Negative regulation of α2-adrenergic receptor-mediated Gi signalling by a novel pathway

Abstract

Chinese hamster ovary (CHO) cells stably expressing α2 adrenergic receptor (α2AR) were pretreated with cholera toxin (CTX) and then treated with or without PMA. The α2AAR-mediated inhibition of forskolin-stimulated cAMP accumulation was completely ablated by CTX pretreatment only after additional treatment with PMA. Although the addition of cycloheximide (protein synthesis inhibitor) and H-89 (cAMP dependent protein kinase inhibitor) did not completely counteract the negative regulation, the elevation of cAMP was a primary factor for negative regulation by treatment with CTX and PMA. In contrast with the cAMP response, the inhibition of membrane adenylate cyclase activity and the agonist competition curve were not influenced by treatment with CTX or PMA, suggesting that a cytosolic factor was involved in this negative regulation. The m2-muscarinic-acetylcholine-receptor-mediated inhibition of the forskolin-stimulated accumulation of cAMP was also attenuated by treatment with CTX and PMA. The ablation of α2AAR-mediated inhibition was not observed when α2AAR was expressed in Rat2 fibroblast cells, suggesting that this negative regulation is not dependent on the receptor type but is instead a phenomenon common to Gi-coupled receptors in CHO cells. Reverse-transcriptase-mediated PCR and Northern blot analysis showed that the expression of GOS8/RGS2 mRNA, which is a member of the regulator of G-protein signalling (RGS) group of proteins, was considerably increased by pretreatment with CTX. These results indicate a novel regulatory pathway, whereby a cytosolic factor induced by the elevation of cellular cAMP levels negatively regulates Gi signalling in a protein-kinase-C-dependent manner.