Tumor microenvironment: a main actor in the metastasis process

Over recent decades, various studies have argued that the metastatic tissue microenvironment is fully controlled by the intrinsic properties of the cancer cells (growth, motility and invasion, angiogenesis, extracellular matrix remodeling, immune escape) and additional cells types. Overall, the extrinsic factors and determinants medi- ate the contribution of the host microenvironment to metastasis formation. The tumor microenvironment carries out these functions by secretion of molecules that can influence and modulate its phenotype, making these complex interactions the basis for support for the progression of a cancer. Here, we undertake a summary of the ‘‘state of the art’’ of the functions and actions of these cells, as the main actors in the promotion of the formation of the microenvi- ronment of the metastatic niche, and the associated network of interactions. The unraveling of the relationships between tumorigenic cells and their microenvironment represents an important issue for the development of new therapeutic agents that can fight both initiation and recurrence of cancer.