CYP1B1 converts procarcinogens into genotoxins in Saccharomyces cerevisiae

CYP1B1 activates many chemical carcinogens into potent genotoxins, and allelic variants are risk factors in lung, breast, and prostate cancer. However, genetic instability phenotypes incurred by CYP1B1-activated metabolites have been investigated for only few compounds. In this study, we expressed human CYP1B1 in yeast strains that measure DNA damage-associated toxicity and frequencies of chromosomal translocations and mutations. DNA damage-associated toxicity was measured in a rad4 rad51 strain, defective in both DNA excision and recombinational repair. Frequencies of chromosomal translocations were measured in diploid yeast strains containing two his3 fragments, and mutation frequencies were measured by selecting for canavanine resistance (CanR) in haploid strains. These strains were exposed to benzo[a]pyrene dihydrodiol (BaP-DHD), aflatoxin B1 (AFB1), and the heterocyclic aromatic amines, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-3-methylimidazo(4,5-f)quinoline (IQ). We observed that AFB1, BaP-DHD, IQ, and MeIQx conferred toxicity in the DNA repair mutant expressing CYP1B1. Translocation frequencies increased eight-fold and three-fold after exposure to 50 μM AFB1 and 33 μM BaP-DHD respectively. Only a two-fold increase in mutation frequency was observed after exposure to 50 μM AFB1. However, a robust DNA damage response was observed after AFB1 exposure, as measured by the induction of the small subunit of ribonucleotide reductase, Rnr3. While CYP1B1-mediated activation of BaP-DHD and heterocyclic aromatic amines was expected, strong activation of AFB1 was not. These studies demonstrate that CYP1B1-mediated activation of carcinogens does not only activate compounds to become mutagens but also can convert compounds to become potent recombinagens.