tBid elicits a conformational alteration in membrane-bound Bcl-2 such that it inhibits Bax pore formation

Structure-destabilizing mutations unleash an intrinsic perforation activity of antiapoptotic Bcl-2 in the mitochondrial membrane enabling apoptotic cell death

Bcl-2 and Bax share a similar structural fold in solution, yet function oppositely in the mitochondrial outer membrane (MOM) during apoptosis. The proapoptotic Bax forms pores in the MOM to trigger cell death, whereas Bcl-2 inhibits the Bax pore formation to prevent cell death. Intriguingly both proteins can switch to a similar conformation after activation by BH3-only proteins, with multiple regions embedded in the MOM. Here we tested a hypothesis that destabilization of the Bcl-2 structure might convert Bcl-2 to a Bax-like perforator. We discovered that mutations of glutamate 152 which eliminate hydrogen bonds in the protein core and thereby reduce the Bcl-2 structural stability. These Bcl-2 mutants induced apoptosis by releasing cytochrome c from the mitochondria in the cells that lack Bax and Bak, the other proapoptotic perforator. Using liposomal membranes made with typical mitochondrial lipids and reconstituted with purified proteins we revealed this perforation activity was intrinsic to Bcl-2 and could be unleashed by a BH3-only protein, similar to the perforation activity of Bax. Our study thus demonstrated a structural conversion of antiapoptotic Bcl-2 to a proapoptotic perforator through a simple molecular manipulation or interaction that is worthy to explore further for eradicating cancer cells that are resistant to a current Bcl-2-targeting drug.

Protective effect of polysaccharides isolated from the seeds of Cuscuta chinensis Lam. on 5-fluorouracil-induced intestinal mucositis in mice

Purpose:

To evaluate the protective effect of Cuscuta chinensis Lam. polysaccharides (PCCL) on 5-fluorouracil-(5-FU)-induced intestinal mucositis (IM) in mice.

Methods:

PCCL was orally administered at a dose of 20 mg·kg^–1 for 7 days and its protective effect on 5-FU-induced IM (5-FU, 50 mg·kg^–1 for 5 days) was evaluated by monitoring changes in body weight, degree of diarrhea, levels of tissue inflammatory factors (tumor necrosis factor α, interleukin 6, and interleukin 1β levels), apoptosis rates, and the expression levels of caspase-3, Bax and Bcl-2.

Results:

The severity of mucosal injury (as reflected by body weight changes, degree of diarrhea, height of villi, and damage to crypts) was significantly attenuated by PCCL administration. PCCL also reduced the levels of tissue inflammatory factors, the apoptosis rate, and the expression of caspase-3 and Bax, and increased Bcl-2 expression.

Conclusions:

PCCL administration may be significantly protective against 5-FU-induced IM by inhibiting apoptosis and regulating the abnormal inflammation associated with it.

Identification of mini-chromosome maintenance 8 as a potential prognostic marker and its effects on proliferation and apoptosis in gastric cancer

Mini-chromosome maintenance (MCM) proteins play important roles in initiating eukaryotic genome replication. The MCM family of proteins includes several members associated with the development and progression of certain cancers. We performed online data mining to assess the expression of MCMs in gastric cancer (GC) and the correlation between their expression and survival in patients with GC. Notably, MCM8 expression was undoubtedly up-regulated in GC, and higher expression correlated with shorter overall survival (OS) and progression-free survival (PFS) in patients with GC. However, the role of MCM8 in GC has not been previously explored. Our in vitro experiments revealed that MCM8 knockdown inhibited cell growth and metastasis. Moreover, MCM8 knockdown induced apoptosis. Mechanistically, the expression levels of Bax and cleaved caspase-3 were increased, whereas Bcl-2 expression decreased. Additionally, we demonstrated that MCM8 knockdown suppressed tumorigenesis in vivo. Overall, these results suggest that MCM8 plays a significant role in GC progression.

[Physiological Function and Structural Basis of Bcl-2 Family Proteins]

Apoptosis is an important biological process that plays a key role in the regulation of cell fate and homeostasis. The B-cell lymphoma-2 (Bcl-2) family proteins are important regulators of the apoptotic pathway, and their dysfunction is associated with a variety of diseases, including cancer, neurodegenerative and autoimmune diseases. In the past decade, a large number of research work on the physiological functions and atomic structures of Bcl-2 family proteins have been reported, which has deepened our understanding of the molecular mechanism and pathological significance of Bcl-2 family proteins. Recently, new drugs targeting different Bcl-2 proteins have been developed and used in clinics or tested in clinical trials. However, the complexity and diversity in functions and structures of Bcl-2 family have left many unsolved problems. This article summarizes current knowledge of the structure and function of Bcl-2 family proteins and discusses the pharmacological significance of Bcl-2 proteins as effective therapeutic targets.

Cell division cycle associated 5 promotes colorectal cancer progression by activating the ERK signaling pathway

Cell division cycle associated 5 (CDCA5) is implicated in the development and progression of a variety of human cancers. Functional significance of CDCA5 in colorectal cancer (CRC), however, has not been investigated. Using a combination of on-line data mining, biochemistry, and molecular biology, we examined the potential oncogenic activity of CDCA5 and the underlying mechanisms. Experiments with human tissue sample showed increased CDCA5 expression in CRC vs. in noncancerous adjacent tissue, and association of CDCA5 upregulation in CRC tissues with shorter patient survival. Also, representative CRC cell-lines had higher CDCA5 expression vs. fetal colonic mucosal cells. CDCA5 knockdown using lentivirus-mediated shRNA inhibited the proliferation and induced apoptosis in cultured HCT116 and HT-29 cells, and suppressed the growth of xenograft in nude mice. CDCA5 knockdown decreased the expression of CDK1 and CyclinB1, increased caspase-3 activity, cleaved PARP and the Bax/Bcl-2 ratio. CDCA5 knockdown also significantly decreased phosphorylation of ERK1/2 and expression of c-jun. Taken together, these findings suggest a significant role in CRC progression of CRC, likely by activating the ERK signaling pathway.

Chemical characterization of cytotoxic indole acetic acid derivative from mulberry fruit (Morus alba L.) against human cervical cancer

The fruit of the white mulberry tree (Morus alba L.) is a multiple fruit with a sweet flavor commonly consumed around the world. Chemical investigation of the fruits led to the isolation of two indole acetic acid derivatives (1 -2) including a new compound, which turned out to be an isolation artifact, 3S-(β-D-glucopyranosyloxy)-2,3-dihydro-2-oxo-1H-indole-3-acetic acid butyl ester (1), along with five known compounds (3 -7). Compounds 2 and 7 were newly identified from mulberry fruit. The new isolation artifact (1) exhibited cytotoxic effect on human cervical cancer Hela cells in a dose-dependent manner. Compound 1 activated caspase-8, caspase-9, and caspase-3, followed by cleavage of PARP, a substrate of caspase-3, in a dose-dependent manner. Simultaneous alterations in protein expression of mitochondrial factors Bax, BID and Bcl-2 were also observed. A comparison between compounds 1 and 2 led to a structure-activity relationship analysis of the cytotoxic effect. These results suggest that compound 1 could be beneficial in human cervical cancer treatment, and provide a theoretical basis for further application of compound 1.

Pien Tze Huang inhibits the growth of hepatocellular carcinoma cells by upregulating miR-16 expression

Hepatocellular carcinoma (HCC) is characterized by uncontrolled proliferation and the deregulation of apoptotic signaling, although its molecular pathogenesis is not fully characterized. The ability to inhibit excessive proliferation and induce the apoptosis of cancer cells are crucial characteristics of anticancer drugs. Pien Tze Huang (PZH) is a widely used traditional Chinese medicine for the treatment of various types of cancer, and has exhibited promising therapeutic effects in clinical trials of HCC. However, the underlying mechanisms for its action are unclear. In the present study, the aim was to explore the effect of PZH on the proliferation and apoptosis of the BEL-7402 HCC cell line, and the associated mechanisms. PZH treatment significantly inhibited BEL-7402 cell viability, confluence and clonogenicity, inducing cell cycle arrest and promoting apoptosis. In addition, PZH treatment suppressed the expression of the pro-proliferative genes cyclin D1 and cyclin-dependent kinase 4, and decreased the expression of the anti-apoptotic gene Bcl-2. PZH treatment also upregulated the expression of a key microRNA (miR), miR-16. The study demonstrated that PZH can effectively inhibit cancer cell proliferation and induce apoptosis in BEL-7402 HCC cells via the upregulation of the tumor suppressor miR-16.

Pien Tze Huang alleviates 5-fluorouracil-induced intestinal mucositis in CT-26 tumor-bearing mice

Chemotherapeutic agents, including 5-fluorouracil (5-FU), frequently cause intestinal mucositis as a side effect, leading to life quality reduction in colorectal cancer (CRC) patients and interruption of CRC treatment. Traditional Chinese medicines (TCMs) have recently received attention due to their relatively few adverse effects. Pien Tze Huang (PZH), a well-known TCM formulation first documented to have been prescribed >450 years ago, has been demonstrated to be clinically effective in treating various types of cancer including CRC. Although previous studies by our group reported that PZH possesses a variety of anti-cancer activities via multiple mechanisms, it has remained elusive whether it is able to reduce intestinal mucositis induced by 5-FU. The present study evaluated the effect of PZH on 5-FU-induced intestinal mucositis in CT-26 tumor-bearing xenograft mice and investigated the possible molecular mechanism. The results indicated that administration of PZH effectively alleviated the severity of 5-FU-induced diarrhea and morphological intestinal damages, but had no significant effect on body weight loss. In addition, a terminal deoxynucleotidyl transferase dUTP nick end labeling assay revealed that PZH treatment significantly inhibited cell apoptosis in the intestinal crypt. Furthermore, immunohistochemical staining showed that PZH treatment reduced the protein expression of pro-apoptotic B-cell lymphoma 2 (Bcl-2)-associated X protein but enhanced that of anti-apoptotic Bcl-2 in the intestinal crypt. Taken together, the results of the present study suggested that PZH effectively attenuates 5-FU-induced intestinal mucositis, which is in part associated with its inhibitory effect on cell apoptosis in the intestinal crypt.

Oleanolic acid modulates multiple intracellular targets to inhibit colorectal cancer growth

Due to drug resistance and unacceptable cytotoxicity of most currently-used cancer chemotherapies, naturally occurring products have gained attention in the field of anticancer treatment. Oleanolic acid (OA) is a natural pentacyclic triterpenoic acid and a principal active compound in many medicinal herbs that have long been used to clinically treat various types of human malignancies. Using a colorectal cancer (CRC) mouse xenograft model and the cell line HT-29, we evaluated the effect of OA on tumor growth in vivo and in vitro, and investigated the underlying molecular mechanisms in the present study. We found that OA significantly inhibited tumor growth in volume and weight in CRC xenograft mice. In addition, OA treatment led to the induction of apoptosis and inhibition of cell proliferation. OA significantly reduced the expression of Bcl-2, Cyclin D1 and CKD4, whereas Bax and p21 expression was profoundly increased after OA treatment. Furthermore, OA significantly suppressed the activation of Akt, p70S6K and MAPK signalings, but promoted p53 pathway activation. Collectively, findings from this study suggest that OA possesses a broad range of anticancer effects via modulation of multiple intracellular targets.

So-Cheong-Ryong-Tang induces apoptosis through activation of the intrinsic and extrinsic apoptosis pathways, and inhibition of the PI3K/Akt signaling pathway in non-small-cell lung cancer A549 cells

Background

So-Cheong-Ryong-Tang (SCRT), a traditional Korean medicine containing eight species of medicinal plant, has been used to treat patients with bronchial asthma and allergic rhinitis for hundreds of years; however, its anti-cancer potential is poorly understood. The present study was designed to evaluate the apoptotic effect of SCRT against human non-small-cell lung cancer (NSCLC) A549 cells.

Methods

The effects of SCRT on cell growth and viability were evaluated by trypan blue dye exclusion and 3-(4, 5-dimethyl-thiazol-2-yl)-2, 5-diphenyl tetrazoliumbromide (MTT) assays, respectively. Apoptosis was detected using 4,6-diamidino-2-phenyllindile (DAPI) staining, agarose gel electrophoresis and flow cytometry. The protein levels were determined by Western blot analysis. Caspase activity was measured using a colorimetric assay.

Results

SCRT treatment resulted in significantly decreased A549 cell growth and viability by induction of apoptosis. SCRT induced the translocation of pro-apoptotic Bax to the mitochondria, mitochondrial membrane permeabilization, cytochrome c release from mitochondria to cytosol, and activated caspase-9 and caspase-3. SCRT also increased death receptor-associated ligands and enhanced the activation of caspase-8 and cleavage of its substrate Bid. However, the pan-caspases inhibitor significantly blocked the SCRT-induced apoptosis, suggesting that it is a caspase-dependent pathway. In addition, SCRT suppressed the phosphorylation of phosphoinositide 3-kinase (PI3K) and Akt, and treatment with a potent inhibitor of PI3K further increased the apoptotic activity of SCRT.

Conclusions

These findings suggest that SCRT may play its anti-cancer actions partly through a suppression of the PI3K/Akt signal pathway in A549 cells, and further in vivo studies on the potential of SCRT for prevention and therapy of NSCLCs are warranted.

Activation of mitochondrial protease OMA1 by Bax and Bak promotes cytochrome c release during apoptosis

Intrinsic apoptotic stimuli initiate mammalian cells' apoptotic program by first activating the proteins that have only Bcl-2 homology domain 3 (BH3), such as Bcl-2 interacting mediator of cell death (Bim) and truncated BH3 interacting death domain agonist (tBid), which in turn trigger conformational changes in BCL2-associated X (Bax) and BCL2-antagonist/killer (Bak) proteins that enable oligomer formation on the mitochondria, causing cytochrome c and other apoptogenic proteins in the intermembrane space to leak out. Leaked cytochrome c then initiates apoptotic caspase activation through a well-defined biochemical pathway. However, how oligomerized Bax and Bak cause cytochrome c release from mitochondria remains unknown. We report here the establishment of cell lines in which Bim or tBid can be inducibly expressed to initiate apoptosis in a controlled, quantitative manner. We used these cell lines to examine apoptotic events after Bax and Bak oligomerization but before cytochrome c release. The mitochondrial metalloprotease OMA1 was activated in this system in a Bax- and Bak-dependent fashion. Activated OMA1 cleaved the dynamin-like GTPase, optical nerve atrophy 1, an event that is critical for remodeling of mitochondrial cristae. Knockdown or knockout of OMA1 in these cells attenuated cytochrome c release. Thus it is clear that oligomerized Bax and Bak trigger apoptosis by causing both the permeabilization of the mitochondrial outer membrane and activation OMA1.

Regulating cell death at, on, and in membranes

Bcl-2 family proteins are central regulators of apoptosis. Various family members are located in the cytoplasm, endoplasmic reticulum, and mitochondrial outer membrane in healthy cells. However during apoptosis most of the interactions between family members that determine the fate of the cell occur at the membranes of intracellular organelles. It has become evident that interactions with membranes play an active role in the regulation of Bcl-2 family protein interactions. Here we provide an overview of various models proposed to explain how the Bcl-2 family regulates apoptosis and discuss how membrane binding affects the structure and function of each of the three categories of Bcl-2 proteins (pro-apoptotic, pore-forming, and anti-apoptotic). We also examine how the Bcl-2 family regulates other aspects of mitochondrial and ER physiology relevant to cell death.

After embedding in membranes antiapoptotic Bcl-XL protein binds both Bcl-2 homology region 3 and helix 1 of proapoptotic Bax protein to inhibit apoptotic mitochondrial permeabilization

Bcl-XL binds to Bax, inhibiting Bax oligomerization required for mitochondrial outer membrane permeabilization (MOMP) during apoptosis. How Bcl-XL binds to Bax in the membrane is not known. Here, we investigated the structural organization of Bcl-XL·Bax complexes formed in the MOM, including the binding interface and membrane topology, using site-specific cross-linking, compartment-specific labeling, and computational modeling. We found that one heterodimer interface is formed by a specific interaction between the Bcl-2 homology 1-3 (BH1-3) groove of Bcl-XL and the BH3 helix of Bax, as defined previously by the crystal structure of a truncated Bcl-XL protein and a Bax BH3 peptide (Protein Data Bank entry 3PL7). We also discovered a novel interface in the heterodimer formed by equivalent interactions between the helix 1 regions of Bcl-XL and Bax when their helical axes are oriented either in parallel or antiparallel. The two interfaces are located on the cytosolic side of the MOM, whereas helix 9 of Bcl-XL is embedded in the membrane together with helices 5, 6, and 9 of Bax. Formation of the helix 1·helix 1 interface partially depends on the formation of the groove·BH3 interface because point mutations in the latter interface and the addition of ABT-737, a groove-binding BH3 mimetic, blocked the formation of both interfaces. The mutations and ABT-737 also prevented Bcl-XL from inhibiting Bax oligomerization and subsequent MOMP, suggesting that the structural organization in which interactions at both interfaces contribute to the overall stability and functionality of the complex represents antiapoptotic Bcl-XL·Bax complexes in the MOM.

tBid undergoes multiple conformational changes at the membrane required for Bax activation

Bid is a Bcl-2 family protein that promotes apoptosis by activating Bax and eliciting mitochondrial outer membrane permeabilization (MOMP). Full-length Bid is cleaved in response to apoptotic stimuli into two fragments, p7 and tBid (p15), that are held together by strong hydrophobic interactions until the complex binds to membranes. The detailed mechanism(s) of fragment separation including tBid binding to membranes and release of the p7 fragment to the cytoplasm remain unclear. Using liposomes or isolated mitochondria with fluorescently labeled proteins at physiological concentrations as in vitro models, we report that the two components of the complex quickly separate upon interaction with a membrane. Once tBid binds to the membrane, it undergoes slow structural rearrangements that result in an equilibrium between two major tBid conformations on the membrane. The conformational change of tBid is a prerequisite for interaction with Bax and is, therefore, a novel step that can be modulated to promote or inhibit MOMP. Using automated high-throughput image analysis in cells, we show that down-regulation of Mtch2 causes a significant delay between tBid and Bax relocalization in cells. We propose that by promoting insertion of tBid via a conformational change at the mitochondrial outer membrane, Mtch2 accelerates tBid-mediated Bax activation and MOMP. Thus the interaction of Mtch2 and tBid is a potential target for therapeutic control of Bid initiated cell death.

Granzyme B triggers a prolonged pressure to die in Bcl-2 overexpressing cells, defining a window of opportunity for effective treatment with ABT-737

Overexpression of Bcl-2 contributes to resistance of cancer cells to human cytotoxic lymphocytes (CL) by blocking granzyme B (GraB)-induced mitochondrial outer membrane permeabilization (MOMP). Drugs that neutralise Bcl-2 (e.g., ABT-737) may therefore be effective adjuvants for immunotherapeutic strategies that use CL to kill cancer cells. Consistent with this we found that ABT-737 effectively restored MOMP in Bcl-2 overexpressing cells treated with GraB or natural killer cells. This effect was observed even if ABT-737 was added up to 16 h after GraB, after which the cells reset their resistant phenotype. Sensitivity to ABT-737 required initial cleavage of Bid by GraB (gctBid) but did not require ongoing GraB activity once Bid had been cleaved. This gctBid remained detectable in cells that were sensitive to ABT-737, but Bax and Bak were only activated if ABT-737 was added to the cells. These studies demonstrate that GraB generates a prolonged pro-apoptotic signal that must remain active for ABT-737 to be effective. The duration of this signal is determined by the longevity of gctBid but not activation of Bax or Bak. This defines a therapeutic window in which ABT-737 and CL synergise to cause maximum death of cancer cells that are resistant to either treatment alone, which will be essential in defining optimum treatment regimens.

Phosphatidic acid mediates the targeting of tBid to induce lysosomal membrane permeabilization and apoptosis

Upon apoptotic stimuli, lysosomal proteases, including cathepsins and chymotrypsin, are released into cytosol due to lysosomal membrane permeabilization (LMP), where they trigger apoptosis via the lysosomal-mitochondrial pathway of apoptosis. Herein, the mechanism of LMP was investigated. We found that caspase 8-cleaved Bid (tBid) could result in LMP directly. Although Bax or Bak might modestly enhance tBid-triggered LMP, they are not necessary for LMP. To study this further, large unilamellar vesicles (LUVs), model membranes mimicking the lipid constitution of lysosomes, were used to reconstitute the membrane permeabilization process in vitro. We found that phosphatidic acid (PA), one of the major acidic phospholipids found in lysosome membrane, is essential for tBid-induced LMP. PA facilitates the insertion of tBid deeply into lipid bilayers, where it undergoes homo-oligomerization and triggers the formation of highly curved nonbilayer lipid phases. These events induce LMP via pore formation mechanisms because encapsulated fluorescein-conjugated dextran (FD)-20 was released more significantly than FD-70 or FD-250 from LUVs due to its smaller molecular size. On the basis of these data, we proposed tBid-PA interactions in the lysosomal membranes form lipidic pores and result in LMP. We further noted that chymotrypsin-cleaved Bid is more potent than tBid at binding to PA, inserting into the lipid bilayer, and promoting LMP. This amplification mechanism likely contributes to the culmination of apoptotic signaling.

Hyaluronan synthase mediates dye translocation across liposomal membranes

Background

Hyaluronan (HA) is made at the plasma membrane and secreted into the extracellular medium or matrix by phospolipid-dependent hyaluronan synthase (HAS), which is active as a monomer. Since the mechanism by which HA is translocated across membranes is still unresolved, we assessed the presence of an intraprotein pore within HAS by adding purified Streptococcus equisimilis HAS (SeHAS) to liposomes preloaded with the fluorophore Cascade Blue (CB).

Results

CB translocation (efflux) was not observed with mock-purified material from empty vector control E. coli membranes, but was induced by SeHAS, purified from membranes, in a time- and dose-dependent manner. CB efflux was eliminated or greatly reduced when purified SeHAS was first treated under conditions that inhibit enzyme activity: heating, oxidization or cysteine modification with N-ethylmaleimide. Reduced CB efflux also occurred with SeHAS K48E or K48F mutants, in which alteration of K48 within membrane domain 2 causes decreased activity and HA product size. The above results used liposomes containing bovine cardiolipin (BCL). An earlier study testing many synthetic lipids found that the best activating lipid for SeHAS is tetraoleoyl cardiolipin (TO-CL) and that, in contrast, tetramyristoyl cardiolipin (TM-CL) is an inactivating lipid (Weigel et al, J. Biol. Chem. 281, 36542, 2006). Consistent with the effects of these CL species on SeHAS activity, CB efflux was more than 2-fold greater in liposomes made with TO-CL compared to TM-CL.

Conclusions

The results indicate the presence of an intraprotein pore in HAS and support a model in which HA is translocated to the exterior by HAS itself.

Natural diterpenoid compound elevates expression of Bim protein, which interacts with antiapoptotic protein Bcl-2, converting it to proapoptotic Bax-like molecule

Overwhelming evidence indicates that Bax and Bak are indispensable for mediating cytochrome c release from mitochondria during apoptosis. Here we report a Bax/Bak-independent mechanism of cytochrome c release and apoptosis. We identified a natural diterpenoid compound that induced apoptosis in bax/bak double knock-out murine embryonic fibroblasts and substantially reduced the tumor growth from these cells implanted in mice. Treatment with the compound significantly increased expression of Bim, which migrated to mitochondria, altering the conformation of and forming oligomers with resident Bcl-2 to induce cytochrome c release and caspase activation. Importantly, purified Bim and Bcl-2 proteins cooperated to permeabilize a model mitochondrial outer membrane; this was accompanied by oligomerization of these proteins and deep embedding of Bcl-2 in the membrane. Therefore, the diterpenoid compound induces a structural and functional conversion of Bcl-2 through Bim to permeabilize the mitochondrial outer membrane, thereby inducing apoptosis independently of Bax and Bak. Because Bcl-2 family proteins play important roles in cancer development and relapse, this novel cell death mechanism can be explored for developing more effective anticancer therapeutics.

Pien Tze Huang induced apoptosis in human colon cancer HT-29 cells is associated with regulation of the Bcl-2 family and activation of caspase 3

OBJECTIVE

To investigate the cellular effects of Pien Tze Huang (PZH) in the HT-29 human colon carcinoma cell line.

METHODS

The viability of HT-29 cells was determined by MTT assay. A fluorescence-activated cell sorting (FACS) analysis with annexin-V/propidium iodide (PI) and JC-1 staining were performed to determine cell apoptosis and the loss of mitochondrial membrane potential, respectively. Activation of caspase 3 was evaluated by a colorimetric assay. The mRNA expression levels of Bcl-2 and Bax were measured by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

PZH, in a dose- and time-dependent manner, reduced viability and induced apoptosis of HT-29 cells. Moreover, PZH treatment resulted in the collapse of the mitochondrial membrane potential, activation of caspase 3, and an increase in the Bax/Bcl-2 ratio.

CONCLUSION

PZH inhibits the growth of HT-29 cells by inducing cancer cell apoptosis via regulation of the Bcl-2 family and activation of caspase 3, which may, in part, explain its anticancer activity.

Amphipathic tail-anchoring peptide and Bcl-2 homology domain-3 (BH3) peptides from Bcl-2 family proteins induce apoptosis through different mechanisms

Bcl-2 homology domain-3 (BH3) peptides are potent cancer therapeutic reagents that target regulators of apoptotic cell death in cancer cells. However, their cytotoxic effects are affected by different expression levels of Bcl-2 family proteins. We recently found that the amphipathic tail-anchoring peptide (ATAP) from Bfl-1, a bifunctional Bcl-2 family member, produced strong pro-apoptotic activity by permeabilizing the mitochondrial outer membrane. Here, we test whether the activity of ATAP requires other cellular factors and whether ATAP has an advantage over the BH3 peptides in targeting cancer cells. Confocal microscopic imaging illustrates specific targeting of ATAP to mitochondria, whereas BH3 peptides show diffuse patterns of cytosolic distribution. Although the pro-apoptotic activities of BH3 peptides are largely inhibited by either overexpression of anti-apoptotic Bcl-2 or Bcl-xL or nullification of pro-apoptotic Bax and Bak in cells, the pro-apoptotic function of ATAP is not affected by these cellular factors. Reconstitution of synthetic ATAP into liposomal membranes results in release of fluorescent molecules of the size of cytochrome c from the liposomes, suggesting that the membrane permeabilizing activity of ATAP does not require additional protein factors. Because ATAP can target to the mitochondrial membrane and its pro-apoptotic activity does not depend on the content of Bcl-2 family proteins, it represents a promising candidate for anti-cancer drugs that can potentially overcome the intrinsic apoptosis-resistant nature of cancer cells.

The C. elegans B-cell lymphoma 2 (Bcl-2) homolog cell death abnormal 9 (CED-9) associates with and remodels LIPID membranes

Bcl-2 proteins associate with and remodel mitochondria to regulate apoptosis. While the C. elegans Bcl-2 homolog CED-9 constitutively associates with mitochondria, it is unclear whether or not this association reflects an innate ability of CED-9 to directly remodel mitochondrial membranes. To address this question, we have characterized the effects of recombinantly expressed and purified CED-9 on synthetic lipid vesicles. We found that CED-9 associates with anionic lipid vesicles at neutral pH, and that association can occur independently of the C-terminal transmembrane domain. Membrane association changes the environment of CED-9 tryptophans and results in an apparent increase in α-helical structure. Upon association, CED-9 alters the permeability of membranes resulting in leakage of encapsulated dyes. Furthermore, this membrane remodeling promotes membrane fusion upon protonation of CED-9. Bypass of this protonation trigger can be achieved by mutating two conserved glutamates (E187K/E190K) or removing the N-terminal 67 residues. Together, these in vitro results suggest that CED-9 retains the amphitropic ability of mammalian Bcl-2 proteins to associate with cellular membranes. We therefore discuss the possibility that CED-9 and other Bcl-2 homologs localize at mitochondria to regulate mitochondrial homeostasis by either modulating mitochondrial membrane permeability or fusion.

BH3-only proteins: Orchestrators of apoptosis

The BH3-only proteins of Bcl-2 family are essential initiators of apoptosis that propagate extrinsic and intrinsic cell death signals. The interaction of BH3-only proteins with other Bcl-2 family members is critical for understanding the core machinery that controls commitment to apoptosis by permeabilizing the mitochondrial outer membrane. BH3-only proteins promote apoptosis by both directly activating Bax and Bak and by suppressing the anti-apoptotic proteins at the mitochondria and the endoplasmic reticulum. To prevent constitutive cell death, BH3-only proteins are regulated by a variety of mechanisms including transcription and post-translational modifications that govern specific protein-protein interactions. Furthermore, BH3-only proteins also control the initiation of autophagy, another important pathway regulating cell survival and death. Emerging evidence indicates that the interaction of BH3-only proteins with membranes regulates binding to other Bcl-2 family members, thereby specifying function. Due to the important role of BH3-only proteins in the regulation of cell death, several promising BH3-mimetic drugs that are active in pre-clinical models are currently being tested as anti-cancer agents. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Still embedded together binding to membranes regulates Bcl-2 protein interactions

The dysregulation of apoptosis is a key step in developing tumours, and mediates resistance to cancer therapy. Many different signals for cell death converge on permeabilization of the outer mitochondrial membrane, which is controlled by the Bcl-2 family of proteins. The importance of this step is becoming increasingly relevant as the first generation of small molecules that inhibit the interaction of Bcl-2 family proteins enters clinical trials as anticancer agents. The Bcl-2 family can be divided into three classes: BH3-only proteins that are activated by various forms of cellular stress, Bax and Bak proteins that mediate mitochondrial membrane permeabilization, and inhibitory proteins such as Bcl-2 and Bcl-XL. The recently proposed embedded together model emphasizes the fact that many of the regulatory interactions between different classes of Bcl-2 family members occur at intracellular membranes, and binding to membranes causes conformational changes in the proteins that dictate functions in a dynamic manner. Within this context, recent results indicate that Bcl-XL functions as a dominant-negative Bax, a concept that resolves the paradox of similar structures but opposite functions of Bcl-XL and Bax. We have also shown that the conformational change that allows Bax to insert into the outer mitochondrial membrane is the rate-limiting step in the multistep process of Bax activation. Nevertheless, investigating the structure of activated Bax or Bak as monomers and as components of the oligomeric structures that mediate membrane permeabilization is the focus of ongoing research (and controversy) at many laboratories worldwide.

Drugs targeting Bcl-2 family members as an emerging strategy in cancer

Inhibiting apoptosis is widely accepted as a necessary step in the transition from normal to cancer cells, and most cancer therapies exert their effects by indirectly reversing this process. Commitment to apoptosis is caused by permeabilisation of the outer mitochondrial membrane – a process regulated by the binding between different members of the Bcl-2 family. Furthermore, Bcl-2 family members also bind to the endoplasmic reticulum, where they modify processes such as the unfolded-protein response and autophagy that also cause or modify different types of cell death. With the growing understanding of the importance of the Bcl-2 family as crucial regulators of the decision to initiate apoptosis, much effort has been directed at developing small molecules that modify function by directly binding to Bcl-2 proteins. Preclinical experiments have confirmed that these agents kill cancer cells and overcome chemotherapy resistance. Two of these drugs are in the initial stages of clinical development (ABT-263 and obatoclax), and early results show clinical efficacy at tolerable doses. Important questions for the future include the role of these drugs as monotherapy versus combination therapy with other anticancer drugs, and the related issue of the relative toxicity to cancerous versus normal cells.

Bcl-2 and Bax interact via the BH1-3 groove-BH3 motif interface and a novel interface involving the BH4 motif

The interaction of Bcl-2 family proteins at the mitochondrial outer membrane controls membrane permeability and thereby the apoptotic program. The anti-apoptotic protein Bcl-2 binds to the pro-apoptotic protein Bax to prevent Bax homo-oligomerization required for membrane permeabilization. Here, we used site-specific photocross-linking to map the surfaces of Bax and Bcl-2 that interact in the hetero-complex formed in a Triton X-100 micelle as a membrane surrogate. Heterodimer-specific photoadducts were detected from multiple sites in Bax and Bcl-2. Many of the interaction sites are located in the Bcl-2 homology 3 (BH3) region of Bax and the BH1-3 groove of Bcl-2 that likely form the BH3-BH1-3 groove interface. However, other interaction sites form a second interface that includes helix 6 of Bax and the BH4 region of Bcl-2. Loss-of-function mutations in the BH3 region of Bax and the BH1 region of Bcl-2 disrupted the BH3-BH1-3 interface, as expected. Surprisingly the second interface was also disrupted by these mutations. Similarly, a loss-of-function mutation in the BH4 region of Bcl-2 that forms part of the second interface also disrupted both interfaces. As expected, both kinds of mutation abolished Bcl-2-mediated inhibition of Bax oligomerization in detergent micelles. Therefore, Bcl-2 binds Bax through two interdependent interfaces to inhibit the pro-apoptotic oligomerization of Bax.

Oligomerization of membrane-bound Bcl-2 is involved in its pore formation induced by tBid

Both pro-apoptotic Bax and anti-apoptotic Bcl-2 are structurally homologous to the pore-forming domain of bacterial toxins. Bax proteins oligomerize in the mitochondrial outer membranes forming pores that release cytochrome c from the mitochondrial intermembrane space. Bcl-2 proteins also form pores that, however, are much smaller than the Bax pore. It is unknown whether Bcl-2 forms monomeric or oligomeric pores. Here, we characterized the Bcl-2 pore formation in liposomes using biophysical and biochemical techniques. The results show that the Bcl-2 pore enlarges as the concentration of Bcl-2 increases, suggesting that the pore is formed by Bcl-2 oligomers. As expected from oligomerization-mediated pore-formation, the small pores are formed earlier than the large ones. Bcl-2 oligomers form pores faster than the monomer, indicating that the oligomerization constitutes an intermediate step of the pore formation. A Bcl-2 mutant with higher affinity for oligomerization forms pores faster than wild type Bcl-2. Bcl-2 oligomers were detected in the liposomal membranes under conditions that Bcl-2 forms pores, and the extent of oligomerization was positively correlated with the pore-forming activity. Therefore, Bcl-2 oligomerizes in membranes forming pores, but the extent of oligomerization and the size of the resulting pores are much smaller than that of Bax, supporting the model that Bcl-2 is a defective Bax.

Mechanisms by which Bak and Bax permeabilise mitochondria during apoptosis

Mitochondrial outer membrane permeabilisation (MOMP) is the point of no return in many forms of apoptotic cell death. The killing effect of MOMP is twofold; it both initiates a proteolytic cascade of pro-apoptotic enzymes and damages mitochondrial function. Accordingly, prevention of MOMP can rescue cells from death. It is clear that either Bak or Bax, which are Bcl-2 family members, are required for MOMP to occur; however, the pore complexes that are formed by Bak and Bax remain poorly defined in terms of their composition, size, number and structure, as well as the mechanism by which they are regulated by other Bcl-2 family members. We recently reported that a key step leading to Bak homo-oligomerisation following an apoptotic stimulus involves transient exposure of the Bak BH3 domain before it binds to the hydrophobic groove of another activated Bak molecule to form a novel symmetric dimer. To form the higher-order oligomers that probably constitute the apoptotic pore complex, Bak dimers then interact via regions away from the BH3 domain and groove. The BH3:groove interaction within Bak homodimers supports a general model to explain the associations between Bcl-2 family members. In this Commentary, we discuss the implications of these findings for the regulation of apoptosis by Bcl-2 family proteins.

Membrane promotes tBID interaction with BCL(XL)

Two important questions on the molecular mechanism of the B cell CLL/lymphoma 2 (BCL2) proteins involve the interaction network between pro- and antiapoptotic members and the role of their translocation to the mitochondrial membrane during apoptosis. We used fluorescence correlation spectroscopy to quantify the molecular interactions of BH3-interacting domain death agonist (BID) and its truncated form tBID with the B cell lymphoma extra-large protein truncated at the C terminus (BCL(XL)DeltaCt) in solution and in membranes, and we found that (i) only the active form tBID binds to BCL(XL)DeltaCt and (ii) that the membrane strongly promotes binding between them. Particularly, a BH3 peptide from BID disrupts the tBID-BCL(XL) complex in solution, but only partially in lipid bilayers. These data indicate that tBID-BCL(XL) interactions in solution and lipid membranes are distinct, and they support a model in which BCL(XL) inhibition of tBID takes place predominantly at the membrane. Our findings imply an active role of the membrane in modulating the interactions between BCL2 proteins that has so far been underestimated.

Structure assembly of Bcl-x(L) through alpha5-alpha5 and alpha6-alpha6 interhelix interactions in lipid membranes

Lipid bilayer membrane is the main site where Bcl-x(L) executes its anti-apoptotic function. Here we used site-directed mutagenesis and cysteine-directed cross-linking to trap the structure of Bcl-x(L) upon membrane insertion. Cys151 on alpha5-helix and Asn185 on alpha6-helix of two neighboring Bcl-x(L) are found in close positions, respectively. The FRET based binding assay indicated that the BH3-peptide binding pocket in Bcl-x(L) is disrupted after its membrane insertion. Co-immunoprecipitation experiments showed that the membrane-bound Bcl-x(L) sequestered tBid by direct interaction at physiological pH. If Bcl-x(L) behaves similarly at low pH as it does at physiological pH, the membrane-bound Bcl-x(L) should bind to tBid through protein regions other than the BH3 domain of tBid and the hydrophobic pocket of Bcl-x(L). Previously, a crystallography study demonstrated that Bcl-x(L) formed homodimers through domain swapping in water, where Cys151 and Asn185 of two monomeric subunits are far apart from each other and the BH3-peptide binding pocket is intact. Our results indicated that Bcl-x(L) dimer trapped by cross-linking in lipids is distinct from the domain swapped dimer, suggesting that Bcl-x(L) transits through a structural change from the water-soluble state to the membrane-bound state and there are multiple possibilities for structural reorganization of Bcl-x(L) protein.

Cyclophilin D interacts with Bcl2 and exerts an anti-apoptotic effect

Cyclophilin D (CypD) is a mitochondrial immunophilin and a key positive regulator of the mitochondrial permeability transition (MPT). Several reports have shown that CypD is overexpressed in various tumors, where it has an anti-apoptotic effect. Because the MPT is a cell death-inducing phenomenon, we hypothesized that the anti-apoptotic effect of CypD is independent of the MPT but is due to its interaction with some key apoptosis regulator, such as Bcl2. Our data indicate that CypD indeed interacts with Bcl2 as confirmed with co-immunoprecipitation, pulldown, and mammalian two-hybrid assays. A cyclophilin D inhibitor, cyclosporine A, disrupts the CypD-Bcl2 interaction. CypD enhances the limiting effect of Bcl2 on the tBid-induced release of cytochrome c from mitochondria, which is not mediated via the MPT. Gain- and loss-of-function experiments confirm that CypD has a limiting effect on cytochrome c release from mitochondria and that such an effect of CypD is cyclosporine A- and Bcl2-dependent. On a cellular level, overexpression or knockdown of CypD respectively decreases or increases cytochrome c release from mitochondria and overall cell sensitivity to apoptosis progressing via the "intrinsic" pathway. Therefore, we here describe a novel function of CypD as a Bcl2 collaborator and an inhibitor of cytochrome c release from mitochondria independent of the MPT. This function of CypD may explain the anti-apoptotic effect of this protein observed in various cancer cells. The fact that some tumors overexpress CypD suggests that this may be an additional mechanism of suppression of apoptosis in cancer.

A conserved hydrophobic core at Bcl-xL mediates its structural stability and binding affinity with BH3-domain peptide of pro-apoptotic protein

Bcl-2 family proteins regulate apoptosis through their homo- and heterodimerization. By protein sequence analysis and structural comparison, we have identified a conserved hydrophobic core at the BH1 and BH2 domains of Bcl-2 family proteins. The hydrophobic core is stabilized by hydrophobic interactions among the residues of Trp137, Ile140, Trp181, Ile182, Trp188 and Phe191 in Bcl-x(L). Destabilization of the hydrophobic core can promote the protein unfolding and pore formation in synthetic lipid vesicles. Interestingly, though the hydrophobic core does not participate in binding with BH3 domain of pro-apoptotic proteins, disruption of the hydrophobic core can reduce the affinity of Bcl-x(L) with BH3-domain peptide by changing the conformation of Bcl-x(L) C-terminal residues that are involved in the peptide interaction. The BH3-domain peptide binding affinity and pore forming propensity of Bcl-x(L) were correlated to its death-repressor activity, which provides new information to help study the regulatory mechanism of anti-apoptotic proteins. Meanwhile, as the tryptophans are conserved in the hydrophobic core, in vitro binding assay based on FRET of "Trp-->AEDANS" can be devised to screen for new modulators targeting anti-apoptotic proteins as well as "multi-BH domains" pro-apoptotic proteins.

A short Nur77-derived peptide converts Bcl-2 from a protector to a killer

Bcl-2 can be converted into a proapoptotic molecule by nuclear receptor Nur77. However, the development of Bcl-2 converters as anticancer therapeutics has not been explored. Here we report the identification of a Nur77-derived Bcl-2-converting peptide with 9 amino acids (NuBCP-9) and its enantiomer, which induce apoptosis of cancer cells in vitro and in animals. The apoptotic effect of NuBCPs and their activation of Bax are not inhibited but rather potentiated by Bcl-2. NuBCP-9 and its enantiomer bind to the Bcl-2 loop, which shares the characteristics of structurally adaptable regions with many cancer-associated and signaling proteins. NuBCP-9s act as molecular switches to dislodge the Bcl-2 BH4 domain, exposing its BH3 domain, which in turn blocks the activity of antiapoptotic Bcl-X(L).

BCL-2 family proteins: critical checkpoints of apoptotic cell death

Apoptosis is a morphologically distinct form of programmed cell death essential for normal development and tissue homeostasis. Aberrant regulation of this pathway is linked to multiple human diseases, including cancer, autoimmunity, neurodegenerative disorders, and diabetes. The BCL-2 family of proteins constitutes a critical control point in apoptosis residing immediately upstream of irreversible cellular damage, where family members control the release of apoptogenic factors from mitochondria. The cardinal member of this family, BCL-2, was originally discovered as the defining oncogene in follicular lymphomas, located at one reciprocal breakpoint of the t(14;18) (q32;q21) chromosomal translocation. Since this original discovery, remarkable efforts marshaled by many investigators around the world have advanced our knowledge of the basic biology, molecular mechanisms, and therapeutic targets in the apoptotic pathway. This review highlights findings from many laboratories that have helped uncover some of the critical control points in apoptosis. The emerging picture is that of an intricate cellular machinery orchestrated by tightly regulated molecular interactions and conformational changes within BCL-2 family proteins that ultimately govern the cellular commitment to apoptotic death.

Two independent positive feedbacks and bistability in the Bcl-2 apoptotic switch

Background

The complex interplay between B-cell lymphoma 2 (Bcl-2) family proteins constitutes a crucial checkpoint in apoptosis. Its detailed molecular mechanism remains controversial. Our former modeling studies have selected the ‘Direct Activation Model’ as a better explanation for experimental observations. In this paper, we continue to extend this model by adding interactions according to updating experimental findings.

Methodology/Principal Findings

Through mathematical simulation we found bistability, a kind of switch, can arise from a positive (double negative) feedback in the Bcl-2 interaction network established by anti-apoptotic group of Bcl-2 family proteins. Moreover, Bax/Bak auto-activation as an independent positive feedback can enforce the bistability, and make it more robust to parameter variations. By ensemble stochastic modeling, we also elucidated how intrinsic noise can change ultrasensitive switches into gradual responses. Our modeling result agrees well with recent experimental data where bimodal Bax activation distributions in cell population were found.

Conclusions/Significance

Along with the growing experimental evidences, our studies successfully elucidate the switch mechanism embedded in the Bcl-2 interaction network and provide insights into pharmacological manipulation of Bcl-2 apoptotic switch as further cancer therapies.

Regulation of apoptosis by C. elegans CED-9 in the absence of the C-terminal transmembrane domain

Bcl-2 proteins regulate apoptosis in organisms as diverse as mammals and nematodes. These proteins are often localized at mitochondria by a C-terminal transmembrane domain. Although the transmembrane domain and mitochondrial localization are centrally involved in specific cases of vertebrate Bcl-2 activity, the significance of this localization is not clear for all species. Studying the Caenorhabditis elegans Bcl-2 homolog CED-9, we found that the transmembrane domain was both necessary and sufficient for localization at mitochondrial outer membranes. Furthermore, we found that in our assays, ced-9 transgenes lacking the transmembrane domain, although somewhat less active than equivalent transgenes derived from wild-type ced-9, rescued embryonic lethality of ced-9(lf) animals and responded properly to upstream signals in controlling the fate of Pn.aap neurons. Both of these apoptotic activities were retained in a construct where CED-9 lacking the transmembrane domain was targeted to the cytosolic surface of the endoplasmic reticulum and derived organelles, suggesting that in wild-type animals, accumulation at mitochondria is not essential for CED-9 to either inhibit or promote apoptosis in C. elegans. Taken together, these data are consistent with a multimodal character of CED-9 action, with an ability to regulate apoptosis through interactions in the cytosol coexisting with additional evolutionarily conserved role(s) at the membrane.

Embedded together: the life and death consequences of interaction of the Bcl-2 family with membranes

Permeabilization of the outer mitochondrial membrane is the point of no return in most programmed cell deaths. This critical step is mainly regulated by the various protein-protein and protein-membrane interactions of the Bcl-2 family proteins. The two main models for regulation of mitochondrial outer membrane permeabilization, direct activation and displacement do not account for all of the experimental data and both largely neglect the importance of the membrane. We propose the embedding together model to emphasize the critical importance of Bcl-2 family protein interactions with and within membranes. The embedding together model proposes that both pro- and anti-apoptotic Bcl-2 family proteins engage in similar dynamic interactions that are governed by membrane dependent conformational changes and culminate in either aborted or productive membrane permeabilization depending on the final oligomeric state of pro-apoptotic Bax and/or Bak.