Endogenous and exogenous glucocorticoid regulation of ENaC mRNA expression in developing kidney and lung

Inhaled β2 Adrenergic Agonists and Other cAMP-Elevating Agents: Therapeutics for Alveolar Injury and Acute Respiratory Disease Syndrome?

Inhaled long-acting β-adrenergic agonists (LABAs) and short-acting β-adrenergic agonists are approved for the treatment of obstructive lung disease via actions mediated by β2 adrenergic receptors (β2-ARs) that increase cellular cAMP synthesis. This review discusses the potential of β2-AR agonists, in particular LABAs, for the treatment of acute respiratory distress syndrome (ARDS). We emphasize ARDS induced by pneumonia and focus on the pathobiology of ARDS and actions of LABAs and cAMP on pulmonary and immune cell types. β2-AR agonists/cAMP have beneficial actions that include protection of epithelial and endothelial cells from injury, restoration of alveolar fluid clearance, and reduction of fibrotic remodeling. β2-AR agonists/cAMP also exert anti-inflammatory effects on the immune system by actions on several types of immune cells. Early administration is likely critical for optimizing efficacy of LABAs or other cAMP-elevating agents, such as agonists of other Gs-coupled G protein-coupled receptors or cyclic nucleotide phosphodiesterase inhibitors. Clinical studies that target lung injury early, prior to development of ARDS, are thus needed to further assess the use of inhaled LABAs, perhaps combined with inhaled corticosteroids and/or long-acting muscarinic cholinergic antagonists. Such agents may provide a multipronged, repurposing, and efficacious therapeutic approach while minimizing systemic toxicity. SIGNIFICANCE STATEMENT: Acute respiratory distress syndrome (ARDS) after pulmonary alveolar injury (e.g., certain viral infections) is associated with ∼40% mortality and in need of new therapeutic approaches. This review summarizes the pathobiology of ARDS, focusing on contributions of pulmonary and immune cell types and potentially beneficial actions of β2 adrenergic receptors and cAMP. Early administration of inhaled β2 adrenergic agonists and perhaps other cAMP-elevating agents after alveolar injury may be a prophylactic approach to prevent development of ARDS.

Glucocorticoids, sodium transport mediators, and respiratory distress syndrome in preterm infants

BACKGROUND

Antenatal glucocorticoids (GCs) reduce respiratory distress syndrome (RDS) in preterm infants and are associated with reduced lung liquid content. Our aim was to assess whether airway gene expression of mediators of pulmonary epithelial sodium and liquid absorption, and further, respiratory morbidity, associate with cord blood GC concentrations.

METHODS

The study included 64 infants delivered <32 weeks gestation. Cortisol and betamethasone in umbilical cord blood were quantified with liquid chromatography-tandem mass spectrometry. The total GC concentration was calculated. Gene expression of the epithelial sodium channel (ENaC), Na,K-ATPase, and serum- and GC-inducible kinase 1 at <2 h and at 1 day postnatally in nasal epithelial cell samples was quantified with reverse transcription-polymerase chain reaction. The mean oxygen supplementation during the first 72 h was calculated.

RESULTS

Concentrations of cord blood betamethasone and total GC were significantly lower in infants with RDS and correlated with mean oxygen supplementation. Expression of αENaC and α1- and β1Na,K-ATPase at <2 h correlated with betamethasone and total GC concentrations. Expression of Na,K-ATPase was lower in infants with RDS.

CONCLUSION

Enhancement of lung liquid absorption via increased expression of sodium transporters may contribute to the beneficial pulmonary effects of antenatal GCs.

IMPACT

RDS is related to lower umbilical cord blood GC concentrations and lower airway expression of sodium transporters. In addition to the timing of antenatal GC treatment, resulting concentrations may be of importance in preventing RDS. Induction of sodium transport may be a factor contributing to the pulmonary response to antenatal GCs.

Glucocorticoids Equally Stimulate Epithelial Na+ Transport in Male and Female Fetal Alveolar Cells

Preterm infants frequently suffer from respiratory distress syndrome (RDS), possibly due to lower expression of epithelial Na⁺ channels (ENaC). RDS incidence is sex-specific, affecting males almost twice as often. Despite the use of antenatal glucocorticoids (GCs), the sex difference persists. It is still controversial whether both sexes benefit equally from GCs. We previously showed that Na⁺ transport is higher in female compared with male fetal distal lung epithelial (FDLE) cells. Since GCs increase Na⁺ transport, we hypothesized that their stimulating effect might be sex-specific. We analyzed FDLE cells with Ussing chambers and RT-qPCR in the presence or absence of fetal serum. In serum-free medium, GCs increased the ENaC activity and mRNA expression, independent of sex. In contrast, GCs did not increase the Na⁺ transport in serum-supplemented media and abolished the otherwise observed sex difference. Inhibition of the GC receptor in the presence of serum did not equalize Na⁺ transport between male and female cells. The GC-induced surfactant protein mRNA expression was concentration and sex-specific. In conclusion, female and male FDLE cells exhibit no sex difference in response to GCs with regard to Na⁺ transport, and GR activity does not contribute to the higher Na⁺ transport in females.

Bioinformatic Analysis of Gene Variants from Gastroschisis Recurrence Identifies Multiple Novel Pathogenetic Pathways: Implication for the Closure of the Ventral Body Wall

We investigated whether likely pathogenic variants co-segregating with gastroschisis through a family-based approach using bioinformatic analyses were implicated in body wall closure. Gene Ontology (GO)/Panther functional enrichment and protein-protein interaction analysis by String identified several biological networks of highly connected genes in UGT1A3, UGT1A4, UGT1A5, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, AOX1, NOTCH1, HIST1H2BB, RPS3, THBS1, ADCY9, and FGFR4. SVS–PhoRank identified a dominant model in OR10G4 (also as heterozygous de novo), ITIH3, PLEKHG4B, SLC9A3, ITGA2, AOX1, and ALPP, including a recessive model in UGT1A7, UGT1A6, PER2, PTPRD, and UGT1A3. A heterozygous compound model was observed in CDYL, KDM5A, RASGRP1, MYBPC2, PDE4DIP, F5, OBSCN, and UGT1A. These genes were implicated in pathogenetic pathways involving the following GO related categories: xenobiotic, regulation of metabolic process, regulation of cell adhesion, regulation of gene expression, inflammatory response, regulation of vascular development, keratinization, left-right symmetry, epigenetic, ubiquitination, and regulation of protein synthesis. Multiple background modifiers interacting with disease-relevant pathways may regulate gastroschisis susceptibility. Based in our findings and considering the plausibility of the biological pattern of mechanisms and gene network modeling, we suggest that the gastroschisis developmental process may be the consequence of several well-orchestrated biological and molecular mechanisms which could be interacting with gastroschisis predispositions within the first ten weeks of development.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines

Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na⁺, K⁺-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

Inflammatory Responses Regulating Alveolar Ion Transport during Pulmonary Infections

The respiratory epithelium is lined by a tightly balanced fluid layer that allows normal O₂ and CO₂ exchange and maintains surface tension and host defense. To maintain alveolar fluid homeostasis, both the integrity of the alveolar–capillary barrier and the expression of epithelial ion channels and pumps are necessary to establish a vectorial ion gradient. However, during pulmonary infection, auto- and/or paracrine-acting mediators induce pathophysiological changes of the alveolar–capillary barrier, altered expression of epithelial Na,K-ATPase and of epithelial ion channels including epithelial sodium channel and cystic fibrosis membrane conductance regulator, leading to the accumulation of edema and impaired alveolar fluid clearance. These mediators include classical pro-inflammatory cytokines such as TGF-β, TNF-α, interferons, or IL-1β that are released upon bacterial challenge with Streptococcus pneumoniae, Klebsiella pneumoniae, or Mycoplasma pneumoniae as well as in viral infection with influenza A virus, pathogenic coronaviruses, or respiratory syncytial virus. Moreover, the pro-apoptotic mediator TNF-related apoptosis-inducing ligand, extracellular nucleotides, or reactive oxygen species impair epithelial ion channel expression and function. Interestingly, during bacterial infection, alterations of ion transport function may serve as an additional feedback loop on the respiratory inflammatory profile, further aggravating disease progression. These changes lead to edema formation and impair edema clearance which results in suboptimal gas exchange causing hypoxemia and hypercapnia. Recent preclinical studies suggest that modulation of the alveolar–capillary fluid homeostasis could represent novel therapeutic approaches to improve outcomes in infection-induced lung injury.

Regulation of epithelial sodium channel expression by oestradiol and progestogen in alveolar epithelial cells

Oestrogen (E) and progestogen (P) exert regulatory effects on the epithelial Na(+) channel (ENaC) in the kidneys and the colon. However, the effects of E and P on the ENaC and on alveolar fluid clearance (AFC) remain unclear, and the mechanisms of action of these hormones are unknown. In this study, we showed that E and/or P administration increased AFC by more than 25% and increased the expression of the α and γ subunits of ENaC by approximately 35% in rats subjected to oleic acid-induced acute lung injury (ALI). A similar effect was observed in the dexamethasone-treated group. Furthermore, E and/or P treatment inhibited 11β-hydroxysteroid dehydrogenase (HSD) type 2 (11β-HSD2) activity, increased corticosterone expression and decreased the serum adrenocorticotrophic hormone (ACTH) levels. These effects were similar to those observed following treatment with carbenoxolone (CBX), a nonspecific HSD inhibitor. Further investigation showed that CBX further significantly increased AFC and α-ENaC expression after treatment with a low dose of E and/or P. In vitro, E or P alone inhibited 11β-HSD2 activity in a dose-dependent manner and increased α-ENaC expression by at least 50%, and E combined with P increased α-ENaC expression by more than 80%. Thus, E and P may augment the expression of α-ENaC, enhance AFC, attenuate pulmonary oedema by inhibiting 11β-HSD2 activity, and increase the active glucocorticoid levels in vivo and in vitro.

Duration of gestation and mode of delivery affect the genes of transepithelial sodium transport in pulmonary adaptation

BACKGROUND

Respiratory distress due to inadequate lung liquid clearance is a significant problem in infants delivered late preterm or early term, especially by elective cesarean delivery (CD). Lung liquid clearance depends on epithelial ion transport and in animals is induced by glucocorticoids.

OBJECTIVES

In newborn late preterm and term infants to study airway epithelial gene expressions of epithelial sodium channel (ENaC), and the serum and glucocorticoid-inducible kinase 1 (SGK1), and their association with cortisol, mode of delivery, and gestational age (GA).

METHODS

Infants were delivered at 35(0/7)-41(6/7) weeks. Cortisol in umbilical cord plasma was analyzed with liquid chromatography-tandem mass spectrometry. ENaC and SGK1 mRNAs in airway epithelial cells obtained within 3 h and at 1 day postnatally were quantified with real-time PCR.

RESULTS

ENaC and SGK1 mRNAs were significantly lower in late preterm and early term infants than in those ≥ 39(0/7) weeks. Significant correlations existed between both ENaC and SGK1 and cord cortisol and GA. In term infants, SGK1 mRNA at 1.5 h was higher after vaginal delivery than elective CD.

CONCLUSIONS

In late preterm and early term infants, low expression of ENaC and SGK1 may parallel insufficient lung liquid clearance predisposing to respiratory distress. Lower SGK1 expression after term CD could translate into insufficient sodium and lung liquid absorption. The findings demonstrate a central role for cortisol in regulation of ENaC and potentially perinatal sodium and lung liquid clearance.

The interaction of glucocorticoids and progesterone distinctively affects epithelial sodium transport

PURPOSE

Glucocorticoids and progesterone exert stimulatory effects on epithelial Na(+) transport, including increased mRNA expression of the participating ion transporters (epithelial Na(+) channels [ENaC] and Na,K-ATPases) and their electrophysiological activity. Fetuses threatened by preterm labor may receive high doses of glucocorticoids to stimulate lung maturation and are naturally exposed to high levels of female sex steroids. However, it is still unknown how the combination of both hormones influences the epithelial Na(+) transport, which is crucial for alveolar fluid clearance.

METHODS

Fetal distal lung epithelial cells were incubated in media supplemented with dexamethasone and progesterone. Real-time qPCR and Ussing chamber analysis were used to determine the effects on ENaC mRNA expression and channel activity. In addition, the specific progesterone receptor antagonist (PF-02367982) and the glucocorticoid receptor antagonist mifepristone were used to identify the involved hormone receptors.

RESULTS

Both dexamethasone and progesterone increased ENaC subunit expression and channel activity. However, the combination of dexamethasone and progesterone reduced the α- and γ-ENaC subunit expression compared to the effect of dexamethasone alone. Furthermore, higher dexamethasone concentrations in combination with progesterone also significantly reduced Na(+) transport in Ussing chamber measurements. Hormone receptor antagonists showed that inhibition of the progesterone receptor increased the mRNA expression of α- and γ-ENaC, whereas mifepristone decreased mRNA expression of all ENaC subunits.

CONCLUSION

Glucocorticoids and progesterone individually increase ENaC mRNA expression; however, the combination of both hormones decreases the stimulatory effects of dexamethasone on Na(+) transport and ENaC mRNA expression.

Mineralocorticoid effects in the late gestation ovine fetal lung

This study was designed to determine the effects of corticosteroids at MR in the late‐gestation fetal lung. Since both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) are expressed at relatively high levels in the fetal lung, endogenous corticosteroids may act at MR as well as GR in the preterm fetal lung. The GR agonist, betamethasone, the MR agonist, aldosterone, or both were infused intravenously for 48 h in ovine fetuses of approximately 130 days gestation. Effects on airway pressures during stepwise inflation of the in situ lung, expression of ENaC alpha (SCNN1A), ENaC beta (SCNN1B), and Na,K ATPase (ATP1A1), and elastin and collagen content were determined after the infusions. We found that aldosterone significantly reduced the airway pressure measured during the initial step in inflation of the lung, although aldosterone had no overall effect on lung compliance, nor did aldosterone induce expression of ENaCα, ENaCβ or Na,K ATPaseα1. Betamethasone significantly increased expression of the epithelial sodium channel (ENaC) subunit mRNAs, and collagen and elastin content in the lungs, although this dose of betamethasone also had no effect on lung compliance. There was no synergy between effects of the MR and GR agonists. Transcriptomic analysis suggested that although aldosterone did not alter genes in pathways related to epithelial sodium transport, aldosterone did alter genes in pathways involved in cell proliferation in the lungs. The results are consistent with corticosteroid‐induced fluid reabsorption at birth through GR rather than MR, but suggest that MR facilitates lung maturation, and may contribute to inflation with the first breaths via mechanisms distinct from known aldosterone effects in other epithelia.

Infusion of the mineralocorticoid receptor agonist, aldosterone, to the ovine fetus resulted in reduced airway pressures with initial lung inflation. However, aldosterone did not alter lung surfactant or epithelial sodium transport genes which are classical MR gene targets. Transcriptomic analysis revealed an aldosterone effect on genes related to cell cycle, suggesting that MR have a role distinct form that of GR in the maturing lung.

Function of cGMP-dependent protein kinase II in volume load-induced diuresis

Atrial natriuretic peptide (ANP)/cGMPs cause diuresis and natriuresis. Their downstream effectors beyond cGMP remain unclear. To elucidate a probable function of cGMP-dependent protein kinase II (cGKII), we investigated renal parameters in different conditions (basal, salt diets, starving, water load) using a genetically modified mouse model (cGKII-KO), but did not detect any striking differences between WT and cGKII-KO. Thus, cGKII is proposed to play only a marginal role in the adjustment of renal concentration ability to varying salt loads without water restriction or starving conditions. When WT mice were subjected to a volume load (performed by application of a 10-mM glucose solution (3% of BW) via feeding needle), they exhibited a potent diuresis. In contrast, urine volume was decreased significantly in cGKII-KO. We showed that AQP2 plasma membrane (PM) abundance was reduced for about 50% in WT upon volume load, therefore, this might be a main cause for the enhanced diuresis. In contrast, cGKII-KO mice almost completely failed to decrease AQP2-PM distribution. This significant difference between both genotypes is not induced by an altered p-Ser256-AQP2 phosphorylation, as phosphorylation at this site decreases similarly in WT and KO. Furthermore, sodium excretion was lowered in cGKII-KO mice during volume load. In summary, cGKII is only involved to a minor extent in the regulation of basal renal concentration ability. By contrast, cGKII-KO mice are not able to handle an acute volume load. Our results suggest that membrane insertion of AQP2 is inhibited by cGMP/cGKII.

The fetal sheep lung does not respond to cortisol infusion during the late canalicular phase of development

The prepartum surge in plasma cortisol concentrations in humans and sheep promotes fetal lung and surfactant system maturation in the support of air breathing after birth. This physiological process has been used to enhance lung maturation in the preterm fetus using maternal administration of betamethasone in the clinical setting in fetuses as young as 24 weeks gestation (term = 40 weeks). Here, we have investigated the impact of fetal intravenous cortisol infusion during the canalicular phase of lung development (from 109- to 116-days gestation, term = 150 ± 3 days) on the expression of genes regulating glucocorticoid (GC) activity, lung liquid reabsorption, and surfactant maturation in the very preterm sheep fetus and compared this to their expression near term. Cortisol infusion had no impact on mRNA expression of the corticosteroid receptors (GC receptor and mineralocorticoid receptor) or HSD11B-2, however, there was increased expression of HSD11B-1 in the fetal lung. Despite this, cortisol infusion had no effect on the expression of genes involved in lung sodium (epithelial sodium channel -α, -β, or -γ subunits and sodium–potassium ATPase-β1 subunit) or water (aquaporin 1, 3, and 5) reabsorption when compared to the level of expression during exposure to the normal prepartum cortisol surge. Furthermore, in comparison to late gestation, cortisol infusion does not increase mRNA expression of surfactant proteins (SFTP-A, -B, and -C) or the number of SFTP-B-positive cells present in the alveolar epithelium, the cells that produce pulmonary surfactant. These data suggest that there may be an age before which the lung is unable to respond biochemically to an increase in fetal plasma cortisol concentrations.

Unimpaired postnatal respiratory adaptation in a preterm human infant with a homozygous ENaC-α unit loss-of-function mutation

The amiloride-sensitive epithelial sodium channel, ENaC, is thought to have a major role in clearing fluid from the alveoli immediately after birth. ENaC-α knockout mice die soon after birth from failure to clear their lungs of liquid. We report on a male infant born after 33 weeks of gestation with uneventful postnatal adaptation (Apgar 9/9/9 at 1, 5 and 10  min after birth) who did not require any respiratory support during his first days of life. At nine days of life, he became lethargic and hyperthermic, displaying low Na(+) (126  mmol l(-1)), high K(+) (8.9  mmol l(-1)), high aldosterone (3000  ng l(-1))and high renin (1000  ng l(-1)) plasma concentrations, commensurate with pseudohypoaldosteronism type I. He was found to be homozygous for the c.1678G>A mutation in the SCNN1A gene that codes for the ENaC-α unit. We conclude that clearance of alveolar fluid after birth in humans does not critically depend on ENaC.

Airway expression of the epithelial sodium channel α-subunit correlates with cortisol in term newborns

BACKGROUND

Glucocorticoids have profound effects on lung maturation and function. In in vitro and animal models, they induce epithelial sodium channels (ENaCs) in the airway epithelium, a process that is important to perinatal lung fluid clearance.

OBJECTIVE

The objective of this study was to determine whether, in newborn infants, airway ENaC expression is associated with cortisol concentrations.

METHODS

Cord blood, saliva, and cells from nasal epithelium were obtained from 69 infants delivered at term. Epithelial and saliva sampling was repeated 3 times: <3, 22 to 29, and 40 to 54 hours postnatally. Cortisol, thyrotropin, and free triiodothyronine concentrations were measured with immunoassays, and expression of α-ENaC and β-ENaC was quantified with real-time reverse-transcriptase polymerase chain reaction.

RESULTS

Expression of α-ENaC <30 minutes postnatally correlated with cord plasma cortisol in infants delivered by elective cesarean delivery. In addition, in the total study population <2 hours postnatally, α-ENaC expression correlated with salivary cortisol concentrations. β-ENaC expression, in contrast, showed no association with cortisol concentrations. A significant decrease in β-ENaC expression during the first postnatal day was revealed, whereas timing of the peak in α-ENaC expression seemed to depend on mode of delivery.

CONCLUSIONS

These results support a role in humans for endogenous glucocorticoids in the regulation of airway ion transport. This finding may be a physiologic mechanism mediating pulmonary adaptation in the newborn infant.

Glucocorticoid activity during lung maturation is essential in mesenchymal and less in alveolar epithelial cells

Corticosteroid treatment is an established therapy for preterm infants, and germline inactivation of the glucocorticoid receptor (GR) gene in the mouse leads to respiratory failure and postnatal lethality. Although glucocorticoids have been thought to critically act in epithelial cells inducing the functional maturation of the lung, inactivation of the GR gene exclusively in the epithelium of the developing murine lung did not impair survival. In contrast, mice lacking GR specifically in mesenchyme-derived cells displayed a phenotype strongly reminiscent of GR knockout animals and died immediately after birth. Detailed analysis of gene expression allows the conclusion that GR acts in cells of the fibroblast lineage controlling their proliferation rate and the composition of the extracellular matrix.

A role for mineralocorticoid receptors in the physiology of the ovine fetus: effects on ACTH and lung liquid composition

In the human and ovine fetus, the presence of 11β-hydroxysteroid dehydrogenase 1 allows cortisol and other corticosteroids to act at mineralocorticoid receptors (MRs) in lung and brain. To test the physiologic role of MRs in the late gestation fetus, fetal lambs were infused with a specific MR antagonist for 12 h. Infusion of the MR antagonist significantly increased plasma ACTH and cortisol concentrations. Infusion of the MR antagonist also significantly increased fetal Pco2 and hematocrit, and decreased fetal pH, but did not alter fetal heart rate or blood pressure. Infusion of the MR antagonist altered the ratio of Na⁺ to K⁺ in lung fluid but did not alter the rate of production of lung liquid or the expression of the epithelial sodium channel α or of the Na,K ATPaseα1 in lung. These results suggest that corticosteroids act at MR to regulate ACTH and blood volume and modulate lung fluid composition in the fetus, but basal levels of corticosteroids do not alter lung liquid production rate through effects on MR.

Regulation of endogenous ENaC functional expression by CFTR and ΔF508-CFTR in airway epithelial cells

The functional expression of the epithelial sodium channel (ENaC) appears elevated in cystic fibrosis (CF) airway epithelia, but the mechanism by which this occurs is not clear. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) alters the trafficking of endogenously expressed human ENaC in the CFBE41o⁻ model of CF bronchial epithelia. Functional expression of ENaC, as defined by amiloride-inhibited short-circuit current (I(sc)) in Ussing chambers, was absent under control conditions but present in CFBE41o⁻ parental and ΔF508-CFTR-overexpressing cells after treatment with 1 μM dexamethasone (Dex) for 24 h. The effect of Dex was mimicked by incubation with the glucocorticoid hydrocortisone but not with the mineralocorticoid aldosterone. Application of trypsin to the apical surface to activate uncleaved, "near-silent" ENaC caused an additional increase in amiloride-sensitive I(sc) in the Dex-treated cells and was without effect in the control cells, suggesting that Dex increased ENaC cell surface expression. In contrast, Dex treatment did not stimulate amiloride-sensitive I(sc) in CFBE41o⁻ cells that stably express wild-type (wt) CFTR. CFBE41o⁻ wt cells also had reduced expression of α- and γ-ENaC compared with parental and ΔF508-CFTR-overexpressing cells. Furthermore, application of trypsin to the apical surface of Dex-treated CFBE41o⁻ wt cells did not stimulate amiloride-sensitive I(sc), suggesting that ENaC remained absent from the surface of these cells even after Dex treatment. We also tested the effect of trafficking-corrected ΔF508-CFTR on ENaC functional expression. Incubation with 1 mM 4-phenylbutyrate synergistically increased Dex-induced ENaC functional expression in ΔF508-CFTR-overexpressing cells. These data support the hypothesis that wt CFTR can regulate the whole cell, functional, and surface expression of endogenous ENaC in airway epithelial cells and that absence of this regulation may foster ENaC hyperactivity in CF airway epithelia.

Reduced viability of mice with lung epithelial-specific knockout of glucocorticoid receptor

Glucocorticoid (GC)-responsive epithelial-mesenchymal interactions regulate lung development. The GC receptor (GR) mediates GC signaling. Mice lacking GR in all tissues die at birth of respiratory failure. To determine the specific need for epithelial GR in lung development, we bred triple transgenic mice that carry SPC/rtTA, tet-O-Cre, and floxed, but not wild-type, GR genes. When exposed to doxycycline in utero, triple transgenic (GRepi⁻) mice exhibit a Cre-mediated recombination event that inactivates the floxed GR gene in airway epithelial cells. Immunofluorescence confirmed the elimination of GR in Cre-positive airway epithelial cells of late gestation GRepi⁻ mice. Embryonic Day 18.5 pups had a relatively immature appearance with increased lung cellularity and increased pools of glycogen in the epithelium. Postnatal Day 0.5 pups had decreased viability. We used quantitative RT-PCR to demonstrate that specific elimination of epithelial immunoreactive GR in GRepi⁻ mice is associated with reduced mRNA expression for surfactant proteins (SPs) A, B, C, and D; β- and γ-ENaC; T1α; the 10-kD Clara cell protein (CCSP); and aquaporin 5 (AQP5). Western blots confirmed reduced levels of AQP5 protein. No reduction in the levels of the GR transport protein importin (IPO)-13 was observed. Our findings demonstrate a requirement for lung epithelial cell GR in normal lung development. We speculate that impaired epithelial differentiation, leading to decreased SPs, transepithelial Na, and liquid absorption at birth, may contribute to the reduced survival of newborn mice with suppressed lung epithelial GR.

Role of calcineurin-mediated dephosphorylation in modulation of an inwardly rectifying K+ channel in human proximal tubule cells

Activity of an inwardly rectifying K(+) channel with inward conductance of about 40 pS in cultured human renal proximal tubule epithelial cells (RPTECs) is regulated at least in part by protein phosphorylation and dephosphorylation. In this study, we examined involvement of calcineurin (CaN), a Ca(2+)/calmodulin (CaM)-dependent phosphatase, in modulating K(+) channel activity. In cell-attached mode of the patch-clamp technique, application of a CaN inhibitor, cyclosporin A (CsA, 5 microM) or FK520 (5 microM), significantly suppressed channel activity. Intracellular Ca(2+) concentration ([Ca(2+)]( i )) estimated by fura-2 imaging was elevated by these inhibitors. Since inhibition of CaN attenuates some dephosphorylation with increase in [Ca(2+)]( i ), we speculated that inhibiting CaN enhances Ca(2+)-dependent phosphorylation, which might result in channel suppression. To verify this hypothesis, we examined effects of inhibitors of PKC and Ca(2+)/CaM-dependent protein kinase-II (CaMKII) on CsA-induced channel suppression. Although the PKC inhibitor GF109203X (500 nM) did not influence the CsA-induced channel suppression, the CaMKII inhibitor KN62 (20 microM) prevented channel suppression, suggesting that the channel suppression resulted from CaMKII-dependent processes. Indeed, Western blot analysis showed that CsA increased phospho-CaMKII (Thr286), an activated CaMKII in inside-out patches, application of CaM (0.6 microM) and CaMKII (0.15 U/ml) to the bath at 10(-6) M Ca(2+) significantly suppressed channel activity, which was reactivated by subsequent application of CaN (800 U/ml). These results suggest that CaN plays an important role in supporting K(+) channel activity in RPTECs by preventing CaMKII-dependent phosphorylation.

Regulation of Epithelial Na+ Channel (ENaC) in the Salivary Cell Line SMG-C6

Glucocorticoids and mineralocorticoids modulate Na+ transport via epithelial Na+ channels (ENaC). The rat submandibular epithelial cell line, SMG-C6, expresses α-ENaC mRNA and protein and exhibits amiloride-sensitive Na+ transport when grown in low-serum (2.5%) defined medium, therefore, we examined the effects of altering the composition of the SMG-C6 cell growth medium on ENaC expression and function. No differences in basal or amiloride-sensitive short-circuit current (Isc) were measured across SMG-C6 monolayers grown in the absence of thyroid hormone, insulin, transferrin, or EGF. In the absence of hydrocortisone, basal and amiloride-sensitive Isc significantly decreased. Similarly, monolayers grown in 10% serum-supplemented medium had lower basal Isc and no response to amiloride. Adding hydrocortisone (1.1 μM) to either the low or 10% serum medium increased basal and amiloride-sensitive Isc, which was blocked by RU486, the glucocorticoid and progesterone receptor antagonist. Aldosterone also induced an increase in α-ENaC expression and Na+ transport, which was also blocked by RU486 but not by the mineralocorticoid receptor antagonist spironolactone. Thus, in the SMG-C6 cell line, hydrocortisone and aldosterone increased ENaC expression and basal epithelial Na+ transport. The absence of endogenous ENaC expression in culture conditions devoid of steroids makes the properties of this cell line an excellent model for investigating pathways regulating ENaC expression and Na+ transport.

Is the fetal lung a mineralocorticoid receptor target organ? Induction of cortisol-regulated genes in the ovine fetal lung, kidney and small intestine

BACKGROUND

Lung, kidney and small intestine are involved in fetal volume regulation and amniotic fluid secretion and play a pivotal role in the transition from intrauterine to extrauterine life.

OBJECTIVE

This study was performed to determine the ontogeny of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), and of MR- and GR-regulated genes and proteins, serum and glucocorticoid-induced kinase (Sgk-1), epithelial sodium channel (ENaC alpha), and Na,K-ATPase alpha1.

METHODS

Lung, renal cortex and medulla, and small intestine were collected from fetuses at 80, 100, 120, 130 and 145 days' gestation and from day 1 and 7 neonatal lambs. Real-time PCR was performed to determine mRNA concentration for MR, GR, the 11 beta-hydroxysteroid dehydrogenases (11 beta-HSD1 and 2), Sgk-1, ENaC alpha, and Na,K-ATPase alpha1. Protein expression of ENaC alpha and Na,K-ATPase alpha1 in whole cell and membrane fractions was determined by immunoblotting.

RESULTS

Expression of corticosteroid-induced genes in renal cortex increases at term; in small intestine the induction occurs postnatally. In contrast, in lung expression of MR and GR mRNAs were greater at 100 days to term than postnatally and 11 beta-HSD1 peaked at 145 days; the corticosteroid-induced genes also increased prenatally: Sgk-1 and ENaC alpha increased by 120 days, peaking at 145 days, and Na,K-ATPase alpha1 was greatest at 130 days.

CONCLUSIONS

The expression of high levels of MR and 11 beta-HSD1 in preterm fetal lung suggest low endogenous fetal cortisol may exert actions at the high affinity MR in vivo, leading to increases in expression of sodium channels important in the regulation of lung liquid secretion and reabsorption.

Discordant effects of corticosteroids and expression of subunits on ENaC activity

In renal distal nephron and airway epithelial cells, adrenocortical steroids increase epithelial Na+ channel (ENaC) activity and also markedly increase the expression of the α-subunit. The present experiments were designed to reconstitute this steroid effect in ENaC-expressing cells by overexpressing the subunits whose expression is enhanced by corticosteroids. In renal collecting duct monolayers, corticosteroids increased ENaC activity 5- to 8-fold, endogenous α-ENaC mRNA and protein ∼10-fold, and β-ENaC protein and mRNA 1.2- to 2-fold. γ-ENaC expression was unchanged. To determine whether this increase in expression was sufficient to increase ENaC activity, we used a regulated adenovirus system to increase expression of each subunit alone and in combination. Unexpectedly, increased expression of the α- and/or β-subunit had no effect on ENaC activity in collecting duct cells or lung epithelial cells. In contrast, a small increase in γ-ENaC expression increased ENaC activity about threefold. This increase in activity was additive to the effect of steroids. Thus, even though corticosteroids strongly increase α-ENaC expression and moderately increase β-ENaC expression, these effects are not, by themselves, sufficient to increase ENaC activity. Knockdown experiments are consistent with the idea that the increased expression of α-ENaC is necessary for the full steroid effect on ENaC. Increased expression of γ-ENaC and corticosteroid treatment enhances ENaC activity by parallel, noninteracting pathways. These results underscore the importance of other actions of steroid hormones for long-term enhancement of ENaC activity and raise new possibilities for regulation of ENaC activity by γ-ENaC expression.

Steroid-mediated regulation of the epithelial sodium channel subunits in mammary epithelial cells

The epithelial sodium channel (ENaC) is a key mediator of sodium transport in epithelia; however, little is known about ENaC expression in mammary epithelia. Using real-time PCR, we demonstrated the expression of the ENaC subunit mRNAs in mouse and human mammary cell lines and in vivo mouse mammary tissue. We determined the effects of glucocorticoids, progesterone, and prolactin on ENaC expression in four mammary cell lines. Dexamethasone induced all detectable ENaC subunits in noncancerous cell lines, HC11 and MCF10A. Interestingly, in cancerous cell lines (T-47D and MCF-7), both beta- and gamma- but not alphaENaC mRNAs were induced by dexamethasone. Progesterone induced ENaC mRNA only in T-47D cells, and prolactin had no effects. gammaENaC was rapidly induced by steroids, whereas induction of alpha- and betaENaC was slower; moreover, the induction of the beta-subunit required de novo protein synthesis. Dexamethasone treatment did not affect ENaC mRNA stability. Western blot analysis revealed immunoreactive bands corresponding to different forms of alpha-, beta-, and gammaENaC; dexamethasone significantly increased the intensity of alphaENaC (85 kDa) and betaENaC (90 kDa). We also showed an in vivo reduction in alphaENaC levels in the mammary tissue of lactating mice as compared with controls, whereas beta- and gammaENaC mRNA levels were significantly increased. Furthermore, dexamethasone in vivo significantly increased alpha-, beta-, and gammaENaC mRNA expression. Our data indicate that both mouse and human mammary cells express all ENaC subunits, and they are regulated by steroid hormones in a temporal and cell-specific manner both in culture and in vivo.

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.

Prenatal glucocorticoids improve lung morphology and partially restores surfactant mRNA expression in lambs with diaphragmatic hernia undergoing fetal tracheal occlusion

In fetal sheep with surgically created diaphragmatic hernia (DH), tracheal occlusion (TO) can restore lung growth but does not ameliorate the increase in inter-alveolar wall thickness (T(W)). We determined whether prenatal exposure to glucocorticoids (GC) could reduce T(w) in fetuses with DH undergoing TO. At 65 days of gestation, DH was created in 12 fetal sheep, and TO subsequently performed at 110 days (DH/TO). Six of these fetuses were exposed to betamethasone (DH/TO + GC; 0.5 mg/kg; maternal, IM) 48 hr before delivery; Sham operated fetuses (n = 7) served as controls. At 139 days, we measured alveolar surface density (S(V)), parenchymal tissue fraction, T(W), alveolar type 2 (AE2) cell density and lung surfactant protein (SP) mRNA expression. Prenatal GC decreased T(W) and S(V) by 33% and 27% respectively, and increased fixed lung volume (by 55%), AE2 cell density and partially restored SPmRNA expression. Our data indicate that prenatal exposure to GC can reverse some of the negative effects of prolonged fetal TO. We hypothesize that a GC-induced reduction in lung liquid volume during TO contributes, in part, to the observed increase in AE2 cell density and SPmRNA expression.

Mice heterozygous for beta-ENaC deletion have defective potassium excretion

The present studies were designed to determine whether mice heterozygous for deletion of beta-ENaC exhibited defects in Na+/K+ transport and blood pressure regulation. In response to an acute KCl infusion, +/-mice developed higher serum [K+] and excreted only 40% of the K+ excreted by +/+mice. After 6 days on a low (0.01%)-Na+ diet, the cumulative Na+ excretion from days 3-6 was greater for +/-mice. This low-Na+ diet caused higher serum [K+] and lower K+ excretion rates in +/-mice than in +/+mice, but the rectal potential differences were not different. Analyses of mRNA from mice on this diet showed the expected approximately 50% reduction of beta-ENaC in kidney and colon of +/-mice. Unexpectedly, the level of gamma-ENaC mRNA was similarly reduced. NHE3 mRNA was approximately 30% higher in +/-mice whereas mRNA of the Na-K-2Cl cotransporter was not different. Also unexpectedly, the amount of beta-ENaC proteins was similar in both groups of mice but there was a reduction of one form of gamma-ENaC in +/-mice. These experiments demonstrate that mice heterozygous for beta-ENaC have a small but detectable defect in their ability to conserve Na+ and a more readily apparent defect in the ability to secrete K+.

C-type natriuretic peptide and its receptor are downregulated in pulmonary epithelium following birth

C-type natriuretic peptide (CNP) is a member of the natriuretic peptide family and acts through the membrane bound guanylyl cyclase linked natriuretic peptide receptor B (NPR-B) to increase intracellular cGMP. Activation of the CNP/NPR-B pathway in pulmonary epithelium has been linked to the inhibition of amiloride-sensitive sodium absorption and to the stimulation of the cystic fibrosis transmembrane conductance regulator (CFTR). Given the importance of ion movement across the pulmonary epithelium of the fetal and newborn lung, we sought to examine the expression of CNP and NPR-B in pulmonary epithelium of the developing fetal lamb and following the transition to air breathing. Lambs were sacrificed at 100 and 136 days of gestation and at 3 days, and 4 weeks after full term delivery. Lung sections were immunostained for CNP and NPR-B. At 100 days of gestation, staining for CNP and NPR-B was absent within all pulmonary epithelium. At 136 days of gestation, prominent staining for both CNP and NPR-B was seen within alveolar type II cells, non-ciliated cells of the distal airways (Clara cells), and ciliated epithelium of the upper airways. At both 3 days and 4 weeks following birth, staining for CNP and NPR-B was absent in alveolar type II cells, ciliated bronchial epithelium and was markedly reduced in Clara cells. The presence of CNP and NPR-B within the pulmonary epithelium in the nearterm fetal period and its rapid downregulation following birth suggests that CNP may contribute to the maintenance of the fluid-filled lung through the regulation of trans-epithelial ion flux.

Sp1: regulation of gene expression by phosphorylation

As the prototype of a family of transcription factors, Sp1 has been extensively studied and widely reported for its role in gene regulation. The first evidence of Sp1 phosphorylation was reported more than a decade ago. Since then, an increasing number of Sp1 phosphorylation events have been characterized. Recent data demonstrate an important role for the phosphorylation state of Sp1 in the regulation of multiple genes. In this article, we review published literature in four specific areas relating to the phosphorylation of Sp1: (1) signal transduction pathways for Sp1 phosphorylation, (2) mechanisms of Sp1 dephosphorylation, (3) the functional implications of Sp1 phosphorylation, and (4) Sp1 phosphorylation in the lung.

Estrogen and progesterone regulate alpha, beta, and gammaENaC subunit mRNA levels in female rat kidney

BACKGROUND

Estrogen and progesterone regulate alpha, beta, and gamma amiloride-sensitive epithelial sodium channel (ENaC) subunit mRNA levels in female rat kidney. Renal Na(+) handling differs between males and females. Further, within females Na(+) metabolism changes during the menstrual cycle and pregnancy. Electrolyte homeostasis and extracellular fluid volume are maintained primarily by regulated transport of Na(+) via the amiloride-sensitive Na(+) channel. This study examines the role of the female gender steroids in the regulation of expression of ENaC.

METHODS

We measured ENaC subunit mRNA levels in rat kidney using Northern blotting. Kidneys were taken from male and females at different ages and from adult ovariectomized rats treated with 17-beta-estradiol benzoate (estrogen) and/or progesterone for 8 or 24 hours.

RESULTS

The abundance of alpha, beta, and gammaENaC mRNA was significantly higher in female compared to male rat kidneys from 10 weeks of age (P= 0.001, P= 0.004, and P= 0.02, N= 10, respectively). These differences were abolished in ovariectomized rats. Treatment of ovariectomized rats with estrogen increased alphaENaC mRNA abundance in the kidney at both 8 and 24 hours (P < 0.05, N= 6; and P < 0.05, N= 7, respectively). Progesterone inhibited the effect of estrogen on alphaENaC mRNA at 8 hours but when given alone increased gammaENaC mRNA (P < 0.05, N= 3). Neither hormone, alone or in combination, had any significant effect on betaENaC mRNA levels at 8 or 24 hours.

CONCLUSION

Female gonadal steroids differentially modulate expression of ENaC subunit mRNA in the rat kidney.

The renin-angiotensin system in kidney development: role of COX-2 and adrenal steroids

Recent data from studies in rodents with targeted gene disruption and pharmacological antagonists have shown that the renin-angiotensin-aldosterone system (RAAS) and cyclooxygenase type-2 (COX-2) are necessary for late stages of kidney development. The present review summarizes data on the developmental changes of RAAS and COX-2 and the pathways by which they are activated; their possible interplay and the mechanisms by which they affect kidney development. Intrarenal and circulating renin and angiotensin II (ANG II) are stimulated at birth in most mammals. In rats, renin and ANG II stay significantly elevated in the suckling period while aldosterone stabilizes at an adult level. COX-2 is stimulated in thick ascending limb of Henle's loop in the suckling period at a time when urine concentrating ability is not developed. Data suggest that this induction is mediated by combined low plasma glucocorticoid concentration and by a low NaCl intake. Studies with selective inhibitors of COX-2 and COX-2 null mice show that COX-2 activity stimulates renin secretion from JG-cells during postnatal kidney development and that lack of COX-2 activity leads to pathological change in cortical architecture and eventually to renal failure. In the postnatal period, ANG II initiates and maintains pelvic and ureteric contractions necessary for urine flow. Lack of ANG II in the neonatal period is thought to cause injury by a chronic increase of renal pelvic pressure. Aldosterone is crucial for survival and growth in the neonatal period through its effects on sodium reabsorption and the intrarenal sensitivity to aldosterone is increased in the postnatal period. Final maturation of the kidney occurs through an intimate interplay between a low dietary sodium intake and a non-responsive HPA-axis which stimulates cortical COX-2 activity. COX-2 supports increased activity of the RAAS and may contribute to a low concentrating ability.

Molecular determinants of sodium and water balance during early human development

The past decade has seen enormous progress in understanding the renal regulation of salt and water homeostasis. Most of the key transporters have been cloned, and their physiological importance has been revealed from studies of children with inherited diseases and from mutagenesis studies on a cellular level. We are beginning to understand the complexity with which the activity of these transporters is regulated by hormones. Studies on experimental animals have uniformly shown that the majority of renal salt and water transporters undergo profound changes in the postnatal period. There is generally a robust increase in the number of transporters expressed in a single tubular cell. Many of the transporters also shift their expression from one isoform to another with a somewhat different function. The short-term regulation of salt and water transporters, the key to a well-functioning homeostatic system, is often blunted in the early postnatal period. Taken together, these findings explain some phenomena well known in infants. The low urinary concentrating capacity can, for example, be at least partially attributed to immaturity of the expression of water channels, sodium losses in preterm infants to low expression of the energy generator for salt transport, Na(+),K(+)-ATPase, and the disposition to acidosis to immaturity of the Na(+)/H(+)exchanger. We propose that further studies on how these transporters are regulated will lead to the improved prevention and treatment of salt water balance disorders in infants.

Low endogenous glucocorticoid allows induction of kidney cortical cyclooxygenase-2 during postnatal rat development

In postnatal weeks 2-4, cyclooxygenase-2 (COX-2) is induced in the rat kidney cortex where it is critically involved in final stages of kidney development. We examined whether changes in circulating gluco- or mineralocorticosteroids or in their renal receptors regulate postnatal COX-2 induction. Plasma corticosterone concentration peaked at birth, decreased to low levels at days 3-13, and increased to adult levels from day 22. Aldosterone peaked at birth and then stabilized at adult levels. Gluco- and mineralocorticoid receptor (GR and MR) mRNAs were expressed stably in kidney before, during, and after COX-2 induction. 11 beta-hydroxysteroid dehydrogenase 2 was induced shortly after birth and was widely distributed in the whole collecting duct system in the suckling period and then returned to an adult pattern. Supplementation with corticosterone (20 mg.kg-1.day-1) or GR-specific dexamethasone (1 mg.kg-1.day-1) during low endogenous corticosterone suppressed renal COX-2 mRNA and protein and led to a restricted distribution of COX-2 immunolabeling. The ability of glucocorticoids to affect COX-2 was reflected in colocalization of GR-alpha and COX-2 immunoreactivity and mRNAs in thick ascending limb of Henle's loop. The MR antagonist potassium canrenoate (20 mg.kg-1.day-1) enhanced COX-2 expression from days 5 to 10, but low MR-specific concentrations of DOCA (1 mg.kg-1.day-1) had no effect on COX-2. Renomedullary interstitial cells expressed GR-alpha and COX-2. Dexamethasone suppressed COX-2 in these cells. Thus low plasma concentrations of corticosterone allowed for cortical and medullary COX-2 induction during postnatal kidney development. Increased circulating glucocorticoid in the postnatal period may damage late renal development through inhibition of COX-2.

Increased collecting duct urea transporter expression in Dahl salt-sensitive rats

Because abnormalities of inner medullary function have been proposed in Dahl salt-sensitive (DS) rats vs. salt-resistant (DR) rats, we performed transporter profiling by semiquantitative immunoblotting to determine whether specific solute transporter abundances are altered in inner medullas of DS rats vs. DR rats. Although none of the expressed Na transporters were upregulated in the inner medullas of DS rats compared with DR rats, there were marked increases in the protein abundances of the collecting duct urea transporters UT-A1 (to 212% of DR) and UT-A3 (to 223% of DR). These differences were confirmed by immunocytochemistry. Quantitative real-time RT-PCR showed higher mRNA abundance in DS rats for both UT-A1 (to 256% of DR) and UT-A3 (to 210% of DR). In isolated, perfused inner medullary collecting ducts, urea permeability was significantly greater in DS rats. Because both UT-A1 and UT-A3 are transcriptionally regulated by glucocorticoids, we measured both plasma corticosterone levels and inner medullary 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) abundances. Although the plasma corticosterone concentrations were not different between DS and DR rats, immunoblotting and immunocytochemistry revealed a marked elevation of 11beta-HSD2 abundance in DS rats. Consistent with the view that an elevated 11beta-HSD2 level is responsible for increased urea transporter expression in the inner medullary collecting duct, administration of the 11beta-HSD2 inhibitor carbenoxolone to DS rats decreased the abundances of UT-A1 and UT-A3 to levels similar to those seen in DR rats.

Expression of alpha-ENaC2 is dependent on an upstream Sp1 binding motif and is modulated by protein phosphatase 1 in lung epithelial cells

The amiloride-sensitive Na(+) channel ENaC is expressed in lung epithelium and plays a pivotal role in lung fluid clearance in the newborn. Multiple splice variants of the ENaC alpha-subunit have been reported. Among them, alpha-ENaC2 accounts for a considerable portion of alpha-ENaC transcripts in human lung and kidney, possesses channel functions similar to alpha-ENaC1, and is driven by a downstream promoter. In the current study, we examine the regulation of alpha-ENaC2 transcription in lung epithelial cells. We found that transcription factors Sp1 and Sp3 activate alpha-ENaC2 transcription through a GC-rich element (Sp1-binding site) in the promoter. Because alpha-ENaC expression and Sp1 phosphorylation are both significantly up-regulated in the perinatal lung, we then examined the possible connection between Sp1/Sp3 phosphorylation and alpha-ENaC2 expression. We found that protein phosphatase 1 (PP1) dephosphorylates Sp1 and Sp3 in lung epithelial cells, reduces their binding to the alpha-ENaC2 promoter, and decreases Sp1/Sp3-mediated promoter activity. Our results suggest that Sp1 and Sp3 are essential for alpha-ENaC2 transcription in lung epithelial cells and that dephosphorylation of the Sp transcription factors by PP1 suppresses alpha-ENaC2 expression. The significance of these findings in the regulation of gene expression in perinatal lung is discussed.