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 E R in breast tumor cytosol are ligand-binding assay (LBA), which detects bot h ER alpha and ERP, and the enzymatic immunoassay (EIA), in which monoclona l antibodies are directed against EP alpha, As shown in several studies, th e 2 assays correlate and both are used routinely. However, some discrepanci es between the 2 assays were found and explanations remain controversial. W e evaluated ER alpha and ER beta mRNA coexpression in breast tumors in orde r to study whether the presence of ER beta could account for differences be tween LBA and EIA in the determination of ER protein level. Using HeLa cell lines transfected with either EP alpha or ER beta, we confirmed that EIA, using H222 and D547 monoclonal antibodies, recognizes only ERa expression, whereas LBA detects both isoforms. In 119 breast tumor cytosols, the correl ation between ER-EIA and ER-LBA was high (r = 0.72), although some discrepa ncies were found. When analyzing ER mRNA expression of samples with higher LBA values, no overexpression of ER beta mRNA relatively to ER alpha mRNA w ere observed, There was a difference in ER beta/ ER alpha ratio between ER- negative and ER-positive samples, with a IO-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 ER alpha at both the protein and mRNA levels. More over, our data do not support the hypothesis that ERP expression could expl ain differences between LBA and EIA in the determination of ER protein leve l. (C) 2001 Wiley-Liss, Inc.