Modulation of intracellular Ca2+ signalling in HeLa cells by the apoptotic cell death enhancer PK11195

1-(2-Chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195) is a proven enhancer of apoptotic cell death in a variety of cellular models. This effect is independent of its established cellular target, the mitochondrial benzodiazepine receptor (mBzR), since it is able to promote cell death also in mBzR knockout cells. Thus recently it was suggested that PK11195 might exert its effect by modulating the expression and function of the oncogene Bcl-2. We have previously demonstrated that Bcl-2 modulates cellular Ca(2+) homeostasis as its overexpression reduces the Ca(2+) concentration in the endoplasmic reticulum (ER) ([Ca(2+)](er)), impairing mitochondrial and cytosolic Ca(2+) overload during cellular stress and therefore inhibiting the induction of the apoptotic cascade. Here, using ER, mitochondria and cytosolic targeted aequorin probes, we show that cellular treatment with PK11195 induces opposite changes in cellular Ca(2+) homeostasis, increasing the [Ca(2+)](er) and amplifying IP(3) induced Ca(2+) transients in mitochondria ([Ca(2+)](m)) and cytosol ([Ca(2+)](c)). This work provides evidence for a novel pharmacological effect of PK11195 on Ca(2+) signalling which may be linked to its effect on Bcl-2 and account for its role in apoptotic cell death. (c) 2008 Elsevier Inc. All rights reserved.