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ABSTRACT:Polycyclic aromatic hydrocarbons (PAHs) and metals are often environmental cocontaminants, yet there have been relatively few studies of combined effects of PAHs and metals on cytochrome P450 (P450)-catalyzed metabolism. We examined the effects of NaAsO 2 in combination with benzo[a]pyrene (BAP) on CYP1A1 and CYP1B1 in T-47D human breast cancer cells by using estrogen metabolism as a probe of their activities. Exposure to BAP caused elevated rates of the 2-and 4-hydroxylation pathways of estrogen metabolism, indicating induction of both CYP1A1, an estradiol 2-hydroxylase, and CYP1B1, an estradiol 4-hydroxylase. BAP-induced metabolism peaked 9 to 16 h after exposure and returned to near-basal levels by 48 h. Concentration-response studies showed maximal induction of the 2-and 4-hydroxylation pathways at 3 M BAP; higher levels caused reduced rates of metabolism due to inhibition of CYP1A1 and CYP1B1. NaAsO 2 caused pronounced decreases in the induction of CYP1A1 and CYP1B1 by 3 M BAP because cotreatment with 10 M NaAsO 2 inhibited the rates of the 2-and 4-hydroxylation pathways by 86 and 92%, respectively. Western immunoblots showed diminished levels of BAP-induced CYP1A1 by coexposure to NaAsO 2 . The levels of the CYP1A1 and CYP1B1 mRNAs induced by BAP were not significantly affected by coexposure to NaAsO 2 ; however, heme oxygenase 1 mRNA levels were markedly induced by coexposure to BAP and NaAsO 2 . These results indicate a post-transcriptional inhibitory effect of arsenite on the expression of CYP1A1 and CYP1B1 in T-47D cells, possibly resulting from reduced heme availability.Among the environmental contaminants of most concern due to their toxicity, including carcinogenicity, are heavy metals, such as arsenic, lead, and mercury, and polycyclic aromatic hydrocarbons (PAHs 1 ) typified by benzo[a]pyrene (BAP). PAHs and heavy metals are often cocontaminants in the environment, yet there have been relatively few studies of the combined toxic and particularly the carcinogenic effects elicited by PAHs and heavy metals. The carcinogenicity of BAP and other PAHs is a consequence of their metabolic activation catalyzed by cytochromes P450 (P450) and epoxide hydrolase (Wood et al., 1976;Kapitulnik et al., 1978). Covalent adducts formed by the reaction of PAH diol epoxide metabolites with guanine in mutational hotspots of critical genes such as that of the p53 tumor suppressor (Denissenko et al., 1996), if not efficiently repaired, may initiate tumorigenesis.Several members of the P450 superfamily in conjunction with epoxide hydrolase have been shown to catalyze the metabolism of BAP to carcinogenic intermediates. In extrahepatic tissues, CYP1A1 and CYP1B1 are thought to be the most important enzymes in catalyzing the formation of mutagenic intermediates from BAP and a number of other PAHs, including several that are potent mammary gland carcinogens in rodents. CYP1B1 appears to be more active than CYP1A1 in the conversion of a number of PAHs to genotoxic intermediates (Shimada et al., 1996). In the presence...

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