The AF-1-deficient estrogen receptor ERα46 isoform is frequently expressed in human breast tumors

Background: To date, all studies conducted on breast cancer diagnosis have focused on the expression of the full-length 66-kDa estrogen receptor alpha (ER alpha 66). However, much less attention has been paid to a shorter 46-kDa isoform (ER alpha 46), devoid of the N-terminal region containing the transactivation function AF-1. Here, we investigated the expression levels of ER alpha 46 in breast tumors in relation to tumor grade and size, and examined the mechanism of its genER alpha tion and its specificities of coregulatory binding and its functional activities. Methods: Using approaches combining immunohistochemistry, Western blotting, and proteomics, antibodies allowing ER alpha 46 detection were identified and the expression levels of ER alpha 46 were quantified in 116 ER alpha-positive human breast tumors. ER alpha 46 expression upon cellular stress was studied, and coregulator bindings, transcriptional, and prolifer alpha tive response were determined to both ER alpha isoforms. Results: ER alpha 46 was expressed in over 70% of breast tumors at variable levels which sometimes were more abundant than ER alpha 66, especially in differentiated, lower-grade, and smaller-sized tumors. We also found that ER alpha 46 can be generated via internal ribosome entry site-mediated translation in the context of endoplasmic reticulum stress. The binding affinities of both unliganded and fully-activated receptors towards co-regulator peptides revealed that the respective potencies of ER alpha 46 and ER alpha 66 differ significantly, contributing to the differential transcriptional activity of target genes to 17 beta estradiol (E2). Finally, increasing amounts of ER alpha 46 decrease the proliferation rate of MCF7 tumor cells in response to E2. Conclusions: We found that, besides the full-length ER alpha 66, the overlooked ER alpha 46 isoform is also expressed in a majority of breast tumors. This finding highlights the importance of the choice of antibodies used for the diagnosis of breast cancer, which are able or not to detect the ER alpha 46 isoform. In addition, since the function of both ER alpha isoforms differs, this work underlines the need to develop new technologies in order to discriminate ER alpha 66 and ER alpha 46 expression in breast cancer diagnosis which could have potential clinical relevance.