Anti-inflammatory effects of tetrahydrobiopterin on early rejection in renal allografts: modulation of inducible nitric oxide synthase

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.

Nitric oxide and long-term outcomes after kidney transplantation: Results of the TransplantLines cohort study

Impaired endogenous nitric oxide (NO) production may contribute to graft failure and premature mortality in kidney transplant recipients (KTR). We investigated potential associations of 24-h urinary NOx (NO₃^- + NO₂^-) excretion (uNOx) with long-term outcomes. uNOx was determined by HPLC and GC-MS in 698 KTR and in 132 kidney donors before and after donation. Additionally, we measured urinary nitroso species (RXNO) by gas-phase chemiluminescence. Median uNOx was lower in KTR compared to kidney donors (688 [393-1076] vs. 1301 [868-1863] before donation and 1312 [982-1853] μmol/24h after donation, P < 0.001). During median follow-up of 5.4 [4.8-6.1] years, 150 KTR died (61 due to cardiovascular disease) and 83 experienced graft failure. uNOx was inversely associated with all-cause mortality (HR per doubling of uNOx: 0.84 [95% CI 0.75-0.93], P < 0.001) and cardiovascular mortality (HR 0.78 [95% CI 0.67-0.92], P = 0.002). The association of uNOx with graft failure was lost when adjusted for renal function (HR per doubling of uNOx: 0.89 [95% CI 0.76-1.05], P = 0.17). There were no significant associations of urinary RXNO with outcomes. Our study suggests that KTR have lower NO production than healthy subjects and that lower uNOx is associated with a higher risk of all-cause and cardiovascular mortality.

The clinical significance of receiving a kidney allograft from deceased donor with chronic histologic changes

Allograft survival of deceased donor kidneys with suboptimal histology (DRTx/suboptimal histology: >10% glomerulosclerosis, >10% tubulointerstitial scarring, or >mild vascular sclerosis) is inferior to both DRTx with optimal histology (DRTx/optimal histology) and living donor kidneys irrespective of histologic changes (LRTx). In this report, we explored the reasons behind this guarded outcome with a special focus on the role of alloimmunity. We initially assessed gene expression in 39 time-zero allograft biopsies using the Nanostring 770 genes PanCancer Immune Profiling Panel. Subsequently, we studied 696 consecutive adult kidney allograft recipients that were grouped according to allograft type and histology at time-zero biopsy [DRTx/suboptimal histology (n = 194), DRTx/optimal histology (n = 166), and LRTx (n = 336)]. Part-1: Several immune pathways were upregulated in time-zero biopsies from DRTx/suboptimal histology (n = 11) compared to LRTx (n = 17) but not to DRTx/optimal histology (n = 11). Part-2: Amongst the three groups of recipients, DRTx/suboptimal histology had the highest incidence of acute rejection episodes, most of which occurred during the first year after transplantation (early rejection). This increase was mainly attributed to T cell mediated rejection, while the incidence of antibody-mediated rejection was similar amongst the three groups. Importantly, early acute T cell mediated rejection was a strong independent predictor for allograft failure in DRTx/suboptimal histology (adjusted HR: 2.13, P = 0.005) but not in DRTx/optimal histology nor in LRTx. Our data highlight an increased baseline immunogenicity in DRTx/suboptimal histology compared to LRTx but not to DRTx/optimal histology. However, our results suggest that donor chronic histologic changes in DRTx may help transfer such increased baseline immunogenicity into clinically relevant acute rejection episodes that have detrimental effects on allograft survival. These findings may provide a rationale for enhanced immunosuppression in recipients of DRTx with baseline chronic histologic changes to minimize subsequent acute rejection and to prolong allograft survival.

Parenteral Nutrition and Cardiotoxicity

Parenteral nutrition (PN) is a life-saving nutritional therapy for those situations when patients are unable to receive enteral nutrition. However, despite a multitude of benefits offered by PN, it is associated with a variety of side effects, most notably parenteral nutrition-associated liver disease (PNALD). Adverse effects of PN on other organ systems, such as brain and cardiovascular system, have been poorly studied. There have been several case reports, studies, and a recent animal study highlighting cardiotoxic effects of PN; however, much remains unclear about the underlying mechanisms causing cardiac damage. In this review, we propose a series of potential mechanisms behind PN-associated heart injury, and we provide an overview of therapeutic strategies and recent scientific advances.

A nitric oxide synthase-like protein from Synechococcus produces NO/NO3- from l-arginine and NADPH in a tetrahydrobiopterin- and Ca2+-dependent manner

Nitric oxide synthases (NOSs) are heme-based monooxygenases that convert l-Arg to l-citrulline and nitric oxide (NO), a key signaling molecule and cytotoxic agent in mammals. Bacteria also contain NOS proteins, but the role of NO production within these organisms, where understood, differs considerably from that of mammals. For example, a NOS protein in the marine cyanobacterium Synechococcus sp. PCC 7335 (syNOS) has recently been proposed to function in nitrogen assimilation from l-Arg. syNOS retains the oxygenase (NOSox) and reductase (NOSred) domains present in mammalian NOS enzymes (mNOSs), but also contains an N-terminal globin domain (NOSg) homologous to bacterial flavohemoglobin proteins. Herein, we show that syNOS functions as a dimer and produces NO from l-Arg and NADPH in a tetrahydrobiopterin (H4B)-dependent manner at levels similar to those produced by other NOSs but does not require Ca2+-calmodulin, which regulates NOSred-mediated NOSox reduction in mNOSs. Unlike other bacterial NOSs, syNOS cannot function with tetrahydrofolate and requires high Ca2+ levels (>200 μm) for its activation. NOSg converts NO to NO3- in the presence of O2 and NADPH; however, NOSg did not protect Escherichia coli strains against nitrosative stress, even in a mutant devoid of NO-protective flavohemoglobin. We also found that syNOS does not have NOS activity in E. coli (which lacks H4B) and that the recombinant protein does not confer growth advantages on l-Arg as a nitrogen source. Our findings indicate that syNOS has both NOS and NO oxygenase activities, requires H4B, and may play a role in Ca2+-mediated signaling.

Direct Observation of Enhanced Nitric Oxide in a Murine Model of Diabetic Nephropathy

Uncoupling of nitric oxide synthase (NOS) secondary to redox signaling is a central mechanism in endothelial and macrophage activation. To date studies on the production of nitric oxide (NO) during the development of diabetic complications show paradoxical results. We previously showed that recoupling eNOS by increasing the eNOS cofactor tetrahydrobiopterin (BH4) could restore endothelial function and prevent kidney injury in experimental kidney transplantation. Here, we employed a diabetic mouse model to investigate the effects of diabetes on renal tissue NO bioavailability. For this, we used in vivo NO trapping, followed by electron paramagnetic resonance spectroscopy. In addition, we investigated whether coupling of NOS by supplying the cofactor BH4 could restore glomerular endothelial barrier function. Our data show that overall NO availability at the tissue level is not reduced sixteen weeks after the induction of diabetes in apoE knockout mice, despite the presence of factors that cause endothelial dysfunction, and the presence of the endogenous NOS inhibitor ADMA. Targeting uncoupled NOS with the BH4 precursor sepiapterin further increases NO availability, but did not modify renal glomerular injury. Notably, glomerular heparanase activity as a driver for loss of glomerular barrier function was not reduced, pointing towards NOS-independent mechanisms. This was confirmed by unaltered increased glomerular presence of cathepsin L, the protease that activates heparanase.

Detection of Nitric Oxide by Electron Paramagnetic Resonance Spectroscopy: Spin-Trapping with Iron-Dithiocarbamates

Electron paramagnetic resonance (EPR) spectroscopy is the ideal methodology to identify radicals (detection and characterization of molecular structure) and to study their kinetics, in both simple and complex biological systems. The very low concentration and short life-time of NO and of many other radicals do not favor its direct detection and spin-traps are needed to produce a new and persistent radical that can be subsequently detected by EPR spectroscopy.In this chapter, we present the basic concepts of EPR spectroscopy and of some spin-trapping methodologies to study NO. The "strengths and weaknesses" of iron-dithiocarbamates utilization, the NO traps of choice for the authors, are thoroughly discussed and a detailed description of the method to quantify the NO formation by molybdoenzymes is provided.

Cinnamaldehyde and nitric oxide attenuate advanced glycation end products-induced the Jak/STAT signaling in human renal tubular cells

Cinnamaldehyde is a major and a bioactive compound isolated from the leaves of Cinnamomum osmophloeum kaneh. It possesses anti-diabetic properties in vitro and in vivo and has anti-inflammatory and anti-cancer effects. To explore whether cinnamaldehyde was linked to altered advanced glycation end products (AGE)-mediated diabetic nephropathy, the molecular mechanisms of cinnamaldehyde responsible for inhibition of AGE-reduced nitric oxide (NO) bioactivity in human renal proximal tubular cells were examined. We found that raising the ambient AGE concentration causes a dose-dependent decrease in NO generation. Cinnamaldehyde significantly reverses AGE-inhibited NO generation and induces high levels of cGMP synthesis and PKG activation. Treatments with cinnamaldehyde, the NO donor S-nitroso-N-acetylpenicillamine, and the JAK2 inhibitor AG490 markedly attenuated AGE-inhibited NOS protein levels and NO generation. Moreover, AGE-induced the JAK2-STAT1/STAT3 activation, RAGE/p27(Kip1) /collagen IV protein levels, and cellular hypertrophy were reversed by cinnamaldehyde. The ability of cinnamaldehyde to suppress STAT activation was also verified by the observation that it significantly increased SCOS-3 protein level. These findings indicate for the first time that in the presence of cinnamaldehyde, the suppression of AGE-induced biological responses is probably mediated by inactivating the JAK2-STAT1/STAT3 cascade or activating the NO pathway.

Regulation of iNOS function and cellular redox state by macrophage Gch1 reveals specific requirements for tetrahydrobiopterin in NRF2 activation

Inducible nitric oxide synthase (iNOS) is a key enzyme in the macrophage inflammatory response, which is the source of nitric oxide (NO) that is potently induced in response to proinflammatory stimuli. However, the specific role of NO production, as distinct from iNOS induction, in macrophage inflammatory responses remains unproven. We have generated a novel mouse model with conditional deletion of Gch1, encoding GTP cyclohydrolase 1 (GTPCH), an essential enzyme in the biosynthesis of tetrahydrobiopterin (BH4) that is a required cofactor for iNOS NO production. Mice with a floxed Gch1 allele (Gch1(fl/fl)) were crossed with Tie2cre transgenic mice, causing Gch1 deletion in leukocytes (Gch1(fl/fl)Tie2cre). Macrophages from Gch1(fl/fl)Tie2cre mice lacked GTPCH protein and de novo biopterin biosynthesis. When activated with LPS and IFNγ, macrophages from Gch1(fl/fl)Tie2cre mice induced iNOS protein in a manner indistinguishable from wild-type controls, but produced no detectable NO, as judged by L-citrulline production, EPR spin trapping of NO, and by nitrite accumulation. Incubation of Gch1(fl/fl)Tie2cre macrophages with dihydroethidium revealed significantly increased production of superoxide in the presence of iNOS expression, and an iNOS-independent, BH4-dependent increase in other ROS species. Normal BH4 levels, nitric oxide production, and cellular redox state were restored by sepiapterin, a precursor of BH4 production by the salvage pathway, demonstrating that the effects of BH4 deficiency were reversible. Gch1(fl/fl)Tie2cre macrophages showed only minor alterations in cytokine production and normal cell migration, and minimal changes in basal gene expression. However, gene expression analysis after iNOS induction identified 78 genes that were altered between wild-type and Gch1(fl/fl)Tie2cre macrophages. Pathway analysis identified decreased NRF2 activation, with reduced induction of archetypal NRF2 genes (gclm, prdx1, gsta3, nqo1, and catalase) in BH4-deficient Gch1(fl/fl)Tie2cre macrophages. These findings identify BH4-dependent iNOS regulation and NO generation as specific requirements for NRF2-dependent responses in macrophage inflammatory activation.

The role of tetrahydrobiopterin in inflammation and cardiovascular disease

The cofactor tetrahydrobiopterin (BH4) is required for nitric oxide (NO) production by all nitric oxide synthase (NOS) enzymes and is a key regulator of cellular redox signalling. When BH4 levels become limiting NOS enzymes become 'uncoupled' and produce superoxide rather than NO. Endothelial cell BH4 is required for the maintenance of vascular function through NO production, and reduced BH4 levels are associated with vascular dysfunction. Evidence increasingly points to important roles for BH4 and NOS enzymes in other vascular cell types. Leukocytes have a fundamental role in atherosclerosis, and new evidence points to a role in the control of hypertension. Leukocytes are a major site of iNOS expression, and the regulation of this isoform is another mechanism by which BH4 availability may modulate disease. This review provides an overview of BH4 control of NOS function in both endothelial cells and leukocytes in the context of vascular disease and current therapeutic evaluations.

Advanced glycation end products-mediated hypertrophy is negatively regulated by tetrahydrobiopterin in renal tubular cells

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease worldwide. The accumulation of advanced glycation end products (AGE) is a key mediator of renal tubular hypertrophy in DN. Elimination of tetrahydrobiopterin (BH(4)) and nitric oxide (NO) bioavailability may contribute to the aggravation of DN. The present study aims to explore any possible beneficial effect of exogenous BH(4) in alleviating the AGE-induced renal tubular hypertrophy in DN. Thus, renal tubular cells were treated with BH(4), BH(2), sepiapterin, or DAHP in the presence of AGE. We found that AGE (but not non-glycated BSA) markedly reduced NO production and increased hypertrophy index in these cells. Exogenous BH(4)/BH(2) and sepiapterin treatments attenuated AGE-inhibited the iNOS/NO/GTPCH I protein synthesis. Moreover, BH(4) and BH(2) significantly reversed AGE-enhanced the JAK2-STAT1/STAT3 activation. The abilities of BH(4) and BH(2) to inhibit AGE-induced renal cellular hypertrophy were verified by the observation that BH(4) and BH(2) inhibited hypertrophic growth and the protein synthesis of p27(Kip1) and α-SMA. These findings indicate for the first time that exogenous BH(4) and BH(2) attenuate AGE-induced hypertrophic effect at least partly by increasing the iNOS/GTPCH I synthesis and NO generation in renal tubular cells.

Effects of sepiapterin infusion on renal oxygenation and early acute renal injury after suprarenal aortic clamping in rats

Acute kidney injury (AKI) can occur after aortic clamping due to microvascular dysfunction leading to renal hypoxia. In this rat study, we have tested the hypothesis that the administration of the precursor of the nitric oxide synthase essential cofactor tetrahydrobiopterin (BH4) could restore renal oxygenation after ischemia reperfusion (I/R) and prevent AKI. We randomly distributed rats into 4 groups: sham group; ischemia-reperfusion group; I/R + sepiapterin, the precursor of BH4; and I/R + sepiapterin + methotrexate, an inhibitor of the pathway generating BH4 from sepiapterin. Cortical and outer medullary microvascular oxygen pressure, renal oxygen delivery, renal oxygen consumption were measured using dual-wavelength oxygen-dependent quenching phosphorescence techniques during ischemia and throughout 3 hours of reperfusion. Kidney injury was assessed using myeloperoxidase staining for leukocyte infiltration and urine neutrophil gelatinase-associated lipocalin levels. Ischemia reperfusion induced a drop in microvascular PO2 (P < 0.01 vs. Sham, both), which was prevented by the infusion of sepiapterin. Sepiapterin partially prevented the rise in renal oxygen extraction (P < 0.001 vs. I/R). Finally, treatment with sepiapterin prevented renal infiltration by inflammatory cells and decreased urine neutrophil gelatinase-associated lipocalin levels indicating a decrease of renal injury. These effects were blunted when adding methotrexate, except for myeloperoxidase. In conclusion, the administration of sepiapterin can prevent renal hypoxia and AKI after suprarenal aortic clamping in rats.

Role of combination of L-arginine and α-tocopherol in renal transplantation ischaemia/reperfusion injury: a randomized controlled experimental study in a rat model

What's known on the subject? and What does the study add? Renal ischaemia/reperfusion (I/R) injury is an inevitable consequence of kidney transplantation. It contributes to delayed graft function (DGF), acute renal failure and graft rejection. The present study investigates for the first time the impact of a combination of L-arginine and alpha tocopherol on the renal ischemia/reperfusion injury in a rodent model of kidney transplantation. We found that concomitant administration of L-arginine and α-tocopherol has a more protective effect and synergistic antioxidant effect on ischaemia/reperfusion injury in transplanted rat kidneys.

OBJECTIVES

To investigate the role of L-arginine and α-tocopherol in ischaemia/reperfusion injury in a kidney transplanted rat model.

MATERIALS AND METHODS

In total, 40 male Sprague-Dawley rats subjected to renal transplantation received FK506 (tacrolimus) to overcome early acute rejection episodes. Animals were divided randomly into four groups (ten rats each). Group I were treated with FK506 (2 mg/kg/bw/day) and served as the control group. Group II were treated with L-arginine 300 mg/kg/bw. Group III were treated with α-tocopherol 30 mg/kg/bw. Group IV were treated with L-arginine and α-tocopherol. Urine and blood samples were taken at 0 (before operation), 2, 7 and 14 days post-transplantation for estimation of urine sodium, creatinine, fractional excretion of sodium, serum creatinine, sodium and blood urea nitrogen. Histological examination and measurement of malondialdehyde in kidney tissues were also performed.

RESULTS

Serum creatinine and blood urea nitrogen significantly decreased in L-arginine and α-tocopherol, as well as combination groups, compared to the control group. Malondialdehyde was significantly decreased in the combination group compared to L-arginine and α-tocopherol alone. Histological examination of the control group showed that acute tubular necrosis was markedly decreased in transplanted kidneys treated with a combination of both L-arginine and α-tocopherol.

CONCLUSIONS

Concomitant administration of l-arginine and α-tocopherol has a more protective effect and synergistic antioxidant effect on ischaemia/reperfusion injury in transplanted rat kidneys.

Reactive oxygen and nitrogen species regulate inducible nitric oxide synthase function shifting the balance of nitric oxide and superoxide production

Inducible NOS (iNOS) is induced in diseases associated with inflammation and oxidative stress, and questions remain regarding its regulation. We demonstrate that reactive oxygen/nitrogen species (ROS/RNS) dose-dependently regulate iNOS function. Tetrahydrobiopterin (BH4)-replete iNOS was exposed to increasing concentrations of ROS/RNS and activity was measured with and without subsequent BH4 addition. Peroxynitrite (ONOO(-)) produced the greatest change in NO generation rate, approximately 95% decrease, and BH4 only partially restored this loss of activity. Superoxide (O2(.-)) greatly decreased NO generation, however, BH4 addition restored this activity. Hydroxyl radical ((.)OH) mildly decreases NO generation in a BH4-dependent manner. iNOS was resistant to H2O2 with only slightly decreased NO generation with up to millimolar concentrations. In contrast to the inhibition of NO generation, ROS enhanced O2(.-) production from iNOS, while ONOO(-) had the opposite effect. Thus, ROS promote reversible iNOS uncoupling, while ONOO(-) induces irreversible enzyme inactivation and decreases both NO and O2(.-) production.

Endothelium-specific GTP cyclohydrolase I overexpression accelerates refractory wound healing by suppressing oxidative stress in diabetes

Refractory wound is a severe complication that leads to limb amputation in diabetes. Endothelial nitric oxide synthase (eNOS) plays a key role in normal wound repair but is uncoupled in streptozotocin (STZ)-induced type 1 diabetes because of reduced cofactor tetrahydrobiopterin (BH(4)). We tested the hypothesis that overexpression of GTP cyclohydrolase I (GTPCH I), the rate-limiting enzyme for de novo BH(4) synthesis, retards NOS uncoupling and accelerates wound healing in STZ mice. Blood glucose levels were significantly increased in both male endothelium-specific GTPCH I transgenic mice (Tg-GCH; via a tie-2 promoter) and wild-type (WT) littermates 5 days after STZ regimen. A full-thickness excisional wound was created on mouse dorsal skin by a 4-mm punch biopsy. Wound closure was delayed in STZ mice, which was rescued in STZ Tg-GCH mice. Cutaneous BH(4) level was significantly reduced in STZ mice vs. WT mice, which was maintained in STZ Tg-GCH mice. In STZ mice, constitutive NOS (cNOS) activity and nitrite levels were decreased compared with WT mice, paralleled by increased superoxide anion (O(2)(-)) level and inducible NOS (iNOS) activity. In STZ Tg-GCH mice, nitrite level and cNOS activity were potentiated and O(2)(-) level and iNOS activity were suppressed compared with STZ mice. Thus endothelium-specific BH(4) overexpression accelerates wound healing in type 1 diabetic mice by enhancing cNOS activity and suppressing oxidative stress.

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.

CCR2-mediated antiinflammatory effects of endothelial tetrahydrobiopterin inhibit vascular injury-induced accelerated atherosclerosis

BACKGROUND

Vascular injury results in loss of endothelial nitric oxide (NO), production of reactive oxygen species (ROS), and the initiation of an inflammatory response. Both NO and ROS modulate inflammation through redox-sensitive pathways. Tetrahydrobiopterin (BH4) is an essential cofactor for endothelial nitric oxide synthase (eNOS) that regulates enzymatic synthesis of either nitric oxide or ROS. We hypothesized that endothelial BH4 is an important regulator of inflammation and vascular remodeling.

METHODS AND RESULTS

Endothelium-targeted overexpression of GTP cyclohydrolase 1 (GCH), the rate limiting enzyme in BH4 synthesis, increased levels of tetrahydrobiopterin (BH4), reduced endothelial superoxide, improved eNOS coupling, and reduced vein graft atherosclerosis in transgenic GCH/ApoE-KO mice compared to ApoE-KO controls. Immunohistochemistry using anti-MAC-3 and MAC-1 antibody staining revealed a marked reduction in vein graft macrophage content, as did RT-PCR expression of macrophage marker CD68 mRNA levels in GCH/ApoE-KO mice. When we investigated the potential mediators of this reduction, we discovered that mRNA and protein levels of MCP-1 (CCL2) but not RANTES (CCL5) were significantly reduced in GCH/ApoE-KO aortic tissue. Consistent with this finding we found a decrease in CCR2-mediated, but not CCR5-mediated, chemotaxis in vascular tissue and plasma samples from GCH/ApoE-KO animals.

CONCLUSIONS

Increased endothelial BH4 reduces vein graft neointimal hyperplasia and atherosclerosis through a reduction in vascular inflammation. These findings highlight the importance of MCP-1/CCR2 signaling in the response to vascular injury and identify novel pathways linking endothelial BH4 to inflammation and vascular remodeling.

The complex role of iNOS in acutely rejecting cardiac transplants

This review summarizes the evidence for a detrimental role of nitric oxide (NO) derived from inducible NO synthase (iNOS) and/or reactive nitrogen species such as peroxynitrite in acutely rejecting cardiac transplants. In chronic cardiac transplant rejection, iNOS may have an opposing beneficial component. The purpose of this review is primarily to address issues related to acute rejection, which is a recognized risk factor for chronic rejection. The evidence for a detrimental role is based upon strategies involving nonselective NOS inhibitors, NO neutralizers, selective iNOS inhibitors, and iNOS gene deletion in rodent models of cardiac rejection. The review is presented in the context of the impact on various components, including graft survival, histological rejection, and cardiac function, which may contribute to the process of graft rejection in toto. Possible limitations of each strategy are discussed in order to understand better the variance in published findings, including issues related to the potential importance of cell localization of iNOS expression. Finally, the concept of a dual role for NO and its downstream product, peroxynitrite, in rejection vs immune regulation is discussed.

Nitric oxide synthase partial uncoupling as a key switching mechanism for the NO/ONOO- cycle

Short-term stressors, capable of increasing nitric oxide levels, act to initiate cases of illnesses including chronic fatigue syndrome, multiple chemical sensitivity, fibromyalgia and posttraumatic stress disorder. These stressors, acting primarily through the nitric oxide product, peroxynitrite, are thought to initiate a complex vicious cycle mechanism, known as the NO/ONOO- cycle that is responsible for chronic illness. The complexity of the NO/ONOO- cycle raises the question as to whether the mechanism that switches on this cycle is this complex cycle itself or whether a simpler mechanism is the primary switch. It is proposed here that the switch involves a combination of two variable switches, the increase of nitric oxide synthase (NOS) activity and the partial uncoupling of the NOS activity, with uncoupling caused by a tetrahydrobiopterin (BH4) deficiency. NOS uncoupling causes the NOS enzymes to produce superoxide, the other precursor of peroxynitrite, in place of nitric oxide. Thus partial uncoupling will cause NOS proteins to act like peroxynitrite synthases, leading, in turn to increased NF-kappaB activity. Peroxynitrite is known to oxidize BH4, and consequently partial uncoupling may initiate a vicious cycle, propagating the partial uncoupling over time. The combination of high NOS activity and BH4 depletion will lead to a potential vicious cycle that may be expected to switch on the larger NO/ONOO- cycle, thus producing the symptoms and signs of chronic illness. The role of peroxynitrite in the NO/ONOO- cycle also implies that such uncoupling is part of the chronic phase cycle mechanism such that agents that lower uncoupling will be useful in treatment.

Tetrahydrobiopterin attenuates microvascular reperfusion injury following murine pancreas transplantation

In this study we investigated the effect of tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide synthases, on ischemia-reperfusion injury (IRI) following murine pancreas transplantation. Pancreatic grafts were exposed to prolonged cold ischemia times (CIT) and different treatment regimens: normal saline (S), S + 16 h CIT, BH4 50 mg/kg + 16 h CIT. Nontransplanted animals served as controls. Graft microcirculation was analyzed by means of functional capillary density (FCD) and capillary diameters (CD) after 2 h reperfusion using intravital microscopy. Quantification of inflammatory responses (mononuclear infiltration) and endothelial disintegration (edema formation) was done by histology (hematoxylin and eosin), and peroxynitrite formation assessed by nitrotyrosine immunostaining. FCD was significantly reduced after prolonged CIT, paralleled by increased peroxynitrite formation as compared with controls (all p < 0.05). Microcirculatory changes correlated significantly with intragraft peroxynitrite generation (Spearman: r = -0.56; p < 0.01). Pancreatic grafts treated with BH4 displayed markedly higher FCD values (p < 0.01) and abrogated nitrotyrosine staining (p = 0.03). CD were not significantly different in any group. Histology showed increased inflammation, interstitial edema, hemorrhage, acinar vacuolization and focal areas of necrosis after 16 h CIT, which was diminished by BH4 administration (p < 0.01). BH4 treatment significantly reduces post-ischemic deterioration of microcirculation as well as histologic damage and might be a promising novel strategy in attenuating IRI following pancreas transplantation.

Impairment of dilator responses of cerebral arterioles during diabetes mellitus: role of inducible NO synthase

BACKGROUND AND PURPOSE

During diabetes, expression of inducible nitric oxide synthase (iNOS) plays an important role in the development of endothelial dysfunction in extracranial blood vessels. Progression of vascular dysfunction after the onset of diabetes differs among vascular beds. In this study, the effects of hyperglycemia/diabetes on vasomotor function were examined in cerebral arterioles at 2 different times in control and iNOS-deficient mice and compared with the effects on carotid arteries.

METHODS

Streptozotocin (150 mg/kg IP) was given to induce diabetes. The diameter of cerebral arterioles was measured through a cranial window in diabetic and nondiabetic mice in vivo. Vasomotor function of the carotid artery was examined in vitro.

RESULTS

In diabetic mice, responses of the cerebral arterioles to acetylcholine (1 mumol/L) were normal after 3 weeks of diabetes but were significantly impaired after 5 to 6 weeks of diabetes (4+/-1% [mean+/-SEM] increase in diameter) compared with control mice (14+/-1; P=0.0002). Responses to sodium nitroprusside were similar in diabetic and nondiabetic mice at both time points. In contrast, the vasomotor function of the carotid artery was not affected after 5 to 6 weeks of diabetes. In diabetic iNOS-deficient mice, cerebral arteriolar vasomotor function was not impaired, even after 4 months of diabetes.

CONCLUSIONS

During diabetes, endothelial dysfunction of cerebral arterioles requires expression of iNOS and develops earlier than in carotid arteries.

Tetrahydrobiopterin compounds prolong allograft survival independently of their effect on nitric oxide synthase activity

BACKGROUND

In previous work, the four-amino analogue of tetrahydrobiopterin, a novel, selective inhibitor of inducible nitric oxide synthase, has been shown to prolong survival of murine cardiac allografts.

METHODS

To further elucidate the underlying molecular immunosuppressive mechanism, we compared the effect of four-amino tetrahydrobiopterin with that of the unsubstituted parent compound tetrahydrobiopterin and of N-(iminoethyl)-L-lysine (L-NIL), a nonpterin inhibitor of inducible nitric oxide synthase using a murine cardiac transplant model. We analyzed allograft survival, intragraft gene expression in grafts by microarray and real-time polymerase chain reaction, graft nitrotyrosine staining by immunohistochemistry and plasma nitrite plus nitrate levels by high-performance liquid chromatography.

RESULTS

Allograft survival was significantly prolonged by tetrahydrobiopterin and cyclosporin A, but not by L-NIL although decreased plasma nitrite plus nitrate levels confirmed nitric oxide synthase inhibition in vivo. As compared to allogeneic untreated controls, intragraft peroxynitrite formation and hence nitrotyrosine staining was lowered in all groups except in cyclosporine A-treated animals. Gene expression profiles obtained by microarray analysis demonstrated that cyclosporine A was able to counteract the expression changes of more than half of the genes differently expressed in syngeneic grafts versus allografts, whereas tetrahydrobiopterin compounds and L-NIL showed only smaller influences on gene expression profiles.

CONCLUSIONS

These results demonstrate that the four-amino substitution, which is essential for inhibition of nitric oxide synthase, is not required for the immunosuppressive effect of tetrahydrobiopterin compounds. We describe a novel immunosuppressive role of pharmacologically applied tetrahydrobiopterin.

Endothelial Nitric Oxide Synthase

This article explores the physiology of superoxide generation by endothelial nitric oxide synthase (eNOS), the so-called "uncoupled" state of the enzyme. The fact that this alternative chemistry of the eNOS enzyme is evolutionary strongly conserved, suggests that it may play a physiological role. It is proposed that this uncoupled state may contribute to defense against infections. As the switch from NO production to reactive oxygen species by eNOS is also the final common pathway in atherogenesis, the uncoupling of eNOS further builds on the hypothesis that atherogenesis is driven by cellular mechanisms that originally serve host defense. The central role of uncoupled eNOS in redox signaling in the endothelium may open up new avenues for therapy to prevent atherosclerosis.

In Vivo Nitric Oxide Production and iNOS Expression in X-Ray Irradiated Mouse Skin

Inducible nitric oxide synthase (iNOS) and NO have been suggested to be involved in acute radiation response in tissues such as the liver, intestine, colon, and brain. However, direct measurement of NO and iNOS in ionizing radiation-induced skin inflammatory reactions is not reported yet. We show here for the first time, by in vivo experiments, that X-ray irradiated mouse skin generates NO with concomitant expression of iNOS at both the mRNA and protein levels. When irradiated at 50 Gy, iNOS mRNA appeared at day 8 post-irradiation, whereas iNOS protein could be detected only at day 14. No iNOS protein was detectable however for the mice receiving 5 or 15 Gy irradiation, even at day 14. Skin inflammatory reactions were observed at day 8 post-irradiation as an increase in skin thickness, which increased further by day 14. Histological observations showed acute inflammatory responses. The parallel relationship between iNOS induction and the onset of skin inflammatory reactions suggests the involvement of iNOS and NO in the skin damage. Immunohistochemical staining showed the localization of iNOS at skin erosion areas, exudate and infiltrating cells. Taken together, these findings suggest that iNOS induction and NO production in X-irradiated skin are relatively early events in skin inflammatory reactions and are probably secondary rather than primary reactions of irradiation.

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.

Cyclosporine-induced coronary endothelial dysfunction: is tetrahydrobiopterin the solution?

BACKGROUND

Coronary endothelial dysfunction after heart transplantation is predictive of cardiac allograft vasculopathy. Immunosuppressive drugs, particularly cyclosporine may contribute to this dysfunction by a direct effect. Tetrahydrobiopterin (BH(4)) is a potent antioxidant and an essential cofactor of nitric oxide biosynthesis. The purpose of this study was to investigate whether BH(4) could reverse the endothelial dysfunction induced by cyclosporine.

METHODS

A previously described in vitro model of drug incubation in Krebs-bicarbonate solution (4 degrees C, 48 hours) of porcine epicardial coronary arteries was used. Coronary endothelial function studies were performed in organ chamber experiments after incubation with cyclosporine (10(-4) mol/L) in the presence or absence of 6-methyltetrahydropterin (MH(4) [0.1 mol/L], a BH(4) analog) to assess its effect on the cyclosporine-induced endothelial dysfunction.

RESULTS

The average doses of PGF2(alpha) required to attain 50% of the maximal contraction to KCl was significantly lower (P < .001) in the cyclosporine group (8.6 +/- 1.94 x 10(-6) mol/L) compared to the control group (24.8 +/- 5.2 x 10(-6) mol/L). Exposure to cyclosporine induced a significant decrease in endothelium-dependent relaxations to serotonin (5HT) (% E(max) [5HT]: 77% +/- 4%; P < .05). Addition of MH(4) significantly reversed this impaired response (% E(max) [5HT]: 62% +/- 4%; P < .05). No alterations of relaxation were observed with bradykinin in both groups. Endothelium-independent relaxations to sodium nitroprussiate were fully preserved.

CONCLUSIONS

These results suggest a significant protective role of BH(4) on coronary endothelial function following exposure to cyclosporine, which could reduce the incidence of endothelial dysfunction and cardiac allograft vasculopathy following cardiac transplantation.

The role of nitric oxide in renal transplantation

This review discusses the concept that nitric oxide synthase (NOS) may orchestrate both the inflammatory response to the renal allograft and anti-inflammatory defense in the graft itself. NO is produced by endothelial, epithelial, as well as inflammatory cells. In the setting of transplantation, the endothelium is the first lining to be subjected to the early response to injury. In turn, activated endothelial cells facilitate leukocyte recruitment, immune-mediated injury, and angiogenesis. On activation by inflammatory stimuli, endothelial cells up-regulate multiple vasoactive substances, oxygen radicals, cytokines, chemokines, and growth factors. Therefore, endothelial integrity, especially the expression of protecting vasoactive agents, such as NO, may be a key factor in resistance or sensitivity to transplantation-mediated injury. Thus, evaluating the mechanisms by which NO is involved in either protecting or injuring the transplanted allogeneic kidney is important for our understanding of renal allograft rejection. This review focuses on the role of NO in the inflammatory endothelial-leukocyte interactions, which are implicated in acute and chronic rejection of the transplanted kidney.

Wine polyphenols and ethanol do not significantly scavenge superoxide nor affect endothelial nitric oxide production

Epidemiological studies have shown that moderate intake of red wine reduces the risk of coronary heart disease. It has been proposed that the antiatherogenic effect be due to the scavenging of reactive oxygen species by polyphenols and ethanol or an effect on endothelial nitric oxide (NO) production. We have determined the reaction rates of superoxide with four different polyphenols and ethanol. The superoxide reaction rates were determined at 37 degrees C and pH 7.4 using competitive spin trapping and electron paramagnetic resonance (EPR) spectroscopy. Ethanol did not scavenge superoxide. For the polyphenols catechin, epicatechin, gallic acid, and quercetin, we find rate constants of respectively 2.3*10(4), 2.2*10(4), 2.3*10(3) and 1.9*10(4)(mole per second)(-1). Polyphenols can only exert a significant scavenging effect, if the plasma concentration reach sufficiently high levels. At concentrations found in vivo (low nanomolar range), the scavenging of superoxide by polyphenols and ethanol is negligible in comparison with endogenous protection against superoxide. Incubation of cultured endothelial cells with 5 micromol/L of catechin, epicatechin, gallic acid, quercetin, or ethanol 0.05% (v/v) did not influence the maximal production of NO by these cells as measured by fluorescent nitric oxide cheletropic traps (FNOCT). The observed antiatherogenic effects must be caused by a mechanism other than direct scavenging of superoxide or influence on maximal endothelial NO production.

Xanthine oxidase converts nitric oxide to nitroxyl that inactivates the enzyme

Xanthine oxidase (XO) was found to convert nitric oxide (NO* ) released from spermine-NONOate to nitroxyl (HNO), the one-electron reduction product of NO*, in the presence of its substrate hypoxanthine under anaerobic conditions. Under these conditions, XO lost its activity. Upon aerobic incubation of XO with its substrate, neither conversion of NO* to HNO nor inactivation of the enzyme was observed. Angeli's salt (an HNO generator) or synthetic peroxynitrite inactivated XO at low concentrations, whereas high concentrations of diethylamine-NONOate (an NO* donor) and SIN-1 (which generates peroxynitrite by releasing both NO* and superoxide) were required to inactivate XO. These results suggest that HNO generated by XO under anaerobic conditions inactivates XO. As both XO and NO* synthase are activated and/or induced in ischemia-reperfusion injury, HNO formed by XO may contribute to pathogenesis by exerting its potent oxidation activity against a variety of biological compounds.

Tetrahydrobiopterin and nitric oxide: mechanistic and pharmacological aspects

In previous minireviews in this journal, we discussed work on induction of tetrahydrobiopterin biosynthesis by cytokines and its significance for nitric oxide (NO) production of intact cells as well as functions of H4-biopterin identified at this time for NO synthases (Proc Soc Exp Biol Med 203: 1-12, 1993; Proc Soc Exp Biol Med 219: 171-182, 1998). Meanwhile, the recognition of the importance of tetrahydrobiopterin for NO formation has led to new insights into complex biological processes and revealed possible novel pharmacological strategies to intervene in certain pathological conditions. Recent work could also establish that tetrahydrobiopterin, in addition to its allosteric effects, is redox-active in the NO synthase reaction. In this review, we summarize the current view of how tetrahydrobiopterin functions in the generation of NO and focus on pharmacological aspects of tetrahydrobiopterin availability with emphasis on endothelial function.

Gene-targeted mice reveal a critical role for inducible nitric oxide synthase in vascular dysfunction during diabetes

BACKGROUND AND PURPOSE

Inducible nitric oxide synthase (iNOS) is a mediator of vascular dysfunction during inflammation. The purpose of this study was to test the hypothesis that vascular dysfunction during diabetes is dependent on expression of iNOS.

METHODS

Diabetes was produced in mice with streptozotocin (150 mg/kg IP). After 4 to 6 months of diabetes, vasomotor function was examined in vitro in carotid arteries from mice with targeted disruption of the gene for iNOS (iNOS-deficient mice) and from normal, wild-type (WT) mice.

RESULTS

Contractile responses of carotid arteries to U46619, a thromboxane A2 analogue, were not altered by diabetes in WT mice. Responses to U46619 were increased in arteries from diabetic iNOS-deficient mice compared with diabetic WT and nondiabetic mice (iNOS-deficient and WT mice). These results indicate that expression of iNOS inhibits an increased vasoconstrictor response during diabetes. Arteries from nondiabetic WT mice relaxed 83+/-2% (mean+/-SE) in response to acetylcholine (1 micromol/L) compared with 58+/-6% in arteries from diabetic WT mice (P<0.05 versus nondiabetic mice). In contrast, relaxation of carotid arteries to acetylcholine was similar (81+/-4% versus 76+/-6%; P>0.05) in iNOS-deficient mice under nondiabetic and diabetic conditions, respectively. Thus, diabetes produced impairment of endothelium-dependent relaxation in arteries from WT but not iNOS-deficient mice. Endothelium-independent relaxation in response to nitroprusside was similar in arteries from all mice.

CONCLUSIONS

These results provide the first direct evidence that impairment of endothelium-dependent relaxation during diabetes is dependent on expression of iNOS.

Ascorbate enhances iNOS activity by increasing tetrahydrobiopterin in RAW 264.7 cells

Studies on the effect of ascorbic acid on inducible nitric oxide synthase (iNOS) activity are few and diverse, likely to be dependent on the species of cells. We investigated a role of ascorbic acid in iNOS induction and nitric oxide (NO) generation in mouse macrophage cell line RAW 264.7. Although interferon- (IFN-) gamma alone produced NO end products, ascorbic acid enhanced NO production only when cells were synergistically stimulated with IFN-gamma plus Escherichia coli lipopolysaccharide (LPS). Ascorbate neither enhanced nor decreased the expression of iNOS protein in RAW 264.7 cells, in contrast to the reports that ascorbic acid augments iNOS induction in a mouse macrophage-like cell line J774.1 and that ascorbate suppresses iNOS induction in rat skeletal muscle endothelial cells. Intracellular levels of tetrahydrobiopterin (BH4), a cofactor for iNOS, were increased by ascorbate in RAW 264.7 cells. However, ascorbate did not increase GTP cyclohydrolase I mRNA, the main enzyme at the critical steps in the BH4 synthetic pathway, expression levels and activity. Sepiapterin, which supplies BH4 via salvage pathway, more efficiently enhanced NO production if ascorbate was added. These data suggest that enhanced activation of iNOS by ascorbic acid is mediated by increasing the stability of BH4 in RAW 264.7 cells.

Chemical basis of inflammation-induced carcinogenesis

Chronic inflammation induced by biological, chemical, and physical factors has been associated with increased risk of human cancer at various sites. Inflammation activates a variety of inflammatory cells, which induce and activate several oxidant-generating enzymes such as NADPH oxidase, inducible nitric oxide synthase, myeloperoxidase, and eosinophil peroxidase. These enzymes produce high concentrations of diverse free radicals and oxidants including superoxide anion, nitric oxide, nitroxyl, nitrogen dioxide, hydrogen peroxide, hypochlorous acid, and hypobromous acid, which react with each other to generate other more potent reactive oxygen and nitrogen species such as peroxynitrite. These species can damage DNA, RNA, lipids, and proteins by nitration, oxidation, chlorination, and bromination reactions, leading to increased mutations and altered functions of enzymes and proteins (e.g., activation of oncogene products and/or inhibition of tumor-suppressor proteins) and thus contributing to the multistage carcinogenesis process. Appropriate treatment of inflammation should be explored further for chemoprevention of human cancers.

Genetic and epigenetic damage induced by reactive nitrogen species: implications in carcinogenesis

Chronic infection and inflammation are recognized risk factors for human cancer at various sites. Infection and inflammation can activate and induce a variety of oxidant-generating enzymes, including NADPH oxidase and inducible nitric oxide synthase. Reactive oxygen and nitrogen species produced by such enzymes react with each other to generate new and more potent reactive species. These oxidants not only can damage DNA and induce mutations, but also can activate oncogene products and/or inactivate tumor-suppressor proteins, thus contributing to most processes of carcinogenesis. Appropriate treatment of inflammation should be further explored for chemoprevention of human cancers, especially those associated with chronic inflammation.

Inhibitory effect of recombinant iNOS gene expression on vasomotor function of canine basilar artery

The present study was designed to determine the effect of recombinant inducible nitric oxide (NO) synthase (iNOS) gene expression on vasomotor function in cerebral arteries. Isolated canine basilar arteries were exposed ex vivo (30 min at 37 degrees C) to an adenoviral vector [10(7), 10(8), or 10(9) plaque-forming units (pfu)/ml] encoding either the iNOS gene or the beta-galactosidase reporter gene. Twenty-four hours after transduction, Western blot analysis demonstrated expression of iNOS protein only in iNOS (10(9) pfu/ml)-transduced arteries. Immunohistochemical analysis localized iNOS expression predominantly in adventitia. Vascular reactivity of isolated basilar arteries was studied by isometric force recording. Concentration-response curves to UTP (10(-9)-10(-3) M) and diethylaminodiazen-1-ium-1,2-dioate (10(-10)-10(-5) M) were significantly shifted to the right in iNOS gene (10(9) pfu/ml)-transduced rings compared with control and beta-galactosidase-transduced rings (P < 0.05, n = 5-6). Endothelium-dependent relaxation to bradykinin was significantly attenuated in iNOS-transduced rings (P < 0.001, n = 8). The basal level of cGMP and superoxide anion (O(2)(-).) production were elevated in iNOS-transduced rings (P < 0.05, n = 7 for cGMP; P < 0.01, n = 6-9 for O(2)(-). production). Our results suggest that expression of recombinant iNOS in cerebral arteries reduces vasomotor reactivity to both vasoconstrictor and vasodilator agonists. Attenuation of contractions is most likely due to functional antagonism between UTP and cGMP. Reduction of endothelium-dependent relaxation to bradykinin appears to be mediated in part by reduced reactivity of smooth muscle cells to NO.

Endothelial injury: cause and effect of alloimmune inflammation

This review discusses the concept that endothelial cells may facilitate inflammation, but are also targets of the inflammatory response. Endothelial cells express several molecules that promote leukocyte recruitment, and other molecules, such as MHC class I that enable endothelial injury. Circulating alloantibodies produced following transplantation may also target the endothelium for injury. It has been shown that the expression of select protective genes within endothelial cells, including anti-apoptotic genes, may provide resistance to immune-mediated injury. Thus, an understanding of the mechanisms by which endothelial cells are injured and by which endothelial cells are protected is important for our understanding of allograft rejection.