Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Intrauterine hyperglycemia impairs endometrial receptivity via up-regulating SGK1 in diabetes

Diabetes is a complex metabolic disorder which can adversely affect reproductive function. SGK1 is found to be up-regulated in multiple tissues of diabetic patients. However, the effects of diabetes on endometrial SGK1 expression and endometrial receptivity remain unknown. In this study, we established a streptozotocin-induced diabetic mouse model and observed reduced implantation sites, retarded development of pinopodes, increased SGK1, and aberrant expression of LIF and MUC1 in the endometrial epithelium. We injected the uterine lumen of normal mice with high-glucose solution and cultured endometrial cells in high-glucose medium to mimic intrauterine hyperglycemia. Both studies provided compelling evidence that hyperglycemia could lead to diminished embryo implantation and dysregulated SGK1, LIF and MUC1. Additionally, through over-expression of SGK1 in vivo and in vitro, we found that enhanced SGK1 also decreased LIF expression, increased MUC1 expression, and attenuated embryo implantation rate. We further identified that hyperglycemia-activated SMAD2/3 might be responsible for the enhancement of SGK1 and verified directly the interaction between SMAD3 and corresponding SMAD binding elements within SGK1 promoter. Taken together, our study confirmed the association between diabetes-related hyperglycemia and endometrial receptivity defects. Hyperglycemia-induced SGK1 has a tremendous role in this pathological process, rendering it as an attractive therapeutic target for diabetes-related reproductive disorders.

Does the early aldosterone-induced SGK1 play a role in early Kaliuresis?

Urinary K⁺ potassium excretion rapidly increases after a potassium-rich meal. The early aldosterone-induced sgk1 gene (encoding serum and glucocorticoid-induced kinase 1), activates potassium clearance, but the role of this kinase in the early activation of K⁺ secretion has not been clearly defined. Here, we challenged inducible renal-tubule-specific Sgk1^{Pax8 / LC1}  knockout mice with an acute high-potassium load (HK:5%K⁺ ) and compared the physiological and molecular responses to control mice. We observe that urinary excretion after a K⁺ load over the first 3 h is not dependent on SGK1 but is coincident with the rapid dephosphorylation of the Na⁺ ,Cl^- -cotransporter (NCC) to increase distal salt delivery. Molecular analyses indicate that whereas SGK1-mediated phosphorylation of the ubiquitin-protein ligase NEDD4-2 begins to increase by 3h, SGK1-dependent proteolytic activation of ENaC only becomes detectable after 6 h of HK intake. Consistent with SGK1-dependent ENaC activation via inhibition of NEDD4-2-mediated ubiquitylation, Sgk1^{Pax8 / LC1}  mice are unable to efficiently inhibit NEDD4-2 or increase ENaC cleavage after 6 h of HK. Nevertheless, no defect in acute K⁺ balance was detected in the mutant mice after 6 h of HK. Moreover, we found that Sgk1^{Pax8 / LC1}  mice reduce NCC phosphorylation and NCC-mediated salt absorption to a greater extent than control mice after a K⁺ load, promoting increased amiloride-sensitive Na⁺ -reabsorption via ENaC to maintain adequate kaliuresis. Together, these data indicate that: (a) during the early 3 h of HK intake, K⁺ excretion is SGK1-independent even under an extreme K⁺ challenge, (b) shortly after, SGK1 inhibits NEDD4-2 and activates ENaC to stimulate K⁺ -secretion, (c) SGK1-dependent phosphorylation of NCC occurs, acting more likely as a brake pedal to prevent excessive K⁺ loss.

Mechanistic insights into the role of serum-glucocorticoid kinase 1 in diabetic nephropathy: A systematic review

Aberrant expression of serum-glucocorticoid kinase 1 (SGK1) contributes to the pathogenesis of multiple disorders, including diabetes, hypertension, obesity, fibrosis, and metabolic syndrome. SGK1 variant is expressed in the presence of insulin and several growth factors, eventually modulating various ion channels, carrier proteins, and transcription factors. SGK1 also regulates the enzymatic activity of Na⁺ K⁺ ATPase, glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, and phosphomannose mutase impacting cell cycle regulation, neuroexcitation, and apoptosis. Ample evidence supports the crucial role of aberrant SGK1 expression in hyperglycemia-mediated secondary organ damage. Diabetic nephropathy (DN), a dreadful microvascular complication of diabetes, is the leading cause of end-stage renal failures with high morbidity and mortality rate. The complex pathogenesis of DN encompasses several influencing factors, including transcriptional factors, inflammatory markers, cytokines, epigenetic modulators, and abnormal enzymatic activities. SGK1 plays a pivotal role by controlling various physiological functions associated with the occurrence and progression of DN; therefore, targeting SGK1 may favorably influence the clinical outcome in patients with DN. This review aimed to provide mechanistic insights into SGK1 regulated DN pathogenesis and summarize the evidence supporting the therapeutic potential of SGK1 inhibition and its consequences on human health.

SGK1 mediates hypotonic challenge-induced proliferation in basilar artery smooth muscle cells via promoting CREB signaling pathway

Hypotonic stimulus enlarges cell volume and increased cell proliferation with the exact mechanisms unknown. Glucocorticoid-induced kinase-1 (SGK1) is a serine/threonine kinase that can be regulated by osmotic pressure. We have revealed that SGK1 was activated by hypotonic solution-induced lowering of intracellular Cl- concentration. Therefore, we further examined whether SGK1 mediated hypotonic solution-induced proliferation and the internal mechanisms in basilar smooth muscle cells (BASMCs). In the present study, BrdU incorporation assay, flow cytometry, western blotting were performed to evaluate cell viability, cell cycle transition, and the expression of cell cycle regulators and other related proteins. We found that silence of SGK1 largely blunted hypotonic challenge-induced increase in cell viability and cell cycle transition from G0/G1 phase to S phase, whereas overexpression of SGK1 showed the opposite effects. The effect of SGK1 on proliferation was related to the upregulation of cyclin D1 and cyclin E1, and the downregulation of p27 and p21, which is mediated by the interaction between SGK1 and cAMP responsive element-binding protein (CREB). Moreover, we overexpressed ClC-3 Cl- channel to further verify the role of SGK1 in low Cl- environment-induced proliferation. The results revealed that overexpression of ClC-3 further enhanced hypotonic solution-induced cell viability, cell cycle transition, and CREB activation, which were alleviated or potentiated by silencing or overexpression of SGK1. In summary, this study provides compelling evidences that SGK1, as a Cl--sensitive kinase, is a critical link between low osmotic pressure and proliferation in BASMCs, and shed a new light on the treatment of proliferation-associated cardiovascular diseases.

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.

Role of SGK1 in the Osteogenic Transdifferentiation and Calcification of Vascular Smooth Muscle Cells Promoted by Hyperglycemic Conditions

In diabetes mellitus, hyperglycemia promotes the osteogenic transdifferentiation of vascular smooth muscle cells (VSMCs) to enhance medial vascular calcification, a common complication strongly associated with cardiovascular disease and mortality. The mechanisms involved are, however, still poorly understood. Therefore, the present study explored the potential role of serum- and glucocorticoid-inducible kinase 1 (SGK1) during vascular calcification promoted by hyperglycemic conditions. Exposure to high-glucose conditions up-regulated the SGK1 expression in primary human aortic VSMCs. High glucose increased osteogenic marker expression and activity and, thus, promoted the osteogenic transdifferentiation of VSMCs, effects significantly suppressed by additional treatment with the SGK1 inhibitor EMD638683. Moreover, high glucose augmented the mineralization of VSMCs in the presence of calcification medium, effects again significantly reduced by SGK1 inhibition. Similarly, SGK1 knockdown blunted the high glucose-induced osteogenic transdifferentiation of VSMCs. The osteoinductive signaling promoted by high glucose required SGK1-dependent NF-kB activation. In addition, advanced glycation end products (AGEs) increased the SGK1 expression in VSMCs, and SGK1 inhibition was able to interfere with AGEs-induced osteogenic signaling. In conclusion, SGK1 is up-regulated and mediates, at least partly, the osteogenic transdifferentiation and calcification of VSMCs during hyperglycemic conditions. Thus, SGK1 inhibition may reduce the development of vascular calcification promoted by hyperglycemia in diabetes.

SGK1 mediates the hypotonic protective effect against H2O2-induced apoptosis of rat basilar artery smooth muscle cells by inhibiting the FOXO3a/Bim signaling pathway

Serum- and glucocorticoid-inducible kinease-1 (SGK1) is a serine/threonine kinase regulated by hypotonic stimuli, which is involved in regulation of cell cycle and apoptosis. Our previous study shows that activation of volume-regulated Cl^- channels (VRCCs) protects rat basilar artery smooth muscle cells (BASMCs) against hydrogen peroxide (H₂O₂)-induced apoptosis. In the present study, we investigated whether SGK1 was involved in the protective effect of VRCCs in BASMCs. We showed that hypotonic challenge significantly reduced H₂O₂-induced apoptosis, and increased SGK1 phosphorylation, but did not affect SGK1 protein expression. The protective effect of hypotonic challenge against H₂O₂-induced apoptosis was mediated through inhibiting mitochondria-dependent apoptotic pathway, evidenced by increased Bcl-2/Bax ratio, stabilizing mitochondrial membrane potential (MMP), decreased cytochrome c release from the mitochondria to the cytoplasm, and inhibition of the activation of caspase-9 and caspase-3. These protective effects of hypotonic challenge against H₂O₂-induced apoptosis was diminished and enhanced, respectively, by SGK1 knockdown and overexpression. We further revealed that SGK1 activation significantly increased forkhead box O3a (FOXO3a) phosphorylation, and then inhibited the translocation of FOXO3a into nucleus and the subsequent expression of Bcl-2 interacting mediator of cell death (Bim). In conclusion, SGK1 mediates the protective effect of VRCCs against H₂O₂-induced apoptosis in BASMCs via inhibiting FOXO3a/Bim signaling pathway. Our results provide compelling evidences that SGK1 is a critical link between VRCCs and apoptosis, and shed a new light on the treatment of vascular apoptosis-associated diseases, such as vascular remodeling, angiogenesis, and atherosclerosis.

Renal Effects of Fetal Reprogramming With Pentaerythritol Tetranitrate in Spontaneously Hypertensive Rats

Aims

Current antihypertensive therapies cannot cure hypertension and a life-long medication is necessary. Maternal treatment may represent a promising strategy for hypertension treatment. We have previously shown that maternal treatment of spontaneously hypertensive rats (SHR) with pentaerythritol tetranitrate (PETN) leads to a persistent blood pressure reduction in the female offspring. The underlying mechanisms include improved endothelial function resulting from long-lasting epigenetic changes. In the present study, we address the renal effects of maternal PETN treatment.

Methods and Results

F0 parental SHR were fed with either normal chow or PETN-containing (1 g/kg) chow ad libitum from the time point of mating to the end of lactation period. The F1 offspring received normal chow without PETN from the time point of weaning (at the age of 3 weeks). At the age of 16 weeks, female PETN offspring showed lower blood pressure than the control. No difference was observed in male offspring. All following experiments were performed with kidneys from 16-week-old female offspring. Maternal PETN treatment reduced the mRNA and protein expression of angiotensin-converting enzyme (ACE) and basic fibroblast growth factor (FGF2), resulting from epigenetic modifications found at the proximal promoter regions. The expression levels of mineralocorticoid receptor (MR) and factors in the MR signalling pathway (Rac1 and Sgk1) were also reduced by PETN. Major profibrotic cytokines, including Wnt4, TNF-alpha, TGF-beta, and MMP9, were downregulated by PETN, which was associated with reduced collagen deposition and glomerulus sclerosis in the kidney of PETN offspring. In addition, PETN treatment also decreased the markers of inflammation and immune cell infiltration in the kidneys.

Conclusions

PETN maternal treatment leads to epigenetic changes in the kidney of female SHR offspring. The reduced renal inflammation, alleviated kidney fibrosis, and decreased MR signalling are potential mechanisms contributing to the observed blood pressure-lowering effect.

The Disease-modifying Drug Candidate, SAK3 Improves Cognitive Impairment and Inhibits Amyloid beta Deposition in App Knock-in Mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the most common form of elderly dementia in the world. At present, acetylcholine inhibitors, such as donepezil, galantamine and rivastigmine, are used for AD therapy, but the therapeutic efficacy is limited. We recently proposed T-type voltage-gated Ca²⁺ channels' (T-VGCCs) enhancer as a new therapeutic candidate for AD. In the current study, we confirmed the pharmacokinetics of SAK3 in the plasma and brain of mice using ultra performance liquid chromatography-tandem mass spectrometry. We also investigated the effects of SAK3 on the major symptoms of AD, such as cognitive dysfunction and amyloid beta (Aβ) accumulation, in App^NL-F knock-in (NL-F) mice, which have been established as an AD model. Chronic SAK3 (0.5 mg/kg/day) oral administration for 3 months from 9 months of age improved cognitive function and inhibited Aβ deposition in 12-month-old NL-F mice. Using microarray and real-time PCR analysis, we discovered serum- and glucocorticoid-induced protein kinase 1 (SGK1) as one of possible genes involved in the inhibition of Aβ deposition and improvement of cognitive function by SAK3. These results support the idea that T-VGCC enhancer, SAK3 could be a novel candidate for disease-modifying therapeutics for AD.

The impact of insulin resistance on the kidney and vasculature

Insulin resistance is a systemic disorder that affects many organs and insulin-regulated pathways. The disorder is characterized by a reduced action of insulin despite increased insulin concentrations (hyperinsulinaemia). The effects of insulin on the kidney and vasculature differ in part from the effects on classical insulin target organs. Insulin causes vasodilation by enhancing endothelial nitric oxide production through activation of the phosphatidylinositol 3-kinase pathway. In insulin-resistant states, this pathway is impaired and the mitogen-activated protein kinase pathway stimulates vasoconstriction. The action of insulin on perivascular fat tissue and the subsequent effects on the vascular wall are not fully understood, but the hepatokine fetuin-A, which is released by fatty liver, might promote the proinflammatory effects of perivascular fat. The strong association of salt-sensitive arterial hypertension with insulin resistance indicates an involvement of the kidney in the insulin resistance syndrome. The insulin receptor is expressed on renal tubular cells and podocytes and insulin signalling has important roles in podocyte viability and tubular function. Renal sodium transport is preserved in insulin resistance and contributes to the salt-sensitivity of blood pressure in hyperinsulinaemia. Therapeutically, renal and vascular insulin resistance can be improved by an integrated holistic approach aimed at restoring overall insulin sensitivity and improving insulin signalling.

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.

Sodium glucose cotransporter SGLT1 as a therapeutic target in diabetes mellitus

INTRODUCTION

Glycemic control is important in diabetes mellitus to minimize the progression of the disease and the risk of potentially devastating complications. Inhibition of the sodium-glucose cotransporter SGLT2 induces glucosuria and has been established as a new anti-hyperglycemic strategy. SGLT1 plays a distinct and complementing role to SGLT2 in glucose homeostasis and, therefore, SGLT1 inhibition may also have therapeutic potential.

AREAS COVERED

This review focuses on the physiology of SGLT1 in the small intestine and kidney and its pathophysiological role in diabetes. The therapeutic potential of SGLT1 inhibition, alone as well as in combination with SGLT2 inhibition, for anti-hyperglycemic therapy are discussed. Additionally, this review considers the effects on other SGLT1-expressing organs like the heart.

EXPERT OPINION

SGLT1 inhibition improves glucose homeostasis by reducing dietary glucose absorption in the intestine and by increasing the release of gastrointestinal incretins like glucagon-like peptide-1. SGLT1 inhibition has a small glucosuric effect in the normal kidney and this effect is increased in diabetes and during inhibition of SGLT2, which deliver more glucose to SGLT1 in late proximal tubule. In short-term studies, inhibition of SGLT1 and combined SGLT1/SGLT2 inhibition appeared to be safe. More data is needed on long-term safety and cardiovascular consequences of SGLT1 inhibition.

Up-regulation of epithelial Na(+) channel ENaC by human parvovirus B19 capsid protein VP1

BACKGROUND

Clinical disorders caused by parvovirus B19 (B19V) infection include endothelial dysfunction with cardiac ischemia. The virus is effective in part by lysophosphatidylcholine-producing phospholipase A2 (PLA2) activity of B19V capsid protein VP1. Mechanisms compromising endothelial function include up-regulation of amiloride sensitive epithelial Na(+)-channel ENaC leading to endothelial cell stiffness. Regulators of ENaC include ubiquitin-ligase Nedd4-2. The present study explored whether VP1 modifies ENaC-activity.

METHODS

cRNA encoding ENaC was injected into Xenopus oocytes without or with cRNA encoding VP1. Experiments were made with or without coexpression of Nedd4-2. ENaC activity was estimated from amiloride (50 μM) sensitive current.

RESULTS

Injection of cRNA encoding ENaC into Xenopus oocytes was followed by appearance of amiloride sensitive current, which was significantly enhanced by additional injection of cRNA encoding VP1, but not by additional injection of cRNA encoding PLA2-negative VP1 mutant (H153A). The effect of VP1 on ENaC was mimicked by treatment of ENaC expressing oocytes with lysophosphatidylcholine (1 μg/ml). The effect of VP1 and lysophosphatidylcholine was not additive. ENaC activity was downregulated by Nedd4-2, an effect not reversed by VP1.

CONCLUSIONS

The B19V capsid protein VP1 up-regulates ENaC, an effect at least partially due to phospholipase A2 (PLA) dependent formation of lysophosphatidylcholine.

Relevance of SGK1 in structural, functional and molecular alterations produced by aldosterone in heart

Aldosterone regulates sodium (Na+) and potassium (K+) transports in epithelial cells. Besides, aldosterone participates in cardiac alterations associated with hypertension, heart failure, diabetes, and other pathological alterations. One of the main cardiac alterations induced by aldosterone is cardiac hypertrophy in which different mechanisms are involved such as increased cardiomyocyte, calcium concentration, oxidative stress, and inflammatory and fibrotic mediators stimulation. Many epidemiological studies have demonstrated that left ventricular hypertrophy is associated with significantly increased risk of heart failure and malignant arrhythmias. SGK1 is a member of the serine/threonine kinase gene family that plays an important role in the absorption of Na+ and water through the Na+ channel in the apical membrane of tubular epithelial cells. SGK1 has been related to fibrotic mediator increase such as connective tissue growth factor (CTGF) and transforming growth factor-β (TGF-β) as well as inflammatory [tumor necrosis factor-α (TNF-α) and interleukin (IL)-1β] and oxidative (NADPH oxidase) species. It has been shown that aldosterone induces SGK1 gene expression not only in kidneys but also in the heart. Supporting the central role of SGK1 in cardiac alterations induced by aldosterone, treatment with the mineralocorticoid antagonist spironolactone is able to reduce the gene expression of SGK1 in aldosterone-treated rats. Taken together, data suggest the involvement of SGK1 in a complex intracellular signaling, involving fibrotic, inflammatory, and oxidative pathways, which lead to cardiac hypertrophy and fibrosis induced by aldosterone.

Disturbance of oligodendrocyte function plays a key role in the pathogenesis of schizophrenia and major depressive disorder

The major psychiatric disorders such as schizophrenia (SZ) and major depressive disorder (MDD) are thought to be multifactorial diseases related to both genetic and environmental factors. However, the genes responsible and the molecular mechanisms underlying the pathogenesis of SZ and MDD remain unclear. We previously reported that abnormalities of disrupted-in-Schizophrenia-1 (DISC1) and DISC1 binding zinc finger (DBZ) might cause major psychiatric disorders such as SZ. Interestingly, both DISC and DBZ have been further detected in oligodendrocytes and implicated in regulating oligodendrocyte differentiation. DISC1 negatively regulates the differentiation of oligodendrocytes, whereas DBZ plays a positive regulatory role in oligodendrocyte differentiation. We have reported that repeated stressful events, one of the major risk factors of MDD, can induce sustained upregulation of plasma corticosterone levels and serum/glucocorticoid regulated kinase 1 (Sgk1) mRNA expression in oligodendrocytes. Repeated stressful events can also activate the SGK1 cascade and cause excess arborization of oligodendrocyte processes, which is thought to be related to depressive-like symptoms. In this review, we discuss the expression of DISC1, DBZ, and SGK1 in oligodendrocytes, their roles in the regulation of oligodendrocyte function, possible interactions of DISC1 and DBZ in relation to SZ, and the activation of the SGK1 signaling cascade in relation to MDD.

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.

Sodium chloride, SGK1, and Th17 activation

The incidence of autoimmune diseases in Western civilizations is increasing rapidly, suggesting an influence of environmental factors, such as diet. The pathogenesis of several of these autoimmune diseases is characterized by aberrant activation of T helper 17 (Th17) cells. Recent reports have shown that the differentiation of Th17 cells is sensitive to changes in local microenvironments, in particular salt (NaCl) concentrations, in a molecular mechanism centered around the serum- and glucocorticoid-inducible kinase 1 (SGK1). In this review, we summarize the recently disclosed mechanisms by which salt has been shown to affect SGK1 and, subsequently, Th17 activation.

Regulation of methylglyoxal-elicited leukocyte recruitment by endothelial SGK1/GSK3 signaling

Excessive levels of the glycolysis metabolite methylglyoxal (MG) elicit enhanced expression of adhesion molecules which foster leukocyte-endothelial cell interactions. The signaling mechanisms involved remain elusive. To address this, we investigated the signal transduction of leukocyte- and endothelial-expressed phosphoinositide 3-kinase (PI3K) effector kinases glycogen synthase kinase 3 (GSK3) and serum- and glucocorticoid-inducible kinase 1 (SGK1) in the regulation of MG-elicited leukocyte recruitment. Using intravital microscopy of mouse cremasteric microvasculature, we demonstrate that GSK3 inhibitors lithium and SB216763 mitigate MG-elicited leukocyte recruitment and microvascular hyperpermeability. In SVEC4-10EE2 endothelial cells, but not in neutrophils, MG transiently activates GSK3 by reducing inhibitory phospho-GSK3α/β (Ser21/9) which parallels decrease of phospho-Akt at early time points (<30min). At later time points (≥1h), MG induces GSK3 deactivation which is dissipated by siRNA silencing of SGK. MG treatment potentiates endothelial SGK1 mRNA, total SGK1, phospho-SGK1 and phospho-NDRG1. The SGK1 inhibitor GSK650394 attenuates MG-elicited leukocyte recruitment. Pharmacological inhibition or silencing endothelial GSK3 or SGK attenuates MG-triggered nuclear factor (NF)-κB activity. Furthermore, silencing SGK blunts MG-triggered redox-sensitive phosphorylation of endothelial transcription factor CREB. Inhibition of SGK1 or GSK3 mitigates the expression of endothelial adhesion molecules P- and E-selectins and ICAM-1. Moreover, SGK1-dependent CREB activation participates in MG-elicited ICAM-1 upregulation. We conclude that temporal activation of endothelial SGK1 and GSK3 is decisive in MG-elicited upregulation of transcription factors, adhesion molecule expression, and leukocyte-vascular endothelium interactions. This novel signaling pathway may link excessive MG levels in vivo to inflammation, thus, unraveling potential therapeutic targets.

Fluvastatin inhibits the expression of fibronectin in human peritoneal mesothelial cells induced by high-glucose peritoneal dialysis solution via SGK1 pathway

BACKGROUND

Previous studies showed that statins may have protective effects on peritoneal mesothelial cells (PMC) cultured in high glucose. However, the mechanisms are not clear yet. Several studies demonstrated that serum- and glucocorticoid-inducible kinase 1 (SGK1) is implicated in tissue fibrosis of liver, lung and kidney by regulating the expression of many profibrogenic cytokines and extracellular matrix (e.g., fibronectin). However, few available reports elucidated whether the SGK1 is involved in the pathogenesis of peritoneal fibrosis (PF) in patients with peritoneal dialysis (PD). So far, there is no study about the interaction between the statins and SGK1 in PMC. The purpose of this study was to identify whether fluvastatin may decrease the expression of fibronectin (FN) in human peritoneal mesothelial cells (HPMC) cultured with high-glucose peritoneal dialysis solution (HGPDS) by affecting SGK1 signal pathway.

METHODS

Cultured HPMC were divided into groups of control, high-glucose peritoneal dialysis solution (HGPDS), HGPDS with fluvastatin (10(-8) mol/L ~ 10(-6) mol/L) or GSK650394 10(-5) mol/L (the competitive inhibitor of SGK1), fluvastatin 10(-6) mol/L or GSK650394 10(-5) mol/L alone. The expression of SGK1 and FN was detected by RT-PCR, western immunoblotting or ELISA.

RESULTS

Compared with the control, the mRNA and protein expression of SGK1 and FN increased significantly in HPMC treated with HGPDS (p < 0.05). GSK650394 significantly decreased the upregulated mRNA and protein expression of SGK1 and FN induced by HGPDS (p < 0.05), and fluvastatin had the same effects as GSK650394 in a dose-dependent manner (p < 0.05).

CONCLUSIONS

Expression of SGK1 and FN increased in HPMC induced by HGPDS. Treated with fluvastatin and the SGK1-inhibitor GSK650394, abnormalities of SGK1 and FN could be corrected partially, which suggested that the SGK1 pathway was implicated in the pathogenesis of PF, and that fluvastatin might decrease the expression of SGK1 so as to meliorate the progression of PF.

Diverse effects of ANXA7 and p53 on LNCaP prostate cancer cells are associated with regulation of SGK1 transcription and phosphorylation of the SGK1 target FOXO3A

Tumor suppressor function of the calcium/phospholipid-binding Annexin-A7 (ANXA7) has been shown in Anxa7-deficient mice and validated in human cancers. In the androgen-resistant prostate cancer cells, ANXA7 and p53 showed similar cytotoxicity levels. However, in the androgen-sensitive LNCaP, ANXA7 greatly exceeded the p53-induced cytotoxicity. We hypothesized that the p53 underperformance in LNCaP could be due to the involvement of p53-responsive SGK1 and FOXO3A. In this study, we show that p53 failed to match programmed cell death (PCD) and G1-arrest that were induced by ANXA7 in LNCaP. WT-ANXA7 preserved total FOXO3A expression with no hyperphosphorylation that could enable FOXO3A nuclear translocation and proapoptotic transcription. In contrast, in the p53-transfected LNCaP cells with maintained cell proliferation, the phosphorylated (but not total) FOXO3A fraction was increased implying a predominantly cytoplasmic localization and, subsequently, a lack of FOXO3A proapoptotic transcription. In addition, p53 reduced the expression of aberrant SGK1 protein form in LNCaP. Using Ingenuity Pathway Analysis and p53-signature genes, we elucidated the role of distinct SGK1/FOXO3A-associated regulation in p53 versus ANXA7 responses and proposed that aberrant SGK1 could affect reciprocal SGK1-FOXO3A-Akt regulation. Thus, the failure of the cell growth regulator p53 versus the phospholipid-binding ANXA7 could be potentially attributed to its diverse effects on SGK1-FOXO3A-Akt pathway in the PTEN-deficient LNCaP.

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.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

SGK1 is regulated by metabolic-related factors in 3T3-L1 adipocytes and overexpressed in the adipose tissue of subjects with obesity and diabetes

AIMS

The present study aimed to investigate the pathophysiological role of SGK1 in the development of metabolic syndrome by investigating the expression and regulation of serum and glucocorticoid-inducible kinase 1 (SGK1) in adipose tissues in obesity and diabetes.

METHODS

SGK1 expression in adipose tissue was investigated using reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. SGK1 regulation in differentiated 3T3-L1 adipocytes by metabolic-related factors was assessed using Northern blot analysis. Humans with obesity and type 2 diabetes and KKAy and db/db mice were used to evaluate SGK1 expression in the adipose tissue of subjects with obesity and diabetes using quantitative real-time PCR and Western blot analysis.

RESULTS

SGK1 was expressed in white adipose tissue as shown by mRNA and protein levels. Aldosterone and glucocorticoids stimulated SGK1 expression in a time- and dose-dependent manner, whereas PPAR-γ agonists inhibited SGK1 expression in differentiated 3T3-L1 adipocytes. Furthermore, SGK1 mRNA and protein were overexpressed in the adipose tissue of mice and humans with obesity and type 2 diabetes.

CONCLUSION

Aldosterone, glucocorticoids and other factors contribute to excessive SGK1 expression in adipose tissue. This excessive SGK1 expression may be related to adipose tissue dysfunction, which may contribute to the development of obesity, diabetes and metabolic syndrome.

TGFβ1 and SGK1-sensitive store-operated Ca2+ entry and Orai1 expression in endometrial Ishikawa cells

The serum-and-glucocorticoid-inducible-kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress, hormones and further mediators. A most powerful stimulator of SGK1 expression is transforming growth factor TGFβ1. SGK1 is activated by insulin and growth factors via phosphatidylinositol-3-kinase and the 3-phosphoinositide-dependent kinase PDK1. As shown recently, SGK1 increases the store-operated Ca(2+) entry (SOCE), which is accomplished by the pore-forming ion channel unit Orai. Most recent observations further revealed that SGK1 plays a critical role in the regulation of fertility. SGK1 is up-regulated in the luminal epithelium of women with unexplained infertility but down-regulated in decidualizing stromal cells of patients with recurrent pregnancy loss. The present study explored whether Orai1 is expressed in endometrium and sensitive to regulation by SGK1 and/or TGFβ1. To this end, Orai1 protein abundance was determined by western blotting and SOCE by fura-2 fluorescence. As a result, Orai1 was expressed in human endometrium and in human endometrial Ishikawa cells. Orai1 expression and SOCE in Ishikawa cells were increased by transfection with constitutively active (S422D)SGK1 but not by transfection with inactive (K127N)SGK1. The difference of SOCE between (S422D)SGK1 and (K127N)SGK1-transfected cells was virtually abrogated in the presence of Orai1 inhibitor 2-aminoethoxydiphenyl borate (2-APB, 50 µM). Similar to (S422D)SGK1 transfection TGFβ1 treatment up-regulated both Orai1 protein abundance and SOCE. In conclusion, Orai1 is expressed in the human endometrium and is up-regulated by SGK1 and TGFβ1. The present observations thus uncover a novel element in SGK1-sensitive regulation of endometrial cells.

Upregulation of intestinal NHE3 following saline ingestion

BACKGROUND

Little is known about the effect of salt content of ingested fluid on intestinal transport processes. Osmosensitive genes include the serum- and glucocorticoid-inducible kinase SGK1, which is up-regulated by hyperosmolarity and cell shrinkage. SGK1 is in turn a powerful stimulator of the intestinal Na(+)/H(+) exchanger NHE3. The present study was thus performed to elucidate, whether the NaCl content of beverages influences NHE3 activity.

METHODS

Mice were offered access to either plain water or isotonic saline ad libitum. NHE3 transcript levels and protein abundance in intestinal tissue were determined by confocal immunofluorescent microscopy, RT-PCR and western blotting, cytosolic pH (pHi) in intestinal cells from 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence and Na(+)/H(+) exchanger activity from the Na(+) dependent realkalinization following an ammonium pulse.

RESULTS

Saline drinking significantly enhanced fluid intake and increased NHE3 transcript levels, NHE3 protein and Na(+)/H(+) exchanger activity.

CONCLUSIONS

Salt content of ingested fluid has a profound effect on intestinal Na(+)/H(+) exchanger expression and activity.

Regulation of ion channels by the serum- and glucocorticoid-inducible kinase SGK1

The ubiquitously expressed serum- and glucocorticoid-inducible kinase-1 (SGK1) is genomically regulated by cell stress (including cell shrinkage) and several hormones (including gluco- and mineralocorticoids). SGK1 is activated by insulin and growth factors through PI3K and 3-phosphoinositide-dependent kinase PDK1. SGK1 activates a wide variety of ion channels (e.g., ENaC, SCN5A, TRPV4-6, ROMK, Kv1.3, Kv1.5, Kv4.3, KCNE1/KCNQ1, KCNQ4, ASIC1, GluR6, ClCKa/barttin, ClC2, CFTR, and Orai/STIM), which participate in the regulation of transport, hormone release, neuroexcitability, inflammation, cell proliferation, and apoptosis. SGK1-sensitive ion channels participate in the regulation of renal Na(+) retention and K(+) elimination, blood pressure, gastric acid secretion, cardiac action potential, hemostasis, and neuroexcitability. A common (∼3-5% prevalence in Caucasians and ∼10% in Africans) SGK1 gene variant is associated with increased blood pressure and body weight as well as increased prevalence of type II diabetes and stroke. SGK1 further contributes to the pathophysiology of allergy, peptic ulcer, fibrosing disease, ischemia, tumor growth, and neurodegeneration. The effect of SGK1 on channel activity is modest, and the channels do not require SGK1 for basic function. SGK1-dependent ion channel regulation may thus become pathophysiologically relevant primarily after excessive (pathological) expression. Therefore, SGK1 may be considered an attractive therapeutic target despite its broad range of functions.

Functional expression of TRPV4 channels in human collecting duct cells: implications for secondary hypertension in diabetic nephropathy

Background. The Vanilloid subfamily of transient receptor potential (TRPV) ion channels has been widely implicated in detecting osmotic and mechanical stress. In the current study, we examine the functional expression of TRPV4 channels in cell volume regulation in cells of the human collecting duct. Methods. Western blot analysis, siRNA knockdown, and microfluorimetry were used to assess the expression and function of TRPV4 in mediating Ca²⁺-dependent mechanical stimulation within a novel system of the human collecting duct (HCD). Results. Native and siRNA knockdown of TRPV4 protein expression was confirmed by western blot analysis. Touch was used as a cell-directed surrogate for osmotic stress. Mechanical stimulation of HCD cells evoked a transient increase in [Ca²⁺]_i that was dependent upon thapsigargin-sensitive store release and Ca²⁺ influx. At 48 hrs, high glucose and mannitol (25 mM) reduced TRPV4 expression by 54% and 24%, respectively. Similar treatment doubled SGK1 expression. Touch-evoked changes were negated following TRPV4 knockdown. Conclusion. Our data confirm expression of Ca²⁺-dependent TRPV4 channels in HCD cells and suggest that a loss of expression in response to high glucose attenuates the ability of the collecting duct to exhibit regulatory volume decreases, an effect that may contribute to the pathology of fluid and electrolyte imbalance as observed in diabetic nephropathy.

The serum- and glucocorticoid-inducible kinase 1 (SGK1) influences platelet calcium signaling and function by regulation of Orai1 expression in megakaryocytes

Platelets are activated on increase of cytosolic Ca2+ activity ([Ca2+]i), accomplished by store-operated Ca2+ entry (SOCE) involving the pore-forming ion channel subunit Orai1. Here, we show, for the first time, that the serum- and glucocorticoid-inducible kinase 1 (SGK1) is expressed in platelets and megakaryocytes. SOCE and agonist-induced [Ca2+]i increase are significantly blunted in platelets from SGK1 knockout mice (sgk1−/−). Similarly, Ca2+-dependent degranulation, integrin αIIbβ3 activation, phosphatidylserine exposure, aggregation, and in vitro thrombus formation were significantly impaired in sgk1−/− platelets, whereas tail bleeding time was not significantly enhanced. Platelet and megakaryocyte Orai1 transcript levels and membrane protein abundance were significantly reduced in sgk1−/− mice. In human megakaryoblastic cells (MEG-01), transfection with constitutively active S422DSGK1 but not with inactive K127NSGK1 significantly enhanced Orai1 expression and SOCE, while effects reversed by the SGK1 inhibitor GSK650394 (1μM). Transfection of MEG-01 cells with S422DSGK1 significantly increased phosphorylation of IκB kinase α/β and IκBα resulting in nuclear translocation of NF-κB subunit p65. Treatment of S422DSGK1-transfected MEG-01 cells with the IκB kinase inhibitor BMS-345541 (10μM) abolished SGK1-induced increase of Orai1 expression and SOCE. The present observations unravel SGK1 as novel regulator of platelet function, effective at least in part by NF-κB–dependent transcriptional up-regulation of Orai1 in megakaryocytes and increasing platelet SOCE.

Sgk1 sensitivity of Na(+)/H(+) exchanger activity and cardiac remodeling following pressure overload

Sustained increase of cardiac workload is known to trigger cardiac remodeling with eventual development of cardiac failure. Compelling evidence points to a critical role of enhanced cardiac Na(+)/H(+) exchanger (NHE1) activity in the underlying pathophysiology. The signaling triggering up-regulation of NHE1 remained, however, ill defined. The present study explored the involvement of the serum- and glucocorticoid-inducible kinase Sgk1 in cardiac remodeling due to transverse aortic constriction (TAC). To this end, experiments were performed in gene targeted mice lacking functional Sgk1 (sgk1 (-/-)) and their wild-type controls (sgk1 (+/+)). Transcript levels have been determined by RT-PCR, cytosolic pH (pH( i )) utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence, Na(+)/H(+) exchanger activity by the Na(+)-dependent realkalinization after an ammonium pulse, ejection fraction (%) utilizing cardiac cine magnetic resonance imaging and cardiac glucose uptake by PET imaging. As a result, TAC increased the mRNA expression of Sgk1 in sgk1 (+/+) mice, paralleled by an increase in Nhe1 transcript levels as well as Na(+)/H(+) exchanger activity, all effects virtually abrogated in sgk1 (-/-) mice. In sgk1 (+/+) mice, TAC induced a decrease in Pgc1a mRNA expression, while Spp1 mRNA expression was increased, both effects diminished in the sgk1 (-/-) mice. TAC was followed by a significant increase of heart and lung weight in sgk1 (+/+) mice, an effect significantly blunted in sgk1 (-/-) mice. TAC increased the transcript levels of Anp and Bnp, effects again significantly blunted in sgk1 (-/-) mice. TAC increased transcript levels of Collagen I and III as well as Ctgf mRNA and CTGF protein abundance, effects significantly blunted in sgk1 (-/-) mice. TAC further decreased the ejection fraction in sgk1 (+/+) mice, an effect again attenuated in sgk1 (-/-) mice. Also, cardiac FDG-glucose uptake was increased to a larger extent in sgk1 (+/+) mice than in sgk1 (-/-) mice after TAC. These observations point to an important role for SGK1 in cardiac remodeling and development of heart failure following an excessive work load.

Transcription factor NF-κB regulates expression of pore-forming Ca2+ channel unit, Orai1, and its activator, STIM1, to control Ca2+ entry and affect cellular functions

The serum and glucocorticoid-inducible kinase SGK1 increases the activity of Orai1, the pore forming unit of store-operated Ca(2+) entry, and thus influences Ca(2+)-dependent cellular functions such as migration. SGK1 further regulates transcription factor nuclear factor κB (NF-κB). This study explored whether SGK1 influences transcription of Orai1 and/or STIM1, the Orai1-activating Ca(2+) sensor. Orai1 and STIM1 transcript levels were decreased in mast cells from SGK1 knock-out mice and increased in HEK293 cells transfected with active (S422D)SGK1 but not with inactive (K127N)SGK1 or in (S422D)SGK1-transfected cells treated with the NF-κB inhibitor Wogonin (100 μm). Treatment with the stem cell factor enhanced transcript levels of STIM1 and Orai1 in sgk1(+/+) but not in sgk1(-/-) mast cells and not in sgk1(+/+) cells treated with Wogonin. Orai1 and STIM1 transcript levels were further increased in sgk1(+/+) and sgk1(-/-) mast cells by transfection with active NF-κB subunit p65 as well as in HEK293 cells by transfection with NF-κB subunits p65/p50 or p65/p52. They were decreased by silencing of NF-κB subunits p65, p50, or p52 or by NF-κB inhibitor Wogonin (100 μm). Luciferase assay and chromatin immunoprecipitation defined NF-κB-binding sites in promoter regions accounting for NF-κB sensitive genomic regulation of STIM1 and Orai1. Store-operated Ca(2+) entry was similarly increased by overexpression of p65/p50 or p65/p52 and decreased by treatment with Wogonin. Transfection of HEK293 cells with p65/p50 or p65/p52 further augmented migration. The present observations reveal powerful genomic regulation of Orai1/STIM1 by SGK1-dependent NF-κB signaling.

Angiotensin II mediates cell survival through upregulation and activation of the serum and glucocorticoid inducible kinase 1

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is known to regulate a wide variety of cellular processes, including renal sodium retention and cell survival. Angiotensin II (Ang II) is one of the many signaling molecules capable of regulating SGK1 expression, and is also known to impact cell survival. Here, we examined the role of SGK1 in Ang II-mediated cell survival. We hypothesized that Ang II protects cells from apoptosis by upregulating and activating SGK1. To test this, we examined the effects of Ang II stimulation on SGK1 expression and downstream signaling. We also examined the effects of Ang II treatment and siRNA-mediated SGK1 knockdown on apoptosis after serum starvation. We found that after 2h of Ang II treatment, SGK1 mRNA expression was increased approximately 2-fold. This induction was sensitive to reductions in intracellular calcium levels after pretreatment with BAPTA-AM, but insensitive to the L-type calcium channel blocker verapamil. SGK1 induction was also sensitive to the tyrosine kinase inhibitor genistein. Ang II treatment also caused a rapid increase in the level of phosphorylation of SGK1 at Ser422 and Thr256, and Ser422 phosphorylation was rapamycin-sensitive. We found that Ang II treatment was protective against serum starvation-induced apoptosis, and this protective effect was significantly blunted when SGK1 was silenced via siRNA. Lastly, Ang II induced FOXO3A phosphorylation in an SGK1-dependent manner, thereby reducing the pro-apoptotic actions of FOXO3A. Overall, these results indicate that Ang II upregulates and activates SGK1, leading to increased cell survival via multiple, non-redundant mechanisms.

The Western-style diet: a major risk factor for impaired kidney function and chronic kidney disease

The Western-style diet is characterized by its highly processed and refined foods and high contents of sugars, salt, and fat and protein from red meat. It has been recognized as the major contributor to metabolic disturbances and the development of obesity-related diseases including type 2 diabetes, hypertension, and cardiovascular disease. Also, the Western-style diet has been associated with an increased incidence of chronic kidney disease (CKD). A combination of dietary factors contributes to the impairment of renal vascularization, steatosis and inflammation, hypertension, and impaired renal hormonal regulation. This review addresses recent progress in the understanding of the association of the Western-style diet with the induction of dyslipidemia, oxidative stress, inflammation, and disturbances of corticosteroid regulation in the development of CKD. Future research needs to distinguish between acute and chronic effects of diets with high contents of sugars, salt, and fat and protein from red meat, and to uncover the contribution of each component. Improved therapeutic interventions should consider potentially altered drug metabolism and pharmacokinetics and be combined with lifestyle changes. A clinical assessment of the long-term risks of whole-body disturbances is strongly recommended to reduce metabolic complications and cardiovascular risk in kidney donors and patients with CKD.

Plasma corticosterone activates SGK1 and induces morphological changes in oligodendrocytes in corpus callosum

Repeated stressful events are known to be associated with onset of depression. Further, stress activates the hypothalamic–pituitary–adrenocortical (HPA) system by elevating plasma cortisol levels. However, little is known about the related downstream molecular pathway. In this study, by using repeated water-immersion and restraint stress (WIRS) as a stressor for mice, we attempted to elucidate the molecular pathway induced by elevated plasma corticosterone levels. We observed the following effects both, in vivo and in vitro: (1) repeated exposure to WIRS activates the 3-phosphoinositide-dependent protein kinase (PDK1)–serum glucocorticoid regulated kinase (SGK1)–N-myc downstream-regulated gene 1 (NDRG1)–adhesion molecule (i.e., N-cadherin, α-catenin, and β-catenin) stabilization pathway via an increase in plasma corticosterone levels; (2) the activation of this signaling pathway induces morphological changes in oligodendrocytes; and (3) after recovery from chronic stress, the abnormal arborization of oligodendrocytes and depression-like symptoms return to the control levels. Our data strongly suggest that these abnornalities of oligodendrocytes are possibly related to depression-like symptoms.

Stimulation of Ca2+-channel Orai1/STIM1 by serum- and glucocorticoid-inducible kinase 1 (SGK1)

Ca(2+) signaling includes store-operated Ca(2+) entry (SOCE) following depletion of endoplasmic reticulum (ER) Ca(2+) stores. On store depletion, the ER Ca(2+) sensor STIM1 activates Orai1, the pore-forming unit of Ca(2+)-release-activated Ca(2+) (CRAC) channels. Here, we show that Orai1 is regulated by serum- and glucocorticoid-inducible kinase 1 (SGK1), a growth factor-regulated kinase. Membrane Orai1 protein abundance, I(CRAC), and SOCE in human embryonic kidney (HEK293) cells stably expressing Orai1 and transfected with STIM1 were each significantly enhanced by coexpression of constitutively active (S422D)SGK1 (by+81, +378, and+136%, respectively) but not by inactive (K127N)SGK1. Coexpression of the ubiquitin ligase Nedd4-2, an established negatively regulated SGK1 target, down-regulated SOCE (by -48%) and I(CRAC) (by -60%), an effect reversed by expression of (S422D)SGK1 (by +175 and +173%, respectively). Orai1 protein abundance and SOCE were significantly lower in mast cells from SGK1-knockout (sgk1(-/-)) mice (by -37% and -52%, respectively) than in mast cells from wild-type (sgk1(+/+)) littermates. Activation of SOCE by sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase-inhibitor thapsigargin (2 μM) stimulated migration, an effect significantly higher (by +306%) in (S422D)SGK1-expressing than in (K127N)SGK1-expressing HEK293 cells, and also significantly higher (by +108%) in sgk1(+/+) than in sgk1(-/-) mast cells. SGK1 is thus a novel key player in the regulation of SOCE.

Second AKT: the rise of SGK in cancer signalling

The serum and glucocorticoid kinase (SGK) family of serine/threonine kinases consists of three isoforms, SGK-1, SGK-2 and SGK-3. This family of kinases is highly homologous to the AKT kinase family, sharing similar upstream activators and downstream targets. SGKs have been implicated in the regulation of cell growth, proliferation, survival and migration: cellular processes that are dysregulated in cancer. Furthermore, SGKs lie downstream of phosphoinositide-3-kinase (PI3Kinase) signalling and interact at various levels with RAS/RAF/ERK signalling, two pathways that are involved in promoting tumorigenesis. Recent evidence suggests that mutant PI3Kinase can induce tumorigenesis through an AKT-independent but SGK3-dependent mechanism, thus implicating SGKs as potential players in malignant transformation. Here, we will review the current state of knowledge on the regulation of the SGKs and their role in normal cell physiology and transformation with a particular focus on SGK3.

Serum- and glucocorticoid-regulated kinase 1 is upregulated following unilateral ureteral obstruction causing epithelial-mesenchymal transition

Obstructive nephropathy leads to chronic kidney disease, characterized by a progressive epithelial-to-mesenchymal cell transition (EMT)-driven interstitial fibrosis. To identify the mechanisms causing EMT, we used the mouse model of unilateral ureteral obstruction and found a rapid and significant increase in serum- and glucocorticoid-regulated kinase-1 (SGK1) expression in the kidneys with an obstructed ureter. Knockout of SGK1 significantly suppressed obstruction-induced EMT, kidney fibrosis, increased glycogen synthase kinase-3β activity, and decreased accumulation of the transcriptional repressor Snail. This caused a reduced expression of the mesenchymal marker α-smooth muscle actin, and collagen deposition in this model. In cultured kidney epithelial cells, mechanical stretch or treatment with transforming growth factor-β not only stimulated the transcription of SGK1, but also stimulated EMT in an SGK1-dependent manner. Activated SGK1 stimulated Snail accumulation and downregulation of the epithelial marker E-cadherin. Hence, our study shows that SGK1 is involved in mediating fibrosis associated with obstructive nephropathy.

Regulation of renal tubular glucose reabsorption by Akt2/PKBβ

Akt/PKB is known to regulate the facilitative glucose carrier GLUT4. Nothing is known, however, of the role of Akt/PKB in the regulation of renal epithelial transport. To explore whether Akt2/PKBβ influences the Na(+)-coupled glucose cotransporter SGLT1, human SGLT1 was expressed in Xenopus laevis oocytes with or without Akt/PKB, and electrogenic glucose transport was determined by dual-electrode voltage clamp. The coexpression of Akt/PKB in SGLT1-expressing oocytes was followed by an increase in glucose-induced currents. To study the functional significance of Akt/PKB-sensitive renal glucose transport, further experiments were performed in gene-targeted mice lacking functional Akt2/PKBβ (akt2(-/-)) and in their wild-type littermates (akt2(+/+)). Plasma glucose concentration was significantly higher in akt2(-/-) mice than in akt2(+/+) mice but was virtually identical to the plasma glucose concentration in fructose-treated akt2(+/+) mice. Urinary glucose excretion was significantly higher in akt2(-/-) mice compared with akt2(+/+) mice with or without fructose treatment. Moreover, the glucose-induced depolarization of proximal tubular cells was significantly smaller in isolated, perfused renal tubules from akt2(-/-) mice than in those from akt2(+/+) mice. In conclusion, Akt2/PKBβ plays a role in the regulation of renal glucose transport.

Review article: importance of the kidney proximal tubular cells in thiazolidinedione-mediated sodium and water uptake

Thiazolidinediones (TZD) such as pioglitazone and rosiglitazone are proxisome proliferator-activated receptor gamma (PPARg) agonists and are widely used clinically to treat type 2 diabetes mellitus. Fluid retention still poses a significant limitation to its use. The primary renal process underlying TZD-associated oedema is reduced urinary sodium and water excretion. Experimental evidence suggests that this is mainly related to the effects of PPARg agonists on the distal nephron and collecting duct. We have recently shown that PPARg agonists upregulate sodium and water transport channels in human proximal tubule cells and that Sgk-1 is involved. In this review, we focus on the importance of the proximal tubular cells in TZD-mediated sodium and water uptake.

Bestrophin 1 promotes epithelial-to-mesenchymal transition of renal collecting duct cells

Bestrophin 1 (Best1) controls intracellular Ca(2+) concentration, induces Ca(2+)-activated Cl(-) conductance, and increases proliferation of colon carcinoma cells. Here, we show that expression of Best1 in mouse renal collecting duct (CD) cells causes i) an increase in cell proliferation, ii) a loss of amiloride-sensitive Na(+) absorption, iii) induction of Ca(2+)-dependent Cl(-) conductance (CaCC), and iv) epithelial-to-mesenchymal transition. During conditions of high proliferation or when we exposed CD cells to serum or TGF-beta1, we observed upregulation of Best1, increased CaCC, redistribution of the epithelial-to-mesenchymal transition marker beta-catenin, and upregulation of vimentin. In contrast, suppression of Best1 by RNAi inhibited proliferation, reduced CaCC, and downregulated markers of EMT. CaCC and expression of Best1 were independent of the cell cycle but clearly correlated to cell proliferation and cell density. During renal inflammation in LPS-treated mice or after unilateral ureteral obstruction, we observed transient upregulation of Best1. These data indicate that repression of cell proliferation, CaCC, and expression of Best1 occurs during mesenchymal-to-epithelial transition once CD cells polarize and terminally differentiate. These results may suggest a role for Best1 in renal fibrosis and tissue repair.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

In various mammalian species, including humans, water restriction leads to an acute increase in urinary sodium excretion. This process, known as dehydration natriuresis, helps prevent further accentuation of hypernatremia and the accompanying rise in extracellular tonicity. Serum- and glucocorticoid-inducible kinase (Sgk1), which is expressed in the renal medulla, is regulated by extracellular tonicity. However, the mechanism of its regulation and the physiological role of hypertonicity-induced SGK1 gene expression remain unclear. Here, we identified a tonicity-responsive enhancer (TonE) upstream of the rat Sgk1 transcriptional start site. The transcription factor NFAT5 associated with TonE in a tonicity-dependent fashion in cultured rat renal medullary cells, and selective blockade of NFAT5 activity resulted in suppression of the osmotic induction of the Sgk1 promoter. In vivo, water restriction of rats or mice led to increased urine osmolality, increased Sgk1 expression, increased expression of the type A natriuretic peptide receptor (NPR-A), and dehydration natriuresis. In cultured rat renal medullary cells, siRNA-mediated Sgk1 knockdown blocked the osmotic induction of natriuretic peptide receptor 1 (Npr1) gene expression. Furthermore, Npr1-/- mice were resistant to dehydration natriuresis, which suggests that Sgk1-dependent activation of the NPR-A pathway may contribute to this response. Collectively, these findings define a specific mechanistic pathway for the osmotic regulation of Sgk1 gene expression and suggest that Sgk1 may play an important role in promoting the physiological response of the kidney to elevations in extracellular tonicity.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Circulating transforming growth factor-beta1 levels and the risk for kidney disease in African Americans

Transforming growth factor-β1 (TGF-β1) is well known to induce progression of experimental renal disease. Here we determined whether there is an association between serum levels of TGF-β1 and the risk factors for progression of clinically relevant renal disorders in 186 black and 147 white adults none of whom had kidney disease or diabetes. Serum TGF-β1 protein levels were positively and significantly associated with plasma renin activity along with the systolic and diastolic blood pressure in blacks but not whites after controlling for age, gender and body mass index. These TGF-β1 protein levels were also significantly associated with body mass index and metabolic syndrome and more predictive of microalbuminuria in blacks than in whites. The differential association between TGF-β1 and renal disease risk factors in blacks and whites suggests an explanation for the excess burden of end-stage renal disease in the black population but this requires validation in an independent cohort. Whether these findings show that it is the circulating levels of TGF-β1 that contributes to renal disease progression or reflects other unmeasured factors will need to be tested in longitudinal studies.