This document reviews molecular alterations that contribute to metastasis in osteosarcoma and potential targets for therapy. It discusses the key steps in osteosarcoma metastasis, including migration/invasion, survival in circulation, immune evasion, arrest/extravasation, adherence, dormancy, and angiogenesis/proliferation. For each step, molecular mechanisms are described, such as the roles of MMPs, Notch, CXCR4, Ezrin, and VEGF. Preclinical and clinical studies targeting these molecules/pathways with drugs like dasatinib, MTP-PE, and IGF-1R inhibitors are summarized. The goal is to design treatments targeting the biology of metastatic osteosarcoma cells.

1. Clin Exp Metastasis (2011) 28:493–503
DOI 10.1007/s10585-011-9384-x
REVIEW
Molecular alterations as target for therapy in metastatic
osteosarcoma: a review of literature
J. PosthumaDeBoer • M. A. Witlox •
G. J. L. Kaspers • B. J. van Royen
Received: 15 July 2010 / Accepted: 18 March 2011 / Published online: 2 April 2011
Ó The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Treating metastatic osteosarcoma (OS) remains Keywords Drug resistance Á Metastasis Á
a challenge in oncology. Current treatment strategies target Osteosarcoma Á Therapy
the primary tumour rather than metastases and have a
limited efficacy in the treatment of metastatic disease. Abbreviations
Metastatic cells have specific features that render them less Bcl-2 B cell lymphoma 2 associated oncogene
sensitive to therapy and targeting these features might Bcl-XL Bcl2-like 1
enhance the efficacy of current treatment. A detailed study CXCR4 Chemokine (C-X-C-motif) receptor 4
of the biological characteristics and behaviour of metastatic CXCL Chemokine (C-X-C-motif) ligand
OS cells may provide a rational basis for innovative ECM Extracellular matrix
treatment strategies. The aim of this review is to give an EGFR Epidermal growth factor receptor
overview of the biological changes in metastatic OS cells ERK Extracellular signal regulated kinase
and the preclinical and clinical efforts targeting the dif- FAK Focal adhesion kinase
ferent steps in OS metastases and how these contribute to GH Growth hormone
designing a metastasis directed treatment for OS. HLA Human leukocyte antigen
IGF-1R Insulin-like growth factor 1 receptor
IFN-a Interferon-alpha
IL Interleukin
JAK Janus kinase
mAB Monoclonal antibody
MAP(K) Mitogen-activated protein (kinase)
MMP Matrix metalloproteinase
J. PosthumaDeBoer Á B. J. van Royen (&) MTP-PE Muramyl tripeptide phosphatidyl ethanolamine
Department of Orthopaedic Surgery, VU University Medical mTOR Mammalian target of rapamycin
Center, De Boelelaan 1117, 1081 HV Amsterdam, The NF-jB Nuclear factor-kappa B
Netherlands
NK cells Natural killer cells
e-mail: bj.vanroyen@vumc.nl
PI3K Phosphatidylinositol 3-kinase
M. A. Witlox PDGF-R Platelet-derived growht factor receptor
Department of Orthopaedic Surgery, Westfries Gasthuis, OS Osteosarcoma
Hoorn, The Netherlands
SARC Sarcoma Alliance for Research through
G. J. L. Kaspers Collaboration
Paediatric Oncology/Haematology, VU University Medical STAT Signal transducer and activator of
Center, Amsterdam, The Netherlands transcription
TGF-b Transforming growth factor-beta
B. J. van Royen
VU University Medical Center, PO Box 7057, VEGF Vascular endothelial growth factor
1007 MB Amsterdam, The Netherlands WIF-1 Wnt inhibitory factor 1
123

2. 494 Clin Exp Metastasis (2011) 28:493–503
Introduction of these specific steps with special attention to the mole-
cules involved in OS metastasis (Table 1) and implications
Osteosarcoma (OS) is the most common primary malignant for therapy. Over the last decade, much research has been
bone tumour in children and adolescents. The estimated performed to try to unravel the biology of OS metastasis
incidence rate worldwide is 4/million/year, with a peak and many (pre)clinical studies have attempted to discover
incidence at the age of 15–19 years [1]. In OS there is a new treatment options for metastatic OS. For example,
high tendency to metastatic spread. Approximately 20% of gene expression profiling of metastatic cells using a cDNA
patients present with lung metastases at initial diagnosis microarray approach has identified genes responsible for
and, additionally, in 40% of patients metastases occur at a metastasis [27, 37–39]. Also, expression levels of specific
later stage. Eighty percent of all metastases arise in the proteins in OS lung metastases have been analysed. In
lungs, most commonly in the periphery of the lungs, and these studies, expression levels of proteins involved in
exhibit resistance to conventional chemotherapy [2–7]. The metastases link the molecular aberrancies to clinical out-
5-year survival rate for OS patients with metastases is 20% come in terms of survival rates. These alterations may also
compared to 65% for patients with localised disease and provide novel drug targets [21, 23, 25, 36, 40–42]. Table 2
most deaths associated with OS are the result of metastatic summarises (pre)clinical studies for treating OS
disease [5, 8–11]. metastases.
For patients with pulmonary metastasis, especially those The aim of this review is to give an overview of the
who have metastasis at initial diagnosis, the combination of biology of metastatic OS cells and of (pre)clinical efforts
radical metastasectomy and chemotherapy offers the best targeting the different steps in OS metastases and how
outcome and even potential cure. Nevertheless, recurrent these may contribute to designing a metastasis directed
development of pulmonary metastases after initial radical treatment for OS.
metastasectomy is reported to be high and repeated
metastasectomies are sometimes performed. As metasta-
sectomy does yield an improved survival in most patients it OS metastasis
should therefore always be performed when feasible [2, 4,
12–14]. (I) Migration and invasion
In order to improve survival, the ultimate questions to be
answered are: Why does OS metastasize, particularly to the Migration of cells away from the primary tumour and
lungs? And, more importantly: Why does therapy fail in invasion through the extracellular matrix (ECM) towards
metastatic disease? In this regard, we hypothesise that drug the bloodstream is considered the first step contributing to
resistance is a key issue in the failure to control metastatic metastasis. In OS, it has been described that MetalloPro-
disease. It has been shown that OS lung metastases display teinases (MMPs) 2 and 9 and m-Calpain play a role in
a biological behaviour different from the primary tumours degradation of the ECM [10, 14, 16, 18, 19, 27, 34, 35,
[2, 14–27]. Metastases are comprised of cell clones that 43–46]. Also, the Wnt/b-catenin pathway and Src-kinase
differ from primary tumours with respect to ploidy, enzyme are implicated as inducers of migration [9, 35, 46, 47]. In
profile, karyotype and chemosensitivity [2, 28–32]. Ther- OS, Notch is a relatively recently identified pathway and
apeutic regimens that target primary tumours are therefore has been identified as a promoter of invasion in OS. In
unlikely to be successful in the treatment of metastatic highly metastatic OS cell lines there is an upregulation of
disease. the Notch1 and Notch2 receptor, as well as the Notch
Metastasis is considered to be the final though most induced gene Hes1. In patient samples, expression of the
critical step in tumorigenesis of malignant tumours [33]. Hes1 gene inversely correlated with survival [41, 48–50].
The metastatic cancer cells subsequently complete the
following steps: Invasion through the extracellular host (Pre)clinical studies: Notch
matrix and entrance into the circulation (I), survival in the
circulation (II) and evasion of the host immune system In a preclinical setting, downregulation of the Notch sig-
(III), arrest and extravasation at a target organ site (IV), nalling pathway has been shown to impair the invasiveness
adherence and survival in the target organ microenviron- of cell lines, but has no effect on cell proliferation or in
ment (V, VI) and finally formation of neovasculature to vitro tumorigenesis. Notch-inhibited cell lines had less
allow growth at the target organ site (VII) [14, 33–36]. potential to form lung metastases in an orthotopic mouse
Each step is of equal importance and must be fully com- model when compared to untreated cell lines. The exact
pleted by the tumour cell to achieve successful metastasis. mechanism through which inhibition of the Notch pathway
The altered biological behaviour in metastatic cells is the and its target gene Hes1 leads to reduced invasion still
result of specific molecular changes. We will discuss each remains unknown [41, 50].
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3. Clin Exp Metastasis (2011) 28:493–503 495
Table 1 Steps of metastasis in
Steps of metastasis Molecular involvement References
OS and molecular alterations
that contribute to each process I Migration and invasion MMPs [10, 14, 16, 18, 19, 27, 34, 35, 44–
46]
m-Calpain [35, 43]
Wnt [9, 35, 46, 47]
Src [35, 44]
Notch [41, 48–50]
II (a) Anoikis resistance PI3K/Akt [9, 14, 16, 51]
Src/PI3k/Akt [9, 14, 16, 44]
Src/Ras/MAPK [18, 35]
NF-jB [27]
Wnt/b-catenin [14]
BcL family [9, 35, 51]
(b) Apoptosis resistance Src [9, 16, 18, 35, 44]
NF-jB [27, 44, 51, 67]
Wnt/b-catenin [14, 17, 19, 46, 47, 53, 54]
Fas/FasL [5, 23, 28, 36, 55, 56]
III Evasion of immune system HLA-1 [14, 60]
IL-10 [14]
Fas [62]
IV Arrest and extravasation CXCR4-CXCL12 [9, 10, 14, 15, 22, 42, 69]
CXCR3-CXCL9-11 [10]
CXCR4/MMPs [9, 10, 15]
CXCR3-4/Erk/NF-jB [10, 67]
V Adherence Ezrin/MAPK/Akt [14, 21, 25, 35]
Ezrin/b4-Integrin/PI3K [70]
CD44/Akt/mTOR [14, 19, 21]
VI Dormancy Integrin-a5b1 [73, 74]
Integrin-a5b1/Erk/p38 [14, 35]
Bcl-XL [14, 35]
IGF/PI3K [75]
ECM [73, 74]
VII Angiogenesis and EGFR. PDGFR, VEGF, IGFR, [9, 14, 35, 40, 68, 77, 78, 81]
proliferation TGF-b
MMPs [9, 44, 78]
VEGF/Erk/NF-jB [35, 68]
VEGF/PI3K [35, 40]
EGFR/Src/Ras/MAPK/STAT3 [9, 18]
Src [14, 35, 44]
Integrin/PI3K/Erk1-2 [9, 35, 67, 75]
Wnt/b-catenin/CyclinD-Survivin [46, 52]
(IIa) Survival in the bloodstream cells. Under adherent conditions, survival is often mediated
through Integrin signalling pathways in which Focal
Dissemination of cancer cells through the body requires the adhesion kinase (FAK) is a central player. FAK activates
cells to survive in the circulation. Non-malignant cells, as the important PI3K/Akt survival pathway. Metastatic
well as non-metastatic tumour cells, become apoptotic after tumour cells evade anoikis by intrinsically activated sur-
loss of cell–cell adhesions or interaction with the ECM in vival pathways via for example PI3K or Akt signalling,
their original tissue. This specific mode of apoptosis is independent of extrinsic Integrin and FAK signalling
called anoikis. One reason for this type of apoptosis to [9, 14, 16, 51]. Src kinase can also activate the PI3K/Akt
occur is that Integrin signalling ceases to exist in solitary and the Ras/MAPK survival pathways independent of FAK
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4. 496 Clin Exp Metastasis (2011) 28:493–503
Table 2 Preclinical and
Steps of metastasis Target Drug References
clinical studies targeting
specific molecules in OS Preclinical
metastasis
I Migration and Invasion Notch [41, 50]
II (a) Anoikis resistance
(b) Apoptosis resistance Preclinical
Wnt [46, 52]
Src [18]
Clincial
Src
Dasatinib [60]
www.clinicaltrials.gov/NCT00752206
III Evasion of Preclinical
immune system Fas
IL12 [36, 56, 62]
IL18 [63]
Gemcitabine [55]
Clinical
Fas
Liposomal MTP-PE [64]
IFN-a [61, 66]
IV Arrest and extravasation Preclinical
CXCR4 [22]
CXCR3 [10]
V Adherence Preclinical
Ezrin [21, 25, 45, 70]
VI Dormancy
VII Proliferation and Preclinical
angiogenesis Endostatin [83]
IGF-1R [79, 84]
Clinical
IGF-1R
OncoLar [78]
R1507 www.clinicaltrials.gov/NCT00642941
SCH717454 www.clinicaltrials.gov/NCT00617890
signalling and thus stimulate ainoikis resistance [9, 14, 16, resistance throughout metastasis include activation of the
18, 35, 44, 51]. Other survival mechanisms are also of Src and NF-jB pathways and the overexpression of anti-
importance in the evasion of ainoikis, such as activation the apoptotic genes [9, 44, 46, 51, 52]. Wnt-signalling is also
nuclear factor-kappa B (NF-jB) pathway [27, 51] and involved in resistance to apoptosis throughout other steps
Wnt-mediated upregulation of the b-catenin activity. High of metastasis and is considered to be important for tumour
levels of b-catenin expression have been shown to be progression in general. Upon binding of Wnt to one of its
associated with a metastatic phenotype in OS [14, 19, 20, receptors, b-catenin degradation in the cytoplasm is pre-
46]. Finally, overexpression of anti-apoptotic genes such as vented. After b-catenin is stabilised it translocates to the
Bcl-2, Bcl-XL or FAK is exploited by solitary metastatic nucleus where it co-regulates oncogene transcription and
cells to obtain a survival benefit [9, 35, 51]. cell cycle progression and hence promotes survival and
proliferation [14, 17, 46, 47, 53, 54].
(IIb) Apoptosis resistance In established metastases, the tumour cells are con-
fronted with receptor-mediated cell death. Binding of Fas
The survival of tumour cells through all stages of metas- on the surface of metastatic cells to its Fas-ligand (FasL)
tasis (not only in the bloodstream) is paramount to suc- expressed constitutively on lung tissue, activates the Fas-
cessful metastasis. Mechanisms involved in apoptosis apoptosis pathway and leads to cell death. Cross-linkage of
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5. Clin Exp Metastasis (2011) 28:493–503 497
Fas with FasL on one cell results in apoptosis as well [36, inhibits Src phosphorylation in primary tumours; however,
55]. Resistance to death-receptor-induced apoptosis is it did not impair the development of pulmonary metastases.
commonly seen and is highly important for the successful Histopathological analysis of both OS primary tumours and
maintenance of metastases. The Fas/Fas-ligand pathway is lung nodules showed minimal Src-kinase phosphorylation
a death receptor pathway that is often down-regulated in after treatment with Dasatinib. However, Src-kinase
metastatic cell populations, rather than in primary tumours phosphorylation was low in untreated lung metastasis as
[23, 28, 36]. The Fas pathway is also of influence on well. This suggests that Dasatinib was effective in inhib-
chemotherapy-induced apoptosis and thus on its therapeu- iting Src-pathway activation in OS cells, but it is not clear
tic efficacy [56]. Much (pre) clinical research has been what the phosphorylation status is during the stages of OS
performed concerning this pathway in metastatic OS and metastasis and how this influences the process [18]. The
promising results have been obtained. These results are use of Dasatinib in patients with advanced (osteo)sarcomas
discussed following the section on immune evasion, since was examined recently by the SARC (Sarcoma Alliance for
the Fas pathway plays a role in that as well. Research through Collaboration) in a phase II clinical trial.
Thus, apoptosis resistance is very much exploited by the Disappointingly, preliminary results show no treatment
metastatic cell and this feature is likely to contribute to effect of Dasatinib as a single agent in patients with overt
resistance to therapy in metastatic OS [51, 52]. The failure lung metastases [60]. The same group is looking into the
to induce apoptosis upon treatment is thought to be the effectiveness of Src inhibition with a more specific Src-
result of a misbalance between pro- and anti-apoptotic kinase inhibitor (Saracatinib) to obtain progression free
signalling. Restoration of this balance, thereby creating an survival among patients with resectable OS lung metasta-
environment in favour of pro-apoptotic signalling could ses (clinicaltrials.gov/NCT00752206).
theoretically enhance treatment with cytotoxic agents [9,
35, 45, 51, 56–58]. (III) Evasion of the immune system
(Pre)clinical studies: Wnt and Src Another important precondition for the survival of meta-
static cells is the evasion of the host immune surveillance
The Wnt-pathway is a putative therapeutic target because a throughout all the steps of metastasis. Tumour cells, either
majority of OS samples show aberrant activation of this circulating or at the site of metastases, can modulate the
pathway, leading to the transcription of oncogenes and cell immune system of the host in order to achieve a survival
cycle progression. This in turn leads to proliferation and advantage. Down-regulation of cell surface receptor HLA
enhanced survival [46, 53, 59]. When targeting the Wnt- class 1 is one of such mechanisms. This impairs the recog-
pathway, activating mutations in downstream molecules for nition of tumour cells by the host cytotoxic T-lymphocytes.
example b-catenin can be of negative influence as it may Tumour cells can also induce the production of immuno-
bypass Wnt inhibition and preserve the invasive phenotype suppressive molecules such as IL-10 [14, 61]. Modulation of
of the metastatic cell [19]. A preclinical study by Leow et al. the immune system such that it recognizes and destroys
[52] has shown that inhibition of the Wnt/b-catenin path- (circulating) tumour cells would be a successful anti-meta-
way resulted in lower levels of nuclear b-catenin, resulting static treatment. Interferons are cytokines that can affect the
in a decreased expression of b-catenin target genes. This led recognition of tumour cells by the immune system by
to an inhibition of migratory potential through downregu- influencing the (re)expression of HLA molecules on the cell
lation of MMP-9, and a decrease in expression of Cyclin-D, surface. Interferons also exert an anti-proliferative effect on
c-myc and Survivin. The latter was responsible for an anti- OS cells through pathways that are yet unknown [61]. The
proliferative effect and an increase in cell death. These balance between the intrinsic downregulation of HLA
results were recently confirmed by Rubin et al. [46], who molecules of the tumour cells and the effect of Interferon
showed that re-expression of Wnt inhibitory factor 1 (WIF-1), stimulation will eventually determine whether the circulat-
a secreted Wnt-antagonist, inhibited Wnt signalling and ing tumour cell is cleared by the immune system or not.
reduced tumour growth and metastasis in OS mouse mod- Fas also plays a role in immune evasion. Fas expression
els. These results show a possible therapeutic benefit of leads to recognition by, and activation of cytotoxic natural
Wnt-pathway disruption in the treatment of metastatic OS. killer (NK) cells and promotes elimination from the cir-
Src-kinase is a kinase that is involved in almost all steps culation by the host immune system. Successful down-
of cancer metastasis, namely in proliferation, adhesion, regulation of the Fas molecule on the cell surface or cor-
migration, survival and angiogenesis [44]. Based on its ruption of downstream elements in the Fas pathway pro-
multi-step involvement in metastasis, it could be an inter- vides metastatic tumour cells with a survival advantage in
esting therapeutic target in OS metastasis. Pre-clinical the circulation and leads to an increase in metastatic
work shows that Src inhibition with Dasatinib effectively potential. Patient samples from pulmonary OS metastases
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6. 498 Clin Exp Metastasis (2011) 28:493–503
have been shown to be Fas-negative [40, 62]. Indeed, (Pre)clinical studies: Immune modulation
absence of Fas expression correlates with disease pro-
gression and poor survival outcome [23, 36, 55, 62]. Modulation of the immune system to exert anti-tumour
activity by the addition of interferon-a (IFN-a) as a
(Pre)clinical studies: Fas maintenance treatment after standard chemotherapeutic
treatment is currently under investigation in the EURA-
As the Fas receptor pathway is so important in the survival MOS-1 trial, which is an initiative of the European and
of metastatic cells, it is an attractive therapeutic target. American Osteosarcoma Study Group [66]. IFN-a is
Restoration of the Fas death pathway has been tried with immunomodulatory and able to stimulate a host-anti-
success in preclinical models. Interleukin-12 (cytokine) tumour immune reaction and induce anti-proliferative
therapy can achieve a dose-dependent upregulation of Fas signalling via the JAK/STAT1 pathway [58]. Accrual of
on the surface of OS cells as well as a stimulation of patients for this worldwide trial is due to be completed in
cytotoxic T-cells and NK-cells. This renders the metastatic July 2011 [61, 66].
cells more sensitive to Fas-induced cell death in the
microenvironment of the lung and enhances clearage of the (IV) Arrest and extravasation
cells from the circulation by the host immune system [36].
A drawback is the potent immunostimulatory effect of The mechanism of arrest of metastatic tumour cells at the
Interleukin-12 that can induce severe adverse effects after distant organ sites remains controversial. One hypothesis is
systemic administration in patients [56, 62]. that metastatic cells are larger than ordinary cells in the
In an in vivo experiment, intranasal administration of circulation and that they become trapped in the microcir-
IL-12 resulted in Fas overexpression on OS lung metasta- culation of a capillary bed. When trapped they form micro-
ses, leading to a decrease in tumour burden. Combination embolisms and start interaction with the local environment.
therapy with Ifosfamide, which induces the expression of It is striking, however, that different tumour types have an
FasL on the tumours, could further augment anti-tumour organ specific preference for metastasis. The metastatic
effect [28, 56]. behaviour of OS is very distinct as over 80% of all
IL-18 was reported to have similar effects on the acti- metastases arise in the lungs and other organs usually
vation of T-cells and NK-cells, as well as induction of the remain unaffected. This suggests that the circulating
expression of FasL on already Fas expressing tumours. tumour cell specifically ‘homes’ to distinct molecules that
This compound did not, however, exert an anti-tumour are expressed on the endothelium of the organ of prefer-
effect in mice bearing OS lung metastases [63]. ence. Although it might be trapped in different capillary
Gemcitabine is an agent that upregulates Fas-expression beds throughout the body, it will interact with the surface
when administered as an aerosol therapy in mice bearing molecules on the endothelium of the organ of interest
OS lung metastases. Gemcitabine aerosol therapy has been rather than with endothelium at other sites [35]. There is
shown to effectively reduce size and number of pulmonary evidence of endothelium-specific tropism in OS [10, 14,
metastases [5, 55]. 15, 22, 42, 67]. The processes of exit of the circulation and
Liposomal MTP-PE (muramyl tripeptide phosphatidyl invasion at the distant organ site are mediated by chemo-
ethanolamine) is a promising agent for clinical use as it can kines and proteinases. Proteinases are responsible for
induce endogenous IL-12 production and thus provide an extravasation whereas chemokines determine the site at
up-regulation of Fas on OS cells but without the systemic which circulating tumour cells adhere [9, 15]. Chemokines
toxicity encountered when exogenous IL-12 is adminis- were initially thought to regulate leukocyte trafficking and
tered to patients [28]. MTP-PE is a synthetic analogue of a homing, but recently they are also known as important
component of bacterial cell walls. As an immunomodula- components in the regulation of site-specific metastasis as
tory agent it can also stimulate monocytes and macro- they bind to G-protein coupled receptors on the plasma
phages to exert anti-tumour activity. The Children’s membrane of specific cells, in the case of OS to receptors
Oncology Group performed a prospective randomised in the lung [14, 42, 67–69]. CXCR-4, a commonly
phase III clinical trial with this compound in patients with expressed chemokine in OS, is involved in site-specific
high-grade conventional OS with metastases at diagnosis. metastasis. Its sole ligand is CXCL12 which is expressed
Treatment with liposomal MTP-PE improved overall sur- abundantly in the lung. Binding of CXCR-4 to CXCL12
vival, irrespective of the chemotherapy regimen. These allows adherence and extravasation of OS cells in the lung
data are promising and suggest that there is a critical role [9, 10, 14, 15, 22, 42, 69]. Laverdiere et al. [42] found that
for the Fas death pathway in chemotherapy response which CXCR-4 expression levels in patient samples inversely
can be exploited in clinical practice to enhance the efficacy correlated to event-free and overall survival. There was a
of chemotherapy in OS [40, 64, 65]. positive correlation between CXCR-4 expression in
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7. Clin Exp Metastasis (2011) 28:493–503 499
primary tumours and the presence of metastases at initial adhesions. Ezrin is a membrane-cytoskeleton linker protein
diagnosis. Interestingly, expression levels of CXCR-4 were that plays an important role in cell–microenvironment
similar in primary tumours and lung metastases. This interaction. It is thought to facilitate anchorage of OS cells
suggests that CXCR-4 expression is not regulated during to lung tissue, as well as to enhance survival mechanisms in
metastasis, but is simply present. It could be of predictive the new environment through Integrin mediated activation
value for the formation of metastasis. of Akt and MAPK survival pathways [14, 21, 25, 35]. The
CXCR-3, another chemokine, is expressed by OS as well exact mechanism through which Ezrin mediates metastasis
as other malignancies. Its ligands are CXCL9, -10 and -11, is not entirely clear, however, recently Wan et al. [70]
all of which are expressed by lung, and this molecule is discovered that b4-Integrin is an important mediator.
thought to co-operate with CXCR-4. Apart from mediating b4-Integrin can bind Ezrin and Ezrin is required for the
adherence, the interactions of CXCR-3 and -4 with their maintenance of this protein. b4-Integrin can activate the
respective ligands also trigger pathways involved in other PI3K pathway and thus stimulate survival and proliferation
necessary events in metastasis, namely in invasion, survival in the newly arrived cells in the lung. b4-Integrin is found to
and proliferation in the secondary tissue [10, 22, 67]. be highly expressed in OS tumour samples from both pri-
For example, hypoxia upregulates CXCR-3 and -4 mary and metastatic lesions. Furthermore, it was shown that
expression, which in turn induce the expression of MMP-2 b4-Integrin knockdown inhibits the formation of OS lung
and -9 on the cell surface and modulate the microenviron- metastasis in vivo, and leads to prolonged survival.
ment into an inflammation-like condition, abundant with High expression of Ezrin correlated with a higher risk of
growth factors and stimulation of angiogenesis. Further- metastatic relapse and poor survival in OS patients [21, 25].
more, binding of CXCR-4 to CXCL12 can activate the Furthermore, it was found to be 3-fold overexpressed in lung
NF-jB survival pathway via ERK (Extracellular-signal- metastases in a murine model for OS lung metastases [38].
Regulating-Kinase)-signalling and stimulate proliferation CD44 is another surface molecule that can form a complex
through MAPK signalling. Thus, apart from facilitating with Ezrin and correlates with metastasis and poor progno-
seeding at the distant organs site, chemokines play a very sis. Apart from influence on the cytoskeleton and cell shape,
important role in the modulation of the microenvironment CD44 controls proliferation, growth arrest and survival via
into a place permissive for the tumour cells to proliferate [9, the Akt/mTOR pathway [14, 19, 21].
10, 15, 67].
(Pre)clinical studies: Ezrin
(Pre)clinical studies: Chemokines
Suppression of Ezrin with a full-length anti-Ezrin construct
CXCR-4 is the most important chemokine-player in OS. did not inhibit primary tumour growth in a mouse model of
Kim et al. [22] have demonstrated a reduction in metastatic OS, but it effectively inhibited the formation of metastases.
tumour burden in an orthotopic mouse model in which cells It was speculated that metastatic OS cells express phos-
were treated with a CXCR-4 inhibitor prior to injection of phorylated Ezrin only early after arrival in the lung, and
tumour cells into the mice. However, reduction of meta- this causes limited efficacy of suppression of Ezrin in
static tumour burden without pre-treatment could not been readily established metastases, since its essential function
shown consistently. The authors argue that the critical in metastasis, namely connecting with the target organ site
event, namely binding of CXCR-4 to CXCL12 with con- had already been fulfille [21]. Recently however, Ren et al.
secutive activation of signalling pathways, granting survival [25] suggested that Ezrin phosphorylation is not only
and proliferation, occurs too early in the establishment of present in the early stage of metastasis, but also late in
metastases for inhibitory therapy of CXCR-4 to be benefi- tumour progression, at the leading edge of large metastasic
cial for the patient with already existing metastasis. To what lesions. This finding was verified on sections of patient OS
extent CXCR-4 inhibition could be beneficial in a pre- metastases.
ventive setting requires additional studies. Pignochino et al. [45] reported that Sorafenib inhibited
CXCR-3 inhibition was tested in an animal model for invasion via reduction in MMP-2 production and inhibited
human OS lung metastases and showed a significant survival via downregulation of Ezrin-activated MAPK/Akt
decrease in the development and progression of pulmonary signalling. Furthermore, Sorafenib could also induce
lesions compared to the non-treated group [10]. apoptosis in OS cells through downregulation of members
of the anti-apoptotic Bcl-2 family. Wan et al. [70] showed
(V) Adherence that inhibition of Ezrin-related b4-Integrin can reduce
metastasis in a mouse model. Taken together, targeting
Establishment at a distant organ requires the metastatic cell Ezrin seems promising in the management of OS lung
to connect to its new environment and re-establish cell–cell metastases.
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(VI) Dormancy The cells that had switched to the proliferative phenotype
had elevated RNA levels of common cancer pathways such
Dormancy refers to a prolonged period of survival of single as PI3K- and IGF-pathways. They also found that Endocan
cells or small micrometastases. OS patients can progress was upregulated in rapidly proliferating cells, a protein that
with metastases after a disease free interval of many years is also expressed on tumour endothelial cells. This might
[71, 72]. This is likely explained by the presence of mi- indicate that endothelial changes support the switch of cells
crometastases in a dormant state, which at some point are from dormancy into the proliferative state.
triggered develop into gross metastases. Dormancy could have a role in therapy resistance in OS
Little is known concerning the biological processes reg- metastases, however, whether this applies to OS and to what
ulating dormancy in OS. The anti-apoptotic gene Bcl-XL is extent remains unknown. In general, dormancy can bring
thought to be involved in the survival of dormant cells, as about drug resistance because non-proliferating cells are not
well as a5b1-Integrin mediated activation of NF-jB. Fur- so susceptible to conventional treatment. Most treatment
thermore the dormant state is thought to be regulated by the modalities induce DNA damage which is usually more
ratio between the ERK and p38-MAPK proteins, also lethal to rapidly proliferating cells [14, 73, 76]. To intervene
steered by Integrin-a5b1 signalling [14, 35, 73, 74]. in this step of metastasis seems difficult. Angiogenesis
The mechanisms by which dormant tumour cells are at seems to be an important factor. Elucidation of mechanisms
one point triggered to start proliferating are yet unac- that steer the switch from dormant to proliferative state may
counted for, however, the microenvironment is thought to give some options. If it would be possible to keep the cells
play a regulatory role. Tumour outgrowth is dependent on locked in the dormant state, it may grant the patient stable
vascularisation, and it has been suggested that endothelial metastatic disease with prolonged survival.
cells in the microenvironment can both activate dormant
tumour cells through cell-to-cell signalling and induce (VII) Angiogenesis and proliferation
angiogenesis for nutrition [74]. The ECM is also thought to
be involved in activation of dormant cells, as it serves as a Tumour growth and progression is often restricted by
source of growth and survival signals. It has been postu- vascularisation and thus nutrition. Hypoxia leads to the
lated that micrometastases that fail to properly connect to upregulation of growth factor receptors, angiogenic cyto-
the ECM remain in the dormant state because they remain kines and proteolytic enzymes, among which EGFR,
deprived of growth- and angiogenic signalling and go into PDGF-R, VEGF, IGF-1R, TGF-b, IL-8 and MMPs, all of
quiescence as a means to survive. Anchorage to the ECM these providing neo-angiogenesis and allowing prolifera-
would stimulate cells to convert to a proliferative state via tion. These molecules can be overexpressed by the tumour
b1-Integrin signalling [73]. The microenvironment can be cell population itself, but can also be provided by host
regulated by the tumour cells themselves, but also by host endothelial (progenitor) cells during neo-angiogenesis [9,
stromal cells. Leucocytes and macrophages can modulate 14, 35, 40, 44, 68, 77]. Apart from induction of neo-
the ECM to either form a pro- or anti-angiogenic micro- angiogenesis, VEGF also provides the tumour cells with a
environment. Apart from this, other mediating factors can survival benefit via activation of ERK-1/2/NF-jB and
also be influenced by stromal cells. For example, Wnt can PI3K pathways [35].
be secreted from macrophages, and cytokines secreted by Players involved in provision of vasculature and nour-
stromal cells can upregulate the intracellular Wnt/b-catenin ishment are often encountered in other processes within OS
signalling pathway and hence induce survival and prolif- metastasis as well. For example, Src-kinase activity is
eration in a late stage in the process of metastasis [9, 35, regulated through various growth factor receptors, such as
54, 73]. Also, bone marrow derived progenitor cells (cre- EGFR and Integrin receptors. Src activation leads to Ras/
ating a ‘pre-metastatic niche’) can modulate the microen- MAPK signalling and activation of the transcription factor
vironment and thus influence whether solitary cells or STAT3, allowing cell cycle progression and production of
micrometastases remain dormant or are allowed to progress angiogenic factors such as fibroblast growth factor, VEGF
[68, 73]. In an effort to elucidate the cellular mechanisms and IL-8. Src phosphorylation by EGFR especially is
that establish the switch of dormant to rapidly growing considered to stimulate the onset of hyper-proliferation of
cells, Almog et al. [75] designed an in vivo model for tumour cells and induction of vascular permeability and
dormancy of various cancers, including OS, and performed neovascularisation [18, 44].
gene-expression analysis of cells in the dormant state Proliferation of OS cells at a distant organ site is often
versus cells in a proliferative state. They found that during mediated by Receptor-Tyrosine-Kinase or Integrin induced
dormancy, there is an upregulation of anti-angiogenic activation of PI3K and ERK1/2 pathways [9, 35, 67].
proteins. In this pre-angiogenic situation, the tumour cells Alterations in cell cycle regulation can also promote pro-
would lack the nutrition and oxygen needed to proliferate. liferation by facilitating progression through the cell cycle
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9. Clin Exp Metastasis (2011) 28:493–503 501
checkpoints and speeding up the cycle. For example, the The SARC-011 clinical trial is evaluating the treatment
Wnt/b-catenin pathway is of influence on both G1/S and effect of R1507, a mAB targeting the IGF-1R in patients
G2/M progression via activation of Cyclin-D by c-myc and with recurrent sarcomas, including OS (clinicaltrials.gov/
activation of Survivin respectively [46, 52]. NCT00642941). In another clinical trial the efficacy of
The Insulin-Like-Growth-Factor 1 (IGF-1) Receptor axis SCH717454, also targeting the IGF-1R, is evaluated in
is also implicated in the development of OS. It is striking relapsed OS patients. In this trial, both inoperable patients
that most OS arise during or shortly after puberty. The and patients in whom metastasectomy is feasible are
influence of GH and IGF-1 on bone growth steer the lon- included. The latter group will be treated pre- and post-
gitudinal growth during the adolescent growth spurt and metastasectomy and might, apart from tumour response
contribute to approximately 50% of bone cell proliferation. rate, give information about progression-free survival
As there is a peak incidence of OS during the adolescent (clinicaltrials.gov/NCT00617890).
growth spurt, it is conceivable that there could be GH/IGF-1
axis involvement in tumour development. IGF-1R signal-
ling can activate the PI3K/Akt/mTOR pathway and stimu- Conclusion
late survival and proliferation in tumour cells [40, 77–79].
In conclusion, this review summarises potential molecular
(Pre)clinical studies: Angiogenesis alterations that contribute to metastasis in OS and gives an
overview of (pre)clinical efforts to develop new therapeutic
As OS is a highly vascularised tumour, a rationale exists to targets for the treatment of metastatic OS. In spite of these
use this feature as a therapeutic target. High serum-VEGF efforts, OS metastasis is not yet well understood and there
levels correlate with metastatic relapse, tumour progres- has been little evolvement in the treatment of this disease
sion, poor response to chemotherapy and a decrease in over the last decade. We hypothesise that certain molecular
survival [40, 77, 80]. Endostatin is an endogenous angio- alterations seen in metastatic cells can also contribute to
genesis inhibitor, produced by tumours itself, involved in resistance to chemotherapy, and targeting these features
repression of neo-angiogenesis and is commonly expressed might enhance the efficacy of current treatments. Further
in human OS samples. It can also induce apoptosis in unravelling the biology of OS metastasis will hopefully
endothelial cells. Given its important role in angiogenesis, provide new insights to be used as a rational basis for
it was hypothesised that Endostatin could impair OS innovative metastasis directed treatments for OS.
tumour growth and metastasis [18, 77, 81, 82]. However, in
a murine model of OS lung metastasis, Endostatin failed to Acknowledgements We would like to acknowledge prof. E.S.
Kleinerman (M.D. Anderson Cancer Center, Houston, TX, USA) for
induce tumour shrinkage in the lungs, although, it did her input and guidance in the preparation of this manuscript. JP is
retard growth of lung nodules. Treatment with this drug supported by the Individualised Musculoskeletal Regeneration and
will not cure OS patients, but it may result in stable met- Reconstruction Network (Danish Research Council) Aarhus, Den-
astatic disease with prolonged survival [83]. mark and by VONK: VUmc Onderzoek naar Kinderkanker (Stichting
Research Fonds Kindergeneeskunde VUmc) Amsterdam, the
Netherlands.
(Pre)clinical studies: Proliferation
Open Access This article is distributed under the terms of the
Pharmacologic inhibition of the GH/IGF-1 axis and thus Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
IGF-1R pathways has been explored. However, whereas medium, provided the original author(s) and source are credited.
there is evidence that IGF-1R signalling is important to
primary OS growth, the extent to which IGF-1R (inhibi-
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