title: Inhibition of procarcinogen-bioactivating human CYP1A1, CYP1A2 and CYP1B1 enzymes by melatonin.
abstract: Administration of melatonin to rodents decreases the incidence of tumorigenesis initiated by benzo[a]pyrene or 7,12-dimethylbenz[a]anthracene, which requires bioactivation by cytochrome P450 enzymes, such as CYP1A1, CYP1A2 and CYP1B1, to produce carcinogenic metabolites. The present study tested the hypothesis that melatonin is a modulator of human CYP1 catalytic activity and gene expression. As a comparison, we also investigated the effect of melatonin on the catalytic activity of CYP2A6, which is also a procarcinogen-bioactivating enzyme. Melatonin (3-300 microm) decreased 7-ethoxyresorufin O-dealkylation catalyzed by human hepatic microsomes and recombinant CYP1A1, CYP1A2 and CYP1B1, whereas it did not affect coumarin 7-hydroxylation catalyzed by hepatic microsomes or recombinant CYP2A6. Melatonin inhibited CYP1 enzymes by mixed inhibition, with apparent K(i) values (mean +/- S.E.M.) of 59 +/- 1 (CYP1A1), 12 +/- 1 (CYP1A2), 14 +/- 2 (CYP1B1) and 46 +/- 8 microm (hepatic microsomes). Additional experiments indicated that melatonin decreased benzo[a]pyrene hydroxylation catalyzed by hepatic microsomes and CYP1A2 but not by CYP1A1 or CYP1B1. Treatment of MCF-10A human mammary epithelial cells with melatonin (up to 300 microm) did not affect basal or benzo[a]pyrene-inducible CYP1A1 or CYP1B1 gene expression. Consistent with this finding, melatonin did not influence reporter activity in aryl hydrocarbon receptor-dependent pGudluc6.1-transfected MCF-10A cells treated with or without benzo[a]pyrene, as assessed in an in vitro cell-based luciferase reporter gene assay. Overall, melatonin is an in vitro inhibitor of human CYP1 catalytic activity, and it may be useful to develop potent analogues of melatonin as potential cancer chemopreventive agents that block CYP1-mediated chemical carcinogenesis.
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, BC, Canada.
authors.forename: Thomas K H
authors.initials: TK
authors.lastname: Chang
authors.forename: Jie
authors.initials: J
authors.lastname: Chen
authors.forename: Guixiang
authors.initials: G
authors.lastname: Yang
authors.forename: Eugene Y H
authors.initials: EY
authors.lastname: Yeung
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism*; D001580:Benzopyrenes / Q000378:metabolism; D002460:Cell Line; D002478:Cells, Cultured; D019363:Cytochrome P-450 CYP1A1 / Q000235:genetics / Q000378:metabolism*; D019388:Cytochrome P-450 CYP1A2 / Q000235:genetics / Q000378:metabolism*; D065633:Cytochrome P-450 CYP1B1; D065637:Cytochrome P-450 CYP2A6; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D008550:Melatonin / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000378:metabolism; D010078:Oxazines / Q000378:metabolism; D010957:Plasmids; D016133:Polymerase Chain Reaction
chemical_list: D001580:Benzopyrenes; D004791:Enzyme Inhibitors; D010078:Oxazines; C007649:ethoxyresorufin; D001189:Aryl Hydrocarbon Hydroxylases; C585496:CYP1B1 protein, human; C585710:CYP2A6 protein, human; D019363:Cytochrome P-450 CYP1A1; D019388:Cytochrome P-450 CYP1A2; D065633:Cytochrome P-450 CYP1B1; D065637:Cytochrome P-450 CYP2A6; D008550:Melatonin