Studies using fluorescent tetrahydrochrysene estrogens for in situ visualization of the estrogen receptor in living cells.

We have analyzed four fluorescent nonsteroidal estrogens for their potential to serve as vital cytological stains to visualize the estrogen receptor (ER) in a model receptor expression system. The novel estrogen fluorophores are based on the rigidified stilbene-like structure of 5,6,11,12-tetrahydrochrysene (THC), and they embody electron-donor (hydroxyl) and electron-acceptor groups (nitrile, amide, ester, or ketone) that afford efficient, long wavelength, and environment-sensitive fluorescence. These probes bind with high affinity to human ER (relative binding affinity, 22-85 vs. estradiol, 100), and they stimulate the transcriptional activity of this receptor. The strong fluorescence of the estrogenic THCs permits visualization, using conventional epifluorescence microscopy, of ER in transfected Cos-7 cells that express elevated levels of receptor. Cell staining by the donor-acceptor THCs characteristically displays a nonuniform pattern of nuclear fluorescence that can be fully inhibited by nonfluorescent estrogens such as estradiol or diethylstilbestrol. Additionally, this staining appears to be specific for ER, since it coincides with the distribution of receptor as determined by indirect immunofluorescence analysis using an ER-specific monoclonal antibody. Using these probes, we have analyzed the intracellular distribution of ER mutants containing a variety of deletions. Evidence is presented to show that removal of amino-terminal sequences within the ER polypeptide results in an altered pattern of intranuclear distribution with preferential accumulation of receptor protein within the nucleolus. These THC fluorophores therefore represent excellent probes for cytological studies involving ER expressed in cultured cells and represent an important advance toward the goal of exploiting fluorescence technology to analyze the expression and distribution of ER within tissue samples.