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

The role of serum/glucocorticoid-regulated kinase 1 in brain function following cerebral ischemia

Cardiopulmonary arrest (CA) is a major cause of death/disability in the U.S. with poor prognosis and survival rates. Current therapeutic challenges are physiologically complex because they involve hypoperfusion (decreased cerebral blood flow), neuroinflammation, and mitochondrial dysfunction. We previously discovered novel serum/glucocorticoid-regulated kinase 1 (SGK1) is highly expressed in brain of neurons that are susceptible to ischemia (hippocampus and cortex). We inhibited SGK1 and utilized pharmacological (specific inhibitor, GSK650394) and neuron-specific genetic approaches (shRNA) in rodent models of CA to determine if SGK1 is responsible for hypoperfusion, neuroinflammation, mitochondrial dysfunctional, and neurological deficits after CA. Inhibition of SGK1 alleviated cortical hypoperfusion and neuroinflammation (via Iba1, GFAP, and cytokine array). Treatment with GSK650394 enhanced mitochondrial function (via Seahorse respirometry) in the hippocampus 3 and 7 days after CA. Neuronal injury (via MAP2, dMBP, and Golgi staining) in the hippocampus and cortex was observed 7 days after CA but ameliorated with SGK1-shRNA. Moreover, SGK1 mediated neuronal injury by regulating the Ndrg1-SOX10 axis. Finally, animals subjected to CA exhibited learning/memory, motor, and anxiety deficits after CA, whereas SGK1 inhibition via SGK1-shRNA improved neurocognitive function. The present study suggests the fundamental roles of SGK1 in brain circulation and neuronal survival/death in cerebral ischemia-related diseases.

Serum Glucocorticoid-Regulated Kinase-1 in Ischemia-Reperfusion Injury: Blessing or Curse

The family of serum-glucocorticoid-regulated kinase (SGK) consists of three paralogs, SGK-1, SGK-2, and SGK-3, with SGK-1 being the better studied. Indeed, recognition of the role of SGK-1 in regulation of cell survival and proliferation has led to introduction of a number of small-molecule inhibitors for some types of cancer. In addition, SGK-1 regulates major physiologic effects, such as renal solute transport, and contributes to the pathogenesis of non-neoplastic conditions involving major organs including the heart and the kidney. These observations raise the prospect for therapeutic modulation of SGK-1 to reduce the burden of such diseases as myocardial infarction and acute kidney injury. Following a brief description of the structure and function of SGK family of proteins, the present review is primarily focused on our current understanding of the role of SGK-1 in pathologies related to ischemia-reperfusion injury involving several organs (e.g., heart, kidney). The essential role of the mitochondrial permeability transition pore in cell death coupled with the pro-survival function of SGK-1 raise the prospect that its therapeutic modulation could beneficially impact conditions associated with ischemia-reperfusion injury. SIGNIFICANCE STATEMENT: Since the discovery of serum glucocorticoid-regulated kinase (SGK)-1, extensive research has unraveled its role in cancer biology and, thus, its therapeutic targeting. Increasingly, it is also becoming clear that SGK-1 is a major determinant of the outcome of ischemia-reperfusion injury to various organs. Thus, evaluation of existing information should help identify gaps in our current knowledge and also determine whether and how its therapeutic modulation could impact the outcome of ischemia-reperfusion injury.

Dexamethasone upregulates mitochondrial Tom20, Tom70, and MnSOD through SGK1 in the kidney cells

Dexamethasone augments mitochondrial protein abundance. The translocase of the outer membrane (Tom) of mitochondria plays a major role in importing largely cytosolically synthesized proteins into mitochondria. We hypothesize that dexamethasone upregulates the Tom transport system, leading to increase of mitochondrial protein localization. Tom20 and Tom70 are the two major subunits. Dexamethasone increased Tom20 and Tom70 mRNA levels by 53 ± 11% and 25 ± 9% and mitochondrial protein abundance by 27 ± 7% and 25 ± 4% (p < 0.05 for all), respectively, in HEK293 cells. In parallel, dexamethasone elevated the SGK1 mRNA by 79 ± 17% and activity by 190 ± 42%, and mitochondrial protein level by 41 ± 2% (all p < 0.05) without significantly affecting the cytosol counterpart. The discovery of the effect of dexamethasone on SGK1 protein restricted in the mitochondria attracted us to examine the effect of the hormone on MnSOD, an enzyme with known mitochondrial localization and function. Similarly, dexamethasone significantly increased MnSOD transcripts by 67 ± 15% and protein level only in the mitochondria dose-dependently. Inhibition of SGK1 by GSK650394 and RNAi significantly attenuated the effects of the hormone on Tom20, Tom70, and MnSOD, indicating that SGK1 relays the effects of dexamethasone. Catalase inhibited the effects of dexamethasone on SGK1 and the subsequent effects of SGK1 on Tom20, Tom70, and MnSOD. Finally, knock-down of Tom20 and Tom70 by their siRNAs reduced dexamethasone-induced increases in the mitochondrial localization of SGK1 and MnSOD proteins. In conclusion, dexamethasone upregulates Tom20, Tom70, and MnSOD, and these effects are dependent on reactive oxygen species and SGK1. Dexamethasone-induced increases of SGK1 and MnSOD mitochondrial localization requires Tom20 and Tom70.

Serum- and glucocorticoid-induced kinase 1, a new therapeutic target for autophagy modulation in chronic diseases

Introduction: Autophagy, a basic cellular degradation pathway essential for survival, is altered both in aging and in many chronic human diseases, including infections, cancer, heart disease, and neurodegeneration. Identifying new therapeutic targets for the control and modulation of autophagy events is therefore of utmost importance in drug discovery. Serum and glucocorticoid activated kinase 1 (SGK1), known for decades for its role in ion channel modulation, is now known to act as a switch for autophagy homeostasis, and has emerged as a novel and important therapeutic target likely to attract considerable research attention in the coming years.Areas covered: In this general review of SGK1 we describe the kinase's structure and its roles in physiological and pathological contexts. We also discuss small-molecule modulators of SGK1 activity. These modulators are of particular interest to medicinal chemists and pharmacists seeking to develop more potent and selective drug candidates for SGK1, which, despite its key role in autophagy, remains relatively understudied.Expert opinion: The main future challenges in this area are (i) deciphering the role of SGK1 in selective autophagy processes (e.g. mitophagy, lipophagy, and aggrephagy); (ii) identifying selective allosteric modulators of SGK1 with specific biological functions; and (iii) conducting first-in-man clinical studies.

EAE-induced upregulation of mitochondrial MnSOD is associated with increases of mitochondrial SGK1 and Tom20 protein in the mouse kidney cortex

Our previous demonstration that severe experimental autoimmune encephalomyelitis (EAE) increases MnSOD protein abundance in the mouse kidney cortex led this study to elucidate the underlying mechanism with monensin-treated HEK293 cells as a model. Severe EAE increases mitochondrial protein abundance of SGK1 kinase and Tom20, a critical subunit of mitochondrial translocase in the renal cortex. In HEK293 cells, catalase inhibits monensin-induced increases of mitochondrial SGK1 and Tom20 protein levels. Further, GSK650394, a specific inhibitor of SGK1 reduces monensin-induced increase of mitochondrial protein abundance of Tom20 and MnSOD. Finally, RNAi of Tom20 reduces the effect of monensin on MnSOD. MnSOD and Tom20 physically associate with each other. In conclusion, in HEK293 cells, mitochondrial reactive oxygen species increase protein abundance of mitochondrial SGK1, which leads to a rise of mitochondrial Tom20, resulting in importing MnSOD protein into the mitochondria. This could be a mechanism by which severe EAE up-regulates mitochondrial MnSOD in the kidney cortex.

Mitochondrial Permeability Uncouples Elevated Autophagy and Lifespan Extension

Autophagy is required in diverse paradigms of lifespan extension, leading to the prevailing notion that autophagy is beneficial for longevity. However, why autophagy is harmful in certain contexts remains unexplained. Here, we show that mitochondrial permeability defines the impact of autophagy on aging. Elevated autophagy unexpectedly shortens lifespan in C. elegans lacking serum/glucocorticoid regulated kinase-1 (sgk-1) because of increased mitochondrial permeability. In sgk-1 mutants, reducing levels of autophagy or mitochondrial permeability transition pore (mPTP) opening restores normal lifespan. Remarkably, low mitochondrial permeability is required across all paradigms examined of autophagy-dependent lifespan extension. Genetically induced mPTP opening blocks autophagy-dependent lifespan extension resulting from caloric restriction or loss of germline stem cells. Mitochondrial permeability similarly transforms autophagy into a destructive force in mammals, as liver-specific Sgk knockout mice demonstrate marked enhancement of hepatocyte autophagy, mPTP opening, and death with ischemia/reperfusion injury. Targeting mitochondrial permeability may maximize benefits of autophagy in aging.

A novel mutation of sgk-1 gene in central serous chorioretinopathy

AIM

To investigate the association of serum glucocorticoid kinase gene-1 (SGK-1) DNA variants with chronic central serous chorioretinopathy (CSC).

METHODS

We enrolled 32 eyes of 32 patients who were diagnosed with chronic CSC and composed 32 normal eyes as a control group. Peripheral blood was used for DNA extraction and polymerase chain reaction (PCR) amplification. SGK1 gene was sequenced by using BigDye(®) Terminator v3.1 cycle sequencing KIT (Applied Biosystems, Foster City, CA, USA). The SGK1 gene and its variants were investigated in CSC patient group and control group.

RESULTS

We identified a new polymorphism M32V in two person in the patient group (Minor allele frequency (MAF)=0.009) on the region of 1-60 amino acids. The rs1057293 was located in the encoder region of the SGK 1 gene but not associated with CSC (P=0.68). An intrinsic rs1743966 is also not associated (P=0.28).

CONCLUSIONS

The new polymorphism M32V is located on the region of 1-60 amino acids which is necessary for localization to the mitochondria in CSC patient. This mutation is probably important for the energy metabolism and plays an important role in the cellular response to hyperosmotic stress and other stress stimuli. Both rs1057293 and rs1743966 are not associated with CSC.

Where is mTOR and what is it doing there?

Target of rapamycin (TOR) forms two conserved, structurally distinct kinase complexes termed TOR complex 1 (TORC1) and TORC2. Each complex phosphorylates a different set of substrates to regulate cell growth. In mammals, mTOR is stimulated by nutrients and growth factors and inhibited by stress to ensure that cells grow only during favorable conditions. Studies in different organisms have reported localization of TOR to several distinct subcellular compartments. Notably, the finding that mTORC1 is localized to the lysosome has significantly enhanced our understanding of mTORC1 regulation. Subcellular localization may be a general principle used by TOR to enact precise spatial and temporal control of cell growth.

Cholera toxin enhances Na(+) absorption across MCF10A human mammary epithelia

Cellular mechanisms to account for the low Na(+) concentration in human milk are poorly defined. MCF10A cells, which were derived from human mammary epithelium and grown on permeable supports, exhibit amiloride- and benzamil-sensitive short-circuit current (Isc; a sensitive indicator of net ion transport), suggesting activity of the epithelial Na(+) channel ENaC. When cultured in the presence of cholera toxin (Ctx), MCF10A cells exhibit greater amiloride-sensitive Isc at all time points tested (2 h to 7 days), an effect that is not reduced with Ctx washout for 12 h. Amiloride-sensitive Isc remains elevated by Ctx in the presence of inhibitors for PKA (H-89, Rp-cAMP), PI3K (LY294002), and protein trafficking (brefeldin A). Additionally, the Ctx B subunit, alone, does not replicate these effects. RT-PCR and Western blot analyses indicate no significant increase in either the mRNA or protein expression for α-, β-, or, γ-ENaC subunits. Ctx increases the abundance of both β- and γ-ENaC in the apical membrane. Additionally, Ctx increases both phosphorylated and nonphosphorylated Nedd4-2 expression. These results demonstrate that human mammary epithelia express ENaC, which can account for the low Na(+) concentration in milk. Importantly, the results suggest that Ctx increases the expression but reduces the activity of the E3 ubiquitin ligase Nedd4-2, which would tend to reduce the ENaC retrieval and increase steady-state membrane residency. The results reveal a novel mechanism in human mammary gland epithelia by which Ctx regulates ENaC-mediated Na(+) transport, which may have inferences for epithelial ion transport regulation in other tissues throughout the body.

The serum- and glucocorticoid-induced protein kinase-1 (Sgk-1) mitochondria connection: identification of the IF-1 inhibitor of the F(1)F(0)-ATPase as a mitochondria-specific binding target and the stress-induced mitochondrial localization of endogenous Sgk-1

The expression, localization and activity of the serum- and glucocorticoid-induced protein kinase, Sgk-1, are regulated by multiple hormonal and environmental cues including cellular stress. Biochemical fractionation and indirect immunofluorescence demonstrated that sorbitol induced hyperosmotic stress stimulated expression and triggered the localization of endogenous Sgk-1 into the mitochondria of NMuMG mammary epithelial cells. The immunofluorescence pattern of endogenous Sgk-1 was similar to that of a green fluorescent linked fusion protein linked to the N-terminal Sgk-1 fragment that encodes the mitochondrial targeting signal. In the presence or absence of cellular stress, exogenously expressed wild type Sgk-1 efficiently compartmentalized into the mitochondria demonstrating the mitochondrial import machinery per se is not stressed regulated. Co-immunoprecipitation and GST-pull down assays identified the IF-1 mitochondrial matrix inhibitor of the F1F0-ATPase as a new Sgk-1 binding partner, which represents the first observed mitochondrial target of Sgk-1. The Sgk-1/IF-1 interaction requires the 122-176 amino acid region within the catalytic domain of Sgk-1 and is pH dependent, occurring at neutral pH but not at slightly acidic pH, which suggests that this interaction is dependent on mitochondrial integrity. An in vitro protein kinase assay showed that the F1F0-ATPase can be directly phosphorylated by Sgk-1. Taken together, our results suggest that stress-induced Sgk-1 localizes to the mitochondria, which permits access to physiologically appropriate mitochondrial interacting proteins and substrates, such as IF-1 and the F1F0-ATPase, as part of the cellular stressed induced program.

SGK regulation of renal sodium transport

PURPOSE OF REVIEW

The serum and glucocorticoid regulated kinase (SGK) family of protein kinases shares similar biochemical and hormonal signaling properties; however, the SGK kinases also exhibit distinct differences in regulating renal sodium (Na(+)) transport. This review will highlight recent advances in our understanding of the specificity of SGK kinase signaling and regulation of renal Na(+) transport.

RECENT FINDINGS

Differential expression of SGK kinases at the cellular and subcellular levels contributes to signaling specificity. New evidence indicates that SGK1 associates with the apical cell membrane of cortical collecting duct cells to regulate open probability of the epithelial Na(+) channel (ENaC). Scaffold proteins can also recruit SGK1 to multiprotein complexes for regulation of ENaC expression in the apical membrane. Recent SGK1 knockout models have implicated the NaCl co-transporter (NCC) as another target of SGK1 regulation. Less is known about the function of SGK2 or SGK3, but both kinases can regulate Na(+)/H(+) exchanger 3 (NHE3) activity.

SUMMARY

The SGK kinases assume distinct roles in regulating Na transport in both proximal and distal elements of the kidney tubule. Future examination of the molecular mechanisms by which the SGK kinases regulate specific substrates will inform our understanding of how these kinases contribute to the physiology of renal Na(+) transport.

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

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

Targeting SGK1 in diabetes

Compelling evidence is accumulating indicating a pathophysiological role of the serum-and-glucocorticoid-inducible-kinase-1 (SGK1) in the development and complications of diabetes. SGK1 is ubiquitously expressed with exquisitely high transcriptional volatility. Stimulators of SGK1 expression include hyperglycemia, cell shrinkage, ischemia, glucocorticoids and mineralocorticoids. SGK1 is activated by insulin and growth factors via PI3K, 3-phosphoinositide dependent kinase PDK1 and mTOR. SGK1 activates ion channels (including ENaC, TRPV5, ROMK, KCNE1/KCNQ1 and CLCKa/Barttin), carriers (including NCC, NKCC, NHE3, SGLT1 and EAAT3), and the Na(+)/K(+)-ATPase. It regulates the activity of several enzymes (e.g., glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g., forkhead-transcription-factor FOXO3a, beta-catenin and NF-kappaB). A common SGK1 gene variant ( approximately 3 - 5% prevalence in Caucasians, approximately 10% in Africans) is associated with increased blood pressure, obesity and type 2 diabetes. In patients suffering from type 2 diabetes, SGK1 presumably contributes to fluid retention and hypertension, enhanced coagulation and increased deposition of matrix proteins leading to tissue fibrosis such as diabetic nephropathy. Accordingly, targeting SGK1 may favourably influence occurrence and course of type 2 diabetes.

The physiological impact of the serum and glucocorticoid-inducible kinase SGK1

PURPOSE OF REVIEW

The role of serum and glucocorticoid-inducible kinase 1 (SGK1) in renal physiology and pathophysiology is reviewed with particular emphasis on recent advances.

RECENT FINDINGS

The mammalian target of rapamycin complex 2 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 programing 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.

The contribution of epithelial sodium channels to alveolar function in health and disease

Amiloride-sensitive epithelial sodium channels (ENaC) play an important role in lung sodium transport. Sodium transport is closely regulated to maintain an appropriate fluid layer on the alveolar surface. Both alveolar type I and II cells have several different sodium-permeable channels in their apical membranes that play a role in normal lung physiology and pathophysiology. In many epithelial tissues, ENaC is formed from three subunit proteins: alpha, beta, and gamma ENaC. Part of the diversity of sodium-permeable channels in lung arises from assembling different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. Thus, lung epithelium has enormous flexibility to alter the magnitude of salt and water transport. In lung, ENaC is regulated by many transmitter and hormonal agents. Regulation depends upon the type of sodium channel but involves controlling the number of apical channels and/or the activity of individual channels.

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

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