Aminoflavone (4H-1-
benzopyran-4-one, 5-amino-2-(4-amino-3-fluorophenyl)-6,8-difluoro-7-methyl;
NSC 686288) demonstrates differential antiproliferative activity in the National Cancer Institute's anticancer
drug screen. We demonstrate here that MCF-7 human
breast cancer cells are sensitive to
aminoflavone both in vitro and when grown in vivo as xenografts in athymic mice. As previous work has indicated that
aminoflavone requires metabolic activation by
cytochrome P450 1A1 (
CYP1A1), we investigated the effect of
aminoflavone on
CYP1A1 expression and on the
aryl hydrocarbon receptor (AhR), a transcriptional regulator of
CYP1A1. In
aminoflavone-sensitive but not
aminoflavone-resistant cells, the
drug caused a 100-fold induction of
CYP1A1 mRNA and a corresponding increase in
ethoxyresorufin-O-deethylase activity. An AhR-deficient variant of the MCF-7
breast carcinoma, AH(R100), with diminished
CYP1A1 inducibility, exhibits cellular resistance to
aminoflavone and is refractory to
CYP1A1 mRNA induction by the
drug. The increase in
CYP1A1 mRNA in the
aminoflavone-sensitive MCF-7
breast tumor cell results from transcriptional activation of
xenobiotic-responsive
element (XRE)-controlled transcription.
Aminoflavone treatment causes a translocation of the AhR from the cytoplasm to the nucleus with subsequent formation of AhR-XRE
protein DNA complexes. In contrast to the
aminoflavone-sensitive MCF-7 cells, the resistant cell lines (MDA-MB-435, PC-3, and AH(R100)) demonstrated constitutive nuclear localization of AhR. Additionally,
aminoflavone failed to induce
ethoxyresorufin-O-deethylase activity,
CYP1A1 transcription, AhR-XRE complex formation, and apoptosis in
aminoflavone-resistant cells. These results suggest that the cytotoxicity of
aminoflavone in a sensitive
breast tumor cell line is the result of the engagement of AhR-mediated signal transduction.