ABSTRACT
Macrophages are mononuclear phagocytes that are derived from precursor stem cells in the bone marrow, and are recruited from the circulation to sites of injury, where they play key roles in both inflammation and tissue repair. As inflammatory cells, macrophages recognize and destroy micro-organisms, either directly or by presenting antigens to lymphocytes, thus activating acquired immunity. As cells mediating tissue repair, macrophages produce chemokines and growth factors that regulate the migration, proliferation and biosynthetic activity of connective tissue cells, as well as angiogenic factors that mediate neovascularization. To mediate these diverse roles, macrophages exhibit exquisite plasticity in terms of their gene expression profile, and adopt markedly different activation phenotypes, depending upon the nature of the micro-environment in which they reside. Classicallyactivated macrophages are induced by interferon gamma (IFNg) and Tolllike receptor (TLR) agonists to express inflammatory cytokines, including tumor necrosis factor-a (TNFa), IL-1, IL-6 and IL-12, as well activated oxygen radicals and nitric oxide (NO) (Nathan 1991; Stein et al. 1992; Ehrt et al. 2001; Domachowske et al. 2002; Ma et al. 2003). This phenotype has been termed “M1”, and M1 macrophages act as strong promoters of the type 1 helper T (Th1) immune response. The persistent presence of M1 macrophages can be detrimental to the host, by causing DNA damage that might contribute to cancer development. In contrast, in response to a different set of microenvironmental signals, macrophages can be activated through distinct pathways to acquire anti-inflammatory, wound healing, and angiogenic phenotypes. These phenotypes have been generically termed “M2”, and M2 macrophages express the anti-inflammatory cytokine IL-10, and the angiogenic growth factor VEGF rather than inflammatory cytokines (Goerdt et al. 1999; Mantovani 2006; Mosser and Edwards 2008; Classen et al. 2009; Martinez et al. 2009; Gordon and Martinez 2010). In an effort to promote survival, tumor cells can evade innate and adaptive immune responses by hijacking macrophages and inducing them to adopt an M2 phenotype. These M2 macrophages then play an anti-inflammatory, immunosuppressive and angiogenic role, thus promoting a pro-tumoral environment. The M2 designation of macrophages is further subdivided into M2a (activated by IL-4 or IL-13 through the IL-4Ra sub-unit), M2b (induced by immune complexes and IL-1b or TLR stimulation), and M2c (stimulated by glucocorticoids, IL-10 or TGF-b) (Martinez et al. 2009). In addition, we have found that production of inflammatory cytokines and VEGF by macrophages is strongly regulated by adenosine, a ubiquitous metabolite which is induced by conditions of ischemia and hypoxia (Hasko et al. 1996; Hasko et al. 2000; Pinhal-Enfield et al. 2003; Hasko and Cronstein 2004). Adenosine synergizes with TLR agonists such as endotoxin (LPS) to strongly down-regulate the expression of TNFa and other inflammatory chemokines and cytokines, and to up-regulate the expression of IL-10,
VEGF and other anti-inflammatory and angiogenic factors (Leibovich et al. 2002). The synergy between TLR and adenosine receptor signaling in these alternatively activated macrophages results in an “angiogenic switch” that converts macrophages from an inflammatory to an angiogenic phenotype (Leibovich 2007). We have termed macrophages activated in this manner “M2d” (Enfield and Leibovich 2011) (Fig. 1). Several studies have shown that TAMs often exhibit an M2-like phenotype, with reduced antimicrobial and anti-tumoral activity and increased production of mediators such as IL-10, TGFb and VEGF, that suppress immunity and promote angiogenesis (Mantovani and Sica 2010). Since tumors exhibit regions of hypoxia and ischemia and thus generate high levels of adenosine, we propose that tumor cells utilize the adenosine receptor-TLR synergy to switch macrophages from an inflammatory to an angiogenic state. This chapter focuses on reviewing the importance of adenosine receptor and TLR signaling in macrophages and its role in regulating angiogenesis and tumor progression. Understanding the mechanisms involved in the angiogenic switch of macrophages may provide insight leading to the development of novel therapeutics for inflammation, wound healing and cancer.
