title: Surface interactions and intracellular localization of cationic ferritin: similarity to 125I-insulin in 3T3-L1 adipocytes.
abstract: We previously reported that in 3T3-L1 adipocytes 125I-insulin associates preferentially with microvilli and coated pits at low temperatures and early times of incubation. At higher temperatures it is internalized through a series of membrane limited intracellular compartments. In the present study, we used a high resolution probe, cationic ferritin (CF), to track adsorptive endocytosis in the 3T3-L1 adipocyte. We find that CF initially associates with coated pits at 2 min of incubation at 37 degrees C. With further incubation at 37 degrees C CF is internalized and after 2 to 10 min of incubation is predominantly localized to coated and non-coated clear vesicles. Approximately 50% of the apparent coated vesicles seen near the plasma membrane on single thin sections are shown by serial sectioning to be true vesicles (i.e., without a surface connection). At later time points CF is localized predominantly to lysosomal structures and, to a much smaller extent, Golgi-related structures. The remarkable similarity between 125I-insulin and CF with respect to post-binding processing suggests that while the membrane receptor confers the initial specificity, post-binding events are common for different types of ligands after they bind to cell surfaces and are subject to adsorptive endocytosis.
mesh_terms: D000273:Adipose Tissue; D000818:Animals; D002451:Cell Compartmentation; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D003034:Coated Pits, Cell-Membrane / Q000378:metabolism; D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism*; D005293:Ferritins / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism*; D051379:Mice; D008854:Microscopy, Electron
chemical_list: D007328:Insulin; C010297:polycationic ferritin; D005293:Ferritins