Characterization of transferrin receptor released by K562 erythroleukemia cells

A soluble form of transferrin receptor has been detected in human serum and has been shown recently to be a truncated form of the intact membrane bound receptor. Mechanisms governing the release of transferrin receptor by cells are poorly understood and could be better defined by tissue culture. The present investigation was undertaken to characterize the transferrin receptor released by K562 erythroleukemic cells. In contrast with maturing sheep reticulocytes, which have been shown to release transferrin receptor in small vesicles termed exosomes, we demonstrated, with a monoclonal enzyme-linked immunoassay, that less than 30% of the transferrin receptor released by K562 cells in log phase growth was in a particulate form. The relative amounts of soluble and particulate receptor released to the supernatant did not change significantly during 48 hr of incubation. Soluble receptor was purified by immunoaffinity chromatography. On polyacrylamide gel electrophoresis, its mobility was the same (85 kDa) as that of the truncated monomeric form recently identified in human serum. Further evidence that serum and soluble receptors released by K562 cells are identical was provided by amino acid sequence analysis, which demonstrated that 16 of the first 19 residues of the N-terminal sequence of soluble K562 receptor are homologous with the serum receptor. The remaining three were not identifiable. K562 cells provide a useful in vitro model for studying the production of membrane-bound and soluble forms of released transferrin receptor.