EFFECT OF CELL-TYPE ON THE SUBCELLULAR-LOCALIZATION OF THE THYROTROPIN-RELEASING-HORMONE RECEPTOR

The localization of an epitope-tagged receptor for thyrotropin-releasi ng hormone (TRH) expressed in different cell contexts was studied with immunofluorescence microscopy. In pituitary lactotrophs, which normal ly express TRH receptors, and in AtT20 pituitary corticotrophs, TRH re ceptor immunoreactivity was primarily confined to the plasma membrane. In HEK 293 and COS7 cells, TRH receptors were predominantly intracell ular. In transiently transfected COS7 cells, the TRH receptor colocali zed with endoplasmic reticulum and Golgi markers. The pattern of TRH r eceptor immunofluorescence was the same over a wide range of receptor expression in transiently transfected COS7 cells, and all cell lines b ound similar amounts of H-3- and rhodamine-labeled TRH analogs, sugges ting that cell-specific differences in TRH receptor localization were not simply the result of overexpression. In all cell contexts, TRH rec eptors on the plasma membrane underwent extensive ligand-driven endocy tosis. Inhibitors of glycosylation did not alter the subcellular distr ibution of receptors. In HEK 293 cells expressing the transfected TRH receptor, protein synthesis inhibitors caused translocation of intrace llular receptors to the cell surface, as shown by a marked increase in cell surface immunofluorescence and [H-3][N-3-methyl-His(2)]TRH bindi ng. These results demonstrate that the subcellular localization of the TRH receptor depends on the cell context in which it is expressed and that intracellular receptors are capable of translocation to the plas ma membrane.