Differences between aldosterone and its antagonists in binding kinetics and ligand-induced hsp90 release from mineralocorticosteroid receptor.

We have previously reported that mineralocorticosteroid receptor (MR) is a 8-9 S heterooligomeric complex that includes the 90 kDa heat shock protein (hsp90). To elucidate how antagonist-receptor complexes are biologically inactive in terms of transcriptional regulation, we analyzed the binding of mineralocorticosteroid agonists and antagonists with MR and the ligand-induced transformation of its heterooligomeric structure. This study was performed in the cytosol of adrenalectomized rat kidney and of COS cells transiently transfected with human MR cDNA. Although aldosterone antagonists (SC9420 and RU26752) bind MR with the same affinity as aldosterone, they dissociate much more rapidly from the 8-9 S form of both rat and human MR than does aldosterone. Using sedimentation gradient analysis, we showed that the interaction between hsp90 and the steroid binding subunit of MR is highly dependent upon the nature of the steroid ligand since the binding of aldosterone antagonists results in an easy release of hsp90. We propose that both rapid dissociation of ligand and weakened hsp90-receptor interaction play a key role in the mechanism of mineralocorticosteroid antagonism. In the COS cell model, cortisol, described as a weak mineralocorticosteroid agonist, dissociates also more rapidly from human MR than does aldosterone. Our results suggest that ligand binding kinetics and ligand dependent modification in receptor structure are important modulators of MR function as a transcriptional regulatory factor.