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Apoptosis, a well-recognized mode of cell death, is usually defined by plasma membrane biebbing, cell volume reduction, chromatin condensation, DNA cleavaged into nucleosomal multiples and formation of apoptotic bodies. Although the cause and execution mechanism are not clearly understood, apoptosis are shown closely related to oxidative stress, proto-oncogene, [Ca2+] and endonuclease activity. Intensive studies demonstrate that bcl-2 and bax genes are involved in regulation of apoptosis, e.g. Bcl-2 may prevent apoptosis induced by a variety of dissimilar causes including radiation, cancer chemotheapeutics, and nerve growth factor withrawal. Since immunocytochemical localization identifies both Bcl-2 and Bak in mitochondria, we propose that mitochondria may play a critical role in apoptosis. Our speculation is further supported by a recent study which demonstrated the reduction of transmembrane potential (Δ φ m) of the mitochondria might induce apoptosis (Zamzami et al. , 1996) . In this proposal, retinoic acid- and TPA- induced apoptosis in human cervical cancer HeLa cells was confirmed by DAPI stain and DNA agarose gel electrophoresis. Intracellular Ca2+ concentration determined by fluorescent ratio of fura-2-acetoxymethyl ester demonstrated that TPA may deprive Ca2+ in endoplasmic reticulum and mitochondria in 4 h, and RA in 48 h. Immunocytochemical localization displayed that Na+/Ca2+ exchanger was expressed in HeLa cells treated with TPA for 8h, and RA for 48 h. In addition, thapsigargin (release Ca2+ from endoplasmic reticulum) and ionomycin (release Ca2+ from endoplasmic reticulum and mitochondria) both induced apoptosis in HeLa cells. These results indicated that abrupt release of Ca2+ from endoplasmic reticulum and mitochondria may play an important role in apoptosis.
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