Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Superoxide Dismutase 1-3

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101647

Synonyms

 CuZnSOD;  EC-SOD;  MnSOD;  SOD1;  SOD2;  SOD3

Historical Background

According to oxygenic theory of evolution, the atmosphere initially contained low concentration of oxygen. The genesis of photosynthetic organisms approximately 2.5 billion years ago initiated the development of plants and other life-forms able to use aerobic metabolism. Reactive oxygen species (ROS), although needed to normal cellular functions, are in aberrant concentrations potentially harmful molecules damaging various cellular structures. Hence, it is probable that already the first living organisms acquired antioxidative defense mechanisms. Superoxide dismutases (SOD) represent a reduction-oxidation (redox) metalloprotein enzyme family, which, according to oxygenic theory of evolution, was connected to the availability of transition metals in the biosphere (Bannister et al. 1991).

There are three members in mammalian superoxide dismutase family: CuZnSOD (SOD1), MnSOD (SOD2), and EC-SOD (SOD3). Superoxide dismutase protein, first reported 1938 by Mann and Keilin, was purified from bovine blood and named as hemocuprein. The same protein was isolated from the liver in 1939, called as hepatocuprein (Mann and Keilin 1939), and from the brain, named as cerebrocuprein (Porter and Folch 1957). McCord and Fridovich discovered at the end of the 1960s the ability of the protein to catalyze dismutase reaction, thus identifying it as an enzyme and naming it as superoxide dismutase (CuZnSOD). Cupper ion located at the active center of the enzyme functions as an electron carrier in the dismutase reaction, whereas zinc participates indirectly the reaction by regulating the three-dimensional conformation of the active center (Tainer et al. 1983). Zimmermann and coworkers published in 1973 the discovery of the second member of SOD family, the mitochondrial SOD (MnSOD, SOD2) (Zimmermann et al. 1973), which was confirmed a few months later by Weisiger and Fridovich (Weisiger and Fridovich 1973). Stefan L. Marklund reported in 1982 the third superoxide dismutase, extracellular superoxide dismutase (EC-SOD, SOD3), demonstrating the ability of the enzyme to bind reversibly to cell membrane proteoglycan structures (Marklund 1982).

Introduction

SOD1, SOD2, and SOD3 enzymes regulate reduction-oxidation (redox) balance by converting superoxide anion (O2-) to hydrogen peroxide (H2O2) in two half reactions:

$$\begin{array}{l} Cu^{2 + } + O_2 ^ - > Cu^{2 + } + O_2 \\ Cu^ + + O_2 ^ - + 2H^ + > Cu^{2 + } + H_2 O_2 \\ \end{array}$$

In the two-step reaction the oxidized form of the enzyme binds to O2-., reacts with proton, and releases molecular oxygen. In the second half-reaction, the reduced form of the enzyme binds a second O2-. and two protons to synthesize H2O2. Consequently the enzyme returns to oxidized state. Hydrogen peroxide is further processed to water by catalase and a large family of peroxidases. Catalase is able to utilize H2O2 itself as a reductant oxidizing it to molecular oxygen, whereas peroxidases dispose H2O2 by oxidizing a secondary reductant, such as glutathione.

The primary location, and thus also cellular response to dismutase reaction, of each SOD varies. SOD1, which is mainly observed in cytoplasm, has a prominent role in amyotrophic lateral sclerosis (ALS). Mitochondrial SOD2 is essentially involved in cellular senescence and survival. Lack of SOD2 expression causes death of newborn mice and induction of cell cycle arrest with consequent cellular senescence in mouse embryonic fibroblasts (MEF). The extracellular cell membrane-bound SOD3 activates cell survival, antiapoptotic, and growth signal transduction resulting in increased cell proliferation and increased healing of tissue damages.

SOD1

SOD1-related signaling has been studied most intensively in amyotrophic lateral sclerosis (ALS) patients. Both increased ROS production and increased inflammation characterize the development of ALS. Several studies have demonstrated correlation between ALS and mutations in SOD1 gene. Thus far approximately 150 mutations increasing sensitivity to ALS pathogenesis have been identified from promoter, exons, introns, and untranslated regions of SOD1 (Hitchler and Domann 2014). Mutant SOD1 has been shown to induce motoneuron degeneration by activating Toll-like receptors 2 and 4 (TLR2, TLR4) involving IRAK-BID-TRAF6 and IκBα-p50/p65 signaling molecules, thus increasing proinflammatory cytokine expression with consequent increased inflammation (Kinsella et al. 2016). In accordance to increased inflammation, mutant SOD1 molecules affect ALS development by increasing ROS production through NADPH oxidase. SOD1 regulates NADPH oxidase activity by directly binding to RAC1, a small GTPase commonly studied in cancer cell migration, causing inhibition of GTP dissociation from RAC1 and therefore maintaining the small GTPase active. Importantly, binding of wild-type SOD1 to RAC1 is redox sensitive: wild-type SOD1-RAC1 is disassembled by increased concentration of H2O2, dismutase reaction end product, whereas mutant SOD1-RAC1 shows insensitivity to self-regulated uncoupling of the molecules. Thus, mutant SOD1 molecules, such as SOD1L8Q and SOD1G10V, have increased affinity to RAC1 maintaining it active by preventing GTP hydrolysis and causing increased NADPH oxidase activity with consequent increased O2-. ROS production (Harraz et al. 2008) (Fig. 1).
Superoxide Dismutase 1-3, Fig. 1

Mutations in SOD1 gene result in increased ROS production, increased inflammatory cytokine expression, and increased risk for ALS development. A number of factors stimulate SOD1 mRNA synthesis. Wild-type SOD1 binds to small GTPase RAC in a redox-sensitive manner, whereas mutations in SOD1 gene result in more persistent RAC binding and ROS production. Increased concentration of H2O2 stimulates dissociation of wild-type SOD1 but not mutant SOD1 from RAC, thus causing accumulation of ROS with consequent activation of Toll-like receptor 4 signaling through IRAK-IKKα/β/γ-p50/p66 pathway. Binding of p50–p65 into DNA stimulates inflammatory cytokine mRNA expression

SOD2

Studies demonstrating lethal phenotype of SOD2 knockout stress the importance of the enzyme in mitochondrial redox balance. Several stimuli increase SOD2 gene expression, such as ROS, inflammatory cytokines, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), specificity protein 1 (SP1), cyclic adenosine 3′,5′-monophosphate (cAMP)-responsive element binding protein (CREB), and nuclear non-phosphorylated forkhead box proteins’ family members FOXO3a and FOXM1, whereas p53 and phosphorylation-mediated inactivation of FOXO3a reduce the mRNA synthesis. SOD2 signaling studies in senescence models suggest phosphatidylinositol-3-kinase (PI3K) – protein kinase B (PKB, AKT)-Forkhead box O3 (FOXO3a) signal transduction pathway as a major route regulating SOD2 gene expression. Nuclear FOXO3a binds to Forkhead transcription factor-binding elements at SOD2 promoter regions, thus increasing SOD2 expression. Phosphorylation of AKT with consequent inactivation of FOXO3a by phosphorylation translocates FOXO3a transcription factor from the nucleus to the cytoplasm causing reduced SOD2 mRNA synthesis, increased mitochondrial ROS production, and concomitant development of senescence. Forkhead box protein M1 (FOXM1) functions parallel to FOXO3a increasing SOD2 expression in proliferating cells but not in non-proliferating cells (Imai et al. 2014) (Fig. 2).
Superoxide Dismutase 1-3, Fig. 2

Decreased SOD2 production is associated to cellular aging. FOXO3a and FOXM1 are the main activators of SOD2 gene expression binding to the same element in the promoter region in quiescent and proliferating cells, respectively. In the presence of mitogenic stimuli, PI3K-AKT signaling phosphorylates FOXO3a, which then is translocated into cytoplasm causing downregulation of FOXO3a-activated SOD2 gene expression allowing FOXM1 binding to SOD2 promoter region. When both FOXO3a and FOXM1 binding is blocked, ROS accumulates in the cells causing increased DNA damages, activation of DNA damage response, growth arrest, and senescence

SOD3

Secreted SOD3 binds reversibly to cell membrane proteoglycan structures, which brings the enzyme at close proximity with membrane-associated signaling molecules. SOD3 activates several cell membrane tyrosine kinase receptors (RTKs) and their downstream signaling molecules, most importantly RAS-RAF-MAPKK (MEK)-MAPK p44/42 (ERK1/ERK2) signaling pathway. Increased MAPK p44/42 expression stimulates SOD3 expression, thus forming a positive feedback loop. Interestingly, the positive feedback look is controlled at the level of small GTPases and correlates to the SOD3 concentration; high SOD3 concentration inhibits small GTPase RAS, RAC, RHO, and CDC42 activation by downregulating guanine nucleotide exchange factor (GEF) gene expression responsible for GTP loading to small GTPases and by upregulating GTPase-activating protein (GAP) expression that functions opposite to GEF. Data demonstrating increased phosphorylation of RAS upstream signaling molecules RTKs, focal adhesion kinase (FAK), and family of SRC proto-oncogenes at high SOD3 concentrations support this conclusion suggesting that regulation of SOD3-stimulated signal transduction occurs at the level of small GTPases. Moreover, moderately overexpressed SOD3 increases GEF and inhibits GAP gene expression, hence resulting in increased GTP loading to small GTPases and consequent activation of downstream signaling (Laukkanen et al. 2015).

Other important signal transduction molecules and pathways involved in SOD3 signaling include p38 MAPK, which inhibits SOD3 expression (Adachi et al. 2006), although the mechanism of inhibition is not completely characterized. Large G-protein-associated thyroid-stimulating hormone (TSH) receptor activation has been demonstrated to increase expression of SOD3 through cyclic AMP – protein kinase A (cAMP-PKA) pathway and through phospholipase C-calcium 2+ (PLC-Ca2+) signaling inducing cell proliferation (Laatikainen et al. 2010). SOD3 has also shown to increase PI3K-AKT signaling causing inactivation of FOXO3a by phosphorylation and consequent translocation from the nucleus into the cytosol (Laatikainen et al. 2011).

SOD3 stimulated increased mitogen signaling results in increased tissue healing, primary cell proliferation, and cancer cell metastasis, as observed in cardiovascular and cancer models. SOD3 expression is upregulated in benign tumors and frequently, although not always, downregulated in transformed cells despite of the growth stimulatory nature of the enzyme. Overexpression of SOD3 in primary cells, such as mouse embryonic fibroblasts (MEF), induces initial proliferative burst followed by growth arrest and senescence suggesting that SOD3 mediates RAS oncogene-induced senescence in tumorigenesis. Rescued SOD3 expression in triple-negative breast cancer cells significantly enhances primary tumor formation and metastasis hence suggesting growth-promoting role for the enzyme (Castellone et al. 2014; Wang et al. 2014; Laukkanen 2016) (Fig. 3).
Superoxide Dismutase 1-3, Fig. 3

SOD3 signal transduction. Cell membrane tyrosine kinase receptor and G-protein-coupled receptor-stimulated signal transduction increase SOD3 expression. Induction of RTK signaling increases SRC proto-oncogene phosphorylation and RAS activation with consequent BRAF-MEK1/MEK2-ERK1/ERK2 signal transduction increasing SOD3 expression. Similarly, ligand binding to GPCRs increases cAMP-PKA and PLC-Ca2+ signaling that both increase SOD3 mRNA synthesis in cells. Secreted enzyme binds to cell membrane proteoglycan structures and unspecifically activates TRKs by increased production of H2O2, therefore forming a positive feedback loop. The expression of SOD3 is controlled at the level of small GTPases by GTP regulatory genes, by SOD3-RAS-p38 MAPK, and by SOD3-PI3K-AKT-activated FOXO3a. Moderately increased SOD3 concentrations increase GEF expression and downregulated GAP and GDI expression causing increased RAS GTP loading. High-level SOD3 expression has an opposite effect thus inhibiting mitogenic signaling and stimulation of SOD3 production. Activation of SOD3-PI3K-AKT signaling causes translocation of FOXO3a into cytoplasm releasing mir21 expression, which then binds to 3′UTR of SOD3 mRNA causing silencing of the gene expression. Increased phosphorylation of p38 MAPK by, e.g., RAS causes progressive downregulation of SOD3 expression. In carcinogenic models both increased synthesis of mir21 and high activation of p38 phosphorylation occur at tenfold RAS activation levels

Summary

ROS are often connected to tissue damages and pathological conditions although they are essential maintainers of normal cellular functions. Once the concentration of redox enzymes, oxidants, and antioxidants is unbalanced by tissue trauma or pathological condition, excess ROS activates aberrant signal transduction or directly injures cellular macromolecules. O2-. and H2O2 are both secondary messengers targeting unspecifically signaling molecules; hence, the function of SOD enzymes goes beyond the balancing of O2-. and H2O concentrations extending to signal transduction regulation. ROS has been shown to mediate RAS oncogene-induced primary cell senescence (Leikam et al. 2008) and cellular transformation (Mitsushita et al. 2004) highlighting the importance of redox system in defending normal tissue functions and in progression of pathological conditions. Cellular response of ROS depends on their quality, quantity, and cellular location. Each ROS has characteristically different function in cellular environment that further depends on the concentration of a specific ROS, which then may directly interact with macromolecules in the cytoplasm, in mitochondria, or at cell membranes. Correspondingly, cellular response of SOD enzymes correlates to their location and level of expression. Because SODs are difficult drug targets, clarification of the signal transduction pathways could potentially reveal a selective group of molecules mediating the cellular response of the enzymes. Hence, efforts elucidating SOD-stimulated signaling routes could lead to the development of novel therapeutic regimens.

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.IRCCS SDNNaplesItaly