Influence of TCDD on Zebrafish CYP1B1 Transcription during Development

Cytochrome P450 1B1 (CYP1B1) is a heme-containing mono-oxygenase that metabolizes various polycyclic aromatic hydrocarbons and aryl amines, as well as retinoic acid and steroid hormones. Here we report the cloning of an ortholog of CYP1B1 from zebrafish and the demonstration that transcription of zebrafish CYP1B1 was modulated by two types of mechanisms during different developmental stage. First in late pharyngula stage before hatching, CYP1B1 was constitutively transcribed in retina, midbrain-hindbrain boundary and diencephalon regions through a close coordination between aryl hydrocarbon receptor 2 (AHR2)-dependent and AHR2-independent pathways. After hatching, the basal transcription was attenuated and it could not be elicited upon 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. In contrast, TCDD exposure induced de novo CYP1B1 transcription in larval branchial arches and heart tissues via an AHR2-dependent pathway. Blocking AHR2 translation completely eliminated the TCDD-mediated CYP1B1 transcription. However, we did not detect any types of CYP1B1 transcription in liver and kidney tissues through the developmental stage. It suggests that the constitutive and TCDD-inducible types of CYP1B1 transcriptions are modulated by distinct pathways with different tissue specificities. Finally, we investigated the role of CYP1B1 in TCDD-mediated embryonic toxicity. Because knockdown of CYP1B1 did not prevent TCDD-induced pericardial edema and cranial defects, it suggests that CYP1B1 is not involved in the developmental toxicity of dioxin.