THAPSIGARGIN-INDUCED PERSISTENT INTRACELLULAR CALCIUM POOL DEPLETION AND APOPTOSIS IN HUMAN HEPATOMA-CELLS

We found that thapsigargin (Tg), a non-phorbol ester type tumor promoter that specifically inhibits endoplasmic reticulum Ca2+-ATPase, transiently increased the level of cytosolic free calcium ([Ca2+](i)) and subsequently induced chromatin condensation, nuclear fragmentation, and internucleosomal DNA cleavage in cultured PLC/PRF/5 human hepatoma cells. These alterations were followed by the loss of plasma membrane integrity and by cell death. Epidermal growth factor (EGF) and vasopressin similarly elevated [Ca2+](i) without causing DNA fragmentation which is characteristic of apoptosis. Consequently, the elevation of [Ca2+](i) itself was not sufficient for causing Tg-induced cell death. On the other hand, preculturing the cells with Tg completely suppressed Ca2+ mobilization induced by EGF and vasopressin; a result that strongly suggests that Tg depleted the endoplasmic reticulum Ca2+ pool. Such depletion is hypothesized to induce apoptotic cell death in this hepatoma cell line by changing the nuclear Ca2+ levels which probably produce a structural change in chromatin.