Metals are essential for the normal functioning of living organisms. Their uses in biological systems are varied, but are frequently associated with sites of critical protein function, such as zinc finger motifs and electron or oxygen carriers. These functions only require essential metals in minute amounts, hence they are termed trace metals. Other metals are, however, less beneficial, owing to their ability to promote a wide variety of deleterious health effects, including cancer. Metals such as arsenic, for example, can produce a variety of diseases ranging from keratosis of the palms and feet to cancers in multiple target organs. The nature and type of metal-induced pathologies appear to be dependent on the concentration, speciation, and length of exposure. Unfortunately, human contact with metals is an inescapable consequence of human life, with exposures occurring from both occupational and environmental sources. A uniform mechanism of action for all harmful metals is unlikely, if not implausible, given the diverse chemical properties of each metal. In this chapter we will review the mechanisms of carcinogenesis of arsenic, cadmium, chromium, and nickel, the four known carcinogenic metals that are best understood. The key areas of speciation, bioavailability, and mechanisms of action are discussed with particular reference to the role of metals in alteration of gene expression and maintenance of genomic integrity.
The association of metal exposure with cancer is a well-documented phenomenon. Metals such as arsenic (As), cadmium (Cd), chromium (Cr), and nickel (Ni) are part of an ever growing list of environmental agents that have been formally classified by the International Agency for Cancer Research (IARC) as being known carcinogens [1–4]. For other metals such as iron, copper, beryllium, lead, and mercury there exists an ever increasing body of evidence to support their inclusion in the IARC listings [5–8]. Iron  and copper , in particular, are carcinogenic in excess, but are highly regulated and generally only produce cancer in animal models or in people with genetic diseases that prevent appropriate metabolic regulation. There is even less information on beryllium carcinogenesis, and no definitive studies that indicate the species, conditions or length of exposure by which lead and mercury metals cause cancer in humans. For these reasons, this review will focus on the known carcinogenic metals: As, Cd, Cr and Ni.
Despite increasing numbers of researchers in the field and the expanding role of metals in environmental health issues, the nature of cancer induction by metals remains a complex and poorly understood process. However, what is known is that metals can promote change in normal cellular functions, leading to aberrant cell growth and development . All metals are now thought to promote cancer by a number of common mechanisms. These include the formation of free radicals, either actively as key players in redox reactions, or through less direct means such as biomethylation [5, 6, 9, 10]. Similarly, many metals can also influence cell control by altering gene regulation [7, 11–14]. In terms of direct damage to DNA, most metals are only weakly mutagenic; however, many are strong co-carcinogens, promoting a synergistic effect in the presence of other cancer-causing agents [5, 6, 15]. Hence, the ability of metals to promote cellular alterations may be far more dynamic than has been classically assumed. Thus, it is the purpose of this review to evaluate mechanisms that are central to the role of metals as carcinogenic agents. This review outlines current evidence related to the mechanisms of genotoxicity and gene expression, as well as other mechanisms unique to specific metals. The principle focus is on those metals for which the IARC has deemed there to be sufficient evidence to classify as carcinogens, and that there is the most information regarding genotoxic mechanisms. Because of the great amount of data now available on this topic, this chapter does not claim to be exhaustive, but will hopefully provide a useful survey of the field, with selected references focusing heavily on recent reviews. ...
Speciation, uptake and health effects of specific metals
... Occupational exposure to arsenic is greatest in mining and metal smelting industries; ...
Arsenic in the environment can take a range of forms, both organic and inorganic. Inorganic arsenic has two possible valencies, arsenite, or As(III) and arsenate, As(V). Arsenite is the more toxic of the two species with cell viability assays indicating that concentrations anywhere from 1 to 10 µM and upwards are able to promote toxicity . Arsenate is approximately three to fivefold less toxic than arsenite, presumably because As(V) requires reduction to As(III) to exert its toxicity. Organoarsenic species can also be formed by biometabolism. Many organoarsenic species are significantly less toxic than inorganic As(III). However, methyl As(III) species can be significantly more toxic than inorganic As(III)  and may contribute to arsenic carcinogenesis. The relative toxicity of the different forms of arsenic is predominantly the result of their different chemical properties, but may also relate to the relative efficiency of their uptake [41, 42], the duration of the exposure, and the time when the toxicity assay is performed [42, 43]. Arsenic excretion rates vary, but it is generally accepted that arsenic, unlike other carcinogenic metals, is rapidly excreted by the body, to the extent that more than 50% is removed within 2 days in acute poisoning cases . ...
Arsenic pathology is complex. ... At very low doses, arsenic appears to have minimal short-term effect however, over longer periods a range of pathologies are seen . Chronic low-dose exposure initially produces blotching of the skin, followed by hyperkeratosis of the palms and soles of the feet. If exposure continues, alterations to peripheral vasculature are seen along with the formation of skin lesions, which left untreated, can become cancerous [47, 48]. Arsenic is also associated with an increased risk for cancer of the lungs, liver and bladder .
Induction of cancer by arsenic is not thought to originate from a single exposure, but rather is the result of gradual changes to a variety of processes within the cell. Different arsenic species enter cells by different mechanisms. Arsenate is able to mimic phosphate, and hence is able to enter cells using phosphate transport proteins. Arsenite, however, is thought to enter through aquaglyceroporins . Once in the blood stream, arsenic is taken to the liver where biometabolism occurs. This process involves the progressive methylation of arsenic, with As(III) converted to the less toxic methyl As(V) species. The ingested arsenic is excreted predominantly in the urine as inorganic As(III) and As(V), methyl As(V), and dimethyl As(V), with the proportions of these being variable and related to arsenic dose [50–52]. Some intermediate trivalent arsenic metabolites are also produced, and can be found in the urine . Despite their greater toxicity, relative to either inorganic As or organic As(V) species, it is yet to be determined whether these methyl As(III) and dimethyl As(III) species play a significant role in carcinogenesis.
Unlike many other metals, cadmium is found in only one valence state, that of Cd(II). Exposure to cadmium has also been far less common than other carcinogenic metals. Of greatest note was the historical use of cadmium as a paint additive giving rise to the bright yellows seen in many paintings, such as those of Claude Monet . Industrial use of cadmium is only a recent phenomenon, beginning in the 1940s. Cadmium is now most commonly encountered in cadmium- nickel battery production , although it continues to be used in paints, as well as in plastic production where it is an effective stabilizing agent. Like arsenic, occupational exposure to cadmium can occur through metal refining processes, where cadmium is often associated with copper and can be released into the atmosphere during heating . The greatest exposure to cadmium, however, comes from cigarette smoke . Particulate cadmium in cigarette smoke collects in the lungs where it can be transported into the bloodstream across the alveoli. Unlike arsenic, cadmium has a long biological half life, considered to be somewhere between 15 and 25 years [4, 56]. This means cadmium can accumulate to levels many times greater than an individual would be subjected to in a single exposure. Cadmium is only a weak mutagen, but is a strong co-mutagen [4, 57, 58]. This is of particular concern for cigarette smokers who simultaneously inhale cadmium and benzo[a]pyrene, as well as a range of other chemicals, including arsenic and other metals.
Health effects of cadmium are quite dissimilar to other metals. Non-toxic doses of cadmium produce a wide variety of effects, many of which are related to bone development and maintenance. Individuals exposed to cadmium can develop osteoporosis, anemia, eosinophilia, emphysema, and renal tubular damage . Long-term cadmium toxicity can produce Itai-Itai disease, in which individuals suffer from bone fractures, severe pain, proteinuria and severe osteomalacia . Acute high-level exposure to cadmium is also able to produce severe lung damage. However, like other metals, prolonged repeated exposures are required to induce carcinogenesis. Target organs for cadmium are varied however, lung cancers predominate . Other tissues subject to malignant transformation by cadmium include the prostate, pancreas and kidney. The testes are also thought to be a site of cadmium carcinogenesis; however, this has only been shown in animal models. Like arsenic, cadmium is only a weak mutagen. This suggests that tumors result from either epigenetic or co-carcinogenic effects, particularly in cases of smoking-induced lung cancer .
Chromium is widely available, complex in action, and used industrially in a myriad of applications including, pigment production, chrome plating, welding, production of ferrochrome metals, leather tanning and as a dietary supplement [3, 60]. Dietary supplementation is of particular interest because of the critical nature of Cr(III) for optimum insulin binding . Occupational exposure to Cr(VI) is a well-established source of human carcinogenesis; however, occupational health initiatives have had a considerable impact in reducing incidence levels. Non-occupational sources of exposure are thought to originate from engine emissions, atmospheric particles released from smelting and refining industries, as well as through cigarette smoke . Chromium speciation is complex, and chromium is often found in compounds with other metals. Environmental chromium is generally found in two principle valency states, the more toxic and carcinogenic Cr(VI)  and the essential Cr(III). Cr(VI) species are readily taken up into cells by phosphate/sulfate anion channels [62–64]. Cr(III), however, cannot move into cells by the same mechanism, and is required at considerably higher concentrations to produce toxicity in cells. It must be noted, however, that not all Cr(VI) species are of equal carcinogenic risk. Animal models have shown that the largely insoluble chromium compounds are far more carcinogenic than their soluble counterparts [3, 65]. It appears that particulate matter containing insoluble chromium is deposited on the epithelial surface of the lung where it accumulates to levels high enough to produce cancer . ...
Unlike arsenic and cadmium, chromium is an essential trace element in its trivalent form. That said, Cr(VI) species can be highly toxic to humans . Inhalation of particulate Cr(VI) can cause irritation to the nasal tissue, leading to nose bleeds, ulceration and formation of lesions in the nasal passage . Damage to lung tissue is also not uncommon [70, 71]. Ingestion of Cr(VI) can cause nausea, vomiting, ulceration of the stomach, damage to the liver and kidney, and finally death . Both species of chromium can cause contact hypersensitivity, leading to rashes, swelling and ulcerations. Cr(VI) is the most carcinogenic form of chromium, with insoluble particulate chromium compounds being the most persistent  and the most hazardous .
Nickel has many common industrial uses, thanks largely to its unique chemical properties. Industrially, it is used in electroplating, electroforming, in circuitry, and in nickel-cadmium batteries. Nickel alloys, including stainless steel, are used in a wide variety of objects, from kitchen knives to building tools . Nickel is also used in jewelry and medical implements. Metallic nickel is non-carcinogenic to humans; however, all other nickel compounds, such as nickel sulfides, oxides, and silicates, and other soluble salts, are known carcinogens . Carcinogenic nickel exposure is greatest through the inhalation of nickel-containing particulates. The burning of fossil fuels, as well as the refining of metals such as copper, introduces considerable amounts of nickel into the atmosphere . Like arsenic, nickel can also be leached from soils and rock, thereby contaminating water supplies. In lower organisms such as bacteria, nickel is an essential trace element found in up to seven different enzymes . Higher organisms, however, have failed to show any definitive role for nickel in normal cellular function. That said, studies in the 1970s and 80s showed that the removal of nickel from the diet of rats had significant effects both physically and mentally, which, with continued exclusion of nickel from the diet, were more profound in the subsequent generations . It may be that nickel is not required for normal cellular function in humans, but rather is essential for our intestinal microflora. Like both arsenic and chromium, nickel occurs in different oxidation states, ranging from I to IV, with Ni(II) being most common in biological systems.
As with chromium, particulate nickel is most harmful to humans, especially in the lung where crystalline nickel becomes lodged in the mucous prior to being phagocytized by both epithelial cells and macrophages . Once inside the cells, the nickel compounds are gradually broken down releasing reactive nickel ions. The phagocytic nature of nickel uptake means considerable amounts of nickel are able to accumulate over time, damaging lung tissue and frequently causing latent effects in individuals who may have been exposed to nickel many years earlier .
Nickel is not overly toxic to individuals at low doses; however, nickel-containing jewelry can produce contact hypersensitivity in many people . This normally results in rashes and inflammation of the region of contact. However, 104 T.R. Durham and E.T. Snow in more extreme reactions, individuals can suffer from asthma attacks. Individuals who inhale nickel fumes for prolonged periods of time frequently develop bronchitis and chronic lung infections. While ingestion of large quantities of nickel is not normally fatal, it can produce stomach aches, kidney pain and blood in the urine . Nickel carcinogenesis is generally limited to the lung, because phagocytosis is necessary to bring the nickel ions to the DNA in the target tissue [12, 77].
Metals and oxidative stress
Most, if not all, of the carcinogenic metals, have the capacity to produce a variety of radical species that can damage cells. Arsenic, chromium, copper, iron, nickel and, to a lesser extent, cadmium, have all been shown to be able to participate in reactions resulting in the formation of reactive oxygen, sulfur or nitrogen species (for reviews see [6, 7, 11, 14, 27, 76–79]). In most cases these metals produce either radicals based on oxygen species or those based on nitrogen species; however, the formation of oxygen species appears to predominate. The formation of radical species can originate from a variety of sources, from redox cycling, through Fenton/Haber-Weiss chemistry, as products of biometabolism, as messengers in signal cascades, and as normal products of cellular metabolism [6, 80, 81]. Essential transition metals, such as iron and copper, are most likely to participate in redox cycling and Fenton/Haber- Weiss chemistry; however, these metals are highly regulated and are of less concern with regard to carcinogenesis. Nevertheless, other carcinogenic metals may also react in similar fashion and thereby produce reactive species that can cause DNA damage and mutations. Some of the key reactions responsible for the metal-related formation of reactive oxygen species (ROS) are described briefly below. ...
Mechanisms of metal induced alterations in DNA repair
DNA is a dynamic molecule, constantly under assault from both endogenous and exogenous agents, which can often facilitate mutational changes to its sequence. DNA replication also causes changes in genetic material through the infidelity of replication enzymes, most notably during bypass of DNA lesions. The error rate of replication and repair of endogenous base damage has been shown to lead to the formation of lesions with a frequency of one in every 104–109 bases per cell per day . To combat this, cells have developed a variety of DNA repair mechanisms. In mammalian cells these repair processes fall within several distinct pathways: mismatch repair (MMR), homologous and non-homologous rejoining, nucleotide excision repair, base excision repair, and direct reversion of damage. Alterations in the regulation and activity of repair processes have been shown to occur through interactions of cells with a variety of agents, including many metals. Interference by metal ions with DNA repair has the capacity to increase the potential for mutations, which then persist in the genome. A major consequence of this is the initiation of carcinogenesis. The following paragraphs outline the repair processes that have been shown to be affected by As, Cr, Cd, and Ni. ...
DNA and protein interactions
The formation of metal complexes with amino acids, proteins and DNA is common in cells. Interactions of this nature have been speculated to have a wide range of consequences, including initiation of signal cascades, constitutive activation or inactivation of enzymes, as well as inhibition of both DNA repair and replication. Arsenic, chromium and nickel all exhibit the capacity to create or become part of a variety of complexes in cells. Cadmium and other metals may also form protein complexes, although the role of these complexes in carcinogenesis is less well understood. ...
Effects on gene regulation: direct and epigenetic changes
Metals have been shown to alter the expression of a great number of genes, too many to cover in detail here. These changes in gene expression are generally 114 T.R. Durham and E.T. Snow transient, and can be produced or caused by a multitude of different factors. Accordingly, this section looks at a limited number of genes that best illustrate the effects of carcinogenic metals on gene expression. For more detailed information on gene expression, the following recent reviews cover each metal in detail [10, 12–14]. Changes in gene expression are often thought to be the indirect result of signal cascades, DNA methylation changes and ROS; however, metals may also be directly responsible for changes in transcription factor activity. Epigenetic mechanisms are heritable changes that can impart effects on the regulation of genes without altering the genomic sequence itself. Hypermethylation generally causes genes to be downregulated or effectively switched off, while hypomethylation often results in increased levels of gene expression. A number of agents that induce carcinogenesis, such as X-rays, have been shown to affect cells in this manner . Similarly, nickel, arsenic, and, to a lesser extent, cadmium and chromium, are able to produce extensive alterations in genomic methylation [10, 23, 25, 97, 225–228]. ...
Agents responsible for human carcinogenesis are grossly varied in their properties, and metals are no exception. However, it seems likely that metals share several common means by which to induce cancer. Critically, the most important of these appears to be the generation of oxidative stress and deregulation of key maintenance genes within cells. That said, the nature of the dose of each of these metals, as well as confounding variables required to produce a carcinogenesis, remain at best an unresolved issue. However, with time, and as research progresses, it is likely that a more complete picture will emerge on metal-induced carcinogenesis.