Antimicrobial actions of dual oxidases and lactoperoxidase

Abstract

The NOX/DUOX family of NADPH oxidases are transmembrane proteins generating reactive oxygen species as their primary enzymatic products. NADPH oxidase (NOX) 1-5 and Dual oxidase (DUOX) 1 and 2 are members of this family. These enzymes have several biological functions including immune defense, hormone biosynthesis, fertilization, cell proliferation and differentiation, extracellular matrix formation and vascular regulation. They are found in a variety of tissues such as the airways, salivary glands, colon, thyroid gland and lymphoid organs. The discovery of NADPH oxidases has drastically transformed our view of the biology of reactive oxygen species and oxidative stress. Roles of several isoforms including DUOX1 and DUOX2 in host innate immune defense have been implicated and are still being uncovered. DUOX enzymes highly expressed in the respiratory and salivary gland epithelium have been proposed as the major sources of hydrogen peroxide supporting mucosal oxidative antimicrobial defenses. In this review, we shortly present data on DUOX discovery, structure and function, and provide a detailed, up-to-date summary of discoveries regarding antibacterial, antiviral, antifungal, and antiparasitic functions of DUOX enzymes. We also present all the literature describing the immune functions of lactoperoxidase, an enzyme working in partnership with DUOX to produce antimicrobial substances.

Keywords: DUOX; LPO; NADPH oxidase; antimicrobial; dual oxidase; lactoperoxidase.

Fig. 1.. Model of the DUOX/H₂O₂/LPO/SCN antimicrobial system in the respiratory tract.

Ciliated (C) airway epithelial cells express DUOX1/DUOXA1 and DUOX2/DUOXA2 complexes in their luminal/apical plasma membrane producing H₂O₂ into the airway surface liquid (ASL). Lactoperoxidase (LPO) uses this H₂O₂ to oxidize its main substrate, thiocyanate (SCN⁻) present in large quantities in the ASL into antimicrobial hypothiocyanite (OSCN⁻). AL, airway lumen; BC, basal cell; BV, blood vessel; GC, Goblet cell.

Fig. 2.. Submucosal tissue localization of lactoperoxidase in mouse trachea.

Trachea of 6–8 week-old C57BL/6J mice were fixed in 4% paraformalde-hyde, embedded in paraffin and subjected to immunohistochemistry to detect lactoperoxidase (LPO). Tracheal sections were blocked with Dako Protein Block (serum-free, code #X0909) before probing with primary mouse LPO antibody (Novus Biological, cat#: NBP1–87010, 1/500, rabbit) followed by horseradish peroxidase-labelled secondary anti-rabbit antibody (GBI Labs, Polymer HRP anti-rabbit IgG, cat#: D13–18). Brown staining indicative of LPO localization was developed with 3,3′-diaminoben zidine substrate (left panel). In the right panel the LPO antibody was omitted. Bar indicates 20 μm. AL, airway lumen; C, cilia; CA, cartilage; EC, epithelial cells; SM, submucosa. One representative result, n = 5.