Blunted DOCA/high salt induced albuminuria and renal tubulointerstitial damage in gene-targeted mice lacking SGK1

Unraveling the Potential of SGK1 in Osteoporosis: From Molecular Mechanisms to Therapeutic Targets

Osteoporosis (OP) is a prevalent metabolic bone disease, with several million cases of fractures resulting from osteoporosis worldwide each year. This phenomenon contributes to a substantial increase in direct medical expenditures and poses a considerable socioeconomic burden. Despite its prevalence, our understanding of the underlying mechanisms remains limited. Recent studies have demonstrated the involvement of serum glucocorticoid-regulated protein kinase 1 (SGK1) in multiple signaling pathways that regulate bone metabolism and its significant role in the development of osteoporosis. Therefore, it is of great significance to deeply explore the mechanism of SGK1 in osteoporosis and its therapeutic potential. In this paper, we present a comprehensive review of the structure and activation mechanism of SGK1, its biological function, the role of SGK1 in different types of osteoporosis, and the inhibitors of SGK1. The aim is to comprehensively assess the latest research progress with regards to SGK1's role in osteoporosis, clarify its role in the regulation of bone metabolism and its potential as a therapeutic target, and lay the foundation for the development of novel therapeutic strategies and personalized treatment in the future. Furthermore, by thoroughly examining the interactions between SGK1 and other molecules or signaling pathways, potential biomarkers may be identified, thereby enhancing the efficacy of early screening and intervention for osteoporosis.

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.

Approaches towards tissue-selective pharmacology of the mineralocorticoid receptor

Mineralocorticoid receptor antagonists (MRAs) are highly effective therapies for cardiovascular and renal disease. However, the widespread clinical use of currently available MRAs in cardiorenal medicine is hampered by an increased risk of hyperkalemia. The mineralocorticoid receptor (MR) is a nuclear receptor responsible for fluid and electrolyte homeostasis in epithelial tissues, whereas pathophysiological MR activation in nonepithelial tissues leads to undesirable pro-inflammatory and pro-fibrotic effects. Therefore, new strategies that selectively target the deleterious effects of MR but spare its physiological function are needed. In this review, we discuss recent pharmacological developments starting from novel non-steroidal MRAs that are now entering clinical use, such as finerenone or esaxerenone, to concepts arising from the current knowledge of the MR signaling pathway, aiming at receptor-coregulator interaction, epigenetics, or downstream effectors of MR.

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.

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.

Serum- and glucocorticoid-inducible kinase 1 and the response to cell stress

Expression of the serum- and glucocorticoid-inducible kinase 1 (SGK1) is up-regulated by several types of cell stress, such as ischemia, radiation and hyperosmotic shock. The SGK1 protein is activated by a signaling cascade involving phosphatidylinositide-3-kinase (PI3K), 3-phosphoinositide-dependent kinase 1 (PDK1) and mammalian target of rapamycin (mTOR). SGK1 up-regulates Na⁺/K⁺-ATPase, a variety of carriers including Na⁺-,K⁺-,2Cl⁻- cotransporter (NKCC), NaCl cotransporter (NCC), Na⁺/H⁺ exchangers, diverse amino acid transporters and several glucose carriers such as Na⁺-coupled glucose transporter SGLT1. SGK1 further up-regulates a large number of ion channels including epithelial Na⁺ channel ENaC, voltagegated Na⁺ channel SCN5A, Ca²⁺ release-activated Ca²⁺ channel (ORAI1) with its stimulator STIM1, epithelial Ca²⁺ channels TRPV5 and TRPV6 and diverse K⁺ channels. Furthermore, SGK1 influences transcription factors such as nuclear factor kappa-B (NF-κB), p53 tumor suppressor protein, cAMP responsive element-binding protein (CREB), activator protein-1 (AP-1) and forkhead box O3 protein (FOXO3a). Thus, SGK1 supports cellular glucose uptake and glycolysis, angiogenesis, cell survival, cell migration, and wound healing. Presumably as last line of defense against tissue injury, SGK1 fosters tissue fibrosis and tissue calcification replacing energy consuming cells.

Two Liters a Day Keep the Doctor Away? Considerations on the Pathophysiology of Suboptimal Fluid Intake in the Common Population

Suboptimal fluid intake may require enhanced release of antidiuretic hormone (ADH) or vasopressin for the maintenance of adequate hydration. Enhanced copeptin levels (reflecting enhanced vasopressin levels) in 25% of the common population are associated with enhanced risk of metabolic syndrome with abdominal obesity, type 2 diabetes, hypertension, coronary artery disease, heart failure, vascular dementia, cognitive impairment, microalbuminuria, chronic kidney disease, inflammatory bowel disease, cancer, and premature mortality. Vasopressin stimulates the release of glucocorticoids which in turn up-regulate the serum- and glucocorticoid-inducible kinase 1 (SGK1). Moreover, dehydration upregulates the transcription factor NFAT5, which in turn stimulates SGK1 expression. SGK1 is activated by insulin, growth factors and oxidative stress via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase PDK1 and mTOR. SGK1 is a powerful stimulator of Na+/K+-ATPase, carriers (e.g. the Na+,K+,2Cl- cotransporter NKCC, the NaCl cotransporter NCC, the Na+/H+ exchanger NHE3, and the Na+ coupled glucose transporter SGLT1), and ion channels (e.g. the epithelial Na+ channel ENaC, the Ca2+ release activated Ca2+ channel Orai1 with its stimulator STIM1, and diverse K+ channels). SGK1 further participates in the regulation of the transcription factors nuclear factor kappa-B NFκB, p53, cAMP responsive element binding protein (CREB), activator protein-1, and forkhead transcription factor FKHR-L1 (FOXO3a). Enhanced SGK1 activity fosters the development of hypertension, obesity, diabetes, thrombosis, stroke, inflammation including inflammatory bowel disease and autoimmune disease, cardiac fibrosis, proteinuria, renal failure as well as tumor growth. The present brief review makes the case that suboptimal fluid intake in the common population may enhance vasopressin and glucocorticoid levels thus up-regulating SGK1 expression and favouring the development of SGK1 related pathologies.

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.

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.

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.

Sweat, the driving force behind normal skin: an emerging perspective on functional biology and regulatory mechanisms

The various symptoms associated with excessive or insufficient perspiration can significantly reduce a patient's quality of life. If a versatile and minimally invasive method could be established for returning sweat activity to normalcy, there is no question that it could be used in the treatment of many diseases that are believed to involve perspiration. For this reason, based on an understanding of the sweat-gland control function and sweat activity, it was necessary to conduct a comprehensive search for the factors that control sweating, such as the central and peripheral nerves that control sweat-gland function, the microenvironment surrounding the sweat glands, and lifestyle. We focused on the mechanism by which atopic dermatitis leads to hypohidrosis and confirmed that histamine inhibits acetylcholinergic sweating. Acetylcholine promotes the phosphorylation of glycogen synthesis kinase 3β (GSK3β) in the sweat-gland secretory cells and leads to sensible perspiration. By suppressing the phosphorylation of GSK3β, histamine inhibits the movement of sweat from the sweat-gland secretory cells through the sweat ducts, which could presumably be demonstrated by dynamic observations of the sweat glands using two-photon microscopy. It is expected that the discovery of new factors that control sweat-gland function can contribute to the treatment of diseases associated with dyshidrosis.

PKB/SGK-resistant GSK-3 signaling following unilateral ureteral obstruction

BACKGROUND/AIMS

Renal tissue fibrosis contributes to the development of end-stage renal disease. Causes for renal tissue fibrosis include obstructive nephropathy. The development of renal fibrosis following unilateral ureteral obstruction (UUO) is blunted in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1. Similar to Akt isoforms, SGK1 phosphorylates and thus inactivates glycogen synthase kinase GSK-3. The present study explored whether PKB/SGK-dependent phoshorylation of GSK-3α/β impacts on pro-fibrotic signaling following UUO.

METHODS

UUO was induced in mice carrying a PKB/SGK-resistant GSK-3α/β (gsk-3(KI)) and corresponding wild-type mice (gsk-3(WT)). Three days after the obstructive injury, expression of fibrosis markers in kidney tissues was analyzed by quantitative RT-PCR and western blotting.

RESULTS

GSK-3α and GSK-3β phosphorylation was absent in both, the non-obstructed and the obstructed kidney tissues from gsk-3(KI) mice but was increased by UUO in kidney tissues from gsk-3(WT) mice. Expression of α-smooth muscle actin, type I collagen and type III collagen in the non-obstructed kidney tissues was not significantly different between gsk-3(KI) mice and gsk-3(WT) mice but was significantly less increased in the obstructed kidney tissues from gsk-3(KI) mice than from gsk-3(WT) mice. After UUO treatment, renal β-catenin protein abundance and renal expression of the β-catenin sensitive genes: c-Myc, Dkk1, Twist and Lef1 were again significantly less increased in kidney tissues from gsk-3(KI) mice than from gsk-3(WT) mice.

CONCLUSIONS

PKB/SGK-dependent phosphorylation of glycogen synthase kinase GSK-3 contributes to the pro-fibrotic signaling leading to renal tissue fibrosis in obstructive nephropathy.

Contribution of aldosterone to cardiovascular and renal inflammation and fibrosis

The steroid hormone aldosterone regulates sodium and potassium homeostasis. Aldosterone and activation of the mineralocorticoid receptor also causes inflammation and fibrosis of the heart, fibrosis and remodelling of blood vessels and tubulointerstitial fibrosis and glomerular injury in the kidney. Aldosterone and mineralocorticoid-receptor activation initiate an inflammatory response by increasing the generation of reactive oxygen species by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and mitochondria. High salt intake potentiates these effects, in part by activating the Rho family member Rac1, a regulatory subunit of reduced NADPH oxidase that activates the mineralocorticoid receptor. Studies in mice in which the mineralocorticoid receptor has been deleted from specific cell types suggest a key role for macrophages in promoting inflammation and fibrosis. Aldosterone can exert mineralocorticoid-receptor-independent effects via the angiotensin II receptor and via G-protein-coupled receptor 30. Mineralocorticoid-receptor antagonists are associated with decreased mortality in patients with heart disease and show promise in patients with kidney injury, but can elevate serum potassium concentration. Studies in rodents genetically deficient in aldosterone synthase or treated with a pharmacological aldosterone-synthase inhibitor are providing insight into the relative contribution of aldosterone compared with the contribution of mineralocorticoid-receptor activation in inflammation, fibrosis, and injury. Aldosterone-synthase inhibitors are under development in humans.

Therapeutic potential of serum and glucocorticoid inducible kinase inhibition

INTRODUCTION

Expression of serum-and-glucocorticoid-inducible kinase-1 (SGK1) is low in most cells, but dramatically increases under certain pathophysiological conditions, such as glucocorticoid or mineralocorticoid excess, inflammation with TGFβ release, hyperglycemia, cell shrinkage and ischemia. SGK1 is activated by insulin and growth factors via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase and mammalian target of rapamycin. SGK1 sensitive functions include activation of ion channels (including epithelial Na(+) channel ENaC, voltage gated Na(+) channel SCN5A transient receptor potential channels TRPV4 - 6, Ca(2+) release activated Ca(2+) channel Orai1/STIM1, renal outer medullary K(+) channel ROMK, voltage gated K(+) channels KCNE1/KCNQ1, kainate receptor GluR6, cystic fibrosis transmembrane regulator CFTR), carriers (including Na(+),Cl(-) symport NCC, Na(+),K(+),2Cl(-) symport NKCC, Na(+)/H(+) exchangers NHE1 and NHE3, Na(+), glucose symport SGLT1, several amino acid transporters), and Na(+)/K(+)-ATPase. SGK1 regulates several enzymes (e.g., glycogen synthase kinase-3, ubiquitin-ligase Nedd4-2) and transcription factors (e.g., forkhead transcription factor 3a, β-catenin, nuclear factor kappa B).

AREAS COVERED

The phenotype of SGK1 knockout mice is mild and SGK1 is apparently dispensible for basic functions. Excessive SGK1 expression and activity, however, contributes to the pathophysiology of several disorders, including hypertension, obesity, diabetes, thrombosis, stroke, fibrosing disease, infertility and tumor growth. A SGK1 gene variant (prevalence ∼ 3 - 5% in Caucasians and ∼ 10% in Africans) is associated with hypertension, stroke, obesity and type 2 diabetes. SGK1 inhibitors have been developed and shown to reduce blood pressure of hyperinsulinemic mice and to counteract tumor cell survival.

EXPERT OPINION

Targeting SGK1 may be a therapeutic option in several clinical conditions, including metabolic syndrome and tumor growth.

Angiotensin II- and salt-induced kidney injury through Rac1-mediated mineralocorticoid receptor activation

Experiments with hyperaldosteronemic animals suggest that, despite lowering plasma aldosterone, salt worsens renal injury by paradoxical activation of the mineralocorticoid receptor (MR). Salt and aldosterone synergistically contribute to renal impairment through Rac1-mediated activation of the MR, but whether angiotensin II also promotes renal injury through this mechanism is unknown. Here, we placed angiotensin II-overproducing double transgenic Tsukuba hypertensive mice on a low- or high-salt intake for 6 weeks and treated some animals with adrenalectomy, the MR antagonist eplerenone, the Rac inhibitor EHT1864, or hydralazine. High-salt intake, but not low-salt intake, led to hypertension and prominent kidney injury. Adrenalectomy prevented angiotensin II/salt-induced nephropathy in mice receiving high-salt intake, which was recapitulated by aldosterone supplementation, suggesting the involvement of aldosterone/MR signaling. Plasma aldosterone levels, however, were lower in high- than low-salt conditions. Instead, angiotensin II/salt-evoked MR activation associated with Rac1 activation and was not dependent on plasma aldosterone level. Both EHT1864 and eplerenone repressed the augmented MR signaling and mitigated kidney injury with partial but significant reduction in BP with high-salt intake. Hydralazine similarly reduced BP, but it neither suppressed the Rac1-MR pathway nor ameliorated the nephropathy. Taken together, these results show that angiotensin II and salt accelerate kidney injury through Rac1-mediated MR activation. Rac inhibition may be a promising strategy for the treatment of CKD.

Aldosterone-induced fibrosis in the kidney: questions and controversies

Over the years, aldosterone has been a favorite topic of renal physiologists given its role in the maintenance of body fluids. Investigators only recently are coming to appreciate a second proinflammatory and profibrotic role for this hormone. Mineralocorticoids such as aldosterone trigger a profibrotic process that in many respects mimics the early phase of wound healing. Depending on the type of cell involved, aldosterone may activate the profibrotic process through classic mineralocorticoid receptors, nonclassic membrane-associated mineralocorticoid receptors, and/or glucocorticoid receptors. In the kidney, the actions of aldosterone can be attenuated by 11-dehydro metabolites of endogenous glucocorticoids generated by isoforms of the enzyme 11β-hydroxysteroid dehydrogenase (11β-HSD-1 and 11β-HSD-2). Thus, the renal 11β-HSD isoforms may have 2 functions: to block the improper activation of mineralocorticoid receptors by binding endogenous glucocorticoids and to synthesize agents that limit the actions of aldosterone. Although sodium in the diet has been implicated in aggravating aldosterone-induced renal fibrotic processes, preliminary findings are consistent with the view that aldosterone alone can initiate matrix production in renal tissue even in the absence of active sodium transport. Thus, there is a growing body of laboratory and clinical evidence supporting the use of inhibitors of aldosterone action in patients with both glomerular and tubular diseases.

Heme arginate therapy enhanced adiponectin and atrial natriuretic peptide, but abated endothelin-1 with attenuation of kidney histopathological lesions in mineralocorticoid-induced hypertension

We investigated the role of heme oxygenase (HO), adiponectin, and atrial natriuretic peptide (ANP) in uninephrectomized (UnX) deoxycorticosterone-acetate (DOCA)-salt hypertensive rats, a volume-overload model characterized by elevated endothelin-1 (ET-1), mineralocorticoid-induced oxidative/inflammatory insults, fibrosis, hypertrophy, and severe renal histopathological lesions that closely mimic end-stage renal disease (ESRD). HO was enhanced with heme arginate (HA) or blocked with chromium mesoporphyrin (CrMP). Histological, morphological/morphometrical, quantitative reverse transcription-polymerase chain reaction, Western blot, enzyme immunoassay, and spectrophotometric analysis were used. Our experimental design included the following groups of rats: A, controls [surgery-free Sprague-Dawley, UnX-sham, UnX-salt (0.9% NaCl + 0.2% KCl), and UnX-DOCA]; B, UnX-DOCA-salt hypertensive; C, UnX-DOCA-salt + HA; D, UnX-DOCA-salt + HA + CrMP; E, UnX-DOCA-salt + CrMP; F, UnX-DOCA-salt + captopril; G, UnX-DOCA-salt + L-arginine; H, UnX-DOCA-salt + spironolactone; and I, UnX-DOCA-salt + vehicle. HA lowered blood pressure and abated kidney hypertrophy and renal lesions, including glomerulosclerosis, tubular dilation, tubular cast formation, interstitial mononuclear cell infiltration, glomerular hypertrophy, and renal-arteriolar thickening in UnX-DOCA hypertension. Correspondingly, HO activity, adiponectin, adenosine monophosphate-activated protein kinase (AMPK), ANP, cGMP, antioxidants such as bilirubin, ferritin, superoxide dismutase, and catalase, and total antioxidant capacity were increased, whereas ET-1, transforming growth factor beta (TGF-beta), fibronectin, and 8-isoprostane were abated. These were accompanied by reduced proteinuria/albuminuria, but increased creatinine clearance. Interestingly, HA was more renoprotective than sipronolactone, L-arginine, and captopril, whereas the HO blocker CrMP exacerbated oxidative injury, aggravating renal lesions and function. Because 8-isoprostane stimulates ET-1 to potentiate oxidative stress and fibrosis, up-regulating HO-1 enhanced tissue antioxidant status alongside cellular targets such as adiponectin, AMPK, ANP, and cGMP to suppress ET-1, TGF-beta, and fibronectin with a corresponding decline of renal lesions, proteinuria/albuminuria, and thus improved renal function. The potent renoprotection of HA could be explored to combat renal hypertrophy and histopathological lesions characteristic of ESRD.

Activation of the aldosterone/mineralocorticoid receptor system in chronic kidney disease and metabolic syndrome

Recent clinical and experimental studies have shown that aldosterone is a potent inducer of proteinuria and that mineralocorticoid receptor (MR) antagonists confer efficient antiproteinuric effects. We identified glomerular epithelial cells (podocytes) as novel targets of aldosterone; activation of MR injures podocytes possibly via oxidative stress, resulting in disruption of glomerular filtration barrier, proteinuria, and progression of chronic kidney disease. We also demonstrated that SHR/cp, a rat model of metabolic syndrome, was susceptible to podocyte injury and proteinuria. Aldosterone excess caused by adipocyte-derived aldosterone-releasing factors was suggested to underlie the nephropathy. High salt intake augmented MR activation in the kidney and exacerbated the nephropathy. Furthermore, we identified an alternative pathway of MR activation by small GTPase Rac1. RhoGDIalpha knockout mice, a model with Rac1 activation in the kidney, showed albuminuria, podocyte injury, and glomerulosclerosis. Renal injury in the knockout mice was accompanied by enhanced MR signaling in the kidney despite normoaldosteronemia, and was ameliorated by an MR antagonist, eplerenone. Moreover, Rac-specific inhibitor significantly reduced the nephropathy, concomitantly with repression of MR activation. In vitro transfection studies provided direct evidence of Rac1-mediated MR activation. In conclusion, our findings suggest that MR activation plays a pivotal role in the pathogenesis of chronic kidney disease in metabolic syndrome, and that MR may be activated both aldosterone dependently (via aldosterone-releasing factors) and independently (via Rac1). MR antagonists are promising antiproteinuric drugs in metabolic syndrome, although long-term effects on renal outcomes, mortality, and safety need to be established.

Heme-arginate suppresses phospholipase C and oxidative stress in the mesenteric arterioles of mineralcorticoid-induced hypertensive rats

Induction of heme-oxygenase (HO) is an important cellular defense mechanism against oxidative and inflammatory insults. We analyzed the effects of the HO inducer, heme-arginate, on the phospholipase C (PLC)/inositol-triphosphate (IP(3)) pathway in the mesenteric arterioles of uninephrectomized (UnX) deoxycorticosterone acetate (DOCA)-salt hypertensive rats, which is a volume-overload model characterized by elevated endothelin (ET-1) and mineralocorticoid-induced oxidative/inflammatory insults. Our study included the following groups: (A) controls [(i) surgery-free Sprague-Dawley (SD) rats, (ii) UnX-Sham, (iii) UnX-Salt (0.9% NaCl+0.2% KCl) and (iv) UnX-DOCA)]; (B) UnX-DOCA-salt hypertensive rats; (C) UnX-DOCA-salt+heme-arginate; (D) UnX-DOCA-salt+heme-arginate+chromium mesoporphyrin (CrMP), the HO inhibitor; (E) UnX-DOCA-salt+CrMP (F); SD+heme-arginate, (G) UnX-DOCA-salt+vehicle dissolving heme-arginate and CrMP and (H) normal-SD+heme-arginate. Quantitative reverse transcriptase PCR, western blot, enzyme immunoassay and spectrophotometric analyses were used. Heme-arginate enhanced mesenteric arteriole HO-1, HO activity, cyclic guanosine monophosphate (cGMP) and anti-oxidants including bilirubin, ferritin, superoxide dismutase with potentiation of the total anti-oxidant capacity. Correspondingly, oxidative/inflammatory mediators such as 8-isoprostane, nuclear-factor kappaB (NF-kappaB) and ET-1 were markedly reduced. Furthermore, heme-arginate suppressed PLC activity, attenuated IP(3) and reduced resting intracellular calcium. The effects of heme-arginate were nullified by the HO inhibitor, with aggravation of oxidative/inflammatory insults. In heme-arginate-treated SD rats, the HO system was potentiated to a lesser magnitude and the suppression of ET-1, PLC, IP(3) and NF-kappaB were less accentuated, suggesting greater selectivity of HO against the ET-1-PLC-IP(3)-NF-kappaB destructive axis in the pathological condition of mineralocorticoid-induced hypertension. Given that ET-1 stimulates PLC and IP(3), which in turn activates NF-kappaB, the concomitant reduction of ET-1, PLC, IP(3) and NF-kappaB alongside the corresponding decline of resting intracellular calcium may account for the reduction of blood pressure and attenuation of oxidative/inflammatory injury by heme-arginate.

Direct fibrogenic effects of aldosterone on normotensive kidney: an effect modified by 11β-HSD activity

Aldosterone (Aldo) can be a profibrotic factor in cardiovascular and renal tissues. This study tests the hypothesis that prolonged Aldo exposure is able to directly induce fibrotic changes in the kidney of a normal nonhypertensive animal. Immortalized rat proximal tubule cells (IRPTC) containing 11β-hydroxysteroid dehydrogenase (11β-HSD1) but no mineralocorticoid receptors (MR) and mouse inner medullary collecting duct cells (IMCD) containing 11β-HSD2 and MR were examined. IRPTC exposed to Aldo or corticosterone (10 nM) for 48 h demonstrated no change in collagen production as assessed by Sirius red staining. In contrast, IMCD treated with Aldo exhibited a marked increase in the expression of collagen, fibronectin, and connective tissue growth factor (CTGF), whereas corticosterone alone had no effect. The Aldo-induced overexperession of collagen, fibronectin, and CTGF was substantially attenuated by the MR antagonist RU-318 and by the 11β-HSD end product 11-dehydrocorticosterone, but not by the glucocorticoid receptor antagonist RU-486. In vivo, early fibrotic changes with elevated collagen, fibronectin, and CTGF expression were observed in kidneys isolated from normotensive adrenalectomized mice receiving a continuous infusion of Aldo (8 μg·kg(-1)·day(-1)) for 1 wk. These changes were not present in corticosterone-treated mice. Aldo-induced changes were attenuated in adrenally intact mice and in mice treated with RU-318 or 11-dehydrocorticosterone. Thus, extended Aldo exposure produces fibrotic changes in cells containing MR and in normal kidneys. MR antagonists and the end products of 11β-HSD attenuate these fibrogenic effects.

Deletion of Mineralocorticoid Receptors From Macrophages Protects Against Deoxycorticosterone/Salt-Induced Cardiac Fibrosis and Increased Blood Pressure

Increased mineralocorticoid levels plus high salt promote vascular inflammation and cardiac tissue remodeling. Mineralocorticoid receptors are expressed in many cell types of the cardiovascular system, including monocytes/macrophages and other inflammatory cell types. Although mineralocorticoid receptors are expressed in monocytes/macrophages, their role in regulating macrophage function to date has not been investigated. We, thus, used the Cre/LoxP-recombination system to selectively delete mineralocorticoid receptors from monocytes/macrophages with the lysozyme M promoter used to drive Cre expression (MR flox/flox /LysM Cre/− mice). Male mice from each genotype (MR flox/flox or wild-type and MR flox/flox /LysM Cre/− mice) were uninephrectomized, given 0.9% NaCl solution to drink, and treated for 8 days or 8 weeks with either vehicle (n=10) or deoxycorticosterone (n=10). Equivalent tissue macrophage numbers were seen for deoxycorticosterone treatment of each genotype at 8 days; in contrast, plasminogen activator inhibitor type 1 and NAD(P)H oxidase subunit 2 levels were increased in wild-type but not in MR flox/flox /LysM Cre/− mice given deoxycorticosterone. Baseline expression of other inflammatory genes was reduced in MR flox/flox /LysM Cre/− mice compared with wild-type mice. At 8 weeks, deoxycorticosterone-induced macrophage recruitment and connective tissue growth factor and plasminogen activator inhibitor type 1 mRNA levels were similar for each genotype; in contrast, MR flox/flox /LysM Cre/− mice showed no increase in cardiac fibrosis or blood pressure, as was seen in wild-type mice at 8 weeks. These data demonstrate the following points: (1) mineralocorticoid receptor signaling regulates basal monocyte/macrophage function; (2) macrophage recruitment is not altered by loss of mineralocorticoid receptor signaling in these cells; and (3) a novel and significant role is seen for macrophage signaling in the regulation of cardiac remodeling and systolic blood pressure in the deoxycorticosterone/salt model.

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.

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.

Hemin therapy attenuates kidney injury in deoxycorticosterone acetate-salt hypertensive rats

Upregulating the heme oxygenase (HO) system removes the prooxidant heme, and thus is cytoprotective. Additionally, the products from the HO pathway including, carbon monoxide, bilirubin, and biliverdin, scavenge reactive oxygen species, inhibit lipid peroxidation, and suppress tissue inflammation, while the iron formed enhances the synthesis of the antioxidant ferritin. Deoxycorticosterone acetate (DOCA)-salt hypertension, a model of human primary aldosteronism, causes oxidative stress and impairs renal function by stimulating inflammatory/oxidative transcription factors such as NF-kappaB and activating protein (AP-1). The effect of the HO system in end-organ damage in mineralocorticoid-induced hypertension has not been fully characterized. In this study, the administration of the HO inducer hemin lowered blood pressure (191 vs. 135 mmHg; n = 22, P < 0.01), increased creatinine clearance, and reduced kidney hypertrophy proteinuria, albuminuria, and histopathological lesions, including glomerular hypertrophy, glomerulosclerosis, tubular dilation, tubular cast formation, and interstitial mononuclear cell infiltration in nephrectomy/DOCA-high-salt-hypertension. The renoprotection was accompanied by reduced levels of NF-kappaB, AP-1, fibronectin, transforming growth factor (TGF)-beta, and 8-isoprostane, a marker of oxidative stress. Correspondingly, a robust increase in total antioxidant capacity, HO activity, cGMP, and an antioxidant like ferritin was observed in hemin-treated animals. Our findings suggest that suppression of oxidative/inflammatory insults alongside the corresponding decline of fibronectin and TGF-beta, an activator of extracellular matrix proteins, may account for the attenuation of renal histopathological lesions and the antihypertrophic effects of hemin. The multifaceted interaction among the HO system, TGF-beta, fibronectin, AP-1, and NF-kappaB may be explored to design new drugs against end-stage-organ damage.

Aldosterone activates NF-kappaB in the collecting duct

Besides its classical effects on salt homeostasis in renal epithelial cells, aldosterone promotes inflammation and fibrosis and modulates cell proliferation. The proinflammatory transcription factor NF-kappaB has been implicated in cell proliferation, apoptosis, and regulation of transepithelial sodium transport. The effect of aldosterone on the NF-kappaB pathway in principal cells of the cortical collecting duct, a major physiologic target of aldosterone, is unknown. Here, in both cultured cells and freshly isolated rat cortical collecting duct, aldosterone activated the canonical NF-kappaB signaling pathway, leading to increased expression of several NF-kappaB-targeted genes (IkappaBalpha, plasminogen activator inhibitor 1, monocyte chemoattractant protein 1, IL-1beta, and IL-6). Small interfering RNA-mediated knockdown of the serum and glucocorticoid-inducible kinase SGK1, a gene induced early in the response to aldosterone, but not pharmacologic inhibition of extracellular signal-regulated kinase and p38 kinase, attenuated aldosterone-induced NF-kappaB activation. Pharmacologic antagonism or knockdown of the mineralocorticoid receptor prevented aldosterone-induced NF-kappaB activity. In addition, activation of the glucocorticoid receptor inhibited the transactivation of NF-kappaB by aldosterone. In agreement with these in vitro findings, spironolactone prevented NF-kappaB-induced transcriptional activation observed in cortical collecting ducts of salt-restricted rats. In summary, aldosterone activates the canonical NF-kappaB pathway in principal cells of the cortical collecting duct by activating the mineralocorticoid receptor and by inducing SGK1.

Serum- and glucocorticoid-inducible kinase 1 in doxorubicin-induced nephrotic syndrome

Doxorubicin-induced nephropathy leads to epithelial sodium channel (ENaC)-dependent volume retention and renal fibrosis. The aldosterone-sensitive serum- and glucocorticoid-inducible kinase SGK1 has been shown to participate in the stimulation of ENaC and to mediate renal fibrosis following mineralocorticoid and salt excess. The present study was performed to elucidate the role of SGK1 in the volume retention and fibrosis during nephrotic syndrome. To this end, doxorubicin (15 mug/g body wt) was injected intravenously into gene-targeted mice lacking SGK1 (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)). Doxorubicin treatment resulted in heavy proteinuria (>100 mg protein/mg crea) in 15/44 of sgk1(+/+) and 15/44 of sgk1(-/-) mice leading to severe nephrotic syndrome with ascites, lipidemia, and hypoalbuminemia in both genotypes. Plasma aldosterone levels increased in nephrotic mice of both genotypes and was followed by increased SGK1 protein expression in sgk1(+/+) mice. Urinary sodium excretion reached signficantly lower values in sgk1(+/+) mice (15 +/- 5 mumol/mg crea) than in sgk1(-/-) mice (35 +/- 5 mumol/mg crea) and was associated with a significantly higher body weight gain in sgk1(+/+) compared with sgk1(-/-) mice (+6.6 +/- 0.7 vs. +4.1 +/- 0.8 g). During the course of nephrotic syndrome, serum urea concentrations increased significantly faster in sgk1(-/-) mice than in sgk1(+/+) mice leading to uremia and a reduced median survival in sgk1(-/-) mice (29 vs. 40 days in sgk1(+/+) mice). In conclusion, gene-targeted mice lacking SGK1 showed blunted volume retention, yet were not protected against renal fibrosis during experimental nephrotic syndrome.

Steroid hormone release as well as renal water and electrolyte excretion of mice expressing PKB/SGK-resistant GSK3

Insulin and insulin-like growth factor (IGF1) participate in the regulation of renal electrolyte excretion. Insulin- and IGF1-dependent signaling includes phosphatidylinositide-3 (PI3)-kinase, phosphoinositide-dependent kinase PDK1 as well as protein kinase B (PKB) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit glycogen synthase kinase GSK3alpha,beta. Replacement of the serines in the PKB/SGK consensus sequences by alanine (gsk3 ( KI )) confers resistance of GSK3 to PKB/SGK. To explore the role of PKB/SGK-dependent inhibition of GSK3 in the regulation of water/electrolyte metabolism, mice carrying the PKB/SGK resistant mutant (gsk3 ( KI )) were compared to their wild-type littermates (gsk3 ( WT ) ). Body weight was similar in gsk3 ( KI ) and gsk3 ( WT ) mice. Plasma aldosterone at 10 A.M: . and corticosterone concentrations at 5 P.M: . were significantly lower, but 24-h urinary aldosterone was significantly higher, and corticosterone excretion tended to be higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. Food and water intake, fecal excretion, glomerular filtration rate, urinary flow rate, urine osmolarity, as well as urinary Na+, K+, urea excretion were significantly larger, and plasma Na+, urea, but not K+ concentration, were significantly lower in gsk3 ( KI ) than in gsk3 ( WT ) mice. Body temperature was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. When allowed to choose between tap water and saline, gsk3 ( WT ) mice drank more saline, whereas gsk3 ( KI ) mice drank similar large volumes of tap water and saline. During high-salt diet, urinary vasopressin excretion increased to significantly higher levels in gsk3 ( KI ) than in gsk3 ( WT ) mice. After water deprivation, body weight decreased faster in gsk3 ( KI ) than in gsk3 ( WT ) mice. Blood pressure, however, was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. The observations disclose a role of PKB/SGK-dependent GSK3 activity in the regulation of steroid hormone release, renal water and electrolyte excretion and blood pressure control.

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.

Aldosterone-stimulated SGK1 activity mediates profibrotic signaling in the mesangium

Several recent reports support the hypothesis that aldosterone contributes to the progression of renal injury. Mineralocorticoids increase the expression of serum- and glucocorticoid-inducible protein kinase 1 (SGK1), which is upregulated in several fibrotic diseases. It was hypothesized that SGK1 may mediate the effects of aldosterone on glomerular fibrosis and inflammation. In primary cultures of rat mesangial cells, aldosterone stimulated the expression, phosphorylation, and kinase activity of SGK1, as well as SGK1-dependent NF-kappaB activity. Furthermore, aldosterone augmented the promoter activity and protein expression of intercellular adhesion molecule-1 (ICAM-1), which modulates the inflammatory response, and the profibrotic cytokine connective tissue growth factor (CTGF) in an SGK1- and NF-kappaB-dependent manner. Similar to the in vitro results, uninephrectomized rats that were treated with aldosterone demonstrated increased glomerular expression of SGK1, ICAM-1, and CTGF proteins than untreated rats; these changes were accompanied by hypertension, glomerulosclerosis, and inflammation. In conclusion, these findings suggest that aldosterone stimulates ICAM-1 and CTGF transcription via the activation of SGK1 and NF-kappaB, effects that may contribute to the progression of aldosterone-induced mesangial fibrosis and inflammation.

SGK1 is not required for regulation of colonic ENaC activity

The serum and glucocorticoid-inducible kinase SGK1 is known to be upregulated by mineralocorticoids and to enhance ENaC activity in several expression systems. Moreover, the amiloride-sensitive transepithelial potential difference in the collecting duct is lower in gene-targeted mice lacking SGK1 (sgk1 (-/-)) than in their wild-type littermates (sgk1 (+/+)). Accordingly, the ability of sgk1 (-/-) mice to decrease urinary sodium output during salt depletion is impaired. These observations highlight the importance of SGK1 in the stimulation of renal ENaC activity. In colonic epithelium, ENaC activity and, thus, transepithelial potential difference (V (te)) are similarly upregulated by mineralocorticoids. The present study thus explored V (te) and the apparent amiloride-sensitive equivalent short circuit current (I (amil)) in the colon from sgk1 (-/-) and sgk1 (+/+) mice before and after treatment with low salt diet, the glucocorticoid dexamethasone [DEXA, 10 mug/g body weight (BW)], or the mineralocorticoid deoxycorticosterone acetate (DOCA, 1.5 mg/day). Surprisingly, V (te) and I (amil) were both significantly (p<0.05) higher in sgk1 (-/-) than in sgk1 (+/+) untreated mice. A 7-day exposure to low salt diet increased V (te) and I (amil) in both genotypes, but did not abrogate the differences of V (te) and I (amil) between sgk1 (-/-) and sgk1 (+/+) mice. Plasma aldosterone levels were significantly higher in sgk1 (-/-) than in sgk1 (+/+) mice both under control conditions and under low salt diet, which may explain the enhanced V (te) in sgk1 (-/-) mice. Treatment with DEXA or DOCA both significantly increased V (te) and I (amil) in sgk1 (+/+) mice and tended to increase V (te) and I (amil) in sgk1 (-/-) mice. Under treatment with DEXA or DOCA, V (te) and I (amil) were similar in sgk1 (-/-) and sgk1 (+/+) mice. Fecal Na(+) excretion was similar in sgk1 (+/+) mice and in sgk1 (-/-) mice and was similarly decreased by low Na(+) diet in both genotypes. In conclusion, transepithelial potential and amiloride-sensitive short circuit current are enhanced in the colonic epithelium of SGK1-deficient mice. Thus, lack of SGK1 does not disrupt colonic ENaC activity and its regulation by salt depletion.