Activation of SGK1 by HGF, Rac1 and integrin-mediated cell adhesion in MDCK cells: PI-3K-dependent and -independent pathways

Dysregulated miRNAs in recurrent miscarriage: A systematic review

Recurrent miscarriage (RM) is a complex reproductive medicine disease that affects many families. The cause of RM is unclear at this time; however, lifestyle and genetic variables may influence the process. The slight alteration in miRNA expression has enormous consequences for a variety of difficulties, one of which may be RM. The target of this systematic study was to provide a framework of the dysregulated miRNAs in RM. The Prisma guidelines were applied to perform current systematic review pertaining to articles in the seven databases. Thirty-nine papers out of 245 received fulfilled all inclusion requirements. From all the mentioned miRNAs, 40 were up-regulated (65.57 %), whereas 21 were down-regulated (34.43 %). These dysregulated miRNAs contributed to the pathophysiology of RM by influencing key pathways and processes such as apoptosis, angiogenesis, epithelial-mesenchymal transition, and the immune system. Understanding the dysregulation of miRNAs, as well as the pathways and processes that engage these miRNAs and impact disease pathogenesis, may aid in clarifying the unknown underlying mechanisms of RM and the development of novel molecular therapeutic targets and medical domains.

Mechanistic insights into the role of serum-glucocorticoid kinase 1 in diabetic nephropathy: A systematic review

Aberrant expression of serum-glucocorticoid kinase 1 (SGK1) contributes to the pathogenesis of multiple disorders, including diabetes, hypertension, obesity, fibrosis, and metabolic syndrome. SGK1 variant is expressed in the presence of insulin and several growth factors, eventually modulating various ion channels, carrier proteins, and transcription factors. SGK1 also regulates the enzymatic activity of Na⁺ K⁺ ATPase, glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, and phosphomannose mutase impacting cell cycle regulation, neuroexcitation, and apoptosis. Ample evidence supports the crucial role of aberrant SGK1 expression in hyperglycemia-mediated secondary organ damage. Diabetic nephropathy (DN), a dreadful microvascular complication of diabetes, is the leading cause of end-stage renal failures with high morbidity and mortality rate. The complex pathogenesis of DN encompasses several influencing factors, including transcriptional factors, inflammatory markers, cytokines, epigenetic modulators, and abnormal enzymatic activities. SGK1 plays a pivotal role by controlling various physiological functions associated with the occurrence and progression of DN; therefore, targeting SGK1 may favorably influence the clinical outcome in patients with DN. This review aimed to provide mechanistic insights into SGK1 regulated DN pathogenesis and summarize the evidence supporting the therapeutic potential of SGK1 inhibition and its consequences on human health.

Knockdown of the long non-coding RNA HOTTIP inhibits colorectal cancer cell proliferation and migration and induces apoptosis by targeting SGK1

More and more long non-coding RNA (lncRNA) might be serve as molecular biomarkers for tumor cell progression. HOTTIP has been recently revealed as oncogenic regulator in several cancers. However, it remains unclear about whether and how HOTTIP regulates Colorectal cancer (CRC). In the present study, we assayed the expression of HOTTIP in CRC tissues and cell lines, and detected CRC cells (HCT-116 and SW620) proliferation, migration, and apoptosis when HOTTIP was knocked down. Furthermore, we discovered the underlying mechanism. The level of HOTTIP was higher in CRC tissues and in CRC cells compared with adjacent normal tissues and normal colon tissue cell. Knockdown of HOTTIP inhibited the cell proliferation migration and induced apoptosis in HCT-116 and SW620 cell lines. In addition, luciferase reporter assay suggested that knockdown of HOTTIP could target decreasing the expression of Serum- and glucocorticoid-inducible kinase 1 (SGK1) gene, and we subsequently verified that up-regulation of the SGK1 gene promoted cell proliferation and migration and inhibited cell apoptosis in HCT-116 and SW620 cell lines. Furthermore, Knockdown of HOTTIP significantly suppressed the expression of GSK3β, β-catenin, c-myc, Vimentin and MMP-7, and increased the expression of E-cadherin, FoxO3a, p27 and Bim proteins in HCT-116 and SW620 cell lines, and up-regulation of the SGK1 emerged the opposite effect with knockdown of HOTTIP. The data described in this study suggest that HOTTIP may be an oncogene and a potential target in CRC.

HGF induces the serine‑phosphorylation and cell surface translocation of ROMK (Kir 1.1) channels in rat kidney cells

Extracellular potassium homeostasis is dependent on the activity of potassium channels, which are expressed on the apical membrane of epithelial tubular cells. The renal outer medullary potassium channel (ROMK) is considered to be the major route for potassium transport into the tubule lumen. Hepatocyte growth factor (HGF) exerts multiple biological activities and is important for maintaining renal homeostasis. It is also anti‑apoptotic and mitogenic for protection and recovery from ARF. Whether HGF regulates the ion channel activities remains to be elucidated, therefore, the present study aimed to investigate the modulation of HGF on the expression of ROMK in cultured renal tubular cells. NRK‑52E cells were treated with recombinant HGF, however, no alterations in the total expression of ROMK were observed by western blot analysis. In examining the serine 44 phosphorylation of ROMK in NRK‑52E cells, the present study observed that HGF enhanced the serine 44 phosphorylation of ROMK. In addition, to investigate whether HGF‑Met signaling induces the movement of ROMK to the cell surface in NRK‑52E cells, the protein constituents of cells were separated into plasma membrane and cytoplasm. Using immunofluorescence assay, the expression of ROMK on the plasma membrane was increased in the HGF‑treated NRK‑52E cells, which suggested that ROMK was translocated to the plasma membrane following the HGF‑induced phosphorylation of serine 44. Therefore, HGF may be important in potassium excretion and perform antihyperkalemic effects through the translocation of potassium channels.

Novel mechanism of miRNA-365-regulated trophoblast apoptosis in recurrent miscarriage

Clinical pregnancies increasingly end in recurrent miscarriage (RM) during the first trimester, with genetic factors shouldering the main responsibility. MicroRNAs (miRNAs) regulate gene expression in a wide array of important biological processes. We examined the potential role of dysregulated miRNAs in RM pathogenesis and trophoblast development as an approach to elucidate the molecular mechanism behind RM. miRNA profiles from clinical specimens of RM and induced abortion (IA) were compared, and several miRNAs were found to be aberrantly expressed in RM samples. Among the miRNAs, miR-365 was significantly differentially expressed in RM decidual tissues. Furthermore, our results demonstrate that miR-365 functions as an upstream regulator of MDM2/p53 expression, cell cycle progression and apoptosis in trophoblasts. Bioinformatic prediction and experimental validation assays identified SGK1 as a direct target of miR-365; consistently, its protein levels were low in decidual tissues. Additionally, functional studies revealed that SGK1 silencing elicits cell cycle arrest and apoptosis in trophoblasts and that SGK1 overexpression attenuates the effects of miR-365 on apoptosis and MDM2/p53 expression. Collectively, our data provide evidence that the up-regulation of miR-365 may contribute to RM by decreasing SGK1 expression, which suggests its potential utility as a prognostic biomarker and therapeutic target for RM.

Akt3 kinase suppresses pinocytosis of low-density lipoprotein by macrophages via a novel WNK/SGK1/Cdc42 protein pathway

Fluid-phase pinocytosis of LDL by macrophages is regarded as a novel promising target to reduce macrophage cholesterol accumulation in atherosclerotic lesions. The mechanisms of regulation of fluid-phase pinocytosis in macrophages and, specifically, the role of Akt kinases are poorly understood. We have found previously that increased lipoprotein uptake via the receptor-independent process in Akt3 kinase-deficient macrophages contributes to increased atherosclerosis in Akt3^-/- mice. The mechanism by which Akt3 deficiency promotes lipoprotein uptake in macrophages is unknown. We now report that Akt3 constitutively suppresses macropinocytosis in macrophages through a novel WNK1/SGK1/Cdc42 pathway. Mechanistic studies have demonstrated that the lack of Akt3 expression in murine and human macrophages results in increased expression of with-no-lysine kinase 1 (WNK1), which, in turn, leads to increased activity of serum and glucocorticoid-inducible kinase 1 (SGK1). SGK1 promotes expression of the Rho family GTPase Cdc42, a positive regulator of actin assembly, cell polarization, and pinocytosis. Individual suppression of WNK1 expression, SGK1, or Cdc42 activity in Akt3-deficient macrophages rescued the phenotype. These results demonstrate that Akt3 is a specific negative regulator of macropinocytosis in macrophages.

Hepatic serum- and glucocorticoid-regulated protein kinase 1 (SGK1) regulates insulin sensitivity in mice via extracellular-signal-regulated kinase 1/2 (ERK1/2)

Insulin resistance is a major hallmark of metabolic syndromes, including Type 2 diabetes. Although numerous functions of SGK1 (serum- and glucocorticoid-regulated kinase 1) have been identified, a direct effect of SGK1 on insulin sensitivity has not been previously reported. In the present study, we generated liver-specific SGK1-knockout mice and found that these mice developed glucose intolerance and insulin resistance. We also found that insulin signalling is enhanced or impaired in Hep1-6 cells infected with adenoviruses expressing SGK1 (Ad-SGK1) or shRNA directed against the coding region of SGK1 (Ad-shSGK1) respectively. In addition, we determined that SGK1 inhibits ERK1/2 (extracellular-signal-regulated kinase 1/2) activity in liver and Ad-shERK1/2-mediated inhibition of ERK1/2 reverses the attenuated insulin sensitivity in Ad-shSGK1 mice. Finally, we found that SGK1 functions are compromised under insulin-resistant conditions and overexpression of SGK1 by Ad-SGK1 significantly ameliorates insulin resistance in both glucosamine-treated HepG2 cells and livers of db/db mice, a genetic model of insulin resistance.

Isoform 5 of PIPKIγ regulates the endosomal trafficking and degradation of E-cadherin

Phosphatidylinositol phosphate kinases (PIPKs) have distinct cellular targeting, allowing for site-specific synthesis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] to activate specific signaling cascades required for cellular processes. Several C-terminal splice variants of PIPKIγ (also known as PIP5K1C) exist, and have been implicated in a multitude of cellular roles. PI(4,5)P2 serves as a fundamental regulator of E-cadherin transport, and PI(4,5)P2-generating enzymes are important signaling relays in these pathways. We present evidence that the isoform 5 splice variant of PIPKIγ (PIPKIγi5) associates with E-cadherin and promotes its lysosomal degradation. Additionally, we show that the endosomal trafficking proteins SNX5 and SNX6 associate with PIPKIγi5 and inhibit PIPKIγi5-mediated E-cadherin degradation. Following HGF stimulation, activated Src directly phosphorylates PIPKIγi5. Phosphorylation of the PIPKIγi5 C-terminus regulates its association with SNX5 and, consequently, E-cadherin degradation. Additionally, this PIPKIγi5-mediated pathway requires Rab7 to promote degradation of internalized E-cadherin. Taken together, the data indicate that PIPKIγi5 and SNX5 are crucial regulators of E-cadherin sorting and degradation. PIPKIγi5, SNX and phosphoinositide regulation of lysosomal sorting represent a novel area of PI(4,5)P2 signaling and research. PIPKIγi5 regulation of E-cadherin sorting for degradation might have broad implications in development and tissue maintenance, and enhanced PIPKIγi5 function might have pathogenic consequences due to downregulation of E-cadherin.

Activation of serum/glucocorticoid-induced kinase 1 (SGK1) underlies increased glycogen levels, mTOR activation, and autophagy defects in Lafora disease

Lafora disease (LD), a fatal genetic form of myoclonic epilepsy, is characterized by abnormally high levels of cellular glycogen and its accumulation as Lafora bodies in affected tissues. Therefore the two defective proteins in LD-laforin phosphatase and malin ubiquitin ligase-are believed to be involved in glycogen metabolism. We earlier demonstrated that laforin and malin negatively regulate cellular glucose uptake by preventing plasma membrane targeting of glucose transporters. We show here that loss of laforin results in activation of serum/glucocorticoid-induced kinase 1 (SGK1) in cellular and animals models and that inhibition of SGK1 in laforin-deficient cells reduces the level of plasma membrane-bound glucose transporter, glucose uptake, and the consequent glycogen accumulation. We also provide evidence to suggest that mammalian target of rapamycin (mTOR) activates SGK1 kinase in laforin-deficient cells. The mTOR activation appears to be a glucose-dependent event, and overexpression of dominant-negative SGK1 suppresses mTOR activation, suggesting the existence of a feedforward loop between SGK1 and mTOR. Our findings indicate that inhibition of SGK1 activity could be an effective therapeutic approach to suppress glycogen accumulation, inhibit mTOR activity, and rescue autophagy defects in LD.

High-glucose-based peritoneal dialysis solution induces the upregulation of VEGF expression in human peritoneal mesothelial cells: The role of pleiotrophin

The aim of the present study was to investigate the effect of a high-glucose-based peritoneal dialysis solution (HGPDS) on the expression of pleiotrophin (PTN) and vascular endothelial growth factor (VEGF) in human peritoneal mesothelial cells (HPMCs) and the mechanisms through which fluvastatin (Flu) protects the peritoneal membrane in continuous ambulatory peritoneal dialysis (CAPD). HPMCs were cultured with HGPDS, Flu (10-8‑10-6 mol/l) and PTN (10‑30 nmol/l). The expression of PTN and VEGF was examined at the mRNA and protein level. To define the role of PTN in the regulation of VEGF expression, HPMCs were cultured with HGPDS in the presence or absence of the blocking peptide of PTN. The signaling pathways involved in PTN synthesis induced by HGPDS were also characterized. The phenotypic characteristics of HPMCs were observed under a light microscope. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetry and the mRNA and protein expression of PTN, VEGF and ERK1/2 was assessed by RT‑PCR and the western blot analysis, respectively. Following incubation with HGPDS for 48 h, the morphology of the HPMCs changed from a typical cobblestone‑like appearance to a fibroblast‑like phenotype. The same alteration in the morphology of the HPMCs also occurred following incubation with 20 nmol/l PTN. Flu (10-6 mol/l), GSK650394 [a competitive inhibitor of serum/glucocorticoid-regulated kinase 1 (SGK1), 10-5 mol/l] and PD98059 (a competitive inhibitor of ERK1/2, 10-5 mol/l) improved the negative changes in cell morphology induced by HGPDS. The results of MTT assay revealed that the reduction in HPMC viability occurred in the groups treated with HGPDS and this reduction was partially restored by Flu, GSK650394 and PD98059. A significant improvement in cell viability, which had been decreased by HGPDS, was observed following treatment with Flu (10-6 mol/l), PD98059 (10-5 mol/l) or GSK650394 (10-5 mol/l) (P<0.05). Compared with the control, the mRNA and protein expression of PTN and VEGF significantly increased in the HPMCs treated with HGPDS (P<0.05). GSK650394 and PD98059 significantly decreased the high mRNA and protein expression levels of PTN and VEGF induced by HGPDS (P<0.05) and Flu had the same inhibitory effect as GSK650394 and PD98059 in a dose‑dependent manner (P<0.05). The mRNA and protein expression of VEGF increased following the incubation of HPMCs with 20 nmol/l PTN. By contrast, the mRNA and protein expression levels of VEGF in the HPMCs decreased in the presence of the blocking peptide of PTN. The results from the present study indicated that HGPDS increased the expression of PTN and VEGF in the HPMCs, and this increase was attenuated by Flu, GSK650394 and PD98059. The protein expression of phosphorylated ERK1/2 (p-ERK1/2) was decreased by GSK650394 in the HPMCs treated with HGPDS. Taken together, the protective effects of Flu in HPMCs may be partially achieved through the SGK1‑ERK1/2 signaling pathway.

Induction of pathogenic TH17 cells by inducible salt-sensing kinase SGK1

Th17 cells are highly proinflammatory cells critical for clearing extracellular pathogens and for induction of multiple autoimmune diseases¹. IL-23 plays a critical role in stabilizing and reinforcing the Th17 phenotype by increasing expression of IL-23 receptor (IL-23R) and endowing Th17 cells with pathogenic effector functions^{2, 3}. However, the precise molecular mechanism by which IL-23 sustains the Th17 response and induces pathogenic effector functions has not been elucidated. Here, we used transcriptional profiling of developing Th17 cells to construct a model of their signaling network and nominate major nodes that regulate Th17 development. We identified serum glucocorticoid kinase-1 (SGK1), a serine-threonine kinase⁴, as an essential node downstream of IL-23 signaling. SGK1 is critical for regulating IL-23R expression and stabilizing the Th17 cell phenotype by deactivation of Foxo1, a direct repressor of IL-23R expression. SGK1 has been shown to govern Na⁺ transport and salt (NaCl) homeostasis in other cells^{5, 6, 7, 8}. We here show that a modest increase in salt concentration induces SGK1 expression, promotes IL-23R expression and enhances Th17 cell differentiation in vitro and in vivo, accelerating the development of autoimmunity. Loss of SGK1 abrogated Na⁺-mediated Th17 differentiation in an IL-23-dependent manner. These data demonstrate that SGK1 plays a critical role in the induction of pathogenic Th17 cells and provides a molecular insight into a mechanism by which an environmental factor such as a high salt diet triggers Th17 development and promotes tissue inflammation.

Phosphorylation of nicastrin by SGK1 leads to its degradation through lysosomal and proteasomal pathways

The gamma-secretase complex is involved in the intramembranous proteolysis of a variety of substrates, including the amyloid precursor protein and the Notch receptor. Nicastrin (NCT) is an essential component of the gamma-secretase complex and functions as a receptor for gamma-secretase substrates. In this study, we determined that serum- and glucocorticoid-induced protein kinase 1 (SGK1) markedly reduced the protein stability of NCT. The SGK1 kinase activity was decisive for NCT degradation and endogenous SGK1 inhibited gamma-secretase activity. SGK1 downregulates NCT protein levels via proteasomal and lysosomal pathways. Furthermore, SGK1 directly bound to and phosphorylated NCT on Ser437, thereby promoting protein degradation. Collectively, our findings indicate that SGK1 is a gamma-secretase regulator presumably effective through phosphorylation and degradation of NCT.

Serum- and glucocorticoid-regulated kinase 1 is upregulated following unilateral ureteral obstruction causing epithelial-mesenchymal transition

Obstructive nephropathy leads to chronic kidney disease, characterized by a progressive epithelial-to-mesenchymal cell transition (EMT)-driven interstitial fibrosis. To identify the mechanisms causing EMT, we used the mouse model of unilateral ureteral obstruction and found a rapid and significant increase in serum- and glucocorticoid-regulated kinase-1 (SGK1) expression in the kidneys with an obstructed ureter. Knockout of SGK1 significantly suppressed obstruction-induced EMT, kidney fibrosis, increased glycogen synthase kinase-3β activity, and decreased accumulation of the transcriptional repressor Snail. This caused a reduced expression of the mesenchymal marker α-smooth muscle actin, and collagen deposition in this model. In cultured kidney epithelial cells, mechanical stretch or treatment with transforming growth factor-β not only stimulated the transcription of SGK1, but also stimulated EMT in an SGK1-dependent manner. Activated SGK1 stimulated Snail accumulation and downregulation of the epithelial marker E-cadherin. Hence, our study shows that SGK1 is involved in mediating fibrosis associated with obstructive nephropathy.

PAK4: a pluripotent kinase that regulates prostate cancer cell adhesion

Hepatocyte growth factor (HGF) is associated with tumour progression and increases the invasiveness of prostate carcinoma cells. Migration and invasion require coordinated reorganisation of the actin cytoskeleton and regulation of cell-adhesion dynamics. Rho-family GTPases orchestrate both of these cellular processes. p21-activated kinase 4 (PAK4), a specific effector of the Rho GTPase Cdc42, is activated by HGF, and we have previously shown that activated PAK4 induces a loss of both actin stress fibres and focal adhesions. We now report that DU145 human prostate cancer cells with reduced levels of PAK4 expression are unable to successfully migrate in response to HGF, have prominent actin stress fibres, and an increase in the size and number of focal adhesions. Moreover, these cells have a concomitant reduction in cell-adhesion turnover rates. We find that PAK4 is localised at focal adhesions, is immunoprecipitated with paxillin and phosphorylates paxillin on serine 272. Furthermore, we demonstrate that PAK4 can regulate RhoA activity via GEF-H1. Our results suggest that PAK4 is a pluripotent kinase that can regulate both actin cytoskeletal rearrangement and focal-adhesion dynamics.

Rap1 promotes multiple pancreatic islet cell functions and signals through mammalian target of rapamycin complex 1 to enhance proliferation

Recent studies have implicated Epac2, a guanine-nucleotide exchange factor for the Rap subfamily of monomeric G proteins, as an important regulator of insulin secretion from pancreatic beta-cells. Although the Epac proteins were originally identified as cAMP-responsive activators of Rap1 GTPases, the role of Rap1 in beta-cell biology has not yet been defined. In this study, we examined the direct effects of Rap1 signaling on beta-cell biology. Using the Ins-1 rat insulinoma line, we demonstrate that activated Rap1A, but not related monomeric G proteins, promotes ribosomal protein S6 phosphorylation. Using isolated rat islets, we show that this signaling event is rapamycin-sensitive, indicating that it is mediated by the mammalian target of rapamycin complex 1-p70 S6 kinase pathway, a known growth regulatory pathway. This newly defined beta-cell signaling pathway acts downstream of cAMP, in parallel with the stimulation of cAMP-dependent protein kinase, to drive ribosomal protein S6 phosphorylation. Activated Rap1A promotes glucose-stimulated insulin secretion, islet cell hypertrophy, and islet cell proliferation, the latter exclusively through mammalian target of rapamycin complex 1, suggesting that Rap1 is an important regulator of beta-cell function. This newly defined signaling pathway may yield unique targets for the treatment of beta-cell dysfunction in diabetes.

Relative resistance of SGK1 knockout mice against chemical carcinogenesis

The serum and glucocorticoid inducible kinase SGK1 was originally cloned from mammary tumor cells. SGK1 was found to be up-regulated in a variety of tumors, but down-regulated in several distinct tumors. Thus, evidence for a role of SGK1 in tumor growth remained conflicting. According to in vitro observations, SGK1 is up-regulated by the oncogene beta-catenin and negatively regulates the proapoptotic transcription factor FOXO3a, which in turn stimulates transcription of the Bcl2-interacting mediator BIM. This study aimed to define the role of SGK1 in colon carcinoma in vivo. SGK1 knockout mice (sgk1(-/-)) and their wild type littermates (sgk1(+/+)) were subjected to chemical cancerogenesis (intraperitoneal injection of 20 mg/kg 1,2-dimethylhydrazine followed by three cycles of 30 g/L synthetic dextran sulfate sodium for 7 days). Moreover, SGK1 was silenced in HEK293 cells. FOXO3a and BIM protein abundance was determined by Western blotting and immunohistochemistry. Following chemical cancerogenesis, sgk1(-/-)mice developed significantly less colonic tumors than sgk1(+/+)mice. According to Western blotting and immunohistochemistry, SGK1 deficiency enhanced the expression of FOXO3a and BIM both, in vitro and in vivo. SGK1 deficiency counteracts the development of colonic tumors, an effect at least in part due to up-regulation of FOXO3a and BIM.

Wnt signaling inhibits Forkhead box O3a-induced transcription and apoptosis through up-regulation of serum- and glucocorticoid-inducible kinase 1

In human cancers, mutations in components of the Wnt signaling pathway lead to beta-catenin stabilization and result in augmented gene transcription. HCT116 colon cancer cells carry stabilizing mutations in beta-catenin and exhibit an elevated activation of Wnt signaling. To clarify the role of an overactive Wnt signaling, we used DNA microarray analysis to search for genes whose expression is up-regulated after knockdown of the wild type adenomatous polyposis coli (APC) tumor suppressor in HCT116 cells, which further enhances Wnt signaling activation. Serum and glucocorticoid-inducible kinase 1 (SGK1) was among the most up-regulated genes following APC knockdown through small interfering RNA. Up-regulation of SGK1 in response to small interfering RNA against APC was inhibited by concomitant knockdown of beta-catenin. Quantitative real time reverse transcription-PCR, Western blot, and chromatin immunoprecipitation analyses confirmed that SGK1 is a direct beta-catenin target gene. SGK1 negatively regulates the pro-apoptotic transcription factor Forkhead box O3a (FoxO3a) via phosphorylation and exclusion from the nucleus. We show that Wnt signaling activation results in FoxO3a exclusion from the nucleus and inhibits expression of FoxO3a target genes. Importantly, FoxO3a mutants that fail to be phosphorylated and therefore are regulated by SGK1 are not influenced by activation of Wnt signaling. In line, knockdown of SGK1 relieves the effects of Wnt signaling on FoxO3a localization and FoxO3a-dependent transcription. Finally, we show that induction of Wnt signaling inhibits FoxO3a-induced apoptosis. Collectively our results indicate that evasion of apoptosis is another feature employed by an overactive Wnt signaling.

Association of SGK1 gene polymorphisms with type 2 diabetes

The serum and glucocorticoid inducible kinase SGK1 is genomically upregulated by glucocorticoids and in turn stimulates a variety of carriers and channels including the renal epithelial Na(+) channel ENaC and the intestinal Na(+) glucose transporter SGLT1. Twin studies disclosed an association of a specific SGK1 haplotype with moderately enhanced blood pressure in individuals who are carrying simultaneously a homozygous genotype for a variant in intron 6 [I6CC] and a homozygous or heterozygous genotype for the C allele of a polymorphism in exon 8 [E8CC/CT] of the SGK1 gene. A subsequent study confirmed the impact of this risk haplotype on blood pressure. SGK1 knockout mice are resistant to the insulin and high salt induced increase of blood pressure, glucocorticoid induced increase of electrogenic glucose transport, and glucocorticoid induced suppression of insulin release. The present study explored whether the I6CC/E8CC/CT haplotype impacts on the prevalence of type 2 diabetes. The prevalence of the I6CC genotype was 3.1% in a healthy German, 2.4 % in a healthy Romanian and 11.6 % in a healthy African population from Ghana (p=0.0006 versus prevalence in Caucasians). Comparison of genotype frequencies between type 2 diabetic patients and the respective control groups revealed significant differences for the intron 6 T>C variant. Carriers of at least one T allele were protected against type 2 diabetes (Romanians: p=0.023; OR 0.29; 95% CI 0.09-0.89; Germans: p=0.01; OR 0.37; 95% CI 0.17-0.81). The SGK1 risk haplotype (I6CC/E8CC/CT) was significantly (p=0.032; OR 4.31, 95% CI 1.19-15.58) more frequent in diabetic patients (7.2 %) than in healthy volunteers from Romania (1.8%). The observations support the view that SGK-1 may participate in the pathogenesis of metabolic syndrome.

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.

Serum and glucocorticoid-regulated protein kinases: variations on a theme

The phosphatidylinositol 3' kinase (PI3K)-signaling pathway plays a critical role in a variety of cellular responses such as modulation of cell survival, glucose homeostasis, cell division, and cell growth. PI3K generates important lipid second messengers-phosphatidylinositides that are phosphorylated at the 3' position of their inositol ring head-group. These membrane restricted lipids act by binding with high affinity to specific protein domains such as the pleckstrin homology (PH) domain. Effectors of PI3K include molecules that harbor such domains such as phosphoinositide-dependent kinase (PDK1) and protein kinase B (PKB), also termed Akt. The mammalian genome encodes three different PKB genes (alpha, beta, and gamma; Akt1, 2, and 3, respectively) and each is an attractive target for therapeutic intervention in diseases such as glioblastoma and breast cancer. A second family of three protein kinases, termed serum and glucocorticoid-regulated protein kinases (SGKs), is structurally related to the PKB family including regulation by PI3K but lack a PH domain. However, in addition to PH domains, a second class of 3' phosphorylated inositol phospholipid-binding domains exists that is termed Phox homology (PX) domain: this domain is found in one of the SGKs (SGK3). Here, we summarize knowledge of the three SGK isoforms and compare and contrast them to PKB with respect to their possible importance in cellular regulation and potential as therapeutic targets.

Regulation of NaCl transport in the renal collecting duct: lessons from cultured cells

The fine control of NaCl absorption regulated by hormones takes place in the distal nephron of the kidney. In collecting duct principal cells, the epithelial sodium channel (ENaC) mediates the apical entry of Na(+), which is extruded by the basolateral Na(+),K(+)-ATPase. Simian virus 40-transformed and "transimmortalized" collecting duct cell lines, derived from transgenic mice carrying a constitutive, conditionally, or tissue-specific promoter-regulated large T antigen, have been proven to be valuable tools for studying the mechanisms controlling the cell surface expression and trafficking of ENaC and Na(+),K(+)-ATPase. These cell lines have made it possible to identify sets of aldosterone- and vasopressin-stimulated proteins, and have provided new insights into the concerted mechanism of action of serum- and glucocorticoid-inducible kinase 1 (Sgk1), ubiquitin ligase Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), and 14-3-3 regulatory proteins in modulating ENaC-mediated Na(+) currents. Epidermal growth factor and induced leucine zipper protein have also been shown to repress and stimulate ENaC-dependent Na(+) absorption, respectively, by activating or repressing the mitogen-activated protein kinase externally regulated kinase(1/2). Overall, these findings have provided evidence suggesting that multiple pathways are involved in regulating NaCl absorption in the distal nephron.

Radiation enhances the invasive potential of primary glioblastoma cells via activation of the Rho signaling pathway

Glioblastoma multiforme (GBM) is among the most treatment-refractory of all human tumors. Radiation is effective at prolonging survival of GBM patients; however, the vast majority of GBM patients demonstrate progression at or near the site of original treatment. We have identified primary GBM cell lines that demonstrate increased invasive potential upon radiation exposure. As this represents a novel mechanism by which radiation-treated GBMs can fail therapy, we further investigated the identity of downstream signaling molecules that enhance the invasive phenotype of irradiated GBMs. Matrigel matrices were used to compare the extent of invasion of irradiated vs. non-irradiated GBM cell lines UN3 and GM2. The in vitro invasive potential of these irradiated cells were characterized in the presence of both pharmacologic and dominant negative inhibitors of extracellular matrix and cell signaling molecules including MMP, uPA, IGFR, EGFR, PI-3K, AKT, and Rho kinase. The effect of radiation on the expression of these signaling molecules was determined with Western blot assays. Ultimately, the in vitro tumor invasion results were confirmed using an in vivo 9L GBM model in rats. Using the primary GBM cell lines UN3 and GM2, we found that radiation enhances the invasive potential of these cells via activation of EGFR and IGFR1. Our findings suggest that activation of Rho signaling via PI-3K is required for radiation-induced invasion, although not required for invasion under physiologic conditions. This report clearly demonstrates that radiation-mediated invasion is fundamentally distinct from invasion under normal cellular physiology and identifies potential therapeutic targets to overcome this phenomenon.

Sgk kinases and their role in epithelial transport

The serum/glucocorticoid-induced kinase Sgk1 plays an important role in the regulation of epithelial ion transport. This kinase is very rapidly regulated at the transcriptional level as well as via posttranslational modifications involving phosphorylation by the MAP or PI-3 kinase pathways and/or ubiquitylation. Although Sgk1 is a cell survival kinase, its primary role likely concerns the regulation of epithelial ion transport, as suggested by the phenotype of Sgk1-null mice, which display a defect in Na( homeostasis owing to disturbed renal tubular Na+ handling. In this review we first discuss the molecular, cellular, and regulatory aspects of Sgk1 and its paralogs. We then discuss its roles in the physiology and pathophysiology of epithelial ion transport.

The serum- and glucocorticoid-inducible kinase Sgk-1 is involved in pulmonary vascular remodeling: role in redox-sensitive regulation of tissue factor by thrombin

The stress-responsive serum- and glucocorticoid-inducible kinase Sgk-1 is involved in osmoregulation and cell survival and may contribute to fibrosis and hypertension. However, the function of Sgk-1 in vascular remodeling and thrombosis, 2 major determinants of pulmonary hypertension (PH), has not been elucidated. We investigated the role of Sgk-1 in thrombin signaling and tissue factor (TF) expression and activity in pulmonary artery smooth muscle cells (PASMC). Thrombin increased Sgk-1 activity and mRNA and protein expression. H2O2 similarly induced Sgk-1 expression. Antioxidants, dominant-negative Rac, and depletion of the NADPH oxidase subunit p22phox diminished thrombin-induced Sgk-1 expression. Inhibition of p38 mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and phosphoinositide-dependent kinase-1 prevented thrombin-induced Sgk-1 expression. Thrombin or Sgk-1 overexpression enhanced TF expression and procoagulant activity, whereas TF upregulation by thrombin was diminished by kinase-deficient Sgk-1 and was not detectable in fibroblasts from mice deficient in sgk-1 (sgk1(-/-)). Similarly, dexamethasone treatment failed to induce TF expression and activity in lung tissue from sgk1(-/-) mice. Transcriptional induction of TF by Sgk-1 was mediated through nuclear factor kappaB. Finally, Sgk-1 and TF proteins were detected in the media of remodeled pulmonary vessels associated with PH. These data show that thrombin potently induces Sgk-1 involving NADPH oxidases, phosphatidylinositol 3-kinase, p38 mitogen-activated protein kinase, and phosphoinositide-dependent kinase-1, and that activation of nuclear factor kappaB by Sgk-1 mediates TF expression and activity by thrombin. Because enhanced procoagulant activity can promote pulmonary vascular remodeling, and Sgk-1 and TF were present in the media of remodeled pulmonary vessels, this pathway may play a critical role in vascular remodeling in PH.

Alternative signaling pathways: when, where and why?

Alternative cell signal transduction pathways have been demonstrated in some experimental systems. The importance of their existence has not been completely appreciated. In this review we present the cases of alternative pathways resulted from a survey of the available experimental data. The alternative pathways could show different relationships, i.e., synergistic, redundant, additive, opposite and competitive effects. They could have distinct time courses and cell, organ, sex or species specification. Further, they could happen during physiological or pathological situations, and display differentiated sensitivity. These case studies together imply that alternative signal pathways could be involved in the regulation of cell functions at the pathway level. In-depth understanding of the importance of the alternative pathways will rely on building and exploration of mathematical models.

Serum- and glucocorticoid-regulated kinase 1 regulates ubiquitin ligase neural precursor cell-expressed, developmentally down-regulated protein 4-2 by inducing interaction with 14-3-3

Serum- and glucocorticoid-regulated kinase 1 (SGK1) is an aldosterone-regulated early response gene product that regulates the activity of several ion transport proteins, most notably that of the epithelial sodium channel (ENaC). Recent evidence has established that SGK1 phosphorylates and inhibits Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), a ubiquitin ligase that decreases cell surface expression of the channel and possibly stimulates its degradation. The mechanistic basis for this SGK1-induced Nedd4-2 inhibition is currently unknown. In this study we show that SGK1-mediated phosphorylation of Nedd4-2 induces its interaction with members of the 14-3-3 family of regulatory proteins. Through functional characterization of Nedd4-2-mutant proteins, we demonstrate that this interaction is required for SGK1-mediated inhibition of Nedd4-2. The concerted action of SGK1 and 14-3-3 appears to disrupt Nedd4-2-mediated ubiquitination of ENaC, thus providing a mechanism by which SGK1 modulates the ENaC-mediated Na(+) current. Finally, the expression pattern of 14-3-3 is also consistent with a functional role in distal nephron Na(+) transport. These results demonstrate a novel, physiologically significant role for 14-3-3 proteins in modulating ubiquitin ligase-dependent pathways in the control of epithelial ion transport.

New insights into the role of serum- and glucocorticoid-inducible kinase SGK1 in the regulation of renal function and blood pressure

PURPOSE OF REVIEW

The serum and glucocorticoid inducible kinase 1 (SGK1) is induced in the aldosterone sensitive distal nephron (ASDN) where it may stimulate Na reabsorption, partly by inhibiting ubiquitin ligase Nedd4-2-mediated retrieval of epithelial Na+ channel ENaC from the luminal membrane. We describe recent advances in our understanding of SGK1 function in the regulation of renal function and blood pressure.

RECENT FINDINGS

Thiazolidinediones, i.e. activators of peroxisome proliferator-activated receptor gamma (PPAR gamma), upregulate SGK1 and ENaC mRNA expression and increase cell-surface expression of ENaC alpha in a human cortical-collecting-duct cell line. cAMP/protein kinase A can induce phosphorylation and inhibition of Nedd4-2-independent of SGK1. Part of ENaC stimulation by SGK1 appears dependent on a SGK1 consensus motif in ENaC alpha and independent of Nedd4-2. SGK1-dependent upregulation of Na+ reabsorption in ASDN contributes to upregulation of renal K+ excretion. In oocytes, SGK1 activates various renal transport proteins including Na+/glucose cotransporter SGLT1, Na+-coupled dicarboxylate transporter NaDC-1, epithelial Ca+ channel TRPV5, renal outer medullary K+ channel ROMK and voltage gated K+ channels KCNE1/KCNQ1 and Kv1.3. A variant of the SGK1 gene associates with increased blood pressure and body mass index.

SUMMARY

PPAR gamma activators may increase renal Na reabsorption by stimulating SGK1 and ENaC. Nedd4-2 integrates influences of cAMP/protein kinase A and SGK1. SGK1 can activate ENaC in part directly and independent of Nedd4-2. K+ homeostasis requires SGK1-dependent Na+ reabsorption in ASDN. SGK1 may affect renal transport mechanisms beyond Na+ reabsorption and K+ secretion in ASDN. Polymorphisms of SGK1 may be relevant to the pathophysiology of hypertension and other diseases.

Distribution and regulation of expression of serum- and glucocorticoid-induced kinase-1 in the rat kidney

The serum- and glucocorticoid-induced kinase-1 (sgk1) increases the activity of a number of epithelial ion channels and transporters. The present study examines the distribution and subcellular localization of sgk1 protein in the rat kidney and the regulation of levels of expression induced by steroids. The results indicate that the kidney expresses predominantly the sgk1 isoform with a distribution restricted to the thick ascending limb of Henle, distal convoluted, connecting and cortical collecting tubules. Within cells, sgk1 strongly associates with the microsomal fraction of homogenates and it colocalizes with the Na+,K+-ATPase to the basolateral membrane. Analysis of the levels of expression of sgk1 by Western blotting and immunohistochemistry indicates constitutive high expression under basal conditions. Approximately half of the basal level is maintained by glucocorticoids whereas physiological fluctuations of aldosterone produce minor changes in sgk1 abundance in adrenal-intact animals. These results do not support the notion that physiological changes of aldosterone concentration turn the expression of sgk1 'on and off' in the mammalian kidney. Additionally, localization of sgk1 to the basolateral membrane indicates that the effects mediated by sgk1 do not require a direct interaction with the ion channels and transporters whose activity is modulated, since most of these proteins are located in the apical membrane of renal epithelial cells.

Hormone-regulated transepithelial Na+ transport in mammalian CCD cells requires SGK1 expression

To study the role of serum and glucocorticoid-inducible kinase-1 (SGK1) in mammalian cells, we compared Na(+) transport rates in wild-type (WT) M1 cortical collecting duct cells with M1 populations stably expressing human full-length SGK1, NH(2)-terminal truncated (DeltaN-60) SGK1, "kinase-dead" (K127M) SGK1, and cells that have downregulated levels of SGK1 mRNA (antisense SGK1). Basal rates of transepithelial Na(+) transport were highest in full-length SGK1 populations, compared among the above populations. Dexamethasone treatment increased Na(+) transport in WT and full-length SGK1 cells 2.7- and 2-fold, respectively. Modest stimulation of Na(+) absorption was detected after dexamethasone treatment in DeltaN-60 SGK1 populations. However, DeltaN-60 SGK1 transport rates remained substantially lower than WT values. Importantly, a combination of high insulin, dexamethasone, and serum failed to significantly stimulate Na(+) transport in antisense or K127M SGK1 cells. Additionally, expression of antisense SGK1 significantly decreased transepithelial resistance values. Overall, we concluded that SGK1 is a critical component in corticosteroid-regulated Na(+) transport in mammalian cortical collecting duct cells. Furthermore, our data suggest that the NH(2) terminus of SGK1 may contain a Phox homology-like domain that may be necessary for effective Na(+) transport.

Role of SGK in hormonal regulation of epithelial sodium channel in A6 cells

The purpose of this study was to examine the role of the serum- and glucocorticoid-induced kinase (SGK) in the activation of the epithelial sodium channel (ENaC) by aldosterone, arginine vasopressin (AVP), and insulin. We used a tetracycline-inducible system to control the expression of wild-type (SGK(wt)(T)), constitutively active (S425D mutation; SGK(S425D)(T)), or inactive (K130M mutation; SGK(K130M)(T)) SGK in A6 cells independently of hormonal stimulation. The effect of SGK expression on ENaC activity was monitored by measuring transepithelial amiloride-sensitive short-circuit current (I(sc)) of transfected A6 cell lines. Expression of SGK(wt)(T) or SGK(S425D)(T) and aldosterone stimulation have additive effects on I(sc). Although SGK could play some role in the aldosterone response, our results suggest that other mechanisms take place. SGK(S425D)(T) abrogates the responses to AVP and insulin; hence, in the signaling pathways of these hormones there is a shared step that is stimulated by SGK. Because AVP and insulin induce fusion of vesicles to the apical membrane, our results support the notion that SGK promotes incorporation of channels in the apical membrane.

Ubiquitin modification of serum and glucocorticoid-induced protein kinase-1 (SGK-1)

The serum and glucocorticoid-induced protein kinase gene (sgk-1) encodes a multifunctional kinase that can be phosphorylated and activated through a phosphatidylinositol 3-kinase-dependent signaling pathway. In many cell types, endogenous SGK-1 steady-state protein levels are very low but can be acutely up-regulated after glucocorticoid receptor-mediated transcriptional activation; in breast epithelial and cancer cell lines, this up-regulation is associated with promotion of cell survival. We and others have noted that ectopically introduced full-length SGK-1 is poorly expressed, although SGK-1 lacking the first 60 amino acids (delta60SGK-1) is expressed at much higher-fold protein levels than wild-type SGK-1 in both human embryonic kidney 293T and MCF10A mammary epithelial cells. In this report, we demonstrate for the first time that the low steady-state expression level of SGK-1 is due to polyubiquitination and subsequent degradation by the 26S proteasome. Deletion of the amino-terminal 60 amino acids of SGK-1 results in a mutant SGK-1 protein that is neither efficiently polyubiquitinated nor degraded by the 26S proteasome, accounting for the higher steady-state levels of the truncated protein. We also demonstrate that a subset of SGK-1 localizes to the plasma membrane and that the polyubiquitin-modified SGK-1 localizes to a membrane-associated fraction of the cell. Taken together, these data suggest that a significant fraction of SGK-1 is membrane-associated and ubiquitinated. These findings are consistent with the recently described role of SGK-1 in phosphorylating the membrane-associated protein Nedd4-2 and the integral membrane Na+/H+ exchanger isoform 3 (NHE3) and suggest a novel mechanism of regulation of SGK-1.

PAK4 is activated via PI3K in HGF-stimulated epithelial cells

The p21-activated kinases (PAKs) are divided into two subgroups based on sequence homology. Group 1 PAKs (PAK1-3) are involved in cell migration, and are activated by pro-migratory stimuli and by Cdc42/Rac GTPases. In contrast,little is known about the regulation of the recently identified group II PAKs(PAK4-6). Here we report that PAK4 is activated by HGF, a migratory stimulus for epithelial cells. In unstimulated MDCK cells, activated PAK4 induces a decrease in stress fibres, and when cells are stimulated with HGF, it induces a loss of focal complexes and cell rounding. This response is dependent on PAK4 kinase activity but does not require Cdc42 interaction. Activated PAK4 localises to the cell periphery but not specifically in lamellipodia, and HGF induces localisation of wild-type PAK4 to the cell periphery. LY294002, a phosphoinositide 3-kinase (PI3K) inhibitor, inhibits HGF-induced PAK4 kinase activation, relocalisation, and cell rounding. However, the isolated C-terminal kinase domain of PAK4 can induce cell rounding in the presence of LY294002, suggesting that the N-terminal region acts as a negative regulator of PAK4 activity. These results indicate that HGF stimulates PAK4 through PI3K, and that PAK4 could contribute to HGF-induced changes in actin organisation and cell-substratum adhesion.