Superfund Research Program
Understanding the Molecular Basis for Metal-Induced Cancers
Arsenic and chromium are naturally occurring metallic elements that enter aquatic, terrestrial, and atmospheric systems from both natural sources and human activities. However, in areas of the country where metal contamination is a serious problem, the contributions from human activities are oftentimes greater than those from natural sources. For instance, in certain locations of the country the two metals are soil and groundwater contaminants as a result of past waste disposal practices. Throughout the United States, arsenic has been detected on at least 781 of 1,300 Superfund hazardous waste sites, while chromium has been detected on at least 115 sites. These sites include locations where arsenic containing pesticides and herbicides were once disposed in pits and buried, and where chromium wastewater was dumped directly onto the ground during metal plating operations. In some cases this contamination infiltrated water supplies, exposing humans to significant levels of the contaminants.
These exposures are a considerable source of public health concern because both metals are now recognized as human carcinogens. Based on epidemiological studies, the trivalent form of arsenic is associated with lung cancer from inhalation and with skin, bladder, and lung cancer from ingestion, while the hexavalent form of chromium is strongly associated with lung cancer from inhalation. In determining the possible health effects of environmental exposures to these agents, there is interest in elucidating how the metal alters normal cellular processes to produce toxicity. Understanding the mechanisms underlying the pathological effects of arsenic and chromium is critical not only for evaluating the impact of past exposures, but also for setting appropriate standards that protect the health of current and future generations.
Although arsenic and chromium are both human carcinogens and share the lung as a common target, they appear to act very differently at the cellular level. Based on bacterial and mammalian mutagenesis assays, chromium is clearly genotoxic and mutagenic; in contrast, arsenic shows little propensity to damage DNA and is generally considered a non-genotoxic metal. Based on this available knowledge, scientists hypothesize these two agents operate through unique mechanisms when they act as carcinogens in the body. Beyond this, however, very little is known about the biological processes underlying metal-induced carcinogenesis.
To develop a greater understanding of how these and other metals act as toxins, researchers at Dartmouth College are examining the molecular basis for the biological effects of arsenic and chromium, focusing on how these metals alter inducible gene expression. The hypothesis under study is that chromium and arsenic each target specific molecular sites, leading to selective alterations in expression of a sub-set of genes that may contribute to the cancer process.
Genes are regulated by an interaction between transcription factor proteins and DNA regulatory elements within the regulatory "promoter" regions of genes. The transcription factors are responsive to cell signaling pathways in the cytoplasm of the cell. This allows the cell to receive information from and interact with its environment. The scientists hypothesize that chromium preferentially targets both specific signaling pathways and the transcription factors they control, as well as interacts directly with certain DNA sites within the promoter regions of sensitive genes. In contrast, arsenic is proposed to interact principally with cell signaling pathways and transcription factors, but not directly with DNA to alter gene expression. By testing these possibilities the researchers may uncover potential mechanisms of metal-induced carcinogenesis.
The Dartmouth scientists have focused on the effects of chromium and arsenic on expression of two model inducible genes, phosphoenolpyruvate carboxykinase (PEPCK) and MDR1. PEPCK is a hormone-regulated gene whose normal regulation has been well studied and whose expression has previously been shown by this group to be strongly affected in vivo by single treatments with very low, non-toxic doses of chromium(VI) or arsenic(III). Thus, this is a useful model for examining the molecular basis for these selective metal effects on inducible gene expression. The MDR1 gene is also highly sensitive to alterations by low dose chromium or arsenic treatments. MDR1 codes for P-glycoprotein, which is one of the major contributors to development of multidrug resistance by human tumors during cancer chemotherapy. Thus, studying metal-induced alterations in expression of MDR1 in pre-malignant or malignant cells may contribute to understanding their overall cancer phenotype and resistance to treatment.
In recent work, the Dartmouth researchers have found that low doses of either chromium or arsenic strongly alter several important cell-signaling pathways important for gene regulation, including the glucocorticoid hormone receptor pathway. However, the patterns of these effects were metal-, dose and cell type-specific. In the PEPCK model system, they have also recently narrowed down the apparent genetic targets for metal-mediated effects to two small DNA elements within the promoter region of the PEPCK gene. These DNA elements also mediate the normal regulation of the PEPCK gene by glucocorticoid hormones. Inclusion of the region of the PEPCK promoter that contains these hormone-regulatory elements into an artificial gene confers chromium and arsenic sensitivity to the artificial gene. Conversely, mutation of these elements abolishes both hormone responsiveness and metal sensitivity. These results further suggest that the glucocorticoid receptor pathway is one of the principal targets for chromium and arsenic effects on expression of the PEPCK gene. In parallel studies with MDR1, other genetic elements appear to mediate that gene's sensitivity to chromium and arsenic treatments. However, the MDR1 gene is also responsive to glucocorticoid hormones; thus, several pathways may contribute to the overall effects of metals on expression of this gene.
These studies demonstrate that arsenic and chromium target specific regulatory pathways within cells to selectively alter gene expression. Some of these pathways are affected by both metals, whereas others respond distinctly to one and not the other. There are also differences in response of cells from different tissues of origin to the same metal treatment, suggesting a possible basis for the tissue-specific toxicity and carcinogenesis caused by these agents in vivo. Identification of specific genes such as MDR1 that are responsive to chromium and arsenic, and that are directly involved in the cancer phenotype of cells may provide important new insights to our overall understanding of how these agents contribute to human cancer. In addition to adding to the body of knowledge on metal-induced pathologies, these studies are significant for investigating doses of metals that are highly relevant to current U.S. exposures. For example, the doses of arsenic used in these studies fall roughly between the current arsenic drinking water standard of 50 ppb and the proposed new range of 2-10 ppb, doses that significantly altered gene expression. Identification of individual genes that are highly sensitive to low dose arsenic or chromium treatments may also provide new molecular markers of heavy metal exposure for use in ecological fieldwork, epidemiological studies, and exposure/risk assessment.
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To learn more about this research, please refer to the following sources:
- Hamilton JW, Kaltreider RC, Bajenova OV, Ihnat MA, McCaffrey J, Turpie BW, Rowell EE, Oh J, Nemeth MJ, Pesce CA, Lariviere JP. 1998. Molecular basis for effects of carcinogenic heavy metals on inducible gene expression. Environ Health Perspect 106Suppl.4:1005-1015. PMID:9703486
- Ihnat MA, Lariviere JP, Warren AJ, La Ronde N, Blaxall JN, Pierre KM, Turpie BW, Hamilton JW. 1997. Suppression of p-glycoprotein expression and multi-drug resistance by DNA crosslinking agents. Clin Cancer Res 3:1339-1346. PMID:9815817
- McCaffrey J, Wolf CM, Hamilton JW. 1994. Effects of the genotoxic carcinogen chromium (VI) on basal and hormone-inducible phosphoenolpyruvate carboxykinase gene expression in vivo: correlation with glucocorticoid- and developmentally-regulated expression. Mol Carcinog 10:189-198. PMID:8068179
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