A supramicromolar elevation of intracellular free calcium ([Ca(2+)](i)) is consistently required to induce the execution phase of apoptosis

Abstract

Many agents, such as the endoplasmic reticulum Ca(2+) ATPase inhibitor, thapsigargin, or the ionophore, ionomycin, induce apoptosis by transiently elevating [Ca(2+)](i). The role of [Ca(2+)](i) in apoptosis induced by agents that do not immediately increase [Ca(2+)](i), such as 5-FdUr, TGF beta-1, doxorubicin, or radiation, is far more controversial. In the present paper, [Ca(2+)](i) was measured continuously for 120 h. in prostate and bladder cancer cell lines exposed to these four agents: 5-FdUR, TGF beta-1, doxorubicin, or radiation. Each of them consistently induced a delayed [Ca(2+)](i) rise associated with the morphological changes that characterize the execution phase of apoptosis (i.e. rounding, blebbing). This [Ca(2+)](i) rise occurred in two consecutive steps (< or = 10 microM and >10 microM) and resulted from a Ca(2+) influx from the extracellular medium. This delayed supramicromolar [Ca(2+)](i) rise was also observed previously in breast, prostate and bladder cancer cell lines exposed to thapsigargin. This influx regulated transcriptional reprogramming of Gadd153 and is required to activate cytochrome c release, caspase-3 activation, loss of clonal survival and DNA fragmentation. When cells were maintained in low extracellular Ca(2+) media, these phenomena were temporarily delayed but occurred on return to normal Ca(2+) medium. Similarly, apoptosis could be delayed by overexpressing the Ca(2+)-binding proteins, Calbindin-D(28K) and parvalbumin. As this delayed >or = 10 microM [Ca(2+)](i) elevation was observed in a number of cell lines exposed to a variety of different agents, we conclude that such elevation constitutes a key and general event of apoptosis in these malignant cells.