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.