Regulation of rat renal (Na(+) + K+)-adenosine triphosphatase mRNA levels by corticosterone

Dexamethasone responsive element in the rat Na, K-ATPase beta1 gene coding region

The Na, K-ATPase plays an essential role in active alveolar epithelial fluid resorption. In fetal and adult alveolar epithelial cells, glucocorticoids (GC) increase Na, K-ATPase activity, mRNA levels, and transcription rate of the beta(1) subunit. In this study, we describe a glucocorticoid responsive element (GRE) in the coding region of the rat Na, K-ATPase beta(1) gene in a rat lung epithelial cell line. Transient transfection experiments with the beta(1) subunit coding region with or without the 5' and 3' untranslated regions demonstrated responsiveness to dexamethasone induction and also identified a GRE at +434 in exon IV. The +434 GRE conferred dexamethasone responsiveness in a heterologous thymidine kinase promoter irrespective of its orientation to the beta(1) promoter. Transcriptional upregulation by dexamethasone was abolished in +434 mutants. Electrophoretic mobility shift assays (EMSA) demonstrated specific binding of nuclear proteins to the +434 GRE and the presence of the GC receptor. This specific binding was inhibited by a GRE previously described in the rat Na, K-ATPase beta(1) gene at -631. In conclusion, we identified a GRE at +434 in the exon IV of the rat Na, K-ATPase beta(1) gene.

Translational regulation of Na-K-ATPase subunit mRNAs by glucocorticoids

Glucocorticoids (GC) regulate Na-K-ATPase-subunit mRNA transcription. However, GC-induced increases in Na-K-ATPase activity are not always paralleled by changes in subunit mRNA abundance. We therefore examined posttranscriptional mechanisms of subunit gene regulation by GC. cDNA-derived mRNAs encoding alpha 1-, alpha 3-, and beta 1-subunits were tested for stability and translation efficiency in a cell-free lysate, in the presence of hydrocortisone (HC) or dexamethasone (Dex). No effect of HC on subunit mRNA stability was noted. Translation efficiency of alpha1- and alpha 3-mRNAs, but not of beta 1-mRNA, was significantly increased by HC and Dex. Deletion of the 5'untranslated region (5'UT) of alpha 1-mRNA abolished this effect. Translation of a chimeric beta 1-mRNA, constructed by transposing the 5'UT of alpha 1 onto the coding region of beta1, was enhanced by HC. Transposition of a putative steroid-modulatory element conserved in the 5'UT of all alpha isoforms (ACAGGACCC) onto the coding region of beta 1-mRNA rendered it responsive to HC. A synthetic primer containing the ACAGGACCC sequence abolished the effect of HC on alpha 1- and chimeric beta 1-mRNAs. Our results indicate that GC can directly enhance Na-K-ATPase translation in vitro in a subunit-specific manner, via a putative GC-modulatory element situated in a predicted loop structure within the 5'UT of alpha-mRNAs.

Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells

Previous studies in kidney, heart, and liver cells have demonstrated that dexamethasone regulates the expression of Na-K-ATPase. In the lungs, Na-K-ATPase has been reported in alveolar epithelial type II (ATII) cells and is thought to participate in active Na+ transport and lung edema clearance. The aim of this study was to determine whether Na-K-ATPase would be regulated by dexamethasone in cultured rat ATII cells. Regulation of the Na-K-ATPase by dexamethasone could lead to a greater understanding of its role in active Na+ transport and lung edema clearance. Rat ATII cells were isolated, plated for 24 h, and exposed to 10(-7) and 10(-8) M dexamethasone. These cells were harvested at 0, 3, 6, 12, and 24 h after dexamethasone exposure for determination of steady-state Na-K-ATPase mRNA transcript levels, protein expression, and function. The steady-state Na-K-ATPase beta1-mRNA transcript levels increased in ATII cells 6, 12, and 24 h after dexamethasone exposure (P < 0.05). However, the steady-state alpha1-mRNA transcript levels were unchanged. The protein expression for the alpha1- and beta1-subunits increased in ATII cells exposed to dexamethasone compared with controls in association with a temporal increase in Na-K-ATPase function after dexamethasone exposure. These results suggest that dexamethasone regulates Na-K-ATPase in ATII cells possibly by transcriptional, translational, and posttranslational mechanisms.

Glucocorticoid regulation of mRNA encoding (Na+K) ATPase alpha 3 and beta 1 subunits in rat brain measured by in situ hybridization

The effect of glucocorticoids on (Na+K)ATPase mRNA synthesis was studied in 19 brain areas of adrenalectomized (ADX) rats untreated or receiving dexamethasone (DEX). For in situ hybridization, we employed a [35S]oligonucleotide probe for the alpha 3-subunit isoform, and a [3H]cDNA coding for the beta 1-subunit of the enzyme. Mean levels of grain density for the alpha 3 subunit mRNA of DEX-treated rats were significantly higher by a 't' test in medial septum, amygdala lateralis (AL) and medialis (AME), gyrus dentatus, CA4 hippocampal area, substantia nigra and periventricular gray, compared to untreated rats. For the beta 1-subunit, mean levels after DEX were significantly higher in AL and lateral preoptic area. In addition, the Kolmogorov-Smirnov test applied to frequency histograms of neuronal densities indicated a coordinate increase in alpha 3 and beta 1-subunit mRNA expression for the CA2 subfield and preoptic area medialis (POA MED). We conclude that (1) glucocorticoids are positive modulators of (Na+K)ATPase mRNA; (2) analysis of frequency histograms suggests that glucocorticoids promote in a few regions (AL, POA MED, CA2 subfield) a coordinate increase in the biosynthesis of the alpha 3 and beta 1-subunit mRNA. In 11 other areas stimulation occurs for one subunit mRNA only, whereas 5 areas were insensitive to glucocorticoid effects on this enzyme.