Oxidative stress and inflammation contribute to traffic noise-induced vascular and cerebral dysfunction via uncoupling of nitric oxide synthases

Abstract

Environmental pollution and non-chemical stressors such as mental stress or traffic noise exposure are increasingly accepted as health risk factors with substantial contribution to chronic noncommunicable diseases (e.g. cardiovascular, metabolic and mental). Whereas the mechanisms of air pollution-mediated adverse health effects are well characterized, the mechanisms of traffic noise exposure are not completely understood, despite convincing clinical and epidemiological evidence for a significant contribution of environmental noise to overall mortality and disability. The initial mechanism of noise-induced cardiovascular, metabolic and mental disease is well defined by the "noise reaction model" and consists of neuronal activation involving the hypothalamic-pituitary-adrenal (HPA) axis as well as the sympathetic nervous system, followed by a classical stress response via cortisol and catecholamines. Stress pathways are initiated by noise-induced annoyance and sleep deprivation/fragmentation. This review highlights the down-stream pathophysiology of noise-induced mental stress, which is based on an induction of inflammation and oxidative stress. We highlight the sources of reactive oxygen species (ROS) involved and the known targets for noise-induced oxidative damage. Part of the review emphasizes noise-triggered uncoupling/dysregulation of endothelial and neuronal nitric oxide synthase (eNOS and nNOS) and its central role for vascular dysfunction.

Keywords: Cardiovascular disease; Environmental risk factors; NOS uncoupling; Oxidative stress; Traffic noise exposure.

Graphical abstract

Fig. 1

Noise exposure induces neuronal activation. First line neuronal events in response to noise exposure are sleep disturbance (when exposed during the sleep phase) and stress response reactions via activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system. This leads to release of stress hormones (glucocorticoids and catecholamines) and secondary activation of the cerebral (and systemic) renin-angiotensin-aldosterone system (RAAS) as well as endothelin-1 expression. These potent triggers of inflammation and oxidative stress will cause activation of NOX-2 via protein kinase C (PKC) and p47^phox phosphorylation in the brain, expression of markers of inflammation, increased lipid peroxidation, down-regulation of neuronal nitric oxide synthase (nNOS) and loss of antioxidant genes such as catalase (Cat) and forkhead box O3 (Foxo3) transcription factor. All of these changes induce a neuroinflammatory phenotype with increased cerebral oxidative stress. These stress hormones and vasoconstrictors lead to similar adverse changes in the cardiovascular (and pulmonary) system, finally resulting in cardiometabolic disease. The HPA axis, sympathetic nervous system, RAAS, endothelin-1 expression and neuroinflammation are redox regulated and vice versa can induce oxidative stress via NOX-2 activation and other sources. AT-II, angiotensin-II; CRH, corticotrophin-releasing hormone; ACTH, adrenocorticotrophic hormone; CVD, cardiovascular disease.

Fig. 2

Noise-induced neuronal activation induces cardiovascular ROS sources and oxidative damage as well as inflammation (summary of animal and human data). The stress hormones and vasoconstrictors have direct effects on the vasculature, the heart and immune cells via their receptors. Noise-induced sleep deprivation causes dysregulation of the circadian clock (e.g. changes in expression of period and cryptochrome genes), in part by adverse redox regulation of clock genes, which negatively affects inflammation and redox balance. In the heart, noise upregulated markers of oxidative stress (3-NT and 8-OH-dG), eNOS and NOX-2 activity, whereas noise down-regulated the antioxidant factor Foxo3 and the structural component connexin43 (Cx43), all of which indicated cardiac remodeling and fibrosis. In endothelial cells isolated from mouse lungs (MLEC), antioxidant defense genes (Hmox1 and Pgc-1α) as well as Nox1 and Nos3 (eNOS) were upregulated in the noise group suggesting counter-regulatory mechanisms. In the aorta, noise increased oxidative protein damage (3-NT and 4-HNE), markers of inflammation (e.g. iNOS, Cd68, Mcp1 and Vcam1) and NOX-2 activity. While eNOS expression was upregulated in aortic tissue of noise-exposed animals, ^•NO bioavailability was decreased as well as phosphorylation of the vasodilator-stimulated phosphoprotein (P-VASP), which is a read-out of the ^•NO/cyclic guanosine-3',5'-monophosphate (cGMP)/protein kinase G (PKG) signaling cascade. Also aortic oxidative stress was directly measured and immune cell infiltration (mainly CD11b⁺ cells, not shown in the scheme) was quantified - both were increased upon noise exposure. Circulating markers of oxidative stress (MDA, 3-NT, 8-isoP) and inflammation (IL-6, IL-1β, IL-12, hsCRP) were increased upon noise exposure, whereas levels of reduced glutathione (GSH) were decreased. Circulating white blood cells had more oxidative DNA damage (8-OH-dG), compensatory increase in glutathione peroxidase 1 (GPx-1) and displayed more pronounced oxidative burst in noise exposed animals or subjects. Mitochondria of noise exposed animals showed increased swelling of the membranes, cristolysis and mtDNA damage, impaired function of the mitochondrial permeability transition pore (mPTP) and calcium handling as well as monoamine oxidase (MAO) activation by high noradrenaline (NA) levels due to noise exposure. Direct impact of hormones besides vasoconstriction is mentioned in the text boxes. The adverse redox changes and inflammatory signaling by noise promote the development of cardiometabolic disease. Pharmacological activation of FOXO3 and genetic Nox2 deletion largely prevent noise-induced adverse health effects. AT-II, angiotensin-II; ET-1, endothelin-1; 3-NT, 3-nitrotyrosine; 4-HNE, 4-hydroxynonenal; MDA, malondialdehyde; 8-isoP, 8-isoprostane; 8-OH-dG, 8-hydroxy-2′deoxyguanosine; hsCRP, high sensitivity C-reactive protein.

Fig. 3

Noise exposure causes eNOS and nNOS uncoupling. Superoxide formation induced by noise (e.g. NOX-2 activation) causes oxidative break-down of ^•NO leading to peroxynitrite formation and providing an explanation for increased protein tyrosine nitration (also iNOS-derived ^•NO contributes) as well as impairment of ^•NO/cGMP signaling. Despite upregulation of eNOS and the tetrahydrobiopterin (BH4)-generating enzymes GTP-cyclohydrolase 1 (GCH-1) and dihydrofolate reductase (DHFR), diminished ^•NO bioavailability was observed in aorta of noise-exposed mice. BH4 is an essential cofactor of eNOS (but also of iNOS) and oxidative depletion by ROS (e.g. peroxynitrite) to dihydrobiopterin (BH2) causes uncoupling of all NOS isoforms. BH4 levels were not measured upon noise exposure but upregulation of GCH-1 and DHFR obviously cannot compensate for the loss of function of eNOS in this setting. Therefore, eNOS was found to be dysfunctional or uncoupled, which was supported by S-glutathionylation (GSS-modification) by either ROS/GSH or S-nitrosoglutathione (GSNO) reaction. eNOS S-glutathionylation is an accepted marker of eNOS uncoupling (see Table 1) and uncoupled eNOS after noise exposure was detected at the molecular level by endothelial ROS formation that was sensitive to inhibition by the NOS inhibitor l-NAME. Although not measured in tissues of noise-exposed animals so far, the inactivating phosphorylation of eNOS at threonine 495 or tyrosine 657 (see Table 2), mediated by redox-activated protein kinase C (PKC) and protein tyrosine kinase 2 (PYK-2) respectively, would be conceivable under noise-induced oxidative stress conditions. In the brain, oxidative stress induction (e.g. NOX-2 activation) by noise caused nNOS uncoupling as envisaged by cerebral ROS formation that was sensitive to inhibition by the selective nNOS inhibitor ARL-17477 as well as phosphorylation at serine 847 (previously reported for uncoupled nNOS). Phospho-Ser847 in nNOS is introduced by calcium/calmodulin-dependent protein kinase (CaMKII) that is activated by ROS. The adverse redox regulation of eNOS and nNOS by noise induced oxidative stress promote the development of cardiometabolic disease and cognitive impairment that were previously reported for noise exposure for clinical/epidemiological studies.