Abstract
NOX/DUOX enzymes are transmembrane proteins that carry electrons through biological membranes generating reactive oxygen species. The NOX family is composed of seven members, which are NOX1 to NOX5 and DUOX1 and 2. DUOX enzymes were initially called thyroid oxidases, based on their high expression level in the thyroid tissue. However, DUOX expression has been documented in several extrathyroid tissues, mostly at the apical membrane of the salivary glands, the airways, and the intestinal tract, revealing additional cellular functions associated with DUOX-related H2O2 generation. In this review, we will briefly summarize the current knowledge regarding DUOX structure and physiological functions, as well as their possible role in cancer biology.
Highlights
Reactive oxygen species (ROS) comprise a large group of radicals and non-radical molecules derived from molecular oxygen (O2)
The DUOX1 gene is at the same locus as DUOX2, and it encodes a protein of 1551 amino acids, which shares with DUOX2 more than 77% sequence identity at the amino acid level (Carvalho and Dupuy, 2017)
The high levels of DUOX2 in gastric cancer were significantly associated with smoking history, while its protein expression levels in colorectal cancers (CRC) were higher in stages II-IV than in stage I (Qi et al, 2016)
Summary
Reactive oxygen species (ROS) comprise a large group of radicals and non-radical molecules derived from molecular oxygen (O2). The DUOX1 gene is at the same locus as DUOX2, and it encodes a protein of 1551 amino acids, which shares with DUOX2 more than 77% sequence identity at the amino acid level (Carvalho and Dupuy, 2017) Both DUOXs have seven transmembrane domains, with two calciumbinding sites in its large intracellular loop, which are located between the first two transmembrane segments. DUOX enzymes are transmembrane proteins, and they are fully active only at the apical plasma membrane. DUOX expression has since been documented in several extrathyroid tissues, mostly at the apical cell membrane of the salivary glands, the airways, and the intestinal tract, revealing additional cellular functions associated with DUOX-related H2O2 generation (Geiszt et al, 2003). The presence of DUOX enzymes in secretory glands and on mucosal surfaces, such as salivary gland, rectum, trachea, and bronchium led to their identification as a source of H2O2, supporting lactoperoxidase (LPO)-catalyzed oxidation of thiocyanate (SCN-) or iodide (I-) to form secondary oxidants, like hypothiocyanous acid (HOSCH) and hypoio-
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