Effects of in ovo exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic AHR/ARNT-CYP1A signaling pathways in common cormorants (Phalacrocorax carbo).

Our previous studies have isolated multiple isoforms of aryl hydrocarbon receptors (AHRs) and AHR nuclear translocators (ARNTs) in avian species. However, roles of such genes on cytochrome P450 1A (CYP1A) expression are not fully understood. To investigate the effects of dioxins on the hepatic expression profiles of AHR1, AHR2, ARNT1 and ARNT2 in avian species, and whether the expression levels of AHRs and ARNTs affect the transcriptions of CYP1A4 and CYP1A5 genes, the eggs of common (great) cormorants (Phalacrocorax carbo) collected from Lake Biwa, Japan, were in ovo administrated with 0, 1500 and 4500pg/g egg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and the hepatic expression levels of AHRs, ARNTs and CYP1As in embryos were monitored with two-step real-time RT-PCR. In young and adult cormorants collected from the same location, the hepatic expressions of these genes were also measured to understand the effects of growth stage. The residue levels of TCDD and other chlorinated dioxin-like congeners (DLCs) in the body of cormorants were quantified with high-resolution gas chromatography equipped with mass-spectrometry. There was no observable effect of in ovo TCDD treatment even at the highest dosage on mortality, body weight and morphology of the liver, heart, spleen, kidney and lung in the embryos. The mRNA expression of ARNT2 was slightly suppressed by the treatment with TCDD, while no alteration was observed for the expression of AHR1, AHR2 and ARNT1. Expressions of CYP1A4 and CYP1A5 were dose-dependently enhanced by TCDD, but CYP1A4 mRNA level increased more prominently than CYP1A5, indicating the difference in induction efficiency between the CYP1A isozymes. Comparison of hepatic mRNA levels of these genes among embryonic, young and adult cormorants revealed that young and adult cormorants had greater CYP1A5 expression levels than embryos, independently of the accumulation levels of DLCs. These results suggest that the hepatic induction of each CYP1A by DLCs in cormorants occurs in an isoform-specific manner and CYP1A5 expression, at least partially, depends on the factors related to the growth of cormorants, but the transcriptional processes of CYP1As are not related to the expression levels of AHRs and ARNTs. This study yielded results supporting our previous observations that in reality, high accumulation of DLCs induces hepatic CYP1A4 and 1A5 expressions in the wild cormorant population.