Receptor-mediated uptake of 3,3',5-triiodo-L-thyronine by cultured fibroblasts.

Abstract

Using video intensification fluorescence microscopy and tetramethylrhodamine (Rho)-labeled 3,3'',5-triiodo-L-thyronine (T3), T3 uptake by cultured mouse fibroblasts was studied. After incubation of cells with Rho-T3 for 30 min at 37.degree. C the fluorescent hormone was concentrated in many small bright accumulations. With a 1000-fold excess of unlabeled T3, only weak background fluorescence was seen. When cells were incubated with Rho or Rho-thyronine only background fluorescence was detected. The cellular uptake of Rho-T3 apparently occurred through a T3-specific receptor-mediated process. Most of these accumulations underwent saltatory motion in living cells, indicating that the T3 was contained within endocytic vesicles. When cultured cells were incubated with Rho-T3 for 60 min at 4.degree. C, only diffuse fluorescence was observed. Rho-T3 became concentrated in vesicles upon warming of the cells to either 23.degree. C or 37.degree. C. Simultaneous incubation of cells with fluorescein-labeled .alpha.2-macroglobulin and Rho-T3 showed that Rho-T3 was internalized in the same vesicles as .alpha.2-macroglobulin. As previously reported for .alpha.2-macroglobulin in the presence of methylamine, dansylcadaverine, or bacitracin, clustering and internalization were inhibited but the overall fluorescence intensity of the cells did not appear to be affected. Because receptor-mediated endocytosis of .alpha.2-macroglobulin occurs through clustering of ligands in coated pits on the cell surface, Rho-T3 apparently follows the same pathway. A low-molecular weight hormone enters cells by this pathway.