The intracellular localization of the mineralocorticoid receptor is regulated by 11 ss-hydroxysteroid dehydrogenase type 2

11 beta -Hydroxysteroid dehydrogenase (11 beta -HSD) type 2 has been consid ered to protect the mineralocorticoid receptor (MR) by converting 11 beta - hydroxyglucocorticoids into their inactive 11-keto forms, thereby providing specificity to the TMR for aldosterone. To investigate the functional prot ection of the MR by 11 beta -HSD2, we coexpressed epitope-tagged MR and 11 beta -HSD2 in HEK-293 cells lacking 11 beta -HSD2 activity and analyzed the ir subcellular localization by fluorescence microscopy. When expressed alon e in the absence of hormones, the MR was both cytoplasmic and nuclear. Howe ver, when coexpressed with 11 beta -HSD2, the MR displayed a reticular dist ribution pattern, suggesting association with 11 beta -HSD2 at the endoplas mic reticulum membrane. The endoplasmic reticulim membrane localization of the AM was observed upon coexpression only with 11 beta -HSD2, but not with 11 beta -HSD1 or other steroid-metabolizing enzymes. Aldosterone induced r apid nuclear translocation of the AM whereas moderate cortisol concentratio ns (10-200 nM) did not activate the, receptor, due to 11 beta -HSD2-depende nt oxidation to cortisone. Compromised 11 beta -HSD2 activity (due to genet ic mutations, the presence of inhibitors, or saturating cortisol concentrat ions) led to cortisol-induced nuclear accumulation of the MR. Surprisingly, the 11 beta -HSD2 product cortisone blocked the aldosterone-induced MR act ivation by a strictly 11 beta -HSD2-dependent mechanism. Our results provid e evidence that 11 beta -HSD2, besides inactivating 11 beta -hydroxyglucoco rticoids, functionally interacts with the MR and directly regulates the mag nitude of aldosterone-induced MR activation.