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

Abstract

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 α1-, α3-, and β1-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 α1- and α3-mRNAs, but not of β1-mRNA, was significantly increased by HC and Dex. Deletion of the 5′untranslated region (5′UT) of α1-mRNA abolished this effect. Translation of a chimeric β1-mRNA, constructed by transposing the 5′UT of α1 onto the coding region of β1, was enhanced by HC. Transposition of a putative steroid-modulatory element conserved in the 5′UT of all α isoforms (ACAGGACCC) onto the coding region of β1-mRNA rendered it responsive to HC. A synthetic primer containing the ACAGGACCC sequence abolished the effect of HC on α1- and chimeric β1-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 α-mRNAs.