Proteins homologous to p47phox and p67phox support superoxide production by NAD(P)H oxidase 1 in colon epithelial cells

Identification of structural elements in Nox1 and Nox4 controlling localization and activity

Nox NADPH oxidases differ in their mode of activation, subcellular localization, and physiological function. Nox1 releases superoxide anions (O(2)(-)) and depends on cytosolic activator proteins, whereas Nox4 extracellularly releases hydrogen peroxide (H(2)O(2)), and its activity does not require cotransfection of additional proteins. We constructed chimeric proteins consisting of Nox1 and Nox4 expressed in HEK293 cells. When the cytosolic tail of Nox4 was fused with the transmembrane part of Nox1, Nox1 became constitutively active. The reciprocal construct was inactive, suggesting that cytosolic subunit-dependent activation requires elements in the transmembrane loops. By TIRF-microscopy, Nox1 was observed in the plasma membrane, whereas Nox4 colocalized with proteins of the endoplasmic reticulum. Fusion proteins of Myc and Nox revealed that the N-terminal part of Nox1 but not Nox4 is cleaved. When the potential signal peptide of Nox4 was inserted into Nox1, plasma-membrane localization was lost, and the protein was retained in vesicle-like structures below the plasma membrane. The potential signal peptide of Nox1 failed to translocate Nox4 to the plasma membrane but switched the extracellularly detectable ROS from H(2)O(2) to O(2)(-). Thus, the very N-terminal part of Nox proteins determines subcellular localization and the ROS type released, whereas the cytosolic tail regulates activity.

NADPH oxidases and angiotensin II receptor signaling

Over the last decade many studies have demonstrated the importance of reactive oxygen species (ROS) production by NADPH oxidases in angiotensin II (Ang II) signaling, as well as a role for ROS in the development of different diseases in which Ang II is a central component. In this review, we summarize the mechanism of activation of NADPH oxidases by Ang II and describe the molecular targets of ROS in Ang II signaling in the vasculature, kidney and brain. We also discuss the effects of genetic manipulation of NADPH oxidase function on the physiology and pathophysiology of the renin-angiotensin system.

A region N-terminal to the tandem SH3 domain of p47phox plays a crucial role in the activation of the phagocyte NADPH oxidase

The superoxide-producing NADPH oxidase in phagocytes is crucial for host defence; its catalytic core is the membrane-integrated protein gp91phox [also known as Nox2 (NADPH oxidase 2)], which forms a stable heterodimer with p22phox. Activation of the oxidase requires membrane translocation of the three cytosolic proteins p47phox, p67phox and the small GTPase Rac. At the membrane, these proteins assemble with the gp91phox–p22phox heterodimer and induce a conformational change of gp91phox, leading to superoxide production. p47phox translocates to membranes using its two tandemly arranged SH3 domains, which directly interact with p22phox, whereas p67phox is recruited in a p47phox-dependent manner. In the present study, we show that a short region N-terminal to the bis-SH3 domain is required for activation of the phagocyte NADPH oxidase. Alanine substitution for Ile152 in this region, a residue that is completely conserved during evolution, results in a loss of the ability to activate the oxidase; and the replacement of Thr153 also prevents oxidase activation, but to a lesser extent. In addition, the corresponding isoleucine residue (Ile155) of the p47phox homologue Noxo1 (Nox organizer 1) participates in the activation of non-phagocytic oxidases, such as Nox1 and Nox3. The I152A substitution in p47phox, however, does not affect its interaction with p22phox or with p67phox. Consistent with this, a mutant p47phox (I152A), as well as the wild-type protein, is targeted upon cell stimulation to membranes, and membrane recruitment of p67phox and Rac normally occurs in p47phox (I152A)-expressing cells. Thus the Ile152-containing region of p47phox plays a crucial role in oxidase activation, probably by functioning at a process after oxidase assembly.

Activation of dual oxidases Duox1 and Duox2: differential regulation mediated by camp-dependent protein kinase and protein kinase C-dependent phosphorylation

Dual oxidases were initially identified as NADPH oxidases producing H(2)O(2) necessary for thyroid hormone biosynthesis. The crucial role of Duox2 has been demonstrated in patients suffering from partial iodide organification defect caused by bi-allelic mutations in the DUOX2 gene. However, the Duox1 function in thyroid remains elusive. We optimized a functional assay by co-expressing Duox1 or Duox2 with their respective maturation factors, DuoxA1 and DuoxA2, to compare their intrinsic enzymatic activities under stimulation of the major signaling pathways active in the thyroid in relation to their membrane expression. We showed that basal activity of both Duox isoenzymes depends on calcium and functional EF-hand motifs. However, the two oxidases are differentially regulated by activation of intracellular signaling cascades. Duox1 but not Duox2 activity is stimulated by forskolin (EC(50) = 0.1 microm) via protein kinase A-mediated Duox1 phosphorylation on serine 955. In contrast, phorbol esters induce Duox2 phosphorylation via protein kinase C activation associated with high H(2)O(2) generation (phorbol 12-myristate 13-acetate EC(50) = 0.8 nm). These results were confirmed in human thyroid cells, suggesting that Duox1 is also involved in thyroid hormonogenesis. Our data provide, for the first time, detailed insights into the mechanisms controlling the activation of Duox1-2 proteins and reveal additional phosphorylation-mediated regulation.

Tumor necrosis factor alpha activates transcription of the NADPH oxidase organizer 1 (NOXO1) gene and upregulates superoxide production in colon epithelial cells

NADPH oxidase 1 (Nox1) is a multicomponent enzyme consisting of p22(phox), Nox organizer 1 (NOXO1), Nox1 activator 1, and Rac1. Interleukin-1beta, flagellin, interferon-gamma, and tumor necrosis factor alpha (TNF-alpha) similarly induced Nox1 in a colon cancer cell line (T84), whereas only TNF-alpha fully induced NOXO1 and upregulated superoxide-producing activity by ninefold. This upregulation was canceled by knockdown of NOXO1 with small interfering RNAs. TNF-alpha rapidly phosphorylated p38 mitogen-activated protein kinase and c-Jun N-terminal kinase 1/2, followed by phosphorylation of c-Jun and c-Fos and appearance of an AP-1 binding activity within 30 min. We cloned the 5' flank of the human NOXO1 gene (-3888 to +263 bp), and found that the region between -585 and -452 bp, which contains consensus elements of YY-1, AP-1, and Ets, and the GC-rich region encoding three putative binding sites for SP-1, was crucial for TNF-alpha-dependent promoter activity. Serial mutation analysis of the elements identified an AP-1 binding site (from -561 to -551 bp, agtAAGtcatg) as a crucial element for TNF-alpha-stimulated transcription of the human NOXO1 gene, which was also confirmed by the AP-1 decoy experiments. Thus, TNF-alpha acts as a potent activator of Nox1-based oxidase in colon epithelial cells, suggesting a potential role of this oxidase in inflammation of the colon.

A possible biochemical link between NADPH oxidase (Nox) 1 redox-signalling and ERp72

Emerging evidence indicates that Nox (NADPH oxidase) 1-generated ROS (reactive oxygen species) play critical regulatory roles in various cellular processes, yet little is known of direct targets for the oxidase. In the present study we show that one of the proteins selectively oxidized in response to Nox1-generated ROS was ERp72 (endoplasmic reticulum protein 72 kDa) with TRX (thioredoxin) homology domains. Oxidation of ERp72 by Nox1 resulted in an inhibition of its reductase activity. EGF treatment of cells stimulated the Nox1 activity and the activated Nox1 subsequently mediated EGF-induced suppression of the ERp72 reductase activity. Co-immunoprecipitation, GST (glutathione transferase) pulldown assays and mutational analysis, indicated that Nox1 associates with ERp72, which involves its N-terminus encompassing a Ca(2+)-binding site and the first TRX-like motif. Furthermore, confocal microscopy showed co-localization between Nox1 and ERp72 at the plasma membrane. These results suggest that Nox1 functionally associates with ERp72, regulating redox-sensitive signalling pathways in a cellular context.

Mutational analysis reveals distinct features of the Nox4-p22 phox complex

The integral membrane protein p22(phox) forms a heterodimeric enzyme complex with NADPH oxidases (Noxs) and is required for their catalytic activity. Nox4, a Nox linked to cardiovascular disease, angiogenesis, and insulin signaling, is unique in its ability to produce hydrogen peroxide constitutively. To date, p22(phox) constitutes the only identified regulatory component for Nox4 function. To delineate structural elements in p22(phox) essential for formation and localization of the Nox4-p22(phox) complex and its enzymatic function, truncation and point mutagenesis was used. Human lung carcinoma cells served as a heterologous expression system, since this cell type is p22(phox)-deficient and promotes cell surface expression of the Nox4-p22(phox) heterodimer. Expression of p22(phox) truncation mutants indicates that the dual tryptophan motif contained in the N-terminal amino acids 6-11 is essential, whereas the C terminus (amino acids 130-195) is dispensable for Nox4 activity. Introduction of charged residues in domains predicted to be extracellular by topology modeling was mostly tolerated, whereas the exchange of amino acids in predicted membrane-spanning domains caused loss of function or showed distinct differences in p22(phox) interaction with various Noxs. For example, the substitution of tyrosine 121 with histidine in p22(phox), which abolished Nox2 and Nox3 function in vivo, preserved Nox4 activity when expressed in lung cancer cells. Many of the examined p22(phox) mutations inhibiting Nox1 to -3 maturation did not alter Nox4-p22(phox) association, further accenting the differences between Noxs. These studies highlight the distinct interaction of the key regulatory p22(phox) subunit with Nox4, a feature which could provide the basis for selective inhibitor development.

Activation of NADPH oxidase 1 in tumour colon epithelial cells

In the plasma membrane fraction from Caco-2 human colon carcinoma cells, active Nox1 (NADPH oxidase 1) endogenously co-localizes with its regulatory components p22phox, NOXO1, NOXA1 and Rac1. NADPH-specific superoxide generating activity was reduced by 80% in the presence of either a flavoenzyme inhibitor DPI (diphenyleneiodonium) or NADP+. The plasma membranes from PMA-stimulated cells showed an increased amount of Rac1 (19.6 pmol/mg), as compared with the membranes from unstimulated Caco-2 cells (15.1 pmol/mg), but other components did not change before and after the stimulation by PMA. Spectrophotometric analysis found approx. 36 pmol of FAD and 43 pmol of haem per mg of membrane and the turnover of superoxide generation in a cell-free system consisting of the membrane and FAD was 10 mol/s per mol of haem. When the constitutively active form of Rac, Rac1(Q61L) or GTP-bound Rac1 was added exogenously to the membrane, O2−-producing activity was enhanced up to 1.5-fold above the basal level, but GDP-loaded Rac1 did not affect superoxide-generating kinetics. A fusion protein [NOXA1N–Rac1(Q61L)] between truncated NOXA1(1–211) and Rac1-(Q61L) exhibited a 6-fold increase of the basal Nox1 activity, but NOXO1N(1–292) [C-terminal truncated NOXO1(1–292)] alone showed little effect on the activity. The activated forms of Rac1 and NOXA1 are essentially involved in Nox1 activation and their interactions might be responsible for regulating the O2−-producing activity in Caco-2 cells.

Anthrax edema toxin inhibits Nox1-mediated formation of reactive oxygen species by colon epithelial cells

One major route of intoxication by Bacillus anthracis (anthrax) spores is via their ingestion and subsequent uptake by the intestinal epithelium. Anthrax edema toxin (ETx) is an adenylate cyclase that causes persistent elevation of cAMP in intoxicated cells. NADPH oxidase enzymes (Nox1-Nox5, Duox1 and 2) generate reactive oxygen species (ROS) as components of the host innate immune response to bacteria, including Nox1 in gastrointestinal epithelial tissues. We show that ETx effectively inhibits ROS formation by Nox1 in HT-29 colon epithelial cells. This inhibition requires the PKA-mediated phosphorylation of the Nox1-regulatory component, NoxA1, and the subsequent binding of 14-3-3zeta. Inhibition of Nox1-mediated ROS formation in the gut epithelium may be a mechanism used by B. anthracis to circumvent the innate immune response.

Essential role of NOXA1 in generation of reactive oxygen species induced by oxidized low-density lipoprotein in human vascular endothelial cells

Oxidative stress induced by superoxide plays an important role in pathogenesis of cardiovascular diseases. NAD(P)H oxidase is a principal enzymatic origin for superoxide in vasculature. Recently, novel homologues of cytosolic components of NAD(P)H oxidase, Nox organizer 1 (NOXO1) and Nox activator 1 (NOXA1), are identified. On the other hand, oxidized low-density lipoprotein (ox-LDL) generates reactive oxygen species (ROS) in endothelial cells via lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1). In the present investigation, the authors examined the expression, the regulation, and the role of NOXA1 in the generation of ROS in endothelial cells. The expression of NOXA1 was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR). Dihydroethidium method showed that ox-LDL and angiotensin II increased the generation of intracellular ROS. Once the expression of p22(phox) or NOXA1 was suppressed by siRNA, the generation of ROS induced by ox-LDL and angiotensin II were potently decreased. Moreover, the expression of NOXA1 was increased by ox-LDL in a time-and dose-dependent manner. In conclusion, endothelial NOXA1 plays an essential role in generation of ROS. Ox-LDL not only increased the generation of ROS via LOX-1, but also enhanced the expression of NOXA1 in endothelial cells. NOXA1 is likely a key player that links ox-LDL with the activation of endothelial NAD(P)H oxidase.

TNFalpha and reactive oxygen species in necrotic cell death

Death receptors, including the TNF receptor-1 (TNF-RI), have been shown to be able to initiate caspase-independent cell death. This form of "necrotic cell death" appears to be dependent on the generation of reactive oxygen species. Recent data have indicated that superoxide generation is dependent on the activation of NADPH oxidases, which form a complex with the adaptor molecules RIP1 and TRADD. The mechanism of superoxide generation further establishes RIP1 as the central molecule in ROS production and cell death initiated by TNFalpha and other death receptors. A role for the sustained JNK activation in necrotic cell death is also suggested. The sensitization of virus-infected cells to TNFalpha indicates that necrotic cell death may represent an alternative cell death pathway for clearance of infected cells.

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.

Expression of NADPH oxidases and enhanced H(2)O(2)-generating activity in human coronary artery endothelial cells upon induction with tumor necrosis factor-alpha

Tumor necrosis factor (TNF)-alpha, which potentiates reactive oxygen species (ROS) generation, is crucial for the development of coronary arteritis and aneurysm in Kawasaki disease. We hypothesized that vascular NADPH oxidase (Nox) enzymes participate in the TNF-alpha-triggered endothelial damage through elevating ROS generation. Thus, we herein examine the expression of Nox enzymes in human coronary artery endothelial cells (HCAEC) and the effects of TNF-alpha on Nox-mediated ROS generation. We show that HCAEC in culture spontaneously generate H(2)O(2) at basal level (0.53 nmol/min/mg protein). In searching for Nox components responsible for the H(2)O(2) generation, two distinct isoforms of Nox4 are found expressed in HCAEC: the prototype Nox4A and the shorter Nox4B, respectively in the postnuclear supernatant and the nuclear fractions. Other expressed Nox family components are: as mRNAs, Nox4C, Nox4D, Nox1, p51(nox), and Racs; as mRNAs and proteins, Nox2, p22(phox), p47(phox), and p67(phox). The H(2)O(2)-generating activity increases up to three-fold upon inclusion of TNF-alpha in culture, concomitantly with augmented expressions of Nox4A, p22(phox), p47(phox) and p67(phox) proteins. Together, these results suggest that Nox2 and Nox4A enzymes are induced by TNF-alpha endowing HCAEC with enhanced ROS-generating activity, which may play a role in the initial endothelial dysfunction through oxidative stress.

Nox enzymes and oxidative stress in the immunopathology of the gastrointestinal tract

Chronic inflammation caused by Helicobacter pylori infection or inflammatory bowel disease (IBD) is closely linked to cancer development. Innate immune abnormalities and enhanced production of reactive oxygen species through a phagocyte NADPH oxidase (Nox2) are key issues in understanding the pathogenesis of inflammation-dependent carcinogenesis. Besides Nox2, functionally distinct homologues (Nox1, Nox3, Nox4, Nox5, Duox1, and Duox2) have been identified. Nox1 and Duox2 are highly expressed in the gastrointestinal tract. Although the functional roles of Nox/Duox in the gastrointestinal tract are still unclear, we will review their potential roles in the gastrointestinal immunopathology, particularly in H. pylori-induced inflammation, IBD, and malignancy.

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.

NOX enzymes and Toll-like receptor signaling

Invading microorganisms are recognized by the host innate immune system through pattern recognition receptors. Among these receptors, Toll-like receptors (TLRs) are able to sense the molecular signatures of microbial pathogens, protozoa, fungi, and virus and activate proinflammatory signaling cascades. In addition to their role in bacterial killing by phagocytes, reactive oxygen species generated by NADPH oxidase (NOX) homologues also play key roles in signaling and host defense in a variety of cell types. Recent studies have demonstrated a link between TLR activation and NOX homologues following microbial recognition highlighting their important role in the innate immune response and host defense.

The involvement of the tyrosine kinase c-Src in the regulation of reactive oxygen species generation mediated by NADPH oxidase-1

NADPH oxidase (Nox) family enzymes are one of the main sources of cellular reactive oxygen species (ROS), which have been shown to function as second messenger molecules. To date, seven members of this family have been reported, including Nox1-5 and Duox1 and -2. With the exception of Nox2, the regulation of the Nox enzymes is still poorly understood. Nox1 is highly expressed in the colon, and it requires two cytosolic regulators, NoxO1 and NoxA1, as well as the binding of Rac1 GTPase, for its activity. In this study, we investigate the role of the tyrosine kinase c-Src in the regulation of ROS formation by Nox1. We show that c-Src induces Nox1-mediated ROS generation in the HT29 human colon carcinoma cell line through a Rac-dependent mechanism. Treatment of HT29 cells with the Src inhibitor PP2, expression of a kinase-inactive form of c-Src, and c-Src depletion by small interfering RNA (siRNA) reduce both ROS generation and the levels of active Rac1. This is associated with decreased Src-mediated phosphorylation and activation of the Rac1-guanine nucleotide exchange factor Vav2. Consistent with this, Vav2 siRNA that specifically reduces endogenous Vav2 protein is able to dramatically decrease Nox1-dependent ROS generation and abolish c-Src-induced Nox1 activity. Together, these results establish c-Src as an important regulator of Nox1 activity, and they may provide insight into the mechanisms of tumor formation in colon cancers.

Mutation of the Cyba gene encoding p22phox causes vestibular and immune defects in mice

In humans, hereditary inactivation of either p22(phox) or gp91(phox) leads to chronic granulomatous disease (CGD), a severe immune disorder characterized by the inability of phagocytes to produce bacteria-destroying ROS. Heterodimers of p22(phox) and gp91(phox) proteins constitute the superoxide-producing cytochrome core of the phagocyte NADPH oxidase. In this study, we identified the nmf333 mouse strain as what we believe to be the first animal model of p22(phox) deficiency. Characterization of nmf333 mice revealed that deletion of p22(phox) inactivated not only the phagocyte NADPH oxidase, but also a second cytochrome in the inner ear epithelium. As a consequence, mice of the nmf333 strain exhibit a compound phenotype consisting of both a CGD-like immune defect and a balance disorder caused by the aberrant development of gravity-sensing organs. Thus, in addition to identifying a model of p22(phox)-dependent immune deficiency, our study indicates that a clinically identifiable patient population with an otherwise cryptic loss of gravity-sensor function may exist. Thus, p22(phox) represents a shared and essential component of at least 2 superoxide-producing cytochromes with entirely different biological functions. The site of p22(phox) expression in the inner ear leads us to propose what we believe to be a novel mechanism for the control of vestibular organogenesis.

NADPH oxidase: recent evidence for its role in erectile dysfunction

Important roles for reactive oxygen species (ROS) in physiology and pathophysiology have been increasingly recognized. Under normal conditions, ROS serve as signaling molecules in the regulation of cellular functions. However, enhanced ROS production as a result of the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase contributes significantly to the pathogeneses of vascular diseases. Although it has become evident that increased ROS is associated with erectile dysfunction (ED), the sources of ROS in the penis remain largely unknown. In recent years, emergent evidence suggests the possible role of NADPH oxidase in inducing ED. In this review, we examine the relationship between ROS and ED in different disease models and discuss the current evidence basis for NADPH oxidase-derived ROS in ED.

Oxidative stress and aging: the potential role of iron

According to the free radical theory of aging proposed by Denham Harman more than 50 years ago, oxidatively modified cellular components accumulate continuously in the cells during the organism's lifespan leading to progressive decline of cellular functions. Since then, it has been shown that proteins, lipids, nucleic acids and other cell components undergo reversible and/or irreversible oxidative modifications during aging. Moreover, oxidized cell components can undergo further oxidative modifications leading to formation of products that cell degradation systems are incapable of removing. Accumulation of such non-degradable aggregates further inhibits the functionality of degradation systems, thus aggravating the effects and leading to a vicious cycle. In this presentation, we propose that the availability of intracellular iron in its redox active form (labile iron) represents the main catalyst that mediates extensive oxidative modifications of cellular components and ultimately leads to their accumulation and consequent cellular dysfunction. It is tempting to speculate that regulated restriction of labile iron may have positive effects on health in general and aging in particular.

Regulation of smooth muscle by inducible nitric oxide synthase and NADPH oxidase in vascular proliferative diseases

Inflammation plays a critical role in promoting smooth muscle migration and proliferation during vascular diseases such as postangioplasty restenosis and atherosclerosis. Another common feature of many vascular diseases is the contribution of reactive oxygen (ROS) and reactive nitrogen (RNS) species to vascular injury. Primary sources of ROS and RNS in smooth muscle are several isoforms of NADPH oxidase (Nox) and the cytokine-regulated inducible nitric oxide (NO) synthase (iNOS). One important example of the interaction between NO and ROS is the reaction of NO with superoxide to yield peroxynitrite, which may contribute to the pathogenesis of hypertension. In this review, we discuss the literature that supports an alternate possibility: Nox-derived ROS modulate NO bioavailability by altering the expression of iNOS. We highlight data showing coexpression of iNOS and Nox in vascular smooth muscle demonstrating the functional consequences of iNOS and Nox during vascular injury. We describe the relevant literature demonstrating that the mitogen-activated protein kinases are important modulators of proinflammatory cytokine-dependent expression of iNOS. A central hypothesis discussed is that ROS-dependent regulation of the serine/threonine kinase protein kinase Cdelta is essential to understanding how Nox may regulate signaling pathways leading to iNOS expression. Overall, the integration of nonphagocytic NADPH oxidase with cytokine signaling in general and in vascular smooth muscle in particular is poorly understood and merits further investigation.

Overexpression of human NOX1 complex induces genome instability in mammalian cells

The production of reactive oxygen species (ROS) in mammalian cells is tightly regulated because of their potential to damage macromolecules, including DNA. To investigate possible links between high ROS levels, oxidative DNA damage, and genomic instability in mammalian cells, we established a novel model of chronic oxidative stress by coexpressing the NADPH oxidase human (h) NOX1 gene together with its cofactors NOXO1 and NOXA1. Transfectants of mismatch repair (MMR)-proficient HeLa cells or MMR-defective Msh2(-/-) mouse embryo fibroblasts overexpressing the hNOX1 complex displayed increased intracellular ROS levels. In one HeLa clone in which ROS were particularly elevated, reactive nitrogen species were also increased and nitrated proteins were identified with an anti-3-nitrotyrosine antibody. Overexpression of the hNOX1 complex increased the steady-state levels of DNA 8-oxo-7,8-dihydroguanine and caused a threefold increase in the HPRT mutation rate in HeLa cells. In contrast, additional oxidatively generated damage did not affect the constitutive mutator phenotype of the Msh2(-/-) fibroblasts. Because no significant changes in the expression of several DNA repair enzymes for oxidative DNA damage were identified, we suggest that chronic oxidative stress can saturate the cell's DNA repair capacity and cause significant genomic instability.

Regulation of NOX1 expression by GATA, HNF-1alpha, and Cdx transcription factors

NOX1, a member of the NOX family of NADPH oxidases, is expressed primarily in colon epithelium, where it may function in host defense and growth regulation. We investigated factors responsible for its transcriptional regulation in vitro and its expression in vivo. Analysis of promoter constructs in the CaCo2 cell line identified a complex element between -422 and -291 critical for promoter activity. This element contained four sites that bound GATA-4, -5, and -6 in vitro with varied affinities. One site also bound the caudal-related homeodomain proteins Cdx1 and Cdx2, whereas another also bound hepatocyte nuclear factor-1alpha (HNF-1alpha). GATA-6, HNF-1alpha, and Cdx2 also bound to this region in the intact chromatin of CaCo2 cells. These factors demonstrated cooperativity when transactivating the NOX1 promoter. NOX1 mRNA was detected in human colon epithelial cells along the crypt-villus axis. A gradient of NOX1 mRNA expression was seen in the colons of normal as well as germ-free mice, with significantly higher levels in distal compared with proximal segments. The expression gradients of NOX1 mRNA in the colon paralleled those of GATA-6, HNF-1alpha, and Cdx1. These data indicate that developmental, tissue-restricted transcription factors play a key role in NOX1 regulation in vivo.

Evidence for cancer-associated expression of NADPH oxidase 1 (Nox1)-based oxidase system in the human stomach

Helicobacter pylori infection has been suggested to stimulate expression of the NADPH oxidase 1 (Nox1)-based oxidase system in guinea pig gastric epithelium, whereas Nox1 mRNA expression has not yet been documented in the human stomach. PCR of human stomach cDNA libraries showed that Nox1 and Nox organizer 1 (NOXO1) messages were absent from normal stomachs, while they were specifically coexpressed in intestinal- and diffuse-type adenocarcinomas including signet-ring cell carcinoma. Immunohistochemistry showed that Nox1 and NOXO1 proteins were absent from chronic atrophic gastritis (15 cases), adenomas (4 cases), or surrounding tissues of adenocarcinomas (45 cases). In contrast, Nox1 and its partner proteins were expressed in intestinal-type adenocarcinomas (19/21 cases), diffuse-type adenocarcinomas (15/15 cases), and signet-ring cell carcinomas (9/9 cases). Confocal microscopy revealed that Nox1, NOXO1, Nox activator 1, and p22(phox) were predominantly associated with Golgi apparatus in these cancer cells, while diffuse-type adenocarcinomas also contained cancer cells having Nox1 and its partner proteins in their nuclei. Nox1-expressing cancer cells exhibited both gastric and intestinal phenotypes, as assessed by expression of mucin core polypeptides. Thus, the Nox1-base oxidase may be a potential marker of neoplastic transformation and play an important role in oxygen radical- and inflammation-dependent carcinogenesis in the human stomach.

Mechanism of angiotensin II-induced superoxide production in cells reconstituted with angiotensin type 1 receptor and the components of NADPH oxidase

The mechanism of angiotensin II (Ang II)-induced superoxide production was investigated with HEK293 or Chinese hamster ovary cells reconstituted with the angiotensin type 1 receptor (AT(1)R) and NADPH oxidase (either Nox1 or Nox2) along with a pair of adaptor subunits (either NOXO1 with NOXA1 or p47(phox) with p67(phox)). Ang II enhanced the activity of both Nox1 and Nox2 supported by either adaptor pair, with more effective activation of Nox1 in the presence of NOXO1 and NOXA1 and of Nox2 in the presence of p47(phox) and p67(phox). Expression of several AT(1)R mutants showed that interaction of the receptor with G proteins but not that with beta-arrestin or with other proteins (Jak2, phospholipase C-gamma1, SH2 domain-containing phosphatase 2) that bind to the COOH-terminal region of AT(1)R, was necessary for Ang II-induced superoxide production. The effects of constitutively active alpha subunits of G proteins and of various pharmacological agents implicated signaling by a pathway comprising AT(1)R, Galpha(q/11), phospholipase C-beta, and protein kinase C as largely, but not exclusively, responsible for Ang II-induced activation of Nox1 and Nox2 in the reconstituted cells. A contribution of Galpha(12/13), phospholipase D, and phosphatidyl-inositol 3-kinase to Ang II-induced superoxide generation was also suggested, whereas Src and the epidermal growth factor receptor did not appear to participate in this effect of Ang II. In reconstituted cells stimulated with Ang II, Nox2 exhibited a more sensitive response than Nox1 to the perturbation of protein kinase C, phosphatidylinositol 3-kinase, or the small GTPase Rac1.

Vascular adaptations to hypoxia: molecular and cellular mechanisms regulating vascular tone

Several molecular and cellular adaptive mechanisms to hypoxia exist within the vasculature. Many of these processes involve oxygen sensing which is transduced into mediators of vasoconstriction in the pulmonary circulation and vasodilation in the systemic circulation. A variety of oxygen-responsive pathways, such as HIF (hypoxia-inducible factor)-1 and HOs (haem oxygenases), contribute to the overall adaptive process during hypoxia and are currently an area of intense research. Generation of ROS (reactive oxygen species) may also differentially regulate vascular tone in these circulations. Potential candidates underlying the divergent responses between the systemic and pulmonary circulations may include Nox (NADPH oxidase)-derived ROS and mitochondrial-derived ROS. In addition to alterations in ROS production governing vascular tone in the hypoxic setting, other vascular adaptations are likely to be involved. HPV (hypoxic pulmonary vasoconstriction) and CH (chronic hypoxia)-induced alterations in cellular proliferation, ionic conductances and changes in the contractile apparatus sensitivity to calcium, all occur as adaptive processes within the vasculature.

Regulation of Nox1 activity via protein kinase A-mediated phosphorylation of NoxA1 and 14-3-3 binding

Nox activator 1 (NoxA1) is a homologue of p67(phox) that acts in conjunction with Nox organizer 1 (NoxO1) to regulate reactive oxygen species (ROS) production by the NADPH oxidase Nox1. The phosphorylation of cytosolic regulatory components by multiple kinases plays important roles in assembly and activity of the phagocyte NADPH oxidase (Nox2) system, but little is known about regulation by phosphorylation in the Nox1 system. Here we identify Ser(172) and Ser(461) of NoxA1 as phosphorylation sites for protein kinase A (PKA). A consequence of this phosphorylation was the enhancement of NoxA1 complex formation with 14-3-3 proteins. Using both a transfected human embryonic kidney 293 cell Nox1 model system and endogenous Nox1 in colon cell lines, we showed that the elevation of cAMP inhibits, whereas the inhibition of PKA enhances, Nox1-dependent ROS production through effects on NoxA1. Inhibition of Nox1 activity was intensified by the availability of 14-3-3zeta protein, and this regulatory interaction was dependent on PKA-phosphorylatable sites at Ser(172) and Ser(461) in NoxA1. We showed that phosphorylation and 14-3-3 binding induce the dissociation of NoxA1 from the Nox1 complex at the plasma membrane, suggesting a mechanism for the inhibitory effect on Nox1 activity. Our data establish that PKA-phosphorylated NoxA1 is a new binding partner of 14-3-3 protein(s) and that this forms the basis of a novel mechanism regulating the formation of ROS by Nox1 and, potentially, other NoxA1-regulated Nox family members.

Molecular evolution of Phox-related regulatory subunits for NADPH oxidase enzymes

Background

The reactive oxygen-generating NADPH oxidases (Noxes) function in a variety of biological roles, and can be broadly classified into those that are regulated by subunit interactions and those that are regulated by calcium. The prototypical subunit-regulated Nox, Nox2, is the membrane-associated catalytic subunit of the phagocyte NADPH-oxidase. Nox2 forms a heterodimer with the integral membrane protein, p22phox, and this heterodimer binds to the regulatory subunits p47phox, p67phox, p40phox and the small GTPase Rac, triggering superoxide generation. Nox-organizer protein 1 (NOXO1) and Nox-activator 1 (NOXA1), respective homologs of p47phox and p67phox, together with p22phox and Rac, activate Nox1, a non-phagocytic homolog of Nox2. NOXO1 and p22phox also regulate Nox3, whereas Nox4 requires only p22phox. In this study, we have assembled and analyzed amino acid sequences of Nox regulatory subunit orthologs from vertebrates, a urochordate, an echinoderm, a mollusc, a cnidarian, a choanoflagellate, fungi and a slime mold amoeba to investigate the evolutionary history of these subunits.

Results

Ancestral p47phox, p67phox, and p22phox genes are broadly seen in the metazoa, except for the ecdysozoans. The choanoflagellate Monosiga brevicollis, the unicellular organism that is the closest relatives of multicellular animals, encodes early prototypes of p22phox, p47phox as well as the earliest known Nox2-like ancestor of the Nox1-3 subfamily. p67phox- and p47phox-like genes are seen in the sea urchin Strongylocentrotus purpuratus and the limpet Lottia gigantea that also possess Nox2-like co-orthologs of vertebrate Nox1-3. Duplication of primordial p47phox and p67phox genes occurred in vertebrates, with the duplicated branches evolving into NOXO1 and NOXA1. Analysis of characteristic domains of regulatory subunits suggests a novel view of the evolution of Nox: in fish, p40phox participated in regulating both Nox1 and Nox2, but after the appearance of mammals, Nox1 (but not Nox2) became independent of p40phox. In the fish Oryzias latipes, a NOXO1 ortholog retains an autoinhibitory region that is characteristic of mammalian p47phox, and this was subsequently lost from NOXO1 in later vertebrates. Detailed amino acid sequence comparisons identified both putative key residues conserved in characteristic domains and previously unidentified conserved regions. Also, candidate organizer/activator proteins in fungi and amoeba are identified and hypothetical activation models are suggested.

Conclusion

This is the first report to provide the comprehensive view of the molecular evolution of regulatory subunits for Nox enzymes. This approach provides clues for understanding the evolution of biochemical and physiological functions for regulatory-subunit-dependent Nox enzymes.

Structure and Function of the PB1 Domain, a Protein Interaction Module Conserved in Animals, Fungi, Amoebas, and Plants

Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like beta-grasp fold, containing two alpha helices and a mixed five-stranded beta sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first beta strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40(phox) and p67(phox) (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40(phox) regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.

Hypoxia-Dependent Regulation of Nonphagocytic NADPH Oxidase Subunit NOX4 in the Pulmonary Vasculature

Nonphagocytic NADPH oxidases have recently been suggested to play a major role in the regulation of physiological and pathophysiological processes, in particular, hypertrophy, remodeling, and angiogenesis in the systemic circulation. Moreover, NADPH oxidases have been suggested to serve as oxygen sensors in the lung. Chronic hypoxia induces vascular remodeling with medial hypertrophy leading to the development of pulmonary hypertension. We screened lung tissue for the expression of NADPH oxidase subunits. NOX1, NOXA1, NOXO1, p22phox, p47phox, p40phox, p67phox, NOX2, and NOX4 were present in mouse lung tissue. Comparing mice maintained for 21 days under hypoxic (10% O(2)) or normoxic (21% O(2)) conditions, an upregulation exclusively of NOX4 mRNA was observed under hypoxia in homogenized lung tissue, concomitant with increased levels in microdissected pulmonary arterial vessels. In situ hybridization and immunohistological staining for NOX4 in mouse lungs revealed a localization of NOX4 mRNA and protein predominantly in the media of small pulmonary arteries, with increased labeling intensities after chronic exposure to hypoxia. In isolated pulmonary arterial smooth muscle cells (PASMCs), NOX4 was localized primarily to the perinuclear space and its expression levels were increased after exposure to hypoxia. Treatment of PASMCs with siRNA directed against NOX4 decreased NOX4 mRNA levels and reduced PASMC proliferation as well as generation of reactive oxygen species. In lungs from patients with idiopathic pulmonary arterial hypertension (IPAH), expression levels of NOX4, which was localized in the vessel media, were 2.5-fold upregulated. These results support an important role for NOX4 in the vascular remodeling associated with development of pulmonary hypertension.

Regulation of Nox and Duox enzymatic activity and expression

In recent years, it has become clear that reactive oxygen species (ROS, which include superoxide, hydrogen peroxide, and other metabolites) are produced in biological systems. Rather than being simply a by-product of aerobic metabolism, it is now recognized that specific enzymes--the Nox (NADPH oxidase) and Duox (Dual oxidase) enzymes--seem to have the sole function of generating ROS in a carefully regulated manner, and key roles in signal transduction, immune function, hormone biosynthesis, and other normal biological functions are being uncovered. The prototypical Nox is the respiratory burst oxidase or phagocyte oxidase, which generates large amounts of superoxide and other reactive species in the phagosomes of neutrophils and macrophages, playing a central role in innate immunity by killing microbes. This enzyme system has been extensively studied over the past two decades, and provides a basis for comparison with the more recently described Nox and Duox enzymes, which generate ROS in a variety of cells and tissues. This review first considers the structure and regulation of the respiratory burst oxidase, and then reviews recent studies relating to the regulation of the activity of the novel Nox/Duox enzymes. The regulation of Nox and Duox expression in tissues and by specific stimuli is also considered here. An accompanying review considers biological and pathological roles of the Nox family of enzymes.

TNF-induced activation of the Nox1 NADPH oxidase and its role in the induction of necrotic cell death

Tumor necrosis factor (TNF) is an important cytokine in immunity and inflammation and induces many cellular responses, including apoptosis and necrosis. TNF signaling enables the generation of superoxide in phagocytic and vascular cells through the activation of the NADPH oxidase Nox2/gp91. Here we show that TNF also activates the Nox1 NADPH oxidase in mouse fibroblasts when cells undergo necrosis. TNF treatment induces the formation of a signaling complex containing TRADD, RIP1, Nox1, and the small GTPase Rac1. TNF-treated RIP1-deficient fibroblasts fail to form such a complex, indicating that RIP1 is essential for Nox1 recruitment. Moreover, the prevention of TNF-induced superoxide generation with dominant-negative mutants of TRADD or Rac1, as well as knockdown of Nox1 using siRNA, inhibits necrosis. Thus our study suggests that activation of Nox1 through forming a complex with TNF signaling components plays a key role in TNF-induced necrotic cell death.

Molecular evolution of the reactive oxygen-generating NADPH oxidase (Nox/Duox) family of enzymes

Background

NADPH-oxidases (Nox) and the related Dual oxidases (Duox) play varied biological and pathological roles via regulated generation of reactive oxygen species (ROS). Members of the Nox/Duox family have been identified in a wide variety of organisms, including mammals, nematodes, fruit fly, green plants, fungi, and slime molds; however, little is known about the molecular evolutionary history of these enzymes.

Results

We assembled and analyzed the deduced amino acid sequences of 101 Nox/Duox orthologs from 25 species, including vertebrates, urochordates, echinoderms, insects, nematodes, fungi, slime mold amoeba, alga and plants. In contrast to ROS defense enzymes, such as superoxide dismutase and catalase that are present in prokaryotes, ROS-generating Nox/Duox orthologs only appeared later in evolution. Molecular taxonomy revealed seven distinct subfamilies of Noxes and Duoxes. The calcium-regulated orthologs representing 4 subfamilies diverged early and are the most widely distributed in biology. Subunit-regulated Noxes represent a second major subdivision, and appeared first in fungi and amoeba. Nox5 was lost in rodents, and Nox3, which functions in the inner ear in gravity perception, emerged the most recently, corresponding to full-time adaptation of vertebrates to land. The sea urchin Strongylocentrotus purpuratus possesses the earliest Nox2 co-ortholog of vertebrate Nox1, 2, and 3, while Nox4 first appeared somewhat later in urochordates. Comparison of evolutionary substitution rates demonstrates that Nox2, the regulatory subunits p47phox and p67phox, and Duox are more stringently conserved in vertebrates than other Noxes and Nox regulatory subunits. Amino acid sequence comparisons identified key catalytic or regulatory regions, as 68 residues were highly conserved among all Nox/Duox orthologs, and 14 of these were identical with those mutated in Nox2 in variants of X-linked chronic granulomatous disease. In addition to canonical motifs, the B-loop, TM6-FAD, VXGPFG-motif, and extreme C-terminal regions were identified as important for Nox activity, as verified by mutational analysis. The presence of these non-canonical, but highly conserved regions suggests that all Nox/Duox may possess a common biological function remained in a long history of Nox/Duox evolution.

Conclusion

This report provides the first comprehensive analysis of the evolution and conserved functions of Nox and Duox family members, including identification of conserved amino acid residues. These results provide a guide for future structure-function studies and for understanding the evolution of biological functions of these enzymes.

VEGF signaling through NADPH oxidase-derived ROS

Angiogenesis is a key process involved in normal development and wound repair, as well as ischemic heart and limb diseases, and atherosclerosis. Vascular endothelial growth factor (VEGF), a potent angiogenesis factor, stimulates proliferation, migration, and tube formation of endothelial cells (ECs), primarily through the VEGF receptor type2 (VEGFR2). Reactive oxygen species (ROS) function as signaling molecules to mediate biological responses. In ECs, NADPH oxidase is one of the major sources of ROS and consists of catalytic subunits (Nox1, Nox2, and Nox4), p22phox, p47phox, p67phox, and the small GTPase Rac1. VEGF stimulates ROS production via activation of gp91phox (Nox2)-based NADPH oxidase, and ROS are involved in VEGFR2-mediated signaling linked to EC migration and proliferation. Moreover, ROS derived from NADPH oxidase are involved in postnatal angiogenesis. Localizing NADPH oxidase and its regulators at the specific subcellular compartment is an important mechanism for activating specific redox signaling events. This review focuses on a role of NADPH oxidase-derived ROS in angiogenesis and critical regulators involved in generation of spatially and temporally restricted ROS-dependent VEGF signaling at leading edge, focal adhesions/complexes, caveolae/lipid rafts, and cell-cell junctions in ECs. Understanding these mechanisms should facilitate the development of new therapeutic strategies to modulate new blood vessel formation.

Role of the small GTPase Rac in p22phox-dependent NADPH oxidases

The superoxide-producing phagocyte NADPH oxidase gp91(phox)/Nox2 and the non-phagocytic oxidases Nox1 and Nox3 each form a complex in the membrane with p22(phox), which provides both stabilization and a docking site for organizer proteins. The p22(phox)-complexed Nox2 and Nox1 are dormant on their own, and their activation requires soluble supportive proteins such as a Nox organizer (p47(phox) or Noxo1) and a Nox activator (p67(phox) or Noxa1). The small GTPase Rac directly binds to the activators, and thus plays an essential role in the Nox2-based oxidase containing p47(phox) and p67(phox) or a positive role in Nox1 activity supported by Noxo1 and Noxa1. Although Nox3 complexed with p22(phox) constitutively produce superoxide, the production can be enhanced by supportive proteins. Here we compare the roles of Rac in these p22(phox)-dependent oxidases using the organizer and activator in different combinations. Expression of constitutively active Rac1(Q61L) is essential for activation of the Nox2- or Nox1-based oxidase containing the organizer p47(phox) and either p67(phox) or Noxa1. When these oxidases use Noxo1 as an organizer instead of p47(phox), they produce a small but significant amount of superoxide without expression of Rac1(Q61L), although the production is enhanced by Rac1(Q61L). Thus p47(phox) is likely related to strict dependence on Rac. The Nox3-based oxidase has a similar tendency in the change of the dependence: Rac plays a positive role in Nox3 activation in the presence of p47(phox) and either p67(phox) or Noxa1, whereas Rac fails to upregulate Nox3 activity when p47(phox) is replaced with Noxo1. We also demonstrate that, in the Nox3-based oxidase containing solely p67(phox) as supportive protein, expression of Rac1(Q61L) enhances not only superoxide production but also membrane translocation of p67(phox). Since the enhancements are not observed with a mutant p67(phox) defective in binding to Rac, this GTPase appear to directly recruit p67(phox) to the membrane.

NAD(P)H Oxidases Regulate HIF-2α Protein Expression

Biallelic inactivation of the von Hippel-Lindau tumor suppressor gene (VHL) is linked to the development of hereditary and sporadic renal cell carcinoma (RCC). In the absence of VHL, the alpha subunits of heterodimeric hypoxia-inducible transcription factors (HIF-1alpha and HIF-2alpha) are stabilized. Reactive oxygen species, generated by NAD(P)H oxidases, are involved in signaling cascades of malignant growth. We show that in VHL-deficient cells p22phox, Nox4 protein levels and NADPH-dependent superoxide generation are increased. Reintroduction of VHL into the VHL-deficient cells down-regulates the expression of p22phox and NADPH-dependent superoxide generation. Inhibition of the 26 S proteasome in VHL-expressing cells increased p22phox protein levels, which correlated with an increase of NADPH-dependent superoxide generation. We also show that p22phox co-immunoprecipitates with VHL in vivo. Moreover, p22phox is a target of ubiquitination. Importantly, in VHL-deficient cells, diphenyleneiodonium chloride (DPI), an inhibitor of Nox oxidases, decreased the expression of HIF-2alpha. Down-regulation of Nox1, Nox4, and p22phox expression by small interfering RNA also decreased HIF-2alpha protein expression and inhibited Akt and 4E-BP1 phosphorylation, suggesting that a translational mechanism is involved in maintaining HIF-2alpha in VHL-deficient cells. Colony formation by RCC 786-O in soft agar was markedly inhibited by DPI. Moreover, DPI significantly inhibited RCC 786-O tumor formation in athymic mice. Collectively, the data demonstrate that VHL protein exerts its tumor suppressor action, at least partially, via inhibition of p22phox-based Nox4/Nox1 NADPH oxidase-dependent reactive oxygen species generation.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

Subcellular localization and function of alternatively spliced Noxo1 isoforms

Nox organizer 1 (Noxo1), a p47(phox) homolog, is produced as four isoforms with unique N-terminal PX domains derived by alternative mRNA splicing. We compared the subcellular distribution of these isoforms or their isolated PX domains produced as GFP fusion proteins, as well as their ability to support Nox1 activity in several transfected models. Noxo1alpha, beta, gamma, and delta show different subcellular localization patterns, determined by their PX domains. In HEK293 cells, Noxo1beta exhibits prominent plasma membrane binding, Noxo1gamma shows plasma membrane and nuclear associations, and Noxo1alpha and delta localize primarily on intracellular vesicles or cytoplasmic aggregates, but not the plasma membrane. Nox1 activity correlates with Noxo1 plasma membrane binding in HEK293 cells, since Noxo1beta supports the highest activity and Noxo1gamma and Noxo1alpha support moderate or low activities, respectively. In COS-7 cells, where Noxo1alpha localizes on the plasma membrane, the activities supported by the three isoforms (alpha, beta, and gamma) do not differ significantly. The PX domains of beta and gamma bind the same phospholipids, including phosphatidic acid. These results indicate that the variant PX domains are unique determinants of Noxo1 localization and Nox1 function. Finally, the overexpressed Noxo1 isoforms do not affect p22(phox) localization, although Nox1 is needed to transport p22(phox) to the plasma membrane.

Genetics, biology and clinical management of myeloid cell primary immune deficiencies: chronic granulomatous disease and leukocyte adhesion deficiency

PURPOSE OF REVIEW

Chronic granulomatous disease and leukocyte adhesion deficiency are the major primary immune deficiencies affecting phagocytic blood cells. Major advances in clinical diagnosis and development of novel treatments for these disorders merit review.

RECENT FINDINGS

Clinically beneficial gene therapy correction of X-linked chronic granulomatous disease in two adult patients was reported. Nonmyeloablative busulfan conditioning before administration of gene corrected autologous hematopoietic stem cells was likely an essential maneuver to achieve successful gene therapy. There is an increased association of autoimmune disorders with chronic granulomatous disease. Preimplantation genetic diagnosis of leukocyte adhesion deficiency-I led to the birth of a normal child. A canine model of leukocyte adhesion deficiency-I facilitated development of new nonmyeloablative hematopoietic stem cell transplant and gene therapy approaches to leukocyte adhesion deficiency. Nonmyeloablative transplantation may provide an effective, but less toxic approach for leukocyte adhesion deficiency in children. There have been advances in understanding the basis of leukocyte adhesion deficiency-II and III.

SUMMARY

The most important subjects reviewed in this chapter include new advances in development of gene therapy for chronic granulomatous disease and leukocyte adhesion deficiency-I; transplantation for leukocyte adhesion deficiency-I; prenatal diagnosis of leukocyte adhesion deficiency-I; and association of autoimmune diseases with chronic granulomatous disease.

Expression of Nox1 in 3T3 cells increases cellular acid production but not proton conductance

The role of the NADPH oxidase homolog 1 (Nox1) in plasma membrane H(+) conductance and cellular H(+) production was investigated in 3T3 cells stably expressing Nox1 (Nox1 3T3) compared to vector-expressing control cells (mock 3T3). In whole cell patch clamp experiments both Nox1 and mock 3T3 expressed a similar H(+) conductance (Nox1 3T3, 13.2+/-8.6 pS/pF; mock 3T3, 16.6+/-13.4 pS/pF) with a number of similar characteristics (e.g., current-voltage relations, current activation kinetics, Zn(2+)-sensitivity). When the intracellular pH of cells was alkalinized with NH(4)Cl, rates of intracellular acidification were significantly higher in Nox1 3T3 compared to mock 3T3. Nox1 3T3 showed a time course of acidification that followed a double-exponential function with a fast and a slow component of, on average, tau=165 s and 1780 s, whereas mock 3T3 showed only a single slow tau of 1560 s. Expression of Nox1 also caused cells to acidify the extracellular medium at higher rates than control cells; Nox1 3T3 released 96+/-19 fmol h(-1)cell(-1) of acid equivalents compared to 19+/-12 fmol h(-1)cell(-1) in mock 3T3. These data show that expression of Nox1 results in a mechanism that has the capacity to rapidly acidify the cytosol and generate significant amounts of acid. No significant effect of Nox1 expression on the plasma membrane H(+) conductance was found.

Interaction between the SH3 domains and C-terminal proline-rich region in NADPH oxidase organizer 1 (Noxo1)

NADPH oxidase organizer 1 (Noxo1), harboring a PX domain, two SH3 domains, and a proline-rich region (PRR), participates in activation of superoxide-producing Nox-family NADPH oxidases. Here, we show that Noxo1 supports superoxide production in a cell-free system for gp91(phox)/Nox2 activation by arachidonic acid. This lipid enhances an SH3-mediated binding of Noxo1 to p22(phox), a protein complexed with Nox oxidases; the binding is known to be required for Nox activation. We also demonstrate that the bis-SH3 domain directly interacts with the Noxo1 PRR. The interaction appears to prevent the bis-SH3 domain and PRR from binding to their target proteins; disruption of the intramolecular interaction facilitates Noxo1 binding to p22(phox) and also allows the PRR to associate with the Nox activator Noxa1, which association is crucial for Nox activation as well. These findings suggest that Nox activation involves a conformational change leading to disruption of the bis-SH3-PRR interaction in Noxo1.

Expression and function of Noxo1gamma, an alternative splicing form of the NADPH oxidase organizer 1

Activation of the superoxide-producing NADPH oxidase Nox1 requires both the organizer protein Noxo1 and the activator protein Noxa1. Here we describe an alternative splicing form of Noxo1, Noxo1gamma, which is expressed in the testis and fetal brain. The Noxo1gamma protein contains an additional five amino acids in the N-terminal PX domain, a phosphoinositide-binding module; the domain plays an essential role in supporting superoxide production by NADPH oxidase (Nox) family oxidases including Nox1, gp91(phox)/Nox2, and Nox3, as shown in this study. The PX domain isolated from Noxo1gamma shows a lower affinity for phosphoinositides than that from the classical splicing form Noxo1beta. Consistent with this, in resting cells, Noxo1gamma is poorly localized to the membrane, and thus less effective in activating Nox1 than Noxo1beta, which is constitutively present at the membrane. On the other hand, cell stimulation with phorbol 12-myristate 13-acetate (PMA), an activator of Nox1-3, facilitates membrane translocation of Noxo1gamma; as a result, Noxo1gamma is equivalent to Noxo1beta in Nox1 activation in PMA-stimulated cells. The effect of the five-amino-acid insertion in the Noxo1 PX domain appears to depend on the type of Nox; in activation of gp91(phox)/Nox2, Noxo1gamma is less active than Noxo1beta even in the presence of PMA, whereas Noxo1gamma and Noxo1beta support the superoxide-producing activity of Nox3 to the same extent in a manner independent of cell stimulation.

Redox-dependent expression of cyclin D1 and cell proliferation by Nox1 in mouse lung epithelial cells

NADPH oxidases produce reactive oxygen species (ROS) that serve as co-stimulatory signals for cell proliferation. In mouse lung epithelial cells that express Nox1, Nox2, Nox4, p22(phox), p47(phox), p67(phox), and Noxo1, overexpression of Nox1 delayed cell cycle withdrawal by maintaining AP-1-dependent expression of cyclin D1 in low serum conditions. In cycling cells, the effects of Nox1 were dose dependent: levels of Nox1 that induced 3- to 10-fold increases in ROS promoted phosphorylation of ERK1/2 and expression of cyclin D1, whereas expression of Nox1 with Noxo1 and Noxa1 (or expression of Nox4 alone) that induced substantial increases in intracellular ROS inhibited cyclin D1 and proliferation. Catalase reversed the effects of Nox1 on cyclin D1 and cell proliferation. Diphenylene iodonium, an inhibitor of NADPH oxidase activity, did not affect dosedependent responses of ERK1/2 or Akt to serum, but markedly inhibited the sequential expression of c-Fos and Fra-1 required for induction of cyclin D1 during cell cycle re-entry. These results indicate that Nox1 stimulates cell proliferation in actively cycling cells by reducing the requirement for growth factors to maintain expression of cyclin D1, whereas during cell cycle re-entry, NADPH oxidase activity is required for transcriptional activation of Fos family genes during the immediate early gene response.

Intracellular redox regulation by the family of small GTPases

There is growing experimental evidence that the production of intracellular reactive oxygen species (ROS) represents a tightly regulated process. In particular, numerous observations have suggested a role for the Ras superfamily of small GTPases in redox regulation. This article reviews the evidence that ROS can serve as important downstream effectors for both Ras and Rac proteins. Given the prominent role these proteins play in regulating growth, senescence, and transformation, understanding the role of the small GTPase family in redox regulation may significantly alter our current concepts as to how free radicals contribute to diverse processes from aging to cancer.

NADPH oxidases in the gastrointestinal tract: a potential role of Nox1 in innate immune response and carcinogenesis

The gastrointestinal epithelium functions as physical and innate immune barriers against commensal or pathogenic microbes. NADPH oxidase 1 (Nox1) and dual oxidase 2 (Duox2), highly expressed in the colon, are suggested to play a potential role in host defense. Guinea-pig gastric pit cells and human colonic epithelial cells (T84 cells) express Nox1. With regard to activation of Nox1, the gastric epithelial cells are primed with Helicobacter pylori lipopolysaccharide, whereas T84 cells preferentially use the Toll-like receptor (TLR) 5, rather than TLR4, against Salmonella enteritidis infection. Thus, gastric and colonic epithelial cells may use different TLR members to discern pathogenicities among bacteria, depending on their environments and to activate Nox1 appropriately for host defense. Nox1-derived reactive oxygen species (ROS) have been implicated in the pathogenesis of inflammation-associated tumor development. The human stomach does not express Nox1. Helicobacter pylori infection alone does not induce it, whereas Nox1 is specifically expressed in gastric adenocarcinomas. In the human colon, Nox1 is differentiation-dependently expressed, and its expression is upregulated in adenomas and well-differentiated adenocarcinomas. Although Nox1 expression may not be directly linked to mitogenic activity, Nox1-derived ROS may exert a cancer-promoting effect by increasing resistance to programmed cell death of tumor cells.