Naturally occurring
coumarins (NOCs) inhibit
polycyclic aromatic hydrocarbon-induced skin
tumor initiation in mice by blocking
cytochrome P450 (P450)-mediated bioactivation of
benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]
anthracene (DMBA).
Bergamottin selectively inhibits
tumor initiation by B[a]P, whereas
imperatorin and
isopimpinellin inhibit
tumor initiation with both
carcinogens. The goals of the current study were to examine the ability of NOCs to inhibit human P450s in vitro and to establish whether NOCs, which are anticarcinogenic in mice, can block
carcinogen bioactivation in cultured human cells. For the initial experiments, incubations containing 5 microM P450, P450 substrate, an
NADPH generating system, and NOCs were used to determine the concentrations of each inhibitor that blocked 50% of P450 activity (IC(50)). These results confirmed that NOCs are capable of inhibiting multiple human P450s and that they exhibit selectivity for certain
isoforms of human P450s. In subsequent experiments, we examined the effects of
bergamottin,
imperatorin, and
isopimpinellin on DMBA and B[a]P
DNA adduct formation in the human breast MCF-7
adenocarcinoma cell line. Coincubation of cells with the three different NOCs significantly inhibited
DMBA DNA adduct formation by 29-82% at doses ranging from 2 to 10 microM and significantly inhibited B[a]P
DNA adduct formation by 37-80% at doses ranging from 20 to 80 microM. HPLC analysis of the
DNA hydrolysates demonstrated that inhibition of
DNA adducts corresponded to inhibition of the major B[a]P and DMBA diol-
epoxide-derived adducts. Although
bergamottin was not effective at blocking DMBA bioactivation in the mouse skin model, it was similar in effectiveness to
imperatorin and
isopimpinellin in MCF-7 cells. These results demonstrate that NOCs, which are present in citrus fruits and other components of the human diet, are capable of inhibiting
carcinogen metabolizing
enzymes and blocking bioactivation of both B[a]P and DMBA in MCF-7 cells.