Mice lacking extracellular superoxide dismutase are more sensitive to hyperoxia

The reductive hotspot hypothesis: an update

The mitochondrial free radical theory of aging is seriously challenged by the finding that mutant mtDNA never becomes abundant in vivo, a result disputed only in experiments using novel PCR variants whose quantitative accuracy is widely doubted. However, evidence continues to mount that mitochondria are the crucial site of free radical damage in vivo, most notably that mice lacking the nonmitochondrial isoforms of superoxide dismutase are healthy. It is thus important to determine whether a low level of mutant mtDNA could have serious systemic effects. This possibility exists because of the observed mosaic distribution of mutant mtDNA: some cells (or muscle fiber segments) lack any aerobic respiration. Such cells are presumed to satisfy their ATP needs by glycolysis. In vitro, however, NADH recycling by transmembrane pyruvate/lactate exchange does not suffice: cells only survive if they can up-regulate the plasma membrane oxidoreductase (PMOR). The PMOR's physiological electron acceptor is unknown. It was proposed recently (de Grey, A. D. N. J. (1998) J. Anti-Aging Med. 1(1), 53-66) that a prominent in vivo acceptor from these mitochondrially mutant cells may be oxygen, forming extracellular superoxide. The mosaic ("hotspot") distribution of this superoxide would limit its dismutation by extracellular superoxide dismutase; it may thus reduce transition metals leading to oxidation of circulating material, such as LDL. This would raise systemic oxidative stress, greatly amplifying the damage done by the originating mitochondrially mutant cells. This model, now known as the "reductive hotspot hypothesis," has recently gained much indirect experimental support; several direct tests of it are also feasible.

Human islets in mixed islet grafts protect mouse pancreatic beta-cells from alloxan toxicity

We have previously shown that human beta-cells are resistant to the toxic effects of alloxan. In order to further clarify this characteristic of human islets, we investigated whether these cells might transfer their alloxan resistance to alloxan-sensitive rat or mouse islets. Islets from two species (human-mouse or rat-mouse) were mixed into one graft, which was implanted into the subcapsular kidney space of nude mice. Alloxan or saline was injected intravenously two weeks after implantation and one week thereafter the mice were killed. The number of grafted and endogenous beta-cells were evaluated by a semi-quantitative method after immunohistochemistry. Human islet production of the scavenging enzymes extracellular superoxide dismutase and plasma glutathione peroxidase were analyzed with ELISA-techniques, and mouse and human islet hydrogen peroxide breakdown activity were monitored with a horseradish peroxidase-dependent assay. Mouse beta-cells transplanted together with human islets were protected against alloxan cytotoxicity. Rat islets did not protect mouse beta-cells against alloxan, suggesting that the mixing procedure as such did not impose the protection. Production of extracellular superoxide dismutase and plasma glutathione peroxidase by human islets was very low. Moreover, H2O2 breakdown in vitro, did not differ between human and mouse islets. Alloxan-insensitive human islets protect mouse beta-cells against alloxan-induced lesions, suggesting that yet to be identified extracellular factors are involved in human islet resistance to alloxan toxicity.

Overexpression of Mn-containing superoxide dismutase in transgenic Drosophila melanogaster

The general objective of this study was to examine the role of mitochondria in the aging process. Two alternative hypotheses were tested: (i) that overexpression of Mn superoxide dismutase (Mn SOD) in the mitochondria of Drosophila melanogaster would slow the accrual of oxidative damage and prolong survival or (ii) that there is an evolved optimum level of superoxide anion radical, such that overexpression of Mn SOD would have deleterious or neutral effects. Microinjection and mobilization of a transgene, which contained a 9-kb genomic sequence encoding Mn SOD, produced 15 experimental lines overexpressing Mn SOD by 5-116% relative to the parental y w strain. Comparisons between these lines and control lines containing inserted vector sequences alone indicated that the mean longevity of the experimental lines was decreased by 4-5% relative to controls. There were no compensatory changes in the metabolic rate, level of physical activity, or the levels of other antioxidants, namely Cu-Zn SOD, catalase, and glutathione. There were no differences between groups in rates of mitochondrial hydrogen peroxide release, protein oxidative damage, or resistance to 100% oxygen or starvation conditions. The experimental lines had a marginally increased resistance to moderate heat stress. These results are consistent with the existence of an optimum level of Mn SOD activity which minimizes oxidative stress. The naturally evolved level of Mn SOD activity in Drosophila appears to be near the optimum required under normal conditions, although the optimum may be shifted to a higher level under more stressful conditions.

Endothelial function in hypertension: the role of superoxide anion

Much attention has been focused on the role of nitric oxide in hypertension and cardiovascular disease. More recently, the role of superoxide anion and its interaction with nitric oxide has been investigated in this context. This review will concentrate on the role of superoxide in human and experimental hypertension, paying particular attention to the potential sources of superoxide within the vasculature and discussing some of the molecular mechanisms surrounding its production and dismutation. We discuss what is known about the human superoxide dismutase enzymes. We conclude that the balance between nitric oxide and superoxide is more important than the absolute levels of either alone.

Hindlimb motor neurons require Cu/Zn superoxide dismutase for maintenance of neuromuscular junctions

The role of oxidative damage in neurodegenerative disease was investigated in mice lacking cytoplasmic Cu/Zn superoxide dismutase (SOD), created by deletion of the SOD1 gene (SOD1(-/-)). SOD1(-/-) mice developed a chronic peripheral hindlimb axonopathy. Mild denervation of muscle was detected at 2 months, and behavioral and physiological motor deficits were present at 5-7 months of age. Ventral root axons were shrunken but were normal in number. The somatosensory system in SOD1(-/-) mice was mildly affected. SOD1(-/-) mice expressing Cu/Zn SOD only in brain and spinal cord were generated using transgenic mice expressing mouse SOD1 driven by the neuron-specific synapsin promoter. Neuron-specific expression of Cu/Zn SOD in SOD1(-/-) mice rescued motor neurons from the neuropathy. Therefore, Cu/Zn SOD is not required for normal motor neuron survival, but is necessary for the maintenance of normal neuromuscular junctions by hindlimb motor neurons.

Extracellular superoxide dismutase deficiency worsens outcome from focal cerebral ischemia in the mouse

The role of endogenous extracellular superoxide dismutase (EC-SOD) was examined in a murine model of transient focal cerebral ischemia. Homozygous EC-SOD deficient (EC-SOD-/-; n = 18) and wild type (EC-SOD+/+; n = 19) littermates were anesthetized with halothane and subjected to 50 min of intraluminal middle cerebral artery occlusion with pericranial temperature maintained at 37.0 degrees C. After 24 h of reperfusion, resultant hemiparesis and cerebral infarct size were measured. Total infarct volume was 81% greater (P = 0.03) and hemiparesis was more severe (P = 0.01) in EC-SOD-/- versus EC-SOD+/+ mice. The worsened ischemic outcome observed in EC-SOD-/- mice is consistent with prior work which found transgenic EC-SOD overexpressing mice to exhibit enhanced tolerance to focal ischemia. The results suggest that endogenous antioxidant activity in the extracellular compartment plays an important role in the histologic/neurologic response to focal cerebral ischemia.

Development of eosinophil peroxidase activity and concomitant alteration of the antioxidant defenses in the liver of mice infected with Schistosoma mansoni

BACKGROUND/AIMS

The tropical parasite Schistosoma mansoni causes granulomatous inflammation following lodging of eggs in portal capillaries. In vitro studies indicated that the host reaction should involve reactive oxygen intermediates; however, it is not known what occurs in vivo at the site of the disease. Moreover, the ultimate pathophysiological effects of oxidative processes depend upon antioxidant factors, which are investigated in this study.

METHODS

We explored the changes in the major enzyme activities involved in liver redox metabolism during the course of infection and, for some of them, the mRNA expression. We also measured the reduced glutathione and lipid peroxide levels in the liver.

RESULTS

We found that the deposition of parasite eggs triggers the release of endogenous eosinophil peroxidase; enzyme activity developed in the immediate vicinity of the eggs and it increased dramatically with time. However, Cu,Zn-superoxide dismutase, catalase and glutathione peroxidase activities decreased drastically. In contrast, glutathione transferase was unaffected. There was no proportional decrease in mRNA levels for the H2O2 scavenging enzymes. Reduced glutathione concentrations also dropped as a result of infection. Lastly, a two-fold increase in the levels of hepatic products generated by lipid peroxidation was observed.

CONCLUSIONS

These results show that on the one hand oxidative processes occurred at the site of granulomatous inflammation and on the other hand the antioxidant capacity of the liver decreased, leading to the generation of lipid peroxides. The resulting imbalance between pro- and anti-oxidant processes may play a central role in the pathology associated with schistosomiasis.

Extracellular superoxide dismutase in the airways of transgenic mice reduces inflammation and attenuates lung toxicity following hyperoxia

Extracellular superoxide dismutase (EC-SOD, or SOD3) is the major extracellular antioxidant enzyme in the lung. To study the biologic role of EC-SOD in hyperoxic-induced pulmonary disease, we created transgenic (Tg) mice that specifically target overexpression of human EC-SOD (hEC-SOD) to alveolar type II and nonciliated bronchial epithelial cells. Mice heterozygous for the hEC-SOD transgene showed threefold higher EC-SOD levels in the lung compared with wild-type (Wt) littermate controls. A significant amount of hEC-SOD was present in the epithelial lining fluid layer. Both Tg and Wt mice were exposed to normobaric hyperoxia (>99% oxygen) for 48, 72, and 84 hours. Mice overexpressing hEC-SOD in the airways attenuated the hyperoxic lung injury response, showed decreased morphologic evidence of lung damage, had reduced numbers of recruited inflammatory cells, and had a reduced lung wet/dry ratio. To evaluate whether reduced numbers of neutrophil infiltration were directly responsible for the tolerance to oxygen toxicity observed in the Tg mice, we made Wt and Tg mice neutropenic using anti-neutrophil antibodies and subsequently exposed them to 72 hours of hyperoxia. Both Wt and Tg neutrophil-depleted (ND) mice have less severe lung injury compared with non-ND animals, thus providing direct evidence that neutrophils recruited to the lung during hyperoxia play a distinct role in the resultant acute lung injury. We conclude that oxidative and inflammatory processes in the extracellular lung compartment contribute to hyperoxic-induced lung damage and that overexpression of hEC-SOD mediates a protective response to hyperoxia, at least in part, by attenuating the neutrophil inflammatory response.

Pertussis toxin-induced lung edema. Role of manganese superoxide dismutase and protein kinase C

The mechanism by which pertussis toxin (Ptx) causes lung edema is not clear. We investigated the role of pulmonary manganese superoxide dismutase (MnSOD) and protein kinase C (PKC) in Ptx-induced lung edema. We demonstrated that intraperitoneal injection of Ptx at a concentration of 5 microg/100 g body weight caused a similar degree of lung edema in 2 d, as measured by lung wet weight/dry weight ratio, in heterozygous MnSOD gene (Sod2)-knockout mice (Sod2(+/-)) and in their wild-type littermates (Sod2(+/+)). The level of lung MnSOD activity in Sod2(+/-) mice was approximately half that of Sod2(+/-) mice. Ptx had no effect on levels of lung MnSOD messenger RNA, immunoreactive protein, or enzyme activity in either Sod2(+/+) or Sod2(+/-) mice. Ptx also had no effect on lung copper-zinc SOD, catalase, and glutathione peroxidase activities in these mice. On the other hand, Ptx caused the activation of lung PKC, for example, by translocation of a 72-kD PKC isoform from the cytosolic fraction to the membrane fraction. Pretreatment of mice with bisindolylmaleimide, a PKC inhibitor, prevented both the Ptx-induced activation of PKC and lung edema. These data suggest that Ptx-induced lung edema in mice is, at least in part, due to the activation of lung PKC.

Mitochondrial disease in superoxide dismutase 2 mutant mice

Oxidative stress has been implicated in many diseases. The chief source of reactive oxygen species within the cell is the mitochondrion. We have characterized a variety of the biochemical and metabolic effects of inactivation of the mouse gene for the mitochondrial superoxide dismutase (CD1-Sod2(tm1Cje)). The Sod2 mutant mice exhibit a tissue-specific inhibition of the respiratory chain enzymes NADH-dehydrogenase (complex I) and succinate dehydrogenase (complex II), inactivation of the tricarboxylic acid cycle enzyme aconitase, development of a urine organic aciduria in conjunction with a partial defect in 3-hydroxy-3-methylglutaryl-CoA lyase, and accumulation of oxidative DNA damage. These results indicate that the increase in mitochondrial reactive oxygen species can result in biochemical aberrations with features reminiscent of mitochondrial myopathy, Friedreich ataxia, and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.

Tissue distribution of immunoreactive mouse extracellular superoxide dismutase

Protein content and mRNA expression of extracellular superoxide dismutase (EC-SOD) were investigated in 16 mouse tissues. We developed a double-antibody sandwich ELISA using the affinity-purified IgG against native mouse EC-SOD. EC-SOD could be detected in all of the tissues examined (lung, kidney, testis, brown fat, liver, adrenal gland, pancreas, colon, white fat, thymus, stomach, spleen, heart, skeletal muscle, ileum, and brain, in decreasing order of content measured as microg/g wet tissue). Lung showed a markedly higher value of EC-SOD than other tissues. Interestingly, white fat had a high content of EC-SOD in terms of micrograms per milligram protein, which corresponded to that of lung. Kidney showed the strongest expression of EC-SOD mRNA. Relatively strong expression of the mRNA was observed in lung, white fat, adrenal gland, brown fat, and testis. Heart and brain showed only weak signals, and no such expression could be detected in either digestive organs or skeletal muscle. Immunohistochemically, EC-SOD was localized mainly to connective tissues and vascular walls in the tissues examined. Deep staining in the cytosol was observed in the cortical tubular cells of kidney. These results suggest that EC-SOD is distributed systemically in mice and that the physiological importance of this enzyme may be a compensatory adaptation to oxidative stress, particularly in lung and kidney.

Experimental developments in movement disorders: update on proposed free radical mechanisms

Free radicals have been implicated in the pathogenesis of movement disorders such as Parkinson's disease and Huntington's disease. Some basic aspects about free radicals as they relate to oxidative stress in neurodegeneration are summarized. Old and new experimental findings pertinent to oxidative damage in movement disorders are reviewed. Finally, the degree to which toxin-induced and genetically engineered experimental models have been useful in delineating parts of the mechanisms involved in the cascade of events that lead to neuronal death is emphasized.

Gene therapy for oxidant injury-related diseases: adenovirus-mediated transfer of superoxide dismutase and catalase cDNAs protects against hyperoxia but not against ischemia-reperfusion lung injury

Hyperoxia and ischemia-reperfusion cause profound lung cellular damage mediated, in part, by generation of oxygen radicals. We hypothesized that gene therapy can be used to overcome oxidant injury by augmenting intracellular antioxidant enzymes. Adult rats were injected intratracheally with an adenovirus (Ad) vector encoding human superoxide dismutase (CuZn-SOD) or catalase cDNA, a mixture of both Ad vectors, or a control Ad vector containing no exogenous gene. Expression of human catalase and CuZn-SOD was demonstrated 3 days later in distal lung epithelial cells and alveolar macrophages, using ELISA and immunochemistry. After exposure to 100% O2 for 62 hr, survival was greater in rats injected with the catalase and/or SOD Ad vectors than in control rats. Ischemia-reperfusion injury was evaluated in the isolated perfused lung model. Overexpression of SOD worsened ischemia-reperfusion injury. Interestingly, concomitant overexpression of catalase prevented this adverse effect, but did not protect against ischemia-reperfusion injury. We conclude that Ad-mediated transfer to lungs of both catalase and SOD cDNAs protects from pulmonary O2 toxicity. Absence of protection against ischemia-reperfusion using intratracheal Ad injections may be related to the lack of endothelial protection, despite epithelial expression of catalase and SOD.

Susceptibility of heterozygous MnSOD gene-knockout mice to oxygen toxicity

Recent studies have shown that homozygous Mn superoxide dismutase (Sod2) gene-knockout mice (Sod2(-/-)) die shortly after birth with extensive myocardial injury, whereas heterozygous mutants (Sod2(+/-)) are phenotypically normal in room air. In the current study, we showed that Sod2(+/-) mice with approximately 50% of normal pulmonary MnSOD activity and normal levels of lung CuZnSOD, catalase, and glutathione peroxidase activities were not substantially more susceptible to 100% O2 toxicity than their normal Sod2(+/+) littermates. The mean (+/- SD) survival of Sod2(+/-) mice in 100% O2 was 101.4 +/- 14.8 h (n = 20) versus 103.2 +/- 11.3 h (n = 20) for Sod2(+/+) littermates (P > 0.60). In addition, Sod2(+/-) mice with approximately 50% of normal heart MnSOD activity and Sod2(+/+) mice did not develop any ultrastructural abnormalities in the myocardium at 75 h or 90 h after 100% O2 exposure. These results suggest that in mice, only 50% of MnSOD activity may be sufficient for normal resistance to 100% O2 toxicity.

Vascular expression of extracellular superoxide dismutase in atherosclerosis

We characterized a novel form of extracellular superoxide dismutase (ecSOD) in atherosclerotic vessels. Specific activity and protein expression of ecSOD was increased two- to threefold in apo E-deficient compared with control aortas. RNase protection assays demonstrated that the expected ecSOD transcript was not increased in either apo E-deficient mice or cholesterol-fed LDL receptor-deficient mice, but that a second, lower molecular weight transcript was present and became predominant as atherosclerosis progressed. Sequence analysis revealed that this novel ecSOD has a 10-bp deletion in the 3' untranslated region and an asparagine to aspartic acid mutation at amino acid 21. Studies of isolated macrophages and immunohistochemistry suggested that the truncated ecSOD transcript was expressed by lipid-laden but not control macrophages. Recombinant wild-type and novel ecSODs expressed in Sf9 cells exhibited similar SOD activities. These experiments show that ecSOD expression is increased in atherosclerotic vessels and that this is characterized by an alteration in mRNA and protein structure. Further, the source of this altered ecSOD is likely the lipid-laden macrophage. The enzymatic properties of this novel ecSOD may have important implications for the function of the lipid-laden macrophage and the atherosclerotic process.

Gene disruption in mice: models of development and disease

Gene targeting technology in mice by homologous recombination has become an important method to generate loss-of-function of genes in a predetermined locus. Although the inactivation is limited to irreversible alteration of chromosomal DNA and a surprising variety of genes have given unexpected and disappointing results, modification of the basic technology now provides additional choices for a more specific and variety of manipulations of the mouse genome. This includes conditional cell-type specific gene targeting, knockin technique and the induction of the specific balanced chromosomal translocations. In the past decade this technique not only generated a wealth of knowledge concerning the roles of growth factors, oncogenes, hormone receptors and Hox genes but also helped to produce animal models for several human genetic disorders. In the future it may provide more powerful and necessary tools to dissect the psychiatric disorders, understanding the complex central nervous system and to correct the inherited disorders.

Inflammatory and epithelial responses in mouse strains that differ in sensitivity to hyperoxic injury

The pulmonary response to various toxicants including bleomycin, ozone, ionizing radiation, and hyperoxia is highly variable among mouse strains. The current study tests the hypothesis that at a similar stage of injury, regardless of strain, expression of inflammatory cytokine and epithelial marker genes would be similar, indicating a common pathway of injury progression. Three strains of mice, C57B1/6J, 129/J, and C3H/HeJ, ranging from sensitive to resistant, were exposed to > 95% O2 for varying times. Ribonuclease protection was used to quantify changes in cytokine mRNA. Despite differences in the kinetics, each strain demonstrated similar hyperoxia-induced changes in the abundance of interleukin (IL)-6, IL-1 beta, IL-3, and tumor neucrosis factor (TNF)-alpha. For each strain, death was accompanied by similar increases in cytokine mRNAs above steady-state control levels. Other inflammatory cytokines, including IL-1 alpha, IL-4, and interferon (IFN)-gamma, were unaltered in all strains at all times. In situ hybridization analysis of the epithelial markers, surfactant protein B (SPB), and clara cell secretory protein (CCSP) at the time of proinflammatory induction showed a similar pattern of expression in all strains. Increased SPB was detected in bronchiolar epithelium, while the number of type II cells expressing this message declined. Both the number of cells expressing CCSP as well as abundance per cell declined. These results suggest that although differences in acute sensitivity to hyperoxia exist between mouse strains, once initiated, acute epithelial cell injury and associated inflammatory changes follow the same pattern in all strains.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Extracellular superoxide dismutase is upregulated with inducible nitric oxide synthase after NF-kappa B activation

Inflammatory cytokines have been shown to upregulate secretion of the antioxidant enzyme extracellular superoxide dismutase (EC-SOD) in dermal fibroblasts and, in other cells, to stimulate production of nitric oxide (.NO). Because superoxide rapidly scavenges .NO, forming the injurious peroxynitrite anion (OONO-), we hypothesize that stimulated cells upregulate EC-SOD expression concurrently with .NO release. To test for coregulation of EC-SOD and .NO within the same cell, the timing of inducible nitric oxide synthase (iNOS) and EC-SOD transcription was measured after exposure of a rate type II pneumocyte analog, the L2 cell line, to a combination of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). Upregulation of iNOS and EC-SOD transcription occurred after 6 h of exposure, and transcription of both genes was linked by activation of the transcription factor nuclear factor-kappa B. Both EC-SOD and iNOS were elevated in rat lung homogenates 24 h after intratracheal instillation with IFN-gamma and TNF-alpha. The observation that EC-SOD and iNOS are temporally coregulated after cytokine exposure suggests the possibility of a critical mechanism by which cells might protect .NO and avoid the formation of OONO- during inflammation.

Mouse extracellular superoxide dismutase: primary structure, tissue-specific gene expression, chromosomal localization, and lung in situ hybridization

Extracellular superoxide dismutase (EC-SOD) is the major extracellular antioxidant enzyme. We have determined the primary structure of mouse EC-SOD by characterization of complementary DNA (cDNA) clones and by amino-acid sequence analysis of purified protein. cDNA sequence analysis indicates that mouse EC-SOD is synthesized as a 251-amino-acid precursor protein with a predicted molecular weight of 27,400 D. Amino-terminal micro sequence analysis of purified mature mouse lung EC-SOD demonstrated the sequence to begin with SSFDLADRLDPV-. These results indicate that EC-SOD as initially synthesized contains a 24-amino-acid precursor peptide, and that the mature protein is 227 amino acids in length. Computer algorithms that predict the most likely site of cotranslational signal peptidase cleavage suggest that processing will occur between amino acids 18 and 19 or 20 and 21, which implies that EC-SOD may be initially synthesized as a pre-pro-protein. Like human EC-SOD, mature mouse EC-SOD is glycosylated. The full-length mouse EC-SOD cDNA is 1,834 base pairs long and is 82% (79% for protein) identical to rat EC-SOD, but only 60% (60% for protein) identical to human EC-SOD. The mouse EC-SOD gene locus (Sod3) was mapped by interspecific backcross haplotype analysis as being 0.9 +/- 0.9 centimorgans distal to the Qdpr locus on mouse Chromosome 5, a position suggesting that the human homologue of EC-SOD will map close to the human QDPR locus (4p15.3). Of nine tissues examined by Northern blot analysis, those of the kidney and lung are by far the major tissues that express EC-SOD messenger RNA. Using in situ hybridization in the mouse lung, we demonstrate EC-SOD gene expression to be highly localized to alveolar Type II epithelial cells. These data suggest that alveolar Type II cells play a central role in mediating EC-SOD antioxidant function in the lung.

Altered pulmonary response to hyperoxia in Clara cell secretory protein deficient mice

Clara cell secretory protein (CCSP) is an abundant component of the extracellular lining fluid of airways. Even though the in vivo function of CCSP is unknown, in vitro studies support a potential role of CCSP in the control of inflammatory responses. CCSP-deficient mice (CCSP -/-) were generated to investigate the in vivo function of this protein (13). In this study, we used hyperoxia exposure as a model to investigate phenotypic consequences of CCSP deficiency following acute lung injury. The pathologic response of the mouse lung to hyperoxia, and recovery of the lung, include inflammatory cell infiltrate and edema. Continuous exposure to > 95% O2 was associated with significantly reduced survival time among CCSP -/- mice as compared with strain-, age-, and sex-matched wild-type control mice. Differences in survival were associated with early onset of lung edema in CCSP -/- mice as compared with wild-type controls. To further investigate these differences in response, mice were exposed to > 95% O2 for either 48 h or 68 h with one group receiving 68 h of hyperoxia followed by room-air recovery. Lung RNA was characterized for changes in the abundance of cytokine messenger RNA (mRNA) using a ribonuclease (RNase) protection assay. After 68 h of hyperoxia, interleukin-6 (IL-6), IL-1beta, and IL-3 mRNAs were 14-, 3-, and 2.5-fold higher, respectively, in CCSP -/- mice than in similarly exposed wild-type control mice. Increased expression of IL-1beta mRNA in hyperoxia-exposed CCSP -/- mice was localized principally within the lung parenchyma, suggesting that the effects of CCSP deficiency were not confined to the airway epithelium. We conclude that CCSP deficiency results in increased sensitivity to hyperoxia-induced lung injury as measured by increased mortality, early onset of lung edema, and induction of proinflammatory cytokine mRNAs.

Safety and pharmacokinetics of multiple doses of recombinant human CuZn superoxide dismutase administered intratracheally to premature neonates with respiratory distress syndrome

OBJECTIVES

To examine the safety and pharmacokinetics of multiple intratracheal (IT) doses of recombinant human CuZn superoxide dismutase (rhSOD) in premature infants with respiratory distress syndrome who are at risk for developing bronchopulmonary dysplasia (BPD). Methods. Thirty-three infants (700 to 1300 g) were randomized and blindly received saline, 2.5 mg/kg or 5 mg/kg rhSOD IT within 2 hours of surfactant administration. Infants were treated every 48 hours (as long as endotracheal intubation was required) up to 7 doses. Serial blood and urine studies, chest radiographs, neurosonograms, SOD concentration and activity measurements, and tracheal aspirate (TA) inflammatory markers were assessed throughout the 28-day study.

RESULTS

SOD concentrations in serum (0.1 [0.05/0.15] microg/mL-geometric mean with lower/upper confidence intervals), tracheal aspirates (TA) (0.2 [0.1/0.3] microg/mL) and urine (0.3 [0.2/0.4] microg/mL) were similar at baseline in all 3 groups and did not change significantly in the placebo group. In the rhSOD treatment groups, SOD concentrations were increased on day 3 and did not change significantly thereafter over the 14-day dosing period (also measured on days 5, 7, and 13). SOD concentrations averaged 0.4 [0.3/0.5] microg/mL in serum, 0.8 [0.6/1.2] microg/mL in TA and 1.1 [1.0/1.3] microg/mL in urine for the low-dose group and 0.6 [0.5/0.7] microg/mL in serum, 1.1 [0.9/1.5] microg/mL in TA, and 2.2 [1.6/2.9] microg/mL in urine for the high-dose group over the 14-day dosing period. Enzyme activity directly correlated with SOD concentration and rhSOD was active even when excreted in urine. TA markers of acute lung injury (neutrophil chemotactic activity, albumin concentration) were lower in the rhSOD agroups compared with placebo. No significant differences in any clinical outcome variable were noted between groups.

CONCLUSIONS

These data indicate that multiple IT doses of rhSOD increase the concentration and activity of the enzyme in serum, TA and urine, reduce TA lung injury markers and are well-tolerated. Further clinical trials examining the efficacy of rhSOD in the prevention of BPD are warranted.

Early embryonic lethality caused by targeted disruption of the mouse selenocysteine tRNA gene (Trsp)

Selenoprotein biosynthesis is mediated by tRNASec, which inserts selenocysteine at UGA codons in a complex, context-specific manner. This opal suppressor serves in the conversion of serine to selenocysteine as well. The mouse tRNASec gene (Trsp) maps to a proximal segment of chromosome 7. We constructed mice carrying a targeted deletion of the Trsp gene. The heterozygous mutants were viable, fertile, and appeared normal. Although the level of tRNASec was reduced to about 50%-80% of the wild type in most organs, one of the selenoproteins, glutathione peroxidase, remained unaffected in the levels of its mRNA, protein, and enzyme activity, indicating that the haploid amount of tRNASec is not limiting in its biosynthesis. In contrast, the homozygous mutants died shortly after implantation, and the embryos were resorbed before 6.5 days post coitum. When the preimplantation embryos were placed in culture, however, the trophoectoderm cells showed outgrowths and the inner cell mass cells of the homozygous embryos were able to proliferate. These results indicate that Trsp expression is essential for early development of the embryo, and its lack causes peri-implantation lethality. However, the lethality does not appear to be due to a cell-autonomous function of tRNASec.

Intestinal motility during hypoxia and reoxygenation in vitro

Ischemia-reperfusion injury leads to profound functional and structural alterations of the gastrointestinal tract. We developed an in vitro model of reperfusion injury to study the changes in intestinal motility during hypoxia followed by reoxygenation. We recorded the spontaneous motor activity of intestinal rings from the proximal mouse jejunum, using force displacement transducers. In addition to the rhythmic contractions, we studied the contractile response to transmural stimulation of intrinsic nerves. During hypoxia, the frequency of the spontaneous contractions and the resting tension decreased. While 29% of the tissues still responded to neural stimulation after 15 min of hypoxia, electrical field stimulation did not evoke any response after 60 min of hypoxia. Reoxygenation resulted in a transient increase in the baseline tension and an initial normalization of the spontaneous rhythmic contractions, which subsequently became irregular. The percentage of tissues that recovered their ability to respond to electrical field stimulation 10 min after reoxygenation decreased from 100% after 15 min of hypoxia to 47% after 60 min of hypoxia. The administration of the antioxidant glutathione prevented the functional abnormalities seen 10 min after reoxygenation. The pharmacological inhibition of Cu,Zn superoxide dismutase exacerbated the functional reoxygenation damage. Conversely, the overexpression of this radical-scavenging enzyme in transgenic mice increased the likelihood of functional recovery. Reoxygenation in a calcium-free solution also prevented prolonged functional damage of the muscle rings. We conclude that hypoxia-reoxygenation significantly alters intestinal motility. The generation of reactive oxygen species and disruptions in the calcium homeostasis play an important role in the pathogenesis of reoxygenation damage. Interventions that alter the intracellular redox state or affect the secondary changes in the intracellular calcium concentration can prevent or blunt the effects of reoxygenation injury on intestinal motility.

Purification and subunit structure of extracellular superoxide dismutase from mouse lung tissue

The first purification of mouse extracellular superoxide dismutase (EC-SOD) and the analysis of the native enzyme are described. Mouse EC-SOD was purified from lung tissues with a high recovery (41%) and a specific polyclonal antibody against the purified enzyme was obtained. The purified enzyme had a strong affinity for, heparin and a molecular mass of 150 kDa (estimated by a gel filtration chromatography). The native mouse EC-SOD was composed of two different sizes of subunits, a M(r) of 33 and 35 kDa (determined by SDS-PAGE). The 35-kDa subunit had an interchain disulfide bond at the C-terminus and existed as a covalent dimer in the molecule, whereas the 33-kDa subunit resulted from the 35-kDa subunit by truncating its C-terminus as a posttranslational modification, with resultant loss of the interchain disulfide bond. These results suggest that the native mouse EC-SOD is a heterotetramer composed of two different dimers, with or without a covalent bond.

Myringotomized mice develop myringosclerosis in the pars flaccida and not in the pars tensa

The development of myringosclerosis has been correlated with increased production of oxygen-derived free radicals. For the present study, we used a null mutant mouse lacking extracellular superoxide dismutase to test the hypothesis that increased production of free radicals can cause the development of myringosclerosis. Null mutant mice and wild-type, control mice were myringotomized and kept in ambient air for 3 weeks. Both groups developed myringosclerosis in the pars flaccida, but not in the pars tensa. The sclerotic lesions were visible in both the light and the electron microscope but not in the otomicroscope. In particular, the localization of the sclerotic deposits was found beneath both the inner and outer epidermal epithelium. No difference concerning the extent or number of sclerotic lesions between the null mutant and the wild-type mice could be distinguished.

Biochemical comparison of the Cu,Zn superoxide dismutases of Cryptococcus neoformans var. neoformans and Cryptococcus neoformans var. gattii

Cu,Zn superoxide dismutases (SODs) have been purified to homogeneity from the two varieties of Cryptococcus neoformans, C. neoformans var. neoformans and var. gattii. The N-terminal amino acid sequences of the two enzymes were similar, though not identical, and demonstrated homology with Cu,Zn SODs from other organisms. SOD activity was present in supernatants from stationary-phase cultures of isolates of C. neoformans var. neoformans and was also present from the mid-log phase onwards in cultures of an acapsular mutant of C. neoformans var. neoformans. SOD activity was practically undetectable in culture supernatants from isolates of C. neoformans var. gattii. The C. neoformans var. neoformans SOD had a reduced relative molecular mass of 19 kDa, and in its nonreduced form the enzyme was present as a 125-kDa species. Isoelectric focusing indicated that four species with pIs of 5.9, 6.15, 6.35, and 6.6 were present. The equivalent reduced molecular mass of the C. neoformans var. gattii enzyme was 19 kDa, with a single species present under nonreducing conditions (relative molecular mass of 145 kDa) with a pI of 7.5. The activities of the enzymes from both varieties were inhibited by KCN; however, the copper chelator diethyldithiocarbamate was inhibitory only against the C. neoformans var. gattii enzyme, as was sodium azide. The C. neoformans var. neoformans SOD was not affected by preincubation for 1 h at 70 degrees C, and it also retained most of its activity when incubated at 37 degrees C relative to its activity when incubated at 20 degrees C, in contrast to the C. neoformans var. gattii enzyme. The pronounced differences in the physical and biochemical characteristics of the Cu,Zn SODs from the two Cryptococcus varieties complement recent reports illustrating the biochemical and genetic differences between C. neoformans var. neoformans and C. neoformans var. gattii, and the successful purification of the two enzymes comprises the first step in determining what role, if any, the cryptococcal Cu,Zn SODs might have in protection against externally generated superoxide.

Overexpression of heme oxygenase-1 in human pulmonary epithelial cells results in cell growth arrest and increased resistance to hyperoxia

Heme oxygenase (HO) catalyzes the rate-limiting step in the degradation of heme to biliverdin, which is reduced by biliverdin reductase to bilirubin. Heme oxygenase-1 (HO-1) is inducible not only by its heme substrate, but also by a variety of agents causing oxidative stress. Although much is known about the regulation of HO-1 expression, the functional significance of HO-1 induction after oxidant insult is still poorly understood. We hypothesize and provide evidence that HO-1 induction serves to protect cells against oxidant stress. Human pulmonary epithelial cells (A549 cells) stably transfected with the rat HO-1 cDNA exhibit marked increases of HO-1 mRNA levels which were correlated with increased HO enzyme activity. Cells that overexpress HO-1 (A549-A4) exhibited a marked decrease in cell growth compared with wild-type A549 (A549-WT) cells or A549 cells transfected with control DNA (A549-neo). This slowing of cell growth was associated with an increased number of cells in G0/G1 phase during the exponential growth phase and decreased entry into the S phase, as determined by flow cytometric analysis of propidium iodide-stained cells and pulse experiments with bromodeoxyuridine. Furthermore, the A549-A4 cells accumulated at the G2/M phase and failed to progress through the cell cycle when stimulated with serum, whereas the A549-neo control cells exhibited normal cell cycle progression. Interestingly, the A549-A4 cells also exhibited marked resistance to hyperoxic oxidant insult. Tin protoporphyrin, a selective inhibitor of HO, reversed the growth arrest and ablated the increased survival against hyperoxia observed in the A549-A4 cells overexpressing HO-1. Taken together, our data suggest that overexpression of HO-1 results in cell growth arrest, which may facilitate cellular protection against non-heme-mediated oxidant insult such as hyperoxia.

Neurodegeneration, myocardial injury, and perinatal death in mitochondrial superoxide dismutase-deficient mice

Manganese superoxide dismutase (SOD2) converts superoxide to oxygen plus hydrogen peroxide and serves as the primary defense against mitochondrial superoxide. Impaired SOD2 activity in humans has been associated with several chronic diseases, including ovarian cancer and type I diabetes, and SOD2 overexpression appears to suppress malignancy in cultured cells. We have produced a line of SOD2 knockout mice (SOD2m1BCM/SOD2m1BCM) that survive up to 3 weeks of age and exhibit several novel pathologic phenotypes including severe anemia, degeneration of neurons in the basal ganglia and brainstem, and progressive motor disturbances characterized by weakness, rapid fatigue, and circling behavior. In addition, SOD2m1BCM/SOD2m1BCM mice older than 7 days exhibit extensive mitochondrial injury within degenerating neurons and cardiac myocytes. Approximately 10% of SOD2m1BCM/SOD2m1BCM mice exhibit markedly enlarged and dilated hearts. These observations indicate that SOD2 deficiency causes increased susceptibility to oxidative mitochondrial injury in central nervous system neurons, cardiac myocytes, and other metabolically active tissues after postnatal exposure to ambient oxygen concentrations. Our SOD2-deficient mice differ from a recently described model in which homozygotes die within the first 5 days of life with severe cardiomyopathy and do not exhibit motor disturbances, central nervous system injury, or ultrastructural evidence of mitochondrial injury.

Blood radicals: reactive nitrogen species, reactive oxygen species, transition metal ions, and the vascular system

Free radicals, such as superoxide, hydroxyl and nitric oxide, and other "reactive species", such as hydrogen peroxide, hypochlorous acid and peroxynitrite, are formed in vivo. Some of these molecules, e.g. superoxide and nitric oxide, can be physiologically useful, but they can also cause damage under certain circumstances. Excess production of reactive oxygen or nitrogen species (ROS, RNS), their production in inappropriate relative amounts (especially superoxide and NO) or deficiencies in antioxidant defences may result in pathological stress to cells and tissues. This oxidative stress can have multiple effects. It can induce defence systems, and render tissues more resistant to subsequent insult. If oxidative stress is excessive or if defence and repair responses are inadequate, cell injury can be caused by such mechanisms as oxidative damage to essential proteins, lipid peroxidation, DNA strand breakage and base modification, and rises in the concentration of intracellular "free" Ca(2+). Considerable evidence supports the view that oxidative damage involving both ROS and RNS is an important contributor to the development of atherosclerosis. Peroxynitrite (derived by reaction of superoxide with nitric oxide) and transition metal ions (perhaps released by injury to the vessel wall) may contribute to lipid peroxidation in atherosclerotic lesions.