Translational activation and repression by distinct elements within the 5'-UTR of ENaC alpha-subunit mRNA

Fast Nongenomic Effect of Aldosterone on the Volume of Principal Cells in Collecting Tube and Genetic Heterogeneity of Epithelial Sodium Channel in the Postnatal Ontogenesis of Rat Kidney

The effects of amiloride, epithelial sodium pump inhibitor, on the fast nongenomic effect of aldosterone in principal cells of an isolated segment of the distal portion of renal collecting tubes were studied in 10-day-old and adult rats. Fluorescent staining with Calcein AM showed various effects of amiloride (10(-5) M) on the stabilizing effect of aldosterone (10 nM) in hypotonic shock (280/140 mOsm/kg). Amiloride attenuated by 30% the effect of aldosterone on the amplitude of principal cell swelling in adult animals and almost completely abolished this effect in 10-day rats (p<0.05). These age-specific differences in the contribution of the distal portion of the collecting tube to the nongenomic effect of aldosterone did not depend on genetic heterogeneity of its α-subunit.

Investigation on genetic heterogeneity of α-subunit of the epithelial sodium channel in rat renal cortex

Genotypic variability of αENaC mRNA in adult rat kidney caused by deletion at the 3'-end of α-subunit DNA was studied by real-time PCR using specific probes with a fluorescent dye (TaqMan). It was found that mRNA deletion forms (a and b), products of alternative splicing of aENaC gene, are absent in the cortex of adult rat kidney.

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.

Sequence analysis of coding and 3' and 5' flanking regions of the epithelial sodium channel alpha, beta, and gamma genes in Dahl S versus R rats

BACKGROUND

To test whether epithelial sodium channel (ENaC) genes' variants contribute to salt sensitive hypertension in Dahl rats, we screened ENaC alpha, beta, and gamma genes entire coding regions, intron-exon junctions, and the 3' and 5' flanking regions in Dahl S, R and Wistar rats using both Denaturing High Performance Liquid Chromatography (DHPLC) and sequencing.

RESULTS

Our analysis revealed no sequence variability in the three genes encoding ENaC in Dahl S versus R rats. One homozygous sequence variation predicted to result in a D75E substitution was identified in Dahl and Wistar rat ENaC alpha compared to Brown Norway. Six and two previously reported polymorphic sites in Brown Norway sequences were lost in Dahl and Wistar rats, respectively. In the 5' flanking regions, we found a deletion of 5GCTs in Dahl and Wistar rat ENaC alpha gene, five new polymorphic sites in ENaC beta and gamma genes, one homozygous sequence variation in Dahl and Wistar rat ENaC gamma gene, as well as one Dahl rat specific homozygous insertion of -1118CCCCCA in ENaC gamma gene. This insertion created additional binding sites for Sp1 and Oct-1. Five and three Brown Norway polymorphic sites were lost in Dahl and Wistar rats, respectively. No sequence variability in ENaC 3' flanking regions was identified in Dahl compared to Brown Norway rats.

CONCLUSION

The first comprehensive sequence analysis of ENaC genes did not reveal any differences between Dahl S and R rats that were isogenic in the regions screened. Mutations in ENaC genes intronic sequence or in ENaC-regulatory genes might possibly account for increased ENaC activity in Dahl S versus R rats.

Dexamethasone inhibits the action of TNF on ENaC expression and activity

We have reported that TNF, a proinflammatory cytokine present in several lung pathologies, decreases the expression and activity of the epithelial Na(+) channel (ENaC) by approximately 70% in alveolar epithelial cells. Because dexamethasone has been shown to upregulate ENaC mRNA expression and is well known to downregulate proinflammatory genes, we tested if it could alleviate the effect of TNF on ENaC expression and activity. In cotreatment with TNF, we found that dexamethasone reversed the inhibitory effect of TNF and upregulated alpha, beta, and gammaENaC mRNA expression. When the cells were pretreated for 24 h with TNF before cotreatment, dexamethasone was still able to increase alphaENaC mRNA expression to 1.8-fold above control values. However, in these conditions, beta and gammaENaC mRNA expression was reduced to 47% and 14%, respectively. The potential role of TNF and dexamethasone on alphaENaC promoter activity was tested in A549 alveolar epithelial cells. TNF decreased luciferase (Luc) expression by approximately 25% in these cells, indicating that the strong diminution of alphaENaC mRNA must be related to posttranscriptional events. Dexamethasone raised Luc expression by fivefold in the cells and augmented promoter activity by 2.77-fold in cotreatment with TNF. In addition to its effect on alphaENaC gene expression, dexamethasone was able to maintain amiloride-sensitive current as well as the liquid clearance abilities of TNF-treated cells within the normal range. All these results suggest that dexamethasone alleviates the downregulation of ENaC expression and activity in TNF-treated alveolar epithelial cells.

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

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

Differential translational efficiency of ENaC subunits during lung development

The amiloride-sensitive epithelial Na(+) channel (ENaC), the rate-limiting step in epithelial Na(+) transport, consists of three subunits: alpha, beta, and gamma. The abundance of mRNA encoding the alpha-subunit far surpasses the amount for other subunits, and considerably exceeds the predicted subunit protein stoichiometry. We evaluated 5'-untranslated region (UTR) expression and found that fetal rat lung uses alternative 5'UTRs for alpha-ENaC during development. Sucrose density gradient analysis of postnuclear supernatants from fetal rat lung homogenates demonstrated that all three ENaC subunits were associated with high molecular weight polysomes, indicating active translation of the mRNAs, but translational efficiency was much lower for the alpha-subunit. Sucrose density gradient distributions were comparable for the endogenously expressed alpha-ENaC 5'UTRs in rat lung at Fetal Day 20 or Postnatal Day 1 using Northern analysis. Although birth resulted in a global decrease in lung mRNA translation, the loading of ribosomes on ENaC subunit mRNAs was largely unaffected. Evaluation of cytokeratin 18 and vimentin mRNAs in these gradients suggested a cell-specific effect. We conclude that there are different translational efficiencies for ENaC subunits and that perinatal processes globally modulate lung mRNA translation.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Oxygen-evoked changes in transcriptional activity of the 5'-flanking region of the human amiloride-sensitive sodium channel (alphaENaC) gene: role of nuclear factor kappaB

Expression of the alpha-subunit of the amiloride-sensitive sodium channel (alphaENaC) is regulated by a number of factors in the lung, including oxygen partial pressure (PO2). As transcriptional activation is a mechanism for raising cellular mRNA levels, we investigated the effect of physiological changes in PO2 on the activity of the redox-sensitive transcription factor nuclear factor kappaB (NF-kappaB) and transcriptional activity of 5'-flanking regions of the human alphaENaC gene using luciferase reporter-gene vectors transiently transfected into human adult alveolar carcinoma A549 cells. By Western blotting we confirmed the presence of NF-kappaB p65 but not p50 in these cells. Transiently increasing PO2 from 23 to 42 mmHg for 24 h evoked a significant increase in NF-kappaB DNA-binding activity and transactivation of a NF-kappaB-driven luciferase construct (pGLNF-kappaBpro), which was blocked by the NF-kappaB activation inhibitor sulphasalazine (5 mM). Transcriptional activity of alphaENaC-luciferase constructs containing 5'-flanking sequences (including the NF-kappaB consensus) were increased by raising PO2 from 23 to 142 mmHg if they contained transcriptional initiation sites (TIS) for exons 1A and 1B (pGL3E2.2) or the 3' TIS of exon 1B alone (pGL3E0.8). Sulphasalazine had no significant effect on the activity of these constructs, suggesting that the PO2-evoked rise in activity was not a direct consequence of NF-kappaB activation. Conversely, the relative luciferase activity of a construct that lacked the 3' TIS, a 3' intron and splice site but still retained the 5' TIS and NF-kappaB consensus sequence was suppressed significantly by raising PO2. This effect was reversed by sulphasalazine, suggesting that activation of NF-kappaB mediated PO2-evoked suppression of transcription from the exon 1A TIS of alphaENaC.