FRET analysis of androgen receptor structure and biochemistry in living cells

Studying Nuclear Receptor Complexes in the Cellular Environment

The ligand-regulated structure and biochemistry of nuclear receptor complexes are commonly determined by in vitro studies of isolated receptors, cofactors, and their fragments. However, in the living cell, the complexes that form are governed not just by the relative affinities of isolated cofactors for the receptor but also by the cell-specific sequestration or concentration of subsets of competing or cooperating cofactors, receptors, and other effectors into distinct subcellular domains and/or their temporary diversion into other cellular activities. Most methods developed to understand nuclear receptor function in the cellular environment involve the direct tagging of the nuclear receptor or its cofactors with fluorescent proteins (FPs) and the tracking of those FP-tagged factors by fluorescence microscopy. One of those approaches, Förster resonance energy transfer (FRET) microscopy, quantifies the transfer of energy from a higher energy "donor" FP to a lower energy "acceptor" FP attached to a single protein or to interacting proteins. The amount of FRET is influenced by the ligand-induced changes in the proximities and orientations of the FPs within the tagged nuclear receptor complexes, which is an indicator of the structure of the complexes, and by the kinetics of the interaction between FP-tagged factors. Here, we provide a guide for parsing information about the structure and biochemistry of nuclear receptor complexes from FRET measurements in living cells.

The fluorescent two-hybrid assay for live-cell profiling of androgen receptor modulators

The androgen receptor (AR) is an important target for drug therapies combating prostate cancer. However, various acquired mutations within the AR sequence often render this receptor resistant to treatment. Ligand-induced interaction between the N- and C-termini of the AR marks the initial step in the AR signaling cascade and can thus serve as an early read-out for analysis of potential antagonists of wt and mutant AR. To measure changes of the N/C interaction in the wt and mutant AR variants upon the addition of inhibitors, we applied our recently developed Fluorescent Two-Hybrid (F2H) assay. The F2H method enables real-time monitoring and quantitative analysis of the interactions between GFP- and RFP-tagged proteins in live mammalian cells, where GFP-tagged proteins are tethered to a specific nuclear location. This anchoring approach provides a local signal enrichment suitable for direct visualization of protein-protein interactions as co-localizations by conventional epifluorescence microscopy. Since the F2H assay is fully reversible, we could monitor dynamics of AR N/C interactions in living cells in real time upon agonistic, as well as antagonistic treatments. In dose-response F2H experiments, we compared the potencies of abiraterone, bicalutamide, enzalutamide, flutamide, and galeterone/TOK-001 to prevent the dihydrotestosterone-induced N/C interaction in wt AR. We further applied the newly developed F2H assay to analyze how the AR N/C interaction is affected by the clinically relevant mutations W741L, F876L, T877A and F876L/T877A. We conclude that F2H is a reliable and technically undemanding approach for straightforward screening of new AR modulators, as well as for monitoring their activity in real time in living cells.

Estrogen receptor alpha/co-activator interaction assay: TR-FRET

Time-resolved fluorescence resonance energy transfer, TR-FRET, is a time-gated fluorescence intensity measurement which defines the relative proximity of two biomolecules (e.g., proteins, peptides, or DNA) based on the extent of non-radiative energy transfer between two fluorophores with overlapping emission/excitation spectra. In these assays, an excited lanthanide ion acts as a "donor" that transfers energy to an "acceptor" fluorophore through dipole-dipole interactions. A FRET signal is reported as the ratio of acceptor to donor emission following donor excitation. When a donor-conjugated protein interacts with an acceptor-conjugated protein, the donor and acceptor fluorophores are brought in close proximity allowing energy transfer from the donor to the acceptor resulting in a FRET signal. Because the lanthanide donors have a long emission half-life, the energy transfer measurement can be time-gated, which dramatically reduces assay interference (due to background autofluorescence and direct acceptor excitation) and thereby increases data quality. Here, we describe a TR-FRET assay that monitors the interaction of the estrogen receptor (ER) α ligand binding domain (labeled with a terbium chelate via a streptavidin-biotin interaction) with a sequence of coactivator protein SRC3 (labeled directly with fluorescein) and the disruption of this interaction with a peptide and a small molecule inhibitor.