Activation of NADPH oxidase-related proton and electron currents in human eosinophils by arachidonic acid

Reactive oxygen species and molecular regulation of renal oxygenation

It has been known since the 1940s that a gradient of renal oxygenation exists in the kidney with the lowest PO2 in the renal inner medulla under physiological conditions. Due to a low PO2 milieu in the renal medulla, the cells in this region are at constant risk of hypoxic injury. Although numerous studies have shown that renal medullary cells adapt well to low PO2, the precise mechanism mediating this adaptive response remains poorly understood. Recently, hypoxia-induced molecular adaptation in mammalian tissues or cells has been studied extensively and many studies have indicated that the molecular regulation of gene expression is importantly involved. This paper focuses on the role of a transcription factor, hypoxia-inducible factor-1 (HIF-1)-mediated molecular adaptation and explores the physiological relevance of molecular activation of HIF-1 and its target genes in the renal medulla. Given that this HIF-1-mediated action is associated with local redox status, evidence is presented to indicate that reactive oxygen species (ROS), especially superoxide (O) is importantly involved in HIF-1-mediated molecular adaptation in renal medullary cells. O degrades HIF-1alpha, an HIF-1 subunit, by activating ubiquitin-proteasome and thereby decreases the transcriptional activation of many oxygen-sensitive genes. This action of O disturbs renal medullary adaptation to low PO2 and produces renal medullary dysfunction, resulting in sodium retention and hypertension. This report also provides evidence indicating the primary source of O, enzymatic pathways for O production and activating mechanism of O production in the kidney. It is concluded that HIF-1-mediated molecular adaptation to low PO2 is of importance in the regulation of renal medullary function and that ROS may target this HIF-1-mediated medullary adaptation to damage renal function.

Arachidonic acid triggers an oxidative burst in leukocytes

The change in cellular reducing potential, most likely reflecting an oxidative burst, was investigated in arachidonic acid- (AA) stimulated leukocytes. The cells studied included the human leukemia cell lines HL-60 (undifferentiated and differentiated into macrophage-like and polymorphonuclear-like cells), Jurkat and Raji, and thymocytes and macrophages from rat primary cultures. The oxidative burst was assessed by nitroblue tetrazolium reduction. AA increased the oxidative burst until an optimum AA concentration was reached and the burst decreased thereafter. In the leukemia cell lines, optimum concentration ranged from 200 to 400 microM (up to 16-fold), whereas in rat cells it varied from 10 to 20 microM. Initial rates of superoxide generation were high, decreasing steadily and ceasing about 2 h post-treatment. The continuous presence of AA was not needed to stimulate superoxide generation. It seems that the NADPH oxidase system participates in AA-stimulated superoxide production in these cells since the oxidative burst was stimulated by NADPH and inhibited by N-ethylmaleimide, diphenyleneiodonium and superoxide dismutase. Some of the effects of AA on the oxidative burst may be due to its detergent action. There apparently was no contribution of other superoxide-generating systems such as xanthine-xanthine oxidase, cytochromes p-450 and mitochondrial electron transport chain, as assessed by the use of inhibitors. Eicosanoids and nitric oxide also do not seem to interfere with the AA-stimulated oxidative burst since there was no systematic effect of cyclooxygenase, lipoxygenase or nitric oxide synthase inhibitors, but lipid peroxides may play a role, as indicated by the inhibition of nitroblue tetrazolium reduction promoted by tocopherol.

Regulation of eosinophil membrane depolarization during NADPH oxidase activation

Protein kinase C (PKC) activation in human eosinophils increases NADPH oxidase activity, which is associated with plasma membrane depolarization. In this study, membrane potential measurements of eosinophils stimulated with phorbol ester (phorbol 12-myristate 13-acetate; PMA) were made using a cell-permeable oxonol membrane potential indicator, diBAC4(3). Within 10 minutes after PMA stimulation, eosinophils depolarized from -32.9+/-5.7 mV to +17.3+/-1.8 mV. The time courses of depolarization and proton channel activation were virtually identical. Blocking the proton conductance with 250 microM ZnCl2 (+43.0+/-4.2 mV) or increasing the proton channel activation threshold by reducing the extracellular pH to 6.5 (+44.4+/-1.4 mV) increased depolarization compared with PMA alone. Additionally, the protein kinase C (PKC) delta-selective blocker, rottlerin, inhibited PMA-stimulated depolarization, indicating that PKCdelta was involved in regulating depolarization associated with eosinophil NADPH oxidase activity. Thus, the membrane depolarization that is associated with NADPH oxidase activation in eosinophils is sufficient to produce marked proton channel activation under physiological conditions.

Temperature dependence of NADPH oxidase in human eosinophils

The phagocyte NADPH oxidase helps kill pathogens by producing superoxide anion, O2-. This enzyme is electrogenic because it translocates electrons across the membrane, generating an electron current, Ie. Using the permeabilized patch voltage-clamp technique, we studied the temperature dependence of Ie in human eosinophils stimulated by phorbol myristate acetate (PMA) from room temperature to >37 degrees C. For comparison, NADPH oxidase activity was assessed by cytochrome c reduction. The intrinsic temperature dependence of the assembled, functioning NADPH oxidase complex measured during rapid temperature increases to 37 degrees C was surprisingly weak: the Arrhenius activation energy Ea was only 14 kcal mol(-1) (Q10, 2.2). In contrast, steady-state NADPH oxidase activity was strongly temperature dependent at 20-30 degrees C, with Ea 25.1 kcal mol(-1) (Q10, 4.2). The maximum Ie measured at 34 degrees C was -30.5 pA. Above 30 degrees C, the temperature dependence of both Ie and O2- production was less pronounced. Above 37 degrees C, Ie was inhibited reversibly. After rapid temperature increases, a secondary increase in Ie ensued, suggesting that high temperature promotes assembly of additional NADPH oxidase complexes. Evidently, about twice as many NADPH oxidase complexes are active near 37 degrees C than at 20 degrees C. Thus, the higher Q10 of steady-state Ie reflects both increased activity of each NADPH oxidase complex and preferential assembly of NADPH oxidase complexes at high temperature. In summary, NADPH oxidase activity in intact human eosinophils is maximal precisely at 37 degrees C.

The voltage dependence of NADPH oxidase reveals why phagocytes need proton channels

The enzyme NADPH oxidase in phagocytes is important in the body's defence against microbes: it produces superoxide anions (O2-, precursors to bactericidal reactive oxygen species). Electrons move from intracellular NADPH, across a chain comprising FAD (flavin adenine dinucleotide) and two haems, to reduce extracellular O2 to O2-. NADPH oxidase is electrogenic, generating electron current (I(e)) that is measurable under voltage-clamp conditions. Here we report the complete current-voltage relationship of NADPH oxidase, the first such measurement of a plasma membrane electron transporter. We find that I(e) is voltage-independent from -100 mV to >0 mV, but is steeply inhibited by further depolarization, and is abolished at about +190 mV. It was proposed that H+ efflux mediated by voltage-gated proton channels compensates I(e), because Zn2+ and Cd2+ inhibit both H+ currents and O2- production. Here we show that COS-7 cells transfected with four NADPH oxidase components, but lacking H+ channels, produce O2- in the presence of Zn2+ concentrations that inhibit O2- production in neutrophils and eosinophils. Zn2+ does not inhibit NADPH oxidase directly, but through effects on H+ channels. H+ channels optimize NADPH oxidase function by preventing membrane depolarization to inhibitory voltages.

The degree of unsaturation of dietary fatty acids and the development of atherosclerosis (review)

Atherosclerosis is the principal contributor to the pathogenesis of myocardial and cerebral infarction, gangrene and loss of function in the extremities. It results from an excessive inflammatory-fibroproliferative response to various forms of insult to the endothelium and smooth muscle of the artery wall. Atherosclerotic lesions develop fundamentally in three stages: dysfunction of the vascular endothelium, fatty streak formation and fibrous cap formation. Each stage is regulated by the action of vasoactive molecules, growth factors and cytokines. This multifactorial etiology can be modulated through the diet. The degree of unsaturation of dietary fatty acids affects lipoprotein composition as well as the expression of adhesion molecules and other pro-inflammatory factors, and the thrombogenicity associated with atherosclerosis development. Thus, the preventive effects of a monounsaturated-fatty acid-rich diet on atherosclerosis may be explained by the enhancement of high-density lipoprotein-cholesterol levels and the impairment of low-density lipoprotein-cholesterol levels, the low-density lipoprotein susceptibility to oxidation, cellular oxidative stress, thrombogenicity and atheroma plaque formation. On the other hand, the increase of high-density lipoprotein cholesterol levels and the reduction of thrombogenicity, atheroma plaque formation and vascular smooth muscle cell proliferation may account for the beneficial effects of polyunsaturated fatty acid on the prevention of atherosclerosis. Thus, the advantages of the Mediterranean diet rich in olive oil and fish on atherosclerosis may be due to the modulation of the cellular oxidative stress/antioxidant status, the modification of lipoproteins and the down-regulation of inflammatory mediators.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Mitochondrial reactive oxygen species regulate spatial profile of proinflammatory responses in lung venular capillaries

Cytokine-induced lung expression of the endothelial cell (EC) leukocyte receptor P-selectin initiates leukocyte rolling. To understand the early EC signaling that induces the expression, we conducted real-time digital imaging studies in lung venular capillaries. To compare receptor- vs nonreceptor-mediated effects, we infused capillaries with respectively, TNF-alpha and arachidonate. At concentrations adjusted to give equipotent increases in the cytosolic Ca(2+), both agents increased reactive oxygen species (ROS) production and EC P-selectin expression. Blocking the cytosolic Ca(2+) increases abolished ROS production; blocking ROS production abrogated P-selectin expression. TNF-alpha, but not arachidonate, released Ca(2+) from endoplasmic stores and increased mitochondrial Ca(2+). Furthermore, Ca(2+) depletion abrogated TNF-alpha responses partially, but arachidonate responses completely. These differences in Ca(2+) mobilization by TNF-alpha and arachidonate were reflected in spatial patterning in the capillary in that the TNF-alpha effects were localized at branch points, while the arachidonate effects were nonlocalized and extensive. Furthermore, mitochondrial blockers inhibited the TNF-alpha- but not the arachidonate-induced responses. These findings indicate that the different modes of Ca(2+) mobilization determined the spatial patterning of the proinflammatory response in lung capillaries. Responses to TNF-alpha revealed that EC mitochondria regulate the proinflammatory process by generating ROS that activate P-selectin expression.

Developmental changes in the management of acid loads during preimplantation mouse development

Intracellular pH recovery in Quackenbush Swiss mouse preimplantation embryos following acid loading was investigated under conditions of H+-monocarboxylate cotransporter inactivity. Isoform-sensitive inhibitors of Na+-H+ exchange (NHE) were used to block the Na+-dependent component of the response. A biphasic dose-response curve for HOE-694 and N-methylisopropylamiloride (MIA) suggested that two isoforms (putatively NHE1 and NHE3) are active in the oocyte, 1-cell, and 2-cell stages. By the blastocyst stage, loss of one of the MIA-sensitive NHE activities (putatively NHE3) was observed in isolated inner cell masses, and an MIA-resistant component of the recovery was identified. The MIA-resistant component was inhibited by 2 mM amiloride and enhanced by external K+ and by 4,4'-diisothiocyanostilbene-2,2'-disulfonate, suggesting NHE4 activity. However, unlike NHE4 in other tissues, the MIA-resistant component did not transport Li+ in exchange for H+, and reverse transcription-polymerase chain reaction detected NHE4 mRNA in the oocyte but not in later stages. Trophoblast, whether in intact or collapsed blastocysts, did not show measurable NHE activity or MIA-sensitive activity during recovery from acid load. Both trophoblast and pluriblast manifested an H+ conductance in response to acid load. This H+ conductance was first detected at the 8-cell stage and was blocked by zinc in the isolated inner cell mass but not in trophoblast. No other effective inhibitors of its activity were found.

Absence of proton channels in COS-7 cells expressing functional NADPH oxidase components

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is an enzyme of phagocytes that produces bactericidal superoxide anion (O(2)(-)) via an electrogenic process. Proton efflux compensates for the charge movement across the cell membrane. The proton channel responsible for the H(+) efflux was thought to be contained within the gp91(phox) subunit of NADPH oxidase, but recent data do not support this idea (DeCoursey, T.E., V.V. Cherny, D. Morgan, B.Z. Katz, and M.C. Dinauer. 2001. J. Biol. Chem. 276:36063-36066). In this study, we investigated electrophysiological properties and superoxide production of COS-7 cells transfected with all NADPH oxidase components required for enzyme function (COS(phox)). The 7D5 antibody, which detects an extracellular epitope of the gp91(phox) protein, labeled 96-98% of COS(phox) cells. NADPH oxidase was functional because COS(phox) (but not COS(WT)) cells stimulated by phorbol myristate acetate (PMA) or arachidonic acid (AA) produced superoxide anion. No proton currents were detected in either wild-type COS-7 cells (COS(WT)) or COS(phox) cells studied at pH(o) 7.0 and pH(i) 5.5 or 7.0. Anion currents that decayed at voltages positive to 40 mV were the only currents observed. PMA or AA did not elicit detectable H(+) current in COS(WT) or COS(phox) cells. Therefore, gp91(phox) does not function as a proton channel in unstimulated cells or in activated cells with a demonstrably functional oxidase.

The gp91phox component of NADPH oxidase is not the voltage-gated proton channel in phagocytes, but it helps

During the "respiratory burst," the NADPH oxidase complex of phagocytes produces reactive oxygen species that kill bacteria and other invaders (Babior, B. M. (1999) Blood 93, 1464-1476). Electron efflux through NADPH oxidase is electrogenic (Henderson, L. M., Chappell, J. B., and Jones, O. T. G. (1987) Biochem. J. 246, 325-329) and is compensated by H(+) efflux through proton channels that reportedly are contained within the gp91(phox) subunit of NADPH oxidase. To test whether gp91(phox) functions as a proton channel, we studied H(+) currents in granulocytes from X-linked chronic granulomatous disease patients lacking gp91(phox) (X-CGD), the human myelocytic PLB-985 cell line, PLB-985 cells in which gp91(phox) was knocked out by gene targeting (PLB(KO)), and PLB-985 knockout cells re-transfected with gp91(phox) (PLB(91)). H(+) currents in unstimulated PLB(KO) cells had amplitude and gating kinetics similar to PLB(91) cells. Furthermore, stimulation with the phorbol ester phorbol 12-myristate 13-acetate increased H(+) currents to a similar extent in X-CGD, PLB(KO), and PLB(91) cells. Thus, gp91(phox) is not the proton channel in unstimulated phagocytes and does not directly mediate the increase of proton conductance during the respiratory burst. Changes in H(+) channel gating kinetics during NADPH oxidase activity are likely crucial to the activation of H(+) flux during the respiratory burst.

Interactions between NADPH oxidase-related proton and electron currents in human eosinophils

1. Proton and electron currents in human eosinophils were studied using the permeabilized-patch voltage-clamp technique, with an applied NH4+ gradient to control pH(i). 2. Voltage-gated proton channels in unstimulated human eosinophils studied with the permeabilized-patch approach had properties similar to those reported in whole-cell studies. 3. Superoxide anion (O2-) release assessed by cytochrome c reduction was compared in human eosinophils and neutrophils stimulated by phorbol myristate acetate (PMA). PMA-stimulated O2 release was more transient and the maximum rate was three times greater in eosinophils. 4. In PMA-activated eosinophils, the H+ current amplitude (I(H)) at +60 mV increased 4.7-fold, activation was 4.0 times faster, deactivation (tail current decay) was 5.4 times slower, the H+ conductance-voltage (g(H)-V) relationship was shifted -43 mV, and diphenylene iodinium (DPI)-inhibitable inward current reflecting electron flow through NADPH oxidase was activated. The data reveal that PMA activates the H+ efflux during the respiratory burst by modulating the properties of H+ channels, not simply as a result of NADPH oxidase activity. 5. The electrophysiological response of eosinophils to PMA resembled that reported in human neutrophils, but PMA activated larger proton and electron currents in eosinophils and the response was more transient. 6. ZnCl2 slowed the activation of H+ currents and shifted the g(H)-V relationship to more positive voltages. These effects occurred at similar ZnCl2 concentrations in eosinophils before and after PMA stimulation. These data are compatible with the existence of a single type of H+ channel in eosinophils that is modulated during the respiratory burst.