Cardiomyocyte tetrahydrobiopterin synthesis regulates fatty acid metabolism and susceptibility to ischaemia-reperfusion injury.

NEW FINDINGS: What is the central question of this study? What are the physiological roles of cardiomyocyte-derived tetrahydrobiopterin (BH4) in cardiac metabolism and stress response? What is the main finding and its importance? Cardiomyocyte BH4 has a physiological role in cardiac metabolism. There was a shift of substrate preference from fatty acid to glucose in hearts with targeted deletion of BH4 synthesis. The changes in fatty-acid metabolic profile were associated with a protective effect in response to ischaemia-reperfusion (IR) injury, and reduced infarct size. Manipulating fatty acid metabolism via BH4 availability could play a therapeutic role in limiting IR injury. ABSTRACT: Tetrahydrobiopterin (BH4) is an essential cofactor for nitric oxide (NO) synthases in which its production of NO is crucial for cardiac function. However, non-canonical roles of BH4 have been discovered recently and the cell-specific role of cardiomyocyte BH4 in cardiac function and metabolism remains to be elucidated. Therefore, we developed a novel mouse model of cardiomyocyte BH4 deficiency, by cardiomyocyte-specific deletion of Gch1, which encodes guanosine triphosphate cyclohydrolase I, a required enzyme for de novo BH4 synthesis. Cardiomyocyte (cm)Gch1 mRNA expression and BH4 levels from cmGch1 KO mice were significantly reduced compared to Gch1flox/flox (WT) littermates. Transcriptomic analyses and protein assays revealed downregulation of genes involved in fatty acid oxidation in cmGch1 KO hearts compared with WT, accompanied by increased triacylglycerol concentration within the myocardium. Deletion of cardiomyocyte BH4 did not alter basal cardiac function. However, the recovery of left ventricle function was improved in cmGch1 KO hearts when subjected to ex vivo ischaemia-reperfusion (IR) injury, with reduced infarct size compared to WT hearts. Metabolomic analyses of cardiac tissue after IR revealed that long-chain fatty acids were increased in cmGch1 KO hearts compared to WT, whereas at 5 min reperfusion (post-35 min ischaemia) fatty acid metabolite levels were higher in WT compared to cmGch1 KO hearts. These results indicate a new role for BH4 in cardiomyocyte fatty acid metabolism, such that reduction of cardiomyocyte BH4 confers a protective effect in response to cardiac IR injury. Manipulating cardiac metabolism via BH4 could play a therapeutic role in limiting IR injury.

Endothelial cell-specific reactive oxygen species production increases susceptibility to aortic dissection.

BACKGROUND: Increased production of reactive oxygen species (ROS) throughout the vascular wall is a feature of cardiovascular disease states, but therapeutic strategies remain limited by our incomplete understanding of the role and contribution of specific vascular cell ROS to disease pathogenesis. To investigate the specific role of endothelial cell (EC) ROS in the development of structural vascular disease, we generated a mouse model of endothelium-specific Nox2 overexpression and tested the susceptibility to aortic dissection after angiotensin II (Ang II) infusion. METHODS AND RESULTS: A specific increase in endothelial ROS production in Nox2 transgenic mice was sufficient to cause Ang II-mediated aortic dissection, which was never observed in wild-type mice. Nox2 transgenic aortas had increased endothelial ROS production, endothelial vascular cell adhesion molecule-1 expression, matrix metalloproteinase activity, and CD45(+) inflammatory cell infiltration. Conditioned media from Nox2 transgenic ECs induced greater Erk1/2 phosphorylation in vascular smooth muscle cells compared with wild-type controls through secreted cyclophilin A (CypA). Nox2 transgenic ECs (but not vascular smooth muscle cells) and aortas had greater secretion of CypA both at baseline and in response to Ang II stimulation. Knockdown of CypA in ECs abolished the increase in vascular smooth muscle cell Erk1/2 phosphorylation conferred by EC conditioned media, and preincubation with CypA augmented Ang II-induced vascular smooth muscle cell ROS production. CONCLUSIONS: These findings demonstrate a pivotal role for EC-derived ROS in the determination of the susceptibility of the aortic wall to Ang II-mediated aortic dissection. ROS-dependent CypA secretion by ECs is an important signaling mechanism through which EC ROS regulate susceptibility of structural components of the aortic wall to aortic dissection.

Cardiomyocyte GTP cyclohydrolase 1 and tetrahydrobiopterin increase NOS1 activity and accelerate myocardial relaxation.

RATIONALE: Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthases (NOS). Oral BH4 supplementation preserves cardiac function in animal models of cardiac disease; however, the mechanisms underlying these findings are not completely understood. OBJECTIVE: To study the effect of myocardial transgenic overexpression of the rate-limiting enzyme in BH4 biosynthesis, GTP cyclohydrolase 1 (GCH1), on NOS activity, myocardial function, and Ca2+ handling. METHODS AND RESULTS: GCH1overexpression significantly increased the biopterins level in left ventricular (LV) myocytes but not in the nonmyocyte component of the LV myocardium or in plasma. The ratio between BH4 and its oxidized products was lower in mGCH1-Tg, indicating that a large proportion of the myocardial biopterin pool was oxidized; nevertheless, myocardial NOS1 activity was increased in mGCH1-Tg, and superoxide release was significantly reduced. Isolated hearts and field-stimulated LV myocytes (3 Hz, 35°C) overexpressing GCH1 showed a faster relaxation and a PKA-mediated increase in the PLB Ser16 phosphorylated fraction and in the rate of decay of the [Ca2+]i transient. RyR2 S-nitrosylation and diastolic Ca2+ leak were larger in mGCH1-Tg and ICa density was lower; nevertheless the amplitude of the [Ca2+]i transient and contraction did not differ between genotypes, because of an increase in the SR fractional release of Ca2+ in mGCH1-Tg myocytes. Xanthine oxidoreductase inhibition abolished the difference in superoxide production but did not affect myocardial function in either group. By contrast, NOS1 inhibition abolished the differences in ICa density, Ser16 PLB phosphorylation, [Ca2+]i decay, and myocardial relaxation between genotypes. CONCLUSIONS: Myocardial GCH1 activity and intracellular BH4 are a limiting factor for constitutive NOS1 and SERCA2A activity in the healthy myocardium. Our findings suggest that GCH1 may be a valuable target for the treatment of LV diastolic dysfunction.

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.

Endothelial Nox2 overexpression potentiates vascular oxidative stress and hemodynamic response to angiotensin II: studies in endothelial-targeted Nox2 transgenic mice.

Vascular disease states are associated with endothelial dysfunction and increased production of reactive oxygen species (ROS) derived from vascular NADPH oxidases in both vascular smooth muscle cells (VSMCs) and endothelial cells. Recent evidence suggests an important role for VSMC NADPH oxidases in vascular ROS production. However, it is unclear whether increased NADPH oxidase activity in endothelial cells alone is sufficient to alter overall vascular ROS production and hemodynamics. We sought to address these questions using transgenic mice with endothelial-targeted overexpression of the catalytic subunit of NADPH oxidase, Nox2. Aortas of Nox2 transgenic (Nox2-Tg) mice had increased total Nox2 mRNA and protein levels compared with wild-type littermates. Both p22phox mRNA and protein levels were also significantly elevated in Nox2-Tg aortas. Aortic superoxide production was significantly increased in Nox2-Tg mice compared with wild-type, but this difference was abolished by endothelial removal. Superoxide dismutase inhibition increased superoxide release and levels of Mn superoxide dismutase protein were significantly elevated in aortas from Nox2-Tg mice compared with wild type. Increased ROS production from endothelial Nox2 overexpression led to increased endothelial nitric oxide synthase protein and extracellular signal-regulated kinase 1/2 phosphorylation in transgenic aortas. Basal blood pressure was similar, however the pressor responses to both acute and chronic angiotensin II administration were significantly increased in Nox2-Tg mice compared with wild type. These results demonstrate that endothelial-targeted Nox2 overexpression is sufficient to increase vascular NADPH oxidase activity, activate downstream signaling pathways, and potentiate the hemodynamic response to angiotensin II, despite compensatory increases in vascular antioxidant enzymes. Endothelial cell Nox2-containing NADPH oxidase plays an important functional role in vascular redox signaling.

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.

Pivotal role for endothelial tetrahydrobiopterin in pulmonary hypertension.

BACKGROUND: Pulmonary hypertension is a fatal disease characterized by vasoconstriction and vascular remodeling. Loss of endothelial nitric oxide bioavailability is implicated in pulmonary hypertension pathogenesis. Recent evidence suggests that the cofactor tetrahydrobiopterin (BH4) is an important regulator of nitric oxide synthase enzymatic function. METHODS AND RESULTS: In the hph-1 mouse with deficient BH4 biosynthesis, BH4 deficiency caused pulmonary hypertension, even in normoxic conditions, and greatly increased susceptibility to hypoxia-induced pulmonary hypertension. In contrast, augmented BH4 synthesis in the endothelium, by targeted transgenic overexpression of GTP-cyclohydrolase I (GCH), prevented hypoxia-induced pulmonary hypertension. Furthermore, specific augmentation of endothelial BH4 in hph-1 mice by crossing with GCH transgenic mice rescued pulmonary hypertension induced by systemic BH4 deficiency. Lung BH4 availability controlled pulmonary vascular tone, right ventricular hypertrophy, and vascular structural remodeling in a dose-dependent manner in both normoxia and hypoxia. Furthermore, BH4 availability had striking effects on the immediate vasoconstriction response to acute hypoxia. These effects of BH4 were mediated through the regulation of nitric oxide compared with superoxide synthesis by endothelial nitric oxide synthase. CONCLUSIONS: Endothelial BH4 availability is essential for maintaining pulmonary vascular homeostasis, is a critical mediator in the pathogenesis of pulmonary hypertension, and is a novel therapeutic target.

Contrasting roles of NADPH oxidase isoforms in pressure-overload versus angiotensin II-induced cardiac hypertrophy.

Increased production of reactive oxygen species (ROS) is implicated in the development of left ventricular hypertrophy (LVH). Phagocyte-type NADPH oxidases are major cardiovascular sources of ROS, and recent data indicate a pivotal role of a gp91phox-containing NADPH oxidase in angiotensin II (Ang II)-induced LVH. We investigated the role of this oxidase in pressure-overload LVH. gp91phox-/- mice and matched controls underwent chronic Ang II infusion or aortic constriction. Ang II-induced increases in NADPH oxidase activity, atrial natriuretic factor (ANF) expression, and cardiac mass were inhibited in gp91phox-/- mice, whereas aortic constriction-induced increases in cardiac mass and ANF expression were not inhibited. However, aortic constriction increased cardiac NADPH oxidase activity in both gp91phox-/- and wild-type mice. Myocardial expression of an alternative gp91phox isoform, Nox4, was upregulated after aortic constriction in gp91phox-/- mice. The antioxidant, N-acetyl-cysteine, inhibited pressure-overload-induced LVH in both gp91phox-/- and wild-type mice. These data suggest a differential response of the cardiac Nox isoforms, gp91phox and Nox4, to Ang II versus pressure overload.

Pivotal role of a gp91(phox)-containing NADPH oxidase in angiotensin II-induced cardiac hypertrophy in mice.

BACKGROUND: Angiotensin II induces both cardiac and vascular smooth muscle (VSM) hypertrophy. Recent studies suggest a central role for a phagocyte-type NADPH oxidase in angiotensin II-induced VSM hypertrophy. The possible involvement of an NADPH oxidase in the development of cardiac hypertrophy has not been studied. Methods and Results- Mice with targeted disruption of the NADPH oxidase subunit gp91(phox) (gp91(phox-/-)) and matched wild-type mice were subjected to subcutaneous angiotensin II infusion at a subpressor dose (0.3 mg/kg/day) for 2 weeks. Systolic blood pressure was unaltered by angiotensin II in either group. Angiotensin II significantly increased heart/body weight ratio, atrial natriuretic factor and beta-myosin heavy chain mRNA expression, myocyte area, and cardiac collagen content in wild-type but not gp91(phox-/-) mice. Angiotensin II treatment increased myocardial NADPH oxidase activity in wild-type but not gp91(phox-/-) mice. CONCLUSIONS: A gp91(phox)-containing NADPH oxidase plays an important role in the development of angiotensin II-induced cardiac hypertrophy, independent of changes in blood pressure.

Role of myocardial neuronal nitric oxide synthase-derived nitric oxide in beta-adrenergic hyporesponsiveness after myocardial infarction-induced heart failure in rat.

BACKGROUND: An emerging concept is that a neuronal isoform of nitric oxide synthase (NOS1) may regulate myocardial contractility. However, a role for NOS1-derived nitric oxide (NO) in heart failure (HF) has not been defined. METHODS AND RESULTS: Using a model of myocardial infarction-induced HF, we demonstrated that cardiac NOS1 expression and activity increased in HF rats (P<0.05 and P<0.001 versus shams, respectively). This was associated with translocation of NOS1 from the ryanodine receptor to the sarcolemma through interactions with caveolin-3 in HF hearts. With ex vivo and in vivo pressure-volume analysis, cardiac NOS1-derived NO was found to be negatively inotropic in shams but not HF hearts. Ventricular elastance (E(es)) was significantly reduced in HF rats (P<0.05), and tau, the time constant of left ventricular relaxation, was prolonged (both P<0.05). Acute NOS1 inhibition significantly increased E(es) by 33+/-3% and tau by 17+/-2% (P<0.05) in shams, although these effects were significantly attenuated in HF hearts. beta-Adrenergic stimulation induced a marked increase in systolic performance in sham hearts, with the responses being significantly blunted in HF hearts. E(es) increased by 163+/-42% (P<0.01) in sham hearts and 56+/-9% in HF hearts, and LV +dP/dt increased by 97+/-9% (P<0.01) in shams and 37+/-7% (P<0.05) in the HF group. Interestingly, preferential NOS1 inhibition enhanced the blunted responses of LV +dP/dt and E(es) to beta-adrenergic stimulation in HF rats but had no effect in shams. CONCLUSIONS: These results provide the first evidence that increased NOS1-derived NO production may play a role in the autocrine regulation of myocardial contractility in HF.

Cardiac specific increase in aldosterone production induces coronary dysfunction in aldosterone synthase-transgenic mice.

BACKGROUND: Elevated circulating aldosterone level is associated with impaired cardiovascular function. Although the mechanisms are not fully understood, aldosterone antagonists decrease total and cardiovascular mortality in heart failure and myocardial infarction. Aldosterone induces cardiac fibrosis in experimental models, and it is synthesized locally in rat heart. These observations suggest pathological effects of aldosterone in heart that remain unclear. METHODS AND RESULTS: Transgenic mice (TG) that overexpress the terminal enzyme of aldosterone biosynthesis, aldosterone synthase (AS), in heart have been raised by gene targeting with the alpha-myosin heavy chain promoter. AS mRNA increased 100-fold and aldosterone concentration 1.7-fold in hearts of male TG mice relative to wild-type. No structural or myocardial alterations were evidenced, because ventricle/body weight, AT1 and AT2 receptor binding, and collagen content were unchanged in TG. No alteration in cardiac function was evidenced by echocardiography, isolated perfused heart, or whole-cell patch clamp experiments. In contrast, coronary function was impaired, because basal coronary flow was decreased in isolated perfused heart (-55% of baseline values), and vasodilatation to acetylcholine, bradykinin, and sodium nitroprusside was decreased by 75%, 60%, and 75%, respectively, in TG mice compared with wild-type, showing that the defect was not related to NO production. CONCLUSIONS: Increased cardiac aldosterone production in male mice induces a major coronary endothelium-independent dysfunction with no detectable alterations in cardiac structure and function. However, coronary dysfunction may be harmful for coronary adaptation to increased flow demand.

Increased myocardial NADPH oxidase activity in human heart failure.

OBJECTIVES: This study was designed to investigate whether nicotinamide adenine dinucleotide 3-phosphate (reduced form) (NADPH) oxidase is expressed in the human heart and whether it contributes to reactive oxygen species (ROS) production in heart failure. BACKGROUND: A phagocyte-type NADPH oxidase complex is a major source of ROS in the vasculature and is implicated in the pathophysiology of hypertension and atherosclerosis. An increase in myocardial oxidative stress due to excessive production of ROS may be involved in the pathophysiology of congestive heart failure. Recent studies have suggested an important role for myocardial NADPH oxidase in experimental models of cardiac disease. However, it is unknown whether NADPH oxidase is expressed in the human myocardium or if it has any role in human heart failure. METHODS: Myocardium of explanted nonfailing (n = 9) and end-stage failing (n = 13) hearts was studied for the expression of NADPH oxidase subunits and oxidase activity. RESULTS: The NADPH oxidase subunits p22(phox), gp91(phox), p67(phox), and p47(phox) were all expressed at messenger ribonucleic acid and protein level in cardiomyocytes of both nonfailing and failing hearts. NADPH oxidase activity was significantly increased in end-stage failing versus nonfailing myocardium (5.86 +/- 0.41 vs. 3.72 +/- 0.39 arbitrary units; p < 0.01). The overall level of oxidase subunit expression was unaltered in failing compared with nonfailing hearts. However, there was increased translocation of the regulatory subunit, p47(phox), to myocyte membranes in failing myocardium. CONCLUSIONS: This is the first report of the presence of NADPH oxidase in human myocardium. The increase in NADPH oxidase activity in the failing heart may be important in the pathophysiology of cardiac dysfunction by contributing to increased oxidative stress.

Up-regulation of cardiac nitric oxide synthase 1-derived nitric oxide after myocardial infarction in senescent rats.

Nitric oxide (NO) has been implicated in the development of heart failure, although the source, significance, and functional role of the different NO synthase (NOS) isoforms in this pathology are controversial. The presence of a neuronal-type NOS isoform (NOS1) in the cardiac sarcoplasmic reticulum has been recently discovered, leading to the hypothesis that NOS1-derived NO may notably alter myocardial inotropy. However, the regulation and role(s) of NOS1 in cardiac diseases remain to be determined. Using an experimental model of myocardial infarction (MI) in senescent rats, we demonstrated a significant increase in cardiac NOS1 expression and activity in MI, coupled with the translocation of this enzyme to the sarcolemma through interactions with caveolin-3. The enhanced NOS1 activity counteracts the decrease in cardiac NOS3 expression and activity observed in heart failure. We demonstrated an increased interaction between NOS1 and its regulatory protein HSP90 in post-MI hearts, a potential mechanism for the higher NOS1 activity in this setting. Finally, preferential in vivo inhibition of NOS1 activity enhanced basal post-MI left ventricular dysfunction in senescent rats. These results provide the first evidence that increased NOS1-derived NO production may play a significant role in the autocrine regulation of myocardial contractility after MI in aging rats.

Tetrahydrobiopterin in cardiovascular health and disease.

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Cell-autonomous role of endothelial GTP cyclohydrolase 1 and tetrahydrobiopterin in blood pressure regulation.

Tetrahydrobiopterin (BH4) is an essential cofactor for endothelial nitric oxide synthase (eNOS) function and NO generation. Augmentation of BH4 levels can prevent eNOS uncoupling and can improve endothelial dysfunction in vascular disease states. However, the physiological requirement for de novo endothelial cell BH4 biosynthesis in eNOS function remains unclear. We generated a novel mouse model with endothelial cell-specific deletion of GCH1, encoding GTP cyclohydrolase 1, an essential enzyme for BH4 biosynthesis, to test the cell-autonomous requirement for endothelial BH4 biosynthesis in vivo. Mice with a floxed GCH1 allele (GCH1(fl/fl)) were crossed with Tie2cre mice to delete GCH1 in endothelial cells. GCH1(fl/fl)Tie2cre mice demonstrated virtually absent endothelial NO bioactivity and significantly greater O2 (•-) production. GCH1(fl/fl)Tie2cre aortas and mesenteric arteries had enhanced vasoconstriction to phenylephrine and impaired endothelium-dependent vasodilatations to acetylcholine and SLIGRL. Endothelium-dependent vasodilatations in GCH1(fl/fl)Tie2cre aortas were, in part, mediated by eNOS-derived hydrogen peroxide (H2O2), which mediated vasodilatation through soluble guanylate cyclase. Ex vivo supplementation of aortic rings with the BH4 analogue sepiapterin restored normal endothelial function and abolished eNOS-derived H2O2 production in GCH1(fl/fl)Tie2cre aortas. GCH1(fl/fl)Tie2cre mice had higher systemic blood pressure than wild-type littermates, which was normalized by NOS inhibitor, NG-nitro-L-arginine methyl ester. Taken together, these studies reveal an endothelial cell-autonomous requirement for GCH1 and BH4 in regulation of vascular tone and blood pressure and identify endothelial cell BH4 as a pivotal regulator of NO versus H2O2 as alternative eNOS-derived endothelial-derived relaxing factors.

Endothelial-specific Nox2 overexpression increases vascular superoxide and macrophage recruitment in ApoE⁻/⁻ mice.

AIMS: Vascular disease states are associated with endothelial dysfunction and increased production of reactive oxygen species derived from NADPH oxidases. However, it remains unclear whether a primary increase in superoxide production specifically in the endothelium alters the initiation or progression of atherosclerosis. METHODS AND RESULTS: Mice overexpressing Nox2 specifically in the endothelium (Nox2-Tg) were crossed with ApoE(-/-) mice to produce Nox2-Tg ApoE(-/-) mice and ApoE(-/-) littermates. Endothelial overexpression of Nox2 in ApoE(-/-) mice did not alter blood pressure, but significantly increased vascular superoxide production compared with ApoE(-/-) littermates, measured using both lucigenin chemiluminescence and 2-hydroxyethidium production (ApoE(-/-), 19.9 ± 6.3 vs. Nox2-Tg ApoE(-/-), 47.0 ± 7.0 nmol 2-hydroxyethidium/aorta, P< 0.05). Increased endothelial superoxide production increased endothelial levels of vascular cell adhesion protein 1 and enhanced macrophage recruitment in early lesions in the aortic roots of 9-week-old mice, indicating increased atherosclerotic plaque initiation. However, endothelial-specific Nox2 overexpression did not alter native or angiotensin II-driven atherosclerosis in either the aortic root or the descending aorta. CONCLUSION: Endothelial-targeted Nox2 overexpression in ApoE(-/-) mice is sufficient to increase vascular superoxide production and increase macrophage recruitment possible via activation of endothelial cells. However, this initial increase in macrophage recruitment did not alter the progression of atherosclerosis. These results indicate that Nox-mediated reactive oxygen species signalling has important cell-specific and distinct temporal roles in the initiation and progression of atherosclerosis.

Targeting redox signaling in the vascular wall: from basic science to clinical practice.

Oxidative stress is a key feature in vascular homeostasis. Reactive oxygen species (ROS) are produced by multiple enzymatic sources located in various anatomical structures of the vascular wall, such as the vascular endothelium, the smooth muscle cells and inflammatory cells infiltrating sub-endothelial space and the rest of the vascular wall. Although ROS behave as signaling molecules regulating important aspects of vascular physiology, their excess generation is harmful. Further to the cytotoxic effect of ROS in the vascular wall, they also activate various redox sensitive transcription pathways, regulating the expression of proinflammatory molecules with strong pro-atherogenic effects. The activation of redox-sensitive enzymatic systems in the vascular wall such as matrix metalloproteinases as well as the impairment of endothelial function have a significant impact on vascular elasticity and vascular mechanics in general. The impairment of vascular mechanics has a significant impact on vascular homeostasis, promoting atherogenesis. It is therefore crucial to regulate vascular redox signaling, by developing therapeutic strategies able to target the effectively intracellular ROS bioavailability. Statins, angiotensin converting enzyme inhibitors, thiazolidinediones, folates, tetrahydrobiopterin and other therapeutic strategies seem promising in targeting vascular redox signaling, although it is still unclear which of these treatments have the potential to effectively prevent atherogenesis. Future studies need to define the key redox sensitive pathways in the vascular wall in order to develop effective therapeutic strategies against atherosclerosis.

Radiochemical HPLC detection of arginine metabolism: measurement of nitric oxide synthesis and arginase activity in vascular tissue.

Nitric oxide (NO) plays a key role in vascular homeostasis. Accurate measurement of NO production by endothelial nitric oxide synthase (eNOS) is critical for the investigation of vascular disease mechanisms using genetically modified animal models. Previous assays of NO production measuring the conversion of arginine to citrulline have required homogenisation of tissue and reconstitution with cofactors including NADPH and tetrahydrobiopterin. However, the activity and regulation of NOS in vivo is critically dependant on tissue levels of these cofactors. Therefore, understanding eNOS regulation requires assays of NO production in intact vascular tissue that do not depend on the addition of exogenous cofactors and have sufficient sensitivity and specificity. We describe a novel technique, using radiochemical detection of arginine to citrulline conversion, to measure NO production within intact mouse aortas, without exogenous cofactors. We demonstrate the presence of arginase activity in mouse aortas which has the potential to confound this assay. Furthermore, we describe the use of N-hydroxy-nor-L-arginine (nor-NOHA) to inhibit arginase and permit specific detection of NO production in intact mouse tissue. Using this technique we demonstrate a 2.4-fold increase in NO production in aortas of transgenic mice overexpressing eNOS in the endothelium, and show that this technique has high specificity and high sensitivity for detection of in situ NO synthesis by eNOS in mouse vascular tissue. These results have important implications for the investigation of NOS regulation in cells and tissues.

EPR quantification of vascular nitric oxide production in genetically modified mouse models.

With increasing use of genetically modified mice to study endothelial nitric oxide (NO) biology, methods for reliable quantification of vascular NO production by mouse tissues are crucial. We describe a technique based on electron paramagnetic resonance (EPR) spectroscopy, using colloid iron (II) diethyldithiocarbamate [Fe(DETC)2], to trap NO. A signal was seen from C57BL/6 mice aortas incubated with Fe(DETC)2, that increased 4.7-fold on stimulation with calcium ionophore A23187 [3.45+/-0.13 vs 0.73+/-0.13au (arbitrary units)]. The signal increased linearly with incubation time (r(2) = 0.93), but was abolished by addition of N(G)-nitro-l-arginine methyl ester (L-NAME) or endothelial removal. Stimulated aortas from eNOS knockout mice had virtually undetectable signals (0.14+/-0.06 vs 3.17+/-0.21 au in littermate controls). However, the signal was doubled from mice with transgenic eNOS overexpression (7.17+/-0.76 vs 3.37+/-0.43 au in littermate controls). We conclude that EPR is a useful tool for direct NO quantification in mouse vessels.

Strain-dependent variation in vascular responses to nitric oxide in the isolated murine heart.

Numerous studies in the literature have employed gene-modified mice to investigate vascular function. However, only very limited information exists on baseline murine vascular physiology or on potential variations between different strains. We therefore compared coronary and aortic vascular responses to endothelium-derived vasodilators and exogenous nitric oxide (NO) in three commonly used mouse strains and correlated these data with expression of eNOS, NADPH oxidase subunits, gp91(phox) and p67(phox), and superoxide production. Isolated perfused hearts from MF1, 129sv and C57BL/6J mice were subjected to: (a) increasing doses of bradykinin, acetylcholine and sodium nitroprusside, and (b) bolus doses of adenosine and the NO synthase inhibitor, N(G)-monomethyl- L -arginine. Vascular responses of thoracic aortic rings were assessed for comparison. Expression of eNOS and NADPH oxidase subunits was assessed by immunoblotting, and superoxide production by lucigenin-enhanced chemiluminescence. Coronary vasodilator responses to bradykinin, acetylcholine and sodium nitroprusside were significantly attenuated in MF1 compared with C57BL/6J and 129sv hearts. Similarly, aortic relaxation to acetylcholine was significantly impaired in MF1 aortic rings compared with in C57BL/6J aortae; these differences were reversed by Tiron. N(G)-monomethyl- L -arginine induced significantly less vasoconstriction in MF1 and 129sv hearts compared with C57BL/6J. No differences in aortic relaxation to A23187 or sodium nitroprusside were observed. Cardiac and aortic superoxide production and cardiac expression of p67(phox) and gp91(phox) were significantly greater in MF1 mice compared with the other strains. There is significant strain-dependent variation in coronary and aortic vascular responsiveness in mice, which may reflect differences in the balance between NO and superoxide generation.