Structural and biochemical basis of apoptotic activation by Smac/DIABLO

Smac therapeutic Peptide nanoparticles inducing apoptosis of cancer cells for combination chemotherapy with Doxorubicin

Smac-conjugated nanoparticle (Smac-NP) was designed to induce the apoptosis of cancer cells and as a drug carrier for combination therapy. It contained three parts, a SmacN7 peptide which could induce apoptosis of cancer cells by interacting with XIAPs, the cell penetrating domain rich in arginine, and four hydrophobic tails for self-assembled Smac-NP. We demonstrated that Smac-NPs exerted an antitumor effect in breast cancer cell MDA-MB-231 and nonsmall lung cancer (NSCLC) cell H460, which efficiently inhibited cancer cells proliferation without influencing normal liver cell lines LO2. Smac-NPs also significantly induced apoptosis of MDA-MB-231 and H460 cells through activating pro-caspase-3, down-regulating the expression of antiapoptotic protein Bcl-2 and up-regulating the pro-apoptotic protein Bax. Furthermore, Smac-NPs could be explored as a drug delivery system to load hydrophobic drug such as DOX for combination therapy. The DOX-loaded nanoparticles (DOX-Smac-NPs) exhibited higher cellular uptake efficiency and antitumor effect. Our work provided a new insight into therapeutic peptides integrated with drug simultaneously in one system for cancer combination treatment.

Activation of a primed RING E3-E2-ubiquitin complex by non-covalent ubiquitin

RING ubiquitin ligases (E3) recruit ubiquitin-conjugate enzymes (E2) charged with ubiquitin (Ub) to catalyze ubiquitination. Non-covalent Ub binding to the backside of certain E2s promotes processive polyUb formation, but the mechanism remains elusive. Here, we show that backside bound Ub (Ub(B)) enhances both RING-independent and RING-dependent UbcH5B-catalyzed donor Ub (Ub(D)) transfer, but with a more prominent effect in RING-dependent transfer. Ub(B) enhances RING E3s' affinities for UbcH5B-Ub, and RING E3-UbcH5B-Ub complex improves Ub(B)'s affinity for UbcH5B. A comparison of the crystal structures of a RING E3, RNF38, bound to UbcH5B-Ub in the absence and presence of Ub(B), together with molecular dynamics simulation and biochemical analyses, suggests Ub(B) restricts the flexibility of UbcH5B's α1 and α1β1 loop. Ub(B) supports E3 function by stabilizing the RING E3-UbcH5B-Ub complex, thereby improving the catalytic efficiency of Ub transfer. Thus, Ub(B) serves as an allosteric activator of RING E3-mediated Ub transfer.

Molecular regulation of auditory hair cell death and approaches to protect sensory receptor cells and/or stimulate repair following acoustic trauma

Loss of auditory sensory hair cells (HCs) is the most common cause of hearing loss. This review addresses the signaling pathways that are involved in the programmed and necrotic cell death of auditory HCs that occur in response to ototoxic and traumatic stressor events. The roles of inflammatory processes, oxidative stress, mitochondrial damage, cell death receptors, members of the mitogen-activated protein kinase (MAPK) signal pathway and pro- and anti-cell death members of the Bcl-2 family are explored. The molecular interaction of these signal pathways that initiates the loss of auditory HCs following acoustic trauma is covered and possible therapeutic interventions that may protect these sensory HCs from loss via apoptotic or non-apoptotic cell death are explored.

Fumonisin B₁ inhibits apoptosis in HepG2 cells by inducing Birc-8/ILP-2

Fumonisin B1 (FB1) is a mycotoxin produced by Fusarium sp., a common contaminant of maize. FB1 inhibits sphingolipid biosynthesis, alters sphingosine/sphinganine ratios and modifies cell survival and cell death processes at varying propensities at both species- and tissue-specific level. We investigated the effect of FB1 on the apoptotic pathway in human hepatoma (HepG2) cells. We measured: (i) the level of cell proliferation and cell death mechanism of HepG2 cells (MTT assay, annexin V and propidium iodide staining, JC-1 assay, γH2AX and cleaved PARP and Hoechst staining); (ii) initiator and executioner caspase activity (luminometric enzyme activity assays); (iii) regulation of mRNA expression of pro- and anti- apoptotic molecules using an apoptosis array (qPCR) and (iv) levels of significantly altered apoptosis-related proteins (Western blotting) following a 24 h incubation. FB1 caused a dose-dependent decrease in cell viability with an inhibitory concentration for 50% of cell growth at 200 μM. FACS data showed FB1 induced a 2.5-fold increase in annexin V staining, however, caspase activity and mitochondrial depolarization was not significantly influenced. Cleaved PARP and γH2AX were significantly lower in treated cells with minimal DNA condensation and fragmentation observed with the Hoechst stain. BIRC-8/ILP-2 was most significantly up-regulated (8-fold; apoptosis array). ILP2 protein levels were elevated (2.3-fold) with a corresponding decrease in Smac/DIABLO protein levels (1.7-fold). Further analysis showed a dose-dependent increase in BIRC-8/ILP-2 mRNA and protein expression in HepG2 cells. We conclude that FB1 modulates apoptosis in a complex dose-dependent regulation of pro- and anti-apoptotic molecules.

The molecular relationships between apoptosis, autophagy and necroptosis

Cells are constantly subjected to a vast range of potentially lethal insults, which may activate specific molecular pathways that have evolved to kill the cell. Cell death pathways are defined partly by their morphology, and more specifically by the molecules that regulate and enact them. As these pathways become more thoroughly characterized, interesting molecular links between them have emerged, some still controversial and others hinting at the physiological and pathophysiological roles these death pathways play. We describe specific molecular programs controlling cell death, with a focus on some of the distinct features of the pathways and the molecular links between them.

Conversion of the LIMA1 tumour suppressor into an oncogenic LMO-like protein by API2-MALT1 in MALT lymphoma

MALT1 is the only known paracaspase and is a critical mediator of B- and T-cell receptor signalling. The function of the MALT1 gene is subverted by oncogenic chimeric fusions arising from the recurrent t(11;18)(q21;q21) aberration, which is the most frequent translocation in mucosa-associated lymphoid tissue (MALT) lymphoma. API2-MALT1-positive MALT lymphomas manifest antibiotic resistance and aggressive clinical behaviour with poor clinical outcome. However, the mechanisms underlying API2-MALT1-induced MALT lymphomagenesis are not fully understood. Here we show that API2-MALT1 induces paracaspase-mediated cleavage of the tumour suppressor protein LIMA1. LIMA1 binding by API2-MALT1 is API2 dependent and proteolytic cleavage is dependent on MALT1 paracaspase activity. Intriguingly, API2-MALT1-mediated proteolysis generates a LIM domain-only (LMO)-containing fragment with oncogenic properties in vitro and in vivo. Importantly, primary MALT lymphomas harbouring the API2-MALT1 fusion uniquely demonstrate LIMA1 cleavage fragments. Our studies reveal a novel paracaspase-mediated oncogenic gain-of-function mechanism in the pathogenesis of MALT lymphoma.

Liver damage and caspase-dependent apoptosis is related to protein malnutrition in mice: effect of methionine

This study aimed to determine whether the effects on the mouse liver caused by three periods of feeding a protein-free diet for 5 days followed by a normal complete diet for 5 days (3PFD-CD) are prevented by a constant methionine supply (3PFD+Met-CD). The expressions of carbonic anhydrase III (CAIII), fatty acid synthase (FAS), glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and glutathione S-transferase P1 (GSTP1) were assessed by proteomics and reverse transcriptase-polymerase chain reactions. The liver redox status was examined by measuring the activities of superoxide dismutase (SOD) and catalase (CAT), as well as protein carbonylation. Because oxidative stress can result in apoptosis, the activity and content of caspase-3, as well as the x-linked inhibitor of the apoptosis protein (XIAP) and mitochondrial caspase-independent apoptosis inducing factor (AIF) contents were assessed. In addition, the liver histomorphology was examined. Compared to the controls fed a normal complete diet throughout, feeding with 3PFD-CD increased the FAS content, decreased the CAIII content, decreased both the SOD and CAT activities, and increased protein carbonylation. It also activated caspase-3, decreased the XIAP content, decreased the AIF content, increased the number of GSTP1-positive foci and caspase-3-positive cells, and caused fatty livers. Conversely, the changes were lessened to varying degrees in mice fed 3PFD+Met-CD. The present results indicate that a regular Met supply lessens the biochemical changes, damage, and caspase-dependent apoptosis provoked by recurrent dietary amino acid deprivation in the mouse liver.

Targeted silencing of inhibitors of apoptosis proteins with siRNAs: a potential anti-cancer strategy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common malignancies, with a very poor prognosis. Despite significant improvements in diagnosis and treatment in recent years, the long-term therapeutic efficacy is poor, partially due to tumor metastasis, recurrence, and resistance to chemo- or radio-therapy. Recently, it was found that a major feature of tumors is a combination of unrestrained cell proliferation and impaired apoptosis. There are now 8 recognized members of the IAP-family: NAIP, c-IAP1, c-IAP2, XIAP, Survivin, Bruce, Livin and ILP-2. These proteins all contribute to inhibition of apoptosis, and provide new potential avenues of cancer treatment. As a powerful tool to suppress gene expression in mammalian cells, RNAi species for inhibiting IAP genes can be directed against cancers. This review will provide a brief introduction to recent developments of the application IAP-siRNA in tumor studies, with the aim of inspiring future treatment of HCC.

IAPs: Modular regulators of cell signalling

Members of the inhibitor of apoptosis (IAP) family are characterised by the presence of at least one baculoviral IAP repeat (BIR) domain. However, during the course of evolution, other globular modules have been adopted to perform distinct functions. Consequently, the IAP family is now recognised as consisting of members that perform critical functions in different aspects of cellular regulation. In this review, the structural diversity present within the IAP protein family is presented. Known structures of individual domains are discussed and their properties are described in light of recent data. In particular the plasticity of BIR domains and their ability to accommodate different binding partners is highlighted, as well as the importance of communication between the domains in regulating the covalent attachment of ubiquitin.

Involvement of NF-κB and HSP70 signaling pathways in the apoptosis of MDA-MB-231 cells induced by a prenylated xanthone compound, α-mangostin, from Cratoxylum arborescens

Background

Cratoxylum arborescens has been used traditionally in Malaysia for the treatment of various ailments.

Methods

α-Mangostin (AM) was isolated from C. arborescens and its cell death mechanism was investigated. AM-induced cytotoxicity was observed with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Acridine orange/propidium iodide staining and annexin V were used to detect cells in early phases of apoptosis. High-content screening was used to observe the nuclear condensation, cell permeability, mitochondrial membrane potential, and cytochrome c release. The role of caspases-3/7, -8, and -9, reactive oxygen species, Bcl-2 and Bax expression, and cell cycle arrest were also investigated. To determine the role of the central apoptosis-related proteins, a protein array followed by immunoblot analysis was conducted. Moreover, the involvement of nuclear factor-kappa B (NF-κB) was also analyzed.

Results

Apoptosis was confirmed by the apoptotic cells stained with annexin V and increase in chromatin condensation in nucleus. Treatment of cells with AM promoted cell death-transducing signals that reduced MMP by downregulation of Bcl-2 and upregulation of Bax, triggering cytochrome c release from the mitochondria to the cytosol. The released cytochrome c triggered the activation of caspase-9 followed by the executioner caspase-3/7 and then cleaved the PARP protein. Increase of caspase-8 showed the involvement of extrinsic pathway. AM treatment significantly arrested the cells at the S phase (P<0.05) concomitant with an increase in reactive oxygen species. The protein array and Western blotting demonstrated the expression of HSP70. Moreover, AM significantly blocked the induced translocation of NF-κB from cytoplasm to nucleus.

Conclusion

Together, the results demonstrate that the AM isolated from C. arborescens inhibited the proliferation of MDA-MB-231 cells, leading to cell cycle arrest and programmed cell death, which was suggested to occur through both the extrinsic and intrinsic apoptosis pathways with involvement of the NF-κB and HSP70 signaling pathways.

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Putting the pieces together: How is the mitochondrial pathway of apoptosis regulated in cancer and chemotherapy?

In order to solve a jigsaw puzzle, one must first have the complete picture to logically connect the pieces. However, in cancer biology, we are still gaining an understanding of all the signaling pathways that promote tumorigenesis and how these pathways can be pharmacologically manipulated by conventional and targeted therapies. Despite not having complete knowledge of the mechanisms that cause cancer, the signaling networks responsible for cancer are becoming clearer, and this information is serving as a solid foundation for the development of rationally designed therapies. One goal of chemotherapy is to induce cancer cell death through the mitochondrial pathway of apoptosis. Within this review, we present the pathways that govern the cellular decision to undergo apoptosis as three distinct, yet connected puzzle pieces: (1) How do oncogene and tumor suppressor pathways regulate apoptosis upstream of mitochondria? (2) How does the B-cell lymphoma 2 (BCL-2) family influence tumorigenesis and chemotherapeutic responses? (3) How is post-mitochondrial outer membrane permeabilization (MOMP) regulation of cell death relevant in cancer? When these pieces are united, it is possible to appreciate how cancer signaling directly impacts upon the fundamental cellular mechanisms of apoptosis and potentially reveals novel pharmacological targets within these pathways that may enhance chemotherapeutic success.

Histone deacetylase inhibitors and cell death

Histone deacetylases (HDACs) are a vast family of enzymes involved in chromatin remodeling and have crucial roles in numerous biological processes, largely through their repressive influence on transcription. In addition to modifying histones, HDACs also target many other non-histone protein substrates to regulate gene expression. Recently, HDACs have gained growing attention as HDAC-inhibiting compounds are being developed as promising cancer therapeutics. Histone deacetylase inhibitors (HDACi) have been shown to induce differentiation, cell cycle arrest, apoptosis, autophagy and necrosis in a variety of transformed cell lines. In this review, we mainly discuss how HDACi may elicit a therapeutic response to human cancers through different cell death pathways, in particular, apoptosis and autophagy.

Small-molecule SMAC mimetics as new cancer therapeutics

Apoptosis is a tightly regulated cellular process and faulty regulation of apoptosis is a hallmark of human cancers. Targeting key apoptosis regulators with the goal to restore apoptosis in tumor cells has been pursued as a new cancer therapeutic strategy. XIAP, cIAP1, and cIAP2, members of inhibitor of apoptosis (IAP) proteins, are critical regulators of cell death and survival and are attractive targets for new cancer therapy. The SMAC/DIABLO protein is an endogenous antagonist of XIAP, cIAP1, and cIAP2. In the last decade, intense research efforts have resulted in the design and development of several small-molecule SMAC mimetics now in clinical trials for cancer treatment. In this review, we will discuss the roles of XIAP, cIAP1, and cIAP2 in regulation of cell death and survival, and the design and development of small-molecule SMAC mimetics as novel cancer treatments.

α-Mangostin from Cratoxylum arborescens demonstrates apoptogenesis in MCF-7 with regulation of NF-κB and Hsp70 protein modulation in vitro, and tumor reduction in vivo

Cratoxylum arborescens is an equatorial plant belonging to the family Guttiferae. In the current study, α-Mangostin (AM) was isolated and its cell death mechanism was studied. HCS was undertaken to detect the nuclear condensation, mitochondrial membrane potential, cell permeability, and the release of cytochrome c. An investigation for reactive oxygen species formation was conducted using fluorescent analysis. To determine the mechanism of cell death, human apoptosis proteome profiler assay was conducted. In addition, using immunofluorescence and immunoblotting, the levels of Bcl-2-associated X protein (Bax) and B-cell lymphoma (Bcl)-2 proteins were also tested. Caspaces such as 3/7, 8, and 9 were assessed during treatment. Using HCS and Western blot, the contribution of nuclear factor kappa-B (NF-κB) was investigated. AM had showed a selective cytotoxicity toward the cancer cells with no toxicity toward the normal cells even at 30 μg/mL, thereby indicating that AM has the attributes to induce cell death in tumor cells. The treatment of MCF-7 cells with AM prompted apoptosis with cell death-transducing signals. This regulated the mitochondrial membrane potential by down-regulation of Bcl-2 and up-regulation of Bax, thereby causing the release of cytochrome c from the mitochondria into the cytosol. The liberation of cytochrome c activated caspace-9, which, in turn, activated the downstream executioner caspace-3/7 with the cleaved poly (ADP-ribose) polymerase protein, thereby leading to apoptotic alterations. Increase of caspace 8 had showed the involvement of an extrinsic pathway. This type of apoptosis was suggested to occur through both extrinsic and intrinsic pathways and prevention of translocation of NF-κB from the cytoplasm to the nucleus. Our results revealed AM prompt apoptosis of MCF-7 cells through NF-κB, Bax/Bcl-2 and heat shock protein 70 modulation with the contribution of caspaces. Moreover, ingestion of AM at (30 and 60 mg/kg) significantly reduced tumor size in an animal model of breast cancer. Our results suggest that AM is a potentially useful agent for the treatment of breast cancer.

Mitochondria-mediated apoptosis in mammals

The mitochondria-mediated caspase activation pathway is a major apoptotic pathway characterized by mitochondrial outer membrane permeabilization (MOMP) and subsequent release of cytochrome c into the cytoplasm to activate caspases. MOMP is regulated by the Bcl-2 family of proteins. This pathway plays important roles not only in normal development, maintenance of tissue homeostasis and the regulation of immune system, but also in human diseases such as immune disorders, neurodegeneration and cancer. In the past decades the molecular basis of this pathway and the regulatory mechanism have been comprehensively studied, yet a great deal of new evidence indicates that cytochrome c release from mitochondria does not always lead to irreversible cell death, and that caspase activation can also have non-death functions. Thus, many unsolved questions and new challenges are still remaining. Furthermore, the dysfunction of this pathway involved in cancer development is obvious, and targeting the pathway as a therapeutic strategy has been extensively explored, but the efficacy of the targeted therapies is still under development. In this review we will discuss the mitochondria-mediated apoptosis pathway and its physiological roles and therapeutic implications.

Small molecules as pro-apoptotic anticancer agents

The quest for potent and selective targeted therapies in anticancer research is taking advantage of apoptosis-related mechanisms of action to identify a number of novel clinical candidates. This review is chemically focused on small molecules and deals with five target families that influence caspase-dependent apoptosis: caspase-3, Bcl-2 and IAP protein family members, p53 and the proteasome. Each target class is briefly described at first in terms of its involvement and relevance in tumor initiation and progression. Drug candidates currently undergoing clinical trials are then presented for each target class, followed by a quick summary of target-modulating chemotypes that have appeared in patent literature since 2006. Finally, future trends likely to become significant in apoptosis-targeted cancer therapies are presented and discussed.

Direct, reagentless electrochemical detection of the BIR3 domain of X-linked inhibitor of apoptosis protein using a peptide-based conducting polymer sensor

In this work, we report a reagentless electrochemical peptide (AVPFAQKG) sensor to directly detect the BIR3 domain of X-linked inhibitor of apoptosis protein (XIAP-BIR3). The bioreceptor was based on a conducting copolymer film electrosynthesized from juglone and a juglone-peptide conjugate (JP) newly designed. The peptide-protein interactions generated an important increase of steric hindrance at the interface and a current decrease (signal off) of the redox reaction from quinone embedded in the polymer backbone as evidenced by Square Wave Voltammetry. This allowed a specific and sensitive detection of XIAP-BIR3 with a detection limit of 1 nM (13 ng mL(-1)). The peptide-protein complex could be then dissociated by adding the free precursor peptide (AVPFAQKG) into solution, causing a shift-back on the signal, i.e. an increase in the current intensity (signal-on). This "off-on" detection sequence was used in this work as a double verification of the specificity and this approach can be employed as a general way to increase the reliability of the results. In general, the approach described in this work may be inspired to develop other direct and reagentless electrochemical protein assays with high specificity and sensitivity.

Effect of Smac and Taxol on non-small-cell lung cancer

A series of structurally unique second mitochondria-derived activator of caspases (Smacs) that act as antagonists of the inhibitor of apoptosis proteins (IAPs) directly have been discovered. They play crucial roles in mitochondrial apoptosis pathways and promote chemotherapy-induced apoptosis. In this study, we constructed a eukaryotic expression vector pcDNA3.1/Smac and transfected it into A549 human lung cancer cells. Then we analyzed the cell invasive and cloning ability, as well as cell apoptosis induced by Taxol. The results showed that over-expressed Smac significantly inhibited A549 cell invasive and cloning ability and promoted apoptosis following Taxol treatment. This finding provides a potential approach for the biological therapy of lung cancer.

Pellino3 targets RIP1 and regulates the pro-apoptotic effects of TNF-α

Tumour necrosis factor-α (TNF) can activate NF-κB to induce pro-inflammatory genes but can also stimulate the caspase cascade to promote apoptosis. Here we show that deficiency of the ubiquitin E3 ligase, Pellino3, sensitizes cells to TNF-induced apoptosis without inhibiting the NF-κB pathway. Suppressed expression of Pellino3 leads to enhanced formation of the death-induced signalling complex, complex II, in response to TNF. We show that Pellino3 targets RIP1, in a TNF-dependent manner, to inhibit TNF-induced complex II formation and caspase 8-mediated cleavage of RIP1 in response to TNF/cycloheximide co-stimulation. Pellino3-deficient mice also show increased sensitivity to TNF-induced apoptosis and greatly increased lethality in response to TNF administration. These findings define Pellino3 as a novel regulator of TNF signalling and an important determining factor in dictating whether TNF induces cell survival or death.

SMAC-armed vaccinia virus induces both apoptosis and necroptosis and synergizes the efficiency of vinblastine in HCC

Hepatocellular carcinoma (HCC) has particularly high incidence rate in Asia and its resistance to the chemotherapeutic drugs and cell death make it intractable. Vaccinia virus (VV) is a potential vehicle and has been widely used in cancer therapy. SMAC/DIABLO is a critical factor in activating caspases and eliminating inhibition of IAPs when the programmed cell death is promoted. In this study, we constructed a tumor-targeted vaccinia virus carrying SMAC/DIABLO gene that was knocked in the region of viral thymidine kinase gene (VV-SMAC). Our results showed that VV-SMAC efficiently infected and destroyed HCC cells via triggering both caspase-dependent apoptosis and necroptosis with depletion of IAPs. Furthermore, ripoptosome, a prerequisite complex of necroptosis, was assembled and induced by VV-SMAC. In addition, the combination of VV-SMAC and vinblastine represented a synergistic effect on HCC cells. In summary, our data suggest that VV-SMAC is a potential candidate and combination of VV-SMAC and vinblastine may provide a new avenue in treatment of HCC.

Targeting the intrinsic apoptosis pathway as a strategy for melanoma therapy

Melanoma drug resistance is often attributed to abrogation of the intrinsic apoptosis pathway. Targeting regulators of apoptosis is thus considered a promising approach to sensitizing melanomas to treatment. The development of small-molecule inhibitors that mimic natural antagonists of either antiapoptotic members of the BCL-2 family or the inhibitor of apoptosis proteins (IAPs), known as BH3- or SMAC-mimetics, respectively, are helping us to understand the mechanisms behind apoptotic resistance. Studies using BH3-mimetics indicate that the antiapoptotic BCL-2 protein MCL-1 and its antagonist NOXA are particularly important regulators of BCL-2 family signaling, while SMAC-mimetic studies show that both XIAP and the cIAPs must be targeted to effectively induce apoptosis of cancer cells. Although most solid tumors, including melanoma, are insensitive to these mimetic drugs as single agents, combinations with other therapeutics have yielded promising results, and tests combining them with BRAF-inhibitors, which have already revolutionized melanoma treatment, are a clear priority.

Saturated hydrogen saline attenuates endotoxin-induced acute liver dysfunction in rats

To determine the effect of saturated hydrogen saline on lipopolysaccharide (LPS)-induced acute liver dysfunction, rats were divided into control, LPS, and LPS plus saturated hydrogen saline (LPS+H(2)) groups. Treatment with saturated hydrogen saline prolonged the median survival time and reduced liver dysfunction. Moreover, saturated hydrogen saline significantly reduced pathological alterations in liver tissues, the number of ballooned hepatocytes, serum tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 levels, and myeloperoxidase (MPO) and malondialdehyde (MDA) levels in liver tissues (P<0.05). Cell apoptosis was detected in liver tissues after LPS treatment, and attenuated by saturated hydrogen saline treatment. Saturated hydrogen saline also decreased phosphorylated extracellular signal-regulated kinase (p-ERK), phosphorylated Jun kinase (p-JNK), nuclear factor-kappa B (NF-kappaB), and second mitochondria-derived activator of caspase (Smac) levels, and increased p38 activation (P<0.05). Thus, saturated hydrogen saline may attenuate LPS-induced acute liver dysfunction in rats, possibly by reducing inflammation and cell apoptosis. Mitogen-activated protein kinase (MAPK), NF-kappaB, and Smac may contribute to saturated hydrogen saline-mediated liver protection.

Survivin as a preferential target for cancer therapy

Cancer is typically a consequence of imbalance between cell death and proliferation in a way favorable to cell proliferation and survival. Most conventional cancer therapies are based on targeting rapidly growing cancerous cells to block growth or enhance cell death, thereby, restoring the balance between these processes. In many instances, malignancies that develop resistance to current treatment modalities, such as chemotherapy, immunotherapy, and radiotherapy often present the greatest challenge in subsequent management of the patient. Studies have shown that under normal circumstances, cells utilize different death mechanisms, such as apoptosis (programmed cell death), autophagy, mitotic catastrophe, and necrosis to maintain homeostasis and physiological integrity of the organism, but these processes often appear to be altered in cancer. Thus, in recent years developing various strategies for administration of cytotoxic chemotherapeutics in combination with apoptosis-sensitizing reagents is receiving more emphasis. Here, we review the properties of the anti-apoptotic protein, survivin, a member of the inhibitor of apoptosis protein (IAP) family and the clinical feasibility and anti-cancer potential of drugs targeting this protein. We also discuss some key points and concerns that should be taken into consideration while developing drugs that target apoptotic proteins, such as survivin.

Targeting the apoptosis pathway in hematologic malignancies

Apoptosis is a cell death program that is well-orchestrated for normal tissue homeostasis and for removal of damaged, old or infected cells. It is regulated by intrinsic and extrinsic pathways. The intrinsic pathway responds to signals such as ultraviolet radiation or DNA damage and activates "executioner" caspases through a mitochondria-dependent pathway. The extrinsic pathway is activated by death signals induced, for example, by an infection that activates the immune system or receptor-mediated pathways. The extrinsic pathway signals also cascade down to executioner caspases that cleave target proteins and lead to cell death. Strict control of cellular apoptosis is important for the hematopoietic system as it has a high turnover rate. However, the apoptosis program is often deregulated in hematologic malignancies leading to the accumulation of malignant cells. Therefore, apoptosis pathways have been identified for the development of anticancer therapeutics. We review here the proteins that have been targeted for anticancer drug development in hematologic malignancies. These include BCL-2 family proteins, death ligands and receptors, inhibitor of apoptosis family proteins and caspases. Except for caspase activators, drugs that target each of these classes of proteins have advanced into clinical trials.

Inhibitor of Apoptosis (IAP) proteins in hematological malignancies: molecular mechanisms and therapeutic opportunities

Inhibitor of Apoptosis (IAP) proteins exert essential functions during tumorigenesis as well as treatment resistance by simultaneously blocking cell death pathways and promoting cell survival. As IAP proteins are typically aberrantly expressed in human cancers including hematological malignancies, they represent in principle promising targets for therapeutic interventions. There are currently exciting opportunities to rationally exploit the therapeutic targeting of IAP proteins for the treatment of leukemia and lymphoma. Further insights into the signaling pathways that are under the control of IAP proteins and into the specific IAP protein-dependent vulnerabilities of hematological neoplasms are expected to pave the avenue to novel treatment strategies.

Inhibitor of apoptosis proteins as targets for anticancer therapy

Cell death by apoptosis plays a critical role in regulating the subtle balance between cell death and proliferation to maintain tissue homeostasis. Accordingly, tipping the balance in either direction may cause human disease. Too little cell death may promote tumor formation and progression. In addition, killing of cancer cells by current therapies is largely due to induction of apoptosis in tumor cells. Since a hallmark of human cancers is their resistance to apoptosis, there is a demand to develop novel strategies that restore the apoptotic machinery in order to overcome cancer resistance. Inhibitor of apoptosis proteins (IAPs) block apoptosis at the core of the apoptotic machinery by inhibiting caspases. Elevated levels of IAPs are found in many human cancers and have been associated with poor prognosis. Recent insights into the role of IAPs have provided the basis for various exciting developments that aim to modulate the expression or function of IAPs in human cancers. Targeting IAPs (e.g., by antisense approaches or small-molecule inhibitors) presents a promising novel approach to either directly trigger apoptosis or to potentiate the efficacy of cytotoxic therapies in cancer cells. Thus, inhibition of IAPs such as X chromosome-linked IAP may prove to be a successful strategy to overcome apoptosis resistance of human cancers that deserves further exploitation.

Inhibitor of apoptosis proteins as therapeutic targets in multiple myeloma

The inhibitor of apoptosis (IAP) proteins plays a critical role in the control of apoptotic machinery, and has been explored as a therapeutic target. Here, we have examined the functional importance of IAPs in multiple myeloma (MM) by using a Smac-mimetic LCL161. We observed that LCL161 was able to potently induce apoptosis in some MM cell lines but not in others. Examining the levels of XIAP, cIAP1 and cIAP2 post LCL161 treatment indicated clear down regulation of both XIAP activity and cIAP1 levels in both the sensitive and less sensitive (resistant) cell lines. cIAP2, however, was not down regulated in the cell line resistant to the drug. siRNA mediated silencing of cIAP2 significantly enhanced the effect of LCL161 indicating the importance of down regulating all IAPs simultaneously for induction of apopotsis in MM cells. LCL161 induced marked up regulation of the Jak2/Stat3 pathway in the resistant MM cell lines. Combining LCL161 with a Jak2 specific inhibitor resulted in synergistic cell death in MM cell lines and patient cells. In addition, combining LCL161 with death inducing ligands clearly showed that LCL161 sensitized MM cells to both FAS-L and TRAIL.

Synthesis and biological evaluation of dual action cyclo-RGD/SMAC mimetic conjugates targeting αvβ3/αvβ5 integrins and IAP proteins

Dual action cyclo-RGD/SMAC mimetic conjugates endowed with in vitro activity against anti-apoptotic IAPs and pro-angiogenetic integrins are reported.

Arrestins in apoptosis

Programmed cell death (apoptosis) is a coordinated set of events eventually leading to the massive activation of specialized proteases (caspases) that cleave numerous substrates, orchestrating fairly uniform biochemical changes than culminate in cellular suicide. Apoptosis can be triggered by a variety of stimuli, from external signals or growth factor withdrawal to intracellular conditions, such as DNA damage or ER stress. Arrestins regulate many signaling cascades involved in life-or-death decisions in the cell, so it is hardly surprising that numerous reports document the effects of ubiquitous nonvisual arrestins on apoptosis under various conditions. Although these findings hardly constitute a coherent picture, with the same arrestin subtypes, sometimes via the same signaling pathways, reported to promote or inhibit cell death, this might reflect real differences in pro- and antiapoptotic signaling in different cells under a variety of conditions. Recent finding suggests that one of the nonvisual subtypes, arrestin-2, is specifically cleaved by caspases. Generated fragment actively participates in the core mechanism of apoptosis: it assists another product of caspase activity, tBID, in releasing cytochrome C from mitochondria. This is the point of no return in committing vertebrate cells to death, and the aspartate where caspases cleave arrestin-2 is evolutionary conserved in vertebrate, but not in invertebrate arrestins. In contrast to wild-type arrestin-2, its caspase-resistant mutant does not facilitate cell death.

Radiation-sensitising effects of antennapedia proteins (ANTP)-SmacN7 on tumour cells

The objective of this study was to investigate the underlying mechanisms behind the radiation-sensitising effects of the antennapedia proteins (ANTP)-smacN7 fusion protein on tumour cells. ANTP-SmacN7 fusion proteins were synthesised, and the ability of this fusion protein to penetrate cells was observed. Effects of radiation on the expression of X-linked inhibitor of apoptosis protein (XIAP) were detected by western blotting. The radiation-sensitising effects of ANTP-SmacN7 fusion proteins were observed by a clonogenic assay. The effects of drugs and radiation on tumour cell apoptosis were determined using Annexin V/FITC double staining. Changes in caspase-8, caspase-9 and caspase-3 were detected by western blot before and after ANTP-SmacN7 inhibition of XIAP. The ANTP-SmacN7 fusion protein could enter and accumulate in cells; in vitro XIAP expression of radiation-induced tumour cells was negatively correlated with tumour radiosensitivity. The ANTP-SmacN7 fusion protein promoted tumour cell apoptosis through the activation of caspase3. ANTP-SmacN7 fusion protein may reduce tumour cell radioresistance by inducing caspase3 activation.

Radiation-induced cytochrome c release and the neuroprotective effects of the pan-caspase inhibitor z-VAD-fmk in the hypoglossal nucleus

Numerous studies have demonstrated that neuronal cell death occurs via extrinsic (death receptors) and intrinsic (mitochondria) pathways. Radiation induces caspase activation fundamentally via the mitochondrial pathway. To investigate the role of caspase, a cell permeable pan-caspase inhibitor, z-VAD-fmk [N-benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone], was used to investigate the effects of caspase blockade in vivo following irradiation. Adult male Sprague-Dawley rats (weight, 250–300 g) underwent irradiation at room temperature with a 4-Gy dose of radiation. Since z-VAD-fmk does not penetrate the blood-brain barrier, it was applied intracerebroventricularly via a bolus injection (0.2 μg/h for 1 h). Terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) demonstrated that z-VAD-fmk reduced the numbers of TUNEL-positive cells within the hypoglossal nucleus, suggesting that intervention in the caspase cascade following radiation may have therapeutic applications. The caspase inhibitor z-VAD-fmk reduced the expression and activation of caspase-3, caspase-8 and caspase-9 in the irradiated rats, indicating that caspase may be a potential therapeutic target in the treatment of brain radiation injury. Treatment with z-VAD-fmk also reduced the appearance of cytochrome c within the cytosolic fraction following radiation. The hypoglossal nucleus may be used as a model of radiation-induced injury in the central nervous system, providing visual information and displaying apoptotic nuclear morphology.

Apoptosis and necrosis in the liver

Because of its unique function and anatomical location, the liver is exposed to a multitude of toxins and xenobiotics, including medications and alcohol, as well as to infection by hepatotropic viruses, and therefore, is highly susceptible to tissue injury. Cell death in the liver occurs mainly by apoptosis or necrosis, with apoptosis also being the physiologic route to eliminate damaged or infected cells and to maintain tissue homeostasis. Liver cells, especially hepatocytes and cholangiocytes, are particularly susceptible to death receptor-mediated apoptosis, given the ubiquitous expression of the death receptors in the organ. In a quite unique way, death receptor-induced apoptosis in these cells is mediated by both mitochondrial and lysosomal permeabilization. Signaling between the endoplasmic reticulum and the mitochondria promotes hepatocyte apoptosis in response to excessive free fatty acid generation during the metabolic syndrome. These cell death pathways are partially regulated by microRNAs. Necrosis in the liver is generally associated with acute injury (i.e., ischemia/reperfusion injury) and has been long considered an unregulated process. Recently, a new form of "programmed" necrosis (named necroptosis) has been described: the role of necroptosis in the liver has yet to be explored. However, the minimal expression of a key player in this process in the liver suggests this form of cell death may be uncommon in liver diseases. Because apoptosis is a key feature of so many diseases of the liver, therapeutic modulation of liver cell death holds promise. An updated overview of these concepts is given in this article.

Small-molecule IAP antagonists sensitize cancer cells to TRAIL-induced apoptosis: roles of XIAP and cIAPs

TNF-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent because it shows apoptosis-inducing activity in transformed, but not in normal, cells. As with most anticancer agents, however, its clinical use is restricted by either inherent or acquired resistance by cancer cells. We demonstrate here that small-molecule SMAC mimetics that antagonize the inhibitor of apoptosis proteins (IAP) potently sensitize previously resistant human cancer cell lines, but not normal cells, to TRAIL-induced apoptosis, and that they do so in a caspase-8-dependent manner. We further show that the compounds have no cytotoxicity as single agents. Also, we demonstrate that several IAP family members likely participate in the modulation of cellular sensitivity to TRAIL. Finally, we note that the compounds that sensitize cancer cells to TRAIL are the most efficacious in binding to X-linked IAP, and in inducing cellular-IAP (cIAP)-1 and cIAP-2 degradation. Our studies thus describe valuable compounds that allow elucidation of the signaling events occurring in TRAIL resistance, and demonstrate that these agents act as potent TRAIL-sensitizing agents in a variety of cancer cell lines.

Xiao-Xu-Ming decoction preserves mitochondrial integrity and reduces apoptosis after focal cerebral ischemia and reperfusion via the mitochondrial p53 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Xiao-Xu-Ming decoction (XXMD) has been used to treat stroke and other neurological diseases for more than 1000 years. The purpose of this study was to investigate the effects of XXMD on mitochondrial damage and apoptosis after cerebral ischemia and reperfusion.

MATERIALS AND METHODS

Male Sprague-Dawley rats were randomly divided into 3 groups: sham, cerebral ischemia and reperfusion (I/R), and cerebral ischemia and reperfusion plus XXMD (60 g/kg/day) (XXMD60). Focal cerebral ischemia and reperfusion models were induced by middle cerebral artery occlusion. Cerebral ischemic injury was evaluated by hematoxylin and eosin staining. Ultrastructural features of mitochondria in the penumbra of the ischemic cortex were analyzed by transmission electron microscopy. Apoptosis was evaluated by terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate nick end labeling (TUNEL) staining and cleaved caspase 3 immunohistochemistry. Proteins in the mitochondrial p53 pathway were detected by western blot and immunofluorescence.

RESULTS

The results showed that XXMD treatment markedly attenuated ischemic changes, preserved mitochondrial integrity, and significantly reduced apoptosis. In addition, we found that XXMD treatment reduced p53 and Bax levels and increased Bcl-2 levels in mitochondrial fractions. XXMD significantly blocked the release of cytochrome c and Smac/Diablo from mitochondria, and inhibited activation of caspase 9 and caspase 3 in cytoplasmic fractions. Increased expression of c-IAP1 was observed in the XXMD60 group.

CONCLUSIONS

The findings demonstrated that XXMD protected mitochondria from ischemic injury and inhibited apoptosis. The mitochondrial p53 pathway could be partially involved in the protective effects.

Smac mimetic compound LCL161 sensitizes esophageal carcinoma cells to radiotherapy by inhibiting the expression of inhibitor of apoptosis protein

Currently, unresectable esophageal squamous cell carcinoma (ESCC) is primarily treated by chemoradiotherapy. However, the outcome has not improved significantly due to radioresistance of cancer cells. This study aimed to determine the radiosensitizing effect of LCL161, a novel second mitochondrial-derived activator of caspase (Smac) mimetic, in ESCC cells. ESCC cell lines were treated with LCL161 or radiation, alone or in combination. Cell proliferation was detected by MTT assay. Radiosensitization was evaluated by clonogenic survival assay. Cell apoptosis was detected by flow cytometry. The results showed that LCL161 potently sensitized ESCC cells to radiation with a sensitization enhancement ratio of 1.4-2.0. LCL161 increased radiation-induced DNA double-stranded breaks and promoted the apoptosis of ESCC cells, which could be abrogated by a pan-caspase inhibitor z-VAD-FMK. Furthermore, LCL161 decreased the level of cIAP1 in ESCC cells in a dose-dependent manner and synthesized with irradiation to promote the activation of caspase 8 and the upregulation of TNFα expression in ESCC cells. In conclusion, LCL161 acts as a strong radiosensitizer in human esophageal cancer cells by inhibiting the expression of cIAP1 and promoting the activation of caspase 8, leading to enhanced apoptosis.

IAP proteins as targets for drug development in oncology

The inhibitors of apoptosis (IAPs) constitute a family of proteins involved in the regulation of various cellular processes, including cell death, immune and inflammatory responses, cell proliferation, cell differentiation, and cell motility. There is accumulating evidence supporting IAP-targeting in tumors: IAPs regulate various cellular processes that contribute to tumor development, such as cell death, cell proliferation, and cell migration; their expression is increased in a number of human tumor samples, and IAP overexpression has been correlated with tumor growth, and poor prognosis or low response to treatment; and IAP expression can be rapidly induced in response to chemotherapy or radiotherapy because of the presence of an internal ribosome entry site (IRES)-dependent mechanism of translation initiation, which could contribute to resistance to antitumor therapy. The development of IAP antagonists is an important challenge and was subject to intense research over the past decade. Six molecules are currently in clinical trials. This review focuses on the role of IAPs in tumors and the development of IAP-targeting molecules for anticancer therapy.

Withanone as an inhibitor of survivin: a potential drug candidate for cancer therapy

Survivin, the smallest inhibitor of apoptosis protein, which has been reported to be highly expressed in almost all known cancers, plays a dual role in survival as well as the proliferation of cancer cells. It inhibits apoptosis by inhibiting caspases as well as facilitating mitosis by becoming a part of chromosomal passenger complex through its BIR5 domain. Docking studies carried out with herbal ligand withanone derived from roots of Withania somnifera have shown strong binding affinity of -19.1088 kJ/mol with BIR5 domain of survivin and in turn interferes with inhibitory action against caspases and may lead to apoptosis. Binding of withanone at BIR5 domain of survivin may also interfere with chromosomal passenger complex and lead to halt the mitotic process within the cancer cell. Docking studies support various experimental outcomes and suggest withanone as a potential candidate molecule in cancer therapy.

RAF inhibition overcomes resistance to TRAIL-induced apoptosis in melanoma cells

Mutated BRAF represents a critical oncogene in melanoma, and selective inhibitors have been approved for melanoma therapy. However, the molecular consequences of RAF inhibition in melanoma cells remained largely elusive. Here, we investigated the effects of the pan-RAF inhibitor L-779,450, which inhibited cell proliferation both in BRAF-mutated and wild-type melanoma cell lines. It furthermore enhanced apoptosis in combination with the death ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and overcame TRAIL resistance in melanoma cells. Enhanced apoptosis coincided with activation of mitochondrial pathways, seen by loss of mitochondrial membrane potential and release of cytochrome c, Smac (second mitochondria-derived activator of caspases), and apoptosis-inducing factor (AIF). Subsequently, caspase-9 and -3 were activated. Apoptosis induction by L-779,450/TRAIL was prevented by Bcl-2 overexpression and was dependent on Bax. Thus, activation of Bax by L-779,450 alone was demonstrated by Bax conformational changes, whereas Bak was not activated. Furthermore, the BH3-only protein Bim was upregulated in response to L-779,450. The significant roles of Smac, Bax, and Bim in this setting were proven by small interfering RNA (siRNA)-mediated knockdown experiments. L-779,450 also resulted in morphological changes indicating autophagy confirmed by the autophagy marker light chain 3-II (LC3-II). The pro-apoptotic effects of L-779,450 may explain the antitumor effects of RAF inhibition and may be considered when evaluating RAF inhibitors for melanoma therapy.

Caspase-dependent regulation of the ubiquitin-proteasome system through direct substrate targeting

Drosophila inhibitor of apoptosis (IAP) 1 (DIAP1) is an E3 ubiquitin ligase that regulates apoptosis in flies, in large part through direct inhibition and/or ubiquitinylation of caspases. IAP antagonists, such as Reaper, Hid, and Grim, are thought to induce cell death by displacing active caspases from baculovirus IAP repeat domains in DIAP1, but can themselves become targets of DIAP1-mediated ubiquitinylation. Herein, we demonstrate that Grim self-associates in cells and is ubiquitinylated by DIAP1 at Lys136 in an UbcD1-dependent manner, resulting in its rapid turnover. K48-linked ubiquitin chains are added almost exclusively to BIR2-bound Grim as a result of its structural proximity to DIAP1's RING domain. However, active caspases can simultaneously cleave Grim at Asp132, removing the lysine necessary for ubiquitinylation as well as any existing ubiquitin conjugates. Cleavage therefore enhances the stability of Grim and initiates a feed-forward caspase amplification loop, resulting in greater cell death. In summary, Grim is a caspase substrate whose cleavage promotes apoptosis by limiting, in a target-specific fashion, its ubiquitinylation and turnover by the proteasome.

Physiologically based pharmacokinetic and pharmacodynamic modeling of an antagonist (SM-406/AT-406) of multiple inhibitor of apoptosis proteins (IAPs) in a mouse xenograft model of human breast cancer

The inhibitors of apoptosis proteins (IAPs) are a class of key apoptosis regulators overexpressed or dysregulated in cancer. SM-406/AT-406 is a potent and selective small molecule mimetic of Smac that antagonizes the inhibitor of apoptosis proteins (IAPs). A physiologically based pharmacokinetic and pharmacodynamic (PBPK-PD) model was developed to predict the tissue concentration-time profiles of SM-406, the related onco-protein levels in tumor, and the tumor growth inhibition in a mouse model bearing human breast cancer xenograft. In the whole body physiologically based pharmacokinetic (PBPK) model for pharmacokinetics characterization, a well stirred (perfusion rate-limited) model was used to describe SM-406 pharmacokinetics in the lung, heart, kidney, intestine, liver and spleen, and a diffusion rate-limited (permeability limited) model was used for tumor. Pharmacodynamic (PD) models were developed to correlate the SM-406 concentration in tumor to the cIAP1 degradation, pro-caspase 8 decrease, CL-PARP accumulation and tumor growth inhibition. The PBPK-PD model well described the experimental pharmacokinetic data, the pharmacodynamic biomarker responses and tumor growth. This model may be helpful to predict tumor and plasma SM-406 concentrations in the clinic.

Smac/DIABLO regulates the apoptosis of hypertrophic scar fibroblasts

In abnormal skin wound healing, hypertrophic scars (HS) are characterized by excessive fibroblast hypercellularity and an overproduction of collagen, leading to atypical extracellular matrix (ECM) remodeling. Although the exact mechanisms of HS remain unclear, decreased HS fibroblast (HSFB) apoptosis and increased proliferation are evident in the development of HS. In this study, the contribution of the second mitochondria-derived activator of caspases/direct inhibitor of apoptosis protein (IAP)-binding protein with a low isoelectric point (pI) (Smac/DIABLO), an apoptosis-promoting protein released from the mitochondria, was investigated in human normal skin and HSFB cultures. The expression of Smac/DIABLO is usually decreased in many malignant tumors compared with normal tissues. Immunohistochemical analysis of skin tissues and the western blot analyses of fibroblasts revealed that the expression of Smac/DIABLO was lower in HS tissues compared with normal skin tissues. Of note, adenovirus-mediated Smac/DIABLO overexpression in the cultured HSFBs significantly reduced cell proliferation, as detected by the cell counting kit-8, and increased caspase-3 and -9 activity, as detected by spectrofluorimetry. In addition, it increased apoptosis, as detected by fluorescence-activated cell sorting (FACS). Furthermore, we found that the silencing of Smac with siRNA in the HSFBs induced a noticeable decrease in caspase-3 and -9 activity, leading to a significant reduction in apoptosis. In addition, the mRNA expression of type I and III pro-collagen detected in the HSFBs was significantly increased following the silencing of Smac with siRNA and was inhibited following Smac/DIABLO overexpression, as shown by real-time RT-PCR. In conclusion, Smac/DIABLO decreases the proliferation and increases the apoptosis of HSFBs. To our knowledge, the data from our study suggest for the first time that Smac/DIABLO is a novel therapeutic target for HS.