A caspase-8-independent component in TRAIL/Apo-2L-induced cell death in human rhabdomyosarcoma cells

Characterization of BV6-Induced Sensitization to the NK Cell Killing of Pediatric Rhabdomyosarcoma Spheroids

Although the overall survival in pediatric rhabdomyosarcoma (RMS) has increased over the last decades, the most aggressive subtype of alveolar RMS is in dire need of novel treatment strategies. RMS cells evade cell death induction and immune control by increasing the expression of inhibitors of apoptosis proteins (IAPs), which can be exploited and targeted with stimulation with Smac mimetics. Here, we used the Smac mimetic BV6 to re-sensitize RMS spheroids to cell death, which increased killing induced by natural killer (NK) cells. Single BV6 treatment of RMS spheroids did not reduce spheroidal growth. However, we observed significant spheroidal decomposition upon BV6 pre-treatment combined with NK cell co-cultivation. Molecularly, IAPs s are rapidly degraded by BV6, which activates NF-κB signal transduction pathways in RMS spheroids. RNA sequencing analysis validated NF-κB activation and identified a plethora of BV6-regulated genes. Additionally, BV6 released caspases from IAP-mediated inhibition. Here, caspase-8 might play a major role, as knockdown experiments resulted in decreased NK cell-mediated attack. Taken together, we improved the understanding of the BV6 mechanism of RMS spheroid sensitization to cytotoxic immune cells, which could be suitable for the development of novel combinatory cellular immunotherapy with Smac mimetics.

Radioresistance in rhabdomyosarcomas: Much more than a question of dose

Management of rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in children, frequently accounting the genitourinary tract is complex and requires a multimodal therapy. In particular, as a consequence of the advancement in dose conformity technology, radiation therapy (RT) has now become the standard therapeutic option for patients with RMS. In the clinical practice, dose and timing of RT are adjusted on the basis of patients' risk stratification to reduce late toxicity and side effects on normal tissues. However, despite the substantial improvement in cure rates, local failure and recurrence frequently occur. In this review, we summarize the general principles of the treatment of RMS, focusing on RT, and the main molecular pathways and specific proteins involved into radioresistance in RMS tumors. Specifically, we focused on DNA damage/repair, reactive oxygen species, cancer stem cells, and epigenetic modifications that have been reported in the context of RMS neoplasia in both in vitro and in vivo studies. The precise elucidation of the radioresistance-related molecular mechanisms is of pivotal importance to set up new more effective and tolerable combined therapeutic approaches that can radiosensitize cancer cells to finally ameliorate the overall survival of patients with RMS, especially for the most aggressive subtypes.

Sphingosine kinase 1 in cancer

The role of sphingolipids as bioactive signaling molecules that can regulate cell fate decisions puts them at center stage for cancer treatment and prevention. While ceramide and sphingosine have been established as antigrowth molecules, sphingosine-1-phosphate (S1P) offers a progrowth message to cells. The enzymes responsible for maintaining the balance between these "stop" or "go" signals are the sphingosine kinases (SK), SK1 and SK2. While the relative contribution of SK2 is still being elucidated and may involve an intranuclear role, a substantial amount of evidence suggests that regulation of sphingolipid levels by SK1 is an important component of carcinogenesis. Here, we review the literature regarding the role of SK1 as an oncogene that can function to enhance cancer cell viability and promote tumor growth and metastasis; highlighting the importance of developing specific SK1 inhibitors to supplement current cancer therapies.

Effects of TRAIL and taurolidine on apoptosis and proliferation in human rhabdomyosarcoma, leiomyosarcoma and epithelioid cell sarcoma

Soft tissue sarcomas (STS) are a heterogeneous group of malignant tumours representing 1% of all malignancies in adults. Therapy for STS should be individualised and multimodal, but complete surgical resection with clear margins remains the mainstay of therapy. Disseminated soft tissue sarcoma still represents a therapeutic dilemma. Commonly used chemotherapeutic agents such as doxorubicin and ifosfamide have proven to be effective in fewer than 30% in these cases. Therefore, we tested the apoptotic and anti-proliferative in vitro effects of TNF-related apoptosis-inducing ligand (TRAIL) and taurolidine (TRD) on rhabdomyosarcoma (A-204), leiomyosarcoma (SK-LMS-1) and epithelioid cell sarcoma (VA-ES-BJ) cell lines. Viability, apoptosis and necrosis were quantified by FACS analysis (propidium iodide/Annexin V staining). Gene expression was analysed by DNA microarrays and the results validated for selected genes by rtPCR. Protein level changes were documented by western blot analysis. Cell proliferation was analysed by BrdU ELISA assay. The single substances TRAIL and TRD significantly induced apoptotic cell death and decreased proliferation in rhabdomyosarcoma and epithelioid cell sarcoma cells. The combined use of TRAIL and TRD resulted in a synergistic apoptotic effect in all three cell lines, especially in rhabdomyosarcoma cells leaving 18% viable cells after 48 h of incubation (p<0.05). Analysis of the differentially regulated genes revealed that TRD and TRAIL influence apoptotic pathways, including the TNF-receptor associated and the mitochondrial pathway. Microarray analysis revealed remarkable expression changes in a variety of genes, which are involved in different apoptotic pathways and cross talk to other pathways at multiple levels. This in vitro study demonstrates that TRAIL and TRD synergise in inducing apoptosis and inhibiting proliferation in different human STS cell lines. Effects on gene expression differ relevantly in the sarcoma entities. These results provide experimental support for in vivo trials assessing the effect of TRAIL and TRD in STS and sustain the approach of individualized therapy.

Staurosporine induces necroptotic cell death under caspase-compromised conditions in U937 cells

For a long time necrosis was thought to be an uncontrolled process but evidences recently have revealed that necrosis can also occur in a regulated manner. Necroptosis, a type of programmed necrosis is defined as a death receptor-initiated process under caspase-compromised conditions. The process requires the kinase activity of receptor-interacting protein kinase 1 and 3 (RIPK1 and RIPK3) and mixed lineage kinase domain-like protein (MLKL), as a substrate of RIPK3. The further downstream events remain elusive. We applied known inhibitors to characterize the contributing enzymes in necroptosis and their effect on cell viability and different cellular functions were detected mainly by flow cytometry. Here we report that staurosporine, the classical inducer of intrinsic apoptotic pathway can induce necroptosis under caspase-compromised conditions in U937 cell line. This process could be hampered at least partially by the RIPK1 inhibitor necrotstin-1 and by the heat shock protein 90 kDa inhibitor geldanamycin. Moreover both the staurosporine-triggered and the classical death ligand-induced necroptotic pathway can be effectively arrested by a lysosomal enzyme inhibitor CA-074-OMe and the recently discovered MLKL inhibitor necrosulfonamide. We also confirmed that the enzymatic role of poly(ADP-ribose)polymerase (PARP) is dispensable in necroptosis but it contributes to membrane disruption in secondary necrosis. In conclusion, we identified a novel way of necroptosis induction that can facilitate our understanding of the molecular mechanisms of necroptosis. Our results shed light on alternative application of staurosporine, as a possible anticancer therapeutic agent. Furthermore, we showed that the CA-074-OMe has a target in the signaling pathway leading to necroptosis. Finally, we could differentiate necroptotic and secondary necrotic processes based on participation of PARP enzyme.

Cytotoxic Capacity of IL-15-Stimulated Cytokine-Induced Killer Cells Against Human Acute Myeloid Leukemia and Rhabdomyosarcoma in Humanized Preclinical Mouse Models

Allogeneic stem cell transplantation (allo-SCT) has become an important treatment modality for patients with high-risk acute myeloid leukemia (AML) and is also under investigation for soft tissue sarcomas. The therapeutic success is still limited by minimal residual disease (MRD) status ultimately leading to patients' relapse. Adoptive donor lymphocyte infusions based on MRD status using IL-15-expanded cytokine-induced killer (CIK) cells may prevent relapse without causing graft-versus-host-disease (GvHD). To generate preclinical data we developed mouse models to study anti-leukemic- and anti-tumor-potential of CIK cells in vivo. Immunodeficient mice (NOD/SCID/IL-2Rγc(-), NSG) were injected intravenously with human leukemic cell lines THP-1, SH-2 and with human rhabdomyosarcoma (RMS) cell lines RH41 and RH30 at minimal doses required for leukemia or tumor engraftment. Mice transplanted with THP-1 or RH41 cells were randomly assigned for analysis of CIK cell treatment. Organs of mice were analyzed by flow cytometry as well as quantitative polymerase chain reaction for engraftment of malignant cells and CIK cells. Potential of CIK cells to induce GvHD was determined by histological analysis. Tissues of the highest degree of THP-1 cell expansion included bone marrow followed by liver, lung, spleen, peripheral blood (PB), and brain. RH30 and RH41 engraftment mainly took place in liver and lung, but was also detectable in spleen and PB. In spite of delayed CIK cell expansion compared with malignant cells, CIK cells injected at equal amounts were sufficient for significant reduction of RH41 cells, whereas against fast-expanding THP-1 cells 250 times more CIK than THP-1 cells were needed to achieve comparable results. Our preclinical in vivo mouse models showed a reliable 100% engraftment of malignant cells which is essential for analysis of anti-cancer therapy. Furthermore our data demonstrated that IL-15-activated CIK cells have potent cytotoxic capacity against AML and RMS cells without causing GvHD.

Cell death pathways as therapeutic targets in rhabdomyosarcoma

Resistance of rhabdomyosarcoma to current therapies remains one of the key issues in pediatric oncology. Since the success of most cytotoxic therapies in the treatment of cancer, for example, chemotherapy, depends on intact signaling pathways that mediate programmed cell death (apoptosis), defects in apoptosis programs in cancer cells may result in resistance. Evasion of apoptosis in rhabdomyosarcoma may be caused by defects in the expression or function of critical mediators of apoptosis or in aberrant expression of antiapoptotic proteins. Therefore, the identification of the molecular mechanisms that confer primary or acquired resistance to apoptosis in rhabdomyosarcoma presents a critical step for the rational development of molecular targeted drugs. This approach will likely open novel perspectives for the treatment of rhabdomyosarcoma.

Platelet-derived growth factor receptor beta inhibition increases tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) sensitivity: imatinib and TRAIL dual therapy

BACKGROUND

There is a crucial need for better therapeutic approaches for the treatment of Ewing sarcoma (EWS). Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in EWS cells in vitro. However, in vivo, acquired resistance to TRAIL is a major limiting factor. Platelet-derived growth factor receptor-beta (PDGFR-beta) is highly expressed on EWS cells. Thus, the authors evaluated whether PDGFR-beta blockade could sensitize EWS cells to TRAIL-induced apoptosis in vitro and in vivo.

METHODS

The effect of combined imatinib mesylate (Gleevec) and TRAIL on EWS cell growth and apoptosis was tested in vitro. Stable PDGFR-beta knockdown in EWS cells was achieved by short-hairpin RNA transduction, and TRAIL sensitivity of these cells was measured. Expression of death receptors was measured by fluorescence-activated cell-sorting (FACS) analysis, and caspase 8 activity was evaluated by Western blot analysis. An orthotopic human xenograft model of EWS growth and spontaneous metastasis in nude mice was used to assess the in vivo affect of combined imatinib mesylate and TRAIL.

RESULTS

Imatinib mesylate induced a significant TRAIL proapoptotic effect in EWS cells in vitro. Specific PDGFR-beta silencing in EWS cells enhanced the effects of TRAIL, possibly through an increase in the expression of death receptors 4 and 5. The combination of imatinib mesylate and TRAIL significantly inhibited the growth of primary tumors and decreased the incidence of spontaneous EWS pulmonary metastasis compared with either drug alone.

CONCLUSIONS

PDGFR-beta blockade combined with TRAIL resulted in antihuman EWS activity in vitro and in vivo, suggesting the possibility that combining these treatments will improve anti-EWS therapy.

Inhibition of NF-kappaB signaling by quinacrine is cytotoxic to human colon carcinoma cell lines and is synergistic in combination with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) or oxaliplatin

Colorectal cancer is the third most common malignancy in the United States. Modest advances with therapeutic approaches that include oxaliplatin (L-OHP) have brought the median survival rate to 22 months, with drug resistance remaining a significant barrier. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is undergoing clinical evaluation. Although human colon carcinomas express TRAIL receptors, they can also demonstrate TRAIL resistance. Constitutive NF-kappaB activation has been implicated in resistance to TRAIL and to cytotoxic agents. We have demonstrated constitutive NF-kappaB activation in five of six human colon carcinoma cell lines; this activation is inhibited by quinacrine. Quinacrine induced apoptosis in colon carcinomas and potentiated the cytotoxic activity of TRAIL in RKO and HT29 cells and that of L-OHP in HT29 cells. Similarly, overexpression of IkappaBalpha mutant (IkappaBalphaM) or treatment with the IKK inhibitor, BMS-345541, also sensitized these cells to TRAIL and L-OHP. Importantly, 2 h of quinacrine pretreatment resulted in decreased expression of c-FLIP and Mcl-1, which were determined to be transcriptional targets of NF-kappaB. Extended exposure for 24 h to quinacrine did not further sensitize these cells to TRAIL- or L-OHP-induced cell death; however, exposure caused the down-regulation of additional NF-kappaB-dependent survival factors. Short hairpin RNA-mediated knockdown of c-FLIP or Mcl-1 significantly sensitized these cells to TRAIL and L-OHP. Taken together, data demonstrate that NF-kappaB is constitutively active in colon cancer cell lines and NF-kappaB, and its downstream targets may constitute an important target for the development of therapeutic approaches against this disease.

Initial response to treatment was highly associated with the prognosis of childhood rhabdomyosarcoma: a retrospective analysis of a single center experience in Korea

PURPOSE

Following the introduction of a multimodal approach to diagnosis and treatment, the prognosis of rhabdomyosarcoma (RMS) has markedly improved over the last three decades. However, there are few data on treatment outcomes in Korean patients.

MATERIALS AND METHODS

We performed a retrospective analysis of 77 patients with RMS diagnosed and treated at Seoul National University Children's Hospital between 1986 and 2005.

RESULTS

The overall 5-year survival and event-free survival rates for all patients were 77% and 59%, respectively. The Intergroup Rhabdomyosarcoma Study clinical grouping and initial response to treatment (20-week response) were important prognostic factors.

CONCLUSIONS

The outcome of childhood RMS was closely associated with the initial staging and the initial response to treatment. Modulating therapies according to initial responses and risk factors is critical, and new treatment strategies for high-risk patients are needed.

Opposing control of rhabdomyosarcoma growth and differentiation by myogenin and interleukin 4

Rhabdomyosarcoma is a tumor of striated muscle origin that displays defective myogenic differentiation. Terminal myogenesis switches off cell proliferation and migration, hence, the promotion of rhabdomyosarcoma differentiation should antagonize tumor growth and metastasis. Terminal myogenesis is controlled by cell-intrinsic myogenic transcription factors like myogenin and environmental mediators like interleukin 4 (IL-4). We studied whether the expression of myogenin or exposure to IL-4 could promote the myogenesis of poorly differentiating human rhabdomyosarcoma cells RD/12. Forced expression of myogenin amplified myosin expression and the formation of myotube-like elements, inhibited cell migration, and reduced the growth of local tumors and liver metastases in immunodepressed mice. In contrast, exposure to IL-4 promoted cell proliferation and survival, especially at high cell density, inhibited myogenin expression, and myogenesis. Moreover, IL-4 stimulated the directed migration of cells with low myogenin levels, but not of cells with higher (spontaneous or forced) levels. Thus, IL-4, which was known to promote late stages of normal myogenesis, favors growth and migration, and inhibits further differentiation of the myogenic stages attained by rhabdomyosarcoma cells. Strategies to increase myogenin expression and block IL-4 could simultaneously reduce growth and migration, and enhance terminal differentiation of rhabdomyosarcoma, thus contributing to the control of tumor growth and metastatic spread.

Ardipusilloside I induces apoptosis in human glioblastoma cells through a caspase-8-independent FasL/Fas-signaling pathway

Ardipusilloside I, a triterpenoid saponin isolated from Ardisia pusilla A. DC, suppresses the growth of a variety of cancer cells, and has certain immunomodulative properties. Herein, we investigated its effect on glioblastoma cell line U87MG cells and primary cultured human glioblastoma cells, and examined the underlying mechanism of action. Ardipusilloside I substantially decreased the number of viable cells of both cell lines in a time- and concentration-dependent manner, with a similar IC(50) of 4.05μM. Microscopy revealed apoptotic characteristics, including chromatin condensation and cell nucleus fragmentation, demonstrating that ardipusilloside I-induced apoptosis. Ardipusilloside I exposure also gradually increased the sub-G1 fraction (the apoptotic cell population) and an S phase-arrest of both glioblastoma cells. Furthermore, ardipusilloside I increased the expression of Fas and its ligand (FasL), and enhanced the activation of caspase-8 and caspase-3. Additionally, we observed a significant decreased apoptosis after the trigger effection of FasL was abolished by the neutralization antibody anti-FasL antibody and an unchanged apoptosis level when the activation of caspase-8 was interrupted by specific inhibitor z-IETD-fmk, which suggested that a casepase-8 independent FasL/Fas-signaling-mediated death receptor pathway is involved. These data suggested that ardipusilloside I could be developed as a chemotherapeutic agent for the management of gliomas.

Actinomycin D enhances TRAIL-induced caspase-dependent and -independent apoptosis in SH-SY5Y neuroblastoma cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has attracted great attention as a promising anti-cancer reagent. Recombinant soluble TRAIL (rsTRAIL) derivatives induce apoptosis in various cancer cells, but not in most normal cells. However, a number of cancerous cell types are resistant to TRAIL cytotoxicity, limiting its application in cancer therapy. In the present study, we report that actinomycin D (Act D) pretreatment increases apoptosis in human neuroblastoma SH-SY5Y cells treated with rsTRAIL. Both caspase-9 and caspase-7, but not caspase-3, were activated during the apoptosis process. z-VAD-fmk, a pan-caspase inhibitor, only partially suppressed apoptosis of the cells, suggesting that the Act D-enhanced apoptosis of SH-SY5Y occurred via caspase-dependent and -independent manners. In cells pretreated with Act D, we found decreased mitochondrial transmembrane potential, high levels of reactive oxygen species (ROS), and up-regulated apoptotic-inducing factor (AIF). Cell death was blocked in cells stably transfected with AIF-siRNA plasmid. Taken together, these data indicate that Act D sensitizes SH-SY5Y cells to rsTRAIL-induced apoptosis via caspase activation, impairment of the mitochondrial membrane, release of ROS, and up-regulation of AIF expression. This study provides a novel strategy for the therapy of malignant neuroblastoma resistant to rsTRAIL cytotoxicity.

Bid is cleaved upstream of caspase-8 activation during TRAIL-mediated apoptosis in human osteosarcoma cells

TRAIL induces apoptosis in many malignant cell types. In this study, we used the human papilloma virus (HPV) 16 E6 protein as a molecular tool to probe the TRAIL pathway in HCT116 colon carcinoma cells and U2OS osteosarcoma cells. Intriguingly, we found that while E6 protected HCT116 cells from TRAIL, U2OS cells expressing E6 remained sensitive to TRAIL. Furthermore, silencing FADD and procaspase-8 expression with siRNA did not prevent TRAIL-induced apoptosis in U2OS cells. However, siBid provided significant protection from TRAIL, and the cleavage kinetics of Bid and caspase-8 revealed that Bid was cleaved prior to the activation of caspase-8. Cathepsin B activity in U2OS cells was significantly activated shortly after exposure to TRAIL, and the cathepsin B inhibitor, CA074Me, inhibited both TRAIL- and anti-DR5-mediated apoptosis and delayed the cleavage of Bid. These findings suggest that TRAIL activates a pathway dependent on Bid, but largely independent of FADD and caspase-8, in U2OS cells.

Sphingosine-induced apoptosis in rhabdomyosarcoma cell lines is dependent on pre-mitochondrial Bax activation and post-mitochondrial caspases

Sphingolipids is the collective term ascribed to components of the sphingomyelin cycle. Modulation of the cellular levels of individual sphingolipids can induce a diverse range of cellular responses including apoptosis, proliferation, and cell cycle arrest. We present data showing that rhabdomyosarcoma cell lines, independent of lineage (alveolar rhabdomyosarcoma and embryonal rhabdomyosarcoma), are particularly sensitive to the induction of apoptosis as a result of an elevation in the cellular levels of sphingosine (D-erythro-sphingosine). Sphingosine-mediated apoptosis does not require its metabolism to the related proapoptotic molecule ceramide and is stereospecific because exposure of the rhabdomyosarcoma cell line RD to the L-erythro and DL-threo isoforms of sphingosine did not induce apoptosis. Importantly, for efficient induction of apoptosis, sphingosine required Bax activation and consequential translocation to the mitochondria. This resulted in selective mitochondrial release of cytochrome c and Smac/Diablo but not other mitochondrial related factors (apoptosis-inducing factor, endonuclease G, and HtrA2/Omi). Using small interfering RNA, reduced Bax expression conferred the impaired release of mitochondrial cytochrome c to the cytoplasm following sphingosine exposure, inhibiting the induction of apoptosis. Furthermore, dissipation of the inner mitochondrial membrane potential and enhanced production of reactive oxygen species were not observed. Bax activation and cytochrome c release were independent of caspases; however, caspase-3 and caspase-9 activity distal to the mitochondria was essential for the execution of apoptosis.

Proteasome inhibitors sensitize colon carcinoma cells to TRAIL-induced apoptosis via enhanced release of smac/DIABLO from the mitochondria

The synergistic interaction between proteasome inhibitors and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising approach to induce cell death in tumor cells. However, the molecular and biochemical mechanisms of this synergism have been proven to be cell type specific. We therefore focused our investigation on TRAIL-resistant colon carcinoma cells in this study. DNA fragmentation, mitochondrial membrane depolarization and increased caspase-3-like enzyme activity was exclusively induced only by combined treatment with proteasome inhibitors (epoxomicin, MG132, bortezomib/PS-341) and TRAIL. The expression level of anti-apoptotic proteins (XIAP, survivin, Bcl-2, Bcl-XL), regulated by NF-kappaB transcription factor, was not effected by any of these treatments. TRAIL alone induced only partial activation of caspase-3 (p20), while the combination of TRAIL and proteasome inhibition led to the full proteolytic activation of caspase-3 (p17). Only the combination treatment induced marked membrane depolarization and the release of cytochrome c, HtrA2/Omi and Smac/DIABLO. Apoptosis-inducing factor (AIF) was not released in any of these conditions. These results are consistent with a model where the full activation of caspase-3 by caspase-8 is dependent on the release of Smac/DIABLO in response to the combined treatment. This molecular mechanism, independent of the inhibition NF-kappaB activity, may provide rationale for the combination treatment of colon carcinomas with proteasome inhibitors and recombinant TRAIL or agonistic antibody of TRAIL receptors.

Rhabdomyosarcoma: new windows of opportunity

Rhabdomyosarcoma is a highly malignant, small blue cell tumor characterized by muscle differentiation. With modern treatment, more than 70% of children and adolescents with this disease are cured. Adequate biopsy to obtain sufficient tissue for accurate diagnosis and molecular characterization is critical. Patients must be assessed for tumor extent; the Intergroup Rhabdomyosarcoma Study (IRS) clinical group and Staging system is universally applied in North America. Multidisciplinary therapy is necessary to maximize cure rates. Local control relies on complete surgical excision when possible; those whose tumors are not completely excised and those with alveolar histology tumors require local irradiation to maximize local control. In North America, vincristine (Oncovin); Eli Lilly and Company, Indianapolis, http://www.lilly.com), dactinomycin (Cosmegen); Merck & Co., Inc., Whitehouse Station, NJ, http://www.merck.com), and cyclophosphamide are the standard chemotherapy agents. The IRS has used therapeutic window studies to confirm the predictive nature of preclinical xenograft models and to identify several new single agents and combinations of agents with activity in high-risk patient groups. Despite these efforts, the outcome for these high-risk patients remains poor. The next generation of Children's Oncology Group studies will evaluate the efficacy of topoisomerase-I inhibitors and dose-compression therapy approaches. New advances in molecular characterization of tumors, including gene-expression analysis, may identify new therapeutic targets that can be exploited by expanded preclinical drug discovery efforts, and hold the promise of revolutionizing risk-based therapies.

Mechanisms of resistance to TRAIL-induced apoptosis in cancer

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is regarded as a potential anticancer agent. However, considerable numbers of cancer cells, especially some highly malignant tumors, are resistant to apoptosis induction by TRAIL, and some cancer cells that were originally sensitive to TRAIL-induced apoptosis can become resistant after repeated exposure (acquired resistance). Understanding the mechanisms underlying such resistance and developing strategies to overcome it are important for the successful use of TRAIL for cancer therapy. Resistance to TRAIL can occur at different points in the signaling pathways of TRAIL-induced apoptosis. Dysfunctions of the death receptors DR4 and DR5 due to mutations can lead to resistance. The adaptor protein Fas-associated death domain (FADD) and caspase-8 are essential for assembly of the death-inducing signaling complex, and defects in either of these molecules can lead to TRAIL resistance. Overexpression of cellular FADD-like interleukin-1beta-converting enzyme-inhibitory protein (cFLIP) correlates with TRAIL resistance in several types of cancer. Overexpression of Bcl-2 or Bcl-X(L), loss of Bax or Bak function, high expression of inhibitor of apoptosis proteins, and reduced release of second mitochondria-derived activator of caspases (Smac/Diablo) from the mitochondria to the cytosol have all been reported to result in TRAIL resistance in mitochondria-dependent type II cancer cells. Finally, activation of different subunits of mitogen-activated protein kinases or nuclear factor-kappa B can lead to development of either TRAIL resistance or apoptosis in certain types of cancer cells.

Caspase-10 sensitizes breast carcinoma cells to TRAIL-induced but not tumor necrosis factor-induced apoptosis in a caspase-3-dependent manner

Although signaling by death receptors involves the recruitment of common components into their death-inducing signaling complexes (DISCs), apoptosis susceptibility of various tumor cells to each individual receptor differs quite dramatically. Recently it was shown that, besides caspase-8, caspase-10 is also recruited to the DISCs, but its function in death receptor signaling remains unknown. Here we show that expression of caspase-10 sensitizes MCF-7 breast carcinoma cells to TRAIL- but not tumor necrosis factor (TNF)-induced apoptosis. This sensitization is most obvious at low TRAIL concentrations or when apoptosis is assessed at early time points. Caspase-10-mediated sensitization for TRAIL-induced apoptosis appears to be dependent on caspase-3, as expression of caspase-10 in MCF-7/casp-3 cells but not in caspase-3-deficient MCF-7 cells overcomes TRAIL resistance. Interestingly, neutralization of TRAIL receptor 2 (TRAIL-R2), but not TRAIL-R1, impaired apoptosis in a caspase-10-dependent manner, indicating that caspase-10 enhances TRAIL-R2-induced cell death. Furthermore, whereas processing of caspase-10 was delayed in TNF-treated cells, TRAIL triggered a very rapid activation of caspase-10 and -3. Therefore, we propose a model in which caspase-10 is a crucial component during TRAIL-mediated apoptosis that in addition actively requires caspase-3. This might be especially important in systems where only low TRAIL concentrations are supplied that are not sufficient for the fast recruitment of caspase-8 to the DISC.

Casein kinase II (CK2) enhances death-inducing signaling complex (DISC) activity in TRAIL-induced apoptosis in human colon carcinoma cell lines

Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli, as well as being elevated in many human cancers examined. We have demonstrated that CK2 is a key survival factor that protects human colon carcinoma cells from TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. We determined that inhibition of CK2 phosphorylation events by DRB (5,6-dichlorobenzimidazole) resulted in dramatic sensitization of tumor cells to TRAIL-induced apoptosis, in the absence of effects in normal cells. Sensitization was caspase dependent, and independent of regulation via NF-kappaB. Further, inhibition of phosphorylation by CK2 did not modify the expression level of antiapoptotic proteins. Analysis of TRAIL-induced death-inducing signaling complex (DISC) formation demonstrated enhanced formation of the DISC, enhanced cleavage of caspase-8 and cleavage of Bid in the presence of DRB, thereby facilitating the release of proapoptotic factors from the mitochondria with subsequent downregulation of the expression of XIAP and c-IAP1. Further, silencing of CK2alpha in HT29 cells following transfection of CK2alpha shRNA abrogated CK2 kinase activity while simultaneously increasing TRAIL sensitivity. These findings demonstrate that CK2 plays a critical antiapoptotic role by conferring resistance to TRAIL at the level of the DISC.

Differential involvement of Bax and Bak in TRAIL-mediated apoptosis of leukemic T cells

TRAIL-induced apoptosis has been considered a promising therapeutic approach for tumors that are resistant to chemotherapy, which is usually mediated via mitochondrial apoptotic cascades. Recent studies have shown that in certain cancer cells, TRAIL-mediated apoptosis is also dependent on mitochondrial involvement, suggesting that similar mechanisms of resistance to chemotherapy might be implicated in the resistance of tumor cells to TRAIL. We have used TRAIL-resistant leukemic cells that are deficient in both Bax and Bak to determine the roles of these Bcl-2 members in TRAIL-mediated apoptosis. Exposure of these cells to TRAIL did not have an impact on cell viability, although it induced the processing of caspase-3 to its active p20 subunit. The activity of the p20 caspase-3 appeared to be inhibited as no autoprocessing of this p20 subunit or cleavage of known caspase-3 substrates were detected. Also, in the absence of Bax and Bak, no release of mitochondrial apoptogenic proteins was observed following TRAIL treatment. Adenoviral transduction of the Bax, but not the Bak gene, to the Bax/Bak-deficient leukemic cells rendered them TRAIL-sensitive as assessed by enhanced apoptotic death and caspase-3 processing. These findings demonstrate preferential utilization of Bax over Bak in leukemic cell response to specific apoptotic stimulation.

Influence of Casein Kinase II in Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand-Induced Apoptosis in Human Rhabdomyosarcoma Cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis via the death receptors DR4 and DR5 in transformed cells in vitro and exhibits potent antitumor activity in vivo with minor side effects. Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli and is aberrantly elevated in a variety of human cancers. Rhabdomyosarcoma tumors are the most common soft-tissue sarcoma in childhood. In this investigation, we demonstrate that CK2 is a key survival factor that protects tumor cells from TRAIL-induced apoptosis. We have demonstrated that inhibition of CK2 phosphorylation events by 5,6-dichlorobenzimidazole (DRB) resulted in dramatic sensitization of tumor cells to TRAIL-induced apoptosis. CK2 inhibition also induced rapid cleavage of caspase-8, -9, and -3, as well as the caspase substrate poly(ADP-ribose) polymerase after TRAIL treatment. Overexpression of Bcl-2 protected cells from TRAIL-induced apoptosis in the presence of the CK2 inhibitor. Death signaling by TRAIL in these cells was Fas-associated death domain and caspase dependent because dominant negative Fas-associated death domain or the cowpox interleukin 1beta-converting enzyme inhibitor protein cytokine response modifier A prevented apoptosis in the presence of DRB. Analysis of death-inducing signaling complex (DISC) formation demonstrated that inhibition of CK2 by DRB increased the level of recruitment of procaspase-8 to the DISC and enhanced caspase-8-mediated cleavage of Bid, thereby increasing the release of the proapoptotic factors cytochrome c, HtrA2/Omi, Smac/DIABLO, and apoptosis inducing factor (AIF) from the mitochondria, with subsequent degradation of X-linked inhibitor of apoptosis protein (XIAP). To further interfere with CK2 function, JR1 and Rh30 cells were transfected with either short hairpin RNA targeted to CK2alpha or kinase-inactive CK2alpha (K68M) or CK2alpha' (K69M). Data show that the CK2 kinase activity was abrogated and that TRAIL sensitivity in both cell lines was increased. Silencing of CK2alpha expression with short hairpin RNA was also associated with degradation of XIAP. These findings suggest that CK2 regulates TRAIL signaling in rhabdomyosarcoma by modulating TRAIL-induced DISC formation and XIAP expression.