DNA damage signals induction of fas ligand in tumor cells

Metabolic changes induced by DNA damage in Ramos cells: exploring the role of mTORC1 complex

DNA damage induces the activation of many different signals associated with repair or cell death, but it is also connected with physiological events, such as adult neurogenesis and B-cell differentiation. DNA damage induces different signaling pathways, some of them linked to important metabolic changes. The mTORC1 pathway has a central role in the regulation of growth processes and cell division in response to environmental changes and also controls protein synthesis, lipid biogenesis, nucleotide synthesis, and expression of glycolytic genes. Here, we report that double-strand breaks induced with etoposide affect the expression of genes encoding different enzymes associated with specific metabolic pathways in Ramos cells. We also analyzed the role of mTOR signaling, demonstrating that double-strand breaks induce downregulation of mTOR signaling. Specific inhibition of mTORC1 using rapamycin also induced changes in the expression of metabolic genes. Finally, we demonstrated that DNA damage and rapamycin can regulate glucose uptake. In summary, our findings show that etoposide and rapamycin affect the expression of metabolic genes as well as apoptotic and proliferation markers in Ramos cells, increasing our understanding of cancer metabolism.

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

Effects of saturated palmitic acid and omega-3 polyunsaturated fatty acids on Sertoli cell apoptosis

Obesity is believed to negatively affect male semen quality and is accompanied by dysregulation of free fatty acid (FFA) metabolism in plasma. However, the implication of dysregulated FFA on semen quality and the involvement of Sertoli cells remain unclear. In the present study, we report obesity decreased Sertoli cell viability through dysregulated FFAs. We observed an increased rate of apoptosis in Sertoli cells, accompanied with elevated FFA levels, in the testes of obese mice that were provided a high-fat diet (HFD). Moreover, the levels of reactive oxygen species were elevated. Furthermore, we demonstrated by in vitro assays that saturated palmitic acid (PA), which is the most common saturated FFA in plasma, led to decreased cell viability of TM4 Sertoli cells in a time- and dose-dependent manner. A similar finding was noted in primary mouse Sertoli cells. In contrast to saturated FFA, omega-3 (ω-3) polyunsaturated fatty acids (PUFAs) protected Sertoli cells from PA-induced lipotoxicity at the physiologically relevant levels. These results indicated that the lipotoxicity of saturated fatty acids might be the cause of obesity-induced Sertoli cell apoptosis, which leads to decreased semen quality. In addition, ω-3 PUFAs could be classified as protective FFAs.

ABBREVIATIONS

FFA: free fatty acid; HFD: high-fat diet; SD: standard diet; PA: palmitic acid; PUFA: polyunsaturated fatty acid; AI: apoptotic index; MTT: 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide; ROS: reactive oxygen species; HE: Hematoxylin and eosin; WT1: Wilm Tumor 1; NAFLD: non- alcoholic fatty liver disease; DCFH-DA: 2', 7' dichlorofluorescin diacetate; 36B4: acidic ribosomal phosphoprotein P0; SD: standard deviation; EPA: eicosapentaenoic acid; PI: propidium iodide; DHA: docosahexenoic acid.

Proteasomal inhibition sensitizes cervical cancer cells to mitomycin C-induced bystander effect: the role of tumor microenvironment

Inaccessibility of drugs to poorly vascularized strata of tumor is one of the limiting factors in cancer therapy. With the advent of bystander effect (BE), it is possible to perpetuate the cellular damage from drug-exposed cells to the unexposed ones. However, the role of infiltrating tumor-associated macrophages (TAMs), an integral part of the tumor microenvironment, in further intensifying BE remains obscure. In the present study, we evaluated the effect of mitomycin C (MMC), a chemotherapeutic drug, to induce BE in cervical carcinoma. By using cervical cancer cells and differentiated macrophages, we demonstrate that MMC induces the expression of FasL via upregulation of PPARγ in both cell types (effector cells) in vitro, but it failed to induce bystander killing in cervical cancer cells. This effect was primarily owing to the proteasomal degradation of death receptors in the cervical cancer cells. Pre-treatment of cervical cancer cells with MG132, a proteasomal inhibitor, facilitates MMC-mediated bystander killing in co-culture and condition medium transfer experiments. In NOD/SCID mice bearing xenografted HeLa tumors administered with the combination of MMC and MG132, tumor progression was significantly reduced in comparison with those treated with either agent alone. FasL expression was increased in TAMs, and the enhanced level of Fas was observed in these tumor sections, thereby causing increased apoptosis. These findings suggest that restoration of death receptor-mediated apoptotic pathway in tumor cells with concomitant activation of TAMs could effectively restrict tumor growth.

The DNA damage response and immune signaling alliance: Is it good or bad? Nature decides when and where

The characteristic feature of healthy living organisms is the preservation of homeostasis. Compelling evidence highlight that the DNA damage response and repair (DDR/R) and immune response (ImmR) signaling networks work together favoring the harmonized function of (multi)cellular organisms. DNA and RNA viruses activate the DDR/R machinery in the host cells both directly and indirectly. Activation of DDR/R in turn favors the immunogenicity of the incipient cell. Hence, stimulation of DDR/R by exogenous or endogenous insults triggers innate and adaptive ImmR. The immunogenic properties of ionizing radiation, a prototypic DDR/R inducer, serve as suitable examples of how DDR/R stimulation alerts host immunity. Thus, critical cellular danger signals stimulate defense at the systemic level and vice versa. Disruption of DDR/R-ImmR cross talk compromises (multi)cellular integrity, leading to cell-cycle-related and immune defects. The emerging DDR/R-ImmR concept opens up a new avenue of therapeutic options, recalling the Hippocrates quote "everything in excess is opposed by nature."

Cytogenetic evidence that DNA topoisomerase II is not involved in radiation induced chromsome-type aberrations

ICRF-187 (Cardioxane™, Chiron) is a catalytic inhibitor of DNA topoisomerase II (Topo II), proposed to act by blocking Topo II-mediated DNA cleavage without stabilizing DNA-Topo II-"cleavable complexes". In this study ICRF-187 was used to evaluate the potential involvement of DNA topoisomerase II in the formation of the radiation-induced chromosome-type aberrations in the G0 phase of the cell cycle in human lymphocytes from three healthy male donors. This is based on many evidences that DNA topoisomerases are involved in DNA recombination, mainly of illegitimate type (non-homologous) both in vitro and in vivo. The results obtained clearly indicated that ICRF-187 did not induce per se any chromosomal damage. When challenged with the non-catalytic Topo II poison VP-16 (etoposide), which acts by stabilizing the "cleavable complex" generating "protein concealed" DSB's and thus chromosomal aberrations, it completely abolished the significant induction of chromosome-type aberrations and formation of dicentric chromosomes. This indicates that ICRF-187 acts effectively as catalytic inhibitor of Topo II. On the other hand, when X-ray treatments were challenged with ICRF-187 using experimental conditions as for VP-16 treatments, no modification of the incidence of chromosome-type aberrations and dicentric chromosomes was observed. On this basis, we conclude that Topo II is not involved in the formation of X-ray-induced chromosome-type aberrations and dicentric chromosomes in human lymphocytes in the G0 phase of the cell cycle.

Potential for modulation of the fas apoptotic pathway by epidermal growth factor in sarcomas

One important mechanism by which cancer cells parasitize their host is by escaping apoptosis. Thus, selectively facilitating apoptosis is a therapeutic mechanism by which oncotherapy may prove highly advantageous. One major apoptotic pathway is mediated by Fas ligand (FasL). The death-inducing signaling Ccmplex (DISC) and subsequent death-domain aggregations are created when FasL is bound by its receptor thereby enabling programmed cell death. Conceptually, if a better understanding of the Fas pathway can be garnered, an oncoselective prodeath therapeutic approach can be tailored. Herein, we propose that EGF and CTGF play essential roles in the regulation of the Fas apoptotic pathway in sarcomas. Tumor and in vitro data suggest viable cells counter the prodeath signal induced by FasL by activating EGF, which in turn induces prosurvival CTGF. The prosurvival attributes of CTGF ultimately predominate over the death-inducing FasL. Cells destined for elimination inhibit this prosurvival response via a presently undefined pathway. This scenario represents a novel role for EGF and CTGF as regulators of the Fas pathway in sarcomas.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Targeting MMP-9, uPAR, and cathepsin B inhibits invasion, migration and activates apoptosis in prostate cancer cells

Prostate cancer is one of the most commonly diagnosed cancers and the second leading cause of cancer deaths in Americans. The high mortality rate is mainly attributed to the invasiveness and metastasis of advanced prostate cancer. Targeting the molecules involved in metastasis could be an effective mode of treatment for prostate cancer. In this study, the therapeutic potential of siRNA-mediated targeting of matrix metalloproteinase-9 (MMP-9), urokinase plasminogen activator receptor (uPAR), and cathepsin B (CB) in prostate cancer was carried out using single and bi-cistronic siRNA-expressing constructs. Downregulation of MMP-9, uPAR, and CB inhibited matrigel invasion, in vitro angiogenesis and wound-healing migration ability of PC3 and DU145 prostate cancer cell lines. In addition, the siRNA treatments induced apoptosis in the tumor cells as determined by TUNEL and DNA laddering assays. An attempt to elucidate the apoptotic pathway showed the involvement of FAS-mediated activation of caspases-8 and -7. Further, mice with orthotopic prostate tumors treated with siRNA-expressing vectors showed significant inhibition in tumor growth and migration. In conclusion, we report that the siRNA-mediated knockdown of MMP-9, uPAR, and CB inhibits invasiveness and migration of prostate cancer cells and leads to apoptosis both in vitro and in vivo.

Alpha-eleostearic acid suppresses proliferation of MCF-7 breast cancer cells via activation of PPARgamma and inhibition of ERK 1 / 2

Alpha-eleostearic acid (alpha-ESA) is known to suppress the growth in cancer cells although its underlying molecular mechanisms have not been fully elucidated. The present study was designed to elucidate and evaluate the anticancer mechanism of alpha-ESA on MCF-7 breast cancer cells. Also, an attempt was made to better understand the anticancer mechanism by which alpha-ESA activated PPARgamma and attenuated the ERK1/2 MAPK phosphorylation state. The MCF-7 breast cancer cell-line and nontumorigenic MCF-10A human mammary epithelial cells were treated with alpha-ESA and compared with negative control (without treatment) and positive control groups (treated with rosiglitazone), and changes of apoptosis-related molecules, PPARgamma and pERK1/2 were examined. In MCF-7 cells treated with alpha-ESA, we found that the expression of p53, p21, and Bax was up-regulated whereas expression of Bcl-2 and procaspase-9 was down-regulated. Moreover, nuclear translocation of PPARgamma by alpha-ESA positively correlated with inhibition of ERK1/2 activation. Our data suggest that alpha-ESA can be considered to be a PPARgamma agonist and thus a candidate for a chemotherapeutic agent against breast cancer.

Protection of mouse bone marrow from etoposide-induced genomic damage by dexrazoxane

PURPOSE

The objective of the current investigation is to determine whether non-toxic doses of the catalytic topoisomerase-II inhibitor, dexrazoxane, have influence on the genomic damage induced by the anticancer topoisomerase-II poison, etoposide, on mice bone marrow cells.

METHOD

The scoring of micronuclei, chromosomal aberrations, and mitotic activity were undertaken as markers of cyto- and genotoxicity. Oxidative damage markers such as reduced glutathione and lipid peroxidation were assessed as a possible mechanism underlying this amelioration.

RESULTS

Dexrazoxane pre-treatment significantly reduced the etoposide-induced micronuclei formation, chromosomal aberrations, and also the suppression of erythroblast proliferation in bone marrow cells of mice. These effects were dose dependent. Etoposide induced marked biochemical alterations characteristic of oxidative stress including enhanced lipid peroxidation and reduction in the reduced glutathione level. Prior administration of dexrazoxane ahead of etoposide challenge ameliorated these biochemical markers.

CONCLUSION

Based on our data presented, strategies can be developed to decrease the etoposide-induced genomic damage in normal cells using dexrazoxane.

Peroxisome proliferator-activated receptor gamma activates fas ligand gene promoter inducing apoptosis in human breast cancer cells

In just over a decade, apart from established metabolic actions, peroxisome proliferator-activated receptor gamma (PPARgamma) has evolved as key therapeutic target in cancer disease. Fas ligand (FasL), a trans-membrane protein, induces apoptosis by crosslinking with the Fas receptor. Despite the FasL relevance, little is available on the regulation of its expression. In the current study, we explored for the first time the potential role of PPARgamma in triggering apoptotic events through the Fas/FasL pathway in breast cancer cells. In MCF7 cells, by reverse transcription-polymerase chain reaction and Western blotting, we showed that the synthetic PPARgamma ligand rosiglitazone (BRL) enhanced FasL expression, that was abrogated by a specific PPARgamma antagonist GW9662. Transient transfection assays demonstrated that BRL transactivated human FasL promoter gene in a PPARgamma-dependent manner. Progressive 5' deletion analysis has identified a minimal promoter fragment spanning nucleotides from -318 to -237 bp, which is still sensitive to BRL treatment. FasL promoter activity was abrogated by mithramycin, suggesting an involvement of Sp1 transcription factor in PPARgamma action. Electrophoretic mobility shift and chromatin immuno-precipitation assays demonstrated that BRL increased the binding of PPARgamma and Sp1 to the Sp1 sequence located within the FasL gene promoter. The role of PPARgamma and Fas/FasL pathways in BRL-induced apoptotic events was assessed by caspase 8 cleavage in the presence of GW as well as PPARgamma and FasL RNA interferences. Our results indicate that PPARgamma positively regulates the FasL gene expression also in MDA-MB231 and in BT20, revealing a new molecular mechanism by which BRL induces apoptosis in breast cancer cells.

Bystander killing of breast cancer MCF-7 cells by MDA-MB-231 cells exposed to 5-fluorouracil is mediated via Fas

The major drawback with cancer therapy is the development of resistant cells within tumors due to their heterogeneous nature and due to inadequate drug delivery during chemotherapy. Therefore, the propagation of injury ("bystander effect" (BE)) from directly damaged cells to other cells may have great implications in cancer chemotherapy. The general advantage of the bystander cell killing phenomenon is the large therapeutic index that can be achieved. Experiments suggest that this phenomenon is detected in radiation therapy as well as in gene therapy in conjunction with chemotherapy. In the present study, we developed an original in vitro model dedicated to the exploration of bystander cytotoxicity induced during breast carcinoma chemotherapy. In brief, we investigated this perpetuation of injury on untreated bystander MCF-7 breast cancer cells which were coplated with 5-fluorouracil (5-FU)-treated MDA-MB-231 breast cancer cells. To achieve this goal, a specific in vitro coculture model which involved mixing of aggressive MDA-MB-231 breast cancer cells with enhanced green fluorescent protein (EGFP) expressing stable clone of non-metastatic MCF-7 breast cancer cells (MCF-EGFP), was used. A bystander killing effect was observed in MCF-EGFP cells cocultured with MDA-MB-231 cells pretreated with 5-FU. The striking decrease in MCF-EGFP cells, as detected by assaying for total GFP intensity, is mediated by activation of Fas/FasL system. The implication of Fas in MCF-EGFP cell death was confirmed by using antagonistic anti-FasL antibody that reverses bystander cell death by blocking FasL on MDA-MB-231 cells. In addition, inhibition of CD95/Fas receptor on the cell surface of MCF-EGFP cells by treatment with Pifithrin-alpha, a p53 specific transactivation inhibitor, partially abrogated the sensitivity of bystander MCF-EGFP cells. Our data, therefore, demonstrates that the Fas/FasL system could be considered as a new determinant for chemotherapy-induced bystander cell death in breast cancers.

Differential cell cycle-specificity for chromosomal damage induced by merbarone and etoposide in V79 cells

Merbarone, a topoisomerase II (topo II) inhibitor which, in contrast to etoposide, does not stabilize topo II-DNA cleavable complexes, was previously shown to be a potent clastogen in vitro and in vivo. To investigate the possible mechanisms, we compared the cell cycle-specificity of the clastogenic effects of merbarone and etoposide in V79 cells. Using flow cytometry and BrdU labeling techniques, etoposide was shown to cause a rapid and persistent G2 delay while merbarone was shown to cause a prolonged S-phase followed by a G2 delay. To identify the stages which are susceptible to DNA damage, we performed the micronucleus (MN) assay with synchronized cells or utilized a combination of BrdU pulse labeling and the cytokinesis-blocked MN assay with non-synchronized cells. Treatment of M phase cells with either agent did not result in increased MN formation. Etoposide but not merbarone caused a significant increase in MN when cells were treated during G2 phase. When treated during S-phase, both chemicals induced highly significant increases in MN. However, the relative proportion of MN induced by merbarone was substantially higher than that induced by etoposide. Both chemicals also caused significant increases in MN in cells that were treated during G1 phase. To confirm the observations in the MN assay, first division metaphases were evaluated in the chromosome aberration assay. The chromosomes of cells treated with merbarone and etoposide showed increased frequencies of both chromatid- and chromosome-type of aberrations. Our findings indicate that while etoposide causes DNA damage more evenly throughout the G1, S and G2 phases of the cell cycle, an outcome which may be closely associated with topo II-mediated DNA strand cleavage, merbarone induces DNA breakage primarily during S-phase, an effect which is likely due to the stalling of replication forks by inhibition of topo II activity.

Topoisomerase II trapping agent teniposide induces apoptosis and G2/M or S phase arrest of oral squamous cell carcinoma

BACKGROUND

Teniposide (VM-26) has been widely used in the treatment of small cell lung cancer, malignant lymphoma, breast cancer, etc. However, there are few reports on VM-26 against oral cancers. The present study was designed to identify the effect of VM-26 against oral squamous cell carcinoma in vitro, and to provide evidence for the feasibility and effectiveness of VM-26 for application to the patients with oral cancer.

METHODS

Human tongue squamous cell carcinoma cell line, Tca8113, was used. Cells were incubated with different concentrations of VM-26 for a variety of time span. Cisplatin (CDDP) was employed as a control reagent. MTT assay was used to assess the inhibitory rate of Tca8113 growth. Flow cytometer (FCM), transmission electronic microscope (TEM) and fluorescence staining were employed for determining the cell apoptotic rate. Cell cycle distribution of Tca8113 incubated with VM-26 was examined by flow cytometer assay. Statistic software (SAS 6.12, USA) was used for one-way ANOVA.

RESULTS

The IC50 of VM-26 against Tca8113 cells was 0.35 mg/l and that of CDDP was 1.1 mg/l. The morphological changes of Tca8113 cells were observed with fluorescence microscope and TEM. Apoptotic morphological feature could be found in the nucleus. Apoptotic rate of Tca8113 cells incubated with 5.0 mg/l of VM-26 for 72 hours was 81.67% and cells were arrested at S phase. However, when exposed to 0.15 mg/l of VM-26 for 72 hours, G2/M phase increased from 12.75% to 98.71%, while the apoptotic rate was 17.38%, which was lower than that exposed to 5.0 mg/l of VM-26.

CONCLUSION

VM-26 could significantly induce apoptosis of oral squamous cell carcinoma and inhibit cell growth. There may be another pathway to induce apoptosis of oral squamous cell carcinoma cells except for G2/M phase arrest.

BN80927

BN80927 belongs to a novel family of camptothecin analogs, the homocamptothecins, developed on the concept of topoisomerase I (Topo I) inhibition and characterized by a stable seven-membered beta-hydroxylactone ring. Preclinical data reported here show that BN80927 retains Topo I poisoning activity in cell-free assay (DNA relaxation) as well as in living cells, in which in vivo complexes of topoisomerase experiments and quantification of DNA-protein-complexes stabilization, have confirmed the higher potency of BN80927 as compared with the Topo I inhibitor SN38. In addition, BN80927 inhibits Topo II-mediated DNA relaxation in vitro but without cleavable-complex stabilization, thus indicating catalytic inhibition. Moreover, a Topo I-altered cell line (KBSTP2), resistant to SN38, remains sensitive to BN80927, suggesting that a part of the antiproliferative effects of BN80927 are mediated by a Topo I-independent pathway. This hypothesis is also supported by in vitro data showing an antiproliferative activity of BN80927 on a model of resistance related to the noncycling state of cells (G(0)-G(1) synchronized). In cell growth assays, BN80927 is a very potent antiproliferative agent as shown by IC(50) values consistently lower than those of SN38 in tumor cell lines as well as in their related drug-resistant lines. BN80927 shows high efficiency in vivo in tumor xenograft studies using human androgen-independent prostate tumors PC3 and DU145. Altogether, these data strongly support the clinical development of BN80927.

Death receptors in chemotherapy and cancer

Apoptosis, the cell's intrinsic death program, is a key regulator of tissue homeostasis. An imbalance between cell death and proliferation may result in tumor formation. Also, killing of cancer cells by cytotoxic therapies such as chemotherapy, gamma-irradiation or ligation of death receptors is predominantly mediated by triggering apoptosis in target cells. In addition to the intrinsic mitochondrial pathway, elements of death receptor signaling pathways have been implied to contribute to the efficacy of cancer therapy. Failure to undergo apoptosis in response to anticancer therapy may lead to resistance. Also, deregulated expression of death receptor pathway molecules may contribute to tumorigenesis and tumor escape from endogenous growth control. Understanding the molecular events that regulate apoptosis induced by anticancer therapy and how cancer cells evade apoptosis may provide new opportunities for pathway-based rational therapy and for drug development.

Function and regulation of the CD95 (APO-1/Fas) ligand in the immune system

CD95/CD95L mediated apoptosis is an important mechanism of immune homeostasis. It is instrumental for termination of an immune response and mainly be involved in peripheral tolerance. Dysregulation of the CD95/CD95L system leads to severe diseases. In this review, we present a survey of the role of the CD95/CD95L system in the immune system and, particularly, focus on the signals and transcription factors (NF-AT, Egr, NF-kappaB, AP-1, c-Myc, Nur77, IRFs, SP-1, ALG-4, ROR(gamma)t, and CIITA) involved in CD95L expression. It should also be evident from this review that a profound insight into the molecular mechanisms of CD95L activation should allow to explore potential therapeutic means to treat CD95/CD95L-dependent diseases.

Synthesis and antiproliferative activity of basic thioanalogues of merbarone

Three series of 5-substituted 1,3-diphenyl-6-(omega-dialkyl- and omega-cyclo-aminoalkyl)thio-2-thiobarbiturates (11-13) were synthesized as polysubstituted thioanalogues of merbarone, a topoisomerase II inhibitor acting on the catalytic site. To better understand pharmacophore requirements, a forth series of conformationally constrained analogues 14 was also prepared. Derivatives 11b,e, 14b,e,h,i,j were active in the low micromolar concentration range (IC(50): 3.3-4.3 microM), whereas compounds 11a,c,d,f,h,j and 13a,b,d,g,j and 14a,d,f showed IC(50) values between 10 and 15.5 microM. In contrast, compounds 12a-c,g-j, 13e,f,h and 14k were inactive. Cytotoxicity data provided from N.C.I. on selected compounds provided evidence that 11b,d, 13d,g and 14b,d,f,h,i,j were endowed with potent antiproliferative activity against leukemia and prostate cell lines (GI(50) up to 0.01 microM). In general, bicyclic derivatives 14 were up to 10-fold more potent than monocyclic counterparts against solid tumor-derived cell lines. SAR studies indicated that, in general, a certain tolerability in length of the alkyl side chains and in shape of distal amines is allowed in the four series, but in the monocyclic derivatives (11-13) antiproliferative activity was strongly affected by the nature of the 5-substituents (COOC(2)H(5)>COCH(3)>>C(6)H(5)). Compounds 11b and 14b were also evaluated against KB cell subclones expressing altered levels of topoisomerases or the multidrug resistance phenotype (MDR). In both cases the above compounds showed a decrease in potency. In enzyme assays, 11b and 14b turned out to be inhibitors of topoisonerase II as merbaron.

Resistance to mitochondrial- and Fas-mediated apoptosis in human leukemic cells with acquired resistance to 9-beta-D-arabinofuranosylguanosine

We have previously reported that in a MOLT-4 leukemia cell line the acquired resistance to 9-beta-D-arabinofuranosylguanine (Ara-G) is due to deficiency of the activating enzymes deoxyguanosine kinase and deoxycytidine kinase [Biochem. Biophys. Res. Commun. 293 (5) (2002) 1489]. In this study we investigated whether apoptotic pathways are affected in two human T-cell lymphoblastic MOLT-4 cell lines with acquired resistance to Ara-G. In contrast to the MOLT-4 wild type cells, Ara-G resistant cells displayed no increase in caspase-3 or caspase-9 activity, DNA fragmentation, cytochrome c release or a drop in the mitochondrial membrane potential (DeltaPsi(mito)) upon Ara-G treatment. A drop in the DeltaPsi(mito) was induced in wild type cells after treatment with tributyltin, an inducer of mitochondrial permeability transition, and with carbonyl cyanide m-chlorophenylhydrazone, an uncoupling agent that reduces the DeltaPsi(mito), although not in Ara-G resistant cells. Ara-G resistant cells displayed higher levels of the anti-apoptotic protein Bcl-xL in immunoblots. A recent study indicates that Ara-G-induced apoptosis is mediated in part via the Fas pathway [Cancer Res. 43 (2047) (2002) 411]. When cells were treated with anti-Fas antibody, the wild type cell line exhibited increased caspase-3-like activity but the Ara-G resistant cells did not. Using FACS analysis and semi-quantitative PCR, 3-6-fold decreased protein levels and almost no detectable mRNA levels of Fas in the resistant cells were recorded. These data indicate that the inability to induce apoptosis via both the apoptosome pathway and the Fas pathway, due to increased levels of Bcl-xL and a lack of Fas, contributes to Ara-G resistance. This resistance to apoptosis in Ara-G resistant cells may serve to explain the overall resistance to a variety of anti-neoplastic drugs.

pRb2/p130 promotes radiation-induced cell death in the glioblastoma cell line HJC12 by p73 upregulation and Bcl-2 downregulation

This study shows that in the glioblastoma hamster cell line HJC12 the retinoblastoma family member pRb2/p130 enhances gamma-radiation-induced cell death. In HJC12 cells the tetracycline-regulated expression of pRb2/p130 increased the percentage of gamma-radiation-induced apoptotic cells from 27 to 47%. pRb2/p130 overexpression was associated with the downregulation of the anti-apoptotic factor Bcl-2 and the upregulation of the steady-state protein levels of the pro-apoptotic transcription factor p73. In particular, RT-PCR showed a significant increase in the expression of the p73delta isoform when pRb2/p130 was overexpressed. The ability of pRb2/p130 to modulate apoptosis was not associated with its role in mediating G0/G1 arrest during cell cycle progression. Our data suggest a role for pRb2/p130 in glioblastoma gamma-radiation-induced cell death, indicating that the antitumoral action of pRb2/p130 can regulate both inhibition of cell cycle progression and induction of cell death.

7,12-Dimethylbenz[a]anthracene induces apoptosis in murine pre-B cells through a caspase-8-dependent pathway

Polycyclic aromatic hydrocarbons (PAHs) have been demonstrated to cause a variety of tumors and immunosuppressive effects. Our laboratory, and others, have demonstrated that coculture of progenitor B lymphocytes (pre-B cells) with bone marrow stromal cells and the model PAH 7,12-dimethylbenz[a]anthracene (DMBA) results in pre-B cell apoptosis. In this study we investigated the molecular events that precede apoptosis in DMBA-treated 70Z/3 cells, a pre-B cell line. Using caspase activity assays and immunoblotting techniques, we determined the temporal pattern of caspase expression in the pre-B cells. Using caspase inhibitors, we demonstrated that DMBA-mediated pre-B cell apoptosis is dependent on activation of caspase-8, whereas caspase-9 activation is essential for maximal apoptosis. We also demonstrated that DMBA activated PKR, an interferon-inducible protein kinase, in pre-B cells. PKR in turn can activate caspase-8 independently of death receptor ligation. As a result of these studies, we propose a novel PKR-dependent pathway for activation of apoptosis in DMBA-treated pre-B cells.

Nurr1 affects pRL-TK but not phRG-B internal control plasmid in genetic reporter system

In transcription assays, Renilla luciferase-expressing plasmids (more specifically pRL-TK) are commonly used as an internal control of transfection efficiency. Normalization of the experimental reporter gene transcription to the internal control reporter gene transcription minimizes variability of obtained results caused by differences in transfection efficiency between different samples of transfected cells. It is obvious that co-transfection with other plasmids or applied treatments should not affect the activity of the control reporter. Here we report that expression of the control Renilla luciferase encoded by pRL-TK plasmid was enhanced by co-transfection with vectors expressing orphan nuclear receptors Nur77 family (Nur77, Nurr1, Nor-1), leading to misinterpretation of the assay results. Further, we show that for Nurr1, phRG-B (a promoterless reporter plasmid containing synthetic Renilla luciferase gene) is a better control reporter vector than HSV-TK containing vectors. Finally, we noted the lack of effect of Nurr1 protein on the Fas Ligand promoter-driven transcription.

Catalytic inhibitors of topoisomerase II are DNA-damaging agents: induction of chromosomal damage by merbarone and ICRF-187

Merbarone is a catalytic inhibitor of topoisomerase II (topo II) that has been proposed to act primarily by blocking topo II-mediated DNA cleavage without stabilizing DNA-topo II-cleavable complexes. In this study merbarone was used as a model compound to investigate the genotoxic effects of catalytic inhibitors of topo II. The clastogenic properties of merbarone were evaluated using in vitro and in vivo micronucleus (MN) assays combined with CREST staining. For the in vitro MN assay, ICRF-187, a different type of catalytic inhibitor, and etoposide, a topo II poison, were used for comparison. Treatment of TK6 cells with all three of these drugs resulted in highly significant dose-related increases in kinetochore-lacking MN and, to a lesser extent, kinetochore-containing MN. In addition, a good correlation between p53 accumulation and MN formation was seen in the drug-treated cells. A mouse MN assay was performed to confirm that similar DNA-damaging effects would occur in vivo. Bone marrow smears from merbarone-treated B6C3F1 mice showed a dose-related increase in micronucleated polychromatic erythrocytes with a mean of 26 MN per 1000 cells being seen at the 60 mg/kg dose. Almost all MN lacked a kinetochore signal, indicating that merbarone was predominantly clastogenic under these conditions in vivo. The present study clearly shows that merbarone is genotoxic both in vitro and in vivo, and demonstrates the inaccuracy of earlier statements that merbarone and other catalytic inhibitors block the enzymatic activity of topo II without damaging DNA.

Cellular stress response and apoptosis in cancer therapy

Anticancer treatment using cytotoxic drugs is considered to mediate cell death by activating key elements of the apoptosis program and the cellular stress response. While proteolytic enzymes (caspases) serve as main effectors of apoptosis, the mechanisms involved in activation of the caspase system are less clear. Two distinct pathways upstream of the caspase cascade have been identified. Death receptors, eg, CD95 (APO-1/Fas), trigger caspase-8, and mitochondria release apoptogenic factors (cytochrome c, Apaf-1, AIF), leading to the activation of caspase-9. The stressed endoplasmic reticulum (ER) contributes to apoptosis by the unfolded protein response pathway, which induces ER chaperones, and by the ER overload response pathway, which produces cytokines via nuclear factor-kappaB. Multiple other stress-inducible molecules, such as p53, JNK, AP-1, NF-kappaB, PKC/MAPK/ERK, and members of the sphingomyelin pathway have a profound influence on apoptosis. Understanding the complex interaction between different cellular programs provides insights into sensitivity or resistance of tumor cells and identifies molecular targets for rational therapeutic intervention strategies.

The role of Fas and FasL as mediators of anticancer chemotherapy

Fas Ligand (FasL) is a member of the TNF superfamily that induces apoptosis in susceptible cells upon cross-linking of its own receptor, Fas (Apo-1/CD95). FasL-induced apoptosis contributes to immune homeostasis and cell-mediated cytotoxicity. Several groups have suggested that it also participates in the mechanism of action of DNA-damaging anticancer drugs. However, others have disputed this hypothesis, based largely on the inability of exogenously added anti-Fas/FasL reagents to attenuate drug-induced apoptosis in their studies. In this minireview, we discuss the most recent evidence for and against the involvement of FasL/Fas in the sensitivity and resistance to chemotherapy in a variety of models. In our own model of Ewing's sarcoma (ES), we have extensively investigated the involvement of the FasL/Fas pathway in doxorubicin (Dox)-induced apoptosis. We have generated clones of the Fas-sensitive, Dox-sensitive ES cell line SK-N-MC that were either Fas-resistant or FasL-deficient, and found that they were significantly resistant to Dox. Cleavage of FasL by MMP-7 (matrilysin) protected the parental SK-N-MC cells from Dox, whereas inhibition of MMP-7 activity increased their sensitivity. Transfection of a construct encoding soluble (decoy) Fas protected SK-N-MC cells from Dox. However, incubation with anti-Fas or anti-FasL neutralizing antibodies or exogenous addition of pre-synthesized recombinant soluble Fas decoy protein had no protective effect. This raises the possibility that the proposed Fas/FasL suicidal interaction may take place in an intracellular compartment and thus is not accessible to exogenously added reagents. Therefore, commercially available Fas/FasL neutralizing reagents may not be a reliable indicator of the involvement of the Fas pathway in anticancer-drug-induced apoptosis and experiments using these agents should be carefully re-evaluated. The combined use of MMP inhibitors with conventional, cytotoxic chemotherapy may hold therapeutic benefit.

Tumor cell death induced by topoisomerase-targeting drugs

DNA topoisomerases are double-edged swords. They are essential for many vital functions of DNA during normal cell growth. However, they are also highly vulnerable under various physiological and nonphysiological stresses because of their delicate act on breaking and rejoining DNA. These stresses (e.g. exposure to topoisomerase poisons, acidic pH, and oxidative stresses) can convert DNA topoisomerases into DNA-breaking nucleases, resulting in cell death and/or genomic instability. The importance of topoisomerase-mediated DNA cleavage in tumor cell death and carcinogenesis has been recognized. This review focuses on recent findings concerning the molecular mechanisms of the stress responses to topoisomerase-mediated DNA damage. The involvement of ubiquitin/26S proteasome and SUMO/UBC9 in these processes, as well as the role of topoisomerase cleavable complexes in apoptotic cell death are discussed.

Western and Chinese antirheumatic drug-induced T cell apoptotic DNA damage uses different caspase cascades and is independent of Fas/Fas ligand interaction

Spontaneous or therapeutic induction of T cell apoptosis plays a critical role in establishing transplantation tolerance and maintaining remission of autoimmune diseases. We investigated the mechanisms of apoptosis induced by Chinese and Western antirheumatic drugs (ARDs) in human T cells. We found that hydroxychloroquine, Tripterygium wilfordii hook F, and tetrandrine (Tet), but not methotrexate, at therapeutic concentrations can cause T cell death. In addition, Tet selectively killed T cells, especially activated T cells. Although ARD-induced cytotoxicity was mediated through apoptotic mechanisms, Fas/Fas ligand interaction was not required. We further demonstrated that the processes of phosphatidylserine externalization and DNA damage along the ARD-induced T cell apoptotic pathway could operate independently, and that selective inhibition of DNA damage by caspase inhibitors did not prevent T cells from undergoing cell death. Moreover, we found that Tet- and Tripterygium wilfordii hook F-induced T cell DNA damage required caspase-3 activity, and hydroxychloroquine-induced T cell DNA damage was mediated through a caspase-3- and caspase-8-independent, but Z-Asp-Glu-Val-Asp-fluomethyl ketone-sensitive, signaling pathway. Finally, the observation that ARD-induced activation of caspase-3 in both Fas-sensitive and Fas-resistant Jurkat T cells indicates that Fas/Fas ligand interaction plays no role in ARD-induced T cell apoptosis. Our observations provide new information about the complex apoptotic mechanisms of ARDs, and have implications for combining Western and Chinese ARDs that have different immunomodulatory mechanisms in the therapy of autoimmune diseases and transplantation rejection.

Pharmacological basis for cladribine resistance in a human acute T lymphoblastic leukaemia cell line selected for resistance to etoposide

Cross-resistance between different classes of anti-neoplastic agents can jeopardize successful combination cancer chemotherapy. In this study, we observed an unexpected cross-resistance between the podophyllotoxine derivative etoposide (VP) and the nucleoside analogue cladribine (CdA) in CCRF-CEM cells developed for resistance to VP. The resistant cells also displayed 14- and twofold resistance to cytarabine (ara-C) and gemcitabine respectively. Closer analysis of these cells showed that they contained lower amounts of topoisomerase (topo) IIalpha (P < 0.001) and beta protein (P < 0.026), formed substantially lower amounts of the topo II-DNA complex, and had a markedly decreased level of Fas (CD95/APO-1)-ligand mRNA expression. Interestingly, Fas expression in the resistant cells did not differ from that in the parental cell line. No differences were observed in the accumulation/efflux of daunorubicin or in the gene expressions of P-glycoprotein, multidrug resistance-associated protein and the lung resistance-related protein. The activity of deoxycytidine kinase (dCK), responsible for activation of CdA and ara-C, was the same for resistant and wild-type cells. However, there was an increase in the activity of the cytosolic 5'-nucleotidases (5'-NT), responsible for deactivation of nucleotides, amounting to 206% (P < 0.001) for the high Km and 134% (P < 0.331) for the low Km 5'-NT in resistant cells. The high Km 5'-NT is probably responsible for the decreased amount of the active metabolite CdA 5'-triphosphate [40% decreased (P < 0.045)], as well as for other purine ribonucleosides and deoxyribonucleosides triphosphates in the resistant cells. In contrast, a significantly higher deoxycytidine triphosphate (dCTP) level (167%, P < 0.001) was observed in the resistant cells. Thus, this study suggests that the major cause of resistance to the nucleoside analogues CdA and ara-C in cells selected for resistance to VP is a result of metabolic alterations producing increased activity of 5'-NT and higher dCTP levels. Furthermore, these results indicate that there is a common factor in the regulation of nucleotide-degrading enzymes and DNA topoisomerases, which may be altered in cross-resistant cells.

Characterization of the biological and biochemical activities of F 11782 and the bisdioxopiperazines, ICRF-187 and ICRF-193, two types of topoisomerase II catalytic inhibitors with distinctive mechanisms of action

F 11782 is a newly identified catalytic inhibitor of topoisomerases I and II, without any detectable interaction with DNA. This study aimed to establish whether its catalytic inhibition of topoisomerase II was mediated by mechanisms similar to those identified for the bisdioxopiperazines. In vitro combinations of F 11782 with etoposide resulted in greater than additive cytotoxicity in L1210 cells, contrasting with marked antagonism for combinations of etoposide with either ICRF-187 or ICRF-193. All three compounds caused a G2/M blockade of P388 cells after an 18-h incubation, but by 40 h polyploidization was evident only with the bisdioxopiperazines. Gel retardation data revealed that only F 11782, and not the bisdioxopiperazines, was capable of completely inhibiting the DNA-binding activity of topoisomerase II, confirming its novel mechanism of action. Furthermore, unlike ICRF-187 and ICRF-193, the cytotoxicity of F 11782 appeared mediated, at least partially, by DNA damage induction in cultured GCT27 human teratoma cells, as judged by a fluorescence-enhancement assay and monitoring p53 activation. Finally, the major in vivo antitumor activity of F 11782 against the murine P388 leukemia (i.v. implanted) and the B16 melanoma (s.c. grafted) contrasted with the bisdioxopiperazines' general lack of activity. Overall, F 11782 and the bisdioxopiperazines appear to function as quite distinctive catalytic topoisomerase II inhibitors.

Acute myeloblastic leukemic cells acquire cellular cytotoxicity under genotoxic stress: implication of granzyme B and perforin

Granzyme B (GrB) and perforin (PFN) are the major components of cytoplasmic granules contained in immune cellular effectors. The granule secretory pathway is one of the mechanisms by which these cells exert their cellular cytotoxicity. Recently, it has been reported that GrB and PFN are also present in circulating hemopoietic CD34+ progenitor cells mobilized by chemotherapy and granulocyte-colony stimulating factor, whereas these proteins are undetected in steady-state peripheral CD34+ cells. In this study, we hypothesized that anticancer agents may increase GrB and PFN expression in immature myeloid leukemic cells and that these treated leukemic cells become cellular effectors through a granule-dependent mechanism. Our results show that KG1a, HEL, and TF-1 CD34+acute myeloblastic leukemia cells expressed both GrB and PFN. Moreover, ionizing radiation, aracytine, and etoposide not only increase GrB and PFN expression but also conferred potent cellular cytotoxicity to these cells toward various cellular targets. Cellular cytotoxicity required cell-cell contact, was not influenced by anti-tumor necrosis factor α or anti-Fas blocking antibodies, and was abrogated by GrB inhibitors or antisense. These results suggest that, when exposed to genotoxic agents, immature leukemic cells acquire potent GrB- and PFN-dependent cellular cytotoxicity that can be potentially directed against normal residual myeloid progenitors or immune effectors.

Acute myeloblastic leukemic cells acquire cellular cytotoxicity under genotoxic stress: implication of granzyme B and perforin

Granzyme B (GrB) and perforin (PFN) are the major components of cytoplasmic granules contained in immune cellular effectors. The granule secretory pathway is one of the mechanisms by which these cells exert their cellular cytotoxicity. Recently, it has been reported that GrB and PFN are also present in circulating hemopoietic CD34+ progenitor cells mobilized by chemotherapy and granulocyte-colony stimulating factor, whereas these proteins are undetected in steady-state peripheral CD34+ cells. In this study, we hypothesized that anticancer agents may increase GrB and PFN expression in immature myeloid leukemic cells and that these treated leukemic cells become cellular effectors through a granule-dependent mechanism. Our results show that KG1a, HEL, and TF-1 CD34+acute myeloblastic leukemia cells expressed both GrB and PFN. Moreover, ionizing radiation, aracytine, and etoposide not only increase GrB and PFN expression but also conferred potent cellular cytotoxicity to these cells toward various cellular targets. Cellular cytotoxicity required cell-cell contact, was not influenced by anti-tumor necrosis factor α or anti-Fas blocking antibodies, and was abrogated by GrB inhibitors or antisense. These results suggest that, when exposed to genotoxic agents, immature leukemic cells acquire potent GrB- and PFN-dependent cellular cytotoxicity that can be potentially directed against normal residual myeloid progenitors or immune effectors.

Topoisomerase II is required for mitoxantrone to signal nuclear factor kappa B activation in HL60 cells

Topoisomerase II is a target for a number of chemotherapeutic agents used in the treatment of cancer. Its essential physiological role in modifying the topology of DNA involves the generation of transient double-strand breaks. Anti-cancer drugs, such as mitoxantrone, that target this enzyme interrupt its catalytic cycle and give rise to persistent double strand breaks, which may be lethal to a cell. We investigated the role of such lesions in signaling the activation of the transcription factor nuclear factor kappaB (NFkappaB) by this drug. Mitoxantrone activated NFkappaB and stimulated IkappaBalpha degradation in the promyelocytic leukemia cell line HL60 but not in the variant cells, HL60/MX2 cells, which lack the beta isoform of topoisomerase II and express a truncated alpha isoform that results in an altered subcellular distribution. Treatment of sensitive HL60 cells with mitoxantrone led to a depletion of both isoforms, suggesting the stabilization of transient DNA-topoisomerase II complexes. This depletion was absent in the variant cells, HL60/MX2. Activation of caspase 3 by mitoxantrone was also impaired in the HL60/MX2 cells. NFkappaB activation in response to tumor necrosis factor and bleomycin, the latter causing topoisomerase II-independent DNA damage, was intact in both cell lines. An inhibitor rather than a poison of topoisomerase II, Imperial Cancer Research Fund 187 (ICRF 187) the mechanism of which does not involve the generation of double strand breaks, did not activate NFkappaB, nor did it induce apoptosis in parental HL60 cells. However, ICRF 187 protected against IkappaB degradation in parental HL60 cells in response to mitoxantrone. This protection was also shown with another topoisomerase II inhibitor, merbarone, which is structurally and functionally distinct from ICRF 187. Their effects were specific, as neither protected against tumor necrosis factor-stimulated IkappaB degradation. The poisoning of topoiso- merase II with resultant DNA damage is therefore a critical signal for NFkappaB activation.

Death and decoy receptors and p53-mediated apoptosis

Recently, several tumor necrosis factor receptor 1 (TNF-R1) and Fas-related death receptors have been discovered and include DR3, DR4, DR5 and DR6. These receptors contain an extracellular region containing varying numbers of cysteine-rich domains and an intracellular region that contains the death domain. The death receptors are activated in a ligand-dependent or independent manner and transduce apoptotic signals via their respective intracellular death domains. In addition to death receptors, several decoy molecules have also been identified and include DcR1/TRID, DcR2/TRUNDD, DcR3 and osteoprotegrin (OPG). The decoy molecules do not transduce apoptotic signals but rather compete with the death receptors for ligand binding and thereby inhibit ligand-induced apoptosis. Recent evidence suggests that p53 upregulates the expression of death receptors Fas and DR5, and thus, may mediate apoptosis in part via Fas and/or DR5. However, p53 also regulates the expression of TRAIL decoy receptors DcR1/TRID and DR2/TRUNDD. Although the significance of p53-dependent regulation of decoy receptors remains unclear, evidence suggests that DcR1/TRUNDD appears to inhibit 53-mediated apoptosis. It is, therefore, possible that p53 may blunt its DR5-dependent apoptotic effects by controlling the levels of decoy receptors.

Role of p53 family members in apoptosis

p53-mediated apoptosis involves multiple mechanisms. A number of p53-regulated apoptosis-related genes have been identified. Some of these genes encode proteins that are important in controlling the integrity of mitochondria while the others code for membrane death receptors. p53 may also induce apoptosis by interfering with the growth factor-mediated survival signals. Although the transactivation-deficient p53 can induce apoptosis, evidence suggests that both the transcription-dependent and independent functions are needed for full apoptotic activity. p73 and p63 are two other members of the p53 family that show homology to p53 in their respective transactivation, DNA-binding and oligomerization domains. Both p73 and p63 transactivate p53-regulated promoters and induce apoptosis. Evidence suggests that both p73 and p63 may mediate apoptosis via some of the same mechanisms that are utilized by p53. However, both p73 and p63 exhibit features that are different from those of p53. Hence, both p73 and p63 are predicted to mediate apoptosis via mechanisms that are completely distinct from those engaged by p53. J. Cell. Physiol. 182:171-181, 2000. Published 2000 Wiley-Liss, Inc.

2-Chloro-2'-deoxyadenosine induces apoptosis through the Fas/Fas ligand pathway in human leukemia cell line MOLT-4

The mechanism of apoptosis induced by 2-chloro-2'-deoxyadenosine (2CdA) in human leukemia cell line MOLT-4 was investigated. 2CdA induced increases of 3'-OH ends of genomic DNA, ladder-like DNA fragmentation and phosphatidylserine translocation to the outer membrane, which are apoptotic characteristics. These apoptotic phenomena induced by 2CdA were inhibited by cycloheximide (CHX; a protein synthesis inhibitor), deoxycytidine (dC; a substrate of deoxycytidine kinase), acetyl Ile-Glu-Thr-Asp aldehyde (Ac-IETD-CHO; a caspase-8 inhibitor) and acetyl Asp-Glu-Val-Asp aldehyde (Ac-DEVD-CHO; a caspase-3 inhibitor). The protein synthesis-dependent expression of Fas and Fas ligand (Fas-L) was detected by treatment with 2CdA. The proteolytic processing of procaspases-8 and -3 to produce active fragments, caspases-8 (p18) and -3 (p17), respectively, was observed after treatment with 2CdA, and suppressed by cycloheximide. Increases in the activities of caspases-8 and -3 were observed after 2CdA treatment. Their activation was also dependent on protein synthesis. These results indicated that 2CdA-induced apoptosis was triggered by phosphorylation of 2CdA followed by the protein synthesis-dependent expression of Fas and Fas-L and activation of caspases-8 and -3.

Ionizing radiation alters Fas antigen ligand at the cell surface and on exfoliated plasma membrane-derived vesicles: implications for apoptosis and intercellular signaling

Resident proteins that reside on the plasma membrane are continually exfoliated from the cell surface. Exfoliation is a selective, energy-dependent process that mediates intercellular communication. Ionizing radiation modulates the expression of many plasma membrane-bound growth regulators, including the "death" ligand, TNFSF6 (formerly known as FasL, CD95L). Here we report that ionizing radiation induces dose-dependent up-regulation of TNFSF6 on plasma membranes purified from SW620 cells, a TNFSF6-expressing colon cancer cell line. Serum-free medium conditioned by exposed and control cells was collected and exfoliated vesicles were obtained by ultracentrifugation. Western blot analysis of vesicles from unexposed cells and from cells treated with 10 Gy showed increased amounts of TNFSF6 compared to that on vesicles from unexposed cells. Cells treated with 4 Gy released vesicles having a low level of TNFSF6 on their surface relative to that on vesicles exfoliated from unexposed cells. When assayed for bioactivity, vesicles from unexposed cells induced the greatest level of apoptosis in TNFRSF6 (formerly known as FAS) receptor-bearing Jurkat cells (cell surviving fraction of 43.7 +/- 6.1; P < 0.05), followed by vesicles collected from cells treated with 4 Gy (79.6 +/- 2.6%; P < 0.05). Despite having a high level of TNFSF6 by Western analysis, vesicles collected from cells exposed to 10 Gy display minimal biological activity (77.9 +/- 3.2%; P < 0.05), suggesting that modification of the vesicle-associated ligand has occurred. Our results indicate that ionizing radiation increases the level of TNFSF6 exfoliated on extracellular vesicles. The data may provide a mechanism for abscopal and bystander effects after irradiation.

Increased expression of death receptors 4 and 5 synergizes the apoptosis response to combined treatment with etoposide and TRAIL

Chemotherapeutic genotoxins induce apoptosis in epithelial-cell-derived cancer cells. The death receptor ligand TRAIL also induces apoptosis in epithelial-cell-derived cancer cells but generally fails to induce apoptosis in nontransformed cells. We show here that the treatment of four different epithelial cell lines with the topoisomerase II inhibitor etoposide in combination with TRAIL (tumor necrosis factor [TNF]-related apoptosis-inducing ligand) induces a synergistic apoptotic response. The mechanism of the synergistic effect results from the etoposide-mediated increase in the expression of the death receptors 4 (DR4) and 5 (DR5). Inhibition of NF-kappaB activation by expression of kinase-inactive MEK kinase 1(MEKK1) or dominant-negative IkappaB (DeltaIkappaB) blocked the increase in DR4 and DR5 expression following etoposide treatment. Addition of a soluble decoy DR4 fusion protein (DR4:Fc) to cell cultures reduced the amount of etoposide-induced apoptosis in a dose-dependent manner. The addition of a soluble TNF decoy receptor (TNFR:Fc) was without effect, demonstrating the specificity of DR4 binding ligands in the etoposide-induced apoptosis response. Thus, genotoxin treatment in combination with TRAIL is an effective inducer of epithelial-cell-derived tumor cell apoptosis relative to either treatment alone.

Tumor cell resistance to DNA topoisomerase II inhibitors: new developments

DNA topoisomerases are critical enzymes involved in replication, transcription, chromatin assembly and other aspects of DNA metabolism. They are also the targets of important anticancer drugs. The type II topoisomerases are specific targets of drug classes that comprise complex-stabilizing (epipodophyllotoxins, anthracyclines) and catalytic (merbarone, bisdioxopiperazines) inhibitors. In this review, we update our current knowledge of resistance to the antitumor inhibitors of the type II DNA topoisomerases, with special emphasis on the catalytic inhibitors, since novel catalytic inhibitor resistant cell lines have only recently been described. Resistance to topoisomerase II inhibitors can manifest as decreased or increased expression of or mutation in the topoisomerase II genes. However, the tumor cell's response to exposure to these inhibitors involves more than the target enzyme, and these other responses are a major focus of this review. Such cellular changes are associated with and may contribute to the drug resistance phenotype. They involve decreased drug accumulation due to expression of membrane 'pump' proteins, altered cytotoxic signaling through stress-activated protein kinases, and alterations in apoptosis and cell cycle proteins (e.g. Bcl-2, Bax, p53, Rb). While it is evident that mutation in or altered expression of the topoisomerase II genes are sufficient to confer resistance to topoisomerase inhibitors, it is not clear whether the other changes are a consequence of the selection or a response to the cytotoxic insult, nor is it clear how these other cellular changes contribute to the drug resistance phenotype. Copyright 1999 Harcourt Publishers Ltd.

NF-kappa B-dependent Fas ligand expression

Apoptosis of lymphocytes is triggered by different stimuli through the induced expression of Fas and Fas ligand (FasL). Using T cell activation-induced Fas/FasL expression as a model system, we observed a differential regulation of the induction of Fas and FasL. cAMP inhibited activation-induced apoptosis by an effective suppression of TCR-coupled FasL expres sion. In contrast, cAMP weakly interfered with activation-induced Fas expression, and the remaining Fas molecules on cAMP-treated T cells still mediated apoptosis. Among the major transcription elements on the FasL promoter, the activation of NF-kappaB, but not of NF-AT and AP-1, was suppressed by cAMP. The prominent role of NF-kappaB was further demonstrated by a better activation of the FasL promoter and an elevated expression of FasL induced by p65 (RelA) overexpression than those induced by AP-1 or NF-AT. Our results demonstrate the essential role of NF-kappaB for the expression of the death receptor ligand FasL, and suggest a direct link between NF-kappaB activation and the expression of FasL. NF-kappaB may be the common mediator in the induction of FasL through TCR activation and by various stress stimuli.