NADPH oxidase in vascular injury: a new insight about its regulation and role in T cells.

Regulation of NADPH Oxidase Activity (Not Expression) Many reports support the critical role of reactive oxygen species (ROS) in vascular injury. The signaling properties of ROS are largely attributable to the reversible oxidation of redox-sensitive target proteins, especially protein tyrosine phosphatases (PTPs). The PTP activity is dependent on the reactive cysteine residues (Cys-SH) that are readily susceptible to oxidation.1 Oxidative inhibition of PTPs by molecules including PTP1B and SHP2 can increase phosphorylation and activation of many receptor tyrosine kinase.2 Therefore, ROS production can be a very important mediator of signal transduction, because activation of receptor tyrosine kinase can initiate many signal transduction pathways. The balance between oxidases and antioxidant enzymes regulates ROS production. One of the prominent sources of vascular reactive oxygen species is NADPH oxidase and its cell-specific expression and localization may have a critical role in regulation of specific and unique ROS-mediated signal transduction pathways by “compartmentalization” of ROS production.2 Therefore, the regulatory mechanism of NADPH oxidase “activation” is of great interest. NADPH oxidases consist of membrane-associated cytochrome b558 comprising of the catalytic gp91phox and regulatory p22phox subunits and cytosolic components including p47phox, p67phox, p40phox, and the small GTPase Rac in phagocytic cells. In nonphagocytic cells, several homologs of gp91phox(Nox2) including Nox1 and Nox3–5, as well as the Dual oxidase (Duox), have been identified.2,3 Two major mechanisms have been reported to regulate NADPH oxidase activity (Figure): (1) Rac GTPase; and (2) p47phox phosphorylation. Rac Activation Rac, a member of the Rho-family small GTPases, plays an essential role in gp91phox/Nox2 activation. Rho GTPase can serve as a molecular switch to cycle between the GTP-bound active form that stimulates downstream effectors and the GDP-inactive form. Rac binds directly to p67phox (but not p47phox) and activates NADPH oxidase activity only in its GTP-bound active form. The importance of Rac GEFs (GDP/GTP exchange factors) such as Vav1 and Tiam1 on NADPH oxidase activation has been reported. Not only Rac/p67phox association but also Rac-mediated PAK (p21activated kinase) are also involved in NADPH oxidase activation by direct phosphorylation of p47phox.4,5 Phosphatidylinositol 3-kinase activation can regulate Rac and p67phox complex formation,6 but the involvement of Akt on NADPH oxidase activity, which is downstream of phosphatidylinositol 3-kinase, is controversial.7