DNA Methylation Patterns Correlate with the Expression of SCNN1A, SCNN1B, and SCNN1G (Epithelial Sodium Channel, ENaC) Genes

The interplay between the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial sodium channel (ENaC) in respiratory epithelia has a crucial role in the pathogenesis of cystic fibrosis (CF). The comprehension of the mechanisms of transcriptional regulation of ENaC genes is pivotal to better detail the pathogenic mechanism and the genotype-phenotype relationship in CF, as well as to realize therapeutic approaches based on the transcriptional downregulation of ENaC genes. Since we aimed to study the epigenetic transcriptional control of ENaC genes, an assessment of their expression and DNA methylation patterns in different human cell lines, nasal brushing samples, and leucocytes was performed. The mRNA expression of CFTR and ENaC subunits α, β and γ (respectively SCNN1A, SCNN1B, and SCNN1G genes) was studied by real time PCR. DNA methylation of 5'-flanking region of SCNN1A, SCNN1B, and SCNN1G genes was studied by HpaII/PCR. The levels of expression and DNA methylation of ENaC genes in the different cell lines, brushing samples, and leukocytes were very variable. The DNA regions studied of each ENaC gene showed different methylation patterns. A general inverse correlation between expression and DNA methylation was evidenced. Leukocytes showed very low expression of all the 3 ENaC genes corresponding to a DNA methylated pattern. The SCNN1A gene resulted to be the most expressed in some cell lines that, accordingly, showed a completely demethylated pattern. Coherently, a heavy and moderate methylated pattern of, respectively, SCNN1B and SCNN1G genes corresponded to low levels of expression. As exceptions, we found that dexamethasone treatment appeared to stimulate the expression of all the 3 ENaC genes, without an evident modulation of the DNA methylation pattern, and that in nasal brushing a considerable expression of all the 3 ENaC genes were found despite an apparent methylated pattern. At least part of the expression modulation of ENaC genes seems to depend on the DNA methylation patterns of specific DNA regions. This points to epigenetics as a controlling mechanism of ENaC function and as a possible therapeutic approach for CF.

Alterations in plasma membrane promote overexpression and increase of sodium influx through epithelial sodium channel in hypertensive platelets

Platelets are small, anucleated cell fragments that activate in response to a wide variety of stimuli, triggering a complex series of intracellular pathways leading to a hemostatic thrombus formation at vascular injury sites. However, in essential hypertension, platelet activation contributes to causing myocardial infarction and ischemic stroke. Reported abnormalities in platelet functions, such as platelet hyperactivity and hyperaggregability to several agonists, contribute to the pathogenesis and complications of thrombotic events associated with hypertension. Platelet membrane lipid composition and fluidity are determining for protein site accessibility, structural arrangement of platelet surface, and response to appropriate stimuli. The present study aimed to demonstrate whether structural and biochemical abnormalities in lipid membrane composition and fluidity characteristic of platelets from hypertensive patients influence the expression of the Epithelial Sodium Channel (ENaC), fundamental for sodium influx during collagen activation. Wb, cytometry and quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) assays demonstrated ENaC overexpression in platelets from hypertensive subjects and in relation to control subjects. Additionally, our results strongly suggest a key role of β-dystroglycan as a scaffold for the organization of ENaC and associated proteins. Understanding of the mechanisms of platelet alterations in hypertension should provide valuable information for the pathophysiology of hypertension.

[Expression of Catechol-O-Methyltransferase (Comt), Mineralocorticoid Receptor (Mlr), and Epithelial Sodium Channel (ENaC) Genes in Kidneys of Hypertensive ISIAH Rats at Rest and during Response to Stress]

Emotional stress plays a significant role in the processes of the development of arterial hypertension, especially in the presence of genetic predisposition. The origin and maintenance of hypertensive status during stress development can be activated by the sympathetic nervous system. An increase in sympathetic stimulation can, in turn, result in a change in the functions of kidneys, which provide fluid and electrolyte balance of the organism. A comparative study of the mRNA expression level of catechol-o-methyltransferase (Comt), mineralocorticoid receptor (Mlr), and β-subunit of epithelial sodium channel (β-ENaC) genes was conducted on the kidneys of hypertensive ISIAH rats and normotensive WAG rats at rest and after the effect of emotional stress. The discovered changes in the expression level of the selected genes confirm their involvement in increased sympathetic stimulation of the kidney, along with changes in the function of kidney regulation of fluid and electrolyte balance, which is an important factor of the development of sustained hypertension in the ISIAH rats strain.

1,25-Dihydroxyvitamin D Enhances Alveolar Fluid Clearance by Upregulating the Expression of Epithelial Sodium Channels

Vitamin D is implicated in the pathogenesis of asthma, acute lung injury, and other respiratory diseases. 1,25-Dihydroxyvitamin D (1,25(OH)2D3), the hormonal form of vitamin D, has been shown to reduce vascular permeability and ameliorate lung edema. Therefore, we speculate that 1,25(OH)2D3 may regulate alveolar Na(+) transport via targeting epithelial Na(+) channels (ENaC), a crucial pathway for alveolar fluid clearance. In vivo total alveolar fluid clearance was 39.4 ± 3.8% in 1,25(OH)2D3-treated mice, significantly greater than vehicle-treated controls (24.7 ± 1.9 %, n = 10, p < 0.05). 1,25(OH)2D3 increased amiloride-sensitive short-circuit currents in H441 monolayers, and whole-cell patch-clamp data confirmed that ENaC currents in single H441 cell were enhanced in 1,25(OH)2D3-treated cells. Western blot showed that the expression of α-ENaC was significantly elevated in 1,25(OH)2D3-treated mouse lungs and 1,25(OH)2D3-treated H441 cells. These observations suggest that vitamin D augments transalveolar fluid clearance, and vitamin D therapy may potentially be used to ameliorate pulmonary edema.

Expression and purification of the alpha subunit of the epithelial sodium channel, ENaC

The epithelial sodium channel (ENaC) plays a critical role in maintaining Na(+) homeostasis in various tissues throughout the body. An understanding of the structure of the ENaC subunits has been developed from homology modeling based on the related acid sensing ion channel 1 (ASIC1) protein structure, as well as electrophysiological approaches. However, ENaC has several notable functional differences compared to ASIC1, thereby providing justification for determination of its three-dimensional structure. Unfortunately, this goal remains elusive due to several experimental challenges. Of the subunits that comprise a physiological hetero-trimeric αβγENaC, the α-subunit is unique in that it is capable of forming a homo-trimeric structure that conducts Na(+) ions. Despite functional and structural interest in αENaC, a key factor complicating structural studies has been its interaction with multiple other proteins, disrupting its homogeneity. In order to address this issue, a novel protocol was used to reduce the number of proteins that associate and co-purify with αENaC. In this study, we describe a novel expression system coupled with a two-step affinity purification approach using NiNTA, followed by a GFP antibody column as a rapid procedure to improve the purity and yield of rat αENaC.

Overexpression of the VSSC-associated CAM, β-2, enhances LNCaP cell metastasis associated behavior

BACKGROUND

Prostate cancer (PCa) is the second-leading cause of cancer death in American men. This is due largely to the "silent" nature of the disease until it has progressed to a highly metastatic and castrate resistant state. Voltage sensitive sodium channels (VSSCs) are multimeric transmembrane protein complexes comprised of a pore-forming α subunit and one or two β subunits. The β-subunits modulate surface expression and gating kinetics of the channels but also have inherent cell adhesion molecule (CAM) functions. We hypothesize that PCa cells use VSSC β-subunits as CAMs during PCa progression and metastasis.

METHODS

We overexpressed the beta-2 isoform as a C-terminal fusion protein with enhanced cyan fluorescence protein (ECFP) in the weakly metastatic LNCaP cells. The effect of beta-2 overexpression on cell morphology was examined using confocal microscopy while metastasis-associated behavior was tested by performing several in vitro metastatic functional assays and in vivo subcutaneous tumor studies.

RESULTS

We found that cells overexpressing beta-2 (2BECFP) converted to a bipolar fibroblastic morphology. 2BECFP cells were more adhesive than control (ECFP) to vitronectin (twofold) and Matrigel® (1.3-fold), more invasive through Matrigel® (3.6-fold in 72 hr), and had enhanced migration (2.1-fold in 96 hr) independent of proliferation in wound-healing assays. In contrast, 2BECFP cells have a reduced tumor-take and tumor volume in vivo even though the overexpression of beta-2 was maintained.

CONCLUSIONS

Functional overexpression of VSSC β-subunits in PCa may be one mechanism leading to increased metastatic behavior while decreasing the ability to form localized tumor masses.

Sildenafil reduces polyuria in rats with lithium-induced NDI

Lithium (Li)-treated patients often develop urinary concentrating defect and polyuria, a condition known as nephrogenic diabetes insipidus (NDI). In a rat model of Li-induced NDI, we studied the effect that sildenafil (Sil), a phosphodiesterase 5 (PDE5) inhibitor, has on renal expression of aquaporin-2 (AQP2), urea transporter UT-A1, Na(+)/H(+) exchanger 3 (NHE3), Na(+)-K(+)-2Cl(-) cotransporter (NKCC2), epithelial Na channel (ENaC; α-, β-, and γ-subunits), endothelial nitric oxide synthase (eNOS), and inducible nitric oxide synthase. We also evaluated cGMP levels in medullary collecting duct cells in suspension. For 4 wk, Wistar rats received Li (40 mmol/kg food) or no treatment (control), some receiving, in weeks 2-4, Sil (200 mg/kg food) or Li and Sil (Li+Sil). In Li+Sil rats, urine output and free water clearance were markedly lower, whereas urinary osmolality was higher, than in Li rats. The cGMP levels in the suspensions of medullary collecting duct cells were markedly higher in the Li+Sil and Sil groups than in the control and Li groups. Semiquantitative immunoblotting revealed the following: in Li+Sil rats, AQP2 expression was partially normalized, whereas that of UT-A1, γ-ENaC, and eNOS was completely normalized; and expression of NKCC2 and NHE3 was significantly higher in Li rats than in controls. Inulin clearance was normal in all groups. Mean arterial pressure and plasma arginine vasopressin did not differ among the groups. Sil completely reversed the Li-induced increase in renal vascular resistance. We conclude that, in experimental Li-induced NDI, Sil reduces polyuria, increases urinary osmolality, and decreases free water clearance via upregulation of renal AQP2 and UT-A1.

Sour ageusia in two individuals implicates ion channels of the ASIC and PKD families in human sour taste perception at the anterior tongue

BACKGROUND

The perception of sour taste in humans is incompletely understood at the receptor cell level. We report here on two patients with an acquired sour ageusia. Each patient was unresponsive to sour stimuli, but both showed normal responses to bitter, sweet, and salty stimuli.

METHODS AND FINDINGS

Lingual fungiform papillae, containing taste cells, were obtained by biopsy from the two patients, and from three sour-normal individuals, and analyzed by RT-PCR. The following transcripts were undetectable in the patients, even after 50 cycles of amplification, but readily detectable in the sour-normal subjects: acid sensing ion channels (ASICs) 1a, 1beta, 2a, 2b, and 3; and polycystic kidney disease (PKD) channels PKD1L3 and PKD2L1. Patients and sour-normals expressed the taste-related phospholipase C-beta2, the delta-subunit of epithelial sodium channel (ENaC) and the bitter receptor T2R14, as well as beta-actin. Genomic analysis of one patient, using buccal tissue, did not show absence of the genes for ASIC1a and PKD2L1. Immunohistochemistry of fungiform papillae from sour-normal subjects revealed labeling of taste bud cells by antibodies to ASICs 1a and 1beta, PKD2L1, phospholipase C-beta2, and delta-ENaC. An antibody to PKD1L3 labeled tissue outside taste bud cells.

CONCLUSIONS

These data suggest a role for ASICs and PKDs in human sour perception. This is the first report of sour ageusia in humans, and the very existence of such individuals ("natural knockouts") suggests a cell lineage for sour that is independent of the other taste modalities.

Respiratory syncytial virus inhibits lung epithelial Na+ channels by up-regulating inducible nitric-oxide synthase

Respiratory syncytial virus (RSV) infection has been shown to reduce Na+-driven alveolar fluid clearance in BALB/c mice in vivo. To investigate the cellular mechanisms by which RSV inhibits amiloride-sensitive epithelial Na+ channels (ENaC), the main pathways through which Na+ ions enter lung epithelial cells, we infected human Clara-like lung (H441) cells with RSV that expresses green fluorescent protein (rRA2). 3-6 days later patch clamp recordings showed that infected cells (i.e. cells expressing green fluorescence; GFP+) had significantly lower whole-cell amiloride-sensitive currents and single channel activity (NPo) as compared with non-infected (GFP-), non-inoculated, or cells infected with UV-inactivated RSV. Both alpha and beta ENaC mRNA levels were significantly reduced in GFP+ cells as measured by real-time reverse transcription-PCR. Infection with RSV increased expression of the inducible nitric-oxide synthase (iNOS) and nitrite concentration in the culture medium; nuclear translocation of NF-kappaB p65 subunit and NF-kappaB activation were also up-regulated. iNOS up-regulation in GFP+ cells was prevented by knocking down IkappaB kinase gamma before infection. Furthermore, pretreatment of H441 cells with the specific iNOS inhibitor 1400W (1 microM) resulted in a doubling of the amiloride-sensitive Na+ current in GFP+ cells. Additionally, preincubation of H441 cells with A77-1726 (20 microM), a de novo UTP synthesis inhibitor, and 1400W completely reversed the RSV inhibition of amiloride-sensitive currents in GFP+ cells. Thus, both UTP- and iNOS-generated reactive species contribute to ENaC down-regulation in RSV-infected airway epithelial cells.

Steroid and Oxygen Effects on eIF4F Complex, mTOR, and ENaC Translation in Fetal Lung Epithelia

Fetal distal lung epithelium (FDLE) must increase amiloride-sensitive epithelial Na(+) channel (ENaC) activity during the perinatal period to increase Na(+) transport and fluid clearance. Glucocorticosteroid (GC) levels increase, there is a 7-fold increase in Po(2) at birth, and we have previously shown that dexamethasone (DEX)-induced alpha-ENaC mRNA is efficiently translated only under postnatal (21%) O(2) (Otulakowski et al., AJRCMB 2006;34:204-212). Translation of mRNAs with long GC-rich 5'UTRs, such as alpha-ENaC mRNA, are sensitive to the amount of eIF4F, the mRNA 5'-cap binding complex composed of eIF4E and eIF4G. We now show, by Western blotting and m(7)GTP-Sepharose pull-down experiments, that in FDLE cultured under 3% O(2), DEX decreases formation of eIF4F and increases association of eIF4E with its inhibitor 4E-BP by changing 4E-BP phosphorylation. Conversely, FDLE cultured at 21% O(2) expressed lower levels of 4E-BP and maintained eIF4E-eIF4G association independent of DEX. Phosphorylation of 4E-BP is regulated by the kinase mTOR. Under 3% O(2), DEX decreased abundance of phosphorylated forms of the mTOR effectors, S6 kinase and ribosomal protein S6. Neither effect was associated with changes in REDD1, an upstream regulator of mTOR. When mTOR was inhibited (3 nM rapamycin) there was reduced 4E-BP phosphorylation, fewer ribosomes on alpha-ENaC mRNA, and decreased amiloride-sensitive short-circuit current, but no change in ribosomal loading onto any of beta- or gamma-ENaC or cytokeratin 18 mRNAs. We speculate that at birth increased Po(2) acts with GC through an mTOR-related pathway to increase alpha-ENaC protein synthesis, thereby promoting lung fluid absorption.

Altered expression of epithelial sodium channel in rats with bilateral or unilateral ureteral obstruction

The roles of epithelial sodium channel (ENaC) subunits (α, β, and γ) in the impaired renal reabsorption of sodium and water were examined in rat models with bilateral (BUO) or unilateral ureteral obstruction (UUO) for 24 h or with BUO followed by release of obstruction and 3 days of observation (BUO-3dR). In BUO rats, plasma osmolality was increased dramatically, whereas plasma sodium concentration was decreased. Immunoblotting revealed a significantly decreased expression of α-ENaC (57 ± 7%), β-ENaC (19 ± 5%), and γ-ENaC (51 ± 10%) as well as 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) in the cortex and outer medulla (C+OM) compared with sham-operated controls. This was confirmed by immunohistochemistry. BUO-3dR was associated with polyuria and impaired renal sodium handling. The protein abundance and the apical labeling of α-ENaC were significantly increased, whereas β- and γ-ENaC as well as 11β-HSD2 expression remained decreased. In UUO rats, expression of α- and β-ENaC and 11β-HSD2 decreased in the C+OM in the obstructed kidney. In contrast, the abundance and the apical labeling of α-ENaC in the nonobstructed kidneys were markedly increased, suggesting compensatory upregulation in this kidney. In conclusion, α-, β-, and γ-ENaC expression levels are downregulated in the obstructed kidney. The expression and apical labeling of α-ENaC were increased in BUO-3dR rats and in the nonobstructed kidneys from UUO rats. These results suggest that altered expression of α-, β-, and γ-ENaC may contribute to impaired renal sodium and water handling in response to ureteral obstruction.

Interleukin-1β suppresses epithelial sodium channel β-subunit expression and ENaC-dependent fluid absorption in human middle ear epithelial cells

Recent reports have shown that cytokines inhibit fluid absorption by suppressing Na(+) channel activity in various epithelia. In this study, we investigated the role of epithelial sodium channel (ENaC) in fluid absorption in normal human middle ear epithelial (NHMEE) cells, as well as the effects of Interleukin (IL)-1beta on ENaC expression and fluid absorption in NHMEE cells. We confirmed that ENaC alpha, beta and gamma were predominantly expressed on the apical surface of the NHMEE cells by immunocytochemistry. Addition of amiloride, a potent ENaC blocker, to apical membranes of NHMEE cells decreased the fluid absorption rate in a dose-dependent manner. Treatment with 10 ng/ml IL-1beta for 24 h suppressed ENaC beta expression, the ENaC-dependent short-circuit current (Isc), and ENaC-dependent fluid absorption. When the NHMEE cells were pretreated with a phospholipase C (PLC)inhibitor (U73122, 10 microM), a protein kinase C (PKC) inhibitor (Calphostin C, 0.1 microM), or extracellular signal regulated kinase (ERK) 1/2 inhibitor (PD98059, 10 microM), the amiloride-sensitive currents in IL-1beta-treated cells were reversed to control levels; an effect not seen with SB202190 (an inhibitor of p38 mitogen-activated protein (MAP) kinase) or SP600125 (a reversible inhibitor of c-Jun N-terminal kinase). In this study we showed that ENaC is essential for fluid absorption in NHMEE cells and that IL-1beta suppresses the ENaC-dependent current via the PLC-PKC-ERK1/2 pathway. These results suggest that IL-1beta may contribute to fluid retention in otitis media with effusion by changing electrolyte transport and reducing middle ear epithelial fluid absorption.

Involvement of p38 MAPK in hypotonic stress-induced stimulation of beta- and gamma-ENaC expression in renal epithelium

We investigated a role of p38 MAPK in the regulation of transepithelial Na(+) reabsorption by chronic application (20-24h) of hypotonicity (hypotonic stress) in renal epithelial A6 cells. Pretreatment with a specific p38 MAPK inhibitor (SB202190) significantly reduced the chronic hypotonicity-stimulated transepithelial Na(+) reabsorption by diminishing the Na(+) entry through epithelial Na(+) channel (ENaC) in the apical membrane and the Na(+) extrusion via the Na(+)/K(+) ATPase (pump), although the rate limiting step was still the Na(+) entry step. We further examined whether the inhibitory effects of SB202190 on the transepithelial Na(+) reabsorption is caused through suppression of mRNA expression of ENaC participating in the transepithelial Na(+) reabsorption as the Na(+) entry pathway. The chronic hypotonicity increased the mRNA expression of alpha-, beta-, and gamma-subunits of ENaC. Moreover, we found that inhibition of p38 MAPK by SB202190 diminished the mRNA expression of beta- and gamma-ENaC but not alpha-ENaC. Based on these observations, it is suggested that the chronic hypotonicity stimulates the renal transepithelial Na(+) reabsorption by upregulating the mRNA expression of beta- and gamma-ENaC via a p38 MAPK-dependent pathway.

Leptospirosis leads to dysregulation of sodium transporters in the kidney and lung

Leptospirosis is a public health problem worldwide. Severe leptospirosis manifests as pulmonary edema leading to acute respiratory distress syndrome and polyuric acute renal failure (ARF). The etiology of leptospirosis-induced pulmonary edema is unclear. Lung edema clearance is largely affected by active sodium transport out of the alveoli rather than by reversal of the Starling forces. The objective of this study was to profile leptospirosis-induced ARF and pulmonary edema. We inoculated hamsters with leptospires and collected 24-h urine samples on postinoculation day 4. On day 5, the animals were killed, whole blood was collected, and the kidneys and lungs were removed. Immunoblotting was used to determine expression and abundance of water and sodium transporters. Leptospirosis-induced ARF resulted in natriuresis, lower creatinine clearance, and impaired urinary concentrating ability. Renal expression of the sodium/hydrogen exchanger isoform 3 and of aquaporin 2 was lower in infected animals, whereas that of the Na-K-2Cl cotransporter NKCC2 was higher. Leptospirosis-induced lesions, predominantly in the proximal tubule, were responsible for the polyuria and natriuresis observed. The polyuria might also be attributed to reduced aquaporin 2 expression and the attendant urinary concentrating defect. In the lungs, expression of the epithelial sodium channel was lower, and NKCC1 expression was upregulated. We found that leptospirosis profoundly influences the sodium transport capacity of alveolar epithelial cells and that impaired pulmonary fluid handling can impair pulmonary function, increasing the chance of lung injury. Greater knowledge regarding sodium transporter dysregulation in the lungs and kidneys can provide new perspectives on leptospirosis treatment.

Increase in weight induced by muraglitazar, a dual PPARalpha/gamma agonist, in db/db mice: adipogenesis/or oedema?

BACKGROUND AND PURPOSE

Muraglitazar, a dual PPARalpha/gamma agonist, caused a robust increase in body weight in db/db mice. The purpose of the study was to see if this increase in weight was due to oedema and/or adipogenesis.

EXPERIMENTAL APPROACH

The affinity of muraglitazar at PPARalpha/gamma receptors was characterized using transactivation assays. Pre-adipocyte differentiation, expression of genes for adipogenesis (aP2), fatty acid oxidation (ACO) and sodium reabsorption (ENaCgamma and Na+, K+-ATPase); haemodilution parameters and serum electrolytes were measured to delineate the role of muraglitazar in causing weight gain vis a vis rosiglitazone.

KEY RESULTS

Treatment with muraglitazar (10 mg kg(-1)) for 14 days significantly reduced plasma glucose and triglycerides. Reduction in plasma glucose was significantly greater than after similar treatment with rosiglitazone (10 mg kg(-1)). A marked increase in weight was also observed with muraglitazar that was significantly greater than with rosiglitazone. Muraglitazar increased aP2 mRNA and caused adipocyte differentiation in 3T3-L1 cells similar to rosiglitazone. It also caused a marked increase in ACO mRNA in the liver of the treated mice. Expression of mRNA for ENaCgamma and Na+, K+-ATPase in kidneys was up-regulated after either treatment. Increased serum electrolytes and decreased RBC count, haemoglobin and haematocrit were observed with both muraglitazar and rosiglitazone.

CONCLUSIONS AND IMPLICATIONS

Although muraglitazar has a better glucose lowering profile, it also has a greater potential for weight gain than rosiglitazone. In conclusion, muraglitazar causes both robust adipogenesis and oedema in a 14-day treatment of db/db mice as observed in humans.

Na channel expression and activity in the medullary collecting duct of rat kidney

The expression and activity of epithelial Na(+) channels (ENaC) in the medullary collecting duct of the rat kidney were examined using a combination of whole cell patch-clamp measurements of amiloride-sensitive currents (I(Na)) in split-open tubules and Western blot analysis of alpha-, beta-, and gamma-ENaC proteins. In the outer medullary collecting duct, amiloride-sensitive currents were undetectable in principal cells from control animals but were robust when rats were treated with aldosterone (I(Na) = 960 +/- 160 pA/cell) or fed a low-Na diet (I(Na) = 440 +/- 120 pA/cell). In both cases, the currents were similar to those measured in principal cells of the cortical collecting duct from the same animals. In the inner medullary collecting duct, currents were much lower, averaging 120 +/- 20 pA/cell in aldosterone-treated rats. Immunoblots showed that all three ENaC subunits were expressed in the cortex, outer medulla, and inner medulla of the rat kidney. When rats were fed a low-Na diet for 1 wk, similar changes in alpha- and gamma-ENaC occurred in all three regions of the kidney; the amounts of full-length as well as putative cleaved alpha-ENaC protein increased, and the fraction of gamma-ENaC protein in the cleaved state increased at the expense of the full-length protein. The appearance of a presumably fully glycosylated form of beta-ENaC in Na-depleted animals was observed mainly in the outer and inner medulla. These findings suggest that the capability of hormone-regulated, channel-mediated Na reabsorption by the nephron extends at least into the outer medullary collecting duct.

Renal sodium retention in cirrhotic rats depends on glucocorticoid-mediated activation of mineralocorticoid receptor due to decreased renal 11β-HSD-2 activity

Downregulation of the renal glucocorticoid-metabolizing enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD-2) during liver cirrhosis may allow activation of the mineralocorticoid receptor (MR) by glucocorticoids and contribute to sodium retention. We tested this hypothesis in male Wistar rats with decompensated liver cirrhosis and ascites 7 wk after bile duct ligation (BDL). Renal 11β-HSD-2 mRNA, protein, and activity were significantly decreased in decompensated rats. The urinary Na+/K+ ratio was reduced by 40%. Renal epithelial sodium channel (ENaC) mRNA and immunostaining were only slightly affected. Complete metabolic studies, including fecal excretion, showed that the BDL rats had avid renal sodium retention. Treatment of the BDL rats with dexamethasone suppressed endogenous glucocorticoid production, normalized total sodium balance and renal sodium excretion, and reduced ascites formation to the same degree as direct inhibition of MR with K-canrenoate. Total potassium balance was negative in the BDL rats, whereas renal potassium excretion was unchanged. In the distal colon, expression of ENaC was increased in BDL rats. Fecal potassium excretion was increased in cirrhotic rats, and this was corrected by treatment with K-canrenoate but not dexamethasone. We conclude that development of sodium retention and decompensation in cirrhotic rats is associated with downregulation of renal 11β-HSD-2 activity and inappropriate activation of renal sodium reabsorption by endogenous glucocorticoids. In addition, the overall potassium loss in the BDL model is due to increased fecal potassium excretion, which is associated with upregulation of ENaC in distal colon.

Enhancement of epithelial sodium channel expression in renal cortical collecting ducts cells by advanced glycation end products

BACKGROUND

The epithelial sodium channel (ENaC) is a complex, and the alphaENaC subunit has a crucial role in sodium uptake induced by aldosterone in the distal nephron. Although experimental animal models of diabetes have demonstrated up-regulation of alphaENaC expression in renal cortical collecting duct (CCD) cells, the molecular mechanism remains unclear. Advanced glycation end products (AGEs) are by-products of long-term hyperglycaemia and comprise a significant pathogenic factor in diabetic nephropathy. We hypothesize that AGEs play a role in regulating alphaENaC gene expression.

METHODS

Mouse CCD cells (mpkCCDcl(4)) were cultured with AGE to determine the effects of AGE on alphaENaC expression and sodium uptake. Gene expressions of ENaC were measured by real-time PCR and sodium uptake was measured with fluorescent dye as a sodium indicator (SBFI-AM). This study analysed mitogen-activated protein kinases signalling pathways by western blotting. Cells co-transfected with plasmids of the alphaENaC promoter carrying a luciferase reporter and plasmids expressing wild-type or mutant serum- and glucocorticoid-induced kinase 1 (Sgk1) mRNA were stimulated with AGE to identify the signalling pathway.

RESULTS

The AGEs, stimulated in a time- and dose-dependent manner, enhanced alphaENaC mRNA expression and sodium uptake in mpkCCDcl(4) cells. The AGEs also significantly stimulated Sgk1 mRNA and Sgk1 activity in a time- and dose-dependent manner. Co-transfected with plasmid expressing mutant Sgk1 significantly limited stimulated alphaENaC promoter-driven luciferase activity by AGEs in mpkCCDcl(4) cells.

CONCLUSION

Experimental results indicate that AGEs induced alphaENaC expression and increased sodium uptake in renal CCD cells. The mechanism through which AGEs activate alphaENaC expression may be via activation of Sgk1 in mpkCCDcl(4) cells.

Maintained ENaC trafficking in aldosterone-infused rats during mineralocorticoid and glucocorticoid receptor blockade

Aldosterone induces redistribution of epithelial sodium channel (ENaC) to the apical plasma membrane from intracellular vesicles in renal connecting tubule (CNT) and cortical collecting duct (CCD). The role of the classical mineralocorticoid receptor (MR) in ENaC trafficking is still debated. We examined whether the MR antagonist spironolactone affects ENaC regulation in the kidney cortex of aldosterone-infused rats. Aldosterone infusion for 7 days resulted in a plasma aldosterone concentration in the high physiological range (3 to 4 nM). Aldosterone infusion decreased plasma K(+) concentration compared with untreated control rats. Cotreatment with spironolactone completely blocked the aldosterone-induced decrease in plasma K(+). Immunoblotting and immunohistochemistry showed increased protein abundance of Na-K-ATPase alpha(1)-subunit and NCC in the kidney cortex, in response to aldosterone infusion that was blocked by spironolactone. In contrast, aldosterone-induced redistribution of ENaC subunits from the cytoplasm to the apical plasma membrane domain in CNT and CCD was unaffected by spironolactone. Immunoblotting of alphaENaC showed increased protein abundance in aldosterone-infused rats that was not blocked by spironolactone treatment. To exclude possible glucocorticoid receptor (GR)-mediated effects of aldosterone, we treated aldosterone-infused rats with both spironolactone and the GR antagonist RU486. Combined MR and GR blockade prevented neither ENaC trafficking nor the upregulation of alphaENaC protein abundance in aldosterone-infused rats. We provide new evidence for ENaC trafficking occurring independent of MR and GR activation in aldosterone-infused rats.