P = 0.002). In multivariable analyses, citrulline levels were associated with ARDS even after adjustment for covariates including severity of illness.

CONCLUSIONS

In severe sepsis, levels of the NOS substrate citrulline are low and are associated with ARDS. Low NOS substrate levels have been shown in other disease states to lead to NOS uncoupling and oxidative injury suggesting a potential mechanism for the association between low citrulline and ARDS. Further studies are needed to determine whether citrulline supplementation could prevent the development of ARDS in patients with severe sepsis and to determine its role in NOS coupling and function.

Evaluation of asymmetric dimethylarginine, arginine, and carnitine metabolism in pediatric sepsis

OBJECTIVE

Increased plasma concentrations of the endogenous nitric oxide synthase inhibitor, asymmetric dimethylarginine, decreased arginine bioavailability, and mitochondrial dysfunction have been reported in adult sepsis. We studied whether asymmetric dimethylarginine, arginine, and carnitine metabolism (a measure of mitochondrial dysfunction) are altered in pediatric sepsis and whether these are clinically useful biomarkers.

DESIGN

: Prospective, observational study.

SETTING

Pediatric intensive care unit at an academic medical center.

PATIENTS

: Ninety patients ≤ 18 yrs old, 30 with severe sepsis or septic shock, compared with 30 age-matched febrile and 30 age-matched healthy control subjects.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Plasma asymmetric dimethylarginine and whole blood arginine, citrulline, ornithine, and acylcarnitine:free carnitine ratio were measured daily for septic patients and once for control subjects using tandem mass spectrometry. Plasma asymmetric dimethylarginine concentration (median; interquartile range µmol/L) on day 1 was lower in severe sepsis and septic shock (0.38; 0.30-0.56) compared with febrile (0.45; 0.40-0.59) and healthy (0.60; 0.54-0.67) control subjects (p < .001), although decreased asymmetric dimethylarginine was predominantly found in neutropenic patients. Day 1 arginine was lower in septic (10; interquartile range, 7-20 µmol/L) compared with healthy patients (32; interquartile range, 23-40; p < .001), and the arginine:ornithine ratio was decreased in sepsis, indicating increased arginase activity (an alternative pathway for arginine metabolism). The arginine:asymmetric dimethylarginine and acylcarnitine:free carnitine ratios did not differ between septic and control patients. Asymmetric dimethylarginine was inversely correlated with organ dysfunction by Pediatric Logistic Organ Dysfunction score (r = -0.50, p = .009), interleukin-6 (r = -0.55, p = .01), and interleukin-8 (r = -0.52, p = .03) on admission. Arginine, arginine:asymmetric dimethylarginine, and acylcarnitine:free carnitine were not associated with organ dysfunction or outcomes.

CONCLUSIONS

Asymmetric dimethylarginine was decreased in pediatric sepsis and was inversely associated with inflammation and organ dysfunction. This suggests that inhibition of nitric oxide synthase by asymmetric dimethylarginine accumulation is unlikely to impact sepsis pathophysiology in septic children despite decreased arginine bioavailability. We did not find an association of asymmetric dimethylarginine with altered carnitine metabolism nor were asymmetric dimethylarginine, arginine, and acylcarnitine:free carnitine useful as clinical biomarkers.

Pollutant particles induce arginase II in human bronchial epithelial cells

Exposure to particulate matter (PM) is associated with adverse pulmonary effects, including induction and exacerbation of asthma. Recently arginase was shown to play an important role in the pathogenesis of asthma. In this study, it was postulated that PM exposure might induce arginase. Human bronchial epithelial cells (HBEC) obtained from normal individuals by endobronchial brushings cultured on an air-liquid interface were incubated with fine Chapel Hill particles (PM₂.₅, 100 μg/ml) for up to 72 h. Arginase activity, protein expression, and mRNA of arginase I and arginase II were measured. PM₂.₅ increased arginase activity in a time-dependent manner. The rise was primarily due to upregulation of arginase II. PD153035 (10 μM), an epidermal growth factor (EGF) receptor antagonist, attenuated the PM₂.₅-induced elevation in arginase activity and arginase II expression. Treatment of HBEC with human EGF increased arginase activity and arginase II expression. Pretreatment with catalase (200 U/ml), superoxide dismutase (100 U/ml), or apocynin (5 μg/ml), an NAD(P)H oxidase inhibitor, did not markedly affect arginase II expression. Treatment of HBEC with arginase II siRNA inhibited the expression of arginase II by 60% and increased IL-8 release induced by PM₂.₅. These results indicate that PM exposure upregulates arginase II activity and expression in human bronchial epithelial cells, in part via EGF-dependent mechanisms independent of oxidative stress. The elevated arginase II activity and expression may be a mechanism underlying adverse effects induced by PM exposure in asthma patients.

Nitrergic response to cyclophosphamide treatment in blood and bone marrow

Daily intraperitoneal injection of cyclophosphamide (CPA) (50 mgkg(-1) of body weight) for 5 days resulted in reduced levels of marrow and blood cellularity, which was most pronounced in 18 days post-treatment (pt). On day 18 after CPA treatment the enhancedlevels of nitric oxide (NO) precursors and metabolites (L-arginine, L-citrulline, reactive nitrogen species (RNS)) of marrow and blood cells (platelet, neutrophil, lymphocyte and monocyte) resulted from up-regulation of Ca(II)/calmodulin(CaM)-independent "inducible" NO synthase (iNOS), with a lessercontribution of Ca(II)/CaM-dependent "constitutive" cNOS isoforms to systemic NO.Biphasic response to CPA of marrow nitrergic system, i.e. both iNOS and cNOS showed significantly depressed activities, as well as diminished levels of NO metabolites on day 9 pt, suggested that signals in addition to NO might be involved in CPA-induced inhibition of hematopoesis, while a gradual increase of neutrophil and platelet NOS activity appeared to be contributed to a CPA-induced development of granulopenia, thrombocytopenia and hemorrhage.

Pros and cons of L-arginine supplementation in disease

The amino acid arginine and one of its metabolites NO have gathered broad attention in the last decade. Although arginine is regarded as a conditionally essential amino acid in disease, L-arginine supplementation in severe illness has not found its way into clinical practice. This might be due to the invalid interpretation of results from studies with immune-enhancing diets containing L-arginine amongst other pharmaconutrients. However, not much attention is given to research using L-arginine as a monotherapy and the possibility of the alternative hypothesis: that L-arginine supplementation is beneficial in disease. The present review will discuss data from studies in healthy and diseased animals and patients with monotherapy of L-arginine to come to an objective overview of positive and negative aspects of L-arginine supplementation in disease with special emphasis on sepsis, cancer, liver failure and wound healing.

Direct detection of reactive nitrogen species in experimental autoimmune uveitis

PURPOSE

Demonstrate unequivocally the generation of nitric oxide in experimental autoimmune uveoretinitis by electron spin resonance spectroscopy (ESR) using ferrous iron complex of N-methyl-D-glucamine dithiocarbamate, (MGD)(2)-Fe(2+), as a spin trap.

METHODS

Experimental autoimmune uveitis was induced in Lewis rats, and at the peak of the intraocular inflammation, the animals received intravitreous injections of the spin trap. The retina and choroid dissected from the enucleated globes were subjected to ESR. Similarly, the retina and choroid obtained at the peak of experimental autoimmune uveo-retinitis (EAU) were placed in a vial containing luminal, and chemiluminescence was counted on a Packard liquid scintillation analyzer.

RESULTS

The ESR three-line spectrum (g=2.04; a(N)=12.5 G) obtained was characteristic of the adduct [(MGD)(2)-Fe(2+)-NO]. The majority of this signal was eliminated by the inducible nitric oxide synthase (iNOS) specific inhibitor aminoguanidine injected inflamed retina was detected when compared with that of the non inflamed controls. The chemiluminescent activity was further increased two-fold by the addition of bicarbonate to the inflamed retina; the phenomenon is attributable only to the presence of a high steady-state concentration of peroxynitrite.

CONCLUSIONS

The study shows an unequivocal presence of nitric oxide in EAU retina and choroid and the generation of peroxynitrite. High levels of these reactive nitrogen species generated in the inflamed retina and choroids are certain to cause irreversible tissue damage, especially at the susceptible sites such as photoreceptors.

Nitric oxide and hormones in breast cancer: allies or enemies?

Unlike other types of cancer, tumors of the breast are greatly influenced by steroid hormones. The effect of estrogen and progesterone depends on the presence of their specific receptors and these constitute important parameters in determining the aggressiveness of the tumor, the feasibility of certain therapies and the prediction of relapse. The molecular mechanisms of steroid hormone action have not been fully elucidated but recent findings implicate the nitric oxide (NO) pathway in some of these effects. Both hormones can regulate the nitric oxide synthases (NOS) and, in turn, the NO produced has profound consequences on tumor cell homeostasis. On one hand, estrogen increases the activity of endothelial NOS (eNOS or NOSIII), while progesterone activates inducible NOS (iNOS or NOSII) expression. The data presented suggest that the low levels of NO produced by NOSIII mediate the proliferative effect of estrogen. On the other hand, the increase in apoptosis in response to progesterone could implicate the high levels of NO produced by induction of NOSII expression. Understanding of the mechanisms and interactions of steroid hormones with the NO pathway could lead to the development of new approaches and strategies for the effective treatment of breast cancer.

Role of free nitrogen and oxygen radicals in the pathogenesis of lipopolysaccharide-induced endotoxemia

We studied molecular mechanisms of changes in oxidative metabolism under conditions of experimental lipopolysaccharide-induced endotoxemia. Generation of reactive nitrogen and oxygen species in mice increased 18 h after treatment with lipopolysaccharide. These changes contributed to inactivation of enzymes and enzyme complexes (ribonucleotide reductase, NADH-ubiquinone oxidoreductase, and cytochrome c oxidase), dysfunction of the mitochondrial electron transport chain, and development of oxidative stress. Plaferon LB protected mice from the toxic effect of lipopolysaccharide.

Sepsis: an arginine deficiency state?

OBJECTIVE

Sepsis is a major health problem considering its significant morbidity and mortality rate. The amino acid L-arginine has recently received substantial attention in relation to human sepsis. However, knowledge of arginine metabolism during sepsis is limited. Therefore, we reviewed the current knowledge about arginine metabolism in sepsis.

DATA SOURCE

This review summarizes the literature on arginine metabolism both in general and in relation to sepsis. Moreover, arginine-related therapies are reviewed and discussed, which includes therapies of both nitric oxide (NO) and arginine administration and therapies directed toward inhibition of NO.

DATA

In sepsis, protein breakdown is increased, which is a key process to maintain arginine delivery, because both endogenous de novo production from citrulline and food intake are reduced. Arginine catabolism, on the other hand, is markedly increased by enhanced use of arginine in the arginase and NO pathways. As a result, lowered plasma arginine levels are usually found. Clinical symptoms of sepsis that are related to changes in arginine metabolism are mainly related to hemodynamic alterations and diminished microcirculation. NO administration and arginine supplementation as a monotherapy demonstrated beneficial effects, whereas nonselective NO synthase inhibition seemed not to be beneficial, and selective NO synthase 2 inhibition was not beneficial overall.

CONCLUSIONS

Because sepsis has all the characteristics of an arginine-deficiency state, we hypothesise that arginine supplementation is a logical option in the treatment of sepsis. This is supported by substantial experimental and clinical data on NO donors and NO inhibitors. However, further evidence is required to prove our hypothesis.

Acute reduction of circulating arginine in mice does not compromise whole body NO production

The amino acid arginine is the sole precursor for nitric oxide (NO) synthesis. We have now studied the role of acutely reducing circulating arginine on whole body NO production in mice. Measurements were performed in 4 groups of mice, treated with saline (SAL) or arginase (ASE), and SAL or bacterial endotoxin (LPS). After 5 h, a 57% reduction in circulating arginine was obtained by intravenous injections of arginase (SAL/SAL: 138+/-7; ASE/SAL: 59+/-10 microM, P<0.05). Reduced circulating arginine caused a reduction in plasma arginine flux (SAL/SAL: 82+/-6; ASE/SAL: 63+/-5 nmol/(10 g b.w. min), P<0.05), but did not change whole body NO production. LPS treatment caused an increase in NO production (SAL/SAL: 1.3+/-0.3 SAL/LPS 2.3+/-0.4 nmol/(10 g b.w. min), P<0.05), presumably by NOS-2 and was unaffected by reducing circulating arginine. Also, intestinal citrulline and renal arginine production were not increased in LPS-challenged mice with reduced circulating arginine levels. The present study indicates that an acute decrease in circulating arginine does not compromise whole body NO production and provides evidence against a role for renal arginine production to counteract an acute reduction of circulating arginine.

Effects of iNOS-related NO on hearts exposed to liposoluble iron

Inducible nitric oxide synthase (iNOS) protects heart against ischemia/reperfusion injury. However, it is unknown whether the beneficial effects of iNOS are mediated by the interaction of NO with radical oxygen species (ROS). To address this issue, we examined the effects of liposoluble iron-induced ROS generation in isolated perfused hearts from rats treated with lipopolysaccharide (LPS). LPS administration (10 mg/kg, i.p., 6 h before heart removal) induced iNOS expression and increased NO production as indicated by a 3-fold elevation of nitrite level in coronary effluents relative to control hearts. An enhanced expression of hemeoxygenase 1 protein was also observed in septic hearts compared to control. Iron-induced perfusion and contractile deficits were ameliorated by LPS with more important coronary than myocardial benefits. In iron-loaded hearts, oxidative stress as measured by the 2,3 dihydroxybenzoic acid/salicylic acid concentration ratio in cardiac tissue was 23% lower in septic than in control heart although the difference did not reach significance. In addition, the presence of the NO synthase inhibitor N-nitro-L-arginine in the perfusion medium totally blocked NO production but did not reverse the protective effects of LPS. The results indicate that LPS protects from iron-induced cardiac dysfunction by mechanisms independent on ex vivo NO production and suggest that NO acts as a trigger rather than a direct mediator of the cardioprotective effects of LPS in heart exposed to iron.

Inducible nitric oxide synthase is expressed in synovial fluid granulocytes

The objective of the study was to evaluate the NO-producing potential of synovial fluid (SF) cells. SF from 15 patients with arthritis was compared with blood from the same individuals and with blood from 10 healthy controls. Cellular expression of inducible nitric oxide synthase (iNOS) was analysed by flow cytometry. High-performance liquid chromatography was used to measure l-arginine and l-citrulline. Nitrite and nitrate were measured colourimetrically utilizing the Griess' reaction. Compared to whole blood granulocytes in patients with chronic arthritis, a prominent iNOS expression was observed in SF granulocytes (P < 0.001). A slight, but statistically significant, increase in iNOS expression was also recorded in lymphocytes and monocytes from SF. l-arginine was elevated in SF compared to serum (257 +/- 78 versus 176 +/- 65 micro mol/l, P = 0.008), whereas a slight increase in l-citrulline (33 +/- 11 versus 26 +/- 9 micro mol/l), did not reach statistical significance. Great variations but no significant differences were observed comparing serum and SF levels of nitrite and nitrate, respectively, although the sum of nitrite and nitrate tended to be elevated in SF (19.2 +/- 20.7 versus 8.6 +/- 6.5 micro mol/l, P = 0.054). Synovial fluid leucocytes, in particular granulocytes, express iNOS and may thus contribute to intra-articular NO production in arthritis.

Reduced arginine availability and nitric oxide production

The precursor for nitric oxide (NO) synthesis is the amino acid arginine. Reduced arginine availability may limit NO production. Arginine availability for NO synthesis may be regulated by de novo arginine production from citrulline, arginine transport across the cell membrane, and arginine breakdown by arginase.

The flux of arginine after ischemia-reperfusion in the rat kidney

BACKGROUND

Renal arginine synthesis is regulated by arginine plasma levels. The amino acid arginine is synthesized in the proximal tubule of the kidney. Renal ischemia reperfusion (I-R) injury as seen after shock, trauma and major vascular surgery, leading to acute tubular necrosis, might reduce arginine production.

METHODS

Wistar rats received either bovine liver arginase (ASE), to lower arginine plasma levels, or saline (SAL). Following the ASE or SAL infusion, rats were randomized to receive a renal artery clamp for 70 minutes, followed by 150 minutes of reperfusion. Renal arteriovenous blood samples were measured and plasma flow was calculated in the I-R kidney (SAL/I-R and ASE/I-R) and the contralateral kidney (SAL/C-L and ASE/C-L) in order to determine renal arginine metabolism.

RESULTS

Arginase infusion resulted in lower arginine plasma levels compared to SAL treatment (SAL/I-R vs. ASE/I-R, P < 0.005, and SAL/C-L vs. ASE/C-L, P < 0.005). Renal plasma flow was similar for all groups. The kidney switched from arginine production into arginine uptake after ischemia reperfusion (SAL/I-R vs. SAL/C-L, P < 0.01, and ASE/I-R vs. ASE/C-L, P < 0.01). Renal uptake of glutamine and citrulline increased after ischemia reperfusion (SAL/I-R vs. SAL/C-L and ASE/I-R vs. ASE/C-L, both P < 0.01). Histopathological slices of the kidney showed significantly higher counts of hyperchromasia, pyknosis, nuclear fragmentation and mitoses in individual kidney cells after ischemia reperfusion.

CONCLUSION

Decreased renal arginine production is observed with unilateral ischemia-reperfusion, and this change in arginine flux could contribute to or slow the recovery from the low plasma levels of arginine seen in conditions like trauma, shock, or after vascular procedures.

A repressor in the proximal human inducible nitric oxide synthase promoter modulates transcriptional activation

The human inducible nitric oxide synthase (iNOS or NOSII) gene is regulated through an extended and complex promoter. In this study, the transcriptional regulation of human NOSII is investigated in the human colon cell line HCT-8R. Stimulation with a cytokine mix (interferon-gamma, interleukin 1-beta, and tumor necrosis factor alpha) induces NOSII mRNA accumulation, as well as promoter activity in these cells. Several random deletions were performed within the proximal 7 kb of the promoter, which led to the identification of a region, whose deletion provokes a marked increase in transcriptional activity upon cytokine stimulation. Furthermore, this region is shown to repress a viral-driven luciferase construct, mainly at basal levels. An AP-1-like sequence present in this region that is specifically recognized by nuclear proteins is shown to be involved in the repressive effect. This element is capable of repressing a viral promoter, and its deletion augments cytokine-stimulated transcription. These findings are confirmed in various cell lines and suggest a general mechanism for the control of basal levels of NOSII expression, to avoid unnecessary toxicity under normal conditions.

Quantitative in vivo assessment of arginine utilization and nitric oxide production in endotoxemia

BACKGROUND

Until recently no methods were available to quantitate nitric oxide (NO) production in vivo. The advent of stable isotope techniques has allowed quantitation of NO production in different animal models and human disease states.

METHODS

In vivo NO production was assessed with the use of stable isotope labeled arginine. Enrichments of metabolites were measured by liquid chromatography-mass spectrometry (LC-MS). Knock-out mice were used to assess the influence of knocking out inducible NOS (iNOS) or constitutively expressed NOS (cNOS) on arginine-NO metabolism. Pig models were used to assess the role of individual organs on arginine-NO fluxes.

RESULTS

In mice under basal conditions cNOS mediates half of the NO production. After endotoxin challenge NO production doubles as a result of iNOS induction and cNOS-mediated NO production is downregulated. In larger animal models (pig) whole body NO production is augmented after endotoxin challenge, largely resulting from NO production in liver, intestine and kidney. Arginine supplementation increases NO production in pigs in liver, intestine and kidney both in the basal state and after endotoxin challenge.

CONCLUSIONS

Stable isotope techniques employing LC-MS allow in vivo assessment of NO production in small and large animal models and in patients. This allows definition of the role that iNOS and cNOS-mediated NO production play in several disease states.

Renal arginine and protein synthesis are increased during early endotoxemia in mice

The kidney has an important function in arginine metabolism, because the kidney is the main endogenous source for de novo arginine production from circulating citrulline. In conditions such as sepsis, nitric oxide (NO) production is increased and is dependent on extracellular arginine availability. To elucidate the adaptive role of renal de novo arginine synthesis in a condition of increased NO production, we studied renal arginine metabolism in a mouse model of endotoxemia. Because arginine flux is largely dependent on protein flux, we also measured protein metabolism in mice. Female mice were injected intraperitoneally with lipopolysaccharide; control mice received 0.9% NaCl. Six hours later, renal blood flow was measured with the use of para-aminohippuric acid. Arginine and protein metabolism were studied using organ-balance, stable-isotope techniques. Systemic NO production was increased in the endotoxin-treated mice. In addition, renal protein synthesis and de novo arginine production from citrulline were increased. However, no effect on renal NO production was observed. In conclusion, increased renal de novo arginine production may serve to sustain systemic NO production. To our knowledge, it was shown for the first time that renal protein synthesis is enhanced in the early response to endotoxemia.

Activation of the stem cell-derived tyrosine kinase/RON receptor tyrosine kinase by macrophage-stimulating protein results in the induction of arginase activity in murine peritoneal macrophages

Regulation of macrophage activities in response to inflammatory stimuli must be finely tuned to promote an effective immune response while, at the same time, preventing damage to the host. Our lab and others have previously shown that macrophage-stimulating protein (MSP), through activation of its receptor RON, negatively regulates NO production in response to IFN-gamma and LPS by inhibiting the expression of inducible NO synthase (iNOS). Furthermore, activated macrophages from mice harboring targeted mutations in RON produce increased levels of NO both in vitro and in vivo, rendering them more susceptible to LPS-induced endotoxic shock. In this study, we demonstrate that stimulation of murine peritoneal macrophages with MSP results in the RON-dependent up-regulation of arginase, an enzyme associated with alternative activation that competes with iNOS for the substrate L-arginine, the products of which are involved in cell proliferation and matrix synthesis. Expression of other genes associated with alternative activation, including scavenger receptor A and IL-1R antagonist, is also up-regulated in MSP-stimulated murine macrophages. Stimulation of cells with IFN-gamma and LPS blocks the ability of MSP to induce arginase activity. However, pretreatment of cells with MSP results in the up-regulation of arginase and inhibits their ability to produce NO in response to IFN-gamma and LPS, even in the presence of excess substrate, suggesting that the inhibition of NO by MSP occurs primarily through its ability to regulate iNOS expression.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Influence of the Ward colon tumor on the host response to endotoxin

Cachexia and a decreased immune function are negative prognostic factors for cancer patients. While the decreased immunity results in a greater susceptibility to bacterial infection, the response of the host to the resulting infection is not clear. The experiments reported here were designed to evaluate the toxicity of endotoxin to rats with a transplantable Ward colon tumor (WCT) and to evaluate the mechanism of the observed increase in lethal toxicity. The lethal toxicity of endotoxin (lipopolysaccharide, LPS) at 5 mg/kg, i.p. was evaluated in the first of two experiments. Rats received LPS and were observed for morbidity and weight loss for a period of 11 days. A second experiment was done to evaluate the effect of LPS on the plasma nitrate/nitrite concentrations and plasma indicators of host tissue dysfunction. LPS was administered as previously described but blood and tissues were collected 5 h after LPS administration. LPS resulted in the death of 1 of 12 nontumor-bearing (NTB) rats and a transient weight loss in the survivors. This same dose of LPS, however, resulted in death for 10 of 12 WCT rats with tumor burdens less than 4% of body weight. The response of WCT rats 5 h after LPS was then compared with that of age-matched NTB rats. Plasma albumin concentrations were not affected by LPS in NTB rats but were significantly decreased in WCT rats. Peripheral blood gases were not consistently affected by LPS in either group. Peripheral blood white cell counts, except monocytes, were significantly decreased by LPS in both groups. Monocyte counts in peripheral blood were further reduced in WCT rats compared with NTB rats receiving LPS. The presence of the WCT significantly enhanced the LPS-associated increase in spleen weight. Liver weights were lower in LPS rats but there was no effect of the presence of WCT. The LPS-associated increase in plasma nitrate/nitrite concentration was enhanced by the WCT. The plasma arginine and citrulline concentrations were altered in a manner consistent with an increase in nitric oxide synthesis. An increase in plasma ornithine concentration suggests an increase in arginine metabolism by arginase. The plasma concentration of alanine aminotransferase was significantly elevated when WCT rats received LPS, suggesting enhanced hepatic dysfunction. The plasma blood urea nitrogen concentration was elevated by LPS to a greater extent in the WCT rats than in the NTB controls, indicating increased renal dysfunction. These results demonstrate that the Ward colon tumor increases the host lethal response to the endotoxin, a toxic product of bacterial infections. The mechanisms of lethality may include an increased nitric oxide synthesis in WCT rats and enhanced liver and renal toxicity.

Renal amino acid metabolism during endotoxemia in the rat

BACKGROUND

The kidney has an important function in the exchange of nitrogenous metabolites. Glutamine is the most important substrate for renal ammoniagenesis and thus plays a crucial role in acid-base homeostasis. Furthermore, the kidney is the main endogenous source for de novo arginine production from citrulline, which in turn is derived from intestinal glutamine metabolism. Sepsis is a condition in which glutamine availability is reduced, whereas the need for arginine biosynthesis may be increased. Limited bioavailability of glutamine may affect arginine synthesis, which may have consequences for nitric oxide (NO) synthesis. Therefore, we studied renal glutamine and arginine metabolism in a rat model of endotoxemia and related this to NO metabolism.

MATERIALS AND METHODS

Rats were subject to double hit endotoxemia, and control rats received 0.9% NaCl. Renal blood flow was measured using para-aminohippuric acid. Concentrations of plasma amino acids and nitrate were measured in the aorta and renal vein to calculate net renal uptake or release of amino acids and address NO production.

RESULTS

The arterial concentrations of glutamine and ammonia were not changed in endotoxemic rats. Although renal glutamine uptake was reduced, total renal ammonia production was not changed during endotoxemia. The arterial concentration of citrulline and renal citrulline uptake was not altered in endotoxin-treated rats, but renal arginine production was increased. However, no effect was observed on nitric oxide production.

CONCLUSIONS

Although the kidney has very important functions in the excretion of waste products and in interorgan metabolism, this study suggests that the kidney has a limited role in glutamine, arginine, and NO metabolism during late endotoxemia in rats.

The effect of mild endotoxemia during low arginine plasma levels on organ blood flow in rats

OBJECTIVE

Arginine is the sole precursor in the generation of the vasodilating agent nitric oxide. Arginine plasma levels are low in situations associated with endotoxemia such as major trauma, sepsis, and experimental obstructive jaundice. The aim of the study was to evaluate hemodynamics at low arginine plasma levels during a low-grade endotoxemia.

DESIGN

Randomized, placebo-controlled animal laboratory investigation.

SUBJECTS

Male Wistar rats (n = 29), anesthetized.

INTERVENTIONS

Rats were randomly assigned to receive (at t = 0 mins) an intravenous infusion of 1.5 mL of 0.9% NaCl (SAL, n = 12) or 1.5 mL of an arginase (3200 IU) solution (ASE, n = 17) over a 20-min period. After the SAL or ASE infusion, rats were randomly assigned to receive an intravenous endotoxin (lipopolysaccharide [LPS], 150 microg/kg in 1.0 mL of 0.9% NaCl; ASE/LPS, n = 10 and SAL/LPS, n = 6) challenge or a control infusion (1.0 mL of 0.9% NaCl; ASE/SAL, n = 7 and SAL/SAL, n = 6) at t = 30 mins.

MEASUREMENTS AND MAIN RESULTS

Organ blood flow was measured at t = 270 mins, using radiolabeled microspheres. At this time point, arginine plasma levels were lower in the ASE-treated rats (ASE/SAL vs. SAL/SAL and ASE/LPS vs. SAL/LPS, both p < .005, respectively). Cardiac output, mean arterial pressure, and therefore total peripheral resistance were similar for all groups. In the LPS-treated animals (SAL/LPS and ASE/LPS), cardiac output was maintained by a higher heart rate compensating the lower stroke volume. Organ blood flow to the small intestine and splanchnic blood flow was lower in the ASE/LPS-treated rats (both p < .05 when compared with other groups). Total liver blood flow was similar for all groups; the lower splanchnic blood flow was compensated for by a higher hepatic arterial blood flow.

CONCLUSION

The present study shows that low arginine plasma levels do not influence organ blood flow, whereas, during a low-grade endotoxemia, low arginine plasma levels result in reduced blood flow to the small intestine.

Redox properties of iron-dithiocarbamates and their nitrosyl derivatives: implications for their use as traps of nitric oxide in biological systems

While the Fe(2+)-dithiocarbamate complexes have been commonly used as NO traps to estimate NO production in biological systems, these complexes can undergo complex redox chemistry. Characterization of this redox chemistry is of critical importance for the use of this method as a quantitative assay of NO generation. We observe that the commonly used Fe(2+) complexes of N-methyl-D-glucamine dithiocarbamate (MGD) or diethyldithiocarbamate (DETC) are rapidly oxidized under aerobic conditions to form Fe(3+) complexes. Following exposure to NO, diamagnetic NO-Fe(3+) complexes are formed as demonstrated by the optical, electron paramagnetic resonance and gamma-resonance spectroscopy, chemiluminescence and electrochemical methods. Under anaerobic conditions the aqueous NO-Fe(3+)-MGD and lipid soluble NO-Fe(2+)-DETC complexes gradually self transform by reductive nitrosylation into paramagnetic NO-Fe(2+)-MGD complexes with yield of up to 50% and the balance is converted to Fe(3+)-MGD and nitrite. In dimethylsulfoxide this process is greatly accelerated. More efficient transformation of NO-Fe(3+)-MGD into NO-Fe(2+)-MGD (60-90% levels) was observed after addition of reducing equivalents such as ascorbate, hydroquinone or cysteine or with addition of excess Fe(2+)-MGD. With isotope labeling of the NO-Fe(3+)-MGD with (57)Fe, it was shown that these complexes donate NO to Fe(2+)-MGD. NO-Fe(3+)-MGD complexes were also formed by reversible oxidation of NO-Fe(2+)-MGD in air. The stability of NO-Fe(3+)-MGD and NO-Fe(2+)-MGD complexes increased with increasing the ratio of MGD to Fe. Thus, the iron-dithiocarbamate complexes and their NO derivatives exhibit complex redox chemistry that should be considered in their application for detection of NO in biological systems.

Glutamine is essential for nitric oxide synthesis by murine macrophages

BACKGROUND

Recent studies suggest an interaction between l-arginine (Arg) and l-glutamine (Gln) in the control of nitric oxide (NO) synthesis. Endotoxemia enhances Gln demand and NO production. This study was initiated to investigate the effects of altered Gln availability on the capacity of macrophages to produce NO and the interaction of Gln with l-citrulline (Cit) and Arg in the regulation of endotoxin-stimulated NO synthesis.

METHODS

Cultures of RAW 264.7 macrophages in MEM containing Gln (0 to 100 mM) or Arg (0 or 0.6 mM) and supplemented or not with Cit (0.31 to 10 mM) were exposed to Escherichia coli lipopolysaccharide (LPS) at 0.001 and 1 microg/ml. After 24-h incubation, supernatants were evaluated for nitrite concentrations by Greiss reaction as a measure of NO synthesis.

RESULTS

LPS stimulated nitrite synthesis in a dose-dependent fashion. Macrophages cultured in Gln-free medium containing Arg (0.6 mM) did not produce NO when stimulated with LPS. In contrast, in the presence of Arg and 0.001 microg/ml LPS, adding as little as 0.31 mM Gln resulted in a 23-fold increase in NO production (from 0.13 +/- 0. 02 to 2.92 +/- 0.06 nmol/ml) (P < 0.0001). Furthermore, a dose-dependent increase in LPS-stimulated nitrite release was observed with increasing amounts of Gln to as much as 1 mM. LPS-stimulated macrophages cultured in Arg-free medium containing Gln (0.31-10 mM) did not produce significant amounts of nitrite. However, in the absence of Arg, increasing extracellular Gln levels to 100 mM in the culture medium resulted in nitrite synthesis (2.39 +/- 0.11 nmol/ml). Detectable levels of nitrite (2.84 +/- 0.21 nmol/ml) were also documented when stimulated macrophages were incubated in culture medium lacking Arg but containing Cit (0.31 mM) and Gln (2 mM). Increasing Cit levels (0.63 to 10 mM) significantly augmented nitrite release (P < 0.05). Once again, no detectable levels of nitrite were observed when macrophages were cultured in Gln-free medium, even when Arg and Cit were present.

CONCLUSION

These results suggest that Gln is an essential amino acid for NO synthesis by macrophages and raise the strong possibility that Gln acts with nitric oxide synthase to catalyze the conversion of Arg to NO. The consumption of Gln during sepsis may represent NO production.

Induction of intracellular arginase activity does not diminish the capacity of macrophages to produce nitric oxide in vitro

The hypothesis was tested that induction of arginase expression in macrophages (M phi) diminishes nitric oxide (NO) synthesis due to intracellular competition between arginase and inducible nitric oxide synthase (iNOS) for L-arginine (L-arg). Murine M phi cell lines and bone marrow-derived M phi (BMM) were stimulated to express either iNOS or arginase or to co-express these two enzymes. The response pattern obtained was complex but allowed the following conclusions: (i) iNOS and arginase are differentially regulated. (ii) High intracellular arginase levels do not limit the capacity of M phi to synthesize NO even when the L-arg concentration in the culture medium is lowered to physiological levels. (iii) Arginase levels in BMM pre-exposed to either M phi colony-stimulating factor (M-CSF) or granulocyte-M phi colony-stimulating factor (GM-CSF) differ markedly, but iNOS expression and NO synthesis by the two BMM types is similar. (iv) Regulation of iNOS and arginase differs between primary murine bone marrow M phi and murine M phi cell lines. (v) Arginase activity appears to be inhibited during high-output NO synthesis. Taken together, our results show that NO production by M phi is not compromised by conditions that increase intracellular arginase activity.

Electron spin resonance detection of nitric oxide generation in major organs from LPS-treated rats

The increased production of nitric oxide (NO) has been implicated as the basis for myocardial dysfunction and the lack of response to vasoconstrictors during endotoxin shock induced by lipopolysaccharide (LPS). Our objective was to evaluate and compare NO production in major organs of rats treated with LPS, 1 or 14 mg/kg. A NO spin-trapping technique using electron spin resonance (ESR) spectroscopy has been used to study NO production in the liver, the kidney, the aorta, and the heart. The method was based on the trapping of NO by a metal-chelator complex consisting of N-methyl-D-glucamine dithiocarbamate (MGD) and reduced iron (Fe2+) to form a stable [(MGD)2-Fe2+-NO] complex, giving rise to a characteristic triplet ESR spectrum with g = 2.04 and aN = 12.65 G: Iron was quantified in the different organs to study the [(MGD)2-Fe2+] complex distribution. Six hours after intravenous injection of 1 or 14 mg/kg of LPS, we observed large increases in the [(MGD)2-Fe2+-NO] adduct signal in the liver, the kidney, and in the aorta, strongly suggesting an increased production of NO in these organs. The [(MGD)2-Fe2+-NO] adduct was also detected in the heart, 6 h after injection of LPS. Moreover, we observed dose-dependent increases in [(MGD)2-Fe2+-NO] adduct in the heart, whereas no changes were observed in the other organs. Concurrently, the [(MGD)2-Fe2+-NO] adduct was not detected in the blood from rats treated with LPS, although circulating nitrosylhemoglobin, nitrite, and nitrate levels increased. The spin-trapping technique allowed us to monitor organ-specific formation of NO after LPS administration and for the first time demonstrated direct NO production in aorta and heart of LPS-treated animals.

Applications of electron paramagnetic resonance spectroscopy to study interactions of iron proteins in cells with nitric oxide

Nitric oxide and species derived from it have a wide range of biological functions. Some applications of electron paramagnetic resonance (EPR) spectroscopy are reviewed, for observing nitrosyl species in biological systems. Nitrite has long been used as a food preservative owing to its bacteriostatic effect on spoilage bacteria. Nitrosyl complexes such as sodium nitroprusside, which are added experimentally as NO-generators, themselves produce paramagnetic nitrosyl species, which may be seen by EPR. We have used this to observe the effects of nitroprusside on clostridial cells. After growth in the presence of sublethal concentrations of nitroprusside, the cells show they have been converted into other, presumably less toxic, nitrosyl complexes such as (RS)2Fe(NO)2. Nitric oxide is cytotoxic, partly due to its effects on mitochondria. This is exploited in the destruction of cancer cells by the immune system. The targets include iron-sulfur proteins. It appears that species derived from nitric oxide such as peroxynitrite may be responsible. Addition of peroxynitrite to mitochondria led to depletion of the EPR-detectable iron-sulfur clusters. Paramagnetic complexes are formed in vivo from hemoglobin, in conditions such as experimental endotoxic shock. This has been used to follow the course of production of NO by macrophages. We have examined the effects of suppression of NO synthase using biopterin antagonists. Another method is to use an injected NO-trapping agent, Fe-diethyldithiocarbamate (Fe-DETC) to detect accumulated NO by EPR. In this way we have observed the effects of depletion of serum arginine by arginase. In brains from victims of Parkinson's disease, a nitrosyl species, identified as nitrosyl hemoglobin, has been observed in substantia nigra. This is an indication for the involvement of nitric oxide or a derived species in the damage to this organ.

Arginase modulates nitric oxide production in activated macrophages

In macrophages and many other cell types, L-arginine is used as a substrate by both nitric oxide synthase (NOS) and arginase to produce nitric oxide (NO) and urea, respectively. Because the availability of L-arginine is a major determinant for NO synthesis in the activated macrophage, we hypothesized that NO production may be reduced by arginase via depleting the common substrate in this cell type. To test this hypothesis, we investigated the effect of an arginase inhibitor, L-norvaline, on NO production in J774A.1 mouse macrophages activated by lipopolysaccharide (LPS, 1.0 microgram/ml) for 22 h. In the absence of LPS, macrophages produced a low level of NO. In contrast, NO production from these cells was significantly increased in the presence of LPS. Increasing extracellular levels of L-arginine (0.01-0.8 mM) produced a concomitant increase in NO production of activated macrophages. L-Norvaline (10 mM), which specifically inhibits arginase activity (i.e., reducing urea production by 50%) without altering NOS activity, enhanced NO production (by 55%) from activated macrophages. The enhancement of NO production by L-norvaline was inversely related to the extracellular level of L-arginine. A more pronounced increase in NO production was observed at the lower level of extracellular L-arginine, i.e., a 55 vs. 28% increase for 0.05 and 0.1 mM extracellular L-arginine, respectively. When the L-arginine concentration exceeded 0.5 mM, the L-norvaline effect was abolished. These results indicate that arginase can compete with NOS for their common substrate and thus inhibit NO production. This regulatory mechanism may be particularly important when the extracellular supply of L-arginine is limited.

Effect of L-lysine on nitric oxide overproduction in endotoxic shock

1. An enhanced production of nitric oxide (NO) from L-arginine, related to the diffuse expression of an inducible NO synthase (iNOS), contributes to the pathogenesis of endotoxic shock. Since iNOS activity depends on extracellular L-arginine, we hypothesized that limiting cellular L-arginine uptake would reduce NO production in endotoxic shock. We investigated the effects of L-lysine, an inhibitor of L-arginine uptake through system y+, on NO production, multiple organ dysfunction and lactate levels, in normal and endotoxaemic rats. 2. Anaesthetized rats challenged with intravenous lipopolysaccharide (LPS, 10 mg kg[-1]) received a 5 h infusion of either L-lysine (500 micromol kg(-1) h(-1), n = 12) or isotonic saline (2 ml kg(-1) h(-1), n = 11). In rats treated with saline, LPS produced a large increase in plasma nitrate and L-citrulline concentrations at 5 h, both markers of enhanced NO production. LPS also caused severe hypotension, low cardiac output and marked hyperlactataemia. All these changes were significantly reduced by L-lysine administration. 3. Endotoxaemia also caused a significant rise in the plasma levels of alanine aminotransferase (ALAT), lipase, urea and creatinine, and hence, liver, pancreatic and renal dysfunction. These changes tended to be less pronounced in rats treated with L-lysine, although the differences did not reach statistical significance. 4. Similar experiments were conducted in 10 rats challenged with LPS vehicle in place of LPS and then treated with L-lysine (500 micromol kg(-1) h(-1), n = 5) or saline (2 ml kg(-1) h(-1), n = 5) for 5 h. In these animals, all the haemodynamic and metabolic variables remained stable and not statistically different between both treatment groups, except for a slight rise in ALAT, which was comparable in L-lysine and saline-treated rats. 5. In conclusion, L-lysine, an inhibitor of cellular L-arginine uptake, reduces NO production and exerts beneficial haemodynamic effects in endotoxaemic rats. L-lysine also reduces hyperlactataemia and tends to blunt the development of organ injury in these animals. Contrastingly, L-lysine has no effects in the absence of endotoxin and thus appears to act as a selective modulator of iNOS activity.

Complexes of Fe2+ with diethyldithiocarbamate or N-methyl-D-glucamine dithiocarbamate as traps of nitric oxide in animal tissues: comparative investigations

In EPR experiments on mice it was demonstrated that a hydrophobic complex Fe2+ with diethyldithiocarbamate (DETC) is a more efficient selective NO trap than a hydrophilic complex Fe2+ with N-methyl-D-glutamine dithiocarbamate (MGD). This difference can be due to the higher stability of paramagnetic nitrosyl iron complex with DETC (MNIC-DETC) formed by NO binding to Fe2+-DETC in animal tissues in vivo. The complex analogue MNIC-MGD is reversibly oxidized in animal blood to transform into the diamagnetic EPR-silent form. The latter is detectable also in urine of animals, especially of those treated with bacterial lipopolysaccharide which initiates the enhanced NO production in the organism. We suggest that NO2 or peroxynitrite formed from endogenous NO can serve as an agent reversibly oxidizing MNIC-MGD in these animals.

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.

The cloning, expression and crystallisation of a thermostable arginase

The gene for the thermostable arginase from the thermophilic bacterium 'Bacillus caldovelox' has been cloned and sequenced. Expression of recombinant arginase at high levels has been achieved in E. coli using an inducible T7 RNA polymerase-based system. A facile purification procedure incorporating a heat-treatment step yielded 0.2 g of recombinant arginase per litre of induced culture. The kinetic properties of the purified recombinant protein are essentially identical to the native enzyme. The recombinant protein has been crystallised and one crystal form is isomorphous to crystals of the native protein.

Cytokines and insulin induce cationic amino acid transporter (CAT) expression in cardiac myocytes. Regulation of L-arginine transport and no production by CAT-1, CAT-2A, and CAT-2B

Cytokine-dependent production of nitric oxide (NO) by rat cardiac myocytes is a consequence of increased expression of the inducible isoform of nitric oxide synthase (iNOS or NOS2) and, in the presence of insulin, depresses the contractile function of these cells in vivo and in vitro. Experiments reported here show that L-lysine, a competitive antagonist of L-arginine uptake, suppressed NO production (detected as nitrite accumulation) by interleukin (IL)-1beta and interferon (IFN) gamma-pretreated cardiac myocytes by 70%, demonstrating that NO production is dependent on L-arginine uptake. Cardiac myocytes constitutively exhibit a high-affinity L-arginine transport system (Km = 125 microM; Vmax = 44 pmol/2 X 10(5) cells/min). Following a 24-h exposure to IL-1beta and IFNgamma, arginine uptake increases Vmax = 167 pmol/2 X 10(5) cells/min) and a second low-affinity L-arginine transporter activity appears (Km = 1.2 mM). To examine the molecular basis for these cytokine-induced changes in arginine transport, we examined expression of three related arginine transporters previously identified in other cell types. mRNA for the high-affinity cationic amino acid transporter-1 (CAT-1) is expressed in resting myocytes and steady-state levels increase by 10-fold following exposure to IL-1beta and IFNgamma. Only cytokine-pretreated myocytes expressed a second high-affinity L-arginine transporter, CAT-2B, as well as a low-affinity L-arginine transporter, CAT-2A. In addition, insulin, which potentiated cytokine-dependent NO production independent of any change in NOS activity, increased myocyte L-arginine uptake by 2-fold and steady-state levels of CAT-1, but not CAT-2A or CAT-2B mRNA. Thus, NO production by cardiac myocytes exposed to IL-1beta plus IFNgamma appears to be dependent on the coinduction of CAT-1, CAT-2A, and CAT-2B, while insulin independently augments L-arginine transport through CAT- 1.

L-arginine depletion inhibits glomerular nitric oxide synthesis and exacerbates rat nephrotoxic nephritis

Nitric oxide (NO) synthesis is induced in glomeruli in glomerulonephritis; its role in the pathogenesis of glomerular injury is unknown. Interpretation of its role using the currently available analogues of L-arginine as in vivo inhibitors of NO is complicated by their lack of specificity for inducible NO synthase (iNOS). As NO synthesis by iNOS depends on extracellular L-arginine, we have here examined effects of L-arginine depletion on glomerular NO synthesis and the course of accelerated nephrotoxic nephritis (NTN). Arginase, which converts L-arginine to urea and L-ornithine, was used to achieve L-arginine depletion. A single dose of i.v. arginase produced complete depletion of plasma arginine for four hours. Two forms of NTN were induced in preimmunised rats by nephrotoxic globulin: (1) the systemic form of the model by intravenous nephrotoxic globulin; or (2) the unilateral form of model by left kidney perfusion with nephrotoxic globulin, which avoids the complications of systemic administration of nephrotoxic globulin. Arginase reduced plasma arginine levels and the synthesis of nitrite (the stable end-product of NO) by NTN glomeruli (95% inhibition). Proteinuria was exacerbated. There was no effect on early (24 hr) leukocyte infiltration. In the systemic form of the model arginine depletion by i.v. arginase increased glomerular thrombosis at 24 hours, and the severity of histological changes at four days, accompanied by systemic hypertension. In the unilateral form of the model, where i.v. arginase did not induce hypertension, there was no increase in thrombosis or histological severity of nephritis. These results show that arginine depletion, which inhibits glomerular NO synthesis in NTN, leads to increased proteinuria. Where injury is severe, or accompanied by systemic hypertension, the disease is further exacerbated by glomerular thrombosis. These results suggest that NO has an important role in limiting acute glomerular injury.