NADPH oxidases: not just for leukocytes anymore!

doi:10.1016/S0968-0004(03)00194-4

Trends in Biochemical Sciences

Volume 28, Issue 9, September 2003, Pages 502-508

https://doi.org/10.1016/S0968-0004(03)00194-4 Get rights and content

Abstract

In addition to their role in bacterial killing by leukocytes, reactive oxygen species (ROS) have been increasingly recognized as important components of signaling and host defense in other cell types. The formation of ROS in both phagocytic- and non-phagocytic cells involves membrane-localized NADPH oxidases (Noxs). Nox proteins show structural homology to the cytochrome b₅₅₈ of leukocytes but, until recently, their regulation has been poorly understood. Here, we describe our current understanding of Nox function, and discuss emerging paradigms for regulation of Nox activity by Rac GTPase and/or other cytosolic components.

Section snippets

Components and regulation of the phagocyte NADPH oxidase

The Nox system of stimulated phagocytic leukocytes catalyzes the one-electron reduction of oxygen to produce superoxide anion using NADPH as substrate. When the phagocyte is activated through the action of soluble chemoattractants and chemokines, or phagocytic particles, the cytosolic components (Rac2, p47^phox and p67^phox) of the oxidase are induced to assemble at the level of the membrane-associated flavocytochrome b₅₅₈ (cyt b) to form the active enzyme. Cyt b has two subunits, gp91^phox (Nox2)

Rac GTPase function in Nox regulation

As indicated earlier, Rac2 GTPase is also a required component of the active phagocyte Nox. Upon cell activation, Rac2 dissociates from a pre-existing cytosolic complex with GDI by an as-yet undetermined mechanism. GDP is exchanged for GTP through the action of membrane-localized GEFs [31] possibly including P-Rex1, a leukocyte-specific PtdIns(3,4,5)P₃- and G-protein βγ subunit-regulated Rac GEF [32] and Vav1 [16]. Rac – now in its GTP-bound active form – becomes membrane-associated. Rac

Mechanisms of Rac action in Nox regulation

Currently, there are three major molecular models describing how Rac GTPase regulates Nox activity (see [46] for a more complete discussion of the data for each). Both Lambeth et al. and Pick et al. (Fig. 2a,b) suggest that p67^phox, which contains the defined activation domain for cyt b, is the only protein influencing the rate-limiting electron-transfer step (step1) of Nox. Rac is considered to act solely as an adaptor molecule that binds to p67^phox through the switch I region and aids in the

Identification of non-phagocyte Nox homologs

Earlier observations describing stimulus-dependent ROS formation at modest levels in non-phagocytic cells, as well as the presence of the known Nox component p22^phox in almost all cell types examined, prompted a search for gp91^phox (also known as Nox2) homologs. Within the past three years, multiple mammalian Nox2 homologs have been identified in various tissues and can be classified into three groups. (1) Nox1, Nox3, Nox4, and a short form of Nox5 resemble Nox2 in that they consist of six

Biological functions of Nox proteins

The biological functions of ROS generated by Nox/Duox proteins are currently based on many hypotheses and little solid data. Their roles might vary depending on the Nox/Duox isoform and the cell type involved. The tissue distribution of the mammalian Nox homologs appears to be restricted, perhaps reflecting specific biological roles of these enzymes. The expression of Nox3 is limited to fetal tissues, where it might play a role in developmental signaling [54]. Nox4 (also known as Renox) has

Regulation of non-phagocyte Nox isoforms

Although the regulation of the phagocyte Nox2 is well-documented, very little is known about the molecular mechanisms involved in Nox isoform regulation. Although the Nox homologs resemble gp91^phox (Nox2) in their basic overall structure, the formation of physical and/or functional complexes with p22^phox for ROS production has not yet been established. Indirect evidence indicates that for Nox4 – the only Nox protein constitutively active in various epithelial cell types – p22^phox is essential

GTPase-mediated regulation of Nox isoforms

It has been known for some time that Ras and Rac GTPases regulate oxidant-signaling pathways that are crucial for mitogenesis and oncogenesis. Transient expression of a constitutively activated form of Ras in NIH3T3 cells induced a significant increase in intracellular ROS that could be inhibited by expression of a dominant negative allele of Rac1 68, 69. Reactive oxygen species production was suppressed by treatment with the flavoprotein inhibitor diphenylene iodonium, suggesting that a Nox

Concluding remarks

Although the observations outlined here strongly indicate a regulatory role for Rac GTPase in Nox function, unfortunately there are little data at this time that directly link Rac to regulation of the activity of the non-phagocytic Nox proteins. Observations from several laboratories, mainly based upon yeast two-hybrid studies and in vitro binding assays using recombinant proteins, point to the potential association of active Rac with Nox carboxyl-terminal residues and with p51, presumably

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Trends in Biochemical Sciences

NADPH oxidases: not just for leukocytes anymore!

Abstract

Section snippets

Components and regulation of the phagocyte NADPH oxidase

Rac GTPase function in Nox regulation

Mechanisms of Rac action in Nox regulation

Identification of non-phagocyte Nox homologs

Biological functions of Nox proteins

Regulation of non-phagocyte Nox isoforms

GTPase-mediated regulation of Nox isoforms

Concluding remarks

Blood

Trends Pharmacol. Sci.

J. Biol. Chem.

J. Biol. Chem.

Immunity

J. Biol. Chem.

Cell

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Cell

J. Biol. Chem.

Mol. Cell

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Blood

Trends Biochem. Sci.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Gene

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Regulation of the phagocyte respiratory burst oxidase by protein interactions

Biochem. Mol. Biol. Int.

Activation of the NADPH oxidase involves the small GTP-binding protein p21rac1

Nature

Activation of the NADPH oxidase involves the small GTP-binding protein p21^rac1