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Short Communication |



From The William Harvey Research Institute,*
St.
Bartholomews and The Royal London School of Medicine and Dentistry,
London, United Kingdom; the Department of
Neuroendocrinology,
Imperial College School of
Medicine, London, United Kingdom; and the Laboratory of Host
Defenses,
National Institute of Allergy and
Infectious Diseases, National Institutes of Health, Bethesda, Maryland
| Abstract |
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| Introduction |
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ANXA1 (previously referred to as lipocortin 1) is a 37-kd member of the annexin superfamily of proteins11 particularly abundant in neutrophils, where it represents up to 4% of the cytosolic proteins.12 On adhesion to monolayers of endothelial cells in vitro ANXA1 is externalized onto the cell surface of neutrophils with the function to down-regulate cell emigration through the endothelial cells.13 Several laboratories, including our own, have demonstrated the anti-migratory action of exogenous and, more importantly, endogenous ANXA1 both in acute14 and chronic15 models of inflammation. The anti-migratory property of the full-length protein is retained by peptides drawn from the N-terminus region, such as peptide Ac2-26.16 The target of endogenous ANXA1 and exogenously administered ANXA1 or peptides seems to be the adherent leukocyte: the net result of their action is leukocyte detachment from the vessel wall rejoining the blood stream.8,17
The cellular mechanism for the anti-migratory action displayed by ANXA1 and its mimetic has been until recently elusive. In a recent in vitro study, Walther and colleagues18 reported the existence of a functional interaction between ANXA1-derived peptides and the receptor for formylated peptides (FPR) on human neutrophils, as measured with calcium flux assay and L-selectin shedding. FPR belongs to the group of seven transmembrane domain G-protein-linked receptors, and it is activated by formylated peptides: the downstream effect is neutrophil or monocyte/macrophage activation.19,20 Importantly, FPR is relatively up-stream of several other receptors for leukocyte activators, and in vitro FPR activation can cause their rapid desensitization.21 Peptide Ac2-26 did not compete with FMLP, however FPR antagonists block its in vitro effects.18 In the present study we have addressed the question of FPR involvement in the inhibitory action of ANXA1-derived peptides on the process of neutrophil extravasation in vivo. FPR knock-out (KO) mice22 were used to test the efficacy and binding capacity also of full-length ANXA1 protein.
| Materials and Methods |
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FPR KO mice, recently described,22 have been back-crossed with C57BL/6 for six generations, and C57BL/6 mice (purchased from Banton and Kingsman, Hull, UK) were used as wild-type controls in the experiments. All animals were fed on a standard chow pellet diet with free access to water, maintained on 12-hour light/dark cycles, and housed for 1 week before experimentation. Animal work was performed according to Home Office regulations (guidance on the operation on animals was from the Scientific Procedures act 1986).
Zymosan peritonitis was induced as previously reported.23 Briefly, mice were injected intraperitoneally with zymosan A (1 mg in 0.5 ml of sterile saline). Animals were sacrificed 4 hours later by carbon-dioxide exposure and peritoneal cavities were lavaged with 3 ml of phosphate-buffered saline (PBS) containing 3 mmol/L ethylenediaminetetraacetic acid. Aliquots of the lavage fluid were then stained with Turks solution (0.01% crystal violet in 3% acetic acid) and differential cell counts performed using a Neubauer hemocytometer and a light microscope (Olympus B061, JENCONS-PLS, Leighton Buzzard, UK).
Reagents
Human recombinant ANXA1, peptide Ac2-26 (Ac-AMVSEFLKQAWFIENEEQEYVQTVK) or Ac2-12 (Ac-AMVSEFLKQAW) were obtained as described.17 The doses used were chosen from a previous study.17 The FPR antagonists Boc1 (N-t-butoxycarbonyl-MLP) and Boc2 (N-t-butoxycarbonyl-PLPLP)18 and other materials were obtained from Sigma-Aldrich Co. (Poole, UK). In vivo, compounds were administered intravenously 15 minutes before zymosan.
ANXA1 Binding
In Vitro
Mice received 20 µg of ANXA1 intravenously at time 0, and were bled by cardiac puncture 5 minutes later. ANXA1 bound on the cell surface was measured using a whole blood staining protocol using 10 µg/ml of monoclonal antibody (mAb) 1B.24-26 Flow cytometry analysis allowed the identification of the monocyte and polymorphonuclear cell population, and the measurement of fluorescence intensity (green channel) associated with either population.
In Vivo
Because radiolabeling protocols cause ANXA1 degradation, we developed an indirect method to assess ANXA1 binding to leukocytes.25 An estimation of binding affinity was made using a flow cytometric approximation of Scatchard analysis, in which free ANXA1 is calculated from total ANXA1 added to each tube less the amount bound to the cells.26 The protocol used has already been described.25 The macrophage population was identified by flow cytometry for the higher values in forward and side scatter characteristics.23 HEK 293 cells expressing mouse FPR have already been fully characterized for their response to FMLP.27,28 The ANXA1 binding assay was performed as described above, and the effect of mouse FPR expression on the binding capacity displayed by the cells was determined.
Statistical Analysis
Comparisons between groups were made using one-way analysis of variance followed by Bonferroni posthoc test. A P value <0.05 was considered significant.
| Results |
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Figure 1A
shows that the intense 4
hours polymorphonuclear leukocyte (PMN) peritoneal infiltration
induced by zymosan was inhibited by peptide Ac2-26 and full-length
ANXA1, as previously reported.16
Co-injection of the FPR
antagonist Boc1 (50 µg) abrogated the inhibition exerted by peptide
Ac2-26, and significantly attenuated that afforded by ANXA1 (Figure 1A)
. The FPR antagonist Boc2 was as active as Boc1 on peptide Ac2-26
(Figure 2A)
.
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ANXA1 Binding to Circulating Leukocytes
The existence of saturable binding sites for ANXA1 on
human29
and mouse30
leukocytes has long been
known, however the nature of these sites has so far remained
elusive.25
ANXA1 injection into the tail vein of wild-type
mice resulted in a detectable binding to circulating monocytes and PMN,
as measured ex vivo (Figure 2, A and B)
. There was a marked
and significant reduction (40 to 60%) in the extent of binding
measured after intravenous injection in FPR KO mice. Figure 2A
shows a
representative histogram with ANXA1 bound on circulating monocytes,
whereas Figure 2B
shows the cumulative data for four distinct mice.
ANXA1 Binding to Mouse FPR
Because residual binding and anti-migratory activity were
displayed by ANXA1 in FPR KO mice, binding studies were performed
in vitro on peritoneal macrophages. Figure 3A
confirms the existence of a saturable
binding to these cells,30
and shows a pronounced reduction
in cells taken from FPR KO mice. Nonetheless residual-binding activity
is present. Out of four distinct experiments, approximate kd 110
± 20 nmol/L and 250 ± 20 nmol/L could be calculated in wild-type
and KO mice. Finally, we sought direct evidence for ANXA1 interaction
with mouse FPR. Importantly, HEK-293 cells transfected with mouse FPR
displayed higher binding capacity for the protein than control cells
(Figure 3B)
.
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| Discussion |
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This study started by testing the effect of two FPR antagonists in an
in vivo model of PMN extravasation. Both Boc1 and Boc2
exerted a potent antagonism on the anti-migratory action displayed by
peptide Ac2-26. Although these antagonists are relatively selective for
FPR (in vitro they do not alter the effect of
platelet-activating factor on human and mouse neutrophils; data not
shown), this finding was validated by using FPR KO mice. These mice are
more susceptible to Listeria monocytogenes infection than
wild-type controls, and succumb at a higher rate after bacterial
infection. In addition, their acute inflammatory reaction to
thioglycollate is not different from that of control
animals.22
Here, the response to peritoneal zymosan
injection was not different between wild-type and FPR KO mice.
Overall these data provide in vivo relevance to the finding
of Walther and colleagues,18
who tested peptide Ac2-26 on
human neutrophils and HeLa cells transfected with human FPR. However,
in vitro the activity was retained by the region spanning
amino acids 19 to 25 (EYVQTVK). In addition, data for full-length ANXA1
were not shown.18
In our hands, peptide Ac2-12 (given at a
dose equimolar to 200 µg of peptide Ac2-26) was able to inhibit
(
40%) neutrophil extravasation, and a more pronounced effect was
measured with recombinant ANXA1. Both these responses were
significantly altered in FPR KO mice, with a significant residual
activity being measured for ANXA1. The efficacy of the shorter peptide
validated our previous study in which direct observation of mesenteric
microvessels was conducted.17
Future studies are required
to address this discrepancy on the sequence of the ANXA1 N-terminus
required for FPR activation.
The fact that ANXA1 retained some degree of efficacy in Boc-treated and
FPR KO mice clearly indicated the existence of another receptor or
mechanism of action. This may not be surprising in view of the length
of ANXA1 (346 amino acids; molecular weight, 37 kd).31
The
existence of specific and saturable ANXA1 binding sites on human
monocytes and neutrophils was originally described by Goulding and
colleagues25
reporting the existence of a binding protein
with an apparent molecular weight of 15 kd. ANXA1 binding to mouse
circulating leukocytes and peritoneal macrophages has also been
characterized.16
To test the hypothesis formulated above
we performed ANXA1 binding assay to leukocytes of FPR KO mice. In
vivo administration of ANXA1 resulted in a detectable binding to
peripheral circulating leukocytes. As expected,26
the
highest binding was observed to monocytes with a lower but still above
basal binding to circulating neutrophils. No binding to lymphocytes was
measured (data not shown). A significant reduction in ANXA1
binding was measured on circulating monocytes and neutrophils of FPR KO
mice. These data are of importance as they link ANXA1 binding to FPR
in vivo to the anti-migratory action of the protein.
Importantly, no changes in neutrophil response to the chemokine
macrophage inflammatory protein-1
22
or to the FPR2
agonist serum amyloid protein type A27,28
have been
reported in FPR KO mice, suggesting (in absence of specific antibodies)
that at least the receptors used by these chemoattractants are not
altered by deletion of the mouse FPR gene.
The apparently modest binding to PMN is expected because adhesion to endothelial cells augments ANXA1 binding capacity to this leukocyte type.26 Importantly FPR can be exported on the neutrophil cell surface on adhesion because it is contained in few cytoplasmic granules and vesicles.32 Because ANXA1 is predominantly contained in gelatinase granules,33 we propose that PMN adhesion to the endothelium triggers externalization of ANXA1 and FPR, and therefore activates the entire pathway. The in vivo data here presented suggest that interaction with FPR is the mechanism responsible for the leukocyte detachment phenomenon promoted by endogenous and exogenous ANXA1, and peptides Ac2-26 and Ac2-12.8,17
In conclusion, this study provides the first in vivo evidence that FPR is functionally involved in the in vivo inhibitory effects of ANXA1 and derived peptido-mimetics. We also report for the first time reduced ANXA1 binding in the absence of a specific receptor. On the other hand, strong indication that other receptor(s) and/or mechanism(s) must be involved in the action of full-length ANXA1 is also provided. For instance direct interaction between ANXA1 and very late antigen-4 on a human monocytic cell line has recently been demonstrated.34 In view of the lack of direct competition between peptide Ac2-26 and formyl-Met-Leu-Phe in vitro binding assays,18 and the fact that formyl-Met-Leu-Phe and ANXA1 are clearly able to produce opposite effects in inflammation, it is likely that we have so far only scraped the surface of a complex and fascinating biological system.
| Acknowledgements |
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| Footnotes |
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Supported by the Arthritis Research Campaign United Kingdom (fellowship P0569 to M. P.).
Accepted for publication March 9, 2001.
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4ß1 integrin. J Immunol 2000, 165:1573-1581This article has been cited by other articles:
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V. Gerke and S. E. Moss Annexins: From Structure to Function Physiol Rev, April 1, 2002; 82(2): 331 - 371. [Abstract] [Full Text] [PDF] |
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M. LA, M. D'AMICO, S. BANDIERA, C. DI FILIPPO, S. M. OLIANI, F. N. E. GAVINS, R. J. FLOWER, and M. PERRETTI Annexin 1 peptides protect against experimental myocardial ischemia-reperfusion: analysis of their mechanism of action FASEB J, October 1, 2001; 15(12): 2247 - 2256. [Abstract] [Full Text] [PDF] |
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