Smoldering inflammation is a component of the tumor microenvironment and represents the seventh hallmark of cancer. Tumor-associated macrophages (TAM) have served as a paradigm for cancer-promoting inflammation. TAM orchestrate various aspects of cancer, including: diversion and skewing of adaptive responses, cell growth; angiogenesis, matrix deposition and remodeling, the construction of a metastatic niche and actual metastasis, and response to hormones and chemotherapeutic agents. Recent studies on human and murine tumors indicate that TAM show a remarkable degree of plasticity and functional heterogeneity, during tumor development. Evidence is accumulating for a switch in macrophage phenotypes during the course of tumor progression. Whereas the functions of classically activated, M1 macrophages during chronic inflammation appear to predispose a given tissue to tumor initiation, in established tumors macrophages exhibit mainly the alternatively activated, M2 phenotype and resemble tolerant macrophages and are engaged in immunosuppression and the promotion of tumor angiogenesis and metastasis. Here we discuss the evidence suggesting that selected members of the NF-κB family of transcription factors play a central role in the functional reprogramming of TAM during the course of tumor progression. Finally, we discuss the implications of this phenomenon for anti-cancer therapies aimed at prompting TAM to mount an effective antitumor response.
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