Molecular basis for the subtype discrimination of the estrogen receptor-beta-selective ligand, diarylpropionitrile

Although the two subtypes of the human estrogen receptor (ER), ERalpha and ERbeta, share only 56% amino acid sequence identity in their ligand binding domain (LBD), the residues that surround the ligand are nearly identical; nevertheless, subtype-selective ligands are known. To understand the molecular basis by which diarylpropionitrile (DPN), an ERbeta-selective ligand, is able to discriminate between the two ERs, we examined its activity on ER mutants and chimeric constructs generated by DNA shuffling. The N-terminal region of the ERbeta LBD (through helix 6) appears to be fully responsible for the ERbeta selectivity of DPN. In fact, a single ERalpha point mutation (L384M) was largely sufficient to switch the DPN response of this ER to that of the ERbeta type, but residues in helix 3 are also important in achieving the full ERbeta selectivity of DPN. Using molecular modeling, we found an energetically favorable fit for the S-DPN enantiomer in ERbeta, in which the proximal phenol mimics the A ring of estradiol, and the nitrile engages in stabilizing interactions with residues in the ligand-binding pocket of ERbeta. Our findings highlight that a limited number of critical interactions of DPN with the ERbeta ligand-binding pocket underlie its ER subtype-selective character.