Role of intracellular free Ca(II) and Zn(II) in dexamethasone‐induced apoptosis and dexamethasone resistance in human leukemic CEM cell lines

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Abstract
The levels of intracellular free Ca(II) and Zn(II) during dexamethasone (dex)‐induced apoptosis in CEM cell lines were determined by 19F nuclear magnetic resonance (NMR), using the fluorinated intracellular chelator 1,2‐bis‐(2‐amino‐5‐fluorophenoxy)ethane‐N, N, N′, N′‐tetraacetic acid (5‐FBAPTA). The effects of these divalent metal ions on growth rate and DNA degradation were evaluated. Measurements were done on one dex‐sensitive (CEM‐C7) and three different dex‐resistant variants (CEM‐C1, CEM‐4R4, and CEM‐ICR27). Dex caused a continuous increase in the Ca(II) level in dex‐sensitive CEM‐C7 cells, while in CEM‐C1 cells dex caused an initial increase in the Ca(II) level which in ≈︁36 h was restored to its normal value. The intracellular Ca(II) level in CEM‐4R4 cells was not significantly affected by dex, while that of CEM‐ICR27 cells decreased after dex incubation. Only the dex‐sensitive CEM‐C7 cells showed dex‐induced DNA degradation. An intracellular free Zn(II) level of ≈︁1 nM was measured for the dex‐resistant CEM‐C1 cells. No detectable level of intracellular Zn(II) was found in the other cell lines. Incubation with <100 μM Zn(II) did not inhibit dex‐induced apoptosis in CEM‐C7 cells (e.g., DNA degradation). Treatment with ≈︁250 μM Zn(II) caused significant decrease in growth rate in all cell lines and prevented dex‐induced DNA degradation in CEM‐C7 cells. A calibrated amount of Ca(II) ionophore (A23187), used to increase Ca(II) concentrations up to the dex‐induced levels, did not induce DNA degradation in CEM‐C7 or CEM‐C1 cells. While elevation of intracellular Ca(II) by itself is not sufficient to initiate apoptosis in CEM‐C7 cells, the results reported here suggest that Ca(II) is involved in the killing mechanism as a secondary factor. The combination of dex and ionophore caused significant DNA degradation in CEM‐C1 cells, which normally showed resistance to each compound individually. The combination of dex and the Zn(II) chelator phenanthroline also caused extensive DNA degradation in the normally dex‐resistant CEM‐C1 cells, suggesting that Zn(II) plays a role in the dex resistance of these cells.