Formation and persistence of estragole-derived DNA adducts in human liver cells and tissue

Estragole (ES) is a phenylpropene naturally occurring in various herbs and spices, such as fennel and basil. Humans are exposed to ES via the diet or herbal medicinal products. Following its gastrointestinal resorption, hepatic bioactivation of ES by Cytochrome P450 monooxygenases (CYPs) and sulfotransferases (SULTs) can lead to DNA adduct formation and liver cancer in rodents. Up to now, no data is available on ES-dependent DNA adducts in human tissue, which would however be important for human risk assessment. Our work thus analyzed whether ES-derived DNA adducts are present in human liver samples. Furthermore, DNA adduct formation after repetitive ES exposure and upon incubation with an ES-containing bitter fennel infusion was studied in human liver cells. Using a sensitive UHPLC-MS/MS method, we detected E3'-N²-dG adducts in a multitude of human liver samples. The adduct levels correlated positively with SULT1A1, but not CYP1A2 protein expression. Noteworthy, the E3'-N⁶-dA adduct was not found in the analyzed human liver tissue. Further experiments revealed that E3'-N²-dG adducts accumulate in metabolically competent human liver cells after repetitive short-term exposure to ES, whereas the E3'-N⁶-dA adduct was not detected at all. Finally, we provided evidence that ES from a bitter fennel tea preparation induces E3'-N²-dG adducts in human liver cells, however at lower levels than expected. Altogether, this study demonstrated the occurrence of ES-derived DNA adducts in human liver tissue at levels comparable to the structurally related methyleugenol. Notably, the found DNA adduct levels are well below those reported to cause mutagenicity and clastogenicity in human cells and rodent in vivo models.