Abstract
Synergism between Gs- and Gi- or Gq-dependent signaling pathways has been demonstrated in the stimulation of type II adenylyl cyclase (AC- II). Provision of activated alpha s is known to allow numerous Gi- coupled receptors to stimulate AC-II and to potentiate the responses to Gq-coupled receptors. To explore possible interactions between Gi- and Gq-coupled receptors that are independent of alpha s, the activity of AC-II was determined after the activation of Gi- and Gq-regulated pathways. Human embryonic kidney 293 cells were transiently cotransfected with cDNAs encoding AC-II and various G-protein-coupled receptors. Agonist-bound Gi-coupled receptors (including the formyl peptide, dopamine-D2, and delta-opioid receptors) stimulated AC-II activity in the absence of activated alpha s, provided that the cells were treated with 100 nM phorbol 12-myristate 13-acetate. Activation of protein kinase C (PKC) thus appears to relieve the requirement for the presence of activated alpha s. Stimulation of PKC via Gq-coupled receptors also allowed Gi-coupled receptors to activate AC-II. Coexpression of the m1 muscarinic receptor with the dopamine-D2 receptor permitted dopamine to stimulate AC-II in the presence of carbachol. The phorbol ester-permissive and alpha s-independent stimulation was mediated by G-protein beta gamma subunits because it was blocked by the beta gamma scavengers alpha t and beta-adrenergic receptor kinase. These results show that AC-II can efficiently integrate signals generated by Gq- and Gi-coupled receptors via a mechanism that is independent of Gs.
