In this study, we used zinc finger nuclease-mediated knockout of the aryl 
hydrocarbon receptor (AHR) or AHR nuclear translocator (ARNT) in MCF7 and AHR 
knockout in MDA-MB-231 human breast cancer cells to investigate cross talk among 
AHR, ARNT, and estrogen receptor α (ERα). Knockout of AHR or ARNT prevented the 
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-dependent induction of all AHR target 
genes examined. Knockout of AHR or ARNT also significantly reduced basal 
cytochrome P4501B1 (CYP1B1) expression levels, which were restored with 
overexpression of either protein but not with a DNA binding-deficient AHR 
mutant. Basal and TCDD-, 17β-estradiol (E2)-, or TCDD + E2-dependent recruitment 
of AHR, ARNT, ERα, NCoA3, and RNA polymerase II to CYP1B1 as well as CYP1B1 mRNA 
levels were abolished in MCF7-AHR((ko)) and MDA-MB-231 AHR(ko) cells. However, 
reduced but significant E2-dependent recruitment of ERα, NCoA3, and RNA 
polymerase II to CYP1B1 and weak increases in CYP1B1 mRNA levels were observed 
in MCF7 ARNT((ko)) cells. Interestingly, E2-dependent increases in trefoil 
factor 1, but not growth regulation by estrogen in breast cancer 1 (GREB1) mRNA 
levels, were dependent on ARNT expression. Moreover, the TCDD-dependent 
increases in the proteolytic degradation of ERα were prevented by the loss of 
AHR or ARNT. Our data show that AHR and ARNT play critical roles in the basal, 
TCDD, and E2-induced regulation of CYP1B1 but also reveal distinct roles for 
both proteins in ERα transactivation.