A MCP1 fusokine with CCR2-specific tumoricidal activity
© Rafei et al; licensee BioMed Central Ltd. 2011
Received: 20 February 2011
Accepted: 24 September 2011
Published: 24 September 2011
The CCL2 chemokine is involved in promoting cancer angiogenesis, proliferation and metastasis by malignancies that express CCR2 receptor. Thus the CCL2/CCR2 axis is an attractive molecular target for anticancer drug development.
We have generated a novel fusion protein using GMCSF and an N-terminal truncated version of MCP1/CCL2 (6-76) [hereafter GMME1] and investigated its utility as a CCR2-specific tumoricidal agent.
We found that distinct to full length CCL2 or its N-truncated derivative (CCL2 5-76), GMME1 bound to CCR2 on mouse lymphoma EG7, human multiple myeloma cell line U266, or murine and human medulloblastoma cell lines, and led to their death by apoptosis. We demonstrated that GMME1 specifically blocked CCR2-associated STAT3 phosphorylation and up-regulated pro-apoptotic BAX. Furthermore, GMME1 significantly inhibited EG7 tumor growth in C57BL/6 mice, and induced apoptosis of primary myeloma cells from patients.
Our data demonstrate that GMME1 is a fusokine with a potent, CCR2 receptor-mediated pro-apoptotic effect on tumor cells and could be exploited as a novel biological therapy for CCR2+ malignancies including lymphoid and central nervous system malignancies.
CC Chemokines and their cognate receptors are involved in the proliferation and metastasis of several tumors . The CCL2/CCR2 axis is a direct example as highlighted by CCL2-driven proliferation and survival of hematological [2, 3] and solid tumors [4, 5]. Thus, inhibiting CCL2 or its receptor may allow a direct interference with tumor biology.
As an alternative to the development of neutralizing or antagonizing antibodies, our group has focussed on the engineering of bifunctional proteins borne from the fusion of two biologically distinct cytokines [6–12]. These fusokines have been shown to lead to novel unheralded pharmacological effects including potent, receptor-specific antitumor effects [6, 11]. Interestingly, granulocyte macrophage-colony stimulating factor (GMCSF)-based fusokines may either lead to pro-inflammatory synergy or profoundly antagonistic properties depending on the influence played by the GMCSF moiety of the fusokine on the C-terminal partner signalling pathway.
We here report its use as a cancer therapeutic agent targeting tumors expressing CCR2 and provide pre-clinical evidence that this pro-apoptotic fusokine could be of great interest as a lead compound of a new class of biological agents targeting CCR2-expressing malignancies.
Mice, cell lines and reagents
All mice used were 6-8 week old C57Bl/6 females (Jackson Laboratory, Bar Harbor, ME). All experimental protocols were approved by the animal ethic committee of the Lady Davis Institute of McGill University. The mouse T-cell lymphoma cell line EG.7 (EL4 cells transfected to express chicken ovalbumin), human medulloblastoma cell line Daoy, and the human myeloma cell line U266 were purchased from ATCC (VA, USA) and propagated according to manufacturer's instructions. The murine medulloblastoma cell line PS125 was established from medulloblastomas derived from the Smo/Smo transgenic mouse  and propagated in DMEM/F-12 supplemented with 10% FBS, 1% L-glutamine, 1% MEM non-essential amino acids, and 1% N-2 at 37°C in 5% CO2. Antibodies for CD19, CD44, CD45, CD73, CD105 and CD138 were purchased from BD Biosciences (San Diego, CA). Mouse recombinant CCL2 protein (CCL2 1-76), ELISA kits for mouse CCL2 and human IL6, anti-human CCR2 antibody, CCR2 primers, and Annexin-V/PI detection kits were purchased from R&D systems (Minneapolis, USA). Antibody for α-tubulin was purchased from Santa Cruz Biotechnology (Santa Cruz, CA). Antibodies for BAX, pSTAT3 and total STAT3 were purchased from Cell Signalling Technology (Danvers, MA). RNA extraction kit was purchased from Qiagen (Mississauga, ON, CANADA). Contigen was purchased from Bard Urological Division (Covington, GA, USA). The 5-76 variant (CCL2 5-76) of murine CCL2 was synthesized from Genecust (Dudelange, Luxembourg). The monocyte enrichment kit was purchased from StemCell Technologies (Vancouver, Canada).
Isolation and characterization of mouse mesenchymal stromal cells
Whole bone marrow was harvested by flushing femurs and tibias bones of female CCL2-/- C57BL/6 mice with DMEM. Collected cells were plated and cultured until the appearance of a homogeneous polyclonal population of mesenchymal stromal cells (MSC). The plasticity of isolated MSCs was tested as previously reported . For CCR2 expression on MSC, RT-PCR was performed on extracted RNA using purchased primers.
Engineering CCL2-/-MSC to express GMME1
We have previously demonstrated that wild-type MSCs could generate in a paracrine fashion truncated CCL2 (5-76) capable of antagonizing CCR2-expressing cells . Therefore, CCL2-/-MSCs were used in this study to avoid any confounding effects arising from endogenous MSC production of CCL2 and derivatives. The generation and concentration of green fluorescent protein (GFP) or GMME1 retroparticles using the bicistronic AP2 vector were generated as previously reported . The level of GMME1 expression was analyzed through the assessment of GFP by flow cytometry, while GMME1 secretion level in the harvested DMEM conditioned medium was quantified using a CCL2 ELISA kit. Alternatively, GMME1 protein was purified from the conditioned medium with an affinity column (Pierce Biotechnology, Rockford, USA, Kit # 26148) loaded with anti-mouse GM-CSF antibodies (R&D, Minneapolis, USA) by following the instruction described in the kit. The purified GMME1 protein was dialyzed with fresh DMEM medium, and concentrated for use.
To test the proliferative property of GMME1, the mouse lymphoma EG7 or human multiple myeloma U266 cell lines were plated at a density of 105 cells/well in a 96-well plate and treated with increasing concentrations of cytokines for 48 hours. The reaction was read at 570 nm after adding 20 μL of 3-(4,5-dimethylhiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). For Apoptosis analysis, the mouse EG7 or human U266 cell lines were cultured for 48 hrs with equimolar concentrations (1.5 pmol) of CCL2 (1-76), CCL2 (5-76), or GMME1 then analyzed by PI/Annexin-V. WT and CCR2-/- monocytes were enriched to 90% purity using negative selection following the bone marrow flush of femurs and tibias. Purified cells were then cultured for 48 hrs in control or GMME1 supernatant. A cell-killing assay was also performed on two medulloblastoma cell lines PS125 (mouse-derived) and Daoy (human-derived), treated with or without GMME1 for 48 hrs and cell death measured by flow-cytometry using PI and Annexin-V. Alternatively, Daoy cells were also treated with GMME1 in conditioned medium or affinity-purified GMME1 protein, and the cell growth was assessed by MTT assay. Western blot was performed on the lysate derived from treated cell lines probed with anti-BAX antibodies, or anti-pSTAT3 or anti-STAT3 antibodies. IL-6 secretion by U266 was quantified with ELISA, following the different cytokine treatments. For signalling analysis, a sandwich ELISA for mouse/human STAT3 was performed.
Cancer induction and treatments
To study the locoregional effect of GMME1 on tumor development, 2 × 106 MSC-GFP were co-implanted with 106 EG7 cells subcutaneously (sc) in immunocompetent C57Bl/6. For systemic efficacy of the fusokine, 106 EG7 cells were injected sc in immunocompetent C57Bl/6 mice on one side, and an sc implant of contigen-embedded gene-engineered MSCs (2 × 106 cells per implant) was injected on the opposite flank as previously described . Tumor appearance and volume were assessed every 48 hrs. To investigate the levels of circulating GMME1 in treated mice, the sera were collected at week 3 post-implantation of the neo-organoid and screened by CCL2 ELISA to detect the CCL2 moiety of the fusokine according to manufacturer's instructions.
GMME1-induced apoptosis of primary myeloma cells from patients
Bone marrow aspirates from consenting myeloma patients (n = 5) were processed as previously described . The IRB protocol was approved by Emory University. White blood cells in the marrow were isolated with lymphocyte separation medium (Mediatech Inc., Manassas, VA), and stained with anti-human CD38 (PE), CD45 (APC-Cy7), CD138 (FITC) (BD Bioscience, San Jose, CA), or CCR2 (PerCP) (Biolegend, San Diego, CA) antibodies or isotype controls (1:100 dilution). CD38+CD45-CD138+ cells were considered as myeloma cells . Alternatively, white blood cells (106 cells/ml) were cultured in RPMI 1640 medium with 10% FBS in presence of GMME1 (20 ng/ml). Condition medium without GMME1 served as control. After 48-hour culture, the cells were harvested and stained with anti-human CD38, CD45 antibodies, and Annexin. FACS analysis was performed on a BD FACSCanto II flow cytometer. Data were analyzed with FlowJo 9.1 software.
P values were calculated using the ANOVA test, or paired t-test in Figure Six F.
Gene engineering of MSC as GMME1 biofactories
GMME1 is a fusion cytokine consisting of granulocyte macrophage colony stimulating factor (GM CSF) in tandem with N-terminal truncated monocyte chemotactic protein-1 (M CP1 6-76) whose design and biochemical analysis were previously reported and summarized in Figure 1A-B. We have also previously demonstrated that gene- modified MSC can serve as robust cytokine biofactories in vitro and in vivo[10, 11, 14, 17–21]. We here sought to exploit this platform to produce recombinant GMME1 in vivo. To obviate any confounding effect arising from naturally produced CCL2 from MSC, MSC isolated from CCL2-/-C57Bl/6 mice were engineered a polyclonal population to produce secreted GMME1 as we previously published [10, 11]. MSC expressing GMME1 are CD44, CD73 and CD105 positive with no detectable CD45 (Figure 1C) or CCR2 (Figure 1D) while retaining their capacity to differentiate into adipocytes or osteoblasts (Figure 1E). Supernatant analysis of these cells revealed a GMME1 secretion level at 30 ng per 1 million cells every 24 hrs (Figure 1F). The conditioned media (CM) from these cells was used as a source of GMME1 for in vitro experiments and the engineered MSC were lately used as in vivo cell factories for drug delivery.
GMME1 is tumoricidal to CCR2+ murine EG7 lymphoma
GMME1 leads to EG7 growth suppression in vivo
GMME1 is tumoricidal to human CCR2+ U266 myeloma cells
GMME1 is tumoricidal to mouse and human CCR2+ medulloblastoma cells
GMME1 induced apoptosis of primary multiple myeloma cells
Interfering with the CCL2/CCR2 ligand/receptor pathway may be of meaningful use in cancer therapy and we here tested whether the CCR2-selective, pro-apoptotic effects of GMME1 we observed previously [10, 11] could be utilized in such a setting. We tested the effect of GMME1 on the CCR2-expressing murine EG7 lymphoid and the human U266 myeloma cell lines in vitro. We found that GMME1 induces their death as previously observed in autoreactive immune cells [10, 11]. A common mechanistic denominator is suppression of phosphorylation of STAT3 and induction of pro-apoptotic effectors such as BAX (none of the remaining Bcl-2 pro-apoptotic molecules were induced). We may speculate that the GMME1/CCR2 interaction leads to recruitment of a GPCR-linked phosphatase or activates an alternate signalling pathway interfering or competing with STAT3 activation. We have previously shown that delivery of GMME1 via a gene-enhanced cellular platform to mice ill with EAE or arthritis, led to immune suppression and clinical remission and we here found that the same platform could serve to treat mice implanted with CCR2+ EG7 lymphoma. These data demonstrate that GMME1's tumoricidal properties can be replicated in vivo as well, demonstrating that its pro-apoptotic effects are unaffected by tumor/host interactions.
Intriguingly, non-hematological epithelial malignancies, such as prostate cancer, have been found to be addicted to CCL2 for their survival and malignant behaviour [25, 26]. Furthermore, an astonishingly high proportion of primary human glial tumors overexpress CCR2 (whilst normal brain structures do not), suggesting possibly a similar mechanistic bonding to CCL2 . We here demonstrate that medulloblastoma cells of both murine and human origins express CCR2 and are susceptible to GMME1-mediated apoptosis in vitro. There have been meaningful attempts at targeting the CCL2/CCR2 pathway for cancer therapy. For example, it was believed that neutralizing CCL2 would be of beneficial potential for the control of CCR2-expressing tumor cells. Unexpectedly, the systemic administration of anti-CCL2 antibodies in prostate cancer mouse models only slightly attenuated the proliferation of tumor cells likely due to the "rescue" of the CCR2+ tumor cells by alternate chemokine ligands . As an alternative, antagonizing CCR2 was suggested as an approach with wider applicability for cancer therapy. A group has investigated the use of a dominant negative CCL2 construct lacking 2-8 amino acids at its N-terminus (7ND) targeting CCR2 in a melanoma mouse model . This study demonstrated modest effects in vivo most likely because of low expression levels and non-efficient delivery method .
In contrast, GMME1 is radically distinct in its tumoricidal CCR2-targeted function since it is not simply a decoy or passive dominant negative, but rather is an active ligand leading to receptor-mediated activation of apoptosis. In essence, GMME1 behaves as an entirely novel chemokine, co-opting CCR2 signaling machinery to compel CCR2+ malignant cells to enter apoptosis. This mechanism of action is attractive as a non-cross-resistant cancer-killing pathway that may complement current therapies for resistant or relapsing CCR2+ hematological malignancies, pediatric medulloblastoma or human multiple myeloma, and other CCR2+ tumors as well. The fact that GMME1 protein can significantly induce cell death of CCR2+ primary myeloma from patients indicates its potential clinical utility.
It should be noted however that CCR2-mediated in vitro killing of tumor cells does not exclude a possibility that in vivo anti-tumor activity of GMME1 is mediated at least in part by killing of CCR2-positive cells within the tumor microenvironment such as macrophages, which are known to express CCR2 and to support tumor growth. We have previously demonstrated  that CCR2+ macrophages harvested from C57BL/6 mice and exposed to GMME1 died by apoptosis 24 h later as shown by activation of caspase-3 as well as annexin-V/PI co-staining. Such macrophage-depleting capacity of the fusokine does not involve the GMCSF moiety since CCR2-/- monocytes expressing the GMCSFR do not die following GMME1 treatment. This set of data suggests that GMME1 can theoretically deplete macrophages in tumour that can potentially play a role in angiogenesis and thus promoting tumor growth.
In conclusion, we have previously demonstrated that the use of GMME1 fusokine could be of significant therapeutic value for depletion of CCR2 autoreactive lymphomyeloid cells. We here further demonstrate that GMME1 also represents a conceptually novel biological approach for eradication of CCR2-expressing malignant cells without noticeable off-target toxicity to the host. The use of innovative chimeric CC-ligand fusokines could serve as a prototype strategy seeking to selectively deplete cancers whose proliferation and survival depends upon CCR-driven signalling.
List of abbreviations
Bcl-2-associated × protein
C-C motif Ligand 2
C-C motif Receptor 2
Green Fluorescent Protein
granulocyte-macrophage colony-stimulating factor
GMCSF-MCP1 fusion protein
Experimental Autoimmune Encephalomyelitis
Enzyme-linked immunosorbent assay
monocyte chemotactic protein-1
Mesenchymal Stromal Cells
Signal transducer and activator of transcription 3
Moutih Rafei is the recipient of a Fonds de Recherches en Santé du Québec (FRSQ), National Cancer Institute (NCI) and Canadian Institute for Health Research (CIHR) Scholarships, and Jacques Galipeau is a Georgia Cancer Coalition Distinguished Cancer Scholar. This work was supported by CIHR MOP-15017 and the Georgia Cancer Coalition, USA.
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