Agonist Regulation of Human β₂-Adrenergic Receptor mRNA Stability Occurs via a Specific AU-rich Element*

Abstract

Prolonged agonist stimulation of β₂-adrenergic receptors results in receptor down-regulation, which is closely associated with a reduction of the corresponding mRNA, an effect mediated in part by changes in mRNA stability. Transfection experiments with human β₂-adrenergic receptor cDNAs bearing or lacking the untranslated regions suggested that the essential agonist sensitivity of the mRNA resides within the 3′-untranslated region. The importance of this region was further confirmed in gel shift experiments; cytosolic preparations from agonist-stimulated DDT₁-MF2 smooth muscle cells caused a shift of β₂-adrenergic receptor mRNAs containing the 3′-untranslated region. Progressive 3′-terminal truncations of the receptor cDNA led to the identification of an AU-rich element at positions 329–337 of the 3′-untranslated region as the responsiblecis-acting element. Substitution of this motif by cytosine residues almost completely abolished mRNA down-regulation and inhibited the formation of the RNA-protein complex. Even though the β₂-adrenergic receptor AU-rich element showed two U → A transitions compared with the recently proposed AU-rich element consensus sequence, it revealed an almost identical destabilizing potency. Fusion of the β₂-adrenergic receptor 3′-untranslated region to the β-globin coding sequence dramatically reduced the half-life of the chimeric transcript in an agonist- and cAMP-dependent manner. This suggests that the agonist-induced β₂-adrenergic receptor mRNA destabilization is regulated by cAMP-dependent RNA-binding protein(s) via a specific AU-rich element.