Purification and sequence of rat extracellular superoxide dismutase B secreted by C6 glioma

Carcinogenesis and Reactive Oxygen Species Signaling: Interaction of the NADPH Oxidase NOX1-5 and Superoxide Dismutase 1-3 Signal Transduction Pathways

SIGNIFICANCE

Reduction/oxidation (redox) balance could be defined as an even distribution of reduction and oxidation complementary processes and their reaction end products. There is a consensus that aberrant levels of reactive oxygen species (ROS), commonly observed in cancer, stimulate primary cell immortalization and progression of carcinogenesis. However, the mechanism how different ROS regulate redox balance is not completely understood. Recent Advances: In the current review, we have summarized the main signaling cascades inducing NADPH oxidase NOX1-5 and superoxide dismutase (SOD) 1-3 expression and their connection to cell proliferation, immortalization, transformation, and CD34+ cell differentiation in thyroid, colon, lung, breast, and hematological cancers.

CRITICAL ISSUES

Interestingly, many of the signaling pathways activating redox enzymes or mediating the effect of ROS are common, such as pathways initiated from G protein-coupled receptors and tyrosine kinase receptors involving protein kinase A, phospholipase C, calcium, and small GTPase signaling molecules.

FUTURE DIRECTIONS

The clarification of interaction of signal transduction pathways could explain how cells regulate redox balance and may even provide means to inhibit the accumulation of harmful levels of ROS in human pathologies.

Redox Regulation of the Superoxide Dismutases SOD3 and SOD2 in the Pulmonary Circulation

When evaluating the role of redox-regulating signaling in pulmonary vascular diseases, it is intriguing to consider the modulation of key antioxidant enzymes like superoxide dismutase (SOD) because SOD isoforms are regulated by redox reactions, and, in turn, modulate downstream redox sensitive processes. The emerging field of redox biology is built upon understanding the regulation and consequences of tightly controlled and specific reduction-oxidation reactions that are critical for diverse cellular processes including cell signaling. Of relevance, both the site of production of specific reactive oxygen and nitrogen species and the site of the antioxidant defenses are highly compartmentalized within the cell. For example, superoxide is generated during oxidative phosphorylation in the mitochondria as well as by a number of enzymatic sources within the cytosol and at the cell membrane. In the pulmonary circulation, these sources include the mitochondrial electron transport chain, NADPH oxidases (NOX1-4, Duox1,2), nitric oxide synthases, and xanthine oxidase; this important topic has been thoroughly reviewed recently [1]. In parallel with these different cellular sites of superoxide production, the three SOD isoforms are also specifically localized to the cytosol (SOD1), mitochondria (SOD2) or extracellular compartment (SOD3). This chapter focuses on the role of redox mechanisms regulating SOD2 and SOD3, with an emphasis on these processes in the setting of pulmonary hypertension.

Homocysteine-induced extracellular superoxide dismutase and its epigenetic mechanisms in monocytes

Although a modest homocysteine (Hcy) elevation is associated with an increased cardiovascular risk, the underlying mechanisms whereby Hcy triggers the accumulation of cholesterol and the roles of the extracellular superoxide dismutase (EC-SOD) in the development of foam cells have not yet been elucidated. In this study, we found both increased numbers of foam cells and an accumulation of cholesterol, and the H(2)O(2) and oxidized low-density lipoprotein content also increased. Levels of EC-SOD were significantly suppressed by Hcy, however, while 5-azacytidine (AZC), a potent DNA methyltransferase (DNMT) inhibitor, increased the expression of EC-SOD. A quantitative real-time PCR of EC-SOD revealed that Hcy (100 micromol l(-1)) accelerates DNA methylation of EC-SOD, but selectively increases the activity of DNA methyl transferase 1 (DNMT1). It showed that Hcy can reduce binding of methyl CpG and binding protein 2 (MeCP2) but has no effect on the activity of DNMT3. Moreover, chromatin immunoprecipitation assays demonstrated that Hcy increased the binding of acetylated histone H3 and H4 in monocytes. Based on the fact that the binding of MeCP2 with the EC-SOD was completely suppressed by AZC and trichostatin A [TSA, a histone deacetylase (HDAC) inhibitor], it is indicated that DNA methylation and HDAC mediate the binding of MeCP2 with EC-SOD gene. In conclusion, the study found that Hcy accelerates the development of foam cells by repressing EC-SOD transcription, and that Hcy exerts this function by upregulating DNA methylation via suppression of HDAC activity and increased DNMT1 activity.

The subunit composition of human extracellular superoxide dismutase (EC-SOD) regulates enzymatic activity

BACKGROUND

Human extracellular superoxide dismutase (EC-SOD) is a tetrameric metalloenzyme responsible for the removal of superoxide anions from the extracellular space. We have previously shown that the EC-SOD subunit exists in two distinct folding variants based on differences in the disulfide bridge pattern (Petersen SV, Oury TD, Valnickova Z, Thøgersen IB, Højrup P, Crapo JD, Enghild JJ. Proc Natl Acad Sci USA. 2003;100(24):13875-80). One variant is enzymatically active (aEC-SOD) while the other is inactive (iEC-SOD). The EC-SOD subunits are associated into covalently linked dimers through an inter-subunit disulfide bridge creating the theoretical possibility of 3 dimers (aa, ai or ii) with different antioxidant potentials. We have analyzed the quaternary structure of the endogenous EC-SOD disulfide-linked dimer to investigate if these dimers in fact exist.

RESULTS

The analyses of EC-SOD purified from human tissue show that all three dimer combinations exist including two homo-dimers (aa and ii) and a hetero-dimer (ai). Because EC-SOD is a tetramer the dimers may combine to generate 5 different mature EC-SOD molecules where the specific activity of each molecule is determined by the ratio of aEC-SOD and iEC-SOD subunits.

CONCLUSION

This finding shows that the aEC-SOD and iEC-SOD subunits combine in all 3 possible ways supporting the presence of tetrameric enzymes with variable enzymatic activity. This variation in enzymatic potency may regulate the antioxidant level in the extracellular space and represent a novel way of modulating enzymatic activity.

Differential expression of EC-SOD, Mn-SOD and CuZn-SOD in rat lung exposed to crystalline silica

Superoxide dismutases (SODs) are antioxidant enzymes that catalyze the dismutation of superoxide into hydrogen peroxide. There are 3 kinds of isozymes: extracellular superoxide dismutase (EC-SOD), manganese-containing superoxide dismutase (Mn-SOD) and copper- and zinc-containing superoxide dismutase (CuZn-SOD). To examine the expression of SOD isozymes in lungs injured by crystalline silica, we intratracheally instilled male Wistar rats with 2 mg (8 mg/kg) of crystalline silica and investigated the mRNA, protein level and distribution of SOD isozymes in the rat lungs using RT-PCR, western blot analysis and immunostaining, respectively at from 3 d to 180 d of recovery following the exposure. EC-SOD mRNA levels significantly increased from 3 d to 90 d and the EC-SOD protein level was significantly higher after 90 and 180 d recovery in the crystalline silica exposed groups than in the control groups. Mn-SOD increased in silica treated rat lungs at both mRNA and protein levels, peaking at 30 d post-exposure. CuZn-SOD mRNA levels were decreased at 3, 7 and 30 d, and CuZn-SOD protein levels were also significantly lower than the control group at 90 and 180 d recovery. There was prominent EC-SOD immunostaining mainly in the plasma and alveolar macrophages and strong Mn-SOD staining in alveolar macrophages and interstitial cells of the proximal and distal portions of the alveolar duct following crystalline silica exposure. There was less CuZn-SOD staining in epithelial cells at terminal bronchioles in the crystalline silica-exposed group. These findings suggest that these SOD isozymes may be related to lung injury induced by crystalline silica.

Superoxide dismutase in the prostate lobes of aging Brown Norway rats

BACKGROUND

Spontaneous age-dependent epithelial cell hyperplasia occurs in the lateral and dorsal, but not the ventral, lobes of aging Brown Norway (BN) rats. Diminished antioxidant enzyme activities and increased formation of reactive oxygen species (ROS) promote the pathology of many aging disorders. We investigated the hypothesis that prostatic epithelial cell hyperplasia in the BN rat was related to age-dependent and/or lobe-specific changes in superoxide dismutase (SOD).

MATERIALS AND METHODS

Using Western blots, immunohistochemistry and enzyme activity assays we determined the levels of protein expression, subcellular localization, and activities, respectively, of the three SOD isoforms, cytoplasmic SOD1, mitochondrial SOD2, and extracellular SOD3 in the ventral, lateral, and dorsal prostate lobes of 4-month-old rats with normal prostate morphology, in 24-month-old rats with lobe-specific hyperplasia and in older 30-month-old rats.

RESULTS

We observed little change in SOD activities as a function of age, although expression of SOD3 increased in the prostatic lobes of older rats. SOD2 levels were higher in the lateral lobe of 4- and 24-month-old rats, but declined by 30 months of age to levels in the ventral and dorsal lobes. SOD1 was localized by immunohistochemistry to the nuclei of epithelial cells in all lobes, but the number of immunopositive nuclei increased in the lateral and dorsal lobes of 24-month-old animals. The concentration of zinc was highest in the prostate lobes of 24-month-old animals.

CONCLUSION

Based upon our data, superoxide dismutase is not significantly altered in the rat prostate during aging and thus is unlikely to be an important factor in the evolution of epithelial cell hyperplasia.

Extracellular superoxide dismutase exists as an octamer

Human extracellular superoxide dismutase (EC-SOD) is involved in the defence against oxidative stress induced by the superoxide radical. The protein is a homotetramer stabilised by hydrophobic interactions within the N-terminal region. During the purification of EC-SOD from human aorta, we noticed that material with high affinity for heparin-Sepharose formed not only a tetramer but also an octamer. Analysis of the thermodynamic stability of the octamer suggested that the C-terminal region is involved in formation of the quaternary structure. In addition, we show that the octamer is composed of both aEC-SOD and iEC-SOD folding variants. The presence of the EC-SOD octamer with high affinity may represent a way to influence the local concentration of EC-SOD to protect tissues specifically sensitive to oxidative damage.

Extracellular superoxide dismutase: structural and functional considerations of a protein shaped by two different disulfide bridge patterns

The effects of reactive oxygen species are detrimental and can cause damage to DNA, protein, and lipids. Hence, the etiology of a large range of diseases resides in the generation of excess reactive oxygen species. However, these species are also involved in the maintenance of physiological functions. In tissues, it is therefore essential to maintain a steady-state level of antioxidant activity to allow both for the physiological functions of reactive oxygen species to proceed and at the same time preventing tissue damage. Extracellular superoxide dismutase (EC-SOD) is the only extracellular scavenger of the superoxide radical. The reactivity of superoxide is promiscuous and it is crucial that EC-SOD is positioned at the site of superoxide production to prevent adventitious reactions. It is therefore likely beneficial to have mechanisms for regulating the EC-SOD tissue distribution and enzymatic activity. The modular architecture of EC-SOD, encompassing three functional regions, is an ideal construction to generate diversity. By intracellular proteolytic processing and generation of active and inactive molecules, EC-SOD represents a flexible protein with the capacity to fine-tune the tissue localization and the antioxidant level in the extracellular space. The present review will address the function and activity of the separate regions of EC-SOD.

Extracellular superoxide dismutase functions as a major repressor of hypoxia-induced erythropoietin gene expression

Hypoxia and biological responses to hypoxia are commonly encountered in both normal and pathologic cellular processes. Here we report that extracellular superoxide dismutase (EC-SOD) plays a major role in regulating the magnitude of hypoxia-induced erythropoietin (Epo) gene expression, thus implicating superoxide as an intermediary signal transduction molecule critical to this process. We found that mice which have the EC-SOD gene inactivated show a marked more than 100-fold elevation in hypoxia-induced Epo gene expression, compared with wild-type controls, which was both dose and time dependent. These mice also showed a significant increase in serum Epo levels after 1 d hypoxia. Interestingly, despite elevated Epo levels, reciprocal changes in hematocrit and reticulocyte counts were not found, suggesting that this newly synthesized Epo lacks functional hematopoietic effects. When EC-SOD was overexpressed in Hep3B cells, we found a significant reduction in Epo gene induction by both CoCl2 (50 microM) and hypoxia (1% O2). Similar findings were noted with another hypoxia-inducible gene, carbonic anhydrase IX. We conclude that EC-SOD functions as a major repressor of hypoxia-induced Epo gene expression, which implicates superoxide as a signaling intermediate whose downstream effects, at least in part, may be mediated by HIF-1alpha.

The Structure of Rabbit Extracellular Superoxide Dismutase Differs from the Human Protein†

The cDNA sequence encoding rabbit, mouse, and rat extracellular superoxide dismutase (EC-SOD) predicts that the protein contains five cysteine residues. Human EC-SOD contains an additional cysteine residue and folds into two forms with distinct disulfide bridge patterns. One form is enzymatically active (aEC-SOD), while the other is inactive (iEC-SOD). Due to the lack of the additional cysteine residue rabbit, mouse, and rat EC-SOD are unable to generate an inactive fold identical to human iEC-SOD. The amino acid sequences predict the formation of aEC-SOD only, but other folding variants cannot be ruled out based on the heterogeneity observed for human EC-SOD. To test this, we purified EC-SOD from rabbit plasma and determined the disulfide bridge pattern. The results revealed that the disulfide bridges are homogeneous and identical to human aEC-SOD. Four cysteine residues are involved in two intra-disulfide bonds while the C-terminal cysteine residue forms an intersubunit disulfide bond. No evidence for other folding variants was detected. These findings show that rabbit EC-SOD exists as an enzymatically active form only. The absence of iEC-SOD in rabbits suggests that the structure and aspects of the physiological function of EC-SOD differs significantly between rabbit and humans. This is an important notion to take when using these animals as model systems for oxidative stress.

The dual nature of human extracellular superoxide dismutase: one sequence and two structures

Human extracellular superoxide dismutase (EC-SOD; EC 1.15.1.1) is a scavenger of superoxide anions in the extracellular space. The amino acid sequence is homologous to the intracellular counterpart, Cu/Zn superoxide dismutase (Cu/Zn-SOD), apart from N- and C-terminal extensions. Cu/Zn-SOD is a homodimer containing four cysteine residues within each subunit, and EC-SOD is a tetramer composed of two disulfide-bonded dimers in which each subunit contains six cysteines. The amino acid sequences of all EC-SOD subunits are identical. It is known that Cys-219 is involved in an interchain disulfide. To account for the remaining five cysteine residues we purified human EC-SOD and determined the disulfide bridge pattern. The results show that human EC-SOD exists in two forms, each with a unique disulfide bridge pattern. One form (active EC-SOD) is enzymatically active and contains a disulfide bridge pattern similar to Cu/Zn-SOD. The other form (inactive EC-SOD) has a different disulfide bridge pattern and is enzymatically inactive. The EC-SOD polypeptide chain apparently folds in two different ways, most likely resulting in different three-dimensional structures. Our study shows that one gene may produce proteins with different disulfide bridge arrangements and, thus, by definition, different primary structures. This observation adds another dimension to the functional annotation of the proteome.

Extracellular superoxide dismutase in biology and medicine

Accumulated evidence has shown that reactive oxygen species (ROS) are important mediators of cell signaling events such as inflammatory reactions (superoxide) and the maintenance of vascular tone (nitric oxide). However, overproduction of ROS such as superoxide has been associated with the pathogenesis of a variety of diseases including cardiovascular diseases, neurological disorders, and pulmonary diseases. Antioxidant enzymes are, in part, responsible for maintaining low levels of these oxygen metabolites in tissues and may play key roles in controlling or preventing these conditions. One key antioxidant enzyme implicated in the regulation of ROS-mediated tissue damage is extracellular superoxide dismutase (EC-SOD). EC-SOD is found in the extracellular matrix of tissues and is ideally situated to prevent cell and tissue damage initiated by extracellularly produced ROS. In addition, EC-SOD is likely to play an important role in mediating nitric oxide-induced signaling events, since the reaction of superoxide and nitric oxide can interfere with nitric oxide signaling. This review will discuss the regulation of EC-SOD and its role in a variety of oxidant-mediated diseases.

Preservation of extracellular glutathione by an astrocyte derived factor with properties comparable to extracellular superoxide dismutase

Cultured rat and human astrocytes and rat neurones were shown to release reduced glutathione (GSH). In addition, GSH oxidation was retarded by the concomitant release of a factor from the cells. One possibility is that this factor is extracellular superoxide dismutase (SOD). In support of this, the factor was found to bind heparin, have a molecular mass estimated to be between 50 and 100 kDa, and CuZn-type SOD protein and cyanide sensitive enzyme activity were demonstrated in the cell-conditioned medium. In addition, supplementation of native medium with exogenous CuZn-type SOD suppressed GSH oxidation. We propose that preservation of released GSH is essential to allow for maximal up-regulation of GSH metabolism in neurones. Furthermore, cytokine stimulation of astrocytes increased release of the extracellular SOD, and enhanced stability of GSH. This may be a protective strategy occurring in vivo under conditions of oxidative stress, and suggests that SOD mimetics may be of therapeutic use.

Superoxide dismutase multigene family: a comparison of the CuZn-SOD (SOD1), Mn-SOD (SOD2), and EC-SOD (SOD3) gene structures, evolution, and expression

Superoxide dismutases are an ubiquitous family of enzymes that function to efficiently catalyze the dismutation of superoxide anions. Three unique and highly compartmentalized mammalian superoxide dismutases have been biochemically and molecularly characterized to date. SOD1, or CuZn-SOD (EC 1.15.1.1), was the first enzyme to be characterized and is a copper and zinc-containing homodimer that is found almost exclusively in intracellular cytoplasmic spaces. SOD2, or Mn-SOD (EC 1.15.1.1), exists as a tetramer and is initially synthesized containing a leader peptide, which targets this manganese-containing enzyme exclusively to the mitochondrial spaces. SOD3, or EC-SOD (EC 1.15.1.1), is the most recently characterized SOD, exists as a copper and zinc-containing tetramer, and is synthesized containing a signal peptide that directs this enzyme exclusively to extracellular spaces. What role(s) these SODs play in both normal and disease states is only slowly beginning to be understood. A molecular understanding of each of these genes has proven useful toward the deciphering of their biological roles. For example, a variety of single amino acid mutations in SOD1 have been linked to familial amyotrophic lateral sclerosis. Knocking out the SOD2 gene in mice results in a lethal cardiomyopathy. A single amino acid mutation in human SOD3 is associated with 10 to 30-fold increases in serum SOD3 levels. As more information is obtained, further insights will be gained.

Natural inhibitors of neutrophil function in acute respiratory distress syndrome

OBJECTIVE

Neutrophils play a key role in the physiopathogenesis of acute lung injury in general and acute respiratory distress syndrome (ARDS) in particular. To identify the anti-inflammatory mediators with a protective effect on lung tissue damage in ARDS, we correlated the concentration of the Clara cell 16-kD protein (CC16; an inhibitor of neutrophil chemotaxis), angiogenin (an inhibitor of degranulation), and the total radical oxygen neutralizing activity with the amount of elastase (a marker of neutrophil activation) and with the Pao2/Fio2 ratio, which is inversely related to lung injury.

SETTING

University hospital.

PATIENTS

Patients with ARDS (n = 12) and patients at risk for developing ARDS (n = 14).

INTERVENTIONS

Patients underwent bronchoalveolar lavage 12 hrs after diagnosis of ARDS or at-risk status.

MEASUREMENTS AND MAIN RESULTS

The amount of CC16 and radical oxygen neutralizing activity was not significantly different in patients with or at risk for ARDS. In contrast, the amount (mean +/- sem) of angiogenin in the bronchoalveolar lavage of ARDS patients (45 +/- 14 ng/mL, n = 12) was increased 11-fold (p <.05) compared with patients at risk for ARDS (4 +/- 1 ng/mL, n = 14). In patients with ARDS, the amount of protein and angiogenin in bronchoalveolar lavage increased with decreasing concentration of CC16 (p <.05). In addition, CC16 correlated with the Pao2/Fio2 ratio (p <.05) and inversely with the amount of elastase (p <.05) and thus may be regarded as a reliable protective agent for lung injury.

CONCLUSION

A high concentration of CC16, a natural inhibitor of neutrophil function, decreases neutrophil-mediated lung damage of patients with ARDS. Strategies to increase natural anti-inflammatory agents, and thus influence the disruption of the balance between natural inflammatory and anti-inflammatory or protective factors, could be useful to modulate the tissue destruction and the course of ARDS.

Taurine prevents the decrease in expression and secretion of extracellular superoxide dismutase induced by homocysteine: amelioration of homocysteine-induced endoplasmic reticulum stress by taurine

BACKGROUND

Hyperhomocysteinemia is an independent risk factor for atherosclerosis. Homocysteine has been shown to induce endoplasmic reticulum (ER) stress in vascular endothelial cells. ER stress is a condition in which glycoprotein trafficking is disrupted and unfolded proteins accumulate in the ER. ER molecular chaperons, such as GRP78, are induced and an ER resident kinase, PERK, is activated when cells are subjected to ER stress. Conversely, taurine is reported to have antiatherogenic effects by unknown mechanisms. To elucidate the mechanisms by which homocysteine induces atherosclerosis and taurine prevents it, we examined whether homocysteine and taurine affect the expression and secretion of extracellular superoxide dismutase (EC-SOD), a glycoprotein secreted from vascular smooth muscle cells (VSMCs) that protects the vascular wall from oxidative stress.

METHODS AND RESULTS

We assessed the expression of EC-SOD and GRP78 mRNA in cultured rat VSMCs by Northern blot analysis. The EC-SOD protein secreted into the culture medium was examined by Western blot analysis. Homocysteine (5 mmol/L) and other ER stress inducers, including A23187, were found to decrease EC-SOD mRNA expression and protein secretion. Furthermore, they upregulated GRP78 mRNA expression and activated PERK. Taurine (0.5 to 10 mmol/L), conversely, prevented these actions induced by homocysteine.

CONCLUSIONS

Homocysteine induces ER stress and reduces the secretion and expression of EC-SOD in VSMCs, leading to increased oxidative stress in the vascular wall. Taurine restores the secretion and expression of EC-SOD by ameliorating ER stress induced by homocysteine.

Local hypomethylation in atherosclerosis found in rabbit ec-sod gene

Extracellular superoxide dismutase (EC-SOD) protects arteries against deleterious effects of superoxide anions and the development of atherosclerosis. In this study, we cloned and characterized rabbit ec-sod gene. We identified 6 rabbit C-elements and 5 CpG clusters in the cloned sequence. One of the CpG clusters is located on the coding sequence. Because CpG clusters are potential sites for methylation and may explain the occurrence of mutations, methylation status of each of the CpG dimers located in the coding sequence CpG cluster was characterized using direct genomic sequencing. Unexpectedly, a marked reduction in the amount of methylated CpG dinucleotides in ec-sod gene was detected in atherosclerotic aortas as compared with normal aortic intima-media. Although alterations in DNA methylation are well characterized in malignant tumors, the presence of methylation changes in atherosclerosis has not been studied even though both diseases are characterized by excess cellular proliferation and alterations in gene expression. Further analysis of the whole genomic methylation by high-pressure liquid chromatography in normal and atherosclerotic aortas revealed a tendency for a decreased 5-methylcytosine (5-mC) content in atherosclerotic aortas as compared with normal arteries. Hypomethylation in atherosclerotic aortas occurred at the same level as has been reported from malignant tumors. Although a causal relationship between the methylation level and expression of EC-SOD cannot be proven, our results show that ec-sod hypomethylation is associated with the development of atherosclerosis and suggest that it may affect structure and function of ec-sod and other genes possibly involved in the development of atherosclerotic lesions.

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.

Inhibition of adhesion and induction of epithelial cell invasion by HAV-containing E-cadherin-specific peptides

The E-cadherin/catenin complex, an organizer of epithelial structure and function, is disturbed in invasive cancer. The HAV (histidine alanine valine) sequence in the first extracellular domain of E-cadherin is crucial for homophilic interactions between cadherins. We report that specific peptides containing an HAV sequence interfere with the functions of the E-cadherin/catenin complex. Cells either expressing specific cadherins or not were challenged with both cadherin and noncadherin peptides comprising a central HAV sequence. Specific E-cadherin peptides inhibited cell aggregation, disturbed the epithelial morphotype and were able to stimulate invasion of cells expressing E-cadherins. Conditioned medium, containing E-cadherin fragments, also stimulated invasion in contrast to conditioned medium from which the E-cadherin fragments were removed. Our studies show that E-cadherin functions are inhibited by homologous proteolytic HAV-containing fragments that are released in an autocrine manner and subsequently inhibit the E-cadherin/catenin complex. In this way such cadherin fragments may induce and support cancer invasion.

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.

Rat testicular extracellular superoxide dismutase: its purification, cellular distribution, and regulation

Using multiple HPLC steps, we have identified and purified a 68-kDa polypeptide (as estimated by gel permeation HPLC) to apparent homogeneity, from primary Sertoli cell-enriched culture medium, that consisted of two monomers of 35 (alpha chain) and 33 kDa (ss chain) on SDS-polyacrylamide gel running under reducing conditions. Partial N-terminal amino acid sequence analysis of these two monomers revealed sequences of NH2-DXGESGVDLADRL (SODEX-alpha) and NH2-XXDTGESGVDLADXL (SODEX-ss), which are identical to rat extracellular superoxide dismutase (SODEX) with the exceptions that SODEX-alpha and SODEX-ss are missing, respectively, four (Trp-Thr-Met-Ser) and two (Trp-Thr) amino acids from their N-termini, compared to rat SODEX, suggesting that the cleavage sites of the SODEX gene in the testis are different from that of other organs. Studies by sequential use of reverse transcription and polymerase chain reaction (PCR) using two SODEX primers have demonstrated the expression of SODEX in the heart, brain, lung, kidney, epididymis, testis, Sertoli, and germ cells, with low expression in the liver and ovary and no expression in the uterus, spleen, or thymus. Nucleotide sequence analysis of this 447-base pair PCR product from Sertoli cells revealed that its sequence is equivalent to the sequence of previously published rat SODEX. During testicular maturation, the SODEX steady-state mRNA level increased significantly from 20 to 60 days of age and then declined at 90 days of age. Such an increase in the testicular SODEX expression during maturation is not likely a result of an up-regulation by germ cells, since germ cells isolated from either 20- or 60-day-old rats when cocultured with Sertoli cells failed to elicit an increase in SODEX expression in the cocultures. Using primary Sertoli cell cultures in vitro, it was found that Sertoli cell SODEX expression was stimulated by interleukin-1alpha but not by either interferon-gamma or basic fibroblast growth factor. These results illustrate that Sertoli cells as well as germ cells synthesize and/or secrete a testicular variant of SODEX that may provide essential clues to understanding superoxide radical-mediated damage in the gonad.

Localization of extracellular superoxide dismutase in rat lung: neutrophils and macrophages as carriers of the enzyme

Immunohistochemistry (IHC) and in situ hybridization (ISH) was used to localize extracellular superoxide dismutase (EC-SOD) and its mRNA in rat lung before and after a lipopolysaccharide (LPS)- and hyperoxia-induced inflammation. In control rats, EC-SOD mRNA was synthesized in macrophages and in cells of the arterial vessel walls and the alveolar septa. The EC-SOD protein was mainly localized in plasma and on the apical side of the epithelial cells located near bronchus-associated lymphoid tissue (BALT). ISH did not reveal major changes in the distribution of EC-SOD mRNA upon induction of inflammation. In contrast, IHC demonstrated a progressive staining of the epithelium of the larger bronchi for the protein. Neutrophils and macrophages invading the lung showed an intensive staining for the EC-SOD protein concomitantly with a decrease of the enzyme in the plasma. Twenty-four hours after LPS stimulation only a spotty positivity remained on neutrophils in and between the alveolar spaces. In the bronchoalveolar lavage fluid (BALF), only macrophages showed a strong positivity for EC-SOD mRNA while the protein was detected in macrophages and neutrophils. Exposure to hyperoxia did not affect the distribution of EC-SOD mRNA and protein. The presented data demonstrated that in lung tissue the EC-SOD enzyme may have a protective function for activated macrophages, neutrophils, and lympoid tissue-associated epithelial cells.

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.

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.

Chromatographic and electrophoretic methods for analysis of superoxide dismutases

A brief overview of the family of superoxide dismutase (SOD) enzymes and their biomedical significance is presented. Methodology for the purification and electrophoretic analysis of superoxide dismutases is reviewed and discussed, with emphasis on the specific problems raised by the separation of individual superoxide dismutase isoenzymes. Purification methods and their performance, as reported in the literature are summarised in table form. Generally used methods for measuring SOD activity in vitro and SOD visualisation after electrophoresis are outlined, particularly those relevant to the monitoring of progress of SOD purification.

Evidence for secretion of cytosolic CuZn superoxide dismutase by Hep G2 cells and human fibroblasts

The role so far ascribed to intracellular CuZn superoxide dismutase is that of an intracellular scavenger of oxygen radicals. However, other functions of cytosolic CuZn superoxide dismutase have been hypothesized. For example, CuZn superoxide dismutase incubated with rat hepatocyte cells in culture inhibits 3-hydroxy-3methylglutaryl CoA reductase, thereby reducing cholesterol synthesis. We recently demonstrated the presence of surface membrane receptors for CuZn superoxide dismutase, suggesting possible autocrine or paracrine activities. The aim of the present study was to investigate whether cytosolic CuZn superoxide dismutase can be secreted by human hepatocarcinoma and fibroblast cells lines. Proteins in human hepatocellular carcinoma (Hep G2) cells and human fibroblasts were biosynthetically labelled with [35S]-cysteine; then cell lysates and media were immunoprecipitated with rabbit polyclonal anti-human CuZn superoxide dismutase antibodies and separated by 12% polyacrylamide gel electrophoresis. Both Hep G2 cells and human fibroblasts produce and secrete CuZn superoxide dismutase which was detectable in cells and medium as a single protein band with the same electrophoretic mobility as human erythrocyte CuZn superoxide dismutase. These data suggest that CuZn superoxide dismutase, an enzyme thus far considered to be located exclusively intracellularly is secreted by at least two cell lines. This is consistent with autocrine or paracrine roles for CuZn superoxide dismutase.

Extracellular superoxide dismutase in vessels and airways of humans and baboons

Extracellular superoxide dismutase (EC SOD) is generally the least abundant SOD isozyme in tissues, while the intracellular Cu,Zn SOD is usually the most abundant isozyme. The biological significance of EC SOD is unknown. Immunolocalization studies show that EC SOD is in the connective tissue surrounding smooth muscle in vessels and airways within the lung. Endothelium derived relaxing factor, thought to be a nitric oxide (NO) species, is a primary mediator of vascular relaxation. During NO.'s diffusion between the endothelium and smooth muscle, extracellular superoxide would be the most efficient scavenger of NO(.). High levels of extracellular superoxide dismutase in vessels could, therefore, be essential to enable NO. to modulate vascular tone. To evaluate the hypothesis that vessel walls are functionally rich in extracellular superoxide scavenging capacity, this study quantitates the EC SOD levels in pulmonary and systemic vessels and in airways. Both pulmonary and systemic arteries in humans and baboons were found to contain high activities of EC SOD. The level of EC SOD in all human and baboon arteries examined is greater than or equal to the level of intracellular Cu,Zn SOD, and EC SOD accounted for over 70% of the total SOD activity in some vessels examined. Immunolocalization of EC SOD in human and baboon vessels show similar distributions of this enzyme in pulmonary and systemic vessels. EC SOD is located beneath the endothelium, surrounding smooth muscle cells, and throughout the adventitia of vessels. The high level of EC SOD in vessels, and its localization between endothelial and smooth muscle cells, suggest that regulation of superoxide may be particularly important in this region, possibly in regulating vascular tone.

Cyclic AMP-induced differentiation increases the synthesis of extracellular superoxide dismutase in rat C6 glioma

The effect of membrane permeable cAMP analogues on the expression of extracellular superoxide dismutase (EC-SOD) was studied in rat C6 glioma. EC-SOD is constitutively expressed but stimulation with cAMP analogues still increased the EC-SOD transcription and the secreted SOD activity. The potency to enhance EC-SOD expression is correlated with the ability of the cAMP analogue to induce cAMP-dependent differentiation in C6. The increase in EC-SOD mRNA and in secreted activity depended on the concentration of the cAMP analogues and on the cultivation time. Twenty-four hours after addition of 0.5 mM N6, O'2-dibutyryl cAMP (dbcAMP) or N6-monobutyryl cAMP (N6-mbcAMP) EC-SOD mRNA expression increased approximately twofold, while stimulation for 68 h with 0.5 mM N6-mbcAMP or 1 mM 8-Chloro cAMP (ClcAMP) and 1 mM dbcAMP enhanced the mean secreted activity/cell three- and fivefold, respectively. O'2-monobutyryl cAMP (O'2-mbcAMP) did not affect EC-SOD synthesis. The enhancement in EC-SOD activity did not require activation of protein kinase A. ATP, TGF-beta, IFN-gamma, and LPS did not affect EC-SOD synthesis. The presented data point to a cAMP-dependent pathway for the enhanced expression of EC-SOD by glial cells in brain.

Cadherin-dependent cell aggregation is affected by decapeptide derived from rat extracellular super-oxide dismutase

A synthetic HAV-containing decapeptide homologous to the amino acid sequence 44R-Q53 in rat extracellular superoxide dismutase B affects cadherin-dependent cell aggregation. Cell lines, some of them transfected, expressing different types of cadherins were tested using in vitro cell aggregation and cell dissociation assays. A concentration-dependent inhibition of aggregation by the EC-SOD-derived HAV-containing peptide was detected only in N-cadherin expressing cells. These results suggest the localisation and possible protective role of EC-SOD B for cells expressing N-cadherin.

Characterization of a rat C6 glioma-secreted follistatin-related protein (FRP). Cloning and sequence of the human homologue

A protein was isolated from rat C6 glioma-conditioned medium and was biochemically characterized. The heparin-binding protein has a native molecular mass of 55-75,000 Da, a molecular mass of 40-48,000 Da under denaturing conditions, and a pI of 5.0-6.0. Based on the determined partial amino acid sequences, the full lenght cDNA encoding the rat and human proteins were cloned. The cDNA sequences identified the isolated rat and human protein as the homologue of a recently reported mouse osteoblast-transforming-growth-factor-beta 1-inducible protein, encoded by the TSC-36 gene [Shibanuma, M., Mashimo, J., Mita, A., Kuroki, T. & Nose, K. (1993) Eur. J. Biochem. 217, 13-19]. Analysis of the human, rat and mouse amino acid sequences indicates that these proteins are highly conserved (> 92% sequence identity). Sequence similarities with follistatin and the follistatin-like domain of agrin are revealed. The relationship with follistatin and agrin points to possible common functions for the cloned follistatin-related proteins (FRP). The protein has no effect on the inhibitory action of transforming growth factor-beta 1, on CCl-64 cell growth.