Chlorinated polycyclic aromatic hydrocarbons (CIPAHs), which are a series of halogenated aromatic hydrocarbons, have been found in the environment. The primary step in their metabolic activation seems to be associated with aryl hydrocarbon receptor (AhR)-mediated induction of the cytochrome P450 (CYP) 1 family, although the evidence remains unclear. In this study, we first investigated the effects of five CIPAHs with three to five rings and the corresponding parent PAHs on the expression of CYP1A1 and 1131 in human breast cancer MCF-7 cells. For the targeted CIPAHs, Western blot analysis of CIPAH-induced CYP1A1 and 1131 showed an enhancement in activities in comparison with induction by the corresponding parent PAHs, and the effects of chlorination were especially prominent in phenanthrene. In a further study, using 6-chlorobenzo[a]pyrene (6-CIBaP), cotreatment with 17 beta-estradiol showed an increase in the expression of CYP1B1 mRNA but not CYP1A1 mRNA. Since the AhR ligand has been reported to induce formation of an AhR-estrogen receptor (ER) complex, which stimulates transcription of ER target genes, the effects of CIPAHs in MCF-7 cells transfected with estrogen response elements-regulated green fluorescent protein (GFP) reporter genes were also investigated in this study. 6-CIBaP induced a dose-dependent increase in GFP expression related to ER signaling through AhR activation in the cells, but 3,9,10-trichlorophenanthrene (3,9,10-Cl(3)CIPhe) did not, despite its ability to activate AhR. Furthermore, we investigated the effect of CIPAHs on the expression of the endogenous ER-responsive genes, cathepsin D, in MCF-7 cells. 6-CIBaP stimulated expression of the ER-responsive genes but 3,9,10-Cl(3)CIPhe did not, as in the GFP expression system. These results suggest that estrogenic action mediated ER signaling through AhR activation does not necessarily occur for every ligand that can activate AhR. (C) 2009 Wiley Periodicals, Inc. Environ Toxicol 25: 180-187, 2010.