Differential mechanism for the cell surface sorting and agonist‐promoted internalization of the α1B‐adrenoceptor

Abstract

α_1B‐adrenoceptors are localized at a steady state in the plasma membrane in untreated cells, and internalize to intracellular vesicles when exposed to agonist. Flow cytometry analysis with an anti‐N‐terminus‐antibody (1B‐N1‐C, (Hirasawa et al., 1996)) facilitated the quantification of cell surface α_1B‐adrenoceptor. Also, the cellular distribution of α_1B‐adrenoceptors was visually monitored by immunocytochemical confocal microscopy. Utilizing this combined approach, we have examined the molecular mechanism for cellular trafficking of α_1B‐adrenoceptors, including the process of sorting of the synthesized receptor protein to the cell surface, and the agonist‐induced internalization. The two processes were separately examined by using α_1B‐adrenoceptor inducible DDT₁MF‐2 cells for the sorting process and CHO cells stably expressing α_1B‐adrenoceptors for the agonist‐promoted internalization. We examined the effects of cytochalasin D and mycalolide B (actin depolymerization agents), demecolcine (a microtubule disrupting agent), brefeldin A (an inhibitor of vesicular transport and Golgi function), bafilomycin A1 (a specific inhibitor of the vacuolar proton pump) or hyperosmotic sucrose treatment (that may inhibit clathrin‐mediated endocytosis) on these processes. We found that the agonist‐promoted internalization was blocked by cytochalasin D and mycalolide B, while the cell surface sorting process was specifically blocked by brefeldin A, indicating that the two processes involve different components of the cellular endocytic machinery. The experimental approach as exemplified in this study would provide a valuable system to study further the molecular mechanism(s) of cellular trafficking of G protein‐coupled receptors.