Elsevier

Toxicology

Volume 222, Issues 1–2, 1 May 2006, Pages 95-102
Toxicology

Relationship between thallium(I)-mediated plasma membrane fluidification and cell oxidants production in Jurkat T cells

https://doi.org/10.1016/j.tox.2006.01.028Get rights and content

Abstract

The effects of thallous cation (Tl+) on: (a) the production of oxidant species and (b) membrane fluidity were evaluated in human leukemia T cells (Jurkat). After 72 h of incubation in the presence of Tl+ (5–100 μM), no significant changes in cell viability were observed, although the average cell size was decreased as evaluated by steady-state light scattering. Tl+ (5–100 μM) caused a significant increase in the concentration of cellular oxidants as measured with the probe 5(6)-carboxy-2′,7′-dichlorodihydrofluorescein diacetate (DCDCDHF). Similarly, a higher lipid oxidation products release was observed as measured by TBARS production. Both Tl+-mediated DCDCDHF oxidation and TBARS production were prevented when cells were supplemented with 2 mM Trolox. Tl+ (5–100 μM) also induced a concentration-dependent increase in plasma membrane fluidity, evaluated with the probe 6-(9-anthroyloxy)stearic acid (6-AS). This effect of Tl+ was neither associated to the externalization of phosphatidylserine, nor observed in Trolox-supplemented cells. Significant correlations were found between the increase in plasma membrane fluidity and TBARS production and DCDCDHF oxidation. Together, the present results suggest that the increase in cellular oxidants caused by Tl+ could oxidize membrane fatty acids, resulting in an increase in membrane fluidity. These effects could underlie the pathology associated with Tl+ toxicity.

Introduction

Thallium (Tl) is a heavy metal present in the earth's crust as salts and minerals, at very low concentrations. This metal is widely used in the manufacture of electronic devices, in smelting plants, in cement factories, and in certain medical procedures (ATSDR, 1999). As a consequence of these human activities, Tl can be released to the environment (Douglas et al., 1990, Repetto et al., 1998, Heim et al., 2002, Xiao et al., 2004), exposing humans to its noxious effects. For example, it has been described that Tl can affect several tissues and systems, including the epidermal, gastrointestinal, cardiovascular, reproductive and renal systems (Douglas et al., 1990, Moore et al., 1993, Galvan-Arzate and Santamaria, 1998, Heim et al., 2002). It can also cross the blood–brain barrier (Galvan-Arzate and Rios, 1994, Galván-Arzate et al., 2000), and deposit in the brain where it causes neurodegeneration, demyelination, and the accumulation of end products of lipid oxidation (Galván-Arzate et al., 2000).

Recently, we demonstrated that both Tl cationic species (Tl+ and Tl3+) can interact with membrane phospholipids causing alterations in their physical properties (Villaverde and Verstraeten, 2003). Opposed to Tl3+, Tl+ has a relatively low charge density and, therefore, it has a poor tendency to establish ionic bonding to phospholipid headgroups. However, we found that Tl+ can induce a modest increase in liposome surface potential, followed by a decrease in the fluidity of lipid domains enriched in negatively charged phospholipids (Villaverde and Verstraeten, 2003).

The aim of the present work was to investigate the hypothesis that, similarly to our previous findings in liposomes, Tl+ could either affect cells plasma membrane fluidity through direct or indirect mechanisms. Working with human leukemia T cells (Jurkat), the effects of increasing Tl+ concentrations (5–100 μM) on plasma membrane integrity and fluidity were investigated. Also, the effects of this cation on the formation of cellular oxidant levels and the release of lipid oxidation products were evaluated, as well as their possible association with plasma membrane fluidity. In the range of concentrations assessed, and after 72 h of Tl+ addition, a significant increase in plasma membrane fluidity was observed. No alterations in cell membrane permeability to bulky molecules were found. In addition, Tl+ increased the content of cytoplasmic oxidant species and oxidation products release to the media. This latter significantly correlated with the increase in plasma membrane fluidity and with the average cells size. Together, the experimental results suggest that in Jurkat T cells, Tl+ exacerbates the production of oxidant species available to oxidize membrane lipids and alter their fluidity, effect that could ultimately lead to a dysfunction of plasma membrane-associated metabolic processes observed in Tl+ toxicity.

Section snippets

Chemicals

Human leukemia T cells (Jurkat) were obtained from the American Type Culture Collection (A.T.C.C., Rockville, MD, USA). RPMI 1640 medium and fetal bovine serum (FBS) were purchased from Gibco BRL (Grand Island, NY, USA). Thallium(I) nitrate was from Fluka (Milwaukee, WI, USA). The fluorescent probes 6-(9-anthroyloxy)stearic acid (6-AS), 5(6)-carboxy-2′,7′-dichlorodihydrofluorescein diacetate (DCDCDHF), merocyanine 540 (MC), and propidium iodide (PI) were purchased from Molecular Probes Inc.

Effects of Tl+ on cell viability

The possibility that Tl+ could affect the functional integrity of Jurkat T cells plasma membrane was investigated 72 h after a single administration of Tl+ (5–100 μM). The exclusion of the dye Trypan Blue, a method commonly used for the determination of cell viability was first assessed. In the range of concentrations studied, Tl+ did not affect cell viability (Fig. 1A). Similarly, the ability of these cells to reduce the dye MTT was comparable to that found in control cells (Fig. 1A). The

Discussion

Tl is a non-essential heavy metal that contaminates soils and water due to its elimination into the environment (Heim et al., 2002, Hoffman, 2003). In fact, the high content of Tl in the soil is an environmental concern in certain countries (Heim et al., 2002, Xiao et al., 2004). It has been described that Tl+ intoxication results in the alteration of a considerable amount of biological processes. Among others, Tl+ can induce a local increase of lipid oxidation products content in five

Acknowledgements

This work was supported by grants from the University of Buenos Aires (B 072) and the Agencia Nacional de Promoción Científica y Tecnológica (PICT 12285), Argentina.

References (33)

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