The PXR-CYP3A response in rainbow trout and interactions with pharmaceuticals

We have isolated the complete coding sequence of the PXR gene from rainbow trout liver and it shares 56% and 46% sequence identity with zebrafish PXR and human PXR, respectively. The PXR is expressed in heart, liver, kidney and intestine in juvenile rainbow trout. We are looking at effects of pharmaceuticals on the PXR-CYP3A response in rainbow trout in vivo and in the rainbow trout hepatoma cell line (RTH-149) in vitro. To date, almost 1/3 of all pharmaceuticals have been classified based on PBT-indices, which determined as: Persistence in the aquatic environment; Bioaccumulation in biota; acute Toxicity in aquatic organisms, ranging from no risk (PBT=0) to high risk (PBT=9). In addition to PBT-index values, it is important to assess metabolism of pharmaceuticals in different species as that further provides information metabolic clearance and drug-interactions in a specific animal species. We have looked at ketoconazole and ethinylestradiol and how these drugs interact with the PXR-CYP3A pathway in rainbow trout. We propose that induction of CYP3A expression is indicative of PXR ligand activation and that dealkylation of 7-benzyloxy-4-[trifluoromethyl]-coumarin is indicative of CYP3A enzyme activity. Ketoconazole treatment resulted in a weak activation of PXR and a profound inhibition of the CYP3A enzyme activity. Though, the ketoconazole treatment appeared to affect both PXR and CYP3A, the inhibitory effect of the CYP3A enzyme overrides the PXR activation. This interaction resulted in impaired metabolic clearance of pharmaceuticals, such as ethinylestradiol. Funded by the Swedish Research Council Formas, grant 21.0/2004-0363 and by the Swedish EPA, grant I-193-03.