Lack of Ca2+ involvement in thymocyte apoptosis induced by chelation of intracellular Zn2+

Chelation of intracellular Zn2+ with N, N, N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) triggers apoptosis predominantly in mature thymocytes. A rise in intracellular-free Ca2+ concentration ([Ca2+]i) has been associated with induction of thymocyte apoptosis by a variety of agents. Zn2+ can affect intracellular Ca2+ homeostasis, so the aim of this study was to investigate whether TPEN-induced apoptosis is mediated by Ca2+ signalling.

The possible role of Ca2+ in TPEN-induced apoptosis was investigated. Apoptotic markers used were DNA cleavage into oligonucleosomal fragments and formation of apoptotic nuclei. The change in [Ca2+]i in thymocytes after TPEN treatment was monitored using the fluorescence Ca2+ indicator dye, fura-2. The requirement of an increase in [Ca2+]i for TPEN-induced apoptosis was examined in thymocytes preloaded with the intracellular Ca2+ buffer, bis-(o-aminophenoxy)-ethane-N, N,N',N'-tetraacetic acid, or incubated in nominally Ca(2+)-free medium supplemented with EGTA. The effect of an increase in [Ca2+]i on TPEN-induced DNA fragmentation was studied by using thapsigargin or ionomycin to elevate [Ca2+]i in thymocytes.

No increase in [Ca2+]i could be detected before DNA fragmentation in thymocytes during TPEN treatment. Buffering intracellular Ca2+ with bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid or incubating cells in nominally Ca(2+)-free medium with EGTA had little effect on TPEN-induced DNA fragmentation and formation of apoptotic nuclei. Increasing thymocyte [Ca2+]i with thapsigargin or ionomycin administration during TPEN treatment resulted in an additive effect on TPEN-induced DNA fragmentation in thymocytes.

Our study shows that TPEN induces apoptosis in thymocytes by Ca(2+)-independent mechanisms and that apoptosis triggered by Zn2+ chelation and [Ca2+]i elevation affects distinct thymocyte subpopulations.