2. Invasion and Metastasis:
Introduction
Cancer seemingly can evolve from hyperplasia through a series of
increasingly disorganized and invasive-appearing tumors that can
then colonize distant organs in a nonrandom fashion. This spread of
cancer from the organ of origin (primary site) to distant tissues is
called metastasis.
Although our understanding about cell proliferation, cell death,
genomic instability, and signal transduction pathways has rapidly
progressed, detailed understanding about the molecular
mechanisms of metastasis has lagged considerably behind.
3. The Evolution and Pathogenesis of
Metastasis
Somatic Evolution of Cancer
Hyperplastic and dysplastic lesions need not always progress to
cancer, but when they do, it can take years if not decades for this to
occur. This protracted course to malignancy is consistent with
epidemiologic studies that show an age-dependent increase in the
incidence of cancer.
Cancer requires several genetic alterations during a course of
somatic evolution.
However, the mutation frequency of human cells is thought to be too
low to explain the high prevalence of the disease if so many
stochastic genetic alterations are needed.
4. The dynamics of tumor progression depend on mutation, selection,
and tissue organization.
Mutations can result in activation of oncogenes or loss of tumor
suppressor genes that increase fitness and cell autonomy.
To oppose the accumulation of cells with tumorigenic mutations,
tissue architecture often limits the spread of mutant cells that have
reached fixation. For example, large compartments containing many
cells accumulate advantageous mutations more rapidly than smaller
compartments.
Similarly, if there are only a limited number of precursor cells that
have self-renewal capabilities (stem cells), this also has the effect of
reducing the risk of enriching for tumorigenic mutations.
However, despite the sequential mutations and steps predicted by
the somatic evolution of cancer, the nature and/or sequence of
genes that are altered during this evolution are mostly unknown.
5. Clinical, Pathologic, and Anatomic Correlations
Metastasis is often associated with several clinical and pathologic
characteristics. Among these, tumor size and regional lymph node
involvement are consistently associated with distant relapse.
For tumor size, no clear threshold exists but trends are clear. For
example, metastatic risk for breast cancer rises sharply after 2
cm, while distant metastasis in sarcoma is more common for tumor
sizes larger than 5 cm.
The involvement of regional lymph nodes is often, but not always, a
harbinger for increased risk of distant metastasis. For head and neck
cancer, the association between lymph node involvement and
metastasis is predictable.
Metastasis rarely occurs without prior involvement of cervical neck
lymph nodes, and the lower down in the neck nodal involvement
occurs, the more likely distant metastasis becomes.
For breast cancer, the presence of positive lymph nodes is the
strongest clinicopathologic prognostic marker for distant relapse.
However, lymph node metastasis does not always precede distant
relapse. In sarcomas, for example, metastasis is often seen in the
absence of nodal disease.
6. Clinical, Pathologic, and Anatomic Correlations
Although many histopathologic traits for different cancer types have been
reported to associate with poor prognosis, there are several that consistently
appear to track with metastatic risk across various tumor types. These traits
include:
(1) Tumor grade. Tumors that are poorly differentiated, or retain few
features of their normal tissue counterparts, are generally considered to be
high grade. High-grade tumors often exhibit infiltrative rather than pushing
borders and show signs of rapid cell division. Breast cancer and sarcomas are
well recognized for displaying a markedly elevated risk of metastasis with higher
tumor grade.
(2) Depth of invasion beyond normal tissue compartmental boundaries.
Some cancers like melanoma and gastrointestinal malignancies are staged by
how deeply they extend beyond the basement membrane. Violation of deeper
layers of the dermis, or invasion through the lamina propria, muscularis mucosa,
and serosa, represent progressively more extensive invasion and higher risk of
metastasis.
(3) Lymphovascular invasion. Tumor emboli seen in the blood or lymphatic
vessels generally carry a poorer prognosis than cancer without these features.
Breast cancer and squamous cell cancers of the head and neck or female cervix
are examples.
7. Tissue Tropism and the Seed and Soil Hypothesis
In 1889, Stephen Paget proposed his “seed and soil” hypothesis .
This stated that the propensity of different cancers to form
metastases in specific organs was due to the dependence of the
seed (the cancer) on the soil (the distant organ).
In contrast, James Ewing and others argued that tissue tropism
could be accounted for based on mechanical factors and circulatory
patterns of the primary tumor.
For example, colorectal cancer can enter the hepatic-portal system,
explaining its propensity for liver metastasis, and prostate cancer
can traverse a presacral plexus that connects the periprostatic and
vertebral veins, explaining its propensity for metastases to the lower
spine and pelvis.
Supporting the arguments for both views, current understanding
would suggest that both seed and soil factors and anatomic
(“plumbing”) considerations contribute to metastatic tropism.
8. Stereotypic Patterns of Metastasis to Distant
Organs By Cancer Type
Breast
Colon
Kidney
Lung
Ovary
Prostate
Stomach
Testis
UrinaryBladder
Uterine Lining
Axillary lymph nodes, other breast, lung, pleura,
liver, bone brain, spleen, adrenals, ovary
Regional lymph nodes, liver, lung, bladder,
stomach
Lung, liver, bone
Regional lymph nodes, pleura, diaphagm, liver,
bone, brain, kidney, adrenal, throid, spleen
Peritoneum, regional lymph nodes, lung, liver
Bones of spine and pelvis, regional lymph nodes
Regional lymph nodes, liver, lung, bone
Regional lymph nodes, lung. Liver
Rectum, colon, prostate, ureter, vagina, bone,
regional lymph nodes, lung, peritoneum, pleura,
liver, brain
Regional lymph nodes, lung, liver, ovary
10. Basic Steps in the Metastatic Cascade
From clinical, anatomic, and pathologic observations of metastasis,
a picture of the steps involved in a metastatic cascade emerges.
Invasion and motility. Normal tissue requires proper adhesions
with basement membrane and/or neighboring cells to signal to each
other that proper tissue compartment size and homeostasis is being
maintained. Tumor cells display diminished cellular adhesion,
allowing them to become motile, a fundamental property of
metastatic cells. Tumor cells use their migratory and invasive
properties in order to burrow through surrounding extracellular
stroma and to gain entry into blood vessels and lymphatics.
Intravasation and survival in the circulation. Once tumor cells
enter the circulation, or intravasate, they must be able to withstand
the physical shear forces and the hostility of sentinel immune cells.
Solid tumors are not accustomed to surviving as single cells without
attachments and often interact with each other or blood elements to
form intravascular tumor emboli.
11. Basic Steps in the Metastatic Cascade
Arrest and extravasation. Once arrested in the capillary system of
distant organs, tumor cells must extravasate, or exit the circulation,
into foreign parenchyma.
Growth in distant organs. Successful adaptation to the new
microenvironment results in sustained growth.
Of all the steps in the metastatic cascade, the ability to grow in
distant organs has the greatest clinical impact and lies at the core of
the seed and soil hypothesis. Accomplishing this step may be rate-
limiting and may determine whether distant relapse occurs rapidly or
dormancy ensues.
13. Heterogeneity in Cancer Metastasis and Rarity of
Metastatic Cells
Because numerous sequential steps are needed for metastasis,
multiple genetic changes are envisioned. A failure in any step would
prevent metastasis altogether.
Accordingly, tumor cells that can accumulate a full complement of
needed alterations to endow them with metastatic ability should be
rare.
Early cell fate studies revealed that less than 0.01% of tumor cells
gave rise to metastases.
Thus, important early studies helped to establish the idea that
primary tumors are heterogeneous in their metastatic ability and that
tumor cells that can successfully metastasize are exceedingly rare.
14. The Traditional Progression Model for Metastasis
and its Implications
In this view, primary tumor cells undergo somatic evolution and
accumulate genetic changes.
Because numerous steps are required for metastasis, the number of
genetic changes that are needed for full metastatic competency is
large; hence, tumor cells that have acquired these changes are rare.
Many clinicopathologic traits such as lymphovascular invasion and
regional lymph node involvement represent successful completion of
some of the steps in the metastatic cascade but not necessarily all.
The clinical observation that metastatic risk increases with tumor
size is explained by mathematical considerations predicting that
genetic changes accumulate faster with increased population size.
Larger tumors are more likely to contain rare cells that are
metastatically competent, making metastasis a late event in
tumorigenesis.
15. The Traditional Progression Model for Metastasis
and its Implications
One of the primary objectives in the clinical management of cancer
is to prevent or decrease the risk of metastasis.
The idea that metastasis occurs as a late event in tumorigenesis
argues that early detection and early eradication of the primary
tumor will prevent metastasis and be sufficient for cure.
Screening programs, radical versus more limited surgical excisions,
and the use of adjuvant radiation to the surgical bed can be justified
based on the idea that cancers caught early have not likely spread.
Metastatic heterogeneity within the primary tumor and the rarity of
tumor cells that can complete all the sequential steps in the
metastatic cascade suggest that the detection of tumor cells caught
in the act of undergoing an early step in the cascade may still
represent an opportunity to stop metastasis in its tracks.
This is a rationale for oncologic surgeries that include regional
lymph node dissections and the use of regional radiation therapy.
16. Alternative Models
1. Halsted model: Although more anatomic than cellular in nature,
the Halsted model looked at breast cancer from a traditional
vantage point and imposed on it an orderly anatomic spread pattern
from primary site, to regional lymph nodes, to distant organs. This
orderly progression would make complete eradication of the primary
and regional tumor burden sufficient to stop metastasis.
2. Fisher model: Fisher hypothesized that whether distant relapse
occurs in breast cancer is predetermined from the onset of
tumorigenesis (discussed in ref. 9). This view emphasizes breast
cancer as a systemic disease for those tumors so fated and the
importance of adjuvant systemic chemotherapy.
3. Clonal dominance model
4. Dynamic heterogeneity model.
17. An Integrated Model for Metastasis
Different concepts on how metastasis progresses have individual
merits and limitations. A clearer understanding of metastasis
requires sophisticated insight on a molecular level.
Recent advances in the field of metastasis research are beginning to
bring together an integrated and more complex paradigm whereby
elements from different models may be interconnected.
During primary tumor growth, the principal functions that are
selected are tumorigenic functions that can be met by a large
repertoire of oncogenic mutations. Examples of these tumorigenic
functions include proliferative and metabolic autonomy, self-renewal
ability, resistance to cell death, resistance to inhibitory signals,
immune evasion, motility, invasion, and angiogenesis.
18. An Integrated Model for Metastasis
Most of these traits were enumerated by Hanahan and
Weinberg as being hallmarks of cancer. Many of these
tumorigenic functions allow transformed cells to attract
supporting stroma and migrate and invade surrounding
tissue, regardless of whether or not cells reside in the
primary tumor.
This subset of tumorigenic functions is a prerequisite for
metastasis because such functions are needed for cells
to invade, penetrate blood vessels, and give rise to
circulating tumor cells.
These functions are shared by primary tumors and
metastasis and are defined as metastasis initiation
functions. A prominent example includes epithelial-to-
mesenchymal transition (EMT).
19. An Integrated Model for Metastasis
It is evident how genes with tumorigenic functions and genes with
metastasis initiation functions can be selected for during primary
tumor growth.
However, how are metastasis-specific functions (i.e. functions that
are not characteristic of general tumorigenesis) selected during
growth at the primary site?
Metastasis-specific functions include survival in the circulation,
extravasation, survival in the microenvironment of distant organs,
and organ-specific colonization.
20. An Integrated Model for Metastasis
Recent experimental evidence reveals that some genes can mediate
tumorigenic functions and secondarily serve metastasis-specific
functions either in a general way or with particular organ selectivity.
These types of functions are called metastasis-progression
functions and genes with this duality are defined as metastasis-
progression genes.
Metastasis-progression genes form the basis for predetermination
models for metastasis. When metastasis-progression genes are
selected for, their expression by the primary tumor will track with
increased risk of metastasis. These genes will also mechanistically
couple certain traits of primary tumor progression (e.g., rapid growth,
invasiveness, resistance to hypoxia) with distant spread.
22. An Integrated Model for Metastasis
Cancer cells that have acquired metastasis-progression genes can
undergo additional selective pressure during life away from the
primary tumor.
Functionally, genes selected by the pressures of a distant site are
similar to metastasis-progression genes but they are not coupled to
tumorigenic genes and so confer no advantage to a primary tumor.
Therefore, altered expression of these genes would be rare or
absent in the primary tumor and discernible only in the metastatic
lesion.
These genes are called macrometastatic-colonization genes and
provide macrometastatic-colonization functions.
Macrometastatic-colonization genes form the basis of traditional
progression models for metastasis.
23. Selective Pressures at the Primary Tumor Driving
Acquisition of Metastasis Functions
Hypoxia
In order to disrupt tissue homeostasis during primary tumorigenesis,
many barriers that can limit growth must be overcome.
A near-universal need is for tumors to respond to hypoxia.
Normal tissue such as epithelium is separated from blood vessels by
a basement membrane. When preinvasive tumor growth occurs,
hypoxia can ensue because oxygen and glucose typically can only
diffuse 100 to 150 microns, resulting in portions of the expanding
mass becoming hypoxic.
This can be seen in comedo-type ductal carcinoma in situ (DCIS) of
the breast, whereby a necrotic center characterizes these
preinvasive breast tumors.
The fact that DCIS can take years to progress to invasive cancer, or
never progresses to cancer, suggests that hypoxia can be a
significant barrier.
24. Selective Pressures at the Primary Tumor Driving
Acquisition of Metastasis Functions
Although there are multiple paths that cancer cells can take to adapt
to hypoxia, the hypoxia-inducible factor (HIF) transcription factors
have a central role.
Under hypoxic conditions, HIF-1α and HIF-2α become stabilized,
resulting in the transcription of over 100 HIF-α regulated genes.
These target genes are involved in angiogenesis, glycolysis, and
invasion, which together help hypoxic cells adapt.
Up-regulated angiogenesis genes include vascular endothelial
growth factor (VEGF) and platelet-derived growth factor (PDGF).
These factors cause quiescent blood vessels to undergo
remodeling, including the laying down of a matrix that activated
endothelial cells use to form newly vascularized areas.
25. Selective Pressures at the Primary Tumor Driving
Acquisition of Metastasis Functions
Various glycolysis genes are expressed and their metabolic by-
products lead to acidification of the extracellular space. This is
normally toxic to cells and requires further adaptation either by up-
regulation of H+transporters or acquired resistance to apoptosis.
To assist in invasion toward newly vascularized areas, HIF-α up-
regulates matrix metalloproteinase 1 and 2 (MMP1, MMP2), lysyl
oxidase (LOX), and the chemokine receptor CXCR4. Degradation of
the basement membrane by MMP2 and alteration of the extracellular
matrix (ECM) by MMP1 and LOX clears away a barrier to migration.
The activation of CXCR4 then stimulates cancer cells to migrate to
regions of angiogenesis.
Thus, if these series of events can be successfully completed, not
only will preinvasive tumors successfully deal with hypoxia, but they
will also likely invade through the basement membrane in the
process. Invasion through the basement membrane defines invasive
carcinomas
26. Selective Pressures at the Primary Tumor Driving
Acquisition of Metastasis Functions
Inflammation
When normal tissue homeostasis and architecture are disrupted, this
can lead to vessel injury, hypoxic zones, extravasation of blood
proteins, and the entry of foreign pathogens.
A rapid response is mounted by a front line composed of immune
and bone marrow-derived cells (BMDCs) such as lymphocytes,
neutrophils, macrophages, dendritic cells, eosinophils, and natural
killer (NK) cells.
The purpose is to restore homeostasis through several phases:
inflammation, tissue formation, and tissue remodeling.
27. Cancer cells are often surrounded by activated fibroblasts and
BMDCs. Because of the resemblances between primary tumors and
normal tissue wound response, cancer has been described as a
“wound that does not heal.”
Although Virchow hypothesized in the 1850s that inflammation was
the cause of cancer, the presence of an inflammatory response has
generally been interpreted as evidence that the immune system
actively fights the cancer as it does with invading bacterial or viral
pathogens.
Under this scenario, the inflammatory response would apply
significant selective pressure on the tumor to evade immune-
mediated attack, and the nonhealing nature of the response
suggests a back-and-forth struggle.
Tumors that progress do so by orchestrating an immunosuppressive
environment, a process known as immunoediting.24
28. To facilitate an immunosuppressive environment, the tumor
microenvironment selects for cells that favor production of
immunomodulatory factors like TGF-β, cycoloxygenase-2 (COX2),
CSF-1 (macrophage growth factor, colony-stimulating factor-1), IL-
10, and IL-6.
These cytokines inhibit maturation of dendritic cells and promote
tumor-associated macrophages (TAMs) that are immunosupressed.
Tumors also recruit BMDCs that have immunosuppressive
properties such as myeloid-derived suppressor cells (MDSCs).
Thus, although the inflammatory response undoubtedly can help to
limit cancer growth, cancers seem to select for cells that create
immunosuppressive surroundings.
29. Figure 10.2. Interactions between cancer and stroma
that promote invasion and metastasis.
Cancerized stroma consists of fibroblasts,
inflammatory cells, and other bone marrow-
derived cells that have been conscripted to
aid the tumor in overcoming hypoxia and in
invasion and migration. Tissue breakdown,
hypoxia, and inflammatory cytokines and
chemokines secreted by the tumor cells result
in recruitment of tumor-associated
macrophages (TAMs), carcinoma-associated
fibroblasts (CAFs), mesenchymal stem cells
(MSCs), and myeloid-derived suppressor cells
(MDSCs). TAMs and MDSCs can be found at
points of basement membrane breakdown
and at the invasive front of the tumor. These
cells produce angiogenic factors to promote
vascularization, proteases to degrade the
extracellular matrix, and growth factors that
stimulate tumor invasion and motility. CAFs
also produce similar angiogenic factors,
protease, and tumor growth factors. In
addition, CAFs recruit bone marrow-derived
endothelial precursors for angiogenesis. The
cytokines and growth factors that TAMs and
CAFs secrete are mutually beneficial to each
other as part of an inflammatory/woundlike
response. Cancers have been described as
“wounds that do not heal.” This chronic state
is maintained by immunomodulatory cytokines
that suppress immune functions to ensure a
protumorigenic environment.
30. Escaping Apoptosis and Senescence
A major mechanism to safeguard against a breakdown in tissue
homeostasis due to cells that stray, become damaged, or spent, is to
have these cells commit programmed cell death, or apoptosis.
Ability of cancer cells to resist cell death likely contributes to
successful establishment of tumors.
In addition to apoptosis, senescence is another important barrier to
cancer. This exit from the pool of proliferating cells results from
telomere erosion, oncogene induction, and DNA damage. Similar to
some forms of apoptosis, senescence is p53-dependent. Thus,
pressure to escape senescence can result in the loss of p53 or
mutations in p53.
31. Self-Renewal Ability
Normal tissues result from the differentiation of precursor cells
called stem cells, which are multipotent cells with self-renewal ability.
In the adult, mature differentiated cells serve specialized tasks and
have limited proliferative potential.
However, adult tissue still undergoes turnover and is maintained
through the self-renewal and multilineage differentiation of adult
stem cells. Examples of this include skin, mucosa, and
hematopoietic cells whereby a limited and spatially restricted pool of
adult stem cells asymmetrically divides. One daughter cell maintains
the stem cell pool by self-renewal, and the other daughter cell starts
the process of terminal differentiation for tissue maintenance.
32. Self-Renewal Ability
The majority of cancers maintain some resemblance to their tissue
of origin by virtue of persistent differentiation, albeit in an abnormal
way. Thus, many cells in a tumor population may have limited
proliferative potential and be incapable of sustained self-renewal,
similar to their normal counterparts. The idea that only a limited
subset of cells in a cancer is capable of self-renewal is called
the cancer stem cell hypothesis.
The existence of cancer stem cells was first demonstrated in acute
myeloid leukemia and recently shown in breast cancer, glioblastoma,
and other cancer
33. Coupling Tumorigenesis with Metastasis Initiation
The selective pressures previously described that are encountered
during primary tumor growth—hypoxia, inflammation, apoptosis,
senescence, and need for proliferative, metabolic, and self-renewal
sufficiency—drive primary tumors to acquire tumorigenic alterations
that support aggressive growth.
These same pressures collaterally support the initial stems of
metastasis, and remain important throughout the subsequent
malignant steps. One of the most striking types of metastasis
initiation functions is EMT.
34. Epithelial-to-Mesenchymal Transition
During development, the generation of many adult tissues and
organs results from a series of EMT events and the reverse process,
a mesenchymal-to-epithelial transition (MET).
Growing evidence points toward EMT as an important characteristic
of metastasis-prone cancers. EMT in cancer is not a concrete and
tidy single process but rather a collection of cell reprogramming
phenomena that share the property of down-regulating epithelial cell
markers and, for convenience, the collective denomination of EMT
35. Epithelial-to-Mesenchymal Transition
Besides hypoxia and inflammation, the need for primary cancers
to resist apoptosis and overcome senescence may be additional
reasons to flip an EMT switch.
Cells that have undergone EMT are associated with increased
resistance to apoptosis, possibly through prosurvival activity
conferred by Snail and Twist.
EMT can also help cancer overcome oncogene-induced
senescence.
36. Epithelial-to-Mesenchymal Transition
What makes EMT cells particularly adept at initiating early
metastatic events?
A careful analysis of the effect of Twist on metastasis revealed a role
for Twist in establishing high levels of circulating tumor cells through
enhancing intravasation and/or survival in the circulation.
The ability of cells undergoing EMT to intravasate is consistent with
observations that EMT occurs at the invasive front of tumors
whereby cells lose E-cadherin, detach, invade, and break down the
basement membrane.
Accordingly, experiments that directly analyzed EMT and non-EMT
cells showed that only the EMT cells were able to penetrate
surrounding stroma and intravasate.43
37.
38. Coupling Tumorigenesis with Metastasis
Progression
The selection of genes that primarily fulfill tumorigenic functions may
also result in genes that secondarily aid cancer cells after they have
found their way into the circulation.
In other words, when genes are selected to help the primary tumor
grow, some of these genes have a collateral effect of benefiting the
cancer after it disseminates by providing metastasis-specific tools.
Such genes can be classified as metastasis-progression genes;
however, a clear distinction with metastasis-initiation genes may not
always be evident.
In this section, we describe functions that can be classified as
metastasis-progression functions.
39. Premetastatic Niche
Even before tumor cells colonize distant organs, they can
help prepare foreign soil for the subsequent arrival of
DTCs(disseminated tumor cells) by remotely coordinating
a “premetastatic niche” from the primary tumor.
These niches are often located within distant organs
around terminal veins and are characterized by newly
recruited hematopoietic progenitor cells of the myeloid
lineage and by stromal cells.
This niche provides an array of cytokines, growth factors,
and adhesion molecules to help support metastatic cells
on their arrival
40. Premetastatic Niche
Once myeloid and activated stromal cells form the
premetastatic niche, the local environment in the distant
organ is altered by the production of inflammatory cytokines
and MMPs, which begins bearing an evolving resemblance to
the primary site.
Consequently, when primary tumor cells start wandering in
the circulation, the target organs with an established
premetastatic niche become a better soil in which to attach,
survive, and grow. Indeed, if the formation of the niche is
disrupted, metastasis is inhibited.
In this way, the shower of cytokines and growth factors that
accompany inflammatory and hypoxic responses at the
primary tumor not only selects for cancer cells that can
flourish in the primary site but has the secondary effect of
creating a more welcoming environment in distant organs
after dissemination..
41. Survival in the Circulation
From experimental model systems, it has been estimated that
approximately one million cancer cells per gram of tumor tissue can
be introduced daily into the circulation.
Direct inoculation of tumor cells into mice demonstrates that
metastasis can be an inefficient process because despite large
numbers of circulating tumor cells (CTCs), relatively few metastases
form.
Thus, even if cancer cells acquire metastasis-initiation functions like
EMT, the ability to merely enter into the circulation often is not a rate-
limiting step in metastasis. Other obstacles must be overcome.
42. Survival in the Circulation
How can CTCs evade cell death to enhance their metastatic
potential? Growth at the primary tumor site will involve a selection
for increased resistance to apoptosis. Antiapoptosis genes such
as BCL2 or BCL-XL, or the loss of proapoptotic genes and genes
downstream of the TNF-related receptor family, can result in
increased metastasis.
Part of this may be the result of survival both in the circulation and
shortly after extravasation. Both CTCs and platelets can also
express the αvβ3 integrin to promote aggregation of these cells to
form tumor emoboli.
This aggregation not only facilitates arrest but can protect against
shear forces and NK cell-mediated killing.
Thus, the ability of a primary tumor to respond to apoptosis,
senescence, and inflammatory signals can secondarily make cancer
cells better able to survive in the circulation once metastasis has
been initiated.
43. Extravasation and Colonization
After arresting in capillaries, tumor cells that are able to survive can
grow intravascularly. This can lead to a physical disruption of the
vessels.
Cancer cells can mimic leukocytes and bind to endothelial E- and P-
selectins. Molecular mediators of extravasation include the
cytoskeletal anchoring protein Ezrin, which links the cell membrane
to the actin cytoskeleton and engages the cell with its
microenvironment.
46. Tumor Self-Seeding
As cancer cells selected by the inflammatory and hypoxic
surroundings of the primary tumor wander through the circulation,
might the most hospitable and likely destination for extravasation be
the primary tumor from which they came?
At least in theory, it would seem that compared with uncharted
foreign environments or even premetastatic niches, the primary
tumor would impose the least resistance to colonization.
The ability to self-seed is promoted by IL-6 and IL-8, common
prometastatic cytokines found in the tumor microenvironment. The
cancer cell expression ofMMP1 and Fascin-1, two previously
identified lung metastasis genes, facilitates transendothelial
migration and tumor self-seeding. In fact, metastatic cells in general
are more efficient seeders, which may result from a direct cause and
effect.
47. From Metastasis Progression to Macrometastatic
Colonization
Because metastasis-initiation functions and metastasis-progression
functions are coupled to tumorigenic functions, the accumulating
evidence that primary tumors can exhibit metastatic traits early on
during primary tumorigenesis and with such high prevalence may
not be surprising.
However, despite the ability of EMT to drive invasion, intravasation,
and self-renewal, and despite the remote influence the primary
tumor has on survival in the circulation, extravasation, and
premetastatic niche formation, the completion of the metastatic
cascade is still relatively infrequent for many cancer types.
This suggests an important requirement for macrometastatic
colonization functions, or functions selected at distant sites for
organ-specific colonization.
48. Dormancy
A major limiting step in metastasis is acquiring the ability to sustain
growth within a distant site after extravasation.
Many cancers such as breast and prostate will not give rise to
metastasis until years or even decades after eradication of the
primary tumor.
Experimentally, it has been shown that the vast majority of
extravasated cancer cells do not form macrometastasis.In
aggregate, these observations of latency are referred to
as metastatic dormancy
49. Early, Multiorgan Metastasis
Most cancers such as breast, prostate, and sarcoma, to name a few,
demonstrate appreciable latency periods. Depending on the onset of
dissemination and colonization within distant organs, this provides
varying and often lengthy periods for acquiring macrometastatic-
colonization genes.
In contrast, some cancers, like adenocarcinomas of the lung and
pancreas, have short latency periods and exhibit early systemic
metastasis often to multiple organs. One explanation for short
versus long periods of latency is related to origin of the cell
population initially targeted for transforming and tumorigenic events.
The cancer stem cell hypothesis and the role of EMT in metastasis
already suggest that early progenitor cells or cells that may have
played a role in developmental processes can be predisposed to
activate metastasis-progression mechanisms. An example of this
has been demonstrated by introducing defined oncogenic alterations
into different cell.
Thus, the transformation of certain unique cell types may predispose
to early metastatic behavior and could explain certain phenomenon
such as cancers of unknown primary.
50. Clinical Application: Therapy Directed Against
Invasion, Angiogenesis, and Metastasis
Molecular-Targeted Agents: Monoclonal Antibodies
Antiangiogenic antibodies.
Bevacizumab is a recombinant humanized version of the murine
antihuman VEGF monoclonal antibody. This VEGF-neutralizing
antibody inhibits VEGF-induced signaling, resulting in reduced
angiogenesis and tumor growth. It has been U.S. FDA-approved for
renal cell, lung, and colon cancers.
Progression-free survival at 6-month rate was 23.9% and 40.4% in
cervical and endometrial cancer , respectively.
51. Anti-EGFR family antibodies.
Cetuximab is a recombinant chimeric antibody to EGFR that is
presently approved for treatment of metastatic colon cancer and
unresectable head and neck cancer, with activity in breast and
non-small cell lung cancer..
Trastuzumab is a monoclonal antibody against EGFR-2 (Her2)
that prolongs survival in Her2-positive breast cancer in both the
adjuvant and metastatic settings.
52. Molecular Targeted Agents: Kinase Inhibitors
Antiangiogenic Agents.
A number of small-molecule inhibitors of the VEGF receptors and
other angiogenic pathways have been developed and reached
clinical trials. VEGFR2 is most commonly targeted with preclinical
data showing this approach to be active in reducing endothelial cell
proliferation, migration, and vascular development, and xenograft
models have confirmed activity in a number of solid tumors .
Selective VEGFR Inhibitors.
Cediranib is an oral tyrosine kinase inhibitor (TKI) that targets all
three VEGFRs and c-kit.
53. Mixed Kinase Inhibitors, Including VEGFRs.
Sorafenib is an oral Raf-kinase and VEGFR-2 inhibitor that has been
approved for treatment of metastatic renal cell and hepatocellular
cancers.
Nintedanib (BIBF1120) inhibits VEGFRs 1, 2, and 3, PDGFR-α and -
β, and FGFRs 1, 2, and 3.
Pazopanib is a TKI that targets all three VEGFRs, both PDGFRs,
and c-kit. The phase II trial of pazopanib in ovarian cancer patients
with initial complete CA125 response yielded a CA-125 response
rate of 31%, where 29% of patients with measurable disease had
stabilization
54. .
Sunitinib, an inhibitor of VEGFRs and PDGFRs , resulted in a
response rate of 16.7% in a noncontinuous arm and 5.4% with
continuous dosing in a phase II trial in platinum-refractory EOC
Cabozantinib is an oral TKI that inhibits c-Met, ALK, and VEGFR2,
and has been shown to reduce tumor growth and angiogenesis. The
agent continues to be evaluated in women with refractory ovarian
cancer.
Vandetanib, a dual VEGFR and EGFR inhibitor, was examined in
lung and ovarian cancers because of the recognized presence and
activity of both targets
55. Table 8.3Metastasis Therapeutic Targets and
Agents
A. Targeted Therapeutics
Target Example Agents Effects
Growth factors C225 (anti-EGFR) Tyrphostins (anti-RTK) Block growth factor signaling
Cell adhesion Anti-αvβ3 (Vitaxin) αvβ3 peptidomimetics Blocks endothelial cell interaction with matrix,
may regulate MP activation
Proteolysis MMPIs uPAR-I Blocks degradation of matrix, blocks activation of
proteases, growth factors
Motility Taxanes Blockade of microtubule cycling
Signaling [See below] Blockade of signals necessary for angiogenesis,
invasion, and metastasis
B. Signal Inhibitors
Agent Target Activity
CAI Calcium influx Inhibits adhesion, motility, angiogenesis
Squalamine Inhibits NHE-3 Anti-angiogenic
PI3 kinase inhibitors — Inhibit motility, proliferation, promote
MAPK inhibitors — Inhibit invasion, proliferation
56. Growth Factor Metastasis Targets
Many growth factors can stimulate both tumor and
endothelial cell behaviors ranging from proliferation to
attachment, motility, and proteolysis. For that reason,
they are a logical target for therapeutic intervention.
Two classes of molecules have been developed against
growth factor receptors: small molecule receptor
antagonists and monoclonal antibodies.
Tyrphostins have been developed that bind to a series of
molecules, tyrosine kinase–containing growth factor
receptors, such as those of EGF, VEGF, and PDGF.This
class of molecules focused on the ligand and ATP–
binding sites of the tyrosine kinase receptors.
Another approach is the use of directed monoclonal
antibodies..
57. Antiadhesive Agents
Limited agents targeted at tumor or endothelial cell adhesion have
entered clinical trials, and several are under development.
Interventions include peptidomimetics and monoclonal antibodies
targeted currently at integrins.
Antagonists of αvβ3, such as the murine monoclonal antibody
LM609 and its newer humanized counterpart Vitaxin, induce
vascular cell apoptosis and potently inhibit angiogenesis. This is
related to the ability of these antagonists to selectively promote
programmed cell death of newly sprouting blood vessels.
Matrix Metalloproteinase Inhibitors
Regulation of the TIMP/MMP balance is critical to the localization
inhibition of matrix breakdown for both physiologic invasion of
angiogenesis and the malignant invasion of metastasis.. The first
class of MMPIs are the hydroxamate molecules, examples of which
are batimastat (BB-94) and marimastat, targeted to interact with the
MMPs at the activation site by blocking chelation of the metal ion,
thus mimicking the physiologic action of TIMPs.Marimastat is now in
phase III clinical trials for ovarian cancer
58. Antimetastasis Signal Transduction Therapy
The loss of balance in the cellular communication process may allow
for dysregulation leading to tumorigenicity, invasion, and metastasis.
Therapeutic efforts in cancer prevention and treatment are being
focused at the level of signaling pathways or selective modulatory
proteins.
Investigations into the signaling pathways underlying metastasis
have suggested that protein kinase activity, calcium homeostasis,
and ras activation are important signals and therefore may be key
regulatory sites for therapeutic intervention.
Several natural products have been found that inhibit protein
tyrosine kinase activity and may possess antiproliferative or anti-
invasive properties. These include genistein, herbimycin, and
lavendustin A.
