Vascular Biology of Superoxide-Generating NADPH Oxidase 5-Implications in Hypertension and Cardiovascular Disease

Abstract

Significance: NADPH oxidases (Noxs), of which there are seven isoforms (Nox1-5, Duox1/Duox2), are professional oxidases functioning as reactive oxygen species (ROS)-generating enzymes. ROS are signaling molecules important in physiological processes. Increased ROS production and altered redox signaling in the vascular system have been implicated in the pathophysiology of cardiovascular diseases, including hypertension, and have been attributed, in part, to increased Nox activity. Recent Advances: Nox1, Nox2, Nox4, and Nox5 are expressed and functionally active in human vascular cells. While Nox1, Nox2, and Nox4 have been well characterized in models of cardiovascular disease, little is known about Nox5. This may relate to the lack of experimental models because rodents lack NOX5. However, recent studies have advanced the field by (i) elucidating mechanisms of Nox5 regulation, (ii) identifying Nox5 variants, (iii) characterizing Nox5 expression, and (iv) discovering the Nox5 crystal structure. Moreover, studies in human Nox5-expressing mice have highlighted a putative role for Nox5 in cardiovascular disease.

Critical issues: Although growing evidence indicates a role for Nox-derived ROS in cardiovascular (patho)physiology, the exact function of each isoform remains unclear. This is especially true for Nox5.

Future directions: Future directions should focus on clinically relevant studies to discover the functional significance of Noxs, and Nox5 in particular, in human health and disease. Two important recent studies will impact future directions. First, Nox5 is the first Nox to be crystallized. Second, a genome-wide association study identified Nox5 as a novel blood pressure-associated gene. These discoveries, together with advancements in Nox5 biology and biochemistry, will facilitate discovery of drugs that selectively target Noxs to interfere in uncontrolled ROS generation.

Keywords: Nox isoforms; atherosclerosis; endothelium; oxidative stress; reactive oxygen species; vascular smooth muscle.

FIG. 1.

Diagram demonstrating the structure and regulation of Nox5. Nox5 possesses a six-transmembrane domain, N-terminal domain with EF-hands, and C-terminal domain with phosphorylation sites. Nox5 is regulated by changes in intracellular Ca²⁺ levels, regulated, in part, by Ca²⁺ influx through Ca²⁺ channels. Nox5 is regulated by various kinases, transcription factors, and binding proteins, which may influence phosphorylation of Nox5. Nox5 activation generates O₂⁻, which is dismutated by SOD to H₂O₂. O₂⁻ and H₂O₂ act as signaling molecules influencing redox-sensitive signaling pathways. H₂O₂, hydrogen peroxide; Nox, NADPH oxidase; O₂⁻, superoxide; SOD, superoxide dismutase. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 2.

Subcellular localization of Nox5. Nox5 has been identified in the plasma membrane as well as various intracellular locations. It is highly expressed in the perinuclear area and is also detected in mitochondria and ER. The site-specific location of Nox5 may influence the function of this isoform through close association with specific signaling molecules. Increased Nox5-induced generation of ROS leads to oxidation of signaling molecules that influence redox signaling. ER, endoplasmic reticulum; ROS, reactive oxygen species. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 3.

Possible role of Nox5 in vascular contraction. Nox5 may be a point of crosstalk between calcium and ROS, placing Nox5 as an important regulator of vascular contraction and development of vascular dysfunction in cardiovascular disease. Vasoactive factors, such as Ang II and ET-1, activate Nox5-induced ROS generation. Once ROS levels are increased in VSMCs, oxidation of calcium channels in the cell membrane or intracellular compartments, for example, ER, leads to dysregulated calcium influx and mobilization and activation of the contractile machinery—inhibition of MYPT1 and activation of MLCK and MLC, leading to contraction. Ang II, angiotensin II; ET-1, endothelin-1; MLC, myosin light chain; MLCK, myosin light chain kinase; MYPT1, myosin light chain phosphatase 1; VSMCs, vascular smooth muscle cells. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 4.

Schematic demonstrating vascular signaling effects of Nox5. Schematic demonstrating putative mechanisms whereby activation of Nox5 leads to vascular dysfunction, contraction, and injury in cardiovascular disease. Vasoactive peptides (Ang II and ET-1), growth factors, cytokines, and hyperglycemia induce Nox5 activation and increased levels of intracellular free Ca²⁺ ([Ca²⁺]_i), which influence redox-sensitive and Ca²⁺-dependent signaling molecules associated with contraction, inflammation, growth, and endothelial function. Increased Nox5-mediated oxidative stress leads to increased protein oxidation (reversible and irreversible forms) and activation of signaling pathways that influence vascular function and structure in cardiovascular disease. PDGF, platelet-derived growth factor. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 5.

Insights from animals expressing Nox5. Studies in animals with inducible expression of human Nox5 in mice in a cell-specific manner demonstrated possible roles of Nox5 in the pathophysiology of cardiovascular diseases. Glomerular damage, kidney dysfunction, and hypertension were observed in mice expressing Nox5 specifically in podocytes. Nox5 expression in mesangial cells influenced processes related to fibrosis and inflammation. In endothelial cells, expression of Nox5 induced changes in blood pressure and increased risk of stroke. Nox5 in vascular smooth muscle cells is associated with vascular hypercontractility and endothelial dysfunction. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 6.

Nox5 and human disease. Nox5 is demonstrated to be a key regulator of calcium influx and signaling in many human cells. Nox5 regulates physiological signaling in cells from the cardiovascular, renal, and reproductive systems. Through ROS generation and oxidation, Nox5 influences signaling kinases, phosphatases, and transcription factors, resulting in contraction, inflammation, proliferation, fibrosis, and remodeling. These effects contribute to development of cardiovascular diseases, renal pathologies, and cancer. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars