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Heavy Metals Toxicity and the Environment
Paul B Tchounwou, Clement G Yedjou, Anita K Patlolla, Dwayne J Sutton
NIH-RCMI Center for Environmental Health, College of Science, Engineering and
Technology, Jackson State University, 1400 Lynch Street, Box 18750, Jackson, MS 39217,
USA
Abstract: Heavy metals are naturally occurring elements that have a high atomic weight and a
density at least 5 times greater than that of water. Their multiple industrial, domestic,
agricultural, medical and technological applications have led to their wide distribution in the
environment; raising concerns over their potential effects on human health and the
environment. Their toxicity depends on several factors including the dose, route of exposure,
and chemical species, as well as the age, gender, genetics, and nutritional status of exposed
individuals. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and
mercury rank among the priority metals that are of public health significance. These metallic
elements are considered systemic toxicants that are known to induce multiple organ damage,
even at lower levels of exposure. They are also classified as human carcinogens (known or
probable) according to the U.S. Environmental Protection Agency, and the International
Agency for Research on Cancer. This review provides an analysis of their environmental
occurrence, production and use, potential for human exposure, and molecular mechanisms of
toxicity, genotoxicity, and carcinogenicity.
Keywords: heavy metals, production and use, human exposure, toxicity, genotoxicity,
carcinogenicity
Introduction
Heavy metals are defined as metallic elements that have a relatively high density compared to
water [1]. With the assumption that heaviness and toxicity are inter-related, heavy metals also
include metalloids, such as arsenic, that are able to induce toxicity at low level of exposure
[2]. In recent years, there has been an increasing ecological and global public health concern
associated with environmental contamination by these metals. Also, human exposure has
risen dramatically as a result of an exponential increase of their use in several industrial,
agricultural, domestic and technological applications [3]. Reported sources of heavy metals in
the environment include geogenic, industrial, agricultural, pharmaceutical, domestic
effluents, and atmospheric sources [4] . Environmental pollution is very prominent in point
source areas such as mining, foundries and smelters, and other metal-based industrial
operations [1, 3, 4].
Although heavy metals are naturally occurring elements that are found throughout the
earth’s crust, most environmental contamination and human exposure result from
anthropogenic activities such as mining and smelting operations, industrial production and
use, and domestic and agricultural use of metals and metal-containing compounds [4-7].
Environmental contamination can also occur through metal corrosion, atmospheric
deposition, soil erosion of metal ions and leaching of heavy metals, sediment re-suspension
and metal evaporation from water resources to soil and ground water [8]. Natural phenomena
such as weathering and volcanic eruptions have also been reported to significantly contribute
to heavy metal pollution [1, 3, 4, 7, 8]. Industrial sources include metal processing in
refineries, coal burning in power plants, petroleum combustion, nuclear power stations and
high tension lines, plastics, textiles, microelectronics, wood preservation and paper
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processing plants [9-11].
It has been reported that metals such as cobalt (Co), copper (Cu), chromium (Cr), iron
(Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se) and
zinc (Zn) are essential nutrients that are required for various biochemical and physiological
functions [12]. Inadequate supply of these micro-nutrients results in a variety of deficiency
diseases or syndromes [12].
Heavy metals are also considered as trace elements because of their presence in trace
concentrations (ppb range to less than 10ppm) in various environmental matrices [13].
Their bioavailability is influenced by physical factors such as temperature, phase association,
adsorption and sequestration. It is also affected by chemical factors that influence speciation
at thermodynamic equilibrium, complexation kinetics, lipid solubility and octanol/water
partition coefficients [14]. Biological factors such as species characteristics, trophic
interactions, and biochemical/physiological adaptation, also play an important role [15].
The essential heavy metals play biochemical and physiological functions in plants and
animals. They are important constituents of several key enzymes and play important roles in
various oxidation-reduction reactions [12]. Copper for example serves as an essential co-
factor for several oxidative stress-related enzymes including catalase, superoxide dismutase,
peroxidase, cytochrome c oxidases, ferroxidases, monoamine oxidase, and dopamine β-
monooxygenase [16-18]. Hence, it is an essential nutrient that is incorporated into a number
of metalloenzymes involved in hemoglobin formation, carbohydrate metabolism,
catecholamine biosynthesis, and cross-linking of collagen, elastin, and hair keratin. The
ability of copper to cycle between an oxidized state, Cu(II), and reduced state, Cu(I), is used
by cuproenzymes involved in redox reactions [16-18]. However, it is this property of copper
that also makes it potentially toxic because the transitions between Cu(II) and Cu(I) can result
in the generation of superoxide and hydroxyl radicals [16-19]. Also, excessive exposure to
copper has been linked to cellular damage leading to Wilson disease in humans [18, 19].
Similar to copper, several other essential elements are required for biologic functioning,
however, excess amount of such metals produces cellular and tissue damage leading to a
variety of adverse effects and human diseases. For some including chromium and copper,
there is a very narrow range of concentrations between beneficial and toxic effects [19, 20] .
Other metals such as aluminium (Al), antinomy (Sb), arsenic (As), barium (Ba), beryllium
(Be), bismuth (Bi), cadmium (Cd), gallium (Ga), germanium (Ge), gold (Au), indium (In),
lead (Pb), lithium (Li), mercury (Hg), nickel (Ni), platinum (Pt), silver (Ag), strontium (Sr),
tellurium (Te), thallium (Tl), tin (Sn), titanium (Ti), vanadium (V) and uranium (U) have no
established biological functions and are considered as non-essential metals [20].
In biological systems, heavy metals have been reported to affect cellular organelles and
components such as cell membrane, mitochondrial, lysosome, endoplasmic reticulum, nuclei, and
some enzymes involved in metabolism, detoxification, and damage repair [21]. Metal ions have
been found to interact with cell components such as DNA and nuclear proteins, causing DNA
damage and conformational changes that may lead to cell cycle modulation, carcinogenesis or
apoptosis [20-22]. Several studies from our laboratory have demonstrated that reactive oxygen
species (ROS) production and oxidative stress play a key role in the toxicity and
carcinogenicity of metals such as arsenic [23, 24, 25], cadmium [26], chromium [27, 28], lead
[29, 30], and mercury [31, 32]. Because of their high degree of toxicity, these five elements
rank among the priority metals that are of great public health significance. They are all
systemic toxicants that are known to induce multiple organ damage, even at lower levels of
exposure. According to the United States Environmental Protection Agency (U.S. EPA), and
the International Agency for Research on Cancer (IARC), these metals are also classified as
known or probable human carcinogens based on epidemiological and experimental studies
showing an association between exposure and cancer incidence in humans and animals.
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Heavy metal-induced toxicity and carcinogenicity involves many mechanistic aspects,
some of which are not clearly elucidated or understood. However, each metal is known to
have unique features that confer to its specific toxicological mechanisms of action. This
review provides an analysis of their environmental occurrence, production and use, potential
for human exposure, and molecular mechanisms of toxicity, genotoxicity, and carcinogenicity
of arsenic, cadmium, chromium, lead and mercury.
Arsenic
Environmental Occurrence, Industrial Production and Use
Arsenic is a ubiquitous element that is detected at low concentrations in virtually all
environmental matrices [33]. The major inorganic forms of arsenic include the trivalent
arsenite and the pentavalent arsenate. The organic forms are the methylated metabolites –
monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and trimethylarsine oxide.
Environmental pollution by arsenic occurs as a result of natural phenomena such as volcanic
eruptions and soil erosion, and anthropogenic activities [33]. Several arsenic-containing
compounds are produced industrially, and have been used to manufacture products with
agricultural applications such as insecticides, herbicides, fungicides, algicides, sheep dips,
wood preservatives, and dye-stuffs. They have also been used in veterinary medicine for the
eradication of tapeworms in sheep and cattle [34]. Arsenic compounds have also been used in
the medical field for at least a century in the treatment of syphilis, yaws, amoebic dysentery,
and trypanosomaiasis [34,35]. Arsenic-based drugs are still used in treating certain tropical
diseases such as African sleeping sickness and amoebic dysentery, and in veterinary medicine
to treat parasitic diseases, including filariasis in dogs and black head in turkeys and chickens
[35]. Recently, arsenic trioxide has been approved by the Food and Drug Administration as an
anticancer agent in the treatment of acute promeylocytic leukemia [36]. Its therapeutic action
has been attributed to the induction of programmed cell death (apoptosis) in leukemia cells
[24].
Potential for Human Exposure
It is estimated that several million people are exposed to arsenic chronically throughout the
world, especially in countries like Bangladesh, India, Chile, Uruguay, Mexico, Taiwan, where
the ground water is contaminated with high concentrations of arsenic. Exposure to arsenic
occurs via the oral route (ingestion), inhalation, dermal contact, and the parenteral route to
some extent [33,34,37]. Arsenic concentrations in air range from 1 to 3 ng/m3 in remote
locations (away from human releases), and from 20 to 100 ng/m3 in cities. Its water
concentration is usually less than 10µg/L, although higher levels can occur near natural
mineral deposits or mining sites. Its concentration in various foods ranges from 20 to 140
ng/kg [38]. Natural levels of arsenic in soil usually range from 1 to 40 mg/kg, but pesticide
application or waste disposal can produce much higher values [25].
Diet, for most individuals, is the largest source of exposure, with an average intake of
about 50 µg per day. Intake from air, water and soil are usually much smaller, but exposure
from these media may become significant in areas of arsenic contamination. Workers who
produce or use arsenic compounds in such occupations as vineyards, ceramics, glass-making,
smelting, refining of metallic ores, pesticide manufacturing and application, wood
preservation, semiconductor manufacturing can be exposed to substantially higher levels of
arsenic [39]. Arsenic has also been identified at 781 sites of the 1,300 hazardous waste sites
that have been proposed by the U.S. EPA for inclusion on the national priority list [33,39].
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Human exposure at these sites may occur by a variety of pathways, including inhalation of
dusts in air, ingestion of contaminated water or soil, or through the food chain [40].
Contamination with high levels of arsenic is of concern because arsenic can cause a
number of human health effects. Several epidemiological studies have reported a strong
association between arsenic exposure and increased risks of both carcinogenic and systemic
health effects [41]. Interest in the toxicity of arsenic has been heightened by recent reports of
large populations in West Bengal, Bangladesh, Thailand, Inner Mongolia, Taiwan, China,
Mexico, Argentina, Chile, Finland and Hungary that have been exposed to high
concentrations of arsenic in their drinking water and are displaying various clinico-
pathological conditions including cardiovascular and peripheral vascular disease,
developmental anomalies, neurologic and neurobehavioural disorders, diabetes, hearing loss,
portal fibrosis, hematologic disorders (anemia, leukopenia and eosinophilia) and carcinoma
[25, 33, 35, 39]. Arsenic exposure affects virtually all organ systems including the
cardiovascular, dermatologic, nervous, hepatobilliary, renal, gastro-intestinal, and respiratory
systems [41]. Research has also pointed to significantly higher standardized mortality rates
for cancers of the bladder, kidney, skin, and liver in many areas of arsenic pollution. The
severity of adverse health effects is related to the chemical form of arsenic, and is also time-
and dose-dependent [42,43]. Although the evidence of carcinogenicity of arsenic in humans
seems strong, the mechanism by which it produces tumors in humans is not completely
understood [44].
Mechanisms of Toxicity and Carcinogenicity
Analyzing the toxic effects of arsenic is complicated because the toxicity is highly influenced
by its oxidation state and solubility, as well as many other intrinsic and extrinsic factors [45].
Several studies have indicated that the toxicity of arsenic depends on the exposure dose,
frequency and duration, the biological species, age, and gender, as well as on individual
susceptibilities, genetic and nutritional factors [46]. Most cases of human toxicity from
arsenic have been associated with exposure to inorganic arsenic. Inorganic trivalent arsenite
(AsIII ) is 2-10 times more toxic than pentavalent arsenate (AsV) [5]. By binding to thiol or
sulfhydryl groups on proteins, As (III) can inactivate over 200 enzymes. This is the likely
mechanism responsible for arsenic’s widespread effects on different organ systems. As (V)
can replace phosphate, which is involved in many biochemical pathways [5, 47].
One of the mechanisms by which arsenic exerts its toxic effect is through impairment
of cellular respiration by the inhibition of various mitochondrial enzymes, and the uncoupling
of oxidative phosphorylation. Most toxicity of arsenic results from its ability to interact with
sulfhydryl groups of proteins and enzymes, and to substitute phosphorous in a variety of
biochemical reactions [48]. Arsenic in vitro reacts with protein sulfhydryl groups to inactivate
enzymes, such as dihydrolipoyl dehydrogenase and thiolase, thereby producing inhibited
oxidation of pyruvate and betaoxidation of fatty acids [49]. The major metabolic pathway for
inorganic arsenic in humans is methylation. Arsenic trioxide is methylated to two major
metabolites via a non-enzymatic process to monomethylarsonic acid (MMA), which is further
methylated enzymatically to dimethyl arsenic acid (DMA) before excretion in the urine [40,
47]. It was previously thought that this methylation process is a pathway of arsenic
detoxification, however, recent studies have pointed out that some methylated metabolites
may be more toxic than arsenite if they contain trivalent forms of arsenic [41].
Tests for genotoxicity have indicated that arsenic compounds inhibit DNA repair, and
induce chromosomal aberrations, sister-chromatid exchanges, and micronuclei formation in
both human and rodent cells in culture [50-52] and in cells of exposed humans [53].
Reversion assays with Salmonella typhimurium fail to detect mutations that are induced by
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arsenic compounds. Although arsenic compounds are generally perceived as weak mutagens
in bacterial and animal cells, they exhibit clastogenic properties in many cell types in vivo
and in vitro [54]. In the absence of animal models, in vitro cell transformation studies become
a useful means of obtaining information on the carcinogenic mechanisms of arsenic toxicity.
Arsenic and arsenical compounds are toxic to and induce morphological transformations of
Syrian hamster embryo (SHE) cells as well as mouse C3H10T1/2 cells and BALB/3T3 cells
[55,56].
Based on the comet assay, it has been reported that arsenic trioxide induces DNA
damage in human lymphophytes [57] and also in mice leukocytes [58]. Arsenic compounds
have also been shown to induce gene amplification, arrest cells in mitosis, inhibit DNA
repair, and induce expression of the c-fos gene and the oxidative stress protein heme
oxygenase in mammalian cells [58, 59]. They have been implicated as promoters and
comutagens for a variety of toxic agents [60]. Recent studies in our laboratory have
demonstrated that arsenic trioxide is cytotoxic and able to transcriptionally induce a
significant number of stress genes and related proteins in human liver carcinoma cells [61].
Epidemiological investigations have indicated that long-term arsenic exposure results
in promotion of carcinogenesis. Several hypotheses have been proposed to describe the
mechanism of arsenic-induced carcinogenesis. Zhao et al. [62] reported that arsenic may act
as a carcinogen by inducing DNA hypomethylation, which in turn facilitates aberrant gene
expression. Additionally, it was found that arsenic is a potent stimulator of extracellular
signal-regulated protein kinase Erk1 and AP-1 transactivational activity, and an efficient
inducer of c-fos and c-jun gene expression [63]. Induction of c-jun and c-fos by arsenic is
associated with activation of JNK [64]. However, the role of JNK activation by arsenite in
cell transformation or tumor promotion is unclear.
In another study, Trouba et al. [65] concluded that long-term exposure to high levels
of arsenic might make cells more susceptible to mitogenic stimulation and that alterations in
mitogenic signaling proteins might contribute to the carcinogenic action of arsenic.
Collectively, several recent studies have demonstrated that arsenic can interfere with cell
signaling pathways (e.g., the p53 signaling pathway) that are frequently implicated in the
promotion and progression of a variety of tumor types in experimental animal models, and of
some human tumors [66, 68]. However, the specific alterations in signal transduction
pathways or the actual targets that contribute to the development of arsenic-induced tumors in
humans following chronic consumption of arsenic remains uncertain.
Recent clinical trials have found that arsenic trioxide has therapeutic value in the
treatment of acute promyelocytic leukemia, and there is interest in exploring its effectiveness
in the treatment of a variety of other cancers [69,70]. In acute promyelocytic leukemia, the
specific molecular event critical to the formation of malignant cells is known. The study by
Puccetti et al. [71] found that forced overexpression of BCR-ABL susceptibility in human
lymphoblasts cells resulted in greatly enhanced sensitivity to arsenic-induced apoptosis. They
also concluded that arsenic trioxide is a tumor specific agent capable of inducing apoptosis
selectively in acute promyelocytic leukemia cells. Several recent studies have shown that
arsenic can induce apoptosis through alterations in other cell signaling pathways [72,73]. In
addition to acute peomyelocytic leukemia, arsenic is thought to have therapeutic potential for
myeloma [74]. In summary, numerous cancer chemotherapy studies in cell cultures and in
patients with acute promyelocytic leukemia demonstrate that arsenic trioxide administration
can lead to cell-cycle arrest and apoptosis in malignant cells.
Previous studies have also examined p53 gene expression and mutation in tumors
obtained from subjects with a history of arsenic ingestion. p53 participates in many cellular
functions, cell-cycle control, DNA repair, differentiation, genomic plasticity and programmed
cell death. Additional support for the hypothesis that arsenic can modulate gene expression
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has been provided by several different studies [75,76]. Collectively, these studies provide
further evidence that various forms of arsenic can alter gene expression and that such changes
could contribute substantially to the toxic and carcinogenic actions of arsenic treatment in
human populations [77].
Several in vitro studies in our laboratory have demonstrated that arsenic modulates
DNA synthesis, gene and protein expression, genotoxicity, mitosis and/or apoptotic
mechanisms in various cell lines including keratinocytes, melanocytes, dendritic cells, dermal
fibroblasts, microvascular endothelial cells, monocytes, and T-cells [78], colon cancer cells
[79], lung cancer cells [80], human leukemia cells [81], Jurkat-T lymphocytes [82], and
human liver carcinoma cells [83]. We have also shown that oxidative stress plays a key role
in arsenic induced cytotoxicity, a process that is modulated by pro- and/or anti-oxidants such
as ascorbic acid and n-acetyl cysteine [84-86]. We have further demonstrated that the toxicity
of arsenic depends on its chemical form, the inorganic form being more toxic than the organic
one [42].
Various hypotheses have been proposed to explain the carcinogenicity of inorganic
arsenic. Nevertheless, the molecular mechanisms by which this arsenical induces cancer are
still poorly understood. Results of previous studies have indicated that inorganic arsenic does
not act through classic genotoxic and mutagenic mechanisms, but rather may be a tumor
promoter that modifies signal transduction pathways involved in cell growth and proliferation
[68]. Although much progress has been recently made in the area of arsenic’s possible
mode(s) of carcinogenic action, a scientific consensus has not yet reached. A recent review
discusses nine different possible modes of action of arsenic carcinogenesis: induced
chromosomal abnormalities, oxidative stress, altered DNA repair, altered DNA methylation
patterns, altered growth factors, enhanced cell proliferation, promotion/progression,
suppression of p53, and gene amplification [87]. Presently, three modes (chromosomal
abnormality, oxidative stress, and altered growth factors) of arsenic carcinogenesis have
shown a degree of positive evidence, both in experimental systems (animal and human cells)
and in human tissues. The remaining possible modes of carcinogenic action (progression of
carcinogenesis, altered DNA repair, p53 suppression, altered DNA methylation patterns and
gene amplification) do not have as much evidence, particularly from in vivo studies with
laboratory animals, in vitro studies with cultured human cells, or human data from case or
population studies. Thus, the mode-of-action studies suggest that arsenic might be acting as a
cocarcinogen, a promoter, or a progressor of carcinogenesis.
Cadmium
Environmental Occurrence, Industrial Production and Use
Cadmium is a heavy metal of considerable environmental and occupational concern. It is
widely distributed in the earth's crust at an average concentration of about 0.1 mg/kg. The
highest level of cadmium compounds in the environment is accumulated in sedimentary
rocks, and marine phosphates contain about 15 mg cadmium/kg [88].
Cadmium is frequently used in various industrial activities. The major industrial
applications of cadmium include the production of alloys, pigments, and batteries [89].
Although the use of cadmium in batteries has shown considerable growth in recent years, its
commercial use has declined in developed countries in response to environmental concerns.
In the United States for example, the daily cadmium intake is about 0.4μg/kg/day, less than
half of the U.S. EPA’s oral reference dose [90]. This decline has been linked to the
introduction of stringent effluent limits from plating works and, more recently, to the
introduction of general restrictions on cadmium consumption in certain countries.
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Potential for Human Exposure
The main route of exposure to cadmium is via inhalation or cigarette smoke or ingestion of
food. Skin absorption is rare. Human exposure to cadmium is possible through a number of
several sources including employment in primary metal industries, eating contaminated food,
smoking cigarettes, and working in cadmium-contaminated work places, with smoking being
a major contributor [91, 92]. Other sources of cadmium include emissions from industrial
activities, including mining, smelting, and manufacturing of batteries, pigments, stabilizers,
and alloys [93]. Cadmium is also present in trace amounts in certain foods such as leafy
vegetables, potatoes, grains and seeds, liver and kidney, and crustaceans and mollusks [94].
In addition, foodstuffs that are rich in cadmium can greatly increase the cadmium
concentration in human bodies. Examples are liver, mushrooms, shellfish, mussels, cocoa
powder and dried seaweed. An important route of exposure is the circulatory system whereas
blood vessels are considered to be main stream organs of cadmium toxicity. Chronic
inhalation exposure to cadmium particulates is generally associated with changes in
pulmonary function and chest radiographs that are consistent with emphysema [95].
Workplace exposure to airborne cadmium particulates has been associated with decreases in
olfactory function [96]. Several epidemiologic studies have documented an association of
chronic low-level cadmium exposure with decreases in bone mineral density and osteoporosis
[97-99].
Exposure to cadmium is commonly determined by measuring cadmium levels in
blood or urine. Blood cadmium reflects recent cadmium exposure (from smoking, for
example). Cadmium in urine (usually adjusted for dilution by calculating the
cadmium/creatinine ratio) indicates accumulation, or kidney burden of cadmium [100, 101].
It is estimated that about 2.3% of the U.S. population has elevated levels of urine cadmium
(>2µg/g creatinine), a marker of chronic exposure and body burden [102]. Blood and urine
cadmium levels are typically higher in cigarette smokers, intermediate in former smokers and
lower in nonsmokers [102, 103]. Because of continuing use of cadmium in industrial
applications, the environmental contamination and human exposure to cadmium have
dramatically increased during the past century [104].
Molecular Mechanisms of Toxicity and Carcinogenicity
Cadmium is a severe pulmonary and gastrointestinal irritant, which can be fatal if inhaled or
ingested. After acute ingestion, symptoms such as abdominal pain, burning sensation, nausea,
vomiting, salivation, muscle cramps, vertigo, shock, loss of consciousness and convulsions
usually appear within 15 to 30 min [105]. Acute cadmium ingestion can also cause
gastrointestinal tract erosion, pulmonary, hepatic or renal injury and coma, depending on the
route of poisoning [105, 106]. Chronic exposure to cadmium has a depressive effect on
levels of norepinephrine, serotonin, and acetylcholine [107]. Rodent studies have shown that
chronic inhalation of cadmium causes pulmonary adenocarcinomas [108, 109]. It can also
cause prostatic proliferative lesions including adenocarcinomas, after systemic or direct
exposure [110].
Although the mechanism of cadmium induced toxicity is poorly understood, it has
been speculated that cadmium causes damage to cells primarily through the generation of
reactive oxygen species [111], which causes single-strand DNA damage and disrupts the
synthesis of nucleic acids and proteins [112]. Studies using two-dimensional gel
electrophoresis have shown that several stress response systems are expressed in response to
cadmium exposure, including those for heat shock, oxidative stress, stringent response, cold
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shock, and SOS [113- 115]. In vitro studies indicate that cadmium induces cytotoxic effects at
the concentrations 0.1 to 10mM and free radical-dependent DNA damage [116, 117]. In vivo
studies have shown that cadmium modulates male reproduction in mice model at a
concentration of 1 mg/kg body weight [118]. However, cadmium is a weak mutagen when
compared with other carcinogenic metals [119]. Previous reports have indicated cadmium
affects signal transduction pathways; inducing inositol polyphosphate formation, increasing
cytosolic free calcium levels in various cell types [120], and blocking calcium channels [121,
122]. At lower concentrations (1-100 μM), cadmium binds to proteins, decreases DNA repair
[123], activates protein degradation, up-regulates cytokines and proto-oncogenes such as c-
fos, c-jun, and c-myc [124], and induces expression of several genes including
metallothioneins [125], heme oxygenases, glutathione transferases, heat-shock proteins,
acute-phase reactants, and DNA polymerase β [126].
Cadmium compounds are classified as human carcinogens by several regulatory
agencies. The International Agency for Research on Cancer [91] and the US National
Toxicology Program have concluded that there is adequate evidence that cadmium is a human
carcinogen. This designation as a human carcinogen is based primarily on repeated findings
of an association between occupational cadmium exposure and lung cancer, as well as on
very strong rodent data showing the pulmonary system as a target site [91]. Thus, the lung is
the most definitively established site of human carcinogenesis from cadmium exposure. Other
target tissues of cadmium carcinogenesis in animals include injection sites, adrenals, testes,
and the hemopoietic system [91, 108, 109]. In some studies, occupational or environmental
cadmium exposure has also been associated with development of cancers of the prostate,
kidney, liver, hematopoietic system and stomach [108, 109]. Carcinogenic metals including
arsenic, cadmium, chromium, and nickel have all been associated with DNA damage through
base pair mutation, deletion, or oxygen radical attack on DNA [126]. Animal studies have
demonstrated reproductive and teratogenic effects. Small epidemiologic studies have noted an
inverse relationship between cadmium in cord blood, maternal blood or maternal urine and
birth weight, and length at birth [127, 128].
Chromium
Environmental Occurrence, Industrial Production and Use
Chromium (Cr) is a naturally occurring element present in the earth’s crust, with oxidation
states (or valence states) ranging from chromium (II) to chromium (VI) [129]. Elemental
Chromium [Cr(0)] does not occur naturally. Chromium compounds are stable in the trivalent
[Cr(III)] form and occur in nature in this state in ores, such as ferrochromite. The hexavalent
[Cr(VI)] form is the second-most stable state [28]. Chromium enters into various
environmental matrices (air, water, and soil) from a wide variety of natural and anthropogenic
sources with the largest release occurring from industrial establishments. Industries with the
largest contribution to chromium release include metal processing, tannery facilities,
chromate production, stainless steel welding, and ferrochrome and chrome pigment
production. In humans and animals, [Cr(III)] is an essential nutrient that plays a role in
glucose, fat and protein metabolism by potentiating the action of insulin [5]. Hexavalent
chromium [Cr(VI)] is a toxic industrial pollutant that is classified as human carcinogen by
several regulatory and non-regulatory agencies [130-132]. The health hazard associated with
exposure to chromium depends on its oxidation state, ranging from the low toxicity of the
metal form to the high toxicity of the hexavalent form. All Cr(VI)-containing compounds
were once thought to be man-made, with only Cr(III) naturally ubiquitous in air, water, soil
and biological materials. Recently, however, naturally occurring Cr(VI) has been found in
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ground and surface waters at values exceeding the World Health Organization limit for
drinking water of 50 µg of Cr(VI) per liter [133]. Chromium is widely used in numerous
industrial processes and as a result, is a contaminant of many environmental systems [134].
Commercially chromium compounds are used in industrial welding, chrome plating, dyes and
pigments, leather tanning and wood preservation. Chromium is also used as anticorrosive in
cooking systems and boilers [135, 136].
Potential for Human Exposure
It is estimated that more than 300,000 workers are exposed annually to chromium and
chromium-containing compounds in the workplace. The increase in the environmental
concentrations of chromium has been linked to air and wastewater release of chromium,
mainly from metallurgical, refractory, and chemical industries. Chromium released into the
environment from anthropogenic activity occurs mainly in the hexavalent form [Cr(VI)]
[130]. Occupational Cr(VI) exposure has been a major concern, as workers in industries that
use Cr(VI) compounds are at higher risk of developing Cr-induced diseases [137]. However,
the general human population and some wildlife may also be at risk. It is estimated that 33
tons of total Cr are released annually into the environment [130]. The U.S. Occupational
Safety and Health Administration (OSHA) recently set a ‘‘safe’’ level of 5µg/m3, for an 8-hr
time-weighted average, even though this revised level may still pose a carcinogenic risk
[138]. For the general human population, atmospheric levels range from 1 to 100 ng/cm3
[139], but can exceed this range in areas that are close to Cr manufacturing.
Non-occupational exposure to the metal occurs via ingestion of chromium containing
food and water whereas occupational exposure occurs via inhalation [140]. Chromium
concentrations range between 1 and 3000 mg/kg in soil, 5 to 800 µg/L in sea water, and 26
µg/L to 5.2 mg/L in rivers and lakes [129]. Chromium content in foods varies greatly and
depends on the processing and preparation. In general, most fresh foods typically contain
chromium levels ranging from <10 to 1,300 µg/kg. Present day workers in chromium-related
industries can be exposed to chromium concentrations two orders of magnitude higher than
the general population [141]. Even though the principal route of human exposure to
chromium is through inhalation, and the lung is the primary target organ, significant human
exposure to chromium has also been reported to take place through the skin [142, 143]. For
example, the widespread incidence of dermatitis noticed among construction workers is
attributed to their exposure to chromium present in cement [143]. Occupational and
environmental exposure to Cr(VI)-containing compounds is known to cause multiorgan
toxicity such as renal damage, allergy and asthma, and cancer of the respiratory tract in
humans [5, 144].
Breathing high levels of chromium (VI) can cause irritation to the lining of the nose,
and nose ulcers. The main health problems seen in animals following ingestion of chromium
(VI) compounds are irritation and ulcers in the stomach and small intestine, anemia, sperm
damage and male reproductive system damage. Chromium (III) compounds are much less
toxic and do not appear to cause these problems. Some individuals are extremely sensitive to
chromium(VI) or chromium(III), allergic reactions consisting of severe redness and swelling
of the skin have been noted. An increase in stomach tumors was observed in humans and
animals exposed to chromium(VI) in drinking water. Accidental or intentional ingestion of
extremely high doses of chromium (VI) compounds by humans has resulted in severe
respiratory, cardiovascular, gastrointestinal, hematological, hepatic, renal, and neurological
effects as part of the sequelae leading to death or in patients who survived because of medical
treatment [141]. Although the evidence of carcinogenicity of chromium in humans and
terrestrial mammals seems strong, the mechanism by which it causes cancer is not completely
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understood [145].
Mechanisms of Toxicity and Carcinogenicity
Major factors governing the toxicity of chromium compounds are oxidation state and
solubility. Cr(VI) compounds, which are powerful oxidizing agents and thus tend to be
irritating and corrosive, appear to be much more toxic systemically than Cr(III) compounds,
given similar amount and solubility [146, 147]. Although the mechanisms of biological
interaction are uncertain, the variation in toxicity may be related to the ease with which
Cr(VI) can pass through cell membranes and its subsequent intracellular reduction to reactive
intermediates. Since Cr(III) is poorly absorbed by any route, the toxicity of chromium is
mainly attributable to the Cr(VI) form. It can be absorbed by the lung and gastrointestinal
tract, and even to a certain extent by intact skin. The reduction of Cr(VI) is considered to
serve as a detoxification process when it occurs at a distance from the target site for toxic or
genotoxic effect while reduction of Cr(VI) may serve to activate chromium toxicity if it takes
place in or near the cell nucleus of target organs [148]. If Cr(VI) is reduced to Cr(III)
extracellularly, this form of the metal is not readily transported into cells and so toxicity is not
observed. The balance that exists between extracellular Cr(VI) and intracellular Cr(III) is
what ultimately dictates the amounts and rates at which Cr(VI) can enter cells and impart its
toxic effects [134].
Cr(VI) enters many types of cells and under physiological conditions can be reduced
by hydrogen peroxide (H2O2), glutathione (GSH) reductase, ascorbic acid, and GSH to
produce reactive intermediates, including Cr(V), Cr(IV), thiylradicals, hydroxyl radicals, and
ultimately, Cr(III). Any of these species could attack DNA, proteins, and membrane lipids,
thereby disrupting cellular integrity and functions [149, 150].
Studies with animal models also found many harmful effects of Cr (VI) on mammals.
Subcutaneous administration of Cr (VI) to rats caused severe progressive proteinuria, urea
nitrogen and creatinine, as well as elevation in serum alanine aminotransferase activity and
hepatic lipid peroxide formation [151]. Similar studies reported by Gumbleton and Nicholls
[152] found that Cr (VI) induced renal damage in rats when administered by single sc
injections. Bagchi et al demonstrated that rats received Cr (VI) orally in water induced
hepatic mitochondrial and microsomal lipid peroxidation, as well as enhanced excretion of
urinary lipid metabolites including malondialdehyde [153, 154]. Moreover, some adverse
health effects induced by Cr (VI) have been reported in humans. Epidemiological
investigations have reported respiratory cancers in workers occupationally exposed to Cr
(VI)-containing compounds [142, 148]. DNA strand breaks in peripheral lymphocytes and
lipid peroxidation products in urine observed in chromium-exposed workers also support the
evidence of Cr (VI)-induced toxicity to humans [155, 156]. Oxidative damage is considered
to be the underlying cause of these genotoxic effects including chromosomal abnormalities
[157, 158], and DNA strand breaks [159]. Nevertheless, recent studies indicate a biological
relevance of non-oxidative mechanisms in Cr(VI) carcinogenesis [160].
Carcinogenicity appears to be associated with the inhalation of the less
soluble/insoluble Cr(VI) compounds. The toxicology of Cr(VI) does not reside with the
elemental form. It varies greatly among a wide variety of very different Cr(VI) compounds
[161]. Epidemiological evidence strongly points to Cr(VI) as the agent in carcinogenesis.
Solubility and other characteristics of chromium, such as size, crystal modification, surface
charge, and the ability to be phagocytized might be important in determining cancer risk
[135].
Studies in our laboratory have indicated that chromium (VI) is cytotoxic and able to
induce DNA damaging effects such as chromosomal abnormalities [162], DNA strand breaks,
11
DNA fragmentation and oxidative stress in Sprague-Dawley rats and human liver carcinoma
cells [27, 28]. Recently, our laboratory has also demonstrated that chromium (VI) induces
biochemical, genotoxic and histopathologic effects in liver and kidney of goldfish, carassius
auratus [163].
Various hypothesis have been proposed to explain the carcinogenicity of chromium
and its salts, however some inherent difficulties exist when discussing metal carcinogenesis.
A metal cannot be classified as carcinogenic per se since its different compounds may have
different potencies. Because of the multiple chemical exposure in industrial establishments, it
is difficult from an epidemiological standpoint to relate the carcinogenic effect to a single
compound. Thus, the carcinogenic risk must often be related to a process or to a group of
metal compounds rather than to a single substance. Differences in carcinogenic potential are
related not only to different chemical forms of the same metal but also to the particle size of
the inhaled aerosol and to physical characteristics of the particle such as surface charge and
crystal modification [164].
Lead
Environmental Occurrence, Industrial Production and Use
Lead is a naturally occurring bluish-gray metal present in small amounts in the earth’s crust.
Although lead occurs naturally in the environment, anthropogenic activities such as fossil
fuels burning, mining, and manufacturing contribute to the release of high concentrations.
Lead has many different industrial, agricultural and domestic applications. It is currently used
in the production of lead-acid batteries, ammunitions, metal products (solder and pipes), and
devices to shield X-rays. An estimated 1.52 million metric tons of lead were used for various
industrial applications in the United Stated in 2004. Of that amount, lead-acid batteries
production accounted for 83 percent, and the remaining usage covered a range of products
such as ammunitions (3.5 percent), oxides for paint, glass, pigments and chemicals (2.6
percent), and sheet lead (1.7 percent) [165, 166].
In recent years, the industrial use of lead has been significantly reduced from paints
and ceramic products, caulking, and pipe solder [167]. Despite this progress, it has been
reported that among 16.4 million United States homes with more than one child younger than
6 years per household, 25% of homes still had significant amounts of lead-contaminated
deteriorated paint, dust, or adjacent bare soil [168]. Lead in dust and soil often re-contaminate
cleaned houses [169] and contributes to elevating blood lead concentrations in children who
play on bare, contaminated soil [170]. Today, the largest source of lead poisoning in children
comes from dust and chips from deteriorating lead paint on interior surfaces [171]. Children
who live in homes with deteriorating lead paint can achieve blood lead concentrations of
20µg/dL or greater [172].
Potential for Human Exposure
Exposure to lead occurs mainly via inhalation of lead-contaminated dust particles or aerosols,
and ingestion of lead-contaminated food, water, and paints [173, 174]. Adults absorb 35 to
50% of lead through drinking water and the absorption rate for children may be greater than
50%. Lead absorption is influenced by factors such as age and physiological status. In the
human body, the greatest percentage of lead is taken into the kidney, followed by the liver
and the other soft tissues such as heart and brain but the lead in the skeleton represents the
major body fraction [175]. The nervous system is the most vulnerable target of lead
12
poisoning. Headache, poor attention spam, irritability, loss of memory and dullness are the
early symptoms of the effects of lead exposure on the central nervous system (CNS).
Since the late 1970’s, lead exposure has decreased significantly as a result of multiple
efforts including the elimination of lead in gasoline, and the reduction of lead levels in
residential paints, food and drink cans, and plumbing systems [173, 174]. Several federal
programs implemented by state and local health governments have not only focused on
banning lead in gasoline, paint and soldered cans, but have also supported screening
programs for lead poisoning in children and lead abatement in housing [167]. Despite the
progress in these programs, human exposure to lead remains a serious health problem [176,
177]. Lead is the most systemic toxicant that affects several organs in the body including the
kidneys, liver, central nervous system, hematopoetic system, endocrine system, and
reproductive system [173].
Lead exposure usually results from lead in deteriorating household paints, lead in the
work place, lead in crystals and ceramic containers that leaches into water and food, lead use
in hobbies, and lead use in some traditional medicines and cosmetics [167, 174]. Several
studies conducted by the National Health and Nutrition Examination surveys (NHANES)
have measured blood lead levels in the U.S. populations and have assessed the magnitude of
lead exposure by age, gender, race, income and degree of urbanization [176]. Although the
results of these surveys have demonstrated a general decline in blood lead levels since the
1970s, they have also shown that large populations of children continue to have elevated
blood lead levels (> 10µg/dL). Hence, lead poisoning remains one of the most common
pediatric health problems in the United States today [167, 173, 174, 176-179]. Exposure to
lead is of special concern among women particularly during pregnancy. Lead absorbed by the
pregnant mother is readily transferred to the developing fetus [180]. Human evidence
corroborates animal findings [181], linking prenatal exposure to Pb with reduced birth weight
and preterm delivery [182], and with neuro-developmental abnormalities in offspring [183].
Molecular Mechanisms of Toxicity and Carcinogenicity
There are many published studies that have documented the adverse effects of lead in
children and the adult population. In children, these studies have shown an association
between blood level poisoning and diminished intelligence, lower intelligence quotient-IQ,
delayed or impaired neurobehavioral development, decreased hearing acuity, speech and
language handicaps, growth retardation, poor attention span, and anti social and diligent
behaviors [178, 179, 184, 185]. In the adult population, reproductive effects, such as
decreased sperm count in men and spontaneous abortions in women have been associated
with high lead exposure [186, 187]. Acute exposure to lead induces brain damage, kidney
damage, and gastrointestinal diseases, while chronic exposure may cause adverse effects on
the blood, central nervous system, blood pressure, kidneys, and vitamin D metabolism [173,
174, 178, 179, 184-187].
One of the major mechanisms by which lead exerts its toxic effect is through
biochemical processes that include lead's ability to inhibit or mimic the actions of calcium
and to interact with proteins [173]. Within the skeleton, lead is incorporated into the mineral
in place of calcium. Lead binds to biological molecules and thereby interfering with their
function by a number of mechanisms. Lead binds to sulfhydryl and amide groups of enzymes,
altering their configuration and diminishing their activities. Lead may also compete with
essential metallic cations for binding sites, inhibiting enzyme activity, or altering the transport
of essential cations such as calcium [188]. Many investigators have demonstrated that lead
intoxication induces a cellular damage mediated by the formation of reactive oxygen species
(ROS) [189]. In addition, Jiun and Hseien [190] demonstrated that the levels of
malondialdehyde (MDA) in blood strongly correlate with lead concentration in the blood of
13
exposed workers. Other studies showed that the activities of antioxidant enzymes, including
superoxide dismutase (SOD), and glutathione peroxidase in erythrocytes of workers exposed
to lead are remarkably higher than that in non-exposed workers [191]. A series of recent
studies in our laboratory demonstrated that lead-induced toxicity and apoptosis in human
cancer cells involved several cellular and molecular processes including induction of cell
death and oxidative stress [29, 192], transcriptional activation of stress genes [30], DNA
damage [29], externalization of phosphatidylserine and activation of caspase-3 [193].
A large body of research has indicated that lead acts by interfering with calcium-
dependent processes related to neuronal signaling and intracellular signal transduction. Lead
perturbs intracellular calcium cycling, altering releasability of organelle stores, such as
endoplasmic reticulum and mitochondria [194, 195]. In some cases lead inhibits calcium-
dependent events, including calcium-dependent release of several neurotransmitters and
receptor-coupled ionophores in glutamatergic neurons [196]. In other cases lead appears to
augment calcium-dependent events, such as protein kinase C and calmodulin [194, 197].
Experimental studies have indicated that lead is potentially carcinogenic, inducing
renal tumors in rats and mice [198, 199], and is therefore considered by the IARC as a
probable human carcinogen [200]. Lead exposure is also known to induce gene mutations and
sister chromatid exchanges [201, 202], morphological transformations in cultured rodent cells
[203], and to enhance anchorage independence in diploid human fibroblasts [204]. In vitro
and in vivo studies indicated that lead compounds cause genetic damage through various
indirect mechanisms that include inhibition of DNA synthesis and repair, oxidative damage,
and interaction with DNA-binding proteins and tumor suppressor proteins. Studies by Roy
and his group showed that lead acetate induced mutagenicity at a toxic dose at the E. coli gpt
locus transfected to V79 cells [205]. They also reported that toxic doses of lead acetate and
lead nitrate induced DNA breaks at the E. coli gpt locus transfected to V79 cells [205].
Another study by Wise and his collaborators found no evidence for direct genotoxic or DNA-
damaging effects of lead except for lead chromate. They pointed out that the genotoxicity
may be due to hexavalent chromate rather than lead [206].
Mercury
Environmental Occurrence, Industrial Production and Use
Mercury is heavy metal belonging to the transition element series of the periodic table. It is
unique in that it exists or is found in nature in three forms (elemental, inorganic, and organic),
with each having its own profile of toxicity [207]. At room temperature elemental mercury
exists as a liquid which has a high vapor pressure and is released into the environment as
mercury vapor. Mercury also exists as a cation with oxidation states of +1 (mercurous) or +2
(mercuric) [208]. Methylmercury is the most frequently encountered compound of the
organic form found in the environment, and is formed as a result of the methylation of
inorganic (mercuric) forms of mercury by microorganisms found in soil and water [209].
Mercury is a widespread environmental toxicant and pollutant which induces severe
alterations in the body tissues and causes a wide range of adverse health effects [210]. Both
humans and animals are exposed to various chemical forms of mercury in the environment.
These include elemental mercury vapor (Hg0), inorganic mercurous (Hg+1), mercuric (Hg+2),
and the organic mercury compounds [211]. Because mercury is ubiquitous in the
environment, humans, plants and animals are all unable to avoid exposure to some form of
mercury [212].
Mercury is utilized in the electrical industry (switches, thermostats, batteries),
dentistry (dental amalgams), and numerous industrial processes including the production of
14
caustic soda, in nuclear reactors, as antifungal agents for wood processing, as a solvent for
reactive and precious metal, and as a preservative of pharmaceutical products [213]. The
industrial demand for mercury peaked in 1964 and began to sharply decline between 1980
and 1994 as a result of federal bans on mercury additives in paints, pesticides, and the
reduction of its use in batteries [214].
Potential for Human Exposure
Humans are exposed to all forms of mercury through accidents, environmental pollution,
food contamination, dental care, preventive medical practices, industrial and agricultural
operations, and occupational operations [215]. The major sources of chronic, low level
mercury exposure are dental amalgams and fish consumption. Mercury enters water as a
natural process of off-gassing from the earth’s crust and also through industrial pollution
[216]. Algae and bacteria methylate the mercury entering the waterways. Methyl mercury
then makes its way through the food chain into fish, shellfish, and eventually into humans
[217].
The two most highly absorbed species are elemental mercury (Hg0) and methyl
mercury (MeHg). Dental amalgams contain over 50% elemental mercury [218]. The
elemental vapor is highly lipophilic and is effectively absorbed through the lungs and tissues
lining the mouth. After it enters the blood, it rapidly passes through cell membranes, which
include both the blood-brain barrier and the placental barrier [219]. Once it gains entry into
the cell, Hg0 is oxidized and becomes highly reactive Hg2+. Methyl mercury derived from
eating fish is readily absorbed in the gastrointestinal tract and because of its lipid solubility,
can easily cross both the placental and blood-brain barriers. Once mercury is absorbed it has a
very low excretion rate. A major proportion of what is absorbed accumulates in the kidneys,
neurological tissue and the liver. All forms of mercury are toxic and their effects include
gastrointestinal toxicity, neurotoxicity, and nephrotoxicity [213].
Molecular Mechanisms of Mercury Toxicity
The molecular mechanisms of toxicity of mercury are based on its chemical activity and
biological features which suggest that oxidative stress is involved in its toxicity [220].
Through oxidative stress mercury has shown mechanisms of sulfhydryl reactivity. Once in
the cell both Hg2+ and MeHg form covalent bonds with cysteine residues of proteins and
deplete cellular antioxidants. Antioxidants enzymes serve as a line of cellular defense against
mercury compounds [221]. The interaction of mercury compounds suggests the production of
oxidative damage through the accumulation of reactive oxygen species which would
normally be eliminated by cellular antioxidants.
In eukaryotic organisms the primary site for the production of reactive oxygen
species occurs in the mitochondria through normal metabolism [222]. Inorganic mercury has
been reported to increase the production of these reactive oxygen species through causing
defects in oxidative phosphorylation and electron transport at the ubiquinone-cytochrome b5
step [223]. Through the acceleration of the rate of electron transfer in the electron transport
chain in the mitochondria, mercury induces the premature shedding of electrons to molecular
oxygen which causes an increase in the generation of reactive oxygen species [224].
Oxidative stress appears to also have an effect on calcium homeostasis. The role of
calcium in the activation of proteases, endonucleases and phospholipases is well established.
The activation of phospholipase A2 has been shown to result in an increase in reactive oxygen
species through the increase generation of arachidonic acid. Arachidonic acid has also been
shown to be an important target of reactive oxygen species [225]. Both organic and inorganic
15
mercury have been shown to alter calcium homeostasis but through different mechanisms.
Organic mercury compounds (MeHg) are believed to increase intracellular calcium through
accelerating the influx of calcium from the extracellular medium and mobilizing intracellular
stores, while inorganic mercury (Hg2+) compounds increase intracellular calcium stores only
through the influx of calcium from the extracellular medium [226]. Mercury compounds have
also been shown to induce increased levels of MDA in both the livers, kidneys, lungs and
testes of rats treated with HgCl2 [227]. This increase in concentration was shown to correlate
with the severity of hepatotoxicity and nephrotoxicity [228]. HgCl2-induced lipid
peroxidation was shown to be significantly reduced by antioxidant pretreatment with
selenium. Selenium has been shown to achieve this protective effect through direct binding
to mercury or serving as a cofactor for glutathione peroxidase and facilitating its ability to
scavenge reactive oxygen species [229]. Vitamin E was also shown to protect against HgCl2-
induced lipid peroxidation in the liver [230].
Molecular Mechanisms of Mercury Carcinogenicity
Metal-induced carcinogenicity has been a research subject of great public health interest.
Generally, carcinogenesis is considered to have three stages including initiation, promotion,
and progression and metastasis. Although mutations of DNA, which can activate oncogenesis
or inhibit tumor suppression, were traditionally thought to be crucial factors for the initiation
of carcinogenesis, recent studies have demonstrated that other molecular events such as
transcription activation, signal transduction, oncogene amplification, and recombination, also
constitute significant contributing factors [231, 232]. Studies have shown that mercury and
other toxic metals effect cellular organelles and adversely affect their biologic functions [231,
233]. Accumulating evidence also suggests that reactive oxygen species play a major role in
the mediation of metal-induced cellular responses and carcinogenesis [234-236].
The connection between mercury exposure and carcinogenesis is very controversial.
While some studies have confirmed its genotoxic potential, others have not shown an
association between mercury exposure and genotoxic damage [237]. In studies implicating
mercury as a genotoxic agent, oxidative stress has been described has the molecular
mechanism of toxicity. Hence, mercury has been shown to induce the formation of reactive
oxygen species known to cause DNA damage in cells, a process which can lead to the
initiation of carcinogenic processes [238, 239]. The direct action of these free radicals on
nucleic acids may generate genetic mutations. Although mercury-containing compounds are
not mutagenic in bacterial assays, inorganic mercury has been shown to induce mutational
events in eukaryotic cell lines with doses as low as 0.5 µM [240]. These free radicals may
also induce conformational changes in proteins that are responsible for DNA repair, mitotic
spindle, and chromosomal segregation [241]. To combat these effects, cells have antioxidant
mechanisms that work to correct and avoid the formation of reactive oxygen species (free
radicals) in excess. These antioxidant mechanisms involve low molecular molecules such as
vitamins C and E, melatonin, glutathione, superoxide dismutase, catalase, glutathione
peroxidase and glutathione reductase. Cells respond to mercury exposures by increasing the
levels of these low molecular weight molecules which protect them through their antioxidant
capacity and the chelation of mercury [242].
Glutathione levels in human populations exposed to methylmercury intoxication by
eating contaminated fish have been shown to be higher than normal. These studies were also
able to confirm a direct and positive correlation between mercury and glutathione levels in
blood. They also confirmed an increased mitotic index and polyploidal aberrations associated
with mercury exposure [243]. Epidemiological studies have demonstrated that enzymatic
activity was altered in populations exposed to mercury; producing genotoxic alterations, and
16
suggesting that both chronic and relatively low level mercury exposures may inhibit enzyme
activity and induce oxidative stress in the cells [244]. There is no doubt that the connection
between mercury exposure and carcinogenesis is very controversial. However, in-vitro
studies suggest that the susceptibility to DNA damage exists as a result of cellular exposure to
mercury. These studies also indicate that metal-induced toxicity and carcinogenicity may be
cell-, organ- and/or species- specific.
Prospects
A comprehensive analysis of published data indicates that heavy metals such as arsenic
cadmium, chromium, lead, and mercury, occur naturally. However, anthropogenic activities
contribute significantly to environmental contamination. These metals are systemic toxicants
known to induce adverse health effects in humans, including cardiovascular diseases,
developmental abnormalities, neurologic and neurobehavioral disorders, diabetes, hearing
loss, hematologic and immunologic disorders, and various types of cancer. The main
pathways of exposure include ingestion, inhalation, and dermal contact. The severity of
adverse health effects is related to the type of heavy metal and its chemical form, and is also
time- and dose-dependent. Among many other factors, speciation plays a key role in metal
toxicokinetics and toxicodynamics, and is highly influenced by factors such as valence state,
particle size, solubility, biotransformation, and chemical form. Several studies have shown
that toxic metals exposure causes long term health problems in human populations. Although
the acute and chronic effects are known for some metals, little is known about the health
impact of mixtures of toxic elements. Recent reports have pointed out that these toxic
elements may interfere metabolically with nutritionally essential metals such as iron, calcium,
copper, and zinc [245, 246]. However, the literature is scarce regarding the combined toxicity
of heavy metals. Simultaneous exposure to multiple heavy metals may produce a toxic effect
that is either additive, antagonistic or synergistic.
A recent review of a number of individual studies that addressed metals interactions
reported that co-exposure to metal/metalloid mixtures of arsenic, lead and cadmium produced
more severe effects at both relatively high dose and low dose levels in a biomarker-specific
manner [247]. These effects were found to be mediated by dose, duration of exposure and
genetic factors. Also, human co-exposure to cadmium and inorganic arsenic resulted in a
more pronounced renal damage than exposure to each of the elements alone [248]. In many
areas of metal pollution, chronic low dose exposure to multiple elements is a major public
health concern. Elucidating the mechanistic basis of heavy metal interactions is essential for
health risk assessment and management of chemical mixtures. Hence, research is needed to
further elucidate the molecular mechanisms and public health impact associated with human
exposure to mixtures of toxic metals.
Acknowledgement
This research was supported in by the National Institutes of Health RCMI Grant No.
2G12RR013459, and in part by the National Oceanic and Atmospheric Administration ECSC
Grant No. NA06OAR4810164 & Subcontract No. 000953.
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