Vol. 303, Issue 1, 247-256, October 2002

Tonic Inhibitory Role for cAMP in _1a-Adrenergic Receptor Coupling to Extracellular Signal-Regulated Kinases 1/2

Xiuxiang Jiao , Pedro J. Gonzalez-Cabrera¹, Lei Xiao², Michael E. Bradley, Peter W. Abel and William B. Jeffries

Department of Pharmacology (X.J., M.E.B., P.W.A., W.B.J.) and Nephrology Research Laboratory (X.J., W.B.J.), Creighton University School of Medicine, Omaha, Nebraska

_1a-Adrenergic receptors (ARs) couple to phosphoinositide hydrolysis, adenylyl cyclase, and mitogen-activated protein kinase (MAPK) pathways. However, the interaction among these signaling pathways in activating extracellular signal-regulated kinase 1/2 (ERK1/2) is not well understood. We investigated the coupling of _1a-ARs to ERK1/2 in Chinese hamster ovary (CHO)-K1 cells stably transfected with mouse _1a-ARs, as well as the interaction between ERK1/2 and norepinephrine-induced cAMP accumulation. _1a-AR activation by norepinephrine increased the cytosolic Ca²⁺ concentration and phosphorylated ERK1/2 in a time- and concentration-dependent manner. ERK1/2 phosphorylation was blocked by the MAPK kinase 1/2 inhibitor 2'-amino-3'-methoxyflavone (PD 98059) and the ₁-AR antagonist prazosin. A transient elevation in intracellular Ca²⁺ was required for the phosphorylation of ERK1/2; however, activation of protein kinase C did not seem to be required for ERK1/2 phosphorylation. Norepinephrine also stimulated cAMP accumulation in transfected CHO-K1 cells in a concentration-dependent manner via _1a-ARs, which was blocked by the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Norepinephrine-induced ERK1/2 phosphorylation was inhibited by the adenylyl cyclase activator forskolin and was enhanced by the adenylyl cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purine-6-amine (SQ 22536) and the protein kinase A inhibitor 4-cyano-3-methylisoquinoline. In conclusion, in transfected CHO-K1 cells, _1a-AR activation activates both phospholipase C and adenylyl cyclase-mediated signaling pathways. _1a-AR-mediated ERK1/2 phosphorylation was dependent on a rise in intracellular Ca²⁺, and this pathway was reciprocally regulated by the concomitant activation of adenylyl cyclase, which inhibits ERK1/2 phosphorylation. Thus, _1a-AR stimulation of cAMP production may play an important role in regulating ERK1/2 phosphorylation in cell lines and native tissues.