RESEARCH ARTICLE The Outgrowth of Micrometastases Is Enabled by the Formation of Filopodium-like Protrusions

ABSTRACT Disseminated cancer cells that have extravasated into the tissue parenchyma must interact productively with its extracellular matrix components to survive, prolifer-ate, and form macroscopic metastases. The biochemical and cell biologic mechanisms enabling this interaction remain poorly understood. We fi nd that the formation of elongated integrin β1–containing adhesion plaques by cancer cells that have extravasated into the lung parenchyma enables the prolif-eration of these cells via activation of focal adhesion kinase. These plaques originate in and appear only after the formation of fi lopodium-like protrusions (FLP) that harbor integrin β1 along their shafts. The cytoskeleton-regulating proteins Rif and mDia2 contribute critically to the formation of these protru-sions and thereby enable the proliferation of extravasated cancer cells. Hence, the formation of FLPs represents a critical rate-limiting step for the subsequent development of macroscopic metastases. SIGNIFICANCE: Although the mechanisms of metastatic dissemination have begun to be uncovered, those involved in the establishment of extravasated cancer cells in foreign tissue microenvironments remained largely obscure. We have studied the behavior of recently extravasated cancer cells in the lungs and identifi ed a series of cell biologic processes involving the formation of fi lopodium-like pro-trusions and the subsequent development of elongated, mature adhesion plaques, which contribut