A novel BH3 mimetic S1 potently induces Bax/Bak-dependent apoptosis by targeting both Bcl-2 and Mcl-1

Duplicate CgCREBL2β involved in the response of oyster upon high-temperature stress through the induction of glycolysis and haemocyte apoptosis

The cAMP response element-binding protein-like 2 (CREBL2) is involved in the regulation of response to environmental stress. A CREBL2 homologue, CgCREBL2β, was identified in the Pacific oyster Crassostrea gigas and considered a paralog derived from CREBL2 duplication. In the present study, its evolutionary characteristics and involvement in the regulation of glucose metabolism and cell apoptosis after 6 h and 60 h of high-temperature stress were investigated. At 6 h after CgCREBL2β dsRNA injection and high-temperature stress, the mRNA expressions of CgENO1 (enolase 1) and CgPGK1L (phosphoglycerate kinase 1-like), the activities of HK (hexokinase) and PK (pyruvate kinase), and the contents of glucose and GLY (glycogen) were 0.55-fold (p < 0.01), 0.44-fold (p < 0.05), 0.60-fold (p < 0.05), 1.35-fold (p < 0.05), 1.29-fold (p < 0.05) and 0.60-fold (p < 0.05) of that in the control group, respectively. CgCREBL2β was suggested to be involved in the regulation of glucose metabolism through glycolysis at very early stage of high-temperature stress. The mRNA expressions of apoptosis-related genes CgBcl-2, CgBax and CgCaspase3 were 1.80-fold (p < 0.05), 0.53-fold (p < 0.05) and 0.62-fold (p < 0.05) of that in the control group at 6 h after high-temperature stress, respectively, and were 1.60-fold (p < 0.05), 0.57-fold (p < 0.05) and 1.00-fold (p > 0.05) of that in the control group at 60 h after high-temperature stress, respectively. The apoptosis rate in the CgCREBL2β-RNAi group was 16.70 % (p < 0.05) and 20.31 % (p > 0.05) at 6 h and 60 h after high-temperature stress, respectively, which was lower than that in the control group. It is indicated that CgCREBL2β transcript was involved in the upregulation of the mRNA expressions of pro-apoptotic genes and the downregulation of the mRNA expressions of anti-apoptotic genes, thereby promoting haemocyte apoptosis. These results collectively demonstrated that the duplicate CgCREBL2β was involved in the response to high-temperature stress through the induction of glycolysis and haemocyte apoptosis.

Overview of BH3 mimetics in ovarian cancer

Ovarian carcinoma is the leading cause of gynecological cancer-related death, still with a dismal five-year prognosis, mainly due to late diagnosis and the emergence of resistance to cytotoxic and targeted agents. Bcl-2 family proteins have a key role in apoptosis and are associated with tumor development/progression and response to therapy in different cancer types, including ovarian carcinoma. In tumors, evasion of apoptosis is a possible mechanism of resistance to therapy. BH3 mimetics are small molecules that occupy the hydrophobic pocket on pro-survival proteins, allowing the induction of apoptosis, and are currently under study as single agents and/or in combination with cytotoxic and targeted agents in solid tumors. Here, we discuss recent advances in targeting anti-apoptotic proteins of the Bcl-2 family for the treatment of ovarian cancer, focusing on BH3 mimetics, and how these approaches could potentially offer an alternative/complementary way to treat patients and overcome or delay resistance to current treatments.

Yeast Bxi1/Ybh3 mediates conserved mitophagy and apoptosis in yeast and mammalian cells: convergence in Bcl-2 family

The process of degrading unwanted or damaged mitochondria by autophagy, called mitophagy, is essential for mitochondrial quality control together with mitochondrial apoptosis. In mammalian cells, pan-Bcl-2 family members including conical Bcl-2 members and non-conical ones are involved in and govern the two processes. We have illustrated recently the BH3 receptor Hsp70 interacts with Bim to mediate both apoptosis and mitophagy. However, whether similar pathways exist in lower eukaryotes where conical Bcl-2 members are absent remained unclear. Here, a specific inhibitor of the Hsp70-Bim PPI, S1g-10 and its analogs were used as chemical tools to explore the role of yeast Bxi1/Ybh3 in regulating mitophagy and apoptosis. Using Om45-GFP processing assay, we illustrated that yeast Ybh3 mediates a ubiquitin-related mitophagy pathway in both yeast and mammalian cells through association with Hsp70, which is in the same manner with Bim. Moreover, by using Bax/Bak double knockout MEF cells, Ybh3 was identified to induce apoptosis through forming oligomerization to trigger mitochondrial outer membrane permeabilization (MOMP) like Bax. We not only illustrated a conserved ubiquitin-related mitophagy pathway in yeast but also revealed the multi-function of Ybh3 which combines the function of BH3-only protein and multi-domain Bax protein as one.

Ectopic BH3-Only Protein Bim Associates with Hsp70 to Regulate Yeast Mitophagy

Mitophagy, a form of selective autophagy, plays an essential role to maintain a population of healthy and functional mitochondria for normal cellular metabolism. Acting mainly as one of the B-cell lymphoma 2 (Bcl-2) family pro-apoptotic members, Bim (also known as BCL2L11) was identified to be a co-chaperone of Hsp70 to promote mitophagy in mammalian cells. Herein, with the help of a specific Hsp70/Bim disruptor and Om45-GFP processing assay, we illustrated that ectopic BimEL is able to promote mitophagy through Hsp70/Bim interaction in yeast, where Bax/Bak is absent. The Hsp70/Bim-mediated mitophagy is conserved in eukaryotes, from yeast to humans.

Hsp70-Bim interaction facilitates mitophagy by recruiting parkin and TOMM20 into a complex

BACKGROUND

For cancer therapy, the identification of both selective autophagy targets and small molecules that specifically regulate autophagy is greatly needed. Heat shock protein 70 (Hsp70) is a recently discovered BH3 receptor that forms a protein‒protein interaction (PPI) with Bcl-2-interacting mediator of cell death (Bim). Herein, a specific inhibitor of the Hsp70-Bim PPI, S1g-2, and its analog S1, which is a Bcl-2-Bim disruptor, were used as chemical tools to explore the role of Hsp70-Bim PPI in regulating mitophagy.

METHODS

Co-immunoprecipitation and immunofluorescence assays were used to determine protein interactions and colocalization patterns. Organelle purification and immunodetection of LC3-II/LC3-I on mitochondria, endoplasmic reticulum (ER) and Golgi were applied to identify specific types of autophagy. Cell-based and in vitro ubiquitination studies were used to study the role of the Hsp70-Bim PPI in parkin-mediated ubiquitination of outer mitochondrial membrane 20 (TOMM20).

RESULTS

We found that after the establishment of their PPI, Hsp70 and Bim form a complex with parkin and TOMM20, which in turn facilitates parkin translocation to mitochondria, TOMM20 ubiquitination and mitophagic flux independent of Bax/Bak. Moreover, S1g-2 selectively inhibits stress-induced mitophagy without interfering with basal autophagy.

CONCLUSIONS

The findings highlight the dual protective function of the Hsp70-Bim PPI in regulating both mitophagy and apoptosis. S1g-2 is thus a newly discovered antitumor drug candidate that drives both mitophagy and cell death via apoptosis.

Identification of bicyclic compounds that act as dual inhibitors of Bcl-2 and Mcl-1

Elevated expression of anti-apoptotic proteins, such as Bcl-2 and Mcl-1 contributes to poor prognosis and resistance to current treatment modalities in multiple cancers. Here, we report the design, synthesis and characterization of benzimidazole chalcone and flavonoid scaffold-derived bicyclic compounds targeting both Bcl-2 and Mcl-1 by optimizing the structural differences in the binding sites of both these proteins. Initial docking screen of Bcl-2 and Mcl-1 with pro-apoptotic protein Bim revealed possible hits with optimal binding energies. All the optimized bicyclic compounds were screened for their in vitro cytotoxic activity against two oral cancer cell lines (AW8507 and AW13516) which express high levels of Bcl-2 and Mcl-1. Compound 4d from the benzimidazole chalcone series and compound 6d from the flavonoid series exhibited significant cytotoxic activity (IC₅₀ 7.12 μM and 17.18 μM, respectively) against AW13516 cell line. Time Resolved-Fluorescence Resonance Energy Transfer (TR-FRET) analysis further demonstrated that compound 4d and compound 6d could effectively inhibit the Bcl-2 and Mcl-1 proteins by displacing their BH3 binding partners. Both compounds exhibited potent activation of canonical pathway of apoptosis evident from appearance of cleaved Caspase-3 and PARP. Further, treatment of oral cancer cells with the inhibitors induced dissociation of the BH3 only protein Bim from Mcl-1 and Bak from Bcl-2 but failed to release Bax from Bcl-xL thereby confirming the nature of compounds as BH3-mimetics selectively targeting Bcl-2 and Mcl-1. Our study thus identifies bicyclic compounds as promising candidates for anti-apoptotic Bcl-2/Mcl-1 dual inhibitors with a potential for further development.

Anti-tumor compounds identification from gossypol Groebke imidazopyridine product

Series of imidazo[1,2-a]pyridines designed from gossypol modification based on Groebke-Blackburn-Bienaymé reaction were discovered as potent Bcl-2 inhibitors. Compound 4 was found to display good anti-proliferative activities for 7 human cancer cell lines (0.33-1.7 µM) among them, which were better than separate gossypol and imidazopyridine moiety compounds. It was capable of suppressing antiapoptotic proteins Bcl-2 and Bcl-X_L demonstrated by mechanism studies, and possible binding model was also illustrated by molecular modelling.

Another structural correction for 1-oxo-1H-phenalene-2,3-dicarbonitriles: Synthesis of a potent BCL-2 inhibiting 7-phenoxy derivative

Aromatic scaffolds are an important part of biologically active compounds and molecular probes used to study biochemical pathways and the involved targeted proteins of interest. 1-Oxo-1H-phenalene-2,3-dicarbonitrile-based compounds have been described as inhibitors of the BCL-2 family of proteins, and this core structure represents numerous possibilities for modifications that could lead to improved inhibitory potencies. Many studies demonstrated intriguing characteristics of these compounds in terms of reactivity and, interestingly, some contradictory literature reports appeared about reaction outcomes to synthesize them. Here, we initially provide a condensed overview of transformations performed on the phenalene scaffold, followed by the resynthesis of a 6-phenoxy-substituted derivative. We show that the initial determination of this particular structure was wrong and provide two-dimensional nuclear magnetic resonance (NMR) evidence to assign the structure properly. When preparing new derivatives using the same synthetic route, we observed 6- and 7-substituted regioisomers. After confirming their structures by NMR experiments, the ability of these compounds to inhibit BCL-2 was evaluated. The most potent 1-oxo-1H-phenalene-2,3-dicarbonitrile derivatives inhibited BCL-2 in the nanomolar range and showed double-digit micromolar cytotoxicity against four different cancer cell lines.

Bim transfer between Bcl-2-like protein and Hsp70 underlines Bcl-2/Hsp70 crosstalk to regulate apoptosis

The chaperone heat shock protein 70 (Hsp70) is crucial for avoiding protein misfolding under stress, but it is also upregulated in many kinds of cancers, where its ability to buffer cellular stress prevents apoptosis. Previous research has suggested that Bim, a BH3-only member of the Bcl-2 family proteins, also serves as a cochaperone for Hsp70, which modulates the folding and stabilization of many Hsp70 oncogenic substrates in tumor cells. However, a definitive demonstration of crosstalk between Bcl-2 and Hsp70 family proteins and molecular mechanism remain unclear. Herein, we examined the effects of pan-Bcl-2 inhibitor S1, Hsp70 inhibitor S1g-6 on the K562, U937, H23, HL-60 cell lines and these inhibitors synergistically induce mitochondrial apoptosis in cancer cell lines. Moreover, we identified that Bim transfer between Bcl-2-like protein and Hsp70 underlines Bcl-2/Hsp70 crosstalk in mitochondrial apoptosis pathway. Thus, the synergy of S1 and S1g-6 to induce a panel of cancer cell lines apoptosis by inhibiting free Bim and facilitating oncogenic client AKT folding and activation. Together, our results demonstrated the combination of Bcl-2 inhibitor and Hsp70 inhibitor showed synergistic effect in cancer cells and the potential to decrease tumor regression.

Synthesis and structure-activity relationship of coumarins as potent Mcl-1 inhibitors for cancer treatment

Myeloid cell leukemia-1 (Mcl-1) is a validated and attractive target for cancer therapy. Over-expression of Mcl-1 in many cancers allows cancer cells to evade apoptosis and contributes to their resistance to current chemotherapeutics. In this study, more than thirty coumarin derivatives with different substituents were designed and synthesized, and their Mcl-1 inhibitory activities evaluated using a fluorescence polarization-based binding assay. The results showed that the catechol group was a key constituent for Mcl-1 inhibitory activity of the coumarins, and methylation of the catechol group led to decreased inhibitory activity. The introduction of a hydrophobic electron-withdrawing group at the C-4 position of 6,7-dihydroxycoumarin, enhanced Mcl-1 inhibitory capacity, and a hydrophilic group in this position was unbeneficial to the inhibitory potency. In addition, the introduction of a nitrogen-containing group to the C-5 or C-8 position, which allowed an intramolecular hydrogen bond, was also unfavorable for Mcl-1 inhibition. Among all coumarins tested, 4-trifluoromethyl-6,7-dihydroxycoumarin (Cpd 4) displayed the most potent inhibitory activity towards Mcl-1 (Ki = 0.21 ± 0.02 μM, IC50 = 1.21 ± 0.56 μM, respectively), for which the beneficial effect on taxol resistance was also validated in A549 cells. A strong interaction between Cpd 4 and Mcl-1 in docking simulations further supported the observed potent Mcl-1 inhibition ability of Cpd 4. 3D-QSAR analysis of all tested coumarin derivatives further provides new insights into the relationships linking the inhibitory effects on Mcl-1 and the steric-electrostatic properties of coumarins. These findings could be of great value for medicinal chemists for the design and development of more potent Mcl-1 inhibitors for biomedical applications.

Development of Mcl-1 inhibitors for cancer therapy

The myeloid leukemia cell differentiation protein (Mcl-1) is an anti-apoptotic protein of the B-cell lymphoma 2 (Bcl-2) family, which regulates cellular apoptosis. Mcl-1 expression plays a key role in survival of cancer cells and therefore serves as a promising target in cancer therapy. Besides, its importance as a cancer target, various peptides and small-molecule inhibitors have been successfully designed and synthesized, yet no Mcl-1 inhibitor is approved for clinical use. However, recent development on the understanding of Mcl-1's role in key cellular processes in cancer and an upsurge of reports highlighting its association in various anticancer drug resistance supports the view that Mcl-1 is a key target in various cancers, especially hematological cancers. This review compiles structures of a variety of inhibitors of Mcl-1 reported to date. These include inhibitors based on a diverse range of heterocycles (e.g. indole, imidazole, thiophene, nicotinic acid, piperazine, triazine, thiazole, isoindoline), oligomers (terphenyl, quaterpyridine), polyphenol, phenalene, anthranilic acid, anthraquinone, macrocycles, natural products, and metal-based complexes. In addition, an effort has been made to summarize the structure activity relationships, based on a variety of assays, of some important classes of Mcl-1 inhibitors, giving affinities and selectivities for Mcl-1 compared to other Bcl-2 family members. A focus has been placed on categorizing the inhibitors based on their core frameworks (scaffolds) to appeal to the chemical biologist or medicinal chemist.

A "Three-in-One" Multifunctional Probe for Bcl-2/Mcl-1 Profiling and Visualizing in Situ

Bcl-2 and Mcl-1, the two arms of the anti-apoptotic Bcl-2 family proteins, have been identified as key regulators of apoptosis and effective therapeutic targets of cancer. However, no small-molecular probe is capable of profiling and visualizing both Bcl-2 and Mcl-1 simultaneously in situ. Herein, we report a multifunctional molecular probe (BnN₃ -OPD-Alk) by a "three-in-one" molecular designing strategy, which integrated the Bcl-2/Mcl-1 binding ligand, fluorescent reporter group and photoreactive group azido into the same scaffold. BnN₃ -OPD-Alk exhibited sub-micromolar affinities to Bcl-2/Mcl-1 and bright green self-fluorescence. It was then successfully applied for Bcl-2/Mcl-1 labeling, capturing, enriching, and bioimaging both in vitro and in cells. This strategy could facilitate the precise early diagnosis and effective therapy of dual Bcl-2/Mcl-1-related diseases.

Molecular Chaperone HSP70 and Key Regulators of Apoptosis - A Review

Identified as a molecular chaperone constitutively being synthesized due to enhanced elevated temperature change, this heat shock protein HSP70 has shown to be intimately involved in many protein biogenesis, facilitating the synthesis and folding of proteins and trafficking of nascent peptides during cell growth. HSP70 also plays a vital role in protein assembly, regulation and interaction with a wide variety of proteins. Stress-induced cell death is under the control of the Bcl-2 family of apoptotic regulators and display either pro-apoptotic or anti-apoptotic activities. Subjected to stress conditions such as heat shock, cells have been reported to express elevated expressions of HSP70. Moreover, this molecular chaperon has shown to act at multiple levels to suppress stressed-induced apoptotic signals of some Bcl-2 members by repairing, re-synthesizing damaged proteins, and stabilizing unfolded proteins. Therefore, HSP70 synthesis can act as an essential recovery mode for cellular survival and adaptation during lethal conditions.

Mcl-1 as a "barrier" in cancer treatment: Can we target it now?

During the last two decades, the study of Mcl-1, an anti-apoptotic member of the Bcl-2 family, attracted researchers due to its important role in cancer cell survival and tumor development. The significance of Mcl-1 protein in resistance to chemotherapeutics makes it an attractive target in cancer therapy. Here, we discuss the diverse possibilities for indirect Mcl-1 inhibition through its downregulation, for example, via targeting for proteasomal degradation or blockage of translation and transcription. We also provide an overview of the direct blocking of protein-protein interactions with pro-apoptotic Bcl-2 family proteins, including examples of the most promising regulators of Mcl-1 and selective BH3-mimetics, which at present are under clinical evaluation. Moreover, several approaches for the co-targeting of Mcl-1 and other proteins (e.g., CDKs) are also presented. In addition, we highlight the broad spectrum of problems that accompanied the discovery and development of effective Mcl-1 inhibitors.

Importance of Hypericin-Bcl2 interactions for biological effects at subcellular levels

Hypericin (Hyp) is a naturally occurring compound used as photosensitizer in photodynamic therapy and diagnosis. Recently, we have shown that Hyp presence alone, without illumination, resulted in substantial biological effects at several sub-cellular levels. Hyp induced changes in cellular ultrastructure, mitochondria function and metabolism, and distribution of Bcl2 proteins in malignant and non-malignant cells. The molecular mechanisms that underlie Hyp light-independent effects are still elusive. We have hypothesized that Bcl2-Hyp interactions might be one possible mechanism. We performed molecular docking studies to determine the Hyp-Bcl2 interaction profile. Based on the interaction profiles small Bcl2 peptide segments were selected for further study. We designed small peptides corresponding to Bcl2 BH3 and BH1 domains and tested the binding of Hyp and Bcl2 known inhibitor, ABT263, to the peptides in computer modeling and in vitro binding studies. We employed endogenous tryptophan and tyrosine in the BH3 and BH1 peptides, respectively, and their fluorescent properties to show interaction with Hyp and ABT263. Overall, our results indicate that Hyp can interact with Bcl2 protein at its BH3-BH1 hydrophobic groove, and this interaction may trigger changes in intracellular distribution of Bcl2 proteins. In addition, our computer modeling results suggest that Hyp also interacts with other anti-apoptotic members of Bcl2 family similar to the known BH3 mimetics. Our findings are novel and might contribute to understanding Hyp light-independent effects. In addition, they may substantiate the therapeutic use of Hyp as a BH3 mimetic molecule to enhance other cancer treatments.

The chemical biology of apoptosis: Revisited after 17 years

A balance of Bcl-2 family proteins dictates cell survival or death, as the interactions between these proteins regulate mitochondrial apoptotic signaling pathways. However, cancer cells frequently show upregulation of pro-survival Bcl-2 proteins and sequester activated pro-apoptotic BH3-only proteins driven by diverse cytotoxic stresses, resulting in tumor progression and chemoresistance. Synthetic molecules from either structure-based design or screening procedures to engage and inactivate pro-survival Bcl-2 proteins and restore apoptotic process represent a chemical biological means of selectively killing malignant cells. 17 years ago, one of us reviewed on the discovery of novel Bcl-2 targeted agents [1]. Here we revisit this area and examine the progress and current status of small molecule Bcl-2 inhibitor development, demonstrating the Bcl-2 family as a valid target for cancer therapy and providing successful examples for the discovery of inhibitors that target protein-protein interactions.

Glucose deprivation promotes apoptotic response to S1 by enhancing autophagy in human cervical cancer cells

Background

S1 is a novel BH3 mimetic that can induce death in some types of cancer cells, such as melanoma B16, ovarian cancer SKOV3, and U251 glioma cells. S1 inhibits Bcl-2 and Mcl-1 expression and induces cancer cell apoptosis. Cancer cell survival is highly dependent on glucose. Here, we observed the effect of glucose deprivation on the apoptotic response to S1 in human cervical cancer (HeLa) cells.

Materials and methods

Earle’s balanced salt solution (EBSS) was used to simulate glucose deprivation. MTT assay was used to analyze the cell survival rate, and Hoechst 33342 dye was used to detect the apoptosis in HeLa cells. Western blotting was used to detect the expression of ER stress and autophagy relative proteins. In addition, lysosomes were observed with Lyso-Tracker staining by confocal microscopy.

Results

S1 decreased cell distribution density and survival rate, and MTT assay showed that EBSS enhanced the inhibitory effects of S1 on HeLa cell growth. Hoechst 33342 dye showed that S1 led to pyknosis, fragmentation, and strong staining in HeLa cell nuclei, and EBSS enhanced these effects. Western blotting indicated that EBSS enhanced the expression of apoptosis-related proteins (cytochrome C, caspase-3, and poly[ADP-ribose] polymerase 1) induced by S1 in HeLa cells. S1 decreased p62 expression and increased the autophagosome-associated protein LC3-II expression, which indicated that S1 induced autophagy in HeLa cells. EBSS enhanced S1-induced autophagy in HeLa cells. Furthermore, autophagy inhibitor chloroquine enhanced S1-induced apoptosis in HeLa cells.

Conclusion

These results indicate that EBSS exacerbates S1-induced autophagy and severe autophagy induced by EBSS and S1 could lead to apoptosis in HeLa cells. The results also suggest that EBSS enhances the sensitivity of HeLa cells to S1-induced apoptosis and that autophagy plays an important role in this process.

Discovery of Mcl-1 inhibitors based on a thiazolidine-2,4-dione scaffold

Inspired by a rhodanine-based dual inhibitor of Bcl-x_L and Mcl-1, a focused library of analogues was prepared wherein the rhodanine core was replaced with a less promiscuous thiazolidine-2,4-dione scaffold. Compounds were initially evaluated for their abilities to inhibit Mcl-1. The most potent compound 12b inhibited Mcl-1 with a K_i of 155 nM. Further investigation revealed comparable inhibition of Bcl-x_L (K_i = 90 nM), indicating that the dual inhibitory profile of the initial rhodanine lead had been retained upon switching the heterocycle core.

Deciphering the crucial residues involved in heterodimerization of Bak peptide and anti-apoptotic proteins for apoptosis

B-cell lymphoma 2 (Bcl-2) family proteins are the central regulators of apoptosis, functioning via mitochondrial outer membrane permeabilization. The family members are involved in several stages of apoptosis regulation. The overexpression of the anti-apoptotic proteins leads to several cancer pathological conditions. This overexpression is modulated or inhibited by heterodimerization of pro-apoptotic BH3 domain or BH3-only peptides to the hydrophobic groove present at the surface of anti-apoptotic proteins. Additionally, the heterodimerization displayed differences in binding affinity profile among the pro-apoptotic peptides binding to anti-apoptotic proteins. In light of discovering the novel peptide/drug molecules that contain the potential to inhibit specific anti-apoptotic protein, it is necessary to understand the molecular basis of recognition between the protein and its binding partner (peptide or ligand) along with its binding energies. Therefore, the present work focused on deciphering the molecular basis of recognition between pro-apoptotic Bak peptide binding to different anti-apoptotic (Bcl-xL, Bfl-1, Bcl-W, Mcl-1, and Bcl-2) proteins using advanced Molecular Dynamics (MD) approach such as Molecular Mechanics-Generalized Born Solvent Accessible. The results from our investigation revealed that the predicted binding free energies showed excellent correlation with the experimental values (r² = .95). The electrostatic (ΔG_ele) contributions are the major component that drives the interaction between Bak peptides and different anti-apoptotic peptides. Additionally, van der Waals (ΔG_vdw) energies also play an indispensible role in determining the binding free energy. Furthermore, the decomposition analysis highlighted the comprehensive information about the energy contributions of hotspot residues involved in stabilizing the interaction between Bak peptide and different anti-apoptotic proteins.

Expanding the Cancer Arsenal with Targeted Therapies: Disarmament of the Antiapoptotic Bcl-2 Proteins by Small Molecules

A hallmark of cancer is the evasion of apoptosis, which is often associated with the upregulation of the antiapoptotic members of the Bcl-2 family of proteins. The prosurvival function of the antiapoptotic Bcl-2 proteins is manifested by capturing and neutralizing the proapoptotic Bcl-2 proteins via their BH3 death domains. Accordingly, strategies to antagonize the antiapoptotic Bcl-2 proteins have largely focused on the development of low-molecular-weight, synthetic BH3 mimetics ("magic bullets") to disrupt the protein-protein interactions between anti- and proapoptotic Bcl-2 proteins. In this way, apoptosis has been reactivated in malignant cells. Moreover, several such Bcl-2 family inhibitors are presently being evaluated for a range of cancers in clinical trials and show great promise as new additions to the cancer armamentarium. Indeed, the selective Bcl-2 inhibitor venetoclax (Venclexta) recently received FDA approval for the treatment of a specific subset of patients with chronic lymphocytic leukemia. This review focuses on the major developments in the field of Bcl-2 inhibitors over the past decade, with particular emphasis on binding modes and, thus, the origins of selectivity for specific Bcl-2 family members.

Caspase-mediated cleavage of Beclin1 inhibits autophagy and promotes apoptosis induced by S1 in human ovarian cancer SKOV3 cells

S1, a novel BH3 mimetic, can induce apoptosis dependent on Bax/Bak through inhibition of Bcl-2 in various tumors. S1 also induces autophagy through interrupting the interaction of Bcl-2 and Beclin1. Our results showed that S1 induces apoptosis in human ovarian cancer SKOV3 cells in a time- and dose-dependent manner. Autophagy precedes apoptosis, in SKOV3 cells treated with S1 (6 μmol/L), autophagy reached the maximum peak at 12 h after treatment and decreased to 24 h. In SKOV3 cells treated with different concentrations of S1 for 24 h, the highest level of autophagy was observed with 5 μmol/L and decreased to 10 μmol/L. Autophagy inhibitors 3-MA and CQ enhanced apoptosis induced by S1 in SKOV3 cells. However, overactivation of caspases in apoptosis induced by S1 may inhibit the autophagy-inducing function of Beclin1. Because the pan-caspase inhibitor Z-VAD recovered the autophagy-inducing function of Beclin1 through reduction of activated caspase-mediated cleavage of Beclin1. Furthermore, the Beclin1 cleavage products could further increase apoptosis induced by S1 in SKOV3 cells. This indicates that apoptosis induced by high doses and long exposure of S1 causes the overactivation of caspases and subsequent cleavage of Beclin1, and inhibits the protection of autophagy. Moreover, the cleaved product of Beclin1 further promotes apoptosis induced by S1 in SKOV3 cells. Our results suggest this may be a molecular mechanism for enhancing the sensitivity of cancer cells to apoptosis induced by small molecular compound targeting Bcl-2.

BCL2 Inhibitors as Anticancer Drugs: A Plethora of Misleading BH3 Mimetics

Antiapoptotic BCL2 proteins play a major role in tumor cell survival. Hence, BCL2 inhibitors have been developed as direct inducers of apoptosis. ABT-199 (venetoclax) received breakthrough therapy designation from the FDA due to its apparent efficacy in CLL and AML. However, resistance to ABT-199 is mediated by other BCL2 proteins including BCLXL and MCL1. Considerable effort has been expended seeking novel "BH3 mimetics" that inhibit all of these BCL2 proteins. While many BH3 mimetics inhibit BCL2 proteins in vitro, they fail to directly inhibit them in intact cells. Many BH3 mimetics induce the unfolded protein response culminating in induction of the proapoptotic protein NOXA, which in turn inhibits MCL1. We propose simple experiments to validate BH3 mimetics in cells. A true BCL2 inhibitor will rapidly induce apoptosis in chronic lymphocytic leukemia cells ex vivo A BCLXL inhibitor will rapidly induce apoptosis in platelets. Finally, a BH3 mimetic targeting MCL1 will inhibit its degradation thereby inducing rapid MCL1 accumulation. Compounds that fail these tests should no longer be called BH3 mimetics. We now have a toolbox of selective inhibitors for most of the BCL2 proteins, and we hope these new tools will lead to effective treatment options for many cancers. Mol Cancer Ther; 15(9); 2011-7. ©2016 AACR.

Small-Molecule and Peptide Inhibitors of the Pro-Survival Protein Mcl-1

The ability of protein-protein interactions to regulate cellular processes in both beneficial and detrimental ways has made them obvious drug targets. The Bcl-2 family of proteins undergo a series of protein-protein interactions which regulate the intrinsic cell-death pathway. The pro-survival members of the Bcl-2 family, including Bcl-2, Bcl-xL , and Mcl-1, are commonly overexpressed in a number of human cancers. Effective modulators of members of the Bcl-2 family have been developed and are undergoing clinical trials, but the efficient modulation of Mcl-1 is still not represented in the clinic. In addition, Mcl-1 is a major cause of resistance to radio- and chemotherapies, including inhibitors that target other Bcl-2 family members. Subsequently, the inhibition of Mcl-1 has become of significant interest to the scientific community. This review covers the progress made to date in modulating the activity of Mcl-1, by both stapled peptides and small molecules. The development of peptides as drug candidates, and the advancement of experimental and computational techniques used to discover small molecules are also highlighted.

Bcl-2/MDM2 Dual Inhibitors Based on Universal Pyramid-Like α-Helical Mimetics

No α-helical mimetic that exhibits Bcl-2/MDM2 dual inhibition has been rationally designed due to the different helicities of the α-helixes at their binding interfaces. Herein, we extracted a one-turn α-helix-mimicking ortho-triarene unit from o-phenylene foldamers. Linking benzamide substrates with a rotatable C-N bond, we constructed a novel semirigid pyramid-like scaffold that could support its two-turn α-helix mimicry without aromatic stacking interactions and could adopt the different dihedral angles of the key residues of p53 and BH3-only peptides. On the basis of this universal scaffold, a series of substituent groups were installed to capture the key residues of both p53TAD and BimBH3 and balance the differences of the bulks between them. Identified by FP, ITC, and NMR spectroscopy, a compound 6e (zq-1) that directly binds to Mcl-1, Bcl-2, and MDM2 with balanced submicromolar affinities was obtained. Cell-based experiments demonstrated its antitumor ability through Bcl-2/MDM2 dual inhibition simultaneously.

Small molecules targeting Mcl-1: the search for a silver bullet in cancer therapy

Progress towards the development of potent and selective inhibitors of the pro-survival protein Mcl-1 is reviewed.

Killing Two Cells with One Stone: Pharmacologic BCL-2 Family Targeting for Cancer Cell Death and Immune Modulation

A crucial component of regulating organismal homeostasis is maintaining proper cell number and eliminating damaged or potentially malignant cells. Apoptosis, or programed cell death, is the mechanism responsible for this equilibrium. The intrinsic apoptotic pathway is also especially important in the development and maintenance of the immune system. Apoptosis is essential for proper positive and negative selection during B- and T-cell development and for efficient contraction of expanded lymphocytes following an immune response. Tight regulation of the apoptotic pathway is critical, as excessive cell death can lead to immunodeficiency while apoptotic resistance can lead to aberrant lymphoproliferation and autoimmune disease. Dysregulation of cell death is implicated in a wide range of hematological malignancies, and targeting various components of the apoptotic machinery in these cases is an attractive chemotherapeutic strategy. A wide array of compounds has been developed with the purpose of reactivating the intrinsic apoptotic pathway. These compounds, termed BH3 mimetics are garnering considerable attention as they gain greater clinical oncologic significance. As their use expands, it will be imperative to understand the effects these compounds have on immune homeostasis. Uncovering their potential immunomodulatory activity may allow for administration of BH3 mimetics for direct tumor cell killing as well as novel therapies for a wide range of immune-based directives. This review will summarize the major proteins involved in the intrinsic apoptotic pathway and define their roles in normal immune development and disease. Clinical and preclinical BH3 mimetics are described within the context of what is currently known about their ability to affect immune function. Prospects for future antitumor immune amplification and immune modulation are then proposed.

Attacking cancer's Achilles heel: antagonism of anti-apoptotic BCL-2 family members

Malignant cells routinely violate cellular checkpoints that should initiate cell death in normal cells by triggering pro-apoptotic members of the BCL-2 family of proteins. To escape such death inducing signals, cancer cells often select for upregulation of anti-apoptotic BCL-2 family members including BCL-2, BCL-XL , BFL-1, BCL-W and MCL-1. These family members prevent death by sequestering pro-apoptotic molecules. To counter this resistance mechanism, small molecule inhibitors of anti-apoptotic BCL-2 family members have been under development. These molecules have shown promise in pre-clinical and clinical testing to overcome apoptotic resistance, prompting cancer cells to undergo apoptosis. Alternatively, other strategies have taken advantage of the normal regulatory machinery controlling anti-apoptotic molecules and have used inhibitors of signaling pathways to down-modulate the expression of anti-apoptotic molecules, thus tilting the balance in cancer cells to cell death. This review explores recent developments and strategies aimed at antagonizing anti-apoptotic BCL-2 family member action to promote the induction of cell death in cancer therapy.

Bcl-2 family of proteins as drug targets for cancer chemotherapy: the long way of BH3 mimetics from bench to bedside

Bcl-2 proteins are key determinants in the life-death balance. In recent years, proteins in this family have been identified as drug targets in the design of new anti-tumor therapies. Advances in the knowledge of the mechanism of action of anti-apoptotic and pro-apoptotic members of the Bcl-2 family have enabled the development of the so-called 'BH3 mimetics'. These compounds act by inhibiting anti-apoptotic proteins of the family, imitating the function of the BH3-only subset of pro-apoptotic members. Combinations of BH3-mimetics with anti-tumor drugs are being evaluated in both preclinical models and clinical trials. Recent advances in these approaches will be reviewed.

New dimension in therapeutic targeting of BCL-2 family proteins

Proteins of the BCL-2 family control the mitochondrial pathway of apoptosis. Targeting these proteins proves to be an attractive strategy for anticancer therapy. The biological context is based on the fact that BH3-only members of the family are specific antagonists of prosurvival members. This prompted the identification of "BH3 mimetic" compounds. These small peptides or organic molecules indeed mimic the BH3 domain of BH3-only proteins: by selectively binding and antagonizing prosurvival proteins, they can induce apoptosis in malignant cells. Some small-molecule inhibitors of prosurvival proteins have already entered clinical trials in cancer patients and two of them have shown significant therapeutic effects. The latest developments in the field of targeting BCL-2 family proteins highlight several new antagonists of prosurvival proteins as well as direct activators of proapoptotic proteins. These compounds open up novel prospects for the development of BH3 mimetic anticancer drugs.

Neuroprotective activities of catalpol against CaMKII-dependent apoptosis induced by LPS in PC12 cells

BACKGROUND AND PURPOSE

Neurodegenerative diseases present progressive neurological disorder induced by cell death or apoptosis. Catalpol, an iridoid glucoside isolated from the root of Rehmannia glutinosa Libosch, is present in a wide range of plant families. Although catalpol is an effective anti-apoptotic agent in LPS-induced neurodegeneration, the underlying mechanism has not been established. Here we have identified some of the mechanisms involved the prevention by catalpol of apoptosis induced by LPS in an experimental model of neurodegeneration in vitro.

EXPERIMENTAL APPROACH

Apoptosis was induced by adding LPS (80 ng·mL(-1)) to pheochromocytoma (PC12) cells, pretreated with catalpol for 12 h. We measured intracellular reactive oxygen species (ROS), apoptosis and intracellular calcium concentration ([Ca(2+)]i) by flow cytometry or laser confocal scanning microscopy. We also analysed the protein expression of Bcl-2, Bax and Ca(2+)-calmodulin-dependent protein kinase II (CaMKII)-dependent apoptosis signal-regulating kinase-1 (ASK-1)/JNK/p38 signalling pathway in PC12 cells by Western blot.

KEY RESULTS

Catalpol stimulated expression of Bcl-2 and inhibited the expression of Bax. Catalpol also attenuated the increase in Ca(2+) concentration induced by LPS in PC12 cells and down-regulated CaMK phosphorylation. The CaMKII-dependent ASK-1/JNK/p38 signalling cascade was blocked by catalpol. All these changes were accompanied by a decrease of apoptosis induced by LPS in PC12 cells.

CONCLUSIONS AND IMPLICATIONS

The data presented here provide new mechanistic insights into the links between the CaMKII-dependent ASK-1/JNK/p38 signalling pathway and the protective effect of catalpol on apoptosis induced by LPS in PC12 cells.

Mechanism of synergy of BH3 mimetics and paclitaxel in chronic myeloid leukemia cells: Mcl-1 inhibition

Paclitaxel is an alternative chemotherapeutic agent for chronic myelogenous leukemia (CML) when primary or secondary resistance of tyrosine kinase inhibitors (TKI) is emerging, because paclitaxel could bypass the apoptotic deficiencies linked to p53 and fas ligand pathways in CML. However, high levels of Bcl-2 family proteins in CML could resist paclitaxel-induced apoptosis. Herein, we utilized two BH3 mimetics ABT-737 and S1 to study the potential of BH3 mimetics in combination with paclitaxel in treatment of CML cells and illustrated the mechanism by which BH3 mimetics synergize with paclitaxel. As a single agent, S1 could induce apoptosis in CML-derived cell line K562, whereas ABT-737 was largely ineffective. However, both of the two agents could efficiently synergize with paclitaxel through intrinsic apoptosis pathway. By using Bcl-2 siRNA, Bcl-XL siRNA or Mcl-1 siRNA, we found although each of the three members exhibited activities to block paclitaxel-induced apoptosis, Mcl-1 was the determinant for the synergistic effect between paclitaxel and ABT-737 or S1. Furthermore, paclitaxel/ABT737 synergized to drastically upregulate Bim to displace Bak from Mcl-1, whereas S1 directly binds Mcl-1 to release both Bim and Bak. As such, ABT-737 and S1 sensitized CML to paclitaxel by Mcl-1 inhibition, indirect inhibition through Bim antagonizing Mcl-1, or direct inhibition through binding to Mcl-1 itself. Finally, activation of JNK/Bim pathway was identified as the apical mechanism for ABT-737/paclitaxel synergism. Together, our results demonstrated potent synergy between BH3 mimetics and paclitaxel in the killing of CML cells and revealed an important role for Mcl-1 in mediating synergism by these agents.

Discovery of a novel family of polycyclic aromatic molecules with unique reactivity and members valuable for fluorescent sensing and medicinal chemistry

We review major achievements in the study of derivatives of 1-oxo-1H-phenalene-2,3-dicarbonitrile.

Resistance to BH3 mimetic S1 in SCLC cells that up-regulate and phosphorylate Bcl-2 through ERK1/2

BACKGROUND AND PURPOSE

B cell lymphoma 2 (Bcl-2) is a central regulator of cell survival that is overexpressed in the majority of small-cell lung cancers (SCLC) and contributes to both malignant transformation and therapeutic resistance. The purpose of this work was to study the key factors that determine the sensitivity of SCLC cells to Bcl-2 homology domain-3 (BH3) mimetic S1 and the mechanism underlying the resistance of BH3 mimetics.

EXPERIMENTAL APPROACHES

Western blot was used to evaluate the contribution of Bcl-2 family members to the cellular response of SCLC cell lines to S1. Acquired resistant cells were derived from initially sensitive H1688 cells. Quantitative PCR and gene silencing were performed to investigate Bcl-2 up-regulation.

KEY RESULTS

A progressive increase in the relative levels of Bcl-2 and phosphorylated Bcl-2 (pBcl-2) characterized the increased de novo and acquired resistance of SCLC cell lines. Furthermore, acute treatment of S1 induced Bcl-2 expression and phosphorylation. We showed that BH3 mimetics, including S1 and ABT-737, induced endoplasmic reticulum (ER) stress and then activated MAPK/ERK pathway. The dual function of MAPK/ERK pathway in defining BH3 mimetics was illustrated; ERK1/2 activation leaded to Bcl-2 transcriptional up-regulation and sustained phosphorylation in naïve and acquired resistant SCLC cells. pBcl-2 played a key role in creating resistance of S1 and ABT-737 not only by sequestrating pro-apoptotic proteins, but also sequestrating a positive feedback to promote ERK1/2 activation.

CONCLUSIONS AND IMPLICATIONS

These results provide significant novel insights into the molecular mechanisms for crosstalk between ER stress and endogenously apoptotic pathways in SCLC following BH3 mimetics treatment.

The putative BH3 mimetic S1 sensitizes leukemia to ABT-737 by increasing reactive oxygen species, inducing endoplasmic reticulum stress, and upregulating the BH3-only protein NOXA

S1 is a putative BH3 mimetic proposed to inhibit BCL2 and MCL1 based on cell-free assays. However, we previously demonstrated that it failed to inhibit BCL2 or induce apoptosis in chronic lymphocytic leukemia (CLL) cells, which are dependent on BCL2 for survival. In contrast, we show here that S1 rapidly increases reactive oxygen species, initiates endoplasmic reticulum stress, and upregulates the BH3-only protein NOXA. The BCL2 inhibitors, ABT-737, ABT-263, and ABT-199, have demonstrated pro-apoptotic efficacy in cell lines, while ABT-263 and ABT-199 have demonstrated efficacy in early clinical trials. Resistance to these inhibitors arises from the upregulation of anti-apoptotic factors, such as MCL1, BFL1, and BCLXL. This resistance can be induced by co-culturing CLL cells on a stromal cell line that mimics the microenvironment found in patients. Since NOXA can inhibit MCL1, BFL1, and BCLXL, we hypothesized that S1 may overcome resistance to ABT-737. Here we demonstrate that S1 induces NOXA-dependent sensitization to ABT-737 in a human promyelocytic leukemia cell line (NB4). Furthermore, S1 sensitized CLL cells to ABT-737 ex vivo, and overcame resistance to ABT-737 induced by co-culturing CLL cells with stroma.

Small molecule Mcl-1 inhibitors for the treatment of cancer

The Bcl-2 family of proteins serves as primary regulators of apoptosis. Myeloid cell leukemia 1 (Mcl-1), a pro-survival member of the Bcl-2 family of proteins, is overexpressed and the Mcl-1 gene is amplified in many tumor types. Moreover, the overexpression of Mcl-1 is the cause of resistance to several chemotherapeutic agents. Thus, Mcl-1 is a promising cancer target. This review highlights the current progress on the discovery of small molecule Mcl-1 inhibitors.

Bcl-2 family proteins are involved in the signal crosstalk between endoplasmic reticulum stress and mitochondrial dysfunction in tumor chemotherapy resistance

Tumor cells overexpress antiapoptotic proteins of the Bcl-2 (B-cell leukemia/lymphoma-2) family, which can lead to both escape from cell death and resistance to chemotherapeutic drugs. Recent studies suggest that the endoplasmic reticulum (ER) can produce proapoptotic signals, amplifying the apoptotic signaling cascade. The crosstalk between mitochondria and ER plays a decisive role in many cellular events but especially in cell death. Bcl-2 family proteins located in the ER and mitochondria can influence not only the function of the two organelles but also the interaction between them. Therefore, the Bcl-2 family of proteins may also be involved in the mechanism of tumor chemotherapy resistance by influencing crosstalk between the ER and mitochondria. In this review we will briefly discuss evidence to support this concept.

A competitive stapled peptide screen identifies a selective small molecule that overcomes MCL-1-dependent leukemia cell survival

Cancer cells hijack BCL-2 family survival proteins to suppress the death effectors and thereby enforce an immortal state. This is accomplished biochemically by an antiapoptotic surface groove that neutralizes the proapoptotic BH3 α helix of death proteins. Antiapoptotic MCL-1 in particular has emerged as a ubiquitous resistance factor in cancer. Although targeting the BCL-2 antiapoptotic subclass effectively restores the death pathway in BCL-2-dependent cancer, the development of molecules tailored to the binding specificity of MCL-1 has lagged. We previously discovered that a hydrocarbon-stapled MCL-1 BH3 helix is an exquisitely selective MCL-1 antagonist. By deploying this unique reagent in a competitive screen, we identified an MCL-1 inhibitor molecule that selectively targets the BH3-binding groove of MCL-1, neutralizes its biochemical lock-hold on apoptosis, and induces caspase activation and leukemia cell death in the specific context of MCL-1 dependence.

The use of a neutral peptide aptamer scaffold to anchor BH3 peptides constitutes a viable approach to studying their function

The B-cell CLL/lymphoma-2 (Bcl-2) family of proteins are important regulators of the intrinsic pathway of apoptosis, and their interactions, driven by Bcl-2 homology (BH) domains, are of great interest in cancer research. Particularly, the BH3 domain is of clinical relevance, as it promotes apoptosis through activation of Bcl-2-associated x protein (Bax) and Bcl-2 antagonist killer (Bak), as well as by antagonising the anti-apoptotic Bcl-2 family members. Although investigated extensively in vitro, the study of the BH3 domain alone inside cells is more problematic because of diminished secondary structure of the unconstrained peptide and a lack of stability. In this study, we report the successful use of a novel peptide aptamer scaffold – Stefin A quadruple mutant – to anchor and present the BH3 domains from Bcl-2-interacting mediator of cell death (Bim), p53 upregulated modulator of apoptosis (Puma), Bcl-2-associated death promoter (Bad) and Noxa, and demonstrate its usefulness in the study of the BH3 domains in vivo. When expressed intracellularly, anchored BH3 peptides exhibit much the same binding specificities previously established in vitro, however, we find that, at endogenous expression levels, Bcl-2 does not bind to any of the anchored BH3 domains tested. Nonetheless, when expressed inside cells the anchored PUMA and Bim BH3 α-helices powerfully induce cell death in the absence of efficient targeting to the mitochondrial membrane, whereas the Noxa helix requires a membrane insertion domain in order to kill Mcl-1-dependent myeloma cells. Finally, the binding of the Bim BH3 peptide to Bax was the only interaction with a pro-apoptotic effector protein observed in this study.

Targeting BCL2-Proteins for the Treatment of Solid Tumours

Due to their central role in the regulation of apoptosis, the antiapoptotic BCL2-proteins are highly promising targets for the development of novel anticancer treatments. To this end, several strategies have been developed to inhibit BCL2, BCL-XL, BCL-w, and MCL1. While early clinical trials in haematological malignancies demonstrated exciting single-agent activity of BCL2-inhibitors, the response in solid tumours was limited, indicating that, in solid tumours, different strategies have to be developed in order to successfully treat patients with BCL2-inhibitors. In this review, the function of the different antiapoptotic BCL2-proteins and their role in solid tumours will be discussed. In addition, a comprehensive analysis of current small molecules targeting these antiapoptotic BCL2-proteins (e.g., ABT-737, ABT-263, ABT-199, TW-37, sabutoclax, obatoclax, and MIM1) will be provided including a discussion of the results of any clinical trials. This analysis will summarise the potential of BCL2-inhibitors for the treatment of solid tumours and will unravel novel approaches to utilise these inhibitors in clinical applications.

Targeting the Bcl-2 family in B-cell chronic lymphocytic leukemia

SUMMARY B-cell chronic lymphocytic leukemia (CLL) is the most common leukemia in human adults of the western world and no definitive cure is yet available. One key factor in CLL pathogenesis and disease progression is misbalanced Bcl-2 cell death machinery that is shifted towards protection from apoptosis. Thus, strategies to counteract the antiapoptotic action of the Bcl-2 family in CLL cells are being explored. The Bcl-2 family is composed of a growing number of proteins related to Bcl-2 by sequence homology and their interactions regulate the cell’s decision to die. The features of one particular subclass, the BH3-only proteins, are being studied and exploited for the development of therapeutic anticancer approaches that specifically target antiapoptotic Bcl-2 proteins overexpressed in tumors, including CLL. Preclinical and clinical efficacy and toxicity of the most effective among these ‘BH3 mimetics’ are presented, together with a model that accounts for the differential sensitivity of CLL and normal cells to Bcl-2 neutralization.