Homodimerization of the androgen receptor (AR) has been confirmed in a study published in Nature Communications.

AR consists of an N-terminal domain, DNA-binding domain (DBD), a hinge region, and a C-terminal ligand-binding domain (LBD), which contains an internal ligand-binding pocket (LBP). The structures of AR-LBD monomers complexed with agonists and antagonists have previously been elucidated, but no structural information of the full multidomain receptor has been shown and dimerization remains controversial. However, consolidation of AR dimerization mechanisms could be key in developing new therapeutics to target the receptor.

The work, led by Eva Estébanez and Pablo Fuentes, both from Barcelona, studied the crystal structure of the AR-LBD homodimer. In contrast to previously reported AR-LBD structures, which contain a monomer in the asymmetric unit (ASU), they observed that their AR-LBD structure presented four independent helically arranged LBDs in the ASU. The contacts between these LBDs had a stabilizing effect, as evidenced by the lower temperature factors of this complex than the monomers previously described. “Until now, all published structures were monomeric AR-LBD and there was little evidence for its dimerization,” author Frank Claessens told Nature Reviews Urology. “This structure now definitively closes the controversy on whether or not the AR-LBD dimerizes.”

Credit: Image courtesy of Eva Estébanez

The team went on to confirm AR-LBD dimerization in solution, by assessing the capacity of DHT-bound AR-LBD for noncovalent 1:1 self-association. Kinetic analysis demonstrated rapid association and dissociation and crosslinking experiments using glutaraldehyde revealed dimer formation. Mass spectrometry of monomeric and dimeric AR-LBD showed combinations of BMOE-crosslinks in the dimeric form only.

Fluorescence resonance energy transfer (FRET) microscopy was used to study dimerization in living cells. When hormone was not present, no FRET signal was noted, but addition of agonists such as DHT, testosterone, and R1881, strongly induced a signal. However, the addition of antagonists such as enzalutamide, bicalutamide, or hydroxyflutamide did not induce a response. Based on these data, the authors concluded that hormone binding to the LBP results in dimerization, and that current antiandrogens could act by blocking this effect.

...some mutations found in androgen insensitivity syndrome or prostate cancer reduce homodimer stability and activity...

The in vivo importance of the dimer interface was indicated by their discovery that some mutations found in androgen insensitivity syndrome (AIS) or prostate cancer reduce homodimer stability and activity, even though ligand binding was still possible. “Mutations that have been correlated with AIS, for which we had no explanation, now fall within the dimer interface,” Claessens explained. “In cellular assays, these mutations are indeed interfering with the dimerization and they are also interfering with the activity of the AR as a transcription factor, while conserving ligand binding.”

These data shed new light on AR biology, and could offer new targets for treating prostate cancer and other AR-related disorders. Instead of targeting the LBD, other areas of the AR structure could offer new avenues of research for therapeutic targeting.