Evidence for a role of G protein βγ subunits in the enhancement of cAMP accumulation and DNA synthesis by adenosine in human cells

The expression of both A 1 ‐ and A 2a ‐adenosine receptors occurs in human foreskin and lung fibroblasts (Ahmed et al., 1995, Biochem. Biophys. Res. Commun. 208:871–878). Studies with highly specific A 1 ‐ and A 2a ‐adenosine receptor agonists provide indirect evidence that binding of adenosine activates G s and G i , after which G sα interacts with βγ subunits released from G i . The interaction of G sα with βγ augments cyclic adenosine monophosphate (cAMP) accumulation, more than does G sα alone. In the present study, we have provided direct evidence for a role of the βγ complex in the augmentation of cAMP accumulation by using a recombinant His 6 fusion protein containing the carboxyl third of βARK1. This portion of βARK1 contains G βγ binding sequences and acts as a specific βγ scavenger (Koch et al.,1994, Proc. Natl. Acad. Sci. USA 1:12706–12710). In permeabilized fibroblasts, the His 6 fusion protein inhibited the augmentation of cAMP accumulation resulting from adenosine binding to both A 1 and A 2a receptors. In addition, the specific G βγ scavenger inhibited the further rise in cellular cAMP levels caused by pretreating cells with pertussis toxin before incubation with adenosine. Finally, we observed that specific A 1 ‐adenosine receptor agonists augmented the cAMP accumulation stimulated by A 2a ‐receptor agonists, and this cAMP augmentation was also suppressed by the G βγ scavenger. Similar results were obtained when the cells were treated with extracellular ATP and lysophosphatidic acid (LPA) to stimulate G s and release G βγ subunits, respectively. J. Cell. Physiol. 170:263–271, 1997. © 1997 Wiley‐Liss, Inc.