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Cytosolic Ca2+ Signal Is Involved in Regulating UV-Induced Apoptosis in HeLa Cells

https://doi.org/10.1006/bbrc.2001.4532Get rights and content

Abstract

Results of recent studies using BAPTA/AM have raised a serious question on whether Ca2+ signal is truly involved in regulating the progression of apoptosis. To resolve this question, we examined the differential effects of three different Ca2+ signaling blockers (BAPTA/AM, membrane-impermeant BAPTA, and heparin) on UV-induced apoptosis in HeLa cells. We found that although the membrane-permeable form of BAPTA (i.e., BAPTA/AM) could not inhibit cell death, the membrane-impermeant form of BAPTA, loaded into the cytosol by electroporation, clearly protected cells from entering apoptosis. Furthermore, when we injected heparin to block Ca2+ release from the endoplasmic reticulum (ER) to cytosol, apoptosis was greatly suppressed. These findings strongly suggest that elevation of cytosolic Ca2+ is part of the signal that drives the progression of apoptosis. The negative result of BAPTA/AM is probably due to its dual effect on subcellular Ca2+ distribution; besides suppressing the Ca2+ elevation in cytosol, BAPTA/AM can also enter into the ER to reduce the free Ca2+ level there. The depletion of Ca2+ in ER is believed to stimulate apoptosis and thus would counterbalance the protection effect of BAPTA/AM in suppressing the cytosolic Ca2+ elevation.

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      BAPTA AM is a well-known intracellular calcium blocker that enters the cell, is cleaved and trapped irreversibly as charged Ca2+ chelator. Even then, at high concentrations BAPTA can trigger apoptosis (Caccamo et al., 2005; Pu and Chang, 2001) and increased CLU promoter activities (Araki et al., 2005). It is not clear, if intracellular Ca2+ depletion, or Ca2+ influx from the outside elicit cell death.

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