(Received for publication, September 25, 1995)
From the
Tumor cell adhesion to the triple-helical domain of basement
membrane (type IV) collagen occurs at several different regions.
Cellular recognition of the sequence spanning 1(IV)531-543
has been proposed to be independent of triple-helical conformation
(Miles, A. J., Skubitz, A. P. N., Furcht, L. T., and Fields, G.
B.(1994) J. Biol. Chem. 269, 30939-30945). In the
present study, integrin interactions with a peptide analog of the
1(IV)531-543 sequence have been analyzed. Tumor cell
adhesion (melanoma, ovarian carcinoma) to the
1(IV)531-543
chemically synthesized peptide was inhibited by a monoclonal antibody
against the
integrin subunit, and to a lesser extent
by monoclonal antibodies against the
and
integrin subunits. An anti-
monoclonal antibody
and normal mouse IgG were ineffective as inhibitors of tumor cell
adhesion to the peptide. Two cell surface proteins of 120 and 150 kDa
bound to an
1(IV)531-543 peptide affinity column and were
eluted with 20 mM EDTA. When the eluted proteins were
incubated with monoclonal antibodies against either the
or
integrin subunit, proteins corresponding in
molecular weight to
and
integrin
subunits were precipitated. No proteins were immunoprecipated with
monoclonal antibodies against the
or
integrin subunits. Thus, the
integrin from two tumor cell types has been shown to bind
directly to the
1(IV)531-543 peptide. The
1(IV)531-543 peptide is the first collagen-like sequence
that has been shown to bind the
integrin.
Basement membranes are specialized extracellular matrices that
delineate tissue boundaries. The metastatic process involves tumor cell
interaction with and invasion through the basement membrane. Basement
membrane (type IV) collagen can promote the adhesion and migration of
diverse cell types including melanoma, corneal epithelial, endothelial,
and neural crest cells (Herbst et al., 1988; Chelberg et
al., 1989; Etoh et al., 1993; Olivero and Furcht, 1993;
Perris et al., 1993). Normal and tumorigenic cellular
interactions with type IV collagen can be mediated by integrins and/or
cell surface proteoglycans (reviewed in Faassen et al., 1992).
The and
integrins can bind directly to type IV collagen (Staatz et
al., 1989; Vandenberg et al., 1991; Kern et al.,
1993; Eble et al., 1993; Carmeliet et al., 1994),
while the
integrin has been
implicated in cell migration on type IV collagen (Yoshinaga et
al., 1993; Vink et al., 1994; Melchiori et al.,
1995). Identification of specific adhesion sites within type IV
collagen for the individual integrins has proven difficult, most likely
due to the conformational dependence of integrin binding to the
collagen triple-helix (Kühn and Eble, 1994). Type
IV collagen cyanogen bromide fragment 3 (CB3(IV)), which includes
1(IV) residues 388-551 and
2(IV) residues
407-570, has been reported to contain the individual binding
sites for the
1
and
2
integrins (Vandenberg et al., 1991; Kern et al.,
1993). The N-terminal region of a trypsin-derived fragment of
CB3(IV), incorporating residues
1(IV)414-452 and
2(IV)432-469, is believed to be a high affinity binding site
for the
1
integrin from fibrosarcoma cells (Eble et al., 1993). The
2
integrin from human
fibroblasts and human platelets adheres to a peptide model of
1(I)430-442 in a Mg
-dependent fashion
(Staatz et al., 1990, 1991). This peptide inhibits platelet
and breast adenocarcinoma cell adhesion to type I collagen (Staatz et al., 1991) but does not inhibit
2
-mediated chondrosarcoma cell adhesion to type
II collagen (Tuckwell et al., 1994).
Other collagen-derived
sequences may also function as integrin binding sites. Peptide models
of the 1(IV)1263-1277 region promote the adhesion,
spreading, and migration of highly metastatic tumor cells (Chelberg et al., 1990; Mayo et al., 1991; Fields et
al., 1993). A peptide incorporating the
1(IV)531-543 (
)(Gly-Glu-Phe-Tyr-Phe-Asp-Leu-Arg-Leu-Lys-Gly-Asp-Lys-Tyr)
sequence promotes keratinocyte, corneal epithelial, melanoma, ovarian
carcinoma, and Jurkat cell adhesion (Wilke and Furcht, 1990; Cameron et al., 1991; Miles et al., 1994) and migration of
corneal epithelial cells and keratinocytes (Cameron et al.,
1991; Kim et al., 1994). Cellular recognition of the
1(IV)531-543 peptide is, in general, independent of
substrate conformation and configuration (chirality) (Miles et
al., 1994). Competition studies suggested that the L- and D-peptides incorporating
1(IV)531-543 are bound by
the same receptor (Miles et al., 1994). Preliminary results
indicated that the
integrin subunit was involved in
mediating cell adhesion to this sequence (Miles et al., 1994).
We have presently examined integrin binding to the
1(IV)531-543 sequence. Since an
1
integrin binding site in type IV collagen has already been
identified (see above), we have focused on the
2
and
integrins as possible
receptors for
1(IV)531-543. In addition, the
integrin has been considered as a
potential receptor for
1(IV)531-543 in this study, as it can
mediate cell adhesion to denatured collagen (Gullberg et al.,
1992; Tuckwell et al., 1994). We have utilized a number of
assays including inhibition of cell adhesion and affinity
chromatography of solubilized cells to characterize the integrin(s)
involved in cellular recognition of the
1(IV)531-543
sequence. Two different tumor cell lines have been utilized, melanoma
and ovarian carcinoma, since both of these cell types adhere well to
type IV collagen and the
1(IV)531-543 peptide (Miles et
al., 1994).
Cells were passaged for 4-5 weeks and then replaced from
frozen stocks of early passage cells to minimize phenotypic drift. All
cells were maintained at 37 °C in a humidified incubator containing
5% CO. All media reagents were purchased from Sigma except
where noted.
We had shown previously that 40-60% maximum cell
adhesion is achieved with 5-10 µM amounts of a
peptide analog of the 1(IV)531-543 sequence (Miles et
al., 1994). Inhibition of cell adhesion studies were thus
performed at a peptide concentration of 5.7 µM. Ovarian
carcinoma cell adhesion to the peptide could be inhibited in a
dose-dependent manner by mAbs against
,
2, or
integrin subunits, with maximum inhibition occurring
at the highest mAb concentration tested (5 µg/ml) (Fig. 1A). The mAb against the
subunit was most effective, producing >60% inhibition of
adhesion at a mAb concentration as low as 2.5 µg/ml (data not
shown). A similar level of inhibition (
50%) was achieved by the
anti-
subunit mAb at a concentration of 5 µg/ml (Fig. 1A). The anti-
2 subunit mAb was less
effective as an inhibitor, causing only
35% inhibition at 5
µg/ml (Fig. 1A). A mAb against the
integrin subunit and normal mouse IgG did not give
dose-dependent inhibition of cell adhesion, even up to a concentration
of 5 µg/ml (Fig. 1A). In a similar fashion,
melanoma cell adhesion to the peptide could be inhibited in a
dose-dependent manner by mAbs against the
,
2, or
integrin subunits, with maximum inhibition occurring
at a mAb concentration of 5 µg/ml (Fig. 1B). The
anti-
subunit mAb was the most effective inhibitor,
causing
60% inhibition. A mAb against the
integrin subunit and normal mouse IgG did not give dose-dependent
inhibition of melanoma cell adhesion up to a concentration of 5
µg/ml (Fig. 1B).
Figure 1:
Inhibition of ovarian carcinoma (A) or melanoma cell (B) adhesion to Immulon I plate
surfaces adsorbed with 5.7 µM 1(IV)531-543
peptide by 5 µg/ml of mAbs to the integrin subunits
,
,
, or
. Normal mouse (n.m.) IgG was used as a
negative control. Cells were preincubated with the mAbs for 1 h, then
added to the wells in the presence of the mAbs for a 1 h incubation.
All assays were repeated at least in triplicate. Conditions are given
under ``Experimental
Procedures.''
The 1(IV)531-543
peptide was immobilized to CH-Sepharose, and affinity chromatography
was performed with an
I-labeled extract of the ovarian
carcinoma cells. Following application of cells, the column was first
washed with extraction buffer, then eluted by 20 mM EDTA.
Eluants were incubated with 5 µg/ml mAbs against
2,
,
, or
integrin
subunits. Precipitated proteins were then analyzed by 7.5%
SDS-polyacrylamide gel electrophoresis with detection by
autoradiography. Immunoprecipitation of the EDTA eluant with the
anti-
integrin subunit mAb resulted in detection of a
135-kDa protein under reducing conditions (Fig. 2).
Immunoprecipitation of the same eluant with the anti-
integrin subunit mAb, followed by reduction with
2-mercaptoethanol, resulted in detection of a 135-kDa protein (Fig. 2). In other studies, the
and
integrin subunits have been shown to have similar
apparent molecular weights under reducing conditions (Gehlsen et
al., 1992; Sonnenberg, 1993). No proteins were seen following
incubation of the EDTA eluant with anti-
2 integrin subunit mAb or
normal mouse IgG (Fig. 2). When the immunoprecipitants were not
reduced, two proteins of 150 and 120 kDa were immunoprecipitated from
the EDTA eluant by the anti-
integrin subunit mAb (Fig. 3). The molecular weights correspond to the
and
integrin subunits, respectively
(Sonnenberg, 1993). In similar fashion, two proteins of 150 and 120 kDa
were immunoprecipitated from the EDTA eluant by the anti-
integrin subunit mAb (Fig. 3). No proteins were seen using
normal mouse IgG or mAbs against the
2 or
integrin subunits (Fig. 3).
Figure 2:
Immunoprecipitation analysis of ovarian
carcinoma cell surface proteins eluted from the 1(IV)531-543
peptide affinity column. The proteins eluted by EDTA from the peptide
column were immunoprecipitated with anti-integrin mAbs and reduced.
Immunoprecipitation of a 135-kDa protein(s) was seen using mAbs against
either the
or
integrin subunit. No
proteins were immunoprecipitated when an anti-
2 mAb or normal
mouse (n.m.) IgG were used.
Figure 3:
Immunoprecipitation analysis of ovarian
carcinoma cell surface proteins eluted from the 1(IV)531-543
peptide affinity column. The proteins eluted by EDTA from the peptide
column were immunoprecipitated with either an anti-
or
anti-
integrin subunit mAb. Two proteins of 120 and
150 kDa, corresponding to the
and
integrin subunits, respectively, were immunoprecipitated. No
proteins were immunoprecipitated when an anti-
2 mAb, an
anti-
mAb, or normal mouse (n.m.) IgG were
used.
Affinity chromatography and
immunoprecipitation experiments were repeated using I-labeled melanoma cells. Incubation of the EDTA eluant
with the anti-
integrin subunit mAb resulted in
immunoprecipitation of a 135-kDa protein under reducing conditions
(data not shown). Similarly, incubation of the eluant with the
anti-
integrin subunit mAb resulted in
immunoprecipitation of a 135-kDa protein under reducing conditions
(data not shown). Under non-reducing conditions, two proteins of 150
and 120 kDa were immunoprecipitated from the EDTA eluant by the
anti-
integrin subunit mAb (Fig. 4). The
apparent molecular weights correspond to the
and
integrin subunits, respectively (Sonnenberg, 1993).
No proteins were seen using normal mouse IgG (Fig. 4) or mAbs
against the
2 (Fig. 4) or
integrin
subunits (data not shown).
Figure 4:
Immunoprecipitation analysis of melanoma
cell surface proteins eluted from the 1(IV)531-543 peptide
affinity column. The proteins eluted by EDTA from the peptide column
were immunoprecipitated with an anti-
integrin subunit
mAb. Two proteins of 120 and 150 kDa, corresponding to the
and
integrin subunits, respectively, were
immunoprecipitated. No proteins were immunoprecipitated when an
anti-
2 mAb or normal mouse (n.m.) IgG were
used.
In an attempt to further refine our knowledge of cellular
receptors for type IV collagen, melanoma and ovarian carcinoma cell
adhesion to the 1(IV)531-543 peptide was examined in the
presence of anti-integrin subunit mAbs. For both cell types, adhesion
to this peptide was most effectively inhibited by the anti-
integrin subunit mAb, followed by the anti-
and
anti-
2 integrin subunit mAbs. Cell surface proteins with molecular
masses of 120 and 150 kDa bound to a
1(IV)531-543 peptide
affinity column in an EDTA-dependent fashion. These proteins could be
immunoprecipitated with mAbs against either the
or
integrin subunit, and the protein molecular weights
corresponded to
and
integrin
subunits. Thus, it would appear that the
integrin binds directly to the
1(IV)531-543 peptide.
The 1(IV)531-543 peptide is the first collagen-like
sequence identified as a specific
integrin/ligand binding site. Two different tumor cell types,
melanoma and ovarian carcinoma, bind this site via the
integrin. At present, it is unclear
as to what role this integrin plays during tumor cell invasion, where
extravasating cells have contact with type IV collagen. The level of
expression varies amongst tumor cell
types. Metastatic melanoma cells up-regulate the
integrin compared with primary
melanoma cells (Yoshinaga et al., 1993), while highly invasive
prostate carcinoma cells have decreased expression of the
integrin compared to the parental
cell line (Dedhar et al., 1993). Transformed fibroblasts
retain the same level of
as
non-transformed cells while decreasing levels of other integrins
(Plantefaber and Hynes, 1989).
The level of
integrin expression may correlate to
the utility of this integrin for cell migration. mAbs to the
subunit inhibit melanocyte and melanoma cell motility
(Morelli et al., 1993; Yoshinaga et al., 1993;
Melchiori et al., 1995) and dysplastic nevus cell spreading
and migration (Vink et al., 1994) on type IV collagen.
Tumorigenic cell types such as metastatic melanoma cells may have
increased levels of
, which enhances
motility on the basement membrane or basement membrane molecules. Other
cell types may use different receptors for migration. For example,
keratinocytes use
2
integrins for migration on
type IV collagen (Chen et al., 1993; Kim et al.,
1994). There are suggestions that cellular interactions with the
basement membrane via
integrins may
also lead to basement membrane degradation. Antibodies to
integrin stimulate the expression of
matrix metalloproteinase-9 (92 kDa type IV collagenase) (Larjava et
al., 1993). Matrix metalloproteinase-9 can be induced in
transformed cells (Wilhelm et al., 1989) or by direct contact
with tumorigenic cells (Himelstein et al., 1994) and has been
localized to the invasion front of oral squamous cell carcinoma
(Kawahara et al., 1993). Matrix metalloproteinase-9
efficiently degrades type IV collagen (Morodomi et al., 1992).
The 2
integrin may have a role in cellular
recognition of the
1(IV)531-543 sequence based on the
inhibition of cell adhesion assays.
2
integrin
binding to the anti-
2 integrin subunit mAb used in this study
(P1E6) does not result in signal transduction (Kapron-Bras et
al., 1993); thus, the function of the
integrin is probably not indirectly altered by
2
interaction with the mAbs used here. More
likely, the
2
integrin has lower affinity for the
1(IV)531-543 peptide than
. An
2
binding
site is found within
1(IV)453-551 (Vandenberg et
al., 1991; Eble et al., 1993). The overlap between the
2
site and the sequence examined in this study
(
1(IV) residues 531-543) may be part of the
2
binding site. There is also a greater
expression of the
integrin than the
2
integrin for the ovarian carcinoma (
)and melanoma (
)cell types studied here,
possibly contributing to more
receptor-ligand binding events and a more avid binding overall.
Other studies indicate that both the 2
and
integrins can participate in
cellular interactions with the
1(IV)531-543 peptide. mAbs
against the
2,
, and
integrin
subunits have been used to demonstrate
2
and
integrin involvement for corneal
epithelial cell adhesion to a peptide incorporating
1(IV)531-543 (Maldonado and Furcht, 1995). Also, the
1(IV)531-543 peptide promotes the motility of keratinocytes
and inhibits keratinocyte migration on type IV collagen via the
2
integrin (Kim et al., 1994).
In
addition to type IV collagen, type I collagen, fibronectin, laminin,
and epiligrin have all been reported to be ligands for the
integrin (see reviews by
Ruoslahti(1991), Hynes(1992), and Kühn and Eble
(1994)). From these various other proteins, only two specific peptide
sequences have been identified as
integrin binding sites. A mAb against the human squamous cell
carcinoma
integrin inhibits cell
adhesion to peptide GD-2
(Lys-Glu-Gly-Tyr-Lys-Val-Arg-Leu-Asp-Leu-Asn-Ile-Thr-Leu-Glu-Phe-Arg-Thr-Thr-Ser-Lys,
which corresponds to mouse laminin A chain residues 2890-2910). (
)The human melanoma cell line C8161
integrin binds to peptide GD-6
(Lys-Gln-Asn-Cys-Leu-Ser-Ser-Arg-Ala-Ser-Phe-Arg-Gly-Cys-Val-Arg-Asn-Leu-Arg-Leu-Ser-Arg,
which encompasses mouse laminin A chain residues 3011-3032)
(Gehlsen et al., 1992). Peptides GD-2 and GD-6 have virtually
no sequence homology to the
1(IV)531-543 peptide (GD-6 and
1(IV)531-543 have a Leu-Arg-Leu overlap). In addition, GD-6
is considerably more basic than
1(IV)531-543. The activity
of
1(IV)531-543 is dependent upon the acidic residues
Asp
and Asp
(Miles et al., 1994),
while peptide GD-6 contains neither Asp nor Glu. These results suggest
that different sites of the
integrin
may be used to bind different ligands. (
)One would predict
this behavior based on the variety of
ligands. Consistent with this notion are prior studies
demonstrating that
binds fibronectin
in an Arg-Gly-Asp-dependent fashion but that laminin binding to the
integrin is Arg-Gly-Asp-independent
(Gehlsen et al., 1988; Elices et al., 1991;
Sonnenberg et al., 1991). There are also different isoforms of
(Tamura et al., 1991), which may result in
different binding specificities and/or affinities of the
integrin. Further investigations are
thus required to better understand the function of the
integrin in tumor cell invasion.