Detection of hydrogen peroxide by lactoperoxidase-mediated dityrosine formation

Functional Characterization of DUOX Enzymes in Reconstituted Cell Models

Biosynthesis of active human dual oxidases (DUOX1 and DUOX2) requires maturation factors, a.k.a. DUOX activator proteins (DUOXA1 and DUOXA2), that form covalent complexes with DUOX; both chains together represent the mature catalytic unit that functions as a dedicated hydrogen peroxide-generating enzyme. Genetic defects in DUOX2 or DUOXA2 can result in congenital hypothyroidism, whereas partial defects in DUOX2 activity also have been associated with very early-onset inflammatory bowel disease. Our understanding of the links between DUOX dysfunction and these diseases remains incomplete. An important challenge in developing a better understanding of the pathogenic roles of DUOX defects requires robust and reliable DUOX reconstitution cell models to examine the functional consequences of candidate DUOX missense mutations and polymorphisms. Here, we describe methods for efficient heterologous DUOX/DUOXA co-expression and functional characterization, including detailed assessments of posttranslational processing and subcellular translocation of DUOX that accompanies the maturation of these enzymes into catalytically active NADPH oxidases.

Interaction between p22phox and Nox4 in the endoplasmic reticulum suggests a unique mechanism of NADPH oxidase complex formation

The p22^phox protein is an essential component of the phagocytic- and inner ear NADPH oxidases but its relationship to other Nox proteins is less clear. We have studied the role of p22^phox in the TGF-β1-stimulated H₂O₂ production of primary human and murine fibroblasts. TGF-β1 induced H₂O₂ release of the examined cells, and the response was dependent on the expression of both Nox4 and p22^phox. Interestingly, the p22^phox protein was present in the absence of any detectable Nox/Duox expression, and the p22^phox level was unaffected by TGF-β1. On the other hand, Nox4 expression was dependent on the presence of p22^phox, establishing an asymmetrical relationship between the two proteins. Nox4 and p22^phox proteins localized to the endoplasmic reticulum and their distribution was unaffected by TGF-β1. We used a chemically induced protein dimerization method to study the orientation of p22^phox and Nox4 in the endoplasmic reticulum membrane. This technique is based on the rapamycin-mediated heterodimerization of the mammalian FRB domain with the FK506 binding protein. The results of these experiments suggest that the enzyme complex produces H₂O₂ into the lumen of the endoplasmic reticulum, indicating that Nox4 contributes to the development of the oxidative milieu within this organelle.

NADPH oxidase 4 is an oncoprotein localized to mitochondria

Reactive oxygen species (ROS) are known to be involved in many physiological and pathological processes. Initially ROS-producing NADPH oxidase (NOX) proteins were thought to be present in phagocytes. However, recent studies have demonstrated that NOX proteins are expressed in many other cell types and tissues. NOX family members' expression and function seems to vary from tissue to tissue. We determined the expression of the NOX family of proteins (NOX1-5) in normal breast tissue and breast tumors. Our study revealed that normal breast tissues express NOX1, 4 and 5 genes. Similar pattern of expression was revealed in a breast epithelial cell line. We found that NOX4 was overexpressed in the majority of breast cancer cell lines and primary breast tumors. NOX4 was also overexpressed in ovarian tumors. Overexpression of NOX4 in normal breast epithelial cells resulted in cellular senescence, resistance to apoptosis, and tumorigenic transformation. Overexpression of NOX4 in already transformed breast tumor cells also showed increased tumorigenicity. Strong evidence suggests that regulation of these processes occurs through NOX4 generation of ROS in the mitochondria. We demonstrate that the NOX4 protein contains a 73 amino acid long mitochondrial localization signal at the N-terminus that is capable of transporting a passenger protein GFP into the mitochondria. Treatment of NOX4 overexpressing cells with catalase resulted in decreased tumorigenic characteristics. Together, this study provides evidence for an oncogenic function for NOX4 protein localized to mitochondria and suggests that NOX4 is a novel source of ROS produced in the mitochondria. This study also identifies a possible treatment of NOX4-induced breast cancer by antioxidant treatment.