Activity of Xenoestrogens at Nanomolar Concentrations in the E-Screen Assay

BACKGROUND: Certain effects induced by endocrine-disrupting chemicals (EDCs) may occur at dose levels lower than those normally tested in toxicology, but few systematic dose–response studies have been carried out in the low-dose range. OBJECTIVES: The high statistical power afforded by a high-throughput in vitro assay such as the E-Screen assay was exploited with the aim of producing low-dose estimates for 24 estrogenic chemicals, including endogenous hormones and xenoestrogens. RESULTS: Unusual dose–response curves with inverted U-shapes were not observed in the low-dose range. Instead, many chemicals exhibited curves with very small gradients at low doses, and this complicated the reliable estimation of low effects. Systematic comparisons between the outcomes of hypothesis-testing procedures (lowest observed effect concentrations—LOECs, no observed effect concentrations—NOECs) and regression modeling approaches (EC(01)—effective concentration causing a 1% effect, EC(05)—effective concentration causing a 5% effect) produced estimates that agreed reasonably well. In many cases, NOECs were shown to be associated with proliferative responses of 1–2%. This is in contrast with the widespread perception of NOECs as values that signal complete absence of effects. For many of the tested xenoestrogens, the NOECs, EC(01), and EC(05) were in the nanomolar range, and comparisons with measured serum and adipose tissue levels in Europe revealed considerable overlaps in some cases. CONCLUSIONS: Our studies illustrate the difficulties that may be encountered during the estimation of low doses in vivo. High statistical power is required when the underlying dose–response curves are shallow. Through the use of large sample sizes and numerous repeats, the experimental power of the E-Screen assay was sufficiently high to measure effect magnitudes of around 1–2% with reliability. However, such resources are usually not available for in vivo testing, with the consequence that the statistical detection limits are considerably higher. If this coincides with shallow dose–response curves in the low-effect range (which is normally not measurable in vivo), the limited resolving power of in vivo assays may seriously constrain low-dose testing.