Activation of caspases and induction of apoptosis by novel ribonucleotide reductase inhibitors amidox and didox

The Antitumor Didox Acts as an Iron Chelator in Hepatocellular Carcinoma Cells

Ribonucleotide reductase (RR) is the rate-limiting enzyme that controls the deoxynucleotide triphosphate synthesis and it is an important target of cancer treatment, since it is expressed in tumor cells in proportion to their proliferation rate, their invasiveness and poor prognosis. Didox, a derivative of hydroxyurea (HU), is one of the most potent pharmaceutical inhibitors of this enzyme, with low in vivo side effects. It inhibits the activity of the subunit RRM2 and deoxyribonucleotides (dNTPs) synthesis, and it seems to show iron-chelating activity. In the present work, we mainly investigated the iron-chelating properties of didox using the HA22T/VGH cell line, as a model of hepatocellular carcinoma (HCC). We confirmed that didox induced cell death and that this effect was suppressed by iron supplementation. Interestingly, cell treatments with didox caused changes of cellular iron content, TfR1 and ferritin levels comparable to those caused by the iron chelators, deferoxamine (DFO) and deferiprone (DFP). Chemical studies showed that didox has an affinity binding to Fe³⁺ comparable to that of DFO and DFP, although with slower kinetic. Structural modeling indicated that didox is a bidentated iron chelator with two theoretical possible positions for the binding and among them that with the two hydroxyls of the catechol group acting as ligands is the more likely one. The iron chelating property of didox may contribute to its antitumor activity not only blocking the formation of the tyrosil radical on Tyr122 (such as HU) on RRM2 (essential for its activity) but also sequestering the iron needed by this enzyme and to the cell proliferation.

Isolation of eudesmanes from Pluchea odorata and evaluation of their effects on cancer cell growth and tumor invasiveness in vitro

The traditionally used Central American medicinal plant Pluchea odorata, known as an anti-inflammatory and cancer cell growth-inhibiting remedy, was subjected to bioassay-guided isolation. Structure elucidation by 1D- and 2D-NMR and MS techniques supported by ECD and UV spectroscopic data revealed seven structurally previously undescribed and eight known eudesmane-type sesquiterpenes. Furthermore, one previously undescribed and one known phytol-like alcohol were identified. All compounds were tested for their cytotoxicity in cancer cells and for their anti-invasive effects. Among the eudesmanes, 3α-(2',3'-epoxy-2'-methylbutyryloxy)-4α-hydroxy-11-hydroperoxy-eudesm-6-en-8-one exhibited the most potent cytotoxic activity with an IC₅₀ value of 8.8 μM (after 48 h). Also in an in vitro model measuring the tumor-triggered breaching of the adjacent lymph endothelial cell barrier (3S*,4R*,5S*,10S*,2'R*,3'R*)-3-(2',3'-epoxy-2'-methylbutyryloxy)-4,7-dihydroxy-eudesm-11-en-8-one (IC₇₅ = 47 μM) and (3S*,4R*,5R*,10S*,2'R*,3'R*)-3-(2',3'-epoxy-2'-methylbutyryloxy)-4-acetyloxy-6-methoxy-11-hydroxy-eudesm-6-en-8-one (IC₇₅ = 73 μM) showed inhibitory activities. Furthermore, preliminary structure-activity relationships (SARs) of the eudesmanes were developed.

Didox and resveratrol sensitize colorectal cancer cells to doxorubicin via activating apoptosis and ameliorating P-glycoprotein activity

Doxorubicin (DOX) has limited efficacy in colorectal cancer due to multi-drug resistance. Resveratrol (RES) and didox (DID) are polyhydroxyphenols with potential chemosensitizing effects. Herein, we assessed the chemomodulatory effects of RES and DID to DOX in colorectal cancer cells. Equitoxic combination of DOX with RES and DID in HCT 116 reduced the IC₅₀ of DOX from 0.96 ± 0.02 μM to 0.52 ± 0.05 μM and 0.4 ± 0.06 μM, respectively. Similarly, combination of DOX with RES and DID in HT-29 decreased the IC₅₀’s of DOX from 0.88 ± 0.03 μM to 0.47 ± 0.02 μM and 0.29 ± 0.04 μM, respectively. The expressions of p53 and Bax genes were markedly elevated in HCT 116 cells after exposure to DOX/DID. In HT-29 cells, the expression of Bcl-XL gene was significantly decreased after exposure to DOX/DID. In addition, combination of DOX with RES significantly increased the expression of Bax gene in HCT 116 cells. RES treatment induced significant S-phase arrest in DOX-treated HCT 116 cells, while DID induced G₂/M- and S-phase arrest in HCT 116 and HT-29, respectively. Both RES and DID significantly enhanced the intracellular entrapment of DOX due to blocking the efflux activity of p-glycoprotein pump. In conclusion, RES and DID sensitize colorectal cancer cells to DOX via facilitating apoptosis and enhancing intracellular entrapment of DOX.

Selective anticancer activity of the novel thiobenzanilide 63T against human lung adenocarcinoma cells

Previously, it has been reported that molecules built on the benzanilide and thiobenzanilide scaffold are the promising groups of compounds with several biological activities including antifungal, antimycotic, antibacterial, spasmolytic, and anticancer ones. In this study the mechanism of action of one selected thiobenzanilide derivative N,N'-(1,2-phenylene)bis3,4,5-trifluorobenzothioamide (63T) with strongest cytotoxic activity has been investigated for the first time in human lung adenocarcinoma (A549) and normal lung derived fibroblast (CCD39Lu) in a cell culture model. The results demonstrated, that 63T can be considered a selective anticancer compound. Based on these results, several experiments including the analysis of cellular morphology, cell phase distribution, cytoplasmic histone-associated DNA fragmentation, apoptosis, necrosis, and autophagy detection were performed to understand better the mechanism underlying the anticancer activity. The data showed that 63T is a small molecule compound, which selectively induces cancer cell death in a caspase independent pathway; moreover, the autophagic dose-dependent processes may be involved in the mechanism of cell death.

A eudesmane-type sesquiterpene isolated from Pluchea odorata (L.) Cass. combats three hallmarks of cancer cells: Unrestricted proliferation, escape from apoptosis and early metastatic outgrowth in vitro

Pluchea odorata is ethno pharmaceutically used to treat inflammation-associated disorders. The dichloromethane extract (DME) was tested in the carrageenan-induced rat paw oedema assay investigating its effect on inflammation that was inhibited by 37%. Also an in vitro anti-neoplastic potential was reported. However, rather limited information about the bio-activity of purified compounds and their cellular mechanisms are available. Therefore, two of the most abundant eudesmanes in P. odorata were isolated and their anti-neoplastic and anti-intravasative activities were studied. HL-60 cells were treated with P. odorata compounds and metabolic activity, cell number reduction, cell cycle progression and apoptosis induction were correlated with relevant protein expression. Tumour cell intravasation through lymph endothelial monolayers was measured and potential causal mechanisms were analyzed by Western blotting. Compound PO-1 decreased the metabolic activity of HL-60 cells (IC50 = 8.9 μM after 72 h) and 10 μM PO-1 induced apoptosis, while PO-2 showed just weak anti-neoplastic activities at concentrations beyond 100 μM. PO-1 arrested the cell cycle in G1 and this correlated with induction of JunB expression. Independent of this mechanism 25 μM PO-1 decreased MCF-7 spheroid intravasation through the lymph endothelial barrier. Hence, PO-1 inhibits an early step of metastasis, impairs unrestricted proliferation and induces apoptosis at low micromolar concentrations. These results warrant further testing in vivo to challenge the potential of PO-1 as novel lead compound.

Epigallocatechin gallate, ellagic acid, and rosmarinic acid perturb dNTP pools and inhibit de novo DNA synthesis and proliferation of human HL-60 promyelocytic leukemia cells: Synergism with arabinofuranosylcytosine

Epigallocatechin gallate (EGCG), ellagic acid (EA) and rosmarinic acid (RA) are natural polyphenols exerting cancer chemopreventive effects. Ribonucleotide reductase (RR; EC 1.17.4.1) converts ribonucleoside diphosphates into deoxyribonucleoside diphosphates being essential for DNA replication, which is why the enzyme is considered an excellent target for anticancer therapy. EGCG, EA, and RA dose-dependently inhibited the growth of human HL-60 promyelocytic leukemia cells, exerted strong free radical scavenging potential, and significantly imbalanced nuclear deoxyribonucleoside triphosphate (dNTP) concentrations without distinctly affecting the protein levels of RR subunits (R1, R2, p53R2). Incorporation of (14)C-cytidine into nascent DNA of tumor cells was also significantly lowered, being equivalent to an inhibition of DNA synthesis. Consequently, treatment with EGCG and RA attenuated cells in the G0/G1 phase of the cell cycle, finally resulting in a pronounced induction of apoptosis. Sequential combination of EA and RA with the first-line antileukemic agent arabinofuranosylcytosine (AraC) synergistically potentiated the antiproliferative effect of AraC, whereas EGCG plus AraC yielded additive effects. Taken together, we show for the first time that EGCG, EA, and RA perturbed dNTP levels and inhibited cell proliferation in human HL-60 promyelocytic leukemia cells, with EGCG and RA causing a pronounced induction of apoptosis. Due to these effects and synergism with AraC, these food ingredients deserve further preclinical and in vivo testing as inhibitors of leukemic cell proliferation.

The efficacy of the ribonucleotide reductase inhibitor Didox in preclinical models of AML

Acute Myeloid Leukemia (AML) is an aggressive malignancy which leads to marrow failure, and ultimately death. There is a desperate need for new therapeutics for these patients. Ribonucleotide reductase (RR) is the rate limiting enzyme in DNA synthesis. Didox (3,4-Dihydroxybenzohydroxamic acid) is a novel RR inhibitor noted to be more potent than hydroxyurea. In this report we detail the activity and toxicity of Didox in preclinical models of AML. RR was present in all AML cell lines and primary patient samples tested. Didox was active against all human and murine AML lines tested with IC₅₀ values in the low micromolar range (mean IC₅₀ 37 µM [range 25.89–52.70 µM]). It was active against primary patient samples at concentrations that did not affect normal hematopoietic stem cells (HSCs). Didox exposure resulted in DNA damage and p53 induction culminating in apoptosis. In syngeneic, therapy-resistant AML models, single agent Didox treatment resulted in a significant reduction in leukemia burden and a survival benefit. Didox was well tolerated, as marrow from treated animals was morphologically indistinguishable from controls. Didox exposure at levels that impaired leukemia growth did not inhibit normal HSC engraftment. In summary, Didox was well tolerated and effective against preclinical models of AML.

Digalloylresveratrol, a novel resveratrol analog inhibits the growth of human pancreatic cancer cells

Digalloylresveratrol (DIG) is a recently synthesized substance aimed to combine the effects of the natural polyphenolic compounds gallic acid and resveratrol, which both are excellent free radical scavengers with anticancer activity. In this study, we investigated the effects of DIG in the human AsPC-1 and BxPC-3 pancreatic adenocarcinoma cell lines. Treatment with DIG dose-dependently attenuated cells in the S phase of the cell cycle and led to a significant depletion of the dATP pool in AsPC-1 cells. The incorporation of (14)C-cytidine into nascent DNA of tumor cells was significantly inhibited at all DIG concentrations due to inhibition of ribonucleotide reductase, a key enzyme of DNA synthesis in tumor cells. Furthermore, Erk1/2 became inactivated and moderated p38 phosphorylation reflecting increased replication stress. DIG also activated ATM and Chk2, and induced the phosphorylation and proteasomal degradation of the proto-oncogene Cdc25A, which contributed to cell cycle attenuation. Taken together, DIG is an excellent free radical scavenger, strongly inhibits RR in situ activity, cell cycle progression, and colony formation in AsPC-1 and BxPC-3 cells thus warranting further investigations.

Mechanisms underlying reductant-induced reactive oxygen species formation by anticancer copper(II) compounds

Intracellular generation of reactive oxygen species (ROS) via thiol-mediated reduction of copper(II) to copper(I) has been assumed as the major mechanism underlying the anticancer activity of copper(II) complexes. The aim of this study was to compare the anticancer potential of copper(II) complexes of Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone; currently in phase II clinical trials) and its terminally dimethylated derivative with that of 2-formylpyridine thiosemicarbazone and that of 2,2'-bipyridyl-6-carbothioamide. Experiments on generation of oxidative stress and the influence of biologically relevant reductants (glutathione, ascorbic acid) on the anticancer activity of the copper complexes revealed that reductant-dependent redox cycling occurred mainly outside the cells, leading to generation and dismutation of superoxide radicals resulting in cytotoxic amounts of H(2)O(2). However, without extracellular reductants only weak intracellular ROS generation was observed at IC(50) levels, suggesting that cellular thiols are not involved in copper-complex-induced oxidative stress. Taken together, thiol-induced intracellular ROS generation might contribute to the anticancer activity of copper thiosemicarbazone complexes but is not the determining factor.

Dynamics of chemosensitivity and chromosomal instability in recurrent glioblastoma

Glioblastoma multiforme is characterised by invasive growth and frequent recurrence. Here, we have analysed chromosomal changes in comparison to tumour cell aggressiveness and chemosensitivity of three cell lines established from a primary tumour and consecutive recurrences (BTL1 to BTL3) of a long-term surviving glioblastoma patient together with paraffin-embedded materials of five further cases with recurrent disease. Following surgery, the BTL patient progressed under irradiation/ lomustine but responded to temozolomide after re-operation to temozolomide. The primary tumour -derived BTL1 cells showed chromosomal imbalances typical of highly aggressive glioblastomas. Interestingly, BTL2 cells established from the first recurrence developed under therapy showed signs of enhanced chromosomal instability. In contrast, BTL3 cells from the second recurrence resembled a less aggressive subclone of the primary tumour. Although BTL2 cells exhibited a highly aggressive phenotype, BTL3 cells were characterised by reduced proliferative and migratory potential. Despite persistent methylation of the O6-methylguanine-DNA methyltransferase promoter, BTL3 cells exhibited the highest temozolomide sensitivity. A comparable situation was found in two out of five glioblastoma patients, both characterised by enhanced survival time, who also relapsed after surgery/chemotherapy with less aggressive recurrences. Taken together, our data suggest that pretreated glioblastoma patients may relapse with highly chemosensitive tumours confirming the feasibility of temozolomide treatment even in case of repeated recurrence.

The synergistic interaction of gemcitabine and cytosine arabinoside with the ribonucleotide reductase inhibitor triapine is schedule dependent

Gemcitabine and ara-C have multiple mechanisms of action: DNA incorporation and for gemcitabine also ribonucleotide reductase (RNR) inhibition. Since dCTP competes with their incorporation into DNA, dCTP depletion can potentiate their cytotoxicity. We investigated whether additional RNR inhibition by Triapine (3-AP), a new potent RNR inhibitor, enhanced cytotoxicity of gemcitabine and ara-C in four non-small-cell-lung-cancer (NSCLC) cell lines, using the multiple-drug-effect analysis. Simultaneous and sequential exposure (preexposure to 3-AP for 24h) in a constant molar ratio of 3-AP and gemcitabine was antagonistic/additive in all cell lines. Preexposure to 3-AP at an IC(25) concentration for 24h before variable concentrations of gemcitabine was synergistic. RNR inhibition by 3-AP resulted in a more synergistic interaction in combination with ara-C, which does not inhibit RNR. Two cell lines with pronounced synergism (SW1573) or antagonism (H460) for gemcitabine/3-AP, were evaluated for accumulation of the active metabolites, dFdCTP and ara-CTP. Simultaneous exposure induced no or a small increase, but ara-CTP increased after pretreatment with 3-AP, 4-fold in SW1573 cells, but not in H460 (<1.5 fold). Ara-C and gemcitabine incorporation into DNA were more pronounced (about 2-fold increased) for sequential treatment in SW1573 compared to H460 cells (<1.5 fold). This was not related to the activity and expression of deoxycytidine kinase and the M2 subunit of RNR. In conclusion, RNR inhibition by 3-AP prior to gemcitabine or ara-C exposure stimulates accumulation of the active metabolites and incorporation into DNA. The combination 3-AP/Ara-C is more synergistic than 3-AP/gemcitabine possibly because gemcitabine already inhibits RNR, but ara-C does not.

Didox, a ribonucleotide reductase inhibitor, induces apoptosis and inhibits DNA repair in multiple myeloma cells

Ribonucleotide reductase (RR) is the enzyme that catalyses the rate-limiting step in DNA synthesis, the production of deoxynucleotides. RR activity is markedly elevated in tumour tissue and is crucial for cell division. It is therefore an excellent target for cancer chemotherapy. This study examined the anti-myeloma activity of Didox (3,4-Dihydroxybenzohydroxamic acid), a novel RR inhibitor (RRI). Our data showed that Didox induced caspase-dependent multiple myeloma (MM) cell apoptosis. Didox, unlike other RRIs that mainly target the pyrimidine metabolism pathway, targets both purine and pyrimidine metabolism pathways in MM, as demonstrated by transcriptional profiling using the Affymetrix U133A 2.0 gene chip. Specifically, a >or=2-fold downregulation of genes in these anabolic pathways was shown as early as 12 h after exposure to Didox. Furthermore, apoptosis was accompanied by downregulation of bcl family proteins including bcl-2, bcl(xl), and XIAP. Importantly, RR M1 component transcript was also downregulated, associated with decreased protein expression. Genes involved in DNA repair mechanisms, specifically RAD 51 homologue, were also downregulated. As Didox acts on MM cells by inhibiting DNA synthesis and repair, combination studies with melphalan, an agent commonly used in MM, were performed. A strong in vitro synergism was shown, with combination indices of <0.7 as determined by the Chou-Talalay method. These studies therefore provide the preclinical rationale for evaluation of Didox, alone and in combination with DNA-damaging agents, to improve patient outcome in MM.

Intracellular nucleotides act as critical prosurvival factors by binding to cytochrome C and inhibiting apoptosome

Cytochrome c (CC)-initiated Apaf-1 apoptosome formation represents a key initiating event in apoptosis. This process can be reconstituted in vitro with the addition of CC and ATP or dATP to cell lysates. How physiological levels of nucleotides, normally at high mM concentrations, affect apoptosome activation remains unclear. Here we show that physiological levels of nucleotides inhibit the CC-initiated apoptosome formation and caspase-9 activation by directly binding to CC on several key lysine residues and thus preventing CC interaction with Apaf-1. We show that in various apoptotic systems caspase activation is preceded or accompanied by decreases in overall intracellular NTP pools. Microinjection of nucleotides inhibits whereas experimentally reducing NTP pools enhances both CC and apoptotic stimuli-induced cell death. Our results thus suggest that the intracellular nucleotides represent critical prosurvival factors by functioning as natural inhibitors of apoptosome formation and a barrier that cells must overcome the nucleotide barrier to undergo apoptosis cell death.

Trimidox Induces Apoptosis via Cytochrome c Release in NALM-6 Human B Cell Leukaemia Cells

Trimidox (3,4,5-trihydroxybenzamidoxime) has been shown to reduce the activity of ribonucleotide reductase accompanied by growth inhibition and the differentiation of mammalian cells. Here we examine the induction of apoptosis by trimidox in several human leukaemia cell lines, focusing on the release of cytochrome c and the activation of caspase proteases in the human B cell line NALM-6. Induction of apoptosis by trimidox (300 microM) was detected in NALM-6, HL-60 (premyelocytic leukaemia cells), MOLT-4 (an acute lymphoblastic leukaemia cells), Jurkat (a T-cell leukaemia cells), U937 (expressing many monocyte-like characteristics), and K562 (erythroleukaemia). NALM-6 was most affected by trimidox among leukaemia cells; therefore, we employed NALM-6 cells in the subsequent experiments. The cells showed a time-dependent increase in DNA damage after trimidox (250 microM) treatment. A significant increase in the amount of cytochrome c release was detected after treatment with trimidox. Bcl-2 and Bax protein expressions were not changed by trimidox. Caspase-3 and -9 were activated by incubation with trimidox, whereas caspase-8 was not. Furthermore, trimidox-induced apoptosis was prevented by a broad-spectrum caspase inhibitor, a caspase-3, and a caspase-9 inhibitor, but not by a caspase-8 inhibitor. Inhibition of c-Jun NH2-terminal kinase (JNK) by SP600125 appreciably protected cells from trimidox-induced apoptosis, but no effect inhibition of p38 mitogen-activated protein kinase (MAPK) by SB203580. In contrast, extracellular signal-regulated kinase (ERK) inhibitors U0126 and PD98059 strongly potentiated the apoptotic effect of trimidox. This report shows that the induction of apoptosis by trimidox occurs through a cytochrome c-dependent pathway, which sequentially activates caspase-3 and caspase-9.

Ribonucleotide reductase inhibitors enhance cidofovir-induced apoptosis in EBV-positive nasopharyngeal carcinoma xenografts

In nasopharyngeal carcinoma (NPC), Epstein-Barr virus (EBV) infection is mainly latent, and the tumor cells contain episomal viral DNA. We have shown that the acyclic nucleoside phosphonate analog, cidofovir [(S)-1-(3-hydroxy-2-(phosphonylmethoxypropyl))cytosine] (HPMPC), inhibits growth of NPC xenografts in nude mice by causing apoptosis. The ribonucleotide reductase (RR) inhibitors, hydroxyurea and didox (3,4-dihydroxybenzohydroxamic acid), have been demonstrated to inhibit neoplastic growth and are used as antiviral and anticancer agents. Here we show that RR inhibitors enhance the antitumor effect of cidofovir in EBV-transformed epithelial cells. MTT assays indicate that hydroxyurea and didox enhance cidofovir-induced cell toxicity in NPC-KT cells, an EBV-positive epithelial cell line derived from NPC. The effect is due to enhancement of apoptosis through the caspase cascade as shown by pronounced cleavage of poly(ADP-ribose) polymerase. Finally, hydroxyurea strikingly enhanced the cidofovir-induced growth-inhibitory effect on NPC grown in athymic mice. The results suggest that RR inhibitors should enhance the antitumor effect of acyclic nucleoside phosphonate analogs on NPC.

Non-parenchymal liver cells support the growth advantage in the first stages of hepatocarcinogenesis

Hepatocellular carcinoma almost always arises in chronically inflamed livers. We developed a culture model to study the role of non-parenchymal cells (NPCs) for inflammation-driven hepatocarcinogenesis. Rats were treated with the carcinogen N-nitrosomorpholine, which induced initiated hepatocytes expressing the marker placental glutathione-S-transferase (GSTp). After 21 days two preparations of hepatocytes were made: (i) conventional ones (Hep-conv) containing NPCs and (ii) hepatocytes purified of NPCs (Hep-pur). Initiated hepatocytes, being positive for GSTp (GSTp-pos) were present in both preparations and were cultured along with normal hepatocytes, being negative for GSTp (GSTp-neg). Under any culture condition DNA synthesis was approximately 4-fold higher in GSTp-pos than in GSTp-neg hepatocytes demonstrating the inherent growth advantage of the first stages of hepatocarcinogenesis. Hepatocytes showed approximately 3-fold lower rates of DNA synthesis in Hep-pur than in Hep-conv, which was elevated above Hep-conv levels by addition of NPC or NPC-supernatant. Pretreatment of NPCs with proinflammatory lipopolysaccharide (LPS) further increased DNA synthesis. Thus, NPCs release soluble growth stimulators. Next we investigated the effect of specific cytokines produced by NPCs. Tumour necrosis factor alpha and interleukin 6 barely altered DNA synthesis, whereas hepatocyte growth factor (HGF), keratinocyte growth factor (KGF) and the heparin-binding epidermal growth factor-like growth factor (HB-EGF) were potent inducers of DNA replication in both, GSTp-neg and GSTp-pos cells. In conclusion, DNA synthesis of hepatocytes is increased by factors released from NPCs, an effect augmented by LPS-stimulation. NPC-derived cytokines, such as KGF, HGF and HB-EGF, stimulate DNA synthesis preferentially in initiated hepatocytes, presumably resulting in tumour promotion. Similar mechanisms may contribute to carcinogenesis in human inflammatory liver diseases.

Amidox, an inhibitor of ribonucleotide reductase, potentiates the action of Ara-C in HL-60 human promyelocytic leukemia cells

Amidox (3,4-dihydroxybenzamidoxime), a new polyhydroxy-substituted benzoic acid derivative, is a potent inhibitor of the enzyme ribonucleotide reductase (RR), which catalyses the de novo synthesis of DNA. RR is considered to be an excellent target for cancer chemotherapy. In the present study we investigated the antineoplastic effects of Amidox alone and in combination with Arabinofuranosylcytosine (Ara-C) in HL-60 human promyelocytic leukemia cells. In growth inhibition experiments Amidox yielded an IC50 of 30 microM, colony formation was inhibited at an IC50 of 20 microM as determined by a soft agar assay. Exposure of the cells to 75 and 100 microM Amidox for 24 hours was shown to significantly decrease intracellular dCTP, dGTP and dATP pools, whereas dTTP concentration increased, as determined by HPLC. The combination of Amidox with Ara-C yielded more than additive cytotoxic effects both in growth inhibition assays and in soft agar assays. We could show that--after preincubating the cells with 75 and 100 microM Amidox and subsequent exposure to Ara-C--intracellular Ara-CTP levels increased by 576% and 1143%, respectively. In conclusion, Amidox might offer an additional option for the treatment of leukemia and thus be further investigated in vitro and in vivo.

Overexpression and significance of prion protein in gastric cancer and multidrug-resistant gastric carcinoma cell line SGC7901/ADR

In our previous work, cellular prion protein (PrPc) was identified as an upregulated gene in adriamycin-resistant gastric carcinoma cell line SGC7901/ADR compared to its parental cell line SGC7901. Here we investigate the expression of PrPc in gastric cancer and whether it was involved in multidrug resistance (MDR) of gastric cancer. We demonstrated that PrPc was ubiquitously expressed in gastric cancer cell lines and tissues. PrPc conferred resistance of both P-glycoprotein (P-gp)-related and P-gp-nonrelated drugs on SGC7901, which was accompanied by decreased accumulation and increased releasing amount of adriamycin in PrPc-overexpressing cell line. Inhibition of PrPc expression by antisense or RNAi technology could partially reverse multidrug-resistant phenotype of SGC7901/ADR. PrPc significantly upregulated the expression of the classical MDR-related molecule P-gp but not multidrug resistance associated protein and glutathione S-transferase pi. The PrPc-induced MDR could be partially reversed by P-gp inhibitor verapamil. PrPc could also suppress adriamycin-induced apoptosis and alter the expression of Bcl-2 and Bax, which might be another pathway contributing to PrPc-related MDR. The further study of the biological functions of PrPc may be helpful for understanding the mechanisms of occurrence and development of clinical gastric carcinoma and PrPc-related MDR and developing possible strategies to treat gastric cancer.

Cisplatin-dependent upregulation of death receptors 4 and 5 augments induction of apoptosis by TNF-related apoptosis-inducing ligand against esophageal squamous cell carcinoma

TNF-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily known to induce apoptosis in a variety of cancers. The purpose of our study was to examine the effects of TRAIL in combination with cisplatin against esophageal squamous cell carcinoma (ESCC) cell lines in vitro and in vivo, and to elucidate underlying molecular mechanisms. Expression profiles of TRAIL receptors were investigated in 19 ESCC (KYSE) cell lines using RT-PCR. Crystal violet staining assays were performed to reveal the sensitivity against TRAIL. Flow cytometric analyses of apoptosis induction and TRAIL receptor expression were performed. Furthermore, Western blot was used to clarify the apoptosis pathway involved, and a nude-mouse xenograft model was used to show effects in vivo. Results show that death receptors (DR) 4 and 5 were expressed in 100% of the cell lines, and 79% (15/19) expressed 4 TRAIL receptors. There was only 1 cell line without decoy receptor expression. Eighteen cell lines were resistant to TRAIL, but in some, the combination treatment with cisplatin could overcome this resistance. They underwent apoptosis via activation of caspase-8 and -3, and cisplatin-dependent upregulation of DR4 and 5 was detected. Furthermore, pretreatment with cisplatin followed by TRAIL resulted in significant tumoricidal effects. Finally, systemic administration of TRAIL with cisplatin synergistically suppressed tumor growth of ESCC xenografts in nude mice. These results provide a significance of cisplatin-induced upregulation of death receptors as apoptosis-inducing machinery, and it was suggested that sequential administration of cisplatin and TRAIL might be a feasible chemotherapeutic regimen against ESCC.

Stimulation of endothelial IL-8 (eIL-8) production and apoptosis by phenolic metabolites of benzene in HL-60 cells and human bone marrow endothelial cells

Benzene toxicity is considered to be elicited by its metabolites and phenolic metabolites of benzene are known to induce apoptosis in leukemia cells in culture and in human bone marrow progenitor cells. One potential mechanism of apoptosis induced by benzene metabolites that has not been examined is the production of pro-apoptotic cytokines such as endothelial IL-8 from endothelial cells in bone marrow stroma. In this study, we utilized HL-60 cells which are known to produce the endothelial form of IL-8 (elL-8) and human bone marrow endothelial cells (HBMEC) as model systems. Hydroquinone (HQ), Catechol (Cat) and benzenetriol (BT) all induced eIL-8 production and apoptosis in HL-60 cells. HQ induced a marked 50-70-fold stimulation of eIL-8 levels and HL-60 cells were shown to have the eIL-8 receptor, CXCR I thus enabling an autocrine pathway of apoptosis. However, treatment with recombinant elL-8 failed to induce apoptosis in HL-60 cells as previously reported and antibodies to either IL-8 or CXCRI did not significantly abrogate benzene metabolite-induced apoptosis. HQ and Cat but not BT also induced stimulation of elL-8 production in HBMEC. These data demonstrate that although metabolites of benzene induce marked stimulation of eIL-8, this is unlikely to be responsible for apoptosis induced in HL-60 cells. Our data also demonstrates that phenolic metabolites of benzene stimulate the production of eIL-8 from HBMEC suggesting that higher levels of endothelial-derived cytokines may occur in bone marrow after benzene exposure.

Stimulation of endothelial IL-8 (eIL-8) production and apoptosis by phenolic metabolites of benzene in HL-60 cells and human bone marrow endothelial cells

Benzene toxicity is considered to be elicited by its metabolites and phenolic metabolites of benzene are known to induce apoptosis in leukemia cells in culture and in human bone marrow progenitor cells. One potential mechanism of apoptosis induced by benzene metabolites that has not been examined is the production of pro-apoptotic cytokines such as endothelial IL-8 from endothelial cells in bone marrow stroma. In this study, we utilized HL-60 cells which are known to produce the endothelial form of IL-8 (eIL-8) and human bone marrow endothelial cells (HBMEC) as model systems. Hydroquinone (HQ), Catechol (Cat) and benzenetriol (BT) all induced eIL-8 production and apoptosis in HL-60 cells. HQ induced a marked 50-70 fold stimulation of eIL-8 levels and HL-60 cells were shown to have the eIL-8 receptor, CXCR1 thus enabling an autocrine pathway of apoptosis. However, treatment with recombinant eIL-8 failed to induce apoptosis in HL-60 cells as previously reported and antibodies to either IL-8 or CXCR1 did not significantly abrogate benzene metabolite-induced apoptosis. HQ and Cat but not BT also induced stimulation of eIL-8 production in HBMEC. These data demonstrate that although metabolites of benzene induce marked stimulation of eIL-8, this is unlikely to be responsible for apoptosis induced in HL-60 cells. Our data also demonstrates that phenolic metabolites of benzene stimulate the production of eIL-8 from HBMEC suggesting that higher levels of endothelial-derived cytokines may occur in bone marrow after benzene exposure.

Biochemical modulation of Ara-C effects by amidox, an inhibitor of ribonucleotide reductase in HL-60 promyelocytic human leukemia cells

Amidox, a new polyhydroxy-substituted benzoic acid derivative, is a potent inhibitor of the enzyme ribonucleotide reductase (RR), which catalyses the de novo synthesis of DNA. RR is considered to be an excellent target for anti cancer chemotherapy. We investigated the biochemical and antineoplastic effects of amidox as a single agent and in combination with Ara-C in human HL-60 promyelocytic leukemia cells. Amidox inhibited the growth of HL-60 cells in a growth inhibition assay with an IC50 of 25 microM. In a soft agar colony forming assay, amidox yielded a 50% inhibition of colony formation at 13 microM. We also investigated the effects of amidox treatment on the formation of deoxynucleosidetriphosphates. Amidox (50 and 75 microM for 24 hours) could significantly decrease intracellular concentrations of dCTP, dATP and dGTP pools, whereas dTTP levels increased. We then tested the combination effects of amidox with Ara-C; this combination yielded additive cytotoxic effects both in growth inhibition and in soft agar colony formation assays. This effect was due to the increased formation of Ara-CTP, the active metabolite of Ara-C, after preincubation with amidox. Preincubation of HL-60 cells with 75 and 100 microM amidox for 24 hours caused an increase in the intracellular Ara-CTP concentrations by 576% and 1143%, respectively. Therefore amidox might offer an additional option for the treatment of leukemia and thus be further investigated in in vivo studies as a single agent and in combination with Ara-C.

Depletion of deoxyribonucleoside triphosphate pools in tumor cells by nitric oxide

Nitric oxide displays pro- and anti-tumor activities, prompting further studies to better understand its precise role. Nitric oxide inhibits ribonucleotide reductase (RnR), the limiting enzyme for de novo dNTP synthesis. We report here the first detailed analysis of dNTP variations induced in tumor cells by NO. NO prodrugs induced a depletion in dNTP pools and an activation of the pyrimidine salvage pathway, as did hydroxyurea, the prototypic RnR inhibitor. In the presence of dipyridamole, which blocked salvaged dNTP synthesis, depletion of dNTP pools was also observed in tumor cells cocultured with macrophages expressing the high-output iNOS activity. This effect was rapid, reversible, blocked by NO scavengers, and cGMP independent. It was quantitatively correlated to iNOS activity. In the absence of dipyridamole, NO still induced a decrease in dATP concentration in tumor cells cocultured with macrophages, whereas surprisingly, concentrations of dCTP and dTTP expanded considerably, resulting in a strong imbalance in dNTP pools. NO prodrugs did not cause such an increase in pyrimidine dNTP, suggesting that pyrimidine nucleosides were released by NO-injured macrophages. Altered dNTP levels have been reported to promote mutagenesis and apoptosis. It is suggested that abnormal changes in dNTP pools in tumors might contribute to NO-dependent toxicity.

Subcellular localisation of Cdc25A determines cell fate

Cell division cycle 25A (Cdc25A) was shown to colocalise both with nuclear and cytoplasmic proteins. Recently, we have demonstrated that overexpressed Cdc25A promoted the survival of rat 423 cells through indirect activation of PKB-protein kinase B. Using a Cdc25A:ER fusion protein, which can be shuttled from the cytoplasm into the nucleus, the present investigation evidences that the antiapoptotic effect of Cdc25A was restricted to its cytoplasmic localisation in rat 423 cells. In contrast, nuclear Cdc25A overexpression caused dephosphorylation and nuclear retention of the proapoptotic transcription factor Forkhead in rhabdomyosarcoma-like 1 (FKHRL1) in human N.1 ovarian carcinoma cells. This resulted in the increased constitutive expression of the FKHRL1 targets Fas ligand and Bim, and promoted apoptosis. Thus, the Cdc25A oncogene, which was found to be frequently overexpressed in certain human cancers, can increase or decrease the susceptibility to apoptosis depending on the cell-type-specific subcellular distribution.

Molecular mechanism of satratoxin-induced apoptosis in HL-60 cells: activation of caspase-8 and caspase-9 is involved in activation of caspase-3

Satratoxins have been recognized as potential immunomodulatory agents in outbreaks of building-related illness. Here we report that satratoxin G-treated human leukemia HL-60 cells underwent apoptosis through the action of caspase-3 which was activated by both caspase-8 and caspase-9. Western blot analysis of caspase-3 in the satratoxin G-treated cells apparently indicated the appearance of a catalytically active fragment of 17 kDa. Increased caspase-3 activity was also detected by using a fluorogenic substrate, DEVD-AMC. Next, exposure to satratoxin G led to cleavage of PARP from its native 116 kDa form to a 85 kDa product. Moreover, DFF-45/ICAD were cleaved into a 12.5 kDa fragment via satratoxin G treatment. Enzymic assay on IETD-AMC revealed that caspase-8 is strongly activated by exposure to satratoxin G while T-2 toxin (T-2) could not activate caspase-8 at an early stage of apoptosis. Furthermore, satratoxin G caused a release of cytochrome c from mitochondria into the cytosol and increased the activity of caspase-9 against LEHD-AMC. These findings indicate that satratoxin G-induced apoptosis involves activation of caspase-3 and DFF-40/CAD through both activation of caspase-8 and cytosolic accumulation of cytochrome c along with activation of caspase-9.

Maintenance of ATP favours apoptosis over necrosis triggered by benzamide riboside

A new synthetic drug, benzamide riboside (BR) exhibited strong oncolytic activity against leukemic cells in the 5-10 microM range. Higher BR-concentrations (20 microM) predominantly induced necrosis which correlated with DNA strand breaks and subsequent depletion of ATP- and dATP levels. Replenishment of the ATP pool by addition of adenosine prevented necrosis and favoured apoptosis. This effect was not a pecularity of BR-treatment, but was reproduced with high concentrations of all trans-retinoic acid (120 microM) and cyanide (20 mM). Glucose was also capable to suppress necrosis and to favour apoptosis of HL-60 cells, which had been treated with necrotic doses of BR and cyanide. Apoptosis eliminates unwanted cells without affecting the microenvironment, whereas necrosis causes severe inflammation of surrounding tissues due to spillage of cell fluids into the peri-cellular space. Thus, the monitoring and maintenance of cellular energy pools during therapeutic drug treatment may help to minimize nonspecific side effects and to improve attempted drug effects.