* Research Institute of Molecular Pathology, University of Vienna, A-1030 Vienna, Austria; Ontario Cancer Institute, Toronto,
Ontario, Canada; § Department of Morphology,
Department of Pathology, University Medical Center, CH-1211 Geneva 4, Switzerland; ¶ Max-Planck Institute for Physiological and Clinical Research, Department of Molecular and Cell Biology, D-61231
Bad Nauheim, Germany; and ** Center for Transgene Technology and Gene Therapy, Flanders Interuniversity Institute for
Biotechnology, B-3000 Leuven, Belgium
The middle T antigen of murine Polyomavirus (PymT) rapidly transforms endothelial cells, leading to the formation of vascular tumors in newborn mice. Transformed endothelial (End.) cell lines established from such tumors exhibit altered proteolytic activity as a result of increased expression of urokinase-type plasminogen activator (uPA) and are capable of inducing vascular tumors efficiently when injected into adult mice. In this study we have used mice lacking components of the PA/plasmin system to analyze the role of this system in the transformation process and in tumor growth. We found that the proteolytic status of the host is not a critical determinant for PymT-induced vascular tumor formation. In addition, the lack of either uPA or tissue-type PA (tPA) activity is not limiting for the establishment and proliferation of End. cells in vitro, although the combined loss of both PA activities leads to a marked reduction in proliferation rates. Furthermore, the in vitro morphogenetic properties of mutant End. cells in fibrin gels could only be correlated with an altered proteolytic status in cells lacking both uPA and tPA. However, in contrast with tumors induced by PymT itself, the tumorigenic potential of mutant and wild-type End. cell lines was found to be highly dependent on the proteolytic status of both the tumor cells and the host. Thus, genetic alterations in the PA/plasmin system affect vascular tumor development, indicating that this system is a causal component in PymTmediated oncogenesis.
Oncogenesis is a multistep process that requires a
long latency period between the initiating event
and the appearance of the tumor. However, a few
viral oncogenes are known to rapidly transform target tissues without the need for additional genetic events; one
such viral oncogene is the Polyoma middle T antigen (PymT).1 The PymT antigen specifically transforms proliferating endothelial cells apparently in a single-step manner, resulting in the formation of hemorrhagic cystic tumors in embryonic and neonatal mice, but not in adult mice
where endothelial cell proliferation has ceased (for review
see Pepper et al., 1997 Tumor formation and subsequent invasion involve the
disruption of anatomical barriers including basement
membranes and penetration of tumor cells into normal adjacent tissues. Several lines of evidence indicate that enhanced invasive and metastatic potential is correlated with
increased protease activity. It has been shown that both tumor and stromal cells express enzymes that degrade components of the extracellular matrix. Many of the relevant
enzymes belong to one of two families: the serine proteases, in particular the plasminogen activator (PA)/plasmin
system, and the matrix metalloproteinases (MMPs). Urokinase and tissue-type PAs (uPA and tPA) are the principal
activators of plasminogen (Plg), the zymogen from which
plasmin is derived. uPA is secreted in the form of an inactive precursor that binds with high affinity to a specific glycosylphosphatidylinositol-anchored cell surface receptor.
Plasmin is a protease of tryptic specificity that either directly, or indirectly through the activation of latent MMPs, hydrolyzes extracellular proteins. The existence of multiple
specific physiological inhibitors of both plasmin, i.e., The recent advent of mice lacking uPA, uPA receptor
(uPAR), tPA, PAI-1, and Plg has questioned much early
dogma concerning the role of the PA/plasmin system in
processes requiring extracellular matrix breakdown. Thus,
uPA In previous studies with PymT-transformed endothelial
(End.) cell lines in a fibrin gel system, we observed that
End. cells formed cystlike structures rather than capillarylike tubes, which correlated with their increased proteolytic activity (Montesano et al., 1990 Mice
The generation of uPA Virus Induced Tumor Induction in Neonatal Mice
Helper-free stocks of the N-TKmT virus were obtained from the viral producer cell line GP+E subclone 108.4.2 as described previously (Williams
et al., 1988 Derivation and Maintenance of End. Cells
Macroscopically visible tumors from mice injected with PymT viral particles were explanted, and End. cell lines were established as previously described (Kiefer et al., 1994b Table I.
Vascular Tumors Induced by PymT Virus and
Establishment of End. Cell Lines
). The potency with which PymT transforms endothelial cells is thought to be a consequence of its interaction with proteins of the signal transduction machinery (for review see Brizuela et al., 1994
;
Kiefer et al., 1994a
). These tumors are functionally reminiscent of human vascular tumors in that they are endothelial specific and organ nonspecific (for review see Wagner
and Risau, 1994
; Pepper et al., 1997
), and that tumor-bearing mice develop features of the Kasabach-Merritt syndrome, i.e., thrombocytopenia, anemia, and associated
splenomegaly (Dubois-Stringfellow et al., 1994a
).
2antiplasmin, and PAs, i.e., PA inhibitors 1 and 2 (PAI-1
and PAI-2), provides additional points of regulation along
this protease cascade (for review see Danø et al., 1985
; Vassalli et al., 1991
; Pepper et al., 1996
).
/
, tPA
/
, and uPA
/
and tPA
/
(utPA
/
) mice, as well as Plg
/
mice, develop normally in utero
and are fertile. However, utPA
/
mice develop multiple
organ failure starting at 2-3 mo of age as a result of extensive extravascular fibrin accumulation as well as generalized microvascular thrombosis (Carmeliet et al., 1994
).
utPA
/
mice thus have retarded postnatal growth, exhibit reduced fertility, and have a shortened life span. A
similar phenotype is observed in Plg-deficient mice
(Bugge et al., 1995a
; Ploplis et al., 1995
). The phenotype of
both utPA
/
and Plg-deficient mice therefore appears
to be due to a systemic defect in fibrinolysis. Although no
overt phenotypic alterations have been observed in
uPAR
/
mice with respect to development, fertility,
and homeostasis (Bugge et al., 1995b
), it has recently been reported that adult mice with a combined deficiency
in tPA and uPAR display hepatic sinusoidal fibrin deposits, demonstrating that uPAR plays a role in physiological
fibrinolysis (Bugge et al., 1996
). Finally, development and
fertility are unaffected in PAI-1-deficient mice, although
the absence of PAI-1 induces a mild hyperfibrinolytic state
(Carmeliet et al., 1993a,b). Therefore, neither PAs nor Plg
are essential for mouse development, postnatal growth, or
fertility, suggesting that the primary role for the PA/plasmin system in the mouse is fibrinolysis.
). Reduction of protease activity by the addition of broad spectrum serine
protease inhibitors led to the formation of capillary-like
tubes, resulting in the hypothesis that tightly controlled extracellular proteolysis is essential for normal capillary morphogenesis (Pepper and Montesano, 1990
). Further support for the role of the PA/plasmin system in aberrant
morphogenesis in vitro came from experiments in which
depletion of Plg from serum or addition of neutralizing antibodies to murine uPA inhibited cyst formation (DuboisStringfellow et al., 1994b). These findings implicated increased proteolysis, and in particular increased uPA activity, in aberrant vascular morphogenesis in vitro and further suggested a causal role for PymT in the elevated
proteolytic activity of End. cells. In this study, mice lacking
uPA, tPA, utPA, PAI-1, or Plg were used to investigate
whether the PA/plasmin system might be important for
PymT- or End. cell-induced tumor formation.
Materials and Methods
/
, tPA
/
, uPA,tPA
/
(utPA
/
), PAI1
/
, and Plg
/
mice has been described (Carmeliet et al., 1993a
, 1994;
Ploplis et al., 1995
). All mice in a C57Bl6/129 background were propagated by brother-sister matings and have been interbred for more than six
generations. CD1 nude mice were purchased from CharlesRiver (Sulzfeld,
Germany).
). Neonatal mice of various genotypes were injected intraperitoneally with 1-2.5 × 105 neoR-transferring particles. The mice were observed carefully and analyzed as soon as the first mouse of an injected litter showed signs of anemia.
). End. cells could be established with relative
ease from heterogeneously growing primary cultures, since they outgrow
contaminating fibroblasts. Within 5-10 passages, a homogeneously growing, mostly clonal population of End. cells was obtained, which was used
for the experiments described. Cell lines were maintained in DME supplemented with 5% FCS, glutamine, and nonessential amino acids and subcultured at confluence every 4-8 d. The origin and nomenclature of these cell lines are summarized in Table I.
Determination of Proliferation Rates of End. Cells
End. cells were plated at a density of 4 × 105-1 × 106 per 25-cm2 flask, counted, and replated at the same density every 6-8 d. Total cumulative cell numbers are represented over 23 d in culture.
Tumor Induction by Injection of End. Cells in Adult Mice
For the induction of End. cell-induced tumors, C57Bl6/129 wild-type and mutant mice, derived from brother-sister matings and interbred for more than six generations (10-14 wk old), and CD1 nude mice (8-10 wk old) were injected subcutaneously into the flanks of the right leg with 1.5-2 × 106 cells, and then scored for tumor formation. The times taken for the onset of tumors and the latency period to form tumors of 125 mm2 (maximum size) were recorded. All mice were killed when they had 125-mm2 tumors. The observation period was until 60 d after injection.
Northern Blot Hybridization
RNA analyses were carried out using standard Northern techniques
(Kiefer et al., 1994b). The probes were kindly provided by D. Belin (University of Geneva, Switzerland) (uPA), S. Strickland (State University of
New York, Stony Brook) (tPA), and R. Bravo (Bristol-Myers Squibb
Pharmaceutical Research Institute, Princeton, NJ) (PAI-1).
Immunohistochemical Analysis and Immunocytochemistry
Tissue specimens were explanted, dissected, fixed overnight in 4%
paraformaldehyde in PBS, and embedded in paraffin; 5-µm-thick sections
were stained with hematoxylin and eosin. Adjacent sections were used for
immunocytochemical reactions with antibody against factor VIII-related
antigen/von Willebrand factor (vWF) (Dakopatts, Copenhagen, Denmark). Some sections were processed using special staining techniques for
fibrin. These included the Lendrum (Lendrum et al., 1962) and Mallory's
phosphotungstic acid-hematoxylin (PTAH) techniques.
Zymography and Reverse Zymography
Confluent monolayers of End. cells in 35-mm tissue-culture dishes were
washed twice with serum-free DME, and 1.5 ml serum-free DME containing 200 KIU/ml Trasylol (Bayer AG, Zurich, Switzerland) was added. 15 h
later, cell extracts and culture supernatants were prepared and analyzed
by zymography and reverse zymography as previously described (Vassalli
et al., 1984; Montesano et al., 1990
). Cell number was determined in a second set of dishes processed in parallel, and cell extract and culture supernatant samples were analyzed on the basis of cell equivalents.
In Vitro Fibrin Gel Assay
End. cells were seeded in suspension into 500-µl fibrin gels at 1 × 104 cells
per gel. 500 µl DME containing 10% FCS was added to each well above
the fibrin gels. Fibrin gels were prepared as previously described (Montesano et al., 1990). All experiments were performed in the absence or presence of 200 KIU/ml Trasylol, which was added both to the gel and to the
medium at the time of embedding. Medium (±Trasylol) was renewed every 2-3 d. Between 4 and 16 d after seeding, cultures were fixed in situ
overnight in 2.5% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.4, and
photographed using a Phase Contrast-2 microscope (Nikon Inc., Garden
City, NY).
PymT Efficiently Induces Vascular Tumors in the Absence of Individual Components of the PA/Plasmin System
To determine if PA/plasmin-dependent proteolytic activity is an essential requirement for the formation of vascular tumors by PymT, we inoculated newborn mice lacking
uPA, tPA, utPA, PAI-1, or Plg with the PymT transducing
retrovirus N-TkmT (Williams et al., 1988). The frequency
of tumor formation was almost 100% in all mutant mice
tested, and the time required for the development of lethal
tumors (latency) varied only slightly (Table I). The high
penetrance in Plg
/
mice was unexpected and no delay
or inhibition of tumor formation was observed. Most mice
were terminally ill within 8-10 d, and only the doublemutant pups lacking uPA and tPA succumbed after a
slightly longer period of 12-14 d (Table I). These utPA
/
tumors retained the characteristic morphologic properties of PymT-induced tumors (Fig. 1, A and B). All tumors appeared as blood-filled/hemorrhagic cysts lined by endothelial cells and were not different from similar tumors induced by PymT in control mice (Kiefer et al., 1994b
). Representative sections through a typical lesion that formed in
a double mutant (utPA
/
) mouse as well as in a Plg
/
mouse are shown in Fig. 1. Staining with anti-vWF antibody shows endothelial cells lining these cysts (Fig. 1 B).
These results indicate that the PA/plasmin-dependent proteolytic status of the host is apparently not a critical determinant in the transformation of endothelial cells by PymT
and in the formation of vascular tumors in newborn mice.
Derivation of End. Cell Lines Lacking uPA, tPA, utPA, and PAI-1
We have previously established transformed End. cell
lines from tumor-bearing wild-type and various Src-kinase
mutant mice, as well as by infection of primary endothelial
cells by PymT in vitro (for review see Wagner and Risau,
1994; Pepper et al., 1997
). To test whether the absence of
components of the PA/plasmin system affects the capacity
to establish permanent End. cell lines in vitro, we derived
a number of independent cell lines from tumors induced in
tPA-, uPA-, utPA-, and PAI-1-deficient mice (Table I).
Each of these cell lines displayed the characteristic spindle-shaped, highly refractile morphology that is typical of
endothelial cells (data not shown). The time required to
establish the different mutant cell lines as homogeneously
growing, characteristic End. cells was not significantly different using single knockout mice. However, the double
mutant utPA
/
End. cells took three times as long to establish when compared with controls (data not shown). With
the exception of ut1 and ut2, all End. cell lines exhibited roughly equivalent growth parameters in vitro as measured by cell doubling times after ~10 passages (Fig. 2).
The proliferation rate of ut1 and ut2 cell lines was <50%
of controls. All End. cell lines obtained expressed the endothelial-specific tyrosine kinase receptor Flk-1 (Fig. 3 A)
and CD31/PECAM-1 (data not shown), confirming their
endothelial origin. Most of these cell lines were clonal,
since only one integration site for the provirus was detected by Southern blotting (data not shown).
To verify the genotype and measure the proteolytic
profile of mutant End. cells, RNA expression and zymographic analysis were performed. As shown in Fig. 3 A,
both uPA-deficient cell lines (u23 and u24) expressed tPA
at significant levels but no detectable uPA. The tPA-deficient End. cells t37 and t38 had high levels of uPA and no
detectable tPA, whereas PAI-1 deficient cells expressed either uPA (p3) or uPA and tPA (p1) at high levels, but no
PAI-1. As expected, the two End. cell lines lacking both
PAs only expressed PAI-1 (Fig. 3 A). Zymographic and
reverse zymographic analysis (Fig. 3 B and Table II) confirmed the genotype of the cells. Thus, uPA, tPA, and
PAI-1 activity was undetectable in cell lines derived from
the corresponding knockout mice. Although we previously
indicated that PAI-1 activity was undetectable by reverse
zymography in two wild-type End. cell lines, namely b1
and e2 (Montesano et al., 1990), in the four additional
wild-type cell lines assessed in this study, PAI-1 activity
was detected (data not shown). Similarly, PAI-1 activity
was detected in virtually all uPA
/
, tPA
/
, and utPA
/
End. cell lines (Table II).
Table II. Proteolytic Activity and Morphogenetic Behavior of End. Cells Lacking uPA, tPA, utPA, and PAI-1 |
Morphogenetic Behavior of Mutant End. Cells in Fibrin Gels
We have previously reported that End. cells form large
cystlike structures lined by a monolayer of endothelial
cells when embedded in three-dimensional fibrin gels in
vitro (Montesano et al., 1990). In the present study we
have observed that although all six wild-type cell lines
have the capacity to form cysts in fibrin gels, there is heterogeneity with respect to the size of the cysts and the frequency with which they form. In addition to cysts, the
presence of narrow or ectatic tubes was observed in four
out of six lines (Fig. 4 and Table II). When grown in fibrin
gels in the presence of Trasylol, a broad-spectrum serine protease inhibitor, cyst formation was inhibited and tubelike structures were observed in most wild-type End. cell
lines (Fig. 4 and data not shown).
Both uPA/
cell lines (u23 and u24) retained their capacity to form medium to small cysts (Fig. 4 and Table II).
Cyst formation was completely inhibited in the presence
of Trasylol, demonstrating the requirement for serine protease activity (data not shown). However, in contrast with
wild-type cells, tube formation was never observed in the
presence of Trasylol in either uPA
/
cell line; instead
these cells either formed multicellular aggregates or remained as single isolated cells (data not shown). The two tPA
/
cell lines (t37 and t38) formed small cysts and occasional tube-like structures (Fig. 4 and Table II). Cyst
and tube formation could be prevented by addition of Trasylol to t37 but not to t38 cells (data not shown). In striking
contrast with most of the cell lines described thus far, both
utPA
/
cell lines (ut1 and ut2) completely lost their capacity to form cysts (Fig. 4 and Table II). In addition, two
clearly distinguishable cell populations could be identified:
cells either organized into a network of cell cords or remained isolated as single rounded cells (Fig. 4 and Table
II). Finally, PAI-1-deficient cells formed cysts, although in
later passage p1 cells, rapid and massive fibrinolysis precluded the formation of either cysts or tubes (Fig. 4 and
Table II). p3 cells appeared to be heterogeneous: in early
passage cells, two cell populations that either formed cysts
or cell cords could be identified. Cyst formation (in p1 and
p3 cells) and fibrinolysis (in late passage p1 cells) could be
inhibited by Trasylol. Cell cord formation by ut1, ut2, and
p3 cells was unaffected by addition of Trasylol (data not
shown). These findings show that, contrary to what might
have been expected, uPA
/
and PAI-1
/
cells do not
display a decreased or increased capacity to form cysts.
The observation that the capacity to form cysts was reduced to a greater extent in tPA
/
cells than in uPA
/
cells suggests that, although both enzymes are capable of
fibrinolysis in this system, tPA is more efficient.
Tumorigenic Potential of uPA-, tPA-, and utPA-deficient End. Cells in Adult Wild-Type Mice
To define a causal role for the PAs in End. cell-induced
vascular tumors, we have used cells lacking uPA, tPA, and
both uPA and tPA (utPA) for tumor formation studies in
adult wild-type mice. We have used immunologically competent adult mice in these studies to avoid variability that
may arise from the use of newborn mice with a developing
immune system. Tumors were visible as small outgrowths
in all mice injected with wild-type End. cells between day 4 and 7 and grew in size to 125 mm2 by 11-18 d (Fig. 5 A).
Histologically, these tumors consisted of a central hemorrhagic/necrotic core surrounded by an intense inflammatory cell infiltrate with neovascularization and very little
fibrin deposition (Fig. 6, A and B). Similar results were obtained in mice injected with tPA-deficient End. cells, and
there was no significant delay in either the onset or growth
rates of these tumors. In contrast, tumor induction with
uPA/
End. cells showed incomplete penetrance and a
delay in formation (Fig. 5 A). Only 50% of the mice developed tumors by days 11-14, and it took between 18 and 25 d
for these tumors to reach maximum size. The remaining
mice did not develop tumors for the entire 60-d observation period. Similarly, about two-thirds of the mice injected with End. cells deficient in both uPA and tPA developed tumors at an extremely slow rate (Fig. 5 A).
Tumors were visible between 19 and 24 d after injection,
and it took between 40 and 60 d to form tumors of maximum size. The remaining mice did not develop tumors.
Histologically, these tumors were similar to those induced by wild-type cells, with the notable exception that extensive fibrin deposition was observed with the Lendrum and
PTAH staining techniques (Fig. 6, E and F; data not
shown). These results indicate that End. cell lines lacking
only tPA activity are not inhibited in their capacity to form
tumors. However, cell lines lacking uPA activity (both
uPA
/
and utPA
/
) display a reduced efficiency in
tumor formation in wild-type mice; when tumors do arise,
their growth rate is significantly retarded, indicating that
uPA activity of tumor cells is critical for efficient tumor
growth.
To rule out the possibility that the observed differences
in tumor formation rates were due to various degrees of
histoincompatibility, we performed the tumor formation
studies in immunoincompetent nude mice. After injection
of wild-type and tPA/
End. cells, tumors were visible
around day 5 and grew in size to 125 mm2 by 10-11 d (Fig.
5 A). In contrast, tumors were only visible between 7 and
14 d after injection with uPA
/
End. cells, and these tumors grew in size to 125 mm2 by 18-22 d (Fig. 5 A). Similar
results were obtained when utPA
/
End. cells were
used, with tumors being visible between 10 and 14 d and
reaching maximum size by 22-26 d (Fig. 5 A). These results in immunoincompetent mice confirm that the lack of
tPA activity in End. cell lines does not affect their tumorigenic potential, whereas lack of uPA activity retards the
tumorigenic potential of both uPA
/
and utPA
/
End. cell lines.
Tumor Induction by Wild-Type and Mutant End. Cells in Adult uPA-, tPA-, and utPA-deficient Mice
We next investigated if adult mice lacking uPA, tPA, or
both utPA are susceptible to tumor induction by wild-type
End. cells. Cells were inoculated into the various mutant
mice and tumor growth rates were scored (Fig. 5 B). No
significant delay was observed in the onset of tumor development in tPA-deficient mice when compared with wildtype controls, although the time taken to reach maximumsized tumors was slightly delayed (between 14 and 20 d in
tPA-deficient mice as compared with 11-18 d in wild-type
mice). However, tumor growth was delayed and variation
in tumor formation rates was observed in uPA-deficient
mice as follows: (a) only 3 out of 11 uPA-deficient mice
developed tumors 12 to 14 d after injection, which grew to
maximum size by day 27; (b) tumors were also visible in
two other uPA-deficient mice between 12 and 14 d, but
these tumors subsequently regressed; (c) the remaining six
mice did not develop tumors throughout the entire 60-d
observation period. Of the utPA-deficient mice injected
with wild-type cells, only two out of four had visible tumors 18-20 d after injection, which then progressed to 125mm2 tumors by days 40-50. The remaining two mice did
not develop tumors for the entire observation period.
Only four utPA/
mice could be used in this study since
most of these mutant mice, even uninjected, became ill as
they grew older (Carmeliet et al., 1994
). Histologically, tumors induced by wild-type cells in mutant mice (Fig. 6, C
and D) were similar to those induced in wild-type mice (Fig. 6, A and B), both at early (3 d; Fig. 6, C and D) and
late (27 d; Fig. 6 G) time points. However, tumors induced
by wild-type cells in utPA
/
mice were consistently associated with extensive fibrin deposition, as seen using the
Lendrum and PTAH staining techniques (Fig. 6 H and
data not shown). These results indicate that the host's PA
status, and in particular the presence of uPA activity, plays
a critical role in determining if proteolytically competent
tumor cells can proliferate in vivo.
Since lack of uPA activity in both the host and the injected tumor cells affected the efficient formation of tumors, we next investigated the potential of utPA/
End.
cells to form tumors in mice lacking either uPA or tPA.
Tumor formation by utPA
/
End. cells was inhibited in
tPA
/
mice, since only three of the seven tPA
/
mice
initially developed tumors between days 16 and 21; however, these tumors regressed by day 40 in all cases (Fig. 5 C). The remaining mice remained tumor free. utPA
/
End. cells did not form tumors in any of the uPA
/
mice
during the entire observation period (Fig. 5 C). These results suggest that, although the lack of tPA alone has no
significant effect on tumor formation, the uPA status of
both the host and invading tumor cell has an important
role in determining the progression of End. cell tumors.
The specific formation of vascular tumors by PymT antigen provides an excellent system to study the molecular mechanisms underlying endothelial cell transformation. One of the components of this process is extracellular matrix degradation and invasion of surrounding tissues, which in the case of PymT results in a profound subversion of normal vasculature. In an attempt to analyze the role of the PA/plasmin system in PymT-mediated oncogenesis, we have investigated its ability to transform endothelial cells in mice lacking uPA, tPA, both uPA and tPA (utPA), or Plg. We found that the proteolytic status (PA/plasmin system) is not a critical determinant, and hence is not limiting for PymT-induced tumor formation. Moreover, the lack of either uPA or tPA activity does not affect the establishment of End. cells in vitro, although the combined loss of both PA activities leads to a reduced proliferation rate. Cyst formation by End. cells in fibrin gels in vitro could be correlated with altered proteolytic activity only in cells lacking uPA and tPA. However, End. cell-induced tumor formation in vivo, as opposed to tumors induced by PymT itself, is dependent on the proteolytic status (PA/plasmin system) of both the tumor cells and the host.
The finding that PymT was able to induce vascular tumors efficiently in the absence of uPA, tPA, utPA, and Plg indicates that the primary transformation process is independent of the PA/plasmin proteolytic status of the endothelial cells. This suggests that PymT-induced tumors can only be used as an in vivo assay for determining the role of this oncogene and associated signaling components in the initial transformation process. To date, the only signaling molecule thus identified is the Src-like kinase, Yes, since PymT could not efficiently transform endothelial cells in Yes-deficient mice in which the latency for tumor formation was also increased (Kiefer et al, 1994b).
Mutant End. cells lacking either uPA or tPA show no proliferation defect in culture. However, End. cell lines lacking both uPA and tPA require a longer time for establishment (three times longer compared with wild-type End. cells) and, when established, exhibit a marked reduction in cell proliferation in vitro. This proliferation defect is also exemplified in End. cell-induced tumor formation in vivo where cells lacking both PAs do not form tumors efficiently, and, when they do, the time taken to reach maximum size is about three times that seen with wild-type End. cells.
Assessment of the morphogenetic behavior of mutant
End. cells in fibrin gels in vitro revealed that, although
tPA appeared to be more efficient, the absence of either
uPA or tPA alone did not render End. cells incapable of
forming cysts, indicating that one PA is sufficient. However, combined loss of both uPA and tPA prevented cyst
formation, clearly indicating that PA activity is required.
The previous prediction that the "proteolytic balance," as
represented by the ratio of PA:PAI-1, would predict the
behavior of End. cells in fibrin gels (Montesano et al.,
1990) therefore appears to hold true only when both PAs
are absent. These findings indicate that the in vitro system
only partially mimics the in vivo situation, and that the behavior of End. cells in fibrin gels is a poor predictor of
End. cell tumorigenesis.
The incidence and rate of tumor formation in wild-type
mice using End. cells lacking tPA activity were not significantly affected. However, there was a marked decrease in
the number of mice that developed tumors when injected
with uPA/
and utPA
/
End. cells. When tumors did
form in a subset of these mice, the onset was delayed and
the time taken to develop to maximum size was increased. In some cases, mice developed tumors initially, but these
subsequently regressed completely. The data from tumor
formation studies with nude mice ruled out the possibility
that various degrees of histoincompatibility may have
arisen during the establishment or the maintenance of mouse strains and tumor cell lines, thus affecting the experimental outcome. While wild-type and tPA
/
cell
lines still formed tumors with 100% penetrance in nude
mice, both uPA
/
and utPA
/
End. cell lines showed
a delay in the onset of tumor formation as well as an increase in the time taken to form 125-mm2 tumors. Furthermore, we have previously generated numerous End. cell
lines lacking functional c-src, fyn, and yes genes in the same genetic background, C57Bl/6 × 129. These cell lines
have been used in similar tumorigenicity studies, and no
variations in the tumorigenic profile were found (Kiefer et
al., 1994). Thus, the observed delay in End. cell-induced
tumor formation and the consequent low incidence of tumor-bearing mice are not likely to result from genetic variations but could be from inefficient matrix degradation
and a reduction in migratory/invasive properties of the tumor cells, which in turn lead to inefficient tumor expansion. This observation that End. cell-induced tumors cannot be efficiently formed when endogenous uPA is lacking
but are independent of tPA may reflect the longstanding
notion that uPA is required in situations of cell migration,
morphogenesis, and tissue remodeling, while tPA is primarily implicated in intravascular thrombolysis.
The finding that mice lacking uPA activity (both uPA and utPA null mice) do not display efficient formation of wild-type End. cell-induced tumors was surprising. There was a marked decrease in the formation of tumors, and, when formed, there was a delay in the onset and latency. This indicates that the host's proteolytic status is also critical for the efficient formation of these tumors. Although the exact role of the host's uPA activity in tumor formation is presently unclear, it could be envisaged that the uPA proteolytic mechanism has to involve concerted interactions between both the invading tumor cells and the host's matrix for efficient fibrinolysis and subsequent matrix degradation. Thus, the lack of uPA activity in either would result in inefficient colonization by the tumor cells, as was shown here.
In conclusion, our studies have revealed that the pathogenesis of PymT-induced and End. cell-induced tumors is
inherently different. In the former, the PymT virus infects
endothelial cells in an existing vascular tree without the requirement for the PA/plasmin system in the initial transformation process (Fig. 7 A). Therefore, PymT-mediated
vascular tumor formation appears to be PA/plasmin independent. On the other hand, in End. cell-induced tumors,
the injected cells first have to invade host tissues and the
basement membrane of existing blood vessels to gain access to the circulation to integrate within host vessels. This
invasive process, which is reminiscent of tumor cell intravasation into the vascular tree, is dependent on both the
host's and invading tumor cell's PA/plasmin status (Fig.
7 B). In addition, proliferation is impaired in cells entirely
lacking PA activity. Defects in both invasion and proliferation, both of which appear to be PA dependent, are therefore likely responsible for the reduction in End. cell-induced
tumor growth. Whether other proteases such as the MMPs,
which have been shown to be involved in vascular tumor
formation in vivo (Taraboletti et al., 1995), may play a role
in this process has to be investigated in future experiments.
Received for publication 26 July 1996 and in revised form 28 February 1997.
1. Abbreviations used in this paper: End., endothelioma; MMP, matrix metalloproteinase; PA, plasminogen activator; uPA, urokinase-type PA; uPAR, uPA receptor; tPA, tissue-type PA; PAI, PA inhibitor; Plg, plasminogen; PTAH, phosphotungstic acid-hemotoxylin; PymT, Polyoma middle T antigen.We thank Dr. V. Ploplis for the Plg/
mice used in this study. We also
thank Drs. H. Beug and M. Sibilia for critical reading of the manuscript,
Drs. J.-D. Vassalli and S. Fisher for helpful comments, and Dr. L. Orci for
continuing support. Technical assistance was provided by C. Di Sanza, M. Quayzin, and J. Stadler, and photographic work was done by B. Favri, G. Negro, and H. Tkadletz. This work was partly supported by the Austrian
Industrial Research Promotion Fund and by grants from the Swiss National Science Foundation (31-34097.92 and 31-43364.95).