The last decade has witnessed tremendous progress in the understanding of the mineralocorticoid receptor (MR), its molecular mechanism of action, and its implications for physiology and pathophysiology. After the initial cloning of MR, and identification of its gene structure and promoters, it now appears as a major actor in protein-protein interaction networks. The role of transcriptional coregulators and the determinants of mineralocorticoid selectivity have been elucidated.Targeted oncogenesis and transgenic mouse models have identified unexpected sites of MR expression and novel roles for MR in non-epithelial tissues. These experimental approaches have contributed to the generation of new cell lines for the characterization of aldosterone signaling pathways, and have also facilitated a better understanding of MR physiology in the heart, vasculature, brain and adipose tissues. This review describes the structure, molecular mechanism of action and transcriptional regulation mediated by MR, emphasizing the most recent developments at the cellular and molecular level. Finally, through insights obtained from mouse models and human disease, its role in physiology and pathophysiology will be reviewed. Future investigations of MR biology should lead to new therapeutic strategies, modulating cell-specific actions in the management of cardiovascular disease, neuroprotection, mineralocorticoid resistance, and metabolic disorders. Received July 20th, 2007; Accepted Novemeber 2nd, 2007; Published Novemeber 30th, 2007 | Abbreviations: 11β-HSD2: 11β-hydroxysteroid dehydrogenase 2; ACE: angiotensin converting enzyme; ACTH: adrenocorticotrophic hormone; ADAMTS1: a disintegrin and metalloproteinase with thrombospondin-like motifs 1; adPHA1: autosomal dominant pseudohypoaldosteronism type 1; AF1: activation function 1; AF2: activation function 2; ANF: atrial natriuretic factor; AR: androgen receptor; ASC2: activating signal cointegrator 2; BMP2: bone morphogenetic protein 2; CBP: CREB binding protein; CHIF: channel-inducing factor; CNS: central nervous system; DAXX: death-associated protein 6; DBD: DNA binding domain; EGF-R: epidermal growth factor receptor; Egr-1: early growth response gene-1; ELL: eleven-nineteen lysine-rich leukemia; ENaC: epithelial sodium channel; ERK: extracellular signal-regulated kinase; ET-1: endothelin-1; FAF1: Fas associated factor 1; FLASH: FLICE associated huge; G6PD: glucose-6-phosphate dehydrogenase; GILZ: glucocorticoid-induced leucine zipper protein; GR: glucocorticoid receptor; GRE: glucocorticoid responsive element; HAS2: hyaluronic acid synthase 2; HDAC: histone deacetylase; hMR: human mineralocorticoid receptor; HRE: hormone responsive element; hsp: heat shock protein; KS-WNK1: kidney specific with no lysine [K] kinase 1; LBD: ligand binding domain; LXRβ: liver X receptor β; MAPK: mitogen-activated protein kinase; MDM2: murine double minute gene 2; MR: mineralocorticoid receptor; MRE: mineralocorticoid responsive element; NAD: nicotinamide adenine dinucleotide; NCoR: nuclear receptor core...

The mineralocorticoid receptor (MR) integrates hormonal signaling and activates the expression of aldosterone target genes, which control various physiological processes. In recent years, evidence has been provided for an important role of MR not only in the regulation of sodium and water homeostasis but also in cardiovascular function, neuronal fate, and adipocyte differentiation. MR belongs to the steroid receptor family that displays common mechanism of action. As a result, some apparent similarities with the glucocorticoid receptor (GR) have shaded MR's own specificities. The description of its gene structure, messenger isoforms, protein variants, functional domains, and posttranslational modifications (phosphorylation, ubiquitinylation, sumoylation, acetylation) as well as a panel of interactions with coregulators, progressively depicted an original portrait of MR and shed light on its specific mechanism of action. In this review, after an overview of MR characteristics, the multiple levels of MR selectivity over other steroid receptors, in particular GR, will be described as well as the consequences for aldosterone-regulated gene expression.

Adiponectin and resistin, two recently identi¢ed adipocyte-speci¢c secretory factors, are able to modulate insulin actions in target tissues. To investigate their expression and hormonal regulation in brown adipocytes, we used the brown adipocyte cell line T37i, which, beside uncoupling protein expression, secretes leptin. Adiponectin and resistin mRNA were detected as a function of cell di¡erentiation. Both transcripts were expressed at relatively high levels in di¡erentiated T37i cells, reaching maximal levels on day 7, while resistin expression drastically fell afterwards. These stable transcripts (t 1=2 s 8 h) were di¡erentially regulated by factors involved in insulin responsiveness. Insulin and thiazolidinedione, a peroxisome proliferator-activated receptor Q Q agonist, stimulated resistin expression two-to four-fold in di¡erentiated T37i cells, whereas adiponectin mRNA levels increased 1.5^2-fold. In contrast, dexamethasone and isoproterenol reduced by two-fold the level of adiponectin and resistin transcripts in di¡erentiated T37i cells. This study provides the ¢rst direct evidence that di¡er-entiated brown adipocytes are endocrine cells capable of expressing adiponectin and resistin. The complex hormonal regulation of their expression in brown adipocytes clearly di¡ers from that reported in white adipose tissue, pointing to di¡er-ential physiological and pathophysiological implications of brown fat in energy homeostasis.

The dynamic and coordinated recruitment of coregulators by steroid receptors is critical for specific gene transcriptional activation. To identify new cofactors of the human (h) mineralocorticoid receptor (MR), its highly specific N-terminal domain was used as bait in a yeast two-hybrid approach. We isolated ELL (eleven-nineteen lysine-rich leukemia), a RNA polymerase II elongation factor which, when fused to MLL (mixed lineage leukemia) contributes to the pathogenesis of acute leukemia. Specific interaction between hMR and ELL was confirmed by glutathione-S-transferase pull-down and coimmunoprecipitation experiments. Transient transfections demonstrated that ELL increased receptor transcriptional potency and hormonal efficacy, indicating that ELL behaves as a bona fide MR coactivator. Of major interest, ELL differentially modulates steroid receptor responses, with striking opposite effects on hMR and glucocorticoid receptor-mediated transactivation, without affecting that of androgen and progesterone receptors. Furthermore, the MLL-ELL fusion protein, as well as several ELL truncated mutants and the ELL L214V mutant, lost their ability to potentiate MR transcriptional activities, suggesting that both the elongation domain and the ELL-associated factor 1 interaction domains are required for ELL to fulfill its selector activity on steroid receptors. This study is the first direct demonstration of a functional interaction between a nuclear receptor and an elongation factor. These results provide further evidence that the selectivity of the mineralo vs. glucocorticoid signaling pathways also occurs at the transcriptional complex level and may have major pathophysiological implications, most notably in leukemogenesis and corticosteroid-induced apoptosis. These findings allow us to propose the concept of "transcriptional selector" for ELL on steroid receptor transcriptional functions.

Molecular mechanisms underlying mineralocorticoid receptor (MR)-mediated gene expression are not fully understood but seem to largely depend upon interactions with specific coregulators. To identify novel human MR (hMR) molecular partners, yeast two-hybrid screenings performed using the N-terminal domain as bait, allowed us to isolate protein inhibitor of activated signal transducer and activator of transcription (PIAS)1 and PIASxbeta, described as SUMO (small ubiquitin-related modifier) E3-ligases. Specific interaction between PIAS1 and hMR was confirmed by glutathione-S-transferase pull-down experiments and N-terminal subdomains responsible for physical contacts were delineated. Transient transfections demonstrated that PIAS1 is a corepressor of aldosterone-activated MR transactivation but has no significant effect on human glucocorticoid receptor transactivation. The agonist or antagonist nature of the bound ligand also determines PIAS1 corepressive action. We provided evidence that PIAS1 conjugated SUMO-1 to hMR both in vitro and in vivo. Deciphering the unique sumoylation pattern of hMR, which possesses five consensus SUMO-1 binding sites, by combinatorial lysine substitutions, revealed a major impact of sumoylation on hMR properties. Using a murine mammary tumor virus promoter, PIAS1 action was independent of sumoylation whereas with glucocorticoid response element promoter, PIAS1 corepressive action depended on hMR sumoylation status. Taken together, our results identify a novel function for PIAS1 which interacts with the N-terminal domain of hMR and represses its ligand-dependent transcriptional activity, at least in part, through SUMO modifications.

Altogether these data indicate that MR is expressed, through alternative translation initiation, as distinct protein variants which possess different functional properties. These MR forms could tightly dictate the modulation of aldosterone responsiveness in various tissues or in pathophysiological situations.

The proopiomelanocortin (POMC) gene is occasionally expressed in nonpituitary tumors leading to Cushing's syndrome. Bronchial carcinoid tumors, one of the most frequent source for ectopic ACTH secretion, often display numerous features of the corticotroph phenotype. To identify new markers of corticotroph differentiation in these tumors, we compared the pattern of gene expression in ACTH-secreting (ACTH+) and nonsecreting (ACTH-) bronchial carcinoids by differential display/RT-PCR. Using groups of ACTH+ and ACTH- tumors, we initially selected approximately 300 differentially expressed genes. Fifteen were considered differentially expressed after further characterization by RT-PCR on a larger series of 8 ACTH+ and 12 ACTH- bronchial carcinoids; 11 were restricted to--or overexpressed in--ACTH+ and four in ACTH- tumors. In ACTH+, beside the expected POMC gene, we identified cFos, and KIAA1775, a large expressed sequence tag encoding a putative protocadherin-related protein. On the other hand, the tetraspanin TM4SF5 gene was specifically expressed in ACTH-. Dot blot analysis confirmed the specific expression of KIAA1775 in ACTH+ bronchial carcinoids. However, the expression of most of the differential genes, including KIAA1775, was detected by RT-PCR in pituitary or lung tumors, whether secreting ACTH or not, excepted for TM4SF5, which was only detected in some nonendocrine lung tumors. Our results show that corticotroph differentiation of bronchial carcinoid tumors is accompanied by induction and repression of specific genes. The nature of some of these genes, identified here, underlines the importance of cell-cell or cell-extracellular matrix interactions in the establishment of neoplastic corticotroph phenotype. These genes should help to better characterize ACTH+ bronchial carcinoids as well as other bronchial carcinoid subtypes.

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