Receptor-mediated uptake and internalization of transthyretin

Evidence of cellular transthyretin (TTR) binding was sought because of the observation that transthyretin can increase the uptake of its hormonal ligand. Transthyretin was bound by human hepatoma (Hep G2) cells in a time- and temperature-dependent manner, reaching equilibrium within 2 h. Scatchard analysis was consistent with a single class of high affinity binding sites with a Kd of approximately 5 nM at 0 and 4 degrees C and 14 nM at 37 degrees C. These dissociation constants are more than 2 orders of magnitude lower than the concentration of transthyretin in human serum. The apparent capacity at 0 degrees C, corrected for internalized TTR, was approximately 20,000 sites/cell. Saturable, high affinity binding of human transthyretin was also demonstrable with rat primary hepatocytes and human renal adenocarcinoma, neuroblastoma, and transformed lung cells. Rat and human transthyretin were equipotent in displacing isotopically labeled, species-specific transthyretin from human hepatoma cells and rat primary hepatocytes, a finding that is consistent with the strong homology between rat and human transthyretin. Eighty-eight percent of the saturable uptake was internalized as determined by proteolytic removal of surface transthyretin. Internalization was dependent on receptor binding and was more markedly inhibited than surface binding at 0 degrees C. Concentrations of thyroxine within a range that saturated a significant proportion of the primary and secondary TTR iodothyronine binding sites increased the uptake and internalization of transthyretin in a dose-dependent manner. By analogy to the function of receptors for other transport proteins, the interaction between transthyretin and its receptor is likely to affect ligand delivery and may have additional metabolic effects.