Growth factor signaling in metastasis: current understanding and future opportunities.

Paget's "seed and soil" hypothesis stated that cancer metastasis requires permissive interactions between tumor cells and secondary organ microenvironments. Many of these "permissive interactions" are now known to be growth factor receptor and ligand interactions by which metastatic tumor cells coopt signaling pathways normally used by host organs. However, although cancer cell signaling pathways responsible for primary cancer growth have been extensively characterized, signaling pathways important in supporting tumor cell-secondary organ heterotypic interactions have been neglected. Even as targeted therapies have shown promise and efficacy in treating myriad primary tumors, metastatic cancer remains incurable. Here, we will discuss several growth factor signaling pathways known to be involved in both general and site-specific metastasis. We will address the complexity in generalizing the role of growth factor signaling in metastasis, as both pro- and antimetastatic roles for the same pathways have been demonstrated depending upon context. We will discuss the limitations of current usage of targeted therapies to pathways known to be dysregulated in metastasis. We propose that the future of cancer metastasis-targeted therapy will lie in better understanding of the interactions between tumor cells and the secondary organ microenvironments that may guide rationally designed personalized combinatorial targeted regimens. We hope to promote research to better understand the complex process of metastasis and ultimately better treatments for the abjectly underserved population of patients with metastatic cancer.