Received for publication September 22, 2006.
Revised October 26, 2006.
Accepted for publication November 14, 2006.

The Synthetic Androgen Methyltrienolone (R1881) Acts as a Potent Antagonist of the Mineralocorticoid Receptor

Abstract

Aldosterone binds to the mineralocorticoid receptor (MR), and exerts fine control over Na⁺ absorption in renal collecting duct cells (CCDs). Many natural and synthetic steroids can also bind to the MR to produce agonist or antagonist effects. Here we investigate whether androgenic hormones act as MR agonist or antagonist ligands in CCDs. Testosterone (T), dihydrotestosterone (DHT) and methyltrienolone (R1881), a synthetic androgen agonist, all bind to the MR. R1881 dysplayed the same affinity for MR as aldosterone. Androgens did not activate the MR transiently expressed in human embryonic kidney HEK 293T cells, but did antagonize aldosterone-induced MR trans-activation activity (R1881>DHT>T). Short-circuit current (I_sc) experiments, used to measure transepithelial Na⁺ transport, revealed that 10⁵ M T and DHT or R1881 prevented the rise in the amiloride-sensitive component of I_sc caused by aldosterone in mouse mpkCCDcl4 collecting duct cells partially and totally, respectively. In contrast, androgens had no effect on stimulated-I_sc elicited by the specific glucocorticoid agonist RU26988. Docking of steroids within the crystal structure of the ligand-binding domain of MR, together with trans-activation studies, revealed that the contacts between the 17-hydroxyl group of androgens and the Asn770, Cys942 and Thr945 residues of the ligand-binding cavity stabilize ligand-binding complexes, but are not strong enough to keep the receptor in its active state. Altogether, these findings indicate that androgen ligands, particularly R1881, act as MR antagonists in aldosterone target cells, and provide new insights into the requirements for MR activation to occur, as well as for the designing of new selective MR antagonists.

Key words: Glucorticoids/Mineralocorticoids, Sex hormones, Structure-activity relationships and modeling, Mutagenesis/Chimeric approaches, Receptor binding studies, Structure/function/mechanism

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