SGK1 kinase upregulates GLUT1 activity and plasma membrane expression

Sodium chloride in the tumor microenvironment enhances T cell metabolic fitness and cytotoxicity

The efficacy of antitumor immunity is associated with the metabolic state of cytotoxic T cells, which is sensitive to the tumor microenvironment. Whether ionic signals affect adaptive antitumor immune responses is unclear. In the present study, we show that there is an enrichment of sodium in solid tumors from patients with breast cancer. Sodium chloride (NaCl) enhances the activation state and effector functions of human CD8+ T cells, which is associated with enhanced metabolic fitness. These NaCl-induced effects translate into increased tumor cell killing in vitro and in vivo. Mechanistically, NaCl-induced changes in CD8+ T cells are linked to sodium-induced upregulation of Na+/K+-ATPase activity, followed by membrane hyperpolarization, which magnifies the electromotive force for T cell receptor (TCR)-induced calcium influx and downstream TCR signaling. We therefore propose that NaCl is a positive regulator of acute antitumor immunity that might be modulated for ex vivo conditioning of therapeutic T cells, such as CAR T cells.

The progression of secondary diabetes: A review of modeling studies

Mathematical modeling has provided quantitative information consistent with experimental data, greatly improving our understanding of the progression of type 1 and type 2 diabetes. However, diabetes is a complex metabolic disease and has been found to be involved in crosstalk interactions with diverse endocrine diseases. Mathematical models have also been developed to investigate the quantitative impact of various hormonal disorders on glucose imbalance, advancing the precision treatment for secondary diabetes. Here we review the models established for the study of dysglycemia induced by hormonal disorders, such as excessive glucocorticoids, epinephrine, and growth hormone. To investigate the influence of hyperthyroidism on the glucose regulatory system, we also propose a hyperthyroid-diabetes progression model. Model simulations indicate that timely thyroid treatment can halt the progression of hyperglycemia and prevent beta-cell failure. This highlights the diagnosis of hormonal disorders, together withblood sugar tests, as significant measures for the early diagnosis and treatment of diabetes. The work recapitulates updated biological research on the interactions between the glucose regulatory system and other endocrine axes. Further mathematical modeling of secondary diabetes is desired to promote the quantitative study of the disease and the development of individualized diabetic therapies.

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.

Upregulation of glucocorticoid receptor-mediated glucose transporter 4 in enzalutamide-resistant prostate cancer

Enzalutamide (Enz) is a second‐generation androgen receptor (AR) antagonist for castration‐resistant prostate cancer (CRPC) therapy, and it prolongs survival time in these patients. However, during Enz treatment, CRPC patients usually acquire resistance to Enz and often show cross‐resistance to other AR signaling inhibitors. Although glucocorticoid receptor (GR) is involved in this resistance, the role of GR has not yet been clarified. Here, we report that chronic Enz treatment induced GR‐mediated glucose transporter 4 (GLUT4) upregulation, and that upregulation was associated with resistance to Enz and other AR signaling inhibitors. Additionally, inhibition of GLUT4 suppressed cell proliferation in Enz‐resistant prostate cancer cells, which recovered from Enz resistance and cross‐resistance without changes in GR expression. Thus, a combination of Enz and a GLUT4 inhibitor could be useful in Enz‐resistant CRPC patients.

SGK1 signaling promotes glucose metabolism and survival in extracellular matrix detached cells

Loss of integrin-mediated attachment to extracellular matrix (ECM) proteins can trigger a variety of cellular changes that affect cell viability. Foremost among these is the activation of anoikis, caspase-mediated cell death induced by ECM detachment. In addition, loss of ECM attachment causes profound alterations in cellular metabolism, which can lead to anoikis-independent cell death. Here, we describe a surprising role for serum and glucocorticoid kinase-1 (SGK1) in the promotion of energy production when cells are detached. Our data demonstrate that SGK1 activation is necessary and sufficient for ATP generation during ECM detachment and anchorage-independent growth. More specifically, SGK1 promotes a substantial elevation in glucose uptake because of elevated GLUT1 transcription. In addition, carbon flux into the pentose phosphate pathway (PPP) is necessary to accommodate elevated glucose uptake and PPP-mediated glyceraldehyde-3-phosphate (G3P) is necessary for ATP production. Thus, our data show SGK1 as master regulator of glucose metabolism and cell survival during ECM-detached conditions.

Decreased GLUT1/NHE1 RNA expression in whole blood predicts disease severity in patients with COVID-19

AIMS

We aimed to assess whether expression of whole-blood RNA of sodium proton exchanger 1 (NHE1) and glucose transporter 1 (GLUT1) is associated with COVID-19 infection and outcome in patients presenting to the emergency department with respiratory infections. Furthermore, we investigated NHE1 and GLUT1 expression in the myocardium of deceased COVID-19 patients.

METHODS AND RESULTS

Whole-blood quantitative assessment of NHE1 and GLUT1 RNA was performed using quantitative PCR in patients with respiratory infection upon first contact in the emergency department and subsequently stratified by SARS-CoV-2 infection status. Assessment of NHE1 and GLUT1 RNA using PCR was also performed in left ventricular myocardium of deceased COVID-19 patients. NHE1 expression is up-regulated in whole blood of patients with COVID-19 compared with other respiratory infections at first medical contact in the emergency department (control: 0.0021 ± 0.0002, COVID-19: 0.0031 ± 0.0003, P = 0.01). The ratio of GLUT1 to NHE1 is significantly decreased in the blood of COVID-19 patients who are subsequently intubated and/or die (severe disease) compared with patients with moderate disease (moderate disease: 0.497 ± 0.083 vs. severe disease: 0.294 ± 0.0336, P = 0.036). This ratio is even further decreased in the myocardium of patients who deceased from COVID-19 in comparison with the myocardium of non-infected donors.

CONCLUSIONS

NHE1 and GLUT1 may be critically involved in the disease progression of SARS-CoV-2 infection. We show here that SARS-CoV-2 infection critically disturbs ion channel expression in the heart. A decreased ratio of GLUT1/NHE1 could potentially serve as a biomarker for disease severity in patients with COVID-19.

NDRG1 Activity in Fat Depots Is Associated With Type 2 Diabetes and Impaired Incretin Profile in Patients With Morbid Obesity

OBJECTIVE

We aimed to investigate insulin-, mTOR- and SGK1-dependent signaling basal states in morbidly obese patients' fat. We analyzed the correlation between the signaling activity, carbohydrate metabolism, and incretin profiles of patients.

METHODS

The omental and subcutaneous fat was obtained in patients with obesity. The omental study included 16 patients with normal glucose tolerance (NGT) and 17 patients with type 2 diabetes mellitus (T2DM); the subcutaneous study included 9 NGT patients and 12 T2DM patients. Insulin resistance was evaluated using the hyperinsulinemic euglycemic clamp test and HOMA-IR index. The oral glucose tolerance test (OGTT) for NGT patients and mixed meal tolerance test (MMTT) for T2DM patients were performed. The levels of incretins (GLP-1, GIP, oxyntomodulin) and glucagon were measured during the tests. Signaling was analyzed by Western blotting in adipose tissue biopsies.

RESULTS

We have shown equal levels of basal phosphorylation of insulin- and mTOR-dependent signaling in omental fat depot in NGT and T2DM obese patients. Nevertheless, pNDRG1-T346 was decreased in omental fat of T2DM patients. Correlation analysis has shown an inverse correlation of pNDRG1-T346 in omental fat and diabetic phenotype (HbA1c, impaired incretin profile (AUC GLP-1, glucagon)). Moreover, pNDRG1-T346 in subcutaneous fat correlated with impaired incretin levels among obese patients (inverse correlation with AUC glucagon and AUC GIP).

CONCLUSIONS

According to results of the present study, we hypothesize that phosphorylation of pNDRG1-T346 can be related to impairment in incretin hormone processing. pNDRG1-T346 in adipose tissue may serve as a marker of diabetes-associated impairments of the systemic incretin profile and insulin sensitivity.

Recent advancements in role of TAM receptors on efferocytosis, viral infection, autoimmunity, and tissue repair

Evolutionarily conserved highly regulated process of apoptosis has been a major physiological process throughout the entire evolutionary history of living beings that has impacted the process of evolution itself. One of the key features of this highly researched field of science is the process of phosphatidylserine (PS) externalization by the different membrane bound enzymes. The process is a result of series of biological events and is associated with various biological outcomes depending on the proper recognition of this ligand. In this review, we will briefly summarize the recent advancement in the field pertaining to the set of receptors, known as TAM (Tyro3, Axl and Mertk) receptors, for their influence in the recognition of various PS externalization events and mediation of pathological outcomes such as autoimmunity, cancer, and tissue repair.

Overfeeding and Substrate Availability, But Not Age or BMI, Alter Human Satellite Cell Function

Satellite cells (SC) aid skeletal muscle growth and regeneration. SC-mediated skeletal muscle repair can both be influenced by and exacerbate several diseases linked to a fatty diet, obesity, and aging. The purpose of this study was to evaluate the effects of different lifestyle factors on SC function, including body mass index (BMI), age, and high-fat overfeeding. For this study, SCs were isolated from the vastus lateralis of sedentary young (18–30 years) and sedentary older (60–80 years) men with varying BMIs (18–32 kg/m²), as well as young sedentary men before and after four weeks of overfeeding (OVF) (55% fat/ + 1000 kcal, n = 4). The isolated SCs were then treated in vitro with a control (5 mM glucose, 10% fetal bovine serum (FBS)) or a high substrate growth media (HSM) (10% FBS, 25 mM glucose, and 400 μM 2:1 oleate–palmitate). Cells were assessed on their ability to proliferate, differentiate, and fuel substrate oxidation after differentiation. The effect of HSM was measured as the percentage difference between SCs exposed to HSM compared to control media. In vitro SC function was not affected by donor age. OVF reduced SC proliferation rates (–19% p < 0.05) but did not influence differentiation. Cellular proliferation in response to HSM was correlated to the donor’s body mass index (BMI) (r² = 0.6121, p < 0.01). When exposed to HSM, SCs from normal weight (BMI 18–25 kg/m²) participants exhibited reduced proliferation and fusion rates with increased fatty-acid oxidation (p < 0.05), while SCs from participants with higher BMIs (BMI 25–32 kg/m²) demonstrated enhanced proliferation in HSM. HSM reduced proliferation and fusion (p < 0.05) in SCs isolated from subjects before OVF, whereas HSM exposure accelerated proliferation and fusion in SCs collected following OVF. These results indicated that diet has a greater influence on SC function than age and BMI. Though age and BMI do not influence in vitro SC function when grown in controlled conditions, both factors influenced the response of SCs to substrate challenges, indicating age and BMI may mediate responses to diet.

The functional duality of SGK1 in the regulation of hyperglycemia

Hyperglycemia is the consequence of blood glucose dysregulation and a driving force of diabetic complications including retinopathy, nephropathy and cardiovascular diseases. The serum and glucocorticoid inducible kinase-1 (SGK1) has been suggested in the modulation of various pathophysiological activities. However, the role of SGK1 in blood glucose homeostasis remains less appreciated. In this review, we intend to summarize the function of SGK1 in glucose level regulation and to examine the evidence supporting the therapeutic potential of SGK1 inhibitors in hyperglycemia. Ample evidence points to the controversial roles of SGK1 in pancreatic insulin secretion and peripheral insulin sensitivity, which reflects the complex interplay between SGK1 activation and blood glucose fluctuation. Furthermore, SGK1 is engaged in glucose absorption and excretion in intestine and kidney and participates in the progression of hyperglycemia-induced secondary organ damage. As a net effect, blockage of SGK1 activation via either pharmacological inhibition or genetic manipulation seems to be helpful in glucose control at varying diabetic stages.

Restoring glucose uptake rescues neutrophil dysfunction and protects against systemic fungal infection in mouse models of kidney disease

Disseminated candidiasis caused by the fungus Candida albicans is a major clinical problem in individuals with kidney disease and accompanying uremia; disseminated candidiasis fatality is twice as common in patients with uremia as those with normal kidney function. Many antifungal drugs are nephrotoxic, making treatment of these patients particularly challenging. The underlying basis for this impaired capacity to control infections in uremic individuals is poorly understood. Here, we show in multiple models that uremic mice exhibit an increased susceptibility to systemic fungal infection. Uremia inhibits Glut1-mediated uptake of glucose in neutrophils by causing aberrant activation of GSK3β, resulting in reduced ROS generation and hence impaired killing of C. albicans in mice. Consequently, pharmacological inhibition of GSK3β restored glucose uptake and rescued ROS production and candidacidal function of neutrophils in uremic mice. Similarly, neutrophils isolated from patients with kidney disease and undergoing hemodialysis showed similar defect in the fungal killing activity, a phenotype rescued in the presence of a GSK3β inhibitor. These findings reveal a mechanism of neutrophil dysfunction during uremia and suggest a potentially translatable therapeutic avenue for treatment of disseminated candidiasis.

WNK1 phosphorylation sites in TBC1D1 and TBC1D4 modulate cell surface expression of GLUT1

Glucose uptake by mammalian cells is a key mechanism to maintain cell and tissue homeostasis and relies mostly on plasma membrane-localized glucose transporter proteins (GLUTs). Two main cellular mechanisms regulate GLUT proteins in the cell: first, expression of GLUT genes is under dynamic transcriptional control and is used by cancer cells to increase glucose availability. Second, GLUT proteins are regulated by membrane traffic from storage vesicles to the plasma membrane (PM). This latter process is triggered by signaling mechanisms and well-studied in the case of insulin-responsive cells, which activate protein kinase AKT to phosphorylate TBC1D4, a RAB-GTPase activating protein involved in membrane traffic regulation. Previously, we identified protein kinase WNK1 as another kinase able to phosphorylate TBC1D4 and regulate the surface expression of the constitutive glucose transporter GLUT1. Here we describe that downregulation of WNK1 through RNA interference in HEK293 cells led to a 2-fold decrease in PM GLUT1 expression, concomitant with a 60% decrease in glucose uptake. By mass spectrometry, we identified serine (S) 704 in TBC1D4 as a WNK1-regulated phosphorylation site, and also S565 in the paralogue TBC1D1. Transfection of the respective phosphomimetic or unphosphorylatable TBC1D mutants into cells revealed that both affected the cell surface abundance of GLUT1. The results reinforce a regulatory role for WNK1 in cell metabolism and have potential impact for the understanding of cancer cell metabolism and therapeutic options in type 2 diabetes.

Low AS160 and high SGK basal phosphorylation associates with impaired incretin profile and type 2 diabetes in adipose tissue of obese patients

OBJECTIVE

To compare basal insulin and mTOR signaling in subcutaneous fat of obese T2DM vs. obese subjects with normal glucose tolerance (NGT), and correlate it with clinical parameters of carbohydrate metabolism and incretin secretion profiles.

METHODS

Recruited were 22 patients with long (>10 years) and morbid (BMI > 35 kg/m²) obesity, 12 of which had NGT and 10 had T2DM. Hyperinsulinemic-euglycemic clamp test and HOMA-IR were used to measure insulin resistance. Blood samples taken at 0, 30 and 120 min of food load test were used to assess incretin profile, insulin and glucose levels. Amount of total and visceral fat was determined by bioelectrical impedance analysis. Subcutaneous fat biopsies were obtained during bariatric surgery for all patients and analyzed by western blots.

RESULTS

As assessed by western blots of insulin receptor substrate (IRS), Akt, Raptor, Rictor, mTOR and S6K1, the basal insulin signaling and mTORC activities were comparable in NGT and T2DM groups, whereas phosphorylation of AS160 was significantly lower and that of serum and glucocorticoid-induced kinase (SGK) was significantly higher in T2DM group. Various correlations were found between the degree of insulin resistance and amount of visceral fat, changes in incretin profile, glucose metabolic parameters and phosphorylation level of AS160, incretin secretion profile and phosphorylated levels of AS160 or SGK1.

CONCLUSION

Altered phosphorylation of AS160 and SGK1 is associated with obese T2DM phenotype.

Glucocorticoid receptor signaling in astrocytes is required for aversive memory formation

Stress elicits the release of glucocorticoids (GCs) that regulate energy metabolism and play a role in emotional memory. Astrocytes express glucocorticoid receptors (GR), but their contribution to cognitive effects of GC's action in the brain is unknown. To address this question, we studied how astrocyte-specific elimination of GR affects animal behavior known to be regulated by stress. Mice with astrocyte-specific ablation of GR presented impaired aversive memory expression in two different paradigms of Pavlovian learning: contextual fear conditioning and conditioned place aversion. These mice also displayed compromised regulation of genes encoding key elements of the glucose metabolism pathway upon GR stimulation. In particular, we identified that the glial, but not the neuronal isoform of a crucial stress-response molecule, Sgk1, undergoes GR-dependent regulation in vivo and demonstrated the involvement of SGK1 in regulation of glucose uptake in astrocytes. Together, our results reveal astrocytes as a central element in GC-dependent formation of aversive memory and suggest their relevance for stress-induced alteration of brain glucose metabolism. Consequently, astrocytes should be considered as a cellular target of therapies of stress-induced brain diseases.

Efferocytosis induces a novel SLC program to promote glucose uptake and lactate release

We turnover billions of apoptotic cells daily, and these are removed by professional and non-professional phagocytes via efferocytosis¹. Characterizing the transcriptional program of phagocytes, we discovered a novel solute carrier family (SLC) gene signature (involving 33 SLC members) that is specifically modified during efferocytosis, but not antibody-mediated phagocytosis. Assessing the functional relevance of these SLCs, we noted a robust induction of an aerobic glycolysis program in efferocytic phagocytes, initiated by SLC2A1-mediated glucose uptake, with concurrent suppression of oxidative phosphorylation program. Interestingly, the different steps of phagocytosis², i.e. smell (‘find-me’ signals/ sensing factors released by apoptotic cells), taste (phagocyte-apoptotic cell contact), and ingestion (corpse internalization), activated different SLCs and other molecules to promote glycolysis. Further, lactate, a natural by-product of aerobic glycolysis³, was released via another SLC (SLC16A1) that was upregulated after corpse uptake. While glycolysis within phagocytes contributed to actin polymerization and the continued uptake of corpses, the lactate released via SLC16A1 influenced the establishment of an anti-inflammatory tissue environment. Collectively, these data reveal a novel SLC program activated during efferocytosis, identify a previously unknown reliance on aerobic glycolysis during apoptotic cell uptake, and that glycolytic byproducts of efferocytosis can also influence other cells in the microenvironment.

Mitochondrial genome modulates myocardial Akt/Glut/HK salvage pathway in spontaneously hypertensive rats adapted to chronic hypoxia

Recently we have shown that adaptation to continuous normobaric hypoxia (CNH) decreases myocardial ischemia/reperfusion injury in spontaneously hypertensive rats (SHR) and in a conplastic strain (SHR-mt^BN). The protective effect was stronger in the latter group characterized by a selective replacement of the SHR mitochondrial genome with that of a more ischemia-resistant Brown Norway strain. The aim of the present study was to examine the possible involvement of the hypoxia inducible factor (HIF)-dependent pathway of the protein kinase B/glucose transporters/hexokinase (Akt/GLUT/HK) in this mitochondrial genome-related difference of the cardioprotective phenotype. Adult male rats were exposed for 3 wk to CNH ([Formula: see text] 0.1). The expression of dominant isoforms of Akt, GLUT, and HK in left ventricular myocardium was determined by real-time RT-PCR and Western blotting. Subcellular localization of GLUTs was assessed by quantitative immunofluorescence. Whereas adaptation to hypoxia markedly upregulated protein expression of HK2, GLUT1, and GLUT4 in both rat strains, Akt2 protein level was significantly increased in SHR-mt^BN only. Interestingly, a higher content of HK2 was revealed in the sarcoplasmic reticulum-enriched fraction in SHR-mt^BN after CNH. The increased activity of HK determined in the mitochondrial fraction after CNH in both strains suggested an increase of HK association with mitochondria. Interestingly, HIF1a mRNA increased and HIF2a mRNA decreased after CNH, the former effect being more pronounced in SHR-mt^BN than in SHR. Pleiotropic effects of upregulated Akt2 along with HK translocation to mitochondria and mitochondria-associated membranes can potentially contribute to a stronger CNH-afforded cardioprotection in SHR-mt^BN compared with progenitor SHR.

DISC1 regulates lactate metabolism in astrocytes: implications for psychiatric disorders

Our knowledge of how genetic risk variants contribute to psychiatric disease is mainly limited to neurons. However, the mechanisms whereby the same genetic risk factors could affect the physiology of glial cells remain poorly understood. We studied the role of a psychiatric genetic risk factor, Disrupted-In-Schizophrenia-1 (DISC1), in metabolic functions of astrocytes. We evaluated the effects of knockdown of mouse endogenous DISC1 (DISC1-KD) and expression of a dominant-negative, C-terminus truncated human DISC1 (DN-DISC1) on the markers of energy metabolism, including glucose uptake and lactate production, in primary astrocytes and in mice with selective expression of DN-DISC1 in astrocytes. We also assessed the effects of lactate treatment on altered affective behaviors and impaired spatial memory in DN-DISC1 mice. Both DISC1-KD and DN-DISC1 comparably decreased mRNA and protein levels of glucose transporter 4 and glucose uptake by primary astrocytes. Decreased glucose uptake was associated with reduced oxidative phosphorylation and glycolysis as well as diminished lactate production in vitro and in vivo. No significant effects of DISC1 manipulations in astrocytes were observed on expression of the subunits of the electron transport chain complexes or mitofilin, a neuronal DISC1 partner. Lactate treatment rescued the abnormal behaviors in DN-DISC1 male and female mice. Our results suggest that DISC1 may be involved in the regulation of lactate production in astrocytes to support neuronal activity and associated behaviors. Abnormal expression of DISC1 in astrocytes and resulting abnormalities in energy supply may be responsible for aspects of mood and cognitive disorders observed in patients with major psychiatric illnesses.

Distinct Akt phosphorylation states are required for insulin regulated Glut4 and Glut1-mediated glucose uptake

Insulin, downstream of Akt activation, promotes glucose uptake into fat and muscle cells to lower postprandial blood glucose, an enforced change in cellular metabolism to maintain glucose homeostasis. This effect is mediated by the Glut4 glucose transporter. Growth factors also enhance glucose uptake to fuel an anabolic metabolism required for tissue growth and repair. This activity is predominantly mediated by the Glut1. Akt is activated by phosphorylation of its kinase and hydrophobic motif (HM) domains. We show that insulin-stimulated Glut4-mediated glucose uptake requires PDPK1 phosphorylation of the kinase domain but not mTORC2 phosphorylation of the HM domain. Nonetheless, an intact HM domain is required for Glut4-mediated glucose uptake. Whereas, Glut1-mediated glucose uptake also requires mTORC2 phosphorylation of the HM domain, demonstrating both phosphorylation-dependent and independent roles of the HM domain in regulating glucose uptake. Thus, mTORC2 links Akt to the distinct physiologic programs related to Glut4 and Glut1-mediated glucose uptake.

DOI:http://dx.doi.org/10.7554/eLife.26896.001

Ras-ling with new therapeutic targets for metastasis

Successful cancer metastasis relies on the ability of cancer cells to survive independently of attachment to the extracellular matrix (ECM) and to overcome ECM-detachment-induced death programs. This can be accomplished through activating mutations in cellular oncogenes that subsequently lead to the inhibition of anoikis and to alterations in productive metabolism. One example of such an oncogene is Ras which is found to be mutated and hyperactivated in a variety of distinct cancers. Despite numerous studies on Ras, the precise molecular mechanisms that facilitate survival during ECM-detachment remain poorly understood. Recently, we discovered that ECM-detached cells harboring oncogenic Ras mutations require signaling through the PI(3)K/SGK1 signaling axis to promote survival. Furthermore, we found that oncogenic Ras can concurrently diminish PHLPP1 phosphatase levels, which results in a decrease in p38 MAPK-mediated activation of anoikis. Thus, our data suggest that cancer cells with activating Ras mutations can survive during ECM-detachment using downstream effector molecules that modulate distinct pathways. Overall, these data suggest that new therapeutic interventions that aim to mitigate SGK1 signaling and activate the p38 MAPK activity may aid in specifically targeting and eliminating metastatic cancer cells.

Colectomy induces an aldosterone-mediated increase in jejunal glucose uptake in rats

AIMS

The main function of the colon is water and electrolyte absorption. Total colectomy eliminates this colonic function and may alter the absorptive capacity of the small intestine for nutrients. This study examines the effect of total colectomy on jejunal glucose absorption and investigates the potential role of aldosterone in mediating the alterations in glucose uptake post-colectomy using the aldosterone antagonist spironolactone.

MAIN METHODS

Total colectomy with ileo-rectal anastomosis was performed on anesthetized rats. Sham rats were identically handled without colon resection. Two days post-surgery, groups of colectomized rats were injected with either a daily subcutaneous dose of spironolactone or sesame oil for 12days. Body weight changes and food and water intake were measured in all experimental groups. Glucose absorption was measured by in-vivo single pass perfusion in the rat jejunum of control, sham, colectomized, colectomized with spironolactone, and colectomized with sesame oil treatment. Na/K ATPase, SGK1, SGLT1 and GLUT2 expressions were determined in jejunal mucosa in control, colectomized and colectomized/spironolactone injected rats by Western blot analysis. Histological assessment was performed on jejunal sections in control and colectomized groups.

KEY FINDINGS

Glucose absorption significantly increased in colectomized rats with an observed increase in Na/K ATPase and SGK1 expression. No significant expression change in SGLT1 and GLUT2 was detected in the jejunum in colectomized rats. Spironolactone, however, significantly decreased the glucose uptake post-colectomy and normalized Na/K ATPase and SGK1 expression.

SIGNIFICANCE

Our results suggest that jejunal glucose uptake increases post-colectomy as a possible consequence of an aldosterone-mediated function.

Insulin and SGK1 reduce the function of Na+/monocarboxylate transporter 1 (SMCT1/SLC5A8)

SMCTs move several important fuel molecules that are involved in lipid, carbohydrate, and amino acid metabolism, but their regulation has been poorly studied. Insulin controls the translocation of several solutes that are involved in energetic cellular metabolism, including glucose. We studied the effect of insulin on the function of human SMCT1 expressed in Xenopus oocytes. The addition of insulin reduced α-keto-isocaproate (KIC)-dependent ²²Na⁺ uptake by 29%. Consistent with this result, the coinjection of SMCT1 with SGK1 cRNA decreased the KIC-dependent ²²Na⁺ uptake by 34%. The reduction of SMCT1 activity by SGK1 depends on its kinase activity, and it was observed that the coinjection of SMCT1 with S442D-SGK1 (a constitutively active mutant) decreased the KIC-dependent ²²Na⁺ uptake by 50%. In contrast, an SMCT1 coinjection with K127M-SGK1 (an inactive mutant) had no effect on the KIC-dependent Na⁺ uptake. The decreasing SMCT1 function by insulin or SGK1 was corroborated by measuring [1-¹⁴C]acetate uptake and the electric currents of SMCT1-injected oocytes. Previously, we found that SMCT2/Slc5a12-mRNA, but not SMCT1/Slc5a8-mRNA, is present in zebrafish pancreas (by in situ hybridization); however, SLC5a8 gene silencing was associated with the development of human pancreatic cancer. We confirmed that the mRNA and protein of both transporters were present in rat pancreas using RT-PCR with specific primers, Western blot analysis, and immunohistochemistry. Additionally, significant propionate-dependent ²²Na⁺ uptake occurred in pancreatic islets and was reduced by insulin treatment. Our data indicate that human SMCT1 is regulated by insulin and SGK1 and that both SMCTs are present in the mammalian pancreas.

Role of SGK1 for fatty acid uptake, cell survival and radioresistance of NCI-H460 lung cancer cells exposed to acute or chronic cycling severe hypoxia

Background

Unsaturated fatty acids (FA) are required for cancer cell growth. In normoxia cells can generate unsaturated FA from saturated stearic and palmitic acid by desaturation. However, since the desaturation step is oxygen-dependent hypoxic cancer cells display an increased dependence on the uptake of unsaturated FA. Up to now the mechanism of increased FA uptake in hypoxia is largely unknown. Here we aimed to study the role of human serum and glucocorticoid-inducible kinase (SGK1) in the regulation of FA uptake in cancer cells exposed to acute or chronic cycling hypoxia and explore its use as target for the radiosensitization of hypoxic cancer cells.

Methods

The effect of SGK1-inhibition (GSK650394) on NCI-H460 lung adenocarcinoma cells exposed to normoxia, acute or chronic cycling hypoxia was analyzed under standard and serum-deprived conditions by short-term proliferation, apoptosis and cell death assays. The impact of SGK1-inhibition on radiation sensitivity was determined by standard colony formation assays. The effect of GSK650394 on FA uptake was quantified by measuring intracellular accumulation of fluorescent FA (C1-BODIPY®-C12).

Results

Exposure to acute or chronic cycling hypoxia was associated with up-regulated expression of SGK1 in NCI-H460 cells, increased uptake of FA from the culture medium, and increased sensitivity to serum deprivation. Survival of serum-deprived hypoxic NCI-H460 cells was rescued by the addition of the unsaturated FA, oleic acid, whereas the saturated FA, palmitic acid was highly toxic to the hypoxic cancer cells. Interestingly, SGK1 inhibition abrogated the rescue effect of oleic acid in serum-deprived hypoxic cancer cells and this effect was associated with a reduction in FA uptake particularly in anoxia-tolerant cancer cells exposed to severe hypoxia. Finally, SKG1 inhibition decreased long-term survival and potently sensitized the parental and anoxia-tolerant NCI-H460 cells to the cytotoxic effects of ionizing radiation in normoxia as well as the anoxia-tolerant cancer cells in severe hypoxia.

Conclusions

Our data suggest that SGK1 plays a role in the regulation of FA uptake that becomes essential under conditions of acute or chronic cycling hypoxia. We assume that SGK1 may represent a promising therapeutic target for the eradication of hypoxic cancer cells.

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.

Serum glucocorticoid inducible kinase (SGK)-1 protects endothelial cells against oxidative stress and apoptosis induced by hyperglycaemia

Diabetic hyperglycaemia causes endothelial dysfunction mainly by impairing endothelial nitric oxide (NO) production. Moreover, hyperglycaemia activates several noxious cellular pathways including apoptosis, increase in reactive oxygen species (ROS) levels and diminishing Na(+)-K(+) ATPase activity which exacerbate vascular damage. Serum glucocorticoid kinase (SGK)-1, a member of the serine/threonine kinases, plays a pivotal role in regulating NO production through inducible NO synthase activation and other cellular mechanisms. Therefore, in this study, we aimed to investigate the protective role of SGK-1 against hyperglycaemia in human umbilical endothelial cells (HUVECs). We used retrovirus to infect HUVECs with either SGK-1, SGK-1Δ60 (lacking of the N-60 amino acids-increase SGK-1 activity) or SGK-1Δ60KD (kinase-dead constructs). We tested our hypothesis in vitro after high glucose and glucosamine incubation. Increase in SGK-1 expression and activity (SGK-1Δ60) resulted in higher production of NO, inhibition of ROS synthesis and lower apoptosis in endothelial cell after either hyperglycaemia or glucosamine treatments. Moreover, in this study, we showed increased GLUT-1 membrane translocation and Na(+)-K(+) ATPase activity in cell infected with SGK-1Δ60 construct. These results suggest that as in endothelial cells, an increased SGK-1 activity and expression reduces oxidative stress, improves cell survival and restores insulin-mediated NO production after different noxae stimuli. Therefore, SGK-1 may represent a specific target to further develop novel therapeutic options against diabetic vascular disease.

Reversal of tumor immune inhibition using a chimeric cytokine receptor

The success of adoptively transferred tumor-directed T cells requires them to survive and expand in vivo. Most tumors, however, employ immune evasion mechanisms, including the production of inhibitory cytokines that limit in vivo T-cell persistence and effector function. To protect tumor-directed T cells from such negative influences, we generated a chimeric cytokine receptor in which the interleukin (IL) 4 receptor exodomain was fused to the IL7 receptor endodomain. We thereby inverted the effects of tumor-derived IL4 so that the proliferation and activation of tumor directed cytotoxic T cells was enhanced rather than inhibited in the tumor microenvironment, resulting in superior antitumor activity. These transgenic T cells were only activated in the tumor environment since triggering required exposure to both tumor antigen (signal 1) and tumor-derived IL4 (signal 2). This selectivity supports future clinical adaptation.

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.

Periodontitis and insulin resistance: casual or causal relationship?

Insulin resistance (IR) is now considered as a chronic and low level inflammatory condition. It is closely related to altered glucose tolerance, hypertriglyceridemia, abdominal obesity, and coronary heart disease. IR is accompanied by the increase in the levels of inflammatory cytokines like interleukin-1 and 6, tumor necrosis factor-α. These inflammatory cytokines also play a crucial part in pathogenesis and progression of insulin resistance. Periodontitis is the commonest of oral diseases, affecting tooth investing tissues. Pro-inflammatory cytokines are released in the disease process of periodontitis. Periodontitis can be attributed with exacerbation of IR. Data in the literature supports a "two way relationship" between diabetes and periodontitis. Periodontitis is asymptomatic in the initial stages of disease process and it often escapes diagnosis. This review presents the blurred nexus between periodontitis and IR, underlining the pathophysiology of the insidious link. The knowledge of the association between periodontitis and IR can be valuable in planning effectual treatment modalities for subjects with altered glucose homeostasis and diabetics. Presently, the studies supporting this association are miniscule. Further studies are mandatory to substantiate the role of periodontitis in the deterioration of IR.

Significance of SGK1 in the regulation of neuronal function

The present brief review highlights the putative role of the serum- and glucocorticoid-inducible-kinase-1 (SGK1) in the regulation of neuronal function. SGK1 is genomically upregulated by cell shrinkage and by a variety of hormones including mineralocorticoids and glucocorticoids. The kinase is activated by insulin and growth factors via phosphatidylinositide-3-kinase (PI3-kinase), phosphoinositide-dependent kinase PDK1 and mammalian target of rapamycin mTORC2. SGK1 upregulates ion channels (e.g. SCN5A, ENaC, ASIC1, TRPV5,6, ROMK, Kv1.1-5, KCNEx/KCNQ1-5, GluR6, VSOAC, ClC2, CFTR), carriers (e.g. NHE3, NKCC2, NCC, NaPiIIb, SMIT, GLUT1,4, SGLT1, NaDC, EAAT1-5, SN1, ASCT2, 4F2/LAT, PepT2), and the Na(+)/K(+)-ATPase. SGK1 regulates enzymes (e.g. glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g. forkhead transcription factor Foxo3a, β-catenin, nuclear factor-kappa-B (NFB)). SGK1 participates in the regulation of transport, hormone release, neuroexcitability, inflammation, coagulation, cell proliferation and apoptosis. SGK1 contributes to regulation of renal Na(+) retention, renal K(+) elimination, salt appetite, gastric acid secretion, intestinal Na(+)/H(+) exchange and nutrient transport, insulin-dependent salt sensitivity of blood pressure, salt sensitivity of peripheral glucose uptake, cardiac repolarization and memory consolidation. Presumably, SGK1 contributes to the regulation of diverse cerebral functions (e.g. memory consolidation, fear retention) and the pathophysiology of several cerebral diseases (e.g. Parkinson's disease, schizophrenia, depression, Alzheimer's disease). Despite multiple SGK1 functions, the phenotype of the SGK1 knockout mouse is mild and becomes only apparent under challenging conditions.

Renal glucose transporters: novel targets for hyperglycemia management

The naturally occurring substance phlorizin has long been recognized to block the reabsorption of glucose from the glomerular ultrafiltrate into the systemic circulation. The poor oral bioavailability and adverse effects associated with this agent, however, have prevented its use in clinical practice and restricted its use to that of a physiological tool. The development of novel agents that are able to block the principal glucose transporter in the kidney has allowed the inhibition of renal glucose reabsorption to be re-evaluated as a therapeutic tool in patients with diabetes mellitus. This Perspectives article summarizes current knowledge pertaining to glucose transport in the kidney and describes the evidence regarding glucose transport blockade as a novel target for the management of hyperglycemia in the context of existing treatment strategies.

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.

Pathomechanisms of type 2 diabetes genes

Type 2 diabetes mellitus is a complex metabolic disease that is caused by insulin resistance and beta-cell dysfunction. Furthermore, type 2 diabetes has an evident genetic component and represents a polygenic disease. During the last decade, considerable progress was made in the identification of type 2 diabetes risk genes. This was crucially influenced by the development of affordable high-density single nucleotide polymorphism (SNP) arrays that prompted several successful genome-wide association scans in large case-control cohorts. Subsequent to the identification of type 2 diabetes risk SNPs, cohorts thoroughly phenotyped for prediabetic traits with elaborate in vivo methods allowed an initial characterization of the pathomechanisms of these SNPs. Although the underlying molecular mechanisms are still incompletely understood, a surprising result of these pathomechanistic investigations was that most of the risk SNPs affect beta-cell function. This favors a beta-cell-centric view on the genetics of type 2 diabetes. The aim of this review is to summarize the current knowledge about the type 2 diabetes risk genes and their variants' pathomechanisms.

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.

SGK1-sensitive renal tubular glucose reabsorption in diabetes

The hyperglycemia of diabetes mellitus increases the filtered glucose load beyond the maximal tubular transport rate and thus leads to glucosuria. Sustained hyperglycemia, however, may gradually increase the maximal renal tubular transport rate and thereby blunt the increase of urinary glucose excretion. The mechanisms accounting for the increase of renal tubular glucose transport have remained ill-defined. A candidate is the serum- and glucocorticoid-inducible kinase SGK1. The kinase has been shown to stimulate Na(+)-coupled glucose transport in vitro and mediate the stimulation of electrogenic intestinal glucose transport by glucocorticoids in vivo. SGK1 expression is confined to glomerula and distal nephron in intact kidneys but may extend to the proximal tubule in diabetic nephropathy. To explore whether SGK1 modifies glucose transport in diabetic kidneys, Akita mice (akita(+/-)), which develop spontaneous diabetes, have been crossbred with gene-targeted mice lacking SGK1 on one allele (sgk1(+/-)) to eventually generate either akita(+/-)/sgk1(-/-) or akita(+/-)/sgk1(+/+) mice. Both akita(+/-)/sgk1(-/-) and akita(+/-)/sgk1(+/+) mice developed profound hyperglycemia (>20 mM) within approximately 6 wk. Body weight and plasma glucose concentrations were not significantly different between these two genotypes. However, urinary excretion of glucose and urinary excretion of fluid, Na(+), and K(+), as well as plasma aldosterone concentrations, were significantly higher in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. Studies in isolated perfused proximal tubules revealed that the electrogenic glucose transport was significantly lower in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. The data provide the first evidence that SGK1 participates in the stimulation of renal tubular glucose transport in diabetic kidneys.

Variance of the SGK1 gene is associated with insulin secretion in different European populations: results from the TUEF, EUGENE2, and METSIM studies

Hypothesis

Serum- and Glucocorticoid-inducible Kinase 1 (SGK1) is involved in the regulation of insulin secretion and may represent a candidate gene for the development of type 2 diabetes mellitus in humans.

Methods

Three independent European populations were analyzed for the association of SGK1 gene (SGK) variations and insulin secretion traits. The German TUEF project provided the screening population (N = 725), and four tagging SNPs (rs1763527, rs1743966, rs1057293, rs9402571) were investigated. EUGENE2 (N = 827) served as a replication cohort for the detected associations. Finally, the detected associations were validated in the METSIM study, providing 3798 non-diabetic and 659 diabetic (type 2) individuals.

Results

Carriers of the minor G allele in rs9402571 had significantly higher C-peptide levels in the 2 h OGTT (+10.8%, p = 0.04; dominant model) and higher AUC_C-Peptide/AUC_Glc ratios (+7.5%, p = 0.04) compared to homozygous wild type TT carriers in the screening population. As interaction analysis for BMI×rs9402571 was significant (p = 0.04) for the endpoint insulin secretion, we stratified the TUEF cohort for BMI, using a cut off point of BMI = 25. The effect on insulin secretion only remained significant in lean TUEF participants (BMI≤25). This finding was replicated in lean EUGENE2 rs9402571 minor allele carriers, who had a significantly higher AUC_Ins/AUC_Glc (TT: 226±7, XG: 246±9; p = 0.019). Accordingly, the METSIM trial revealed a lower prevalence of type 2 diabetes (OR: 0.85; 95%CI: 0.71–1.01; p = 0.065, dominant model) in rs9402571 minor allele carriers.

Conclusions

The rs9402571 SGK genotype associates with increased insulin secretion in lean non-diabetic TUEF/EUGENE2 participants and with lower diabetes prevalence in METSIM. Our study in three independent European populations supports the conclusion that SGK variability affects diabetes risk.

SGK1 dependence of insulin induced hypokalemia

Insulin stimulates cellular K+ uptake leading to hypokalemia. Cellular K+ uptake is accomplished by parallel stimulation of Na+/H+ exchange, Na+,K+,2Cl- co-transport, and Na+/K+ ATPase and leads to cell swelling, a prerequisite for several metabolic effects of the hormone. Little is known about underlying signaling. Insulin is known to activate the serum and glucocorticoid-inducible kinase SGK1, which in turn enhances the activity of all three transport proteins. The present study thus explored the contribution of SGK1 to insulin-induced hypokalemia. To this end, gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+) have been infused with insulin (2 mU kg(-1) min(-1)) and glucose at rates leaving the plasma glucose concentration constant. Moreover, isolated liver perfusion experiments have been performed to determine stimulation of cellular K+ uptake by insulin (100 nM). As a result, combined glucose and insulin infusion significantly decreased plasma K+ concentration despite a significant decrease of urinary K+ excretion in sgk1+/+ but not in sgk1-/- mice. Accordingly, the plasma K+ concentration was within 60 min significantly lower in sgk1+/+ than in sgk1-/- mice. In isolated liver perfusion experiments, cellular K+ uptake was stimulated by insulin (100 nM), an effect blunted by 72% in sgk1-/- mice as compared to sgk1+/+ mice. Accordingly, insulin-induced cell hydration was 63% lower in sgk1-/- mice than in sgk1+/+ mice. Moreover, volume regulatory K+ release was 31% smaller in sgk1-/- mice than in sgk1+/+ mice. In conclusion, the serum and glucocorticoid-inducible kinase SGK1 participates in the signaling mediating the hypokalemic effect of insulin.

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.

Akt mediates the effect of insulin on epithelial sodium channels by inhibiting Nedd4-2

The epithelial sodium channel (ENaC) plays an important role in transepithelial Na(+) absorption; hence its function is essential for maintaining Na(+) and fluid homeostasis and regulating blood pressure. Insulin is one of the hormones that regulates activity of ENaC. In this study, we investigated the contribution of two related protein kinases, Akt (also known as protein kinase B) and the serum- and glucocorticoid-dependent kinase (Sgk), on insulin-induced ENaC activity in Fisher rat thyroid cells expressing ENaC. Overexpression of Akt1 or Sgk1 significantly increased ENaC activity, whereas expression of a dominant-negative construct of Akt1, Akt1(K179M), decreased basal activity of ENaC. Inhibition of the endogenous expression of Akt1 and Sgk1 by short interfering RNA not only inhibited ENaC but also disrupted the stimulatory effect on ENaC of insulin and of the downstream effectors of insulin, phosphatidylinositol 3-kinase and PDK1. Conversely, overexpression of Akt1 or Sgk1 increased expression of ENaC at the cell membrane and overcame the inhibitory effect of Nedd4-2 on ENaC. Furthermore, mutation of consensus phosphorylation sites on Nedd4-2 for Akt1 and Sgk1, Ser(342) and Ser(428), completely abolished the inhibitory effect of Sgk1 and Akt1 on Nedd4-2 action. Together these data suggest that both Akt and Sgk are components of an insulin signaling pathway that increases Na(+) absorption by up-regulating membrane expression of ENaC via a regulatory system that involves inhibition of Nedd4-2.

Controlling cell growth and survival through regulated nutrient transporter expression

Although all cells depend upon nutrients they acquire from the extracellular space, surprisingly little is known about how nutrient uptake is regulated in mammalian cells. Most nutrients are brought into cells by means of specific transporter proteins. In yeast, the expression and trafficking of a wide variety of nutrient transporters is controlled by the TOR (target of rapamycin) kinase. Consistent with this, recent studies in mammalian cells have shown that mTOR (mammalian TOR) and the related protein, PI3K (phosphoinositide 3-kinase), play central roles in coupling nutrient transporter expression to the availability of extrinsic trophic and survival signals. In the case of lymphocytes, it has been particularly well established that these extrinsic signals stimulate cell growth and proliferation in part by regulating nutrient transporter expression. The ability of growth factors to control nutrient access may also play an important role in tumour suppression: the non-homoeostatic growth of tumour cells requires that nutrient transporter expression is uncoupled from trophic factor availability. Also supporting a link between nutrient transporter expression levels and oncogenesis, several recent studies demonstrate that nutrient transporter expression drives, rather than simply parallels, cellular metabolism. This review summarizes the evidence that regulated nutrient transporter expression plays a central role in cellular growth control and highlights the implications of these findings for human disease.

Role of SGK1 kinase in regulating glucose transport via glucose transporter GLUT4

Insulin stimulates glucose transport into muscle and fat cells by enhancing GLUT4 abundance in the plasma membrane through activation of phosphatidylinositol 3-kinase (PI3K). Protein kinase B (PKB) and PKCzeta are known PI3K downstream targets in the regulation of GLUT4. The serum- and glucocorticoid-inducible kinase SGK1 is similarly activated by insulin and capable to regulate cell surface expression of several metabolite transporters. In this study, we evaluated the putative role of SGK1 in the modulation of GLUT4. Coexpression of the kinase along with GLUT4 in Xenopus oocytes stimulated glucose transport. The enhanced GLUT4 activity was paralleled by increased transporter abundance in the plasma membrane. Disruption of the SGK1 phosphorylation site on GLUT4 ((S274A)GLUT4) abrogated the stimulating effect of SGK1. In summary, SGK1 promotes glucose transporter membrane abundance via GLUT4 phosphorylation at Ser274. Thus, SGK1 may contribute to the insulin and GLUT4-dependent regulation of cellular glucose uptake.

EAAT4 phosphorylation at the SGK1 consensus site is required for transport modulation by the kinase

EAAT4 (SLC1A6) is a Purkinje-Cell-specific post-synaptic excitatory amino acid transporter that plays a major role in clearing synaptic glutamate. EAAT4 abundance and function is known to be modulated by the serum and glucocorticoid inducible kinase (SGK) 1 but the precise mechanism of kinase action has not been defined yet. The present work aims to identify the molecular mechanism of EAAT4 modulation by the kinase. The EAAT4 sequence bears two putative SGK1 consensus sites (at Thr40 and Thr504) at the amino and carboxy terminus that are conserved among species. Expression studies in Xenopus oocytes demonstrated that EAAT4-mediated [(3)H] glutamate uptake and cell surface abundance are enhanced by co-expression of SGK1. Disruption of the SGK1 phosphorylation site at threonine 40 ((T40A)EAAT4) or of both phosphorylation sites ((T40AT504A)EAAT4) abrogated the effect of SGK1 on transporter function and expression. SGK1 modulates several transport proteins via inhibition of the ubiquitin ligase Nedd4-2. Co-expression of Nedd4-2 inhibited wild-type EAAT4 but not the (T40AT504A)EAAT4 mutant. Besides, RNA interference-mediated reduction of endogenous Nedd4-2 (xNedd4-2) expression increased the activity of the transporter. In conclusion, maximal glutamate transport modulation by SGK1 is accomplished by direct EAAT4 stimulation and to a lesser extent by inhibition of intrinsic Nedd4-2.

Resistance of mice lacking the serum- and glucocorticoid-inducible kinase SGK1 against salt-sensitive hypertension induced by a high-fat diet

Mineralocorticoids enhance expression and insulin stimulates activity of the serum- and glucocorticoid-inducible kinase SGK1, which activates the renal epithelial Na+ channel (ENaC). Under a salt-deficient diet, SGK1 knockout mice ( sgk1−/−) excrete significantly more NaCl than their wild-type littermates ( sgk1 +/+) and become hypotensive. The present experiments explored whether SGK1 participates in the hypertensive effects of a high-fat diet and high-salt intake. Renal SGK1 protein abundance of sgk1 +/+ mice was significantly elevated after a high-fat diet. Under a control diet, fluid intake, blood pressure, urinary flow rate, and urinary Na+, K+, and Cl− excretion were similar in sgk1−/− and sgk1 +/+ mice. Under a standard diet, high salt (1% NaCl in the drinking water for 25 days) increased fluid intake, urinary flow rate, and urinary Na+, K+, and Cl− excretion similarly in sgk1−/− and sgk1 +/+ mice without significantly altering blood pressure. A high-fat diet alone (17 wk) did not significantly alter fluid intake, urinary flow rate, urinary Na+, K+, or Cl− excretion, or plasma aldosterone levels but increased plasma insulin, total cholesterol, triglyceride concentrations, and systolic blood pressure to the same extent in both genotypes. Additional salt intake (1% NaCl in the drinking water for 25 days) on top of a high-fat diet did not affect hyperinsulinemia or hyperlipidemia but increased fluid intake, urinary flow rate, and urinary NaCl excretion significantly more in sgk1−/− than in sgk1 +/+mice. Furthermore, in animals receiving a high-fat diet, additional salt intake increased blood pressure only in sgk1 +/+ mice (to 132 ± 3 mmHg) but not in sgk1−/− mice (120 ± 4 mmHg). Thus lack of SGK1 protects against the hypertensive effects of a combined high-fat/high-salt diet.

(Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms

The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.

Reduced intestinal and renal amino acid transport in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 activates the serum- and glucocorticoid-inducible kinase isoforms SGK1, SGK2, and SGK3 and protein kinase B, which in turn are known to up-regulate a variety of sodium-coupled transporters. The present study was performed to explore the role of PDK1 in amino acid transport. As mice completely lacking functional PDK1 are not viable, mice expressing 10-25% of PDK1 (pdk1(hm)) were compared with their wild-type (WT) littermates (pdk1(wt)). Body weight was significantly less in pdk1(hm) than in pdk1(wt) mice. Despite lower body weight of pdk1(hm) mice, food and water intake were similar in pdk1(hm) and pdk1(wt) mice. According to Ussing chamber experiments, electrogenic transport of phenylalanine, cysteine, glutamine, proline, leucine, and tryptophan was significantly smaller in jejunum of pdk1(hm) mice than in pdk1(wt) mice. Similarly, electrogenic transport of phenylalanine, glutamine, and proline was significantly decreased in isolated perfused proximal tubules of pdk1(hm) mice. The urinary excretion of proline, valine, guanidinoacetate, methionine, phenylalanine, citrulline, glutamine/glutamate, and tryptophan was significantly larger in pdk1(hm) than in pdk1(wt) mice. According to immunoblotting of brush border membrane proteins prepared from kidney, expression of the Na+-dependent neutral amino acid transporter B(0)AT1 (SLC6A19), the glutamate transporter EAAC1/EAAT3 (SLC1A1), and the transporter for cationic amino acids and cystine b(0,+)AT (SLC7A9) was decreased but the Na+/proline cotransporter SIT (SLC6A20) was increased in pdk1(hm) mice. In conclusion, reduction of functional PDK1 leads to impairment of intestinal absorption and renal reabsorption of amino acids. The combined intestinal and renal loss of amino acids may contribute to the growth defect of PDK1-deficient mice.