PTEN counteracts FBXL2 to promote IP3R3– and Ca(2+)–mediated apoptosis limiting tumour growth

In response to environmental cues that promote IP3 (inositol 1,4,5-trisphosphate) generation, IP3 receptors (IP3Rs) located on the endoplasmic reticulum allow the ‘quasisynaptical’ feeding of calcium to the mitochondria to promote oxidative phosphorylation(1). However, persistent Ca(2+) release results in mitochondrial Ca(2+) overload and consequent apoptosis(2). Among the three mammalian IP3Rs, IP3R3 appears to be the major player in Ca(2+)-dependent apoptosis. Here we show that the F-box protein FBXL2 (the receptor subunit of one of 69 human SCF (SKP1, CUL1, F-box protein) ubiquitin ligase complexes(3)) binds IP3R3 and targets it for ubiquitin-, p97- and proteasome-mediated degradation to limit Ca(2+) influx into mitochondria. FBXL2-knockdown cells and FBXL2-insensitive IP3R3 mutant knock-in clones display increased cytosolic Ca(2+) release from the endoplasmic reticulum and sensitization to Ca(2+)-dependent apoptotic stimuli. The phosphatase and tensin homologue (PTEN) gene is frequently mutated or lost in human tumours and syndromes that predispose individuals to cancer(4). We found that PTEN competes with FBXL2 for IP3R3 binding, and the FBXL2-dependent degradation of IP3R3 is accelerated in Pten(−/−) mouse embryonic fibroblasts and PTEN-null cancer cells. Reconstitution of PTEN-null cells with either wild-type PTEN or a catalytically dead mutant stabilizes IP3R3 and induces persistent Ca(2+) mobilization and apoptosis. IP3R3 and PTEN protein levels directly correlate in human prostate cancer. Both in cell culture and xenograft models, a non-degradable IP3R3 mutant sensitizes tumour cells with low or no PTEN expression to photodynamic therapy, which is based on the ability of photosensitizer drugs to cause Ca(2+)-dependent cytotoxicity after irradiation with visible light(5,6). Similarly, disruption of FBXL2 localization with GGTi-2418, a geranylgeranyl transferase inhibitor(7), sensitizes xenotransplanted tumours to photodynamic therapy. In summary, we identify a novel molecular mechanism that limits mitochondrial Ca(2+) overload to prevent cell death. Notably, we provide proof-of-principle that inhibiting IP3R3 degradation in PTEN-deregulated cancers represents a valid therapeutic strategy.