Cell-type specific effects of endocytosis inhibitors on 5-hydroxytryptamine_2A receptor desensitization and resensitization reveal an arrestin-, GRK2-, and GRK5-independent mode of regulation in human embryonic kidney 293 cells

The effect of endocytosis inhibitors on 5-hydroxytryptamine_2A (5-HT_2A) receptor desensitization and resensitization was examined in transiently transfected human embryonic kidney (HEK) 293 cells and in C6 glioma cells that endogenously express 5-HT_2A receptors. In HEK-293 cells, 5-HT_2A receptor desensitization was unaffected by cotransfection with a dominant-negative mutant of dynamin (DynK44A), a truncation mutant of arrestin-2 [Arr2(319-418)], or by two well-characterized chemical inhibitors of endocytosis: concanavalin A (conA) and phenylarsine oxide (PAO). In contrast, β2-adrenergic receptor desensitization was significantly potentiated by each of these treatments in HEK-293 cells. In C6 glioma cells, however, DynK44A, Arr2(319- 418), conA, and PAO each resulted in the potentiation of 5-HT_2A and β-adrenergic receptor desensitization. The cell-type-specific effect of Arr2(319-418) on 5-HT_2A receptor desensitization was not related to the level of GRK2 or GRK5 expression. Interestingly, although β2-adrenergic receptor resensitization was potently blocked by cotransfection with DynK44A, 5-HT_2A receptor resensitization was enhanced, suggesting the existence of a novel cell-surface mechanism for 5-HT_2A receptor resensitization in HEK-293 cells. In addition, Arr2(319-418) had no effect on 5-HT_2A receptor resensitization in HEK-293 cells, although it attenuated the resensitization of the β2-adrenergic receptor. However, in C6 glioma cells, both DynK44A and Arr2(319-418) significantly reduced 5-HT_2A receptor resensitization. Taken together, these results provide the first convincing evidence of cell-type-specific roles for endocytosis inhibitors in regulating GPCR activity. Additionally, these results imply that novel GRK and arrestin-independent mechanisms of 5-HT_2A receptor desensitization and resensitization exist in HEK-293 cells.