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

The structure-function relationships and physiological roles of MnSOD mutants

In this review, we focus on understanding the structure–function relationships of numerous manganese superoxide dismutase (MnSOD) mutants to investigate the role that various amino acids play to maintain enzyme quaternary structure or the active site structure, catalytic potential and metal homeostasis in MnSOD, which is essential to maintain enzyme activity. We also observe how polymorphisms of MnSOD are linked to pathologies and how post-translational modifications affect the antioxidant properties of MnSOD. Understanding how modified forms of MnSOD may act as tumor promoters or suppressors by altering the redox status in the body, ultimately aid in generating novel therapies that exploit the therapeutic potential of mutant MnSODs or pave the way for the development of synthetic SOD mimics.

Substitution of histidine 30 by asparagine in manganese superoxide dismutase alters biophysical properties and supports proliferation in a K562 leukemia cell line

We have generated a mutant of C. elegans manganese superoxide dismutase at histidine 30 by site-directed mutagenesis. The structure was solved at a resolution of 1.52 Å by X-ray crystallography (pdb: 6S0D). His30 was targeted, as it forms as a gateway residue at the top of the solvent access funnel to the active site, together with Tyr34. In the wild-type protein, these gateway residues are involved in the hydrogen-bonding network providing the protons necessary for the catalytic reaction at the metal center. However, biophysical characterization and cell viability experiments reveal that a mutation from histidine to asparagine in the H30N mutant modifies metal selectivity in the protein, favoring the uptake of iron over manganese in minimal media conditions, alters active-site coordination from the characteristic trigonal bipyramidal to octahedral geometry, and encourages cellular proliferation in K562 cells, when added exogenously to the cells.

Bystander signaling via oxidative metabolism

The radiation-induced bystander effect (RIBE) is the initiation of biological end points in cells (bystander cells) that are not directly traversed by an incident-radiation track, but are in close proximity to cells that are receiving the radiation. RIBE has been indicted of causing DNA damage via oxidative stress, besides causing direct damage, inducing tumorigenesis, producing micronuclei, and causing apoptosis. RIBE is regulated by signaling proteins that are either endogenous or secreted by cells as a means of communication between cells, and can activate intracellular or intercellular oxidative metabolism that can further trigger signaling pathways of inflammation. Bystander signals can pass through gap junctions in attached cell lines, while the suspended cell lines transmit these signals via hormones and soluble proteins. This review provides the background information on how reactive oxygen species (ROS) act as bystander signals. Although ROS have a very short half-life and have a nanometer-scale sphere of influence, the wide variety of ROS produced via various sources can exert a cumulative effect, not only in forming DNA adducts but also setting up signaling pathways of inflammation, apoptosis, cell-cycle arrest, aging, and even tumorigenesis. This review outlines the sources of the bystander effect linked to ROS in a cell, and provides methods of investigation for researchers who would like to pursue this field of science.

The structure of the Caenorhabditis elegans manganese superoxide dismutase MnSOD-3-azide complex

C. elegans MnSOD-3 has been implicated in the longevity pathway and its mechanism of catalysis is relevant to the aging process and carcinogenesis. The structures of MnSOD-3 provide unique crystallographic evidence of a dynamic region of the tetrameric interface (residues 41-54). We have determined the structure of the MnSOD-3-azide complex to 1.77-Å resolution. Analysis of this complex shows that the substrate analog, azide, binds end-on to the manganese center as a sixth ligand and that it ligates directly to a third and new solvent molecule also positioned within interacting distance to the His30 and Tyr34 residues of the substrate access funnel. This is the first structure of a eukaryotic MnSOD-azide complex that demonstrates the extended, uninterrupted hydrogen-bonded network that forms a proton relay incorporating three outer sphere solvent molecules, the substrate analog, the gateway residues, Gln142, and the solvent ligand. This configuration supports the formation and release of the hydrogen peroxide product in agreement with the 5-6-5 catalytic mechanism for MnSOD. The high product dissociation constant k4 of MnSOD-3 reflects low product inhibition making this enzyme efficient even at high levels of superoxide.

Consequences of age on ischemic wound healing in rats: altered antioxidant activity and delayed wound closure

Advertisements targeted at the elderly population suggest that antioxidant therapy will reduce free radicals and promote wound healing, yet few scientific studies substantiate these claims. To better understand the potential utility of supplemental antioxidant therapy for wound healing, we tested the hypothesis that age and tissue ischemia alter the balance of endogenous antioxidant enzymes. Using a bipedicled skin flap model, ischemic and non-ischemic wounds were created on young and aged rats. Wound closure and the balance of the critical antioxidants superoxide dismutase and glutathione in the wound bed were determined. Ischemia delayed wound closure significantly more in aged rats. Lower superoxide dismutase 2 and glutathione in non-ischemic wounds of aged rats indicate a basal deficit due to age alone. Ischemic wounds from aged rats had lower superoxide dismutase 2 protein and activity initially, coupled with decreased ratios of reduced/oxidized glutathione and lower glutathione peroxidase activity. De novo glutathione synthesis, to restore redox balance in aged ischemic wounds, was initiated as evidenced by increased glutamate cysteine ligase. Results demonstrate deficiencies in two antioxidant pathways in aged rats that become exaggerated in ischemic tissue, culminating in profoundly impaired wound healing and prolonged inflammation.

The functional role of MnSOD as a biomarker of human diseases and therapeutic potential of a new isoform of a human recombinant MnSOD

Reactive oxygen species (ROS) are generated as a consequence of metabolic reactions in the mitochondria of eukaryotic cells. This work describes the role of the manganese superoxide dismutase (MnSOD) as a biomarker of different human diseases and proposes a new therapeutic application for the prevention of cancer and its treatment. The paper also describes how a new form of human MnSOD was discovered, its initial application, and its clinical potentials. The MnSOD isolated from a human liposarcoma cell line (LSA) was able to kill cancer cells expressing estrogen receptors, but it did not have cytotoxic effects on normal cells. Together with its oncotoxic activity, the recombinant MnSOD (rMnSOD) exerts a radioprotective effect on normal cells irradiated with X-rays. The rMnSOD is characterized by the presence of a leader peptide, which allows the protein to enter cells: this unique property can be used in the radiodiagnosis of cancer or chemotherapy, conjugating radioactive substances or chemotherapic drugs to the leader peptide of the MnSOD. Compared to traditional chemotherapic agents, the drugs conjugated with the leader peptide of MnSOD can selectively reach and enter cancer cells, thus reducing the side effects of traditional treatments.

Modulation of MnSOD in Cancer:Epidemiological and Experimental Evidence

Since it was first observed in late 1970s that human cancers often had decreased manganese superoxide dismutase (MnSOD) protein expression and activity, extensive studies have been conducted to verify the association between MnSOD and cancer. Significance of MnSOD as a primary mitochondrial antioxidant enzyme is unquestionable; results from in vitro, in vivo and epidemiological studies are in harmony. On the contrary, studies regarding roles of MnSOD in cancer often report conflicting results. Although putative mechanisms have been proposed to explain how MnSOD regulates cellular proliferation, these mechanisms are not capitulated in epidemiological studies. This review discusses most recent epidemiological and experimental studies that examined the association between MnSOD and cancer, and describes emerging hypotheses of MnSOD as a mitochondrial redox regulatory enzyme and of how altered mitochondrial redox may affect physiology of normal as well as cancer cells.

Curbing cancer's sweet tooth: is there a role for MnSOD in regulation of the Warburg effect?

Reactive oxygen species (ROS), while vital for normal cellular function, can have harmful effects on cells, leading to the development of diseases such as cancer. The Warburg effect, the shift from oxidative phosphorylation to glycolysis, even in the presence of adequate oxygen, is an important metabolic change that confers many growth and survival advantages to cancer cells. Reactive oxygen species are important regulators of the Warburg effect. The mitochondria-localized antioxidant enzyme manganese superoxide dismutase (MnSOD) is vital to survival in our oxygen-rich atmosphere because it scavenges mitochondrial ROS. MnSOD is important in cancer development and progression. However, the significance of MnSOD in the regulation of the Warburg effect is just now being revealed, and it may significantly impact the treatment of cancer in the future.

The peroxidase activity of mitochondrial superoxide dismutase

Manganese superoxide dismutase (MnSOD) is an integral mitochondrial protein known as a first-line antioxidant defense against superoxide radical anions produced as by-products of the electron transport chain. Recent studies have shaped the idea that by regulating the mitochondrial redox status and H(2)O(2) outflow, MnSOD acts as a fundamental regulator of cellular proliferation, metabolism, and apoptosis, thereby assuming roles that extend far beyond its proposed antioxidant functions. Accordingly, allelic variations of MnSOD that have been shown to augment levels of MnSOD in mitochondria result in a 10-fold increase in prostate cancer risk. In addition, epidemiologic studies indicate that reduced glutathione peroxidase activity along with increases in H(2)O(2) further increase cancer risk in the face of MnSOD overexpression. These facts led us to hypothesize that, like its Cu,ZnSOD counterpart, MnSOD may work as a peroxidase, utilizing H(2)O(2) to promote mitochondrial damage, a known cancer risk factor. Here we report that MnSOD indeed possesses peroxidase activity that manifests in mitochondria when the enzyme is overexpressed.

Clair DK. Manganese superoxide dismutase: beyond life and death

Manganese superoxide dismutase (MnSOD) is a nuclear-encoded antioxidant enzyme that localizes to the mitochondria. Expression of MnSOD is essential for the survival of aerobic life. Transgenic mice expressing a luciferase reporter gene under the control of the human MnSOD promoter demonstrate that the level of MnSOD is reduced prior to the formation of cancer. Overexpression of MnSOD in transgenic mice reduces the incidences and multiplicity of papillomas in a DMBA/TPA skin carcinogenesis model. However, MnSOD deficiency does not lead to enhanced tumorigenicity of skin tissue similarly treated because MnSOD can modulate both the p53-mediated apoptosis and AP-1-mediated cell proliferation pathways. Apoptosis is associated with an increase in mitochondrial levels of p53 suggesting a link between MnSOD deficiency and mitochondrial-mediated apoptosis. Activation of p53 is preventable by application of a SOD mimetic (MnTE-2-PyP(5+)). Thus, p53 translocation to mitochondria and subsequent inactivation of MnSOD explain the observed mitochondrial dysfunction that leads to transcription-dependent mechanisms of p53-induced apoptosis. Administration of MnTE-2-PyP(5+) following apoptosis but prior to proliferation leads to suppression of protein carbonyls and reduces the activity of AP-1 and the level of the proliferating cellular nuclear antigen, without reducing the activity of p53 or DNA fragmentation following TPA treatment. Remarkably, the incidence and multiplicity of skin tumors are drastically reduced in mice that receive MnTE-2-PyP(5+) prior to cell proliferation. The results demonstrate the role of MnSOD beyond its essential role for survival and suggest a novel strategy for an antioxidant approach to cancer intervention.

Comparison of two yeast MnSODs: mitochondrial Saccharomyces cerevisiae versus cytosolic Candida albicans

Human MnSOD is significantly more product-inhibited than bacterial MnSODs at high concentrations of superoxide (O(2)(-)). This behavior limits the amount of H(2)O(2) produced at high [O(2)(-)]; its desirability can be explained by the multiple roles of H(2)O(2) in mammalian cells, particularly its role in signaling. To investigate the mechanism of product inhibition in MnSOD, two yeast MnSODs, one from Saccharomyces cerevisiae mitochondria (ScMnSOD) and the other from Candida albicans cytosol (CaMnSODc), were isolated and characterized. ScMnSOD and CaMnSODc are similar in catalytic kinetics, spectroscopy, and redox chemistry, and they both rest predominantly in the reduced state (unlike most other MnSODs). At high [O(2)(-)], the dismutation efficiencies of the yeast MnSODs surpass those of human and bacterial MnSODs, due to very low level of product inhibition. Optical and parallel-mode electron paramagnetic resonance (EPR) spectra suggest the presence of two Mn(3+) species in yeast Mn(3+)SODs, including the well-characterized 5-coordinate Mn(3+) species and a 6-coordinate L-Mn(3+) species with hydroxide as the putative sixth ligand (L). The first and second coordination spheres of ScMnSOD are more similar to bacterial than to human MnSOD. Gln154, an H-bond donor to the Mn-coordinated solvent molecule, is slightly further away from Mn in yeast MnSODs, which may result in their unusual resting state. Mechanistically, the high efficiency of yeast MnSODs could be ascribed to putative translocation of an outer-sphere solvent molecule, which could destabilize the inhibited complex and enhance proton transfer from protein to peroxide. Our studies on yeast MnSODs indicate the unique nature of human MnSOD in that it predominantly undergoes the inhibited pathway at high [O(2)(-)].

Clair DK. Manganese superoxide dismutase: guardian of the powerhouse

The mitochondrion is vital for many metabolic pathways in the cell, contributing all or important constituent enzymes for diverse functions such as β-oxidation of fatty acids, the urea cycle, the citric acid cycle, and ATP synthesis. The mitochondrion is also a major site of reactive oxygen species (ROS) production in the cell. Aberrant production of mitochondrial ROS can have dramatic effects on cellular function, in part, due to oxidative modification of key metabolic proteins localized in the mitochondrion. The cell is equipped with myriad antioxidant enzyme systems to combat deleterious ROS production in mitochondria, with the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) acting as the chief ROS scavenging enzyme in the cell. Factors that affect the expression and/or the activity of MnSOD, resulting in diminished antioxidant capacity of the cell, can have extraordinary consequences on the overall health of the cell by altering mitochondrial metabolic function, leading to the development and progression of numerous diseases. A better understanding of the mechanisms by which MnSOD protects cells from the harmful effects of overproduction of ROS, in particular, the effects of ROS on mitochondrial metabolic enzymes, may contribute to the development of novel treatments for various diseases in which ROS are an important component.

Manganese superoxide dismutase (Sod2) and redox-control of signaling events that drive metastasis

Manganese superoxide dismutase (Sod2) has emerged as a key enzyme with a dual role in tumorigenic progression. Early studies were primarily directed at defining the tumor suppressive function of Sod2 based on its low level expression in many tumor types. It is now commonly held that loss of Sod2 expression is likely an early event in tumor progression allowing for further propagation of the tumorigenic phenotype resulting from steady state increases in free radical production. Increases in free radical load have also been linked to defects in mitochondrial function and metastatic disease progression. It was initially believed that Sod2 loss may propagate metastatic disease progression, in reality both epidemiologic and experimental evidence indicate that Sod2 levels increase in many tumor types as they progress from early stage non-invasive disease to late stage metastatic disease. Sod2 overexpression in many instances enhances the metastatic phenotype that is reversed by efficient H(2)O(2) scavenging. This review evaluates the many sequelae associated with increases in Sod2 that impinge on the metastatic phenotype. The ability to use Sod2 to modulate the cellular redox-environment has allowed for the identification of redox-responsive signaling events that drive malignancy, such as invasion, migration and prolonged tumor cell survival. Further studies of these redox-driven events will help in the development of targeted therapeutic strategies to efficiently restrict redox-signaling essential for malignant progression.

Investigation of the highly active manganese superoxide dismutase from Saccharomyces cerevisiae

Manganese superoxide dismutase (MnSOD) from different species differs in its efficiency in removing high concentrations of superoxide (O(2)(-)), due to different levels of product inhibition. Human MnSOD exhibits a substantially higher level of product inhibition than the MnSODs from bacteria. In order to investigate the mechanism of product inhibition and whether it is a feature common to eukaryotic MnSODs, we purified MnSOD from Saccharomyces cerevisiae (ScMnSOD). It was a tetramer with 0.6 equiv of Mn per monomer. The catalytic activity of ScMnSOD was investigated by pulse radiolysis and compared with human and two bacterial (Escherichia coli and Deinococcus radiodurans) MnSODs. To our surprise, ScMnSOD most efficiently facilitates removal of high concentrations of O(2)(-) among these MnSODs. The gating value k(2)/k(3) that characterizes the level of product inhibition scales as ScMnSOD > D. radiodurans MnSOD > E. coli MnSOD > human MnSOD. While most MnSODs rest as the oxidized form, ScMnSOD was isolated in the Mn(2+) oxidation state as revealed by its optical and electron paramagnetic resonance spectra. This finding poses the possibility of elucidating the origin of product inhibition by comparing human MnSOD with ScMnSOD.

Modulation of mammary cancer cell migration by 15-deoxy-delta(12,14)-prostaglandin J(2): implications for anti-metastatic therapy

Recently, a number of steps in the progression of metastatic disease have been shown to be regulated by redox signalling. Electrophilic lipids affect redox signalling through the post-translational modification of critical cysteine residues in proteins. However, the therapeutic potential as well as the precise mechanisms of action of electrophilic lipids in cancer cells is poorly understood. In the present study, we investigate the effect of the electrophilic prostaglandin 15d-PGJ2 (15-deoxy-Delta12,14-prostaglandin J2) on metastatic properties of breast cancer cells. 15d-PGJ2 was shown to decrease migration, stimulate focal-adhesion disassembly and cause extensive F-actin (filamentous actin) reorganization at low concentrations (0.03-0.3 microM). Importantly, these effects seem to be independent of PPARgamma (peroxisome-proliferator-activated receptor gamma) and modification of actin or Keap1 (Kelch-like ECH-associated protein 1), which are known protein targets of 15d-PGJ2 at higher concentrations. Interestingly, the p38 inhibitor SB203580 was able to prevent both 15d-PGJ2-induced F-actin reorganization and focal-adhesion disassembly. Taken together, the results of the present study suggest that electrophiles such as 15d-PGJ2 are potential anti-metastatic agents which exhibit specificity for migration and adhesion pathways at low concentrations where there are no observed effects on Keap1 or cytotoxicity.

Complexity of free radical Metabolism in human Erythrocytes

The auto-oxidation of oxyhaemoglobin to methaemoglobin generating superoxide anion radical (O2.-) represents the main source of free radicals in the erythro-cytes. Hydrogen peroxide is produced by O2.-dismutation or originates from the circulation. Human erythrocytes are also exposed to the prooxidative actions of nitric oxide (NO) from circulation. Free radicals that may induce reactions with direct dangerous consequences to erythrocytes are also preceded by the reaction of O2.-and NO producing peroxynitrite. In physiological settings, erythrocytes show a self-sustaining activity of antioxidative defence (AD) enzymes, such as: superoxide dismutase (SOD, EC 1.11.16), catalase (CAT, EC 1.11.1.6), glutathione peroxidase (GSHPx, EC 1.11.1.9) and glutathione reductase (GR, EC 1.6.4.2), as well as low molecular weight antioxidants: glutathione and vitamins E and C. Their coordinate actions protect the erythrocyte's bio-macromolecules from free radical-mediated damage. Since there is node novosynthesis of AD enzymes in mature erythrocytes, their defence capacity is limited. Free radicals influence antioxidative enzymes capacities and relative share of particular components in the whole antioxidative system. Therefore, by measuring changes in the activity of individual AD components, as well as their interrelations by statistical canonical discriminant methods, valuable data about the complexity, overall relations and coordinated actions in the AD system in erythrocytes and its relevance for systemic effects can be acquired.

The structural biochemistry of the superoxide dismutases

The discovery of superoxide dismutases (SODs), which convert superoxide radicals to molecular oxygen and hydrogen peroxide, has been termed the most important discovery of modern biology never to win a Nobel Prize. Here, we review the reasons this discovery has been underappreciated, as well as discuss the robust results supporting its premier biological importance and utility for current research. We highlight our understanding of SOD function gained through structural biology analyses, which reveal important hydrogen-bonding schemes and metal-binding motifs. These structural features create remarkable enzymes that promote catalysis at faster than diffusion-limited rates by using electrostatic guidance. These architectures additionally alter the redox potential of the active site metal center to a range suitable for the superoxide disproportionation reaction and protect against inhibition of catalysis by molecules such as phosphate. SOD structures may also control their enzymatic activity through product inhibition; manipulation of these product inhibition levels has the potential to generate therapeutic forms of SOD. Markedly, structural destabilization of the SOD architecture can lead to disease, as mutations in Cu,ZnSOD may result in familial amyotrophic lateral sclerosis, a relatively common, rapidly progressing and fatal neurodegenerative disorder. We describe our current understanding of how these Cu,ZnSOD mutations may lead to aggregation/fibril formation, as a detailed understanding of these mechanisms provides new avenues for the development of therapeutics against this so far untreatable neurodegenerative pathology.

Superoxide dismutases-a review of the metal-associated mechanistic variations

Superoxide dismutases are enzymes that function to catalytically convert superoxide radical to oxygen and hydrogen peroxide. These enzymes carry out catalysis at near diffusion controlled rate constants via a general mechanism that involves the sequential reduction and oxidation of the metal center, with the concomitant oxidation and reduction of superoxide radicals. That the catalytically active metal can be copper, iron, manganese or, recently, nickel is one of the fascinating features of this class of enzymes. In this review, we describe these enzymes in terms of the details of their catalytic properties, with an emphasis on the mechanistic differences between the enzymes. The focus here will be concentrated mainly on two of these enzymes, copper, zinc superoxide dismutase and manganese superoxide dismutase, and some relatively subtle variations in the mechanisms by which they function.

Product inhibition of secreted phospholipase A2 may explain lysophosphatidylcholines' unexpected therapeutic properties

Background

Lysophosphatidylcholines (lysoPCs) are products of phospholipase A2 (PLA2) enzyme activity, and like the enzyme, have a direct role in toxic inflammatory responses in variety of organ systems. Paradoxically, reduced plasma lysoPC levels have been noted in sepsis patients and systemic treatment with lysoPCs is therapeutic in rodent models of sepsis and ischemia. These observations suggest that elevation of plasma levels of these lipids can actually help to relieve serious inflammatory conditions. We demonstrate that specific lysoPCs act as uncompetitive product inhibitors of plasma secreted PLA2 enzymes (sPLA2s), especially under conditions of elevated enzyme activity, thus providing a feedback mechanism for the observed anti-inflammatory effects of these compounds.

Methods

Thin layer chromatography and mass spectroscopy were used to estimate total lysoPC concentration and the relative contributions of different lysoPC species in rat plasma samples. Kinetic studies of sPLA2 enzyme activity were conducted on these samples ex vivo and on purified group IA sPLA2 in vitro after addition of specific lysoPC species to the reaction mixture. Enzyme activity was also measured in plasma samples of rats injected with these same lysoPCs.

Results

Palmitoyl (16:0), stearoyl (18:0) are the most abundant lysoPCs in rat plasma consistent with other reports. Kinetic studies demonstrated that both were uncompetitive inhibitors of plasma sPLA2 enzyme activity. In vitro experiments with group IA sPLA2 confirmed the inhibition and the kinetic properties of these lysoPC species. Decanoyl lysoPC (10:0), which was not detected in plasma, did not inhibit enzyme activity in vitro. LysoPC injections into normal rats resulted in "buffering" of plasma sPLA2 activity in a narrow low range, consistent with the activity-dependent inhibition suggested by the ex vivo and in vitro experiments.

Conclusion

The results may explain the efficacy of lysoPC therapy during periods of elevated inflammatory activity and further highlight the utility uncompetitive enzyme inhibitors. In this case, the inhibitor is a product of the enzyme reaction, and therefore represents an example of activity-driven feedback inhibition.

Distinct functions of CCAAT enhancer-binding protein isoforms in the regulation of manganese superoxide dismutase during interleukin-1beta stimulation

The mitochondrial antioxidant enzyme manganese superoxide dismutase (Mn-SOD) is crucial in maintaining cellular and organismal homeostasis. Mn-SOD expression is tightly regulated in a manner that synchronizes its cytoprotective functions during inflammatory challenges. Induction of Mn-SOD gene expression by the proinflammatory cytokine IL-1beta is mediated through a complex intronic enhancer element. To identify and characterize the transcription factors required for Mn-SOD enhancer function, a yeast one-hybrid assay was utilized, and two CCAAT enhancer-binding protein (C/EBP) members, C/EBP beta and C/EBP delta, were identified. These two transcription factors responded to IL-1beta treatment with distinct expression profiles, different temporal yet inducible interactions with the endogenous Mn-SOD enhancer, and also opposite effects on Mn-SOD transcription. C/EBP beta is expressed as three isoforms, LAP* (liver-activating protein), LAP, and LIP (liver-inhibitory protein). Our functional analysis demonstrated that only the full-length C/EBP beta/LAP* served as a true activator for Mn-SOD, whereas LAP, LIP, and C/EBP delta functioned as potential repressors. Finally, our systematic mutagenesis of the unique N-terminal 21 amino acids further solidified the importance of LAP* in the induction of Mn-SOD and emphasized the crucial role of this isoform. Our data demonstrating the physiological relevance of the N-terminal peptide also provide a rationale for revisiting the role of LAP* in the regulation of other genes and in pathways such as lipogenesis and development.

Isogenic normal basal and luminal mammary epithelial isolated by a novel method show a differential response to ionizing radiation

Epithelial cells within the normal breast duct seem to be the primary target for neoplastic transformation events that eventually produce breast cancer. Normal epithelial cells are easily isolated and propagated using standard techniques. However, these techniques almost invariably result in populations of cells that are largely basal in character. Because only approximately 20% of human breast cancers exhibit a basal phenotype, our understanding of the disease may be skewed by using these cells as the primary comparator to cancer. Further, because germ line mutations in BRCA1 yield breast cancers that are most often of the basal type, a comparison of normal basal and luminal cells could yield insight into the tissue and cell type specificity of this hereditary cancer susceptibility gene. In this report, we describe a simplified and efficient method for isolating basal and luminal cells from normal human breast tissue. These isogenic cells can be independently propagated and maintain phenotypic markers consistent with their respective lineages. Using these cultured cells, we show that basal and luminal cells exhibit distinct responses to ionizing radiation. Basal cells undergo a rapid but labile cell cycle arrest, whereas luminal cells show a much more durable arrest, primarily at the G(2)-M boundary. Molecular markers, including p53 protein accumulation, p53-activated genes, and BRCA1 nuclear focus formation all correlate with the respective cell cycle responses. Further, we show that short-term cultures of human breast tissue fragments treated with ionizing radiation show a similar phenomenon as indicated by the biphasic accumulation of p53 protein in the basal versus luminal layer. Together, these results indicate that normal basal cells have a transitory cell cycle arrest after DNA damage that may underlie their increased susceptibility to transformation after the loss of functional BRCA1.

Engineering and characterization of human manganese superoxide dismutase mutants with high activity and low product inhibition

Human manganese superoxide dismutase is a mitochondrial metalloenzyme that is involved in protecting aerobic organisms against superoxide toxicity, and has been implicated in slowing tumor growth. Unfortunately, this enzyme exhibits strong product inhibition, which limits its potential biomedical applications. Previous efforts to alleviate human manganese superoxide dismutase product inhibition utilized rational protein design and site-directed mutagenesis. These efforts led to variants of human manganese superoxide dismutase at residue 143 with dramatically reduced product inhibition, but also reduced catalytic activity and efficiency. Here, we report the use of a directed evolution approach to engineer two variants of the Q143A human manganese superoxide dismutase mutant enzyme with improved catalytic activity and efficiency. Two separate activity-restoring mutations were found--C140S and N73S--that increase the catalytic efficiency of the parent Q143A human manganese superoxide dismutase enzyme by up to five-fold while maintaining low product inhibition. Interestingly, C140S is a context-dependent mutation, and the C140S-Q143A human manganese superoxide dismutase did not follow Michaelis-Menten kinetics. The re-engineered human manganese superoxide dismutase mutants should be useful for biomedical applications, and our kinetic and structural studies also provide new insights into the structure-function relationships of human manganese superoxide dismutase.

Prevention of human cancer by modulation of chronic inflammatory processes

Chronic inflammation induced by biological, chemical and physical factors has been associated with increased risk of human cancer at various sites. Inflammation facilitates the initiation of normal cells and their growth and progression to malignancy through production of pro-inflammatory cytokines and diverse reactive oxygen and nitrogen species. These also activate signaling molecules involved in inflammation and carcinogenesis such as nuclear transcription factor (NF-kappaB), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). Several chemopreventive agents act through inhibition of signaling pathways (e.g. NF-kappaB), inhibition of oxidant-generating enzymes (e.g. iNOS) and mediators of inflammation (e.g. COX-2), scavenging reactive oxygen and nitrogen species, and modulation of xenobiotic-metabolizing enzymes (especially phase II enzyme induction). Some anti-inflammatory drugs have been tested in clinical trials to prevent human cancer at several sites. Better understanding of the molecular mechanisms by which chronic inflammation increases cancer risk will lead to further development of new strategies for cancer prevention at many sites.

Mechanism of the tumor suppressive effect of MnSOD overexpression

The mitochondrial antioxidant protein manganese-containing superoxide dismutase (MnSOD) has been shown to be a new type of tumor suppressor protein. Overexpression of MnSOD protein inhibits growth in a wide variety of cancer types. This review examines the molecular mechanism of the tumor suppressive effect of MnSOD. Three species have been proposed to cause the tumor suppressive effect: superoxide radical, hydrogen peroxide and nitric oxide. At the present time, the evidence appears strongest that hydrogen peroxide is the effector molecule since both catalase and glutathione peroxidase has been shown to modulate the effect. Surprisingly, in different cancer cell lines, overexpression of GPx has been found to both decrease and increase the growth inhibitory effect of MnSOD overexpression. Knowledge of which molecule causes the tumor suppressive effect of MnSOD and the mechanism of action will likely lead to new therapies for the treatment of cancer.

Mitochondrial redox state regulates transcription of the nuclear-encoded mitochondrial protein manganese superoxide dismutase: a proposed adaptive response to mitochondrial redox imbalance

Overexpression of human manganese superoxide dismutase (MnSOD) in mouse NIH/3T3 cells using an inducible retroviral system led to alterations in the mitochondrial redox state since levels of reactive oxygen species rapidly increased after induction of human MnSOD (Antioxid. Redox Signal.6:489-500; 2004). Alterations in exogenous human MnSOD led to large increases in levels of endogenous mouse MnSOD (sod2) and thioredoxin 2 (txn2) mRNAs, but smaller increases in MnSOD and thioredoxin 2 protein expression. Tight regulation of mitochondrial protein levels seems to be necessary for optimal cellular function, since mitochondrial antioxidant protein levels did not increase to the same extent as antioxidant protein mRNA levels. We hypothesize that these changes in antioxidant proteins are adaptations to the altered mitochondrial redox state elicited by MnSOD overexpression. The mitochondrial-specific antioxidant MitoQ reversed cell growth inhibition, and greatly decreased levels of endogenous sod2 and txn2 transcripts following induction of exogenous MnSOD. Elevated levels of mouse sod2 transcripts resulted from transcriptional activation of the endogenous sod2 gene since actinomycin D prevented transcription of this gene. Therefore, the mitochondrial redox state appears to modulate a nuclear-driven biochemical event, i.e., transcriptional activation of a nuclear gene encoding a protein targeted to mitochondria.

Particulate phase cigarette smoke increases MnSOD, NQO1, and CINC-1 in rat lungs

Loss of antioxidant/oxidant homeostasis perpetuates inflammation in the lungs and may contribute to the development of COPD and lung cancer. Cigarette smoke (CS) is a primary source of airway oxidative stress and recruits inflammatory cells into smokers' lungs. However, whether these consequences are attributable to a specific or the collective fraction of CS is unknown. We investigated whether the particulate or the gas phase of CS would alter expression of the antioxidant enzymes MnSOD and NQO1 or CINC-1. Sprague Dawley rats were exposed to sham (n = 10) or the particulate phase (PP; n = 10) or gas phase (n = 10) of a Kentucky reference cigarette (1R4F) for 2 h/d for 28 d, after which animals were sacrificed and the lower left lobe of the lung was removed. Immunoblots for SOD and NQO1 revealed that lungs exposed to PP had higher MnSOD/actin and NQO1/actin ratios than either sham-or gas phase-treated animals. In contrast, CuZnSOD remained unchanged. In PP-exposed animals, CINC-1 was 3-fold higher than in sham-exposed animals. The increases in MnSOD and NQO1 protein were associated with increases in total SOD, NQO1, and MPO activities. These data provide evidence that the PP of CS alters oxidant/antioxidant homeostasis in the lungs and participates in the pathogenesis of CS-induced lung diseases such as COPD and cancer.

Superoxide inhibits 4Fe-4S cluster enzymes involved in amino acid biosynthesis. Cross-compartment protection by CuZn-superoxide dismutase

Among the phenotypes of Saccharomyces cerevisiae mutants lacking CuZn-superoxide dismutase (Sod1p) is an aerobic lysine auxotrophy; in the current work we show an additional leaky auxotrophy for leucine. The lysine and leucine biosynthetic pathways each contain a 4Fe-4S cluster enzyme homologous to aconitase and likely to be superoxide-sensitive, homoaconitase (Lys4p) and isopropylmalate dehydratase (Leu1p), respectively. We present evidence that direct aerobic inactivation of these enzymes in sod1 Delta yeast results in the auxotrophies. Located in the cytosol and intermembrane space of the mitochondria, Sod1p likely provides direct protection of the cytosolic enzyme Leu1p. Surprisingly, Lys4p does not share a compartment with Sod1p but is located in the mitochondrial matrix. The activity of a second matrix protein, the tricarboxylic acid cycle enzyme aconitase, was similarly lowered in sod1 Delta mutants. We measured only slight changes in total mitochondrial iron and found no detectable difference in mitochondrial "free" (EPR-detectable) iron making it unlikely that a gross defect in mitochondrial iron metabolism is the cause of the decreased enzyme activities. Thus, we conclude that when Sod1p is absent a lysine auxotrophy is induced because Lys4p is inactivated in the matrix by superoxide that originates in the intermembrane space and diffuses across the inner membrane.

Superoxide dismutases: active sites that save, but a protein that kills

Protection from oxidative damage is sufficiently important that biology has evolved three independent enzymes for hastening superoxide dismutation: the Cu- and Zn-containing superoxide dismutases (Cu,Zn-SODs), the SODs that are specific for Fe or Mn or function with either of the two (Fe-SODs, Mn-SODs or Fe/Mn-SODs), and the SODs that use Ni (Ni-SODs). Despite the overwhelming similarities between the active sites of Fe-SOD and Mn-SOD, the mechanisms and redox tuning of these two sites appear to incorporate crucial differences consistent with the differences between Fe3+/2+ and Mn3+/2+. Ni-SOD is revealed by spectroscopy to employ completely different ligation to that of the other SODs while nonetheless incorporating a device also found in Cu,Zn-SOD. Finally, the protein of human Cu,Zn-SOD appears to be an important contributor to the development of amyotrophic lateral sclerosis, possibly because of its propensity for extended beta-sheet formation.