| Human cytochrome P-450 1A2 (hCYP1A2) is a biotransformation enzyme that activates several procarcinogens, including aflatoxin B1 (AFB). AFB is a metabolite of grain mold and a primary factor in hepatocellular carcinoma development. Worldwide, hepatocellular carcinoma is the fifth most common cancer. Consumption of apiaceous vegetables (carrot-family) inhibits CYP1A2 in humans, although the mechanism is unknown. Phytochemicals in high concentrations in apiaceous vegetables may be responsible for CYP1A2 inhibition, but the duration of the inhibitory effect and influence of polymorphisms in CYP1A2 are likewise unknown.; Apiaceous vegetable constituents were screened for inhibition of CYP1A2-mediated methoxyresorufin O-demethylase (MROD) activity using microsomes from yeast expressing hCYP1A2. Apigenin, psoralen, 5-methoxypsoralen, and 8-methoxypsoralen were potent inhibitors of MROD activity in low (<10) micromolar concentrations. A mutagenicity assay was then used to test whether apigenin, psoralen, 5-methoxypsoralen, and 8-methoxypsoralen inhibit hCYP1A2 activation of AFB and resulting mutagenicity in intact yeast cells expressing hCYP1A2. Yeast cells underwent 2-hour pretreatment with each phytochemical at low micromolar concentrations followed by 4-hour treatment with 100 muM AFB. Psoralen, 5-methoxypsoralen, and 8-methoxypsoralen significantly reduced AFB-induced cytotoxicity; apigenin, psoralen, 5-methoxypsoralen, and 8-methoxypsoralen significantly reduced AFB-induced mutagenicity. As part of a larger randomized crossover feeding trial in humans, it was tested whether CYP1A2 activity rebounds after apiaceous vegetables are removed from the diet and whether response varies by CYP1A2*1F genotype (confers decreased inducibility). After 13 days of consuming a low-phytochemical diet supplemented with either cruciferous plus apiaceous vegetables or just cruciferous vegetables, all vegetables were removed from the diet and CYP1A2 activity was assessed on days 14 and 15. CYP1A2 activity recovered after apiaceous vegetables were removed with no significant influence from CYP1A2*1F genotype on this response.; Taken together, apigenin, psoralen, 5-methoxypsoralen, and 8-methoxypsoralen may be responsible for in vivo CYP1A2 inhibition from apiaceous consumption; apiaceous consumption may be chemopreventive with regard to CYP1A2-activated procarcinogens such as AFB; recovery of CYP1A2 activity after vegetable removal suggests that apiaceous-mediated inhibition of CYP1A2 is short and primarily through effects on the protein, not gene expression suggesting that the timing of apiaceous exposure relative to procarcinogen exposure might influence any chemopreventive efficacy. |
