Serum And Glucocorticoid-inducible Kinase 1 Research Articles

Proline Absorption and SGK1 Expression are Inhibited in Intestinal Tis7 Transgenic Mice

Background/Aims: Expression of the transcriptional co-regulator tis7 is markedly increased in the adaptive small intestine in a mouse model of short bowel syndrome. Transgenic mice with enterocytic overexpression of tis7 (tis7^tg) have accelerated triglyceride absorption, with increased adiposity yet reduced skeletal muscle mass. To further explore this phenotype, we examined whether tis7 also regulates amino acid and carbohydrate absorption. Methods: Small intestinal glucose and amino acid uptake were quantified in wild type (WT) and tis7^tg mice. Amino acid transporter expression was assessed by qRT-PCR and immunoblot. Apical cell surface transporter expression was quantified by cell surface biotinylation. Results: Active glucose uptake rates were unchanged. Uptake of proline but not leucine was significantly reduced in tis7^tg vs. WT jejunum. Expression of serum and glucocorticoid-induced kinase 1 (SGK1), a solute carrier activator, was inhibited in tis7^tg jejunum. Apical membrane expression of the proline transporter SLC6A20 was reduced in tis7^tg jejunum. Conclusions: Tis7 overexpression in enterocytes inhibits proline uptake, associated with decreased expression of activated SGK1 and reduced cell surface expression of SLC6A20. Consistent with the observed tis7^tg phenotype, tis7 overexpression increases triglyceride absorption but has adverse effects on the uptake of selected amino acids. Tis7 has pleiotropic effects on nutrient absorption.

Possible divergence of serum- and glucocorticoid-inducible kinase function in ischemic brain injury.

As recent medical progress decreases the incidence of certain diseases, ischemic brain injury remains one of the major diseases that threaten human lives, especially in western countries. Ischemic brain injury occurs as a result of lack of oxygen and nutrients due to obstruction of blood flow in the brain, and often leads to neurological disorders such as cerebral palsy, depression, and ultimately, death. Around 800,000 Americans suffer a new or recurrent stroke, and more than 130,000 people die annually in the United States (Goldstein et al., 2011). Despite much effort in searching for an effective treatment, at most a few reagents are approved for therapeutic medication in many countries. One of them, tissue plasminogen activator, is able to positively recover blood flow but unfortunately has a limited therapeutic time window and the potential side effect of intracranial hemorrhage. Therefore, identifying new targets for this disorder (and subsequent development of therapeutic reagents) may shed light on future solutions for treating ischemic brain injury. Incidentally, the incidence rate is higher among patients with hypertension and/or diabetes mellitus (diabetes) (Goldstein et al., 2011), with both conditions involved in metabolic diseases. Moreover, these two conditions and ischemic stroke together represent the main causes of morbidity and mortality in western countries after cancer. Notably, given that the prevalence of hypertension and diabetes is rising as a result of the modern lifestyle, the incidence rate of ischemic brain injury may further increase interactively. To resolve this complication, it would be preferable if anti-hypertensive and/or anti-diabetic reagents also work against ischemic brain injury. In this regards, one suitable candidate is serum- and glucocorticoid-inducible kinase (SGK) inhibitors, which we have recently reported the effect of in rodent models (Inoue et al., 2016).

Role of Serum and Glucocorticoid-Inducible Kinase (SGK)-1 in Senescence: A Novel Molecular Target Against Age-Related Diseases.

Senescence is a phenomenon characterized by a progressive decline of body homeostasis. Premature senescence acts when the cellular system is not able to adequately respond to noxious stimuli by synthesis of stressor molecules. Among those, serum-and-glucocorticoidinducible kinase-1 (SGK-1) dramatically increases under typical physiopathological conditions, such as glucocorticoid or mineralcorticoids exposure, inflammation, hyperglycemia, and ischemia. SGK-1 has been implicated in mechanism regulating oxidative stress, apoptosis, and DNA damage, which are all leading to a state of accelerating aging. Moreover, SGK-1-sensitive ion channels participate in the regulation of renal Na(+)/K(+) regulation, blood pressure, gastric acid secretion, cardiac action potential, and neuroexcitability. Recently, we demonstrated in endothelial cells as an increase in SGK-1 activity and expression reduces oxidative stress, improves cell survival and restores insulin-mediated nitric oxide production after hyperglycemia. Moreover, we showed as SGK-1 delays the onset of senescence by increasing telomerase activity, significantly decreasing reactive oxygen species (ROS) production, and by directly interacting with hTERT. Therefore, SGK-1 may represent a specific target to further develop novel therapeutic options against chronic diseases such as diabetes typical of aging. SGK-1 has been also associated with cancer, neurodegenerative diseases, and cardiovascular disease, among other age-related diseases. However, to date, the data available on SGK-1 and aging, are sparse, controversial, and only from C. elegans experimental models. In this review we sought to discuss the possible implication of SGK-1 in mechanisms regulating senescence and age-related diseases. Moreover, we aimed to discuss and identify the possible role of SGK-1 as possible molecular target to counteract and prevent aging.

STIMULATION OF RAT TYPE IIA SODIUM DEPENDENT PHOSPHATE COTRANSPORTER (NAPI-2) BY SERUM AND GLUCOCORTICOID-INDUCED KINASE SGK1 AND NA+/H+ EXCHANGE REGULATING FACTOR NHERF2

Inorganic phosphate (Pi) is reabsorbed in the renal proximal convoluted tubules mainly via the electrogenic sodium dependent phosphate cotransporter NaPi Type IIa (NaPi-IIa). The isoforms of NaPi-IIa have been cloned from different species such as rat (NaPi-2). Serum and glucocorticoid-induced kinase 1 (SGK1) and rat sodium dependent phosphate cotransporter (NaPi-2) are highly expressed in the brush border membrane (BBM) of proximal tubule cells. The significance of the kinase in regulation of sodium dependent phosphate cotransporter (NaPi-2) has, however, remained elusive. On the other hand, the carboxyl-terminal tail of NaPi-2 contains information for apical expression, and interacts by means of its three terminal amino acids with several PSD95/DglA/ZO-1-like domains (PDZ)-containing proteins such as Na+/H+ exchanger 3 regulatory factors NHERF1 or NHERF2. Both, NHERF1 and NHERF2 modulate the targeting and trafficking of several proteins into the plasma membrane. Trafficking of the Na+/H+ exchanger NHE3 is controlled by NHE regulating factor NHERF2 and serum and glucocorticoid-inducible kinase SGK1. To test for a possible involvement in NaPi-2 regulation, cRNA encoding NaPi-2 was injected into Xenopuslaevis oocytes with or without additional injection of cRNA encoding SGK1 and/or NHERF2. Using two-electrode voltage-clamp, the transport activity was quantified as the substrate-induced current. Exposure to 1 mM phosphate induced an inward current (IP) in NaPi-2 expressing oocytes but not in water injected oocytes. Coexpression of SGK1 in NaPi-2 expressing oocytes significantly stimulated the phosphate-induced inward current. Moreover, coexpression of NHERF2 also significantly stimulated the phosphate-induced inward current in NaPi-2 expressing oocytes. The effect of SGK1 on NaPi-2 is mimicked by additional coexpression of NHERF2. The observations suggest that SGK1 and NHERF2 regulate NaPi-2 activity and are thus likely to participate in the stimulatory effect of some hormones, such as growth hormone and insulin, on renal phosphate transport. The present results thus disclose novel signaling mechanisms regulating NaPi-2 activity and renal phosphate transport, which may be important for regulation of phosphate homeostasis.

Glucocorticoids suppress GLP-1 secretion: possible contribution to their diabetogenic effects.

Evidence indicates that subtle abnormalities in GC (glucocorticoid) plasma concentrations and/or in tissue sensitivity to GCs are important in the metabolic syndrome, and it is generally agreed that GCs induce insulin resistance. In addition, it was recently reported that short-term exposure to GCs reduced the insulinotropic effects of the incretin GLP-1 (glucagon-like peptide 1). However, although defective GLP-1 secretion has been correlated with insulin resistance, potential direct effects of GCs on GLP-1-producing L-cell function in terms of GLP-1 secretion and apoptosis have not been studied in any greater detail. In the present study, we sought to determine whether GCs could exert direct effects on GLP-1-producing L-cells in terms of GLP-1 secretion and cell viability. We demonstrate that the GR (glucocorticoid receptor) is expressed in GLP-1-producing cells, where GR activation in response to dexamethasone induces SGK1 (serum- and glucocorticoid-inducible kinase 1) expression, but did not influence preproglucagon expression or cell viability. In addition, dexamethasone treatment of enteroendocrine GLUTag cells reduced GLP-1 secretion induced by glucose, 2-deoxy-D-glucose, fructose and potassium, whereas the secretory response to a phorbol ester was unaltered. Furthermore, invivo administration of dexamethasone to rats reduced the circulating levels of GLP-1 concurrent with induction of insulin resistance and glucose intolerance. We can conclude that GR activation in GLP-1-producing cells will diminish the secretory responsiveness of these cells to subsequent carbohydrate stimulation. These effects may not only elucidate the pathogenesis of steroid diabetes, but could ultimately contribute to the identification of novel molecular targets for controlling incretin secretion.

Tumor Necrosis Factor-α Underlies Loss of Cortical Dendritic Spine Density in a Mouse Model of Congestive Heart Failure.

BackgroundHeart failure (HF) is a progressive disorder characterized by reduced cardiac output and increased peripheral resistance, ultimately leading to tissue perfusion deficits and devastating consequences for several organs including the brain. We previously described a tumor necrosis factor-α (TNF-α)–dependent enhancement of posterior cerebral artery tone and concomitant reduced cerebral blood flow in a mouse model of early HF in which blood pressure remains minimally affected. HF is often associated with cognitive impairments such as memory deficits, even before any overt changes in brain structure and function occur. The pathophysiology underlying the development of cognitive impairments in HF is unknown, and appropriate treatment strategies are lacking.Methods and ResultsWe used a well-established mouse model in which HF was induced by experimental myocardial infarction produced by permanent surgical ligation of the left anterior descending coronary artery (infarct size ≈25% of the left ventricular wall). Ligated mice developed enlarged hearts, congested lungs, and reduced cardiac output and blood pressure, with elevated peripheral resistance within 6 to 8 weeks after ligation. In this study, we demonstrated the significance of the proinflammatory cytokine TNF-α during HF-mediated neuroinflammation and associated impaired hippocampus-independent nonspatial episodic memory function. Augmented cerebral TNF-α expression and microglial activation in HF mice, indicative of brain inflammation, were accompanied by morphological changes and significant reduction of cortical dendritic spines (61.39±8.61% for basal and 61.04±9.18% for apical spines [P<0.001]). The significance of TNF-α signaling during the observed HF-mediated neurodegenerative processes is supported by evidence showing that sequestration or genetic deletion of TNF-α ameliorates the observed reduction of cortical dendritic spines (33.51±7.63% for basal and 30.13±6.98% for apical spines in wild-type mice treated with etanercept; 17.09±6.81% for basal and 17.21±7.29% for apical spines in TNF-α−/−). Moreover, our data suggest that alterations in cerebral serum and glucocorticoid-inducible kinase 1 (SgK1) expression and phosphorylation during HF may be TNF-α dependent and that an increase of SgK1 phosphorylation potentially plays a role in the HF-associated reduction of dendritic spine density.ConclusionsOur findings demonstrate that TNF-α plays a pivotal role in HF-mediated neuroinflammation and associated alterations of cortical dendritic spine density and has the potential to reveal novel treatment strategies for HF-associated memory deficits.

SGK1 inhibits cellular apoptosis and promotes proliferation via the MEK/ERK/p53 pathway in colitis.

To investigate the role of serum-and-glucocorticoid-inducible-kinase-1 (SGK1) in colitis and its potential pathological mechanisms. SGK1 expression in mucosal biopsies from patients with active Crohn's disease (CD) and normal controls was detected by immunohistochemistry. We established an acute colitis model in mice induced by 2,4,6-trinitrobenzene sulfonicacid, and demonstrated the presence of colitis using the disease activity index, the histologic activity index and hematoxylin and eosin staining. The cellular events and potential mechanisms were implemented with small interference RNA and an inhibitor of signaling molecule (i.e., U0126) in intestinal epithelial cells (IECs). The interaction between SGK1 and the signaling molecule was assessed by co-immunoprecipitation. SGK1 expression was significantly increased in the inflamed epithelia of patients with active CD and TNBS-induced colitis model (0.58 ± 0.055 vs 0.85 ± 0.06, P < 0.01). At the cellular level, silencing of SGK1 by small interference RNA (siSGK1) significantly inhibited the phosphorylation of mitogen-activated protein kinase kinase 1 (MEK1) and the downstream molecule extracellular signal regulated protein kinase (ERK) 1/2, which induced the upregulation of p53 and Bcl-2-associated X protein, mediating the subsequent cellular apoptosis and proliferation in IECs. Cells treated with MEK1 inhibitor (i.e., U0126) before siSGK1 transfection showed a reversal of the siSGK1-induced cellular apoptosis. Our data suggested that SGK1 may protect IECs in colitis from tumor necrosis factor-α-induced apoptosis partly by triggering MEK/ERK activation.

Duration of Gestation and Mode of Delivery Affect the Genes of Transepithelial Sodium Transport in Pulmonary Adaptation

Background: Respiratory distress due to inadequate lung liquid clearance is a significant problem in infants delivered late preterm or early term, especially by elective cesarean delivery (CD). Lung liquid clearance depends on epithelial ion transport and in animals is induced by glucocorticoids. Objectives: In newborn late preterm and term infants to study airway epithelial gene expressions of epithelial sodium channel (ENaC), and the serum and glucocorticoid-inducible kinase 1 (SGK1), and their association with cortisol, mode of delivery, and gestational age (GA). Methods: Infants were delivered at 35^0/7-41^6/7 weeks. Cortisol in umbilical cord plasma was analyzed with liquid chromatography-tandem mass spectrometry. ENaC and SGK1 mRNAs in airway epithelial cells obtained within 3 h and at 1 day postnatally were quantified with real-time PCR. Results: ENaC and SGK1 mRNAs were significantly lower in late preterm and early term infants than in those ≥39^0/7 weeks. Significant correlations existed between both ENaC and SGK1 and cord cortisol and GA. In term infants, SGK1 mRNA at 1.5 h was higher after vaginal delivery than elective CD. Conclusions: In late preterm and early term infants, low expression of ENaC and SGK1 may parallel insufficient lung liquid clearance predisposing to respiratory distress. Lower SGK1 expression after term CD could translate into insufficient sodium and lung liquid absorption. The findings demonstrate a central role for cortisol in regulation of ENaC and potentially perinatal sodium and lung liquid clearance.

Insulin upregulates the expression of epithelial sodium channel in vitro and in a mouse model of acute lung injury: Role of mTORC2/SGK1 pathway

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are characterized by proteinaceous pulmonary edema and severe arterial hypoxemia with a mortality of approximately 40%. Stimulation of epithelial sodium channel (ENaC) promotes Na+ transport, a rate-limiting step for pulmonary edema reabsorption. Insulin is known to participate in the ion transport; however, its role in pulmonary edema clearance and the regulatory mechanism involved have not been fully elucidated. In the current study, in a lipopolysaccharide-based mouse model of ALI, we found that insulin alleviated pulmonary edema by promoting ENaC-mediated alveolar fluid clearance through serum and glucocorticoid induced kinase-1 (SGK1). In alveolar epithelial cells, insulin increased the expression of α-, β-, and γ-ENaC, which was blocked by the mammalian target of rapamycin complex 2 (mTORC2) inhibitor or knockdown of Rictor (a necessary component of mTORC2), and SGK1 inhibitor, respectively. In addition, an immunoprecipitation study demonstrated that SGK1(Ser422) phosphorylation, the key step for complete SGK1 activation by insulin, was conducted through PI3K/mTORC2 pathway. Finally, we testified the role of mTORC2 in vivo by demonstrating that PP242 prevented insulin-stimulated SGK1 activation and ENaC increase during ALI. The data revealed that during ALI, insulin stimulates alveolar fluid clearance by upregulating the expression of α-, β-, and γ-ENaC at the cell surface, which was, at least, partially through activating mTROC2/SGK1 signaling pathway.

Novel mechanism of intra‑renal angiotensin II-induced sodium/proton exchanger 3 expression by losartan in spontaneously hypertensive rats.

The present study aimed to investigate the molecular pharmacodynamic mechanisms of losartan used in the treatment of hypertension. A total of 12 spontaneously hypertensive rats (SHR) were divided randomly into an SHR group treated with saline and LOS group treated with losartan. Six Wistar‑kyoto rats (WKY) were enrolled as the WKY group with saline in the study. The LOS group received 30mg/kg/day losartan by intragastric injection, while the SHR and WKY were fed the same volume of saline. The dosage was modulated according to the weekly weight. Changes in blood pressure were measured by the indirect tail cuff method. Angiotensin (Ang) II production in the plasma and renal tissue was measured by an immunoradiometric method. Na+/H+ exchanger (NHE)3 and serum and glucocorticoid‑inducible kinase (SGK)1 were assessed by quantitative polymerase chain reaction (qPCR) and western blot analysis. When compared with the WKY group, the blood pressure of the SHR and LOS groups were higher prior to treatment with losartan. Following two weeks, blood pressure was reduced and the trend continued to decrease over the following six weeks. The plasma and renal tissue levels of Ang II in the SHR and LOS groups were significantly higher than those in the WKY group. NHE3 and SGK1 were increased at the mRNA and protein level in the SHR group, and losartan reduced the expression of both of them. The results suggested that in hypertensive rats, the circular and tissue renin angiotensin systems were activated, and the increased Ang II stimulated the expression of NHE3 and SGK1, which was reduced by losartan. Therefore, the effects of losartan in hypertension may be associated with the AngII‑SGK1‑NHE3 of intra‑renal tissue.

Increased expression and activation of serum- and glucocorticoid-inducible kinase-1 (SGK1) by cadmium in HK-2 renal proximal tubular epithelial cells

In HK-2 cells exposed to cadmium chloride (CdCl2), the level of serum- and glucocorticoid-inducible kinase-1 (SGK1) protein is increased, but the levels of SGK2 and SGK3 proteins are not. Phosphorylation of SGK1 protein is also observed. Treatment with actinomycin D abolished CdCl2-induced elevation of SGK1 mRNA level. Treatment with actinomycin D or cycloheximide suppressed SGK1 protein levels in cells exposed to CdCl2. Treatment with SGK1 inhibitor EMD638683 or knockdown of SGK1 with siRNA suppressed CdCl2-induced phosphorylation of N-Myc downstream-regulated kinase 1 (NDRG1). These results indicate that cadmium induces the transcriptional upregulation of SGK1 expression and regulates NDRG1 in HK-2 cells.

Revealing the role of SGK1 in the dynamics of medulloblastoma using a mathematical model

Deregulation of signaling pathways and subsequent abnormal interactions of downstream genes very often results in carcinogenesis. In this paper, we propose a two-compartment model describing intricate dynamics of the target genes of the Wnt signaling pathway in medulloblastoma. The system of nine nonlinear ordinary differential equations accounts for the formation and dissociation of complexes as well as for the transcription, translation and transport between the cytoplasm and the nucleus. We focus on the interplay between MYC and SGK1 (serum and glucocorticoid-inducible kinase 1), which are the products of Wnt/β-catenin signaling pathway, and GSK3β (glycogen synthase kinase). Numerical simulations of the model solutions yield a better understanding of the process and indicate the importance of the SGK1 gene in the development of medulloblastoma, which has been confirmed in our recent experiments. The model is calibrated based on the gene expression microarray data for two types of medulloblastoma, characterized by monosomy and trisomy of chromosome 6q to highlight the difference between diagnoses.

Serum and Glucocorticoid-Inducible Kinase1 Increases Plasma Membrane wt-CFTR in Human Airway Epithelial Cells by Inhibiting Its Endocytic Retrieval

BackgroundChloride (Cl) secretion by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) located in the apical membrane of respiratory epithelial cells plays a critical role in maintenance of the airway surface liquid and mucociliary clearance of pathogens. Previously, we and others have shown that the serum and glucocorticoid-inducible kinase-1 (SGK1) increases wild type CFTR (wt-CFTR) mediated Cl transport in Xenopus oocytes by increasing the amount of wt-CFTR protein in the plasma membrane. However, the effect of SGK1 on the membrane abundance of wt-CFTR in airway epithelial cells has not been examined, and the mechanism whereby SGK1 increases membrane wt-CFTR has also not been examined. Thus, the goal of this study was to elucidate the mechanism whereby SGK1 regulates the membrane abundance of wt-CFTR in human airway epithelial cells.Methods and ResultsWe report that elevated levels of SGK1, induced by dexamethasone, increase plasma membrane abundance of wt-CFTR. Reduction of SGK1 expression by siRNA (siSGK1) and inhibition of SGK1 activity by the SGK inhibitor GSK 650394 abrogated the ability of dexamethasone to increase plasma membrane wt-CFTR. Overexpression of a constitutively active SGK1 (SGK1-S422D) increased plasma membrane abundance of wt-CFTR. To understand the mechanism whereby SGK1 increased plasma membrane wt-CFTR, we examined the effects of siSGK1 and SGK1-S442D on the endocytic retrieval of wt-CFTR. While siSGK1 increased wt-CFTR endocytosis, SGK1-S442D inhibited CFTR endocytosis. Neither siSGK1 nor SGK1-S442D altered the recycling of endocytosed wt-CFTR back to the plasma membrane. By contrast, SGK1 increased the endocytosis of the epidermal growth factor receptor (EGFR).ConclusionThis study demonstrates for the first time that SGK1 selectively increases wt-CFTR in the plasma membrane of human airway epithelia cells by inhibiting its endocytic retrieval from the membrane.

TGF 1 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.

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

AimsThe 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. MethodsSGK1 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. ResultsSGK1 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. ConclusionAldosterone, 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.

PKB/SGK-dependent GSK3-phosphorylation in the regulation of LPS-induced Ca2+ increase in mouse dendritic cells

The function of dendritic cells (DCs) is modified by glycogen synthase kinase GSK3 and GSK3 inhibitors have been shown to protect against inflammatory disease. Regulators of GSK3 include the phosphoinositide 3 kinase (PI3K) pathway leading to activation of protein kinase B (PKB/Akt) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit GSK3. The present study explored, whether PKB/SGK-dependent inhibition of GSK3 contributes to the regulation of cytosolic Ca2+ concentration following stimulation with bacterial lipopolysaccharides (LPS). To this end DCs from mutant mice, in which PKB/SGK-dependent GSK3α,β regulation was disrupted by replacement of the serine residues in the respective SGK/PKB-phosphorylation consensus sequence by alanine (gsk3KI), were compared to DCs from respective wild type mice (gsk3WT). According to Western blotting, GSK3 phosphorylation was indeed absent in gsk3KI DCs. According to flow cytometry, expression of antigen-presenting molecule major histocompatibility complex II (MHCII) and costimulatory molecule CD86, was similar in unstimulated and LPS (1μg/ml, 24h)-stimulated gsk3WT and gsk3KI DCs. Moreover, production of cytokines IL-6, IL-10, IL-12 and TNFα was not significantly different in gsk3KI and gsk3WT DCs. In gsk3WT DCs, stimulation with LPS (1μg/ml) within 10min led to transient phosphorylation of GSK3. According to Fura2 fluorescence, LPS (1μg/ml) increased cytosolic Ca2+ concentration, an effect significantly more pronounced in gsk3KI DCs than in gsk3WT DCs. Conversely, GSK3 inhibitor SB216763 (3-[2,4-Dichlorophenyl]-4-[1-methyl-1H-indol-3-yl]-1H-pyrrole-2,5-dione, 10μM, 30min) significantly blunted the increase of cytosolic Ca2+ concentration following LPS exposure. In conclusion, PKB/SGK-dependent GSK3α,β activity participates in the regulation of Ca2+ signaling in dendritic cells.

Serum and glucocorticoid inducible kinase, metabolic syndrome, inflammation, and tumor growth

Serum-and-glucocorticoid-inducible-kinase-1 (SGK1) is under regulation of several hormones, mediators and cell stressors. More specifically, SGK1 expression is particularly sensitive to glucocorticoids, mineralocorticoids, and TGFβ. Moreover, SGK1 expression is exquisitely sensitive to hypertonicity, hyperglycemia, and ischemia. SGK1 is activated by insulin and growth factors via phosphatidylinositol-3-kinase, 3-phosphoinositide dependent-kinase PDK1, and mTOR. SGK1 up-regulates the Na⁺/K⁺-ATPase, a variety of carriers (e.g. NCC, NKCC, NHE1, NHE3, SGLT1, several amino acid transporters) and many ion channels (e.g. ENaC, SCN5A, TRPV4-6, Orai1/STIM1, ROMK, KCNE1/KCNQ1, GluR6, CFTR). SGK1 further up-regulates a number of enzymes (e.g. glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2), and transcription factors (e.g. forkhead-transcription-factor FOXO3a, β-catenin, nuclear-factor-kappa-B NFκB). SGK1 sensitive functions contribute to regulation of epithelial transport, excitability, degranulation, matrix protein deposition, coagulation, platelet aggregation, migration, cell proliferation, and apoptosis. Apparently, SGK1 is not required for housekeeping functions, as the phenotype of SGK1 knockout mice is mild. However, excessive SGK1 expression and activity participates in the pathophysiology of several disorders, including hypertension, obesity, diabetes, thrombosis, stroke, inflammation, autoimmune disease, fibrosis, and tumor growth. A SGK1 gene variant (prevalence ~3-5% prevalence in Caucasians, ~10% in Africans) predisposes to hypertension, stroke, obesity, and type 2 diabetes. Moreover, excessive salt intake and/or excessive release of glucocorticoids, mineralocorticoids, and TGFβ up-regulates SGK1 expression thus predisposing to SGK1-related diseases.

SGK1 Increases Plasma Membrane CFTR in Human Airway Epithelial Cells by Inhibiting Its Endocytic Retrieval

Chloride secretion by CFTR located in the apical membrane of airway epithelial cells (AECs) plays a critical role in maintenance of the airway surface liquid and mucociliary clearance of respiratory pathogens and debris. Previously, others and we have shown that the serum and glucocorticoid‐inducible kinase1 (SGK1) increases the amount of wt‐CFTR protein in the plasma membrane of Xenopus oocytes by an unknown mechanism. The goal of this study was to elucidate the mechanism whereby SGK1 regulates the membrane abundance of CFTR in human AECs. We report that elevated levels of SGK1, induced by dexamethasone, increases plasma membrane abundance of CFTR. Reduction of SGK1 expression by siRNA (siSGK1) or inhibition of SGK1 activity by GSK‐650394 abrogated the ability of dexamethasone to increase plasma membrane CFTR. By contrast, overexpression of a constitutively active SGK1 (SGK1–422D) increased plasma membrane abundance of CFTR. To examine the mechanism whereby SGK1 increased plasma membrane CFTR the effects of siSGK1 and SGK1‐S442D on the endocytic retrieval of CFTR was examined. Whereas siSGK1 increased CFTR endocytosis, SGK1‐ S442D inhibited CFTR endocytosis. By contrast, neither siSGK1 nor SGK1‐S442D altered the recycling of endocytosed CFTR back to the plasma membrane. This work demonstrates that SGK1 increases CFTR in the plasma membrane of human airway epithelia by inhibiting its endocytic retrieval.

Serum and glucocorticoid inducible kinase regulates hERG channels through inhibition of Nedd4–2 and activation of Rab11 recycling endosomes

The human ether‐a‐go‐go‐related gene (hERG) encodes the rapidly activating delayed rectifier potassium channel (IKr). Loss of function of hERG channels due to mutations or drug‐induced blockage can cause long QT syndrome, leading to ventricular arrhythmias or sudden death. Physical or emotional stress is known to affect ion channel activity. Stress hormones such as cortisol regulate the expression of various genes including the serum‐ and glucocorticoid‐inducible kinase (SGK). Using patch clamp, Western blot, co‐immunoprecipitation, and immunocytochemistry methods, we demonstrate that SGK isoforms SGK1 and SGK3 increased hERG current (IHERG) and the expression level of mature hERG proteins. We recently showed that mature hERG channels are degraded by ubiquitin ligase Nedd4–2 via enhanced ubiquitination. We observed that SGK1 and SGK3 overexpression enhanced Nedd4–2 phosphorylation which is known to inhibit Nedd4–2 activity. However, removal of the Nedd4–2 effect on hERG channels by disrupting Nedd4–2 target motif in hERG reduced but did not eliminate the SGK‐induced increase in hERG expression. Additional disruption of Rab11 proteins led to a complete elimination of SGK‐mediated hERG increase. We conclude that SGK isoforms 1 and 3 enhance hERG stability by inhibiting Nedd4–2 ubiquitin ligase and activating Rab11 recycling endosomes.Supported by: Canadian Institutes of Health Research (CIHR)

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 conditons, 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, Ca2+ release activated Ca2+ 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.