PTEN counteracts FBXL2 to promote IP3R3- and Ca2+-mediated apoptosis limiting tumour growth

Abstract

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 phosphorylation1. However, persistent Ca2+ release results in mitochondrial Ca2+ overload and consequent apoptosis2. Among the three mammalian IP3Rs, IP3R3 appears to be the major player in Ca2+-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 complexes3) binds IP3R3 and targets it for ubiquitin-, p97- and proteasome-mediated degradation to limit Ca2+ influx into mitochondria. FBXL2-knockdown cells and FBXL2-insensitive IP3R3 mutant knock-in clones display increased cytosolic Ca2+ release from the endoplasmic reticulum and sensitization to Ca2+-dependent apoptotic stimuli. The phosphatase and tensin homologue (PTEN) gene is frequently mutated or lost in human tumours and syndromes that predispose individuals to cancer4. 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 Ca2+ 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 Ca2+-dependent cytotoxicity after irradiation with visible light5,6. Similarly, disruption of FBXL2 localization with GGTi-2418, a geranylgeranyl transferase inhibitor7, sensitizes xenotransplanted tumours to photodynamic therapy. In summary, we identify a novel molecular mechanism that limits mitochondrial Ca2+ overload to prevent cell death. Notably, we provide proof-of-principle that inhibiting IP3R3 degradation in PTEN-deregulated cancers represents a valid therapeutic strategy.

NS5A Promotes Constitutive Degradation of IP3R3 to Counteract Apoptosis Induced by Hepatitis C Virus

“…FBXL2 is a member of the family of F-box proteins (Kuchay et al, 2013, 2017), which are substrate-targeting subunits for SCF (SKP1, CUL1, F-box protein) ubiquitin ligase complexes (Skaar et al, 2013). In humans, there are 69 SCF ligases, each utilizing a different F-box protein subunit (Skaar et al, 2009).…”

Tumor suppressive Ca2+ signaling is driven by IP3 receptor fitness

“…Instead, in this work, the effects of FBXL2 on IP 3 R3 are tumor promoting by increasing IP 3 R3 degradation and making the cells more resistant to cell death [36]. These effects are neutralized by PTEN, which prevents FBLX2 binding to IP 3 R3 channels.…”

“…It is nonetheless likely that FBXL2 binding to IP 3 Rs does not occur by targeting its calmodulin-binding motif even though a role for calmodulin in regulating IP 3 R3/FBXL2-complex formation may not be ruled out. In any case, access to the degron region in the ligand-binding core of IP 3 R3 for FBXL2 is facilitated by deletion of the suppressor domain and loss of PTEN is associated with increased FBXL2 binding to IP 3 R3 and degradation of IP 3 R3, contributing to the apoptotic resistance of cells [36]. In cancer cells lacking PTEN, FBXL2 knockdown could therefore restore IP 3 R3 levels and apoptotic sensitivity.…”

“…Recent work from Kuchay et al revealed an unexpected role for the tumor suppressor lipid/protein phosphatase PTEN, an allele frequently lost in cancer [35] and wellknown negative regulator of PKB/Akt signaling, stabilizing IP 3 R channels by protecting them from proteasomal degradation [36] (Fig. 1).…”

“…In wild-type p53 tumors, direct p53 activators might be of use. Other solace may come from SERCA-activating small molecules like CDN1163 [54], provided these agents can be delivered to tumor cells while sparing healthy cells [52].These recent papers highlight that deregulation of IP 3 R3 ubiquitination homeostasis not only impacts death and survival of cells but also contributes to the oncogenic behavior of cells with dysfunctional tumor suppressors by either (i) lacking PTEN [36] or by (ii) displaying deficiencies in BAP1 [47]. They underpin the emerging role of altered Ca 2+ signaling at the MAM level as a key event in apoptosis resistance that contributes early events associated with oncogenesis and tumor formation.…”