Evaluation of ligand-dependent changes in AR structure using peptide probes

Mutations in the AR are frequently found in relapsed prostate cancers, some of which permit antiandrogens as well as nonandrogenic compounds to function as androgens. However, the molecular mechanism(s) by which these mutations enable this aberrant AR pharmacology is still unknown. To explore this issue, we used a series of LxxLL-containing peptides (L, leucine; x, any amino acid) to probe the conformation of the AF-2/coactivator binding pocket of AR and AR mutants when complexed with different ligands. We have identified in a previous study two peptides that bind to the wild-type AR in an agonist-dependent manner. Interestingly, we found these same peptides also interacted with several AR variants that are frequently found in antihormone refractory prostate cancers, in the presence of either androgens or antiandrogens. This suggests that the agonist activity of antiandrogens and other physiologically relevant ligands occurs because they, in the background of these mutations, allow AR-AF2 to adopt an active conformation. Initially, this result appeared to contradict the findings of others that suggest that coactivator binding to AR-AF2 is not required for AR activity. In probing this paradox further, however, we determined that the role of AR-AF2 appears to be to stabilize the overall structure of the receptor, allowing the amino terminus to interact with appropriate coactivators. This conclusion is supported by our finding that overexpression of the AF2-binding peptides blocks the interaction between the amino and carboxyl termini of AR but does not attenuate AR transcriptional activity. This can be explained by the fact that overexpression of the LxxLL-containing peptide or the amino terminus of AR appears to have a similar effect on the AR-ligand binding domain, as both have the ability to stabilize agonist binding by decreasing ligand off-rate. Thus, we believe that resistance in certain prostate cancers occurs as a consequence of receptor mutations that enable antagonist-and/or nonclassical ligand-bound AR to present a wild-type-like AF-2 conformation.