Background
Pancreatic cancers are one of the most serious oncological diseases, for which novel treatment options are urgently needed. TRAIL is a cytokine that is involved in natural tumour surveillance mechanisms and as recombinant protein has been shown to exert specific anti-tumour effects by induction of apoptosis in cancer cells [
1‐
5]. Apoptosis is triggered after binding of TRAIL to one of its two receptors, TRAIL-receptor 1 (TRAIL-R1) or TRAIL-receptor 2 (TRAIL-R2), also known as DR4 and DR5, respectively [
6‐
8]. Binding of TRAIL to these two receptors stimulates the formation of a protein complex called the death-inducing signaling complex (DISC). It consists of TRAIL-R1 and/or TRAIL-R2, the adaptor protein Fas-associated death domain (FADD) and procaspase-8. At the DISC, caspase-8 is activated by a mechanism that involves dimerisation and proteolytic cleavage [
9,
10]. Active caspase-8 can then, either directly, or indirectly via the BH3-only protein Bid, activate effector caspases, such as caspase-3, which in turn cleave many cellular substrates resulting in the biochemical and morphological features characteristic of apoptosis [
11]. Aside from the two death domain (DD)-containing, apoptosis-inducing receptors, TRAIL-R1 and TRAIL-R2, three additional decoy receptors exist, TRAIL-R3 (DcR1), TRAIL-R4 (DcR2) and Osteoprogerin (OPG) [
6,
7,
12‐
14]. These decoy receptors can inhibit the apoptosis-inducing function of TRAIL [
15]. To address this issue, agonistic antibodies against either TRAIL-R1 or TRAIL-R2 have been developed and have been tested in pre-clinical and as well as clinical studies [
16‐
21].
In addition, engineered variants of TRAIL, containing specific amino acid changes leading to specific targeting of TRAIL-R1 or TRAIL-R2 have been designed and have shown improved anti-tumour effects
in-vitro and
in-vivo when compared to wild-type TRAIL [
22‐
27]. Such TRAIL-receptor variants have been studied in the context of various specific cancer types as well as in the context of combination treatments [
28‐
32]. TRAIL variants might hold important advantages over TRAIL-receptor specific antibodies as they are smaller than antibodies and might therefore be better able to reach and infiltrate growing tumours. In addition, such proteins can be further optimised to increase activity, specificity and stability and they can be used as part of gene and cell therapeutic approaches [
31,
33‐
38]. This way of potentially improving the therapeutic efficacy of TRAIL by using TRAIL-receptor specific agents is of particular interest for pancreatic cancer, as previous studies have shown that pancreatic tumour cells preferentially use TRAIL-R1 to execute TRAIL-induced apoptosis [
39,
40]. Thus, agonistic TRAIL-R1 specific antibodies or TRAIL-R1 targeting variants of TRAIL were regarded as having a higher therapeutic potential than normal TRAIL in the treatment of pancreatic carcinoma.
We wondered, given the molecular heterogeneity of tumours, how such a uniform TRAIL response with respect to receptor preferences could be possible. Therefore, we set out to examine an array of pancreatic cancer cells for their TRAIL-receptor preferences. We found that a number of pancreatic cancer cells used TRAIL-R2 rather than TRAIL-R1 to initiate apoptosis signalling. These results demonstrate that, while TRAIL-receptor specific variants constitute a potentially substantial improvement to conventional TRAIL therapies, generalised predictions according to cancer type are difficult. Therefore, additional research is needed to identify factors that determine the optimal TRAIL variant (or antibody) on a case-by-case basis for each individual tumour.
Methods
Reagents and cell culture
All reagents were purchased from Sigma (St. Louis, MO) unless otherwise stated. The human pancreatic cancer cell lines Panc1 and PancTu1, the human embryonic kidney cell line HEK293, the human colon cancer cell line Colo205 and the human cervix carcinoma cell line HeLa were maintained in Dulbecco’s modified Eagle’s medium (DMEM). The human pancreatic cancer cell lines AsPC-1, BxPC-3 and Colo357 were cultured in RPMI-1640 medium. The human colorectal cancer cell line HCT116 was cultured in McCoy’s medium and the human prostate cancer PC-3 cells were grown in Ham’s F12 medium. All media were supplemented with 10 % FBS, 100 U/ml penicillin and 100 μg/ml streptomycin. Cells were cultured in a humidified incubator at 37 °C and 5 % CO2.
Generation of sTRAIL constructs
Generation of sTRAIL constructs and site-directed mutagenesis have been previously described [
31]. Briefly, the soluble portion of human TRAIL (amino acids 114–281) was first subcloned into the NheI/NotI sites of a pcDNA3 plasmid (Invitrogen) giving rise to pcDNA3.sTRAIL. Then an exogenous signal peptide sequence of the human fibrillin protein, the Furin cleavage site (Furin CS) and Isoleucine-zipper sequence (ILZ) cassette was cloned into the BamHI/NheI sites of the pcDNA3.sTRAIL vector. The resulting vector was termed sTRAIL
wt. The two sTRAIL
DR5 and three sTRAIL
DR4 constructs were generated using the Quick-Change site-directed mutagenesis kit (Stratagene, La Jolla, CA) and confirmed by DNA sequencing.
TRAIL Enzyme-linked Immunosorbent Assay (ELISA)
TRAIL concentrations were measured by a human TRAIL/TNFSF10 Quantikine ELISA Kit as recommended by the manufacturer (R&D Systems, Minneapolis, MN). Before the measurement the medium supernatants were pipetted off the respective HEK293 producer cells and then centrifuged to clear them of any cellular debris.
TRAIL receptor surface stain
For the TRAIL receptor stain we used monoclonal anti-TRAIL-R1 (DJR1) and anti-TRAIL-R2 (DJR2-4) antibodies (1 μg/10
6 cells; BioLegend, San Diego, CA) that were conjugated to Phycoerythrin (PE). The isotype control antibody (MOPC-21) (1 μg/10
6 cells) was also purchased from BioLegend. The surface expression of TRAIL receptors was measured by incubating cells with the PE-conjugated mouse anti-human TRAIL-R1 and mouse anti-human TRAIL-R2 antibodies as described previously [
41].
Transfection of HEK293 cells
HEK293 cells were transfected using the Calcium-phosphate method. Briefly, before transfection, fresh 2 % FBS containing medium was added to the cells. For each well of a 6-well plate, 0.5 ml HBS were aliquoted into a sterile 1.5 ml Eppendorf tube. In a separate tube 5 μg of plasmid DNA were mixed with 250 μl CaCl2 (2.5 mM) and sterile water added to 0.5 ml. The CaCl2/DNA mix was then added to the HBS in a drop-wise fashion and constant vortexing at slow speed. After 45 min of incubation at room temperature, the mixture was slowly added to the cells. After 4 h, the medium was removed and the cells were washed with PBS and fresh growth medium added.
Apoptosis assay
Apoptosis was measured according to Nicoletti et al. (DNA hypoploidy assay) and has been described previously [
42,
43]. Trypsinised cells including the supernatant medium and PBS wash-solution were directly transferred into FACS tubes and centrifuged at 1,300 rpm for 7 min at 4 °C. After washing the cell pellet with PBS, Nicoletti buffer (Sodium citrate 0.1 % (w/v) supplemented with 0.1 % Triton X-100 (w/v) and propidium iodide at 50 μg/ml) was added. Then the tubes were vortexed for 10 s at medium speed and left for 5 h in a refrigerator. The fluorescence intensity was then measured by flow cytometry and analysed using the Venturi One software package (Applied Cytometry, Sheffield, UK). Where specified, untreated cells were taken as reference to calculate specific apoptosis by subtraction of the basal cell death values from the apoptosis levels of treated cells.
RNAi knock-down constructs and stable cell line generation
The following small hairpin (sh) RNA motifs were used to silence: DR5 (5′-GCTAGAAGGTAATGCAGACTCTGCCATGTC -3’), DR4 (5′-GCTGTTCTTTGACAAGTTGC-3’) and XIAP (5′-GTGGTAGTCCTGTTTCAGC-3’). Sense and antisense oligos containing the sh-sequence and a 5’ overhang representing a restricted BbsI site and EcoRI site on the 3’ side were hybridised to generate double-stranded DNA fragments. These fragments were then cloned into a BbsI/EcoRI opened up pU6.ENTR plasmid (Life Technologies, Carlsbad, CA). The resulting pU6.ENTR plasmids (pU6.ENTR.shDR5, pU6.ENTR.shDR4, pU6.ENTR.shXIAP) were used to generate the pBlock-iT.shDR5, pBlock-iT.shDR4 and pBlock-iT.shXIAP plasmids using the pBLOCK-iT6-DEST vector (Life Technologies) and LR Clonase II. This was used to generate stable DR5 and DR4 knock-down clones of HCT116 cells and stable XIAP knock-down clones of PancTu1 and Panc1 cells. For this, the pBlock-iT.shDR5, pBlock-iT.shDR4 and pBlock-iT.shXIAP plasmids were FuGeneHD-transfected (Roche, Basle, Switzerland) into HCT116, PancTu1 and Panc1 cells, respectively. Three days later, the transfected cells were split into Blasticidin containing selection medium. Clones were then picked, transferred to 24 well-plates and analysed for DR5, DR4 and XIAP knock-down, respectively. Clones that did not show a knock-down were used as controls and labelled PancTu1.shctrl and Panc1.shctrl, respectively. These control clones were tested and shown to behave like parental cells.
Statistical analysis
Experimental values are expressed as mean value ± standard error (SEM). For significance analyses, analysis of variance (ANOVA) between groups was used and P < 0.05 (*) was considered significant.
Discussion
Initially it was thought that TRAIL-R2 is the main apoptosis-inducing receptor for the death ligand TRAIL [
27]. This led to the development and testing of agonistic antibodies against this receptor as potential anti-cancer agents [
16,
18,
46,
47]. However, more recently reports showed that TRAIL-R1 has a more prominent role, than first thought, in specific types of cancer such as lymphoid malignancies [
29] and leukaemic cells [
30,
48]. Additionally, it was suggested that pancreatic cancer cells also trigger TRAIL-induced apoptosis mainly through TRAIL-R1 [
39,
40]. However, when we analysed a wider array of pancreatic cancer cell lines we found that 2 out of 3 pancreatic cancer cells preferred the TRAIL-R2 pathway in response to TRAIL. In addition, Panc1 cells also showed higher apoptosis levels when treated with sTRAIL
DR5 and XIAP was silenced concomitantly (Table
1).
Table 1
TRAIL-R preference of different cancer cell types
HCT116 | colorectal carcinoma | DR5 |
Colo205 | colorectal carcinoma | DR5 |
HeLa | cervical carcinoma | DR4 |
Colo357 | pancreatic carcinoma | DR4 |
BxPC-3 | pancreatic carcinoma | DR5 |
AsPC-1 | pancreatic carcinoma | DR5 |
Panc1 | pancreatic carcinoma | DR5 |
PancTu1 | pancreatic carcinoma | DR4 |
While these results appear to contrast the two afore mentioned publications [
39,
40], it is important to point out that we used, at least in part, different cell lines and sTRAIL variant proteins instead of agonistic antibodies. Interestingly, the results in one of the reports indicate that both TRAIL-R1 and TRAIL-R2 agonistic antibodies can trigger apoptosis in pancreatic cells and that the TRAIL-R1 preference was only detected when one of the two receptors was inhibited by blocking antibodies followed by treatment with TRAIL [
39]. In contrast, the second study found clear differences between the apoptosis-inducing activities of the two agonistic antibodies, with a clear preference for TRAIL-R1. It is therefore possible that sTRAIL variant proteins and TRAIL-receptor specific antibodies have distinct effects owing to their different modes of action with regard to their receptor engagement. Notwithstanding, the notion that pancreatic cancer cells and possibly other tumour types have a general TRAIL receptor preference needs to be re-visited, re-examined and possibly refined. Furthermore, we tested whether the expression profile of TRAIL-R1 and TRAIL-R2 could determine receptor preference, but failed to observe any clear correlation. These findings are generally in line with results reported earlier [
39]. Thus, other factors and mechanisms than surface expression levels of the TRAIL-receptors must determine their apoptosis-inducing function.
Potential molecular mechanisms that could determine whether a receptor can be activated are O-glycosylation of both receptors [
49] as well as S-palmitoylation, S-nitrosylation, N-glycosylation and ubiquitination of TRAIL-R1 [
50‐
53]. Thus, despite being present on the cell surface a receptor might be relatively inactive, making it impossible to determine receptor preferences based solely on expression levels.
An area where specific TRAIL variants and/or agonistic antibodies can be used with good predictability is in combination treatments, in which up-regulation of either TRAIL-R1 or TRAIL-R2 can be targeted by the respective variant. For example, pre-treatment with the anti-cancer drug doxorubicin gave rise to significantly increased cell death when treated with the agonistic TRAIL-R2 antibody lexatumumab [
54]. In addition, combined treatment of colorectal tumours with lexatumumab and radiotherapy had similar sensitising effects [
55]. Soluble TRAIL
DR5 also showed better apoptosis inducing effects after priming with 5-Fluorouracil as compared to sTRAIL
wt or sTRAIL
DR4, because the drug caused p53-independent upregulation of TRAIL-R2 [
31]. In contrast, HDAC inhibition has been shown to result in sensitisation to TRAIL-R1 specific apoptosis [
48,
56]. Of note in this context is that the individual activation of TRAIL-R1 and -R2 could be an advantage, since it was shown that combined exposure to DR4- and DR5-selective TRAIL variants in cells, sensitive for both receptors, was more potent in triggering apoptosis when compared to single agent treatment [
22]. Other factors that can influence TRAIL receptor preferences are so called non-canonical pathways including the activation of NF-κB, p38 and JNK [
57]. The issue with these pathways is that they have been reported to have opposing effects and different apoptosis factor requirements depending on cell type and cellular context [
57]. For example, TRAIL-induced JNK activation has been reported to be caspase-dependent in HeLa human cervical cancer cells, but caspase-independent in the human rhabdomyosarcoma Kym-1 cell line [
58]. These findings illustrate that the TRAIL receptors have varying, cell type-specific and in parts receptor specific capabilities to recruit different signalling complexes to their intracellular domain. These complexes and their individual constituents might have an impact on the apoptosis-inducing function of the receptors and thereby may contribute to TRAIL-receptor preferences in TRAIL-triggered cell death.
Consequently, further research is needed to better understand potential differences between TRAIL agonistic antibodies and recombinant TRAIL proteins and variants. Additionally, it is important to elucidate the molecular components that determine TRAIL-receptor preferences in order to be able to select the best TRAIL agents to potentially treat pancreatic cancer and other tumour types in the future.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AM and RY designed the study; performed experiments; analysed and interpreted data; wrote the manuscript. RMZ conceived and designed this study; analysed and interpreted data; wrote the manuscript. All authors read and approved the final manuscript.