J. Cell Sci. 33, 121-132 (1978)
Printed in Great Britain © Company of Biologists Limited igy8
I 2 I
SELECTIVE ADHESION AND IMPAIRED
ADHESIVE PROPERTIES OF
TRANSFORMED CELLS
MIA BRUGMANS, JEAN-JACQUES CASSIMAN AND
HERMAN VAN DEN BERGHE
Division of Human Genetics, Department of Human Biology,
Minderbroederstraat 12 JS-3000, Leuven, Belgium
SUMMARY
Quantitative studies on the adhesive properties of transformed cells have yielded inconclusive
and sometimes contradictory results. The present investigation has examined adhesive interactions between normal human fibroblasts, established as well as virus-transformed animal
cell lines, and human tumour-derived cell lines by the cell—cell layer binding assay.
The results of these investigations indicate that adhesive selectivity can be observed
between normal human fibroblasts and 2 human tumour-derived cell lines, providing an
in vitro system to study cell surface components involved in cellular interactions between
normal and malignant cells.
In addition it is demonstrated that cell layers of transformed cells form a poorly adhesive
substratum for both trypsinized normal and transformed cells. Furthermore, it is confirmed
that the adhesive properties of transformed cells, including adhesive selectivity, are affected
by the dissociation procedure (trypsin or EDTA). In view of the observations made by other
investigators, the present results suggest that transformed cells display adhesive properties
which can be quantitatively and reproducibly measured but which are modulated by the
dissociation procedure as well as by the configuration in which the cells are at the time of the
assay.
INTRODUCTION
Since Coman (1944) first proposed that malignant cells demonstrate impaired
adhesive properties, a large number of investigations using virus-transformed or
tumour-derived cell lines have attempted to verify the validity of this view. The
advent of more reliable and refined methods to examine and quantitate the adhesive
interactions between 'normal' and 'malignant' cells in vitro have not been able,
however, to confirm or refute this original hypothesis. Variable as well as contradictory results have been published on this issue. Impaired, unaltered and increased
intercellular adhesiveness have been reported for transformed cells (Edwards, Campbell & Williams, 1971; Dorsey & Roth, 1973; Walther, Ohman & Roseman, 1973;
Cassiman & Bernfield, 1975, 1976a; Vosbeck & Roth, 1976; Winkelhake & Nicolson,
1976).
Such variables as the type of the assay, the cell lines under investigation, the
in vitro culture conditions and the dissociation procedure used, are considered to
be the major causes for these discrepancies.
122
M. Brugmans, J.-J. Cassiman and H. Van Den Berghe
Recently however, it was demonstrated that a definite correlation existed between
the neoplastic nature of a cell line and the rate of its aggregation following EDTA
dissociation (Wright, Ukena, Campbell & Karnovsky, 1977). These results confirmed
and extended to a large number of cell lines previous observations (Cassiman &
Bernfield, 1975). Although at variance with Coman's original hypothesis (Coman,
1944) these results are the first to demonstrate altered adhesive properties common
to most transformed cells and clearly distinct from those of untransformed cells.
The present investigation will attempt to define adhesive properties equally
representative of a variety of transformed cells. Using the single cell-cell layer
binding assay (Walther et al. 1973) it will be demonstrated that cell layers of most
transformed or tumour-derived cell lines tested have impaired adhesive properties
in this assay. Furthermore, 2 tumour-derived cell lines of human origin will be
shown to demonstrate selective adhesion with normal human fibroblasts. It will
be shown that these adhesive properties are modulated or altered as well by the
dissociation procedure (trypsin or EDTA) as by the configuration the cells are in
at the time of the assay (cell layer, single cell).
It will be proposed that the present results as well as results obtained by other
investigators support the idea that transformed cells can express high stickiness as
well as poor cohesion (Coman, 1961) and that this dual property might be responsible
for the apparent contradiction found in the literature when results of different assays
are compared.
MATERIALS AND METHODS
Cell lines
Normal human fibroblasts (NHF) were derived from skin biopsies of normal individuals
and assayed between the 10th and 20th passage.
MG 63 cells, derived from a human osteosarcoma, were a gift from H. Heremans (Heremans
et al. In Press). This cell line was cloned by plating 100 cells in 10-cm Petri dishes (Falcon Co.).
After 10 days of undisturbed growth, plates containing 2-3 clones were selected; the clones
were removed from the plate by gentle scraping and transferred to T 25 flasks (Falcon). After
a few days in culture each clone was recloned using the above-described procedure.
Two other human tumour-derived cell lines, CCL 121 (fibrosarcoma) and CCL 30 (lung
carcinoma) were obtained from the American Type Culture Collection (Rockville, Md).
MOH is a C3H mouse embryo-derived established cell line and MO4 its Kirsten-virus
transformant (Billiau, Sobis, Eyssen & Van Den Berghe, 1973).
L 929 cells and a rat hepatoma cell line were obtained from E. De Clercq, Rega Instituut,
Division of Virology, Leuven.
Growth of the cells
Cells were grown in Dulbecco's Modified Eagle's medium (DME, Grand Island Biological
Co., Grand Island, N.Y.), supplemented with 10% Newborn Calf Serum (NCS, Sera-Lab,
Plasma Lab Limited, Crawley Down, England), 200 U./ml penicillin, 100 fig/ml streptomycin
and 1 g/1. NaHCO3, buffered with TES (Af-tris-(hydroxymethyl)-methyl-2-aminoethanesulphonic Acid) and HEPES (iV-2-hydroxyethyl-piperazine-iV-2-ethanesulphonic acid) (Calbiochem, San Diego, Calif.) at 15 mM each (pH 7-4) (growth medium).
The cultures were kept in an atmosphere of 5 % CO2, 95 % air and 100 % humidity at
37 °C.
Cells were subcultured at confluency. After a single wash with a solution containing
Selective adhesion of tumour cells
123
0-02 % EDTA in 2 mM Tris (tris (hydroxymethyl)-aminoethane) (Calbiochem)-buffered saline,
°'°S % crude trypsin (Difco, 1/250) dissolved in the same solution, was added for less than
5 min. The cultures were divided one in two, twice a week.
Assay procedure
The assay for intercellular adhesion described by Walther et al. (1973) was used with
minor modifications.
Each well of a tissue culture tray (Linbro, Flow Laboratories) was seeded 48 h prior to
testing with 1-5 x io 5 cells in 1 ml growth medium.
CCL 30 cells were plated at 4X10 5 cells. At the same time cells were seeded in 75-cm 2
flasks (Falcon) at 5 x io 5 cells per flask; after 24 h the medium was replaced by medium
containing 10/tCi/ml [3H]leucine (sp.act. 1 Ci/mM) (The Radiochemical Centre, Amersham).
Two hours before the assay, the labelled cell cultures were washed once with phosphatebuffered saline (PBS) and reincubated with fresh medium.
Twenty minutes prior to the assay the cell layers in the wells were washed 3 times with
assay medium (growth medium without NCS and without NaHCO 3 ) and the trays were
floated on a 37 °C waterbath.
Single-cell suspensions were prepared by washing the flasks once with Ca-Mg-free PBS
(CMF-PBS) containing 0-02 % EDTA, followed by an incubation for 10 min in either 0-05 %
twice-crystallysed trypsin (Sigma) or 0-02 % E D T A in CMF-PBS. The cells were washed
twice in assay medium, passed through a double layer of 20-fim pore-size nylon filter (Nytex
Co.) and diluted to 2 x io 4 cells/ml. Viability as monitored by trypan-blue exclusion was
over 9 0 % . O'S ml of this suspension (1 x io 4 cells) was added to each well using an automatic
dispenser (1 ml Biopette, Schwarz/Mann). The trays were floated on a 37 °C waterbath and
at regular time intervals, up to 60 min, the cell layers were washed twice with 0-5 ml prewarmed
PBS and the non-adherent single cells were removed with the PBS by vacuum aspiration.
Additional washes could not remove more cells. Monolayers and adherent cells were dissolved
in 3 changes of 0-5 ml 1 N NH 4 OH, transferred to counting vials containing 15 ml Instagel
(Packard) and counted in a liquid scintillation counter (Packard Tricarb).
The results of the assay are expressed as the percent cells of the maximum. The maximum
(taken as 100 %) was determined by seeding 0-5 ml of the same cell suspension in triplicate
in untreated Linbro wells, followed by an incubation for at least 90 min at 37 °C; microscopic
examination revealed that over 90 % of the cells had attached to the plastic at that time.
Maximum cpm per well ranged between 7000 and 15000 per io 4 cells, depending on the cell
line used. Leakage of radioactivity from the cells amounted to + 40 % of the total. The label
leaked by the cells in the supernatant was not taken up by cell layers incubated with this
supernatant for 90 min.
Reagents
Trasylol (Bayer, Batch no. SMV 760, activity 6380 keV/mg, a gift from Dr E. Phillip) was
used at 0-41 mg per mg crystalline trypsin. Soybean trypsin inhibitor (STI) was used at
2-5 mg per mg trypsin. These concentrations effectively inhibited the hydrolysis of BANA
(Serva) (Erlanger, Kokowsky & Cohen, 1961). Neuraminidase (500 U./ml) and STI were
obtained from Calbiochem.
Electron microscopy
Cell layers to which single cells had attached were fixed for 2 h in 2-5 % glutaraldehyde
(Fluka AG, Chemische Fabrik, Buchs, Schweiz) in o-i M cacodylate buffer (pH 74), and
postfixed for 1 h in 1 % osmium tetroxide solution in o-i M phosphate buffer (pH 7"4). After
dehydration in a series of ethanol baths the cells were embedded in Epon (Epikote 812, Gurr
Ltd, London). The plastic dishes were removed, i-/tm-thick sections were made perpendicular
to the substratum on a Reichert OM U2 ultramicrotome and either stained with a 2 %
toluidine-blue solution or contrasted with uranyl acetate and lead citrate and examined with
a Philips (TEM 201) electron microscope.
I24
M. Brugmans, J.-J. Cassiman and H. Van Den Berghe
For scanning electron microscopy, the cell layers were grown on glass coverslips, fixed as
described for TEM, dried by the critical-point method (E-3000 Polaron) with carbon dioxide,
coated with gold in a Polaron sputtering system (E 5000 diode) and examined in a Philips
PSEM 500.
Autoradiography
To demonstrate that the [3H]leucine-labelled cells adhered to the cell layer and not to the
plastic, autoradiographs were performed on i-/«n-sections of Epon-embedded material. The
sections were overlaid with emulsion (Ilford K-2) and kept in the dark for 7 days before
photographic development (Fig. 1).
1A
10//m
B
10//m
Fig. 1. Representative autoradiography on thin section of a normal human fibroblast
adhering to a NHF cell layer, A, After 10-min; B, after 40 min binding of labelled
cells. The added cells adhere to the cell layer and are able to spread on top of it;
cell processes force their way in between cells of the cell layer, x 1100.
Selective adhesion of tumour cells
125
RESULTS
The ability of this assay to measure quantitative differences in cellular interactions was first examined with embryonic cells. Once it was established that previously
published results (Walther et al. 1973) could be reproduced, the adhesive properties
of various 'normal' and 'transformed' cell lines were compared and the effect of
exogenous factors such as the dissociation procedure and serum components was
examined.
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Fig. 2. Adhesion kinetics of A, trypsin-dispersed, and B, EDTA-dispersed cells to
a confluent cell layer. The assays were carried out in serum-free medium. Each
, MOH/MOH;
point represents the mean + s.E. of at least 5 determinations.
—, MO4/MO4;
, MOH/MO 4 .
Adhesive properties of various normal and transformed cell lines
The adhesion of various trypsin-dissociated single-cell suspensions to homologous
or heterologous cell layers was examined at regular time intervals during the first
60 min.
Established mouse-embryo derived fibroblasts (MOH) adhered to MOH cell
layers at a rapid rate reaching a plateau around 20 min and an extent of 60% after
60 min (Fig. 2A). Normal human diploid fibroblasts adhered at a rapid rate to
homologous cell layers during the first 30 min of the assay, reaching 55%. During
the next 30 min a level of 80% was reached (Fig. 3 A). Fibroblasts derived from
different normal adults and from different human foetuses (aged 16-22 weeks),
showed the same adhesive rates in the homologous or in the heterologous combinations.
The Kirsten virus-transformed MO4 cells, derived from the MOH cells, adhered
to MO4 cell layers at a slow but constant rate (Fig. 2A). Similar slow homotypic
binding rates were also obtained for L 929 cells, a rat hepatoma cell line and for the
9
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33
M. Brugmans, jf.-J. Cassiman and H. Van Den Berghe
126
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50 60
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30 40
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Time, mm
Time, mm
Fig. 3. Adhesion kinetics of A, trypsin-dispersed, and B, EDTA-dispersed single
cells to a confluent cell layer. The assays were carried out in serum-free medium.
, NHF/NHF;
Each point represents the mean + S.E. of at least 9 determinations.
— , 7.10/7.10;
, NHF/7.10.
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Fig. 4. Adhesion kinetics of trypsin-dissociated NHF and CCL 30 cells to CCL 30
cell layers. The assays were carried out in serum-free medium. Each point represents
the mean with the S.E. of at least 5 determinations. —, CCL 30/CCL 30; —,
NHF/CCL 30.
human tumour-derived cell lines CCL 121, CCL 30 (Fig. 4) and MG 63 (Fig. 3 A
shows clone 7.10 of this cell line). After 60 min the extent never exceeded 50%.
The adhesion of MOH cells to MO4 (Fig. 2 A) or to MG 63 cell layers occurred at a
slower rate than to MOH cell layers. Binding of NHF to L 929, rat hepatoma, CCL 30
(Fig. 4), MG 63 (Fig. 3 A) and MO4 cell layers occurred always at a lower rate than to
NHF cell layers.
Reciprocal experiments were performed for only a few of these combinations
(MG 63 to NHF, MO4 to MOH cell layers); in these cases transformed cells adhered
to the normal cell layers at a fast rate, similar to the homotypic normal cell adhesion
(results not shown).
Selective adhesion of tumour cells
127
Adhesion of NHF to 2 of the transformed cell layers - the tumour-derived cell
lines MG 63 (an osteosarcoma) (Fig. 3 A) and CCL 30 (a lung carcinoma) (Fig. 4) —
was characterized by an initial lag of approximately 20 min, after which the cells
started to adhere at a more rapid rate. These interactions therefore can be considered
as demonstrating selective adhesion, defined as the ability of both homologous
combinations to adhere at a higher rate than at least one of the heterologous combinations.
Various clones of the MG 63 cell line showed the same general adhesive properties
as the original tumour cell line in homotypic as well as heterotypic combination. One
clone, 7.10, was used in further experiments instead of the parent MG 63 culture.
To determine whether the adhesive selectivity of these cell lines was based upon
properties common to both cell lines or upon properties distinct for each cell type,
the 2 cell lines which demonstrated selectivity were tested against each other.
Adhesive selectivity could not be demonstrated between these cell lines.
Adhesive selectivity between normal and tumour-derived cell lines
The adhesion of normal fibroblasts to MG 63 and CCL 30, which conferred to
these interactions adhesive selectivity, was characterized by 2 distinct phases; a
pronounced lag or low adhesive period of 20 min was followed by a period during
which rather rapid stable adhesions were formed. The question was therefore asked
what the nature was of the modifications that occurred either in the cell layer, or in
the cell suspension, or in both, which might cause the change observed in the adhesive
interactions at the end of the lag period. Various causes of cell-surface modifications
which might alter the adhesive behaviour of the cells were examined.
A change in the ultrastructure of the cell layer such as the formation of surface
villi or blebs during the first phase of the assay was ruled out by examining the
cells by SEM and TEM at various periods of time after the addition of normal
fibroblasts (Fig. 5). In an attempt to make the cell layer more adhesive for the
normal cells, neuraminidase (5 /tg/ml) at a concentration sufficient to remove sialic
acid residues (Deman & Bruyneel, 1975; De Ridder, Mareel & Vakaet, 1975) was
added for 20 min to the cell layer at 37 °C. No effect of this treatment was observed.
Extensive washings of the cell layers or the cell suspension before the assay to
remove any cell released material remained equally ineffective in altering the adhesion
kinetics. Other experiments were aimed at detecting a change in the adhesive properties of either the cell layer or the cell suspension during the initial 20-min lag
period. Single cells were plated on cell layers for 15 min. At that time (a) the nonadherent cells were removed and a fresh suspension was added, (b) the non-adherent
cells were left undisturbed and a fresh suspension of cells was added in addition
to the original one, (c) the non-adherent cells were removed and the cell layers
were incubated for another 60 min at which time a fresh suspension was plated
on them, or (d) the non-adherent cells were plated on fresh cell layers.
The adhesion of a fresh or preincubated cell suspension on these pretreated cell
layers showed the same lag period and the same kinetics as the untreated controls.
Since serum components are known to affect the adhesion of cells to a substratum
9-2
128
M. Brugmans, J.-Jf. Cassiman and H. Van Den Berghe
(Grinnell, 1976) the following experiments were aimed at determining the possible
role of serum in the observed selective interactions.
Pretreatment of the cell layer (clone 7.10) for 20 min with 10% serum followed by
a serum-free wash as wellaspretreatment of the cell suspension (NHF) with io%NCS
had no effect on the observed 20-min lag. Incubation of the 7.10 cell layer for 4 days
in serum-free medium remained equally ineffective in altering the adhesive properties
of the normal cells to it.
Fig. 5. Scanning electron micrographs of an adherent normal human skin fibroblast
to a normal human fibroblast cell layer (A, 10 min; B, 40 min) and to a cell layer of
MG 63 clone 7.10 cells (c, 10 min; D, 40 min). No major changes in the ultrastructural appearance of the cell layer could be detected during the assay, x 3225.
Effect of the dissociation procedure
The observed differences in adhesive properties between normal and transformed
cell lines disappeared completely when the cell suspensions were prepared using
EDTA only.
Fig. 2B shows the MOH-MO4 system. Whereas the rate of adhesion of the MOH
cells to homologous cell layers remained practically unchanged, as compared to
trypsin-dissociated cells (Fig. 2 A), the rate of adhesion of EDTA-dissociated transformed cells increased to attain the same rate as the untransformed cells. Binding
of MOH cells to MO4 cell layers was much less affected by EDTA dissociation than
MO4-MO4 binding. When the adhesive properties of the normal and 7.10 (clone
Selective adhesion of tumour cells
129
MG 63 cell line) cells were examined using EDTA-prepared single cell suspensions
(Fig. 3B), no differences in homotypic adhesiveness between the normal and tumour
cells could be detected. This treatment abolishes, in addition, the adhesive selectivity
between the NHF and 7.10 cells. In an attempt to examine whether remnant trypsin
on the cell suspension or protease released by the cell layer might be responsible
for the observed adhesive interactions, the effect of known protease inhibitors was
examined on MO4-MO4 binding and on the selective NHF-7.10 interaction.
STI or Trasylol was added to the cell suspension for iomin or to the cell layer
for 5 min in concentrations known to inhibit the total amount of trypsin added
(Avineri-Goldman, Snir, Blauer & Rigbi, 1967). No effect was observed for any of
these treatments. It should be noted that pretreatment with 10% serum, as mentioned
above, should also have affected the activity of most proteases present.
DISCUSSION
In the present study, adhesion is quantitated by determining the rate of formation
of stable adhesion of [3H]leucine-labelled cells to a confluent cell layer.
Adhesive selectivity between 2 cell types was operationally defined as the ability
of both homologous combinations to adhere at a faster rate than at least one of the
heterologous combinations. Adhesion as well as selectivity, as defined here, therefore
do not imply anything about the mechanisms of the interactions. Using autoradiographic techniques, it could be demonstrated that cell-cell layer binding was measured
as suggested (Walther et al. 1973) and that the results were not affected by labelled
cells bound to the plastic substratum; leakage of label was ruled out as a source of
error.
The results obtained using various in vitro cultured cell lines demonstrate that:
(a) selective adhesion can be detected between in vitro cultured normal human
skin fibroblasts and 2 human tumour-derived cell lines; (b) virus-transformed or
tumour-derived cell layers are consistently less adhesive for homotypic as well as
heterotypic cells as compared to untransformed cell layers; and (e) the adhesive
interactions of transformed or tumour cell lines are affected by the dissociation
procedure to a greater extent than those of the normal cell lines.
The cell lines, MG 63 and CCL 30 have similar adhesive properties to the other
transformed cell lines (MO4, L 929, rat hepatoma), as will be discussed below. In
addition however, these cell lines demonstrate adhesive selectivity with NHF as
denned. This is the first indication that cell lines grown for prolonged periods of
time in vitro can maintain properties necessary for selective interactions. In addition,
these cell lines provide an in vitro model to study surface properties involved in
cellular interaction between normal and malignant cells. At variance with the adhesive
selectivity observed with embryonic cells the adhesive behaviour of normal human
fibroblasts on these tumour cell lines is characterized by a pronounced lag. This lag
could not be altered by various treatments which were aimed at modifying the single
cells or the cell layer. It can only be concluded from the results obtained that, during
the lag period, an interaction between the cells of the suspension and of the cell
130
M. Brugmans, J.-J. Cassiman and H. Van Den Berghe
layer must occur before stable adhesion can be formed which will now resist removal
by the exerted washings. The relevance of these observations for the tumoural
properties of these cells is unknown. It is not clear either whether tissue- or tumourspecific components are involved, although both cell lines do not demonstrate
selectivity between them. Fibroblastic cell lines from different normal adult or foetal
donors do not show adhesive selectivity. Therefore individual specific molecules,
such as transplantation antigens, are not responsible for differences in the adhesive
behaviour of the cells as measured in the present assay.
The present investigation has also shown that the binding of several trypsindissociated 'normal' cells (NHF, MOH) as well as transformed cells (CCL 30,
MG 63, L 929, rat hepatoma, MO4) to a cell layer of transformed cells always occurs
at a slower rate than their binding to a cell layer of normal cells. Various cell suspensions therefore consistently discriminate between a normal and a transformed
cell layer. This low adhesivity of a transformed cell layer was also observed in
aggregate-cell layer binding experiments with SV 3T3 cells (Cassiman & Bernfield,
1976 a) whereas Vosbeck & Roth (1976) observed the lowest collection rate for DEAE
beads coated with transformed cells. Therefore, since the same transformed cells
are capable of adhering at a rapid rate as single cells (Dorsey & Roth, 1973; Vosbeck
& Roth, 1976; present results) or as aggregates, it is suggested that the transformed
cells demonstrate consistently poor adhesive properties only in the cell layer configuration. The causes for the low adhesivity of the surface of transformed cell layers
as compared to 'normal' cell layers remain unknown. They may or may not be
related to the many differences in surface composition and architecture which have
been described for these cells, such as differences in their surface composition
(reviewed by Poste & Weiss, 1976) in the thickness of the surface coat (MartinezPalomo, 1970; Shigematsu & Dmochowski, 1973) in the relative glycosaminoglycan
composition of their surfaces (Roblin, Albert, Gelb & Black, 1975; Cohn, Cassiman
& Bernfield, 1976) and in the presence of large surface molecules which modulate
some adhesive properties (Hynes, 1973).
In addition to having a poor adhesive surface in the cell layer configuration,
transformed cells express different adhesive properties as single cells, depending
on the mode in which these single cells are obtained and on the system used to
test their adhesive properties. The effect of the dissociation procedure is much less
pronounced on normal cells whether the cell-cell layer binding is measured or whether
the initial rate of aggregation is examined.
The adhesion rate of EDTA-dissociated transformed cells to a transformed cell
layer is comparable to the adhesion rate of normal single cells to a normal cell layer
(present results). The aggregation rate of EDTA-dissociated transformed cells is
higher than the aggregation rate of EDTA-dissociated normal cells (Cassiman &
Bernfield, 1975; Wright et al. 1977).
These results might suggest that transformed cells, in contrast to the normal cells,
are much more sensitive to EDTA treatment and that the high adhesivity might be
due to the release of intracellular components or to cellular damage (Snow & Allen,
1970; Waymouth, 1974). Alternatively they could suggest that transformed cell
Selective adhesion of tumour cells
131
surfaces are more sensitive to trypsin than normal cell surfaces, which would decrease
their adhesiveness. Winkelhake & Nicolson (1976) did indeed observe that undissociated, ascites-grown melanoma cells, are highly adhesive when aggregated or
when plated on a cell layer, and Coman (1961) found mechanically dissociated transformed cells very sticky. An explanation for the observed adhesive behaviour of the
transformed cells in suspension might therefore be that these cells are indeed rather
insensitive to EDTA dissociation and that the highly adhesive properties which
they demonstrate are characteristic of single transformed cells in suspension.
The present results as well as previously published observations suggest that the
adhesive properties of transformed cells or tumour-derived cells might be dependent
not only on the configurations in which the cells are (cell layer, aggregate, single
cell) but also on the mode of dissociation. As a result, the same cells can display high
as well as poor adhesive properties.
The lack of reciprocity observed with fibroblastic cell lines and with some embryonic cells (Roth, 1968; Cassiman & Bernfield, 19766) in the different assays used to
measure adhesion might also partially result from this configuration effect.
It can also be speculated that this ability to display differences in adhesive properties
might be important in understanding the behaviour of tumour cells in vivo. Whereas
poor intercellular adhesion might be one of the mechanisms by which tumour cells
can leave the original tumour cell mass, the stickiness of the single cells might
explain their tendency to adhere firmly at new and sometimes specific sites in the
host.
We are indebted to Dr B. Van Der Schueren and Ms. G. De Geest for the light- and
electron-microscopic studies, to Ms G. Deferme for cloning the MG 63 cell line and Mr J. De
Boer for help in processing the data.
This work was supported by a grant from the Belgian Cancer Fund (ASLK) and by grant
no. 3.0025.75 (FGWO).
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(Received 6 March 1978 - Revised 25 April 1978)