© 2005 Oxford University Press
ARTICLE |
Formylpeptide Receptor FPR and the Rapid Growth of Malignant Human Gliomas
Affiliations of authors: Laboratory of Molecular Immunoregulation (YZ, YL, JH, PI, OMZH, WF, JMW) and Laboratory of Experimental Immunology (XZ, RS), Center for Cancer Research, and Basic Research Program (WG, LH), SAIC-Frederick, Inc., NCI–Frederick, Frederick, MD; Institute of Pathology, Southwest Hospital, The Third Military Medical University, Chongqing, 400038, China (XB)
Correspondence to: Ji Ming Wang, MD, PhD, LMI, CCR, NCI-Frederick, Building 560, Room 31-40, Frederick, MD 21702–1201 (e-mail: wangji{at}mail.ncifcrf.gov).
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ABSTRACT |
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Background: The formylpeptide receptor (FPR) is a G-protein–coupled receptor (GPCR) that mediates chemotaxis of phagocytic leukocytes induced by bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLF). We previously showed that selected human glioma cell lines also express functional FPR. We therefore investigated the relationship between FPR expression and the biologic behavior of glioma cells. Methods: Expression and function of FPR in the human glioblastoma cell line U-87 were examined by reverse transcription–polymerase chain reaction (RT-PCR) and chemotaxis assays, respectively. FPR protein expression was detected in specimens from 33 human primary gliomas by immunohistochemistry. FPR short interfering (si) RNA was used to block FPR expression in U-87 cells. Cell proliferation was assessed by measuring DNA synthesis. Xenograft tumor formation and growth were measured in nude mice. Endogenous FPR agonist activity released by necrotic tumor cells was assessed by measuring FPR activation in an FPR-transfected basophil leukemia cell line and live U-87 cells. Vascular endothelial growth factor (VEGF) mRNA was assessed by RT-PCR, and VEGF protein was assessed by enzyme-linked immunosorbent assay. All statistical tests were two-sided. Results: FPR was selectively expressed by the highly malignant human glioblastoma cell line U-87 and most primary grade IV glioblastomas multiforme and grade III anaplastic astrocytomas. U-87 cells responded to the FPR agonist fMLF by chemotaxis (i.e., increased motility), increased cell proliferation, and increased production of VEGF protein. FPR siRNA substantially reduced the tumorigenicity of U-87 cells in nude mice (38 days after implantation, mean tumor volume from wild-type U-87 cells = 842 mm3, 95% confidence interval [CI] = 721 to 963 mm3; and from FPR-siRNA transfected U-87 cells = 225 mm3, 95% CI = 194 to 256 mm3; P = .001). Necrotic glioblastoma cells released a factor(s) that activated FPR in live U-87 cells. Conclusions: FPR is expressed by highly malignant human glioma cells and appears to mediate motility, growth, and angiogenesis of human glioblastoma by interacting with host-derived agonists. Thus, FPR may represent a molecular target for the development of novel antiglioma therapeutics.
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INTRODUCTION |
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TopNotesAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Chemoattractant receptors are seven transmembrane G-protein–coupled receptors (GPCRs) that mediate cell migration in response to a variety of chemotactic factors. Chemoattractant GPCRs also participate in essential pathophysiologic processes, including inflammation, hematopoiesis, development, wound healing, human immunodeficiency virus infection, and, most intriguingly, in the progression of malignant tumors. In fact, some GPCRs for chemokines promote angiogenesis, thus contributing to neovascularization and tumor outgrowth (1,2). In addition, some chemokines interact with their GPCRs to induce chemotaxis of malignant human and animal tumor cells and direct their organ-specific metastasis (3,4). Consequently, GPCRs that respond to chemoattractants may also increase the motility and dissemination of malignant tumor cells.
Glioma is the most common malignant neoplasm of the central nervous system. These tumors range in degree of aggressiveness from slowly growing low-grade tumors to rapidly growing high-grade tumors, such as anaplastic astrocytoma and glioblastoma multiforme. High-grade tumors contain necrotic foci and are richly vascularized, presumably as a result of aberrant expression of angiogenic factors, such as vascular endothelial growth factor (VEGF), by tumor cells. Established human glioma cell lines and normal astrocytes express several GPCRs for chemokines and respond in vitro to selected ligands by chemotaxis and intracellular calcium mobilization (5,6). In addition, we previously reported (7) that some human glioma cell lines express another GPCR, the formylpeptide receptor (FPR). FPR, originally identified in phagocytic leukocytes, mediates the chemotaxis and activation of these cells in response to the bacterial chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLF) (7) and potential agonist peptides derived from mitochondria of ruptured cells. Agonist binding to FPR in phagocytic leukocytes leads to the activation of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinases (MAPKs), and the transcription factor nuclear factor (NF)-
B [for review, see (1)]. Because FPR was believed to mainly engage in proinflammatory and antibacterial host responses (8,9), the unexpected expression of this GPCR by glioma cells (7) prompted us, in this study, to investigate the contribution of FPR to glioma cell motility, proliferation, and tumorigenicity and to the progression of tumors in vivo. In addition, we explored whether FPR interacted with agonists derived from necrotic tumor cells.
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MATERIALS AND METHODS |
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TopNotesAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Reagents
fMLF was from Sigma-Aldrich (St. Louis, MO). The inhibitor of MAPK kinase 1 (MEK1) PD98059, antibodies against phosphorylated extracellular signal-regulated kinases 1/2 (ERK1/2), p38 MAPK, Jun-N-terminal kinase (JNK), Akt, or signal transducers and activators of transcription 3 (STAT3), and antibodies against total ERK1/2, p38, JNK, Akt, STAT3, Bcl-2, or Bcl-xL were from Cell Signaling Technology (Beverly, MA). The tyrosine kinase inhibitor Tyrphostin AG490 was from Calbiochem (San Diego, CA). Anti-hypoxia inducible factor (HIF)-1
antibody was from Novus Biologicals (Littleton, CO). Anti-VEGF antibody was from R&D Systems (Minneapolis, MN). Anti-
-actin antibody was from Santa Cruz (Santa Cruz, CA). Anti-glial fibrillary acidic protein (GFAP) and anti-vimentin antibodies and the EnVision system were from DAKO (Carpinteria, CA). t-Butyloxycarbonyl-methionyl-leucyl-phenylalanine (tBoc-MLF) was from MP Biomedicals (Irvine, CA). Anti-FPR antibody was from BD Biosciences Pharmingen (San Diego, CA). Cyclosporin H (CsH) was a kind gift from Novartis (Basel, Switzerland).
Cells
Human glioblastoma cell lines U-87 and SNB75 were from the American Type Culture Collection (ATCC, Manassas, VA). SHG-44 cells were established from a surgically removed human astroglioma (Suzhou Medical University, Suzhou, China). These cells were grown in Dulbecco's modified Eagle medium (DMEM) containing 10% fetal calf serum. A rat basophil leukemia cell line transfected with the FPR gene (ETFR cells), from Dr. R. Snyderman (Duke University, Durham, NC), was maintained in DMEM with 10% fetal calf serum and G418 (Invitrogen, Carlsbad, CA) at 0.8 mg/mL. The collection of human umbilical cord vein endothelial cells (HUVECs) was previously described (10). Human peripheral blood mononuclear cells were isolated from leukopacks obtained from the Transfusion Medicine Department, National Institutes of Health Clinical Center (Bethesda, MD), by Ficoll-Hypaque (Sigma-Aldrich) density gradient centrifugation. Monocytes were purified from human peripheral blood mononuclear cells by Percoll gradient centrifugation (Amersham Biosciences, Little Chalfont, UK) to yield preparations that were more than 90% pure (11).
Immunohistochemistry
Tumor specimens from 33 glioma patients were examined for FPR expression by immunohistochemical staining. All patients received diagnosis and surgical therapy and provided written informed consent at the Southwestern Hospital, Third Military Medical University, Chongqing, China. The histologic diagnoses were based on the 2000 edition of World Health Organization (WHO) classification of nervous system tumors. The tumor specimens studied included 13 grade II astrocytomas, 14 grade III anaplastic astrocytomas, and six grade IV glioblastomas multiforme. Tumor specimens were fixed in 10% formalin and embedded in paraffin. Sections were incubated with 0.3% H2O2 for 10 minutes, blocked with 10% normal goat serum for 10 minutes at room temperature, and reacted with anti-human FPR antibody for 2 hours at 37 °C, followed by biotinylated secondary antibody (30 minutes, at room temperature) and streptavidin-peroxidase complex (30 minutes, at room temperature). Tumor sections were stained for antibody against FPR protein by use of diaminobenzidine as substrate to give a brown color and were then counterstained with hematoxylin. To detect GFAP and vimentin, glioma cells that had been cultured on chamber slides were fixed in ethanol, permeabilized with Triton X-100, and incubated with normal goat serum to block potential nonspecific binding of antibodies to the cells. Antibodies against GFAP or vimentin were added, and slides were treated with a horseradish-conjugated secondary antibody for visualization of the cellular proteins by use of an EnVision system (DAKO).
RT-PCR
Total RNA was extracted from cells with RNeasy Mini Kit (QIAGEN, Valencia, CA), and 0.5 µg was used for reverse transcription–polymerase chain reaction (RT-PCR). For human FPR, sense primer 5'-CTCCAGTTGGACTAGCCACA-3' and antisense primer 5'-CCATCACCCAGGGCCCAATG-3' generated a 500-bp product. For human VEGF, primers were designed to amplify four isoforms of the VEGF gene. Sense primer 5'-ATGAACTTTCTGCTGTCTTGGG-3' and antisense primer 5'-CTGTATCAGTCTTTCCTGGTGAG-3' generated 514-bp VEGF121, 646-bp VEGF165, 718-bp VEGF189, and 769-bp VEGF206 products (12). RT-PCR was performed with a ProSTAR High Fidelity single-tube RT-PCR system (Stratagene, La Jolla, CA), consisting of a 30-minute reverse transcription at 42 °C; a 1-minute inactivation of Moloney murine leukemia virus reverse transcriptase at 95 °C; 40 cycles (30 cycles for VEGF) of denaturing at 95 °C for 30 seconds, annealing at 60 °C for 30 seconds, and extension at 68 °C for 2 minutes; and a final 10-minute extension at 68 °C. Primers for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were used as controls (Stratagene). PCR products were electrophoresed on 2% agarose gels and visualized with ethidium bromide staining.
Directional Cell Migration (Chemotaxis)
Chemotaxis assays were performed in 48-well chemotaxis chambers (NeuroProbe, Gaithersburg, MD) (7,10). The upper and lower compartments of the chemotaxis chambers were separated by a 10-µm (pore-sized) (8 µm for HUVECs) polycarbonate filter (GE Osmonics Labstore, Minnetonka, MN) coated with collagen type I (BD Biosciences, San Jose, CA) at 50 µg/mL. A 27-µL aliquot of chemoattractants was placed in the wells of the lower compartment, and 50 µL of tumor cells, ETFR cells, or HUVECs (each at 1 x 106 cells per mL of RPMI 1640 medium containing 1% bovine serum albumin and 25 mM HEPES) were placed in the wells of the upper compartment. A 270-minute incubation at 37 °C was used to measure tumor and ETFR cell migration, and a 120-minute incubation was used to measure HUVEC migration. After incubation, the filters were removed and stained, and cells that migrated across the filters were counted under light microscopy. The results were expressed as the means, and 95% confidence intervals (CIs), of migrated cells in three high-powered fields (x400 magnification) in triplicate samples.
Immunoblot Analysis
Tumor cells were lysed in 150 µL of 1 x sodium dodecyl sulfate sample buffer (62.5 mM Tris-HCl at pH 6.8, 2% sodium dodecyl sulfate, 10% glycerol, and 50 mM dithiothreitol), sonicated for 3 seconds, and then boiled for 5 minutes to produce a cell lysate. The cell lysate was then centrifuged at 10 000 x g at 4 °C for 10 minutes. Immunoblot analysis of Bcl-2 and Bcl-xL and of phosphorylated ERK1/2, p38, JNK, Akt, and STAT3 proteins was performed with antibodies for the specific proteins and phosphorylated proteins, respectively. Total cell proteins were electrophoresed on 4%–12% gradient Tris-Glycine precast gels (Invitrogen) and transferred onto Immobilon P membranes (Millipore, Billerica, MA). The membranes were blocked by incubation in 3% nonfat dry milk for 3 hours at room temperature and then incubated with primary antibodies in phosphate-buffered saline (PBS) containing 0.01% Tween-20 overnight at 4 °C. After incubation with a horseradish peroxidase–conjugated secondary antibody, the protein bands were detected with Super Signal Chemiluminescent Substrate Stable Peroxide Solution (Pierce, Rockford, IL) and BIOMAX-MR film (Eastman Kodak, Rochester, NY). To detect -actin, total ERK1/2, p38, JNK, Akt, or STAT3, the membranes were stripped with Restore Western Blot Stripping Buffer (Pierce) and then incubated with the corresponding specific antibodies. To detect HIF-1, glioblastoma cells were washed with five volumes of hypotonic buffer (10 mM HEPES, 10 mM KCl, 1.5 mM MgCl2, and 0.5 mM dithiothreitol at pH 7.9) and lysed in the same buffer supplemented with 1% Nonidet P-40. After centrifugation at 10 000 x g at 4 °C for 1 hour, the nuclei-containing pellet was resuspended in 150 µL of low-salt buffer (10 mM HEPES, 25% glycerol, 1.5 mM MgCl2, 20 mM KCl, 0.5 mM dithiothreitol, and 0.2 mM EDTA) and 150 µL of high-salt buffer (low-salt buffer containing 800 mM KCl). The nuclear extracts were then centrifuged as described above, and supernatants were electrophoresed on a 4%–12% gradient Tris-Glycine precast gel (Invitrogen). The proteins were transferred to Immobilon P membranes, and the blot was probed with anti-HIF-1 antibody.
Electrophoretic Mobility Shift Assay
The electrophoretic mobility shift assay was used to detect the formation of complexes between a [32P]dATP-labeled nucleotide element (5'-AGTTGAGGGGACTTTCCAGGC-3') and NF-B contained in the nuclear proteins of tumor cells. The labeled nucleotide element was incubated with 5 µg of nuclear proteins in 20 µL of binding mixture (50 mM Tris-HCl at pH 7.4, 25 mM MgCl2, 5 mM dithiothreitol, and 50% glycerol) at 4 °C for 2 hours. For supershift assays, nuclear extracts were incubated with 1 µg of normal rabbit serum or antiserum specific for p50/p65 NF-B at 4 °C for 1 hour and then with 32P-labeled oligonucleotide for 15 minutes at room temperature. Nucleotide–protein complexes were resolved on 5% polyacrylamide gels containing 0.25x TBE (Tris–borate–EDTA) buffer at room temperature for 2 hours at 150 V. The gels were then heat-dried under vacuum and exposed to X-Omat films (Eastman Kodak) at –70 °C.
VEGF Production and the Formation of Capillary-Like Structures by HUVECs
VEGF in the supernatants of tumor cells was quantified by commercial enzyme-linked immunosorbent assay kits (Lymphokine Testing Laboratory, SAIC Frederick, Frederick, MD). The formation of capillary-like structures by HUVECs was measured after culturing the cells on 24-well plates coated with Matrigel (BD Biosciences) at 0.3 mL per well. Matrigel in the wells was solidified at 37 °C for 1 hour, and 4 x 104 HUVECs in 0.5 mL of conditioned medium from tumor cells were added to each well. The capillary-like structure formed by HUVECs after 20 hours was photographed at x200 magnification under a phase–contrast microscope.
Tumor Cell Proliferation
Tumor cell proliferation was assessed by measuring the incorporation of [3H]thymidine during DNA synthesis. Briefly, U-87 cells were cultured in 96-well tissue culture plates at 8000 cells per well in DMEM containing 10% fetal calf serum for 12 hours. After a 1-hour treatment with various inhibitors, the cells were incubated in DMEM containing 0.5% fetal calf serum alone or 0.5% fetal calf serum in the presence of 1 µM fMLF. Cells were then pulse-labeled with 1 µCi (in a 5-µL volume containing 20 nmol) of [3H]thymidine (Amersham Pharmacia Biotech, Piscataway, NJ) per well for 16 hours and harvested onto membranes by use of an Inotech Harvester (Inotech Biosystems, Rockville, MD). The incorporated [3H]thymidine was measured with a Wallac Microbeta Counter (Perkin-Elmer Life Sciences, Gaithersburg, MD), and the results were expressed as mean counts per minute (cpm) and 95% confidence interval of four replicates.
Xenografts
Approximately 5 x 106 human glioma cells (in 100 µL of PBS) were implanted by subcutaneous injection into the flank of each 4-week-old (20–22 g) female athymic Ncr-nu/nu mouse (NCI-Frederick Cancer Research Facility). Tumor size was calculated by the formula lw2/2, where l is the length of the tumor in millimeters and w is the width in millimeters. Each group contained at least five mice. To generate xenografts with glioblastoma cells transfected with FPR short interfering RNA (siRNA), 1 x 106 cells were injected into the flank of each nude mouse. Nontransfected U-87 cells and cells transfected with random siRNA (mock) were used as controls. Animal care was provided in accordance with the Guide for the Care and Use of Laboratory Animals.
FPR siRNA and Transfection of U-87 Cells
Construction of hairpin siRNA expression cassettes was performed as described (13). Briefly, three 19-nucleotide sequences were targeted to FPR mRNA at nucleotides 392 to 410 (in the second transmembrane region of the putative protein), nucleotides 605 to 623 (in the third transmembrane region), and nucleotides 926 to 944 (in the third extracellular loop) (GenBank sequence no. NM_002029). Retroviral vector stocks were produced by transient transfection of Phoenix-Ampho cells with the Superfect Transfection Reagent (QIAGEN) and 5 µg of FPR siRNA expression plasmid. The virus was collected from the culture supernatants on day 2 after transfection, and U-87 cells were transfected with a combination of three retroviral vectors, each containing a separate siRNA construct in the presence of Polybrene at 5 µg/mL. The U-87 cells stably transfected with FPR siRNA were selected and maintained by continual incubation with puromycin (BD Biosciences Clontech, Palo Alto, CA) at 2 µg/mL.
Ca2+ Flux
Ca2+ mobilization was measured by incubating 2 x 107 cells in 1 mL of loading medium (DMEM containing 10% fetal calf serum and 2 mM glutamine) with 7 µM Fura-2 acetoxymethyl ester (Molecular Probes, Eugene, OR) for 45 minutes at room temperature. The dye-loaded cells were washed and resuspended in saline buffer (138 mM NaCl, 6 mM KCl, 1 mM CaCl2, 10 mM HEPES, 5 mM glucose, and 0.1% bovine serum albumin at pH 7.4) at a density of 0.5 x 106 cells per mL. The cells were then transferred into quartz cuvettes (1 x 106 cells in 2 mL of saline buffer), and cuvettes were placed in a fluorescence spectrometer (Perkin-Elmer, Beaconsfield, UK). Stimulants were added to a cuvette in a volume of 20 µL, and the intensity of the fluorescence was measured by use of the ratio of the absorbance at 340 nm to the absorbance at 380 nm, calculated with an FL WinLab program (Perkin-Elmer).
Generation of Tumor Cell Supernatant and Tissue Extracts
Necrotic U-87 cells were generated by subjecting 20 x 106 cells in 1 mL of PBS to three cycles of freezing and thawing followed by centrifugation. Apoptotic U-87 cells were generated by treating 20 x 106 cells in 1 mL of PBS with 0.5 µM staurosporine for 6 hours. Supernatants from necrotic U-87 tumor tissues were obtained by mincing tumor tissues from xenograft nude mice at a concentration of 1 g of tissue in 4 mL of PBS and then subjecting the mixture to repeated cycles of freezing and thawing followed by centrifugation. For both types of cells, centrifugation was at 10 000 x g for 1 hour (4 °C), and supernatants were collected and stored at –70 °C for further assays.
Statistical Analyses
All experiments were performed at least three times. A t test, with the computer-aided program Prism (Version IV) (GraphPad Software, Inc. San Diego, CA), was used to determine the statistical significance of the difference between cell responses to testing materials and to controls in chemotaxis and cell proliferation experiments as well as comparison of tumor volumes. P values equal to or less than .05 were considered statistically significant. All statistical tests were two-sided. Mouse survival curves were plotted as Kaplan–Meier plots (Prism Version IV, GraphicPad Software, Inc.).
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RESULTS |
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TopNotesAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Functional Characteristics of FPR in Glioblastoma Cells
As previously described (7), we confirmed that an established human glioblastoma cell line, U-87 (14), expressed FPR transcripts (Fig. 1, A) and responded to stimulation with the FPR agonist fMLF in nanomolar concentration range by chemotaxis (Fig. 1, B). To examine whether FPR transcripts were expressed by all astroglial cells, we used another human glioma cell line, SHG-44, and normal human astroglial cells. Neither SHG-44 nor normal human astrocytes expressed FPR transcripts (Fig. 1, A), and these cells did not respond to fMLF at any concentration tested (data not shown), suggesting that functional FPR is expressed by some human glioma cells.
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The selective expression of FPR by U-87 cells prompted us to examine FPR signaling pathways by probing for increased phosphorylation of signaling components that may contribute to the malignant behavior of these tumor cells, including those that may promote tumor cell proliferation and gene transcription. We found that the levels of phosphorylation of ERK1/2, p38 MAPK, and JNK in U-87 glioblastoma cells increased after the cells were stimulated with fMLF (Fig. 1, C). Activation of FPR in U-87 cells also promoted the phosphorylation of Akt (Fig. 1, C), also known as protein kinase B, which is located downstream of the PI3K pathway and has been reported to support tumor cell survival and proliferation (15). Therefore, PI3K, which controls the activation of Akt, and MAPK appear to be coupled to FPR in U-87 cells.
Because activation of FPR in myeloid cells apparently induces nuclear translocation of NF-B (16), which regulates the transcription of diverse genes coding for cytokines and growth factors, we investigated whether NF-B contained in the nuclear proteins isolated from fMLF-stimulated U-87 cells formed complexes with an NF-B–binding nucleotide element. As early as 5 minutes after fMLF stimulation, we detected the formation of complexes that contained the NF-B subunits p50 and p65 (Fig. 2, A). In addition, in fMLF-stimulated U-87 cells under normal oxygenated culture conditions, we also detected the nuclear translocation of HIF-1. HIF-1 has been reported to increase the transcription of the VEGF gene, followed by increased production of VEGF protein. VEGF protein recruits vascular endothelial cells and thus promotes angiogenesis (17). Addition of the MEK1 inhibitor PD98059 inhibited the fMLF-stimulation of HIF-1 translocation in glioblastoma cells (Fig. 2, B), suggesting that the ERK1/2 MAPK pathway is involved in fMLF-induced HIF-1 translocation in U-87 cells.
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The transcription factor STAT3 has also been implicated in increasing angiogenesis in malignant tumors and is coupled to the signaling pathways of some GPCRs for chemoattractants (18,19). We investigated whether fMLF could activate STAT3 in glioblastoma cells by measuring the level of STAT3 phosphorylation and found that fMLF induced a rapid and transient phosphorylation of STAT3 in U-87 cells at both tyrosine-705 (Tyr-705) and serine-727 (Ser-727) residues (Fig. 2, C). In fMLF-treated monocytes, transiently increased phosphorylation was observed only at Ser-727 of STAT3. In untreated monocytes, Tyr-705 was constitutively phosphorylated at a relatively high level, which was not further increased after fMLF stimulation (Fig. 2, D). The reason for the difference observed in FPR-induced STAT3 activation between glioblastoma cells and monocytes is not clear. It should be noted, however, that U-87 cells express approximately 500 high-affinity binding sites for fMLF per cell (7), whereas monocytes express approximately 4500 sites per cell. Whether this could account for the difference in FPR-induced signaling between monocytes and U-87 glioblastoma cells requires further investigation. Thus, FPR in U-87 cells can apparently stimulate the phosphorylation of STAT3, and phosphorylation at both Ser-727 and Tyr-705 has been reported to be required for maximal transcriptional activity of STAT3 (20,21).
Activation of FPR and VEGF Production by U-87 Cells
Our finding that fMLF activated the transcription factors NF-B, HIF-1, and STAT3 in U-87 cells by activating FPR prompted us to examine whether fMLF could also stimulate the production of VEGF in these cells. Untreated U-87 glioblastoma cells expressed a low level of VEGF mRNA, whereas fMLF-treated cells expressed an increased level of VEGF mRNA (Fig. 3, A). fMLF-treated U-87 cells also secreted elevated levels of VEGF protein, which reached a maximum at 72 hours after stimulation with fMLF (Fig. 3, B). There was no further increase in the production of VEGF by U-87 cells stimulated by fMLF for longer periods of time (data not shown).
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The VEGF contained in the culture medium of fMLF-stimulated U-87 cells was biologically active, because the conditioned medium increased the chemotaxis of HUVECs, and the chemotactic activity in the conditioned medium was completely neutralized by the addition of a monoclonal antibody against human VEGF (Fig. 3, C). When HUVECs were cultured with conditioned medium from fMLF-treated U-87 cells, HUVECs formed capillary-like structures on a Matrigel surface, and formation of these structures was inhibited by the addition of anti-VEGF antibody (Fig. 3, D). Thus, VEGF in conditioned medium from FPR-activated U-87 cells appears to induce endothelial cells to migrate and to form capillary-like structures, two key events associated with neovascularization.
FPR Activation and U-87 Cell Proliferation
We measured DNA synthesis as a reflection of cell proliferation. When U-87 cells were cultured under suboptimal conditions (i.e., culture medium containing only 0.5% fetal calf serum), DNA synthesis in fMLF-treated cells was higher than that in untreated cells (e.g., at 72 hours, [3H]thymidine incorporation in cells cultured in the medium alone = 29 000 cpm [taken as 100%], 95% CI = 25 700 to 32 300 cpm; [3H]thymidine incorporation in cells cultured with fMLF = 40 000 cpm [a 37% increase compared with the medium-alone group], 95% CI = 36 500 to 43 500 cpm; P<.001) (Fig. 4, A). Addition of CsH, a well-characterized FPR inhibitor (1), to the cells blocked the effect of fMLF ([3H]thymidine incorporation in cells cultured in medium alone = 25 000 cpm [taken as 100%], 95% CI = 21 200 to 28 800 cpm; [3H]thymidine incorporation in cells cultured in the presence of fMLF = 34 000 cpm [36% increase compared with the medium-alone group], 95% CI = 28 000 to 40 000 cpm; [3H]thymidine incorporation in cells cultured with both fMLF and CsH for 72 hours = 22 000 cpm [89% of the medium-alone group], 95% CI = 18 200 to 25 800 cpm; P = .013 for the CsH+fMLF group versus the fMLF-alone group) (Fig. 4, B). Similarly, the tyrosine kinase inhibitor Tyrphostin AG490, which disrupts JAK/STAT signaling (22,23), also inhibited fMLF-stimulated U-87 cell growth (at 72 hours, medium alone = 31 000 cpm [taken as 100%], 95 % CI = 25 000 to 37 000 cpm; the fMLF group = 50 000 cpm [a 61% increase compared with the medium-alone group], 95% CI = 46 200 to 53 800 cpm; AG490+fMLF group = 28 000 cpm [90% of the medium control], 95% CI = 25 100 to 30 900 cpm; P = .001 between fMLF group and AG490+fMLF group) (Fig. 4, C), suggesting that FPR regulates U-87 cell growth by activation of the JAK/STAT cascade.
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We next investigated whether fMLF treatment altered the expression of the antiapoptotic molecules Bcl2 and Bcl-xL. We found that fMLF-treated U-87 cells had higher levels of Bcl-2 than untreated cells but had the same levels of Bcl-xL (Fig. 4, D). fMLF-activated FPR increased the level of Bcl-2 via the MEK1/ERK pathway, because addition of the MEK1 inhibitor PD98059, but not of the p38 inhibitor SB202190 or JAK/STAT inhibitor Tyrphostin AG490, blocked the fMLF-induced increase in expression of Bcl-2 (Fig. 4, D, and data not shown).
FPR and Tumorigenesis of Glioma Cell Lines
We next investigated the relationship between FPR and the degree of glioma cell malignancy by examining the expression of vimentin, a marker for poorly differentiated astroglial cells (24), and of GFAP, a glial differentiation marker (25). We found that U-87 cells contained higher levels of vimentin (Fig. 5, A) and lower levels of GFAP than another glioma cell line, SHG-44, which does not express functional FPR (Fig. 5, A). These results led us to hypothesize that highly malignant human glioblastomas selectively express FPR.
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We then compared tumor growth of U-87 cells, which express FPR, and SHG-44 cells, which do not express FPR, in a xenograph model. We injected U-87 and SHG-44 cells subcutaneously into the flanks of athymic mice and measured the rate of tumor formation and growth. Tumor nodules appeared in all mice on day 4 after U-87 cell implantation and in all mice on day 10 after SHG-44 cell implantation. Tumors formed by U-87 cells grew more rapidly than did those formed by SHG-44 cells. By day 29 after implantation, all mice bearing U-87 tumors had become moribund. In contrast, all mice bearing SHG-44 tumors had become moribund by day 44 after implantation (Fig. 5, B). Thus, FPR-expressing glioblastoma cells appear to have a higher rate of tumorigenicity and growth in vivo in athymic mice than do glioma cells that do not express a functional FPR.
To determine the clinical relevance of this result, we examined specimens derived from 33 surgically removed gliomas with various grades. FPR protein staining was detected in 11 of 14 grade III anaplastic astrocytoma specimens and six of six grade IV glioblastoma multiforme specimens. Microvessels and necrotic tumor cells were readily visible among FPR-positive intact tumor cells, as shown in a representative section from a grade IV glioblastoma multiforme tumor (Fig. 5, C). In contrast, only two of 13 less aggressive grade II astrocytoma specimens showed positive FPR staining. Thus, FPR expression appears to be associated with a majority of poorly differentiated primary human gliomas of grades III and IV.
FPR Knockdown by siRNA and Tumorigenicity of Glioblastoma Cells
To more precisely evaluate the role of FPR in glioblastoma tumorigenicity, we used siRNA to inhibit the expression and function of FPR in U-87 cells. After stable transfection of FPR siRNA into U-87 cells, the expression of FPR mRNA (Fig. 6, A) and in vitro fMLF-induced chemotaxis (Fig. 6, B) were almost completely abolished, compared with those of cells transfected with random siRNA (i.e., mock-transfected cells). In addition, the ability of fMLF to induce phosphorylation of ERK1/2 (Fig. 6, C) and of STAT3 (Fig. 6, D) was abrogated, the cell proliferation rate was slowed, and the cells did not respond to the growth-stimulating activity of fMLF, compared with mock-transfected cells (Fig. 6, E).
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To examine whether FPR contributed to the tumorigenicity of U-87 cells in vivo, we injected U-87 cells that had been transfected with FPR siRNA into the flanks of athymic mice. Tumor nodules formed by U-87 cells transfected with FPR siRNA appeared later (Fig. 7, A), and the corresponding tumors grew more slowly than those formed by wild-type U-87 cells or by mock-transfected cells (Fig. 7, B). By day 42 after implantation, all mice implanted with wild-type or mock-transfected U-87 cells had died or had to be sacrificed because they carried large necrotic tumors. In contrast, all mice bearing tumors formed by FPR siRNA-transfected U-87 cells survived to at least day 72 after implantation (Fig. 7, C). These results indicate that depletion of FPR from U-87 cells markedly reduced their ability to form tumors in athymic mice and improved the survival rate of tumor-bearing mice. In addition, we transfected another human glioblastoma cell line, SNB75, which expresses functional FPR (7), with FPR siRNA and determined the effect of FPR siRNA on the tumorigenicity of these cells. SNB75 cells transfected with FPR siRNA lost the ability to chemotactically respond to fMLF (Fig. 7, D). Tumors formed by SNB75 cells that were transfected with FPR siRNA and injected in nude mice grew more slowly than those formed by mock-transfected SNB75 cells. For example, 14 days after implantation, tumors formed by SNB75 cells transfected with FPR siRNA had a size of 12 mm3 (95% CI = 8 to 16 mm3), and tumors formed by mock-transfected SNB75 cells had a size of 33 mm3 (95% CI = 28 to 38 mm3) (P<.001). Thus, FPR appears to contribute to the growth of SNB75 glioblastoma in nude mice.
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Production of Molecules with FPR Agonist Activity by Necrotic Glioblastoma Cells
Because a characteristic feature of malignant glioma is the presence of necrosis, even in relatively small lesions with vigorous neovascularization (26), and because mitochondria of ruptured cells contain chemotactic formylpeptides that appear to activate FPR in myeloid cells (27), we investigated whether necrotic glioblastoma cells and tissues produced a natural agonist(s) recognized by FPR on glioblastoma cells. We found that U-87 cells and U-87 tumors formed in athymic mice released potent chemotactic activity for live U-87 cells (Fig. 8, A and B) and for ETFR cells, which overexpress FPR (data not shown). The FPR agonist activity released by necrotic U-87 cells and U-87 tumor tissues was blocked by an anti-FPR antibody or by the FPR-specific antagonist tBoc-MLF (28) (Fig. 8, B, and data not shown). Necrotic glioblastoma cell supernatant also induced a robust intracellular Ca2+ mobilization in live U-87 cells (Fig. 8, C) and attenuated the ability of U-87 cells to respond to fMLF administered subsequently (Fig. 8, D). These results suggest that fMLF and the natural agonist activity contained in the supernatants of necrotic U-87 cells may share the common GPCR, FPR, on U-87 (11). We additionally observed that necrotic tumor supernatants inhibited the expression of FPR on cell surface of ETFR cells with an efficacy comparable to that of bacterial fMLF at 103 nM (Fig. 8, E). Thus, necrotic glioblastoma cells appear to produce an FPR agonist activity that interacts with the FPR on live tumor cells.
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In this article, we show that FPR is selectively expressed by highly malignant glioma cells but not by less aggressive glioma cells or by normal human astrocytes. To our knowledge, this is the first demonstration that activated FPR may contribute to the progression of highly malignant gliomas by mediating tumor cell chemotaxis, proliferation, and production of VEGF in response to an agonist(s) potentially produced by necrotic tumor cells.
Because FPR was originally detected in cells of the immune system and interacts with bacterial chemotactic peptides, this receptor was hypothesized to participate in a host defense mechanism against microbial infection (1). This hypothesis was supported by reduced antibacterial responses in mice depleted of the counterpart murine receptor FPR1 (29). However, FPR has been more recently reported to also interact with host-derived chemotactic peptides, including formylpeptides potentially released by mitochondria, annexin I produced by activated epithelia, and a neutrophil granule protein, cathepsin G (1,9,27,30,31). In addition, functional FPR has been detected in cells of nonhematopoietic origin, such as lung epithelial cells and hepatocytes (30,32). These findings have indicated that FPR may be involved in a broader spectrum of pathophysiologic processes that also include inflammation and immunity. Our present study further extended the functional scope of FPR to its potential role in promoting the growth of malignant human glioma.
A hallmark in the progression of malignant tumors is increased angiogenesis, which has been attributed to the aberrant production of angiogenic factors. One of the most potent angiogenic factors produced in solid tumors is VEGF, which not only induces endothelial cell migration, proliferation, and tubule formation but also increases microvascular permeability, which may facilitate dissemination of malignant tumor cells (33–35). Antiangiogenic intervention with VEGF antibodies or by VEGF withdrawal results in endothelial cell apoptosis and inhibition of tumor growth (36,37). Malignant gliomas, notably glioblastoma multiforme, are characterized by a high degree of vascularity and the production of copious amounts of VEGF. Hypoxia may further increase the production of VEGF in tumor cells that surround regions of necrosis, even in the early stages of tumor progression (38,39). Hypoxia promotes VEGF gene transcription through nuclear translocation of HIF-1. HIF-1 protein is overexpressed and undergoes enhanced nuclear translocation in a variety of human malignant tumors, and increased levels of HIF-1 are associated with vigorous vascularization and tumor progression (17,40). Although hypoxia induces the nuclear translocation of HIF-1 in many cell types, growth factors and genetic abnormalities frequently detected in human cancer can also increase the level of HIF-1 protein, its DNA binding activity, and the expression of VEGF (41,42). In this study, we have demonstrated, to our knowledge for the first time, that activation of FPR in glioblastoma cells can promote the nuclear translocation of HIF-1 and also increase the expression of VEGF mRNA and protein.
We found that MAPKs, including ERK1/2, p38, and JNK, were phosphorylated in FPR-expressing glioblastoma cells that were activated by peptide agonists, consistent with previously reported results obtained with myeloid cells (43). ERK1/2 MAPK may play an important role in promoting endothelial cell proliferation, the expression of VEGF, and the resultant angiogenic process (44). We showed that inhibition of FPR-mediated ERK1/2, but not p38, activity in U-87 cells reduced the levels of VEGF mRNA induced by FPR agonists and that inhibition of MEK1, but not p38, activity completely blocked FPR agonist–triggered nuclear translocation of HIF-1. Thus, the ERK1/2 pathway appears to be crucial for FPR agonist–induced VEGF expression in U-87 cells. However, it has also been reported (45) that a p38 inhibitor blocks prostaglandin E1–stimulated VEGF synthesis in osteoblast-like cells, suggesting that in different cell types ERK1/2 and p38 may be differentially associated with the signaling cascade that promotes transcription of the VEGF gene. Further research to more clearly define the identity of signaling molecules associated with activated FPR in glioma cells that may promote the production of angiogenic factors will be important in the design of antiangiogenic therapy for malignant human gliomas.
We found that FPR may increase the survival of glioblastoma cells under suboptimal culture conditions by increasing the levels of Bcl-2. In a variety of cell types, the effect of growth factors on survival is dependent on the relative levels of pro- versus antiapoptotic members of the Bcl-2 family. Bcl-2, a widely studied antiapoptotic protein, is thought to interfere with the release of cytochrome c from mitochondria and the activation of procaspase 9 (46,47). In a murine B-cell lymphoma cell line (48), activation of the MEK/ERK pathway is associated with Bcl-2 expression. This result is in accordance with our findings that a MEK/ERK inhibitor reduced the level of fMLF-induced Bcl-2 in human U-87 cells. Thus, we identified an additional role of FPR, i.e., mediation of tumor cell survival through the MEK/ERK-dependent signaling cascade. Our results suggest that FPR may also be considered a target for the design of agents that inhibit glioblastoma cell survival and proliferation. In fact, the ability of the MEK1 inhibitor PD98059 to inhibit the growth and clonogenicity of acute myeloid leukemia cells has been attributed to its reduction of the level of Bcl-2 in these cells (49,50).
We also demonstrated that FPR agonists, such as fMLF, activate the transcriptional factor STAT3 in U-87 cells. STAT3 is a key signaling molecule downstream of receptors for many cytokines and growth factors (51). In malignant tumor cells, STAT3 is in an activated state and plays a critical role in oncogenesis, the production of angiogenic factors, and cell survival (52–54). Phosphorylation of STAT3 at Tyr-705 induces the dimerization of STAT3, the nuclear translocation of the STAT3 dimer, and its binding to the promoter regions of target genes (55). Phosphorylation of STAT3 at Ser-727 in the carboxyl-terminal region maximizes the transcriptional activity of STAT3 (20,56). Although disruption of the STAT3 pathway does not cause death of normal cells in in vitro (57) and in animal (22) models, activated STAT3 is essential for tumor cell survival (54). Although we did not detect high levels of constitutively active STAT3 in U-87 cells, the FPR agonist fMLF increased the level of phosphorylation at both Tyr-705 and Ser-727 residues. In addition, because the JAK/STAT inhibitor Tyrphostin AG490 inhibited the growth-stimulating activity of fMLF, the JAK/STAT pathway appears to be associated with activated FPR, as also reported for selected chemokine GPCRs (18,19).
Malignant tumors exploit their microenvironment to favor their survival, growth, invasion, and metastasis (58). For instance, tumor cells often produce aberrant levels of growth factors that stimulate cell surface receptors to increase cell proliferation in an autocrine manner. Tumor cells also produce high levels of VEGF constitutively or in response to stimulation that recruits endothelial cells and promotes endothelial cell proliferation and vascularization. In addition, malignant tumor cells express receptors that interact with agonists that are present in the vicinity of the tumor or produced by distant organs to increase tumor cell motility and thus to favor tumor cell invasion and metastasis. Chemokine receptors, such as CXCR4 and CCR7, have been implicated in promoting tumor metastasis, presumably by increasing the chemotaxis and extravasation of tumor cells in response to locally produced chemokine ligands (2,4,59). The chemokine receptor CXCR4, expressed by a majority of glioma cell lines, mediates tumor cell migration and supports cell survival, presumably in response to the ligand SDF-1, which is present in tumor tissues (60). However, because CXCR4 is also expressed in normal astrocytes, it may not be a good biomarker for differentiating normal astrocytes from malignant astrocytes or for distinguishing less aggressive tumor cells from highly aggressive malignant tumor cells. In contrast, FPR is not widely expressed in glioma cell lines or in normal glial cells but, rather, is expressed in more highly malignant glioblastoma cells and contributes to their tumorigenicity in vivo. FPR is also detected in a majority of primary grade III anaplastic astrocytoma and grade IV glioblastoma multiforme specimens that we examined. Identification of FPR agonist activity in the supernatants of necrotic tumor cells provides evidence that this receptor may interact with host-derived agonists produced in tumor lesions, presumably in the necrotic area frequently associated with highly malignant gliomas or in surrounding tissues that are compressed by growing tumor in a limited anatomical compartment. It is thus plausible that FPR in live tumor cells may serve as a sensor for the agonists produced in a "paracrine" manner in the tumor microenvironment to promote cell migration, to support cell survival and proliferation, to activate transcription of VEGF mRNA, and to increase the production of VEGF.
Further study is required to more precisely define the relationship between the FPR expression and the progression of human primary gliomas and to identify the mechanistic basis for the control of FPR expression in highly malignant human glioma cells. In addition, the pathogenesis of human gliomas is likely to be complex, and FPR may not be the sole factor that regulates the progression of malignant gliomas. Indeed, our observation that there were three FPR-protein-negative tumors of the 14 primary grade III anaplastic astrocytoma specimens examined suggests that factors other than FPR also participate in the development of malignant human gliomas. Also, the relationship between FPR expression and the survival of glioma patients after treatment remains to be established. Nevertheless, the present study implicates the role of FPR in the rapid progression of highly malignant human gliomas and thus raises the possibility that FPR may be a candidate molecular target for developing novel therapeutics to treat gliomas.
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Present affiliation: Institute for Nutritional Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China (YL).
Ye Zhou and Xiuwu Bian contributed equally to the study. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. government. The publisher or recipient acknowledges the right of the U.S. government to retain a nonexclusive, royalty-free license in and to any copyright covering the article.
This project was funded in part with federal funds from the National Cancer Institute, National Institutes of Health, under Contract No. NO1-CO-12400. We thank Dr. J. J. Oppenheim for critical review of this manuscript.
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Manuscript received August 31, 2004; revised March 18, 2005; accepted April 20, 2005.
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