In vivo evidence against the existence of antiprogestins disrupting receptor binding to DNA.

The binding of a steroid hormone to its receptor elicits a sequence of events: activation of the receptor (probably through dissociation from a complex of heat shock proteins), dimerization, binding to hormone responsive elements, and finally modulation of gene transcription. RU 486, the first antiprogestin studied, has been shown to act at the last step of this sequence: provoking an inefficient binding of the receptor to hormone responsive elements. Recently, based on in vitro studies, it has been proposed that ZK 98299 was the prototype of a second class of antiprogestins that were supposed to act through disruption of the binding to DNA. We have devised methods allowing us to study the various steps of agonist or antagonist action in vivo. We show here that RU 486 and ZK 98299 have the same effects on receptor activation, dimerization, and binding to hormone responsive elements; differences in their action are explained by the 10-fold difference in their affinity for the receptor (ZK 98299 having the lower affinity).