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CUTTING EDGE |




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Laboratory of Molecular Immunoregulation, Division of Basic Sciences, National Cancer Institute-Frederick and
Intramural Research Support Program, Science Applications International Corporation-Frederick, Frederick, MD 21702;
Pathology Section, National Heart, Lung and Blood Institute and
Laboratory of Host Defenses, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892
| Abstract |
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| Introduction |
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| Materials and Methods |
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PrP106126 was purchased from Bachem Bioscience (King of Prussia, PA) or synthesized and purified by the Department of Biochemistry, Colorado State University (Fort Collins, CO), according to the published sequence (4). The purity was >90% and the amino acid composition was verified by mass spectrometry. The endotoxin levels in the dissolved peptide were undetectable. PrP106126 was dissolved in DMSO at 10 mM as stock solution and was diluted in RPMI 1640 containing 1% BSA for experiments. The final concentration of DMSO in a solution of 50 µM PrP106126 was 0.5%. This concentration of DMSO was used in control medium and we found no effect on cell responsiveness. The synthetic N-formyl-methionyl-leucyl-phenylalanine (fMLF) was purchased from Sigma (St. Louis, MO). Human peripheral blood monocytes were isolated from buffy coats (Transfusion Medicine Department, National Institutes of Health Clinical Center, Bethesda, MD) enriched for mononuclear cells by using an iso-osmotic Percoll gradient. The purity of the cell preparations was examined by morphology and was >90%. Rat basophilic leukemia cell line (RBL-2H3) transfected with epitope-tagged formyl peptide receptor (FPR) (designated ETFR) was a kind gift from R. Snyderman (Duke University, Durham, NC). cDNA cloning and establishment of FPRL1-transfected HEK/293 cells (FPRL1/293) have been described previously (11). All of the transfected cells were maintained in DMEM supplemented with 10% FBS (HyClone, Logan, UT), 1 mM glutamine (Life Technologies, Grand Island, NY), and 800 µg/ml geneticin (G418; Life Technologies).
Chemotaxis assays
Chemotaxis assays were performed using 48-well chemotaxis chambers (Neuroprobe, Cabin John, MD) as described previously (12). The chemotaxis index was used which represented the fold increase in the number of cells migrated in response to chemoattractants over the spontaneous cell migration (in response to control medium).
Calcium mobilization
Cells (2 x 107 cells/ml) were incubated with 5 µM fura-2-acetoxymethyl ester (Molecular Probes, Eugene, OR) in loading medium (DMEM, 10% FBS, 2 mM glutamine) for 30 min at room temperature. Ca2+ mobilization induced by stimulants was measured with a luminescence spectrometer (LS-50B; Perkin-Elmer, Beaconsfield, U.K.) as previously described (12).
FPRL1 internalization
FPRL1/293 cells cultured on chamber slides (Nalge Nunc International, Naperville, IL) were stimulated for 15 min at 37°C with peptides. Cells were fixed in 4% paraformaldehyde for 10 min at room temperature. After washing with PBS, cells were incubated with PBS containing 0.05% Tween 20 and 5% normal goat serum for 1 h to block nonspecific binding sites and to permeabilize the cells. The slides were incubated for 1 h at room temperature with a rabbit polyclonal Ab that recognizes the C-terminal 20 aa of FPRL1 (a kind gift from C.-C. Li, Science Applications International Corporation-Frederick, National Cancer Institute-Frederick; 1:50 dilution in PBS-Tween 20-normal goat serum). The slides were then washed three times with PBS and further incubated with a FITC-conjugated goat anti-rabbit IgG (Sigma; 1:150 dilution in TBS containing 3% BSA) for 30 min. The slides finally were mounted with an anti-fade water-based mounting medium with 4',6'-diamidino-2-phenylindole (DAPI; Vector Laboratories, Burlingame, CA) and analyzed with a laser scanning confocal fluorescence microscope (Leica TCS-4D DMIRBE; Leica, Heidelberg, Germany). Excitation wavelengths of 365 nm (for DAPI) and 488 nm (for FITC) were used to generate fluorescence emission in blue and green, respectively.
Measurements of cytokine production
Monocytes were first preincubated with or without 50 ng/ml pertussis toxin (PT) at 37°C for 4 h and then incubated with stimulants for another 24 h in RPMI 1640 without FCS or BSA. Supernatants were collected and secreted cytokines were measured by ELISA using QuantiGlo ELISA kits (R&D Systems, Minneapolis, MN).
Statistical analysis
All experiments were performed at least three times and the results presented are from representative experiments. The significance of the difference between test and control groups was analyzed with Students t test.
| Results and Discussion |
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We further investigated the capacity of ligand activation of FPRL1 to
enhance the production of proinflammatory cytokines in monocytes.
PrP106126 at concentrations that induced
FPRL1-mediated cell migration stimulated a PT-sensitive production of
proinflammatory cytokines in monocytes (Table I
). The FPRL1 ligand MMK-1 had similar
effects. In contrast, cytokine production in response to bacterial LPS,
which does not use G protein-coupled receptor, was resistant to PT.
Thus, in addition to mediating cell migration, FPRL1 activation by
PrP106126 induced the release of
proinflammatory cytokines that have been implicated in the neurotoxic
effect of monocyte supernatants (10).
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FPRL1 is a G protein-coupled receptor that is capable of interacting
with a number of agonists, including peptide domains derived from HIV-1
envelope proteins (13) and at least four host-derived
molecules, serum amyloid A (15), lipid metabolite lipoxin
A4 (16),
amyloid
peptide,5, and
cathelicidin (17). FPRL1 mediates the chemotactic activity
of peptide agonists and serum amyloid A, which is an acute phase
protein and forms amyloid deposit during chronic inflammation
(18). Consequently, FPRL1 behaves as a "pattern
recognition" receptor that is activated to transduce signals by a
wide variety of unrelated ligands. Stimulation of FPRL1 by its
chemotactic agonists triggers a series of G protein-mediated signaling
cascades leading to cell adhesion, migration, protein tyrosine
phosphorylation, release of reactive oxygen
intermediates, as well as gene activation and production of
proinflammatory cytokines (13). These properties of FPRL1
account for various biological activities reported for
PrP106126. FPRL1 is expressed in a number of
cell types including cells of the nonhematopoietic origin such as
epithelia (13). We found that human astrocytoma cells
express FPRL1 and can be activated by FPRL1-specific agonists
(19). We also detected FPRL1 gene expression in a
neuroblastoma cell line and in a murine microglial cell line N9 (Y. Le,
unpublished observation). Whether normal human neurons and astroglial
cells express the functional FPRL1 and its role in glial activation and
neuronal destruction are under further investigation.
A relatively high concentration (50 µM) of PrP106126 was required to induce an FPRL1-dependent maximal monocyte chemotactic response. Therefore, PrP106126 appears to interact with FPRL1 with a relatively low affinity. However, many chemoattractants, including some chemokines, also show low-affinity interaction with their receptors and such interactions nevertheless contribute to the recruitment of leukocytes to the sites of inflammation (20). Although our results are based on in vitro models, they have repeatedly been shown to correlate directly with in vivo disease states (2, 3) in which infiltration of mononuclear phagocytes (monocytes and microglia) was found in and around prion disease lesions in association with proinflammatory reactions (2, 3). Thus, the low-affinity PrP106126 and FPRL1 interaction may help direct monocytes/microglia migrate to the vicinity of prion lesions which contain high concentrations of these amyloidogenic precursors and aggregated fragments including PrP106126 (4, 21, 22, 23, 24). Interestingly, a recent study revealed that PrP106126 was detected in brain lesions of some Alzheimers disease patients, suggesting the coexistence of prion disease pathology in Alzheimers disease (25). Our observation of FPRL1 as a functional receptor, used by PrP106126 to chemoattract and activate mononuclear phagocytes, should promote further assessment of this receptor as a mediator of proinflammatory responses in neurodegenerative diseases and as a potential therapeutic target.
| Acknowledgments |
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| Footnotes |
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2 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 right of the U.S. government to retain a nonexclusive, royalty-free license in and to any copyright covering the article. ![]()
3 Address correspondence and reprint requests to Dr. Ji Ming Wang, Laboratory of Molecular Immunoregulation, Division of Basic Sciences, National Cancer Institute-Frederick, Building 560, Room 31-40, Frederick, MD 21702. ![]()
4 Abbreviations used in this paper: FPRL1, formyl peptide receptor-like 1; fMLF, N-formyl-methionyl-leucyl-phenylalanine; DAPI, 4',6'-diamidino-2-phenylindole; PT, pertussis toxin; FPR, formyl peptide receptor. ![]()
5 Y. Le, et al. Amyloid
42 activates a G-protein-coupled chemoattractant receptor FPR-like 1. Submitted for publication. ![]()
Received for publication October 18, 2000. Accepted for publication November 30, 2000.
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