Antiglucocorticosteroid effects suggest why steroid hormone is required for receptors to bind DNA in vivo but not in vitro

Abstract

Sequence-specific interaction between steroid hormone receptors (R) and DNA hormone-responsive elements (HRE) takes place in vitro irrespective of the presence of hormone and even when R is liganded with an antagonist^1,2. In vivo, in contrast, the presence of hormone is mandatory for glucocorticosteroid (G) receptor-HRE interaction to occur and no HRE occupancy is detected in the presence of an antagonist³. One possible explanation is that in vivo R is originally complexed with a protein that prevents its binding to target HREs. The hormone would then induce the dissociation of the oligomer, thus unmasking the functional DNA binding domain of the receptor. The unliganded, non DNA-binding 8S-f orm of the chick GR is a hetero-oligomer including the relative molecular mass (M_r) 94,000 steroid-binding unit (4S-GR), and the non-steroid-binding, non-DNA-binding 90,000 protein⁴ common to all classes of 8S-R⁵ and identified as heat-shock protein (hsp90)^6–8. We report here that triamcinolone acetonide (TA) promotes the transformation of 8S-GR to 4S-GR complexes both in explants and in cell-free conditions and that the high-affinity antiglucocorticosteroid RU 486⁹ stabilizes the 8S-GR, as assessed by gradient sedimentation and HPLC. However, in vitro TA- and RU 486- 4S-GR showed comparable DNA-binding activity. These results suggest that the lack of affinity for DNA of the 8S form of GR may be attributable in vivo to the interaction of the 4S-GR protein with hsp 90, and that hormone binding might trigger a conformational change which results in the release of active 4S-GR.