The Intracellular Localization of the Mineralocorticoid Receptor is Regulated by 11β-Hydroxysteroid Dehydrogenase Type 2

Abstract

11β-Hydroxysteroid dehydrogenase (11β-HSD) type 2 has been considered to protect the mineralocorticoid receptor (MR) by converting 11β-hydroxyglucocorticoids into their inactive 11-keto forms, thereby providing specificity to the MR for aldosterone. To investigate the functional protection of the MR by 11β-HSD2, we coexpressed epitope-tagged MR and 11β-HSD2 in HEK-293 cells lacking 11β-HSD2 activity and analyzed their subcellular localization by fluorescence microscopy. When expressed alone in the absence of hormones, the MR was both cytoplasmic and nuclear. However, when coexpressed with 11β-HSD2, the MR displayed a reticular distribution pattern, suggesting association with 11β-HSD2 at the endoplasmic reticulum membrane. The endoplasmic reticulum membrane localization of the MR was observed upon coexpression only with 11β-HSD2, but not with 11β-HSD1 or other steroid-metabolizing enzymes. Aldosterone induced rapid nuclear translocation of the MR, whereas moderate cortisol concentrations (10-200 nM) did not activate the receptor, due to 11β-HSD2-dependent oxidation to cortisone. Compromised 11β-HSD2 activity (due to genetic mutations, the presence of inhibitors, or saturating cortisol concentrations) led to cortisol-induced nuclear accumulation of the MR. Surprisingly, the 11β-HSD2 product cortisone blocked the aldosterone-induced MR activation by a strictly 11β-HSD2-dependent mechanism. Our results provide evidence that 11β-HSD2, besides inactivating 11β-hydroxyglucocorticoids, functionally interacts with the MR and directly regulates the magnitude of aldosterone-induced MR activation.