Expression of human inducible nitric oxide synthase in a tetrahydrobiopterin (H4B)-deficient cell line: H4B promotes assembly of enzyme subunits into an active dimer

Unveiling the significance of inducible nitric oxide synthase: Its impact on cancer progression and clinical implications

The intricate role of inducible nitric oxide synthase (iNOS) in cancer pathophysiology has garnered significant attention, highlighting the complex interplay between tumorigenesis, immune response, and cellular metabolism. As an enzyme responsible for producing nitric oxide (NO) in response to inflammatory stimuli. iNOS is implicated in various aspects of cancer development, including DNA damage, angiogenesis, and evasion of apoptosis. This review synthesizes the current findings from both preclinical and clinical studies on iNOS across different cancer types, reflecting the variability depending on cellular context and tumor microenvironment. We explore the molecular mechanisms by which iNOS modulates cancer cell growth, survival, and metastasis, emphasizing its impact on immune surveillance and response to treatment. Additionally, the potential of targeting iNOS as a therapeutic strategy in cancer treatment is examined. By integrating insights from recent advances, this review aims to elucidate the significant role of iNOS in cancer and pave the way for novel diagnostic and therapeutic approaches.

BH4 supplementation reduces retinal cell death in ischaemic retinopathy

Dysregulation of nitric oxide (NO) production can cause ischaemic retinal injury and result in blindness. How this dysregulation occurs is poorly understood but thought to be due to an impairment in NO synthase function (NOS) and nitro-oxidative stress. Here we investigated the possibility of correcting this defective NOS activity by supplementation with the cofactor tetrahydrobiopterin, BH4. Retinal ischaemia was examined using the oxygen-induced retinopathy model and BH4 deficient Hph-1 mice used to establish the relationship between NOS activity and BH4. Mice were treated with the stable BH4 precursor sepiapterin at the onset of hypoxia and their retinas assessed 48 h later. HPLC analysis confirmed elevated BH4 levels in all sepiapterin supplemented groups and increased NOS activity. Sepiapterin treatment caused a significant decrease in neuronal cell death in the inner nuclear layer that was most notable in WT animals and was associated with significantly diminished superoxide and local peroxynitrite formation. Interestingly, sepiapterin also increased inflammatory cytokine levels but not microglia cell number. BH4 supplementation by sepiapterin improved both redox state and neuronal survival during retinal ischaemia, in spite of a paradoxical increase in inflammatory cytokines. This implicates nitro-oxidative stress in retinal neurones as the cytotoxic element in ischaemia, rather than enhanced pro-inflammatory signalling.

Sec22b Regulates Inflammatory Responses by Controlling the Nuclear Translocation of NF-κB and the Secretion of Inflammatory Mediators

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) regulate the vesicle transport machinery in phagocytic cells. Within the secretory pathway, Sec22b is an endoplasmic reticulum-Golgi intermediate compartment (ERGIC)-resident SNARE that controls phagosome maturation and function in macrophages and dendritic cells. The secretory pathway controls the release of cytokines and may also impact the secretion of NO, which is synthesized by the Golgi-active inducible NO synthase (iNOS). Whether ERGIC SNARE Sec22b controls NO and cytokine secretion is unknown. Using murine bone marrow-derived dendritic cells, we demonstrated that inducible NO synthase colocalizes with ERGIC/Golgi markers, notably Sec22b and its partner syntaxin 5, in the cytoplasm and at the phagosome. Pharmacological blockade of the secretory pathway hindered NO and cytokine release, and inhibited NF-κB translocation to the nucleus. Importantly, RNA interference-mediated silencing of Sec22b revealed that NO and cytokine production were abrogated at the protein and mRNA levels. This correlated with reduced nuclear translocation of NF-κB. We also found that Sec22b co-occurs with NF-κB in both the cytoplasm and nucleus, pointing to a role for this SNARE in the shuttling of NF-κB. Collectively, our data unveiled a novel function for the ERGIC/Golgi, and its resident SNARE Sec22b, in the production and release of inflammatory mediators.

Inducible nitric oxide synthase: Regulation, structure, and inhibition

A considerable number of human diseases have an inflammatory component, and a key mediator of immune activation and inflammation is inducible nitric oxide synthase (iNOS), which produces nitric oxide (NO) from l-arginine. Overexpressed or dysregulated iNOS has been implicated in numerous pathologies including sepsis, cancer, neurodegeneration, and various types of pain. Extensive knowledge has been accumulated about the roles iNOS plays in different tissues and organs. Additionally, X-ray crystal and cryogenic electron microscopy structures have shed new insights on the structure and regulation of this enzyme. Many potent iNOS inhibitors with high selectivity over related NOS isoforms, neuronal NOS, and endothelial NOS, have been discovered, and these drugs have shown promise in animal models of endotoxemia, inflammatory and neuropathic pain, arthritis, and other disorders. A major issue in iNOS inhibitor development is that promising results in animal studies have not translated to humans; there are no iNOS inhibitors approved for human use. In addition to assay limitations, both the dual modalities of iNOS and NO in disease states (ie, protective vs harmful effects) and the different roles and localizations of NOS isoforms create challenges for therapeutic intervention. This review summarizes the structure, function, and regulation of iNOS, with focus on the development of iNOS inhibitors (historical and recent). A better understanding of iNOS' complex functions is necessary before specific drug candidates can be identified for classical indications such as sepsis, heart failure, and pain; however, newer promising indications for iNOS inhibition, such as depression, neurodegenerative disorders, and epilepsy, have been discovered.

Regulation of mycobacterial infection by macrophage Gch1 and tetrahydrobiopterin

Inducible nitric oxide synthase (iNOS) plays a crucial role in controlling growth of Mycobacterium tuberculosis (M.tb), presumably via nitric oxide (NO) mediated killing. Here we show that leukocyte-specific deficiency of NO production, through targeted loss of the iNOS cofactor tetrahydrobiopterin (BH4), results in enhanced control of M.tb infection; by contrast, loss of iNOS renders mice susceptible to M.tb. By comparing two complementary NO-deficient models, Nos2-/- mice and BH4 deficient Gch1fl/flTie2cre mice, we uncover NO-independent mechanisms of anti-mycobacterial immunity. In both murine and human leukocytes, decreased Gch1 expression correlates with enhanced cell-intrinsic control of mycobacterial infection in vitro. Gene expression analysis reveals that Gch1 deficient macrophages have altered inflammatory response, lysosomal function, cell survival and cellular metabolism, thereby enhancing the control of bacterial infection. Our data thus highlight the importance of the NO-independent functions of Nos2 and Gch1 in mycobacterial control.

Gasotransmitter Heterocellular Signaling

SIGNIFICANCE

The family of gasotransmitter molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), has emerged as an important mediator of numerous cellular signal transduction and pathophysiological responses. As such, these molecules have been reported to influence a diverse array of biochemical, molecular, and cell biology events often impacting one another. Recent Advances: Discrete regulation of gasotransmitter molecule formation, movement, and reaction is critical to their biological function. Due to the chemical nature of these molecules, they can move rapidly throughout cells and tissues acting on targets through reactions with metal groups, reactive chemical species, and protein amino acids.

CRITICAL ISSUES

Given the breadth and complexity of gasotransmitter reactions, this field of research is expanding into exciting, yet sometimes confusing, areas of study with significant promise for understanding health and disease. The precise amounts of tissue and cellular gasotransmitter levels and where they are formed, as well as how they react with molecular targets or themselves, all remain poorly understood.

FUTURE DIRECTIONS

Elucidation of specific molecular targets, characteristics of gasotransmitter molecule heterotypic interactions, and spatiotemporal formation and metabolism are all important to better understand their true pathophysiological importance in various organ systems. Antioxid. Redox Signal. 26, 936-960.

My Continuing Evolution as a Surgeon-Scientist: A Decade after the Jacobson Promising Investigator Award

THE SECOND JOAN L AND JULIUS H JACOBSON PROMISING INVESTIGATOR AWARDEE, EDITH TZENG MD, FACS: In 2005, the Surgical Research Committee of the American College of Surgeons was tasked with selecting the recipient of a newly established award, "The Joan L and Julius H Jacobson Promising Investigator Award." According to the Jacobsons, the award funded by Dr Jacobson should be given at least once every 2 years to a surgeon investigator at "the tipping point," who can demonstrate that his or her research shows the promise of leading to a significant contribution to the practice of surgery and patient safety. Every year, the Surgical Research Committee receives many excellent nominations and has the difficult task of selecting one awardee. The first awardee was Michael Longaker MD, FACS, who 10 years later reflected on the award and the impact it had on his career.¹ This year, Edith Tzeng, MD, FACS, the second Jacobson awardee, reflects on her 10-year journey after receiving the award. Dr Tzeng is now a national and international figure in the field of vascular surgery and has studied the effect of nitric oxide and carbon monoxide on intimal hyperplasia. Kamal MF Itani, MD, FACS and Leigh Neumayer, MD, FACS, on behalf of the Surgical Research Committee of the American College of Surgeons.

Nitric oxide in liver fibrosis: The role of inducible nitric oxide synthase

The inducible form of nitric oxide synthase (iNOS) is expressed in hepatic cells in pathological conditions. Its induction is involved in the development of liver fibrosis, and thus iNOS could be a therapeutic target for liver fibrosis. This review summarizes the role of iNOS in liver fibrosis, focusing on 1) iNOS biology, 2) iNOS-expressing liver cells, 3) iNOS-related therapeutic strategies, and 4) future directions.

Characterization of cerebral microvasculature in transgenic mice with endothelium targeted over-expression of GTP-cyclohydrolase I

Tetrahydrobiopterin (BH4) is a critical determinant of nitric oxide (NO) production by nitric oxide synthase (NOS) in the vascular endothelium and its biosynthesis is regulated by the enzymatic activity of GTP-cyclohydrolase I (GTPCH I). The present study was designed to determine the effects of endothelium-targeted overexpression of GTPCH I (eGCH-Tg) on murine cerebral vascular function. Endothelium targeted over-expression of GTPCH I was associated with a significant increase in levels of BH4, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH2) in cerebral microvessels. Importantly, ratio of BH4 to 7,8-BH2, indicative of BH4 available for eNOS activation, was significantly increased in eGCH-Tg mice. However, expression of endothelial NOS, levels of nitrate/nitrite--indicative of NO production--remained unchanged between cerebral microvessels of wild-type and eGCH-Tg mice. Furthermore, increased BH4 biosynthesis neither affected production of superoxide anion nor expression of antioxidant proteins. Moreover, endothelium-specific GTPCH I overexpression did not alter intracellular levels of cGMP, reflective of NO signaling in cerebral microvessels. The obtained results suggest that, despite a significant increase in BH4 bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGCH-Tg mice. We conclude that under physiological conditions the levels of BH4 in the cerebral microvessels are optimal for activation of endothelial NOS and NO/cGMP signaling.

Up-regulation of thrombospondin-2 in Akt1-null mice contributes to compromised tissue repair due to abnormalities in fibroblast function

Vascular remodeling is essential for tissue repair and is regulated by multiple factors, including thrombospondin-2 (TSP2) and hypoxia/VEGF-induced activation of Akt. In contrast to TSP2 knock-out (KO) mice, Akt1 KO mice have elevated TSP2 expression and delayed tissue repair. To investigate the contribution of increased TSP2 to Akt1 KO mice phenotypes, we generated Akt1/TSP2 double KO (DKO) mice. Full-thickness excisional wounds in DKO mice healed at an accelerated rate when compared with Akt1 KO mice. Isolated dermal Akt1 KO fibroblasts expressed increased TSP2 and displayed altered morphology and defects in migration and adhesion. These defects were rescued in DKO fibroblasts or after TSP2 knockdown. Conversely, the addition of exogenous TSP2 to WT cells induced cell morphology and migration rates that were similar to those of Akt1 KO cells. Akt1 KO fibroblasts displayed reduced adhesion to fibronectin with manganese stimulation when compared with WT and DKO cells, revealing an Akt1-dependent role for TSP2 in regulating integrin-mediated adhesions; however, this effect was not due to changes in β1 integrin surface expression or activation. Consistent with these results, Akt1 KO fibroblasts displayed reduced Rac1 activation that was dependent upon expression of TSP2 and could be rescued by a constitutively active Rac mutant. Our observations show that repression of TSP2 expression is a critical aspect of Akt1 function in tissue repair.

Comparison of oxygen-induced radical intermediates in iNOS oxygenase domain with those from nNOS and eNOS

Inducible nitric-oxide synthase (iNOS) produces the reactive oxygen and nitrogen species (ROS/RNS) involved in bacteria killing and is crucial in the host defense mechanism. However, high level ROS/RNS can also be detrimental to normal cells and thus their production has to be tightly controlled. Availability or deficiency of tetrahydrobiopterin (BH4) cofactor and l-arginine substrate controls coupling or uncoupling of NOS catalysis. Fully coupled reaction, with abundant BH4 and l-arginine, produces NO whereas the uncoupled NOS (in the absence of BH4 and/or l-arginine) generates ROS/RNS. In the current work we focus on direct rapid freeze EPR to characterize the structure and kinetics of oxygen-induced radical intermediates produced by ferrous inducible NOS oxygenase domain (iNOSox) in the presence or absence of BH4 and/or l-arginine. Fully reconstituted iNOSox (+BH4, +L-Arg) forms a dimer and yields a typical BH4 radical that indicates coupled reaction. iNOSox (-BH4) remains mainly monomeric and produces exclusively superoxide, that is only marginally affected by the presence of l-arginine. iNOSox (+BH4, -L-Arg) exists as a monomer/dimer mixture and yields both BH4 radical and superoxide. Present study is a natural extension of our previous work on the ferrous endothelial NOSox (eNOSox) [V. Berka, G. Wu, H.C. Yeh, G. Palmer, A.L. Tsai, J. Biol. Chem. 279 (2004) 32243-32251] and ferrous neuronal NOSox (nNOSox) [V. Berka, L.H. Wang, A.L. Tsai, Biochemistry 47 (2008) 405-420]. Overall, our data suggests different regulatory roles of l-arginine and BH4 in the production of oxygen-induced radical intermediates in NOS isoforms which nicely serve individual functional role.

A pivotal role for tryptophan 447 in enzymatic coupling of human endothelial nitric oxide synthase (eNOS): effects on tetrahydrobiopterin-dependent catalysis and eNOS dimerization

Tetrahydrobiopterin (BH4) is a required cofactor for the synthesis of NO by NOS. Bioavailability of BH4 is a critical factor in regulating the balance between NO and superoxide production by endothelial NOS (eNOS coupling). Crystal structures of the mouse inducible NOS oxygenase domain reveal a homologous BH4-binding site located in the dimer interface and a conserved tryptophan residue that engages in hydrogen bonding or aromatic stacking interactions with the BH4 ring. The role of this residue in eNOS coupling remains unexplored. We overexpressed human eNOS W447A and W447F mutants in novel cell lines with tetracycline-regulated expression of human GTP cyclohydrolase I, the rate-limiting enzyme in BH4 synthesis, to determine the importance of BH4 and Trp-447 in eNOS uncoupling. NO production was abolished in eNOS-W447A cells and diminished in cells expressing W447F, despite high BH4 levels. eNOS-derived superoxide production was significantly elevated in W447A and W447F versus wild-type eNOS, and this was sufficient to oxidize BH4 to 7,8-dihydrobiopterin. In uncoupled, BH4-deficient cells, the deleterious effects of W447A mutation were greatly exacerbated, resulting in further attenuation of NO and greatly increased superoxide production. eNOS dimerization was attenuated in W447A eNOS cells and further reduced in BH4-deficient cells, as demonstrated using a novel split Renilla luciferase biosensor. Reduction of cellular BH4 levels resulted in a switch from an eNOS dimer to an eNOS monomer. These data reveal a key role for Trp-447 in determining NO versus superoxide production by eNOS, by effects on BH4-dependent catalysis, and by modulating eNOS dimer formation.

Modulation of NO and ROS production by AdiNOS transduced vascular cells through supplementation with L-Arg and BH4: implications for gene therapy of restenosis

OBJECTIVE

Gene therapy with viral vectors encoding for NOS enzymes has been recognized as a potential therapeutic approach for the prevention of restenosis. Optimal activity of iNOS is dependent on the intracellular availability of L-Arg and BH4 via prevention of NOS decoupling and subsequent ROS formation. Herein, we investigated the effects of separate and combined L-Arg and BH4 supplementation on the production of NO and ROS in cultured rat arterial smooth muscle and endothelial cells transduced with AdiNOS, and their impact on the antirestenotic effectiveness of AdiNOS delivery to balloon-injured rat carotid arteries.

METHODS AND RESULTS

Supplementation of AdiNOS transduced endothelial and vascular smooth muscle cells with L-Arg (3.0 mM), BH4 (10 μM) and especially their combination resulted in a significant increase in NO production as measured by nitrite formation in media. Formation of ROS was dose-dependently increased following transduction with increasing MOIs of AdiNOS. Exposure of RASMC to AdiNOS tethered to meshes via a hydrolyzable cross-linker, modeling viral delivery from stents, resulted in increased ROS production, which was decreased by supplementation with BH4 but not L-Arg or L-Arg/BH4. Enhanced cell death, caused by AdiNOS transduction, was also preventable with BH4 supplementation. In the rat carotid model of balloon injury, intraluminal delivery of AdiNOS in BH4-, L-Arg-, and especially in BH4 and L-Arg supplemented animals was found to significantly enhance the antirestenotic effects of AdiNOS-mediated gene therapy.

CONCLUSIONS

Fine-tuning of iNOS function by L-Arg and BH4 supplementation in the transduced vasculature augments the therapeutic potential of gene therapy with iNOS for the prevention of restenosis.

Dipterinyl calcium pentahydrate inhibits intracellular mycobacterial growth in human monocytes via the C-C chemokine MIP-1β and nitric oxide

Tuberculosis remains one of the top three leading causes of morbidity and mortality worldwide, complicated by the emergence of drug-resistant Mycobacterium tuberculosis strains and high rates of HIV coinfection. It is important to develop new antimycobacterial drugs and immunomodulatory therapeutics and compounds that enhance antituberculous immunity. Dipterinyl calcium pentahydrate (DCP), a calcium-complexed pterin compound, has previously been shown to inhibit human breast cancer cells and hepatitis B virus (HBV). DCP inhibitory effects were attributed to induction of apoptosis and/or increased production of interleukin 12 (IL-12) and granulocyte-macrophage colony-stimulating factor (GM-CSF). In this study, we tested the ability of DCP to mediate inhibition of intracellular mycobacteria within human monocytes. DCP treatment of infected monocytes resulted in a significant reduction in viability of intracellular but not extracellular Mycobacterium bovis BCG. The antimicrobial activity of DCP was comparable to that of pyrazinamide (PZA), one of the first-line antituberculosis drugs currently used. DCP potentiated monocyte antimycobacterial activity by induction of the cysteine-cysteine (C-C) chemokine macrophage inflammatory protein 1β (MIP-1β) and inducible nitric oxide synthase 2. Addition of human anti-MIP-1β neutralizing antibody or a specific inhibitor of the l-arginase-nitric oxide pathway (N(G)-monomethyl l-arginine [l-NMMA] monoacetate) reversed the inhibitory effects of DCP on intracellular mycobacterial growth. These findings indicate that DCP induced mycobacterial killing via MIP-1β- and nitric oxide-dependent effects. Hence, DCP acts as an immunoregulatory compound enhancing the antimycobacterial activity of human monocytes.

PEX7 and EBP50 target iNOS to the peroxisome in hepatocytes

iNOS localizes to both the cytosol and peroxisomes in hepatocytes in vitro and in vivo. The structural determinants for iNOS localization are not known. One plausible mechanism for iNOS localization to the peroxisome is through the interaction with peroxisomal import proteins PEX5 or PEX7. siRNA knockdown of PEX7 reduced iNOS colocalization with the peroxisomal protein PMP70. Proteomic studies using MALDI-MS identified iNOS association with the 50-kD ezrin binding PDZ protein (EBP50). Confocal microscopy studies and immunoelectron microscopy confirmed iNOS association with EBP50, with greatest colocalization occurring at 8h of cytokine exposure. EBP50 associated with peroxisomes in a PEX5 and PEX7-dependent manner. iNOS localization to peroxisomes was contingent on EBP50 expression in LPS-treated mice. Thus, iNOS targeting to peroxisomes in hepatocytes involves interaction with PEX7 and EBP50. The targeting of iNOS protein to the peroxisome may shift the balance of metabolic processes that rely on heme proteins susceptible to modification by radical oxygen and nitrogen radicals.

Structural basis for isoform-selective inhibition in nitric oxide synthase

Nitric oxide synthase (NOS) converts l-arginine into l-citrulline and releases the important signaling molecule nitric oxide (NO). In the cardiovascular system, NO produced by endothelial NOS (eNOS) relaxes smooth muscle which controls vascular tone and blood pressure. Neuronal NOS (nNOS) produces NO in the brain, where it influences a variety of neural functions such as neural transmitter release. NO can also support the immune system, serving as a cytotoxic agent during infections. Even with all of these important functions, NO is a free radical and, when overproduced, it can cause tissue damage. This mechanism can operate in many neurodegenerative diseases, and as a result the development of drugs targeting nNOS is a desirable therapeutic goal. However, the active sites of all three human isoforms are very similar, and designing inhibitors specific for nNOS is a challenging problem. It is critically important, for example, not to inhibit eNOS owing to its central role in controlling blood pressure. In this Account, we summarize our efforts in collaboration with Rick Silverman at Northwestern University to develop drug candidates that specifically target NOS using crystallography, computational chemistry, and organic synthesis. As a result, we have developed aminopyridine compounds that are 3800-fold more selective for nNOS than eNOS, some of which show excellent neuroprotective effects in animal models. Our group has solved approximately 130 NOS-inhibitor crystal structures which have provided the structural basis for our design efforts. Initial crystal structures of nNOS and eNOS bound to selective dipeptide inhibitors showed that a single amino acid difference (Asp in nNOS and Asn in eNOS) results in much tighter binding to nNOS. The NOS active site is open and rigid, which produces few large structural changes when inhibitors bind. However, we have found that relatively small changes in the active site and inhibitor chirality can account for large differences in isoform-selectivity. For example, we expected that the aminopyridine group on our inhibitors would form a hydrogen bond with a conserved Glu inside the NOS active site. Instead, in one group of inhibitors, the aminopyridine group extends outside of the active site where it interacts with a heme propionate. For this orientation to occur, a conserved Tyr side chain must swing out of the way. This unanticipated observation taught us about the importance of inhibitor chirality and active site dynamics. We also successfully used computational methods to gain insights into the contribution of the state of protonation of the inhibitors to their selectivity. Employing the lessons learned from the aminopyridine inhibitors, the Silverman lab designed and synthesized symmetric double-headed inhibitors with an aminopyridine at each end, taking advantage of their ability to make contacts both inside and outside of the active site. Crystal structures provided yet another unexpected surprise. Two of the double-headed inhibitor molecules bound to each enzyme subunit, and one molecule participated in the generation of a novel Zn(2+) site that required some side chains to adopt alternate conformations. Therefore, in addition to achieving our specific goal, the development of nNOS selective compounds, we have learned how subtle differences in dynamics and structure can control protein-ligand interactions and often in unexpected ways.

Integrated redox sensor and effector functions for tetrahydrobiopterin- and glutathionylation-dependent endothelial nitric-oxide synthase uncoupling

Endothelial nitric-oxide synthase (eNOS) is a critical regulator of vascular homeostasis by generation of NO that is dependent on the cofactor tetrahydrobiopterin (BH4). When BH4 availability is limiting, eNOS becomes "uncoupled," resulting in superoxide production in place of NO. Recent evidence suggests that eNOS uncoupling can also be induced by S-glutathionylation, although the functional relationships between BH4 and S-glutathionylation remain unknown. To address a possible role for BH4 in S-glutathionylation-induced eNOS uncoupling, we expressed either WT or mutant eNOS rendered resistant to S-glutathionylation in cells with Tet-regulated expression of human GTP cyclohydrolase I to regulate intracellular BH4 availability. We reveal that S-glutathionylation of eNOS, by exposure to either 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or glutathione reductase-specific siRNA, results in diminished NO production and elevated eNOS-derived superoxide production, along with a concomitant reduction in BH4 levels and BH4:7,8-dihydrobiopterin ratio. In eNOS uncoupling induced by BH4 deficiency, BCNU exposure further exacerbates superoxide production, BH4 oxidation, and eNOS activity. Following mutation of C908S, BCNU-induced eNOS uncoupling and BH4 oxidation are abolished, whereas uncoupling induced by BH4 deficiency was preserved. Furthermore, BH4 deficiency alone is alone sufficient to reduce intracellular GSH:GSSG ratio and cause eNOS S-glutathionylation. These data provide the first evidence that BH4 deficiency- and S-glutathionylation-induced mechanisms of eNOS uncoupling, although mechanistically distinct, are functionally related. We propose that uncoupling of eNOS by S-glutathionylation- or by BH4-dependent mechanisms exemplifies eNOS as an integrated redox "hub" linking upstream redox-sensitive effects of BH4 and glutathione with redox-dependent targets and pathways that lie downstream of eNOS.

Tetrahydrobiopterin ameliorates hepatic ischemia-reperfusion Injury by coupling with eNOS in mice

BACKGROUND

In the liver, eNOS appears to have a central role in protecting against ischemia/reperfusion (I/R) injury. We hypothesized that tetrahydrobiopterin (BH4) would protect livers subjected to I/R injury by coupling with eNOS.

METHODS

Chinese Kun Ming (KM) mice were subjected to 60 min of 70% hepatic ischemia 30 min after the administration of BH4 or saline. After reperfusion, survival was evaluated. The histologic appearance and ALT, BH4, nitrite/nitrate, 8-isoprostane, and eNOS protein expression levels were measured.

RESULTS

The 1-wk survival rate was 66.67% in the BH4 group and 33.33% in the saline group. The serum ALT values in the BH4 group 1, 3, 6, 12, and 24 h after reperfusion were significantly lower than those of the saline group. A histologic examination of the liver revealed only a small necrotic area in the BH4 group as opposed to massive necrosis in the saline group. The percentage values of the hepatic necrotic area 24 h after reperfusion were significantly less for the BH4 group than for the saline group. The nitrite/nitrate levels in the liver tissue were significantly increased by ~2-fold in the BH4 group compared with the saline group. The free radical indicator 8-isoprostane was reduced approximately 50% in the BH4 group compared with the saline group. Western blotting showed that the level of eNOS protein between the groups was not significantly different.

CONCLUSIONS

BH4 significantly improved the survival rate by reducing liver failure. This was supported by the histologic findings, and the mechanism was explored. According to the results, we suggest that BH4 prevents liver damage from I/R injury by attenuating reactive oxygen species and increasing NO synthesis, and might provide a novel and promising therapeutic option for preventing I/R injury.

A novel cryo-reduction method to investigate the molecular mechanism of nitric oxide synthases

Nitric oxide synthases (NOSs) are hemoproteins responsible for the biosynthesis of NO in mammals. They catalyze two successive oxidation reactions. The mechanism of oxygen activation is based on the transfer of two electrons and two protons. Despite structural analogies with cytochromes P450, the molecular mechanism of NOS remains yet to be elucidated. Because of extremely high reaction rates, conventional kinetics methods failed to trap and characterize the major reaction intermediates. Cryo-reduction methods offer a possibility to circumvent this technological lock, by triggering oxygen activation at cryogenic temperatures by using water radiolysis. However, this method is not adapted to the NOS mechanism because of the high instability of the initial Fe(II)O2 complex (extremely fast autoxidation and/or reaction with the cofactor H4B). This imposed a protocol with a stable Fe(II)O2 complex (observed only for one NOS-like protein) and that excludes any redox role for H4B. A relevant approach to the NOS mechanism would use H4B to provide the (second) electron involved in oxygen activation; water radiolysis would thus provide the first electron (heme reduction). In this context, we report here an investigation of the first electron transfer by this alternative approach, i.e., the reduction of native NOS by water radiolysis. We combined EPR and resonance Raman spectroscopies to analyze NOS reduction for a combination of different substrates, cofactor, and oxygen concentrations, and for different NOS isoforms. Our results show that cryo-reduction of native NOS is achieved for all conditions that are relevant to the investigation of the NOS mechanism.

Chronic exercise preserves renal structure and hemodynamics in spontaneously hypertensive rats

AIMS

Exercise training (ExT) is a recommended adjunct to many pharmaceutical antihypertensive therapies. The effects of chronic ExT on the development of hypertension-induced renal injury remain unknown. We examined whether ExT would preserve renal hemodynamics and structure in the spontaneously hypertensive rat (SHR), and whether these effects were mediated by improved redox status and decreased inflammation. Normotensive WKY rats and SHR underwent moderate-intensity ExT for 16 weeks. One group of SHR animals was treated with hydralazine to investigate the pressure-dependent/independent effects of ExT. Acute renal clearance experiments were performed prior to sacrifice. Tissue free radical production rates were measured by electron paramagnetic resonance; gene and protein expression were measured by real time RT-PCR and Western blot or immunofluorescence, respectively. Plasma angiotensin II levels and kidney antioxidants were assessed. Training efficacy was assessed by citrate synthase activity assay in hind-limb muscle.

RESULTS

ExT delayed hypertension, prevented oxidative stress and inflammation, preserved antioxidant status, prevented an increase in circulating AngII levels, and preserved renal hemodynamics and structure in SHR. In addition, exercise-induced effects, at least, in part, were found to be pressure-independent.

INNOVATION

This study is the first to provide mechanistic evidence for the renoprotective benefits of ExT in a model of hypertension. Our results demonstrate that initiation of ExT in susceptible patients can delay the development of hypertension and provide renoprotection at the functional and ultrastructural level.

CONCLUSION

Chronic ExT preserves renal hemodynamics and structure in SHR; these effects are partially mediated by improved redox status and decreased inflammation.

Differential effects of substrate-analogue inhibitors on nitric oxide synthase dimerization

Nitric oxide synthase (NOS) isoforms are hemoenzymes that are only active as homodimers. We have examined the effect of the substrate-analogue inhibitors, N(G)-monomethyl-L-arginine (L-NMA), N(G)-nitro-L-arginine (L-NNA), N(G)-nitro-L-arginine methyl ester (L-NAME), N(5)-(1-iminoethyl)-L-ornithine (L-NIO), and N(6)-(1-iminoethyl)-L-lysine (L-NIL), the guanidine-containing inhibitor aminoguanidine (AG), and the amidine moiety-containing iNOS-specific inhibitor 1400W, on the formation of NOS dimer. Of these inhibitors, L-NMA effectively not only inhibited iNOS dimerization, but also destabilized its dimeric form in RAW264.7 cells stimulated with lipopolysaccharide plus interferon-γ, but not eNOS dimerization in endothelial cells. Importantly, this inhibition was highly correlated with NO production. These inhibitory effects were significantly reversed by addition of L-arginine. However, L-NNA, L-NAME, and AG in part or significantly increased dimerization of iNOS and eNOS in intact cells, and the other inhibitors assessed did not alter dimerization of iNOS and eNOS. These data taken together suggest that substituted groups of an arginine guanidino moiety play an important role in NOS dimerization as well as its catalytic activity. Our results indicate that l-NMA can inhibit iNOS-dependent NO production by preventing iNOS dimerization and destabilizing its dimeric form.

Oxygen-dependent regulation of nitric oxide production by inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) catalyzes the reaction that converts the substrates O(2) and l-arginine to the products nitric oxide (NO) and l-citrulline. Macrophages, and many other cell types, upregulate and express iNOS primarily in response to inflammatory stimuli. Physiological and pathophysiological oxygen tension can regulate NO production by iNOS at multiple levels, including transcriptional, translational, posttranslational, enzyme dimerization, cofactor availability, and substrate dependence. Cell culture techniques that emphasize control of cellular PO(2), and measurement of NO or its stable products, have been used by several investigators for in vitro study of the O(2) dependence of NO production at one or more of these levels. In most cell types, prior or concurrent exposure to cytokines or other inflammatory stimuli is required for the upregulation of iNOS mRNA and protein by hypoxia. Important transcription factors that target the iNOS promoter in hypoxia include hypoxia-inducible factor 1 and/or nuclear factor κB. In contrast to the upregulation of iNOS by hypoxia, in most cell types NO production is reduced by hypoxia. Recent work suggests a prominent role for O(2) substrate dependence in the short-term regulation of iNOS-mediated NO production.

Leishmania-macrophage interactions: insights into the redox biology

Leishmaniasis is a neglected tropical disease that affects about 350 million individuals worldwide. The protozoan parasite has a relatively simple life cycle with two principal stages: the flagellated mobile promastigote living in the gut of the sandfly vector and the intracellular amastigote within phagolysosomal vesicles of the vertebrate host macrophage. This review presents a state-of-the-art overview of the redox biology at the parasite-macrophage interface. Although Leishmania species are susceptible in vitro to exogenous superoxide radical, hydrogen peroxide, nitric oxide, and peroxynitrite, they manage to survive the endogenous oxidative burst during phagocytosis and the subsequent elevated nitric oxide production in the macrophage. The parasite adopts various defense mechanisms to cope with oxidative stress: the lipophosphoglycan membrane decreases superoxide radical production by inhibiting NADPH oxidase assembly and the parasite also protects itself by expressing antioxidant enzymes and proteins. Some of these enzymes could be considered potential drug targets because they are not expressed in mammals. In respect to antileishmanial therapy, the effects of current drugs on parasite-macrophage redox biology and its involvement in the development of drug resistance and treatment failure are presented.

Regulation of Inducible Nitric Oxide Synthase (iNOS) and its Potential Role in Insulin Resistance, Diabetes and Heart Failure

Nitric oxide synthases (NOS) are the enzymes responsible for nitric oxide (NO) generation. NO is a reactive oxygen species as well as a reactive nitrogen species. It is a free radical which mediates several biological effects. It is clear that the generation and actions of NO under physiological and pathophysiological conditions are regulated and extend to almost every cell type and function within the circulation. In mammals 3 distinct isoforms of NOS have been identified: neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS). The important isoform in the regulation of insulin resistance (IR) is iNOS. Understanding the molecular mechanisms regulating the iNOS pathway in normal and hyperglycemic conditions would help to explain some of vascular abnormalities observed in type 2 diabetes mellitus (T2DM). Previous studies have reported increased myocardial iNOS activity and expression in heart failure (HF). This review considers the recent animal studies which focus on the understanding of regulation of iNOS activity/expression and the role of iNOS agonists as potential therapeutic agents in treatment of IR, T2DM and HF.

Nitric oxide and redox mechanisms in the immune response

The role of redox molecules, such as NO and ROS, as key mediators of immunity has recently garnered renewed interest and appreciation. To regulate immune responses, these species trigger the eradication of pathogens on the one hand and modulate immunosuppression during tissue-restoration and wound-healing processes on the other. In the acidic environment of the phagosome, a variety of RNS and ROS is produced, thereby providing a cauldron of redox chemistry, which is the first line in fighting infection. Interestingly, fluctuations in the levels of these same reactive intermediates orchestrate other phases of the immune response. NO activates specific signal transduction pathways in tumor cells, endothelial cells, and monocytes in a concentration-dependent manner. As ROS can react directly with NO-forming RNS, NO bioavailability and therefore, NO response(s) are changed. The NO/ROS balance is also important during Th1 to Th2 transition. In this review, we discuss the chemistry of NO and ROS in the context of antipathogen activity and immune regulation and also discuss similarities and differences between murine and human production of these intermediates.

The molecular mechanism of mammalian NO-synthases: a story of electrons and protons

Since its discovery, nitric oxide synthase (NOS), the enzyme responsible for NO biosynthesis in mammals, has been the subject of extensive investigations regarding its catalytic and molecular mechanisms. These studies reveal the high degree of sophistication of NOS functioning and regulation. However, the precise description of the NOS molecular mechanism and in particular of the oxygen activation chemistry is still lacking. The reaction intermediates implicated in NOS catalysis continue to elude identification and the current working paradigm is increasingly contested. Consequently, the last three years has seen the emergence of several competing models. All these models propose the same global reaction scheme consisting of two successive oxidation reactions but they diverge in the details of their reaction sequence. The major discrepancies concern the number, source and characteristics of proton and electron transfer processes. As a result each model proposes distinct reaction pathways with different implied oxidative species. This review aims to examine the different experimental evidence concerning NOS proton and electron transfer events and the role played by the substrates and cofactors in these processes. The resulting discussion should provide a comparative picture of all potential models for the NOS molecular mechanism.

GAPDH regulates cellular heme insertion into inducible nitric oxide synthase

Heme proteins play essential roles in biology, but little is known about heme transport inside mammalian cells or how heme is inserted into soluble proteins. We recently found that nitric oxide (NO) blocks cells from inserting heme into several proteins, including cytochrome P450s, hemoglobin, NO synthases, and catalase. This finding led us to explore the basis for NO inhibition and to identify cytosolic proteins that may be involved, using inducible NO synthase (iNOS) as a model target. Surprisingly, we found that GAPDH plays a key role. GAPDH was associated with iNOS in cells. Pure GAPDH bound tightly to heme or to iNOS in an NO-sensitive manner. GAPDH knockdown inhibited heme insertion into iNOS and a GAPDH mutant with defective heme binding acted as a dominant negative inhibitor of iNOS heme insertion. Exposing cells to NO either from a chemical donor or by iNOS induction caused GAPDH to become S-nitrosylated at Cys152. Expressing a GAPDH C152S mutant in cells or providing a drug to selectively block GAPDH S-nitrosylation both made heme insertion into iNOS resistant to the NO inhibition. We propose that GAPDH delivers heme to iNOS through a process that is regulated by its S-nitrosylation. Our findings may uncover a fundamental step in intracellular heme trafficking, and reveal a mechanism whereby NO can govern the process.

Regulation of the expression of inducible nitric oxide synthase

Nitric oxide (NO) generated by the inducible isoform of nitric oxide synthase (iNOS) is involved in complex immunomodulatory and antitumoral mechanisms and has been described to have multiple beneficial microbicidal, antiviral and antiparasital effects. However, dysfunctional induction of iNOS expression seems to be involved in the pathophysiology of several human diseases. Therefore iNOS has to be regulated very tightly. Modulation of expression, on both the transcriptional and post-transcriptional level, is the major regulation mechanism for iNOS. Pathways resulting in the induction of iNOS expression vary in different cells or species. Activation of the transcription factors NF-kappaB and STAT-1alpha and thereby activation of the iNOS promoter seems to be an essential step for the iNOS induction in most human cells. However, at least in the human system, also post-transcriptional mechanisms involving a complex network of RNA-binding proteins build up by AUF1, HuR, KSRP, PTB and TTP is critically involved in the regulation of iNOS expression. Recent data also implicate regulation of iNOS expression by non-coding RNAs (ncRNAs).

Tetrahydrobiopterin supplementation reduces atherosclerosis and vascular inflammation in apolipoprotein E-knockout mice

BH4 (tetrahydrobiopterin) supplementation improves endothelial function in models of vascular disease by maintaining eNOS (endothelial nitric oxide synthase) coupling and NO (nitric oxide) bioavailability. However, the cellular mechanisms through which enhanced endothelial function leads to reduced atherosclerosis remain unclear. We have used a pharmaceutical BH4 formulation to investigate the effects of BH4 supplementation on atherosclerosis progression in ApoE-KO (apolipoprotein E-knockout) mice. Single oral dose pharmacokinetic studies revealed rapid BH4 uptake into plasma and organs. Plasma BH4 levels returned to baseline by 8 h after oral dosing, but remained markedly increased in aorta at 24 h. Daily oral BH4 supplementation in ApoE-KO mice from 8 weeks of age, for a period of 8 or 12 weeks, had no effect on plasma lipids or haemodynamic parameters, but significantly reduced aortic root atherosclerosis compared with placebo-treated animals. BH4 supplementation significantly reduced VCAM-1 (vascular cell adhesion molecule 1) mRNA levels in aortic endothelial cells, markedly reduced the infiltration of T-cells, macrophages and monocytes into plaques, and reduced T-cell infiltration in the adjacent adventitia, but importantly had no effect on circulating leucocytes. GCH (GTP cyclohydrolase I)-transgenic mice, with a specific increase in endothelial BH4 levels, exhibited a similar reduction in vascular immune cell infiltration compared with BH4-deficient controls, suggesting that BH4 reduces vascular inflammation via endothelial cell signalling. In conclusion, BH4 supplementation reduces vascular immune cell infiltration in atherosclerosis and may therefore be a rational therapeutic approach to reduce the progression of atherosclerosis.

Mass spectroscopy and molecular modeling predict endothelial nitric oxide synthase dimer collapse by hydrogen peroxide through zinc tetrathiolate metal-binding site disruption

Endothelial nitric oxide synthase (eNOS) is inhibited by hydrogen peroxide (H(2)O(2)), but the mechanism has not been determined. Thus, the purpose of this study was to delineate the mechanism by which H(2)O(2) inhibits eNOS activity. Using mass spectroscopy, we found that the tetrathiolate cysteine residues 94 and 99 were susceptible to oxidation by H(2)O(2). Molecular modeling predicted that these cysteic acid modifications would disrupt the van der Waals interactions and the hydrogen bonding network mediated by the tetrathiolate cysteines 94 and 99 resulting in changes in quaternary structure, zinc release, and dimer collapse. Using recombinant human eNOS (heNOS) to test the predictions of the molecular modeling we found that H(2)O(2) caused disruption of the heNOS dimer and this was accompanied by zinc release and decreased NO generation. We also found that H(2)O(2) increased the oxidation of tetrahydrobiopterin (BH(4)) to dihydrobiopterin (BH(2)), whereas preincubation of heNOS with excess BH(4) prevented the destruction of zinc tetrathiolate and dimer collapse and preserved activity. Interestingly, we found that the dimmer-stabilizing effect of BH(4) is due to its ability to act as a catalase mimetic. Further, we confirmed that, in ovine aortic endothelial cells, H(2)O(2) could also induce dimer collapse and that increasing cellular BH(4) levels could maintain eNOS in its dimeric form and NO signaling when cells were challenged with H(2)O(2). This study links the inhibitory action of H(2)O(2) on heNOS through the destruction of zinc tetrathiolate metal-binding site and dimer collapse both in vitro and in vivo.

Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways

Tetrahyrobiopterin (BH4) is a required cofactor for the synthesis of nitric oxide by endothelial nitric-oxide synthase (eNOS), and BH4 bioavailability within the endothelium is a critical factor in regulating the balance between NO and superoxide production by eNOS (eNOS coupling). BH4 levels are determined by the activity of GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in de novo BH4 biosynthesis. However, BH4 levels may also be influenced by oxidation, forming 7,8-dihydrobiopterin (BH2), which promotes eNOS uncoupling. Conversely, dihydrofolate reductase (DHFR) can regenerate BH4 from BH2, but the functional importance of DHFR in maintaining eNOS coupling remains unclear. We investigated the role of DHFR in regulating BH4 versus BH2 levels in endothelial cells and in cell lines expressing eNOS combined with tet-regulated GTPCH expression in order to compare the effects of low or high levels of de novo BH4 biosynthesis. Pharmacological inhibition of DHFR activity by methotrexate or genetic knockdown of DHFR protein by RNA interference reduced intracellular BH4 and increased BH2 levels resulting in enzymatic uncoupling of eNOS, as indicated by increased eNOS-dependent superoxide but reduced NO production. In contrast to the decreased BH4:BH2 ratio induced by DHFR knockdown, GTPCH knockdown greatly reduced total biopterin levels but with no change in BH4:BH2 ratio. In cells expressing eNOS with low biopterin levels, DHFR inhibition or knockdown further diminished the BH4:BH2 ratio and exacerbated eNOS uncoupling. Taken together, these data reveal a key role for DHFR in eNOS coupling by maintaining the BH4:BH2 ratio, particularly in conditions of low total biopterin availability.

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.

Tetrahydrobiopterin recycling, a key determinant of endothelial nitric-oxide synthase-dependent signaling pathways in cultured vascular endothelial cells

Tetrahydrobiopterin (BH4) is a key redox-active cofactor in endothelial isoform of NO synthase (eNOS) catalysis and is an important determinant of NO-dependent signaling pathways. BH4 oxidation is observed in vascular cells in the setting of the oxidative stress associated with diabetes. However, the relative roles of de novo BH4 synthesis and BH4 redox recycling in the regulation of eNOS bioactivity remain incompletely defined. We used small interference RNA (siRNA)-mediated "knockdown" GTP cyclohydrolase-1 (GTPCH1), the rate-limiting enzyme in BH4 biosynthesis, and dihydrofolate reductase (DHFR), an enzyme-recycling oxidized BH4 (7,8-dihydrobiopterin (BH2)), and studied the effects on eNOS regulation and biopterin metabolism in cultured aortic endothelial cells. Knockdown of either DHFR or GTPCH1 attenuated vascular endothelial growth factor (VEGF)-induced eNOS activity and NO production; these effects were recovered by supplementation with BH4. In contrast, supplementation with BH2 abolished VEGF-induced NO production. DHFR but not GTPCH1 knockdown increased reactive oxygen species (ROS) production. The increase in ROS production seen with siRNA-mediated DHFR knockdown was abolished either by simultaneous siRNA-mediated knockdown of eNOS or by supplementing with BH4. In contrast, addition of BH2 increased ROS production; this effect of BH2 was blocked by BH4 supplementation. DHFR but not GTPCH1 knockdown inhibited VEGF-induced dephosphorylation of eNOS at the inhibitory site serine 116; these effects were recovered by supplementation with BH4. These studies demonstrate a striking contrast in the pattern of eNOS regulation seen by the selective modulation of BH4 salvage/reduction versus de novo BH4 synthetic pathways. Our findings suggest that the depletion of BH4 is not sufficient to perturb NO signaling, but rather that concentration of intracellular BH2, as well as the relative concentrations of BH4 and BH2, together play a determining role in the redox regulation of eNOS-modulated endothelial responses.

Proteomic analysis of GTP cyclohydrolase 1 multiprotein complexes in cultured normal adult human astrocytes under both basal and cytokine-activated conditions

GTP cyclohydrolase 1 (GCH1) is the rate-limiting enzyme of a metabolic pathway synthesizing tetrahydrobiopterin (BH(4)), the cofactor dimerizing and activating inducible nitric oxide synthase (NOS-2). GCH1 protein expression and enzyme activity are minimal in cultured, phenotypically stable, untreated normal adult human astrocytes (NAHA), but are strongly induced, together with NOS-2, by a mixture of three proinflammatory cytokines (IL-1beta, TNF-alpha, and IFN-gamma--the CM-trio) released by microglia under brain-damaging conditions. The resulting hyper-production of NO severely harms neurons. In this study, using MALDI-TOF/MS, PMF, Western immunoblotting (WB), and antibody microarrays we identified several proteins coimmunoprecipitating with GCH1. Under basal conditions, GCH1 was associated with various adaptor/regulator molecules involved in G-protein-coupled receptors signalling, protein serine/threonine phosphatase 2Cbeta (PP2Cbeta), and serine-threonine kinases like Ca(2+) calmodulin kinases (CaMKs), casein kinases (CKs), cAMP-dependent kinases (PKAs), and mitogen-activated protein kinases (MAPKs). Exposure to the three cytokines' mixture (CM-trio) significantly changed, within the 48-72 h required for the induction and activation of GCH1, the levels and identities of some of the 0 h-associated proteins: after 72 h CK-IIalpha tended to dissociate from, whereas MAPK12 and JNK3 were strongly associated with fully active GCH1. These findings provide a first enticing glimpse into the intricate mechanisms regulating GCH1 activation by proinflammatory cytokines in NAHA, and may have therapeutic implications.

Nitrosative stress suppresses checkpoint activation after DNA synthesis inhibition

DNA synthesis is promoted by the dephosphorylation and activation of cyclin-dependent kinase 2 (Cdk2) complexes by Cdc25A. Nitrosative stress suppresses Cdk2 dephosphorylation by Cdc25A in vitro and inhibits Cdc25A protein translation in cells, but the effects on S-phase progression remain unexamined. Herein we report that nitrosative stress catalyzed by inducible nitric oxide (*NO) synthase (iNOS) or the chemical nitrosant S-nitrosocysteine ethyl ester (SNCEE) rapidly inhibited DNA synthesis concomitant with Cdc25A loss. Surprisingly, this inhibition of DNA synthesis was refractory to ectopic expression of Cdc25A or a Cdc25-independent Cdk2 mutant. Nitrosative stress inhibited DNA synthesis without activating checkpoint signaling, thus distinguishing it from S-phase arrest mediated by other reactive *NO-derived species. The apparent lack of checkpoint activation was due to an active suppression because accumulation of pSer345-Chk1, pThr68-Chk2 and gammaH2AX was inhibited by nitrosative stress in cells exposed to DNA damage or replication inhibitors. We speculate that failure to activate the S-phase checkpoint in precancerous cells undergoing nitrosative stress may elevate the risk of transmitting damaged genomes to daughter cells upon cell cycle reentry.

Quantitative regulation of intracellular endothelial nitric-oxide synthase (eNOS) coupling by both tetrahydrobiopterin-eNOS stoichiometry and biopterin redox status: insights from cells with tet-regulated GTP cyclohydrolase I expression

Tetrahydrobiopterin (BH4) is a critical determinant of endothelial nitric-oxide synthase (eNOS) activity. In the absence of BH4, eNOS becomes "uncoupled" and generates superoxide rather than NO. However, the stoichiometry of intracellular BH4/eNOS interactions is not well defined, and it is unclear whether intracellular BH4 deficiency alone is sufficient to induce eNOS uncoupling. To address these questions, we developed novel cell lines with tet-regulated expression of human GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in BH4 synthesis, to selectively induce intracellular BH4 deficiency by incubation with doxycycline. These cells were stably co-transfected to express a human eNOS-green fluorescent protein fusion protein, selecting clones expressing either low (GCH/eNOS-LOW) or high (GCH/eNOS-HIGH) levels. Doxycycline abolished GTPCH mRNA expression and GTPCH protein, leading to markedly diminished total biopterin levels and a decreased ratio of BH4 to oxidized biopterins in cells expressing eNOS. Intracellular BH4 deficiency induced superoxide generation from eNOS, as assessed by N-nitro-L-arginine methyl ester inhibitable 2-hydroxyethidium generation, and attenuated NO production. Quantitative analysis of cellular BH4 versus superoxide production between GCH/eNOS-LOW and GCH/eNOS-HIGH cells revealed a striking linear relationship between eNOS protein and cellular BH4 stoichiometry, with eNOS uncoupling at eNOS:BH4 molar ratio >1. Furthermore, increasing the intracellular BH2 concentration in the presence of a constant eNOS:BH4 ratio was sufficient to induce eNOS-dependent superoxide production. This specific, reductionist approach in a cell-based system reveals that eNOS:BH4 reaction stoichiometry together with the intracellular BH4:BH2 ratio, rather than absolute concentrations of BH4, are the key determinants of eNOS uncoupling, even in the absence of exogenous oxidative stress.

Deficient BH4 production via de novo and salvage pathways regulates NO responses to cytokines in adult cardiac myocytes

Adult rat cardiac myocytes typically display a phenotypic response to cytokines manifested by low or no increases in nitric oxide (NO) production via inducible NO synthase (iNOS) that distinguishes them from other cell types. To better characterize this response, we examined the expression of tetrahydrobiopterin (BH4)-synthesizing and arginine-utilizing genes in cytokine-stimulated adult cardiac myocytes. Intracellular BH4 and 7,8-dihydrobiopterin (BH2) and NO production were quantified. Cytokines induced GTP cyclohydrolase and its feedback regulatory protein but with deficient levels of BH4 synthesis. Despite the induction of iNOS protein, cytokine-stimulated adult cardiac myocytes produced little or no increase in NO versus unstimulated cells. Western blot analysis under nonreducing conditions revealed the presence of iNOS monomers. Supplementation with sepiapterin (a precursor of BH4) increased BH4 as well as BH2, but this did not enhance NO levels or eliminate iNOS monomers. Similar findings were confirmed in vivo after treatment of rat cardiac allograft recipients with sepiapterin. It was found that expression of dihydrofolate reductase, required for full activity of the salvage pathway, was not detected in adult cardiac myocytes. Thus, adult cardiac myocytes have a limited capacity to synthesize BH4 after cytokine stimulation. The mechanisms involve posttranslational factors impairing de novo and salvage pathways. These conditions are unable to support active iNOS protein dimers necessary for NO production. These findings raise significant new questions about the prevailing understanding of how cytokines, via iNOS, cause cardiac dysfunction and injury in vivo during cardiac inflammatory disease states since cardiac myocytes are not a major source of high NO production.

Cytokines and lipopolysaccharides induce inducible nitric oxide synthase but not enzyme activity in adult rat cardiomyocytes

There is evidence that nitric oxide (NO) formation in adult cardiomyocytes stimulated with lipopolysaccharide (LPS) is not commensurate with iNOS levels. Tetrahydrobiopterin (BH(4)) is a key factor in the stabilization and NO production by iNOS homodimer. Thus we hypothesized that BH(4) is a limiting factor for NO production in adult cardiomyocytes in response to LPS and cytokines (TNF-alpha, IL-1, IFN-gamma alone, or mixed). It was verified that LPS and cytokines induced iNOS expression which did not translate into increased nitrite or [(14)C]citrulline production. This response coincided with defective BH(4) synthesis and low GTP cyclohydrolase activity. Furthermore, supplementation with BH(4) and ascorbate failed to increase iNOS activity. This effect was related to preferential accumulation of BH(2) rather than BH(4) in these cells. Uncoupled iNOS activity in stimulated cells was examined using mitochondrial aconitase activity as an endogenous marker of superoxide anion radical (O(2)(-)) formation, and found not to be significantly inhibited. 2-Hydroxyethidium also was not significantly increased. We conclude that adult cardiomyocytes are an unlikely source of NO and O(2)(-) in inflammatory conditions. This finding adds a new and unexpected layer of complexity to our understanding of the responses of the adult heart to inflammation.

Identification of differentially expressed genes in dorsal root ganglion in early diabetic rats

OBJECTIVE

To screen and identify differentially expressed genes in the dorsal root ganglion (DRG) in early experimental diabetic rats.

METHODS

Diabetic model rats were induced by single intraperitoneal injection of streptozotocin (STZ). At the second week after STZ injection, the sensory nerve conduction velocities (SNCV) of sciatic nerve were measured as an indicator of neuropathy. The technique of silver-staining mRNA differential display polymerase chain reaction (DD-PCR) was used to detect the levels of differentially expressed genes in rat DRG. The cDNA fragments that displayed differentially were identified by reverse-hybridization, cloned and sequenced subsequently, and then confirmed by Northern blot.

RESULTS

The SNCV in the diabetic model group [n = 9, (45.25+/-10.38) m/s] reduced obviously compared with the control group [n = 8, (60.10+/-11.92) m/s] (P < 0.05). Seven distinct cDNA clones, one was up-regulated gene and the others were down-regulated ones, were isolated by silver-staining mRNA differential display method and confirmed by Northern blot. According to the results of sequence alignment with GenBank data, majority of the clones had no significant sequence similarity to previously reported genes except only one that showed high homology to 6-pyruvoyl-tetrahydropterin synthase mRNA (accession No. BC059140), which had not been reported to relate to diabetic neuropathy.

CONCLUSION

These differentially expressed genes in the diabetic DRG may contribute to the pathogenesis of diabetic peripheral neuropathy.

Direct effect of cholesterol on insulin secretion: a novel mechanism for pancreatic beta-cell dysfunction

OBJECTIVE

Type 2 diabetes is often accompanied by abnormal blood lipid and lipoprotein levels, but most studies on the link between hyperlipidemia and diabetes have focused on free fatty acids (FFAs). In this study, we examined the relationship between cholesterol and insulin secretion from pancreatic beta-cells that is independent of the effects of FFAs.

RESEARCH DESIGN AND METHODS

Several methods were used to modulate cholesterol levels in intact islets and cultured beta-cells, including a recently developed mouse model that exhibits elevated cholesterol but normal FFA levels. Acute and metabolic alteration of cholesterol was done using pharmacological reagents.

RESULTS

We found a direct link between elevated serum cholesterol and reduced insulin secretion, with normal secretion restored by cholesterol depletion. We further demonstrate that excess cholesterol inhibits secretion by downregulation of metabolism through increased neuronal nitric oxide synthase dimerization.

CONCLUSIONS

This direct effect of cholesterol on beta-cell metabolism opens a novel set of mechanisms that may contribute to beta-cell dysfunction and the onset of diabetes in obese patients.

Mechanisms for the role of tetrahydrobiopterin in endothelial function and vascular disease

NO produced by eNOS (endothelial nitric oxide synthase) is a key mediator of vascular homoeostasis. NO bioavailability is reduced early in vascular disease states, such as hypercholesterolaemia, diabetes and hypertension, and throughout the progression of atherosclerosis. This is a result of both reduced NO synthesis and increased NO consumption by reactive oxygen species. eNOS enzymatic activity appears to be determined by the availability of its cofactor BH4 (tetrahydrobiopterin). When BH4 levels are adequate, eNOS produces NO; when BH4 levels are limiting, eNOS becomes enzymatically uncoupled and generates superoxide, contributing to vascular oxidative stress and endothelial dysfunction. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus oxidative degradation in dysfunctional endothelium. Augmenting vascular BH4 levels by pharmacological supplementation, by enhancing the rate of de novo biosynthesis or by measures to reduce BH4 oxidation have been shown in experimental studies to enhance NO bioavailability. Thus BH4 represents a potential therapeutic target for preserving eNOS function in vascular disease.

Manganese modulates pro-inflammatory gene expression in activated glia

Redox-active metals are of paramount importance for biological functions. Their impact and cellular activities participate in the physiological and pathophysiological processes of the central nervous system (CNS), including inflammatory responses. Manganese is an essential trace element and it is required for normal biological activities and ubiquitous enzymatic reactions. However, excessive chronic exposure to manganese results in neurobehavioral deficits. Recent evidence suggests that manganese neurotoxicity involves activation of microglia or astrocytes, representative CNS immune cells. In this study, we assessed the molecular basis of the effects of manganese on the modulation of pro-inflammatory cytokines and nitric oxide (NO) production in primary rat cortical glial cells. Cultured glial cells consisted of 85% of astrocytes and 15% of microglia. Within the assayed concentrations, manganese was unable to induce tumor necrosis factor alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) expression, whereas it potentiated iNOS and TNF-alpha gene expression by lipopolysaccharide/interferon-gamma-activated glial cells. The enhancement was accompanied by elevation of free manganese, generation of oxidative stress, activation of mitogen-activated protein kinases, and increased NF-kappaB and AP-1 binding activities. The potentiated degradation of inhibitory molecule IkappaB-alpha was one of underlying mechanisms for the increased activation of NF-kappaB by manganese. However, manganese decreased iNOS enzymatic activity possibly through the depletion of cofactor since exogenous tetrahydrobiopterin reversed manganese's action. These data indicate that manganese could modulate glial inflammation through variable strategies.

Increased vascular biosynthesis of tetrahydrobiopterin in apolipoprotein E-deficient mice

Previous studies suggested that loss of tetrahydrobiopterin (BH(4)) may play an important role in the pathogenesis of vascular endothelial dysfunction induced by diabetes and hypertension. In contrast, controversial results have been reported regarding BH(4) metabolism in experimental models of atherosclerosis. Therefore, the present study was designed to characterize the expression and activity of GTP-cyclohydrolase I, a rate-limiting enzyme in biosynthesis of BH(4), during atherogenesis. BH(4) levels were significantly increased in atherosclerotic aortas of apolipoprotein E (apoE)-deficient mice as compared with wild-type mice after 5 mo of Western diet treatment. This increase was further significantly enhanced in apoE-deficient mice fed for 9 and 14 mo. Removal of the endothelium almost eliminated BH(4) in wild-type mice but not in apoE-deficient mice, suggesting that a major component of increased BH(4) synthesis is localized in the vascular media of apoE-deficient mice. Oxidative products of BH(4) were low and did not differ between wild-type and apoE-deficient mice over the course of this study. Increased protein expression and enzymatic activity of GTP-cyclohydrolase I were detected in aortas of apoE-deficient mice (P < 0.05), providing molecular mechanisms responsible for elevation of vascular BH(4). In contrast to aortas, we did not detect any change in levels of BH(4) and in GTP-cyclohydrolase I expression in the brain. Our results demonstrate selective increase of intracellular BH(4) levels via elevation of GTP-cyclohydrolase I activity in vascular tissue of apoE-deficient mice.

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.

Detection of nitrosothiols and other nitroso species in vitro and in cells

The nitric oxide (NO)-mediated nitrosation of peptides and proteins may play important roles in normobiology and pathobiology. With the realization that S-nitrosothiols (RSNOs) participate in the transport, storage, and delivery of NO, as well as posttranslational modifications in cell signaling and inflammatory processes, there is an increasing need for the detection of nitrosothiols (RSNOs) and other nitroso species in cells and tissues. In this chapter, we describe the utilization of a gas phase chemiluminescence-based assay and "biotin switch" method for the detection of nitroso species in cells. These methods are sensitive enough to quantify and contrast the different pools of nitroso species that may coexist under physiologically relevant conditions. They also provide the means to characterize and identify proteins that may represent specific targets for nitrosation reactions.

Epigenetic basis for the transcriptional hyporesponsiveness of the human inducible nitric oxide synthase gene in vascular endothelial cells

A marked difference exists in the inducibility of inducible NO synthase (iNOS) between humans and rodents. Although important cis and trans factors in the murine and human iNOS promoters have been characterized using episomal-based approaches, a compelling molecular explanation for why human iNOS is resistant to induction has not been reported. In this study we present evidence that the hyporesponsiveness of the human iNOS promoter is based in part on epigenetic silencing, specifically hypermethylation of CpG dinucleotides and histone H3 lysine 9 methylation. Using bisulfite sequencing, we demonstrated that the iNOS promoter was heavily methylated at CpG dinucleotides in a variety of primary human endothelial cells and vascular smooth muscle cells, all of which are notoriously resistant to iNOS induction. In contrast, in human cell types capable of iNOS induction (e.g., A549 pulmonary adenocarcinoma, DLD-1 colon adenocarcinoma, and primary hepatocytes), the iNOS promoter was relatively hypomethylated. Treatment of human cells, such as DLD-1, with a DNA methyltransferase inhibitor (5-azacytidine) induced global and iNOS promoter DNA hypomethylation. Importantly, 5-azacytidine enhanced the cytokine inducibility of iNOS. Using chromatin immunoprecipitation, we found that the human iNOS promoter was basally enriched with di- and trimethylation of H3 lysine 9 in endothelial cells, and this did not change with cytokine addition. This contrasted with the absence of lysine 9 methylation in inducible cell types. Importantly, chromatin immunoprecipitation demonstrated the selective presence of the methyl-CpG-binding transcriptional repressor MeCP2 at the iNOS promoter in endothelial cells. Collectively, our work defines a role for chromatin-based mechanisms in the control of human iNOS gene expression.

Stoichiometric relationships between endothelial tetrahydrobiopterin, endothelial NO synthase (eNOS) activity, and eNOS coupling in vivo: insights from transgenic mice with endothelial-targeted GTP cyclohydrolase 1 and eNOS overexpression

Endothelial dysfunction in vascular disease states is associated with reduced NO bioactivity and increased superoxide (O2*-) production. Some data suggest that an important mechanism underlying endothelial dysfunction is endothelial NO synthase (eNOS) uncoupling, whereby eNOS generates O2*- rather than NO, possibly because of a mismatch between eNOS protein and its cofactor tetrahydrobiopterin (BH4). However, the mechanistic relationship between BH4 availability and eNOS coupling in vivo remains undefined because no studies have investigated the regulation of eNOS by BH4 in the absence of vascular disease states that cause pathological oxidative stress through multiple mechanisms. We investigated the stoichiometry of BH4-eNOS interactions in vivo by crossing endothelial-targeted eNOS transgenic (eNOS-Tg) mice with mice overexpressing endothelial GTP cyclohydrolase 1 (GCH-Tg), the rate-limiting enzyme in BH4 synthesis. eNOS protein was increased 8-fold in eNOS-Tg and eNOS/GCH-Tg mice compared with wild type. The ratio of eNOS dimer:monomer was significantly reduced in aortas from eNOS-Tg mice compared with wild-type mice but restored to normal in eNOS/GCH-Tg mice. NO synthesis was elevated by 2-fold in GCH-Tg and eNOS-Tg mice but by 4-fold in eNOS/GCH-Tg mice compared with wild type. Aortic BH4 levels were elevated in GCH-Tg and maintained in eNOS/GCH-Tg mice but depleted in eNOS-Tg mice compared with wild type. Aortic and cardiac O2*- production was significantly increased in eNOS-Tg mice compared with wild type but was normalized after NOS inhibition with Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME), suggesting O2*- production by uncoupled eNOS. In contrast, in eNOS/GCH-Tg mice, O2*- production was similar to wild type, and L-NAME had no effect, indicating preserved eNOS coupling. These data indicate that eNOS coupling is directly related to eNOS-BH4 stoichiometry even in the absence of a vascular disease state. Endothelial BH4 availability is a pivotal regulator of eNOS activity and enzymatic coupling in vivo.