Sensitivity of a Tier I Screening Battery Compared to an in Utero Exposure for Detecting the Estrogen Receptor Agonist 17β-Estradiol

Abstract

A Tier I screening battery for detecting endocrine active compounds (EACs) has been evaluated for its ability to identify 17β-estradiol, a pure estrogen receptor agonist. In addition, the responses obtained with the Tier I battery were compared to the responses obtained from F1 generation rats from a 90-day/one-generation reproduction study with 17β-estradiol in order to characterize the sensitivity of the Tier I battery against the sensitivity of an in utero exposure for detecting EACs. The Tier I battery incorporates two short-term in vivo tests (5-day ovariectomized female battery; 15-day Intact male battery) and an in vitro yeast transactivation system (YTS) for identifying compounds that alter endocrine homeostasis. The Tier I female battery consists of traditional uterotrophic endpoints coupled with biochemical and hormonal endpoints. It is designed to identify compounds that are estrogenic/antiestrogenic or modulate dopamine levels. The Tier I male battery consists of organ weights coupled with microscopic evaluations and a comprehensive hormonal assessment It is designed to identify compounds that have the potential to act as agonists or antagonists to the estrogen, androgen, progesterone, or dopamine receptor; steroid biosynthesis inhibitors (aromatase, 5 α-reductase, and testosterone biosynthesis); or compounds that alter thyroid function. The YTS is designed to identify compounds that bind to steroid hormone receptors (estrogen, androgen, and progesterone) and activate gene transcription. The profile generated for 17β-estradiol was characteristic of the responses expected with a pure estrogen receptor agonist. In the female battery, responses to 17β-estradiol included increases in uterine fluid imbibition, uterine weight, estrus conversion, uterine stromal cell proliferation, uterine epithelial cell height, uterine progesterone receptor content, serum prolactin and estradiol levels, and decreases in uterine estrogen receptor content and follicle stimulating hormone and luteinizing hormone levels. In the male battery, responses to 17β-estradiol included decreases in absolute testis and epididymi-des weights, decreases in relative weights for androgen-dependent tissues (prostate, seminal vesicles, and accessory sex gland unit), hormonal alterations (decreased serum testosterone, dihydrotest-osterone, and LH and increased serum prolactin levels), and microscopic alterations of the testis and epididymides. In the YTS for the estrogen receptor, 17β-estradiol had an EC50, value of 7.2 × 10−9 M, while DHT and progesterone had little cross-activation. The androgen and progesterone receptor systems were less selective in that 17β-estradiol activated these systems within 3 orders of magnitude of the primary ligand. In the 90-day/one-generation reproduction study, responses to dietary administration of 17β-estradiol included alterations in organ weights, developmental landmarks, and hormonal levels. Comparison of the responses obtained with our Tier I battery and an in utero exposure demonstrates that the Tier I screening battery is as sensitive as an in utero exposure for detecting 17β-estradiol-induced alterations in hormonal homeostasis.