**1. Introduction**

In the past decades the major focus of cancer research has been the transformed tumor cells itself, while the role of cellular microenvironment in tumorigenesis has not been widely explored. Several studies have demonstrated the ability of stroma to regulate the growth and differentiation state of breast cancer cells, and the invasive behaviour, and polarity of normal mammary epithelial and breast carcinomas are influenced by tumor microenvironment, immune and stromal cells (Bissell, et al., 2002, Radisky & Radisky, 2007, Tlsty, 2001, Tlsty & Hein, 2001). In addition, genetic abnormalities, such as loss of heterozygosity, occur not only in cancer cells, but in stromal cells as well (Kurose, et al., 2002, Kurose, et al., 2001, Moinfar, et al., 2000).

It is believed that a better understanding of the tumor microenvironment could help render more accurate diagnostics or assist in predicting tumor aggressiveness (i.e., bad prognosis) thus facilitating the design of personalized treatments.

By the end of the nineteenth century, the English surgeon S. Paget suggested the idea that, in order for breast cancer to develop, a specific "seeding" process must occur and, for this primary onset to metastasize to a specific distant organ, particular stromal features would be required postulating his "seed and soil" hypothesis (Paget, 1889). His work greatly contributed to somewhat earlier observations by T. Langhans who first used the word stroma to describe the connective tissue, vessels and other components between tumors (Langhans, 1879) and to the theory postulated by R. Virchow suggesting a possible origin of cancer at sites of chronic inflammation (Balkwill& Mantovani, 2001). A century later, researchers such as B. Mintz and K. Illmensee in general, as well as M. Bisell, in breast cancer in particular, pointed to the tumor milieu as an essential component of neoplasias, not only for cancer evolution but also for cancer instigation (Mintz & Illmensee, 1975; Lochter & Bissell, 1995). Together these and additional findings had painted a broad picture of the complexity of tumor microenvironment, where diverse stromal cells interact with

Novel Insights Into the Role of Inflammation in Promoting Breast Cancer Development 131

The presence and significance of leukocyte infiltrates in developing neoplasms is now undisputed (Allen, et al., 2007, Moser & Loetscher, 2001, Moser & Willimann, 2004). It has been demonstrated that leukocyte infiltration in developing tumors is one of the host´s main immune mechanisms to eradicate malignant cells. However, while some leukocytes certainly have this potential, i.e., cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells (Luster, 1998), other leukocyte cell types, most notably innate immune cells, i.e., mast cells (MCs), immature myeloid cells, granulocytes, and macrophages, instead potentiate tumor progression (Baggiolini, et al., 1997, Chen, et al., 2006, Joyce & Pollard, 2009), and enhance neoplastic cell survival. Upon entry into the neoplastic microenvironment, infiltrating leukocytes become alternatively activated and manifest a pro-tumor phenotype as defined by activation of cellular programs involved in immune tolerance and tissue remodelling (Mishra, et al., 2011, Strieter, et al., 2006). During premalignant progression, a consequence of alternative activation of leukocytes is promotion and elaboration of a microenvironment rich in extracellular matrix (ECM) remodelling proteases, and increased presence of pro-survival, pro-growth and pro-angiogenic factors that further enhance proliferative and invasive capacities of neoplastic cells (Li, et al., 2007, Orimo, et al., 2005). Such pro-tumor inflammatory microenvironments promote not only malignant conversion and development of solid tumors, but also dissemination of neoplastic cells into blood vasculature by driving invasive capacity of malignant cells, expansion of angiogenic vasculature, and neoplastic cell entry into blood vessels (and lymphatics) (Keeley, et al.,

Breast carcinomas are highly infiltrated by different types of host leukocytes, including primarily T cells, and monocytes that differentiate into tumor-associated macrophages (TAM) at the tumor site (Ben-Baruch, 2003, Crowther, et al., 2001). The presence of the cellular infiltrate in breast tumors was initially regarded as evidence for the potential activity of immune mechanisms against the growing neoplasm. As explained above, several studies suggest that T-cell antitumor responses are impaired in advanced stages of breast carcinoma, and there is no definite conclusion regarding the efficacy of T-cell-dependent immune mechanisms, or regarding the correlation between the type of T-cell infiltration and tumor progression in most subtypes of breast carcinoma (Hsiao, et al., 2010). The only exception is the relatively infrequent type of medullary carcinoma, in which favourable prognosis was correlated with intensive lymphoid infiltration (Hadden, 1999). In contrast to T lymphocytes, large evidence suggests that high levels of TAM are correlated with poor prognosis in breast carcinoma. Many studies have shown a positive relationship between high levels of TAM and lymph node metastases, and suggested that the density of TAM is associated with clinical aggressiveness (Crowther, et al., 2001, O'Sullivan & Lewis, 1994). Again, the potential contribution of TAM to tumor elimination, in view of several potential antimalignant activities that may be exerted by these cells, such as antigen presentation, cytotoxicity, or/and phagocytosis, was contradictory with the promalignant activities of TAM in breast carcinoma. These promalignant activities of TAM are the result of their ability to express numerous tumor-promoting mediators, such as growth factors for breast tumor cells, angiogenic molecules, ECM degrading enzymes, inflammatory cytokines, and chemokines (Balkwill & Mantovani, 2001, Colotta, et al., 2009). In addition, TAM might contribute to tumor progression by the release of reactive oxygen intermediates, which may induce mutagenic changes that could result in increased DNA damage and generation of new subtypes of cancer cells within the tumor (Colotta, et al., 2009). A major TAM-derived

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each other and with the cancer cells playing important roles in tumorigenesis (Soto & Sonnenschein, 2004; Egeblad et al., 2010).

It is clear now that metastatic tumors represent the greatest threat to cancer patient mortality. Indeed, when breast cancer is diagnosed early and metastases are not present, 5 year survival is >88%; however, if metastases are also present, long-term survival is significantly diminished (~10%) (Jemal, et al., 2011). Thereby, the major cause of mortality of breast cancer and different types of cancer is due to metastasis to distant organs, such as lung, bone, liver and brain (Lu & Kang, 2007). A notable feature of this process is the variation in metastatic organ tropism displayed by different types of cancer (Chambers, et al., 2002, Fidler, 2002). A classic view has proposed that purely mechanical factors regulate the fate of blood-borne metastasis tumor cells (MacDonald, et al., 2002); however, this does not fully explain the non-random distribution and distinct pattern of metastasis in each tumor type (Lu & Kang, 2007). However, tumor microenvironment has also shown an important role in the regulation of this process (Valdivia-Silva, et al., 2009). A number of different molecules present in the microenvironment have been associated to the metastasis of breast cancer, among them, chemokines, which have been associated with regulation of cell migration and invasion of tumor cells into specific organs (Muller, et al., 2001, Zlotnik, 2006). Chemokines are a superfamily of chemotactic cytokines characterized by their ability to induce directed migration of leukocytes, during haematopoiesis, lymphoid organ development, and in disease (Sallusto, et al., 2000); their expression may be inducible, primarily by pro-inflammatory cytokines such as TNF-a and IL-1-b (Ben-Baruch, 2003). Chemokine receptor expression in many cancer cells have shown to be a non-random process (Shields, et al., 2007, Zlotnik, 2006) and to have a role in organ-specific metastasis: for example, CXCR4 expression and metastasis to lung, bone and lymph nodes (Muller, et al., 2001), CCR7 to lymph nodes (Shields, et al., 2007), CX3CR1 to brain (Mourad, et al., 2005), CCR9 to liver and small bowel (Amersi, et al., 2008, Letsch, et al., 2004), and CCR5 and CXCR2 to lung, liver, vessel endothelial cells and bone (Gross & Meier, 2009, Keeley, et al., 2010, Miller, et al., 1998).

Here, we will discuss the ability of the chemokines to affect tumor cell–microenvironment interactions, increasing the invasive behaviour and metastasis, confirming the importance of the host inflammatory response that may differ between tumor types, disease stages, and/or many other host factors; and the role of stromal contribution of the inflammatory microenvironment to cancer progression and metastasis.
