Delayed micromolar elevation in intracellular calcium precedes induction of apoptosis in thapsigargin-treated breast cancer cells

Thapsigargin (TG), a highly specific inhibitor of the sarcoplasmic reticulu m and endoplasmic reticulum Ca2+-ATPase pump, can induce apoptosis in a var iety of epithelial and lymphoid cell types. In prostate cancer cell lines, TG induces an initial 5- to 10-fold elevation of intracellular calcium ([Ca 2+](i)) within a few minutes of exposure. With prolonged exposure times (i. e., 12-36 h) a second elevation of [Ca2+](i) to >10 mu M is observed. In th is study, the human breast carcinoma cell lines MCF-7 and MDA MB 468 cells were used to determine the temporal relationship between TG-induced elevati on of [Ca2+](i) and activation of programmed cell death. Using a microinjec tion method that allows for long-term analysis of [Ca2+](i) changes, we fou nd that after TG exposure, calcium measurements in these cells demonstrated an initial rise (4-fold) in [Ca2+](i) that occurred within minutes and ret urned to baseline within a few hours. With prolonged TG exposure, the cells underwent a second elevation (>5 mu M) of [Ca2+](i) occurring stochastical ly between 12 and 36 h after the initial exposure to TG. Both of the cell l ines were growth-inhibited by 100 nM TG after only 1 h of exposure, but clo nogenic ability in the MCF-7 cells was significantly reduced only after 48 h of exposure, The induction of apoptosis by TG was demonstrated by morphol ogical changes typical for programmed cell death and DNA fragmentation (bot h high molecular weight and oligonucleosomal-sized fragments were detected) after 48 h of treatment. TG induction of apoptosis in these breast cancer cells occurred subsequent to the secondary rise in [Ca2+](i), which confirm ed that this secondary rise in [Ca2+](i) is not prostate cancer-specific. T he secondary rise in [Ca2+](i) to micromolar levels may directly activate t he endonucleases responsible for DNA fragmentation that occurs as Dart of t he apoptotic process. These studies indicate that TG is an active agent in vitro against breast cancer cells. Inactive prodrug analogues of TG are cur rently being developed that can be activated by tissue-specific proteases, and further pursuit of this strategy as a potential treatment for breast ca ncer is warranted.