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

ABSTRACT

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.