The Physiological Impact of the Serum- and Glucocorticoid-inducible Kinase SGK1

Abstract

Purpose of review

The role of SGK1 in renal physiology and pathophysiology is reviewed with particular emphasis of recent advances.

Recent findings

The mammalian target of rapamycin complex 2 (mTORC2) has been shown to phosphorylate SGK1 at Ser⁴²² (the so-called hydrophobic motif). Ser³⁹⁷ and Ser⁴⁰¹ are two additional SGK1-phosphorylation sites required for maximal SGK1-activity. A 5′ variant alternate transcript of human Sgk1 has been identified that is widely expressed and shows improved stability, enhanced membrane association, and greater stimulation of epithelial Na⁺ transport. SGK1 is essential for optimal processing of the epithelial sodium channel and also regulates the expression of the Na⁺-Cl⁻ cotransporter. With regard to pathophysiology, SGK1 participates in the stimulation of renal tubular glucose transport in diabetes, the renal profibrotic effect of both angiotensin II and aldosterone, and in fetal programming of arterial hypertension.

Summary

The outlined recent findings advanced our understanding of the molecular regulation of SGK1 as well as the role of the kinase in renal physiology and the pathophysiology of renal disease and hypertension. Future studies using pharmacological inhibitors of SGK1 will reveal the utility of the kinase as a new therapeutic target.

Introduction

The serum- and glucocorticoid-inducible kinase 1 (SGK1) was originally cloned as an immediate early gene transcriptionally stimulated by serum and glucocorticoids in rat mammary tumor cells [1]. The human isoform has been discovered as a gene upregulated by cell shrinkage [2]. As listed in Table 1, SGK1 is under transcriptional control of a variety of further stimuli.

Table 1.

Regulation of SGK1 transcription (due to space limits, only recent original papers are cited, for previous citations see the cited reviews)

Stimulators	References
glucocorticoids	[1,3-7]
mineralocorticoids	[7-9]
gonadotropins	[7]
progestin	[10]
medroxyprogesterone	[11]
1,25-dyhydroxyvitamin D3 (1,25(OH)2D3)	[7]
transforming growth factor β (TGFβ)	[7]
interleukin 6	[12]
fibroblast and platelet-derived growth factor	[7]
thrombin	[13]
endothelin	[14]
advanced glycation end products (AGE)	[15]
further cytokines	[7]
activation of peroxisome proliferator-activated receptor γ	[7]
cell shrinkage	[7,16]
chelation of Ca2+	[17]
A6 cell swelling	[17]
excessive glucose concentrations	[7,18]
metabolic acidosis	[19]
salt loading of spontaneously hypertensive mice	[20]
heat shock, UV radiation and oxidative stress	[21,22]
DNA damage	[7]
ischemia	[23]
neuronal injury	[7]
neuronal excitotoxicity	[7]
neuronal challenge by exposure to microgravity	[24]
fear conditioning	[25]
plus maze exposure	[26]
enrichment training	[7]
amphetamine	[7]
lysergic acid dimethylamide LSD	[7]
electroconvulsive therapy	[27]
sleep deprivation	[27]
fluoxetine	[27]
Rett syndrome	[28]
organ rejection	[29]
dialysis	[30]
wound healing	[7]
diabetic nephropathy	[7,31]
glomerulonephritis	[7]
liver cirrhosis	[7]
fibrosing pancreatitis	[7]
Crohn’s disease	[7]
lung fibrosis	[7]
cardiac fibrosis	[32]
Inhibitors
heparin	[7]
mutations in the gene MECP2	[7,28]
dietary iron	[7]
nucleosides	[33]
Signaling molecules
protein kinase C	[7]
protein kinase Raf	[7]
mitogen-activated protein kinase (BMK1)	[7]
mitogen-activated protein kinase (MKK1)	[7]
stress-activated protein kinase-2 (SAPK2, p38 kinase)	[7]
Nuclear factor of activated T cells (NFAT) 5	[16]
phosphatidylinositol-(PI)-3-kinase	[7]
cyclic AMP	[7,10]
extracellular signal-regulated kinase (ERK1/2)	[7]
P53	[7]
cytosolic Ca2+	[7]
nitric oxide	[7,34]
EWS/NOR1(NR4A3) fusion protein	[35]

The gene encoding human SGK1 has been localised to chromosome 6q23 [36]. Distinct translational isoforms of SGK1 have been disclosed differing in regulation of expression, subcellular localization and function [37-39]. The kinase forms dimers by two intermolecular disulfide bonds between Cys258 in the activation loop and Cys193 [40]. SGK kinases were detected in a variety of species including shark and Caenorhabditis elegans [7]. Yeast express two orthologues, Ypk1 and Ypk2, kinases involved in the regulation of endocytosis and required for survival [7].

Regulation of SGK1 expression and activity

SGK1 expression is virtually ubiquitous [2], but varies between different cell types, as observed in brain [7,41,42], eye [7], inner ear [3,4,43], semicircular canal duct epithelium [4], lung [7,44-48], kidney [7,49], liver [7], intestine [7], pancreas [7] and ovary [7]. Moreover, typical expression patterns are found during embryonic as well as postnatal development [41,50-52]. The subcellular localisation of SGK1 may depend on the functional state of the cell. Activation of SGK1 following exposure of cells to serum has been suggested to trigger importin-alpha mediated entry of SGK1 into the nucleus [7] whereas activation by hyperosmotic shock or glucocorticoids enhances cytosolic localization of the kinase [1]. SGK1 may further localize to the mitochondrial membrane [53,54].

SGK1 transcription is rapidly regulated by a wide variety of stimulators and inhibitors (Table 1). Transcription factor binding sites have been identified in the promoter of the rat SGK1 gene for the glucocorticoid receptor (GR), the mineralocorticoid receptor (MR), the progesterone receptor (PR), the vitamin D receptor (VDR), the retinoid X receptor (RXR), the farnesoid X receptor (FXR), the sterol regulatory element binding protein (SREBP), peroxisome proliferator-activated receptor γ (PPARγ), the cAMP response element binding protein (CREB), the p53 tumor suppressor protein, the Sp1 transcription factor, the activating protein 1 (AP1), the activating transcription factor 6 (ATF6), the heat shock factor (HSF), reticuloendotheliosis viral oncogene homolog (c-Rel), nuclear factor κB (NFκB), signal transducers and activators of transcription (STAT), TGFβ dependent transcription factors SMAD3 and SMAD4, and fork-head activin signal transducer (FAST) [1].

SGK1 is activated by the phosphatidylinositol-3-kinase (PI3-kinase) pathway involving the 3-phosphoinositide (PIP3)-dependent kinase PDK1 [7]. PIP3 is degraded and thus SGK1 activation discontinued by the phosphatase and tensin homolog PTEN [55]. SGK1 activation by PDK1 may involve the scaffold protein Na⁺/H⁺ exchanger regulating factor 2 (NHERF2), which mediates the assembly of SGK1 and PDK1 via its PDZ domains and PIF consensus sequence [7]. Activation of SGK1 by PDK1 may further involve the mammalian target of rapamycin mTOR [56-59] and the serine/threonine kinase WNK1 (with no lysine kinase 1) [60-62].

PI3-kinase pathway dependent activation of SGK1 is triggered by insulin, IGF1 , hepatic growth factor (HGF), and follicle stimulating hormone (FSH) [7]. SGK1 can further be activated by bone marrow kinase/extracellular signal-regulated kinase 5 (BK/ERK5) or by p38α [7], by feeding [63], by an increase of cytosolic Ca²⁺ activity with subsequent activation of calmodulin-dependent protein kinase kinase (CaMKK) [7], and by the small G-protein Rac1 [7]. SGK1 is further activated by neuronal depolarization, cAMP, lithium, oxidation and adhesion to fibronectin [7].

SGK1 is degraded with a half-life of 30 minutes [7]. SGK1 may be ubiquitinated [64,65] by the ubiquitin ligase Nedd4-2 (neuronal precursor cells expressed developmentally downregulated) [66].

SGK1 dependent regulation of cellular functions

The SGK1 kinase consensus sequence R-X-R-X-X-(S/T)-phi (X stands for any amino acid, R for arginine and phi indicates a hydrophobic amino acid) is shared by other kinases [7] and the only exclusive SGK1 targets known are the N-myc downregulated genes NDRG1 and NDRG2 [7,67]. Thus, most SGK1 sensitive functions are similarly regulated by SGK and protein kinase B isoforms or other related kinases. As listed in Table 2, SGK1 regulates a wide variety of channels and carriers.

Table 2.

Regulation of carriers and transporters by SGK1 (due to space limits, only recent original papers are cited, for previous citations see the cited reviews)

Channel	Function	References
ENaC	Epithelial Na+ channel	[7,68-77]
ROMK1	Renal outer medullary K+ channel	[7]
TRPV5	Ca2+ channel	[7]
TRPV6	Ca2+ channel	[78]
ClCKa/barttin	Cl− channel	[7,79]
ClC2	Cl− channel	[7]
CFTR	Cystic fibrosis transmembrane conductance regulator	[80,81]
VSOAC	volume-sensitive osmolyte and anion channel	[7]
SCN5A	Na+ channel	[7]
KCNE1/KCNQ1	K+ channel	[7,82]
KCNQ4	K+ channel	[83]
Kv1.3	K+ channel	[7,84]
Kv1.5	K+ channel	[85,86]
Kv4.3	K+ channel	[87]
ASIC1	acid sensing ion channel	[88]
GluR6	glutamate receptor	[89]
4F2/LAT	cation channel	[7]
NCC	NaCl cotransport	[90]
NKCC2	Na+,K+,2Cl- cotransport	[7]
NHE3	Na+/H+ exchanger	[7,91-93]
SGLT1	glucose transporters	[7,94,95]
GLUT1	glucose transporter	[96]
GLUT4	glucose transporter	[97]
ASCT2	amino acid transporter	[97]
SN1	glutamine transporter	[7]
EAAT1	glutamate transporter	[7]
EAAT2	glutamate transporter	[7,98]
EAAT3	glutamate transporter	[7]
EAAT4	glutamate transporter	[7,99]
EAAT5	glutamate transporter	[100]
PepT2	peptide transporter	[101]
NaDC-1	Na+,dicarboxylate cotransporter	[7]
CreaT	creatine transporter	[102,103]
SMIT	Na+, myoinositol cotransporter	[104]
NaPiIIb	phosphate carrier	[7,105,106]
Na+/K+-ATPase	Na+/K+-pump	[7,107]

SGK1 regulates the channels and carriers partially by phosphorylating the target proteins [7]. Alternatively, SGK1 phosphorylates the ubiquitin ligase Nedd4-2 [7], which otherwise ubiquitinates channel and transport proteins thus preparing them for clearance from the cell membrane and subsequent degradation [7]. Phosphoprylated Nedd4-2 is bound to 14-3-3, and is thus unable to ubiquitinate its targets [108-111]. The ^P355LNedd4-2 variant, found in patients with end-stage renal disease (ESRD) suffering from arterial hypertension, was shown to be more sensitive to phosphorylation by SGK1 and, accordingly, to exert a weaker negative effect on ENaC [7]. Furthermore, SGK1 may phosphorylate WNK4, a kinase inhibiting ENaC activity [112], and inhibit inducible nitric oxide synthase, and the decreased formation of nitric oxide may then disinhibit ENaC [114]. SGK1 may also be effective through modification of channel or carrier expression [113]. SGK1 may further regulate carriers and channels through activation of the phosphatidylinositol-3-phosphate-5-kinase PIKfyve and subsequent formation of PIP2 [95,115,116].

SGK1 phosphorylates and thus inhibits several enzymes, including the ubiquitin ligase Nedd4-2 [7,66], the mitogen-activated protein kinase/ERK kinase kinase 3 (MEKK3) [7], the kinase SEK1 [21], the B-Raf kinase [7], the phosphomannose mutase 2 [117], inducible nitric oxide (NO) synthase [114] and the glycogen synthase kinase 3 GSK3 [7,118], which, however, may be regulated by PKB/Akt rather than SGK1 [7].

SGK1 regulates several transcription factors, including the cAMP responsive element binding protein (CREB) [119], nuclear factor kappa B (NFκB) [32,120,121], and the forkhead transcription factor FKHR-L1 (FOXO3a) [7,122].

SGK1 phosphorylates a variety of further proteins such as the type A natriuretic peptide receptor (NPR-A) [7], Ca²⁺ regulated heat-stable protein of apparent molecular mass 24 kDa CRHSP24 [123], the adaptor precursor (APP) Fe65 [72], NDRG1 and NDRG2 [67], mosinVc [124], filamin C [7], microtubule-associated protein tau [125] and huntingtin [7]. In most cases, the functional significance of SGK1 dependent phorphorylation of those proteins is still elusive. A wide variety of functions have, however, been described, which are regulated by SGK1 (Table 3). Most of those functions have been reviewed previously [7].

Table 3.

SGK1 sensitive functions (due to space limits, only recent original papers are cited, for previous citations see the cited reviews)

Function	References
Cell volume	[7,126]
Cell survival	[7,127,128]
Cell proliferation	[1,7]
Aldosterone release	[1,7]
Insulin release	[86,86,129]
Glucose metabolism	[130]
Function of decidualizing cells	[10,10]
Gastric acid secretion	[131]
Intestinal transport	[94]
Renal transport	[7,16,90,132]
Proteinuria & nephrotic syndrome	[133-135]
Blood pressure regulation	[7,136,137]
Coagulation	[13]
Pulmonary hypertension	[13]
Cardiac excitation	[7,82,138]
Cardiac hypertrophy	[139,140]
Memory consolidation	[7,24,25,125,141,142]
Pain perception	[143]
Tumor growth and metastasis	[7,39,144-146]
Inflammation and Fibrosis	[7,32,147-150]

In the following the role of SGK1 in the regulation of electrolyte metabolism and blood pressure will be discussed in more detail.

The role of SGK1 in renal function and salt appetite

SGK1 stimulates a variety of renal tubular ion channels and transporters (Table 2) and thus participates in the regulation of renal electrolyte excretion (for earlier references see [7,151-154]).

Specifically, SGK1 participates in the regulation of renal Na⁺ excretion by aldosterone, insulin and IGF1 [91,155]. The effect of aldosterone is only partially dependent on the presence of SGK1 and effects of aldosterone and SGK1 are additive. In contrast, antidiuretic hormone (ADH) or insulin do not further stimulate ENaC in cells expressing active SGK1 [156]. Experiments in SGK1-knockout (sgk1^−/−) mice indeed revealed subtle but relevant impairment of renal salt retention [90]. In salt replete conditions arterial blood pressure and renal salt output is similar in sgk1^−/− mice and their wild-type littermates (sgk1^+/+). The maintenance of salt balance and blood pressure requires, however, increased plasma aldosterone levels in sgk1^−/− mice [7]. Under NaCl-deficient diet, NaCl excretion decreases in both sgk1^−/− and sgk1^+/+ mice, the renal NaCl retention remains, however, insufficient in sgk1^−/− mice, despite an increase of plasma aldosterone concentration, decrease of arterial blood pressure, decrease of glomerular filtration rate and enhanced proximal tubular Na⁺ reabsorption [7]. The renal salt loss of sgk1^−/− mice may not be only due to decreased stimulation of ENaC as enhanced ENaC activity has been observed in salt depleted sgk1^−/− mice [90], presumably as a result of stimulation of ENaC expression by hyperaldosteronism [90]. Similarly, despite the lack of SGK1 ENaC activity is enhanced in the colon of sgk1^−/− mice, which is again likely due to hyperaldosteronism [157]. The enhanced renal salt loss despite increased ENaC activity may be due to decreased expression of the NaCl cotransport in early distal tubule [90]. Inhibition of ENaC by triamterene leads to excessive, eventually lethal salt loss in sgk1^−/− but not in sgk1^+/+ mice, an observation again pointing to a renal transport defect other than ENaC in sgk1^−/− mice [158]. SGK1 is particularly important for the antinatriuretic effect of insulin, which is significantly blunted in sgk1^−/− mice [136]. Renal salt loss does not appear to be more pronounced in mice lacking both SGK1 and SGK3 [159].

In dehydration, the enhanced medullary osmolarity upregulates the expression of SGK1, which in turn stimulates the expression of the natriuretic peptide receptor type A thus sensitizing the epithelial cells to natriuretic peptides and triggering natriuresis [16]. The renal loss of Na⁺ counteracts hyperosomolarity during water deprivation [16].

Lack of SGK1 further impairs the ability of the kidney to excrete a K⁺ load. Despite enhanced plasma levels of aldosterone, which should increase renal K⁺ excretion, sgk1^−/− mice do not adequately increase the renal K⁺ excretion following an acute K⁺ load [7]. Following a chronic K⁺ load, sgk1^−/− mice enhance renal K⁺ excretion only upon marked increase of plasma K⁺ and aldosterone concentrations [7]. SGK1 is further critically important for the effect of insulin on cellular K⁺ uptake [160].

The sgk1^−/− mice express significantly less TRPV5 Ca²⁺ channels [132], an observation consistent with a stimulating effect of SGK1 on TRPV5 [7]. However, despite decreased TRPV5 expression, the renal Ca²⁺ excretion is decreased in sgk1^−/− mice [132]. The extracellular volume contraction following renal salt loss due to impaired stimulation of NaCl cotransport and ENaC enhances Na⁺ [7] and presumably Ca²⁺ reabsorption in the proximal tubule and in the thick ascending limb of the loop of Henle, thus decreasing renal Ca²⁺ excretion. It is noteworthy that inhibition of NaCl cotransport by thiazide diuretics regularly leads to anticalciuria [161] by a mechanism that involves upregulation of proximal tubular Ca²⁺ reabsorption [162].

In normal kidneys, the proximal tubular SGK1 expression is low [7] and SGK1 is thus not considered to participate in the regulation of proximal tubular transport. In diabetic nephropathy, however, SGK1 is expressed throughout the kidney including proximal renal tubules and thus stimulates renal tubular glucose transport [163]. The effect may be due to stimulation of SGLT1 [7] or due to activation of KCNQ1/KCNE1 [7], which maintains the electrical driving force for electrogenic glucose transport [164].

SGK1 is expressed in glomerular podocytes [22,165] and is upregulated in those cells by aldosterone and oxidative stress [20,22]. Thus, SGK1 may participate in the development of proteinuria during mineralocorticoid excess and inflammation. As a matter of fact, the proteinuria following DOCA treatment is significantly blunted in SGK1 knockout mice [133]. The sgk1^−/− mice are, however, not protected against doxorubicin induced glomerular injury [135]. SGK1-dependent renal salt retention favor the development of edema during treatment with PPARγ agonists [166] or in nephrotic syndrome [135]. Furthermore, increased SGK1 expression has been found during ascites formation in cirrhotic rats [167].

SGK1 does not only participate in the regulation of salt excretion but contributes to the regulation of salt intake [168]. Treatment of animals with the mineralocorticoid DOCA is followed by excessive salt intake in sgk1^+/+ but not in sgk1^−/− mice. Thus, SGK1 plays a dual role in mineralocorticoid-regulated NaCl homeostasis, stimulating both, intake and renal retention of salt.

Role of SG1 in hypertension, obesity and metabolic syndrome

The effect of SGK1 on renal salt excretion and salt intake is expected to impact on blood pressure control. As a matter of fact, a certain variant of the SGK1 gene (the combined presence of distinct polymorphisms in intron 6 [I6CC] and in exon 8 [E8CC/CT]) is associated with moderately enhanced blood pressure [7]. The gene variant is common, affecting 3-5 % of a Caucasian population [7]. While one study failed to detect a correlation of the gene variant with blood pressure in patients with renal failure [7], a subsequent study on more than four thousand individuals confirmed the association of the gene variant with increased blood pressure [169]. This latter study revealed a particularly strong correlation between insulinemia and blood pressure in individuals carrying the SGK1 gene variant unravelling the decisive role of SGK1 in the hypertension paralleling hyperinsulinemia [169]. Hyperinsulinemia by pretreatment with a high-fructose diet [136] or a high fat diet [137] sensitizes arterial blood pressure to high-salt intake in sgk1^+/+ but not in sgk1^−/− mice. This observation underscores the role of SGK1 in insulin induced antinatriuresis and blood pressure control. Moreover, SGK1 may participate in the hypertensive effects of glucocorticoids [170]. Lack of Nedd4-2 leads to hypertension in mice thus mimicking overactivity of SGK1 [171]. Beyond the contribution of SGK1 to blood pressure increase in a given individual, maternal SGK1 appears to be critical for the fetal programming of hypertension in the offspring by stress of the mother [172].

As SGK1 is a powerful stimulator of the Na⁺ coupled glucose transporter SGLT1 [7], the SGK1 gene variant may further accelerate the intestinal glucose absorption. Enhanced SGLT1 activity accelerates intestinal glucose absorption leading to excessive insulin release, fat deposition, and subsequent decrease of plasma glucose concentration triggering repeated glucose uptake and thus obesity [7]. Conversely, inhibitors of SGLT1 have been shown to counteract obesity [7]. Accordingly, the same SGK1 gene variant associated with enhanced blood pressure proved to be associated with increased body mass index [7]. Moreover, this SGK1 gene variant is more prevalent in patients with type 2 diabetes than in individuals without family history of diabetes [173]

Hypertension, obesity and susceptibility to develop type II diabetes are hallmarks of metabolic syndrome, which is associated with enhanced morbidity and mortality from cardiovascular disease [7]. Metabolic syndrome and Cushing’s syndrome share common clinical features , but the plasma cortisol levels are not typically elevated in metabolic syndrome [7]. The disorder is rather caused by inappropriate activity of a downstream signaling element. To the extend that SGK1 mediates effects of glucocorticoids on blood pressure and body weight, a SGK1 gene variant leading to increased SGK1 activity would trigger glucocorticoid actions without enhanced plasma glucocorticoid concentrations. Metabolic syndrome is further typically associated with enhanced coagulation [7], which is again stimulated by SGK1 [13]. Taken together, compelling evidence suggests a participation of SGK1 in the pathophysiology of metabolic syndrome.

Conclusions

The serum- and glucocorticoid-inducible kinase SGK1 is a powerful regulator of metabolism, transport, transcription and enzyme activity and thus participates in the regulation of a wide variety of physiological functions including epithelial transport, excitability, cell proliferation and apoptosis. The phenotype of SGK1 knockout mice is mild and obviously, the function of this kinase is involved in but not critically important for the maintenance of house keeping functions. However, following appropriate challenges the lack of SGK1 becomes obvious and physiologically or pathophysiologically relevant. SGK1 plays a particularly important role in electrolyte balance, extracellular volume regulation, metabolic syndrome, tumor growth, inflammation, and fibrosing disease.

Fig. 1. Renal function in SGK1 deficient mice.

SGK1 deficiency decreases NCC and ENaC activity and blunts the stimulation of renal Na⁺ reabsorption by insulin and aldosterone as well as the stimulation of salt appetite by mineralocorticoids. The extracellular fluid volume (ECV) contraction decreases cardiac output (C.O.) thus compromising maintenance of mean arterial pressure (MAP) with subsequent stimulation of the renin-angiotensin and aldosterone system. The hyperaldosteronism at least partially reverses the decrease of ENaC, ROMK and Na⁺/K⁺ ATPase activity in principle cells. Nevertheless, SGK1 deficiency impairs stimulation of renal K⁺ excretion during K⁺ excess. Despite its stimulation of the distal tubular Ca²⁺ channel TRPV5, lack of SGK1 leads to anticalciuria due compensatory stimulation of Na⁺ (and Ca²⁺) reabsorption in proximal tubules. (A II = angiotensin II; R= total peripheral vascular resistance).