Low-Dose Transplacental Arsenic Exposure Alters Cell Cycle Gene Expression in Mouse Lungs

Arsenic exposure has been linked to a number of human diseases including diabetes, skin lesions, cardiovascular disease and cancer. Arsenic toxicity is a worldwide problem. The mechanism of arsenic toxicity, however, is not well understood. Drinking water is a major source of arsenic exposure in south-east Asia and western South America where high levels of arsenic occur naturally in groundwater. Some areas of the United States, especially in the southwest and New England, also have arsenic in the groundwater, but at much lower levels. The Environmental Protection Agency (EPA) and the World Health Organization (WHO) have set the maximum allowable safe level of arsenic in public drinking water at 10ppb (parts per billion; μg/L). This level is often exceeded in private wells in the state of Maine. Therefore, studying the effects of drinking water arsenic exposure is crucial for Maine residents and their future generations. The current study examined the effect of low-dose, environmentally relevant levels of arsenic (10 to 500 ppb) on transplacentally exposed mice (F1) and the next generation (F2) which focused on altered regulation of the normal cell cycle. Foxm1, Cdc6 and Cdc25a are three critical genes involved in regulating a normal cell cycle. Altered expression of these genes may lead to abnormalities in the cell cycle and eventually cause genomic instability. Female mice (F0 generation) were exposed to 0, 10, 50 or 500 ppb of arsenic via drinking water throughout mating and pregnancy. Their pups (F1) were only exposed to arsenic via the placenta before birth and via milk from their mothers for 21 days post-birth. F2 generation mice were not directly exposed to arsenic. Gene and protein expression were examined in the lungs of first generation (F1) and second generation (F2) mice with Quantitative Real Time Polymerase Chain Reaction (qRTPCR), and Immunoblotting. Statistically significant induction of Foxm1, Cdc6 and Cdc25a expression was seen even at concentrations as low as l0ppb of arsenic. Evidence of cell proliferation was obtained by evaluating PCNA (Proliferating Cell Nuclear Antigen) expression using immunohistochemistry. Arsenic concentrations in lung tissues from F0, F1 and F2 mice were detected by inductively coupled plasma mass spectroscopy (ICP- MS). It was interesting to detect higher expression of cell cycle genes and proteins in offspring of dams exposed to 10 ppb, especially because it is the maximum allowable level in drinking water. Data from the F1 generation support in vitro studies of arsenic-related cell cycle dysregulation done in other laboratories. F1 and F2 males show increased cell proliferation at concentrations as low as 10ppb suggesting a transgenerational effect of arsenic. Molecular changes in lung tissues of F1 generation females suggest increased cell proliferation while the F2 generation shows an inhibition or delay in cell cycle progression. These data suggest a gender-specific double role of transplacental arsenic exposure. Findings from this study suggest transplacental arsenic exposure alters the expression of genes involved in regulation of the cell cycle and may be a probable mechanism involved in causing human diseases