Carcinogenicity of dimethylarsinic acid in Ogg1-deficient mice

Abstract

Oxidative stress to DNA is recognized as a mechanism underlying carcinogenic effects of some environmental agents. Here, we hypothesized that dimethylarsinic acid (DMA(V)), an organic metabolite of inorganic arsenic in humans, might exert carcinogenic potential in a mouse line carrying a mutant Mmh allele of the Mmh/OGG1 gene encoding the enzyme 8-hydroxyguanine DNA glycosylase 1 (OGG1). Ogg1 mutant and wild type mice were treated with DMA(V) in their drinking water at a dose of 200 p.p.m. for up to 72 weeks. All DMA(V)-treated Ogg1(-/-)animals developed tumors, with a tendency for lower total incidences in the Ogg1(+/+) cases. Lung tumors in particular were induced as compared to the lack in non-carcinogen controls and were significantly more frequent in the homozygotes. At week 4, the levels of DNA 8-OH-dG and cell proliferation were significantly elevated in the lungs of non-treated Ogg1(-/-) as compared to Ogg1(+/+) mice and were strongly enhanced by DMA(V) treatment. Marked induction of Pola1, Cyp7b1, Ndfua3, Mmp13 and other genes specific to cell proliferation, cell signaling and xenobiotic metabolism in the lungs of DMA(V)-treated Ogg1(-/-) mice was found. Electron microscopic examination revealed the growth of microvilli, with increased numbers of mitochondria only in lungs and lung tumors of DMA(V)-exposed Ogg1(-/-) mice. Therefore, we strongly suggest that DMA(V) exerts carcinogenicity in the lungs of Ogg1(-/-) mutant mice, with a possible role for persistent accumulation of DNA oxidative adducts.

Figure 1

Survival curves for Ogg1 ^−/– (a) and Ogg1 ^+/+ (b) mice. Non‐treated Ogg1 ^−/– mice were found to be more healthy and long‐lived as compared to the Ogg1 ^+/+ control mice. The number of surviving Ogg1 ^−/– mice administered dimethylarsinic acid (DMA^V) started to decrease at week 58 due to tumor development.

Figure 2

Results of histopathological (H&E) staining and transmission electron microscopy (TEM) in the lungs of Ogg1 ^−/– mice maintained on 200 p.p.m. dimethylarsinic acid (DMA^V). (a) (× 400) and (b) (× 400) Adenocarcinoma and adenoma, respectively, in the lung of the DMA^V‐treated Ogg1 ^−/– mouse (H&E). (c) TEM of alveoli from a non‐tumorous area in a Ogg1 ^−/– animal treated with DMA^V and (d) alveoli of a non‐treated Ogg1 ^−/– animal. (e, f) TEM of a lung adenocarcinoma from an Ogg1 ^−/– animal treated with DMA^V. Note the numerous mitochondria, microvilli and abundant vesicles. Those changes were absent in Ogg1 ^+/+ mice (data not shown). Co, collagen fibres; CV, clatrin vesicles; D, desmosomes; Ec, euchromatin; GA, Golgy apparatus; Hc, heterochromatin; LB, lamella bodies; Mi, mitochondria; Mv, microvilli; N, nucleus; Nc, nucleolus; RER, rough endoplasmic reticulum; Vs, vesicles. Original magnifications: × 400 (a); × 400 (b); × 25 000 (c–f), Scale bar: 860 nm (c); 1.2 µm (d); 670 nm (e); 565 nm (f).

Figure 3

Formation of 8‐hydroxy‐2′‐deoxyguanosine (8‐OH‐dG) and cellular proliferation and apoptosis indices in the lungs of Ogg1 ^−/– and Ogg1 ^+/+ mice treated with dimethylarsinic acid (DMA^V) at 200 p.p.m. for 72 weeks. (a) 8‐OH‐dG (c) proliferating cell nuclear antigen (PCNA) and (d) apoptosis indices. (b) 8‐OH‐dG staining pattern in DMA^V‐treated and control Ogg1 ^−/– mouse. †Normal‐appearing area; *Significant at P < 0.05 versus Ogg1 ^−/– control group; **Significant at P < 0.001 versus Ogg1 ^−/– control group; ***Significant at P < 0.0001 and ****Significant at P < 0.05 versus Ogg1 ^+/+ control group.

Figure 4

Gene Spring clustering analysis of the Affymetrix oligonucleotide microarray data (a) Ingenuity pathway analysis (b) and conformation of microarray results by real‐time quantitative polymerase chain reaction (Q‐PCR) (c–f). (c) Polymerase (DNA‐directed) alpha 1 (d) Mmp13 (e) nicotineamide adenine dinucleotide reduced form (NADH) dehydrogenase (ubiquinone) 1 alpha, and (f) CYP7b1 mRNA expression. *P < 0.05 and **P < 0.0001 versus Ogg1 ^+/+ control and dimethylarsinic acid (DMA^V)‐treated groups.

Figure 5

The putative pathway of arsenic metabolism in rodents. As^III, arsenite; As^V, arsenate; DMA^III, monomethylarsinous acid; DMA^V, dimethylarsinic acid; hGSTO1‐1, human glutathione‐S‐transferase omega 1‐1; MMA^III, monomethylarsonous acid; MMA^V, monomethylarsonic acid; SAHC, S‐adenosyl‐L‐homocysteine; SAM, S‐adenosyl‐L‐methionine; TMAO, trimethylarsine oxide.