Role of macrophages in tumour progression.

It is now becoming clear that the inflammatory cells that exist in the tumour microenvironment play an indispensable role in cancer progression. Tumour associated macrophages (TAMs) represent a prominent component of the mononuclear leukocyte population of solid tumours, which displays an ambivalent relationship with tumours. They originate in the circulation and are recruited to the tumour site by tumour-derived attractants such as chemokines and interact with the tumour cells and preferentially localize at the tumour-host tissue interface, in regions often associated with low oxygen tensions. The tumour microenvironment, including cytokines and hypoxia, regulates the localization and function of TAMs. Upon activated by cancer cells, the TAMs can release a vast diversity of growth factors, proteolytic enzymes, cytokines, and inflammatory mediators. Many of these factors are key agents in cancer metastasis. Substantial evidence suggests that TAMs can interact with cancer cells, modify the ECM, and promote cancer cell invasion and metastasis. Several natural products have shown ability to inhibit the production of proinflammatory cytokines and growth factors by TAMs. The presence of extensive TAM infiltration has been shown to correlate with cancer metastasis and poor prognosis in a variety of human carcinomas.