MicroRNA let-7 establishes expression of β2-adrenergic receptors and dynamically down-regulates agonist-promoted down-regulation

Although beta(2)-adrenergic receptors (beta(2)AR) are expressed on most cell types, mechanisms that establish expression levels and regulate expression by chronic agonist remain unclear. The 3' UTR of ADRB2 has a conserved 8-nucleotide seed region that we hypothesized is targeted by the let-7 family of miRNAs leading to translational repression. In luciferase assays with transfected cells, luc-beta(2)WT3'UTR had decreased expression when cotransfected with let-7f, but a mutated luc-beta(2)3'UTR lacking the seed was unaffected by let-7f; a mutated let-7f also had no effect on luc-beta(2)WT3'UTR expression. ADRB2 mRNA was in greater abundance in immunoprecipitates of Ago2, a core component of the miRNA-induced silencing complex, when cells were transfected with let-7f, but not with a mutated let-7f, indicating a direct interaction with the silencing mechanism. H292 cells transfected with let-7f caused similar to 60% decrease in native beta(2)AR expression, but transfection with let-7f-specific locked nucleic acid anti-miRNA increased beta(2)AR expression by similar to twofold. We considered that an increase in let-7f leading to greater repression of translation contributes to agonist-promoted down-regulation. Paradoxically, in cells and in lungs from mice treated in vivo, an similar to 50% decrease in let-7f occurs during long-term agonist exposure, indicating a counterregulatory event. Consistent with this notion, let-7f locked nucleic acid transfection caused depressed agonist-promoted down-regulation. Thus, let-7f miRNA regulates baseline beta(2)AR expression and decreases in let-7f evoked by agonist attenuate down-regulation. This positive feedback loop has not previously been described for a G protein-coupled receptor and its miRNA. Methods to decrease let-7f expression in targeted cells may increase therapeutic responses to beta-agonist by increasing beta(2)AR expression or minimizing tachyphylaxis.