Harry S. Nick

Catalytic Properties of Human Manganese Superoxide Dismutase

The depletion of superoxide catalyzed by human manganese superoxide dismutase (MnSOD) was observed spectrophotometrically by measuring the absorbance of superoxide at 250-280 nm following pulse radiolysis and by stopped-flow spectrophotometry. Catalysis showed an initial burst of activity lasting approximately 1 ms followed by the rapid emergence of a greatly inhibited catalysis of zero-order rate. These catalytic properties of human MnSOD are qualitatively similar to those reported for MnSOD from Thermus thermophilus (Bull, C., Niederhoffer, E. C., Yoshida, T., and Fee, J. A. (1991) J. Am. Chem. Soc. 113, 4069-4076). However, there are significant quantitative differences; the emergence of the inhibited form is approximately 30-fold more rapid for human MnSOD. The turnover number for human MnSOD at pH 9.4 and 20°C was kcat = 4 × 104 s−1 and kcat/Km = 8 × 108 M−1 s−1, determined by a simulated fit of the model of Bull et al. (1991) to the pulse radiolysis data. We also report that the maximum of the visible absorption spectrum of human MnSOD (ε480 = 525 M−1 cm−1) showed a strong dependence on pH that could be described by an ionization of pKa 9.4 ± 0.1 with a maximum at low pH.

Smad7-dependent Regulation of Heme Oxygenase-1 by Transforming Growth Factor-β in Human Renal Epithelial Cells

Heme oxygenase-1 (HO-1), a 32-kDa microsomal enzyme, is induced as a beneficial and adaptive response in cells/tissues exposed to oxidative stress. Transforming growth factor-β1 (TGF-β1) is a regulatory cytokine that has been implicated in a variety of renal diseases where it promotes extracellular matrix deposition and proinflammatory events. We hypothesize that the release of TGF-β1 via autocrine and/or paracrine pathways may induce HO-1 and serve as a protective response in renal injury. To understand the molecular mechanism of HO-1 induction by TGF-β1, we exposed confluent human renal proximal tubule cells to TGF-β1 and observed a significant induction of HO-1 mRNA at 4 h with a maximal induction at 8 h. This induction was accompanied by increased expression of HO-1 protein. TGF-β1 treatment in conjunction with actinomycin D or cycloheximide demonstrated that induction of HO-1 mRNA requires de novo transcription and, in part, protein synthesis. Exposure to TGF-β1 resulted in marked induction of Smad7 mRNA with no effect on Smad6 expression. Overexpression of Smad7, but not Smad6, inhibited TGF-β1-mediated induction of endogenous HO-1 gene expression. We speculate that the induction of HO-1 in the kidney is an adaptive response to the inflammatory effects of TGF-β1 and manipulations of the Smad pathway to alter HO-1 expression may serve as a potential therapeutic target.

Tumor Necrosis Factor-α Selectively InducesMnSOD Expression via Mitochondria-to-Nucleus Signaling, whereas Interleukin-1β Utilizes an Alternative Pathway

Mitochondrial levels of the anti-oxidant enzyme, manganese superoxide dismutase (MnSOD), are dramatically elevated in response to stimulation with tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and lipopolysaccharide (LPS). However, the precise intracellular signaling pathways responsible for this inducible expression are poorly understood. MnSODexpression in pulmonary epithelial and endothelial cells, treated with inflammatory mediators and various inhibitors, was studied by Northern analysis. The mitochondrial electron transport chain inhibitors, antimycin A and myxothiazol, selectively blocked TNF-α-inducible expression of MnSOD but not that of IL-1β or LPS, indicating different signaling pathways. N-Acetylcysteine could reliably decrease inducible MnSOD expression by TNF-α, but not IL-1, linking reactive oxygen species (ROS) to the TNF-α signaling pathway. Elevated levels of arachidonic acid have been demonstrated previously to generate mitochondrial ROS. A specific cytoplasmic phospholipase A2 inhibitor reduced stimulatedMnSOD expression by TNF-α, but not by IL-1β, further supporting the role of ROS. Other investigators have shown thatMnSOD expression may be regulated by NF-κB. Our results with a specific inhibitory κ-kinase inhibitor indicate that NF-κB modulates IL-1β signaling but not the TNF-α pathway. Thus, we have demonstrated that although inducible MnSOD transcription may appear similar at the messenger RNA level, the intracellular signaling pathways are differentially regulated.

Regulation of the manganese superoxide dismutase and inducible nitric oxide synthase gene in rat neuronal and glial cells

Abstract: Bidirectional communication occurs between neuroendocrine and immune systems through the action of various cytokines. Responses to various inflammatory mediators include increases in intracellular reactive oxygen species (ROS), notably, superoxide anion (O2−) and nitric oxide (NO•). Neurotoxicity mediated by NO• may result from the reaction of NO• with O2, leading to formation of peroxynitrite (ONOO−). ROS are highly toxic, potentially contributing to extensive neuronal damage. We, therefore, evaluated the effects of a variety of inflammatory mediators on the regulation of mRNA levels for manganese superoxide dismutase (MnSOD) and inducible nitric oxide synthase (iNOS) in primary cultures of rat neuronal and glial cells. To determine age‐dependent variation of mRNA expression, we used glial cells derived from newborn, 3‐, 21‐, and 95‐day‐old rat brains. Interleukin‐1β, interferon‐γ (IFN‐γ), bacterial lipopolysaccharide (LPS), and tumor necrosis factor‐α showed significant induction of MnSOD in both glial and neuronal cells. However, only LPS and IFN‐γ increased iNOS mRNA. These data demonstrate that these two genes are similarly regulated in two cells of the nervous system, further suggesting that the oxidative state of a cell may dictate a neurotoxic or neuroprotective outcome.

Regulation of manganese superoxide dismutase: IL-1 and TNF induction in pulmonary artery and microvascular endothelial cells

IL-1 and TNF are important mediators in the inflammatory response, and have been associated with endothelial cell damage in the lung. TNF and IL-1 cell-mediated injury has been proposed to occur through an increase in intracellular oxygen free radical production. However, these cytokines have also been shown to protect the lung from hyperoxia-mediated oxidant injury. In this paper we evaluated the response of the antioxidant enzymes, MnSOD and Cu/ZnSOD to IL-1, TNF, and LPS in both rat pulmonary artery and microvascular endothelial cells. These mediators produced an increase in MnSOD but not Cu/ZnSOD expression in both rat pulmonary endothelial cells. An additive effect was observed with co-treatment by the cytokines with LPS. The MnSOD mRNA induction is dependent upon a transcriptional event, but did not require de novo protein synthesis.

Activation of the Unfolded Protein Response Pathway Induces Human Asparagine Synthetase Gene Expression

The gene for the amino acid biosynthetic activity asparagine synthetase (AS) is induced by both amino acid and glucose deprivation of cells. The data reported here document that the human AS gene is induced following activation of the UnfoldedResponse Pathway (UPR), also known as theEndoplasmic Reticulum StressResponse (ERSR) in mammals. Increased AS transcription occurs in response to glucose deprivation, tunicamycin, or azetidine-2-carboxylate, all known to activate the UPR/ERSR pathway. Previously identified ERSR target genes contain multiple copies of a single highly conserved cis-element. In contrast, the human AS gene does not contain the ERSR element, as it has been described for other responsive genes. Instead, AS induction requires an Sp1-like sequence, a sequence previously shown to be associated with amino acid control of transcription, and possibly, a third region containing no consensus sequences for known transcription factors. Oligonucleotides covering each of these regions form DNA-protein complexes in vitro, and for some the amount of these complexes is greater when nuclear extracts from glucose-starved cells are tested. These results document that a wider range of metabolic activities are activated by the UPR/ERSR pathway than previously recognized and that genomic elements other than those already described can serve to enhance transcription of specific target genes.

Characterization of the Product-inhibited Complex in Catalysis by Human Manganese Superoxide Dismutase

The reduction with excess H2O2 of human Mn(III) superoxide dismutase (SOD) and the active-site mutant Y34F Mn(III)SOD was measured by scanning stopped-flow spectrophotometry and revealed the presence of an intermediate in the reduction of the manganese. The visible absorption spectrum of this intermediate closely resembled that of the enzyme in the inhibited, zero-order phase of the catalyzed disproportionation of superoxide. The decay of the visible spectrum of this intermediate was 2-fold faster for the wild-type compared with the mutant Y34F Mn-SOD. This correlates with the enhanced product inhibition of Y34F during the catalysis of O⨪2 dismutation. The visible spectrum of the product-inhibited complex resembles that of the azide-Mn-SOD complex, suggesting that the inhibited complex has expanded geometry about the metal to octahedral. This study shows that the inhibited complex responsible for the zero-order phase in the catalysis by Mn-SOD of superoxide dismutation can be reached through both the forward (O⨪2) and reverse (H2O2) reactions, supporting a mechanism in which the zero-order phase results from product inhibition.

Acute-phase induction of manganese superoxide dismutase in intestinal epithelial cell lines

Cellular protection from immune-generated oxygen free radicals is initiated by the reduction of oxygen radicals by manganese superoxide dismutase (MnSOD) and copper/zinc superoxide dismutase (Cu/ZnSOD). Using rat adult (IEC-6) and fetal (IRD-98) intestinal epithelial cell lines, factors involved in the regulation of the SODs at the messenger RNA (mRNA) level were examined. Exposure of IEC-6 and IRD-98 to Escherichia coli lipopolysaccharide (LPS) or tumor necrosis factor alpha (TNF-alpha) results in a marked increase in MnSOD mRNA as early as at 1 hour. Cotreatment of cells exposed to LPS or TNF-alpha with actinomycin D or cycloheximide showed that de novo transcription but not protein synthesis is required for the LPS- and TNF-alpha-dependent induction in MnSOD mRNA. Treatment with interleukin 1 beta results in a 12-fold increase in MnSOD mRNA, but no change was observed with interleukin 6 or interferon alpha. No change was observed in the level of Cu/ZnSOD mRNA under any condition tested. The results indicate that MnSOD functions as a cytokine-regulated acute phase protein involved in cellular protection from free radical-mediated damage.

Potent anti-tumor effects of an active site mutant of human manganese-superoxide dismutase evolutionary conservation of product inhibition

Mn-SOD serves as the primary cellular defense against oxidative damage by converting superoxide radicals (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{{\dot{2}}}^{-}\) \end{document}) to O2 and H2O2. A unique characteristic of this mitochondrial anti-oxidant enzyme is the conservation from bacteria to man of a rapidly formed product inhibited state. Using site-directed mutagenesis, we have generated an active site mutant (H30N) of human Mn-SOD, which exhibits significantly reduced product inhibition and increased enzymatic efficiency. Overexpression of the H30N enzyme causes anti-proliferative effects in vitro and anti-tumor effects in vivo. Our results provide a teleological basis for the phylogenetically invariant nature of position His-30 and the evolutionary conservation of product inhibition. These data also provide more direct intracellular evidence for the signaling role associated with H2O2.

A cis-Acting Region Regulates Oxidized Lipid-Mediated Induction of the Human Heme Oxygenase-1 Gene in Endothelial Cells

Objective—Several proatherogenic agents including oxidized LDL and its major component, 13-hydroperoxyoctadecadienoic acid (13-HPODE), upregulate heme oxygenase-1 (HO-1). Our previous studies have demonstrated that 13-HPODE-mediated HO-1 induction occurs via transcriptional mechanisms. The purpose of this study was to evaluate the molecular regulation and identify the signaling pathways involved in 13-HPODE-mediated HO-1 induction in human aortic endothelial cells. Methods and Results—The half-life of HO-1 mRNA after stimulation with 13-HPODE was ≈1.8 hours. Antioxidants such as N-acetylcysteine, iron chelation with deferoxamine mesylate, and protein kinase C inhibition with Gö6976 blocked HO-1 induction. Using promoter constructs up to 9.1 kb, no significant reporter activity was observed in response to 13-HPODE. A 13-HPODE-inducible DNase I hypersensitive site was identified that maps to a region ≈10 to 11 kb from the transcription start site of the human HO-1 gene. Based on the DNase I analysis, a −11.6-kb human HO-1 promoter construct was generated and elicited a 2.5-fold increase in reporter activity, indicating that 13-HPODE-mediated human HO-1 induction requires, at least in part, sequences that reside between 9.1 and 11.6 kb of the human HO-1 promoter. Conclusions—Elucidation of the molecular mechanisms which control HO-1 gene expression will allow us to develop therapeutic strategies to enhance the cytoprotective potential of HO-1 in atherosclerosis.