The dynamic and elusive membrane estrogen receptor-alpha

Many studies have demonstrated the nuclear forms of steroid receptors and their activities, while fewer investigators have identified and described the membrane forms of these receptors. Our immuno-identification approaches for the qualitative and quantitative comparison of the membrane form of the estrogen receptor-alpha (mER alpha) to its nuclear counterpart now allow us to address questions about the comparative levels and regulation of these receptor forms. ER alpha-specific antisense oligonucleotides eliminate mER alpha expression, while only mildly reducing the nuclear ER alpha. Success of immuno-identification for the mER alpha is very sensitive to different fixation protocols, affecting cell permeability (and thus distinction from the intracellular form) and differential epitope preservation. All such identifications must be accompanied by proof of cell membrane integrity and focal plane assessments. The mER alpha expression on selected cells declines rapidly with cell passage number and cell density. Expression of mER alpha is enhanced by serum starvation and selection for specific phases of the cell cycle. The hinge region of the protein is sensitive to ligand-induced epitope masking and to antibody-induced changes in receptor-mediated responses. Responsive cells are often diluted within cell populations by loss of the membrane receptor form. The bimodality of the rapid estrogen action, with inhibitory doses between picomolar and nanomolar stimulatory concentrations, requires detailed dose-response curves. Finally, responsive cells can be lost from assays, as upon estrogen treatment they rapidly round up and leave the substrates to which they are attached. These regulatory phenomena demonstrate that levels of the membrane form of the estrogen receptor are very dynamic.