Estrogen Receptor Imaging Using Intrinsically Fluorescent Ligands

Abstract

This article will summarize recent advances in the development of fluorescent probes that bind as ligands to the estrogen receptor (ER). While fluorescent steroid derivatives have been investigated for decades as potential probes for cytofluorometric analysis of steroid receptors, only recently has it been possible to convincingly demonstrate their utility for receptor visualization within cells by in situ fluorescence microscopy. We have recently shown that inherently fluorescent ligands such as the naturally occurring plant estrogen coumestrol¹ and synthetic tetrahydochrysene (THC) estrogens² are able to visualize ER within cells in a cell culture model for ER expression. Reversibly binding fluorescent ligands have several distinct advantages compared to using chemically conjugated antibodies for the analysis of receptor expression and distribution. Notable among these are an improvement in localization of the target protein due to the small size of these probes and the fact that upon binding, they become effectively immobilized at the ligand binding site of their receptor. In contrast, analysis by immunohistochemical methods involves the deposition of a potentially diffusible chromophore in proximity to the target protein, directed by an antigen-antibody interaction on the protein surface. An increase in resolution is afforded by immunofluorescence analysis; however, this technique is still indirect since the imaging fluorochrome is coupled to either a primary or a secondary antibody rather than to the target protein under investigation.