Simultaneous α2B- and β2-Adrenoceptor Activation Sensitizes the α2B-Adrenoceptor for Agonist-Induced Down-Regulation

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 311; no. 2; p. 794
• Main Authors: Aarti N. Desai, Kelly M. Standifer, Douglas C. Eikenburg
• Format: Journal Article
• Language: English
• Published: American Society for Pharmacology and Experimental Therapeutics 01.11.2004
• ISSN: 0022-3565; 1521-0103
• DOI: 10.1124/jpet.104.069674

We recently reported that α 2A -adrenoceptor (AR) desensitization and down-regulation occurs after 24-h treatment with epinephrine (EPI) (0.3 μM) in BE(2)-C cells that express both α 2 - and β 2 -ARs. The same concentration of norepinephrine (NE) has no effect. The effect of EPI is prevented by β 2 -AR blockade and is associated with an increase in G protein-coupled receptor kinase 3 (GRK3) expression. Because differences in agonist-induced down-regulation of the α 2A -versus α 2B -ARs have been reported, the present study examines the effects of simultaneous activation of α 2B - and β 2 -ARs on α 2B -AR number and signaling. We studied NG108 cells that naturally express α 2B -ARs, and BN17 cells, NG108 cells transfected to express the human β 2 -AR. In NG108 cells, α 2B -AR desensitization and down-regulation require treatment with 20 μM EPI or NE; GRK expression was not changed. In BN17 cells expressing β 2 -ARs, the threshold EPI concentration for α 2B -AR desensitization and down-regulation was reduced to 0.3 μM; 10 μM NE was required for the same effect. Furthermore, 24-h EPI or NE treatments that produced desensitization also resulted in a selective 2-fold up-regulation of GRK3; GRK2 was unchanged. The β-AR antagonist alprenolol (1 μM) and GRK3 antisense (but not sense) DNA blocked 0.3 μM EPI- and 10 μM NE-induced desensitization and down-regulation of the α 2B -AR as well as GRK3 up-regulation. In conclusion, simultaneous activation of α 2B - and β 2 -ARs results in a 67-fold decrease in the threshold concentration of EPI required for α 2B -AR down-regulation. This lower threshold for down-regulation is associated with α 2B - and β 2 -AR dependent up-regulation of GRK3 expression.