A common binding site mediates heterodimerization and homodimerization of Bcl-2 family members

Sulfatase-Induced In Situ Formulation of Antineoplastic Supra-PROTACs

Proteolysis targeting chimera (PROTAC) technology is an innovative strategy for cancer therapy, which, however, suffers from poor targeting delivery and limited capability for protein of interest (POI) degradation. Here, we report a strategy for the in situ formulation of antineoplastic Supra-PROTACs via intracellular sulfatase-responsive assembly of peptides. Coassembling a sulfated peptide with two ligands binding to ubiquitin VHL and Bcl-xL leads to the formation of a pro-Supra-PROTAC, in which the ratio of the two ligands is rationally optimized based on their protein binding affinity. The resulting pro-Supra-PROTAC precisely undergoes enzyme-responsive assembly into nanofibrous Supra-PROTACs in cancer cells overexpressing sulfatase. Mechanistic studies reveal that the pro-Supra-PROTACs selectively cause apparent cytotoxicity to cancer cells through the degradation of Bcl-xL and the activation of caspase-dependent apoptosis, during which the rationally optimized ligand ratio improves the bioactivity for POI degradation and cell death. In vivo studies show that in situ formulation enhanced the tumor accumulation and retention of the pro-Supra-PROTACs, as well as the capability for inhibiting tumor growth with excellent biosafety when coadministrating with chemodrugs. Our findings provide a new approach for enzyme-regulated assembly of peptides in living cells and the development of PROTACs with high targeting delivering and POI degradation efficiency.

Phylogenetic analysis of the MCL1 BH3 binding groove and rBH3 sequence motifs in the p53 and INK4 protein families

B-cell lymphoma 2 (Bcl-2) proteins are central, conserved regulators of apoptosis. Bcl-2 family function is regulated by binding interactions between the Bcl-2 homology 3 (BH3) motif in pro-apoptotic family members and the BH3 binding groove found in both the pro-apoptotic effector and anti-apoptotic Bcl-2 family members. A novel motif, the reverse BH3 (rBH3), has been shown to interact with the anti-apoptotic Bcl-2 homolog MCL1 (Myeloid cell leukemia 1) and have been identified in the p53 homolog p73, and the CDK4/6 (cyclin dependent kinase 4/6) inhibitor p18INK4c, (p18, cyclin-dependent kinase 4 inhibitor c). To determine the conservation of rBH3 motif, we first assessed conservation of MCL1's BH3 binding groove, where the motif binds. We then constructed neighbor-joining phylogenetic trees of the INK4 and p53 protein families and analyzed sequence conservation using sequence logos of the rBH3 locus. This showed the rBH3 motif is conserved throughout jawed vertebrates p63 and p73 sequences and in chondrichthyans, amphibians, mammals, and some reptiles in p18. Finally, a potential rBH3 motif was identified in mammalian and osteichthyan p19INK4d (p19, cyclin dependent kinase 4 inhibitor d). These findings demonstrate that the interaction between MCL1 and other cellular proteins mediated by the rBH3 motif may be conserved throughout jawed vertebrates.

Lipid accumulation and oxidative stress in the crop tissues of male and female pigeons during incubation and chick-rearing periods

The current study aimed to evaluate the changes in lipid accumulation and oxidative status in pigeon crops during different breeding stages. Forty-two pairs of adult pigeons were randomly assigned to 7 groups. Lipid droplet accumulation in pigeon crops was visualized by using oil red O staining from d 17 of incubation (I17) to d 7 of chick rearing (R7). Transmission electron microscopy analysis showed swollen mitochondria with disintegration of cristae and typical characteristics of endoplasmic reticulum stress in crop tissues at R1 compared with those at I4. During the peak of pigeon milk formation, the concentrations of reactive oxygen species, and oxidative damage markers (advanced oxidation protein products, 8-hydroxy-2 deoxyguanosine, and malondialdehyde) and the enzyme activities of total superoxide dismutase and glutathione peroxidase were all elevated significantly (P < 0.05). The protein concentration of B-cell lymphoma-2 associated X in crop tissues was significantly higher at R1, while the level of B-cell lymphoma-2 protein in males was the highest at I4 (P < 0.05). The ratio of B-cell lymphoma-2 associated X protein (Bax)/B-cell lymphoma-2 (Bcl-2) in both male and female crops peaked at R1 (P < 0.05). Gene expression of the key enzymes involved in mitochondrial and peroxisomal fatty acid β-oxidation was investigated in crops. In males, the gene expression of carnitine palmitoyltransferase 1a peaked at R15, and that of carnitine palmitoyltransferase 2 increased significantly from R1 to R15 (P < 0.05). The mRNA abundance of long chain 3-hydroxyacyl-CoA dehydrogenase increased to the maximum value at R1 and I17 in males and females, respectively. From I17 to R7, the mRNA levels of acyl-CoA oxidase 1 and acyl-CoA oxidase 2 were decreased in pigeon crops (P < 0.05). Conclusively, lipid droplet accumulation was found in male and female pigeon crops from the end of incubation to the early stage of chick rearing. Although antioxidant defence and mitochondrial fatty acid β-oxidation were both mobilized, oxidative stress in crop tissues still occurred during the peak of milk formation.

Experimental, molecular docking and molecular dynamic studies of natural products targeting overexpressed receptors in breast cancer

Natural compounds are proper tools for inhibiting cancer cell proliferation. Hence, the search for these ligands of overexpressed receptors in breast cancer has been a competitive challenge recently and opens new avenues for drug discovery. In this research, we have investigated molecular interactions between natural products and overexpressed receptors in breast cancer using molecular docking and dynamic simulation approaches followed by extraction of the best ligand from Citrus limetta and developing for nanoscale encapsulation composed of soy lecithin using a sonicator machine. The encapsulation process was confirmed by DLS and TEM analyses. Anticancer activity was also examined using MTT method. Among the investigated natural compounds, hesperidin was found to bind to specific targets with stronger binding energy. The molecular dynamics results indicated that the hesperidin-MCL-1 complex is very stable at 310.15 K for 200 ns. The RP-HPLC analysis revealed that the purity of extracted hesperidin was 98.8% with a yield of 1.72%. The results of DLS and TEM showed a strong interaction between hesperidin and lecithin with an entrapped efficiency of 92.02 ± 1.08%. Finally, the cytotoxicity effect of hesperidin was increased against the MDA-MB-231 cell line with an IC50 value of 62.93 μg/mL after encapsulation, whereas no significant effect against the MCF10A cell line. We showed for the first time that hesperidin is a flexible and strong ligand for the MCL-1 receptor. Also, it has the in vitro ability to kill the MDA-MB-231 cell lines without having a significant effect on the MCF10A cell lines. Therefore, hesperidin could be used as a food ingredient to generate functional foods.

Coumarin-Palladium(II) Complex Acts as a Potent and Non-Toxic Anticancer Agent against Pancreatic Carcinoma Cells

Pancreatic carcinoma still represents one of the most lethal malignant diseases in the world although some progress has been made in treating the disease in the past decades. Current multi-agent treatment options have improved the overall survival of patients, however, more effective treatment strategies are still needed. In this paper we have characterized the anticancer potential of coumarin-palladium(II) complex against pancreatic carcinoma cells. Cells viability, colony formation and migratory potential of pancreatic carcinoma cells were assessed in vitro, followed by evaluation of apoptosis induction and in vivo testing on zebrafish. Presented results showed remarkable reduction in pancreatic carcinoma cells growth both in vitro and in vivo, being effective at micromolar concentrations (0.5 μM). Treatments induced apoptosis, increased BAX/BCL-2 ratio and suppressed the expression of SOX9 and SOX18, genes shown to be significantly up-regulated in pancreatic ductal adenocarcinoma. Importantly, treatments of the zebrafish-pancreatic adenocarcinoma xenografts resulted in significant reduction in tumor mass, without provoking any adverse toxic effects including hepatotoxicity. Presented results indicate the great potential of the tested compound and the perspective of its further development towards pancreatic cancer therapy.

Alternative splicing of apoptosis genes promotes human T cell survival

Alternative splicing occurs in the vast majority of human genes, giving rise to distinct mRNA and protein isoforms. We, and others, have previously identified hundreds of genes that change their isoform expression upon T cell activation via alternative splicing; however, how these changes link activation input with functional output remains largely unknown. Here, we investigate how costimulation of T cells through the CD28 receptor impacts alternative splicing in T cells activated through the T cell receptor (TCR, CD3) and find that while CD28 signaling alone has minimal impact on splicing, it enhances the extent of change for up to 20% of TCR-induced alternative splicing events. Interestingly, a set of CD28-enhanced splicing events occur within genes encoding key components of the apoptotic signaling pathway; namely caspase-9, Bax, and Bim. Using both CRISPR-edited cells and antisense oligos to force expression of specific isoforms, we show for all three of these genes that the isoform induced by CD3/CD28 costimulation promotes resistance to apoptosis, and that changes in all three genes together function combinatorially to further promote cell viability. Finally, we show that the JNK signaling pathway, induced downstream of CD3/CD28 costimulation, is required for each of these splicing events, further highlighting their co-regulation. Together, these findings demonstrate that alternative splicing is a key mechanism by which costimulation of CD28 promotes viability of activated T cells.

Effect of Akt activation on apoptosis-related gene expression in the crop tissues of male and female pigeons (Columba livia)

The current study investigated whether the expression of apoptosis genes in the pigeon crops was affected by the Akt signaling pathway during crop milk formation. First, 78 pairs of adult White King pigeons were randomly assigned to 7 groups, and the expression of apoptosis-related genes and Akt signaling pathway-related proteins in the crop tissues during different breeding stages were examined. The results showed that the mRNA levels of Bak, caspase-3, caspase-6, and caspase-9 in female crops all increased and reached their highest levels at d 17 of incubation (I17). In male crops, the levels of caspase-3 and caspase-9 gene expression peaked at d 1 of chick rearing (R1). The lowest level of Bcl-2 gene expression in females was observed at I17. The expression ratios of p-Akt (Ser473)/Akt and p-Akt (Thr308)/Akt in male crops decreased to their minimum at R1, while it was observed at d 7 of chick rearing (R7) in females. Second, 36 pairs of adult pigeons were divided into 3 groups and were subjected to SC79 injections with dosages of 0, 0.02, or 0.04 mg/kg bodyweight. The SC79 injections resulted in a considerable decrease in growth performance of pigeon squabs. In male crops, the expression ratios of p-Akt (Ser473)/Akt and p-Akt (Thr308)/Akt were significantly elevated in the 0.02 mg/kg SC79 group, while in female crops, they were higher in the 0.04 mg/kg SC79 group (P < 0.05). The SC79 injection inhibited the gene expression of Bak in female crops, but enhanced the gene expression of Bcl-2 in both male and female crops. In the 0.04 mg/kg SC79 group, a 50.7 to 75.7% decrease was observed in the expression of caspase-3, caspase-6, and caspase-9 in male and female pigeon crops. Expression of the caspase-8 gene and total Akt protein in pigeon crops was not changed in different breeding stages or after SC79 injection. In conclusion, the expression of genes related to mitochondria-dependent apoptosis can be regulated by the Akt signaling pathway, which may play a potential role in pigeon milk formation.

Supramolecular Antagonists Promote Mitochondrial Dysfunction

Mitochondrion-targeting therapy exhibits great potential in cancer therapy but significantly suffers from limited therapeutic efficiency. Here we report on mitochondrion-targeting supramolecular antagonist-inducing tumor cell death via simultaneously promoting cellular apoptosis and preventing survival. The supramolecular antagonist was created via coassembly of a mitochondrion-targeting pentapeptide with its two derivatives functionalized with a BH3 domain or the drug camptothecin (CPT). While drug CPT released from the antagonist induced cellular apoptosis via decreasing the mitochondrial membrane potential, the BH3 domain prevented cellular survival through facilitating the association between the supramolecular antagonists and antiapoptotic proteins, thereby initiating mitochondrial permeabilization. Both in vitro and in vivo studies confirmed the combinatorial therapeutic effect arising from the BH3 domain and CPT drug within the supramolecular antagonist on cell death and thereby inhibiting tumor growth. Our findings demonstrate an efficient combinatorial mechanism for mitochondrial dysfunction, thus potentially serving as novel organelle-targeting medicines.

A novel polyethylene glycol (PEG)-drug conjugate of Venetoclax, a Bcl-2 inhibitor, for treatment of acute myeloid leukemia (AML)

Background

Venetoclax (VTX) is an anticancer drug. It is a selective Bcl‐2 inhibitor that is clinically used for the treatment of patients with lymphomas and leukemias. Treatment with VTX, however, is accompanied by severe adverse events such as tumor lysis syndrome and neutropenia, because VTX readily binds to serum proteins, which results in poor pharmacokinetics and poor tumor tissue concentration. To avoid such adverse events, VTX is administered using a daily or weekly ramp‐up schedule that is cumbersome in clinical situations.

Aims

To overcome these shortcomings, we prepared a novel polyethylene glycol (PEG)‐drug conjugate of VTX (PEG‐VTX) and evaluated its cytotoxic effects on acute myeloid leukemia (AML) both in vitro and in vivo.

Methods and results

VTX and 4‐armed PEG derivatives were covalently attached through an amide bond linker. In a series of in vitro studies, PEG‐VTX selectively induced potent growth inhibition of MV4‐11 human AML cells via the inducement of Bcl‐2‐mediated apoptosis. PEG‐VTX had the effect of free VTX, presumably due to the protease‐mediated release of VTX from the conjugates. In in vivo studies with AML tumor‐xenograft mice models, intravenous PEG‐VTX promoted sufficient tumor growth suppression. Compared with a regimen of oral free VTX, the intravenous regimen in those studies used a VTX dosage that was 15–30 times smaller for an OCI‐AML‐2 xenograft model and a dosing regimen that was less frequent for an MV4‐11 xenograft model. The most important development, however, was the absence of weight loss related to severe side effects throughout the treatments. An increase in water solubility and the resultant hydrodynamic size of VTX via PEGylation improved the pharmacokinetics of VTX by avoiding protein interactions and lessening the extravasation from blood. The result was an increase in tumor accumulation and a decrease in the nonspecific distribution of VTX.

Conclusion

The results of this study suggest that PEG‐VTX could be an alternative therapeutic option for the safe and effective treatment of patients with AML.

Conformational Switching in Bcl-xL: Enabling Non-Canonic Inhibition of Apoptosis Involves Multiple Intermediates and Lipid Interactions

The inhibition of mitochondrial permeabilization by the anti-apoptotic protein Bcl-xL is crucial for cell survival and homeostasis. Its inhibitory role requires the partitioning of Bcl-xL to the mitochondrial outer membrane from an inactive state in the cytosol, leading to its extensive refolding. The molecular mechanisms behind these events and the resulting conformations in the bilayer are unclear, and different models have been proposed to explain them. In the most recently proposed non-canonical model, the active form of Bcl-xL employs its N-terminal BH4 helix to bind and block its pro-apoptotic target. Here, we used a combination of various spectroscopic techniques to study the release of the BH4 helix (α1) during the membrane insertion of Bcl-xL. This refolding was characterized by a gradual increase in helicity due to the lipid-dependent partitioning-coupled folding and formation of new helix αX (presumably in the originally disordered loop between helices α1 and α2). Notably, a comparison of various fluorescence and circular dichroism measurements suggested the presence of multiple Bcl-xL conformations in the bilayer. This conclusion was explicitly confirmed by single-molecule measurements of Förster Resonance Energy Transfer from Alexa-Fluor-488-labeled Bcl-xL D189C to a mCherry fluorescent protein attached at the N-terminus. These measurements clearly indicated that the refolding of Bcl-xL in the bilayer is not a two-state transition and involves multiple membranous intermediates of variable compactness.

Self-Assembly of Human Profilin-1 Detected by Carr-Purcell-Meiboom-Gill Nuclear Magnetic Resonance (CPMG NMR) Spectroscopy

Protein oligomerization in the cell has important implications for both health and disease, and an understanding of the mechanisms by which proteins can self-associate is, therefore, of critical interest. Initial stages of the oligomerization process can be hard to detect, as they often involve the formation of sparsely populated and transient states that are difficult to characterize by standard biophysical approaches. Using relaxation dispersion nuclear magnetic resonance spectroscopy, we study the oligomerization of human profilin-1, a protein that regulates the polymerization of actin. We show that in solution and at millimolar concentrations profilin-1 is predominantly monomeric. However, fits of concentration-dependent relaxation data are consistent with the formation of a higher-order oligomer that is generated via a multistep process. Together with crystallographic data for profilin-2, a homologue of the protein studied here, our results suggest that profilin-1 forms a sparsely populated tetrameric conformer in solution.

Bcl-xL inhibition by molecular-targeting drugs sensitizes human pancreatic cancer cells to TRAIL

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis in various types of cancer cells without damaging normal cells. However, in terms of pancreatic cancer, not all cancer cells are sensitive to TRAIL. In this study, we examined a panel of human pancreatic cancer cell lines for TRAIL sensitivity and investigated the effects of Bcl-2 family inhibitors on their response to TRAIL. Both ABT-263 and ABT-737 inhibited the function of Bcl-2, Bcl-xL, and Bcl-w. Of the nine pancreatic cancer cell lines tested, six showed no or low sensitivity to TRAIL, which correlated with protein expression of Bcl-xL. ABT-263 significantly sensitized four cell lines (AsPC-1, Panc-1, CFPAC-1, and Panc10.05) to TRAIL, with reduced cell viability and increased apoptosis. Knockdown of Bcl-xL, but not Bcl-2, by siRNA transfection increased the sensitivity of AsPC-1 and Panc-1 cells to TRAIL. ABT-263 treatment had no effect on protein expression of Bcl-2, Bcl-xL, or c-FLIPs. In Panc-1 cells, ABT-263 increased the surface expression of death receptor (DR) 5; the NF-κB pathway, but not endoplasmic reticulum stress, participated in the increase. In xenograft mouse models, the combination of TRAIL and ATB-737 suppressed the in vivo tumor growth of AsPC-1 and Panc-1 cells. These results indicate that Bcl-xL is responsible for TRAIL resistance in human pancreatic cancer cells, and that Bcl-2 family inhibitors could represent promising reagents to sensitize human pancreatic cancers in DR-targeting therapy.

Dynamic Bcl-xL (S49) and (S62) Phosphorylation/Dephosphorylation during Mitosis Prevents Chromosome Instability and Aneuploidy in Normal Human Diploid Fibroblasts

Bcl-xL proteins undergo dynamic phosphorylation/dephosphorylation on Ser49 and Ser62 residues during mitosis. The expression of Bcl-xL(S49A), (S62A) and dual (S49/62A) phosphorylation mutants in tumor cells lead to severe mitotic defects associated with multipolar spindle, chromosome lagging and bridging, and micro-, bi- and multi-nucleated cells. Because the above observations were made in tumor cells which already display genomic instability, we now address the question: will similar effects occur in normal human diploid cells? We studied normal human diploid BJ foreskin fibroblast cells expressing Bcl-xL (wild type), (S49A), (S49D), (S62A), (S62D) and the dual-site (S49/62A) and (S49/62D) mutants. Cells expressing S49 and/or S62 phosphorylation mutants showed reduced kinetics of cell population doubling. These effects on cell population doubling kinetics correlated with early outbreak of senescence with no impact on the cell death rate. Senescent cells displayed typical senescence-associated phenotypes including high-level of senescence-associated β-galactosidase activity, interleukin-6 (IL-6) secretion, tumor suppressor p53 and cyclin-dependent kinase inhibitor p21Waf1/Cip1 activation as well as γH2A.X-associated nuclear chromatin foci. Fluorescence in situ hybridization analysis and Giemsa-banded karyotypes revealed that the expression of Bcl-xL phosphorylation mutants in normal diploid BJ cells provoked chromosome instability and aneuploidy. These findings suggest that dynamic Bcl-xL(S49) and (S62) phosphorylation/dephosphorylation cycles are important in the maintenance of chromosome integrity during mitosis in normal cells. They could impact future strategies aiming to develop and identify compounds that could target not only the anti-apoptotic domain of Bcl-xL protein, but also its mitotic domain for cancer therapy.

The deadly landscape of pro-apoptotic BCL-2 proteins in the outer mitochondrial membrane

Apoptosis is a biological process that removes damaged, excess or infected cells through a genetically controlled mechanism. This process plays a crucial role in organismal development, immunity and tissue homeostasis, and alterations in apoptosis contribute to human diseases including cancer and auto-immunity. In the past two decades, significant efforts have focused on understanding the function of the BCL-2 proteins, a complex family of pro-survival and pro-apoptotic α-helical proteins that directly control the mitochondrial pathway of apoptosis. Diverse structural investigations of the BCL-2 family members have broadened our mechanistic understanding of their individual functions. However, an often over-looked aspect of the mitochondrial pathway of apoptosis is how the BCL-2 family specifically interacts with and targets the outer mitochondrial membrane to initiate apoptosis. Structural information on the relationship between the BCL-2 family and the outer mitochondrial membrane is missing; likewise, knowledge of the biophysical mechanisms by which the outer mitochondrial membrane affects and effects apoptosis is lacking. In this mini-review, we provide a current overview of the BCL-2 family members and discuss the latest structural insights into BAK/BAX activation and oligomerization in the context of the outer mitochondrial membrane and mitochondrial biology.

Bcl-2 family inhibition sensitizes human prostate cancer cells to docetaxel and promotes unexpected apoptosis under caspase-9 inhibition

Docetaxel (DTX) is a useful chemotherapeutic drug for the treatment of hormone-refractory prostate cancer. However, emergence of DTX resistance has been a therapeutic hurdle. In this study, we investigated the effect of combining DTX with Bcl-2 family inhibitors using human prostate cancer cell lines (PC3, LNCaP, and DU145 cells). PC3 cells were less sensitive to DTX than were the other two cell lines. In contrast to ABT-199, which inhibits Bcl-2 and Bcl-w, both ABT-263 and ABT-737, which inhibit Bcl-2, Bcl-xL, and Bcl-w, significantly augmented the antitumor effect of DTX on PC3 cells. ABT-263 also enhanced the antitumor effect of DTX on a DTX-resistant PC3 variant cell line. The antitumor effect of ABT-263 was due mainly to its inhibitory effect on Bcl-xL. In a xenograft mouse model, DTX and ABT-737 combination therapy significantly inhibited PC3 tumor growth. Interestingly, although ABT-263 activated caspase-9 in PC3 cells, inhibition of caspase-9 unexpectedly promoted ABT-263-induced apoptosis in a caspase- 8-dependent manner. This augmented apoptosis was also observed in LNCaP cells. These findings indicate that Bcl-xL inhibition can sensitize DTX-resistant prostate cancer cells to DTX, and they reveal a unique apoptotic pathway in which antagonism of Bcl-2 family members in caspase-9-inhibited prostate cancer cells triggers caspase-8-dependent apoptosis.

Behavior of solvent-exposed hydrophobic groove in the anti-apoptotic Bcl-XL protein: clues for its ability to bind diverse BH3 ligands from MD simulations

Bcl-XL is a member of Bcl-2 family of proteins involved in the regulation of intrinsic pathway of apoptosis. Its overexpression in many human cancers makes it an important target for anti-cancer drugs. Bcl-XL interacts with the BH3 domain of several pro-apoptotic Bcl-2 partners. This helical bundle protein has a pronounced hydrophobic groove which acts as a binding region for the BH3 domains. Eight independent molecular dynamics simulations of the apo/holo forms of Bcl-XL were carried out to investigate the behavior of solvent-exposed hydrophobic groove. The simulations used either a twin-range cut-off or particle mesh Ewald (PME) scheme to treat long-range interactions. Destabilization of the BH3 domain-containing helix H2 was observed in all four twin-range cut-off simulations. Most of the other major helices remained stable. The unwinding of H2 can be related to the ability of Bcl-XL to bind diverse BH3 ligands. The loss of helical character can also be linked to the formation of homo- or hetero-dimers in Bcl-2 proteins. Several experimental studies have suggested that exposure of BH3 domain is a crucial event before they form dimers. Thus unwinding of H2 seems to be functionally very important. The four PME simulations, however, revealed a stable helix H2. It is possible that the H2 unfolding might occur in PME simulations at longer time scales. Hydrophobic residues in the hydrophobic groove are involved in stable interactions among themselves. The solvent accessible surface areas of bulky hydrophobic residues in the groove are significantly buried by the loop LB connecting the helix H2 and subsequent helix. These observations help to understand how the hydrophobic patch in Bcl-XL remains stable in the solvent-exposed state. We suggest that both the destabilization of helix H2 and the conformational heterogeneity of loop LB are important factors for binding of diverse ligands in the hydrophobic groove of Bcl-XL.

Bcl-xL phosphorylation at Ser49 by polo kinase 3 during cell cycle progression and checkpoints

Functional analysis of a Bcl-xL phosphorylation mutant series has revealed that cells expressing Bcl-xL(Ser49Ala) mutant are less stable at G2 checkpoint after DNA damage and enter cytokinesis more slowly after microtubule poisoning, than cells expressing wild-type Bcl-xL. These effects of Bcl-xL(Ser49Ala) mutant seem to be separable from Bcl-xL function in apoptosis. Bcl-xL(Ser49) phosphorylation is cell cycle-dependent. In synchronized cells, phospho-Bcl-xL(Ser49) appears during the S phase and G2, whereas it disappears rapidly in early mitosis during prometaphase, metaphase and early anaphase, and re-appears during telophase and cytokinesis. During DNA damage-induced G2 arrest, an important pool of phospho-Bcl-xL(Ser49) accumulates in centrosomes which act as essential decision centers for progression from G2 to mitosis. During telophase/cytokinesis, phospho-Bcl-xL(Ser49) is found with dynein motor protein. In a series of in vitro kinase assays, specific small interfering RNA and pharmacological inhibition experiments, polo kinase 3 (PLK3) was implicated in Bcl-xL(Ser49) phosphorylation. These data indicate that, during G2 checkpoint, phospho-Bcl-xL(Ser49) is another downstream target of PLK3, acting to stabilize G2 arrest. Bcl-xL phosphorylation at Ser49 also correlates with essential PLK3 activity and function, enabling cytokinesis and mitotic exit.

PPARgamma and Apoptosis in Cancer

Peroxisome proliferator-activated receptors (PPARs) are ligand binding transcription factors which function in many physiological roles including lipid metabolism, cell growth, differentiation, and apoptosis. PPARs and their ligands have been shown to play a role in cancer. In particular, PPARγ ligands including endogenous prostaglandins and the synthetic thiazolidinediones (TZDs) can induce apoptosis of cancer cells with antitumor activity. Thus, PPARγ ligands have a potential in both chemoprevention and therapy of several types of cancer either as single agents or in combination with other antitumor agents. Accordingly, the involvement of PPARγ and its ligands in regulation of apoptosis of cancer cells have been extensively studied. Depending on cell types or ligands, induction of apoptosis in cancer cells by PPARγ ligands can be either PPARγ-dependent or -independent. Through increasing our understanding of the mechanisms of PPARγ ligand-induced apoptosis, we can develop better strategies which may include combining other antitumor agents for PPARγ-targeted cancer chemoprevention and therapy. This review will highlight recent research advances on PPARγ and apoptosis in cancer.

Possible mechanisms of trichosanthin-induced apoptosis of tumor cells

Trichosanthin (TCS) is a type I ribosome-inactivating protein that is isolated from the root tubers of the Chinese medicinal herb Trichosanthes kirilowii Maximowicz. TCS has been used as an abortifacient for 1,500 years in China because of its high toxicity on trophoblasts. Over the past 20 years, TCS has been the subject of much research because of its potential antitumor activities. Many reports have revealed that TCS is cytotoxic in a variety of tumor cell lines in vitro and in vivo. Monoclonal antibody-conjugated TCS could enhance its antitumor efficacy; thus, TCS is considered to be a potential biological agent for cancer treatment. TCS is able to inhibit protein synthesis and consequently induce necrosis. Recent studies have demonstrated that TCS does indeed induce apoptosis in several tumor cell lines. Although TCS-induced apoptosis of tumor cell lines is now well known, the underlying mechanisms remain to be elucidated. The purpose of this review was to investigate the effects of TCS and its possible mechanisms of action, based on published literature and the results of our own studies.

p34SEI-1 inhibits doxorubicin-induced senescence through a pathway mediated by protein kinase C-delta and c-Jun-NH2-kinase 1 activation in human breast cancer MCF7 cells

In this study, we describe a novel function of the p34(SEI-1) protein, which is both an oncogenic protein and a positive regulator of the cell cycle. The p34(SEI-1) protein was found to inhibit doxorubicin-induced senescence. We investigated the molecular mechanisms of the inhibitory effect of p34(SEI-1) on senescence. First, we found that the activation of protein kinase C-delta (PKC-delta), which is cleaved into a 38 kDa active form from a 78 kDa pro-form, induced after doxorubicin treatment, was inhibited by p34(SEI-1). Furthermore, p34(SEI-1) induced the ubiquitination of PKC-delta. Yet, there is no interaction between p34(SEI-1) and PKC-delta. We also found that the phosphorylation of c-Jun-NH(2)-kinase 1 (JNK1) induced after doxorubicin treatment was suppressed by p34(SEI-1), but not in JNK2. Consistently, pharmacologic or genetic inactivation of either PKC-delta or JNK1 was found to inhibit doxorubicin-induced senescence. In addition, the genetic inactivation of PKC-delta by PKC-delta small interfering RNA resulted in an inhibition of JNK1 activation, but PKC-delta expression was not inactivated by JNK1 small interfering RNA, implying that the activation of JNK1 could be dependently induced by PKC-delta. Therefore, p34(SEI-1) inhibits senescence by inducing PKC-delta ubiquitination and preventing PKC-delta-dependent phosphorylation of JNK1.

Protective function of taurine in glutamate-induced apoptosis in cultured neurons

Previously, we showed that taurine protects neurons against glutamate-induced excitotoxicity by inhibiting the glutamate-induced increase of [Ca2+](i). In this study, we report that taurine prevents glutamate-induced chromosomal condensation, indicating that taurine inhibits glutamate-induced apoptosis. We found that Bcl-2 was down-regulated while Bax was up-regulated by glutamate treatment, and these changes were prevented in the presence of taurine. We have also shown that taurine inhibits glutamate-induced activation of calpain. Furthermore, calpastatin, a specific calpain inhibitor, also prevented glutamate-induced cell death. Here we propose the mechanisms underlying glutamate-induced apoptosis and taurine's inhibition of glutamate-induced apoptosis to be as follows: glutamate stimulation induces [Ca2+](i) elevation, which in turn activates calpain; activation of calpain leads to a reduction of Bcl-2:Bax ratios; with decreased Bcl-2:Bax ratios Bax homodimers form, Bax homodimerization, and translocation to the mitochondria result in the release of cytochrome c; released cytochrome c in turn activates a downstream caspase cascade leading to apoptosis. The antiapoptotic function of taurine is due to its inhibition of glutamate-induced membrane depolarization.

Peptides and peptide mimics as modulators of apoptotic pathways

Programmed cell death is an important and stringently controlled process. Aberrancies in its control mechanisms can lead to disease; overactive apoptosis can cause neurodegenerative disorders, whereas deficient apoptotic activity can lead to cancer. Therefore, controlling apoptotic pathways with peptides is showing increasing promise as a strategy in drug development.Programmed cell death or apoptosis is a noninvasive and strictly regulated cellular process required for organism development and tissue homeostasis. Deficiencies in apoptotic pathways are the source of many diseases such as cancer, neurodegenerative and autoimmune diseases, and disorders related to an inappropriate loss of cells such as heart failure, stroke, and liver injury. Validation of the various points of intervention as targets for drug development has been the subject of a vast number of studies. Peptides are essential tools for drug discovery, as well as preclinical and pharmaceutical drug development.

Evolution of regulatory sequences in 12 Drosophila species

Characterization of the evolutionary constraints acting on cis-regulatory sequences is crucial to comparative genomics and provides key insights on the evolution of organismal diversity. We study the relationships among orthologous cis-regulatory modules (CRMs) in 12 Drosophila species, especially with respect to the evolution of transcription factor binding sites, and report statistical evidence in favor of key evolutionary hypotheses. Binding sites are found to have position-specific substitution rates. However, the selective forces at different positions of a site do not act independently, and the evidence suggests that constraints on sites are often based on their exact binding affinities. Binding site loss is seen to conform to a molecular clock hypothesis. The rate of site loss is transcription factor–specific and depends on the strength of binding and, in some cases, the presence of other binding sites in close proximity. Our analysis is based on a novel computational method for aligning orthologous CRMs on a tree, which rigorously accounts for alignment uncertainties and exploits binding site predictions through a unified probabilistic framework. Finally, we report weak purifying selection on short deletions, providing important clues about overall spatial constraints on CRMs. Our results present a complex picture of regulatory sequence evolution, with substantial plasticity that depends on a number of factors. The insights gained in this study will help us to understand the combinatorial control of gene regulation and how it evolves. They will pave the way for theoretical models that are cognizant of the important determinants of regulatory sequence evolution and will be critical in genome-wide identification of non-coding sequences under purifying or positive selection.

The spatial–temporal expression pattern of a gene, which is crucial to its function, is controlled by cis-regulatory DNA sequences. Forming the basic units of regulatory sequences are transcription factor binding sites, often organized into larger modules that determine gene expression in response to combinatorial environmental signals. Understanding the conservation and change of regulatory sequences is critical to our knowledge of the unity as well as diversity of animal development and phenotypes. In this paper, we study the evolution of sequences involved in the regulation of body patterning in the Drosophila embryo. We find that mutations of nucleotides within a binding site are constrained by evolutionary forces to preserve the site's binding affinity to the cognate transcription factor. Functional binding sites are frequently destroyed during evolution and the rate of loss across evolutionary spans is roughly constant. We also find that the evolutionary fate of a site strongly depends on its context; a pair of interacting sites are more likely to survive mutational forces than isolated sites. Together, these findings provide new insights and pose new challenges to our understanding of cis-regulatory sequences and their evolution.

Protein kinase C isoforms: Multi-functional regulators of cell life and death

The protein kinase C (PKC) family consists of 10 related serine/threonine protein kinases some of which are critical regulators of cell proliferation, survival and cell death. While early studies relied on broad spectrum chemical activators or inhibitors of this family, the generation of isoform specific tools has greatly facilitated our understanding of the contribution of specific PKC isoforms to cell proliferation and apoptosis. These studies suggest that PKC-alpha, PKC-epsilon, and the atypical PKC's, PKC-lambda/iota and PKC-zeta, preferentially function to promote cell proliferation and survival, while the novel isoform, PKC-delta is an important regulator of apoptosis. The essential role of this kinase family in both cell survival and apoptosis suggests that specific isoforms may function as molecular sensors, promoting cell survival or cell death depending on environmental cues. Given their central role in cell and tissue homeostasis, it is not surprising that the expression or activity of some of these kinases is altered in human diseases, particularly cancer.

Targeting antiapoptotic Bcl-2 family members with cell-permeable BH3 peptides induces apoptosis signaling and death in head and neck squamous cell carcinoma cells

Head and neck squamous cell carcinomas (HNSCC) are frequently characterized by chemotherapy and radiation resistance, and by overexpression of Bcl-XL, an antiapoptotic member of the Bcl-2 protein family. In this report we examined whether cell-permeable peptides derived from the BH3 domains of proapoptotic Bax, Bad, or Bak could be used to target Bcl-XL and/or Bcl-2 in HNSCC cells, and induce apoptotic death in these cells. To render the peptides cell permeable, Antennapedia (Ant) or polyarginine (R8) peptide transduction domains were fused to the amino termini. Fluorescence microscopy of peptide-treated HNSCC cells revealed that the BH3 peptides colocalized with mitochondria, the site of Bcl-XL and Bcl-2 expression. By contrast, a mutant peptide (BaxE BH3) which cannot bind Bcl-XL or Bcl-2 was diffusely localized throughout the cytoplasm. Treatment of three HNSCC cell lines (1483, UM-22A, UM-22B) with the wild-type BH3 peptides resulted in loss of viability and induction of apoptosis, as assessed by MTS assays and annexin V staining. In general, Ant-conjugated peptides were more potent than R8-conjugated peptides, and Bad BH3 peptide was typically more potent than Bax BH3 or Bak BH3. Treatment of purified HNSCC mitochondria with BH3 peptides resulted in robust release of cytochrome c. Thus, the relative apoptosis resistance of HNSCC cells is not due to a deficit in this step of the intrinsic, mitochondrial-mediated apoptosis pathway. We conclude that cell-permeable BH3 peptides can be used to target Bcl-XL and/or Bcl-2 in HNSCC, and targeting of these proteins may have therapeutic value in the treatment of this disease.

Bcl-xL forms two distinct homodimers at non-ionic detergents: implications in the dimerization of Bcl-2 family proteins

As the key regulator of apoptosis, Bcl-2 family protein controls the cell death by forming homo- or heterodimers among anti-apoptotic and pro-apoptotic members of this family. Here we have studied Bcl-x(L) homodimerization at different pH in the presence of various detergents and organic solvents. We found that both acidic and basic pHs are beneficial for Bcl-x(L) dimerization. High concentrations of non-ionic detergents and some organic solvents can significantly promote this event. In addition to non-covalently linked acidic-dimer as that formed at acidic pH, Bcl-x(L) formed disulphide-bonded detergent-dimer at neutral and basic pH when incubated with high concentrations of non-ionic detergents. The acidic-dimer retains the BH3 peptide binding activity, whereas the detergent-dimer does not. The formation of acidic-dimer and detergent-dimer implies that Bcl-x(L) may dimerize via two different pathways under certain conditions. The implications of these findings has been discussed with previous experimental results, which provides some new insight into the events and would help the experiment design and data interpretation when non-ionic detergents are used to study the dimerization and pore formation of Bcl-2 family proteins.

Association of primary varicose veins with dysregulated vein wall apoptosis

BACKGROUND

Disordered programmed cell death may play a role in the development of superficial venous incompetence. We have determined the number of cells in apoptosis, and the mediators regulating the intrinsic and extrinsic pathways in specimens of varicose vein.

METHODS

Venous segments were obtained from 46 patients undergoing surgical treatment for primary varicose veins. Controls samples were obtained from 20 patients undergoing distal arterial bypass grafting surgery. Segments of the distal and proximal saphenous trunk as well as tributaries were studied. Cell apoptoses and mediators of the mitochondrial and trans membrane pathway were evaluated with peroxidase in situ apoptosis detection, Bax and Fas detection, caspase-9 and 8 detection in the medial layer.

RESULTS

Disorganised histological architecture was observed in varicose veins. Primary varicose veins also contained fewer peroxidase in situ-positive cells than control veins (2.6% S.D. 0.2% versus 12% S.D. 0.93%, P=.0001, Mann-Whitney u test), fewer Bax positive cells (2.1.% S.D. 0.3% versus 13% S.D. 0.9%, P=.0001) and fewer Caspase 9 positive cells (3.2% S.D. 1% versus 12% S.D. 1.3%, P=.0001). Similar findings were observed in saphenous trunk, main tributaries and accessory veins. In patients with recurrent varicose veins in whom the saphenous trunk had been preserved showed similar findings to primary varicose veins. Residual varicose veins contained fewer peroxidase in situ-positive cells than healthy veins (3.2% S.D. 0.6% versus 11% S.D. 2%, P=.0001), fewer Bax positive cells (2.2% S.D. 0.3% versus 12% S.D. 0.7%, P=.0001) and fewer Caspase 9 positive cells (2.6% S.D. 0.6% versus 12% S.D. 1%, P=.0001). Immunohistochemical detection for Fas and caspase 8 remained equal was the same in the varicose vein and control groups.

CONCLUSION

Apoptosis is down regulated in the medial layer of varicose veins. This dysregulation is attributable to a disorder of the intrinsic pathway and involves the great saphenous vein trunk, major tributaries and accessory veins. This process may be among the causes of primary varicose veins.

MEK activation suppresses CPT11-induced apoptosis in rat intestinal epithelial cells through a COX-2-dependent mechanism

Resistance to chemotherapeutic agents is one of the distinct features of cancer cells. We evaluate the role of activated MEK-ERK signaling in Camptotecin/irinotecan (CPT-11)-induced cell death using constitutively activated MEK1-transfected normal rat intestinal epithelial cells (IEC-caMEK cells). A CPT-11-induced inhibitory concentration of 50% was determined by WST assay. Apoptosis was evaluated by DNA staining and fragmented DNA analysis. Protein expressions were analyzed by western blotting. We also examined the role of cyclooxygenase-2 in the cell systems. IEC-caMEK cells possessed survival advantages compared to control cells. Apoptosis was remarkably suppressed in IEC-caMEK cells. Western blot analysis revealed increased expression of Bcl-2, Bcl-xL, Mcl-1, and COX-2 and decreased expression of Bak in IEC-caMEK cells. The COX-2 selective inhibitor ameliorated the antiapoptotic nature of IEC-caMEK cells. MEK activation suppressed CPT-11-induced apoptosis in IEC-caMEK cells via a COX-2- dependent mechanism. Therefore, MEK-ERK signaling may contribute to the drug-resistant nature of cancer cells.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

D-beta-hydroxybutyrate inhibits the apoptosis of PC12 cells induced by 6-OHDA in relation to up-regulating the ratio of Bcl-2/Bax mRNA

D-beta-hydroxybutyrate (DbetaHB) is a predominant member of ketone bodies produced by hepatocytes and, to a lesser extent, by astrocytes. It is an alternative source of energy in the brain when glucose supply is depleted such as during starvation. It has been reported that ketone bodies could protect dopaminergic culture. However, the biological function of DbetaHB in Parkinson disease (PD) is still unclear. In the present work, we investigated the role of DbetaHB in protecting rat pheochromocytoma (PC12) cells from apoptosis induced by 6-Hydroxydopamine (6-OHDA). DbetaHB rescued PC12 cells from apoptotic death induced by 6-OHDA by MTT assay, acridine orange (AO) staining, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining and the activity of caspase-3. DbetaHB prevented the decrease of cell viability and the increase of caspase-3 activity induced by 6-OHDA in a dose-dependent manner in PC12 cells. AO and TUNEL staining showed that DbetaHB prevented the apoptosis of PC12 cells induced by 6-OHDA. The ratio of Bcl-2/Bax at mRNA levels, which regulates the apoptosis of PC12 cells when exposed to 6-OHDA, increased when DbetaHB was preincubated. The data showed that DbetaHB inhibited the apoptosis of PC12 cells induced by 6-OHDA in relation to up-regulating the ratio of Bcl-2/Bax mRNA.

Bcl-2 Expression as a Predictive Marker of Hormone-Refractory Prostate Cancer Treated with Taxane-Based Chemotherapy

PURPOSE

Bcl-2 inhibits apoptosis, and its overexpression is associated with hormone refractory prostate cancer (HRPC). Bak and Bax are in the Bcl-2 family and counteract the antiapoptotic function of Bcl-2. Taxane-induced (paclitaxel and its analogue docetaxel) phosphorylation of Bcl-2 abolishes the potential antiapoptotic effect of Bcl-2. We hypothesized that (a) survival benefit in HRPC patients treated with taxanes is determined by the presence of Bcl-2 protein and (b) altered expression of Bak and Bax protein caused by genetic mutation is associated with biological aggressiveness of prostate cancer.

EXPERIMENTAL DESIGN

Forty localized prostate cancer and 30 HRPC cases were used in this study. Surgical specimens of localized prostate cancer and biopsy specimens of HRPC were used for immunostaining of Bcl-2, Bak, and Bax as well as DNA extraction. Mutations in the Bak and Bax genes were screened by single-strand conformational polymorphism, and confirmed by direct DNA sequencing.

RESULTS

Bcl-2-positive HRPC showed longer cause-specific survival in comparison with the counterparts. Multivariate analysis revealed that the level of Bcl-2 expression before treatment with taxane-based chemotherapy was an independent predictor for cause-specific survival (P < 0.01) and baseline prostate-specific antigen level was an independent predictor for progression-free survival (P < 0.01). Bax gene mutation was found in only one HRPC specimen.

CONCLUSIONS

Bcl-2 expression in addition to prostate-specific antigen measurement before treatment could identify HRPC patients who may benefit from taxane-based chemotherapy. Mutation of the Bak and Bax genes is a rare event in prostate cancer.

BCL-2 in the crosshairs: tipping the balance of life and death

The discovery of B-cell lymphoma-2 (BCL-2) over 20 years ago revealed a new paradigm in cancer biology: the development and persistence of cancer can be driven by molecular roadblocks along the natural pathway to cell death. The subsequent identification of an expansive family of BCL-2 proteins provoked an intensive investigation of the interplay among these critical regulators of cell death. What emerged was a compelling tale of guardians and executioners, each participating in a molecular choreography that dictates cell fate. Ten years into the BCL-2 era, structural details defined how certain BCL-2 family proteins interact, and molecular targeting of the BCL-2 family has since become a pharmacological quest. Although many facets of BCL-2 family death signaling remain a mechanistic mystery, small molecules and peptides that effectively target BCL-2 are eliminating the roadblock to cell death, raising hopes for a medical breakthrough in cancer and other diseases of deregulated apoptosis.

Partial Beclin 1 silencing aggravates doxorubicin- and Fas-induced apoptosis in HepG2 cells

AIM: To investigate the role of Beclin 1 on the susceptibility of HepG2 cells to undergo apoptosis after anti-Fas antibody or doxorubicin treatment.

METHODS: Beclin 1 silencing was achieved using RNA interference. DNA ploidy, the percentage of apoptotic cells and the mitochondrial membrane potential were assessed by flow cytometry. Levels of Beclin 1, Bcl-X_L and cytochrome c, and the cleavage of poly (ADP-ribose) polymerase (PARP) were assayed by using Western blots.

RESULTS: Beclin 1 expression decreased by 75% 72 h after Beclin 1 siRNA transfection. Partial Beclin 1 silencing significantly increased the percentage of subG1 cells 24 and 40 h after treatment with doxorubicin or anti-Fas antibody, respectively, and this potentiation was abrogated by treatment with a pan-caspase inhibitor. Partial Beclin 1 silencing also increased PARP cleavage, mitochondrial membrane depolarization and cytosolic cytochrome c. The pro-apoptotic consequences of partial Beclin 1 silencing were not associated with a decline in Bcl-X_L expression.

CONCLUSION: Partial Beclin 1 silencing aggravates mitochondrial permeabilization and apoptosis in HepG2 cells treated with an anti-Fas antibody or with doxorubicin.

Vinblastine-induced apoptosis is mediated by discrete alterations in subcellular location, oligomeric structure, and activation status of specific Bcl-2 family members

To gain a broader insight into the role of Bcl-2 proteins in apoptosis induced after mitotic arrest, we investigated the subcellular location, oligomeric structure, and protein interactions of Bax, Bcl-2, and Bcl-xL in vinblastine-treated KB-3 cells. Vinblastine induced the translocation of Bax from the cytosol to the mitochondria, which was accompanied by conformational activation and oligomerization of Bax. Bcl-2 was located in the mitochondria, underwent multisite phosphorylation after vinblastine treatment, and was strictly monomeric under all conditions. In contrast, in control cells, Bcl-xL existed in both monomeric (30 kDa) and oligomeric (150 kDa) forms. Treatment with agents that induced Bcl-xL phosphorylation (microtubule inhibitors) caused loss of the 150-kDa form, but this species was unaffected by apoptotic stimuli that did not stimulate phosphorylation. Vinblastine also promoted Bax activation and Bax oligomerization in HCT116 colon cancer cells. Both wild-type and Bax-deficient HCT116 cells expressed the 150-kDa form of Bcl-xL, which was depleted similarly in both cell lines upon vinblastine treatment. Co-immunoprecipitation studies revealed that in untreated KB-3 cells inactive cytosolic Bax interacted with Bcl-xL, whereas in vinblastine-treated cells, activated mitochondrial Bax did not interact with Bcl-xL. Interaction of Bcl-2 with Bax was not observed under any condition. Overexpression of Bcl-xL inhibited vinblastine-induced Bax activation and Bax dimerization and in parallel inhibited apoptosis. The results indicate that vinblastine-induced apoptosis requires translocation, activation, and oligomerization of Bax and is associated with specific changes in the oligomeric properties of Bcl-xL, which occur independently of Bax.

Auto-activation of the apoptosis protein Bax increases mitochondrial membrane permeability and is inhibited by Bcl-2

Interactions among Bcl-2 family proteins mediated by Bcl-2 homology (BH) regions transform apoptosis signals into actions. The interactions between BH3 region-only proteins and multi-BH region proteins such as Bax and Bcl-2 have been proposed to be the dominant interactions required for initiating apoptosis. Experimental evidence also suggests that both homo- and hetero-interactions are mediated primarily by the BH3 regions in all Bcl-2 family proteins and contribute to commitment to or inhibition of apoptosis. We found that a peptide containing the BH3 helix of Bax was not sufficient to activate recombinant Bax to permeabilize mitochondria. However, an extended peptide containing the BH3 helix and additional downstream sequences activated Bax to permeabilize mitochondria and liposomes. Bcl-2 inhibited the membrane-permeabilizing activity of peptide-activated Bax. This activity of Bcl-2 was inhibited by the extended but not the BH3-only peptide despite both peptides binding to Bcl-2 with similar affinity. Further, membrane-bound Bax activation intermediates directly activated soluble Bax further permeabilizing the membrane. Bcl-2 inhibited Bax auto-activation. We therefore propose that Bax auto-activation amplifies the initial death signal produced by BH3-only proteins and that Bcl-2 functions as an inhibitor of Bax auto-activation.

Cell death regulation by B-cell lymphoma protein

Bcl-2 (B Cell Lymphoma) protein is an anti-apoptotic member of Bcl-2 family, which is comprised of pro- and anti-apoptotic members. It regulates cellular proliferation and death by inter- and intra-family interactions. It has a potential to suppress apoptotic cell death under variety of stress conditions by modulating mitochondrial transmembrane potential. However, prevalence of constitutively activated Bcl-2 cellular activity is not always required in cells; a mechanism likely exists in cells, which controls its activity. When expression of Bcl-2 is unregulated, it generates lymphoma like, follicular B-cell lymphoma. This article reviews the structural and functional regulation of Bcl-2 activity at transcriptional, translational, domain, structural and post-translational level, which also accounts for the effects of its deletion and site-directed mutants in the regulation of cellular proliferation and differentiation in vitro and in vivo. This concisely reviewed information on Bcl-2 helps us to update our understanding of cell death and its modulation by Bcl-2 and its mutant's interaction, which has gained therapeutic benefits in cell growth and proliferation, particularly for sensitive human hematopoietic stem cells.

BCL-XL Dimerization by Three-dimensional Domain Swapping

Dimeric interactions among anti- and pro-apoptotic members of the BCL-2 protein family are dynamically regulated and intimately involved in survival and death functions. We report the structure of a BCL-X(L) homodimers a 3D-domain swapped dimer (3DDS). The X-ray crystal structure demonstrates the mutual exchange of carboxy-terminal regions including BH2 (Bcl-2 homology 2) between monomer subunits, with the hinge region occurring at the hairpin turn between the fifth and sixth alpha helices. Both BH3 peptide-binding hydrophobic grooves are unoccupied in the 3DDS dimer and available for BH3 peptide binding, as confirmed by sedimentation velocity analysis. BCL-X(L) 3DDS dimers have increased pore-forming activity compared to monomers, suggesting that 3DDS dimers may act as intermediates in membrane pore formation. Chemical crosslinking studies of Cys-substituted BCL-X(L) proteins demonstrate that 3DDS dimers form in synthetic lipid vesicles.

A novel Bcl-x splice product, Bcl-xAK, triggers apoptosis in human melanoma cells without BH3 domain

Pro- and antiapoptotic proteins of the large Bcl-2 family are critical regulators of apoptosis via the mitochondrial pathway. Whereas antiapoptotic proteins of the family share all four Bcl-2 homology domains (BH1-BH4), proapoptotic members may lack some of these domains, but all so far described proapoptotic Bcl-2 proteins enclose BH3. The bcl-x gene gives rise to several alternative splice products resulting in proteins with distinct functions as the antiapoptotic Bcl-xL and proapoptotic Bcl-xS. Here, we describe a novel Bcl-x splice product of 138 amino acids termed Bcl-xAK (Atypical Killer), which encloses the Bcl-2 homology domains BH2 and BH4 as well as the transmembrane domain, but lacks BH1 and BH3. Weak endogenous expression of Bcl-xAK was seen in melanoma and other tumor cells. Interestingly, its overexpression by applying a tetracycline-inducible expression system resulted in significant induction of apoptosis in melanoma cells, which occurred in synergism with drug-induced apoptosis. After exogenous overexpression, Bcl-xAK was localized both in mitochondrial and in cytosolic cell fractions. By these findings, a completely new class of Bcl-2-related proteins is introduced, which promotes apoptosis independently from the BH3 domain and implies additional, new mechanisms for apoptosis regulation in melanoma cells.

Disruption of protein-protein interactions: towards new targets for chemotherapy

Protein-protein interactions play a key role in various mechanisms of cellular growth and differentiation, and in the replication of pathogen organisms in host cells. Thus, inhibition of these interactions is a promising novel approach for rational drug design against a wide number of cellular and microbial targets. In the past few years, attempts to inhibit protein-protein interactions using antibodies, peptides, and synthetic or natural small molecules have met with varying degrees of success, and these will be the focus of this review.

Structural conservation of residues in BH1 and BH2 domains of Bcl-2 family proteins

The sequence of Bcl-2 homology domains, BH1 and BH2, is known to be conserved among anti- and pro-apoptotic members of Bcl-2 family proteins. But structural conservation of these domains with respect to functionally active residues playing role in heterodimerization-mediated regulation of apoptosis has never been elucidated. Here, we have suggested the formation of an active site by structurally conserved residues in BH1 (glycine, arginine) and BH2 (tryptophan) domains of Bcl-2 family members, which also accounts for the functional effect of known mutations in BH1 (G145A, G145E) and BH2 (W188A) domains of Bcl-2.

An N-terminal region of translationally controlled tumor protein is required for its antiapoptotic activity

Bcl-xL plays a critical role in maintaining cell survival. However, the relationship between the potential interaction of Bcl-xL with other cytosolic proteins and the regulation of cell survival remains incompletely defined. We have identified translationally controlled tumor protein (TCTP), a multifunctional protein, as a novel antiapoptotic Bcl-xL-interacting protein. TCTP interacted in vivo and in vitro with Bcl-xL, and their sites have been mapped to an N-terminal region of TCTP and the Bcl-2 homology domain 3 of Bcl-xL. Consistent with a role in maintaining T-cell survival during activation, TCTP was significantly upregulated in murine T cells activated by T-cell antigen receptor (TCR) ligation and CD28 costimulation, which was correlated with the upregulation of Bcl-xL in activated T cells. Moreover, downregulation of TCTP expression by antisense technology in T cells results in the increase of T-cell apoptosis. Furthermore, the N-terminal region of TCTP was required for its ability to inhibit apoptosis. In conclusion, this study has demonstrated that an N-terminal region of a cytosolic protein, TCTP, is required for its binding to Bcl-xL and for its antiapoptotic activity.

Dysregulated apoptosis in primary varicose veins

OBJECTIVE

Programmed cell death plays a critical role in various physiological processes. To investigate its possible pathogenic role in primary varicose veins we studied histological changes in surgical specimens from human varicose veins. In varicose and healthy veins, we also determined the number of cells in apoptosis, and investigated mediators regulating the intrinsic apoptotic mitochondrial pathway (Bax and caspase 9).

METHODS

A total 23 varicose veins were obtained from 18 patients undergoing lower-extremity varicose vein surgery for primary varicose disorders. We used nine healthy veins obtained from nine patients undergoing distal arterial bypass grafting surgery as controls. The venous segment analysed was the distal part of the greater saphenous vein. Specimens for histological examination were stained with hematoxylin and eosin, trichromic and Victoria blue. Cell apoptoses and mediators of the mitochondrial pathway were detected in the media by immunohistochemistry using antibodies to peroxidase in situ apoptosis, Bax and caspase 9. Results were expressed as indexes for the three antibodies tested. The Mann-Whitney test was used to compare the results obtained in the two groups.

RESULTS

Varicose vein specimens exhibited a more disorganised architecture than healthy veins and showed an increased number of collagen fibres and a decrease in the density and size of elastic fibres. All anti-apoptotic antibodies tested detected significantly fewer immunoreactive cells in tissue sections from the media of varicose veins than of healthy veins (peroxidase in situ, varicose veins (VV) median 2.4% (inter-quartile range 1.6-3.9) versus control (C) 14% (IQR 8.8-19); Bax, VV 1.4% (IQR 0.36-2.4) versus C 11% (IQR 7.6-15); and caspase 9, VV 1.7% (IQR 0.06-3.4) versus C 10% (IQR 9.1-12), P=0.0001 (Mann-Whitney test).

CONCLUSION

Apoptosis is down regulated in the medial layer of varicose veins. This dysregulation of the cellular mechanism that maintains normal tissue integrity is mediated through the intrinsic apoptotic pathway and may be among the causes of primary varicose veins.

Thiazolidenediones mediate apoptosis in prostate cancer cells in part through inhibition of Bcl-xL/Bcl-2 functions independently of PPARgamma

Certain members of the thiazolidenedione family of the peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, such as troglitazone and ciglitazone, exhibit antitumor effects; however, the underlying mechanism remains inconclusive. This study shows that the effect of these thiazolidenedione members on apoptosis in prostate cancer cells is independent of PPARgamma activation. First, close structural analogues of thiazolidenediones, whereas devoid of PPARgamma activity, retain the ability to induce apoptosis with equal potency. Second, both PC-3 (PPARgamma-expressing) and LNCaP (PPARgamma-deficient) cells are sensitive to apoptosis induction by troglitazone and its PPARgamma-inactive analogue irrespective of their PPARgamma expression status. Third, rosiglitazone and pioglitazone, potent PPARgamma agonists, show marginal effects on apoptosis even at high concentrations. Evidence indicates that the apoptotic effect of troglitazone, ciglitazone, and their PPARgamma-inactive analogues 5-[4-(6-hydroxy-2,5,7,8-tetramethyl-chroman-2-ylmethoxy)-benzylidene]-2,4-thiazolidine-dione (Delta2-TG) and 5-[4-(1-methyl-cyclohexylmethoxy)-benzylidene]-thiazolidine-2,4-dione, respectively, is in part attributable to their ability to inhibit the anti-apoptotic functions of Bcl-xL and Bcl-2. Treatment of PC-3 cells with troglitazone or Delta2-TG led to reduced association of Bcl-2 and Bcl-xL with Bak, leading to caspase-dependent apoptosis. Bcl-xL overexpression protects LNCaP cells from apoptosis induction by troglitazone and Delta2-TG in an expression level-dependent manner. Considering the pivotal role of Bcl-xL/Bcl-2 in regulating mitochondrial integrity, this new mode of mechanism provides a framework to account for the PPARgamma-independent action of thiazolidenediones in inducing apoptosis in cancer cells. Moreover, dissociation of these two pharmacologic activities provides a molecular basis to develop novel Bcl-xL/Bcl-2 inhibitors, of which the proof of principle is illustrated by a Delta2-TG analogue with potent in vivo antitumor activities.

Anti-apoptotic effect of nerve growth factor is lost in congenital insensitivity to pain with anhidrosis (CIPA) B lymphocytes

Congenital insensitivity to pain with anhidrosis (CIPA) is identified as a genetic disorder of mutations in the human TrkA known as high affinity receptor of nerve growth factor (NGF). NGF signal through TrkA promotes anti-apoptotic activity in hematopoietic cells including B lymphocytes. Here we studied the effect of NGF on anti-apoptotic activity by using human EBV-immortalized B lymphoblastoid cell lines (EB-LCLs) derived from a patient with CIPA and the associated carriers of CIPA. The TrkA(mt/mt) EB-LCL derived from the CIPA patient and the TrkA(wt/mt) EB-LCL derived from the carrier with the heterozygous TrkA mutation did not show any responses to NGF on anti-apoptotic activity. We concluded that this phenomenon is one of the pathogeneses of CIPA.

(−)-Gossypol acts directly on the mitochondria to overcome Bcl-2- and Bcl-XL-mediated apoptosis resistance

Aberrant overexpression of antiapoptotic members of the Bcl-2 protein family, including Bcl-2 and Bcl-XL, contributes to malignant transformation and subsequent resistance to traditional chemotherapeutics. Thus, these proteins represent attractive targets for novel anticancer agents. The small molecule, gossypol, was initially investigated as a contraceptive agent, but subsequently has been shown to possess anticancer properties in vitro and in vivo. Recently gossypol has been found to bind to Bcl-XL and, with less affinity, to Bcl-2. Here we investigate the ability of the (−) enantiomer of gossypol, (−)-gossypol, to overcome the apoptosis resistance conferred by Bcl-2 or Bcl-XL overexpression in Jurkat T leukemia cells. (−)-Gossypol potently induced cell death in Jurkat cells overexpressing Bcl-2 (IC50, 18.1 ± 2.6 μmol/L) or Bcl-XL (IC50, 22.9 ± 3.7 μmol/L). Vector-transfected control cells were also potently killed by (−)-gossypol (IC50, 7.0 ± 2.7 μmol/L). By contrast, the chemotherapy drug etoposide only induced efficient killing of vector-transfected cells (IC50, 9.6 ± 2.3μmol/L). Additionally, (−)-gossypol was more efficient than etoposide at inducing caspase-3 activation and phosphatidylserine externalization in the setting of Bcl-2 or Bcl-XL overexpression. (−)-Gossypol-induced apoptosis was associated with Bak activation and release of cytochrome c from mitochondria, suggesting a mitochondrial-mediated apoptotic mechanism. Moreover, (−)-gossypol treatment of isolated mitochondria purified from Bcl-2-overexpressing cells also resulted in cytochrome c release, indicating a possible direct action on Bcl-2 present in the mitochondrial outer membrane. Taken together, these results suggest that (−)-gossypol is a potent and novel therapeutic able to overcome apoptosis resistance by specifically targeting the activity of antiapoptotic Bcl-2 family members. (−)-Gossypol may be a promising new agent to treat malignancies that are resistant to conventional therapies.

Breast Cancer Cells Can Evade Apoptosis-Mediated Selective Killing by a Novel Small Molecule Inhibitor of Bcl-2

Proteins of the Bcl-2 family are key regulators of caspase activation and apoptosis. Some members of this family, notably Bcl-2 and Bcl-x(L), are overexpressed in cancer cells, which have been associated with chemoresistance. We have designed and synthesized a small molecule inhibitor of Bcl-2, named YC137, and studied its role in cancer cells. In vitro studies showed that YC137 inhibits the binding of the Bid BH3 peptide to Bcl-2, thus disrupting an interaction essential for the antiapoptotic activity of Bcl-2. This inhibitor induces apoptosis of hematopoietic progenitors overexpressing Bcl-2 but not Bcl-x(L) and breast cancer cells that express high levels of Bcl-2. On the contrary, a variety of normal primary cells, including CD34(+) progenitors, myoblasts, and peripheral blood mononuclear cells, do not respond to the inhibitor. A breast cancer cell line resistant to YC137 was generated. Analysis of resistant cells revealed a reduced expression of Bcl-2, which correlated with low activation of signal transducer and activator of transcription-3 (Stat3) and reduced expression of the human epidermal growth factor receptor-2 (HER2). Of note, YC137-resistant cells were more sensitive to apoptosis induced by chemotherapy. Because HER2 has not been linked previously to the Stat3-Bcl-2 transcriptional pathway, we additionally confirmed that specific blockade of HER2 in breast cancer cells resulted in down-regulation of Stat3 activity and reduced levels of Bcl-2. Consistently, HER2 blockade led to YC137 resistance. These data provide evidence for the selective killing of tumor cells by YC137 and represent the first example of in vitro selection of cancer cells refractory to a Bcl-2 inhibitor.