Non-monotonic dose responses in studies of endocrine disrupting chemicals: bisphenol a as a case study

Abstract

Non-monotonic dose response curves (NMDRCs) have been demonstrated for natural hormones and endocrine disrupting chemicals (EDCs) in a variety of biological systems including cultured cells, whole organ cultures, laboratory animals and human populations. The mechanisms responsible for these NMDRCs are well known, typically related to the interactions between the ligand (hormone or EDC) and a hormone receptor. Although there are hundreds of examples of NMDRCs in the EDC literature, there are claims that they are not 'common enough' to influence the use of high-to-low dose extrapolations in risk assessments. Here, we chose bisphenol A (BPA), a well-studied EDC, to assess the frequency of non-monotonic responses. Our results indicate that NMDRCs are common in the BPA literature, occurring in greater than 20% of all experiments and in at least one endpoint in more than 30% of all studies we examined. We also analyzed the types of endpoints that produce NMDRCs in vitro and factors related to study design that influence the ability to detect these kinds of responses. Taken together, these results provide strong evidence for NMDRCs in the EDC literature, specifically for BPA, and question the current risk assessment practice where 'safe' low doses are predicted from high dose exposures.

Keywords: NOAEL; U-shaped; biphasic; extrapolation; molecular mechanism; reference dose.

FIGURE 1.

Characteristics of the 250 experiments identified for additional analyses. A) Graph illustrating the breakdown of the number of doses examined in these studies. The majority of studies examined only 2, 3 or 4 different concentrations of BPA. B) Graph illustrating the number of different log doses examined in these studies. This graph shows that the majority of studies examined doses from 2, 3 or 4 log M. C) Graph illustrating the span of log doses from the highest to lowest concentrations examined in these studies. Most studies examined doses that spanned 2–5 log M.

FIGURE 2.

Comparisons of characteristics in experiments that report NMDRCs versus those that report monotonic responses. A) Comparisons of the number of doses examined. B) Comparisons of the number of different log doses examined. C) Comparisons of the span of log doses examined. All graphs represent means ± SEM, ^*p<0.05, T-test.

FIGURE 3.

Percent of experiments displaying monotonic or non-monotonic responses depend on experimental design factors. A) Experiments were analyzed based on the number of doses examined and the percent displaying monotonic and non-monotonic responses is shown. B) Experiments were analyzed based on the number of different log doses tested and the percent displaying monotonic and non-monotonic responses is shown. C) Experiments were analyzed based on the range of log doses examined and the percent displaying monotonic and non-monotonic responses is shown. In all panels, each group was compared as an ‘observed’ value to the total number of experiments (59 non-monotonic, 191 monotonic; the ‘expected’ value) using a Chi Square test; ^*p<0.05.