Puma indirectly activates Bax to cause apoptosis in the absence of Bid or Bim

Recent advances and applications of peptide-agent conjugates for targeting tumor cells

BACKGROUND

Cancer, being a complex disease, presents a major challenge for the scientific and medical communities. Peptide therapeutics have played a significant role in different medical practices, including cancer treatment.

METHOD

This review provides an overview of the current situation and potential development prospects of anticancer peptides (ACPs), with a particular focus on peptide vaccines and peptide-drug conjugates for cancer treatment.

RESULTS

ACPs can be used directly as cytotoxic agents (molecularly targeted peptides) or can act as carriers (guiding missile) of chemotherapeutic agents and radionuclides by specifically targeting cancer cells. More than 60 natural and synthetic cationic peptides are approved in the USA and other major markets for the treatment of cancer and other diseases. Compared to traditional cancer treatments, peptides exhibit anticancer activity with high specificity and the ability to rapidly kill target cancer cells. ACP's target and kill cancer cells via different mechanisms, including membrane disruption, pore formation, induction of apoptosis, necrosis, autophagy, and regulation of the immune system. Modified peptides have been developed as carriers for drugs, vaccines, and peptide-drug conjugates, which have been evaluated in various phases of clinical trials for the treatment of different types of solid and leukemia cancer.

CONCLUSIONS

This review highlights the potential of ACPs as a promising therapeutic option for cancer treatment, particularly through the use of peptide vaccines and peptide-drug conjugates. Despite the limitations of peptides, such as poor metabolic stability and low bioavailability, modified peptides show promise in addressing these challenges. Various mechanism of action of anticancer peptides. Modes of action against cancer cells including: inducing apoptosis by cytochrome c release, direct cell membrane lysis (necrosis), inhibiting angiogenesis, inducing autophagy-mediated cell death and immune cell regulation.

Deregulation and epigenetic modification of BCL2-family genes cause resistance to venetoclax in hematologic malignancies

The BCL2 inhibitor venetoclax has been approved to treat different hematological malignancies. Because there is no common genetic alteration causing resistance to venetoclax in chronic lymphocytic leukemia (CLL) and B-cell lymphoma, we asked if epigenetic events might be involved in venetoclax resistance. Therefore, we employed whole-exome sequencing, methylated DNA immunoprecipitation sequencing, and genome-wide clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 screening to investigate venetoclax resistance in aggressive lymphoma and high-risk CLL patients. We identified a regulatory CpG island within the PUMA promoter that is methylated upon venetoclax treatment, mediating PUMA downregulation on transcript and protein level. PUMA expression and sensitivity toward venetoclax can be restored by inhibition of methyltransferases. We can demonstrate that loss of PUMA results in metabolic reprogramming with higher oxidative phosphorylation and adenosine triphosphate production, resembling the metabolic phenotype that is seen upon venetoclax resistance. Although PUMA loss is specific for acquired venetoclax resistance but not for acquired MCL1 resistance and is not seen in CLL patients after chemotherapy-resistance, BAX is essential for sensitivity toward both venetoclax and MCL1 inhibition. As we found loss of BAX in Richter's syndrome patients after venetoclax failure, we defined BAX-mediated apoptosis to be critical for drug resistance but not for disease progression of CLL into aggressive diffuse large B-cell lymphoma in vivo. A compound screen revealed TRAIL-mediated apoptosis as a target to overcome BAX deficiency. Furthermore, antibody or CAR T cells eliminated venetoclax resistant lymphoma cells, paving a clinically applicable way to overcome venetoclax resistance.

Proteasome and Neuroprotective Effect of Hyperbaric Oxygen Preconditioning in Experimental Global Cerebral Ischemia in Rats

Objectives

We investigated the involvement of the proteasome in the mechanism of preconditioning with hyperbaric oxygen (HBO-PC).

Methods

The experiments were performed on male Wistar rats subjected to a transient global cerebral ischemia of 5 min duration (2-vessel occlusion model) and preconditioned or not with HBO for 5 preceding days (1 h HBO at 2.5 atmosphere absolute [ATA] daily). In subgroups of preconditioned rats, the proteasome inhibitor MG132 was administered 30 min prior to each preconditioning session. Twenty-four hours and 7 days post-ischemia, after neurobehavioral assessment, the brains were collected and evaluated for morphological changes and quantitative immunohistochemistry of cell markers and apoptosis-related proteins.

Results

We observed reduced damage of CA1 pyramidal cells in the HBO preconditioned group only at 7 days post-ischemia. However, both at early (24 h) and later (7 days) time points, HBO-PC enhanced the tissue expression of 20S core particle of the proteasome and of the nestin, diminished astroglial reactivity, and reduced p53, rabbit anti-p53 upregulated modulator of apoptosis (PUMA), and rabbit anti-B cell lymphoma-2 interacting mediator of cell death (Bim) expressions in the hippocampus and cerebral cortex. HBO-PC also improved T-maze performance at 7 days. Proteasome inhibitor abolished the beneficial effects of HBO-PC on post-ischemic neuronal injury and functional impairment and reduced the ischemic alterations in the expression of investigated proteins.

Significance

Preconditioning with hyperbaric oxygen-induced brain protection against severe ischemic brain insult appears to involve the proteasome, which can be linked to a depletion of apoptotic proteins and improved regenerative potential.

Genome-wide mRNA profiling identifies X-box-binding protein 1 (XBP1) as an IRE1 and PUMA repressor

Accumulation of misfolded proteins in ER activates the unfolded protein response (UPR), a multifunctional signaling pathway that is important for cell survival. The UPR is regulated by three ER transmembrane sensors, one of which is inositol-requiring protein 1 (IRE1). IRE1 activates a transcription factor, X-box-binding protein 1 (XBP1), by removing a 26-base intron from XBP1 mRNA that generates spliced XBP1 mRNA (XBP1s). To search for XBP1 transcriptional targets, we utilized an XBP1s-inducible human cell line to limit XBP1 expression in a controlled manner. We also verified the identified XBP1-dependent genes with specific silencing of this transcription factor during pharmacological ER stress induction with both an N-linked glycosylation inhibitor (tunicamycin) and a non-competitive inhibitor of the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) (thapsigargin). We then compared those results to the XBP1s-induced cell line without pharmacological ER stress induction. Using next‐generation sequencing followed by bioinformatic analysis of XBP1-binding motifs, we defined an XBP1 regulatory network and identified XBP1 as a repressor of PUMA (a proapoptotic gene) and IRE1 mRNA expression during the UPR. Our results indicate impairing IRE1 activity during ER stress conditions accelerates cell death in ER-stressed cells, whereas elevating XBP1 expression during ER stress using an inducible cell line correlated with a clear prosurvival effect and reduced PUMA protein expression. Although further studies will be required to test the underlying molecular mechanisms involved in the relationship between these genes with XBP1, these studies identify a novel repressive role of XBP1 during the UPR.

Gene Expression Profiling to Delineate the Anticancer Potential of a New Alkaloid Isopicrinine From Rhazya stricta

Background:Rhazya stricta has been used as a folkloric medicinal herb for treating various diseases such as diabetes, inflammatory disorders, and sore throat. Several studies have revealed the potential of this plant as an important source of phytochemicals with anticancer properties. Objective: The present study was designed to isolate a novel anticancer compound from Rhazya stricta and elucidate its mechanism of action using genomics approach. Methods:Rhazya stricta leaves extract was prepared, and several alkaloids were purified and characterized. These alkaloids were screened for their anticancer potential. One of the alkaloids, termed as isopicrinine, showed efficient cytotoxicity against MCF7 breast cancer cell line and was selected for further analysis. RNA-Seq transcription profiling was conducted to identify the affected genes and cellular pathways in MCF7 cells after treatment with isopicrinine alkaloid. Results: In vitro studies revealed that newly identified isopicrinine alkaloid possess efficient anticancer activity. Exposure of MCF7 cells with isopicrinine affected the expression of various genes involved in p53 signaling pathway. One of the crucial proapoptotic genes, significantly upregulated in MCF7 after exposure to alkaloid, was PUMA (p53 upregulated modulator of apoptosis), which is involved in p53-dependent and -independent apoptosis. Moreover, exposure of sublethal dose of isopicrinine alkaloid in breast cancer cell line led to the downregulation of survivin, which is involved in negative regulation of apoptosis. Besides, several genes involved in mitosis and cell proliferation were significantly downregulated. Conclusion: In this article, we report the determination of a new alkaloid isopicrinine from the aerial parts of Rhazya stricta with anticancer property. This compound has the potential to be developed as a drug for curing cancer.

BECN1 modulates hematopoietic stem cells by targeting Caspase-3-GSDME-mediated pyroptosis

Hematopoietic stem cells (HSCs) maintain the blood system throughout the lifespan. However, the molecular mechanism maintaining HSC character remains not fully understood. In this study, we observed that the targeted deletion of Becn1 disrupts the blood system and impairs the reconstitution capacity of HSCs. Interestingly, Becn1 deletion did not lead to dysfunction of autophagy in HSCs, indicating a non-classical role of BECN1 in regulating HSCs function. While we observed the increase of Caspase-3-GSDME-mediated pyroptosis in Becn1 deficient hematopoietic stem and progenitor cells. Forced expression of the full-length GSDME compromises the function of HSCs. In brief, we identified a novel role of Becn1 in modulating HSCs by regulating pyroptosis, but not through autophagy. This study provides a new link between BECN1-Caspase-3-GSDME signaling and HSC maintenance.

Application of Mito-Priming to Generate BCL-2 Addicted Cells

The majority of apoptotic stimuli trigger cell death through the mitochondrial pathway of apoptosis. Invariably, mitochondrial apoptosis requires engagement of mitochondrial outer membrane permeabilization or MOMP to initiate cell death. We have developed a new method, called mito-priming, that allows for rapid and synchronous induction of mitochondrial apoptosis in an on-target manner. Mito-priming uses coexpression of pro- and antiapoptotic Bcl-2 proteins to render cells sensitive to the addition of Bcl-2 targeting BH3-mimetic drugs. This chapter describes how to design mito-priming constructs and apply them to generate mito-primed lines. Second, we describe how to validate cell death sensitivity of mito-primed lines using different methods. Finally, we describe how to generate MOMP-resistant cell lines, using CRISPR-Cas9 mediated deletion of BAX and BAK. Facilitating the investigation of mitochondrial apoptosis, mito-priming provides a clean, robust way to induce mitochondrial apoptosis both in vitro and in vivo.

Role of pigment epithelium-derived factor (PEDF) on arsenic-induced neuronal apoptosis

Chronic exposure to high levels of arsenic is closely associated with nervous system disorders that harm learning, memory, and intelligence. Studies have shown that the primary characteristic of brain damage is neuronal apoptosis. Arsenic induces apoptosis in a variety of nerve cells. Therefore, substance that inhibit apoptosis promise to mitigate arsenic toxicity. Pigment epithelium-derived factor (PEDF) is widely distributed in brain tissues and has various effects on neurons, including induction of apoptosis. Our previous study suggested that PEDF might augment arsenic-induced apoptosis in rat brains. In this study of 151 adults with normal, mild, moderate, and high exposure to arsenic, the measured serum PEDF levels were 15.46 ± 5.87 ng/mL, 17.33 ± 8.22 ng/mL, 19.43 ± 9.51 ng/mL and 21.65 ± 14.46 ng/mL, respectively. Multiple linear regression analysis revealed an independent positive correlation between serum PEDF levels and arsenic exposure in drinking water. To study the underlying mechanism of arsenic-induced apoptosis, we exposed PEDF-transfected PC12 cells to NaAsO₂. We discovered that NaAsO_2--induced mitochondrial apoptosis was enhanced in cells that over expressed PEDF. Moreover, p53 up regulated modulator of apoptosis (PUMA) gene and B-cell lymphoma 2 (Bcl-2) protein were primary factors in the progression of arsenic-induced apoptosis. Taken together, our results suggest that PEDF inhibition might mitigate arsenic toxicity to nerve cells.

Nano-chemotherapeutic efficacy of (-) -epigallocatechin 3-gallate mediating apoptosis in A549 cells: Involvement of reactive oxygen species mediated Nrf2/Keap1signaling

Chemotherapeutic drugs exert systemic toxicity in lung cancer cells and therefore novel treatment strategies are warranted. Epigallocatechin 3-gallate (EGCG), though possessing beneficial effects in alleviating cancer, its effect has been limited due to ineffective systemic delivery, toxicity and bioavailability. To attain the maximum therapeutic response of EGCG, we have synthesized bovine serum albumin (BSA) encapsulated magnetite nanoparticle (MNPs) loaded with EGCG (nano EGCG). The synthesized nano EGCG was characterized using HR-TEM, XRD and FT-IR. Cytotoxicity analysis of BSA-MNP and nano EGCG using flow cytometry was evaluated in lung adenocarcinoma A549 cells. The effect of native and nano EGCG modulating apoptosis and Nrf2/Keap1 signaling was analysed. Nano EGCG exhibited increased ROS/RNS levels and decreased mitochondrial membrane potential, as evaluated by DCFH and JC1 staining, respectively. Expression of pro-apoptotic Bcl-2 family proteins (Bcl-2, Bax, Bak, Bim and Puma) was evaluated. This study demonstrates that native and nano EGCG induces apoptosis through the involvement of ROS leading to loss in mitochondrial membrane potential. EGCG exhibited an increased expression of Nrf2 and Keap1 that could regulate apoptosis in A549 cells. This study, for the first time reveals the potential of BSA-MNPs loaded EGCG as drug target and renders better efficacy against lung cancer cells.

Cycloheximide Can Induce Bax/Bak Dependent Myeloid Cell Death Independently of Multiple BH3-Only Proteins

Apoptosis mediated by Bax or Bak is usually thought to be triggered by BH3-only members of the Bcl-2 protein family. BH3-only proteins can directly bind to and activate Bax or Bak, or indirectly activate them by binding to anti-apoptotic Bcl-2 family members, thereby relieving their inhibition of Bax and Bak. Here we describe a third way of activation of Bax/Bak dependent apoptosis that does not require triggering by multiple BH3-only proteins. In factor dependent myeloid (FDM) cell lines, cycloheximide induced apoptosis by a Bax/Bak dependent mechanism, because Bax^-/-Bak^-/- lines were profoundly resistant, whereas FDM lines lacking one or more genes for BH3-only proteins remained highly sensitive. Addition of cycloheximide led to the rapid loss of Mcl-1 but did not affect the expression of other Bcl-2 family proteins. In support of these findings, similar results were observed by treating FDM cells with the CDK inhibitor, roscovitine. Roscovitine reduced Mcl-1 abundance and caused Bax/Bak dependent cell death, yet FDM lines lacking one or more genes for BH3-only proteins remained highly sensitive. Therefore Bax/Bak dependent apoptosis can be regulated by the abundance of anti-apoptotic Bcl-2 family members such as Mcl-1, independently of several known BH3-only proteins.

Homocysteine induces PUMA-mediated mitochondrial apoptosis in SH-SY5Y cells

Previous studies have reported that homocysteine induced endoplasmic reticulum (ER) stress in neuronal cells, proposing the underlying mechanism by which it could induce neurotoxicity. Induction of pro-apoptotic transcription factor C/EBP homologous protein (CHOP) and activation of caspase-4 by calpain have been suggested to be an important route in inducing apoptosis in response to ER stress. In this study, we investigated the molecular pathway of homocysteine-induced apoptosis in caspase-4 deficient SH-SY5Y human neuroblastoma cells. Homocysteine significantly increased mRNA levels of CHOP and p53, resulting in the upregulation of their downstream target gene, p53 up-regulated modulator of apoptosis (PUMA). In cells treated with homocysteine, Bcl-2-associated X protein (BAX) protein levels, cytochrome c release from the mitochondria, and caspase-9 activation were significantly increased. Consistently, a caspase-9 inhibitor significantly alleviated homocysteine-induced cytotoxicity. Significantly lower BAX mRNA levels and caspase-9 activation were observed in cells transfected with siRNA for PUMA. Taken together, our findings suggest that PUMA would be involved in the possible crosstalk between the ER and the mitochondria in the homocysteine-induced apoptosis of caspase-4 deficient SH-SY5Y cells.

Bok is a genuine multi-BH-domain protein that triggers apoptosis in the absence of Bax and Bak

The pro-apoptotic multidomain Bcl-2 proteins Bax and Bak (also known as BAK1) are considered the gatekeepers of the intrinsic pathway of apoptosis by triggering the mitochondrial release of cytochrome c The role of the third Bax- and Bak-homologous multidomain protein Bok, however, is still unresolved. As cells doubly deficient for Bax and Bak are largely resistant to various apoptotic stimuli, it has been proposed that Bok is either dispensable for apoptosis or that its role is dependent on Bax and Bak. Here, we demonstrate, in several cell systems, that Bok efficiently induces cytochrome c release and apoptosis even in the complete absence of both Bak and Bax. Moreover, modulation of endogenous Bok levels affects the apoptosis response. By RNA interference and targeted deletion of the Bok gene, we demonstrate that Bok can significantly influence the apoptotic response to chemotherapeutic drugs in ovarian carcinoma cells. Hence, our results not only establish Bok as a Bak- and Bax-independent apoptosis inducer, but also suggest a potential impact of Bok expression in ovarian cancer therapy.

Mito-priming as a method to engineer Bcl-2 addiction

Most apoptotic stimuli require mitochondrial outer membrane permeabilization (MOMP) in order to execute cell death. As such, MOMP is subject to tight control by Bcl-2 family proteins. We have developed a powerful new technique to investigate Bcl-2-mediated regulation of MOMP. This method, called mito-priming, uses co-expression of pro- and anti-apoptotic Bcl-2 proteins to engineer Bcl-2 addiction. On addition of Bcl-2 targeting BH3 mimetics, mito-primed cells undergo apoptosis in a rapid and synchronous manner. Using this method we have comprehensively surveyed the efficacy of BH3 mimetic compounds, identifying potent and specific MCL-1 inhibitors. Furthermore, by combining different pro- and anti-apoptotic Bcl-2 pairings together with CRISPR/Cas9-based genome editing, we find that tBID and PUMA can preferentially kill in a BAK-dependent manner. In summary, mito-priming represents a facile and robust means to trigger mitochondrial apoptosis.

Data on the DNA damaging and mutagenic potential of the BH3-mimetics ABT-263/Navitoclax and TW-37

Unfortunately, the mutagenic activities of chemotherapy and radiotherapy can provoke development of therapy-induced malignancies in cancer survivors. Non-mutagenic anti-cancer therapies may be less likely to trigger subsequent malignant neoplasms. Here we present data regarding the DNA damaging and mutagenic potential of two drugs that antagonize proteins within the Bcl-2 family: ABT-263/Navitoclax and TW-37. Our data reveal that concentrations of these agents that stimulated Bax/Bak-dependent signaling provoked little DNA damage and failed to trigger mutations in surviving cells. The data supplied in this article is related to the research work entitled "Inhibition of Bcl-2 or IAP proteins does not provoke mutations in surviving cells" [1].

Mechanisms of microRNAs in regulation of apoptosis in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) has become a common liver disease in Western developed countries, and the prevalence is also continuously increasing in developing countries. NAFLD comprises a spectrum of disease stages, in which simple steatosis is a benign course and steatohepatitis can progress to liver fibrosis, cirrhosis and even hepatocellular carcinoma. The pathogenesis of NAFLD has not been fully understood. As noncoding RNA molecules, microRNAs (miRNAs) regulate the pathophysiological processes including development, metabolism, cell proliferation, differentiation, apoptosis and carcinogenesis, as well as the pathogenesis of NAFLD. miRNAs extensively participate in insulin resistance, lipid metabolic disorder, endoplasmic injury and cell apoptosis in NAFLD. This review highlights the roles of miRNAs in cell apoptosis in NAFLD.

Inhibition of Bcl-2 or IAP proteins does not provoke mutations in surviving cells

Chemotherapy and radiotherapy can cause permanent damage to the genomes of surviving cells, provoking severe side effects such as second malignancies in some cancer survivors. Drugs that mimic the activity of death ligands, or antagonise pro-survival proteins of the Bcl-2 or IAP families have yielded encouraging results in animal experiments and early phase clinical trials. Because these agents directly engage apoptosis pathways, rather than damaging DNA to indirectly provoke tumour cell death, we reasoned that they may offer another important advantage over conventional therapies: minimisation or elimination of side effects such as second cancers that result from mutation of surviving normal cells. Disappointingly, however, we previously found that concentrations of death receptor agonists like TRAIL that would be present in vivo in clinical settings provoked DNA damage in surviving cells. In this study, we used cell line model systems to investigate the mutagenic capacity of drugs from two other classes of direct apoptosis-inducing agents: the BH3-mimetic ABT-737 and the IAP antagonists LCL161 and AT-406. Encouragingly, our data suggest that IAP antagonists possess negligible genotoxic activity. Doses of ABT-737 that were required to damage DNA stimulated Bax/Bak-independent signalling and exceeded concentrations detected in the plasma of animals treated with this drug. These findings provide hope that cancer patients treated by BH3-mimetics or IAP antagonists may avoid mutation-related illnesses that afflict some cancer survivors treated with conventional DNA-damaging anti-cancer therapies.

RNAi-mediated inhibition of apoptosis fails to prevent cationic nanoparticle-induced cell death in cultured cells

Background: Nanoparticles are increasingly being considered as a novel and potent tool for drug delivery, and, therefore, concerns regarding the safety of their use in humans are pertinent. It has been shown that nanoparticles displaying unsaturated amines at their surface are toxic to cells, but the molecular and cellular mechanisms elicited in this response have yet to be described. Aims: In this work we identify key proteins involved in the cytotoxicity of amine-modified polystyrene nanoparticles. We also demonstrate the suitability of RNAi to provide a molecular description of how nanoparticles and cells interact. Materials & methods: We have used a focused RNAi strategy in 1321N1 cells to identify key proteins involved in the cytotoxicity induced by amine-modified polystyrene nanoparticles. Results: We show that the apoptosome is central to the observed mechanism of toxicity and that, although the proapoptotic proteins BAX, BAK, BID, BIM and PUMA are critical modulators of the process, their cellular depletion is insufficient to protect cells from nanoparticle-induced cell death. Conclusion: We conclude that the apoptosome, together with proapoptotic proteins of the Bcl-2 family of proteins, is central to amine-modified polystyrene nanoparticle-induced cell death. We further demonstrate that RNAi is a powerful and suitable tool to study the effects of nanoparticles on cellular processes, in particular apoptosis.

Original submitted 18 March 2013; Revised submitted 22 July 2013

Multimodal interaction with BCL-2 family proteins underlies the proapoptotic activity of PUMA BH3

PUMA is a proapoptotic BCL-2 family member that drives the apoptotic response to a diversity of cellular insults. Deciphering the spectrum of PUMA interactions that confer its context-dependent proapoptotic properties remains a high priority goal. Here, we report the synthesis of PUMA SAHBs, structurally stabilized PUMA BH3 helices that, in addition to broadly targeting antiapoptotic proteins, directly bind to proapoptotic BAX. NMR, photocrosslinking, and biochemical analyses revealed that PUMA SAHBs engage an α1/α6 trigger site on BAX to initiate its functional activation. We further demonstrated that a cell-permeable PUMA SAHB analog induces apoptosis in neuroblastoma cells and, like expressed PUMA protein, engages BCL-2, MCL-1, and BAX. Thus, we find that PUMA BH3 is a dual antiapoptotic inhibitor and proapoptotic direct activator, and its mimetics may serve as effective pharmacologic triggers of apoptosis in resistant human cancers.

The indirect activation model of mitochondrial outer membrane permeabilisation (MOMP) initiation requires a trade-off between robustness in the absence of and sensitivity in the presence of stress

Mitochondrial outer membrane permeabilisation (MOMP) is a hallmark of apoptosis and is controlled by antagonising members of the BCL 2 protein family. However, whether the effector proteins for MOMP, BAX and BAK, require a separate activation step (direct activation model) or if the sole inhibition of otherwise active effectors by anti-apoptotic BCL 2 proteins suffices (indirect activation model) is controversial. To address this question, we recently identified that, in most cells, effector proteins were more abundant than their inhibitors. We now employ systems modelling to elucidate fundamental consequences of this imbalance for the indirect activation model by integrating its key features into the recently validated implementation of the direct activation model. First, by modelling constitutively active BAK, several cell lines were predicted to be unstable against MOMP even in the absence of stress. This instability could be rectified by assuming BAK pores to be subject to degradation, which however demanded BAK to underlie a high and biologically unreasonable protein turnover. Stability against MOMP in the absence of stress was similarly reconstituted by assuming BAK oligomerisation to be weak, but predicted some cells to be MOMP resistant even under high stress. Assuming also BAX to be constitutively active required highly effective BAX re-translocation to the cytosol along with weak BAX oligomerisation, which again rendered some cells insensitive to induce MOMP in the presence of stress. Our results suggest that an activation step of the two effectors is required to allow stability against MOMP in the absence of stress and execution of MOMP under stress.

MicroRNAs affect BCL-2 family proteins in the setting of cerebral ischemia

The BCL-2 family is centrally involved in the mechanism of cell death after cerebral ischemia. It is well known that the proteins of the BCL-2 family are key regulators of apoptosis through controlling mitochondrial outer membrane permeabilization. Recent findings suggest that many BCL-2 family members are also directly involved in controlling transmission of Ca(2+) from the endoplasmic reticulum (ER) to mitochondria through a specialization called the mitochondria-associated ER membrane (MAM). Increasing evidence supports the involvement of microRNAs (miRNAs), some of them targeting BCL-2 family proteins, in the regulation of cerebral ischemia. In this mini-review, after highlighting current knowledge about the multiple functions of BCL-2 family proteins and summarizing their relationship to outcome from cerebral ischemia, we focus on the regulation of BCL-2 family proteins by miRNAs, especially miR-29 which targets multiple BCL-2 family proteins.

Selective BCL-2 inhibition by ABT-199 causes on-target cell death in acute myeloid leukemia

B-cell leukemia/lymphoma 2 (BCL-2) prevents commitment to programmed cell death at the mitochondrion. It remains a challenge to identify those tumors that are best treated by inhibition of BCL-2. Here, we demonstrate that acute myeloid leukemia (AML) cell lines, primary patient samples, and murine primary xenografts are very sensitive to treatment with the selective BCL-2 antagonist ABT-199. In primary patient cells, the median IC50 was approximately 10 nmol/L, and cell death occurred within 2 hours. Our ex vivo sensitivity results compare favorably with those observed for chronic lymphocytic leukemia, a disease for which ABT-199 has demonstrated consistent activity in clinical trials. Moreover, mitochondrial studies using BH3 profiling demonstrate activity at the mitochondrion that correlates well with cytotoxicity, supporting an on-target mitochondrial mechanism of action. Our protein and BH3 profiling studies provide promising tools that can be tested as predictive biomarkers in any clinical trial of ABT-199 in AML.

Evaluation of the BH3-only protein Puma as a direct Bak activator

Interactions among Bcl-2 family proteins play critical roles in cellular life and death decisions. Previous studies have established the BH3-only proteins Bim, tBid, and Noxa as "direct activators" that are able to directly initiate the oligomerization and activation of Bak and/or Bax. Earlier studies of Puma have yielded equivocal results, with some concluding that it also acts as a direct activator and other studies suggesting that it acts solely as a sensitizer BH3-only protein. In the present study we examined the interaction of Puma BH3 domain or full-length protein with Bak by surface plasmon resonance, assessed Bak oligomerization status by cross-linking followed by immunoblotting, evaluated the ability of the Puma BH3 domain to induce Bak-mediated permeabilization of liposomes and mitochondria, and determined the effect of wild type and mutant Puma on cell viability in a variety of cellular contexts. Results of this analysis demonstrate high affinity (KD = 26 ± 5 nM) binding of the Puma BH3 domain to purified Bak ex vivo, leading to Bak homo-oligomerization and membrane permeabilization. Mutations in Puma that inhibit (L141E/M144E/L148E) or enhance (M144I/A145G) Puma BH3 binding to Bak also produce corresponding alterations in Bak oligomerization, Bak-mediated membrane permeabilization and, in a cellular context, Bak-mediated killing. Collectively, these results provide strong evidence that Puma, like Bim, Noxa, and tBid, is able to act as a direct Bak activator.

Cardiomyocyte protection by GATA-4 gene engineered mesenchymal stem cells is partially mediated by translocation of miR-221 in microvesicles

Introduction

microRNAs (miRs), a novel class of small non-coding RNAs, are involved in cell proliferation, differentiation, development, and death. In this study, we found that miR-221 translocation by microvesicles (MVs) plays an important role in cardioprotection mediated by GATA-4 overexpressed mesenchymal stem cells (MSC).

Methods and Results

Adult rat bone marrow MSC and neonatal rat ventricle cardiomyocytes (CM) were harvested as primary cultures. MSC were transduced with GATA-4 (MSC^GATA-4) using the murine stem cell virus (pMSCV) retroviral expression system. Empty vector transfection was used as a control (MSC^Null). The expression of miRs was assessed by real-time PCR and localized using in situ hybridization (ISH). MVs collected from MSC cultures were characterized by expression of CD9, CD63, and HSP70, and photographed with electron microscopy. Cardioprotection during hypoxia afforded by conditioned medium (CdM) from MSC cultures was evaluated by lactate dehydrogenase (LDH) release, MTS uptake by CM, and caspase 3/7 activity. Expression of miR-221/222 was significantly higher in MSC than in CM and miR-221 was upregulated in MSC^GATA-4. MSC overexpression of miR-221 significantly enhanced cardioprotection by reducing the expression of p53 upregulated modulator of apoptosis (PUMA). Moreover, expression of PUMA was significantly decreased in CM co-cultured with MSC. MVs derived from MSC expressed high levels of miR-221, and were internalized quickly by CM as documented in images obtained from a Time-Lapse Imaging System.

Conclusions

Our results demonstrate that cardioprotection by MSC^GATA-4 may be regulated in part by a transfer of anti-apoptotic miRs contained within MVs.

Specific acetylation of p53 by HDAC inhibition prevents DNA damage-induced apoptosis in neurons

Histone deacetylase (HDAC) inhibitors have been used to promote neuronal survival and ameliorate neurological dysfunction in a host of neurodegenerative disease models. The precise molecular mechanisms whereby HDAC inhibitors prevent neuronal death are currently the focus of intensive research. Here we demonstrate that HDAC inhibition prevents DNA damage-induced neurodegeneration by modifying the acetylation pattern of the tumor suppressor p53, which decreases its DNA-binding and transcriptional activation of target genes. Specifically, we identify that acetylation at K382 and K381 prevents p53 from associating with the pro-apoptotic PUMA gene promoter, activating transcription, and inducing apoptosis in mouse primary cortical neurons. Paradoxically, acetylation of p53 at the same lysines in various cancer cell lines leads to the induction of PUMA expression and death. Together, our data provide a molecular understanding of the specific outcomes of HDAC inhibition and suggest that strategies aimed at enhancing p53 acetylation at K381 and K382 might be therapeutically viable for capturing the beneficial effects in the CNS, without compromising tumor suppression.

Hoxb8 regulates expression of microRNAs to control cell death and differentiation

Hoxb8 overexpression immortalises haematopoietic progenitor cells in a growth-factor-dependant manner and co-operates with interleukin-3 (IL-3) to cause acute myeloid leukaemia. To further understand how Hoxb8 contributes to myeloid cell immortalisation, we generated IL-3-dependant myeloid cells expressing Hoxb8 under the control of an inducible promoter. Downregulation of Hoxb8, in the presence of IL-3, caused cell-cycle arrest and apoptosis in the majority of cells. Apoptosis was dependant on Bax and Bak and, in part, on Bim, which was repressed by Hoxb8. Deletion of the miR-17∼92 seed sequences in the Bim 3'UTR abolished Hoxb8-dependant regulation of Bim reporter constructs. Expression of all six miRNAs from this cluster were elevated when Hoxb8 was overexpressed. The miR-17∼92 cluster was required for repression of Bim in Hoxb8-immortalised cells and deletion of the miR-17∼92 cluster substantially inhibited Hoxb8, but not Hoxa9, mediated survival and proliferation. Hoxb8 appears to promote miR-17∼92 expression through c-Myc, a known transcriptional regulator of the miR-17∼92 cluster. We have uncovered a previously unrecognised link between Hoxb8 expression and microRNAs that provides a new insight into the oncogenic functions of Hoxb8.

Akt1 is the principal Akt isoform regulating apoptosis in limiting cytokine concentrations

The activation of the Akt signalling in response to cytokine receptor signalling promotes protein synthesis, cellular growth and proliferation. To determine the role of Akt in interleukin-3 (IL-3) signalling, we generated IL-3-dependent myeloid cell lines from mice lacking Akt1, Akt2 or Akt3. Akt1 deletion resulted in accelerated apoptosis at low concentrations of IL-3. Expression of constitutively active Akt1 was sufficient to delay apoptosis in response to IL-3 withdrawal, but not sufficient to induce proliferation in the absence of IL-3. Akt1 prolonged survival of Bim- or Bad-deficient cells, but not cells lacking Puma, indicating that Akt1-dependent repression of apoptosis was in part dependent on Puma and independent of Bim or Bad. Our data show that a key role of Akt1 during IL-3 signalling is to repress p53-dependent apoptosis pathways, including transcriptional upregulation of Puma. Moreover, our data indicate that regulation of BH3-only proteins by Akt is dispensable for Akt-dependent cell survival.

Flavokawain B, a kava chalcone, inhibits growth of human osteosarcoma cells through G2/M cell cycle arrest and apoptosis

BACKGROUND

Osteosarcoma (OS) is the most common primary bone malignancy with a high propensity for local invasion and distant metastasis. Limited by the severe toxicity of conventional agents, the therapeutic bottleneck of osteosarcoma still remains unconquered. Flavokawain B (FKB), a kava extract, has been reported to have significant anti-tumor effects on several carcinoma cell lines both in vitro and in vivo. Its efficacy and low toxicity profile make FKB a promising agent for use as a novel chemotherapeutic agent.

RESULTS

In the current study, we investigated the anti-proliferative and apoptotic effects of FKB against human osteosarcomas. Exposure of OS cells to FKB resulted in apoptosis, evidenced by loss of cell viability, morphological changes and the externalization of phosphatidylserine. Apoptosis induced by FKB resulted in activation of Caspase-3/7, -8 and -9 in OS cell lines, 143B and Saos-2. FKB also down-regulated inhibitory apoptotic markers, including Bcl-2 and Survivin and led to concomitant increases in apoptotic proteins, Bax, Puma and Fas. Therefore, the induction of apoptosis by FKB involved both extrinsic and intrinsic pathways. FKB also caused G2/M phase cell cycle arrest, which was observed through reductions in the levels of cyclin B1, cdc2 and cdc25c and increases in Myt1 levels. Furthermore, migration and invasion ability was decreased by FKB in a dose-dependent manner. The cytotoxicity profile showed FKB had significant lower side effects on bone marrow cells and small intestinal epithelial cells compared with Adriamycin.

CONCLUSIONS

Taken together, our evidence of apoptosis and cell cycle arrest by FKB treatment with less toxicity than the standard treatments provides an innovative argument for the use of FKB as a chemotherapeutic and chemopreventive compound. In vivo experiments utilizing FKB to reduce tumorigenesis and metastatic potential will be crucial to further justify clinical application.

Mechanisms of action of Bcl-2 family proteins

The Bcl-2 family of proteins controls a critical step in commitment to apoptosis by regulating permeabilization of the mitochondrial outer membrane (MOM). The family is divided into three classes: multiregion proapoptotic proteins that directly permeabilize the MOM; BH3 proteins that directly or indirectly activate the pore-forming class members; and the antiapoptotic proteins that inhibit this process at several steps. Different experimental approaches have led to several models, each proposed to explain the interactions between Bcl-2 family proteins. The discovery that many of these interactions occur at or in membranes as well as in the cytoplasm, and are governed by the concentrations and relative binding affinities of the proteins, provides a new basis for rationalizing these models. Furthermore, these dynamic interactions cause conformational changes in the Bcl-2 proteins that modulate their apoptotic function, providing additional potential modes of regulation.

Getting away with murder: how does the BCL-2 family of proteins kill with immunity?

The adult human body produces approximately one million white blood cells every second. However, only a small fraction of the cells will survive because the majority is eliminated through a genetically controlled form of cell death known as apoptosis. This review places into perspective recent studies pertaining to the BCL-2 family of proteins as critical regulators of the development and function of the immune system, with particular attention on B cell and T cell biology. Here we discuss how elegant murine model systems have revealed the major contributions of the BCL-2 family in establishing an effective immune system. Moreover, we highlight some key regulatory pathways that influence the expression, function, and stability of individual BCL-2 family members, and discuss their role in immunity. From lethal mechanisms to more gentle ones, the final portion of the review discusses the nonapoptotic functions of the BCL-2 family and how they pertain to the control of immunity.

17β-estradiol impedes Bax-involved mitochondrial apoptosis of retinal nerve cells induced by oxidative damage via the phosphatidylinositol 3-kinase/Akt signal pathway

Oxidative stress leading to retinal nerve cells (RNCs) apoptosis is a major cause of neurodegenerative disorders of the retina. 17β-Estradiol (E2) has been suggested to be a neuroprotective agent in the central nervous system; however, at present, the underlying mechanisms are not well understood, and the related research on the RNCs is less reported. Here, in order to investigate the protective role and mechanism of E2 against oxidative stress-induced damage on RNCs, the transmission electron microscopy and annexin V-FITC/propidium iodide assay were applied to detect the RNCs apoptosis. Western blot and real-time PCR were used to determine the expression of the critical molecules in Bcl-2 and caspase family associated with apoptosis. The transmission electron microscopy results showed that H(2)O(2) could induce typical features of apoptosis in RNCs, including formation of the apoptosome. E2 could, however, suppress the H(2)O(2)-induced morphological changes of apoptosis. Intriguingly, we observed E2-mediated phagocytic scavenging of apoptosome. In response to H(2)O(2)-induced apoptosis, Bax, acting as one of the pivotal pro-apoptotic members of Bcl-2 family, increased significantly, which directly resulted in an increased ratio of Bax to anti-apoptotic protein Bcl-2 (Bax/Bcl-2). Additionally, caspases 9 and 3, which are the critical molecules of the mitochondrial apoptosis pathway, were activated by H(2)O(2). In contrast, E2 exerted anti-apoptotic effects by reducing the expression of Bax to decrease the ratio of Bax/Bcl-2 and impeded the caspases 9/3 activation. Moreover, LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor, could sharply block the effect of E2 in reducing the percentage of apoptotic cells resistance to H(2)O(2). And the attenuation of Bax, the reduced activities of caspases 9/3 and the impeded release of mitochondrial cytochrome c mediated by E2 resistance to H(2)O(2) damage were significantly retrieved by LY294002 administration. Taken together, E2 protects the RNCs against H(2)O(2)-induced apoptosis by significantly inhibiting the Bax-involved mitochondrial apoptosis via the activation of PI3K/Akt signal pathway.

Direct interaction of Bax and Bak proteins with Bcl-2 homology domain 3 (BH3)-only proteins in living cells revealed by fluorescence complementation

The key event in the mitochondrial pathway of apoptosis is the activation of Bax and Bak by BH3-only proteins through a molecular mechanism that is still a matter of debate. Here we studied interactions among anti- and proapoptotic proteins of the Bcl-2 family in living cells by using bimolecular fluorescence complementation analysis. Our results indicate that the antiapoptotic proteins Mcl-1 and Bcl-x(L) bind preferably to the BH3-only proteins Bim, PUMA, and Noxa but can also bind to Bak and Bax. We also found a direct interaction between Bim, PUMA, or Noxa with either Bax or Bak during apoptosis induction. In HeLa cells, interaction of Bim with Bax occurs in cytosol, and then Bim-Bax complexes translocate to mitochondria. Complexes of either PUMA or Noxa with Bax or Bak were always detected at mitochondria. Overexpression of Bcl-x(L) or Mcl-1 delayed Bim/Bax translocation to mitochondria. These results reveal the ability of main BH3-only proteins to directly activate Bax and Bak in living cells and suggest that a complex network of interactions regulate the function of Bcl-2 family members during apoptosis.

PUMA, a critical mediator of cell death--one decade on from its discovery

PUMA (p53 upregulated modulator of apoptosis) is a pro-apoptotic member of the BH3-only subgroup of the Bcl-2 family. It is a key mediator of p53-dependent and p53-independent apoptosis and was identified 10 years ago. The PUMA gene is mapped to the long arm of chromosome 19, a region that is frequently deleted in a large number of human cancers. PUMA mediates apoptosis thanks to its ability to directly bind known anti-apoptotic members of the Bcl-2 family. It mainly localizes to the mitochondria. The binding of PUMA to the inhibitory members of the Bcl-2 family (Bcl-2-like proteins) via its BH3 domain seems to be a critical regulatory step in the induction of apoptosis. It results in the displacement of the proteins Bax and/or Bak. This is followed by their activation and the formation of pore-like structures on the mitochondrial membrane, which permeabilizes the outer mitochondrial membrane, leading to mitochondrial dysfunction and caspase activation. PUMA is involved in a large number of physiological and pathological processes, including the immune response, cancer, neurodegenerative diseases and bacterial and viral infections.

Critical role of p53 upregulated modulator of apoptosis in benzyl isothiocyanate-induced apoptotic cell death

Benzyl isothiocyanate (BITC), a constituent of edible cruciferous vegetables, decreases viability of cancer cells by causing apoptosis but the mechanism of cell death is not fully understood. The present study was undertaken to determine the role of Bcl-2 family proteins in BITC-induced apoptosis using MDA-MB-231 (breast), MCF-7 (breast), and HCT-116 (colon) human cancer cells. The B-cell lymphoma 2 interacting mediator of cell death (Bim) protein was dispensable for proapoptotic response to BITC in MCF-7 and MDA-MB-231 cells as judged by RNA interference studies. Instead, the BITC-treated MCF-7 and MDA-MB-231 cells exhibited upregulation of p53 upregulated modulator of apoptosis (PUMA) protein. The BITC-mediated induction of PUMA was relatively more pronounced in MCF-7 cells due to the presence of wild-type p53 compared with MDA-MB-231 with mutant p53. The BITC-induced apoptosis was partially but significantly attenuated by RNA interference of PUMA in MCF-7 cells. The PUMA knockout variant of HCT-116 cells exhibited significant resistance towards BITC-induced apoptosis compared with wild-type HCT-116 cells. Attenuation of BITC-induced apoptosis in PUMA knockout HCT-116 cells was accompanied by enhanced G2/M phase cell cycle arrest due to induction of p21 and down regulation of cyclin-dependent kinase 1 protein. The BITC treatment caused a decrease in protein levels of Bcl-xL (MCF-7 and MDA-MB-231 cells) and Bcl-2 (MCF-7 cells). Ectopic expression of Bcl-xL in MCF-7 and MDA-MB-231 cells and that of Bcl-2 in MCF-7 cells conferred protection against proapoptotic response to BITC. Interestingly, the BITC-treated MDA-MB-231 cells exhibited induction of Bcl-2 protein expression, and RNA interference of Bcl-2 in this cell line resulted in augmentation of BITC-induced apoptosis. The BITC-mediated inhibition of MDA-MB-231 xenograft growth in vivo was associated with the induction of PUMA protein in the tumor. In conclusion, the results of the present study indicate that Bim-independent apoptosis by BITC in cancer cells is mediated by PUMA.

Mechanisms of lysophosphatidylcholine-induced hepatocyte lipoapoptosis

Isolated hepatocytes undergo lipoapoptosis, a feature of hepatic lipotoxicity, on treatment with saturated free fatty acids (FFA) such as palmitate (PA). However, it is unknown if palmitate is directly toxic to hepatocytes or if its toxicity is indirect via the generation of lipid metabolites such as lysophosphatidylcholine (LPC). PA-mediated hepatocyte lipoapoptosis is associated with endoplasmic reticulum (ER) stress, c-Jun NH(2)-terminal kinase (JNK) activation, and a JNK-dependent upregulation of the potent proapoptotic BH3-only protein PUMA (p53 upregulated modulator of apoptosis). Our aim was to determine which of these mechanisms of lipotoxicity are activated by PA-derived LPC. We employed Huh-7 cells and isolated murine and human primary hepatocytes. Intracellular LPC concentrations increase linearly as a function of the exogenous, extracellular PA, stearate, or LPC concentration. Incubation of Huh-7 cells or primary hepatocytes with LPC induced cell death by apoptosis in a concentration-dependent manner. Substituting LPC for PA resulted in caspase-dependent cell death that was accompanied by activating phosphorylation of JNK with c-Jun phosphorylation and an increase in PUMA expression. LPC also induced ER stress as manifest by eIF2α phosphorylation and CAAT/enhancer binding homologous protein (CHOP) induction. LPC cytotoxicity was attenuated by pharmacological inhibition of JNK or glycogen synthase kinase-3 (GSK-3). Similarly, short-hairpin RNA (shRNA)-targeted knockdown of CHOP protected Huh-7 cells against LPC-induced toxicity. The LPC-induced PUMA upregulation was prevented by JNK inhibition or shRNA-targeted knockdown of CHOP. Finally, genetic deficiency of PUMA rendered murine hepatocytes resistant to LPC-induced apoptosis. We concluded that LPC-induced lipoapoptosis is dependent on mechanisms largely indistinguishable from PA. These data suggest that FFA-mediated cytotoxicity is indirect via the generation of the toxic metabolite, LPC.

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.

Cytokine receptor signaling activates an IKK-dependent phosphorylation of PUMA to prevent cell death

P53-upregulated modifier of apoptosis (PUMA), a pro-apoptotic member of the Bcl-2 family, is transcriptionally activated by p53 and is a key effector of p53-dependent apoptosis. We show that PUMA protein is subject to rapid post-translational regulation by phosphorylation at a conserved residue, serine 10, following serum or interleukin-3 (IL-3) stimulation. Serine 10 is not within the Bcl-2 homology (BH3) domain, and PUMA phosphorylated at serine 10 retained the ability to co-immunoprecipitate with antiapoptotic Bcl-2 family members. However, phosphorylated PUMA was targeted for proteasomal degradation indicating that it is less stable than unphosphorylated PUMA. Importantly, we identified IKK1/IKK2/Nemo as the kinase complex that interacts with and phosphorylates PUMA, thereby also demonstrating that IL-3 activates NFκB signaling. The identification and characterization of this novel survival pathway has important implications for IL-3 signaling and hematopoietic cell development.

Regulation of memory B-cell survival by the BH3-only protein Puma

Apoptosis is crucial for immune system homeostasis, including selection and survival of long-lived antibody-forming cells and memory cells. The interactions between proapoptotic and pro-survival proteins of the Bcl-2 family are critical for this process. In this report, we show that expression of the proapoptotic BH3-only Bcl-2 family member Puma was selectively up-regulated on in vitro activation with antigens or mitogens of both human and mouse B cells. Puma expression coincided in vivo, with the prosurvival Bcl-2 family member Mcl-1 within the germinal centers and its expression correlates with the germinal center like phenotype of Burkitt lymphoma. Experiments performed in Puma-deficient mice revealed that Puma is essential for apoptosis of mitogen-activated B cells in vitro and for the control of memory B-cell survival. In conclusion, using both human and murine models, our data show that Puma has a major role in the T cell- dependent B-cell immune response. These data demonstrate that Puma is a major regulator of memory B lymphocyte survival and therefore a key molecule in the control of the immune response.

Mechanisms of lipotoxicity in NAFLD and clinical implications

With the epidemic of childhood obesity, nonalcoholic fatty liver disease (NAFLD) has become the most common cause of chronic liver disease in pediatrics. NAFLD is strongly associated with insulin resistance and increased level of serum free fatty acids (FFAs). FFAs have direct hepatotoxicity through the induction of an endoplasmic reticulum stress response and subsequently activation of the mitochondrial pathway of cell death. FFAs may also result in lysosomal dysfunction and alter death receptor gene expression. Lipoapoptosis is a key pathogenic process in NAFLD, and correlates with progressive inflammation, and fibrosis. Accumulation of triglyceride in the liver results from uptake and esterification of FFAs by the hepatocyte, and is less likely to be hepatotoxic per se. To date, there are no proven effective therapies that halt NAFLD progression or unfortunately improve prognosis in children. The cellular mechanisms of lipotoxicity are complex but provide potential therapeutic targets for NAFLD. In this review we discuss several potential therapeutic opportunities in detail including inhibition of apoptosis, c-Jun-N-terminal kinase, and endoplasmic reticulum stress pathways.

A role for miR-296 in the regulation of lipoapoptosis by targeting PUMA

Saturated free fatty acids (FFA) induce hepatocyte lipoapoptosis, a key mediator of liver injury in nonalcoholic fatty liver disease (NAFLD). Lipoapoptosis involves the upregulation of the BH3-only protein PUMA, a potent pro-apoptotic protein. Given that dysregulation of hepatic microRNA expression has been observed in NAFLD, we examined the role of miRNA in regulating PUMA expression during lipotoxicity. By in silico analysis, we identified two putative binding sites for miR-296-5p within the 3' untranslated region (UTR) of PUMA mRNA. Enforced miR-296-5p levels efficiently reduced PUMA protein expression in Huh-7 cells, while antagonism of miR-296-5p function increased PUMA cellular levels. Reporter gene assays identified PUMA 3'UTR as a direct target of miR-296-5p. The saturated FFA, palmitate, repressed miR-296-5p expression; and Huh-7 cells were sensitized to palmitate-induced lipotoxicity by antagonism of miR-296-5p function using a targeted locked nucleic acid (LNA). Finally, miR-296-5p was reduced in liver samples from nonalcoholic steatohepatitis (NASH) patients compared with patients with simple steatosis (SS) or controls. Also miR-296-5p levels inversely varied with PUMA mRNA levels in human liver specimens. Our results implicate miR-296-5p in the regulation of PUMA expression during hepatic lipoapoptosis. We speculate that enhancement of miR-296-5p expression may represent a novel approach to minimize apoptotic damage in human fatty liver diseases.

Bax: Addressed to kill

The pro-apoptototic protein Bax (Bcl-2 Associated protein X) plays a central role in the mitochondria-dependent apoptotic pathway. In healthy mammalian cells, Bax is essentially cytosolic and inactive. Following a death signal, the protein is translocated to the outer mitochondrial membrane, where it promotes a permeabilization that favors the release of different apoptogenic factors, such as cytochrome c. The regulation of Bax translocation is associated to conformational changes that are under the control of different factors. The evidences showing the involvement of different Bax domains in its mitochondrial localization are presented. The interactions between Bax and its different partners are described in relation to their ability to promote (or prevent) Bax conformational changes leading to mitochondrial addressing and to the acquisition of the capacity to permeabilize the outer mitochondrial membrane.

In vivo contributions of BH3-only proteins to neuronal death following seizures, ischemia, and traumatic brain injury

The Bcl-2 homology (BH) domain 3-only proteins are a proapoptotic subgroup of the Bcl-2 gene family, which regulate cell death via effects on mitochondria. The BH3-only proteins react to various cell stressors and promote cell death by binding and inactivating antiapoptotic Bcl-2 family members and direct activation of proapoptotic multi-BH domain proteins such as Bax. Here, we review the in vivo evidence for their involvement in the pathophysiology of status epilepticus and contrast it to ischemia and traumatic brain injury. Seizures in rodents activate three potent proapoptotic BH3-only proteins: Bid, Bim, and Puma. Analysis of damage after seizures in mice singly deficient for each BH3-only protein supports a causal role for Puma and to a lesser extent Bim but, surprisingly, not Bid. In ischemia and trauma, where core aspects of the pathophysiology of cell death overlap, multiple BH3-only proteins are also activated and Bid has been shown to be required for neuronal death. The findings suggest that while each neurologic insult activates multiple BH3-only proteins, there may be specificity in their functional contribution. Future challenges include evaluating the remaining BH3-only proteins, explaining different causal contributions, and, if possible, exploring neurologic outcomes in mouse models deficient for multiple BH3-only proteins.

Caspase-3 triggers a TPCK-sensitive protease pathway leading to degradation of the BH3-only protein puma

The protein Puma (p53-upregulated modulator of apoptosis) belongs to the BH3-only group of the Bcl-2 family and is a major regulator of apoptosis. Although the transcriptional regulation of Puma is clearly established, little is known about the regulation of its expression at the protein levels. We show here that various signals--including the cytokine TGFβ, the death effector TRAIL or chemical drugs such as anisomycin--downregulate Puma protein levels via a novel pathway based on the sequential activation of caspase-3 and a protease inhibited by the serpase inhibitor N-tosyl-L-phenylalanine chloromethyl ketone. This pathway is specific for Puma because (1) the levels of other BH3-only proteins, such as Bim and Noxa were not modified by these stimuli and (2) this caspase-mediated degradation was dependent on both the BH3 and C-terminal domains of Puma. Our data also show that Puma is regulated during the caspase-3-dependent differentiation of murine embryonic stem cells and suggest that this pathway may be relevant and important during caspase-mediated cell differentiation not associated with apoptosis.

BH3-only activator proteins Bid and Bim are dispensable for Bak/Bax-dependent thrombocyte apoptosis induced by Bcl-xL deficiency: molecular requisites for the mitochondrial pathway to apoptosis in platelets

A pivotal step in the mitochondrial pathway of apoptosis is activation of Bak and Bax, although the molecular mechanism remains controversial. To examine whether mitochondrial apoptosis can be induced by just a lack of antiapoptotic Bcl-2-like proteins or requires direct activators of the BH3-only proteins including Bid and Bim, we studied the molecular requisites for platelet apoptosis induced by Bcl-xL deficiency. Severe thrombocytopenia induced by thrombocyte-specific Bcl-xL knock-out was fully rescued in a Bak and Bax double knock-out background but not with single knock-out of either one. In sharp contrast, deficiency of either Bid, Bim, or both did not alleviate thrombocytopenia in Bcl-xL knock-out mice. An in vitro study revealed that ABT-737, a Bad mimetic, induced platelet apoptosis in association with a conformational change of the amino terminus, translocation from the cytosol to mitochondria, and homo-oligomerization of Bax. ABT-737-induced Bax activation and apoptosis were also observed in Bid/Bim-deficient platelets. Human platelets, upon storage, underwent spontaneous apoptosis with a gradual decline of Bcl-xL expression despite a decrease in Bid and Bim expression. Apoptosis was attenuated in Bak/Bax-deficient or Bcl-xL-overexpressing platelets but not in Bid/Bim-deficient platelets upon storage. In conclusion, platelet lifespan is regulated by a fine balance between anti- and proapoptotic multidomain Bcl-2 family proteins. Despite residing in platelets, BH3-only activator proteins Bid and Bim are dispensable for Bax activation and mitochondrial apoptosis.

Snail1 suppresses TGF-beta-induced apoptosis and is sufficient to trigger EMT in hepatocytes

Although TGF-β suppresses early stages of tumour development, it later contributes to tumour progression when cells become resistant to its suppressive effects. In addition to circumventing TGF-β-induced growth arrest and apoptosis, malignant tumour cells become capable of undergoing epithelial-to-mesenchymal transition (EMT), favouring invasion and metastasis. Therefore, defining the mechanisms that allow cancer cells to escape from the suppressive effects of TGF-β is fundamental to understand tumour progression and to design specific therapies. Here, we have examined the role of Snail1 as a suppressor of TGF-β-induced apoptosis in murine non-transformed hepatocytes, rat and human hepatocarcinoma cell lines and transgenic mice. We show that Snail1 confers resistance to TGF-β-induced cell death and that it is sufficient to induce EMT in adult hepatocytes, cells otherwise refractory to this transition upon exposure to TGF-β. Furthermore, we show that Snail1 silencing prevents EMT and restores the cell death response induced by TGF-β. As Snail1 is a known target of TGF-β signalling, our data indicate that Snail1 might transduce the tumour-promoting effects of TGF-β, namely the EMT concomitant with the resistance to cell death.

Role of Bcl-2 family proteins and caspases in the regulation of apoptosis

Apoptosis, or programmed cell death, plays a pivotal role in the elimination of unwanted, damaged, or infected cells in multicellular organisms and also in diverse biological processes, including development, cell differentiation, and proliferation. Apoptosis is a highly regulated form of cell death, and dysregulation of apoptosis results in pathological conditions including cancer, autoimmune and neurodegenerative diseases. The Bcl-2 family proteins are key regulators of apoptosis, which include both anti- and pro-apoptotic proteins, and a slight change in the dynamic balance of these proteins may result either in inhibition or promotion of cell death. Execution of apoptosis by various stimuli is initiated by activating either intrinsic or extrinsic pathways which lead to a series of downstream cascade of events, releasing of various apoptotic mediators from mitochondria and activation of caspases, important for the cell fate. In view of recent research advances about underlying mechanism of apoptosis, this review highlights the basics concept of apoptosis and its regulation by Bcl-2 family of protein. Furthermore, this review discusses the interplay of various apoptotic mediators and caspases to decide the fate of the cell. We expect that this review will add to the pool of basic information necessary to understand the mechanism of apoptosis which may implicate in designing better strategy to develop biomedical therapy to control apoptosis.

Anti-apoptotic Bcl-2 fails to form efficient complexes with pro-apoptotic Bak to protect from Celecoxib-induced apoptosis

The non-steroidal anti-inflammatory drug Celecoxib is a specific inhibitor of cyclooxygenase-2. Apart from its inhibitor function, Celecoxib induces apoptosis through the intrinsic pathway which is controlled by the Bcl-2 family members. In Jurkat T lymphoma cells, treatment with Celecoxib results in a rapid decline of the anti-apoptotic Bcl-2-related protein Mcl-1. The depletion of Mcl-1 is sufficient for apoptosis induction and can be blocked by overexpression of Bcl-xL but not by the close homologue Bcl-2. The present investigation analyzed the mechanism by which Bcl-xL prevents apoptosis induction whereas Bcl-2 failed to. Our data show that the involvement of the orphan nuclear receptor Nur77/TR3 specifically targeting Bcl-2 but not Bcl-xL was not involved in Celecoxib-induced apoptosis. Surprisingly, BH3-only proteins Bid, Bim, and Puma of the Bcl-2 family were not needed either. However, unlike Bcl-2, Mcl-1, and Bcl-xL sequestered Bak preventing it from activation through a direct interaction. Thus, when abundantly expressed, Bcl-xL can substitute for the loss of Mcl-1 whereas Bcl-2, incapable of forming a high affinity complex with Bak, could not.