N-WASP-mediated invadopodium formation is
involved in intravasation and lung metastasis of
Bojana Gligorijevic1,2,*,`, Jeffrey Wyckoff1,*, Hideki Yamaguchi3, Yarong Wang1, Evanthia T. Roussos1and
1Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
2Gruss-Lipper Biophotonics Center, Albert Einstein College of Medicine, Bronx, NY 10461, USA
3Division of Metastasis and Invasion Signaling, National Cancer Center Research Institute, Chuo-ku, Tokyo 104-0045, Japan
*These authors contributed equally to this work
`Authors for correspondence (email@example.com; firstname.lastname@example.org)
Accepted 11 November 2011
Journal of Cell Science 125, 724–734
? 2012. Published by The Company of Biologists Ltd
Invadopodia are proteolytic membrane protrusions formed by highly invasive cancer cells, commonly observed on substrate(s) mimicking
extracellular matrix. Although invadopodia are proposed to have roles in cancer invasion and metastasis, direct evidence has not been
available. We previously reported that neural Wiskott–Aldrich syndrome protein (N-WASP), a member of WASP family proteins that
regulate reorganization of the actin cytoskeleton, is an essential component of invadopodia. Here, we report that N-WASP-mediated
invadopodium formation is essential in breast cancer invasion, intravasation and lung metastasis. We established stable cell lines based on
MTLn3 rat mammary adenocarcinoma cells that either overexpressed a dominant-negative (DN) N-WASP construct or in which N-WASP
expression was silenced by a pSuper N-WASP shRNA. Both the N-WASP shRNA and DN N-WASP cells showed a markedly decreased
ability to form invadopodia and degrade extracellular matrix. In addition, formation of invadopodia in primary tumors and collagen I
degradation were reduced in the areas of invasion (collagen-rich areas in the invasive edge of the tumor) and in the areas of intravasation
(blood-vessel-rich areas). Our results suggest that tumor cells in vivo that have a decreased activity of N-WASP also have a reduced ability
to form invadopodia, migrate, invade, intravasate and disseminate to lung compared with tumor cells with parental N-WASP levels.
Key words: N-WASP, Invadopodia, Breast cancer, Intravasation, Metastasis
In primary tumors, cell migration is essential for metastasis
(Friedl and Gilmour, 2009; Roussos et al., 2011a; Sahai, 2005).
To migrate through barriers carcinoma cells need to remodel
extracellular matrix (ECM) (Even-Ram and Yamada, 2005),
which is a process controlled by proteases, mainly matrix
metalloproteases (MMPs) (Egeblad et al., 2008; Koblinski et al.,
2000). In vitro studies have shown that invasive cancer cells,
when cultured on components of ECM or basement membranes
(collagen I-, fibronectin- and laminin-based substrates) form
invadopodia, membrane protrusions specialized to degrade ECM
(Buccione et al., 2004). Invadopodia are enriched with a variety
of proteins, including actin and actin regulatory proteins, matrix-
proteins. Many of these proteins are present in other structures
linked with tumor cell motility but the defining feature of
invadopodia comes from their high proteolytic activity. This
directly leads to a hypothesis that invadopodia have important
roles in one or more steps of the metastatic cascade (Eckert et al.,
2011). In addition, invadopodium formation in breast cancer cells
is regulated by the epidermal growth factor (EGF) signaling
pathway (Yamaguchi et al., 2005a). The EGF receptor (EGFR)
and its signaling pathway are also involved in invasion,
intravasation and metastasis of breast cancer (Xue et al., 2006).
Intravital imaging studies revealed that carcinoma cells
polarize and form protrusions towards blood vessels in primary
mammary tumors, which are believed to be involved in invasion
and intravasation (Condeelis and Segall, 2003; Sidani et al.,
2006; Wang et al., 2002; Yamaguchi et al., 2005b). Additionally,
neural Wiskott–Aldrich syndrome protein (N-WASP), has been
implicated in cell invasion (Lorenz et al., 2004; Oser et al., 2010;
Snapper et al., 2001; Wang et al., 2005; Yamaguchi et al.,
N-WASP (Miki et al., 1996) is a member of the WASP family
of proteins that are key regulators of reorganization of the
actin cytoskeleton in several signaling pathways (Stradal et al.,
2004; Takenawa and Suetsugu, 2007). N-WASP activates
Arp2/3-complex-mediated actin polymerization and this activity
phosphoinositides. N-WASP was previously shown to be an
essential component of invadopodia (Lorenz et al., 2004;
Yamaguchi et al., 2005a), but not lamellipodia (Desmarais
et al., 2009) or filopodia (Li et al., 2010) in metastatic carcinoma
cells. In other cell types present in primary tumors, macrophages
are known to develop invasive and adhesive structures very
similar to invadopodia (Oser et al., 2011) called podosomes. In
these structures the primary controller of actin polymerization is
WASP (Linder et al., 1999).
Journal of Cell Science
Given this background, we have studied the importance of N-
WASP in the invasion and migration of tumor cells in vivo and its
requirement for intravasation and metastasis of mammary tumors.
In particular, we have studied N-WASP involvement in the
formation of invadopodia in vivo in tumor cells and the direct link
between invadopodium formation and remodeling of stroma by
cancercells duringinvasion,intravasation and lung metastasis. We
examined the phenotypes of MTLn3 cells with impaired function
or silencing of N-WASP, and the ability of these cells to form
tumors, invade, intravasate and metastasize. We report that N-
WASP-mediated invadopodium formation is necessary for the
invasion and intravasation steps in breast cancer metastasis.
Expression DN N-WASP and N-WASP shRNA reduces
number of invadopodia and matrix degradation levels
In order to study the role of N-WASP in tumor cell behavior, we
established stable MTLn3 rat mammary adenocarcinoma cell
lines in which we interfered with the endogenous N-WASP
protein. One approach was to overexpress the dominant-negative
(DN) form of N-WASP that lacks essential amino acids for
activation of the Arp2/3 complex and therefore acts as a
competitive inhibitor of endogenous N-WASP proteins as
shown previously (Banzai et al., 2000). We also used an
alternative approach to block N-WASP activity by silencing N-
WASP expression with N-WASP small hairpin RNA (shRNA).
The control cells used were parental MTLn3 cells transduced
with control vectors (either pMX vector or scrambled shRNA in
pSuper vector). All constructs were introduced by retroviral
transfections. Immunoblotting analysis demonstrated that DN N-
WASP-transfected cells overexpressed ectopic DN N-WASP
protein 6.7-fold, whereas N-WASP shRNA resulted in 75%
silencing compared with control-shRNA-treated cells (Fig. 1A).
All cell lines showed similar morphology and growth rates and
no changes in random cell motility were observed (data not
We previously reported that transient interference of N-WASP
by siRNA or overexpression of DN N-WASP transfection
Fig. 1. Establishment of MTLn3 cell lines expressing DN N-WASP and shRNA constructs and effects on invadopodium formation and activity.
(A) Relative expression levels of N-WASP protein measured in lysates of control MTLn3 (pMX and scrambled shRNA) cells or MTLn3 cells stably
overexpressing DN N-WASP or N-WASP shRNA. Immunoblotting was done with anti-N-WASP or anti-b-actin antibodies. (B) Representative images of MTLn3
control (pMX and scrambled shRNA) cells and cells expressing DN N-WASP or N-WASP shRNA cultured on a fluorescent gelatin-based matrix. Fixed cells were
labeled for anti-cortactin (green) and actin (phalloidin, red). Upper panels show invadopodia (anti-cortactin and phalloidin colocalization – yellow inside cells);
lower panels show sites of degraded gelatin substrate (white). Scale bar: 20 mm. The arrowhead points to an invadopodium and its corresponding hole in the
matrix. (C) The number of cells with invadopodia (dots where cortactin and actin are colocalized) and the percentage of degraded matrix were calculated in
control, DN -N-WASP and N-WASP shRNA cell lines. Error bars indicate the s.e.m. of three separate experiments. In each determination, $20 different fields of
view with $100 cells were analyzed; error bars indicate the s.e.m.; *P,0.05, **P,0.01, Student’s t-test.
N-WASP regulates breast cancer metastasis725
Journal of Cell Science
suppressed invadopodium precursor formation in MTLn3 cells in
vitro (Oser et al., 2009; Yamaguchi et al., 2005a). The original
phenotype was then rescued in vitro by insertion of N-WASP–
GFP construct (Yamaguchi et al., 2005a). In the present study
we examined the ability of cell lines stably expressing either
DN N-WASP or N-WASP shRNA constructs to form mature
invadopodia and degrade matrix. Cells were stained for cortactin
and actin and colocalization was observed at the cell periphery
and in invadopodia as punctate dots, and matrix degradation was
present on the ventral surface of the cell (Fig. 1B. upper panels;
arrowhead points to an invadopodium). Corresponding images of
fluorescent gelatin-based matrix underneath the cells showed
degradation (white) patterns that matched the actin–cortactin dots
(Fig. 1B, lower panels; supplementary material Fig. S1).
Analysis of invadopodium frequency and matrix degradation in
images shows that the expression of either DN N-WASP or N-
WASP shRNA dramatically reduced the number of cells capable
of forming invadopodia and suppressed their proteolytic activity
(Fig. 1C). In addition, invadopodia show a decrease F-actin
assembly,a result that suggests
invadopodium formation occurs through defects in actin
assembly [supplementary material Fig. S2 and Desmarais et al.
(Desmarais et al., 2009)].
These data lead us to hypothesis that N-WASP-deficient tumor
cells are incapable of accomplishing metastatic steps that require
invadopodium formation and matrix proteolysis by invadopodia.
N-WASP is required for the MMP-dependent in vivo
Using allograft mammary tumors derived by injecting tumor cells
into mammary glands of rats we next examined the invasive
ability of tumor cells in an in vivo invasion assay (Fig. 2)
(Wyckoff et al., 2000b). Control pMX (empty vector), DN N-
WASP, control (scrambled) shRNA and N-WASP shRNA cell
lines were injected into mammary glands of rat and grown for 4
weeks (tumors ,1 cm in diameter). As previously described
(Wyckoff et al., 2010; Wyckoff et al., 2000a), micromanipulator-
guided microneedles containing Matrigel with or without EGF
were inserted into the primary tumors, and the number of cells
that entered the microneedles was determined. Both DN N-
WASP and N-WASP shRNA cells showed a markedly decreased
invasive ability in vivo compared with controls. Next, we were
interested in whether the invasive ability, which is N-WASP
sensitive, was also MMP dependent. If so, a similar defect of
invasive ability would be observed by inhibiting MMPs. To this
end, we introduced pan-metalloprotease inhibitor GM6001 into
the control shRNA tumors. The addition of GM6001 resulted in a
significant (P,0.01) decrease of the number of cells entering the
microneedles (Fig. 2). By contrast, the number of invading cells
in control shRNA tumors (with and without dimethyl sulfoxide;
DMSO) was not statistically different. These results demonstrate
that N-WASP activity is necessary for MMP-dependent in
vivo invasion by mammary adenocarcinoma MTLn3 cells,
strengthening the hypothesis that N-WASP has a role in early
steps of metastasis and that this role is through formation of
Functional N-WASP is required for early steps of
metastasis in mouse and rat mammary tumors
In order to test whether N-WASP function is important for tumor
metastasis, stable MTLn3 cell lines, all labeled with cytoplasmic
(untargeted) GFP, were injected into mammary glands of severe
combined immunodeficiency disease (SCID) mice and rats and
allowed to form tumors. The mean volumes of DN N-WASP and
N-WASP shRNA tumors were not significantly different from
those of control tumors (P.0.25; data not shown) indicating no
change in primary tumor growth. As reported previously, primary
tumors derived from control MTLn3–GFP cells spontaneously
metastasize to lung and form micro- and macrometastases (Farina
et al., 1998; Neri et al., 1982). Therefore, we examined the effects
of DN N-WASP and the knockdown of N-WASP compared with
controls on tumor metastasis to lung, and whether steps of
Fig. 2. Tumor cell invasion in vivo in response to EGF requires
functional N-WASP. MTLn3 control cells (pMX or scrambled
shRNA) or MTLn3 cells stably expressing DN N-WASP or N-WASP
shRNA constructs were injected into the mammary glands of rats and
formed orthotopic primary tumors. The ability of these cells to
invade from primary tumors into microneedles filled with Matrigel
with or without (+/–) 25 nM EGF (see Materials and Methods) was
measured by counting the cells recovered from the needle after
staining with DAPI. In addition, inhibition of invasion in the absence
or presence of 10 mM GM6001, an MMP inhibitor, was measured.
**P,0.01, Student’s t-test.
Journal of Cell Science 125 (3)726
Journal of Cell Science
dissemination were affected. The number of circulating tumor
cells present in blood was determined by collecting blood from
the right atrium of the heart, culturing the blood, and later
counting the number of tumor cell colonies formed (Wyckoff
et al., 2000a) (supplementary material Fig. S3A). This analysis
revealed that DN N-WASP and N-WASP shRNA cells have
markedly fewer circulating tumor cells than control tumors
(Fig. 3A). Moreover, primary tumors derived from DN N-WASP
and N-WASP shRNA cells have reduced metastasis to the lungs
compared with control tumors (Fig. 3B and supplementary
material Fig. S3B). Similar results were seen for both
(Fig. 3C,D). These results indicate that expression of DN N-
WASP or the N-WASP inhibits lung metastasis of breast tumors
and this may be due to reduced entry into the blood vessel
(invasion and intravasation).
To determine whether the role of N-WASP in metastasis is
important for dissemination prior to blood vessel entry, control
and N-WASP-deficient MTLn3 cells were labeled with the
photoconvertible protein Dendra2 (Chudakov et al., 2007).
Previously, we reported that the fate of Dendra2–MTLn3 cell
populations, when photoconverted from the green to the red state
by exposure to 405 nm light, can be followed over days, and
dissemination of tumor cells from the primary tumor can be
observed directly (Gligorijevic, 2009; Kedrin et al., 2008).
Although (red) cells photoconverted in the areas populated with
microvessels showed some dispersion and increase in numbers
(as a result of cell division) 24 hours later, cells photoconverted
in the vicinity of major blood vessels showed a significant
(P50.01220.028) decrease in cell numbers 24 hours later. Here
we used this assay on both control tumors and N-WASP-deficient
tumors in order to directly compare their abilities to intravasate.
Squares of 2506250 mm next to or around major blood vessels
were photoconverted at 0 hours, and three-dimensional (3D)
(supplementary material Fig. S4). The number of red cells in
the control tumors decreased by 14–37% over 24 hours, which
was shown previously to be due to dissemination of cells by the
blood stream (Kedrin et al., 2008; Roussos et al., 2011b), there
was a slight increase (due to cell division) or no change in the
number of red cells in DN N-WASP and N-WASP shRNA
tumors (Fig. 4). This result further suggested an important role
for N-WASP in the early steps of metastasis by invasion and
intravasation. Combined with the data from cells cultured on thin
matrices and the in vivo invasion assay, these results suggest that
the formation of invadopodia in vivo is N-WASP sensitive and
directly involved in tumor cell migration through the tissue and
at 0 hoursand24 hours
N-WASP function is required for cell protrusion and
migration in vivo
In order to directly assess the role of N-WASP in cell motility
within primary tumors, multiphoton intravital imaging of GFP-
labeled tumors (pMX, DN N-WASP, control shRNA and N-
WASP shRNA) was performed following skin flap surgery
(Farina et al., 1998; Wang et al., 2002). Tumor cells with
suppressed N-WASP were rounded and less polarized than cells
with parental levels of N-WASP (Condeelis and Segall, 2003) in
the living tumors (Fig. 5). As previously described, cells with
parental N-WASP levels are often motile (Fig. 5A) and form
protrusions (Fig. 5B), in contrast to tumor cells deficient in N-
Fig. 3. The presence of circulating
tumor cells and formation of lung
metastasis requires N-WASP in rat
and mouse mammary tumors.
(A,B) Rat tumor-cell-derived
mammary tumors in rat. (A) Number
of viable circulating tumor cells per
milliliter of blood; (B) Number of
lung micrometastases. (C,D) Rat
tumor-cell-derived mammary tumors
in SCID mice. (C) Number of viable
circulating tumor cells per milliliter
of blood; (D) number of lung
metastases. Error bars indicate the
s.e.m.; *P,0.05, **P,0.01, verified
by Mann–Whitney test.
N-WASP regulates breast cancer metastasis 727
Journal of Cell Science
Tumor cell motility was then monitored in 3D by time-lapse
imaging (over 30-minute periods). Fig. 5A show stills from
representative time-lapse images where dots point to the cell
front (yellow at 0 minutes, red at 30 minutes) and arrows indicate
the cell trajectory. In N-WASP-deficient tumors, there was a
dramatic decrease in motile cells compared with control tumors
(Fig. 5C). The few motile cells in the N-WASP-deficient tumors
had a similar speed and directionality as the control cells
(Fig. 5D,E). In addition, protrusions seen forming in control
tumors (Fig. 5B) were absent from N-WASP-deficient tumors
(Fig. 5B,E), demonstrating that some of the protrusion formation
in vivo was N-WASP dependent. Because the N-WASP-deficient
cells are unable to form invadopodia, we hypothesized that some
of the protrusions seen in vivo are invadopodia and that the lack
of proteolytic activity associated with invadopodia is essential for
extracellular matrix remodeling and cell migration through the
tissue. Our next step was, therefore, to test whether N-WASP-
dependent protrusions in tumors cells in vivo have structural
components typical for invadopodia and whether they are linked
to proteolytic activity of tumor cells.
N-WASP-sensitive protrusions observed in tumors have
structural components and proteolytic activity typical
Invadopodial assays, such as the one used in Fig. 1B, commonly
fluorescent substrate such as fibronectin, gelatin or collagen.
They have become a standard technique for cell cultures grown
on thin matrices, but standardized invadopodial assays for 3D
cell culture or tumor tissue (ex vivo and in vivo) have not
been established yet. Previously, quenched substrates and
immunofluorescence have been used to show areas of matrix
proteolysis (Fisher et al., 2009; Packard et al., 2009; Sloane et al.,
2006) in 3D culture, whereas proteolysis in tissue sections has
been visualized by in situ zymography (Frederiks and Mook,
2004). To compare levels of proteolytic degradation in control
and N-WASP-suppressed tumor cells, we applied an antibody
against degraded collagen, Collagen 3/4 (i.e. Col2 3/4 short),
which recognizes collagen fragments created by the proteolytic
activity of MMP-1, MMP-2 or MMP-13 (Tolde et al., 2011; Wolf
et al., 2007) in tumor cryosections (Fig. 6A). Results were
confirmed by direct comparison of the proteolysis detected as
the absence of gelatin (supplementary material Fig. S5A) and
association with cortactin as a structural invadopodium marker
(supplementary material Fig. S5B). When combined with DAPI,
cortactin and actin, the Collagen 3/4 antibody indicated the
presence of invadopodial protrusions in cells adjacent to the
tumor edge or tumor blood vessels (invasive edge areas, Fig. 6B,
and perivascular areas, Fig. 6C; also supplementary material Fig.
S6). In cryosections of N-WASP-deficient tumors there was only
one third of the number of invadopodia compared with control
sections, which is similar to the results of in vitro cultures.
However, the degraded area in N-WASP-deficient tumors was
approximately one-tenth than in control tumors whereas in the in
vitro experiments there was an approximately one third than in
Finally, we tested whether the N-WASP is localized to
invasive protrusions and, therefore, whether it has a direct
function in invadopodium assembly in tumor cells in vivo,
followed by matrix degradation. To this end, we sectioned
tumors from MTLn3 cells expressing Cerulean-labeled N-
protrusions, aimed towards an adjacent blood vessel (white
Fig. 4. Intravasation of tumor cells in the primary tumor requires N-WASP. (A) At 0 hours, a region of cells was photoconverted (red) in highly vascular
regions of the tumors grown from either Dendra2–control-shRNA MTLn3 (top) or Dendra2–N-WASP-shRNA MTLn3 (bottom). At 0 hours and 24 hours z-stacks
of 0–100 mm were collected in the green and red channels, and maximum projections are shown. This intravasation assay documents that there is less
movement of tumor cells into blood vessels from tumors derived from Dendra2–N-WASP shRNA MTLn3 cells. Scale bar: 70 mm. (B) Number of red tumor cells
remaining around blood vessels when normalized to the number at 0 hours. Measurements were based on three or four animals per cell line, 5–11 regions per
animal. Error bars indicate the s.e.m. *P,0.05, Student’s t-test.
Journal of Cell Science 125 (3) 728
Journal of Cell Science
dashed line) that are enriched in cortactin, actin and N-WASP
(Fig. 7B). Further quantification of .15 protrusions showed
similar levels throughoutthe tissue(Fig. 7C).Finally,
supplementary material Fig. S7 demonstrates that degradation
of collagen I occurs around protrusions rich in cortactin, actin
Fig. 5. Cell motility and protrusion formation in vivo requires N-WASP. (A) Motility of control and N-WASP-deficient GFP-labeled cells was monitored
over time at different regions of the tumor using time-lapse images taken at 0, 15 and 30 minutes (images at same z-sections are shown). Small colored
squares indicate the cell fronts (yellow at 0 minutes, red at 30 minutes) and arrows show the length of the trajectory of the migrating cells. (B) Higher
magnifications of regions where cells are sparsely distributed showing that more protrusions are present in control tumors. (C–F) Data from both control and
N-WASP-deficient tumors showing the number of motile cells per 4D stack (C), velocity of motile cells (D), directionality of motile cells (E) and number of
protrusions per 4D stack (F). *P,0.05, **P,0.01, Student’s t-test, from three or more different tumors. The 4D stacks were 51265126100 mm630
N-WASP regulates breast cancer metastasis729
Journal of Cell Science
Breast cancer metastasis is the sum of multistep processes,
including migration and invasion through tumor stroma,
intravasation insidetheprimary tumor, tumorcell
dissemination, extravasation and cell growth at the secondary
sites. Our data demonstrates that N-WASP activity is essential for
the formation of proteolytic protrusions (invadopodia) in vivo,
which in turn drive the invasive steps of metastasis. Silencing of
Fig. 6. N-WASP-sensitive protrusions observed in tumors contain invadopodium markers and have degradation activity. (A) Matrix degradation sites
detected by immunofluorescence (Collagen 3/4; red) in control shRNA tissue slices (anti-cortactin, green; DAPI, blue). Scale bar: 10 mm. (B) Quantification of the
average number of protrusions, and collagen-I-degraded area in control tumors, DN N-WASP and N-WASP-shRNA tumors, as determined with the Collagen 3/4
assay. *P,0.05, Student’s t-test. (C,D) Immunohistofluorescence of control shRNA tumors and N-WASP shRNA tumors in (C) the invasive edge and (D) areas
next to major blood vessels (perivascular). DAPI is blue, anti-cortactin is green, phalloidin is yellow, Collagen 3/4 is red. The bottom row of images are merged
images of all the markers. White arrowheads point to protrusions associated with degradation activity. Scale bars: 10 mm.
Journal of Cell Science 125 (3)730
Journal of Cell Science
N-WASP was shown to substantially reduce the formation of
invasive protrusions, and consequently invasive migration in
vivo, intravasation, the number of circulating tumor cells and
metastasis to the lungs.
Invasive migration was reduced to a similar level in tumors
deficient in N-WASP and when using an MMP inhibitor. These
results show that N-WASP-sensitive migration in tumors is
MMP-dependent but it also implicates a possible feedback loop
in invasive cell migration. Several studies have suggested the
possibility of an invadopodial cycle (Magalhaes et al., 2011) in
which the assembly of invadopodial proteins (cortactin, N-
WASP, cofilin, etc.) is the first and MMP proteolysis is the last
stage in the cycle (Oser et al., 2009), and the feedback signals
(Clark et al., 2007) initiate a new cycle to extend invadopodia
further into the matrix. If this is the mechanism of invadopodial
extension, breaking the cycle either by inhibiting assembly of
proteins or by MMP inhibition should result in a similar
Detachment of cancer cells from primary tumors requires loss
of cell–cell adhesion and enhanced cell motility. It is unclear
whether N-WASP functions in this process. A previous study has
shown that the overexpression of N-WASP or DN N-WASP does
not affect cell–cell junctions in epithelial cells (Yamaguchi et al.,
2002) and that DN N-WASP overexpression had little effect on
random cell migration in vitro, indicating that N-WASP is not
involved in general cell locomotion. A different study (Martin
et al., 2008) reported that N-WASP overexpression increased
adhesiveness in breast cancer cells and reduced tumor growth
To intravasate, carcinoma cells need to break down basement
membrane and the endothelial barrier surrounding blood vessels.
It has been shown that upregulation of EGFR and its sensitivity to
EGF (Roussos et al., 2011b) is closely correlated with enhanced
intravasation and lung metastasis of breast tumors (Xue et al.,
2006). We also reported that EGFR signaling is essential for
invadopodium formation in MTLn3 cells and that N-WASP is an
essential mediator of invadopodium formation downstream of
EGFR signaling (Oser et al., 2009; Yamaguchi et al., 2005a).
Therefore, it is probable that EGF-stimulated formation of
invadopodia promotes carcinoma cell intravasation. This is
supported by our observations here and those obtained by
intravital imaging previously that carcinoma cells around blood
vessels form vessel-directed protrusions before they intravasate
(Condeelis and Segall, 2003; Wang et al., 2002; Yamaguchi
et al., 2005b). In this study, we found that perturbing the function
of N-WASP by either a dominant negative construct or shRNA
Fig. 7. Protrusions observed in tumors are
enriched in actin, cortactin and N-WASP.
(A) Cerulean–N-WASP–MTLn3 tumor sections
(purple) in the area next to a blood vessel (dashed
white line), additionally stained by anti-cortactin
(green) and phalloidin (yellow). Scale bar: 7 mm.
(B) Intensity profiles of cortactin, actin and N-WASP
along the red line shown in A illustrating enrichment
of these proteins in the protrusion. (C) Levels of
cortactin (green), actin (yellow) and N-WASP
(purple) in protrusions relative to the average
cytoplasmic levels illustrating enrichment of these
proteins in protrusions in Cerulean–N-WASP–
MTLn3 tumors. Error bars indicate the s.e.m. for .15
protrusions; **P,0.01, Student’s t-test.
N-WASP regulates breast cancer metastasis 731
Journal of Cell Science
inhibits invadopodium-mediated ECM degradation and in vivo
invasion, as well as the formation of protrusions, by carcinoma
cells. These results, together with our previous studies, strongly
suggest that N-WASP-mediated invadopodium formation is
specifically involved in the invasion and intravasation steps of
It is well established that WASP family proteins, including
WASP,N-WASP and WAVE1, 2 and 3, are key regulators of actin
polymerization, during cell migration and invasion (Takenawa and
Suetsugu, 2007). WAVE proteins regulate the formation of
lamellipodia that determine cell direction and protrusive force
for general cell migration. It has been previously reported that
WAVE2 knockdown in mammary adenocarcinoma and melanoma
cells blocks formation of lamellipodia in vitro and lung metastasis
in experimental metastasis assays, respectively (Kurisu et al.,
2005; Sarmiento et al., 2008). Additionally, the expression level of
WAVE2protein is increasedas melanoma cells become metastatic
(Yamaguchi and Condeelis, 2007). These results strongly suggest
that WAVE2-mediated cell migration is necessary for cancer cells
to acquire a metastatic potential.
In contrast to WAVE proteins, N-WASP regulates the
formation of more specialized protrusions such as invadopodia,
which degrade ECM (Li et al., 2010; Yamaguchi et al., 2005a).
Our data agree with those of Li et al., who showed that siRNA N-
WASP knockdown reduces collagen I degradation by ,60% (Li
et al., 2010). In addition, the total number of protrusions in N-
WASP-deficient fibroblasts from N-WASP null embryos is
substantially reduced (Snapper et al., 2001). Carcinoma cells
overexpressing DN N-WASP have no obvious defect in
lamellipodium formation and random cell migration in vitro
(data not shown), confirming that N-WASP is not involved in
general cell migration driven by lamellipodial protrusions.
However, N-WASP-dependent invadopodium assembly has
been implicated in chemotaxis to EGF in tumor cells
(Desmarais et al., 2009), and WASP is required for both
podosome assembly and chemotaxis by macrophages (Zicha
et al., 1998), suggesting that N-WASP- and WASP-dependent
invasive protrusions are involved in directional migration toward
growth factors. These results are consistent with our observations
that N-WASP-dependent protrusions that extend toward blood
vessels are correlated with intravasation and metastasis.
It has been reported that lower levels of expression of N-WASP
in whole breast tumor tissueis correlatedwithpoor outcome, using
disease-free survival as the end point (Martin et al., 2008).
However, this study did not evaluate the behavior of the migratory
and disseminating population of breast cancer cells in the primary
It is not clear whether the lower levels of N-WASP expression
were confined to the migratory tumor cells. Our results indicate
that the invasive and migratory population of tumor cells in vivo
requires N-WASP for migration, intravasation and metastasis.
Taken together, all of the above cited studies establish that
WASP-family proteins have distinct roles in cell migration
during tumor invasion and metastasis: WAVE proteins regulate
lamellipodium-mediated general cell migration and N-WASP
regulates invadopodium-mediated directional cell migration
through stroma and toward blood vessels, which requires
matrix remodeling. In conclusion, the potential of N-WASP as
a target for anti-invasion and anti-metastasis therapeutics should
be investigated further.
Materials and Methods
Establishment of stable cell lines
MTLn3 cells derived from rat mammary adenocarcinoma (Neri et al., 1982),
which are a basal breast cancer cell type (Roussos et al., 2011b), were cultured and
maintained in a-MEM supplemented with 5% FBS and antibiotics as described
previously (Neri et al., 1982; Segall et al., 1996). A control pMX cell line was
created by infection of MTLn3 cells stably expressing GFP (Farina et al., 1998)
with a pMX virus. To generate DN N-WASP, the same cells were infected with
pMX retrovirus containing DN N-WASP cDNA. A control for N-WASP
knockdown (control shRNA), was created by infection of MTLn3 cells with
pSuper retrovirus expressing scrambled shRNA (TCCACACCACTACACGAGC)
(Gevrey et al., 2005). To generate N-WASP knockdown cells (N-WASP shRNA),
MTLn3 cells were infected with pSuper retrovirus expressing shRNA targeted to
rat N-WASP cDNA (GACGAGATGCTCCAAATGG-TTCAAGAGA-CCAT-
TTGGAGCATCTCG-TC) (Yamaguchi et al., 2005a).
Cerulean–N-WASP–MTLn3 cells were a gift from Ved Sharma (Albert Einstein
College of Medicine, NY). Dendra2-expressing cell lines (control pMX, control
shRNA, DN N-WASP and N-WASP shRNA) were generated by transfecting cells
with Dendra2 using Lipofectamine 2000. Retroviruses were expanded by
transfection in Phoenix packaging cells. Under these conditions, .95% of cells
were infected with the stably expressing or knockdown N-WASP.
Invadopodia assay, immunofluorescence and immunohistofluorescence
Counting of invadopodia and analysis of the degradation area in cell culture was
performed as described previously (Yamaguchi et al., 2005a). Briefly, cells were
cultured on Alexa-Fluor-568–fibronectin-coated or Alexa-Fluor-405–gelatin-
coated coverslips for 8–16 hours, fixed in 3.7% formaldehyde for 15 minutes
and permeabilized with 0.1% Triton X-100 for 10 minutes. Measurement of
degradation area in tumors was performed on frozen tissue slices. Tumors grown in
SCID mice were excised and lightly fixed (1 hour in 3.7% paraformaldehyde),
washed for 1 hour in cold PBS and dehydrated overnight in 30% sucrose. They
were then embedded in OCT and 6 mm sections cut using a cryostat. Sections were
permeabilized using 0.1% Triton X-100 or cold acetone for 10 minutes.
Cultures or tissue slices were blocked in 1% BSA and 1% FBS for 1 hour,
incubated with primary antibodies that were diluted in blocking buffer for 1 hour,
and then with Alexa-Fluor-conjugated secondary antibodies diluted in blocking
buffer (Invitrogen; 1:300) mixed with Alexa Fluor 488, 543 or 633 and/or Alexa-
Fluor-633–phalloidin (Invitrogen; 1:300) for 1 hour. Primary antibodies used were
against cortactin (Abcam ab-33333; 1:100) or degraded collagen (Ibex
Pharmaceuticals, C1 C2 antibody or Col2 3/4 short antibody, 1:100; we refer to
this antibody as Collagen 3/4). In addition, tissue samples were incubated with
DAPI for 10 minutes. Cells were observed with an Olympus widefield microscope
equipped with a cooled charge coupled device (CCD) camera, or with a Leica SP5
confocal laser-scanning microscope. ImageJ was used for image processing and
quantification of invadopodial degradation areas. Briefly, 3D stacks (0–5 mm) of
images of degraded fluorescent gelatin or degraded collagen staining were
combined into a maximum projection, processed using the smoothing filter and
thresholded to the top (or bottom) 20% of the signal (supplementary material
Fig. S1). Processing removes heterogeneities in matrix thickness and brightness
and nonspecific antibody binding and delineates degradation borders. In addition,
gelatin matrix images were inverted (black to white) for easier comparison with
In vivo invasion, circulating tumor cells and lung metastases
All procedures involving animals were conducted in accordance with NIH
regulations, and approved by the Albert Einstein College of Medicine Animal Use
MTLn3 cells (control pMX, control shRNA, DN N-WASP and N-WASP
shRNA) were injected into the mammary glands of 7-week-old female Fischer 344
rats or 5- to 7-week-old SCID mice and allowed to form tumors for 4–5 weeks. The
in vivo invasion assay with MTLn3-derived mammary tumors in rats was
performed as described previously (Wyckoff et al., 2000b). In brief, the in vivo
invasion assay uses microneedles filled with Matrigel and growth factor (EGF) to
collect the invasive tumor cells from primary tumors. Microneedles were held in a
clamping devise and positioned in the primary tumor with a micromanipulator.
Cells were collected over 2.5–4 hours and animals were anesthetized with 2–5%
isoflurane throughout. The number of tumor cells collected was counted on a
widefield microscope after expelling them on a glass slide and incubating them for
10 minutes with DAPI.
Circulating tumor cell count was determined as described previously (Wyckoff
et al., 2000a; Roussos et al., 2011b). Briefly, 1–2 ml blood was drawn from the
right ventricle of anesthetized mice and plated in a-MEM. Following 7 days in cell
culture, tumor cells and/or cell colonies were counted and calculated as the number
of circulating tumor cells surviving per milliliter of drawn blood. All cells counted
were GFP positive, confirming their identity as tumor cells (supplementary
material Fig. S3A). After blood was drawn from the rats and mice they were killed
and the lungs were removed. The number of micrometastasis on and near the
surface of the lungs were counted in twenty random fields using a 606 objective
Journal of Cell Science 125 (3) 732
Journal of Cell Science
on a widefield microscope (supplementary material Fig. S3B). Tumor size was
determined after excision as an average of the x-, y- and z-dimensions.
Multiphoton Intravital Imaging
Multiphoton imaging of GFP-labeled tumors was done as described previously
(Wyckoff et al., 2010). Briefly, GFP-labeled MTLn3 (control pMX, control
shRNA, DN N-WASP and N-WASP shRNA) were injected into the mammary fat
pads of 5- to 7-week-old SCID mice. After 4–5 weeks, skin flap surgery was
performed on anesthetized animals. The exposed area of the tumor was positioned
on top of a coverslip on an inverted microscope and imaged using an Olympus
FV1000-MPE multiphoton system with 880 nm excitation. Cell velocity and
directionality were calculated using a custom-written ROI tracking ImageJ plugin
(Entenberg et al., 2011).
In vivo Intravasation assay
constitutively expressed Dendra2 was performed as described previously
(Gligorijevic et al., 2009; Kedrin et al., 2008). Briefly, Dendra2-expressing
MTLn3 cells (control pMX, control shRNA, DN N-WASP and N-WASP shRNA)
were injected in SCID mice. A window was implanted on top of the growing tumor
after 17–20 days (5–7 mm in diameter). Following a 3-day recovery, the mouse
was anesthetized and placed in an imaging box (Gligorijevic et al., 2009).
Photoconversion was done by exposing a 2506250 mm square to 405 nm laser
light using a Leica SP5 confocal system with a 206 objective. Photoconversion
sites were chosen to be either far from (.200 mm) or close (,10 mm) to a major
blood vessel. All further imaging was done using a custom-built multiphoton
system, which contained two Ti-Sapphire lasers and an inverted Olympus IX71
microscope (Entenberg et al., 2011). Lasers were tuned to 880 nm (for imaging of
the green species of Dendra2) and 1035 nm (for the red species of Dendra2)
(Kedrin et al., 2008). All photoconversion sites were imaged at 0 hours and
24 hours post-photoconversion, collecting 3D image stacks 0–100 mm from the
tumor surface. The number of photoconverted Dendra2 cells was counted in five
20-mm-thick z-sections (supplementary material Fig. S4) using the ROI plugin for
ImageJ (Entenberg et al., 2011).
We are grateful to Ved Sharma, Dianne Cox and Jose Javier Bravo
for gifts of cell lines and constructs. We thank all members of the
John Condeelis, Jeffrey Segall and Dianne Cox laboratories and the
Analytical Imaging Facility at Albert Einstein College of Medicine
for helpful discussions.
This work was supported by National Institutes of Health [grant
number CA150344 to H.Y., E.R. and J.C.]; National Institutes of
Health [grant number CA100324 to J.W. and Y.W.]; National
Institutes of Health Mouse Models of Human Cancers Consortium
[grant number UO1-1105490 to J.C.]; and a Charles H. Revson
Fellowship in Biomedical Science 2010 to B.G. Deposited in PMC
for release after 12 months.
Supplementary material available online at
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