Tumor Microenvironment and Differential
Responses to Therapy
Eishu Hirata
1
and Erik Sahai
2
1
Department of Oncologic Pathology, Kanazawa Medical University, Ishikawa 920-0293, Japan
2
Tumor Cell Biology Laboratory, Francis Crick Institute, London WC2A 3LY, United Kingdom
Correspondence: ehirata@kanazawa-med.ac.jp; erik.sahai@crick.ac.uk
Cancer evolution plays a key role in both the development of tumors and their response to
therapy. Like all evolutionary processes, tumor evolution is shaped by the environment. In
tumors, this consists of a complex mixture of nontransformed cell types and extracellular
matrix. Chemotherapy or radiotherapy imposes further strong selective pressures on cancer
cells during cancer treatment. Here, we review how different components of the tumor
microenvironment can modulate the response to chemo- and radiotherapy. We further de-
scribe how therapeutic strategies directly alter the composition, or function, of the tumor
microenvironment, thereby further altering the selective pressures to which cancer cells are
exposed. Last, we explore the consequences of these interactions for therapy outcomes and
how to exploit our increasing understanding of the tumor microenvironment for therapeutic
benefit.
S
olid tumors contain a complex mixture of
noncancerous cell types and matrix compo-
nents. Collectively, this is referredtoasthe tumor
microenvironment or tumor stroma. The mi-
croenvironment plays a critical role in many as-
pects of tumorigenesis. It generates the tumor
vasculature and it is highly implicated in the
progression to metastasis. More recently, it has
become clear that the tumor microenvironment
affects the response to therapies. Further, mod-
ulating the tumor stroma may improve the effi-
cacy of existing therapies and could present new
opportunities for therapeutic targeting. In this
article, we introduce the key features of the tu-
mor microenvironment and then discuss how
they influence the selective pressures on cancer
cells during targeted, chemo- and radiotherapy.
COMPOSITION OF TUMOR
MICROENVIRONMENT
Tumors contain various noncancerous cells in-
cluding fibroblasts, vascular endothelial cells,
and immune cells, including T-cells, macro-
phages, and neutrophils (Fig. 1) (Hanahan
and Coussens 2012). In many cases, organ-spe-
cific interstitial cells are also present, for exam-
ple, osteoblasts in bone tissue and astrocytes in
the central nervous system. Collectively, these
cells are often termed the tumor stroma and,
together with factors such as the extracellular
matrix, oxygen levels, and pH, they make up
the tumor microenvironment. Because of space
constraints, we will only briefly outline the role
of stromal cells here. Endothelial cells form the
Editors: Charles Swanton, Alberto Bardelli, Kornelia Polyak, Sohrab Shah, and Trevor A. Graham
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Cell type Roles within tumor
Endothelial cells
Generate blood vessels that provide
nutrients and oxygen. Provide escape
route for metastatic cells. Local
“angiocrine” signals can protect cancer
cells.
Fibroblasts
Produce HGF, CXCL12, TGF-β, and many
other soluble factors. Produce and
physically remodel the tumor extracellular
matrix
Macrophages
Depending on subtype, can either favor
or antagonize T-cell function. Promote
cancer cell migration via EGF and vessel
leakiness via VEGF.
Neutrophils
Can be both pro- and antitumorigenic.
Can boost stem cells.
Dendritic cells
Gather antigens to present to T cells
Cytotoxic T cells
Kill tumor cells expressing tumor neo-
antigens. Activity can be limited by PD-
1, CTLA-4, and other microenvironmental
factors.
Cancer cell
Figure 1. Major components of the tumor microenvironment. Illustration of the main cellular types found within tumors
alongside a table listing their main roles within the tumor.
E. Hirata and E. Sahai
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tumor blood vessels and are critical for the deliv-
ery of oxygen, nutrients, and drugs to the tumor.
Further, they provide an exit route for metabolic
waste products and metastatic cancer cells (Rey-
mond et al. 2013). Unlike normal vasculature,
tumor vessels are often disorganized leading to
local variations in tumor oxygenation and other
environmental factors (Harney et al. 2015; Eales
et al. 2016). Switching from oxidative phosphor-
ylation to glycolysis is considered to be one of
the adaptation strategies of cancercells to survive
in hypoxic conditions (Gatenby and Gillies
2004), although it also works advantageously
to produce nucleic acids and nicotinamide ade-
nine dinucleotide phosphate (NADPH) for cell
proliferation (Vander Heiden et al. 2009). A by-
product of this is increased lactate levels and
therefore lower extracellular pH can be a feature
of tumors (Damaghi et al. 2015).
Cells from both the innate and adaptive im-
mune system are found within the tumors (Ha-
nahan and Coussens 2012). The adaptive im-
mune system can be capable of recognizing
tumor cells as “not normal” and CD8
þ
cytotox-
ic T lymphocytes (CTLs) can target them for
killing, a process called tumor immune-surveil-
lance (Grivennikov et al. 2010). It is increasingly
appreciated that overcoming immune surveil-
lance is a critical part of tumorigenesis (Mittal
et al. 2014) and reactivating the process by sup-
pressing “checkpoints” that limit T-cell func-
tion is a potent anticancer strategy (Melero
et al. 2015; Miller and Sadelain 2015). Innate
immune cells, including macrophages and neu-
trophils are recruited into tumors by similar
mechanisms to those that attract them to
wounds. They can be both anti- and protumori-
genic and cross talk extensively with endothelial
cells and the innate immune system (Qian and
Pollard 2010). Fibroblastic cells, including resi-
dent tissue fibroblasts, pericytes, and mesen-
chymal stem cells can become activated in
tumors. Activated fibroblasts, termed cancer-
associated fibroblasts (CAFs), produce and re-
model much of the extracellular matrix within
tumors (Bhowmick et al. 2004; Kalluri and Zeis-
berg 2006; Hanahan and Coussens 2012). This
can often lead to elevated levels of tissue stiff-
ness in tumors (Levental et al. 2009). CAFs are
generally proinvasive and proangiogenic (Ma-
dar et al. 2013), although recent evidence shows
that they are not universally protumorigenic
(Ozdemir et al. 2014; Rhim et al. 2014). Readers
are directed to several excellent reviews describe
the various components of the tumor microen-
vironment in detail (Joyce and Pollard 2009;
Hanahan and Weinberg 2011; Hanahan and
Coussens 2012; McAllister and Weinberg 2014).
To summarize a large body of work, cancer
cells and stromal cells can interact in ways that
may either favor or hinder tumor progression
(Fig. 2). These environmental influences signif-
icantly shape tumor evolution. Cancer cells are
under selective pressure to maximally exploit
favorable microenvironmental conditions and
overcome unfavorable ones. The former situa-
tion is exemplified by the gain of chemokine
receptor expression in various carcinomas. The
tumor microenvironment can often contain
high levels of chemokines, such as C-X-C che-
mokine ligand (CXCL) 12/stromal derived fac-
tor (SDF) 1 (Orimo et al. 2005). The availability
of CXCL12 then means that cancer cells express-
ing the relevant receptor, C-X-C motif chemo-
kine receptor (CXCR) 4, will be at an advantage.
Indeed, high levels of CXCR4 expression are as-
sociated with high CXCL12 levels in the primary
tumor and metastasis to tissues with high
CXCL12 levels (Zhang et al. 2013). Conversely,
The gain of immune-suppressive molecules,
such as programmed death ligand 1 (PD-L1),
can lead to cancer cells overcoming the presence
of CTLs that express PD-1 in the tumor micro-
environment (Iwai et al. 2002). In addition,there
is selective pressure for cancer cells to gain traits
that promote the recruitment of protumorigenic
stroma. This is perhaps best exemplified by the
gain-of-expression of the proangiogenic ligands
by cancer cells (Carmeliet and Jain 2000).
Variations in physical factors, such as tissue
stiffness, matrix geometry, and electromagnetic
fields are also features of the tumor microenvi-
ronment. For example, stiffened extracellular
matrix prepared by CAFs enhances integrin-
mediated mechanotransduction related signals,
which strongly support cancer cell survival,
proliferation, and invasion (Paszek et al. 2005;
Butcher et al. 2009; Sulzmaier et al. 2014). Elec-
Tumor Microenvironment and Therapeutic Responses
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tric/magnetic fields created in the tumor mi-
croenvironment strongly affect cancer cell mi-
tosis, which is already used in the treatment of
glioblastoma (Stupp et al. 2012; Swanson et al.
2016). Thus, the tumor microenvironment
contains a complex mixture of biochemical
and biophysical cues that modulate cell behav-
ior and provide the environment in which the
fittest cancer cells are selected in the absence of
therapy. These same factors can also modulate
the strong selective pressures applied by chemo-
and radiotherapy. Cancer treatments can also
directly affect many of the cellular components
of the microenvironment and further alter the
context in which cancer evolution occurs.
CANCER–STROMA INTERPLAY UNDER
CYTOTOXIC REAGENTS
The majority of cytotoxic chemotherapy agents
either cause DNA damage, which is more diffi-
cult for cells that are replicating their DNA to
resolve, or perturb mitosis. Although these
agents can cause levels of damage and structural
defects that are incompatible with cell viability
and lead to rapid cell death in vitro, the situa-
tion in vivo is more nuanced. Imaging studies
have revealed that the kinetics of cell death are
much slower in vivo and it is more likely that
cytotoxic agents trigger cell death through in-
teraction with various “checkpoints” and en-
gagement of the apoptotic machinery (Janssen
et al. 2013). These latter processes can be affect-
ed by external cues; indeed, several studies have
indicated how the tumor microenvironment
can modulate responses to cytotoxic drugs
(Sherman-Baust et al. 2003; Gilbert and He-
mann 2010; Nakasone et al. 2012; Sun et al.
2012; Dijkgraaf et al. 2013).
The cell death caused by chem otherapeutic
agents can act as a trigger for the recruitment of
myeloid cells (Ruffell and Coussens 2015). This
Macrophage CAF
Reduced leukocyte numbers
Immune suppression
TGF-
β
activation
Cytokines and
chemokines
ECM
Endothelial cell
Integrins
Localized protection
NF-
κ
B and JAK-STAT
Systemic protection
Increased numbers of surviving cancer cells leading to higher
probabilit
y
of
g
enetic solutions to therapeutic challen
g
e
Growth factors
RTKs
Figure 2. Major mechanisms by which the tumor microenvironment modulates the response to therapy. Stromal
cells, including macrophages, endothelial cells, and fibroblasts, can produce grow th factors, cytokines, and
chemokines that locally promote cancer cell survival. Fibroblasts also play a major role in shaping the tumor
extracellular matrix and this can promote prosurvival signals via integrins. The production and activation of
transforming growth factor b (TGF-b) by stromal cells can lead to immune suppression that further protects
cancer cells; this can be both local and systemic. In addition, the reduction in leukocyte numbers caused by
cytotoxic and radiotherapy can lead to further immune suppression.
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is potentially because the dying cells generate
similar signals to a wound. Indeed, the C-C
motif chemokine ligand (CCL) 2/C-C motif
chemokine receptor (CCR) 2 and colony-stim-
ulating factor (CSF)1/CSF1R axes play an im-
portant role in this process (DeNardo et al.
2011; Qian et al. 2011; Hughes et al. 2015). In
the longer term, cytotoxic drugs may systemi-
cally reduce leukocyte numbers or skew the di-
versity of leukocytes produced, because they
disrupt the expansion of hematopoietic stem
cells. It is unclear what effect this has on the
efficacy of cancer cell killing; however, it is as-
sociated with significant side effects, including
neutropenia. To counteract this, granulocyte
(G)-CSF is often given to patients to boost neu-
trophil numbers (Bennett et al. 2013). However,
recent work has suggested that boosting neutro-
phil numbers in this way may actually lead to
more aggressive tumor phenoty pes (Antonio
et al. 2015; Wculek and Malanchi 2015a,b).
The presence of tumor-associated macro-
phages can have several consequences for the
tumor. This is partly attributable to the presence
of different macrophages subtypes within tu-
mors (Mantovani et al. 2005). In reductionist
coculture experiments, macrophage s can reduce
the sensitivity of cancer cells to paclitaxel, eto-
poside, and doxorubicin (Shree et al. 2011). Sig-
nal transducer and activator of transcription
(STAT) 3 and downstream transcription from
inflammatory modulators are required in mac-
rophages for the protection of pancreatic ductal
adenocarcinoma cells (Mitchem et al. 2013).
The production of cathepsin B by macrophages
is important to protect breast tumors from the
effect of paclitaxel (Bruchard et al. 2013). This
may be caused by cathepsin B activating the in-
flammasome and thereby elevating the produc-
tion of a range of cytokines. Interleukin (IL)-6 is
a possible mediator of this chemoprotection in
both contexts. In addition to the production of
growth factors and proteases, macrophages can
affect cancer cell behavior through the pro-
duction of exosomes, which are 150-nm
lipid-enclosed cell fragments. The exosome-
mediated transfer of miR-155 from monocytes
to neuroblastoma cells reduces the cancer cells’
sensitivity to cisplatin (Challagundla et al.
2015). Activated “M2-like” macrophages re-
cruited to tumors following chemotherapy can
also affect other aspects of the tumor pheno-
typ e. They express CXCL12/SDF1a, which is a
promigratory cue for many cancer cells, and
they produce vascular endothelial growth factor
A (VEGF-A), which modulates the tumor vas-
culature and its leakiness (Du et al. 2008). The
combination of these events may contribute to
increased dissemination of tumor cells follow-
ing chemotherapy. Tumor-associated macro-
phages are also capable of immune suppression
(Doedens et al. 2010; Ruffell et al. 2014). To-
gether, these mechanisms tend to favor cancer
cell survival following chemotherapy and sup-
port the idea that targeting macrophages may
enhance the ability of conventional chemother-
apy to eliminate tumors.
DNA damaging agents, such as doxorubi-
cin, will also trigg er DNA damage in stromal
cells. Notably, triggering DNA damage in endo-
thelial cells leads to increased NF-kB activit y
and the elevated production of numerous cyto-
kines, including the antiapoptotic cytokine IL-
6, IL-1a, and granulocyte-macrophage (GM)-
CSF (Tavora et al. 2014). These factors then help
to protect tumor cells from DNA damage. DNA
damaging agents can drive certain stromal cell
typ es, notably fibroblasts, into a state of irrevers-
ible cell-cycle arrest called senescence (Krtolica
et al. 2001). Interestingly, this state is associated
with a characteristic secretome, rich in chemo-
kines and growth factors, such as CCL2, VEGF,
and transforming growth factor (TGF)-b, which
is capable of reducing the effects of chemothera-
py (Acosta et al. 2013). Mesenchymal stem cells
can protect gastric cancer cells from cytotoxic
therapies through producing exosomes that acti-
vate Ca
2þ
/calmodulin-dependent protein kinase
and extracellular signal-regulated kinase (ERK)/
mitogen-activated protein kinase (MAPK) (Ji
et al. 2015). The various mechanisms highlighted
here are from a variety of model systems and may
not act at once; nonetheless, it is clear that there
are a multitude of mechanisms by which the ef-
ficacy of cytotoxic agents can be reduced by cells
within the tumor microenvironment.
Stromal cells can also modulate the efficacy
of therapy by influencing drug access to the
Tumor Microenvironment and Therapeutic Responses
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