tBID Homooligomerizes in the mitochondrial membrane to induce apoptosis

Canonical and Noncanonical Functions of the BH3 Domain Protein Bid in Apoptosis, Oncogenesis, Cancer Therapeutics, and Aging

Effective cancer therapy with limited adverse effects is a major challenge in the medical field. This is especially complicated by the development of acquired chemoresistance. Understanding the mechanisms that underlie these processes remains a major effort in cancer research. In this review, we focus on the dual role that Bid protein plays in apoptotic cell death via the mitochondrial pathway, in oncogenesis and in cancer therapeutics. The BH3 domain in Bid and the anti-apoptotic mitochondrial proteins (Bcl-2, Bcl-XL, mitochondrial ATR) it associates with at the outer mitochondrial membrane provides us with a viable target in cancer therapy. We will discuss the roles of Bid, mitochondrial ATR, and other anti-apoptotic proteins in intrinsic apoptosis, exploring how their interaction sustains cellular viability despite the initiation of upstream death signals. The unexpected upregulation of this Bid protein in cancer cells can also be instrumental in explaining the mechanisms behind acquired chemoresistance. The stable protein associations at the mitochondria between tBid and anti-apoptotic mitochondrial ATR play a crucial role in maintaining the viability of cancer cells, suggesting a novel mechanism to induce cancer cell apoptosis by freeing tBid from the ATR associations at mitochondria.

Mesenchymal stromal cells for improvement of cardiac function following acute myocardial infarction: a matter of timing

Acute myocardial infarction (AMI) is the leading cause of cardiovascular death and remains the most common cause of heart failure. Reopening of the occluded artery, i.e., reperfusion, is the only way to save the myocardium. However, the expected benefits of reducing infarct size are disappointing due to the reperfusion paradox, which also induces specific cell death. These ischemia-reperfusion (I/R) lesions can account for up to 50% of final infarct size, a major determinant for both mortality and the risk of heart failure (morbidity). In this review, we provide a detailed description of the cell death and inflammation mechanisms as features of I/R injury and cardioprotective strategies such as ischemic postconditioning as well as their underlying mechanisms. Due to their biological properties, the use of mesenchymal stromal/stem cells (MSCs) has been considered a potential therapeutic approach in AMI. Despite promising results and evidence of safety in preclinical studies using MSCs, the effects reported in clinical trials are not conclusive and even inconsistent. These discrepancies were attributed to many parameters such as donor age, in vitro culture, and storage time as well as injection time window after AMI, which alter MSC therapeutic properties. In the context of AMI, future directions will be to generate MSCs with enhanced properties to limit cell death in myocardial tissue and thereby reduce infarct size and improve the healing phase to increase postinfarct myocardial performance.

Mechanisms of BCL-2 family proteins in mitochondrial apoptosis

The proteins of the BCL-2 family are key regulators of mitochondrial apoptosis, acting as either promoters or inhibitors of cell death. The functional interplay and balance between the opposing BCL-2 family members control permeabilization of the outer mitochondrial membrane, leading to the release of activators of the caspase cascade into the cytosol and ultimately resulting in cell death. Despite considerable research, our knowledge about the mechanisms of the BCL-2 family of proteins remains insufficient, which complicates cell fate predictions and does not allow us to fully exploit these proteins as targets for drug discovery. Detailed understanding of the formation and molecular architecture of the apoptotic pore in the outer mitochondrial membrane remains a holy grail in the field, but new studies allow us to begin constructing a structural model of its arrangement. Recent literature has also revealed unexpected activities for several BCL-2 family members that challenge established concepts of how they regulate mitochondrial permeabilization. In this Review, we revisit the most important advances in the field and integrate them into a new structure-function-based classification of the BCL-2 family members that intends to provide a comprehensive model for BCL-2 action in apoptosis. We close this Review by discussing the potential of drugging the BCL-2 family in diseases characterized by aberrant apoptosis.

Mechanisms of ferroptosis regulating oxidative stress and energy metabolism in myocardial ischemia-reperfusion injury and a novel perspective of natural plant active ingredients for its treatment

Acute myocardial infarction remains the leading cause of death in humans. Timely restoration of blood perfusion to ischemic myocardium remains the most effective strategy in the treatment of acute myocardial infarction, which can significantly reduce morbidity and mortality. However, after restoration of blood flow and reperfusion, myocardial injury will aggravate and induce apoptosis of cardiomyocytes, a process called myocardial ischemia-reperfusion injury. Studies have shown that the loss and death of cardiomyocytes caused by oxidative stress, iron load, increased lipid peroxidation, inflammation and mitochondrial dysfunction, etc., are involved in myocardial ischemia-reperfusion injury. In recent years, with the in-depth research on the pathology of myocardial ischemia-reperfusion injury, people have gradually realized that there is a new form of cell death in the pathological process of myocardial ischemia-reperfusion injury, namely ferroptosis. A number of studies have found that in the myocardial tissue of patients with acute myocardial infarction, there are pathological changes closely related to ferroptosis, such as iron metabolism disorder, lipid peroxidation, and increased reactive oxygen species free radicals. Natural plant products such as resveratrol, baicalin, cyanidin-3-O-glucoside, naringenin, and astragaloside IV can also exert therapeutic effects by correcting the imbalance of these ferroptosis-related factors and expression levels. Combining with our previous studies, this review summarizes the regulatory mechanism of natural plant products intervening ferroptosis in myocardial ischemia-reperfusion injury in recent years, in order to provide reference information for the development of targeted ferroptosis inhibitor drugs for the treatment of cardiovascular diseases.

Apoptosis Regulation in Osteoarthritis and the Influence of Lipid Interactions

Osteoarthritis (OA) is one of the most common chronic diseases in human and animal joints. The joints undergo several morphological and histological changes during the development of radiographically visible osteoarthritis. The most discussed changes include synovial inflammation, the massive destruction of articular cartilage and ongoing joint destruction accompanied by massive joint pain in the later stadium. Either the increased apoptosis of chondrocytes or the insufficient apoptosis of inflammatory macrophages and synovial fibroblasts are likely to underly this process. In this review, we discuss the current state of research on the pathogenesis of OA with special regard to the involvement of apoptosis.

Orchestration of Mitochondrial Function and Remodeling by Post-Translational Modifications Provide Insight into Mechanisms of Viral Infection

The regulation of mitochondria structure and function is at the core of numerous viral infections. Acting in support of the host or of virus replication, mitochondria regulation facilitates control of energy metabolism, apoptosis, and immune signaling. Accumulating studies have pointed to post-translational modification (PTM) of mitochondrial proteins as a critical component of such regulatory mechanisms. Mitochondrial PTMs have been implicated in the pathology of several diseases and emerging evidence is starting to highlight essential roles in the context of viral infections. Here, we provide an overview of the growing arsenal of PTMs decorating mitochondrial proteins and their possible contribution to the infection-induced modulation of bioenergetics, apoptosis, and immune responses. We further consider links between PTM changes and mitochondrial structure remodeling, as well as the enzymatic and non-enzymatic mechanisms underlying mitochondrial PTM regulation. Finally, we highlight some of the methods, including mass spectrometry-based analyses, available for the identification, prioritization, and mechanistic interrogation of PTMs.

Anti-apoptotic and anti-fibrotic efficacy of exercise training in hypertensive hearts: A systematic review

Background

This review aims to summarize the antiapoptotic, pro-survival, and antifibrotic effects of exercise training in hypertensive hearts.

Methods

Keyword searches were conducted in PubMed, Web of Science, and Scopus in May 2021. Research published in English on the effects of exercise training on the apoptosis, survival, and fibrosis pathways in hypertension was included. The CAMARADES checklist was used to determine the quality of the studies. Two reviewers independently implemented predesigned protocols for the search and selection of studies, the assessment of study quality, and the evaluation of the strength of evidence.

Results

Eleven studies were included after selection. The duration of the exercise training ranged from 5 to 27 weeks. Nine studies showed that exercise training improved cardiac survival rates by increasing IGF-1, IGF-1 receptor, p-PI3K, Bcl-2, HSP 72, and p-Akt. Furthermore, 10 studies showed that exercise training reduced apoptotic pathways by downregulating Bid, t-Bid, Bad, Bak, Bax, TNF, and FADD. Finally, two studies reported the modification and subsequent improvement of physiological characteristics of fibrosis and decreased MAPK p38 and PTEN levels by exercise training in the left ventricle of the heart.

Conclusions

The findings of the review showed that exercise training could improve cardiac survival rates and attenuate cardiac apoptotic and fibrotic pathways in hypertension, suggesting that exercise training could act as a therapeutic approach to prevent hypertension-induced cardiac apoptosis and fibrosis.

Systematic Review Registration

https://www.crd.york.ac.uk, identifier: CRD42021254118.

Apoptotic stress-induced FGF signalling promotes non-cell autonomous resistance to cell death

Damaged or superfluous cells are typically eliminated by apoptosis. Although apoptosis is a cell-autonomous process, apoptotic cells communicate with their environment in different ways. Here we describe a mechanism whereby cells under apoptotic stress can promote survival of neighbouring cells. We find that upon apoptotic stress, cells release the growth factor FGF2, leading to MEK-ERK-dependent transcriptional upregulation of pro-survival BCL-2 proteins in a non-cell autonomous manner. This transient upregulation of pro-survival BCL-2 proteins protects neighbouring cells from apoptosis. Accordingly, we find in certain cancer types a correlation between FGF-signalling, BCL-2 expression and worse prognosis. In vivo, upregulation of MCL-1 occurs in an FGF-dependent manner during skin repair, which regulates healing dynamics. Importantly, either co-treatment with FGF-receptor inhibitors or removal of apoptotic stress restores apoptotic sensitivity to cytotoxic therapy and delays wound healing. These data reveal a pathway by which cells under apoptotic stress can increase resistance to cell death in surrounding cells. Beyond mediating cytotoxic drug resistance, this process also provides a potential link between tissue damage and repair.

Potential application of two cobalt (III) Schiff base complexes in cancer chemotherapy: Leads from a study using breast and lung cancer cells

Cobalt (III) Schiff base complexes are of attraction in the context of their potential application in cancer therapy. The aim of this study has been to find the mechanism of action of cobalt (III) Schiff base complexes 1 and 2, the synthesis and characterization of which have already been reported, in inhibiting growth of human breast cancer cell MCF-7 and lung cancer cell A549. The already proclaimed anti-proliferative effect of the cobalt complexes was ascertained using MTT cytotoxicity assay. More assays such as Acridine orange & Ethidium bromide staining, AnnexinV-Cy3 staining, Hoechst staining, comet assay, and Reactive Oxygen Species (ROS) assay- all supported the cytotoxic property of the complexes. Moreover, the expression levels of mRNA of pro- and antiapoptotic genes also supported the effectiveness of cobalt complexes by modifying the ratio of Bax: Bcl-2. In addition, the cobalt complexes induced apoptosis in MCF- 7 and A549 cells through modulation of pro-apoptotic, anti-apoptotic, and ROS modulatory gene expressions. The present study validates the scientific evidence for antiproliferative efficacy of cobalt complexes against MCF-7 and A549 cells. Thus, this study takes cobalt complexes 1 and 2 to a step higher towards their use as anticancer agents.

Glucose deprivation-induced endoplasmic reticulum stress response plays a pivotal role in enhancement of TRAIL cytotoxicity

Abnormalities of the tumor vasculature result in insufficient blood supply and development of a tumor microenvironment that is characterized by low glucose concentrations, low extracellular pH, and low oxygen tensions. We previously reported that glucose-deprived conditions induce metabolic stress and promote tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced cytotoxicity. In this study, we examined whether the metabolic stress-associated endoplasmic reticulum (ER) stress response pathway plays a pivotal role in the enhancement of TRAIL cytotoxicity. We observed no significant cytotoxicity when human colorectal cancer SW48 cells were treated with various doses of TRAIL (2-100 ng/ml) for 4 h or glucose (0-25 mM) for 24 h. However, a combination of TRAIL and low glucose-induced dose-dependent apoptosis through activation of caspases (-8, -9, and -3). Studies with activating transcription factor 4 (ATF4), C/EBP-homologous protein (CHOP), p53 upregulated modulator of apoptosis (PUMA), or death receptor 5 (DR5)-deficient mouse embryonic fibroblasts or HCT116 cells suggest that the ATF4-CHOP-PUMA axis and the ATF4-CHOP-DR5 axis are involved in the combined treatment-induced apoptosis. Moreover, the combined treatment-induced apoptosis was completely suppressed in BH3 interacting-domain death agonist (Bid)- or Bcl-2-associated X protein (Bax)-deficient HCT116 cells, but not Bak-deficient HCT116 cells. Interestingly, the combined treatment-induced Bax oligomerization was suppressed in PUMA-deficient HCT116 cells. These results suggest that glucose deprivation enhances TRAIL-induced apoptosis by integrating the ATF4-CHOP-PUMA axis and the ATF4-CHOP-DR5 axis, consequently amplifying the Bid-Bax-associated mitochondria-dependent pathway.

Voluntary exercise training attenuated the middle-aged maturity-induced cardiac apoptosis

AIMS

Voluntary exercise training has cardioprotective effects in humans, but the underlying mechanism is unknown. This research was done to estimate the effect of voluntary exercise training to attenuate middle-aged maturity-induced cardiac apoptosis.

MATERIALS AND METHODS

The study was designed to divide 64 male mice randomly into four groups, consisting of a 9-month sedentary pre-middle-aged group (9M), 15-month sedentary middle-aged group (15M), and two exercise groups using a voluntary wheel running respectively (9M+EX, 15M+EX). After 3 months, the condition of cardiac apoptosis in different groups was measured by HE dying, TUNEL and DAPI staining, and Western Blot analysis.

KEY FINDINGS

TUNEL-positive cells were increased in 15M group compared with 9M group, while decreased in 9M+EX and 15M+EX groups compared with their control groups respectively. Protein levels of AIF, Endo G, TNF-α, TNFR1, TRAF2, TRADD, Fas, FasL, FADD, activated caspase 8, 3, 9, Bax/Bcl2, Bak/BclxL, and tBid were decreased in 9M+EX and 15M+EX groups compared with their control groups respectively. The protein levels of pBad/Bad, 14-3-3, IGF1, IGFR1, pPI3K/PI3K, and pAKT/AKT were more activated in the 9M+EX and 15M+EX groups than those in their control groups respectively. Significant differences were found between 9M group and 15M group for the protein levels of TRAF2, FADD, Bax/Bcl2, tBid and pAKT/AKT.

SIGNIFICANCE

Voluntary exercise training as an important lifestyle modification may prevent cardiac widely dispersed apoptosis and enhance cardiac survival at middle-aged maturity.

Stoichiometry and regulation network of Bcl-2 family complexes quantified by live-cell FRET assay

The stoichiometry and affinity of Bcl-2 family complexes are essential information for understanding how their interactome network is orchestrated to regulate mitochondrial permeabilization and apoptosis. Based on over-expression model system, FRET analysis was used to quantify the protein-protein interactions among Bax, Bcl-xL, Bad and tBid in healthy and apoptotic cells. Our data indicate that the stoichiometry and affinity of Bcl-2 complexes are dependent on their membrane environment. Bcl-xL, Bad and tBid can form hetero-trimers in mitochondria. Bcl-xL binds preferentially to Bad, then to tBid and Bax in mitochondria, whilst Bcl-xL displays higher affinity to Bad or tBid than to itself. Strikingly, Bax can bind to Bcl-xL in cytosol. In cytosol of apoptotic cells, Bcl-xL associates with Bax to form hetero-trimer with 1:2 stoichiometry, while Bcl-xL associates with Bad to form hetero-trimer with 2:1 stoichiometry and Bcl-xL associates with tBid to form hetero-dimer. In mitochondria, Bcl-xL associates with Bax/Bad to form hetero-dimer in healthy cells, while Bcl-xL associates with Bad to form hetero-tetramer with 3:1 stoichiometry in apoptotic cells.

Detection of tBid Oligomerization and Membrane Permeabilization by Graphene-Based Single-Molecule Surface-Induced Fluorescence Attenuation

The permeabilization of organelle membranes by BCL-2 family proteins is a pivotal step during the regulation of apoptosis; the underlying mechanisms remain unclear. Based on the fluorescence attenuation by graphene oxide, we developed a single-molecule imaging method termed surface-induced fluorescence attenuation (smSIFA), which enabled us to track both vertical and lateral kinetics of singly labeled BCL-2 family protein tBid during membrane permeabilization. We found that tBid monomers lie shallowly on the lipid bilayer, where they self-assemble to form oligomers. During the initiation phase of self-assembly, the two central hydrophobic helices (α₆ and α₇) of tBid insert halfway into the phospholipid core, while the other helices remain on the surface. In oligomerized tBid clusters, α₆ and α₇ prefer to float up, and the other helices may sink to the bottom of the membrane and cause the formation of transient two-dimensional, micelle-like pore structures, which are responsible for the permeabilization of membranes and the induction of apoptosis. Our results shed light on the understanding of tBid-induced apoptosis, and this nanotechnology-based smSIFA approach could be used to dissect the kinetic interaction between membrane protein and lipid bilayer at the single-molecule level with subnanometer precision.

Antiapoptotic and mitochondrial biogenetic effects of exercise training on ovariectomized hypertensive rat hearts

This study was to investigate the effects of exercise training on antiapoptotic pathways and mitochondrial biogenesis in ovariectomized hypertensive rats. Histopathological analysis, TUNEL assay, and Western blotting were performed on the excised hearts from female spontaneously hypertensive rats (SHR), which were divided into a sham-operated sedentary hypertensive (SHR-S), a sedentary hypertensive ovariectomized (SHR-O), and hypertensive ovariectomized rats that underwent treadmill exercise training (SHR-OT; 60 min/day, 5 days/wk) for 8 wk, along with normotensive Wistar Kyoto rats (WKY). When compared with the WKY group, the SHR-S group exhibited decreased protein levels of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and mitochondrial OPA-1 (mitochondrial biogenesis) and decreased further in the SHR-O group. The protein levels of p-PI3K, p-Akt, Bcl-2, Bcl-xL (prosurvival pathways), and the protein levels of PGC-1α and mitochondrial OPA1 (mitochondrial biogenesis) were increased in the SHR-OT group, but estrogen receptor (ER)α and ERβ were not changed when compared with the SHR-O group. The protein levels of t-Bid, Bad, Bax, cytosolic cytochrome c, activated caspase 9, and activated caspase 3 (mitochondria-dependent apoptotic pathways), as well as Fas ligand, TNF-α, Fas receptors, Fas-associated death domain, activated caspase 8 (Fas receptor-dependent apoptotic pathways) were decreased in the SHR-OT group, when compared with the SHR-O group. Exercise training protection on the coexistence of hypertension and ovariectomy-induced cardiac mitochondria-dependent and Fas receptor-dependent apoptotic pathways by enhancing the Bcl2-related and mitochondrial biogenetic prosurvival pathways might provide a new therapeutic effect on cardiac protection in oophorectomized early postmenopausal hypertensive women. NEW & NOTEWORTHY Widely dispersed cardiac apoptosis was found in the coexistence of hypertension and ovariectomy. Exercise training on a treadmill could prevent ovariectomized hypertension-induced widely dispersed cardiac apoptosis via mitochondria-dependent apoptotic pathway (t-Bid, Bad, Bax, cytosolic cytochrome c, activated caspase 9, and activated caspase 3) and Fas receptor-dependent apoptotic pathway (Fas ligand, tumor necrosis factor-α, Fas receptors, Fas-associated death domain, activated caspase 8, and activated caspase 3) through enhancing the Bcl2-related (p-PI3K, p-Akt, Bcl-2, Bcl-xL) and mitochondrial biogenetic (PGC-1α and mitochondrial optic atrophy 1) prosurvival pathways.

Rhynchophylline ameliorates myocardial ischemia/reperfusion injury through the modulation of mitochondrial mechanisms to mediate myocardial apoptosis

Rhynchophylline (RP), the primary active ingredient of Uncaria rhynchophylla, has an anti-hypertensive effect and protects against ischemia-induced neuronal damage. The present study aimed to examine the roles and mechanisms of RP in myocardial ischemia-reperfusion (MI/R) injury of rat cardiomyocytes. Cell viability, reactive oxygen species, mitochondrial membrane potential (MMP) and cell apoptosis were examined by a Cell Counting Kit-8 assay and flow cytometry, respectively. An ELISA was performed to assess the expression of oxidative stress markers. Spectrophotometry was used to detect the degree of mitochondrial permeability transition pore (mPTP) openness. Western blotting and reverse transcription- quantitative polymerase chain reaction assays were used to evaluate the associated protein and mRNA expression, respectively. The present results demonstrated that RP increased the cell viability of MI/R-induced cardiomyocytes, and suppressed the MI/R-induced apoptosis of cardiomyocytes. Additionally, RP modulated the Ca²⁺ and MMP levels in MI/R-induced cardiomyocytes. Furthermore, RP decreased the oxidative stress and mPTP level of MI/R-induced cardiomyocytes. It was additionally observed that RP affected the apoptosis-associated protein expression and regulated the mitochondrial-associated gene expression in MI/R-induced cardiomyocytes. In conclusion, RP ameliorated MI/R injury through the modulation of mitochondrial mechanisms. The potential effects of RP on the protection of MI/R-induced apoptosis of cardiomyocytes suggest that RP may be an effective target for MI/R therapy.

Ferroptosis-inducing agents enhance TRAIL-induced apoptosis through upregulation of death receptor 5

Ferroptosis is considered genetically and biochemically distinct from other forms of cell death. In this study, we examined whether ferroptosis shares cell death pathways with other types of cell death. When human colon cancer HCT116, CX-1, and LS174T cells were treated with ferroptotic agents such as sorafenib (SRF), erastin, and artesunate, data from immunoblot assay showed that ferroptotic agents induced endoplasmic reticulum (ER) stress and the ER stress response-mediated expression of death receptor 5 (DR5), but not death receptor 4. An increase in the level of DR5, which is activated by binding to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and initiates apoptosis, was probably responsible for synergistic apoptosis when cells were treated with ferroptotic agent in combination with TRAIL. This collateral effect was suppressed in C/EBP (CCAAT-enhancer-binding protein)-homologous protein (CHOP)-deficient mouse embryonic fibroblasts or DR5 knockdown HCT116 cells, but not in p53-deficient HCT116 cells. The results from in vitro studies suggest the involvement of the p53-independent CHOP/DR5 axis in the synergistic apoptosis during the combinatorial treatment of ferroptotic agent and TRAIL. The synergistic apoptosis and regression of tumor growth were also observed in xenograft tumors when SRF and TRAIL were administered to tumor-bearing mice.

Erythrocyte-Derived Theranostic Nanoplatforms for Near Infrared Fluorescence Imaging and Photodestruction of Tumors

Nanoparticles activated by near-infrared (NIR) excitation provide a capability for optical imaging and photodestruction of tumors. We have engineered optical nanoconstructs derived from erythrocytes, which are doped with the FDA-approved NIR dye, indocyanine green (ICG). We refer to these constructs as NIR erythrocyte-mimicking transducers (NETs). Herein, we investigate the phototheranostic capabilities of NETs for fluorescence imaging and photodestruction of SKBR3 breast cancer cells and subcutaneous xenograft tumors in mice. Our cellular studies demonstrate that NETs are internalized by these cancer cells and localized to their lysosomes. As evidenced by NIR fluorescence imaging and in vivo laser irradiation studies, NETs remain available within tumors at 24 h postintravenous injection. In response to continuous wave 808 nm laser irradiation at intensity of 680 mW/cm² for 10-15 min, NETs mediate the destruction of cancer cells and tumors in mice through synergistic photochemical and photothermal effects. We demonstrate that NETs are effective in mediating photoactivation of Caspase-3 to induce tumor apoptosis. Our results provide support for the effectiveness of NETs as theranostic agents for fluorescence imaging and photodestruction of tumors and their role in photoinduced apoptosis initiated by their localization to lysosomes.

tert-Butylhydroperoxide induces apoptosis in RAW264.7 macrophages via a mitochondria-mediated signaling pathway

Macrophage apoptosis occurs throughout all stages of atherosclerosis, mainly induced by oxidized low density lipoproteins (Ox LDLs), leading to the formation of necrotic cores. Nevertheless, the mechanism of macrophage apoptosis induced by Ox LDLs is not yet clearly understood. In this study, a model of RAW264.7 macrophages exposed to an Ox LDL analogue, i.e. tert-butylhydroperoxide (t-BHP), was established. We thoroughly evaluated the viability and apoptosis of RAW264.7 cells treated with t-BHP at different time intervals. t-BHP treatment decreases the viability of RAW264.7 cells in a dose- and time-dependent manner (IC₅₀: 400 μM) and also induces a loss of the mitochondrial membrane potential (MMP) in RAW264.7 cells. Moreover, the activation of Bid, up-regulation of Bcl-2, and down-regulation of Bax, as well as the proteolysis of pro-caspase 3 and cleavage of PARP, were all also observed in t-BHP treated RAW264.7 cells. Finally, we concluded that t-BHP induces the apoptosis of macrophages via a mitochondria-mediated signaling pathway.

Humanin decreases mitochondrial membrane permeability by inhibiting the membrane association and oligomerization of Bax and Bid proteins

Humanin (HN) is a 24-residue peptide identified from the brain of a patient with Alzheimer's disease (AD). HN has been found to protect against neuronal insult caused by Aβ peptides or transfection of familial AD mutant genes. In order to elucidate the molecular mechanisms of HN neuroprotection, we explored the effects of HN on the association of Bax or Bid with lipid bilayers and their oligomerization in the membrane. By using single-molecule fluorescence and Förster resonance energy transfer techniques, we showed that Bax was mainly present as monomers, dimers and tetramers in lipid bilayers, while truncated Bid (tBid) enhanced the membrane association and tetramerization of Bax. HN (100 nmol/L) inhibited the self-association and tBid-activated association of Bax with the bilayers, and significantly decreased the proportion of Bax in tetramers. Furthermore, HN inhibited Bid translocation to lipid bilayers. HN could bind with Bax and Bid either in solution or in the membrane. However, HN could not pull the proteins out of the membrane. Based on these results, we propose that HN binds to Bax and cBid in solution and inhibits their translocation to the membrane. Meanwhile, HN interacts with the membrane-bound Bax and tBid, preventing the recruitment of cytosolic Bax and its oligomerization in the membrane. In this way, HN inhibits Bax pore formation in mitochondrial outer membrane and suppresses cytochrome c release and mitochondria-dependent apoptosis.

Ferroptosis-Induced Endoplasmic Reticulum Stress: Cross-talk between Ferroptosis and Apoptosis

Since its discovery in 2012, ferroptosis has been well characterized by the accumulation of lipid peroxides due to the failure of glutathione-dependent antioxidant defenses. It is known as an iron-dependent form of programmed cell death, which is distinct from other forms of cell death such as apoptosis and necrosis. Nonetheless, little is known about the ferroptotic agent-induced endoplasmic reticulum (ER) stress response and its role in cell death. Recent studies reveal that the ferroptotic agent-induced ER stress response plays an important role in the cross-talk between ferroptosis and other types of cell death. Ferroptotic agents induce the unfolded protein response and subsequently ER stress-mediated activation of the PERK-eIF2α-ATF4-CHOP pathway. CHOP (C/EBP homologous protein) signaling pathway-mediated p53-independent PUMA (p53 upregulated modulator of apoptosis) expression is involved in the synergistic interaction between ferroptosis and apoptosis. This review highlights the recent literature on ferroptotic and apoptotic agent interactions through the ER stress-mediated PERK-eIF2α-ATF4-CHOP-PUMA pathway and implicates combined treatment to effectively enhance tumoricidal efficacy as a novel therapeutic strategy for cancer. Mol Cancer Res; 16(7); 1073-6. ©2018 AACR.

Rab5 and Alsin regulate stress-activated cytoprotective signaling on mitochondria

Mitochondrial stress response is essential for cell survival, and damaged mitochondria are a hallmark of neurodegenerative diseases. Thus, it is fundamental to understand how mitochondria relay information within the cell. Here, by investigating mitochondrial-endosomal contact sites we made the surprising observation that the small GTPase Rab5 translocates from early endosomes to mitochondria upon oxidative stress. This process is reversible and accompanied by an increase in Rab5-positive endosomes in contact with mitochondria. Interestingly, activation of Rab5 on mitochondria depends on the Rab5-GEF ALS2/Alsin, encoded by a gene mutated in amyotrophic lateral sclerosis (ALS). Alsin-deficient human-induced pluripotent stem cell-derived spinal motor neurons are defective in relocating Rab5 to mitochondria and display increased susceptibility to oxidative stress. These findings define a novel pathway whereby Alsin catalyzes the assembly of the Rab5 endocytic machinery on mitochondria. Defects in stress-sensing by endosomes could be crucial for mitochondrial quality control during the onset of ALS.

The inside of a human cell is divided into compartments called organelles, which are surrounded by membranes. Each organelle plays a specific role in keeping the cell healthy and also has unique mix of molecular markers on its surface. These markers allow other molecules to identify the different organelles, meaning that specific organelles can communicate with each other and coordinate their activities. One way that organelles can do this is via so-called membrane contact sites, which are small areas where the compartments’ outer membranes come close together.

Mitochondria are organelles that release energy inside human cells. These compartments also work to keep the levels of toxic chemicals called reactive oxygen species in the cell within a safe range. This is important because cells can die if these levels become too high – a state known as oxidative stress. Mitochondria also communicate with other organelles called endosomes, which receive materials from the cell surface, sort and direct them to different destinations throughout the cell.

In many diseases affecting the nervous system, the mitochondria and endosomes in nerve cells do not work properly. These cells also have higher than normal levels of oxidative stress. Hsu et al. therefore wanted to find out if mitochondria and endosomes worked together to help cells to cope with this kind of stress.

Hsu et al. triggered oxidative stress in human cancer cells by exposing them first to a dye that stained the mitochondria and then to intense light. In stressed cells, a subset of endosomes called early endosomes formed many more membrane contact sites with mitochondria than in non-stressed cells. At the same time, the protein Rab5, usually found on early endosomes, relocated to the surface of mitochondria. Human cells previously engineered to produce larger than normal amounts of Rab5 were also more likely to survive oxidative stress. These experiments suggested that early endosomes cooperate with mitochondria, via Rab5, to protect cells from oxidative stress.

So, how is Rab5 relocated to mitochondria? Hsu et al. searched for activators of Rab5 and found that Alsin also migrated to mitochondria in stressed cells. The gene for Alsin is also mutated in amyotrophic lateral sclerosis (ALS), a degenerative nerve disorder that remains poorly understood. Next, Hsu et al. deleted the gene for Alsin from human stem cells growing in the laboratory and coaxed these cells into becoming nerve cells. Experiments with these cells showed that the absence of Alsin prevented Rab5 from moving to the mitochondria. Nerve cells lacking Alsin were also more susceptible to oxidative stress than normal cells.

Together, these findings show that early endosomes work with mitochondria to sense and ward off oxidative stress. They also reveal an unexpected connection between this process and a gene mutated in ALS. Further experiments are now needed to explore if problems with endosomes or mitochondria, and specifically with molecules like Alsin and Rab5, are responsible for other neurodegenerative disorders, like Parkinson’s disease and Huntington’s disease.

Molecular crosstalk between ferroptosis and apoptosis: emerging role of ER stress-induced p53-independent PUMA expression

Ferroptosis is a type of programmed cell death that depends on iron and is characterized by the accumulation of lipid peroxides. In the present study, we investigated the nature of the interplay between ferroptosis and other forms of cell death such as apoptosis. Human pancreatic cancer PANC-1 and BxPC-3 and human colorectal cancer HCT116 cells were treated with ferroptotic agents such as erastin and artesunate (ART) in combination with the apoptotic agent tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). We observed synergistic interaction of erastin or ART with TRAIL as determined by cell death assay, caspase activation, poly [ADP-ribose] polymerase 1 (PARP-1) cleavage, flow cytometry analysis, and lipid peroxidation assay. Moreover, erastin and ART induced endoplasmic reticulum (ER) stress and promoted p53 upregulated modulator of apoptosis (PUMA) expression via C/EBP-homologous protein (CHOP). Synergy of erastin/ART and TRAIL was abolished in PUMA-deficient HCT116 cells and CHOP-deficient mouse embryonic fibroblasts, but not in p53-deficient HCT116 cells. The results suggest the involvement of the p53-independent CHOP/PUMA axis in response to ferroptosis inducers, which may play a key role in ferroptotic agent-mediated sensitization to TRAIL-induced apoptosis.

The Coexistence of Hypertension and Ovariectomy Additively Increases Cardiac Apoptosis

To investigate whether the coexistence of hypertension and ovariectomy will increase cardiac Fas receptor and mitochondrial-dependent apoptotic pathways, histopathological analysis, the TUNEL assay and Western blotting were performed on the excised hearts from three groups of female spontaneously hypertensive rats (SHR), which were divided into a sham-operated group (SHR-Sham), bilaterally ovariectomized group (SHR-OVX) and normotensive Wistar Kyoto rats (WKY). Compared with the WKY group, the SHR-Sham group exhibited decreased protein levels of ERα, ERβ, p-Akt/Akt, Bcl-2, Bcl-xL and p-Bad and decreased further in the SHR-OVX group, as well as protein levels of t-Bid, Bak, Bad, Bax, cytochrome c, activated caspase-9 and activated caspase-3 (mitochondria-dependent apoptosis) increased in the SHR-Sham group and increased further in the SHR-OVX group. Compared with the WKY group, protein levels of Fas ligand, TNF-α, Fas death receptors, TNFR1, FADD and activated caspase-8 (Fas receptor-dependent apoptosis) increased in the SHR-Sham group, but did not increase in the SHR-OVX group, except Fas ligand and TNF-α. The coexistence of hypertension and ovariectomy attenuated the estrogen receptor survival pathway and appeared to additively increase the cardiac mitochondria-dependent, but not the Fas receptor-dependent apoptosis pathway, which might provide one possible mechanism for the development of cardiac abnormalities in hypertensive postmenopausal women.

Bcl-2 proteins bid and bax form a network to permeabilize the mitochondria at the onset of apoptosis

The most critical step in the initiation of apoptosis is the activation of the Bcl-2 family of proteins to oligomerize and permeabilize the outer-mitochondrial membrane (OMM). As this step results in the irreversible release of factors that enhance cellular degradation, it is the point of no return in programmed cell death and would be an ideal therapeutic target. However, the arrangement of the Bcl-2 proteins in the OMM during permeabilization still remains unknown. It is also unclear whether the Bcl-2 protein, Bid, directly participates in the formation of the oligomers in live cells, even though it is cleaved and translocates to the OMM at the initiation of apoptosis. Therefore, we utilized confocal microscopy to measure Förster resonance energy transfer (FRET) efficiencies in live cells to determine the conformation(s) and intermolecular contacts of Bid within these Bcl-2 oligomers. We found that Bid adopts an extended conformation, which appears to be critical for its association with the mitochondrial membrane. This conformation is also important for intermolecular contacts within the Bid oligomer. More importantly for the first time, direct intermolecular contacts between Bid and Bax were observed, thereby, confirming Bid as a key component of these oligomers. Furthermore, the observed FRET efficiencies allowed us to propose an oligomeric arrangement of Bid, Bax, and possibly other members of the Bcl-2 family of proteins that form a self-propagating network that permeabilizes the OMM.

Onto better TRAILs for cancer treatment

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), also known as Apo-2 ligand (Apo2L), is a member of the TNF cytokine superfamily. By cross-linking TRAIL-Receptor (TRAIL-R) 1 or TRAIL-R2, also known as death receptors 4 and 5 (DR4 and DR5), TRAIL has the capability to induce apoptosis in a wide variety of tumor cells while sparing vital normal cells. The discovery of this unique property among TNF superfamily members laid the foundation for testing the clinical potential of TRAIL-R-targeting therapies in the cancer clinic. To date, two of these therapeutic strategies have been tested clinically: (i) recombinant human TRAIL and (ii) antibodies directed against TRAIL-R1 or TRAIL-R2. Unfortunately, however, these TRAIL-R agonists have basically failed as most human tumors are resistant to apoptosis induction by them. It recently emerged that this is largely due to the poor agonistic activity of these agents. Consequently, novel TRAIL-R-targeting agents with increased bioactivity are currently being developed with the aim of rendering TRAIL-based therapies more active. This review summarizes these second-generation novel formulations of TRAIL and other TRAIL-R agonists, which exhibit enhanced cytotoxic capacity toward cancer cells, thereby providing the potential of being more effective when applied clinically than first-generation TRAIL-R agonists.

Altered Traffic of Cardiolipin during Apoptosis: Exposure on the Cell Surface as a Trigger for "Antiphospholipid Antibodies"

Apoptosis has been reported to induce changes in the remodelling of membrane lipids; after death receptor engagement, specific changes of lipid composition occur not only at the plasma membrane, but also in intracellular membranes. This paper focuses on one important aspect of apoptotic changes in cellular lipids, namely, the redistribution of the mitochondria-specific phospholipid, cardiolipin (CL). CL predominantly resides in the inner mitochondrial membrane, even if the rapid remodelling of its acyl chains and the subsequent degradation occur in other membrane organelles. After death receptor stimulation, CL appears to concentrate into mitochondrial “raft-like” microdomains at contact sites between inner and outer mitochondrial membranes, leading to local oligomerization of proapoptotic proteins, including Bid. Clustering of Bid in CL-enriched contacts sites is interconnected with pathways of CL remodelling that intersect membrane traffic routes dependent upon actin. In addition, CL association with cytoskeleton protein vimentin was observed. Such novel association also indicated that CL molecules may be expressed at the cell surface following apoptotic stimuli. This observation adds a novel implication of biomedical relevance. The association of CL with vimentin at the cell surface may represent a “new” target antigen in the context of the apoptotic origin of anti-vimentin/CL autoantibodies in Antiphospholipid Syndrome.

The proapoptotic protein tBid forms both superficially bound and membrane-inserted oligomers

Bid is a proapopotic activator protein of the Bcl-2 family that plays a pivotal role in controlling mitochondrial outer membrane permeabilization during apoptosis. Here, we characterized the interaction of fluorescently labeled truncated Bid (tBid) with a mitochondria-like supported lipid bilayer at the single-molecule level. The proteins observed at the membrane exhibited a very wide range of mobility. Confocal images of the membrane displayed both diffraction-limited Gaussian spots and horizontal streaks, corresponding to immobile and mobile tBid species, respectively. We observed 1), fast-diffusing proteins corresponding to a loosely, probably electrostatically bound state; 2), slowly diffusing proteins, likely corresponding to a superficially inserted state; and 3), fully immobilized proteins, suggesting a fully inserted state. The stoichiometry of these proteins was determined by normalizing their fluorescence intensity by the brightness of a tBid monomer, measured separately using fluorescence fluctuation techniques. Strikingly, the immobile species were found to be mainly tetramers and higher, whereas the mobile species had on average a significantly lower stoichiometry. Taken together, these results show that as soluble Bid progresses toward a membrane-inserted state, it undergoes an oligomerization process similar to that observed for Bax.

Structural insights of tBid, the caspase-8-activated Bid, and its BH3 domain

The Bcl-2 family proteins regulate mitochondria-mediated apoptosis through intricate molecular mechanisms. One of the pro-apoptotic proteins, tBid, can induce apoptosis by promoting Bax activation, Bax homo-oligomerization, and mitochondrial outer membrane permeabilization. Association of tBid on the mitochondrial outer membrane is key to its biological function. Therefore knowing the conformation of tBid on the membrane will be the first step toward understanding its crucial role in triggering apoptosis. Here, we present NMR characterization of the structure and dynamics of human tBid in 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-RAC-(1-glycerol)] micelles. Our data showed that tBid is monomeric with six well defined α-helices in the micelles. Compared with the full-length Bid structure, a longer flexible loop between tBid helix α4 and α5 was observed. Helices in tBid do not pack into a compact-fold but form an extended structure with a C-shape configuration in the micelles. All six tBid helices were shown to interact with LPPG micelles, with helix α6 and α7 being more embedded. Of note, the BH3-containing helix α3, which was previously believed to be exposed above the membrane surface, is also membrane associated, suggesting an "on the membrane" binding mode for tBid interaction with Bax. Our data provided structural details on the membrane-associated state of tBid and the functional implications of its membrane-associated BH3 domain.

Regulation of tumor necrosis factor-α-induced microvascular endothelial cell hyperpermeability by recombinant B-cell lymphoma-extra large

BACKGROUND

Tumor necrosis factor-α (TNF-α), a cytotoxic cytokine, induces endothelial cell barrier dysfunction and microvascular hyperpermeability, leading to tissue edema, a hallmark of traumatic injuries. The objective of the present study was to determine whether B-cell lymphoma-extra large (Bcl-xL), an antiapoptotic protein, would regulate and protect against TNF-α-mediated endothelial cell barrier dysfunction and microvascular hyperpermeability.

METHODS

Rat lung microvascular endothelial cells were grown as monolayers on Transwell membranes, and fluorescein isothiocyanate-bovine albumin flux (5 mg/mL) across the monolayer was measured fluorometrically to indicate changes in monolayer permeability. The rat lung microvascular endothelial cell adherens junctional integrity and actin cytoskeleton was studied using β-catenin immunofluorescence and rhodamine phalloidin dye, respectively. Pretreatment of caspase-8 inhibitor (Z-IETD-FMK, 100 μM) for 1 hour and transfection of Bcl-2-homology domain 3-interacting domain death agonist small interfering RNA (10 μM) for 48 hours were performed to study their respective effects on TNF-α-induced (10 ng/mL; 1-hour treatment) monolayer permeability. Recombinant Bcl-xL protein (2.5 μg/ml) was transfected in rat lung microvascular endothelial cells for 1 hour, and its effect on permeability was demonstrated using a permeability assay. Caspase-3 activity was assayed fluorometrically.

RESULTS

Z-IETD-FMK pretreatment protected the adherens junctions and decreased TNF-α-induced monolayer hyperpermeability. Bcl-2-homology domain 3-interacting domain death agonist small interfering RNA transfection attenuated the TNF-α-induced increase in monolayer permeability. Recombinant Bcl-xL protein showed protection against TNF-α-induced actin stress fiber formation, an increase in caspase-3 activity, and monolayer hyperpermeability.

CONCLUSIONS

Our results have demonstrated the protective effects of recombinant Bcl-xL protein against TNF-α-induced endothelial cell adherens junction damage and microvascular endothelial cell hyperpermeability. These findings support the potential for Bcl-xL-based drug development against microvascular hyperpermeability and tissue edema.

Programmed necrosis and autophagy in immune function

It has long been known that apoptosis is vital to the generation and maintenance of proper adaptive immune function. An example is the essential requirement for apoptotic signaling during the generation of self-tolerant lymphocytes: the apoptotic death of B and T cells with overt autoreactivity is essential to central tolerance. More recently, the contributions of additional processes including cellular autophagy and programmed necrosis have been implicated in controlling both innate and adaptive immune functions. Evidence has been provided to demonstrate that the death of cells following ligation of death receptors (DRs), a subfamily of cell surface molecules related to tumor necrosis factor receptor 1, is not exclusively the domain of caspase-dependent apoptosis. In cells lacking the capacity to activate caspase-8 following DR ligation, cell death instead occurs via programmed necrosis, or as it has been recently termed, 'necroptosis'. This death process depends on RIP1 and RIP3, serine/threonine kinases that are recruited by DRs, and likely by other cellular signals including DNA damage and antigen receptor ligation. The generation of RIP1/RIP3 containing 'necrosomes' activates downstream necroptotic signaling that ultimately targets cellular energetic metabolism. Also related to cellular metabolic regulation, cellular autophagy has also been found to play unique and important roles in immunity. In this review, we describe the roles of necroptosis and autophagy in innate and adaptive immunity and speculate on the intriguing interplay between these two cellular processes.

Endosomes and lysosomes are involved in early steps of Tl(III)-mediated apoptosis in rat pheochromocytoma (PC12) cells

The mechanisms that mediate thallium (Tl) toxicity are still not completely understood. The exposure of rat pheochromocytoma (PC12) cells to Tl(I) or Tl(III) activates both mitochondrial (Tl(I) and Tl(III)) and extrinsic (Tl(III)) pathways of apoptosis. In this work we evaluated the hypothesis that the effects of Tl(III) may be mediated by the damage to lysosomes, where it might be incorporated following the route of iron uptake. PC12 cells exposed for 3 h to 100 μM Tl(III) presented marked endosomal acidification, effect that was absent when cells were incubated in a serum-free medium and that was fully recovered when the latter was supplemented with transferrin. After 6 h of incubation the colocalization of cathepsins D and B with the lysosomal marker Lamp-1 was decreased together with an increase in the total activity of the enzymes. A permanent damage to lysosomes after 18 h of exposure was evidenced from the impairment of acridine orange uptake. Cathepsin D caused the cleavage of pro-apoptotic protein BID that is involved in the activation of the intrinsic pathway of apoptosis. Supporting that, BID cleavage and the activation of caspase 3 by Tl(III) were fully prevented when cells were preincubated with cathepsin D inhibitor (pepstatin A) and only partially prevented when cathepsin B inhibitor (E64d) was used. None of these inhibitors affected BID cleavage or caspase 3 activation in Tl(I)-treated cells. Together, experimental results support the role of Tl(III) uptake by the acidic cell compartments and their involvement in the early steps of Tl(III)-mediated PC12 cells apoptosis.

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.

Sorafenib and pemetrexed toxicity in cancer cells is mediated via SRC-ERK signaling

The present studies sought to further understand how the anti-folate pemetrexed and the multi-kinase inhibitor sorafenib interact to kill tumor cells. Sorafenib activated SRC, and via SRC the drug combination activated ERK1/2. Expression of dominant negative SRC or dominant negative MEK1 abolished drug-induced ERK1/2 activation, together with drug-induced autophagy, acidic lysosome formation, and tumor cell killing. Protein phosphatase 2A is an important regulator of the ERK1/2 pathway. Fulvestrant resistant MCF7 cells expressed higher levels of the PP2A inhibitor SET/I2PP2A, had lower endogenous PP2A activity, and had elevated basal ERK1/2 activity compared with their estrogen dependent counterparts. Overexpression of I2PP2A blocked drug-induced activation of ERK1/2 and tumor cell killing. PP2A can be directly activated by ceramide and SET/I2PP2A can be inhibited by ceramide. Inhibition of the de novo ceramide synthase pathway blocked drug-induced ceramide generation, PP2A activation and tumor cell killing. Collectively these findings demonstrate that ERK1/2 plays an essential role downstream of SRC in pemetrexed and sorafenib lethality and that PP2A plays an important role in regulating this process.

Infection of neutrophil granulocytes with Leishmania major activates ERK 1/2 and modulates multiple apoptotic pathways to inhibit apoptosis

Neutrophil granulocytes provide the first line of defense against bacterial, fungal, and parasitic infections. They phagocytose and kill many invading pathogens. Certain pathogenic microorganisms such as the intracellular protozoan parasite Leishmania major (L. major) can survive inside neutrophils. Mature neutrophils have a very short life span due to spontaneous apoptosis. Previously, we have reported that infections with L. major are able to delay spontaneous apoptosis. In the present study, we addressed the underlying mechanisms of regulation of both extrinsic and intrinsic apoptosis. We show that interaction with L. major transiently activates ERK1/2 phosphorylation. Pharmacological inhibition of ERK1/2 phosphorylation reversed the apoptosis delay. Moreover, infection leads to the enhanced and sustainable expression of the anti-apoptotic proteins Bcl-2 and Bfl-1, respectively. As downstream events, the release of cytochrome c from mitochondria and processing of caspase-6 were inhibited. We also confirm that infection with L. major results in reduced FAS expression on the surface of neutrophils. The presented data indicate that infection with L. major affects both intrinsic as well as extrinsic pathways of neutrophil apoptosis. Enhanced life span of host neutrophils enables the parasite to survive within neutrophils.

Molecular mechanisms underlying delayed apoptosis in neutrophils from multiple trauma patients with and without sepsis

Delayed neutrophil apoptosis and overshooting neutrophil activity contribute to organ dysfunction and subsequent organ failure in sepsis. Here, we investigated apoptotic signaling pathways that are involved in the inhibition of spontaneous apoptosis in neutrophils isolated from major trauma patients with uneventful outcome as well as in those with sepsis development. DNA fragmentation in peripheral blood neutrophils showed an inverse correlation with the organ dysfunction at d 10 after trauma in all patients, supporting the important role of neutrophil apoptosis regulation for patient's outcome. The expression of the antiapoptotic Bcl-2 protein members A1 and Mcl-1 were found to be diminished in the septic patients at d 5 and d 10 after trauma. This decrease was also linked to an impaired intrinsic apoptosis resistance, which has been previously shown to occur in neutrophils during systemic inflammation. In patients with sepsis development, delayed neutrophil apoptosis was found to be associated with a disturbed extrinsic pathway, as demonstrated by reduced caspase-8 activity and Bid truncation. Notably, the expression of Dad1 protein, which is involved in protein N-glycosylation, was significantly increased in septic patients at d 10 after trauma. Taken together, our data demonstrate that neutrophil apoptosis is regulated by both the intrinsic and extrinsic pathway, depending on patient's outcome. These findings might provide a molecular basis for new strategies targeting cell death pathways in apoptosis-resistant neutrophils during systemic inflammation.

Dynamic interaction of cBid with detergents, liposomes and mitochondria

The BH3-only protein Bid plays a key role in the induction of mitochondrial apoptosis, but its mechanism of action is still not completely understood. Here we studied the two main activation events of Bid: Caspase-8 cleavage and interaction with the membrane bilayer. We found a striking reversible behaviour of the dissociation-association events between the Bid fragments p15 and p7. Caspase-8 cleavage does not induce per se separation of the two Bid fragments, which remain in a stable complex resembling the full length Bid. Detergents trigger a complete dissociation, which can be fully reversed by detergent removal in a range of protein concentrations from 100 µM down to 500 nM. Incubation of cBid with cardiolipin-containing liposomes leads to partial dissociation of the complex. Only p15 (tBid) fragments are found at the membrane, while p7 shows no tendency to interact with the bilayer, but complete removal of p7 strongly increases the propensity of tBid to become membrane-associated. Despite the striking structural similarities of inactive Bid and Bax, Bid does not form oligomers and reacts differently in the presence of detergents and membranes, highlighting clear differences in the modes of action of the two proteins. The partial dissociation of cBid triggered by the membrane is suggested to depend on the strong and specific interaction between p15 and p7. The reversible disassembly and re-assembly of the cBid molecules at the membrane was as well proven by EPR using spin labeled cBid in the presence of isolated mitochondria. The observed dynamic dissociation of the two Bid fragments could allow the assistance to the pore-forming Bax to occur repeatedly and may explain the proposed “hit-and-run" mode of action of Bid at the bilayer.

Mitochondrial carrier homolog 2 (MTCH2): the recruitment and evolution of a mitochondrial carrier protein to a critical player in apoptosis

Recent studies report mitochondrial carrier homolog 2 (MTCH2) as a novel and uncharacterized protein that acts as a receptor-like protein for the truncated BH3-interacting domain death agonist (tBID) protein in the outer membrane of mitochondria. These studies, using mouse embryonic stem cells and fibroblasts as well as mice with a conditional knockout of MTCH2 in the liver, showed that deletion of MTCH2 hindered recruitment of tBID to the mitochondria with subsequent reductions in the activation of pro-apoptotic proteins, mitochondrial outer membrane permeabilization and apoptosis. Sequence analysis shows that MTCH2 is present in all examined multicellular Metazoa as well as unicellular Choanoflagellata, and is a highly derived member of the mitochondrial carrier family. Mitochondrial carriers are monomeric transport proteins that are usually found in the inner mitochondrial membrane, where they exchange small substrates between the mitochondrial matrix and intermembrane space. There are extensive differences between the protein sequences of MTCH2 and other mitochondrial carriers that may explain the ability of MTCH2 to associate with tBID and thus its role in apoptosis. We review the experimental evidence for the role of MTCH2 in apoptosis and suggest that the original transport function of the ancestral MTCH2 mitochondrial carrier has been co-opted by the apoptotic machinery to provide a receptor and signaling mechanism.

Alteration in the apoptosis process of rat esophageal epithelium with hyperproliferation of indigenous bacteria under a physiological condition

The apoptosis process in rat esophageal epithelium was investigated using enzyme-immunohistochemistry and transmission electron microscopy. As a result, Fas and Fas-L were expressed in the epithelial cell membrane and cytoplasm from the stratum spinosum (SS) to the stratum granulosum (SG). No TNF-R1 show immunopositivity in the cell membranes. TNF-α and caspase-8 were not observed in any layer. Caspase-10, cleaved caspase-3, XIAP and DNase-1 were found in the epithelial cytoplasm from the SS to the SG, whereas Bid, Apaf-1 and cleaved caspase-9 were detected only in the SG. Cytochrome c was observed as cytoplasmic granular positivity from the stratum basale (SB) and altered into homogeneous immunopositivity in the SG. Bcl-2 and Bcl-X immunopositivity was detected in cytoplasm from the SB to the SG. Immunoreactions of Bak in the cytoplasm and Bax beneath the cell membrane were observed from the upper portion of the SS with increasing intensity toward the SG. In the sites with the hyperproliferation of indigenous bacteria, TNF-R1, TNF-α and caspase-8 were detected in the SG and the immunopositive intensities of Bid, Apaf-1 and cleaved caspase-9 were altered to be strong. Prominently swollen cells and decreased mitochondria were ultrastructurally confirmed in the uppermost layers of stratum corneum. These findings suggest that the Fas-Fas-L-interaction initially induces apoptosis through a mitochondria-independent pathway and secondarily through a mitochondria-dependent pathway, leading to eventual epithelial cell death in the rat esophageal epithelium. The bacterial stimuli probably enhance the mitochondria-dependent pathway through the TNF-R1-TNF-α interaction.

Effects of phospholipids on the functional regulation of tBID in membranes

The functional interplay between tBID and phospholipids was investigated in this study. The binding of tBID to model membranes was increased by an incorporation of phosphatidylserine (PS) into the liposomes. Using limited proteolysis and mass spectrometry, two peptide regions, which correspond to Ser(100)-Arg(114) and His(89)-Arg(114) in BID, revealed the specific PS-binding site. tBID also decreased the light scattering values of PS-containing liposomes and increased the leakage of fluorescent dye encapsulated in vesicles, which suggest that tBID reduces membrane integrity by fragmentation. The membrane fragmentation by tBID was also observed using confocal and transmission electron microscopy. The activity of tBID paralleled results that were obtained with cardiolipin (CL)-containing membranes. However, other anionic phospholipids had little effect. CL- and PS-induced conformational changes of tBID were observed by circular dichroism and intrinsic fluorescence. CL and PS also stimulated the insertion of BID into lipid monolayers. tBID stimulated the leakage of Ca(2+) from purified microsomes and mitochondria in a protein concentration-dependent manner. In contrast, BID showed significantly reduced effects when compared to tBID in all of the experiments performed. These results suggest that tBID specifically interacts with PS as well as CL and decreases membrane integrity without the aid of other pro-apoptotic proteins.

The Role of BH3-Only Proteins in Tumor Cell Development, Signaling, and Treatment

Tumor cells have devised several strategies to block the mitochondrial pathway of apoptosis despite endogenous or pharmacological cues to die. This process of cell death proceeds through the coordinated regulation of multiple anti-apoptotic and pro-apoptotic BCL-2 family proteins that ultimately impinge on the integrity of the outer mitochondrial membrane. Once compromised, mitochondria release pro-apoptotic factors to promote caspase activation and the apoptotic phenotype. Within the BCL-2 family exists a subclass of pro-apoptotic members termed the BH3-only proteins, which directly and/or indirectly functionally regulate the remaining anti- and pro-apoptotic BCL-2 proteins to compromise mitochondria and engage apoptosis. The focus of this review is to discuss the cellular and pharmacological regulation of the BH3-only proteins to gain a better understanding of the signaling pathways and agents that regulate this class of proteins. As the BH3-only proteins increase cellular sensitivity to pro-apoptotic agents such as chemotherapeutics, numerous small-molecule BH3 mimetics have been developed and are currently in various phases of clinical trials. Toward the end of the review, the discovery and application of the small-molecule BH3 mimetics will be discussed.

Sorafenib enhances pemetrexed cytotoxicity through an autophagy-dependent mechanism in cancer cells

Pemetrexed (ALIMTA, Lilly) is a folate antimetabolite that has been approved by the U.S. Food and Drug Administration for the treatment of non-small cell lung cancer and has been shown to stimulate autophagy. In the present study, we sought to further understand the role of autophagy in response to pemetrexed and to test if combination therapy could enhance the level of toxicity through altered autophagy in tumor cells. The multikinase inhibitor sorafenib (Nexavar, Bayer), used in the treatment of renal and hepatocellular carcinoma, suppresses tumor angiogenesis and promotes autophagy in tumor cells. We found that sorafenib interacted in a greater than additive fashion with pemetrexed to increase autophagy and to kill a diverse array of tumor cell types. Tumor cell types that displayed high levels of cell killing after combination treatment showed elevated levels of AKT, p70 S6K, and/or phosphorylated mTOR, in addition to class III receptor tyrosine kinases such as platelet-derived growth factor receptor beta and VEGF receptors, known in vivo targets of sorafenib. In xenograft and in syngeneic animal models of mammary carcinoma and glioblastoma, the combination of sorafenib and pemetrexed suppressed tumor growth without deleterious effects on normal tissues or animal body mass. Taken together, the data suggest that premexetred and sorafenib act synergistically to enhance tumor killing via the promotion of a toxic form of autophagy that leads to activation of the intrinsic apoptosis pathway, and predict that combination treatment represents a future therapeutic option in the treatment of solid tumors.

Minute kinetics of proapoptotic proteins: BAX and Smac/DIABLO in living tumor cells revealed by homeostatic confocal microscopy

Traditional methods of visualization and analysis based on fixed cell populations treated with the drug for a different time give the limited possibility of time-sequence analysis. In time-lapse microscopy where the whole cell is observed regardless to intracellular structure, precise localization of events and differentiation between colocalization and overlapping of the fluorescence is impossible. Furthermore prolonged experiments with living cells increased the influence of improper environmental conditions. Homeostatic confocal microscopy gives an exceptional insight into minute pattern of changes occurring in the same living cell maintained in stable conditions during whole experimental period. It is built on a confocal system equipped with the homeostatic chamber providing constant, monitored heating and moisturized, CO(2)-enriched atmosphere during long period observations. In the present study 2D/time and 4D homeostatic confocal microscopy were applied for analysis of minute pattern of changes occurring at the mitochondria. The release of Smac/DIABLO from mitochondria in tumor cells under the apoptogenic stimulus, consist of two phases: the first immediately after drug administration, and the major second one after 15 min. Furthermore the time-pattern of BAX translocation to the mitochondria and Smac/DIABLO release coincide, suggesting that the release of Smac/DIABLO is correlated with BAX translocation to the mitochondria.

Bcl-2 proteins and mitochondria--specificity in membrane targeting for death

The localization and control of Bcl-2 proteins on mitochondria is essential for the intrinsic pathway of apoptosis. Anti-apoptotic Bcl-2 proteins reside on the outer mitochondrial membrane (OMM) and prevent apoptosis by inhibiting the activation of the pro-apoptotic family members Bax and Bak. The Bcl-2 subfamily of BH3-only proteins can either inhibit the anti-apoptotic proteins or directly activate Bax or Bak. How these proteins interact with each other, the mitochondrial surface and within the OMM are complex processes we are only beginning to understand. However, these interactions are fundamental for the transduction of apoptotic signals to mitochondria and the subsequent release of caspase activating factors into the cytosol. In this review we will discuss our knowledge of how Bcl-2 proteins are directed to mitochondria in the first place, a crucial but poorly understood aspect of their regulation. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Apoptosis-induced changes in mitochondrial lipids

Apoptosis is an active and tightly regulated form of cell death, which can also be considered a stress-induced process of cellular communication. Recent studies reveal that the lipid network within cells is involved in the regulation and propagation of death signalling. Despite the vast growth of our current knowledge on apoptosis, little is known of the specific role played by lipid molecules in the central event of apoptosis-the piercing of mitochondrial membranes. Here we review the information regarding changes in mitochondrial lipids that are associated with apoptosis and discuss whether they may be involved in the permeabilization of mitochondria to release their apoptogenic factors, or just lie downstream of this permeabilization leading to the amplification of caspase activation. We focus on the earliest changes that physiological apoptosis induces in mitochondrial membranes, which may derive from an upstream alteration of phospholipid metabolism that reverberates on the mitochondrial re-modelling of their characteristic lipid, cardiolipin. Hopefully, this review will lead to an increased understanding of the role of mitochondrial lipids in apoptosis and also help revealing new stress sensing mechanisms in cells. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

New insights into apoptosis signaling by Apo2L/TRAIL

Apoptosis ligand 2 tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) belongs to a small subset of proapoptotic protein ligands in the TNF superfamily. This subset, which also includes Fas ligand and TNF-alpha, can activate the extrinsic apoptotic cell death pathway on binding to cognate death receptors at the cell surface. Over the past 10 years, Apo2L/TRAIL has emerged as a promising candidate for cancer therapy, on the basis of its unique ability to trigger apoptosis in various types of cancer cells without significant toxicity toward normal cells. Herein, we review key advances in understanding the molecular events that control apoptosis signaling by Apo2L/TRAIL, which may aid in the development of cancer therapies based on the extrinsic apoptotic pathway.