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Departments of
*
Cardiothoracic Surgery and
Pediatrics, Stanford University, Stanford, CA 94305
| Abstract |
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-helices
separated by short loop regions. In this report, synthetic peptides
corresponding to the linear granulysin sequence were characterized for
lytic activity. Peptides corresponding to the central region of
granulysin lyse bacteria, human cells, and synthetic liposomes, while
peptides corresponding to the amino or carboxyl regions are not lytic.
Peptides corresponding to either helix 2 or helix 3 lyse bacteria,
while lysis of human cells and liposomes is dependent on the helix 3
sequence. Peptides in which positively charged arginine residues are
substituted with neutral glutamine exhibit reduced lysis of all three
targets. While reduction of recombinant 9-kDa granulysin increases
lysis of Jurkat cells, reduction of cysteine-containing granulysin
peptides decreases lysis of Jurkat cells. In contrast, lysis of
bacteria by recombinant granulysin or by cysteine-containing granulysin
peptides is unaffected by reducing conditions. Jurkat cells transfected
with either CrmA or Bcl-2 are protected from lysis by recombinant
granulysin or the peptides. Differential activity of granulysin
peptides against tumor cells and bacteria may be exploited to develop
specific antibiotics without toxicity for mammalian
cells. | Introduction |
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Granulysin is highly homologous to NK-lysin, a cytotoxic and
anti-microbial molecule expressed in porcine CTL and NK cells
(6). NK-lysin has a high
-helical content and is folded
tightly through three intramolecular disulfide bonds formed by six
conserved cysteine residues (7, 8). By analogy to
NK-lysin, granulysin is predicted to adopt a similar structure
(1), although the 9-kDa form of granulysin contains only
four cysteine residues that form two rather than three predicted
intramolecular disulfide bonds. In contrast to NK-lysin, which loses
its lytic activity against tumor cells and bacteria after reduction of
disulfide bonds (9), we observed that reduction of
recombinant granulysin enhanced its lytic activity against tumor
targets and did not affect its activity against bacteria. This
observation prompted us to design a panel of synthetic peptides
corresponding to linear regions of granulysin and to assess their lytic
activity. Peptides from the central region of granulysin exhibit most
of the lytic activity. Substitution analysis revealed that positively
charged residues in these peptides are critical for lysis of both tumor
cells and bacteria. Reduction of cysteine residues affects lysis of
tumor targets but not bacteria. Peptides that selectively lyse
bacterial targets may provide the basis for development of antibiotics
with limited toxicity for human cells.
| Materials and Methods |
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Peptides were synthesized using F-moc chemistry on an Applied Biosystems (Foster City, CA) automatic peptide synthesizer, purified to >95% homogeneity by reverse-phase HPLC, and peptide composition confirmed by mass spectrometry and amino acid analysis. Stock peptide solutions (515 mM) were prepared in DMSO and diluted into assay medium at 150 µM.
Cell lines and Abs
The human EBV-transformed B lymphoma cell line JY and T cell tumor line Jurkat were maintained in RPMI 1640 (Irvine Scientific, Irvine, CA) supplemented with 10% heat-activated FCS (HyClone, Logan, UT) and 2 mM L-glutamine. Jurkat cells transfected with Bcl-2 or CrmA were maintained in medium supplemented with 0.5 mg/ml G418 as described (10). Anti-Fas mAb (CH-11) was purchased from Medical and Biological Laboratories (Nagoya, Japan).
Granulysin
Recombinant refolded granulysin was prepared as described (3, 4, 5). Briefly, recombinant 9-kDa granulysin was expressed using the vector pET28a (Novagen, Madison, WI). The fusion protein was purified in 6 M guanidine HCl on nickel affinity resin. After refolding and dialysis, the histidine tag was removed by thrombin cleavage and the material was further purified by reverse-phase HPLC. After lyophilization, the protein was suspended in PBS, and the concentration was determined using a protein assay (Bio-Rad, Hercules, CA) with lysozyme as a standard. The recombinant refolded granulysin exhibits two features that indicate that it assumes a similar tertiary structure to granulysin produced in CTL. First, both types of granulysin migrate faster in SDS-PAGE under nonreducing than reducing conditions. Second, while the granulysin-specific mAbs DH2 and DH4 recognize both recombinant and cellular granulysin, treatment of both types of granulysin with DTT abrogates recognition by DH2 but does not affect recognition by DH4 (3).
Liposome disruption assay
Granulysin-induced lysis of liposomes was measured using the fluorescent probe 1-aminonaphtlene-3,6,8-trisulfonic acid and its collisional quencher N,N'-(p-phenylenedimethylene)bis(pridiniumbromide) (Molecular Probes, Eugene, OR). Liposomes were prepared using palmitoyl-oleoyl-phosphatidyl-glycerol or a combination of dipalmitoyl-phosphatidylcholine and egg phosphatidylglycerol. Large unilamellar vesicles were prepared as previously described (11). In experiments comparing reduced and nonreduced granulysin, palmitoyl-oleoyl-phosphatidyl-glycerol liposomes (25 µM phospholipids) were treated with 550 nM granulysin. For reducing conditions, liposomes were suspended in TBS supplemented with 1 mM DTT. DTT had no effect on baseline or final fluorescence. In experiments testing granulysin peptides, dipalmitoyl-phosphatidylcholine plus egg phosphatidylglycerol liposomes (20 µM phospholipids) were treated with 0.5 µM granulysin peptides. Release of fluorescent probe was measured on a spectrophotometer as described (11). Data is plotted as percent specific lysis recorded over time.
Granulysin-mediated human cell lysis
Jurkat T cells in log-phase culture or freshly isolated PBMC were washed once with RPMI 1640 medium supplemented with 0.01% BCS and resuspended at a density of 1 x 106/ml. Cells (50 µl) were incubated with 50 µl of peptide in a 96-well plate at 37°C for 4 h. Then 50 µl of PBS containing 3 µl of 50 µg/ml of propidium iodide (PI)4 was added to each well, and the PI-containing cells were enumerated by FACScan (Becton Dickinson). Data was analyzed with CellQuest software (Becton Dickinson). The PI-negative cell population in control sample was set as a reference to determine the percentage of PI-negative cells in all other samples. Similar results were obtained using both PBMC and Jurkat cells.
Granulysin-mediated lysis of Salmonella
A log-phase culture of Salmonella typhimurium was diluted to 2 x 105/ml in 10 mM sodium phosphate buffer (pH 7.4) containing 0.03% Luria-Bertani broth. A 25-µl aliquot of Salmonella was incubated with 25 µl of peptide at 37°C for 3 h. The bacteria and peptide mixture was diluted 100-fold with the cold phosphate buffer and plated on LB plates that were incubated at 37°C overnight. Bacterial colonies were enumerated the following day.
Treatment with DTT
Recombinant granulysin or peptide (500 µM) was incubated with 1 mM DTT and, where indicated, boiled for 10 min and then diluted to the indicated concentrations in assay medium supplemented with 250 µM DTT. This concentration of DTT had no effect on PI uptake of Jurkat cells.
| Results |
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A hallmark of SAPLIP family members is the characteristic spacing
of conserved cysteine residues. These cysteine residues are predicted
to form intramolecular disulfide bonds critical for protein structure
and function. Reduction of disulfide bonds in NK-lysin, the SAPLIP
member most similar to granulysin, eliminated its lytic activity
against bacteria and tumor cells (9). To test whether the
disulfide bonds in granulysin contribute to its function, recombinant
9-kDa granulysin was treated with 1 mM DTT and boiled for 10 min.
Surprisingly, boiled and reduced granulysin is
25 times more potent
than refolded granulysin at lysing Jurkat cells, as judged by PI uptake
(Fig. 1
A). Consistent with
this observation, disruption of liposomes is more complete using boiled
and reduced rather than refolded granulysin (Fig. 1
B). A
mutant construct of granulysin was generated in which all four cysteine
residues were changed to serine. After expression, purification, and
refolding, the serine-substituted recombinant granulysin exhibited
increased lytic activity relative to the cysteine-containing molecule
(not shown). In contrast, boiled and reduced granulysin was equipotent
to refolded granulysin in lysis of Salmonella (Fig. 1
C). These findings indicate that lytic activity of
recombinant granulysin does not depend on disulfide bonds or three
dimensional structure and that short linear regions of the granulysin
sequence might mediate lysis. Therefore, we prepared a panel of
synthetic peptides corresponding to linear sequences of granulysin and
assessed their lytic activity.
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As a first step, five overlapping synthetic peptides (G1 to G5)
and two longer peptides (G6 and G7) were synthesized (Table I
). In addition, all cysteine residues
were substituted with serine. Only peptides G3 and G7, which correspond
to the central region of granulysin, lyse Jurkat and
Salmonella (not shown). Peptides corresponding to the amino
(G1, G2, G6) or carboxyl (G4, G5) regions of granulysin do not lyse
either Jurkat or Salmonella at concentrations up to 100 µM
(not shown).
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Two of the five residues in the loop region between helix 2 and
helix 3 (designated loop 2) are positively charged arginine residues
(Table I
). In other bactericidal peptides, positive charges are
critical for lytic activity (12, 13). To analyze the
contribution of the loop 2 region to lysis, peptides G13, G14, and G15
(Table I
) were synthesized and assayed against all three targets (Table II
and Fig. 3
). Peptide G13 contains
helix 2 plus loop 2, while G14 contains loop 2 plus helix 3. Both
peptides exhibit higher lytic activity against bacteria than their
shorter counterparts, G10 and G11, which lack the loop 2 sequence. G13
does not lyse Jurkat cells or liposomes, while G14 shows intermediate
activity against Jurkat targets and efficiently lyses liposomes. G15,
in which the two positively charged arginine residues in loop 2 are
replaced with glutamine, shows reduced activity against bacteria and
liposomes and does not lyse Jurkat cells at all. Thus, lysis of Jurkat
and liposomes by these peptides is dependent on the presence of helix
3, and the positively charged residues in loop 2 are critical for the
lytic function of these peptides.
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The wild-type peptide G8 and its serine substitute counterpart,
G9, both are potent anti-bacterial agents, but G9 exhibits
significantly less activity against tumor cells than G8 (Table II
and
Fig. 2
). This indicates that disulfide bonds are important for
anti-tumor activity but are not required for anti-bacterial
activity. To confirm this, granulysin peptides were diluted in medium
containing 1 mM DTT and then incubated with bacteria or tumor cells in
medium supplemented with DTT. These conditions had no detectable effect
on the viability of Jurkat cells, but did slightly decrease the
viability of Salmonella (data not shown). In the presence of
DTT, G8 (cysteines at residues 34 and 45) and G14 (cysteine at residue
45) lose most of their anti-tumor activity (Fig. 4
). DTT does not affect the
anti-bacterial activity of G8 or G14. These findings suggest that
both intra- and intermolecular disulfide bonds likely affect the
activity of peptides derived from granulysin.
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Previously, we found that granulysin-induced apoptosis of Jurkat
cells is inhibited by Bcl-2 and CrmA.3 CrmA
protection implicates caspases in the death pathway (14),
while Bcl-2 protection suggests that the apoptotic pathway induced by
granulysin involves mitochondrial damage (15, 16). To
determine whether apoptosis induced by granulysin peptides was similar
to that induced by recombinant granulysin, we assessed PI uptake in
Jurkat cells transfected with Bcl-2, CrmA, or vector control (Fig. 5
). Treatment of vector control cells
with anti-Fas mAb or recombinant granulysin caused PI uptake in
>80% of cells, while both Bcl-2 and CrmA reduced granulysin and
anti-Fas mAb-induced death. PI uptake
mediated by G8 and G14 was significantly reduced in the Bcl-2 and CrmA
transfectants, suggesting that these peptides induce apoptosis by a
similar mechanism to intact 9-kDa granulysin.
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| Discussion |
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In addition to the highly specific, cell-mediated immune system, vertebrates and other organisms fight infections by releasing broad-spectrum anti-microbial peptides (19, 20, 21). Currently, several hundred structures with varying degrees of anti-microbial activity have been described, but very few are of human origin. These "peptide antibiotics" can be divided into three major groups based on gross motifs and three-dimensional structure; sequence similarities within each group are almost negligible. Group I consists of linear peptides that lack cysteine residues, including cecropins, magainins, bombinins, and temporins. Group II consists of peptides with an even number of cysteines intralinked by disulfide bridges, including insect and mammalian defensins. Group III consists of peptides containing a very high content of unusual amino acids, most often proline and arginine together, including porcine PR-39 and bovine Bac5. Peptide antibiotics are usually synthesized as inactive precursors, two to five times the size of active effector molecules.
Granulysin is a member of group II, as it contains four cysteine residues that can form disulfide bonds. NK-lysin, the nearest homologue of granulysin, loses all activity when its disulfide bonds are reduced (9). In contrast, reduction of recombinant granulysin increases lysis of tumors while not changing activity against bacteria. Surprisingly, DTT treatment of peptide G8, which comprises the central helices of granulysin, decreases lysis of Jurkat targets but does not affect lysis of Salmonella. The cytolytic activity of G14, which contains only one cysteine and therefore cannot form intramolecular disulfide bridges, is also sensitive to DTT, indicating that both inter- and intramolecular bonds affect the activity of granulysin and peptides derived from it.
A high content of positively charged amino acids spread throughout the peptide is another characteristic of many anti-microbial peptides (19, 20). In granulysin, nine of the 29 residues that comprise helix 2 through helix 3 are arginines. However, a high number of positively charged amino acids is not sufficient for lytic activity: seven of the 24 residues in peptide G6 are positive, but it does not exhibit activity against either bacteria or tumor targets. Substitution of two arginine residues with glutamine in G15 abrogates lytic activity against Jurkat and somewhat reduces activity against Salmonella.
The mechanism by which peptides G8 and G14 cause apoptosis shares critical features with that of recombinant granulysin. Transfectants expressing Bcl-2 or CrmA are protected from lysis by all three molecules. The anti-apoptotic effect of Bcl-2 on granulysin-induced apoptosis indicates that mitochondrial damage is a critical step in granulysin-induced apoptosis (15, 16). Using the caspase inhibitor z-VAD-fmk, we previously showed that caspases are involved in granulysin-mediated apoptosis (4). This observation is extended here, as overexpression of CrmA, which binds and inactivates certain processed caspases, including activated caspase 8 (22), protects from granulysin- and peptide-mediated apoptosis.
Our findings suggest that the mechanism of lysis of bacteria and tumors by granulysin peptides are different. Peptides containing either helix 2 or helix 3 lyse bacteria, while only peptides containing helix 3 lyse tumor targets. Both intra- and intermolecular disulfide bonds are necessary for peptide lysis of tumor targets, whereas lysis of bacteria is unaffected by reduction of disulfide bonds. In contrast, peptide lysis of both tumor targets and bacteria is sensitive to substitution of arginine residues in loop 2. Collectively, these differences may provide the basis for development of novel selective antibiotics.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Carol Clayberger, Department of Cardiothoracic Surgery, Stanford University, Falk Cardiovascular Research Building, Room 116, Stanford, CA 94305-5247. ![]()
3 A. Kaspar, F. R. Poulain, Kelekar, D. Hanson, Hitoshi, D. E. Johnson, C. J. Froelich, C. B. Thompson, A. Anel, C. Clayberger, and A. M. Krensky. Granulysin-induced apoptosis involves membrane disruption, mitochondrial damage, and capsase 9 activation. Submitted for publication. ![]()
4 Abbreviation used in this paper: PI, propidium iodide. ![]()
Received for publication November 22, 1999. Accepted for publication May 16, 2000.
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