Different roles for G sub(i) and G sub(o) proteins in modulation of adenylyl cyclase type-2 activity

The effect of G sub(i/o) protein-coupled receptors on adenylyl cyclase type 2 (AC2) has been studied in Sf9 insect cells. Stimulation of cells expressing AC2 with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) led to a twofold stimulation of cAMP synthesis that could be blocked with the protein kinase C inhibitor GF109203X. Activation of a coexpressed alpha sub(2A)-adrenoceptor or muscarinic M4 receptor inhibited the stimulation by TPA almost completely in a pertussis toxin-sensitive manner. Activation of G sub(s) proteins switched the response of the alpha sub(2A)-adrenoceptor to potentiation of prestimulated AC2 activity. The potentiation, but not the inhibition, could be blocked by a G beta gamma scavenger. A novel methodological approach, whereby signalling through endogenous G proteins was ablated, was used to assess specific G protein species in the signal pathway. Expression of G sub(o) proteins ( alpha sub(o1) + beta sub(1) gamma sub(2)) restored both the inhibition and the potentiation, whereas expression of Gi proteins ( alpha sub(i1) + beta sub(1) gamma sub(2)) resulted in a potentiation of both the TPA- and the G sub(s)-stimulated AC2 activity. The data presented supports the view of AC2 as a molecular switch and implicates this isoform as a target for G sub(o) protein-linked signalling.