Expression of an estrogen receptor variant with transcriptional regulatory activity in human breast cancer cell lines: A possible model system for the development of hormone resistance
Description
Accurate identification of prognostic markers in primary breast cancer is critical for selecting the most beneficial mode of therapy for the affected patients. The estrogen receptor (ER) is a major prognostic marker, indicating the estrogen-responsive nature of breast tumors and the likelihood of response to hormonal therapy. Estrogen receptor variants may possess outlaw functions, acting in a dominant-positive (transcriptionally active in the absence of estrogen) or dominant-negative (transcriptionally inactive, while inhibiting the function of wild-type ER) manner. Developing a strong correlation between particular ER variants and altered estrogen-responsiveness suggests that over-expression of these ER variants or an altered expression ratio of these variants to the wild-type ER may be an important biologic event leading to the formation of hormone-independent, antiestrogen-resistant breast cancer Using a variety of approaches including RNase protection analysis and RT-PCR followed by cloning and sequencing, variant ER mRNAs with complete deletions of exons 5 and 7, in addition to the wild-type ER transcript, have been identified in the ER-negative BT-20 and ER-positive MCF-7 breast tumor cell lines. We have also been able to identify these two variant ER transcripts in a number of breast tumor cell lines both ER-positive and ER-negative. It appears that the exon 5 deletion variant $(\Delta 5)$ is expressed at higher levels than the exon 7 deletion variant $(\Delta 7)$ in each cell line, and the ratio of expression of variant to wild-type ER transcript differed among the cell lines examined. Immunoprecipitation of ER followed by Western blot analysis revealed a truncated receptor in the BT-20 cell line that is the expected size of the purported $\Delta 5$ variant ER protein and which possesses constitutive transcriptional regulatory activity in a yeast expression system. RNase protection analysis also demonstrated a variation in the levels of expression of $\Delta 5$ variant transcripts among stocks of MCF-7 cell lines obtained from different sources. Perhaps more importantly, a preliminary study has suggested that the expression of the $\Delta 5$ variant is modulated in culture by the presence or absence of estradiol