Activation of the NADPH-dependent H2O2-generating system in pig thyroid particulate fraction by limited proteolysis and Zn2+ treatment

Structural basis of human NOX5 activation

NADPH oxidase 5 (NOX5) catalyzes the production of superoxide free radicals and regulates physiological processes from sperm motility to cardiac rhythm. Overexpression of NOX5 leads to cancers, diabetes, and cardiovascular diseases. NOX5 is activated by intracellular calcium signaling, but the underlying molecular mechanism of which - in particular, how calcium triggers electron transfer from NADPH to FAD - is still unclear. Here we capture motions of full-length human NOX5 upon calcium binding using single-particle cryogenic electron microscopy (cryo-EM). By combining biochemistry, mutagenesis analyses, and molecular dynamics (MD) simulations, we decode the molecular basis of NOX5 activation and electron transfer. We find that calcium binding to the EF-hand domain increases NADPH dynamics, permitting electron transfer between NADPH and FAD and superoxide production. Our structural findings also uncover a zinc-binding motif that is important for NOX5 stability and enzymatic activity, revealing modulation mechanisms of reactive oxygen species (ROS) production.

Dual Oxidase System Genes Defects in Children With Congenital Hypothyroidism

PURPOSE

The objectives of this study were to analyze the distribution of dual oxidase (DUOX) system genes (containing DUOX2, DUOX1, DUOXA2, and DUOXA1) variants in children with congenital hypothyroidism (CH) and their phenotypes.

METHODS

Target region sequencing technology was performed on DUOX system genes among 606 CH subjects covering all the exon and intron regions. Detailed clinical data were collected for statistical analysis.

RESULTS

A total of 95 suspected pathogenic variants were detected in the DUOX system genes, showing a 39.11% rate in variant carrying (237/606). DUOX2 had the highest rate in this study. There were statistical differences in maximum adjusted dose and current dose of levothyroxine between the DUOX system genes nonmutated group with the mutated group (both Ps < 0.001). The cases in the DUOX system genes mutated group were more likely to develop into transient CH (χ 2 = 23.155, P < 0.001) and more likely to manifested as goiter or gland-in-situ (χ 2 = 66.139, P < 0.001). In addition, there was no significant difference in clinical characteristics between DUOX system genes monoallelic and non-monoallelic. Although 20% of the variants affected the functional domain regions (EF hand, flavin adenine dinucleotide and nicotinamide adenine dinucleotide binding sites), there was no significant effect on the phenotype severity whether the variation is located in the functional domain regions.

CONCLUSIONS

Our results showed the high variation rate of DUOX2 in the DUOX system genes among Chinese CH patients. The complex genotype-phenotype relationship of DUOX system genes broadened the understanding of CH phenotype spectrum.

Proton channel blockers inhibit Duox activity independent of Hv1 effects

The NADPH oxidase reaction produces protons. In the case of the NADPH oxidase, NOX2, activity depends on secretion of these protons and is inhibited by blockade of the voltage-gated proton channel (Hv1). Duox1 and Duox2 activities similarly produce intracellular protons but synthesize hydrogen peroxide directly instead of superoxide. Hv1 contributes to acid secretion in some epithelia that express Duox. To test the hypothesis that Duox activity is also sensitive to Hv1 channel blockers, Duox was assayed in the presence of either Zn²⁺ or 5-chloro-2-guanidinobenzimidazole (ClGBI). Both compounds inhibited Duox activity in normal human bronchial epithelial cells but with an IC50 over 10-fold higher than that reported for Hv1 (IC50 Zn²⁺ = 0.68 mM; IC50 ClGBI = 0.07–0.14 mM). Homogenized HEK293T cells expressing either Duox1 or Duox2 showed similar IC50 values for ClGBI suggesting these compounds inhibit the enzymes through alternate mechanisms independent of Hv1 proton secretion. Inclusion of superoxide dismutase did not restore Duox hydrogen peroxide synthesis. Addition of nigericin to eliminate any possible transmembrane pH gradients in intracellular membrane-localized Duox did not alter activity in HEK293T homogenates. Extracellular Zn²⁺ blocked intracellular Ca²⁺ increases needed for Duox activity. Together the data suggest that Duox enzyme activities in epithelia are inhibited by compounds that block Hv1 but inhibition occurs through Hv1-independent mechanisms and support the idea that Hv1 is not required for Duox activity.

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Hv1 proton channel inhibitors block Duox in differentiated bronchial epithelial cells.

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Zinc blocks Duox activity concurrently with reduction of calcium transients.

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ClGBI, an inhibitor of Hv1, blocks Duox activity in homogenates of cells lacking Hv1.

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In differentiated bronchial epithelia, Hv1 blockers did not alter intracellular pH.

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H⁺/K⁺ ATPase inhibition acidified cytoplasm but did not block Duox activity.

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.

Inhibitory action of NoxA1 on dual oxidase activity in airway cells

Imbalance between pro- and antioxidant mechanisms in the lungs can compromise pulmonary functions, including blood oxygenation, host defense, and maintenance of an anti-inflammatory environment. Thus, tight regulatory control of reactive oxygen species is critical for proper lung function. Increasing evidence supports a role for the NADPH oxidase dual oxidase (Duox) as an important source for regulated H2O2 production in the respiratory tract epithelium. In this study Duox expression, function, and regulation were investigated in a fully differentiated, mucociliary airway epithelium model. Duox-mediated H2O2 generation was dependent on calcium flux, which was required for dissociation of the NADPH oxidase regulatory protein Noxa1 from plasma membrane-bound Duox. A functional Duox1-based oxidase was reconstituted in model cell lines to permit mutational analysis of Noxa1 and Duox1. Although the activation domain of Noxa1 was not required for Duox function, mutation of a proline-rich domain in the Duox C terminus, a potential interaction motif for the Noxa1 Src homology domain 3, caused up-regulation of basal and stimulated H2O2 production. Similarly, knockdown of Noxa1 in airway cells increased basal H2O2 generation. Our data indicate a novel, inhibitory function for Noxa1 in Duox regulation. This represents a new paradigm for control of NADPH oxidase activity, where second messenger-promoted conformational change of the Nox structure promotes oxidase activation by relieving constraint induced by regulatory components.

Structure, regulation and evolution of Nox-family NADPH oxidases that produce reactive oxygen species

NADPH oxidases of the Nox family exist in various supergroups of eukaryotes but not in prokaryotes, and play crucial roles in a variety of biological processes, such as host defense, signal transduction, and hormone synthesis. In conjunction with NADPH oxidation, Nox enzymes reduce molecular oxygen to superoxide as a primary product, and this is further converted to various reactive oxygen species. The electron-transferring system in Nox is composed of the C-terminal cytoplasmic region homologous to the prokaryotic (and organelle) enzyme ferredoxin reductase and the N-terminal six transmembrane segments containing two hemes, a structure similar to that of cytochrome b of the mitochondrial bc(1) complex. During the course of eukaryote evolution, Nox enzymes have developed regulatory mechanisms, depending on their functions, by inserting a regulatory domain (or motif) into their own sequences or by obtaining a tightly associated protein as a regulatory subunit. For example, one to four Ca(2+)-binding EF-hand motifs are present at the N-termini in several subfamilies, such as the respiratory burst oxidase homolog (Rboh) subfamily in land plants (the supergroup Plantae), the NoxC subfamily in social amoebae (the Amoebozoa), and the Nox5 and dual oxidase (Duox) subfamilies in animals (the Opisthokonta), whereas an SH3 domain is inserted into the ferredoxin-NADP(+) reductase region of two Nox enzymes in Naegleria gruberi, a unicellular organism that belongs to the supergroup Excavata. Members of the Nox1-4 subfamily in animals form a stable heterodimer with the membrane protein p22(phox), which functions as a docking site for the SH3 domain-containing regulatory proteins p47(phox), p67(phox), and p40(phox); the small GTPase Rac binds to p67(phox) (or its homologous protein), which serves as a switch for Nox activation. Similarly, Rac activates the fungal NoxA via binding to the p67(phox)-like protein Nox regulator (NoxR). In plants, on the other hand, this GTPase directly interacts with the N-terminus of Rboh, leading to superoxide production. Here I describe the regulation of Nox-family oxidases on the basis of three-dimensional structures and evolutionary conservation.

Molecular composition and regulation of the Nox family NAD(P)H oxidases

Reactive oxygen species (ROS) are conventionally regarded as inevitable deleterious by-products in aerobic metabolism with a few exceptions such as their significant role in host defense. The phagocyte NADPH oxidase, dormant in resting cells, becomes activated during phagocytosis to deliberately produce superoxide, a precursor of other microbicidal ROS, thereby playing a crucial role in killing pathogens. The catalytic center of this oxidase is the membrane-integrated protein gp91(phox), tightly complexed with p22(phox), and its activation requires the association with p47(phox), p67(phox), and the small GTPase Rac, which normally reside in the cytoplasm. Since recent discovery of non-phagocytic gp91(phox)-related enzymes of the NAD(P)H oxidase (Nox) family--seven homologues identified in humans--deliberate ROS production has been increasingly recognized as important components of various cellular events. Here, we describe a current view on the molecular composition and post-translational regulation of Nox-family oxidases in animals.

Dual oxidase-2 has an intrinsic Ca2+-dependent H2O2-generating activity

Duox2 (and probably Duox1) is a glycoflavoprotein involved in thyroid hormone biosynthesis, as the thyroid H2O2 generator functionally associated with Tpo (thyroperoxidase). So far, because of the impairment of maturation and of the targeting process, transfecting DUOX into nonthyroid cell lines has not led to the expression of a functional H2O2-generating system at the plasma membrane. For the first time, we investigated the H2O2-generating activity in the particulate fractions from DUOX2- and DUOX1-transfected HEK293 and Chinese hamster ovary cells. The particulate fractions of these cells stably or transiently transfected with human or porcine DUOX cDNA demonstrate a functional NADPH/Ca2+-dependent H2O2-generating activity. The immature Duox proteins had less activity than pig thyrocyte particulate fractions, and their activity depended on their primary structures. Human Duox2 seemed to be more active than human Duox1 but only half as active as its porcine counterpart. TPO co-transfection produced a slight increase in the enzymatic activity, whereas p22(phox), the 22-kDa subunit of the leukocyte NADPH oxidase, had no effect. In previous studies on the mechanism of H2O2 formation, it was shown that mature thyroid NADPH oxidase does not release O2*- but H2O2. Using a spin-trapping technique combined with electron paramagnetic resonance spectroscopy, we confirmed this result but also demonstrated that the partially glycosylated form of Duox2, located in the endoplasmic reticulum, generates superoxide in a calcium-dependent manner. These results suggest that post-translational modifications during the maturation process of Duox2 could be implicated in the mechanism of H2O2 formation by favoring intramolecular superoxide dismutation.

Dual oxidase2 is expressed all along the digestive tract

The dual oxidase (Duox)2 flavoprotein is strongly expressed in the thyroid gland, where it plays a critical role in the synthesis of thyroid hormones by providing thyroperoxidase with H2O2. DUOX2 mRNA was recently detected by RT-PCR and in-situ hybridization experiments in other tissues, such as rat colon and rat and human epithelial cells from the salivary excretory ducts and rectal glands. We examined Duox2 expression at the protein level throughout the porcine digestive tract and in human colon. Western blot analysis identified Duox2 as the same two molecular species (M(r) 165 and 175 kDa) as detected in the thyroid. It was expressed in all the tissues tested, but the highest levels were found in the cecum and sigmoidal colon. Immunohistochemical studies showed that Duox2 protein is mainly present in these parts of the gut and located at the apical membrane of the enterocytes in the brush border, indicating that it is expressed only in highly differentiated cells. A Ca2+/NADPH-dependent H2O2-generating system was associated with Duox2 protein expression, which had the same biochemical characteristics as the NADPH oxidase in the thyroid. Indeed, treatment of the thyroid and cecum particulate fractions with phenylarsine oxide resulted in complete calcium desensitization of both enzymes. A marked increase in DUOX2 expression was also found during spontaneous differentiation of postconfluent Caco-2 cells. The discovery of Duox2 as a novel source of H2O2 in the digestive tract, particularly in the cecum and colon, makes it a new candidate mediator of physiopathological processes.

Biochemical characterization of a Ca2+/NAD(P)H-dependent H2O2 generator in human thyroid tissue

An NAD(P)H-dependent H2O2 forming activity has been evidenced in thyroid tissue from patients with Grave's disease. Its biochemical properties were compared to those of the NADPH oxidase previously described in pig thyroid gland. Both were Ca2+-dependent and activated by inorganic phosphate anions in the same range of concentrations. Both are flavoproteins using FAD as cofactor, but the human enzyme was also able to utilize FMN. The apparent Km for NADPH of the human enzyme (100 microM) was 5-10 times higher than that of porcine enzyme. Vm was 3 to 10 times higher in pig (150 nmol x h(-1) x mg(-1)) than in man (14 to 45). Total content in human tissue was 7 to 9% of that in porcine tissue. An unidentified inhibitor has been detected in the 3000 g particulate fraction from most patients, which could account for this apparently low enzyme content. An NADH-dependent H2O2 production has also been observed in porcine and human thyroid tissues. This activity was only partly Ca2+-dependent (man, 50-70%; pig, 80-90%) and presented similar apparent Km values for NADH (man, 100 microM; pig, 200 microM). In pig thyrocytes, the expression of the Ca2+-dependent part of the NADH-oxidase activity was induced by TSH and down-regulated by TGFbeta, as was the NADPH oxidase activity. Furthermore, NADPH and NADH-dependent activities were not additive. We conclude that a single, inducible, NAD(P)H-oxidase can use NADPH or NADH as substrate to catalyse H2O2 formation, and that human and porcine NAD(P)H-oxidases are highly similar. Differences observed could be attributed to minor differences in enzyme structure and/or in membrane microenvironment. The NADH-dependent Ca2+-independent activity observed in human and porcine thyroid fractions could be attributed to a distinct and constitutive enzyme.

Regulation of the thyroid NADPH-dependent H2O2 generator by Ca2+: studies with phenylarsine oxide in thyroid plasma membrane

Pig thyroid plasma membranes contain a Ca(2+)-dependent NADPH:O2 oxidoreductase, the thyroid NADPH-dependent H2O2 generator. This provided the H2O2 for the peroxidase-catalysed synthesis of thyroid hormones. The effect of the tervalent arsenical, phenylarsine oxide (PAO), on the NADPH oxidase was studied. PAO caused two directly related dose-dependent effects with similar half-effect concentrations of PAO (3 nmol of PAO/mg of protein): (i) partial inactivation of H2O2 formation by the Ca(2+)-stimulated enzyme, and (ii) desensitization of the enzyme activity to Ca2+. PAO had no effect on membranes that had been Ca(2+)-desensitized by alpha-chymotrypsin treatment. The NADPH oxidase in membranes treated with excess PAO had the same Vmax with and without Ca2+. This value was half the Vmax of the native enzyme. However, the K(m) for NADPH determined with Ca2+ (18 microM, identical with that of the native enzyme) was approx, one-third of the K(m) measured without Ca2+, showing the direct action of Ca2+ on the PAO-enzyme complex. PAO had the same effects, partial inactivation and Ca2+ desensitization, on the NADPH: ferricyanide oxidoreductase activity of the NADPH oxidase, suggesting that PAO acts on the flavodehydrogenase entity of the enzyme. Both partial inactivation and Ca2+ desensitization were completely and specifically reversed by 2.3-dimercaptopropanol, partly reversed by dithiothreitol and not reversed by 2-mercaptoethanol, indicating that PAO binds to vicinal thiol groups. These results suggest that thiol groups are involved in the control of thyroid NADPH oxidase by Ca2+; PAO bound to vicinal thiols might alter the structure of the enzyme so that electron transfer occurs without Ca2+ but more slowly.

Solubilization and characterization of a thyroid Ca(2+)-dependent and NADPH-dependent K3Fe(CN)6 reductase. Relationship with the NADPH-dependent H2O2-generating system

The thyroid plasma membrane contains a Ca(2+)-regulated NADPH-dependent H2O2-generating system which provides H2O2 for the thyroid-peroxidase-catalyzed biosynthesis of thyroid hormones. The molecular nature of the membrane-associated electron transport chain that generates H2O2 in the thyroid is unknown, but recent observations indicate that a flavoprotein containing a FAD prosthetic group is involved. Solubilization was reinvestigated using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps), Triton X-100, and high salt concentrations. Chaps eliminated about 30% of the proteins, which included a ferricyanide reductase, without affecting the H2O2-generating system. Similarly, Triton X-100 alone did not extract the NADPH oxidase. An NADPH-oxidase activity, which was measured in the presence of the artificial electron acceptor potassium ferricyanide, was solubilized by increasing the ionic strength to 2 M KCl. This NADPH-ferricyanide reductase activity was shown to belong to the H2O2-generating system, although it did not produce H2O2. It was still Ca2+ dependent and H2O2 production was restored by decreasing the ionic strength by overnight dialysis. No H2O2 production activity was detected after sucrose density gradient centrifugation of the dialyzed solubilized enzyme, but a well-defined peak of NADPH oxidation activity with a sedimentation coefficient of 3.71 S was found in the presence of K3Fe(CN)6. These results suggest that some unknown component(s) (phospholipid or protein) is removed during sucrose density gradient centrifugation. Finally, thyrotropin, which induces NADPH oxidase and regulates H2O2 production in porcine thyrocytes in primary culture, also induced the NADPH-K3Fe(CN)6 reductase activity associated with the H2O2-generating system. Thus, this enzyme seems to be another marker of thyroid differentiation.

The Ca2+/NADPH-dependent H2O2 generator in thyroid plasma membrane: inhibition by diphenyleneiodonium

The thyroid plasma membrane contains a Ca(2+)-regulated NADPH-dependent H2O2-generating system which provides H2O2 for the peroxidase-catalysed biosynthesis of thyroid hormones. The electron transfer from NADPH to O2 catalysed by this system was studied by using diphenyleneiodonium (DPI), an inhibitor of flavo- and haemo-proteins. The prosthetic group of the H2O2 generator was removed by incubation with 5 mM CHAPS at 40 degrees C, and an active holoenzyme was reconstituted with FAD, but not with FMN. The H2O2-generating system also had an intrinsic Ca(2+)-dependent NADPH:ferricyanide reductase activity which is probably linked to its flavodehydrogenase component (or domain). Both activities, H2O2 production and ferricyanide reductase activity, were inhibited by DPI, with similar K1/2 (2.5 nmol/mg of protein). DPI only inhibited a system reduced with NADPH in the presence of Ca2+. NADPH could not be replaced by NADP+, NADH or sodium dithionite, suggesting the need for specific mild reduction of a redox centre in a particular conformation. Ferricyanide protected both activities against inhibition by DPI; the NADPH:ferricyanide reductase activity was completely protected at a ferricyanide concentration 20 times lower than that needed to protect the H2O2 formation, implying at least two target sites for DPI. One might be the flavodehydrogenase component; the other was beyond, on the entity which transfers the electrons to O2. This second site has not been identified.