Desensitization of α_2A‐adrenoceptor signalling by modest levels of adrenaline is facilitated by β₂‐adrenoceptor‐dependent GRK3 up‐regulation

Abstract

Adrenaline (ADR) and noradrenaline (NA) can simultaneously activate inhibitory α2‐ and stimulatory β‐adrenoceptors (AR). However, ADR and NA differ significantly in that ADR is a potent β2‐AR agonist while NA is not. Only recently has the interaction resulting from the simultaneous activation of α2‐ and β2‐AR been examined at the cellular level to determine the mechanisms of α2‐AR regulation following concomitant activation of both α2‐ and β2‐ARs by chronic ADR. This study evaluates β2‐AR regulation of α2A‐AR signalling following chronic ADR (300 nM) and NA (1 and 30 μM) treatments of BE(2)‐C human neuroblastoma cells that natively express both β2‐ and α2A‐ARs. Chronic (24 h) treatment with ADR (300 nM) desensitized the response to the α2A‐AR agonist, brimonidine, in BE(2)‐C cells. Addition of the β‐AR antagonist, propranolol, blocked the ADR‐induced α2A‐AR desensitization. Unlike ADR, chronic NA (1 μM) treatment had no effect on the α2A‐AR response. However if NA was increased to 30 μM for 24 h, α2A‐AR desensitization was observed; this desensitization was partially reversed by propranolol. Chronic ADR (300 nM) treatment reduced α2A‐AR binding levels, contributing to the α2A‐AR desensitization. This decrease was prevented by addition of propranolol during ADR treatment. Chronic NA (30 μM), like ADR, treatment lowered specific binding, whereas 1 μM NA treatment was without effect. Chronic ADR treatment produced a significant increase in GRK3 levels and this was blocked by propranolol or GRK2/3 antisense DNA treatment. This antisense DNA, common to both GRK2 and GRK3, also blocked chronic ADR‐induced α2A‐AR desensitization and down‐regulation. Acute (1 h) ADR (300 nM) or NA treatment (1 μM) produced α2A‐AR desensitization. The desensitization produced by acute treatment was β‐AR independent, as it was not blocked by propranolol. We conclude that chronic treatment with modest levels of ADR produces α2A‐AR desensitization by mechanisms that involve up‐regulation of GRK3 and down‐regulation of α2A‐AR levels through interactions with the β2‐AR. British Journal of Pharmacology (2003) 138, 921–931. doi:

Involvement of G Protein-Coupled Receptor Kinase (GRK) 3 and GRK2 in Down-Regulation of the α2B-Adrenoceptor

“…The human ␣ 2A -AR mutant (Del 293-304) that lacks the four serine residues in the third intracellular loop thought to be phosphorylated by GRKs does not acutely desensitize but does undergo down-regulation (Jewell-Motz et al, 1997). In contrast to these observations, recent studies in BE(2)-C and BN17 cells and in GRK3-and GRK2-overexpressing NG108 cells suggest that the ␣ 2A -and ␣ 2B -AR are rendered more sensitive to agonist-induced down-regulation because of a 2-to 3-fold up-regulation of GRK3 (Bawa et al, 2003;Desai et al, 2004Desai et al, , 2005. In addition, mutation of potential GRK phosphorylation sites ( 322 SSTS 325 changed to AAVA) in the third intracellular loop of opossum ␣ 2C -AR prevents receptor down-regulation after 24-h exposure to 0.3 M NE (Deupree et al, 2002).…”

“…We have previously shown an increase in sensitivity of the ␣ 2A -or ␣ 2B -AR to undergo agonist-induced down-regulation after an increase in GRK levels (Bawa et al, 2003;Desai et al, 2004Desai et al, , 2005. In BN17 cells, coactivation of ␣ 2B -and ␤ 2 -AR results in a modest selective up-regulation of GRK3 and increases the sensitivity of the ␣ 2B -AR to undergo agonist-induced down-regulation (Desai et al, 2004).…”

Cellular G Protein-Coupled Receptor Kinase Levels Regulate Sensitivity of the α2B-Adrenergic Receptor to Undergo Agonist-Induced Down-Regulation

“…Previous results indicate that the ␣ 2 -AR may be more susceptible to the action of GRK3 than GRK2 (Diverse-Pierluissi et al, 1996;Bawa et al, 2003;Desai et al, 2004). Therefore, the sensitivity of the ␣ 2B -AR to increasing levels of GRK2 also was determined.…”

“…We have shown that GRK is not only required for ␣ 2A -and ␣ 2B -AR down-regulation, but that the sensitivity of the ␣ 2 -AR to undergo down-regulation is increased if the level of GRK is increased (Bawa et al, 2003;Desai et al, 2004). This observation is interesting for two reasons.…”

Arrestins and two receptor kinases are upregulated in Parkinson's disease with dementia

“…The concentrations of arrestin/GRKs in cultured cells and in vivo are altered by drugs that directly or indirectly cause persistent stimulation or blockade of GPCRs [7,22,25,29,42,45,72,80,99], which implies the involvement of signaling mechanism in the regulation of the arrestin/GRK expression. The concentration GRK2, a short-lived protein, is regulated by synthesis as well as degradation, both of which are responsive to various signaling pathways (reviewed in [85,86]).…”