The inability of cells to grow in low iron correlates with increasing activity of their iron regulatory protein (IRP)

Sensitivity of cells to apoptosis induced by iron deprivation can be reversibly changed by iron availability

We tested the effect of iron deprivation on cell death induction in human Raji cells pre-adapted to differing availability of extracellular iron. Iron deprivation was achieved by incubation in a defined iron-free medium. Original Raji cells have previously been adapted to long-term culture in a defined medium with 5 microg/ml of iron-saturated human transferrin as a source of iron. Raji/lowFe cells were derived from original Raji cells by subsequent adaptation to culture in the medium with 50 microm ferric citrate as a source of iron. Raji/lowFe-re cells were derived from Raji/lowFe cells by re-adaptation to the transferrin-containing (5 microg/ml) medium. Iron deprivation induced cell death in both Raji cells and Raji/lowFe-re cells; that is, cells pre-adapted to a near optimum source of extracellular iron (5 microg/ml of transferrin). However, Raji/lowFe cells preadapted to a limited source of extracellular iron (50 microm ferric citrate) became resistant to the induction of cell death by iron deprivation. We demonstrated that cell death induction by iron deprivation in Raji cells correlates with the activation of executioner caspase-3 and the cleavage of caspase-3 substrate, poly-ADP ribose polymerase. Two other executioner caspases, caspase-7 and caspase-6, were not activated. Taken together, we suggest that in human Raji cells, iron deprivation induces apoptotic cell death related to caspase-3 activation. However, the sensitivity of the cells to death induction by iron deprivation can be reversibly changed by extracellular iron availability. The cells pre-adapted to a limited source of extracellular iron became resistant.

Stimulation of non-transferrin iron uptake by iron deprivation in K562 cells

We tested the effect of iron deprivation on the uptake of iron from ferric citrate by human erythroleukemia K562 cells. The iron uptake after 24-h preincubation in defined iron-free medium was approximately 2-3x higher than after the preincubation in control transferrin-containing medium. The preincubation of K562 cells in iron-free medium together with the inhibitor of protein synthesis cycloheximide completely abrogated the stimulation of the iron uptake. The preincubation in iron-free medium resulted in a slight decrease (20%) of DMT1 mRNA level. The level of Dcytb, ferroportin and hephaestin mRNA did not exert any significant change. We also did not find any significant effect on the protein level of DMT1, Dcytb, ferroportin and hephaestin. We conclude that iron deprivation stimulates the uptake of non-transferrin iron in K562 cells and that this stimulation depends on protein synthesis. It seems that the expression of an unknown or seemingly unrelated protein(s) is involved.

Additive stimulatory effect of extracellular calcium and potassium on non-transferrin ferric iron uptake by HeLa and K562 cells

We studied the effects of Ca(2+) and K(+) on non-transferrin iron uptake from ferric citrate complex by HeLa and K562 cells. Uptake experiments in Na-HEPES buffer (137 mM NaCl, 4 mM KCl) showed that extracellular Ca(2+) stimulated the iron uptake. The rate of iron uptake in 4 mM Ca(2+) was about 3-5 times higher than without Ca(2+). The iron uptake in K-HEPES buffer (68 mM NaCl, 75 mM KCl) with a high K(+) level was transiently stimulated during the first 10 min. The rate of iron uptake for 0.4 mM Ca(2+) was approximately 3 times higher in K-HEPES buffer than in Na-HEPES buffer. The calcium channel blockers verapamil (50 microM) and nifedipine (5 microM) had no effect on the uptake either in control Na-HEPES buffer or after K(+) stimulation in K-HEPES buffer. The sodium channel blocker lidocaine (50 microM) also had no effect on the uptake of iron in Na-HEPES buffer as well as after K(+) stimulation. Furthermore, the iron uptake was not significantly affected when Na(+) in the Na-HEPES and K-HEPES buffers was replaced by isotonic saccharose. We conclude that extracellular calcium per se, and not intracellular calcium or Ca(2+) transport, stimulates ferric iron uptake by both HeLa and K562 cells. A high level of extracellular K(+) also stimulates the uptake, probably via cell membrane depolarization. Na(+) is not involved in these stimulations of iron uptake. The transient K(+) effect and continuous Ca(2+) effect seem to be additive.

Screening of transcriptionally regulated genes following iron chelation in human astrocytoma cells

Deferoxamine is an effective iron chelator and a potential therapeutic agent for use in minimizing free radical-mediated injury following trauma. Iron chelation may also be an effective means of limiting tumor growth by decreasing bioavailable iron. Deferoxamine can modulate gene expression through manipulation of intracellular iron levels; specifically at the posttranscription level by changing the activity of iron regulatory proteins (IRPs). The effect of iron chelation on the transcription of genes is still unclear, but iron-binding sites on DNA have been reported. Here we investigate the influence of deferoxamine on gene transcription. Two-directional (forward and backward) suppression subtraction hybridization (SSH) was performed on human astrocytoma cells (SW1088) cultured in either standard media or treated for 48 hours with deferoxamine. To restrict the number of false-positive clones, reverse Northern blotting was used to further verify the differentially expressed cDNA clones. Positive clones were sequenced and the mRNAs were re-examined on Northern blots for changes in expression over time of deferoxamine exposure. The results of these analyses have identified both novel and known genes whose expression is influenced by iron chelation. The known genes include a group related to energy production and a group related to protease function. These results provide examples of genes not previously known to be directly influenced by iron availability, and as such may be potential targets for iron chelation therapy.