Oxidative stress and metal carcinogenesis.
ABSTRACT Occupational and environmental exposures to metals are closely associated with an increased risk of various cancers. Although carcinogenesis caused by metals has been intensively investigated, the exact mechanisms of action are still unclear. Accumulating evidence indicates that reactive oxygen species (ROS) generated by metals play important roles in the etiology of degenerative and chronic diseases. This review covers recent advances in (1) metal-induced generation of ROS and the related mechanisms; (2) the relationship between metal-mediated ROS generation and carcinogenesis; and (3) the signaling proteins involved in metal-induced carcinogenesis, especially intracellular reduction-oxidation-sensitive molecules.
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ABSTRACT: An extended exposure of the retina to visible light may lead to photochemical damage in retinal photoreceptor cells. The exact mechanism of retinal light damage remains unknown, and an effective therapy is still unavailable. Here, we demonstrated that rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), markedly protected 661W photoreceptor cells from visible light exposure-induced damage at the nanomolar level. We also observed by transmission electron microscopy that light exposure led to severe endoplasmic reticulum (ER) stress in 661W cells as well as abnormal endomembranes and ER membranes. In addition, obvious upregulated ER stress markers were monitored by western blot at the protein level and by quantitative reverse transcription-polymerase chain reaction (RT-PCR) at the mRNA level. Interestingly, rapamycin pretreatment significantly suppressed light-induced ER stress and all three major branches of the unfolded protein response (UPR), including the RNA-dependent protein kinase-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6) pathways both at the protein and mRNA levels. Additionally, the inhibition of ER stress by rapamycin was further confirmed with a dithiothreitol (DTT; a classical ER stress inducer)-damaged 661W cell model. Meanwhile, our results also revealed that rapamycin was able to remarkably inhibit the activation of mTOR and its downstream factors eukaryotic translation initiation factor 4E-binding protein 1 (4EBP1), p-4EBP1, p70, p-p70, and phosphorylated ribosomal protein S6 kinase (p-S6K) in the light-injured 661W cells. Thus, these data indicate that visible light induces ER stress in 661W cells; whereas the mTOR inhibitor, rapamycin, effectively protects 661W cells from light injury through suppressing the ER stress pathway.Brain research 03/2014; · 2.83 Impact Factor
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ABSTRACT: Oropharyngeal cancer is a significant public health issue in the world. The incidence of oropharyngeal cancer has been increased among people who have habit of chewing smokeless tobacco (SLT) in Pakistan. The aim of present study was to evaluate the concentration of nickel (Ni) in biological samples (whole blood, serum) of oral (n = 95) and pharyngeal (n = 84) male cancer patients. For comparison purposes, the biological samples of healthy age-matched referents (n = 150), who consumed and did not consumed SLT products, were also analyzed for Ni levels. As the Ni level is very low in biological samples, a preconcentration procedure has been developed, prior to analysis of analyte by flame atomic absorption spectrometry (FAAS). The Ni in acid-digested biological samples was complexed with ammonium pyrrolidinedithio carbamate (APDC), and a resulted complex was extracted in a surfactant Triton X-114. Acidic ethanol was added to the surfactant-rich phase prior to its analysis by FAAS. The chemical variables, such as pH, amounts of reagents (APDC, Triton X-114), temperature, incubation time, and sample volume were optimized. The resulted data indicated that concentration of Ni was higher in blood and serum samples of cancer patients as compared to that of referents who have or have not consumed different SLT products (p = 0.012-0.001). It was also observed that healthy referents who consumed SLT products have two to threefold higher levels of Ni in both biological samples as compared to those who were not chewing SLT products (p < 0.01).Environmental Science and Pollution Research 06/2014; · 2.76 Impact Factor
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ABSTRACT: Lead exposure is known to cause apoptotic neurodegeneration and neurobehavioral abnormalities in developing and adult brain by impairing cognition and memory. Coriandrum sativum is an herb belonging to Umbelliferae and is reported to have a protective effect against lead toxicity. In the present investigation, an attempt has been made to evaluate the protective activity of the hydroalcoholic extract of C. sativum seed against lead-induced oxidative stress. Male Wistar strain rats (100-120 g) were divided into four groups: control group: 1,000 mg/L of sodium acetate; exposed group: 1,000 mg/L lead acetate for 4 weeks; C. sativum treated 1 (CST1) group: 250 mg/kg body weight/day for seven consecutive days after 4 weeks of lead exposure; C. sativum treated 2 (CST2) group: 500 mg/kg body weight/day for seven consecutive days after 4 weeks of lead exposure. After the exposure and treatment periods, rats were sacrificed by cervical dislocation, and the whole brain was immediately isolated and separated into four regions: cerebellum, hippocampus, frontal cortex, and brain stem along with the control group. After sacrifice, blood was immediately collected into heparinized vials and stored at 4 °C. In all the tissues, reactive oxygen species (ROS), lipid peroxidation products (LPP), and total protein carbonyl content (TPCC) were estimated following standard protocols. An indicator enzyme for lead toxicity namely delta-amino levulinic acid dehydratase (δ-ALAD) activity was determined in the blood. A significant (p < 0.05) increase in ROS, LPP, and TPCC levels was observed in exposed rat brain regions, while δ-ALAD showed a decrease indicating lead-induced oxidative stress. Treatment with the hydroalcoholic seed extract of C. sativum resulted in a tissue-specific amelioration of oxidative stress produced by lead.Biological trace element research 05/2014; · 1.92 Impact Factor
Oxidative stress and metal carcinogenesis
Jeong-Chae Leea,b, Young-Ok Sona, Poyil Pratheeshkumara, Xianglin Shia,n
aGraduate Center for Toxicology, College of Medicine, University of Kentucky, Lexington, KY 40536, USA
bInstitute of Oral Biosciences (BK 21 program) and School of Dentistry, Research Center of Bioactive Materials, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
Received 1 May 2012
Received in revised form
31 May 2012
Accepted 2 June 2012
Available online 15 June 2012
Reactive oxygen species
a b s t r a c t
Occupational and environmental exposures to metals are closely associated with an increased risk of
various cancers. Although carcinogenesis caused by metals has been intensively investigated, the exact
mechanisms of action are still unclear. Accumulating evidence indicates that reactive oxygen species (ROS)
generated by metals play important roles in the etiology of degenerative and chronic diseases. This review
covers recent advances in (1) metal-induced generation of ROS and the related mechanisms; (2) the
relationship between metal-mediated ROS generation and carcinogenesis; and (3) the signaling proteins
involved in metal-induced carcinogenesis, especially intracellular reduction–oxidation-sensitive molecules.
& 2012 Elsevier Inc. All rights reserved.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743
Metals and ROS generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743
General sources of ROS production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743
Common mechanisms of metal-mediated ROS generation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743
ROS and antioxidant defense systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
ROS, oxidative stress, and diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
Hydroxyl radicals and oxidative stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
Oxidative DNA damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
Chronic and degenerative diseases induced by oxidative stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
Metal-induced oxidative stress and carcinogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745
Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745
Beryllium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745
Cadmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745
Chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746
Cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746
Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746
Iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
Selenium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
Vanadium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
Zinc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/freeradbiomed
Free Radical Biology and Medicine
0891-5849/$-see front matter & 2012 Elsevier Inc. All rights reserved.
Abbreviations: AP-1, activator protein-1; ARE, antioxidant response element; CAT, catalase; CBD, chronic beryllium disease; DFX, desferoxamine; EGF, epidermal growth
factor; ERK, extracellular signal-regulated kinase; ESR, electron spin resonance; GF, growth factor; GPx, glutathione peroxidase; GSH, reduced glutathione; HIF-1, hypoxia-
inducible factor 1; IL, interleukin; JNK, c-Jun-NH2-terminal kinase; MAPK, mitogen-activated protein kinase; MDA, malondialdehyde; NADPH, reduced nicotine adenine
dinucleotide phosphate; NFAT, nuclear factor of activated T cells; NF-kB, nuclear transcription factor kB; 8-OHdG, 8-hydroxydeoxy guanosine; PDGF, platelet-derived
growth factor; PI3K, phosphoinositide 3-kinase; PKC, protein kinase C; RNS, reactive nitrogen species; ROS, reactive oxygen species; SOD, superoxide dismutase; VEGF,
vascular endothelial growth factor; XO, xanthine oxidase.
nCorresponding author. Fax: þ1 859 323 1059.
E-mail address: firstname.lastname@example.org (X. Shi).
Free Radical Biology and Medicine 53 (2012) 742–757
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
Metals and redox-sensitive signal transduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
Growth factor receptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
Protein kinases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749
Nuclear transcription factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750
Membrane-bound G-protein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
Discussion and concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752
Carcinogenic metals present in occupational and general
environments are believed to be critical factors involved in the
increased incidence of cancers over the last half century .
Potential sources of metal exposure include groundwater con-
tamination, metal working, leather tanning, and mining [1–4].
In addition to environmental and occupational settings, a variety
of uses in medicine can result in exposure to different forms of
metals [5,6]. Many metals, such as arsenic (As), beryllium (Be),
cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury
(Hg), Nickel (Ni), and vanadium (V), are toxic even at low levels of
exposure [1,7–9]. These metals are known to induce cellular
damage, inflammation, and cancers mainly in the kidney, liver,
lung, prostate, and skin [8,10,11]. Even though metals, such as
copper (Cu), iron (Fe), selenium (Se), and zinc (Zn), are essential to
living organisms in trace amounts, chronic and extensive expo-
sure causes detrimental effects to tissues and organs, eventually
resulting in carcinogenesis [8,12,13].
Although the molecular mechanisms are not completely
understood, the potential of metals to generate reactive oxygen
species (ROS) and thus to alter cellular reduction–oxidation
(redox) states is considered the most important mechanism
involved in metal-induced carcinogenicity [11,13–16]. Recent
research suggests that chronic exposure to ROS causes oxidative
stress by disrupting the balance between the levels of ROS
produced and the potential of cellular antioxidant systems to
remove them. Prolonged and persistent oxidative stress causes
changes in cellular redox homeostasis and leads to abnormal
activation of redox-sensitive signaling molecules . Oxidative
stress also damages biomacromolecules, such as DNA, proteins,
and lipids, and eventually induces a variety of chronic and
degenerative diseases including cancer, cardiovascular disorders,
diabetes, rheumatoid arthritis, and Alzheimer’s and Parkinson’s
disease [18–20]. Most carcinogenic metals have been shown to
produce the superoxide anion radical (O2
(dOH) mostly via the Fenton reaction . Metal-induced ROS
production has also been implicated in the initiation of cellular
injury and the stimulation of inflammatory processes, which can
lead to cancer development . Further, oxidative stress causes
genetic and epigenetic changes, uncontrolled cell growth, and
abnormal cellular signaling, all of which are primary mechanisms
involved in metal-mediated carcinogenesis [22–24].
Accumulating evidence provides a correlation between metal-
induced oxidative stress and increased cancer risk. Due to the
increasing utilization of toxic metals in industry and medicine as
well as their inefficient recycling, environmental accumulation of
carcinogenic metals may result in subsequent increases in cancer
incidence [8,25]. This makes understanding the relationships
among metals, oxidative stress, and carcinogenicity of great
interest. Such knowledge could improve risk assessment and
the design of anticancer therapeutics. This review offers a brief
overview of the current knowledge regarding oxidative damage
d?) and hydroxyl radical
and carcinogenicity induced by metals. This review also covers
recent evidence for the involvement of oxidative stress in the
unregulated activation of redox-sensitive signal transduction and
gene expression, especially in metal-induced carcinogenesis.
Metals and ROS generation
General sources of ROS production
Cellular ROS can be distinguished by whether they are endo-
genously or exogenously generated. The mitochondrial respira-
tory chain, the cytochrome P450 metabolic pathway, and the
inflammatory responseare important
plexes I and III of the electron transport chain in mitochondria by
the addition of one electron to molecular oxygen. The mitochon-
dria produce approximately 2–3 nmol of superoxide/min per
milligram of protein [28,29]. This radical reacts with cellular
molecules to generate hydrogen peroxide (H2O2) as well as
reactive radicals such as hydroxyl radicals (dOH) and peroxyl
radicals (ROOd) [28,29]. Xanthine oxidase (XO), a highly versatile
enzyme, is also an important source of oxygen-free radicals .
XO catalyzes the reaction of hypoxanthine to xanthine and to uric
acid by forming O2
. Immune cells including macrophages and neutrophils, as
well as microsomes, generate intracellular ROS [31,32]. Peroxi-
somes are also capable of producing H2O2, but not O2
physiological conditions . In addition to these sources, various
chemicals and xenobiotics, such as chlorinated compounds, metal
ions, and radiation, are other important exogenous sources of
cellular ROS generation. While the generation of ROS induced by
endogenous sources is mostly related to normal metabolism
and/or functions of immune cells, exogenous sources, especially
metals, not only produce ROS directly and/or stimulate ROS
generation by endogenous sources, but also have toxic and
carcinogenic properties [8,11,15,22–25].
d?, the simplest form of ROS, is produced from com-
d?in the first step and H2O2in the second step
Common mechanisms of metal-mediated ROS generation
Metal ions produce intracellular ROS in a direct and indirect
manner, where the Fenton-type reaction is one of the most well-
known mechanisms. During this reaction, a transition metal ion
reacts with H2O2to generate the highly toxicdOH and an oxidized
Many metals, such as Fe(II), Cu, Cr(III), (V), and (IV), Co(II),
Ni(II), and V(IV), can generate free radicals via the Fenton-type
reaction, although their abilities to generate free radicals differ
. While the efficiencies of Co(II) and Ni(II) to generatedOH are
very low due to their high redox potentials, Fe(II) produces
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
the toxic radical more readily [34–36]. Neither the significance of
the Fenton-type reaction under physiological conditions nor
in vivo mechanisms by which free Fe or Cu ions mediate the
Another key mechanism in metal-induced ROS generation is
the Haber–Weiss reaction [37–39]. In this reaction, O2
dOH generation from H2O2and also participates in the reduction
of Fe(III) leading to the Fenton reaction.
dOH via the Fenton-like reaction are completely
metaln þ=metalnþ 1
The Haber–Weiss reaction can involve metals, such as Cr(III),
(IV), (V), and (VI), V(IV), and Co(I) and Co(II) [35,40,41]. The
Haber–Weiss-type mechanism ofdOH generation is likely to be
more predominant in vivo than the Fenton-type reaction, based
on ROS production in the immune function of macrophages
during phagocytosis [42,43]. It is commonly accepted that the
step to convert O2
ion is present as a catalyst for the Haber–Weiss reaction .
In vivo, intracellular free iron is present in very low quantities
under normal physiological conditions. However, the release of
excess free iron from iron-containing molecules can occur in
response to stress, which stimulates in vivo ROS production .
Without the process of the Fenton or Haber–Weiss reactions,
a few metal ions can generate cellular free radicals via a direct
reaction with cellular molecules. Reaction of Cr(VI) with cysteine
or penicillamine generates thiol radicals [46,47]. These radicals
can react with other thiol molecules to generate O2
d?to H2O2is too slow, unless a suitable metal
The generation of Cr or V intermediates by flavoenzymes,
glutathione reductase (GR), lipoyl dehydrogenase, and ferroxin-
NADPþreductase also causes the formation of O2
In this process, reduced nicotine adenine dinucleotide phosphate
(NADPH) functions as a cofactor. In addition, As leads to the
formation of free radicals by stimulating radical-producing sys-
tems within cells; at low levels it activates NADPH oxidase and
vascular endothelial cells .
d?in both vascular smooth muscle cells  and
ROS and antioxidant defense systems
A low level of ROS is constantly generated during normal
aerobic metabolism in living organisms . ROS overproduction
is primarily suppressed by endogenous antioxidants, such as
superoxide dismutase (SOD), catalase (CAT), and glutathione
peroxidase (GPx) as well as by exogenous secondary antioxidants,
such as vitamins and polyphenolic compounds. As the mitochon-
dria are the most potent ROS source, they are predominantly
enriched with abundant GSH and antioxidant enzymes within
both the matrix and the intermembrane space . While Cu,
Zn-SOD (SOD1) is present in the intermembrane space, Mn-SOD
(SOD2) is located in the matrix . The O2
mitochondrial inner membranes is detoxified initially to H2O2by
SOD and then to water by CAT.
d?formed in the
ROS, oxidative stress, and diseases
Hydroxyl radicals and oxidative stress
While the balance between cellular antioxidant defense and
ROS generation is maintained under normal metabolic conditions,
excessive ROS generation can cause oxidative stress. Among the
various types of ROS, H2O2is generated from almost all sources of
oxidative stress and can diffuse freely in and out of cells and
tissues. This indicates that H2O2 is a key oxygen metabolite
involved in oxidative stress. However, H2O2itself is not a reactive
radical, rather the agent is a precursor to the most highly toxic
dOH radical .
Intracellular biomolecules, such as protein kinases, lipids, and
DNA, are potential targets ofdOH [58,59]. In particular, DNA is
sensitive to damage from exposure to H2O2, which induces cell
death by apoptosis, pyknosis, and/or necrosis [60–62]. Numerous
oxidative DNA adducts have been identified and their frequency is
estimated to be 104lesions/cell/day in humans [58,59,63]. Thus,
oxidative DNA damage induced bydOH is a key factor influencing
the mutagenic and carcinogenic load in living organisms.
Oxidative DNA damage
Oxidative DNA damage can result in single- or double-strand
breakage, base modifications, deoxyribose modification, and DNA
cross-linking. DNA mutation, replication errors, genomic instabil-
ity, and eventual cell death also occur unless ROS-induced DNA
damage is repaired prior to DNA replication [63,64].
8-Hydroxydeoxy guanosine (8-OHdG) is the most abundant
mutagenic adduct produced by oxidative DNA damage in bacter-
ial and mammalian cells . Considerable evidence shows the
increase of 8-OHdG levels in various human cancers [66,67] and
in animal tumor models . An 8-OHdG-mediated transversion
of G:C to T:A is the most well-known event found in mutated
oncogenes and tumor suppressor genes . During DNA replica-
tion, the formation of 8-OHdG by the reaction of ROS with dGTP
mediates transversion of A:T to C:G . Thus, 8-OHdG is
considered a biomarker of ROS-induced DNA damage and is also
widely used to examine the mutagenic and carcinogenic loads
derived from oxidative stress [65,67].
Chronic inflammation can produce reactive nitrogen species
(RNS), which cause nitrative DNA damage leading to the forma-
tion of 8-nitroguanine . As 8-nitroguanine, a mutagenic DNA
lesion, is known to lead to G-T transversion, its levels are also
used to evaluate the conditions of oxidative DNA damage [72,73].
Chronic and degenerative diseases induced by oxidative stress
Genetic alterations, chronic inflammation, and malignant
transformation are the main events leading to cancer and all of
these conditions are associated with excessive ROS generation
and the resulting oxidative stress. The ulcerative colitis-mediated
incidence of colorectal cancer is a good example of the association
among chronic inflammation, ROS production, and carcinogeni-
city [74,75]. Oxidative stress also causes lipid peroxidation and
the formation of reactive aldehydes , although there are no
conclusive findings to support the involvement of these alde-
hydes in human mutagenesis. In addition, chronic degeneration in
neurons is caused by oxidative damage to protein-coding or -
noncoding RNA and by eventual dysregulation of gene expression
and protein synthesis . Thus, prolonged and excessive oxida-
tive stress mediates a variety of chronic and degenerative diseases
including cancers, inflammation, aging, and neuronal disorders.
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
Metal-induced oxidative stress and carcinogenesis
Although the mitochondria are the main endogenous source of
ROS generation, metals are also important sources of ROS. Numer-
ous studies have focused on elucidating the mechanisms involved in
metal-induced toxicity and carcinogenicity and demonstrated that
metals play critical roles in generating ROS and RNS in biological
systems and in modulating basic cellular events in living organisms
[78–80]. Highly toxic
comprises the majority of ROS generated by metals [78,81].
Metals and/or metal-induced ROS cause various modifications
to DNA bases, enhanced lipid peroxidation, and changes in
calcium and sulfhydryl homeostasis, which facilitate carcinogenic
responses. Recent emerging evidence indicates that occupational
and environmental pollutants affect genes specific to relevant
xenobiotic metabolizing enzymes and alter the expression of
downstream target genes, thus increasing the risk of cancer. This
section discusses recent advances in knowledge, emphasizing the
involvement of metals and/or metal-induced oxidative stress in
increased cancer risk and the mechanisms involved.
dOH derived from the breakdown of H2O2
Arsenic has been found in many natural sources, such as air, soil,
water, and food. It exists mainly in two oxidative forms, trivalent
arsenite [As(III)] and pentavalent arsenate [As(V)]. In combination
with other elements such as oxygen, sulfur, and chlorine, this element
is referred to as inorganic arsenic, and when combined with hydrogen
and carbon, it is known as organic arsenic. While arsenic trioxide
(As2O3) is the most prevalent form of inorganic arsenic found in the
air, a variety of inorganic arsenates (AsO4
present in water, soil, or food [82,83]. Epidemiological studies have
shown that chronic exposure to As or its compounds at low doses is
linked to an increased risk of lung, skin, liver, bladder, kidney, and
brain cancers, in addition to cardiovascular disorders, peripheral
neuropathy, coronary disease, and anemia [84–86]. Inorganic As is
known to be more toxic than methylated organic As . The
trivalent [As(III)] forms are the most toxic and react with thiol groups
of proteins, while the pentavalent [As(V)] forms are less toxic. The
toxicity of pentavalent inorganic As is as a result of its reduction to
trivalent As .
Oxidative stress has long been considered an important mechan-
ism of As-induced toxicity and carcinogenicity. Many studies have
reported the generation of various types of ROS during As metabo-
lism in cells, as well as the activation of redox-sensitive signal
transduction [89,90]. Electron spin resonance (ESR) spin trapping
As-treated cells . Arsenite toxicity in the brain is also believed
to be connected to the generation ofdOH . These results suggest
that As increases the production of H2O2and subsequently the toxic
dOH radical. The production of H2O2via the oxidation of As(III) to
As(V) is believed to be an important source ofdOH radicals under
3?) or arsenites (AsO2
cellular levels ofH2O2
In addition todOH, RNS are thought to be directly involved in
oxidative damage to biomolecules in As-exposed cells . The
element inhibited the synthesis of GSH, one of the most powerful
cellular antioxidants. All of these results strongly suggest the involve-
ment of oxidative stress in the As-induced increased risk of cancer
Be has only limited industrial applications due to its material
properties and approximately 20% of the world’s production is
used in nuclear reactors . The major sources of Be exposure in
occupational and environmental contexts are extraction plants
and ceramic plants. Acute and excessive exposure to soluble Be
compounds irritates the entire respiratory tract and causes con-
tact dermatitis . It may also induce acute pneumonitis and
pulmonary edema . Chronic exposure to insoluble Be com-
pounds leads to chronic Be disease (CBD) and this is the most
common health problem caused by Be [97,98]. Although current
epidemiologic data are insufficient to confirm Be-mediated
human carcinogenesis, the accumulated data strongly suggest
that Be is associated with human cancer.
Be is believed to stimulate the formation of ROS, leading to Be-
induced macrophage apoptosis
mechanisms of Be-induced toxicity have yet to be elucidated,
macrophage apoptosis is thought to contribute to the metal-
induced CBD [99,100]. Be is also suspected to induce oxidative
stress through the depletion of endogenous thiol antioxidants and
the subsequent increase in ROS generation . Be-stimulated
proliferation of blood CD4þT cells appears to be related to
increased ROS levels . These findings suggest that oxidative
stress specific to CD4þT cells is associated with chronic inflam-
mation induced by Be. Cellular responses, such as caspase-3
activation and interferon-gamma and interleukin (IL)-2 increases,
are associated with Be-induced apoptosis . More detailed
experiments will be needed in order to understand the possible
mechanisms of Be-induced ROS generation as well as the exact
mode of action of the element in terms of carcinogenicity.
Cd is a toxic heavy metal that is widely distributed in the
earth’s crust, air, and water. Major sources of Cd exposure are
food, cigarette smoke, and Cd-related industries [78,104]. Cd has a
very long biological half-life, around 20–35 years. The human
body does not contain an excretion pathway for the element. This
results in its accumulation and the attendant toxicity to cells and
tissues. Cd is accumulated primarily in the lungs, liver, pancreas,
and kidney and the skin is also easily exposed to the toxic metal
[105,106]. Cd is predominantly bound to metallothioneins in the
body  and this is believed to be critical for preventing Cd
toxicity . The Cd–metallothionein complex is distributed to
various tissues and organs and is ultimately reabsorbed in the
ROS-induced oxidative damage is believed to be an important
mechanism of Cd-mediated toxicity in various types of tissues
and organs . While Cd itself is thought to be unable to
generate free radicals directly, indirect ROS production has been
reported in Cd-exposed cells . Some experiments revealed
the generation of H2O2by Cd, indicatingdOH-mediated oxidative
stress via a Fenton-type reaction. It was recently supported that
H2O2, but not O2
in Cd-exposed cells . The ability of Cd to replace iron and
copper in various cytoplasmic and membrane proteins, thereby
stimulating the Fenton reaction, has been proposed as an indirect
mechanism of Cd-induced ROS generation . ROS generation
induced by Cd also activates the mTOR pathway, leading to
neuronal cell death . Decreased SOD activity and increased
malondialdehyde (MDA) were found in mouse testis after Cd
exposure . Cd treatment activates the mitogen-activated
protein kinase (MAPK) pathway via production of ROS and
inhibition of protein phosphatases 2A and 5 . These reports
strongly suggest that Cd not only increases ROS generation, but
also dysregulates the primary antioxidant systems, which might
mediate cellular DNA damage, leading to an increased risk of
cancer. Moreover, Cd disrupts cell–cell adhesion, and this is an
important event in Cd-stimulated tumor induction and promotion
d-, plays an important role in mediating apoptosis
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
. There are many studies suggesting possible clinical appli-
cations of exogenous antioxidants in the treatment of toxicity
induced by Cd exposure. For example, either ascorbic acid or
a-tocopherol was found to reduce the level of ROS-initiated
testicular damage and their combined administration restored
normal testicular function in Cd-supplemented rats .
A combination of quercetin and a-tocopherol ameliorated
Cd-induced oxidative stress and alterations in lipid metabolism
in Cd-intoxicated rats . Treatment with coenzyme G(10) and
a-tocopherol reversed Cd-induced alterations of the antioxidant
defense system and significantly prevented Cd-induced testes
damage in rats .
Cr and Cr-containing compounds are widely used in industry
for the production of stainless steel, chromate, chromium plating,
and spray painting. The health effects and toxicity/carcinogenicity
of Cr are primarily related to its oxidation state at the time of
exposure. The trivalent [Cr(III)] form in low doses is an essential
dietary mineral and found in drinking water as well as in most
fresh foods, including breads, meats, and vegetables . Cr(III)
deficiency in humans has been associated with cardiovascular
disease and diabetes [122,123]. Chromate compounds having the
hexavalent [Cr(VI)] form are well known to be relatively more
toxic and carcinogenic in high doses than other forms [124,125].
The carcinogenicity induced by a prolonged exposure of Cr(VI) is
site specific, mainly to the lungs and nasal cavity [126,127].
In addition to their carcinogenic effects, Cr(VI) compounds can
cause ulcers and allergic symptoms on the skin [128,129].
The toxicity/carcinogenicity of Cr compounds has motivated
the extensive exploration of the cellular mechanisms of Cr(VI)-
induced carcinogenesis. Cr(VI) enters cells through an anion
transport system . Upon entry, Cr(VI) is reduced by cellular
reductants such as GSH, ascorbate, and NADPH, to its lower
oxidation states, Cr(V) and Cr(IV) [131,132]. These intermediate
states of Cr are reactive and participate in the production of ROS
, thereby causing several types of oxidative DNA damage,
such as base modification, single-strand breaks, DNA–DNA inter-
adducts, and oxidative nucleotide changes, as well as chromoso-
mal aberrations and lipid peroxidation [134,135]. These findings
indicate that the key process involved in Cr-mediated carcino-
genicity is the reduction of Cr(VI) to Cr(V) and Cr(IV) by cellular
There are also many reports demonstrating that Cr(VI) induces
cell death by oxidative stress [132,136]. Our research group,
as well as other groups, have shown that Cr(VI) generatesdOH,
[132,134,137]. SOD did not protect cells from Cr(VI)-mediated
toxicity, whereas CAT and desferoxamine (DFX) significantly
inhibited Cr(VI)-induced cell death . It has been suggested
thatdOH, which is produced from H2O2by the Fenton reaction, is
a direct mediator of Cr(VI)-induced toxicity, where O2
participate both in the production of H2O2 and in part in
the conversion of H2O2 to
Consequently, the carcinogenic role of Cr(VI), evidenced by the
induction of chromium–DNA adducts and the production of muta-
tion-inducing DNA damage, might be derived from its redox cycling
activity . The schemes below describe ROS production, especially
dOH, through reactions of Cr(VI) with biological reductants.
d?, and H2O2 via both Haber–Weiss and Fenton reactions
dOH by the Haber–Weiss reaction.
Co, a natural trace element, forms a number of organic and
inorganic salts with the most stable oxidation numbers being
trivalent [Co(III)] and divalent [Co(II)]. Co is found in soil, water,
plants, and animals in many different chemical forms . It is
essential to humans as it is a necessary component of vitamin B12
. However, at higher concentrations, the element may have
carcinogenic properties. Recent studies have demonstrated the
carcinogenicity of Co powder in experimental models [140,141].
Inhalation of Co at high concentrations affects mainly the lungs,
leading to asthma, pneumonia, wheezing, and cancer .
The carcinogenic effects of Co are based on its ability to inhibit
DNA repair mechanisms and to induce DNA damage and DNA
exchange between sister-chromatids and aneuploidy, and ROS are
considered to play key roles in these processes . The first
step in Co-induced ROS generation is the formation of the radical
intermediate, Co(I)–OOd, from the reaction of [Co(0)] with oxy-
gen, as illustrated in Reactions (13)–(15) [78,90]. Second, SOD
catalyzes the decomposition of Co(I)–OOdspecies to H2O2 and
Co(I), where H2O2 is produced from O2
reaction and also by one-electron reduction of molecular dioxy-
gen catalyzed by Co . ESR spin trapping experiments indicated
that the Fenton reaction was responsible for the generation of
both Co(I) and Co(II) forms . Under physiological conditions,
the catalytic activity of the divalent Co(II) form depends on
cellular chelators, where GSH, cysteine, or NADPH participates
in the generation ofdOH and other oxygen- and carbon-centered
d?via the dismutation
In addition to ROS generation according to the reactions
shown above, Co(II) exposure can deplete intracellular ascorbate
, indicating that Co compounds are able to change the
intracellular redox state via an interaction with ascorbate.
There are also contrasting findings showing the beneficial roles
of Co. Shukla et al.  revealed that hypoxic preconditioning
with Co diminishes hypobaric hypoxia-induced oxidative damage
in rat lungs. Co(II) b-ketoaminato complexes exhibited a prospec-
tive pharmacological benefit as novel inhibitors of neuroinflam-
significantly increased serum leptin, adiponectin, and high-den-
sity lipoprotein levels and normalized the glucose level and
adipose cell size in mice fed a high-fat diet, suggesting a new
potential application of Co in the adipose tissue of patients with
obesity-related diseases . However, most studies emphasize
the fact that Co at higher concentrations is toxic and carcinogenic,
with oxidative stress playing a crucial role. Further detailed
experiments will be needed to evaluate the clinical implications
of Co compounds.
Cu is an essential trace element, and Cu(I) and Cu(II) are the
primary forms found in living organisms. Cu is a cofactor of many
enzymes, such as cytochrome c oxidase, ascorbate oxidase, or
SOD, all of which are involved in cellular redox mechanisms.
In addition to its enzymatic roles, Cu is used in biological systems
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
for electron transport. Cu is readily absorbed from the diet across
the small intestine (?2 mg/day) and stored in the liver via the
high-affinity human copper transporter, hCtr1 [148,149]. Roughly
80% of Cu excretion is via the biliary pathway. Once entered into
the cell, Cu is escorted to a (i) metallothionein pool, (ii) trans-
ported to the mitochondria for cytochrome c oxidase incorporation
for delivery to emerging Cu, Zn-SOD, or (iii) transported to the
Wilson disease P-type ATPase in the trans-Golgi network [148,149].
Many studies have shown higher Cu levels in both serum and tumor
tissues in cancer patients compared to their healthy counterparts.
Cancer patients suffering from breast, cervical, ovarian, lung, pros-
tate, stomach cancer, or leukemia were found to exhibit relatively
high Cu levels . Cu is also linked to Alzheimer’s disease,
Parkinson’s disease, and other neurodegenerative diseases with
increased oxidative stress in the brain , as well as chronic
diseases, such as diabetes , cardiovascular disease , and
ROS generation by Cu is one of the most well-defined mechan-
isms in metal-induced free radical generation. Cu can induce
oxidative stress by two mechanisms; first, it can catalyze ROS
formation via the Fenton or Haber–Weiss reaction [8,153];
second, exposure to elevated levels of Cu significantly decreases
GSH levels . Cupric ion [Cu(II)] can be reduced to cuprous ion
[Cu(I)] in the presence of O2
ascorbic acid, NADPH, or GSH. This ion catalyzes the formation of
dOH through the decomposition of H2O2via the Fenton reaction
. Cu is capable of causing DNA strand breaks and oxidation of
bases mainly by a site-specific Fenton reaction .
d?or biological reductants, such as
Cu causes lipid peroxidation, whereas ascorbic acid or a-toco-
pherol inhibits such oxidation by preserving cellular antioxidant
activity associated with this lipoprotein fraction . Thus, in the
presence of excess free Cu ions, vitamins may act predominantly as
antioxidants to scavenge ROS or protect cells against oxidative
damage. Although the combination of ascorbate, Cu, and H2O2is
widely used as an efficientdOH generating system, where ascorbate
acts as an electron donor , ascorbate might act as an antiox-
idant preventing lipid peroxidation and protein oxidation even in
the presence of redox-active Cu and H2O2[159,160]. Collectively,
effective Cu(II) chelation or elevated cellular antioxidation is the
most useful way of treating Cu-induced carcinogenicity, neurode-
generative disorders, and chronic diseases.
Iron is present primarily in two oxidation states, ferrous ions
[Fe(II)] and ferric ions [Fe(III)]. Ferrous ions are soluble in
biological fluids and react with molecular oxygen to form ferric
ions and free radicals. The oxidized form of iron, Fe(III), is
insoluble in water at neutral pH and precipitates in the formation
of ferric hydroxide . Like Cu, Fe is thought to affect glucose
metabolism  and insulin sensitivity . While homeo-
static mechanisms prevent excessive Fe uptake and regulate the
rate of Fe release involved in recycling, overload of this metal in
the body causes severe damage to organs and tissues, leading
to hypoxia, inflammation, and cancer [164,165]. Oxidative
damage-induced changes in genetic material are the initial step
involved in Fe-induced mutagenesis and carcinogenesis .
In addition, Fe metabolism is involved in the etiology of hyper-
tension, atherosclerosis, neurodegenerative disorders, and rheu-
matoid arthritis [167,168].
Similar to Cu, the generation of ROS via a Fenton-type reaction
is the most important mechanism of Fe-mediated toxicity [8,78].
Fe(II) present in the cells catalyzes the production of highly toxic
dOH via the Fenton reaction from H2O2produced mainly by the
dismutation of SOD . As inefficient chelation of Fe can result
in the formation of toxic free radicals, proper chelation is critical
for preventing its harmful effects . Recent investigations
have focused on the development of novel Fe chelators, where
di-2-pyridylketone thiosemicarbazone and 2-benzoylpyridine thio-
semicarbazone were found to have potential applications in the
treatment of cancer . There are also many kinds of synthetic
and natural products with the potential to serve as iron chelators
[171,172]. Taken together, the results of these studies suggest that
Fe-overload diseases can be controlled efficiently by treating with
Pb is a heavy metal and has been widely used in various
industrial applications because of its unusual physico–chemical
properties . Similar to As, Cd, and Hg, Pb is a persistent toxic
metal to living organisms and persists for long periods in water,
soil, and dust, as well as in manufactured products containing the
metal . Paints or soil containing Pb pose more serious health
risks to children than adults because gastrointestinal absorption
of Pb is higher in children (40–50%) than in adults (3–10%)
[174,175]. Concentrations of 10 mg/dl Pb (equivalent to 0.48
mmol/L) or higher in the blood are considered toxic and can result
in cancer, cardiovascular disease, neurological disorders, cognitive
impairments, hypertension, and other disorders [175–177].
Similar to other carcinogenic metals, Pb damages cellular
components by inducing oxidative stress . One of the main
mechanisms of Pb-mediated oxidative damage is due to the direct
formation of ROS including singlet oxygen, H2O2, and hydroper-
oxides . Another mechanism is involved in Pb-mediated
depletion of the cellular antioxidant pool; Pb not only decreases
GSH level and GR activity but also inhibits recycling of oxidized
glutathione and reduces CAT, SOD, and GPx levels [180,181].
This mechanism is also related to the ability of Pb to enhance
the toxic potential of other carcinogenic metals. Further, these
two mechanisms can occur simultaneously, leading to an increase
in ROS and depletion of cellular antioxidant potential . In
addition, the formation of RNS, such as highly reactive peroxyl
nitrite (ONOO?), has been shown to play a significant role in
the increased incidence of Pb-mediated diseases . While
Pb-induced toxicity is involved in the formation of reactive
oxidants, a large number of exogenous antioxidants have been
found to diminish Pb-induced disease development [81,184,185].
Chelators specific to Pb are used for the medicinal treatment of Pb
toxicity [186,187]. Collectively, efficient chelation of the element
and/or supplementing antioxidative materials is the preferred
medical treatment for reducing various toxic and carcinogenic
effects followed by Pb exposure.
Ni is an essential trace nutrient in plants and several animal
species. Ni and its compounds are widely used in industry. Ni
compounds include insoluble Ni compounds, such as Ni sulfide
(NiS), Ni subsulfide (Ni3S2), and Ni oxide (NiO), and soluble Ni
salts, such as Ni acetate, Ni sulfate (NiSO4), and Ni chloride
(NiCl2). Epidemiological studies of workers at Ni refineries sug-
gest a close relationship between Ni exposure and an increased
risk of lung and nasal cancer . Most Ni compounds also have
strong oncogenicity in animal models.
Although Ni produces relatively low levels of ROS in cells
compared with other redox-active metals, including Cu, Fe, and
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
Co , accumulating evidence has implicated the critical roles of
ROS in Ni-mediated carcinogenesis . Actually, Ni(II) chloride
or Ni3S2was found to enhance the formation of ROS within cells
. Ni also diminished cellular GSH levels in cells, suggesting
the disruption of antioxidant defense systems in Ni-mediated
toxicity. The oxidative effects of Ni depend on its ability to form
the Ni(III)/Ni(II) redox couple because Ni(II) forms square planar
Ni complexes with peptides or proteins .
The water-insoluble Ni particles (e.g., Ni3S2and NiO) enter the
cell via phagocytosis and Ni(II) is then released from the phago-
cytic vesicles into the cytoplasm and the nucleus. Ni(II) from
soluble compounds (e.g., NiSO4, NiCl2, Ni(II) acetate) is trans-
ported into the cell via Ca2þ
transporter system DMT-1 (Nramp 2), and diffusion. Once Ni(II)
enters the cells, it forms a variety of complexes with different
ligands and catalyzes
The nuclear Ni(II) and Ni(II)-generated ROS interact with DNA
and histones, resulting in promutagenic DNA damage. The reac-
tive intermediates, such as Ni3S2and NiS, are sensitive to oxida-
tion by ambient oxygen, which facilitates their dissolution in
biological fluids and also leads to intracellular Ni(II) formation
[189–191]. Consequently, epigenetic alterations, disruption of
cellular iron homeostasis, and activation of the hypoxia signaling
pathway, all of which are closely associated with oxidative stress,
are considered to be the major molecular mechanisms involved in
Ni-induced carcinogenesis .
channels, the divalent cation
dOH production from H2O2 [189,190].
Se, an essential trace element, is widely used in industry. It is
an integral component of GPx, where the complex of Se–cysteine
is the active site of the enzyme. Based on its beneficial roles, Se is
believed to be required for human health. Numerous studies have
shown the positive effects of Se compounds and selenoproteins in
the prevention of several cancers . The chemopreventive
potential of Se compounds on cancers is related to their ability to
regulate cell cycle progression, to stimulate apoptosis, and to
inhibit tumor cell migration and invasion . Se also protects
against cell death induced by toxic metals by decreasing oxidative
stress or regulating signal transduction pathways involved in
While many studies emphasize the preventative effect of Se on
cancers due to its antioxidant properties, excessive exposure to
the element irritates the eyes, skin, nose, and throat, resulting in
liver atrophy, necrosis, and hemorrhage [185,195]. Moderate
genotoxic activity of Se compounds, such as selenite, selenate,
selenide, selenocysteine, and selenosulfide, has also been reported
. In addition, chromosome damage was observed in bone
marrow  and in human peripheral lymphocytes after
selenium salt treatment . One report revealed that low
levels of Se compounds are selectively toxic to a human neuron
cell line by inducing an increase in ROS and activation of the
apoptotic process . Therefore, excess exposure to Se or its
compounds is thought to be toxic and carcinogenic in that it
stimulates ROS formation and oxidative damage, although it is an
essential trace element required for antioxidative protection of
V is a major transition element that is released primarily by
the burning of fossil fuels, including petroleum, oil, coal, tar,
bitumen, and asphaltite. The cytotoxicity of V was found to be
more likely to cause DNA damage and cell death than Co, Cr,
molybdenum, and Cu . Among V compounds, V pentoxide is
highly toxic . Similar to Ni, the pentavalent forms, such as V
and vanadate, have carcinogenic potential via ROS generation,
DNA damage, and activation of hypoxia signaling . ROS
generation and MAPK phosphorylation have been shown to play
important roles in cell cycle arrest induced by vanadate .
These results suggest that oxidative damage is closely related to
vanadate-mediated toxic and carcinogenic processes.
Similar to Ni, several mechanisms are involved in V-mediated
ROS generation within cells. Upon entering the body, vanadate is
rapidly reduced to V(IV) in the plasma by reductants such as
NADPH and ascorbic acid. The reduced vanadium is bound to
plasma proteins and mediates the generation of toxic radicals,
such as peroxovanadyl radicals [V(IV)–OOd] and vanadyl hydro-
peroxide [V(IV)–HOd] . The following reactions may take
place inside the cells in the presence of V .
V(IV) also mediates ROS generation from H2O2 and lipid
hydroperoxide via a Fenton-type reaction and this is the major
mechanism explaining V(V)-induced cellular injury under phy-
siological conditions . These findings strongly suggest that V
might be cytotoxic and carcinogenic due to its generation of ROS
and the resulting oxidative damage.
Zn is an essential trace element in humans. While the amounts
of Zn in adults range from 1.5 to 2.5 g, the intracellular level of the
divalent [Zn(II)] form, the most common and stable oxidation
state, is estimated to be 0.5 nM or lower [78,81]. Zn is present in
all organs and tissues in the form of complexes with over 70
different enzymes involved in the cellular metabolism of proteins,
lipids, and carbohydrates. Zn not only enhances the action of
insulin and manages blood glucose concentration, but also plays
an essential role in the development and maintenance of the
immune system . In contrast, Zn deficiency causes growth
retardation and hypogonadism, loss of appetite, dermatitis,
reduced taste acuity, delayed wound healing, impaired reproduc-
tion, and poor immune function . Zn deficiency is related to
poor dietary intake, excessive dietary phytate intake, chronic
illness, or oversupplementation with F or Cu.
Unlike other carcinogenic metals, Zn deficiency, rather than
excessive exposure, is often implicated in detrimental effects
including oxidative damage to biomolecules such as lipids,
proteins, and DNA. This is because Zn is a redox inert metal and
thus does not participate in redox reactions . Rather, Zn
protects sulfhydryl groups of proteins against free radical attack
and reduces free radical formation by inhibiting redox-active
transition metals, such as iron and copper . Zn was found
to inhibit DNA strand breaks and ROS formation and also resulted
in increases in GSH levels in cells and other experimental systems
[206,207]. Many studies using Zn-deficient animals showed an
increase in ROS production, lipid and lipoprotein oxidation, and
MDA generation, thereby supporting the correlation of Zn defi-
ciency with oxidative damage . Taken together, these findings
indicate that Zn could act as a protective antioxidant in chronic
diseases, cancers, and metabolic disorders. The protective action
of Zn is derived from its ability to inhibit NADPH oxidase and
induce metallothionein, in addition to its antioxidant potential on
the integral Cu metal, Zn-SOD, and the suppression of inflamma-
tory responses via up-regulation of Zn-finger proteins [208,209].
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
While many metals play essential roles in humans, excessive
exposure or deficiency can cause dysregulation of basic cellular
functions, eventually leading to degenerative and chronic dis-
eases. Physiological disorders caused by metal exposure are
closely related to the generation of ROS and subsequent oxidative
stress, which is thought to be the main mechanism involved in
metal-induced carcinogenicity. Fig. 1 summarizes the types of
metal-induced oxidative damage and carcinogenicity and the
Metals and redox-sensitive signal transduction
Carcinogenic metals increase cellular ROS levels via various
mechanisms, as described above. ROS can act as signal transduc-
tion messengers and/or activators of redox-sensitive signal trans-
duction pathways, affecting the expression of genes that regulate
a variety of cellular events. Oxidative stress can cause dysregu-
lated gene expression by aberrant activation of signaling
molecules and this is believed to be a potential mechanism of
metal-induced carcinogenicity. Although the molecular mechan-
isms by which metals act as carcinogenic have not been com-
pletely explored, numerous studies have proposed that metals
modulate a diversity of signaling molecules, including growth
factor (GF) receptors, proteins kinases, and nuclear transcription
factors. In this regard, this section discusses recent advances in
the characterization of the cellular mechanisms related to metal-
induced carcinogenesis, focusing on the roles of redox-sensitive
Growth factor receptors
GF receptors play key roles in signal transmission from outside
the cell into the cytoplasm and the nucleus. Among GF receptors,
epidermal growth factor (EGF) receptor, platelet-derived growth
factor (PDGF) receptor, and vascular endothelial growth factor
(VEGF) receptor have shown to be sensitive to carcinogenic metal
exposure. Many metals such as As, Be, Cd, Co, Ni, and Pb have
been shown to differentially affect these receptor pathways and
the expression of their protein components [8,14,210]. As these
receptors can act as upstream effectors of various signaling
molecules involved in cell proliferation, differentiation, and sur-
vival, changes in their expression as a result of metal exposure are
of great importance when considering targets for controlling
There are many types of protein kinases within cells, where
MAPKs, phosphatidylinositol 3-kinase (PI3K)/Akt pathway, pro-
tein kinase C (PKC), and Src are the critical kinases involved in the
signal transduction induced by metals. MAPKs are serine/threo-
nine kinases consisting mainly of c-Jun-NH2-terminal kinase
(JNK), extracellular signal-regulated kinase (ERK), and p38 MAPK.
These kinases play critical roles in signal transduction pathways
by acting as upstream mediators of GF receptors, G-proteins, and
tyrosine kinases, and as downstream mediators of transcription
factors. Thus, dysregulation of MAPK function is closely related to
carcinogenicity . Almost all metals noted in this review have
been reported to affect MAPK signaling. For example, As(III) can
activate all three classes of MAPKs in a variety of human cell lines,
whereas the resulting effects differ according to the time, dose,
and oxidative form of the element, and the type of target cell .
Similarly, other metals such as Be, Cd, Co, Cr(VI), and Ni, and their
compounds, act as MAPK activators, although down- or upstream
effectors of MAPKs differ depending on the exposure conditions
The PI3K/Akt pathway interacts with several genes and recep-
tors involved in metal-induced carcinogenesis . Similar to
MAPK pathways, the activation of PI3K and its downstream
mediator, Akt, involves numerous phosphorylation events. Akt
activation results in the subsequent activation of signaling path-
ways involved in cell growth and the inhibition of apoptotic
pathways . Metals including Cu(II), Zn(II), and Ni(II) have
been shown to activate the PI3K/Akt pathway via ROS- and non-
ROS-dependent mechanisms in various types of cells . There
have also been reports of metals showing opposite effects on the
PI3K/Akt pathway depending on the cell type studied [216,217].
The PI3K pathway also appears to be associated with the
CdCl2-induced phosphorylation of p53 in MCF7 cells and with
the CoCl2-induced VEGF transcription in HepG2 cells .
In addition, vanadate-mediated alterations of cell cycle progression
Fig. 1. General scheme of metal-induced reactive oxygen species (ROS) formation and increased carcinogenic risk.
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
and hypoxia-inducible factor (HIF) expression are affected by the
PI3K/Akt pathway [219,220].
PKC, a family of serine/threonine kinases, induces the phos-
phorylation of other proteins, thereby regulating a variety of cell
functions including proliferation, cell cycle, differentiation, cytos-
keletal organization, cell migration, and apoptosis . ROS
induce the release of calcium from intracellular stores resulting
in the activation of PKC. The activation of PKC is also required for
generating ROS as well as for the ROS-mediated migration of cells
 and HIF-1a activation . Thus, uncontrolled activation of
PKC is a potential mechanism of metal and/or ROS-induced
carcinogenesis. It is known that nuclear accumulation of Nrf2 by
Cd involves PKCd activation in human astrocytoma cells . Ni
induced the generation of O2
led to NF-kB activation and IL-8 production via activation of PKC in
human neutrophils . PKC is also involved in As(III)-induced
apoptosis and inhibits proliferation of human bladder cancer cells
 and in Fe-dependent oxidative stress with the subsequent
development of neurodegenerative diseases . Lead acetate
causes immediate activation of the EGF receptor, leading to
activation of the Src family tyrosine kinases, PKCa and the Ras-
Raf1-ERK1/2 signaling cascade . Activation of PKC is also
required for V-induced phosphorylation of Akt in mouse epidermal
JB6 cells .
Src is a nonreceptor tyrosine kinase consisting of the p56lck,
p59fyn, or p53/56lyn protein and is commonly overexpressed in
cancers of the colon, breast, pancreas, bladder, head, and neck
. Activated Src binds to cell membranes by myristoylation
and initiates MAPK, nuclear transcription factor-kB (NF-kB), and
PI3K signaling pathways . While ROS and ultraviolet (UV)
radiation are known as critical activators of Src, several metals
have been shown to activate Src. As(III) induced the activation of
EGF and ERK, and Cr compound-induced ROS formation, apopto-
sis, and JNK activation are also known to be dependent on the
Src . Environmental toxins such as Cd, mercury, biophenol, and
dioxin, are known to cause male infertility by inducing oxidative
stress and cell junction disruption, which is tightly regulated by
the activation of PI3K/c-Src/focal adhesion kinase and MAPK
signaling pathways .
d–, H2O2, and hypochlorous acid and
Nuclear transcription factors
Metals regulate a variety of cellular events by affecting gene
transcription via the expression and activation of numerous
signaling proteins. Nuclear transcription factors including activa-
tor protein-1 (AP-1), HIF-1, nuclear factor of activated T cells
(NFAT), NF-kB, Nrf2, and p53 are critical for the transduction of
signals into the nucleus and activation of target-specific gene
expression in responses to metal exposure. AP-1 was the first
nuclear transcription factor found to contribute to both basal
gene expression and phorbol ester-inducible gene expression
. AP-1 is a homodimerized or heterodimerized protein with
the Jun (c-Jun, JunB, JunD) and Fos (c-Fos, FosB, Fra-1, Fra-2)
subfamily proteins. AP-1 functions as a transcriptional regulator
in signal transduction processes leading to transcription, prolif-
eration, or apoptosis. Oxidative stress stimulates this factor as
well as the genes encoding AP-1 . As AP-1 is sensitive to the
intracellular redox state, most carcinogenic metals, such as As, Be,
Cd, Cr, Co, Cu, Fe, Pb, Ni, and V, have been shown to affect gene
expression via the activation of AP-1 . Among numerous
signaling proteins, MAPKs are known as key upstream effectors
of AP-1, although the patterns of AP-1 activation differ according
to the redox cycling activity of metals to generate ROS or the cell
types examined [211,212].
HIF-1 is a transcription factor composed of heterodimers of
two subunits, HIF-1a and HIF-1b. HIF-1a is a unique subunit
tightly regulated in response to hypoxia, whereas HIF-1b is
expressed constitutively in all types of cells . HIF-1 regulates
the expression of many cancer-related genes including VEGF,
heme-oxygenase 1, aldolase, enolase, and lactate dehydrogenase
A. HIF-1 is induced by the expression of oncogenes, such as v-Src
and Ras, and the levels of HIF activity in cells correlate with
tumorigenicity . HIF-1 has been implicated in ROS-induced
carcinogenesis in a variety of human tumors, including bladder,
breast, colon, glial, hepatocellular, ovarian, pancreatic, prostate,
and renal tumors . Considerable evidence also indicates that
intracellular ROS are required for stabilization and hypoxic
activation of HIF-1a . There are many carcinogenic metals
that affect HIF-1 activity. The metals Cd, Co(II), Cr(VI), Ni(II), and V
are known to stimulate HIF-1 activity and expression via genera-
tion of ROS and subsequent activation of PI3K/Akt signaling
[220,237,238]. The previous findings that metal-induced expres-
sion or activation of HIF-1 can be inhibited by antioxidants
support the involvement of oxidative stress in metal-mediated
HIF-1 activation [239,240].
Members of the NFAT family of nuclear transcription factors
are activated mostly in a calcium-dependent manner and regulate
a diversity of cellular events including cytokine production,
muscle growth and differentiation, angiogenesis, chondrogenesis,
and adipogenesis [241,242]. Like NF-kB, inactive NFAT compo-
nents are present in the cytoplasm and translocate to the nucleus
upon activation. NFAT interacts with both AP-1 and NF-kB and
coregulates the expression of various cytokines as well as cell
surface proteins . Of the metals, Ni, V(V), and Fe have been
shown to activate NFAT through generation of H2O2 [8,244].
Particles composed of Co, Cr, and molybdenum upregulate tumor
necrosis factor-a production by MLO-Y4 osteocytes via activation
of calcineurin–NFAT signaling . Combined treatment with
CdCl2and mercury increased NFAT activation, whereas PKC and
calcium mobilization were found to be upstream events in human
Jurkat T lymphoma cells .
NF-kB is a redox-sensitive and dimeric transcription factor
composed of different members of the Rel family, such as p50
(NF-kB1), p52 (NF-kB2), c-Rel, v-Rel, Rel A (p65), and Rel B. Once
activated, NF-kB is translocated into the nucleus by the degrada-
tion of inhibitory IkB proteins (IkBa, IkBb, and IkBe). Activated
NF-kB binds to specific DNA sequences in target genes and
regulates the transcription of genes regulating basic cellular
events. NF-kB activation has been linked to carcinogenesis
because of its critical roles in inflammation, differentiation, and
cell growth . It is important to note that NF-kB has two
levels of redox regulation: one in the cytoplasm and another in
the nucleus . The former involves phosphorylation of two
serine residues (S32 and S36) on IkBa, which is mediated by ROS.
This process permits unmasking of the nuclear location signal and
translocation of activated NF-kB into the nucleus. Another model
of redox regulation of NF-kB takes place in the nucleus and is a
direct redox modification of specific cysteine residues in the DNA
binding domain of NF-kB . Like AP-1, there are a number of
studies supporting the involvement of carcinogenic metals on the
activation of NF-kB. However, the way in which metals affect NF-
kB differs according to the metals applied and the cell types used.
As(III) has dual effects on NF-kB, i.e., inducer or inhibitor
depending on its concentration, treatment duration, and the cell
type examined . While Cr(VI) induced NF-kB activation in
Jurkat cells through its reduction to lower oxidation states, this
activation was decreased by treatment with a metal chelator, CAT,
ordOH scavengers, but not by SOD . On the contrary, the
resulting effects of Cr on NF-kB differed depending on the dose
and cell type studied [250,251]. In addition, the metals including
Ni(II), Co(II), Be, Cd(II), Pb, and Fe stimulated the expression or
activation of NF-kB in various cells [11,14,15]. These reports
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
revealed the relationship between metal-mediated ROS genera-
tion and NF-kB activation, suggesting that NF-kB may serve as a
potential molecular target for the clinical treatment of metal-
induced chronic diseases. However, it has been suggested that
caution should be exercised with regard to blocking the NF-kB
pathway when treating cancers because of the suppressive effect
NF-kB has on carcinogenesis . NF-kB functions in DNA repair
to preserve genome integrity and maintain cells in the senescent
state in mouse and human fibroblast senescence models .
The predominant role of NF-kB might be to serve as a tumor
promoting transcription factor, at least in ROS and/or metal-
Nuclear factor (erythroid-derived 2)-like 2, also known as
NFE2L2 or Nrf2, is a basic region-leucine zipper-type transcription
factor that is encoded by the NFE2L2 gene in humans .
Nrf2 is a highly conserved cellular defense mechanism found in
most vertebrates. Nrf2 binds to antioxidant response element
(ARE) in the nucleus and leads to the transcriptional expression of
ARE-regulated genes . Various exogenous and endogenous
stresses, such as ROS, RNS, lipid aldehydes, and electrophilic
xenobiotics and their metabolites, can activate the binding of
Nrf2 to ARE and the subsequent gene transcription [253–255].
Nrf2 was reported to be activated as a result of exposure to metals
such as Cr (VI)  and Cd . Based on these results, Nrf2
appears to have a protective role against metal-induced toxicity
and carcinogenicity by activating the gene expression specific to
antioxidant defense systems. Interestingly, however, emerging
evidence suggests dual roles of Nrf2, emphasizing that elevated
Nrf2 activity may play a role in the evolution of cancer [258–260].
Further experiments will be needed in order to understand the
exact roles of this transcription factor in the increased incidence
of metal-induced carcinogenicity.
The nuclear transcription factor p53 plays an important role as
a tumor suppressor in protecting cells from tumorigenesis by
stopping cell cycle progression or by initiating apoptosis. Muta-
tional inactivation of p53 has been found to be involved in 450%
of human cancers, which indicates the importance of p53 in
human carcinogenesis . This factor is activated in response
to a variety of stimuli, such as UV radiation, hypoxia, and
nucleotide deprivation. Most metals have been shown to affect
p53 expression depending on their chemical composition and/or
oxidation state. Among various As compounds, As(III) was the
strongest inducer of p53 in cells . Cr(VI) induces p53-
mediated apoptosis in multiple ways, where reduction to its
oxidized forms and the resultant formation of ROS are the most
critical events . Few studies have presented evidence refut-
ing the fact that the p53 gene should be considered a major target
for Cr(VI)-induced carcinogenesis in humans . Other metals,
such as NiCl2, Ni(II) acetate, Cd acetate, CdCl2, Co(II), and Fe, also
affected the transactivation of p53 activity depending on the cell
type [90,202,265]. Se and its compounds exert toxic effects by
inducing ROS production, which leads to p53 activation [104,185].
N-acetylcysteine, CAT, and DFX inhibited p53 activation and
mitochondrial damage caused by metals, thereby indicating the
involvement of oxidative stress in p53 activation [185,202].
Ras oncogenes (H-Ras, N-Ras, and K-Ras) are the membrane-
bound G-proteins that function to regulate cell growth and inhibit
apoptotic effects. These oncogenes comprise the most frequently
mutated classes of oncogenes in human cancers (around 33%) and
their mutation has been reported in most cancers of the lung,
skin, liver, bladder, and colon . Thus, efficient blocking of Ras
membrane association or downstream effector signaling using
anti-Ras inhibitors has been considered in the development of
cancer treatments [266,267]. Ras is activated in response to ROS,
cell stress, UV radiation, and mitogenic stimuli. In addition to
these activators, many metals including As, Ni, Fe, Be, Cr, and Cu
have been shown to be associated with Ras mutations and cancer
development via generation of ROS [8,268–272]. Inorganic Cd
compounds were genotoxic in human lung fibroblasts in the
absence of a significant point mutation of the K-Ras proto-
oncogene, although cadmium salt treatment induced a weak
point mutation in K-Ras . Consequently, the mutation of
Ras is one of the important events involved in metal-induced
carcinogenicity, and thus the development of a potent Ras
inhibitory drug is of great interest.
This section described the signal transduction pathways and
related molecules involved in metal-induced carcinogenicity.
Most carcinogenic metals affect the activity of biomolecules that
function as key regulators of proliferation, differentiation, and
survival mainly by a mechanism involving ROS. Fig. 2 summarizes
the signaling proteins affected by metal-induced changes of
intracellular redox states and the associated mechanisms.
Discussion and concluding remarks
While numerous studies have been highlighted the possible
roles of ROS in metal-induced carcinogenesis, there are critical
issues and/or challenges facing the field on the topic of ROS and
metal carcinogenesis. First, there are no suitable animal models
for metal-induced carcinogenesis study. It is difficult to induce
tumors in a target organ after metal exposure as a single agent to
animals . Developments of adequate animal models for
metal-induced carcinogenesis might be necessary at the most.
Second, in the natural environment, humans could be exposed to
metals or metal-containing compounds in combination with
other carcinogens and/or toxic agents for a long time . Thus,
it will be hard to elucidate the real carcinogenic mechanisms of
metals using one or two compounds. Third, it is difficult to
understand the microenvironment occurring in the process
of carcinogenesis . Even though animals are exposed to
the same carcinogenic metals, cancer development can differ
Fig. 2. Signal transduction pathways activated by metal and metal-induced ROS
J.-C. Lee et al. / Free Radical Biology and Medicine 53 (2012) 742–757
depending on the target organ’s microenvironment. Cancer devel-
opment stages, such as initiation, promotion, and progression, are
also affected differently according to an organ’s microenviron-
ment. Fourth, there are inconsistent roles of ROS in metal-induced
carcinogenesis. Although cellular ROS levels are increased after
metal exposure and this is believed to play important roles in
metal-induced carcinogenesis, a few of reports highlight that the
cells transformed by a prolonged metal exposure show lower ROS
levels than untransformed cells [276,277]. An explanation on why
metal-mediated transformed cells have lower levels of cellular
ROS than untransformed ones is needed.
To clarify how ROS induce cellular response and signal
transduction is quite important for an understanding of the
mechanisms of metal-induced carcinogenesis. Of course, many
researchers have implicated the involvement of ROS signaling in
metal-induced carcinogenesis over the last decade. However, they
did not provide a direct evidence of the correlation between ROS
and metal-induced carcinogenesis. Our research group high-
lighted an involvement of ROS signaling in carcinogenesis
mediated by As [276,278], Cr , or Cd (unpublished data),
supporting the critical roles of ROS in metal-induced carcinogen-
esis. Additional studies will be needed in order to verify the roles
of other carcinogenic metals. It is also important to elucidate how
ROS signaling induces transformation of normal cells to cancer
cells. Until recently, a critical downstream signaling of ROS in
metal-induced carcinogenesis is not completely understood. Our
previous findings suggest that the potential ROS target molecules
are Akt, GSK-3b, and b-catenin. Exposure to As and Cr through
drinking water resulted in the increases in ROS levels, b-catenin
expression, and GSK phosphorylation in a mouse colorectal cancer
model . Chronic exposure to cadmium appeared to induce
carcinogenesis in BEAS-2B cells through ROS-dependent activa-
tion of AKT/GSK-3b/b-catenin signaling (unpublished data).
Increases in phosphorylation of AKT and GSK-3b were shown in
increased ROS levels and b-catenin expression in DLD1 cells
. Further detailed work will be needed to fully elucidate
the cellular response and signal transduction of ROS signaling in
In summary, metals are capable of generating ROS mostly
through Fenton and Haber–Weiss reactions. ROS generated by
metals are key mediators responsible for DNA damage, lipid
peroxidation, and protein modification in cells and tissues.
Metals also result in activation of nuclear transcription factors
and various signaling proteins, in addition to cell cycle arrest
and apoptosis through ROS-dependent or -independent mechan-
isms. Numerous biomolecules are quite sensitive to intracellular
redox states. Although metal-induced oxidative stress does
not explain all of the carcinogenicity caused by metals, accumu-
lating evidence emphasizes the protective effect of antioxidants
in the setting of metal-induced toxicity and carcinogenicity.
The effectiveness of an antioxidant-based treatment approach
is dependent on understanding the mechanisms by which
metals cause cancer and other health conditions. In this regard,
future studies should focus on defining cellular networks of
response genes, identifying target molecules of metal and
metal-induced oxidative stress, developing efficient biomarkers,
and identifying individuals with increased susceptibility to metal
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