Direct activation of the apoptosis machinery as a mechanism to target cancer cells

Conjugation of a novel Apaf-1 inhibitor to peptide-based cell-membrane transporters: effective methods to improve inhibition of mitochondria-mediated apoptosis

We have identified a family of peptoids that inhibits in vitro the activity of the apoptosome, a macromolecular complex that activates mitochondrial-dependent apoptosis pathways. The analysis of peptide-based cell compatible delivery systems of the most active peptoid is presented. The active peptoid was then fused to cell penetrating peptides (CPP) as penetratin (PEN-peptoid) and HIV-1 TAT (TAT-peptoid). PEN-peptoid showed greater cell viability and as a consequence better efficiency as an apoptosis inhibitor than the TAT-peptoid. The intracellular trafficking of both inhibitors was studied by flow cytometry and confocal fluorescence microscopy. Finally, the influence of the cargo (peptoid) molecules on the conformational behavior of the CPP in buffers and in membrane mimetic environments was analyzed using circular dichroism (CD) spectroscopy.

Therapeutic targets in the mitochondrial apoptotic pathway

Every cell in the human body has most of the components of the apoptotic apparatus and is thus principally equipped to die by apoptosis. Situations of increased or decreased apoptosis contribute to many forms of human disease, making this pathway an attractive target of therapeutic intervention. The past few years have seen an enormous refinement in the understanding how apoptosis works on a molecular level and the role of mitochondria as a central element in apoptotic signal transduction has become obvious. Here, the authors consider the events that are critical in this mitochondrial pathway, in particular at mitochondria but also upstream and downstream. The authors' opinion is presented on the merits and feasibility of approaches that aim at treating disease by interfering with the mitochondrial apoptotic pathway.

Loss of APAF-1 expression is associated with tumour progression and adverse prognosis in colorectal cancer

The aim of this study was to determine the prognostic value of APAF-1 in colorectal cancer (CRC). Immunohistochemistry for APAF-1 was performed on a tissue microarray of 1015 mismatch-repair (MMR) proficient and 130 sporadic MLH1-negative CRCs. The association of APAF-1 with clinico-pathological features including 10-year survival time was analysed. Methylation specific PCR was performed on a subset of MMR-proficient and MLH1-negative CRC. Loss of APAF-1 was associated with advanced T stage (p-value=0.022), N stage (p-value=0.009), vascular invasion (p-value=0.001) and worse survival (p-value=0.017) in MMR-proficient CRC. In MLH1-negative CRC, loss of APAF-1 was associated with metastasis (p-value=0.041), worse prognosis (p-value<0.001) and independently predicted shorter survival time (p-value<0.001). No methylation was found in the selected region of APAF-1. APAF-1 is a marker of tumour progression in MMR-proficient CRC and an independent adverse prognostic factor in MLH1-negative CRC.

Cutting-Edge Apoptosis-Based Therapeutics

Defects in programmed cell death or apoptosis are major hallmarks of cancer contributing to tumorigenesis, tumor progression, and therapy resistance. In the past decade, many of the pathways leading to apoptosis, as well as the molecular mechanisms blocking the death of tumor cells, have been elucidated. This detailed knowledge of the core apoptosis machinery is now being exploited for translation into novel cancer therapies in order to restore apoptosis induction in tumor cells. Strategies include activation of proapoptotic mediators such as death receptors, tumor protein p53, and second mitochondria-derived activator of caspases (SMAC)/DIABLO as well as inhibition of endogenous apoptosis inhibitors such as IAPs (inhibitor of apoptosis proteins) and BCL-2 (B-cell chronic lymphoid leukemia/lymphoma) proteins. Several approaches employing gene therapy and antisense strategies, recombinant biologics, or classic organic and combinatorial chemistry, have advanced into clinical trials or are already approved. This review looks at recent developments in apoptosis-based cancer therapies and highlights some very promising advances in drug design.

Role of XIAP in inhibiting cisplatin-induced caspase activation in non-small cell lung cancer cells: a small molecule Smac mimic sensitizes for chemotherapy-induced apoptosis by enhancing caspase-3 activation

X-linked IAP (XIAP) suppresses apoptosis by binding to initiator caspase-9 and effector caspases-3 and -7. Smac/DIABLO that is released from mitochondria during apoptosis can relieve its inhibitory activity. Here we investigated the role of XIAP in the previously found obstruction of chemotherapy-induced caspase-9 activation in non-small cell lung cancer (NSCLC) cells. Endogenously expressed XIAP bound active forms of both caspase-9 and caspase-3. However, downregulation of XIAP using shRNA or disruption of XIAP/caspase-9 interaction using a small molecule Smac mimic were unable to significantly induce caspase-9 activity, indicating that despite a strong binding potential of XIAP to caspase-9 it is not a major determinant in blocking caspase-9 in NSCLC cells. Although unable to revert caspase-9 blockage, the Smac mimic was able to enhance cisplatin-induced apoptosis, which was accompanied by increased caspase-3 activity. Additionally, a more detailed analysis of caspase activation in response to cisplatin indicated a reverse order of activation, whereby caspase-3 cleaved caspase-9 yielding an inactive form. Our findings indicate that the use of small molecule Smac mimic, when combined with an apoptotic trigger, may have therapeutic potential for the treatment of NSCLC.

Strategies for targeting protein-protein interactions with synthetic agents

The development of small-molecule modulators of protein-protein interactions is a formidable goal, albeit one that possesses significant potential for the discovery of novel therapeutics. Despite the daunting challenges, a variety of examples exists for the inhibition of two large protein partners with low-molecular-weight ligands. This review discusses the strategies for targeting protein-protein interactions and the state of the art in the rational design of molecules that mimic the structures and functions of their natural targets.

Mathematical modeling identifies inhibitors of apoptosis as mediators of positive feedback and bistability

The intrinsic, or mitochondrial, pathway of caspase activation is essential for apoptosis induction by various stimuli including cytotoxic stress. It depends on the cellular context, whether cytochrome c released from mitochondria induces caspase activation gradually or in an all-or-none fashion, and whether caspase activation irreversibly commits cells to apoptosis. By analyzing a quantitative kinetic model, we show that inhibition of caspase-3 (Casp3) and Casp9 by inhibitors of apoptosis (IAPs) results in an implicit positive feedback, since cleaved Casp3 augments its own activation by sequestering IAPs away from Casp9. We demonstrate that this positive feedback brings about bistability (i.e., all-or-none behaviour), and that it cooperates with Casp3-mediated feedback cleavage of Casp9 to generate irreversibility in caspase activation. Our calculations also unravel how cell-specific protein expression brings about the observed qualitative differences in caspase activation (gradual versus all-or-none and reversible versus irreversible). Finally, known regulators of the pathway are shown to efficiently shift the apoptotic threshold stimulus, suggesting that the bistable caspase cascade computes multiple inputs into an all-or-none caspase output. As cellular inhibitory proteins (e.g., IAPs) frequently inhibit consecutive intermediates in cellular signaling cascades (e.g., Casp3 and Casp9), the feedback mechanism described in this paper is likely to be a widespread principle on how cells achieve ultrasensitivity, bistability, and irreversibility.

Small-molecule activation of procaspase-3 to caspase-3 as a personalized anticancer strategy

Mutation and aberrant expression of apoptotic proteins are hallmarks of cancer. These changes prevent proapoptotic signals from being transmitted to executioner caspases, thereby averting apoptotic death and allowing cellular proliferation. Caspase-3 is the key executioner caspase, and it exists as an inactive zymogen that is activated by upstream signals. Notably, concentrations of procaspase-3 in certain cancerous cells are significantly higher than those in noncancerous controls. Here we report the identification of a small molecule (PAC-1) that directly activates procaspase-3 to caspase-3 in vitro and induces apoptosis in cancerous cells isolated from primary colon tumors in a manner directly proportional to the concentration of procaspase-3 inside these cells. We found that PAC-1 retarded the growth of tumors in three different mouse models of cancer, including two models in which PAC-1 was administered orally. PAC-1 is the first small molecule known to directly activate procaspase-3 to caspase-3, a transformation that allows induction of apoptosis even in cells that have defective apoptotic machinery. The direct activation of executioner caspases is an anticancer strategy that may prove beneficial in treating the many cancers in which procaspase-3 concentrations are elevated.

p53 and its downstream proteins as molecular targets of cancer

The p53 tumor suppressor gene plays a key role in prevention of tumor formation through transcriptional dependent and independent mechanisms. Transcriptional-dependent mechanisms are mainly mediated by p53 regulation of downstream targets, leading to growth arrest and apoptosis. Mutational inactivation of the p53 gene is detected in more than 50% of human cancers. Mutation of p53 renders cancer cells more resistant to current cancer therapies due to lack of p53-mediated apoptosis. Extensive studies have been conducted to identify small molecules that manipulate p53, including restoration of mutant p53 conformation to wild-type, disruption of murine double minute-2 (Mdm2)-p53 binding to increase p53 level and inhibition of Mdm2 E3 ubiquitin ligase activity to prevent p53 degradation. Another approach was to identify and validate "drugable" target(s) in p53 signaling pathways that modulate p53-induced apoptosis. We profiled a p53 temperature-sensitive lung cancer cell model with the Affymetrix human HG-U133 GeneChip, covering the entire human transcriptome. We identified thousands of unique genes that were either induced or repressed in response to p53-induced apoptosis. A follow-up study characterized a p53-repressed gene, SAK, a polo-like kinase (PLK) family member, as an appealing cancer drug target. Snk/Plk-akin kinase (SAK) silencing via small interfering RNA (siRNA) induced apoptosis, whereas SAK overexpression attenuated p53-induced apoptosis. Thus, SAK repression by p53 contributes to p53-induced apoptosis. Future work is directed at determining the normal cell response to SAK silencing. If a therapeutic window is obtained, a SAK inhibitor identified from high throughput screening (HTS) could serve as a lead compound for development of a novel class of apoptosis-inducing anticancer drugs.

Intrinsic apoptotic pathway in anaplastic large cell lymphoma

Anaplastic large cell lymphoma (ALCL) includes a subset of tumors that has abnormalities of chromosome 2p23, resulting in overexpression of anaplastic lymphoma kinase (ALK). Previous studies have reported differences in apoptotic rate and expression levels of apoptosis regulatory proteins between ALK+ and ALK- ALCL. In this study, we assessed for expression of the intrinsic apoptotic pathway proteins cytochrome c, apoptosis protease-activating factor 1, and procaspase 9 in 2 ALK+ ALCL cell lines and 42 ALCL tumors (17 ALK+, 25 ALK-). We used the Karpas 299 and SU-DHL-1 cell lines, and the inhibitors Z-LEHD-FMK (specific for caspase 9) and Boc-D-FMK (general caspase inhibitor) to investigate the role of caspase 9 activation in chemotherapy-induced apoptotic cell death. Caspase 9 activity was significantly increased in Karpas-299 and SU-DHL-1 cells after chemotherapy treatment, but remained as low as control levels with addition of either caspase inhibitor. Both caspase inhibitors rescued a substantial fraction of Karpas 299 and SU-DHL-1 cells from drug-induced cell death. In ALCL tumors, expression of cytochrome c, apoptosis protease-activating factor 1, and procaspase 9 was also assessed and correlated with apoptotic rate and activated caspase 3 levels. Cytochrome c was expressed in all 13 (100%) ALK+ and 18 (95%) of 19 ALK- ALCL tumors. Apoptosis protease-activating factor 1 was detected in 14 (88%) of 16 ALK+ and 19 (79%) of 24 ALK- ALCL tumors. Procaspase 9 was expressed in 5 (30%) of 17 ALK+ and 2 (8%) of 25 ALK- ALCL tumors (P = .09). In the entire study group (ALK+ and ALK- ALCL), procaspase 9 expression levels significantly correlated with apoptotic rate (P = .02) and activated caspase 3 levels (P = .05). This correlation could not be shown in the ALK+ or ALK- ALCL subgroups, presumably because of the small sample size. In conclusion, chemotherapy-induced cell death in ALK+ ALCL cells involves the intrinsic apoptotic pathway, and apoptosome function may be an important determinant of apoptosis in ALCL tumors.

The apoptosome: physiological, developmental, and pathological modes of regulation

Apoptosis, a form of programmed cell death, is executed by a family of zymogenic proteases known as caspases, which cleave an array of intracellular substrates in the dying cell. Many proapoptotic stimuli trigger cytochrome c release from mitochondria, promoting the formation of a complex between Apaf-1 and caspase-9 in a caspase-activating structure known as the apoptosome. In this review, we describe knockout and knockin studies of apoptosome components, elegant structural and biochemical experiments, and analyses of the apoptosome in various cancers and other disease states, all of which have provided new insight into this critical locus of apoptotic control.

Apoptosis-based therapies and drug targets

The pathogenesis of many diseases is most closely connected with aberrantly regulated apoptotic cell death. The past 15 years have witnessed an explosion in the basic knowledge of mechanisms that regulate apoptosis and the mediators that either trigger or inhibit cell death. Consequently, great interest has emerged in devising therapeutic strategies for modulating the key molecules of life-and-death decisions. Numerous novel approaches are currently being followed employing gene therapy and antisense strategies, recombinant biologics or classical organic and combinatorial chemistry in order to target specific apoptotic regulators. Although drug development is still in its infancy, several therapeutics have progressed to clinical testing or have even been approved in record time. This review outlines the recent advances in the field of apoptosis-based therapies and explores some highlights of a very active field of drug development.

Knockdown of Sox4 expression by RNAi induces apoptosis in ACC3 cells

Microarray RNA gene expression profiling analysis has shown that Sox4 (Sry-related high mobility group (HMG) box 4) is one of the most upregulated genes in adenoid cystic carcinoma (ACC), relative to non-neoplastic tissue of origin. Here, we show that Sox4 protein is similarly upregulated in ACC by immunohistochemistry of 28 primary cancers and 20 normal tissues. To elucidate the functional significance of these findings, RNA interference (RNAi)-mediated RNA silencing was used to downregulate Sox4 expression in the ACC-derived cell line, ACC3. With confirmed knockdown of Sox4 protein, cell viability was reduced by 51%, with a corresponding increase of apoptosis to 85% as compared to 12% in controls. Apoptosis was confirmed by cell morphology, DNA fragmentation and flow cytometry. Cells could be rescued from the proapoptotic effects of Sox4 RNAi by co-transfection with a construct expressing functional Sox4. Microarray gene expression profiling of RNAi knockdown experiments shows that downregulation of Sox4-modulated expression of critical genes involved in apoptosis and cell cycle control. Overall, our findings suggest that Sox4 contributes to the malignant phenotype of ACC cells by promoting cell survival.

Oncolytic activity of vesicular stomatitis virus in primary adult T-cell leukemia

Treatments for hematological malignancies have improved considerably over the past decade, but the growing therapeutic arsenal has not benefited adult T-cell leukemia (ATL) patients. Oncolytic viruses such as vesicular stomatitis virus (VSV) have recently emerged as a potential treatment of solid tumors and leukemias in vitro and in vivo. In the current study, we investigated the ability of VSV to lyse primary human T-lymphotropic virus type 1 (HTLV-1)-infected T-lymphocytes from patients with ATL. Ex vivo primary ATL cells were permissive for VSV and underwent rapid oncolysis in a time-dependent manner. Importantly, VSV infection showed neither viral replication nor oncolysis in HTLV-1-infected, nonleukemic cells from patients with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and in naive CD4(+) T-lymphocytes from normal individuals or in ex vivo cell samples from patients with chronic lymphocytic leukemia (CLL). Interestingly, activation of primary CD4(+) T-lymphocytes with anti-CD3/CD28 monoclonal antibody, and specifically with anti-CD3, was sufficient to induce limited viral replication and oncolysis. However, at a similar level of T-cell activation, VSV replication was increased fourfold in ATL cells compared to activated CD4(+) T-lymphocytes, emphasizing the concept that VSV targets genetic defects unique to tumor cells to facilitate its replication. In conclusion, our findings provide the first essential information for the development of a VSV-based treatment for ATL.

Enhanced Sensitivity to Cytochrome c–Induced Apoptosis Mediated by PHAPI in Breast Cancer Cells

Apoptotic signaling defects both promote tumorigenesis and confound chemotherapy. Typically, chemotherapeutics stimulate cytochrome c release to the cytoplasm, thereby activating the apoptosome. Although cancer cells can be refractory to cytochrome c release, many malignant cells also exhibit defects in cytochrome c-induced apoptosome activation, further promoting chemotherapeutic resistance. We have found that breast cancer cells display an unusual sensitivity to cytochrome c-induced apoptosis when compared with their normal counterparts. This sensitivity, not observed in other cancers, resulted from enhanced recruitment of caspase-9 to the Apaf-1 caspase recruitment domain. Augmented caspase activation was mediated by PHAPI, which is overexpressed in breast cancers. Furthermore, cytochrome c microinjection into mammary epithelial cells preferentially killed malignant cells, suggesting that this phenomenon might be exploited for chemotherapeutic purposes.

Catechin derivatives: Specific inhibitor for caspases-3, 7 and 2, and the prevention of apoptosis at the cell and animal levels

Tea-catechin derivatives are shown to inhibit activities of caspases-3, 2 and 7 in vitro, and prevented experimental apoptosis at the cell and animal levels. Epigallo-catechin-gallate showed the strongest inhibition at 1 x 10(-7)M to these caspases, but cysteine cathepsins and caspase-8 were not inhibited. Caspase-3 inhibition showed a 2nd-order allosteric-type, but the inhibition of caspases-2 and 7 showed a non-competitive-type. The apoptosis-test using cultured HeLa cells was inhibited by these catechins. In rat hepatocytes, apoptosis was induced by d-galactosamine in vivo. In this case, caspase-3 activity in the cytoplasm, the serum aminotransferases and dUTP nick formation detected by TUNNEL-staining were effects, and these elevations were suppressed by administration of catechin.

Caspases: pharmacological manipulation of cell death

Caspases, a family of cysteine proteases, play a central role in apoptosis. During the last decade, major progress has been made to further understand caspase structure and function, providing a unique basis for drug design. This Review gives an overview of caspases and their classification, structure, and substrate specificity. We also describe the current knowledge of how interference with caspase signaling can be used to pharmacologically manipulate cell death.

Defects of the apoptotic pathway as therapeutic target against cancer

Over the past 10 years evidence has been accumulating that antitumour agents induce apoptosis in cancer cells and that abnormalities in apoptosis signaling pathways often occur in cancer cells and are associated with drug resistance. The implication is that factors regulating the apoptotic process play a critical role in tumour sensitivity to chemotherapy, and hence may be rational molecular targets for novel antitumour agents. Significantly, oncogenic signals make cancer cells intrinsically more susceptible to apoptosis; defects in the pathway occur subsequent to cancer development. Important emerging questions are the pattern of alterations in the apoptotic pathway in a particular tumour (cell) and the best strategy to exploit them and induce selective tumour cell death. Here, we review recent progress in this field.

Small molecule inhibitors of Apaf-1-related caspase- 3/-9 activation that control mitochondrial-dependent apoptosis

Apoptosis is a biological process relevant to human disease states that is strongly regulated through protein-protein complex formation. These complexes represent interesting points of chemical intervention for the development of molecules that could modulate cellular apoptosis. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated Apaf-1 (apoptotic protease-activating factor), dATP and procaspase-9 that link mitochondria disfunction with activation of the effector caspases and in turn is of interest for the development of apoptotic modulators. In the present study we describe the identification of compounds that inhibit the apoptosome-mediated activation of procaspase-9 from the screening of a diversity-oriented chemical library. The active compounds rescued from the library were chemically optimised to obtain molecules that bind to both recombinant and human endogenous Apaf-1 in a cytochrome c-noncompetitive mechanism that inhibits the recruitment of procaspase-9 by the apoptosome. These newly identified Apaf-1 ligands decrease the apoptotic phenotype in mitochondrial-mediated models of cellular apoptosis.

A small molecule Smac-mimic compound induces apoptosis and sensitizes TRAIL- and etoposide-induced apoptosis in breast cancer cells

Inhibitor of apoptosis protein (IAP) suppresses apoptosis through binding and inhibiting active caspases-3, -7 and -9 via its baculoviral IAP repeat (BIR) domains. During apoptosis the caspase inhibition by IAPs can be negatively regulated by a mitochondrial protein second mitochondrial-derived activator of caspase (Smac). Smac physically interacts with multiple IAPs and relieves their inhibitory effect on caspases-3, -7 and -9. Recently, a small molecule Smac-mimic compound (Smac-mimic), which potentiates TNF-related apoptosis-inducing ligand (TRAIL) and tumor necrosis factor (TNF)-alpha mediated cell death in glioblastoma T98G cells and HeLa cells, was identified and characterized. To determine the efficacy of this compound in breast cancer cells, we first measured protein expression of three IAPs: XIAP, cIAP-1, and cIAP-2 in nine independent breast cancer cell lines. Three cell lines were chosen: a high IAPs expressing line MDA-MB-231, and two low IAPs expressing lines, T47D and MDA-MB-453. The cell lines were tested for their sensitivity to Smac-mimic alone or in combination with TRAIL or etoposide. Acting alone, Smac-mimic was quite potent with a cytotoxic IC50 of 3.8 nM in high IAPs expressing MDA-MB-231 cells, but was inactive at a much higher concentration in low IAPs expressing T47D and MDA-MB-453 cells. In fact, as low as 2.5 nM of Smac-mimic alone was sufficient to activate caspase-3 and induce apoptosis in MDA-MB-231 cells. In combinational treatments with TRAIL or etoposide, Smac-mimic significantly sensitized cells to growth suppression in MDA-MB-231 cells, but to a lesser extent in T47D and MDA-MB-453 cells. Furthermore, it significantly synergized MDA-MB-231, but not T47D cells to apoptosis induced by either TRAIL or etoposide. Thus, in these cell lines, Smac-mimic acts in an apparent IAPs dependent manner to induce apoptosis alone as well as sensitizes breast cancer cells to TRAIL or etoposide induced apoptosis via caspase-3 activation.

Hsp70 inhibits heat-induced apoptosis upstream of mitochondria by preventing Bax translocation

Hsp70 overexpression can protect cells from stress-induced apoptosis. Our previous observation that Hsp70 inhibits cytochrome c release in heat-stressed cells led us to examine events occurring upstream of mitochondrial disruption. In this study we examined the effects of heat shock on the proapoptotic Bcl-2 family member Bax because of its central role in regulating cytochrome c release in stressed cells. We found that heat shock caused a conformational change in Bax that leads to its translocation to mitochondria, stable membrane association, and oligomerization. All of these events were inhibited in cells that had elevated levels of Hsp70. Hsp70 did not physically interact with Bax in control or heat-shocked cells, indicating that Hsp70 acts to suppress signals leading to Bax activation. Hsp70 inhibited stress-induced JNK activation and inhibition of JNK with SP600125 or by expression of a dominant negative mutant of JNK-blocked Bax translocation as effectively as Hsp70 overexpression. Hsp70 did not protect cells expressing a mutant form of Bax that has constitutive membrane insertion capability or cells treated with a small molecule activator of apoptosome formation, indicating that it is unable to prevent cell death after mitochondrial disruption and caspase activation have occurred. These results indicate that Hsp70 blocks heat-induced apoptosis primarily by inhibiting Bax activation and thereby preventing the release of proapoptotic factors from mitochondria. Hsp70, therefore, inhibits events leading up to mitochondrial membrane permeabilization in heat-stressed cells and thereby controls the decision to die but does not interfere with cell death after this event has occurred.

New approaches and therapeutics targeting apoptosis in disease

Apoptosis, the major form of cellular suicide, is central to various physiological processes and the maintenance of homeostasis in multicellular organisms. Presumably, even more important is a causative or contributing role of apoptosis to various human diseases. These include situations with unwanted cell accumulation (cancer) and failure to eradicate aberrant cells (autoimmune diseases) or disorders with an inappropriate loss of cells (heart failure, stroke, AIDS, neurodegenerative diseases, and liver injury). The past decade has witnessed a tremendous progress in the knowledge of the molecular mechanisms that regulate apoptosis and the mediators that either prevent or trigger cell death. Consequently, apoptosis regulators have emerged as key targets for the design of therapeutic strategies aimed at modulating cellular life-and-death decisions. Numerous novel approaches are currently being followed employing gene therapy and antisense strategies, recombinant biologics, or classical organic and combinatorial chemistry to target specific apoptotic regulators. Convincing proof-of-principle evidence obtained in several animal models confirms the validity of strategies targeting apoptosis and revealed an enormous potential for therapeutic intervention in a variety of illnesses. Although numerous apoptotic drugs are currently being developed, several therapeutics have progressed to clinical testing or are already approved and marketed. Here we review the recent progress of apoptosis-based therapies and survey some highlights in a very promising field of drug development.

p53-defective tumors with a functional apoptosome-mediated pathway: a new therapeutic target

BACKGROUND

Although cancer cells appear to maintain the machinery for intrinsic apoptosis, defects in the pathway develop during malignant transformation, preventing apoptosis from occurring. How to specifically induce apoptosis in cancer cells remains unclear.

METHODS

We determined the apoptosome activity and p53 status of normal human cells and of lung, colon, stomach, brain, and breast cancer cells by measuring cytochrome c-dependent caspase activation and by DNA sequencing, respectively, and we used COMPARE analysis to identify apoptosome-specific agonists. We compared cell death, cytochrome c release, and caspase activation in NCI-H23 (lung cancer), HCT-15 (colon cancer), and SF268 (brain cancer) cells treated with Triacsin c, an inhibitor of acyl-CoA synthetase (ACS), or with vehicle. The cells were mock, transiently, or stably transfected with genes for Triacsin c-resistant ACSL5, dominant negative caspase-9, or apoptotic protease activating factor-1 knockdown. We measured ACS activity and levels of cardiolipin, a mitochondrial phospholipid, in mock and ACSL5-transduced SF268 cells. Nude mice carrying NCI-H23 xenograft tumors (n = 10) were treated with Triacsin c or vehicle, and xenograft tumor growth was assessed. Groups were compared using two-sided Student t tests.

RESULTS

Of 21 p53-defective tumor cell lines analyzed, 17 had higher apoptosome activity than did normal cells. Triacsin c selectively induced apoptosome-mediated death in tumor cells (caspase activity of Triacsin c-treated versus untreated SF268 cells; means = 1020% and 100%, respectively; difference = 920%, 95% CI = 900% to 940%; P<.001). Expression of ACSL5 suppressed Triacsin c-induced cytochrome c release and subsequent cell death (cell survival of Triacsin c-treated mock- versus ACSL5-transduced SF268 cells; means = 40% and 83%, respectively; difference = 43%, 95% CI = 39% to 47%; P<.001). ACS was also essential to the maintenance of cardiolipin levels. Finally, Triacsin c suppressed growth of xenograft tumors (relative tumor volume on day 21 of Triacsin c-treated versus untreated mice; means = 4.6 and 9.6, respectively; difference = 5.0, 95% CI = 2.1 to 7.9; P = .006).

CONCLUSIONS

Many p53-defective tumors retain activity of the apoptosome, which is therefore a potential target for cancer chemotherapy. Inhibition of ACS may be a novel strategy to induce the death of p53-defective tumor cells.

Plasma membrane sequestration of apoptotic protease-activating factor-1 in human B-lymphoma cells: a novel mechanism of chemoresistance

Burkitt lymphoma (BL) is a highly aggressive B-cell neoplasm harboring chromosomal rearrangements of the c-myc oncogene. BL cells frequently resist apoptosis induction by chemotherapeutic agents; however, the mechanism of unresponsiveness has not been elucidated. Here, we show that cytochrome c fails to stimulate apoptosome formation and caspase activation in cytosolic extracts of human BL-derived cell lines, due to insufficient levels of apoptotic protease-activating factor-1 (Apaf-1). Enforced expression of Apaf-1 increased its concentration in the cytosolic compartment, restored cytochrome c-dependent caspase activation, and rendered the prototypic Raji BL cell line sensitive to etoposide- and staurosporine-induced apoptosis. Surprisingly, in nontransfected BL cells, the bulk of Apaf-1 was found to associate with discrete domains in the plasma membrane. Disruption of lipid raft domains or the actin cytoskeleton of Raji cells liberated Apaf-1 and restored sensitivity to cytochrome c–dependent apoptosis, indicating that constitutive Apaf-1 retained its ability to promote caspase activation. Moreover, disruption of lipid rafts sensitized BL cells to apoptosis induced by etoposide. Together, our findings suggest that ectopic (noncytosolic) localization of Apaf-1 may constitute a novel mechanism of chemoresistance in B lymphoma.

Elimination of the differential chemoresistance between the murine B-cell lymphoma LY-ar and LY-as cell lines after arsenic (As2O3) exposure via the overexpression of gsto1 (p28)

PURPOSE

Arsenic, in the form of As(2)O(3), has gained therapeutic importance because it has been shown to be very effective clinically in the treatment of acute promyelocytic leukemia (APL). Via numerous pathways arsenic induces cellular alterations such as induction of apoptosis, inhibition of cellular proliferation, stimulation of differentiation, and inhibition of angiogenesis. Responses vary depending on cell type, dose and the form of arsenic. GSTO1, a member of the glutathione S-transferase superfamily omega, has recently been shown to be identical to the rate-limiting enzyme, monomethyl arsenous (MMA(V)) reductase which catalyzes methylarsonate (MMA(V)) to methylarsenous acid (MMA(III)) during arsenic biotransformation. In this study, we investigated whether arsenic trioxide (As(2)O(3)) induces apoptosis in both chemosensitive and chemoresistant cell lines that varied in their expression of p28 (gsto1), the mouse homolog of GSTO1.

METHODS

The cytotoxicity of arsenic in the gsto1- and bcl-2-expressing chemoresistant and radioresistant LY-ar mouse lymphoma cell line, was compared with that of the LY-ar's parental cell line, LY-as. LY-as cells are radiosensitive, apoptotically permissive, and do not express gsto1 or bcl-2. Cell survival, glutathione (GSH) levels, mitochondrial membrane potential, and stress-activated kinase status after arsenic treatment were examined in these cell lines.

RESULTS

As(2)O(3) induced an equivalent dose- and time-dependent increase in apoptosis in these cell lines. Cellular survival, as measured after a 24-h exposure, was also the same in each cell line. Reduced GSH was modulated in a similar time- and dose-dependent manner. Apoptosis was preceded by loss of mitochondrial membrane potential that triggered caspase-mediated pathways associated with apoptosis. With a prolonged exposure of As(2)O(3), both cell lines showed decreased activation of ERK family members, ERK1, ERK2 and ERK5. As(2)O(3) enhanced the death signals in LY-ar cells through a decrease in GSH, loss of mitochondrial membrane potential, and abatement of survival signals. This effect is similar to that seen when LY-ar cells are treated with thiol-depleting agents or by the removal of methionine and cysteine (GSH precursor) from the growth medium. This response is also completely contrary to that seen for radiation, actinomycin D, VP-16 and other agents, where LY-ar cells do not succumb to apoptosis.

CONCLUSIONS

The overexpression of gsto1 in normally chemoresistant and radioresistant LY-ar cells renders them vulnerable to the cytotoxic effects of As(2)O(3), despite the 30-fold overexpression of the survival factor bcl-2. Gsto1 and its human homolog, GSTO1, may serve as a marker for arsenic sensitivity, particularly in cells that are resistant to other chemotherapeutic agents.

Nuclear localization of apoptosis protease activating factor-1 predicts survival after tumor resection in early-stage non-small cell lung cancer

The proapoptotic protein apoptosis protein activating factor-1 (Apaf-1), which is normally located in the cytoplasm, can translocate to the nucleus before non-small cell lung carcinoma (NSCLC) cells manifest signs of apoptosis such as mitochondrial damage, caspase activation, or chromatin condensation. This may indicate a stage of imminent apoptosis. Importantly, we found that 24% (15 of 62) of resected stage I NSCLC (T(1)N(0)M(0) or T(2)N(0)M(0)), manifested a marked nuclear localization of Apaf-1 (Apaf-1(Nuc)), as compared with the mostly cytoplasmic localization of Apaf-1 found in the remaining tumors (Apaf-1(Cyt)). After a median follow-up of 6.31 years, the actuarial 5-year overall survival rates were 89% (56-98%) in the Apaf-1(Nuc) group and 54% (36-71%) in the Apaf-1(Cyt) group (P = 0.039). No correlation between the subcellular localization of Apaf-1 and that of p53 and Hsp70 could be established. Thus, the subcellular location of Apaf-1 (but not that of p53 or Hsp70) constitutes an accurate prognostic factor for overall survival in NSCLC.

Highly specific and accurate selection of siRNAs for high-throughput functional assays

MOTIVATION

Small interfering RNA (siRNA) is widely used in functional genomics to silence genes by decreasing their expression to study the resulting phenotypes. The possibility of performing large-scale functional assays by gene silencing accentuates the necessity of a software capable of the high-throughput design of highly specific siRNA. The main objective sought was the design of a large number of siRNAs with appropriate thermodynamic properties and, especially, high specificity. Since all the available procedures require, to some extent, manual processing of the results to guarantee specific results, specificity constitutes to date, the major obstacle to the complete automation of all the steps necessary for the selection of optimal candidate siRNAs.

RESULT

Here, we present a program that for the first time completely automates the search for siRNAs. In SiDE, the most complete set of rules for the selection of siRNA candidates (including G+C content, nucleotides at determined positions, thermodynamic properties, propensity to form internal hairpins, etc.) is implemented and moreover, specificity is achieved by a conceptually new method. After selecting possible siRNA candidates with the optimal functional properties, putative unspecific matches, which can cause cross-hybridization, are checked in databases containing a unique entry for each gene. These truly non-redundant databases are constructed from the genome annotations (Ensembl). Also intron/exon boundaries, presence of polymorphisms (single nucleotide polymorphisms) specificity for either gene or transcript, and other features can be selected to be considered in the design of siRNAs.

AVAILABILITY

The program is available as a web server at http://side.bioinfo.cnio.es. The program was written under the GPL license.

CONTACT

jdopazo@cnio.es.

Discovery of an allosteric site in the caspases

Allosteric regulation of proteins by conformational change is a primary means of biological control. Traditionally it has been difficult to identify and characterize novel allosteric sites and ligands that freeze these conformational states. We present a site-directed approach using Tethering for trapping inhibitory small molecules at sites away from the active site by reversible disulfide bond formation. We screened a library of 10,000 thiol-containing compounds against accessible cysteines of two members of the caspase family of proteases, caspase-3 and -7. We discovered a previously unreported and conserved allosteric site in a deep cavity at the dimer interface 14 A from the active site. This site contains a natural cysteine that, when disulfide-bonded with either of two specific compounds, inactivates these proteases. The allosteric site is functionally coupled to the active site, such that binding of the compounds at the allosteric site prevents peptide binding at the active site. The x-ray crystal structures of caspase-7 bound by either compound demonstrates that they inhibit caspase-7 by trapping a zymogen-like conformation. This approach may be useful to identify new allosteric sites from natural or engineered cysteines, to study allosteric transitions in proteins, and to nucleate drug discovery efforts.

Chemical-induced apoptosis: formation of the Apaf-1 apoptosome

Many environmental and therapeutic agents initiate apoptotic cell death by inducing the release of cytochrome c from the mitochondria, which activates Apaf-1 (apoptotic protease-activating factor-1). This large (approximately 130kD) protein is a mammalian homologue of CED-4, an essential protein involved in programmed cell death in the nematode C. elegans. Cytochrome c activates Apaf-1, which oligomerizes to form an approximately 700-1400-kDa caspase-activating complex known as the Apaf-1 apoptosome. Caspase-9, an initiator caspase, is then recruited to the complex by binding to Apaf-1 through CARD-CARD (caspase recruitment domain) interactions to form a holoenzyme complex. Subsequently, the Apaf-1/caspase-9 holoenzyme complex recruits the effector caspase-3 via an interaction between the active site cysteine in caspase-9 and the critical aspartate, which is the cleavage site for generating the large and small subunits of caspase-3 that constitute the activated form of caspase-3. This initiates the caspase cascade that is responsible for the execution phase of apoptosis. Intracellular levels of K+, XIAP an inhibitor of apoptosis protein, and at least two mitochondrial released proteins, Smac/DIABLO and Omi/Htra 2 a serine protease, tightly regulate formation and function of the apoptosome. Thus, a number of physiological mechanisms ensure that the apoptosome complex is only fully assembled and functional when the cell is irrevocably committed to die. It is interesting that more recent studies show that a variety of small molecules can directly activate or inhibit caspase activation by interfering with the formation and function of the apoptosome complex. The cytotoxicity of many conventional chemotherapeutic drugs rests on their ability to induce apoptosome formation and apoptosis. Defects in this pathway can result in drug resistance, and the discovery that small molecules can directly activate or inhibit the apoptosome may provide new alternative treatments for cancer.

Apo cytochrome c inhibits caspases by preventing apoptosome formation

Caspases are cysteine proteases and potent inducers of apoptosis. Their activation and activity is therefore tightly regulated. There are several mechanisms by which caspases can be activated but one key pathway involves release of holo cytochrome c from mitochondria into the cytoplasm. Cytoplasmic holo cytochrome c binds to apoptotic protease activating factor-1 (Apaf-1), driving the formation of an Apaf-1 oligomer (the apoptosome) which in turn binds and activates caspase-9. Previously we showed that the apo form of cytochrome c (lacking heme) can bind Apaf-1 and block both holo-dependent caspase activation in cell extracts and Bax-induced apoptosis in cells. Here we tested the ability of apo cytochrome c to inhibit caspase-9 activation induced by recombinant Apaf-1. Furthermore, using purified proteins and size exclusion chromatography we show that apo cytochrome c prevents holo cytochrome c-dependent apoptosome formation.

Apoptosis in human cancer cells

PURPOSE OF REVIEW

Apoptosis, or programmed cell death, is a vital physiologic process to eliminate damaged or unwanted cells. Defects in apoptosis promote tumor formation and make cancer cells resistant to therapy. This review provides an overview of recent advances in the understanding of apoptosis in human cancer cells.

RECENT FINDINGS

Recent studies revealed that the apoptotic machinery in humans consists of a molecular network of a large number of proteins. These proteins regulate a cascade of events in signaling, commitment and execution stages of apoptosis through multiple parallel pathways. Delineation of the basic mechanisms of apoptosis has shed light on how apoptosis is deregulated in human cancer cells. Therapeutic strategies based on apoptosis have also been designed to selectively target tumor cells.

SUMMARY

Understanding the basic mechanisms of apoptosis and determining how cancer cells evade apoptosis will afford discoveries of new molecular targets and better cancer therapies.