Structure-dependent activation and inhibition of nuclear receptors by hepatotoxic pyrrolizidine alkaloids

1,2-Unsaturated pyrrolizidine alkaloids (PA) belong to the most toxic compounds. These substances are found in several plants such as Asteraceae and Boraginaceae families. Acute PA poisoning by food contamination causes severe damage to liver; long-term, sub-lethal doses may cause cumulative damage or cancer. A previous whole genome μ-array analysis of PA in primary human hepatocytes revealed potential interactions of PA with certain nuclear receptors acting or working as transcription factors. Employing reporter gene assays it was analyzed whether PA can activate or inhibit nuclear receptors such as RARα, RXRα, LXRα, FXR, PPARα, PPARγ, PPARδ, PXR, ERα and ERβ. As PXR and VDR mediate CYP3A4 promoter activity a potential induction or inhibition of CYP3A4 promoter activity was additionally investigated. To cover the most frequently occurring PA structures (retronecine, heliotridine and otonecine type as well as monoester, diester and cyclic diester) the four PA senecionine, heliotrine, echimidine and senkirkine were selected as representative PA. Heliotrine was found to inhibit the transactivation of LXRα, RXRα, PPARα, PPARγ, PPARδ and PXR. Senecionine activated ERα and β. Echimidine was the only PA that activated PXR. Consequently, it also exclusively induced CYP3A4 promoter activity. Neither interactions nor transactivation could be observed for senkirkine. In conclusion, the prediction from the μ-array data suggesting an interaction of PA with different nuclear receptors could be confirmed. However, the interactions were found to be heterogeneous between the structurally different PA. This suggests a specific structure–activity relationship of PA concerning their interaction with nuclear receptors.