The Ca2+ concentration of the endoplasmic reticulum is a key determinant of ceramide-induced apoptosis: significance for the molecular mechanism of Bcl-2 action

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Abstract
The mechanism of action of the anti‐apoptotic oncogene Bcl‐2 is still largely obscure. We have recently shown that the overexpression of Bcl‐2 in HeLa cells reduces the Ca2+ concentration in the endoplasmic reticulum ([Ca2+]er) by increasing the passive Ca2+ leak from the organelle. To investigate whether this Ca2+ depletion is part of the mechanism of action of Bcl‐2, we mimicked the Bcl‐2 effect on [Ca2+]er by different pharmacological and molecular approaches. All conditions that lowered [Ca2+]er protected HeLa cells from ceramide, a Bcl‐2‐sensitive apoptotic stimulus, while treatments that increased [Ca2+]er had the opposite effect. Surprisingly, ceramide itself caused the release of Ca2+ from the endoplasmic reticulum and thus [Ca2+] increased both in the cytosol and in the mitochondrial matrix, paralleled by marked alterations in mitochondria morphology. The reduction of [Ca2+]er levels, as well as the buffering of cytoplasmic [Ca2+] changes, prevented mitochondrial damage and protected cells from apoptosis. It is therefore concluded that the Bcl‐2‐dependent reduction of [Ca2+]er is an important component of the anti‐apoptotic program controlled by this oncogene.