Ca2+ but not H2O2 modulates GRE-element activation by the human mineralocorticoid receptor in HEK cells

Glomerular Mesangial Cell pH Homeostasis Mediates Mineralocorticoid Receptor-Induced Cell Proliferation

Mineralocorticoids (e.g., aldosterone) support chronic inflammatory tissue damage, including glomerular mesangial injury leading to glomerulosclerosis. Furthermore, aldosterone leads to activation of the extracellular signal-regulated kinases (ERK1/2) in rat glomerular mesangial cells (GMC). Because ERK1/2 can affect cellular pH homeostasis via activation of Na⁺/H⁺-exchange (NHE) and the resulting cellular alkalinization may support proliferation, we tested the hypothesis that aldosterone affects pH homeostasis and thereby cell proliferation as well as collagen secretion also in primary rat GMC. Cytoplasmic pH and calcium were assessed by single-cell fluorescence ratio imaging, using the dyes BCECF or FURA2, respectively. Proliferation was determined by cell counting, thymidine incorporation and collagen secretion by collagenase-sensitive proline incorporation and ERK1/2-phosphorylation by Western blot. Nanomolar aldosterone induces a rapid cytosolic alkalinization which is prevented by NHE inhibition (10 µmol/L EIPA) and by blockade of the mineralocorticoid receptor (100 nmol/L spironolactone). pH changes were not affected by inhibition of HCO₃^- transporters and were not dependent on HCO₃^-. Aldosterone enhanced ERK1/2 phosphorylation and inhibition of ERK1/2-phosphorylation (10 µmol/L U0126) prevented aldosterone-induced alkalinization. Furthermore, aldosterone induced proliferation of GMC and collagen secretion, both of which were prevented by U0126 and EIPA. Cytosolic calcium was not involved in this aldosterone action. In conclusion, our data show that aldosterone can induce GMC proliferation via a MR and ERK1/2-mediated activation of NHE with subsequent cytosolic alkalinization. GMC proliferation leads to glomerular hypercellularity and dysfunction. This effect presents a possible mechanism contributing to mineralocorticoid receptor-induced pathogenesis of glomerular mesangial injury during chronic kidney disease.

The selective mineralocorticoid receptor antagonist eplerenone prevents decompensation of the liver in cirrhosis

BACKGROUND AND PURPOSE

The mineralocorticoid receptor (MR) contributes to fibrosis in various tissues, and MR antagonists, like eplerenone, are used to prevent fibrosis. The role of MR antagonists in hepatic fibrosis and cirrhosis is unknown. Here, we investigated the role of MRs and eplerenone in cirrhosis development.

EXPERIMENTAL APPROACH

Liver fibrosis (5 weeks) and cirrhosis, without (8 weeks) and with ascites (12 weeks), were induced by CCl₄ in rats and comprehensively analysed. The effect of eplerenone on the development of cirrhosis with ascites was assessed. MR expression, cellular and subcellular distribution and impact of hypoxia were investigated in vivo and ex vivo. Primary rat hepatocytes and cell lines were used to investigate MR trafficking and transcriptional activity mechanistically.

KEY RESULTS

In cirrhosis with ascites, MR mRNA and protein expressions were reduced in hepatocytes of hypoxic areas. While in normoxic areas MRs were mainly cytosolic, the remaining MRs in hypoxic areas were mainly localized in the nuclei, indicating activation followed by translocation and degradation. Accordingly, eplerenone treatment prevented nuclear MR translocation and the worsening of cirrhosis. Exposing hepatocytes ex vivo to hypoxia induced nuclear MR translocation and enhanced transcriptional MR activity at response elements of the NF-κB pathway.

CONCLUSIONS AND IMPLICATIONS

We showed for the first time that hypoxia leads to a pathogenetic ligand-independent activation of hepatic MRs during cirrhosis resulting in their nuclear translocation and transcriptional activation of the NF-κB pathway. Treatment with eplerenone prevented the worsening of cirrhosis by blocking this ligand-independent activation of the MR.

Nuclear shuttling precedes dimerization in mineralocorticoid receptor signaling

The mineralocorticoid receptor (MR), a member of the steroid receptor superfamily, regulates water-electrolyte balance and mediates pathophysiological effects in the renocardiovascular system. Previously, it was assumed that after binding aldosterone, the MR dissociates from HSP90, forms homodimers, and then translocates into the nucleus where it acts as a transcription factor (Guiochon-Mantel et al., 1989; Robertson et al., 1993; Savory et al., 2001). We found that, during aldosterone-induced nuclear translocation, MR is bound to HSP90 both in the cytosol and the nucleus. Homodimerization measured by eBRET and FRET takes place when the MR is already predominantly nuclear. In vitro binding of MR to DNA was independent of ligand but could be partially inhibited by geldanamycin. Overall, here we provide insights into classical MR signaling necessary for elucidating the mechanisms of pathophysiological MR effects and MR specificity.

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.

Impact of extracellular acidosis on intracellular pH control and cell signaling in tumor cells

Cells in solid tumors generate an extracellular acidosis due to the Warburg effect and tissue hypoxia. Acidosis can affect the functional behavior of tumor cells, causing, e.g., multidrug resistance. In this process ERK1/2 and p38 mitogen-activated protein kinases (MAPK) seem to play a key role. However, the underlying mechanism of MAPK activation by extracellular acidosis remains unclear. Experiments were performed in three tumor and three normal tissue cell lines in which the cells were exposed to an extracellular pH of 6.6 for 3 h. Intracellular pH (pHi), protein expression and activation, acidosis-induced transactivation, and reactive oxygen species (ROS) formation were measured. Extracellular acidosis resulted in a rapid and sustained decrease of pHi leading to a reversal of the extra-/intracellular pH gradient. Extracellular acidosis led to p38 phosphorylation in all cell types and to ERK1/2 phosphorylation in three of six cell lines. Furthermore, p38 phosphorylation was also observed during sole intracellular lactacidosis at normal pHe. Acidosis-enhanced formation of ROS, probably originating from mitochondria, seems to trigger MAPK phosphorylation. Finally, acidosis increased phosphorylation of the transcription factor CREB and resulted in increased transcriptional activity. Thus, an acidic tumor microenvironment can induce a longer-lasting p38 CREB-mediated change in the transcriptional program.

Modulation of transcriptional mineralocorticoid receptor activity by nitrosative stress

The mineralocorticoid receptor (MR) plays an important role in salt and water homeostasis and pathological tissue modifications, such as cardiovascular and renal fibrosis. Importantly, MR activation by aldosterone per se is not sufficient for the deleterious effects but requires the additional presence of a certain pathological milieu. Phenomenologically, this milieu could be generated by enhanced nitrosative stress. However, little is known regarding the modulation of MR transcriptional activity in a pathological milieu. The glucocorticoid receptor (GR), the closest relative of the MR, binds to the same hormone-response element but elicits protective effects on the cardiovascular system. To investigate the possible modulation of MR and GR by nitrosative stress under controlled conditions we used human embryonic kidney (HEK) cells and measured MR and GR transactivation after stimulation with the nitric oxide (NO)-donor SNAP and the peroxynitrite-donor Sin-1. In the presence of corticosteroids NO led to a general reduced corticosteroid receptor activity by repression of corticosteroid receptor-DNA interaction. The NO-induced diminished transcriptional MR activity was most pronounced during stimulation with physiological aldosterone concentrations, suggesting that NO treatment prevented its pathophysiological overactivation. In contrast, single peroxynitrite administration specifically induced the MR transactivation activity whereas genomic GR activity remained unchanged. Mechanistically, peroxynitrite permitted nuclear MR translocation whereas the cytosolic GR distribution was unaffected. Consequently, peroxynitrite represents a MR-specific aldosterone mimetic. In summary, our data indicate that the genomic function of corticosteroid receptors can be modulated by nitrosative stress which may induce the shift from physiological toward pathophysiological MR effects.

Differential ubiquitylation of the mineralocorticoid receptor is regulated by phosphorylation

Aldosterone stimulation of the mineralocorticoid receptor (MR) is involved in numerous physiological responses, including Na+ homeostasis, blood pressure control, and heart failure. Aldosterone binding to MR promotes different post-translational modifications that regulate MR nuclear translocation, gene expression, and finally receptor degradation. Here, we show that aldosterone stimulates rapid phosphorylation of MR via ERK1/2 in a dose-dependent manner (from 0.1 to 10 nM) in renal epithelial cells. This phosphorylation induces an increase of MR apparent molecular weight, with a maximal upward shift of 30 kDa. Strikingly, these modifications are critical for the regulation of the MR ubiquitylation state. Indeed, we find that MR is monoubiquitylated in its basal state, and this status is sustained by the tumor suppressor gene 101 (Tsg101). Phosphorylation leads to disruption of MR/Tsg101 association and monoubiquitin removal. These events prompt polyubiquitin-dependent destabilization of MR and degradation. Preventing MR phosphorylation by ERK1/2 inhibition or mutation of target serines affects the sequential mechanisms of MR ubiquitylation and inhibits the aldosterone-mediated degradation. Our data provide a novel model of negative feedback of aldosterone signaling, involving sequential phosphorylation, monoubiquitin removal and subsequent polyubiquitylation/degradation of MR.

Tau phosphorylation and μ-calpain activation mediate the dexamethasone-induced inhibition on the insulin-stimulated Akt phosphorylation

Evidence has suggested that insulin resistance (IR) or high levels of glucocorticoids (GCs) may be linked with the pathogenesis and/or progression of Alzheimer's disease (AD). Although studies have shown that a high level of GCs results in IR, little is known about the molecular details that link GCs and IR in the context of AD. Abnormal phosphorylation of tau and activation of μ-calpain are two key events in the pathology of AD. Importantly, these two events are also related with GCs and IR. We therefore speculate that tau phosphorylation and μ-calpain activation may mediate the GCs-induced IR. Akt phosphorylation at Ser-473 (pAkt) is commonly used as a marker for assessing IR. We employed two cell lines, wild-type HEK293 cells and HEK293 cells stably expressing the longest human tau isoform (tau-441; HEK293/tau441 cells). We examined whether DEX, a synthetic GCs, induces tau phosphorylation and μ-calpain activation. If so, we examined whether the DEX-induced tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. The results showed that DEX increased tau phosphorylation and induced tau-mediated μ-calpain activation. Furthermore, pre-treatment with LiCl prevented the effects of DEX on tau phosphorylation and μ-calpain activation. Finally, both LiCl pre-treatment and calpain inhibition prevented the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. In conclusion, our study suggests that the tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation.

Acidic environment leads to ROS-induced MAPK signaling in cancer cells

Tumor micromilieu often shows pronounced acidosis forcing cells to adapt their phenotype towards enhanced tumorigenesis induced by altered cellular signalling and transcriptional regulation. In the presents study mechanisms and potential consequences of the crosstalk between extra- and intracellular pH (pH_e, pH_i) and mitogen-activated-protein-kinases (ERK1/2, p38) was analyzed. Data were obtained mainly in AT1 R-3327 prostate carcinoma cells, but the principle importance was confirmed in 5 other cell types. Extracellular acidosis leads to a rapid and sustained decrease of pH_i in parallel to p38 phosphorylation in all cell types and to ERK1/2 phosphorylation in 3 of 6 cell types. Furthermore, p38 phosphorylation was elicited by sole intracellular lactacidosis at normal pH_e. Inhibition of ERK1/2 phosphorylation during acidosis led to necrotic cell death. No evidence for the involvement of the kinases c-SRC, PKC, PKA, PI3K or EGFR nor changes in cell volume in acidosis-induced MAPK activation was obtained. However, our data reveal that acidosis enhances the formation of reactive oxygen species (ROS), probably originating from mitochondria, which subsequently trigger MAPK phosphorylation. Scavenging of ROS prevented acidosis-induced MAPK phosphorylation whereas addition of H₂O₂ enhanced it. Finally, acidosis increased phosphorylation of the transcription factor CREB via p38, leading to increased transcriptional activity of a CRE-reporter even 24 h after switching the cells back to a normal environmental milieu. Thus, an acidic tumor microenvironment can induce a longer lasting p38-CREB-medited change in the transcriptional program, which may maintain the altered phenotype even when the cells leave the tumor environment.

The living microarray: a high-throughput platform for measuring transcription dynamics in single cells

Background

Current methods of measuring transcription in high-throughput have led to significant improvements in our knowledge of transcriptional regulation and Systems Biology. However, endpoint measurements obtained from methods that pool populations of cells are not amenable to studying time-dependent processes that show cell heterogeneity.

Results

Here we describe a high-throughput platform for measuring transcriptional changes in real time in single mammalian cells. By using reverse transfection microarrays we are able to transfect fluorescent reporter plasmids into 600 independent clusters of cells plated on a single microscope slide and image these clusters every 20 minutes. We use a fast-maturing, destabilized and nuclear-localized reporter that is suitable for automated segmentation to accurately measure promoter activity in single cells. We tested this platform with synthetic drug-inducible promoters that showed robust induction over 24 hours. Automated segmentation and tracking of over 11 million cell images during this period revealed that cells display substantial heterogeneity in their responses to the applied treatment, including a large proportion of transfected cells that do not respond at all.

Conclusions

The results from our single-cell analysis suggest that methods that measure average cellular responses, such as DNA microarrays, RT-PCR and chromatin immunoprecipitation, characterize a response skewed by a subset of cells in the population. Our method is scalable and readily adaptable to studying complex systems, including cell proliferation, differentiation and apoptosis.

Interaction between mineralocorticoid receptor and cAMP/CREB signaling

Besides regulating water and electrolyte homeostasis, the mineralocorticoid receptor (MR) elicits pathophysiological effects in the renocardiovascular system. Although the MR's closest relative, the glucocorticoid receptor (GR), acts as a transcription factor at the same hormone-response-element (HRE), activated glucocorticoid receptor mediates very different effects. One explanation for this discrepancy is that the MR interacts with additional signaling pathways in the cytosol. In the literature, there are several indications for an interaction between aldosterone/MR and the cAMP/CREB signaling. Here we summarize the current knowledge of the cross-talk between the two signaling pathways, including some unpublished observations of our own that indicate that MR/CREB signaling is mediated by calcineurin and has potentially pathophysiological consequences.

Mineralocorticoid receptor inhibits CREB signaling by calcineurin activation

We investigated the interaction of MR with cAMP-response element binding protein (CREB) and provide a mechanistic explanation and insights into the cellular relevance. MR --> CREB crosstalk was assessed in vascular smooth muscle cells and heterologous expression systems. Experiments were designed in a way that only one variable changed at a time and the respective vehicles served as controls. MR, but not GR, activation (aldosterone or hydrocortisone, IC(50), approximately 0.3 nM) inhibits CREB transcriptional activity induced by stimulation of beta1/2-adrenoceptors and adenylyl cyclase or addition of membrane-permeable cAMP up to 70% within 2 h after addition. The MR DNA-binding domain is not required for this inhibition. cAMP formation is virtually unchanged, whereas MR exerts a robust inhibition of CREB(S133) phosphorylation via calcineurin/PP2B activation without changes in PP2B-Aalpha or beta expression. In parallel, the PP2B-sensitive NFaT-pathway is activated. The inhibitory crosstalk attenuates CREB-induced glucose-6-phosphate dehydrogenase expression. Overall, transcriptional relevant MR --> CREB crosstalk occurs at the level of CREB phosphorylation by enhanced calcineurin activity, enables GRE-independent genomic signaling of MR, and is of potential pathophysiological relevance.

Targeting SGK1 in diabetes

Compelling evidence is accumulating indicating a pathophysiological role of the serum-and-glucocorticoid-inducible-kinase-1 (SGK1) in the development and complications of diabetes. SGK1 is ubiquitously expressed with exquisitely high transcriptional volatility. Stimulators of SGK1 expression include hyperglycemia, cell shrinkage, ischemia, glucocorticoids and mineralocorticoids. SGK1 is activated by insulin and growth factors via PI3K, 3-phosphoinositide dependent kinase PDK1 and mTOR. SGK1 activates ion channels (including ENaC, TRPV5, ROMK, KCNE1/KCNQ1 and CLCKa/Barttin), carriers (including NCC, NKCC, NHE3, SGLT1 and EAAT3), and the Na(+)/K(+)-ATPase. It regulates the activity of several enzymes (e.g., glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g., forkhead-transcription-factor FOXO3a, beta-catenin and NF-kappaB). A common SGK1 gene variant ( approximately 3 - 5% prevalence in Caucasians, approximately 10% in Africans) is associated with increased blood pressure, obesity and type 2 diabetes. In patients suffering from type 2 diabetes, SGK1 presumably contributes to fluid retention and hypertension, enhanced coagulation and increased deposition of matrix proteins leading to tissue fibrosis such as diabetic nephropathy. Accordingly, targeting SGK1 may favourably influence occurrence and course of type 2 diabetes.

The physiological impact of the serum and glucocorticoid-inducible kinase SGK1

PURPOSE OF REVIEW

The role of serum and glucocorticoid-inducible kinase 1 (SGK1) in renal physiology and pathophysiology is reviewed with particular emphasis on recent advances.

RECENT FINDINGS

The mammalian target of rapamycin complex 2 has been shown to phosphorylate SGK1 at Ser422 (the so-called hydrophobic motif). Ser397 and Ser401 are two additional SGK1-phosphorylation sites required for maximal SGK1 activity. A 5' variant alternate transcript of human Sgk1 has been identified that is widely expressed and shows improved stability, enhanced membrane association, and greater stimulation of epithelial Na+ transport. SGK1 is essential for optimal processing of the epithelial sodium channel and also regulates the expression of the Na+-Cl- cotransporter. With regard to pathophysiology, SGK1 participates in the stimulation of renal tubular glucose transport in diabetes, the renal profibrotic effect of both angiotensin II and aldosterone, and in fetal programing of arterial hypertension.

SUMMARY

The outlined recent findings advanced our understanding of the molecular regulation of SGK1 as well as the role of the kinase in renal physiology and the pathophysiology of renal disease and hypertension. Future studies using pharmacological inhibitors of SGK1 will reveal the utility of the kinase as a new therapeutic target.

An obligatory heterodimer of 14-3-3beta and 14-3-3epsilon is required for aldosterone regulation of the epithelial sodium channel

Increased distal nephron sodium absorption in response to aldosterone involves Nedd4-2 phosphorylation, which blocks its ability to ubiquitylate ENaC and increases apical membrane channel density by reducing its endocytosis. Our prior work (Liang, X., Peters, K. W., Butterworth, M. B., and Frizzell, R. A. (2006) J. Biol. Chem. 281, 16323-16332) showed that aldosterone selectively increased 14-3-3 protein isoform expression and that the association of 14-3-3beta with phospho-Nedd4-2 was required for sodium transport stimulation. The knockdown of 14-3-3beta alone nearly eliminated the response to aldosterone, despite the expression of other 14-3-3 isoforms in cortical collecting duct (CCD) cells. To further examine this marked effect of 14-3-3beta knockdown, we evaluated the hypothesis that phospho-Nedd4-2 binding prefers a heterodimer composed of two different 14-3-3 isoforms. We tested this concept in polarized CCD cells using RNA interference and assays of sodium transport and of the interaction of Nedd4-2 with 14-3-3epsilon, a second aldosterone-induced isoform. As observed previously for 14-3-3beta knockdown, small interfering RNA-induced reduction of 14-3-3epsilon markedly attenuated aldosterone-stimulated ENaC expression and sodium transport and increased the interaction of Nedd4-2 with ENaC toward prealdosterone levels. After aldosterone induction, 14-3-3beta and 14-3-3epsilon were quantitatively co-immunoprecipitated from CCD cell lysates, and the association of both isoforms with Nedd4-2 increased. Finally, the knockdown of either 14-3-3beta or 14-3-3epsilon reduced the association of Nedd4-2 with the other isoform. We conclude that the two aldosterone-induced 14-3-3 isoforms, beta and epsilon, interact with phospho-Nedd4-2 as an obligatory heterodimer, blocking its interaction with ENaC and thereby increasing apical ENaC density and sodium transport.

Non-classical actions of the mineralocorticoid receptor: misuse of EGF receptors?

The mineralocorticoid receptor (MR) plays a key role in cardiovascular and renal injury. The underlying mechanisms seem to involve the epidermal growth factor receptor (EGFR) for the development of fibrosis and vascular dysfunction. Both enhanced EGFR transactivation by activated MR as well as upregulation of EGFR expression by aldosterone-bound MR have been described. While the former seems to be mediated by the tyrosine kinase cSrc, reporter gene assays and chromatin immunoprecipitation data indicate that the latter is caused by an interaction between MR and the EGFR promoter. Pharmacological inhibition of EGFR function prevents some of MR's pathological actions in cell culture systems, like vascular smooth muscle cells. Thus, transactivation as well as enhanced expression of EGFR may be an important switch for the pathophysiological actions in the reno-cardiovascular continuum. Furthermore, EGFR signaling may serve as a negative feedback loop to limit sodium retention. Overall, MR's "misuse" of the EGFR is one possible explanation for the pathophysiological effects of aldosterone, making the EGFR a potential target for therapeutical interventions against reno-cardiovascular remodelling.