Hydroxylated Benzo[a]pyrene Metabolites Are Responsible for in Vitro Estrogen Receptor-Mediated Gene Expression Induced by Benzo[a]pyrene, but Do Not Elicit Uterotrophic Effects in Vivo

The estrogenic activities of benzo[a]pyrene (B[a]P) and 10 metabolites (1, 3-, 7-, and 9-hydroxy-B[a]P; 4,5-, 7,8-, and 9,10-dihydrodihydroxy-B[a]P; and 1,6-, 3,6-, and 6,12-B[a]P-dione) were investigated. In vitro, B[a]P did not displace tritiated 17beta-estradiol ([3H]E2) from either a bacterially expressed fusion protein consisting of glutathione-S:-transferase linked to the D, E, and F domains of human ERalpha (GST-hERalphadef), or from full-length human ERbeta (hERbeta) at concentrations as high as 60 microM. However, 10 microM B[a]P demonstrated partial agonist activity in human Gal4-ERalphadef and mouse Gal4-ERbetadef reporter gene assays in transiently transfected MCF-7 cells, relative to 10 nM E2. 1-, 3-, 7-, and 9-hydroxy-B[a]P were found to bind to both receptor isoforms, each showing a higher affinity for the beta isoform. At 10 microM the four monohydroxylated metabolites were able to induce Gal4-hERalphadef- and Gal4-mERbetadef-mediated reporter gene expression to levels 20-100% of that caused by 10 nM E2, suggesting that these metabolites, and not the parent compound, induced reporter gene expression following B[a]P treatment of transiently transfected MCF-7 cells. In addition, the effect of B[a]P on two estrogen-inducible end points, uterine weight and lactoferrin mRNA levels, was determined in ovariectomized DBA/2 and C57BL/6 mice. Neither orally administered B[a]P at doses as high as 10 mg/kg body weight nor subcutaneously injected 3- or 9-hydroxy-B[a]P at doses as high as 20 mg/kg induced effects on uterine wet weight or uterine lactoferrin mRNA levels in either strain. These data suggest that B[a]P metabolites that are estrogenic at high concentrations in vitro do not induce estrogenic effects in the mouse uterus.