cAMP stimulates transcription of the beta 2-adrenergic receptor gene in response to short-term agonist exposure.

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Abstract
In addition to conveying cellular responses to an effector molecule, receptors are often themselves regulated by their effectors. We have demonstrated that epinephrine modulates both the rate of transcription of the .beta.2-adrenergic receptor (.beta.2AR) gene and the steady-state level of .beta.2AR mRNA in DDT1MF-2 cells. Short-term (30 min) exposure to epinephrine (100 nM) stimulates the rate of .beta.2AR gene transcription, resulting in a 3- to 4-fold increase in steady-state .beta.2AR mRNA levels. These effects are mimicked by 1 mM N6,O2''-dibutyryladenosine 3'',5''-cyclic monophosphate (Bt2cAMP) or forskolin but not by phorbol esters. The half-life of the .beta.2AR mRNA after addition of actinomycin D (46.7 .+-. 10.2 min; mean .+-. SEM; n = 5) remained unchanged after 30 min of epinephrine treatment (46.8 .+-. 10.6 min; mean .+-. SEM; n = 4), indicating that a change in transcription rate is the predominant factor responsible for the increase of .beta.2AR mRNA. Whereas brief exposure to epinephrine or Bt2cAMP does not significantly affect the total number of cellular .beta.2ARs (assessed by ligand binding), continued exposure results in a gradual decline in .beta.2AR number to .apprxeq. 20% (epinephrine) or .apprxeq. 45% (Bt2 cAMP) of the levels in control cells by 24 hr. Similar decreases in agonist-stimulated adenylyl cyclase activity are observed. This loss of receptors with prolonged agonist exposure is accompanied by a 50% reduction in .beta.2AR mRNA. Transfection of the .beta.2AR promoter region cloned onto a reporter gene (bacterial chloramphenicol acetyltransferase) allowed demonstration of a 2- to 4-fold induction of transcription by agents that elevate cAMP levels, such as forskolin or phosphodiesterase inhibitors. These results establish the presence of elements within the proximal promoter region of the .beta.2AR gene responsible for the transcriptional enhancing activity of cAMP and demonstrte that .beta.2AR gene expression is regulated by a type of feedback mechanism involving the second messenger cAMP.