Post-translational regulation of nadph oxidase 5 (NOX5) mediated via phosphorylation and sumoylation

Abstract

the protein: protein interactions that coordinate the activation of other Nox isoforms [4]. Instead, the primary driving force for Nox5 activity is calcium [5]. While calcium is absolutely required for Nox5 activity, discrepancies between the ~mount of calcium needed to initiate ROS production versus that measured inside cells has led to the discovery by our laboratory and others that the calcium sensitivity of Nox5 can be modified by the specific phosphorylation of serine/threonine residues in response to the protein kinase C (PKC)-agonist, PMA resulting in a sustained activation of Nox5 at resting levels of calcium [6, 7]. However, the specific kinase(s) mediating the phosphorylation and activation of Nox5 are not known and their identification was the goal of our study. Using pharmacological inhibitors, dominant negative mutants and knockdown of endogenous genes (MEKl, MEK2 and CAMKIIa) using siRNA approach, we demonstrated that MEKl/2-ERKl/2 and CAMKIIa signaling pathways can positively regulate Nox5 activity by inducing the specific phosphorylation of S498 and S475, respectively. While much attention has been given to the mechanisms that positively regulate Nox activity, little is known about mechanisms that suppress Nox function. Cellular stress arising from changes in osmotic pressure, heat, cold etc are potent stimuli for protein SUMOylation. Importantly, oxidative stress arising from increased ROS is one of the best recognized stimuli for regulating protein SUMOylation [8, 9]. Hence, we investigated whether SUMO could influence· the activity of Nox and thus limit the damaging effects of these molecules. We found that SUMO-1 and the SUMO-specific conjugating enzyme, UBC-9 potently suppressed the activity of Nox5 as well as other Nox isoforms (Noxl, 2, 3 and 4). We also found that co-expression of SUMO-1 does not result in the SUMOylation ofNox5 and that mutation of predicted sites of SUMOylation and conserved lysines on Nox5 failed to prevent the SUMO-I driven inhibition of ROS production. In summary, we have identified the expression ofNox5 and more specifically the and p splice variants in human blood vessels and tissues. Our data suggest that Nox5 a and p are the only variants capable of producing ROS in human blood vessels, but also that the inactive variants can function as dominant negatives. Additionally, we have shown that MAPK and CAMKIIa signaling pathways positively regulate Nox5 activity via changes in phosphorylation whereas SUMO-I negatively regulates activity through a yet to be defined mechanism.