Immunoprecipitation experiments showed that during starvation Beclin 1, released from Bcl-2, first bound with increasing efficiency to Ins(1,4,5)P 3 Rs; after reaching a maximal binding after 3 h, binding, however, decreased again. The interaction site of Beclin 1 was determined to be present in the N-terminal Ins(1,4,5)P 3 -binding domain of the Ins(1,4,5) P 3 R. The starvation-induced Ins(1,4,5)P 3 R sensitization was abolished in cells treated with BECN1 siRNA, but not with ATG5 siRNA, pointing toward an essential role of Beclin 1 in this process. Moreover, recombinant Beclin 1 sensitized Ins(1,4,5) P 3 Rs in 45 Ca 2+ -flux assays, indicating a direct regulation of Ins(1,4,5)P 3 R activity by Beclin 1. Finally, we found that Ins(1,4,5) P 3 R-mediated Ca 2+ signaling was critical for starvation-induced autophagy stimulation, since the Ca 2+ chelator BAPTA-AM as well as the Ins(1,4,5)P 3 R inhibitor xestospongin B abolished the increase in LC3 lipidation and GFP-LC3-puncta formation. Hence, our results indicate a tight and essential interrelation between intracellular Ca 2+ signaling and autophagy stimulation as a proximal event in response to starvation.

Disrupting inositol 1,4,5-trisphosphate (IP3) receptor (IP3R)/B-cell lymphoma 2 (Bcl-2) complexes using a cell-permeable peptide (stabilized TAT-fused IP3R-derived peptide (TAT-IDPS)) that selectively targets the BH4 domain of Bcl-2 but not that of B-cell lymphoma 2-extra large (Bcl-Xl) potentiated pro-apoptotic Ca2+ signaling in chronic lymphocytic leukemia cells. However, the molecular mechanisms rendering cancer cells but not normal cells particularly sensitive to disrupting IP3R/Bcl-2 complexes are poorly understood. Therefore, we studied the effect of TAT-IDPS in a more heterogeneous Bcl-2-dependent cancer model using a set of ‘primed to death' diffuse large B-cell lymphoma (DL-BCL) cell lines containing elevated Bcl-2 levels. We discovered a large heterogeneity in the apoptotic responses of these cells to TAT-IDPS with SU-DHL-4 being most sensitive and OCI-LY-1 being most resistant. This sensitivity strongly correlated with the ability of TAT-IDPS to promote IP3R-mediated Ca2+ release. Although total IP3R-expression levels were very similar among SU-DHL-4 and OCI-LY-1, we discovered that the IP3R2-protein level was the highest for SU-DHL-4 and the lowest for OCI-LY-1. Strikingly, TAT-IDPS-induced Ca2+ rise and apoptosis in the different DL-BCL cell lines strongly correlated with their IP3R2-protein level, but not with IP3R1-, IP3R3- or total IP3R-expression levels. Inhibiting or knocking down IP3R2 activity in SU-DHL-4-reduced TAT-IDPS-induced apoptosis, which is compatible with its ability to dissociate Bcl-2 from IP3R2 and to promote IP3-induced pro-apoptotic Ca2+ signaling. Thus, certain chronically activated B-cell lymphoma cells are addicted to high Bcl-2 levels for their survival not only to neutralize pro-apoptotic Bcl-2-family members but also to suppress IP3R hyperactivity. In particular, cancer cells expressing high levels of IP3R2 are addicted to IP3R/Bcl-2 complex formation and disruption of these complexes using peptide tools results in pro-apoptotic Ca2+ signaling and cell death.

Anti-apoptotic Bcl-2 proteins are upregulated in different cancers, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL), enabling survival by inhibiting pro-apoptotic Bcl-2-family members and inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R)-mediated Ca 2+ -signaling. A peptide tool (Bcl-2/IP 3 R Disruptor-2; BIRD-2) was developed to abrogate the interaction of Bcl-2 with IP 3 Rs by targeting Bcl-2′s BH4 domain. BIRD-2 triggers cell death in primary CLL cells and in DLBCL cell lines. Particularly, DLBCL cells with high levels of IP 3 R2 were sensitive to BIRD-2. Here, we report that BIRD-2-induced cell death in DLBCL cells does not only depend on high IP 3 R2-expression levels, but also on constitutive IP 3 signaling, downstream of the tonically active B-cell receptor. The basal Ca 2+ level in SU-DHL-4 DLBCL cells was significantly elevated due to the constitutive IP 3 production. This constitutive IP 3 signaling fulfilled a prosurvival role, since inhibition of phospholipase C (PLC) using U73122 (2.5 µM) caused cell death in SU-DHL-4 cells. Milder inhibition of IP 3 signaling using a lower U73122 concentration (1 µM) or expression of an IP 3 sponge suppressed both BIRD-2-induced Ca 2+ elevation and apoptosis in SU-DHL-4 cells. Basal PLC/IP 3 signaling also fulfilled a pro-survival role in other DLBCL cell lines, including Karpas 422, RI-1 and SU-DHL-6 cells, whereas PLC inhibition protected these cells against BIRD-2-evoked apoptosis. Finally, U73122 treatment also suppressed BIRD-2-induced cell death in primary CLL, both in unsupported systems and in co-cultures with CD40L-expressing fibroblasts. Thus, constitutive IP 3 signaling in lymphoma and leukemia cells is not only important for cancer cell survival, but also represents a vulnerability, rendering cancer cells dependent on Bcl-2 to limit IP 3 R activity. BIRD-2 seems to switch constitutive IP 3 signaling from pro-survival into pro-death, presenting a plausible therapeutic strategy.

The anti-apoptotic Bcl-2 protein is emerging as an efficient inhibitor of IP3R function, contributing to its oncogenic properties. Yet, the underlying molecular mechanisms remain not fully understood. Using mutations or pharmacological inhibition to antagonize Bcl-2's hydrophobic cleft, we excluded this functional domain as responsible for Bcl-2-mediated IP3Rs inhibition. In contrast, the deletion of the C-terminus, containing the trans-membrane domain, which is only present in Bcl-2α, but not in Bcl-2β, led to impaired inhibition of IP3R-mediated Ca2+ release and staurosporine-induced apoptosis. Strikingly, the trans-membrane domain was sufficient for IP3R binding and inhibition. We therefore propose a novel model, in which the Bcl-2's C-terminus serves as a functional anchor, which beyond mere ER-membrane targeting, underlies efficient IP3R inhibition by (i) positioning the BH4 domain in the close proximity of its binding site on IP3R, thus facilitating their interaction; (ii) inhibiting IP3R-channel openings through a direct interaction with the C-terminal region of the channel downstream of the channel-pore. Finally, since the hydrophobic cleft of Bcl-2 was not involved in IP3R suppression, our findings indicate that ABT-199 does not interfere with IP3R regulation by Bcl-2 and its mechanism of action as a cell-death therapeutic in cancer cells likely does not involve Ca2+ signaling.

Autophagy is a lysosomal degradation pathway important for cellular homeostasis and survival. Inhibition of the mammalian target of rapamycin (mTOR) is the best known trigger for autophagy stimulation. In addition, intracellular Ca2+ regulates autophagy, but its exact role remains ambiguous. Here, we report that the mTOR inhibitor rapamycin, while enhancing autophagy, also remodeled the intracellular Ca2+-signaling machinery. These alterations include a) an increase in the endoplasmic-reticulum (ER) Ca2+-store content, b) a decrease in the ER Ca2+-leak rate, and c) an increased Ca2+ release through the inositol 1,4,5-trisphosphate receptors (IP3Rs), the main ER-resident Ca2+-release channels. Importantly, buffering cytosolic Ca2+ with BAPTA impeded rapamycin-induced autophagy. These results reveal intracellular Ca2+ signaling as a crucial component in the canonical mTOR-dependent autophagy pathway.

Bcl-2 protein has emerged as a critical regulator of intracellular Ca 2+ dynamics by directly targeting and inhibiting the IP3 receptor (IP3R), a major intracellular Ca 2+ -release channel. Here, we demonstrate that such inhibition occurs under conditions of basal, but not high IP3R activity, since overexpressed and purified Bcl-2 (or its BH4 domain) can inhibit IP3R function provoked by low concentration of agonist or IP3, while fails to attenuate against high concentration of agonist or IP3. Surprisingly, Bcl-2 remained capable of inhibiting IP3R1 channels lacking the residues encompassing the previously identified Bcl-2-binding site (a.a. 1380-1408) located in the ARM2 domain, part of the modulatory region. Using a plethora of computational, biochemical and biophysical methods, we demonstrate that Bcl-2 and more particularly its BH4 domain bind to the ligand-binding domain (LBD) of IP3R1. In line with this finding, the interaction between the LBD and Bcl-2 (or its BH4 domain) was sensitive to IP3 and adenophostin A, ligands of the IP3R. Consistent with this, the BH4 domain of Bcl-2 counteracted the binding of IP3 to the LBD. Collectively, our work reveals a novel mechanism by which Bcl-2 influences IP3R activity at the level of the LBD. This allows for exquisite modulation of Bcl-2's inhibitory properties on IP3Rs that is tunable to the level of IP3 signaling in cells.

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Previous work revealed that intracellular Ca signals and the inositol 1,4,5-trisphosphate (IP) receptors (IPR) are essential to increase autophagic flux in response to mTOR inhibition, induced by either nutrient starvation or rapamycin treatment. Here, we investigated whether autophagy induced by resveratrol, a polyphenolic phytochemical reported to trigger autophagy in a non-canonical way, also requires IPRs and Ca signaling. Resveratrol augmented autophagic flux in a time-dependent manner in HeLa cells. Importantly, autophagy induced by resveratrol (80μM, 2h) was completely abolished in the presence of 10μM BAPTA-AM, an intracellular Ca-chelating agent. To elucidate the IPR's role in this process, we employed the recently established HEK 3KO cells lacking all three IPR isoforms. In contrast to the HEK293 wt cells and to HEK 3KO cells re-expressing IPR1, autophagic responses in HEK 3KO cells exposed to resveratrol were severely impaired. These altered autophagic responses could not be attributed to alterations in the mTOR/p70S6K pathway, since resveratrol-induced inhibition of S6 phosphorylation was not abrogated by chelating cytosolic Ca or by knocking out IPRs. Finally, we investigated whether resveratrol by itself induced Ca release. In permeabilized HeLa cells, resveratrol neither affected the sarco- and endoplasmic reticulum Ca ATPase (SERCA) activity nor the IP-induced Ca release nor the basal Ca leak from the ER. Also, prolonged (4 h) treatment with 100μM resveratrol did not affect subsequent IP-induced Ca release. However, in intact HeLa cells, although resveratrol did not elicit cytosolic Ca signals by itself, it acutely decreased the ER Ca-store content irrespective of the presence or absence of IPRs, leading to a dampened agonist-induced Ca signaling. In conclusion, these results reveal that IPRs and cytosolic Ca signaling are fundamentally important for driving autophagic flux, not only in response to mTOR inhibition but also in response to non-canonical autophagy inducers like resveratrol. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.