Comparative evaluation of alkylphenolic compounds on estrogenic activity in vitro and in vivo

This study was undertaken to compare the sensitivity of screening test methods and to investigate the structure-activity relationships of the estrogenic activity of alkylphenolic compounds (APs) using in vitro and in vivo assays. Two in vitro systems, MCF-7 cell proliferation (E-screen assay) and competitive binding assay to estrogen receptor (ER), were selected to evaluate the estrogenic effects. Uterotrophic assay and Calbindin-D_9K (CaBP-9K) mRNA expression were also examined in ovariectomized Sprague-Dawley female rats. A series of APs with various alkyl groups were examined, namely, 4-propylphenol, 4-butylphenol, 4-t-butylphenol, 4-pentylphenol, 4-nonylphenol, 4-octylphenol, 4-t-octylphenol, and 4-phenylphenol, and 17 β-estradiol (E2) was used as a positive control. In the E-screen assay, E2 was found to induce maximum proliferation of MCF-7 cells at 1 nM. Among the APs, 4-t-octylphenol and 4-nonylphenol were found to be considerably more potent than any other compound and estrogenic effects were detectable at 1 and 10 μM, respectively. 4-t-Octylphenol and 4-nonylphenol inhibited the binding of E2 to the ER of MCF-7 cells in a competitive ER binding assay. The uterotrophic effects to APs (10, 50, 200, and 400 mg/kg/d) were compared to E2 (1 μg/kg) in ovariectomized rats after treatment for 3 d. 4-Nonylphenol, 4-t-octylphenol, and 4-phenylphenol produced dose-dependent increases in the uterine weights of ovariectomized rats. In the CaBP-9K mRNA expression test, CaBP-9K mRNA levels were detected in the uteri of ovariectomized rats treated with 4-pentylphenol (400 mg/kg), 4-nonylphenol, 4-phenylphenol (200 and 400 mg/kg), and 4-t-octylphenol (50 mg/kg and above), respectively. In the dot blot assay, CaBP-9K mRNA levels were significantly increased in rats exposed to 4-t-octylphenol (200 and 400 mg/kg), 4-pentylphenol, 4-nonylphenol, and 4-phenylphenol (400 mg/kg), respectively. Among the APs, compounds with bulky alkyl groups or higher carbon numbers possessed higher estrogenic capacity. In addition, the pattern of CaBP-9K expression correlated with that of the 3-d uterotrophic assay. Therefore, our results suggest that the CaBP-9K gene might be used as a potential biomarker for the screening of endocrine disruptors.