The role of 11beta-hydroxysteroid dehydrogenase in steroid hormone specificity.
11Beta-hydroxysteroid dehydrogenase (11beta-HSD) is thought to confer aldosterone specificity to mineralocorticoid target cells by protecting the mineralocorticoid receptor (MR) from occupancy by endogenous glucocorticoids. In aldosterone target cells the type 2 11beta-HSD is present, which, in contrast to the type 1 11beta-HSD, has very high affinity for its substrate, is unidirectional and prefers NAD as cofactor. cDNAs encoding 11beta-HSD2 have been recently cloned from different species, and the cell-specific expression of its mRNA and protein were determined. 11Beta-HSD2 is expressed in every aldosterone target tissue. Northern analysis revealed that the rabbit 11beta-HSD2 is expressed at high levels in the renal collecting duct and at much lower levels in the colon. RT-PCR experiments demonstrated that 11beta-HSD2 mRNA is present only in aldosterone target cells within the kidney. We determined the subcellular localization of the rabbit 11beta-HSD2 using a chimera encoding 11beta-HSD2 and the green fluorescent protein (GFP). This construct was stably transfected into CHO and MDCK cells. The expressed 11beta-HSD2/GFP protein retained high enzymatic activity, and its characteristics were undistinguishable from those of the native enzyme. The intracellular localization of this protein was determined by fluorescence microscopy. 11Beta-HSD2-associated fluorescence was observed as a reticular network over the cytoplasm whereas the plasma membrane and the nucleus were negative, suggesting endoplasmic reticulum (ER) localization. Co-staining with markers for ER proteins, the Golgi membrane, mitochondria and nucleus confirmed that 11beta-HSD2 is localized exclusively to the ER. To determine what structural motifs are responsible for the ER localization, we generated deletion mutants missing the C-terminal 42 and 118 amino acids, and fused them to GFP. Similarly as with the intact 11beta-HSD2, these mutants localized exclusively to the ER. Both C-terminal deletion mutants completely lost dehydrogenase activity, independently whether activity was determined in intact cells or homogenates. These results indicate that 11beta-HSD2 has a novel ER retrieval signal which is not localized to the C-terminal region. In addition, the C-terminal 118 amino acids are essential for NAD-dependent 11beta-HSD activity.
Náray-Fejes-Tóth
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Department of Physiology, Dartmouth Medical School, Lebanon, NH 03756, USA.
Colombowala
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Fejes-Tóth
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D042842:11-beta-Hydroxysteroid Dehydrogenases; D000450:Aldosterone / Q000494:pharmacology*; D000818:Animals; D002451:Cell Compartmentation; D003001:Cloning, Molecular; D004721:Endoplasmic Reticulum / Q000201:enzymology; D006913:Hydroxysteroid Dehydrogenases / Q000235:genetics / Q000302:isolation & purification*; D011817:Rabbits; D018161:Receptors, Mineralocorticoid / Q000378:metabolism*; D014018:Tissue Distribution
D018161:Receptors, Mineralocorticoid; D000450:Aldosterone; D006913:Hydroxysteroid Dehydrogenases; D042842:11-beta-Hydroxysteroid Dehydrogenases