Gallium compound GaQ3-induced Ca2+signalling triggers p53-dependent and -independent apoptosis in cancer cells

BACKGROUND AND PURPOSE A novel anti-neoplastic gallium complex GaQ3 (KP46), earlier developed by us, is currently in phase I clinical trial. GaQ3 induced S-phase arrest and apoptosis via caspase/PARP cleavage in a variety of cancers. However, the underlying mechanism of apoptosis is unknown. Here, we have explored the mechanism(s) of GaQ3-induced apoptosis in cancer cells, focusing on p53 and intracellular Ca2+ signalling. EXPERIMENTAL APPROACH GaQ3-induced cytotoxicity and apoptosis were determined in cancer cell lines, with different p53 status (p53+/+, p53-/-and p53 mutant). Time course analysis of intracellular Ca2+ calcium release, p53 promoter activation, p53-nuclear/ cytoplasmic movements and reactive oxygen species (ROS) were conducted. Ca2+-dependent formation of the p53-p300 transcriptional complex was analysed by co-immunoprecipitation and chromatin immunoprecipitation. Ca2+ signalling, p53, p300 and ROS were serially knocked down to study Ca2+-p53-ROS ineractions in GaQ3-induced apoptosis. KEY RESULTS GaQ3 triggered intracellular Ca2+ release stabilizing p53-p300 complex and recruited p53 to p53 promoter, leading to p53 mRNA and protein synthesis. p53 induced higher intracellular Ca2+ release and ROS followed by activation of p53 downstream genes including those for the micro RNA mir34a. In p53-/-and p53 mutant cells, GaQ3-induced Ca2+-signalling generated ROS. ROS further increased membrane translocation of FAS and FAS-mediated extrinsic apoptosis. CONCLUSIONS AND IMPLICATIONS This study disclosed a novel mechanism of Ca2+-signalling-mediated p53 activation and ROS up-regulation. Understanding the mechanism of GaQ3-induced apoptosis will help establish this gallium-based organic compound as a potent anti-cancer drug.