Nox proteins in signal transduction

The NADPH oxidase (Nox) family of superoxide (O(2)(*-)) and hydrogen peroxide (H(2)O(2))-producing proteins has emerged as an important source of reactive oxygen species (ROS) in signal transduction. ROS produced by Nox proteins Nox1-5 and Duox1/2 are now recognized to play essential roles in the physiology of the brain, the immune system, the vasculature, and the digestive tract as well as in hormone synthesis. Nox-derived ROS have been implicated in regulation of cytoskeletal remodeling, gene expression, proliferation, differentiation, migration, and cell death. These processes are tightly controlled and reversible. In this review, we will discuss recent literature on Nox protein tissue distribution, subcellular localization, activation, and the resulting signal transduction mechanisms.

Epithelial nitration by a peroxidase/NOX5 system mediates mosquito antiplasmodial immunity

Plasmodium ookinetes traverse midgut epithelial cells before they encounter the complement system in the mosquito hemolymph. We identified a heme peroxidase (HPX2) and NADPH oxidase 5 (NOX5) as critical mediators of midgut epithelial nitration and antiplasmodial immunity that enhance nitric oxide toxicity in Anopheles gambiae. We show that the two immune mechanisms that target ookinetes-epithelial nitration and thioester-containing protein 1 (TEP1)-mediated lysis-work sequentially, and we propose that epithelial nitration works as an opsonization-like system that promotes activation of the mosquito complement cascade.

Nox5 mediates PDGF-induced proliferation in human aortic smooth muscle cells

The proliferation of vascular smooth muscle cells is important in the pathogenesis of many vascular diseases. Reactive oxygen species (ROS) produced by NADPH oxidases in smooth muscle cells have been shown to participate in signaling cascades regulating proliferation induced by platelet-derived growth factor (PDGF), a powerful smooth muscle mitogen. We sought to determine the role of Nox5 in the regulation of PDGF-stimulated human aortic smooth muscle cell (HASMC) proliferation. Cultured HASMC were found to express four isoforms of Nox5. When HASMC stimulated with PDGF were pretreated with N-acetyl cysteine (NAC), proliferation was significantly reduced. Proliferation induced by PDGF was also heavily dependent on JAK/STAT activation, as the JAK inhibitor, AG490, was able to completely abolish PDGF-stimulated HASMC growth. Specific knockdown of Nox5 with a siRNA strategy reduced PDGF-induced HASMC ROS production and proliferation. Additionally, siRNA to Nox5 inhibited PDGF-stimulated JAK2 and STAT3 phosphorylation. ROS produced by Nox5 play an important role in PDGF-induced JAK/STAT activation and HASMC proliferation.

Novel redox-dependent regulation of NOX5 by the tyrosine kinase c-Abl

We investigated the mechanism of H(2)O(2) activation of the Ca(2+)-regulated NADPH oxidase NOX5. H(2)O(2) induced a transient, dose-dependent increase in superoxide production in K562 cells expressing NOX5. Confocal studies demonstrated that the initial calcium influx generated by H(2)O(2) is amplified by a feedback mechanism involving NOX5-dependent superoxide production and H(2)O(2). H(2)O(2) NOX5 activation was inhibited by extracellular Ca(2+) chelators, a pharmacological inhibitor of c-Abl, and overexpression of kinase-dead c-Abl. Transfected kinase-active GFP-c-Abl colocalized with vesicular sites of superoxide production in a Ca(2+)-dependent manner. In contrast to H(2)O(2), the Ca(2+) ionophore ionomycin induced NOX5 activity independent of c-Abl. Immunoprecipitation of cell lysates revealed that active GFP-c-Abl formed oligomers with endogenous c-Abl and that phosphorylation of both proteins was increased by H(2)O(2) treatment. Furthermore, H(2)O(2)-induced NOX5 activity correlated with increased localization of c-Abl to the membrane fraction, and NOX5 proteins could be coimmunoprecipitated with GFP-Abl proteins. Our data demonstrate for the first time that NOX5 is activated by c-Abl through a Ca(2+)-mediated, redox-dependent signaling pathway and suggest a functional association between NOX5 NADPH oxidase and c-Abl.

Nitric oxide reduces NADPH oxidase 5 (Nox5) activity by reversible S-nitrosylation

The NADPH oxidases (Noxs) are a family of transmembrane oxidoreductases that produce superoxide and other reactive oxygen species (ROS). Nox5 was the last of the conventional Nox isoforms to be identified and is a calcium-dependent enzyme that does not depend on accessory subunits for activation. Recently, Nox5 was shown to be expressed in human blood vessels and therefore the goal of this study was to determine whether nitric oxide (NO) can modulate Nox5 activity. Endogenously produced NO potently inhibited basal and stimulated Nox5 activity and this inhibition was reversible with chronic, but not acute, exposure to L-NAME. Nox5 activity was reduced by NO donors, iNOS, and eNOS and in endothelial cells and LPS-stimulated smooth muscle cells in a manner dependent on NO concentration. ROS production was diminished by NO in an isolated enzyme activity assay replete with surplus calcium and NADPH. There was no evidence for NO-dependent changes in tyrosine nitration, glutathiolation, or phosphorylation of Nox5. In contrast, there was evidence for the increased nitrosylation of Nox5 as determined by the biotin-switch assay and mass spectrometry. Four S-nitrosylation sites were identified and of these, mutation of C694 dramatically lowered Nox5 activity, NO sensitivity, and biotin labeling. Furthermore, coexpression of the denitrosylation enzymes thioredoxin 1 and GSNO reductase prevented NO-dependent inhibition of Nox5. The potency of NO against other Nox enzymes was in the order Nox1 ≥ Nox3 > Nox5 > Nox2, whereas Nox4 was refractory. Collectively, these results suggest that endogenously produced NO can directly S-nitrosylate and inhibit the activity of Nox5.