GTP cyclohydrolase I is coinduced in hepatocytes stimulated to produce nitric oxide

Toll-Like Receptor 4 Promotes NO Synthesis by Upregulating GCHI Expression under Oxidative Stress Conditions in Sheep Monocytes/Macrophages

Many groups of Gram-negative bacteria cause diseases that are harmful to sheep. Toll-like receptor 4 (TLR4), which is critical for detecting Gram-negative bacteria by the innate immune system, is activated by lipopolysaccharide (LPS) to initiate inflammatory responses and oxidative stress. Oxidation intermediates are essential activators of oxidative stress, as low levels of free radicals form a stressful oxidative environment that can clear invading pathogens. NO is an oxidation intermediate and its generation is regulated by nitric oxide synthase (iNOS). Guanosine triphosphate cyclohydrolase (GCHI) is the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis, which is essential for the production of inducible iNOS. Previously, we made vectors to overexpress the sheep TLR4 gene. Herein, first generation (G1) of transgenic sheep was stimulated with LPS in vivo and in vitro, and oxidative stress and GCHI expression were investigated. Oxidative injury caused by TLR4 overexpression was tightly regulated in tissues. However, the transgenic (Tg) group still secreted nitric oxide (NO) when an iNOS inhibitor was added. Furthermore, GCHI expression remained upregulated in both serum and monocytes/macrophages. Thus, overexpression of TLR4 in transgenic sheep might accelerate the clearance of invading microbes through NO generation following LPS stimulation. Additionally, TLR4 overexpression also enhances GCHI activation.

Attenuation of biopterin synthesis prevents Escherichia coli K1 invasion of brain endothelial cells and the development of meningitis in newborn mice

Elevated levels of pterins and nitric oxide (NO) are observed in patients with septic shock and bacterial meningitis. We demonstrate that Escherichia coli K1 infection of human brain microvascular endothelial cells (HBMECs) induces the expression of guanosine triphosphate cyclohydrolase (GCH1), the rate-limiting enzyme in pterin synthesis, thereby elevating levels of biopterin. DAHP (2,4-diamino hydroxyl pyrimidine), a specific inhibitor of GCH1, prevented biopterin and NO production and invasion of E. coli K1 in HBMECs. GCH1 interaction with Ecgp96, the receptor for outer membrane protein A of E. coli K1, also increases on infection, and suppression of Ecgp96 expression prevents GCH1 activation and biopterin synthesis. Pretreatment of newborn mice with DAHP prevented the production of biopterin and the development of meningitis. These results suggest a novel role for biopterin synthesis in the pathogenesis of E. coli K1 meningitis.

Inhibition of CDKS by roscovitine suppressed LPS-induced *NO production through inhibiting NFkappaB activation and BH4 biosynthesis in macrophages

In inflammatory diseases, tissue damage is critically associated with nitric oxide ((*)NO) and cytokines, which are overproduced in response to cellular release of endotoxins. Here we investigated the inhibitory effect of roscovitine, a selective inhibitor of cyclin-dependent kinases (CDKs) on (*)NO production in mouse macrophages. In RAW264.7 cells, we found that roscovitine abolished the production of (*)NO induced by lipopolysaccharide (LPS). Moreover, roscovitine significantly inhibited LPS-induced inducible nitric oxide synthase (iNOS) mRNA and protein expression. Our data also showed that roscovitine attenuated LPS-induced phosphorylation of IkappaB kinase beta (IKKbeta), IkappaB, and p65 but enhanced the phosphorylation of ERK, p38, and c-Jun NH(2)-terminal kinase (JNK). In addition, roscovitine dose dependently inhibited LPS-induced expression of cyclooxygenase-2 (COX)-2, IL-1beta, and IL-6 but not tumor necrosis factor (TNF)-alpha. Tetrahydrobiopterin (BH(4)), an essential cofactor for iNOS, is easily oxidized to 7,8-dihydrobiopterin (BH(2)). Roscovitine significantly inhibited LPS-induced BH(4) biosynthesis and decreased BH(4)-to-BH(2) ratio. Furthermore, roscovitine greatly reduced the upregulation of GTP cyclohydrolase-1 (GCH-1), the rate-limiting enzyme for BH(4) biosynthesis. Using other CDK inhibitors, we found that CDK1, CDK5, and CDK7, but not CDK2, significantly inhibited LPS-induced (*)NO production in macrophages. Similarly, in isolated peritoneal macrophages, roscovitine strongly inhibited (*)NO production, iNOS, and COX-2 upregulation, activation of NFkappaB, and induction of GCH-1 by LPS. Together, our data indicate that roscovitine abolishes LPS-induced (*)NO production in macrophages by suppressing nuclear factor-kappaB activation and BH(4) biosynthesis, which might be mediated by CDK1, CDK5, and CDK7. Our results also suggest that roscovitine may inhibit inflammation and that CDKs may play important roles in the mechanisms by which roscovitine attenuates inflammation.

GTP cyclohydrolase I expression is regulated by nitric oxide: role of cyclic AMP

Our previous studies have demonstrated that nitric oxide (NO) leads to nitric oxide synthase (NOS) uncoupling and an increase in NOS-derived superoxide. However, the cause of this uncoupling has not been adequately resolved. The pteridine cofactor tetrahydrobiopterin (BH(4)) is a critical determinant of endothelial NOS (eNOS) activity and coupling, and GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme in its generation. Thus the initial purpose of this study was to determine whether decreases in BH(4) could underlie, at least in part, the NO-mediated uncoupling of eNOS we have observed both in vitro and in vivo. Initially we evaluated the effect of inhaled NO levels on GCH1 expression and BH(4) levels in the intact lamb. Contrary to our hypothesis, we found that there was a significant increase in both plasma BH4 levels and peripheral lung GCH1 protein levels. Furthermore, in vitro, we found that exposure to the NO donor spermine NONOate (SPNONO) led to an increase in GCH1 protein and BH(4) levels in both COS-7 and pulmonary arterial endothelial cells. However, SPNONO treatment also caused a significant increase in phospho-cAMP response element binding protein (CREB) levels, as detected by Western blot analysis, and significantly increased cAMP levels, as detected by enzyme immunoassay. Furthermore, utilizing GCH1 promoter fragments fused to a luciferase reporter gene, we found that GCH1 promoter activity was enhanced by SPNONO in a CREB-dependent manner, and electromobility shift assays revealed an NO-dependent increase in the nuclear binding of CREB. These data suggest that NO increases BH(4) levels through a cAMP/CREB-mediated increase in GCH1 transcription and that the eNOS uncoupling associated with exogenous NO does not involved reduced BH(4) levels.

Sepiapterin decreases acute rejection and apoptosis in cardiac transplants independently of changes in nitric oxide and inducible nitric-oxide synthase dimerization

Tetrahydrobiopterin (BH(4)), a cofactor of inducible nitric-oxide synthase (iNOS), is an important post-translational regulator of NO bioactivity. We examined whether treatment of cardiac allograft recipients with sepiapterin [S-(-)-2-amino-7,8-dihydro-6-(2-hydroxy-1-oxopropyl)-4-(1H)-pteridinone], a precursor of BH(4), inhibited acute rejection and apoptosis in cardiac transplants. Heterotopic cardiac transplantation was performed in Wistar-Furth donor to Lewis recipient strain rats. Recipients were treated daily after transplantation with 10 mg/kg sepiapterin. Grafts were harvested on post-transplant day 6 for analysis of BH(4) (high-performance liquid chromatography), expression of inflammatory cytokines (reverse transcription- and real-time polymerase chain reaction), iNOS (Western blots), and NO (Griess reaction and NO analyzer). Histological rejection grade was scored, and graft function was determined by echocardiography. Apoptosis, protein nitration, and oxidative stress were determined by immunohistochemistry. Treatment of allografts with sepiapterin increased cardiac BH(4) levels by 3-fold without changing protein levels of GTP cyclohydrolase, the enzyme that regulates de novo BH(4) synthesis. Sepiapterin decreased inflammatory cell infiltrate and significantly inhibited histological rejection scores and apoptosis similar in magnitude to cyclosporine. Sepiapterin also decreased nitrative and oxidative stress. Sepiapterin caused a smaller increase in left ventricular mass versus untreated allografts but without improving fractional shortening. Sepiapterin did not alter tumor necrosis factor-alpha and interferon-gamma expression, whereas it decreased interleukin (IL)-2 expression. Sepiapterin did not change total iNOS protein or monomer levels, or plasma and tissue NO metabolites levels. It is concluded that the mechanism(s) of antirejection are due in part to decreased apoptosis, protein nitration, and oxidation of cardiomyocytes, which seems to be mediated at the immune level by limiting inflammatory cell infiltration via decreased IL-2-mediated T-lymphocyte expansion.

Analysis of two promoters that control the expression of the GTP cyclohydrolase I gene in Drosophila melanogaster

GTP cyclohydrolase I (GTPCH) is a key enzyme in the de novo synthesis of tetrahydrobiopterin. Previously, the Drosophila melanogaster GTPCH gene has been shown to be expressed from two different promoters (P1 and P2). In our study, the 5'-flanking DNA regions required for P1 and P2 promoter activities were characterized using transient expression assay. The DNA regions between -98 and +31, and between -73 and +35 are required for efficient P1 and P2 promoter activities, respectively. The regions between -98 and -56 and between -73 and -41 may contain critical elements required for the expression of GTPCH in Drosophila. By aligning the nucleotide sequences in the P1 and P2 promoter regions of the Drosophila melanogaster and Drosophila virilis GTPCH genes, several conserved elements including palindromic sequences in the regions critical for P1 and P2 promoter activities were identified. Western blot analysis of transgenic flies transformed using P1 or P2 promoter-lacZ fusion plasmids further revealed that P1 promoter expression is restricted to the late pupae and adult developmental stages but that the P2 promoter driven expression of GTPCH is constitutive throughout fly development. In addition, X-gal staining of the embryos and imaginal discs of transgenic flies suggests that the P2 promoter is active from stage 13 of embryo and is generally active in most regions of the imaginal discs at the larval stages.

Altered plasma versus vascular biopterins in human atherosclerosis reveal relationships between endothelial nitric oxide synthase coupling, endothelial function, and inflammation

BACKGROUND

Tetrahydrobiopterin (BH4) is a key regulator of endothelial nitric oxide synthase (eNOS) activity and coupling. However, the extent to which vascular and/or systemic BH4 levels are altered in human atherosclerosis and the importance of BH4 bioavailability in determining endothelial function and oxidative stress remain unclear. We sought to define the relationships between plasma and vascular biopterin levels in patients with coronary artery disease and to determine how BH4 levels affect endothelial function, eNOS coupling, and vascular superoxide production.

METHODS AND RESULTS

Samples of saphenous veins and internal mammary arteries were collected from 219 patients with coronary artery disease undergoing coronary artery bypass grafting. We determined plasma and vascular levels of biopterins, vasomotor responses to acetylcholine, and vascular superoxide production in the presence and absence of the eNOS inhibitor N(G)-nitro-L-arginine methyl ester. High vascular BH4 was associated with greater vasorelaxations to acetylcholine (P<0.05), whereas high plasma BH4 was associated with lower vasorelaxations in response to acetylcholine (P<0.05). Furthermore, an inverse association was observed between plasma and vascular biopterins (P<0.05 for both saphenous veins and internal mammary arteries). High vascular (but not plasma) BH4 was associated with reduced total and N(G)-nitro-L-arginine methyl ester-inhibitable superoxide, suggesting improved eNOS coupling. Finally, plasma but not vascular biopterin levels were correlated with plasma C-reactive protein levels (P<0.001).

CONCLUSIONS

An inverse association exists between plasma and vascular biopterins in patients with coronary artery disease. Vascular but not plasma BH4 is an important determinant of eNOS coupling, endothelium-dependent vasodilation, and superoxide production in human vessels, whereas plasma biopterins are a marker of systemic inflammation.

Dissociation and unfolding of inducible nitric oxide synthase oxygenase domain identifies structural role of tetrahydrobiopterin in modulating the heme environment

The oxygenase domain of the inducible nitric oxide synthase, Delta65 iNOSox is a dimer that binds heme, L-Arginine (L-Arg), and tetrahydrobiopterin (H(4)B) and is the site for NO synthesis. The role of H(4)B in iNOS structure-function is complex and its exact structural role is presently unknown. The present paper provides a simple mechanistic account of interaction of the cofactor tetrahydrobiopterin (H(4)B) with the bacterially expressed Delta65 iNOSox protein. Transverse urea gradient gel electrophoresis studies indicated the presence of different conformers in the cofactor-incubated and cofactor-free Delta65 iNOSox protein. Dynamic Light Scattering (DLS) studies of cofactor-incubated and cofactor-free Delta65 iNOSox protein also showed two distinct populations of two different diameter ranges. Cofactor tetrahydrobiopterin (H(4)B) shifted one population, with higher diameter, to the lower diameter ranges indicating conformational changes. The additional role played by the cofactor is to elevate the heme retaining capacity even in presence of denaturing stress. Together, these findings confirm that the H(4)B is essential in modulating the iNOS heme environment and the protein environment in the dimeric iNOS oxygenase domain.

Nitric oxide formation in acutely rejecting cardiac allografts correlates with GTP cyclohydrolase I activity

Inducible nitric oxide synthase (iNOS) is a prominent component of the complex array of mediators in acute graft rejection. While NO production is determined by iNOS expression, BH4 (tetrahydrobiopterin), a cofactor of iNOS synthesized by GTP cyclohydrolase I, has been considered critical in sustaining NO production. In the present study, we examined time-dependent changes in iNOS and GTP cyclohydrolase I in rat cardiac allografts. The increase in iNOS protein and mRNA in allografts was similar at POD4 (post-operative day 4) and POD6. However, the peak increase in intragraft NO level at POD4 was not sustained at POD6. This disparity could not be explained by any decrease in iNOS enzyme activity measured ex vivo with optimal amounts of substrate and cofactors. Lower iNOS activity could be explained by changes in total biopterin levels in allografts at POD4 that was decreased to baseline at POD6. Changes in biopterin production correlated with lower GTP cyclohydrolase I protein levels but not by any change in GTP cyclohydrolase I mRNA. Functionally, allografts displayed bradycardia and distended diastolic and systolic dimensions at POD6 but not at POD4. Likewise, histological rejection scores were increased at POD4 but with a secondary increased stage at POD6. It is hypothesized that the dissimilar amounts of NO at early and later stages of rejection is due to uncoupling of iNOS arising from disproportionate synthesis of BH4. These findings provide insight into a potential pathway regulating NO bioactivity in graft rejection. Such knowledge may potentially assist in the design of newer strategies to prevent acute graft rejection.

Monomeric inducible nitric oxide synthase localizes to peroxisomes in hepatocytes

Hepatocytes are capable of repeated inducible NO synthase (iNOS) expression, which occurs under inflammatory and stress conditions. This iNOS expression regulates a number of cellular functions as well as cell viability. To better understand the posttranslational mechanisms that regulate the fate of iNOS in these cells, we characterized the iNOS distributed within peroxisomes. The selective permeabilization of membranes (plasma vs. peroxisomal) confirmed that there are cytosolic and peroxisomal pools of iNOS in cytokine-stimulated hepatocytes and that the iNOS protein associates with peroxisome. Detergent solubilization of the membrane fraction released iNOS to the soluble fraction. iNOS localized to membrane fraction is predominantly monomeric, but dimerization is partially reconstituted rapidly upon incubation with tetrahydrobiopterin. The reconstituted iNOS exhibits a lower specific activity than iNOS isolated from the soluble pool. Depletion of intracellular tetrahydrobiopterin with an inhibitor of de novo pterin synthesis resulted in a predominance of monomeric iNOS without a greater relative distribution of iNOS to the peroxisomal pool. Thus, iNOS exists in a least two pools in hepatocytes: a soluble pool composed of both active dimer and monomer and a peroxisomal pool of monomeric iNOS. iNOS might localize to peroxisomes in long-lived cells such as hepatocytes as a protective mechanism to remove incompetent enzyme.

Regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper transcription factors

Tetrahydrobiopterin is an essential cofactor for the phenylalanine, tyrosine and tryptophan hydroxylases, and the family of nitric oxide synthases. The initial and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin is GTP cyclohydrolase I. The proximal promoter of the human GTP cyclohydrolase I gene contains the sequence motif 5'-TGACGCGA-3', resembling a cAMP response element (CRE). The objective of this study was to analyze the regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper (bZIP) transcription factors. A constitutively active mutant of the cAMP response element binding (CREB) protein strongly stimulated GTP cyclohydrolase I promoter activity, indicating that the CRE in the context of the GTP cyclohydrolase I gene is functional. Likewise, GTP cyclohydrolase I promoter/luciferase gene transcription was stimulated following nuclear expression of the catalytic subunit of cAMP-dependent protein kinase. Constitutively active mutants of activating transcription factor 2 (ATF2) and c-Jun additionally stimulated GTP cyclohydrolase I promoter activity, but to a lesser extent than the constitutively active CREB mutant. The fact that stress-activated protein kinases target the GTP cyclohydrolase I gene was corroborated by expression experiments involving p38 and MEKK1 protein kinases. We conclude that signaling pathways involving either the cAMP-dependent protein kinase or stress-activated protein kinases converge to the GTP cyclohydrolase I gene. Hence, enzymatic reactions that require tetrahydrobiopterin as cofactor are therefore indirectly controlled by signaling cascades involving the signal-responsive transcription factors CREB, c-Jun, and ATF2.

Evaluation of glutathione deficiency in rat livers by microarray analysis

Hepatic glutathione content was measured and gene expression data were obtained using an Affymetrix RG U34 array after treatment with tap water containing 20mM l-buthionine (S, R)-sulfoximine (BSO) to male F344 rats for four consecutive days. Both Spearman's and Pearson's correlation coefficients were calculated between the glutathione content and the mRNA content level obtained from the microarray analysis individually. Sixty-nine gene probes, which were statistically significant (Spearman's correlation, P < 0.05) and showed a Pearson's correlation coefficients (Pearson's r) less than -0.8 between mRNA content and hepatic glutathione content, were identified as glutathione deficiency-correlated probes. By comparing the hepatic gene expression profiles between BSO- and butylated hydroxyanisole (BHA)-treated rats, 14 probes of genes that showed an increase in the corresponding gene mRNA levels only after the BSO treatment were thought to be good indicators of glutathione deficiency. A principal component analysis successfully illustrated the time-course of hepatic gene expression after the treatment with acetaminophen, phenobarbital and clofibrate, and the expression profiles were thought to reflect the changes in hepatic glutathione levels. The identified gene probes in the present study would be useful as markers for assessing hepatocellular glutathione deficiency, or oxidative stress level, based on microarray data.

Nitric oxide (NO) pretreatment increases cytokine-induced NO production in cultured rat hepatocytes by suppressing GTP cyclohydrolase I feedback inhibitory protein level and promoting inducible NO synthase dimerization

Nitric oxide (NO) regulates the biological activity of many enzymes and other functional proteins as well as gene expression. In this study, we tested whether pretreatment with NO regulates NO production in response to cytokines in cultured rat hepatocytes. Hepatocytes were recovered in fresh medium for 24 h following pretreatment with the NO donor S-nitroso-N-acetyl-d,l-penicillamine (SNAP) and stimulated to express the inducible NO synthase (iNOS) with interleukin-1beta and interferon-gamma or transfected with the human iNOS gene. NO pretreatment resulted in a significant increase in NO production without changing iNOS expression for both conditions. This effect, which did not occur in macrophages and smooth muscle cells, was inhibited when NO was scavenged using red blood cells. Pretreatment with oxidized SNAP, 8-Br-cGMP, NO(2)(-), or NO(3)(-) did not increase the cytokine-induced NO production. SNAP pretreatment increased cytosolic iNOS activity measured only in the absence of exogenous tetrahydrobiopterin (BH(4)). SNAP pretreatment suppressed the level of GTP cyclohydrolase I (GTPCHI) feedback regulatory protein (GFRP) and increased GTPCHI activity without changing GTPCHI protein level. SNAP pretreatment also increased total cellular levels of biopterin and active iNOS dimer. These results suggest that SNAP pretreatment increased NO production from iNOS by elevating cellular BH(4) levels and promoting iNOS subunit dimerization through the suppression of GFRP levels and subsequent activation of GTPCHI.

Distinct dimer interaction and regulation in nitric-oxide synthase types I, II, and III

Homodimer formation activates all nitric-oxide synthases (NOSs). It involves the interaction between two oxygenase domains (NOSoxy) that each bind heme and (6R)-tetrahydrobiopterin (H4B) and catalyze NO synthesis from L-Arg. Here we compared three NOSoxy isozymes regarding dimer strength, interface composition, and the ability of L-Arg and H4B to stabilize the dimer, promote its formation, and protect it from proteolysis. Urea dissociation studies indicated that the relative dimer strengths were NOSIIIoxy >> NOSIoxy > NOSIIoxy (endothelial NOSoxy (eNOSoxy) >> neuronal NOSOXY (nNOSoxy) > inducible NOSoxy (iNOSoxy)). Dimer strengths of the full-length NOSs had the same rank order as judged by their urea-induced loss of NO synthesis activity. NOSoxy dimers containing L-Arg plus H4B exhibited the greatest resistance to urea-induced dissociation followed by those containing either molecule and then by those containing neither. Analysis of crystallographic structures of eNOSoxy and iNOSoxy dimers showed more intersubunit contacts and buried surface area in the dimer interface of eNOSoxy than iNOSoxy, thus revealing a potential basis for their different stabilities. L-Arg plus H4B promoted dimerization of urea-generated iNOSoxy and nNOSoxy monomers, which otherwise was minimal in their absence, and also protected both dimers against trypsin proteolysis. In these respects, L-Arg alone was more effective than H4B alone for nNOSoxy, whereas for iNOSoxy the converse was true. The eNOSoxy dimer was insensitive to proteolysis under all conditions. Our results indicate that the three NOS isozymes, despite their general structural similarity, differ markedly in their strengths, interfaces, and in how L-Arg and H4B influence their formation and stability. These distinguishing features may provide a basis for selective control and likely help to regulate each NOS in its particular biologic milieu.

The nitric oxide system in glucocorticoid-induced hypertension

The blood pressure-raising effects of adrenocortical steroids with predominantly glucocorticoid activity, both naturally occurring and synthetic, are well known. Recent evidence suggests that the nitric oxide system plays a key role in the hypertension produced by glucocorticoids. Glucocorticoid actions at various sites in the nitric oxide synthase (NOS) pathway may result in elevated blood pressure. These include: alterations in l-arginine availability or transport; NOS2 and NOS3 downregulation; reduced cofactor bioavailability; NOS uncoupling; a concomitant elevation in reactive oxygen species and removal of nitric oxide (NO) from the vascular environment; alterations in whole body antioxidant status; and erythropoietin induced resistance to NO.

Catecholamines decrease nitric oxide production by cytokine-stimulated hepatocytes

BACKGROUND

Catecholamines are significantly elevated in inflammatory responses and play a regulatory role in sepsis. Nitric oxide (NO), also a key inflammatory mediator in sepsis, is produced in large amounts by the inducible nitric oxide synthase (iNOS) in the liver. The purpose of this study was to test the hypothesis that catecholamines play a role in the regulation of NO production by hepatocytes.

METHODS

Primary hepatocytes were isolated from healthy male Sprague-Dawley rats and either cultured with normal medium or stimulated with cytomix (interleukin-1 beta, interferon-gamma, and tumor necrosis factor-alpha) in the presence or absence of epinephrine or norepinephrine at varying concentrations. Total RNA was isolated 6 hours after treatment and analyzed by Northern blotting for iNOS mRNA. Protein extracts were obtained at 12 hours and were analyzed by Western immunoblotting for iNOS. Cell culture supernatants were analyzed for NO, determined as the stable end-product NO(2)(-), at 24 hours.

RESULTS

Epinephrine and norepinephrine significantly decreased NO(2)(-) levels in stimulated hepatocytes but had no effect on iNOS mRNA or protein levels. The decrease in NO(2)(-) was reproduced by the adenylate cyclase stimulator, forskolin. The catecholamine-induced decrease in NO(2)(-) was completely reversed by the protein kinase A inhibitor Rp-8-Br-cyclic adenosine monophosphate.

CONCLUSIONS

Catecholamines decrease hepatocyte production of NO in response to cytokine stimulation. This effect seems to be due to post-translational events and appears to be mediated in part by cyclic adenosine monophosphate.