The sphingosine kinase 2 inhibitor ABC294640 displays anti‐non‐small cell lung cancer activities in vitro and in vivo

Non‐small cell lung cancer (NSCLC) accounts for about 85‐90% of lung cancer cases, and is the number one killer among cancers in the United States. The majorities of lung cancer patients do not respond well to conventional chemo‐ and/or radio‐therapeutic regimens, and have a dismal 5‐year survival rate of ～15%. The recent introduction of targeted therapy and immunotherapy gives new hopes to NSCLC patients, but even with these agents, not all patients respond, and responses are rarely complete. Thus, there is still an urgent need to identify new therapeutic targets in NSCLC and develop novel anti‐cancer agents. Sphingosine kinase 2 (SphK2) is one of the key enzymes in sphingolipid metabolism. SphK2 expression predicts poor survival in NSCLC patients, and is associated with Gefitinib‐resistance. In this study, the anti‐NSCLC activities of ABC294640, the only first‐in‐class orally available inhibitor of SphK2, were explored. The results obtained indicate that ABC294640 treatment causes significant NSCLC cell apoptosis, cell cycle arrest and suppression of tumor growth in vitro and in vivo. Moreover, lipidomics analyses revealed the complete signature of ceramide and dihydro(dh)‐ceramide species in the NSCLC cell‐lines with or without ABC294640 treatment. These findings indicate that sphingolipid metabolism targeted therapy may be developed as a promising strategy against NSCLC.     

Targeting sphingosine kinase 2 (SphK2) by ABC294640 inhibits colorectal cancer cell growth in vitro and in vivo

Background

Colorectal cancer (CRC) is a major health problem in China and around the world. It is one of the leading causes of cancer-related deaths. Research groups are thus searching for novel and more efficient anti-CRC agents.

Results

Here we demonstrated that ABC294640, a novel SphK2 inhibitor, induced growth inhibition and apoptosis in transformed and primary CRC cells. The SphK activity was remarkably inhibited by ABC294640, accompanied by sphingosine-1-phosphate (S1P) depletion and ceramide incensement in CRC cells. Exogenously-added S1P inhibited ABC294640-induced HT-29 cell lethality. While C6 ceramide and SphK1 inhibitor SKI-II facilitated ABC294640-induced cytotoxicity against HT-29 cells. ABC294640 inhibited AKT-S6K1, but activated JNK signaling in transformed and primary CRC cells. JNK inhibitors (SP600125 and JNKi-II) alleviated ABC294640-induced CRC cell apoptosis. Moreover, a low concentration of ABC294640 sensitized the activity of 5-FU and cisplatin in vitro. In vivo, ABC294640 oral administration dramatically inhibited HT-29 xenografts growth in nude mice.

Conclusions

Targeting of SphK2 by ABC294640 potently inhibits CRC cell growth both in vitro and in vivo, ABC294640 could be developed as a novel therapeutic for the treatment of CRC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-015-0205-y) contains supplementary material, which is available to authorized users.     

Targeting NFĸB mediated breast cancer chemoresistance through selective inhibition of sphingosine kinase-2

Resistance to chemotherapy remains a significant obstacle in the treatment of hormone-independent breast cancer. Recent evidence suggests that altered sphingolipid signaling through increased sphingosine kinase activity may be an important mediator of breast cancer drug resistance. Sphingosine kinase-1 (Sphk1) is a proposed key regulator of breast cancer tumorigenesis, proliferation and resistance. There is, however, conflicting data on the role of sphingosine kinase-2 (Sphk2) in cancer biology and resistance, with some suggesting that Sphk2 has an opposing role to that of Sphk1. Here, we studied the effects of the novel selective Sphk2 inhibitor, ABC294640 (3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl) amide), on human breast cancer. ABC294640 blocked both viability and survival at low micromolar IC₅₀ concentrations in the endocrine therapy-resistant MDA-MB-231 and chemoresistant MCF-7TN-R cell systems. Treatment with the inhibitor significantly reduced proliferation, as seen in immunofluorescence staining of Ki-67 in vitro. Interestingly, pharmacological inhibition of Sphk2 induced apoptosis through the intrinsic programmed cell death pathway. Furthermore, ABC294640 also diminished NFκB survival signaling, through decreased activation of the Ser536 phosphorylation site on the p65 subunit. Xenografts of MCF-7TN-R cells growing in immunocompromised mice were utilized to validate the therapeutic efficacy of the sphingosine kinase-2 inhibitor. Treatment with 50 mg of ABC294640/kg completely blocked tumor volume in this model. These results indicate that pharmacological inhibition of Sphk2 with the orally bioavailable selective inhibitor, ABC294640, has therapeutic potential in the treatment of chemoand endocrine therapy-resistant breast cancer.Key words: sphingolipids, chemoresistance, sphingosine kinase, NFkappaB, breast cancer, ceramide, TNF, sphingosine-1-phosphate     

TRAIL, FasL and a blocking anti-DR5 antibody augment paclitaxel-induced apoptosis in human non-small-cell lung cancer.

Lung carcinoma is one of the most frequent causes of malignancy-related mortality in the world. Paclitaxel (PA) is an antineoplastic agent used in the treatment of non-small-cell lung cancer (NSCLC) and possesses a single-agent response rate approaching 25%. PA kills tumor cells by inducing both cellular necrosis and apoptosis. Fas and Trail receptors (DR4 and DR5) are TNF family members and act as death signal transduction proteins in the apoptosis cascade. Despite the importance of PA in lung cancer treatment, the function of Fas, DR4 and DR5 in PA-induced apoptosis, as well as the effect of their respective ligands FasL and TRAIL alone or in combination with PA, remains poorly understood. We show here that 10 microM PA induces a significant 10- to 57-fold increase in primary lung cancer cell apoptosis and is associated with 20-215% increases in caspase-3 activity in various NSCLC cell types. All the lung cancer cells express Fas, FasL, DR4 and DR5; however PA did not significantly modify their levels. We provide here the first time evidence that TRAIL is a potent inducer of apoptosis in multiple NSCLC cell lines. Noticeably, CH11, the Fas receptor cross-linking and the antagonistic anti-DR5 antibody enhance considerably the spontaneous apoptotic rate in 3 out of 5 cell types. The combination treatments, FasL+PA, TRAIL+PA or PA+anti-DR5 antibody, greatly enhance PA-apoptotic effect in most cell lines. These data suggest that the use of new combination treatment with PA and ligands targeting Fas or TRAIL receptors would be particularly efficacious.     

Tunicamycin sensitizes human prostate cells to TRAIL-induced apoptosis by upregulation of TRAIL receptors and downregulation of cIAP2

The addition of tunicamycin to prostate cancer cells enhances cell death mediated by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study, we investigated whether tunicamycin, an endoplasmic reticulum stress inducer, can potentiate TRAIL-induced apoptosis in human prostate cancer cells. We evaluated the combination of tunicamycin and TRAIL and found synergistic promotion of apoptosis in prostate cancer cells. The combined treatment with tunicamycin and TRAIL significantly induced apoptosis, and stimulated caspase-3, -8 and -9 activity, as well as the cleavage of poly (ADP-ribose) polymerase. We found that tunicamycin promoted TRAIL-induced apoptosis by the upregulation of death receptor (DR)4 and DR5 and the downregulation of cellular inhibitor of apoptosis 2 (cIAP2). In addition, downregulation of cIAP2 expression using small interfering RNA significantly attenuated the apoptosis induced by TRAIL. Taken together, our results demonstrate that the combination of tunicamycin and TRAIL may provide a novel strategy for treating prostate cancer by overcoming critical mechanisms of apoptosis resistance.     

The proteasome inhibitor PS-341 (bortezomib) up-regulates DR5 expression leading to induction of apoptosis and enhancement of TRAIL-induced apoptosis despite up-regulation of c-FLIP and survivin expression in human NSCLC cells

The proteasome inhibitor PS-341 (bortezomib or Velcade), an approved drug for treatment of patients with multiple myeloma, is currently being tested in clinical trials against various malignancies, including lung cancer. Preclinical studies have shown that PS-341 induces apoptosis and enhances tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in human cancer cells with undefined mechanisms. In the present study, we show that PS-341 induced caspase-8-dependent apoptosis, cooperated with TRAIL to induce apoptosis, and up-regulated death receptor 5 (DR5) expression in human non-small cell lung cancer (NSCLC) cells. DR5 induction correlated with the ability of PS-341 to induce apoptosis. Blockage of PS-341-induced DR5 up-regulation using DR5 small interfering RNA (siRNA) rendered cells less sensitive to apoptosis induced by either PS-341 or its combination with TRAIL, indicating that DR5 up-regulation mediates PS-341-induced apoptosis and enhancement of TRAIL-induced apoptosis in human NSCLC cells. We exclude the involvement of c-FLIP and survivin in mediating these events because c-FLIP (i.e., FLIP(S)) and survivin protein levels were actually elevated on exposure to PS-341. Reduction of c-FLIP with c-FLIP siRNA sensitized cells to PS-341-induced apoptosis, suggesting that c-FLIP elevation protects cells from PS-341-induced apoptosis. Thus, the present study highlights the important role of DR5 up-regulation in PS-341-induced apoptosis and enhancement of TRAIL-induced apoptosis in human NSCLC cells.     

Ceramides promote apoptosis for virus-infected lymphoma cells through induction of ceramide synthases and viral lytic gene expression

Kaposi''s sarcoma-associated herpesvirus (KSHV) is the etiologic agent for several human cancers including primary effusion lymphoma (PEL), a rapidly progressive malignancy arising preferentially in immunocompromised patients. With conventional chemotherapy, PEL continues to portend high mortality, dictating the development of novel therapeutic strategies. Sphingosine kinase 2 (SphK2) represents a key gatekeeper for sphingolipid metabolism, responsible for conversion of ceramides to sphingosine-1-phosphate (S1P). We have previously demonstrated that targeting SphK2 using a novel selective inhibitor, ABC294640, leads to intracellular accumulation of ceramides and induces apoptosis for KSHV-infected PEL cells, while suppressing tumor progression in vivo. In the current study, we sought to determine whether specific ceramide/dh-ceramide species and related ceramide synthases (CerS) impact viability for KSHV-infected PEL cells during targeting of SphK2. We found that several specific ceramide and dihydro(dh)-ceramide species and their associated CerS reduce PEL survival and tumor expansion in vitro and in vivo. Moreover, we found that dhC16-Cer induces PEL apoptosis in part through activation of KSHV lytic gene expression. These data further implicate bioactive sphingolipids in regulation of PEL survival, and provide justification for future studies evaluating clinically relevant ceramide analogs or mimetics for their potential as therapeutic agents for PEL.     

The promise of cancer therapeutics targeting the TNF-related apoptosis-inducing ligand and TRAIL receptor pathway

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily and has been shown to induce apoptosis in cancer cells but not normal cells. TRAIL triggers apoptosis through binding to its receptors DR4 and KILLER/DR5. Chemo or radiotherapy induces apoptosis through activation of p53 in response to cellular damage, whereas TRAIL induces apoptosis independent of p53. Mutations or deletions of p53 occurred in more than half of human tumors confer resistance to chemo-radiotherapy. Treatment of TRAIL-resistant tumors with agents targeting death receptors, intrinsic Bcl-2 family members, inhibitor of apoptosis proteins or PI3K/Akt pathway restores the sensitivity to TRAIL-induced apoptosis. Combination of rhTRAIL or the agonist antibody for TRAIL receptor with conventional chemotherapeutic agents results in enhanced efficacy in preventing tumor progression and metastasis. Therefore, the rational design of TRAIL-based therapy combining with other modality that either synergizes to apoptosis induction or overcomes the resistance represents a challenging strategy to achieve the systemic tumor targeting and augment the antitumor activity of cancer therapeutics.     

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.     

Cardiac glycosides initiate Apo2L/TRAIL-induced apoptosis in non-small cell lung cancer cells by up-regulation of death receptors 4 and 5

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) belongs to the TNF family known to transduce their death signals via cell membrane receptors. Because it has been shown that Apo2L/TRAIL induces apoptosis in tumor cells without or little toxicity to normal cells, this cytokine became of special interest for cancer research. Unfortunately, cancer cells are often resistant to Apo2L/TRAIL-induced apoptosis; however, this can be at least partially negotiated by parallel treatment with other substances, such as chemotherapeutic agents. Here, we report that cardiac glycosides, which have been used for the treatment of cardiac failure for many years, sensitize lung cancer cells but not normal human peripheral blood mononuclear cells to Apo2L/TRAIL-induced apoptosis. Sensitization to Apo2L/TRAIL mediated by cardiac glycosides was accompanied by up-regulation of death receptors 4 (DR4) and 5 (DR5) on both RNA and protein levels. The use of small interfering RNA revealed that up-regulation of death receptors is essential for the demonstrated augmentation of apoptosis. Blocking of up-regulation of DR4 and DR5 alone significantly reduced cell death after combined treatment with cardiac glycosides and Apo2L/TRAIL. Combined silencing of DR4 and DR5 abrogated the ability of cardiac glycosides and Apo2L/TRAIL to induce apoptosis in an additive manner. To our knowledge, this is the first demonstration that glycosides up-regulate DR4 and DR5, thereby reverting the resistance of lung cancer cells to Apo2/TRAIL-induced apoptosis. Our data suggest that the combination of Apo2L/TRAIL and cardiac glycosides may be a new interesting anticancer treatment strategy.     

Lipoxygenase inhibitors induce death receptor 5/TRAIL-R2 expression and sensitize malignant tumor cells to TRAIL-induced apoptosis

Lipoxygenases induce malignant tumor progression and lipoxygenase inhibitors have been considered as promising anti-tumor agents. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is one of the most promising candidates for new cancer therapeutics. Combined treatment with nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, and TRAIL markedly induced apoptosis in Jurkat T-cell leukemia cells at suboptimal concentrations for each agent. The combined treatment efficiently activated caspase-3, -8 and -10, and Bid. The underling mechanism by which NDGA enhanced TRAIL-induced apoptosis was examined. NDGA did not change the expression levels of anti-apoptotic factors, Bcl-x(L), Bcl-2, cIAP-1, XIAP and survivin. The expression of death receptor-related genes was investigated and it was found that NDGA specifically up-regulated the expression of death receptor 5 (DR5) at mRNA and protein levels. Down-regulation of DR5 by small interfering RNA prevented the sensitizing effect of NDGA on TRAIL-induced apoptosis. Furthermore, NDGA sensitized prostate cancer and colorectal cancer cells to TRAIL-induced apoptosis. In contrast, NDGA neither enhanced TRAIL-induced apoptosis nor up-regulated DR5 expression in normal peripheral blood mononuclear cells. Another lipoxygenase inhibitor, AA861, also up-regulated DR5 and sensitized Jurkat and DU145 cells to TRAIL. These results indicate that lipoxygenase inhibitors augment the apoptotic efficiency of TRAIL through DR5 up-regulation in malignant tumor cells, and raise the possibility that the combination of lipoxygenase inhibitor and TRAIL is a promising strategy for malignant tumor treatment.     

Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand cooperates with chemotherapy to inhibit orthotopic lung tumor growth and improve survival

Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is a tumor necrosis factor superfamily member that induces apoptosis through the death receptors DR4 and/or DR5 in various cancer cell types but not in most normal cells. Several lung cancer cell lines express DR4 and DR5 and undergo apoptosis in vitro in response to Apo2L/TRAIL. We investigated the efficacy of recombinant soluble human Apo2L/TRAIL and its interaction with chemotherapy in xenograft models based on human NCI-H460 non-small cell lung carcinoma cells. In vitro, Taxol enhanced caspase activation and apoptosis induction by Apo2L/TRAIL. In vivo, Apo2L/TRAIL or Taxol plus carboplatin chemotherapy partially delayed progression of established subcutaneous tumor xenografts, whereas combined treatment caused tumor regression and a substantially longer growth delay. Apo2L/TRAIL, chemotherapy, or the combination of both inhibited growth of preformed orthotopic lung parenchymal tumors versus control by 60%, 57%, or 97%, respectively (all P < 0.01; n = 8-10). Furthermore, combination treatment improved day-90 survival relative to control (7 of 15 versus 1 of 15; P = 0.0003 by Mantel-Cox) as well as to Apo2L/TRAIL (3 of 14; P = 0.031) or chemotherapy (3 of 15; P = 0.035). These studies provide evidence for in vivo activity of Apo2L/TRAIL against lung tumor xenografts and underscore the potential of this ligand for advancing current lung cancer treatment strategies.     

Functional roles of tumor necrosis factor‐related apoptosis‐inducing ligand–DR5 interaction in B16F10 cells by activating the nuclear factor‐κB pathway to induce metastatic potential

Tumor necrosis factor‐related apoptosis‐inducing ligand (TRAIL) has been recognized as a promising target for cancer therapy because it can induce apoptotic cell death in tumor cells but not normal cells. Although TRAIL shows specific tumoricidal activity, resistance to TRAIL‐induced apoptosis in some tumor cells has been considered a clinical obstacle of its application. It has been shown that TRAIL provides inflammatory signals that may contribute to the TRAIL‐resistance of cancer cells; however, it is not known whether TRAIL itself is involved in malignant cancer cell behavior. In the present study, we examined the functional role of TRAIL in B16F10 mouse melanoma cells, which are totally insensitive to TRAIL‐induced apoptosis. By establishing B16F10 cells stably expressing the nuclear factor‐κB (NFκB)‐luciferase reporter gene, we found that TRAIL can activate NFκB through its death receptor DR5 in B16F10 cells. Furthermore, TRAIL–DR5 interaction not only promoted malignant behaviors of B16F10 cells, such as cell proliferation and MMP‐9 production, but also induced lung metastasis of B16F10 cells in vivo. These findings may imply a contrary role for the TRAIL–DR5 pathway in the inflammatory tumor microenvironment, in its ability to induce the metastatic potential of B16F10 melanoma cells instead of inducing apoptosis.     

Thymoquinone Crosstalks with DR5 to Sensitize TRAIL Resistance and Stimulate ROS-Mediated Cancer Apoptosis

Objective: Cancer treatment using a targeted inducer of apoptosis like tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) faced the obstacle of resistance, thus providing a plus drug like Thymoquinone (TQ) could be of great interest to tackle breast cancer cells. The aim of the present work is to examine the genetic modulation impacts of the TRAIL receptors and apoptotic markers upon the combinatorial remedy of TRAIL plus TQ on human breast cancer cell lines. Methods: To achieve this rationale, the protein content-based cytotoxicity using SRB assay, as well as the genetic expressions of the TRAIL receptors (DR4 and DR5) and apoptotic markers (Bcl-2, Cas-8, and FADD) using real time qRT-PCR technique were preceded against breast cancer MCF-7 and MDA-MB-231 cancerous cell lines. Results: The current study showed that the combination therapy of TQ+TRAIL significantly inhibited the protein content-based proliferation of MDA-MB-231 cells more than MCF-7 cells. The synergistic effect of them significantly up-regulated the genetic expressions of DR4, DR5, Cas-8, and FADD genes and inhibited the genetic expression of the Bcl-2 gene in the proposed cell lines treated for 24 h. The induction of the apoptotic genes using the combined therapy was stimulated by the elevation of the reactive oxygen species (ROS); nitric oxide (NO) and malondialdehyde (MDA) levels. Conclusions: The synergistic influence between TQ which induced the DR5 and TRAIL, facilitating the connection between TRAIL and its receptors on the cancerous cell membrane. Hence, the proposed combination therapy induced the ROS-mediated apoptotic stimulus.     

Chemotherapeutic agents sensitize osteogenic sarcoma cells, but not normal human bone cells, to Apo2L/TRAIL-induced apoptosis

Apo2L/TRAIL is a member of the tumor necrosis factor (TNF) family of cytokines that induces death of cancer cells but not normal cells. Its potent apoptotic activity is mediated through its cell surface death domain-containing receptors, DR4 and DR5. Apo2L/TRAIL interacts also with 3 "decoy" receptors that do not induce apoptosis, DcR1, DcR2, which lack functional death domains, and osteoprotegerin (OPG). The aim of our study was to investigate the cytotoxic activity of Apo2L/TRAIL on established osteogenic sarcoma cell lines (BTK-143, HOS, MG-63, SJSA-1, G-292 and SAOS2) and in primary cultures of normal human bone (NHB) cells. When used alone, Apo2L/TRAIL at 100 ng/ml for 24 hr induced greater than 80% cell death in only 1 (BTK-143) of the 6 osteogenic sarcoma cell lines. In contrast, Apo2L/TRAIL-resistant cells were susceptible to Apo2L/TRAIL-mediated apoptosis in the presence of the anticancer drugs, Doxorubicin (DOX), Cisplatin (CDDP) and Etoposide (ETP) but not Methotrexate (MTX) or Cyclophosphamide (CPM). Importantly, neither Apo2L/TRAIL alone nor in combination with any of these drugs affected primary normal human bone cells under equivalent conditions. Apo2L/TRAIL-induced apoptosis, and its augmentation by chemotherapy in the resistant cell lines was mediated through caspase-8 and caspase-3 activation. Furthermore, Apo2L/TRAIL-induced apoptosis and its augmentation by chemotherapy was effectively inhibited by caspase-8 zIETD-fmk and caspase-3 zDEVD-fmk protease inhibitors and by the pan-caspase inhibitor zVAD-fmk. The pattern of basal Apo2L/TRAIL receptor mRNA expression, or expression of the intracellular caspase inhibitor FLICE-inhibitory protein, FLIP, could not be readily correlated with resistance or sensitivity to Apo2L/TRAIL-induced apoptosis. However, the augmentation of Apo2L/TRAIL effects by chemotherapy was associated with drug-induced up-regulation of death receptors DR4 and DR5 mRNA and protein. No obvious correlation was seen between the expression of OPG mRNA or protein and susceptibility of cells to Apo2L/TRAIL-induced apoptosis. Stable over-expression of a dominant negative form of the Fas-associated death domain protein (FADD) in the Apo2L/TRAIL-sensitive BTK-143 cells completely inhibited Apo2L/TRAIL-induced cell death. Our results indicate that chemotherapy and Apo2L/TRAIL act synergistically to kill cancer cells but not normal bone-derived osteoblast-like cells, which has implications for future therapy of osteosarcoma.     

An antibody against DR4 (TRAIL-R1) in combination with doxorubicin selectively kills malignant but not normal prostate cells

Prostate cancer is a major health problem among American men and new treatment approaches are needed. Tumor necrosis factor related apoptosis-inducing ligand (TRAIL/Apo2L) is a death ligand that can induce apoptosis in some but not all cancer cells. Resistance to TRAIL-mediated apoptosis can be overcome by radiation or chemotherapy. The effect of doxorubicin/TRAIL combination therapy was compared among PC3, normal prostate epithelial (PrEC) and stromal (PrSC) cells and cell viability measured by MTS assay. Combination of doxorubicin and TRAIL caused cytotoxicity in all cells tested, although PrSC were more resistant. There was no correlation between TRAIL phenotype and expression of c-FLIP, caspases or TRAIL decoy receptors, although PrSC failed to express DR4. A DR4-specific antibody, which behaved as an agonist in combination with doxorubicin, selectively induced cell death in malignant but not normal prostate cells. Although normal PrEC expressed DR4 as determined by western blot, flow cytometry revealed that only maligant prostate cancer cells (PC3, JCA-1) and not PrEC's exhibited DR4 surface expression. Therefore, combination of doxorubicin and an antibody to DR4 might have therapeutic potential for the treatment of prostate cancer by selectively targeting malignant prostate cells.     

Differential susceptibility of metastatic and primary oral cancer cells to TRAIL-induced apoptosis

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) preferentially induces apoptosis of cancer cells without toxicity in normal cells. TRAIL plays an important role in host immune surveillance against tumor metastasis. Cathepsin B (CB) is a mediator of apoptosis whose activity is regulated by its inhibitors, known as cystatins. We examined the TRAIL-mediated cytotoxicity rates of clonally-related primary and metastatic oral cancer (OC) cells and correlated them with the expression levels of TRAIL receptors, cathepsin B and cystatins A, B, C and M. Two pairs of primary (686Tu and 101A) and metastatic (686Ln and 101B) OC cell lines were treated with various concentrations (5 to 1000 ng/ml) of recombinant human TRAIL protein for 14 h, and cell viability and apoptotic rate were determined. In both pairs of cell lines, primary OC cells revealed greater susceptibility to TRAIL than their metastatic counterparts. The protein synthesis inhibitor cycloheximide markedly increased the TRAIL sensitivity of these cell lines, whereas the CB-specific chemical inhibitor CA-074 markedly reduced the sensitivity of primary OC cells to TRAIL. DNA laddering and M30 CytoDEATH immunodetection assays confirmed that TRAIL-induced OC cell death is an apoptotic process. Expression levels of TRAIL death (DR4 and DR5) and decoy (DcR1 and DcR2) receptors were not different between primary and metastatic OC cells. However, expression levels of cystatins were higher in metastatic OC cells than in their respective primary cells, whereas CB levels remain unchanged. Cathepsin B is a mediator of TRAIL-induced apoptosis in OC cells. Elevated levels of cystatins in metastatic OC cells may cause their greater resistance to TRAIL-induced apoptosis. Our data suggest that high expression of cystatins in OC cells may confer a metastatic phenotype by enhancing their resistance to TRAIL.