A synthetic peptide targeting the BH4 domain of Bcl-2 induces apoptosis in multiple myeloma and follicular lymphoma cells alone or in combination with agents targeting the BH3-binding pocket of Bcl-2

Recent advances in canonical versus non-canonical Ca2+-signaling-related anti-apoptotic Bcl-2 functions and prospects for cancer treatment

Cell fate is tightly controlled by a continuous balance between cell survival and cell death inducing mechanisms. B-cell lymphoma 2 (Bcl-2)-family members, composed of effectors and regulators, not only control apoptosis at the level of the mitochondria but also by impacting the intracellular Ca2+ homeostasis and dynamics. On the one hand, anti-apoptotic protein Bcl-2, prevents mitochondrial outer membrane permeabilization (MOMP) by scaffolding and neutralizing proapoptotic Bcl-2-family members via its hydrophobic cleft (region composed of BH-domain 1-3). On the other hand, Bcl-2 suppress pro-apoptotic Ca2+ signals by binding and inhibiting IP3 receptors via its BH4 domain, which is structurally exiled from the hydrophobic cleft by a flexible loop region (FLR). As such, Bcl-2 prevents excessive Ca2+ transfer from ER to mitochondria. Whereas regulation of both pathways requires different functional regions of Bcl-2, both seem to be connected in cancers that overexpress Bcl-2 in a life-promoting dependent manner. Here we discuss the anti-apoptotic canonical and non-canonical role, via calcium signaling, of Bcl-2 in health and cancer and evolving from this the proposed anti-cancer therapies with their shortcomings. We also argue how some cancers, with the major focus on diffuse large B-cell lymphoma (DLBCL) are difficult to treat, although theoretically prime marked for Bcl-2-targeting therapeutics. Further work is needed to understand the non-canonical functions of Bcl-2 also at organelles beyond the mitochondria, the interaction partners outside the Bcl-2 family as well as their ability to target or exploit these functions as therapeutic strategies in diseases.

Expanding roles of BCL-2 proteins in apoptosis execution and beyond

The proteins of the BCL-2 family are known as key regulators of apoptosis, with interactions between family members determining permeabilisation of the mitochondrial outer membrane (MOM) and subsequent cell death. However, the exact mechanism through which they form the apoptotic pore responsible for MOM permeabilisation (MOMP), the structure and specific components of this pore, and what roles BCL-2 proteins play outside of directly regulating MOMP are incompletely understood. Owing to the link between apoptosis dysregulation and disease, the BCL-2 proteins are important targets for drug development. With the development and clinical use of drugs targeting BCL-2 proteins showing success in multiple haematological malignancies, enhancing the efficacy of these drugs, or indeed developing novel drugs targeting BCL-2 proteins is of great interest to treat cancer patients who have developed resistance or who suffer other disease types. Here, we review our current understanding of the molecular mechanism of MOMP, with a particular focus on recently discovered roles of BCL-2 proteins in apoptosis and beyond, and discuss what implications these functions might have in both healthy tissues and disease.

CYD0281, a Bcl-2 BH4 domain antagonist, inhibits tumor angiogenesis and breast cancer tumor growth

BACKGROUND

B-cell lymphoma 2 (Bcl-2) family proteins are key regulators of apoptosis, which possess four conserved Bcl-2 homologies (BH) domains. Among the BH domains, the BH3 domain is considered as a potent 'death domain' while the BH4 domain is required for anti-apoptotic activity. Bcl-2 can be converted to a pro-apoptotic molecule through the removal or mutation of the BH4 domain. Bcl-2 is considered as an inducer of angiogenesis, which can promote tumor vascular network formation and further afford nutrients and oxygen to promote tumor progression. However, whether disrupting the function of the BH4 domain to convert Bcl-2 into a pro-apoptotic molecule could make Bcl-2 possess the potential for anti-angiogenic therapy remains to be defined.

METHODS

CYD0281 was designed and synthesized according to the lead structure of BDA-366, and its function on inducing a conformational change of Bcl-2 was further evaluated via immunoprecipitation (IP) and immunofluorescence (IF) assays. Moreover, the function of CYD0281 on apoptosis of endothelial cells was analyzed via cell viability, flow cytometry, and western blotting assays. Additionally, the role of CYD0281 on angiogenesis in vitro was determined via endothelial cell migration and tube formation assays and rat aortic ring assay. Chick embryo chorioallantoic membrane (CAM) and yolk sac membrane (YSM) models, breast cancer cell xenograft tumor on CAM and in mouse models as well as the Matrigel plug angiogenesis assay were used to explore the effects of CYD0281 on angiogenesis in vivo.

RESULTS

We identified a novel potent small-molecule Bcl-2-BH4 domain antagonist, CYD0281, which exhibited significant anti-angiogenic effects both in vitro and in vivo, and further inhibited breast cancer tumor growth. CYD0281 was found to induce conformational changes in Bcl-2 through the exposure of the BH3 domain and convert Bcl-2 from an anti-apoptotic molecule into a cell death inducer, thereby resulting in the apoptosis of vascular endothelial cells.

CONCLUSIONS

This study has revealed CYD0281 as a novel Bcl-2-BH4 antagonist that induces conformational changes of Bcl-2 to convert to a pro-apoptotic molecule. Our findings indicate that CYD0281 plays a crucial role in anti-angiogenesis and may be further developed as a potential anti-tumor drug candidate for breast cancer. This work also provides a potential anti-angiogenic strategy for breast cancer treatment.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Balancing life and death: BCL-2 family members at diverse ER-mitochondrial contact sites

The outer mitochondrial membrane is a busy place. One essential activity for cellular survival is the regulation of membrane integrity by the BCL-2 family of proteins. Another critical facet of the outer mitochondrial membrane is its close approximation with the endoplasmic reticulum. These mitochondrial-associated membranes (MAMs) occupy a significant fraction of the mitochondrial surface and serve as key signaling hubs for multiple cellular processes. Each of these pathways may be considered as forming their own specialized MAM subtype. Interestingly, like membrane permeabilization, most of these pathways play critical roles in regulating cellular survival and death. Recently, the pro-apoptotic BCL-2 family member BOK has been found within MAMs where it plays important roles in their structure and function. This has led to a greater appreciation that multiple BCL-2 family proteins, which are known to participate in numerous functions throughout the cell, also have roles within MAMs. In this review, we evaluate several MAM subsets, their role in cellular homeostasis, and the contribution of BCL-2 family members to their functions.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

A non-canonical role for pyruvate kinase M2 as a functional modulator of Ca2+ signalling through IP3 receptors

Pyruvate kinase isoform M2 (PKM2) is a rate-limiting glycolytic enzyme that is widely expressed in embryonic tissues. The expression of PKM2 declines in some tissues following embryogenesis, while other pyruvate kinase isozymes are upregulated. However, PKM2 is highly expressed in cancer cells and is believed to play a role in supporting anabolic processes during tumour formation. In this study, PKM2 was identified as an inositol 1,4,5-trisphosphate receptor (IP₃R)-interacting protein by mass spectrometry. The PKM2:IP₃R interaction was further characterized by pull-down and co-immunoprecipitation assays, which showed that PKM2 interacted with all three IP₃R isoforms. Moreover, fluorescence microscopy indicated that both IP₃R and PKM2 localized at the endoplasmic reticulum. PKM2 binds to IP₃R at a highly conserved 21-amino acid site (corresponding to amino acids 2078-2098 in mouse type 1 IP₃R isoform). Synthetic peptides (denoted 'TAT-D5SD' and 'D5SD'), based on the amino acid sequence at this site, disrupted the PKM2:IP₃R interaction and potentiated IP₃R-mediated Ca²⁺ release both in intact cells (TAT-D5SD peptide) and in a unidirectional ⁴⁵Ca²⁺ flux assay on permeabilized cells (D5SD peptide). The TAT-D5SD peptide did not affect the enzymatic activity of PKM2. Reducing PKM2 protein expression using siRNA increased IP₃R-mediated Ca²⁺ signalling in intact cells without altering the ER Ca²⁺ content. These data identify PKM2 as an IP₃R-interacting protein that inhibits intracellular Ca²⁺ signalling. The elevated expression of PKM2 in cancer cells is therefore not solely connected to its canonical role in glycolytic metabolism, rather PKM2 also has a novel non-canonical role in regulating intracellular signalling.

Deregulated calcium signaling in blood cancer: Underlying mechanisms and therapeutic potential

Intracellular calcium signaling regulates diverse physiological and pathological processes. In solid tumors, changes to calcium channels and effectors via mutations or changes in expression affect all cancer hallmarks. Such changes often disrupt transport of calcium ions (Ca²⁺) in the endoplasmic reticulum (ER) or mitochondria, impacting apoptosis. Evidence rapidly accumulates that this is similar in blood cancer. Principles of intracellular Ca²⁺ signaling are outlined in the introduction. We describe different Ca²⁺-toolkit components and summarize the unique relationship between extracellular Ca²⁺ in the endosteal niche and hematopoietic stem cells. The foundational data on Ca²⁺ homeostasis in red blood cells is discussed, with the demonstration of changes in red blood cell disorders. This leads to the role of Ca²⁺ in neoplastic erythropoiesis. Then we expand onto the neoplastic impact of deregulated plasma membrane Ca²⁺ channels, ER Ca²⁺ channels, Ca²⁺ pumps and exchangers, as well as Ca²⁺ sensor and effector proteins across all types of hematologic neoplasms. This includes an overview of genetic variants in the Ca²⁺-toolkit encoding genes in lymphoid and myeloid cancers as recorded in publically available cancer databases. The data we compiled demonstrate that multiple Ca²⁺ homeostatic mechanisms and Ca²⁺ responsive pathways are altered in hematologic cancers. Some of these alterations may have genetic basis but this requires further investigation. Most changes in the Ca²⁺-toolkit do not appear to define/associate with specific disease entities but may influence disease grade, prognosis, treatment response, and certain complications. Further elucidation of the underlying mechanisms may lead to novel treatments, with the aim to tailor drugs to different patterns of deregulation. To our knowledge this is the first review of its type in the published literature. We hope that the evidence we compiled increases awareness of the calcium signaling deregulation in hematologic neoplasms and triggers more clinical studies to help advance this field.

Bcl-2 Family of Proteins in the Control of Mitochondrial Calcium Signalling: An Old Chap with New Roles

Bcl-2 family proteins are considered as one of the major regulators of apoptosis. Indeed, this family is known to control the mitochondrial outer membrane permeabilization (MOMP): a central step in the mitochondrial pathway of apoptosis. However, in recent years Bcl-2 family members began to emerge as a new class of intracellular calcium (Ca²⁺) regulators. At mitochondria-ER contacts (MERCs) these proteins are able to interact with major Ca²⁺ transporters, thus controlling mitochondrial Ca²⁺ homeostasis and downstream Ca²⁺ signalling pathways. Beyond the regulation of cell survival, this Bcl-2-dependent control over the mitochondrial Ca²⁺ dynamics has far-reaching consequences on the physiology of the cell. Here, we review how the Bcl-2 family of proteins mechanistically regulate mitochondrial Ca²⁺ homeostasis and how this regulation orchestrates cell death/survival decisions as well as the non-apoptotic process of cell migration.

Promising Anti-Mitochondrial Agents for Overcoming Acquired Drug Resistance in Multiple Myeloma

Multiple myeloma (MM) remains an incurable tumor due to the high rate of relapse that still occurs. Acquired drug resistance represents the most challenging obstacle to the extension of survival and several studies have been conducted to understand the mechanisms of this phenomenon. Mitochondrial pathways have been extensively investigated, demonstrating that cancer cells become resistant to drugs by reprogramming their metabolic assessment. MM cells acquire resistance to proteasome inhibitors (PIs), activating protection programs, such as a reduction in oxidative stress, down-regulating pro-apoptotic, and up-regulating anti-apoptotic signals. Knowledge of the mechanisms through which tumor cells escape control of the immune system and acquire resistance to drugs has led to the creation of new compounds that can restore the response by leading to cell death. In this scenario, based on all literature data available, our review represents the first collection of anti-mitochondrial compounds able to overcome drug resistance in MM. Caspase-independent mechanisms, mainly based on increased oxidative stress, result from 2-methoxyestradiol, Artesunate, ascorbic acid, Dihydroartemisinin, Evodiamine, b-AP15, VLX1570, Erw-ASNase, and TAK-242. Other agents restore PIs' efficacy through caspase-dependent tools, such as CDDO-Im, NOXA-inhibitors, FTY720, GCS-100, LBH589, a derivative of ellipticine, AT-101, KD5170, SMAC-mimetics, glutaminase-1 (GLS1)-inhibitors, and thenoyltrifluoroacetone. Each of these substances improved the efficacy rates when employed in combination with the most frequently used antimyeloma drugs.

Cancer cell death strategies by targeting Bcl-2's BH4 domain

The Bcl-2-family proteins have long been known for their role as key regulators of apoptosis. Overexpression of various members of the family is associated with oncogenesis. Its founding member, anti-apoptotic Bcl-2 regulates cell death at different levels, whereby Bcl-2 emerged as a major drug target to eradicate cancers through cell death. This resulted in the development of venetoclax, a Bcl-2 antagonist that acts as a BH3 mimetic. Venetoclax already entered the clinic to treat relapse chronic lymphocytic leukemia patients. Here, we discuss the role of Bcl-2 as a decision-maker in cell death with focus on the recent advances in anti-cancer therapeutics that target the BH4 domain of Bcl-2, thereby interfering with non-canonical functions of Bcl-2 in Ca²⁺-signaling modulation. In particular, we critically discuss previously developed tools, including the peptide BIRD-2 (Bcl-2/IP₃R-disrupter-2) and the small molecule BDA-366. In addition, we present a preliminary analysis of two recently identified molecules that emerged from a molecular modeling approach to target Bcl-2's BH4 domain, which however failed to induce cell death in two Bcl-2-dependent diffuse large B-cell lymphoma cell models. Overall, antagonizing the non-canonical functions of Bcl-2 by interfering with its BH4-domain biology holds promise to elicit cell death in cancer, though improved tools and on-target antagonizing small molecules remain necessary and ought to be designed.

BIRD-2, a BH4-domain-targeting peptide of Bcl-2, provokes Bax/Bak-independent cell death in B-cell cancers through mitochondrial Ca2+-dependent mPTP opening

Anti-apoptotic Bcl-2 critically controls cell death by neutralizing pro-apoptotic Bcl-2-family members at the mitochondria. Bcl-2 proteins also act at the endoplasmic reticulum, the main intracellular Ca²⁺-storage organelle, where they inhibit IP₃ receptors (IP₃R) and prevent pro-apoptotic Ca²⁺-signaling events. IP₃R channels are targeted by the BH4 domain of Bcl-2. Some cancer types rely on the IP₃R-Bcl-2 interaction for survival. We previously developed a cell-permeable, BH4-domain-targeting peptide that can abrogate Bcl-2's inhibitory action on IP₃Rs, named Bcl-2 IP₃ receptor disrupter-2 (BIRD-2). This peptide kills several Bcl-2-dependent cancer cell types, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukaemia (CLL) cells, by eliciting intracellular Ca²⁺ signalling. However, the exact mechanisms by which these excessive Ca²⁺ signals triggered by BIRD-2 provoke cancer cell death remain elusive. Here, we demonstrate in DLBCL that although BIRD-2 activates caspase 3/7 and provokes cell death in a caspase-dependent manner, the cell death is independent of pro-apoptotic Bcl-2-family members, Bim, Bax and Bak. Instead, BIRD-2 provokes mitochondrial Ca²⁺ overload that is rapidly followed by opening of the mitochondrial permeability transition pore (mPTP). Inhibiting mitochondrial Ca²⁺ overload using Ru265, an inhibitor of the mitochondrial Ca²⁺ uniporter complex counteracts BIRD-2-induced cancer cell death. Finally, we validated our findings in primary CLL patient samples where BIRD-2 provoked mitochondrial Ca²⁺ overload and Ru265 counteracted BIRD-2-induced cell death. Overall, this work reveals the mechanisms by which BIRD-2 provokes cell death, which occurs via mitochondrial Ca²⁺ overload but acts independently of pro-apoptotic Bcl-2-family members.

Inhibition of InsP3R with Xestospongin B Reduces Mitochondrial Respiration and Induces Selective Cell Death in T Cell Acute Lymphoblastic Leukemia Cells

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy whose chemoresistance and relapse persist as a problem despite significant advances in its chemotherapeutic treatments. Mitochondrial metabolism has emerged as an interesting therapeutic target given its essential role in maintaining bioenergetic and metabolic homeostasis. T-ALL cells are characterized by high levels of mitochondrial respiration, making them suitable for this type of intervention. Mitochondrial function is sustained by a constitutive transfer of calcium from the endoplasmic reticulum to mitochondria through the inositol 1,4,5-trisphosphate receptor (InsP3R), making T-ALL cells vulnerable to its inhibition. Here, we determine the bioenergetic profile of the T-ALL cell lines CCRF-CEM and Jurkat and evaluate their sensitivity to InsP3R inhibition with the specific inhibitor, Xestospongin B (XeB). Our results show that T-ALL cell lines exhibit higher mitochondrial respiration than non-malignant cells, which is blunted by the inhibition of the InsP3R. Prolonged treatment with XeB causes T-ALL cell death without affecting the normal counterpart. Moreover, the combination of XeB and glucocorticoids significantly enhanced cell death in the CCRF-CEM cells. The inhibition of InsP3R with XeB rises as a potential therapeutic alternative for the treatment of T-ALL.

IP3 Receptor Biology and Endoplasmic Reticulum Calcium Dynamics in Cancer

Intracellular Ca²⁺ signaling regulates a plethora of cellular functions. A central role in these processes is reserved for the inositol 1,4,5-trisphosphate receptor (IP₃R), a ubiquitously expressed Ca²⁺-release channel, mainly located in the endoplasmic reticulum (ER). Three IP₃R isoforms (IP₃R1, IP₃R2 and IP₃R3) exist, encoded respectively by ITPR1, ITPR2 and ITPR3. The proteins encoded by these genes are each about 2700 amino acids long and assemble into large tetrameric channels, which form the target of many regulatory proteins, including several tumor suppressors and oncogenes. Due to the important role of the IP₃Rs in cell function, their dysregulation is linked to multiple pathologies. In this review, we highlight the complex role of the IP₃R in cancer, as it participates in most of the so-called "hallmarks of cancer". In particular, the IP₃R directly controls cell death and cell survival decisions via regulation of autophagy and apoptosis. Moreover, the IP₃R impacts cellular proliferation, migration and invasion. Typical examples of the role of the IP₃Rs in these various processes are discussed. The relative levels of the IP₃R isoforms expressed and their subcellular localization, e.g. at the ER-mitochondrial interface, is hereby important. Finally, evidence is provided about how the knowledge of the regulation of the IP₃R by tumor suppressors and oncogenes can be exploited to develop novel therapeutic approaches to fight cancer.

BDA-366, a putative Bcl-2 BH4 domain antagonist, induces apoptosis independently of Bcl-2 in a variety of cancer cell models

Several cancer cell types, including chronic lymphocytic leukemia (CLL) and diffuse large B-cell lymphoma (DLBCL) upregulate antiapoptotic Bcl-2 to cope with oncogenic stress. BH3 mimetics targeting Bcl-2's hydrophobic cleft have been developed, including venetoclax as a promising anticancer precision medicine for treating CLL patients. Recently, BDA-366 was identified as a small molecule BH4-domain antagonist that could kill lung cancer and multiple myeloma cells. BDA-366 was proposed to switch Bcl-2 from an antiapoptotic into a proapoptotic protein, thereby activating Bax and inducing apoptosis. Here, we scrutinized the therapeutic potential and mechanism of action of BDA-366 in CLL and DLBCL. Although BDA-366 displayed selective toxicity against both cell types, the BDA-366-induced cell death did not correlate with Bcl-2-protein levels and also occurred in the absence of Bcl-2. Moreover, although BDA-366 provoked Bax activation, it did neither directly activate Bax nor switch Bcl-2 into a Bax-activating protein in in vitro Bax/liposome assays. Instead, in primary CLL cells and DLBCL cell lines, BDA-366 inhibited the activity of the PI3K/AKT pathway, resulted in Bcl-2 dephosphorylation and reduced Mcl-1-protein levels without affecting the levels of Bcl-2 or Bcl-xL. Hence, our work challenges the current view that BDA-366 is a BH4-domain antagonist of Bcl-2 that turns Bcl-2 into a pro-apoptotic protein. Rather, our results indicate that other mechanisms beyond switching Bcl-2 conformation underlie BDA-366's cell-death properties that may implicate Mcl-1 downregulation and/or Bcl-2 dephosphorylation.

Lessons from the Endoplasmic Reticulum Ca2+ Transporters-A Cancer Connection

Ca2+ is an integral mediator of intracellular signaling, impacting almost every aspect of cellular life. The Ca2+-conducting transporters located on the endoplasmic reticulum (ER) membrane shoulder the responsibility of constructing the global Ca2+ signaling landscape. These transporters gate the ER Ca2+ release and uptake, sculpt signaling duration and intensity, and compose the Ca2+ signaling rhythm to accommodate a plethora of biological activities. In this review, we explore the mechanisms of activation and functional regulation of ER Ca2+ transporters in the establishment of Ca2+ homeostasis. We also contextualize the aberrant alterations of these transporters in carcinogenesis, presenting Ca2+-based therapeutic interventions as a means to tackle malignancies.

Bcl-2-Protein Family as Modulators of IP3 Receptors and Other Organellar Ca2+ Channels

The pro- and antiapoptotic proteins belonging to the B-cell lymphoma-2 (Bcl-2) family exert a critical control over cell-death processes by enabling or counteracting mitochondrial outer membrane permeabilization. Beyond this mitochondrial function, several Bcl-2 family members have emerged as critical modulators of intracellular Ca²⁺ homeostasis and dynamics, showing proapoptotic and antiapoptotic functions. Bcl-2 family proteins specifically target several intracellular Ca²⁺-transport systems, including organellar Ca²⁺ channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs), Ca²⁺-release channels mediating Ca²⁺ flux from the endoplasmic reticulum, as well as voltage-dependent anion channels (VDACs), which mediate Ca²⁺ flux across the mitochondrial outer membrane into the mitochondria. Although the formation of protein complexes between Bcl-2 proteins and these channels has been extensively studied, a major advance during recent years has been elucidating the complex interaction of Bcl-2 proteins with IP₃Rs. Distinct interaction sites for different Bcl-2 family members were identified in the primary structure of IP₃Rs. The unique molecular profiles of these Bcl-2 proteins may account for their distinct functional outcomes when bound to IP₃Rs. Furthermore, Bcl-2 inhibitors used in cancer therapy may affect IP₃R function as part of their proapoptotic effect and/or as an adverse effect in healthy cells.

Partners in Crime: Towards New Ways of Targeting Calcium Channels

The characterization of calcium channel interactome in the last decades opened a new way of perceiving ion channel function and regulation. Partner proteins of ion channels can now be considered as major components of the calcium homeostatic mechanisms, while the reinforcement or disruption of their interaction with the channel units now represents an attractive target in research and therapeutics. In this review we will focus on the targeting of calcium channel partner proteins in order to act on the channel activity, and on its consequences for cell and organism physiology. Given the recent advances in the partner proteins’ identification, characterization, as well as in the resolution of their interaction domain structures, we will develop the latest findings on the interacting proteins of the following channels: voltage-dependent calcium channels, transient receptor potential and ORAI channels, and inositol 1,4,5-trisphosphate receptor.

The role of mitochondria-associated membranes in cellular homeostasis and diseases

Mitochondria and endoplasmic reticulum (ER) are fundamental in the control of cell physiology regulating several signal transduction pathways. They continuously communicate exchanging messages in their contact sites called MAMs (mitochondria-associated membranes). MAMs are specific microdomains acting as a platform for the sorting of vital and dangerous signals. In recent years increasing evidence reported that multiple scaffold proteins and regulatory factors localize to this subcellular fraction suggesting MAMs as hotspot signaling domains. In this review we describe the current knowledge about MAMs' dynamics and processes, which provided new correlations between MAMs' dysfunctions and human diseases. In fact, MAMs machinery is strictly connected with several pathologies, like neurodegeneration, diabetes and mainly cancer. These pathological events are characterized by alterations in the normal communication between ER and mitochondria, leading to deep metabolic defects that contribute to the progression of the diseases.

Creating a New Cancer Therapeutic Agent by Targeting the Interaction between Bcl-2 and IP3 Receptors

Bcl-2 is a member of a family of proteins that regulate cell survival. Expression of Bcl-2 is aberrantly elevated in many types of cancer. Within cells of the immune system, Bcl-2 has a physiological role in regulating immune responses. However, in cancers arising from cells of the immune system Bcl-2 promotes cell survival and proliferation. This review summarizes discoveries over the past 30 years that have elucidated Bcl-2's role in the normal immune system, including its actions in regulating calcium (Ca²⁺) signals necessary for the immune response, and for Ca²⁺-mediated apoptosis at the end of an immune response. How Bcl-2 modulates the release of Ca²⁺ from intracellular stores via inositol 1,4,5-trisphosphate receptors (IP₃R) is discussed, and in particular, the role of Bcl-2/IP₃R interactions in promoting the survival of cancer cells by preventing Ca²⁺-mediated cell death. The development and usage of a peptide, referred to as TAT-Pep8, or more recently, BIRD-2, that induces death of cancer cells by inhibiting Bcl-2's control over IP₃R-mediated Ca²⁺ elevation is discussed. Studies aimed at discovering a small molecule that mimics BIRD-2's anticancer mechanism of action are summarized, along with the prospect of such a compound becoming a novel therapeutic option for cancer.

ER Ca2+ release and store-operated Ca2+ entry - partners in crime or independent actors in oncogenic transformation?

Ca²⁺ is a pleiotropic messenger that controls life and death decisions from fertilisation until death. Cellular Ca²⁺ handling mechanisms show plasticity and are remodelled throughout life to meet the changing needs of the cell. In turn, as the demands on a cell alter, for example through a change in its niche environment or its functional requirements, Ca²⁺ handling systems may be targeted to sustain the remodelled cellular state. Nowhere is this more apparent than in cancer. Oncogenic transformation is a multi-stage process during which normal cells become progressively differentiated towards a cancerous state that is principally associated with enhanced proliferation and avoidance of death. Ca²⁺ signalling is intimately involved in almost all aspects of the life of a transformed cell and alterations in Ca²⁺ handling have been observed in cancer. Moreover, this remodelling of Ca²⁺ signalling pathways is also required in some cases to sustain the transformed phenotype. As such, Ca²⁺ handling is hijacked by oncogenic processes to deliver and maintain the transformed phenotype. Central to generation of intracellular Ca²⁺ signals is the release of Ca²⁺ from the endoplasmic reticulum intracellular (ER) Ca²⁺ store via inositol 1,4,5-trisphosphate receptors (InsP₃Rs). Upon depletion of ER Ca²⁺, store-operated Ca²⁺ entry (SOCE) across the plasma membrane occurs via STIM-gated Orai channels. SOCE serves to both replenish stores but also sustain Ca²⁺ signalling events. Here, we will discuss the role and regulation of these two signalling pathways and their interplay in oncogenic transformation.

Interorganellar calcium signaling in the regulation of cell metabolism: A cancer perspective

Organelles were originally considered to be individual cellular compartments with a defined organization and function. However, recent studies revealed that organelles deeply communicate within each other via Ca²⁺ exchange. This communication, mediated by specialized membrane regions in close apposition between two organelles, regulate cellular functions, including metabolism and cell fate decisions. Advances in microscopy techniques, molecular biology and biochemistry have increased our understanding of these interorganelle platforms. Research findings suggest that interorganellar Ca²⁺ signaling, which is altered in cancer, influences tumorigenesis and tumor progression by controlling cell death programs and metabolism. Here, we summarize the available data on the existence and composition of interorganelle platforms connecting the endoplasmic reticulum with mitochondria, the plasma membrane, or endolysosomes. Finally, we provide a timely overview of the potential function of interorganellar Ca²⁺ signaling in maintaining cellular homeostasis.

Therapeutic implications of novel peptides targeting ER-mitochondria Ca2+-flux systems

Intracellular Ca²⁺-flux systems located at the ER-mitochondrial axis govern mitochondrial Ca²⁺ balance and cell fate. Multiple yet incurable pathologies are characterized by insufficient or excessive Ca²⁺ fluxes toward the mitochondria, in turn leading to aberrant cell life or death dynamics. The discovery and ongoing molecular characterization of the main interorganellar Ca²⁺ gateways have resulted in a novel class of peptide tools able to regulate relevant protein-protein interactions (PPIs) underlying this signaling scenario. Here, we review peptides, molecularly derived from Ca²⁺-flux systems or their accessory proteins. We discuss how they alter Ca²⁺-signaling protein complexes and modulate cell survival in light of their forthcoming therapeutic applications.

Extracellular and ER-stored Ca2+ contribute to BIRD-2-induced cell death in diffuse large B-cell lymphoma cells

The anti-apoptotic protein Bcl-2 is upregulated in several cancers, including diffuse large B-cell lymphoma (DLBCL) and chronic lymphocytic leukemia (CLL). In a subset of these cancer cells, Bcl-2 blocks Ca²⁺-mediated apoptosis by suppressing the function of inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) located at the endoplasmic reticulum (ER). A peptide tool, called Bcl-2/IP₃ receptor disruptor-2 (BIRD-2), was developed to disrupt Bcl-2/IP₃R complexes, triggering pro-apoptotic Ca²⁺ signals and killing Bcl-2-dependent cancer cells. In DLBCL cells, BIRD-2 sensitivity depended on the expression level of IP₃R2 channels and constitutive IP₃ signaling downstream of the B-cell receptor. However, other cellular pathways probably also contribute to BIRD-2-provoked cell death. Here, we examined whether BIRD-2-induced apoptosis depended on extracellular Ca²⁺ and more particularly on store-operated Ca²⁺ entry (SOCE), a Ca²⁺-influx pathway activated upon ER-store depletion. Excitingly, DPB162-AE, a SOCE inhibitor, suppressed BIRD-2-induced cell death in DLBCL cells. However, DPB162-AE not only inhibits SOCE but also depletes the ER Ca²⁺ store. Treatment of the cells with YM-58483 and GSK-7975A, two selective SOCE inhibitors, did not protect against BIRD-2-induced apoptosis. Similar data were obtained by knocking down STIM1 using small interfering RNA. Yet, extracellular Ca²⁺ contributed to BIRD-2 sensitivity in DLBCL, since the extracellular Ca²⁺ buffer ethylene glycol tetraacetic acid (EGTA) blunted BIRD-2-triggered apoptosis. The protective effects observed with DPB162-AE are likely due to ER Ca²⁺-store depletion, since a similar protective effect could be obtained using the sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin. Thus, both the ER Ca²⁺-store content and extracellular Ca²⁺, but not SOCE, are critical factors underlying BIRD-2-provoked cell death.

New concepts in follicular lymphoma biology: From BCL2 to epigenetic regulators and non-coding RNAs

The molecular pathogenesis of follicular lymphoma (FL) was partially revealed 3 decades ago, with the discovery of the translocation that brings BCL2 under the influence of immunoglobulin heavy chain enhancers in a vast majority of cases. Despite the importance of this seminal observation, it has become increasingly clear that additional genetic alterations need to occur to trigger neoplastic transformation and disease progression. The evolution of FL involves developmental arrest and disruption of the normal function of one or more of epigenetic regulators including KMT2D/MLL2, EZH2, CBP/CREBBP, p300/EP300, and HIST1H1 in >95% of cases. B-cells "arrested" in germinal centers acquire dozens of additional genetic aberrations that influence key pathways controlling their physiological development including B Cell Receptor (BCR) signaling, PI3K/AKT, TLR, mTOR, NF-κB, JAK/STAT, MAPK, CD40/CD40L, chemokine, and interleukin signaling. Additionally, most cases of FL do not result from linear accumulation of genomic aberrations, but rather evolve from a common progenitor cell population by diverse evolution, creating multiple FL subclones in one patient. Moreover, one of the subclones might acquire a combination of aberrations involving genes controlling cell survival and proliferation including MDM2, CDKN2A/B, BCL6, MYC, TP53, β2M, FOXO1, MYD88, STAT3, or miR-17-92, and this can lead to the transformation of an initially indolent FL to an aggressive lymphoma (2%-3% risk per year). The complexity of the disease is also underscored by the importance of its interactions with the microenvironment that can substantially influence disease development and prognosis. Interpreting individual aberrations in relation to their impact on normal processes, their frequency, position in the disease evolution, and the consequences of their (co)occurrence, are the basis for understanding FL pathogenesis. This is necessary for the identification of patients with risk of early progression or transformation, for the development of novel targeted therapies, and for personalized treatment approaches. In this review, we summarize recent knowledge of molecular pathways and microenvironmental components involved in FL biology, and discuss them in the context of physiological B-cell development, FL evolution, and targeted therapies.

The mitochondrial pathway is involved in sodium fluoride (NaF)-induced renal apoptosis in mice

The objective of the present study was to explore the molecular mechanism of apoptosis induced by sodium fluoride (NaF) in the mouse kidney by using the methods of flow cytometry, quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and experimental pathology. 240 four-week-old ICR mice were randomly divided into 4 groups and exposed to different concentrations of NaF (0 mg kg-1, 12 mg kg-1, 24 mg kg-1 and 48 mg kg-1) for a period of 42 days. The results demonstrated that NaF increased cell apoptosis and the depolarization of the mitochondrial membrane potential (MMP), and that the mitochondrial pathway was involved in NaF-induced apoptosis. Alteration of the mitochondrial pathway was characterized by significantly increasing mRNA and protein expression levels of cytosolic cytochrome c (Cyt c), the second mitochondrial activator of caspases/direct inhibitors of the apoptosis binding protein with low pI (Smac/Diablo), the serine protease high-temperature-requirement protein A2/Omi (HtrA2/Omi), the apoptosis inducing factor (AIF), endonuclease G (Endo G), cleaved-cysteine aspartate specific protease-9 (cleaved-caspase-9), cleaved-cysteine aspartate specific protease-3 (cleaved-caspase-3), Bcl-2 antagonist killer (Bak), Bcl-2 associated X protein (Bax), Bcl-2 interacting mediator of cell death (Bim), cleaved-poly-ADP-ribose polymerase (cleaved-PARP), p-p53, and decreasing mRNA and protein expression levels of B-cell lymphoma-2 (Bcl-2), Bcl-extra large (Bcl-xL), and X chromosome-linked inhibitors of apoptosis proteins (XIAPs). To our knowledge, the mitochondrial pathway is reported for the first time in NaF-induced apoptosis of the human or animal kidney. Also, this study provides novel insights for further studying fluoride-induced nephrotoxicity.

Probing Gallate-Mediated Selectivity and High-Affinity Binding of Epigallocatechin Gallate: a Way-Forward in the Design of Selective Inhibitors for Anti-apoptotic Bcl-2 Proteins

Selective inhibition is a key focus in the design of chemotherapeutic compounds that can abrogate the oncogenic activities of anti-apoptotic Bcl-2 proteins. Although recent efforts have led to the development of highly selective BH3 mimetics, setbacks such as toxicities have limited their use in cancer therapy. Epigallocatechingallate (EGCG) has been widely reported to selectively inhibit Bcl-2 and Bcl-xL compared to other green tea phenols due to its gallate group. Herein, we investigate the interaction dynamics of EGCG at the hydrophobic grooves of Bcl-2 and Bcl-xL and the consequential effects on their BH4 domains. Arg143 and Asp108 (Bcl-2), and Glu96 and Tyr195 (Bcl-xL) formed high-affinity hydrogen interactions with the gallate group while non-gallate groups of EGCG formed weak interactions. EGCG-bound proteins showed systemic perturbations of BH4 domains coupled with the burial of crucial surface-exposed residues such as Lys17 (Bcl-2) and Asp11 (Bcl-xL); hence, a distortion of non-canonical domain interactions. Interactions of gallate group of EGCG with key hydrophobic groove residues underlie EGCG selectivity while concurrent BH4 domain perturbations potentiate EGCG inhibitory activities. Findings will aid the optimization and design of selective inhibitors that could suppress anti-apoptotic activities of Bcl2-family proteins with minimal toxicities.

IP3 Receptor Properties and Function at Membrane Contact Sites

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a ubiquitously expressed Ca²⁺-release channel localized in the endoplasmic reticulum (ER). The intracellular Ca²⁺ signals originating from the activation of the IP₃R regulate multiple cellular processes including the control of cell death versus cell survival via their action on apoptosis and autophagy. The exact role of the IP₃Rs in these two processes does not only depend on their activity, which is modulated by the cytosolic composition (Ca²⁺, ATP, redox status, …) and by various types of regulatory proteins, including kinases and phosphatases as well as by a number of oncogenes and tumor suppressors, but also on their intracellular localization, especially at the ER-mitochondrial and ER-lysosomal interfaces. At these interfaces, Ca²⁺ microdomains are formed, in which the Ca²⁺ concentration is finely regulated by the different ER, mitochondrial and lysosomal Ca²⁺-transport systems and also depends on the functional and structural interactions existing between them. In this review, we therefore discuss the most recent insights in the role of Ca²⁺ signaling in general, and of the IP₃R in particular, in the control of basal mitochondrial bioenergetics, apoptosis, and autophagy at the level of inter-organellar contact sites.

Targeting Bcl-2-IP3 receptor interaction to treat cancer: A novel approach inspired by nearly a century treating cancer with adrenal corticosteroid hormones

Bcl-2 inhibits cell death by at least two different mechanisms. On the one hand, its BH3 domain binds to pro-apoptotic proteins such as Bim and prevents apoptosis induction. On the other hand, the BH4 domain of Bcl-2 binds to the inositol 1,4,5-trisphosphate receptor (IP₃R), preventing Ca²⁺ signals that mediate cell death. In normal T-cells, Bcl-2 levels increase during the immune response, protecting against cell death, and then decline as apoptosis ensues and the immune response dissipates. But in many cancers Bcl-2 is aberrantly expressed and exploited to prevent cell death by inhibiting IP₃R-mediated Ca²⁺ elevation. This review summarizes what is known about the mechanism of Bcl-2's control over IP₃R-mediated Ca²⁺ release and cell death induction. Early insights into the role of Ca²⁺ elevation in corticosteroid-mediated cell death serves as a model for how targeting IP₃R-mediated Ca²⁺ elevation can be a highly effective therapeutic approach for different types of cancer. Moreover, the successful development of ABT-199 (Venetoclax), a small molecule targeting the BH3 domain of Bcl-2 but without effects on Ca²⁺, serves as proof of principle that targeting Bcl-2 can be an effective therapeutic approach. BIRD-2, a synthetic peptide that inhibits Bcl-2-IP₃R interaction, induces cell death induction in ABT-199 (Venetoclax)-resistant cancer models, attesting to the value of developing therapeutic agents that selectively target Bcl-2-IP₃R interaction, inducing Ca²⁺-mediated cell death.

Constitutive IP3 signaling underlies the sensitivity of B-cell cancers to the Bcl-2/IP3 receptor disruptor BIRD-2

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 (IP₃) receptor (IP₃R)-mediated Ca²⁺-signaling. A peptide tool (Bcl-2/IP₃R Disruptor-2; BIRD-2) was developed to abrogate the interaction of Bcl-2 with IP₃Rs 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 IP₃R2 were sensitive to BIRD-2. Here, we report that BIRD-2-induced cell death in DLBCL cells does not only depend on high IP₃R2-expression levels, but also on constitutive IP₃ signaling, downstream of the tonically active B-cell receptor. The basal Ca²⁺ level in SU-DHL-4 DLBCL cells was significantly elevated due to the constitutive IP₃ production. This constitutive IP₃ signaling fulfilled a pro-survival role, since inhibition of phospholipase C (PLC) using U73122 (2.5 µM) caused cell death in SU-DHL-4 cells. Milder inhibition of IP₃ signaling using a lower U73122 concentration (1 µM) or expression of an IP₃ sponge suppressed both BIRD-2-induced Ca²⁺ elevation and apoptosis in SU-DHL-4 cells. Basal PLC/IP₃ 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 IP₃ 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 IP₃R activity. BIRD-2 seems to switch constitutive IP₃ signaling from pro-survival into pro-death, presenting a plausible therapeutic strategy.

Integration of Ca2+ signaling regulates the breast tumor cell response to simvastatin and doxorubicin

Recent studies have suggested that the lipid-lowering agent simvastatin holds great promise as a cancer therapeutic; it inhibits the growth of multiple tumors, including triple-negative breast cancer. Doxorubicin- and simvastatin-induced cytotoxicity has been associated with the modulation of Ca²⁺ signaling, but the underlying mechanisms remain incompletely understood. Here we identify how Ca²⁺ signaling regulates the breast tumor cell response to doxorubicin and simvastatin. These two drugs inhibit cell survival while increasing apoptosis in two human breast cancer cell lines and five primary breast tumor specimens through the modulation of Ca²⁺ signaling. Signal transduction and functional studies revealed that both simvastatin and doxorubicin trigger persistent cytosolic Ca²⁺ release, thereby stimulating the proapoptotic BIM pathway and mitochondrial Ca²⁺ overload, which are responsible for metabolic dysfunction and apoptosis induction. Simvastatin and doxorubicin suppress the prosurvival ERK1/2 pathway in a Ca²⁺-independent and Ca²⁺-dependent manner, respectively. In addition, reduction of the Ca²⁺ signal by chelation or pharmacological inhibition significantly prevents drug-mediated anticancer signaling. Unexpectedly, a scratch-wound assay indicated that these two drugs induce rapid cell migration, while inhibiting cell invasion and colony formation in a Ca²⁺-dependent manner. Further, the in vivo data for MDA-MB-231 xenografts demonstrate that upon chelation of Ca²⁺, the ability of both drugs to reduce the tumor burden was significantly reduced via caspase-3 deactivation. Our results establish a calcium-based mechanism as crucial for executing the cell death process triggered by simvastatin and doxorubicin, and suggest that combining simvastatin with doxorubicin may be an effective regimen for the treatment of breast cancer.

A polysaccharide from Lentinus edodes inhibits human colon cancer cell proliferation and suppresses tumor growth in athymic nude mice

The antitumor effect of Lentinan is thought rely on the activation of immune responses; however, little is known about whether Lentinan also directly attacks cancer cells. We therefore investigated the direct antitumor activity of SLNT (a water-extracted polysaccharide from Lentinus edodes) and its probable mechanism. We showed that SLNT significantly inhibited proliferation of HT-29 colon cancer cells and suppressed tumor growth in nude mice. Annxein V-FITC/PI, DAPI, AO/EB and H&E staining assays all showed that SLNT induced cell apoptosis both in vitro and in vivo. SLNT induced apoptosis by activating Caspase-3 via both intrinsic and extrinsic pathways, which presented as the activation of Caspases-9 and -8, upregulation of cytochrome c and the Bax/Bcl-2 ratio, downregulation of NF-κB, and overproduction of ROS and TNF-α in vitro and in vivo. Pretreatment with the caspase-3 inhibitor Ac-DEVD-CHO or antioxidant NAC blocked SLNT-induced apoptosis. These findings suggest that SLNT exerts direct antitumor effects by inducing cell apoptosis via ROS-mediated intrinsic and TNF-α-mediated extrinsic pathways. SLNT may thus represent a useful candidate for colon cancer prevention and treatment.

Emerging molecular mechanisms in chemotherapy: Ca2+ signaling at the mitochondria-associated endoplasmic reticulum membranes

Inter-organellar communication often takes the form of Ca2+ signals. These Ca2+ signals originate from the endoplasmic reticulum (ER) and regulate different cellular processes like metabolism, fertilization, migration, and cell fate. A prime target for Ca2+ signals are the mitochondria. ER-mitochondrial Ca2+ transfer is possible through the existence of mitochondria-associated ER membranes (MAMs), ER structures that are in the proximity of the mitochondria. This creates a micro-domain in which the Ca2+ concentrations are manifold higher than in the cytosol, allowing for rapid mitochondrial Ca2+ uptake. In the mitochondria, the Ca2+ signal is decoded differentially depending on its spatiotemporal characteristics. While Ca2+ oscillations stimulate metabolism and constitute pro-survival signaling, mitochondrial Ca2+ overload results in apoptosis. Many chemotherapeutics depend on efficient ER-mitochondrial Ca2+ signaling to exert their function. However, several oncogenes and tumor suppressors present in the MAMs can alter Ca2+ signaling in cancer cells, rendering chemotherapeutics ineffective. In this review, we will discuss recent studies that connect ER-mitochondrial Ca2+ transfer, tumor suppressors and oncogenes at the MAMs, and chemotherapy.

Breast Cancer Targeting through Inhibition of the Endoplasmic Reticulum-Based Apoptosis Regulator Nrh/BCL2L10

Drug resistance and metastatic relapse remain a top challenge in breast cancer treatment. In this study, we present preclinical evidence for a strategy to eradicate advanced breast cancers by targeting the BCL-2 homolog Nrh/BCL2L10, which we discovered to be overexpressed in >45% of a large cohort of breast invasive carcinomas. Nrh expression in these tumors correlated with reduced metastasis-free survival, and we determined it to be an independent marker of poor prognosis. Nrh protein localized to the endoplasmic reticulum. Mechanistic investigations showed that Nrh made BH4 domain-dependent interactions with the ligand-binding domain of the inositol-1,4,5-triphosphate receptor (IP3R), a type 1/3 Ca2⁺ channel, allowing Nrh to negatively regulate ER-Ca2⁺ release and to mediate antiapoptosis. Notably, disrupting Nrh/IP3R complexes by BH4 mimetic peptides was sufficient to inhibit the growth of breast cancer cells in vitro and in vivo Taken together, our results highlighted Nrh as a novel prognostic marker and a candidate therapeutic target for late stage breast cancers that may be addicted to Nrh.Significance: These findings offer a comprehensive molecular model for the activity of Nrh/BCL2L10, a little studied antiapoptotic molecule, prognostic marker, and candidate drug target in breast cancer. Cancer Res; 78(6); 1404-17. ©2018 AACR.

Remodeling of Ca2+ signaling in cancer: Regulation of inositol 1,4,5-trisphosphate receptors through oncogenes and tumor suppressors

The calcium ion (Ca²⁺) is a ubiquitous intracellular signaling molecule that regulates diverse physiological and pathological processes, including cancer. Increasing evidence indicates that oncogenes and tumor suppressors regulate the Ca²⁺ transport systems. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are IP₃-activated Ca²⁺ release channels located on the endoplasmic reticulum (ER). They play pivotal roles in the regulation of cell death and survival by controlling Ca²⁺ transfer from the ER to mitochondria through mitochondria-associated ER membranes (MAMs). Optimal levels of Ca²⁺ mobilization to mitochondria are necessary for mitochondrial bioenergetics, whereas excessive Ca²⁺ flux into mitochondria causes loss of mitochondrial membrane integrity and apoptotic cell death. In addition to well-known functions on outer mitochondrial membranes, B-cell lymphoma 2 (Bcl-2) family proteins are localized on the ER and regulate IP₃Rs to control Ca²⁺ transfer into mitochondria. Another regulatory protein of IP₃R, IP₃R-binding protein released with IP₃ (IRBIT), cooperates with or counteracts the Bcl-2 family member depending on cellular states. Furthermore, several oncogenes and tumor suppressors, including Akt, K-Ras, phosphatase and tensin homolog (PTEN), promyelocytic leukemia protein (PML), BRCA1, and BRCA1 associated protein 1 (BAP1), are localized on the ER or at MAMs and negatively or positively regulate apoptotic cell death through interactions with IP₃Rs and regulation of Ca²⁺ dynamics. The remodeling of Ca²⁺ signaling by oncogenes and tumor suppressors that interact with IP₃Rs has fundamental roles in the pathology of cancers.

BCL2-BH4 antagonist BDA-366 suppresses human myeloma growth

Multiple myeloma (MM) is a heterogeneous plasma cell malignancy and remains incurable. B-cell lymphoma-2 (BCL2) protein correlates with the survival and the drug resistance of myeloma cells. BH3 mimetics have been developed to disrupt the binding between BCL2 and its pro-apoptotic BCL2 family partners for the treatment of MM, but with limited therapeutic efficacy. We recently identified a small molecule BDA-366 as a BCL2 BH4 domain antagonist, converting it from an anti-apoptotic into a pro-apoptotic molecule. In this study, we demonstrated that BDA-366 induces robust apoptosis in MM cell lines and primary MM cells by inducing BCL2 conformational change. Delivery of BDA-366 substantially suppressed the growth of human MM xenografts in NOD-scid/IL2Rγnull mice, without significant cytotoxic effects on normal hematopoietic cells or body weight. Thus, BDA-366 functions as a novel BH4-based BCL2 inhibitor and offers an entirely new tool for MM therapy.

Reciprocal sensitivity of diffuse large B-cell lymphoma cells to Bcl-2 inhibitors BIRD-2 versus venetoclax

Bcl-2 is often upregulated in cancers to neutralize the BH3-only protein Bim at the mitochondria. BH3 mimetics (e.g. ABT-199 (venetoclax)) kill cancer cells by targeting Bcl-2's hydrophobic cleft and disrupting Bcl-2/Bim complexes. Some cancers with elevated Bcl-2 display poor responses towards BH3 mimetics, suggesting an additional function for anti-apoptotic Bcl-2 in these cancers. Indeed, Bcl-2 via its BH4 domain prevents cytotoxic Ca²⁺ release from the endoplasmic reticulum (ER) by directly inhibiting the inositol 1,4,5-trisphosphate receptor (IP₃R). The cell-permeable Bcl-2/IP₃R disruptor-2 (BIRD-2) peptide can kill these Bcl-2-dependent cancers by targeting Bcl-2's BH4 domain, unleashing pro-apoptotic Ca²⁺-release events. We compared eight "primed to death" diffuse large B-cell lymphoma cell lines (DLBCL) for their apoptotic sensitivity towards BIRD-2 and venetoclax. By determining their IC₅₀ using cytometric cell-death analysis, we discovered a reciprocal sensitivity towards venetoclax versus BIRD-2. Using immunoblotting, we quantified the expression levels of IP₃R2 and Bim in DLBCL cell lysates, revealing that BIRD-2 sensitivity correlated with IP₃R2 levels but not with Bim levels. Moreover, the requirement of intracellular Ca²⁺ for BIRD-2- versus venetoclax-induced cell death was different. Indeed, BAPTA-AM suppressed BIRD-2-induced cell death, but promoted venetoclax-induced cell death in DLBCL cells. Finally, compared to single-agent treatments, combining BIRD-2 with venetoclax synergistically enhanced cell-death induction, correlating with a Ca²⁺-dependent upregulation of Bim after BIRD-2 treatment. Our findings suggest that some cancer cells require Bcl-2 proteins at the mitochondria, preventing Bax activation via its hydrophobic cleft, while others require Bcl-2 proteins at the ER, preventing cytotoxic Ca²⁺-signaling events via its BH4 domain.

Non-canonical roles of Bcl-2 and Bcl-xL proteins: relevance of BH4 domain

Bcl-2 protein family is constituted by multidomain members originally identified as modulators of programmed cell death and whose expression is frequently misbalanced in cancer cells. The lead member Bcl-2 and its homologue Bcl-xL proteins are characterized by the presence of all four conserved BH domain and exert their antiapoptotic role mainly through the involvement of BH1, BH2 and BH3 homology domains, that mediate the interaction with the proapoptotic members of the same Bcl-2 family. The N-terminal BH4 domain of Bcl-2 and Bcl-xL is responsible for the interaction with other proteins that do not belong to Bcl-2 protein family. Beyond a classical role in inhibiting apoptosis, BH4 domain has been characterized as a crucial regulator of other important cellular functions attributed to Bcl-2 and Bcl-xL, including proliferation, autophagy, differentiation, DNA repair, cell migration, tumor progression and angiogenesis. During the last two decades a strong effort has been made to dissect the molecular pathways involved the capability of BH4 domain to regulate the canonical antiapoptotic and the non-canonical activities of Bcl-2 and Bcl-xL, creating the basis for the development of novel anticancer agents targeting this domain. Indeed, recent evidences obtained on in vitro and in vivo model of different cancer histotypes are confirming the promising therapeutic potential of BH4 domain inhibitors supporting their future employment as a novel anticancer strategy.

Pathways and mechanisms of venetoclax resistance

The approval of venetoclax, a 'BH3-mimetic' antagonist of the BCL-2 anti-apoptotic protein, for chronic lymphocytic leukemia represents a major milestone in translational apoptosis research. Venetoclax has already received 'breakthrough' designation for acute myeloid leukemia, and is being studied in many other tumor types. However, resistance to BCL-2 inhibitor monotherapy may rapidly ensue. Several studies have shown that the other two major anti-apoptotic BCL-2 family proteins, BCL-XL and MCL-1, are the main determinants of resistance to venetoclax. This opens up possibilities for rationally combining venetoclax with other targeted agents to circumvent resistance. Here, we summarize the most promising combinations, and highlight those already in clinical trials. There is also increasing recognition that different tumors display different degrees of addiction to individual BCL-2 family proteins, and of the need to refine current 'BH3 profiling' techniques. Finally, the successful clinical development of potent and selective antagonists of BCL-XL and MCL-1 is eagerly awaited.

Regulation of Endoplasmic Reticulum-Mitochondria Ca2+ Transfer and Its Importance for Anti-Cancer Therapies

Inter-organelle membrane contact sites are emerging as major sites for the regulation of intracellular Ca²⁺ concentration and distribution. Here, extracellular stimuli operate on a wide array of channels, pumps, and ion exchangers to redistribute intracellular Ca²⁺ among several compartments. The resulting highly defined spatial and temporal patterns of Ca²⁺ movement can be used to elicit specific cellular responses, including cell proliferation, migration, or death. Plasma membrane (PM) also can directly contact mitochondria and endoplasmic reticulum (ER) through caveolae, small invaginations of the PM that ensure inter-organelle contacts, and can contribute to the regulation of numerous cellular functions through scaffolding proteins such as caveolins. PM and ER organize specialized junctions. Here, many components of the receptor-dependent Ca²⁺ signals are clustered, including the ORAI1-stromal interaction molecule 1 complex. This complex constitutes a primary mechanism for Ca²⁺ entry into non-excitable cells, modulated by intracellular Ca²⁺. Several contact sites between the ER and mitochondria, termed mitochondria-associated membranes, show a very complex and specialized structure and host a wide number of proteins that regulate Ca²⁺ transfer. In this review, we summarize current knowledge of the particular action of several oncogenes and tumor suppressors at these specialized check points and analyze anti-cancer therapies that specifically target Ca²⁺ flow at the inter-organelle contacts to alter the metabolism and fate of the cancer cell.

Investigation of the neuroprotective effects of a novel synthetic compound via the mitochondrial pathway

The present study aimed to investigate the neuroprotective effect of a novel synthetic compound (5zou) on differentiated PC12 cells against 6-hydroxydopamine (6-OHDA) and L-glutamic acid (L-Glu) neurotoxin-induced cell injury and the potential mechanisms involved. 5zou is a 2, 2-disubstituted 1,2-dihydropyridine. PC12 cells were treated with 6-OHDA and L-Glu to establish neurotoxic cell models. MTT assay, DCFH-DA staining, Fluo-4-AM staining, JC-1 staining and western blotting were used to determine the changes in cell viability, intracellular reactive oxygen species concentration, Ca²⁺ influx, mitochondrial membrane potential and the protein expressions of B-cell lymphoma-2 (Bcl-2) and B-cell lymphoma-extra large (Bcl-xL). Morphological analysis demonstrated the effect of 5zous on neuritogenesis and differentiation in PC12 cells. The results suggested that 5zou rescued the cell viability, intracellular ROS level, Ca²⁺ influx, mitochondrial membrane potential, and expression of Bcl-2 and Bcl-xL, which were altered by 6-OHDA and L-Glu. The study confirmed that 5zou has neuroprotective effects on neurotoxin-induced differentiated PC12 cells injury, potentially via the mitochondrial apoptosis pathway.

Modulation of Ca2+ Signaling by Anti-apoptotic B-Cell Lymphoma 2 Proteins at the Endoplasmic Reticulum-Mitochondrial Interface

Mitochondria are important regulators of cell death and cell survival. Mitochondrial Ca²⁺ levels are critically involved in both of these processes. On the one hand, excessive mitochondrial Ca²⁺ leads to Ca²⁺-induced mitochondrial outer membrane permeabilization and thus apoptosis. On the other hand, mitochondria need Ca²⁺ in order to efficiently fuel the tricarboxylic acid cycle and maintain adequate mitochondrial bioenergetics. For obtaining this Ca²⁺, the mitochondria are largely dependent on close contact sites with the endoplasmic reticulum (ER), the so-called mitochondria-associated ER membranes. There, the inositol 1,4,5-trisphosphate receptors are responsible for the Ca²⁺ release from the ER. It comes as no surprise that this Ca²⁺ release from the ER and the subsequent Ca²⁺ uptake at the mitochondria are finely regulated. Cancer cells often modulate ER-Ca²⁺ transfer to the mitochondria in order to promote cell survival and to inhibit cell death. Important regulators of these Ca²⁺ signals and the onset of cancer are the B-cell lymphoma 2 (Bcl-2) family of proteins. An increasing number of reports highlight the ability of these Bcl-2-protein family members to finely regulate Ca²⁺ transfer from ER to mitochondria both in healthy cells and in cancer. In this review, we focus on recent insights into the dynamic regulation of ER-mitochondrial Ca²⁺ fluxes by Bcl-2-family members and how this impacts cell survival, cell death and mitochondrial energy production.

Alterations in Ca2+ Signalling via ER-Mitochondria Contact Site Remodelling in Cancer

Inter-organellar contact sites establish microdomains for localised Ca²⁺-signalling events. One of these microdomains is established between the ER and the mitochondria. Importantly, the so-called mitochondria-associated ER membranes (MAMs) contain, besides structural proteins and proteins involved in lipid exchange, several Ca²⁺-transport systems, mediating efficient Ca²⁺ transfer from the ER to the mitochondria. These Ca²⁺ signals critically control several mitochondrial functions, thereby impacting cell metabolism, cell death and survival, proliferation and migration. Hence, the MAMs have emerged as critical signalling hubs in physiology, while their dysregulation is an important factor that drives or at least contributes to oncogenesis and tumour progression. In this book chapter, we will provide an overview of the role of the MAMs in cell function and how alterations in the MAM composition contribute to oncogenic features and behaviours.

The selective Bcl-2 inhibitor venetoclax, a BH3 mimetic, does not dysregulate intracellular Ca2+ signaling

Anti-apoptotic B cell-lymphoma-2 (Bcl-2) proteins are emerging as therapeutic targets in a variety of cancers for precision medicines, like the BH3-mimetic drug venetoclax (ABT-199), which antagonizes the hydrophobic cleft of Bcl-2. However, the impact of venetoclax on intracellular Ca²⁺ homeostasis and dynamics in cell systems has not been characterized in detail. Here, we show that venetoclax did not affect Ca²⁺-transport systems from the endoplasmic reticulum (ER) in permeabilized cell systems. Venetoclax (1μM) did neither trigger Ca²⁺ release by itself nor affect agonist-induced Ca²⁺ release in a variety of intact cell models. Among the different cell types, we also studied two Bcl-2-dependent cancer cell models with a varying sensitivity towards venetoclax, namely SU-DHL-4 and OCI-LY-1, both diffuse large B-cell lymphoma cell lines. Acute application of venetoclax did also not dysregulate Ca²⁺ signaling in these Bcl-2-dependent cancer cells. Moreover, venetoclax-induced cell death was independent of intracellular Ca²⁺ overload, since Ca²⁺ buffering using BAPTA-AM did not suppress venetoclax-induced cell death. This study therefore shows that venetoclax does not dysregulate the intracellular Ca²⁺ homeostasis in a variety of cell types, which may underlie its limited toxicity in human patients. Furthermore, venetoclax-induced cell death in Bcl-2-dependent cancer cells is not mediated by intracellular Ca²⁺ overload. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.