Agonist-induced modulation of inverse agonist efficacy at the beta 2-adrenergic receptor

Sustained stimulation of several G protein-coupled receptors is known to lead to a reduction in the signaling efficacy. This phenomenon, named agonist-induced desensitization, has been best studied for the beta 2-adrenergic receptor (AR) and is characterized by a decreased efficacy of beta-adrenergic agonists to stimulate the adenylyl cyclase activity. Recently, several beta-adrenergic ligands were found to inhibit the spontaneous agonist-independent activity of the beta 2AR. These compounds, termed inverse agonists, have different inhibitory efficacies, ranging from almost neutral antagonists to full inverse agonists. The current study was undertaken to determine whether, as is the case for agonists, desensitization can affect the efficacies of inverse agonists. Agonist-promoted desensitization of the human beta 2AR expressed in Sf9 cells potentiated the inhibitory actions of the inverse agonists, with the extent of the potentiation being inversely proportional to their intrinsic activity. For example, desensitization increased the inhibitory action of the weak inverse agonist labetalol by 29%, whereas inhibition of the spontaneous activity by the strong inverse agonist timolol was not enhanced by the desensitizing stimuli. Interestingly, dichloroisoproterenol acted stochastically as either a weak partial agonist or a weak inverse agonist in control conditions but always behaved as an inverse agonist after desensitization. These data demonstrate that like for agonists, the efficacies of inverse agonists can be modulated by a desensitizing treatment. Also, the data show that the initial state of the receptor can determine whether a ligand behaves as a partial agonist or an inverse agonist.