Naturally occurring
coumarins (NOCs) are anti-carcinogenic in the mouse skin model. To characterize the chemopreventive potential of NOCs against
breast cancer, we first examined their effects on 7,12-dimethylbenz[a]
anthracene (
DMBA)-DNA adduct formation in mouse mammary gland. We hypothesized that those NOCs that both inhibited
cytochrome P450 1A1/1B1 and induced hepatic
glutathione S-
transferases (
GSTs) would be the most effective in blocking
DMBA-DNA adduct formation in mouse mammary gland. To address this hypothesis, simple
coumarins (e.g.
coumarin and
limettin, which induced mouse hepatic
GSTs but had little effect on P4501A1/1B1) and linear
furanocoumarins (e.g.
imperatorin and
isopimpinellin, which induced hepatic
GSTs and were potent inhibitors of P4501A1/1B1) were compared. Mice were pretreated with NOCs (150 mg/kg body wt, by gavage) prior to either a single dose of DMBA (50 microg) or multiple doses of DMBA (20 microg daily for 3 and 6 weeks). Mammary
DMBA-DNA adduct formation was quantitated by the nuclease P1-enhanced 32P-postlabeling assay. With the single dose of DMBA,
coumarin,
limettin,
imperatorin and
isopimpinellin inhibited
DMBA-DNA adduct formation by 50, 41, 79 and 88%, respectively.
Coumarin,
limettin and
imperatorin blocked
DMBA-DNA adduct formation by 36, 60, and 66% at 3 weeks, and by 0, 49 and 55% at 6 weeks of DMBA dosing, respectively. In a 6 week dose-response study of select NOCs and
7,8-benzoflavone (a potent P4501 inhibitor that had little effect on
GSTs),
DMBA-DNA adduct formation was inhibited by 0, 43 and 24% in the
limettin groups; by 26, 26 and 69% in the
isopimpinellin groups; and by 80, 96 and 97% in the 7,8- benzoflavone groups at 35, 70 and 150 mg/kg, respectively. Taken together, these results suggest that linear
furanocoumarins had a greater inhibitory effect on
DMBA-DNA adduct formation in mouse mammary glands compared with simple
coumarins, and that the predominant effect may be P4501 inhibition.