Cross-regulation between G-protein-mediated Pathways activation of the inhibitory pathway of adenylylcyclase increases the expression of β₂-adrenergic receptors

Cross-regulation from the stimulatory (G_sα-mediated) to the inhibitory (G_iα-mediated) pathways controlling adenylylcyclase has been described (Hadcock, J. R., Ros, M., Watkins, D. C., and Malbon, C. C. (1990) J. Biol. Chem. 265, 14784-14790). The extent to which cross-regulation occurs from inhibitory to stimulatory pathways for adenylylcyclase was explored. Persistent activation of the inhibitory pathway of adenylylcyclase by the A₁-adenosine receptor agonist (-)N⁶(R-phenylisopropyl)adenosine (PIA) in hamster smooth muscle DDT₁ MF-2 cells enhanced the stimulatory pathway of adenylylcyclase and its activation by the β₂-adrenergic receptor agonist isoproterenol. PIA treatment (48 h) of cells increased isoproterenol-stimulated adenylylcyclase by 2-fold. In addition, the ED₅₀ for stimulation of adenylylcyclase by isoproterenol decreased 50-fold to ∼1 nM. Persistent activation of cells with PIA increased β₂-adrenergic receptor number in a time- and dose-dependent manner. The steady-state levels of β₂-adrenergic receptors (radioligand binding and immunoblotting) and receptor mRNA levels increased by more than 70%, while the half-life of the receptor (24 h) was unaltered. Both A₁-adenosine receptor binding and G_iα2 levels declined by half in cells persistently activated with PIA. Although G_iα2 mRNA levels and the relative rate of synthesis of G_iα2 protein upon persistent activation of the inhibitory pathway were found to increase, a decrease in the half-life of G_iα2 from ∼75 h in naive cells to ∼40 h in cells provides the basis for the decline in G_iα2 levels. The steady-state level of mRNA and half-life of G_sα protein were unaltered in persistently activated cells. Thus, activation of the inhibitory pathway of adenylylcyclase cross-regulates the stimulatory, hormone-sensitive adenylylcyclase system by: (i) up-regulating β₂-adrenergic receptors and enhancing the activation of the stimulatory adenylylcyclase pathway and (ii) down-regu-lating elements of the inhibitory adenylylcyclase pathway (G_iα2 and A₁-adenosine receptor binding).