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.