Abstract
1730
The aryl hydrocarbon receptor (AhR), a ligand-activated helix loop helix transcription factor, binds environmental poly aromatic hydrocarbons (PAH) such as dioxin, and mediates their toxicity, including carcinogenesis. This study was designed to investigate the functional significance of AhR in breast carcinogenesis. The AhR expression and its transcriptional activity was analyzed in a battery of human breast carcinoma (HBC) cell lines with varying degrees of malignancy in comparison to immortalized normal and primary human mammary epithelial cells (HMEC). Western immuno-blotting revealed dramatic elevated levels of AhR proteins in tumorigenic HBC of advanced malignancy (MD231, MDA468, ZR-75, MDA435, MCF-7), while less levels were expressed in HBC typically categorized as early stages of maliganacy (T-47D, HBL100, BT-549), normal immortalized (H16N2 and MCF10A) and primary HMEC. RT-PCR analysis of mRNA revealed a similar trend although not as dramatic high levels, suggesting a role for protein stabilization in these elevated levels. These results were further confirmed in Sager’s 21T series, which are closely matched pairs of HBC derived from a single patient and are characterized by exhibiting a gradient order of malignancy (21MT2>21NT>21PT). The 21MT-2 lines showed the highest expression of protein and mRNA and 21NT was medium between 21MT-2 and 21PT lines. This high expression of AhR is independent of estrogen receptor (ER) status, as verified in MDA 231 (ER negative) and its variant S30 (ER positive), as well as T47D-A18 (ER positive) and its variant T47D-C4:2W (ER negative), as well as in 21T series which are all ER negative. The AhR is usually localized in the cytoplasm of most normal cells including HMEC and is translocated to the nucleus upon ligand (dioxin) activation. However, both sub-cellular fractionation experiments and fluorescence immuno-cytochemical staining has shown that the AhR in the HBC are predominantly localized to the nuclear compartments in absence of ligand treatment. Similarly, immuno-histochemical analysis of human breast tumors has shown an increase in AhR expression in the invasive carcinomas, where the AhR staining was predominantly or exclusively nuclear. Notably, this nuclear accumulation of AhR in untreated HBC was transcriptionally active as evidenced by the substantial expression of CYP1A1 mRNA, which is exclusively regulated transcriptionally by the activated AhR. In conclusion, this study reports a novel finding of elevated levels of AhR in human breast carcinomas in direct proportion to their degree of malignancy. These data identify the AhR as a possible regulator of breast cancer progression and its possible consideration as a candidate prognostic factor for survival as well as its potential as a target for breast cancer therapeutic intervention.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]