An assessment of the tumorigenic properties of a Hudson County soil sample heavily contaminated with hexavalent chromium

Carcinogenic Mechanisms of Hexavalent Chromium: From DNA Breaks to Chromosome Instability and Neoplastic Transformation

PURPOSE OF REVIEW

Hexavalent chromium [Cr(VI)] is a well-established human carcinogen, yet the mechanisms by which it leads to carcinogenic outcomes is still unclear. As a driving factor in its carcinogenic mechanism, Cr(VI) causes DNA double strand breaks and break-repair deficiency, leading to the development of chromosome instability. Therefore, the aim of this review is to discuss studies assessing Cr(VI)-induced DNA double strand breaks, chromosome damage and instability, and neoplastic transformation including cell culture, experimental animal, human pathology and epidemiology studies.

RECENT FINDINGS

Recent findings confirm Cr(VI) induces DNA double strand breaks, chromosome instability and neoplastic transformation in exposed cells, animals and humans, emphasizing these outcomes as key steps in the mechanism of Cr(VI) carcinogenesis. Moreover, recent findings suggest chromosome instability is a key phenotype in Cr(VI)-neoplastically transformed clones and is an inheritable and persistent phenotype in exposed cells, once more suggesting chromosome instability as central in the carcinogenic mechanism. Although limited, some studies have demonstrated DNA damage and epigenetic modulation are also key outcomes in biopsies from chromate workers that developed lung cancer. Additionally, we also summarized new studies showing Cr(VI) causes genotoxic and clastogenic effects in cells from wildlife, such as sea turtles, whales, and alligators. Overall, across the literature, it is clear that Cr(VI) causes neoplastic transformation and lung cancer. Many studies measured Cr(VI)-induced increases in DNA double strand breaks, the most lethal type of breaks clearly showing that Cr(VI) is genotoxic. Unrepaired or inaccurately repaired breaks lead to the development of chromosome instability, which is a common phenotype in Cr(VI) exposed cells, animals, and humans. Indeed, many studies show Cr(VI) induces both structural and numerical chromosome instability. Overall, the large body of literature strongly supports the conclusion that Cr(VI) causes DNA double strand breaks, inhibits DNA repair and chromosome instability, which are key to the development of Cr(VI)-induced cell transformation.

Is hexavalent chromium carcinogenic via ingestion? A weight-of-evidence review

Hexavalent chromium [Cr(VI)] is recognized as a human carcinogen via inhalation, based on elevated rates of lung cancer among occupationally exposed workers in certain industries. Cr(VI) is also genotoxic in bacterial and mammalian cell lines. In contrast, scientific panels in the United States and abroad have reviewed the weight of evidence (WOE) and decided that the available data are insufficient to conclude that Cr(VI) is an oral carcinogen. A criterion of 0.2 ppb was established by a California agency for Cr(VI) in drinking water to prevent cancer, however, this criterion was withdrawn in November, 2001. This criterion was remarkably lower than the promulgated California and federal drinking-water standards for total chromium of 50 ppb and 100 ppb, respectively. Both of the promulgated standards are designed to be protective of humans who ingest Cr(VI). This article describes a WOE analysis to examine the likelihood that Cr(VI) in drinking water poses a cancer hazard at the current U.S. drinking-water standard. The results indicate that: (1) From the historical epidemiological studies, there are a few reports of increased rates of digestive system cancer among Cr(VI)-exposed workers, although most are not statistically significant; (2) the preponderance of evidence from recent epidemiological studies of Cr(VI)-exposed workers does not support an increased risk of cancer outside of the respiratory system; (3) studies of four environmentally exposed populations are negative; (4) there is only one lifetime animal feeding study, and the findings from that study are considered to be flawed and inconclusive; and (5) recent kinetics and in vivo genotoxicity data demonstrate that Cr(VI) is reduced to nontoxic Cr(III) in saliva, in the acidic conditions of the stomach, and in blood. In short, at concentrations at least as high as the current U.S. maximum contaminant level (100 ppb), and probably at least an order of magnitude higher, Cr(VI) is reduced to Cr(III) prior to or upon systemic absorption. The weight of scientific evidence supports that Cr(VI) is not carcinogenic in humans via the oral route of exposure at permissible drinking-water concentrations.

Threshold mechanisms and site specificity in chromium(VI) carcinogenesis

Ten years have elapsed since the International Agency for Research on Cancer (IARC) evaluated the carcinogenicity of chromium and chromium compounds. Further studies performed during the last decade have provided further epidemiological, experimental and mechanistic data which support the IARC conclusions. A wealth of results indicate that, at variance with chromium(0) and chromium(III), chromium(VI) can induce a variety of genetic and related effects in vitro. The lack of carcinogenicity of chromium(0) and chromium(III) compounds in experimental animals is well established, and only a minority of animal carcinogenicity data with chromium(VI) compounds were positive (30 out of 70, i.e. 42.9%). Moreover, most positive studies used administration routes which do not mimic any human exposure and by-pass physiological defense mechanisms. Typically, positive results were only obtained at implantation sites and at the highest dose tested. Exposure to chromium(VI) has been known for more than a century to be associated with induction of cancer in humans. Carcinogenicity requires massive exposures, as is only encountered in well defined occupational settings, and is site specific, being specifically targeted to the lung and, in some cases, to the sinonasal cavity. Increased death rates for cancers at other sites, which were occasionally reported in some epidemiological studies, were almost invariably not statistically significant, and inconsistent (being counterbalanced by other studies which apparently showed decreased rates for the same cancers). As we recently quantified in human body compartments, chromium(VI) can be reduced in body fluids and non-target cells, which results in its detoxification, due to the poor ability of chromium(III) to cross cell membranes. In target cells, chromium(VI) tends to be metabolized by a network of mechanisms leading to generation of reduced chromium species and reactive oxygen species, which will result either in activation or in detoxification depending on the site of the intracellular reduction and its proximity to DNA. When introduced by the oral route, chromium(VI) is efficiently detoxified upon reduction by saliva and gastric juice, and sequestration by intestinal bacteria. If some chromium(VI) is absorbed by the intestine, it is massively reduced in the blood of the portal system and then in the liver. These mechanisms explain the lack of genotoxicity, carcinogenicity, and induction of other long-term health effects of chromium (VI) by the oral route. Within the respiratory tract, chromium(VI) is reduced in the epithelial-lining fluid, pulmonary alveolar macrophages, bronchial tree and peripheral lung parenchyma cells. Hence, lung cancer can only be induced when chromium(VI) doses overwhelm these defense mechanisms. The efficient uptake and reduction of chromium(VI) in red blood cells explains its lack of carcinogenicity at a distance from the portal of entry into the body. All experimental and epidemiological data, and the underlying mechanisms, point to the occurrence of thresholds in chromium(VI) carcinogenesis.

Field method for the determination of hexavalent chromium by ultrasonication and strong anion-exchange solid-phase extraction

A simple, fast, sensitive, and economical field method was developed and evaluated for the determination of hexavalent chromium (CrVI) in environmental and workplace air samples. By means of ultrasonic extraction in combination with a strong anion-exchange solid-phase extraction (SAE-SPE) technique, the filtration, isolation, and determination of CrVI in the presence of trivalent chromium (CrIII) and potential interferents was achieved. The method entails (1) ultrasonication in basic ammonium buffer solution to extract CrVI from environmental matrixes; (2) SAE-SPE to separate CrVI from CrIII and interferences; (3) elution/acidification of the eluate; (4) complexation of chromium with 1,5-diphenylcarbazide; and (5) spectrophotometric determination of the colored chromium-diphenylcarbazone complex. Several critical parameters were optimized in order to effect the extraction of both soluble (K2CrO4) and insoluble (PbCrO4) forms of CrVI without inducing CrIII oxidation or CrVI reduction. The method allowed for the dissolution and purification of CrVI from environmental and workplace air sample matrixes for up to 24 samples simultaneously in less than 90 min (including ultrasonication). The results demonstrated that the method was simple, fast, quantitative, and sufficiently sensitive for the determination of occupational exposures of CrVI. The method is applicable for on-site monitoring of CrVI in environmental and industrial hygiene samples.