Interference of Endocrine Disrupters with Thyroid Hormone Receptor–Dependent Transactivation

Thyroid hormones regulate critical developmental processes and key metabolic pathways. A number of natural and synthetic substances have been identified which adversely interfere with the endocrine system. These so-called endocrine disrupters (ED) have mainly been studied for their impact on the gonadal hormone axis. The aim of this work was to develop a novel sensitive and convenient in vitro screening assay for the detection and characterization of potential ED of thyroid hormone (TH)–dependent transactivation of gene transcription and to apply this tool to test relevant environmental and nutritive ED compounds. We constructed a TH-responsive luciferase–based reporter plasmid and established a reporter gene assay in a 96 well microplate format using the human hepatocarcinoma cell line HepG2 as host system. Both the synthetic TH receptor (TR) agonist GC-1 and the antagonist NH-3 were used to evaluate the assay. Concentration-response data of test compounds (food constituents, isoflavones, ultraviolet-absorbers, pesticides, industrial chemicals) were recorded in activation assays. In addition, interference with TH-mediated transactivation was tested by coincubation of the ED with triiodothyronine (T3) in competition assays. Most ED tested affected T3 reporter gene activity at concentrations of 1μM or higher and displayed either agonistic or mixed agonistic/antagonistic activities. Effects of relevant ED occurred only at relatively high concentrations compared with the endogenous TR ligand T3. However, on basis of their high production volumes and potential bioaccumulation of some fat-soluble ED our data indicate the need to carefully monitor certain ED for potential disruption of the TH system in intact organisms and humans.