Role of macrophage polarization in tumor angiogenesis and vessel normalization: implications for new anticancer therapies.

Angiogenesis, the formation of new capillary blood vessels from preexisting vasculature, is one of the hallmarks of cancer that is pivotal for tumor growth and metastasis. Tumor vessels are known to be abnormal, with typically aberrant, leaky and disordered vessels. Thus, the combination of angiogenesis inhibition and vessel normalization is a potential strategy for anticancer therapy. The solid tumor is composed of not only cancer cells, but also the nonmalignant resident stromal cells, such as bone-marrow-derived cells (BMDCs) and cancer-associated fibroblasts (CAFs). Tumor-associated macrophages (TAMs) are the most abundant cell components of BMDCs, which play a significant role in promoting tumor progression. Accumulating evidences from both patient biopsies and experimental animal models have shown that TAMs function in tumor angiogenesis and vessel abnormalization in a density- and phenotype-dependent manner. This chapter will discuss the evidence for the factors and signaling pathways that are involved in macrophage recruitment and polarization in the tumor microenvironment, and it summarizes the role and underlying molecular mechanisms of macrophage polarization in tumor angiogenesis and vessel normalization. In addition, an overview of the potential of targeting TAM polarization for anticancer therapy will be provided.