SGK1 increases Na,K-ATP cell-surface expression and function in Xenopus laevis oocytes

Association between Decreased SGK1 and Increased Intestinal α-Synuclein in an MPTP Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is a globally common progressive neurodegenerative disease resulting from the loss of dopaminergic neurons in the brain. Increased α-synuclein (α-syn) is associated with the degeneration of dopaminergic neurons and non-motor symptoms like gastrointestinal disorders. In this study, we investigated the association between serum/glucocorticoid-related kinase 1 (SGK1) and α-syn in the colon of a PD mouse model. SGK1 and α-syn expression patterns were opposite in the surrounding colon tissue, with decreased SGK1 expression and increased α-syn expression in the PD group. Immunofluorescence analyses revealed the colocation of SGK1 and α-syn; the PD group demonstrated weaker SGK1 expression and stronger α-syn expression than the control group. Immunoblotting analysis showed that Na+/K+ pump ATPase α1 expression levels were significantly increased in the PD group. In SW480 cells with SGK1 knockdown using SGK1 siRNA, decreasing SGK1 levels corresponded with significant increases in the expression levels of α-syn and ATPase α1. These results suggest that SGK1 significantly regulates Na+/K+ pump ATPase, influencing the relationship between electrolyte balance and fecal formation in the PD mouse model. Gastrointestinal disorders are some of the major prodromal symptoms of PD. Therefore, modulating SGK1 expression could be an important strategy for controlling PD.

Paracrine and endocrine regulation of renal potassium secretion

The seminal studies conducted by Giebisch and colleagues in the 1960s paved the way for understanding the renal mechanisms involved in K+ homeostasis. It was demonstrated that differential handling of K+ in the distal segments of the nephron is crucial for proper K+ balance. Although aldosterone had been classically ascribed as the major ion transport regulator in the distal nephron, thereby contributing to K+ homeostasis, it became clear that aldosterone per se could not explain the kidney's ability to modulate kaliuresis in both acute and chronic settings. The existence of alternative kaliuretic and antikaliuretic mechanisms was suggested by physiological studies in the 1980s but only gained form and shape with the advent of molecular biology. It is now established that the kidneys recruit several endocrine and paracrine mechanisms for adequate kaliuretic response. These mechanisms include the direct effects of peritubular K+, a gut-kidney regulatory axis sensing dietary K+ levels, the kidney secretion of kallikrein during postprandial periods, the upregulation of angiotensin II receptors in the distal nephron during chronic changes in the K+ diet, and the local increase of prostaglandins by low K+ diet. This review discusses recent advances in the understanding of endocrine and paracrine mechanisms underlying the modulation of K+ secretion and how these mechanisms impact kaliuresis and K+ balance. We also highlight important unknowns about the regulation of renal K+ excretion under physiological circumstances.

Mineralocorticoid regulation of cell function: the role of rapid signalling and gene transcription pathways

The mineralocorticoid receptor (MR) and mineralocorticoids regulate epithelial handling of electrolytes, and induces diverse effects on other tissues. Traditionally, the effects of MR were ascribed to ligand-receptor binding and activation of gene transcription. However, the MR also utilises a number of intracellular signalling cascades, often by transactivating unrelated receptors, to change cell function more rapidly. Although aldosterone is the physiological mineralocorticoid, it is not the sole ligand for MR. Tissue-selective and mineralocorticoid-specific effects are conferred through the enzyme 11β-hydroxysteroid dehydrogenase 2, cellular redox status and properties of the MR itself. Furthermore, not all aldosterone effects are mediated via MR, with implication of the involvement of other membrane-bound receptors such as GPER. This review will describe the ligands, receptors and intracellular mechanisms available for mineralocorticoid hormone and receptor signalling and illustrate their complex interactions in physiology and disease.

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.

Coordinated Control of ENaC and Na+,K+-ATPase in Renal Collecting Duct

Tubular reabsorption of filtered sodium is tightly controlled to maintain body volume homeostasis. The rate of sodium transport by collecting duct (CD) cells varies widely in response to dietary sodium intake, GFR, circulating hormones, neural signals, and local regulatory factors. Reabsorption of filtered sodium by CD cells occurs via a two-step process. First, luminal sodium crosses the apical plasma membrane along its electrochemical gradient through epithelial sodium channels (ENaC). Intracellular sodium is then actively extruded into the interstitial space by the Na(+),K(+)-ATPase located along the basolateral membrane. Mismatch between sodium entry and exit induces variations in sodium intracellular concentration and cell volume that must be maintained within narrow ranges for control of vital cell functions. Therefore, renal epithelial cells display highly coordinated apical and basolateral sodium transport rates. We review evidence from experiments conducted in vivo and in cultured cells that indicates aldosterone and vasopressin, the two major hormones regulating sodium reabsorption by CD, generate a coordinated stimulation of apical ENaC and basolateral Na(+),K(+)-ATPase. Moreover, we discuss evidence suggesting that variations in sodium entry per se induce a coordinated change in Na(+),K(+)-ATPase activity through the signaling of protein kinases such as protein kinase A and p38 mitogen-activated protein kinase.

Dietary potassium and the renal control of salt balance and blood pressure

Dietary potassium (K(+)) intake has antihypertensive effects, prevents strokes, and improves cardiovascular outcomes. The underlying mechanism for these beneficial effects of high K(+) diets may include vasodilation, enhanced urine flow, reduced renal renin release, and negative sodium (Na(+)) balance. Indeed, several studies demonstrate that dietary K(+) intake induces renal Na(+) loss despite elevated plasma aldosterone. This review briefly highlights the epidemiological and experimental evidences for the effects of dietary K(+) on arterial blood pressure. It discusses the pivotal role of the renal distal tubule for the regulation of urinary K(+) and Na(+) excretion and blood pressure and highlights that it depends on the coordinated interaction of different nephron portions, epithelial cell types, and various ion channels, transporters, and ATPases. Moreover, we discuss the relevance of aldosterone and aldosterone-independent factors in mediating the effects of an altered K(+) intake on renal K(+) and Na(+) handling. Particular focus is given to findings suggesting that an aldosterone-independent downregulation of the thiazide-sensitive NaCl cotransporter significantly contributes to the natriuretic and antihypertensive effect of a K(+)-rich diet. Last but not least, we refer to the complex signaling pathways enabling the kidney to adapt its function to the homeostatic needs in response to an altered K(+) intake. Future work will have to further address the underlying cellular and molecular mechanism and to elucidate, among others, how an altered dietary K(+) intake is sensed and how this signal is transmitted to the different epithelial cells lining the distal tubule.

WNK3 abrogates the NEDD4-2-mediated inhibition of the renal Na+-Cl- cotransporter

The serine/threonine kinase WNK3 and the ubiquitin-protein ligase NEDD4-2 are key regulators of the thiazide-sensitive Na+-Cl- cotransporter (NCC), WNK3 as an activator and NEDD2-4 as an inhibitor. Nedd4-2 was identified as an interacting partner of WNK3 through a glutathione-S-transferase pull-down assay using the N-terminal domain of WNK3, combined with LC-MS/MS analysis. This was validated by coimmunoprecipitation of WNK3 and NEDD4-2 expressed in HEK293 cells. Our data also revealed that the interaction between Nedd4-2 and WNK3 does not involve the PY-like motif found in WNK3. The level of WNK3 ubiquitylation did not change when NEDD4-2 was expressed in HEK293 cells. Moreover, in contrast to SGK1, WNK3 did not phosphorylate NEDD4-2 on S222 or S328. Coimmunoprecipitation assays showed that WNK3 does not regulate the interaction between NCC and NEDD4-2. Interestingly, in Xenopus laevis oocytes, WNK3 was able to recover the SGK1-resistant NEDD4-2 S222A/S328A-mediated inhibition of NCC and further activate NCC. Furthermore, elimination of the SPAK binding site in the kinase domain of WNK3 (WNK3-F242A, which lacks the capacity to bind the serine/threonine kinase SPAK) prevented the WNK3 NCC-activating effect, but not the Nedd4-2-inhibitory effect. Together, these results suggest that a novel role for WNK3 on NCC expression at the plasma membrane, an effect apparently independent of the SPAK kinase and the aldosterone-SGK1 pathway.

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.

Non-genomic effect of glucocorticoids on cardiovascular system

Glucocorticoids (GCs) are essential steroid hormones for homeostasis, development, metabolism, and cognition and possess anti-inflammatory and immunosuppressive actions. Since glucocorticoid receptor II (GR) is nearly ubiquitous, chronic activation or depletion of GCs leads to dysfunction of diverse organs, including the heart and blood vessels, resulting predominantly from changes in gene expression. Most studies, therefore, have focused on the genomic effects of GC to understand its related pathophysiological manifestations. The nongenomic effects of GCs clearly differ from well-known genomic effects, with the former responding within several minutes without the need for protein synthesis. There is increasing evidence that the nongenomic actions of GCs influence various physiological functions. To develop a GC-mediated therapeutic target for the treatment of cardiovascular disease, understanding the genomic and nongenomic effects of GC on the cardiovascular system is needed. This article reviews our current understanding of the underlying mechanisms of GCs on cardiovascular diseases and stress, as well as how nongenomic GC signaling contributes to these conditions. We suggest that manipulation of GC action based on both GC and GR metabolism, mitochondrial impact, and the action of serum- and glucocorticoid-dependent kinase 1 may provide new information with which to treat cardiovascular diseases.

Regulation of KCNQ1/KCNE1 by β-catenin

β-catenin, a multifunctional protein expressed in all tissues including the heart stimulates the expression of several genes important for cell proliferation. Signaling involving ß-catenin participates in directing cardiac development and in the pathophysiology of cardiac hypertrophy. Nothing is known, however, on the role of β-catenin in the regulation of cardiac ion channels. The present study explored the functional interaction of β-catenin and KCNE1/KCNQ1, the K⁺ channel complex underlying the slowly activating outwardly rectifying K⁺ current. To this end, KCNE1/KCNQ1 was expressed in Xenopus oocytes with and without β-catenin and the depolarization (up to + 80 mV) induced current (I(Ks)) was determined using the two-electrode voltage clamp. As a result, β-catenin enhanced I(Ks) by 30%. The effect of β-catenin on I(Ks) was not affected by actinomycin D (10 μM), an inhibitor of transcription, indicating that β-catenin was not effective as transcription factor. Confocal microscopy revealed that β-catenin enhanced the KCNE1/KCNQ1 protein abundance in the cell membrane. Exposure of the oocytes to brefeldin A (5 μM), an inhibitor of vesicle insertion, was followed by a decline of I(Ks), which was then similar in oocytes expressing KCNE1/KCNQ1 together with β-catenin and in oocytes expressing KCNE1/KCNQ1 alone. In conclusion, β-catenin enhances I(Ks) by increasing the KCNE1/KCNQ1 protein abundance in the cell membrane, an effect requiring vesicle insertion into the cell membrane.

Morpholino gene knockdown in adult Fundulus heteroclitus: role of SGK1 in seawater acclimation

The Atlantic killifish (Fundulus heteroclitus) is an environmental sentinel organism used extensively for studies on environmental toxicants and salt (NaCl) homeostasis. Previous research in our laboratory has shown that rapid acclimation of killifish to seawater is mediated by trafficking of CFTR chloride channels from intracellular vesicles to the plasma membrane in the opercular membrane within the first hour in seawater, which enhances chloride secretion into seawater, thereby contributing to salt homeostasis. Acute transition to seawater is also marked by an increase in both mRNA and protein levels of serum glucocorticoid kinase 1 (SGK1) within 15 minutes of transfer. Although the rise in SGK1 in gill and its functional analog, the opercular membrane, after seawater transfer precedes the increase in membrane CFTR, a direct role of SGK1 in elevating membrane CFTR has not been established in vivo. To test the hypothesis that SGK1 mediates the increase in plasma membrane CFTR we designed two functionally different vivo-morpholinos to knock down SGK1 in gill, and developed and validated a vivo-morpholino knock down technique for adult killifish. Injection (intraperitoneal, IP) of the splice blocking SGK1 vivo-morpholino reduced SGK1 mRNA in the gill after transition from fresh to seawater by 66%. The IP injection of the translational blocking and splice blocking vivo-morpholinos reduced gill SGK1 protein abundance in fish transferred from fresh to seawater by 64% and 53%, respectively. Moreover, knock down of SGK1 completely eliminated the seawater induced rise in plasma membrane CFTR, demonstrating that the increase in SGK1 protein is required for the trafficking of CFTR from intracellular vesicles in mitochondrion rich cells to the plasma membrane in the gill during acclimation to seawater. This is the first report of the use of vivo-morpholinos in adult killifish and demonstrates that vivo-morpholinos are a valuable genetic tool for this environmentally relevant model organism.

Epithelial sodium channel regulation by cell surface-associated serum- and glucocorticoid-regulated kinase 1

Serum- and glucocorticoid-regulated kinase 1 (sgk1) participates in diverse biological processes, including cell growth, apoptosis, and sodium homeostasis. In the cortical collecting duct of the kidney, sgk1 regulates sodium transport by stimulating the epithelial sodium channel (ENaC). Control of subcellular localization of sgk1 may be an important mechanism for modulating specificity of sgk1 function; however, which subcellular locations are required for sgk1-regulated ENaC activity in collecting duct cells has yet to be established. Using cell surface biotinylation studies, we detected endogenous sgk1 at the apical cell membrane of aldosterone-stimulated mpkCCD(c14) collecting duct cells. The association of sgk1 with the cell membrane was enhanced when ENaC was co-transfected with sgk1 in kidney cells, suggesting that ENaC brings sgk1 to the cell surface. Furthermore, association of endogenous sgk1 with the apical cell membrane of mpkCCD(c14) cells could be modulated by treatments that increase or decrease ENaC expression at the apical membrane; forskolin increased the association of sgk1 with the apical surface, whereas methyl-β-cyclodextrin decreased the association of sgk1 with the apical surface. Single channel recordings of excised inside-out patches from the apical membrane of aldosterone-stimulated A6 collecting duct cells revealed that the open probability of ENaC was sensitive to the sgk1 inhibitor GSK650394, indicating that endogenous sgk1 is functionally active at the apical cell membrane. We propose that the association of sgk1 with the apical cell membrane, where it interacts with ENaC, is a novel means by which sgk1 specifically enhances ENaC activity in aldosterone-stimulated collecting duct cells.

Nedd4-2 modulates renal Na+-Cl- cotransporter via the aldosterone-SGK1-Nedd4-2 pathway

Regulation of renal Na(+) transport is essential for controlling blood pressure, as well as Na(+) and K(+) homeostasis. Aldosterone stimulates Na(+) reabsorption by the Na(+)-Cl(-) cotransporter (NCC) in the distal convoluted tubule (DCT) and by the epithelial Na(+) channel (ENaC) in the late DCT, connecting tubule, and collecting duct. Aldosterone increases ENaC expression by inhibiting the channel's ubiquitylation and degradation; aldosterone promotes serum-glucocorticoid-regulated kinase SGK1-mediated phosphorylation of the ubiquitin-protein ligase Nedd4-2 on serine 328, which prevents the Nedd4-2/ENaC interaction. It is important to note that aldosterone increases NCC protein expression by an unknown post-translational mechanism. Here, we present evidence that Nedd4-2 coimmunoprecipitated with NCC and stimulated NCC ubiquitylation at the surface of transfected HEK293 cells. In Xenopus laevis oocytes, coexpression of NCC with wild-type Nedd4-2, but not its catalytically inactive mutant, strongly decreased NCC activity and surface expression. SGK1 prevented this inhibition in a kinase-dependent manner. Furthermore, deficiency of Nedd4-2 in the renal tubules of mice and in cultured mDCT(15) cells upregulated NCC. In contrast to ENaC, Nedd4-2-mediated inhibition of NCC did not require the PY-like motif of NCC. Moreover, the mutation of Nedd4-2 at either serine 328 or 222 did not affect SGK1 action, and mutation at both sites enhanced Nedd4-2 activity and abolished SGK1-dependent inhibition. Taken together, these results suggest that aldosterone modulates NCC protein expression via a pathway involving SGK1 and Nedd4-2 and provides an explanation for the well-known aldosterone-induced increase in NCC protein expression.

SGK1-dependent stimulation of intestinal SGLT1 activity by vitamin D

The serum- and glucocorticoid-inducible kinase SGK1 has previously been shown to mediate the glucocorticoid-dependent stimulation of several intestinal transport systems including the electrogenic glucose transporter SGLT1. In squamous carcinoma cells, SGK1 expression is stimulated by 1,25(OH)₂D₃, the biologically active metabolite of vitamin D. The present study explored whether vitamin D influences the intestinal SGLT1 activity. Jejunal SGLT1 activity was determined by Ussing chamber experiments. Under a normal diet, the electrogenic glucose transport was similar in SGK1 knockout (sgk1 ( -/- )) and wild type mice (sgk1 ( +/+ )). Following a vitamin D-rich diet (14 days 10,000 I.U. vitamin D), the SGK1 transcript levels as well as the SGLT1 protein abundance were increased in sgk1(+/+) mice. Moreover, SGLT1 activity was increased in sgk1(+/+) mice but not in sgk1(-/-) mice following a vitamin D-rich diet. Furthermore, an oral glucose load was followed by an increase in the plasma glucose concentration to significantly higher values in sgk1(+/+) mice treated with a vitamin D-rich diet than in untreated sgk1(+/+) mice. In conclusion, vitamin D treatment upregulates the expression of SGK1, which in turn enhances SGLT1 activity.

Role of epithelial sodium channels and their regulators in hypertension

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

Stimulation of electrogenic intestinal dipeptide transport by the glucocorticoid dexamethasone

According to recent in vitro experiments, the peptide transporter PepT2 is stimulated by the serum- and glucocorticoid-inducible kinase SGK1. The present study explored the contribution of SGK1 to the regulation of electrogenic intestinal peptide transport. Intestinal PepT1 was expressed in Xenopus oocytes, and peptide transport was determined by dual electrode voltage clamping. Peptide transport in intestinal segments was determined utilizing Ussing chamber. Cytosolic pH (pH( i )) was determined by BCECF fluorescence and Na(+)/H(+) exchanger activity was estimated from Na(+)-dependent pH recovery (pH ( i )) following an ammonium pulse. In PepT1-expressing Xenopus oocytes, coexpression of SGK1 enhanced electrogenic peptide transport. Intestinal transport and pH( i ) of untreated mice were similar in SGK1 knockout mice (sgk1 ( -/- )) and their wild-type littermates (sgk1 ( +/+ )). Glucocorticoid treatment (4 days 10 microg/g body weight (bw)/day dexamethasone) increased peptide transport in sgk1 ( +/+ ) but not in sgk1 (-/-) mice. Irrespective of dexamethasone treatment, luminal peptide (5 mM glycyl-glycine) led to a similar early decrease of pH( i ) in sgk1 (-/-) and sgk1 (+/+) mice, but to a more profound and sustained decline of pH( i ) in sgk1 (-/-) than in sgk1 ( +/+ ) mice. In the presence and absence of glycyl-glycine, pH ( i ) was significantly enhanced by dexamethasone treatment in sgk1 ( +/+ ) mice, an effect significantly blunted in sgk1 ( -/- ) mice. During sustained exposure to glycyl-glycine, pH ( i ) was significantly larger in sgk1 (+/+) mice than in sgk1 (-/-) mice, irrespective of dexamethasone treatment. In conclusion, basal intestinal peptide transport does not require stimulation by SGK1. Glucocorticoid treatment stimulates both Na(+)/H(+) exchanger activity and peptide transport, effects partially dependent on SGK1. Moreover, chronic exposure to glycyl-glycine stimulates Na(+)/H(+) exchanger activity, an effect again involving SGK1.

Orphan transporter SLC6A18 is renal neutral amino acid transporter B0AT3

The orphan transporter Slc6a18 (XT2) is highly expressed at the luminal membrane of kidney proximal tubules and displays approximately 50% identity with Slc6a19 (B(0)AT1), which is the main neutral amino acid transporter in both kidney and small intestine. As yet, the amino acid transport function of XT2 has only been experimentally supported by the urinary glycine loss observed in xt2 null mice. We report here that in Xenopus laevis oocytes, co-expressed ACE2 (angiotensin-converting enzyme 2) associates with XT2 and reveals its function as a Na(+)- and Cl(-)-de pend ent neutral amino acid transporter. In contrast to its association with ACE2 observed in Xenopus laevis oocytes, our experiments with ace2 and collectrin null mice demonstrate that in vivo it is Collectrin, a smaller homologue of ACE2, that is required for functional expression of XT2 in kidney. To assess the function of XT2 in vivo, we reanalyzed its knock-out mouse model after more than 10 generations of backcrossing into C57BL/6 background. In addition to the previously published glycinuria, we observed a urinary loss of several other amino acids, in particular beta-branched and small neutral ones. Using telemetry, we confirmed the previously described link of XT2 absence with hypertension but only in physically restrained animals. Taken together, our data indicate that the formerly orphan transporter XT2 functions as a sodium and chloride-de pend ent neutral amino acid transporter that we propose to rename B(0)AT3.

SGK1-sensitive renal tubular glucose reabsorption in diabetes

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

Aldosterone regulation of intestinal Na absorption involves SGK-mediated changes in NHE3 and Na+ pump activity

Aldosterone-induced intestinal Na(+) absorption is mediated by increased activities of apical membrane Na(+)/H(+) exchange (aNHE3) and basolateral membrane Na(+)-K(+)-ATPase (BLM-Na(+)-K(+)-ATPase) activities. Because the processes coordinating these events were not well understood, we investigated human intestinal Caco-2BBE cells where aldosterone increases within 2-4 h of aNHE3 and alpha-subunit of BLM-Na(+)-K(+)-ATPase, but not total abundance of these proteins. Although aldosterone activated Akt2 and serum glucorticoid kinase-1 (SGK-1), the latter through stimulation of phosphatidylinositol 3-kinase (PI3K), only the SGK-1 pathway mediated its effects on Na(+)-K(+)-ATPase. Ouabain inhibition of the early increase in aldosterone-induced Na(+)-K(+)-ATPase activation blocked most of the apical NHE3 insertion, possibly by inhibiting Na(+)-K(+)-ATPase-induced changes in intracellular sodium concentration ([Na](i)). Over the next 6-48 h, further increases in aNHE3 and BLM-Na(+)-K(+)-ATPase activity and total protein expression were observed to be largely mediated by aldosterone-activated SGK-1 pathway. Aldosterone-induced increases in NHE3 mRNA, for instance, could be inhibited by RNA silencing of SGK-1, but not Akt2. Additionally, aldosterone-induced increases in NHE3 promoter activity were blocked by silencing SGK-1 as well as pharmacological inhibition of PI3K. In conclusion, aldosterone-stimulated intestinal Na(+) absorption involves two phases. The first phase involves stimulation of PI3K, which increases SGK-dependent insertion and function of BLM-Na(+)-K(+)-ATPase and subsequent increased membrane insertion of aNHE3. The latter may be caused by Na(+)-K(+)-ATPase-induced changes in [Na] or transcellular Na flux. The second phase involves SGK-dependent increases in total NHE3 and Na(+)-K(+)-ATPase protein expression and activities. The coordination of apical and BLM transporters after aldosterone stimulation is therefore a complex process that requires multiple time- and interdependent cellular processes.

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

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

SGK1 dependence of insulin induced hypokalemia

Insulin stimulates cellular K+ uptake leading to hypokalemia. Cellular K+ uptake is accomplished by parallel stimulation of Na+/H+ exchange, Na+,K+,2Cl- co-transport, and Na+/K+ ATPase and leads to cell swelling, a prerequisite for several metabolic effects of the hormone. Little is known about underlying signaling. Insulin is known to activate the serum and glucocorticoid-inducible kinase SGK1, which in turn enhances the activity of all three transport proteins. The present study thus explored the contribution of SGK1 to insulin-induced hypokalemia. To this end, gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+) have been infused with insulin (2 mU kg(-1) min(-1)) and glucose at rates leaving the plasma glucose concentration constant. Moreover, isolated liver perfusion experiments have been performed to determine stimulation of cellular K+ uptake by insulin (100 nM). As a result, combined glucose and insulin infusion significantly decreased plasma K+ concentration despite a significant decrease of urinary K+ excretion in sgk1+/+ but not in sgk1-/- mice. Accordingly, the plasma K+ concentration was within 60 min significantly lower in sgk1+/+ than in sgk1-/- mice. In isolated liver perfusion experiments, cellular K+ uptake was stimulated by insulin (100 nM), an effect blunted by 72% in sgk1-/- mice as compared to sgk1+/+ mice. Accordingly, insulin-induced cell hydration was 63% lower in sgk1-/- mice than in sgk1+/+ mice. Moreover, volume regulatory K+ release was 31% smaller in sgk1-/- mice than in sgk1+/+ mice. In conclusion, the serum and glucocorticoid-inducible kinase SGK1 participates in the signaling mediating the hypokalemic effect of insulin.

NF-kappaB inhibits sodium transport via down-regulation of SGK1 in renal collecting duct principal cells

Tubulointerstitial inflammation is a common feature of renal diseases. We have investigated the relationship between inflammation and Na(+) transport in the collecting duct (CD) using the mCCD(cl1) and mpkCDD(cl4) principal cell models. Lipopolysaccharide (LPS) decreased basal and aldosterone-stimulated amiloride-sensitive transepithelial current in a time-dependent manner. This effect was associated with a decrease in serum and glucocorticoid-regulated kinase 1 (SGK1) mRNA and protein levels followed by a decrease in epithelial sodium channel (ENaC) alpha-subunit mRNA levels. The LPS-induced decrease in SGK1 expression was confirmed in isolated rat CD. This decreased expression of either SGK1 or the ENaC alpha-subunit was not due to enhanced degradation of mRNA. In contrast, LPS inhibited transcriptional activity of the SGK1 promoter measured by luciferase-reporter gene assay. The effect of LPS was not mediated by inhibition of mineralocorticoid or glucocorticoid receptor, because expression of both receptors was unchanged and blockade of either receptor by spironolactone or RU486, respectively, did not prevent the down-regulation of SGK1. The effect of LPS was mediated by the canonical NF-kappaB pathway, as overexpression of a constitutively active mutant, IKKbeta (inhibitor of nuclear factor kappaB kinase-beta) decreased SGK1 mRNA levels, and knockdown of p65 NF-kappaB subunit by small interfering RNA increased SGK1 mRNA levels. Chromatin immunoprecipitation showed that LPS increased p65 binding to two NF-kappaB sites along the SGK1 promoter. In conclusion, we show that activation of the NF-kappaB pathway down-regulates SGK1 expression, which might lead to decreased ENaC alpha-subunit expression, ultimately resulting in decreased Na(+) transport.

Regulation of epithelial Na+ channels by aldosterone: role of Sgk1

1. The epithelial sodium channel (ENaC) is tightly regulated by hormonal and humoral factors, including cytosolic ion concentration and glucocorticoid and mineralocorticoid hormones. Many of these regulators of ENaC control its activity by regulating its surface expression via neural precursor cell-expressed developmentally downregulated (gene 4) protein (Nedd4-2). 2. During the early phase of aldosterone action, Nedd4-2-dependent downregulation of ENaC is inhibited by the serum- and glucocorticoid-induced kinase 1 (Sgk1). 3. Sgk1 phosphorylates Nedd4-2. Subsequently, phosphorylated Nedd4-2 binds to the 14-3-3 protein and, hence, reduces binding of Nedd4-2 to ENaC. 4. Nedd4-2 is also phosphorylated by protein kinase B (Akt1). Both Sgk1 and Akt1 are part of the insulin signalling pathway that increases transepithelial Na(+) absorption by inhibiting Nedd4-2 and activating ENaC.

Association of SGK1 gene polymorphisms with type 2 diabetes

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

Differential regulation of serum- and glucocorticoid-inducible kinase 1 (SGK1) splice variants based on alternative initiation of transcription

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is a key-regulator of transport, cell volume and cell survival. SGK1 transcription is under genomic control of a wide variety of hormones and cell stressors. Little is known, however, about sequence variation in SGK1 transcripts. Thus, we took an in silico approach to determine sequence variations in the N-terminal region of SGK1, which is considered particularly important for subcellular SGK1 localization. Expressed Sequence Tag analysis revealed two novel phylogenetically highly conserved SGK1 mRNAs with different promoter sites based on alternative initiation of transcription at -2981, -850 upstream of the transcription initiation site (+1) of the reference mRNA. RT-PCR in various human cell lines and tissues confirmed the expression of the 3 alternative splice variants, which differed exclusively in their first exons. The two novel variants were devoid of the localization and degradation signal with otherwise unchanged and intact open reading frames. Spatial distribution of transcription factor binding sites among the three promoter sites indicated common responsiveness to glucocorticoids but different responsiveness to hypoxia and cellular differentiation. Differential expression under those conditions was confirmed for all variants in cultured myoblasts and myotubes. p53 and ETF-1 binding sites were overrepresented at the promoter site of the reference sequence variant SGK1(+1). Transcript levels were 4.1-fold [SGK1(+1)] and 3.1-fold [SGK1(-850)] higher in renal clear cell carcinoma than in remote tissue. The transcript levels were 42-fold [SGK1(+1)], 26-fold [SGK1(-850)] and 17-fold [SGK1(-2981)] higher in highly malignant human glioma cells than in non-neoplastic brain tissue. SGK1 transcript levels were differentially increased by differentiation or hypoxia (treatment with CoCl(2)). In conclusion, the present observations disclose the transcription of three distinct SGK1 splice variants, which are all markedly upregulated in tumor tissue but differentially upregulated following differentiation or hypoxia.

Crystal structure of the kinase domain of serum and glucocorticoid-regulated kinase 1 in complex with AMP PNP

Serum and glucocorticoid-regulated kinase 1 (SGK1) is a serine/threonine protein kinase of the AGC family which participates in the control of epithelial ion transport and is implicated in proliferation and apoptosis. We report here the 1.9 A crystal structure of the catalytic domain of inactive human SGK1 in complex with AMP-PNP. SGK1 exists as a dimer formed by two intermolecular disulfide bonds between Cys258 in the activation loop and Cys193. Although most of the SGK1 structure closely resembles the common protein kinase fold, the structure around the active site is unique when compared to most protein kinases. The alphaC helix is not present in this inactive form of SGK1 crystal structure; instead, the segment corresponding to the C helix forms a beta-strand that is stabilized by the N-terminal segment of the activation loop through a short antiparallel beta-sheet. Since the differences from other kinases occur around the ATP binding site, this structure can provide valuable insight into the design of selective and highly potent ATP-competitive inhibitors of SGK1 kinase.

Role of the serum and glucocorticoid inducible kinase SGK1 in glucocorticoid stimulation of gastric acid secretion

Glucocorticoids stimulate gastric acid secretion, an effect favoring the development of peptic ulcers. Putative mechanisms involved include the serum- and glucocorticoid-inducible kinase (SGK1), which stimulates a variety of epithelial channels and transporters. The present study explored the contribution of SGK1 to effects of glucocorticoids on gastric acid secretion. In isolated gastric glands from gene-targeted mice lacking functional SGK1 (sgk1 (-/-)) and their wild-type littermates (sgk1 (+/+)), H(+)-secretion (DeltapH/min) was determined utilizing 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF)-fluorescence, SGK1 transcript levels by in situ hybdridization, and expression of KCNQ1 channels by immunohistochemistry and real-time polymerase chain reaction. SGK1 transcript levels were enhanced by a 4-day treatment with 10 mug/g body weight (BW)/day dexamethasone (DEX). Before treatment, DeltapH/min was similar in sgk1 (-/-) and sgk1 (+/+)mice. DEX increased DeltapH/min approximately fourfold in sgk1 (+/+)mice and approximately twofold in sgk1 (-/-)mice, effects abolished in the presence of K(+)/H(+)ATPase-inhibitor omeprazole (50 microM). Increase in local K(+) concentrations to 35 mM (replacing Na(+)) enhanced DeltapH/min, which could not be further stimulated by DEX and was not significantly different between sgk1 (-/-) and sgk1 (+/+)mice. Carbachol (100 microM) and forskolin (5 microM) stimulated gastric acid secretion to a similar extent in sgk1 (-/-) and sgk1 (+/+)mice. In conclusion, SGK1 is not required for basal and cyclic AMP-stimulated gastric H(+) secretion but participates in the stimulation of gastric H(+) secretion by glucocorticoids. The effects of glucocorticoids and SGK1 are not additive to an increase in extracellular K(+) concentration and may thus involve stimulation of K(+) channels.

Molecular pathogenesis of renal pseudohypoaldosteronism type 1

Pseudohypoaldosteronism is a rare heterogeneous syndrome of mineralocorticoid resistance resulting in insufficient potassium and hydrogen secretion. Pseudohypoaldosteronism type 1 is characterized by mineralocorticoid resistance leading to neonatal salt loss, dehydration and failure to thrive. At least two different forms of pseudohypoaldosteronism type 1 can be distinguished, showing either a systemic or renal form of mineralocorticoid resistance. This review offers an overview on transepithelial sodium reabsorption and pseudohypoaldosteronism in general, and focuses on the underlying molecular pathology of the renal-restricted pseudohypoaldosteronism type 1 form caused by heterozygous mutations in the mineralocorticoid receptor-coding gene NR3C2. The investigation of several NR3C2 mutants in vitro has resulted in important progress in the understanding of the physiology of the mineralocorticoid receptor. However, there are still some families or individuals suffering from renal pseudohypoaldosteronism type 1 in whom no genetic defect was found in the NR3C2 or other genes such as SCNN1A, SCNN1B, SCNN1G, NEDD4 or SGK1 that are involved in the epithelial salt transport machinery. Further research in these cases may enable the identification of other pathologies leading to renal pseudohypoaldosteronism type 1 and permit deeper insights into the epithelial sodium reabsorption process.

Exclusion of serum- and glucocorticoid-induced kinase 1 (SGK1) as a candidate gene for genetically heterogeneous renal pseudohypoaldosteronism type I in eight families

OBJECTIVE

Autosomal-dominant pseudohypoaldosteronism type 1 (PHA1) is mostly caused by mutations in the mineralocorticoid receptor (NR3C2) gene. However, several kindreds with clinical signs of PHA1 but without NR3C2 gene mutations or deletions are reported. Serum- and glucocorticoid-induced kinase 1 (Sgk1) is involved in epithelial sodium reabsorption by facilitating the accumulation of the epithelial sodium channel in the plasma membrane. Therefore variations in the SGK1 gene may aggravate renal salt loss or cause PHA1.

METHODS

The SGK1 genewas sequenced in 10 patients with the typical clinical signs of PHA1 but without NR3C2 or SCNN1A, SCNN1B and SCNN1C gene mutation.

RESULTS

No disease-causing SGK1 gene mutation was detected in the studied PHA1 patient group. Two novel intronic SNPs which were also present in the normal population were detected in 2 patients.

CONCLUSION

Our data do not support that SGK1 gene variations are disease-causing factors in genetically unexplained PHA1. Therefore, systematic investigation of other factors downstream of the MR involved in epithelial sodium reabsorption is warranted in patients with autosomal-dominant PHA1.

Dexamethasone increases Na+/K+ ATPase activity in insulin secreting cells through SGK1

Glucocorticoids blunt insulin release, an effect partially due to activation of Kv channels. Similar to those channels Na+/K+ ATPase activity repolarizes the plasma membrane. The present study explored whether glucocorticoids increase the Na+/K+ ATPase activity in pancreatic beta-cells. The glucocorticoid dexamethasone (100 nmol/l for 1 day) significantly increased Na+/K+ ATPase alpha1/beta1-subunit transcript levels and ouabain-sensitive outward current reflecting Na+/K+ ATPase activity in INS-1 cells, effects blunted by glucocorticoid-receptor-blocker RU487 (1 micromol/l). Dexamethasone (100 nmol/l) increased K+ current in beta-cells from wild type mice but not from knockout mice lacking functional serum and glucocorticoid inducible kinase SGK1. Thus, glucocorticoids indeed up-regulate Na+/K+ ATPase activity, an effect requiring SGK1.

Resistance of mice lacking the serum- and glucocorticoid-inducible kinase SGK1 against salt-sensitive hypertension induced by a high-fat diet

Mineralocorticoids enhance expression and insulin stimulates activity of the serum- and glucocorticoid-inducible kinase SGK1, which activates the renal epithelial Na+ channel (ENaC). Under a salt-deficient diet, SGK1 knockout mice ( sgk1−/−) excrete significantly more NaCl than their wild-type littermates ( sgk1 +/+) and become hypotensive. The present experiments explored whether SGK1 participates in the hypertensive effects of a high-fat diet and high-salt intake. Renal SGK1 protein abundance of sgk1 +/+ mice was significantly elevated after a high-fat diet. Under a control diet, fluid intake, blood pressure, urinary flow rate, and urinary Na+, K+, and Cl− excretion were similar in sgk1−/− and sgk1 +/+ mice. Under a standard diet, high salt (1% NaCl in the drinking water for 25 days) increased fluid intake, urinary flow rate, and urinary Na+, K+, and Cl− excretion similarly in sgk1−/− and sgk1 +/+ mice without significantly altering blood pressure. A high-fat diet alone (17 wk) did not significantly alter fluid intake, urinary flow rate, urinary Na+, K+, or Cl− excretion, or plasma aldosterone levels but increased plasma insulin, total cholesterol, triglyceride concentrations, and systolic blood pressure to the same extent in both genotypes. Additional salt intake (1% NaCl in the drinking water for 25 days) on top of a high-fat diet did not affect hyperinsulinemia or hyperlipidemia but increased fluid intake, urinary flow rate, and urinary NaCl excretion significantly more in sgk1−/− than in sgk1 +/+mice. Furthermore, in animals receiving a high-fat diet, additional salt intake increased blood pressure only in sgk1 +/+ mice (to 132 ± 3 mmHg) but not in sgk1−/− mice (120 ± 4 mmHg). Thus lack of SGK1 protects against the hypertensive effects of a combined high-fat/high-salt diet.

Reduced intestinal and renal amino acid transport in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 activates the serum- and glucocorticoid-inducible kinase isoforms SGK1, SGK2, and SGK3 and protein kinase B, which in turn are known to up-regulate a variety of sodium-coupled transporters. The present study was performed to explore the role of PDK1 in amino acid transport. As mice completely lacking functional PDK1 are not viable, mice expressing 10-25% of PDK1 (pdk1(hm)) were compared with their wild-type (WT) littermates (pdk1(wt)). Body weight was significantly less in pdk1(hm) than in pdk1(wt) mice. Despite lower body weight of pdk1(hm) mice, food and water intake were similar in pdk1(hm) and pdk1(wt) mice. According to Ussing chamber experiments, electrogenic transport of phenylalanine, cysteine, glutamine, proline, leucine, and tryptophan was significantly smaller in jejunum of pdk1(hm) mice than in pdk1(wt) mice. Similarly, electrogenic transport of phenylalanine, glutamine, and proline was significantly decreased in isolated perfused proximal tubules of pdk1(hm) mice. The urinary excretion of proline, valine, guanidinoacetate, methionine, phenylalanine, citrulline, glutamine/glutamate, and tryptophan was significantly larger in pdk1(hm) than in pdk1(wt) mice. According to immunoblotting of brush border membrane proteins prepared from kidney, expression of the Na+-dependent neutral amino acid transporter B(0)AT1 (SLC6A19), the glutamate transporter EAAC1/EAAT3 (SLC1A1), and the transporter for cationic amino acids and cystine b(0,+)AT (SLC7A9) was decreased but the Na+/proline cotransporter SIT (SLC6A20) was increased in pdk1(hm) mice. In conclusion, reduction of functional PDK1 leads to impairment of intestinal absorption and renal reabsorption of amino acids. The combined intestinal and renal loss of amino acids may contribute to the growth defect of PDK1-deficient mice.

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

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

Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse

In vitro experiments have demonstrated the stimulating effect of serum- and glucocorticoid-inducible kinase (SGK)1 on the activity of the Na+/H+ exchanger (NHE3). SGK1 requires activation by phosphoinositide-dependent kinase (PDK)1, which may thus similarly play a role in the regulation of NHE3-dependent epithelial electrolyte transport. The present study was performed to explore the role of PDK1 in the regulation of NHE3 activity. Because mice completely lacking functional PDK1 are not viable, hypomorphic mice expressing approximately 20% of PDK1 (pdk1(hm)) were compared with their wild-type littermates (pdk1(wt)). NHE3 activity in the intestine and PDK1-overexpressing HEK-293 cells was estimated by utilizing 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein fluorescence for the determination of intracellular pH. NHE activity was reflected by the Na+-dependent pH recovery from an ammonium prepulse (DeltapH(NHE)). The pH changes after an ammonium pulse allowed the calculation of cellular buffer capacity, which was not significantly different between pdk1(hm) and pdk1(wt) mice. DeltapH(NHE) was in pdk1(hm) mice, only 30 +/- 6% of the value obtained in pdk1(wt) mice. Conversely, DeltapH(NHE) was 32 +/- 7% larger in PDK1-overexpressing HEK-293 cells than in HEK-293 cells expressing the empty vector. The difference between pdk1(hm) and pdk1(wt) mice and between PDK1-overexpressing and empty vector-transfected HEK cells, respectively, was completely abolished in the presence of the NHE3 inhibitor S3226 (10 microM). In conclusion, defective PDK1 expression leads to significant impairment of NHE3 activity in the intestine, pointing to a role of PDK1-dependent signaling in the regulation of NHE-mediated electrolyte transport.

PI3-kinase-dependent electrogenic intestinal transport of glucose and amino acids

Intestinal glucose and amino acid transport is stimulated by the serum- and glucocorticoid-inducible kinase isoforms SGK1, SGK2, and SGK3 and protein kinase B which are, in turn, stimulated following activation of the phosphoinositol-3 kinase (PI3 kinase). The present study has been performed to explore whether pharmacological inhibition of the PI3 kinase affects electrogenic jejunal transport of glucose and amino acids. In Ussing chamber experiments, glucose (20 mM), phenylalanine (20 mM), glutamine (20 mM), cysteine (20 mM), and proline (20 mM) generated lumen negative currents (I (glc), I (phe), I (gln), I (cys), and I (pro)), respectively, which gradually declined following application of the PI3 kinase inhibitor Wortmannin (1 muM). Within 40 min, Wortmannin treatment significantly decreased I (glc) by 39 +/- 10% (n = 5), I (phe) by 70 +/- 7% (n = 4), I (gln) by 69 +/- 8% (n = 4), I (cys) by 67 +/- 8% (n = 6), and I (prol) by 79 +/- 12% (n = 3). A similar decline of I (glc) was observed following application of the PI3 kinase inhibitor LY294002 (50 microM). Exposure to the inhibitors did not significantly alter transepithelial potential difference and resistance in the absence of substrates for electrogenic transport. The observations suggest that the electrogenic transport of glucose and several amino acids requires the continued activity of PI3 kinase.

Renal function of gene-targeted mice lacking both SGK1 and SGK3

Serum- and glucocorticoid-inducible kinase (SGK) 1 and SGK3 share the ability to upregulate several ion channels, including the epithelial Na(+) channel. Whereas SGK1 is under genomic control of mineralocorticoids and glucocorticoids, SGK3 is constitutively expressed. The SKG1-knockout (sgk1(-/-)) mouse is seemingly normal when it is fed a standard diet, but its ability to retain NaCl is impaired when it is fed a salt-deficient diet. In the SGK3-knockout (sgk3(-/-)) mouse fed standard and salt-deficient diets, hair growth is strikingly delayed but NaCl excretion is normal. Thus the possibility was considered that SGK1 and SGK3 could mutually replace each other, thus preventing severe NaCl loss in sgk1(-/-) and sgk3(-/-) mice. We crossed SGK1- and SGK3-knockout mice and compared renal electrolyte excretion of the double mutants (sgk1(-/-)/sgk3(-/-)) with that of their wild-type littermates (sgk1(+/+)/sgk3(+/+)). Similar to sgk3(-/-) mice, the sgk1(-/-)/sgk3(-/-) mice display delayed hair growth. Blood pressure was slightly, but significantly (P < 0.03), lower in sgk1(-/-)/sgk3(-/-) (102 +/- 4 mmHg) than in sgk1(+/+)/sgk3(+/+) (114 +/- 3 mmHg) mice, a difference that was maintained in mice fed low- and high-salt diets. Plasma aldosterone concentrations were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) than in sgk1(+/+)sgk3(+/+) mice fed control (511 +/- 143 vs. 143 +/- 32 pg/ml) and low-salt (1,325 +/- 199 vs. 362 +/- 145 pg/ml) diets. During salt depletion, absolute and fractional excretions of Na(+) were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) (1.2 +/- 0.2 micromol/24 h g body wt, 0.12 +/- 0.03%) than in sgk1(+/+)/sgk3(+/+) (0.4 +/- 0.1 micromol/24 h g body wt, 0.04 +/- 0.01%) mice. The sgk1(-/-)/sgk3(-/-) mice share the delayed hair growth with sgk3(-/-) mice and the modestly impaired renal salt retention with sgk1(-/-) mice. Additional lack of the isoform kinase does not substantially compound the phenotype for either property.

Sgk kinases and their role in epithelial transport

The serum/glucocorticoid-induced kinase Sgk1 plays an important role in the regulation of epithelial ion transport. This kinase is very rapidly regulated at the transcriptional level as well as via posttranslational modifications involving phosphorylation by the MAP or PI-3 kinase pathways and/or ubiquitylation. Although Sgk1 is a cell survival kinase, its primary role likely concerns the regulation of epithelial ion transport, as suggested by the phenotype of Sgk1-null mice, which display a defect in Na( homeostasis owing to disturbed renal tubular Na+ handling. In this review we first discuss the molecular, cellular, and regulatory aspects of Sgk1 and its paralogs. We then discuss its roles in the physiology and pathophysiology of epithelial ion transport.

Intestinal function of gene-targeted mice lacking serum- and glucocorticoid-inducible kinase 1

In vitro experiments have revealed the ability of serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate intestinal Na(+)-coupled glucose cotransporter 1 (SGLT1) and intestinal Na(+)/H(+) exchanger 3 (NHE3). The present study explored the contribution of SGK1 to the regulation of intestinal transport in vivo. SGK1 transcript levels were determined by real-time PCR and glucose-induced currents (I(g)) reflecting SGLT1 activity by Ussing chamber experiments. BCECF fluorescence was utilized for the determination of Na(+)-dependent pH recovery from an ammonium pulse (DeltapH(NHE)) reflecting NHE activity. As a result, intestinal SGK1 transcript levels were significantly enhanced by a 4-day treatment with 10 microg.mg body wt(-1).day(-1) dexamethasone (Dex). I(g) was, under control conditions, virtually identical in sgk1 knockout mice (sgk1(-/-)) and their wild type littermates (sgk1(+/+)). A 4-day treatment with Dex, however, increased I(g) approximately threefold in sgk1(+/+) mice but not in sgk1(-/-) mice. DeltapH(NHE) was similar in sgk1(-/-) and sgk1(+/+) mice before treatment. Dex increased DeltapH(NHE) approximately threefold in sgk1(+/+) mice and approximately twofold in sgk1(-/-)mice, an effect significantly blunted in the presence of the specific NHE3 blocker S-3226 (10 microM). According to Western blot analysis, Dex significantly enhanced SGLT1 and NHE3 protein abundance in brush-border membranes of sgk1(+/+) mice but not of sgk1(-/-)mice. In conclusion, basic functions of SGLT1 and NHE3 in the intestine do not require stimulation by SGK1. However, the effects of glucocorticoids on SGLT1 are fully, and on NHE3 partially, dependent on SGK1.

The Role of Aldosterone in Renal Sodium Transport

Aldosterone is the body's major hormone involved in volume homeostasis because of its effects on sodium reabsorption in the distal nephron. Our comprehension of the signaling pathways that this mineralocorticoid unleashes has been enhanced through the convergence of bedside physiologic observations with advances in medical genetics and molecular biology. This overview updates our current understanding of the aldosterone-initiated pathways throughout the distal nephron to promote sodium retention. Three essential features of the pathways are explored: how the mineralocorticoid gains specificity and targets gene transcription in distal tubular cells; how the key endpoints of aldosterone action in these cells-the epithelial sodium channel, the thiazide-sensitive sodium chloride cotransporter, and Na,K,ATPase-are regulated; and how 3 kinases, directly or indirectly, are activated by aldosterone and serve as critical intermediaries in regulating the sodium transporters. Remarkably, perturbations in many genes integral to aldosterone-induced pathways result in blood-pressure abnormalities. The familial disorders of hypertension and hypotension that follow from these mutated genes are presented with their molecular and physiologic consequences. The clustering of so many genetic disorders within the aldosterone-sensitive distal nephron supports the hypothesis that renal sodium regulation plays a pivotal role in long-term blood-pressure control. Identifying and characterizing other components of the pathways that modulate these sodium transporters represent the core challenges in this scientific field. It is posited that meeting these challenges will help elucidate the pathogenesis of human hypertension and provide new therapeutic options for its treatment.

SGK1 activates Na+-K+-ATPase in amphibian renal epithelial cells

Serum- and glucocorticoid-induced kinase 1 (SGK1) is thought to be an important regulator of Na+reabsorption in the kidney. It has been proposed that SGK1 mediates the effects of aldosterone on transepithelial Na+transport. Previous studies have shown that SGK1 increases Na+transport and epithelial Na+channel (ENaC) activity in the apical membrane of renal epithelial cells. SGK1 has also been implicated in the modulation of Na+-K+-ATPase activity, the transporter responsible for basolateral Na+efflux, although this observation has not been confirmed in renal epithelial cells. We examined Na+-K+-ATPase function in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible promoter. The results showed that expression of a constitutively active mutant of SGK1 (SGK1TS425D) increased the transport activity of Na+-K+-ATPase 2.5-fold. The increase in activity was a direct consequence of activation of the pump itself. The onset of Na+-K+-ATPase activation was observed between 6 and 24 h after induction of SGK1 expression, a delay that is significantly longer than that required for activation of ENaC in the same cell line (1 h). SGK1 and aldosterone stimulated the Na+pump synergistically, indicating that the pathways mediated by these molecules operate independently. This observation was confirmed by demonstrating that aldosterone, but not SGK1TS425D, induced an ∼2.5-fold increase in total protein and plasma membrane Na+-K+-ATPase α1-subunit abundance. We conclude that aldosterone increases the abundance of Na+-K+-ATPase, whereas SGK1 may activate existing pumps in the membrane in response to chronic or slowly acting stimuli.

SGK1 kinase upregulates GLUT1 activity and plasma membrane expression

Phosphatidylinositol 3-kinase (PI3 kinase) inhibition disrupts the ability of insulin to stimulate GLUT1 and GLUT4 translocation into the cell membrane and thus glucose transport. The effect on GLUT4 but not on GLUT1 is mediated by activation of protein kinase B (PKB). The serum- and glucocorticoid-inducible kinase SGK1, a further kinase downstream of PI3 kinase, regulates several transporters by enhancing their plasma membrane abundance. GLUT1 contains a consensus site ((95)Ser) for phosphorylation by SGK1. Thus, the present study investigated whether GLUT1 is regulated by the kinase. Tracer-flux studies in Xenopus oocytes and HEK-293 cells demonstrated that GLUT1 transport is enhanced by constitutively active (S422D)SGK1. The effect requires the kinase catalytical activity since the inactive mutant (K127N)SGK1 failed to modulate GLUT1. GLUT1 stimulation by (S422D)SGK1 is not due to de novo protein synthesis but rather to an increase of the transporter's abundance in the plasma membrane. Kinetic analysis revealed that SGK1 enhances maximal transport rate without altering GLUT1 substrate affinity. These observations suggest that SGK1 regulates GLUT1 and may contribute to or account for the PI3 kinase-dependent but PKB-independent stimulation of GLUT1 by insulin.

Glucocorticoid regulation of genes in the amiloride-sensitive sodium transport pathway by semicircular canal duct epithelium of neonatal rat

The lumen of the inner ear has an unusually low concentration of endolymphatic Na+, which is important for transduction processes. We have recently shown that glucocorticoid receptors (GR) stimulate absorption of Na+by semicircular canal duct (SCCD) epithelia. In the present study, we sought to determine the presence of genes involved in the control of the amiloride-sensitive Na+transport pathway in rat SCCD epithelia and whether their level of expression was regulated by glucocorticoids using quantitative real-time RT-PCR. Transcripts were present for α-, β-, and γ-subunits of the epithelial sodium channel (ENaC); the α1-, α3-, β1-, and β3-isoforms of Na+-K+-ATPase; inwardly rectifying potassium channels [IC50of short circuit current ( Isc) for Ba2+: 210 μM] Kir2.1, Kir2.2, Kir2.3, Kir2.4, Kir3.1, Kir3.3, Kir4.1, Kir4.2, Kir5.1, and Kir7.1; sulfonyl urea receptor 1 (SUR1); GR; mineralocorticoid receptor (MR); 11β-hydroxysteroid dehydrogenase (11β-HSD) types 1 and 2; serum- and glucocorticoid-regulated kinase 1 (Sgk1); and neural precursor cell-expressed developmentally downregulated 4-2 (Nedd4-2). On the other hand, transcripts for the α4-subunit of Na+-K+-ATPase, Kir1.1, Kir3.2, Kir3.4, Kir6.1, Kir6.2, and SUR2 were found to be absent, and Iscwas not inhibited by glibenclamide. Dexamethasone (100 nM for 24 h) not only upregulated the transcript expression of α-ENaC (∼4-fold), β2-subunit (∼2-fold) and β3-subunit (∼8-fold) of Na+-K+-ATPase, Kir2.1 (∼5-fold), Kir2.2 (∼9-fold), Kir2.4 (∼3-fold), Kir3.1 (∼ 3- fold), Kir3.3 (∼2-fold), Kir4.2 (∼3-fold ), Kir7.1 (∼2-fold), Sgk1 (∼4-fold), and Nedd4-2 (∼2-fold) but also downregulated GR (∼3-fold) and 11β-HSD1 (∼2-fold). Expression of GR and 11β-HSD1 was higher than MR and 11β-HSD2 in the absence of dexamethasone. Dexamethasone altered transcript expression levels (α-ENaC and Sgk1) by activation of GR but not MR. Proteins were present for the α-, β-, and γ-subunits of ENaC and Sgk1, and expression of α- and γ-ENaC was upregulated by dexamethasone. These findings are consistent with the genomic stimulation by glucocorticoids of Na+absorption by SCCD and provide an understanding of the therapeutic action of glucocorticoids in the treatment of Meniere's disease.

Hormonal and nonhormonal mechanisms of regulation of the NA,K-pump in collecting duct principal cells

In the kidney, the collecting duct (CD) is the site of final Na+ reabsorption, according to Na+ balance requirements. In this segment of the renal tubule, principal cells may reabsorb up to 5% of the filtered sodium. The driving force for this process is provided by the basolateral Na,K-adenosine triphosphatase (ATPase) (sodium pump). Na,K-ATPase activity and expression in the CD are modulated physiologically by hormones (aldosterone, vasopressin, and insulin) and nonhormonal factors including intracellular [Na+] and extracellular osmolality. In this article, we review the short- and long-term hormonal regulation of Na,K-ATPase in CD principal cells, and we analyze the integrated network of implicated signaling pathways with an emphasis on the latest findings.

SGK1-mediated Fibronectin Formation in Diabetic Nephropathy

The serum and glucocorticoid inducible kinase SGK1 has been shown to be up regulated in fibrosing tissue including diabetic nephropathy. The present study has been performed to determine the time course of SGK1 transcription in mouse kidneys following induction of diabetes by streptozotocin (STZ). Moreover, the study aimed to explore whether SGK1 may play an active role in the stimulation of matrix protein formation during hyperglycemia. The induction of diabetes in 8 weeks old male C57Bl/6 mice was indeed followed by a significant (p< 0.001) increase of SGK1 transcript levels (up to 2.5-fold) and protein abundance (up to 2.8-fold) both peaking 4 weeks after STZ treatment. The SGK1 transcript levels and protein abundance declined thereafter but remained significantly elevated up to 12 weeks (p<0.05). Exposure to high extracellular glucose concentration (25 mM) significantly increased SGK1 transcript levels in human mesangial cells (HMCs). At low extracellular glucose concentration (5.5 mM), transfection with constitutively active (S422D)SGK1 and transdominant inhibitory (K127N)SGK1 did not significantly modify fibronectin formation by HMCs. Exposure to high extracellular glucose concentration stimulated fibronectin formation (by 2.2 fold), an effect abrogated by transfection with inactive (K127N)SGK1 (1.2 fold) and markedly enhanced by transfection with (S422D)SGK1 (4.7 fold). In conclusion, hyperglycemia of diabetes mellitus leads to partially transient increase of SGK1 transcription and translation. SGK1 overexpression alone has little effect on fibronectin formation but potentiates the effect of hyperglycemia. Thus, SGK1 is upregulated in diabetic nephropathy and actively participates in the stimulation of matrix protein deposition in this common deleterious complication of diabetic hyperglycemia.

Blunted hypertensive effect of combined fructose and high-salt diet in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1

Serum- and glucocorticoid-inducible kinase (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase stimulates the renal epithelial Na(+) channel and may thus participate in blood pressure regulation. Hyperinsulinemia is triggered by dietary fructose, which sensitizes blood pressure for salt intake. The role of SGK1 in hypertensive effects of combined fructose and high-salt intake was thus explored in SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)). Renal SGK1 transcript levels of sgk1(+/+) mice were significantly elevated after fructose diet. Under control diet, fluid intake, urinary flow rate, urinary Na(+), K(+), and Cl(-) excretion, and blood pressure were similar in sgk1(-/-) and sgk1(+/+) mice. Addition of 10% fructose to drinking water increased fluid intake and urinary flow rate in both genotypes, and did not significantly alter urinary Na(+), K(+), and Cl(-) output in either genotype. Additional high NaCl diet (4% NaCl) did not significantly alter fluid intake and urine volume but markedly increased urinary output of Na(+) and Cl(-), approaching values significantly (P < 0.05) larger in sgk1(-/-) than in sgk1(+/+) mice (Na(+): 2,572 +/- 462 vs. 1,428 +/- 236; Cl(-): 2,364 +/- 388 vs. 1,379 +/- 225 micromol/24 h). Blood pressure was similar in sgk1(+/+) and sgk1(-/-) mice at control diet or fructose alone but increased only in sgk1(+/+) mice (115 +/- 1 vs. 103 +/- 0.7 mmHg, P < 0.05) after combined fructose and high-salt intake. Acute intravenous insulin infusion (during glucose clamp) caused antinatriuresis in sgk1(+/+) mice, an effect significantly blunted in sgk1(-/-) mice. The observations reveal a pivotal role of SGK1 in insulin-mediated sodium retention and the salt-sensitizing hypertensive effect of high fructose intake.

Decreased intestinal glucose transport in the sgk3-knockout mouse

Xenopus oocyte coexpression experiments revealed the capacity of the serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) to up-regulate a variety of transport systems including the sodium-dependent glucose transporter SGLT1. The present study explored the functional significance of SGK3-dependent regulation of intestinal transport. To this end, experiments were performed in gene targeted mice lacking functional sgk3 (sgk3(-/-)) and their wild type littermates (sgk3(+/+)). Oral food intake and fecal dry weight were significantly larger in sgk3(-/-) than in sgk3(+/+) mice. Glucose-induced current (I(g)) in Ussing chamber as a measure of Na(+) coupled glucose transport was significantly smaller in sgk3(-/-) than in sgk3(+/+) mouse jejunal segments. Fasting plasma glucose concentrations were significantly lower in sgk3(-/-) than in sgk3(+/+) mice. Intestinal electrogenic transport of phenylalanine, cysteine, glutamine and proline were not significantly different between sgk3(-/-) and sgk3(+/+) mice. In conclusion, SGK3 is required for adequate intestinal Na(+) coupled glucose transport and impaired glucose absorption may contribute to delayed growth and decreased plasma glucose concentrations of SGK3 deficient mice. The hypoglycemia might lead to enhanced food intake to compensate for impaired intestinal absorption.

New insights into the role of serum- and glucocorticoid-inducible kinase SGK1 in the regulation of renal function and blood pressure

PURPOSE OF REVIEW

The serum and glucocorticoid inducible kinase 1 (SGK1) is induced in the aldosterone sensitive distal nephron (ASDN) where it may stimulate Na reabsorption, partly by inhibiting ubiquitin ligase Nedd4-2-mediated retrieval of epithelial Na+ channel ENaC from the luminal membrane. We describe recent advances in our understanding of SGK1 function in the regulation of renal function and blood pressure.

RECENT FINDINGS

Thiazolidinediones, i.e. activators of peroxisome proliferator-activated receptor gamma (PPAR gamma), upregulate SGK1 and ENaC mRNA expression and increase cell-surface expression of ENaC alpha in a human cortical-collecting-duct cell line. cAMP/protein kinase A can induce phosphorylation and inhibition of Nedd4-2-independent of SGK1. Part of ENaC stimulation by SGK1 appears dependent on a SGK1 consensus motif in ENaC alpha and independent of Nedd4-2. SGK1-dependent upregulation of Na+ reabsorption in ASDN contributes to upregulation of renal K+ excretion. In oocytes, SGK1 activates various renal transport proteins including Na+/glucose cotransporter SGLT1, Na+-coupled dicarboxylate transporter NaDC-1, epithelial Ca+ channel TRPV5, renal outer medullary K+ channel ROMK and voltage gated K+ channels KCNE1/KCNQ1 and Kv1.3. A variant of the SGK1 gene associates with increased blood pressure and body mass index.

SUMMARY

PPAR gamma activators may increase renal Na reabsorption by stimulating SGK1 and ENaC. Nedd4-2 integrates influences of cAMP/protein kinase A and SGK1. SGK1 can activate ENaC in part directly and independent of Nedd4-2. K+ homeostasis requires SGK1-dependent Na+ reabsorption in ASDN. SGK1 may affect renal transport mechanisms beyond Na+ reabsorption and K+ secretion in ASDN. Polymorphisms of SGK1 may be relevant to the pathophysiology of hypertension and other diseases.