Internalization and recycling pathways of the thyrotropin receptor

Scant information is available to date on the intracellular trafficking of the TSH receptor. In the present study we have used stably transfected L cells that express the TSH receptor, I-125-labeled TSH, and antireceptor antibodies as well as gold-conjugated antireceptor monoclonal antibodies and hormone. The latter allowed us to study, by electron microscopy, the cellular distribution and endocytosis of TSH receptor. The receptor was initially localized on the plasmalemma proper and in clathrin-coated pits but was excluded from smooth vesicles open to the cell surface. It was internalized through clathrin-coated vesicles. Constitutive endocytosis represented 10% of cell surface receptor molecules. Endocytosis was increased 3-fold by incubation with hormone. The majority of internalized receptor molecules (90%) was recycled to the cell surface, whereas the hormone was degraded in lysosomes. This recycling of receptor was inhibited by administration of monensin. Electron microscopic and confocal microscopic studies were repeated in primary cultures of human thyroid cells and showed a distribution, and endocytosis pathways, very similar to those observed in transfected L cells. A previous study has shown the LH receptor to be endocytosed in high proportion and to be degraded in lysosomes. Confocal microscopy and colocalization studies with transferrin receptor confirmed that the highly homologous LH and TSH receptors exhibit, when expressed in the same cells, very different cellular trafficking properties. The use of LH/TSH receptor chimeras showed that transmembrane-intracellular domains contain information orienting the protein toward recycling or degradative pathways. The extracellular domain seems to play a role in the extent of internalization. These observations should now allow the identification of the molecular signals involved.