Biochemistry, physiology, and pathophysiology of NADPH oxidases in the cardiovascular system

Abstract

The NADPH oxidase (Nox) enzymes are critical mediators of cardiovascular physiology and pathophysiology. These proteins are expressed in virtually all cardiovascular cells, and regulate such diverse functions as differentiation, proliferation, apoptosis, senescence, inflammatory responses and oxygen sensing. They target a number of important signaling molecules, including kinases, phosphatases, transcription factors, ion channels, and proteins that regulate the cytoskeleton. Nox enzymes have been implicated in many different cardiovascular pathologies: atherosclerosis, hypertension, cardiac hypertrophy and remodeling, angiogenesis and collateral formation, stroke, and heart failure. In this review, we discuss in detail the biochemistry of Nox enzymes expressed in the cardiovascular system (Nox1, 2, 4, and 5), their roles in cardiovascular cell biology, and their contributions to disease development.

Figure 1. Structure and activation of Nox enzymes

Catalytic Nox subunits, represented in blue, include an N-terminal domain composed of six transmembrane helices, numbered I-VI. Four histidine residues in helices III and V coordinate two heme iron atoms. A cytosolic C-terminal dehydrogenase domain includes an FAD cofactor and an NADPH substrate binding site. Upon activation, electrons are transferred from NADPH to FAD and across the membrane, via heme irons, to molecular oxygen, thus producing superoxide anion, which can be dismutated into hydrogen peroxide. Upper panel. Both Nox1 and Nox2 (blue) form a complex with p22phox (green), with its two transmembrane domains and C-terminal proline-rich region (PRR). A cytosolic complex (orange), is composed of an organizer (Noxo1 or p47phox), an activator (Noxa1 or p67phox) and p40phox (only with p67phox). The organizer, stimulated by phosphorylation (red dots) in the case of p47phox, binds the proline-rich region of p22phox and membrane lipids. Likewise, p40phox binds lipids in endosomal membranes. Rac, activated by GTP (brown dot), binds membrane, Nox and Noxa. The latter subunit triggers FAD reduction. Middle panel. Nox4 (blue) also forms a complex with p22phox (green). Its activity, constitutive in the absence of cytosolic subunits, can be increased by binding of Poldip2 to the cytosolic C-terminal of p22phox. Lower panel. While Nox5S is composed of a catalytic subunit similar to the other oxidases (blue), Nox5L includes an additional N-terminal segment (red) with four EF-hands. Binding of cytosolic calcium to the EF hands triggers Nox5L activation.

Figure 2. Physiological and pathophysiological responses mediated by Nox homologues in cardiovascular cells

This summary of the roles of Nox enzymes in different cell types of the cardiovascular system illustrates the paradox of their mediating both specialized and redundant functions. VSMC, vascular smooth muscle cells; EC, endothelial cells; FB, fibroblasts; CM, cardiomyocytes, MΦ, macrophages; and EPC, endothelial progenitor cells.