芳香烴接受器之內生性配體於神經發育之角色

Abstract

芳香烴受器 (Aryl hydrocarbon receptor, AHR) 為須經配體 (ligand) 活化的轉錄因子。當 AHR 在細胞質與其外源性配體，如戴奧辛結合時，活化的 AHR 會入核與目標基因結合，並開啟轉錄與轉譯，進而影響許多生理功能如胚胎發育、癌症生成及發炎反應等。然而，關於 AHR 內生性配體之相關研究至今仍不完整。本實驗室利用先前建立的戴奧辛偵測系統在斑馬魚胚胎初萃物中，找到了新的AHR之內生性配體—四氫皮質酮 (tetrahydrocorticosterone, THB)，並發現其中的 5β-THB 可透過活化 AHR 調控斑馬魚的神經分化。然而，同為此受器內生性配體的 5α-THB 是否也透過 AHR 調控特定生理反應仍有待探討，因此，本研究的實驗目標為了解 5α-THB 在 AHR 所影響的生理功能中所扮演的角色。實驗顯示 THB 會活化 AHR，使之入核並調控下游目標基因 Cyp1a1 的表現。先前的研究中已證實 AHR 大量表達會導致神經纖維母細胞瘤細胞 (neuroblastoma) 分化；本研究結果顯示 5α-THB 也具有促進此細胞神經分化的能力。在斑馬魚實驗中，5α/5β-THB 可透過活化 AHR 刺激 zCyp1a 的表現，也對髓鞘相關基因 zSox10、zMbp 有正向調控的作用，且更進一步地影響斑馬魚的泳動能力。綜合以上結果，我們認為 AHR 內生性配體—5α/5β-THB 在斑馬魚的神經分化中扮演了重要角色。

Aryl hydrocarbon receptor (AHR) is a cytosolic ligand-activated transcriptional factor. Once binding to xenobiotic toxic chemicals such as dioxin, AHR triggers downstream signaling and regulates a variety of physiological functions which include embryogenesis, tumorigenesis, and inflammation. However, the characteristic of endogenous AHR ligands remains elusive. In our previous study, tetrahydrocorticosterone (THB) was identified as a potential AHR ligand in zebrafish embryos using a well-established cell-free bioassay for dioxin-like compounds. We also demonstrated that one of the THB, 5β-THB, plays a critical role in neural differentiation in zebrafish. Nevertheless, the role of 5α-THB, a cis-trans isomer of 5β-THB, has not been illustrated. The aim of this study is to investigate the effects of 5α-THB in AHR activation and clarify its physiological functions. Similar to our previous results, 5α-THB was found to induce nucleus translocation of AHR, which subsequently up-regulated the mRNA expression of cytochrome P450 1A1 (CYP1A1) in AHR-overexpressing cells. Besides, 5α-THB promoted neuronal differentiation of neuroblastoma (NB) cells, suggesting that activation of AHR by 5α-THB leads to NB differentiation. Furthermore, 5α/5β-THB both enhanced the expressions of zCyp1a, myelin-associated protein myelin basic protein (zMbp), and sex determining region Y- box 10 (zSox10), and improved the mobility of zebrafish larvae via the Ahr2 pathway. In conclusion, the results in this study suggested that endogenous AHR ligands 5α/5β-THB play critical roles during neural development of zebrafish.