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

Inactivation of epithelial sodium ion channel molecules serves as effective diagnostic biomarkers in clear cell renal cell carcinoma

BACKGROUND

Non-voltage-gated sodium channel, also known as the epithelial sodium channel (ENaC), formed by heteromeric complexes consisting of SCNN1A, SCNN1B, and SCNN1G, is responsible for maintaining sodium ion and body fluid homeostasis in epithelial cells. However, no systematic study of SCNN1 family members has been conducted in renal clear cell carcinoma (ccRCC) to date.

OBJECTIVE

To investigate the abnormal expression of SCNN1 family in ccRCC and its potential correlation with clinical parameters.

METHODS

The transcription and protein expression levels of SCNN1 family members in ccRCC were analyzed based on the TCGA database, and were confirmed by quantitative RT-PCR and immunohistochemical staining assays, respectively. The area under curve (AUC) was used to evaluate the diagnostic value of SCNN1 family members for ccRCC patients.

RESULTS

The mRNA and protein expression of SCNN1 family members was significantly downregulated in ccRCC compared with normal kidney tissues, which might be due to DNA hypermethylation in the promoter region. It is worth noting that the AUC of SCNN1A, SCNN1B, and SCNN1G were 0.965, 0.979, and 0.988 based on the TCGA database (p < 0.0001), respectively. The diagnostic value was even higher when combing these three members together (AUC = 0.997, p < 0.0001). Intriguingly, the mRNA level of SCNN1A was significantly lower in females compared with males, while SCNN1B and SCNN1G were increased with the progression of ccRCC and remarkably associated with a worse outcome for patients.

CONCLUSION

The aberrantly decrease of SCNN1 family members might serve as valuable biomarkers for the diagnosis of ccRCC.

Submersion and hypoxia inhibit alveolar epithelial Na+ transport through ERK/NF-κB signaling pathway

BACKGROUND

Hypoxia is associated with many respiratory diseases, partly due to the accumulation of edema fluid and mucus on the surface of alveolar epithelial cell (AEC), which forms oxygen delivery barriers and is responsible for the disruption of ion transport. Epithelial sodium channel (ENaC) on the apical side of AEC plays a crucial role to maintain the electrochemical gradient of Na⁺ and water reabsorption, thus becomes the key point for edema fluid removal under hypoxia. Here we sought to explore the effects of hypoxia on ENaC expression and the further mechanism related, which may provide a possible treatment strategy in edema related pulmonary diseases.

METHODS

Excess volume of culture medium was added on the surface of AEC to simulate the hypoxic environment of alveoli in the state of pulmonary edema, supported by the evidence of increased hypoxia-inducible factor-1 expression. The protein/mRNA expressions of ENaC were detected, and extracellular signal-regulated kinase (ERK)/nuclear factor κB (NF-κB) inhibitor was applied to explore the detailed mechanism about the effects of hypoxia on epithelial ion transport in AEC. Meanwhile, mice were placed in chambers with normoxic or hypoxic (8%) condition for 24 h, respectively. The effects of hypoxia and NF-κB were assessed through alveolar fluid clearance and ENaC function by Ussing chamber assay.

RESULTS

Hypoxia (submersion culture mode) induced the reduction of protein/mRNA expression of ENaC, whereas increased the activation of ERK/NF-κB signaling pathway in parallel experiments using human A549 and mouse alveolar type 2 cells, respectively. Moreover, the inhibition of ERK (PD98059, 10 µM) alleviated the phosphorylation of IκB and p65, implying NF-κB as a downstream pathway involved with ERK regulation. Intriguingly, the expression of α-ENaC could be reversed by either ERK or NF-κB inhibitor (QNZ, 100 nM) under hypoxia. The alleviation of pulmonary edema was evidenced by the administration of NF-κB inhibitor, and enhancement of ENaC function was supported by recording amiloride-sensitive short-circuit currents.

CONCLUSIONS

The expression of ENaC was downregulated under hypoxia induced by submersion culture, which may be mediated by ERK/NF-κB signaling pathway.

Regulation of hyperoxia-induced neonatal lung injury via post-translational cysteine redox modifications

Preterm infants and patients with lung disease often have excess fluid in the lungs and are frequently treated with oxygen, however long-term exposure to hyperoxia results in irreversible lung injury. Although the adverse effects of hyperoxia are mediated by reactive oxygen species, the full extent of the impact of hyperoxia on redox-dependent regulation in the lung is unclear. In this study, neonatal mice overexpressing the beta-subunit of the epithelial sodium channel (β-ENaC) encoded by Scnn1b and their wild type (WT; C57Bl6) littermates were utilized to study the pathogenesis of high fraction inspired oxygen (FiO2)-induced lung injury. Results showed that O2-induced lung injury in transgenic Scnn1b mice is attenuated following chronic O2 exposure. To test the hypothesis that reversible cysteine-redox-modifications of proteins play an important role in O2-induced lung injury, we performed proteome-wide profiling of protein S-glutathionylation (SSG) in both WT and Scnn1b overexpressing mice maintained at 21% O2 (normoxia) or FiO2 85% (hyperoxia) from birth to 11-15 days postnatal. Over 7700 unique Cys sites with SSG modifications were identified and quantified, covering more than 3000 proteins in the lung. In both mouse models, hyperoxia resulted in a significant alteration of the SSG levels of Cys sites belonging to a diverse range of proteins. In addition, substantial SSG changes were observed in the Scnn1b overexpressing mice exposed to hyperoxia, suggesting that ENaC plays a critically important role in cellular regulation. Hyperoxia-induced SSG changes were further supported by the results observed for thiol total oxidation, the overall level of reversible oxidation on protein cysteine residues. Differential analyses reveal that Scnn1b overexpression may protect against hyperoxia-induced lung injury via modulation of specific processes such as cell adhesion, blood coagulation, and proteolysis. This study provides a landscape view of protein oxidation in the lung and highlights the importance of redox regulation in O2-induced lung injury.

Effects of Hypoxia on Respiratory Diseases: Perspective View of Epithelial Ion Transport

The balance of gas exchange and lung ventilation is essential for the maintenance of body homeostasis. There are many ion channels and transporters in respiratory epithelial cells, including epithelial sodium channel, Na,K-ATPase, cystic fibrosis transmembrane conductance regulator, and some transporters. These ion channels/transporters maintain the capacity of liquid layer on the surface of respiratory epithelial cells, and provide an immune barrier for the respiratory system to clear off foreign pathogens. However, in some harmful external environment and/or pathological conditions, the respiratory epithelium is prone to hypoxia, which would destroy the ion transport function of the epithelium and unbalance the homeostasis of internal environment, triggering a series of pathological reactions. Many respiratory diseases associated with hypoxia manifest an increased expression of hypoxia-inducible factor-1, which mediates the integrity of the epithelial barrier and affects epithelial ion transport function. It is important to study the relationship between hypoxia and ion transport function, whereas the mechanism of hypoxia-induced ion transport dysfunction in respiratory diseases is not clear. This review focuses on the relationship of hypoxia and respiratory diseases, as well as dysfunction of ion transport and tight junctions in respiratory epithelial cells under hypoxia.

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.

The effect of labour pain in caesarean delivery on neonatal and maternal outcomes in a term low-risk obstetric population

Planned caesarean delivery (CD) frequency is increasing in many countries, and elective CD accounts for 50% of preventable causes of neonatal respiratory morbidity. The clearing of fluid in the lungs is important for the establishment of normal gas exchange with inspired air. A defect in this clearance can result in respiratory problems in neonate, but on the other hand patients waiting for the onset of spontaneous labour pain, the increase in emergency CD, together with severe mortality and morbidity are causes of concern and cause anxiety and distrust of obstetricians. In the current study, it was observed that foetal respiratory morbidity was reduced in the group with labour pain. The caesarean applied after the onset of labour pain could be a reason for the significant increase in foetal cord cortisol values in term low-risk neonates. A major advantage of this study was the inclusion of a low-risk patient group, and the increase in foetal cortisol secondary to pain in term neonates was shown objectively. Impact statement What is already known on this subject: The timing of CD is reported to be critical for neonatal morbidity, and elective CD accounts for 50% of preventable causes of neonatal respiratory morbidity. The clearing of fluid in the lungs is important for the establishment of normal gas exchange with inspired air. Cortisol secretion from the foetal adrenal gland is stimulated by stress. What do the results of this study add: Labour pain-related stress in the foetus might be a cause for elevated physiological cortisol, and secondary to that, lower respiratory morbidity. Caesarean delivery commenced after the onset of labour pain could be a reason for the significant increase in foetal cord cortisol values in term neonates. A major advantage of this study was the inclusion of a low-risk patient group, and the increase in foetal cortisol secondary to pain in term neonates was shown objectively. What are the implications of these findings for clinical practice and/or further research: Although it would seem to be rational to perform a caesarean section after the onset of labour pain in cases with a planned caesarean and a history of C-section delivery, with respect to maternal complications, especially in those with a history of more than one caesarean or those who have started active labour; there is a need for further prospective studies.

Increased TMEM16A Involved in Alveolar Fluid Clearance After Lipopolysaccharide Stimulation

Transmembrane protein 16A (TMEM16A) regulates a wide variety of cellular activities, including epithelial fluid secretion and maintenance of ion homeostasis. Lipopolysaccharide (LPS), an outer membrane component of Gram-negative bacteria, is one of the major causes of acute lung injury (ALI). In this study, we investigated the effects of LPS on the expression of TMEM16A in LA795 cells and mouse lung tissue and the potential mechanism.

RESULT

We detected the expression of TMEM16A in LA795 cells and mouse lung tissue by RT-PCR, Western blot, and RNA interference techniques. TMEM16A expression was significantly increased by LPS stimulation in LA795 cells and in mouse lung tissue. Moreover, the LPS-induced TMEM16A expression enhancement in lung tissue was much more prominent in the alveolar epithelial region than in bigger airway epithelial cells. The typical TMEM16A current was recorded, and LPS treatment significantly enhances the current amplitude in LA795 cells. TMEM16A shRNA or TMEM16A inhibitor (T16Ainh-A01) did not affect alveolar fluid clearance (AFC), while co-application of T16Ainh-A01 induced a stronger AFC inhibition than LPS alone. LPS notably and synchronously enhanced Akt phosphorylation (p-Akt) and TMEM16A expression in a time-dependent manner in LA795 cells. Taken together, our results suggest that TMEM16A maybe plays an important role in pathological conditions of LPS-induced ALI as a protective protein.

ENaC/DEG in Tumor Development and Progression

The epithelial Na+ channel/degenerin (ENaC/DEG) superfamily, including the acid-sensing ion channels (ASICs), is characterized by a high degree of similarity in structure but highly diverse in physiological functions. These ion channels have been shown to be important in several physiological functions of normal epithelial cells, including salt homeostasis, fluid transportation and cell mobility. There is increasing evidence suggesting that ENaC/DEG channels are critically engaged in cancer cell biology, such as proliferation, migration, invasion and apoptosis, playing a role in tumor development and progression. In this review, we will discuss recent studies showing the role of ENaC and ASIC channels in epithelial cells and its relationship to the oncogenesis.

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.

Nadph oxidase regulates alveolar epithelial sodium channel activity and lung fluid balance in vivo via O⁻₂ signaling

To define roles for reactive oxygen species (ROS) and epithelial sodium channel (ENaC) in maintaining lung fluid balance in vivo, we used two novel whole animal imaging approaches. Live X-ray fluoroscopy enabled quantification of air space fluid content of C57BL/6J mouse lungs challenged by intratracheal (IT) instillation of saline; results were confirmed by using conventional lung wet-to-dry weight ratios and Evans blue as measures of pulmonary edema. Visualization and quantification of ROS produced in lungs was performed in mice that had been administered a redox-sensitive dye, hydro-Cy7, by IT instillation. We found that inhibition of NADPH oxidase with a Rac-1 inhibitor, NSC23766, resulted in alveolar flooding, which correlated with a decrease in lung ROS production in vivo. Consistent with a role for Nox2 in alveolar fluid balance, Nox2(-/-) mice showed increased retention of air space fluid compared with wild-type controls. Interestingly, fluoroscopic analysis of C57BL/6J lungs IT instilled with LPS showed an acute stimulation of lung fluid clearance and ROS production in vivo that was abrogated by the ROS scavenger tetramethylpiperidine-N-oxyl (TEMPO). Acute application of LPS increased the activity of 20 pS nonselective ENaC channels in rat type 1 cells; the average number of channel and single-channel open probability (NPo) increased from 0.14 ± 0.04 to 0.62 ± 0.23. Application of TEMPO to the same cell-attached recording caused an immediate significant decrease in ENaC NPo to 0.04 ± 0.03. These data demonstrate that, in vivo, ROS has the capacity to stimulate lung fluid clearance by increasing ENaC activity.

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.

Aldosterone and vasopressin affect {alpha}- and {gamma}-ENaC mRNA translation

Vasopressin and aldosterone play key roles in the fine adjustment of sodium and water re-absorption in the nephron. The molecular target of this regulation is the epithelial sodium channel (ENaC) consisting of α-, β- and γ-subunits. We investigated mRNA-specific post-transcriptional mechanisms in hormone-dependent expression of ENaC subunits in mouse kidney cortical collecting duct cells. Transcription experiments and polysome gradient analysis demonstrate that both hormones act on transcription and translation. RNA-binding proteins (RBPs) and mRNA sequence motifs involved in translational control of γ-ENaC synthesis were studied. γ-ENaC–mRNA 3′-UTR contains an AU-rich element (ARE), which was shown by RNA affinity chromatography to interact with AU-rich element binding proteins (ARE-BP) like HuR, AUF1 and TTP. Some RBPs co-localized with γ-ENaC mRNA in polysomes in a hormone-dependent manner. Reporter gene co-expression experiments with luciferase γ-ENaC 3′-UTR constructs and ARE-BP expression plasmids demonstrate the importance of RNA–protein interaction for the up-regulation of γ-ENaC synthesis. We document that aldosterone and the V₂ receptor agonist dDAVP act on synthesis of α- and γ-ENaC subunits mediated by RBPs as effectors of translation but not by mRNA stabilization. Immunoprecipitation and UV-crosslinking analysis of γ-ENaC–mRNA/HuR complexes document the significance of γ-ENaC–mRNA–3′-UTR/HuR interaction for hormonal control of ENaC synthesis.

Article Commentary: The Alternatively Spliced Form “b” of the Epithelial Sodium Channel α subunit (α ENaC): Any Prior Evidence of its Existence?

The epithelial sodium channel (ENaC) is critical in maintaining sodium balance across aldosterone-responsive epithelia. ENaC is a combined channel formed of three subunits (αβγ) with α ENaC subunit being the most critical for channel functionality. In a previous report, we have demonstrated the existence and mRNA expression levels of four alternatively spliced forms of the α ENaC subunit denoted by -a, -b, -c and -d in kidney cortex of Dahl S and R rats. Of the four alternatively spliced forms presently identified, α ENaC-b is considered the most interesting for the following reasons: Aside from being a salt-sensitive transcript, α ENaC-b mRNA expression is ∼32 fold higher than α ENaC wildtype in kidney cortex of Dahl rats. Additionally, the splice site used to generate α ENaC-b is conserved across species. Finally, α ENaC-b mRNA expression is significantly higher in salt-resistant Dahl R rats versus salt-sensitive Dahl S rats. As such, this commentary aims to highlight some of the previously published research articles that described the existence of an additional protein band on α ENaC western blots that could account for α ENaC-b in other rat species.

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.

Long-term terbutaline exposure stimulates alpha1-Na+-K+-ATPase expression at posttranscriptional level in rat fetal distal lung epithelial cells

Transepithelial Na(+) transport through epithelial Na(+) channels (ENaC) on the apical membrane and Na(+)-K(+)-ATPase activity on the basolateral membrane of distal lung epithelial cells are critical for alveolar fluid clearance. Acute exposure to beta-adrenergic agonists stimulates lung fluid clearance by increasing Na(+) transport. We investigated the effects of chronic exposure to the beta(2)-adrenergic agonist terbutaline on the transepithelial Na(+) transport in rat fetal distal lung epithelia (FDLE). FDLE monolayers exposed to 10(-4) M terbutaline for 48 h had significantly increased propanolol-blockable transepithelial total and amiloride-sensitive short-circuit current (I(sc)); however, when these chronically exposed monolayers were acutely exposed to additional beta-agonists and intracellular cAMP upregulators, there was no further increase in I(sc). Monolayers exposed to terbutaline for >48 h had I(sc) similar to control cells. Ouabain-sensitive Na(+)-K(+)-ATPase activity was increased in 48-h terbutaline-exposed FDLE whose apical membranes were permeabilized with nystatin. In contrast, terbutaline did not increase amiloride-sensitive apical membrane I(sc) in FDLE whose basolateral membranes were permeabilized with nystatin. Terbutaline treatment did not affect alpha-, beta-, or gamma-ENaC mRNA or alpha-ENaC protein steady-state levels, but increased total cellular levels and rate of synthesis of alpha(1)-Na(+)-K(+)-ATPase protein in FDLE in the absence of any change in alpha(1)-Na(+)-K(+)-ATPase mRNA. Total cellular beta(1)-Na(+)-K(+)-ATPase mRNA and protein levels were not affected by terbutaline. These data suggest that FDLE have different responses from adult type II epithelial cells when chronically exposed to terbutaline, and their increased transepithelial Na(+) transport occurs via a posttranscriptional increase in alpha(1)-Na(+)-K(+)-ATPase expression.

The contribution of epithelial sodium channels to alveolar function in health and disease

Amiloride-sensitive epithelial sodium channels (ENaC) play an important role in lung sodium transport. Sodium transport is closely regulated to maintain an appropriate fluid layer on the alveolar surface. Both alveolar type I and II cells have several different sodium-permeable channels in their apical membranes that play a role in normal lung physiology and pathophysiology. In many epithelial tissues, ENaC is formed from three subunit proteins: alpha, beta, and gamma ENaC. Part of the diversity of sodium-permeable channels in lung arises from assembling different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. Thus, lung epithelium has enormous flexibility to alter the magnitude of salt and water transport. In lung, ENaC is regulated by many transmitter and hormonal agents. Regulation depends upon the type of sodium channel but involves controlling the number of apical channels and/or the activity of individual channels.

Expression of ENaC subunits, chloride channels, and aquaporins in ovine fetal lung: ontogeny of expression and effects of altered fetal cortisol concentrations

Transition of the epithelium of the fetal lung from fluid secretion to fluid reabsorption requires changes in the expression of ion channels. Corticosteroids regulate expression of several of these channels, including the epithelium sodium channel (ENaC) subunits and aquaporins (AQP). We investigated the ontogenetic changes in these ion channels in the ovine fetal lung during the last half of gestation, a time of increasing adrenal maturation. Expression of the mRNAs for the chloride channels, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride channel 2 (CLCN2) decreased with age. Expression of mRNAs for AQP1, AQP5, and for subunits of ENaC (alpha, beta, gamma) increased with age. In the fetal sheep the expression of ENaCbeta mRNA was dramatically higher than the expression of ENaCalpha or ENaCgamma, but expression of ENaCbeta protein decreased with maturation, although the ratio of the mature (112 kDa) to immature (102 kDa) ENaCbeta protein increased with age, particularly in the membrane fraction. In contrast, ENaCalpha mRNA and protein both increase with maturation, and the mature form of ENaCalpha (68 kDa) predominates at all ages. A modest increase in fetal cortisol, within the range expected to occur naturally in late gestation but prior to active labor, increased ENaCalpha mRNA but not ENaCbeta, ENaCgamma, or AQP mRNAs. We conclude that in the ovine fetal lung, appearance of functional sodium channels is associated with induction of ENACalpha and ENaCgamma, and that ENaCalpha expression may be induced by even small, preterm increases in fetal cortisol.

Ibuprofen-induced patent ductus arteriosus closure: physiologic, histologic, and biochemical effects on the premature lung

OBJECTIVE

The goal was to study the pulmonary, biochemical, and morphologic effects of a persistent patent ductus arteriosus in a preterm baboon model of bronchopulmonary dysplasia.

METHODS

Preterm baboons (treated prenatally with glucocorticoids) were delivered at 125 days of gestation (term: 185 days), given surfactant, and ventilated for 14 days. Twenty-four hours after birth, newborns were randomly assigned to receive either ibuprofen (to close the patent ductus arteriosus; n = 8) or no drug (control; n = 13).

RESULTS

After treatment was started, the ibuprofen group had significantly lower pulmonary/systemic flow ratio, higher systemic blood pressure, and lower left ventricular end diastolic diameter, compared with the control group. There were no differences in cardiac performance indices between the groups. Ventilation index and dynamic compliance were significantly improved with ibuprofen. The improved pulmonary mechanics in ibuprofen-treated newborns were not attributable to changes in levels of surfactant protein B, C, or D, saturated phosphatidylcholine, or surfactant inhibitory proteins. There were no differences in tracheal concentrations of cytokines commonly associated with the development of bronchopulmonary dysplasia. The groups had similar messenger RNA expression of genes that regulate inflammation and remodeling in the lung. Lungs from ibuprofen-treated newborns were significantly drier (lower wet/dry ratio) and expressed 2.5 times more epithelial sodium channel protein than did control lungs. By 14 days after delivery, control newborns had morphologic features of arrested alveolar development (decreased alveolar surface area and complexity), compared with age-matched fetuses. In contrast, there was no evidence of alveolar arrest in the ibuprofen-treated newborns.

CONCLUSIONS

Ibuprofen-induced patent ductus arteriosus closure improved pulmonary mechanics, decreased total lung water, increased epithelial sodium channel expression, and decreased the detrimental effects of preterm birth on alveolarization.

Developmental regulation of lumenal lung fluid and electrolyte transport

In the fetus, there is a net secretion of liquid (LL) by the lung as a result of active transport of chloride ions. The rate of secretion and the resulting volume of LL are vital for normal lung growth but how volume is sensed and how secretion may be regulated are still unknown. Towards term under the influence of thyroid and adrenocorticoid hormones, the epithelial sodium channel (ENaC) is increasingly expressed in the pulmonary epithelium. Adrenaline released by the fetus during labour activates ENaC and produces rapid absorption of liquid in preparation for air breathing; absence of ENaC is incompatible with survival. There may be other mechanisms involved in aiding liquid clearance including changes in epithelial permeability, an effect of oxygen on both ENaC and Na/K ATPase and perhaps the influence of additional hormones on ENaC activity. Some time after birth there are further developmental changes with the appearance of other cation channels (CNG1 and perhaps NSCC) which contribute to the liquid absorptive side of the balance existing across the epithelium between secretion and absorption to produce essentially almost no net liquid movement in the postnatal lung. The evidence for these processes is discussed and areas of uncertainty indicated.

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.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

Glucocorticoids stimulate ENaC upregulation in bovine mammary epithelium

Mammary epithelia produce an isotonic, low-Na(+) fluid that is rich in nutrients. Mechanisms that account for the low electrolyte concentration have not been elucidated, although amiloride-sensitive ion transport has been reported in some situations. We hypothesized that corticosteroid exposure modulates epithelial Na(+) channel (ENaC) expression and/or activity in bovine mammary epithelial cells. BME-UV cells were grown to confluent monolayers on permeable supports with a standard basolateral medium and apical medium of low-electrolyte, high-lactose composition that resembles the ionic composition of milk. Ion transport was assessed in modified Ussing flux chambers. Exposure to glucocorticoids (dexamethasone, cortisol, or prednisolone), but not aldosterone, increased short-circuit current (I(sc)), a sensitive measure of net ion transport, whereas apical exposure to amiloride or benzamil reduced corticosteroid-induced I(sc) close to basal levels. Quantitative RT-PCR indicated a glucocorticoid-induced increase in mRNA for beta- and gamma-ENaC, whereas alpha-ENaC mRNA expression was only mildly affected. Exposure to mifepristone (a glucocorticoid receptor antagonist), but not spironolactone (a mineralocorticoid receptor antagonist), precluded both the corticosteroid-induced elevation in amiloride-sensitive I(sc) and the induced changes in beta- and gamma-ENaC mRNA. We conclude that Na(+) movement across mammary epithelia is modulated by corticosteroids via a glucocorticoid receptor-mediated mechanism that regulates the expression of the beta- and gamma-subunits of ENaC. ENaC expression and activity could account for the low Na(+) concentration that is typical of milk.