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

The in vitro arsenite (As^III) 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 As^III 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 P_o of relatively resistant cells, and a remaining (1-P-P_o) fraction of typical-sensitivity (TS) cells exhibiting the intermediate level of As^III sensitivity characteristic of most cells in each assay. The estimated fractions P and P_o were used to adjust data from all 54 plates (and from the 28 plates yielding the best fits) to reflect the condition that P = P_o = 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.