Background
The metastatic process consists of a number of sequential interrelated steps, all of which must be completed successfully to give rise to a secondary tumor [
1‐
3]. In particular, the adhesion of cancer cells to endothelial cells is a prerequisite for extravasation of circulating cancer cells and for their metastatic dissemination. This adhesive event requires specific interactions between adhesion receptors present on vascular endothelial cells and their ligands or counter-receptors on cancer cells. E-selectin is a specific endothelial adhesion receptor that is induced by pro-inflammatory stimuli. Its natural function is to mediate the adhesion of leukocytes to the endothelium allowing their extravasation into inflamed tissues [
4]. Intriguingly, cancer cells hijack the inflammatory system and interact with E-selectin to extravasate [
5,
6]. For example, colon carcinoma cells adhere to and roll on both purified E-selectin and cytokine-stimulated endothelial cells either in static or dynamic conditions
in vitro [
7‐
9]. Moreover, several studies strongly support the role of E-selectin-mediated adhesion of cancer cells to endothelial cells as an important determinant of metastasis, especially of colon carcinoma cells. In particular, the binding efficiency of clonal colon cancer cell lines to E-selectin is directly proportional to their respective metastatic potential [
10]. In contrast, anti-E-selectin antibodies and antisense oligonucleotides that inhibit E-selectin expression impair experimental liver metastasis of murine and human tumor cells [
11,
12]. Similarly inhibiting the expression of E-selectin with cimetidine, an antagonist of histamine H2 receptors, inhibits the adhesion of cancer to endothelial cells and impairs metastatic dissemination [
13].
The binding of cancer cells to E-selectin involves a counter-receptor for E-selectin that is composed of sialyl Lewis-a/x carbohydrate determinants that are borne by a carrier protein or lipids on cancer cells. The binding is Ca
2+-dependent and is mediated through the N-terminal lectin domain of E-selectin. Sialyl Lewis-a on carrier proteins plays a major role in E-selectin binding of cancer cells derived from the lower digestive organs, such as the colon and rectum, as well as from the pancreas and biliary tract [
14]. On the other hand, sialyl Lewis-x is the representative carbohydrate involved in the E-selectin binding of breast, ovarian and pulmonary cancer cells [
1]. Little is known about the proteins that bear these carbohydrates and that serve as the E-selectin counter-receptor backbone on cancer cells. LAMP-1, LAMP-2, CD44, CEA and podocalyxin-like proteins were all identified as E-selectin counter-receptors on colon cancer cells [
15‐
19]. However, the signaling events that stem from these receptors in the cancer cells bound to E-selectin are still ill defined. Several studies have shown that the adhesion of cancer cells to E-selectin initiates a reverse signaling in the cancer cells, which raises the possibility that this signaling modulates the metastatic potential of cancer cells [
20‐
22]. We previously reported that Death receptor-3 (DR3) is a functional and signaling sialylated ligand that binds E-selectin on colon cancer cells [
20,
23]. The subsequent DR3 activation induced by E-selectin increases the motile potentials of the cancer cells through activation of the p38 MAP kinase pathway [
20,
23].
DR3 is a member of the second group of the TNF receptor (TNFR) superfamily that includes TNFR1, DR4, DR5, DR6, and Fas [
17]. These receptors contain a common 70- to 80-amino acid homologous region in the cytoplasmic tail called the death domain [
24]. The signaling pathways leading to cell death in response to these receptors are similar and rely on trimerization and oligomerization of the receptors upon ligand binding followed by the recruitment of death domain proteins, such as TRADD, FAD, or RIP1, and subsequently, activation of the apoptotic cascade [
25]. More recently, it was reported that CD95/Fas, a member of the TNFR family, induces signaling to phosphatidylinositol 3-kinase (PI3K) via phosphorylation of Tyr residues present in its death domain [
26]. Several splice isoforms of DR3 exists, some of which such as, isoforms 1, 2, 3, 4 and 7, contain a death domain, while others, such as the truncated DR3 isoform 12, do not [
27]. Among these variants, DR3 isoform 2 (DR3v2) is the major and parental member of the family and is referred to hereafter as DR3. Interestingly, the splicing profile of DR3 may be altered in cancer. Notably, DR3β differs from DR3 by the inclusion of a 28 amino-acid stretch in the extracellular domain. Whereas DR3 is expressed in all cell lines and lymphoma samples tested, DR3β expression is restricted to lymphoid T-cell and immature B-cell lines and to some cases of follicular lymphoma. This suggests that several receptor isoforms can participate in lymphoid cell homeostasis [
28]. The functions of DR3 in a physiopathologic context are unclear. However, its ectopic expression in mammalian cells induces apoptosis or activates the pro-survival transcription factor NFκB, depending on the cytoplasmic effectors engaged in the signaling complexes downstream of the death domain [
29,
30]. Intriguingly, the activation of DR3 by TL1A/VEGI, the cognate ligand for DR3 is not followed by apoptosis in human erythroleukemic TF-1 cells. This is presumably because it is associated with the expression of the apoptosis-inhibiting protein c-IAP2 [
31,
32]. More recently, we found that activation of DR3 by E-selectin increased the survival of LoVo colon cancer cells, in part by activating the ERK pathway [
23].
In this study, we further investigated the mechanisms by which activation of DR3 by E-selectin increases the survival of colon carcinoma cells. Our major finding is that metastatic colon cancer cells do not enter into apoptosis in response to E-selectin in part because they bind to DR3 to activate the PI3K/NFκB survival pathway and in part because they generate an alternative splice variant of DR3 that lacks trans-membrane and death domains, thus rendering it unable to induce apoptosis.
Methods
Reagents and antibodies
Recombinant human E-selectin/Fc (rhE-selectin/Fc) was obtained from R&D Systems (Minneapolis, MN). Phenylethylisothiocyanate (PEITC) and LY294002 were purchased from Sigma (St Louis, MO). Calcein-AM was obtained from Invitrogen-Molecular Probes (Burlington, ON, Canada). Dimethylsulfoxyde was purchased from Fisher (Montreal, QC, Canada). Protein G-sepharose was purchased from GE Healthcare (Mississauga, ON, Canada). PP2 and PD098059 were purchased from Calbiochem (Mississauga, ON, Canada). Rabbit anti-DR3 clone H300 was obtained from Santa Cruz biotechnology, mouse anti-DR3 extracellular domain (DR3ecd), mouse anti-vinculin (hVIN-1), rabbit anti-active caspase 3, and irrelevant mouse IgG1κ (MOPC21) were purchased from Sigma (St Louis, MO). Mouse anti-DR3 clone B65 was obtained from Millipore (Nepean ON, Canada). Mouse anti-DR3 (hDR3) was purchased from R&D Systems (Minneapolis, MN). Rabbit anti-phospho Akt (Ser 473), rabbit anti-Akt, rabbit anti-NFκB p65 and mouse anti-caspase 8 (1C12) were all obtained from Cell Signaling Technology, (Beverly, MA). Mouse anti-TATA Binding Protein (TBP) antibody was purchased from AbCam (Cambridge, MA). Goat anti-mouse IgG (H+L) and goat anti-rabbit IgG (H+L) conjugated with horseradish peroxidase were from Jackson Immunoresearch (West Grove, PA).
Cells
HT29 colorectal adenocarcinoma cells were cultivated in McCoy 5A medium supplemented with 10% foetal bovine serum (FBS) and antibiotics. HT29LMM (highly metastatic HT29 cells) and Jurkat T cells were cultivated in RPMI medium containing 10% FBS. Caco2 colorectal adenocarcinoma cells were grown in DMEM high glucose medium supplemented with 10% FBS and Glutamax 1X. SW480 and SW620 are colorectal adenocarcinoma cells isolated from the primary site and lymph node secondary site from the same patient. They were cultivated in Leibovitz medium L15 containing 10% FBS. LoVo colorectal adenocarcinoma cells grade IV were grown in Ham F12K medium supplemented with 10% FBS. HIEC cells are normal human intestinal epithelial cells that were cultivated in OptiMEM containing 5% FBS and 5 ng/ml EGF [
33]. HEK293, HeLa, MDA MB231 and MCF7 cells were cultivated in DMEM containing 10% foetal calf serum. All these cell lines were obtained from ATCC.
Human umbilical vein endothelial cells (HUVEC) were isolated by collagenase digestion of umbilical veins from undamaged sections of fresh cords, as described [
34]. The cells used at passages ≤ 5 were grown to confluence in gelatin-coated tissue culture flasks in medium 199 containing 20% heat-inactivated FBS, endothelial cell growth supplement (60 μg/ml), glutamine (2 mM), heparin (25000 IU). Human micro-capillary endothelial cells (HMEC) were cultivated in MCDB medium containing 10% FBS, 1 μg/ml hydrocortisone and 10 ng/ml EGF. All cells lines were cultivated in the presence of antibiotics and maintained at 37°C in a 5% CO
2 humidified atmosphere.
Adhesion assays in a laminar flow chamber
HUVEC were trypsinized and grown for 24 hrs on gelatin-coated slides. These endothelial cells were treated with 20 ng/ml IL-1β for 4 h to induce the expression of E-selectin. The cultures were then placed in the laminar flow chamber GlycoTech (Gaithersburg, MD, USA) under a shear stress of 1 dyne/cm
2. In certain experiments, anti-human DR3 monoclonal Ab clone B65 or MOPC21 irrelevant antibody were added in the culture medium of HT29 cells, 30 min before their injection in the chamber. In other experiments, a knockdown of DR3 was performed by small interfering RNA, as previously described [
9,
23]. Briefly, HT29 cells were transfected by electroporation with human DR3 siRNA (siRNA; sense, 5'-CCGUCCAGUUGGUGGGUAA-3', and antisense, 5'-UUACCCACCAACUGGACGG-3') or control siRNA purchased from Qiagen (Mississauga, ON, Canada). Tumor cells in suspension (2 × 10
6 per assay) were labeled for 30 min with Calcein AM and washed twice with M199 medium before being added into the flow chamber. Videos were taken directly using a camera mounted on a TE2000 fluorescence microscope at ×20 magnification (Nikon, Melville, NY, USA).
Survival assay
Twenty-four hours after being plated, HT29 cells were left to grow for 96 hours with or without E-selectin or with the apoptosis inducer curcumin (75 μM) [
35]. At the end of the treatments, the cell survival was evaluated with the Quick Cell Proliferation Assay Kit from BioVision (Mountain View, CA). The test evaluates the ability of viable cells to convert tetrazolium salt into formazan (WST-1 assay), which can be monitored at 450 nm.
PI3 kinase and NFκB activation
Cells were washed twice and incubated in serum-free medium for 2 hours in the presence or not of the inhibitors (LY294002 or PP2). Thereafter, rhE-selectin was added for different periods of time. Cell extracts were prepared and PI3K and NFκB activation were assayed in western blotting by determining the phosphorylation of Akt at Ser 473 and nuclear translocation of p65NFκB, respectively.
The protocol was adapted from Andrews and Faller [
36]. Cells were washed 3 times in PBS and were re-suspended in 1.6 ml of PBS. The cell suspension was briefly vortexed and 100 μl of total extract were collected and mixed in 20 μl of extraction buffer. The rest of the cell suspension was centrifuged (16,000 × g) for 10 seconds at 4°C, and the pellet was resuspended in 400 μl of buffer A (10 mM HEPES-NaOH pH 7.9, 1.5 mM MgCl
2, 10 mM KCl, 0.5 mM DDT, 0.2 mM PMSF). The extract was left on ice for 10 min, vortexed for 10 seconds and centrifuged for 10 seconds at 4°C. The supernatant was removed and discarded, and the pellet was resuspended in 70 μl of buffer C (20 mM HEPES-NaOH pH 7.9, 25% glycerol, 420 mM NaCl, 1.5 mM MgCl
2, 0.2 mM EDTA, 0.5 mM DDT, 0.2 mM PMSF). The samples were incubated on ice for 20 minutes and centrifuged for 2 min at 4°C. Extraction buffer was added in each extract before heating. The amount of proteins was quantified by the Lowry method.
DR3 sequencing
Total RNA was extracted from (1 × 10
6) cells using Qiagen RNeasy kit (Mississauga, ON, Canada). All RNA samples were stored at -80°C until assay. The mRNA was reverse-transcribed with Qiagen Sensiscript reverse transcription kit using random hexamers. Nested PCRs were used to amplify a fragment of the
tnfrsf25 gene using specific pairs of primers and the Qiagen Hotstart taq DNA polymerase kit according to the manufacturer protocol (PCR1-Forward primers: 5'-CGTCGGAGGGCTATGGAGCAGC-Reverse primers: 5'-GGCCGGCTGGTGCTGCTACGC. PCR2-Forward primers: 5'-GAGGATCCATGCAGGGCGGCACTCGTAGC-Reverse primers: 5'-ACCTCGAGTCACGGGCCGCGCTGCAG). PCR products were cloned in pcDNA3 (Invitrogen, Burlington ON, Canada) vector and were sequenced by CRCHUQ/CHUL sequencing platform (Québec Qc, Canada). The DR3 sequences were compared with those found in the BLAST database and analyzed with the Human Genome Browser Gateway
http://genome.ucsc.edu/cgi-bin/hgGateway and the AceView genes
http://www.ncbi.nlm.nih.gov/IEB/Research/Acembly/ databases.
Analysis of DR3 variants
Total RNA was extracted from (1 × 106) cells using Qiagen RNeasy kit and one μg was used for a reverse transcription using Omniscript reverse transcriptase (Qiagen). Then, the full length DR3 was amplified by PCR using Qiagen Hotstart polymerase and the following primers: 5'-CGTCGGAGGGCTATGGAGCAGC and 5'-GGCCGGCTGGTGCTGCTACGC following the manufacturer's instructions. Thereafter, the region from exon 5 to exon 7 of DR3 was amplified by PCR, as previously described, using DR3 full length PCR product as a template and the following primers: 5'-CCCGCAGAGATACTGACTGTGGGAC and 5'-GTAGCCAGGGGTCCAGCTGTTACC. The resulting products were separated by agarose gel electrophoresis.
For more precise quantification, targeted PCR reactions were carried out, and the amplified products were analyzed by automated chip-based microcapillary electrophoresis on an Agilent 2100 Bioanalyzer instrument (Agilent Technologies, Santa Clara, CA) as previously described [
37]. Amplicon sizing and relative quantification was performed by the manufacturer's software. The primers used were Forward: 5'-TTCCCGCAGAGA TACTGACTG and Reverse: 5'-AGCACCTGG ACCCAGAACA.
Western blotting
Cells lysis was done at 4°C in extraction buffer (60 mM Tris-base, pH 6.8, 10% glycerol, and 3% sodium dodecyl sulphate) added with 5% β-mercaptoethanol just before use. Then, lysates were boiled, vortexed twice and centrifuged at 13,000 g for 5 minutes. Proteins were separated by SDS-PAGE and transferred to a nitrocellulose membrane. Each antibody was used according to the manufacturer's protocol. Blots were then revealed with Super signal West pico kit obtained from Pierce Biotechnology Inc (Rockford IL). If necessary, the membrane was reprobed for normalization.
Apoptosis evaluation
1) by DNA fragmentation . HT29 cells were treated with rhE-Selectin/Fc at 10 μg/ml for 4 hours or 24 hours, or were treated with phenethyl isothiocyanate at 50 μM for 24 hours. Cells were washed twice with PBS, fixed with 3,7% formaldehyde and stained with Hoechst for 60 min at room temperature in the dark. The cells were examined with a Nikon Eclipse 800 equipped with a 40 × objective lens. 2) by caspase activation . Caspase 8 and 3 activities were evaluated by western blotting using anti-caspase 8 and anti active-caspase-3 antibodies. The assays were performed on pools of cells containing both floating and adhering cells.
Nicolas Porquet, Andrée Poirier, François Houle, Anne-Laure Pin, Stéphanie Gout, Pierre-Luc Tremblay, Éric Paquet and Jacques Huot are from « Le Centre de recherche en cancérologie de l'Université Laval et Centre de recherche du CHUQ, l'Hôtel-Dieu de Québec, 9 rue McMahon, Québec G1R 2J6 Canada ». Nicolas Porquet is now at « The Institute of Developmental Biology and Cancer, CNRS UMR6543, Université Nice Sophia Antipolis 06108 Nice Cedex 2, France ». Stéphanie Gout is now at « Le Centre de recherche Inserm/UJF U823 Equipe 2, Institut Albert Bonniot BP 170, 38052 Grenoble Cedex 09, France ». François A Auger and Pierre-Luc Tremblay are both from « Le Laboratoire d'Organogenèse Expérimentale, Centre hospitalier affilié Universitaire de Québec, 1401, 18e rue Québec, G1J 1Z4, Canada ». Roscoe Klinck is from « Le Laboratoire de Génomique Fonctionnelle de l'Université de Sherbrooke, 3201, rue Jean Mignault, Sherbrooke J1E 4K8 Canada ».
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
NP, AP and FH have performed the signaling and survival experiments. ALP and AP have performed PCR assays and NP, AP and ALP have equally contributed to the identification of DR3Δ6. RK did the quantification of the DR3 variants. SG, PLT and FAA realized the experiments done in flow chambers. EP contributed to the statistics and bioinformatics analysis. FH finalized the figures. NP and JH wrote the manuscript with the contribution of the other authors. All authors read and approved the final version.