Molecular Mechanisms of Selective Estrogen Receptor Modulator Activity in Human Breast Cancer Cells: Identification of Novel Nuclear Cofactors of Antiestrogen–ERα Complexes by Interaction Proteomics

Estrogen receptor alpha (ERα) is a ligand-activated
transcription
factor that controls key cellular pathways via protein–protein
interactions involving multiple components of transcriptional coregulator
and signal transduction complexes. Natural and synthetic ERα
ligands are classified as agonists (17β-estradiol/E2), selective estrogen receptor modulators (SERMs: Tamoxifen/Tam and
Raloxifene/Ral), and pure antagonists (ICI 182,780-Fulvestrant/ICI),
according to the response they elicit in hormone-responsive cells.
Crystallographic analyses reveal ligand-dependent ERα conformations,
characterized by specific surface docking sites for functional protein–protein
interactions, whose identification is needed to understand antiestrogen
effects on estrogen target tissues, in particular breast cancer (BC).
Tandem affinity purification (TAP) coupled to mass spectrometry was
applied here to map nuclear ERα interactomes dependent upon
different classes of ligands in hormone-responsive BC cells. Comparative
analyses of agonist (E2)- vs antagonist (Tam, Ral or ICI)-bound
ERα interacting proteins reveal significant differences among
ER ligands that relate with their biological activity, identifying
novel functional partners of antiestrogen–ERα complexes
in human BC cell nuclei. In particular, the E2-dependent
nuclear ERα interactome is different and more complex than those
elicited by Tam, Ral, or ICI, which, in turn, are significantly divergent
from each other, a result that provides clues to explain the pharmacological
specificities of these compounds.