Arsenic is a well-documented human
carcinogen, and contamination with this
heavy metal is of global concern, presenting a major issue in environmental health. However, the mechanism by which
arsenic induces
cancer is unknown, in large part due to the lack of an appropriate animal model. In the present set of experiments, we focused on
dimethylarsinic acid (DMA), a major metabolite of
arsenic in most mammals including humans. We provide, for the first time, the full data, including detailed pathology, of the carcinogenicity of DMA in male F344 rats in a 2-year bioassay, along with the first assessment of the genetic alteration patterns in the induced rat urinary bladder
tumors. Additionally, to test the hypothesis that
reactive oxygen species (ROS) may play a role in DMA
carcinogenesis,
8-hydroxy-2'-deoxyguanosine (8-OHdG) formation in urinary bladder was examined. In experiment 1, a total of 144 male F344 rats
at 10 weeks of age were randomly divided into four groups that received DMA at concentrations of 0, 12.5, 50 and 200 p.p.m. in the
drinking water, respectively, for 104 weeks. From weeks 97-104, urinary bladder
tumors were observed in 8 of 31 and 12 of 31 rats in groups treated with 50 and 200 p.p.m. DMA, respectively, and the preneoplastic lesion, papillary or nodular
hyperplasias (PN
hyperplasia), was noted in 12 and 14 rats, respectively. DMA treatment did not cause
tumors in other organs and no urinary bladder
tumors or preneoplastic lesions were evident in the 0 and 12.5 p.p.m.-treated groups. Urinary levels of
arsenicals increased significantly in a dose-responsive manner except for
arsenobetaine (AsBe). DMA and
trimethylarsine oxide (
TMAO) were the major compounds detected in the urine, with small amounts of
monomethylarsonic acid (MMA) and
tetramethylarsonium (TeMa) also detected. Significantly increased
5-bromo-2'-deoxyuridine (
BrdU) labeling indices were observed in the morphologically normal epithelium of the groups treated with 50 and 200 p.p.m. DMA. Mutation analysis showed that DMA-induced rat urinary bladder
tumors had a low rate of H-ras mutations (2 of 20, 10%). No alterations of the p53, K-ras or
beta-catenin genes were detected. Only one TCC (6%) demonstrated nuclear accumulation of p53
protein by immunohistochemistry. In 16 of 18 (89%) of the TTCs and 3 of 4 (75%) of the
papillomas, decreased p27(kip1) expression could be demonstrated.
Cyclin D1 overexpression was observed in 26 of 47 (55%) PN
hyperplasias, 3 of 4 (75%)
papillomas, and 10 of 18 (56%) TCCs. As a molecular marker of oxidative stress, increased COX-2 expression was noted in 17 of 18 (94%) TCCs, 4 of 4 (100%)
papillomas, and 39 of 47 (83%) PN
hyperplasias. In experiment 2, 8-OHdG formation in urinary bladder was significantly increased
after treatment with 200 p.p.m. DMA in the
drinking water for 2 weeks compared with the controls. The studies demonstrated DMA to be a
carcinogen for the rat urinary bladder and suggested that DMA exposure may be relevant to the carcinogenic risk of inorganic
arsenic in humans. Diverse genetic alterations observed in DMA-induced urinary bladder
tumors imply that multiple genes are involved in stages of DMA-induced
tumor development. Furthermore, generation of ROS is likely to play an important role in the early stages of DMA
carcinogenesis.