title: Modulation by iron loading and chelation of the uptake of non-transferrin-bound iron by human liver cells.
abstract: Hepatic non-transferrin-bound Fe (NTBI) flux and its regulation were characterized by measuring the uptake of Fe from [59Fe]/nitrilotriacetate (NTA) complexes in control and Fe-loaded cultures of human hepatocellular carcinoma cells (HepG2). Exposure to ferric ammonium citrate (FAC) for 1 to 7 days resulted in a time- and dose-dependent increase in the rate of NTBI uptake. In contrast to previous studies showing a dependence of the rate of Fe uptake on extracellular Fe, this was positively correlated with total cellular Fe content. The Fe3+ chelating agents deferoxamine (DFO), 1,2-dimethyl-3-hydroxypyrid-4-one (CP 020) and 1,2-diethyl-3-hydroxypyrid-4-one (CP 094) prevented or diminished the increase in NTBI transport when present during Fe loading and reversed the stimulation in pre-loaded cells in relation to their abilities to decrease intracellular iron. Although saturation of the Fe uptake process was not achieved in control cells, kinetic modelling to include linear diffusion-controlled processes yielded estimated parameters of Km = 4.3 microM and Vmax = 2.6 fmol/micrograms protein/min for the underlying process. There was a significant increase in the apparent Vmax (31.2 fmol/micrograms protein per min) for NTBI uptake in Fe-loaded cells, suggesting that Fe loading increases the number of a rate-limiting carrier site for Fe. Km also increased to 15.2 microM, comparable to values reported when whole liver is perfused with FeSO4. We conclude that HepG2 cells possess a transferrin-independent mechanism of Fe accumulation that responds reversibly to a regulatory intracellular Fe pool.
authors.affiliation: Department of Clinical Biochemistry, University of Toronto, Ontario, Canada.
authors.forename: J G
authors.initials: JG
authors.lastname: Parkes
authors.forename: E W
authors.initials: EW
authors.lastname: Randell
authors.forename: N F
authors.initials: NF
authors.lastname: Olivieri
authors.forename: D M
authors.initials: DM
authors.lastname: Templeton
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D006528:Carcinoma, Hepatocellular; D016923:Cell Death / Q000187:drug effects; D000077543:Deferiprone; D003676:Deferoxamine / Q000494:pharmacology; D004058:Diffusion; D005290:Ferric Compounds / Q000494:pharmacology; D006801:Humans; D007501:Iron / Q000378:metabolism* / Q000494:pharmacology*; D007502:Iron Chelating Agents / Q000378:metabolism* / Q000494:pharmacology; D007504:Iron Radioisotopes; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008113:Liver Neoplasms; D009571:Nitrilotriacetic Acid / Q000378:metabolism; D011728:Pyridones / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D014168:Transferrin / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D005290:Ferric Compounds; D007502:Iron Chelating Agents; D007504:Iron Radioisotopes; D011728:Pyridones; D000644:Quaternary Ammonium Compounds; D014168:Transferrin; C068543:1,2-diethyl-3-hydroxypyridin-4-one; D000077543:Deferiprone; D007501:Iron; D003676:Deferoxamine; D009571:Nitrilotriacetic Acid; C013531:ferric ammonium citrate