Cellular Zn depletion by metal ion chelators (TPEN, DTPA and chelex resin) and its application to osteoblastic MC3T3-E1 cells

Trace mineral studies involving metal ion chelators have been conducted in investigating the response of gene and protein expressions of certain cell lines but a few had really focused on how these metal ion chelators could affect the availability of important trace minerals such as Zn, Mn, Fe and Cu. The aim of the present study was to investigate the availability of Zn for the treatment of MC3T3-E1 osteoblast-like cells and the availability of some trace minerals in the cell culture media components after using chelexing resin in the FBS and the addition of N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN, membrane-permeable chelator) and diethylenetriaminepentaacetic acid (DTPA, membrane-impermeable chelator) in the treatment medium. Components for the preparation of cell culture medium and Zn-treated medium have been tested for Zn, Mn, Fe and Cu contents by atomic absorption spectrophotometer or inductively coupled plasma spectrophotometer. Also, the expression of bone-related genes (ALP, Runx2, PTH-R, ProCOL I, OPN and OC) was measured on the cellular Zn depletion such as chelexing or TPEN treatment. Results have shown that using the chelexing resin in FBS would significantly decrease the available Zn (p<0.05) (39.4 +/- 1.5 micrometer vs 0.61 +/- 10.15 micrometer) and Mn (p<0.05) (0.74 +/- 0.01 micrometer vs 0.12 +/- 0.04 micrometer). However, levels of Fe and Cu in FBS were not changed by chelexing FBS. The use of TPEN and DTPA as Zn-chelators did not show significant difference on the final concentration of Zn in the treatment medium (0, 3, 6, 9, 12 micrometer) except for in the addition of higher 15 micrometer ZnCl2 which showed a significant increase of Zn level in DTPA-chelated treatment medium. Results have shown that both chelators gave the same pattern for the expression of the five bone-related genes between Zn- and Zn+, and TPEN-treated experiments, compared to chelex-treated experiment, showed lower bone-related gene expression, which may imply that TPEN would be a stronger chelator than chelex resin. This study showed that TPEN would be a stronger chelator compared to DTPA or chelex resin and TPEN and chelex resin exerted cellular zinc depletion to be enough for cell study for Zn depletion.

Zinc Depletion-induced Apoptosis of Human Retinal Pigment Epithelial Cells in Culture

Zinc is essential for the survival of all cells. Retinal pigment epithelium(RPE)and chorioretinal complex contains unusually high concentration of zinc. Recently, zinc has received special attention of clinicians for its possible association with certain ocular diseases such as age-related macular degeneration(ARMD). However, the cellular and molecular pathological correlates of zinc deficiency in retinal cells and RPE are scanty. Thus, we examined zinc deficiency-induced RPE pathology using a cell-permeant zinc chelator, TPEN. Exposure of human RPE cultures to TPEN induced death of RPE between 1 micrometer to 2 micrometer concentrations within 48h exposure. TPEN-induced cell death was completely blocked by coaddition of zinc. Addition of a broad-spectrum caspase inhibitor, zVAD-fmk and a caspase 3 inhibitor, DEVD-fmk markedly attenuated TPEN induced RPE death.Addition of a protein synthesis inhibitor cycloheximide attenuated TPEN-induced RPE death significantly. On the other hand, EGF, IGF-1, PDGF, MK-801, or trolox did not show any protective effect. Additionally, TUNEL and Hoechst staining revealed conspicuous internucleosomal DNA fragmentation. Furthermore, ultrastructural changes supported that TPEN-induced RPE death was apoptosis. The present study using human RPE cells provides a model for zinc depletion-induced apoptosis. Considering the clinical importance of ARMD, the model may provide useful insights into the pathogenic mechanisms of zinc deficiency-related degenerative eye conditions.

Cultured Human Retinal Pigment Epithelial Cell Injury Induced by Ultraviolet: Potentiation with Subtoxic Intracellular Zinc Depletion

We studied the mechanism and inhibition of cell death by exposure to UV alone or combination of UV exposure and intracellular zinc depletion with TPEN in cultured human retinal pigment epithelial cells (RPE). Cell death was quantified by measuring LDH release from injured cells. RPE were exposed to UV at 253.7 nm for 1~20 minutes.The 2~5 minutes UV exposure was duration-dependently cytotoxic, whereas 1minute exposure was minimally so.And exposure to TPEN induced concentration-dependent cell death at 1~4 micrometer range ;0.5 micrometer TPEN was minimally toxic.Then, cultures were exposed to varying exposure durations of UV in the absence or presence of 0.5 micrometer TPEN.At any point, the presence of TPEN markedly increased UV toxicity.In contrast, cell membrane-impermeable zinc chelator showed no toxicity.On the other hand, addition of a protein synthesis inhibitor or caspase inhibitor was markedly protective.In addition, RPE injury with exposure to combination of UV 1 min and TPEN 0.25 micrometer was accompanied by TUNEL and Hoechst staining positivity indicating that the toxicity is mainly apoptosis.Electron microscopic examinations revealed that nuclear fragmentation occurred even in sublethal UV or TPEN exposure, suggesting that the injury process already began at these condition consistently with the death being apoptosis. The present study has shown that combination of known risk factors may act synergistically to induce ARMD etc.The RPE injury induced by low dose UV and minimal intracellular zinc depletion was inhibited with protein synthesis inhibitor or caspase inhibitor, so these results suggested the possibility of prevention or treatment of RPE dysfunction.