Apoptosis which plays an important role in tumorigenesis as well as embryo development
has been known to be induced by the sustained rise in intracellular free Ca2+. However,
many studies have shown that apoptosis is not always related with increased intracellular
free calcium concentration. In the present study, the effect of several intracellular Ca2+
inhibitors on cell death was investigated in order to evaluate the role of intracellular Ca2+
signal inhibition in apoptotic cell death using HL-60 human promyelocytic leukemia cells
as a model cellular system. Plasma membrane Ca2+ channel blockers (verapamil,
nifedipine and diltiazem), intracellular Ca2+ release blockers (dantrolene, TMB-8 and
ruthenium red), an extracellular Ca2+ chelator (EGTA) and an intracellular Ca2+ chelator
(BAPTA/AM) were used as intracellular Ca2+ inhibitors in the experiments. Treatment with
these agents resulted in a concentration-dependent decreased cell viability assessed by
MTT assay. These agents also induced genomic DNA fragmentation, a hallmark of
apoptosis, indicating that the mechanism by which these agents induce cell death was
through apoptosis. No effect of cycloheximide, a protein synthesis inhibitor, on the cell
death was observed, implying that new protein synthesis may not be required for the
apoptosis caused by these intracellular Ca2+ inhibitors. These results suggest that the
inhibition of intracellular Ca2+ signals may be involved in the induction of apoptosis in HL-60
cells.

Keywords: HL-60 cell, Apoptosis, Cycloheximide, BATA/AM, Dantrolene, Diltiazem,
EGTA, Nifedipine, Ruthenium red, Verapamil