Ligand- and coactivator-mediated transactivation function (AF2) of the androgen receptor ligand-binding domain is inhibited by the cognate hinge region

Transactivation functions (AF2) in the ligand-binding domains (LBD) of many steroid receptors are well characterized, but there is little evidence to support such a function for the LED of the androgen receptor (AR). We repor t a mutant AR, with residues 628-646 in the hinge region deleted, which exh ibited transactivation activity that was more than double that of the wild type (WT) AR. Although no androgen-dependent AF2 activity could be observed for the WT ARLSD fused to a heterologous DNA-binding domain, the mutant AR LBD(Delta 628-646) was 30-40 times more active than the WT ARLBD. In the pr esence of the p160 coactivator TIF2, AR(Delta 628-646) was significantly mo re active than similarly treated WT AR. Deletion of residues 628-646 also e nhanced TIF2-ARLBD activity 8-fold, an effect not present when the LED-inte racting LXXLL motifs of TIF2 were mutated, suggesting that the negative mod ulatory activity of residues 628-646 were exerted via coactivator pathways. Although the AP-1 (c-Jun/c-Fos) system and NcoR have been reported to inte ract with and repress the activity of some steroid receptors, c-jun, c-Fos, c-Jun/c-Fos, nor NcoR function was consistently affected by the absence or presence of residues 628-646, implying that the AR hinge region exerts its silencing effects in a manner independent of these corepressors, Our data provide evidence for the novel finding that strong androgen-dependent AF2 e xists in the ARLBD and is the first report of a negative regulatory domain in the AR. Because mutations in this region are commonly associated with pr ostate cancer, it is important to characterize the mechanisms by which the hinge region exerts its repressor effect on ligand-activated and coactivato r-mediated AF2 activity of the ARLBD.