Expression of citrulline-nitric oxide cycle in lipopolysaccharide and cytokine-stimulated rat astroglioma C6 cells

In Vitro Studies of Prednisolone Loaded PLGA Nanoparticles-Surface Functionalized With Folic Acid on Glioma and Macrophage Cell Lines

Background: Glucocorticoids are employed for their anti-inflammatory effects in treatingglioma, whose cells are known to overexpress the folate receptors. Some glucocorticoids haveshown inhibitory effects, but the efficacy of prednisolone when delivered via folate receptormediateduptake, has not been attempted. The study aimed to assess the efficacy of targeteddelivery of prednisolone on glioma cell lines like C6 and U87 via the folate receptors. Methods: Targeted delivery of prednisolone was achieved by initially conjugating folic acid (FA)to the di-block copolymer of polylactic acid (PLA) – polyethylene glycol (PEG). This moietycarrying di-block copolymer was incorporated on the surface of the drug-loaded poly lactic-coglycolicacid (PLGA) nanoparticle (NP) by employing the Interfacial Activity Assisted SurfaceFunctionalization (IAASF) technique. The NPs were evaluated for size, zeta potential, and drugloading. It was characterized using particle size analyser, SEM, 1H-NMR, and XRD. cell uptake,cytotoxicity, and anti-inflammatory activities were studied for various formulations. Results: The cytotoxicity assay indicated a high cell growth inhibitory effect of drug encapsulatedNPs with FA moiety as compared to free drug and NPs without the moiety for an incubationperiod of three, five, and six days. The growth-inhibitory effect of the free drug was short-lived,whereas FA functionalized NPs showed higher uptake and sustained inhibitory effect, and werealso able to significantly control the release of pro-inflammatory cytokines like tumour necrosisfactor-alpha (TNF-α) and nitric oxide (NO). Conclusion: Uptake, attenuation of pro-inflammatory signals, and the inhibitory effect ofprednisolone on the cells were more effective when targeted with the FA moiety on the surfaceof NPs as compared to free drug and NPs without the moiety.

Protective Effect of Citrulline on the Hearts of Rats with Sepsis Induced by Cecal Ligation and Puncture

Purpose

To investigate the protective effect of citrulline (Cit) on the hearts of rats with sepsis.

Methods

Wistar rats were divided into the normal, sham-operated, CLP, Cit, and CLP+Cit groups. Routine blood tests were performed, and the blood biochemical indexes were measured. Pathological changes in the cardiac tissues were observed. The levels of NO and iNOS in blood and SOD activity and MDA levels in the heart were measured.

Results

Less inflammatory cell infiltration of the myocardial fibers and significantly decreased white blood cell count, absolute neutrophil count, neutrophil percentage, CK, HBDH, and NO (all P<0.05) were detected in the CLP+Cit group compared with the CLP group. In addition, SOD activity and MDA levels in heart tissues were, respectively, higher and lower in the CLP+Cit group than in the CLP group (both P<0.05).

Conclusions

Cit reduces pathological damage in the heart and enhances the heart's antioxidant capacity, thereby protecting cardiomyocytes.

Citrulline uptake in rat cerebral cortex slices: modulation by Thioacetamide -Induced hepatic failure

L-citrulline (Cit) is a co-product of NO synthesis and a direct L-arginine (Arg) precursor for de novo NO synthesis. Acute liver failure (ALF) is associated with increased nitric oxide (NO) and cyclic GMP (cGMP) synthesis in the brain, indirectly implicating a role for active transport of Cit. In the present study we characterized [(3)H]Cit uptake to the cortical brain slices obtained from control rats and rats with thioacetamide (TAA)-induced ALF ("TAA slices"). In both control and TAA slices the uptake was partially Na(+)-dependent and markedly inhibited by substrates of systems L and N, including L-glutamine (Gln), which accumulates in excess in brain during ALF. Cit uptake was not affected by Arg, the y(+)/y(+)L transport system substrate, nor by amino acids taken up by systems A, xc (-)or XAG. The Vmax of the uptake in TAA slices was ~60 % higher than in control slices. Chromatographic (HPLC) analysis revealed a ~30 % increase of Cit concentration in the cerebral cortical homogenates of TAA rats. The activity of argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL), the two enzymes of Cit-NO cycle catalyzing synthesis of Arg, showed an increase in TAA rats, consistent with increased ASS and ASL protein expression, by ~30 and ~20 %, respectively. The increased Cit-NO cycle activity was paralleled by increased expression of mRNA coding for inducible nitric oxide synthase (iNOS). Taken together, the results suggest a role for Cit in the activation of cerebral NO synthesis during ALF.

Efficient subtractive cloning of genes activated by lipopolysaccharide and interferon γ in primary-cultured cortical cells of newborn mice

Innate immune responses play a central role in neuroprotection and neurotoxicity during inflammatory processes that are triggered by pathogen-associated molecular pattern-exhibiting agents such as bacterial lipopolysaccharide (LPS) and that are modulated by inflammatory cytokines such as interferon γ (IFNγ). Recent findings describing the unexpected complexity of mammalian genomes and transcriptomes have stimulated further identification of novel transcripts involved in specific physiological and pathological processes, such as the neural innate immune response that alters the expression of many genes. We developed a system for efficient subtractive cloning that employs both sense and antisense cRNA drivers, and coupled it with in-house cDNA microarray analysis. This system enabled effective direct cloning of differentially expressed transcripts, from a small amount (0.5 µg) of total RNA. We applied this system to isolation of genes activated by LPS and IFNγ in primary-cultured cortical cells that were derived from newborn mice, to investigate the mechanisms involved in neuroprotection and neurotoxicity in maternal/perinatal infections that cause various brain injuries including periventricular leukomalacia. A number of genes involved in the immune and inflammatory response were identified, showing that neonatal neuronal/glial cells are highly responsive to LPS and IFNγ. Subsequent RNA blot analysis revealed that the identified genes were activated by LPS and IFNγ in a cooperative or distinctive manner, thereby supporting the notion that these bacterial and cellular inflammatory mediators can affect the brain through direct but complicated pathways. We also identified several novel clones of apparently non-coding RNAs that potentially harbor various regulatory functions. Characterization of the presently identified genes will give insights into mechanisms and interventions not only for perinatal infection-induced brain damage, but also for many other innate immunity-related brain disorders.

Proteomic analysis of the NOS2 interactome in human airway epithelial cells

The cytokine-inducible isoform of nitric oxide synthase (NOS2) is constitutively expressed in human respiratory epithelia and is upregulated in inflammatory lung disease. Here, we sought to better define the protein interactions that may be important for NOS2 activity and stability, as well as to identify potential targets of NOS2-derived NO, in the respiratory epithelium. We overexpressed Flag-tagged, catalytically-inactive NOS2 in A549 cells and used mass spectrometry to qualitatively identify NOS2 co-immunoprecipitating proteins. Stable isotope labeling of amino acids in cell culture (SILAC) was used to quantify the coordinate effects of cytokine stimulation on NOS2-protein interactions. Multi-protein networks dominated the NOS2 interactome, and cytokine-inducible interactions with allosteric activators and with the ubiquitin-proteasome system were correlated with cytokine-dependent increases in NO metabolites and in NOS2 ubiquitination. The ubiquitin ligase scaffolding protein, FBXO45, was identified as a novel, direct NOS2 interactor. Similar to the SPRY domain-containing SOCS box (SPSB) proteins, FBXO45 requires Asn27 in the (23)DINNN(27) motif of NOS2 for its interaction. However, FBXO45 is unique from the SPSBs in that it recruits a distinct E3 ligase complex containing MYCBP2 and SKP1. Collectively, these findings demonstrate the general utility of interaction proteomics for defining new aspects of NOS2 physiology.

Hyperammonemia increases the expression and activity of the glutamine/arginine transporter y+ LAT2 in rat cerebral cortex: implications for the nitric oxide/cGMP pathway

The pathogenesis of hepatic encephalopathy (HE) is associated with hyperammonemia (HA) and subsequent exposure of the brain to excess of ammonia. Alterations of the NO/cGMP pathway and increased glutamine (Gln) content are collectively responsible for many HE symptoms, but how the two events influence each other is not clear. Previously we had shown that Gln administered intracerebrally inhibited the NO/cGMP pathway in control rats and even more so in rats with HA, and we speculated that this effect is due to inhibition by Gln of arginine (Arg) transport (Hilgier et al., 2009). In this study we demonstrate that a 3-day HA in the ammonium acetate model increases the expression in the brain of y(+)LAT2, the heteromeric transporter which preferentially stimulates Arg efflux from the cells in exchange for Gln. The expression of the basic amino acid transporter CAT1, transporting Arg but not Gln remained unaffected by HA. Multiple parameters of Arg or Gln uptake and/or efflux and their mutual dependence were altered in the cerebral cortical slices obtained from HA rats, in a manner indicating enhanced y(+)LAT2-mediated transport. HA elevated Gln content and decreased cGMP content as measured both in the cerebral cortical tissue and microdialysates. Intracortical administration of 6-diazo-5-oxo-L-norleucine (DON), which inhibits Gln fluxes between different cells of the CNS, attenuated the HA-induced decrease of cGMP in the microdialysates of HA rats, but not of control rats. The results suggest that, reduced delivery of Arg due to enhanced y(+)LAT2-mediated exchange of extracellular Gln for intracellular Arg may contribute to the decrease of NO/cGMP pathway activity evoked in the brain by HA.

Regulation of nitric oxide synthesis and apoptosis by arginase and arginine recycling

Nitric oxide (NO) is synthesized from arginine and O2 by NO synthase (NOS). Citrulline formed as a by-product of the NOS reaction can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). We found that AS and sometimes AL are coinduced with inducible NOS (iNOS) in various cells. In these cells, NO was synthesized from citrulline (via arginine) as well as from arginine, indicating operation of the citrulline-NO cycle. On the other hand, we found that arginase isoforms (types I and II) are coinduced with iNOS by LPS in rodent tissues and cultured macrophages. Km values for arginine of arginase I and II (approximately 10 mmol/L) are much higher than that of iNOS (approximately 5 micromol/L), whereas Vmax of arginase I and II were 10(3)-10(4) times higher than that of iNOS in activated macrophages. Thus, Vmax/Km values of arginases were close to that of iNOS, and these enzymes were expected to compete for arginine in the cells. In fact, NO production by iNOS in activated macrophages was decreased by coinduction of arginase I or arginase II. Low concentrations of NO protect cells from apoptosis, whereas excessive NO causes apoptosis. We found that NO causes endoplasmic reticulum (ER) stress, induces a transcription factor, CAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), and leads to apoptosis. These results suggest that the arginine metabolic enzymes and the ER stress-CHOP pathway can be good targets to regulate NO production and NO-mediated apoptosis in diseases associated with overproduction or impaired production of NO.

Arginine metabolic enzymes, nitric oxide and infection

Nitric oxide (NO) is synthesized from arginine by NO synthase (NOS), and the availability of arginine is one of the rate-limiting factors in cellular NO production. Citrulline that is formed as a by-product of the NOS reaction can be recycled to arginine by successive actions of argininosuccinate synthetase (AS) and argininosuccinate lyase (AL), forming the citrulline-NO cycle. AS and sometimes AL have been shown to be coinduced with inducible NOS (iNOS) in various cell types including activated macrophages, microglia, vascular smooth muscle cells, glial cells, neuronal PC12 cells, retinal pigment epithelial cells, and pancreatic beta-cells. Coinduction of endothelial NOS (eNOS), AS, and AL are observed in human umbilical vein endothelial cells. In contrast, arginase can downregulate NO production by decreasing intracellular arginine concentrations. iNOS and arginase activities are regulated reciprocally in macrophages by cytokines, and this may guarantee the efficient production of NO. In contrast, iNOS and arginase isoforms (type I and/or II) are coinduced in immunostimulated macrophages, but not in PC12 cells and glial cells. These results indicate that NO production is modulated by the recycling and degradation of arginine. Arginase also plays an important role in regulation of polyamine and proline synthesis.

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

Argininosuccinate synthetase (ASS, EC 6.3.4.5) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues. Indeed, discovery of the citrulline-NO cycle has increased interest in this enzyme that was found to represent a potential limiting step in NO synthesis. Depending on arginine utilization, location and regulation of ASS are quite different. In the liver, where arginine is hydrolyzed to form urea and ornithine, the ASS gene is highly expressed, and hormones and nutrients constitute the major regulating factors: (a) glucocorticoids, glucagon and insulin, particularly, control the expression of this gene both during development and adult life; (b) dietary protein intake stimulates ASS gene expression, with a particular efficiency of specific amino acids like glutamine. In contrast, in NO-producing cells, where arginine is the direct substrate in the NO synthesis, ASS gene is expressed at a low level and in this way, proinflammatory signals constitute the main factors of regulation of the gene expression. In most cases, regulation of ASS gene expression is exerted at a transcriptional level, but molecular mechanisms are still poorly understood.

Induction of citrulline-nitric oxide (NO) cycle enzymes and NO production in immunostimulated rat RPE-J cells

Nitric oxide (NO) has been implicated in many physiological and pathological conditions in the eyes. The induction of inducible NO synthase (iNOS) and NO production have been noted in immunostimulated retinal pigment epithelial (RPE) cells. Cellular NO production depends on the availability of arginine, a substrate for NOS. Arginine can be regenerated from citrulline, another product of the NOS reaction, by argininosuccinate synthetase and argininosuccinate lyase, forming the citrulline-NO cycle. When rat RPE-J cells were treated with interferon-gamma (IFNgamma), tumor necrosis factor-alpha (TNFalpha) and lipopolysaccharide (LPS), and expression of the citrulline-NO cycle enzymes and related enzymes was analyzed, iNOS and argininosuccinate synthetase were highly induced at both mRNA and protein levels. On the other hand, argininosuccinate lyase was not induced. Among other related enzymes and transporters, mRNA for cationic amino acid transporter (CAT)-1 was weakly induced, whereas those for CAT-2, arginase I and II, ornithine aminotransferase and ornithine decarboxylase remained little changed. NO was produced by cells after stimulation with TNFalpha, IFNgamma and LPS. The induction of iNOS mRNA and the production of NO by these immunostimulated cells was further enhanced by cAMP. NO was produced from citrulline as well as from arginine. Our findings indicate that in activated RPE-J cells citrulline-arginine recycling is important for NO production.

Coinduction of nitric oxide synthase and arginine metabolic enzymes in endotoxin-induced uveitis rats

The regulation of expression of the arginine-recycling enzymes and arginase isoforms in association with inducible nitric oxide synthase (iNOS) in the eye of endotoxin-induced uveitis (EIU) rats is investigated. An animal model of EIU was created in Wistar rats by intravitreal injection of lipopolysaccharide (LPS). mRNAs for argininosuccinate synthase (AS) and arginase I as well as for iNOS, measured by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR), were induced in the eye of EIU rats. iNOS mRNA increased markedly 3 hr after injection, reached a maximum at 6-12 hr, and then decreased at 24 hr. AS mRNA remained little change at 3 hr and increased maximally at 6 hr (by about 3.3-fold), whereas arginase I mRNA increased later and reached a maximum at 12 hr (by about 4.2-fold). iNOS, AS, and arginase I proteins were also induced. AL and arginase II mRNAs remained little changed. In immunohistochemical analysis, iNOS, AS and arginase I were almost colocalized in infiltrated inflammatory cells in the vitreous, iris, ciliary body and inner layers of the retina. In conclusion, AS and arginase I are coinduced with iNOS in infiltrated inflammatory cells in the eyes of EIU rats, and may regulate NO production by changing intracellular concentration of arginine.

Regulation of enzymes of the urea cycle and arginine metabolism

The urea cycle is comprised of five enzymes but also requires other enzymes and mitochondrial amino acid transporters to function fully. The complete urea cycle is expressed in liver and to a small degree also in enterocytes. However, highly regulated expression of several enzymes present in the urea cycle occurs also in many other tissues, where these enzymes are involved in synthesis of nitric oxide, polyamines, proline and glutamate. Glucagon, insulin, and glucocorticoids are major regulators of the expression of urea cycle enzymes in liver. In contrast, the "urea cycle" enzymes in nonhepatic cells are regulated by a wide range of pro- and antiinflammatory cytokines and other agents. Regulation of these enzymes is largely transcriptional in virtually all cell types. This review emphasizes recent information regarding roles and regulation of urea cycle and arginine metabolic enzymes in liver and other cell types.

The in vitro effects of Pb acetate on NO production by C6 glial cells

The CNS neurotoxic effects of lead (Pb) are well documented; however, the molecular toxicity targets have not been clearly delineated. Astroglial cells, which are the most abundant cells in the brain and provide critical support to the neurons, are known to accumulate Pb. Although NO generated by inducible NO synthase (iNOS) in glial cells has been associated with many neurotoxic events, it can also serve to protect by modulating blood flow, increase antimicrobial and tumoricidal activities, and promote immune responses following injury or insult. The present investigations were designed to test the hypothesis that Pb exposure may perturb cytokine signal transduction pathways leading to NO production by astroglial cells. Pretreatment with Pb acetate (500 nM-10 microM) attenuated the generation of NO in a concentration-dependent manner up to 90%, and suppressed iNOS protein expression, as well as interfered with the homeostatic functions of calcium in the cytokine-induced NO signal transduction pathway. In addition, pretreatment with staurosporine, a serine-threonine kinase inhibitor, or KT5720, a specific protein kinase A inhibitor (PKA), inhibited cytokine-induced NO production in a concentration-dependent manner with IC(50) values of 26.3 and 346.7 nM, respectively. Therefore, Pb may impede events within the PKA signal transduction pathway; although, based on results from a gel shift assay, Pb does not directly affect PKA enzyme activity. Taken together, these results suggest the possibility that the suppressive effect of Pb acetate on cytokine-induced NO production in glial cells may be implicated in the neurophysiologic changes noted following occupational or environmental exposure to Pb.

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.

Inducible nitric oxide synthase and argininosuccinate synthetase: co-induction in brain tissue of patients with Alzheimer's dementia and following stimulation with beta-amyloid 1-42 in vitro

In Alzheimer's disease (AD), amyloid plaques within the brain are surrounded by activated glial cells (microglia and astrocytes). The mechanisms of glial activation and its effect on disease progression are not fully understood. Growing evidence suggests that beta-amyloid (Abeta) peptide, a major constituent of the amyloid plaque, is critically involved in the induction of an inflammatory response. The goal of this study was to examine the role of Abeta in the pathogenesis of local inflammation and neuronal cell death. We found increased mRNA levels of inducible nitric oxide synthase (iNOS) and the arginine regenerating enzyme argininosuccinate synthetase (ASS) within cortices of AD patients suggesting high output NO production. In vitro, synthetic Abeta1-42 and to a lesser extent Abeta1-40 induced iNOS and ASS transcription with consecutive NO overproduction in mixed rat neuronal-glial cultures. Furthermore, Abeta-stimulation lead to an increased release of inflammatory cytokines interleukin (IL)-1beta, IL-6 and tumor necrosis factor-alpha. Again, Abeta1-42 had a much more pronounced effect as compared to Abeta1-40. Our data suggest that Abeta1-42 is a key mediator of glial activation and via the induction of inflammatory mediators may be a critical component of the neurodegenerative process in AD.

Expression, regulation and function of carrier proteins for cationic amino acids

Different carrier proteins exhibiting distinct transport properties participate in cationic amino acid transport. There are sodium-independent systems, such as b+, y+, y+L and b0,+, and a sodium-dependent system B0,+, most of which have now been identified at the molecular level. In most non-epithelial cells, members of the cationic amino acid transporter (CAT) family mediating system y+ activity seem to be the major entry pathway for cationic amino acids. CAT proteins underlie complex regulation at the transcriptional, post-transcriptional and activity levels. Recent evidence indicates that individual CAT isoforms are necessary for providing the substrate for nitric oxide synthesis, for example CAT-1 for Ca2+-independent nitric oxide production in endothelial cells and CAT-2B for sustained nitric oxide production in macrophages.

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.

Co-induction of argininosuccinate synthetase, cationic amino acid transporter-2, and nitric oxide synthase in activated murine microglial cells

Nitric oxide (NO) produced by activated microglia has been implicated in many pathophysiological events in the brain including neurodegenerative diseases. Cellular NO production depends absolutely on the availability of arginine, a substrate of NO synthase (NOS). Murine microglial MG5 cells were treated with bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma), and expression of inducible NO synthase (iNOS) and arginine-supplying enzymes was investigated by RNA blot analysis. iNOS mRNA was strongly induced after treatment and reached a maximum at 6-12 h. mRNA for argininosuccinate synthetase (AS), a citrulline-arginine recycling enzyme, increased at 6 h and reached a maximum at 12 h. Immunoblot analysis showed that iNOS and AS proteins were also induced. In addition, mRNA encoding the cationic amino acid transporter-2 (CAT-2) was strongly induced shortly after treatment. Induction of mRNAs for iNOS, AS, and CAT-2 by LPS/IFN-gamma was also observed following stimulation of rat primary microglial cells. These results strongly suggest that both arginine transport by CAT-2 and citrulline-arginine recycling are important for high-output production of NO in activated microglial cells.

Coinduction of endothelial nitric oxide synthase and arginine recycling enzymes in aorta of diabetic rats

Decreased availability of arginine and impaired production of NO (nitric oxide) have been implicated in the development of endothelial dysfunction. Citrulline formed by the NOS reaction is recycled to arginine by the citrulline-NO cycle, which is composed of NOS, argininosuccinate synthetase (AS), and argininosuccinate lyase. Therefore, we investigated the alterations of these enzymes in the aorta of streptozotocin (STZ)-induced diabetic rats. eNOS and AS mRNAs were increased by three- to fourfold 1-2 weeks after STZ treatment and decreased at 4 weeks. AL mRNA was weakly induced. Induction of eNOS and AS proteins was also observed. Cationic amino acid transporter (CAT)-1 mRNA remained little changed, and CAT-2 mRNA was not detected. The plasma nitrogen oxide levels were increased 1-2 weeks after STZ treatment and decreased at 4 weeks. Transforming growth factor-beta1 (TGF-beta1) mRNA in the aorta was also induced. TGF-beta1 induced eNOS and AS mRNAs in human umbilical vein endothelial cells but inhibited the proliferation of HUVEC. These results indicate that eNOS and AS are coinduced in the aorta in early stages of STZ-induced diabetic rats and that the induction is mediated by TGF-beta1. The results also suggest that TGF-beta1 works antiatherogenically at early stages of diabetes by increasing NO production, whereas prolonged elevation of TGF-beta1 functions atherogenically by inhibiting endothelial cell growth.

Coinduction of inducible nitric oxide synthase and arginine recycling enzymes in cytokine-stimulated PC12 cells and high output production of nitric oxide

Nitric oxide (NO) is involved in many physiological and pathological processes in the brain. NO is synthesized from arginine by nitric oxide synthase (NOS), and the citrulline generated as a by-product can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL) via the citrulline-NO cycle. When neuronal PC12 cells differentiated with nerve growth factor were treated with interferon-gamma (IFNgamma) and tumor necrosis factor-alpha (TNFalpha), iNOS and AS mRNAs and proteins were markedly induced, with AL mRNA and protein being weakly induced. Cationic amino acid transporter-1 and -2 were not induced. IFNgamma or TNFalpha alone was ineffective. A large amount of NO (190 microM NO(2)(-) plus NO(3)(-) in culture medium in 24 h) was produced from arginine by cytokine-stimulated cells, and arginine could be replaced by citrulline. iNOS induction and NO production were attenuated by dexamethasone and dibutyryl cAMP and even more strongly so when combined. Therefore, a large amount of NO is produced in cytokine-stimulated PC12 cells following to induction of iNOS and citrulline-arginine recycling is important for NO production.

Regulation of nitric oxide production by arginine metabolic enzymes

Nitric oxide (NO) is synthesized from arginine by NO synthase (NOS), and the availability of arginine is one of the rate-limiting factors in cellular NO production. Citrulline, which is formed as a by-product of the NOS reaction, can be recycled to arginine by successive actions of argininosuccinate synthetase (AS) and argininosuccinate lyase (AL), forming the citrulline-NO cycle. AS and sometimes AL have been shown to be coinduced with inducible NOS (iNOS) in various cell types including activated macrophages, vascular smooth muscle cells, glial cells, neuronal PC12 cells, and pancreatic beta-cells. Cationic amino acid transporter (CAT)-2 is induced in activated macrophages but not in PC12 cells. On the other hand, arginase can downregulate NO production by decreasing intracellular arginine concentrations. iNOS and arginase activities are regulated reciprocally in macrophages by cytokines, and this may guarantee the efficient production of NO. In contrast, iNOS and arginase isoforms (type I and II) are coinduced in lipopolysaccharide (LPS)-activated macrophages. These results indicate that NO production is modulated by the uptake, recycling, and degradation of arginine.