Developmental Regulation of Nuclear Factor Erythroid-2 Related Factors (nrfs) by AHR1b in zebrafish (Danio rerio)

Interactions between regulatory pathways allow organisms to adapt to their environment and respond to stress. One interaction that has been recently identified occurs between the aryl hydrocarbon receptor (AHR) and the nuclear factor erythroid-2 related factor (NRF) family. Each transcription factor regulates numerous downstream genes involved in the cellular response to toxicants and oxidative stress; they are also implicated in normal developmental pathways. The zebrafish model was used to explore the role of AHR regulation of nrf genes during development and in response to toxicant exposure. To determine if AHR1b is responsible for transcriptional regulation of six nrf genes during development, a loss-of-function experiment using morpholino-modified oligonucleotides was conducted followed by a Chromatin Immunoprecipitation (ChIP) study at the beginning of the pharyngula period (24 hours post fertilization; hpf). The expression of nrf1a was AHR1b dependent and its expression was directly regulated through specific XREs in its cis-promoter. However, nrf1a expression was not altered by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a toxicant and prototypic AHR agonist. The expression of nrf1b, nrf2a, and nfe2 was induced by TCDD, and AHR1b directly regulated their expression by binding to cis-XRE promoter elements. Lastly, nrf2b and nrf3 were neither induced by TCDD nor regulated by AHR1b. These results show that AHR1b transcriptionally regulates nrf genes under toxicant modulation via binding to specific XREs. These data provide a better understanding of how combinatorial molecular signaling potentially protects embryos from embryotoxic events following toxicant exposure.