Cell type specific involvement of death receptor and mitochondrial pathways in drug-induced apoptosis

Lysosomal-mitochondrial axis in zoledronic acid-induced apoptosis in human follicular lymphoma cells

Bisphosphonates (BPs) are potent inhibitors of osteoclast function, widely used to treat excessive bone resorption associated with bone metastases, that also have anti-tumor activity. Zoledronic acid (ZOL) represents a potential chemotherapeutic agent for the treatment of cancer. ZOL is the most potent nitrogen-containing BPs, and it inhibits cell growth and induces apoptosis in a variety of cancer cells. Recently we demonstrated that accumulation of isopentenyl pyrophosphate and the consequent formation of a new type of ATP analog (ApppI) after mevalonate pathway inhibition by nitrogen-containing BPs strongly correlates with ZOL-induced cell death in cancer cells in vitro. In this study we show that ZOL-induced apoptosis in HF28RA human follicular lymphoma cells occurs exclusively via the mitochondrial pathway, involves lysosomes, and is dependent on mevalonate pathway inhibition. To define the exact signaling pathway connecting them, we used modified HF28RA cell lines overexpressing either BclXL or dominant-negative caspase-9. In both mutant cells, mitochondrial and lysosomal membrane permeabilization (MMP and LMP) were totally prevented, indicating signaling between lysosomes and mitochondria and, additionally, an amplification loop for MMP and/or LMP regulated by caspase-9 in association with farnesyl pyrophosphate synthetase inhibition. Additionally, the lysosomal pathway in ZOL-induced apoptosis plays an additional/amplification role of the intrinsic pathway independently of caspase-3 activation. Moreover, we show a potential regulation by Bcl-XL and caspase-9 on cell cycle regulators of S-phase. Our findings provide a molecular basis for new strategies concomitantly targeting cell death pathways from multiple sites.

Arsenate-induced apoptosis in murine embryonic maxillary mesenchymal cells via mitochondrial-mediated oxidative injury

BACKGROUND

Arsenic is a ubiquitous element that is a potential carcinogen and teratogen and can cause adverse developmental outcomes. Arsenic exerts its toxic effects through the generation of reactive oxygen species (ROS) that include hydrogen peroxide (H(2)O(2)), superoxide-derived hydroxyl ion, and peroxyl radicals. However, the molecular mechanisms by which arsenic induces cytotoxicity in murine embryonic maxillary mesenchymal (MEMM) cells are undefined.

METHODS

MEMM cells in culture were treated with different concentrations of pentavalent sodium arsenate [As (V)] for 24 or 48 hr and various end points measured.

RESULTS

Treatment of MEMM cells with the pentavalent form of inorganic arsenic resulted in caspase-mediated apoptosis, accompanied by generation of ROS and disruption of mitochondrial membrane potential. Treatment with caspase inhibitors markedly blocked apoptosis. In addition, the free radical scavenger N-acetylcysteine dramatically attenuated arsenic-mediated ROS production and apoptosis, and exposure to arsenate increased Bax and decreased Bcl protein levels in MEMM cells.

CONCLUSIONS

Taken together, these findings suggest that in MEMM cells arsenate-mediated oxidative injury acts as an early and upstream initiator of the cell death cascade, triggering cytotoxicity, mitochondrial dysfunction, altered Bcl/Bax protein ratios, and activation of caspase-9.

Targeting specific cell signaling transduction pathways by dietary and medicinal phytochemicals in cancer chemoprevention

Natural phytochemicals derived from dietary sources or medicinal plants have gained significant recognition in the potential management of several human clinical conditions. Much research has also been geared towards the evaluation of plant extracts as effective prophylactic agents since they can act on specific and/or multiple molecular and cellular targets. Plants have been an abundant source of highly effective phytochemicals which offer great potential in the fight against cancer by inhibiting the process of carcinogenesis through the upregulation of cytoprotective genes that encode for carcinogen detoxifying enzymes and antioxidant enzymes. The mechanistic insight into chemoprevention further includes induction of cell cycle arrest and apoptosis or inhibition of signal transduction pathways mainly the mitogen-activated protein kinases (MAPK), protein kinases C (PKC), phosphoinositide 3-kinase (PI3K), glycogen synthase kinase (GSK) which lead to abnormal cyclooxygenase-2 (COX-2), activator protein-1 (AP-1), nuclear factor-kappaB (NF-κB) and c-myc expression. Effectiveness of chemopreventive agents reflects their ability to counteract certain upstream signals that leads to genotoxic damage, redox imbalances and other forms of cellular stress. Targeting malfunctioning molecules along the disrupted signal transduction pathway in cancer represent a rational strategy in chemoprevention. NF-κB and AP-1 provide mechanistic links between inflammation and cancer, and moreover regulate tumor angiogenesis and invasiveness, indicating that signaling pathways that mediate their activation provide attractive targets for new chemotherapeutic approaches. Thus cell signaling cascades and their interacting factors have become important targets of chemoprevention and phenolic phytochemicals and plant extracts seem to be promising in this endeavor.

Proapoptotic effects of dietary (n-3) fatty acids are enhanced in colonocytes of manganese-dependent superoxide dismutase knockout mice

We recently demonstrated that (n-3) PUFA trigger the induction of apoptosis in the colon by enhancing phospholipid oxidation and mitochondrial Ca2+ accumulation. To further elucidate the mechanisms regulating oxidative stress-induced apoptosis in vivo, a 2 x 2 experiment was designed using both wild type (control) and manganese-dependent superoxide dismutase (SOD2) heterozygous knockout mice (SOD2(+/-)), which exhibit increased mitochondrial oxidative stress. Mice were fed diets differing only in the type of fat [corn oil or fish oil containing (n-3) PUFA] at 15% by weight for 4 wk. Dietary (n-3) PUFA treatment enhanced (22%) apoptosis in colonic crypts. In addition, SOD2 haploinsufficiency enhanced (20%) apoptosis, which was further increased (36%) by (n-3) PUFA feeding. Dietary lipid source and genotype interactively modulated nitrotyrosine levels (P = 0.027) and inflammation (P = 0.032). These findings demonstrate that the proapoptotic effects of (n-3) PUFA are enhanced in oxidatively stressed SOD2(+/-) mice. Thus, (n-3) PUFA appear to promote an oxidation-reduction imbalance in the intestine, which may directly or indirectly trigger apoptosis and thereby reduce colon cancer risk.

Evidence for programmed cell death and activation of specific caspase-like enzymes in the tomato fruit heat stress response

The tomato (Lycopersicon esculentum) fruit is the best available model to study the stress response of fleshy fruit. Programmed cell death (PCD) plays an important role in stress responses in mammals and plants. In this study, we provide evidence that PCD is triggered in the tomato fruit heat stress response by detection of the sequential diagnostic PCD events, including release of cytochrome c, activation of caspase-like proteases and the presence of TUNEL-positive nuclei. Investigating the time course of these events for 12 h after heat treatment indicated that cytochrome c release and caspase-like protease activation occurred rapidly and were consistent with the onset of DNA fragmentation. In addition, LEHDase and DEVDase enzymes were specifically activated in tomato fruit pericarp during the heat treatment and recovery time. There was no significant activation of YVADase or IETDase proteases. Preincubation of pericarp discs with the broad-spectrum, cell-permeable caspase inhibitor Z-VAD-FMK, suppressed heat-induced cell death measured by trypan blue, accompanied by a decrease in LEHDase and DEVDase activities.

Neuromodulatory activities of CD4+CD25+ regulatory T cells in a murine model of HIV-1-associated neurodegeneration

HIV-1-associated neurocognitive impairments are intrinsically linked to microglial immune activation, persistent viral infection, and inflammation. In the era of antiretroviral therapy, more subtle cognitive impairments occur without adaptive immune compromise. We posit that adaptive immunity is neuroprotective, serving in both the elimination of infected cells through CD8(+) cytotoxic T cell activities and the regulation of neuroinflammatory responses of activated microglia. For the latter, little is known. Thus, we studied the neuromodulatory effects of CD4(+) regulatory T cells (Treg; CD4(+)CD25(+)) or effector T cells in HIV-1-associated neurodegeneration. A newly developed HIV-1 encephalitis mouse model was used wherein murine bone marrow-derived macrophages are infected with a full-length HIV-1(YU2)/vesicular stomatitis viral pseudotype and injected into basal ganglia of syngeneic immunocompetent mice. Adoptive transfer of CD3-activated Treg attenuated astrogliosis and microglia inflammation with concomitant neuroprotection. Moreover, Treg-mediated anti-inflammatory activities and neuroprotection were associated with up-regulation of brain-derived neurotrophic factor and glial cell-derived neurotrophic factor expression and down-regulation of proinflammatory cytokines, oxidative stress, and viral replication. Effector T cells showed contrary effects. These results, taken together, demonstrate the importance of Treg in disease control and raise the possibility of their utility for therapeutic strategies.

Knockdown of human bid gene expression enhances survival of CD8+ T cells

Tumor cells have developed immune evasion mechanisms such as considerably heterogenous FasL expression on their surface via which they could induce apoptosis of tumor-specific cytotoxic T lymphocytes (CTLs) in the immune system. Meanwhile, the competition of normal immune cells with tumor cells results in relative growth factors shortage for growth and proliferation of nontumor cells, which improves a susceptibility to early apoptosis of CTL. In an attempt to develop strategies for prolonging the survival of adoptively transferred T cells in a hostile pro-apoptotic tumor microenvironment, we used synthetic siRNA and vector-based shRNA to suppress the expression of Bid in human uterocervical carcinoma HeLa cells, followed by the further achievement of Bid gene silencing in human primary cells-CD8(+) lymphocytes via retrovirus-delivered siRNAs. Our results indicated that Bid knockdown HeLa cells are partially resistant to Fas antibody- or serum deprivation-induced apoptosis. Additionally, the blockade of Bid expression in CD8(+) lymphocytes resulted in a less susceptiveness to Fas antibody-induced apoptosis and a survival advantage following recombinant human interleukin-2 (rhIL-2) withdrawal or under lower rhIL-2 concentrations compared with control lymphocytes. These data suggest that knockdown of Bid might serve as an approach to enhancing the survival and tumoricidal activity of T lymphocytes in adoptive immunotherapy.

Differences in doxorubicin-induced apoptotic signaling in adult and immature cardiomyocytes

A proposed mechanism for the cardiotoxicity of doxorubicin (DOX) involves apoptosis in cardiomyocytes. In the study described here, we investigated the molecular basis for the differences in DOX-induced toxicity in adult rat cardiomyocytes (ARCM), neonatal rat cardiomyocytes (NRCM), and rat embryonic H9c2 cardiomyoblasts. Activation of caspase-9 and -3 was considerably lower in DOX-treated ARCM as compared with NRCM and H9c2 cardiomyoblasts. Addition of cytochrome c caused the activation of caspase-9 and -3 in permeabilized NRCM and H9c2 cardiomyoblasts but not in permeabilized ARCM. Expression of proapoptotic proteins, apoptotic protease activating factor-1 (Apaf1), and procaspase-9 was significantly lower, and abundance of antiapoptotic X-linked inhibitor of apoptosis protein (XIAP) was higher in ARCM, as compared with immature cardiac cells. Despite the abundance of XIAP in ARCM, its role in the inhibition of apoptosome function was dismissed, as second mitochondria-derived activator of caspases (Smac)-N7 peptide, had no effect on caspase activation in response to cytochrome c in these cells. Adenoviral expression of Apaf1 exacerbated the activation of caspase-9 and -3 in DOX-treated NRCM, but did not increase their activities in DOX-treated ARCM. This finding points to a major difference in the apoptotic signaling between immature and adult cardiomyocytes. The mitochondrial apoptotic pathway is limited in ARCM treated with DOX.

Construction and analysis of a modular model of caspase activation in apoptosis

Background

A key physiological mechanism employed by multicellular organisms is apoptosis, or programmed cell death. Apoptosis is triggered by the activation of caspases in response to both extracellular (extrinsic) and intracellular (intrinsic) signals. The extrinsic and intrinsic pathways are characterized by the formation of the death-inducing signaling complex (DISC) and the apoptosome, respectively; both the DISC and the apoptosome are oligomers with complex formation dynamics. Additionally, the extrinsic and intrinsic pathways are coupled through the mitochondrial apoptosis-induced channel via the Bcl-2 family of proteins.

Results

A model of caspase activation is constructed and analyzed. The apoptosis signaling network is simplified through modularization methodologies and equilibrium abstractions for three functional modules. The mathematical model is composed of a system of ordinary differential equations which is numerically solved. Multiple linear regression analysis investigates the role of each module and reduced models are constructed to identify key contributions of the extrinsic and intrinsic pathways in triggering apoptosis for different cell lines.

Conclusion

Through linear regression techniques, we identified the feedbacks, dissociation of complexes, and negative regulators as the key components in apoptosis. The analysis and reduced models for our model formulation reveal that the chosen cell lines predominately exhibit strong extrinsic caspase, typical of type I cell, behavior. Furthermore, under the simplified model framework, the selected cells lines exhibit different modes by which caspase activation may occur. Finally the proposed modularized model of apoptosis may generalize behavior for additional cells and tissues, specifically identifying and predicting components responsible for the transition from type I to type II cell behavior.

Selenocystine induces S-phase arrest and apoptosis in human breast adenocarcinoma MCF-7 cells by modulating ERK and Akt phosphorylation

Selenocystine (SeC) is a nutritionally available selenoamino acid with selective anticancer effects on a number of human cancer cell lines. The present study shows that SeC inhibited the proliferation of human breast adenocarcinoma MCF-7 cells in a time- and dose-dependent manner, through the induction of cell cycle arrest and apoptotic cell death. SeC-induced S-phase arrest was associated with a marked decrease in the protein expression of cyclins A, D1, and D3 and cyclin-dependent kinases (CDKs) 4 and 6, with concomitant induction of p21waf1/Cip1, p27Kip1, and p53. Exposure of MCF-7 cells to SeC resulted in apoptosis as evidenced by caspase activation, PARP cleavage, and DNA fragmentation. SeC treatment also triggered the activation of JNK, p38 MAPK, ERK, and Akt. Inhibitors of ERK (U0126) and Akt (LY294002), but not JNK (SP600125) and p38 MAPK (SB203580), suppressed SeC-induced S-phase arrest and apoptosis in MCF-7 cells. The findings establish a mechanistic link between the PI3K/Akt pathway, MAPK pathway, and SeC-induced cell cycle arrest and apoptosis in MCF-7 cells.

Melanoma genetics and therapeutic approaches in the 21st century: moving from the benchside to the bedside

Metastatic melanoma is notoriously one of the most difficult cancers to treat. Although many therapeutic regimens have been tested, very few achieve response rates greater than 25%. Given the rising incidence of melanoma and the paucity of effective treatments, there is much hope and excitement in leveraging recent genetic and molecular insights for therapeutic advantage. Over the past 30 years, elegant studies by many groups have helped decipher the complex genetic networks involved in melanoma proliferation, progression and survival, as well as several genes involved in melanocyte development and survival. Many of these oncogenic loci and pathways have become crucial targets for pharmacological development. In this article we review: (1) our current understanding of melanoma genetics within the context of signaling networks; (2) targeted therapies, including an extensive discussion of promising agents that act in the Bcl-2 signaling network; (3) future areas of research.

Osmium carbonyl clusters: a new class of apoptosis inducing agents

Osmium carbonyl clusters, especially the cluster [Os(3)(CO)(10)(NCCH(3))(2)], were found to be active against four cancer cell lines, namely, ER+ breast carcinoma (MCF-7), ER- breast carcinoma (MDA-MB-231), metastatic colorectal adenocarcinoma (SW620), and hepatocarcinoma (Hep G2). The mode of action was studied in MCF-7 and MDA-MB-231 cell lines by a number of morphological and apoptosis assays, all of which pointed to the induction of apoptosis.

Global sequencing of proteolytic cleavage sites in apoptosis by specific labeling of protein N termini

The nearly 600 proteases in the human genome regulate a diversity of biological processes, including programmed cell death. Comprehensive characterization of protease signaling in complex biological samples is limited by available proteomic methods. We have developed a general approach for global identification of proteolytic cleavage sites using an engineered enzyme to selectively biotinylate free protein N termini for positive enrichment of corresponding N-terminal peptides. Using this method to study apoptosis, we have sequenced 333 caspase-like cleavage sites distributed among 292 protein substrates. These sites are generally not predicted by in vitro caspase substrate specificity but can be used to predict other physiological caspase cleavage sites. Structural bioinformatic studies show that caspase cleavage sites often appear in surface-accessible loops and even occasionally in helical regions. Strikingly, we also find that a disproportionate number of caspase substrates physically interact, suggesting that these dimeric proteases target protein complexes and networks to elicit apoptosis.

Molecular targeted therapies for pancreatic cancer

BACKGROUND

Pancreatic cancer cells express different mutations that increase the aggressiveness and confer resistance to conventional chemotherapy and radiotherapy. Molecules that selectively bind and inhibit these mutations are effective in other solid tumors and are now emerging as a complementary therapy in pancreatic cancer. The objective of this review is to describe the effect of drugs that inhibit specific mutations present in pancreatic cancer with special emphasis on clinical trials.

DATA SOURCES

We reviewed the English-language literature (MedLine) addressing the role of drugs that target mutations present in pancreatic cancer. Both preclinical and clinical studies were included.

CONCLUSIONS

Preclinical evidence supports the combination of conventional approved therapies plus drugs that block epidermal growth factor receptor and vascular growth endothelial factor or induce apoptosis. However, most of the current clinical evidence is limited to small phase I trials evaluating the toxicity and safety of these regimens. The results of additional randomized trials that are still undergoing will clarify the role of these drugs in pancreatic cancer.

Dysregulation of apoptotic signaling in cancer: molecular mechanisms and therapeutic opportunities

Apoptosis is a tightly regulated cell suicide program that plays an essential role in the maintenance of tissue homeostasis by eliminating unnecessary or harmful cells. Defects in this native defense mechanism promote malignant transformation and frequently confer chemoresistance to transformed cells. Indeed, the evasion of apoptosis has been recognized as a hallmark of cancer. Given that multiple mechanisms function at many levels to orchestrate the regulation of apoptosis, a multitude of opportunities for apoptotic dysregulation are present within the intricate signaling network of cell. Several of the molecular mechanisms by which cancer cells are protected from apoptosis have been elucidated. These advances have facilitated the development of novel apoptosis-inducing agents that have demonstrated single-agent activity against various types of cancers cells and/or sensitized resistant cancer cells to conventional cytotoxic therapies. Herein, we will highlight several of the central modes of apoptotic dysregulation found in cancer. We will also discuss several therapeutic strategies that aim to reestablish the apoptotic response, and thereby eradicate cancer cells, including those that demonstrate resistance to traditional therapies.

Mechanisms by which docosahexaenoic acid and related fatty acids reduce colon cancer risk and inflammatory disorders of the intestine

A growing body of epidemiological, clinical, and experimental evidence has underscored both the pharmacological potential and the nutritional value of dietary fish oil enriched in very long chain n-3 PUFAs such as docosahexaenoic acid (DHA, 22:6, n-3) and eicosapentaenoic acid (EPA, 20:5, n-3). The broad health benefits of very long chain n-3 PUFAs and the pleiotropic effects of dietary fish oil and DHA have been proposed to involve alterations in membrane structure and function, eicosanoid metabolism, gene expression and the formation of lipid peroxidation products, although a comprehensive understanding of the mechanisms of action has yet to be elucidated. In this review, we present data demonstrating that DHA selectively modulates the subcellular localization of lipidated signaling proteins depending on their transport pathway, which may be universally applied to other lipidated protein trafficking. An interesting possibility raised by the current observations is that lipidated proteins may exhibit different subcellular distribution profiles in various tissues, which contain a distinct membrane lipid composition. In addition, the current findings clearly indicate that subcellular localization of proteins with a certain trafficking pathway can be subjected to selective regulation by dietary manipulation. This form of regulated plasma membrane targeting of a select subset of upstream signaling proteins may provide cells with the flexibility to coordinate the arrangement of signaling translators on the cell surface. Ultimately, this may allow organ systems such as the colon to optimally decode, respond, and adapt to the vagaries of an ever-changing extracellular environment.

Apoptotic pathways in tumor progression and therapy

Apoptosis is a cell suicide program that plays a critical role in development and tissue homeostasis. The ability of cancer cells to evade this programmed cell death (PCD) is a major characteristic that enables their uncontrolled growth. The efficiency of chemotherapy in killing such cells depends on the successful induction of apoptosis, since defects in apoptosis signaling are a major cause of drug resistance. Over the past decades, much progress has been made in our understanding of apoptotic signaling pathways and their dysregulation in cancer progression and therapy. These advances have provided new molecular targets for proapoptotic cancer therapies that have recently been used in drug development. While most of those therapies are still at the preclinical stage, some of them have shown much promise in the clinic. Here, we review our current knowledge of apoptosis regulation in cancer progression and therapy, as well as the new molecular targeted molecules that are being developed to reinstate cancer cell death.

Deglycosylated bleomycin induces apoptosis in lymphoma cell via c-jun NH2-terminal kinase but not reactive oxygen species

Bleomycin (BLM) has demonstrated potent activity in treating malignant lymphomas but its therapeutic efficacy is hampered by induction of lung fibrosis. This side effect is related to the ability of the drug to generate reactive oxygen species in lung cells. In the present study, we evaluated the consequences of deglycosylation of BLM in term of cytotoxic activity and generation of reactive oxygen species. When tested on U937 human lymphoma cells, both compounds generated a typical apoptotic phenotype. Cell death induction was associated with Bax oligomerization, dissipation of the mitochondrial membrane potential, release of cytochrome c, caspase activation, chromatin condensation and internucleosomal degradation. Whereas both reactive oxygen species and c-jun NH(2)-terminal kinase (JNK) inhibitors prevented BLM-induced U937 cell death, only JNK inhibition prevented deglycosylated BLM-mediated cell death. Both compounds induced clustering of TRAIL receptors (DR4 and DR5) and Fas at the cell surface but neither a chimeric soluble DR5 receptor that inhibits TRAIL-induced cell death nor a dominant negative version of the adaptor molecule Fas-associated death domain prevented BLM-induced cytotoxicity. These observations indicate that deglycosylation of BLM does not impair the ability of the drug to trigger cell death through activation of the intrinsic pathway but prevents induction of reactive oxygen species. This observation suggests that deglycosylated BLM could exhibit less toxic side effects and could warrant its use in clinic.

Apoptosis and chemo-resistance in colorectal cancer

Systemic chemotherapy plays an integral part in treating advanced colorectal cancer. However 50% of patients respond poorly or have disease progression due to resistance to chemotherapeutic agents. This article reviews the pathways that regulate apoptosis, apoptotic mechanisms through which chemotherapeutic agents mediate their effect and how deregulation of apoptotic proteins may contribute to chemo-resistance. Also discussed are potential therapeutic strategies designed to target these proteins and thereby improve response rates to chemotherapy in colorectal cancer.

Nuclear caspase-3 and caspase-7 activation, and poly(ADP-ribose) polymerase cleavage are early events in camptothecin-induced apoptosis

Chemotherapy-induced apoptosis by DNA-damaging drugs is thought to be generally dependent on the release of cytochrome c and the subsequent activation of caspase-9 and -3. However, the molecular mechanism of how damaged DNA triggers the apoptotic process is not clear. To better understand the mechanisms underlying this process, we examined drug-induced apoptosis in cultured H-460 cells. Using cell fractionation, western blotting, and immunofluorescence assays, we show that the activation of nuclear caspases-7 and -3, and poly(ADP-ribose) polymerase (PARP) cleavage, are early events in camptothecin-induced apoptosis. Moreover, we demonstrate that these events precede the release of cytochrome c and apoptotic inducing factor, and the activation of caspases 2, 8, 9 and 12. Together our results suggest that drugs acting at the DNA level can initiate apoptosis via nuclear caspase activation.

Doxorubicin enhances TRAIL-induced cell death via ceramide-enriched membrane platforms

Previous studies indicated that signalling via CD95 and DR5 is greatly enhanced by the formation of ceramide-enriched membrane platforms. Here, we employed this concept to convert doses of subtherapeutic TRAIL that were unable to release ceramide and kill leukemic B-cells or ex vivo T lymphocytes, into a very effective apoptotic stimulus. Ceramide production was induced by application of sub-toxic doses of doxorubicin that resulted in an activation of the acid sphingomyelinase (ASM), release of ceramide and formation of ceramide-enriched membrane platforms. The latter served DR5 to cluster after application of very low doses of TRAIL in combination with doxorubicin. Genetic deficiency of the ASM abrogated doxorubicin-induced ceramide release, as well as clustering of DR5 and apoptosis induced by the combined treatment of doxorubicin and TRAIL. These data show that local release of ceramide potentiates very low, otherwise inactive doses of TRAIL that may represent a novel therapeutic concept to treat tumors.

Involvement of caspase-8 in chemotherapy-induced apoptosis of patient derived leukemia cell lines independent of the death receptor pathway and downstream from mitochondria

Resistance of leukemic cells to chemotherapy frequently occurs in patients with acute leukemia, which may be caused by alterations in common apoptotic pathways. Controversy exists whether cytostatic agents induce the mitochondrial or death receptor pathway of apoptosis. In the mitochondrial pathway cytochrome C release and caspase-9 activation play a central role in the induction of apoptosis, while formation of a Death Inducing Signaling Complex (DISC) and caspase-8 activation have been reported to be essential in death receptor-induced apoptosis. Here, we show in human derived myeloid and lymphoblastic leukemia cell lines that caspase-8 plays a more important role than previously expected in apoptosis mediated via the mitochondrial pathway. We demonstrated in these malignant cells chemotherapy-induced apoptosis independent of the death receptor pathway, since blocking this pathway using a retroviral construct encoding Flice inhibitory protein (FLIP) did not inhibit drug-induced apoptosis or caspase-8 activation, while overexpression of Bcl-2 completely inhibited both events. Furthermore, we showed that activation of caspase-8 by cytostatic agents occurred downstream from mitochondria. Since caspase-8 plays a central role in both death receptor- and chemotherapy-induced apoptosis of malignant cells from patients with acute leukemia, therapeutic strategies focusing at modulation and activation of caspase-8 may be successful in the treatment of drug-resistant malignancies.

Doxorubicin-induced cardiomyopathy from the cardiotoxic mechanisms to management

First isolated in the early 1960s, doxorubicin (DOX) remains among the most effective anticancer drug ever developed. However, this drug has proven to be a double-edged sword because it also causes a cardiomyopathy that leads to a form of congestive heart failure that is usually refractory to common medications. It is hoped that a better understanding of the mechanisms underlying DOX's cardiotoxicity will enable development of therapies with which to prevent and/or treat the heart failure it causes. Suggested contributors to DOX-induced cardiomyopathy include formation of reactive oxygen species, apoptosis, inhibited expression of cardiomyocyte-specific genes, and altered molecular signaling. And taking these various contributors into consideration, a variety of approaches aimed at preventing or mitigating the cardiotoxicity of DOX have been tried, but so far, the ability of these treatments to protect the heart from damage has been limited. That said, one recent approach that shows promise is adjuvant therapy with a combination of hematopoietic cytokines, including erythropoietin, granulocyte colony-stimulating factor, and thrombopoietin. We suggest this approach to preventing DOX-induced cardiomyopathy is worthy of serious consideration for clinical use.

Induction of mitochondria-dependent apoptosis through the inhibition of mevalonate pathway in human breast cancer cells by YM529, a new third generation bisphosphonate

YM529, a new third generation bisphosphonate, induced apoptosis of a human breast cancer cell line, MX-1. Cytotoxic activity of YM529 was more potent than that of incadronate. YM529 activated caspase-9, but not caspase-8, and induced the release of cytochrome c into cytosol. YM529 increased Bax expression and decreased Bcl-2 expression, while it did not induce caspase-8-dependent Bid truncation. Farnesyl pyrophosphate prevented YM529-mediated cytotoxicity. These results suggest that YM529 is a potent therapeutic agent for human breast cancers, activating the mitochondria-dependent apoptotic pathway through the inhibition of protein farnesylation.

Cytosolic activation of cathepsins mediates parvovirus H-1-induced killing of cisplatin and TRAIL-resistant glioma cells

Gliomas are often resistant to the induction of apoptotic cell death as a result of the development of survival mechanisms during astrocyte malignant transformation. In particular, the overexpression of Bcl-2-family members interferes with apoptosis initiation by DNA-damaging agents (e.g., cisplatin) or soluble death ligands (e.g., TRAIL). Using low-passage-number cultures of glioma cells, we have shown that parvovirus H-1 is able to induce death in cells resistant to TRAIL, cisplatin, or both, even when Bcl-2 is overexpressed. Parvovirus H-1 triggers cell death through both the accumulation of lysosomal cathepsins B and L in the cytosol of infected cells and the reduction of the levels of cystatin B and C, two cathepsin inhibitors. The impairment of either of these effects protects glioma cells from the viral lytic effect. In normal human astrocytes, parvovirus H-1 fails to induce a killing mechanism. In vivo, parvovirus H-1 infection of rat glioma cells intracranially implanted into recipient animals triggers cathepsin B activation as well. This report identifies for the first time cellular effectors of the killing activity of parvovirus H-1 against malignant brain cells and opens up a therapeutic approach which circumvents their frequent resistance to other death inducers.

TRAIL-mediated apoptosis in HIV-1-infected macrophages is dependent on the inhibition of Akt-1 phosphorylation

HIV-1 uses mononuclear phagocytes (monocytes, tissue macrophages, and dendritic cells) as a vehicle for its own dissemination and as a reservoir for continuous viral replication. The mechanism by which the host immune system clears HIV-1-infected macrophages is not understood. TRAIL may play a role in this process. TRAIL is expressed on the cell membrane of peripheral immune cells and can be cleaved into a soluble, secreted form. The plasma level of TRAIL is increased in HIV-1-infected patients, particularly those with high viral loads. To study the effect of elevated TRAIL on mononuclear phagocytes, we used recombinant human (rh) TRAIL and human monocyte-derived macrophages (MDM) as an in vitro model. Our results demonstrated rhTRAIL-induced apoptosis in HIV-1-infected MDM and inhibited viral replication, while having a reduced effect on uninfected MDM. HIV-1 infection significantly decreased Akt-1 phosphorylation; rhTRAIL exposure further decreased Akt-1 phosphorylation. Infection with a dominant-negative Akt-1 adenovirus potentiated rhTRAIL-induced apoptosis, while constitutively active Akt-1 blocked rhTRAIL-induced apoptosis in HIV-1-infected MDM. From this data we conclude the death ligand TRAIL preferentially provokes apoptosis of HIV-1-infected MDM, and the mechanism is reliant upon the inhibition of Akt-1 phosphorylation. Understanding this mechanism may facilitate the elimination of HIV-1-infected macrophages and lead to new therapeutic avenues for treatment of HIV-1 infection.

Extrinsic versus intrinsic apoptosis pathways in anticancer chemotherapy

Apoptosis or programmed cell death is a key regulator of physiological growth control and regulation of tissue homeostasis. One of the most important advances in cancer research in recent years is the recognition that cell death mostly by apoptosis is crucially involved in the regulation of tumor formation and also critically determines treatment response. Killing of tumor cells by most anticancer strategies currently used in clinical oncology, for example, chemotherapy, gamma-irradiation, suicide gene therapy or immunotherapy, has been linked to activation of apoptosis signal transduction pathways in cancer cells such as the intrinsic and/or extrinsic pathway. Thus, failure to undergo apoptosis may result in treatment resistance. Understanding the molecular events that regulate apoptosis in response to anticancer chemotherapy, and how cancer cells evade apoptotic death, provides novel opportunities for a more rational approach to develop molecular-targeted therapies for combating cancer.

Rejection severity directly correlates with myocyte apoptosis in pig-to-baboon cardiac xenotransplantation

BACKGROUND

The process by which cardiac myocytes die during xenograft rejection is incompletely understood. The presence of cardiac myocyte apoptosis in discordant xenotransplant models has been noted, yet no investigators have examined whether a relationship between myocyte apoptosis and rejection severity exists. Thus, we chose to further investigate this observation.

METHODS

Eight explanted pig-to-baboon cardiac grafts with varying severities of rejection, as determined by hematoxylin and eosin histology, were examined for apoptosis by transmission electron microscopy (TEM) and TUNEL (terminal deoxynucleotide transferase-mediated digoxigenin-dUTP nick-end labeling) immunohistochemistry. In addition, Western blot analysis for the cleavage of the apoptosis regulatory proteins pro-caspase 8 and 3 was performed.

RESULTS

Transmission electron microscopy revealed that a severely rejected graft displayed widespread condensation of nuclear chromatin, which is a characteristic morphologic feature of apoptosis. TUNEL staining verified this observation and allowed for the quantification of myocyte apoptosis in each graft. Subsequent linear regression analysis of the extent of myocyte apoptosis and rejection severity revealed a direct correlation (R(2)=0.757, p=0.005). In addition, Western blot analysis demonstrated that myocyte apoptosis involves the cleavage of pro-caspase 8 and 3.

CONCLUSIONS

Myocyte death in rejecting pig-to-baboon cardiac xenografts occurs through an apoptotic pathway and directly correlates with the severity of graft rejection. Further studies aimed at elucidating the apoptotic stimulus are therefore warranted. Moreover, our data suggest that antiapoptotic strategies may be of benefit in the treatment of xenograft rejection.

Caspase-10 involvement in cytotoxic drug-induced apoptosis of tumor cells

Anticancer drugs can induce tumor cell death by caspase-dependent apoptosis. The observation that procaspase-10 expression decreased in leukemic cells from acute myeloblastic leukemia patients at first relapse led us to explore the role of caspase-10 in cytotoxic drug-induced apoptosis. We show that caspase-10 is activated in etoposide-treated cells in a dose- and time-dependent manner. A caspase-10 peptide inhibitor, a caspase-10 dominant-negative mutant or a small interfering RNA (siRNA)-mediated downregulation of the enzyme negatively interfere with drug-induced cell death and caspase-2, -3, -8 and -9 activation. The extrinsic pathway to apoptosis is not involved in drug-induced caspase-10 activation that occurs downstream of Bax redistribution to mitochondria and cytochrome c release from this organelle. siRNA-mediated downregulation of Apaf-1 prevents etoposide-mediated activation of caspase-10. In a cell-free assay, cytochrome c and dATP treatment of cell extracts after immunodepletion of either caspase-3 or caspase-9 indicates that caspase-10 is activated downstream of caspase-9. Then, caspase-10 is involved in a feedback amplification loop that amplifies caspase-9 and -3 activities. Altogether, these data indicate an active role for caspase-10 in cytotoxic drug-induced tumor cell death, downstream of the mitochondria.

TNF-related apoptosis-inducing ligand as a therapeutic agent in autoimmunity and cancer

Recombinant, soluble TNF-related apoptosis-inducing ligand (TRAIL) is currently being developed as a promising natural immune molecule for trial in cancer patients because it selectively induces apoptosis in transformed or stressed cells but not in most normal cells. In cancer patients, phase 1 and 2 clinical trials using agonistic mAbs that engage the human TRAIL receptors DR4 and DR5 have also provided encouraging results. It is now evident that TRAIL suppresses autoimmune disease in various experimental animal models, suggesting that the therapeutic value of recombinant TRAIL and agonistic DR4 and DR5 mAbs might also extend to the suppression of autoimmune disease. This review provides an insight into our current understanding of the role(s) of TRAIL in disease, with a specific focus on cancer and autoimmunity. We also emphasize biological agents and drugs that sensitize tumour cells to TRAIL-mediated apoptosis and discuss the potential molecular basis for their sensitization.

Upregulation of Bcl-2 is associated with cisplatin-resistance via inhibition of Bax translocation in human bladder cancer cells

The efficacy of cisplatin in cancer chemotherapy is limited by the development of resistance. To elucidate the molecular basis of resistance to cisplatin, we compared cisplatin-induced apoptotic responses of the parental human bladder cancer cell line, T24 and its resistant subclone, T24R2. In T24 cells, cisplatin induce apoptosis and the activation of caspase-8, -9 and -3 and poly(ADP-ribose) polymerase cleavage. The expression levels of Fas, FasL, and FADD were not changed by the treatment with cisplatin. Furthermore, neither Fas-neutralizing antibody nor dominant negative mutant of FADD affected cisplatin-induced apoptosis. Western blot analysis of subcellular fractions showed that cisplatin induced redistribution of Bax and cytochrome c. Thus, cisplatin causes apoptosis in a death receptor-independent and mitochondria-dependent fashion in T24 cells. In contrast, overexpressed Bcl-2 protein inhibited cisplatin-induced Bax translocation and its downstream events in T24R2. Downregulation of Bcl-2 by RNAi potentiated the redistribution of Bax and cytochrome c and reversed cisplatin-resistance. Our results indicate that upregulation of Bcl-2 contributes to the development of cisplatin-resistance and usage of siRNA which targets the Bcl-2 gene may offer a potential tool to reverse the resistance to cisplatin in bladder cancer.

HPMA copolymer-bound doxorubicin induces apoptosis in ovarian carcinoma cells by the disruption of mitochondrial function

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymer-bound doxorubicin has showed greater potency than free doxorubicin in the treatment of ovarian cancer in vivo and in vitro. The promising activity of the conjugate demonstrated in clinical trials has generated considerable interest in understanding the mechanism of action of this macromolecular therapeutic. In this study, the involvement of the mitochondrial pathway in HPMA copolymer-bound doxorubicin-induced apoptosis in the human ovarian cancer cell line A2780 was investigated. Through a series of in vitro assays, including confocal microscopy, flow cytometry, and spectrofluorimetry, a significant decrease in mitochondrial membrane potential in A2780 cells treated with HPMA copolymer-bound doxorubicin was found. The most dramatic changes in mitochondrial membrane potential were observed between 2 and 12 h of continuous drug exposure. The potential of the mitochondrial membrane remained collapsed when drug treatment continued up to 24 h. For the first time, it was shown that HPMA copolymer-bound doxorubicin induces apoptosis in ovarian cancer cells by simultaneous activation of both caspase-dependent and caspase-independent pathways of DNA damage. This was determined by monitoring the translocation of the mitochondrial proteins cytochrome c and apoptosis-inducing factor to cytosol. The altered balance between anti-apoptotic and pro-apoptotic members of the Bcl-2 family of proteins was responsible for the mitochondrial function distraction. HPMA copolymer-bound doxorubicin induced a time-dependent decrease in the expression of the anti-apoptotic Bcl-2 and Bcl-xL proteins, which control cell survival. At the same time, the expression level of pro-apoptotic members (Bax, Bad) of the Bcl-2 family was increased under the chosen experimental conditions. Altogether, these results indicate that HPMA copolymer-bound doxorubicin induced apoptosis in ovarian cancer cells through the mitochondrial pathway.

Protection of betulin against cadmium-induced apoptosis in hepatoma cells

The protective effects of betulin (BT) against cadmium (Cd)-induced cytotoxicity have been previously reported. However, the mechanisms responsible for these protective effects are unclear. Therefore, this study investigated the mechanisms responsible for the protection of BT against Cd-induced cytotoxicity in human hepatoma cell lines. The protection of BT against Cd cytotoxicity was more effective in the HepG2 than in the Hep3B cells. The protection of BT on Cd-induced cytotoxicity in the HepG2 cells appeared to be related to the inhibition of apoptosis, as determined by PI staining and DNA fragmentation analysis. The anti-apoptosis exerted by BT involved the blocking of Cd-induced reactive oxygen species (ROS) generation, the abrogation of the Cd-induced Fas upregulation, the blocking of caspase-8-dependent Bid activation, and subsequent inhibition of mitochondrial pathway. The BT pretreatment did not affect the p21 and p53 expression levels, when compared with those of the treated cells with Cd alone. BT induced the transient S phase arrest at an early stage and the G0/G1 arrest at a relatively late stage, but it did not observe the sub-G1 apoptotic peak. In the Hep3B cells, Cd did not induce ROS generation. The BT pretreatment partially inhibited the Cd-induced apoptosis, which was related with the incomplete blockage in caspase-9 or -3 activation, as well as in Bax activation. Taken together, it was found that Cd can induce apoptosis via the Fas-dependent and -independent apoptosis pathways. However, the observed protective effects of BT were clearly more sensitive to Fas-expressing HepG2 cells than to Fas-deficient Hep3B cells.

Induction of apoptosis and cell cycle arrest by a chalcone panduratin A isolated from Kaempferia pandurata in androgen-independent human prostate cancer cells PC3 and DU145

Because of unsatisfactory treatment options for prostate cancer (CaP) there is a need to develop novel preventive approaches for this malignancy. One such strategy is through chemoprevention by the use of non-toxic dietary substances and botanical products. We have shown previously that panduratin A isolated from the extract of Kaempferia pandurata (Zingiberaceae) is a strong inhibitor of cyclooxygenase-2 in RAW264.7 cells and induces apoptosis in HT-29 cells. In the present study, we provide evidence that panduratin A treatment to androgen-independent human CaP cells PC3 and DU145 result in a time and dose-dependent inhibition of cell growth with an IC50 of 13.5-14 microM and no to little effect on normal human prostate epithelial cells. To define the mechanism of these anti-proliferative effects of panduratin A, we determined its effect on critical molecular events known to regulate the cell cycle and the apoptotic machinery. Annexin V/propidium iodide staining provided the evidence for the induction of apoptosis which was further confirmed by the observation of cleavage of poly (ADP-ribose) polymerase and degradation of acinus. Panduratin A treatment to cells was found to result in inhibition of procaspases 9, 8, 6 and 3 with significant increase in the ratio of Bax:Bcl-2, suggesting the involvement of a mitochondrial-dependent apoptotic pathway. Panduratin A-mediated apoptosis was accompanied with upregulation of Fas death receptor and TNF-related apoptosis-inducing ligand (TRAIL). Furthermore, cell cycle analysis using flow cytometry showed that panduratin A treatment of cells resulted in a G2/M arrest in a dose-dependent manner. The immunoblot analysis data revealed that in both cell lines panduratin A treatment resulted in a dose-dependent (i) induction of p21WAF1/Cip1 and p27Kip1, (ii) downregulation of cdks 2, 4 and 6 and (iii) decrease in cyclins D1 and E. These findings suggest that panduratin A may be an effective chemopreventive or therapeutic agent against CaP.

Collapse of mitochondrial membrane potential and caspases activation are early events in okadaic acid-treated Caco-2 cells

Diarrhetic Shellfish Poisoning (DSP) results from the consumption of shellfish contaminated with okadaic acid (OA) or one of the dinophysistoxins (DTX). It has been reported that this toxin induces apoptosis in several cell models, but the molecular events involved in this process have not been clarified. In this report we studied intracellular signals induced by OA in Caco-2 cells: mitochondrial membrane potential, F-actin depolymerization, caspases activation, cell proliferation and cell membrane integrity. Results indicate that caspases-8 and -9 increased their activity after 30 min of OA treatment according to their role as initiator caspases. In contrast, activation of the downstream caspase-3 is a later event in the execution phase of apoptosis. Mitochondrial membrane potential changes are detected at 30 min of OA exposure indicating that this is an early response in the apoptotic cascade. F-actin depolymerization occurs after 24h of incubation with OA and this effect is significant at low doses of the toxin. LDH is released into the culture medium, although there is not PI uptake, indicative of a significant cell death in addition to apoptosis. Moreover, OA led to a dose- and time-dependent decrease in cellular proliferation.

In vitro and In vivo Antitumor Effects of the Dual Insulin-Like Growth Factor-I/Insulin Receptor Inhibitor, BMS-554417

The insulin-like growth factor receptor (IGF-IR) and insulin receptor are either overactivated and/or overexpressed in a wide range of tumor types and contribute to tumorigenicity, proliferation, metastasis, and drug resistance. Here, we show that BMS-554417, a novel small molecule developed as an inhibitor of IGF-IR, inhibits IGF-IR and insulin receptor kinase activity and proliferation in vitro, and reduces tumor xenograft size in vivo. In a series of carcinoma cell lines, the IC50 for proliferation ranged from 120 nmol/L (Colo205) to >8.5 micromol/L (OV202). The addition of stimulatory ligands was unnecessary for the antiproliferative effect in MCF-7 and OV202 cells. BMS-554417 treatment inhibited IGF-IR and insulin receptor signaling through extracellular signal-related kinase as well as the phosphoinositide 3-kinase/Akt pathway, as evidenced by decreased Akt phosphorylation at Ser473. At doses that inhibited proliferation, the compound also caused a G0-G1 arrest and prevented nuclear accumulation of cyclin D1 in response to LR3 IGF-I. In Jurkat T-cell leukemia cells, this agent triggered apoptotic cell death via the mitochondrial pathway. BMS-554417 was orally bioavailable and significantly inhibited the growth of IGF1R-Sal tumor xenografts in vivo. BMS-554417 is a member of a novel class of IGF-IR/insulin receptor inhibitors that have potential clinical applications because of their antiproliferative and proapoptotic activity in vitro and in vivo.

Pyrimethamine (2,4-diamino-5-p-chlorophenyl-6-ethylpyrimidine) induces apoptosis of freshly isolated human T lymphocytes, bypassing CD95/Fas molecule but involving its intrinsic pathway

Pyrimethamine (2,4-diamino-5-p-chlorophenyl-6-ethyl-pyrimidine), a folic acid antagonist, may exert, in addition to antiprotozoan effects, immunomodulating activities, including induction of peripheral blood lymphocyte apoptosis. However, the molecular mechanisms underlying this proapoptotic activity remain to be elucidated. Here we show that pyrimethamine, used at a pharmacologically relevant concentration, induced per se apoptosis of activated lymphocytes via the activation of the caspase-8- and caspase-10-dependent cascade and subsequent mitochondrial depolarization. Importantly, this seems to occur independently from CD95/Fas engagement. The proapoptotic activity of pyrimethamine was further confirmed in a patient with autoimmune lymphoproliferative syndrome, an immune disorder associated with a defect of Fas-induced apoptosis. In this patient, pyrimethamine treatment resulted in a "normalization" of lymphocyte apoptosis with a significant amelioration of laboratory parameters. Altogether, these results suggest a mechanism for pyrimethamine-mediated apoptosis that seems to bypass CD95/Fas engagement but fully overlaps CD95/Fas-induced subcellular pathway. On these bases, a reappraisal of the use of pyrimethamine in immune lymphoproliferative disorders characterized by defects in CD95/Fas-mediated apoptosis should be taken into account.

Phosphodiesterase-5 inhibition with sildenafil attenuates cardiomyocyte apoptosis and left ventricular dysfunction in a chronic model of doxorubicin cardiotoxicity

BACKGROUND

Sildenafil, a phosphodiesterase-5 inhibitor, induces cardioprotection against ischemia/reperfusion injury via opening of mitochondrial K(ATP) channels. It is unclear whether sildenafil would provide similar protection from doxorubicin-induced cardiotoxicity.

METHODS AND RESULTS

Male ICR mice were randomized to 1 of 4 treatments: saline, sildenafil, doxorubicin (5 mg/kg IP), and sildenafil (0.7 mg/kg IP) plus doxorubicin (n=6 per group). Apoptosis was assessed with the use of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling and in situ oligo ligation methods. Desmin distribution was determined via immunofluorescence. Bcl-2 expression was analyzed by Western blot. Left ventricular function was assessed by measuring developed pressure and rate pressure product in Langendorff mode. ECG changes indicative of doxorubicin cardiotoxicity were also measured. For in vitro studies, adult ventricular cardiomyocytes were exposed to doxorubicin (1 micromol/L), sildenafil (1 micromol/L) with or without N(G)-nitro-L-arginine methyl ester (L-NAME) (100 micromol/L), or 5-hydroxydecanoate (100 micromol/L) 1 hour before doxorubicin and incubated for 18 hours. Doxorubicin-treated mice demonstrated increased apoptosis and desmin disruption, which was attenuated in the sildenafil+doxorubicin group. Bcl-2 was decreased in the doxorubicin group but was maintained at basal levels in the sildenafil+doxorubicin group. Left ventricular developed pressure and rate pressure product were significantly depressed in the doxorubicin group but were attenuated in the sildenafil+doxorubicin group. ST interval was significantly increased in the doxorubicin group over 8 weeks. In the sildenafil+doxorubicin group, ST interval remained unchanged from baseline. Doxorubicin caused a significant increase in apoptosis, caspase-3 activation, and disruption of mitochondrial membrane potential in vitro. In contrast, sildenafil significantly protected against doxorubicin cardiotoxicity; however, this protection was abolished by both L-NAME and 5-hydroxydecanoate.

CONCLUSIONS

Prophylactic treatment with sildenafil prevented apoptosis and left ventricular dysfunction in a chronic model of doxorubicin-induced cardiomyopathy.

APAF-1 signaling in human melanoma

Acquired resistance to mechanisms of programmed cell death is one of the hallmarks of cancer. Human melanoma, in advanced stage, is hardly curable, due to development of several strategies that impair apoptosis induced by the death receptor and the mitochondrial pathways of apoptosis. Among these apoptosis escape strategies, one is based on inactivation of pro-apoptotic factors such as Apoptotic Protease Activating Factor-1 (APAF-1). APAF-1 couples cytochrome c release from the mitochondria to caspase-9 activation and has been considered a central adaptor in the intrinsic pathway of programmed cell death. Inactivation of APAF-1 in human melanoma may impair the mitochondrial pathway of apoptosis induced by chemotherapeutic drugs that activate the p53 pathway, thus contributing to the development of chemoresistance. In-vivo, loss of expression of APAF-1 is associated with tumor progression, suggesting that APAF-1 inactivation may provide a selective survival advantage to neoplastic cells. However, recent results have indicated the existence of APAF-1-independent pathways of caspase activation and apoptosis in normal and neoplastic cells. Moreover, it has been found that expression of APAF-1 is not necessary for the apoptotic response of melanoma cells to different pro-apoptotic drugs. The emerging picture from results obtained in melanoma and other human tumors is that the relevance of the APAF-1 pathway in programmed cell death is cell-context-dependent and related to the specificity of the pro-apoptotic-stimuli.