The Mineralocorticoid Receptor—How to Get Away with Promiscuity

Isidore Edelman (1) had difficulty publishing his groundbreaking paper on isolation and characterization of the mineralocorticoid receptor (2), because reviewers argued—not without good reasons—that he might have gotten the wrong receptor. They objected that its affinity was admittedly greatest for aldosterone, but considering the fact that the intracellular concentration of glucocorticoids was higher by a factor of 103, the receptor should be stimulated in humans mainly by cortisol in vivo. When the human mineralocorticoid receptor was cloned, the structural and functional kinship with the glucocorticoid receptor was readily apparent (3). Today we know that one trick to resolve the dilemma of shared affinities is preceptor metabolism, i.e., local conversion of cortisol to inactive 11-keto congeners, cortisone, and 11-dehydrocorticosterone by the 11β hydroxysteroid dehydrogenase isoenzyme type 2. These products have markedly lower affinity to the mineralocorticoid receptor. This is the main, potentially not the only, mechanism that protects the promiscuous receptor from stimulation by the “wrong” ligand and guarantees relative ligand specificity, at least in the classic sodium transporting target organs (4–6). Nature obviously has difficulty keeping apart within the cell the ligands that trigger mineralocorticoid and glucocorticoid actions respectively (7, 8). Part of the answer may be found in the evolution. Recently the hypothesis had been proposed that the mineralocorticoid and the glucocorticoid receptors are the result of a gene duplication step in the earliest stages of the evolution of the vertebrate lineage (9). Hidden behind this curiosity is a more fundamental question. It was addressed in the above paper Bridgham et al. (10) and goes to the heart of Darwin’s hypothesis of selection: In systems comprising a corresponding specific ligand (hormones) and receptor (ligand-activated regulators) respectively, how can selection drive the selectivity of the receptor’s affinity if the hormone is not yet present? Can a ligand, e.g., a hormone, contribute to the evolution of a receptor “in absentia”? This question is pertinent since the mineralocorticoid receptor existed (10) well before aldosterone synthase, i.e., CYP11B2, was present (8). In the evolution of vetebrates, the ancestral corticoid receptor (from which both the glucocorticoid and the mineralocorticoir receptors are thought to have descended by gene duplication) is found only in the most primitive species, i.e., jawless fish (Petromyzon marinus) and hagfish (Myxina gelatinosa). Even in the relatively primitive cartilaginous fish (elasmobranch), e.g., Raja erinacea, distinct glucocorticoid and mineralocorticoid receptors already exist. Phylogenetic analysis indicated that the duplication which separated the two receptors occurred approximately 450 million years ago, the time when cartilaginous fish had separated from more primitive jawless fish and hagfish, but before the split between cartilaginous fish (elasmobranch) and bony fish (teleosts) had occured. In a next step, the investigators assessed the hormone specificities of the receptors. The data showed that in higher species, i.e., teleosts (bony fish) and tetrapods (quadripeds), the “basal” receptors were activated by low concentrations of both aldosterone (mineralocorticoids) and cortisol (glucocorticoids) or 11-deoxycorticosterone. In contrast in higher species, i.e., teleosts and tetrapods, the only receptor which is insensitive to aldosterone is the glucocorticoid receptor. Applying Occam’s razor (i.e., the postulate by the 14th century friar that for the explanation of a phenomenon one should make as few assumptions as possible), the authors concluded that the ancient receptor was activated by aldosterone as well as by glucocorticoids, while in the higher species of bony vertebrates the aldosterone sensitivity was restricted to the remaining ancient steroid receptor, this sensitivity having been lost by the novel glucocorticoid receptor. This sounds like a nice hypothesis, but how can this postulate be proven? In a methodologic tour de force, the authors resurrected the ancestral receptor with its broad glucocorticoid and mineralocorticoid specificity, aligning extant receptor sequences (full discussion of which is beyond the scope of this communication) and inferring the most likely amino acid sequence of the ancient ligand binding domain (maximum likelihood approach). To provide further evidence they expressed in cultured cells the molecule with the sequence they had thus inferred. In a next step, they monitored the effect of ligand binding using the reporter assay technique. The resurrected ancient receptor was activated by low doses of desoxycorticosterone (DOC) and, to a lesser extent, by cortisol. Using Bayesian phylogenetics, plausible ancestral trees were reconstructed and the plausiblity of the constructs was tested by mathematic modelling. Nature is parsimonious and the same or modified molecules are used over and over again—for related or completely different purposes. A telling example is a remarkable finding in Argentina: The cause of a serious problem of farmers, entque secco, i.e., hypercalcemia of cows which then fail to produce milk, results from the synthesis of 1,25(OH)2D3 by the plant solanum malacoxylon (11, 12)—obviously, as a plant, without bones, parathyroids, or need of calcemia control. Other examples are ligands interacting with the mammalian estrogen receptor in plants ( 13, 14), although plants obviously have no female reproductive organs akin to the mammalian ones. Why should nephrologists now care about steroid receptors in a lamprey? The complexity of the evolution of this system may help explain the enormous complexity of the mammalian mineralocorticoid system which, beyond the classic role in vectorial transepithelial sodium transport, plays an increasingly important pathogenetic role in nonepithelial tissues such as heart (15), vessels (16, 17), or kidney (18). Such insight has recently led to novel treatment strategies in cardiac (19, 20) and renal (21–23) diseases. What is interesting, and still unresolved, is the issue of why in nonepithelial tissue where 11β hydroxysteroid dehydrogenase isoenzyme type 2 is not coexpressed with the mineralocorticoid receptor, glucocorticoids ordinarily fail to mimic the effect of aldosterone (7). And even more puzzling is why the deleterious effect of aldosterone on nonepithelial tissues is dependent upon, and provoked by, high salt intake, although high salt suppresses endogenous aldosterone. A recent study suggests that the missing link is generation of reactive oxygen species under high salt intake and stimulation of the (unprotected) promiscuous mineralocorticoid receptor by glucocorticoids, despite the low aldosterone concentration (24). Against this background some more simple paradigms of the past may require revision. The Nestor of nephrology, Homer W. Smith, would be delighted to see how studying issues “from fish to philosopher” helps us better understand human pathophysiology.