Assessment of the effects of organic vs. inorganic arsenic and mercury in Caenorhabditis elegans

Strengths and limitations of the worm development and activity test (wDAT) as a chemical screening tool for developmental hazards

The worm Development and Activity Test (wDAT) measures C. elegans developmental milestone acquisition timing and stage-specific spontaneous locomotor activity (SLA). Previously, the wDAT identified developmental delays and SLA level changes in C. elegans with mammalian developmental toxicants arsenic, lead, and mercury. 5-fluorouracil (5FU), cyclophosphamide (CP), hydroxyurea (HU), and ribavirin (RV) are teratogens that also induce growth retardation in developing mammals. In at least some studies on each of these chemicals, fetal weight reductions were seen at mammalian exposures below those that had teratogenic effects, suggesting that screening for developmental delay in a small alternative whole-animal model could act as a general toxicity endpoint to identify chemicals for further testing for more specific adverse developmental outcomes. Consistent with mammalian developmental effects, 5FU, HU, and RV were associated with developmental delays with the wDAT. Exposures associated with developmental delay induced hypoactivity with 5FU and HU, but slight hyperactivity with RV. CP is a prodrug that requires bioactivation by cytochrome P450s for both therapeutic and toxic effects. CP tests as a false negative in several in vitro assays, and it was also a false negative with the wDAT. These results suggest that the wDAT has the potential to identify some developmental toxicants, and that a positive wDAT result with an unknown may warrant further testing in mammals. Further assessment with larger panels of positive and negative controls will help qualify the applicability and utility of this C. elegans wDAT assay within toxicity test batteries or weight of evidence approaches for developmental toxicity assessment.

Arsenic bioaccumulation and biotransformation in the marine copepod Tigriopus japonicus under chronic dietborne and waterborne exposure

Arsenic (As) can be transferred along the food chain, while little is known about the toxic effects of dietborne As on marine copepods. In this study, we investigated the short-term and long-term effects of waterborne and dietborne As exposure on the bioaccumulation and biotransformation, as well as developmental toxicity of Tigriopus japonicus. Under acute As exposure, As bioaccumulation increased and reached a plateau with increasing exposure concentration. Moreover, As accumulation at dietborne exposure was 4.3 and 5.7 times greater than that at control group for AsIII and AsV, respectively. At chronic As exposure, As accumulation increased continuously with exposure time, with a 2.8-day extension of development time and a 45% reduction in 10-d fecundity under dietborne exposure compared to control, whereas 2.3-day extension of development time and a 20% reduction in 10-d fecundity were observed under waterborne exposure. Among As species, inorganic As had the highest concentrations, but the proportion of inorganic As decreased from 89% to 63% during 4 to 21 d of exposure, suggesting the conversion of inorganic As to organic As. The organic As was dominated by arsenobetaine (AsB, 13-25%), followed by monomethylarsenic (MMA, 8-25%). These results suggest that dietborne exposure has more pronounced toxic effects on T. japonicus, but the toxicity of As could be reduced through biotransformation under chronic exposure. Therefore, the arsenic species should be considered when assessing As toxicity.

The worm Adult Activity Test (wAAT): A de novo mathematical model for detecting acute chemical effects in Caenorhabditis elegans

We have adapted a semiautomated method for tracking Caenorhabditis elegans spontaneous locomotor activity into a quantifiable assay by developing a sophisticated method for analyzing the time course of measured activity. The 16-h worm Adult Activity Test (wAAT) can be used to measure C. elegans activity levels for efficient screening for pharmacological and toxicity-induced effects. As with any apical endpoint assay, the wAAT is mode of action agnostic, allowing for detection of effects from a broad spectrum of response pathways. With caffeine as a model mild stimulant, the wAAT showed transient hyperactivity followed by reversion to baseline. Mercury chloride (HgCl2 ) produced an early dose-response hyperactivity phase followed by pronounced hypoactivity, a behavior pattern we have termed a toxicant "escape response." Methylmercury chloride (meHgCl) produced a similar pattern to HgCl2 , but at much lower concentrations, a weaker hyperactivity response, and more pronounced hypoactivity. Sodium arsenite (NaAsO2 ) and dimethylarsinic acid (DMA) induced hypoactivity at high concentrations. Acute toxicity, as measured by hypoactivity in C. elegans adults, was ranked: meHgCl > HgCl2  > NaAsO2  = DMA. Caffeine was not toxic with the wAAT at tested concentrations. Methods for conducting the wAAT are described, along with instructions for preparing C. elegans Habitation Medium, a liquid nutrient medium that allows for developmental timing equivalent to that found with C. elegans grown on agar with OP50 Escherichia coli feeder cultures. A de novo mathematical parametric model for adult C. elegans activity and the application of this model in ranking exposure toxicity are presented.

Reproductive-Toxicity-Related Endpoints in C. elegans Are Consistent with Reduced Concern for Dimethylarsinic Acid Exposure Relative to Inorganic Arsenic

Exposures to arsenic and mercury are known to pose significant threats to human health; however, the effects specific to organic vs. inorganic forms are not fully understood. Caenorhabditis elegans' (C. elegans) transparent cuticle, along with the conservation of key genetic pathways regulating developmental and reproductive toxicology (DART)-related processes such as germ stem cell renewal and differentiation, meiosis, and embryonic tissue differentiation and growth, support this model's potential to address the need for quicker and more dependable testing methods for DART hazard identification. Organic and inorganic forms of mercury and arsenic had different effects on reproductive-related endpoints in C. elegans, with methylmercury (meHgCl) having effects at lower concentrations than mercury chloride (HgCl2), and sodium arsenite (NaAsO2) having effects at lower concentrations than dimethylarsinic acid (DMA). Progeny to adult ratio changes and germline apoptosis were seen at concentrations that also affected gravid adult gross morphology. For both forms of arsenic tested, germline histone regulation was altered at concentrations below those that affected progeny/adult ratios, while concentrations for these two endpoints were similar for the mercury compounds. These C. elegans findings are consistent with corresponding mammalian data, where available, suggesting that small animal model test systems may help to fill critical data gaps by contributing to weight of evidence assessments.

Sodium arsenite and dimethylarsenic acid induces apoptosis in OC3 oral cavity cancer cells

Although arsenic is an environmental toxicant, arsenic trioxide (ATO) is used to treat acute promyelocytic leukemia (APL) with anticancer effects. Studies have demonstrated oral cancer is in the top 10 cancers in Taiwan. High rate of oral cancers is linked to various behaviors, such as excessive alcohol consumption and tobacco use. Similarly, betel chewing is a strong risk factor in oral cancer. In the present study, oral squamous carcinoma OC3 cells were investigated with the treatments of sodium arsenite (NaAsO2) and dimethylarsenic acid (DMA), respectively, to examine if arsenic compounds have anti‑cancer efforts. It was found that 1 µM NaAsO2 and 1 mM DMA for 24 h induced rounded contours with membrane blebbing phenomena in OC3 cells, revealing cell apoptotic characteristics. In addition, NaAsO2 (10‑100 µM) and DMA (1‑100 mM) significantly decreased OC3 cell survival. In cell cycle regulation detected by flow cytometry, NaAsO2 and DMA significantly augmented percentage of subG1 and G2/M phases in OC3 cells, respectively. Annexin V/PI double staining assay was further used to confirm NaAsO2 and DMA did induce OC3 cell apoptosis. In mechanism investigation, western blotting assay was applied and the results showed that NaAsO2 and DMA significantly induced phosphorylation of JNK, ERK1/2 and p38 and then the cleavages of caspase‑8, ‑9, ‑3 and poly ADP‑ribose polymerase (PARP) in OC3 cells, dynamically. In conclusion, NaAsO2 and DMA activated MAPK pathways and then apoptotic pathways to induce OC3 oral cancer cell apoptosis.