Signal transduction and hormone-dependent internalization of the thyrotropin-releasing hormone receptor in cells lacking Gq and G11

The thyrotropin-releasing hormone (TRH) receptor was expressed in embryonic fibroblasts from mice lacking the a subunits of G(q) and G(11) (Fq/11 cells) to determine whether G protein coupling is necessary for agonist-dependent receptor internalization. Neither TRH nor agonists acting on endogenous receptors increased intracellular calcium unless the cells were co-transfected with the alpha subunit of G(q). In contrast, temperature-dependent internalization of [H-3]MeTRH in Fq/11 cells was the same whether G(q)alpha was expressed or not. A rhodamine-labeled TRH analog and fluorescein-labeled transferrin co-localized in endocytic vesicles in Fq/11 cells, indicating that endocytosis took place via the normal clathrin pathway. Cotransfection with beta-arrestin or V53D beta-arrestin increased TRH-dependent receptor sequestration. Fq/11 cells were co-transfected with the TRH receptor and a green fluorescent protein (GFP)-beta-arrestin conjugate. GFP-beta-arrestin was uniformly distributed in the cytoplasm of untreated cells and quickly translocated to the periphery of the cells when TRH was added. A truncated TRH receptor that lacks potential phosphorylation sites in the cytoplasmic carboxyl terminus signaled but did not internalize or cause membrane localization of GFP-beta-arrestin. These results prove that calcium signaling by the TRH receptor requires coupling to a G protein in the G(q) family, but TRH-dependent binding of beta-arrestin and sequestration do not.