Estrogen receptors in medaka (Oryzias latipes) and estrogenic environmental contaminants: An in vitro–in vivo correlation

In many vertebrates, estrogens are necessary to promote the growth and differentiation of the female reproductive system during development, and have important reproductive roles in both males and females. Medaka (Oryzias latipes) has three estrogen receptor (ER) subtypes, ERα, ERβ1 and ERβ2. To evaluate the three medaka ER (mER)-ligand interactions, we applied the ERE-luciferase reporter assay system to characterize each ER subtype. In this transient transfection assay system using mammalian cells, the mER proteins displayed estrogen-dependent activation. 17β-Estradiol (E2) and op’-DDT showed high activation irrespective of ERs. Endosulfan also exhibited activation; with less/no transactivity measured using other pesticides, i.e., heptachlor, carbendazim, deltamethrin, acephate, dimethoate and amitraz. It was generally observed that ERβ2 had higher activation potential than ERα and ERβ1. To understand the molecular mechanism of estrogen action via ER, we also conducted E2 treatment where we observed a trigger in ERβ2 expression upon E2 exposure. The present data suggest that ERβ2 is essential for female gonad maintenance. The data were supported by induction of vitellogenin (VTG) mRNA in the liver and reduced VTG receptor mRNA expression in the gonad of both sexes. The present work will provide a basic tool allowing future studies to examine the receptor–ligand interactions and endocrine disrupting mechanisms, and also expands our knowledge of estrogen action on reproductive development in medaka.

▶ In fish, as in other vertebrates, estrogen, primarily estradiol-17β(E2), plays an important role in the regulation of various processes of reproduction. These actions of E2 are known to be mediated by estrogen receptors (ERs). Medaka (Oryzias latipes) has three estrogen receptor (ER) subtypes, ERα, ERβ1 and ERβ2. In this investigation, we focused on characterization of each subtype of medaka ER via documenting trans-activation capacity of different ERs to various chemicals and effect of E2 treatment during adulthood. 17β-Estradiol (E2) and op’-DDT showed high activation irrespective of ERs. Endosulfan also exhibited activation; with less/no transactivity measured using other pesticides, i.e., heptachlor, carbendazim, deltamethrin, acephate, dimethoate and amitraz. It was generally observed that ERβ2 has higher activation potential than other two. The present data suggest that ERβ2 is essential for female gonad maintenance. This was further supported by ERβ2 co-relative induction of vitellogenin (VTG) mRNA in the liver and reduced VTG receptor mRNA expression in the gonad of both sexes. The present work will provide a basic tool allowing future studies to examine the receptor–ligand interactions and endocrine disrupting mechanisms and also expands our knowledge of estrogen action on reproductive development in medaka.