The influence of the pre-metastatic niche on breast cancer metastasis.

The emergence of metastatic disease constitutes a significant life-threatening development during cancer progression. To date, intensive efforts have been focused on understanding the intrinsic properties that confer malignant potential to cancer cells, as well as the role of the primary tumour microenvironment in promoting cancer metastasis. Beyond events occurring at the primary site, the metastatic cascade is composed of numerous barriers that must be overcome in order for disseminating cancer cells to form distal metastases. The most formidable of these is the ability of cancer cells to seed and grow in a completely foreign microenvironment. Interestingly, solid malignancies often display a particular tropism for specific tissue sites. For example, breast patients with metastatic disease will often develop bone, lung, liver or brain metastases. This mini-review will explore aspects of pre-existing and induced metastatic niches and focus on how the unique composition and function of diverse niche components, within common sites of breast cancer metastasis, enable the survival and growth of disseminated cancer cells. These common supportive functions of the niche are provided by a complex array of stromal components and molecular mechanisms that are, in part, reflective of the tissue in which the metastases arise. Finally, the metastatic niche is a dynamic structure that is continually altered and sculpted by the cancer cells during progression of the metastatic lesion.