Priming the 'soil' for breast cancer metastasis: the pre-metastatic niche

The long prevailing model of metastasis recognizes the importance of both "seed" and "soil" for metastatic progression [1]. Much attention has focused on understanding the molecular and genetic factors that confer an intrinsic metastatic advantage to certain tumor cells. Meanwhile, changes occurring within distant tissues, creating a "soil" conducive for tumor invasion, have been largely neglected. Bone marrow-derived hematopoietic progenitor cells (HPCs) recently emerged as key players in initiating these early changes, creating a receptive microenvironment at designated sites for distant tumor growth and establishing the "Pre-Metastatic Niche" [2]. This insight into the earliest stages in the metastatic cascade revises our concept of the metastatic "microenvironment" to include physiological cells recruited from the bone marrow. Moreover, the concept of pre-metastatic tissues as 'niches' similar to physiological stem cell niches establishes a paradigm in which disseminated tumor cells may reside within a highly defined microcosm, both supportive and regulatory, and which may confer specific functions on indwelling cells. Understanding the cellular and molecular cross-talk between "seed" and "soil" may further our understanding of the factors that govern both site-specific patterning in metastasis and the phenomenon of tumor dormancy. This may lead to therapeutic strategies to detect and prevent metastasis at its earliest inception.