2,3,7,8-Tetrachlorodibenzo-p-dioxin increases steady-state estrogen receptor-β mRNA levels after CYP1A1 and CYP1B1 induction in rat granulosa cells in vitro11Part of the work communicated in this paper was presented in the 82nd (Toronto, Canada) Annual Meetings of the Endocrine Society. All cDNA clones used in the present experiments are available from the laboratory of Professor Hutz.

Previous in-vitro investigations of rat granulosa cells (GC) have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits estrogen secretion and FSH-induced aromatase activity. Although TCDD exerted no effect on basal aromatase enzyme activity, TCDD did reduce steady-state aromatase mRNA levels in GC using competitive RT-PCR. TCDD is hypothesized to induce these changes through aromatic hydrocarbon receptor(AHR)-mediated gene transcription and the modulation of the estrogen receptor (ER)-signaling pathway. In this study we show that rat GC express mRNA for AHR and the AHR nuclear translocator (ARNT) as well as biomarkers of TCDD action, CYP1A1 and CYP1B1 mRNA. Basal CYP1A1 and ER-α mRNAs were present only in trace amounts. By relative RT-PCR analysis we showed that CYP1A1 and CYP1B1 mRNA were induced significantly by TCDD at 6 h and that induction of CYP1A1 was maintained throughout the experiment. Using competitive RT-PCR, we observed no significant change in the mRNA levels of ARNT between control and TCDD-treated GC. Both AHR and ER-β mRNA levels increased significantly at 48 h with TCDD compared with controls. Since ER-β mRNA was not increased significantly until 48 h in culture, we suggest that in rat GC, the observed ER-β mRNA increase by TCDD might be a result of CYP1A1/CYP1B1 catalyzed estrogen metabolism and aromatase mRNA inhibition via AHR.