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

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 approximately 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 approximately 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 microM Zn(II) did not inhibit dex-induced apoptosis in CEM-C7 cells (e.g., DNA degradation). Treatment with approximately 250 microM 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.