Oxidative stress induced by L-buthionine-(S,R)-sulfoximine, a selective inhibitor of glutathione metabolism, abrogates mouse kidney mineralocorticoid receptor function

Importance of Micromilieu for Pathophysiologic Mineralocorticoid Receptor Activity-When the Mineralocorticoid Receptor Resides in the Wrong Neighborhood

The mineralocorticoid receptor (MR) is a member of the steroid receptor family and acts as a ligand-dependent transcription factor. In addition to its classical effects on water and electrolyte balance, its involvement in the pathogenesis of cardiovascular and renal diseases has been the subject of research for several years. The molecular basis of the latter has not been fully elucidated, but an isolated increase in the concentration of the MR ligand aldosterone or MR expression does not suffice to explain long-term pathologic actions of the receptor. Several studies suggest that MR activity and signal transduction are modulated by the surrounding microenvironment, which therefore plays an important role in MR pathophysiological effects. Local changes in micromilieu, including hypoxia, ischemia/reperfusion, inflammation, radical stress, and aberrant salt or glucose concentrations affect MR activation and therefore may influence the probability of unphysiological MR actions. The surrounding micromilieu may modulate genomic MR activity either by causing changes in MR expression or MR activity; for example, by inducing posttranslational modifications of the MR or novel interaction with coregulators, DNA-binding sites, or non-classical pathways. This should be considered when developing treatment options and strategies for prevention of MR-associated diseases.

Posttranslational Modifications of the Mineralocorticoid Receptor and Cardiovascular Aging

During aging, the cardiovascular system is especially prone to a decline in function and to life-expectancy limiting diseases. Cardiovascular aging is associated with increased arterial stiffness and vasoconstriction as well as left ventricular hypertrophy and reduced diastolic function. Pathological changes include endothelial dysfunction, atherosclerosis, fibrosis, hypertrophy, inflammation, and changes in micromilieu with increased production of reactive oxygen and nitrogen species. The renin-angiotensin-aldosterone-system is an important mediator of electrolyte and blood pressure homeostasis and a key contributor to pathological remodeling processes of the cardiovascular system. Its effects are partially conveyed by the mineralocorticoid receptor (MR), a ligand-dependent transcription factor, whose activity increases during aging and cardiovascular diseases without correlating changes of its ligand aldosterone. There is growing evidence that the MR can be enzymatically and non-enzymatically modified and that these modifications contribute to ligand-independent modulation of MR activity. Modifications reported so far include phosphorylation, acetylation, ubiquitination, sumoylation and changes induced by nitrosative and oxidative stress. This review focuses on the different posttranslational modifications of the MR, their impact on MR function and degradation and the possible implications for cardiovascular aging and diseases.

A Spatiotemporal Characterisation of Redox Molecules in Planarians, with a Focus on the Role of Glutathione during Regeneration

A strict coordination between pro- and antioxidative molecules is needed for normal animal physiology, although their exact function and dynamics during regeneration and development remains largely unknown. Via in vivo imaging, we were able to locate and discriminate between reactive oxygen species (ROS) in real-time during different physiological stages of the highly regenerative planarian Schmidtea mediterranea. All ROS signals were strong enough to overcome the detected autofluorescence. Combined with an in situ characterisation and quantification of the transcription of several antioxidant genes, our data showed that the planarian gut and epidermis have a well-equipped redox system. Pharmacological inhibition or RNA interference of either side of the redox balance resulted in alterations in the regeneration process, characterised by decreased blastema sizes and delayed neurodevelopment, thereby affecting tails more than heads. Focusing on glutathione, a central component in the redox balance, we found that it is highly present in planarians and that a significant reduction in glutathione content led to regenerative failure with tissue lesions, characterised by underlying stem cell alterations. This exploratory study indicates that ROS and antioxidants are tightly intertwined and should be studied as a whole to fully comprehend the function of the redox balance in animal physiology.

Dibenzoylthiamine Has Powerful Antioxidant and Anti-Inflammatory Properties in Cultured Cells and in Mouse Models of Stress and Neurodegeneration

Thiamine precursors, the most studied being benfotiamine (BFT), have protective effects in mouse models of neurodegenerative diseases. BFT decreased oxidative stress and inflammation, two major characteristics of neurodegenerative diseases, in a neuroblastoma cell line (Neuro2a) and an immortalized brain microglial cell line (BV2). Here, we tested the potential antioxidant and anti-inflammatory effects of the hitherto unexplored derivative O,S-dibenzoylthiamine (DBT) in these two cell lines. We show that DBT protects Neuro2a cells against paraquat (PQ) toxicity by counteracting oxidative stress at low concentrations and increases the synthesis of reduced glutathione and NADPH in a Nrf2-independent manner. In BV2 cells activated by lipopolysaccharides (LPS), DBT significantly decreased inflammation by suppressing translocation of NF-κB to the nucleus. Our results also demonstrate the superiority of DBT over thiamine and other thiamine precursors, including BFT, in all of the in vitro models. Finally, we show that the chronic administration of DBT arrested motor dysfunction in FUS transgenic mice, a model of amyotrophic lateral sclerosis, and it reduced depressive-like behavior in a mouse model of ultrasound-induced stress in which it normalized oxidative stress marker levels in the brain. Together, our data suggest that DBT may have therapeutic potential for brain pathology associated with oxidative stress and inflammation by novel, coenzyme-independent mechanisms.

Modulation of Immunity and Inflammation by the Mineralocorticoid Receptor and Aldosterone

The mineralocorticoid receptor (MR) is a ligand dependent transcription factor. MR has been traditionally associated with the control of water and electrolyte homeostasis in order to keep blood pressure through aldosterone activation. However, there is growing evidence indicating that MR expression is not restricted to vascular and renal tissues, as it can be also expressed by cells of the immune system, where it responds to stimulation or antagonism, controlling immune cell function. On the other hand, aldosterone also has been associated with proinflammatory immune effects, such as the release of proinflammatory cytokines, generating oxidative stress and inducing fibrosis. The inflammatory participation of MR and aldosterone in the cardiovascular disease suggests an association with alterations in the immune system. Hypertensive patients show higher levels of proinflammatory mediators that can be modulated by MR antagonism. Although these proinflammatory properties have been observed in other autoimmune and chronic inflammatory diseases, the cellular and molecular mechanisms that mediate these effects remain unknown. Here we review and discuss the scientific work aimed at determining the immunological role of MR and aldosterone in humans, as well as animal models.

Post-translational modifications of the mineralocorticoid receptor: How to dress the receptor according to the circumstances?

Aldosterone or glucocorticoid stimulation of the mineralocorticoid receptor (MR) is involved in numerous physiological responses, including ions and water homeostasis, blood pressure control and metabolism. The understanding of MR signaling regulation in the patho/physiological context took a new direction the last few years with a focus on the post-translational modifications of MR. Depending on its environment, cellular expression, activity or its binding partners, the MR is submitted to several post-translational modifications such as phosphorylation, ubiquitylation, sumoylation and acetylation that regulate its localization, activity and/or stability. A complex interplay between all these modifications allows a fine tuning of MR signaling depending on the physiological context. This review reports recent knowledge about post-translational modifications of MR and describes the enzymes and the molecular mechanisms involved.

Rapid mineralocorticoid receptor trafficking

The mineralocorticoid receptor (MR) is a ligand-dependent transcription factor that physiologically regulates water-electrolyte homeostasis and controls blood pressure. The MR can also elicit inflammatory and remodeling processes in the cardiovascular system and the kidneys, which require the presence of additional pathological factors like for example nitrosative stress. However, the underlying molecular mechanism(s) for pathophysiological MR effects remain(s) elusive. The inactive MR is located in the cytosol associated with chaperone molecules including HSP90. After ligand binding, the MR monomer rapidly translocates into the nucleus while still being associated to HSP90 and after dissociation from HSP90 binds to hormone-response-elements called glucocorticoid response elements (GREs) as a dimer. There are indications that rapid MR trafficking is modulated in the presence of high salt, oxidative or nitrosative stress, hypothetically by induction or posttranslational modifications. Additionally, glucocorticoids and the enzyme 11beta hydroxysteroid dehydrogenase may also influence MR activation. Because MR trafficking and its modulation by micro-milieu factors influence MR cellular localization, it is not only relevant for genomic but also for nongenomic MR effects.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

Nuclear import of the glucocorticoid receptor-hsp90 complex through the nuclear pore complex is mediated by its interaction with Nup62 and importin beta

Glucocorticoid receptor (GR) is cytoplasmic in the absence of ligand and localizes to the nucleus after steroid binding. Previous evidence demonstrated that the hsp90-based heterocomplex bound to GR is required for the efficient retrotransport of the receptor to the nuclear compartment. We examined the putative association of GR and its associated chaperone heterocomplex with structures of the nuclear pore. We found that importin beta and the integral nuclear pore glycoprotein Nup62 interact with hsp90, hsp70, p23, and the TPR domain proteins FKBP52 and PP5. Nup62 and GR were able to interact in a more efficient manner when chaperoned by the hsp90-based heterocomplex. Interestingly, the binding of hsp70 and p23 to Nup62 does not require the presence of hsp90, whereas the association of FKBP52 and PP5 is hsp90 dependent, as indicated by the results of experiments where the hsp90 function was disrupted with radicicol. The ability of both FKBP52 and PP5 to interact with Nup62 was abrogated in cells overexpressing the TPR peptide. Importantly, GR cross-linked to the hsp90 heterocomplex was able to translocate to the nucleus in digitonin-permeabilized cells treated with steroid, suggesting that GR could pass through the pore in its untransformed state.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

Mineralocorticoid receptors: emerging complexity and functional diversity

Mineralocorticoid receptor (MR) activation in renal epithelial cells in response to the binding of aldosterone has long been implicated in the maintenance of body salt and fluid homeostasis and blood pressure control. 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) is believed to confer specificity on aldosterone to activate MR by inactivating 11beta-hydroxyglucocorticoids (corticosterone, cortisol) that are 100-1000 times more abundant in plasma than aldosterone and that can also bind and activate MR. Increasing evidence, however, challenges such a simple view of MR activation as well as its interaction with glucocorticoids and 11beta-HSDs. In non-epithelial tissues including brain, cardiomyocytes and macrophages, 11beta-hydroxyglucocorticoids seem to act as MR antagonists, and redox changes and signaling events may play pivotal roles for receptor activation in these tissues. This review addresses the emerging new view of the complex mechanisms underlying MR specificity of action, with a diversity of physiological roles and functions in different mineralocorticoid-responsive tissues.

Corticosteroids and redox potential modulate spontaneous contractions in isolated rat ventricular cardiomyocytes

The mineralocorticoid receptor has been implicated in the development of several cardiac pathologies and could participate in the high incidence of lethal ventricular arrhythmias associated with hyperaldosteronism. We have observed previously that aldosterone markedly increases in vitro the rate of spontaneous contractions of isolated neonate rat ventricular myocytes, a putative proarrhythmogenic condition if occurring in vivo. In the present study, we investigated the effect of glucocorticoids, the involvement of the glucocorticoid receptor, and the modulation of their action by redox agents. Aldosterone and glucocorticoids exerted in vitro a similar, concentration-dependent chronotropic action on cardiomyocytes, which was mediated by both the mineralocorticoid and glucocorticoid receptors. However, the relative contribution of each receptor was different for each agonist, at each concentration. Angiotensin II induced a similar response that was entirely dependent on the activity of the glucocorticoid receptor. Corticosteroid action was modulated by the redox state of the cells, with oxidation increasing the response while reducing conditions partially preventing it. When only the mineralocorticoid receptor was functionally present in the cells, oxidation was necessary to reveal glucocorticoid action, but no obvious competition with mineralocorticoids was observed when both agonists where simultaneously present. In conclusion, corticosteroids exert a strong chronotropic action in ventricular cardiomyocytes, mediated by both the mineralocorticoid and glucocorticoid receptors and modulated by the redox state of the cell. This phenomenon is believed to be because of cell electric remodeling and could contribute in vivo to the deleterious consequence of inappropriate receptor activation, leading to increased susceptibility of patients to arrhythmias.

Aging reduces responsiveness to BSO- and heat stress-induced perturbations of glutathione and antioxidant enzymes

Aging alters cellular responses to both heat and oxidative stress. Thiol-mediated metabolism of reactive oxygen species (ROS) is believed to be important in aging. To begin to determine the role of thiols in aging and heat stress, we depleted liver glutathione (GSH) by administering l-buthionine sulfoximine (BSO) in young (6 mo) and old (24 mo) Fisher 344 rats before heat stress. Animals were given BSO (4 mmol/kg ip) or saline (1 ml ip) 2 h before heat stress and subsequently heated to a core temperature of 41°C over a 90-min period. Liver tissue was collected before and 0, 30, and 60 min after heat stress. BSO inhibited glutamate cysteine ligase (GCL, the rate-limiting enzyme in GSH synthesis) catalytic activity and resulted in a decline in liver GSH and GSSG that was more pronounced in young compared with old animals. Catalase activity did not change between groups until 60 min after heat stress in young BSO-treated rats. Young animals experienced a substantial and persistent reduction in Cu,Zn-SOD activity with BSO treatment. Mn-SOD activity increased with BSO but declined after heat stress. The differences in thiol depletion observed between young and old animals with BSO treatment may be indicative of age-related differences in GSH compartmentalization that could have an impact on maintenance of redox homeostasis and antioxidant balance immediately after a physiologically relevant stress. The significant changes in antioxidant enzyme activity after GSH depletion suggest that thiol status can influence the regulation of other antioxidant enzymes.

The Mineralocorticoid Receptor and Oxidative Stress

Reactive oxygen species are profoundly important for many physiologic functions and are also pivotal to numerous disease processes, particularly those involving inflammation. Much evidence has accrued demonstrating that aldosterone acts locally in many cells aside from those in the cortical collecting duct. Peripheral blood monocytes and vascular smooth muscle cells are both influenced by aldosterone to produce reactive oxygen species. This production contributes to nuclear factor kappaB (NF-kappaB) activation and the genes regulated by this transcription factor. Aldosterone thereby plays an important role in atherosclerosis and hypertension-induced vascular injury. Aldosterone interacts with angiotensin (Ang) II-induced signaling. Both aldosterone and Ang II initiate ERK1/2 and JNK signaling; the effects of the two compounds is additive and involves the epidermal growth factor receptor. Recent data suggest that reactive oxygen species, might contribute to aldosterone production in nonadrenal tissues. A novel oxidized derivative of linoleic acid is a prime candidate in this regard. Oxidative stress may impair mineralocorticoid receptor function by inhibiting aldosterone binding. The latter finding has particularly important implications for elderly persons who exhibit increased oxidative stress and who are at risk for diminished aldosterone function in the distal nephron and subsequent hyperkalemia.

Activation of the ligand-mineralocorticoid receptor functional unit by ancient, classical, and novel ligands. Structure-activity relationship

The mineralocorticoid effect on epithelial cells is the resultant of an intricate net of biochemical regulations that ultimately leads to the maintenance of electrolyte homeostasis. Two key protagonists in this plot are the ligand, which broadcasts the information, and the receptor, which functions as a receiver and transducer. Therefore, the responsibility for the final biological effect is not limited to each individual component but to both of them, so they constitute a functional unit. In addition, several prereceptor regulatory mechanisms are also determinant factors for the final biological response. Because steroids are present in both animals and plants and are derived from common precursors, it is intriguing how these simple molecules have acquired specialization to shape biological development and differentiation. This is particularly true for the function of aldosterone in mammals, which is mimicked by glucocorticoids or progesterone in some particular cases. Inasmuch as the most potent mineralocorticoid in nature, aldosterone, shows a poorly angled steroid nucleus at the A?B-ring junction, and because steroids that possess identical functional groups and different steroidal frames elicit different mineralocorticoid effects, we postulate that a planar conformation of the ligand is a key requirement to acquire potent sodium retention properties. The model takes into consideration all the mechanisms involved in the regulation of the final biological effect, although it does not provide a definitive answer to the original question. It is also discussed how the use of novel mineralocorticoid ligands may shed light on the still obscure mechanism of action of the mineralocorticoid receptor.

Mass spectrometric characterization of the oxidation of the fluorescent lipid peroxidation reporter molecule C11-BODIPY(581/591)

C11-BODIPY(581/591) is a fluorescent lipid peroxidation reporter molecule that shifts its fluorescence from red to green when challenged with oxidizing agents, i.e., reactive oxygen species (ROS) or reactive nitrogen species (RNS). To understand the molecular mechanism responsible for this shift, we studied the molecular rearrangements leading to the shift in fluorescence in C11-BODIPY(581/591). Furthermore, we aimed to determine if these rearrangements were dependent on the nature of the applied ROS, in homogenous solution, bilayer vesicles, and living cells. C11-BODIPY(581/591) was challenged with various ROS- or RNS-generating systems, including peroxynitrite, NO(2)(?), peroxides, and hydroxyl, alkoxyl, tyrosyl, and peroxyl radicals. The reaction products were subsequently analyzed by means of mass spectrometry. Our results show that the initial target for free radical-mediated oxidation is the conjugated diene interconnection between the BODIPY core and the terminal phenyl moiety, which already explains the shift in fluorescence properties of the probe. After oxidative challenge, three different stable products were identified, one of which was specific for oxidation by peroxynitrite. The two other stable end products had lost the entire phenyl moiety, irrespective of the type of radical generating system used. These products were also recovered from Rat-1 fibroblasts stressed either by GSH depletion/serum starvation or by exposure to peroxynitrite, and were the only C11-BODIPY(581/591) oxidation products detectable in these cells.

Molecular mechanism of activation and nuclear translocation of the mineralocorticoid receptor upon binding of pregnanesteroids

The mineralocorticoid receptor (MR) is primarily localized in the cytoplasm of the cell in the absence of ligand. The first step in the genomic-dependent mechanism of action of mineralocorticoids is the binding of steroid to the MR, which in turn triggers MR nuclear translocation. The regulation of hormone-binding to MR is complex and involves a multifactorial mechanism, making it difficult to determine the optimal structure of a steroid for activating the MR and promoting its nuclear translocation. Here we review the structure-activity relationship for several pregnanesteroids that possess various functional groups, and suggest that a flat conformation of the ligand rather than the presence of particular chemical groups is a critical parameter for the final biological effect in vivo. We also discuss how the MR undergoes differential conformational changes according to the nature of the bound ligand, which in turn affects the dynein-dependent retrograde rate of movement for the steroid/receptor complex.

Evidence for altered ETB receptor characteristics during development and progression of ventricular cardiomyocyte hypertrophy

The hypothesis that endothelin (ET) receptor mechanisms are altered during development and progression of left ventricular hypertrophy (LVH) in vivo was tested using spontaneously hypertensive rats (SHRs). Ventricular cardiomyocytes were isolated from SHRs before onset (8 and 12 wk) and during progression (16, 20, and 24 wk) of LVH and compared with age-matched normotensive Wistar-Kyoto (WKY) rats. PreproET-1 mRNA expression was elevated in SHR (P < 0.05) relative to WKY cardiomyocytes at 20-24 wk. ET binding-site density was twofold greater in SHR than WKY cells at 12 wk (P < 0.05) but normalized at 20 wk. ET(B) receptors were detected on SHR cardiomyocytes as early as 8 wk and their affinity increased progressively with age (P < 0.05), whereas ET(B) receptors were not detected on WKY cells until 20 wk. ET-1 stimulated protein synthesis with similar maximum responses between strains (21-30%), in contrast with sarafotoxin 6c, which stimulated protein synthesis in SHR (13-20%) but not WKY cells at 12-20 wk. In SHR but not WKY cells, the ET(B) receptor-selective ligand A-192621 increased protein synthesis progressively with the development of LVH (15% maximum effect). In conclusion, the presence of ET(B) receptors (8-12 wk) coupled with functional responsiveness of SHR cells but not WKY cells to sarafotoxin 6c at 12 wk supports the involvement of ET(B) receptors before the onset of cardiomyocyte hypertrophy, whereas altered ET(B) receptor characteristics during active hypertrophy (16-24 wk) indicate that ET(B) receptor mechanisms may also contribute to disease progression.

1-Cys peroxiredoxin overexpression protects cells against phospholipid peroxidation-mediated membrane damage

1-Cys peroxiredoxin (1-cysPrx) is a novel antioxidant enzyme able to reduce phospholipid hydroperoxides in vitro by using glutathione as a reductant. This enzyme is widely expressed and is enriched in lungs. A fusion protein of green fluorescent protein with 1-cysPrx was stably expressed in a lung-derived cell line (NCI-H441) lacking endogenous enzyme. Overexpressing cells (C17 or C48) degraded H(2)O(2) and t-butylhydroperoxide more rapidly and showed decreased sensitivity to oxidant stress as measured by (51)Cr release. On exposure to (*)OH generated by Cu(2+)-ascorbate (Asc), overexpressing cells compared with H441 showed less increase in thiobarbituric acid-reactive substance and phosphatidylcholine hydroperoxide content. This effect was reversed by depletion of cellular glutathione. Diphenyl-1-pyrenoylphosphonium fluorescence, used as a real-time probe of membrane phospholipid peroxidation, increased immediately on exposure to Cu(2+)-Asc and was abolished by preincubation of cells with Trolox (a soluble vitamin E) or Tempol (a radical scavenger). The rate of diphenyl-1-pyrenoylphosphonium fluorescence increase with Cu(2+)-Asc exposure was markedly attenuated in C17 and C48 cells as compared with H441. Annexin V-Cy3 was used to detect phosphatidylserine translocation from the inner to outer leaflet of the plasma membrane. Cu(2+)-Asc treatment induced phosphatidylserine translocation within 2 h in H441 cells but none was observed in C48 cells up to 24 h. These results indicate that 1-cysPrx can scavenge peroxides but in addition can reduce peroxidized membrane phospholipids. Thus, the enzyme can protect cells against oxidant-induced plasma membrane damage, thereby playing an important role in cellular defense against oxidant stress.

Impairment of mineralocorticoid receptor (MR)-dependent biological response by oxidative stress and aging: correlation with post-translational modification of MR and decreased ADP-ribosylatable level of elongating factor 2 in kidney cells

Acute and chronic treatments of mice with the glutathione-depleting agent, L-buthionine-(SR)-sulfoximine (BSO), impaired the mineralocorticoid receptor (MR)-dependent biological response by inhibiting aldosterone binding. This steroid-binding inhibition was fully reversed when reducing agents were added to kidney cytosol obtained from mice treated for 5 h, but it was only partially reversed in cytosol obtained from mice treated for 10 days. Although the oligomeric structure of the MR-hsp90 heterocomplex was always unaffected, a decreased amount of MR protein was evidenced after the long term treatment. Such a deleterious effect was correlated with a post-translational modification of MR, as demonstrated by an increased level of receptor carbonylation. In addition, a failure at the elongation/termination step was also observed during the receptor translation process in a reticulocyte lysate system. Thus, a high polyribosomes/monomers ratio and both increased proteolysis and decreased ADP-ribosylatable concentration of elongation factor 2 (EF-2) were shown. Importantly, similar observations were also performed in vivo after depletion of glutathione. Notwithstanding the EF-2 functional disruption, not all renal proteins were equally affected as the MR. Interestingly, both EF-2 and MR expressed in old mice were similarly affected as in L-buthionine-(SR)-sulfoximine-treated young mice. We therefore propose that a dramatic depletion of glutathione in kidney cells mimics the cumulative effect of aging which, at the end, may lead to a renal mineralocorticoid dysfunction.

Modification of an essential amino group in the mineralocorticoid receptor evidences a differential conformational change of the receptor protein upon binding of antagonists, natural agonists and the synthetic agonist 11,19-oxidoprogesterone

The alkylation of amino groups of the mineralocorticoid receptor (MR) with pyridoxal 5'-phosphate or 2,4,6-trinitrobenzenesulphonate (TNBS) under controlled conditions modifies only one lysyl residue, which accounts for a 70% inhibition of steroid binding capacity. The Kd of aldosterone for MR is not affected by the treatment, but the total number of binding sites is greatly decreased. The modified receptor is capable of dynamically conserving its association with the hsp90-based heterocomplex. Importantly, the binding of natural agonists protects the hormone binding capacity of the MR from the inactivating action of alkylating agents. In contrast, antagonistic steroids are totally incapable of providing such protection. Like the antagonistic ligands, and despite its potent mineralocorticoid biological effect, the sole MR specific synthetic agonist known to date, 11,19-oxidoprogesterone (11-OP), shows no protective effect upon treatment of the MR with pyridoxal 5'-phosphate or TNBS. Limited digestion of the MR with alpha-chymotrypsin generates a 34 kDa fragment, which becomes totally resistant to digestion upon binding of natural agonists, but not upon binding of antagonists. Interestingly, the synthetic 21-deoxypregnanesteroid 11-OP exhibits an intermediate pattern of proteolytic degradation, suggesting that the conformational change generated in the MR is not equivalent to that induced by antagonists or natural agonists. We conclude that in the first steps of activation, the MR changes its conformation upon binding of the ligand. However, the nature of this conformational change depends on the nature of the ligand. The experimental evidence shown in this work suggests that a single lysyl group can determine the hormone specificity of the MR.