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1 Pediatric Nephrology, Yale University School of Medicine, New Haven 06520, and Albert Einstein College of Medicine, Bronx, New York 10467; and 2 Department of Medicine, Yale University School of Medicine, New Haven, Connecticut 06520; and Johns Hopkins School of Medicine, Baltimore, Maryland 21205
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 
1-,
3-, and 
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 1- and 3-mRNAs, but not of
1-mRNA, was significantly increased by HC and Dex. Deletion of the
5'untranslated region (5'UT) of 1-mRNA abolished this effect.
Translation of a chimeric 1-mRNA, constructed by transposing the
5'UT of 1 onto the coding region of 1, was enhanced by HC. Transposition of a putative steroid-modulatory element conserved in the
5'UT of all isoforms (ACAGGACCC) onto the coding region of
1-mRNA rendered it responsive to HC. A synthetic primer containing the ACAGGACCC sequence abolished the effect of HC on 1- and chimeric 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 -mRNAs.
mRNA stability; 5'untranslated region; hormonal regulation of sodium-potassium-adenosinetriphosphatase; glucocorticoid modulatory element; mRNA secondary structure.
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