Conformationally constrained mimetics of laminin peptide YIGSR as precursors for antimetastatic disintegrins

Article abstract of DOI:10.1021/jm901212n

Conformationally constrained mimetics of the laminin cell-adhesion site, YIGSR, are described. The site is the natural antagonist of the integrin-associated laminin receptor 1 (LAMR1) known to mediate metastatic tumor adhesion. The attachment of selected

Full text of DOI:10.1021/jm901212n

7966 J. Med. Chem. 2009, 52, 7966–7969
DOI: 10.1021/jm901212n
have been shown to mediate metastatic tumor cell adhesion
and to inhibit experimental metastasis without affecting other
cellular activities implicated in cell invasion.13 The peptides
neither influence proliferation rate or density of tumor cells
nor alter their tumorigenicity, which supports a nontoxic
mechanism of action.13,14 Thus, they act as disintegrins,
receptor antagonists that specifically block cells from binding
to the extracellular matrix.15 Consequently, designing syn-
thetic disintegrins can confer a very attractive strategy for
anticancer therapy.16 However, progress in this direction is
precluded by the requirement for the folding of an intact
protein to be “fixed” within a much smaller molecular space,
that is, within its receptor-recognition site.16 In turn, this
necessitates a conformation-oriented approach to ligand
mimicry, which yet remains hampered by the conformational
ambiguity of short peptide sequences. As a result, lower
selectivity and poor activity of isolated recognition sites when
compared with those of native proteins or, in fact, of the same
peptides but structured within the native proteins are obser-
ved. In this context, conformational constraining of cell-
adhesion motifs presents an obvious place to start.
Among different approaches, backbone or head-to-tail
cyclization is intrinsically compatible with native folding
and, therefore, is seen as the most efficient means toward
conformational mimetics.16 One notable example of the ap-
proach is cilengitide, a superpotent Rvβ3 integrin antagonist
developed by the Kessler group.17,18 This is a cyclic β-turn
mimetic derived from the RGD sequence presenting a com-
mon andthe mostabundant motif formany adhesionproteins
including snake venom disintegrins.15 RGD motif is located in
loop regions of the proteins where it is arranged into a cyclic
form by oxidized cysteine residues.15,16 Unlike RGD, the
LAMR1-binding peptides are primarily found in the III
domain of the β1 chain of laminin 1, which renders their
mimetics potentially more selective than RGD.13,14,19 The III
domain contains homologous regions outfitted with consen-
sus cysteine repeats that assemble into loop-rich structures
through the formation of disulfide bonds.11,13,20 Additionally,
1 has three turn-inducing residues, one proline (P) and two
glycines (G) which can provide two potential bending sites,
DPG and IGS. Therefore, it can be assumed that 1 and 2 have
inherent tendency for bend conformations. The assumption is
further strengthened by the fact that in the native setting 1 and
2 can be spatially fixed by flanking cysteine residues20 via
oxidation.
Conformationally Constrained Mimetics of
Laminin Peptide YIGSR as Precursors for
Antimetastatic Disintegrins
Angelo Bella,† Helen Lewis,† Jennifer Phu,‡
Andrew R. Bottrill,§ Sharad C. Mistry,§ Christine E. Pullar,
and Maxim G. Ryadnov*,†
†Department of Chemistry, University of Leicester, Leicester LE1
7RH, U.K., ‡School of Chemistry, University of Bristol, Cantock’s
Close, Bristol BS8 1TS, U.K., §Protein and Nucleic Acid Laboratory,
University of Leicester, Leicester LE1 7RH, U.K., and Department
of Cell Physiology and Pharmacology, School of Medical Sciences,
University of Leicester, Leicester, LE1 9HN, U.K.
Received August 14, 2009
Abstract: Conformationally constrained mimetics of the laminin
cell-adhesion site, YIGSR, are described. The site is the natural
antagonist of the integrin-associated laminin receptor 1 (LAMR1)
known to mediate metastatic tumor adhesion. The attachment of
selected metastatic cell lines toward the constrained antagonists has
been assessed. Observed differential responses prompted by folding
preferences of the mimetics revealed stronger attachment activities
for turnlike structures. The results permit the conformational design
of antimetastatic disintegrins.
The survival and metastasis of malignant cells strongly
depend on their ability to invade and migrate through base-
ment membranes.1 The process is regulated by cell-surface
receptors, integrins, and related non-integrin receptors, which
mediate cellular adhesion to basement proteins.2,3 One of such
proteins, laminin 1, iscritical forthe invasionand migrationof
metastasizing cells.4 A high-affinity receptor for laminin 1 has
been identified as LAMR1, a 67 kDa protein.5 The protein
appears to be associated with the R6 integrin subunit of R6β1
and R6β4 integrins.6,7 Malignant cells display increased ex-
pression of LAMR1 and the integrins compared to those
observed in normal or dysplastic cells.2-4,8 Tumor cells also
shed LAMR1 in a soluble form in amounts exceeding those
proportional to the overexpression of the receptor.9 This
abundance of free LAMR1 on cellular surfaces correlates
well with the enhanced invasiveness of tumor cells reflected in
their promoted aggressive malignant behavior.1-4 This is
complementary to tumor blood vessels expressing receptors
or receptor-associated proteins as specific markers for adhe-
sion proteins during angiogenesis.10 Taken together, the find-
ingsimplythatthe site-specific targeting of LAMR1-laminin1
interactions may sustain an efficient inhibition of metastasis.
In this regard, a recognition site responsible for the receptor
binding and identified in the β1 chain of laminin as a nine
amino acid sequence, CDPGYIGSR (1),11 with the pharma-
cophore segment comprising five residues, YIGSR (2),12
provides a straightforward platform for the specific inhibition
of laminin-promoted cell adhesion. Indeed, synthetic 1 and 2
On the basis of these conventions, we set out to generate a
series of 1/2 cyclopeptide mimetics and to explore correlations
between their induced cell-adhesion responses and conforma-
tional preferences. In designing the mimetics the main em-
phasis was made on imposing conformational constraints
within the pharmacophore site, YIGSR. Previous studies of
the sequence by others revealed that tyrosine and arginine are
essential for the cell-adhesion activity of the peptide.12,19,21
Indeed, substitutions at these positions invariably led to a
marked loss of the biological activity. Isoleucine and serine
residues are less defining, and their substitutions can give
comparably active derivatives. Glycine may be important for
a bend structure supporting the bioactive conformation.12,19,21
Collectively, the findings suggest that the spacing between
*To whom correspondence should be addressed. Phone: þ44 (0)116
252 2105. Fax: þ44 (0)116 252 3789. E-mail: max.ryadnov@le.ac.uk.
r
pubs.acs.org/jmc
Published on Web 11/24/2009
2009 American Chemical Society

Letter
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 24 7967
tions, questioning the interpretation of its activity. To circumvent
this, X2 was made D-serine to give c(NYIsSR) (6). Not only
was this peptide readily soluble in the medium but it also
showed dose-dependent increases in the activity (nearly 20%
and 40% at the lowest and higher peptide concentrations
respectively) (Figure 2a).
To reveal if the effect was conformation-driven, we probed
the peptides using circular dichroism (CDa) spectroscopy
(Supporting Information). Intriguingly, spectra for 3, 4, and
5 peptides were similar and characteristic of random coil
conformations (5 not shown) (Figure S1 and Table S2). In
marked contrast, spectra typical of the β-form with minimum
and maximum at 216 and 196-200 nm, respectively, were
recorded for 6 (Figure S1). Although certain ambiguity
remains in the assessment of bend conformations, the spectra
were consistent with those reported for β-turns.22-24 There-
fore, it was reasonable to assume that the side chains of polar
D-amino acids incorporated at the X2 position can be involved
in intramolecular interactions whereby more active confor-
mations (presumably turns) are induced. To follow this, we
made two other peptides, 7 and 8, with X2 positions occupied
byanionicglutamateandcationic lysineresidues, respectively.
8 was noticeably less active for cell attachment than 6, but the
activity of 7 was almost comparable with that of 6. Interest-
ingly, CD spectroscopy revealed characteristic β-structure
bands at ∼216 nm for 7 and 8. However, unlike for 6 the
maximum at 196 nm was not apparent in both peptides,
suggesting a contribution of an open conformation.24 The
latter is featured by a distinctive minimum at 198-200 nm, the
emergence of which from 6 to 8 was particularly evident
(Figure S1). This suggests that the side chain of D-lysine
may relieve the induced constraints, for example, as a result
of eliminated intramolecular interactions that are otherwise
stabilizing for 6 and 7. One way to confirm this is to extend 6
into a more open cycle. With this in mind, a 6 dimer (9), in
which two sequential copies of NYIsSR are cyclized, was
made (Figure 1b, Tables S1 and S2). In this peptide both
copies are in the ring, but constraints imposed in each are
weaker allowing thus a greater, yet still limited, degree of
flexibility. This resulted in a substantial increase in CD signals
at 198 nm for 9 compared with 6 and 8. The signals were
similar to those of 3 and 4 and so were comparably weak cell-
adhesion activities of the peptides (Figures 2 and S1, Table
S2). Furthermore, the behavior of the 4 dimer (10) was almost
identical to that of 2: no appreciable secondary structure and
comparable cell adhesion activity. Thus, taken together, the
results emphasize the strong correlation between the cell
attachment and conformational constraining of 2 within the
cyclic template. In this respect, the ring size may play an
equally important role. Indeed, it is generally proposed that
small cycles provide more constrained and therefore more
stable conformations.16 These are not necessarily bioactive, as
the requirement for matching structure with activity remains
dependent on mimicking active sites in native environments
rather than on mimicking elementary motifs.16,21
Figure 1. Designed LAMR1 antagonists: (a) cyclopeptide template
c(X1YIX2SR) with variables X1 and X2; (b) table summary of
studied peptides. X0 denotes the extra X positions (X1YIX2-
SRX1 YIX2 SR) for dimers 9 and 10. D-Amino acids are in lower-
case.
0
0
tyrosinyl and arginyl side chains and their spatial orientation
are critical for the activity.
To establish whether these can be best matched in a
restricted framework, we designed a cyclic hexapeptide tem-
plate, c(X1YIX2SR) (Figure 1). In this template tyrosine and
arginine residues are separated by three (IX2S) residues as in
the native sequence and one (X1) residue serving as a spacer.
The only smaller cyclic arrangement for the peptide is cyclo-
pentapeptide. However, in the cyclopentapeptide arrange-
ment tyrosine and arginine residues are directly linked,
which may compromise the match. By contrast, hexapeptide
templates permit the degree of flexibility shown to be neces-
sarily sufficient for conformational design.16-18 For example,
single glycine, proline, or D-amino acid residues in cyclohex-
apeptides induce turn arrangements and can be incorporated
to allow for the conformational screening of bioactive
forms.16 In our template, X1 and X2 are variables. X2 is
occupied by the native glycine, proline, or a D-amino acid to
favor bending. X1 is reserved for asparagine as a polar and
neutral spacer and to afford the on-resin cyclization of the
peptides (Scheme S1, Supporting Information). X1 is also to
be used to optimize the bend around X2, for which bend-
promoting residues or motifs can be used.
Initial screening for active conformations of 2 was per-
formed using the c(NYIX2SR) template. In the assignment
of X2 position, two main criteria were applied. First, the site
should bend; hence, glycine, proline, or D-amino acid is
used. Second, the site should not interfere with the autono-
mous folding of the template; hence, sterically hindered or
hydrophobic residues that are potentially conducive to
insolubility or aggregation problems had to be avoided
and only small and polar residues can be used. Following
these criteria, the library comprising individually con-
structed peptides was generated. The ability of each peptide
to mediate cell adhesion was tested at four concentrations
using HT-1080 human fibrosarcoma cells. These cells are
highly metastatic and exhibit a high affinity to laminin 1,
which makes this cell line a standard model system for cell
attachment.11-14
a Abbreviations: OAll, allyl; CD, circular dichroism; DCM, dichro-
methane; DIPEA, N,N-diisopropylethylamine; DMF, dimethylforma-
mide; FAB, fast atom bombardment; HCTU, (2-(6-chloro-1H-benzo-
triazole-1-yl)-1,1,3,3-tetramethylaminium hexafluorophosphate); RP-HPLC,
reversed-phase high performance liquid chromatography; MALDI-
ToF, matrix-assisted laser desorption ionization time-of-flight; MOPS,
3-(N-morpholino)propanesulfonic acid; Pbf, 2,2,4,6,7-pentamethylben-
zofuran-5-sulfonyl; PBS, phosphate buffered saline; TFA, trifluoroace-
tic acid; TFE, trifluoroethanol; TIS, triisopropysilane.
The first peptide in the series, c(NYIGSR) (3), was made to
test the effect of the template on the activity of 2. No increase
in cell attachment was observed for the peptide when com-
pared to 2. The substitutions of glycine for proline (4) and
D-alanine (5) led to similar activities. However, 5 proved to be
poorly soluble in the cell culture medium at higher concentra-

7968 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 24
Bella et al.
differences in the activity were reflected in the conformational
preferences of the peptides (Figure S1 and Table S2). CD
signals for 14 were associated with an increased minimum of
∼200 nm and represented a typical random coil conforma-
tion. In the case of 13 the minimum was also apparent but
shifted to 208 nm which with another minimum at 222 nm can
be characteristic of a helical conformation. In this light, the
transition from a β-structure in 6 to a helix in 13 indicates that
7 maybe pronetoadoptinga helical structure, which is proven
to lead to the impairing of adhesion. This is also supported by
the folding of 12 into a β-form with retention of the cell
attachment activity of 6. Further, CD spectra of 11 may
represent an intermediate state including the features of
a β-form and a helix. The activity of the peptides also seems
to be intermediate compared to those of 3 and 6. Although not
fully conclusive with regard to which structural type is re-
sponsible for the increase in the activity of 3, this is clearly
attributable to DPG. Nevertheless, being fairly moderate, the
effect poses a question of whether more efficient constraints
can be afforded by a single bending residue in X1 position. To
clarify this, we made two peptides derived from 6, with
asparagine replaced with bend-promoting glycine and D-pro-
line residues, 15 and 16, respectively (Figure 1b, Scheme S2,
and Table S1). Gratifyingly, CD signatures for 15 and 16 were
remarkably similar to those for 11 and 12 and so were the
corresponding cell attachment activities (Figures 2b and S1).
To this end, the ability of the designed mimetics to mediate
adhesion of HT-1080 cells that are known to adhere well to
laminin and laminin-like peptides was studied. To test if the
same applies to other cell types, we probed mimetics using
mouse melanoma B16-F10 cells. Cells of this type are highly
metastatic and have been used as tumor cell adhesion and
metastasis models.11-13 The cells have reportedly been shown
to readily adhere to laminin 1. However, their attachment to 2
or laminin-like peptides is less pronounced and often lacks
consistency between different design series.11-13,19,25 This can
yet be beneficial for estimating the strength of the effect
observed for HT-1080 cells and their selective response to
the designed mimetics. With this in mind, we tested several
mimetics for their ability to support the adhesion of B16-F10
cells. Generally, the extent of the attachment was found to be
comparable to that for HT-1080. Yet a few distinctive features
in the behavior of B16-F10 cells were observed. Consistent
with the data for HT-1080, B16-F10 gave dose response
attachments (Figure 2c). However, these were only for 3, 4,
8, and 2, the activities of which increased with increasing
concentrations. In contrast, responses to 6 and 7 were not
concentration-dependent, with 6 being notably less active for
B16-F10 than for HT-1080. Structurally similar 11 and 15
exhibited different attachment patterns that appear to be only
in part consistent with those observed for HT-1080 cells: 11 can
be grouped with 3, 4, and 8, whereas 15 gave responses compar-
able with those of 6 and 7 (Figures 2c and S1). Additionally, the
activities within the 6 < 7 < 15 row were reverse to those found
for HT-1080 cells (Figure 2c and Table S2).
Figure 2. Attachment of HT-1080 (a, b), B16-F10 (c), and HeLa
(d) cells to peptide-coated plates. Cell attachments to laminin 1 and
BSA are taken as 100% and 0%. Each bar represents the mean of
two independent experiments done in triplicate.
Admittedly, this varies from one case to another. However,
one main principle in finding matched mimetics is always to
consider the secondary structure within and near the active
site. This may require more extended cyclic systems providing
the stabilizing contribution of added residues. Related to the
LAMR1 antagonists, the aforementioned DPG in 1 may
impose additional constraints on 2 to tighten the bioactive
conformation. To probe this, X1 was made DPG to give a
series of cyclooctapeptide derivatives c(DPGYIX2SR), where
X2=glycine (11), D-serine (12), D-glutamate (13), and D-lysine
(14) (Figure 1). All peptides, except 11, were found to be less
active for cell attachment than their cyclohexapeptide coun-
terparts (Figure 2b, Tables S1 and S2). 13 and 14 showed
weaker activities than 7 and 8. The activities of 11 and 12 were
higher and similar to those of 3 and 6, correspondingly. The
Overall, the observed differences were not substantial
prompting the conclusion that B16-F10 cells are less respon-
sive to conformational changes in 2. Thus, consistent with
earlier observations,12,19,25 HT-1080 and B16-F10 readily
adhere to laminin 1 but respond to 2-derived peptides differ-
ently. Related to our case, it can be speculated that HT-1080
cells provide differential attachment responses to the designed
mimetics, while B16-F10 cells react nonspecifically, that is,
strong adherence to laminin 1 and nondifferential response to

Letter
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 24 7969
the mimetics. In this vein, a cell type with less specific adhesion
properties toward laminin 1 may give a lower cut-off in the
activity, within which differential or no response to the
conformational mimetics may be observed. To establish this,
we performed the same set of tests using HeLa cells. These are
human cervical carcinoma cells that were shown to be con-
ditionally adherent to laminin and its components.26,27 Little
has been reported on the binding of HeLa cells to 2-like
peptides, which may partly be attributed to the moderate
affinity of the cells to the peptides.28 In our case, HeLa
adhered to laminin 1 less efficiently than did HT-1080 and
B16-F10 cells (Figure 2d). Likewise, lower but medially dose-
dependent attachments were observed for the mimetics. On
the basis of the expressed activities, two groupings could be
identified. In one, peptides that gave more appreciable β-form
CD spectra (6, 7, 15) also gave stronger attachment responses
(Figures 2d and S1, Table S2). In the other, structurally
amorphous 3, 4, and 8 yielded lower responses that were also
comparable with those for 2. 11, which showed intermediate
activities in HT-1080 and B16-F10, could also be placed
between the two groups in HeLa.
(8) Hand, P. H.; Thor, A.; Schlom, J.; Rao, C. N.; Liotta, L. Expres-
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tissues as defined by a monoclonal antibody. Cancer Res. 1985,
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integrin ligands. Methods Enzymol. 2007, 426, 463–503.
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In summary, the obtained results reveal the general ten-
dency of the designed mimetics with more appreciable β-con-
formations to generate stronger cell attachment responses.
Although the observed effect is not sufficiently discriminative
to single out one particular construct, the findings stress the
conformational constraining of isolated cell-adhesion sites as
an efficient means toward the development of selective anti-
metastatic disintegrins.
€
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Acknowledgment. We thank Bristol Cancer Research
Fund and University of Leicester for financial support and
Harry Mellor, Ruth Rollason, and George Banting (Univer-
sity of Bristol) for help with biological assays.
Supporting Information Available: Experimental procedures
including synthesis, characterization, spectroscopic data, and
biological assays. This material is available free of charge via the
Internet at http://pubs.acs.org.
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