Study of molecular mechanisms of pro-apoptotic activity of NCX 4040, a novel nitric oxide-releasing aspirin, in colon cancer cell lines
© Tesei et al; licensee BioMed Central Ltd. 2007
Received: 02 October 2007
Accepted: 30 October 2007
Published: 30 October 2007
Despite numerous studies aimed at verifying the antitumor activity of nitric oxide-releasing nonsteroidal antiflammatory drugs (NO-NSAIDs), little is known about the molecular targets responsible for their antineoplastic properties. In the present study, we investigated the mechanisms underlying the cytotoxicity of NCX 4040, a novel NO-aspirin with promising antineoplastic action, in in vitro human colon cancer models.
The effect on tumor growth was evaluated in four human colon cancer cell lines (LoVo, LRWZ, WiDr and LoVo Dx) by sulforhodamine B assay, oxidative stress by immunohistochemistry, apoptosis by laddering assay, mitochondrial membrane potential (ΔΨm) by flow cytometry, and apoptosis- and chemoresistance-related markers by western-blot and real-time method, respectively. Prostaglandin E2 levels were determined by ELISA.
NCX 4040 produced a higher cytotoxic effect in all the cell lines than that produced by other NO donors tested. In particular, in LoVo and LRWZ cells, NCX 4040 induced a cytocidal effect and apoptosis through p53 and NAG-1 expression, an early ΔΨm collapse, and a sequential release of cytoplasmatic cytochrome c and caspase -9 and -3 active forms. 8-hydroxyguanine lesions, indicative of oxidative stress, were also observed. Conversely, in WiDr line, the drug caused a cytocidal effect, albeit not through apoptosis, and a concomitant increase in COX-2 activity. In LoVo Dx line, characterized by high levels drug resistance and DNA repair-related markers, only a cytostatic effect was observed, again in concomitance with the increase in COX-2 enzyme activity.
This study highlights the multiplicity of mechanisms involved in sensitivity or resistance to NCX 4040 and could provide useful indications for tailored therapy by identifying potentially drug-responsive tumors.
Nitric oxide-releasing non-steroidal anti-inflammatory drugs (NO-NSAIDS), which consist of a traditional NSAID linked to an -NO2 moiety via a molecular spacer, represent a promising class of compounds. These drugs were developed to overcome gastrointestinal and renal toxicities, the main limitations in the long-term use of traditional NSAIDs as chemoprotective agents against colon cancer. The rationale behind NO-NSAID development was based on the protective properties of nitric oxide on the gastric mucosa, similar to that exerted by prostaglandins, whose biosynthesis is inhibited by traditional NSAIDs.
Emerging data indicate that these compounds, in addition to maintaining the chemopreventive properties of traditional NSAIDs, show enhanced safety and efficacy [1–3]. Furthermore, recent studies by our group [4, 5] and by other researchers  have shown that the NO-aspirin derivative, NCX 4040, inhibits tumor cell growth and induces apoptosis in both in vitro and in vivo experimental systems. The efficacy of this compound seems to be independent of tumor histotypes [7, 8].
For many years the anti-tumorigenic activity of NSAIDs was ascribed to the inhibition of cyclooxygenase (COX) enzymes, in particular, the inducible isoform COX-2. However, in a previous study  we showed the inability of NCX 4040 to induce apoptosis in cells with high COX-2 expression, and there is increasing evidence to suggest that NSAIDs may also have a non COX-mediated effect . In the present study we explored other pathways of apoptosis induction and, in particular, investigated the role of the so-called NSAID-activated gene, NAG-1, which belongs to the TGF-β superfamily and is characterized by pro-apoptotic and anti-tumorigenic activities [10, 11], in a panel of human colon cancer cell lines.
The study was performed on four cell lines: LoVo and WiDr, obtained from the American Type Culture Collection (Rockville, MD), LRWZ, isolated in our laboratory from a human colon adenocarcinoma, and the multidrug-resistant LoVo Dx, derived from LoVo cells, and kindly provided by Dr. Mario Colombo (Istituto Nazionale Tumori, Milan, Italy). Tumor colon cell lines were maintained as a monolayer at 37°C and subcultured weekly. Culture medium was composed of DMEM/HAM F12 (1:1) supplemented with fetal calf serum (10%), glutamine (2 mM), non-essential aminoacids (1%) (Mascia Brunelli s.p.a., Milan, Italy), and insulin (10 μg/ml) (Sigma Aldrich, Milan, Italy). Cells were used in the exponential growth phase in all the experiments.
In vitro chemosensitivity assay
SRB (sulforhodamine B) assay was used according to Skehan et al.'s method . Briefly, cells were collected by trypsinization, counted and plated at a density of 10,000 cells/well in 96-well flat-bottomed microtiter plates (100 μl of cell suspension per well). Experiments were run in octuplet, and each experiment was repeated three times. The optical density of treated cells was determined at a wavelength of 540 nm using a fluorescence plate reader. In the chemosensitivity assay, NCX 4040 and NO donors were tested at scalar concentrations ranging from 1 to 50 μM for 24 hours followed by a 24-hour drug wash-out.
Growth inhibition and cytocidal effect of drugs were calculated according to the formula reported by Monks : [(ODtreated - ODzero)/(ODcontrol - ODzero)] × 100%, when ODtreated was > to ODzero. In case ODtreated was below ODzero, cell killing had occurred. The ODzero depicts the cell number at the moment of drug addition, the ODcontrol reflects the cell number of untreated wells and the ODtreated reflects the cell number in treated wells on the day of the assay.
Mitochondrial membrane depolarization analysis
After different exposure times to NCX 4040 tested at a concentration of 10 μM in LoVo and 50 μM in LRWZ lines, cells were harvested, washed once in PBS and then immediately incubated in JC-1 Working solution (BD Biosciences Pharmingen, San Diego, CA) for 10 minutes in a humidified atmosphere at 37°C in the dark. Cells were then washed and resuspended in 1X Assay Buffer (BD Biosciences Pharmingen) and analyzed (FACS Vantage, Becton Dickinson, San Jose, CA). Data acquisition and analysis were performed using CELLQuest software (Becton Dickinson). For each sample, 15,000 events were recorded
After different exposure times to NCX 4040 tested at a concentration of 10 μM in LoVo and 50 μM in LRWZ lines, the cells were harvested by trypsinization and their DNA content was isolated using GenElute™ Mammalian Genomic DNA Miniprep Kit (Sigma Aldrich), according to the manufacturer's instructions. The DNA samples were loaded on 1.5 % agarose gel and run at 70 mV for about 1 hour. Internucleosomal DNA fragments were visualized by ethidium bromide under UV light.
Fluorescence microscopy analysis
Cell-permeable DNA dye 4',6-DAPI and a fluorescence photomicroscope (Zeiss, Axioscope 40) were used to visualize chromatin condensation and/or fragmentation typical of apoptotic cells.
The percentage of apoptotic cells was evaluated by flow cytometric analysis, according to the previously described TUNEL assay procedure , in WiDr and LoVo Dx cell lines after exposure to NCX 4040 (10 μM), to the selective COX-2 inhibitor NS 398 (50 μM), or to both drugs. Briefly, after treatment, cells were trypsinized, fixed, exposed to the TUNEL reaction mixture, counterstained with propidium iodide, and then analyzed by FACS.
Mitochondrial and cytoplasmic fractionations
The ApoAlert Cell Fractionation Kit supplied by BD Bioscience Clontech (Palo Alto, CA) was used according to the manufacturer's instructions, as previously described . Briefly, cells were centrifuged at 600 × g for 5 minutes at 4°C and resuspended in 800 μl of fractionation buffer (BD Bioscience Clontech). Cells were passed 20 times through a 22-gauge needle and the homogenates were centrifuged at 700 × g for 10 minutes and at 10,000 × g for 25 minutes at 4°C.
Western blot analysis
Cells were treated according to the previously described Western blot procedure . Antibodies used were anti-caspase-3 (polyclonal antibody, Cell Signaling Technology Inc., Beverly, MA, dilution 1:500); anti-caspase-9 (polyclonal antibody, Cell Signaling Technology Inc., dilution 1:500); anti-NAG-1/PTGF-β (polyclonal antibody, Upstate Biotechnology Inc., Lake Placid, NY, dilution 1:500); anti-p53 (PAb 1801, monoclonal antibody, Bioptica, Milan, Italy, dilution 1:400); anti-cytochrome c (ApoAlert Cell Fractionation Kit, BD Bioscience Clontech, dilution 1:100), and anti-COX 4 antibody (BD Bioscience Clontech, dilution 1:500).
The bound antibody was detected by enhanced chemiluminescence (ECL) using an ECL kit (Amersham Pharmacia Biotech, Cologno Monzese, Italy).
Oxidative damage determination
After a 6-hour exposure of LoVo to 10 μM of NCX 4040 or a 14-hour exposure of LRWZ to 50 μM of NCX 4040, cells were trypsinized and washed in PBS. Approximately 0.5-1 × 105 cells were plated onto microscope slides using a Cytospin cell preparation system (Shandon, Pittsburgh, PA). Fixed samples, either untreated or treated with NCX 4040, were stained with hematoxylin-eosin and exposed to the antibody that specifically recognizes the presence of 8-hydroxyguanine lesions (anti-8-oxo-dG monoclonal antibody, Trevigen Inc., MD, dilution 1:1000), which are almost exclusively elicited by oxidative stress.
Prostaglandin E2 determination
The determination of prostaglandin E2 (PGE2) in culture medium was performed using High Sensitivity Prostaglandin E2 Enzyme Immunoassay Kit (Assay Designs, Inc., Ann Arbor, MI), according to the manufacturer's instructions.
mRNA RT-PCR analysis
Total cellular RNA was isolated using RNeasy Minikit (Qiagen, Hilden, Germany). One microgram of RNA was reverse-transcribed into cDNA using iScript (BioRad, Hercules, CA), according to the manufacturer's instructions, and analyzed by real-time RT-PCR (MyiQ System, BioRad) to detect the expression of human breast cancer-resistance protein (BCRP-1), excision repair cross-complementing 1 and 2 (ERCC1-2), topoisomerase I (TOPO I), and thymidylate synthase (TYMS) chemosensitivity markers.
The standard reaction volume was 25 μl and contained 1× SYBR GREEN SuperMix (BioRad), 200 mM of each primer and 2 ul of cDNA template. The mixture was subjected to the following cycling: 95°C for 3 minutes followed by 40 cycles of denaturation at 95°C for 15 seconds, and annealing and extension at 60°C for 30 seconds. Primer sequences were as follows: forward primer, 5'-ACCAACCCTGACGACAGAAGAATC-3' and reverse primer, 5'-GGCGATGTTGAAAGGCACACC-3' for TYMS; forward primer, 5'-AGTCCAAGCATAGCAACAGTGAAC-3' and reverse primer, 5'-CCATCTTTGTGTTTGGTCTTCTCC -3' for TOPO 1; forward primer, 5'-TCAGTCAACAAAACGGACAGTCAG-3' and reverse primer, 5'-TCCTTGGGTTCTTTCCCAGAGC-3' for ERCC1v1; forward primer, 5'-TCATCGCCGCATCAAGAGAAG-3' and reverse primer, 5'-TCATCAGGGTACTTTCAAGAAGGG-3' for ERCC1v2; forward primer, 5'-ATGCTTGGTGGTCTTGTTAAGTGG -3' and reverse primer, 5'-AAGGCTCAGGATCTCAGGATGC-3' for BCRP-1.
The amount of mRNA of each marker was normalized to the endogenous reference β2-microglobulin using Gene Expression Macro Software, Version 1.1 (BioRad).
The experimental values of PGE2 determination and mRNA RT-PCR analysis represent the median from three independent experiments. Student's t test for paired samples was performed. Differences were considered significant at p values < 0.05 (two-sided).
Cell sensitivity to NO-donors and to NCX 4040
Pro-apoptotic activity of NCX 4040
Following pro-apoptotic protein induction and ΔΨm collapse, the release of cytoplasmatic cytochrome c and the presence of caspase-9 and -3 active forms were observed after a 6-hour exposure in LoVo and a 14-hour exposure in LRWZ cells (Fig. 5C and 5D).
NSAIDs, which have long been used for pain relief, fever, and inflammation, are currently under intense investigation to determine the extent and nature of their anti-cancer properties. There is mounting evidence to suggest that the new chemical entities, NO-NSAIDs, show enhanced anticancer activity compared to native NSAIDs. We recently demonstrated that NCX 4040 is highly effective in inhibiting the growth of human colon cancer cells in vitro through apoptotic pathways in two of the four cell lines investigated . In the present work, we showed that apoptosis occurs mainly via the mitochondrial pathway, with early mitochondrial membrane depolarization followed by cytochrome c release into the cytoplasm and caspase-9 and -3 activation. This is in agreement with results previously reported by our group for other tumor histotypes [7, 8]. Furthermore, the different susceptibility of the various colon cancer cell lines to apoptosis makes this experimental system an interesting model to improve our understanding of the molecular mechanisms that are determinants in triggering cell death.
In the two cell lines prone to apoptosis, we observed the induction of the specific NSAID-activated gene (NAG-1) after a short exposure to NCX 4040, highlighting the role of the aspirin component in the pro-apoptotic activity of the NO-derivative. NAG-1 is a divergent member of the transforming growth factor-β superfamily. Several studies have shown that the anti-tumorigenic and pro-apoptotic role of NAG-1 protein [11, 15, 16] is regulated by several NSAIDs, such as aspirin, indomethacin and ibuprofen, providing new clues to explain the anti-cancer activities of these anti-inflammatory agents.
The NAG-1 promoter has been characterized and many transcription factors, including p53 , are known to regulate this gene. However, the original finding that NAG-1 expression is also induced by NSAIDs in p53 null cell lines  seems to indicate the existence of non p53-mediated activation mechanisms. Consistently with this hypothesis, we observed an increment of NAG-1 expression in both LoVo and LRWZ cells and an increment of p53 in only the former cell line.
Our group also hypothesized that the NO component of the molecule plays a pivotal role in the cytotoxic and pro-apoptotic activity of NCX 4040 [4, 7, 8]. NO is synthesized by normal and neoplastic tissues [18, 19] and it has been shown that low endogenous NO levels increase cell proliferation, whereas high levels cause cytotoxicity and apoptosis [20, 21]. Moreover, NO released from chemical agents, such as SNP, and other NO-generating molecules has shown anti-proliferative and pro-apoptotic properties in cells from different tumor histotypes in vitro and in vivo [22–24]. In the present study, we showed that NCX 4040 is more effective in inhibiting cancer cell proliferation than the other three NO-donors, SNP, SNAP and NONOATE. The enhanced effect of NCX 4040 with respect to the classic NO donor compounds is probably largely due to the high -NO release capacity of the spacer component, whose important role in the efficacy of the drug was also highlighted in the work by Kashfi and Rigas . The pivotal role of the -NO molecule in the anti-tumor activity of the drug was further confirmed by the detection, for the first time, of 8-hydroxyguanine lesions, an index of oxidative stress , in the DNA of cells that underwent apoptosis after a short exposure to NCX 4040.
The results of the present study also provided some potential explanations for the inability of NCX 4040 to trigger cell death machinery in LoVo Dx and WiDr cells.
In a previous study we showed that COX-2 expression did not change in either WiDr or LoVo Dx cell lines after NCX 4040 exposure . In the present work we analyzed the effect of drugs on the catalytic activity of the COX-2 enzyme, as expressed by PGE2 levels. As expected, a considerable inhibition was observed following exposure to NS-398, a specific inhibitor of COX-2 catalytic activity, whereas, surprisingly, a significant increment was seen after exposure to NCX 4040. This finding, already reported by other authors in cancer cell lines with an elevated resistance to apoptosis induced by traditional NSAIDs [27–29] and NO-NSAIDs [30, 31], together with the observation that LoVo and LRWZ lines did not express this cyclooxygenase isoform , raises intriguing questions about the biological role of COX-2 in the natural history of cancer and also about the mechanisms by which NSAIDs, including NO-NSAIDs, are capable of preventing cancer. Moreover, the onset of apoptosis in cell death-resistant WiDr and LoVo Dx lines after COX-2 inhibition and exposure to NCX 4040 clearly indicates the close correlation between this enzyme and resistance to apoptosis.
In addition to basal elevated COX-2 expression, the high expression of chemoresistance- and DNA repair-related markers in LoVo Dx may explain both the weak cytotoxic effect of NCX 4040 and its failure to trigger apoptosis in this cell line.
In conclusion, our results indicate that NCX 4040, a novel NO-aspirin, exerts a different cytotoxic activity as a function of the molecular profile of human colon cancer cells. Furthermore, the data we obtained strongly suggest that the NO-releasing moiety (-NO2) is responsible for the apoptotic process, which is also enhanced by the aspirin component of the molecule, as shown by its specific induction of the pro-apoptotic protein, NAG-1. In particular, it would seem that the cytocidal effect of NCX 4040 is obtained via mitochondrial pathway-mediated apoptosis in the cells not expressing the COX-2 enzyme, which, conversely, is upregulated after drug exposure in cells failing to undergo apoptosis. These observations, together with the finding that the most resistant cell line was characterized by high levels of drug resistance and DNA repair-related markers, highlight the multiplicity of mechanisms involved in sensitivity or resistance to the drug and could provide useful indications for tailored therapy by identifying potentially drug-responsive tumors.
nitric oxide-releasing non-steroidal anti-inflammatory drug
- NCX 4040:
breast cancer-resistance protein
excision repair cross-complementing 1 and 2
- TOPO I:
The authors wish to thank Prof. Rosella Silvestrini for her invaluable scientific contribution and Gráinne Tierney for editing the manuscript.
This work was supported by Istituto Oncologico Romagnolo, Forlì, and the Italian Ministry of Health.
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