Oxidases and peroxidases in cardiovascular and lung disease: new concepts in reactive oxygen species signaling

Abstract

Reactive oxygen species (ROS) are involved in numerous physiological and pathophysiological responses. Increasing evidence implicates ROS as signaling molecules involved in the propagation of cellular pathways. The NADPH oxidase (Nox) family of enzymes is a major source of ROS in the cell and has been related to the progression of many diseases and even environmental toxicity. The complexity of this family's effects on cellular processes stems from the fact that there are seven members, each with unique tissue distribution, cellular localization, and expression. Nox proteins also differ in activation mechanisms and the major ROS detected as their product. To add to this complexity, mounting evidence suggests that other cellular oxidases or their products may be involved in Nox regulation. The overall redox and metabolic status of the cell, specifically the mitochondria, also has implications on ROS signaling. Signaling of such molecules as electrophilic fatty acids has an impact on many redox-sensitive pathologies and thus, as anti-inflammatory molecules, contributes to the complexity of ROS regulation. This review is based on the proceedings of a recent international Oxidase Signaling Symposium at the University of Pittsburgh's Vascular Medicine Institute and Department of Pharmacology and Chemical Biology and encompasses further interaction and discussion among the presenters.

Figure 1. Current models of the active Nox complexes

Each of the 7 members of the Nox family requires a different set of conditions and protein associations. This figure shows the cytosolic subunits that are beleived to assemble onto each Nox isoform in the active oxidase state.

Figure 2. Schematic representation of common Nox features

The “Nox” portion of each of the Nox family members contains a similar “Nox” component. Shown here are the key features in common for all Nox family members. The letter designations A – E signify the cytosolic/extracellular loop domains. Shown also are the heme binding sites (Fe) and the FAD and NADPH binding domains.

Figure 3. Outline of the structure of Nox family members

Shown are the structural elements of the different members of the Nox family. The transmembrane domains are numbered I to VII. Shown are the calcium binding motifs (EF hands) and the peroxidase homology domain of Duox1/2.

Figure 4. The mitochondrial electron transport chain

This simplified diagram of the electron transport chain on the inner mitochondrial membrane shows the direction of electron (e⁻) flow along the chain (black arrows) and the direction of flow (red arrows) of hydrogen ion (H⁺) across the mitochondrial membrane. Dotted arrows show ROS production as a result of the electron leak. UQ refers to ubiquinone; Cyt C refers to cytochrome c.

Figure 5. Arsenic-stimulated capillarization of fenestrated liver sinusoidal microvessels

A model for the proposed mechanism of arsenic (As) action via Nox2 to induce phenotypic changes. SEM images are of liver sinusoids from mice drinking normal water or water containing 100 ppb arsenite for two weeks (modified from [322]).

Figure 6. Scheme of xanthine oxidase-derived reactive intermediates

Xanthine oxidase (XO) may produce ROS or ^•NO thereby contributing to either tissue dysfunction or protection, or both.

Figure 7. Electrophilic fatty acids are potent anti-inflammatory cell signaling mediators

They are generated from fatty acids through enzymatic and non-enzymatic pathways, and modulate transcription factors, regulating inflammation and metabolism. Electrophilic fatty acids 1) adduct to specific cysteine residues on NFκB, resulting in inhibition of pro-inflammatory cytokine expression; 2) bind to PPARγ and transactivate downstream responsive genes; and 3) adduct to highly reactive cysteines on KEAP1 releasing Nrf2, causing its nuclear translocation, binding to ARE and activation of phase II genes.