The stroma consists of the extracellular matrix (ECM), which is composed of proteoglycans, hyaluronic acid, and fibrous proteins such as collagen, fibronectin, and laminin; growth factors, chemokines, cytokines, antibodies, and metabolites; and mesenchymal supporting cells (e.g., fibroblasts and adipocytes), cells of the vascular system, and cells of the immune system (Fig. 1). As tumors develop, the stroma also evolves.1–6
COMPOSITION OF THE STROMA
Cancer cells produce factors that activate and recruit carcinoma-associated fibroblasts, which are an activated fibroblast subtype (myofibroblasts).7 Carcinoma-associated fibroblasts resemble mesenchymal progenitors or embryonic fibroblasts8 and are able to stimulate cancer cell growth and invasion as well as inflammation and angiogenesis.7,9,10 In some systems, they may also be tumor inhibiting.7,11 Carcinoma-associated fibroblasts activated by the tumor microenvironment are largely responsible for tumor-associated changes in the ECM including increased ECM synthesis and remodeling of ECM proteins by proteinases, for example, matrix metalloproteinases (MMPs).12,13 The altered ECM then influences tumor progression by architectural and signaling interactions.14
Several ECM proteins such as tenascin C and an alternatively spliced version of fibronectin expressed embryonically during organ development are re-expressed during tumor progression.15 Fibrillar type I collagen also increases in tumors.16 Fragments of type I collagen or laminin 332 produced as a result of MMP cleavage may be tumor promoting by stimulating cellular migration and survival.17,18 The biophysical characteristics of tissues, such as stiffness, may affect cellular function. Mammary epithelial cells cultured in compliant collagen matrices form polarized acini, whereas in rigid matrices, they lose polarity and become proliferative and invasive.4,19
Inflammatory responses are associated with many cancers and may facilitate tumor progression.20 Both adaptive and innate immune cells infiltrate into tissues and are critical players.21–25 Whereas the innate immune compartment is primarily tumor promoting, the adaptive immune compartment (B and T cells) can be tumor suppressing.
The adaptive immune compartment (B and T cells) carries out immune surveillance, keeping initiated cancer cells in check.22,26 Indeed, patients with a suppressed adaptive immune system have an increased risk of developing cancers.27 CD4+ T cells are key regulators of the immune system and differentiate into various T-helper cell lineages: interferon γ–producing TH1 cells that promote cell-mediated immunity and interleukin 4 (IL-4)–producing T helper 2 (TH2) cells that support humoral immune responses.28 Both TH1 and TH2 cells can enhance antitumor immunity by expanding the cytotoxic CD8+ T-cell (CTC) population. In contrast, regulatory T (Tregs) cells suppress antitumor immunity by inhibiting cytotoxic T cells. TH17 cells secrete IL-17. Whereas TH1 cells are primarily antitumor, TH2 cells promote tumors through their cytokines, which polarize tumor-associated macrophages (TAMs) to promote cancer progression.21 CD4+ Tregs are immune suppressive, directly suppressing antitumor immunity of CD8+ cytotoxic T cells via secretion of IL-10 and transforming growth factor β. Depletion of Tregs enhances tumor growth.28 CD4+ TH17 cells play roles in inflammation and tumor immunity.29 TH17 cells develop from naive CD4+ T cells in the presence of transforming growth factor β, IL-6, and IL-1β. Whether TH17 cells adopt a protumorigenic or antitumorigenic role depends on the stimuli encountered by the cells.
Myeloid-derived innate cells (e.g., macrophages, neutrophils, and mast cells) are largely responsible for inflammatory reactions (Fig. 2). Monocytes are polarized to M1 macrophages by cytokines secreted from TH1 cells such as interferon γ, tumor necrosis factor α, and granulocyte-monocyte colony-stimulating factor; produce reactive oxygen and nitrogen intermediates and inflammatory cytokines; and are antitumor.30,31 In contrast, monocytes exposed to cytokines secreted from TH2 cells such as IL-4 and IL-13 become polarized toward the M2 macrophage phenotype. However, this classification does not accurately define the differentiated state of macrophages exposed to the complex in vivo environments. Tumor-associated macrophages mostly resemble M2 macrophages. Accumulation of TAMs is associated with poor prognosis.32,33
Neutrophils can inhibit or promote cancer development.24,34 With increased tumor burden, activated neutrophils accumulate in bone marrow, spleen, and peripheral blood and, at the invasive tumor front,24,35,36 promote cancer metastasis by inhibiting cytotoxic T cells and promoting angiogenesis. Mast cells can also drive tumor progression.37,38
Recruiting vasculature is a critical step in tumor development. Tumor-infiltrating myeloid-derived cells are major sources of proangiogenic factors.39,40 These myeloid cells also regulate the resistance of tumors to antiangiogenic therapy.41
Tumors have specialized niches, or microenvironments regulate functions of the cancer cells. Genes involved in regulation of stem cell niches also play a role in cancer.42,43 Like stem cell–like niches that require Wnt signaling for self-renewal of intestinal stem cells, activation of the Wnt pathway by inactivating mutations in the APC gene results in colorectal cancers.44 Alteration of the microenvironment through genetic mutation in SMAD4 in the stroma also results in gastrointestinal epithelial cancer.45
How the microenvironment directs tumor development is just beginning to be elucidated. One clue is that the percentage of cancer cells that express stem cell or basal markers increases when the cells are grown on type I collagen.46,47 Moreover, invasive cells are often associated with fibrillar collagen in vivo,47 and cancer stem-like cells may be enriched at the invasive front where the highest levels of type I collagen are found.48
Certain microenvironments can restrict tumor progression. In the presence of a basement membrane–like matrix, breast tumor cells behave more like normal epithelial cells.14 Antibodies to β1 integrins that increase in malignant cells can normalize the malignant phenotype of cancer cells both in culture and in vivo.49 In essence, ECM molecules that maintain normal tissue architecture and cancer cells quiescence may be tumor suppressors (Fig. 3).
The organ specificity of metastasis correlates with specific gene expression of the disseminating cancer cells.50 This raises the question of whether the specific organ microenvironments are matched to specific needs of cancer cells. Moreover, primary tumors alter the distant microenvironmental niches through secreted factors and exosomes, making them more amenable for colonization.24,51,52 Because exosomes and other microvesicles may be taken up in a cell-specific manner, they may be involved in selecting specific metastatic sites.53
THE TUMOR MICROENVIRONMENT AND RESPONSE TO CHEMOTHERAPY
The tumor microenvironment is critical in the response chemotherapy.54 Cancer cells may be more distant from blood vessels, impairing drug penetration. Tumors may have incompetent blood vessels and decreased lymphatics, resulting in increased interstitial fluid pressure, which inhibits diffusion of drugs into the tumor. The properties of the microenvironment including ECM composition and hypoxia may alter the phenotype of the tumor cells, resulting in decreased drug uptake. This raises the question of whether chemoresistance may be overcome by targeting the microenvironment. Altering vascular permeability by affecting MMPs may increase drug delivery.55,56 Blocking fibroblast-activating protein, which is made by carcinoma-associated fibroblasts, results in reduced type I collagen and improved drug delivery.57
Components of the tumor microenvironment contribute to both the establishment of primary tumors as well as to the initiation, establishment, and growth of metastases. However, there may be different requirements for the primary tumor compared with the distant environment. In many ways, the tumor microenvironment resembles the microenvironments used in development and tissue repair. The tumor-associated stromal cells (e.g., macrophages and fibroblasts) are different from their counterparts in the normal tissue.58 They may be newly recruited from the bone marrow and have more embryonic character, but they are also misregulated. Altering the tumor microenvironment therapeutically has promise for improving the cancer therapy on a widespread basis.
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