Translational regulation of the serum- and glucocorticoid-inducible kinase-1 (SGK1) in platelets

GPR21 is an orphan G-protein-coupled receptor. We found that mice deficient for the GPR21 gene were resistant to diet-induced obesity. Knockout mice were leaner than their wildtype counterpart, despite that no difference was observed in food intake. No differences were observed in the respiratory exchange rate and thermogenesis. However, knockout mice were more active than wildtype littermates, and this level of activity may be an underlying reason for the difference in energy balance. Mutant mice were more sensitive to insulin than their wildtype control and showed an improved glucose tolerance. Several inflammatory markers MCP-1, CRP and IP-10 were decreased in mutant animals, suggesting that GPR21 may also mediate its effect through anti-inflammatory mechanisms. We found that GPR21 is widely expressed in all tissues, with the highest levels found in the brain and in the spleen. Overall, these findings suggest that GPR21 may play an important role in regulating body weight and glucose metabolism.     

Subcellular location of serum- and glucocorticoid-induced kinase-1 in renal and mammary epithelial cells

Serum- and glucocorticoid-induced kinase-1 (SGK1) is involved in aldosterone-induced Na⁺ reabsorption by increasing epithelial Na⁺ channel (ENaC) activity in cortical collecting duct (CCD) cells, but its exact mechanisms of action are unknown. Although several potential targets such as Nedd4-2 have been described in expression systems, endogenous substrates mediating SGK1's physiological effects remain to be identified. In addition, subcellular localization studies of SGK1 have provided controversial results. We determined the subcellular location of SGK1 using SGK1-autofluorescent protein (AFP) fusion proteins. Rabbit CCD (RCCT-28A) cells were transiently transfected with a construct encoding for SGK1-AFP and were stained or cotransfected with markers for various subcellular compartments. In live cells, transiently expressed SGK1-AFP clearly colocalized with the mitochondrial marker rhodamine 123. Similarly, SGK1-AFP colocalized with the mitochondrial marker MitoTracker when stably expressed using a retroviral system in either RCCT-28A cells or the mammary epithelial cell line MCF10A. To determine which region of SGK1 is responsible for this subcellular localization, we generated RCCT-28A cell lines stably expressing SGK1 mutants. The results indicate that the NH₂-terminal 60-amino acid region of SGK1 is necessary and sufficient for its subcellular localization. Localization of SGK1 to the mitochondria raises the possibility that SGK1 may play a role in regulating energy metabolism. mitochondria; localization     

mTOR complex 2 (mTORC2) controls hydrophobic motif phosphorylation and activation of serum- and glucocorticoid-induced protein kinase 1 (SGK1)

SGK1 (serum- and glucocorticoid-induced protein kinase 1) is a member of the AGC (protein kinase A/protein kinase G/protein kinase C) family of protein kinases and is activated by agonists including growth factors. SGK1 regulates diverse effects of extracellular agonists by phosphorylating regulatory proteins that control cellular processes such as ion transport and growth. Like other AGC family kinases, activation of SGK1 is triggered by phosphorylation of a threonine residue within the T-loop of the kinase domain and a serine residue lying within the C-terminal hydrophobic motif (Ser(422) in SGK1). PDK1 (phosphoinositide-dependent kinase 1) phosphorylates the T-loop of SGK1. The identity of the hydrophobic motif kinase is unclear. Recent work has established that mTORC1 [mTOR (mammalian target of rapamycin) complex 1] phosphorylates the hydrophobic motif of S6K (S6 kinase), whereas mTORC2 (mTOR complex 2) phosphorylates the hydrophobic motif of Akt (also known as protein kinase B). In the present study we demonstrate that SGK1 hydrophobic motif phosphorylation and activity is ablated in knockout fibroblasts possessing mTORC1 activity, but lacking the mTORC2 subunits rictor (rapamycin-insensitive companion of mTOR), Sin1 (stress-activated-protein-kinase-interacting protein 1) or mLST8 (mammalian lethal with SEC13 protein 8). Furthermore, phosphorylation of NDRG1 (N-myc downstream regulated gene 1), a physiological substrate of SGK1, was also abolished in rictor-, Sin1- or mLST8-deficient fibroblasts. mTORC2 immunoprecipitated from wild-type, but not from mLST8- or rictor-knockout cells, phosphorylated SGK1 at Ser(422). Consistent with mTORC1 not regulating SGK1, immunoprecipitated mTORC1 failed to phosphorylate SGK1 at Ser(422), under conditions which it phosphorylated the hydrophobic motif of S6K. Moreover, rapamycin treatment of HEK (human embryonic kidney)-293, MCF-7 or HeLa cells suppressed phosphorylation of S6K, without affecting SGK1 phosphorylation or activation. The findings of the present study indicate that mTORC2, but not mTORC1, plays a vital role in controlling the hydrophobic motif phosphorylation and activity of SGK1. Our findings may explain why in previous studies phosphorylation of substrates, such as FOXO (forkhead box O), that could be regulated by SGK, are reduced in mTORC2-deficient cells. The results of the present study indicate that NDRG1 phosphorylation represents an excellent biomarker for mTORC2 activity.     

Serum- and glucocorticoid-inducible kinase 1 (SGK1) increases neurite formation through microtubule depolymerization by SGK1 and by SGK1 phosphorylation of tau

Serum- and glucocorticoid-inducible kinase 1 (SGK1) is a member of the Ser/Thr protein kinase family that regulates a variety of cell functions. Recently, SGK1 was shown to increase dendritic growth but the mechanism underlying the increase is unknown. Here we demonstrated that SGK1 increased the neurite formation of cultured hippocampal neurons through microtubule (MT) depolymerization via two distinct mechanisms. First, SGK1 directly depolymerized MTs. In vitro MT depolymerization experiments revealed that SGK1, especially N-truncated SGK1, directly disassembled self-polymerized MTs and taxol-stabilized MTs in a dose-dependent and ATP-independent manner. The transfection of sgk1 to HeLa cells also inhibited MT assembly in vivo. Second, SGK1 indirectly depolymerized MTs through the phosphorylation of tau at Ser214. An in vitro kinase assay revealed that active SGK1 phosphorylated tau Ser214 specifically. In vivo transfection of sgk1 also phosphorylated tau Ser214 in HEK293T cells and hippocampal neurons. Further, sgk1 transfection significantly increased the number of primary neurites and shortened the length of the total process in cultured hippocampal neurons. These effects were antagonized by the cotransfection of the tauS214A mutant plasmid. Dexamethasone, a synthetic glucocorticoid, mimics the effect of sgk1 overexpression. Together, these results suggest that SGK1 enhances neurite formation through MT depolymerization by a direct action of SGK1 and by the SGK1 phosphorylation of tau.     

Regulation of human cystic fibrosis transmembrane conductance regulator (CFTR) by serum- and glucocorticoid-inducible kinase (SGK1).

BACKGROUND: Serum- and glucocorticoid-inducible kinase-1 (SGK1) increases CFTR Cl currents in Xenopus oocytes by an unknown mechanism. Because SGK increases the plasma membrane expression of other ion channels, the goal of this paper was to test the hypothesis that SGK1 stimulates CFTR Cl currents by increasing the number of CFTR Cl channels in the plasma membrane. METHODS: CFTR Cl currents were measured in Xenopus oocytes by the two-electrode voltage clamp technique, and CFTR in the plasma membrane was determined by laser scanning confocal microscopy. RESULTS: wt-SGK1 stimulated CFTR Cl currents by 42% and increased the amount of CFTR in the plasma membrane by 35%. A kinase-dead SGK mutant (K127N) had a dominant-negative effect on CFTR, reducing CFTR Cl currents by 38%. In addition, deletion of the C-terminal PDZ-interacting motif (SGK1-DeltaSFL) increased CFTR Cl currents by 108%. Thus, SGK1-DeltaSFL was more effective than wt-SGK1 in stimulating CFTR Cl currents. Neither wt-SGK nor the K127N mutant had any effect on Cl currents in oocytes when expressed alone in the absence of CFTR. CONCLUSION: SGK1 stimulates CFTR Cl currents in Xenopus oocytes by increasing the number of channels in the plasma membrane. Moreover, the effect of SGK may be mediated by protein-protein interactions involving the PDZ interacting motif.     

Ubiquitin-proteasome degradation of serum- and glucocorticoid-regulated kinase-1 (SGK-1) is mediated by the chaperone-dependent E3 ligase CHIP

SGK-1 (serum- and glucocorticoid-regulated kinase-1) is a stress-induced serine/threonine kinase that is phosphorylated and activated downstream of PI3K (phosphoinositide 3-kinase). SGK-1 plays a critical role in insulin signalling, cation transport and cell survival. SGK-1 mRNA expression is transiently induced following cellular stress, and SGK-1 protein levels are tightly regulated by rapid proteasomal degradation. In the present study we report that SGK-1 forms a complex with the stress-associated E3 ligase CHIP [C-terminus of Hsc (heat-shock cognate protein) 70-interacting protein]; CHIP is required for both the ubiquitin modification and rapid proteasomal degradation of SGK-1. We also show that CHIP co-localizes with SGK-1 at or near the endoplasmic reticulum. CHIP-mediated regulation of SGK-1 steady-state levels alters SGK-1 kinase activity. These data suggest a model that integrates CHIP function with regulation of the PI3K/SGK-1 pathway in the stress response.     

Identification of a novel serum and glucocorticoid regulated kinase-1 (SGK1) ligand from virtual screening

The serum and glucocorticoid regulated kinase-1 (SGK1) is part of the serine/threonine kinase family and has therapeutic potential in several neurodegenerative diseases such as ischemic stroke and Parkinson's disease. Here we use structure-based virtual screening to identify a novel ligand which inhibits SGK1 activity. The data presented here can be used for future scaffold hopping and possible drug development efforts.     

Expression of ENaC and serum- and glucocorticoid-induced kinase 1 in the rat intestinal epithelium

Increase in epithelium sodium channel (ENaC) activity induced by aldosterone in the distal tubule of the kidney has been attributed to serum- and glucocorticoid-induced kinase 1 (sgk1). The distal colon constitutes another classical aldosterone-responsive epithelium that expresses both ENaC and sgk1 in an aldosterone-dependent manner. However, the site of expression and the temporal relationship of the aldosterone induction of these two proteins have not been investigated. Here, we examined the distribution and abundance of sgk1 in the rat intestine under basal conditions and after changes in the concentration of aldosterone and glucocorticoids. Results indicate that sgk1 is expressed in the distal colon and also in the ileum and jejunum. Abundance of sgk1 was high in control animals, and it did not change significantly after sodium depletion or after a single dose of aldosterone; however, it decreased after adrenalectomy. In contrast, the three subunits of ENaC were markedly induced in the distal colon by acute and chronic increases in aldosterone levels. Results indicate differential regulation of sgk and ENaC subunits by aldosterone in the distal colon. Distribution of sgk1 in the intestine beyond the aldosterone-responsive segments suggests that sgk1 may additionally regulate other sodium transporters in the intestinal epithelium.     

Deregulation of the serum- and glucocorticoid-inducible kinase SGK1 in the endometrium causes reproductive failure

Infertility and recurrent pregnancy loss (RPL) are prevalent but distinct causes of reproductive failure that often remain unexplained despite extensive investigations. Analysis of midsecretory endometrial samples revealed that SGK1, a kinase involved in epithelial ion transport and cell survival, is upregulated in unexplained infertility, most prominently in the luminal epithelium, but downregulated in the endometrium of women suffering from RPL. To determine the functional importance of these observations, we first expressed a constitutively active SGK1 mutant in the luminal epithelium of the mouse uterus. This prevented expression of certain endometrial receptivity genes, perturbed uterine fluid handling and abolished embryo implantation. By contrast, implantation was unhindered in Sgk1-/- mice, but pregnancy was often complicated by bleeding at the decidual-placental interface and fetal growth retardation and subsequent demise. Compared to wild-type mice, Sgk1-/- mice had gross impairment of pregnancy-dependent induction of genes involved in oxidative stress defenses. Relative SGK1 deficiency was also a hallmark of decidualizing stromal cells from human subjects with RPL and sensitized these cells to oxidative cell death. Thus, depending on the cellular compartment, deregulated SGK1 activity in cycling endometrium interferes with embryo implantation, leading to infertility, or predisposes to pregnancy complications by rendering the feto-maternal interface vulnerable to oxidative damage.     

Inhibition of mitogen-activated kinase kinase kinase 3 activity through phosphorylation by the serum- and glucocorticoid-induced kinase 1

The mitogen-activated protein kinase kinase kinase 3 (MEKK3) is a member of the MAP kinase family whose cellular activity is elevated in response to growth factors, oxidative stress, and hyperosmolar conditions. MEKK3 regulates MKK3 and MKK5/6/7. MEKK3 is involved distinctively in the signal pathway for blocking cell proliferation and cell cycle progression, contradictory to the biological responses commonly associated with other members of MEKKs. Based information concerning the substrate specificity of serum- and glucocorticoid-induced kinase 1 (SGK1), R-x-R-x-x-(S/T)-phi, where phi indicates a hydrophobic amino acid, two putative phosphorylation sites (Ser(166) and Ser(337)) were found in MEKK3. It was shown that the recombinant MEKK3 protein and fluorescein-labeled MEKK3 peptides (FITC-(159)epRsRhlSVi(168) and FITC-(330)dpRgRlpSAd(339)) are phosphorylated by SGK1 in vitro. It was also observed that the intrinsic kinase activity of MEKK3 on Ser(189) of MKK3 (equivalent to Ser(207) of MKK6) decreased along with phosphorylation of Ser(166) and Ser(337) in MEKK3 in vitro and in vivo. Therefore, it is suggested that SGK1 inhibits MEKK3-MKK3/6 signal transduction by phosphorylation of MEKK3.     

Modulation of alpha-ENaC and alpha1-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells

cAMP and dexamethasone are known to modulate Na+ transport in epithelial cells. We investigated whether dibutyryl cAMP (DBcAMP) and dexamethasone modulate the mRNA expression of two key elements of the Na+ transport system in isolated rat alveolar epithelial cells: alpha-, beta-, and gamma-subunits of the epithelial Na+ channel (ENaC) and the alpha1- and beta1-subunits of Na+-K+-ATPase. The cells were treated for up to 48 h with DBcAMP or dexamethasone to assess their long-term impact on the steady-state level of ENaC and Na+-K+-ATPase mRNA. DBcAMP induced a twofold transient increase of alpha-ENaC and alpha1-Na+-K+-ATPase mRNA that peaked after 8 h of treatment. It also upregulated beta- and gamma-ENaC mRNA but not beta1-Na+-K+-ATPase mRNA. Dexamethasone augmented alpha-ENaC mRNA expression 4.4-fold in cells treated for 24 h and also upregulated beta- and gamma-ENaC mRNA. There was a 1.6-fold increase at 8 h of beta1-Na+-K+-ATPase mRNA but no significant modulation of alpha1-Na+-K+-ATPase mRNA expression. Because DBcAMP and dexamethasone did not increase the stability of alpha-ENaC mRNA, we cloned 3.2 kb of the 5' sequences flanking the mouse alpha-ENaC gene to study the impact of DBcAMP and dexamethasone on alpha-ENaC promoter activity. The promoter was able to drive basal expression of the chloramphenicol acetyltransferase (CAT) reporter gene in A549 cells. Dexamethasone increased the activity of the promoter by a factor of 5.9. To complete the study, the physiological effects of DBcAMP and dexamethasone were investigated by measuring transepithelial current in treated and control cells. DBcAMP and dexamethasone modulated transepithelial current with a time course reminiscent of the profile observed for alpha-ENaC mRNA expression. DBcAMP had a greater impact on transepithelial current (2.5-fold increase at 8 h) than dexamethasone (1.8-fold increase at 24 h). These results suggest that modulation of alpha-ENaC and Na+-K+-ATPase gene expression is one of the mechanisms that regulates Na+ transport in alveolar epithelial cells.     

Up-regulation of serum- and glucocorticoid-induced protein kinase 1 in the brain tissue of human and experimental epilepsy

Several studies have shown that serum- and glucocorticoid-induced protein kinase 1(SGK1) can regulate both glutamate receptors and glutamate transporters and may participate in the regulation of neuroexcitability in neuronal diseases. In our previous study, we analyzed differential gene expression in the anterior temporal neocortex of drug-refractory epilepsy patients relative to control patients using a complementary DNA microarray and found that the SGK1 gene was up-regulated more than twofold in the brain tissues of epileptic patients. In the current study, we measured SGK1 expression in the brain tissues of humans and in an experimental model of rat epilepsy in order to explore the relationship between SGK1 expression and epilepsy. The SGK1 expression was detected in thirty human brain tissues derived from patients undergoing operation for drug-refractory epilepsy and was also detected in eight samples from autopsies. Meanwhile, we investigated SGK1 expression during the epileptic process in rats using immunofluorescence, RT-PCR and western blot analysis. SGK1 expression was enhanced in the temporal neocortex of patients with drug-refractory epilepsy and was also highly expressed in the rat brain during different phases of the epileptic process. SGK1 expression was also related with the elevation of EAAT3, which expression reduced after knockdown SGK1. These results provide new insight into the potential role of SGK1 in the pathophysiology of epilepsy.