In Situ evaluation of estrogen receptor dimers in breast carcinoma cells: Visualization of protein-protein interactions

The estrogen receptor (ER) functions as a dimer and is involved in several different biological functions. However ER dimeric proteins have not been identified by in situ methodologies. Structured illumination microscopy (SIM) has been recently developed, which enabled the localization of protein and protein interaction. Therefore, in this study, we firstly demonstrated that ERs formed both homodimers and heterodimers in breast carcinoma cell lines using Nikon’s SIM (N-SIM). ERa/a homodimers were detected in the nuclei of both ERa-positive MCF-7 and T-47D cells; 23.0% and 13.4% of ERa proteins formed ERa/a homodimers, respectively. ERa/β heterodimers were also detected in MCF-7 and T-47D. Approximately 6.6% of both ERα and ERβ1 proteins formed ERa/β1 heterodimers in MCF-7. In addition, 18.1% and 22.4% of ERa and ERβ proteins formed ERa/β2 heterodimers and ERa/β5 heterodimers in MCF-7, respectively. In addition, by using proximity ligation assay (PLA) in MCF-7, estradiol-induced ERa/a homodimers and ERa/β1 heterodimers were both detected after 15 to 45 min of treatment and at 15 min, respectively. The percentage of total ER proteins could also be determined using N-SIM. By using both methods, it has become possible to evaluate precise localization and ratio of ER dimers among different cell types.