SGK1 as a determinant of kidney function and salt intake in response to mineralocorticoid excess

Control of sodium appetite by hindbrain aldosterone-sensitive neurons

Mineralocorticoids play a key role in hydromineral balance by regulating sodium retention and potassium wasting. Through favoring sodium, mineralocorticoids can cause hypertension from fluid overload under conditions of hyperaldosteronism, such as aldosterone-secreting tumors. An often-overlooked mechanism by which aldosterone functions to increase sodium is through stimulation of salt appetite. To drive sodium intake, aldosterone targets neurons in the hindbrain which uniquely express 11β-hydroxysteroid dehydrogenase type 2 (HSD2). This enzyme is a necessary precondition for aldosterone-sensing cells as it metabolizes glucocorticoids - preventing their activation of the mineralocorticoid receptor. In this review, we will consider the role of hindbrain HSD2 neurons in regulating sodium appetite by discussing HSD2 expression in the brain, regulation of hindbrain HSD2 neuron activity, and the circuitry mediating the effects of these aldosterone-sensitive neurons. Reducing the activity of hindbrain HSD2 neurons may be a viable strategy to reduce sodium intake and cardiovascular risk, particularly for conditions of hyperaldosteronism.

Endothelial cell serum and glucocorticoid regulated kinase 1 (SGK1) mediates vascular stiffening

BACKGROUND

Excessive dietary salt intake increases vascular stiffness in humans, especially in salt-sensitive populations. While we recently suggested that the endothelial sodium channel (EnNaC) contributes to salt-sensitivity related endothelial cell (EC) and arterial stiffening, mechanistic understanding remains incomplete. This study therefore aimed to explore the role of EC-serum and glucocorticoid regulated kinase 1 (SGK1), as a reported regulator of sodium channels, in EC and arterial stiffening.

METHODS AND RESULTS

A mouse model of salt sensitivity-associated vascular stiffening was produced by subcutaneous implantation of slow-release deoxycorticosterone acetate (DOCA) pellets, with salt (1 % NaCl, 0.2 % KCl) administered via drinking water. Preliminary data showed that global SGK1 deletion caused significantly decreased blood pressure (BP), EnNaC activity and aortic endothelium stiffness as compared to control mice following DOCA-salt treatment. To probe EC signaling pathways, selective deletion of EC-SGK1 was performed by cross-breeding cadherin 5-Cre mice with sgk1flox/flox mice. DOCA-salt treated control mice had significantly increased BP, EC and aortic stiffness in vivo and ex vivo, which were attenuated by EC-SGK1 deficiency. To demonstrate relevance to humans, human aortic ECs were cultured in the absence or presence of aldosterone and high salt with or without the SGK1 inhibitor, EMD638683 (10uM or 25uM). Treatment with aldosterone and high salt increased intrinsic stiffness of ECs, which was prevented by SGK1 inhibition. Further, the SGK1 inhibitor prevented aldosterone and high salt induced actin polymerization, a key mechanism in cellular stiffening.

CONCLUSION

EC-SGK1 contributes to salt-sensitivity related EC and aortic stiffening by mechanisms appearing to involve regulation of actin polymerization.

Dietary High Salt Intake Exacerbates SGK1-Mediated T Cell Pathogenicity in L-NAME/High Salt-Induced Hypertension

Sodium chloride (NaCl) activates Th17 and dendritic cells in hypertension by stimulating serum/glucocorticoid kinase 1 (SGK1), a sodium sensor. Memory T cells also play a role in hypertension by infiltrating target organs and releasing proinflammatory cytokines. We tested the hypothesis that the role of T cell SGK1 extends to memory T cells. We employed mice with a T cell deletion of SGK1, SGK1fl/fl × tgCD4cre mice, and used SGK1fl/fl mice as controls. We treated the mice with L-NAME (0.5 mg/mL) for 2 weeks and allowed a 2-week washout interval, followed by a 3-week high-salt (HS) diet (4% NaCl). L-NAME/HS significantly increased blood pressure and memory T cell accumulation in the kidneys and bone marrow of SGK1fl/fl mice compared to knockout mice on L-NAME/HS or groups on a normal diet (ND). SGK1fl/fl mice exhibited increased albuminuria, renal fibrosis, and interferon-γ levels after L-NAME/HS treatment. Myography demonstrated endothelial dysfunction in the mesenteric arterioles of SGK1fl/fl mice. Bone marrow memory T cells were adoptively transferred from either mouse strain after L-NAME/HS administration to recipient CD45.1 mice fed the HS diet for 3 weeks. Only the mice that received cells from SGK1fl/fl donors exhibited increased blood pressure and renal memory T cell infiltration. Our data suggest a new therapeutic target for decreasing hypertension-specific memory T cells and protecting against hypertension.

Endothelial sodium channel activation mediates DOCA-salt-induced endothelial cell and arterial stiffening

INTRODUCTION

High salt intake and aldosterone are both associated with vascular stiffening in humans. However, our preliminary work showed that high dietary salt alone did not increase endothelial cell (EC) or vascular stiffness or endothelial sodium channel (EnNaC) activation in mice, presumably because aldosterone production was significantly suppressed as a result of the high salt diet. We thus hypothesized that high salt consumption along with an exogenous mineralocorticoid would substantially increase EC and vascular stiffness via activation of the EnNaC.

METHODS AND RESULTS

Mice were implanted with slow-release DOCA pellets and given salt in their drinking water for 21 days. Mice with either specific deletion of the alpha subunit of EnNaC or treated with a pharmacological inhibitor of mTOR, a downstream signaling molecule involved in mineralocorticoid receptor activation of EnNaC, were studied. DOCA-salt treated control mice had increased blood pressure, EC Na⁺ transport activity, EC and arterial stiffness, which were attenuated in both the αEnNaC^-/- and mTOR inhibitor treated groups. Further, depletion of αEnNaC prevented DOCA-salt-induced impairment in EC-dependent vascular relaxation.

CONCLUSION

While high salt consumption alone does not cause EC or vascular stiffening, the combination of EC MR activation and high salt causes activation of EnNaC which increases EC and arterial stiffness and impairs vascular relaxation. Underlying mechanisms appear to include mTOR signaling.

Effects of Black Garlic Extract and Nanoemulsion on the Deoxy Corticosterone Acetate-Salt Induced Hypertension and Its Associated Mild Cognitive Impairment in Rats

Organosulfur compounds, phenolic acids and flavonoids in raw and black garlic were determined, and followed by preparation of black garlic nanoemulsion for studying their effects on deoxycorticosterone acetate-salt-induced hypertension and associated mild cognitive impairment in rats. Three organosulfur compounds, including diallyl sulfide (87.8 μg/g), diallyl disulfide (203.9 μg/g) and diallyl trisulfide (282.6 μg/g) were detected in black garlic by GC-MS, while gallic acid (19.19 μg/g), p-coumaric acid (27.03 μg/g) and quercetin (22.77 μg/g) were detected by UPLC-MS/MS. High doses of both black garlic extract and nanoemulsion prepared using Tween-80, glycerol, grapeseed oil and water could decrease systolic blood pressure through the elevation of bradykinin and nitric oxide levels as well as diminish aldosterone and angiotensin II levels in rats. In Morris water maze test, they could significantly decrease escape latency and swimming distance and increase the time spent in the target quadrant, accompanied by a decline of acetylcholinesterase activity and malondialdehyde level in the hippocampus as well as a rise in glutathione level and activities of superoxide dismutase, catalase and glutathione peroxidase. In addition, the levels of tumor necrosis factor, interleukin-6 and interleukin-1β were reduced. Effects of lowering blood pressure and improving learning/memory ability in rats followed the order: lisinopril > black garlic nanoemulsion > black garlic extract.

Empagliflozin Inhibits IL-1β-Mediated Inflammatory Response in Human Proximal Tubular Cells

SGLT2 inhibitor-related nephroprotection is—at least partially—mediated by anti-inflammatory drug effects, as previously demonstrated in diabetic animal and human studies, as well as hyperglycemic cell culture models. We recently presented first evidence for anti-inflammatory potential of empagliflozin (Empa) under normoglycemic conditions in human proximal tubular cells (HPTC) by demonstrating Empa-mediated inhibition of IL-1β-induced MCP-1/CCL2 and ET-1 expression on the mRNA and protein level. We now add corroborating evidence on a genome-wide level by demonstrating that Empa attenuates the expression of several inflammatory response genes in IL-1β-induced (10 ng/mL) normoglycemic HPTCs. Using microarray-hybridization analysis, 19 inflammatory response genes out of >30.000 human genes presented a consistent expression pattern, that is, inhibition of IL-1β (10 ng/mL)-stimulated gene expression by Empa (500 nM), in both HK-2 and RPTEC/TERT1 cells. Pathway enrichment analysis demonstrated statistically significant clustering of annotated pathways (enrichment score 3.64). Our transcriptomic approach reveals novel genes such as CXCL8/IL8, LOX, NOV, PTX3, and SGK1 that might be causally involved in glycemia-independent nephroprotection by SGLT2i.

Increased SGK1 activity potentiates mineralocorticoid/NaCl-induced kidney injury

Serum and glucocorticoid-regulated kinase 1 (SGK1) stimulates aldosterone-dependent renal Na⁺ reabsorption and modulates blood pressure. In addition, genetic ablation or pharmacological inhibition of SGK1 limits the development of kidney inflammation and fibrosis in response to excess mineralocorticoid signaling. In this work, we tested the hypothesis that a systemic increase in SGK1 activity would potentiate mineralocorticoid/salt-induced hypertension and kidney injury. To that end, we used a transgenic mouse model with increased SGK1 activity. Mineralocorticoid/salt-induced hypertension and kidney damage was induced by unilateral nephrectomy and treatment with deoxycorticosterone acetate and NaCl in the drinking water for 6 wk. Our results show that although SGK1 activation did not induce significantly higher blood pressure, it produced a mild increase in glomerular filtration rate, increased albuminuria, and exacerbated glomerular hypertrophy and fibrosis. Transcriptomic analysis showed that extracellular matrix- and immune response-related terms were enriched in the downregulated and upregulated genes, respectively, in transgenic mice. In conclusion, we propose that systemically increased SGK1 activity is a risk factor for the development of mineralocorticoid-dependent kidney injury in the context of low renal mass and independently of blood pressure.NEW & NOTEWORTHY Increased activity of the protein kinase serum and glucocorticoid-regulated kinase 1 may be a risk factor for accelerated renal damage. Serum and glucocorticoid-regulated kinase 1 expression could be a marker for the rapid progression toward chronic kidney disease and a potential therapeutic target to slow down the process.

Serum and Glucocorticoid-Inducible Kinase 1 (SGK1) in NSCLC Therapy

Non-small cell lung cancer (NSCLC) remains the most prevalent and one of the deadliest cancers worldwide. Despite recent success, there is still an urgent need for new therapeutic strategies. It is also becoming increasingly evident that combinatorial approaches are more effective than single modality treatments. This review proposes that the serum and glucocorticoid-inducible kinase 1 (SGK1) may represent an attractive target for therapy of NSCLC. Although ubiquitously expressed, SGK1 deletion in mice causes only mild defects of ion physiology. The frequent overexpression of SGK1 in tumors is likely stress-induced and provides a therapeutic window to spare normal tissues. SGK1 appears to promote oncogenic signaling aimed at preserving the survival and fitness of cancer cells. Most importantly, recent investigations have revealed the ability of SGK1 to skew immune-cell differentiation toward pro-tumorigenic phenotypes. Future studies are needed to fully evaluate the potential of SGK1 as a therapeutic target in combinatorial treatments of NSCLC. However, based on what is currently known, SGK1 inactivation can result in anti-oncogenic effects both on tumor cells and on the immune microenvironment. A first generation of small molecules to inactivate SGK1 has already been already produced.

Sodium butyrate ameliorates deoxycorticosterone acetate/salt-induced hypertension and renal damage by inhibiting the MR/SGK1 pathway

Our recent work demonstrates that infusion of sodium butyrate (NaBu) into the renal medulla blunts angiotensin II-induced hypertension and improves renal injury. The present study aimed to test whether oral administration of NaBu attenuates salt-sensitive hypertension in deoxycorticosterone acetate (DOCA)/salt-treated rats. Uninephrectomized male Sprague-Dawley (SD) rats were treated with DOCA pellets (150 mg/rat) plus 1% NaCl drinking water for 2 weeks. Animals received oral administration of NaBu (1 g/kg) or vehicle once per day. Our results showed that NaBu administration significantly attenuated DOCA/salt-increased mean arterial pressure from 156 ± 4 mmHg to 136 ± 1 mmHg. DOCA/salt treatment markedly enhanced renal damage as indicated by an increased ratio of kidney weight/body weight, elevated urinary albumin, extensive fibrosis, and inflammation, whereas kidneys from NaBu-treated rats exhibited a significant reduction in these renal damage responses. Compared to the DOCA/salt group, the DOCA/salt-NaBu group had ~30% less salt water intake and decreased Na⁺ and Cl^- excretion in urine but no alteration in 24-h urine excretion. Mechanistically, NaBu inhibited the protein levels of several sodium transporters stimulated by DOCA/salt in vivo, such as βENaC, γENaC, NCC, and NKCC-2. Further examination showed that NaBu downregulated the expression of mineralocorticoid receptor (MR) and serum and glucocorticoid-dependent protein kinase 1 (SGK1) in DOCA/salt-treated rats or aldosterone-treated human renal tubular duct epithelial cells. These results provide evidence that NaBu may attenuate DOCA/salt-induced hypertension and renal damage by inhibiting the MR/SGK1 pathway.

The role of sodium in modulating immune cell function

Sodium intake is undoubtedly indispensable for normal body functions but can be detrimental when taken in excess of dietary requirements. The consequences of excessive salt intake are becoming increasingly clear as high salt consumption persists across the globe. Salt has long been suspected to promote the development of hypertension and cardiovascular diseases and is now also recognized as a potential modulator of inflammatory and autoimmune diseases through its direct and indirect effects on immune cells. The finding that, in addition to the kidneys, other organs such as the skin regulate sodium levels in the body prompted new hypotheses, including the concept that skin-resident macrophages might participate in tissue sodium regulation through their interactions with lymphatic vessels. Moreover, immune cells such as macrophages and different T cell subsets are found in sodium-rich interstitial microenvironments, where sodium levels modulate their function. Alterations to the intestinal bacterial community induced by excess dietary salt represent another relevant axis whereby salt indirectly modulates immune cell function. Depending on the inflammatory context, sodium might either contribute to protective immunity (for example, by enhancing host responses against cutaneous pathogens) or it might contribute to immune dysregulation and promote the development of cardiovascular and autoimmune diseases.

Aldosterone Mediated Regulation of Epithelial Sodium Channel (ENaC) Subunits in the Rat Hypothalamus

Current evidence suggests that the epithelial Na⁺ channel (ENaC) in the brain plays a significant role in the development of hypertension. ENaC is present in vasopressin (VP) neurons in the hypothalamus, suggesting that ENaC in VP neurons is involved in the regulation of blood pressure. Our recent study demonstrated that high dietary salt intake caused an increase in the expression and activity of ENaC that were responsible for the more depolarized basal membrane potential in VP neurons. A known regulator of ENaC expression, the mineralocorticoid receptor (MR), is present in VP neurons, suggesting that ENaC expression in VP neurons is regulated by aldosterone. In this study, the effects of aldosterone and corticosterone on ENaC were examined in acute hypothalamic slices. Real-time PCR and Western blot analysis showed that aldosterone and corticosterone treatment resulted in a significant increase in the expression of γENaC, but not α- or βENaC, and that this expression was attenuated by MR and glucocorticoid receptor (GR) antagonists. Moreover, chromatin immunoprecipitation demonstrated that the aldosterone-MR complex directly interacts with the promoter region of the γENaC gene. However, the treatment with aldosterone did not cause subcellular translocation of ENaC toward the plasma membrane nor an increase in ENaC Na⁺-leak current. These results indicate that expression of γENaC in VP neurons is induced by aldosterone and corticosterone through their MR and GR, respectively; however, aldosterone or corticosterone alone is not sufficient enough to increase ENaC current when they are applied to hypothalamic slices in vitro.

Serum and Glucocorticoid Regulated Kinase 1 in Sodium Homeostasis

The ubiquitously expressed serum and glucocorticoid regulated kinase 1 (SGK1) is tightly regulated by osmotic and hormonal signals, including glucocorticoids and mineralocorticoids. Recently, SGK1 has been implicated as a signal hub for the regulation of sodium transport. SGK1 modulates the activities of multiple ion channels and carriers, such as epithelial sodium channel (ENaC), voltage-gated sodium channel (Nav1.5), sodium hydrogen exchangers 1 and 3 (NHE1 and NHE3), sodium-chloride symporter (NCC), and sodium-potassium-chloride cotransporter 2 (NKCC2); as well as the sodium-potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) and type A natriuretic peptide receptor (NPR-A). Accordingly, SGK1 is implicated in the physiology and pathophysiology of Na⁺ homeostasis. Here, we focus particularly on recent findings of SGK1’s involvement in Na⁺ transport in renal sodium reabsorption, hormone-stimulated salt appetite and fluid balance and discuss the abnormal SGK1-mediated Na⁺ reabsorption in hypertension, heart disease, edema with diabetes, and embryo implantation failure.

SGK1-dependent ENaC processing and trafficking in mice with high dietary K intake and elevated aldosterone

We examined renal Na and K transporters in mice with deletions in the gene encoding the aldosterone-induced protein SGK1. The knockout mice were hyperkalemic, and had altered expression of the subunits of the epithelial Na channel (ENaC). The kidneys showed decreased expression of the cleaved forms of the γENaC subunit, and the fully glycosylated form of the βENaC subunits when animals were fed a high-K diet. Knockout animals treated with exogenous aldosterone also had reduced subunit processing and diminished surface expression of βENaC and γENaC. Expression of the three upstream Na transporters NHE3, NKCC2, and NCC was reduced in both wild-type and knockout mice in response to K loading. The activity of ENaC measured as whole cell amiloride-sensitive current (I_Na) in principal cells of the cortical collecting duct (CCD) was minimal under control conditions but was increased by a high-K diet to a similar extent in knockout and wild-type animals. I_Na in the connecting tubule also increased similarly in the two genotypes in response to exogenous aldosterone administration. The activities of both ROMK channels in principal cells and BK channels in intercalated cells of the CCD were unaffected by the deletion of SGK1. Acute treatment of animals with amiloride produced similar increases in Na excretion and decreases in K excretion in the two genotypes. The absence of changes in ENaC activity suggests compensation for decreased surface expression. Altered K balance in animals lacking SGK1 may reflect defects in ENaC-independent K excretion.

Determinants of water and sodium intake and output

Physiological regulation of sodium and water intake and output is required for the maintenance of homeostasis. The behavioral and neuroendocrine mechanisms that govern fluid and salt balance are highly interdependent, with acute and chronic alterations in renal output tightly balanced by appropriate changes in thirst and, to a lesser extent in humans, sodium appetite. In healthy individuals, these tightly coupled mechanisms maintain extracellular fluid volume and body tonicity within a narrow homeostatic range by initiating ingestive behaviors and the release of hormones necessary to conserve water and sodium within the body. In this review, the factors that determine output of sodium and fluid and those that determine "normal" input (i.e., matched to output) are addressed. For output, individual variability accompanied by dysregulation of homeostatic mechanisms may contribute to acute and/or chronic disease. To illustrate that point, the specific condition of salt-sensitive hypertension is discussed. For input, physical characteristics, physiological phenotypes, genetic and developmental influences, and cultural and environmental factors combine to result in a wide range of individual variability that, in humans, is compensated for by alterations in excretion.

Hyperuricemia induces hypertension through activation of renal epithelial sodium channel (ENaC)

OBJECTIVES

The mechanisms leading to hypertension associated with hyperuricemia are still unclear. The activity of the distal nephron epithelial sodium channel (ENaC) is an important determinant of sodium balance and blood pressure. Our aim was to investigate whether the effect of hyperuricemia on blood pressure is related to ENaC activation.

METHODS

A hyperuricemic model was induced in rats by 2% oxonic acid and 6 mg/dl uric acid (UA). The hyperuricemic rats were co-treated with either 10mg/kg/d benzbromarone (Ben) or 1 mg/kg/d amiloride (Ami). Blood pressure was monitored using a tail-cuff, and blood, urine, and kidney samples were taken. Western blotting and immunohistochemical staining were performed to determine the expressions of ENaC subunits and components of the ENaC Regulatory Complex (ERC) in kidney tissue or mCCD cells.

RESULTS

Serum uric acid (SUA) was increased 2.5-3.5 times above normal in hyperuricemic rats after 3 weeks and remained at these high levels until 6 weeks. The in vivo rise in SUA was followed by elevated blood pressure, renal tubulointerstitial injury, and increased expressions of ENaC subunits, SGK1, and GILZ1, which were prevented by Ben treatment. The decrease in urinary Na(+) excretion in hyperuricemic rats was blunted by Ami. UA induced the expression of all three ENaC subunits, SGK1, and GILZ1, and increased Na(+) transport in mCCD cells. Phosphorylation of ERK was significantly decreased in both UA-treated mCCD cells and hyperuricemic rat kidney; this effect was prevented by Ben co-treatment.

CONCLUSION

Our findings suggest that elevated serum uric acid could induce hypertension by activation of ENaC and regulation of ERC expression.

The mineralocorticoid receptor (MR) regulates ENaC but not NCC in mice with random MR deletion

Aldosterone binds to the mineralocorticoid receptor (MR) and increases renal Na(+) reabsorption via up-regulation of the epithelial Na(+) channel (ENaC) and the Na(+)-K(+)-ATPase in the collecting system (CS) and possibly also via the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). However, whether aldosterone directly regulates NCC via MR or indirectly through systemic alterations remains controversial. We used mice with deletion of MR in ∼20 % of renal tubule cells (MR/X mice), in which MR-positive (MR(wt)) and -negative (MR(ko)) cells can be studied side-by-side in the same physiological context. Adult MR/X mice showed similar mRNA and protein levels of renal ion transport proteins to control mice. In MR/X mice, no differences in NCC abundance and phosphorylation was seen between MR(wt) and MR(ko) cells and dietary Na(+) restriction up-regulated NCC to similar extent in both groups of cells. In contrast, MR(ko) cells in the CS did not show any detectable alpha-ENaC abundance or apical targeting of ENaC neither on control diet nor in response to dietary Na(+) restriction. Furthermore, Na(+)-K(+)-ATPase expression was unaffected in MR(ko) cells of the DCT, while it was lost in MR(ko) cells of the CS. In conclusion, MR is crucial for ENaC and Na(+)-K(+)-ATPase regulation in the CS, but is dispensable for NCC and Na(+)-K(+)-ATPase regulation in the DCT.

Regulation of αENaC transcription

Aldosterone is a major regulator of Na(+) absorption and acts primarily by controlling the epithelial Na(+) channel (ENaC) function at multiple levels including transcription. ENaC consists of α, β, and γ subunits. In the classical model, aldosterone enhances transcription primarily by activating mineralocorticoid receptor (MR). However, how aldosterone induces chromatin alternation and thus leads to gene activation or repression remains largely unknown. Emerging evidence suggests that Dot1a-Af9 complex plays an important role in repression of αENaC by directly binding and modulating targeted histone H3 K79 hypermethylation at the specific subregions of αENaC promoter. Aldosterone impairs Dot1a-Af9 formation by decreasing expression of Dot1a and Af9 and by inducing Sgk1, which, in turn, phosphorylates Af9 at S435 to weaken Dot1a-Af9 interaction. MR counterbalances Dot1a-Af9 action by competing with Dot1a for binding Af9. Af17 derepresses αENaC by competitively interacting with Dot1a and facilitating Dot1a nuclear export. Consistently, MR(-/-) mice have impaired ENaC expression at day 5 after birth, which may contribute to progressive development of pseudohypoaldosteronism type 1 in a later stage. Af17(-/-) mice have decreased ENaC expression, renal Na(+) retention, and blood pressure. In contrast, Dot1l(AC) mice have increased αENaC expression, despite a 20% reduction of the principal cells. This chapter reviews these findings linking aldosterone action to ENaC transcription through chromatin modification. Future direction toward the understanding the role of Dot1a-Af9 complex beyond ENaC regulation, in particular, in renal fibrosis is also briefly discussed.

Urinary serum- and glucocorticoid-inducible kinase SGK1 reflects renal injury in patients with immunoglobulin A nephropathy

AIM

Serum- and glucocorticoid-inducible kinase SGK1 functions as an important regulator of transepithelial sodium transport by activating epithelial sodium channel in renal tubules. Considerable evidence demonstrated that SGK1 was associated with hypertension and fibrosing diseases, such as diabetic nephropathy and glomerulonephritis. The present study was performed to evaluate the role of SGK1 played in immunoglobulin A (IgA) nephropathy.

METHODS

Seventy-six patients of biopsy-proven IgA nephropathy and 33 healthy volunteers were enrolled in this study. All patients and healthy volunteers' urinary and serum samples were tested for SGK1 expression by indirect enzyme-linked immunosorbent assay. Meanwhile all patients' renal tissues were semi-quantified for SGK1 expression by immunohistochemistry assay. The relationships between SGK1 expressions and clinical or pathological parameters were also assessed.

RESULTS

SGK1 expression was upregulated in urine and renal tubules in patients of Oxford classification T1 and T2, whereas its expression in serum did not increase significantly. Relationship analysis indicated that urinary and tissue SGK1 expressions were associated with heavy proteinuria and renal insufficiency in patients with IgA nephropathy. On the other hand, RAS blockades would reduce the SGK1 levels both in urine and renal tissues.

CONCLUSION

These results suggested that urinary SGK1 should be a good indicator of tubulointerstitial damage in patients of IgA nephropathy. SGK1 expressions in urine and renal tissues were associated with the activity of renin-angiotensin-aldosterone system.

Mineralocorticoid and SGK1-sensitive inflammation and tissue fibrosis

Effects of mineralocorticoids are not restricted to regulation of epithelial salt transport, extracellular volume and blood pressure; mineralocorticoids also influence a wide variety of seemingly unrelated functions such as inflammation and fibrosis. The present brief review addresses the role of mineralocorticoids in the orchestration of these latter processes. Mineralocorticoids foster inflammation as well as vascular, cardiac, renal and peritoneal fibrosis. Mechanisms involved in mineralocorticoid-sensitive inflammation and fibrosis include the serum- and glucocorticoid-inducible kinase 1 (SGK1), which is genomically upregulated by mineralocorticoids and transforming growth factor β (TGF-β), and stimulated by mineralocorticoid-sensitive phosphatidylinositide 3-kinase. SGK1 upregulates the inflammatory transcription factor nuclear factor-κB, which in turn stimulates the expression of diverse inflammatory mediators including connective tissue growth factor. Moreover, SGK1 inhibits the degradation of the TGF-β-dependent transcription factors Smad2/3. Mineralocorticoids foster the development of TH17 cells, which is compromised following SGK1 deletion. Excessive SGK1 expression is observed in a wide variety of fibrosing diseases including lung fibrosis, diabetic nephropathy, glomerulonephritis, obstructive kidney disease, experimental nephrotic syndrome, obstructive nephropathy, liver cirrhosis, fibrosing pancreatitis, peritoneal fibrosis, Crohn's disease and celiac disease. The untoward inflammatory and fibrosing effects of mineralocorticoids could be blunted or even reversed by mineralocorticoid receptor blockers, which may thus be considered in the treatment of inflammatory and/or fibrosing disease.

Mineralocorticoid-induced sodium appetite and renal salt retention: evidence for common signaling and effector mechanisms

An increase in renal sodium chloride (salt) retention and an increase in sodium appetite are the body's responses to salt restriction or depletion in order to restore salt balance. Renal salt retention and increased sodium appetite can also be maladaptive and sustain the pathophysiology in conditions like salt-sensitive hypertension and chronic heart failure. Here we review the central role of the mineralocorticoid aldosterone in both the increase in renal salt reabsorption and sodium appetite. We discuss the working hypothesis that aldosterone activates similar signaling and effector mechanisms in the kidney and brain, including the mineralocorticoid receptor, the serum- and glucocorticoid-induced kinase SGK1, the ubiquitin ligase NEDD4-2, and the epithelial sodium channel ENaC. The latter also mediates the gustatory salt sensing in the tongue, which is required for the manifestation of increased salt intake. Effects of aldosterone on both the brain and kidney synergize with the effects of angiotensin II. Thus, mineralocorticoids appear to induce similar molecular pathways in the kidney, brain, and possibly tongue, which could provide opportunities for more effective therapeutic interventions. Inhibition of renal salt reabsorption is compensated by stimulation of salt appetite and vice versa; targeting both mechanisms should be more effective. Inhibiting the arousal to consume salty food may improve a patient's compliance to reducing salt intake. While a better understanding of the molecular mechanisms is needed and will provide new therapeutic options, current pharmacological interventions that target both salt retention and sodium appetite include mineralocorticoid receptor antagonists and potentially inhibitors of angiotensin II and ENaC.

On the pleotropic actions of mineralocorticoids

Classical effects of mineralocorticoids include stimulation of Na(+) reabsorption and K(+) secretion in the kidney and other epithelia including colon and several glands. Moreover, mineralocorticoids enhance the excretion of Mg(2+) and renal tubular H(+) secretion. The renal salt retention following mineralocorticoid excess leads to extracellular volume expansion and hypertension. The increase of blood pressure following mineralocorticoid excess is, however, not only the result of volume expansion but may result from stiff endothelial cell syndrome impairing the release of vasodilating nitric oxide. Beyond that, mineralocorticoids are involved in the regulation of a wide variety of further functions, including cardiac fibrosis, platelet activation, neuronal function and survival, inflammation as well as vascular and tissue fibrosis and calcification. Those functions are briefly discussed in this short introduction to the special issue. Beyond that, further contributions of this special issue amplify on mineralocorticoid-induced sodium appetite and renal salt retention, the role of mineralocorticoids in the regulation of acid-base balance, the involvement of aldosterone and its receptors in major depression, the mineralocorticoid stimulation of inflammation and tissue fibrosis and the effect of aldosterone on osteoinductive signaling and vascular calcification. Clearly, still much is to be learned about the various ramifications of mineralocorticoid-sensitive physiology and pathophysiology.

Upregulation of store operated Ca channel Orai1, stimulation of Ca(2+) entry and triggering of cell membrane scrambling in platelets by mineralocorticoid DOCA

BACKGROUND/AIMS

Mineralocorticoid excess leads to vascular injury, which is partially due to hypertension but in addition involves increased concentration of cytosolic Ca(2+) concentration in platelets, key players in the pathophysiology of occlusive vascular disease. Mineralocorticoids are in part effective by rapid nongenomic mechanisms including phosphatidylinositide-3-kinase (PI3K) signaling, which involves activation of the serum & glucocorticoid inducible kinase (SGK) isoforms. SGK1 has in turn been shown to participate in the regulation of the pore forming Ca(2+) channel protein Orai1 in platelets. Orai1 accomplishes entry of Ca(2+), which is in turn known to trigger cell membrane scrambling. Platelets lack nuclei but are able to express protein by translation, which is stimulated by PI3K signaling. The present study explored whether the mineralocorticoid desoxycorticosterone acetate (DOCA) influences platelet Orai1 protein abundance, cytosolic Ca(2+)-activity ([Ca(2+)]i), phosphatidylserine abundance at the cell surface and/or cell volume.

METHODS

Orai1 protein abundance was estimated utilizing CF™488A conjugated antibodies, [Ca(2+)]i utilizing Fluo3-fluorescence, phosphatidylserine abundance utilizing FITC-labelled annexin V, and cell volume utilizing forward scatter in flow cytometry.

RESULTS

DOCA (10 µg/ml) treatment of murine platelets was followed by a significant increase of Orai1 protein abundance, [Ca(2+)]i, percentage of phosphatidylserine exposing platelets and platelet swelling. The effect on [Ca(2+)]i, phosphatidylserine abundance and cell volume were completely abrogated by addition of the specific SGK inhibitor EMD638683 (50 µM) CONCLUSIONS: The mineralocorticoid DOCA upregulates Orai1 protein abundance in the cell membrane, thus increasing [Ca(2+)]i and triggering phosphatidylserine abundance, effects paralleled by platelet swelling.

Genetic dissection of sodium and potassium transport along the aldosterone-sensitive distal nephron: importance in the control of blood pressure and hypertension

In this review, we discuss genetic evidence supporting Guyton's hypothesis stating that blood pressure control is critically depending on fluid handling by the kidney. The review is focused on the genetic dissection of sodium and potassium transport in the distal nephron and the collecting duct that are the most important sites for the control of sodium and potassium balance by aldosterone and angiotensin II. Thanks to the study of Mendelian forms of hypertension and their corresponding transgenic mouse models, three main classes of diuretic receptors (furosemide, thiazide, amiloride) and the main components of the aldosterone- and angiotensin-dependent signaling pathways were molecularly identified over the past 20 years. This will allow to design rational strategies for the treatment of hypertension and for the development of the next generation of diuretics.

DOCA sensitive pendrin expression in kidney, heart, lung and thyroid tissues

BACKGROUND/AIMS

Pendrin (SLC26A4), a transporter accomplishing anion exchange, is expressed in inner ear, thyroid gland, kidneys, lung, liver and heart. Loss or reduction of function mutations of SLC26A4 underlie Pendred syndrome, a disorder invariably leading to hearing loss with enlarged vestibular aqueducts and in some patients to hypothyroidism and goiter. Renal pendrin expression is up-regulated by mineralocorticoids such as aldosterone or deoxycorticosterone (DOCA). Little is known about the impact of mineralocorticoids on pendrin expression in extrarenal tissues.

METHODS

The present study utilized RT-qPCR and Western blotting to quantify the transcript levels and protein abundance of Slc26a4 in murine kidney, thyroid, heart and lung prior to and following subcutaneous administration of 100 mg/kg DOCA.

RESULTS

Slc26a4 transcript levels as compared to Gapdh transcript levels were significantly increased by DOCA treatment in kidney, heart, lung and thyroid. Accordingly pendrin protein expression was again significantly increased by DOCA treatment in kidney, heart, lung and thyroid.

CONCLUSION

The observations reveal mineralocorticoid sensitivity of pendrin expression in kidney, heart, thyroid and lung.

Oxidative stress-induced glomerular mineralocorticoid receptor activation limits the benefit of salt reduction in Dahl salt-sensitive rats

BACKGROUND

Mineralocorticoid receptor (MR) antagonists attenuate renal injury in salt-sensitive hypertensive rats with low plasma aldosterone levels. We hypothesized that oxidative stress causes MR activation in high-salt-fed Dahl salt-sensitive rats. Furthermore, we determined if MR activation persisted and induced renal injury, even after switching from a high- to a normal-salt diet.

METHODS AND FINDINGS

High-salt feeding for 4 weeks increased dihydroethidium fluorescence (DHE, an oxidant production marker), p22phox (a NADPH oxidase subunit) and serum and glucocorticoid-regulated kinase-1 (SGK1, an MR transcript) in glomeruli, compared with normal-salt feeding, and these changes persisted 4 weeks after salt withdrawal. Tempol treatment (0.5 mmol/L) during high-salt feeding abolished the changes in DHE fluorescence, p22phox and SGK1. Dietary salt reduction after a 4-week high-salt diet decreased both blood pressure and proteinuria, but was associated with significantly higher proteinuria than in normal control rats at 4 weeks after salt reduction. Administration of tempol during high-salt feeding, or eplerenone, an MR antagonist (100 mg/kg/day), started after salt reduction, recovered proteinuria to normal levels at 4 weeks after salt reduction. Paraquat, a reactive oxygen species generator, enhanced MR transcriptional activity in cultured rat mesangial cells and mouse podocytes.

CONCLUSIONS

These results suggest that oxidative stress plays an important role in glomerular MR activation in Dahl salt-sensitive rats. Persistent MR activation even after reducing salt intake could limit the beneficial effects of salt restriction.

Role of mineralocorticoid receptor/Rho/Rho-kinase pathway in obesity-related renal injury

OBJECTIVE:

We examined whether aldosterone/Rho/Rho-kinase pathway contributed to obesity-associated nephropathy.

SUBJECTS:

C57BL/6J mice were fed a high fat or low fat diet, and mice on a high fat diet were treated with a mineralocorticoid receptor antagonist, eplerenone.

RESULTS:

The mice on a high fat diet not only developed obesity, but also manifested renal histological changes, including glomerular hypercellularity and increased mesangial matrix, which paralleled the increase in albuminuria. Furthermore, enhanced Rho-kinase activity was noted in kidneys from high fat diet-fed mice, as well as increased expressions of inflammatory chemokines. All of these changes were attenuated by eplerenone. In high fat diet-fed mice, mineralocorticoid receptor protein levels in the nuclear fraction and SGK1, an effector of aldosterone, were upregulated in kidneys, although serum aldosterone levels were unaltered. Furthermore, aldosterone and 3β-hydroxysteroid dehydrogenase in renal tissues were upregulated in high fat diet-fed mice. Finally, in cultured mesangial cells, stimulation with aldosterone enhanced Rho-kinase activity, and pre-incubation with eplerenone prevented the aldosterone-induced activation of Rho kinase.

CONCLUSION:

Excess fat intake causes obesity and renal injury in C57BL/6J mice, and these changes are mediated by an enhanced mineralocorticoid receptor/Rho/Rho-kinase pathway and inflammatory process. Mineralocorticoid receptor activation in the kidney tissue and the subsequent Rho-kinase stimulation are likely to participate in the development of obesity-associated nephropathy without elevation in serum aldosterone levels.

EMD638683, a novel SGK inhibitor with antihypertensive potency

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase enhances renal tubular Na(+)-reabsorption and accounts for blood pressure increase following high salt diet in mice made hyperinsulinemic by dietary fructose or fat. The present study describes the in vitro and in vivo efficacy of a novel SGK1 inhibitor (EMD638683). EMD638683 was tested in vitro by determination of SGK1-dependent phosphorylation of NDRG1 (N-Myc downstream-regulated gene 1) in human cervical carcinoma HeLa-cells. In vivo EMD638683 (4460 ppm in chow, i.e. approx. 600 mg/kg/day) was administered to mice drinking tap water or isotonic saline containing 10% fructose. Blood pressure was determined by the tail cuff method, and urinary electrolyte (flame photometry) concentrations determined in metabolic cages. In vitro testing disclosed EMD638683 as a SGK1 inhibitor with an IC50 of 3 μM. Within 24 hours in vivo EMD638683 treatment significantly decreased blood pressure in fructose/saline-treated mice but not in control animals or in SGK1 knockout mice. EMD638683 failed to alter the blood pressure in SGK1 knockout mice. Following chronic (4 weeks) fructose/high salt treatment, additional EMD638683 treatment again decreased blood pressure. EMD638683 thus abrogates the salt sensitivity of blood pressure in hyperinsulinism without appreciably affecting blood pressure in the absence of hyperinsulinism. EMD638683 tended to increase fluid intake and urinary excretion of Na(+), significantly increased urinary flow rate and significantly decreased body weight.

CONCLUSION

EMD638683 could serve as a template for drugs counteracting hypertension in individuals with type II diabetes and metabolic syndrome.

SGK1-dependent salt appetite in pregnant mice

AIM

Pregnancy is typically paralleled by substantial increase in maternal extracellular fluid volume, requiring net accumulation of water and NaCl. The positive water and salt balance is accomplished at least in part by increased uptake of salt secondary to enhanced salt appetite. Little is known about the underlying cellular mechanisms. Stimulation of salt appetite by mineralocorticoids, however, is known to be dependent on the serum- and glucocorticoid-inducible kinase SGK1.

METHODS

To test for a role of SGK1 in the stimulation of salt appetite during pregnancy, fluid intake was recorded in pregnant SGK1 knockout mice (sgk1(-/-) ) and their wild type littermates (sgk1(+/+) ). The mice were offered two bottles, one with plain water and the other with isotonic saline.

RESULTS

In early pregnancy, i.e. up to 10 days prior to parturition, the sgk1(+/+) mice displayed a significant preference for saline, whereas the sgk1(-/-) mice preferred water. Accordingly, the water intake was significantly smaller and saline intake was significantly larger in sgk1(+/+) mice than in sgk1(-/-) mice and the preference for water was significantly stronger in sgk1(-/-) mice than in sgk1(+/+) mice. Plasma aldosterone levels were higher in sgk1(-/-) mice than in sgk1(+/+) mice, a difference contrasting the enhanced salt appetite of sgk1(+/+) mice.

CONCLUSIONS

SGK1 participates in the stimulation of salt appetite during pregnancy.

Insulin activates epithelial sodium channel (ENaC) via phosphoinositide 3-kinase in mammalian taste receptor cells

Diabetes is a profound disease that results in a severe lack of regulation of systemic salt and water balance. From our earlier work on the endocrine regulation of salt taste at the level of the epithelial sodium channel (ENaC), we have begun to investigate the ability of insulin to alter ENaC function with patch-clamp recording on isolated mouse taste receptor cells (TRCs). In fungiform and vallate TRCs that exhibit functional ENaC currents (e.g., amiloride-sensitive Na(+) influx), insulin (5-20 nM) caused a significant increase in Na(+) influx at -80 mV (EC(50) = 7.53 nM). The insulin-enhanced currents were inhibited by amiloride (30 μM). Similarly, in ratiometric Na(+) imaging using SBFI, insulin treatment (20 nM) enhanced Na(+) movement in TRCs, consistent with its action in electrophysiological assays. The ability of insulin to regulate ENaC function is dependent on the enzyme phosphoinositide 3-kinase since treatment with the inhibitor LY294002 (10 μM) abolished insulin-induced changes in ENaC. To test the role of insulin in the regulation of salt taste, we have characterized behavioral responses to NaCl using a mouse model of acute hyperinsulinemia. Insulin-treated mice show significant avoidance of NaCl at lower concentrations than the control group. Interestingly, these differences between groups were abolished when amiloride (100 μM) was added into NaCl solutions, suggesting that insulin was regulating ENaC. Our results are consistent with a role for insulin in maintaining functional expression of ENaC in mouse TRCs.

Role of epithelial sodium channels and their regulators in hypertension

The kidney has a central role in the regulation of blood pressure, in large part through its role in the regulated reabsorption of filtered Na(+). Epithelial Na(+) channels (ENaCs) are expressed in the most distal segments of the nephron and are a target of volume regulatory hormones. A variety of factors regulate ENaC activity, including several aldosterone-induced proteins that are present within an ENaC regulatory complex. Proteases also regulate ENaC by cleaving the channel and releasing intrinsic inhibitory tracts. Polymorphisms or mutations within channel subunits or regulatory pathways that enhance channel activity may contribute to an increase in blood pressure in individuals with essential hypertension.

mPGES-1 deletion impairs aldosterone escape and enhances sodium appetite

Aldosterone (Aldo) is a major sodium-retaining hormone that reduces renal sodium excretion and also stimulates sodium appetite. In the face of excess Aldo, the sodium-retaining action of this steroid is overridden by an adaptive regulatory mechanism, a phenomenon termed Aldo escape. The underlying mechanism of this phenomenon is not well defined but appeared to involve a number of natriuretic factors such prostaglandins (PGs). Here, we investigated the role of microsomal prostaglandin E synthase-1 (mPGES-1) in the response to excess Aldo. A 14-day Aldo infusion at 0.35 mg x kg(-1) x day(-1) via an osmotic minipump in conjunction with normal salt intake did not produce obvious disturbances in fluid metabolism in WT mice as suggested by normal sodium and water balance, plasma sodium concentration, hematocrit, and body weight, despite the evidence of a transient sodium accumulation on days 1 or 2. In a sharp contrast, the 14-day Aldo treatment in mPGES-1 knockoute (KO) mice led to increased sodium and water balance, persistent reduction of hematocrit, hypernatremia, and body weight gain, all evidence of fluid retention. The escaped wild-type (WT) mice displayed a remarkable increase in urinary PGE(2) excretion in parallel with coinduction of mPGES-1 in the proximal tubules, accompanied by a remarkable, widespread downregulation of renal sodium and water transporters. The increase in urinary PGE(2) excretion together with the downregulation of renal sodium and water transporters were all significantly blocked in the KO mice. Interestingly, compared with WT controls, the KO mice exhibited consistent increases in sodium and water intake during Aldo infusion. Together, these results suggest an important role of mPGES-1 in antagonizing the sodium-retaining action of Aldo at the levels of both the central nervous system and the kidney.

Dietary Na+ inhibits the open probability of the epithelial sodium channel in the kidney by enhancing apical P2Y2-receptor tone

Apical release of ATP and UTP can activate P2Y(2) receptors in the aldosterone-sensitive distal nephron (ASDN) and inhibit the open probability (P(o)) of the epithelial sodium channel (ENaC). Little is known, however, about the regulation and physiological relevance of this system. Patch-clamp studies in freshly isolated ASDN provide evidence that increased dietary Na(+) intake in wild-type mice lowers ENaC P(o), consistent with a contribution to Na(+) homeostasis, and is associated with increased urinary concentrations of UTP and the ATP hydrolytic product, ADP. Genetic deletion of P2Y(2) receptors in mice (P2Y(2)(-/-); littermates to wild-type mice) or inhibition of apical P2Y-receptor activation in wild-type mice prevents dietary Na(+)-induced lowering of ENaC P(o). Although they lack suppression of ENaC P(o) by dietary NaCl, P2Y(2)(-/-) mice do not exhibit NaCl-sensitive blood pressure, perhaps as a consequence of compensatory down-regulation of aldosterone levels. Consistent with this hypothesis, clamping mineralocorticoid activity at high levels unmasks greater ENaC activity and NaCl sensitivity of blood pressure in P2Y(2)(-/-) mice. The studies indicate a key role of the apical ATP/UTP-P2Y(2)-receptor system in the inhibition of ENaC P(o) in the ASDN in response to an increase in Na(+) intake, thereby contributing to NaCl homeostasis and blood pressure regulation.

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.

Aldosterone mediates activation of the thiazide-sensitive Na-Cl cotransporter through an SGK1 and WNK4 signaling pathway

Aldosterone regulates volume homeostasis and blood pressure by enhancing sodium reabsorption in the kidney's distal nephron (DN). On the apical surface of these renal epithelia, aldosterone increases expression and activity of the thiazide-sensitive Na-Cl cotransporter (NCC) and the epithelial sodium channel (ENaC). While the cellular mechanisms by which aldosterone regulates ENaC have been well characterized, the molecular mechanisms that link aldosterone to NCC-mediated Na+/Cl- reabsorption remain elusive. The serine/threonine kinase with-no-lysine 4 (WNK4) has previously been shown to reduce cell surface expression of NCC. Here we measured sodium uptake in a Xenopus oocyte expression system and found that serum and glucocorticoid-induced kinase 1 (SGK1), an aldosterone-responsive gene expressed in the DN, attenuated the inhibitory effect of WNK4 on NCC activity. In addition, we showed--both in vitro and in a human kidney cell line--that SGK1 bound and phosphorylated WNK4. We found one serine located within an established SGK1 consensus target sequence, and the other within a motif that was, to our knowledge, previously uncharacterized. Mutation of these target serines to aspartate, in order to mimic phosphorylation, attenuated the effect of WNK4 on NCC activity in the Xenopus oocyte system. These data thus delineate what we believe to be a novel mechanism for aldosterone activation of NCC through SGK1 signaling of WNK4 kinase.

Aldosterone modulates steroid receptor binding to the endothelin-1 gene (edn1)

Aldosterone and endothelin-1 (ET-1) act on collecting duct cells of the kidney and are important regulators of renal sodium transport and cardiovascular physiology. We recently identified the ET-1 gene (edn1) as a novel aldosterone-induced transcript. However, aldosterone action on edn1 has not been characterized at the present time. In this report, we show that aldosterone stimulated edn1 mRNA in acutely isolated rat inner medullary collecting duct cells ex vivo and ET-1 peptide in rat inner medulla in vivo. Aldosterone induction of edn1 mRNA occurred in cortical, outer medullary, and inner medullary collecting duct cells in vitro. Inspection of the edn1 promoter revealed two putative hormone response elements. Levels of heterogeneous nuclear RNA synthesis demonstrated that edn1 mRNA stimulation occurred at the level of transcription. RNA knockdowns corroborated pharmacological studies and demonstrated both mineralocorticoid receptor and glucocorticoid receptor participated in this response. Aldosterone resulted in dose-dependent nuclear translocation and binding of mineralocorticoid receptor and glucocorticoid receptor to the edn1 hormone response elements. Hormone receptors mediated the association of chromatin remodeling complexes, histone modification, and RNA polymerase II at the edn1 promoter. Direct interaction between aldosterone and ET-1 has important implications for renal and cardiovascular function.

Regulation of a third conserved phosphorylation site in SGK1

SGK1 (serum- and glucocorticoid-induced kinase 1) is a member of the AGC branch of the protein kinase family. Among well described functions of SGK1 is the regulation of epithelial transport through phosphorylation of the ubiquitin protein ligase Nedd4-2 (neuronal precursor cell expressed developmentally down-regulated 4-2). The activation of SGK1 has been widely accepted to be dependent on the phosphorylation of Thr256 in the activation loop and Ser422 in the hydrophobic motif near the C terminus. Here, we report the identification of two additional phosphorylation sites, Ser397 and Ser401. Both are required for maximum SGK1 activity induced by extracellular agents or by coexpression with other protein kinases, with the largest loss of activity from mutation of Ser397. Coexpression with active Akt1 increased the phosphorylation of Ser397 and thereby SGK1 kinase activity. SGK1 activation was further augmented by coexpression with the protein kinase WNK1 (with no lysine kinase 1). These findings reveal further complexity underlying the regulation of SGK1 activity.

SGK1-sensitive renal tubular glucose reabsorption in diabetes

The hyperglycemia of diabetes mellitus increases the filtered glucose load beyond the maximal tubular transport rate and thus leads to glucosuria. Sustained hyperglycemia, however, may gradually increase the maximal renal tubular transport rate and thereby blunt the increase of urinary glucose excretion. The mechanisms accounting for the increase of renal tubular glucose transport have remained ill-defined. A candidate is the serum- and glucocorticoid-inducible kinase SGK1. The kinase has been shown to stimulate Na(+)-coupled glucose transport in vitro and mediate the stimulation of electrogenic intestinal glucose transport by glucocorticoids in vivo. SGK1 expression is confined to glomerula and distal nephron in intact kidneys but may extend to the proximal tubule in diabetic nephropathy. To explore whether SGK1 modifies glucose transport in diabetic kidneys, Akita mice (akita(+/-)), which develop spontaneous diabetes, have been crossbred with gene-targeted mice lacking SGK1 on one allele (sgk1(+/-)) to eventually generate either akita(+/-)/sgk1(-/-) or akita(+/-)/sgk1(+/+) mice. Both akita(+/-)/sgk1(-/-) and akita(+/-)/sgk1(+/+) mice developed profound hyperglycemia (>20 mM) within approximately 6 wk. Body weight and plasma glucose concentrations were not significantly different between these two genotypes. However, urinary excretion of glucose and urinary excretion of fluid, Na(+), and K(+), as well as plasma aldosterone concentrations, were significantly higher in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. Studies in isolated perfused proximal tubules revealed that the electrogenic glucose transport was significantly lower in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. The data provide the first evidence that SGK1 participates in the stimulation of renal tubular glucose transport in diabetic kidneys.

Proteinuria in mice expressing PKB/SGK-resistant GSK3

SGK1 is critically important for mineralocorticoid/salt-induced glomerular injury. SGK1 inactivates GSK3, which downregulates Snail, a DNA-binding molecule repressing the transcription of nephrin, a protein critically important for the integrity of the glomerular slit membrane. PKB/SGK-dependent GSK regulation is disrupted in mice carrying a mutation, in which the serine in the SGK/PKB-phosphorylation consensus sequence is replaced by alanine. The present study explored whether PKB/SGK-dependent GSK3 regulation influences glomerular proteinuria. Gene-targeted knockin mice with mutated and thus PKB/SGK-resistant GSK3alpha,beta (gsk3(KI)) were compared with their wild-type littermates (gsk3(WT)). gsk3(KI) and gsk3(WT) mice were implanted with DOCA release pellets and offered 1% saline as drinking water for 21 days. Under standard diet, tap water intake and absence of DOCA, urinary flow rate, glomerular filtration rate, and urinary albumin excretion were significantly larger and blood pressure was significantly higher in gsk3(KI) than in gsk3(WT) mice. Within 18 days, DOCA/salt treatment significantly increased fluid intake and urinary flow rate, urinary protein and albumin excretion, and blood pressure in both genotypes but the respective values were significantly higher in gsk3(KI) than in gsk3(WT) mice. Plasma albumin concentration was significantly lower in gsk3(KI) than in gsk3(WT) mice. Proteinuria was abrogated by lowering of blood pressure with alpha(1)-blocker prazosin (1 microg/g body wt) in 8-mo-old mice. According to immunofluorescence, nephrin at 3 and 8 mo and podocin expression at 3 mo were significantly lower in gsk3(KI) than in gsk3(WT) mice. After 18 days, DOCA/salt treatment renal glomerular sclerosis and tubulointerstitial damage were significantly more pronounced in gsk3(KI) than in gsk3(WT) mice. The observations reveal that disruption of PKB/SGK-dependent regulation of GSK3 leads to glomerular injury with proteinuria, which may at least partially be secondary to enhanced blood pressure.

Organic anion transporter 3 contributes to the regulation of blood pressure

Renal organic anion transporters (OAT) are known to mediate the excretion of many drugs, but their function in normal physiology is not well understood. In this study, mice lacking organic anion transporter 3 (Oat3) had a 10 to 15% lower BP than wild-type mice, raising the possibility that Oat3 transports an endogenous regulator of BP. The aldosterone response to a low-salt diet was blunted in Oat3-null mice, but baseline aldosterone concentration was higher in these mice, suggesting that aldosterone dysregulation does not fully explain the lower BP in the basal state; therefore, both targeted and global metabolomic analyses of plasma and urine were performed, and several potential endogenous substrates of Oat3 were found to accumulate in the plasma of Oat3-null mice. One of these substrates, thymidine, was transported by Oat3 expressed in vitro. In vivo, thymidine, as well as two of the most potent Oat3 inhibitors that were characterized, reduced BP by 10 to 15%; therefore, Oat3 seems to regulate BP, and Oat3 inhibitors might be therapeutically useful antihypertensive agents. Moreover, polymorphisms in human OAT3 might contribute to the genetic variation in susceptibility to hypertension.

Mineralocorticoid receptor blockade ameliorates peritoneal fibrosis in new rat peritonitis model

Peritoneal fibrosis (PF) is an important complication of long-term peritoneal dialysis. Although mineralocorticoid and mineralocorticoid receptor (MR) have attracted increasing attention in the field of vascular injury, including the heart, kidney, and vessels, little is known about the role of mineralocorticoid in PF. This work was designed to explore the effects of MR blockade on PF. We developed a new model of PF in rats based on mechanical scraping of the peritoneum. This model is characterized by acute-phase inflammation (neutrophil and macrophage infiltration on days 0–3) and late-phase PF (α-smooth muscle actin-positive fibroblast infiltration, type III collagen accumulation, and neoangiogenesis on days 7–14). Peritoneal thickening peaked on day 14. MR was expressed in rat peritoneum and a rat fibroblast cell line. Expression of its effector kinase [serum- and glucocorticoid-induced kinase-1 (Sgk1)], transforming growth factor-β (TGF-β), plasminogen activator inhibitor-1 (PAI-1), and CD31-positive vessels increased during the course of PF. Rats were treated with spironolactone, angiotensin receptor blockade (ARB), or angiotensin-converting enzyme inhibitor (ACEI)-ARB-spironolactone starting at 6 h after peritoneal scraping. All parameters, including peritoneal thickening, number of macrophages and CD31-positive vessels, and expression of monocyte chemoattractant protein-1, TGF-β, PAI-1, and Sgk1, were significantly suppressed by spironolactone (10 mg·kg−1·day−1). The effects of spironolactone (10 and 20 mg·kg−1·day−1) were very similar to those of triple blockade. ARB, but not ACEI, significantly reduced peritoneal thickening. Furthermore, peritoneal function assessed by peritoneal equilibration test was significantly improved by spironolactone. Our results suggest that MR is a potential target to prevent inflammation-induced PF in patients on peritoneal dialysis.

Association of SGK1 gene polymorphisms with type 2 diabetes

The serum and glucocorticoid inducible kinase SGK1 is genomically upregulated by glucocorticoids and in turn stimulates a variety of carriers and channels including the renal epithelial Na(+) channel ENaC and the intestinal Na(+) glucose transporter SGLT1. Twin studies disclosed an association of a specific SGK1 haplotype with moderately enhanced blood pressure in individuals who are carrying simultaneously a homozygous genotype for a variant in intron 6 [I6CC] and a homozygous or heterozygous genotype for the C allele of a polymorphism in exon 8 [E8CC/CT] of the SGK1 gene. A subsequent study confirmed the impact of this risk haplotype on blood pressure. SGK1 knockout mice are resistant to the insulin and high salt induced increase of blood pressure, glucocorticoid induced increase of electrogenic glucose transport, and glucocorticoid induced suppression of insulin release. The present study explored whether the I6CC/E8CC/CT haplotype impacts on the prevalence of type 2 diabetes. The prevalence of the I6CC genotype was 3.1% in a healthy German, 2.4 % in a healthy Romanian and 11.6 % in a healthy African population from Ghana (p=0.0006 versus prevalence in Caucasians). Comparison of genotype frequencies between type 2 diabetic patients and the respective control groups revealed significant differences for the intron 6 T>C variant. Carriers of at least one T allele were protected against type 2 diabetes (Romanians: p=0.023; OR 0.29; 95% CI 0.09-0.89; Germans: p=0.01; OR 0.37; 95% CI 0.17-0.81). The SGK1 risk haplotype (I6CC/E8CC/CT) was significantly (p=0.032; OR 4.31, 95% CI 1.19-15.58) more frequent in diabetic patients (7.2 %) than in healthy volunteers from Romania (1.8%). The observations support the view that SGK-1 may participate in the pathogenesis of metabolic syndrome.

Lack of the serum and glucocorticoid-inducible kinase SGK1 attenuates the volume retention after treatment with the PPARgamma agonist pioglitazone

PPARgamma-agonists enhance insulin sensitivity and improve glucose utilization in diabetic patients. Adverse effects of PPARgamma-agonists include volume retention and edema formation. Recent observations pointed to the ability of PPARgamma agonists to enhance transcription of the serum and glucocorticoid-inducible kinase SGK1, a kinase that is genomically upregulated by mineralocorticoids and stimulates various renal channels and transporters including the renal epithelial Na+ channel ENaC. SGK1 has been proposed to mediate the volume retention after treatment with PPARgamma agonists. To test this hypothesis, food containing the PPARgamma agonist pioglitazone (0.02%, i.e., approximately 25 mg/kg bw/day) was administered to gene-targeted mice lacking SGK1 (sgk1-/-, n=12) and their wild-type littermates (sgk1+/+), n=12). According to in situ hybridization, quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence, treatment with pioglitazone significantly increased renal SGK1 mRNA and protein expression in sgk1+/+ mice. The treatment increased body weight significantly in both, sgk1+/+ mice (+2.2+/-0.3 g) and sgk-/- mice (+1.3+/-0.2 g), and decreased hematocrit significantly in sgk1+/+ mice (-6.5+/-1.0%) and sgk1-/- mice (-3.1+/-0.6%). Both effects were significantly (p<0.05) more pronounced in sgk1+/+ mice. According to Evans Blue distribution, pioglitazone increased plasma volume only in sgk1+/+ mice (from 50.9+/-3.9 to 63.7+/-2.5 microl/g bw) but not in sgk-/- mice (from 46.8+/-3.8 to 48.3+/-5.2 microl/g bw). Pioglitazone decreased aldosterone plasma levels and blood pressure and increased leptin plasma levels in both genotypes. We conclude that SGK1 contributes to but does not fully account for the volume retention during treatment with the PPARgamma agonist pioglitazone.

Salt-induced nephropathy in obese spontaneously hypertensive rats via paradoxical activation of the mineralocorticoid receptor: role of oxidative stress

Aldosterone is implicated in the pathogenesis of proteinuria and chronic kidney disease. We previously demonstrated the contribution of elevated serum aldosterone in the early nephropathy of SHR/NDmcr-cp (SHR/cp), a rat model of metabolic syndrome. In the present study, we investigated the effect of salt loading on renal damage in SHR/cps and explored the underlying mechanisms. SHR/cps fed a high-sodium diet for 4 weeks developed severe hypertension, massive proteinuria, and advanced renal lesions. High salt also worsened glomerular podocyte impairment. Surprisingly, selective mineralocorticoid receptor (MR) antagonist eplerenone dramatically ameliorated the salt-induced proteinuria and renal injury in SHR/cps. Although salt loading reduced circulating aldosterone, it increased nuclear MR and expression of aldosterone effector kinase Sgk1 in the kidney. Gene expressions of transforming growth factor-beta1 and plasminogen activator inhibitor-1 were also enhanced in the kidneys of salt-loaded SHR/cps, and eplerenone completely inhibited these injury markers. To clarify the discrepancy between decreased aldosterone and enhanced MR signaling by salt, we further investigated the role of oxidative stress, a putative key factor mediating salt-induced tissue damage. Interestingly, antioxidant Tempol attenuated the salt-evoked MR upregulation and Sgk1 induction and alleviated proteinuria and renal histological abnormalities, suggesting the involvement of oxidative stress in salt-induced MR activation. MR activation by salt was not attributed to increased serum corticosterone or reduced 11beta-hydroxysteroid dehydrogenase type 2 activity. In conclusion, sodium loading exacerbated proteinuria and renal injury in metabolic syndrome rats. Salt reduced circulating aldosterone but caused renal MR activation at least partially via induction of oxidative stress, and eplerenone effectively improved the nephropathy.

Mice lacking P2Y2 receptors have salt-resistant hypertension and facilitated renal Na+ and water reabsorption

Extracellular nucleotides (e.g., ATP) regulate many physiological and pathophysiological processes through activation of nucleotide (P2) receptors in the plasma membrane. Here we report that gene-targeted (knockout) mice that lack P2Y2 receptors have salt-resistant arterial hypertension in association with an inverse relationship between salt intake and heart rate, indicating intact baroreceptor function. Knockout mice have multiple alterations in their handling of salt and water: these include suppressed plasma renin and aldosterone concentrations, lower renal expression of the aldosterone-induced epithelial sodium channel alpha-ENaC, greater medullary expression of the Na-K-2Cl-cotransporter NKCC2, and greater furosemide-sensitive Na+ reabsorption in association with greater renal medullary expression of aquaporin-2 and vasopressin-dependent renal cAMP formation and water reabsorption despite similar vasopressin levels compared with wild type. Of note, smaller increases in plasma aldosterone were required to adapt renal Na+ excretion to restricted intake in knockout mice, suggesting a facilitation in renal Na+ retention. The results thus identify a previously unrecognized role for P2Y2 receptors in blood pressure regulation that is linked to an inhibitory influence on renal Na+ and water reabsorption. Based on these findings in knockout mice, we propose that a blunting in P2Y2 receptor expression or activity is a new mechanism for salt-resistant arterial hypertension.

Aldosterone-induced Sgk1 relieves Dot1a-Af9-mediated transcriptional repression of epithelial Na+ channel alpha

Aldosterone plays a major role in the regulation of salt balance and the pathophysiology of cardiovascular and renal diseases. Many aldosterone-regulated genes--including that encoding the epithelial Na+ channel (ENaC), a key arbiter of Na+ transport in the kidney and other epithelia--have been identified, but the mechanisms by which the hormone modifies chromatin structure and thus transcription remain unknown. We previously described the basal repression of ENaCalpha by a complex containing the histone H3 Lys79 methyltransferase disruptor of telomeric silencing alternative splice variant a (Dot1a) and the putative transcription factor ALL1-fused gene from chromosome 9 (Af9) as well as the release of this repression by aldosterone treatment. Here we provide evidence from renal collecting duct cells and serum- and glucocorticoid-induced kinase-1 (Sgk1) WT and knockout mice that Sgk1 phosphorylated Af9, thereby impairing the Dot1a-Af9 interaction and leading to targeted histone H3 Lys79 hypomethylation at the ENaCalpha promoter and derepression of ENaCalpha transcription. Thus, Af9 is a physiologic target of Sgk1, and Sgk1 negatively regulates the Dot1a-Af9 repressor complex that controls transcription of ENaCalpha and likely other aldosterone-induced genes.

Podocyte as the target for aldosterone: roles of oxidative stress and Sgk1

Accumulating evidence suggests that mineralocorticoid receptor blockade effectively reduces proteinuria in hypertensive patients. However, the mechanism of the antiproteinuric effect remains elusive. In this study, we investigated the effects of aldosterone on podocyte, a key player of the glomerular filtration barrier. Uninephrectomized rats were continuously infused with aldosterone and fed a high-salt diet. Aldosterone induced proteinuria progressively, associated with blood pressure elevation. Notably, gene expressions of podocyte-associated molecules nephrin and podocin were markedly decreased in aldosterone-infused rats at 2 weeks, with a gradual decrease thereafter. Immunohistochemical studies and electron microscopy confirmed the podocyte damage. Podocyte injury was accompanied by renal reduced nicotinamide-adenine dinucleotide phosphate oxidase activation, increased oxidative stress, and enhanced expression of aldosterone effector kinase Sgk1. Treatment with eplerenone, a selective aldosterone receptor blocker, almost completely prevented podocyte damage and proteinuria, with normalization of elevated reduced nicotinamide-adenine dinucleotide phosphate oxidase activity. In addition, proteinuria, podocyte damage, and Sgk1 upregulation were significantly alleviated by tempol, a membrane-permeable superoxide dismutase, suggesting the pathogenic role of oxidative stress. Although hydralazine treatment almost normalized blood pressure, it failed to improve proteinuria and podocyte damage. In cultured podocytes with consistent expression of mineralocorticoid receptor, aldosterone stimulated membrane translocation of reduced nicotinamide-adenine dinucleotide phosphate oxidase cytosolic components and oxidative stress generation in podocytes. Furthermore, aldosterone enhanced the expression of Sgk1, which was inhibited by mineralocorticoid receptor antagonist and tempol. In conclusion, podocytes are injured at the early stage in aldosterone-infused rats, resulting in the occurrence of proteinuria. Aldosterone can directly modulate podocyte function, possibly through the induction of oxidative stress and Sgk1.

(Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms

The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.