Regulation of transport across cell membranes by the serum- and glucocorticoid-inducible kinase SGK1

Abstract

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is genomically upregulated by cell stress including energy depletion and hyperosmotic shock as well as a variety of hormones including glucocorticoids, mineralocorticoids and TGFβ. SGK1 is activated by insulin, growth factors and oxidative stress via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase PDK1 and mTOR. SGK1 is a powerful stimulator of Na⁺/K⁺-ATPase, carriers (e.g., NCC, NKCC, NHE1, NHE3, SGLT1, several amino acid transporters) and ion channels (e.g., ENaC, SCN5A, TRPV4-6, ORAI1/STIM1, ROMK, KCNE1/KCNQ1, GluR6, CFTR). Mechanisms employed by SGK1 in transport regulation include direct phosphorylation of target transport proteins, phosphorylation and thus activation of other transport regulating kinases, stabilization of membrane proteins by phosphorylation and thus inactivation of the ubiquitin ligase NEDD4-2, as well as stimulation of transport protein expression by upregulation transcription factors (e.g., nuclear factor kappa-B [NFκB]) and by fostering of protein translation. SGK1 sensitivity of pump, carrier and channel activities participate in the regulation of epithelial transport, cardiac and neuronal excitability, degranulation, platelet function, migration, cell proliferation and apoptosis. SGK1-sensitive functions do not require the presence of SGK1 but are markedly upregulated by SGK1. Accordingly, the phenotype of SGK1 knockout mice is mild. The mice are, however, less sensitive to excessive activation of transport by glucocorticoids, mineralocorticoids, insulin and inflammation. Moreover, excessive SGK1 activity contributes to the pathophysiology of hypertension, obesity, diabetes, thrombosis, stroke, inflammation, autoimmune disease, fibrosis and tumor growth.

• Ca²⁺ release activated Ca²⁺ channel ORAI/STIM
• Epithelial Na⁺ channel ENaC
• Na⁺-coupled glucose transporter SGLT1
• Na⁺/H⁺ exchanger NHE1
• Na⁺/K⁺ ATPase
• renal outer medullary K⁺ channel ROMK
• voltage-gated K⁺ channel KCNQ1

Introduction

The ubiquitously expressed serum- and glucocorticoid-inducible kinase 1 (SGK1) has originally been discovered as a serum and glucocorticoid-sensitive gene in rat mammary tumor cells (Firestone et al. 2003) and subsequently found to be under the powerful genomic control of a wide variaty of hormones including glucocorticoids, mineralocorticoids, gonadotropins, progestin, progesterone, 1,25-dyhydroxyvitamin D₃ (1,25(OH)₂D₃), erythropoietin, morphine, transforming growth factor β (TGFβ), interleukin 6, fibroblast growth factor, platelet-derived growth factor, thrombin, endothelin, further cytokines, advanced glycation end products (AGE) and activators of peroxisome proliferator-activated receptor γ (Lang and Stournaras 2013). SGK1 expression is further upregulated by a wide variety of cell stressors, such as hyperosmotic or isotonic cell shrinkage, excessive glucose concentrations, A6 and M1 cell swelling, mechanical stress, Ca²⁺ chelation, metabolic acidosis, oxidative stress, heat shock, UV radiation, DNA damage, ischemia, neuronal injury, and neuronal excitotoxicity (Lang and Stournaras 2013). Along those lines, SGK1 expression is upregulated following dehydration, saline ingestion, high salt diet, high fat diet, exposure to microgravity, fear conditioning, plus maze exposure, enrichment training, amphetamine, lysergic acid dimethylamide (LSD), electroconvulsive therapy, sleep deprivation, antidepressant fluoxetine, and testicular torsion (Lang and Stournaras 2013, Pasham et al. 2013, Wu et al. 2013).

Factors down-regulating SGK1 expression include serum, starvation, heparin, dietary iron, nucleosides and nephrilin (Lang and Stournaras 2013). SGK1 expression further declines with age (Harries et al. 2012).

Signalling in transcriptional SGK1 regulation involves cytosolic Ca²⁺, cyclic AMP, stress-activated protein kinase-2 (SAPK2, p38 kinase), protein kinase C, protein kinase Raf, big mitogen-activated protein kinase 1 (BMK1), extracellular signal-regulated kinase (ERK1/2), mitogen-activated protein kinase 14 (MAPK14), phosphatidylinositide-(PI)-3-kinase, reactive oxygen species, NADPH oxidases, nitric oxide, EWS/NOR1(NR4A3) fusion protein, methyl-CpG-binding protein 2 (MECP2), TonE binding protein (NFAT5) and p53 (Lang et al. 2006, 2009b, Notch et al. 2012, Lang and Stournaras 2013).

SGK1 expression is enhanced in several clinical conditions including liver cirrhosis, fibrosing pancreatitis, Crohn’s disease, lung fibrosis, cardiac fibrosis, diabetes, dialysis, glomerulonephritis, wound healing, organ rejection, and Rett syndrome (Lang and Stournaras 2013).

SGK1 translation is stimulated by phosphosphoinositide 3-kinase (PI3-kinase) and dependent on actin polymerization (Pelzl et al. 2012).

SGK1 is activated by several growth factors, insulin, follicle stimulating hormone (FSH), thrombin and corticosterone (Lang and Stournaras 2013). Signaling involved in the activation of SGK1 include PI3-kinase and 3-phosphoinositide (PIP3)-dependent kinase PDK1 (Lang and Stournaras 2013), mammalian target of rapamycin mTOR complex-2 (mTORC2) and WNK1 (with no lysine kinase 1) (Lang and Stournaras 2013). The SGK1-activating mTOR complex-2 (mTORC2) is composed of mTOR, Rictor (rapamycin-insensitive companion of mTOR), Sin1 (stress-activated protein kinase-interacting protein 1), mLST8 and Protor-1 (Pearce et al. 2011). Further signaling activating SGK1 include p38α kinase, bone marrow kinase/extracellular signal-regulated kinase 5 (BK/ERK5), cAMP, lithium, Ca²⁺-sensitive calmodulin-dependent protein kinase kinase (CaMKK) and G-protein Rac1 (Lang and Stournaras 2013). SGK1 is further activated by neuronal depolarization, oxidation, hypertonicity, adhesion to fibronectin and feeding (Lang and Stournaras 2013).

Degradation of SGK1 is initiated by ubiquitination involving NEDD4-2 (neuronal precursor cells expressed developmentally downregulated) and Rictor/Cullin-1. SGK1 ubiquitinylation and degradation are inhibited by glucocorticoid-induced leucine zipper protein-1 (Lang and Stournaras 2013).

The gene encoding human SGK1 is located in chromosome 6q23 (Lang et al. 2006). Several SGK1 gene variants have been identified which differ in regulation of expression, subcellular localization and function (Lang et al. 2009b, Raikwar et al. 2012).

The present brief review focuses on the role of SGK1 in transport regulation. The following examples are provided to illustrate the (patho)physiological relevance of SGK1-sensitive transport regulation. In view of the limited number of references allowed, recent reviews are cited instead of earlier publications (Lang et al. 2006, 2009b, 2010a, 2012, Pao 2012, Lang and Shumilina 2013, Lang and Stournaras 2013, Lang and Voelkl 2013).

SGK1-sensitive channels

SGK1 up-regulates a myriad of ion channels (Lang and Shumilina 2013). The first channel shown to be stimulated by SGK1 is the epithelial Na⁺ channel ENaC (Lang and Stournaras 2013). SGK1 sensitivity of ENaC has been shown in cortical collecting duct cells, A6 cells, respiratory epithelial cells and neurons (Lang and Shumilina 2013). SGK1 and ENaC are further coexpressed in epithelial cells of the inner ear and in human adrenocortical cells (Lang and Shumilina 2013). The stimulation of ENaC participates in the regulation of renal Na⁺ reabsorption and blood pressure (see below). SGK1-dependent regulation of ENaC further contributes to stimulation of fluid clearance from lung tissue (Lang and Shumilina 2013).

SGK1 further upregulates the renal outer medullary K⁺ channel ROMK1 (Lang and Shumilina 2013), a K⁺ channel in the apical cell membrane of principal cells, which are located in the aldosterone-sensitive distal nephron (Lang and Shumilina 2013). Besides increasing ROMK protein abundance in the cell membrane, SGK1 shifts the pH sensitivity of ROMK to more acidic values, which increases the current at the normally prevailing cytosolic pH (Lang and Shumilina 2013). SGK1-dependent ROMK regulation contributes to regulation of renal K⁺ elimination. Along those lines the renal ability to eliminate an oral K⁺ load is impaired in gene-targeted mice lacking functional SGK1 (Lang and Shumilina 2013).

SGK1 up-regulates the cardiac voltage-gated Na⁺ channel SCN5A (Lang and Shumilina 2013). Besides increasing SCN5A protein abundance in the cell membrane, SGK1 modifies the gating kinetics of SCN5A channels (Lang and Shumilina 2013). The stimulation of SCN5A is expected to accelerate the rapid depolarization of cardiomyocytes.

SGK1 up-regulates several voltage-gated K⁺ channels including KCNE1/KCNQ1, KCNQ4 and hERG (Lamothe and Zhang 2013, Lang and Shumilina 2013, Pakladok et al. 2013). KCNE1/KCNQ1 is expressed in a variety of tissues including heart, inner ear and epithelia, hERG in the heart and KCNQ4 in inner ear (Lang and Shumilina 2013). KCNE1/KCNQ1 and hERG are critically important for cardiac repolarization and up-regulation of cardiac KCNE1/KCNQ1 and hERG by SGK1 presumably contributes to shortening of the action potential during tachycardia in physical or psychological stress (Lang and Shumilina 2013). Mutations of KCNQ1 (KCNQ1^Y111C, KCNQ1^L114P) may turn the stimulating effect of SGK1 into an inhibitory effect of the kinase. Carriers of those mutations are presumably unable to adequately shorten the cardiac action potential during stress conditions and may thus develop potentially fatal cardiac arrhythmias in stress situations (Lang and Stournaras 2013). KCNQ1 is further required for recycling of K⁺ across the apical cell membrane of the parietal cell and thus for gastric acid secretion (Lang and Shumilina 2013). SGK1 is thus required for up-regulation of KCNQ1 and gastric acid secretion by glucocorticoids (Lang and Shumilina 2013).

SGK1 up-regulates the voltage-gated K⁺ channels Kv1.3, Kv1.5, Kv4.3 and Kv7.2/3 (Lang and Shumilina 2013, Miranda et al. 2013). Kv1.5 rapidly repolarizes pancreatic beta cells and thus curtails Ca²⁺ entry through voltage-gated Ca²⁺ channels and thus insulin release (Lang and Shumilina 2013). SGK1-dependent regulation of Kv channels may participate in a wide variety of functions including regulation of cell proliferation and tumor growth (Lang and Shumilina 2013).

SGK1 up-regulates the kainate receptor GluR6, an effect possibly contributing to regulation of neuronal excitability (Lang et al. 2010b). Along those lines, SGK1 may contribute to memory consolidation and fear retention and to the pathophysiology of Parkinson’s disease, schizophrenia, depression and Alzheimer’s disease (Lang et al. 2010b).

SGK1 up-regulates the Cl⁻ channel complexes ClCKa or CLCKb/barttin, which are involved in Cl⁻ transport of renal thick ascending limb and stria vascularis of the inner ear (Lang and Shumilina 2013). SGK1 further up-regulates the cell volume regulatory Cl⁻ channel ClC2 and the volume-sensitive osmolyte and anion channel VSOAC (Lang and Shumilina 2013). Stimulation of those channels is counterintuitive, as SGK1 is upregulated by cell shrinkage and not by cell swelling. Clearly, SGK1 is not the kinase stimulating rapid regulatory cell volume decrease following cell swelling. SGK1 up-regulates the cystic fibrosis transmembrane conductance regulator CFTR and thus participates in the regulation of epithelial Cl⁻ secretion (Lang and Shumilina 2013).

SGK1 up-regulates several Ca²⁺ channels including the Ca²⁺-permeable transient receptor potential channels TRPV4, TRPV5 and TRPV6 (Lang and Shumilina 2013). SGK1-dependent phosphorylation of TRPV4 facilitates the interaction of the channel protein with F-actin. SGK1-sensitive regulation of TRPV4 participates in the tuning of mechanoreception (Verma et al. 2010), the stimulation of TRPV5 and TRPV6 contributes to Ca²⁺ transport across the apical cell membrane of renal (TRPV5) or intestinal (TRPV6) epithelia (Lang and Shumilina 2013).

SGK1 further up-regulates ORAI1, the pore-forming Ca²⁺ channel subunit contributing to the Ca²⁺-release activated channel (I_CRAC), which accomplishes store-operated Ca²⁺ entry (SOCE) (Lang and Shumilina 2013). The activity of ORAI1 is stimulated by STIM1, which senses the Ca²⁺ content of the intracellular stores and activates ORAI1 upon store depletion. STIM1 is similarly up-regulated by SGK1 (Lang and Shumilina 2013). The functions of ORAI1/STIM1 are shared by the respective isofoms ORAI2, ORAI3 and STIM2. At present it is uncertain, whether the isoforms are similarly upregulated by SGK1. SGK1-dependent regulation of ORAI1/STIM1 contributes to a variety of SGK1-sensitive functions including stimulation of cell proliferation, cell migration, degranulation (e.g., of mast cells) and platelet activation (Lang and Shumilina 2013).

SGK1 up-regulates an unselective cation channel generated by oxidation of the amino acid transporter subunits 4F2/LAT (Lang et al. 2006). The functional significance of this effect remained elusive. SGK1 further stimulates the acid-sensing ion channel ASIC1 (Lang et al. 2009b). Again, the functional significance of this effect remains to be shown.

SGK1-sensitive carriers and Na⁺/K⁺ ATPase

SGK1 stimulates a large number of carriers (Lang and Stournaras 2013) thus affecting the function of epithelia and nonpolarized cells. SGK1 stimulates the Na⁺,K⁺,2Cl⁻ transporter NKCC2 accomplishing the apical uptake of NaCl and KCl in the thick ascending limb of Henle’s loop (Lang et al. 2006). SGK1 further up-regulates the NaCl cotransporter NCC in the apical cell membrane of the early distal tubule (Arroyo et al. 2011, Lang et al. 2009b, Pao 2012, Rotin and Staub 2012, Lang and Stournaras 2013). Disrupted up-regulation of NCC substantially contributes to the salt-losing phenotype of the SGK1 knockout mice (Lang et al. 2009b, Arroyo et al. 2011, Pao 2012, Rotin and Staub 2012, Lang and Stournaras 2013).

SGK1 stimulates the ubiquitously expressed Na⁺/H⁺ exchanger NHE1 (Rotte et al. 2011, Voelkl et al. 2012, 2013) and the epithelial Na⁺/H⁺ exchanger NHE3 (Lang et al. 2009b, Dynia et al. 2010, Pao et al. 2010, He et al. 2011, Panchapakesan et al. 2011, Pasham et al. 2013). The up-regulation of NHE1 participates in the regulation of cell volume, migration and cell proliferation (Lang et al. 1998), the stimulation of NHE3 participate in the regulation of Na⁺ transport in intestine and presumably kidney (Lang et al. 2006).

SGK1 up-regulates Na⁺-coupled glucose transporter SGLT1 as well as the glucose uniporters GLUT1 and GLUT4 (Lang et al. 2009b, Rexhepaj et al. 2011). Thus, SGK1 stimulates interstinal and renal glucose transport as well glucose uptake in nonpolarized cells. SGK1 further up-regulates a number of amino acid transporters, such as ASCT2 (Lang et al. 2009b), SN1 (Lang et al. 2006), B(0)AT1 (Lang and Stournaras 2013), EAAT1 (Lang et al. 2006), EAAT2 (Lang et al. 2006, Lang and Stournaras 2013), EAAT3 (Lang et al. 2006, Lang and Stournaras 2013), EAAT4 (Lang et al. 2009b, Lang and Stournaras 2013) and EAAT5 (Lang et al. 2006). Moreover, SGK1 up-regulates the peptide transporter PepT (Lang et al. 2009b, Lang and Stournaras 2013) as well as albumin uptake (Slattery et al. 2011, Hryciw et al. 2012). Thus, SGK1 fosters cellular uptake of amino acids, peptides and proteins. The effects could again contribute to cellular osmoregulation (Lang et al. 1998) and to the delivery of building blocks for protein synthesis in proliferating cells.

SGK1 further stimulates the Na⁺-dicarboxylate cotransporter NaDC-1 (Lang et al. 2006), the creatine transporter CreaT (Lang et al. 2006) and the Na⁺-myoinositol cotransporter SMIT (Lang et al. 2009b). In particular, the effect on SMIT may contribute to cellular osmoregulation (Lang et al. 2006).

SGK1 further up-regulates the phosphate carriers NaPi-IIa (Andrukhova et al. 2012) and NaPi-IIb (Lang et al. 2006). To which extent this effect contributes to phosphate metabolism, remains, however, uncertain.

SGK1 further up-regulates the Na⁺/K⁺-ATPase (Lang et al. 2006, 2009b), an effect contributing to the regulation of epithelial salt transport and to cellular K⁺ uptake (Lang et al. 2009b).

Mechanisms of SGK1-dependent regulation of transport proteins

SGK1 may influence transport proteins by direct phosphorylation of the channel or carrier protein (Diakov et al. 2010, Lang and Stournaras 2013, Voelkl et al. 2013). The SGK1 phosphorylation consensus sequence is R-X-R-X-X-(S/T)-phi (X = any amino acid, R = arginine, phi = hydrophobic amino acid) (Lang et al. 2006). The only hitherto identified specific SGK1 targets are N-myc downregulated genes NDRG1 and NDRG2 (Lang et al. 2006, Lang and Stournaras 2013, McCaig et al. 2011). All other known SGK1 targets are shared by other SGK isoforms, protein kinase B (PKB/Akt) isoforms and/or other kinases (Lang and Stournaras 2013).

The full effect of SGK1 on some channels or carriers requires the presence of the Na⁺/H⁺ exchanger regulatory factor 2 (NHERF2) (Lang and Stournaras 2013).

SGK1 further regulates channels and carriers indirectly by phosphorylating neuronal precursor cells expressed developmentally down-regulated (Nedd4-2) an ubiquitin ligase ubiquitinating target proteins thus preparing them for clearance from the cell membrane and subsequent degradation (Hallows et al. 2010, Ke et al. 2010, Lang and Stournaras 2013). The phosphorylation by SGK1 fosters interaction of Nedd4-2 with the chaperone protein 14-3-3 thus precluding Nedd4-2-dependent target ubiquitination (Lang and Stournaras 2013). On the other hand, SGK1 stimulates the ubiquitin ligase MDM2 (Lang and Stournaras 2013), an effect, however, not known to participate in transport regulation.

SGK1 may further regulate channels and carriers via other transport regulating kinases (Lang and Shumilina 2013). For instance, SGK1 phosphorylates and thus inhibits the serine/threonine kinase WNK (with no lysine) 4 WNK4 (Lang et al. 2009b, Na et al. 2013), a kinase in turn inhibiting ENaC (Lang and Stournaras 2013) and ROMK (Lin et al. 2012) activity. SGK further phosphorylates PIP2 forming phosphatidylinositol-3-phosphate-5-kinase PIKfyve, which in turn participates in the regulation of carrier and channel trafficking (Pakladok et al. 2012). SGK1 phosphorylates B-Raf kinase and and glycogen synthase kinase-3 GSK-3 (Lang et al. 2006), which in turn up-regulate SGLT1 (Rexhepaj et al. 2010, Pakladok et al. 2012). Further kinases phosphorylated by SGK1 include extracellular signal-regulated kinase ERK2 (Lang and Stournaras 2013), mitogen-activated protein kinase/ERK kinase kinase 3 MEKK3 and stress-activated kinase SEK1 (Lang et al. 2009b), which could, at least in theory, influence channels and carriers.

In addition, SGK1 may influence the transcription of channels or transport proteins (Lang and Shumilina 2013). SGK1- sensitive transcription of ENaC is accomplished by phosphorylation and thus inhibition of the putative transcription factor ALL1-fused gene from chromosome 9 Af9, a suppressor of ENaCα expression (Lang and Stournaras 2013). SGK1 regulates expression of ORAI1 by upregulationg the transcription factor nuclear factor kappa B (NFκB) (Lang et al. 2009b, Rotte et al. 2011, Borst et al. 2012, Eylenstein et al. 2012, Lang and Stournaras 2013). On the other hand, SGK1 activates NDRG1, which in turn downregulates NFκB signaling (Murakami et al. 2010). SGK1 further influences gene expression by modifying further transcription factors, such as p53 (Lang and Stournaras 2013), cAMP responsive element binding protein (CREB) (Lang et al. 2006, Reiter et al. 2011), activator protein-1 (Reiter et al. 2011), and forkhead transcription factor FKHR-L1 (FOXO3a) (Lang et al. 2006, 2009b, Lang and Stournaras 2013, Sahin et al. 2013). Whether or not those transcription factors participate in SGK1-sensitive transport regulation, remains to be shown.

SGK1 down-regulates the inducible nitric oxide synthase, which in turn inhibits ENaC by formation of nitric oxide (Lang and Stournaras 2013).

(Patho)physiological significance of SGK1-sensitive transport regulation

SGK1-sensitive basic cellular functions

SGK1 participates in the regulation of a wide variety of cellular functions including organization of the cytoskeleton (Schmid et al. 2013), cell volume regulation (Lang et al. 2009b), cell survival and cell proliferation (Lang et al. 2010a, Chen et al. 2012b, Lang and Stournaras 2013), cell migration (Schmidt et al. 2012a, 2012b), degranulation (Hua 2013, Schmid et al. 2013) and hormone release (Lang et al. 2006, 2009b). The effect on migration may not only result from regulation of transport but as well from downregulation of vinculin phosphorylation, which in turn may enhance migration via actin cytoskeleton redistribution (Schmidt et al. 2012a, 2012b). SGK1 sensitivity of channels, carriers, and Na⁺/K⁺ ATPase contributes to the regulation of epithelial transport, such as renal tubular Na⁺ reabsorption (Lang et al. 2006, 2009b, Faresse et al. 2012, Rotin and Staub 2012, Soundararajan et al. 2012), renal tubular K⁺ transport (Lang and Vallon 2012), gastric acid secretion (Lang et al. 2009b, Lang and Stournaras 2013) and intestinal Na⁺ and nutrient transport (Lang et al. 2006).

Renal transport and hypertension

SGK1 stimulates NKCC, NCC, NHE3 and ENaC, effects supporting renal tubular salt reabsorption (Lang et al. 2009b, Resch et al. 2010, Faresse et al. 2012, Soundararajan et al. 2012, Lang and Stournaras 2013). SGK1 further enhances salt appetite and thus salt intake (Lang et al. 2006, Umbach et al. 2011, Lang and Stournaras 2013). As a result, increased SGK1 activity predisposes to extracellular volume expansion and hypertension (Resch et al. 2010, Kawarazaki et al. 2012, Nakagaki et al. 2012, Nakano et al. 2013, Pao 2012, Rao et al. 2013). As a matter of fact, several blood pressure-associated SGK1 gene variants have been identified (Rao et al. 2013) including combined polymorphisms in intron 6 [I6CC] and exon 8 [E8CC/CT] (Lang et al. 2006, 2009b). The gene variant is common in Caucasians (3–5%) and particularly in Africans (10%) (Lang et al. 2006, 2009b). The SGK1 knockout mice maintain a normal blood pressure at regular diet (Lang et al. 2006), but, contrast to their wild-type littermates, does not develop hypertension following induction of hyperinsulinism with high-fructose diet or high-fat diet (Huang et al. 2006a, 2006b, Lang et al. 2006, Ackermann et al. 2011). Accordingly, hperinsulinism leads to hypertension presumably by SGK1-dependent stimulation of renal tubular salt reabsorption (Lang et al. 2006, 2009b). Moreover, SGK1-sensitive transport contributes to glucocorticoid-induced hypertension (Lang et al. 2009b).

Intestinal transport and obesity

Enhanced SGLT1 activity fosters the development of obesity presumably by accelerating the postprandial increase of plasma glucose concentration leading to excessive insulin release and subsequent fat deposition (Lang et al. 2006, 2009b). Thus, stimulation of SGLT1 by SGK1 predisposes to development of obesity (Lang et al. 2006, 2009b). Moreover, SGK1 stimulates adipocyte differentiation and adipogenesis (Di Pietro et al. 2010). Accordingly, carriers of the I6CC/E8CC/CT SGK1 gene variant are heavier and at enhanced risk to develop diabetes (Lang et al. 2009b). Since hyperglycemia stimulates intestinal SGK1 expression (Lang et al. 2006, 2009b), diabetic individuals may up-regulate intestinal SGLT1 activity and thus gain further weight.

Ca²⁺ channel activity and platelet function

The SGK1-sensitive upregulation of the platelet Ca²⁺ channel ORAI1/STIM1 increases the reagibility of blood platelets (Borst et al. 2012). Moreover, SGK1 stimulates coagulation by stimulating tissue factor expression (Lang et al. 2009b). As a result, enhanced SGK1 activity may predispose to thrombosis (Borst et al. 2012) and stroke (Dahlberg et al. 2011, Nakano et al. 2013).

Tumor growth

SGK1 is heavily expressed in a variaty of tumors (Lang et al. 2010a) including non-small cell lung cancer (Abbruzzese et al. 2012), colon cancer (Lang et al. 2010a), prostate cancer (Szmulewitz et al. 2012), ovarian tumors (Lang and Stournaras 2013), myeloma (Fagerli et al. 2011), and medulloblastoma (Lang and Stournaras 2013). SGK1 presumably supports the survival of those tumor cells (Lang et al. 2006, 2009a, Lang and Stournaras 2013, Towhid et al. 2013). Specifically, SGK1 has indeed been shown to confer interleukin 6 (IL6)-induced survival of cholangiocarcinoma cells (Lang et al. 2006, 2010a), interleukin 2 (IL2)-dependent survival of kidney cancer cells (Lang and Stournaras 2013), angiotensin II-induced survival of fibrosarcoma-derived cells (Baskin and Sayeski 2012) and androgen receptor-dependent survival of prostate cancer cells (Lang and Stournaras 2013). Moreover, SGK1 renders breast cancer cells resistant to chemotherapy and SGK1 silencing enhances the efficacy of chemotherapeutic drugs (Lang et al. 2006, 2010a, Sommer et al. 2013). SGK1 inhibition counteracts androgen-induced growth of prostate cancer cells (Lang et al. 2009b) and SGK1 contributes to glucocorticoid- or colony stimulating factor 1 (CSF1)-induced stimulation of invasiveness, motility and adhesiveness (Lang et al. 2006, 2010a).

SGK1 counteracts the proapoptotic effect of membrane androgen receptors (mAR) (Lang and Stournaras 2013) and thus interferes with the effect of mAR on actin cytoskeleton architecture and migration of colon carcinoma cells (Gu et al. 2011, Schmidt et al. 2012a, 2012b).

SGK1 counteracts apoptosis by multiple mechanisms not directly reloated to transport, including phosphorylation and thus inactivation of the proapoptotic forkhead transcription factor Foxo3a/FKRHL1 (Lang and Stournaras 2013), phosphorylation and thus inhibition of glycogen synthase kinase GSK-3 with subsequent up-regulation of oncogenic β-catenin (Lang et al. 2006, 2009a), phosphorylation of IKKβ, with subsequent phosphorylation and degradation of the inhibitory protein IκB and translocation of NFκB into the nucleus (Lang et al. 2010a), phosphorylation of the ubiquitin ligase MDM2 with subsequent MDM2-dependent ubiquitination and proteosomal degradation of proapoptotic transcription factor p53 (Lang and Stournaras 2013), as well as phosphorylation of SEK1 and thus interference of SEK1 binding to JNK1 and MEKK1 (Lang et al. 2006, 2010a). Moreover, SGK1 up-regulates Ran binding protein (RanBP), which in turn modifies the microtubule network and decreases taxol sensitivity of cancer cells (Amato et al. 2013). Reports on the effect of SGK1 on extracellular signal-regulated kinase ERK2 have been conflicting suggesting either downregulation or stimulation of kinase activity (Lang et al. 2009b, 2010a, Lang and Stournaras 2013).

SGK1 influences cell proliferation and cell death further by up-regulating channels and transporters, such as the Ca²⁺ release-activated channels (I_CRAC) ORAI1/STIM1 (Eylenstein et al. 2011, 2012, Borst et al. 2012). Ca²⁺ entry via I_CRAC is followed by oscillations of cytosolic Ca²⁺ activity, which are required for depolymerization of the actin filament network, a prerequisite for cell proliferation (Lang et al. 2006, 2010a). The Ca²⁺ entry via I_CRAC requires the maintenance of the cell membrane potential, a function of K⁺ channels, which are similarly up-regulated by SGK1 (Lang et al. 2006, 2010a).

SGK1-dependent up-regulation of cellular glucose uptake provides the tumor cell with fuel for the excessive glycolytic flux due to aerobic glycolysis in tumor cells (Lang et al. 2006). Glycolytic flux requires mantainance of alkaline pH (Lang and Stournaras 2013), which is accomplished by SGK1-dependent stimulation of Na⁺/H⁺ ion exchanger (Rotte et al. 2011). Those effects may be particularly important during ischemia, which up-regulates SGK1 expression (Lang et al. 2006, 2009b, 2010a, Rusai et al. 2010).

However, SGK1 is apparently not uniformly supporting the growth of all tumor cells. In adrenocortical carcinoma, a positive correlation was observed between SGK1 abundance and patient survival (Ronchi et al. 2012a, 2012b). Downregulation of SGK1 abundance has been observed in several tumor cell types, such as prostate cancer, ovarian tumors, hepatocellular carcinoma and adenomatous polyposis coli (APC) (Lang et al. 2006, 2010a, Lang and Stournaras 2013). On the other hand, SGK1 knockout has been shown to counteract the development of spontaneous tumors in APC deficient mice (Lang et al. 2009b) and chemically induced colonic tumours in wild-type mice (Lang and Stournaras 2013). Presumably tumor cells with high activity of related kinases such as PKB/Akt isoforms or SGK3 do not require SGK1 for growth and survival.

Despite the multiple effects of SGK1 on cell growth and cell survival the phenotype of SGK1 knockout mice is mild, indicating that SGK1 is not critically important for cell proliferation and survival (Lang et al. 2006, 2010a). Accordingly, inhibition of SGK1 alone does presumably not kill tumor cells. Parallel inhibition of SGK1 may, however, enhance the efficacy of treatment with cytotoxic drugs or radiation (Lang and Voelkl 2013).

Inflammation and fibrosis

SGK1 participates in the pathophysiology of inflammation, an effect not attributed to altered transport. SGK1 is required for the inactivation of the transcription factor Foxo1, which is in turn a prerequisite for the expression of the IL23 receptor (Wu et al. 2013) and thus for the stimulating effect of interlekin 23 (IL-23) on the generation of interleukin (IL)-17-producing CD4⁺ helper T cells (T_H17 cells) (Kleinewietfeld et al. 2013). The T_H17 cells upregulate the pro-inflammatory cytokines GM-CSF; TNF-α and IL-2 and play a pivotal role in autoimmune disease (Kleinewietfeld et al. 2013). T_H17 cells are up-regulated by modest increases of local NaCl concentrations (Wu et al. 2013), which are effective through activation of the p38/MAPK pathway, SGK1 and nuclear factor of activated T cells 5 (NFAT5, TONEBP) (Kleinewietfeld et al. 2013). Presumably due to upregulation of SGK1 high-salt diet predisposes to a particularly severe form of experimental autoimmune encephalomyelitis with enhanced infiltration of T_H17 cells into the central nervous system (Kleinewietfeld et al. 2013).

SGK1 further participates in the machinery leading to tissue fibrosis. SGK1 expression is excessive in inflammatory and fibrosing tissues, such as lung fibrosis, diabetic nephropathy, glomerulonephritis, experimental nephrotic syndrome, obstructive nephropathy, liver scirrhosis, fibrosing pancreatitis, peritoneal fibrosis, Crohn’s disease and coeliac disease (Lang et al. 2006, Cheng et al. 2010, Szebeni et al. 2010). Expression of SGK1 is stimulated by TGFβ (Lang et al. 2006), a key stimulator of fibrosis (Roos et al. 2011, Akhurst and Hata 2012, Chen et al. 2012a, Li et al. 2012, MacDonald and Cohn 2012, Rassler et al. 2012, Wang et al. 2012). TGFβ is partially effective through the transcription factors Smad2/3 (Lang and Stournaras 2013), which is degraded by the ubiquitin ligase Nedd4L (Lang and Stournaras 2013). SGK1 phosphorylates and thus inactivates Nedd4L and thus enhances the efficacy of TGBβ (Lang and Stournaras 2013). SGK1 further stimulates expression of connective tissue growth factor (CTGF), an effect mediated by nuclear translocation of NFκB (Lang et al. 2006), a transcrition factor fostering inflammation and fibrosis (Shih et al. 2011, Stone et al. 2011, Lang and Stournaras 2013). Along those lines SGK1 mediates the up-regulation of CTGF formation and cardiac fibrosis (Lang et al. 2006) as well aging of the skin (Tsai et al. 2012) following mineralocorticooid excess. SGK1 further contributes to angiotensin II-induced cardiac CTGF formation and fibrosis (Yang et al. 2012, Chilukoti et al. 2013) and in cardiac remodelling following increased afterload (Das et al. 2012, Voelkl et al. 2012, Lang and Stournaras 2013). Overexpression of SGK1 alone has little effect on the formation of the matrix protein fibronectin but SGK1 augments the formation of fibronectin at excessive extracellular glucose concentrations (Lang and Stournaras 2013). Thus, additional glucose-dependent mechanisms are required for the induction of fibrosis by hyperglycemia (Lang and Stournaras 2013). Little is known about the contribution of SGK1-sensitive channel and transporter activity to inflammation and fibrosis. There is little doubt, however, that the proliferation of inflammatory cells and the production of matrix proteins involve transport across the cell membrane.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Acknowledgements

The study was supported by the Deutsche Forschungsgemeinschaft (DFG 315/15-1). The authors acknowledge the meticulous preparation of the manuscript by Tanja Loch.