Sgk, a putative serine/threonine kinase, is differentially expressed in the kidney of diabetic mice and humans

Mineralocorticoid Receptor Pathway Is a Key Mediator of Carfilzomib-induced Nephrotoxicity: Preventive Role of Eplerenone

Carfilzomib is an irreversible proteasome inhibitor indicated for relapsed/refractory multiple myeloma. Carfilzomib toxicity includes renal adverse effects (RAEs) of obscure pathobiology. Therefore, we investigated the mechanisms of nephrotoxicity developed by Carfilzomib. In a first experimental series, we used our previously established in vivo mouse models of Carfilzomib cardiotoxicity, that incorporated 2 and 4 doses of Carfilzomib, to identify whether Carfilzomib affects renal pathways. Hematology and biochemical analyses were performed, while kidneys underwent histological and molecular analyses. In a second and third experimental series, the 4 doses protocol was repeated for 24 hours urine collection and proteomic/metabolomic analyses. To test an experimental intervention, primary murine collecting duct tubular epithelial cells were treated with Carfilzomib and/or Eplerenone and Metformin. Finally, Eplerenone was orally co-administered with Carfilzomib daily (165 mg/kg) in the 4 doses protocol. We additionally used material from 7 patients to validate our findings and patients underwent biochemical analysis and assessment of renal mineralocorticoid receptor (MR) axis activation. In vivo screening showed that Carfilzomib-induced renal histological deficits and increased serum creatinine, urea, NGAL levels, and proteinuria only in the 4 doses protocol. Carfilzomib decreased diuresis, altered renal metabolism, and activated MR axis. This was consistent with the cytotoxicity found in primary murine collecting duct tubular epithelial cells, whereas Carfilzomib + Eplerenone co-administration abrogated Carfilzomib-related nephrotoxic effects in vitro and in vivo. Renal SGK-1, a marker of MR activation, increased in patients with Carfilzomib-related RAEs. Conclusively, Carfilzomib-induced renal MR/SGK-1 activation orchestrates RAEs and water retention both in vivo and in the clinical setting. MR blockade emerges as a potential therapeutic approach against Carfilzomib-related nephrotoxicity.

Renal Inflammation Induces Salt Sensitivity in Male db/db Mice through Dysregulation of ENaC

BACKGROUND

Hypertension is considered a major risk factor for the progression of diabetic kidney disease. Type 2 diabetes is associated with increased renal sodium reabsorption and salt-sensitive hypertension. Clinical studies show that men have higher risk than premenopausal women for the development of diabetic kidney disease. However, the renal mechanisms that predispose to salt sensitivity during diabetes and whether sexual dimorphism is associated with these mechanisms remains unknown.

METHODS

Female and male db/db mice exposed to a high-salt diet were used to analyze the progression of diabetic kidney disease and the development of hypertension.

RESULTS

Male, 34-week-old, db/db mice display hypertension when exposed to a 4-week high-salt treatment, whereas equivalently treated female db/db mice remain normotensive. Salt-sensitive hypertension in male mice was associated with no suppression of the epithelial sodium channel (ENaC) in response to a high-salt diet, despite downregulation of several components of the intrarenal renin-angiotensin system. Male db/db mice show higher levels of proinflammatory cytokines and more immune-cell infiltration in the kidney than do female db/db mice. Blocking inflammation, with either mycophenolate mofetil or by reducing IL-6 levels with a neutralizing anti-IL-6 antibody, prevented the development of salt sensitivity in male db/db mice.

CONCLUSIONS

The inflammatory response observed in male, but not in female, db/db mice induces salt-sensitive hypertension by impairing ENaC downregulation in response to high salt. These data provide a mechanistic explanation for the sexual dimorphism associated with the development of diabetic kidney disease and salt sensitivity.

Lack of serum- and glucocorticoid-inducible kinase 3 leads to podocyte dysfunction

Serum- and glucocorticoid-inducible kinase 3 (SGK3) is a downstream mediator of PI3K, which is essential for maintaining the functional integrity of podocytes. However, little is known about the role of SGK3 in podocyte function. Herein, we demonstrated that SGK3 contributes to the maintenance of podocyte integrity. Conditionally immortalized mouse podocyte cells (MPCs) were treated with puromycin aminonucleoside (PAN). PAN treatment inhibited the activity of SGK3 and the expression of podocin. Short hairpin RNA (shRNA)-mediated knockdown of SGK3 also reduced podocin expression in the absence of PAN. Adriamycin (ADR)-treated mice developed proteinuria and had decreased renal glomerular SGK3 expression in comparison to control mice. Consistent with a role for SGK3 in the ADR effect, SGK3 knockout (KO) mice had markedly reduced kidney podocin expression and significantly elevated proteinuria compared with wild-type mice. Electron microscopy revealed that SGK3 KO mice displayed partial effacement of podocyte foot processes. Further, a SGK3 target protein, glycogen synthase kinase-3 (GSK3), was discovered to be dramatically activated in PAN and SGK3 shRNA-treated MPCs and in SGK3 KO mice. Taken together, these data strongly suggest that SGK3 plays a significant role in regulating podocyte function, likely by controlling the expression and activity of GSK3.-Peng, L.-Q., Zhao, H., Liu, S., Yuan, Y.-P., Yuan, C.-Y., Mwamunyi, M.-J., Pearce, D., Yao, L.-J. Lack of serum- and glucocorticoid-inducible kinase 3 leads to podocyte dysfunction.

SGK1 is regulated by metabolic-related factors in 3T3-L1 adipocytes and overexpressed in the adipose tissue of subjects with obesity and diabetes

AIMS

The present study aimed to investigate the pathophysiological role of SGK1 in the development of metabolic syndrome by investigating the expression and regulation of serum and glucocorticoid-inducible kinase 1 (SGK1) in adipose tissues in obesity and diabetes.

METHODS

SGK1 expression in adipose tissue was investigated using reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. SGK1 regulation in differentiated 3T3-L1 adipocytes by metabolic-related factors was assessed using Northern blot analysis. Humans with obesity and type 2 diabetes and KKAy and db/db mice were used to evaluate SGK1 expression in the adipose tissue of subjects with obesity and diabetes using quantitative real-time PCR and Western blot analysis.

RESULTS

SGK1 was expressed in white adipose tissue as shown by mRNA and protein levels. Aldosterone and glucocorticoids stimulated SGK1 expression in a time- and dose-dependent manner, whereas PPAR-γ agonists inhibited SGK1 expression in differentiated 3T3-L1 adipocytes. Furthermore, SGK1 mRNA and protein were overexpressed in the adipose tissue of mice and humans with obesity and type 2 diabetes.

CONCLUSION

Aldosterone, glucocorticoids and other factors contribute to excessive SGK1 expression in adipose tissue. This excessive SGK1 expression may be related to adipose tissue dysfunction, which may contribute to the development of obesity, diabetes and metabolic syndrome.

TGFβ1 and SGK1-sensitive store-operated Ca2+ entry and Orai1 expression in endometrial Ishikawa cells

The serum-and-glucocorticoid-inducible-kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress, hormones and further mediators. A most powerful stimulator of SGK1 expression is transforming growth factor TGFβ1. SGK1 is activated by insulin and growth factors via phosphatidylinositol-3-kinase and the 3-phosphoinositide-dependent kinase PDK1. As shown recently, SGK1 increases the store-operated Ca(2+) entry (SOCE), which is accomplished by the pore-forming ion channel unit Orai. Most recent observations further revealed that SGK1 plays a critical role in the regulation of fertility. SGK1 is up-regulated in the luminal epithelium of women with unexplained infertility but down-regulated in decidualizing stromal cells of patients with recurrent pregnancy loss. The present study explored whether Orai1 is expressed in endometrium and sensitive to regulation by SGK1 and/or TGFβ1. To this end, Orai1 protein abundance was determined by western blotting and SOCE by fura-2 fluorescence. As a result, Orai1 was expressed in human endometrium and in human endometrial Ishikawa cells. Orai1 expression and SOCE in Ishikawa cells were increased by transfection with constitutively active (S422D)SGK1 but not by transfection with inactive (K127N)SGK1. The difference of SOCE between (S422D)SGK1 and (K127N)SGK1-transfected cells was virtually abrogated in the presence of Orai1 inhibitor 2-aminoethoxydiphenyl borate (2-APB, 50 µM). Similar to (S422D)SGK1 transfection TGFβ1 treatment up-regulated both Orai1 protein abundance and SOCE. In conclusion, Orai1 is expressed in the human endometrium and is up-regulated by SGK1 and TGFβ1. The present observations thus uncover a novel element in SGK1-sensitive regulation of endometrial cells.

Functional expression of TRPV4 channels in human collecting duct cells: implications for secondary hypertension in diabetic nephropathy

Background. The Vanilloid subfamily of transient receptor potential (TRPV) ion channels has been widely implicated in detecting osmotic and mechanical stress. In the current study, we examine the functional expression of TRPV4 channels in cell volume regulation in cells of the human collecting duct. Methods. Western blot analysis, siRNA knockdown, and microfluorimetry were used to assess the expression and function of TRPV4 in mediating Ca²⁺-dependent mechanical stimulation within a novel system of the human collecting duct (HCD). Results. Native and siRNA knockdown of TRPV4 protein expression was confirmed by western blot analysis. Touch was used as a cell-directed surrogate for osmotic stress. Mechanical stimulation of HCD cells evoked a transient increase in [Ca²⁺]_i that was dependent upon thapsigargin-sensitive store release and Ca²⁺ influx. At 48 hrs, high glucose and mannitol (25 mM) reduced TRPV4 expression by 54% and 24%, respectively. Similar treatment doubled SGK1 expression. Touch-evoked changes were negated following TRPV4 knockdown. Conclusion. Our data confirm expression of Ca²⁺-dependent TRPV4 channels in HCD cells and suggest that a loss of expression in response to high glucose attenuates the ability of the collecting duct to exhibit regulatory volume decreases, an effect that may contribute to the pathology of fluid and electrolyte imbalance as observed in diabetic nephropathy.

Collagen VIII influences epithelial phenotypic changes in experimental diabetic nephropathy

Epithelial-to-mesenchymal transition (EMT) is an important mechanism of renal tubulo-interstitial fibrosis in diabetic nephropathy (DN). Inducers of EMT, among others, are transforming growth factor-β(1) (TGF-β(1)) as well as extracellular collagens. In renal cells of diabetic mice and in kidneys of patients with DN, the expression of collagen VIII (gene: Col8α1/α2) is enhanced and characteristic features of DN in streptozotocin (STZ)-induced diabetic Col8α1/α2 knockout-(KO) mice are attenuated compared with diabetic wild-type mice. This study aimed to investigate whether collagen type VIII may influence the induction of EMT. DN was induced in wild-type and Col8α1/α2-KO mice using the established and widely accepted low-dose STZ model [treatment for 5 consecutive days (50 mg/kg)]. Healthy and diabetic mice were analyzed for changes in renal function and the expression of EMT-related genes and proteins. Renal morphology, fibrosis, and various EMT markers were studied in kidneys using immunohistological and molecular biological methods. Knockout of Col8α1/α2 attenuated albuminuria, extracellular matrix production, as well as fibrosis. Furthermore, the kidneys of diabetic Col8α1/α2-KO mice showed a marked reduction in interstitial myofibroblasts, and in tubular cells the inhibition of the expression of epithelial markers as well as the expression of typical mesenchymal markers was reduced. The present study demonstrates that in contrast to diabetic wild-type mice EMT-like changes were attenuated in diabetic Col8α1/α2-KO mice, which indicates that either collagen VIII may be one of the major inducers of EMT-like changes in kidneys of diabetic wild-type mice or/possibly the lack of Col8α1/α2 disrupts TGF-β(1)-induced EMT-like changes.

The role of TGF-β and epithelial-to mesenchymal transition in diabetic nephropathy

Transforming Growth Factor-beta (TGF-β) is a pro-sclerotic cytokine widely associated with the development of fibrosis in diabetic nephropathy. Central to the underlying pathology of tubulointerstitial fibrosis is epithelial-to-mesenchymal transition (EMT), or the trans-differentiation of tubular epithelial cells into myofibroblasts. This process is accompanied by a number of key morphological and phenotypic changes culminating in detachment of cells from the tubular basement membrane and migration into the interstitium. Ultimately these cells reside as activated myofibroblasts and further exacerbate the state of fibrosis. A large body of evidence supports a role for TGF-β and downstream Smad signalling in the development and progression of renal fibrosis. Here we discuss a role for TGF-β as the principle effector in the development of renal fibrosis in diabetic nephropathy, focusing on the role of the TGF-β1 isoform and its downstream signalling intermediates, the Smad proteins. Specifically we review evidence for TGF-β1 induced EMT in both the proximal and distal regions of the nephron and describe potential therapeutic strategies that may target TGF-β1 activity.

Structural, functional, and molecular alterations produced by aldosterone plus salt in rat heart: association with enhanced serum and glucocorticoid-regulated kinase-1 expression

We aimed to evaluate the structural, functional, inflammatory, and oxidative alterations, as well as serum and glucocorticoid-regulated kinase-1 (SGK-1) expression, produced in rat heart by aldosterone + salt administration. Fibrosis mediators such as connective tissue growth factor, matrix metalloproteinase 2, and tissue inhibitor of metalloproteinases 2 were also evaluated. Treatment with spironolactone was evaluated to prove mineralocorticoid mediation. Male Wistar rats received aldosterone (1 mg[middle dot]kg-1[middle dot]d-1) + 1% NaCl for 3 weeks. Half of the animals were treated with spironolactone (200 mg[middle dot]kg-1[middle dot]d-1). Systolic and diastolic blood pressures, left ventricle (LV) systolic pressure, and LV end-diastolic pressure were elevated (P < 0.05) in aldosterone + salt-treated rats. In aldosterone + salt-treated rats, -dP/dt decreased (P < 0.05), but +dP/dt was similar in all groups. Spironolactone normalized (P < 0.05) systolic blood pressure, diastolic blood pressure, LV systolic pressure, LV end-diastolic pressure, and -dP/dt. Relative heart weight, collagen content, messenger RNA expression of transforming growth factor beta, connective tissue growth factor, matrix metalloproteinase 2, tissue inhibitor of metalloproteinases 2, tumor necrosis factor alpha, interleukin-1[beta], p22phox, endothelial nitric oxide synhtase, and SGK-1 were increased (P < 0.05) in aldosterone + salt-treated rats, being reduced by spironolactone (P < 0.05). SGK-1 might be a key mediator in the structural, functional, and molecular cardiac alterations induced by aldosterone + salt in rats. All the observed changes and mediators are related with the activation of mineralocorticoid receptors.

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.

Activation of the aldosterone/mineralocorticoid receptor system in chronic kidney disease and metabolic syndrome

Recent clinical and experimental studies have shown that aldosterone is a potent inducer of proteinuria and that mineralocorticoid receptor (MR) antagonists confer efficient antiproteinuric effects. We identified glomerular epithelial cells (podocytes) as novel targets of aldosterone; activation of MR injures podocytes possibly via oxidative stress, resulting in disruption of glomerular filtration barrier, proteinuria, and progression of chronic kidney disease. We also demonstrated that SHR/cp, a rat model of metabolic syndrome, was susceptible to podocyte injury and proteinuria. Aldosterone excess caused by adipocyte-derived aldosterone-releasing factors was suggested to underlie the nephropathy. High salt intake augmented MR activation in the kidney and exacerbated the nephropathy. Furthermore, we identified an alternative pathway of MR activation by small GTPase Rac1. RhoGDIalpha knockout mice, a model with Rac1 activation in the kidney, showed albuminuria, podocyte injury, and glomerulosclerosis. Renal injury in the knockout mice was accompanied by enhanced MR signaling in the kidney despite normoaldosteronemia, and was ameliorated by an MR antagonist, eplerenone. Moreover, Rac-specific inhibitor significantly reduced the nephropathy, concomitantly with repression of MR activation. In vitro transfection studies provided direct evidence of Rac1-mediated MR activation. In conclusion, our findings suggest that MR activation plays a pivotal role in the pathogenesis of chronic kidney disease in metabolic syndrome, and that MR may be activated both aldosterone dependently (via aldosterone-releasing factors) and independently (via Rac1). MR antagonists are promising antiproteinuric drugs in metabolic syndrome, although long-term effects on renal outcomes, mortality, and safety need to be established.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

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.

SGK1-dependent upregulation of connective tissue growth factor by angiotensin II

Angiotensin II has previously been shown to trigger fibrosis, an effect involving connective tissue growth factor (CTGF). The signaling pathways linking angiotensin II to CTGF formation are, however, incompletely understood. A gene highly expressed in fibrosing tissue is the serum- and glucocorticoid-inducible kinase SGK1. The present study explored whether SGK1 is transcriptionally regulated by angiotensin II and participates in the angiotensin II-dependent regulation of CTGF expression. To this end, experiments have been performed in human kidney fibroblasts and mouse lung fibroblasts from gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+). In human renal fibroblasts, SGK1 and CTGF protein expression were enhanced by angiotensin II (10 nM) within 4 h. In sgk1+/+ mouse fibroblasts, SGK1 transcript levels were significantly increased after 4 h of angiotensin II treatment. Angiotensin II stimulated both transcript and protein abundance of CTGF in fibroblasts from sgk1+/+ mice, effects significantly blunted in fibroblasts of sgk1-/- mice. In conclusion, angiotensin II stimulates the expression of SGK1, which is in turn required for the stimulating effect of angiotensin II on the expression of CTGF. Thus, SGK1 presumably contributes to the profibrotic effect of angiotensin II.

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.

The stimulus-dependent co-localization of serum- and glucocorticoid-regulated protein kinase (Sgk) and Erk/MAPK in mammary tumor cells involves the mutual interaction with the importin-alpha nuclear import protein

In Con8 rat mammary epithelial tumor cells, indirect immunofluorescence revealed that Sgk (serum- and glucocorticoid-regulated kinase) and Erk/MAPK (extracellular signal-regulated protein kinase/mitogen activated protein kinase) co-localized to the nucleus in serum-treated cells and to the cytoplasmic compartment in cells treated with the synthetic glucocorticoid dexamethasone. Moreover, the subcellular distribution of the importin-alpha nuclear transport protein was similarly regulated in a signal-dependent manner. In vitro GST-pull down assays revealed the direct interaction of importin-alpha with either Sgk or Erk/MAPK, while RNA interference knockdown of importin-alpha expression disrupted the localization of both Sgk and Erk into the nucleus of serum-treated cells. Wild type or kinase dead forms of Sgk co-immunoprecipitated with Erk/MAPK from either serum- or dexamethasone-treated mammary tumor cells, suggesting the existence of a protein complex containing both kinases. In serum-treated cells, nucleus residing Sgk and Erk/MAPK were both hyperphosphorylated, indicative of their active states, whereas, in dexamethasone-treated cells Erk/MAPK, but not Sgk, was in its inactive hypophosphorylated state. Treatment with a MEK inhibitor, which inactivates Erk/MAPK, caused the relocalization of both Sgk and ERK to the cytoplasm. We therefore propose that the signal-dependent co-localization of Sgk and Erk/MAPK mediated by importin-alpha represents a new pathway of signal integration between steroid and serum/growth factor-regulated pathways.

Expression of serum and glucocorticoid-inducible kinase1 in diabetic rats and its modulation by fluvastatin

The expression of serum and glucocorticoid-induced protein kinase in the renal cortex of diabetic rats was examined, and the function of signal transduction mediated by SGK1 in diabetic nephropathy and its modulatiqn by fluvastatin were also investigated. 24 male Wistar rats were randomly divided into normal control group (n = 8), diabetic nephropathy group (n = 8) and fluvastatin-treated diabetic nephropathy group (15 mg/kg/d, n = 8). The metabolic parameters were measured at the 8th week. The expression of transforming growth factor beta1 (TGF-beta1) and fibronectin (FN) was immunohistochemically examined. The expression of SGK1 was detected by RT-PCR and Western blot, and CTGF mRNA was assessed by RT-PCR. As compared to DN, blood glucose, 24-h urinary protein, Cer and kidney weight index were all decreased and the weight was increased obviously in group F. At the same time, mesangial cells and extracellular matrix proliferation were relieved significantly. The levels of cortex SGK1 mRNA and protein were up-regulated, and both TGF-beta1 and FN were down-regulated by fluvastatin. The mRNA of SGK1 was positively correlated with the CTGF, TGF-beta1 and FN. SGK1 expression is markedly up-regulated in the renal cortex of DN group and plays an important role in the development and progress of diabetic nephropathy by means of signal transduction. Fluvastatin suppressed the increased SGK1mRNA expression in renal cortex and postponed the development of diabetic nephropathy.

The interdependence of EGF-R and SGK-1 in fibronectin expression in primary kidney cortical fibroblast cells

BACKGROUND

Epidermal growth factor (EGF) has been shown to play a role in the nephromegaly and enhanced sodium reabsorption observed in diabetic nephropathy. This is recognized to be dependent on activation of serine threonine glucocorticoid kinase-1 (SGK-1). However, the roles of EGF and SGK-1 in renal fibrogenesis observed under high glucose conditions have not been established.

METHODS

Primary cultures of human cortical fibroblasts (CFs) were used as the model in which to study the dependent and independent effects of high glucose, EGF and SGK-1 on the expression of the extracellular matrix protein (ECM) fibronectin. Wild type CFs expressing SGK-1, or cells in which SGK-1 was effectively silenced using siRNA methodology, were exposed to normal (5mM) or high (25mM) glucose, or EGF (10ng/ml) for 48hr and fibronectin assessed. The role of the EGF-receptor and its relationship to SGK-1 signaling was studied using concurrent treatment with PKI166, a specific inhibitor of EGF-receptor.

RESULTS

Exposure of CF to high glucose and EGF increased phosphorylated EGF-R, SGK-1, and fibronectin expression in wild-type cells. Inhibition of the EGF-R reduced SGK-1 and fibronectin expression in control, and following exposure to EGF and high glucose conditions. In cells in which SGK-1 was silenced, fibronectin was reduced and there was no significant increase in pEGF-R, suggesting that SGK-1 is downstream of the EGF-R and negatively inhibits EGF-R activation.

CONCLUSION

These results suggest that high glucose induced fibronectin expression is mediated through the EGF-R and downstream expression of SGK-1.

Podocyte as the target for aldosterone: roles of oxidative stress and Sgk1

Accumulating evidence suggests that mineralocorticoid receptor blockade effectively reduces proteinuria in hypertensive patients. However, the mechanism of the antiproteinuric effect remains elusive. In this study, we investigated the effects of aldosterone on podocyte, a key player of the glomerular filtration barrier. Uninephrectomized rats were continuously infused with aldosterone and fed a high-salt diet. Aldosterone induced proteinuria progressively, associated with blood pressure elevation. Notably, gene expressions of podocyte-associated molecules nephrin and podocin were markedly decreased in aldosterone-infused rats at 2 weeks, with a gradual decrease thereafter. Immunohistochemical studies and electron microscopy confirmed the podocyte damage. Podocyte injury was accompanied by renal reduced nicotinamide-adenine dinucleotide phosphate oxidase activation, increased oxidative stress, and enhanced expression of aldosterone effector kinase Sgk1. Treatment with eplerenone, a selective aldosterone receptor blocker, almost completely prevented podocyte damage and proteinuria, with normalization of elevated reduced nicotinamide-adenine dinucleotide phosphate oxidase activity. In addition, proteinuria, podocyte damage, and Sgk1 upregulation were significantly alleviated by tempol, a membrane-permeable superoxide dismutase, suggesting the pathogenic role of oxidative stress. Although hydralazine treatment almost normalized blood pressure, it failed to improve proteinuria and podocyte damage. In cultured podocytes with consistent expression of mineralocorticoid receptor, aldosterone stimulated membrane translocation of reduced nicotinamide-adenine dinucleotide phosphate oxidase cytosolic components and oxidative stress generation in podocytes. Furthermore, aldosterone enhanced the expression of Sgk1, which was inhibited by mineralocorticoid receptor antagonist and tempol. In conclusion, podocytes are injured at the early stage in aldosterone-infused rats, resulting in the occurrence of proteinuria. Aldosterone can directly modulate podocyte function, possibly through the induction of oxidative stress and Sgk1.

(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.

Enhanced aldosterone signaling in the early nephropathy of rats with metabolic syndrome: possible contribution of fat-derived factors

Metabolic syndrome is an important risk factor for proteinuria and chronic kidney disease independent of diabetes and hypertension; however, the underlying mechanisms have not been elucidated. Aldosterone is implicated in target organ injury of obesity-related disorders. This study investigated the role of aldosterone in the early nephropathy of 17-wk-old SHR/NDmcr-cp, a rat model of metabolic syndrome. Proteinuria was prominent in SHR/NDmcr-cp compared with nonobese SHR, which was accompanied by podocyte injury as evidenced by foot process effacement, induction of desmin and attenuation of nephrin. Serum aldosterone level, renal and glomerular expressions of aldosterone effector kinase Sgk1, and oxidative stress markers all were elevated in SHR/NDmcr-cp. Mineralocorticoid receptors were expressed in glomerular podocytes. Eplerenone, a selective aldosterone blocker, effectively improved podocyte damage, proteinuria, Sgk1, and oxidant stress. An antioxidant tempol also alleviated podocyte impairment and proteinuria, along with inhibition of Sgk1. As for the mechanisms of aldosterone excess, visceral adipocytes that were isolated from SHR/NDmcr-cp secreted substances that stimulate aldosterone production in adrenocortical cells. The aldosterone-releasing activity of adipocytes was not inhibited by candesartan. Adipocytes from nonobese SHR did not show such activity. In conclusion, SHR/NDmcr-cp exhibit enhanced aldosterone signaling, podocyte injury, and proteinuria, which are ameliorated by eplerenone or tempol. The data also suggest that adipocyte-derived factors other than angiotensin II might contribute to the aldosterone excess of this model.

A novel N-terminal hydrophobic motif mediates constitutive degradation of serum- and glucocorticoid-induced kinase-1 by the ubiquitin-proteasome pathway

Serum- and glucocorticoid-induced protein kinase-1 (SGK-1) plays a critical role in regulation of the epithelial sodium channel, ENaC. SGK-1 also shares significant catalytic domain homology with protein kinase B (PKB/AKT-1) and is a downstream effector of antiapoptotic phosphoinositide 3-kinase signaling. Steady-state levels of an active SGK-1 are tightly regulated by rapid transcriptional activation and post-translational modification including phosphorylation. We show here that endogenous SGK-1 protein is polyubiquitinated and rapidly degraded by the 26S proteasome. In contrast to other rapidly degraded kinases, neither the catalytic activity of SGK-1 nor activation site phosphorylation was required for its ubiquitin modification and degradation. Instead, SGK-1 degradation required a lysine-less six-amino-acid (amino acids 19-24) hydrophobic motif (GMVAIL) within the N-terminal domain. Deletion of amino acids 19-24 significantly increased the half-life of SGK1 and prevented its ubiquitin modification. Interestingly, this minimal region was also required for the association of SGK-1 with the endoplasmic reticulum. Ubiquitin modification and degradation of SGK-1 were increasingly inhibited by the progressive mutation of six N-terminal lysine residues surrounding the GMVAIL motif. Mutation of all six lysines to arginine did not disrupt the subcellular localization of SGK-1 despite a significant decrease in ubiquitination, implying that this modification per se was not required for targeting to the endoplasmic reticulum. These results suggest that constitutive ubiquitin-mediated degradation of SGK-1 is an important mechanism regulating its biological activity.

Enhanced Expression of Serum and Glucocorticoid-Inducible Kinase-1 in Kidneys of L-NAME-Treated Rats

OBJECTIVE

The serum and glucocorticoid-inducible kinase-1 (SGK1) has previously been shown to be highly expressed in renal injury such as glomerulonephritis and diabetic nephropathy. Inhibition of nitric oxide synthase with NG-nitro-L-arginine methyl ester (L-NAME) leads to arterial hypertension with subsequent renal injury. The present study explored whether chronic treatment with L-NAME affected renal SGK1 expression.

METHODS

36 Sprague-Dawley rats were divided into a control group and an experimental group, in which hypertension was induced by oral administration of L-NAME (100 mg/kg/day). The rats were sacrificed 4 and 8 weeks, respectively, after initiation of the treatment. Blood pressure was determined with the tail-cuff method, urinary albumin and beta2-microglobulin concentration were measured using an immunoturbidimetric assay, and SGK1 expression in renal cortex was quantified by real-time PCR and Western blotting.

RESULTS

The administration of L-NAME increased systolic blood pressure significantly from 107 to 135 mm Hg within 4 weeks and to 155 mm Hg within 8 weeks. It further enhanced urinary excretion of albumin and beta2-microglobulin. Histology revealed marked fibrosis of glomerular and tubular tissue. The 4- and 8-week L-NAME treatment increased significantly (p < 0.01) SGK1 mRNA and protein abundance in renal cortex.

CONCLUSIONS

L-NAME treatment leads to hypertension, proteinuria and renal fibrosis. It increases renal transcription and expression of SGK1, which has previously been shown to foster matrix protein deposition and could thus contribute to renal injury.

Blunted DOCA/high salt induced albuminuria and renal tubulointerstitial damage in gene-targeted mice lacking SGK1

Mineralocorticoids stimulate renal tubular Na(+) reabsorption, enhance salt appetite, increase blood pressure, and favor the development of renal fibrosis. The effects of mineralocorticoids on renal tubular Na(+) reabsorption and salt appetite involve the serum- and glucocorticoid-inducible kinase 1 (SGK1). The kinase is highly expressed in fibrosing tissue. The present experiments thus explored the involvement of SGK1 in renal fibrosis. To this end, SGK1-knockout mice (sgk1 (-/-)) and their wild-type littermates (sgk1 (+/+)) were implanted with desoxycorticosterone acetate (DOCA)-release pellets and offered 1% saline as drinking water for 12 weeks. The treatment led to significant increases in fluid and Na(+) intake and urinary output of fluid and Na(+) in sgk1 (+/+) mice, effects blunted in sgk1 (-/-) mice. Blood pressure increased within the first 7 weeks to a similar extent in both genotypes, but within the next 5 weeks, it increased further only in sgk1 (+/+) mice. Creatinine clearance did not change significantly but albuminuria increased dramatically in sgk1 (+/+) mice, an effect significantly blunted in sgk1 (-/-) mice. Histology after 12 weeks treatment revealed marked glomerular sclerosis and tubulointerstitial damage with interstitial fibrosis and inflammation in kidneys from sgk1 (+/+) mice, but not from sgk1 (-/-) mice. In conclusion, a lack of SGK1 protects against DOCA/high-salt-induced albuminuria and renal fibrosis.

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.

DOCA-induced phosphorylation of glycogen synthase kinase 3beta

Mineralocorticoid excess leads to cardiac fibrosis, a leading cause of morbidity and mortality. Cardiac hypertrophy and fibrosis are inhibited by the glycogen synthase kinase GSK3 which itself is a target of protein kinase B (PKB) and the serum and glucocorticoid inducible kinase SGK1. Phosphorylation of GSK3 by PKB or SGK1 inhibits GSK3 activity and should thus favour the development of cardiac hypertrophy and fibrosis. As SGK1 is transcriptionally upregulated by mineralocorticoids and has been recently shown to play an important role in the pathogenesis of mineralocorticoid-induced cardiac fibrosis, the present study explored whether mineralocorticoid excess had any effect on the phosphorylation status of the a and beta isoforms of GSK3. Western blotting using an antibody specific for the PKB/SGK1 consensus phosphorylation site in GSK3a/beta (serine 21 and 9 respectively) revealed an increase in GSK3a/beta phosphorylation in human embryonic kidney 293 (HEK293) cells overexpressing wild type SGK1, constitutively active SGK1, but not catalytically inactive SGK1. The effect of SGK1 was mimicked by PKB and SGK3. Furthermore, DOCA/high salt treatment of wild type mice induced a robust increase in cardiac GSK3beta phosphorylation and, to a much lesser extent, GSK3a phosphorylation. However, under this treatment GSK3beta phosphorylation was apparent even in mice lacking functional SGK1, indicating that the phosphorylation of GSK3beta was not exclusively mediated by this kinase. Despite similar cardiac GSK3beta phosphorylation cardiac fibrosis following DOCA/high salt treatment was significantly blunted in SGK1 knockout mice. In conclusion, mineralocorticoid excess leads to phosphorylation and thus inactivation of GSK3beta, an effect not only due to upregulation of SGK1 but as well due to activation of additional kinases. The inactivation of GSK3 may play a permissive role in the stimulation of cardiac fibrosis but may by itself not be sufficient to trigger cardiac fibrosis.

Stimulation of serum- and glucocorticoid-regulated kinase-1 gene expression by endothelin-1

The serum- and glucocorticoid-regulated kinase-1 (SGK1) participates in the regulation of sodium homeostasis and blood pressure by mineralocorticoids. Aldosterone rapidly induces SGK1 transcription, which contributes to the activation of renal epithelial sodium channels. Another important regulator of blood pressure is the vasoactive hormone endothelin-1 (ET-1) that is systemically upregulated in chronic renal failure. In the present study, we investigated whether ET-1 modulates SGK1 expression, and thereby might explain some of its hypertensive effects. As assessed by real-time PCR analysis, ET-1 triggered the rapid increase of SGK1 mRNA levels in A-10 smooth muscle cells and also in intact aortas of adult rats. In A-10 cells transcriptional activation was associated with a more than 6-fold upregulation of SGK1 protein expression and in similar range as found after treatment with aldosterone. A stimulatory effect of ET-1 was not only observed in isolated cells, but also in an animal model. Upon subtotal nephrectomy (SNX) of rats, myocardial ET-1 levels strongly increased, which was followed by a more than 2-fold induction of SGK1 expression in the left ventricle. The myocardial upregulation of SGK1 was completely abrogated by a specific ET(A) receptor antagonist, thereby substantiating the in vivo role of ET-1 in SGK1 expression. Thus, these data demonstrate that ET-1 increases expression of SGK1 in vivo and in vitro, and therefore indicate that SGK1 upregulation might be involved in ET-1-dependent regulation of blood pressure and cardiac modelling during mild renal failure.

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.

SGK1-mediated Fibronectin Formation in Diabetic Nephropathy

The serum and glucocorticoid inducible kinase SGK1 has been shown to be up regulated in fibrosing tissue including diabetic nephropathy. The present study has been performed to determine the time course of SGK1 transcription in mouse kidneys following induction of diabetes by streptozotocin (STZ). Moreover, the study aimed to explore whether SGK1 may play an active role in the stimulation of matrix protein formation during hyperglycemia. The induction of diabetes in 8 weeks old male C57Bl/6 mice was indeed followed by a significant (p< 0.001) increase of SGK1 transcript levels (up to 2.5-fold) and protein abundance (up to 2.8-fold) both peaking 4 weeks after STZ treatment. The SGK1 transcript levels and protein abundance declined thereafter but remained significantly elevated up to 12 weeks (p<0.05). Exposure to high extracellular glucose concentration (25 mM) significantly increased SGK1 transcript levels in human mesangial cells (HMCs). At low extracellular glucose concentration (5.5 mM), transfection with constitutively active (S422D)SGK1 and transdominant inhibitory (K127N)SGK1 did not significantly modify fibronectin formation by HMCs. Exposure to high extracellular glucose concentration stimulated fibronectin formation (by 2.2 fold), an effect abrogated by transfection with inactive (K127N)SGK1 (1.2 fold) and markedly enhanced by transfection with (S422D)SGK1 (4.7 fold). In conclusion, hyperglycemia of diabetes mellitus leads to partially transient increase of SGK1 transcription and translation. SGK1 overexpression alone has little effect on fibronectin formation but potentiates the effect of hyperglycemia. Thus, SGK1 is upregulated in diabetic nephropathy and actively participates in the stimulation of matrix protein deposition in this common deleterious complication of diabetic hyperglycemia.

High glucose transactivates the EGF receptor and up-regulates serum glucocorticoid kinase in the proximal tubule

BACKGROUND

Serum glucocorticoid regulated kinase (SGK-1) is induced in the kidney in diabetes mellitus. However, its role in the proximal tubule is unclear. This study determined the expression and functional role of SGK-1 in PTCs in high glucose conditions. As the epidermal growth factor (EGF) receptor is activated by both EGF and other factors implicated in diabetic nephropathy, the relationship of SGK-1 with EGFR activity was assessed.

METHODS

mRNA and protein expression of SGK-1 and mRNA expression of the sodium hydrogen exchanger NHE3 were measured in human PTCs exposed to 5 mmol/L (control) and 25 mmol/L (high) glucose. The effects of SGK-1 on cell growth, apoptosis, and progression through the cell cycle and NHE3 mRNA were examined following overexpression of SGK-1 in PTCs. The role of EGFR activation in observed changes was assessed by phospho-EGFR expression, and response to the EGFR blocker PKI166. SGK-1 expression was then assessed in vivo in a model of streptozotocin-induced diabetes mellitus type 2.

RESULTS

A total of 25 mmol/L glucose and EGF (10 ng/mL) increased SGK-1 mRNA (P < 0.005 and P < 0.002, respectively) and protein (both P < 0.02) expression. High glucose and overexpression of SGK-1 increased NHE3 mRNA (P < 0.05) and EGFR phosphorylation (P < 0.01), which were reversed by PKI166. SGK-1 overexpression increased PTC growth (P < 0.0001), progression through the cell cycle (P < 0.001), and increased NHE3 mRNA (P < 0.01), which were all reversed with PKI166. Overexpression of SGK-1 also protected against apoptosis induced in the PTCs (P < 0.0001). Up-regulation of tubular SGK-1 mRNA in diabetes mellitus was confirmed in vivo. Oral treatment with PKI166 attenuated this increase by 51%. No EGF protein was detectable in PTCs, suggestive of phosphorylation of the EGFR by high glucose and downstream induction of SGK-1.

CONCLUSION

The effects of high glucose on PTC proliferation, reduced apoptosis and increased NHE3 mRNA levels are mediated by EGFR-dependent up-regulation of SGK-1.

SGK1 as a determinant of kidney function and salt intake in response to mineralocorticoid excess

Mineralocorticoids modify salt balance by both stimulating salt intake and inhibiting salt loss. Renal salt retention is accomplished by upregulation of reabsorption, an effect partially mediated by serum- and glucocorticoid-inducible kinase 1 (SGK1). The present study explored the contribution of SGK1 to the regulation of renal function, salt intake, and blood pressure during mineralocorticoid excess. DOCA/1% NaCl treatment increased blood pressure and creatinine clearance to a similar extent in SGK1-deficient sgk1−/−and wild-type sgk1+/+mice but led to more pronounced increase of proteinuria in sgk1+/+mice (by 474 ± 89%) than in sgk1−/−mice (by 154 ± 31%). DOCA/1% NaCl treatment led to significant increase of kidney weight (by 24%) and to hypokalemia (from 3.9 ± 0.1 to 2.7 ± 0.1 mmol/l) only in sgk1+/+mice. The treatment enhanced renal Na+excretion significantly more in sgk1+/+mice (from 3 ± 1 to 134 ± 32 μmol·24 h−1·g body wt−1) than in sgk1−/−mice (from 4 ± 1 to 49 ± 8 μmol·24 h−1·g body wt−1), pointing to SGK1-dependent stimulation of salt intake. With access to two drinking bottles containing 1% NaCl or water, DOCA treatment did not significantly affect water intake in either genotype but increased 1% NaCl intake in sgk1+/+mice (within 9 days from 3.5 ± 0.9 to 16.5 ± 2.4 ml/day) consistent with DOCA-induced salt appetite. This response was significantly attenuated in sgk1−/−mice (from 2.6 ± 0.6 to 5.9 ± 0.9 ml/day). Thus SGK1 contributes to the stimulation of salt intake, kidney growth, proteinuria, and renal K+excretion during mineralocorticoid excess.

Glucose-induced serum- and glucocorticoid-regulated kinase activation in oncofetal fibronectin expression

Preferential expression of oncofetal extra domain-B fibronectin (EDB(+) FN), a proposed angiogenic marker, has been shown in proliferative diabetic retinopathy. High levels of glucose also increase EDB(+) FN expression in endothelial cells (ECs) via transforming growth factor-beta1 (TGF-beta1) and endothelin-1 (ET-1). The present study was aimed at elucidating the role of serum- and glucocorticoid-regulated kinase (SGK-1) in glucose-induced EDB(+) FN expression. Using human macro- and microvascular ECs, we show that high levels of glucose, TGF-beta1, and ET-1 increase the EDB(+) FN expression via SGK-1 alteration at the mRNA, protein, and activity levels. Inhibition of TGF-beta1 and ET-1 prevented glucose-induced SGK-1 activation and the EDB(+) FN expression. Furthermore, using siRNA-mediated SGK-1 gene silencing, we show that glucose-induced EDB(+) FN expression can be completely prevented. These findings provide first evidence of glucose-induced SGK-1 activation in altered EDB(+) FN expression and provide novel avenues for therapeutic modalities.

Role of Sgk1 in salt and potassium homeostasis

Aldosterone plays a pivotal role in NaCl and K(+) homeostasis by stimulation of Na(+) reabsorption and K(+) secretion in the aldosterone-sensitive distal nephron (ASDN). Recent studies demonstrated that the serum- and glucocorticoid-regulated kinase 1 (Sgk1) is induced by aldosterone in the ASDN and that polymorphisms of the kinase associate with arterial blood pressure in normotensive subjects. This review discusses the role of Sgk1 in NaCl and K(+) homeostasis as evidenced by in vivo studies, including those in Sgk1-deficient mice. The studies indicate that Sgk1 is not absolutely required for Na(+) reabsorption and K(+) secretion in the ASDN. On a standard NaCl and K(+) diet, modestly enhanced plasma aldosterone concentrations appear sufficient to establish a compensated phenotype in the absence of Sgk1. The kinase is necessary, however, for upregulation of transcellular Na(+) reabsorption in the ASDN. This may involve Sgk1-mediated stimulation of basolateral Na(+)-K(+)-ATPase as well as retention of epithelial Na(+) channel, ENaC, in the apical membrane. Such an upregulation is a prerequisite for adequate adaptation of 1) renal NaCl reabsorption during restricted dietary NaCl intake, as well as 2) K(+) secretion in response to enhanced K(+) intake. Thus gain-of-function mutations of Sgk1 are expected to result in renal NaCl retention and enhanced K(+) secretion. Further studies are required to elucidate renal and nonrenal aldosterone-induced effects of Sgk1, the role of other Sgk1 activators, as well as the link of Sgk1 polymorphisms to arterial hypertension in humans.

Tissue-specific transcriptome responses in rats with early streptozotocin-induced diabetes

The understanding of common and tissue-specific molecular alterations in diabetes, particularly at early stages, is limited and fragmental. In the present study, we systematically compared transcriptome responses in four important diabetic target tissues in rats with 2 wk of streptozotocin (STZ)-induced diabetes. At this stage of diabetes, the skeletal muscle exhibited the highest transcriptome sensitivity to the STZ treatment with nearly 17% of the transcriptome being altered (false discovery rate, 1.6%) compared with approximately 3% in the cardiac left ventricle, renal cortex, and retina. Similarity in transcriptome response among tissues was low, with the highest similarity being 2.2% between skeletal muscle and the left ventricle. Several biological processes or cellular components, such as lipid metabolism in the left ventricle and collagen in the renal cortex, were significantly overrepresented in the responsive genes than in the entire array. Particularly interesting cases of common or tissue-specific regulation included decorin and CD36, which were upregulated in several tissues, and serum/glucocorticoid-regulated kinase and four and a half LIM domains 2, which were upregulated only in the renal cortex. Further biochemical analyses indicated that the thiol and oxidative stress pathway was altered in a tissue-specific manner at several levels including transcript abundance, content of reduced thiols, and lipid peroxidation, providing an example of the potential biological relevance of tissue-specific transcript regulation. These results provided a transcriptome-wide view of the molecular alterations across several key tissues in early diabetes. It appears that both common pathways and, perhaps more importantly, tissue-specific mechanisms are involved in the adaptation to diabetes or the initiation of diabetic complications.

Transport regulation by the serum- and glucocorticoid-inducible kinase SGK1

The serum- and glucocorticoid-inducible kinase SGK1 is an ubiquitously expressed kinase with the ability to regulate a variety of transport systems. Recent observations point to a role of SGK1 in the regulation of diverse physiological functions such as epithelial transport and cardiac and neuronal excitability. At least partially through its effect on transport, SGK1 contributes to a number of pathophysiological conditions including metabolic syndrome and fibrosing disease.

Significance and expression of serum and glucocorticoid-inducible kinase in kidney of mice with diabetic nephropathy

To investigate the expression and the role of three isoforms of Serum and Glucocorticoid-inducible Kinase (SGK) in experimental diabetic nephropathy (DN), 12 male C57BL/6 mice of 8-weeks-old were divided into two groups. Streptozotocin (STZ)-induced diabetic nephropathy and normal controls were analyzed at the end of the 4th week after the induction of diabetes. Renal hemodynamics and histological studies were performed. The expression of SGK1 mRNA, SGK2 mRNA and SGK3 mRNA of kidney cortex were measured by RT-PCR, and the cortical SGK1 protein was detected with Western blotting. Our results showed that the blood glucose, blood HbA1c, 24h urinary protein, creatinine clearance and the renal index were all increased in DN group. More extracellular matrix (ECM) accumulation was observed. The level of cortical SGK1 mRNA and protein were up-regulated in DN group in comparison with control group. SGK2 and SGK3 mRNA were elevated in DN mice. In DN, mRNA level of three SGK isoforms and SGK1 protein were increased significantly. It is concluded that SGKs may contribute to the early renal injury of DN.

Importin-alpha mediates the regulated nuclear targeting of serum- and glucocorticoid-inducible protein kinase (Sgk) by recognition of a nuclear localization signal in the kinase central domain

The transcriptionally regulated serum and glucocorticoid inducible protein kinase (Sgk) is localized to the nucleus in a serum-dependent manner, and a yeast two-hybrid genetic screen uncovered a specific interaction between Sgk and the importin-alpha nuclear import receptor. In vitro GST pull down assays demonstrated a strong and direct association of importin-alpha with endogenous Sgk and exogenously expressed HA-tagged Sgk, whereas both components coimmunoprecipitate and colocalize to the nucleus after serum stimulation. Consistent with an active mechanism of nuclear localization, the nuclear import of HA-Sgk in permeabilized cells required ATP, cytoplasm, and a functional nuclear pore complex. Ectopic addition of a 107 amino acid carboxy-terminal fragment of importin-alpha, which contains the Sgk binding region, competitively inhibited the ability of endogenous importin-alpha to import Sgk into nuclei in vitro. Mutagenesis of lysines by alanine substitution defined a KKAILKKKEEK sequence within the central domain of Sgk between amino acids 131-141 that functions as a nuclear localization signal (NLS) required for the in vitro interaction with importin-alpha and for nuclear import of full-length Sgk in cultured cells. The serum-induced nuclear import of Sgk requires the NLS-dependent recognition of Sgk by importin-alpha as well as the PI3-kinase-dependent phosphorylation of Sgk. Our results define a new role importin-alpha in the stimulus-dependent control of signal transduction by nuclear localized protein kinases.

Glucocorticoid activation of Na(+)/H(+) exchanger isoform 3 revisited. The roles of SGK1 and NHERF2

The stimulative effect of glucocorticoids on intestinal salt and water absorption has been known for more than two decades. However, molecular mechanisms underlying this activation remain elusive. Previous studies showed that methylprednisolone specifically increased Na(+)/H(+) exchanger isoform (NHE) 3 mRNA in ileum and kidney without affecting NHE1 mRNA levels. These results suggest that glucocorticoids activate NHE3 activity by induction of NHE3 transcripts. We recently found in PS120 and opossum kidney cells that chronic incubation with dexamethasone activated NHE3 independent of gene induction, indicating that the transcriptional activation may not be the only determining factor in the NHE3 activation. Furthermore, dexamethasone activated NHE3 activity only in the presence of a NHE3 regulatory protein, NHERF2, which was previously shown to confer cAMP-dependent inhibition of NHE3. This activation of NHE3 could not be duplicated by NHERF1. We identified serum- and glucocorticoid-induced protein kinase, SGK1, as the protein interacting with PDZ domains of NHERF2 to regulate NHE3 activity. The expression of SGK1 enhanced NHE3 transport in PS120 fibroblasts. In addition, the "kinase-dead" SGK1 blocked activation of NHE3 by dexamethasone in opossum kidney cells. These data demonstrated that glucocorticoid activation of NHE3 requires the activation of SGK1 and the presence of NHERF2 acting as a scaffold protein.

Harnessing the mouse to unravel the genetics of human disease

Complex traits, i.e. those with multiple genetic and environmental determinants, represent the greatest challenge for genetic analysis, largely due to the difficulty of isolating the effects of any one gene amid the noise of other genetic and environmental influences. Methods exist for detecting and mapping the Quantitative Trait Loci (QTLs) that influence complex traits. However, once mapped, gene identification commonly involves reduction of focus to single candidate genes or isolated chromosomal regions. To reach the next level in unraveling the genetics of human disease will require moving beyond the focus on one gene at a time, to explorations of pleiotropism, epistasis and environment-dependency of genetic effects. Genetic interactions and unique environmental features must be as carefully scrutinized as are single gene effects. No one genetic approach is likely to possess all the necessary features for comprehensive analysis of a complex disease. Rather, the entire arsenal of behavioral genomic and other approaches will be needed, such as random mutagenesis, QTL analyses, transgenic and knockout models, viral mediated gene transfer, pharmacological analyses, gene expression assays, antisense approaches and importantly, revitalization of classical genetic methods. In our view, classical breeding designs are currently underutilized, and will shorten the distance to the target of understanding the complex genetic and environmental interactions associated with disease. We assert that unique combinations of classical approaches with current behavioral and molecular genomic approaches will more rapidly advance the field.

Gene expression and identification of gene therapy targets in diabetic nephropathy

A number of novel genes that are up-regulated in diabetic kidneys have been identified. Recently, transforming growth factor-beta (TGF-beta)--driven secreted proteins, i.e., connective tissue growth factor (CTGF) and gremlin, were identified. They are up-regulated in kidneys of diabetic animals and modulate the biology of mesangial cells. CTGF mediates TGF-beta--induced matrix overproduction by the mesangial cells. Gremlin is a putative antagonist of bone morphogenetic protein-2 that blocks mesangial cell proliferation. Thus, gremlin may modulate the biology of mesangium by stimulating mesangial cell proliferation and in turn production of matrix. In addition, transcriptionally regulated kinases, serum glucocorticoid-regulated kinase and munc-13 have been identified. The former stimulates renal tubular Na+ transport and is involved in hyperfiltraion of diabetic kidneys by a Na+ transport feedback mechanism. Munc-13 has been shown to induce apoptosis in hyperglycemic state via diacylglycerol-activated, PKC-independent signaling pathway. Another pathway relevant to diabetic nephropathy is polyol pathway, where glucose is reduced to sorbitol by aldose reductase. Recently, a renal-specific reductase of the aldo-keto reductase family was isolated. It is up-regulated in diabetic mice, and this could serve as a suitable target for gene therapy in renal complications of diabetes. Several mitochondrial genome-encoded genes, such as, cytochrome oxidase and NADH dehydrogenase, are up-regulated in diabetic kidneys. A novel nuclear-encoded mitochondrial gene, i.e., translocase inner mitochondrial membrane 44 (Tim44), is up-regulated in diabetic kidneys, and it may also serve as another target for molecular therapeutic intervention at the core storage energy sites, i.e., mitochondria. In this review, these novel differentially regulated genes that respond to hyperglycemic stress are described, and they may serve as possible targets for gene therapy in the treatment of diabetic nephropathy.

Identification of up-regulated Ras-like GTPase, Rap1b, by suppression subtractive hybridization

BACKGROUND

Diabetic nephropathy accounts for over 30% of the end-stage renal disease (ESRD). A number of defined mechanisms and molecules that are involved in its pathogenesis are known, while others remain to be identified.

METHODS

Suppression subtraction hybridization (SSH)-polymerase chain reaction (PCR) was employed to search for new genes that may be relevant to the pathogenesis of diabetic nephropathy during embryonic development, the time when the kidney is most susceptible to various forms of stress. A diabetic state was induced in pregnant mice at day-13 of gestation by administration of streptozotocin. The kidneys of newborn mice with blood glucose level> 200 mg/dL were harvested, mRNA isolated and subjected to SSH-PCR. Several differentially expressed cDNA fragments with up-regulated expression were isolated. One of the cDNA fragments had homology with human Ras-like guanine 5'-triphosphate (GTPase), Rap1b gene. By utilizing the lambdaZAP II mouse cDNA library and SMART RACE amplification, a full-length Rap1b cDNA was isolated. A recombinant protein was generated in pET15b bacterial expression system. An anti-Rap1b antibody was raised in rabbits by immunizing them with the fusion protein, and its specificity was confirmed by Western blot analysis.

RESULTS

Rap1b cDNA had an open reading frame of 552 bp with a predicted putative protein size of approximately 21 kD. In vitro translation verified the authentication of the Rap1b cDNA clone. Northern blot analyses revealed a single approximately 2.3 kb Rap1b mRNA transcript. Its expression was up-regulated in several tissues, including the kidney of newborn diabetic mice. The degree of up-regulation of Rap1b mRNA expression was proportional to the blood glucose levels. Western blot analyses confirmed the hyperglycemia-induced up-regulation of the Rap1b expression. In situ hybridization and immunofluorescence studies revealed that Rap1b was expressed in the inner medullary collecting tubules. During hyperglycemia, its expression was accentuated and extended into the outer medullary and cortical collecting tubules. Similar up-regulation of Rap1b was observed when embryonic kidneys, harvested at day-13 of gestation, were exposed to high glucose ambience.

CONCLUSION

The data suggest that Rap1b, a GTP-binding protein that plays a critical role in various signaling intracellular events, is another molecule that may be relevant to the pathobiology of diabetic nephropathy.

Regulation and physiological roles of serum- and glucocorticoid-induced protein kinase isoforms

Serum- and glucocorticoid-induced protein kinase 1 (SGK1) was identified in 1993 as an immediate early gene whose mRNA levels increase dramatically within 30 minutes when cells are exposed to serum or glucocorticoids, or both. Subsequently, many other agonists, acting through a variety of signal transduction pathways, have been shown to induce SGK1 gene transcription in cells and tissues. SGK1 is a member of the "AGC" subfamily, which includes protein kinases A, G, and C, and its catalytic domain is most similar to protein kinase B (PKB). Like PKB, SGK1 is activated by phosphorylation in response to signals that stimulate phosphatidylinositol 3-kinase, and this is mediated by 3-phosphoinositide-dependent protein kinase 1 (PDK1) and another protein kinase that has yet to be identified. Thus, SGK1 is remarkable in being activated at both the transcriptional and posttranslational levels by a huge number of extracellular signals. In contrast, little is known about the transcriptional regulation of the two closely related isoforms SGK2 and SGK3, although they can be activated by phosphorylation. The substrate specificity of SGK isoforms superficially resembles that of PKB in that serine and threonine residues lying in Arg-Xaa-Arg-Xaa-Xaa-Ser/Thr sequences (where Xaa is a variable amino acid) are phosphorylated. However, although they may have some substrates in common, evidence is emerging that SGK1 and PKB phosphorylate distinct proteins and have different functions in vivo. In particular, SGK1 plays an important role in activating certain potassium, sodium, and chloride channels, suggesting an involvement in the regulation of processes such as cell survival, neuronal excitability, and renal sodium excretion. Moreover, sustained high levels of SGK1 protein and activity may contribute to conditions such as hypertension and diabetic nephropathy. This raises the possibility that specific inhibitors of SGK1 may have therapeutic potential for the treatment of several diseases.

Serum- and glucocorticoid-dependent kinase, cell volume, and the regulation of epithelial transport

Ample pharmacological evidence points to a role of kinases in the regulation of cell volume. Given the limited selectivity of most inhibitors, however, the specific molecules involved have remained largely elusive. The search for cell volume regulated genes in liver HepG2 cells led to the discovery of the human serum- and glucocorticoid-dependent serine/threonine kinase hsgk1. Transcription and expression of hsgk1 is markedly and rapidly upregulated by osmotic and isotonic cell shrinkage. The effect of osmotic cell shrinkage on hsgk1 is mediated by p38 kinase. Further stimuli of hsgk1 transcription include glucocorticoids, aldosterone, TGF-beta1, serum, increase of intracellular Ca2+ and phorbolesters, whereas cAMP downregulates hsgk1 transcription. The hsgk1 protein is expressed in several epithelial tissues including human pancreas, intestine, kidney, and shark rectal gland. Co-expression of hsgk1 with the renal epithelial Na+-channel ENaC or the Na+/K+/2Cl(-)-cotransporter NKCC2 (BSC1) in Xenopus oocytes, accelerates insertion of the transport proteins into the cell membrane and thus, stimulates channel or transport activity. Thus, hsgk1 participates in the regulation of transport by steroids and secretagogues increasing intracellular Ca2+-activity. The stimulation of hsgk1 transcription by TGF-beta1 may further bear pathophysiological relevance.