Inhibition of vitellogenin gene induction by 2,3,7,8–tetrachlorodibenzo–p–dioxin is mediated by aryl hydrocarbon receptor 2 (AHR2) in zebrafish (Danio rerio)

Vitellogenins are hepatically derived yolk–protein precursors required for oogenesis in all oviparous teleosts. Altered gene–regulation of vitellogenesis by environmental contaminants can have profound effects on reproductive success, and ultimately population sustainability. To better understand chemical effects on vitellogenin gene regulation, we tested the hypothesis that activation of the aryl hydrocarbon receptor 2 (AHR2) by dioxin inhibits the estrogen receptor pathway regulation of 3 vitellogenin genes (vtg1–3) in vivo, using zebrafish (Danio rerio) as a model teleost. Using an embryo–larval bioassay, embryos were either treated with 1000 pptr (parts–per–trillion, pg/mL) 17α–ethynylestradiol (EE2) alone from 6 hours post fertilization (hpf) to 4 days post fertilization (dpf), or pre–treated with dioxin (4–5 hpf) prior to EE2. Pre–treatment with 400 pptr 2,3,7,8–tetrachlorodibenzo–p–dioxin (2,3,7,8–TCDD) or 1,2,3,7,8–pentachlorodibenzo–p–dioxin inhibited the EE2 induction of vtg1, vtg2 and vtg3 by >95% (p ≤ 0.05). In comparison, a splice–blocking AHR2 morpholino used to down–regulate ahr2 expression significantly reduced the inhibition of vtg1, vtg2 and vtg3 by 400 pptr 2,3,7,8–TCDD (20.7–27.4% rescue). These studies demonstrate that 2,3,7,8–TCDD directly inhibits the vitellogenin pathway in vivo through activation of the AHR2. This work provides evidence for AHR2 dependent cross–talk inhibition of vitellogenin genes and offers insight into anti–estrogenic reproductive effects observed in oviparous species exposed to AHR agonist contaminants.