CRM 1-mediated degradation and agonist-induced down-regulation of β-adrenergic receptor mRNAs

The β₁-adrenergic receptor (β₁-AR) mRNAs are post-transcriptionally regulated at the level of mRNA stability and undergo accelerated agonist-mediated degradation via interaction of its 3′ untranslated region (UTR) with RNA binding proteins, including the HuR nuclear protein. In a previous report [Kirigiti et al. (2001). Mol. Pharmacol. 60:1308-1324], we examined the agonist-mediated down-regulation of the rat β₁-AR mRNAs, endogenously expressed in the rat C6 cell line and ectopically expressed in transfectant hamster DDT₁MF2 and rat L6 cells. In this report, we determined that isoproterenol treatment of neonatal rat cortical neurons, an important cell type expressing β₁-ARs in the brain, results in significant decreases in β₁-AR mRNA stability, while treatment with leptomycin B, an inhibitor of the nuclear export receptor CRM 1, results in significant increases in β₁-AR mRNA stability and nuclear retention. UV-crosslinking/immunoprecipitation and glycerol gradient fractionation analyses indicate that the β₁-AR 3′ UTR recognize complexes composed of HuR and multiple proteins, including CRM 1. Cell-permeable peptides containing the leucine-rich nuclear export signal (NES) were used as inhibitors of CRM 1-mediated nuclear export. When DDT₁MF2 transfectants were treated with isoproterenol and peptide inhibitors, only the co-addition of the NES inhibitor reversed the isoproterenol-induced reduction of β₁-AR mRNA levels. Our results suggest that CRM 1-dependent NES-mediated mechanisms influence the degradation and agonist-mediated down-regulation of the β₁-AR mRNAs.