Naturally occurring coumarins possess anti-carcinogenic activities in part by inducing
carcinogen-detoxifying enzymes glutathione S-transferase (GST) and/or NAD(P)H quinone
oxidoreductase (NQO1). Our goal was to determine whether citrus coumarins induce hepatic
GST and/or NQO1 via activation of Nrf2 and the antioxidant response element (ARE).
First, HepG2 cells stably transfected with the ARE and a green fluorescent protein
(GFP) reporter were treated with increasing concentrations of coumarins and compared
to positive controls. tert-Butylhydroquinone (TBHQ) and oltipraz increased GFP fluorescence,
as did coumarin, limettin, auraptene, imperatorin, and 7,8-benzoflavone, suggesting
that they activate the ARE, whereas isopimpinellin did not increase GFP fluorescence.
Next, the effects of orally administered coumarins and oltipraz on hepatic GST and
NQO1 activities were compared in Nrf2 knockout mice or Nrf2 heterozygous mice exhibiting
the wild-type phenotype. Oltipraz, auraptene, imperatorin, isopimpinellin, and auraptene
all significantly increased liver cytosolic GST activities in Nrf2 heterozygous mice.
This effect was abrogated in Nrf2(-/-) mice dosed with oltipraz, attenuated in mice
Nrf2(-/-) mice treated with auraptene and imperatorin, and still significant in Nrf2(-/-)
mice treated with isopimpinellin. Of these compounds, only isopimpinellin significantly
increased liver cytosolic NQO1 activities, and this effect was not attenuated in Nrf2(-/-)
mice. These results strongly suggest that imperatorin and auraptene induce murine
liver cytosolic GST activities via the Nrf2/ARE mechanism. Although structurally similar,
isopimpinellin did not appear to activate HepG2-ARE-GFP and the Nrf2 knockout mouse
study suggests that isopimpinellin may induce GST and NQO1 via additional mechanisms.