NADPH oxidases: an overview from structure to innate immunity-associated pathologies

Abstract

Oxygen-derived free radicals, collectively termed reactive oxygen species (ROS), play important roles in immunity, cell growth, and cell signaling. In excess, however, ROS are lethal to cells, and the overproduction of these molecules leads to a myriad of devastating diseases. The key producers of ROS in many cells are the NOX family of NADPH oxidases, of which there are seven members, with various tissue distributions and activation mechanisms. NADPH oxidase is a multisubunit enzyme comprising membrane and cytosolic components, which actively communicate during the host responses to a wide variety of stimuli, including viral and bacterial infections. This enzymatic complex has been implicated in many functions ranging from host defense to cellular signaling and the regulation of gene expression. NOX deficiency might lead to immunosuppression, while the intracellular accumulation of ROS results in the inhibition of viral propagation and apoptosis. However, excess ROS production causes cellular stress, leading to various lethal diseases, including autoimmune diseases and cancer. During the later stages of injury, NOX promotes tissue repair through the induction of angiogenesis and cell proliferation. Therefore, a complete understanding of the function of NOX is important to direct the role of this enzyme towards host defense and tissue repair or increase resistance to stress in a timely and disease-specific manner.

Figure 1

Assembly and activation of NOX enzymes: NOX1 and NOX2 activation involves the phosphorylation of NOXO1 and p47^phox, respectively, the translocation of the entire multidomain complex, including p40^phox, p67^phox and Rac from the cytosol to the membrane and the transfer of electrons from the substrate to oxygen. Similar to NOX1 and NOX2, NOX3 is p22^phox-dependent, but it does not bind to Rac. NOX4 activation involves p22^phox and Poldip2. In contrast, NOX5 and Duox activation is calcium-dependent. Duox proteins isolated from the thyroid gland possess a peroxidase-like domain. The mature form of Duox generates hydrogen peroxide, which may be the ultimate end product, likely reflecting the rapid dismutation of superoxide. NOX, NADPH oxidase.

Figure 2

The NOX system is central to host defense. When induced through various triggering elements (top-left block), NOX family members become activated, producing ROS (central block). The block arrows represent the main signal pathway. While ROS directly kills microorganisms, additional downstream cellular factors work in an orchestrated manner to mount pressure against the invading pathogens or the cells harboring these microbes (right block). However, when non-pathogenic inducers, such as cigarette smoke or alcohol, induce the NOX system, ROS exerts damaging effects on the host, causing pathogenesis. The signaling mechanisms illustrated in this figure include the induction of TLR4-mediated signaling through cigarette smoke and LPS, leading to the activation of NOX4. TLR5 recognizes flagellin and activates NOX1 and Duox2. Virus-like HCV also activates NOX4, leading to ROS production. TLR5 and NOD2, when activated through flagellin or peptidoglycan, respectively, activate Duox2, leading to NLRP3 binding and activation. Furthermore, the NLRP3-mediated induction of caspase-1 activity leads to the maturation of pro-IL-1β and pro-IL-18, promoting the influx and activation of lymphocytes. ROS, produced through NOX, activates NF-κB through various intermediates, including PKC, MEKK and NIK, thereby inducing the upregulation of IL-8, ICAM-1 and VCAM-1 and promoting the lymphocyte influx important for host defense/pathogenesis. HCV, hepatitis C virus; ICAM-1, intercellular adhesion molecule-1; MEKK, MEK kinase; NF-κB, nuclear factor kappa B; NIK, NF-κB-inducing kinase; NOX, NADPH oxidase; PRC, protein kinase C; ROS, reactive oxygen species; TLR, Toll-like receptor; VCAM-1, vascular cell adhesion molecule-1.