Regulation of beta 2-adrenergic receptor mRNA and gene transcription in rat C6 glioma cells: effects of agonist, forskolin, and protein synthesis inhibition.

Incubation of rat C6 glioma cells with beta-adrenergic receptor (beta AR) agonist or with agents that increase cAMP levels results in down-regulation of the beta 2AR, as measured by the loss of radioligand binding sites. In the present study, the role of beta 2AR mRNA expression and stability in the down-regulation of beta 2AR sites in C6 cells was examined. Isoproterenol or forskolin treatment decreased beta 2AR mRNA levels in a time-dependent manner, with maximal loss of approximately 50% being observed after 2 hr. Pretreatment of the cells with a potent protein synthesis inhibitor, Pseudomonas exotoxin A, completely blocked isoproterenol- and forskolin-mediated down-regulation of beta 2AR mRNA. Exposure to agonist did not significantly influence the half-life of beta 2AR mRNA, which was approximately 60 min. In contrast, isoproterenol treatment for 2 hr significantly decreased the rate of beta 2AR gene transcription, as determined by nuclear run-on analysis. Based on these results, we propose that agonist regulation of beta 2AR mRNA in C6 cells is mediated by activation of the cAMP system and occurs at the level of beta 2AR gene transcription, not mRNA stability. In addition, the observed requirement for protein synthesis indicates that down-regulation of beta 2AR mRNA may be mediated by expression of a repressor of beta 2AR gene transcription.