h-sgk serine-threonine protein kinase as transcriptional target of p38/MAP kinase pathway in HepG2 human hepatoma cells

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

Interleukin-13 affects the epithelial sodium channel in the intestine by coordinated modulation of STAT6 and p38 MAPK activity

KEY POINTS

Interleukin-13 (IL-13) causes intestinal epithelial barrier dysfunction, and is implicated in the pathogenesis of Th2-driven intestinal inflammation (e.g. ulcerative colitis). However, it is unclear whether the epithelial sodium channel (ENaC) - the main limiting factor for sodium absorption in the distal colon - is also influenced by IL-13 and if so, by what mechanism(s). We demonstrate in an intestinal cell model as well as in mouse distal colon that IL-13 causes reduced ENaC activity. We show that IL-13 impairs ENaC-dependent sodium transport by activating the JAK1/2-STAT6 signalling pathway. These results improve our understanding of the mechanisms through which IL-13 functions as a key effector cytokine in ulcerative colitis, thereby contributing to the distinct pathology of this disease.

ABSTRACT

Interleukin-13 (IL-13) has been strongly implicated in the pathogenesis of ulcerative colitis, possibly by disrupting epithelial integrity. In the distal colon, the epithelial sodium channel (ENaC) is an important factor in the regulation of sodium absorption, and therefore plays a critical role in minimizing intestinal sodium and water losses. In the present study, we investigated whether IL-13 also acts as a potent modulator of epithelial sodium transport via ENaC, and the signalling components involved. The effect of IL-13 on ENaC was examined in HT-29/B6-GR/MR human colon cells, as well as in mouse distal colon, by measuring amiloride-sensitive short-circuit current (ISC ) in Ussing chambers. The expression levels of ENaC subunits and the cellular components that contribute to ENaC activity were analysed by qRT-PCR and promoter gene assay. We show that IL-13, in both the cell model and in native intestinal tissue, impaired epithelial sodium absorption via ENaC (JNa ) as a result of decreased transcription levels of β- and γ-ENaC subunits and SGK1, a post-translational regulator of ENaC activity, due to impaired promoter activity. The reduction in JNa was prevented by inhibition of JAK1/2-STAT6 signalling. This inhibition also affected the IL-13-induced decrease in p38 MAPK phosphorylation. The contribution of STAT6 to IL-13-mediated ENaC inactivation was confirmed in a STAT6(-/-) mouse model. In conclusion, these results indicate that IL-13, the levels of which are elevated in ulcerative colitis, contributes to impaired ENaC activity via modulation of the STAT6/p38 MAPK pathways.

Regulation of αENaC transcription

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

Sodium chloride drives autoimmune disease by the induction of pathogenic TH17 cells

There has been a marked increase in the incidence of autoimmune diseases in the past half-century. Although the underlying genetic basis of this class of diseases has recently been elucidated, implicating predominantly immune-response genes, changes in environmental factors must ultimately be driving this increase. The newly identified population of interleukin (IL)-17-producing CD4(+) helper T cells (TH17 cells) has a pivotal role in autoimmune diseases. Pathogenic IL-23-dependent TH17 cells have been shown to be critical for the development of experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis, and genetic risk factors associated with multiple sclerosis are related to the IL-23-TH17 pathway. However, little is known about the environmental factors that directly influence TH17 cells. Here we show that increased salt (sodium chloride, NaCl) concentrations found locally under physiological conditions in vivo markedly boost the induction of murine and human TH17 cells. High-salt conditions activate the p38/MAPK pathway involving nuclear factor of activated T cells 5 (NFAT5; also called TONEBP) and serum/glucocorticoid-regulated kinase 1 (SGK1) during cytokine-induced TH17 polarization. Gene silencing or chemical inhibition of p38/MAPK, NFAT5 or SGK1 abrogates the high-salt-induced TH17 cell development. The TH17 cells generated under high-salt conditions display a highly pathogenic and stable phenotype characterized by the upregulation of the pro-inflammatory cytokines GM-CSF, TNF-α and IL-2. Moreover, mice fed with a high-salt diet develop a more severe form of EAE, in line with augmented central nervous system infiltrating and peripherally induced antigen-specific TH17 cells. Thus, increased dietary salt intake might represent an environmental risk factor for the development of autoimmune diseases through the induction of pathogenic TH17 cells.

The plant-derived glucocorticoid receptor agonist Endiandrin A acts as co-stimulator of colonic epithelial sodium channels (ENaC) via SGK-1 and MAPKs

In a search for secondary plant compounds that bind to the glucocorticoid receptor (GR), the cyclobutane lignan endiandrin A was discovered from the rainforest tree Endiandra anthropophagorum Domin. Our present study aims to characterize the effect of endiandrin A on GR-dependent induction of colonic sodium transport. The effect of endiandrin A was analyzed in GR-expressing colonic HT-29/B6 cells (HT-29/B6-GR). GR transactivation and subcellular localization were investigated by reporter gene assay and immunofluorescence. Epithelial sodium channel (ENaC) was analyzed by qRT-PCR and by measuring amiloride-sensitive short-circuit current (I_sc) in Ussing chambers. Endiandrin A (End A) has been identified as GR receptor binder. However, it did not cause significant GR transactivation as pGRE-luciferase activity was only 7% of that of the maximum effect of dexamethasone. Interestingly, endiandrin A had a significant impact on dexamethasone-dependent sodium absorption in cells co-exposed to tumor necrosis factor (TNF)-α. This was in part due to up-regulation of β- and γ-ENaC subunit expression. Endiandrin A potentiated GR-mediated transcription by increasing GR protein expression and phosphorylation. It inhibited c-Jun N-terminal kinase (JNK) activation induced by dexamethasone and/or TNF-α and increased levels of GR localized to the nucleus. Additionally, endiandrin A increased the serum- and glucocorticoid-induced kinase (sgk)-1 via activation of p38. Finally, the regulation of ENaC function by endiandrin A was confirmed in rat native colon. In conclusion, endiandrin A potentiates glucocorticoid-driven activation of colonic epithelial sodium channels via JNK inhibition and p38 activation due to transcriptional up-regulation of β- and γ-ENaC-subunits along with induction of sgk-1.

p38 MAPK activation and function following osmotic shock of erythrocytes

p38 protein kinase is activated by hyperosmotic shock, participates in the regulation of cell volume sensitive transport and metabolism and is involved in the regulation of various physiological functions including cell proliferation and apoptosis. Similar to apoptosis of nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include hyperosmotic shock, which increases cytosolic Ca(2+) activity and ceramide formation. The present study explored whether p38 kinase is expressed in human erythrocytes, is activated by hyperosmotic shock and participates in the regulation of eryptosis. Western blotting was utilized to determine phosphorylation of p38 kinase, forward scatter to estimate cell volume, annexin V binding to depict phosphatidylserine exposure and Fluo3 fluorescence to estimate cytosolic Ca(2+) activity. As a result, erythrocytes express p38 kinase, which is phosphorylated upon osmotic shock (+550 mM sucrose). Osmotic shock decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence, all effects significantly blunted by the p38 kinase inhibitors SB203580 (2 μM) and p38 Inh III (1 μM). In conclusion, p38 kinase is expressed in erythrocytes and participates in the machinery triggering eryptosis following hyperosmotic shock.

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

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

Modulation of Gene Expression Profiles by Hyperosmolarity and Insulin

Cell hydration changes play a key role in the regulation of cell function and critically affect insulin sensitivity of carbohydrate- and protein metabolism. Here, the modulation of gene expression profiles by hyperosmolarity and insulin was examined in H4IIE rat hepatoma cells by cDNA/oligonucleotiode array-, Northern- and Western blot analysis. Osmosensitive expression of the insulin-like growth factor binding protein Igfbp1, the multidrug resistance protein Mrp5 (Abcc5a) and cyclin D1 (Ccnd1) was established at the mRNA and protein level. Despite a hyperosmotic increase of cyclin D1 mRNA induction by insulin, the cyclin D1 protein expression was decreased by hyperosmolarity, suggesting a hyperosmotic interference with cyclin D1 mRNA translation. Hyperosmolarity at the mRNA level blunted the insulin response of betaine homocysteine-S-methyl transferase, the multidrug resistance proteins Mdr1a (Abcb1a) and 2 (Abcb4), the Igfbp 2 and 5, cyclin G1, dual specificity phosphatase Dusp1, signal transducers and activators of transcription Stat3 and 5, catalase and the bile salt export pump Bsep (Abcb11), whereas the insulin response was increased for Mrp5, cyclin D1 and the phosphoenolpyruvate carboxykinase. Insulin effects on the mRNA expression of the eukaryotic initiation factor 4E binding protein 4e-bp1, tubulin, gene 33, growth hormone receptor, keratin18, ornithine decarboxylase and heme oxygenase 1 were largely insensitive to hyperosmolarity. The data indicate that hyperosmolarity differentially modulates insulin sensitivity at the level of gene expression.

Negative regulation of SEK1 signaling by serum- and glucocorticoid-inducible protein kinase 1

Serum- and glucocorticoid-inducible protein kinase 1 (SGK1) has been implicated in diverse cellular activities including the promotion of cell survival. The molecular mechanism of the role of SGK1 in protection against cellular stress has remained unclear, however. We have now shown that SGK1 inhibits the activation of SEK1 and thereby negatively regulates the JNK signaling pathway. SGK1 was found to physically associate with SEK1 in intact cells. Furthermore, activated SGK1 mediated the phosphorylation of SEK1 on serine 78, resulting in inhibition of the binding of SEK1 to JNK1, as well as to MEKK1. Replacement of serine 78 of SEK1 with alanine abolished SGK1-mediated SEK1 inhibition. Oxidative stress upregulated SGK1 expression, and depletion of SGK1 by RNA interference potentiated the activation of SEK1 induced by oxidative stress in Rat2 fibroblasts. Moreover, such SGK1 depletion prevented the dexamethasone-induced increase in SGK1 expression, as well as the inhibitory effects of dexamethasone on paclitaxel-induced SEK1-JNK signaling and apoptosis in MDA-MB-231 breast cancer cells. Together, our results suggest that SGK1 negatively regulates stress-activated signaling through inhibition of SEK1 function.

Upregulation of HERG channels by the serum and glucocorticoid inducible kinase isoform SGK3

Human ether-a-go-go (HERG) channels participate in the repolarization of the cardiac action potential. Loss of function mutations of HERG lead to delayed cardiac repolarization reflected by prolonged QT interval. HERG channels are regulated through a signaling cascade involving phosphatidylinositol 3 (PI3) kinase. Downstream targets of PI3 kinase include the serum and glucocorticoid inducible kinase (SGK) and protein kinase B (PKB) isoforms. The present study has been performed to explore whether SGK1 and SGK3 participate in the regulation of HERG channel activity. HERG was expressed in Xenopus oocytes with or without additional expression of SGK1 or SGK3. Chemiluminescence was employed to determine HERG plasma membrane protein abundance. Coexpression of SGK3 but not of SGK1 in Xenopus oocytes resulted in an increase of steady state current (I(HERG)) and enhanced cell membrane protein abundance without affecting gating kinetics of the channel. Replacement of serine by alanine at the two SGK consensus sites decreased I(HERG) but neither mutation abolished the stimulating effect of SGK3. In conclusion, SGK3 participates in the regulation of HERG by increasing HERG protein abundance in the plasma membrane and may thus modify the duration of the cardiac action potential.

(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.

Stimulation of serum- and glucocorticoid-regulated kinase-1 gene expression by endothelin-1

The serum- and glucocorticoid-regulated kinase-1 (SGK1) participates in the regulation of sodium homeostasis and blood pressure by mineralocorticoids. Aldosterone rapidly induces SGK1 transcription, which contributes to the activation of renal epithelial sodium channels. Another important regulator of blood pressure is the vasoactive hormone endothelin-1 (ET-1) that is systemically upregulated in chronic renal failure. In the present study, we investigated whether ET-1 modulates SGK1 expression, and thereby might explain some of its hypertensive effects. As assessed by real-time PCR analysis, ET-1 triggered the rapid increase of SGK1 mRNA levels in A-10 smooth muscle cells and also in intact aortas of adult rats. In A-10 cells transcriptional activation was associated with a more than 6-fold upregulation of SGK1 protein expression and in similar range as found after treatment with aldosterone. A stimulatory effect of ET-1 was not only observed in isolated cells, but also in an animal model. Upon subtotal nephrectomy (SNX) of rats, myocardial ET-1 levels strongly increased, which was followed by a more than 2-fold induction of SGK1 expression in the left ventricle. The myocardial upregulation of SGK1 was completely abrogated by a specific ET(A) receptor antagonist, thereby substantiating the in vivo role of ET-1 in SGK1 expression. Thus, these data demonstrate that ET-1 increases expression of SGK1 in vivo and in vitro, and therefore indicate that SGK1 upregulation might be involved in ET-1-dependent regulation of blood pressure and cardiac modelling during mild renal failure.

High Glucose Up-Regulates ENaC and SGK1 Expression in HCD-Cells

BACKGROUND/AIM

Diabetic nephropathy is associated with progressive renal damage, leading to impaired function and end-stage renal failure. Secondary hypertension stems from a deranged ability of cells within the kidney to resolve and appropriately regulate sodium resorption in response to hyperglycaemia. However, the mechanisms by which glucose alters sodium re-uptake have not been fully characterised.

METHODS

Here we present RT-PCR, western blot and immunocytochemistry data confirming mRNA and protein expression of the serum and glucocorticoid inducible kinase (SGK1) and the alpha conducting subunit of the epithelial sodium channel (ENaC) in a model in vitro system of the human cortical collecting duct (HCD). We examined changes in expression of these elements in response to glucose challenge, designed to mimic hyperglycaemia associated with type 2 diabetes mellitus. Changes in Na+ concentration were assessed using single-cell microfluorimetry.

RESULTS

Incubation with glucose, the Ca2+-ionophore ionomycin and the cytokine TGF-beta1 were all found to evoke significant and time-dependent increases in both SGK1 and alphaENaC protein expression. These molecular changes were correlated to an increase in Na+-uptake at the single-cell level.

CONCLUSION

Together these data offer a potential explanation for glucose-evoked Na+-resorption and a potential contributory role of SGK1 and ENaCs in development of secondary hypertension, commonly linked to diabetic nephropathy.

IL-6 activates serum and glucocorticoid kinase via p38alpha mitogen-activated protein kinase pathway

Interleukin-6 (IL-6) has been implicated as an autocrine factor involved in growth of several human cancers, such as tumors arising from the biliary tract or cholangiocarcinoma. In malignant biliary tract epithelia, IL-6 activates the p38 MAPK pathway, which mediates a dominant survival signaling pathway. Serum and glucocorticoid-stimulated kinase (SGK) has been implicated as a survival kinase, but its role in survival signaling by IL-6 is unknown. After IL-6 stimulation, p38 MAPK activation preceded phosphorylation of SGK at Ser78. Pretreatment with the pharmacological inhibitors of p38 MAPK SB-203580 or SB-202190 blocked IL-6-induced SGK phosphorylation at Ser78 and SGK activation. Overexpression of p38alpha increased constitutive SGK phosphorylation at Ser78, whereas dominant negative p38alpha MAPK blocked IL-6-induced SGK phosphorylation and nuclear translocation. Interestingly, in addition to stimulating SGK phosphorylation, both IL-6 stimulation and p38alpha MAPK overexpression increased SGK mRNA and protein expression. An increase in p38 MAPK and SGK occurred following enforced expression of IL-6 in vivo. Furthermore, inhibition of SGK expression by siRNA increased toxicity due to chemotherapeutic drugs. Taken together, these data identify SGK as both a downstream kinase substrate as well as a transcriptionally regulated gene target of p38 MAPK in response to IL-6 and support a role of SGK during survival signaling by IL-6 in human cancers, such as cholangiocarcinoma.

Regulation of CLC-Ka/barttin by the ubiquitin ligase Nedd4-2 and the serum- and glucocorticoid-dependent kinases

BACKGROUND

ClC-Ka and ClC-Kb, chloride channels participating in renal tubular Cl- transport, require the coexpression of barttin to become functional. Mutations of the barttin gene lead to the Bartter's syndrome variant BSND, characterized by congenital deafness and severe renal salt wasting. Barttin bears a proline-tyrosine motif, a target structure for the ubiquitin ligase Nedd4-2, which mediates the clearance of channel proteins from the cell membrane. Nedd4-2 is, in turn, a target of the serum- and glucocorticoid-inducible kinase SGK1, which phosphorylates and, thus, inactivates the ubiquitin ligase. ClC-Ka also possesses a SGK1 consensus site in its sequence. We hypothesized that ClC-Ka/barttin is stimulated by SGK1, and down-regulated by Nedd4-2, an effect that may be reversed by SGK1 and its isoforms, SGK2 or SGK3.

METHODS

To test this hypothesis, ClC-Ka/barttin was heterologously expressed in Xenopus oocytes with or without the additional expression of Nedd4-2, SGK1, SGK2, SGK3, constitutively active S422DSGK1, or inactive K127NSGK1.

RESULTS

Expression of ClC-Ka/barttin induced a slightly inwardly rectifying current that was significantly decreased upon coexpression of Nedd4-2, but not the catalytically inactive mutant C938SNedd4-2. The coexpression of S422DSGK1, SGK1, or SGK3, but not SGK2 or K127NSGK1 significantly stimulated the current. Moreover, S422DSGK1, SGK1, and SGK3 also phosphorylated Nedd4-2 and thereby inhibited Nedd4-2 binding to its target. The down-regulation of ClC-Ka/barttin by Nedd4-2 was abolished by elimination of the PY motif in barttin.

CONCLUSION

ClC-Ka/barttin channels are regulated by SGK1 and SGK3, which may thus participate in the regulation of transport in kidney and inner ear.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Activation of Na+/K+-ATPase by the serum and glucocorticoid-dependent kinase isoforms

BACKGROUND/AIM

Expression of the constitutively active form of serum and glucocorticoid-dependent kinase ((S422D)SGK1) in Xenopus oocytes has recently been shown to upregulate endogenous Na(+)/K(+)-ATPase activity, an effect presumably participating in the regulation of cellular K(+) uptake and transepithelial Na(+) transport. SGK1 and the two isoforms SGK2 and SGK3 are stimulated by insulin and insulin-like growth factor-1 (IGF-1), which have been shown to enhance Na(+)/K(+)-ATPase activity in a variety of cells. The present experiments have been performed to elucidate whether or not wild-type SGK1, SGK2 and SGK3 are similar to (S422D)SGK1 in being effective regulators of Na(+)/K(+)-ATPase.

METHODS

To this end, dual-electrode voltage clamp experiments were performed in Xenopus oocytes injected either with water or with mRNA of constitutively active (S422D)SGK1 and wild-type SGK1, SGK2 or SGK3. Na(+)/K(+)-ATPase activity was estimated from the outward-directed current created by readdition of extracellular K(+) in the presence of K(+) channel blocker Ba(2+) following a 10-min exposure to K(+)-free extracellular fluid.

RESULTS

The outward-directed current was fully abolished by incubation with 1 mM ouabain and was significantly larger in oocytes expressing (S422D)SGK1, SGK1, SGK2 or SGK3, as compared to those injected with water.

CONCLUSION

The stimulating effect of SGK1 on the Xenopus oocyte Na(+)/K(+)-ATPase is mimicked by the isoforms SGK2 and SGK3. Thus, all three kinases may participate in the regulation of Na(+)/K(+)-ATPase activity by hormones such as insulin and IGF-1.

The serine/threonine kinases SGK2 and SGK3 are potent stimulators of the epithelial Na+ channel alpha,beta,gamma-ENaC

The serum- and glucocorticoid-inducible kinase 1 (SGK1) has been identified as a signalling molecule up-regulated by aldosterone, which stimulates the renal epithelial Na(+) channel ENaC. It is therefore thought to participate in the antinatriuretic action of this hormone. More recently, two isoforms, SGK2 and SGK3, have been cloned. The present study was performed to establish whether SGK2 and SGK3 influence ENaC activity similarly to SGK1. Dual-electrode voltage-clamp experiments in Xenopus laevis oocytes expressing alpha,ss,gamma-ENaC with or without SGK1, SGK2 or SGK3 revealed a stimulatory effect of all three kinases on the amiloride-sensitive current (I(Na)). To establish whether the SGK isoforms exert their effects through direct phosphorylation, we replaced the serine at the SGK consensus site of alphaENaC (alpha(S622A)ENaC) by site-directed mutagenesis. alpha(S622A),beta,gamma-ENaC was up-regulated similar to wild-type ENaC, suggesting that SGK isoforms do not act via direct phosphorylation of the transport proteins. In conclusion, SGK2 and SGK3 mimic the function of SGK1 and are likely to participate in the regulation of ENaC activity.

Expression of the serum- and glucocorticoid-inducible protein kinase, Sgk, is a cell survival response to multiple types of environmental stress stimuli in mammary epithelial cells

The effects of multiple stress stimuli on the cellular utilization of the serum- and glucocorticoid-inducible protein kinase (Sgk) were examined in NMuMg mammary epithelial cells exposed to hyperosmotic stress induced by the organic osmolyte sorbitol, heat shock, ultraviolet irradiation, oxidative stress induced by hydrogen peroxide, or to dexamethasone, a synthetic glucocorticoid that represents a general class of physiological stress hormones. Each of the stress stimuli induced Sgk protein expression with differences in the kinetics and duration of induction and in subcellular localization. The environmental stresses, but not dexamethasone, stimulated Sgk expression through a p38/MAPK-dependent pathway. In each case, a hyperphosphorylated active Sgk protein was produced under conditions in which Akt, the close homolog of Sgk, remained in its non-phosphorylated state. Ectopic expression of wild type Sgk or of the T256D/S422D mutant Sgk that mimics phosphorylation conferred protection against stress-induced cell death in NMuMg cells. In contrast, expression of the T256A/S422A Sgk phosphorylation site mutant has no effect on cell survival. Sgk is known to phosphorylate and negatively regulate pro-apoptotic forkhead transcription factor FKHRL1. The environmental stress stimuli that induce Sgk, but not dexamethasone, strongly inhibited the nuclear transcriptional activity and increased the cytoplasmic retention of FKHRL1. Also, the conditional IPTG inducible expression of wild type Sgk, but not of the kinase dead T256A mutant Sgk, protected Con8 mammary epithelial tumor cells from serum starvation-induced apoptosis. Taken together, our study establishes that induction of enzymatically active Sgk functions as a key cell survival component in response to different environmental stress stimuli.

Regulation of KCNE1-dependent K(+) current by the serum and glucocorticoid-inducible kinase (SGK) isoforms

The slowly activating K(+) channel subunit KCNE1 is expressed in a variety of tissues including proximal renal tubules, cardiac myocytes and stria vascularis of inner ear. The present study has been performed to explore whether the serum- and glucocorticoid-inducible kinase family members SGK1, SGK2, or SGK3 and/or protein kinase B (PKB) influence K(+) channel activity in Xenopus oocytes expressing KCNE1. cRNA encoding KCNE1 was injected with or without cRNA encoding wild-type SGK1, constitutively active (S422D)SGK1, inactive (K127 N)SGK1, wild-type SGK2, wild-type SGK3 or constitutively active (T308D,S473D)PKB. In oocytes injected with KCNE1 cRNA but not in water-injected oocytes a depolarization from -80 mV to -10 mV led to the appearance of a slowly activating K(+) current. Coexpression of SGK1,( S422D)SGK1, SGK2, SGK3 or (T308D,S473D)PKB but not (K127 N)SGK1 significantly stimulated KCNE1-induced current. The effect did not depend on Na(+)/K(+)-ATPase activity. KCNE1-induced current was markedly upregulated by coexpression of KCNQ1 and further increased by additional expression of (S422D)SGK1, SGK2, SGK3 or (T308D,S473D)PKB. In conclusion, all three members of the SGK family of kinases SGK1-3 and protein kinase B stimulate the slowly activating K(+) channel KCNE1/KCNQ1. The kinases may thus participate in the regulation of KCNE1-dependent transport and excitability.

Inhibition of interferon-gamma expression by osmotic shrinkage of peripheral blood lymphocytes

A hypertonic environment, as it prevails in renal medulla or in hyperosmolar states such as hyperglycemia of diabetes mellitus, has been shown to impair the immune response, thus facilitating the development of infection. The present experiments were performed to test whether hypertonicity influences activation of T lymphocytes. To this end, peripheral blood lymphocytes (PBL) of cytomegalovirus (CMV)-positive donors were stimulated by human leukocyte antigen (HLA)-A2-restricted CMV epitope NLVPMVATV to produce interferon (IFN)-gamma at varying extracellular osmolarity. As a result, increasing extracellular osmolarity during exposure to the CMV antigen indeed decreased IFN-gamma formation. Addition of NaCl was more effective than urea. A 50% inhibition was observed at 350 mosM by addition of NaCl. The combined application of the Ca(2+) ionophore ionomycin (1 microg/ml) and the phorbol ester phorbol 12-myristate 13-acetate (PMA; 5 microg/ml) stimulated IFN-gamma production, an effect again reversed by hyperosmolarity. Moreover, hyperosmolarity abrogated the stimulating effect of ionomycin (1 microg/ml) and PMA (5 microg/ml) on the transcription factors activator protein (AP)-1, nuclear factor of activated T cells (NFAT), and NF-kappaB but not Sp1. In conclusion, osmotic cell shrinkage blunts the stimulatory action of antigen exposure on IFN-gamma production, an effect explained at least partially by suppression of transcription factor activation.

Serum- and glucocorticoid-regulated kinase (SGK1) gene and blood pressure

The serum- and glucose-regulated kinase (SGK1) gene has recently been identified as an important aldosterone-induced protein kinase that mediates trafficking of the renal epithelial Na(+) channel (ENaC) to the cell membrane. Thus, SGK1 is an appealing candidate for blood pressure regulation and possibly essential hypertension. To test this hypothesis, we recruited monozygotic (126 pairs) and dizygotic (70 pairs) normotensive twin subjects and parents of dizygotic twins. Blood pressure was measured in a controlled fashion: recumbent, sitting, and upright. We documented genetic variance on blood pressure in all positions. We then relied on microsatellite markers at the SGK1 gene locus (D6S472, D6S1038, and D6S270) and 2 single nucleotide polymorphisms within the SGK1 gene. We found significant linkage of the SGK1 gene locus to diastolic blood pressure (P<0.0002) and suggestive evidence for linkage for systolic blood pressure (P<0.04), documenting the locus as a quantitative trait locus for blood pressure. We next performed association, using all dizygotic twins and a monozygotic member from each pair. We found significant associations between both single nucleotide polymorphism variants and blood pressure, as well as a significant interaction between the single nucleotide polymorphisms enhancing the effect. This combined effect of the polymorphisms was confirmed in an independent sample of 260 young normotensive men. We conclude that the SGK1 gene is relevant to blood pressure regulation and probably to hypertension in man.

Hypotonic induction of SGK1 and Na+ transport in A6 cells

Serum and glucocorticoid-regulated kinase-1 (SGK1) is a serine-threonine kinase that is regulated at the transcriptional level by numerous regulatory inputs, including mineralocorticoids, glucocorticoids, follicle-stimulating hormone, and osmotic stress. In the distal nephron, SGK1 is induced by aldosterone and regulates epithelial Na+ channel-mediated transepithelial Na+ transport. In other tissues, including liver and shark rectal gland, SGK1 is regulated by hypertonic stress and is thought to modulate epithelial Na+ channel- and Na+-K+-2Cl- cotransporter-mediated Na+ transport. In this report, we examined the regulation of SGK1 mRNA and protein expression and Na+ currents in response to osmotic stress in A6 cells, a cultured cell line derived from Xenopus laevis distal nephron. We found that in contrast to hepatocytes and rectal gland cells, hypotonic conditions stimulated SGK1 expression and Na+ transport in A6 cells. Moreover, a correlation was found between SGK1 induction and the later phase of activation of Na+ transport in response to hypotonic treatment. When A6 cells were pretreated with an inhibitor of phosphatidylinositol 3-kinase (PI3K), Na+ transport was blunted and only inactive forms of SGK1 were expressed. Surprisingly, these results demonstrate that both hypertonic and hypotonic stimuli can induce SGK1 gene expression in a cell type-dependent fashion. Moreover, these data lend support to the view that SGK1 contributes to the defense of extracellular fluid volume and tonicity in amphibia by mediating a component of the hypotonic induction of distal nephron Na+ transport.

Serum- and glucocorticoid-dependent kinase, cell volume, and the regulation of epithelial transport

Ample pharmacological evidence points to a role of kinases in the regulation of cell volume. Given the limited selectivity of most inhibitors, however, the specific molecules involved have remained largely elusive. The search for cell volume regulated genes in liver HepG2 cells led to the discovery of the human serum- and glucocorticoid-dependent serine/threonine kinase hsgk1. Transcription and expression of hsgk1 is markedly and rapidly upregulated by osmotic and isotonic cell shrinkage. The effect of osmotic cell shrinkage on hsgk1 is mediated by p38 kinase. Further stimuli of hsgk1 transcription include glucocorticoids, aldosterone, TGF-beta1, serum, increase of intracellular Ca2+ and phorbolesters, whereas cAMP downregulates hsgk1 transcription. The hsgk1 protein is expressed in several epithelial tissues including human pancreas, intestine, kidney, and shark rectal gland. Co-expression of hsgk1 with the renal epithelial Na+-channel ENaC or the Na+/K+/2Cl(-)-cotransporter NKCC2 (BSC1) in Xenopus oocytes, accelerates insertion of the transport proteins into the cell membrane and thus, stimulates channel or transport activity. Thus, hsgk1 participates in the regulation of transport by steroids and secretagogues increasing intracellular Ca2+-activity. The stimulation of hsgk1 transcription by TGF-beta1 may further bear pathophysiological relevance.

Mediators of aldosterone action in the renal tubule

The aldosterone-sensitive distal nephron extends from the second part of the distal convoluted tubule to the inner medullary collecting duct. As recently shown, aldosterone increases within two hours the abundance of the alpha-subunit of the epithelial sodium channel along the entire aldosterone-sensitive distal nephron, whereas it induces only in an initial portion of the aldosterone-sensitive distal nephron an apical translocation of all three epithelial sodium channel subunits. This suggests that another factor or factors determines the length of the aldosterone-sensitive distal nephron portion in which aldosterone controls epithelial sodium channel surface expression. Since the glucocorticoid-induced kinase SGK1 was identified as aldosterone-induced protein in 1999, it has been postulated to play a key regulatory role. The in-vivo localization of its induction to segment-specific cells of the aldosterone-sensitive distal nephron, and the in-vitro correlation of the amount of its hyperphosphorylated form with transepithelial sodium transport, support this hypothesis. Other recent studies unravel pathways other than those activated by aldosterone and insulin that impact on SGK1 expression and/or function, and thus shed some light onto the complex network that appears to control sodium transport. In view of the ongoing research, the question of how, and formally also whether, SGK1 acts on the epithelial sodium channel should be resolved in the near future.