Agonist-induced down-regulation of α1B-adrenergic receptor in HEK293 cells transfected with α1B cDNA

HEK293 cells stably expressing hamster ala-adrenergic receptor (alpha(1B)-AR) were used to observe the effect of norepinephrine (NE) on alpha(1B)-AR gene expression. Radioligand binding assays and RNase protection assays were used to determine alpha(1B)-AR number and the mRNA level, respectively. Exposure (2-24 h) of HEK293 cells to NE (10 mu mol) caused a decrease in alpha(1B)-AR mRNA with maximum change found at the 4th hour, and in alpha(1B)-AR density at the 24th hour. NE-induced decrease in alpha(1B)-AR mRNA was inhibited by protein kinase C (PKC) inhibitor calphostin C (0.1 mu mol) and mimicked by PKC activator PMA (1 mu mol). Nuclear run-off transcription assay showed that treatment of the cells with NE (10 mu mol) exerted no effect on the transcription rate of alpha(1B)-AR After the synthesis of new RNAs was inhibited by actinomycin D, NE could not accelerate the degradation of alpha(1B)-AR mRNA. The results suggested that in the HEK293 cells NE could induce the down-regulation of alpha(1B)-AR, and the effects were mediated by PKC pathway. NE could not alter the transcription rate of alpha(1B)-AR mRNA, but it might induce the synthesis of some factors and indirectly accelerate the degradation.