Gallium compound GaQ(3) -induced Ca(2+) signalling triggers p53-dependent and -independent apoptosis in cancer cells

BACKGROUND AND PURPOSE A novel anti-neoplastic gallium complex GaQ(3) (KP46), earlier developed by us, is currently in phase I clinical trial. GaQ(3) 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 GaQ(3) -induced apoptosis in cancer cells, focusing on p53 and intracellular Ca(2+) signalling. EXPERIMENTAL APPROACH GaQ(3) -induced cytotoxicity and apoptosis were determined in cancer cell lines, with different p53 status (p53(+/+) , p53(-/-) and p53 mutant). Time course analysis of intracellular Ca(2+) calcium release, p53 promoter activation, p53-nuclear/cytoplasmic movements and reactive oxygen species (ROS) were conducted. Ca(2+) -dependent formation of the p53-p300 transcriptional complex was analysed by co-immunoprecipitation and chromatin immunoprecipitation. Ca(2+) signalling, p53, p300 and ROS were serially knocked down to study Ca(2+) -p53-ROS ineractions in GaQ(3) -induced apoptosis. KEY RESULTS GaQ(3) triggered intracellular Ca(2+) release stabilizing p53-p300 complex and recruited p53 to p53 promoter, leading to p53 mRNA and protein synthesis. p53 induced higher intracellular Ca(2+) release and ROS followed by activation of p53 downstream genes including those for the micro RNA mir34a. In p53(-/-) and p53 mutant cells, GaQ(3) -induced Ca(2+) -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 Ca(2+) -signalling-mediated p53 activation and ROS up-regulation. Understanding the mechanism of GaQ(3) -induced apoptosis will help establish this gallium-based organic compound as a potent anti-cancer drug.