Sgk1 Gene Expression in Kidney and Its Regulation by Aldosterone: Spatio-Temporal Heterogeneity and Quantitative Analysis

SGK1, autophagy and cancer: an overview

BACKGROUND

The serum and glucocorticoid-induced kinase-1 (SGK1) belonging to the AGC protein kinase family phosphorylates serine and threonine residues of target proteins. It regulates numerous ion channels and transporters and promotes survival under cellular stress. Unique to SGK1 is a tight control at transcriptional and post-transcriptional levels. SGK1 regulates multiple signal transduction pathways related to tumor development. Several studies have reported that SGK1 is upregulated in different types of human malignancies and induces resistance against inhibitors, drugs, and targeted therapies.

RESULTS AND CONCLUSION

This review highlights the cellular functions of SGK1, its crucial role in cancer development, and clinical insights for SGK1 targeted therapies. Furthermore, the role of SGK1-mediated autophagy as a potential therapeutic target for cancer has been discussed.

Sodium butyrate ameliorates deoxycorticosterone acetate/salt-induced hypertension and renal damage by inhibiting the MR/SGK1 pathway

Our recent work demonstrates that infusion of sodium butyrate (NaBu) into the renal medulla blunts angiotensin II-induced hypertension and improves renal injury. The present study aimed to test whether oral administration of NaBu attenuates salt-sensitive hypertension in deoxycorticosterone acetate (DOCA)/salt-treated rats. Uninephrectomized male Sprague-Dawley (SD) rats were treated with DOCA pellets (150 mg/rat) plus 1% NaCl drinking water for 2 weeks. Animals received oral administration of NaBu (1 g/kg) or vehicle once per day. Our results showed that NaBu administration significantly attenuated DOCA/salt-increased mean arterial pressure from 156 ± 4 mmHg to 136 ± 1 mmHg. DOCA/salt treatment markedly enhanced renal damage as indicated by an increased ratio of kidney weight/body weight, elevated urinary albumin, extensive fibrosis, and inflammation, whereas kidneys from NaBu-treated rats exhibited a significant reduction in these renal damage responses. Compared to the DOCA/salt group, the DOCA/salt-NaBu group had ~30% less salt water intake and decreased Na⁺ and Cl^- excretion in urine but no alteration in 24-h urine excretion. Mechanistically, NaBu inhibited the protein levels of several sodium transporters stimulated by DOCA/salt in vivo, such as βENaC, γENaC, NCC, and NKCC-2. Further examination showed that NaBu downregulated the expression of mineralocorticoid receptor (MR) and serum and glucocorticoid-dependent protein kinase 1 (SGK1) in DOCA/salt-treated rats or aldosterone-treated human renal tubular duct epithelial cells. These results provide evidence that NaBu may attenuate DOCA/salt-induced hypertension and renal damage by inhibiting the MR/SGK1 pathway.

RNA sequencing of kidney distal tubule cells reveals multiple mediators of chronic aldosterone action

The renal aldosterone-sensitive distal tubule (ASDT) is crucial for sodium reabsorption and blood pressure regulation. The ASDT consists of the late distal convoluted tubule (DCT2), connecting tubule (CNT), and collecting duct. Due to difficulties in isolating epithelial cells from the ASDT in large quantities, few transcriptome studies have been performed on this segment. Moreover, no studies exist on isolated DCT2 and CNT cells (excluding intercalated cells), and the role of aldosterone for regulating the transcriptome of these specific cell types is largely unknown. A mouse model expressing eGFP in DCT2/CNT/initial cortical collecting duct (iCCD) principal cells was exploited to facilitate the isolation of these cells in high number and purity. Combined with deep RNA sequencing technology, a comprehensive catalog of chronic aldosterone-regulated transcripts from enriched DCT2/CNT/iCCD principal cells was generated. There were 257 significantly downregulated and 290 upregulated transcripts in response to aldosterone ( P < 0.05). The RNA sequencing confirmed aldosterone regulation of well-described aldosterone targets including Sgk1 and Tsc22d3. Changes in selected transcripts such as S100a1 and Cldn4 were confirmed by RT-qPCR. The RNA sequencing showed downregulation of Nr3c2 encoding the mineralocorticoid receptor (MR), and cell line experiments showed a parallel decrease in MR protein. Furthermore, a large number of transcripts encoding transcription factors were downregulated. An extensive mRNA transcriptome reconstruction of an enriched CNT/iCCD principal cell population was also generated. The results provided a comprehensive database of aldosterone-regulated transcripts in the ASDT, allowing development of novel hypotheses for the action of aldosterone.

The E3 ubiquitin ligase Siah1 regulates adrenal gland organization and aldosterone secretion

Primary and secondary hypertension are major risk factors for cardiovascular disease, the leading cause of death worldwide. Elevated secretion of aldosterone resulting from primary aldosteronism (PA) is a key driver of secondary hypertension. Here, we report an unexpected role for the ubiquitin ligase Siah1 in adrenal gland development and PA. Siah1a-/- mice exhibit altered adrenal gland morphology, as reflected by a diminished X-zone, enlarged medulla, and dysregulated zonation of the glomerulosa as well as increased aldosterone levels and aldosterone target gene expression and reduced plasma potassium levels. Genes involved in catecholamine biosynthesis and cAMP signaling are upregulated in the adrenal glands of Siah1a-/- mice, while genes related to retinoic acid signaling and cholesterol biosynthesis are downregulated. Loss of Siah1 leads to increased expression of the Siah1 substrate PIAS1, an E3 SUMO protein ligase implicated in the suppression of LXR, a key regulator of cholesterol levels in the adrenal gland. In addition, SIAH1 sequence variants were identified in patients with PA; such variants impaired SIAH1 ubiquitin ligase activity, resulting in elevated PIAS1 expression. These data identify a role for the Siah1-PIAS1 axis in adrenal gland organization and function and point to possible therapeutic targets for hyperaldosteronism.

Serum and Glucocorticoid Regulated Kinase 1 in Sodium Homeostasis

The ubiquitously expressed serum and glucocorticoid regulated kinase 1 (SGK1) is tightly regulated by osmotic and hormonal signals, including glucocorticoids and mineralocorticoids. Recently, SGK1 has been implicated as a signal hub for the regulation of sodium transport. SGK1 modulates the activities of multiple ion channels and carriers, such as epithelial sodium channel (ENaC), voltage-gated sodium channel (Nav1.5), sodium hydrogen exchangers 1 and 3 (NHE1 and NHE3), sodium-chloride symporter (NCC), and sodium-potassium-chloride cotransporter 2 (NKCC2); as well as the sodium-potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) and type A natriuretic peptide receptor (NPR-A). Accordingly, SGK1 is implicated in the physiology and pathophysiology of Na⁺ homeostasis. Here, we focus particularly on recent findings of SGK1’s involvement in Na⁺ transport in renal sodium reabsorption, hormone-stimulated salt appetite and fluid balance and discuss the abnormal SGK1-mediated Na⁺ reabsorption in hypertension, heart disease, edema with diabetes, and embryo implantation failure.

SGK1 regulation by miR-466g in cortical collecting duct cells

Micro-RNAs (miRNAs) are noncoding RNAs that bind target mRNA transcripts and modulate gene expression. In the cortical collecting duct (CCD), aldosterone stimulates the expression of genes that increase activity of the epithelial sodium channel (ENaC); in the early phase of aldosterone induction, one such gene is serum and glucocorticoid regulated kinase 1 (SGK1). We hypothesized that aldosterone regulates the expression of miRNAs in the early phase of induction to control the expression of target genes that stimulate ENaC activity. We treated mpkCCDc14 cells with aldosterone or vehicle for 1 h and used a miRNA microarray to analyze differential miRNA expression. We identified miR-466g as a miRNA that decreased by 57% after 1 h of aldosterone treatment. Moreover, we identified a putative miR-466g binding site in the 3'-untranslated region of SGK1. We constructed an SGK1 3'-untranslated region luciferase reporter and found that cotransfection of miR-466g suppressed luciferase activity in human embryonic kidney-293 cells in a dose-dependent manner. Deletion or introduction of point mutations that disrupt the miR-466g target site attenuated miR-466g-directed suppression of luciferase activity. Finally, we generated stably transduced mpkCCDc14 cell lines overexpressing miR-466g. Cells overexpressing miR-466g demonstrated 12.9-fold lower level of SGK1 mRNA compared with control cells after 6 h of aldosterone induction; moreover, cells overexpressing miR-466g exhibited 25% decrease in amiloride-sensitive current after 6 h of aldosterone induction and complete loss of amiloride-sensitive current after 24 h of aldosterone induction. Our findings implicate miR-466g as a novel early-phase aldosterone responsive miRNA that regulates SGK1 and ENaC in CCD cells.

Regulation of αENaC transcription

Aldosterone is a major regulator of Na(+) absorption and acts primarily by controlling the epithelial Na(+) channel (ENaC) function at multiple levels including transcription. ENaC consists of α, β, and γ subunits. In the classical model, aldosterone enhances transcription primarily by activating mineralocorticoid receptor (MR). However, how aldosterone induces chromatin alternation and thus leads to gene activation or repression remains largely unknown. Emerging evidence suggests that Dot1a-Af9 complex plays an important role in repression of αENaC by directly binding and modulating targeted histone H3 K79 hypermethylation at the specific subregions of αENaC promoter. Aldosterone impairs Dot1a-Af9 formation by decreasing expression of Dot1a and Af9 and by inducing Sgk1, which, in turn, phosphorylates Af9 at S435 to weaken Dot1a-Af9 interaction. MR counterbalances Dot1a-Af9 action by competing with Dot1a for binding Af9. Af17 derepresses αENaC by competitively interacting with Dot1a and facilitating Dot1a nuclear export. Consistently, MR(-/-) mice have impaired ENaC expression at day 5 after birth, which may contribute to progressive development of pseudohypoaldosteronism type 1 in a later stage. Af17(-/-) mice have decreased ENaC expression, renal Na(+) retention, and blood pressure. In contrast, Dot1l(AC) mice have increased αENaC expression, despite a 20% reduction of the principal cells. This chapter reviews these findings linking aldosterone action to ENaC transcription through chromatin modification. Future direction toward the understanding the role of Dot1a-Af9 complex beyond ENaC regulation, in particular, in renal fibrosis is also briefly discussed.

Biological Effects Induced by Specific Advanced Glycation End Products in the Reconstructed Skin Model of Aging

Advanced glycation end products (AGEs) accumulate in the aging skin. To understand the biological effects of individual AGEs, skin reconstructed with collagen selectively enriched with N^ɛ-(carboxymethyl)-lysine (CML), N^ɛ-(carboxyethyl)-lysine (CEL), methylglyoxal hydroimidazolone (MG-H1), or pentosidine was studied. Immunohistochemistry revealed increased expression of α6 integrin at the dermal epidermal junction by CEL and CML (p<0.01). Laminin 5 was diminished by CEL and MG-H1 (p<0.05). Both CML and CEL induced a robust increase (p<0.01) in procollagen I. In the culture medium, IL-6, VEGF, and MMP1 secretion were significantly decreased (p<0.05) by MG-H1. While both CEL and CML decreased MMP3, only CEL decreased IL-6 and TIMP1, while CML stimulated TIMP1 synthesis significantly (p<0.05). mRNA expression studies using qPCR in the epidermis layer showed that CEL increased type 7 collagen (COL7A1), β1, and α6 integrin, while CML increased only COL7A1 (p<0.05). MG-H1-modified collagen had no effect. Importantly, in the dermis layer, MMP3 mRNA expression was increased by both CML and MG-H1. CML also significantly increased the mRNAs of MMP1, TIMP1, keratinocyte growth factor (KGF), IL-6, and monocyte chemoattractant protein 1 (MCP1) (p<0.05). Mixed effects were present in CEL-rich matrix. Minimally glycoxidized pentosidine-rich collagen suppressed most mRNAs of the genes studied (p<0.05) and decreased VEGF and increased MCP1 protein expression. Taken together, this model of the aging skin suggests that a combination of AGEs tends to counterbalance and thus minimizes the detrimental biological effects of individual AGEs.

Senescence-dependent impact of anti-RAGE antibody on endotoxemic liver failure

Aging often restricts the capacity of the immune system. Endotoxemia is characterized by an immune response initiated by a group of pattern recognition receptors including the receptor for advanced glycation end products (RAGE). The aim of this study was to clarify to which extent RAGE and its signaling pathways such as the so called mitogen-activated protein kinase (MAPK) pathways can contribute to the perpetuation of inflammation in the aging organism. We used senescence-accelerated-prone (SAMP8) and senescence-accelerated-resistant (SAMR1) mice and studied them at the age of 2 and 6 months. Livers of SAMP8 mice had significantly higher malondialdehyde concentrations and a modest reduction of glyoxalase-I expression. Consequently, the abundance of highly modified advanced glycation end products was increased in the liver and plasma of these mice. After galactosamine/lipopolysaccharide-induced acute liver injury, significant activation of the MAPK cascade was observed in both mouse strains. Administration of an anti-RAGE antibody diminished p42/44-phosphorylation as well as tissue injury in SAMP8 mice, whereas the identical treatment in SAMR1 mice leads to a significant increase in p42/44-phosphorylation and intensified liver injury. This observation suggests that dependent on the senescence of the organism, anti-RAGE antibody can have differential effects on the progression of endotoxemic liver failure.

SGK regulation of renal sodium transport

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Protein phosphatase 1 modulates the inhibitory effect of With-no-Lysine kinase 4 on ROMK channels

With-no-Lysine kinase 4 (WNK4) inhibited ROMK (Kir1.1) channels and the inhibitory effect of WNK4 was abolished by serum-glucocorticoid-induced kinase 1 (SGK1) but restored by c-Src. The aim of the present study is to explore the mechanism by which Src-family tyrosine kinase (SFK) modulates the effect of SGK1 on WNK4 and to test the role of SFK-WNK4-SGK1 interaction in regulating ROMK channels in the kidney. Immunoprecipitation demonstrated that protein phosphatase 1 (PP1) binds to WNK4 at amino acid (aa) residues 695-699 (PP1(#1)) and at aa 1211-1215 (PP1(#2)). WNK4(-PP1#1) and WNK4(-PP1#2), in which the PP1(#1) or PP1(#2) binding site was deleted or mutated, inhibited ROMK channels as potently as WNK4. However, c-Src restored the inhibitory effect of WNK4 but not WNK4(-PP1#1) on ROMK channels in the presence of SGK1. Moreover, expression of c-Src inhibited SGK1-induced phosphorylation of WNK4 but not WNK4(-PP1#1) at serine residue 1196 (Ser(1196)). In contrast, coexpression of c-Src restored the inhibitory effect of WNK4(-PP1#2) on ROMK in the presence of SGK1 and diminished SGK1-induced WNK4 phosphorylation at Ser(1196) in cells transfected with WNK4(-PP1#2). This suggests the possibility that c-Src regulates the interaction between WNK4 and SGK1 through activating PP1 binding to aa 695-9 thereby decreasing WNK4 phosphorylation and restoring the inhibitory effect of WNK4. This mechanism plays a role in suppressing ROMK channel activity during the volume depletion because inhibition of SFK or serine/threonine phosphatases increases ROMK channel activity in the cortical collecting duct of rats on a low-Na diet. We conclude that regulation of phosphatase activity by SFK plays a role in determining the effect of aldosterone on ROMK channels and on renal K secretion.

Role of mineralocorticoid receptor/Rho/Rho-kinase pathway in obesity-related renal injury

OBJECTIVE:

We examined whether aldosterone/Rho/Rho-kinase pathway contributed to obesity-associated nephropathy.

SUBJECTS:

C57BL/6J mice were fed a high fat or low fat diet, and mice on a high fat diet were treated with a mineralocorticoid receptor antagonist, eplerenone.

RESULTS:

The mice on a high fat diet not only developed obesity, but also manifested renal histological changes, including glomerular hypercellularity and increased mesangial matrix, which paralleled the increase in albuminuria. Furthermore, enhanced Rho-kinase activity was noted in kidneys from high fat diet-fed mice, as well as increased expressions of inflammatory chemokines. All of these changes were attenuated by eplerenone. In high fat diet-fed mice, mineralocorticoid receptor protein levels in the nuclear fraction and SGK1, an effector of aldosterone, were upregulated in kidneys, although serum aldosterone levels were unaltered. Furthermore, aldosterone and 3β-hydroxysteroid dehydrogenase in renal tissues were upregulated in high fat diet-fed mice. Finally, in cultured mesangial cells, stimulation with aldosterone enhanced Rho-kinase activity, and pre-incubation with eplerenone prevented the aldosterone-induced activation of Rho kinase.

CONCLUSION:

Excess fat intake causes obesity and renal injury in C57BL/6J mice, and these changes are mediated by an enhanced mineralocorticoid receptor/Rho/Rho-kinase pathway and inflammatory process. Mineralocorticoid receptor activation in the kidney tissue and the subsequent Rho-kinase stimulation are likely to participate in the development of obesity-associated nephropathy without elevation in serum aldosterone levels.

Af17 deficiency increases sodium excretion and decreases blood pressure

The putative transcription factor AF17 upregulates the transcription of the epithelial sodium channel (ENaC) genes, but whether AF17 modulates sodium homeostasis and BP is unknown. Here, we generated Af17-deficient mice to determine whether deletion of Af17 leads to sodium wasting and low BP. Compared with wild-type mice, Af17-deficient mice had lower BP (11 mmHg), higher urine volume, and increased sodium excretion despite mildly increased plasma concentrations of aldosterone. Deletion of Af17 led to increased dimethylation of histone H3 K79 and reduced ENaC function. The attenuated function of ENaC resulted from decreased ENaC mRNA and protein expression, fewer active channels, lower open probability, and reduced effective activity. In contrast, inducing high levels of plasma aldosterone by a variety of methods completely compensated for Af17 deficiency with respect to sodium handling and BP. Taken together, these data identify Af17 as a potential locus for the maintenance of sodium and BP homeostasis and suggest that a particular histone modification is directly linked to these processes. Af17-mediated regulation of BP is largely, but not exclusively, the result of modulating ENaC, suggesting it has potential as a therapeutic target for the control of BP.

Expression and role of serum and glucocorticoid-regulated kinase 2 in the regulation of Na+/H+ exchanger 3 in the mammalian kidney

Serum and glucocorticoid-regulated kinase 2 (sgk2) is 80% identical to the kinase domain of sgk1, an important mediator of mineralocorticoid-regulated sodium (Na(+)) transport in the distal nephron of the kidney. The expression pattern and role in renal function of sgk2 are virtually uncharacterized. In situ hybridization and immunohistochemistry of rodent kidney coupled with real-time RT-PCR of microdissected rat kidney tubules showed robust sgk2 expression in the proximal straight tubule and thick ascending limb of the loop of Henle. Sgk2 expression was minimal in distal tubule cells with aquaporin-2 immunostaining but significant in proximal tubule cells with Na(+)/H(+) exchanger 3 (NHE3) immunostaining. To ascertain whether mineralocorticoids regulate expression of sgk2 in a manner similar to sgk1, we examined sgk2 mRNA expression in the kidneys of adrenalectomized rats treated with physiological doses of aldosterone together with the glucocorticoid receptor antagonist RU486. Northern blot analysis and in situ hybridization showed that, unlike sgk1, sgk2 expression in the kidney was not altered by aldosterone treatment. Based on the observation that sgk2 is expressed in proximal tubule cells that also express NHE3, we asked whether sgk2 regulates NHE3 activity. We heterologously expressed sgk2 in opossum kidney (OKP) cells and measured Na(+)/H(+) exchange activity by Na(+)-dependent cell pH recovery. Constitutively active sgk2, but not sgk1, stimulated Na(+)/H(+) exchange activity by >30%. Moreover, the sgk2-mediated increase in Na(+)/H(+) exchange activity correlated with an increase in cell surface expression of NHE3. Together, these results suggest that the pattern of expression, regulation, and role of sgk2 within the mammalian kidney are distinct from sgk1 and that sgk2 may play a previously unrecognized role in the control of transtubular Na(+) transport through NHE3 in the proximal tubule.

Angiotensin II induces phosphorylation of the thiazide-sensitive sodium chloride cotransporter independent of aldosterone

We studied here the independent roles of angiotensin II and aldosterone in regulating the sodium chloride cotransporter (NCC) of the distal convoluted tubule. We adrenalectomized three experimental and one control group of rats. Following surgery, the experimental groups were treated with either a high physiological dose of aldosterone, a non-pressor, or a pressor dose of angiotensin II for 8 days. Aldosterone and both doses of angiotensin II lowered sodium excretion and significantly increased the abundance of NCC in the plasma membrane compared with the control. Only the pressor dose of angiotensin II caused hypertension. Thiazides inhibited the sodium retention induced by the angiotensin II non-pressor dose. Both aldosterone and the non-pressor dose of angiotensin II significantly increased phosphorylation of NCC at threonine-53 and also increased the intracellular abundance of STE20/SPS1-related, proline alanine-rich kinase (SPAK). No differences were found in other modulators of NCC activity such as oxidative stress responsive protein type 1 or with-no-lysine kinase 4. Thus, our in vivo study shows that aldosterone and angiotensin II independently increase the abundance and phosphorylation of NCC in the setting of adrenalectomy; effects are likely mediated by SPAK. These results may explain, in part, the hormonal control of renal sodium excretion and the pathophysiology of several forms of hypertension.

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.

Cyclophilin D deficiency attenuates mitochondrial and neuronal perturbation and ameliorates learning and memory in Alzheimer's disease

Cyclophilin D (CypD, encoded by Ppif) is an integral part of the mitochondrial permeability transition pore, whose opening leads to cell death. Here we show that interaction of CypD with mitochondrial amyloid-beta protein (Abeta) potentiates mitochondrial, neuronal and synaptic stress. The CypD-deficient cortical mitochondria are resistant to Abeta- and Ca(2+)-induced mitochondrial swelling and permeability transition. Additionally, they have an increased calcium buffering capacity and generate fewer mitochondrial reactive oxygen species. Furthermore, the absence of CypD protects neurons from Abeta- and oxidative stress-induced cell death. Notably, CypD deficiency substantially improves learning and memory and synaptic function in an Alzheimer's disease mouse model and alleviates Abeta-mediated reduction of long-term potentiation. Thus, the CypD-mediated mitochondrial permeability transition pore is directly linked to the cellular and synaptic perturbations observed in the pathogenesis of Alzheimer's disease. Blockade of CypD may be a therapeutic strategy in Alzheimer's disease.

An obligatory heterodimer of 14-3-3beta and 14-3-3epsilon is required for aldosterone regulation of the epithelial sodium channel

Increased distal nephron sodium absorption in response to aldosterone involves Nedd4-2 phosphorylation, which blocks its ability to ubiquitylate ENaC and increases apical membrane channel density by reducing its endocytosis. Our prior work (Liang, X., Peters, K. W., Butterworth, M. B., and Frizzell, R. A. (2006) J. Biol. Chem. 281, 16323-16332) showed that aldosterone selectively increased 14-3-3 protein isoform expression and that the association of 14-3-3beta with phospho-Nedd4-2 was required for sodium transport stimulation. The knockdown of 14-3-3beta alone nearly eliminated the response to aldosterone, despite the expression of other 14-3-3 isoforms in cortical collecting duct (CCD) cells. To further examine this marked effect of 14-3-3beta knockdown, we evaluated the hypothesis that phospho-Nedd4-2 binding prefers a heterodimer composed of two different 14-3-3 isoforms. We tested this concept in polarized CCD cells using RNA interference and assays of sodium transport and of the interaction of Nedd4-2 with 14-3-3epsilon, a second aldosterone-induced isoform. As observed previously for 14-3-3beta knockdown, small interfering RNA-induced reduction of 14-3-3epsilon markedly attenuated aldosterone-stimulated ENaC expression and sodium transport and increased the interaction of Nedd4-2 with ENaC toward prealdosterone levels. After aldosterone induction, 14-3-3beta and 14-3-3epsilon were quantitatively co-immunoprecipitated from CCD cell lysates, and the association of both isoforms with Nedd4-2 increased. Finally, the knockdown of either 14-3-3beta or 14-3-3epsilon reduced the association of Nedd4-2 with the other isoform. We conclude that the two aldosterone-induced 14-3-3 isoforms, beta and epsilon, interact with phospho-Nedd4-2 as an obligatory heterodimer, blocking its interaction with ENaC and thereby increasing apical ENaC density and sodium transport.

Up-regulation of hypertonicity-activated myo-inositol transporter SMIT1 by the cell volume-sensitive protein kinase SGK1

Mechanisms of regulatory cell volume increase following cell shrinkage include accumulation of organic osmolytes such as betaine, taurine, sorbitol, glycerophosphorylcholine (GPC) and myo-inositol. Myo-inositol is taken up by the sodium-myo-inositol-transporter SMIT1 (SLC5A3) expressed in a wide variety of cell types. Hypertonicity induces the transcription of the SMIT1 gene upon binding of the transcription factor tonicity enhancer binding protein (TonEBP) to tonicity responsive enhancers (TonE) in the SMIT1 promoter region. However, little is known about post-translational regulation of the carrier protein. In this study we show that SMIT1 is modulated by the serum- and glucocorticoid-inducible kinase SGK1, a protein genomically up-regulated by hypertonicity. As demonstrated by two-electrode voltage-clamp in the Xenopus oocyte expression system, SMIT1-mediated myo-inositol-induced currents are up-regulated by coexpression of wild type SGK1 and constitutively active (S422D)SGK1 but not by inactive (K127N)SGK1. The increase in SMIT1 activity is due to an elevated cell surface expression of the carrier while its kinetic properties remain unaffected. According to the decay of SMIT1 activity in the presence of brefeldin A, SGK1 stabilizes the SMIT1 protein in the plasma membrane. The SGK isoforms SGK2, SGK3 and the closely related protein kinase B (PKB) are similarly capable of activating SMIT1 activity. SMIT1-mediated currents are decreased by coexpression of the ubiquitin-ligase Nedd4-2, an effect counteracted by additional coexpression of SGK1. In conclusion, the present observations disclose SGK isoforms and protein kinase B as novel regulators of SMIT1 activity.

Aldosterone-stimulated SGK1 activity mediates profibrotic signaling in the mesangium

Several recent reports support the hypothesis that aldosterone contributes to the progression of renal injury. Mineralocorticoids increase the expression of serum- and glucocorticoid-inducible protein kinase 1 (SGK1), which is upregulated in several fibrotic diseases. It was hypothesized that SGK1 may mediate the effects of aldosterone on glomerular fibrosis and inflammation. In primary cultures of rat mesangial cells, aldosterone stimulated the expression, phosphorylation, and kinase activity of SGK1, as well as SGK1-dependent NF-kappaB activity. Furthermore, aldosterone augmented the promoter activity and protein expression of intercellular adhesion molecule-1 (ICAM-1), which modulates the inflammatory response, and the profibrotic cytokine connective tissue growth factor (CTGF) in an SGK1- and NF-kappaB-dependent manner. Similar to the in vitro results, uninephrectomized rats that were treated with aldosterone demonstrated increased glomerular expression of SGK1, ICAM-1, and CTGF proteins than untreated rats; these changes were accompanied by hypertension, glomerulosclerosis, and inflammation. In conclusion, these findings suggest that aldosterone stimulates ICAM-1 and CTGF transcription via the activation of SGK1 and NF-kappaB, effects that may contribute to the progression of aldosterone-induced mesangial fibrosis and inflammation.

Aldosterone-induced Sgk1 relieves Dot1a-Af9-mediated transcriptional repression of epithelial Na+ channel alpha

Aldosterone plays a major role in the regulation of salt balance and the pathophysiology of cardiovascular and renal diseases. Many aldosterone-regulated genes--including that encoding the epithelial Na+ channel (ENaC), a key arbiter of Na+ transport in the kidney and other epithelia--have been identified, but the mechanisms by which the hormone modifies chromatin structure and thus transcription remain unknown. We previously described the basal repression of ENaCalpha by a complex containing the histone H3 Lys79 methyltransferase disruptor of telomeric silencing alternative splice variant a (Dot1a) and the putative transcription factor ALL1-fused gene from chromosome 9 (Af9) as well as the release of this repression by aldosterone treatment. Here we provide evidence from renal collecting duct cells and serum- and glucocorticoid-induced kinase-1 (Sgk1) WT and knockout mice that Sgk1 phosphorylated Af9, thereby impairing the Dot1a-Af9 interaction and leading to targeted histone H3 Lys79 hypomethylation at the ENaCalpha promoter and derepression of ENaCalpha transcription. Thus, Af9 is a physiologic target of Sgk1, and Sgk1 negatively regulates the Dot1a-Af9 repressor complex that controls transcription of ENaCalpha and likely other aldosterone-induced genes.

Aldosterone impairs vascular reactivity by decreasing glucose-6-phosphate dehydrogenase activity

Hyperaldosteronism is associated with impaired vascular reactivity; however, the mechanisms by which aldosterone promotes endothelial dysfunction remain unknown. Glucose-6-phosphate dehydrogenase (G6PD) modulates vascular function by limiting oxidant stress to preserve bioavailable nitric oxide (NO(*)). Here we show that aldosterone (10(-9)-;10(-7) mol/l) decreased endothelial G6PD expression and activity in vitro, resulting in increased oxidant stress and decreased NO(*) levels-similar to what is observed in G6PD-deficient endothelial cells. Aldosterone decreased G6PD expression by increasing expression of the cyclic AMP-response element modulator (CREM) to inhibit cyclic AMP-response element binding protein (CREB)-mediated G6PD transcription. In vivo, infusion of aldosterone decreased vascular G6PD expression and impaired vascular reactivity. These effects were abrogated by spironolactone or vascular gene transfer of G6pd. These findings demonstrate that aldosterone induces a G6PD-deficient phenotype to impair endothelial function; aldosterone antagonism or gene transfer of G6pd improves vascular reactivity by restoring G6PD activity.

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.

Dietary electrolyte-driven responses in the renal WNK kinase pathway in vivo

WNK1 and WNK4 are unusual serine/threonine kinases with atypical positioning of the catalytic active-site lysine (WNK: With-No-K[lysine]). Mutations in these WNK kinase genes can cause familial hyperkalemic hypertension (FHHt), an autosomal dominant, hypertensive, hyperkalemic disorder, implicating this novel WNK pathway in normal regulation of BP and electrolyte balance. Full-length (WNK1-L) and short (WNK1-S) kinase-deficient WNK1 isoforms previously have been identified. Importantly, WNK1-S is overwhelmingly predominant in kidney. Recent Xenopus oocyte studies implicate WNK4 in inhibition of both thiazide-sensitive co-transporter-mediated Na+ reabsorption and K+ secretion via renal outer medullary K+ channel and now suggest that WNK4 is inhibited by WNK1-L, itself inhibited by WNK1-S. This study examined WNK pathway gene expression in mouse kidney and its regulation in vivo. Expression of WNK1-S and WNK4 is strongest in distal tubule, dropping sharply in collecting duct and with WNK4 also expressed in thick ascending limb and the macula densa. These nephron segments that express WNK1-S and WNK4 mRNA have major influence on long-term NaCl reabsorption, BP, K+, and acid-base balance, processes that all are disrupted in FHHt. In vivo, this novel WNK pathway responds with significant upregulation of WNK1-S and WNK4 with high K+ intake and reduction in WNK1-S on chronic lowering of K+ or Na+ intake. A two-compartment distal nephron model explains these in vivo findings and the pathophysiology of FHHt well, with WNK and classic aldosterone pathways responding to drivers from K+ balance, extracellular volume, and aldosterone and cross-talk through distal Na+ delivery regulating electrolyte balance and BP.

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.

14-3-3 isoforms are induced by aldosterone and participate in its regulation of epithelial sodium channels

Aldosterone increases sodium absorption across renal collecting duct cells primarily by increasing the apical membrane expression of ENaC, the sodium entry channel. Nedd4-2, a ubiquitin-protein isopeptide ligase, tags ENaC with ubiquitin for internalization and degradation, but when it is phosphorylated by the aldosterone-induced kinase, SGK1, Nedd4-2 is inhibited and apical ENaC density and sodium absorption increase. We evaluated the hypothesis that 14-3-3 proteins participate in the aldosterone-mediated regulation of ENaC by associating with phosphorylated Nedd4-2. Mouse cortical collecting duct (mCCD) epithelia cultured on filters expressed several 14-3-3 isoforms; this study focused on an isoform whose expression was induced 3-fold by aldosterone, 14-3-3beta. In polarized mCCD epithelia, aldosterone elicited significant, time-dependent increases in the expression of alpha-ENaC, SGK1, phospho-Nedd4-2, and 14-3-3beta without altering total Nedd4-2. Aldosterone decreased the interaction of alpha-ENaC with Nedd4-2, and with similar kinetics increased the association of 14-3-3beta with phospho-Nedd4-2. Short interfering RNA-induced knockdown of 14-3-3beta blunted the aldosterone-induced increase in alpha-ENaC expression, returned alpha-ENaC-Nedd4-2 binding toward prealdosterone levels, and blocked the aldosterone-stimulated increase in transepithelial sodium transport. Incubation of cell extracts with a selective phospho-Nedd4-2 antibody blocked the aldosterone-induced association of 14-3-3beta with Nedd4-2, implicating SGK1 phosphorylation at Ser-328 as the primary site of 14-3-3beta binding. Our studies show that aldosterone increases the expression of 14-3-3beta, which interacts with phospho-Nedd4-2 to block its interaction with ENaC, thus enhancing sodium absorption by increasing apical membrane ENaC density.

Dietary sodium intake regulates the ubiquitin-protein ligase nedd4-2 in the renal collecting system

The activity of the epithelial sodium (Na(+)) channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) needs to be tightly regulated to match urinary Na(+) excretion with dietary Na(+) intake. The ubiquitin-protein ligase Nedd4-2, which in vitro interacts with ENaC subunits and reduces ENaC cell surface abundance and activity by ubiquitylation of the channel, may participate in the control of ENaC. This study confirms in vivo by reverse-transcriptase-PCR that Nedd4-2 is expressed throughout the nephron and is detectable by immunoblotting in kidney extracts. By immunohistochemistry, Nedd4-2 was found to be strongly expressed in the ASDN, with low staining intensity in the late distal convoluted tubule and early connecting tubule (where apical ENaC is high) and gradually increasing detection levels toward the collecting duct (CD; where apical ENaC is low). Compared with high-Na(+) diet (5% Na(+)), 2 wk of low-Na(+) diet (0.01% Na(+)) drastically reduces Nedd4-2 immunostaining and increases apical ENaC abundance in ASDN. Reduced Nedd4-2 immunostaining is not dependent on increased apical Na(+) entry in the CD, because it is similarly observed in mice with intact and with suppressed apical ENaC activity in the CD. Consistent with a role of mineralocorticoid hormones in the long-term regulation of Nedd4-2, 5-d treatment of cultured CD (mpkCCD(cl4)) cells with 1 microM aldosterone leads to reduction of Nedd4-2 protein expression. It is concluded that Nedd4-2 abundance is regulated by Na(+) diet, by a mechanism that likely involves aldosterone. This regulation may contribute to adaptation of apical ENaC activity to altered Na(+) intake.

Increased expression of receptor for advanced glycation end products by synovial tissue macrophages in rheumatoid arthritis

OBJECTIVE

The accumulation of advanced glycation end products (AGEs), S100A12, and high mobility group box chromosomal protein 1 has been associated with joint inflammation in rheumatoid arthritis (RA). This study was undertaken to determine the induction of the receptor for these proteins, termed receptor for AGEs (RAGE), in synovial tissue (ST) macrophages from RA patients.

METHODS

RAGE and CD68 expression in ST were determined by 2-color immunofluorescence labeling. Cell surface and messenger RNA (mRNA) expression of RAGE were examined by flow cytometry and reverse transcriptase-polymerase chain reaction (PCR) or real-time PCR, respectively.

RESULTS

CD68+ lining macrophages, like the vasculature, expressed high levels of RAGE in inflamed ST from RA patients. RAGE mRNA expression was significantly higher in RA ST than in ST from patients with osteoarthritis. RAGE mRNA levels were significantly higher in ST macrophages and normal endothelial cells than in ST CD4+ T cells and synovial fibroblasts stimulated with tumor necrosis factor alpha and interleukin-1beta (IL-1beta). Cell surface RAGE was highly induced on normal monocytes after a 24-hour incubation with a 20% concentration of RA ST cell culture supernatants. RAGE mRNA expression in adherent monocytes was augmented by various cytokines, most potently by IL-1beta.

CONCLUSION

These results indicate that RAGE overexpression in lining macrophages may be induced, at least in part, by cytokines such as IL-1, leading to the amplification of inflammatory responses mediated by RAGE ligands that are abundant in RA joints.

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.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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.

Activation of receptor for advanced glycation end products in osteoarthritis leads to increased stimulation of chondrocytes and synoviocytes

OBJECTIVE

The major risk factor for osteoarthritis (OA) is aging, but the mechanisms underlying this risk are only partly understood. Age-related accumulation of advanced glycation end products (AGEs) could be one of these mechanisms. We undertook this study to investigate the role of the receptor for AGEs (RAGE) in mediating the cellular effects of AGEs on chondrocytes and fibroblast-like synoviocytes (FLS).

METHODS

AGE levels in human cartilage were determined by fluorescence, browning, and pentosidine levels. Chondrocyte activation by AGEs was assessed as the release of proteoglycans and the synthesis of matrix metalloproteinase 1 (MMP-1) and type II collagen messenger RNA (mRNA). The activation of FLS by AGEs was measured by MMP-1 production and invasion through matrix proteins.

RESULTS

Patients with focal degeneration of cartilage showed increased AGE levels in their healthy cartilage compared with the levels in healthy cartilage from donors without cartilage degeneration (P < 0.01 for both fluorescence and browning; P not significant for pentosidine content). Stimulation of bovine chondrocytes with glycated albumin increased the release of proteoglycans by 110% (P < 0.001) and the production of MMP-1 mRNA by 200% (P = 0.028). In addition, OA FLS produced 240% more MMP-1 when stimulated with glycated albumin (P < 0.001). Glycated matrix or albumin increased the catabolic activity of OA FLS, which was assessed as invasive behavior, by 150% and 140% (P = 0.001 and P = 0.010), respectively. Effects of stimulation with AGEs were blocked by a neutralizing antibody against RAGE, but not by an isotype control.

CONCLUSION

This study shows that AGEs trigger RAGE on chondrocytes and FLS, leading to increased catabolic activity and therefore to cartilage degradation. AGEs, via RAGE, could therefore contribute to the development and/or progression of OA.

Receptor for advanced glycation end products (RAGE) regulates sepsis but not the adaptive immune response

While the initiation of the adaptive and innate immune response is well understood, less is known about cellular mechanisms propagating inflammation. The receptor for advanced glycation end products (RAGE), a transmembrane receptor of the immunoglobulin superfamily, leads to perpetuated cell activation. Using novel animal models with defective or tissue-specific RAGE expression, we show that in these animal models RAGE does not play a role in the adaptive immune response. However, deletion of RAGE provides protection from the lethal effects of septic shock caused by cecal ligation and puncture. Such protection is reversed by reconstitution of RAGE in endothelial and hematopoietic cells. These results indicate that the innate immune response is controlled by pattern-recognition receptors not only at the initiating steps but also at the phase of perpetuation.

In vitro characterization of aldosterone and cAMP effects in mouse distal convoluted tubule cells

The distal nephron plays a capital role in the fine regulation of sodium reabsorption. Compared with the cortical collecting duct, much less information is available on the hormonal regulation of sodium transporter genes in the distal convoluted tubule (DCT), where the thiazide-sensitive Na+-Cl-cotransporter (NCC) is the major entry pathway for Na+. The purpose of this study was to characterize the in vitro effects of aldosterone (Aldo; 1 μM) and cAMP (8-BrcAMP; 0.5 mM) on mouse DCT (mDCT) by using an immortalized mDCT cell line. Western blot analysis and semiquantitative RT-PCR were performed to analyze the expression of genes involved in sodium transport. The mDCTcell line expressed the 11β-hydroxysteroid dehydrogenase type 2 gene and both the mineralocorticoid and glucocorticoid receptor genes, suggesting Aldo responsiveness. In this sense, we found that mDCT cells expressed the amiloride-sensitive Na+channel (ENaC) and responded to Aldo by upregulating the α-subunit protein. Similarly, α1Na+-K+-ATPase protein was upregulated by Aldo and 8-BrcAMP. In addition, the Aldo intermediate gene sgk1 mRNA was increased in response to both Aldo and 8-BrcAMP, and the transcription factor HNF–3α mRNA was induced by 8-BrcAMP. With respect to NCC regulation, although Aldo induced NCC protein levels in mice in vivo, neither Aldo nor 8-BrcAMP significantly induced the NCC mRNA or protein levels in mDCT cells. These results suggest that in mDCT, Aldo and cAMP modulate some downstream mediators and effectors in vitro but do not influence the expression of NCC in this cell model.

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

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

Mineralocorticoid effects in the kidney: correlation between alphaENaC, GILZ, and Sgk-1 mRNA expression and urinary excretion of Na+ and K+

Aldosterone exerts its effects through interactions with two types of binding sites, the mineralocorticoid (MR) and the glucocorticoid (GR) receptors. Although both receptors are known to be involved in the anti-natriuretic response to aldosterone, the mechanisms of signal transduction leading to modulation of electrolyte transport are not yet fully understood. This study measured the Na(+) and K(+) urinary excretion and the mRNA levels of three known aldosterone-induced transcripts, the serum and glucocorticoid-induced kinase (Sgk-1), the alpha subunit of the epithelial Na(+) channel (alphaENaC), and the glucocorticoid-induced-leucine-zipper protein (GILZ) in the whole kidney and in isolated cortical collecting tubules of adrenalectomized rats treated with low doses of aldosterone and/or dexamethasone. The resulting plasma concentrations of both steroids were close to 1 nmol/L. Aldosterone, given with or without dexamethasone, induced anti-natriuresis and kaliuresis, whereas dexamethasone alone did not. GILZ and alphaENaC transcripts were higher after treatment with either or both hormones, whereas the mRNA abundance of Sgk-1 was increased in the cortical collecting tubule by aldosterone but not by dexamethasone. We conclude the increased expression of Sgk-1 in the cortical collecting tubules is a primary event in the early antinatriuretic and kaliuretic responses to physiologic concentrations of aldosterone. Induction of alphaENaC and/or GILZ mRNAs may play a permissive role in the enhancement of the early and/or late responses; these effects may be necessary for a full response but do not by themselves promote early changes in urinary Na(+) and K(+) excretion.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.

Advanced glycation endproducts and osteoarthritis

Non-enzymatic glycation of proteins, such as collagen, results in the formation of advanced glycation endproducts (AGE). Advanced glycation endproducts result in pathologic stiffening of cartilage and extracellular matrix and accumulate with age. Pentosidine, an AGE, is present in serum, synovial fluid, and articular cartilage from patients with osteoarthritis (OA). However, AGE levels are not always increased, and may be decreased locally, in association with osteoarthritic pathology. The finding of pentosidine in articular cartilage of individuals with OA may not be specific for that disease, independent of chronologic age. Advanced glycation endproduct modification of normal articular cartilage increases its stiffness, increases chondrocyte-mediated proteoglycan degradation, reduces its susceptibility to matrix metalloproteinase-mediated degradation, and decreases proteoglycan synthesis by chondrocytes. These observations parallel findings in osteoarthritic cartilage, which suggests that AGE modification could contribute to the pathogenesis of OA. However, a causative link between AGEs and OA has not yet been established.