Metabolic capacity regulates iron homeostasis in endothelial cells

The sensitivity of endothelial cells to oxidative stress and the high concentrations of iron in mitochondria led us to test the hypotheses that (1) changes in respiratory capacity alter iron homeostasis, and (2) lack of aerobic metabolism decreases labile iron stores and attenuates oxidative stress. Two respiration-deficient (rho(o)) endothelial cell lines with selective deletion of mitochondrial DNA (mtDNA) were created by exposing a parent endothelial cell line (EA) to ethidium bromide. Surviving cells were cloned and mtDNA-deficient cell lines were demonstrated to have diminished oxygen consumption. Total cellular and mitochondrial iron levels were measured, and iron uptake and compartmentalization were measured by inductively coupled plasma atomic emission spectroscopy. Iron transport and storage protein expression were analyzed by real-time polymerase chain reaction and Western blot or ELISA, and total and mitochondrial reactive oxygen species (ROS) generation was measured. Mitochondrial iron content was the same in all three cell lines, but both rho(o) lines had lower iron uptake and total cellular iron. Protein and mRNA expressions of major cytosolic iron transport constituents were down-regulated in rho(o) cells, including transferrin receptor, divalent metal transporter-1 (-IRE isoform), and ferritin. The mitochondrial iron-handling protein, frataxin, was also decreased in respiration-deficient cells. The rho(o) cell lines generated less mitochondrial ROS but released more extracellular H(2)O(2), and demonstrated significantly lower levels of lipid aldehyde formation than control cells. In summary, rho(o) cells with a minimal aerobic capacity had decreased iron uptake and storage. This work demonstrates that mitochondria regulate iron homeostasis in endothelial cells.

Lunar exploration and the advancement of biomedical research: a physiologist's view

Over the next few years, it will become apparent just how important lunar exploration is to biomedical research and vice versa, and how critical both are to the future of human spaceflight. NASA's Project Constellation should put a new lunar-capable vehicle into service by 2014 that will rely on proven Space Shuttle components and allow four astronauts to spend 7 d on the lunar surface. A modern space transportation system opens up a unique opportunity in the space sciences--the establishment of a permanent lunar laboratory for the physical and life sciences. This commentary presents a rationale for focusing American efforts in space on such a Moon base in order to promote understanding of the long-term physiological effects of living on a planetary body outside the Van Allen belts.

Genetic basis of murine responses to hyperoxia-induced lung injury

To evaluate the effect of genetic background on oxygen (O2) toxicity, nine genetically diverse mouse strains (129/SvIm, A/J, BALB/cJ, BTBR+(T)/tf/tf, CAST/Ei, C3H/HeJ, C57BL/6J, DBA/2J, and FVB/NJ) were exposed to more than 99% O2 for 72 h. Immediately following the hyperoxic challenge, the mouse strains demonstrated distinct pathophysiologic responses. The BALB/cJ and CAST/Ei strains, which were the only strains to demonstrate mortality from the hyperoxic challenges, were also the only strains to display significant neutrophil infiltration into their lower respiratory tract. In addition, the O2-challenged BALB/cJ and CAST/Ei mice were among six strains (A/J, BALB/cJ, CAST/Ei, BTBR+(T)/tf/tf, DBA/2J, and C3H/HeJ) that had significantly increased interleukin 6 concentrations in the whole lung lavage fluid and were among all but one strain that had large increases in lung permeability compared with air-exposed controls. In contrast, the DBA/2J strain was the only strain not to have any significant alterations in lung permeability following hyperoxic challenge. The expression of the extracellular matrix proteins, including collagens I, III, and IV, fibronectin I, and tenascin C, also varied markedly among the mouse strains, as did the activities of total superoxide dismutase (SOD) and manganese-SOD (Mn-SOD or SOD2). These data suggest that the response to O2 depends, in part, on the genetic background and that some of the strains analyzed can be used to identify specific loci and genes underlying the response to O2.

Duodenal cytochrome b: a novel ferrireductase in airway epithelial cells

Catalytically active iron in the lung causes oxidative stress and promotes microbial growth that can be limited by intracellular sequestration of iron within ferritin. Because cellular iron uptake requires membrane ferrireductase activity that in the gut can be provided by duodenal cytochrome b (Dcytb), we sought Dcytb in the lung to test the hypothesis that it contributes to epithelial iron regulation by reducing Fe3+ for cellular iron transport. Dcytb expression was found in respiratory epithelium in vitro and in vivo and was responsive to iron concentration. Iron transport was measured in human bronchial epithelial (HBE) cells using inductively coupled plasma atomic emission spectroscopy and was demonstrated to be partially inhibited in the presence of Dcytb-blocking antibody, suggesting that Dcytb reduces Fe3+ for cellular iron transport. A definite source of reducing equivalents for Dcytb was sought but not identified. We found no evidence that ascorbate was involved but did demonstrate that O2−· production decreased when Dcytb function was blocked. The presence of Dcytb in airway epithelial cells and its regulation by iron therefore may contribute to pulmonary cytoprotection.

How do red blood cells cause hypoxic vasodilation? The SNO-hemoglobin paradigm

One of the most intriguing areas of research in erythrocyte physiology is the interaction of hemoglobin with nitric oxide (NO). These two molecules independently fulfill diverse and complex physiological roles, while together they subtly modulate microvascular perfusion in response to second-by-second changes in local metabolic demand, contributing to hypoxic vasodilation. It is through an appreciation of the temporal and structural constraints of the microcirculation that the principal requirements of the physiological interplay between NO and hemoglobin are revealed, elucidating the role of the erythrocyte in hypoxic vasodilation. Among the candidate molecular mechanisms, only S-nitrosohemoglobin (SNO-hemoglobin) directly fulfills the physiological requirements. Thus, NO is transported by red blood cells to microvascular sites of action in protected form as an S-nitrosothiol on the highly conserved hemoglobin beta-93 Cys residue, invariant in birds and mammals. SNO-hemoglobin dispenses NO bioactivity to microvascular cells on the release of oxygen, physiologically coupling hemoglobin deoxygenation to vasodilation. SNO-hemoglobin is the archetype for the role of S-nitrosylation in a newly identified class of biological signals, and disturbances in SNO-hemoglobin activity are associated with the pathogenesis of several important vascular diseases.

Nitric oxide amplifies the excitatory to inhibitory neurotransmitter imbalance accelerating oxygen seizures

CNS O2 toxicity is manifested most profoundly by generalized motor convulsions. The hypothesis was tested that HBO2 triggers seizures by an excitatory to inhibitory neurotransmitter imbalance produced by neuronal nitric oxide (NO) activity. Anesthetized rats were exposed to 5 ATA HBO2 for 75 min with or without prior inhibition of nNOS. Interstitial NO and amino acids: aspartate (Asp), glutamate (Glu) and gamma-aminobutyric acid (GABA) were determined in the striatum by microdialysis coupled with HPLC. Blood flow and EEG in the same striatal region were measured simultaneously. Rats treated with 7-NI showed no EEG spikes of O2 toxicity, while seizure latency for untreated rats was 63 +/- 7 min. Significant increases in NO metabolites and blood flow were observed in control rats before seizures. HBO2 did not change Glu significantly and increased Asp slightly whereas GABA decreased progressively by 37 +/- 7%. Pretreatment with 7-NI led to a significantly smaller decline in GABA. Overall, the simplified excitotoxicity index Glu/GABA increased significantly after 60 min of HBO2 in control but fell in rats treated with 7-NI. We conclude that HBO2-stimulated neuronal NO production promotes an imbalance between glutamatergic and GABAergic synaptic function implicated in the genesis of oxygen-induced seizures.

Carbon monoxide, oxidative stress, and mitochondrial permeability pore transition

The cellular effects of carbon monoxide (CO) are produced primarily by CO binding to iron or other transition metals, which may also promote prooxidant activities of the more reactive gases, oxygen and nitric oxide. We tested the hypothesis that prooxidant effects of CO deregulate the calcium-dependent mitochondrial pore transition (MPT), which disrupts membrane potential and releases apoptogenic proteins. Rats were exposed to either CO (50 ppm) or hypobaric hypoxia (HH) for 1, 3, or 7 days, and liver mitochondria harvested to study protein expression and sensitivity to MPT by calcium and oxidants. Both exposures induced hypoxia-sensitive protein expression: hypoxia-inducible factor 1alpha (HIF-1alpha), heme oxygenase-1 (HO-1), and manganese SOD (SOD2), but SOD2 induction was greater by CO than by HH, especially at 7 days. Relative to HH, CO also caused significant early mitochondrial oxidative and nitrosative stress shown by decreases in GSH/GSSG and increases in protein 3-nitrotyrosine (3-NT) and protein mixed disulfide formation. This altered MPT sensitivity to calcium through an effect on the "S-site," causing loss of pore protection by adenine nucleotides. By 7 days, despite continued CO, nitrosative stress decreased and adenine nucleotide protection was restored to preexposure levels. This is the first evidence of functional mitochondrial pore stress caused by CO independently of its hypoxic effect, as well as a compensatory response exemplifying a mitochondrial phenotype shift. The implications are that cellular CO can activate or deactivate mitochondria for initiation of apoptosis in vivo.

Glutathione regulates susceptibility to oxidant-induced mitochondrial DNA damage in human lymphocytes

Oxidative damage to mitochondrial DNA (mtDNA) interferes with the expression of mitochondrial-encoded subunits of the electron transport complexes of oxidative phosphorylation. MtDNA is protected by several mitochondrial antioxidant systems, but the specific importance of glutathione is unknown. We hypothesized that glutathione protects mtDNA from oxidative damage in human blood lymphocytes and that glutathione depletion increases susceptibility to mtDNA depletion, which increases vulnerability to apoptosis. MtDNA damage was measured in human blood lymphocytes exposed to tert-butyl-hydroperoxide (t-BOOH) or t-BOOH plus the glutathione analog, glutathione ethyl ester (GEE). Mitochondrial oxidative stress, mtDNA damage, and susceptibility to apoptosis were analyzed after glutathione depletion with buthionine sulfoximine (BSO). The data show selective damage to lymphocyte mtDNA at low concentrations of tBOOH that is attenuated by glutathione supplementation. Moreover, inhibition of glutathione synthesis led to lymphocyte ROS generation and mtDNA damage, and increased susceptibility to receptor-mediated apoptosis. These findings implicate the glutathione system in maintaining mtDNA integrity and resistance to apoptosis in lymphocytes and suggest that assessment of mtDNA damage in blood lymphocytes may be a useful marker of oxidative stress in humans.

A comparison of hyperbaric oxygen versus hypoxic cerebral preconditioning in neonatal rats

The potency of hyperbaric preconditioning (HBO-PC) is uncertain compared to well-validated ischemic or hypoxic models and no studies have directly compared HBO-PC to hypoxic preconditioning (HPC). We subjected rat pups to unilateral carotid cauterization followed by 90 min (min) of hypoxia using 8% O(2). Three HBO-PC regimes (maximum 2.5 atmospheres for 150 min) were compared to HPC (150 min of 8% O(2)) for changes in mortality and brain weight. Preconditioning-induced oxidative stress was assessed using aconitase activity and manganese superoxide dismutase (MnSOD) transcript levels. Initial brain weight data revealed a large coefficient of variation and compelled an examination of the temperature sensitivity of the model that revealed a narrow optimal range of 35 to 37 degrees C of variability in brain injury and mortality. With rigorous temperature control, high dose HBO-PC and HPC showed comparable anatomic (mean hemispheric weight decrease: control 42%, HPC 25% (P=0.01), HBO-PC 26% (P=0.01) and mortality protection (control 14.7%, HPC 5.9% HBO-PC 5.7%, P=0.001). High dose HBO-PC, but not HPC, suppressed aconitase activity by 65% at 24 h after the preconditioning stimulus (P=0.001). In contrast, MnSOD mRNA increased 2.5-fold at 24 h after HPC (P=0.007) but not after high dose HBO-PC. Thus, when temperature variability is eliminated, HBO-PC and HPC elicit similar preconditioning efficacy in neonatal brain but invoke different defenses against oxidative stress.

Electrochemical detection of nitric oxide in biological fluids

The challenges that must be overcome in order to detect nitric oxide (NO) in biological fluids include its low physiological concentration (1-nM) and its short half-life (a few seconds or less). Electrochemistry is capable of making such measurements, if certain principles, both biological and electrochemical, are kept in mind. We discuss these principles and demonstrate an example of practical measurement by detecting NO release in a drop of blood suspended within the reference electrode of an electrochemical cell. We elicit the NO release by decreasing the oxygen concentration in the gaseous atmosphere surrounding the drop.

Oxygen-induced mitochondrial biogenesis in the rat hippocampus

The hypothesis that damage to mitochondrial DNA by reactive oxygen species increases the activity of nuclear and mitochondrial transcription factors for mitochondrial DNA replication was tested in the in vivo rat brain. Mitochondrial reactive oxygen species generation was stimulated using pre-convulsive doses of hyperbaric oxygen and hippocampal mitochondrial DNA content and neuronal and mitochondrial morphology and cell proliferation were evaluated at 1, 5 and 10 days. Gene expression was subsequently evaluated to assess nuclear and mitochondrial-encoded respiratory genes, mitochondrial transcription factor A, and nuclear respiratory transcription factors-1 and -2. After 1 day, a mitochondrial DNA deletion emerged involving Complex I and IV subunit-encoding regions that was independent of overt neurological or cytological O(2) toxicity, and resolved before the onset of cell proliferation. This damage was attenuated by blockade of neuronal nitric oxide synthase. Compensatory responses were found in nuclear gene expression for manganese superoxide dismutase, mitochondrial transcription factor A, and nuclear respiratory transcription factor-2. Enhanced nuclear respiratory transcription factor-2 binding activity in hippocampus was accompanied by a nearly three-fold boost in mitochondrial DNA content over 5 days. The finding that O(2) activates regional mitochondrial DNA transcription, replication, and mitochondrial biogenesis in the hippocampus may have important implications for maintaining neuronal viability after brain injury.

Oxidant generation promotes iron sequestration in BEAS-2B cells exposed to asbestos

Lung injury after asbestos exposure is associated with an oxidative stress that is catalyzed by iron in the fiber matrix, complexed to the surface, or both. We tested the hypothesis that the cellular response to asbestos includes the transport and sequestration of this iron through (1) generation of superoxide for ferrireduction, (2) up-regulation of divalent metal transporter-1 (DMT1) for intracellular transport of Fe2+, and (3) increased production of cellular ferritin where the metal is stored in a catalytically less reactive state. BEAS-2B cells with normal and elevated Cu,Zn superoxide dismutase (SOD) expression were employed for in vitro investigations. After exposure of these cells to asbestos, we demonstrated by fluorescence methodology a significantly increased generation of SOD with ferrireductive capacity. Fiber exposure also increased DMT1 protein and mRNA expression in the BEAS-2B cells. Incubation with asbestos elevated cellular iron and ferritin concentrations, and these responses were diminished in cells with an enhanced expression of SOD. Finally, fiber exposure increased supernatant concentrations of interleukin 8, but this inflammatory mediator was actually increased in cells with elevated SOD expression. We conclude that the response of respiratory epithelial cells to asbestos includes oxidant-mediated mechanisms to sequester catalytically active iron associated with the fiber.

Mitochondrial transcription factor A induction by redox activation of nuclear respiratory factor 1

The nuclear expression of mitochondrial transcription factor A (Tfam), which is required for mitochondrial DNA (mtDNA) transcription and replication, must be linked to cellular energy needs. Because respiration generates reactive oxygen species as a side-product, we tested the idea that reactive oxygen species regulate Tfam expression through phosphorylation of nuclear respiratory factor (NRF-1) and binding to the Tfam promoter. In mitochondria-rich rat hepatoma cells that overexpress NRF-1, basal and oxidant-induced increases were found in Tfam expression and mtDNA content. Specific binding of NRF-1 to Tfam promoter was demonstrated by electrophoretic mobility shift assay and chromatin immunoprecipitation. NRF-1-Tfam binding was augmented under pro-oxidant conditions. NRF-1 gene silencing produced 1:1 knockdown of Tfam expression and decreased mtDNA content. To evaluate oxidation-reduction (redox) regulation of NRF-1 in Tfam expression, blockade of upstream phosphatidylinositol 3-kinase was used to demonstrate loss of oxidant stimulation of NRF-1 phosphorylation and Tfam expression. The oxidant response was also abrogated by specific inhibition of Akt/protein kinase B. Examination of the NRF-1 amino acid sequence revealed an Akt phosphorylation consensus at which site-directed mutagenesis abolished NRF-1 phosphorylation by Akt. Finally, Akt phosphorylation and NRF-1 translocation predictably lacked oxidant regulation in a cancer line having no PTEN tumor suppressor (HCC1937 cells). This study discloses novel redox regulation of NRF-1 phosphorylation and nuclear translocation by phosphatidylinositol 3,4,5-triphosphate kinase/Akt signaling in controlling Tfam induction by an anti-oxidant pro-survival network.

A nitric oxide processing defect of red blood cells created by hypoxia: deficiency of S-nitrosohemoglobin in pulmonary hypertension

The mechanism by which hypoxia [low partial pressure of O(2) (pO(2))] elicits signaling to regulate pulmonary arterial pressure is incompletely understood. We considered the possibility that, in addition to its effects on smooth muscle, hypoxia may influence pulmonary vascular tone through an effect on RBCs. We report that exposure of native RBCs to sustained hypoxia is accompanied by a buildup of heme iron-nitrosyl (FeNO) species that are deficient in pO(2-)governed intramolecular transfer of NO to cysteine thiol, yielding a deficiency in the vasodilator S-nitrosohemoglobin (SNO-Hb). S-nitrosothiol (SNO)-deficient RBCs produce impaired vasodilator responses in vitro and exaggerated pulmonary vasoconstrictor responses in vivo and are defective in oxygenating the blood. RBCs from hypoxemic patients with elevated pulmonary arterial pressure (PAP) exhibit a similar FeNO/SNO imbalance and are thus deficient in pO(2)-coupled vasoregulation. Chemical restoration of SNO-Hb levels in both animals and patients restores the vasodilator activity of RBCs, and this activity is associated with improved oxygenation and lower PAPs.

Cerebral blood flow and brain oxygenation in rats breathing oxygen under pressure

Hyperbaric oxygen (HBO(2)) increases oxygen tension (PO(2)) in blood but reduces blood flow by means of O(2)-induced vasoconstriction. Here we report the first quantitative evaluation of these opposing effects on tissue PO(2) in brain, using anesthetized rats exposed to HBO(2) at 2 to 6 atmospheres absolute (ATA). We assessed the contribution of regional cerebral blood flow (rCBF) to brain PO(2) as inspired PO(2) (PiO(2)) exceeds 1 ATA. We measured rCBF and local PO(2) simultaneously in striatum using collocated platinum electrodes. Cerebral blood flow was computed from H(2) clearance curves in vivo and PO(2) from electrodes calibrated in vitro, before and after insertion. Arterial PCO(2) was controlled, and body temperature, blood pressure, and EEG were monitored. Scatter plots of rCBF versus PO(2) were nonlinear (R(2)=0.75) for rats breathing room air but nearly linear (R(2)=0.88-0.91) for O(2) at 2 to 6 ATA. The contribution of rCBF to brain PO(2) was estimated at constant inspired PO(2), by increasing rCBF with acetazolamide (AZA) or decreasing it with N-nitro-L-arginine methyl ester (L-NAME). At basal rCBF (78 mL/100 g min), local PO(2) increased 7- to 33-fold at 2 to 6 ATA, compared with room air. A doubling of rCBF increased striatal PO(2) not quite two-fold in rats breathing room air but 13- to 64-fold in those breathing HBO(2) at 2 to 6 ATA. These findings support our hypothesis that HBO(2) increases PO(2) in brain in direct proportion to rCBF.

Toll-like receptor 4 mediates mitochondrial DNA damage and biogenic responses after heat-inactivated E. coli

An important site of cellular damage in bacterial sepsis is mitochondrial DNA (mtDNA), which we proposed is caused by reactive oxygen and nitrogen species generated by activation of signaling through specific toll-like receptors (TLR). In wild-type (Wt) mice injected with heat-inactivated E. coli, hepatic TLR4 and TLR2 proteins were up-regulated with TLR-dependent increases in transcript levels for tumor necrosis factor (TNF-alpha), interleukin 6, nitric oxide synthase-II (iNOS), and NADPH oxidase 2 (Nox2). The accompanying stress significantly depleted hepatic mtDNA despite eight- and fourfold increases in manganese superoxide dismutase (MnSOD) and mitochondrial transcription factor A (Tfam) expression, respectively. The identical E. coli dose generated significantly less TNF-alpha, NO, and Nox2 in TLR4-/- and TLR2/4-/- but not in TLR2-/- mice. TLR4-/- and TLR2/4-/- compared with Wt mice were protected from mtDNA oxidation but showed no Tfam up-regulation and little copy number restoration. A critical role in the mtDNA damage was determined for TLR4-mediated iNOS transcription through the MyD88 pathway. In Wt mice, mtDNA depletion was avoided by selective iNOS blockade, and residual mtDNA loss was linked to NF-kappaB-dependent TNF-alpha expression. These data disclose the dual role of TLR4 in mtDNA damage and compensatory mitochondrial biogenic responses after innate immune activation.

S-nitrosoglutathione inhibits alpha1-adrenergic receptor-mediated vasoconstriction and ligand binding in pulmonary artery

Endogenous nitric oxide donor compounds (S-nitrosothiols) contribute to low vascular tone by both cGMP-dependent and -independent pathways. We have reported that S-nitrosoglutathione (GSNO) inhibits 5-hydroxytryptamine (5-HT)-mediated pulmonary vasoconstriction via a cGMP-independent mechanism likely involving S-nitrosylation of its G protein-coupled receptor (GPCR) system. Because catecholamines, like 5-HT, constrict lung vessels via a GPCR coupled to G(q), we hypothesized that S-nitrosothiols modify the alpha1-adrenergic GPCR system to inhibit pulmonary vasoconstriction by receptor agonists, e.g., phenylephrine (PE). Rat pulmonary artery rings were pretreated for 30 min with and without an S-nitrosothiol, either GSNO or S-nitrosocysteine (CSNO), and constricted with sequential concentrations of PE (10(-8)-10(-6) M). Effective cGMP-dependence was tested in rings pretreated with soluble guanylate cyclase inhibitors {either 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or LY-83583} or G kinase inhibitor (KT-5823), and a thiol reductant [dithiothreitol (DTT)] was used to test reversibility of S-nitrosylation. Both S-nitrosothiols attenuated the PE dose response. The GSNO effect was not prevented by LY-83583, ODQ, or KT-5823, indicating cGMP independence. GSNO inhibition was reversed by DTT, consistent with S-nitrosylation or other GSNO-mediated cysteine modifications. In CSNO-treated lung protein, the alpha1-adrenergic receptor was shown to undergo S-nitrosylation in vitro using a biotin switch assay. Studies of alpha1-adrenergic receptor subtype expression and receptor density by saturation binding with 125I-HEAT showed that GSNO decreased alpha1-adrenergic receptor density but did not alter affinity for antagonist or agonist. These data demonstrate a novel cGMP-independent mechanism of reversible alpha1-adrenergic receptor inhibition by S-nitrosothiols.

Blockade of tissue factor-factor X binding attenuates sepsis-induced respiratory and renal failure

Tissue factor expression in sepsis activates coagulation in the lung, which potentiates inflammation and leads to fibrin deposition. We hypothesized that blockade of factor X binding to the tissue factor-factor VIIa complex would prevent sepsis-induced damage to the lungs and other organs. Acute lung injury was produced in 15 adult baboons primed with killed Escherichia coli [1 x 10(9) colony-forming units (CFU)/kg], and then 12 h later, they were given 1 x 10(10) CFU/kg live E. coli by infusion. Two hours after live E. coli, animals received antibiotics with or without monoclonal antibody to tissue factor intravenously to block tissue factor-factor X binding. The animals were monitored physiologically for 34 h before being killed and their tissue harvested. The antibody treatment attenuated abnormalities in gas exchange and lung compliance, preserved renal function, and prevented tissue neutrophil influx and bowel edema relative to antibiotics alone (all P < 0.05). It also attenuated fibrinogen depletion (P < 0.01) and decreased proinflammatory cytokines, e.g., IL-6 and -8 (P < 0.01), in systemic and alveolar compartments. Similar protective effects of the antibody on IL-6 and -8 expression and permeability were found in lipopolysaccharide-stimulated endothelial cells. Blockade of factor X binding to the tissue factor-factor VIIa complex attenuates lung and organ injuries in established E. coli sepsis by attenuating the neutrophilic response and inflammatory pathways.

Extracellular superoxide dismutase

The extracellular space is protected from oxidant stress by the antioxidant enzyme extracellular superoxide dismutase (EC-SOD), which is highly expressed in selected tissues including blood vessels, heart, lungs, kidney and placenta. EC-SOD contains a unique heparin-binding domain at its carboxy-terminus that establishes localization to the extracellular matrix where the enzyme scavenges superoxide anion. The EC-SOD heparin-binding domain can be removed by proteolytic cleavage, releasing active enzyme into the extracellular fluid. In addition to protecting against extracellular oxidative damage, EC-SOD, by scavenging superoxide, preserves nitric oxide bioactivity and facilitates hypoxia-induced gene expression. Loss of EC-SOD activity contributes to the pathogenesis of a number of diseases involving tissues with high levels of constitutive extracellular superoxide dismutase expression. A thorough understanding of the biological role of EC-SOD will be invaluable for developing novel therapies to prevent stress by extracellular oxidants.

TNF, IFN-γ, and endotoxin increase expression of DMT1 in bronchial epithelial cells

Regulation of the metal transport protein divalent metal transporter-1 (DMT1) may contribute to the uptake and detoxification of iron by cells resident in the respiratory tract. Inflammation has been associated with an increased availability of this metal resulting in an oxidative stress. Because proinflammatory cytokines and LPS have been demonstrated to affect an elevated expression of DMT1 in a macrophage cell line, we tested the hypothesis that tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and LPS increase DMT1 expression in airway epithelial cells. We used RT-PCR to detect mRNA for both −IRE DMT1 and +IRE DMT1 in BEAS-2B cells. Treatment with TNF-α, IFN-γ, or LPS increased both forms. Western blot analysis also demonstrated an increase in the expression of both isoforms of DMT1 after these treatments. Twenty-four hours after exposure of an animal model to TNF-α, IFN-γ, or LPS, a significant increase in pulmonary expression of −IRE DMT1 was seen by immunohistochemistry; the level of +IRE DMT1 was too low in the lung to be visualized using this methodology. Finally, iron transport into BEAS-2B cells was increased after inclusion of TNF-α, IFN-γ, or LPS in the media. We conclude that proinflammatory cytokines and LPS increase mRNA and protein expression of DMT1 in airway cells in vitro and in vivo. Furthermore, both −IRE and +IRE isoforms are elevated after exposures. Increased expression of this protein appears to be included in a coordinated response of the cell and tissue where the function might be to diminish availability of metal.

Toll-like receptor 4 mediates mitochondrial DNA damage and biogenic responses after heat-inactivated E. coli

An important site of cellular damage in bacterial sepsis is mitochondrial DNA (mtDNA), which we proposed is caused by reactive oxygen and nitrogen species generated by activation of signaling through specific toll-like receptors (TLR). In wild-type (Wt) mice injected with heat-inactivated E. coli, hepatic TLR4 and TLR2 proteins were up-regulated with TLR-dependent increases in transcript levels for tumor necrosis factor (TNF-alpha), interleukin 6, nitric oxide synthase-II (iNOS), and NADPH oxidase 2 (Nox2). The accompanying stress significantly depleted hepatic mtDNA despite eight- and fourfold increases in manganese superoxide dismutase (MnSOD) and mitochondrial transcription factor A (Tfam) expression, respectively. The identical E. coli dose generated significantly less TNF-alpha, NO, and Nox2 in TLR4-/- and TLR2/4-/- but not in TLR2-/- mice. TLR4-/- and TLR2/4-/- compared with Wt mice were protected from mtDNA oxidation but showed no Tfam up-regulation and little copy number restoration. A critical role in the mtDNA damage was determined for TLR4-mediated iNOS transcription through the MyD88 pathway. In Wt mice, mtDNA depletion was avoided by selective iNOS blockade, and residual mtDNA loss was linked to NF-kappaB-dependent TNF-alpha expression. These data disclose the dual role of TLR4 in mtDNA damage and compensatory mitochondrial biogenic responses after innate immune activation.

Divalent metal transporter-1 decreases metal-related injury in the lung

Exposure to airborne particulates makes the detoxification of metals a continuous challenge for the lungs. Based on the fate of iron in airway epithelial cells, we postulated that divalent metal transporter-1 (DMT1) participates in detoxification of metal associated with air pollution particles. Homozygous Belgrade rats, which are functionally deficient in DMT1, exhibited diminished metal transport from the lower respiratory tract and greater lung injury than control littermates when exposed to oil fly ash. Preexposure of normal rats to iron in vivo increased expression of the isoform of DMT1 protein that lacked an iron-response element (-IRE), accelerated metal transport out of the lung, and decreased injury after particle exposure. In contrast, normal rats preexposed to vanadium showed less expression of the -IRE isoform of DMT1, decreased metal transport, and greater pulmonary injury after particle instillation. Respiratory epithelial cells in culture gave similar results. Also, DMT1 mRNA and protein expression for the -IRE isoform increased or decreased in these cells when exposed to iron or vanadium, respectively. These results thus demonstrate for the first time a primary role for DMT1 in lung metal transport and detoxification.