Phagocytes (polymorphonuclear neutrophils, monocytes and macrophages) play an important role in innate immunity and inflammation. During phagocytosis, phagocytes are activated to produce high quantities of superoxide anion which generate other reactive oxygen species (ROS) such hydrogen peroxide, hydroxyl radical and hypochlorous acid. The enzyme responsible for superoxide anion production by neutrophils is called the phagocyte NADPH oxidase. This enzyme complex is composed of two membrane-bound proteins, p22phox and gp91phox/NOX2 and four cytosolic proteins, p47phox, p67phox, p40phox and Rac1/2. In resting cells, these proteins are distributed between the cytosol and membranes of granules and the plasma membrane. Upon activation, p47phox, p67phox, p40phox, p22phox and gp91phox are phosphorylated, Rac2 is activated and the system assembles into an active complex.
ROS production is required for host defense, as illustrated by the genetic immunodeficiency disease called chronic granulomatous disease (CGD) in which phagocytes do not produce ROS and affected patients have more fatal infections than healthy individuals. In addition to host defense, ROS production can participate to tissue injury and excessive inflammation if not well controlled. Thus, activation of the phagocyte NADPH oxidase must be tightly regulated to ensure that ROS are produced only when and where required. One mechanism by which the NADPH oxidase is regulated is the phosphorylation of p47phox. However the pathways which regulate p47phox phosphorylation are not completely known especially in monocytes. In the study published in PNAS*, we showed that p22phox interacts with the protein kinase
ROCK2 in human monocytes and in vitro. ROCK2 was not able to phosphorylate p22phox, gp91phox/NOX2, p67phox neither p40phox in vitro but was able to phosphorylate p47phox on several sites (Ser304, Ser315, Ser320, Ser328 et Ser345). Inhibition of ROCK2 activity and its expression inhibited the phosphorylation of p47phox and ROS production in monocytes. Our results suggest that p22phox is a docking site for ROCK2 which phosphorylates p47phox and thus controls ROS production by human monocytes.