Response to the Waalkes et al., Letter to the editor concerning our "letter to the editor, Re: Lung tumors in mice induced by "whole-life" inorganic arsenic exposure at human relevant doses, Waalkes et al., Arch Toxicol, 2014"

Dose and Diet - Sources of Arsenic Intake in Mouse in Utero Exposure Scenarios

In humans, early life exposure to inorganic arsenic is associated with adverse health effects. Inorganic arsenic in utero or in early postnatal life also produces adverse health effects in offspring of pregnant mice that consumed drinking water containing low part per billion levels of inorganic arsenic. Because aggregate exposure of pregnant mice to inorganic arsenic from both drinking water and food has not been fully evaluated in experimental studies, quantifying arsenic exposure of the developing mouse is problematic. Here, we determined levels of total arsenic and arsenic species in natural ingredient rodent diets that are composed of many plant and animal-derived foodstuffs and in a purified ingredient rodent diet that is composed of a more restricted mixture of foodstuffs. In natural ingredient diets, total arsenic levels ranged from ∼60 to ∼400 parts per billion, and in the purified ingredient diet, total arsenic level was 13 parts per billion. Inorganic arsenic was the predominant arsenic species in trifluoroacetic acid extracts of each diet. Various exposure scenarios were evaluated using information on inorganic arsenic levels in diet and drinking water and on daily food and water consumption of pregnant mice. In a scenario in which pregnant mice consumed drinking water with 10 parts per billion of inorganic arsenic and a natural ingredient diet containing 89 parts per billion of inorganic arsenic, drinking water contributed only ∼20% of inorganic arsenic intake. Quantitation of arsenic species in diets used in studies in which drinking water is the nominal source of arsenic exposure provides more accurate dosimetry and improves understanding of dose-response relations. Use of purified ingredient diets will minimize the discrepancy between the target dosage level and the actual dosage level attained in utero exposure studies designed to evaluate effects of low level exposure to inorganic arsenic.

Low-dose dose-response for reduced cell viability after exposure of human keratinocyte (HEK001) cells to arsenite

The in vitro arsenite (AsIII) cytotoxicity dose-response (DR) of human keratinocytes (HEK001) was examined at greater statistical resolution than ever previously reported using the MTT assay to determine cell viability. Fifty-four 96-well plates were treated with AsIII concentrations of 0.25, 0.5, 1, 2, 3, 4, 5, 7, 10, 15, 20, 25, or 30 μM. Because of unexpected variation in viability response patterns, a two-stage DR analysis was used in which data on plate-specific viability (%), estimated as 100% times the ratio of measured viability in exposed to unexposed cells, were fit initially to a generalized lognormal response function positing that HEK001 cells studied consisted of: a proportion P of relatively highly sensitive (HS) cells, a proportion Po of relatively resistant cells, and a remaining (1-P-Po) fraction of typical-sensitivity (TS) cells exhibiting the intermediate level of AsIII sensitivity characteristic of most cells in each assay. The estimated fractions P and Po were used to adjust data from all 54 plates (and from the 28 plates yielding the best fits) to reflect the condition that P = Po = 0 to provide detailed DR analysis specifically for TS cells. Four DR models fit to the combined adjusted data were each very predictive (R2 > 0.97) overall but were inconsistent with at least one of the data set examined (p < 10-5). Adjusted mean responses at ≤3 μM were approximately equal (p > 0.30) and exceeded 100% significance (p ≤ 10-6). A low-dose hormetic model provided the best fit to the combined adjusted data for TS cells (R2 = 0.995). Marked variability in estimates of P (the proportion of apparent HS cells) was unexpected, not readily explained, and warrants further study using additional cell lines and assay methods, and in vivo.