3ß1 Is an Alternative Cellular Receptor for Adenovirus Serotype 5
Department of Genetics and Molecular Biology, University of Rome "La Sapienza,",1 Institute of Cell Biology and Pathology, CNR,3 Laboratory of Cell Biology, IDI, Rome,5 Parco Scientifico Biomedico di Roma, San Raffaele, Italy,2 Laboratoire de Virologie et Pathogénèse Virale, CNRS UMR-5537, Faculté de Médecine Laennec, Lyon, France4
Received 16 May 2003/ Accepted 5 September 2003
| ABSTRACT |
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3ß1 for adenovirus serotype 5
(Ad5). We found that penton base and integrin
3ß1
could interact in vitro. In vivo, both Ad5-cell binding and
virus-mediated transduction were inhibited in the presence of
anti-
3 and anti-ß1 function-blocking antibodies, and
this occurred in both CAR-positive and CAR-negative cell lines. Peptide
library screenings and data from binding experiments with wild-type and
mutant penton base proteins suggest that the Arg-Gly-Asp (RGD) in the
penton base protein, the best known integrin binding motif, is only
part of the binding interface with
3ß1, which involved
multiple additional contact
sites. | TEXT |
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vß1, -3, -5,
-6, and -8 integrins, the
5ß1 and
8ß1
integrins, and the
IIbß3 integrins form a subgroup
that primarily recognizes ligands containing Arg-Gly-Asp (RGD) motifs
(see reference 13 and
references therein). Many microorganisms utilize integrins to gain
entry into cells: the SA11 rotavirus binds to
2ß1 and
4ß1 (9),
vß3 and
vß1 integrins are receptors
of the human parechovirus 1
(30), and
vß5 has been proposed, although not conclusively, as a
coreceptor in adeno-associated virus type 2 infection
(27,
29). The foot-and-mouth
disease virus uses different integrins for cell infection
(14,
15,
16). Integrin
3ß1 is a cellular receptor for Kaposi's
sarcoma-associated herpesvirus
(1). Yersinia
pseudotuberculosis binds to members of the ß1 integrin
family in order to enter eukaryotic cells
(22).
Several Ad
serotypes contain an RGD motif in the penton base protein. This
feature, and the Ad cell-detaching property, suggested an interaction
of the virus with the integrin receptors. Indeed,
vß3
and
vß5 are receptors for human Ad2 and Ad5, and
direct binding to isolated
vß5 was shown for human
Ad2, Ad3, Ad4, Ad5, and Ad37
(24,
31). In hematopoietic and
melanoma cells, respectively, the
Mß2 and
b1 integrins were found to be implicated in human Ad5
infection (3,
12). More recent evidence
indicates
vß1 as an Ad2 and Ad5 coreceptor in the
human embryonic kidney (HEK293) cell line
(23). Ad interaction with
the
vß1, -3, and -5 integrin subtypes is efficiently
competed by RGD-containing peptides
(23,
31). A second integrin
binding motif is present in the penton base protein of several Ad
serotypes, the triplet Leu-Asp-Val (LDV). Its functional role in the
interaction with the target cell was demonstrated by Karayan and
coworkers, who showed a significant reduced effect of cell detachment
of the Ad5 D288K penton base mutant protein
(17).
Previous work
has shown that a recombinant filamentous phage displaying the human Ad2
penton base protein bound not only to integrins
vß3
and
vß5 but also to the subtype
3ß1
(4). This
integrin is primarily a receptor for laminin, although it recognizes
additional ligands, such as collagens, epiligrin, thrombospondin, and
fibronectin.
3ß1 is widely expressed on nearly all
tissue types and is particularly abundant on endothelial and epithelial
cells. It is also found on nearly all rapidly growing adherent cell
lines (21).
3ß1 is an enigmatic integrin subtype, since it can
recognize ligands in both RGD-dependent and RGD-independent manners
(5,
8).
In this study,
we investigated the interaction between human Ad and the integrin
subtype
3ß1. We present evidence that this surface
molecule binds to the capsid protein penton base. In addition,
3ß1 seems to be involved in viral infection in both
CAR-positive and CAR-negative cells. To identify the amino acid
residues implicated in the interaction, we screened a random peptide
library for integrin
3ß1. The results of this
screening and the results of binding and binding competition
experiments suggest that the LDV tripeptide is not involved in the
penton base-
3ß1 interaction and that the RGD motif is
only part of multiple binding sites between the penton base and
integrin
3ß1.
The Ad penton
base specifically binds to
3ß1 in vitro.
In order to validate the biological
significance of the data obtained with
3ß1 with
recombinant phage (4), we
analyzed the integrin binding pattern of purified wild-type (WT) penton
base produced as a recombinant protein in insect cells by using
recombinant baculovirus. The penton base protein produced in this
system keeps its biological properties, i.e., its ability to form
pentamers, to bind to fiber, to form penton capsomers, to bind to
integrin receptors, to undergo endocytosis, and to migrate to the
nuclear pore complex
(10). The
3ß1 integrin was coated, and plates were incubated
with recombinant penton base protein alone or in the presence of
different competitors. Bound penton base was detected by the addition
of primary anti-Ad antiserum and secondary horseradish
peroxidase-conjugated anti-rabbit monoclonal antibody (MAb). For
comparison, penton base binding was tested on the immobilized Ad
receptor
vß3. As shown in Fig.
1A,
penton base binding to
3ß1 was inhibited in the
presence of the GRGDSP peptide.
However, the competition was substantially weaker than that observed
with the
vß3 receptor. The control peptide
GRGESP was not able to significantly
affect the binding of the penton base to either of the two integrins
tested. The LDV motif has been suggested to act as a possible
alternative integrin ligand in Ad penton base, and LDV occurs in the
human Ad2 and Ad5 penton bases at positions 287 to 289. In the presence
of the peptide PALLDVDA, we observed a
60 to 70% reduction of penton base binding to
vß3 but only a 25% reduction for
3ß1. These data suggest that RGD motifs are involved
to a limited extent only in the physical interaction between the penton
base and
3ß1 and that the LDV motifs have an even more
minor role, if any, in this interaction. To check for specificity,
binding to both integrins was tested in the presence of anti-
3
antibody. As expected, anti-
3 antibody competed with the
penton base for binding to
3ß1only. As a negative
control, anti-
5ß1 antibody was used. The binding of
the penton base to
vß3 and
3ß1 was
not affected by the addition of this antibody.
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3ß1
integrin, we analyzed the binding to
vß3 and
3ß1 of penton base protein mutants modified in the RGD
or LDV peptides. As shown in Fig.
1B, the R340E mutation
only partially impaired the penton base-
3ß1
interaction, whereas the D288K mutation had no detectable effect. Both
mutants bound to
vß3 with lower affinity than did the
WT penton base, as previously reported
(17).
It has been
suggested that
3ß1 may have multiple binding sites, or
at least multiple ligand binding mechanisms. In particular, it has been
demonstrated that binding to laminin or collagen depends on
Ca2+. Conversely, the interaction with fibronectin
is favored in the presence of Mn2+
(5,
8). We found that the
binding of the penton base to integrin
3ß1, as well as
to
vß3, was completely abolished by the chelating
agent EDTA (20 mM), indicating that both interactions are ion dependent
(Fig. 1A). However, the
pattern of binding in the presence of different ions was different for
the two receptors. For
3ß1, the binding that was
observed in the presence of Ca2+ was better than
that in Mg2+, while Mn2+ was
unable to support binding (Fig.
1C). On
vß3, Ca2+, Mn2+,
and Mg2+ all allowed penton base binding. Taken
together, these results suggest that there is a specific, ion-dependent
interaction between Ad2 penton base and integrin
3ß1.
Peptide ligands of
integrin
3ß1 searched by phage biopanning.
A previous study showed that a
filamentous phage displaying the full-length Ad2 penton base bound more
efficiently to
3ß1 than a phage displaying only the
region spanning residues 286 to 392
(4). Considering that both
RGD and LDV motifs are included in the residue 286-to-392 fragment,
this result suggests that other amino acid residues may be implicated
in the interaction between Ad and the
3ß1 integrin.
This is in accordance with the data of competition experiments depicted
in Fig. 1. First,
GRGDSP and
PALLDVDA peptides are inefficient
competitors of the penton base-
3ß1 interaction.
Second, Ca2+ favors the binding to the penton base,
a condition that, for natural
3ß1 ligands, is related
to RGD-independent reactions
(5,
8). Third, the R340E
mutation only slightly impaired penton base binding to
3ß1 integrin, and the D288K mutation showed no
effect.
To gain insight into the molecular nature of the
3ß1 binding reaction, we biopanned a phage-displayed
hexapeptide library for immobilized
3ß1 integrin
(28). Several rounds of
panning were performed on
3ß1, and elution of bound
phages was performed under different conditions. It was first performed
with acidic buffer to identify the major peptide ligand(s) of this
integrin (28), and then
it was performed with the penton base protein used as a competitor, a
method which provides mimotopes of interacting sites in partner
proteins (11). No RGD- or
LDV-containing phagotopes were isolated by elution with the acidic
buffer or the penton base protein. This suggests that if the RGD motif
of the penton base is involved in its binding to
3ß1
integrin, as suggested by our binding data and peptide competition
experiments, this motif does not play a major role in this interaction.
It is noteworthy to emphasize that RGD-containing peptides have been
predominantly isolated from phage libraries selected for purified
vß3,
5ß1, and
IIbß3
integrin subtypes (7,
19,
20).
Out of 22
independent phages recovered by acid elution and sequenced, 14 carried
the phagotope sequence NNAGFL and one carried the
sequence NGGVKS (Table
1, left column). This suggests that NNAGFL, and more
generally the pentapeptide motif NNAG(F/V), is one
of the preferred ligands of the
3ß1 integrin. Scanning
of the Ad2 or Ad5 peptide sequence showed that three regions shared
some homology with this
3ß1 ligand: NNAIV and NNSG are
found at positions 185 to 189 and 311 to 314, respectively, and,
considering the high homology between N and Q, the motif QNGVL is
present at positions 198 to 202.
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3ß1-bound phages eluted by displacement with the
penton base protein (Table
1, middle column), but
NNAFGL was found only once out of 30 phages.
Likewise, some homology could be observed between penton base-eluted
phagotopes ASLLG, LGTLLN, and FGMLLN
and the 253SRLSNLLGIRKR264 motif
present in the Ad2 or Ad5 penton base sequence. The conserved sequence
within residues 253 to 264 has been identified as the fiber-binding
region in the penton base
(11). This would suggest
that one of the
3ß1-binding sites in the penton base
would neighbor or overlap the fiber-binding domain. However, none of
these sequences was enriched by an extra cycle of adsorption and penton
base elution (Table 1,
right column), suggesting that they all represent minor binding sites
with low affinity for
3ß1.
Only one type of
phagotope, PELRIH, was significantly enriched
between the second and third cycle of phage adsorption and elution with
the penton base protein (Table
1, compare the middle and
right columns). Interestingly, the PELRIH motif
shows significant homology with the pentapeptide motif DLRLK found in
the
3ß1-binding peptide GEFYFDLRLKGDKY
of basement membrane collagen
(25) and with hexapeptide
NLRLSR found in the laminin-derived KQNCLSSRASFRGCVRNLRLSR
peptide, identified as an
3ß1 binding
sequence (6). As no
homologous sequence was found in the Ad2 or Ad5 penton base, this
implies that the PELRIH motif does not occur in
the linear sequence but is constituted of discontinuous amino acid
residues.
Integrin
3ß1
plays a role in Ad binding to mammalian cells.
The next step of this work was aimed at
defining whether a specific interaction takes place between human Ad
particles and
3ß1 receptors displayed on the cell
surface. For this purpose, we analyzed the binding of adenovirions and
recombinant penton base to HeLa and SCC-25 human cell lines, which are
known to express ß1 integrins. HeLa cells also display CAR
receptors but are poor in
vß3 and
vß5
integrin subtypes (4,
18). The tongue
squamous-cell carcinoma-derived cell line SCC-25 is characterized by
high levels of
2ß1 and
3ß1 integrins
but displays low levels of CAR,
vß3, and
vß5 receptors
(18).
As shown in
Fig. 2,
when 96-well plates were coated with soluble penton base or purified Ad5
particles, specific adhesion of HeLa cells was observed. Binding to the
penton base and to Ad5 was not substantially competed by preincubation
with function-blocking antibodies directed towards
5ß1,
vß3, and
vß5
integrin subtypes (Fig. 2A and
B). However, significant competition was observed when HeLa
cell adhesion to Ad5 or the penton base was tested after preincubation
with anti-
v, anti-ß1, or anti-
3 antibodies.
SCC-25 adhesion to Ad5-coated plates, following preincubation with
different anti-integrin antibodies, resulted in a binding pattern
similar to that observed with HeLa cells, and no inhibition of binding
was observed in the presence of anti-
vß3 and
anti-
vß5 or anti-
5ß1 antibodies (Fig.
2B). However, binding to
coated plates was significantly inhibited in the presence of
anti-
3, anti-ß1, and anti-
v antibodies (Fig.
2B). In contrast to HeLa
cells and consistent with the low level of CAR receptors on SCC-25
cells, binding was not altered by the addition of anti-CAR antibodies
(Fig. 2B). For comparison,
we analyzed HeLa and SCC-25 binding to the
3ß1 natural
ligand fibronectin. This protein also interacts with other integrin
subtypes, including
vß1/3 and
vß5. As
shown in Fig. 2C, the
anti-
3 and anti-ß1 antibodies inhibited the
interaction between the cells and fibronectin to an extent comparable
to that observed for Ad (Fig.
2B). By contrast, cell
binding to fibronectin was only slightly impaired by the addition of
anti-
v and anti-
vß3/5 antibodies. These data
validate the significance of competition studies performed with the
penton base and with Ad but also indicate that, under the conditions
tested, viral affinity for the different integrin receptors does not
completely overlap that of fibronectin. For a negative control, we used
an antibody directed to the anti-
6 integrin chain, which is
not an
3ß1 ligand and, as shown in Fig.
2C, which did not inhibit
cell binding to fibronectin.
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3 and ß1
integrin subunits in Ad interaction with human cells, in both the
presence and absence of the adenoviral primary receptor CAR. In
addition, our data are in accordance with those from Li and
collaborators, suggesting that
vß1 may act as a
coreceptor for Ad
(23).
Integrin
3ß1 plays a role in Ad infection of mammalian
cells.
The results obtained
in in vitro binding experiments involving the binding of Ad to living
cells and to
3ß1 integrin suggested that
3ß1 may play a role in Ad infection. To verify this
hypothesis, HeLa and SCC-25 cells were infected at a multiplicity of
infection of 50 particles/cell with a recombinant Ad5 carrying genes
encoding the green fluorescent protein (Ad5-GFP). Infection was
performed in the presence or absence of function-blocking anti-integrin
antibodies, and GFP-positive cells were analyzed by flow cytometry at
42 h after infection. As expected from binding competition
experiments with both cell lines, gene transduction was inhibited in
the presence of anti-
v, anti-ß1, and anti-
3
antibodies, whereas it was not significantly altered in the presence of
anti-
vß3, anti-
vß5, and
anti-
5ß1 antibodies (Fig.
3). It should be noted that transduction was inhibited in a more dramatic
way in the presence of competitive antibodies than was binding. These
results suggest, as previously described
(26), that integrin
receptors play key roles in viral internalization rather than in
particle binding to cell membranes. The anti-CAR antibody completely
inhibited GFP transduction of HeLa cells but had no effect on SCC-25
cells, a result which was consistent with the receptor
patterns.
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3ß1 in vitro and in vivo. We also
provide data indicating that this integrin subtype plays a role in Ad
infection. Furthermore, we present several independent evidences, i.e.,
binding experiments with penton base mutants, peptide binding
competition, ion dependency, and peptide selection, showing that the
interaction of the penton base with
3ß1 is not
dependent upon RGD motifs but is rather mediated by
multiple discrete binding sites, some of them probably being formed
from discontinuous amino acid residues. To our knowledge, this is the
first study reporting the involvement of
3ß1 in Ad
infection and suggesting the peptide ligand preferences of this
integrin receptor. Moreover, our results suggest that Ad exploits the
integrin
3ß1 not only as a primary receptor for
anchoring its virions at the cell membrane but also to enter the cell
and transduce its DNA, thus acting as a secondary, endocytosis
receptor. These findings, together with those already published on
other integrin subtypes
(23,
31), support the
hypothesis of a multiple receptor choice for Ad-cell interaction. This
creates a biological advantage for the virus, since different integrin
receptors are expressed on different tissues. Even though it is
difficult to predict the specific role of the various integrins in the
virus infection process, the basolateral membrane of epithelial and
endothelial cells, where
3ß1 is highly expressed
(21), may be a secondary
infection site relative to those tissues where CAR and
v
receptors are major players of viral entry. Our findings are also
important in view of the use of Ad vectors for gene therapy
applications. Although a current opinion in the field assumes that
RGD-deleted Ads would be incapable of interaction with integrins, our
studies on
3ß1 suggest that this assumption should be
more cautiously
evaluated.
| ACKNOWLEDGMENTS |
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We are grateful to Saw See Hong for providing us with anti-Ad antibodies and the hexapeptide library.
This work was supported by the contributions of CNR, Progetto Finalizzato Biotecnologie, Istituto Pasteur Cenci Bolognetti, Università di Roma La Sapienza, and of Consorzio Interuniversitario Biotecnologie. Part of this study was financed by the French Centre National de la Recherche Scientifique (CNRS-SDV, Chercheur Associé 2002).
| FOOTNOTES |
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