Vascular Redox Signaling, Endothelial Nitric Oxide Synthase Uncoupling, and Endothelial Dysfunction in the Setting of Transportation Noise Exposure or Chronic Treatment with Organic Nitrates

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  FIG. 1.. The chemical basis of vascular oxidative stress and redox signaling by superoxide and nitric oxide in animal models of nitrate tolerance and traffic noise exposure. Upper part: Superoxide (•O2−) formation from Nox, the mitochondrial respiratory chain (Mito), XO, a ucNOS, and P450 side reactions confer redox signaling mainly upon breakdown by self-dismutation or catalyzed by SODs to hydrogen peroxide. Reaction with thiol groups is a major route of detoxification for H2O2 via Prx and Trx or the selenol in GPx—these systems require energy-consuming recycling by NADPH-coupled reductases (GR, TR). Another route for H2O2 decomposition is catalyzed by catalase (Cat). When H2O2 accumulates, it may lead to the Fenton reaction (hydroxyl radical formation) upon reaction with ferrous iron (Fe2+), triggering severe oxidative damage at the protein, lipid, and DNA levels. Likewise, nitric oxide (•NO, previously termed EDRF) is formed from eNOS, nNOS, iNOS, or reduction of nitrite from nutrition conferring the diffusion-controlled reaction with superoxide to yield peroxynitrite anion (ONOO−). This rapid kinetics even outcompetes the extremely fast SOD-catalyzed breakdown of superoxide. Upon…

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  FIG. 2.. Proposed mechanisms underlying GTN-induced and traffic noise-mediated eNOS uncoupling and diminished •NO bioavailability. Upper part: GTN treatment causes a decrease in Ser1177 (1) and an increase in Thr495 (2) phosphorylation of the eNOS (Knorr et al, 2011), leading to a decreased activity and uncoupling, respectively. In addition, the key enzyme of the de novo synthetic pathway of the eNOS cofactor BH4 GCH-1 is downregulated by chronic GTN treatment (3), also leading to a dysfunctional, superoxide (•O2−)-producing nitric oxide synthase. S-Glutathionylation represents another regulatory modification of eNOS, which is increased in the setting of tolerance (Knorr et al, 2011) and upon noise exposure (Munzel et al, 2017a; Steven et al, 2020) (4). Lower part: All of these adverse regulatory pathways that are activated by GTN treatment (50 or 100 mg/kg/day for 3.5 days) (Jabs et al, 2015; Knorr et al, 2011) or aircraft noise exposure [72 dB(A) around-the-clock for 1, 2, or 4 days] (Munzel et al, 2017a), including direct scavenging of •NO by •O2−, lead to diminished •NO bioavailability and enhanced endothelial •O2− formation that was blocked by the eNOS inhibitor L-NAME, supporting…

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  FIG. 3.. S-glutathionylation of eNOS in GTN-treated nitrate-tolerant rats and aircraft noise-exposed mice. Upper part: Oxidative stress causes S-glutathionylation of eNOS at cysteine 689 and 908, a surrogate marker for uncoupling of the protein. In the “coupled” eNOS homodimer, electrons are usually transferred from the NADPH and flavins to the heme iron. When cysteine residues 689 and/or 908 undergo S-glutathionylation, structural changes in eNOS are initiated that are followed by misdirection of the electrons to molecular oxygen with subsequent •O2− formation, termed “uncoupled” state of eNOS. Lower part: S-glutathionylation of eNOS was determined by eNOS immunoprecipitation from aortic tissue, followed by antiglutathione staining and normalization on eNOS. S-glutathionylation of eNOS was increased in nitrate-tolerant rats (GTN with a dose of 50 mg/kg/day for 3.5 days), which was normalized by telmisartan cotherapy (T8, 8 mg/kg/day for 10 days) (Knorr et al, 2011). *p < 0.05 versus Ctr, #p < 0.05 versus GTN group. S-glutathionylation of eNOS was also increased by aircraft noise exposure [72 dB(A) around-the-clock for 1, 2, or 4 days] (Munzel et al, 2017a). Disappearance of the…

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  FIG. 4.. Prevention of vascular ROS formation and endothelial dysfunction by GTN or traffic noise exposure by pharmacological or genetic NADPH oxidase inhibition. Left part: Oxidative stress assessed by DHE staining (red fluorescence indicates reactive oxygen species [ROS] formation, whereas green fluorescence represents the autofluorescence of the basal lamina). E=Endothelium, M = Media, A = Adventitia. GTN (50 mg/kg/day for 3.5 days) treatment leads to aortic oxidative stress in rats, which was prevented by the PKC inhibitor chelerythrine (chele) (Knorr et al, 2011). Aircraft noise exposure [72 dB(A) around-the-clock for 1, 2, or 4 days] leads to aortic oxidative stress in rats, which was prevented by genetic deficiency of gp91phox (Kroller-Schon et al, 2018). The phagocytic NADPH oxidase (NOX-2) plays a key role in the pathomechanisms underlying nitrate tolerance and noise exposure-mediated cardiovascular damage. NOX-2 is activated by ATII receptor activation, leading to DAG formation, a strong PKC activator. PKC activation can be envisaged by phosphorylation of its target protein MARCKS. Activated PKC will cause phosphorylation of p47phox at Ser328 leading to translocation of this…

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  FIG. 5.. Impairment of endothelial dysfunction by GTN therapy or traffic noise exposure and improvement by vitamin C coadministration. Left part: Endothelial function was determined by forearm plethysmography, ultrasound-dependent assessment of forearm blood flow in response to increasing doses of infused ACh. FBF was expressed as the ratio of infused to noninfused arm. Each column presents the percent change in FBF from baseline in response to each infused concentration of ACh (7.5, 15, and 30 μg/min) in the untreated control group, chronic GTN treatment for 7 days, and the chronic GTN+vitamin C administration group (Gori et al, 2010). *p < 0.05 versus lowest ACh dose (7.5 μg/min), #p < 0.05 versus GTN therapy without vitamin C. Right part: Endothelial function was determined by FMD, ultrasound-dependent assessment of vasodilation (widening) of a large vessel in the arm upon reactive hyperemia after vascular occlusion for 5 min. FMD was determined in subjects without aircraft noise exposure [Leq 35.4 dB(A)] and 30 or 60 aircraft noise events for 1 night [Leq 43.1 or 46.3 dB(A)] (Schmidt et al, 2013). Likewise, FMD was determined in subjects without train noise exposure [Leq 33 dB(A)] and…

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  FIG. 6.. Summarizing schemes on the similarities and differences of pathomechanisms underlying nitrate tolerance (upper part) and transportation noise exposure (lower part)-mediated cardiovascular damage. (1) Chronic GTN therapy leads to activation of the RAAS via nitrovasodilator-induced hypotension with subsequent angiotensin-II release, PKC and NOX-2 activation (also called pseudotolerance). In addition, GTN therapy causes substantial mtROS formation also causing inhibition of ALDH-2, the GTN-bioactivating enzyme (2), followed by redox-dependent NOX-2 activation (3) and peroxynitrite (ONOO−) formation with subsequent dysregulation of other critical regulators of vascular tone by nitration of prostacyclin synthase (PGI-S) (4), oxidation of sGC (5), oxidative induction of ET-1 (6), and uncoupling of eNOS by BH4 depletion and S-glutathionylation (7), all of which leads to endothelial dysfunction and true vascular tolerance (8). In contrast, traffic noise exposure causes neuronal stress responses via the hypothalamic–pituitary–adrenal axis (release of cortisol/corticosterone) and the sympathetic nervous system (release of NA) (I) with subsequent RAAS and NOX-2 activation (II) along with…