Real-time three-dimensional imaging of lipid signal transduction: apical membrane insertion of epithelial Na(+) channels

Two adjacent phosphorylation sites in the C-terminus of the channel's α-subunit have opposing effects on epithelial sodium channel (ENaC) activity

How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.

Interactive Actions of Aldosterone and Insulin on Epithelial Na+ Channel Trafficking

Epithelial Na⁺ channel (ENaC) participates in renal epithelial Na⁺ reabsorption, controlling blood pressure. Aldosterone and insulin elevate blood pressure by increasing the ENaC-mediated Na⁺ reabsorption. However, little information is available on the interactive action of aldosterone and insulin on the ENaC-mediated Na⁺ reabsorption. In the present study, we tried to clarify if insulin would modify the aldosterone action on the ENaC-mediated Na⁺ reabsorption from a viewpoint of intracellular ENaC trafficking. We measured the ENaC-mediated Na⁺ transport as short-circuit currents using a four-state mathematical ENaC trafficking model in renal A6 epithelial cells with or without aldosterone treatment under the insulin-stimulated and -unstimulated conditions. We found that: (A) under the insulin-stimulated condition, aldosterone treatment (1 µM for 20 h) significantly elevated the ENaC insertion rate to the apical membrane (kI) 3.3-fold and the ENaC recycling rate (kR) 2.0-fold, but diminished the ENaC degradation rate (kD) 0.7-fold without any significant effect on the ENaC endocytotic rate (kE); (B) under the insulin-unstimulated condition, aldosterone treatment decreased kE 0.5-fold and increased kR 1.4-fold, without any significant effect on kI or kD. Thus, the present study indicates that: (1) insulin masks the well-known inhibitory action of aldosterone on the ENaC endocytotic rate; (2) insulin induces a stimulatory action of aldosterone on ENaC apical insertion and an inhibitory action of aldosterone on ENaC degradation; (3) insulin enhances the aldosterone action on ENaC recycling; (4) insulin has a more effective action on diminution of ENaC endocytosis than aldosterone.

Regulation of actin-based apical structures on epithelial cells

Cells of transporting epithelia are characterized by the presence of abundant F-actin-based microvilli on their apical surfaces. Likewise, auditory hair cells have highly reproducible rows of apical stereocilia (giant microvilli) that convert mechanical sound into an electrical signal. Analysis of mutations in deaf patients has highlighted the critical components of tip links between stereocilia, and related structures that contribute to the organization of microvilli on epithelial cells have been found. Ezrin/radixin/moesin (ERM) proteins, which are activated by phosphorylation, provide a critical link between the plasma membrane and underlying actin cytoskeleton in surface structures. Here, we outline recent insights into how microvilli and stereocilia are built, and the roles of tip links. Furthermore, we highlight how ezrin is locally regulated by phosphorylation, and that this is necessary to maintain polarity. Localized phosphorylation is achieved through an intricate coincidence detection mechanism that requires the membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and the apically localized ezrin kinase, lymphocyte-oriented kinase (LOK, also known as STK10) or Ste20-like kinase (SLK). We also discuss how ezrin-binding scaffolding proteins regulate microvilli and how, despite these significant advances, it remains to be discovered how the cell polarity program ultimately interfaces with these processes.

Hydrogen sulfide prevents hydrogen peroxide-induced activation of epithelial sodium channel through a PTEN/PI(3,4,5)P3 dependent pathway

Sodium reabsorption through the epithelial sodium channel (ENaC) at the distal segment of the kidney plays an important role in salt-sensitive hypertension. We reported previously that hydrogen peroxide (H₂O₂) stimulates ENaC in A6 distal nephron cells via elevation of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) in the apical membrane. Here we report that H₂S can antagonize H₂O₂-induced activation of ENaC in A6 cells. Our cell-attached patch-clamp data show that ENaC open probability (P_O) was significantly increased by exogenous H₂O₂, which is consistent with our previous finding. The aberrant activation of ENaC induced by exogenous H₂O₂ was completely abolished by H₂S (0.1 mM NaHS). Pre-treatment of A6 cells with H₂S slightly decreased ENaC P_O; however, in these cells H₂O₂ failed to elevate ENaC P_O. Confocal microscopy data show that application of exogenous H₂O₂ to A6 cells significantly increased intracellular reactive oxygen species (ROS) level and induced accumulation of PI(3,4,5)P₃ in the apical compartment of the cell membrane. These effects of exogenous H₂O₂ on intracellular ROS levels and on apical PI(3,4,5)P₃ levels were almost completely abolished by treatment of A6 cells with H₂S. In addition, H₂S significantly inhibited H₂O₂-induced oxidative inactivation of the tumor suppressor phosphatase and tensin homolog (PTEN) which is a negative regulator of PI(3,4,5)P_3. Moreover, BPV_(pic), a specific inhibitor of PTEN, elevated PI(3,4,5)P₃ and ENaC activity in a manner similar to that of H₂O₂ in A6 cells. Our data show, for the first time, that H₂S prevents H₂O₂-induced activation of ENaC through a PTEN-PI(3,4,5)P₃ dependent pathway.

ROS production as a common mechanism of ENaC regulation by EGF, insulin, and IGF-1

The epithelial Na(+) channel (ENaC) is a key transporter participating in the fine tuning of Na(+) reabsorption in the nephron. ENaC activity is acutely upregulated by epidermal growth factor (EGF), insulin, and insulin-like growth factor-1 (IGF-1). It was also proposed that reactive oxygen species (ROS) have a stimulatory effect on ENaC. Here we studied whether effects of EGF, insulin, and IGF-1 correlate with ROS production in the mouse cortical collecting duct (mpkCCD(c14)) cells. Western blotting confirmed the expression of the NADPH oxidase complex subunits in these cells. Treatment of mpkCCD(c14) cells with EGF, insulin, or IGF-1 evoked an increase in ROS production as measured by CM-H(2)DCF-DA fluorescence. ROS production caused by a xanthine-xanthine oxidase reaction also resulted in a significant elevation in short-circuit current through the mpkCCD(c14) monolayer. Transepithelial current measurements showed an acute increase of amiloride-sensitive current through the mpkCCD(c14) monolayer in response to EGF, insulin, or IGF-1. Pretreatment with the nonselective NADPH oxidase activity inhibitor apocynin blunted both ROS production and increase in ENaC-mediated current in response to these drugs. To further test whether NADPH oxidase subunits are involved in the effect of EGF, we used a stable M-1 cell line with a knockdown of Rac1, which is one of the key subunits of the NADPH oxidase complex, and measured amiloride-sensitive currents in response to EGF. In contrast to control cells, EGF had no effect in Rac1 knockdown cells. We hypothesize that EGF, insulin, and IGF-1 have a common stimulatory effect on ENaC mediated by ROS production.

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.

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.

Hydrogen peroxide stimulates the epithelial sodium channel through a phosphatidylinositide 3-kinase-dependent pathway

Recent studies indicate that oxidative stress mediates salt-sensitive hypertension. To test the hypothesis that the renal epithelial sodium channel (ENaC) is a target of oxidative stress, patch clamp techniques were used to determine whether ENaC in A6 distal nephron cells is regulated by hydrogen peroxide (H(2)O(2)). In the cell-attached configuration, H(2)O(2) significantly increased ENaC open probability (P(o)) and single-channel current amplitude but not the unit conductance. High concentrations of exogenous H(2)O(2) are required to elevate intracellular H(2)O(2), probably because catalase, the enzyme that promotes the decomposition of H(2)O(2) to H(2)O and O(2), is highly expressed in A6 cells. The effect of H(2)O(2) on ENaC P(o) was enhanced by 3-aminotriazole, a catalase inhibitor, and abolished by overexpression of catalase, indicating that intracellular H(2)O(2) levels are critical to produce the effect. However, H(2)O(2) did not directly activate ENaC in inside-out patches. The effects of H(2)O(2) on ENaC P(o) and amiloride-sensitive Na(+) current were abolished by inhibition of phosphatidylinositide 3-kinase (PI3K). Confocal microscopy data showed that H(2)O(2) elevated phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) in the apical membrane by stimulating PI3K. Because ENaC is stimulated by PI(3,4,5)P(3), these data suggest that H(2)O(2) stimulates ENaC via PI3K-mediated increases in apical PI(3,4,5)P(3).

Protein kinase B alpha (PKBα) stimulates the epithelial sodium channel (ENaC) heterologously expressed in Xenopus laevis oocytes by two distinct mechanisms

Kinases contribute to the regulation of the epithelial sodium channel (ENaC) in a complex manner. For example, SGK1 (serum- and glucocorticoid-inducible kinase type 1) enhances ENaC surface expression by phosphorylating Nedd4-2, thereby preventing ENaC retrieval and degradation. An additional mechanism of ENaC activation by SGK1 involves an SGK consensus motif ((616)RSRYWS(621)) in the C-terminus of the channel's α-subunit. This consensus motif may also be a target for ENaC regulation by protein kinase B α (PKBα) known to be activated by insulin and growth factors. Therefore, we investigated a possible role of PKBα in the regulation of rat ENaC heterologously expressed in Xenopus laevis oocytes. We found that recombinant PKBα included in the pipette solution increased ENaC currents in outside-out patches by about 4-fold within 15-20 min. Replacing the serine residue S621 of the SGK consensus motif by an alanine (S621A) abolished this stimulatory effect. In co-expression experiments active PKBα but not catalytically inactive PKBα significantly increased ENaC whole-cell currents and surface expression by more than 50 % within 24 hours of co-expression. Interestingly, this stimulatory effect was preserved in oocytes expressing ENaC with the S621A mutation. We conclude that the acute stimulatory effect of PKBα involves a specific kinase consensus motif in the C-terminus of the channel's α-subunit. In contrast, the increase in channel surface expression caused by co-expression of PKBα does not depend on this site in the channel and is probably mediated by an effect on channel trafficking.

Insulin activates epithelial sodium channel (ENaC) via phosphoinositide 3-kinase in mammalian taste receptor cells

Diabetes is a profound disease that results in a severe lack of regulation of systemic salt and water balance. From our earlier work on the endocrine regulation of salt taste at the level of the epithelial sodium channel (ENaC), we have begun to investigate the ability of insulin to alter ENaC function with patch-clamp recording on isolated mouse taste receptor cells (TRCs). In fungiform and vallate TRCs that exhibit functional ENaC currents (e.g., amiloride-sensitive Na(+) influx), insulin (5-20 nM) caused a significant increase in Na(+) influx at -80 mV (EC(50) = 7.53 nM). The insulin-enhanced currents were inhibited by amiloride (30 μM). Similarly, in ratiometric Na(+) imaging using SBFI, insulin treatment (20 nM) enhanced Na(+) movement in TRCs, consistent with its action in electrophysiological assays. The ability of insulin to regulate ENaC function is dependent on the enzyme phosphoinositide 3-kinase since treatment with the inhibitor LY294002 (10 μM) abolished insulin-induced changes in ENaC. To test the role of insulin in the regulation of salt taste, we have characterized behavioral responses to NaCl using a mouse model of acute hyperinsulinemia. Insulin-treated mice show significant avoidance of NaCl at lower concentrations than the control group. Interestingly, these differences between groups were abolished when amiloride (100 μM) was added into NaCl solutions, suggesting that insulin was regulating ENaC. Our results are consistent with a role for insulin in maintaining functional expression of ENaC in mouse TRCs.

Inhibition of insulin-stimulated hydrogen peroxide production prevents stimulation of sodium transport in A6 cell monolayers

Insulin-stimulated sodium transport across A6 cell (derived from amphibian distal nephron) monolayers involves the activation of a phosphatidylinositol (PI) 3-kinase. We previously demonstrated that exogenous addition of H2O2 to the incubation medium of A6 cell monolayers provokes an increase in PI 3-kinase activity and a subsequent rise in sodium transport (Markadieu N, Crutzen R, Blero D, Erneux C, Beauwens R. Am J Physiol Renal Physiol 288: F1201-F1212, 2005). We therefore questioned whether insulin would produce an intracellular burst of H2O2 leading to PI 3-kinase activation and subsequent increase in sodium transport. An acute production of reactive oxygen species (ROS) in A6 cells incubated with the oxidation-sensitive fluorescent probe 5,6-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate was already detected after 2 min of insulin stimulation. This fluorescent signal and the increase in sodium transport were completely inhibited in monolayers incubated with peggylated catalase, indicating that H2O2 is the main intracellular ROS produced upon insulin stimulation. Similarly, preincubation of monolayers with different chelators of either superoxide (O2(*-); nitro blue tetrazolium, 100 microM) or H2O2 (50 microM ebselen), or blockers of NADPH oxidase (Nox) enzymes (diphenyleneiodonium, 5 microM; phenylarsine oxide, 1 microM and plumbagin, 30 microM) prevented both insulin-stimulated H2O2 production and insulin-stimulated sodium transport. Furthermore, diphenyleneiodonium pretreatment inhibited the recruitment of the p85 PI 3-kinase regulatory subunit in an anti-phosphotyrosine immunoprecipitate in insulin-stimulated cells. In contrast, PI-103, an inhibitor of class IA PI 3-kinase, inhibited insulin-stimulated sodium transport but did not significantly reduce insulin-stimulated H2O2 production. Taken together, our data suggest that insulin induces an acute burst of H2O2production which participates in an increase in phosphatidylinositol 3,4,5-trisphosphate production and subsequently stimulation of sodium transport.

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

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

SGK1-sensitive renal tubular glucose reabsorption in diabetes

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

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.

Steroid hormone release as well as renal water and electrolyte excretion of mice expressing PKB/SGK-resistant GSK3

Insulin and insulin-like growth factor (IGF1) participate in the regulation of renal electrolyte excretion. Insulin- and IGF1-dependent signaling includes phosphatidylinositide-3 (PI3)-kinase, phosphoinositide-dependent kinase PDK1 as well as protein kinase B (PKB) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit glycogen synthase kinase GSK3alpha,beta. Replacement of the serines in the PKB/SGK consensus sequences by alanine (gsk3 ( KI )) confers resistance of GSK3 to PKB/SGK. To explore the role of PKB/SGK-dependent inhibition of GSK3 in the regulation of water/electrolyte metabolism, mice carrying the PKB/SGK resistant mutant (gsk3 ( KI )) were compared to their wild-type littermates (gsk3 ( WT ) ). Body weight was similar in gsk3 ( KI ) and gsk3 ( WT ) mice. Plasma aldosterone at 10 A.M: . and corticosterone concentrations at 5 P.M: . were significantly lower, but 24-h urinary aldosterone was significantly higher, and corticosterone excretion tended to be higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. Food and water intake, fecal excretion, glomerular filtration rate, urinary flow rate, urine osmolarity, as well as urinary Na+, K+, urea excretion were significantly larger, and plasma Na+, urea, but not K+ concentration, were significantly lower in gsk3 ( KI ) than in gsk3 ( WT ) mice. Body temperature was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. When allowed to choose between tap water and saline, gsk3 ( WT ) mice drank more saline, whereas gsk3 ( KI ) mice drank similar large volumes of tap water and saline. During high-salt diet, urinary vasopressin excretion increased to significantly higher levels in gsk3 ( KI ) than in gsk3 ( WT ) mice. After water deprivation, body weight decreased faster in gsk3 ( KI ) than in gsk3 ( WT ) mice. Blood pressure, however, was significantly higher in gsk3 ( KI ) than in gsk3 ( WT ) mice. The observations disclose a role of PKB/SGK-dependent GSK3 activity in the regulation of steroid hormone release, renal water and electrolyte excretion and blood pressure control.

Insulin internalizes GLUT2 in the enterocytes of healthy but not insulin-resistant mice

OBJECTIVES

A physiological adaptation to a sugar-rich meal is achieved by increased sugar uptake to match dietary load, resulting from a rapid transient translocation of the fructose/glucose GLUT2 transporter to the brush border membrane (BBM) of enterocytes. The aim of this study was to define the contributors and physiological mechanisms controlling intestinal sugar absorption, focusing on the action of insulin and the contribution of GLUT2-mediated transport.

RESEARCH DESIGN AND METHODS

The studies were performed in the human enterocytic colon carcinoma TC7 subclone (Caco-2/TC7) cells and in vivo during hyperinsulinemic-euglycemic clamp experiments in conscious mice. Chronic high-fructose or high-fat diets were used to induce glucose intolerance and insulin resistance in mice.

RESULTS AND CONCLUSIONS

In Caco-2/TC7 cells, insulin action diminished the transepithelial transfer of sugar and reduced BBM and basolateral membrane (BLM) GLUT2 levels, demonstrating that insulin can target sugar absorption by controlling the membrane localization of GLUT2 in enterocytes. Similarly, in hyperinsulinemic-euglycemic clamp experiments in sensitive mice, insulin abolished GLUT2 (i.e., the cytochalasin B-sensitive component of fructose absorption), decreased BBM GLUT2, and concomitantly increased intracellular GLUT2. Acute insulin treatment before sugar intake prevented the insertion of GLUT2 into the BBM. Insulin resistance in mice provoked a loss of GLUT2 trafficking, and GLUT2 levels remained permanently high in the BBM and low in the BLM. We propose that, in addition to its peripheral effects, insulin inhibits intestinal sugar absorption to prevent excessive blood glucose excursion after a sugar meal. This protective mechanism is lost in the insulin-resistant state induced by high-fat or high-fructose feeding.

Regulation of cell polarity during epithelial morphogenesis

Epithelial cells have an apical surface facing a lumen or outside of the organism, and a basolateral surface facing other cells and extracellular matrix. The identity of the apical surface is determined by phosphatidylinositol 4,5-bisphosphate, while phosphatidylinositol 3,4,5-trisphophosphate determines the identity of the basolateral surface. The Par3/Par6/atypical protein kinase C complex, as well as the Crumbs and Scribble complexes, controls epithelial polarity. Par4 and AMP kinase regulate polarity during conditions of energy depletion. Lumens are formed in hollow cysts and tubules by fusions of apical vesicles, such as the vacuolar apical compartment, with the plasma membrane. The polarity of individual cells is oriented and coordinated with other cells by interactions with the extracellular matrix.

Lack of the serum and glucocorticoid-inducible kinase SGK1 attenuates the volume retention after treatment with the PPARgamma agonist pioglitazone

PPARgamma-agonists enhance insulin sensitivity and improve glucose utilization in diabetic patients. Adverse effects of PPARgamma-agonists include volume retention and edema formation. Recent observations pointed to the ability of PPARgamma agonists to enhance transcription of the serum and glucocorticoid-inducible kinase SGK1, a kinase that is genomically upregulated by mineralocorticoids and stimulates various renal channels and transporters including the renal epithelial Na+ channel ENaC. SGK1 has been proposed to mediate the volume retention after treatment with PPARgamma agonists. To test this hypothesis, food containing the PPARgamma agonist pioglitazone (0.02%, i.e., approximately 25 mg/kg bw/day) was administered to gene-targeted mice lacking SGK1 (sgk1-/-, n=12) and their wild-type littermates (sgk1+/+), n=12). According to in situ hybridization, quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence, treatment with pioglitazone significantly increased renal SGK1 mRNA and protein expression in sgk1+/+ mice. The treatment increased body weight significantly in both, sgk1+/+ mice (+2.2+/-0.3 g) and sgk-/- mice (+1.3+/-0.2 g), and decreased hematocrit significantly in sgk1+/+ mice (-6.5+/-1.0%) and sgk1-/- mice (-3.1+/-0.6%). Both effects were significantly (p<0.05) more pronounced in sgk1+/+ mice. According to Evans Blue distribution, pioglitazone increased plasma volume only in sgk1+/+ mice (from 50.9+/-3.9 to 63.7+/-2.5 microl/g bw) but not in sgk-/- mice (from 46.8+/-3.8 to 48.3+/-5.2 microl/g bw). Pioglitazone decreased aldosterone plasma levels and blood pressure and increased leptin plasma levels in both genotypes. We conclude that SGK1 contributes to but does not fully account for the volume retention during treatment with the PPARgamma agonist pioglitazone.

Chapter 5: Imaging in depth: controversies and opportunities

One enduring challenge of biological imaging is achieving depth of penetration-into cells, tissues, and animals. How deeply can we probe and with what resolution and efficacy? These are critical issues as microscopists seek to push ever deeper, while resolving structural details and observing specific molecular events. In this guide to depth-appropriate modalities, standard optical platforms such as confocal and two-photon microscopes are considered along with complementary imaging modalities that range in depth of penetration. After an introduction to basic techniques, the trade-offs and limitations that distinguish competing technologies are considered, with emphasis on the visualization of subcellular structures and dynamic events. Not surprisingly, there are differences of opinion regarding imaging technologies, as highlighted in a section on point-scanning and Nipkow-disk style confocal microscopes. Confocal microscopy is then contrasted with deconvolution and multi-photon imaging modalities. It is also important to consider the detectors used by current instruments (such as PMTs and CCD cameras). Ultimately specimen properties, in conjunction with instrumentation, determine the depth at which subcellular operations and larger-scale biological processes can be visualized. Relative advantages are mentioned in the context of experiment planning and instrument-purchase decisions. Given the rate at which new optical techniques are being invented, this report should be viewed as a snapshot of current capabilities, with the goal of providing a framework for thinking about new developments.

Intracellular calcium plays a role as the second messenger of hypotonic stress in gene regulation of SGK1 and ENaC in renal epithelial A6 cells

In A6 cells, a renal cell line derived from Xenopus laevis, hypotonic stress stimulates the amiloride-sensitive Na(+) transport. Hypotonic action on Na(+) transport consists of two phases, a nongenomic early phase and a genomic delayed phase. Although it has been reported that, during the genomic phase, hypotonic stress stimulates transcription of Na(+) transport-related genes, such as serum- and glucocorticoid-inducible kinase 1 (SGK1) and subunits of the epithelial Na(+) channel (ENaC), increasing Na(+) transport, the mechanism remains unknown. We focused the present study on the role of intracellular Ca(2+) in hypotonicity-induced SGK1 and ENaC subunit transcription. Since hypotonic stress raises intracellular Ca(2+) concentration in A6 cells, we hypothesized that Ca(2+)-dependent signals participate in the genomic action. Using real-time quantitative RT-PCR and Western blot techniques and measuring short-circuit currents, we observed that 1) BAPTA-AM and W7 blunted the hypotonicity-induced expression of SGK1 mRNA and protein, 2) ionomycin dose dependently stimulated expression of SGK1 mRNA and protein under an isotonic condition and the time course of the stimulatory effect of ionomycin on SGK1 mRNA was remarkably similar to that of hypotonic action on SGK1 mRNA, 3) hypotonic stress stimulated transcription of three ENaC subunits in an intracellular Ca(2+)-dependent manner, and 4) BAPTA-AM retarded the delayed phase of hypotonic stress-induced Na(+) transport but had no effect on the early phase. These observations indicate for the first time that intracellular Ca(2+) plays a role as the second messenger in hypotonic stress-induced Na(+) transport by stimulating transcription of SGK1 and ENaC subunits.

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Acute regulation of the epithelial Na+ channel by phosphatidylinositide 3-OH kinase signaling in native collecting duct principal cells

Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) reabsorption in the aldosterone-sensitive distal nephron. Hormones, including aldosterone and insulin, increase ENaC activity, in part by stimulating phosphatidylinositide 3-OH kinase (PI3-K) signaling. Recent studies in heterologous expression systems reveal a close spatiotemporal coupling between PI3-K signaling and ENaC activity with the phospholipid product of this kinase, PI(3,4,5)P(3), in some cases, directly binding the channel and increasing open probability (P(o)). This study tested whether this tight coupling plays a physiologic role in modulating ENaC activity in native tissue and polarized epithelial cells. IGF-I was found to increase Na(+) reabsorption across mpkCCD(c14) principal cell monolayers in a PI3-K-sensitive manner. Inhibition of PI3-K signaling, moreover, rapidly decreased Na(+) reabsorption and ENaC activity in mpkCCD(c14) cells that were treated with corticosteroids and IGF-I. These decreases paralleled changes in apical membrane PI(3,4,5)P(3) levels, demonstrating tight spatiotemporal coupling between ENaC activity and PI3-K/PI(3,4,5)P(3) signaling within this membrane. For further probing of the mechanism underpinning this coupling, cortical collecting ducts (CCD) were isolated from rat and split open to expose the apical membrane for patch-clamp analysis. Inhibition of PI3-K signaling with wortmannin and LY294002 but not its inactive analogue rapidly and markedly decreased the P(o) of ENaC. Moreover, IGF-I acutely increased P(o) of ENaC in CCD principal cells in a PI3-K-sensitive manner. Together, these observations stress the importance of tight spatiotemporal coupling between PI3-K signaling and ENaC within the apical membrane of principal cells to the physiologic control of this ion channel.

Trafficking of ENaC subunits in response to acute insulin in mouse kidney

Studies done in cell culture have demonstrated that insulin activates the epithelial sodium channel (ENaC) via a variety of mechanisms. However, to date, upregulation of ENaC in native renal tissue by in vivo administration of insulin has not been demonstrated. To address this, we injected 6-mo-old male C57BL/CBA mice (n = 14/group) intraperitoneally with vehicle or 0.5 U/kg body wt insulin and examined short-term (1-2 h) sodium excretion and kidney ENaC subunits (alpha, beta, and gamma) and serum and glucocorticoid-induced kinase (SGK-1) regulation. Insulin resulted in a significant reduction in urine sodium (by approximately 80%) that was restored by intraperitoneal administration of the ENaC antagonist, benzamil (1.4 mg/kg body wt). Differential centrifugation followed by Western blotting of whole kidney revealed significantly increased band densities (by 26-103%) for insulin- relative to vehicle-treated mice for alpha- and gamma-ENaC in the homogenate (H), and plasma membrane-enriched fraction (MF), with no difference in the vesicle-enriched fraction (VF). Similarly, beta-ENaC was significantly increased in MF (by 45%) but no change in the H. It was, however, significantly decreased in the VF (by 28%) with insulin. In agreement, immunoperoxidase labeling demonstrated relatively stronger apical, relative to cytosolic, localization of alpha-, beta-, and gamma-ENaC with insulin, whereas, with vehicle, labeling was fairly evenly dispersed throughout collecting duct principal cells. Furthermore, Western blotting showed insulin increased SGK-1 (by 75%) and phosphorylated-SGK band densities (by 30%) but only in the MF. These studies demonstrate novel in vivo regulation of renal ENaC activity and subunit proteins and SGK-1 by insulin in the acute time frame in the mouse.

Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na(+) from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na(+) channel (ENaC), Na(+)/K(+)-pump, and basolateral K(+) channels, which regulate Na(+) transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 microg/ml, PRL had no effect on the SCC. PRL (1 microg/ml) was sufficient to stimulate the SCC since 1 and 10 microg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 microg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na(+)/K(+)-pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers' solution on each side. PRL (10 microg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na(+)/K(+)-pump [although PRL (1 microg/ml) had no effect on this SCC]. The effect of PRL on basolateral K(+) channels was investigated using apically nystatin-permeabilized skin with high-K Ringer's solution on the apical side. PRL (10 microg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K(+) channels. Thus, although PRL stimulates the Na(+)/K(+)-pump, this effect probably contributes less than that on ENaC to the regulation of Na(+) transport across adult bullfrog skin.

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

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

Serum- and glucocorticoid-inducible kinase 1 mediates salt sensitivity of glucose tolerance

Excess salt intake decreases peripheral glucose uptake, thus impairing glucose tolerance. Stimulation of cellular glucose uptake involves phosphatidylinositide-3-kinase (PI-3K)-dependent activation of protein kinase B/Akt. A further kinase downstream of PI-3K is serum- and glucocorticoid-inducible kinase (SGK)1, which is upregulated by mineralocorticoids and, thus, downregulated by salt intake. To explore the role of SGK1 in salt-dependent glucose uptake, SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)) were allowed free access to either tap water (control) or 1% saline (high salt). According to Western blotting, high salt decreased and deoxycorticosterone acetate (DOCA; 35 mg/kg body wt) increased SGK1 protein abundance in skeletal muscle and fat tissue of sgk1(+/+) mice. Intraperitoneal injection of glucose (3 g/kg body wt) into sgk1(+/+) mice transiently increased plasma glucose concentration approaching significantly higher values ([glucose]p,max) in high salt (281 +/- 39 mg/dl) than in control (164 +/- 23 mg/dl) animals. DOCA did not significantly modify [glucose]p,max in control sgk1(+/+) mice but significantly decreased [glucose]p,max in high-salt sgk1(+/+) mice, an effect reversed by spironolactone (50 mg/kg body wt). [Glucose]p,max was in sgk1(-/-) mice insensitive to high salt and significantly higher than in control sgk1(+/+) mice. Uptake of 2-deoxy-d-[1,2-(3)H]glucose into skeletal muscle and fat tissue was significantly smaller in sgk1(-/-) mice than in sgk1(+/+) mice and decreased by high salt in sgk1(+/+) mice. Transfection of HEK-293 cells with active (S422D)SGK1, but not inactive (K127N)SGK, stimulated phloretin-sensitive glucose uptake. In conclusion, high salt decreases SGK1-dependent cellular glucose uptake. SGK1 thus participates in the link between salt intake and glucose tolerance.

Renal function of gene-targeted mice lacking both SGK1 and SGK3

Serum- and glucocorticoid-inducible kinase (SGK) 1 and SGK3 share the ability to upregulate several ion channels, including the epithelial Na(+) channel. Whereas SGK1 is under genomic control of mineralocorticoids and glucocorticoids, SGK3 is constitutively expressed. The SKG1-knockout (sgk1(-/-)) mouse is seemingly normal when it is fed a standard diet, but its ability to retain NaCl is impaired when it is fed a salt-deficient diet. In the SGK3-knockout (sgk3(-/-)) mouse fed standard and salt-deficient diets, hair growth is strikingly delayed but NaCl excretion is normal. Thus the possibility was considered that SGK1 and SGK3 could mutually replace each other, thus preventing severe NaCl loss in sgk1(-/-) and sgk3(-/-) mice. We crossed SGK1- and SGK3-knockout mice and compared renal electrolyte excretion of the double mutants (sgk1(-/-)/sgk3(-/-)) with that of their wild-type littermates (sgk1(+/+)/sgk3(+/+)). Similar to sgk3(-/-) mice, the sgk1(-/-)/sgk3(-/-) mice display delayed hair growth. Blood pressure was slightly, but significantly (P < 0.03), lower in sgk1(-/-)/sgk3(-/-) (102 +/- 4 mmHg) than in sgk1(+/+)/sgk3(+/+) (114 +/- 3 mmHg) mice, a difference that was maintained in mice fed low- and high-salt diets. Plasma aldosterone concentrations were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) than in sgk1(+/+)sgk3(+/+) mice fed control (511 +/- 143 vs. 143 +/- 32 pg/ml) and low-salt (1,325 +/- 199 vs. 362 +/- 145 pg/ml) diets. During salt depletion, absolute and fractional excretions of Na(+) were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) (1.2 +/- 0.2 micromol/24 h g body wt, 0.12 +/- 0.03%) than in sgk1(+/+)/sgk3(+/+) (0.4 +/- 0.1 micromol/24 h g body wt, 0.04 +/- 0.01%) mice. The sgk1(-/-)/sgk3(-/-) mice share the delayed hair growth with sgk3(-/-) mice and the modestly impaired renal salt retention with sgk1(-/-) mice. Additional lack of the isoform kinase does not substantially compound the phenotype for either property.

Regulation of the epithelial Na+ channel (ENaC) by phosphatidylinositides

The epithelial Na(+) channel (ENaC) is an end-effector of diverse cellular signaling cascades, including those with phosphatidylinositide second messengers. Recent evidence also suggests that in some instances, phospatidylinositides can directly interact with ENaC to increase channel activity by increasing channel open probability and/or membrane localization. We review here findings relevant to regulation of ENaC by phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-triphosphate (PIP(3)). Similar to its actions on other ion channels, PIP(2) is permissive for ENaC openings having a direct effect on gating. The PIP(2) binding site in ENaC involved in this regulation is most likely localized to the NH(2) terminus of beta-ENaC. PIP(3) also affects ENaC gating but, rather than being permissive, augments open probability. The PIP(3) binding site in ENaC involved in this regulation is localized to the proximal region of the COOH terminus of gamma-ENaC just following the second transmembrane domain. In complementary pathways, PIP(3) also impacts ENaC membrane levels through both direct actions on the channel and via a signaling cascade involving phosphoinositide 3-OH kinase (PI3-K) and the aldosterone-induced gene product serum and glucocorticoid-inducible kinase. The putative PIP(3) binding site in ENaC involved in direct regulation of channel membrane levels has not yet been identified.

Hormonal regulation of the epithelial Na+ channel: from amphibians to mammals

High-resistance epithelia derived from amphibian sources such as frog skin, toad urinary bladder, and the A6 Xenopus laevis kidney cell line have been widely used to elucidate the underlying mechanisms involved in the regulation of vectorial ion transport. More recently, the isolation of high-resistance mammalian cell lines has provided model systems in which to study differences and similarities between the regulation of ion transporter function in amphibian and mammalian renal epithelia. In the present study, we have compared the natriferic (Na+ retaining) responses to aldosterone, insulin, and vasotocin/vasopressin in the A6 and mpkCCDcl4 (mouse principal cells of the kidney cortical collecting duct) cell lines. The functional responses of the epithelial Na+ channel (ENaC) to hormonal stimulation were remarkably similar in both the amphibian and mammalian lines. In addition, insulin- and aldosterone-stimulated, reabsorptive Na+ transport in both cell lines requires the presence of functional PI3-kinase.

Modulation of basal and peptide hormone-stimulated Na transport by membrane cholesterol content in the A6 epithelial cell line

These studies examined the effect of altering plasma membrane cholesterol on basal Na+ flux as well as on the natriferic responses to the peptide hormones, insulin and anti-diuretic hormone (ADH) in the A6 model renal cell line. Membrane cholesterol concentrations were depleted or enriched using methyl-beta-cyclodextrin (MbetaCD) or a MbetaCD/cholesterol inclusion complex respectively. Effects of changes in the apical and basolateral plasma membranes were examined independently. Apical membrane cholesterol removal or supplementation had no effect on the basal Na+ transport rate. Short-term apical membrane cholesterol supplementation also had no effect on insulin-stimulated Na+ transport or on the initial phase of the ADH response. Interestingly, the additional apical membrane cholesterol had an inhibitory effect on the ADH response after 30 minutes. Apical membrane cholesterol depletion partially inhibited the responses to both insulin and ADH. Conversely, supplementation of basolateral cholesterol caused a significant increase in basal Na+ flux. Removal of cholesterol from the basolateral plasma membrane caused a decrease in basal Na+ flux with a time course analogous to channel turnover and completely inhibited peptide hormone responses. None of the changes in membrane cholesterol content decreased transcellular resistance. These results indicate an important role for membrane cholesterol content in the regulation of ENaC-mediated Na+ uptake.

Forskolin-induced cell shrinkage and apical translocation of functional enhanced green fluorescent protein-human alphaENaC in H441 lung epithelial cell monolayers

Elevation of intracellular cAMP increases fluid re-absorption in the lung by raising amiloride-sensitive Na+ transport through the apically localized epithelial, amiloride-sensitive Na+ channel (ENaC). However, the signaling pathways mediating this response are still not fully understood. We show that inhibition of protein-tyrosine kinase (PTK) with Genistein and protein kinase A (PKA) with KT5720, decreased forskolin-stimulated amiloride-sensitive short circuit current (I(sc)) across H441 adult human lung epithelial cell monolayers. KT5720 also decreased basal I(sc). Stable expression of green fluorescent protein (GFP)-labeled human alphaENaC in H441 cells was used to investigate dynamic changes in the cellular localization of this protein in response to forskolin. Reverse transcription-PCR and immunoblotting analysis revealed two clones expressing a truncated (alphaC3-5) and full-length (alphaC3-3) EGFP-halphaENaC protein. Only the alphaC3-3 clone displayed dome formation and exhibited a 50% increase in basal and forskolin-stimulated amiloride-sensitive I(sc) indicating that the full-length protein was required for functional activity. Apical surface biotinylation and real-time confocal microscopy demonstrated that EGFP-halphaENaC (alphaC3-3) translocated to the apical membrane in response to forskolin in a Brefeldin A-sensitive manner. This effect was completely inhibited by Genistein but only partially inhibited by KT5720. Forskolin also induced a reduction in the height of cells within alphaC3-3 monolayers, indicative of cell shrinkage. This effect was inhibited by KT5720 but not by Genistein or Brefeldin A. These data show that forskolin activates PKA-sensitive cell shrinkage in adult human H441 lung epithelial cell monolayers, which induces a PTK-sensitive translocation of EGFP-halphaENaC subunits to the apical membrane and increases amiloride-sensitive Na+ transport.

Blunted hypertensive effect of combined fructose and high-salt diet in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1

Serum- and glucocorticoid-inducible kinase (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase stimulates the renal epithelial Na(+) channel and may thus participate in blood pressure regulation. Hyperinsulinemia is triggered by dietary fructose, which sensitizes blood pressure for salt intake. The role of SGK1 in hypertensive effects of combined fructose and high-salt intake was thus explored in SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)). Renal SGK1 transcript levels of sgk1(+/+) mice were significantly elevated after fructose diet. Under control diet, fluid intake, urinary flow rate, urinary Na(+), K(+), and Cl(-) excretion, and blood pressure were similar in sgk1(-/-) and sgk1(+/+) mice. Addition of 10% fructose to drinking water increased fluid intake and urinary flow rate in both genotypes, and did not significantly alter urinary Na(+), K(+), and Cl(-) output in either genotype. Additional high NaCl diet (4% NaCl) did not significantly alter fluid intake and urine volume but markedly increased urinary output of Na(+) and Cl(-), approaching values significantly (P < 0.05) larger in sgk1(-/-) than in sgk1(+/+) mice (Na(+): 2,572 +/- 462 vs. 1,428 +/- 236; Cl(-): 2,364 +/- 388 vs. 1,379 +/- 225 micromol/24 h). Blood pressure was similar in sgk1(+/+) and sgk1(-/-) mice at control diet or fructose alone but increased only in sgk1(+/+) mice (115 +/- 1 vs. 103 +/- 0.7 mmHg, P < 0.05) after combined fructose and high-salt intake. Acute intravenous insulin infusion (during glucose clamp) caused antinatriuresis in sgk1(+/+) mice, an effect significantly blunted in sgk1(-/-) mice. The observations reveal a pivotal role of SGK1 in insulin-mediated sodium retention and the salt-sensitizing hypertensive effect of high fructose intake.

Oxygen and glucocorticoids modulate alphaENaC mRNA translation in fetal distal lung epithelium

Glucocorticoid hormones play an important role in fetal lung maturation. It is unknown how they interact with changes in O2 tension, which play an important role in converting the lung from a fluid-secreting to a fluid-absorbing organ at birth. Airspace fluid absorption arises from active transepithelial Na+ transport with the amiloride-sensitive epithelial Na channel (ENaC), consisting of alpha, beta, and gamma subunits, representing the rate-limiting step under nonpathologic conditions. We investigated the individual and combined effects of dexamethasone (DEX) and PO2 on alphaENaC mRNA levels, rate of alphaENaC protein synthesis, and amiloride-sensitive short-circuit current in primary cultures of rat fetal distal lung epithelial cells. DEX significantly induced alphaENaC mRNA in fetal (3%) and postnatal (21%) O2, but increases in alphaENaC protein synthesis and function occurred only when epithelia were grown under a postnatal PO2. Sucrose density gradient analyses showed that DEX treatment of cells cultured at 3% O2 decreased the association of alphaENaC mRNA with large polysomes and enhanced the association with small polysomes. Conversely, incubation of DEX-treated cells in 21% O2 restored alphaENaC mRNA association with large polysomes. No significant changes were seen in the overall polyribosome profiles or in the distribution of mRNAs encoding beta and gamma subunits of ENaC or cytokeratin 18, indicating specific modulation of alphaENaC mRNA translation. These data suggest that postnatal O2 exposure may be important for efficient translation of the alphaENaC mRNA.

Acute regulation of epithelial sodium channel by anionic phospholipids

Anionic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) are normally located in the inner leaflet of the plasma membrane, where these anionic phospholipids can regulate transmembrane proteins, including ion channels and transporters. Recent work has demonstrated that (1) ATP inhibits the renal epithelial sodium channel (ENaC) via a phospholipase C-dependent pathway that reduces PIP(2), (2) aldosterone stimulates ENaC via phosphoinositide 3-kinase, and (3) PIP(2) and PIP(3) regulate ENaC. Several lines of evidence show that ATP stimulation of purinergic P2Y receptors hydrolyzes PIP(2) and that aldosterone stimulation of steroid receptors induces PIP(3) formation. These studies together suggest that one primary mechanism for regulating ENaC is by alteration of anionic phospholipids and that the receptor-mediated and hormonal regulation of ENaC works through a variety of signaling pathways, but many of these pathways finally alter ENaC activity by regulating the formation or degradation of anionic phospholipids. Therefore, changes in the concentration of PIP(2) and PIP(3) are hypothesized to participate in the regulation of ENaC by purinergic and corticoid receptors. The underlying mechanism may be associated with a physical interaction of the positively charged cytoplasmic domains of the beta- and gamma-ENaC with the negatively charged membrane phospholipids. The exact nature of this interaction will require further investigation.

Identification of a functional phosphatidylinositol 3,4,5-trisphosphate binding site in the epithelial Na+ channel

Membrane phospholipids, such as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), are signaling molecules that can directly modulate the activity of ion channels, including the epithelial Na(+) channel (ENaC). Whereas PI(3,4,5)P(3) directly activates ENaC, its binding site within the channel has not been identified. We identify here a region of gamma-mENaC just following the second trans-membrane domain (residues 569-583) important to PI(3,4,5)P(3) binding and regulation. Deletion of this track decreases activity of ENaC heterologously expressed in Chinese hamster ovary cells. K-Ras and its first effector phosphoinositide 3-OH kinase (PI3-K), as well as RhoA and its effector phosphatidylinositol 4-phosphate 5-kinase increase ENaC activity. Whereas the former, via generation of PI(3,4,5)P(3), increases ENaC open probability, the latter increases activity by increasing membrane levels of the channel. Deletion of the region just distal to the second trans-membrane domain disrupted regulation by K-Ras and PI3-K but not RhoA and phosphatidylinositol 4-phosphate 5-kinase. Moreover, PI(3,4,5)P(3) binds ENaC with deletion of the region following the second transmembrane domain disrupting this interaction and disrupting direct activation of the channel by PI(3,4,5)P(3). Mutation analysis revealed the importance of conserved positive and negative charged residues as well as bulky amino acids within this region to modulation of ENaC by PI3-K. The current results identify the region just distal to the second trans-membrane domain within gamma-mENaC as being part of a functional PI(3,4,5)P(3) binding site that directly impacts ENaC activity. Phospholipid binding to this site is probably mediated by the positively charged amino acids within this track, with negatively charged and bulky residues also influencing specificity of interactions.

Synergic action of insulin and genistein on Na+/K+/2Cl − cotransporter in renal epithelium

Transepithelial Cl(-) secretion in polarized renal A6 cells is composed of two steps: (1) Cl(-) entry step across the basolateral membrane mediated by Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and (2) Cl(-) releasing step across the apical membrane via cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. We estimated CFTR Cl(-) channel activity and transcellular Cl(-) secretion by measuring 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB, a blocker of CFTR Cl(-) channel)-sensitive transepithelial conductance (Gt) and short-circuit current (Isc), respectively. Pretreatment with 1 microM insulin for 24 h had no effects on NPPB-sensitive Gt or Isc. On the other hand, in A6 cells treated with carbobenzoxy-L-leucyl-leucyl-L-leucinal (MG132; 100 microM for 2 h) that inhibits endocytosis of proteins at the plasma membrane into the cytosolic space, insulin pretreatment increased the NPPB-sensitive Isc with no effects on NPPB-sensitive Gt. Genistein (100 microM) induced sustained increases in NPPB-sensitive Gt and Isc, which were diminished by brefeldin A (a blocker of protein translocation to Golgi apparatus from endoplasmic reticulum). Co-application of insulin and genistein synergically stimulated the NPPB-sensitive Isc without any effects on NPPB-sensitive Gt. These observations suggest that: (1) insertion and endocytosis of NKCC are stimulated by insulin, (2) the insulin-induced stimulation of NKCC insertion into the basolateral membrane is offset by the stimulatory action on NKCC endocytosis from the basolateral membrane, (3) genistein stimulates insertion of both CFTR Cl(-) channel into the apical membrane and NKCC into the basolateral membrane, and (4) insulin and genistein synergically stimulated NKCC insertion into the basolateral membrane.

Hydrogen peroxide and epidermal growth factor activate phosphatidylinositol 3-kinase and increase sodium transport in A6 cell monolayers

Activation of phosphatidylinositol 3-kinase (PI 3-kinase) is required for insulin stimulation of sodium transport in A6 cell monolayers. In this study, we investigate whether stimulation of the PI 3-kinase by other agents also provoked an increase in sodium transport. Both epidermal growth factor (EGF) and H2O2provoked a rise in sodium transport that was inhibited by LY-294002, an inhibitor of PI 3-kinase activity. PI 3-kinase activity was estimated in extracts from A6 cell monolayers directly by performance of a PI 3-kinase assay. We also estimated the relative importance of the PI 3-kinase pathway by two different methods: 1) coprecipitation of the p85 regulatory subunit with anti-phosphotyrosine antibodies and 2) phosphorylation of PKB on both Ser 473 and Thr 308 residues observed by Western blotting. Since the mitogen-activated protein kinase (MAPK) pathway has also been implicated in the regulation of sodium transport, we also investigated whether this pathway is turned on by insulin, H2O2, or EGF. Phosphorylation of ERK1/2 was increased only transiently by insulin and H2O2but quite sustainedly by EGF. Inhibitors of this pathway (U-0126 and PD-98059) failed to affect the insulin and H2O2stimulation of sodium transport but increased substantially the stimulation induced by EGF. The latter effect was associated with an increase in PKB phosphorylation, thus suggesting that the stimulation of the MAPK pathway prevents, in part, the stimulation of the PI 3-kinase pathway in the transport of sodium stimulated by EGF.

Phosphoinositide lipid second messengers: new paradigms for transepithelial signal transduction

Multiple forms of phosphatidylinositol are generated by differential phosphorylation of the inositol headgroup. These phosphoinositides, specifically PI(4,5)P2, have been implicated as modulators in a variety of transport processes. The data indicate that phosphoinositides can modulate transporters directly or via the activation of down-stream signaling components. The phosphoinositide pathway has been linked to changes in transporter kinetics, intracellular signaling, membrane targeting and membrane stability. Recent results obtained for several of the well-characterized transport systems suggest the need to reassess the role of PI(4,5)P2 and question whether lower abundance forms of the phosphoinositides, notably PI(3,4,5)P3 (PIP3) and PI(3,4)P2, are the pertinent transport regulators. In contrast to PI(4,5)P2, these latter forms represent a dynamic, regulated pool, the characteristics of which are more compatible with the nature of signaling intermediates. A recently described, novel transepithelial signaling pathway has been demonstrated for PIP3 in which a signal initiated on the basolateral membrane is transduced to the apical membrane entirely within the planar face of the inner leaflet of the plasma membrane. The new paradigms emerging from recent studies may be widely applicable to transporter regulation in other cell types and are particularly relevant for signaling in polarized cells.