The NADPH oxidase Nox3 constitutively produces superoxide in a p22 ^phox-dependent manner: Its regulation by oxidase organizers and activators

Nox3, a member of the superoxide-producing NADPH oxidase (Nox) family, participates in otoconia formation in mouse inner ears, which is required for perception of balance and gravity. The activity of other Nox enzymes such as gp91^phox/Nox2 and Nox1 is known to absolutely require both an organizer protein (p47^phox or Noxo1) and an activator protein (p67^phox or Noxa1); for the p47^phox-dependent activation of these oxidases, treatment of cells with stimulants such as phorbol 12-myristate 13-acetate is also indispensable. Here we show that ectopic expression of Nox3 in various types of cells leads to phorbol 12-myristate 13-acetate-independent constitutive production of a substantial amount of superoxide under the conditions where gp91^phox and Nox1 fail to generate superoxide, i.e. in the absence of the oxidase organizers and activators. Nox3 likely forms a functional complex with p22^phox; Nox3 physically interacts with and stabilizes p22^phox, and the Nox3-dependent superoxide production is totally dependent on p22^phox. The organizers p47^phox and Noxo1 are capable of enhancing the superoxide production by Nox3 in the absence of the activators, and the enhancement requires the interaction of the organizers with p22^phox, further indicating a link between Nox3 and p22^phox. The p47 ^phox-enhanced Nox3 activity is further facilitated by p67 ^phox or Noxa1, whereas the activators cancel the Noxo1-induced enhancement. On the other hand, the small GTPase Rac, essential for the gp91^phox activity, is likely dispensable to the Nox3 system. Thus Nox3 functions together with p22^phox as an enzyme constitutively producing superoxide, which can be distinctly regulated by combinatorial use of the organizers and activators.