The Wise Laboratory of Environmental and Genetic Toxicology

Arsenic Toxicology Studies

Background Experimental Studies | References | Wise Laboratory Publications |Collaborators | Funding

 

 

Background


Arsenic is a health concern and a major environmental problem in the United States and worldwide (1). Inorganic arsenic is known to be a human carcinogen causing skin, lung, digestive tract, liver, bladder, kidney and lymphatic and hematopoietic cancers (1,2). The main cause of arsenic-related death is lung cancer (3). Currently, the United States is the world's leading consumer of arsenic. Arsenic has been considered the number one priority on the United States Environmental Protection Agency's (EPA) and the Agency for Toxic Substances and Disease Registry's (ATSDR) hazard lists for chemicals in the environment since 1997. Estimates now indicate that the majority of the hazardous waste sites on the National Priority List contain high levels of arsenic (4). The general population is exposed to arsenic mainly through consumption of foods with the estimated daily dietary intake of inorganic arsenic being from 1 to 20 ug. Additively, exposure occurs from drinking water contaminated with arsenic from pesticides, natural mineral deposits or improper disposal of arsenic compounds. The general population also may be exposed to arsenic compounds emitted to the air by pesticide manufacturing facilities, smelters, cotton gins, glass manufacturing operations, cigarette smoking, burning of fossil fuels, and other sources. Thus, exposure of the general public to significant levels of arsenic is widespread.

Arsenic bottle

Inorganic Arsenic dissolved in water is colorless and odorless.


Arsenic is a major concern for public health in Maine. More than half of Maine households obtain their drinking water from domestic household wells, which is the second highest per-capita rate in the U.S. Based on a random sample of households with private wells, 12 percent have arsenic levels above the new federal standard of 10 ppb and arsenic levels have even been reported to reach as high as 5000ppb. This new standard of 10ppb has been estimated to be associated with a 1 per 1000 excess bladder cancer risk from lifetime exposure. Arsenic levels as high as 5000 ppb have been reported. Significantly, Maine ranks first and fifth in the U.S. for bladder cancer mortality rates among females and males respectively. The contribution of drinking water arsenic to elevated bladder cancer rates in Maine, New Hampshire and Vermont, is currently the subject of a National Cancer Institute study.


Arsenic-induced lung cancers target human bronchial cells (HBC). However, despite these observations, the effects of arsenic in HBC and bladder cells are largely unknown. To fully understand how arsenic causes cancer, it is essential that we study its effects in its targets: HBC, and human bladder cells. Our knowledge of arsenic carcinogenicity is currently inadequate because of an absence of appropriate models of its target cells and very little data about how it damages DNA. Our research addresses these critical shortcomings. When completed it will provide: 1) a better understanding of how arsenic causes cancer; 2) essential information to better assess the relative risk of exposure to arsenic; and 3) key information about the basics of DNA repair.

Experimental Studies


Our main arsenic project is aimed at determining the effects of arsenic on mitotic progression and chromatid cohesion. We have found that arsenic destabilizes the structures of chromosomes causing centromere spreading and a lack of chromatid separation. Remarkably, these aberrant metaphase cells are able to prematurely enter anaphase and, consequently, cells with a polyploid number of cells emerge after arsenic exposure. These emergent cells with too many chromosomes may be a key mechanism for arsenic-induced tumors.


Figure 1 illustrates the effects of arsenic on the centromere. Figure 1A shows a normal metaphase from a human lung cell that has not been exposed to arsenic. Note the red arrow pointing to a typical normal centromere (the narrow restriction point between the arms). Figure 1B shows a metaphase from a human lung cell treated with arsenic. Note the separated centromere.

Arsenic chromosomes

Figure 1: Effects of Arsenic on the centromere. a) Normal metaphase spread b) Separated centromere


Figure 2 illustrates a cell that has entered anaphase prematurely. The cells were also treated during the last hour of treatment with demecolchicine, an agent known to stop cells in metaphase and prevent their progression to anaphase. Figure 1A shows a metaphase cell that has not been treated with arsenic. Note that the 2 sister chromatids are held together at the centromere. In Figure 2, the sister chromatids have separated, indicating that the cells have gone on to enter anaphase. In order for this to happen and for the cells to then enter anaphase, arsenic must have disrupted the spindle checkpoint that regulates mitotic progression. Thus, this project is also investigating these potential effects from arsenic exposure, as well.


Figure 3 illustrates the increase in chromosome number in human lung cells after arsenic exposure. Normal human lung cells have 46 chromosomes. Figure 3A shows that arsenic can induce a doubling in chromosome number as in the case of this cell with 92 chromosomes. Figure 3B shows a metaphase cell that is endoreduplicated, which is normally an unusual occurrence, but is a frequent one after arsenic exposure. Note how the chromosomes have doubled, but the two chromosomes are still held together at the centromere. This phenomenon is called endoreduplication and may be responsible for the overall increase in polyploidy. The processes that underlie this increase may be a key factor in arsenic-induced tumorigenesis as abnormal chromosome numbers are a hallmark of tumors.

arsenic premature anaphase

Figure 2: Arsenic Induces Premature Anaphase

aneuploidy
endoreduplication

Figure 3: Arsenic Causes Alteration of Chromosome Number a) Doubling of chromosome number b) Endoreduplication

References

Agency for Toxic Substances and Disease Research (2001) Top 20 hazardous substances: ATSDR/EPA priority list for 2001. U.S. Department of Health and Human Services Public Health Service/ U.S. Environmental Protection Agency.

Report on Carcinogens, Eleventh Edition.

Mead, M.N. (2005) Arsenic: A Global Poison, Environmental Health Perspectives. 113:6:A378-86.

Johnson, B.L. and DeRosa, C.T. (1997) The toxicological hazard of superfund hazardous waste sites. Rev. Environ. Health.,12(4): 235-251.

Collaborators and Cooperators

The Wise Laboratory is assisted in this work by an important number of collaborators and cooperators. In particular, the following prominent scientists and their teams provide significant support and input:

Dr. Douglas Currie is an Assistant Professor of Biology at the University of Southern Maine. He and Dr. Wise are collaborating on a project considering the effects of arsenic exposure on neuronal development.

Dr. Vincent Markowski is an Associate Professor of Psychology at the University of Southern Maine. He and Dr. Wise are collaborating on a project considering the effects of arsenic exposure during development on behavior.

Dr. Stephen Pelsue is an Associate Professor of Applied Immunology at USM. He provides expert advice and guidance on flow cytometry. He and Dr. Wise are collaborating on a project considering the effects of arsenic exposure on the immune system.

Dr. Douglas Thompson is a Professor of Epidemiology and Associate Director of the Maine Center for Toxicology and Environmental Health at the University of Southern Maine. He provides expert advice and guidance on statistical analysis and study design and also assists with the marine mammal studies.

Funding

This work was generously supported by a grant from the Maine Cancer Foundation. It is currently supported by the U.S. Environmental Protection Agency through the STAR and GRO Graduate Fellowship and by the Maine Center for Toxicology and Environmental Health.