ABSTRACT
Although endothelin-1 can elicit prolonged physiologic responses, accumulating evidence suggests that rapid desensitization affects the primary G protein-coupled receptors mediating these responses, the endothelin A and B receptors (ETA-R and ETB-R). The mechanisms by which this desensitization proceeds remain obscure, however. Because some intracellular domain sequences of the ETA-R and ETB-R differ substantially, we tested the possibility that these receptor subtypes might be differentially regulated by G protein-coupled receptor kinases (GRKs). Homologous, or receptor-specific, desensitization occurred within 4 min both in the ETA-R-expressing A10 cells and in 293 cells transfected with either the human ETA-R or ETB-R. In 293 cells, this desensitization corresponded temporally with agonist-induced phosphorylation of each receptor, assessed by receptor immunoprecipitation from 32Pi-labeled cells. Agonist-induced receptor phosphorylation was not substantially affected by PKC inhibition but was reduced 40% (p << 0.03) by GRK inhibition, effected by a dominant negative GRK2 mutant. Inhibition of agonist-induced phosphorylation abrogated agonist-induced ETA-R desensitization. Overexpression of GRK2, -5, or -6 in 293 cells augmented agonist-induced ET-R phosphorylation approximately 2-fold (p << 0.02), but each kinase reduced receptor-promoted phosphoinositide hydrolysis differently. While GRK5 inhibited ET-R signaling by only approximately 25%, GRK2 inhibited ET-R signaling by 80% (p << 0.01). Congruent with its superior efficacy in suppressing ET-R signaling, GRK2, but not GRK5, co-immunoprecipitated with the ET-Rs in an agonist-dependent manner. We conclude that both the ETA-R and ETB-R can be regulated indistinguishably by GRK-initiated desensitization. We propose that because of its affinity for ET-Rs demonstrated by co-immunoprecipitation, GRK2 is the most likely of the GRKs to initiate ET-R desensitization.
