A rt ic le n u m b e r 1 2 Arti cle titl e Ye Information ar of pu bli ca tio n Fre e rad ical s, rea ctiv e oxy gen spe cie s, oxi dat ive str ess an d its clas sifi cati on 28 Oc to be r 20 14 Rea ctiv e oxy gen spe cie s: Rol Free radicals were first described by Moses Gomberg more than a century ago. In 1950th, free radicals were found in biological systems. Discovery of McCord and Fridovich who described the first protective enzyme against free radicals called superoxide dismutase. Complicated study due to following factors: (i) low stability and high reactivity resulting in low steady-state concentrations; (ii) high diversity of reactions they can participate in; (iii) complicated spatiotemporal distribution in cell space and extracellularly; (iv) dependence on physiological state of the organism, and (v) absence of technical tools for reliable evaluation of their absolute and even relative levels. Generation, conversion, and elimination of ROS in living systems . Living organisms possess multilevel and complicated antioxidant system operating either to eliminate ROS, or minimize their negative effects. There are several approaches to classify these systems and here we will use the mostly appreciated one based on molecular masses. According to this system, antioxidants are placed in two groups: low molecular mass antioxidants (usually with molecular masses below one kilodalton) and high molecular mass antioxidants (with molecular mass higher than one or actually higher than ten kilodaltons). The group of low molecular mass antioxidants includes chemically different compounds usually well known to readers such as vitamins C (ascorbic acid) and E (tocopherol), carotenoids, anthocyanins, polyphenols, and uric acid. one very important antioxidant glutathione (tripeptide c-glutamyl-cysteinylglycine, GSH) is synthesized by most living organisms and used to control ROS level either via direct interaction with them, or serving as a cofactor for ROSdetoxifying enzymes Oxidative damage is one of the main contributors to DNA mutations and disruptions in the pro-oxidant/ antioxidant balance have been reported in more than 200 clinical disorders, including chronic diseases like neurodegenerative diseases, inflammatory bowel disease and cancer. Both ROS and RNS participate in cell development, proliferation, differentiation, oxygen sensing and adaptive immunity through reversible oxidative modifications of macromolecules; but when present in excess, they can cause cellular oxidative damage. e in car cin oge nes is, can cer cell sig nali ng an d tu mo r pro gre ssi on 3 Ass oci 3 Ju The main sources of ROS in mammalian cells are the mitochondrial electron transport chain, NOX enzymes and H2O2 production during protein folding at the endoplasmic reticulum. H2O2 is a more potent oxidant with a longer life than O2 •− since it is not a radical. H2O2 is the oxidation product of monooxygenases and oxidases and it can be converted to H2O by catalase (CAT) or Glutathione Peroxidase (GPX). n. A relative change in the balance between oxidants and antioxidants in favor of oxidants is called oxidative stress. This state can lead to alterations in redox signaling, damage to proteins, phospholipids, nucleic acids and in many cases, to cell death. bronchial epithelial cells in culture exposed to cigarette smoke extract have been shown to induce mitochondrial depolarization, increased mitochondrial ROS production and activation of mitophagy. The GSH system is one of the main antioxidant systems in the cell, participating in the maintenance of intracellular redox balance and the regulation of signaling pathways by disulfide bond reduction to cysteines in redox-regulated proteins. ROS play different roles in lung cancer depending on the genetic background to effectively manipulate ROS for lung cancer prevention or therapy and if these effects can be applied to other cancer types. Exposure of cells to UV radiation is known to directly induce DNA oxidation by inducing the formation of 1 O2 and of 8-hydroxy-2´deoxyguanosine (8OHdG), the main product of guanine oxidation, by activating photosensitizers such as bilirubin which can then lead to ROS formation, by activating ROS/ RNS producing enzymes like NOX or NOS or by inhibiting antioxidant enzyme activity, such as CAT. cancer preventive effects are mostly mediated by their antioxidant properties or a combination of cancer suppressive effects, to determine if new molecules or therapeutic options are needed for effective cancer prevention. elevated levels of ROS in cancer cells contribute to the maintenance of protumorigenic signaling, where ROS activate pathways related to avoidance of cell death, proliferation, angiogenesis, invasion and metastasis, as well as to the activation of antioxidant and detoxifying systems. The high antioxidant capacity in cancer cells has been suggested to balance the high levels of oxidative stress and has been related to increased resistance to oxidizing chemotherapy or radiation as well as to the activation of drug detoxification mechanisms. a high oxidation level has been proposed to be necessary for cancer stem cell (CSC) maintenance in different models. ROS induction has also been proposed to be useful for cancer therapy. ROS have a controversial and context-dependent role in cancer, where an antioxidant defense would be needed mostly for cancer prevention and supporting the idea that a healthy lifestyle and a high diet intake of fruits and vegetables to be associated with lower risks of cancer or death and cardiovascular disease This enzyme catalyzes the dismutation of superoxide radicals into hydrogen peroxide and oxygen [5]. Given that mitochondria are the major sites for 4 ati on bet we en ma nga nes e sup ero xid e dis mu tas e pol ym orp his m an d risk of lun g can cer Ma nga nes e sup ero xid e dis mu tas e reg ula tio n an d ne 20 08 25 Ap ril 20 12 cellular metabolism and thus of ROS production, MnSOD is very important in protecting cells from ROS-induced oxidative damage. Manganese superoxide dismutase, as a single superoxide radical scavenger in mitochondria, may have a big role in preventing cells as an antioxidant and tumor suppressor. Structural change can result in altered ROS scavenger activity in the mitochondria, leading to a positive association between MnSOD Val/Val genotype and lung cancer risk. MnSOD is a nuclear-encoded enzyme that is very highly regulated. The expression of MnSOD can be regulated at multiple levels from transcription and translation to posttranslational modifications. Sequence analysis of 50 - and 30 -flanking regions of the human MnSOD gene reveals multiple potential regulatory motifs for Sp1. Epigenetic regulation of genes, which has attracted increasing attention in cancer research, has made possible modifications in DNA molecules without changing DNA sequence, resulting in alterations in gene expression. Another known epigenetic alteration of MnSOD gene expression is mediated by histone acetylation in cancer cells. It has been reported that high basal expression of MnSOD in aggressive breast cancer is associated with the hyperacetylation of H3 histone. The molecular changes that occur after transcription include mRNA stability, mRNA processing, and mRNA translation. These processes contribute to efficient translation for the optimum level of protein expression. Because human MnSOD has a typical splice junction. 5 can cer San jit Ku ma r Dh ar, Dar et K. St. Clai rn Exp an din g rol es of sup ero xid e dis mu tas es in cell reg ula tio n an d can cer Under normal physiological conditions, they participate in redox reactions and serve as second messengers for regulatory functions. SODs are increasingly recognized for their regulatory functions in growth, metabolism and oxidative stress responses, which are also crucial for cancer development and survival. Excessive ROS oxidize macromolecules such as DNA, proteins and lipids, causing elevated mutations, damage to cellular organelles and other structures and, in extreme circumstances, apoptotic cell death. Such a condition is called oxidative stress. When ROS reach a cytotoxic level, an oxidative stress response is triggered that, through transcription factors, upregulates antioxidant and cellular repair genes. SOD3 is the secreted form of SOD with expression restricted mainly to the lung, kidney and adipose tissues to prevent oxidative tissue damage. Based on the rate of mitochondrial respiration, SOD1 controls the level of H2O2, which in turn sets a threshold for mitogen signalling such as receptor tyrosine kinase (RTK) signalling to determine the rate of cell proliferation. SOD1 is the major intracellular form of SOD, accounting for 80 % total SOD protein. SOD1 represses respiration in the presence of glucose and oxygen to promote glycolysis [26]. SOD1 binds to, and regulates the stability of, two casein kinases involved in glucose-mediated respiratory repression through localized production of H2O2 [26]. In this fashion, SOD1 engages in control of metabolic switches between aerobic glycolysis and oxidative phosphorylation (OXYPHOS). SOD1 plays a key part in maintaining genomic DNA stability. SOD1 is found to bind to the promoters and regulates a large set of genes involved in oxidative stress, replication stress, DNA damage response, general stress response and Cu/Fe homeostasis. loss of SOD1 increases ROS level, which is naturally thought to cause oxidative DNA damage and promote carcinogenesis. 6 A Rev iew of the Cat alyt ic Me cha nis m of Hu ma n Ma nga nes e Sup ero xid e Dis mu 30 Ja nu ar y 20 18 Liver is the major organ for iron transport and storage, and is prone to injury in the absence of SOD1, which could explain why only hepatocellular carcinomas are favored. Reduced SOD2 expression tends to be lower in early-stage tumors, suggesting that SOD2 loss is associated with tumor initiation, which is consistent with the general theme of reduced antioxidants being associated with increased ROS and oxidative genomic DNA damage, and thus carcinogenesis. Increased SOD2 expression in cancer cells sustains the flow of H2O2 originating from mitochondria, which is required for maintaining AMPactivated protein kinase (AMPK) activity, causing a metabolic switch from mitochondrial respiration to glycolysis [50], a phenomenon commonly seen in human cancer known as the Warburg effect. The general consensus of several studies is that the SOD3 level is reduced in human cancer, which has a pro-tumorigenic effect. Overexpression of SOD3 in PDA cells results in decreased growth and invasiveness. cancer cells accumulate excessive ROS that can cause severe cellular damage and induce apoptotic cell death. To date, several compounds targeting SODs have been identified that show promising anticancer activity in preclinical studies, demonstrating the feasibility of this approach. Mutations of SODs can cause cancer of the lung, colon, and lymphatic system, as well as neurodegenerative diseases such as Parkinson’s disease and amyotrophic lateral sclerosis. MnSOD, the most significant enzyme in protecting against ROS in the human body. Human MnSOD resides in the mitochondrial matrix, the location of up to 90% of cellular ROS generation. Chronic inflammation can lead to disease states such as atherosclerosis and type two diabetes whereas mitochondrial degeneration leads to neurological disorders such as cerebral palsy and Parkinson’s disease. superoxide acting as a signaling molecule to induce cell division and proliferation MnSOD overexpression by vector delivery showed significant toxicity for tumor cells and radio-protective properties for healthy cells in vitro and in vivo in mice Mn porphyrins are attractive because they (1) lack antigenicity; (2) are extremely stable; (3) can penetrate subcellular membranes; (4) scavenge other ROS such as peroxynitrite; and (5) are modifiable to optimize efficacy for their desired approach, all while approaching the catalytic rate and efficiency of MnSOD. These amino acids and ligands, termed the “inner sphere” residues, form a direct interaction with the manganese. The next layer of contacting amino acids, called the “outer sphere” residues, are essential for efficient dismutation. Substrate is thought to diffuse into the active site through residues His30 and Tyr34 where it most likely binds to the manganese ion in the position opposite Asp159 tas e 6 Ma nga nes e Sup ero xid e Dis mu tas e in Ca nce r Pre ven tio n superoxide is a negatively charged substrate, MnSOD probably achieves rapid catalysis with the aid of electrostatic guidance. indicate that the active site with azide adducts exists in a dynamic equilibrium between five and six coordinate states at 296 K. At 273 K, the coordination shifts to pred. During the active state, superoxide gains a proton from the solvent ligand to form hydroperoxyl radical, whereas the inactive complex forms due to proton donation from the solvent ligand to an alternative proton acceptor. Oxidized MnSOD in the presence of excess hydrogen peroxide is known to instigate a backwards enzymatic reaction of MnSOD, in which hydrogen peroxide is converted to superoxide. The MnSOD mechanism includes three categories: (1) the means of highly efficient substrate and product diffusion; (2) the mode of superoxide binding to the active site; and (3) the proton-shuttling mechanism for proton-assisted electron transfer. the best way to fight against cancer is to prevent or suppress cancer development. Cancer is preventable as indicated by human papilloma virus (HPV) vaccination and tamoxifen/raloxifen treatment in breast cancer prevention. Dietary supplement-based SOD cancer prevention provides another opportunity for antioxidant-based cancer prevention. SODs, providing the first defending line against superoxide radicals, have been studied in human cancers In human esophageal cancers, studies have shown that decreased MnSOD levels are associated with increased incidences of esophageal adenocarcinoma (EAC). In a human tissue microarray analysis, MnSOD expression was higher in malignant tissues compared with normal tissues. H2O2 is considered as a metabolic by-product of oxidative metabolism; however, its ability to oxidatively inactivate regulatory proteins shifts the paradigm of H2O2 from being a simple by-product to an important oncogenic regulator in carcinogenesis. It has been demonstrated that metalloporphyrins with higher SOD activity have higher CAT activity suggesting the utility of SOD mimetics in ROSmediated in vitro and in vivo models of disease. In a chemically induced mouse skin carcinogenesis study, MnSOD expression was suppressed in the early stage but increased at late stages of skin carcinogenesis. Overexpression of antioxidant enzymes has been shown to suppress carcinogenesis both in vitro and in vivo, suggesting that the induction of endogenous antioxidant enzymes may be a potential target for cancer prevention. Increasing SOD activity is a suitable strategy for cancer prevention. It was demonstrated that deficiencies in mitochondrial respiration lead to reduced nicotinamide adenine dinucleotide (NADH) accumulation resulting in inactivation of PTEN and activation of the Akt survival pathway. 7 The str uct ure – fun ctio n rel ati ons hip s an d phy siol ogi cal rol es of Mn SO D mu tan ts. Mitochondria serve as scaffolds for a diverse array of cellular signaling pathways, such as adenosine triphosphate (ATP) and fatty acid synthesis, ROS generation, b-oxidation, calcium (Ca2 + ) homeostasis and apoptosis. The presence of SODs can decrease the efficacy of anticancer drugs with a ROS-dependent mechanism of action, such as mofarotene, adriamycin and mitomycin C. In targeting ROS generation, it is known that oxidative stress is associated with mitochondrial oxidative pathways and energy homeostasis. SOD mimetics have been shown to suppress oxidative injury and biomarkers of cell proliferation.
