Caspase-8 levels affect necessity for mitochondrial amplification in death ligand-induced glioma cell apoptosis

Executioner caspases and CAD are essential for mutagenesis induced by TRAIL or vincristine

Chemotherapy drugs interfere with cellular processes to generate genotoxic lesions that activate cell death pathways. Sustained DNA damage induced by these drugs can provoke mutations in surviving non-cancerous cells, potentially increasing the risk of therapy-related cancers. Ligation of death receptors by ligands such as TRAIL, and subsequent activation of extrinsic apoptotic pathways, also provokes mutations. In this study, we show that executioner caspase activation of the apoptotic nuclease CAD/DFF40 is essential for TRAIL-induced mutations in surviving cells. As exposure to chemotherapy drugs also activates apoptotic caspases and presumably CAD, we hypothesized that these pathways may also contribute to the mutagenesis induced by conventional chemotherapy drugs, perhaps augmenting the mutations that arise from direct DNA damage provoked by these agents. Interestingly, vincristine-mediated mutations were caspase and CAD dependent. Executioner caspases accounted for some of the mutations caused by the topoisomerase poisons doxorubicin and SN38, but were dispensable for mutagenesis following treatment with cisplatin or temozolomide. These data highlight a non-apoptotic role of caspases in mutagenesis mediated by death receptor agonists, microtubule poisons and topoisomerase inhibitors, and provide further evidence for a potential carcinogenic consequence of sublethal apoptotic signaling stimulated by anticancer therapies.

Mitochondria-targeted liposomes improve the apoptotic and cytotoxic action of sclareol

Current efforts toward improving the effectiveness of drug therapy are increasingly relying on drug-targeting strategies to effectively deliver bioactive molecules to their molecular targets. Pharmaceutical nanocarriers represent a major tool toward this aim, and our efforts have been directed toward achieving nanocarrier-mediated subcellular delivery of drug molecules with mitochondria as the primary subcellular target. Meeting the need for specific subcellular delivery is essential to realizing the full potential of many poorly soluble anticancer drugs. In this article, we report that mitochondria-targeted liposomes significantly improve the apoptotic and cytotoxic action of sclareol, a poorly soluble potential anticancer drug. The results support the broad applicability of our nanocarrier-mediated subcellular targeting approach as a means to improve the effectiveness of certain anticancer therapeutics.

TRAIL treatment provokes mutations in surviving cells

Chemotherapy and radiotherapy commonly damage DNA and trigger p53-dependent apoptosis through intrinsic apoptotic pathways. Two unfortunate consequences of this mechanism are resistance due to blockade of p53 or intrinsic apoptosis pathways, and mutagenesis of non-malignant surviving cells which can impair cellular function or provoke second malignancies. Death ligand-based drugs, such as tumor necrosis factor-related apoptosis inducing ligand (TRAIL), stimulate extrinsic apoptotic signaling, and may overcome resistance to treatments that induce intrinsic apoptosis. As death receptor ligation does not damage DNA as a primary mechanism of pro-apoptotic action, we hypothesized that surviving cells would remain genetically unscathed, suggesting that death ligand-based therapies may avoid some of the adverse effects associated with traditional cancer treatments. Surprisingly, however, treatment with sub-lethal concentrations of TRAIL or FasL was mutagenic. Mutations arose in viable cells that contained active caspases, and overexpression of the caspase-8 inhibitor crmA or silencing of caspase-8 abolished TRAIL-mediated mutagenesis. Downregulation of the apoptotic nuclease caspase-activated DNAse (CAD)/DNA fragmentation factor 40 (DFF40) prevented the DNA damage associated with TRAIL treatment. Although death ligands do not need to damage DNA in order to induce apoptosis, surviving cells nevertheless incur DNA damage after treatment with these agents.

A Smac-mimetic sensitizes prostate cancer cells to TRAIL-induced apoptosis via modulating both IAPs and NF-kappaB

Background

Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising agent for human cancer therapy, prostate cancer still remains resistant to TRAIL. Both X-linked inhibitor of apoptosis (XIAP) and nuclear factor-kappaB function as key negative regulators of TRAIL signaling. In this study, we evaluated the effect of SH122, a small molecule mimetic of the second mitochondria-derived activator of caspases (Smac), on TRAIL-induced apoptosis in prostate cancer cells.

Methods

The potential of Smac-mimetics to bind XIAP or cIAP-1 was examined by pull-down assay. Cytotoxicity of TRAIL and/or Smac-mimetics was determined by a standard cell growth assay. Silencing of XIAP or cIAP-1 was achieved by transient transfection of short hairpin RNA. Apoptosis was detected by Annexin V-PI staining followed by flow cytometry and by Western Blot analysis of caspases, PARP and Bid. NF-kappaB activation was determined by subcellular fractionation, real time RT-PCR and reporter assay.

Results

SH122, but not its inactive analog, binds to XIAP and cIAP-1. SH122 significantly sensitized prostate cancer cells to TRAIL-mediated cell death. Moreover, SH122 enhanced TRAIL-induced apoptosis via both the death receptor and the mitochondrial pathway. Knockdown of both XIAP and cIAP-1 sensitized cellular response to TRAIL. XIAP-knockdown attenuated sensitivity of SH122 to TRAIL-induced cytotoxicity, confirming that XIAP is an important target for IAP-inhibitor-mediated TRAIL sensitization. SH122 also suppressed TRAIL-induced NF-kappaB activation by preventing cytosolic IkappaB-alpha degradation and RelA nuclear translocation, as well as by suppressing NF-kappaB target gene expression.

Conclusion

These results demonstrate that SH122 sensitizes human prostate cancer cells to TRAIL-induced apoptosis by mimicking Smac and blocking both IAPs and NF-kappaB. Modulating IAPs may represent a promising approach to overcoming TRAIL-resistance in human prostate cancer with constitutively active NF-kappaB signaling.

In vitro sensitivity testing of minimally passaged and uncultured gliomas with TRAIL and/or chemotherapy drugs

TRAIL/Apo-2L has shown promise as an anti-glioma drug, based on investigations of TRAIL sensitivity in established glioma cell lines, but it is not known how accurately TRAIL signalling pathways of glioma cells in vivo are reproduced in these cell lines in vitro. To replicate as closely as possible the in vivo behaviour of malignant glioma cells, 17 early passage glioma cell lines and 5 freshly resected gliomas were exposed to TRAIL-based agents and/or chemotherapeutic drugs. Normal human hepatocytes and astrocytes and established glioma cell lines were also tested. Cross-linked TRAIL, but not soluble TRAIL, killed both normal cell types and cells from three tumours. Cells from only one glioma were killed by soluble TRAIL, although only inefficiently. High concentrations of cisplatin were lethal to glioma cells, hepatocytes and astrocytes. Isolated combinations of TRAIL and chemotherapy drugs were more toxic to particular gliomas than normal cells, but no combination was generally selective for glioma cells. This study highlights the widespread resistance of glioma cells to TRAIL-based agents, but suggests that a minority of high-grade glioma patients may benefit from particular combinations of TRAIL and chemotherapy drugs. In vitro sensitivity assays may help identify effective drug combinations for individual glioma patients.

Lovastatin sensitized human glioblastoma cells to TRAIL-induced apoptosis

Synergy study with chemotherapeutic agents is a common in vitro strategy in the search for effective cancer therapy. For non-chemotherapeutic agents, efficacious synergistic effects are uncommon. Here, we have examined two non-chemotherapeutic agents for synergistic effects: lovastatin and Tumor Necrosis Factor (TNF)-related apoptosis-inducing ligand (TRAIL) for synergistic effects; on three human malignant glioblastoma cell lines, M059K, M59J, and A172. Cells treated with lovastatin plus TRAIL for 48 h showed 50% apoptotic cell death, whereas TRAIL alone (1,000 ng/ml) did not, suggesting that lovastatin sensitized the glioblastoma cells to TRAIL attack. Cell cycle analysis indicated that lovastatin increased G₀–G₁ arrest in these cells. Annexin V study demonstrated that apoptosis was the predominant mode of cell death. We conclude that the combination of lovastatin and TRAIL enhances apoptosis synergistically. Moreover, lovastatin sensitized glioblastoma cells to TRAIL, suggesting a new strategy to treat glioblastoma.

The extrinsic and intrinsic apoptotic pathways are differentially affected by temperature upstream of mitochondrial damage

It is well known that mild hypothermia prevents neuronal cell death following cerebral ischemia, although it can also cause apoptosis in other cell types. Thus, incubation at room temperature (RT) has been shown to induce apoptosis in hematopoietic cells, including Jurkat T leukemia cells. To further understand the apoptotic events that can be activated at RT, we compared the induction of apoptosis by several apoptotic insults in Jurkat cells stimulated at 37 degrees C or RT. Retinoid-related molecules, which induce apoptosis via the intrinsic pathway, failed to induce apoptosis when cells were treated at RT, as determined by various apoptotic parameters including cytochrome c release and activation of caspase 3. In contrast, most apoptotic events were enhanced by lower temperatures when cells were stimulated with anti-Fas antibody via the extrinsic pathway. Ultraviolet radiation produced partial effects at RT, correlating with its capacity to activate both pathways. Our results indicate that the core caspase machinery is operational under mild hypothermia conditions. Experiments using purified recombinant caspases and cell-free assays confirmed that caspases are fully functional at RT. Other hallmark events of apoptosis, such as phosphatidylserine externalization and formation of apoptotic bodies were variably affected by RT in a stimulus-dependent manner, suggesting the existence of critical steps that are sensitive to temperature. Thus, analysis of apoptosis at RT might be useful to (i) discriminate between the extrinsic and intrinsic pathways in Jurkat cells treated with prospective stimuli, and (ii) to unravel temperature-sensitive steps of apoptotic signaling cascades.

Caspase 8 is absent or low in many ex vivo gliomas

BACKGROUND

Better treatments are required urgently for patients with malignant glioma, which currently is incurable. Death ligands, such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), may offer promise for the treatment high-grade glioma if such ligands induce apoptotic signaling in vivo in glioma cells. Caspase 8 is required for death ligand signaling, and its levels may influence the sensitivity of glioma cells to death ligands. It also may act as a tumor suppressor protein. The authors analyzed caspase 8 expression levels in ex vivo glioma specimens and explored potential mechanisms of its regulation.

METHODS

Eleven glioblastomas, 5 anaplastic astrocytomas, and 3 low-grade astrocytomas were studied. The levels of caspase 8, caspase 10, cellular FLICE inhibitory protein (c-FLIP), and signal transducer and activator of transcription (STAT)-1 were assayed using quantitative immunoblotting. Caspase 8 mRNA was measured by Northern blot analysis. The methylation status of the caspase 8 gene was determined by bisulfate modification of genomic DNA, cloning, and sequencing. Statistical analyses were performed using nonparametric (Spearman) correlations.

RESULTS

Some ex vivo glioma samples lacked detectable caspase 8, with many expressing barely detectable levels. No tumors expressed significant amounts of caspase 10 or c-FLIP. A strong association was found between caspase 8 mRNA and protein levels. Neither expression of the transcription factor STAT-1 nor caspase 8 gene methylation correlated with caspase 8 levels.

CONCLUSIONS

The absence of caspase 8 protein in many resected glioma samples implied that many patients with glioma may not benefit from death ligand-based treatments, unless caspase 8 (or caspase 10) protein expression can be elevated. Demethylating agents are unlikely to boost caspase 8 levels in glioma cells, but treatments that increase caspase 8 mRNA levels may up-regulate expression of the protein.

Monolysocardiolipins accumulate in Barth syndrome but do not lead to enhanced apoptosis

Barth syndrome (BTHS) is an X-linked recessive disorder that is biochemically characterized by low cellular levels of the mitochondrial phospholipid cardiolipin (CL). Previously, we discovered that the yeast disruptant of the TAZ ortholog in Saccharomyces cerevisiae not only displays CL deficiency but also accumulates monolysocardiolipins (MLCLs), which are intermediates in CL remodeling. Therefore, we set out to investigate whether MLCL accumulation also occurs in BTHS. Indeed, we observed MLCL accumulation in heart, muscle, lymphocytes, and cultured lymphoblasts of BTHS patients; however, only very low levels of these lysophospholipids were found in platelets and fibroblasts of these patients. Although the fatty acid composition of the MLCLs was different depending on the tissue source, it did parallel the fatty acid composition of the (remaining) CLs. The possible implications of these findings for the two reported CL remodeling mechanisms, transacylation and deacylation/reacylation, are discussed. Because MLCLs have been proposed to be involved in the initiation of apoptosome-mediated cell death by the sequestration of the proapoptotic protein (t)BH3-interacting domain death agonist (Bid) to the mitochondrial membrane, we used control and BTHS lymphoblasts to investigate whether the accumulation of MLCLs results in higher levels of apoptosis. We found no differences in susceptibility to death receptor-mediated apoptosis or in cellular distribution of Bid, cytochrome c, and other parameters, implying that MLCL accumulation does not lead to enhanced apoptosis in cultured BTHS lymphoblasts.

Caspase-2 is resistant to inhibition by inhibitor of apoptosis proteins (IAPs) and can activate caspase-7

Caspases are a family of cysteine proteases with roles in cytokine maturation or apoptosis. Caspase-2 was the first pro-apoptotic caspase identified, but its functions in apoptotic signal transduction are still being elucidated. This study examined the regulation of the activity of caspase-2 using recombinant proteins and a yeast-based system. Our data suggest that for human caspase-2 to be active its large and small subunits must be separated. For maximal activity its prodomain must also be removed. Consistent with its proposed identity as an upstream caspase, caspase-2 could provoke the activation of caspase-7. Caspase-2 was not subject to inhibition by members of the IAP family of apoptosis inhibitors.

TRAIL and Malignant Glioma

Encouragingly, some types of cancer can now be considered treatable, with patients reasonably expecting their disease to be cured. Chemotherapy and radiation therapy are effective against these cancers because they activate the so-called intrinsic apoptosis pathways within the cancer cells. Unfortunately currently available treatments are only effective against a subset of tumor types. In contrast, other cancers, such as malignant glioma, typically do not respond to currently available therapies. Some of this resistance can be attributed to these tumor cells failing to undergo apoptosis upon anticancer treatment. Recently, considerable research attention has focused on triggering apoptosis in chemotherapy- and radiation-therapy-resistant cancer cells via an alternative route-the "extrinsic" pathway, as a means of bypassing this block in apoptosis. Binding of members of the tumor necrosis factor-alpha (TNF-alpha) family of death ligands to their receptors on the cell surface triggers this pathway. Death ligands can kill some cancer cells that are resistant to the apoptotic pathway triggered by conventional anticancer treatments. Some death ligands, such as TNF-alpha and FasL, cause unacceptable toxicity to normal cells and are therefore not suitable anticancer agents. However another death ligand, TNF-related apoptosis-inducing ligand (TRAIL)/Apo-2L, and antibodies that emulate its actions, show greater promise as candidate anticancer drugs because they have negligible effects on normal cells. This review will discuss the ability of TRAIL to induce apoptosis in malignant glioma cells and the potential clinical applications of TRAIL-based agents for glioma treatment.