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From 1 The Eye Research Laboratories, Brookwood Medical Center, Birmingham, Alabama; and the 2 University of Alabama at Birmingham.
Abstract
PURPOSE. The release of N-acetyl-proline-glycine-proline (PGP), a chemoattractant resulting from direct alkaline hydrolysis of corneal proteins, is believed to be the initial trigger for neutrophil invasion into the alkali-injured cornea. The purpose of this study is twofold: (1) to compare the activity of N-acetyl-PGP with the bioactivities of other similar synthetic peptides in an effort to uncover information about this chemoattractant molecule, and (2) to test these peptide analogs as potential antagonists of N-acetyl-PGP.
METHODS. The polarization assay was used to measure the potential chemotactic response of human neutrophils to peptides. Bioactivity was expressed as the peptide concentration required to produce 50% neutrophil polarization (EC50). Antagonist activity was expressed as the peptide concentration required to produce 50% inhibition (ID50) of polarization activated by N-acetyl-PGP.
RESULTS. Peptide bioactivities (EC50) were ranked as follows: APGPR (0.34 mM) > N-acetyl-PGP (0.5 mM) > N-(PGP)4-PGLG (3 mM) = t-Boc-PGP (3 mM) > N-acetyl-PG (3.4 mM) > N-methyl-PGP (15 mM) = PGP (15 mM) > peptides without detectable activity (t-Boc-PGP-OMe, N-acetyl-P, PG, PGG, GP, GG and gly-pro-hyp). Peptides with no detectable bioactivity were tested as potential antagonists of neutrophil polarization induced by N-acetyl-PGP. Gly-Pro-Hyp inhibited N-acetyl-PGP activation of polarization at 20 mM (ID50). No other synthetic peptide demonstrated a capacity for inhibition.
CONCLUSIONS. The minimum requirement to elicit bioactivity was the presence of PGP alone or derivatives of PG in which the N-terminal proline is blocked. Using this approach, active and inactive mimetic peptides of N-acetyl-PGP were produced. The most active peptide, APGPR, was equal to or slightly greater than N-acetyl-PGP, suggesting that more potent analogs might be designed. Gly-pro-hyp was the only inactive peptide analog to inhibit the chemoattractant.
Alkali-injury of the eye provokes a severe inflammatory reaction, largely composed of neutrophils.1 2 This acute inflammatory response is responsible for corneal ulcerations and perforations, so characteristic of this disease. The release of N-acetyl-proline-glycine-proline (PGP), a chemoattractant resulting from direct alkaline hydrolysis of corneal proteins, is believed to be the initial trigger for neutrophil invasion into the alkali-injured cornea.3 4 This finding created an opportunity to determine the active portions of the N-acetyl-PGP molecule. One avenue of approach is to investigate the activity of peptide analogs, which are structurally related to N-acetyl-PGP. To develop these structure–activity relationships, we synthesized a series of peptide analogs and evaluated them in a system that measures neutrophil polarization, the earliest stage of chemotaxis.
The major purpose of this study was to compare the activity of these peptides to the chemoattractant in an effort to uncover information about the relative importance of different portions of the N-acetyl-PGP molecule to biological activity. Additionally, these peptide analogs were tested as potential antagonists of N-acetyl-PGP.
Materials and Methods
Materials
Hanks’ balanced salt solution (HBSS) was purchased from
Gibco Laboratories (Chagrin Falls, OH). Calcium chloride, magnesium
chloride, sodium chloride, glutaraldehyde, and Ficoll (type 400) were
purchased from Sigma Chemical Co. (St. Louis, MO). Sodium phosphate
monobasic and sodium phosphate dibasic were obtained from Fisher
Scientific (Fair Lawn, NJ). Hypaque-76 was acquired from Winthrope
Laboratories (New York, NY). Leukotriene B4
(LTB4) was purchased from Biomol Research
Laboratories (Plymouth Meeting, PA). Solvents and reagents for peptide
synthesis were purchased from VWR Scientific Products (West Chester,
PA). Resin and suitably derivatized amino acids were purchased from
Advanced Chem Tech (Louisville, KY) or PerSeptive Biosystem
(Framingham, MA).
Peptide Synthesis and Isolation
Several peptide analogs, N-acetyl-P, GG, PGG,
gly-pro-hyp, and APGPR, were obtained from Sigma Chemical Co. Other
peptides, PG, GP, PGP, N-acetyl-PG, t-Boc-PGP,
and t-Boc-PGP-OMe, were synthesized by conventional solution
phase peptide chemistry. However, for large-scale synthesis of
N-acetyl-PGP, an alternative method was used to increase the
yield of the product. In this method, the dipeptide t-Boc-PG
was coupled to Pro-Merrifield resin (Nova Biochem, San Diego, CA) using
the dicyclohexylcarbodiimide/1-hydroxybenzotriazole procedure. After
the removal of the N-terminal protection and acetylation using acetic
anhydride, the peptide was cleaved from the resin, using anhydrous
hydrofluoric acid. The product was purified on a silica gel column,
using chloroform:methanol (90:10, vol/vol) as the eluent.
The starting compound for the synthesis of N-methyl-PGP was t-Boc-PGP. The removal of N terminus t-Boc and the addition of N-methyl group were accomplished by a modified Mannich reaction.5 Briefly, the reaction mixture consisted of 30% formaldehyde, 98% formic acid (50 times molar excess to t-Boc-PGP), and freshly prepared palladium black. The reaction mixture was incubated overnight at 50°C, and the reaction was followed for completion by thin-layer chromatography. After the filtration of palladium black, the reaction mixture was diluted with water and lyophilized.
The homogeneity of each peptide was confirmed by reversed-phase high-performance liquid chromatography (RP-HPLC) on a Vydac C18-analytical column equilibrated at a flow rate of 1.2 ml/min and eluted with a linear gradient from 0% to 30% acetonitrile in water (0.1% trifluoroacetic acid) in 30 minutes. Characterization was done using Electrospray Mass Spectrometry (Perkin-Elmer-Sciex API-3, Norwalk, CT). Quantitative amino acid analysis was performed to show the correct ratio of amino acids and to determine the peptide content for calculation of the final concentration.
The PGP polymer, N-(PGP)4-PGLG, was synthesized on a PerSeptive Biosystem 9050 Peptide synthesizer, using flow solid-phase peptide synthesis with Fmoc chemistry. This technique used preactivated Opfp amino acids with HOAt and preloaded PEG-PS resin. The polymer was purified by RP-HPLC on a Waters Delta Pack C18 A (300 x 39 mm ID). Purity was determined by RP-HPLC on a Dynamax C18 column (300 x 4.8 mm ID) equilibrated at a flow rate of 1 ml/min and eluted with a linear gradient from 5% to 80% CH3CN (0.1% TFA) in 40 minutes The identity of the polymer was confirmed by TOF-MALDI MS (PerSeptive Biosystem).
Preparation of Solutions
LTB4 was dissolved in ethanol and diluted
with HBSS (pH 7.3) to a final ethanol concentration of 0.001%.
Synthetic peptides were dissolved in HBSS (pH 7.3). When necessary, the
osmolality was adjusted between 280 and 320 mOsm by adding a small
amount of distilled water.
Neutrophil Isolation
These experiments followed the tenets of the Declaration of
Helsinki and were approved by the Human Research Committee at Brookwood
Medical Center. All donors signed written consent forms explaining the
nature and possible consequences of the study. Blood was collected from
only one donor each day. Following the technique of Ferrante and Thong,
neutrophils were isolated from fresh heparinized human blood by
centrifugation on Hypaque–Ficoll (density = 1.114), as previously
described.6
7
Isolated neutrophils (96%–99% viability)
were resuspended in HBSS with 15 mM phosphate buffer at room
temperature and gently agitated on a shaker. The purity of this cell
suspension was 
85% neutrophils and 
5% mononuclear
cells and platelets, with the remaining percentage consisting of red
blood cells. Purified neutrophils were used in the polarization assay.
All incubation mixtures were maintained between an osmolality of 280 to
320, a pH of 7.2 to 7.6, 15 mM phosphate buffer, and 50 µM
Ca2+ and 50 µM Mg2+.
Neutrophil Polarization Assay
The polarization assay was performed in a blind fashion. This
assay was used to measure the neutrophil response to
LTB4 or synthetic peptides by determining the
polarization index, a measure of the frequency and degree of cellular
shape change after exposure to a chemoattractant.8
Briefly, preincubated neutrophils (2 x 105)
were mixed with preincubated synthetic peptides in HBSS in a reaction
chamber (total volume = 100 µl) at 37°C for 5 minutes. At the
end of the incubation period, an aliquot was collected and mixed with
an equal volume of 4% glutaraldehyde for microscopic observation. The
remaining volume of each cell suspension was centrifuged immediately at
15,000g for 5 seconds to remove cells. The resulting
supernatant was analyzed for lactic dehydrogenase (LDH)
activity.9
All incubations generated LDH activity,
correlating with <5% cell death. Neutrophils in each sample were
observed microscopically and assigned scores of 0 (resting =
spherical cell with a smooth membrane), 1 (activated = irregular
cell with uneven membranes), or 2 (polarized = cell length
width x 2). Scores of 100 neutrophils for each sample were added
and corrected by subtracting negative control values (neutrophils in
HBSS only), which produced a polarization index. Bioactivity
(EC50) was expressed as the peptide concentration
required to produce a neutrophil polarization index equal to 50% of
the positive control (2 x 10-9 M
LTB4). Inhibition (ID50)
was expressed as the peptide concentration required to produce 50%
inhibition of the EC50 level of polarization.
Results
Polarization Activity of Peptide Analogs of
N-acetyl-PGP
Several peptide analogs of N-acetyl-PGP were
synthesized and evaluated in the neutrophil polarization assay (Table 1)
. The importance of the terminal amino acid residues was investigated
first. Biological activity decreased in a systematic manner when the
terminal amino acid(s) was omitted from the structure.
N-acetyl-PG was sevenfold less active than
N-acetyl-PGP, and activity was eliminated for the simple
blocked proline derivative, N-acetyl-P. Simple dipeptides,
GG, PG, or GP, and tripeptides, PGG and gly-pro-hyp, also were
inactive. Two tripeptides, t-Boc-PGP and
t-Boc-PGP-OMe, were synthesized to determine whether a free
carboxyl terminus is needed for biological activity. Although
t-Boc-PGP demonstrated sixfold less activity than
N-acetyl-PGP, it did retain substantial activity. The
tripeptide with blocking groups at both ends was inactive, suggesting
that a free carboxyl group is needed for activity. These results
demonstrated the importance of the terminal residues to induce
neutrophil polarization.
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Finally, we investigated if an extended PGP sequence could induce neutrophil polarization. A PGP polymer [N-(PGP)4-PGLG] containing 4 repeats of the PGP sequence was prepared. The activity of this peptide was 10-fold less than the simpler N-acetyl-blocked tripeptide, suggesting that these extended sequences are less effective than a shorter blocked PGP sequence. The most interesting discovery reported here deals with a pentapeptide, APGPR, containing the PGP sequence. This pentapeptide displayed activity equal to or slightly greater than N-acetyl-PGP.
Inhibitory Properties of Synthetic Analogs
Synthetic analogs showing no detectable bioactivity were tested as
potential inhibitors of N-acetyl-PGP–activated neutrophils.
Gly-pro-hyp (ID50 = 20 mM) was the only analog
showing measurable inhibition when concentrations up to 70 mM were
tested (Table 2)
.
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N-acetyl-PGP is generated directly by alkali degradation of corneal proteins.3 The synthesis of peptide analogs of this chemoattractant identified several similar, new bioactive peptides for neutrophils and a number of inactive peptides. The small size of the N-acetyl-PGP molecule and the ability to manipulate the amino acid sequence lended itself to studies that differentiated those components of the chemoattractant responsible for its activity. This was accomplished by selecting specific groups on the N-acetyl-PGP molecule to add, substitute, and/or omit during synthesis of the peptide analog.
For substantial bioactivity the analog required the presence of PGP alone or derivatives of PG in which the N-terminal proline is blocked. The presence of an acetyl- or t-Boc–blocking group on the N terminus of the tripeptide produces an uncharged amide at physiological pH. In contrast, PGP and N-methyl-PGP are very similar peptides in that both contain a positively charged amino N terminus at physiological pH. The favorable activity with a neutral amide elucidates a fuller understanding of the determinants of chemoattraction. This information is anticipated to prove useful in the structure-based design of antagonists.
In this study APGPR, an activation pentapeptide for procolipase, produced equal or slightly greater polarization activity than N-acetyl-PGP. The potent activity of this agonist, containing the PGP sequence, gives promise that more agonists could be designed to further enhance biological activity. Pursuit of this line of investigation might be in the direction of other tetrapeptides or pentapeptides containing the PGP sequence but with an amide-protected N terminus, such as N-acetyl-APGPR. These findings might further studies of the inflammatory process and receptor recognition.
The current approach generated one antagonist of neutrophil chemotaxis. High concentrations of gly-pro-hyp showed measurable inhibition of the chemoattractant. The potency of the gly-pro-hyp antagonist might be enhanced by further alteration of the molecule. A more feasible and powerful approach to secure a potent inhibitor might be the more time and cost efficient technology of using antisense peptides.10 This technology has yielded promising results in preliminary studies.
Acknowledgements
Amino acid analysis was performed by Kelly Morrison at the UAB Glycoprotein Analysis Core Facility.
Footnotes
Supported by National Eye Institute Grant EY04716. Mass spectrometry was performed by Marion Kirk at the UAB Comprehensive Cancer Center Mass Spectrometry Shared Facility, which is supported in part by a National Cancer Institute Core Research Support Grant P30CA13148. The mass spectrometer was purchased by funds from a National Institutes of Health Instrumentation Grant S10RR06487 and from UAB.
Submitted for publication September 28, 1998; accepted November 6, 1998.
Proprietary interest category: N.
Corresponding author: Roswell R. Pfister, Suite 504, 513 Brookwood Boulevard, Birmingham, AL 35209. E-mail: jhaddox@scott.net
References
This article has been cited by other articles:
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  A. Gaggar, P. L. Jackson, B. D. Noerager, P. J. O'Reilly, D. B. McQuaid, S. M. Rowe, J. P. Clancy, and J. E. Blalock A Novel Proteolytic Cascade Generates an Extracellular Matrix-Derived Chemoattractant in Chronic Neutrophilic Inflammation J. Immunol., April 15, 2008; 180(8): 5662 - 5669. [Abstract] [Full Text] [PDF]
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