Cloning of two human thyroid cDNAs encoding new members of the NADPH oxidase family

Sexual dimorphism of thyroid reactive oxygen species production due to higher NADPH oxidase 4 expression in female thyroid glands

BACKGROUND

Dual oxidases (DUOX1 and DUOX2) are NADPH oxidases (NOX) involved in hydrogen peroxide production necessary for thyroid hormonogenesis, but recently, the NOX4 has also been described in the thyroid gland. The prevalence of thyroid disease is higher in women, and the basis for this difference might involve a higher oxidative stress level in the female thyroid gland. Hence, we aimed at evaluating whether the function and the expression of enzymes involved in the thyroid redox balance differ between females and males.

METHODS

DUOX1, DUOX2, NOX4, glutathione peroxidase (GPx), and catalase activities and expression levels were evaluated in the thyroids of prepubertal and adult male and female rats. The mRNA levels of DUOXA1 and DUOXA2, the DUOX maturation factors, and of p22phox and Poldip2 (subunits of NOX4) were also determined.

RESULTS

A higher calcium-independent H(2)O(2) production was detected in the adult female rat thyroid, being higher in the estrous phase of the cycle. Moreover, the expression of NOX4 and Poldip2 mRNA was higher in the thyroids of adult female rats, as well as in PCCL3 cells treated with 17β-estradiol. The GPx1 mRNA expression was higher in adult female thyroids, while GPx2 and GPx3 mRNA and total GPx activity were not significantly different. Catalase mRNA expression and activity, together with thyroid thiol levels were significantly lower in the adult female rat thyroid.

CONCLUSIONS

Taken together, our results show that the thyroid gland of female rats is exposed to higher oxidative stress levels due both to increased reactive oxygen species (ROS) production through NOX4, and decreased ROS degradation.

Expression of dual oxidases and secreted cytokines in chronic rhinosinusitis

BACKGROUND

The airway epithelium generates reactive oxygen species (ROS) as a first line of defense. Dual oxidases (DUOX1 and DUOX2) are the H2 O2 -producing isoforms of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family in the airway epithelium. The purpose of this study was to explore the molecular expression, function, and regulation of DUOXs in chronic rhinosinusitis (CRS).

METHODS

Human nasal tissue samples and nasal secretions were collected from 3 groups of patients undergoing sinus surgery (normal, n = 7; CRS with polyposis [CRSwP], n = 6; CRS without polyposis [CRSsP], n = 6). Nasal secretions were studied for cytokine and H2 O2 content. Tissue samples were used to determine DUOX mRNA and protein expression.

RESULTS

DUOX1 mRNA level (80.7 ± 60.5) was significantly increased in CRSwP compared to normal (2.7 ± 1.2) and CRSsP (2.3 ± 0.5, p = 0.042). DUOX2 mRNA levels were increased in both CRSwP (18.6 ± 9.9) and CRSsP (4.0 ± 1.3) compared to normal (1.1 ± 0.3; p = 0.008). DUOX protein was found in the apical portion of the nasal epithelium and protein expression was increased in CRSwP and CRSsP. H2 O2 production was significantly higher in CRSwP (160.9 ± 59.4 nM) and CRSsP (81.7 ± 5.6 nM) compared to normal (53.5 ± 11.5 nM, p = 0.032). H2 O2 content of nasal secretions correlated tightly with DUOX expression (p < 0.001). Cytokines (eotaxin, monokine-induced by interferon γ [MIG], tumor necrosis factor [TNF]-α, interleukin [IL]-8) showed significantly higher levels in nasal secretions from CRSwP compared to normal (p < 0.05). Levels of eotaxin, MIG, and TNF-α correlated closely with DUOX expression.

CONCLUSION

DUOX1 and DUOX2 were identified as factors upregulated in CRS. Close correlations between DUOX expression and H2 O2 release, and correlation between key inflammatory cytokines and DUOX expression, indicate DUOX in the inflammatory response in CRS.

Dual oxidase-1 is required for airway epithelial cell migration and bronchiolar reepithelialization after injury

The respiratory epithelium plays a critical role in innate defenses against airborne pathogens and pollutants, and alterations in epithelial homeostasis and repair mechanisms are thought to contribute to chronic lung diseases associated with airway remodeling. Previous studies implicated the nicotinamide adenine dinucleotide phosphate-reduced oxidase dual oxidase-1 (DUOX1) in redox signaling pathways involved in in vitro epithelial wound responses to infection and injury. However, the importance of epithelial DUOX1 in in vivo epithelial repair pathways has not been established. Using small interfering (si)RNA silencing of DUOX1 expression, we show the critical importance of DUOX1 in wound responses in murine tracheal epithelial (MTE) cells in vitro, as well as its contribution to epithelial regeneration in vivo in a murine model of epithelial injury induced by naphthalene, a selective toxicant of nonciliated respiratory epithelial cells (club cells [Clara]). Whereas naphthalene-induced club-cell injury is normally followed by epithelial regeneration after 7 and 14 days, such airway reepithelialization was significantly delayed after the silencing of airway DUOX1 by oropharyngeal administration of DUOX1-targeted siRNA. Wound closure in MTE cells was related to DUOX1-dependent activation of the epidermal growth factor receptor (EGFR) and the transcription factor signal transducer and activator of transcription-3 (STAT3), known mediators of epithelial cell migration and wound responses. Moreover, in vivo DUOX1 silencing significantly suppressed naphthalene-induced activation of STAT3 and EGFR during early stages of epithelial repair. In conclusion, these experiments demonstrate for the first time an important function for epithelial DUOX1 in lung epithelial regeneration in vivo, by promoting EGFR-STAT3 signaling and cell migration as critical events in initial repair.

Experimental evidence validating the computational inference of functional associations from gene fusion events: a critical survey

More than a decade ago, a number of methods were proposed for the inference of protein interactions, using whole-genome information from gene clusters, gene fusions and phylogenetic profiles. This structural and evolutionary view of entire genomes has provided a valuable approach for the functional characterization of proteins, especially those without sequence similarity to proteins of known function. Furthermore, this view has raised the real possibility to detect functional associations of genes and their corresponding proteins for any entire genome sequence. Yet, despite these exciting developments, there have been relatively few cases of real use of these methods outside the computational biology field, as reflected from citation analysis. These methods have the potential to be used in high-throughput experimental settings in functional genomics and proteomics to validate results with very high accuracy and good coverage. In this critical survey, we provide a comprehensive overview of 30 most prominent examples of single pairwise protein interaction cases in small-scale studies, where protein interactions have either been detected by gene fusion or yielded additional, corroborating evidence from biochemical observations. Our conclusion is that with the derivation of a validated gold-standard corpus and better data integration with big experiments, gene fusion detection can truly become a valuable tool for large-scale experimental biology.

Redox regulation of epidermal growth factor receptor signaling through cysteine oxidation

Epidermal growth factor receptor (EGFR) exemplifies the family of receptor tyrosine kinases that mediate numerous cellular processes, including growth, proliferation, and differentiation. Moreover, gene amplification and EGFR mutations have been identified in a number of human malignancies, making this receptor an important target for the development of anticancer drugs. In addition to ligand-dependent activation and concomitant tyrosine phosphorylation, EGFR stimulation results in the localized generation of H(2)O(2) by NADPH-dependent oxidases. In turn, H(2)O(2) functions as a secondary messenger to regulate intracellular signaling cascades, largely through the modification of specific cysteine residues within redox-sensitive protein targets, including Cys797 in the EGFR active site. In this review, we highlight recent advances in our understanding of the mechanisms that underlie redox regulation of EGFR signaling and how these discoveries may form the basis for the development of new therapeutic strategies for targeting this and other H(2)O(2)-modulated pathways.

Disruption of the melanin-concentrating hormone receptor 1 (MCH1R) affects thyroid function

Melanin-concentrating hormone (MCH) is a peptide produced in the hypothalamus and the zona incerta that acts on one receptor, MCH receptor 1 (MCH1R), in rodents. The MCH system has been implicated in the regulation of several centrally directed physiological responses, including the hypothalamus-pituitary-thyroid axis. Yet a possible direct effect of the MCH system on thyroid function has not been explored in detail. We now show that MCH1R mRNA is expressed in thyroid follicular cells and that mice lacking MCH1R [MCH1R-knockout (KO)] exhibit reduced circulating iodothyronine (T(4), free T(4), T(3), and rT(3)) levels and high TRH and TSH when compared with wild-type (WT) mice. Because the TSH of MCH1R-KO mice displays a normal bioactivity, we hypothesize that their hypothyroidism may be caused by defective thyroid function. Yet expression levels of the genes important for thyroid hormones synthesis or secretion are not different between the MCH1R-KO and WT mice. However, the average thyroid follicle size of the MCH1R-KO mice is larger than that of WT mice and contained more free and total T(4) and T(3) than the WT glands, suggesting that they are sequestered in the glands. Indeed, when challenged with TSH, the thyroids of MCH1R-KO mice secrete lower amounts of T(4). Similarly, secretion of iodothyronines in the plasma upon (125)I administration is significantly reduced in MCH1R-KO mice. Therefore, the absence of MCH1R affects thyroid function by disrupting thyroid hormone secretion. To our knowledge, this study is the first to link the activity of the MCH system to the thyroid function.

Regulation of dual oxidase expression and H2O2 production by thyroglobulin

BACKGROUND

Thyroglobulin (Tg) is a macromolecular precursor in thyroid hormone synthesis to which iodine is stably bound. Tg, which is stored in the follicular space, is also a potent negative feedback regulator of follicular function, and this is achieved by suppressing mRNA levels of thyroid-specific genes such as the sodium/iodide symporter (Slc5a5), Tg, and thyroid peroxidase. Dual oxidase 1 (DUOX1) and DUOX2, originally identified in the thyroid, are nicotinamide adenine dinucleotide phosphate (NADPH) oxidases that are necessary to produce the H2O2 required for thyroid hormone biosynthesis. Since follicular Tg regulates the expression of genes that are essential for thyroid hormone synthesis, we hypothesized that Tg might also regulate DUOX expression and H2O2 production.

METHODS

Rat thyroid FRTL-5 cells were treated with Tg, and the mRNA expression of Duox1 and Duox2 and their corresponding maturation factors Duoxa1 and Duoxa2 were evaluated by DNA microarray and real-time PCR. Duox2 promoter activity was examined by luciferase reporter gene assay. Protein levels of DUOX2 were also examined by Western blot analysis. Intracellular H2O2 generation was quantified by a fluorescent dye, 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, and acetyl ester (CM-H2DCFDA).

RESULTS

mRNA levels of Duox2 and its activation factor Duoxa2 (but not Duox1 or Duoxa1) were significantly suppressed by Tg in a dose-dependent manner and a time-dependent fashion in rat thyroid FRTL-5 cells. DUOX2 promoter activity was significantly suppressed by Tg in a dose-dependent manner. Protein levels of DUOX2 and H2O2 generation in cells were also reduced by Tg treatment.

CONCLUSIONS

We show that physiological concentrations of Tg suppressed the expression and function of DUOX2 in thyroid cells. These results suggest that Tg is a strong suppressor of the expression and the activity of DUOX2/DUOXA2, thereby regulating iodide organification and hormone synthesis in the thyroid. The evidence supports a reported model in which accumulated Tg in thyroid follicles plays important roles in autoregulating the function of individual follicles, which produces the basis of follicular heterogeneity.

Inflammation and oxidative stress induced by cigarette smoke in Lewis rat brains

Exposure to cigarette smoke has been associated with an increased risk of neurological diseases such as stroke, Alzheimer's disease and multiple sclerosis. In these studies, serum and brain sections from Lewis rats or those exposed to cigarette smoke and control rats were examined for evidence of increased inflammation and oxidative stress. Immunocytochemical staining of brain sections from CS-exposed rats showed increased expression of class II MHC and, in ELISA, levels of IFN-gamma and TNF-α were higher than for non-exposed rats. In polymerase chain reaction assays there was increased interferon-gamma, TNF-α, IL-1α, IL-1β, IL-23, IL-6, IL-23, IL-17, IL-10, TGF-β, T-bet and FoxP3 gene expression with CS exposure. There was also markedly elevated MIP-1α/CCL3, less prominent MCP-1/CCL2 and no elevation of SDF-1α gene expression. Analysis of samples from CS-exposed and control rats for anti-oxidant expression showed no significant difference in serum levels of glutathione and, in brain, similar levels of superoxide dismutase and decreased thioredoxin gene expression. In contrast, there was increased brain gene expression for the pro-oxidants iNOS and the NADPH components NOX4, dual oxidase 1 and p22(phox). Nrf2 expression, which is typically triggered as a secondary response to oxidative stress, was also increased in brains from CS-exposed rats with nuclear translocation of this protein from cytoplasm demonstrated in astrocytes in association with increased expression of the aryl hydrocarbon receptor gene, an Nrf2 target. These studies, therefore, demonstrate that CS exposure in these animals can trigger multiple immune and oxidative responses that may have important roles in the pathogenesis of CNS inflammatory neurological diseases.

Tetraspanin is required for generation of reactive oxygen species by the dual oxidase system in Caenorhabditis elegans

Reactive oxygen species (ROS) are toxic but essential molecules responsible for host defense and cellular signaling. Conserved NADPH oxidase (NOX) family enzymes direct the regulated production of ROS. Hydrogen peroxide (H₂O₂) generated by dual oxidases (DUOXs), a member of the NOX family, is crucial for innate mucosal immunity. In addition, H₂O₂ is required for cellular signaling mediated by protein modifications, such as the thyroid hormone biosynthetic pathway in mammals. In contrast to other NOX isozymes, the regulatory mechanisms of DUOX activity are less understood. Using Caenorhabditis elegans as a model, we demonstrate that the tetraspanin protein is required for induction of the DUOX signaling pathway in conjunction with the dual oxidase maturation factor (DUOXA). In the current study, we show that genetic mutation of DUOX (bli-3), DUOXA (doxa-1), and peroxidase (mlt-7) in C. elegans causes the same defects as a tetraspanin tsp-15 mutant, represented by exoskeletal deficiencies due to the failure of tyrosine cross-linking of collagen. The deficiency in the tsp-15 mutant was restored by co-expression of bli-3 and doxa-1, indicating the involvement of tsp-15 in the generation of ROS. H₂O₂ generation by BLI-3 was completely dependent on TSP-15 when reconstituted in mammalian cells. We also demonstrated that TSP-15, BLI-3, and DOXA-1 form complexes in vitro and in vivo. Cell-fusion-based analysis suggested that association with TSP-15 at the cell surface is crucial for BLI-3 activation to release H₂O₂. This study provides the first evidence for an essential role of tetraspanin in ROS generation.

ROS are highly reactive molecules, which can be inappropriately produced during aerobic metabolism or by exogenous stresses such as exposure to UV light and radiation. ROS interact with cellular components including nucleic acids, lipids, and proteins and irreversibly inhibit their functions. However, ROS are essential for innate host defense and multiple physiological processes and are generated by conserved NADPH oxidase (NOX) family enzymes. The release of ROS by ROS generator enzymes must be properly controlled, as chronic oxidative stress can cause an imbalance of the redox state and is often associated with disease and aging. Using C. elegans as a model, we identified a tetraspanin (TSP-15) protein as a new key component of the ROS generation system controlled by dual oxidase (BLI-3), a unique NOX isozyme in C. elegans. Mutants of both bli-3 and tsp-15 developed the same defects in extracellular matrix cross-linking. Using a combination of genetics and reconstitution experiments in mammalian cells, we have demonstrated a novel requirement of tetraspanin for dual oxidase-dependent ROS generation via complex formation at the cell surface.

The type of DUOX-dependent ROS production is dictated by defined sequences in DUOXA

A deliberate generation of ROS is now recognized to be achieved by specific NADPH oxidases (NOX). Dual oxidases (DUOXs) are Ca(2+)-activated NOXs and operate as H(2)O(2)-generators in various tissues. A tight regulation is however required to avoid ROS overproduction that can rapidly be harmful to biological systems. DUOX activator (DUOXA) proteins act as organizing elements for surface expression and activity of the DUOX enzymes. To study DUOX activation by the maturation factors, chimeric DUOXA proteins were generated by replacing particular domains between DUOXA1 and DUOXA2. Their impact on DUOX function and membrane expression were explored in a reconstituted heterologous cell system composed of COS-7 cells. We have shown that the COOH-terminal end of DUOXA1 is responsible for DUOX1-dependent H(2)O(2) generation. The NH(2)-terminal tail of DUOXA2 is critical to specify the type of ROS released by DUOX2, hydrogen peroxide or superoxide. Native DUOXA2 would constrain DUOX2 to produce H(2)O(2). However, alterations of the DUOXA2 NH(2)-terminal domain modify DUOX2 activity triggering superoxide leaking. Our results demonstrate that specific domains of the DUOX maturation factors promote the activation of DUOXs as well as the type of ROS generated by the oxidases.

Implication of NADPH oxidases in the early inflammation process generated by cystic fibrosis cells

In cystic fibrosis (CF) patients, pulmonary inflammation is a major cause of morbidity and mortality. The aim of this study was to further investigate whether oxidative stress could be involved in the early inflammatory process associated with CF pathogenesis. We used a model of CFTR defective epithelial cell line (IB3-1) and its reconstituted CFTR control (S9) cell line cultured in various ionic conditions. This study showed that IB3-1 and S9 cells expressed the NADPH oxidases (NOXs) DUOX1/2 and NOX2 at the same level. Nevertheless, several parameters participating in oxidative stress (increased ROS production and apoptosis, decreased total thiol content) were observed in IB3-1 cells cultured in hypertonic environment as compared to S9 cells and were inhibited by diphenyleneiodonium (DPI), a well-known inhibitor of NOXs; besides, increased production of the proinflammatory cytokines IL-6 and IL-8 by IB3-1 cells was also inhibited by DPI as compared to S9 cells. Furthermore, calcium ionophore (A23187), which upregulates DUOX and NOX2 activities, strongly induced oxidative stress and IL-8 and IL-6 overexpression in IB3-1 cells. All these events were suppressed by DPI, supporting the involvement of NOXs in the oxidative stress, which can upregulate proinflammatory cytokine production by the airway CFTR-deficient cells and trigger early pulmonary inflammation in CF patients.

Towards specific NADPH oxidase inhibition by small synthetic peptides

Reactive oxygen species (ROS) production by the phagocyte NADPH oxidase is essential for host defenses against pathogens. ROS are very reactive with biological molecules such as lipids, proteins and DNA, potentially resulting in cell dysfunction and tissue insult. Excessive NADPH oxidase activation and ROS overproduction are believed to participate in disorders such as joint, lung, vascular and intestinal inflammation. NADPH oxidase is a complex enzyme composed of six proteins: gp91phox (renamed NOX2), p22phox, p47phox, p67phox, p40phox and Rac1/2. Inhibitors of this enzyme could be beneficial, by limiting ROS production and inappropriate inflammation. A few small non-peptide inhibitors of NADPH oxidase are currently used to inhibit ROS production, but they lack specificity as they inhibit NADPH oxidase homologues or other unrelated enzymes. Peptide inhibitors that target a specific sequence of NADPH oxidase components could be more specific than small molecules. Here we review peptide-based inhibitors, with particular focus on a molecule derived from gp91phox/NOX2 and p47phox, and discuss their possible use as specific phagocyte NADPH oxidase inhibitors.

NADPH oxidases as regulators of tumor angiogenesis: current and emerging concepts

SIGNIFICANCE

Reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and peroxynitrite are generated ubiquitously by all mammalian cells and have been understood for many decades as inflicting cell damage and as causing cancer by oxidation and nitration of macromolecules, including DNA, RNA, proteins, and lipids.

RECENT ADVANCES

A current concept suggests that ROS can also promote cell signaling pathways triggered by growth factors and transcription factors that ultimately regulate cell proliferation, differentiation, and apoptosis, all of which are important hallmarks of tumor cell proliferation and angiogenesis. Moreover, an emerging concept indicates that ROS regulate the functions of immune cells that infiltrate the tumor environment and stimulate angiogenesis, such as macrophages and specific regulatory T cells.

CRITICAL ISSUES

In this article, we highlight that the NADPH oxidase family of ROS-generating enzymes are the key sources of ROS and, thus, play an important role in redox signaling within tumor, endothelial, and immune cells thereby promoting tumor angiogenesis.

FUTURE DIRECTIONS

Knowledge of these intricate ROS signaling pathways and identification of the culprit NADPH oxidases is likely to reveal novel therapeutic opportunities to prevent angiogenesis that occurs during cancer and which is responsible for the revascularization after current antiangiogenic treatment.

NADPH oxidase 1-mediated oxidative stress leads to dopamine neuron death in Parkinson's disease

AIM

Oxidative stress has long been considered as a major contributing factor in the pathogenesis of Parkinson's disease. However, molecular sources for reactive oxygen species in Parkinson's disease have not been clearly elucidated. Herein, we sought to investigate whether a superoxide-producing NADPH oxidases (NOXs) are implicated in oxidative stress-mediated dopaminergic neuronal degeneration.

RESULTS

Expression of various Nox isoforms and cytoplasmic components were investigated in N27, rat dopaminergic cells. While most of Nox isoforms were constitutively expressed, Nox1 expression was significantly increased after treatment with 6-hydroxydopamine. Rac1, a key regulator in the Nox1 system, was also activated. Striatal injection of 6-hydroxydopamine increased Nox1 expression in dopaminergic neurons in the rat substantia nigra. Interestingly, it was localized into the nucleus, and immunostaining for DNA oxidative stress marker, 8-oxo-dG, was increased. Nox1 expression was also found in the nucleus of dopaminergic neurons in the substantia nigra of Parkinson's disease patients. Adeno-associated virus-mediated Nox1 knockdown or Rac1 inhibition reduced 6-hydroxydopamine-induced oxidative DNA damage and dopaminergic neuronal degeneration significantly.

INNOVATION

Nox1/Rac1 could serve as a potential therapeutic target for Parkinson's disease.

CONCLUSION

We provide evidence that dopaminergic neurons are equipped with the Nox1/Rac1 superoxide-generating system. Stress-induced Nox1/Rac1 activation causes oxidative DNA damage and neurodegeneration. Reduced dopaminergic neuronal death achieved by targeting Nox1/Rac1, emphasizes the impact of oxidative stress caused by this system on the pathogenesis and therapy in Parkinson's disease.

NADPH oxidase as an important source of reactive oxygen species at the mouse maternal-fetal interface: putative biological roles

Oxygen derivatives that comprise the large family of reactive oxygen species (ROS) are actively involved in placental biology. They are generated at the maternal-fetal interface at the level of decidual, trophoblast and mesenchymal components. In normal conditions, ROS produced in low concentrations participate in different functions as signalling molecules, regulating activation of redox-sensitive transcription factors and protein kinases involved in cell survival, proliferation and apoptosis, hence much of cell functioning. Physiological ROS generation is also associated with such defence mechanisms as phagocytosis and microbiocidal activities. In mice, particularly but not exclusively, trophoblast cells phagocytose intensively during implantation and post-implantation periods and express enzymic machinery to address a ROS-producing response to changes in the environment. The cells directly associated with ROS production are trophoblast giant cells, which mediate each and every relationship with the maternal organism. In this review, the production of ROS by the implanting mouse trophoblast is discussed, focusing on NADPH oxidase expression, regulatory mechanisms and similarities with NOX2 from phagocytes. Some of the current controversies are assessed by attempting to integrate data from studies in human trophoblast and mouse models.

Molecular cloning and characterisation of ovine dual oxidase 2

The dual oxidases (DUOX1 and DUOX2) are NADPH-dependent hydrogen peroxide-producing enzymes that are reported to function in a physiological capacity and as a component of the mucosal immune response. We have previously reported increased expression of the DUOX2 gene in the gut mucosa of sheep in response to gastrointestinal nematode (GIN) challenge. In this paper, we report the cloning of the full-length ovine DUOX2 transcript, using a PCR based strategy. The ovine DUOX2 transcript includes an ORF of 4644 bases, and encodes a protein with 97% identity to the bovine sequence. We also cloned a fragment of DUOX1 (encompassing nucleotides 2692-2829), and the proximal promoter sequence of DUOX2. Through analysis of sequence data we have confirmed that DUOX1 and DUOX2 are co-located in a head to tail arrangement conserved across many species. Alignment of the sequences to the ovine genome predicts a location of this gene cluster on ovine chromosome 7. We quantified the expression of ovine DUOX1 and DUOX2 transcripts in 24 different sheep tissues, and discovered tissue specific expression signatures. DUOX2 was found to be most highly expressed in tissues of the gastrointestinal tract, while expression of DUOX1 predominated in the bladder. Rapid amplification of cDNA ends (RACE) analysis identified the existence of multiple 5' UTR variants in DUOX2, ranging in size from 32 to 242 nucleotides, with 3 distinct transcribed regions. Real time PCR quantification of the DUOX2 UTR variants revealed that these were differentially expressed between tissues, and at various stages of the response to GIN parasite infection. The collective evidence suggested a complex regulation of DUOX2, prompting a bioinformatic analysis of the proximal promoter regions of ovine DUOX2 to identify potential transcription factor binding sites (TFBS) that may explain the differences in the observed expression of the transcript variants of DUOX2. Possible transcription factor families that may regulate this process were identified as Kruppel-like factors (KLF), ETS-factors, erythroid growth receptor factors (EGRF) and myogenic differentiation factors (MYOD).

NADPH oxidase-derived ROS and the regulation of pulmonary vessel tone

Pulmonary vessel constriction results from an imbalance between vasodilator and vasoconstrictor factors released by the endothelium including nitric oxide, endothelin, prostanoids, and reactive oxygen species (ROS). ROS, generated by a variety of enzymatic sources (such as mitochondria and NADPH oxidases, a.k.a. Nox), appear to play a pivotal role in vascular homeostasis, whereas elevated levels effect vascular disease. The pulmonary circulation is very sensitive to changes in the partial pressure of oxygen and differs from the systemic circulation in its response to this change. In fact, the pulmonary vessels contract in response to low oxygen tension, whereas systemic vessels dilate. Growing evidence suggests that ROS production and ROS-related pathways may be key factors that underlie this differential response to oxygen tension. A major emphasis of our laboratory is the role of Nox isozymes in cardiovascular disease. In this review, we will focus our attention on the role of Nox-derived ROS in the control of pulmonary vascular tone.

Normoxic cyclic GMP-independent oxidative signaling by nitrite enhances airway epithelial cell proliferation and wound healing

The airway epithelium provides important barrier and host defense functions. Recent studies reveal that nitrite is an endocrine reservoir of nitric oxide (NO) bioactivity that is converted to NO by enzymatic reductases along the physiological oxygen gradient. Nitrite signaling has been described as NO dependent activation mediated by reactions with deoxygenated redox active hemoproteins, such as hemoglobin, myoglobin, neuroglobin, xanthine oxidoreductase (XO) and NO synthase at low pH and oxygen tension. However, nitrite can also be readily oxidized to nitrogen dioxide (NO(2)·) via heme peroxidase reactions, suggesting the existence of alternative oxidative signaling pathways for nitrite under normoxic conditions. In the present study, we examined normoxic signaling effects of sodium nitrite on airway epithelial cell wound healing. In an in vitro scratch injury model under normoxia, we exposed cultured monolayers of human airway epithelial cells to various concentrations of sodium nitrite and compared responses to NO donor. We found sodium nitrite potently enhanced airway epithelium wound healing at physiological concentrations (from 1 μM). The effect of nitrite was blocked by the NO and NO(2)· scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO). Interestingly, nitrite treatment did not increase cyclic guanosine monophosphate (cGMP) levels under these normoxic conditions, even in the presence of a phosphodiesterase 5 inhibitor, suggesting cGMP independent signaling. Consistent with an oxidative signaling pathway requiring hydrogen peroxide (H(2)O(2))/heme-peroxidase/NO(2)· signaling, the effects of nitrite were potentiated by superoxide dismutase (SOD) and low concentration H(2)O(2), whereas inhibited completely by catalase, followed by downstream extracellular-signal-regulated kinase (ERK) 1/2 activation. Our data represent the first description of normoxic nitrite signaling on lung epithelial cell proliferation and wound healing and suggest novel oxidative signaling pathways involving nitrite-H(2)O(2) reactions, possibly via the intermediary, NO(2)·.

Targeting NADPH oxidases in vascular pharmacology

Oxidative stress is a molecular dysregulation in reactive oxygen species (ROS) metabolism, which plays a key role in the pathogenesis of atherosclerosis, vascular inflammation and endothelial dysfunction. It is characterized by a loss of nitric oxide (NO) bioavailability. Large clinical trials such as HOPE and HPS have not shown a clinical benefit of antioxidant vitamin C or vitamin E treatment, putting into question the role of oxidative stress in cardiovascular disease. A change in the understanding of the molecular nature of oxidative stress has been driven by the results of these trials. Oxidative stress is no longer perceived as a simple imbalance between the production and scavenging of ROS, but as a dysfunction of enzymes involved in ROS production. NADPH oxidases are at the center of these events, underlying the dysfunction of other oxidases including eNOS uncoupling, xanthine oxidase and mitochondrial dysfunction. Thus NADPH oxidases are important therapeutic targets. Indeed, HMG-CoA reductase inhibitors (statins) as well as drugs interfering with the renin-angiotensin-aldosterone system inhibit NADPH oxidase activation and expression. Angiotensin-converting enzyme (ACE) inhibitors, AT1 receptor antagonists (sartans) and aliskiren, as well as spironolactone or eplerenone, have been discussed. Molecular aspects of NADPH oxidase regulation must be considered, while thinking about novel pharmacological targeting of this family of enzymes consisting of several homologs Nox1, Nox2, Nox3, Nox4 and Nox5 in humans. In order to properly design trials of antioxidant therapies, we must develop reliable techniques for the assessment of local and systemic oxidative stress. Classical antioxidants could be combined with novel oxidase inhibitors. In this review, we discuss NADPH oxidase inhibitors such as VAS2870, VAS3947, GK-136901, S17834 or plumbagin. Therefore, our efforts must focus on generating small molecular weight inhibitors of NADPH oxidases, allowing the selective inhibition of dysfunctional NADPH oxidase homologs. This appears to be the most reasonable approach, potentially much more efficient than non-selective scavenging of all ROS by the administration of antioxidants.

NOX5: from basic biology to signaling and disease

In mammals, the NADPH oxidase family of enzymes comprises seven members: NOXs 1-5, DUOX1, and DUOX2. All of these enzymes function to move an electron across cellular membranes, transferring it to oxygen to generate the superoxide anion. This generation of reactive oxygen species has important physiological and pathophysiological roles. NOX5 is perhaps the least well understood of these NOX isoforms, in part because the gene is not present in mice or rats. In recent years, however, there has been a rapid increase in our understanding of the NOX5 gene, the structural and biochemical aspects of the NOX5 enzyme, the role NOX5 plays in health and disease, and the development of novel NOX inhibitors. This review takes a look back at some historical aspects of the discovery of NOX5 and summarizes our current understanding of the enzyme.

Mice deficient in dual oxidase maturation factors are severely hypothyroid

Dual oxidases (DUOX1 and DUOX2) are evolutionary conserved reduced nicotinamide adenine dinucleotide phosphate oxidases responsible for regulated hydrogen peroxide (H(2)O(2)) release of epithelial cells. Specific maturation factors (DUOXA1 and DUOXA2) are required for targeting of functional DUOX enzymes to the cell surface. Mutations in the single-copy Duox and Duoxa genes of invertebrates cause developmental defects with reduced survival, whereas knockdown in later life impairs intestinal epithelial immune homeostasis. In humans, mutations in both DUOX2 and DUOXA2 can cause congenital hypothyroidism with partial iodide organification defects compatible with a role of DUOX2-generated H(2)O(2) in driving thyroid peroxidase activity. The DUOX1/DUOXA1 system may account for residual iodide organification in patients with loss of DUOX2, but its physiological function is less clear. To provide a murine model recapitulating complete DUOX deficiency, we simultaneously targeted both Duoxa genes by homologous recombination. Knockout of Duoxa genes (Duoxa(-/-) mice) led to a maturation defect of DUOX proteins lacking Golgi processing of N-glycans and to loss of H(2)O(2) release from thyroid tissue. Postnatally, Duoxa(-/-) mice developed severe goitreous congenital hypothyroidism with undetectable serum T4 and maximally disinhibited TSH levels. Heterozygous mice had normal thyroid function parameters. (125)I uptake and discharge studies and probing of iodinated TG epitopes corroborated the iodide organification defect in Duoxa(-/-) mice. Duoxa(-/-) mice on continuous T4 replacement from P6 showed normal growth without an overt phenotype. Our results confirm in vivo the requirement of DUOXA for functional expression of DUOX-based reduced nicotinamide adenine dinucleotide phosphate oxidases and the role of DUOX isoenzymes as sole source of hormonogenic H(2)O(2).

Identifying quantitative trait loci affecting resistance to congenital hypothyroidism in 129/SvJcl strain mice

Tyrosylprotein sulfotransferase 2 (TPST2) is one of the enzymes responsible for tyrosine O-sulfation and catalyzes the sulfation of the specific tyrosine residue of thyroid stimulating hormone receptor (TSHR). Since this modification is indispensable for the activation of TSH signaling, a non-functional TPST2 mutation (Tpst2(grt)) in DW/J-grt mice leads to congenital hypothyroidism (CH) characterized by severe thyroid hypoplasia and dwarfism related to TSH hyporesponsiveness. Previous studies indicated that the genetic background of the 129(+Ter)/SvJcl (129) mouse strain ameliorates Tpst2(grt)-induced CH. To identify loci responsible for CH resistance in 129 mice, we performed quantitative trait locus (QTL) analysis using backcross progenies from susceptible DW/J and resistant 129 mice. We used the first principal component calculated from body weights at 5, 8 and 10 weeks as an indicator of CH, and QTL analysis mapped a major QTL showing a highly significant linkage to the distal portion of chromosome (Chr) 2; between D2Mit62 and D2Mit304, particularly close to D2Mit255. In addition, two male-specific QTLs showing statistically suggestive linkage were also detected on Chrs 4 and 18, respectively. All QTL alleles derived from the 129 strain increased resistance to growth retardation. There was also a positive correlation between recovery from thyroid hypoplasia and the presence of the 129 allele at D2Mit255 in male progenies. These results suggested that the major QTL on Chr 2 is involved in thyroid development. Moreover, since DW/J congenic strain mice carrying both a Tpst2(grt) mutation and 129 alleles in the major QTL show resistance to dwarfism and thyroid hypoplasia, we confirmed the presence of the resistant gene in this region, and that it is involved in thyroid development. Further genetical analysis should lead to identification of genes for CH tolerance and, from a better understanding of thyroid organogenesis and function, the subsequent development of new treatments for thyroid disorders.

Dual oxidase 2 is essential for the toll-like receptor 5-mediated inflammatory response in airway mucosa

AIMS

Airway mucosa is constantly exposed to various airborne microbes, and epithelial host defense requires a robust innate immunity. Recently, it has been suggested that NADPH oxidase (NOX) isozymes serve functional roles in toll-like receptor (TLR)-mediated innate immune responses. However, the molecular mechanism between TLR and NOX-mediated reactive oxygen species (ROS) production in human airway mucosa has been poorly understood.

RESULTS

Here, we show that flagellin-induced ROS generation is dependent on dual oxidase 2 (DUOX2) activation, which is regulated by [Ca(2+)](i) mobilization in primary normal human nasal epithelial (NHNE) cells. Interestingly, we observed that silencing of DUOX2 expression in NHNE cells and nasal epithelium of Duox2 knockout mice failed to trigger mucin and MIP-2? production upon challenging flagellin.

INNOVATION

Our observation in this study reveals that flagellin-induced hydrogen peroxide (H(2)O(2)) generation is critical for TLR5-dependent innate immune responses, including IL-8 production and MUC5AC expression in the nasal epithelium. Furthermore, DUOX2-mediated H(2)O(2) generation activated by the flagellin-TLR5 axis might serve as a novel therapeutic target for infectious inflammation diseases in the airway tract.

CONCLUSION

Taken together, we propose that DUOX2 plays pivotal roles in TLR5-dependent inflammatory response of nasal airway epithelium.

Reactive oxygen species and the neuronal fate

Aberrant or elevated levels of reactive oxygen species (ROS) can mediate deleterious cellular effects, including neuronal toxicity and degeneration observed in the etiology of a number of pathological conditions, including Alzheimer's and Parkinson's diseases. Nevertheless, ROS can be generated in a controlled manner and can regulate redox sensitive transcription factors such as NFκB, AP-1 and NFAT. Moreover, ROS can modulate the redox state of tyrosine phosphorylated proteins, thereby having an impact on many transcriptional networks and signaling cascades important for neurogenesis. A large body of literature links the controlled generation of ROS at low-to-moderate levels with the stimulation of differentiation in certain developmental programs such as neurogenesis. In this regard, ROS are involved in governing the acquisition of the neural fate-from neural induction to the elaboration of axons. Here, we summarize and discuss the growing body of literature that describe a role for ROS signaling in neuronal development.

Regulation of reactive oxygen species generation in cell signaling

Reactive oxygen species (ROS) including superoxide anion and hydrogen peroxide (H(2)O(2)) are thought to be byproducts of aerobic respiration with damaging effects on DNA, protein, and lipid. A growing body of evidence indicates, however, that ROS are involved in the maintenance of redox homeostasis and various cellular signaling pathways. ROS are generated from diverse sources including mitochondrial respiratory chain, enzymatic activation of cytochrome p450, and NADPH oxidases further suggesting involvement in a complex array of cellular processes. This review summarizes the production and function of ROS. In particular, how cytosolic and membrane proteins regulate ROS generation for intracellular redox signaling will be detailed.

Nox enzymes in allergic airway inflammation

Chronic airway diseases such as asthma are linked to oxidative environmental factors and are associated with increased production of reactive oxygen species (ROS). Therefore, it is commonly assumed that oxidative stress is an important contributing factor to asthma disease pathogenesis and that antioxidant strategies may be useful in the treatment of asthma. A primary source of ROS production in biological systems is NADPH oxidase (NOX), originally associated primarily with inflammatory cells but currently widely appreciated as an important enzyme system in many cell types, with a wide array of functional properties ranging from antimicrobial host defense to immune regulation and cell proliferation, differentiation and apoptosis. Given the complex nature of asthma disease pathology, involving many lung cell types that all express NOX homologs, it is not surprising that the contributions of NOX-derived ROS to various aspects of asthma development and progression are highly diverse and multifactorial. It is the purpose of the present review to summarize the current knowledge with respect to the functional aspects of NOX enzymes in various pulmonary cell types, and to discuss their potential importance in asthma pathogenesis. This article is part of a Special Issue entitled: Biochemistry of Asthma.

Oxidative stress and lectin-like ox-LDL-receptor LOX-1 in atherogenesis and tumorigenesis

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) has been identified as a major receptor for oxidized low-density lipoprotein (ox-LDL) in endothelial cells, monocytes, platelets, cardiomyocytes, and vascular smooth muscle cells. Its expression is minimal under physiological conditions but can be induced under pathological conditions. The upregulation of LOX-1 by ox-LDL appears to be important for physiologic processes, such as endothelial cell proliferation, apoptosis, and endothelium remodeling. Pathophysiologic effects of ox-LDL in atherogenesis have also been firmly established, including endothelial cell dysfunction, smooth muscle cell growth and migration, monocyte transformation into macrophages, and finally platelet aggregation-seen in atherogenesis. Recent studies show a positive correlation between increased serum ox-LDL levels and an increased risk of colon, breast, and ovarian cancer. As in atherosclerosis, ox-LDL and its receptor LOX-1 activate the inflammatory pathway through nuclear factor-kappa B, leading to cell transformation. LOX-1 is important for maintaining the transformed state in developmentally diverse cancer cell lines and for tumor growth, suggesting a molecular connection between atherogenesis and tumorigenesis.

Thyroid iodide efflux: a team effort?

The thyroid hormones thyroxine (T(4)) and triiodothyronine (T(3)) play key roles in regulating development, growth and metabolism in pre- and postnatal life. Iodide (I(-)) is an essential component of the thyroid hormones and is accumulated avidly by the thyroid gland. The rarity of elemental iodine and I(-) in the environment challenges the thyroid to orchestrate a remarkable series of transport processes that ultimately ensure sufficient levels for hormone synthesis. In addition to actively extracting circulating I(-), thyroid follicular epithelial cells must also translocate I(-) into a central intrafollicular compartment, where thyroglobulin is iodinated to form the protein precursor to T(4) and T(3). In the last decade, several bodies of evidence render questionable the notion that I(-) exits thyrocytes solely via the Cl(-)/I(-) exchanger Pendrin (SLC26A4), therefore necessitating reconsideration of several other candidate I(-) conduits: the Cl(-)/H(+) antiporter, CLC-5, the cystic fibrosis transmembrane conductance regulator (CFTR) and the sodium monocarboxylic acid transporter (SMCT1).

Oxidases and peroxidases in cardiovascular and lung disease: new concepts in reactive oxygen species signaling

Reactive oxygen species (ROS) are involved in numerous physiological and pathophysiological responses. Increasing evidence implicates ROS as signaling molecules involved in the propagation of cellular pathways. The NADPH oxidase (Nox) family of enzymes is a major source of ROS in the cell and has been related to the progression of many diseases and even environmental toxicity. The complexity of this family's effects on cellular processes stems from the fact that there are seven members, each with unique tissue distribution, cellular localization, and expression. Nox proteins also differ in activation mechanisms and the major ROS detected as their product. To add to this complexity, mounting evidence suggests that other cellular oxidases or their products may be involved in Nox regulation. The overall redox and metabolic status of the cell, specifically the mitochondria, also has implications on ROS signaling. Signaling of such molecules as electrophilic fatty acids has an impact on many redox-sensitive pathologies and thus, as anti-inflammatory molecules, contributes to the complexity of ROS regulation. This review is based on the proceedings of a recent international Oxidase Signaling Symposium at the University of Pittsburgh's Vascular Medicine Institute and Department of Pharmacology and Chemical Biology and encompasses further interaction and discussion among the presenters.

Cell biology of H2O2 generation in the thyroid: investigation of the control of dual oxidases (DUOX) activity in intact ex vivo thyroid tissue and cell lines

H2O2 generation by dual oxidase (DUOX) at the apex of thyroid cells is the limiting factor in the oxidation of iodide and the synthesis of thyroid hormones. Its characteristics have been investigated using different in vitro models, from the most physiological thyroid slices to the particulate fraction isolated from transfected DUOX expressing CHO cells. Comparison of the models shows that some positive controls are thyroid specific (TSH) or require the substructure of the in vivo cells (MβCD). Other controls apply to all intact cell models such as the stimulation of the PIP(2) phospholipase C pathway by ATP acting on purinergic receptors, the activation of the Gq protein downstream (NaF), or surrogates of the intracellular signals generated by this cascade (phorbol esters for protein kinase C, Ca(++) ionophore for Ca(++)). Still, other controls, exerted by intracellular Ca(++) or its substitute Mn(++), the intracellular pH, or arachidonate bear directly on the enzyme. Iodide acts at the apical membrane of the cell through an oxidized form, presumably iodohexadecanal. Cooling of the cells to 22°C blocks the activation of the PIP(2) phospholipase C cascade. All these effects are reversible. Their kinetics and concentration-effect characteristics have been defined in the four models. A general scheme of the thyroid signaling pathways regulating this metabolism is proposed. The probes characterized could be applied to other H2O2 producing cells and to pathological material.

Structural stability and heme binding potential of the truncated human dual oxidase 2 (DUOX2) peroxidase domain

The essential role of human dual oxidase 2 (hDUOX2) in thyroid hormone biosynthesis defines this member of the NOX/DUOX family, whose absence due to mutation has been directly related to disease, specifically hypothyroidism. Both human DUOX isoforms, hDUOX1 and hDUOX2, are expressed in thyroid tissue; however, hDUOX1 cannot compensate for inactivation of hDUOX2, suggesting that each enzyme is differentially regulated and/or functions in a unique manner. In efforts to uncover relevant structural and functional differences we have expressed and purified the peroxidase domain of hDUOX2(1-599) for direct comparison with the previously studied hDUOX1(1-593). As was shown for hDUOX1, the truncated hDUOX2 domain purifies without a bound heme co-factor and displays no peroxidase activity. However, hDUOX2(1-599) displays greater stability than hDUOX1(1-593). Surprisingly, upon titration with heme, both isoforms bind heme with a low micromolar affinity, demonstrating that they retain a heme binding site. A conformational difference in the full-length protein and/or a protein-protein interaction may be required to increase the heme binding affinity.

Genetic causes of congenital hypothyroidism due to dyshormonogenesis

PURPOSE OF REVIEW

Overview of congenital hypothyroidism caused by thyroid hormone synthesis defects, the current understanding of their pathophysiology, and clinical implications of molecular diagnoses.

RECENT FINDINGS

Genetic defects in all known thyroid-specific factors required for thyroid hormone synthesis have been described. These include defects in iodide trapping (NIS), in the facilitated iodide efflux across the apical membrane (PDS), the organification of iodide within the follicular lumen (thyroid peroxidase, DUOX2, DUOXA2), the substrate for thyroid hormone synthesis (thyroglobulin) and the ability to recover and retain intrathyroidal iodine (iodotyrosine deiodinase). Clinical and biochemical evaluation aids in selecting the most appropriate candidate gene(s). A definite molecular diagnosis of thyroid dyshormonogenesis allows genetic counseling and has prognostic value in differentiating transient from permanent congenital hypothyroidism and predicting the response of patients to iodine supplementation as adjunct or alternative treatment to L-T4 replacement.

SUMMARY

Congenital hypothyroidism due to thyroid dyshormonogenesis is a heterogenic disorder that may be caused by mutations in any of the known steps in the thyroid hormone biosynthesis pathway. An exact molecular diagnosis allows genetic counseling and the identification of asymptomatic mutation carriers at risk of recurrent hypothyroidism, and provides a rationale for adjunct iodide supplementation.

Essential role of Duox in stabilization of Drosophila wing

NADPH oxidase produces reactive oxygen species (ROS). Drosophila melanogaster has two homologs of NADPH oxidase, dNox and dDuox, with functions that remain unclear in vivo. To clarify these functions, two independent transgenic fly lines expressing dsRNA targeted for different portions of dDuox mRNA were used. In both flies, en-GAL4> UAS-dDuoxIR(976-1145) and en-GAL4> UAS-dDuoxIR(370-518), in which dDuox was knocked down selectively in the posterior area of the wing disc, the posterior compartment of the adult wings became paler and more fragile with wing veins that were indistinct by comparison with the anterior one. Fluorescence staining of the en-GAL4> UAS-dDuoxIR(976-1145) adult wings revealed that the ROS concentration in the posterior compartment was significantly lower than that in the anterior compartment. Moreover, in these flies, the posterior compartment of the wing imaginal disc showed a greater number of apoptotic cells detected by immunostaining with anti-cleaved caspase-3 antibody than those in the anterior compartment. Respective knockdown of tyrosine hydroxylase or dopa-decarboxylase showed paler wing blades in the posterior compartment similar to the phenotype of dDuox-knockdown files. Along with this observation, analysis of the catecholic and dityrosine components in the wings of adult flies proved that dDuox plays important roles in the stabilization of the cuticle structure of the wings via tyrosine cross-linking, the sclerotization and melanization processes possibly through ROS production. These dDuox-knockdown fly lines would be useful tools for further studying dDuox functions during the development of Drosophila.

Impact of flavonoids on thyroid function

Flavonoids are polyphenolic compounds of natural occurrence produced by plants that are largely consumed both for therapeutic purposes and as food. Experimental data have shown that many flavonoids could inhibit thyroperoxidase activity, decreasing thyroid hormones levels thus increasing TSH and causing goiter. In humans, infants fed with soy formula have been shown to develop goiter. However, in post-menopausal women soy intake did not affect thyroid function. In thyroid tumor cell line, flavonoids were shown to inhibit cell growth, but they can also decrease radioiodine uptake, that could reduce the efficacy of radioiodine therapy. Flavonoids could also affect the availability of thyroid hormones to target tissues, by inhibiting deiodinase activity or displacing T4 from transthyretin. Thus, flavonoids have been shown to interfere with many aspects of the thyroid hormones synthesis and availability in in vivo and in vitro models. In the present article, we review and synthesize the literature on the effects of flavonoids on thyroid and discuss the possible relevance of these effects for humans.

Physiological roles of NOX/NADPH oxidase, the superoxide-generating enzyme

NADPH oxidase is a superoxide (O₂^•−)-generating enzyme first identified in phagocytes, essential for their bactericidal activities. Later, in non-phagocytes, production of O₂^•− was also demonstrated in an NADPH-dependent manner. In the last decade, several non-phagocyte-type NADPH oxidases have been identified. The catalytic subunit of these oxidases, NOX, constitutes the NOX family. There are five homologs in the family, NOX1 to NOX5, and two related enzymes, DUOX1 and DUOX2. Transgenic or gene-disrupted mice of the NOX family have also been established. NOX/DUOX proteins possess distinct features in the dependency on other components for their enzymatic activities, tissue distributions, and physiological functions. This review summarized the characteristics of the NOX family proteins, especially focused on their functions clarified through studies using gene-modified mice.

Trans-plasma membrane electron transport in mammals: functional significance in health and disease

Trans-plasma membrane electron transport (t-PMET) has been established since the 1960s, but it has only been subject to more intensive research in the last decade. The discovery and characterization at the molecular level of its novel components has increased our understanding of how t-PMET regulates distinct cellular functions. This review will give an update on t-PMET, with particular emphasis on how its malfunction relates to some diseases, such as cancer, abnormal cell death, cardiovascular diseases, aging, obesity, neurodegenerative diseases, pulmonary fibrosis, asthma, and genetically linked pathologies. Understanding these relationships may provide novel therapeutic approaches for pathologies associated with unbalanced redox state.

A single copy of the recently identified dual oxidase maturation factor (DUOXA) 1 gene produces only mild transient hypothyroidism in a patient with a novel biallelic DUOXA2 mutation and monoallelic DUOXA1 deletion

CONTEXT

Dual oxidases (DUOX1 and DUOX2) play a crucial role in the generation of hydrogen peroxide required in the oxidation of iodide and the synthesis of thyroid hormone. Heterodimerization with specific maturation factors (DUOXA1 and DUOXA2) is essential for the maturation and function of the DUOX enzyme complexes. Biallelic loss-of-function mutations of DUOX2 result in congenital hypothyroidism (CH), whereas a single reported case of homozygous DUOXA2 mutation (Y246X) has been associated with mild CH.

OBJECTIVE

We now report an infant with transient CH due to a complex genetic alteration of the DUOX/DUOXA system.

RESULTS

Our patient was born to euthyroid nonconsanguineous parents and presented with an elevated TSH and enlarged thyroid gland at neonatal screening. Genetic analysis revealed a missense mutation (C189R) on the maternal DUOXA2 allele. The mutant DUOXA2 protein showed complete loss-of-function in reconstituting DUOX2 in vitro. The apparent C189R homozygosity of the proband in the absence of the same mutation in the father led to detailed gene mapping, revealing an approximately 43-kb pair deletion encompassing DUOX2, DUOXA1, and DUOXA2. Thus, in addition to being deficient in DUOXA2, the proband lacks one allele of DUOX2 and DUOXA1 but has two functioning DUOX1 alleles.

CONCLUSION

The transient CH in the presence of only one functional maturation factor allele indicates a high level of functional redundancy in the DUOX/DUOXA system.

Enhancement of respiratory mucosal antiviral defenses by the oxidation of iodide

Recent reports postulate that the dual oxidase (DUOX) proteins function as part of a multicomponent oxidative pathway used by the respiratory mucosa to kill bacteria. The other components include epithelial ion transporters, which mediate the secretion of the oxidizable anion thiocyanate (SCN(-)) into airway surface liquid, and lactoperoxidase (LPO), which catalyzes the H(2)O(2)-dependent oxidation of the pseudohalide SCN(-) to yield the antimicrobial molecule hypothiocyanite (OSCN(-)). We hypothesized that this oxidative host defense system is also active against respiratory viruses. We evaluated the activity of oxidized LPO substrates against encapsidated and enveloped viruses. When tested for antiviral properties, the LPO-dependent production of OSCN(-) did not inactivate adenovirus or respiratory syncytial virus (RSV). However, substituting SCN(-) with the alternative LPO substrate iodide (I(-)) resulted in a marked reduction of both adenovirus transduction and RSV titer. Importantly, well-differentiated primary airway epithelia generated sufficient H(2)O(2) to inactivate adenovirus or RSV when LPO and I(-) were supplied. The administration of a single dose of 130 mg of oral potassium iodide to human subjects increased serum I(-) concentrations, and resulted in the accumulation of I(-) in upper airway secretions. These results suggest that the LPO/I(-)/H(2)O(2) system can contribute to airway antiviral defenses. Furthermore, the delivery of I(-) to the airway mucosa may augment innate antiviral immunity.

NADPH oxidase-mediated redox signaling: roles in cellular stress response, stress tolerance, and tissue repair

NADPH oxidase (Nox) has a dedicated function of generating reactive oxygen species (ROS). Accumulating evidence suggests that Nox has an important role in signal transduction in cellular stress responses. We have reviewed the current evidence showing that the Nox system can be activated by a collection of chemical, physical, and biological cellular stresses. In many circumstances, Nox activation fits to the cellular stress response paradigm, in that (1) the response can be initiated by various forms of cellular stresses; (2) Nox-derived ROS may activate mitogen-activated protein kinases (extracellular signal-regulated kinase, p38) and c-Jun NH(2)-terminal kinase, which are the core of the cell stress-response signaling network; and (3) Nox is involved in the development of stress cross-tolerance. Activation of the cell survival pathway by Nox may promote cell adaptation to stresses, whereas Nox may also convey signals toward apoptosis in irreversibly injured cells. At later stage after injury, Nox is involved in tissue repair by modulating cell proliferation, angiogenesis, and fibrosis. We suggest that Nox may have an integral role in cell stress responses and the subsequent tissue repair process. Understanding Nox-mediated redox signaling mechanisms may be of prominent significance at the crossroads of directing cellular responses to stress, aiming at either enhancing the stress resistance (in such situations as preventing ischemia-reperfusion injuries and accelerating wound healing) or sensitizing the stress-induced cytotoxicity for proliferative diseases such as cancer. Therefore, an optimal outcome of interventions on Nox will only be achieved when this is dealt with in a timely and disease-and stage-specific manner.

Redox regulation of Nox proteins

The generation of reactive oxygen species (ROS) plays a major role in endothelial signaling and function. Of the several potential sources of ROS in the vasculature, the endothelial NADPH oxidase (Nox) family of proteins, Nox1, Nox2, Nox4 and Nox5, are major contributors of ROS. Excess generation of ROS contributes to the development and progression of vascular disease. While hyperoxia stimulates ROS production through Nox proteins, hypoxia appears to involve mitochondrial electron transport in the generation of superoxide. ROS generated from Nox proteins and mitochondria are important for oxygen sensing mechanisms. Physiological concentrations of ROS function as signaling molecule in the endothelium; however, excess ROS production leads to pathological disorders like inflammation, atherosclerosis, and lung injury. Regulation of Nox proteins is unclear; however, antioxidants, MAP Kinases, STATs, and Nrf2 regulate Nox under normal physiological and pathological conditions. Studies related to redox regulation of Nox should provide a better understanding of ROS and its role in the pathophysiology of vascular diseases.

Perturbed heme binding is responsible for the blistering phenotype associated with mutations in the Caenorhabditis elegans dual oxidase 1 (DUOX1) peroxidase domain

Dual oxidase (DUOX) enzymes support a wide variety of essential reactions, from cellular signaling to thyroid hormone biosynthesis. In Caenorhabditis elegans, the DUOX system (CeDUOX1/2) plays a crucial role in innate immunity and in stabilizing the cuticle by forming tyrosine cross-links. The current model suggests that superoxide generated by CeDUOX1 at the C-terminal NADPH oxidase domain is rapidly converted to H(2)O(2). The H(2)O(2) is then utilized by the N-terminal peroxidase-like domain to cross-link tyrosines. We have now created a series of mutations in the isolated peroxidase domain, CeDUOX1(1-589). One set of mutations investigate the roles of a putative distal tyrosine (Tyr(105)) and Glu(238), a proposed covalent heme-binding residue. The results confirm that Glu(238) covalently binds to the heme group. A second set of mutations (G246D and D392N) responsible for a C. elegans blistering cuticle phenotype was also investigated. Surprisingly, although not among the catalytic residues, both mutations affected heme co-factor binding. The G246D mutant bound less total heme than the wild type, but a higher fraction of it was covalently bound. In contrast, the D392N mutant appears to fold normally but does not bind heme. Molecular dynamics simulations of a CeDUOX1(1-589) homology model implicate displacements of the proximal histidine residue as the likely cause. Both enzymes are structurally stable and through altered heme interactions exhibit partial or complete loss of tyrosine cross-linking activity, explaining the blistering phenotype. This result argues that the CeDUOX peroxidase domain is primarily responsible for tyrosine cross-linking.

Functional consequences of dual oxidase-thyroperoxidase interaction at the plasma membrane

CONTEXT

Thyroperoxidase (TPO) and dual oxidase (DUOX) are present at the apical membrane of thyrocytes, where TPO catalyzes thyroid hormone biosynthesis in the presence of H2O2 produced by DUOX. Both enzymes are colocalized and associated, but the consequences of this interaction remain obscure.

OBJECTIVE

The objective of this study was to evaluate the functional consequences of TPO-DUOX interaction at the plasma membrane.

DESIGN

The functional consequences of DUOX-TPO interaction were studied by measuring extracellular H2O2 concentration and TPO activity in a heterologous system. For this purpose, HEK293 cells were transiently transfected with a combination of human TPO with human DUOX1 or DUOX2 in the presence of their respective maturation factors, DUOXA1 or DUOXA2. The effect of human DUOX2 mutants in which cysteine residues in the N-terminal domain were replaced by glycines was also analyzed.

RESULTS

We observed that production of H2O2 decreases both TPO and DUOX activities. We show that TPO presents a catalase-like effect that protects DUOX from inhibition by H2O2. This catalase-like effect depends on the association between both enzymes, which probably occurs through the DUOX peroxidase-like domain because this effect was not observed with human DUOX2 mutants.

CONCLUSION

The DUOX-TPO association at the plasma membrane is relevant for normal enzyme properties. Normally, TPO consumes H2O2 produced by DUOX, decreasing the availability of this substance at the apical membrane of thyrocytes and, in turn, probably decreasing the oxidative damage of macromolecules.

Nox enzymes from fungus to fly to fish and what they tell us about Nox function in mammals

The production of reactive oxygen species (ROS) in a highly regulated fashion is a hallmark of members of the NADPH oxidase (Nox) family of enzymes. Nox enzymes are present in most eukaryotic groups such as the amebozoid, fungi, algae and plants, and animals, in which they are involved in seemingly diverse biological processes. However, a comprehensive survey of Nox functions throughout biology reveals common functional themes. Noxes are often activated in response to stressful conditions such as nutrient starvation, physical damage, or pathogen attack. Although the end result varies depending on the organism and tissue, Nox-produced ROS mediate the response to the adverse stimuli, such as innate immunity responses in plants and animals or cell differentiation in Dictyostelium, fungi, and plants. These responses involve ROS-mediated signaling mechanisms occurring at intracellular or cell-to-cell levels and sometimes involve cell wall or extracellular matrix cross-linking. Indeed, Noxes are involved in local and systemic signaling from plants to fish and in cross-linking of the plant hair-cell wall, synthesis of the nematode cuticle, and formation of the sea urchin fertilization envelope. The extensive use of Nox enzymes in biology to regulate cell-to-cell signaling and morphogenesis suggests that additional functions in mammalian signaling and development remain to be discovered.

NOX/NADPH oxidase, the superoxide-generating enzyme: its transcriptional regulation and physiological roles

NADPH oxidase is a superoxide (O(2)(-))-generating enzyme first identified in phagocytes that shows bactericidal activities. It has been reported that O(2)(-) is also produced in an NADPH-dependent manner in non-phagocytes. In the last decade, non-phagocyte-type NADPH oxidases have been identified, and the catalytic subunit NOX family has been found to be composed of five homologs, NOX1 to NOX5, and two related enzymes, DUOX1 and DUOX2. These NOX proteins have distinct features in dependency on other components for maximal enzymatic activity, tissue distribution, expressional regulation, and physiological functions. This review summarized the distinct characteristics of NOX family proteins, especially focusing on their functions and mechanisms of their expressional regulation.

Mice deficient in MCT8 reveal a mechanism regulating thyroid hormone secretion

The mechanism of thyroid hormone (TH) secretion from the thyroid gland into blood is unknown. Humans and mice deficient in monocarboxylate transporter 8 (MCT8) have low serum thyroxine (T4) levels that cannot be fully explained by increased deiodination. Here, we have shown that Mct8 is localized at the basolateral membrane of thyrocytes and that the serum TH concentration is reduced in Mct8-KO mice early after being taken off a treatment that almost completely depleted the thyroid gland of TH. Thyroid glands in Mct8-KO mice contained more non-thyroglobulin-associated T4 and triiodothyronine than did those in wild-type mice, independent of deiodination. In addition, depletion of thyroidal TH content was slower during iodine deficiency. After administration of 125I, the rate of both its secretion from the thyroid gland and its appearance in the serum as trichloroacetic acid-precipitable radioactivity was greatly reduced in Mct8-KO mice. Similarly, the secretion of T4 induced by injection of thyrotropin was reduced in Mct8-KO in which endogenous TSH and T4 were suppressed by administration of triiodothyronine. To our knowledge, this study is the first to demonstrate that Mct8 is involved in the secretion of TH from the thyroid gland and contributes, in part, to the low serum T4 level observed in MCT8-deficient patients.

Receptor activation of NADPH oxidases

Reactive oxygen species (ROS) have been implicated in many intra- and intercellular processes. High levels of ROS are generated as part of the innate immunity in the respiratory burst of phagocytic cells. Low levels of ROS, however, are generated in a highly controlled manner by various cell types to act as second messengers in redox-sensitive pathways. A NADPH oxidase has been initially described as the respiratory burst enzyme in neutrophils. Stimulation of this complex enzyme system requires specific signaling cascades linking it to membrane-receptor activation. Subsequently, a family of NADPH oxidases has been identified in various nonphagocytic cells. They mainly differ in containing one out of seven homologous catalytic core proteins termed NOX1 to NOX5 and DUOX1 or 2. NADPH oxidase activity is controlled by regulatory subunits, including the NOX regulators p47phox and p67phox, their homologs NOXO1 and NOXA1, or the DUOX1 or 2 regulators DUOXA1 and 2. In addition, the GTPase Rac modulates activity of several of these enzymes. Recently, additional proteins have been identified that seem to have a regulatory function on NADPH oxidase activity under certain conditions. We will thus summarize molecular pathways linking activation of different membrane-bound receptors with increased ROS production of NADPH oxidases.