Discovery and Characterization of Halogenated Xanthene Inhibitors of DUSP5 as Potential Photodynamic Therapeutics.

Dual specific phosphatases (DUSPs) are an important class of mitogen-activated protein kinase (MAPK) regulators, and are drug targets for treating vascular diseases. Previously we had shown that DUSP5 plays a role in embryonic vertebrate vascular patterning. Herein, we screened a library of FDA-approved drugs and related compounds, using a para-nitrophenylphosphate substrate (pNPP)-based assay. This assay identified merbromin (also known as mercurochrome) as targeting DUSP5; and, we subsequently identified xanthene-ring based merbromin analogs eosin Y, erythrosin B, and rose bengal, all of which inhibit DUSP5 in vitro. Inhibition was time-dependent for merbromin, eosin Y, 2',7'-dibromofluorescein, and 2',7'-dichlorofluorescein, with enzyme inhibition increasing over time. Reaction progress curve data fit best to a slow-binding model of irreversible enzyme inactivation. Potency of the time-dependent compounds, except for 2',7'-dichlorofluorescein, was diminished when dithiothreitol (DTT) was present, suggesting thiol reactivity. Two additional merbromin analogs, erythrosin B and rose bengal also inhibit DUSP5, but have the therapeutic advantage of being less sensitive to DTT and exhibiting little time dependence for inhibition. Inhibition potency is correlated with the xanthene dye's LUMO energy, which affects ability to form light-activated radical anions, a likely active inhibitor form. Consistent with this hypothesis, rose bengal inhibition is light-dependent and demonstrates the expected red shifted spectrum upon binding to DUSP5, with a Kd of 690 nM. These studies provide a mechanistic foundation for further development of xanthene dyes for treating vascular diseases that respond to DUSP5 inhibition, with the following relative potencies: rose bengal > merbromin > erythrosin B > eosin Y.

Dual Specificity Phosphatase 5-Substrate Interaction: A Mechanistic Perspective.

The mammalian genome contains approximately 200 phosphatases that are responsible for catalytically removing phosphate groups from proteins. In this review, we discuss dual specificity phosphatase 5 (DUSP5). DUSP5 belongs to the dual specificity phosphatase (DUSP) family, so named after the family members' abilities to remove phosphate groups from serine/threonine and tyrosine residues. We provide a comparison of DUSP5's structure to other DUSPs and, using molecular modeling studies, provide an explanation for DUSP5's mechanistic interaction and specificity toward phospho-extracellular regulated kinase, its only known substrate. We also discuss new insights from molecular modeling studies that will influence our current thinking of mitogen-activated protein kinase signaling. Finally, we discuss the lessons learned from identifying small molecules that target DUSP5, which might benefit targeting efforts for other phosphatases. © 2017 American Physiological Society. Compr Physiol 7:1449-1461, 2017.

Serendipitous discovery of light-induced (In Situ) formation of an Azo-bridged dimeric sulfonated naphthol as a potent PTP1B inhibitor.

BACKGROUND: Protein tyrosine phosphatases (PTPs) like dual specificity phosphatase 5 (DUSP5) and protein tyrosine phosphatase 1B (PTP1B) are drug targets for diseases that include cancer, diabetes, and vascular disorders such as hemangiomas. The PTPs are also known to be notoriously difficult targets for designing inihibitors that become viable drug leads. Therefore, the pipeline for approved drugs in this class is minimal. Furthermore, drug screening for targets like PTPs often produce false positive and false negative results. RESULTS: Studies presented herein provide important insights into: (a) how to detect such artifacts, (b) the importance of compound re-synthesis and verification, and (c) how in situ chemical reactivity of compounds, when diagnosed and characterized, can actually lead to serendipitous discovery of valuable new lead molecules. Initial docking of compounds from the National Cancer Institute (NCI), followed by experimental testing in enzyme inhibition assays, identified an inhibitor of DUSP5. Subsequent control experiments revealed that this compound demonstrated time-dependent inhibition, and also a time-dependent change in color of the inhibitor that correlated with potency of inhibition. In addition, the compound activity varied depending on vendor source. We hypothesized, and then confirmed by synthesis of the compound, that the actual inhibitor of DUSP5 was a dimeric form of the original inhibitor compound, formed upon exposure to light and oxygen. This compound has an IC50 of 36 µM for DUSP5, and is a competitive inhibitor. Testing against PTP1B, for selectivity, demonstrated the dimeric compound was actually a more potent inhibitor of PTP1B, with an IC50 of 2.1 µM. The compound, an azo-bridged dimer of sulfonated naphthol rings, resembles previously reported PTP inhibitors, but with 18-fold selectivity for PTP1B versus DUSP5. CONCLUSION: We report the identification of a potent PTP1B inhibitor that was initially identified in a screen for DUSP5, implying common mechanism of inhibitory action for these scaffolds.

Critical Role of the Secondary Binding Pocket in Modulating the Enzymatic Activity of DUSP5 toward Phosphorylated ERKs.

DUSP5 is an inducible nuclear dual-specificity phosphatase that specifically interacts with and deactivates extracellular signal-regulated kinases ERK1 and ERK2, which are responsible for cell proliferation, differentiation, and survival. The phosphatase domain (PD) of DUSP5 has unique structural features absent from other nuclear DUSPs, such as the presence of a secondary anion-binding site in the proximity of the reaction center and a glutamic acid E264 positioned next to the catalytic cysteine C263, as well as a remote intramolecular disulfide linkage. The overall 400 ns molecular dynamics simulations indicate that the secondary binding site of DUSP5 PD acts as an allosteric regulator of the phosphatase activity of DUSP5. Our studies have identified E264 as a critical constituent of the dual binding pocket, which regulates the catalytic activity of DUSP5 by forming a salt bridge with arginine R269. Molecular dynamics studies showed that initial occupation of the secondary binding pocket leads to the breakage of the salt bridge, which then allows the occupation of the active site. Indeed, biochemical analysis using the pERK assay on mutant E264Q demonstrated that mutation of glutamic acid E264 leads to an increase in the DUSP5 catalytic activity. The role of the secondary binding site in assembling the DUSP5-pERK pre-reactive complex was further demonstrated by molecular dynamics simulations that showed that the remote C197-C219 disulfide linkage controls the structure of the secondary binding pocket based on its redox state (i.e., disulfide/dithiol) and, in turn, the enzymatic activity of DUSP5.

The effects of mitochondrial complex I blockade on ATP and permeability in rat pulmonary microvascular endothelial cells in culture (PMVEC) are overcome by coenzyme Q1 (CoQ1).

In isolated rat lung perfused with a physiological saline solution (5.5mM glucose), complex I inhibitors decrease lung tissue ATP and increase endothelial permeability (Kf), effects that are overcome using an amphipathic quinone (CoQ1) [Free Radic. Biol. Med.65:1455-1463; 2013]. To address the microvascular endothelial contribution to these intact lung responses, rat pulmonary microvascular endothelial cells in culture (PMVEC) were treated with the complex I inhibitor rotenone and ATP levels and cell monolayer permeability (PS) were measured. There were no detectable effects on ATP or permeability in experimental medium that, like the lung perfusate, contained 5.5mM glucose. To unmask a potential mitochondrial contribution, the glucose concentration was lowered to 0.2mM. Under these conditions, rotenone decreased ATP from 18.4±1.6 (mean±SEM) to 4.6±0.8nmol/mg protein, depolarized the mitochondrial membrane potential (Δψm) from -129.0±3.7 (mean±SEM) to -92.8±5.5mV, and decreased O2 consumption from 2.0±0.1 (mean±SEM) to 0.3±0.1nmol/min/mg protein. Rotenone also increased PMVEC monolayer permeability (reported as PS in nl/min) to FITC-dextran (~40kDa) continually over a 6 h time course. When CoQ1 was present with rotenone, normal ATP (17.4±1.4nmol/mg protein), O2 consumption (1.5±0.1nmol/min/mg protein), Δψm (-125.2±3.3mV), and permeability (PS) were maintained. Protective effects of CoQ1 on rotenone-induced changes in ATP, O2 consumption rate, Δψm, and permeability were blocked by dicumarol or antimycin A, inhibitors of the quinone-mediated cytosol-mitochondria electron shuttle [Free Radic. Biol. Med.65:1455-1463; 2013]. Key rotenone effects without and with CoQ1 were qualitatively reproduced using the alternative complex I inhibitor, piericidin A. We conclude that, as in the intact lung, PMVEC ATP supply is linked to the permeability response to complex I inhibitors. In contrast to the intact lung, the association in PMVEC was revealed only after decreasing the glucose concentration in the experimental medium from 5.5 to 0.2mM.

Coenzyme Q supplementation in pulmonary arterial hypertension.

Mitochondrial dysfunction is a fundamental abnormality in the vascular endothelium and smooth muscle of patients with pulmonary arterial hypertension (PAH). Because coenzyme Q (CoQ) is essential for mitochondrial function and efficient oxygen utilization as the electron carrier in the inner mitochondrial membrane, we hypothesized that CoQ would improve mitochondrial function and benefit PAH patients. To test this, oxidized and reduced levels of CoQ, cardiac function by echocardiogram, mitochondrial functions of heme synthesis and cellular metabolism were evaluated in PAH patients (N=8) in comparison to healthy controls (N=7), at baseline and after 12 weeks oral CoQ supplementation. CoQ levels were similar among PAH and control individuals, and increased in all subjects with CoQ supplementation. PAH patients had higher CoQ levels than controls with supplementation, and a tendency to a higher reduced-to-oxidized CoQ ratio. Cardiac parameters improved with CoQ supplementation, although 6-minute walk distances and BNP levels did not significantly change. Consistent with improved mitochondrial synthetic function, hemoglobin increased and red cell distribution width (RDW) decreased in PAH patients with CoQ, while hemoglobin declined slightly and RDW did not change in healthy controls. In contrast, metabolic and redox parameters, including lactate, pyruvate and reduced or oxidized gluthathione, did not change in PAH patients with CoQ. In summary, CoQ improved hemoglobin and red cell maturation in PAH, but longer studies and/or higher doses with a randomized placebo-controlled controlled design are necessary to evaluate the clinical benefit of this simple nutritional supplement.

Depleted energy charge and increased pulmonary endothelial permeability induced by mitochondrial complex I inhibition are mitigated by coenzyme Q1 in the isolated perfused rat lung.

Mitochondrial dysfunction is associated with various forms of lung injury and disease that also involve alterations in pulmonary endothelial permeability, but the relationship, if any, between the two is not well understood. This question was addressed by perfusing isolated intact rat lung with a buffered physiological saline solution in the absence or presence of the mitochondrial complex I inhibitor rotenone (20 µM). Compared to control, rotenone depressed whole lung tissue ATP from 5.66 ± 0.46 (SEM) to 2.34 ± 0.15 µmol · g(-1) dry lung, with concomitant increases in the ADP:ATP and AMP:ATP ratios. Rotenone also increased lung perfusate lactate (from 12.36 ± 1.64 to 38.62 ± 3.14 µmol · 15 min(-1) perfusion · g(-1) dry lung) and the lactate:pyruvate ratio, but had no detectable impact on lung tissue GSH:GSSG redox status. The amphipathic quinone coenzyme Q1 (CoQ1; 50 µM) mitigated the impact of rotenone on the adenine nucleotide balance, wherein mitigation was blocked by NAD(P)H-quinone oxidoreductase 1 or mitochondrial complex III inhibitors. In separate studies, rotenone increased the pulmonary vascular endothelial filtration coefficient (Kf) from 0.043 ± 0.010 to 0.156 ± 0.037 ml · min(-1) · cm H2O(-1) · g(-1) dry lung, and CoQ1 protected against the effect of rotenone on Kf. A second complex I inhibitor, piericidin A, qualitatively reproduced the impact of rotenone on Kf and the lactate:pyruvate ratio. Taken together, the observations imply that pulmonary endothelial barrier integrity depends on mitochondrial bioenergetics as reflected in lung tissue ATP levels and that compensatory activation of whole lung glycolysis cannot protect against pulmonary endothelial hyperpermeability in response to mitochondrial blockade. The study further suggests that low-molecular-weight amphipathic quinones may have therapeutic utility in protecting lung barrier function in mitochondrial insufficiency.

Coenzyme Q(1) as a probe for mitochondrial complex I activity in the intact perfused hyperoxia-exposed wild-type and Nqo1-null mouse lung.

Previous studies showed that coenzyme Q(1) (CoQ(1)) reduction on passage through the rat pulmonary circulation was catalyzed by NAD(P)H:quinone oxidoreductase 1 (NQO1) and mitochondrial complex I, but that NQO1 genotype was not a factor in CoQ(1) reduction on passage through the mouse lung. The aim of the present study was to evaluate the complex I contribution to CoQ(1) reduction in the isolated perfused wild-type (NQO1(+/+)) and Nqo1-null (NQO1(-)/(-)) mouse lung. CoQ(1) reduction was measured as the steady-state pulmonary venous CoQ(1) hydroquinone (CoQ(1)H(2)) efflux rate during infusion of CoQ(1) into the pulmonary arterial inflow. CoQ(1)H(2) efflux rates during infusion of 50 µM CoQ(1) were not significantly different for NQO1(+/+) and NQO1(-/-) lungs (0.80 ± 0.03 and 0.68 ± 0.07 µmol·min(-1)·g lung dry wt(-1), respectively, P > 0.05). The mitochondrial complex I inhibitor rotenone depressed CoQ(1)H(2) efflux rates for both genotypes (0.19 ± 0.08 and 0.08 ± 0.04 µmol·min(-1)·g lung dry wt(-1) for NQO1(+/+) and NQO1(-/-), respectively, P < 0.05). Exposure of mice to 100% O(2) for 48 h also depressed CoQ(1)H(2) efflux rates in NQO1(+/+) and NQO1(-/-) lungs (0.43 ± 0.03 and 0.11 ± 0.04 µmol·min(-1)·g lung dry wt(-1), respectively, P < 0.05 by ANOVA). The impact of rotenone or hyperoxia on CoQ(1) redox metabolism could not be attributed to effects on lung wet-to-dry weight ratios, perfusion pressures, perfused surface areas, or total venous effluent CoQ(1) recoveries, the latter measured by spectrophotometry or mass spectrometry. Complex I activity in mitochondria-enriched lung fractions was depressed in hyperoxia-exposed lungs for both genotypes. This study provides new evidence for the potential utility of CoQ(1) as a nondestructive indicator of the impact of pharmacological or pathological exposures on complex I activity in the intact perfused mouse lung.

Genetic evidence for NAD(P)H:quinone oxidoreductase 1-catalyzed quinone reduction on passage through the mouse pulmonary circulation.

The quinones duroquinone (DQ) and coenzyme Q(1) (CoQ(1)) and quinone reductase inhibitors have been used to identify reductases involved in quinone reduction on passage through the pulmonary circulation. In perfused rat lung, NAD(P)H:quinone oxidoreductase 1 (NQO1) was identified as the predominant DQ reductase and NQO1 and mitochondrial complex I as the CoQ(1) reductases. Since inhibitors have nonspecific effects, the goal was to use Nqo1-null (NQO1(-)/(-)) mice to evaluate DQ as an NQO1 probe in the lung. Lung homogenate cytosol NQO1 activities were 97 ± 11, 54 ± 6, and 5 ± 1 (SE) nmol dichlorophenolindophenol reduced·min(-1)·mg protein(-1) for NQO1(+/+), NQO1(+/-), and NQO1(-/-) lungs, respectively. Intact lung quinone reduction was evaluated by infusion of DQ (50 µM) or CoQ(1) (60 µM) into the pulmonary arterial inflow of the isolated perfused lung and measurement of pulmonary venous effluent hydroquinone (DQH(2) or CoQ(1)H(2)). DQH(2) efflux rates for NQO1(+/+), NQO1(+/-), and NQO1(-/-) lungs were 0.65 ± 0.08, 0.45 ± 0.04, and 0.13 ± 0.05 (SE) µmol·min(-1)·g dry lung(-1), respectively. DQ reduction in NQO1(+/+) lungs was inhibited by 90 ± 4% with dicumarol; there was no inhibition in NQO1(-/-) lungs. There was no significant difference in CoQ(1)H(2) efflux rates for NQO1(+/+) and NQO1(-/-) lungs. Differences in DQ reduction were not due to differences in lung dry weights, wet-to-dry weight ratios, perfusion pressures, perfused surface areas, or total DQ recoveries. The data provide genetic evidence implicating DQ as a specific NQO1 probe in the perfused rodent lung.

Quantifying mitochondrial and plasma membrane potentials in intact pulmonary arterial endothelial cells based on extracellular disposition of rhodamine dyes.

Our goal was to quantify mitochondrial and plasma potential (Δψ(m) and Δψ(p)) based on the disposition of rhodamine 123 (R123) or tetramethylrhodamine ethyl ester (TMRE) in the medium surrounding pulmonary endothelial cells. Dyes were added to the medium, and their concentrations in extracellular medium ([R(e)]) were measured over time. R123 [R(e)] fell from 10 nM to 6.6 ± 0.1 (SE) nM over 120 min. TMRE [R(e)] fell from 20 nM to a steady state of 4.9 ± 0.4 nM after ∼30 min. Protonophore or high K(+) concentration ([K(+)]), used to manipulate contributions of membrane potentials, attenuated decreases in [R(e)], and P-glycoprotein (Pgp) inhibition had the opposite effect, demonstrating the qualitative impact of these processes on [R(e)]. A kinetic model incorporating a modified Goldman-Hodgkin-Katz model was fit to [R(e)] vs. time data for R123 and TMRE, respectively, under various conditions to obtain (means ± 95% confidence intervals) Δψ(m) (-130 ± 7 and -133 ± 4 mV), Δψ(p) (-36 ± 4 and -49 ± 4 mV), and a Pgp activity parameter (K(Pgp), 25 ± 5 and 51 ± 11 µl/min). The higher membrane permeability of TMRE also allowed application of steady-state analysis to obtain Δψ(m) (-124 ± 6 mV). The consistency of kinetic parameter values obtained from R123 and TMRE data demonstrates the utility of this experimental and theoretical approach for quantifying intact cell Δψ(m) and Δψ(p.) Finally, steady-state analysis revealed that although room air- and hyperoxia-exposed (95% O(2) for 48 h) cells have equivalent resting Δψ(m), hyperoxic cell Δψ(m) was more sensitive to depolarization with protonophore, consistent with previous observations of pulmonary endothelial hyperoxia-induced mitochondrial dysfunction.

Characterization of the threshold for NAD(P)H:quinone oxidoreductase activity in intact sulforaphane-treated pulmonary arterial endothelial cells.

Treatment of bovine pulmonary arterial endothelial cells in culture with the phase II enzyme inducer sulforaphane (5µM, 24h; sulf-treated) increased cell-lysate NAD(P)H:quinone oxidoreductase (NQO1) activity by 5.7 ± 0.6 (mean ± SEM)-fold, but intact-cell NQO1 activity by only 2.8 ± 0.1-fold compared to control cells. To evaluate the hypothesis that the threshold for sulforaphane-induced intact-cell NQO1 activity reflects a limitation in the capacity to supply NADPH at a sufficient rate to drive all the induced NQO1 to its maximum activity, total KOH-extractable pyridine nucleotides were measured in cells treated with duroquinone to stimulate maximal NQO1 activity. NQO1 activation increased NADP(+) in control and sulf-treated cells, with the effect more pronounced in the sulf-treated cells, in which the NADPH was also decreased. Glucose-6-phosphate dehydrogenase (G-6-PDH) inhibition partially blocked NQO1 activity in control and sulf-treated cells, but G-6-PDH overexpression via transient transfection with the human cDNA alleviated neither the restriction on intact sulf-treated cell NQO1 activity nor the impact on the NADPH/NADP(+) ratios. Intracellular ATP levels were not affected by NQO1 activation in control or sulf-treated cells. An increased dependence on extracellular glucose and a rightward shift in the K(m) for extracellular glucose were observed in NQO1-stimulated sulf-treated vs control cells. The data suggest that glucose transport in the sulf-treated cells may be insufficient to support the increased metabolic demand for pentose phosphate pathway-generated NADPH as an explanation for the NQO1 threshold.

Preferential utilization of NADPH as the endogenous electron donor for NAD(P)H:quinone oxidoreductase 1 (NQO1) in intact pulmonary arterial endothelial cells.

The goal was to determine whether endogenous cytosolic NAD(P)H:quinone oxidoreductase 1 (NQO1) preferentially uses NADPH or NADH in intact pulmonary arterial endothelial cells in culture. The approach was to manipulate the redox status of the NADH/NAD(+) and NADPH/NADP(+) redox pairs in the cytosolic compartment using treatment conditions targeting glycolysis and the pentose phosphate pathway alone or with lactate, and to evaluate the impact on the intact cell NQO1 activity. Cells were treated with 2-deoxyglucose, iodoacetate, or epiandrosterone in the absence or presence of lactate, NQO1 activity was measured in intact cells using duroquinone as the electron acceptor, and pyridine nucleotide redox status was measured in total cell KOH extracts by high-performance liquid chromatography. 2-Deoxyglucose decreased NADH/NAD(+) and NADPH/NADP(+) ratios by 59 and 50%, respectively, and intact cell NQO1 activity by 74%; lactate restored NADH/NAD(+), but not NADPH/NADP(+) or NQO1 activity. Iodoacetate decreased NADH/NAD(+) but had no detectable effect on NADPH/NADP(+) or NQO1 activity. Epiandrosterone decreased NQO1 activity by 67%, and although epiandrosterone alone did not alter the NADPH/NADP(+) or NADH/NAD(+) ratio, when the NQO1 electron acceptor duroquinone was also present, NADPH/NADP(+) decreased by 84% with no impact on NADH/NAD(+). Duroquinone alone also decreased NADPH/NADP(+) but not NADH/NAD(+). The results suggest that NQO1 activity is more tightly coupled to the redox status of the NADPH/NADP(+) than NADH/NAD(+) redox pair, and that NADPH is the endogenous NQO1 electron donor. Parallel studies of pulmonary endothelial transplasma membrane electron transport (TPMET), another redox process that draws reducing equivalents from the cytosol, confirmed previous observations of a correlation with the NADH/NAD(+) ratio.

Coenzyme Q1 redox metabolism during passage through the rat pulmonary circulation and the effect of hyperoxia.

The objective was to evaluate the pulmonary disposition of the ubiquinone homolog coenzyme Q(1) (CoQ(1)) on passage through lungs of normoxic (exposed to room air) and hyperoxic (exposed to 85% O(2) for 48 h) rats. CoQ(1) or its hydroquinone (CoQ(1)H(2)) was infused into the arterial inflow of isolated, perfused lungs, and the venous efflux rates of CoQ(1)H(2) and CoQ(1) were measured. CoQ(1)H(2) appeared in the venous effluent when CoQ(1) was infused, and CoQ(1) appeared when CoQ(1)H(2) was infused. In normoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 58 and 33% in the presence of rotenone (mitochondrial complex I inhibitor) and dicumarol [NAD(P)H-quinone oxidoreductase 1 (NQO1) inhibitor], respectively. Inhibitor studies also revealed that lung CoQ(1)H(2) oxidation was via mitochondrial complex III. In hyperoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 23% compared with normoxic lungs. Based on inhibitor effects and a kinetic model, the effect of hyperoxia could be attributed predominantly to 47% decrease in the capacity of complex I-mediated CoQ(1) reduction, with no change in the other redox processes. Complex I activity in lung homogenates was also lower for hyperoxic than for normoxic lungs. These studies reveal that lung complexes I and III and NQO1 play a dominant role in determining the vascular concentration and redox status of CoQ(1) during passage through the pulmonary circulation, and that exposure to hyperoxia decreases the overall capacity of the lung to reduce CoQ(1) to CoQ(1)H(2) due to a depression in complex I activity.

Role of mitochondrial electron transport complex I in coenzyme Q1 reduction by intact pulmonary arterial endothelial cells and the effect of hyperoxia.

The objective was to determine the impact of intact normoxic and hyperoxia-exposed (95% O(2) for 48 h) bovine pulmonary arterial endothelial cells in culture on the redox status of the coenzyme Q(10) homolog coenzyme Q(1) (CoQ(1)). When CoQ(1) (50 microM) was incubated with the cells for 30 min, its concentration in the medium decreased over time, reaching a lower level for normoxic than hyperoxia-exposed cells. The decreases in CoQ(1) concentration were associated with generation of CoQ(1) hydroquinone (CoQ(1)H(2)), wherein 3.4 times more CoQ(1)H(2) was produced in the normoxic than hyperoxia-exposed cell medium (8.2 +/- 0.3 and 2.4 +/- 0.4 microM, means +/- SE, respectively) after 30 min. The maximum CoQ(1) reduction rate for the hyperoxia-exposed cells, measured using the cell membrane-impermeant redox indicator potassium ferricyanide, was about one-half that of normoxic cells (11.4 and 24.1 nmol x min(-1) x mg(-1) cell protein, respectively). The mitochondrial electron transport complex I inhibitor rotenone decreased the CoQ(1) reduction rate by 85% in the normoxic cells and 44% in the hyperoxia-exposed cells. There was little or no inhibitory effect of NAD(P)H:quinone oxidoreductase 1 (NQO1) inhibitors on CoQ(1) reduction. Intact cell oxygen consumption rates and complex I activities in mitochondria-enriched fractions were also lower for hyperoxia-exposed than normoxic cells. The implication is that intact pulmonary endothelial cells influence the redox status of CoQ(1) via complex I-mediated reduction to CoQ(1)H(2), which appears in the extracellular medium, and that the hyperoxic exposure decreases the overall CoQ(1) reduction capacity via a depression in complex I activity.

Influence of pulmonary arterial endothelial cells on quinone redox status: effect of hyperoxia-induced NAD(P)H:quinone oxidoreductase 1.

The objective of this study was to examine the impact of chronic hyperoxic exposure (95% O2 for 48 h) on intact bovine pulmonary arterial endothelial cell redox metabolism of 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ). DQ or durohydroquinone (DQH2) was added to normoxic or hyperoxia-exposed cells in air-saturated medium, and the medium DQ concentrations were measured over 30 min. DQ disappeared from the medium when DQ was added and appeared in the medium when DQH2 was added, such that after approximately 15 min, a steady-state DQ concentration was approached that was approximately 4.5 times lower for the hyperoxia-exposed than the normoxic cells. The rate of DQ-mediated reduction of the cell membrane-impermeant redox indicator, potassium ferricyanide [Fe(CN)6(3-)], was also approximately twofold faster for the hyperoxia-exposed cells. Inhibitor studies and mathematical modeling suggested that in both normoxic and hyperoxia-exposed cells, NAD(P)H:quinone oxidoreductase 1 (NQO1) was the dominant DQ reductase and mitochondrial electron transport complex III the dominant DQH2 oxidase involved and that the difference between the net effects of the cells on DQ redox status could be attributed primarily to a twofold increase in the maximum NQO1-mediated DQ reduction rate in the hyperoxia-exposed cells. Accordingly, NQO1 protein and total activity were higher in hyperoxia-exposed than normoxic cell cytosolic fractions. One outcome for hyperoxia-exposed cells was enhanced protection from cell-mediated DQ redox cycling. This study demonstrates that exposure to chronic hyperoxia increases the capacity of pulmonary arterial endothelial cells to reduce DQ to DQH2 via a hyperoxia-induced increase in NQO1 protein and total activity.

Effect of chronic hyperoxic exposure on duroquinone reduction in adult rat lungs.

NAD(P)H:quinone oxidoreductase 1 (NQO1) plays a dominant role in the reduction of the quinone compound 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) to durohydroquinone (DQH2) on passage through the rat lung. Exposure of adult rats to 85% O2 for > or =7 days stimulates adaptation to the otherwise lethal effects of >95% O2. The objective of this study was to examine whether exposure of adult rats to hyperoxia affected lung NQO1 activity as measured by the rate of DQ reduction on passage through the lung. We measured DQH2 appearance in the venous effluent during DQ infusion at different concentrations into the pulmonary artery of isolated perfused lungs from rats exposed to room air or to 85% O2. We also evaluated the effect of hyperoxia on vascular transit time distribution and measured NQO1 activity and protein in lung homogenate. The results demonstrate that exposure to 85% O2 for 21 days increases lung capacity to reduce DQ to DQH2 and that NQO1 is the dominant DQ reductase in normoxic and hyperoxic lungs. Kinetic analysis revealed that 21-day hyperoxia exposure increased the maximum rate of pulmonary DQ reduction, Vmax, and the apparent Michaelis-Menten constant for DQ reduction, Kma. The increase in Vmax suggests a hyperoxia-induced increase in NQO1 activity of lung cells accessible to DQ from the vascular region, consistent qualitatively but not quantitatively with an increase in lung homogenate NQO1 activity in 21-day hyperoxic lungs. The increase in Kma could be accounted for by approximately 40% increase in vascular transit time heterogeneity in 21-day hyperoxic lungs.

Impact of pulmonary arterial endothelial cells on duroquinone redox status.

The study objective was to use pulmonary arterial endothelial cells to examine kinetics and mechanisms contributing to the disposition of the quinone 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) observed during passage through the pulmonary circulation. The approach was to add DQ, durohydroquinone (DQH2), or DQ with the cell membrane-impermeant oxidizing agent, ferricyanide (Fe(CN)6(3)-), to the cell medium, and to measure the medium concentrations of substrates and products over time. Studies were carried out under control conditions and with dicumarol, to inhibit NAD(P)H:quinone oxidoreductase 1 (NQO1), or cyanide, to inhibit mitochondrial electron transport. In control cells, DQH2 appears in the extracellular medium of cells incubated with DQ, and DQ appears when the cells are incubated with DQH2. Dicumarol blocked the appearance of DQH2 when DQ was added to the cell medium, and cyanide blocked the appearance of DQ when DQH2 was added to the cell medium, suggesting that the two electron reductase NQO1 dominates DQ reduction and mitochondrial electron transport complex III is the predominant route of DQH2 oxidation. In the presence of cyanide, the addition of DQ also resulted in an increased rate of appearance of DQH2 and stimulation of cyanide-insensitive oxygen consumption. As DQH2 does not autoxidize-comproportionate over the study time course, these observations suggest a cyanide-stimulated one-electron DQ reduction and durosemiquinone (DQ*-) autoxidation. The latter processes are apparently confined to the cell interior, as the cell membrane impermeant oxidant, ferricyanide, did not inhibit the DQ-stimulated cyanide-insensitive oxygen consumption. Thus, regardless of whether DQ is reduced via a one- or two-electron reduction pathway, the net effect in the extracellular medium is the appearance of DQH2. These endothelial redox functions and their apposition to the vessel lumen are consistent with the pulmonary endothelium being an important site of DQ reduction to DQH2 observed in the lungs.

Duroquinone reduction during passage through the pulmonary circulation.

The lungs can substantially influence the redox status of redox-active plasma constituents. Our objective was to examine aspects of the kinetics and mechanisms that determine pulmonary disposition of redox-active compounds during passage through the pulmonary circulation. Experiments were carried out on rat and mouse lungs with 2,3,5,6-tetramethyl-1,4-benzoquinone [duroquinone (DQ)] as a model amphipathic quinone reductase substrate. We measured DQ and durohydroquinone (DQH2) concentrations in the lung venous effluent after injecting, or while infusing, DQ or DQH2 into the pulmonary arterial inflow. The maximum net rates of DQ reduction to DQH2 in the rat and mouse lungs were approximately 4.9 and 2.5 micromol. min(-1).g dry lung wt(-1), respectively. The net rate was apparently the result of freely permeating access of DQ and DQH2 to tissue sites of redox reactions, dominated by dicumarol-sensitive DQ reduction to DQH2 and cyanide-sensitive DQH2 reoxidation back to DQ. The dicumarol sensitivity along with immunodetectable expression of NAD(P)H-quinone oxidoreductase 1 (NQO1) in the rat lung tissue suggest cytoplasmic NQO1 as the dominant site of DQ reduction. The effect of cyanide on DQH2 oxidation suggests that the dominant site of oxidation is complex III of the mitochondrial electron transport chain. If one envisions DQ as a model compound for examining the disposition of amphipathic NQO1 substrates in the lungs, the results are consistent with a role for lung NQO1 in determining the redox status of such compounds in the circulation. For DQ, the effect is conversion of a redox-cycling, oxygen-activating quinone into a stable hydroquinone.

Pulmonary arterial endothelial cells affect the redox status of coenzyme Q0.

The pulmonary endothelium is capable of reducing certain redox-active compounds as they pass from the systemic venous to the arterial circulation. This may have important consequences with regard to the pulmonary and systemic disposition and biochemistry of these compounds. Because quinones comprise an important class of redox-active compounds with a range of physiological, toxicological, and pharmacological activities, the objective of the present study was to determine the fate of a model quinone, coenzyme Q0 (Q), added to the extracellular medium surrounding pulmonary arterial endothelial cells in culture, with particular attention to the effect of the cells on the redox status of Q in the medium. Spectrophotometry, electron paramagnetic resonance (EPR), and high-performance liquid chromatography (HPLC) demonstrated that, when the oxidized form Q is added to the medium surrounding the cells, it is rapidly converted to its quinol form (QH2) with a small concentration of semiquinone (Q*-) also detectable. The isolation of cell plasma membrane proteins revealed an NADH-Q oxidoreductase located on the outer plasma membrane surface, which apparently participates in the reduction process. In addition, once formed the QH2 undergoes a cyanide-sensitive oxidation by the cells. Thus, the actual rate of Q reduction by the cells is greater than the net QH2 output from the cells.