β-Arrestin 1 and 2 differentially regulate heptahelical receptor signaling and trafficking

The two widely coexpressed isoforms of β-arrestin (termed βarrestin 1 and 2) are highly similar in amino acid sequence. The β-arrestins bind phosphorylated heptahelical receptors to desensitize and target them to clathrin-coated pits for endocytosis. To better define differences in the roles of β-arrestin 1 and 2, we prepared mouse embryonic fibroblasts from knockout mice that lack one of the β-arrestins (βarr1-KO and βarr2-KO) or both (βarr1/2-KO), as well as their wild-type (WT) littermate controls. These cells were analyzed for their ability to support desensitization and sequestration of the β 2 -adrenergic receptor (β 2 -AR) and the angiotensin II type 1A receptor (AT 1A -R). Both βarr1-KO and βarr2-KO cells showed similar impairment in agonist-stimulated β 2 -AR and AT 1A -R desensitization, when compared with their WT control cells, and the βarr1/2-KO cells were even further impaired. Sequestration of the β 2 -AR in the βarr2-KO cells was compromised significantly (87% reduction), whereas in the βarr1-KO cells it was not. Agonist-stimulated internalization of the AT 1A -R was only slightly reduced in the βarr1-KO but was unaffected in the βarr2-KO cells. In the βarr1/2-KO cells, the sequestration of both receptors was dramatically reduced. Comparison of the ability of the two β-arrestins to sequester the β 2 -AR revealed β-arrestin 2 to be 100-fold more potent than β-arrestin 1. Down-regulation of the β 2 -AR was also prevented in the βarr1/2-KO cells, whereas no change was observed in the single knockout cells. These findings suggest that sequestration of various heptahelical receptors is regulated differently by the two β-arrestins, whereas both isoforms are capable of supporting receptor desensitization and down-regulation.