**6. Redox modulation of toxicants — Heavy metals-induced toxicity**

The essential metals are very important for the maintenance of cell homeostasis. Among the 23 chemical elements with physiological functions in humans, half of them are metals, including heavy metals [42]. The heavy metals, generally defined as metallic elements with a relative density above 5 mg/ml, have the potential to cause human toxicity; the main examples are: Pb, Hg, Fe, Cd, Tl, Bi, Mn, and As. The intoxications produced by metals, characterized mainly by neurotoxicity, genotoxicity, or carcinogenicity, are widely known [43]. After their absorption into organism, the metals bind to proteins and lead to impaired enzymatic activity; the result being the damage of many organs.

Cellular redox processes are controlled by two systems (thioredoxin, Trx, and gluta‐ thione, GSH) [44]. Exposure to ions of heavy metals can amplify the production of reactive oxygen species (ROS), which can react with cellular components followed by the debut of many physiological processes [45]. ROS have a double character as both deleterious and useful compounds: on one hand they act within cells as 2nd messengers in intracellular signaling cascades, inducing and keeping the oncogenic phenotype of cancer cells, and on the other hand, ROS induce the cellular senescence and apoptosis and can be considered anticancer species. The cumulative production of ROS (called oxidative stress) is common for many types of cancer cell which are linked with altered redox regulation of the cellular signaling pathways [46].

Metal-induced formation of ROS causes changes to DNA, increased lipid peroxidation, and altered Ca and -SH homeostasis. Lipid peroxides, formed by ROS attack on phospholipids, can react with redox metals finally producing carcinogenic products [27].

Redox active metals (Fe, Cu, Cr, Co) are part of redox cycling reactions and have the ability to produce ROS. Perturbation of metal ion homeostasis can lead to oxidative stress, a state where increased production of ROS overcome the body antioxidant protection and induces DNA damage, lipid peroxidation, and proteins changes. The action mechanism of these metals involves formation of ROS, finally producing mutagenic and carcinogenic products. Redox inactive metals (Cd, As and Pb) show their toxic effects via bonding to proteins -SH groups and depletion of GSH [47].

Lead (Pb) is a chemical element from group 14, and period 6 (p-block). Pb was removed from alimentary cans, paints, and petrol because it was the most common cause of heavy metal poisoning; an important problem remains the water pipes from older houses, some occupa‐ tions, and traditional remedies. Pb causes toxicity to mitochondria by depletion of GSH, which results in excessive ROS production and mitochondrial damage. It was discovered that Pb toxicity leads to cellular damage via two pathways: (1) the production of ROS, and (2) the direct reduction of antioxidant reserves. Mitochondrial antioxidant enzymes play an important role in cellular defense mechanism against oxidative damage [48]. A possible molecular mechanism of Pb toxicity is represented by the oxidative stress, which appears when ROS production exceed the capacity of antioxidant defense mechanisms. Pb is capable of causing oxidative damage to heart, liver, brain, and erythrocytes [49].

Mercury (Hg) is a chemical element from group 12, and period 6 (d-block); it is poorly absorbed from bowels and the ingestion is usually harmless. Hg compounds have the ability to provoke cellular damage through an increase of ROS levels (the molecular mechanism involved in its genotoxicity). In response to Hg exposure, the amount of intracellular GSH increase to chelate Hg in order to protect the cells by its antioxidant role. Tchounwou and *colab*. already demon‐ strated that GSH levels are higher in human populations exposed to methylmercury intoxi‐ cation by a fish-rich diet [50].

Iron (Fe) is a chemical element from group 8, and period 4 (d-block) and it is one of the most abundant elements in the crust of earth. ROS can play a role in Fe-induced cell toxicity because of its salts' powerful prooxidant activity. In the presence of cellular reductants, Fe from low molecular weight salts can be an initiator of free radical reactions. In Fe overload, hepatocel‐ lular Ca homeostasis may be spoiled through mitochondrial damage and microsomal Ca sequestration. DNA has also been reported to be a target of Fe-induced damage in the liver; this may lead to malignant transformation [51].

Due to its oxidation states (3+ and 2+), Fe is considered an intrinsic producer of ROS, leading to neuronal oxidative stress. Paradoxically, Fe redox properties determine its participation in potentially cytotoxic reactions: bivalent form catalyze the formation of hydroxyl radical, considered the most reactive and damaging intermediate of cellular metabolism, while trivalent form can be reduced to Fe2+ after reacting with superoxide anion. Both forms are also involved in the propagation of lipid peroxidation, by a complex mechanism; however, it likely involves the direct interaction of Fe with ROS [52].

the other hand, ROS induce the cellular senescence and apoptosis and can be considered anticancer species. The cumulative production of ROS (called oxidative stress) is common for many types of cancer cell which are linked with altered redox regulation of the cellular

Metal-induced formation of ROS causes changes to DNA, increased lipid peroxidation, and altered Ca and -SH homeostasis. Lipid peroxides, formed by ROS attack on phospholipids,

Redox active metals (Fe, Cu, Cr, Co) are part of redox cycling reactions and have the ability to produce ROS. Perturbation of metal ion homeostasis can lead to oxidative stress, a state where increased production of ROS overcome the body antioxidant protection and induces DNA damage, lipid peroxidation, and proteins changes. The action mechanism of these metals involves formation of ROS, finally producing mutagenic and carcinogenic products. Redox inactive metals (Cd, As and Pb) show their toxic effects via bonding to proteins -SH groups

Lead (Pb) is a chemical element from group 14, and period 6 (p-block). Pb was removed from alimentary cans, paints, and petrol because it was the most common cause of heavy metal poisoning; an important problem remains the water pipes from older houses, some occupa‐ tions, and traditional remedies. Pb causes toxicity to mitochondria by depletion of GSH, which results in excessive ROS production and mitochondrial damage. It was discovered that Pb toxicity leads to cellular damage via two pathways: (1) the production of ROS, and (2) the direct reduction of antioxidant reserves. Mitochondrial antioxidant enzymes play an important role in cellular defense mechanism against oxidative damage [48]. A possible molecular mechanism of Pb toxicity is represented by the oxidative stress, which appears when ROS production exceed the capacity of antioxidant defense mechanisms. Pb is capable of causing oxidative

Mercury (Hg) is a chemical element from group 12, and period 6 (d-block); it is poorly absorbed from bowels and the ingestion is usually harmless. Hg compounds have the ability to provoke cellular damage through an increase of ROS levels (the molecular mechanism involved in its genotoxicity). In response to Hg exposure, the amount of intracellular GSH increase to chelate Hg in order to protect the cells by its antioxidant role. Tchounwou and *colab*. already demon‐ strated that GSH levels are higher in human populations exposed to methylmercury intoxi‐

Iron (Fe) is a chemical element from group 8, and period 4 (d-block) and it is one of the most abundant elements in the crust of earth. ROS can play a role in Fe-induced cell toxicity because of its salts' powerful prooxidant activity. In the presence of cellular reductants, Fe from low molecular weight salts can be an initiator of free radical reactions. In Fe overload, hepatocel‐ lular Ca homeostasis may be spoiled through mitochondrial damage and microsomal Ca sequestration. DNA has also been reported to be a target of Fe-induced damage in the liver;

Due to its oxidation states (3+ and 2+), Fe is considered an intrinsic producer of ROS, leading to neuronal oxidative stress. Paradoxically, Fe redox properties determine its participation in

can react with redox metals finally producing carcinogenic products [27].

signaling pathways [46].

14 Toxicology Studies - Cells, Drugs and Environment

and depletion of GSH [47].

cation by a fish-rich diet [50].

damage to heart, liver, brain, and erythrocytes [49].

this may lead to malignant transformation [51].

Cadmium (Cd) is a chemical element from group 12, and period 5 (d-block); it was discovered in the 19th century and the first studies upon its toxicological properties were initiated shortly after. The smoke and food are considered the main sources of Cd. Cd inhibits the activity of antioxidant enzymes; it displaces Zn and Cu leading to a decreased level of these two metals in the enzymes and an increased level in the cytoplasm. Thus appear conformational changes and inhibition of enzyme activity, and deregulation of Cu homeostasis which can lead to ROS production via the Fenton reaction [53].

Thallium (Tl) is a chemical element from group 13, and period 6 (p-block); Tl and its com‐ pounds must be manipulated with an increased attention due to their important toxicity. Different authors indicate that Tl induce ROS formation, GSH oxidation, and membrane lipid peroxidation; the liver mitochondria seems to be the main targets of its toxicity because liver is its storage site [54].

Bismuth (Bi) is a chemical element from group 15, and period 6 (p-block); it has a few industrial uses in pigments, ceramics and alloys with low melting points. Bi causes kidney damage and the promotion of a reversible encephalopathy; chelating agents may be used as treatment.

In a few studies of Woods and Fowler there were evaluated Bi effects on organelle structure and heme biosynthetic parameters in liver and cells; their study revealed that action of the metal on membrane enzymes only partially accounts for deterioration of the membrane enzymes' activity. They showed that Bi initial acute effects in liver and kidney cells include deformation of mitochondrial membranes and inhibition of specific heme pathway enzymes. Both effects contribute to deterioration of membrane-associated enzymatic functions [55].

D. Bagchi and *colab*. investigated the effects of acute and chronic stress on the enhanced production of ROS; the precautionary ability of bismuth subsalicylate (BSS) was evaluated against the gastrointestinal mucosal injury induced by oxidative stress. Their findings revealed that BSS decreased chronic stress-induced lipid peroxidation, DNA fragmentation, and membrane microviscosity by approx. 40-50% in gastric and in the intestinal mucosa. It was found that oxidative stress produce gastrointestinal mucosal injury through improved production of ROS, and that BSS protect against gastrointestinal mucosal injury [56].

Manganese (Mn) is a chemical element from group 7, and period 4 (d-block); it is an essential dietary nutrient, but its excess lead to an accumulation with toxic effects (the manganism is a disease associated with Mn accumulation and it is due to ROS production. The bivalent ion is a central component of some enzymes and an activator of many metal-enzyme complexes. On the other hand, the trivalent ion is found in the essential enzymes manganese catalase and Mnsuperoxide dismutase (SOD), both of which break down oxidants using the Mn3+ in their reactive catalytic center. The bivalent ion (Mn2+) intends to bind to almost all Ca2+ and Mg2+ binding sites leading to substitutions of these ions in many biological processes; this is due to the similarities of their electron structure [57].

Arsenic (As) is a chemical element from group 15, and period 4 (p-block); it is contained in many minerals, but it also appear as a pure elemental crystal. As inhibits lipoic acid, which is a cofactor for pyruvate dehydrogenase into the citric acid cycle. Another important aspect is the fact that AsO4 3- decouples the oxidative phosphorylation leading to the inhibition of energy-linked reduction of NAD+ , mitochondrial respiration, and ATP synthesis. The produc‐ tion of H2O2 is also increased, which lead to ROS production and oxidative stress. The frequency of human cancers is increased in the case of long term exposure at As probably due to the ROS production [58].

Zhang Z and *colab*. showed that As can activate p47(phox) and p67(phox), proteins which activate NADPH oxidase and it generate ROS in DLD1 cells. It was found that tumor volumes of group treated with As were much larger than those without As treatment. Many researchers found that ROS have a role in the initiation of cellular injury induced by As, which can lead to cancer development. ROS induce direct cellular injury, which may start a set of radical reactions leading to an increase of secondary ROS generation. More than that, the increased ROS production may stimulate the inflammatory processes involving secretion of chemotactic factors, growth factors, proteolytic enzymes, lipoxygenases, and cyclooxygenase, inactivation of anti-proteolytic enzymes, and the release of signaling proteins. NADPH oxidase complex is an important physiological system for ROS production; As is highly capable of activating NADPH oxidase and disrupting of mitochondrias' membrane, leading to the generation of different ROS. It has been generally accepted that ROS are critical regulators for a wide range of cellular responses, from kinase activation, gene expression, DNA damage, cell proliferation, to cell migration in the arsenic treated cells [59].
