Variable-length poly-C tract polymorphisms of the β₂- adrenergic receptor 3′-UTR alter expression and agonist regulation

β₂-Adrenergic receptors (β₂-AR) expressed on airway epithelial and smooth muscle cells regulate mucociliary clearance and relaxation and are the targets for β-agonists in the treatment of obstructive lung disease. However, the clinical responses display extensive interindividual variability, which is not adequately explained by genetic variability in the 5′-flanking or coding region of the intronless β₂-AR gene. The nonsynonymous coding polymorphism most often associated with a bronchodilator phenotype (Arg16) is found within three haplotypes that differ by the number of Cs (11, 12, or 13) within a 3′-untranslated region (UTR) poly-C tract. To examine potential effects of this variability on receptor expression, BEAS-2B cells were transfected with constructs containing the β₂-AR (Arg16) coding sequence followed by its 3′-UTR with the various polymorphic poly-C tracts. β₂Arg16-11C had 25% lower mRNA expression and 33% lower β₂-AR protein expression compared with the other two haplotypes. Consistent with this lower steady-state expression, β₂Arg16-11C mRNA displayed more rapid and extensive degradation after actinomycin D treatment compared with β₂Arg16-12C and -13C. However, β₂Arg16-12C underwent 50% less downregulation of receptor expression during β-agonist exposure compared with the other two haplotypes. Thus these haplotypes direct a potential low-response phenotype due to decreased steady-state receptor expression combined with wild-type agonist-promoted downregulation (β₂Arg16-11C) and a high-response phenotype due to increased baseline expression combined with decreased agonist-promoted downregulation (β₂Arg16-12C). This heterogeneity may contribute to the variability of clinical responses to β-agonist, and genotyping to identify these 3′-UTR polymorphisms may improve predictive power within the context of β₂-AR haplotypes in pharmacogenetic studies.