Sensitization of vitellogenin gene expression by low doses of octylphenol is mediated by estrogen receptor autoregulation in the Bombina orientalis (Boulenger) male liver

Abstract

This study aimed to elucidate the mechanisms by which alkylphenols disrupt endocrine function in wild amphibians in Korea. To this end, the effects of 4-tert-octylphenol (OP),17 beta-estradiol (E-2), and estrogen receptor (ER) agonists on the expression profiles of vitellogenin (VTG) and ERs were examined in livers obtained from male Bombina orientalis toads. A single injection of E-2 (10 mu g/kg; 0.03 mu mol/kg) induced transcription of VTG mRNA at 2 days post injection; however, injection of either the ER alpha-selective agonist propyl-(1H)-pyrazole-1,3,5-triyl-trisphenol (PPT, 50 mu g/kg; 0.12 mu mol/kg) or the ER beta-selective agonist 2,3-bis-(4-hydroxyphenyl)-propionitrile (DPN, 50 mu g/kg; 0.20 mu mol/kg) did not affect the expression of VTG. This finding suggests that both ER alpha and ER beta are required to induce transcription of VTG in the male B. orientalis liver. Interestingly, E2, PPT, and DPN induced transcription of ERa., which was also reflected on the protein level; however, these alkylphenols did not affect ER beta transcription. Similarly, VTG transcription was induced by a single injection of 1-100 mg/kg (0.04-484.66 mu mol/kg) OP, while 0.1 mg/kg (0.48 mu mol/kg) OP had no effect on VTG transcription. This result suggests that the lowest observable effect concentration (LOEC) of OP for induction of VTG transcription in the male liver is 1 mg/kg (4.84 mu mol/kg). Furthermore, treatment with E-2 (10 mu g/kg; 0.03 mu mol/kg) or OP (1 mg/kg; 4.84 mu mol/kg) significantly upregulated ER alpha transcription, and a 10 mg/kg (48.46 mu mol/kg) dose of OP significantly upregulated ER beta transcription. The ER antagonist ICI 182,780 decreased the basal levels of ER alpha and ER beta mRNA, and also prevented E-2-mediated and OP-mediated induction of VTG, ER alpha, and ER beta transcription. A second injection of 0.1 mg/kg (0.48 mu mol/kg) OP after a two-day interval significantly upregulated the transcription of VTG and ERa, but not of ER beta. These results suggest that sensitization of VTG transcription by repeated exposure to OP is mediated by the induction of ERa. Different combinations of alkylphenols that are ubiquitous in the freshwater system in Korea could potentially exert a synergistic effect on endocrine disruption. Thus, chronic exposure to alkylphenols, even at their NOECs, could still disrupt endocrine function in B. orientalis.