The presence of beta2-adrenoceptors sensitizes alpha2A-adrenoceptors to desensitization after chronic epinephrine treatment.
BACKGROUND
In addition to the regulation of blood pressure, alpha2- and beta-adrenoceptor (AR) subtypes play an important role in the modulation of noradrenergic neurotransmission in the human CNS and PNS. Several studies suggest that the alpha2-AR responsiveness in cells and tissues after chronic epinephrine (EPI) or norepinephrine (NE) exposure may vary, depending on the beta-AR activity present there. Recently, we reported that in BE(2)-C human neuroblastoma cells (endogenously expressing alpha2A- and beta2-AR), chronic EPI treatment (300 nM) produced a dramatic beta-adrenoceptor-dependent desensitization of the alpha2A-AR response. The aim of this study is to determine if stable addition of a beta2-AR to a second neuroblastoma cell line (SH-SY5Y), that normally expresses only alpha2A-ARs that are not sensitive to 300 nM EPI exposure, would suddenly render alpha2A-ARs in that cell line sensitive to treatment with the same EPI concentration.


METHODS
These studies employed RT-PCR, receptor binding and inhibition of cAMP accumulation to confirm alpha2-AR subtype expression. Stable clones of SH-SY5Y cells transfected to stably express functional beta2-ARs (SHbeta2AR4) were selected to compare sensitivity of alpha2-AR to EPI in the presence or absence of beta2-ARs.


RESULTS
A series of molecular, biochemical and pharmacological studies indicated that the difference between the cell lines could not be attributed to alpha2-AR heterogeneity. We now report that after transfection of functional beta2-AR into SH-SY5Y cells (SHbeta2AR4), chronic treatment with modest levels of EPI desensitizes the alpha2A-AR. This effect results from a beta2-AR dependent down-regulation of native alpha2A-ARs by EPI accompanied by enhanced translocation of GRK2 and GRK3 to the membrane (required for GRK-mediated phosphorylation of agonist-occupied receptors).


CONCLUSIONS
This study further supports the hypothesis that the presence of the beta-AR renders the alpha2A-AR more susceptible to desensitization with physiological levels of EPI.
Bawa-Khalfe
Tasneem
T
Research Center for Cardiovascular Diseases, Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases, The University of Texas-Houston Health Science Center, Houston, TX, USA. Tasneem.Bawa-Khalfe@uth.tmc.edu
Altememi
Ghazi F
GF
Mandyam
Chitra D
CD
Schwarz
Lindsay A
LA
Eikenburg
Douglas C
DC
Standifer
Kelly M
KM
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D058672:Adrenergic beta-2 Receptor Antagonists; D000068438:Brimonidine Tartrate; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000378:metabolism; D004837:Epinephrine / Q000494:pharmacology*; D054769:G-Protein-Coupled Receptor Kinase 2 / Q000378:metabolism; D054770:G-Protein-Coupled Receptor Kinase 3 / Q000378:metabolism; D006801:Humans; D009638:Norepinephrine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000235:genetics / Q000378:metabolism*; D018343:Receptors, Adrenergic, beta-2 / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
C547768:ADRA2A protein, human; D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D058672:Adrenergic beta-2 Receptor Antagonists; D011810:Quinoxalines; D012333:RNA, Messenger; D018341:Receptors, Adrenergic, alpha-2; D018343:Receptors, Adrenergic, beta-2; D000068438:Brimonidine Tartrate; D011433:Propranolol; D000242:Cyclic AMP; D054770:G-Protein-Coupled Receptor Kinase 3; C496567:GRK2 protein, human; C517848:GRK3 protein, human; D054769:G-Protein-Coupled Receptor Kinase 2; D009638:Norepinephrine; D004837:Epinephrine