Background
Malignant pleural mesothelioma (MPM) is a generally fatal thoracic neoplasia that arises from the pleural lining. In the majority of the patients, a history of occupational exposure to asbestos can be elicited [
1]. Taking into account a latency period of 20–50 years and a decline in workplace exposure to asbestos in Europe since the 1970s, it is estimated that the number of men dying from MPM in Europe will double each year until a peak is reached in about between 2015 and 2020 [
2,
3].
No chemotherapy regimen for mesothelioma has proven curative, although several treatments are valuable for palliation. The clinically best documented chemotherapy is a combination of cisplatin with an antifolate. A large phase III study comparing the combination of cisplatin and pemetrexed with cisplatin alone demonstrated a superior response, survival and a better quality of life for the combination [
4,
5]. For earlier stages of disease, specialized centers offer multimodality therapy with adjuvant or neoadjuvant chemotherapy, radical surgery with or without radiotherapy [
6]. However, despite such aggressive treatment most patients have disease recurrence within 2 years. Therefore, new therapeutic options are needed for more effective treatment of this malignancy. As demonstrated by our
in vitro investigations, the combination of cisplatin-based chemotherapy with agonistic TRAIL receptor antibodies might be a promising option.
Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a type II transmembrane protein belonging to the TNF family of death ligands. Four TRAIL receptors have been identified of which two, TRAIL-R1/DR4 and TRAIL-R2/DR5, are capable of transducing an apoptotic signal whereas the other two receptors (TRAIL-R3/DcR1, TRAIL-R4/DcR2) act as antagonists since they lack death domains and thus cannot engage the apoptotic machinery [
7,
8]. An additional receptor, osteoprotegrin, has been identified but its activity is still matter of debate because of its low affinity for TRAIL at 37°C [
9]. TRAIL can preferentially induce apoptosis in a variety of tumor cell types, whereas normal cells do not appear to be sensitive [
10]. This property suggests TRAIL-R targeting is an excellent strategy for selective cancer therapy and oncology trials with TRAIL and TRAIL-R human agonistic antibodies have been initiated [
11,
12].
Apoptosis-inducing mechanisms by human agonistic TRAIL-R antibodies Mapatumumab and Lexatumumab are thought to be similar to TRAIL-mediated apoptosis [
13]. TRAIL-induced cell death is triggered by the interaction of the ligand with TRAIL-R1 or TRAIL-R2 to assemble the death-inducing signaling complex. The latter forms when death receptor ligation triggers association of the intracellular adaptor, Fas-associated death domain (FADD) with the cytoplasmic tail of the receptor. FADD then recruits procaspase-8, which undergoes spontaneous autoactivation. Activated caspase-8, in turn, cleaves and activates the effector caspases-3, -6 and -7 which cleave cellular substrates to execute cell death [
7,
8]. Recent data suggest the existence of considerable cross-talk between the extrinsic and intrinsic death signalling pathways. Caspase-8, a key player of this communication platform, can proteolytically activate the BH3 only family member Bid, which induces Bax- and Bak-mediated release of cytochrome c and Smac/DIABLO from mitochondria [
14]. Resistance to TRAIL can occur by different mechanisms, including lack of TRAIL apoptosis receptors, death receptor mutations [
15], and enhanced expression of TRAIL-decoy receptors [
16]. FLIP, which bears structural similarity to caspase-8, but lacks caspase-8 activity, can inhibit death receptor-mediated signalling by binding to FADD [
17]. Both forms of FLIP, the long form c-FLIP
L and the short form c-FLIP
S can compete for apical caspase recruitment to the DISC, whereas FLIP
L can also inhibit the full processing of caspase-8 [
18].
MPM cells have been found by others to be resistant or to have a low susceptibility to TRAIL-induced apoptosis, and require either FLIP
L siRNA, chemotherapeutic drugs, α-tocopheryl succinate or cycloheximide to be combined with TRAIL for apoptosis to occur [
19‐
22]. However, these studies were performed with a small number of established human MPM cell lines only and it remains unknown whether the majority of MPM cell lines and primary cultures are indeed resistant to TRAIL combined with chemotherapy. In addition, no information exists on the sensitivity of MPM cells to two fully human agonistic monoclonal antibodies which target TRAIL-R1 (Mapatumumab) and TRAIL-R2 (Lexatumumab) although they have the advantage over TRAIL of a longer plasma half-life and a higher specificity [
23].
In the present study, we compared the sensitivity of 13 MPM cell lines or primary cultures to TRAIL and to two fully human agonistic monoclonal antibodies which target TRAIL-R1 and TRAIL-R2, and examined the apoptosis sensitization of the MPM cell lines with different sensitivity to Mapatumumab or Lexatumumab by the cytotoxic drug cisplatin.
Discussion
With TRAIL and TRAIL-receptor agonistic antibodies entering clinical trials for the treatment of patients with cancer [
11], the question has arisen whether molecular markers can be identified, which would allow to select patients benefiting from such therapy.
Our results indicate that the majority of MPM cell lines (nine of thirteen) are sensitive to TRAIL. This is in contrast to previous studies reporting on a small number of cell lines only [
19‐
22,
34]. The potential advantage of targeting TRAIL pathway via agonistic antibodies for its receptors are the longer half-life compared to TRAIL and the higher specificity [
23]. In agreement with a previous report [
20], we have observed that MPM cells express membrane TRAIL-R1 and TRAIL-R2, with higher TRAIL-R2 expression. Studies based on receptor-blocking antibodies indicate that TRAIL can induce apoptosis through either TRAIL-R1 or -R2 or both receptors, but the relative contribution of each death receptor to apoptosis induction in cells expressing both receptors is unknown [
10]. TRAIL-R2 was shown to contribute more that TRAIL-R1 to TRAIL-induced apoptosis in cells that express both death receptors [
35]. However, a recent study [
28] has shown that chronic lymphocytic leukaemia cells signal apoptosis exclusively via TRAIL-R1 despite surface expression of TRAIL-R2. Moreover, the apoptosis-inducing ability of different TRAIL preparations in various cells demonstrated an unanticipated preferential signalling via either TRAIL-R1 or -R2 [
28]. We found that the majority of MPM cell lines were more sensitive to Lexatumumab (46%) than to Mapatumumab, while a minority (15%) were more sensitive to Mapatumumab than to Lexatumumab. The activity of both antibodies was similar in 2 cell lines (15%) and only few MPM cell lines were resistant to both (23%). Taken together, these results suggest that there is a permissive environment for preferential signalling via TRAIL-R2 to death-receptor mediated apoptosis in MPM cell lines expressing both receptors.
Comparison of TRAIL receptors expression levels and TRAIL sensitivity of the MPM cell lines used in this study did not reveal any consistent pattern, suggesting that TRAIL sensitivity is not dependent on TRAIL-receptor expression levels, thus indicating that other intracellular mechanisms control TRAIL signal transduction in resistant cells. TRAIL sensitivity can be regulated by anti-apoptotic proteins such as Bcl-2, Bcl-XL or FLIP [
20]. We have shown in previous studies that Bcl-2 or Bcl-XL are abundantly expressed in MPM and it has already been demonstrated in MPM cells that downregulation of Bcl-XL is associated to sensitization to TRAIL apotosis [
36]. This indicates that contrarily to what has been described in melanoma cell lines [
37], expression level of TRAIL death receptors is not sufficient to identify MPM patients who may respond to TRAIL or to TRAIL-receptor agonistic antibodies.
No chemotherapy regimen for MPM has proven curative [
5,
6,
38,
39]. Therefore, new therapeutic options for the treatment of this malignancy need to be investigated. There is accumulating evidence indicating a synergism between anti-death receptor pathway and chemotherapy in the induction of apoptosis, although the synergistic mechanisms are not fully understood [
40,
41]. MPM cells have been found by others to be resistant to TRAIL-induced apoptosis, and require either chemotherapeutic drugs or cycloheximide to be combined with TRAIL for apoptosis to occur [
19‐
21]. Our studies show that cisplatin synergistically enhances Mapatumumab- or Lexatumumab-mediated apoptosis in a caspase-dependent fashion and is also effective at promoting apoptosis when used in combination with either Mapatumumab or Lexatumumab in MPM tumor cells that are resistant to cisplatin, Mapatumumab or Lexatumumab single-agent therapy. We observed a high heterogeneity in the response of cell lines and primary cultures to treatment with TRAIL, Mapatumumab, Lexatumumab or cisplatin or a combination thereof. Cytotoxic chemotherapeutic drugs sensitize cultured cancer cells to TRAIL by different mechanisms including up-regulation of the receptors [
42], enhanced death-inducing signalling complex formation or alteration of the expression of pro-apoptotic/anti-apoptotic proteins [
31,
43,
44]. In previous studies, we showed that the expression of the anti-apoptotic proteins Bcl-XL and Bcl2 is highly variable in several MPM cell lines. This is consistent with data observed in a recent study using a large panel of MPM cell lines and tumors where highly variable expression levels of five inhibitor of apoptosis proteins, including survivin were found [
45]. It is therefore likely that the observed differences in cell survival in cell lines and primary cultures upon treatment with cisplatin and TRAIL receptor agonistic antibodies are due to variation in basal expression levels of anti-apoptotic proteins.
The synergistic cytotoxicity between cisplatin and Mapatumumab or Lexatumumab is associated with an increase of caspase-mediated apoptosis. Indeed, the combination of cisplatin with Mapatumumab or Lexatumumab synergistically enhanced caspase-8 and Bid activation in MPM cells sensitive to antibody treatment and caspase-3 activation in all cells treated with a combination of cisplatin and Mapatumumab or Lexatumumab. In a previous study we have demonstrated that p53 is functional in MPM cells and that it negatively regulates the anti-apoptotic protein survivin [
24,
32]. Combination of cisplatin with Mapatumumab or Lexatumumab further increased the expression of p53 transcriptional targets Bax and decreased survivin, compared to the treatment with either agent alone. Similarly, decrease in anti-apoptotic Mcl-1 expression was observed upon exposing MPM cells to the combination of cisplatin with Mapatumumab or Lexatumumab, confirming the significant role of these proteins in the enhancement of death receptor-mediated apoptosis by cisplatin in MPM cells. Based on the scavenging effect of the antioxidant antioxidant NAC on cell death induced either by cisplatin and the combination of Mapatumumab and cisplatin, we infer that the molecular mechanism responsible for the synergism is linked to the production of reactive oxygen species, which act as positive regulator of apoptosis. The role of cisplatin-induced oxidative stress in the enhancement of the efficiency of TNF family members has already been described in MPM cell lines after treatment with cisplatin, FasL or the combination thereof [
33]. Our findings are also in agreement with a study showing that the generation of ROS sensitizes colon cancer cells to death-inducing ligand TRAIL [
46] and with a study showing that ROS generation by Sulforaphane is pivotal for the sensitization of hepatoma cells to TRAIL-induced apoptosis [
47].
Contrary to what has been observed in squamous cell carcinoma and the ligand TRAIL [
31], we have observed that pre-treatment with cisplatin followed by treatment with Mapatumumab or Lexatumumab resulted in significantly higher cytotoxic effects in MPM cell lines than when the sequence was reversed. The reasons for such a difference are not clear and can include different kinetics of apoptotic pathways in different cell lines [
48] and/or dosage.
Methods
Cell culture and reagents
The MPM cell lines SCP212, ZL34,, SPC111, ZL5, ZL55 and the primary cultures SDM4 SDM6 and SDM13 were generated in our laboratory and have been described previously [
24,
25]. The MPM cell lines H2052, H226, H2452, H28 and MST0-211H were obtained from ATCC (LGC Promochem Sarl, France). All cells were maintained in RPMI 1640 (Sigma, St. Louis, MO, USA) supplemented with 2 mM L-glutamine, 1 mM sodium pyruvate, 10% FBS and 1% (w/v) penicillin/streptomycin. Jurkat cells were obtained from ATCC and maintained in RPMI 1640 medium (Sigma) supplemented with 5% FBS (Invitrogen, Paisley, UK), 15 mM HEPES, 2 mM L-glutamine, 50 μM β-mercaptoethanol and 1% (w/v) penicillin/streptomycin (Invitrogen). All cells were grown at 37°C in a humidified atmosphere containing 5% CO
2.
Reagents
Recombinant human polyhistidine-tagged TRAIL (His-TRAIL) and antibodies to TRAIL-R1, TRAIL-R2, TRAIL-R3 and TRAIL-R4 were purchased from Alexis (Lausen, Switzerland). The agonistic monoclonal antibodies against TRAIL-R1 (Mapatumumab) and TRAIL-R2 (Lexatumumab) were provided by Human Genome Sciences (Rockville, MD, USA). The following antibodies were purchased: caspase-8 (Alexis, Lausen, Switzerland), Bid (R&D Systems, Minneapolis, MN, USA), Mcl-1 (Santa Cruz Biotechnology, Santa Cruz, CA, USA), ICAD (Santa Cruz Biotechnology, Santa Cruz, CA, USA), caspase-3 (BD PharMingen, San Diego, CA, USA), Bax (Santa Cruz Biotechnology, Santa Cruz, CA, USA), survivin (R&D Systems, Minneapolis, MN, USA), and actin (ICN Biomedicals, Irvine, CA, USA). Where indicated, either cisplatin (Bristol-Myers Squibb AG, Baar, Switzerland) and/or N-acetyl-L-cysteine (ALEXIS Corporation, Lausen, Switzerland) were added.
Measurement of cell growth
Cell growth was determined using colorimetric cell viability assay based on the reduction of tetrazolium salt MTT, as described [
49]. Cells were plated in quadruplicate in 96-well plates (7500 cells/well) and absorbance was measured using a SPECTRAmax 340 microplate reader. Cell growth was calculated as a percentage of the absorbance signal obtained with wells of untreated (viable) cells kept under identical conditions. Dose curve plots, IC50 and Combination Index (CI) were calculated by using CalcuSyn software from BIOSOFT.
Immunoblotting
Cells were lysed for 30 min with 1× RIPA buffer (Upstate) containing 0.1% SDS, 1 mM PMSF and complete protease inhibitor cocktail (ROCHE). Lysates were clarified by centrifugation (10,000
g for 30 min at 4°C) and protein concentrations were determined using BCA (Pierce/Perbio Science S.A., Lausanne, Switzerland). After SDS/PAGE separation, the protein was transferred to nitrocellulose membrane and immunoblotting was performed as described [
50] using the specific antibodies mentioned above.
Flow cytometric analysis
Immunostaining of intact cells was performed as described previously. Surface expression of TRAIL receptors was evaluated by indirect immunostaining using the primary antibodies mentioned above followed by PE-conjugated anti-mouse secondary antibodies (Alexis Biochemicals). Nonspecific fluorescence was assessed using normal mouse immunoglobulin G (IgG) followed by secondary antibodies. Flow cytometric analyses were performed using a FACSCalibur (FACScan, BD Biosciences, San Jose, CA, USA).
Statistical analysis
Data are presented as the mean ± SE of at least three independent experiments. Statistical differences were assessed using two-sided unpaired Student's t test and P values < 0.05 were considered significant.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
LLB and TMM carried out the molecular biology studies and participated in the interpretation of the data and drafted the manuscript. SK carried out the survival- and immunoassays. SHD and EFB were involved in drafting the manuscript and revising it critically for important intellectual content. RAS was involved in drafting the manuscript, revising it critically for important intellectual content and has given final approval of the version to be published. All authors read and approved the final manuscript.