Mouse ER+/PIK3CAH1047R breast cancers caused by exogenous estrogen are heterogeneously dependent on estrogen and undergo BIM-dependent apoptosis with BH3 and PI3K agents.

Estrogen dependence is major driver of ER + breast cancer, which is associated with PI3K mutation. PI3K inhibition (PI3Ki) can restore dependence on ER signaling for some hormone therapy-resistant ER + breast cancers, but is ineffective in others. Here we show that short-term supplementation with estrogen strongly enhanced Pik3caH1047R-induced mammary tumorigenesis in mice that resulted exclusively in ER + tumors, demonstrating the cooperation of the hormone and the oncogene in tumor development. Similar to human ER + breast cancers that are endocrine-dependent or endocrine-independent at diagnosis, tumor lines from this model retained ER expression but were sensitive or resistant to hormonal therapies. PI3Ki did not induce cell death but did cause upregulation of the pro-apoptotic gene BIM. BH3 mimetics or PI3Ki were unable to restore hormone sensitivity in several resistant mouse and human tumor lines. Importantly however, combination of PI3Ki and BH3 mimetics had a profound, BIM-dependent cytotoxic effect in PIK3CA-mutant cancer cells while sparing normal cells. We propose that addition of BH3 mimetics offers a therapeutic strategy to markedly improve the cytotoxic activity of PI3Ki in hormonal therapy-resistant and ER-independent PIK3CA-mutant breast cancer.

Sensitization of glioblastoma cells to TRAIL-induced apoptosis by IAP- and Bcl-2 antagonism.

Due to the lack of effective treatments for glioblastoma (GBM), we here studied the responsiveness of GBM cell lines to the combination of death ligand, TRAIL and the IAP antagonist, TL32711 (Birinapant). Responses were highly heterogeneous, with synergistic apoptosis as well as treatment resistance observed. Caspase-8 and Bid, together with caspase-3, form a nonlinear signalling hub that efficiently induced apoptosis in responder cell lines. Cells resistant to TRAIL/TL32711 expressed low amounts of procaspase-8 and Bid and poorly activated caspase-3. We therefore hypothesised that improving caspase-8 activation or sensitising mitochondria to truncated Bid (tBid) could convert non-responder GBM cell lines to responders. Mathematical simulations of both strategies predicted mitochondrial sensitization to tBid would outperform enhancing caspase-8 activation. Indeed, antagonising Bcl-2 by ABT-199 allowed TRAIL/TL32711 response synergies to manifest in otherwise TRAIL resistant cell lines. These findings were further corroborated in experiments with a translationally relevant hexavalent TRAIL variant. Our study therefore demonstrates that a high caspase-8/Bid signature is associated with synergistic TRAIL/TL32711-induced apoptosis in GBM cells and outlines Bcl-2 antagonism as a highly potent intervention to sensitize highly TRAIL-resistant GBM cells to TRAIL/TL32711 combination treatment.

Combination therapy with BH3 mimetic and hyperthermia tends to be more effective on anti-melanoma treatment.

Malignant melanoma has shown increased incidence and high mortality rate in the last three decades. In this study, we investigated whether combination therapy with ch282-5 (a novel BH3 mimetic) and microwave hyperthermia could display synergistic antitumor effects against melanoma. Our results indicated that combination therapy reduced the viability and proliferation of melanoma cells. Through inhibiting the expression of anti-apoptotic proteins of Bcl-2 and IAP family and activating MAPK proteins, combined hyperthermia enhanced ch282-5-induced apoptosis. Combination therapy also synergistically disturbed the mTOR/p70S6k signaling pathway, which is important for cell survival and migration. Moreover, our results showed that combination therapy remarkably suppressed melanoma cell migration in vitro and significantly reduced experimental pulmonary metastasis in vivo. In conclusion, our results indicate that combination therapy with ch282-5 and hyperthermia has synergistic antitumor effects and provides a possible therapeutic strategy for advanced melanoma.

MAC inhibitors antagonize the pro-apoptotic effects of tBid and disassemble Bax / Bak oligomers.

Mitochondrial Apoptotic Channel inhibitors or iMACs are di-bromocarbazole derivatives with anti-apoptotic function which have been tested and validated in several mouse models of brain injury and neurodegeneration. Owing to the increased therapeutic potential of these compounds, we sought to expand our knowledge of their mechanism of action. We investigated the kinetics of MAC inhibition in mitochondria from wild type, Bak, and Bax knockout cell lines using patch clamp electrophysiology, fluorescence microscopy, ELISA, and semiquantitative western blot analyses. Our results show that iMACs work through at least two mechanisms: 1) by blocking relocation of the cytoplasmic Bax protein to mitochondria and 2) by disassembling Bax and Bak oligomers in the mitochondrial outer membrane. iMACs exert comparable effects on channel conductance of Bax or Bak and similarly affect cytochrome c release from Bax or Bak-containing mitochondria. Interestingly, wild type mitochondria were more susceptible to inhibition than the Bak or Bax knockouts. Western blot analysis showed that wild type mitochondria had lower steady state levels of Bak in the absence of apoptotic stimulation.

Screening of Small-Molecule Inhibitors of Protein-Protein Interaction with Capillary Electrophoresis Frontal Analysis.

A simple and effective method for identifying inhibitors of protein-protein interactions (PPIs) was developed by using capillary electrophoresis frontal analysis (CE-FA). Antiapoptotic B-cell-2 (Bcl-2) family member Bcl-XL protein, a 5-carboxyfluorescein labeled peptide truncated from the BH3 domain of Bid (F-Bid) as the ligand, and a known Bcl-XL-Bid interaction inhibitor ABT-263 were employed as an experimental model for the proof of concept. In CE-FA, the free ligand is separated from the protein and protein-ligand complex to permit the measurement of the equilibrium concentration of the ligand, hence the dissociation constant of the protein-ligand complex. In the presence of inhibitors, formation of the protein-ligand complex is hindered, thereby the inhibition can be easily identified by the raised plateau height of the ligand and the decayed plateau of the complex. Further, we proposed an equation used to convert the IC50 value into the inhibition constant Ki value, which is more useful than the former for comparison. In addition, the sample pooling strategy was employed to improve the screening throughput more than 10 times. A small chemical library composed of synthetic compounds and natural extracts were screened with the method, two natural products, namely, demethylzeylasteral and celastrol, were identified as new inhibitors to block the Bcl-XL-Bid interaction. Cell-based assay was performed to validate the activity of the identified compounds. The result demonstrated that CE-FA represents a straightforward and robust technique for screening of PPI inhibitors.

Liver Bid suppression for treatment of fibrosis associated with non-alcoholic steatohepatitis.

BACKGROUND & AIMS: Liver fibrosis is the most worrisome feature of non-alcoholic steatohepatitis (NASH). Growing evidence supports a link between hepatocyte apoptosis and liver fibrogenesis. Our aim was to determine the therapeutic efficacy and safety of liver Bid, a key pro-apoptotic molecule, suppression using RNA interference (RNAi) for the treatment of fibrosis. METHODS: First, we optimized the delivery system for Bid siRNA in mice using ten different stealth RNAi siRNAs and two lipid formulations -Invivofectamine2.0 and a newly developed Invivofectamine3.0 - that have been designed for high efficacy accumulation in the liver, assessed via real-time PCR of Bid mRNA. Next, C57BL/6 mice were placed on a choline-deficient L-amino acid defined (CDAA) diet. After 19weeks of the CDAA diet, a time point that results in severe fibrotic NASH, mice were injected with the selected Bid siRNA-Invivofectamine3.0 biweekly for three weeks. Additionally hepatocyte-specific Bid deficient (Bid(Δhep)) mice were placed on CDAA diet for 20weeks. RESULTS: A maximum Bid knockdown was achieved at 1.5mg/kg siRNA with Invivofectamine3.0, whereas it was at 7mg/kg with Invivofectamine2.0. In NASH mice, after 3weeks of treatment, BID protein was reduced to 10% and this was associated with an improvement in liver fibrosis and inflammation associated with a marked reduction in TUNEL positive cells, caspase 3 activation, and a reduction in mitochondrial BAX and BAK. Bid(Δhep) mice showed similar protection from fibrotic changes. CONCLUSION: Our data demonstrate that liver Bid suppression by RNAi technology, as well as hepatocyte-specific Bid deficiency, improves liver fibrosis coupled with a reduction of inflammation in experimental NASH. These findings are consistent with existing evidence that hepatocyte apoptosis triggers hepatic stellate cell activation and liver fibrosis and suggest that Bid inhibition may be useful as an antifibrotic NASH therapy.

Phosphatidylinositol Induces Caspase-Independent Apoptosis of Malignant Pleural Mesothelioma Cells by Accumulating AIF in the Nucleus.

BACKGROUND/AIMS: Phosphatidylinositol (PI) regulates a variety of cell processes. The present study investigated the antitumor action of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-myo-inositol)(DOPI) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-myo-inositol)(DPPI) on human malignant pleural mesothelioma (MPM) cell lines such as NCI-H28, NCI-H2052, NCI-H2452, and MSTO-211H cells. METHODS: MTT assay, TUNEL staining, flow cytometry using propidium iodide (PI) and annexin V (AV), enzymatic caspase assay, and nuclear staining using DAPI were carried out, and mitochondrial membrane potentials and intracellular distribution of apoptosis-inducing factor (AIF) were monitored in cells with and without the siRNA silencing the Bid-targeted gene. RESULTS: Both DOPI and DPPI reduced cell viability for all the investigated MPM cell lines in a concentration (0.01-100 µM)-dependent manner. DOPI and DPPI significantly increased TUNEL-positive cells and the population of PI-negative/AV-positive and PI-positive/AV-positive cells, corresponding to early apoptosis and late apoptosis/secondary necrosis, respectively. DOPI and DPPI perturbed mitochondrial membrane potentials in MSTO-211H cells, but no significant activation of caspase-3, -4, -8, and -9 was obtained. DOPI and DPPI upregulated expression of Bid in MSTO-211H cells. DOPI and DPPI significantly increased nuclear localization of AIF without affecting expression of the mRNAs and proteins in MSTO-211H cells, which was inhibited by knocking-down Bid. In the DAPI staining, nuclear fragmentation and condensation were found. CONCLUSION: The results of the present study indicate that DOPI and DPPI facilitate Bid-mediated AIF release from the mitochondria, to accumulate AIF in the nucleus and induce caspase-independent apoptosis of MPM cells.

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

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

Development of an efficient route toward meiogynin A-inspired dual inhibitors of Bcl-xL and Mcl-1 anti-apoptotic proteins.

The synthesis, on a large scale, with very good yield and er via an efficient strategy, of a chiral 4-substituted 2-cyclohexenone intermediate, was a milestone in the synthesis of seven analogues of meiogynin A, a natural sesquiterpenoid dimer. These compounds were elaborated in ten linear steps. Their binding affinities for Bcl-xL and Mcl-1, two proteins of the Bcl-2 family, overexpressed in various types of cancers, were evaluated. This enabled to further SAR studies en route to the elaboration of potent dual inhibitors of anti-apoptotic proteins of the Bcl-2 family.

Inhibition of JNK3 promotes apoptosis induced by BH3 mimetic S1 in chemoresistant human ovarian cancer cells.

Previous studies have suggested that the novel BH3 mimetic S1 could induce apoptosis in diverse tumor cell lines through endoplasmic reticulum (ER) stress or mitochondrial cell death pathways. The activation of c-Jun N-terminal kinase (JNK) through inositol requiring enzyme-1 (IRE1) is closely connected to ER stress-induced apoptosis. However, the role of JNK is complex, as there are different JNK subtypes and the function of each subtype is still not entirely clear. Here we found that the mRNA expression of JNK3 was continuously high in S1-treated human ovarian cancer SKOV3/DDP cells using a human unfolded protein response (UPR) pathway PCR array. Pharmacological inhibition of JNK3 increased cell sensitivity to apoptosis induced by S1. Furthermore, inhibition of JNK3 induced accumulation of both acidic compartment and p62, and upregulated ROS production. Our results suggest that JNK3 plays a pro-survival role during ER stress through preventing the block of autophagic flux and reducing oxidative stress in SKOV3/DDP cells. Inhibition of JNK3 may be a potential method to enhance the killing effect of the Bcl-2 inhibitor S1.

ATM regulates cell fate choice upon p53 activation by modulating mitochondrial turnover and ROS levels.

Despite extensive study, the mechanisms of cell fate choice upon p53 activation remain poorly understood. Using genome-wide shRNA screening, we recently identified the ATM kinase as synthetic lethal with Nutlin-3, an MDM2 inhibitor that leads to non-genotoxic p53 activation. Here, we demonstrate that while this synthetic lethal interaction relies upon components of both the intrinsic and extrinsic apoptotic pathways (e.g., BAX and BID), it is not due to significant ATM effects on the expression of p53 target genes. Instead, loss of ATM activity results in increased mitochondria and reactive oxygen species that drive apoptosis. Finally, we provide evidence that pharmacologic inhibition of ATM blocks autophagy in direct opposition to p53, which activates this process, and that inhibition of autophagy is sufficient to elicit an apoptotic response when combined with Nutlin-3.

BID mediates selective killing of APC-deficient cells in intestinal tumor suppression by nonsteroidal antiinflammatory drugs.

Colorectal tumorigenesis is driven by genetic alterations in the adenomatous polyposis coli (APC) tumor suppressor pathway and effectively inhibited by nonsteroidal antiinflammatory drugs (NSAIDs). However, how NSAIDs prevent colorectal tumorigenesis has remained obscure. We found that the extrinsic apoptotic pathway and the BH3 interacting-domain death agonist (BID) are activated in adenomas from NSAID-treated patients. Loss of BID abolishes NSAID-mediated tumor suppression, survival benefit, and apoptosis in tumor-initiating stem cells in APC(Min/+) mice. BID-mediated cross-talk between the extrinsic and intrinsic apoptotic pathways is responsible for selective killing of neoplastic cells by NSAIDs. We further demonstrate that NSAIDs induce death receptor signaling in both cancer and normal cells, but only activate BID in cells with APC deficiency and ensuing c-Myc activation. Our results suggest that NSAIDs suppress intestinal tumorigenesis through BID-mediated synthetic lethality triggered by death receptor signaling and gatekeeper mutations, and provide a rationale for developing more effective cancer prevention strategies and agents.

Pro-apoptotic meiogynin A derivatives that target Bcl-xL and Mcl-1.

The biological evaluation of a natural sesquiterpene dimer meiogynin A 1, is described as well as that of five non-natural analogues. Although active on a micromolar range on the inhibition of Bcl-xL/Bak and Mcl-1/Bid interaction, meiogynin A 1 is not cytotoxic on three cell lines that overexpress Bcl-xL and Mcl-1. Contrarily, one of its analogues 6 with an inverted configuration on the side chain and an aromatic moiety replacing the cyclohexane ring was active on both target proteins, cytotoxic on a micromolar range and was found to induce apoptosis through a classical pathway.

The BCL-2 family protein Bid is critical for pro-inflammatory signaling in astrocytes.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of motoneurons in the spinal cord, brainstem and motor cortex. Mutations in the superoxide dismutase 1 (SOD1) gene represent a frequent genetic determinant and recapitulate a disease phenotype similar to ALS when expressed in mice. Previous studies using SOD1(G93A) transgenic mice have suggested a paracrine mechanism of neuronal loss, in which cytokines and other toxic factors released from astroglia or microglia trigger motoneuron degeneration. Several pro-inflammatory cytokines activate death receptors and may downstream from this activate the Bcl-2 family protein, Bid. We here sought to investigate the role of Bid in astrocyte activation and non-cell autonomous motoneuron degeneration. We found that spinal cord Bid protein levels increased significantly during disease progression in SOD1(G93A) mice. Subsequent experiments in vitro indicated that Bid was expressed at relatively low levels in motoneurons, but was enriched in astrocytes and microglia. Bid was strongly induced in astrocytes in response to pro-inflammatory cytokines or exposure to lipopolysaccharide. Experiments in bid-deficient astrocytes or astrocytes treated with a small molecule Bid inhibitor demonstrated that Bid was required for the efficient activation of transcription factor nuclear factor-κB in response to these pro-inflammatory stimuli. Finally, we found that conditioned medium from wild-type astrocytes, but not from bid-deficient astrocytes, was toxic when applied to primary motoneuron cultures. Collectively, our data demonstrate a new role for the Bcl-2 family protein Bid as a mediator of astrocyte activation during neuroinflammation, and suggest that Bid activation may contribute to non-cell autonomous motoneuron degeneration in ALS.

Phosphorylation of the proapoptotic BH3-only protein bid primes mitochondria for apoptosis during mitotic arrest.

Mitosis is a moment of exquisite vulnerability for a metazoan cell. Failure to complete mitosis accurately can lead to aneuploidy and cancer initiation. Therefore, if the exit from mitosis is delayed, normal cells are usually removed by apoptosis. However, how failure to complete mitosis activates apoptosis is still unclear. Here, we demonstrate that a phosphorylated form of the BH3-only protein Bid regulates apoptosis if mitotic exit is delayed. Bid is phosphorylated on serine 66 as cells enter mitosis, and this phosphorylation is lost during the metaphase-to-anaphase transition. Cells expressing a nonphosphorylatable version of Bid or a BH3-domain mutant were resistant to mitotic-arrest-induced apoptosis. Thus, we show that Bid phosphorylation primes cells to undergo mitochondrial apoptosis if mitotic exit is delayed. Avoidance of this mechanism may explain the selective pressure for cancer cells to undergo mitotic slippage.

Inhibition of cysteine cathepsin B and L activation in astrocytes contributes to neuroprotection against cerebral ischemia via blocking the tBid-mitochondrial apoptotic signaling pathway.

The roles of cathepsins in the ischemic astrocytic injury remain unclear. Here, we test the hypothesis that activation of cathepsin B and L contributes to the ischemic astrocyte injury via the tBid-mitochondrial apoptotic signaling pathways. In the rat models of pMCAO, CA-074Me or Clik148, a selective inhibitor of cathepsin B or cathepsin L, reduced the infarct volume, improved the neurological deficits and increased the MAP2 and GFAP levels. In OGD-induced astrocyte injury, CA-074Me or Clik148 decreased the LDH leakage and increased the GFAP levels. In the ischemic cortex or OGD-induced astrocytes injury, Clik148 or CA-074Me reversed pMCAO or OGD-induced increase in active cathepsin L or cathepsin B at 3 h or 6 h, increase in tBid, reduction in mitochondrial cytochrome-c (Cyt-c) and increase in cytoplastic Cyt-c and active caspase-3 at 12-24 h of the late stage of pMCAO or OGD. CA-074Me or Clik148 also reduced cytosolic and mitochondrial tBid, increased mitochondrial Cyt-c and decreased cytoplastic Cyt-c and active caspase-3 at 6 h of the early stage of Bid activation. CA-074Me or Clik148 blocked the pMCAO-induced release of cathepsin B or L from the lysosomes into the cytoplasm and activation of caspase-3 in ischemic astrocytes at 12 h after ischemia. Concurrent inhibition of cathepsin B and cathepsin L provided better protection on the OGD-induced astrocytic apoptosis than obtained with separate use of each inhibitor. These results suggest that inhibition of the cysteine cathepsin B and cathepsin L activation in ischemic astrocytes contributes to neuroprotection via blocking the tBid-mitochondrial apoptotic signaling pathway.

FTY720 induces apoptosis of chronic myelogenous leukemia cells via dual activation of BIM and BID and overcomes various types of resistance to tyrosine kinase inhibitors.

PP2A activator FTY720 has been shown to possess the anti-leukemic activity for chronic myelogenous leukemia (CML), however, the cell killing mechanism underlying its anti-leukemic activity has remained to be verified. We investigated the precise mechanisms underlying the apoptosis induction by FTY720, especially focusing on the roles of BH3-only proteins, and the therapeutic potency of FTY720 for CML. Enforced expression of either BCL2 or the dominant-negative protein of FADD (FADD.DN) partly protected CML cells from apoptosis by FTY720, indicating the involvement of both cell extrinsic and intrinsic apoptosis pathways. FTY720 activates pro-apoptotic BH3-only proteins: BIM, which is essential for apoptosis by BCR-ABL1 tyrosine kinase inhibitors (TKIs), and BID, which accelerates the extrinsic apoptosis pathway. Gene knockdown of either BIM or BID partly protected K562 cells from apoptosis by FTY720, but the extent of cell protection was not as much as that by overexpression of either BCL2 or FADD.DN. Moreover, knockdown of both BIM and BID did not provide additional protection compared with knockdown of only BIM or BID, indicating that BIM and BID complement each other in apoptosis by FTY720, especially when either is functionally impaired. FTY720 can overcome TKI resistance caused by ABL kinase domain mutations, dysfunction of BIM resulting from gene deletion polymorphism, and galectin-3 overexpression. In addition, ABT-263, a BH3-mimetic, significantly augmented cell death induction by FTY720 both in TKI-sensitive and -resistant leukemic cells. These results provide the rationale that FTY720, with its unique effects on BIM and BID, could lead to new therapeutic strategies for CML.

Quercetin-3-O-(2"-galloyl)-α-l-rhamnopyranoside prevents TRAIL-induced apoptosis in human keratinocytes by suppressing the caspase-8- and Bid-pathways and the mitochondrial pathway.

Quercetin and its derivatives have antioxidant and anti-inflammatory effects. Nevertheless, in human keratinocytes, compared to the reports on other toxic insults, researches on the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis that may be involved in skin diseases are rare. Furthermore, the effect of quercetin-3-O-(2"-galloyl)-α-l-rhamnopyranoside (QGR), a new quercetin derivative, on TRAIL-induced apoptosis in keratinocytes has not been studied. In this respect, we investigated the effect of QGR on TRAIL-induced apoptosis in human keratinocytes. TRAIL triggers apoptosis by inducing a decrease in Bid, Bcl-2, Bcl-xL and survivin protein levels, increase in Bax and VDAC1 levels, loss of the mitochondrial transmembrane potential, release of cytochrome c, activation of caspases (-8, -9 and -3), cleavage of PARP-1, and an increase in the tumor suppressor p53 levels. Treatment with QGR prevented TRAIL-induced apoptosis-related protein activation, formation of reactive oxygen species, nuclear damage, and cell death. In contrast, quercetin induces cytotoxicity and had an additive effect on TRAIL-induced apoptosis-related protein activation and cell death. These results suggest that the QGR, unlike quercetin, may reduce TRAIL-induced apoptosis in human keratinocytes by suppressing the activation of the caspase-8- and Bid-pathways and the mitochondria-mediated cell death pathway, which is associated with the formation of reactive oxygen species. These data suggest that QGR could be effective in the prevention of TRAIL-induced apoptosis-mediated skin diseases.

Sensitization of melanoma cells for TRAIL-induced apoptosis by BMS-345541 correlates with altered phosphorylation and activation of Bax.

Resistance to TRAIL (TNF-related apoptosis-inducing ligand)- induced apoptosis limits its therapeutic use. Different strategies of TRAIL sensitization and a dependency on Bax have been reported, but common principles of TRAIL resistance and the way of Bax activation remained poorly understood. Applying a melanoma model of TRAIL-sensitive and -resistant cell lines, efficient sensitization for TRAIL-induced apoptosis is demonstrated by the kinase inhibitor BMS-345541 (N-(1,8-dimethylimidazo(1,2-a)quinoxalin-4-yl)-1,2-ethanediamine hydrochloride), which targets IκB (inhibitor of κB proteins) kinase ß (IKKß). This effect was completely abrogated by Bax knockout as well as by Bcl-2 overexpression, in accordance with a Bax dependency. Early loss of the mitochondrial membrane potential, release of cytochrome c and Smac (second mitochondria-derived activator of caspases) clearly indicated the activation of mitochondrial apoptosis pathways. Of note, BMS-345541 alone resulted in an early Bax activation, seen by conformational changes and by Bax translocation. The synergistic effects can be explained by Bid activation through TRAIL, which inhibits Bcl-2, and the activation of Bax through BMS-345541. The critical roles of XIAP (X-chromosome-linked inhibitor of apoptosis protein), Smac and Bid were clearly proven by overexpression and siRNA knockdown, respectively. The way of Bax activation by BMS-345541 was unraveled by establishing new assays for Bax activation. These showed reduction of the inactivating Bax phosphorylation at serine-184, while the activating Bax phosphorylation at threonine-167 was enhanced. Thus, modulation of Bax phosphorylation appeared as tightly related to TRAIL sensitivity/resistance in melanoma cells, and therapeutic strategies may be considered.

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.