Inactivation of BACE1 increases expression of endothelial nitric oxide synthase in cerebrovascular endothelium

Cerebrovascular effects of β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) inactivation have not been systematically studied. In the present study we employed cultured human brain microvascular endothelial cells (BMECs), BACE1-knockout (BACE1^-/-) mice and conditional (tamoxifen-induced) endothelium-specific BACE1-knockout (eBACE1^-/-) mice to determine effect of BACE1 inhibition on expression and function of endothelial nitric oxide synthase (eNOS). Deletion of BACE1 caused upregulation of eNOS and glypican-1 (GPC1) in human BMECs treated with BACE1-siRNA, and cerebral microvessels of male BACE1^-/- mice and male eBACE1^-/- mice. In addition, BACE1siRNA treatment increased NO production in human BMECs. These effects appeared to be independent of amyloid β-peptide production. Furthermore, adenoviral-mediated overexpression of BACE1 in human BMECs down-regulated GPC1 and eNOS. Treatment of human BMECs with GPC1siRNA suppressed mRNA and protein levels of eNOS. In basilar arteries of male eBACE1^-/- mice, endothelium-dependent relaxations to acetylcholine and endothelium-independent relaxations to NO donor, DEA-NONOate, were not affected, consistent with unchanged expression of eNOS and phosphorylation of eNOS at Ser1177 in large cerebral arteries. In aggregate, our findings suggest that under physiological conditions, inactivation of endothelial BACE1 increases expression of eNOS in cerebral microvessels but not in large brain arteries. This effect appears to be mediated by increased GPC1 expression.

Endothelium-specific deletion of amyloid-β precursor protein exacerbates endothelial dysfunction induced by aging

The physiological function of amyloid precursor protein (APP) in the control of endothelial function during aging is unclear. Aortas of young (4-6 months old) and aged (23-26 months old) wild-type (WT) and endothelium-specific APP-deficient (eAPP^−/−) mice were used to study aging-induced changes in vascular phenotype. Unexpectedly, aging significantly increased protein expression of APP in aortas of WT mice but not in aortas of eAPP^−/− mice thereby demonstrating selective upregulation APP expression in vascular endothelium of aged aortas. Most notably, endothelial dysfunction (impairment of endothelium-dependent relaxations) induced by aging was significantly exacerbated in aged eAPP^−/− mice aortas as compared to age-matched WT mice. Consistent with this observations, endothelial nitric oxide synthase (eNOS) protein expression was significantly decreased in aged eAPP^−/− mice as compared to age matched WT mice. In addition, protein expression of cyclooxygenase 2 and release of prostaglandins were significantly increased in both aged WT and eAPP^−/− mice. Notably, treatment with cyclooxygenase inhibitor, indomethacin, normalized endothelium-dependent relaxations in aged WT mice, but not in aged eAPP^−/− mice. In aggregate, our findings support the concept that aging-induced upregulation of APP in vascular endothelium is an adaptive response designed to protect and preserve expression and function of eNOS.

Vascular phenotype of amyloid precursor protein-deficient mice

The amyloid precursor protein (APP) is expressed in the blood vessel wall, but the physiological function of APP is not completely understood. Previous studies established that APP has amine oxidase activity responsible for degradation of catecholamines. In the present study, we characterized the vascular phenotype of APP-knockout (APP-/-) mice. We demonstrate that circulating levels of catecholamines are significantly increased in male as compared with female APP-/- mice. Studies of vasomotor function in isolated aortas revealed that contractions to the α1-receptor agonist phenylephrine were significantly reduced in male APP-/- mice but not in females. In addition, contractions to G protein activation with sodium fluoride were reduced exclusively in male APP-/- mice aortas. The endothelium-dependent relaxations to acetylcholine were not affected by the loss of APP in mice of both sexes. Further analysis of the mechanisms underlying endothelium-dependent relaxations revealed that inhibition of cyclooxygenase by indomethacin significantly impaired relaxations to acetylcholine exclusively in male APP-/- mice. Furthermore, acetylcholine-induced production of cyclic guanosine monophosphate (cGMP) was significantly reduced in male APP-/- mice aortas while acetylcholine-induced production of cyclic adenosine monophosphate (cAMP) was enhanced. We concluded that altered vascular reactivity to phenylephrine appears to be in part the result of chronic exposure of male APP-/- aorta to high circulating levels of catecholamines. The mechanisms responsible for the impairment of endothelium-dependent cGMP signaling and adaptive enhancement of endothelium-dependent production of cAMP remain to be defined. NEW & NOTEWORTHY Male amyloid precursor protein (APP)-deficient mice have higher circulating levels of catecholamines as compared with female APP-deficient mice. As a consequence, endothelium-dependent and endothelium-independent vasomotor functions of male APP-deficient mice are significantly altered. Under physiological conditions, expression of APP appears to play an important role in vascular function.

Impairment of amyloid precursor protein alpha-processing in cerebral microvessels of type 1 diabetic mice

The mechanisms underlying dysfunction of cerebral microvasculature induced by type 1 diabetes (T1D) are not fully understood. We hypothesized that in cerebral microvascular endothelium, α-processing of amyloid precursor protein (APP) is impaired by T1D. In cerebral microvessels derived from streptozotocin (STZ)-induced T1D mice protein levels of APP and its α-processing enzyme, a disintegrin and metalloprotease 10 (ADAM10) were significantly decreased, along with down-regulation of adenylate cyclase 3 (AC3) and enhanced production of thromboxane A₂ (TXA₂). In vitro studies in human brain microvascular endothelial cells (BMECs) revealed that knockdown of AC3 significantly suppressed ADAM10 protein levels, and that activation of TXA₂ receptor decreased APP expression. Furthermore, levels of soluble APPα (sAPPα, a product of α-processing of APP) were significantly reduced in hippocampus of T1D mice. In contrast, amyloidogenic processing of APP was not affected by T1D in both cerebral microvessels and hippocampus. Most notably, studies in endothelial specific APP knockout mice established that genetic inactivation of APP in endothelium was sufficient to significantly reduce sAPPα levels in the hippocampus. In aggregate, our findings suggest that T1D impairs non-amyloidogenic processing of APP in cerebral microvessels. This may exert detrimental effect on local concentration of neuroprotective molecule, sAPPα, in the hippocampus.

Endothelium-specific amyloid precursor protein deficiency causes endothelial dysfunction in cerebral arteries

The exact physiological function of amyloid-β precursor protein (APP) in endothelial cells is unknown. Endothelium-specific APP-deficient (eAPP^-/-) mice were created to gain new insights into the role of APP in the control of vascular endothelial function. Endothelium-dependent relaxations to acetylcholine were significantly impaired in basilar arteries of global APP knockout (APP^-/-) and eAPP^-/- mice ( P < 0.05). In contrast, endothelium-independent relaxations to nitric oxide (NO)-donor diethylamine-NONOate were unchanged. Western blot analysis revealed that protein expression of endothelial nitric oxide synthase (eNOS) was significantly downregulated in large cerebral arteries of APP^-/- mice and eAPP^-/- mice as compared to respective wild-type littermates ( P < 0.05). Furthermore, basal levels of cyclic guanosine monophosphate (cGMP) were also significantly reduced in large cerebral arteries of APP-deficient mice ( P < 0.05). In contrast, protein expression of prostacyclin synthase as well as levels of cyclic adenosine monophosphate (cAMP) was not affected by genetic inactivation of APP in endothelial cells. By using siRNA to knockdown APP in cultured human brain microvascular endothelial cells we also found a significant downregulation of eNOS mRNA and protein expressions in APP-deficient endothelium ( P < 0.05). These findings indicate that under physiological conditions, expression of APP in cerebral vascular endothelium plays an important protective function by maintaining constitutive expression of eNOS .

Abstract P274: Endothelial Dysfunction of Conduit Arteries in Amyloid Precursor Protein-Deficient Mice

Amyloid precursor protein (APP) is an integral membrane protein expressed in the peripheral arteries. However, the exact vascular physiological function of APP is unknown. Male APP-deficient (APP –/– ) and their wild-type littermates (WT) mice were used to characterize the phenotype of APP in the control of vascular function. Isometric force of isolated aortic rings was recorded in organ chambers. Circulating levels of norepinephrine and epinephrine were significantly enhanced in APP –/– mice (4723±566 pg/mL and 854±98 pg/mL, respectively P<0.05 vs. WT: 1999±319 pg/mL and 429±71 pg/mL, respectively; n=13). The efficacy of phenylephrine induced contractions were significantly reduced in the aorta of APP –/– mice (21±3%, P<0.05 vs. WT: 47±4%; n=10) while contractions to prostaglandin F 2α were unchanged (135±4%, P=n.s. vs. WT: 133±3%; n=9). Western blot analysis revealed that protein expression of alpha1D adrenergic receptors was significantly downregulated in APP –/– mice aortas (0.21±0.05 O.D.; P<0.05 vs. WT: 0.48±0.11 O.D.; n=6). In contrast, endothelium-dependent relaxations to β-agonist isoproterenol were significantly enhanced in APP –/– mice aortas (P<0.05; n=10) while endothelium-dependent relaxations to acetylcholine were unaltered (P=n.s.; n=12). Incubation of aortic rings with indomethacin significantly impaired relaxations to isoproterenol as well as acetylcholine in APP –/– mice (P<0.05; n=8) while concomitant treatment with NOS inhibitor L-NAME completely abolished relaxations to both agonists (P<0.05; n=6-7). Incubation of aortic rings with isoproterenol significantly increased cAMP in the aortas of APP –/– mice (16.2±4.1 pmol/mg; P<0.05 vs. WT: 6.6±4.1 pmol/mg; n=7). Furthermore, cAMP levels were significantly enhanced by acetylcholine in APP –/– mice aortas (38±9 pmol/mg; P<0.05 vs. WT: 14±3 pmol/mg; n=8) while acetylcholine stimulated cGMP levels were reduced (59±5 pmol/mg; P<0.05 vs. WT: 83±7 pmol/mg; n=12). Our results suggest that increased circulating levels of catecholamines in APP –/– mice are responsible for observed vascular phenotype. These findings indicate that under physiological conditions, APP expression plays an important role in control of vascular endothelial function .

Expression and Processing of Amyloid Precursor Protein in Vascular Endothelium

Amyloid precursor protein (APP) is evolutionary conserved protein expressed in endothelial cells of cerebral and peripheral arteries. In this review, we discuss mechanisms responsible for expression and proteolytic cleavage of APP in endothelial cells. We focus on physiological and pathological implications of APP expression in vascular endothelium.

Role of prostacyclin signaling in endothelial production of soluble amyloid precursor protein-α in cerebral microvessels

We tested hypothesis that activation of the prostacyclin (PGI2) receptor (IP receptor) signaling pathway in cerebral microvessels plays an important role in the metabolism of amyloid precursor protein (APP). In human brain microvascular endothelial cells activation of IP receptor with the stable analogue of PGI2, iloprost, stimulated expression of amyloid precursor protein and a disintegrin and metalloprotease 10 (ADAM10), resulting in an increased production of the neuroprotective and anticoagulant molecule, soluble APPα (sAPPα). Selective agonist of IP receptor, cicaprost, and adenylyl cyclase activator, forskolin, also enhanced expression of amyloid precursor protein and ADAM10. Notably, in cerebral microvessels of IP receptor knockout mice, protein levels of APP and ADAM10 were reduced. In addition, iloprost increased protein levels of peroxisome proliferator-activated receptor δ (PPARδ) in human brain microvascular endothelial cells. PPARδ-siRNA abolished iloprost-augmented protein expression of ADAM10. In contrast, GW501516 (a selective agonist of PPARδ) upregulated ADAM10 and increased production of sAPPα. Genetic deletion of endothelial PPARδ (ePPARδ-/-) in mice significantly reduced cerebral microvascular expression of ADAM10 and production of sAPPα. In vivo treatment with GW501516 increased sAPPα content in hippocampus of wild type mice but not in hippocampus of ePPARδ-/- mice. Our findings identified previously unrecognized role of IP-PPARδ signal transduction pathway in the production of sAPPα in cerebral microvasculature.

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.

Abstract T P246: Phenotypic Characterization of Cerebral Microvasculature in Transgenic Mice With Endothelium Targeted Over-expression of GTP Cyclohydrolase I

Background: Optimal availability of tetrahydrobiopterin (BH4) is a critical determinant of nitric oxide (NO) production by endothelial nitric oxide synthase (eNOS) in the vascular endothelium. Biosynthesis of BH4 is regulated by the enzymatic activation of GTP cyclohydrolase I (GTPCH-I). While the physiological role of GTPCH-I and BH4 have been extensively characterized in peripheral vasculature, their role in regulation of cerebral vascular function has not been investigated.

Methods: The role of GTPCH-I in regulation of cerebral vascular function was studied in cerebral microvessels isolated from wild-type (WT) mice and from mice with endothelium-targeted overexpression of GTPCH-I (eGTPCH-I Tg) mice. Vascular protein expression, intracellular levels of biopterin and cGMP (second messenger of NO) as well as production of NO and superoxide anions were determined.

Results: Endothelium targeted over-expression of GTPCH-I resulted in significant increase in levels of BH4, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH2). Importantly, ratio of BH4 to 7,8-BH2, indicative of BH4 available for eNOS activation, was significantly increased in cerebral microvessels of eGTPCH-I Tg mice. However, protein expression of eNOS, levels of nitrate/nitrite - indicative of NO production remained unchanged between cerebral microvessels of WT mice and eGTPCH-I Tg mice. Furthermore, increased BH4 biosynthesis did not affect production of superoxide anions or expression of antioxidant enzymes. Moreover, intracellular levels of cGMP, reflective of NO signaling and activation of soluble guanylate cyclase, were not affected in eGTPCH-I Tg mice.

Conclusion: Our results suggest that, despite a significant increase in BH4 bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGTPCH-I Tg mice. We conclude that under physiological conditions the levels of BH4 are optimal for activation of eNOS and NO/cGMP signaling in wild-type mice.

Abstract W P230: Tetrahydrobiopterin Attenuates Cerebrovascular Oxidative Stress in Tg2576 Mouse Model of Alzheimer’s Disease

Background: The present study was designed to test the hypothesis that supplementation of tetrahydrobiopterin (BH4) to transgenic mice expressing the Swedish double mutation of human amyloid precursor protein (Tg2576 mice) results in restoration of BH4 levels required for activation of endothelial nitric oxide synthase (eNOS), and in turn, prevents oxidative stress in cerebral microvasculature.

Methods: Cerebral microvessels were obtained from 4-5 months old female wild-type and Tg2576 mice. Biopterin levels, enzymatic activity of GTP cyclohydrolase I (GTPCH-I) and superoxide production were measured by HPLC. The effects of supplementation of BH4 on oxidative stress were studied by injecting wild-type and Tg2576 mice subcutaneously with 100 mol/kg (b.w.) of BH 4 ([ 6R ]-5,6,7,8-tetrahydro-L-biopterin dihydrochloride; [ 6R ]-BH4).

Results: Enzymatic activity of GTPCH-I, rate limiting enzyme in BH4 biosynthesis, was not different between cerebral microvessels of wild-type and Tg2576 mice. However, bioavailability of BH4, was significantly reduced in cerebral microvessels of Tg2576 mice (P<0.05, n=8). Production of superoxide anions was significantly elevated in cerebral microvessels of Tg2576 mice (P<0.01, n=6), indicative of oxidative stress. This increased superoxide anion production was abolished by L-NAME, a NOS inhibitor, suggestive of eNOS uncoupling (P<0.05, n=6). Supplementation of [ 6R ]-BH4 to wild-type and Tg2576 mice resulted in significant increase in BH4 bioavailability (P<0.05, n=6). Notably, supplementation of [ 6R ]-BH4 abrogated the increase in superoxide anion production in cerebral microvessels of Tg2576 mice (P<0.05, n=5), while superoxide anion production remained unchanged in cerebral microvessels of WT mice. Furthermore, the inhibitory effects of L-NAME on superoxide anion production in cerebral microvessels of Tg2576 mice were abolished following [ 6R ]-BH4 supplementation (P<0.05, n=4).

Conclusion: Supplementation of [ 6R ]-BH4 restored bioavailability of BH4, thereby abrogating superoxide anion production derived from eNOS. Our results suggest that uncoupling of eNOS contributes to oxidative stress in cerebral microvessels of Tg2576 mice.

Erythropoietin increases bioavailability of tetrahydrobiopterin and protects cerebral microvasculature against oxidative stress induced by eNOS uncoupling

This study was designed to determine whether treatment with erythropoietin (EPO) could protect cerebral microvasculature against the pathological consequences of endothelial nitric oxide (NO) synthase uncoupling. Wild-type and GTP cyclohydrolase I (GTPCH-I)-deficient hph1 mice were administered EPO (1000 U/kg/day, s.c., 3 days). Cerebral microvessels of hph1 mice demonstrated reduced tetrahydrobiopterin (BH4) bioavailability, increased production of superoxide anions and impaired endothelial NO signaling. Treatment of hph1 mice with EPO attenuated the levels of 7,8-dihydrobiopterin, the oxidized product of BH4, and significantly increased the ratio of BH4 to 7,8-dihydrobiopterin. Moreover, EPO decreased the levels of superoxide anions and increased NO bioavailability in cerebral microvessels of hph1 mice. Attenuated oxidation of BH4 and inhibition of endothelial NO synthase uncoupling were explained by the increased expression of antioxidant proteins, manganese superoxide dismutase, and catalase. The protective effects of EPO observed in cerebral microvessels of hph1 mice were also observed in GTPCH-I siRNA-treated human brain microvascular endothelial cells exposed to EPO (1 U/mL or 10 U/mL; 3 days). Our results suggest that EPO might protect the neurovascular unit against oxidative stress by restoring bioavailability of BH4 and endothelial NO in the cerebral microvascular endothelium. We demonstrate that treatment with erythropoietin (EPO) could protect cerebral microvasculature against the pathological consequences of endothelial nitric oxide (NO) synthase uncoupling. Our results suggest that EPO might protect the neurovascular unit against oxidative stress by restoring bioavailability of tetrahydrobiopterin (BH4) and endothelial nitric oxide.

Mechanisms of vascular dysfunction in mice with endothelium-specific deletion of the PPAR-δ gene

Peroxisome proliferator-activated receptor (PPAR)-δ is a nuclear hormone receptor that is mainly involved in lipid metabolism. Recent studies have suggested that PPAR-δ agonists exert vascular protective effects. The present study was designed to characterize vascular function in mice with genetic inactivation of PPAR-δ in the endothelium. Mice with vascular endothelial cell-specific deletion of the PPAR-δ gene (ePPARδ(-/-) mice) were generated using loxP/Cre technology. ePPARδ(-/-) mice were normotensive and did not display any sign of metabolic syndrome. Endothelium-dependent relaxations to ACh and endothelium-independent relaxations to the nitric oxide (NO) donor diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate were both significantly impaired in the aorta and carotid arteries of ePPARδ(-/-) mice (P < 0.05). In ePPARδ(-/-) mouse aortas, phosphorylation of endothelial NO synthase at Ser(1177) was significantly decreased (P < 0.05). However, basal levels of cGMP were unexpectedly increased (P < 0.05). Enzymatic activity of GTP-cyclohydrolase I and tetrahydrobiopterin levels were also enhanced in ePPARδ(-/-) mice (P < 0.05). Most notably, endothelium-specific deletion of the PPAR-δ gene significantly decreased protein expressions of catalase and glutathione peroxidase 1 and resulted in increased levels of H2O2 in the aorta (P < 0.05). In contrast, superoxide anion production was unaltered. Moreover, treatment with catalase prevented the endothelial dysfunction and elevation of cGMP detected in aortas of ePPARδ(-/-) mice. The findings suggest that increased levels of cGMP caused by H2O2 impair vasodilator reactivity to endogenous and exogenous NO. We speculate that chronic elevation of H2O2 predisposes PPAR-δ-deficient arteries to oxidative stress and vascular dysfunction.

Abstract 130: Effects of Genetic Inactivation of Amyloid Precursor Protein in Cerebral Microvasculature

Background: Amyloid precursor protein (APP) is expressed in neuronal and non-neuronal tissues in the brain, including cerebrovascular endothelium. However, the physiological role of APP in cerebral vasculature is not completely understood. The present study was designed to determine the effects of inactivation of APP in cerebral microvasculature.

Methods: Effect of genetic inactivation of APP was studied both in vitro and in vivo . Cultured human brain microvascular endothelial cells (hBMECs) were incubated with APP-siRNA in vitro , while control-siRNA treated hBMECs served as controls. To study the effect of genetic inactivation of APP in vivo , cerebral microvessels were obtained from APP-deficient (APPKO) mice. Cerebral microvessels from wild-type (C57BL/6) littermates served as controls.

Results: Silencing APP expression in hBMECs resulted in selective reduction in endothelial nitric oxide synthase (eNOS) expression (P<0.05, n=6), while expressions of inducible NOS and prostacyclin (PGI2) synthase remained unchanged. Furthermore, loss of APP in hBMECs resulted in significantly increased production (P<0.05, n=5) of superoxide anions, as determined by quantitation of 2-hydroxyethidium from dihydroethidium using HPLC. In line with the results obtained from in vitro studies, cerebral microvessels of APPKO mice also demonstrated increased production of superoxide anions. Furthermore, levels of cGMP, second messenger of endothelial NO, were significantly attenuated in cerebral microvessels of APPKO mice, while levels of cAMP remained unchanged.

Conclusions: Our results suggest that genetic inactivation of APP results in oxidative stress and impairment of endothelial NO signaling. We speculate that APP exerts vascular protective effects in the cerebral circulation under physiological conditions.

Abstract W MP84: Erythropoietin Inhibits Oxidation of Tetrahydrobiopterin and Restores Bioavailability of Endothelial Nitric Oxide in Cerebral Microvessels of Hph-1 Mice

Background: While reported neuroprotective effects of erythropoietin (EPO) make it an appealing candidate for its evaluation in protection of neurovascular unit during cerebrovascular or neurodegenerative disorders, effects of EPO on cerebral microvessels, the vascular component of neurovascular unit, have not been studied to date. The present study was designed to determine the effects of EPO in cerebral microvessels derived from wild-type mice and also from hph-1 mice, a genetic mouse model of BH4 deficiency.

Methods: Hph-1 mice and wild-type littermates (C57BL/6 background) were administered recombinant human EPO (1000 U/kg/day) intraperitoneally for 3 days. Following treatment, mice were killed by injection of an overdose of pentobarbital, brains were removed and cerebral microvessels were isolated.

Results: Treatment of wild-type mice with EPO did not affect BH4 bioavailability, superoxide anion production or basal cGMP levels. We have reported that cerebral microvessels of hph-1 mice demonstrated reduced bioavailability of BH4, increased production of superoxide anions and impaired endothelial NO/cGMP signaling. Treatment of hph-1 mice with EPO attenuated the levels of 7,8-dihydrobiopterin (7,8-BH2; P<0.05, n=5), oxidized product of BH4, and significantly increased the ratio of BH4 to 7,8-BH2 (P<0.05, n=5), indicative of increased bioavailability of BH4 for eNOS activation. Increased superoxide anion production in cerebral microvessels of hph-1 mice were attenuated by EPO treatment (P<0.05, n=5). While eNOS expression remained unchanged, levels of cGMP were significantly increased on EPO treatment in hph-1 mice (P< 0.05, n=6). Furthermore, EPO treatment selectively increased expression of manganese superoxide dismutase.

Conclusion: The ability of EPO to attenuate oxidative stress and restore bioavailability of endothelial NO in cerebral microvessels may help to explain mechanisms responsible for cerebrovascular protective effects of EPO.

Supplementation of nitric oxide attenuates AβPP and BACE1 protein in cerebral microcirculation of eNOS-deficient mice

Recently, we demonstrated in endothelial nitric oxide synthase deficient (eNOS-/-) mice that loss of endothelial NO led to increased protein levels of amyloid-β protein precursor (AβPP), β-site AβPP cleaving enzyme 1 (BACE1), and amyloid-β (Aβ) peptide. Therefore, our aim was to determine if NO supplementation in vivo would attenuate AβPP and BACE1 protein levels. cGMP levels were significantly increased while AβPP and BACE1 protein levels were statistically lower in cerebral microvessels from nitroglycerin-treated eNOS-/- mice as compared to vehicle-treated mice. Our findings support the concept that preservation of NO/cGMP signaling is an important modulator of expression and processing of AβPP.

Deficient eNOS phosphorylation is a mechanism for diabetic vascular dysfunction contributing to increased stroke size

BACKGROUND AND PURPOSE

Phosphorylation of eNOS, an important post-translational modulator of its enzymatic activity, is reduced in diabetes mellitus. We hypothesized that modulation of eNOS phosphorylation could overcome diabetic vascular dysfunction and improves the outcome to stroke.

METHODS

We used the db/db mouse model of type 2 diabetes mellitus. We mated db/db mice with eNOS knock-in mice that carry single amino acid mutations at the S1176 phosphorylation site; the phosphomimetic SD mutation (serine replaced by aspartate) shows increased eNOS enzymatic activity, whereas the unphosphorylatable SA mutation (serine replaced by alanine) shows decreased eNOS activity. We characterized the vascular anatomy, baseline physiological parameters, and vascular reactivity. We used the middle cerebral artery occlusion model of stroke and measured infarct volume and neurological deficits.

RESULTS

db/db mice showed diminished eNOS phosphorylation at S1176. eNOS SD and SA mutations do not change the vascular anatomy at the Circle of Willis, brain capillary density, heart rate, or arterial blood gases of db/db mice. The eNOS SD mutation, but not the SA mutation, lowers blood pressure and improves vascular reactivity to acetylcholine in db/db mice. The eNOS SD mutation reduces stroke size and neurological deficit after middle cerebral artery occlusion.

CONCLUSIONS

Diminished eNOS phosphorylation is a mechanism of vascular dysfunction in db/db mice. We show here that modulation of the eNOS S1176 phosphorylation site in db/db mice is associated with improved vascular reactivity and improved outcome to stroke after middle cerebral artery occlusion.

Increased production of superoxide anion contributes to dysfunction of the arteriovenous fistula

Vascular access dysfunction causes morbidity in hemodialysis patients. This study examined the generation and pathobiological significance of superoxide anion in a rat femoral arteriovenous fistula (AVF). One week after AVF creation, there was increased production of superoxide anion accompanied by decreased total superoxide dismutase (SOD) and Cu/Zn SOD activities and induction of the redox-sensitive gene heme oxygenase-1. Immunohistochemical studies of nitrotyrosine formation demonstrated that peroxynitrite, a product of superoxide anion and nitric oxide, was present in increased amounts in endothelial and smooth muscle cells in the AVF. Because uncoupled NOS isoforms generate superoxide anion, and NOS coupling requires tetrahydrobiopterin (BH(4)) as a cofactor, we assessed NOS uncoupling by determining the ratio of BH(4) to dihydrobiopterin (BH(2)); the BH(4)-to-BH(2) ratio was markedly attenuated in the AVF. Because Src is a vasculopathic signaling species upstream and downstream of superoxide anion, such expression was evaluated; expression of Src and phosphorylated Src was both markedly increased in the AVF. Expression of NADPH oxidase (NOX) 1, NOX2, NOX4, cyclooxygenase (COX) 1, COX2, p47(phox), and p67(phox) was all unchanged, as assessed by Western analyses, thereby suggesting that these proteins may not be involved in increased production of superoxide anion. Finally, administration of tempol, a superoxide anion scavenger, decreased neointima formation in the juxta-anastomotic venous segment and improved AVF blood flow. We conclude that the AVF exhibits increased superoxide anion generation that may reflect the combined effects of decreased scavenging by SOD and increased generation by uncoupled NOS, and that enhanced superoxide anion production promotes juxta-anastomotic stenosis and impairs AVF function.

Activation of PPARδ prevents endothelial dysfunction induced by overexpression of amyloid-β precursor protein

AIMS

Existing evidence suggests that amyloid-β precursor protein (APP) causes endothelial dysfunction and contributes to pathogenesis of atherosclerosis. In the present study, experiments were designed to: (1) determine the mechanisms underlying endothelial dysfunction and (2) define the effects of peroxisome proliferator-activated receptor delta (PPARδ) ligand on endothelial function in transgenic Tg2576 mice overexpressing mutated human APP.

METHODS AND RESULTS

Confocal microscopy and western blot analyses of wild-type mice aortas provided evidence that APP protein is mainly present in endothelial cells. Overexpression of APP significantly impaired endothelium-dependent relaxations to acetylcholine and phosphorylation of endothelial nitric oxide synthase at Ser(1177) in aortas. HPLC analysis revealed that tetrahydrobiopterin (BH(4)) levels were reduced in Tg2576 mice aortas. This was caused by increased oxidation of BH(4) and reduced expression and activity of GTP-cyclohydrolase I. Furthermore, gp91phox protein expression and superoxide anion production were increased in aortas of Tg2576 mice. This augmented superoxide formation was completely prevented by the NADPH oxidase inhibitor VAS2870. Expression of copper-/zinc-superoxide dismutase (Cu/ZnSOD) and extracellular SOD was downregulated. Treatment with PPARδ ligand GW501516 (2 mg/kg/day) for 14 days significantly increased BH(4) bioavailability and improved endothelium-dependent relaxations in Tg2576 mice aortas. GW501516 also normalized protein expression of gp91(phox) and SODs, thereby reducing production of superoxide anion in the aortas.

CONCLUSION

Our results suggest that in APP transgenic mice loss of nitric oxide and increased oxidative stress are the major causes of endothelial dysfunction. The vascular protective effects of GW501516 in Tg2576 mice appear to be critically dependent on prevention of superoxide anion production.

Uncoupling of eNOS causes superoxide anion production and impairs NO signaling in the cerebral microvessels of hph-1 mice

In this study, we used the GTP cyclohydrolase I-deficient mice, i.e., hyperphenylalaninemic (hph-1) mice, to test the hypothesis that the loss of tetrahydrobiopterin (BH(4)) in cerebral microvessels causes endothelial nitric oxide synthase (eNOS) uncoupling, resulting in increased superoxide anion production and inhibition of endothelial nitric oxide signaling. Both homozygous mutant (hph-1(-/-)) and heterozygous mutant (hph-1(+/-) mice) demonstrated reduction in GTP cyclohydrolase I activity and reduced bioavailability of BH(4). In the cerebral microvessels of hph-1(+/-) and hph-1(-/-) mice, increased superoxide anion production was inhibited by supplementation of BH(4) or NOS inhibitor- L- N(G) -nitro arginine-methyl ester, indicative of eNOS uncoupling. Expression of 3-nitrotyrosine was significantly increased, whereas NO production and cGMP levels were significantly reduced. Expressions of antioxidant enzymes namely copper and zinc superoxide dismutase, manganese superoxide dismutase, and catalase were not affected by uncoupling of eNOS. Reduced levels of BH(4), increased superoxide anion production, as well as inhibition of NO signaling were not different between the microvessels of male and female mice. The results of our study are the first to demonstrate that, regardless of gender, reduced BH(4) bioavailability causes eNOS uncoupling, increases superoxide anion production, inhibits eNOS/cGMP signaling, and imposes significant oxidative stress in the cerebral microvasculature.

Reactive oxygen species signaling facilitates FOXO-3a/FBXO-dependent vascular BK channel β1 subunit degradation in diabetic mice

Activity of the vascular large conductance Ca(2+)-activated K(+) (BK) channel is tightly regulated by its accessory β(1) subunit (BK-β(1)). Downregulation of BK-β(1) expression in diabetic vessels is associated with upregulation of the forkhead box O subfamily transcription factor-3a (FOXO-3a)-dependent F-box-only protein (FBXO) expression. However, the upstream signaling regulating this process is unclear. Overproduction of reactive oxygen species (ROS) is a common finding in diabetic vasculopathy. We hypothesized that ROS signaling cascade facilitates the FOXO-3a/FBXO-mediated BK-β(1) degradation and leads to diabetic BK channel dysfunction. Using cellular biology, patch clamp, and videomicroscopy techniques, we found that reduced BK-β(1) expression in streptozotocin (STZ)-induced diabetic mouse arteries and in human coronary smooth muscle cells (SMCs) cultured with high glucose was attributable to an increase in protein kinase C (PKC)-β and NADPH oxidase expressions and accompanied by attenuation of Akt phosphorylation and augmentation of atrogin-1 expression. Treatment with ruboxistaurin (a PKCβ inhibitor) or with GW501516 (a peroxisome proliferator-activated receptor δ activator) reduced atrogin-1 expression and restored BK channel-mediated coronary vasodilation in diabetic mice. Our results suggested that oxidative stress inhibited Akt signaling and facilitated the FOXO-3a/FBXO-dependent BK-β(1) degradation in diabetic vessels. Suppression of the FOXO-3a/FBXO pathway prevented vascular BK-β(1) degradation and protected coronary function in diabetes.

Differential effects of eNOS uncoupling on conduit and small arteries in GTP-cyclohydrolase I-deficient hph-1 mice

In the present study, we used the hph-1 mouse, which displays GTP-cyclohydrolase I (GTPCH I) deficiency, to test the hypothesis that loss of tetrahydrobiopterin (BH(4)) in conduit and small arteries activates compensatory mechanisms designed to protect vascular wall from oxidative stress induced by uncoupling of endothelial nitric oxide synthase (eNOS). Both GTPCH I activity and BH(4) levels were reduced in the aortas and small mesenteric arteries of hph-1 mice. However, the BH(4)-to-7,8-dihydrobiopterin ratio was significantly reduced only in hph-1 aortas. Furthermore, superoxide anion and 3-nitrotyrosine production were significantly enhanced in aortas but not in small mesenteric arteries of hph-1 mice. In contrast to the aorta, protein expression of copper- and zinc-containing superoxide dismutase (CuZnSOD) was significantly increased in small mesenteric arteries of hph-1 mice. Protein expression of catalase was increased in both aortas and small mesenteric arteries of hph-1 mice. Further analysis of endothelial nitric oxide synthase (eNOS)/cyclic guanosine monophosphate (cGMP) signaling demonstrated that protein expression of phosphorylated Ser(1177)-eNOS as well as basal cGMP levels and hydrogen peroxide was increased in hph-1 aortas. Increased production of hydrogen peroxide in hph-1 mice aortas appears to be the most likely mechanism responsible for phosphorylation of eNOS and elevation of cGMP. In contrast, upregulation of CuZnSOD and catalase in resistance arteries is sufficient to protect vascular tissue from increased production of reactive oxygen species generated by uncoupling of eNOS. The results of our study suggest that anatomical origin determines the ability of vessel wall to cope with oxidative stress induced by uncoupling of eNOS.

Antihypertensive therapy increases tetrahydrobiopterin levels and NO/cGMP signaling in small arteries of angiotensin II-infused hypertensive rats

We previously reported that small mesenteric arteries from hypertensive rats have increased NOS-derived H(2)O(2) and reduced NO/cGMP signaling. We hypothesized that antihypertensive therapy lowers blood pressure through a tetrahydrobiopterin (BH(4))-dependent mechanism restoring NO/cGMP signaling and endothelial NOS (NOS3; eNOS) phosphorylation in small arteries. To test this hypothesis, small mesenteric arteries from normotensive rats (NORM), angiotensin II-infused rats (ANG), ANG rats with triple therapy (reserperine, hydrochlorothiazide, and hydralazine), or ANG rats with oral BH(4) therapy were studied. Both triple therapy and oral BH(4) therapy attenuated the rise in systolic blood pressure in ANG rats and restored NO/cGMP signaling in small arteries similarly. Triple therapy significantly increased vascular BH(4) levels and BH(4)-to-BH(2) ratio similar to ANG rats with BH(4) supplementation. Furthermore, triple therapy (but not oral BH(4) therapy) significantly increased GTP cyclohydrolase I (GTPCH I) activity in small arteries without a change in expression. NOS3 phosphorylation at Ser1177 was reduced in small arteries from ANG compared with NORM, while NOS3 phosphorylation at Ser633 and Thr495 were similar in ANG and NORM. NOS3 phosphorylation at Ser1177 was restored with triple therapy or oral BH(4) in ANG rats. In conclusion, antihypertensive therapy regulates NO/cGMP signaling in small arteries through increasing BH(4) levels and NOS3 phosphorylation at Ser1177.

Erythropoietin increases expression and function of vascular copper- and zinc-containing superoxide dismutase

Previous studies have shown that treatment with erythropoietin (EPO) exerts vascular protective effects. The exact mechanisms responsible for these effects are not completely understood. In the present study, we hypothesized that EPO stimulates expression and activity of copper- and zinc-containing superoxide dismutase (SOD1), thus protecting vascular tissue from oxidative stress induced by excessive concentrations of superoxide anions. EPO treatment of wild-type mice for 2 weeks (1000 U/kg, SC, biweekly) significantly increased aortic expression of SOD1. This effect resulted in a significant reduction of superoxide anion concentrations in aorta of treated mice. The ability of EPO to reduce vascular production of superoxide anions was abolished in SOD1-deficient mice. In a mouse model of wire-induced injury of the common carotid artery, treatment of wild-type mice with EPO prevented pathological remodeling, whereas the vascular effect of EPO was absent in SOD1-deficient mice. Our findings demonstrate that treatment with EPO increases vascular expression of SOD1. This effect appears to be an important molecular mechanism underlying vascular protection by EPO.

Cardiovascular effects of erythropoietin an update

Erythropoietin (EPO) is a therapeutic product of recombinant DNA technology and it has been in clinical use as stimulator of erythropoiesis over the last two decades. Identification of EPO and its receptor (EPOR) in the cardiovascular system expanded understanding of physiological and pathophysiological role of EPO. In experimental models of cardiovascular and cerebrovascular disorders, EPO exerts protection either by preventing apoptosis of cardiac myocytes, smooth muscle cells, and endothelial cells, or by increasing endothelial production of nitric oxide. In addition, EPO stimulates mobilization of progenitor cells from bone marrow thereby accelerating repair of injured endothelium and neovascularization. A novel signal transduction pathway involving EPOR--β-common heteroreceptor is postulated to enhance EPO-mediated tissue protection. A better understanding of the role of β-common receptor signaling as well as development of novel analogs of EPO with enhanced nonhematopoietic protective effects may expand clinical application of EPO in prevention and treatment of cardiovascular and cerebrovascular disorders.

Vascular protection by tetrahydrobiopterin: progress and therapeutic prospects

Tetrahydrobiopterin (BH4) is an essential cofactor required for the activity of endothelial nitric oxide (NO) synthase. Suboptimal concentrations of BH4 in the endothelium reduce the biosynthesis of NO, thus contributing to the pathogenesis of vascular endothelial dysfunction. Supplementation with exogenous BH4 or therapeutic approaches that increase endogenous amounts of BH4 can reduce or reverse endothelial dysfunction by restoring production of NO. Improvements in formulations of BH4 for oral delivery have stimulated clinical trials that test the efficacy of BH4 in the treatment of systemic hypertension, peripheral arterial disease, coronary artery disease, pulmonary arterial hypertension, and sickle cell disease. This review discusses ongoing progress in the translation of knowledge, accumulated in preclinical studies, into the clinical application of BH4 in the treatment of vascular diseases. This review also addresses the emerging roles of BH4 in the regulation of endothelial function and their therapeutic implications.

Erythropoietin increases endothelial biosynthesis of tetrahydrobiopterin by activation of protein kinase B alpha/Akt1

Tetrahydrobiopterin (BH(4)) is an essential cofactor required for enzymatic activity of endothelial NO synthase. Recently, it has been shown that vascular protective effects of erythropoietin (EPO) are dependent on activation of endothelial NO synthase. Therefore, our objective was to characterize the effect of EPO on the biosynthesis of BH(4) in the vascular wall. Incubation of isolated C57BL/6J mouse aortas for 18 hours with recombinant human EPO (1 to 50 U/mL) caused a concentration-dependent increase in intracellular BH(4) levels and activity of GTP-cyclohydrolase I. Maximal biosynthesis of BH(4) was detected at therapeutic concentrations of 5 U/mL. Removal of the endothelium abolished EPO-induced biosynthesis of BH(4) demonstrating that the vascular endothelium is a major source of BH(4). Treatment with a selective phosphatidylinositol 3-kinase inhibitor wortmannin significantly reduced BH(4) biosynthesis stimulated by EPO. The stimulatory effect of EPO on vascular GTP-cyclohydrolase I activity, BH(4) production, and phosphorylation of endothelial NO synthase was also detected in vivo in mice treated with recombinant human EPO. These effects of EPO were abolished in protein kinase Balpha/Akt1-deficient mice. In addition, EPO significantly increased systolic blood pressure and the number of circulating platelets in Akt1-deficient mice. Our results demonstrate that EPO stimulates biosynthesis of BH(4) in vascular endothelium and that the increase in BH(4) levels is caused by de novo biosynthesis of BH(4) via the phosphatidylinositol 3-kinase/Akt1 pathway. This effect is most likely designed to provide optimal intracellular concentration of the cofactor necessary for EPO-induced elevation of endothelial NO synthase activity.

Angiogenic function of prostacyclin biosynthesis in human endothelial progenitor cells

The role of prostaglandin production in the control of regenerative function of endothelial progenitor cells (EPCs) has not been studied. We hypothesized that activation of cyclooxygenase (COX) enzymatic activity and the subsequent production of prostacyclin (PGI(2)) is an important mechanism responsible for the regenerative function of EPCs. In the present study, we detected high levels of COX-1 protein expression and PGI(2) biosynthesis in human EPCs outgrown from blood mononuclear cells. Expression of COX-2 protein was almost undetectable under basal conditions but significantly elevated after treatment with tumor necrosis factor-alpha. Condition medium derived from EPCs hyperpolarized human coronary artery smooth muscle cells, similar to the effect of the PGI(2) analog iloprost. The proliferation and in vitro tube formation by EPCs were inhibited by the COX inhibitor indomethacin or by genetic inactivation of COX-1 or PGI(2) synthase with small interfering (si)RNA. Impaired tube formation and cell proliferation induced by inactivation of COX-1 were rescued by the treatment with iloprost or the selective peroxisome proliferator-activated receptor (PPAR)delta agonist GW501516 but not by the selective PGI(2) receptor agonist cicaprost. Downregulation of PPARdelta by siRNA also reduced angiogenic capacity of EPCs. Iloprost failed to reverse PPARdelta siRNA-induced impairment of angiogenesis. Furthermore, transfection of PGI(2) synthase siRNA, COX-1 siRNA, or PPARdelta siRNA into EPCs decreased the capillary formation in vivo after transplantation of human EPCs into the nude mice. These results suggest that activation of COX-1/PGI(2)/PPARdelta pathway is an important mechanism underlying proangiogenic function of EPCs.

Essential role of endothelial nitric oxide synthase in vascular effects of erythropoietin

Erythropoietin (EPO) fosters tissue oxygenation by stimulating erythropoiesis. More recently, EPO has been recognized as a tissue-protective cytokine. In this study, we tested the hypothesis that endothelial NO synthase (eNOS) plays a key role in the vascular protective effect of EPO. A murine model of wire-induced injury of carotid artery was used to examine the effect of EPO on endothelial repair and arterial wall architecture. Recombinant human EPO (1000 U/kg, SC, biweekly) was administered for 2 weeks in wild-type and eNOS-deficient mice after which reactivity of isolated carotid arteries was studied in vitro, and the vasculature was histologically assessed. Injured arteries exhibited impairment of endothelium-dependent relaxations to acetylcholine (P<0.05). This was associated with increased medial cross-sectional area (P<0.05). EPO upregulated expression of phosphorylated Ser1177-eNOS and normalized the vasodilator response to acetylcholine (P<0.05). Furthermore, EPO prevented the injury-induced increase in medial cross-sectional area (P<0.05). The vascular protective effects of EPO were abolished in eNOS-deficient mice. Most notably, EPO significantly increased systolic blood pressure and enhanced medial thickening of injured carotid arteries in eNOS-deficient mice (P<0.05). Our results demonstrate that EPO prevents aberrant remodeling of the injured carotid artery. The protective effects of EPO are critically dependent on activation of eNOS.

An analysis of the DOCA-salt model of hypertension in HO-1-/- mice and the Gunn rat

Heme oxygenase-1 (HO-1) is induced in the vasculature in the DOCA-salt model of hypertension in rats. Whereas the HO system and its products may exert vasodilator effects, recent studies have suggested that the HO system may predispose to hypertension. The present study examined the effects of selected components of the HO system, specifically, the HO-1 isozyme and the product bilirubin in the DOCA-salt model of systemic hypertension; the experimental approach employed mutant rodent models, namely, the HO-1(-/-) mouse and the hyperbilirubinemic Gunn rat. DOCA-salt induced HO-1 protein in the aorta in HO-1(+/+) mice and provoked a significant rise in systolic arterial pressure in HO-1(-/-) mice but not in HO-1(+/+) mice; this effect could not be ascribed to impaired urinary sodium excretion or impaired glomerular filtration rate in the DOCA-salt-treated HO-1(-/-) mice. The administration of DOCA salt to uninephrectomized rats significantly increased systolic arterial pressure in wild-type rats, an effect that was attenuated in the mutant Gunn rat; this reduction in systemic hypertension in the DOCA-salt-treated Gunn rat was not due to a greater induction of HO-1 in the vasculature or to a more avid urinary sodium excretion. DOCA-salt impaired endothelium-dependent and endothelium-independent vasorelaxation in wild-type rats but not in Gunn rats; prior exposure to bilirubin repaired the defect in endothelium-dependent vasorelaxation in aortic rings in DOCA-salt-treated rats. DOCA salt stimulated vascular production of superoxide anion in wild-type but not in Gunn rats. We suggest that HO-1 and the product bilirubin may exert a countervailing effect in the DOCA-salt model of systemic hypertension.

Aging-associated vascular phenotype in mutant mice with low levels of BubR1

BACKGROUND AND PURPOSE

Aging is a major risk for stroke and a highly complex biological process believed to involve multiple mechanisms. Mutant mice that express low levels of the spindle assembly checkpoint protein BubR1 are known to develop several aging-associated phenotypes at a very young age, including cataracts, lordokyphosis, loss of subcutaneous fat, and impaired wound healing. However, whether BubR1 acts to prevent vascular aging has not yet been established. The present study was designed to investigate the vascular phenotype of mutant mice with low levels of BubR1.

METHODS

Morphological, functional, and biochemical analyses were performed on aortas and carotid arteries of 3- to 5-month-old BubR1 mutant mice and wild-type littermates.

RESULTS

Arterial wall thickness and inner diameter were significantly reduced in BubR1 mutant mice. Arterial walls of BubR1 mutant mice had low numbers of medial smooth muscle cells. Masson trichrome staining showed profound fibrosis in arterial walls of BubR1 mutant. In agreement with these morphological changes, functional analysis of pressurized isolated carotid arteries of BubR1 mutant mice demonstrated reduced elastic properties. Endothelium-dependent relaxations to acetylcholine and endothelium-independent relaxations to the nitric oxide donor DEA-NONOate were significantly reduced in carotid arteries of BubR1 mutant mice. Furthermore, enzymatic activity of nitric oxide synthase and levels of cyclic GMP were significantly reduced in aortas of mutant mice, but production of superoxide anions was significantly increased.

CONCLUSIONS

These findings demonstrate that BubR1 insufficiency in mice results in phenotypic changes reminiscent of vascular aging in humans and suggest a role for BubR1 in suppressing the vascular aging process.

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.

Increased blood flow causes coordinated upregulation of arterial eNOS and biosynthesis of tetrahydrobiopterin

Shear stress, imposed on the vascular endothelium by circulating blood, critically sustains vascular synthesis of nitric oxide (NO). Endothelial NO synthase (eNOS) activity is determined by heat shock protein 90 (HSP90), caveolin-1, and the cofactor tetrahydrobiopterin (BH4). To determine whether increased blood flow concomitantly upregulates eNOS and GTP cyclohydrolase I (GTPCH I, the rate-limiting enzyme in BH4 biosynthesis), an aortocaval fistula model in the rat was employed wherein aortic blood flow is enhanced proximal but decreased distal to the fistula. Eight weeks after the creation of the aortocaval fistula, the proximal and distal aortic segments were harvested; sham-operated rats served as controls. Vasomotor function was assessed by isometric force recording. Expression of eNOS, HSP90, caveolin-1, Akt, phosphorylated eNOS (eNOS-Ser1177), and GTPCH I were determined by Western blot analysis. Biosynthesis of BH4 and GTPCH-I activity was examined by HPLC. In the aortic segments exposed to increased flow, contractions to KCl and phenylephrine were reduced, whereas endothelium-dependent relaxations were not affected compared with sham-operated or aortic segments with reduced blood flow. Expression of eNOS, caveolin-1, phosphorylated Akt, and eNOS-Ser1177 was enhanced in aortas exposed to increased blood flow. High flow augmented levels of cGMP and BH4 and increased expression of GTPCH I. In aggregate, these findings provide the first demonstration in vivo that coordinated vascular upregulation of eNOS, and GTPCH I accompanies increased blood flow. This induction of GTPCH I increases BH4 production, thereby optimizing the generation of NO by eNOS and thus the adaptive, vasorelaxant response required in sustaining increased blood flow.

Analysis of superoxide anion production in tissue

Endothelial production of oxygen free radicals, especially superoxide anion (O(2)-), is an important mechanism of vascular dysfunction in hypertension. Overproduction of oxygen free radicals, mainly O(2)- occurs in human hypertension and in a wide variety of animal models. Thus, analysis of O(2)- generation represents a useful tool for identifying oxidative stress in hypertension. Among the methods used for O(2)- detection, the chemiluminescent probe lucigenin has been widely shown to be a useful method for detecting and quantifying the O(2)- formation. On the other hand, staining by the oxidative fluorescent probe dihydroethidine, which is freely permeable to cell membranes, is suitable to monitor in situ production of O(2)- and to provide a reliable marker of its intracellular presence. Dihydroethidine is oxidized in the presence of O(2)- to a fluorescent marker product, which is rapidly intercalated into DNA. Thus, nuclei are the primary fluorescent structures labeled. By simply incubating experimental samples in the presence of dihydroethidine followed by analysis of fluorescence, this method allows rapid and specific detection of intracellular oxidative stress due to superoxide anion generation.

Increased tolerance to hypoxic metabolic inhibition and reoxygenation of cardiomyocytes from apolipoprotein E-deficient mice

Although hypercholesterolemia is a strong risk factor for cardiovascular disease, it has in some instances paradoxically been associated with reduced infarct size and preserved contractile function in isolated hearts after ischemia and reperfusion. To elucidate potential cellular protective mechanisms, myocytes of hypercholesterolemic apolipoprotein E-deficient (ApoE−/−) and wild-type mice were subjected to hypoxic metabolic inhibition (I) with subsequent reoxygenation (R). Intracellular Ca2+concentration ([Ca2+]i) and pH (pHi) were monitored as well as cell length and arrhythmic events. Force measurements in papillary muscles were also recorded, and myocardial expression of Na+/H+exchanger 1 (NHE1) and three Ca2+handling proteins [sarco(endo)plasmic reticulum Ca2+-ATPase, Na+/Ca2+exchanger, and plasma membrane Ca2+-ATPase] was quantified. After 30 min of I and 35 min of R, Ca2+overload was more pronounced in wild-type cells ( P < 0.05). In these myocytes, pHialso dropped faster and remained below those values determined in ApoE−/−cells ( P < 0.05). Furthermore, more wild-type myocytes remained in a contracted state ( P < 0.05). This group also showed a higher incidence of arrhythmic events during R ( P < 0.05). No group difference was found in the expression of the Ca2+handling proteins. However, NHE1 protein was downregulated in hearts of ApoE−/−mice ( P < 0.05). Histological results depict hyperplasia in ApoE−/−hearts without atherosclerosis of the coronaries. Contractile dysfunction was not observed in papillary muscles from ApoE−/−hearts. Our results suggest that downregulated myocardial NHE1 expression in hypercholesterolemic ApoE−/−mice could have contributed to increased tolerance to I/R. It remains to be elucidated whether NHE1 downregulation is a unique feature of these genetically altered animals.

In vivo expression of recombinant vascular endothelial growth factor in rabbit carotid artery increases production of superoxide anion

OBJECTIVE

Vascular endothelial growth factor (VEGF) is one of the most important pro-angiogenic cytokines. Ability of VEGF to stimulate formation of superoxide anion in vivo has not been studied. We hypothesized that in vivo expression of recombinant VEGF in the rabbit carotid artery increases production of superoxide anion.

METHODS AND RESULTS

Plaque-forming units (10(9)) of adenovirus-encoding human VEGF165 (AdVEGF) or beta-galactosidase (AdLacZ) were delivered into the lumen of rabbit carotid arteries. Three days after gene delivery, expression of recombinant proteins was detected in endothelium and smooth muscle cells. Endothelium-dependent relaxations to acetylcholine were impaired in AdVEGF-transduced arteries (P<0.01; n=5). Treatment with superoxide dismutase mimetic, Mn(III) tetra(4-benzoic acid) porphyrin chloride (10(-5) mol/L), improved relaxations to acetylcholine (P<0.01; n=5). Western blot analysis demonstrated increased expression of p47(phox) in AdVEGF-transduced arteries (P<0.05; n=8). Lucigenin chemiluminescence showed significantly higher production of superoxide anion in AdVEGF-transduced arteries (P<0.05; n=5 to 10).

CONCLUSIONS

Our results suggest that in vivo expression of recombinant VEGF in the vascular endothelium increases local production of superoxide anion. Superoxide anion appears to be an important mediator of vascular effects of VEGF in vivo.

The hyperbilirubinemic Gunn rat is resistant to the pressor effects of angiotensin II

ANG II induces vasoconstriction, at least in part, by stimulating NADPH oxidase and generating reactive oxygen species. ANG II also induces heme oxygenase activity, and bilirubin, a product of such activity, possesses antioxidant properties. We hypothesized that bilirubin, because of its antioxidant properties, may reduce the pressor and prooxidant effects of ANG II. Our in vivo studies used the hyperbilirubinemic Gunn rat which is deficient in the enzyme uridine diphosphate glucuronosyl transferase, the latter enabling the excretion of bilirubin into bile. ANG II (0.5 mg x kg(-1) x day(-1)) or saline vehicle was administered by osmotic minipump to control and Gunn rats for 4 wk. The rise in systolic blood pressure induced by ANG II, as observed in control rats, was markedly reduced in Gunn rats, the latter approximately 50% less at 3 and 4 wk after the initiation of ANG II infusion. The chronic administration of ANG II also impaired endothelium-dependent relaxation responses in control rats but not in Gunn rats. As assessed by the tetrahydrobiopterin/dihydrobiopterin ratio, ANG II induced oxidative stress in the aorta in control rats but not in Gunn rats. Heightened generation of superoxide anion in aortic rings in ANG II-infused rats and by vascular smooth muscle cells exposed to ANG II was normalized by bilirubin in vitro. We conclude that the pressor and prooxidant effects of ANG II are attenuated in the hyperbilirubinemic Gunn rat, an effect which, we speculate, may reflect, at least in part, the scavenging of superoxide anion by bilirubin.

In vivo gene transfer of inducible nitric oxide synthase to carotid arteries from hypercholesterolemic rabbits

BACKGROUND AND PURPOSE

Hypercholesterolemia is associated with endothelial dysfunction. Inducible nitric oxide synthase (iNOS) is upregulated in atherosclerotic vessels. However, its role in the regulation of vascular function is not completely understood. We examined the effect of adenovirus-mediated gene transfer of iNOS to the hypercholesterolemic rabbit carotid artery in vivo.

METHODS

Rabbits were fed a high-cholesterol or chow diet for 10 weeks. Two doses (1x10(8) and 1x10(9) plaque-forming units [pfu]/mL) of adenoviral vectors encoding iNOS (AdiNOS) or beta-galactosidase (Ad(beta)gal) were luminally delivered to the carotid arteries from chow- and cholesterol-fed animals. Vascular reactivity and superoxide levels were assessed in Ad(beta)gal- and AdiNOS-transduced vessels from chow- and cholesterol-fed animals after 3 days.

RESULTS

Endothelium-dependent vasorelaxation was impaired in the carotid artery from cholesterol-fed animals. In AdiNOS-transduced arteries, transgene expression was demonstrated by positive immunostaining in the endothelium. Transduction with low-dose (1x10(8) pfu/mL) AdiNOS did not affect vascular reactivity in arteries from chow- and cholesterol-fed animals. In contrast, high-dose (1x10(9) pfu/mL) AdiNOS significantly reduced endothelium-dependent relaxation in vessels from cholesterol- but not chow-fed rabbits. After both low- and high-dose iNOS gene transfer, levels of O2*(-) were significantly (P<0.05) elevated in carotid arteries from cholesterol-fed animals. Incubation with an O2*(-) scavenger did not reverse vascular dysfunction in these arteries.

CONCLUSIONS

Adenoviral-mediated overexpression of iNOS results in increased production of O2*(-) in carotid arteries from cholesterol- but not chow-fed animals. High-dose AdiNOS gene transfer is associated with reduced endothelium-dependent and -independent relaxation in vessels from cholesterol-fed animals.

Long-term vitamin C treatment increases vascular tetrahydrobiopterin levels and nitric oxide synthase activity

In cultured endothelial cells, the antioxidant, L-ascorbic acid (vitamin C), increases nitric oxide synthase (NOS) enzyme activity via chemical stabilization of tetrahydrobiopterin. Our objective was to determine the effect of vitamin C on NOS function and tetrahydrobiopterin metabolism in vivo. Twenty-six to twenty-eight weeks of diet supplementation with vitamin C (1%/kg chow) significantly increased circulating levels of vitamin C in wild-type (C57BL/6J) and apolipoprotein E (apoE)--deficient mice. Measurements of NOS enzymatic activity in aortas of apoE-deficient mice indicated a significant increase in total NOS activity. However, this increase was mainly due to high activity of inducible NOS, whereas eNOS activity was reduced. Significantly higher tetrahydrobiopterin levels were detected in aortas of apoE-deficient mice. Long-term treatment with vitamin C restored endothelial NOS activity in aortas of apoE-deficient mice, but did not affect activity of inducible NOS. In addition, 7,8-dihydrobiopterin levels, an oxidized form of tetrahydrobiopterin, were decreased and vascular endothelial function of aortas was significantly improved in apoE-deficient mice. Interestingly, vitamin C also increased tetrahydrobiopterin and NOS activity in aortas of C57BL/6J mice. In contrast, long-term treatment with vitamin E (2000 U/kg chow) did not affect vascular NOS activity or metabolism of tetrahydrobiopterin. In vivo, beneficial effect of vitamin C on vascular endothelial function appears to be mediated in part by protection of tetrahydrobiopterin and restoration of eNOS enzymatic activity.

Chronic ET(A) receptor blockade prevents endothelial dysfunction of small arteries in apolipoprotein E-deficient mice

OBJECTIVE

This study investigated whether endothelial dysfunction occurs in mesenteric arteries of apoE-deficient mice and determined the role of endothelin (ET)-1, which is increased in human atherosclerosis, using an orally active endothelin ET(A) receptor antagonist.

METHODS

ApoE-deficient and C57BL/6J control mice were fed for 30 weeks with normal chow or high-fat Western-type diet alone or in combination with darusentan (LU135252; 50 mg/kg/day). Vasomotor reactivity of isolated small mesenteric arteries (I.D. 200-250 microm) was studied in vitro under perfused and pressurized conditions.

RESULTS

In both mouse strains, about one fourth of the endothelium-dependent relaxant response to acetylcholine was insensitive to inhibition by L-NAME and indomethacin. In mesenteric arteries of apoE-deficient mice on Western-type diet, increased intima-media thickness and levels of endothelin-1 protein were observed. In addition, NO-mediated endothelium-dependent relaxation to acetylcholine was reduced without affecting L-NAME/indomethacin insensitive relaxation and contractions to endothelin-1 and serotonin were enhanced. Treatment with darusentan normalized vascular structure, NO-mediated relaxation to acetylcholine and contractions to endothelin-1 and serotonin without affecting blood pressure or plasma cholesterol levels.

CONCLUSIONS

Severe hypercholesterolemia in apoE-deficient mice is associated with attenuation of NO-mediated relaxation to acetylcholine and increased vascular endothelin-1 content. Chronic ET(A) receptor blockade may provide a new therapeutic approach to improve NO-mediated endothelium-dependent vasomotion in small arteries.

Vasopeptidase inhibition and endothelial function in hypertension

Vasopeptidase inhibitors are a new class of drugs capable of inhibiting both angiotensin-converting enzyme and neutral endopeptidase 24.11. This involves simultaneous inhibition with a single molecule of two key enzymes, ACE and NEP, which are both involved in the regulation of cardiovascular homeostasis in many ways. This includes metabolism of several vasoactive peptides and their clearance from the circulation, therefore contributing to neurohumoral modulation, which might have therapeutic advantages in the prevention of endothelial dysfunction in hypertension.

Hypercholesterolemia impairs endothelium-dependent relaxations in common carotid arteries of apolipoprotein e-deficient mice

BACKGROUND AND PURPOSE

The effects of Western-type fat diet on endothelium-dependent relaxations and vascular structure in carotid arteries from a mouse model of human atherosclerosis are not known. Our objective was to characterize the mechanisms underlying endothelial dysfunction in apoE-deficient mice.

METHODS

C57BL/6J and apoE-deficient mice were fed for 26 weeks with a lipid-rich Western-type diet. Changes in the intraluminal diameter of pressurized common carotid arteries (ID 450 micrometer) were measured in vitro with a video dimension analyzer. Endothelial NO synthase protein content was evaluated by Western blotting. Intracellular cGMP and cAMP levels were determined by radioimmunoassay.

RESULTS

No morphological changes were observed in the carotid arteries of apoE-deficient mice. However, endothelium-dependent relaxations to acetylcholine (10(-9) to 10(-5) mol/L) were impaired (maximal relaxation 52+/-7% versus 83+/-5% for control mice, P<0.05). Treatment of arteries with NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester inhibited relaxations to acetylcholine to the same extent in apoE-deficient mice as in control mice. Preincubation of carotid arteries with cell-permeable superoxide dismutase mimetic Mn(III) tetra(4-benzoic acid)porphyrin chloride almost normalized NO-mediated relaxations to acetylcholine (75+/-5%, P<0.05). Endothelium-dependent relaxations to calcium ionophore and endothelium-independent relaxations to NO donor diethylammonium(Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate were unchanged in apoE-deficient mice. In addition, no changes in endothelial NO synthase protein expression and cGMP/cAMP levels were found in carotid arteries of apoE-deficient mice.

CONCLUSIONS

In carotid arteries of apoE-deficient mice, hypercholesterolemia causes impairment of receptor-mediated activation of eNOS. Increased superoxide anion production in endothelial cells appears to be coupled to activation of cholinergic receptors and is responsible for hypercholesterolemia-induced endothelial dysfunction. The apoE-deficient mouse carotid artery is a valuable new experimental model of endothelial dysfunction.

Chronic vasopeptidase inhibition restores endothelin-converting enzyme activity and normalizes endothelin levels in salt-induced hypertension

BACKGROUND

Vasopeptidase inhibition (VPI) represents a new therapeutic principle including both inhibition of angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP). The present study investigated the effect of the vasopeptidase inhibitor omapatrilat on endothelin-1 (ET-1)-mediated vascular function in salt-induced hypertension.

METHODS

Dahl salt-sensitive rats (n=6/group) on standard or salt-enriched (4% NaCl) chow were treated for 8 weeks with either omapatrilat (36+/-4 mg/kg/day), captopril (94+/-2 mg/kg/day) or placebo. Aortic and renal artery segments were isolated and suspended in organ chambers for isometric tension recording. Functional endothelin-converting enzyme (ECE) activity was assessed in native segments and after preincubation with omapatrilat. Furthermore, vascular ECE protein levels as well as plasma and tissue ET-1 levels were determined.

RESULTS

The increase in systolic blood pressure of salt-fed rats was prevented by omapatrilat and captopril to a comparable degree. In salt-induced hypertension, functional ECE activity (calculated as the ratio of the contraction to big ET-1 divided by the contraction to ET-1) in renal arteries (0.46+/-0.05) and in aorta (0.68+/-0.05) was reduced as compared with control animals (0.9+/-0.05 and 0.99+/-0.04, respectively; P<0.05). While omapatrilat in vitro blunted the response to big endothelin-1 (big ET-1) and diminished ECE activity further (P<0.01 vs native segments), chronic treatment with omapatrilat in vivo restored contractions to ET-1 (120+/-6%) and big ET-1 (98+/-9%) in renal arteries, and therefore normalized renovascular ECE activity. In addition, omapatrilat normalized plasma ET-1 concentrations (12.9+/-1.2 vs 16.6+/-1.4 pg/ml on high salt diet; P<0.05) and renovascular ECE protein levels.

CONCLUSIONS

In salt-induced hypertension, vasopeptidase inhibition restores alterations in the endothelin system, such as renovascular ECE activity and responsiveness to ET-1 and big ET-1 with chronic but not acute in vitro application. Thus, the beneficial effects of vasopeptidase inhibition may reflect a resetting of cardiovascular control systems and therefore may be particularly suited to treat hypertension and heart failure.

Mechanism of endothelial dysfunction in apolipoprotein E-deficient mice

Endothelium-dependent relaxations mediated by NO are impaired in a mouse model of human atherosclerosis. Our objective was to characterize the mechanisms underlying endothelial dysfunction in aortas of apolipoprotein E (apoE)-deficient mice, treated for 26 to 29 weeks with a lipid-rich Western-type diet. Aortic rings from apoE-deficient mice showed impaired endothelium-dependent relaxations to acetylcholine (10(-)(9) to 10(-)(5) mol/L) and Ca(2+) ionophore (10(-)(9) to 10(-)(6) mol/L) and endothelium-independent relaxations to diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (DEA-NONOate, 10(-)(10) to 10(-)(5) mol/L) compared with aortic rings from C57BL/6J mice (P<0.05). By use of confocal microscopy of an oxidative fluorescent probe (dihydroethidium), increased superoxide anion (O(2)(-)) production was demonstrated throughout the aortic wall but mainly in smooth muscle cells of apoE-deficient mice. CuZn-superoxide dismutase (SOD) and Mn-SOD protein expressions were unaltered in the aorta exposed to hypercholesterolemia. A cell-permeable SOD mimetic, Mn(III) tetra(4-benzoic acid) porphyrin chloride (10(-)(5) mol/L), reduced O(2)(-) production and partially normalized relaxations to acetylcholine and DEA-NONOate in apoE-deficient mice (P<0.05). [(14)C]L-Citrulline assay showed a decrease of Ca(2+)-dependent NOS activity in aortas from apoE-deficient mice compared with C57BL/6J mice (P<0.05), whereas NO synthase protein expression was unchanged. In addition, cGMP levels were significantly reduced in the aortas of apoE-deficient mice (P<0.05). Our results demonstrate that in apoE-deficient mice on a Western-type fat diet, impairment of endothelial function is caused by increased production of O(2)(-) and reduced endothelial NO synthase enzyme activity. Thus, chemical inactivation of NO with O(2)(-) and reduced biosynthesis of NO are key mechanisms responsible for endothelial dysfunction in aortas of atherosclerotic apoE-deficient mice.