Iron accumulation and iron-regulatory protein activity in human hepatoma (HepG2) cells.
Iron may populate distinct hepatocellular iron pools that differentially regulate expression of proteins such as ferritin and transferrin receptor (TfR) through iron-regulatory mRNA-binding proteins (IRPs), and may additionally regulate uptake and accumulation of non-transferrin-bound iron (NTBI). We examined iron-regulatory protein (IRP) binding activity and ferritin/TfR expression in human hepatoma (HepG2) cells exposed to iron at different levels for different periods. Several iron-dependent RNA-binding activities were identified, but only IRP increased with beta-mercaptoethanol. With exposures between 0 and 20 microg/ml iron, decreases in IRP binding accompanied large changes in TfR and ferritin expression, while chelation of residual iron with deferoxamine (DFO) caused a large increase in IRP binding with little additional effect on TfR or ferritin expression. Cellular iron content increased beyond 4 days of exposure to iron at 20 microg/ml, when IRP binding, TfR, and ferritin had all reached stable levels. However, iron content of the cells plateaued by 7 days, or decreased with 24 h exposure to very high concentrations (>50 microg/ml) of iron. These results indicate that iron-replete HepG2 cells exhibit a narrow range of maximal responsiveness of the IRP-regulatory mechanism, whose functional response is blunted both by excessive iron exposure and by removal of iron from a chelatable pool. HepG2 cells are able to limit iron accumulation upon higher or prolonged exposure to NTBI, apparently independent of the IRP mechanism.
Popovic
Zvezdana
Z
Laboratory Medicine and Pathobiology, University of Toronto, 1 King's College Circle, Toronto, Ont., Canada.
Templeton
Douglas M
DM
D015153:Blotting, Western; D002460:Cell Line; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D003593:Cytoplasm / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004492:Edetic Acid / Q000494:pharmacology; D005293:Ferritins / Q000737:chemistry / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D007501:Iron / Q000737:chemistry / Q000378:metabolism*; D035925:Iron-Regulatory Proteins / Q000096:biosynthesis*; D012313:RNA / Q000737:chemistry; D011990:Receptors, Transferrin / Q000096:biosynthesis; D011994:Recombinant Proteins / Q000737:chemistry; D013997:Time Factors
D035925:Iron-Regulatory Proteins; D011990:Receptors, Transferrin; D011994:Recombinant Proteins; D012313:RNA; D005293:Ferritins; D004492:Edetic Acid; D007501:Iron