Reduction site of transferrin-dependent and transferrin-independent iron in cultured human fibroblasts.
Mammalian cells internalize iron as diferric transferrin (Fe2Tf) iron via receptor-mediated endocytosis (RME) and a redox mechanism under physiological condition and as an iron salt through the Tf-independent iron uptake (Tf-IU) system under morbid conditions. We have previously shown that Tf iron is reduced at the Tf molecule through a redox system on the plasma membrane prior to uptake [Oshiro, S., Nakajima, H., Markello, T., Krasnewich, D., Bernadini, I., & Gahl, W. (1993) J. Biol. Chem. 268, 21586-21591]. In the present study, the reduction site for Tf iron uptake via RME and for Tf-independent iron uptake via the Tf-IU system were examined using specific iron sources and well-characterized ferric or ferrous iron chelators in cultured human fibroblasts. At 4 degrees C for 1 h, although [55Fe]2Tf was not internalized into the cells, 55Fe from [55Fe]2Tf-Sepharose was taken up. However, under the same conditions, EDTA or dipyridyl removed the radioactive iron as a ferric or ferrous iron-chelator complex from [55Fe]2Tf bound to the Tf receptor on the cell surface. Moreover, after 55Fe-citrate was loaded into the cells at 4 degrees C for 1 h, 55Fe was removed from the cell surface by dipyridyl just as observed for [55Fe]2Tf-Sepharose. In inhibition experiments, ferric citrate showed a dose-dependent inhibition of the iron uptake of both Tf iron and Tf-independent iron. Conversely, Fe2Tf dose-dependently inhibited the uptake of 55Fe-citrate.(ABSTRACT TRUNCATED AT 250 WORDS)
Oshiro
S
S
Department of Biochemical Genetics, Tokyo Medical and Dental University.
Nakamura
Y
Y
Ishige
R
R
Hori
M
M
Nakajima
H
H
Gahl
W A
WA
D002462:Cell Membrane / Q000502:physiology; D002478:Cells, Cultured; D004705:Endocytosis / Q000502:physiology; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D007501:Iron / Q000378:metabolism*; D010084:Oxidation-Reduction; D014168:Transferrin / Q000502:physiology*
D014168:Transferrin; D007501:Iron