**Executive Summary of Inflammation and Cancer Think Tank**

 

Inflammation is a response to acute tissue damage, whether resulting
from physical injury, ischemic injury, infection, exposure to toxins, or
other types of trauma. It can play a role in tumor suppression by
stimulating an antitumor immune response, but more often it appears to
stimulate tumor development. Epidemiologic and clinical research
indicates an increased risk of certain cancers in the setting of chronic
inflammation.  For example, two inflammatory bowel diseases, ulcerative
colitis and Crohn's disease, predispose to cancers of the intestinal
tract.  Basic research, in turn, has shown that many of the processes
involved in inflammation (e.g., leukocyte migration, dilatation of local
vasculature with increased permeability and blood flow, angiogenesis),
when found in association with tumors, are more likely to contribute to
tumor growth, progression, and metastasis than to elicit an effective
host anti-tumor response.

Interestingly, inflammation functions at all three stages of tumor
development: initiation, progression and metastasis. Inflammation
contributes to initiation by inducing the release of a variety of
cytokines and chemokines that alert the vasculature to release
inflammatory cells and factors into the tissue milieu, thereby causing
oxidative damage, DNA mutations, and other changes in the
microenvironment, making it more conducive to cell transformation,
increased survival and proliferation.

Chronic inflammation appears to contribute to tumor progression by
establishing a milieu conducive to development of different cancers.
However the precise mechanism by which it does so remains to be
determined. Infection is a common cause of inflammation, and evidence
indicates that the presence of microbes can be a cofactor in the tumor
promoting effects of inflammation.

Tumor cells produce various substances that attract inflammatory cells,
which then secrete an array of soluble mediators. These further
stimulate proliferation of the initiated cell, tissue disruption in the
stroma, and tumor growth. Leukocyte infiltration, and particularly
macrophages, can lead to enhanced angiogenesis, which is associated with
a poor prognosis in some tissues. 

The role of inflammation in metastasis is less well defined than its
roles in cancer initiation and progression. The soluble mediators
secreted by tumor-associated leukocytes promote cell motility, and
induce angiogenesis, vascular dilation and extravasation of tumor cells.
Particularly interesting is the recent finding that metastatic cells
leave the tumor as microcolonies, containing lymphocytes and platelets
as well as the tumor cell. Inflammation continues to play a role at
metastatic sites by creating a cytokine milieu conducive to tumor
growth.

Although there is a strong association between chronic inflammation and
cancer, investigators have not yet uncovered all the molecules,
pathways, and mechanisms involved, and numerous questions remain to be
resolved about the mechanisms and targets of pro-inflammatory mediators
of tumor development. These are articulated in the body of the report
and the recommendations that follow. Furthermore, to understand the role
of inflammation in tumor formation and progression, we also need to
understand its role in maintaining homeostasis and responding to damage
in normal tissue. An appreciation of the importance of inflammation has
already led to clinical trials of anti-inflammatory drugs (e.g., COX-2
inhibitors) for cancer prophylaxis and treatment. The results obtained
will provide clues to the dominant mechanisms at work, and will help in
the design of a new generation of interventions.

**Introduction**

Inflammation involves a complex set of interactions between soluble
mediators and immunocytes, triggered in response to tissue injuries that
include trauma, infection, toxic agents and autoimmune responses. Such
injuries trigger a cascade of cellular infiltrations and cytokine
releases that result in local cellular proliferation and repair of
tissue damaged. While sustained proliferation alone is insufficient to
initiate cancer, a functional relationship between inflammation and
cancer has been recognized for a long time. The current discussions
centered on our current understanding of the role of inflammation in
cancer initiation, progression and metastasis and highlighted areas in
which there are major, unresolved questions.

# Discussion Themes

***I. Inflammation and Cancer Initiation***

Although inflammation is a necessary response to clear viral infections,
to repair tissue insults - either chemical exposure or injury- and
suppress tumor initiation/progression, chronic inflammation is also
clearly correlated with increased risk of developing cancer.
Inflammation may become chronic either because an inflammatory stimulus
persists or because of dysregulation in the control mechanisms that
normally turn the process off. Many of the cells, cytokines and systems
(e.g., leukocyte migration, dilatation of the local vasculature and
angiogenesis) involved in inflammation are also found in a variety of
tumors. Chronic inflammation caused by intestinal flora leading to the
inflammatory bowel diseases, ulcerative colitis and Crohn's disease, is
clearly linked with a higher incidence of colon cancer. The use of mouse
models has furthered our understanding of the contributing cellular and
molecular factors in colon cancer. Similarly, dietary intake of
proinflammatory carcinogens has been associated with prostate cancer.
Chronic inflammation resulting from esophageal reflux gives rise to
gastroesophageal reflux disease (GERD) and Barrett's esophagus, also
linked with a higher incidence of cancer. In the case of Barrett's,
chronic inflammation leads to the production of TNF. This, in turn,
induces the nuclear translocation of -catenin and transcriptional
activation of proliferative signals.

The molecular basis for the increased risk is thought to be two-fold: 1)
generation by inflammatory macrophages of reactive oxygen (ROS) and
nitrogen (RNS) species leads to DNA damage in the surrounding epithelial
cells and 2) enhanced proliferative signals mediated by cytokines
released by inflammatory cells increase the number of cells at risk for
mutations. In combination, DNA damage and proliferative signals create a
circumstance conducive to the development of cancer. ROS and RNS can
cause extensive damage to essential proteins (e.g., DNA repair enzymes),
to DNA and to the mitochondria through a series or cascade of reactions.
Among the many possible mutations that may result from oxidative DNA
damage are the formation of single- and/or double-stranded breaks and
the stimulation of recombination events. Free-radical damage can be
caused by the pro-inflammatory prostaglandin enzyme, cyclooxygenase 2
(COX-2), which leads to the production of highly reactive peroxide
intermediates at high levels in a local tissue environment. Drugs that
selectively inhibit the COX-2 enzyme, including NSAIDs, are being
studied to determine their impact on local tumor biology and
development, and in clinical trials. Recent studies have suggested
protective effects of COX-2 inhibitors in colorectal cancer and breast
cancer. Several small studies of colorectal, non-small cell lung cancer,
breast, cervical and esophageal tumors have shown that increased COX-2
levels are associated with poor clinical prognosis. Animal models for
colorectal cancer show similar patterns of COX-2 expression and response
to COX-2 inhibitors as human neoplasias.

Inflammation results in the recruitment of leukocytes secreting a
variety of proliferative cytokines and angiogenic factors to the site of
tissue insult. These cytokines, necessary for proper wound healing,
stimulate epithelial proliferation, which if unchecked could lead to
dysplasias and ultimately cancer. Paradoxically, cytokine deficiency
(e.g., GM-CSF, IL-2 and IFN) can also lead to tumor development. Immune
homeostasis consists of a succession of pro- and anti-inflammatory
signals. Loss of the anti-inflammatory signals leads to chronic
inflammation and proliferative signaling. The mechanisms involved in the
interplay of microbes and defective immune homeostasis is an area that
requires further delineation. Future steps will involve clinical studies
to determine whether individuals have polymorphisms or genetic
variations that affect specific cytokine pathways.

The discussion highlighted the duality of inflammation in controlling
and promoting tumor development. While chronic inflammation can
establish conditions conducive to tumor initiation and progression,
compelling data also suggest that the presence of lymphocytic
infiltrates in a variety of tumors is associated with a good clinical
outcome. The major challenge in this area is to understand the balance
between inflammatory tumor suppression and promotion and how to control
it.

***II. Inflammation and Cancer Progression***

It is generally accepted that chronic inflammation -- triggered by
toxins, microbes or autoimmune reactions -- plays a major role as a
tumor promoter. However, the precise function of inflammation in tumor
progression remains to be elucidated. Tumor cells produce various
cytokines and chemokines that attract leukocytes, which in turn produce
cytokines and chemokines that stimulate further tumor cell
proliferation; the inflammatory tumor microenvironment is characterized
by the presence of host leukocytes both in the stroma and around the
tumor. A developing neoplasm can contain a diverse leukocyte population,
including neutrophils, dendritic cells, macrophages, eosinophils, mast
cells and lymphocytes. These inflammatory cells secrete an array of
cytokines, interleukins, interferons and other soluble mediators and
further induce secretion of cytokines by resident stromal cells.

Interestingly, both cytokines that promote and suppress proliferation of
the tumor cells are produced. As in the case of cancer initiation, it is
the imbalance between the effects of these two classes of activity that
results in tumor promotion. For example, in the presence of GM-CSF and
IL-4, monocytes differentiate into immature dendritic cells, which
migrate into inflamed peripheral tissue, capture antigens and then
migrate to lymph nodes to stimulate T lymphocyte activation. Deletion of
GM-CSF from Polyoma T transgenic mice reduces cancer progression. This
is correlated with reduced macrophage infiltrates, which play a major
role in the transition from adenoma to carcinoma. GM-CSF vaccines that
stimulate dendritic cell and T cell responses are being combined with
anti-CTLA4 treatment in clinical trials to potentiate the anti-tumor
response.

In contrast, IL-6 and CSF-1 secreted by tumor cells can skew monocyte
differentiation towards the macrophage lineage. Although tumor
associated macrophages can kill tumor cells when activated by IL-2,
IL-12 or interferon, they also produce a host of compounds -- angiogenic
factors, growth factors, proteases and cytokines -- that either
contribute to cancer progression or blunt the anti-tumor response.
Macrophages generate a variety of proteases, including cathepsin B,
which contribute to tumor growth. Stromal fibroblasts and monocytes
enhance this proteolysis. During tumor progression, the degradation of
the matrix and stromal fibroblasts appears to be focal, suggesting that
widespread degradation may not be necessary for tumor growth. In mammary
cancer models, a variety of leukocytes are found at the tumor-stroma
interface. A complex interaction between tumor, stroma and inflammatory
cells results in the secretion of protease and matrix degradation and
the entrapment and degradation of fibroblasts by the tumor.

Other studies have indicated that hypoxia signaling pathways are engaged
very early in cancer development. Hypoxia stabilizes HIF-1 which in
turn induces VEGF secretion by epithelial cells that stimulates
microvascularization and angiogenesis.

The spatial relationship between inflammatory cells and tumors is now
being investigated by imaging of a range of live human tumor and
associated cells. Examination of the interaction reveals that although T
cells will home to the tumor, they stay on the periphery and do not
enter. Real-time images of mammary tumors in mice show that the tumor
regions are metabolically active, compared with surrounding stroma and
fat cells. These regions also have a much greater inflammatory response;
associated immune cells, particulary T cells, are very active and
mobile. In mammary cancer models, imaging reveals that tumor associated
macrophages preferentially line up along the lumenal side of the
tumor-associated vessels. Macrophages in this region are relatively
static, whereas those at the stromal interface are very active and
motile, suggesting differential behavior within the tumor. The
functional significance of this remains to be determined.

Although the role of inflammatory cells and soluble mediators in tumor
progression is now well documented, the details of the cellular and
molecular interplay between stroma and tumor progression remain to be
elucidated.

***III. Inflammation and Metastasis***

Unlike tumor progression, where the role of inflammation in promoting
cancer cell proliferation and stromal/matrix degradation is reasonably
well understood, the role of inflammation in metastasis is less well
defined, although appears to be important. The cytokines and chemokines
secreted by tumor associated macrophages and leukocytes promote cell
motility and induce angiogenesis and the growth of tumor-associated
vessels, providing an egress route for metastatic tumor cells. The
leukocytes also promote vessel dilation and extravasation of tumor
cells. Particularly intriguing is the observation that metastatic cells
leave the tumor as microcolonies, containing lymphocytes and platelets,
the latter allowing attachment to distal organ sites. Tumors that are
unable to form such microcolonies are not malignant.

At the distal, metastatic sites, evidence suggests that inflammation
continues to play a role in the establishment of metastases. At the
sites of prostate cancer metastases in the bone, inflammation triggers
the secretion of TGF-β by osteoclasts. TGF-β in turn induces the cancer
cells to secrete PDGF which further stimulates the osteoclasts, leading
to bone degradation and stimulation of cancer cell growth. PDGFR on
tumor-associated endothelial cells increases their levels of bcl-2 and
bcl-xl, rendering them resistant to apoptosis and chemotherapy.
Similarly, in brain metastases of melanoma, astrocytic infiltrates
upregulate MDR (multiple drug resistance) in the tumor cells, making
them more resistant to chemotherapy.

**Next Steps and Important Questions**

-   Despite the evidence for the role of inflammation in cancer
    intiation, progression and metastases, other evidence suggests that
    cancer proceeds through inflammation-dependent and independent
    stages. These stages need to be defined and characterized.

-   To better understand the critical role of inflammation in initiating
    and modulating tumor behavior, host-pathogen interactions need to be
    defined at a molecular level, the phenotypes of hematopoeitic cells
    (leukocytes, monocytes, platelets, etc) involved in wound repair and
    tumor initiation need to be characterized and the roles of
    endocrinological mediators on inflammation need to be examined.

-   To better understand the role of inflammation in tumor progression,
    tumor stage should be correlated with intensity and repertoire of
    hematopoietic infiltrates and with the levels of cytokines and
    proteases present; biomarkers for pre-malignant and malignant
    lesions need to be identified and validated.

-   To better understand the role of inflammation in metastasis, better
    pre-clinical models need to be developed, the molecular relationship
    between primary and metastatic tumor cells needs to be resolved, the
    nature of the inflammatory responses that influence primary versus
    metastatic tumor need to be determined, and the role of the
    hematopoietic network in tumor extravasation and migration needs to
    be elucidated.

-   Real-time imaging models need to be refined and extended to allow
    better definition of the relationship between inflammatory and tumor
    cells that influence the cancer initiation, progression and
    metastasis process.

> Tumor immunotherapy approaches should include targeted intervention of
> inflammation-mediated growth, pairing molecular information of
> inflammatory infiltrates with that of specific tumors. Therapy should
> be specifically directed to both the organ microenvironment and the
> tumor.
>
> **Specific Recommendations for the NCI:**

-   Continue to support basic research aimed at characterizing the role
    of inflammation at all stages of cancer and specifically at
    determining the role of hematopoeitic cells in both metastasis and
    normal development.

-   Encourage collaborative studies between basic and clinical
    investigators. Support studies that characterize inflammatory cells
    in the tumor microenvironment and correlate these with clinical
    outcomes and prognosis

-   Consider mechanisms to stimulate multi-agency, multi-institutional
    and transdisciplinary collaborations to more rigorously define
    critical interactions that occur between tumor cells and their
    inflammatory microenvironment.

-   Sponsor a series of workshops and/or interactive fora on
    inflammation and cancer that interface experts from different
    disciplines (i.e., toxicologists, cellular and tumor immunologists,
    systems biologists, cancer biologists).

-   Develop and standardize reagents and protocols for analysis of
    archived tissues. Establish a uniform database of existing reagents
    and well-defined and catalogued tumor types that are or are not
    associated with inflammation.

-   Establish and make available conditional tissue- or cell-specific
    pre-clinical models to study the biology of inflammation-dependent
    cancers and to stimulate novel prevention, diagnostic and
    therapeutic strategies.

-   Consider mechanisms to provide investigators with

    -   access to imaging tools for in vitro and in vivo analysis to
        characterize immunocyte/tumor interactions.

    -   more sophisticated and cheaper imaging modalities, perhaps by
        encouraging development in the private sector through the SBIR
        mechanism

    -   training in the use and application of imaging techniques