The calcium ion (Ca²⁺), as an important second messenger, is known to be involved in many cellular functions. The present study is aimed at investigating the roles of Ca²⁺ signaling in regulating apoptosis of mammalian cells. We tried to answer the following questions: (1) Is the mitochondrial Ca²⁺ signal involved in the early stage to drive the progression of apoptosis? (2) What are the characteristics and transportation mechanisms of the mitochondrial Ca²⁺ signal in apoptosis? (3) What are the mechanisms by which the Bcl-2 family proteins such as Bcl-2 and Bax regulate the Ca²⁺ mobilization from endoplasmic reticulum (ER) to cytosol and mitochondria during apoptosis? (4) Are there any other regulators such as Bcl-2 family proteins binding partners affecting the Ca²⁺ signals? Using sin...[ Read more ]

The calcium ion (Ca²⁺), as an important second messenger, is known to be involved in many cellular functions. The present study is aimed at investigating the roles of Ca²⁺ signaling in regulating apoptosis of mammalian cells. We tried to answer the following questions: (1) Is the mitochondrial Ca²⁺ signal involved in the early stage to drive the progression of apoptosis? (2) What are the characteristics and transportation mechanisms of the mitochondrial Ca²⁺ signal in apoptosis? (3) What are the mechanisms by which the Bcl-2 family proteins such as Bcl-2 and Bax regulate the Ca²⁺ mobilization from endoplasmic reticulum (ER) to cytosol and mitochondria during apoptosis? (4) Are there any other regulators such as Bcl-2 family proteins binding partners affecting the Ca²⁺ signals? Using single living cell Ca²⁺ imaging techniques, we found that mitochondrial Ca²⁺ signal plays a positive role in the early stage of UV- or TNFα- induced apoptosis in HeLa cells. The transient mitochondrial Ca²⁺ spikes are synchronous with the cytosolic Ca²⁺ changes. Our results suggested that the Ca²⁺ release from ER plays an important role on regulating the apoptotic process since the cytosolic Ca²⁺ elevation during ATP treatment and TNFα-induced apoptosis are highly correlated. We then investigated the mechanisms by which Bcl-2 family proteins affect the IP₃ dependent Ca²⁺ mobilization. We found that pro-apoptotic protein Bax can counteract with anti-apoptotic Bcl-2 to facilitate the ER Ca²⁺ release by interacting with IP₃ receptor. Our results suggested that Bax could sensitize the IP₃ receptor, while Bcl-2 could inhibit IP₃ receptor during apoptosis. Finally, we examined the effect of other Bcl-2 family proteins and their binding partners on Ca²⁺ signaling. Our results indicated that the Bcl-2 family proteins may regulate the apoptotic process through modulating the Ca²⁺ mobilization from ER to cytosol and mitochondria.