Novel in vitro, ex vivo and in vivo assays elucidating the effects of endocrine disrupting compounds (EDCs) on thyroid hormone action

The last years, both scientific and public concern about the possible threat of compounds in the environment that may affect endocrine functions has increased. Thus far, the majority of endocrine disruptor research has focused on the interference of compounds with the sex hormone homeostasis. Less attention has been paid to disruption of the hypothalamus-pituitary-thyroid (HPT) axis, despite the fact that several lines of evidence suggest that this system is also susceptible to disruption by compounds present in the environment. Disturbances in thyroid hormone (TH) homeostasis may lead to mosaic effects on development, growth patterns and metabolism in vertebrates including mammals and amphibians. Recent studies now indicate that the Thyroid hormone Receptors (TRs) also are targets of industrial compounds, but the effects and mechanisms are difficult to establish since suitable test assays are very limited. In part, this may be because research has generally focused on the ability of compounds to affect TH transport, TH metabolism and TH blood plasma levels but not on disruption of TR mediated TH-action.The research presented in this thesis aims to enhance insight into the mechanisms underlying the effects of endocrine disrupting compounds (EDCs) on TR-mediated TH-action. To this end, newly developed and validated in vitro and in vivo assays are applied in addition to an ex vivo assay using isolated tail tissue. This ex vivo model more resembles the natural situation for cells than in vitro assays, but excludes the feedback mechanisms from the TH axis orextracellularmetabolism which may obscure TR-mediated responses. In the assays exposure is performed in combination with TH in order to closer approach the exposure of cells under physiological conditions, where TH is present during important vertebrate fetal developmental periods. The in vitro , ex vivo and in vivo assay responses also are compared to study to what degree the in vitro and ex vivo assays can predict the in vivo thyroid hormone disrupting potency of compounds. Apart from the development of newtestsystemsfor thyroid hormone disruption, the studies presented in this thesis also provide indications for the mechanism ofEDCson the level of the TR. Furthermore, the results illustrate thatthe in vitro results are highly predictive for the effects as observed in vivo . Finally, it can be concluded that testing of compounds for potential TH disrupting effects, should be carried out in presence and absence of TH at its EC 50 .