The right to choose: multiple pathways for activating copper,zinc superoxide dismutase

Identification of [CuCl(acac)(tmed)], a copper(II) complex with mixed ligands, as a modulator of Cu,Zn superoxide dismutase (Sod1p) activity in yeast

Superoxide dismutases (SODs) stand in the prime line of enzymatic antioxidant defense in nearly all eukaryotic cells exposed to oxygen, catalyzing the breakdown of the superoxide anionic radical to O(2) and H(2)O(2). Overproduction of superoxide correlates with numerous pathophysiological conditions, and although the native enzyme can be used as a therapeutic agent in superoxide-associated conditions, synthetic low molecular weight mimetics are preferred in terms of cost, administration mode, and bioavailability. In this study we make use of the model eukaryote Saccharomyces cerevisiae to investigate the SOD-mimetic action of a mononuclear mixed-ligand copper(II) complex, [CuCl(acac)(tmed)] (where acac is acetylacetonate anion and tmed is N,N,N',N'-tetramethylethylenediamine). Taking advantage of an easily reproducible phenotype of yeast cells which lack Cu-Zn SOD (Sod1p), we found that the compound could act either as a superoxide scavenger in the absence of native Sod1p or as a Sod1p modulator which behaved differently under various genetic backgrounds.

Models for the mechanism for activating copper-zinc superoxide dismutase in the absence of the CCS Cu chaperone in Arabidopsis

Copper-zinc superoxide dismutase (CuZnSOD; CSD) is an important antioxidant enzyme for oxidative stress protection. To date, two activation pathways have been identified in many species. One requiring the CCS, Cu chaperone for SOD, to insert Cu and activate CSD (referred to as CCS-dependent pathway), and the other works independently of CCS (referred to as CCS-independent pathway). In our previous study, we suggest an unidentified factor will work with glutathione (GSH) for CSD activation in the absence of the CCS. Here, two models of the CCS-independent mechanism are proposed. The role of the unidentified factor may work as a scaffold protein, which provides a platform for the CSD protein and Cu-GSH to interact, or as a Cu carrier, which itself can bind Cu and interact with CSD proteins. We also suggest that the CSD protein conformation at C-terminal is important in providing a docking site for unidentified factor to access.

Charting the travels of copper in eukaryotes from yeast to mammals

Throughout evolution, all organisms have harnessed the redox properties of copper (Cu) and iron (Fe) as a cofactor or structural determinant of proteins that perform critical functions in biology. At its most sobering stance to Earth's biome, Cu biochemistry allows photosynthetic organisms to harness solar energy and convert it into the organic energy that sustains the existence of all nonphotosynthetic life forms. The conversion of organic energy, in the form of nutrients that include carbohydrates, amino acids and fatty acids, is subsequently released during cellular respiration, itself a Cu-dependent process, and stored as ATP that is used to drive a myriad of critical biological processes such as enzyme-catalyzed biosynthetic processes, transport of cargo around cells and across membranes, and protein degradation. The life-supporting properties of Cu incur a significant challenge to cells that must not only exquisitely balance intracellular Cu concentrations, but also chaperone this redox-active metal from its point of cellular entry to its ultimate destination so as to avert the potential for inappropriate biochemical interactions or generation of damaging reactive oxidative species (ROS). In this review we chart the travels of Cu from the extracellular milieu of fungal and mammalian cells, its path within the cytosol as inferred by the proteins and ligands that escort and deliver Cu to intracellular organelles and protein targets, and its journey throughout the body of mammals. This article is part of a Special Issue entitled: Cell Biology of Metals.

Silver(I), mercury(II), cadmium(II), and zinc(II) target exposed enzymic iron-sulfur clusters when they toxify Escherichia coli

The toxicity of soft metals is of broad interest to microbiologists, both because such metals influence the community structures in natural environments and because several metals are used as antimicrobial agents. Their potency roughly parallels their thiophilicity, suggesting that their primary biological targets are likely to be enzymes that contain key sulfhydryl moieties. A recent study determined that copper poisons Escherichia coli in part by attacking the exposed [4Fe-4S] clusters of dehydratases. The present investigation sought to test whether other soft metals also target these enzymes. In vitro experiments revealed that low-micromolar concentrations of Ag(I) and Hg(II) directly inactivated purified fumarase A, a member of the dehydratase family. The enzyme was also poisoned by higher levels of Cd(II) and Zn(II), but it was unaffected by even millimolar concentrations of Mn(II), Co(II), Ni(II), and Pb(II). Electron paramagnetic resonance analysis and measurements of released iron confirmed that damage was associated with destruction of the [4Fe-4S] cluster, and indeed, the reconstruction of the cluster fully restored activity. Growth studies were then performed to test whether dehydratase damage might underlie toxicity in vivo. Barely toxic doses of Ag(I), Hg(II), Cd(II), and Zn(II) inactivated all tested members of the [4Fe-4S] dehydratase family. Again, activity was recovered when the clusters were rebuilt. The metals did not diminish the activities of other sampled enzymes, including NADH dehydrogenase I, an iron-sulfur protein whose clusters are shielded by polypeptide. Thus, the data indicate that dehydratases are damaged by the concentrations of metals that initiate bacteriostasis.

Cu(I) affinities of the domain 1 and 3 sites in the human metallochaperone for Cu,Zn-superoxide dismutase

The delivery of copper by the human metallochaperone CCS is a key step in the activation of Cu,Zn-superoxide dismutase (SOD1). CCS is a three-domain protein with Cu(I)-binding CXXC and CXC motifs in domains 1 and 3, respectively. A detailed analysis of the binding of copper to CCS, including variants in which the Cys residues from domains 1 and 3 have been mutated to Ser, and also using separate domain 1 and 3 constructs, demonstrates that CCS is able to bind 1 equiv of Cu(I) in both of these domains. The Cu(I) affinity of domain 1 is approximately 5 × 10(17) M(-1) at pH 7.5, while that of domain 3 is at least 1 order of magnitude weaker. The CXXC site will therefore be preferentially loaded with Cu(I), suggesting that domain 1 plays a role in the acquisition of the metal. The delivery of copper to the target occurs via domain 3 whose structural flexibility and ability to be transiently metalated during copper delivery appear to be more important than the Cu(I) affinity of its CXC motif. The Cu(I) affinity of domain 1 of CCS is comparable to that of HAH1, another cytosolic copper metallochaperone. CCS and HAH1 readily exchange Cu(I), providing a mechanism whereby cross-talk can occur between copper trafficking pathways.

Copper chaperone-dependent and -independent activation of three copper-zinc superoxide dismutase homologs localized in different cellular compartments in Arabidopsis

Superoxide dismutases (SODs) are important antioxidant enzymes that catalyze the disproportionation of superoxide anion to oxygen and hydrogen peroxide to guard cells against superoxide toxicity. The major pathway for activation of copper/zinc SOD (CSD) involves a copper chaperone for SOD (CCS) and an additional minor CCS-independent pathway reported in mammals. We characterized the CCS-dependent and -independent activation pathways for three CSDs localized in different cellular compartments in Arabidopsis (Arabidopsis thaliana). The main activation pathway for CSD1 in the cytoplasm involved a CCS-dependent and -independent pathway, which was similar to that for human CSD. Activation of CSD2 in chloroplasts depended totally on CCS, similar to yeast (Saccharomyces cerevisiae) CSD. Peroxisome-localized CSD3 via a CCS-independent pathway was similar to nematode (Caenorhabditis elegans) CSD in retaining activity in the absence of CCS. In Arabidopsis, glutathione played a role in CCS-independent activation, as was reported in humans, but an additional factor was required. These findings reveal a highly specific and sophisticated regulation of CSD activation pathways in planta relative to other known CCS-independent activation.

Copper chaperone-dependent and -independent activation of three copper-zinc superoxide dismutase homologs localized in different cellular compartments in Arabidopsis

Superoxide dismutases (SODs) are important antioxidant enzymes that catalyze the disproportionation of superoxide anion to oxygen and hydrogen peroxide to guard cells against superoxide toxicity. The major pathway for activation of copper/zinc SOD (CSD) involves a copper chaperone for SOD (CCS) and an additional minor CCS-independent pathway reported in mammals. We characterized the CCS-dependent and -independent activation pathways for three CSDs localized in different cellular compartments in Arabidopsis (Arabidopsis thaliana). The main activation pathway for CSD1 in the cytoplasm involved a CCS-dependent and -independent pathway, which was similar to that for human CSD. Activation of CSD2 in chloroplasts depended totally on CCS, similar to yeast (Saccharomyces cerevisiae) CSD. Peroxisome-localized CSD3 via a CCS-independent pathway was similar to nematode (Caenorhabditis elegans) CSD in retaining activity in the absence of CCS. In Arabidopsis, glutathione played a role in CCS-independent activation, as was reported in humans, but an additional factor was required. These findings reveal a highly specific and sophisticated regulation of CSD activation pathways in planta relative to other known CCS-independent activation.

Post-translational modification of Cu/Zn superoxide dismutase under anaerobic conditions

In eukaryotic organisms, the largely cytosolic copper- and zinc-containing superoxide dismutase (Cu/Zn SOD) enzyme represents a key defense against reactive oxygen toxicity. Although much is known about the biology of this enzyme under aerobic conditions, less is understood regarding the effects of low oxygen levels on Cu/Zn SOD enzymes from diverse organisms. We show here that like bakers' yeast (Saccharomyces cerevisiae), adaptation of the multicellular Caenorhabditis elegans to growth at low oxygen levels involves strong downregulation of its Cu/Zn SOD. Much of this regulation occurs at the post-translational level where CCS-independent activation of Cu/Zn SOD is inhibited. Hypoxia inactivates the endogenous Cu/Zn SOD of C. elegans Cu/Zn SOD as well as a P144 mutant of S. cerevisiae Cu/Zn SOD (herein denoted Sod1p) that is independent of CCS. In our studies of S. cerevisiae Sod1p, we noted a post-translational modification to the inactive enzyme during hypoxia. Analysis of this modification by mass spectrometry revealed phosphorylation at serine 38. Serine 38 represents a putative proline-directed kinase target site located on a solvent-exposed loop that is positioned at one end of the Sod1p β-barrel, a region immediately adjacent to residues previously shown to influence CCS-dependent activation. Although phosphorylation of serine 38 is minimal when the Sod1p is abundantly active (e.g., high oxygen level), up to 50% of Sod1p can be phosphorylated when CCS activation of the enzyme is blocked, e.g., by hypoxia or low-copper conditions. Serine 38 phosphorylation can be a marker for inactive pools of Sod1p.

Battles with iron: manganese in oxidative stress protection

The redox-active metal manganese plays a key role in cellular adaptation to oxidative stress. As a cofactor for manganese superoxide dismutase or through formation of non-proteinaceous manganese antioxidants, this metal can combat oxidative damage without deleterious side effects of Fenton chemistry. In either case, the antioxidant properties of manganese are vulnerable to iron. Cellular pools of iron can outcompete manganese for binding to manganese superoxide dismutase, and through Fenton chemistry, iron may counteract the benefits of non-proteinaceous manganese antioxidants. In this minireview, we highlight ways in which cells maximize the efficacy of manganese as an antioxidant in the midst of pro-oxidant iron.

Redox regulation in respiring Saccharomyces cerevisiae

BACKGROUND

In biological systems, redox reactions are central to most cellular processes and the redox potential of the intracellular compartment dictates whether a particular reaction can or cannot occur. Indeed the widespread use of redox reactions in biological systems makes their detailed description outside the scope of one review.

SCOPE OF THE REVIEW

Here we will focus on how system-wide redox changes can alter the reaction and transcriptional landscape of Saccharomyces cerevisiae. To understand this we explore the major determinants of cellular redox potential, how these are sensed by the cell and the dynamic responses elicited.

MAJOR CONCLUSIONS

Redox regulation is a large and complex system that has the potential to rapidly and globally alter both the reaction and transcription landscapes. Although we have a basic understanding of many of the sub-systems and a partial understanding of the transcriptional control, we are far from understanding how these systems integrate to produce coherent responses. We argue that this non-linear system self-organises, and that the output in many cases is temperature-compensated oscillations that may temporally partition incompatible reactions in vivo.

GENERAL SIGNIFICANCE

Redox biochemistry impinges on most of cellular processes and has been shown to underpin ageing and many human diseases. Integrating the complexity of redox signalling and regulation is perhaps one of the most challenging areas of biology. This article is part of a Special Issue entitled Systems Biology of Microorganisms.

Identification of a therapeutic strategy targeting amplified FGF19 in liver cancer by Oncogenomic screening

We screened 124 genes that are amplified in human hepatocellular carcinoma (HCC) using a mouse hepatoblast model and identified 18 tumor-promoting genes, including CCND1 and its neighbor on 11q13.3, FGF19. Although it is widely assumed that CCND1 is the main driving oncogene of this common amplicon (15% frequency in HCC), both forward-transformation assays and RNAi-mediated inhibition in human HCC cells established that FGF19 is an equally important driver gene in HCC. Furthermore, clonal growth and tumorigenicity of HCC cells harboring the 11q13.3 amplicon were selectively inhibited by RNAi-mediated knockdown of CCND1 or FGF19, as well as by an anti-FGF19 antibody. These results show that 11q13.3 amplification could be an effective biomarker for patients most likely to respond to anti-FGF19 therapy.

Characterization of the recombinant copper chaperone (CCS) from the plant Glycine (G.) max

The goal of the present work was to characterize the recombinant copper chaperone (CCS) from soybean. Very little is known about plant copper chaperones, which makes this study of current interest, and allows for a comparison with the better known homologues from yeast and humans. To obtain sizeable amounts of pure protein suitable for spectroscopic characterization, we cloned and overexpressed the G. max CCS chaperone in E. coli in the presence of 0.5 mM CuSO(4) and 0.5 mM ZnSO(4) in the broth. A pure protein preparation was obtained by using two IMAC steps and pH gradient chromatography. Most of the proteins were obtained as apo-form, devoid of copper atoms. The chaperone showed a high content (i.e., over 40%) of loops, turns and random coil as determined both by circular dichroism and homology modelling. The homology 3-D structural model suggests the protein might fold in three structural protein domains. The 3-D model along with the primary structure and spectroscopic data may suggest that copper atoms occupy the two metal binding sites, MKCEGC and CTC, within the N-terminal domain I and C-terminal domain III, respectively. But only one Zn-binding site was obtained spectroscopically.

Bax regulates production of superoxide in both apoptotic and nonapoptotic neurons: role of caspases

A Bax- and, apparently, mitochondria-dependent increase in superoxide (O(2)(·-)) and other reactive oxygen species (ROS) occurs in apoptotic superior cervical ganglion (SCG) and cerebellar granule (CG) neurons. Here we show that Bax also lies upstream of ROS produced in nonapoptotic neurons and present evidence that caspases partially mediate the pro-oxidant effect of Bax. We used the O(2)(·-)-sensitive dye MitoSOX to monitor O(2)(·-) in neurons expressing different levels of Bax and mitochondrial superoxide dismutase (SOD2). Basal and apoptotic O(2)(·-) levels in both SCG and CG neurons were reduced in SOD2 wild-type (WT) cells having lower Bax concentrations. Apoptotic and nonapoptotic neurons from Bax-WT/SOD2-null but not Bax-null/SOD2-null mice had increased O(2)(·-) levels. A caspase inhibitor inhibited O(2)(·-) in both apoptotic and nonapoptotic SCG neurons. O(2)(·-) production increased when WT, but not Bax-null, SCG neurons were permeabilized and treated with active caspase 3. There was no apoptosis and little increase in O(2)(·-) in SCG neurons from caspase 3-null mice exposed to an apoptotic stimulus. O(2)(·-) levels in nonapoptotic caspase 3-null SCG neurons were lower than in WT cells but not as low as in caspase inhibitor-treated cells. These data indicate that Bax lies upstream of most O(2)(·-) produced in neurons, that caspase 3 is required for increased O(2)(·-) production during neuronal apoptosis, that caspase 3 is partially involved in O(2)(·-) production in nonapoptotic neurons, and that other caspases may also be involved in Bax-dependent O(2)(·-) production in nonapoptotic cells.

Zebrafish sod1 and sp1 expression are modulated by the copper ATPase gene atp7a in response to intracellular copper status

Copper is an essential trace metal for physiological functions, whereas copper overload causes cytotoxicity in living organisms. Genetically determined systems regulate acquisition, distribution and storage for copper maintenance and homeostasis. The Human ATP7A copper transport ATPase modulates intracellular copper distribution, which is critical for copper-dependent enzymes such as superoxide dismutase (SOD1). To investigate the role of zebrafish ATP7A in copper homeostasis, zebrafish atp7a gene expression was reduced for analysis of downstream cellular function. The results demonstrated that zebrafish sod1 has lower expression in atp7a-knockdown fish. Similarly, zebrafish sp1, a transcriptional regulator of sod1, also shows reduced expression in atp7a-knockdown fish. The lower expression of sod1 resulting from atp7a knockdown is independent to p53 gene activation. The knockdown of atp7a and copper chelator NeoC results in hypopigmentation and notochord deformation in zebrafish. Addition of exogenous copper alleviated the impaired development. Interestingly, both sod1 and sp1 transcripts are reduced in the presence of NeoC and increased with exogenous copper, suggesting that the expression of sod1 and sp1 are directly affected by copper status. This is the first report to demonstrate a hierarchic gene expression of copper homeostatic genes between atp7a, sp1 and sod1 in zebrafish.

Import, maturation, and function of SOD1 and its copper chaperone CCS in the mitochondrial intermembrane space

Cu, Zn, superoxide dismutase (SOD1) is a ubiquitous enzyme localized in multiple cellular compartments, including mitochondria, where it concentrates in the intermembrane space (IMS). Similar to other small IMS proteins, the import and retention of SOD1 in the IMS is linked to its folding and maturation, involving the formation of critical intra- and intermolecular disulfide bonds. Therefore, the cysteine residues of SOD1 play a fundamental role in its IMS localization. IMS import of SOD1 involves its copper chaperone, CCS, whose mitochondrial distribution is regulated by the Mia40/Erv1 disulfide relay system in a redox-dependent manner: CCS promotes SOD1 maturation and retention in the IMS. The function of SOD1 in the IMS is still unknown, but it is plausible that it serves to remove superoxide released from the mitochondrial respiratory chain. Mutations in SOD1 cause familial amyotrophic lateral sclerosis (ALS), whose pathologic features include mitochondrial bioenergetic dysfunction. Mutant SOD1 localization in the IMS is not dictated by oxygen concentration and the Mia40/Erv1 system, but is primarily dependent on aberrant protein folding and aggregation. Mutant SOD1 localization and aggregation in the IMS might cause the mitochondrial abnormalities observed in familial ALS and could play a significant role in disease pathogenesis.

Cellular copper distribution: a mechanistic systems biology approach

Copper is an essential but potentially harmful trace element required in many enzymatic processes involving redox chemistry. Cellular copper homeostasis in mammals is predominantly maintained by regulating copper transport through the copper import CTR proteins and the copper exporters ATP7A and ATP7B. Once copper is imported into the cell, several pathways involving a number of copper proteins are responsible for trafficking it specifically where it is required for cellular life, thus avoiding the release of harmful free copper ions. In this study we review recent progress made in understanding the molecular mechanisms of copper transport in cells by analyzing structural features of copper proteins, their mode of interaction, and their thermodynamic and kinetic parameters, thus contributing to systems biology of copper within the cell.

Cu,Zn superoxide dismutase maturation and activity are regulated by COMMD1

The maturation and activation of the anti-oxidant Cu,Zn superoxide dismutase (SOD1) are highly regulated processes that require several post-translational modifications. The maturation of SOD1 is initiated by incorporation of zinc and copper ions followed by disulfide oxidation leading to the formation of enzymatically active homodimers. Our present data indicate that homodimer formation is a regulated final step in SOD1 maturation and implicate the recently characterized copper homeostasis protein COMMD1 in this process. COMMD1 interacts with SOD1, and this interaction requires CCS-mediated copper incorporation into SOD1. COMMD1 does not regulate disulfide oxidation of SOD1 but reduces the level of SOD1 homodimers. RNAi-mediated knockdown of COMMD1 expression results in a significant induction of SOD1 activity and a consequent decrease in superoxide anion concentrations, whereas overexpression of COMMD1 exerts exactly the opposite effects. Here, we identify COMMD1 as a novel protein regulating SOD1 activation and associate COMMD1 function with the production of free radicals.

Oxidant sensing by protein kinases a and g enables integration of cell redox state with phosphoregulation

The control of vascular smooth muscle contractility enables regulation of blood pressure, which is paramount in physiological adaptation to environmental challenges. Maintenance of stable blood pressure is crucial for health as deregulation (caused by high or low blood pressure) leads to disease progression. Vasotone is principally controlled by the cyclic nucleotide dependent protein kinases A and G, which regulate intracellular calcium and contractile protein calcium sensitivity. The classical pathways for activation of these two kinases are well established and involve the formation and activation by specific cyclic nucleotide second messengers. Recently we reported that both PKA and PKG can be regulated independently of their respective cyclic nucleotides via a mechanism whereby the kinases sense cellular oxidant production using redox active thiols. This novel redox regulation of these kinases is potentially of physiological importance, and may synergise with the classical regulatory mechanisms.

Regulation of the copper chaperone CCS by XIAP-mediated ubiquitination

In order to balance the cellular requirements for copper with its toxic properties, an elegant set of mechanisms has evolved to regulate and buffer intracellular copper. The X-linked inhibitor of apoptosis (XIAP) protein was recently identified as a copper-binding protein and regulator of copper homeostasis, although the mechanism by which XIAP binds copper in the cytosol is unclear. Here we describe the identification of the copper chaperone for superoxide dismutase (CCS) as a mediator of copper delivery to XIAP in cells. We also find that CCS is a target of the E3 ubiquitin ligase activity of XIAP, although interestingly, ubiquitination of CCS by XIAP was found to lead to enhancement of its chaperone activity toward its physiologic target, superoxide dismutase 1, rather than proteasomal degradation. Collectively, our results reveal novel links among apoptosis, copper metabolism, and redox regulation through the XIAP-CCS complex.

PM(10) impairs the antioxidant defense system and exacerbates oxidative stress driven cell death

The aim of this study was to investigate the effect of airborne particulate matter with a mean aerodynamic diameter of < or =10microm (PM(10)) on oxidative stress markers and antioxidant enzymatic activity and its relevance in the face of acute oxidative challenge in a human lung epithelial cell line (A549). PM(10)-induced reactive oxygen species (ROS) generation and oxidative damage with no changes in cellular viability. In addition, PM(10) decreased glutathione (GSH) levels (54.9%) and the activity of the antioxidant enzymes superoxide dismutase (65%), catalase (31.2%), glutathione reductase (61.5%) and glutathione-S-transferase (42.39%). Trolox, a scavenger of reactive species, prevented the increase of ROS generation and the decrease in GSH levels but partially prevented PM(10)-induced oxidative damage. Interestingly, it was unable to avoid the decrease in the activity of antioxidant enzymes. Finally, the survival of the cells previously exposed to PM(10) and challenged with hydrogen peroxide was significantly lower. We conclude that the impairment in the antioxidant defense system induced by PM(10) weaken ROS detoxification which exacerbates cell death when these cells are exposed to an acute oxidative challenge.