The enigma of arsenic carcinogenesis : Role of metabolism

Inorganic arsenic is considered a high-priority hazard, particu- larly because of its potential to be a human carcinogen. In exposed human populations, arsenic is associated with tumors of the lung, skin, bladder, and liver. While it is known to be a human carcin- ogen, carcinogenesis in laboratory animals by this metalloid has never been convincingly demonstrated. Therefore, no animal mod- els exist for studying molecular mechanisms of arsenic carcino- genesis. The apparent human sensitivity, combined with our in- complete understanding about mechanisms of carcinogenic action, create important public health concerns and challenges in risk assessment, which could be met by understanding the role of metabolism in arsenic toxicity and carcinogenesis. This sympo- sium summary covers three critical major areas involving arsenic metabolism: its biodiversity, the role of arsenic metabolism in molecular mechanisms of carcinogenesis, and the impact of ar- senic metabolism on human risk assessment. In mammals, arsenic is metabolized to mono- and dimethylated species by methyltrans- ferase enzymes in reactions that require S-adenosyl-methionine (SAM) as the methyl donating cofactor. A remarkable species diversity in arsenic methyltransferase activity may account for the wide variability in sensitivity of humans and animals to arsenic toxicity. Arsenic interferes with DNA methyltransferases, result- ing in inactivation of tumor suppressor genes through DNA hy- permethylation. Other studies suggest that arsenic-induced malig- nant transformation is linked to DNA hypomethylation subsequent to depletion of SAM, which results in aberrant gene activation, including oncogenes. Urinary profiles of arsenic me- tabolites may be a valuable tool for assessing human susceptibility to arsenic carcinogenesis. While controversial, the idea that unique arsenic metabolic properties may explain the apparent non-linear threshold response for arsenic carcinogenesis in humans. In order to address these outstanding issues, further efforts are required to identify an appropriate animal model to elucidate carcinogenic mechanisms of action, and to define dose-response relationships.