SGK integrates insulin and mineralocorticoid regulation of epithelial sodium transport

Renal tubular SGK1 is required to achieve blood pressure surge and circadian rhythm

Blood pressure (BP) follows a circadian pattern that rises during the active phase of the day (morning surge) and decreases during the inactive (night dipping) phase of the day. The morning surge coincides with increased circulating glucocorticoids and aldosterone, ligands for glucocorticoid receptors and mineralocorticoid receptors, respectively. Serum- and glucocorticoid-induced kinase 1 (SGK1), a clock-controlled and glucocorticoid receptor- and mineralocorticoid receptor-induced gene, plays a role in BP regulation in human and animal models. However, the role of SGK1 in BP circadian regulation has not yet been demonstrated. Using telemetry, we analyzed BP in the inducible renal tubule-specific Sgk1Pax8/LC1 model under basal K+ diet (1% K+) and high-K+ diet (HKD; 5% K+). Our data revealed that, under basal conditions, renal SGK1 plays a minor role in BP regulation; however, after 1 wk of HKD, Sgk1Pax8/LC1 mice exhibited significant defects in diastolic BP (DBP), including a blunted surge, a decreased amplitude, and reduced day/night differences. After prolonged HKD (7 wk), Sgk1Pax8/LC1 mice had lower BP than control mice and exhibited reduced DBP amplitude, together with decreased DBP day/night differences and midline estimating statistic of rhythm (MESOR). Interestingly, renal SGK1 deletion increased pulse pressure, likely secondary to an increase in circulating aldosterone. Taken together, our data suggest that 1) the kidney plays a significant role in setting the BP circadian rhythm; 2) renal tubule SGK1 mediates the BP surge and, thus, the day/night BP difference; 3) long-term renal SGK1 deletion results in lower BP in mutant compared with control mice; and 4) renal SGK1 indirectly regulates pulse pressure due to compensatory alterations in aldosterone levels.NEW & NOTEWORTHY Dysregulation of blood pressure (BP) circadian rhythm is associated with metabolic, cardiovascular, and kidney diseases. Our study provides experimental evidence demonstrating, for the first time, that renal tubule serum- and glucocorticoid-induced kinase 1 (SGK1) plays an essential role in inducing the BP surge. Inhibitors and activators of SGK1 signaling are parts of several therapeutic strategies. Our findings highlight the importance of the drug intake timing to be in phase with SGK1 function to avoid dysregulation of BP circadian rhythm.

The Post-Translational Modification Networking in WNK-Centric Hypertension Regulation and Electrolyte Homeostasis

The with-no-lysine (WNK) kinase family, comprising four serine-threonine protein kinases (WNK1-4), were first linked to hypertension due to their mutations in association with pseudohypoaldosteronism type II (PHAII). WNK kinases regulate crucial blood pressure regulators, SPAK/OSR1, to mediate the post-translational modifications (PTMs) of their downstream ion channel substrates, such as sodium chloride co-transporter (NCC), epithelial sodium chloride (ENaC), renal outer medullary potassium channel (ROMK), and Na/K/2Cl co-transporters (NKCCs). In this review, we summarize the molecular pathways dysregulating the WNKs and their downstream target renal ion transporters. We summarize each of the genetic variants of WNK kinases and the small molecule inhibitors that have been discovered to regulate blood pressure via WNK-triggered PTM cascades.

Selective Deletion of the Mechanistic Target of Rapamycin From the Renal Collecting Duct Principal Cell in Mice Down-Regulates the Epithelial Sodium Channel

The mechanistic target of rapamycin (mTOR), a serine-threonine-specific kinase, is a cellular energy sensor, integrating growth factor and nutrient signaling. In the collecting duct (CD) of the kidney, the epithelial sodium channel (ENaC) essential in the determination of final urine Na+ losses, has been demonstrated to be upregulated by mTOR, using cell culture and mTOR inhibition in ex vivo preparations. We tested whether CD-principal cell (PC) targeted deletion of mTOR using Cre-lox recombination would affect whole-body sodium homeostasis, blood pressure, and ENaC regulation in mice. Male and female CD-PC mTOR knockout (KO) mice and wild-type (WT) littermates (Cre-negative) were generated using aquaporin-2 (AQP2) promoter to drive Cre-recombinase. Under basal conditions, KO mice showed a reduced (∼30%) natriuretic response to benzamil (ENaC) antagonist, suggesting reduced in vivo ENaC activity. WT and KO mice were fed normal sodium (NS, 0.45% Na+) or a very low Na+ (LS, <0.02%) diet for 7-days. Switching from NS to LS resulted in significantly higher urine sodium losses (relative to WT) in the KO with adaptation occurring by day 2. Blood pressures were modestly (∼5-10 mm Hg) but significantly lower in KO mice under both diets. Western blotting showed KO mice had 20-40% reduced protein levels of all three subunits of ENaC under LS or NS diet. Immunohistochemistry (IHC) of kidney showed enhanced apical-vs.-cellular localization of all three subunits with LS, but a reduction in this ratio for γ-ENaC in the KO. Furthermore, the KO kidneys showed increased ubiquitination of α-ENaC and reduced phosphorylation of the serum and glucocorticoid regulated kinase, type 1 [serum glucocorticoid regulated kinase (SGK1)] on serine 422 (mTOR phosphorylation site). Taken together this suggests enhanced degradation as a consequence of reduced mTOR kinase activity and downstream upregulation of ubiquitination may have accounted for the reduction at least in α-ENaC. Overall, our data support a role for mTOR in ENaC activity likely via regulation of SGK1, ubiquitination, ENaC channel turnover and apical membrane residency. These data support a role for mTOR in the collecting duct in the maintenance of body sodium homeostasis.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Obesity, Adipose Tissue and Vascular Dysfunction

Cardiovascular diseases are the leading cause of death worldwide. Overweight and obesity are strongly associated with comorbidities such as hypertension and insulin resistance, which collectively contribute to the development of cardiovascular diseases and resultant morbidity and mortality. Forty-two percent of adults in the United States are obese, and a total of 1.9 billion adults worldwide are overweight or obese. These alarming numbers, which continue to climb, represent a major health and economic burden. Adipose tissue is a highly dynamic organ that can be classified based on the cellular composition of different depots and their distinct anatomical localization. Massive expansion and remodeling of adipose tissue during obesity differentially affects specific adipose tissue depots and significantly contributes to vascular dysfunction and cardiovascular diseases. Visceral adipose tissue accumulation results in increased immune cell infiltration and secretion of vasoconstrictor mediators, whereas expansion of subcutaneous adipose tissue is less harmful. Therefore, fat distribution more than overall body weight is a key determinant of the risk for cardiovascular diseases. Thermogenic brown and beige adipose tissue, in contrast to white adipose tissue, is associated with beneficial effects on the vasculature. The relationship between the type of adipose tissue and its influence on vascular function becomes particularly evident in the context of the heterogenous phenotype of perivascular adipose tissue that is strongly location dependent. In this review, we address the abnormal remodeling of specific adipose tissue depots during obesity and how this critically contributes to the development of hypertension, endothelial dysfunction, and vascular stiffness. We also discuss the local and systemic roles of adipose tissue derived secreted factors and increased systemic inflammation during obesity and highlight their detrimental impact on cardiovascular health.

Role of the vascular endothelial sodium channel activation in the genesis of pathologically increased cardiovascular stiffness

Cardiovascular (CV) stiffening represents a complex series of events evolving from pathological changes in individual cells of the vasculature and heart which leads to overt tissue fibrosis. While vascular stiffening occurs naturally with ageing it is accelerated in states of insulin (INS) resistance, such as obesity and type 2 diabetes. CV stiffening is clinically manifested as increased arterial pulse wave velocity and myocardial fibrosis-induced diastolic dysfunction. A key question that remains is how are these events mechanistically linked. In this regard, heightened activation of vascular mineralocorticoid receptors (MR) and hyperinsulinaemia occur in obesity and INS resistance states. Further, a downstream mediator of MR and INS receptor activation, the endothelial cell Na+ channel (EnNaC), has recently been identified as a key molecular determinant of endothelial dysfunction and CV fibrosis and stiffening. Increased activity of the EnNaC results in a number of negative consequences including stiffening of the cortical actin cytoskeleton in endothelial cells, impaired endothelial NO release, increased oxidative stress-meditated NO destruction, increased vascular permeability, and stimulation of an inflammatory environment. Such endothelial alterations impact vascular function and stiffening through regulation of vascular tone and stimulation of tissue remodelling including fibrosis. In the case of the heart, obesity and INS resistance are associated with coronary vascular endothelial stiffening and associated reductions in bioavailable NO leading to heart failure with preserved systolic function (HFpEF). After a brief discussion on mechanisms leading to vascular stiffness per se, this review then focuses on recent findings regarding the role of INS and aldosterone to enhance EnNaC activity and associated CV stiffness in obesity/INS resistance states. Finally, we discuss how coronary artery-mediated EnNaC activation may lead to cardiac fibrosis and HFpEF, a condition that is especially pronounced in obese and diabetic females.

Membrane trafficking pathways regulating the epithelial Na+ channel

Renal Na⁺ reabsorption, facilitated by the epithelial Na⁺ channel (ENaC), is subject to multiple forms of control to ensure optimal body blood volume and pressure through altering both the ENaC population and activity at the cell surface. Here, the focus is on regulating the number of ENaCs present in the apical membrane domain through pathways of ENaC synthesis and targeting to the apical membrane as well as ENaC removal, recycling, and degradation. Finally, the mechanisms by which ENaC trafficking pathways are regulated are summarized.

Trichostatin A blocks aldosterone-induced Na+ transport and control of serum- and glucocorticoid-inducible kinase 1 in cortical collecting duct cells

BACKGROUND AND PURPOSE

Aldosterone stimulates epithelial Na⁺ channel (ENaC)-dependent Na⁺ retention in the cortical collecting duct (CCD) of the kidney by activating mineralocorticoid receptors that promote expression of serum and glucocorticoid-inducible kinase 1 (SGK1). This response is critical to BP homeostasis. It has previously been suggested that inhibiting lysine deacetylases (KDACs) can post-transcriptionally disrupt this response by promoting acetylation of the mineralocorticoid receptor. The present study critically evaluates this hypothesis.

EXPERIMENTAL APPROACH

Electrometric and molecular methods were used to define the effects of a pan-KDAC inhibitor, trichostatin A, on the responses to a physiologically relevant concentration of aldosterone (3 nM) in murine mCCD_cl1 cells.

KEY RESULTS

Aldosterone augmented ENaC-induced Na⁺ absorption and increased SGK1 activity and abundance, as expected. In the presence of trichostatin A, these responses were suppressed. Trichostatin A-induced inhibition of KDAC was confirmed by increased acetylation of histone H3, H4, and α-tubulin. Trichostatin A did not block the electrometric response to insulin, a hormone that activates SGK1 independently of increased transcription, indicating that trichostatin A has no direct effect upon the SGK1/ENaC pathway.

CONCLUSIONS AND IMPLICATIONS

Inhibition of lysine de-acetylation suppresses aldosterone-dependent control over the SGK1-ENaC pathway but does not perturb post-transcriptional signalling, providing a physiological basis for the anti-hypertensive action of KDAC inhibition seen in vivo.

Postprandial Effects on ENaC-Mediated Sodium Absorption

Recent studies have suggested that postprandial increases in insulin directly contribute to reduced urinary sodium excretion. An abundance of research supports the ability of insulin to augment epithelial sodium channel (ENaC) transport. This study hypothesized that ENaC contributes to the increase in renal sodium reabsorption following a meal. To test this, we used fasted or 4 hour postprandial Sprague Dawley rats to analyze ENaC expression and activity. We also assessed total expression of additional sodium transporters (Na⁺-Cl⁻ cotransporter (NCC), Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2), and Na⁺-K⁺-ATPase (NKA)) and circulating hormones involved in the renin-angiotensin-aldosterone system (RAAS). We found that after carbohydrate stimulus, ENaC open probability increased in split-open isolated collecting duct tubules, while ENaC protein levels remained unchanged. This was supported by a lack of change in phosphorylated Nedd4-2, an E3 ubiquitin ligase protein which regulates the number of ENaCs at the plasma membrane. Additionally, we found no differences in total expression of NCC, NKCC2, or NKA in the postprandial rats. Lastly, there were no significant changes in RAAS signaling between the stimulated and fasted rats, suggesting that acute hyperinsulinemia increases ENaC activity independent of the RAAS signaling cascade. These results demonstrate that insulin regulation of ENaC is a potential mechanism to preserve sodium and volume loss following a meal, and that this regulation is distinct from classical ENaC regulation by RAAS.

Renal tubule insulin receptor modestly promotes elevated blood pressure and markedly stimulates glucose reabsorption

Although the cause of hypertension among individuals with obesity and insulin resistance is unknown, increased plasma insulin, acting in the kidney to increase sodium reabsorption, has been proposed as a potential mechanism. Insulin may also stimulate glucose uptake, but the contributions of tubular insulin signaling to sodium or glucose transport in the setting of insulin resistance is unknown. To directly study the role of insulin signaling in the kidney, we generated inducible renal tubule-specific insulin receptor-KO mice and used high-fat feeding and mineralocorticoids to model obesity and insulin resistance. Insulin receptor deletion did not alter blood pressure or sodium excretion in mice on a high-fat diet alone, but it mildly attenuated the increase in blood pressure with mineralocorticoid supplementation. Under these conditions, KO mice developed profound glucosuria. Insulin receptor deletion significantly reduced SGLT2 expression and increased urinary glucose excretion and urine flow. These data demonstrate a direct role for insulin receptor-stimulated sodium and glucose transport and a functional interaction of insulin signaling with mineralocorticoids in vivo. These studies uncover a potential mechanistic link between preserved insulin sensitivity and renal glucose handling in obesity and insulin resistance.

Mechanistic target of rapamycin: integrating growth factor and nutrient signaling in the collecting duct

The renal collecting duct and other postmacula densa sites are the primary tubular regions for fine-tuning of electrolyte homeostasis in the body. A role for the mechanistic target of rapamycin (mTOR), a serine-threonine kinase, has recently been appreciated in this regulation. mTOR exists in two distinct multiprotein functional complexes, i.e., mTORC1 and mTORC2. Upregulation of mTORC1, by growth factors and amino acids, is associated with cell cycle regulation and hypertrophic changes. In contrast, mTORC2 has been demonstrated to have a role in regulating Na⁺ and K⁺ reabsorptive processes, including those downstream of insulin and serum- and glucocorticoid-regulated kinase (SGK). In addition, mTORC2 can upregulate mTORC1. A number of elegant in vitro and in vivo studies using cell systems and genetically modified mice have revealed mechanisms underlying activation of the epithelial Na⁺ channel (ENaC) and the renal outer medullary K⁺ channel (ROMK) by mTORC2. Overall, mTOR in its systematic integration of phosphorylative signaling facilitates the delicate balance of whole body electrolyte homeostasis in the face of changes in metabolic status. Thus, inappropriate regulation of renal mTOR has the potential to result in electrolyte disturbances, such as acidosis/alkalosis, hyponatremia, and hypertension. The goal of this minireview is to highlight the physiological role of mTOR in its complexes in regulating electrolyte homeostasis in the aldosterone-sensitive distal nephron.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

Cardioprotective effects of pectin-insulin patch in streptozotocin-induced diabetic rats

BACKGROUND

Cardiovascular complications are among the leading causes of morbidity and mortality in diabetes mellitus. Despite the beneficial effects of subcutaneous insulin, reports suggest that the therapy itself precipitates cardiovascular risks due to the high insulin concentration administered. It is therefore necessary to seek alternative routes of insulin administration that may bypass the undesirable effects associated with high plasma insulin concentrations. Accordingly, the present study investigated the effects of a novel transdermal pectin-insulin patch on selected markers of cardiovascular function in diabetes.

METHODS

Pectin-insulin matrix patches (20.0, 40.8, and 82.9 μg/kg) were prepared as described previously. The three formulations were applied to streptozotocin-induced diabetic rats thrice daily. Blood glucose concentrations and mean arterial pressure (MAP) were monitored weekly for 5 weeks. Rats were then killed and blood collected for analysis of the lipid profile, cardiotropin-1, tumor necrosis factor (TNF)-α, and high-sensitivity C-reactive protein (hsCRP).

RESULTS

The patches decreased blood glucose concentrations and diabetes-induced disturbances in lipid profile were attenuated by patch application (82.9 μg/kg). The diabetes-induced increase in MAP was also attenuated in patch (82.9 μg/kg)-treated rats. Patch treatment resulted in a decreased heart weight: body weight ratio, as well as reductions in cardiotropin-1, TNF-α, and hsCRP concentrations.

CONCLUSIONS

Application of the pectin-insulin patch protects against the debilitating cardiovascular effects associated with conventional diabetes treatment. This suggests that the pectin-insulin patch may provide an effective alternative therapeutic approach to the commonly used subcutaneous insulin injections in the management of diabetes.

Fluid Retention Caused by Rosiglitazone Is Related to Increases in AQP2 and αENaC Membrane Expression

Peroxisome proliferator activated receptor-γ (PPARγ) is a ligand-activated transcription factor of the nuclear hormone receptor superfamily. The decreased phosphorylation of PPARγ due to rosiglitazone (ROS) is the main reason for the increased insulin sensitivity caused by this antidiabetic drug. However, there is no clear evidence whether the nuclear translocation of p-PPARγ stimulated by ROS is related to fluid retention. It is also unclear whether the translocation of p-PPARγ is associated with the change of aquaporin-2 (AQP2) and epithelial sodium channel α subunit (αENaC) in membranes, cytoplasm, and nucleus. Our experiments indicate that ROS significantly downregulates nuclear p-PPARγ and increases membrane AQP2 and αENaC; however, SR1664 (a nonagonist PPARγ ligand) reduces p-PPARγ and has no effect on AQP2 and αENaC. Therefore, we conclude that in vitro the fluid retention caused by ROS is associated with the increases in membrane αENaC and AQP2 but has little relevance to the phosphorylation of PPARγ.

Dexamethasone and insulin activate serum and glucocorticoid-inducible kinase 1 (SGK1) via different molecular mechanisms in cortical collecting duct cells

Serum and glucocorticoid-inducible kinase 1 (SGK1) is a protein kinase that contributes to the hormonal control of renal Na(+) retention by regulating the abundance of epithelial Na(+) channels (ENaC) at the apical surface of the principal cells of the cortical collecting duct (CCD). Although glucocorticoids and insulin stimulate Na(+) transport by activating SGK1, the responses follow different time courses suggesting that these hormones act by different mechanisms. We therefore explored the signaling pathways that allow dexamethasone and insulin to stimulate Na(+) transport in mouse CCD cells (mpkCCDcl4). Dexamethasone evoked a progressive augmentation of electrogenic Na(+) transport that became apparent after ~45 min latency and was associated with increases in SGK1 activity and abundance and with increased expression of SGK1 mRNA Although the catalytic activity of SGK1 is maintained by phosphatidylinositol-OH-3-kinase (PI3K), dexamethasone had no effect upon PI3K activity. Insulin also stimulated Na(+) transport but this response occurred with no discernible latency. Moreover, although insulin also activated SGK1, it had no effect upon SGK1 protein or mRNA abundance. Insulin did, however, evoke a clear increase in cellular PI3K activity. Our data are consistent with earlier work, which shows that glucocorticoids regulate Na(+) retention by inducing sgk1 gene expression, and also establish that this occurs independently of increased PI3K activity. Insulin, on the other hand, stimulates Na(+) transport via a mechanism independent of sgk1 gene expression that involves PI3K activation. Although both hormones act via SGK1, our data show that they activate this kinase by distinct physiological mechanisms.

There and back again: insulin, ENaC, and the cortical collecting duct

Cell culture models suggest mechanisms by which insulin stimulates ENaC in the cortical collecting duct. These mechanisms still need to be tested for physiological significance in animal models of insulin resistance.

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.

Quantitative proteomics and bioinformatic analysis provide new insight into the dynamic response of porcine intestine to Salmonella Typhimurium

The enteropathogen Salmonella Typhimurium (S. Typhimurium) is the most commonly non-typhoideal serotype isolated in pig worldwide. Currently, one of the main sources of human infection is by consumption of pork meat. Therefore, prevention and control of salmonellosis in pigs is crucial for minimizing risks to public health. The aim of the present study was to use isobaric tags for relative and absolute quantification (iTRAQ) to explore differences in the response to Salmonella in two segment of the porcine gut (ileum and colon) along a time course of 1, 2, and 6 days post infection (dpi) with S. Typhimurium. A total of 298 proteins were identified in the infected ileum samples of which, 112 displayed significant expression differences due to Salmonella infection. In colon, 184 proteins were detected in the infected samples of which 46 resulted differentially expressed with respect to the controls. The higher number of changes in protein expression was quantified in ileum at 2 dpi. Further biological interpretation of proteomics data using bioinformatics tools demonstrated that the expression changes in colon were found in proteins involved in cell death and survival, tissue morphology or molecular transport at the early stages and tissue regeneration at 6 dpi. In ileum, however, changes in protein expression were mainly related to immunological and infection diseases, inflammatory response or connective tissue disorders at 1 and 2 dpi. iTRAQ has proved to be a proteomic robust approach allowing us to identify ileum as the earliest response focus upon S. Typhimurium in the porcine gut. In addition, new functions involved in the response to bacteria such as eIF2 signaling, free radical scavengers or antimicrobial peptides (AMP) expression have been identified. Finally, the impairment at of the enterohepatic circulation of bile acids and lipid metabolism by means the under regulation of FABP6 protein and FXR/RXR and LXR/RXR signaling pathway in ileum has been established for the first time in pigs. Taken together, our results provide a better understanding of the porcine response to Salmonella infection and the molecular mechanisms underlying Salmonella-host interactions.

Cross-talk between insulin and IGF-1 receptors in the cortical collecting duct principal cells: implication for ENaC-mediated Na+ reabsorption

Insulin and IGF-1 are recognized as powerful regulators of the epithelial Na+ channel (ENaC) in the aldosterone-sensitive distal nephron. As previously described, these hormones both acutely increase ENaC activity in freshly isolated split open tubules and cultured principal cortical collecting duct cells. The present study was aimed at differentiating the effects of insulin and IGF-1 on Na+ transport in immortalized mpkCCDcl4 cells and defining their interrelations. We have shown that both insulin and IGF-1 applied basolaterally, but not apically, enhanced transepithelial Na+ transport in the mpkCCDcl4 cell line with EC50 values of 8.8 and 14.5 nM, respectively. Insulin treatment evoked phosphorylation of both insulin and IGF-1 receptors, whereas the effects of IGF-1 were more profound on its own receptor rather than the insulin receptor. AG-1024 and PPP, inhibitors of IGF-1 and insulin receptor tyrosine kinase activity, diminished insulin- and IGF-1-stimulated Na+ transport in mpkCCDcl4 cells. The effects of insulin and IGF-1 on ENaC-mediated currents were found to be additive, with insulin likely stimulating both IGF-1 and insulin receptors. We hypothesize that insulin activates IGF-1 receptors in addition to its own receptors, making the effects of these hormones interconnected.

mTORC2 regulates renal tubule sodium uptake by promoting ENaC activity

The epithelial Na+ channel (ENaC) is essential for Na+ homeostasis, and dysregulation of this channel underlies many forms of hypertension. Recent studies suggest that mTOR regulates phosphorylation and activation of serum/glucocorticoid regulated kinase 1 (SGK1), which is known to inhibit ENaC internalization and degradation; however, it is not clear whether mTOR contributes to the regulation of renal tubule ion transport. Here, we evaluated the effect of selective mTOR inhibitors on kidney tubule Na+ and K+ transport in WT and Sgk1-/- mice, as well as in isolated collecting tubules. We found that 2 structurally distinct competitive inhibitors (PP242 and AZD8055), both of which prevent all mTOR-dependent phosphorylation, including that of SGK1, caused substantial natriuresis, but not kaliuresis, in WT mice, which indicates that mTOR preferentially influences ENaC function. PP242 also substantially inhibited Na+ currents in isolated perfused cortical collecting tubules. Accordingly, patch clamp studies on cortical tubule apical membranes revealed that mTOR inhibition markedly reduces ENaC activity, but does not alter activity of K+ inwardly rectifying channels (ROMK channels). Together, these results demonstrate that mTOR regulates kidney tubule ion handling and suggest that mTOR regulates Na+ homeostasis through SGK1-dependent modulation of ENaC activity.

Camelid genomes reveal evolution and adaptation to desert environments

Bactrian camel (Camelus bactrianus), dromedary (Camelus dromedarius) and alpaca (Vicugna pacos) are economically important livestock. Although the Bactrian camel and dromedary are large, typically arid-desert-adapted mammals, alpacas are adapted to plateaus. Here we present high-quality genome sequences of these three species. Our analysis reveals the demographic history of these species since the Tortonian Stage of the Miocene and uncovers a striking correlation between large fluctuations in population size and geological time boundaries. Comparative genomic analysis reveals complex features related to desert adaptations, including fat and water metabolism, stress responses to heat, aridity, intense ultraviolet radiation and choking dust. Transcriptomic analysis of Bactrian camels further reveals unique osmoregulation, osmoprotection and compensatory mechanisms for water reservation underpinned by high blood glucose levels. We hypothesize that these physiological mechanisms represent kidney evolutionary adaptations to the desert environment. This study advances our understanding of camelid evolution and the adaptation of camels to arid-desert environments.

Growth hormone, insulin-like growth factor-1, and the kidney: pathophysiological and clinical implications

Besides their growth-promoting properties, GH and IGF-1 regulate a broad spectrum of biological functions in several organs, including the kidney. This review focuses on the renal actions of GH and IGF-1, taking into account major advances in renal physiology and hormone biology made over the last 20 years, allowing us to move our understanding of GH/IGF-1 regulation of renal functions from a cellular to a molecular level. The main purpose of this review was to analyze how GH and IGF-1 regulate renal development, glomerular functions, and tubular handling of sodium, calcium, phosphate, and glucose. Whenever possible, the relative contributions, the nephronic topology, and the underlying molecular mechanisms of GH and IGF-1 actions were addressed. Beyond the physiological aspects of GH/IGF-1 action on the kidney, the review describes the impact of GH excess and deficiency on renal architecture and functions. It reports in particular new insights into the pathophysiological mechanism of body fluid retention and of changes in phospho-calcium metabolism in acromegaly as well as of the reciprocal changes in sodium, calcium, and phosphate homeostasis observed in GH deficiency. The second aim of this review was to analyze how the GH/IGF-1 axis contributes to major renal diseases such as diabetic nephropathy, renal failure, renal carcinoma, and polycystic renal disease. It summarizes the consequences of chronic renal failure and glucocorticoid therapy after renal transplantation on GH secretion and action and questions the interest of GH therapy in these conditions.

Ethanol regulation of serum glucocorticoid kinase 1 expression in DBA2/J mouse prefrontal cortex

Background

We previously identified a group of glucocorticoid-responsive genes, including Serum Glucocorticoid kinase 1 (Sgk1), regulated by acute ethanol in prefrontal cortex of DBA2/J mice. Acute ethanol activates the hypothalamic pituitary adrenal axis (HPA) causing release of glucocorticoids. Chronic ethanol dysregulates the HPA response in both humans and rodents, possibly contributing to important interactions between stress and alcoholism. Because Sgk1 regulates ion channels and learning and memory, we hypothesized that Sgk1 contributes to HPA-dependent acute and adaptive neuronal responses to ethanol. These studies characterized acute and chronic ethanol regulation of Sgk1 mRNA and protein and their relationship with ethanol actions on the HPA axis.

Results

Acute ethanol increased Sgk1 mRNA expression in a dose and time dependent manner. Three separate results suggested that ethanol regulated Sgk1 via circulating glucocorticoids: acute ethanol increased glucocorticoid receptor binding to the Sgk1 promoter; adrenalectomy blocked ethanol induction of Sgk1 mRNA; and chronic ethanol exposure during locomotor sensitization down-regulated HPA axis activation and Sgk1 induction by acute ethanol. SGK1 protein had complex temporal responses to acute ethanol with rapid and transient increases in Ser422 phosphorylation at 15 min. following ethanol administration. This activating phosphorylation had functional consequences, as suggested by increased phosphorylation of the known SGK1 target, N-myc downstream-regulated gene 1 (NDRG1). After repeated ethanol administration during locomotor sensitization, basal SGK1 protein phosphorylation increased despite blunting of Sgk1 mRNA induction by ethanol.

Conclusions

These results suggest that HPA axis and glucocorticoid receptor signaling mediate acute ethanol induction of Sgk1 transcription in mouse prefrontal cortex. However, acute ethanol also causes complex changes in SGK1 protein expression and activity. Chronic ethanol modifies both SGK1 protein and HPA-mediated induction of Sgk1 mRNA. These adaptive molecular responses of glucocorticoid-responsive gene expression and SGK1 in prefrontal cortex may contribute to mechanisms underlying behavioral responses to chronic ethanol exposure.

Insulin increases the functional activity of the renal NaCl cotransporter

OBJECTIVES

Insulin is recognized to increase renal salt reabsorption in the distal nephron and hyperinsulinemic states have been shown to be associated with increased expression of the renal NaCl cotransporter (NCC). However, the effect of insulin on NCC functional activity has not been reported.

METHODS

Using a heterologous expression system of Xenopus laevis oocytes, a mouse distal convoluted cell line, mDCT15 cells, endogenously expressing NCC, and an ex-vivo kidney perfusion technique, we assessed the effect of insulin on the activity and phosphorylation of NCC. The signaling pathway involved was analyzed.

RESULTS

In Xenopus oocytes insulin increases the activity of NCC together with its phosphorylation at threonine residue 58. Activation of NCC by insulin was also observed in mDCT15 cells. Additionally, insulin increased the NCC phosphorylation in kidney under the ex-vivo perfusion technique. In oocytes and mDCT15 cells, insulin effect on NCC was prevented with inhibitors of phosphatidylinositol 3-kinase (PI3K), mTORC2, and AKT1 kinases, but not by inhibitors of MAP or mTORC1 kinases, suggesting that PI3K-mTORC2-AKT1 is the intracellular pathway required. Additionally, activation of NCC by insulin was not affected by wild-type or mutant versions of with no lysine kinase 1, with no lysine kinase 4, or serum glucocorticoid kinase 1, but it was no longer observed in the presence of wild-type or the dominant negative, catalytically inactive with no lysine kinase 3, implicating this kinase in the process.

CONCLUSION

Insulin induces activation and phosphorylation of NCC. This effect could play an important role in arterial hypertension associated with hyperinsulinemic states, such as obesity, metabolic syndrome, or type 2 diabetes mellitus.

Hypokalemia during treatment of diabetic ketoacidosis: clinical evidence for an aldosterone-like action of insulin

OBJECTIVES

To investigate whether the development of hypokalemia in patients with diabetic ketoacidosis (DKA) treated in the pediatric critical care unit (PCCU) could be caused by increased potassium (K(+)) excretion and its association with insulin treatment.

STUDY DESIGN

In this prospective observational study of patients with DKA admitted to the PCCU, blood and timed urine samples were collected for measurement of sodium (Na(+)), K(+), and creatinine concentrations and for calculations of Na(+) and K(+) balances. K(+) excretion rate was expressed as urine K(+)-to-creatinine ratio and fractional excretion of K(+).

RESULTS

Of 31 patients, 25 (81%) developed hypokalemia (plasma K(+) concentration <3.5 mmol/L) in the PCCU at a median time of 24 hours after therapy began. At nadir plasma K(+) concentration, urine K(+)-to-creatinine ratio and fractional excretion of K(+) were greater in patients who developed hypokalemia compared with those without hypokalemia (19.8 vs 6.7, P = .04; and 31.3% vs 9.4%, P = .004, respectively). Patients in the hypokalemia group received a continuous infusion of intravenous insulin for a longer time (36.5 vs 20 hours, P = .015) and greater amount of Na(+) (19.4 vs 12.8 mmol/kg, P = .02). At peak kaliuresis, insulin dose was higher in the hypokalemia group (median 0.07, range 0-0.24 vs median 0.025, range 0-0.05 IU/kg; P = .01), and there was a significant correlation between K(+) and Na(+) excretion (r = 0.67, P < .0001).

CONCLUSIONS

Hypokalemia was a delayed complication of DKA treatment in the PCCU, associated with high K(+) and Na(+) excretion rates and a prolonged infusion of high doses of insulin.

Functional interaction of COMMD3 and COMMD9 with the epithelial sodium channel

The epithelial sodium channel (ENaC) plays an important role in controlling Na⁺ homeostasis, extracellular fluid volume, and blood pressure. Copper metabolism Murr1 domain-containing protein 1 (COMMD1) interacts with ENaC and downregulates ENaC. COMMD1 belongs to the COMMD family consisting of COMMD1-10, and all COMMD family members share a C-terminal COMM domain. Here, we report that COMMD2-10 also interacts with ENaC, and COMMD3 and COMMD9 were selected for further study. Amiloride-sensitive current in mammalian epithelia expressing ENaC was significantly reduced by COMMD3 or COMMD9, and ENaC expression at the cell surface was significantly decreased in the presence of COMMD3 or COMMD9. COMMD3 and COMMD9 retained their ability to reduce current when COMMD1 was knocked down. COMMD3 and COMMD9 were widely expressed in kidney and were colocalized with ENaC in renal collecting duct cells. These data suggest that COMMD3 and COMMD9 may be endogenous regulators of ENaC to regulate Na⁺ transport through altering ENaC cell surface expression.

Regulation of ENaC in mice lacking renal insulin receptors in the collecting duct

The epithelial sodium channel (ENaC) is one of the central effectors involved in regulation of salt and water homeostasis in the kidney. To study mechanisms of ENaC regulation, we generated knockout mice lacking the insulin receptor (InsR KO) specifically in the collecting duct principal cells. Single-channel analysis in freshly isolated split-open tubules demonstrated that the InsR-KO mice have significantly lower ENaC activity compared to their wild-type (C57BL/6J) littermates when animals were fed either normal or sodium-deficient diets. Immunohistochemical and Western blot assays demonstrated no significant changes in expression of ENaC subunits in InsR-KO mice compared to wild-type littermates. Insulin treatment caused greater ENaC activity in split-open tubules isolated from wild-type mice but did not have this effect in the InsR-KO mice. Thus, these results suggest that insulin increases ENaC activity via its own receptor affecting the channel open probability. To further determine the mechanism of the action of insulin on ENaC, we used mouse mpkCCDc14 principal cells. Insulin significantly augmented amiloride-sensitive transepithelial flux in these cells. Pretreatment of the mpkCCDc14 cells with phosphatidylinositol 3-kinase (LY294002; 10 μM) or mTOR (PP242; 100 nM) inhibitors precluded this effect. This study provides new information about the importance of insulin receptors expressed in collecting duct principal cells for ENaC activity.

Insulin signalling to the kidney in health and disease

Ninety-one years ago insulin was discovered, which was one of the most important medical discoveries in the past century, transforming the lives of millions of diabetic patients. Initially insulin was considered only important for rapid control of blood glucose by its action on a restricted number of tissues; however, it has now become clear that this hormone controls an array of cellular processes in many different tissues. The present review will focus on the role of insulin in the kidney in health and disease.

Ethanol stimulates epithelial sodium channels by elevating reactive oxygen species

Alcohol affects total body sodium balance, but the molecular mechanism of its effect remains unclear. We used single-channel methods to examine how ethanol affects epithelial sodium channels (ENaC) in A6 distal nephron cells. The data showed that ethanol significantly increased both ENaC open probability (P(o)) and the number of active ENaC in patches (N). 1-Propanol and 1-butanol also increased ENaC activity, but iso-alcohols did not. The effects of ethanol were mimicked by acetaldehyde, the first metabolic product of ethanol, but not by acetone, the metabolic product of 2-propanol. Besides increasing open probability and apparent density of active channels, confocal microscopy and surface biotinylation showed that ethanol significantly increased α-ENaC protein in the apical membrane. The effects of ethanol on ENaC P(o) and N were abolished by a superoxide scavenger, 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPOL) and blocked by the phosphatidylinositol 3-kinase inhibitor LY294002. Consistent with an effect of ethanol-induced reactive oxygen species (ROS) on ENaC, primary alcohols and acetaldehyde elevated intracellular ROS, but secondary alcohols did not. Taken together with our previous finding that ROS stimulate ENaC, the current results suggest that ethanol stimulates ENaC by elevating intracellular ROS probably via its metabolic product acetaldehyde.

Sodium-retaining effect of insulin in diabetes

Insulin has long been hypothesized to cause sodium retention, potentially of enough magnitude to contribute to hypertension in obesity, metabolic syndrome, and Type II diabetes. There is an abundance of supportive evidence from correlational analyses in humans, acute insulin infusion studies in humans and animals, and chronic insulin infusion studies in rats. However, the absence of hypertension in human insulinoma patients, and negative results for sodium-retaining or blood pressure effects of chronic insulin infusion in a whole series of dog studies, strongly refute the insulin hypothesis. We recently questioned whether the euglycemic, hyperinsulinemia model used for most insulin infusion studies, including the previous chronic dog studies, was the most appropriate model to test the renal actions of insulin in obesity, metabolic syndrome, and Type II diabetes. In those circumstances, hyperinsulinemia coexists with hyperglycemia. Therefore, we tested the sodium-retaining effect of insulin in chronically instrumented, alloxan-treated diabetic dogs. We used 24 h/day intravenous insulin infusion to regulate plasma insulin concentration. Induction of diabetes (∼400 mg/dl) caused sustained natriuresis and diuresis. However, if we clamped insulin at baseline, control levels, i.e., prevented it from decreasing, then the sustained natriuresis and diuresis were completely reversed, despite the same level of hyperglycemia. We also found that 24 h/day intrarenal insulin infusion had the same effect in diabetic dogs but had no sodium-retaining action in normal dogs. This new evidence that insulin has a sodium-retaining effect during hyperglycemia may have implications for maintaining sodium balance in uncontrolled Type II diabetes.

Regulation of ion channels by the serum- and glucocorticoid-inducible kinase SGK1

The ubiquitously expressed serum- and glucocorticoid-inducible kinase-1 (SGK1) is genomically regulated by cell stress (including cell shrinkage) and several hormones (including gluco- and mineralocorticoids). SGK1 is activated by insulin and growth factors through PI3K and 3-phosphoinositide-dependent kinase PDK1. SGK1 activates a wide variety of ion channels (e.g., ENaC, SCN5A, TRPV4-6, ROMK, Kv1.3, Kv1.5, Kv4.3, KCNE1/KCNQ1, KCNQ4, ASIC1, GluR6, ClCKa/barttin, ClC2, CFTR, and Orai/STIM), which participate in the regulation of transport, hormone release, neuroexcitability, inflammation, cell proliferation, and apoptosis. SGK1-sensitive ion channels participate in the regulation of renal Na(+) retention and K(+) elimination, blood pressure, gastric acid secretion, cardiac action potential, hemostasis, and neuroexcitability. A common (∼3-5% prevalence in Caucasians and ∼10% in Africans) SGK1 gene variant is associated with increased blood pressure and body weight as well as increased prevalence of type II diabetes and stroke. SGK1 further contributes to the pathophysiology of allergy, peptic ulcer, fibrosing disease, ischemia, tumor growth, and neurodegeneration. The effect of SGK1 on channel activity is modest, and the channels do not require SGK1 for basic function. SGK1-dependent ion channel regulation may thus become pathophysiologically relevant primarily after excessive (pathological) expression. Therefore, SGK1 may be considered an attractive therapeutic target despite its broad range of functions.

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.

Functional mechanism of pingchuanning decoction on adjustment of Clara cell secretory protein in airway remodeling of asthmatic rats

OBJECTIVE

To study the functional mechanism of pingchuanning decoction in treatment of airway remodeling in asthmatic rats.

METHODS

Eighty healthy Wistar male rats were randomized into eight groups (n=10 rats each): Normal group, asthma model group, dexamethasone group, guilong kechuanning group, xiaoqinglong decoction group, and pingchuanning decoction low-, middle-, and high-dose groups. The rats of all but the normal group were made into asthma models through intraperitoneal injection and aerosol inhalation of ovalbumin. All treatments were administered at the first stimulation of asthma onset (third week of modeling), and the rats were killed after stimulating asthma attacks for 4 weeks. The general conditions of rats and pathomorphological changes of the lung tissues were observed. The expression of nerve growth factor (NGF) of the lung tissues was measured with immunohistochemical methods, and the content of clara cell secretory protein (CCSP) mRNA was determined with RT-PCR.

RESULTS

Compared with the normal group, the contents of NGF and CCSP mRNA in the lung tissues of the model group were significantly changed (P < 0.01). Compared with the model group, the indices of pingchuanning decoction and other treatment groups were improved to some extent (P < 0.05 or P < 0.01).

CONCLUSIONS

Pathological changes of airway inflammation and remodeling were present in these rat asthma models. Pingchuanning decoction had an intervention effect on these experimental models. Its functional mechanism may be related to multiple factors, including alleviation of airway inflammation, relief of bronchial smooth muscle spasm, and inhibition of airway remodeling.

Scaffold protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3) coordinates assembly of a multiprotein epithelial sodium channel (ENaC)-regulatory complex

Hormone regulation of ion transport in the kidney tubules is essential for fluid and electrolyte homeostasis in vertebrates. A large body of evidence has suggested that transporters and channels exist in multiprotein regulatory complexes; however, relatively little is known about the composition of these complexes or their assembly. The epithelial sodium channel (ENaC) in particular is tightly regulated by the salt-regulatory hormone aldosterone, which acts at least in part by increasing expression of the serine-threonine kinase SGK1. Here we show that aldosterone induces the formation of a 1.0-1.2-MDa plasma membrane complex, which includes ENaC, SGK1, and the ENaC inhibitor Nedd4-2, a key target of SGK1. We further show that this complex contains the PDZ domain-containing protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3). CNK3 physically interacts with ENaC, Nedd4-2, and SGK1; enhances the interactions among them; and stimulates ENaC function in a PDZ domain-dependent, aldosterone-induced manner. These results strongly suggest that CNK3 is a molecular scaffold, which coordinates the assembly of a multiprotein ENaC-regulatory complex and hence plays a central role in Na(+) homeostasis.

Salt sensitivity of nitric oxide generation and blood pressure in mice with targeted knockout of the insulin receptor from the renal tubule

To elucidate the role of the insulin receptor (IR) on kidney nitric oxide generation and blood pressure (BP) control, we generated mice with targeted deletion of renal tubule IR using loxP recombination driven by a Ksp-cadherin promoter. Male knockout (KO) and wild-type (WT) littermates (~4 mo old) were transitioned through three 1-wk treatments: 1) low-NaCl diet (0.085%); 2) high-NaCl diet (HS; 5%); and 3) HS diet plus 3 mM tempol, a superoxide dismutase mimetic, in the drinking water. Mice were then switched to medium-NaCl (0.5%) diet for 5 days and kidneys harvested under pentobarbital anesthesia. Twenty-four-hour urinary nitrates plus nitrites were significantly higher in the WT mice under HS (2,067 ± 280 vs. 1,550 ± 230 nmol/day in WT and KO, respectively, P < 0.05). Tempol attenuated genotype differences in urinary nitrates plus nitrites. A rise in BP with HS was observed only in KO mice and not affected by tempol (mean arterial pressure, dark period, HS, 106 ± 5 vs. 119 ± 4 mmHg, for WT and KO, respectively, P < 0.05). Renal outer medullary protein levels of nitric oxide synthase (NOS) isoforms by Western blot (NOS1-3 and phosphorylated-S1177-NOS3) revealed significantly lower band density for NOS1 (130-kDa isoform) in the KO mice. A second study, when mice were euthanized under HS conditions, confirmed significantly lower NOS1 (130 kDa) in the KO, with an even more substantial (>50%) reduction of the 160-kDa NOS1 isoform. These studies suggest that the loss of renal IR signaling impairs renal nitric oxide production. This may be important in BP control, especially in insulin-resistant states, such as the metabolic syndrome.

Epithelial Na⁺ channel activity in human airway epithelial cells: the role of serum and glucocorticoid-inducible kinase 1

BACKGROUND AND PURPOSE

Glucocorticoids appear to control Na⁺ absorption in pulmonary epithelial cells via a mechanism dependent upon serum and glucocorticoid-inducible kinase 1 (SGK1), a kinase that allows control over the surface abundance of epithelial Na⁺ channel subunits (α-, β- and γ-ENaC). However, not all data support this model and the present study re-evaluates this hypothesis in order to clarify the mechanism that allows glucocorticoids to control ENaC activity.

EXPERIMENTAL APPROACH

Electrophysiological studies explored the effects of agents that suppress SGK1 activity upon glucocorticoid-induced ENaC activity in H441 human airway epithelial cells, whilst analyses of extracted proteins explored the associated changes to the activities of endogenous protein kinase substrates and the overall/surface expression of ENaC subunits.

KEY RESULTS

Although dexamethasone-induced (24 h) ENaC activity was dependent upon SGK1, prolonged exposure to this glucocorticoid did not cause sustained activation of this kinase and neither did it induce a coordinated increase in the surface abundance of α-, β- and γ-ENaC. Brief (3 h) exposure to dexamethasone, on the other hand, did not evoke Na⁺ current but did activate SGK1 and cause SGK1-dependent increases in the surface abundance of α-, β- and γ-ENaC.

CONCLUSIONS AND IMPLICATIONS

Although glucocorticoids activated SGK1 and increased the surface abundance of α-, β- and γ-ENaC, these responses were transient and could not account for the sustained activation of ENaC. The maintenance of ENaC activity did, however, depend upon SGK1 and this protein kinase must therefore play an important but permissive role in glucocorticoid-induced ENaC activation.

Organization of the ENaC-regulatory machinery

The control of fluid and electrolyte homeostasis in vertebrates requires the integration of a diverse set of signaling inputs, which control epithelial Na(+) transport, the principal ionic component of extracellular fluid. The key site of regulation is a segment of the kidney tubules, frequently termed the aldosterone-sensitive distal nephron, wherein the epithelial Na(+) channel (or ENaC) mediates apical ion entry. Na(+) transport in this segment is strongly regulated by the salt-retaining hormone, aldosterone, which acts through the mineralocorticoid receptor (MR) to influence the expression of a selected set of target genes, most notably the serine-threonine kinase SGK1, which phosphorylates and inhibits the E3 ubiquitin ligase Nedd4-2. It has long been known that ENaC activity is tightly regulated in vertebrate epithelia. Recent evidence suggests that SGK1 and Nedd4-2, along with other ENaC-regulatory proteins, physically associate with each other and with ENaC in a multi-protein complex. The various components of the complex are regulated by diverse signaling networks, including steroid receptor-, PI3-kinase-, mTOR-, and Raf-MEK-ERK-dependent pathways. In this review, we focus on the organization of the targets of these pathways by multi-domain scaffold proteins and lipid platforms into a unified complex, thereby providing a molecular basis for signal integration in the control of ENaC.

The role of the ENaC-regulatory complex in aldosterone-mediated sodium transport

The mineralocorticoid aldosterone is indispensable for the control of blood pressure and fluid volume in mammals. It acts in large part to increase the abundance and activity of the epithelial Na(+) channel (ENaC), which mediates apical Na(+) entry in the distal parts of the kidney tubules. Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1. Recent evidence suggests that these key aldosterone-regulated factors function within a unique multi-protein ENaC-regulatory-complex that governs the net cell surface expression and activity of the channel. Another aldosterone-induced protein, CNK3 (connector enhancer of kinase suppressor of Ras 3), also stimulates ENaC and has all of the features of a scaffolding protein. With these observations in mind, we discuss the possibility that CNK3 coordinates the dynamic assembly of the ENaC-regulatory-complex, and promotes context-appropriate aldosterone signal transduction in the regulation of epithelial Na(+) transport.

Insulin resistance: Is it time for primary prevention?

Insulin resistance is a clinical condition characterized by a decrease in sensitivity and responsiveness to the metabolic actions of insulin, so that a given concentration of insulin produces a less-than-expected biological effect. As a result, higher levels of insulin are needed to maintain normal glucose tolerance. Hyperinsulinemia, indeed, is one of the principal characteristics of insulin resistance states. This feature is common in several pathologic conditions, such as type 2 diabetes, obesity, and dyslipidemia, and it is also a prominent component of hypertension, coronary heart disease, and atherosclerosis. The presence of endothelial dysfunction, related to insulin resistance, plays a key role in the development and progression of atherosclerosis in all of these disorders. Insulin resistance represents the earliest detectable abnormality in type 2 diabetes, and is one of the major underlying mechanisms of hypertension and cardiovascular diseases. Its early detection could be of great importance, in order to set a therapeutic attack and to counteract the higher risk of diabetes and cardiovascular diseases.

Chronic intrarenal insulin replacement reverses diabetes mellitus-induced natriuresis and diuresis

We showed recently that sustained natriuresis in type 1 diabetic dogs was attributed to the decrease in insulin rather than the hyperglycemia alone. The sodium-retaining action of insulin appeared to require hyperglycemia, and it completely reversed the diabetic natriuresis and diuresis. This study tested whether the sodium-retaining effect was attributed to direct intrarenal actions of insulin. Alloxan-treated dogs (D; n=7) were maintained normoglycemic using 24-h/d IV insulin replacement. After control measurements, IV insulin was decreased to begin a 6-day diabetic period. Blood glucose increased from 84±6 mg/dL to an average of 428 mg/dL on days 5 and 6, sodium excretion increased from 74±8 to 98±7 meq/d over the 6 days, and urine volume increased from 1645±83 to 2198±170 mL/d. Dir dogs (n=7) were subjected to the same diabetic regimen, but, in addition, insulin was infused continuously into the renal artery at 0.3 mU/kg per minute during the 6-day period. This did not affect plasma insulin. Blood glucose increased from 94±10 mg/dL to an average of 380 mg/dL on days 5 and 6, but sodium excretion averaged 76±5 and 69±8 meq/d during control and diabetes mellitus, respectively. The diuresis also was prevented. Glomerular filtration rate increased only in Dir dogs, and there was no change in mean arterial pressure in either group. This intrarenal insulin infusion had no effect on sodium or volume excretion in normal dogs. Intrarenal insulin replacement in diabetic dogs caused a sustained increase in tubular reabsorption that completely reversed diabetic natriuresis. Insulin plus glucose may work to prevent salt wasting in uncontrolled type 2 diabetes mellitus.

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

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

EMD638683, a novel SGK inhibitor with antihypertensive potency

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase enhances renal tubular Na(+)-reabsorption and accounts for blood pressure increase following high salt diet in mice made hyperinsulinemic by dietary fructose or fat. The present study describes the in vitro and in vivo efficacy of a novel SGK1 inhibitor (EMD638683). EMD638683 was tested in vitro by determination of SGK1-dependent phosphorylation of NDRG1 (N-Myc downstream-regulated gene 1) in human cervical carcinoma HeLa-cells. In vivo EMD638683 (4460 ppm in chow, i.e. approx. 600 mg/kg/day) was administered to mice drinking tap water or isotonic saline containing 10% fructose. Blood pressure was determined by the tail cuff method, and urinary electrolyte (flame photometry) concentrations determined in metabolic cages. In vitro testing disclosed EMD638683 as a SGK1 inhibitor with an IC50 of 3 μM. Within 24 hours in vivo EMD638683 treatment significantly decreased blood pressure in fructose/saline-treated mice but not in control animals or in SGK1 knockout mice. EMD638683 failed to alter the blood pressure in SGK1 knockout mice. Following chronic (4 weeks) fructose/high salt treatment, additional EMD638683 treatment again decreased blood pressure. EMD638683 thus abrogates the salt sensitivity of blood pressure in hyperinsulinism without appreciably affecting blood pressure in the absence of hyperinsulinism. EMD638683 tended to increase fluid intake and urinary excretion of Na(+), significantly increased urinary flow rate and significantly decreased body weight.

CONCLUSION

EMD638683 could serve as a template for drugs counteracting hypertension in individuals with type II diabetes and metabolic syndrome.

mSIN1 protein mediates SGK1 protein interaction with mTORC2 protein complex and is required for selective activation of the epithelial sodium channel

The mammalian target of rapamycin (mTOR) plays a central role in the regulation of a number of cellular processes including growth, metabolism, and ion transport. mTOR is found in two multiprotein complexes, mTORC1 and mTORC2, which phosphorylate distinct substrates and regulate distinct cellular processes. SGK1 is an mTORC2 substrate, which is a key regulator of epithelial Na(+) transport mediated by the epithelial sodium channel. Although it is known that SGK1 physically interacts with mTORC2, it is unknown which mTORC2 component mediates this interaction or whether this interaction plays a physiologically relevant role in specific activation of SGK1. Here we identify mSIN1 as the mTORC2 component that mediates interaction with SGK1 and demonstrate that this interaction is required for SGK1 phosphorylation and epithelial sodium channel activation. We used the yeast two-hybrid system coupled with random mutagenesis to identify a mutant mSIN1 (mSIN1/Q68H), which does not interact with SGK1. Expression of this mutant does not restore SGK1 phosphorylation to wild-type levels in mSIN1-deficient murine embryo fibroblasts. Furthermore, in kidney epithelial cells, mSIN1/Q68H has a dominant-negative effect on SGK1 phosphorylation and on SGK1-dependent Na(+) transport. Interestingly, this interaction appears to be specific in that another mTORC2 substrate, Akt, does not interact with mSIN1, and its phosphorylation and activity are unaffected by the Q68H mutation. These data support the conclusion that mTORC2 uses distinct strategies to phosphorylate different substrates and suggest a mechanism for mTORC2 specificity in the regulation of diverse cellular processes.

Dysregulation of epithelial Na+ absorption induced by inhibition of the kinases TORC1 and TORC2

BACKGROUND AND PURPOSE

Although the serum and glucocorticoid-inducible protein kinase 1 (SGK1) appears to be involved in controlling epithelial Na(+) absorption, its role in this physiologically important ion transport process is undefined. As SGK1 activity is dependent upon target of rapamycin complex 2 (TORC2)-catalysed phosphorylation of SGK1-Ser(422) , we have explored the effects of inhibiting TORC2 and/or TORC1 upon the hormonal control of Na(+) absorption.

EXPERIMENTAL APPROACH

Na(+) absorption was quantified electrometrically in mouse cortical collecting duct cells (mpkCCD) grown to confluence on permeable membranes. Kinase activities were assessed by monitoring endogenous protein phosphorylation, with or without TORC1/2 inhibitors (TORIN1 and PP242) and the TORC1 inhibitor: rapamycin.

KEY RESULTS

Inhibition of TORC1/2 (TORIN1, PP242) suppressed basal SGK1 activity, prevented insulin- and dexamethasone-induced SGK1 activation, and caused modest (10-20%) inhibition of basal Na(+) absorption and substantial (∼80%) inhibition of insulin/dexamethasone-induced Na(+) transport. Inhibition of TORC1 did not impair SGK1 activation or insulin-induced Na(+) transport, but did inhibit (∼80%) dexamethasone-induced Na(+) absorption. Arginine vasopressin stimulated Na(+) absorption via a TORC1/2-independent mechanism.

CONCLUSION AND IMPLICATIONS

Target of rapamycin complex 2, but not TORC1, is important to SGK1 activation. Signalling via phosphoinositide-3-kinase/TORC2/SGK1 can explain insulin-induced Na(+) absorption. TORC2, but not TORC1, is also involved in glucocorticoid-induced SGK1 activation but its role is permissive. Glucocorticoid-induced Na(+) transport displayed a requirement for TORC1 activity. Therefore, TORC1 and TORC2 contribute to the regulation of Na(+) absorption. Pharmacological manipulation of TORC1/2 signalling may provide novel therapies for Na(+)-sensitive hypertension.

Effects of nominally selective inhibitors of the kinases PI3K, SGK1 and PKB on the insulin-dependent control of epithelial Na+ absorption

BACKGROUND AND PURPOSE

Insulin-induced Na(+) retention in the distal nephron may contribute to the development of oedema/hypertension in patients with type 2 diabetes. This response to insulin is usually attributed to phosphatidylinositol-3-kinase (PI3K)/serum and glucocorticoid-inducible kinase 1 (SGK1) but a role for protein kinase B (PKB) has been proposed. The present study therefore aimed to clarify the way in which insulin can evoke Na(+) retention.

EXPERIMENTAL APPROACH

We examined the effects of nominally selective inhibitors of PI3K (wortmannin, PI103, GDC-0941), SGK1 (GSK650394A) and PKB (Akti-1/2) on Na(+) transport in hormone-deprived and insulin-stimulated cortical collecting duct (mpkCCD) cells, while PI3K, SGK1 and PKB activities were assayed by monitoring the phosphorylation of endogenous proteins.

KEY RESULTS

Wortmannin substantially inhibited basal Na(+) transport whereas PI103 and GDC-0941 had only very small effects. However, these PI3K inhibitors all abolished insulin-induced Na(+) absorption and inactivated PI3K, SGK1 and PKB fully. GSK650394A and Akti-1/2 also inhibited insulin-evoked Na(+) absorption and while GSK650394A inhibited SGK1 without affecting PKB, Akti-1/2 inactivated both kinases.

CONCLUSION AND IMPLICATIONS

While studies undertaken using PI103 and GDC-0941 show that hormone-deprived cells can absorb Na(+) independently of PI3K, PI3K seems to be essential for insulin induced Na(+) transport. Akti-1/2 does not act as a selective inhibitor of PKB and data obtained using this compound must therefore be treated with caution. GSK650394A, on the other hand, selectively inhibits SGK1 and the finding that GSK650394A suppressed insulin-induced Na(+) absorption suggests that this response is dependent upon signalling via PI3K/SGK1.

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.