Temporal characteristics of cAMP response element-mediated gene transcription: requirement for sustained cAMP...

Many clinically used drugs are G-protein-coupled receptor (GPCR) antagonists and are given long-term to prevent receptor activation by endogenous agonists. Most GPCR antagonists are considered to have little agonist efficacy of their own. However, many beta antagonists do stimulate very small beta(2) adrenoceptor-mediated cAMP responses, but these responses become substantial at the level of cAMP response element (CRE)-gene transcription. Here, we compared the temporal characteristics of these beta(2) adrenoceptor-mediated cAMP and CRE-gene transcription responses with ligands of differing agonist efficacy. Within a minute, full agonists (e.g., isoprenaline) stimulated large increases in intracellular and exported cAMP. Very weak partial agonists (e.g., alprenolol) did not increase intracellular cAMP (only stimulating a small export). However, all agonists (regardless of efficacy) stimulated an increase in CRE-gene transcription after a 2-h incubation. An initial 30-min continual stimulation was required to initiate the process of CRE-gene transcription for all ligands. Longer agonist incubations resulted in larger gene transcription responses in a proportional manner for both weak and full agonists alike, and this was despite the lack of intracellular cAMP detection for the weaker ligands. Thus, the major initiator for CRE-gene transcription was not cAMP concentration or total quantity generated but a sustained turnover of intracellular cAMP and hence sustained stimulation of CREB phosphorylation. Thus, long-acting agonists and long-term treatments with very weak partial agonists (including many drugs classified previously as antagonists based on traditional second-messenger assays, e.g., several clinically used "beta-blockers") may cause more substantial gene transcription than previously believed.