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Mineralocorticoid selectivity: Molecular and cellular aspects

2000; Elsevier BV; Volume: 57; Issue: 4 Linguagem: Inglês

10.1046/j.1523-1755.2000.00976.x

1523-1755

Nicolette Farman, Brigitte Bocchi,

Coenzyme Q10 studies and effects

Mineralocorticoid selectivity: Molecular and cellular aspects. Aldosterone acts in mineralocorticoid-sensitive cells by binding to the mineralocorticoid receptor (MR). Because the MR displays similar affinity for aldosterone and glucocorticoid hormones and because these latter hormones are 100- to 1000-fold more abundant than aldosterone in the plasma, mechanisms are required to avoid permanent illicit occupancy of MR by glucocorticoid hormones. The main mechanism of mineralocorticoid selectivity is enzymatic: the 11β hydroxysteroid dehydrogenase (HSD2) metabolizes glucocorticoid hormones into derivatives with a low affinity for MR. The cell biology and regulation of HSD2 are reviewed in this article, as well as its implications in human hypertension. Other factors play a role in mineralocorticoid selectivity: the MR itself, the possibility to form homodimers (MR-MR), or heterodimers (with the glucocorticoid receptor). All of these cellular events participate to successive dynamic equilibriums, which allow fine tuning of transcriptional regulation of target genes, depending on the target tissue and the hormonal status. Mineralocorticoid selectivity: Molecular and cellular aspects. Aldosterone acts in mineralocorticoid-sensitive cells by binding to the mineralocorticoid receptor (MR). Because the MR displays similar affinity for aldosterone and glucocorticoid hormones and because these latter hormones are 100- to 1000-fold more abundant than aldosterone in the plasma, mechanisms are required to avoid permanent illicit occupancy of MR by glucocorticoid hormones. The main mechanism of mineralocorticoid selectivity is enzymatic: the 11β hydroxysteroid dehydrogenase (HSD2) metabolizes glucocorticoid hormones into derivatives with a low affinity for MR. The cell biology and regulation of HSD2 are reviewed in this article, as well as its implications in human hypertension. Other factors play a role in mineralocorticoid selectivity: the MR itself, the possibility to form homodimers (MR-MR), or heterodimers (with the glucocorticoid receptor). All of these cellular events participate to successive dynamic equilibriums, which allow fine tuning of transcriptional regulation of target genes, depending on the target tissue and the hormonal status. It has long been known that corticosteroid hormones are important for the maintenance of renal sodium reabsorption, volemia, and blood pressure levels1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar,2Verrey F. Transcriptional control of sodium transport in tight epithelia by adrenal steroids.J Membr Biol. 1995; 144: 93-110Crossref PubMed Scopus (152) Google Scholar. The mineralocorticoid hormone aldosterone promotes sodium absorption in the renal distal nephron, the colon, and the ducts of sweat and salivary glands. Glucocorticoid hormones (cortisol in humans and corticosterone in rodents) also play an important role in the control of ion reabsorption and the glomerular filtration rate. These two hormones appear to act in a complex, distinct but complementary pattern, which is not fully understood at the present time. Both bind intracellular receptors1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar, 2Verrey F. Transcriptional control of sodium transport in tight epithelia by adrenal steroids.J Membr Biol. 1995; 144: 93-110Crossref PubMed Scopus (152) Google Scholar, 3Funder J.W. Glucocorticoid and mineralocorticoid receptors: Biology and clinical relevance.Annu Rev Med. 1997; 48: 231-240Crossref PubMed Scopus (245) Google Scholar, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR; Figure 1). These receptors are members of the superfamily of steroid hormone receptors that act as ligand-dependent transcription factors4Mangelsdorf D.J. Thummel C. Beato M. Herrlich P. Schutz G. Umesono K. Blumberg B. Kastner P. Mark M. Chambon P. Evans R.M. Overview: The nuclear receptor superfamily: The second decade.Cell. 1995; 83: 835-839Abstract Full Text PDF PubMed Scopus (5844) Google Scholar. They interact (directly or indirectly) with glucocorticoid response elements (GREs) in the promoter regions of regulated genes. GR is found in all aldosterone target cells and largely prevails over MR5Farman N. Vandewalle A. Bonvalet J.P. Aldosterone binding in isolated tubules. I. Biochemical determination in proximal and distal parts of the rabbit nephron.Am J Physiol. 1982; 242: F63-F68PubMed Google Scholar,6Farman N. Oblin M.E. Lombes M. Delahaye F. Westphal H.M. Bonvalet J.P. Gasc J.M. Immunolocalization of gluco and mineralocorticoid receptors in the rabbit kidney.Am J Physiol. 1991; 260: C226-C233PubMed Google Scholar: There are about 100,000 GRs per cell in the renal collecting duct and 10,000 MRs Figure 1. Because the MR displays the same affinity for aldosterone and glucocorticoid hormones (about 0.5 to 1 nmol/L) and because these latter hormones are much more abundant (100- to 1000-fold) in the plasma, as compared with aldosterone Figure 1, it is obvious that some mechanisms are necessary to protect the MR against permanent occupancy by glucocorticoid hormones7Edwards C.R.W. Stewart P.M. Burt D. Bret L. McIntyre M.A. Sutanto W.S. de Kloet E.R. Monder C. Localisation of 11β-hydroxysteroid dehydrogenase-tissue specific protector of the mineralocorticoid receptor.Lancet. 1988; 2: 986-989Abstract PubMed Scopus (999) Google Scholar,8Funder J.W. Pearce P.T. Smith R. Smith A.I. Mineralocorticoid action: Target tissue specificity is enzyme, not receptor, mediated.Science. 1988; 242: 583-585Crossref PubMed Scopus (1447) Google Scholar. The main mechanism of mineralocorticoid selectivity depends on an enzyme7Edwards C.R.W. Stewart P.M. Burt D. Bret L. McIntyre M.A. Sutanto W.S. de Kloet E.R. Monder C. Localisation of 11β-hydroxysteroid dehydrogenase-tissue specific protector of the mineralocorticoid receptor.Lancet. 1988; 2: 986-989Abstract PubMed Scopus (999) Google Scholar,8Funder J.W. Pearce P.T. Smith R. Smith A.I. Mineralocorticoid action: Target tissue specificity is enzyme, not receptor, mediated.Science. 1988; 242: 583-585Crossref PubMed Scopus (1447) Google Scholar, 11β hydroxysteroid dehydrogenase (HSD), which metabolizes cortisol (or corticosterone) into derivatives (cortisone or dehydrocorticosterone) with little or no affinity for MR (and GR as well). Thus, in the presence of HSD2, glucocorticoid hormones entering the cell are metabolized into 11-dehydro derivatives, and the MR will be occupied by aldosterone as a function of its plasma levels. In the absence of HSD2, the MR will be occupied by glucocorticoid hormones, and a permanent maximal sodium reabsorption will occur, without any possible regulation by aldosterone. Such a situation is achieved when the enzyme is inhibited (hypertension caused by licorice ingestion) or genetically inactive [hypertension from the syndrome of apparent mineralocorticoid excess (AME)]. Several recent reviews have been published on the subject9Farman N. Molecular and cellular determinants of mineralocorticoid selectivity.Curr Opin Nephrol Hypertens. 1999; 8: 45-51Crossref PubMed Scopus (27) Google Scholar, 10Funder J.W. Apparent mineralocorticoid excess, 11 beta hydroxysteroid dehydrogenase and aldosterone action: Closing one loop, opening another.Trends Endocrinol Metab. 1995; 6: 248-251Abstract Full Text PDF PubMed Scopus (9) Google Scholar, 11Oppermann U.C.T. Persson B. Jornvall H. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Function, gene organization and protein structures of 11-beta-hydroxysteroid dehydrogenase isoforms.Eur J Biochem. 1997; 249: 355-360Crossref PubMed Scopus (69) Google Scholar, 12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar, 13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar. 11β-hydroxysteroid dehydrogenase is coexpressed with MR and GR in renal aldosterone target cells1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar, as illustrated in Figure 2. The MR is expressed selectively in distal tubules and all along the collecting duct of the rabbit5Farman N. Vandewalle A. Bonvalet J.P. Aldosterone binding in isolated tubules. I. Biochemical determination in proximal and distal parts of the rabbit nephron.Am J Physiol. 1982; 242: F63-F68PubMed Google Scholar,6Farman N. Oblin M.E. Lombes M. Delahaye F. Westphal H.M. Bonvalet J.P. Gasc J.M. Immunolocalization of gluco and mineralocorticoid receptors in the rabbit kidney.Am J Physiol. 1991; 260: C226-C233PubMed Google Scholar and presumably in the mouse and in humans. In the rat, however, MR expression extends to the cortical part of the thick ascending limb of Henle's loop (TAL)14Farman N. Bonvalet J.P. Aldosterone binding in isolated tubules. III. Autoradiography along the rat nephron.Am J Physiol. 1983; 245: F606-F614PubMed Google Scholar. HSD2 activity is high in distal tubules and collecting ducts of the rabbit, the mouse, and humans15Bonvalet J.P. Doignon I. Blot-Chabaud M. Pradelles P. Farman N. Distribution of 11beta-hydroxysteroid dehydrogenase along the rabbit nephron.J Clin Invest. 1990; 86: 832-837Crossref PubMed Scopus (83) Google Scholar, 16Hirasawa G. Sasano H. Takahashi K.I. Fukushima K. Suzuki T. Hiwatashi N. Toyota T. Krozowski Z.S. Nagura H. Colocalization of 11-beta-hydroxysteroid dehydrogenase type II and mineralocorticoid receptor in human epithelia.J Clin Endocrinol Metab. 1997; 82: 3859-3863Crossref PubMed Google Scholar, 17Kenouch S. Coutry N. Farman N. Bonvalet J.P. Multiple patterns of 11β-hydroxysteroid dehydrogenase catalytic activity along the mammalian nephron.Kidney Int. 1992; 42: 56-60Abstract Full Text PDF PubMed Scopus (35) Google Scholar, 18Kenouch S. Alfaidy N. Bonvalet J.P. Farman N. Expression of 11-HSD along the nephron of mammals and humans.Steroids. 1994; 59: 100-104Crossref PubMed Scopus (36) Google Scholar, 19Naray-Fejes-Toth A. Watlington C.O. Fejes-Toth G. 11beta-hydroxysteroid dehydrogenase activity in the renal target cells of aldosterone.Endocrinology. 1991; 129: 17-21Crossref PubMed Scopus (125) Google Scholar, 20Naray-Fejes-Toth A. Fejes-Toth G. 11-Beta-hydroxysteroid dehydrogenase-2 is a high affinity corticosterone-binding protein.Mol Cell Endocrinol. 1997; 134: 157-161Crossref Scopus (11) Google Scholar, 21Rusvai E. Naray-Fejes-Toth A. A new isoform of 11-beta-hydroxysteroid dehydrogenase in aldosterone target cells.J Biol Chem. 1993; 268: 10717-10720Abstract Full Text PDF PubMed Google Scholar. As was observed for MR, high HSD2 activity extends to the TAL in rats17Kenouch S. Coutry N. Farman N. Bonvalet J.P. Multiple patterns of 11β-hydroxysteroid dehydrogenase catalytic activity along the mammalian nephron.Kidney Int. 1992; 42: 56-60Abstract Full Text PDF PubMed Scopus (35) Google Scholar,18Kenouch S. Alfaidy N. Bonvalet J.P. Farman N. Expression of 11-HSD along the nephron of mammals and humans.Steroids. 1994; 59: 100-104Crossref PubMed Scopus (36) Google Scholar. Besides renal coexpression of MR and HSD2, other (putative) aldosterone target tissues have been examined. Coexpression has been documented, as expected, in sweat gland ducts22Kenouch S. Lombes M. Delahaye F. Eugene E. Bonvalet J.P. Farman N. Human skin as target for aldosterone: Coexpression of mineralocorticoid receptors and 11 beta-hydroxysteroid dehydrogenase.J Clin Endocrinol Metab. 1994; 79: 1334-1341PubMed Google Scholar and in the epithelium of distal colon23Shimojo M. Ricketts M.L. Petrelli M.D. Moradi P. Johnson G.D. Bradwell A.R. Hewison M. Howie A.J. Stewart P.M. Immunodetection of 11 beta-hydroxysteroid dehydrogenase type 2 in human mineralocorticoid target tissues: Evidence for nuclear localization.Endocrinology. 1997; 138: 1305-1311Crossref PubMed Scopus (80) Google Scholar. In nonepithelial cells, however, the situation is more complex. Evidence for MR expression (and specific aldosterone binding) has been provided in cardiac myocytes24Lombes M. Oblin M.E. Gasc J.M. Baulieu E.E. Farman N. Bonvalet J.P. Immunohistochemical and biochemical evidence for a cardiovascular mineralocorticoid receptor.Circ Res. 1992; 71: 503-510Crossref PubMed Scopus (267) Google Scholar,25Lombes M. Alfaidy N. Eugene E. Lessana A. Farman N. Bonvalet J.P. Prerequisite for cardiac aldosterone action: Mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase in the human heart.Circulation. 1995; 92: 175-182Crossref PubMed Scopus (282) Google Scholar in the rabbit and humans, and HSD2 activity has been evidenced in cardiac biopsies25Lombes M. Alfaidy N. Eugene E. Lessana A. Farman N. Bonvalet J.P. Prerequisite for cardiac aldosterone action: Mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase in the human heart.Circulation. 1995; 92: 175-182Crossref PubMed Scopus (282) Google Scholar, at levels lower than those observed in classic aldosterone cells such as the collecting duct Table 126Alfaidy N. Blot-Chabaud M. Robic D. Kenouch S. Bourbouze R. Bonvalet J.P. Farman N. Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron.Biochim Biophys Acta. 1995; 1243: 461-468Crossref Scopus (26) Google Scholar. A comparison of the amounts of HSD2 available to protect MR in these two cell types shows that the collecting duct cell MR is much better protected by HSD2 that the cardiomyocyte MR (50 times less efficient protection; Table 1). One can wonder whether the cardiac MR-HSD2 system is sufficiently tight to avoid binding of glucocorticoid hormones or whether a significant proportion of MR may be occupied by these latter hormones in vivo. In the brain (particularly in the hippocampus), MR is expressed at high levels27Arriza J.L. Simerly R.B. Swanson L.W. Evans R.M. The neuronal mineralocorticoid receptor as a mediator of glucocorticoid response.Neuron. 1988; 1: 887-900Abstract Full Text PDF PubMed Scopus (495) Google Scholar, 28Herman J.P. Patel P.D. Akil H. Watson S.J. Localization and regulation of glucocorticoid and mineralocorticoid receptor messenger RNAs in the hippocampal formation of the rat.Mol Endocrinol. 1989; 3: 1886-1894Crossref PubMed Scopus (292) Google Scholar, 29Joels M. de Kloet E.R. Corticosteroid hormones: Endocrine messengers in the brain.N Physiol Sci. 1995; 10: 71-76Google Scholar, 30Reul J.M.H.M. Pearce P.T. Funder J.W. Krozowski Z.S. Type I and type II corticosteroid receptor gene expression in the rat: Effect of adrenalectomy and dexamethasone administration.Mol Endocrinol. 1989; 3: 1674-1680Crossref PubMed Scopus (165) Google Scholar, and there is a general agreement to consider that HSD2 is very low in these tissues. This would mean that MR in brain is permanently occupied by glucocorticoid hormones. However, it remains to be understood why aldosterone and corticosterone exert distinct effects, at least in some regions of the brain (article by de Kloet et al, in this issue of Kidney International, p. 1329).Table 1Comparison of mineralocorticoid receptor (MR) and 11β-dehydroxysteroid dehydrogenase-2 (HSD2) abundanceMR receptor per cellHSD2 activity fmol/mg protein/10 minHSD2 MR ratioCollecting duct cells10,00050,0005Cardiac myocytes500–1000700.1The HSD2:MR ratio shows that cardiac myocytes MR are 50 times less protected (by HSD2) than collecting duct cells. Data are from5Farman N. Vandewalle A. Bonvalet J.P. Aldosterone binding in isolated tubules. I. Biochemical determination in proximal and distal parts of the rabbit nephron.Am J Physiol. 1982; 242: F63-F68PubMed Google Scholar, 18Kenouch S. Alfaidy N. Bonvalet J.P. Farman N. Expression of 11-HSD along the nephron of mammals and humans.Steroids. 1994; 59: 100-104Crossref PubMed Scopus (36) Google Scholar, 24Lombes M. Oblin M.E. Gasc J.M. Baulieu E.E. Farman N. Bonvalet J.P. Immunohistochemical and biochemical evidence for a cardiovascular mineralocorticoid receptor.Circ Res. 1992; 71: 503-510Crossref PubMed Scopus (267) Google Scholar, 25Lombes M. Alfaidy N. Eugene E. Lessana A. Farman N. Bonvalet J.P. Prerequisite for cardiac aldosterone action: Mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase in the human heart.Circulation. 1995; 92: 175-182Crossref PubMed Scopus (282) Google Scholar, 26Alfaidy N. Blot-Chabaud M. Robic D. Kenouch S. Bourbouze R. Bonvalet J.P. Farman N. Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron.Biochim Biophys Acta. 1995; 1243: 461-468Crossref Scopus (26) Google Scholar. Open table in a new tab The HSD2:MR ratio shows that cardiac myocytes MR are 50 times less protected (by HSD2) than collecting duct cells. Data are from5Farman N. Vandewalle A. Bonvalet J.P. Aldosterone binding in isolated tubules. I. Biochemical determination in proximal and distal parts of the rabbit nephron.Am J Physiol. 1982; 242: F63-F68PubMed Google Scholar, 18Kenouch S. Alfaidy N. Bonvalet J.P. Farman N. Expression of 11-HSD along the nephron of mammals and humans.Steroids. 1994; 59: 100-104Crossref PubMed Scopus (36) Google Scholar, 24Lombes M. Oblin M.E. Gasc J.M. Baulieu E.E. Farman N. Bonvalet J.P. Immunohistochemical and biochemical evidence for a cardiovascular mineralocorticoid receptor.Circ Res. 1992; 71: 503-510Crossref PubMed Scopus (267) Google Scholar, 25Lombes M. Alfaidy N. Eugene E. Lessana A. Farman N. Bonvalet J.P. Prerequisite for cardiac aldosterone action: Mineralocorticoid receptor and 11β-hydroxysteroid dehydrogenase in the human heart.Circulation. 1995; 92: 175-182Crossref PubMed Scopus (282) Google Scholar, 26Alfaidy N. Blot-Chabaud M. Robic D. Kenouch S. Bourbouze R. Bonvalet J.P. Farman N. Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron.Biochim Biophys Acta. 1995; 1243: 461-468Crossref Scopus (26) Google Scholar. 11β-hydroxysteroid dehydrogenase (HSD) activity has been studied for a long time and was characterized first from liver microsomal preparations31Seckl J.R. Review: 11-Beta-hydroxysteroid dehydrogenase isoforms and their implications for blood pressure regulation.Eur J Clin Invest. 1993; 23: 589-601Crossref PubMed Scopus (104) Google Scholar. Initial cloning revealed a form of the enzyme (named HSD1) that was absent from mineralocorticoid-sensitive tissues32Agarwal A.K. Monder C. Eckstein B. White P.C. Cloning and expression of rat cDNA encoding corticosteroid 11β-dehydrogenase.J Biol Chem. 1989; 264: 18939-18943Abstract Full Text PDF PubMed Google Scholar. The MR-protecting enzyme (named HSD2) was subsequently cloned33Agarwal A.K. Mune T. Monder C. White P.C. NAD(+)-dependent isoform of 11 beta-hydroxysteroid dehydrogenase: Cloning and characterization of cDNA from sheep kidney.J Biol Chem. 1994; 269: 25959-25962Abstract Full Text PDF PubMed Google Scholar,34Albiston A.L. Obeyesekere V.R. Smith R.E. Krozowski Z.S. Cloning and tissue distribution of the human 11 beta-hydroxysteroid dehydrogenase type 2 enzyme.Mol Cell Endocrinol. 1994; 105: R11-R17Crossref PubMed Scopus (718) Google Scholar, and the differences between these two forms are listed in Table 2. Several excellent reviews have been published on this subject11Oppermann U.C.T. Persson B. Jornvall H. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Function, gene organization and protein structures of 11-beta-hydroxysteroid dehydrogenase isoforms.Eur J Biochem. 1997; 249: 355-360Crossref PubMed Scopus (69) Google Scholar, 12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar, 13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar, 31Seckl J.R. Review: 11-Beta-hydroxysteroid dehydrogenase isoforms and their implications for blood pressure regulation.Eur J Clin Invest. 1993; 23: 589-601Crossref PubMed Scopus (104) Google Scholar, 35Penning T.M. Molecular endocrinology of hydroxysteroid dehydrogenases.Endocr Rev. 1997; 18: 281-305Crossref PubMed Scopus (373) Google Scholar. Here, we would like to emphasize some aspects of the cell biology of HSD2. First, the enzyme HSD2 is consistently found as a unidirectional enzyme (dehydrogenase activity only); this unexpected behavior is unexplained. Another peculiarity of the HSD2 is the wide range of substrate concentrations (0.1 nmol/L to 1 μmol/L) where the enzyme is active26Alfaidy N. Blot-Chabaud M. Robic D. Kenouch S. Bourbouze R. Bonvalet J.P. Farman N. Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron.Biochim Biophys Acta. 1995; 1243: 461-468Crossref Scopus (26) Google Scholar, which is not compatible with simple michaelian enzyme kinetics. This aspect has been misregarded by most investigators, who considered it as a michaelian enzyme12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar, 13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar, 19Naray-Fejes-Toth A. Watlington C.O. Fejes-Toth G. 11beta-hydroxysteroid dehydrogenase activity in the renal target cells of aldosterone.Endocrinology. 1991; 129: 17-21Crossref PubMed Scopus (125) Google Scholar, 21Rusvai E. Naray-Fejes-Toth A. A new isoform of 11-beta-hydroxysteroid dehydrogenase in aldosterone target cells.J Biol Chem. 1993; 268: 10717-10720Abstract Full Text PDF PubMed Google Scholar. However, its functioning may be more complex (nonmichaelian), involving a series of equilibriums between multimers of the enzyme (for example, dimers and tetramers). It has been suggested that HSD2 exists as a dimer, and other members of the short-chain alcohol dehydrogenase (SCAD) superfamily also function as multimers13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar. The SCAD superfamily, to which both HSD1 and HSD2 belong, includes more than 100 members; the three-dimensional structures of some of them have been solved by x-ray crystallography. Interestingly, the 3α-20β-HSD is a tetramer, and it has been suggested that the 11β-HSD enzymes exhibit protein folding very similar to those of 3α-20β-HSD and 17β-HSD113Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar,35Penning T.M. Molecular endocrinology of hydroxysteroid dehydrogenases.Endocr Rev. 1997; 18: 281-305Crossref PubMed Scopus (373) Google Scholar.Table 2Differences between HSD1 and HSD2HSD121% identityHSD2Affinity for cortisolLow (100 nm)High (1–10 nm)CofactorNADPNADEnzymatic propertiesDehydrogenase and reductaseDehydrogenaseExpressionUbiquitousAldosterone-sensitive cellsFunctionLocal regulation of glucocorticoid actionProtection of MRMolecular wt34 kD (290 aa)44 kD (400 aa)⇐GlycosylationImportant for activityNot required for activityAbbreviations are: HSD, 11β-hydroxysteroid dehydrogenase; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; MR, mineralocorticoid receptor. Open table in a new tab Abbreviations are: HSD, 11β-hydroxysteroid dehydrogenase; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; MR, mineralocorticoid receptor. HSD2 has a hydrophobic N-terminal domain that is considered important for anchoring in the membrane of the endoplasmic reticulum13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar. However, it has been shown that deletion of this region does not modify its activity and does not release the enzyme in the cytoplasm13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar,36Obeyesekere V.R. Li K.X.Z. Ferrari P. Krozowski Z. Truncation of the N- and C-terminal regions of the human 11 beta-hydroxysteroid dehydrogenase type 2 enzyme and effects on solubility and bidirectional enzyme activity.Mol Cell Endocrinol. 1997; 131: 173-182Crossref PubMed Scopus (26) Google Scholar. Elegant studies with green fluorescent protein (GFP)-tagged HSD2 transfected in Chinese hamster ovary (CHO) cells indicated that HSD2 is localized in the endoplasmic reticulum37Naray-Fejes-Toth A. Fejes-Toth G. Subcellular localization of the type 2, 11 beta-hydroxysteroid dehydrogenase: A green fluorescent protein study.J Biol Chem. 1996; 271: 15436-15442Crossref PubMed Scopus (69) Google Scholar, whereas other studies13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar,23Shimojo M. Ricketts M.L. Petrelli M.D. Moradi P. Johnson G.D. Bradwell A.R. Hewison M. Howie A.J. Stewart P.M. Immunodetection of 11 beta-hydroxysteroid dehydrogenase type 2 in human mineralocorticoid target tissues: Evidence for nuclear localization.Endocrinology. 1997; 138: 1305-1311Crossref PubMed Scopus (80) Google Scholar found nuclear localization of the enzyme, as demonstrated by immunolocalization. Further studies are needed to clarify the subcellular localization of HSD2. Little information is available on putative post-translational modifications of HSD2. Despite the presence of one potential N-glycosylation site, such a phenomenon does not seem to be effective, since the molecular weight of HSD2 is not influenced by tunicamycin or N-glycosidase treatment13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar,38Kyossev Z.N. Reeves W.B. N-glycosylation is not essential for enzyme activity of 11 beta-hydroxysteroid dehydrogenase type 2.Kidney Int. 1997; 52: 682-686Abstract Full Text PDF Scopus (7) Google Scholar. Other post-translational modifications, such as phosphorylation or myristoylation may exist, but experimental investigations have not yet been performed to test these possibilities. Clinical evidence for the major role of HSD2 came from observations of some forms of arterial hypertension, where the enzyme appeared to be abnormal12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar, 13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar, 39White P.C. Mune T. Agarwal A.K. 11 beta-hydroxysteroid dehydrogenase and the syndrome of apparent mineralocorticoid excess.Endocr Rev. 1997; 18: 135-156Crossref PubMed Scopus (528) Google Scholar. The syndrome of apparent mineralocorticoid excess (AME) is a rare form of congenital hypertension, with severe hypertension, hypokalemia, and low levels of renin and aldosterone. Genetic analyses of the kindreds showed mutations in HSD2 as being responsible for inactivation of the enzyme13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar,39White P.C. Mune T. Agarwal A.K. 11 beta-hydroxysteroid dehydrogenase and the syndrome of apparent mineralocorticoid excess.Endocr Rev. 1997; 18: 135-156Crossref PubMed Scopus (528) Google Scholar; in these children, cortisol permanently occupies the MR and promotes sustained sodium reabsorption. A mouse model for AME has been obtained recently by inactivation of HSD2 using homologous recombination40Kotelevtsev Y. Brown R.W. Fleming S. Kenyon C. Edwards C.R.W. Seckl J.R. Mullins J.J. Hypertension in mice lacking 11β-hydroxysteroid dehydrogenase type 2.J Clin Invest. 1999; 103: 683-689Crossref PubMed Scopus (233) Google Scholar; these knockout mice develop hypertension, hypotonic polyuria, and low levels of plasma aldosterone. The enzyme may also be inhibited by glycyrrhetinic acid, the active metabolite of licorice12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar,13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar. Hypertension, which develops after chronic ingestion of licorice, is due to HSD2 inhibition, and normalization of blood pressure occurs after licorice withdrawal. Other situations (for example, subgroups of essential hypertension) may be reminiscent of abnormal HSD2 activity, although definite information is actually lacking12Seckl J.R. Chapman K.E. The 11-beta-hydroxysteroid dehydrogenase system, a determinant of glucocorticoid and mineralocorticoid action: Medical and physiological aspects of the 11-beta-hydroxysteroid dehydrogenase system.Eur J Biochem. 1997; 249: 361-364Crossref PubMed Scopus (80) Google Scholar,13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar, probably because of the difficulty to estimate the in vivo activity of the enzyme. A good estimate of the efficiency of the enzyme can be obtained by the plasma or urine ratio of the THE/THF, that is, the tetrahydro-derivatives of cortisone (E) and cortisol (F). Abnormally low ratios have been reported in patients with AME39White P.C. Mune T. Agarwal A.K. 11 beta-hydroxysteroid dehydrogenase and the syndrome of apparent mineralocorticoid excess.Endocr Rev. 1997; 18: 135-156Crossref PubMed Scopus (528) Google Scholar, but a search for milder involvement of the enzyme deficiency in essential hypertension has been disappointing using this test (as well as the search for mutations of HSD2). It should be taken into consideration that the ratio of reduced versus dehydrogenated cortisol metabolites indeed depends on the activity of renal HSD2 (dehydrogenase) as well as the hepatic HSD1, which is a bidirectional enzyme (dehydrogenase and reductase). Thus, a partial defect in renal HSD2 (leading to abnormal sodium reabsorption) may be masked in blood or in urine by a partial compensation by hepatic HSD1. In order to gain more precise information from human aldosterone target cells, a microassay of HSD2 activity has been developed using sweat gland ducts22Kenouch S. Lombes M. Delahaye F. Eugene E. Bonvalet J.P. Farman N. Human skin as target for aldosterone: Coexpression of mineralocorticoid receptors and 11 beta-hydroxysteroid dehydrogenase.J Clin Endocrinol Metab. 1994; 79: 1334-1341PubMed Google Scholar. These ducts may be obtained by microdissection from skin biopsies and allow an accurate measurement of the catalytic activity of the enzyme. Sweat gland ducts also express the MR, and their level of HSD2 activity is similar to that of the collecting duct22Kenouch S. Lombes M. Delahaye F. Eugene E. Bonvalet J.P. Farman N. Human skin as target for aldosterone: Coexpression of mineralocorticoid receptors and 11 beta-hydroxysteroid dehydrogenase.J Clin Endocrinol Metab. 1994; 79: 1334-1341PubMed Google Scholar. A search for an abnormal function of HSD2 in hypertensive patients is currently under investigation using this approach. An important issue is to know whether or not HSD2 is subject to regulation. Surprisingly little information is available on this point. Some studies showed inhibition of the enzyme by several natural components. Besides glycyrrhetinic acid and carbenoxolone, inhibitors were found in grapefruit juice, cotton seeds, and others13Stewart P.M. Krozowski Z.S. 11 beta-hydroxysteroid dehydrogenase.Vitam Horm. 1999; 57: 249-324Crossref PubMed Scopus (440) Google Scholar. Some bile acids also reduce the activity of HSDs, and recently it has been suggested that endogenous inhibitors of HSD2, named GALF (for glycyrrhetinic acid-like factor) exist in the plasma and may be increased in essential hypertension (discussed in the article by D. Morris in this issue of Kidney International). The search for stimulatory factors has been scarce. In vivo changes in corticosteroid status in rats (deprivation by adrenalectomy and selective treatment by aldosterone or glucocorticoid hormones) showed a moderate increase41Alfaidy N. Blot-Chabaud M. Bonvalet J.P. Farman N. Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct.J Clin Invest. 1997; 100: 2437-2442Crossref PubMed Scopus (39) Google Scholar or no change26Alfaidy N. Blot-Chabaud M. Robic D. Kenouch S. Bourbouze R. Bonvalet J.P. Farman N. Characteristics and regulation of 11 beta-hydroxysteroid dehydrogenase of proximal and distal nephron.Biochim Biophys Acta. 1995; 1243: 461-468Crossref Scopus (26) Google Scholar in HSD activity. No effect of corticosteroid hormones was evidenced on HSD2 transcripts. Protein kinase A may be involved in the regulation of HSD2, since arginine-vasopressin (or cAMP) stimulates HSD2 activity in vitro in isolated cortical collecting ducts41Alfaidy N. Blot-Chabaud M. Bonvalet J.P. Farman N. Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct.J Clin Invest. 1997; 100: 2437-2442Crossref PubMed Scopus (39) Google Scholar. Interestingly, such an effect requires aldosterone; the effect is observed only in tubules from normal animals or adrenalectomized rats receiving a substitutive aldosterone treatment. Thus, it appears that the two main hormones that exert a fine tuning of renal sodium reabsorption42Hawk C.T. Li L. Schafer J.A. AVP and aldosterone at physiological concentrations have synergistic effects on Na transport in rat CCD.Kidney Int. 1996; 50: S35-S41Google Scholar—aldosterone and vasopressin—act coordinately to enhance HSD2 activity, thereby reinforcing mineralocorticoid selectivity. The MR itself has intrinsic properties that discriminate between aldosterone and glucocorticoid hormones43Couette B. Fagart J. Jalaguier S. Lombes M. Souque A. Rafestin-Oblin M.E. The ligand induced conformational change of the mineralocorticoid receptor occurs within its heterooligomeric structure.Biochem J. 1996; 315: 421-427Crossref PubMed Scopus (63) Google Scholar, 44Funder J. Myles K. Exclusion of corticosterone from epithelial mineralocorticoid receptors is insufficient for selectivity of aldosterone action: In vivo binding studies.Endocrinology. 1996; 137: 5264-5268PubMed Google Scholar, 45Lombes M. Kenouch S. Souque A. Farman N. Rafestin-Oblin M.E. The mineralocorticoid receptor discriminates aldosterone from glucocorticoids independently of the 11 beta-hydroxysteroid dehydrogenase.Endocrinology. 1994; 135: 834-840Crossref PubMed Google Scholar, 46Lombes M. Binart N. Delahaye F. Baulieu E.E. Rafestin-Oblin M.E. Differential intracellular localization of human mineralocorticosteroid receptor on binding of agonists and antagonists.Biochem J. 1994; 302: 191-197Crossref PubMed Scopus (68) Google Scholar. Indeed, the MR displays the same apparent affinity for these ligands. However, it is important to remember that the affinity constant at equilibrium (Kd) is the ratio of Kon and Koff; if both are increased (for example, in the case of glucocorticoid hormones binding to the MR), the apparent affinity is unchanged. It has been shown that the interaction of the MR with aldosterone has a more prolonged half-life45Lombes M. Kenouch S. Souque A. Farman N. Rafestin-Oblin M.E. The mineralocorticoid receptor discriminates aldosterone from glucocorticoids independently of the 11 beta-hydroxysteroid dehydrogenase.Endocrinology. 1994; 135: 834-840Crossref PubMed Google Scholar, as compared with glucocorticoid hormone–MR complexes, reflecting distinct molecular ligand–MR interactions. The compaction of the MR bound to aldosterone47Fagart J. Wurtz J.M. Souque A. Hellal-Levy C. Moras D. Rafestin-Oblin M.E. Antagonism in the human mineralocorticoid receptor.EMBO J. 1998; 17: 3317-3325Crossref PubMed Scopus (140) Google Scholar differs from that occurring when MR is bound to glucocorticoid hormones (discussed in the article by M.E. Oblin in this issue of Kidney International). Such distinct properties of MR, depending on its ligand, ultimately modify its transactivation capacities45Lombes M. Kenouch S. Souque A. Farman N. Rafestin-Oblin M.E. The mineralocorticoid receptor discriminates aldosterone from glucocorticoids independently of the 11 beta-hydroxysteroid dehydrogenase.Endocrinology. 1994; 135: 834-840Crossref PubMed Google Scholar,48Arriza J.L. Weinberger C. Cerelli G. Glaser T.M. Handelin B.L. Housman D.E. Evans R.M. Cloning of human mineralocorticoid receptor complementary DNA: Structural and functional kinship with the glucocorticoid receptor.Science. 1987; 237: 268-275Crossref PubMed Scopus (1578) Google Scholar. As already noted by Arriza et al, MR-induced transactivation is much more efficient in the presence of aldosterone, the half maximal transactivation occurring at 5 × 10-10 mol/L with aldosterone and 5 × 10-8 mol/L with dexamethazone48Arriza J.L. Weinberger C. Cerelli G. Glaser T.M. Handelin B.L. Housman D.E. Evans R.M. Cloning of human mineralocorticoid receptor complementary DNA: Structural and functional kinship with the glucocorticoid receptor.Science. 1987; 237: 268-275Crossref PubMed Scopus (1578) Google Scholar. It should be noted that this kind of study is usually performed using a classic GRE [from mouse mammary tumor virus (MMTV)] linked to a reporter gene. It will be interesting to determine whether the aldosterone-bound MR (and the cortisol-bound MR) behaves similarly on the promoters of endogenously regulated genes, such as early aldosterone-induced proteins (most of which have not yet been characterized). Despite the remarkable efficiency of HSD2 to metabolize most of the glucocorticoid hormones entering the aldosterone target cell, it is likely that some glucocorticoids escape this inactivating mechanism and thus are able to occupy MR (and GR) to some extent1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar,9Farman N. Molecular and cellular determinants of mineralocorticoid selectivity.Curr Opin Nephrol Hypertens. 1999; 8: 45-51Crossref PubMed Scopus (27) Google Scholar. This is probably a very significant regulatory pathway in these cells because it may allow the variation of the proportion of each receptor bound to each ligand. Furthermore, because of the possibility to form homodimers (MR-MR or GR-GR) or heterodimers (MR-GR), which have distinct transactivation properties49Trapp T. Holsboer F. Heterodimerization between mineralocorticoid and glucocorticoid receptors increases the functional diversity of corticosteroid action.Trends Pharmacol Sci. 1996; 17: 145-149Abstract Full Text PDF PubMed Scopus (128) Google Scholar, transcription of target genes can be controlled in a very specific pattern, which will vary with the corticosteroid status1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar,9Farman N. Molecular and cellular determinants of mineralocorticoid selectivity.Curr Opin Nephrol Hypertens. 1999; 8: 45-51Crossref PubMed Scopus (27) Google Scholar. Interesting information should be provided by systematic analysis of the transactivation efficiency of these homodimers/heterodimers in the presence of varying proportions of aldosterone and glucocorticoid hormones as ligands. Because inactivation of the bulk of glucocorticoid hormones by HSD2 precedes the formation of liganded homodimers/heterodimers, any change in HSD2 activity will likely modify this series of dynamic equilibriums by affecting the downstream steps, that is, the relative proportion of receptors occupied by each ligand9Farman N. Molecular and cellular determinants of mineralocorticoid selectivity.Curr Opin Nephrol Hypertens. 1999; 8: 45-51Crossref PubMed Scopus (27) Google Scholar. Transcription of target genes will also depend on interactions between MR (or MR-GR) and other transcription factors9Farman N. Molecular and cellular determinants of mineralocorticoid selectivity.Curr Opin Nephrol Hypertens. 1999; 8: 45-51Crossref PubMed Scopus (27) Google Scholar such as tissue-specific factors, cAMP response element-binding proteins, or members of the Jun-Fos family. Such interactions have not yet been fully documented, but they are highly probable in view of the differential effects of aldosterone in distinct tissues. For example, aldosterone up-regulates the transcripts encoding for the sole α subunit of the epithelial sodium channel (ENaC) in renal collecting duct cells (β and γ unchanged), while affecting the β and γ subunits (not α) of ENaC in the epithelial cells of the distal colon50Escoubet B. Coureau C. Bonvalet J.P. Farman N. Noncoordinate regulation of epithelial Na channel and Na pump subunit mRNAs in kidney and colon by aldosterone.Am J Physiol. 1997; 272: C1482-C1491PubMed Google Scholar. Along the same line, aldosterone and vasopressin cooperate to up-regulate ENaC subunits transcripts in kidney cells, with aldosterone affecting the α subunit, and vasopressin the β and γ subunit50Escoubet B. Coureau C. Bonvalet J.P. Farman N. Noncoordinate regulation of epithelial Na channel and Na pump subunit mRNAs in kidney and colon by aldosterone.Am J Physiol. 1997; 272: C1482-C1491PubMed Google Scholar,51Djelidi S. Fay M. Cluzeaud F. Escoubet B. Eugene E. Capurro C. Bonvalet J.P. Farman N. Blot-Chabaud M. Transcriptional regulation of sodium transport by vasopressin in renal cells.J Biol Chem. 1997; 272: 32919-32924Crossref PubMed Scopus (77) Google Scholar. As stated earlier, 11-dehydro metabolites also have little affinity for GR. Thus, in cells expressing MR, GR, and HSD2 (such as those of the renal collecting duct), mineralocorticoid selectivity is indeed ensured, but it is difficult to understand how glucocorticoid hormones can act through their own receptor1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar. Specific actions of glucocorticoid hormones on renal distal tubular function have been described, such as an increase in potassium and proton excretion52Johnson J.P. Cellular mechanisms of action of mineralocorticoid hormones.Pharmacol Ther. 1992; 53: 1-29Crossref PubMed Scopus (24) Google Scholar. Some of these effects (K+ movements) may be secondary to the glucocorticoid-induced increase in glomerular filtration rate or its effects on proximal tubule and Henle's loop and may represent flux-dependent changes rather than primary effects1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar,52Johnson J.P. Cellular mechanisms of action of mineralocorticoid hormones.Pharmacol Ther. 1992; 53: 1-29Crossref PubMed Scopus (24) Google Scholar. In addition, it has repeatedly been found that glucocorticoid hormones potentiate the effect of aldosterone, for example, for the stimulation of Na,K-ATPase1Bonvalet J.P. Regulation of sodium transport by steroid hormones.Kidney Int. 1998; 53: S49-S56Google Scholar,52Johnson J.P. Cellular mechanisms of action of mineralocorticoid hormones.Pharmacol Ther. 1992; 53: 1-29Crossref PubMed Scopus (24) Google Scholar. Further studies should help us understand how the direct effects of glucocorticoid hormones can develop in cells expressing HSD2. Reduction of HSD2 activity is clearly responsible for an increase in blood pressure. We have seen that HSD2 may be mutated (AME) or its activity may be impaired by exogenous factors, such as licorice derivatives, or endogenous factors (GALFs). An important clinical issue will be to determine whether some drugs used for other purposes could reduce HSD2 activity after long-term treatment (for example immunosuppressive drugs) and thus participate in the maintenance of high blood pressure levels. Alternatively, it can be proposed that HSD2 may be stimulated, under some circumstances, and that this could protect against hypertension. Activation of the cAMP-protein kinase A pathway, as observed in vitro after incubation with vasopressin41Alfaidy N. Blot-Chabaud M. Bonvalet J.P. Farman N. Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct.J Clin Invest. 1997; 100: 2437-2442Crossref PubMed Scopus (39) Google Scholar, leads to a transient rise in HSD2 activity. Whether such a phenomenon is effective over the long-term is unknown. The search for HSD2-regulatory proteins should also provide important information. Finally, activating mutations of HSD2 (or of regulatory proteins) may exist and may be important for preventing the development and/or the maintenance of high blood pressure levels (protection from hypertension).