Macrophage-dependent tumor cell transendothelial migration is mediated by Notch1/Mena-initiated invadopodium formation.

The process of intravasation involving transendothelial migration is a key step in metastatic spread. How the triple cell complex composed of a macrophage, Mena over-expressing tumor cell and endothelial cell, called the tumor microenvironment of metastasis (TMEM), facilitates tumor cell transendothelial migration is not completely understood. Previous work has shown that the physical contact between a macrophage and tumor cell results in the formation of invadopodia, actin-rich matrix degrading protrusions, important for tumor cell invasion and transendothelial migration and tumor cell dissemination. Herein, we show that the macrophage-induced invadopodium is formed through a Notch1/Mena signaling pathway in the tumor cell upon macrophage contact. This heterotypic tumor cell - macrophage interaction results in the upregulation of Mena through the activation of MENA transcription. Notch1 and Mena expression are required for tumor cell transendothelial migration, a necessary step during intravasation. Inhibition of the Notch signaling pathway blocked macrophage-induced invadopodium formation in vitro and the dissemination of tumor cells from the primary tumor in vivo. Our findings indicate a novel role for Notch1 signaling in the regulation of Mena expression and transendothelial migration and provide mechanistic information essential to the use of therapeutic inhibitors of metastasis.