Special delivery: microRNA-200-containing extracellular vesicles provide metastatic message to distal tumor cells.

An emerging view is that breast cancer is a systemic disease that utilizes intrinsic and extrinsic tumor cell processes to support both primary tumor growth and metastatic dissemination into distal tissue. Delineation of factors involved in these processes should facilitate a better understanding for both assessing and preventing disease relapse. In this issue of the JCI, Le et al. investigate whether intrinsic properties of metastatic breast cancer cell growth can be regulated through an extrinsic process--contact with tumor cell-derived extracellular vesicles containing microRNAs of the miR-200 family. The authors provide compelling evidence that miR-200s within extracellular vesicles secreted from highly metastatic tumor cells can be internalized by weakly metastatic cells. Thus, internalization and delivery of this metastatic "donor" cell-derived message provide plausible mechanisms by which oncogenic and regulatory factors confer the capability of tumor growth at metastatic lesions. This study provides a strong rationale for detailed assessment of the prognostic and predictive value of circulating extracellular vesicle-bound miR-200s in breast cancer progression and treatment.