DIFFERENTIAL LOCALIZATION OF 2 TYPES OF TRANSFERRIN - PRODUCED BY HUMAN FIBROBLASTS OR INCORPORATED FROM CULTURE-MEDIUM

In a previous paper, we demonstrated that cultured human fibroblasts s ynthesize transferrin (Tf). Two types of Tf are present; one is produc ed by the cells and the other is internalized from the culture medium. To study the metabolism of intracellular Tf, we investigated the subc ellular localization of the two types of Tf in human fibroblasts by im munocytochemical and fluorescence-labeling techniques. The internalize d Tf was found to be localized in the perinuclear area, and the synthe sized Tf was associated with microtubules, forming a fibrous structure in the cytoplasm. When the cells were treated with colchicine which d epolymerizes microtubules irreversibly, the synthesized Tf lost its fi brous structure and spread out in cytoplasm, but the internalized Tf r emained around the nucleus. These results suggest that the two types o f Tf are regulated differently in the cells.