M. Krishnamohan

Urinary arsenic speciation profiles in mice subchronically exposed to low concentrations of sodium arsenate in drinking water.

Arsenic is a proven human carcinogen. Although the mechanism of its carcinogenicity is still largely unknown, methylation is thought to have an important role to play in arsenic toxicity. In this study, urinary methylation profiles were investigated in female C57BL/6J black mice given drinking water containing 500 μg arsenate (AsV)/L, 250 μg AsV/L, or 100 μg AsV/L as sodium arsenate for 2 months. The concentrations of arsenic chosen reflected those in the drinking water often encountered in arsenic‐endemic areas. Urine samples were collected from the mice at the end of the exposure period, and the arsenic species were analyzed by high performance liquid chromatography‐inductively coupled plasma‐mass spectrometry. All detectable arsenic species showed strong linear correlation with the administered dosage. The methylation patterns were similar in all three groups with a slight decrease of dimethylarsinic acid/AsV ratio in the 500‐μg/L group, which corresponded to the significantly higher arsenic retention in the tissue. The results indicate that urinary arsenic could be used as a good biomarker for internal dose and potential biological effects. Different doses of arsenic exposure could result in different degrees of methylation, excretion, and tissue retention, and this may contribute to the understanding of arsenic carcinogenicity.

Carcinogenesis in female C57Bl/6J mice chronically exposed to sodium arsenate (Asv) in drinking water for 2 years

Arsenic is a ubiquitous element in the environment and has been classified as a human carcinogen primarily based on epidemiological evidence. It has been estimated there are over 100 million people globally being exposed to elevated arsenic from both natural and anthropogenic sources. Surprisingly, positive carcinogenicity animal studies were lacking until recent years. We aim to validate inorganic arsenate carcinogenic effect in C57Bl/6J mice, and establish the dose-response relationship using environmental concentrations of arsenic similar to those found in typical endemic-areas. Mice were given 0, 100, 250 or 500 μg As/L in the form of sodium arsenate in drinking water ad libitum over 2 years. Tumours occurred after about 18 months of arsenic exposure otherwise the animals appeared to be normal in their appearance and behaviour. Incidences of all types of tumours and non-tumourous lesions in the treated groups were higher than those observed in the control group. The induction of tumours was in a dose-response manner for some tumour types. Enlargement of the mesenteric lymph node due to hyperplasia or neoplasia of lymphoid elements was commonly observed. Apart from abdominal cavity lymph nodes, tumours were frequently observed in the liver, spleen and intestinal wall, and to a lesser extent in the lung with various other tissues also occasionally affected. Of the non-tumourous lesions, haemorrhagic ovarian cysts occurred more frequently in the treated groups than in the control group. Our results suggest that the C57Bl/6J mouse model can be a useful adjunct for further mechanistic studies of arsenic carcinogenesis. This bioassay data may also be considered for the risk evaluation of chronic exposure to inorganic arsenic.

Use of urinary porphyrin profile as an early warning biomarker for Monomethylarsonous Acid (MMAIII) exposure

Although it is well known that arsenic is toxic and that Arsenic is carcinogenic, the mechanism underlying this carcinogenesis is unknown. Our laboratories have established a model that produces multi-organ tumours in mice following extended exposure to arsenic in drinking water. Until recently the metabolism of arsenic was thought to be a detoxification process. Recent studies have shown that Monomethylarsonous acid (MMA(III)) is the toxic intermediate of arsenic metabolism. It is a more potent cytotoxin and genotoxin than As-III and As-V, and is believed to be the proximal carcinogen. Exposure to arsenic is known to affect the activity of the enzymes of haem biosynthesis. We evaluated the use of urinary porphyrin profiles as an early warning biomarker for arsenic carcinogenicity. Young female mice were given drinking water containing arsenic as MMA(III) ad libitum. 24h urine samples were collected at various time intervals for up to 48 weeks for urinary arsenic accumulation by HPLC-ICPMS and urinary porphyrin measurement by HPLC. Dimethylarsinic acid (DMA(V)) was the major metabolite excreted and it showed significant dose-dependent increase at each time point and exposure dependent increase over 48 weeks. Porphyrin levels appeared to be age dependent. The results indicate that the urinary porphyrin concentration has the potential for use as an early warning indicator of chronic arsenic exposure prior to the onset of arsenic carcinogenesis.