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Advances in Brief |
Departments of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461
| ABSTRACT |
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| Introduction |
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| Materials and Methods |
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In Vitro Cell Collection.
MTLn3 cells were plated in a 35-mm dish 18 h before the experiment
to be 6080% confluent at the time of the experiment. On the day of
the experiment, cells were starved using MEM-BSA, the isotonic
equivalent of 5% FBS, for 2 h. During this time, 26-gauge syringe
needles were prepared by filling them with 10 µl of Matrigel mixed
1:1 with MEM-BSA or MEM-BSA containing EGF for a final concentration of
0.5 nM, 2.5 nM, 5 nM, 25
nM, 50 nM, or 250 nM EGF. After
starvation, the needles were attached to the side of the plate using
paraffin to hold them in place with the bevel of the needle facing the
bottom of the plate so that the matrigel was in contact with the
surface of the plate. Dishes were placed into a 37°C/5.0%
CO2 incubator for up to 6 h. After this
time, the contents of each needle was extruded into a new 35-mm dish
containing MEM with 5% FBS (growth medium). Cells that had entered the
needle were allowed to grow into clones for 6 days to determine cell
count and viability. Positive clones, checked by GFP fluorescence and
cell morphology, were then counted.
To image the cells moving toward the needle, a dish was plated for 4050% confluency before the experiment. Cells were starved, and a needle was prepared as above containing matrigel mixed 1:1 with MEM-BSA containing 25 nM EGF. Images as single frames were taken using the heated microscope and NIH Image every 30 min, as described above. The dish was kept in a 37°C/5% CO2 incubator between images.
In Vivo Cell Collection.
MTLn3-GFP and MTC-GFP cells were injected into female Fischer 344 rats,
as described before (8
, 9)
, and tumors were allowed to
grow for 2.5 weeks. On the day of the experiment, 33-gauge needles were
prepared as above by filling them with matrigel and MEM-BSA, MEM-BSA
with a final EGF concentration of 25 nM, or
MEM-BSA with a final FBS concentration of 10%. All needles included
0.01 mM EDTA (pH 7.4) to sequester heavy metals that might
be released by the needle. A rat was anesthetized using 5% isoflurane
and laid on its back. The isoflurane was reduced to 2%, and a small
patch of skin over the tumor was removed. Three 25-gauge needles (guide
needles) with blocking wires were inserted to a depth of 2 mm. The
blocking wire was removed, and one of the matrigel-filled needles was
inserted into each guide needle (as shown in Fig. 3
). The needle was
then left in the tumor for 6 h. The isoflurane concentration was
slowly lowered to 0.5% during the course of the experiment to keep the
rats breathing even and unlabored. After 6 h, the needles were
withdrawn, extruded into 35-mm dishes containing growth medium, and all
cells were counted immediately. The percentage of cells with GFP
fluorescence was determined.
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| Results |
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To establish the concentration necessary to attract MTLn3 cells into
the needle, needles were filled with a range of EGF concentrations from
0.5250 nM and inserted into a cell culture. At times up
to 6 h of collection, the needles were withdrawn from the culture
and the contents were extruded into a new dish with growth medium, and
the cells were allowed to grow for 67 days to determine cell counts
and test viability. The number of cells entering the needle was
determined by the number of GFP fluorescent clones that grew during
this time. At the peak concentration of 25 nM EGF, an
8-fold increase in the number of cells entering the needle was seen,
when compared with buffer alone (Fig. 1)
. The number of cells collected decreased at 50 nM EGF, and
by 250 nM EGF the number of cells collected returned to
near background.
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By using a needle loaded with matrigel and 25 nM EGF in
MEM-BSA, we were able to capture images of the cells moving toward the
pipette, using time-lapse video-microscopy. In Fig. 2
, the matrigel surface at the edge of the needle is delineated by the
white line and colored gray. At time zero, cells
1 and 2 are seen as nonpolarized cells with no discernable leading
edge. After 1.5 h, cells 1 and 2 have oriented themselves toward
and moved in the direction of the needle-induced EGF gradient,
extending a leading lamellapod toward the needle. Cell 3 has also moved
into the field. After 3 h, all three cells can be seen to have
moved measurably closer to the needle. The cells move toward the needle
at a velocity of 0.32 µm/min, which is comparable with the
velocities reported previously (10)
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The number of cells collected for each condition was normalized to the
number of cells collected from the MTC-GFP tumors using needles
containing matrigel plus buffer (MEM-BSA) only (Fig. 4)
. For the needle with 25 nM EGF, 15.3 times more MTLn3
cells were collected from metastatic MTLn3 tumors compared with MTC
cells from nonmetastatic MTC tumors under the same conditions of
collection. In this case, a maximum of 100 cells was collected. Needles
containing 10% FBS showed only a 6.0-fold difference between the two
tumor types under the same conditions (Fig. 4)
. There was a 2-fold
increase in the number of MTC cells entering the 10% FBS needle from
the MTC tumors compared with the number of cells that entered the
needle containing only buffer. This difference was shown to be
significant (t test value, 0.027) and is consistent with the
increase in motility of MTC cells when stimulated with 10% FBS
in vitro (data not shown). We did not attempt to establish
long-term cultures of tumor cells collected from the tumors in
vivo under any of these conditions.
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The collection of cells from the MTln3 tumors was maximal with needles
containing 25 nM EGF, resulting in the collection of about
100 cells in 6 h. Because the diameter of the 33-gauge collecting
needle is 100 um and the average cell diameter is 25 um, the calculated
average velocity of cell motility required to account for the
collection of 100 cells in 6 h is 0.3 um/min. This value is very
close to the velocity of cell locomotion observed in vitro
during chemotaxis (Fig. 2)
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| Discussion |
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Both EGF and transforming growth factor
are growth factors found in
mammary tissue. MTLn3 cells have around 50,000 EGF receptors/cell,
whereas EGF receptors on the MTC cells are not detectable
(9)
. By using EGF as the chemoattractant, we were able to
selectively collect 15 times as many metastatic MTLn3 cells from
MTLn3-derived metastatic primary tumors as MTC cells from MTC-derived
nonmetastatic tumors. Serum, which contains many growth factors with
potential chemotactic activity, also stimulated the collection of tumor
cells from MTLn3 tumors. Although MTC cells show a motility response to
serum, they were not collected with high efficiency into needles in
response to serum, indicating that additional factors besides motility
are required for the large increase in the number of MTLn3 cells
collected in response to serum.
Morphologically, MTC cells are elongated and polarized both in vivo and in vitro, whereas the MTLn3 cells are generally unpolarized both in culture and in the primary tumor (8 , 11) . This difference is most dramatically illustrated by using intravital imaging techniques where GFP-expressing tumor cells are imaged directly in the primary tumor (8 , 9) . In vivo, MTLn3 cells are highly polarized around and oriented toward the blood vessels running through the primary tumor. MTC cells, on the other hand, are polarized throughout the tumor, but the polarity is randomly oriented relative to vessels (8 , 11) .
Characterization of the cells in vitro confirms the differences between the two cell lines. In cultures that have not been stimulated with a chemoattractant, MTC cells locomote in a linear direction at approximately twice the velocity of MTLn3 cells. MTLn3 cells, under these conditions, are unpolarized and move in random directions or not at all (11) . On stimulation with an EGF gradient, the MTLn3 cells become polarized and move linearly at approximately the same speed as the MTC cells, yet have the ability to reorient themselves to follow an EGF gradient with precision (10) , a property not seen in MTC cells.
In vivo, in the primary tumor, both cell types move linearly at approximately the same speeds, but the MTLn3 cells tend to move only when they are polarized and in association with a vessel, whereas MTC cells can be seen moving throughout the tumor (8 , 9) . The ability of the MTLn3 cells to invade into a needle filled with matrigel in response to growth factors is fully consistent with the chemotactic motility exhibited by these cells in vitro, their polarity and locomotion toward vessels in vivo, and with the dramatically increased efficiency of intravasation measured as blood burden of tumor cells in vivo (8) . This suggests that chemotaxis may be the key aspect of cell motility that contributes to invasion and intravasation. It also suggests that needles filled with growth factors and matrigel, when inserted into the primary tumor, can faithfully mimic the environment that supports invasion and intravasation in vivo, and that the same cell behaviors that contribute to chemotaxis in vitro also contribute to invasion in vivo.
An advantage of using the needle collection technique described here for the collection of cells for genomic/proteomic analysis is that the cell behavior can be characterized during the collection process. This can be done by varying the conditions required for cell collection such as the extracellular matrix composition and/or cytokines used as chemoattractants, determining how these changes affect efficiency of cell collection, and then relating these observations to the gene expression and protein composition patterns subsequently obtained from array analysis of the collected cells. Furthermore, cells can also be characterized by intravital imaging during collection to directly visualize the cell-cell and cell-extracellular matrix interactions that contribute to the invasion of the needle under these different conditions. In addition, cells could be cultured and transplanted into other host animals to determine whether they stably retain differential characteristics that contribute to metastatic potential. Finally, by comparing the gene expression patterns of cells collected by invasion into needles with that of cells obtained from the whole primary tumor, the blood, and whole metastatic tumors, genes that contribute to the invasive process uniquely may be identified.
| FOOTNOTES |
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1 To whom requests for reprints should be
addressed, at Departments of Anatomy and Structural Biology, Albert
Einstein College of Medicine, 1300, Morris Park Avenue, Bronx, NY
10461. Phone: (718) 430-4068; Fax: (718) 430-8996; E-mail: condeeli{at}aecom.yu.edu ![]()
2 The abbreviations used are: FBS, fetal bovine
serum; MEM-BSA, MEM with 0.35% BSA; EGF, epidermal growth factor;
i.d., inside diameter; GFP, green fluorescent protein. ![]()
Received 4/19/00. Accepted 8/16/00.
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