Regulation of Nox and Duox enzymatic activity and expression

Abstract

In recent years, it has become clear that reactive oxygen species (ROS, which include superoxide, hydrogen peroxide, and other metabolites) are produced in biological systems. Rather than being simply a by-product of aerobic metabolism, it is now recognized that specific enzymes--the Nox (NADPH oxidase) and Duox (Dual oxidase) enzymes--seem to have the sole function of generating ROS in a carefully regulated manner, and key roles in signal transduction, immune function, hormone biosynthesis, and other normal biological functions are being uncovered. The prototypical Nox is the respiratory burst oxidase or phagocyte oxidase, which generates large amounts of superoxide and other reactive species in the phagosomes of neutrophils and macrophages, playing a central role in innate immunity by killing microbes. This enzyme system has been extensively studied over the past two decades, and provides a basis for comparison with the more recently described Nox and Duox enzymes, which generate ROS in a variety of cells and tissues. This review first considers the structure and regulation of the respiratory burst oxidase, and then reviews recent studies relating to the regulation of the activity of the novel Nox/Duox enzymes. The regulation of Nox and Duox expression in tissues and by specific stimuli is also considered here. An accompanying review considers biological and pathological roles of the Nox family of enzymes.

Fig. 1. Schematic Structure of Nox2 and p22^phox

Indicated are Nox2 and p22^phox, with predicted α-helical transmembrane regions depicted as cylinders, with the cytosol-facing B loop connecting helices 2 and 3, and hemes indicated in red, with conserved histidine residues in helices 3 and 5 providing axial and distal ligands to the heme irons (red circles). Helix 6 is connected to the predicted globular flavoprotein domain containing flavin adenine nucleotide (FAD) and the NADPH binding site, located on the cytosolic side. Nox2 is associated in the membrane with p22^phox, which has 2 predicted α-helical transmembrane segments and whose C-terminus contains a proline-rich region (PRR) which is the target for binding by the bis-SH3 domain of p47^phox.

Fig. 2. Regulatory Subunits for Nox2

Indicated are domains of the cytosolic factors required for Nox2 activation along with arrows indicating the domains or components with which they interact. Abbreviations are: AD, activation domain; TPR, tetratricopeptide repeats; SH3, Src Homology 3 domain; bis-SH3 refers to the tandem SH3 domains that function as a single domain; PX, ^phox domain; PB1, a domain originally described in p47^phox and Bem1p; AIR, autoinhibitory region.

Fig. 3. Regulatory Subunits for Nox1

Indicated are domains of the regulatory factors required for Nox1 activation along with arrows indicating the domains or components with which they interact. Abbreviations are as in Fig. 2.

Fig. 4. Promoter regions of human Nox1 and Nox2

Shown are schematic drawings of promoter regions of human Nox1 and Nox2. White letters in a filled circle indicate the region identified as regulating responsible gene expression. Black letters in an open circle indicate the region containing a consensus sequence for a given transcription factor. Boxes indicate transcription factors. The abbreviations of responsive element in promoter regions are: IRSE, IFN-γ-responsive stimulated elements; GAS, γ-activated sequence; C/ERB, CCAAT/enhancer binding protein; CDX, caudal-related homeobox; HAF-1, hematopoietic-associated factor 1; CCAAT, CCAAT box binding, BID, binding increased during differentiation.