Removal of the carboxy terminus of the A2A-adenosine receptor blunts constitutive activity: differential effect on cAMP accumulation and MAP kinase stimulation.
The A(2A)-adenosine receptor has an extended carboxy terminus (approximately 120 amino acids), the role of which is poorly defined. In human endothelial cells and in HEK293 cells, the A(2A)-receptor controls at least two independent signalling pathways, i.e. increased cyclic adenosine 3',5'-monophosphate (cAMP) formation via its cognate G protein G(s) and increased phosphorylation of mitogen-activated protein kinase (MAP kinase) by recruiting p21(ras). In order to address the role of the carboxy terminus in signal transfer, we generated HEK293 cells that stably expressed the full-length (wt) receptor and truncated versions [A(2A)-R(1-360) and A(2A)-R(1-311)] at comparable levels (approximately 0.5 pmol/mg) in the plasma membrane. The effects of truncation were divergent with respect to the two effectors regulated by the receptor. In intact cells carrying A(2A)-R(wt) and A(2A)-R(1-360), cAMP accumulation was more potently activated by an A(2A)-agonist than in cells expressing A(2A)-R(1-311). Similarly, A(2A)-R(wt) and A(2A)-R(1-360)--but not A(2A)-R(1-311)--caused constitutive (=agonist-independent) elevation of cAMP which was reversed by the addition of A(2A)-antagonists. In membranes prepared from these cells, however, the three receptors displayed no constitutive activity in stimulating adenylyl cyclase and they did not differ in apparent agonist affinity. Truncation of the A(2A)-receptor did also not decrease the potency of an A(2A)-agonist to stimulate MAP kinase in intact cells. We conclude that the carboxy terminus defines both (a) the level of constitutive activity, i.e. the equilibrium R<--> R*, in intact cells only, indicating a role for a component that is lost upon cell lysis, and (b) the efficiency of signal transfer in alternative pathways.
Klinger
Markus
M
Institute of Pharmacology, University of Vienna, 1090 Vienna, Austria.
Kuhn
Mandy
M
Just
Herwig
H
Stefan
Eduard
E
Palmer
Timothy
T
Freissmuth
Michael
M
Nanoff
Christian
C
D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D020928:Mitogen-Activated Protein Kinases / Q000096:biosynthesis*; D058915:Purinergic P1 Receptor Antagonists; D043705:Receptor, Adenosine A2A; D018047:Receptors, Purinergic P1 / Q000235:genetics* / Q000378:metabolism* / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014162:Transfection / Q000379:methods
D058915:Purinergic P1 Receptor Antagonists; D043705:Receptor, Adenosine A2A; D018047:Receptors, Purinergic P1; D000242:Cyclic AMP; D020928:Mitogen-Activated Protein Kinases