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 Ca²⁺ homeostasis as its overexpression reduces the Ca²⁺ concentration in the endoplasmic reticulum (ER) ([Ca²⁺]_er), impairing mitochondrial and cytosolic Ca²⁺ 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 Ca²⁺ homeostasis, increasing the [Ca²⁺]_er and amplifying IP₃ induced Ca²⁺ transients in mitochondria ([Ca²⁺]_m) and cytosol ([Ca²⁺]_c). This work provides evidence for a novel pharmacological effect of PK11195 on Ca²⁺ signalling which may be linked to its effect on Bcl-2 and account for its role in apoptotic cell death.