THE HUMAN BETA-3-ADRENERGIC RECEPTOR IS RESISTANT TO SHORT-TERM AGONIST-PROMOTED DESENSITIZATION

The human beta3-adrenergic receptor (beta3AR) lacks most of the struct ural determinants that, in the beta2AR, contribute to agonist-induced receptor desensitization. To evaluate the effect of these structural d ifferences on the beta3AR desensitization profile, the human beta2- an d beta3AR were stably expressed in Chinese hamster fibroblasts (CHW) a nd murine Ltk- cells (L cells). Incubation of CHW-beta2 or L-beta2 cel ls with 10 muM isoproterenol for 30 min induced a decrease in the maxi mal agonist-stimulated adenylyl cyclase activity and a cAMP-dependent reduction in the potency of isoproterenol to stimulate the receptor. I n addition, this pretreatment impaired the formation of the high affin ity heterotrimeric agonist-receptor-guanine nucleotide-binding protein complex and induced the sequestration of approximately 30% of the bet a2AR away from the cell surface. In contrast, similar treatment of CHW -beta3 and L-beta3 cells did not affect the maximal receptor-stimulate d adenylyl cyclase activity, nor did it induce any significant sequest ration of the beta3AR. In fact, only a modest cAMP-independent decreas e in the potency of isoproterenol to stimulate the receptor could be o bserved after isoproterenol treatment. The rapid desensitization patte rn of a chimeric beta3AR, in which the third cytoplasmic loop and the carboxyl-terminal tail were exchanged with those of the beta2AR (which include potential phosphorylation sites and other possible molecular determinants of desensitization), was found to be intermediate between those of the two original receptor subtypes. These results demonstrat e that (i) the beta3AR is less prone than the beta2AR to undergo rapid agonist-promoted desensitization and, (ii) in addition to the phospho rylation sites located in the third cytoplasmic loop and the carboxyl- terminal tail of the beta2AR, other molecular determinants contribute to short term desensitization.