Analysis of the potential contribution of estrogen receptor (ER) beta in ER cytosolic assay of breast cancer

Estrogen receptor (ER) content is the most useful parameter for predicting hormone response therapy in breast cancer. Assays available for detecting ER in breast tumor cytosol are ligand-binding assay (LBA), which detects both ERalpha and ERbeta, and the enzymatic immunoassay (EIA), in which monoclonal antibodies are directed against ERalpha. As shown in several studies, the 2 assays correlate and both are used routinely. However, some discrepancies between the 2 assays were found and explanations remain controversial. We evaluated ERalpha and ERbeta mRNA coexpression in breast tumors in order to study whether the presence of ERbeta could account for differences between LBA and EIA in the determination of ER protein level. Using HeLa cell lines transfected with either ERalpha or ERbeta, we confirmed that EIA, using H222 and D547 monoclonal antibodies, recognizes only ERalpha expression, whereas LBA detects both isoforms. In 119 breast tumor cytosols, the correlation between ER-EIA and ER-LBA was high (r = 0.72), although some discrepancies were found. When analyzing ER mRNA expression of samples with higher LBA values, no overexpression of ERbeta mRNA relatively to ERalpha mRNA were observed. There was a difference in ERbeta/ERalpha ratio between ER-negative and ER-positive samples, with a 10-fold increased median ratio in ER-negative samples (p = 0.01). We thus confirmed that the major form of ER in breast cancer is the ERalpha at both the protein and mRNA levels. Moreover, our data do not support the hypothesis that ERbeta expression could explain differences between LBA and EIA in the determination of ER protein level.