The labile iron pool: characterization, measurement, and participation in cellular processes

doi:10.1016/S0891-5849(02)01006-7

Free Radical Biology and Medicine

Volume 33, Issue 8, 15 October 2002, Pages 1037-1046

https://doi.org/10.1016/S0891-5849(02)01006-7 Get rights and content

Abstract

The cellular labile iron pool (LIP) is a pool of chelatable and redox-active iron, which is transitory and serves as a crossroad of cell iron metabolism. Various attempts have been made to analyze the levels of LIP following cell disruption. The chemical identity of this pool has remained poorly characterized due to the multiplicity of iron ligands present in cells. However, the levels of LIP recently have been assessed with novel nondisruptive techniques that rely on the application of fluorescent metalosensors. Methodologically, a fluorescent chelator loaded into living cells binds to components of the LIP and undergoes stoichiometric fluorescence quenching. The latter is revealed and quantified in situ by addition of strong permeating iron chelators. Depending on the intracellular distribution of the sensing and chelating probes, LIP can be differentially traced in subcellular structures, allowing the dynamic assessment of its levels and roles in specific cell compartments. The labile nature of LIP was also revealed by its capacity to promote formation of reactive oxygen species (ROS), whether from endogenous or exogenous redox-active sources. LIP and ROS levels were shown to follow similar “rise and fall” patterns as a result of changes in iron import vs. iron chelation or ferritin (FT) degradation vs. ferritin synthesis. Those patterns conform with the accepted role of LIP as a self-regulatory pool that is sensed by cytosolic iron regulatory proteins (IRPs) and feedback regulated by IRP-dependent expression of iron import and storage machineries. However, LIP can also be modulated by biochemical mechanisms that override the IRP regulatory loops and, thereby, contribute to basic cellular functions. This review deals with novel methodologies for assessing cellular LIP and with recent studies in which changes in LIP and ROS levels played a determining role in cellular processes.

Section snippets

General properties of the labile iron pool (LIP)

The cellular labile iron pool (LIP) is defined as a pool of redox-active iron complexes. It was first suggested by Jacobs [1] as an intermediate or transitory pool between extracellular iron and cellular iron associated with proteins. Operationally it is defined as a cell chelatable pool that comprises both ionic forms of iron (Fe⁺² and Fe⁺³) associated with a diverse population of ligands such as organic anions (phosphates and carboxylates), polypeptides, and surface components of membranes

Quantification of LIP

A more recent approach to estimate the LIP assumed it to be composed of relatively low-affinity iron complexes in which iron is in dynamic equilibrium and amenable to chelation ([13]; detailed review [14], [15]). This approach relied on chelators that could function as cellular iron sensors without affecting cell integrity or seriously depleting the cells from essential LIP. A given chelator would be considered suitable as a cell iron sensor, namely capable of reporting the amount of iron in

LIP and the regulation of cell iron level

Measurements of LIP levels in culture cells have indicated that those levels are maintained within a relatively narrow range of concentrations [13], [14]. It is assumed those levels are maintained homeostatically for cells to meet the metabolic demands for iron while minimizing its potential engagement in ROS formation. The form of LIP germane to cell iron regulation is likely to be in dynamic equilibrium with various intracellular iron pools and duly reporting the cellular iron status. The

Depletion of LIP can protect against oxidative damage

It is widely accepted that major forms of oxidant-induced damage involve catalytically active metals [37]. A cellular protection strategy against such oxidative damage has been attributed to the upregulation of expression of the iron scavenger FT [37]. Cells pre-exposed to hemin acquired the ability to withstand subsequent hemin insults and other oxidative challenges, apparently by increasing first the expression of heme oxygenase 1 and subsequently the endogenous FT levels [38]. A protective

Other cell properties affected by LIP

Changes in LIP can affect various cell properties by various mechanisms, as indicated in the following examples:

1)
Agents that are likely to modify the LIP of yeasts, such as chelators, alter the expression of components of the high-affinity iron uptake system, the ferrireductase Fre1, the ferroxidase Fet3, and the iron permease Ftr1 [47].
2)
Most published studies of cell modulation of LIP are related to upregulation of growth. The main contribution of iron to growth is apparently associated with

Conclusions

Although the double-edged sword properties of iron have classically been attributed to components of the cellular LIP, the latter has been determined experimentally in a limited number of studies. In most cases, the participation of the LIP in oxidative damage has been deduced from studies in which cells were redox challenged and iron chelators were used as chemical agents for attenuating a cellular response or a cell-modified parameter. Recent measurements of changes in LIP levels in living

Acknowledgements

The authors thank Breno Esposito, William Breuer, Silvina Epsztejn, and Hava Glickstein for allowing the use of some of their data for the preparation of the figures.

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¹: This article is part of a series of reviews on “Iron and Cellular Redox Status.” The full list of papers may be found on the homepage of the journal.

View full text

Serial review: iron and cellular redox statusThe labile iron pool: characterization, measurement, and participation in cellular processes1

Abstract

Section snippets

General properties of the labile iron pool (LIP)

Quantification of LIP

LIP and the regulation of cell iron level

Depletion of LIP can protect against oxidative damage

Other cell properties affected by LIP

Conclusions

Acknowledgements

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J. Biol. Chem

Free Radic. Biol. Med

Biochim. Biophys. Acta

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Trends Biochem. Sci

Neurochem. Int

Free Radic. Biol. Med

J. Hepatol

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Biochim. Biophys. Acta

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J. Biol. Chem

FEBS Lett

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FEBS Lett

Blood

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Serial review: iron and cellular redox status
The labile iron pool: characterization, measurement, and participation in cellular processes¹