**1. Introduction**

Solid tumors comprise not only cancer cells but also host stromal cells, such as vascular cells, inflammatory/immune cells, and cancer-associated fibroblasts. The crosstalk between cancer cells and stromal cells plays an important role in tumor growth, metastasis, and response to antitumor therapy (Hanahan and Weinberg, 2011; Joyce and Pollard, 2009; Petrulio et al., 2006). Cancer cells with oncogenic mutations are central to tumor formation. Endothelial cells in tumors form new blood vessels (angiogenesis) which bring oxygen and nutrients to the growing tumor (Ferrara and Kerbel, 2005), and also regulate leukocyte infiltration and tumor cell metastasis (Chouaib et al., 2010). Inflammatory cells have both tumor-promoting and tumor-preventing effects (Grivennikov et al., 2010; Hanahan and Weinberg, 2011). Fibroblasts are the most abundant cells in the tumor stroma and have been demonstrated to have tumor-promoting activities (Bhowmick et al., 2004). Moreover, cancer cells within tumors are heterogeneous and composed of distinct subpopulations with different states of tumorigenicity. One subpopulation of cells that has recently been extensively studied is the cancer initiating cell or cancer stem cell (CSC) (Cho and Clarke, 2008), which exhibits high capacity of generating new tumors.

The microenvironment in solid tumors is very distinct from that in normal tissues. Due to deregulated cancer cell metabolism, highly heterogeneous vasculature and defective blood perfusion, the tumor microenvironment is characterized by hypoxia and acidosis (Cairns et al., 2006; Gatenby et al., 2006; Gatenby and Gillies, 2004). The uncontrolled proliferation of tumor cells results in a growing mass that rapidly consumes oxygen, glucose and nutrients (Gatenby and Gillies, 2004). When an oxygen diffusion limit is reached, some regions of a tumor become hypoxic. Cancer cells rely heavily upon glycolysis ('Warburg effect') to generate ATP and metabolic intermediates for biosynthesis (Gatenby and Gillies, 2004; Vander Heiden et al., 2009). There is much evidence to link the connection between the adaptation to hypoxia and the development of an aggressive tumor phenotype in both experimental and clinical settings (Chang et al., 2011; Gatenby and Gillies, 2004). In addition to hypoxia, the existence of acidosis is a defining hallmark of the tumor microenvironment.

Targeting Tumor Microenvironments for Cancer Prevention and Therapy 5

TECs are different from endothelial cells in normal tissues at several aspects. It has been reported that human hepatocellular carcinoma-derived endothelial cells, when compared to the ones from adjacent normal liver tissue, show increased apoptosis resistance, enhanced angiogenic activity and acquire more resistance to the combination of angiogenesis inhibitor with chemotherapeutic drugs (Xiong et al., 2009). Studies have also revealed distinct gene expression profiles of TECs and identified cell-surface markers distinguishing tumor versus

In blood vessels, pericytes are smooth muscle cell-like cells that cover the vascular tube. They are intimately associated with endothelial cells and embedded within the vascular basement membrane, and play an important role in the maintenance of blood vessel integrity. Pericytes in tumors are different from normal ones: in tumors, pericytes are often less abundant and more loosely attached to the endothelial layer (Abramsson et al., 2002; Morikawa et al., 2002). The abnormality in pericytes weakens the vessel wall and increases vessel leakiness. Pericytes express several markers, though none is pericyte-exclusive, including α-smooth muscle actin (αSMA), platelet-derived growth factor receptor-β (PDGFR-β) and NG2 (Gerhardt and Betsholtz, 2003; McDonald and Choyke, 2003). PDGF-B signaling is important for pericyte recruitment and attachment to endothelial cells during

Tumors are often infiltrated by inflammatory cells, such as macrophages, neutrophils, lymphocytes, mast cells, and myeloid progenitors. This phenomenon was initially observed by Rudolf Virchow more than a century ago and thought as an immunological response attempting to eliminate cancer cells. Whereas immune cells play a role in recognizing and eradicating early cancer cells (Kim et al., 2007), mounting evidence has also shown that inflammatory cells within tumors can enhance tumor initiation and progression by helping cancer cells acquire hallmark capabilities (Grivennikov et al., 2010; Hanahan and Weinberg, 2011). Inflammation is considered as an 'enabling characteristic' of tumor biology (Hanahan

Pathological studies show that the abundance of certain types of infiltrating inflammatory cells, such as macrophages, neutrophils and mast cells, is correlated with poor prognosis of cancer patients (Murdoch et al., 2008). Tumor associated macrophages (TAMs), along with mast cells, neutrophils and other immune cells, produce cytokines (e.g. TNFα and IL-1), chemokines (e.g. CCL2 and CXCL12), angiogenic factors (e.g. VEGF, PDGF, FGF and IL-8), and matrix-degrading enzymes (e.g. MMPs, cathepsin proteases and heparanase) (Grivennikov et al., 2010; Karnoub and Weinberg, 2006). Some inflammatory cells, particularly neutrophils, also generate reactive oxygen and nitrogen species. These bioactive factors promote cancer cell proliferation, invasion and resistance to apoptosis through, for instance, the interleukin-JAK/STAT pathway (Ara and Declerck, 2010), and induce new blood vessel formation in the tumor. Extracelluar matrix-degrading enzymes promote cancer cell invasion and metastasis, whereas accumulation of reactive oxygen and nitrogen species can cause DNA mutagenesis, suppress DNA repair enzymes, increase genomic

vascular development (Abramsson et al., 2002; Abramsson et al., 2003).

normal endothelial cells (Seaman et al., 2007).

**2.1.2 Inflammatory/immune cells** 

instability, and aggravate cancer progression.

and Weinberg, 2011).

This condition arises mainly due to an increase in the production of lactic acid by glycolysis along with other proton sources (Gatenby and Gillies, 2004; Helmlinger et al., 2002; Yamagata et al., 1998). Acidosis is a selection force for cancer cell somatic evolution, modulates cancer cell invasion and metastasis, and affects the efficacy of some chemotherapeutic drugs (Cairns et al., 2006; Gatenby et al., 2006; Gatenby and Gillies, 2004).

Here we will describe cellular heterogeneity, hypoxia, and acidosis in the tumor microenvironment, and discuss some recent progresses in targeting tumor angiogenesis, inflammation, hypoxia and acidosis-related pathways for cancer prevention and therapy.
