You are here:
Rapid effects of the aromatase inhibitor fadrozole on steroid production and gene expression in the ovary of female fathead minnows (Pimephales promelas)
Citation:
Schroeder, A., G. Ankley, T. Habib, N. Garcia-Reyero, B. Escalon, K. Jensen, M. Kahl, E. Durhan, E. Makynen, J. Cavallin, D. Martinovic-Weigelt, E. Perkins, AND Dan Villeneuve. Rapid effects of the aromatase inhibitor fadrozole on steroid production and gene expression in the ovary of female fathead minnows (Pimephales promelas). GENERAL AND COMPARATIVE ENDOCRINOLOGY. Academic Press Incorporated, Orlando, FL, 252:79-87, (2017).
Impact/Purpose:
Aromatase inhibition is one of the chemical modes of action of concern to the Endocrine Disruptor Screening Program (EDSP). In vitro bioassays that can detect aromatase inhibition are part of both the EDSP tier 1 screening program and are included subset of ToxCast assays employed for EDSP21 screening. An adverse outcome pathway (AOP) linking aromatase inhibition to reproductive dysfunction in fish has been described and endorsed by the OECD, establishing a scientifically sound connection between aromatase inhibition and adverse apical outcomes relevant to risk assessment and regulatory decision-making. Further, computational models that allow for quantitative prediction of dose-response time-course behaviors and the potential severity of the adverse outcome based on in vitro screening data have been developed. The present study provides further weight of evidence to support this AOP and its use in regulatory decision-making. In particular, it identifies rapid responses to aromatase inhibition that can be expected to occur within the first 24 h of exposure, examines the dynamic stability of gene expression responses over that period to help identify appropriate time periods in which characteristic gene expression responses may serve as effective biomarkers of exposure to aromatase inhibitors, and provides insights into different gene regulatory mechanisms that may be operating over the first few hours of exposure versus more systemic endocrine-related regulation that appear to take over after 6-12 h of exposure. These data continue to refine our understanding of this important mode of endocrine disruption and how to more efficiently and effectively both model and test for it to support regulatory decision-making. This research supports CSS Project 17.01, AOP Discovery and Development and the EDSP program within OCSPP/OSCP.
Description:
Cytochrome P450 aromatase catalyzes conversion of C19 androgens to C18 estrogens and is critical for normal reproduction in female vertebrates. Fadrozole is a well-studied aromatase inhibitor that has been shown to suppress estrogen production in the ovaries of fish. However, little is known about the early impacts of aromatase inhibition on steroid production and gene expression in fish. Adult female fathead minnows (Pimephales promelas) were exposed via water to 0, 5, or 50 µg fadrozole/L for a time-course of 0, 0.5, 1, 2, 4, and 6 h, or exposed to 0 or 50 µg fadrozole/L for a time-course of 6, 12, and 24 h. We examined ex vivo ovarian 17â-estradiol (E2) and testosterone (T) production, and plasma E2 from each study. Expression profiles of genes known or hypothesized to be impacted by fadrozole including aromatase (cytochrome P450 [cyp] 19a1a), steriodogenic acute regulatory protein (star), cytochrome P450 side-chain cleavage (Cyp11), cytochrome P450 17 alpha hydroxylase/17,20 lyase (cyp17), and follicle stimulating hormone receptor (fshr) were measured in the ovaries by quantitative real-time polymerase chain reaction (QPCR). In addition, broader ovarian gene expression was examined using a 15k fathead minnow microarray. The 5 µg/L exposure significantly reduced ex vivo E2 production by 6 h. In the 50 µg/L treatment, ex vivo E2 production was significantly reduced after just 2 h of exposure and remained depressed at all time-points examined through 24 h. Plasma E2 concentrations were significantly reduced as early as 4 h after initiation of exposure to either 5 or 50 µg fadrozole/L and remained depressed throughout 24 h in the 50 µg/L exposure. Ex vivo T concentrations remained unchanged throughout the time-course. Transcripts involved in steroidogenesis examined by QPCR showed increases in expression within the first 24 suggesting a rapid compensatory response to the fadrozole exposure. Microarray results also showed concentration- and time-dependent changes in gene expression profiles associated with the chemical exposure with many HPG-axis pathways being effected within the first 4 h. This study provides further insights into the rapid, direct, effects of aromatase inhibition steroidogenic processes in fish as well as their ability to compensate for the inhibition.
