Bone marrow cells in the 'pre-metastatic niche': within bone and beyond

Metastasis, the spread of invasive carcinoma to sites distant from the primary tumor, is responsible for the majority of cancer-related deaths (Weigelt, B., Peterse, J. L., & van 't Veer, L. J. (2005). Breast cancer metastasis: Markers and models. Nature Reviews. Cancer, 5, 591-602). Despite progress in other areas of cancer therapeutics, the complexities of this process remain poorly understood. Consequently, there are few successful treatments that directly target this stage of carcinogenesis. Particularly enigmatic is the tissue-specificity of different tumor types observed in metastatic spread. One example is the predilection of colon cancer to spread to liver whereas breast, prostate, and lung carcinomas have a particular affinity to target and proliferate in bone. In 1889, Stephen Paget observed that circulating tumour cells would only "seed" where there was "congenial soil". Since then, attention has focused on explaining the dynamic adhesive and migratory capabilities intrinsic to tumor cells. Meanwhile, the earliest changes occurring within distant tissues that prime the "soil" to receive incoming cancer cells have largely been neglected. Recent work characterizing the importance of bone marrow-derived hematopoietic progenitor cells (HPC) in initiating these early changes has opened new avenues for cancer research and chemotherapeutic targeting (Kaplan, R. N., Riba, R. D., Zacharoulis, S., Bramley, A. H., Vincent, L., Costa, C., et al. (2005). VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature, 438, 820-827). This review discusses the inextricable relationship between bone stromal components, metastasizing cells, and bone marrow-derived hematopoietic cells, and their roles in carcinogenesis and metastasis. Understanding these dynamics may help explain the tissue-specific tropism seen in metastasis. Moreover, exploring the earliest events promoting circulating cancer cells to engraft and establish at secondary sites may expose new targets for diagnostic and therapeutic strategies and reduce the morbidity and mortality from metastatic disease.