Update on the Health Effects of Bisphenol A: Overwhelming Evidence of Harm

Abstract

In 1997, the first in vivo bisphenol A (BPA) study by endocrinologists reported that feeding BPA to pregnant mice induced adverse reproductive effects in male offspring at the low dose of 2 µg/kg/day. Since then, thousands of studies have reported adverse effects in animals administered low doses of BPA. Despite more than 100 epidemiological studies suggesting associations between BPA and disease/dysfunction also reported in animal studies, regulatory agencies continue to assert that BPA exposures are safe. To address this disagreement, the CLARITY-BPA study was designed to evaluate traditional endpoints of toxicity and modern hypothesis-driven, disease-relevant outcomes in the same set of animals. A wide range of adverse effects was reported in both the toxicity and the mechanistic endpoints at the lowest dose tested (2.5 µg/kg/day), leading independent experts to call for the lowest observed adverse effect level (LOAEL) to be dropped 20 000-fold from the current outdated LOAEL of 50 000 µg/kg/day. Despite criticism by members of the Endocrine Society that the Food and Drug Administration (FDA)'s assumptions violate basic principles of endocrinology, the FDA rejected all low-dose data as not biologically plausible. Their decisions rely on 4 incorrect assumptions: dose responses must be monotonic, there exists a threshold below which there are no effects, both sexes must respond similarly, and only toxicological guideline studies are valid. This review details more than 20 years of BPA studies and addresses the divide that exists between regulatory approaches and endocrine science. Ultimately, CLARITY-BPA has shed light on why traditional methods of evaluating toxicity are insufficient to evaluate endocrine disrupting chemicals.

Keywords: biomonitoring; endocrine disruptor; guideline studies; low dose; nonmonotonic dose response; threshold.

Figure 1.

BPA interaction with estrogen receptor subtypes. The bioactive concentration of BPA, acting via each of the subtypes, GPER (19), estrogen related receptor γ (ERRγ) (20), ERα (21), and ERβ (22), is indicated, along with the cell type in which the effect has been observed.

Figure 2.

Data on the number of bisphenol A (BPA) citations from animal experiments (thousands) and human epidemiological studies (>100) as of September 2018. The numbers do not include in vitro mechanistic studies. The figure is from an FDA webinar about the findings from the guideline portion of the Consortium Linking Academic and Regulatory Insights on BPA Toxicity (CLARITY‐BPA), from (66): FDA 2018 Bisphenol A (BPA): Toxicology and pharmacokinetic data to inform on-going safety assessments. FDA Grand Rounds, K.B. Delclos, September 13, 2018. https://www.fda.gov/science-research/about-science-research-fda/bisphenol-toxicology-and-pharmacokenetic-data-inform-ongoing-safety-assessments (public domain).

Figure 3.

Adverse effects due to exposure to BPA during development or in adulthood. (A) Adult control (CTL) vehicle-only exposed adult male mouse. (B) Adult male mouse treated perinatally with 20 µg/kg/day BPA and then in adulthood administered physiologically relevant doses of testosterone and estradiol (second hit) via Silastic capsules that together caused obstructive voiding disorder and hydronephrosis. (C) The dorsolateral prostate from the same BPA-treated male in (B) showing epithelial hyperplasia and prostatitis (arrows); from: Taylor JA, Jones MB, Besch-Williford CL, Berendzen AF, Ricke WA, vom Saal FS 2020 Interactive Effects of Perinatal BPA or diethylstilbestrol and Adult Testosterone and Estradiol Exposure on Adult Urethral Obstruction and Bladder, Kidney, and Prostate Pathology in Male Mice. Int J Mol Sci 21:3902 (open access). (D) Whole-mount mammary glands from adult female mice showing ducts (D), terminal ducts (TD) and a significant increase in alveolar buds (AB) due to perinatal exposure to 0.25 µg BPA/kg/day from Alzet osmotic pumps; from: Vandenberg LN, Maffini MV, Schaeberle CM, et al. 2008 Perinatal exposure to the xenoestrogen bisphenol-A induces mammary intraductal hyperplasias in adult CD-1 mice. Reprod Toxicol 26:210–219, (open access). (E) Confocal images of intact mouse oocytes immunostained to visualize the meiotic spindle (green) and the chromosomes (red). Normal metaphase I (CTL) configuration and representative meiotic abnormality from female mice exposed to BPA leaching from polycarbonate cages and water bottles; from: Hunt PA, Koehler KE, Susiarjo M, et al. 2003 Bisphenol A exposure causes meiotic aneuploidy in the female mouse. Curr Biol 13:546–53, (open access). (F) Decreased daily sperm production (significant effect [*] at and above 20 µg BPA/kg/day) in adult male rats fed BPA for 6 days; from: Sakaue M, Ohsako S, Ishimura R, et al. 2001 Bisphenol A affects spermatogenesis in the adult rat even at a low dose. J Occupational Health 43:185–190, with permission from the publisher. (G) Inhibition by BPA (at and above 40 µg/kg/day) of estrogen-stimulated hippocampal synaptic spine formation in adult female rats; from: MacLusky NJ, Hajszan T, Leranth C 2005 The environmental estrogen bisphenol A inhibits estradiol-induced hippocampal synaptogenesis. Environ Health Perspect 113:675–679, (public domain). (H) Rapid phosphorylation of extracellular signal-regulated kinases (pERK1/2) following administration of doses of BPA ranging from 10^–14 to 10^–7 M or in combination with 1 nM estradiol-17β in rat pituitary cells showing oscillating and agonist/antagonist effects across a wide range of doses beginning with a significant stimulating effect (*) of BPA alone at 10^–14 M (0.01 pM = 2.3 fg/mL BPA), and also antiestrogenic effects when BPA was administered together with 1 nM (0.27 pg/mL 17β-estradiol); from: Jeng YJ, Watson CS 2011 Combinations of physiologic estrogens with xenoestrogens alter ERK phosphorylation profiles in rat pituitary cells. Environ Health Perspect 119:104–112, (public domain).

Figure 4.

Mechanisms of nonmonotonic dose–response (NMDR) relationships. (A) BPA at the low (1 nM) dose acts via ERβ to decrease Cacna1e expression and decrease calcium entry into mouse pancreatic β cells in primary culture in the presence of 11 mM glucose, (B) At 100 nM BPA acts via ERα to enhances the calcium currents in a PI3K-dependent manner; Panels A and B from: Villar-Pazos S, Martinez-Pinna J, Castellano-Mu.oz M, et al. Molecular mechanisms involved in the non-monotonic effect of bisphenol-a on Ca2+ entry in mouse pancreatic β-cells. Sci Rep. 2017;7(1):11770, (open access). (C) The consequence of BPA acting via these two pathways at high doses versus just via ERβ at low doses is the appearance of a non-monotonic dose-response curve for calcium entry into β cells. Panel C was supplied by Dr. Angel Nadal with permission. (D) Up- and downregulation of estrogen receptors in the rat uterus in response to increasing doses (from low to high) of 17β-estradiol administered to ovariectomized females via Silastic capsules; from: Medlock KL, Lyttle CR, Kelepouris N, Newman ED, Sheehan DM 1991 Estradiol down-regulation of the rat uterine estrogen receptor. Proc Soc Exp Biol Med 196:293–300, with permission from the publisher. (E) Up- and downregulation of 8 genes involved in glucose metabolism in fetal mouse prostate mesenchyme cells in primary culture as the dose of estradiol increases from a physiological range (10 pM) to above a physiological range (100 pM); from: Taylor JA, Richter CA, Suzuki A, et al. 2012 Dose-Related Estrogen Effects on Gene Expression in Fetal Mouse Prostate Mesenchymal Cells. PLoS One 7(10) e48311, (open access). (F) Receptor specificity for BPA at low but not high doses: BPA binds to different receptors as dose increases, referred to as receptor crosstalk, which can result in nonmonotonic dose–response relationships.

Figure 5.

Significant effects of BPA reported in the CLARITY-BPA study. (A) Summary of effects from the core guideline study conducted by FDA, and (B) summary of effects from academic studies. The presence of the organ indicates significant differences for one or more measurement in that organ, at the dose indicated, compared to the concurrent negative controls.

Figure 6.

Calculation of the reference dose (or acceptable daily intake dose) from the old and new LOAEL. (A) Prior to the CLARITY-BPA study, the LOAEL calculated from guideline studies was 50 000 µg/kg/day. Using adjustment factors to account for uncertainties in the data (also called safety factors), the reference dose/ADI was determined to be 50 μg/kg/day. (B) Because effects were observed in the CLARITY-BPA study, even in the guideline endpoints evaluated by FDA, at 2.5 μg/kg/day, the new reference dose/ADI would be 2.5 ng/kg/day.

Figure 7.

A timeline of major developments in the history of BPA.