NOX1, 2, 4, 5: counting out oxidative stress

Abstract

For decades, oxidative stress has been discussed as a key mechanism of endothelial dysfunction and cardiovascular disease. However, attempts to validate and exploit this hypothesis clinically by supplementing antioxidants have failed. Nevertheless, this does not disprove the oxidative stress hypothesis. As a certain degree of reactive oxygen species (ROS) formation appears to be physiological and beneficial. To reduce oxidative stress therapeutically, two alternative approaches are being developed. One is the repair of key signalling components that are compromised by oxidative stress. These include uncoupled endothelial nitric oxide (NO) synthase and oxidized/heme-free NO receptor soluble guanylate cyclase. A second approach is to identify and effectively inhibit the relevant source(s) of ROS in a given disease condition. A highly likely target in this context is the family of NADPH oxidases. Animal models, including NOX knockout mice and new pharmacological inhibitors of NADPH oxidases have opened up a new era of oxidative stress research and have paved the way for new cardiovascular therapies.

Figure 1

Balance between oxidative and reductive stress. In arterial hypertension (AHT) and stroke, physiological NO-sGC signalling and vasodilatation can be affected in three ways by NADPH (NOX)-induced oxidative stress (i.e. increased superoxide, O₂⁻, and hydrogen peroxide, H₂O₂, levels): (i) scavenging of NO (with intermediate peroxynitrite, ONOO⁻, formation); (ii) uncoupling of eNOS; and (iii) oxidation and heme-loss of NO-receptor Fe(II)sGC. Reactive oxygen species (ROS) and NO can also react to form reactive nitrogen species (RNS), which modify different cell components, including protein tyrosine nitration (prot-NO₂), correlating with cellular apoptosis and fibrosis. These pathways can be assessed by using biomarkers such as phospho-VASP (P-VASP) for physiologic NO signalling, and nitro-tyrosine for RNS chemical biology. Therapeutic options include inhibition of NADPH oxidases (NOX1 in AHT and NOX4 in stroke), eNOS recoupling (eNOSR), sGC stimulation (sGCS), sGC activation (sGCA) and phosphodiesterase (PDE) inhibition (PDEI). However, reductants or antioxidants are no therapeutic alternative. They are ineffective or even harmful, possibly via causing reductive stress, that is, unphysiological high glutathione (GSH) levels due to activation of glutathione reductase (GRx), by heat shock protein (Hsp)27 or glucose-6-phosphate dehydrogenase (G6PD)-derived NADPH. Increased GSH in turn results in glutathione peroxidase (GPx)-dependent reduction of ROS to unphysiologically low levels and leads to S-(nitroso) glutathionylation, leading eventually resulting in cardiomyopathy and reduced life span.