Sequence-specific
small interfering RNA (
siRNA) duplexes can be used for gene silencing in mammalian cells and as mechanistic probes for determining gene function. Transfection of siRNAs for the
aryl hydrocarbon receptor (AhR) and
AhR nuclear translocator (ARNT) mRNAs in MCF-7
breast cancer cells resulted in a 60 to 80% decrease in levels of AhR and ARNT
proteins in whole-
cell extracts and decreased binding of nuclear extracts to 32P-labeled
dioxin-responsive
element.
siRNA for the AhR also decreased
2,3,7,8-tetrachlorodibenzo-p-dioxin (
TCDD)-induced
CYP1A1 protein, CYP1A1-dependent activity, and
luciferase activity in cells transfected with an Ah-responsive construct. 17beta-estradiol (E2) induces proliferation of MCF-7 cells through enhanced G0/G1 --> S phase progression, and this response is inhibited in cells cotreated with E2 plus
TCDD. The effects of
TCDD on E2-induced cell-cycle progress were partially blocked in MCF-7 cells transfected with
siRNA for AhR. The results also indicated that
siRNA-dependent decreases in AhR
protein in MCF-7 cells were accompanied by increased G0/G1 --> S phase progression, suggesting a growth-inhibitory role for the "endogenous" AhR. Surprisingly,
TCDD alone induced G0/G1 --> S phase progression and exhibited estrogenic activity in MCF-7 cells transfected with
siRNA for the AhR. In contrast, degradation of the AhR in HepG2
liver cancer cells resulted in decreased G0/G1 --> S phase progression, and this was accompanied by decreased expression of
cyclin D1,
cyclin E,
cyclin-dependent kinase 2 (cdk2), and cdk4. In the absence of
ligand, the AhR exhibits growth-inhibitory (MCF-7) and growth-promoting (HepG2) activity that is cell context-dependent.