Multiple receptors shape the estrogen response pathway and are critical considerations for the future of in vitro-based risk assessment efforts

Current in life toxicity testing paradigms are being challenged as the future of risk assessment moves towards more comprehensive mode of action/adverse outcome pathway based approaches. In particular, endocrine disruption screening is now a global activity and key initiatives in the United States focus on the use of high throughput in vitro assays to prioritize compounds for further testing of estrogen, androgen or thyroid disruption. Of these pathways, much of the emphasis to date has been on high-throughput methods for estrogenic activity primarily using ligand binding and trans-activation assays. However, as the knowledge regarding estrogen receptor signaling pathways continues to evolve, it is clear that the assumption of a simple one-receptor pathway underlying current in vitro screening assays is out of date. To develop more accurate models for estrogen-initiated pathways useful for quantitative safety assessments, we must design assays that account for the key signaling processes driving cellular dose response based on up-to-date understanding of the biological network. In this review, we summarize the state of the science for the estrogen receptor signaling network, particularly with regard to proliferative effects, and highlight gaps in current high throughput approaches. From the sum of this literature, we propose a model for the estrogen-signaling pathway that should serve as a starting point for development of new in vitro methods fit for the purpose of predicting dose response for estrogenic chemicals in the human.