The presence of β(2)-adrenoceptors sensitizes α(2A)-adrenoceptors to desensitization after chronic epinephrine treatment

BACKGROUND: In addition to the regulation of blood pressure, α(2)- and β-adrenoceptor (AR) subtypes play an important role in the modulation of noradrenergic neurotransmission in the human CNS and PNS. Several studies suggest that the α(2)-AR responsiveness in cells and tissues after chronic epinephrine (EPI) or norepinephrine (NE) exposure may vary, depending on the β-AR activity present there. Recently, we reported that in BE(2)-C human neuroblastoma cells (endogenously expressing α(2A)- and β(2)-AR), chronic EPI treatment (300 nM) produced a dramatic β-adrenoceptor-dependent desensitization of the α(2A)-AR response. The aim of this study is to determine if stable addition of a β(2)-AR to a second neuroblastoma cell line (SH-SY5Y), that normally expresses only α(2A)-ARs that are not sensitive to 300 nM EPI exposure, would suddenly render α(2A)-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 α(2)-AR subtype expression. Stable clones of SH-SY5Y cells transfected to stably express functional β(2)-ARs (SHβ(2)AR4) were selected to compare sensitivity of α(2)-AR to EPI in the presence or absence of β(2)-ARs. RESULTS: A series of molecular, biochemical and pharmacological studies indicated that the difference between the cell lines could not be attributed to α(2)-AR heterogeneity. We now report that after transfection of functional β(2)-AR into SH-SY5Y cells (SHβ(2)AR4), chronic treatment with modest levels of EPI desensitizes the α(2A)-AR. This effect results from a β(2)-AR dependent down-regulation of native α(2A)-ARs by EPI accompanied by enhanced translocation of GRK2 and GRK3 to the membrane (required for GRK-mediated phosphorylation of agonist-occupied receptors). CONCLUSION: This study further supports the hypothesis that the presence of the β-AR renders the α(2A)-AR more susceptible to desensitization with physiological levels of EPI.